The Structure and Mechanical Properties of Hemp Fibers-Reinforced Poly(ε-Caprolactone) Composites Modified by Electron Beam Irradiation

Rafał Malinowski; Aneta Raszkowska-Kaczor; Krzysztof Moraczewski; Wojciech Głuszewski; Volodymyr Krasinskyi; Lauren Wedderburn

doi:10.3390/app11125317

The Structure and Mechanical Properties of Hemp Fibers-Reinforced Poly(ε-Caprolactone) Composites Modified by Electron Beam Irradiation

Malinowski, Rafał;Raszkowska-Kaczor, Aneta;Moraczewski, Krzysztof;Głuszewski, Wojciech;Krasinskyi, Volodymyr;Wedderburn, Lauren 2021-06-08 00:00:00 applied sciences Article The Structure and Mechanical Properties of Hemp Fibers-Reinforced Poly("-Caprolactone) Composites Modiﬁed by Electron Beam Irradiation 1 , 1 2 3 Rafał Malinowski * , Aneta Raszkowska-Kaczor , Krzysztof Moraczewski , Wojciech Głuszewski , 4 1 Volodymyr Krasinskyi and Lauren Wedderburn Łukasiewicz Research Network—Institute for Engineering of Polymer Materials and Dyes, 55 M. Skłodowska-Curie Street, 87-100 Torun, ´ Poland; aneta.kaczor@impib.lukasiewicz.gov.pl (A.R.-K.); lauren.wedderburn@impib.lukasiewicz.gov.pl (L.W.) Institute of Materials Engineering, Kazimierz Wielki University, 30 Chodkiewicza Street, 85-064 Bydgoszcz, Poland; kmm@ukw.edu.pl Institute of Nuclear Chemistry and Technology, 16 Dorodna Street, 03-195 Warsaw, Poland; w.gluszewski@ichtj.waw.pl Department of Chemical Technology of Plastics Processing, Lviv Polytechnic National University, 12 Stepan Bandera Street, 79-013 Lviv, Ukraine; vkrasinsky82@gmail.com * Correspondence: malinowskirafal@gmail.com; Tel.: +48-53-060-0220 Abstract: The need for the development of new biodegradable materials and modiﬁcation of the properties the current ones possess has essentially increased in recent years. The aim of this study Citation: Malinowski, R.; was the comparison of changes occurring in poly("-caprolactone) (PCL) due to its modiﬁcation by Raszkowska-Kaczor, A.; high-energy electron beam derived from a linear electron accelerator, as well as the addition of natural Moraczewski, K.; Głuszewski, W.; ﬁbers in the form of cut hemp ﬁbers. Changes to the ﬁbers structure in the obtained composites and Krasinskyi, V.; Wedderburn, L. The the geometrical surface structure of sample fractures with the use of scanning electron microscopy Structure and Mechanical Properties were investigated. Moreover, the mechanical properties were examined, including tensile strength, of Hemp Fibers-Reinforced elongation at break, ﬂexural modulus and impact strength of the modiﬁed PCL. It was found that Poly("-Caprolactone) Composites PCL, modiﬁed with hemp ﬁbers and/or electron radiation, exhibited enhanced ﬂexural modulus Modiﬁed by Electron Beam but the elongation at break and impact strength decreased. Depending on the electron radiation Irradiation. Appl. Sci. 2021, 11, 5317. dose and the hemp ﬁbers content, tensile strength decreased or increased. It was also found that https://doi.org/10.3390/app11125317 hemp ﬁbers caused greater changes to the mechanical properties of PCL than electron radiation. Academic Editor: The prepared composites exhibited uniform distribution of the dispersed phase in the polymer matrix Alessandro Pegoretti and adequate adhesion at the interface between the two components. Received: 30 April 2021 Keywords: poly("-caprolactone); hemp ﬁbers; composites; irradiation; mechanical properties; Accepted: 4 June 2021 biodegradable polymers Published: 8 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional afﬁl- One of the most important biodegradable polymers, produced from petroleum re- iations. sources, is poly("-caprolactone) (PCL) [1–5]. Its physico-chemical properties are generally beneﬁcial and the material may play a key role in many branches of the economy (both in specialised and mass applications). However, the PCL properties need to be modiﬁed for some applications. It concerns, for example, properties such as its thermal resistance Copyright: © 2021 by the authors. due to low melt temperatures (58–60 C) or its barrier properties for some gases [6–8]. Licensee MDPI, Basel, Switzerland. PCL mechanical properties can also be modiﬁed with the use of mineral or vegetable ﬁllers This article is an open access article that simultaneously lower the price of the polymer. It is important because PCL in its distributed under the terms and pristine form is generally more expensive than conventional polymers and even some conditions of the Creative Commons biodegradable ones such as PLA [9–13] or TPS [14–17]. Attribution (CC BY) license (https:// PCL properties can be modiﬁed by different methods. Various types of PCL polymer creativecommons.org/licenses/by/ blends as well as composites and nanocomposites are most often obtained. The most well- 4.0/). Appl. Sci. 2021, 11, 5317. https://doi.org/10.3390/app11125317 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 5317 2 of 14 known PCL blends are: (i) PCL/TPS [18–20], (ii) PLA/PCL [21–24] or (iii) PCL/PBAT [25]. PCL blends containing non-biodegradable polymers also exist, e.g., PCL/PVC [26] or PCL/PBT [27]. Another signiﬁcant group is PCL composites with natural ﬁllers, includ- ing plant ﬁbers [28–35]. Other known methods of PCL property modiﬁcation refers to processes such as copolymerization, grafting and crosslinking [36–41]. In this study, we proposed a combination of two methods to modify PCL proper- ties. The ﬁrst method refers to obtaining PCL composites with hemp ﬁbers (HF) and the second method is related to irradiation of such obtained composites with the use of elec- tron radiation. Our literature review revealed only a few studies on PCL/HF composites. For example, the experimental work carried out by Dhakal et al. [42] investigated the reinforcement efﬁciency of hemp ﬁbers with different ﬁber aspect ratios on PCL-based biocomposites and compared the inﬂuence of these aspect ratios on moisture absorption behavior along with the tensile, ﬂexural and impact properties of hemp ﬁber/PCL biocom- posites. In another study by this author [43], the effects of variable mean hemp ﬁber content on nano-mechanical (hardness, modulus, elasticity and plasticity), surface and thermal properties of hemp ﬁber/polycaprolactone biocomposites were studied. Dhakal et al. [44] presented some test results of PCL/HF composites and compared them to the results of similar composites, i.e., those containing date palm ﬁbers. Studies on PCL/HF com- posites have also been carried out by Beaugrand et al. [45], who examined, among other parameters, the effect of processing conditions on deﬁbration and mechanical properties. Many more studies on PCL composites with other types of natural ﬁbers have been con- ducted. Examples of such studies are presented in a review publication by Wahit et al. [46]. The authors of this publication concluded that green composites with enhanced mechanical properties can be successfully prepared. They also showed that natural ﬁbers reduce the price of these composites and expand their application areas, which in turn could attract public interest. Key studies on the modiﬁcation of PCL properties also concern its radiation modiﬁca- tion, especially in cases of modiﬁed PCL to be applied in medicine or tissue engineering. Studies in this area were conducted mainly with the use of gamma radiation or electron radiation [47–49]. These studies indicated that PCL, when subjected solely to irradiation, does not form a signiﬁcant amount of gel fraction and the degradation processes are not dominant at the same periods of time. Good crosslinking of PCL may be successfully carried out by using crosslinking agents such as triallyl isocyanurate (TAIC) [49–51]. Then, with the appropriately selected irradiation conditions, PCL can be completely crosslinked. As a consequence of this, it will not ﬂow and its degree of crosslinking will be more than 90%. Literature analysis revealed that, there are no known studies on the combined effect of both electron radiation and natural hemp ﬁbers on PCL properties, in particular, mechanical ones. Admittedly, some similar studies can be found in the literature, however, they concern either other types of natural ﬁbers or different polymers. For example, Rytlewski et al. investigated the effect of electron radiation on the properties of PLA [52] or PCL [53] containing 20% ﬂax ﬁbers with the addition of 1% to 3% TAIC. Understanding the combined effect of electron radiation and hemp ﬁbers on the structure and mechanical properties of PCL is interesting from both cognitive and utilisation aspects. From the cognitive aspect it is important to know the relationship between the radiation dose, the amount of ﬁbers in the polymer matrix and the properties of the obtained materials. From the utilisation aspect, modiﬁcation of PCL with hemp ﬁbers and electron irradiation has been carried out to obtain new materials susceptible to biodegradation. Furthermore, the use of hemp ﬁbers in our study may assist in reducing the adverse effects of PCL’s partial crosslinking and on its biodegradability. This inspired the authors of the present article to undertake investigations aimed at the comparison of the structure and mechanical properties of PCL/HF composites modiﬁed by electron radiation with doses in the range of 10–90 kGy. The comparison of changes in (i) the structure of the ﬁbers extracted from obtained composites; (ii) the surface geometrical Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 14 Appl. Sci. 2021, 11, 5317 3 of 14 changes in (i) the structure of the fibers extracted from obtained composites; (ii) the sur‐ face geometrical structure of sample fractures; (iii) the tensile strength; (iv) the elongation structure of sample fractures; (iii) the tensile strength; (iv) the elongation at break; (v) the at break; (v) the flexural modulus; and (vi) the impact strength have been presented in ﬂexural modulus; and (vi) the impact strength have been presented in this paper. this paper. 2. Experimental 2. Experimental 2.1. Materials 2.1. Materials The following materials were used in this study: The following materials were used in this study: Poly("-caprolactone) (PCL), type CAPA6800 with a measured melt ﬂow rate (MFR) of  Poly(ε‐caprolactone) (PCL), type CAPA6800 with a measured melt flow rate (MFR) 5.1 g/10 min (2.16 kg, 190 C), density of 1.15 g/cm and average molecular weight of of 5.1 g/10 min (2.16 kg, 190 °C), density of 1.15 g/cm and average molecular weight ca. 82 kDa was supplied by Ingevity UK Ltd. (Warrington, UK); of ca. 82 kDa was supplied by Ingevity UK Ltd. (Warrington, UK); Natural hemp ﬁbers (HF), at 6 mm long, were purchased from the Institute of Natural  Natural hemp fibers (HF), at 6 mm long, were purchased from the Institute of Nat‐ Fibers and Medicinal Plants in Poznan (Poland). The cut hemp ﬁbers’ structure is ural Fibers and Medicinal Plants in Poznan (Poland). The cut hemp fibers’ structure shown in Figure 1 were applied as the PCL ﬁller. is shown in Figure 1 were applied as the PCL filler. Figure 1. SEM image of hemp fibers (HF) structure. Figure 1. SEM image of hemp ﬁbers (HF) structure. 