Preparation and Characterization of Novel Poly(Lactic Acid) Composites Reinforced with “Latxa” Sheep Wool Fibers: The Effect of Peroxide Surface Treatments and Fiber Content

Citation: Arbelaiz, A.; Yurramendi, T.; Larruscain, A.; Arrizabalaga, A.; Eceiza, A.; Peña-Rodriguez, C. Preparation and Characterization of Novel Poly(Lactic Acid) Composites Reinforced with “Latxa” Sheep Wool Fibers: The Effect of Peroxide Surface Treatments and Fiber Content. Materials 2024,17, 4912. https:// doi.org/10.3390/ma17194912 Academic Editor: Sukhoon Pyo Received: 11 September 2024 Revised: 2 October 2024 Accepted: 4 October 2024 Published: 8 October 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Preparation and Characterization of Novel Poly(Lactic Acid) Composites Reinforced with “Latxa” Sheep Wool Fibers: The Effect of Peroxide Surface Treatments and Fiber Content Aitor Arbelaiz *, Telmo Yurramendi, Ander Larruscain, Ane Arrizabalaga , Arantxa Eceiza and Cristina Peña-Rodriguez * ‘Materials + Technologies’ Group (GMT), Chemical & Environmental Engineering Department, Faculty of Engineering, Gipuzkoa, University of the Basque Country UPV/EHU, 20018 Donostia-San Sebastian, Spain; [email protected] (T.Y.); ander[email protected] (A.L.); [email protected] (A.A.); [email protected] (A.E.) *Correspondence: aitor[email protected] (A.A.); [email protected] (C.P.-R.) Abstract: “Latxa” sheep wool is rough, and it is not used in the textile industry because the fiber diameter is high compared with other wool fibers. Nowadays, this wool is considered as disposal and, with the aim to give it value, new uses must be explored. In the current work, the “Latxa” sheep wool fiber was evaluated as poly(lactic acid) (PLA) polymer reinforcement. With the objective to optimize fiber/matrix adhesion, fibers were surface modified with peroxide. Oxidation treatment with peroxide led to chemical modifications of the wool fibers that improved the fiber/PLA adhesion, but the strength values achieved for the composites were lower compared to the neat PLA ones. The mechanical properties obtained in the current work were compared with the literature data of the PLA composites reinforced with vegetable fibers. The wool fibers showed inferior mechanical properties compared to the vegetable fiber counterparts. However, the preliminary results indicated that the incorporation of wool fibers to PLA reduced the flammability of composites. Keywords: wool fiber; poly(lactic acid); composite; mechanical properties; fire resistance 1. Introduction The Latxa sheep breed, originally from Basque Country and Navarra, in the northeast of Spain, is used to produce milk, from which different dairy products are produced such as Idiazabal Cheese, among others. After the sheep are sheared, about 2000 tons of wool are obtained per year, being this wool considered as disposable. Latxa sheep wool cannot be used in textile because it is rough and the fiber diameter is very high compared with other wool fibers used in textile industry [ 1 , 2 ], and consequently other uses must discovered to add value to “Latxa” sheep wool. The value of the wool depends on fiber coarseness, because fibers with 35 µ m diameter or higher hardly have applications [ 3 ]. In the literature, there are reported works where wool fibers are used for the preparation of biocomposites [4–15] . Wool fiber shows interesting mechanical properties and can be used as reinforcement in composites. For example, Alzeer and MacKenzie [ 4 ] incorporated long wool fibers into a geopolymer matrix. One limitation of geopolymer when used in the construction and building sectors is its brittle failure mode under applied force. After the incorporation of long wool fibers the prepared composites showed, in addition to an improvement in flexural strength, around a 40% more stable fracture mode. Fiore et al. [5] prepared and studied cement mortar reinforced with wool fibers. They concluded that after the incorporation of wool fibers into mortars, the thermal insulation properties were enhanced but the compressive strength decreased. Salama et al. [ 6 ] prepared and characterized polymer composites based on polypropylene and recycled wool micro-powder. They concluded that the composite based on polypropylene (PP) and wool powder showed Materials 