2.2. Apparatus 2.2. Apparatus The following devices were utilised to prepare and investigate the irradiated PCL The following devices were utilised to prepare and investigate the irradiated PCL composites: composites: Co-rotating twin screw extruder of type BTSK 20/40D (Bühler, Germany), equipped with  Co‐rotating twin screw extruder of type BTSK 20/40D (Bühler, Germany), equipped 20-mm diameter screws and L/D ratio of 40 intended to produce granulated with 20‐mm diameter screws and L/D ratio of 40 intended to produce granulated PCL composites; PCL composites; Screw injection molding machine of type Battenfeld Plus 35/75 (Battenfeld GmbH,  Screw injection molding machine of type Battenfeld Plus 35/75 (Battenfeld GmbH, Solingen, Germany), equipped with 22-mm diameter screw and L/D ratio of 17 Solingen, Germany), equipped with 22‐mm diameter screw and L/D ratio of 17 de‐ designed to produce standard dumbbell-shaped and bar-shaped specimens; signed to produce standard dumbbell‐shaped and bar‐shaped specimens; Linear accelerator of type Elektronika 10/10 (Institute of Nuclear Chemistry and  Linear accelerator of type Elektronika 10/10 (Institute of Nuclear Chemistry and Technology in Warsaw, Poland) with energy of electrons 10 MeV and electron beam Technology in Warsaw, Poland) with energy of electrons 10 MeV and electron beam power 10 kW for irradiation of PCL composites; power 10 kW for irradiation of PCL composites; Shaker of type Unimax 1010 (Heidoplh, Germany) was used to extract ﬁbers from  Shaker of type Unimax 1010 (Heidoplh, Germany) was used to extract fibers from obtained composites; obtained composites; Scanning electron microscope (SEM) of type Hitachi SU8010 (Hitachi, Japan) was  Scanning electron microscope (SEM) of type Hitachi SU8010 (Hitachi, Japan) was used to examine the structure of HF as well as surface geometrical structure of used to examine the structure of HF as well as surface geometrical structure of sample fractures; sample fractures; Tensile testing machine of type TIRAtest 27025 (TIRA Maschinenbau GmbH, Schalkau,  Tensile testing machine of type TIRAtest 27025 (TIRA Maschinenbau GmbH, Germany) was used to examine mechanical properties under static tension and static Schalkau, Germany) was used to examine mechanical properties under static ten‐ three-point bending; sion and static three‐point bending; Pendulum Impact Tester of type IMPats-15 (ATS FAAR, Novegro-Tregarezzo, Italy)  Pendulum Impact Tester of type IMPats‐15 (ATS FAAR, Novegro‐Tregarezzo, Italy) was intended for the determination of Charpy impact strength. was intended for the determination of Charpy impact strength. 2.3. Methods 2.3. Methods Extrusion of the pristine PCL and granulated PCL composites was performed at the following temperatures of particular cylinder zones of the co-rotating twin screw extruder: Appl. Sci. 2021, 11, 5317 4 of 14 140 C, 150 C, 160 C and 165 C. The temperature of the extrusion die head was 160 C and the screw rotational speed was constant at 250 min . The screws were in a speciﬁc conﬁguration that facilitated adequate dispersion and distribution of HF in the PCL matrix. Moreover, the screws consisted of elements providing intensive HF grinding to obtain ﬁne fractions. Prior to extrusion, HF was dried at 110 C for 24 h. PCL was not subjected to drying. Free degassing of any possible released residual moisture was applied at the screws with lengths of L/D = 22 and L/D = 32, respectively. PCL and HF were introduced into the extruder from volume feeders in such proportions that PCL composites with HF contents equal to 2.5, 5, 10 and 20 wt% were obtained. The percentages of ﬁbers added were chosen based on our previous studies [52,53]. The extrudate was cooled in a water bath at 15 C and then dried on a conveyor belt in a stream of dry air and ﬁnally granulated. The obtained composites in the form of cylindrical granules (height 5 mm and base diameter 3 mm) were dried at 50 C for 24 h to eliminate residual moisture. The prepared samples of the granulated composites were denoted as CF2.5, CF5, CF10 and CF20, where C denotes poly("-caprolactone), F denotes natural hemp ﬁbers and the numbers represent the ﬁbers percentage content. A reference sample, made of the pristine PCL, was also prepared and denoted as C. During extrusion, basic parameters that include the following were recorded: (i) extruder motor torque (M), (ii) energy consumption (E) by the extruder drive system, (iii) temperature (T) of the material being processed that is measured in the extruder die-head and (iv) pressure (P) of the material being processed that is measured in the extruder die-head. The dumbbell-shaped and bar-shaped specimens were prepared according to a relevant standard (PN-EN ISO 527-2:2012) using a screw injection molding machine (Battenfeld Plus 35/75). The temperatures of the plasticising zones I and II of the barrel were 130 C and 140 C, respectively, and the temperature of the injection molding die head was 140 C. The temperature of the injection mold was 23 C and the injection pressure was 157–163 MPa dependent on the kind of composite. The screw rotational speed was constant at 150 min . Sample irradiation was carried out with a linear accelerator type Elektronika 10/10 producing scanned beam of electrons with energy of 10 MeV and of 10 kW power with the possibility of changing the dose rate by controlling the speed of the conveyer. The acceler- ator is located at the Institute of Nuclear Chemistry and Technology in Warsaw, Poland. The samples (granulated samples and injection-moulded pieces) were irradiated with doses of 10, 20, 40, 60 or 90 kGy. Maximum single doses were 20 kGy. This limitation was caused by an increase in temperature of the irradiated material that was equal to ca. 4–7 C for single doses of 10 kGy. Large single doses cause intensive heating of the material, therefore some additional structural changes may occur. Thus, samples were irradiated with lower doses multiple times. During the irradiation procedure, all the granulated samples and moulded pieces were placed in aluminium containers in single layers of the thickness of up to 2 cm. The containers were put on the conveyor belt moving at the speed of 0.3–1.2 m/min under the accelerator scanner. The actual speed was related to the radiation dose absorbed by the polymer material being modiﬁed. The HF structure in the obtained composites was determined by SEM analysis after dissolving the composites in methylene chloride for 24 h at room temperature and then ﬁltering the ﬁbers fraction and ﬁnally drying them at 50 C for 24 h. The HF structure (before and after extrusion) as well as the surface geometrical struc- ture of the sample fractures, including distribution of the dispersed HF phase, were evalu- ated by SEM using the secondary electron (SE) detector and accelerating voltage of 10 kV. The fractures were made in liquid nitrogen. A 5-nm thick gold layer was sputtered on all of the samples to be analysed by SEM. For that purpose, a cathode sputtering apparatus was used, which was equipped with a coating thickness gauge based on a quartz crystal of varying conductivity. Tensile strength ( ) and elongation at break (" ) were evaluated according to the M B PN-EN ISO 527-1:2012 standard by using the extension rate of 10.0 mm/min. Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 14 Appl. Sci. 2021, 11, 5317 5 of 14 Tensile strength (σM) and elongation at break (εB) were evaluated according to the PN‐EN ISO 527‐1:2012 standard by using the extension rate of 10.0 mm/min. Flexural modulus (Mf) was measured by the three‐point bend test at a bending de‐ flection rate of 5.0 mm/min. The measurements were carried out in accordance to the Flexural modulus (M ) was measured by the three-point bend test at a bending PN‐EN ISO 178:2011 standard. deﬂection rate of 5.0 mm/min. The measurements were carried out in accordance to the Impact strength (acN) was evaluated according to the PN‐EN ISO 179‐1:2010 stand‐ PN-EN ISO 178:2011 standard. ard. Impact strength (a ) was evaluated according to the PN-EN ISO 179-1:2010 standard. cN 3. Results and Discussion 3. Results and Discussion 3.1. HF Structure in Obtained Composites 3.1. HF Structure in Obtained Composites The structure of the HF taken from obtained composites (Figure 2) is signiﬁcantly The structure of the HF taken from obtained composites (Figure 2) is significantly different compared to that shown in Figure 1. First of all, the ﬁbers in the obtained different compared to that shown in Figure 1. First of all, the fibers in the obtained composites possess a signiﬁcantly reduced length and diameter (even 10 times their length). composites possess a significantly reduced length and diameter (even 10 times their This is due to two reasons. Firstly, the lengths of the ﬁbers were shortened by the use length). This is due to two reasons. Firstly, the lengths of the fibers were shortened by the of a plasticising system containing elements that induced intense mixing and shearing, use of a plasticising system containing elements that induced intense mixing and shear‐ as well as retracting elements. These elements divided the ﬁbers into smaller particles, ing, as well as retracting elements. These elements divided the fibers into smaller parti‐ including single macroﬁbrils and, as a result, a ﬁne fraction with high dispersion in the cles, including single macrofibrils and, as a result, a fine fraction with high dispersion in polymer matrix was obtained. Secondly, some of the ﬁbers were additionally shortened in the polymer matrix was obtained. Secondly, some of the fibers were additionally short‐ the granulation process. ened in the granulation process. Figure 2. HF structure after extraction from PCL/HF composites ((A) CF2.5; (B) CF5; (C) CF10; (D) Figure 2. HF structure after extraction from PCL/HF composites ((A) CF2.5; (B) CF5; (C) CF10; CF20). (D) CF20). The fibers in the CF2.5 composite (Figure 2A) have a larger diameter and a more The ﬁbers in the CF2.5 composite (Figure 2A) have a larger diameter and a more uniform structure compared to the fibers in the other composites. Here and there, a single uniform structure compared to the ﬁbers in the other composites. Here and there, a single fiber bundle can be seen. As the composite fiber content increases, their structure ﬁber bundle can be seen. As the composite ﬁber content increases, their structure changes. changes. At the highest percentage filling, there are a few coarse fibers with intact struc‐ At the highest percentage filling, there are a few coarse fibers with intact structure and the ture and the dominant fraction is composed of single fiber bundles or individual macro‐ dominant fraction is composed of single fiber bundles or individual macrofibrils (Figure 2D). fibrils (Figure 2D). The change in the ﬁber structure in individual composites is related to the changing processing The change parameters, in the which fiber str inucture turn ar ine ind a derivative ividual comp of the osites incr iseasing related ﬁber to the content changing in processing parameters, which in turn are a derivative of the increasing fiber content in the plasticised polymer mass. As the content of HF in the composite increases, all the measur the pla ed sticised parameters polymer of the maextr ss. As usion the pr co ocess, ntent i.e., of HF M, in E, T the and composite P also incr incre easeas(T es, able all 1the ). The mea incr sured ease parameters in these parameters, of the extrusion which ar process, e indirectly i.e., rM, elated E, Tto and the P incr also ease incre in a shear se (Tfor able ces, 1). result The incre in changes ase in these in the parameters, structure of whic the ﬁbers h are as indir shown ectlyin related Figur eto 2 .the Thus, increa these str in uctur sheear of the obtained ﬁbers may have a signiﬁcant impact on the mechanical properties of the forces, result in changes in the structure of the fibers as shown in Figure 2. Thus, the irradiated PCL/HF composites, which are discussed in Section 3.3. Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 14 Appl. Sci. 2021, 11, 5317 6 of 14 structure of the obtained fibers may have a significant impact on the mechanical proper‐ ties of the irradiated PCL/HF composites, which are discussed in Section 3.3. Table 1. The extrusion process parameters for PCL and its composites with HF. Table 1. The extrusion process parameters for PCL and its composites with HF. Sample M (Nm) E (kW) T ( C) P (MPa) Sample M (Nm) E (kW) T (°C) P (MPa) C 16–17 0.95–0.97 165 10–11 C 16–17 0.95–0.97 165 10–11 CF2.5 18–20 0.98–1.02 166 12–14 CF2.5 18–20 0.98–1.02 166 12–14 CF5 20–21 1.01–1.05 168 14–17 CF5 20–21 1.01–1.05 168 14–17 CF10 21–22 1.04–1.08 170 16–19 CF10 21–22 1.04–1.08 170 16–19 CF20 22–23 1.06–1.11 171 20–24 CF20 22–23 1.06–1.11 171 20–24 3.2. Composites Structure 3.2. Composites Structure The structural analysis results of two selected composites (CF2.5 and CF20) by scan- The structural analysis results of two selected composites (CF2.5 and CF20) by ning electron microscopy (SEM) are shown in Figure 3. scanning electron microscopy (SEM) are shown in Figure 3. Figure 3. Structure of selected fracture surfaces of bar‐shaped specimens obtained from Figure 3. Structure of selected fracture surfaces of bar-shaped specimens obtained from non- non‐irradiated and irradiated CF2.5 (left) and CF20 (right) composites. irradiated and irradiated CF2.5 (left) and CF20 (right) composites. Based on the images of the structures shown in Figure 3, it can be concluded that all fractures are brittle fractures. Moreover, the nature of these breakthroughs does not change Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 14 Based on the images of the structures shown in Figure 3, it can be concluded that all fractures are brittle fractures. Moreover, the nature of these breakthroughs does not change with an increasing radiation dose. In general, all images obtained for composites with the same degree of fiber filling, i.e., 2.5% or 20%, have comparable structures. This proves that an increase in the radiation dose does not cause significant changes in the fracture structure of composites with a specific HF content. Subtle changes can only be found when assessing the adhesion at the polymer–fiber interface. With an increasing radiation dose, the distance between the phases slightly decreases, which is especially visible in the case of sample CF2.5. On one hand, it may be caused by an increase in thermal expansion of the irradiated material, mainly of the polymer phase, as a result of Appl. Sci. 2021, 11, 5317 7 of 14 the temperature increase. On the other hand, it may be related to lower secondary shrinkage due to the changes in the structure of macromolecules after irradiation. Figure 3 also shows that the fibers in composites with a higher percentage filling are finer, which with an increasing radiation dose. In general, all images obtained for composites with the is consistent with the data shown in Figure 2. The craters seen in some photos may come same degree of ﬁber ﬁlling, i.e., 2.5% or 20%, have comparable structures. This proves that an increase in the radiation dose does not cause signiﬁcant changes in the fracture structure from the extraction of the fibers from the matrix during the fractures. These craters do not of composites with a speciﬁc HF content. Subtle changes can only be found when assessing disadvantage the material, but are merely a replica of the other half of the sample. The the adhesion at the polymer–ﬁber interface. With an increasing radiation dose, the distance surface structures of the other composites (CF5 and CF10) are very similar and therefore between the phases slightly decreases, which is especially visible in the case of sample CF2.5. On one hand, it may be caused by an increase in thermal expansion of the irradiated they are not shown here. material, mainly of the polymer phase, as a result of the temperature increase. On the other hand, it may be related to lower secondary shrinkage due to the changes in the structure of 3.3. Mechanical Properties macromolecules after irradiation. Figure 3 also shows that the ﬁbers in composites with a higher percentage ﬁlling are ﬁner, which is consistent with the data shown in Figure 2. The mechanical properties test results of non‐modified PCL and PCL/HF compo‐ The craters seen in some photos may come from the extraction of the ﬁbers from the matrix sites, as well as irradiated PCL and irradiated PCL/HF composites are illustrated in 3D during the fractures. These craters do not disadvantage the material, but are merely a three replica‐dimen of the other sional half of tgr hea sample. phs The (Fig surface ures str4–7 uctur)es ofand the other summ composites arized (CF5 in Table 2. The and CF10) are very similar and therefore they are not shown here. three‐dimensional graphs (3D) were made using STATISTICA version 10.0 software (StatSoft). 3.3. Mechanical Properties The data from Figure 4 indicates that the tensile strength (σM) for PCL significantly The mechanical properties test results of non-modiﬁed PCL and PCL/HF composites, as well as irradiated PCL and irradiated PCL/HF composites are illustrated in 3D three- decreases when the dose of electron radiation is 60 or 90 kGy. The decrease is 3% and dimensional graphs (Figures 4–7) and summarized in Table 2. The three-dimensional 13%, respectively. Smaller doses of irradiation do not affect the σM of that polymer. graphs (3D) were made using STATISTICA version 10.0 software (StatSoft). Figure 4. Tensile strength results. Figure 4. Tensile strength results. Other dependencies can be observed in the case of non‐irradiated composites con‐ taining HF. In their case, the σM initially decreases with the rising HF content and then increases. In Figure 4 there is a clear extreme of strength, the lowest value of which can be observed in the 10% HF content composite. When compared to pristine PCL, filling composites with 10% HF causes a decrease in σM by 27%. Larger amounts of HF do not cause a further decrease in σM, but they are the reason for its increase of 20%, compared to Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 14 Appl. Sci. 2021, 11, 5317 8 of 14 Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14 Figure 5. Elongation at break results. Figure 5. Elongation at break results. Figure 5 shows that both types of modification applied (irradiation and/or addition of HF) significantly reduces the εB value. Elongation decreases both as the electron radi‐ ation dose increases and when HF content increases and especially when both of these factors occur simultaneously. Elongation is least influenced by electron radiation, which reduces its value by a maximum of 17% for samples irradiated with 90 kGy. The decrease in elongation at break following electron radiation may result from elongation of the macromolecules due to mutual linking at chain ends or the formation of branched structures in notable amounts [50]. HF induces a significantly higher effect on the εB value, where a maximum decrease of 95% is noted. The highest influence on the εB value of the studied polymer is the combined action of electron radiation and HF, which re‐ duces εB by as much as 99%. There is, at least, a 100 times decrease in the εB value in comparison to non‐modified PCL and this occurs in composites with 20% HF content and that were irradiated with doses of 40, 60 or 90 kGy. The data in Figure 6 indicates that electron radiation in a dose of 90 kGy causes a growth in PCL’s flexural modulus (Mf) value by up to 13%. Smaller doses cause similar increases where the growth is ca. 11–12%. The flexural modulus of PCL/HF composites increases as the HF content increases. This rate of growth can be considered an exponen‐ Figure 6. Flexural modulus results. Figure 6. Flexural modulus results. tial function, i.e., at low HF concentration (2.5 or 5%) the modulus increases slightly. However, in the case of a filling with more than 10%, a significant growth in its value is The impact strength (acN) of PCL shown in Figure 7 decreases significantly as the observed. The Mf value of the composite containing 20% HF is over two times greater radiation dose increases. The maximum decrease in the PCL’s impact strength, 56%, oc‐ than the modulus of pristine PCL. This knowledge has significant importance on the curs upon irradiation of the polymer with 90 kGy. This may result from the formation of eventual suitable range of application for these types of composites. a branched or partially crosslinked structure of PCL macromolecules that hinders their Irradiation of PCL/HF composites affects their Mf values. When the radiation dose is conformational changes. This, in turn, limits the amount of energy absorbed by the irra‐ 10 kGy, a significant increase in the Mf value is observed. Higher doses do not have a diated PCL. Possible limitations for the rotation of macromolecular fragments can also considerable influence on the further growth of modulus values. Differences in Mf values affect other mechanical properties, including Mf or εB, which were presented earlier. of composites irradiated with doses of 20, 40 or 60 kGy do not differ notably from each other (Table 2). An exception to this is the case of when the CF20 composite is irradiated with 90 kGy and its Mf value decreases slightly. This may be due to degradation of both composite phases and, in particular, the HF phase. We can assume this because no de‐ crease in the Mf value was observed for lower fiber contents. Figure 7. Impact strength results. The PCL/HF composites are also characterised by significantly reduced impact strength. It is also worth noting that the highest impact strength of non‐irradiated com‐ posite (sample CF2.5) is considerably lower than the lowest impact strength value of ir‐ radiated PCL. By introducing the amount of just 2.5% of fibers (sample CF2.5) into the polymer matrix causes a reduction in impact strength by 60% and the addition of 20% fibers (sample CF20) reduces its value by 78%. Therefore, it can be concluded that the impact strength of PCL is more significantly influenced by even a minimal amount (2.5 Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14 Figure 6. Flexural modulus results. The impact strength (acN) of PCL shown in Figure 7 decreases significantly as the radiation dose increases. The maximum decrease in the PCL’s impact strength, 56%, oc‐ curs upon irradiation of the polymer with 90 kGy. This may result from the formation of a branched or partially crosslinked structure of PCL macromolecules that hinders their conformational changes. This, in turn, limits the amount of energy absorbed by the irra‐ Appl. Sci. 2021, 11, 5317 9 of 14 diated PCL. Possible limitations for the rotation of macromolecular fragments can also affect other mechanical properties, including Mf or εB, which were presented earlier. Figure 7. Impact strength results. Figure 7. Impact strength results. Table 2. Mechanical properties of pristine PCL, PCL/HF composites, irradiated PCL and irradiated The PCL/HF composites are also characterised by significantly reduced impact PCL/HF composites. strength. It is also worth noting that the highest impact strength of non‐irradiated com‐ posite (sample CF2.5) is considerably lower than the lowest impact strength value of ir‐ Sample Dose (kGy) (MPa) " (%) M (MPa) a (kJ/m ) M B cN radiated PCL. By introducing the amount of just 2.5% of fibers (sample CF2.5) into the C 0 27.0 0.9 1091 24 351 26 39.4 0.2 polymer matrix causes a reduction in impact strength by 60% and the addition of 20% CF2.5 0 27.6 0.5 1105 52 362 35 15.7 0.1 fibers CF5 (sample CF20 0) reduces23.4 its value 0.8 by 78% 910 . 