2024,17, 4912. https://doi.org/10.3390/ma17194912 https://www.mdpi.com/journal/materials Materials 2024,17, 4912 2 of 17 improved properties. Gama et al. [ 7 ] prepared composites using polyurethane residues and textile fiber residues, among them being wool fibers. They observed that after the incorporation of textile fiber residues into polyurethane, the modulus value increased considerably, although the strength and deformation at break diminished. They suggested that these composites can find many applications, in construction or automotive sectors, with the advantage of being produced from 100% recycled raw materials. Alkateb et al. [ 8 ] used wool as an energy absorber. They fabricated elliptical tubes with composites based on wool woven and epoxy resin. They observed that the wool composite with 30 wt% fiber content absorbed the highest specific energy. The flammability and low thermomechanical stability of PLA limits the expansion of this polymer in many applications. In a previous work, the thermomechanical stability of PLA was improved by the addition of vegetable fibers in combination with an annealing process [ 16 ]. However, the performance against the fire of composites based on polymeric matrices and vegetable fibers is poor, and fire-retardant and intumescent systems should be incorporated [ 17 – 20 ]. The main drawbacks of most of commercial flame retardants are that the preparation process involves healthy risk and that it is expensive, in addition to not being environmentally sustainable. Furthermore, usually, adding them to the polymeric matrix deteriorated the mechanical properties of the biocomposites [ 19 ]. Shumao et al. [ 17 ] added ammonium polyphosphate flame retardant to biocomposites of PLA and ramie fibers. They observed that when flame-retardant loading was 10.5 wt%, the strength decreases considerably. They suggested that flame retardant could hinder the adhesion between PLA and ramie fibers. Bocz et al. [18] prepared flax fiber reinforced PLA/Thermoplastic starch (TPS) biocomposites with glycerol phosphate plasticizer. This plasticizer had a flame-retardant effect; however, after the addition of glycerol phosphate plasticizer, the strength value decreased. They suggested that the presence of glycerol phosphate reduced the compatibility between the cellulosic fibers and the biopolymer matrix. Shukor et al. [ 20 ] observed that the flexural strength was reduced in kenaf fiber/PLA biocomposites after the addition of ammonium polyphosphate flame retardant. Therefore, it is a challenge to prepare novel biocomposites with good mechanical properties and flame-retardant properties. In the literature, it was observed that wool fibers have better fire resistance than vegetable ones, and that the presence in wool fiber of sulfur and nitrogen atoms, around 3 and 15 wt%, respectively, leads to a higher fire resistance than vegetable fiber ones [ 21 ]. Wool fiber forms intumescent char during the combustion resulting in low heat of combustion and also high limiting oxygen index [ 22 ]. Wool showed the limiting oxygen index (LOI) of 25, whereas plant fibers showed values between 18 and 20 [ 23 ]. Najmah et al. [ 9 ] prepared building blocks based on wool, sulfur, and canola oil. They suggested that the presence of wool gives to the composite a considerable flame resistance as well as the ability to resist higher temperatures. Moreover, after the incorporation of wool, the modulus of elasticity increased compared to the unreinforced counterpart. On the other hand, Kim et al. [ 10 ] observed that the fire-retardant behavior was enhanced in the PP/wool fiber composites in comparison with neat PP. Vasina et al.