9Therefore, 395 it 69 can be 12.9 concluded 0.3 that the CF10 0 19.7 0.4 518 71 635 68 11.4 0.1 impact strength of PCL is more significantly influenced by even a minimal amount (2.5 CF20 0 23.6 0.3 48 8 880 64 8.8 0.1 C 10 27.7 0.8 1128 24 381 21 33.8 0.2 CF2.5 10 26.2 0.4 1077 43 396 42 12.0 0.2 CF5 10 23.6 0.3 836 25 487 38 10.7 0.2 CF10 10 20.0 0.1 527 45 635 68 8.7 0.1 CF20 10 23.4 0.9 28 1 1158 68 7.1 0.3 C 20 27.3 0.9 1096 26 392 21 29.5 0.1 CF2.5 20 28.1 0.4 1064 70 435 32 10.4 0.1 CF5 20 24.2 0.7 820 21 499 41 9.5 0.2 CF10 20 20.0 1.1 364 41 639 78 7.8 0.1 CF20 20 23.3 0.8 26 2 1220 22 6.5 0.3 C 40 27.7 1.1 1076 43 399 31 25.2 0.1 CF2.5 40 24.8 1.2 966 76 433 12 9.0 0.1 CF5 40 23.1 0.6 811 7 502 29 8.3 0.2 CF10 40 20.6 0.5 194 70 640 73 6.8 0.2 CF20 40 23.8 1.2 9 4 1080 72 5.6 0.2 C 60 26.1 1.1 1073 74 394 16 21.0 0.1 CF2.5 60 25.8 0.7 951 71 457 20 7.9 0.1 CF5 60 20.4 0.3 648 35 507 55 7.4 0.2 CF10 60 21.4 0.7 141 49 636 61 6.2 0.1 CF20 60 24.3 0.4 6 1 1146 81 5.0 0.1 C 90 23.6 1.3 903 65 404 22 17.3 0.3 CF2.5 90 24.5 1.9 954 38 419 30 6.8 0.1 CF5 90 19.8 0.1 633 26 498 49 6.4 0.3 CF10 90 22.0 0.3 41 25 687 52 5.3 0.1 CF20 90 24.6 0.4 5 1 951 72 4.6 0.1 Appl. Sci. 2021, 11, 5317 10 of 14 The data from Figure 4 indicates that the tensile strength ( ) for PCL signiﬁcantly decreases when the dose of electron radiation is 60 or 90 kGy. The decrease is 3% and 13%, respectively. Smaller doses of irradiation do not affect the of that polymer. Other dependencies can be observed in the case of non-irradiated composites con- taining HF. In their case, the initially decreases with the rising HF content and then increases. In Figure 4 there is a clear extreme of strength, the lowest value of which can be observed in the 10% HF content composite. When compared to pristine PCL, ﬁlling com- posites with 10% HF causes a decrease in by 27%. Larger amounts of HF do not cause a further decrease in , but they are the reason for its increase of 20%, compared to the CF10 sample. However, the value of the non-irradiated CF20 composite is not larger than the value of any PCL sample irradiated with doses ranging from 10 to 90 kGy and it is at most equal to the strength of the PCL irradiated with the dose of 90 kGy. Similar observations are seen in the case of irradiated PCL/HF composites, i.e., for a given dose, as the content of HF in polymer matrix increases, the initially decreases and then increases. However, in this case, the minimum value of irradiated composites depends on the irradiation dose. For doses of 10, 20 and 40 kGy, the minimum occurs in composites with 10% HF ﬁlling. In the case of doses of 60 and 90 kGy, the minimum occurs in composites containing 5% HF. Moreover, in composites containing 10 or 20% HF, an increase in with an increasing dose is observed, which is not observed in composites with smaller ﬁber content. Elongation at break (" ) results for the same samples analysed in tests are presented B M in Figure 5. Figure 5 shows that both types of modiﬁcation applied (irradiation and/or addition of HF) signiﬁcantly reduces the " value. Elongation decreases both as the electron radiation dose increases and when HF content increases and especially when both of these factors occur simultaneously. Elongation is least inﬂuenced by electron radiation, which reduces its value by a maximum of 17% for samples irradiated with 90 kGy. The decrease in elongation at break following electron radiation may result from elongation of the macromolecules due to mutual linking at chain ends or the formation of branched structures in notable amounts [50]. HF induces a signiﬁcantly higher effect on the " value, where a maximum decrease of 95% is noted. The highest inﬂuence on the " value of the studied polymer is the combined action of electron radiation and HF, which reduces " by as much as 99%. There is, at least, a 100 times decrease in the " value in comparison to non-modiﬁed PCL and this occurs in composites with 20% HF content and that were irradiated with doses of 40, 60 or 90 kGy. The data in Figure 6 indicates that electron radiation in a dose of 90 kGy causes a growth in PCL’s ﬂexural modulus (M ) value by up to 13%. Smaller doses cause similar increases where the growth is ca. 11–12%. The ﬂexural modulus of PCL/HF composites increases as the HF content increases. This rate of growth can be considered an expo- nential function, i.e., at low HF concentration (2.5 or 5%) the modulus increases slightly. However, in the case of a ﬁlling with more than 10%, a signiﬁcant growth in its value is observed. The M value of the composite containing 20% HF is over two times greater than the modulus of pristine PCL. This knowledge has signiﬁcant importance on the eventual suitable range of application for these types of composites. Irradiation of PCL/HF composites affects their M values. When the radiation dose is 10 kGy, a signiﬁcant increase in the M value is observed. Higher doses do not have a considerable inﬂuence on the further growth of modulus values. Differences in M values of composites irradiated with doses of 20, 40 or 60 kGy do not differ notably from each other (Table 2). An exception to this is the case of when the CF20 composite is irradiated with 90 kGy and its M value decreases slightly. This may be due to degradation of both composite phases and, in particular, the HF phase. We can assume this because no decrease in the M value was observed for lower ﬁber contents. The impact strength (a ) of PCL shown in Figure 7 decreases significantly as the radia- cN tion dose increases. The maximum decrease in the PCL’s impact strength, 56%, occurs upon Appl. Sci. 2021, 11, 5317 11 of 14 irradiation of the polymer with 90 kGy. This may result from the formation of a branched or partially crosslinked structure of PCL macromolecules that hinders their conforma- tional changes. This, in turn, limits the amount of energy absorbed by the irradiated PCL. Possible limitations for the rotation of macromolecular fragments can also affect other mechanical properties, including M or " , which were presented earlier. f B The PCL/HF composites are also characterised by signiﬁcantly reduced impact strength. It is also worth noting that the highest impact strength of non-irradiated compos- ite (sample CF2.5) is considerably lower than the lowest impact strength value of irradiated PCL. By introducing the amount of just 2.5% of ﬁbers (sample CF2.5) into the polymer matrix causes a reduction in impact strength by 60% and the addition of 20% ﬁbers (sample CF20) reduces its value by 78%. Therefore, it can be concluded that the impact strength of PCL is more signiﬁcantly inﬂuenced by even a minimal amount (2.5 wt%) of ﬁbers introduced into the polymer matrix than by electron radiation with the highest applied dose (90 kGy). In the case of irradiated PCL/HF composites, further decreases in impact strength are observed. However, while present, the changes in impact strength of the composite after ir- radiation are less signiﬁcant. For example, the impact strength of the CF2.5 composite after irradiation with the maximum dose (90 kGy) decreases by a maximum of 83% compared to the impact strength of pristine PCL. Thus, irradiation decreases the impact strength of PCL by an additional 23% compared to the non-irradiated CF2.5 composite. In turn, the impact strength of the composite containing the maximum amount of ﬁbers (sample CF20) and irradiated with the maximum dose (90 kGy) decreases by a maximum of 88% compared to the impact strength of pristine PCL. For this composite (CF20), irradiation de- creases the impact strength of PCL by an additional 10% compared to the non-irradiated CF20 composite. The data in Table 2 indicates that large relative changes occur in the impact strength values for individual composites after irradiation. For example, doses of 10 and 90 kGy led to a decrease in the impact strength of the CF2.5 composite by 24% and 57%, respectively. Moreover, an increase in the HF weight fraction in the PCL matrix causes a decrease in the relative differences in impact strength for composites after irradiation. For example, the doses of 10 and 90 kGy decreases the impact strength of CF20 composite by 19% and 48%, respectively. The signiﬁcant reduction in the impact strength of non-irradiated and irradiated PCL/HF composites results mainly from the stiffening of the material and the perpendicular orientation of the natural ﬁbers (Figure 3) in relation to the impact force applied by the hammer during strength testing. 4. Conclusions The structure and mechanical properties of the investigated materials depended on three main factors: (i) processing conditions, (ii) qualitative and quantitative contribu- tion of dispersed phase and (iii) the magnitude of the electron radiation dose. The ﬁnal ﬁbers’ structures in the obtained composites were closely linked to processing condi- tions, which also depended on the percentage ﬁlling of these ﬁbers in the polymer matrix. Electron radiation did not signiﬁcantly change the structure of the prepared composites. Only small changes could be observed in the case of adhesion between the dispersed phase and the polymer matrix. The structure of the composites, however, depended signiﬁcantly on the HF content. Mechanical properties of the study’s composites were greatly inﬂuenced by electron radiation. Furthermore, as the radiation dose increased, the values of impact strength, elongation at break and in part tensile strength decreased while the ﬂexural modulus increased. An especially interesting synergistic effect on PCL properties was observed and was related to the combined action of electron radiation and natural ﬁbers addition. For example, depending on the used dose and ﬁbers amount, elongation at break can be reduced from 1% to 99%, while the ﬂexural modulus can increase from 1% to 248%. Hemp ﬁbers may be used as an additive in the PCL matrix to manufacture biodegrad- able materials. Ecological advantages and the low price of these ﬁbers compared to the Appl. Sci. 2021, 11, 5317 12 of 14 price of PCL highlight a direction for further research in this ﬁeld. Moreover, the ap- plied ﬁbers should additionally accelerate the biodegradation process of such materials. This statement refers, speciﬁcally, to radiation-modiﬁed biodegradable polymers that can be partially crosslinked and, as consequence of this, the biodegradation process may be slowed down. Thus, hemp ﬁbers should lessen the adverse effect that crosslinking has on biodegradability. In this case, further studies determining the combined action of electron radiation and natural ﬁbers on the biodegradation of these materials may be interesting. Furthermore, the post-radiation oxidation processes of both the matrix and HF as well as the protective effect of lignin in the obtained materials may also be the subject of further research. The radiation-modiﬁed composites presented in this article can be applied pri- marily for the production of biodegradable packaging, including rigid and ﬂexible ones, depending on the applied radiation dose and the ﬁbers content. Author Contributions: Conceptualization, R.M. and V.K.; methodology, R.M., K.M. and W.G.; sample preparation, R.M. and W.G.; formal analysis, A.R.-K. and V.K.; investigation, R.M., A.R.-K. and K.M.; data curation, R.M.; writing—original draft preparation, R.M. and A.R.-K.; writing—review and editing, R.M. and L.W. All authors have read and agreed to the published version of the manuscript. Funding: The project has been ﬁnanced by statutory funds (project no. 110034). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All experimental data to support the ﬁndings of this study are available upon request by contacting the corresponding author. Acknowledgments: The authors of this work would like to express their special thanks to the Institute of Natural Fibers and Medicinal Plants in Poznan (Poland) for the preparation of hemp ﬁbers. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. 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ISSN: 2076-3417
DOI: 10.3390/app11125317
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Abstract

applied sciences Article The Structure and Mechanical Properties of Hemp Fibers-Reinforced Poly("-Caprolactone) Composites Modiﬁed by Electron Beam Irradiation 1 , 1 2 3 Rafał Malinowski * , Aneta Raszkowska-Kaczor , Krzysztof Moraczewski , Wojciech Głuszewski , 4 1 Volodymyr Krasinskyi and Lauren Wedderburn Łukasiewicz Research Network—Institute for Engineering of Polymer Materials and Dyes, 55 M. Skłodowska-Curie Street, 87-100 Torun, ´ Poland; aneta.kaczor@impib.lukasiewicz.gov.pl (A.R.-K.); lauren.wedderburn@impib.lukasiewicz.gov.pl (L.W.) Institute of Materials Engineering, Kazimierz Wielki University, 30 Chodkiewicza Street, 85-064 Bydgoszcz, Poland; kmm@ukw.edu.pl Institute of Nuclear Chemistry and Technology, 16 Dorodna Street, 03-195 Warsaw, Poland; w.gluszewski@ichtj.waw.pl Department of Chemical Technology of Plastics Processing, Lviv Polytechnic National University, 12 Stepan Bandera Street, 79-013 Lviv, Ukraine; vkrasinsky82@gmail.com * Correspondence: malinowskirafal@gmail.com; Tel.: +48-53-060-0220 Abstract: The need for the development of new biodegradable materials and modiﬁcation of the properties the current ones possess has essentially increased in recent years. The aim of this study Citation: Malinowski, R.; was the comparison of changes occurring in poly("-caprolactone) (PCL) due to its modiﬁcation by Raszkowska-Kaczor, A.; high-energy electron beam derived from a linear electron accelerator, as well as the addition of natural Moraczewski, K.; Głuszewski, W.; ﬁbers in the form of cut hemp ﬁbers. Changes to the ﬁbers structure in the obtained composites and Krasinskyi, V.; Wedderburn, L. The the geometrical surface structure of sample fractures with the use of scanning electron microscopy Structure and Mechanical Properties were investigated. Moreover, the mechanical properties were examined, including tensile strength, of Hemp Fibers-Reinforced elongation at break, ﬂexural modulus and impact strength of the modiﬁed PCL. It was found that Poly("-Caprolactone) Composites PCL, modiﬁed with hemp ﬁbers and/or electron radiation, exhibited enhanced ﬂexural modulus Modiﬁed by Electron Beam but the elongation at break and impact strength decreased. Depending on the electron radiation Irradiation. Appl. Sci. 2021, 11, 5317. dose and the hemp ﬁbers content, tensile strength decreased or increased. It was also found that https://doi.org/10.3390/app11125317 hemp ﬁbers caused greater changes to the mechanical properties of PCL than electron radiation. Academic Editor: The prepared composites exhibited uniform distribution of the dispersed phase in the polymer matrix Alessandro Pegoretti and adequate adhesion at the interface between the two components. Received: 30 April 2021 Keywords: poly("-caprolactone); hemp ﬁbers; composites; irradiation; mechanical properties; Accepted: 4 June 2021 biodegradable polymers Published: 8 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional afﬁl- One of the most important biodegradable polymers, produced from petroleum re- iations. sources, is poly("-caprolactone) (PCL) [1–5]. Its physico-chemical properties are generally beneﬁcial and the material may play a key role in many branches of the economy (both in specialised and mass applications). However, the PCL properties need to be modiﬁed for some applications. It concerns, for example, properties such as its thermal resistance Copyright: © 2021 by the authors. due to low melt temperatures (58–60 C) or its barrier properties for some gases [6–8]. Licensee MDPI, Basel, Switzerland. PCL mechanical properties can also be modiﬁed with the use of mineral or vegetable ﬁllers This article is an open access article that simultaneously lower the price of the polymer. It is important because PCL in its distributed under the terms and pristine form is generally more expensive than conventional polymers and even some conditions of the Creative Commons biodegradable ones such as PLA [9–13] or TPS [14–17]. Attribution (CC BY) license (https:// PCL properties can be modiﬁed by different methods. Various types of PCL polymer creativecommons.org/licenses/by/ blends as well as composites and nanocomposites are most often obtained. The most well- 4.0/). Appl. Sci. 2021, 11, 5317. https://doi.org/10.3390/app11125317 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 5317 2 of 14 known PCL blends are: (i) PCL/TPS [18–20], (ii) PLA/PCL [21–24] or (iii) PCL/PBAT [25]. PCL blends containing non-biodegradable polymers also exist, e.g., PCL/PVC [26] or PCL/PBT [27]. Another signiﬁcant group is PCL composites with natural ﬁllers, includ- ing plant ﬁbers [28–35]. Other known methods of PCL property modiﬁcation refers to processes such as copolymerization, grafting and crosslinking [36–41]. In this study, we proposed a combination of two methods to modify PCL proper- ties. The ﬁrst method refers to obtaining PCL composites with hemp ﬁbers (HF) and the second method is related to irradiation of such obtained composites with the use of elec- tron radiation. Our literature review revealed only a few studies on PCL/HF composites. For example, the experimental work carried out by Dhakal et al. [42] investigated the reinforcement efﬁciency of hemp ﬁbers with different ﬁber aspect ratios on PCL-based biocomposites and compared the inﬂuence of these aspect ratios on moisture absorption behavior along with the tensile, ﬂexural and impact properties of hemp ﬁber/PCL biocom- posites. In another study by this author [43], the effects of variable mean hemp ﬁber content on nano-mechanical (hardness, modulus, elasticity and plasticity), surface and thermal properties of hemp ﬁber/polycaprolactone biocomposites were studied. Dhakal et al. [44] presented some test results of PCL/HF composites and compared them to the results of similar composites, i.e., those containing date palm ﬁbers. Studies on PCL/HF com- posites have also been carried out by Beaugrand et al. [45], who examined, among other parameters, the effect of processing conditions on deﬁbration and mechanical properties. Many more studies on PCL composites with other types of natural ﬁbers have been con- ducted. Examples of such studies are presented in a review publication by Wahit et al. [46]. The authors of this publication concluded that green composites with enhanced mechanical properties can be successfully prepared. They also showed that natural ﬁbers reduce the price of these composites and expand their application areas, which in turn could attract public interest. Key studies on the modiﬁcation of PCL properties also concern its radiation modiﬁca- tion, especially in cases of modiﬁed PCL to be applied in medicine or tissue engineering. Studies in this area were conducted mainly with the use of gamma radiation or electron radiation [47–49]. These studies indicated that PCL, when subjected solely to irradiation, does not form a signiﬁcant amount of gel fraction and the degradation processes are not dominant at the same periods of time. Good crosslinking of PCL may be successfully carried out by using crosslinking agents such as triallyl isocyanurate (TAIC) [49–51]. Then, with the appropriately selected irradiation conditions, PCL can be completely crosslinked. As a consequence of this, it will not ﬂow and its degree of crosslinking will be more than 90%. Literature analysis revealed that, there are no known studies on the combined effect of both electron radiation and natural hemp ﬁbers on PCL properties, in particular, mechanical ones. Admittedly, some similar studies can be found in the literature, however, they concern either other types of natural ﬁbers or different polymers. For example, Rytlewski et al. investigated the effect of electron radiation on the properties of PLA [52] or PCL [53] containing 20% ﬂax ﬁbers with the addition of 1% to 3% TAIC. Understanding the combined effect of electron radiation and hemp ﬁbers on the structure and mechanical properties of PCL is interesting from both cognitive and utilisation aspects. From the cognitive aspect it is important to know the relationship between the radiation dose, the amount of ﬁbers in the polymer matrix and the properties of the obtained materials. From the utilisation aspect, modiﬁcation of PCL with hemp ﬁbers and electron irradiation has been carried out to obtain new materials susceptible to biodegradation. Furthermore, the use of hemp ﬁbers in our study may assist in reducing the adverse effects of PCL’s partial crosslinking and on its biodegradability. This inspired the authors of the present article to undertake investigations aimed at the comparison of the structure and mechanical properties of PCL/HF composites modiﬁed by electron radiation with doses in the range of 10–90 kGy. The comparison of changes in (i) the structure of the ﬁbers extracted from obtained composites; (ii) the surface geometrical Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 14 Appl. Sci. 2021, 11, 5317 3 of 14 changes in (i) the structure of the fibers extracted from obtained composites; (ii) the sur‐ face geometrical structure of sample fractures; (iii) the tensile strength; (iv) the elongation structure of sample fractures; (iii) the tensile strength; (iv) the elongation at break; (v) the at break; (v) the flexural modulus; and (vi) the impact strength have been presented in ﬂexural modulus; and (vi) the impact strength have been presented in this paper. this paper. 2. Experimental 2. Experimental 2.1. Materials 2.1. Materials The following materials were used in this study: The following materials were used in this study: Poly("-caprolactone) (PCL), type CAPA6800 with a measured melt ﬂow rate (MFR) of  Poly(ε‐caprolactone) (PCL), type CAPA6800 with a measured melt flow rate (MFR) 5.1 g/10 min (2.16 kg, 190 C), density of 1.15 g/cm and average molecular weight of of 5.1 g/10 min (2.16 kg, 190 °C), density of 1.15 g/cm and average molecular weight ca. 82 kDa was supplied by Ingevity UK Ltd. (Warrington, UK); of ca. 82 kDa was supplied by Ingevity UK Ltd. (Warrington, UK); Natural hemp ﬁbers (HF), at 6 mm long, were purchased from the Institute of Natural  Natural hemp fibers (HF), at 6 mm long, were purchased from the Institute of Nat‐ Fibers and Medicinal Plants in Poznan (Poland). The cut hemp ﬁbers’ structure is ural Fibers and Medicinal Plants in Poznan (Poland). The cut hemp fibers’ structure shown in Figure 1 were applied as the PCL ﬁller. is shown in Figure 1 were applied as the PCL filler. Figure 1. SEM image of hemp fibers (HF) structure. Figure 1. SEM image of hemp ﬁbers (HF) structure. 2.2. Apparatus 2.2. Apparatus The following devices were utilised to prepare and investigate the irradiated PCL The following devices were utilised to prepare and investigate the irradiated PCL composites: composites: Co-rotating twin screw extruder of type BTSK 20/40D (Bühler, Germany), equipped with  Co‐rotating twin screw extruder of type BTSK 20/40D (Bühler, Germany), equipped 20-mm diameter screws and L/D ratio of 40 intended to produce granulated with 20‐mm diameter screws and L/D ratio of 40 intended to produce granulated PCL composites; PCL composites; Screw injection molding machine of type Battenfeld Plus 35/75 (Battenfeld GmbH,  Screw injection molding machine of type Battenfeld Plus 35/75 (Battenfeld GmbH, Solingen, Germany), equipped with 22-mm diameter screw and L/D ratio of 17 Solingen, Germany), equipped with 22‐mm diameter screw and L/D ratio of 17 de‐ designed to produce standard dumbbell-shaped and bar-shaped specimens; signed to produce standard dumbbell‐shaped and bar‐shaped specimens; Linear accelerator of type Elektronika 10/10 (Institute of Nuclear Chemistry and  Linear accelerator of type Elektronika 10/10 (Institute of Nuclear Chemistry and Technology in Warsaw, Poland) with energy of electrons 10 MeV and electron beam Technology in Warsaw, Poland) with energy of electrons 10 MeV and electron beam power 10 kW for irradiation of PCL composites; power 10 kW for irradiation of PCL composites; Shaker of type Unimax 1010 (Heidoplh, Germany) was used to extract ﬁbers from  Shaker of type Unimax 1010 (Heidoplh, Germany) was used to extract fibers from obtained composites; obtained composites; Scanning electron microscope (SEM) of type Hitachi SU8010 (Hitachi, Japan) was  Scanning electron microscope (SEM) of type Hitachi SU8010 (Hitachi, Japan) was used to examine the structure of HF as well as surface geometrical structure of used to examine the structure of HF as well as surface geometrical structure of sample fractures; sample fractures; Tensile testing machine of type TIRAtest 27025 (TIRA Maschinenbau GmbH, Schalkau,  Tensile testing machine of type TIRAtest 27025 (TIRA Maschinenbau GmbH, Germany) was used to examine mechanical properties under static tension and static Schalkau, Germany) was used to examine mechanical properties under static ten‐ three-point bending; sion and static three‐point bending; Pendulum Impact Tester of type IMPats-15 (ATS FAAR, Novegro-Tregarezzo, Italy)  Pendulum Impact Tester of type IMPats‐15 (ATS FAAR, Novegro‐Tregarezzo, Italy) was intended for the determination of Charpy impact strength. was intended for the determination of Charpy impact strength. 2.3. Methods 2.3. Methods Extrusion of the pristine PCL and granulated PCL composites was performed at the following temperatures of particular cylinder zones of the co-rotating twin screw extruder: Appl. Sci. 2021, 11, 5317 4 of 14 140 C, 150 C, 160 C and 165 C. The temperature of the extrusion die head was 160 C and the screw rotational speed was constant at 250 min . The screws were in a speciﬁc conﬁguration that facilitated adequate dispersion and distribution of HF in the PCL matrix. Moreover, the screws consisted of elements providing intensive HF grinding to obtain ﬁne fractions. Prior to extrusion, HF was dried at 110 C for 24 h. PCL was not subjected to drying. Free degassing of any possible released residual moisture was applied at the screws with lengths of L/D = 22 and L/D = 32, respectively. PCL and HF were introduced into the extruder from volume feeders in such proportions that PCL composites with HF contents equal to 2.5, 5, 10 and 20 wt% were obtained. The percentages of ﬁbers added were chosen based on our previous studies [52,53]. The extrudate was cooled in a water bath at 15 C and then dried on a conveyor belt in a stream of dry air and ﬁnally granulated. The obtained composites in the form of cylindrical granules (height 5 mm and base diameter 3 mm) were dried at 50 C for 24 h to eliminate residual moisture. The prepared samples of the granulated composites were denoted as CF2.5, CF5, CF10 and CF20, where C denotes poly("-caprolactone), F denotes natural hemp ﬁbers and the numbers represent the ﬁbers percentage content. A reference sample, made of the pristine PCL, was also prepared and denoted as C. During extrusion, basic parameters that include the following were recorded: (i) extruder motor torque (M), (ii) energy consumption (E) by the extruder drive system, (iii) temperature (T) of the material being processed that is measured in the extruder die-head and (iv) pressure (P) of the material being processed that is measured in the extruder die-head. The dumbbell-shaped and bar-shaped specimens were prepared according to a relevant standard (PN-EN ISO 527-2:2012) using a screw injection molding machine (Battenfeld Plus 35/75). The temperatures of the plasticising zones I and II of the barrel were 130 C and 140 C, respectively, and the temperature of the injection molding die head was 140 C. The temperature of the injection mold was 23 C and the injection pressure was 157–163 MPa dependent on the kind of composite. The screw rotational speed was constant at 150 min . Sample irradiation was carried out with a linear accelerator type Elektronika 10/10 producing scanned beam of electrons with energy of 10 MeV and of 10 kW power with the possibility of changing the dose rate by controlling the speed of the conveyer. The acceler- ator is located at the Institute of Nuclear Chemistry and Technology in Warsaw, Poland. The samples (granulated samples and injection-moulded pieces) were irradiated with doses of 10, 20, 40, 60 or 90 kGy. Maximum single doses were 20 kGy. This limitation was caused by an increase in temperature of the irradiated material that was equal to ca. 4–7 C for single doses of 10 kGy. Large single doses cause intensive heating of the material, therefore some additional structural changes may occur. Thus, samples were irradiated with lower doses multiple times. During the irradiation procedure, all the granulated samples and moulded pieces were placed in aluminium containers in single layers of the thickness of up to 2 cm. The containers were put on the conveyor belt moving at the speed of 0.3–1.2 m/min under the accelerator scanner. The actual speed was related to the radiation dose absorbed by the polymer material being modiﬁed. The HF structure in the obtained composites was determined by SEM analysis after dissolving the composites in methylene chloride for 24 h at room temperature and then ﬁltering the ﬁbers fraction and ﬁnally drying them at 50 C for 24 h. The HF structure (before and after extrusion) as well as the surface geometrical struc- ture of the sample fractures, including distribution of the dispersed HF phase, were evalu- ated by SEM using the secondary electron (SE) detector and accelerating voltage of 10 kV. The fractures were made in liquid nitrogen. A 5-nm thick gold layer was sputtered on all of the samples to be analysed by SEM. For that purpose, a cathode sputtering apparatus was used, which was equipped with a coating thickness gauge based on a quartz crystal of varying conductivity. Tensile strength ( ) and elongation at break (" ) were evaluated according to the M B PN-EN ISO 527-1:2012 standard by using the extension rate of 10.0 mm/min. Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 14 Appl. Sci. 2021, 11, 5317 5 of 14 Tensile strength (σM) and elongation at break (εB) were evaluated according to the PN‐EN ISO 527‐1:2012 standard by using the extension rate of 10.0 mm/min. Flexural modulus (Mf) was measured by the three‐point bend test at a bending de‐ flection rate of 5.0 mm/min. The measurements were carried out in accordance to the Flexural modulus (M ) was measured by the three-point bend test at a bending PN‐EN ISO 178:2011 standard. deﬂection rate of 5.0 mm/min. The measurements were carried out in accordance to the Impact strength (acN) was evaluated according to the PN‐EN ISO 179‐1:2010 stand‐ PN-EN ISO 178:2011 standard. ard. Impact strength (a ) was evaluated according to the PN-EN ISO 179-1:2010 standard. cN 3. Results and Discussion 3. Results and Discussion 3.1. HF Structure in Obtained Composites 3.1. HF Structure in Obtained Composites The structure of the HF taken from obtained composites (Figure 2) is signiﬁcantly The structure of the HF taken from obtained composites (Figure 2) is significantly different compared to that shown in Figure 1. First of all, the ﬁbers in the obtained different compared to that shown in Figure 1. First of all, the fibers in the obtained composites possess a signiﬁcantly reduced length and diameter (even 10 times their length). composites possess a significantly reduced length and diameter (even 10 times their This is due to two reasons. Firstly, the lengths of the ﬁbers were shortened by the use length). This is due to two reasons. Firstly, the lengths of the fibers were shortened by the of a plasticising system containing elements that induced intense mixing and shearing, use of a plasticising system containing elements that induced intense mixing and shear‐ as well as retracting elements. These elements divided the ﬁbers into smaller particles, ing, as well as retracting elements. These elements divided the fibers into smaller parti‐ including single macroﬁbrils and, as a result, a ﬁne fraction with high dispersion in the cles, including single macrofibrils and, as a result, a fine fraction with high dispersion in polymer matrix was obtained. Secondly, some of the ﬁbers were additionally shortened in the polymer matrix was obtained. Secondly, some of the fibers were additionally short‐ the granulation process. ened in the granulation process. Figure 2. HF structure after extraction from PCL/HF composites ((A) CF2.5; (B) CF5; (C) CF10; (D) Figure 2. HF structure after extraction from PCL/HF composites ((A) CF2.5; (B) CF5; (C) CF10; CF20). (D) CF20). The fibers in the CF2.5 composite (Figure 2A) have a larger diameter and a more The ﬁbers in the CF2.5 composite (Figure 2A) have a larger diameter and a more uniform structure compared to the fibers in the other composites. Here and there, a single uniform structure compared to the ﬁbers in the other composites. Here and there, a single fiber bundle can be seen. As the composite fiber content increases, their structure ﬁber bundle can be seen. As the composite ﬁber content increases, their structure changes. changes. At the highest percentage filling, there are a few coarse fibers with intact struc‐ At the highest percentage filling, there are a few coarse fibers with intact structure and the ture and the dominant fraction is composed of single fiber bundles or individual macro‐ dominant fraction is composed of single fiber bundles or individual macrofibrils (Figure 2D). fibrils (Figure 2D). The change in the ﬁber structure in individual composites is related to the changing processing The change parameters, in the which fiber str inucture turn ar ine ind a derivative ividual comp of the osites incr iseasing related ﬁber to the content changing in processing parameters, which in turn are a derivative of the increasing fiber content in the plasticised polymer mass. As the content of HF in the composite increases, all the measur the pla ed sticised parameters polymer of the maextr ss. As usion the pr co ocess, ntent i.e., of HF M, in E, T the and composite P also incr incre easeas(T es, able all 1the ). The mea incr sured ease parameters in these parameters, of the extrusion which ar process, e indirectly i.e., rM, elated E, Tto and the P incr also ease incre in a shear se (Tfor able ces, 1). result The incre in changes ase in these in the parameters, structure of whic the ﬁbers h are as indir shown ectlyin related Figur eto 2 .the Thus, increa these str in uctur sheear of the obtained ﬁbers may have a signiﬁcant impact on the mechanical properties of the forces, result in changes in the structure of the fibers as shown in Figure 2. Thus, the irradiated PCL/HF composites, which are discussed in Section 3.3. Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 14 Appl. Sci. 2021, 11, 5317 6 of 14 structure of the obtained fibers may have a significant impact on the mechanical proper‐ ties of the irradiated PCL/HF composites, which are discussed in Section 3.3. Table 1. The extrusion process parameters for PCL and its composites with HF. Table 1. The extrusion process parameters for PCL and its composites with HF. Sample M (Nm) E (kW) T ( C) P (MPa) Sample M (Nm) E (kW) T (°C) P (MPa) C 16–17 0.95–0.97 165 10–11 C 16–17 0.95–0.97 165 10–11 CF2.5 18–20 0.98–1.02 166 12–14 CF2.5 18–20 0.98–1.02 166 12–14 CF5 20–21 1.01–1.05 168 14–17 CF5 20–21 1.01–1.05 168 14–17 CF10 21–22 1.04–1.08 170 16–19 CF10 21–22 1.04–1.08 170 16–19 CF20 22–23 1.06–1.11 171 20–24 CF20 22–23 1.06–1.11 171 20–24 3.2. Composites Structure 3.2. Composites Structure The structural analysis results of two selected composites (CF2.5 and CF20) by scan- The structural analysis results of two selected composites (CF2.5 and CF20) by ning electron microscopy (SEM) are shown in Figure 3. scanning electron microscopy (SEM) are shown in Figure 3. Figure 3. Structure of selected fracture surfaces of bar‐shaped specimens obtained from Figure 3. Structure of selected fracture surfaces of bar-shaped specimens obtained from non- non‐irradiated and irradiated CF2.5 (left) and CF20 (right) composites. irradiated and irradiated CF2.5 (left) and CF20 (right) composites. Based on the images of the structures shown in Figure 3, it can be concluded that all fractures are brittle fractures. Moreover, the nature of these breakthroughs does not change Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 14 Based on the images of the structures shown in Figure 3, it can be concluded that all fractures are brittle fractures. Moreover, the nature of these breakthroughs does not change with an increasing radiation dose. In general, all images obtained for composites with the same degree of fiber filling, i.e., 2.5% or 20%, have comparable structures. This proves that an increase in the radiation dose does not cause significant changes in the fracture structure of composites with a specific HF content. Subtle changes can only be found when assessing the adhesion at the polymer–fiber interface. With an increasing radiation dose, the distance between the phases slightly decreases, which is especially visible in the case of sample CF2.5. On one hand, it may be caused by an increase in thermal expansion of the irradiated material, mainly of the polymer phase, as a result of Appl. Sci. 2021, 11, 5317 7 of 14 the temperature increase. On the other hand, it may be related to lower secondary shrinkage due to the changes in the structure of macromolecules after irradiation. Figure 3 also shows that the fibers in composites with a higher percentage filling are finer, which with an increasing radiation dose. In general, all images obtained for composites with the is consistent with the data shown in Figure 2. The craters seen in some photos may come same degree of ﬁber ﬁlling, i.e., 2.5% or 20%, have comparable structures. This proves that an increase in the radiation dose does not cause signiﬁcant changes in the fracture structure from the extraction of the fibers from the matrix during the fractures. These craters do not of composites with a speciﬁc HF content. Subtle changes can only be found when assessing disadvantage the material, but are merely a replica of the other half of the sample. The the adhesion at the polymer–ﬁber interface. With an increasing radiation dose, the distance surface structures of the other composites (CF5 and CF10) are very similar and therefore between the phases slightly decreases, which is especially visible in the case of sample CF2.5. On one hand, it may be caused by an increase in thermal expansion of the irradiated they are not shown here. material, mainly of the polymer phase, as a result of the temperature increase. On the other hand, it may be related to lower secondary shrinkage due to the changes in the structure of 3.3. Mechanical Properties macromolecules after irradiation. Figure 3 also shows that the ﬁbers in composites with a higher percentage ﬁlling are ﬁner, which is consistent with the data shown in Figure 2. The mechanical properties test results of non‐modified PCL and PCL/HF compo‐ The craters seen in some photos may come from the extraction of the ﬁbers from the matrix sites, as well as irradiated PCL and irradiated PCL/HF composites are illustrated in 3D during the fractures. These craters do not disadvantage the material, but are merely a three replica‐dimen of the other sional half of tgr hea sample. phs The (Fig surface ures str4–7 uctur)es ofand the other summ composites arized (CF5 in Table 2. The and CF10) are very similar and therefore they are not shown here. three‐dimensional graphs (3D) were made using STATISTICA version 10.0 software (StatSoft). 3.3. Mechanical Properties The data from Figure 4 indicates that the tensile strength (σM) for PCL significantly The mechanical properties test results of non-modiﬁed PCL and PCL/HF composites, as well as irradiated PCL and irradiated PCL/HF composites are illustrated in 3D three- decreases when the dose of electron radiation is 60 or 90 kGy. The decrease is 3% and dimensional graphs (Figures 4–7) and summarized in Table 2. The three-dimensional 13%, respectively. Smaller doses of irradiation do not affect the σM of that polymer. graphs (3D) were made using STATISTICA version 10.0 software (StatSoft). Figure 4. Tensile strength results. Figure 4. Tensile strength results. Other dependencies can be observed in the case of non‐irradiated composites con‐ taining HF. In their case, the σM initially decreases with the rising HF content and then increases. In Figure 4 there is a clear extreme of strength, the lowest value of which can be observed in the 10% HF content composite. When compared to pristine PCL, filling composites with 10% HF causes a decrease in σM by 27%. Larger amounts of HF do not cause a further decrease in σM, but they are the reason for its increase of 20%, compared to Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 14 Appl. Sci. 2021, 11, 5317 8 of 14 Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14 Figure 5. Elongation at break results. Figure 5. Elongation at break results. Figure 5 shows that both types of modification applied (irradiation and/or addition of HF) significantly reduces the εB value. Elongation decreases both as the electron radi‐ ation dose increases and when HF content increases and especially when both of these factors occur simultaneously. Elongation is least influenced by electron radiation, which reduces its value by a maximum of 17% for samples irradiated with 90 kGy. The decrease in elongation at break following electron radiation may result from elongation of the macromolecules due to mutual linking at chain ends or the formation of branched structures in notable amounts [50]. HF induces a significantly higher effect on the εB value, where a maximum decrease of 95% is noted. The highest influence on the εB value of the studied polymer is the combined action of electron radiation and HF, which re‐ duces εB by as much as 99%. There is, at least, a 100 times decrease in the εB value in comparison to non‐modified PCL and this occurs in composites with 20% HF content and that were irradiated with doses of 40, 60 or 90 kGy. The data in Figure 6 indicates that electron radiation in a dose of 90 kGy causes a growth in PCL’s flexural modulus (Mf) value by up to 13%. Smaller doses cause similar increases where the growth is ca. 11–12%. The flexural modulus of PCL/HF composites increases as the HF content increases. This rate of growth can be considered an exponen‐ Figure 6. Flexural modulus results. Figure 6. Flexural modulus results. tial function, i.e., at low HF concentration (2.5 or 5%) the modulus increases slightly. However, in the case of a filling with more than 10%, a significant growth in its value is The impact strength (acN) of PCL shown in Figure 7 decreases significantly as the observed. The Mf value of the composite containing 20% HF is over two times greater radiation dose increases. The maximum decrease in the PCL’s impact strength, 56%, oc‐ than the modulus of pristine PCL. This knowledge has significant importance on the curs upon irradiation of the polymer with 90 kGy. This may result from the formation of eventual suitable range of application for these types of composites. a branched or partially crosslinked structure of PCL macromolecules that hinders their Irradiation of PCL/HF composites affects their Mf values. When the radiation dose is conformational changes. This, in turn, limits the amount of energy absorbed by the irra‐ 10 kGy, a significant increase in the Mf value is observed. Higher doses do not have a diated PCL. Possible limitations for the rotation of macromolecular fragments can also considerable influence on the further growth of modulus values. Differences in Mf values affect other mechanical properties, including Mf or εB, which were presented earlier. of composites irradiated with doses of 20, 40 or 60 kGy do not differ notably from each other (Table 2). An exception to this is the case of when the CF20 composite is irradiated with 90 kGy and its Mf value decreases slightly. This may be due to degradation of both composite phases and, in particular, the HF phase. We can assume this because no de‐ crease in the Mf value was observed for lower fiber contents. Figure 7. Impact strength results. The PCL/HF composites are also characterised by significantly reduced impact strength. It is also worth noting that the highest impact strength of non‐irradiated com‐ posite (sample CF2.5) is considerably lower than the lowest impact strength value of ir‐ radiated PCL. By introducing the amount of just 2.5% of fibers (sample CF2.5) into the polymer matrix causes a reduction in impact strength by 60% and the addition of 20% fibers (sample CF20) reduces its value by 78%. Therefore, it can be concluded that the impact strength of PCL is more significantly influenced by even a minimal amount (2.5 Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14 Figure 6. Flexural modulus results. The impact strength (acN) of PCL shown in Figure 7 decreases significantly as the radiation dose increases. The maximum decrease in the PCL’s impact strength, 56%, oc‐ curs upon irradiation of the polymer with 90 kGy. This may result from the formation of a branched or partially crosslinked structure of PCL macromolecules that hinders their conformational changes. This, in turn, limits the amount of energy absorbed by the irra‐ Appl. Sci. 2021, 11, 5317 9 of 14 diated PCL. Possible limitations for the rotation of macromolecular fragments can also affect other mechanical properties, including Mf or εB, which were presented earlier. Figure 7. Impact strength results. Figure 7. Impact strength results. Table 2. Mechanical properties of pristine PCL, PCL/HF composites, irradiated PCL and irradiated The PCL/HF composites are also characterised by significantly reduced impact PCL/HF composites. strength. It is also worth noting that the highest impact strength of non‐irradiated com‐ posite (sample CF2.5) is considerably lower than the lowest impact strength value of ir‐ Sample Dose (kGy) (MPa) " (%) M (MPa) a (kJ/m ) M B cN radiated PCL. By introducing the amount of just 2.5% of fibers (sample CF2.5) into the C 0 27.0 0.9 1091 24 351 26 39.4 0.2 polymer matrix causes a reduction in impact strength by 60% and the addition of 20% CF2.5 0 27.6 0.5 1105 52 362 35 15.7 0.1 fibers CF5 (sample CF20 0) reduces23.4 its value 0.8 by 78% 910 . 9Therefore, 395 it 69 can be 12.9 concluded 0.3 that the CF10 0 19.7 0.4 518 71 635 68 11.4 0.1 impact strength of PCL is more significantly influenced by even a minimal amount (2.5 CF20 0 23.6 0.3 48 8 880 64 8.8 0.1 C 10 27.7 0.8 1128 24 381 21 33.8 0.2 CF2.5 10 26.2 0.4 1077 43 396 42 12.0 0.2 CF5 10 23.6 0.3 836 25 487 38 10.7 0.2 CF10 10 20.0 0.1 527 45 635 68 8.7 0.1 CF20 10 23.4 0.9 28 1 1158 68 7.1 0.3 C 20 27.3 0.9 1096 26 392 21 29.5 0.1 CF2.5 20 28.1 0.4 1064 70 435 32 10.4 0.1 CF5 20 24.2 0.7 820 21 499 41 9.5 0.2 CF10 20 20.0 1.1 364 41 639 78 7.8 0.1 CF20 20 23.3 0.8 26 2 1220 22 6.5 0.3 C 40 27.7 1.1 1076 43 399 31 25.2 0.1 CF2.5 40 24.8 1.2 966 76 433 12 9.0 0.1 CF5 40 23.1 0.6 811 7 502 29 8.3 0.2 CF10 40 20.6 0.5 194 70 640 73 6.8 0.2 CF20 40 23.8 1.2 9 4 1080 72 5.6 0.2 C 60 26.1 1.1 1073 74 394 16 21.0 0.1 CF2.5 60 25.8 0.7 951 71 457 20 7.9 0.1 CF5 60 20.4 0.3 648 35 507 55 7.4 0.2 CF10 60 21.4 0.7 141 49 636 61 6.2 0.1 CF20 60 24.3 0.4 6 1 1146 81 5.0 0.1 C 90 23.6 1.3 903 65 404 22 17.3 0.3 CF2.5 90 24.5 1.9 954 38 419 30 6.8 0.1 CF5 90 19.8 0.1 633 26 498 49 6.4 0.3 CF10 90 22.0 0.3 41 25 687 52 5.3 0.1 CF20 90 24.6 0.4 5 1 951 72 4.6 0.1 Appl. Sci. 2021, 11, 5317 10 of 14 The data from Figure 4 indicates that the tensile strength ( ) for PCL signiﬁcantly decreases when the dose of electron radiation is 60 or 90 kGy. The decrease is 3% and 13%, respectively. Smaller doses of irradiation do not affect the of that polymer. Other dependencies can be observed in the case of non-irradiated composites con- taining HF. In their case, the initially decreases with the rising HF content and then increases. In Figure 4 there is a clear extreme of strength, the lowest value of which can be observed in the 10% HF content composite. When compared to pristine PCL, ﬁlling com- posites with 10% HF causes a decrease in by 27%. Larger amounts of HF do not cause a further decrease in , but they are the reason for its increase of 20%, compared to the CF10 sample. However, the value of the non-irradiated CF20 composite is not larger than the value of any PCL sample irradiated with doses ranging from 10 to 90 kGy and it is at most equal to the strength of the PCL irradiated with the dose of 90 kGy. Similar observations are seen in the case of irradiated PCL/HF composites, i.e., for a given dose, as the content of HF in polymer matrix increases, the initially decreases and then increases. However, in this case, the minimum value of irradiated composites depends on the irradiation dose. For doses of 10, 20 and 40 kGy, the minimum occurs in composites with 10% HF ﬁlling. In the case of doses of 60 and 90 kGy, the minimum occurs in composites containing 5% HF. Moreover, in composites containing 10 or 20% HF, an increase in with an increasing dose is observed, which is not observed in composites with smaller ﬁber content. Elongation at break (" ) results for the same samples analysed in tests are presented B M in Figure 5. Figure 5 shows that both types of modiﬁcation applied (irradiation and/or addition of HF) signiﬁcantly reduces the " value. Elongation decreases both as the electron radiation dose increases and when HF content increases and especially when both of these factors occur simultaneously. Elongation is least inﬂuenced by electron radiation, which reduces its value by a maximum of 17% for samples irradiated with 90 kGy. The decrease in elongation at break following electron radiation may result from elongation of the macromolecules due to mutual linking at chain ends or the formation of branched structures in notable amounts [50]. HF induces a signiﬁcantly higher effect on the " value, where a maximum decrease of 95% is noted. The highest inﬂuence on the " value of the studied polymer is the combined action of electron radiation and HF, which reduces " by as much as 99%. There is, at least, a 100 times decrease in the " value in comparison to non-modiﬁed PCL and this occurs in composites with 20% HF content and that were irradiated with doses of 40, 60 or 90 kGy. The data in Figure 6 indicates that electron radiation in a dose of 90 kGy causes a growth in PCL’s ﬂexural modulus (M ) value by up to 13%. Smaller doses cause similar increases where the growth is ca. 11–12%. The ﬂexural modulus of PCL/HF composites increases as the HF content increases. This rate of growth can be considered an expo- nential function, i.e., at low HF concentration (2.5 or 5%) the modulus increases slightly. However, in the case of a ﬁlling with more than 10%, a signiﬁcant growth in its value is observed. The M value of the composite containing 20% HF is over two times greater than the modulus of pristine PCL. This knowledge has signiﬁcant importance on the eventual suitable range of application for these types of composites. Irradiation of PCL/HF composites affects their M values. When the radiation dose is 10 kGy, a signiﬁcant increase in the M value is observed. Higher doses do not have a considerable inﬂuence on the further growth of modulus values. Differences in M values of composites irradiated with doses of 20, 40 or 60 kGy do not differ notably from each other (Table 2). An exception to this is the case of when the CF20 composite is irradiated with 90 kGy and its M value decreases slightly. This may be due to degradation of both composite phases and, in particular, the HF phase. We can assume this because no decrease in the M value was observed for lower ﬁber contents. The impact strength (a ) of PCL shown in Figure 7 decreases significantly as the radia- cN tion dose increases. The maximum decrease in the PCL’s impact strength, 56%, occurs upon Appl. Sci. 2021, 11, 5317 11 of 14 irradiation of the polymer with 90 kGy. This may result from the formation of a branched or partially crosslinked structure of PCL macromolecules that hinders their conforma- tional changes. This, in turn, limits the amount of energy absorbed by the irradiated PCL. Possible limitations for the rotation of macromolecular fragments can also affect other mechanical properties, including M or " , which were presented earlier. f B The PCL/HF composites are also characterised by signiﬁcantly reduced impact strength. It is also worth noting that the highest impact strength of non-irradiated compos- ite (sample CF2.5) is considerably lower than the lowest impact strength value of irradiated PCL. By introducing the amount of just 2.5% of ﬁbers (sample CF2.5) into the polymer matrix causes a reduction in impact strength by 60% and the addition of 20% ﬁbers (sample CF20) reduces its value by 78%. Therefore, it can be concluded that the impact strength of PCL is more signiﬁcantly inﬂuenced by even a minimal amount (2.5 wt%) of ﬁbers introduced into the polymer matrix than by electron radiation with the highest applied dose (90 kGy). In the case of irradiated PCL/HF composites, further decreases in impact strength are observed. However, while present, the changes in impact strength of the composite after ir- radiation are less signiﬁcant. For example, the impact strength of the CF2.5 composite after irradiation with the maximum dose (90 kGy) decreases by a maximum of 83% compared to the impact strength of pristine PCL. Thus, irradiation decreases the impact strength of PCL by an additional 23% compared to the non-irradiated CF2.5 composite. In turn, the impact strength of the composite containing the maximum amount of ﬁbers (sample CF20) and irradiated with the maximum dose (90 kGy) decreases by a maximum of 88% compared to the impact strength of pristine PCL. For this composite (CF20), irradiation de- creases the impact strength of PCL by an additional 10% compared to the non-irradiated CF20 composite. The data in Table 2 indicates that large relative changes occur in the impact strength values for individual composites after irradiation. For example, doses of 10 and 90 kGy led to a decrease in the impact strength of the CF2.5 composite by 24% and 57%, respectively. Moreover, an increase in the HF weight fraction in the PCL matrix causes a decrease in the relative differences in impact strength for composites after irradiation. For example, the doses of 10 and 90 kGy decreases the impact strength of CF20 composite by 19% and 48%, respectively. The signiﬁcant reduction in the impact strength of non-irradiated and irradiated PCL/HF composites results mainly from the stiffening of the material and the perpendicular orientation of the natural ﬁbers (Figure 3) in relation to the impact force applied by the hammer during strength testing. 4. Conclusions The structure and mechanical properties of the investigated materials depended on three main factors: (i) processing conditions, (ii) qualitative and quantitative contribu- tion of dispersed phase and (iii) the magnitude of the electron radiation dose. The ﬁnal ﬁbers’ structures in the obtained composites were closely linked to processing condi- tions, which also depended on the percentage ﬁlling of these ﬁbers in the polymer matrix. Electron radiation did not signiﬁcantly change the structure of the prepared composites. Only small changes could be observed in the case of adhesion between the dispersed phase and the polymer matrix. The structure of the composites, however, depended signiﬁcantly on the HF content. Mechanical properties of the study’s composites were greatly inﬂuenced by electron radiation. Furthermore, as the radiation dose increased, the values of impact strength, elongation at break and in part tensile strength decreased while the ﬂexural modulus increased. An especially interesting synergistic effect on PCL properties was observed and was related to the combined action of electron radiation and natural ﬁbers addition. For example, depending on the used dose and ﬁbers amount, elongation at break can be reduced from 1% to 99%, while the ﬂexural modulus can increase from 1% to 248%. Hemp ﬁbers may be used as an additive in the PCL matrix to manufacture biodegrad- able materials. Ecological advantages and the low price of these ﬁbers compared to the Appl. Sci. 2021, 11, 5317 12 of 14 price of PCL highlight a direction for further research in this ﬁeld. Moreover, the ap- plied ﬁbers should additionally accelerate the biodegradation process of such materials. This statement refers, speciﬁcally, to radiation-modiﬁed biodegradable polymers that can be partially crosslinked and, as consequence of this, the biodegradation process may be slowed down. Thus, hemp ﬁbers should lessen the adverse effect that crosslinking has on biodegradability. In this case, further studies determining the combined action of electron radiation and natural ﬁbers on the biodegradation of these materials may be interesting. Furthermore, the post-radiation oxidation processes of both the matrix and HF as well as the protective effect of lignin in the obtained materials may also be the subject of further research. The radiation-modiﬁed composites presented in this article can be applied pri- marily for the production of biodegradable packaging, including rigid and ﬂexible ones, depending on the applied radiation dose and the ﬁbers content. Author Contributions: Conceptualization, R.M. and V.K.; methodology, R.M., K.M. and W.G.; sample preparation, R.M. and W.G.; formal analysis, A.R.-K. and V.K.; investigation, R.M., A.R.-K. and K.M.; data curation, R.M.; writing—original draft preparation, R.M. and A.R.-K.; writing—review and editing, R.M. and L.W. All authors have read and agreed to the published version of the manuscript. Funding: The project has been ﬁnanced by statutory funds (project no. 110034). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All experimental data to support the ﬁndings of this study are available upon request by contacting the corresponding author. Acknowledgments: The authors of this work would like to express their special thanks to the Institute of Natural Fibers and Medicinal Plants in Poznan (Poland) for the preparation of hemp ﬁbers. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. 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Applied Sciences – Multidisciplinary Digital Publishing Institute

Published: Jun 8, 2021

Keywords: poly(ε-caprolactone); hemp fibers; composites; irradiation; mechanical properties; biodegradable polymers

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The Structure and Mechanical Properties of Hemp Fibers-Reinforced Poly(ε-Caprolactone) Composites Modified by Electron Beam Irradiation

The Structure and Mechanical Properties of Hemp Fibers-Reinforced Poly(ε-Caprolactone) Composites Modified by Electron Beam Irradiation

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The Structure and Mechanical Properties of Hemp Fibers-Reinforced Poly(ε-Caprolactone) Composites Modified by Electron Beam Irradiation

The Structure and Mechanical Properties of Hemp Fibers-Reinforced Poly(ε-Caprolactone) Composites Modified by Electron Beam Irradiation

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