[ 11 ] prepared and characterized different polymer/sheep wool composites. They observed that an increase in the wool content in the composites resulted in the enhancement of sound absorption properties due to the higher conversion of acoustic energy into heat. Tusnim eta al. [ 12 ] studied the properties of jute and sheep wool fiber-reinforced hybrid polypropylene composites. They concluded that the mechanical properties increased as the fiber loading was increased and that the best results were obtained at 15% fiber loading with jute and wool fiber ratio of 3:1. In the literature, there is one study where authors investigated wool fiber embedded additive manufacturing-based PLA structures for biomedical applications [14]. Even though some recent publications treat the wool fiber-reinforced polymer composites [ 11 , 13 , 15 ], to the best of our knowledge no study has dealt with PLA/wool fiber biocomposites prepared by injection molding. Materials 2024,17, 4912 3 of 17 In the current work, novel PLA/wool fiber composites prepared by injection molding were characterized. On the other hand, the mechanical results of prepared composites were compared with a literature survey of PLA-based composites reinforced with vegetable fibers. Finally, preliminary flammability results of PLA/wool fiber composites were compared with PLA composites reinforced with vegetable fibers. 2. Materials and Methods 2.1. Materials PLA used in the current work was IngeoTM 3051D (Plymouth, MN, USA) purchased by NatureWorks. According to the supplier, the melt flow index is of 6 g/10 min at 210 ◦C and it has a density of 1.24 g/cm 3 . “Latxa” sheep wool fiber was provided by a local farmer from Urnieta (Gipuzkoa, Spain). The diameter of wool fibers can be higher than 100 µ m [ 2 ] and the density is around 1.24 g/cm3. 2.2. Wool Fiber Treatments The raw fibers were cleaned with a neutral soap in water at the temperature of 55 ◦ C. After drying the cleaned wool fibers, they were dipped in hydrogen peroxide solution with a concentration of 33%. Around 35 g of wool fiber was dipped in 1 L of hydrogen peroxide solution and different treating times were selected, 30 min and 24 h. After the peroxide treatment, fibers were washed with abundant water and finally they were dried. 2.3. Compounding and Processing of Materials PLA and wool fibers were dried in an oven at 100 ◦ C for 12 h. The fiber loading varied in the composites from 5 to 30 wt%. First, dried PLA pellets were molten in a HAAKE Rheomix 600 internal mixer (Thermo Scientific, Karlsruhe, Germany) at 185 ◦ C. Once the polymer was molten, the dried wool fibers were incorporated, and the mixture was processed for 10 min at 80 rpm. The obtained blends were pelletized and dried in an oven prior to process by injection molding technique using a HAAKE Minijet II machine. Injection was carried out at 185 ◦ C applying a pressure of 650 bar. Tensile test specimens (ASTM-D638-10, type V) were obtained. 2.4. Characterization Techniques 2.4.1. Fourier Transform Infrared Characterization Differences in chemical composition between raw wool fiber and treated fibers were observed by Fourier transform infrared spectroscopy (FTIR). FTIR spectra were obtained in a Nexus 670 spectrometer (Nicolet, Markham, ON, Canada) equipped with a MKII Golden Gate accessory (Specac, Orpington, UK). The measurements were taken in the range between 4000 and 650 cm−1with a resolution of 4 cm−1. 2.4.2. Thermogravimetric Analysis Thermogravimetric analysis (TGA) was performed using a TGA/SDTA 851 analyzer (Mettler Toledo, Greifensee, Switzerland). Samples with weights between 5 and 10 mg were heated from 25 to 800 ◦C at a heating rate of 10 ◦C/min in nitrogen atmosphere. 2.4.3. Contact Angle Measurements Contact angle (CA) values of raw wool fibers and treated sisal fibers were measured with OCA 20 (Data Physics Instruments, Filderstadt, Germany) using HPLC water as test liquid. Samples for CA were obtained by compressing short wool fibers in a mold. The water contact angle of a water droplet deposited on the sample surface was measured. 2.4.4. Differential Scanning Calorimetry The thermal properties of neat PLA and composites with 30 wt.% of wool fiber were determined by differential scanning calorimetry (DSC). A Mettler Toledo DSC 3+ equipment Materials 2024,17, 4912 4 of 17 was used and samples (5–10 mg) were heated from room temperature to 170 ◦ C at a scanning rate of 10 ◦C/min in nitrogen atmosphere. 2.4.5. Tensile Test To determine the tensile properties of wool fibers, the cross-section of fiber must be determined. Even though wool fibers show irregular cross-section, for simplicity, fibers with cylindrical shape were considered. The cross-section area was calculated measuring the wool fiber diameter by optical microscopy. For each fiber system, tensile tests were performed using 10 mm gauge length and the rate of 1 mm/min. Fifteen wool fibers were tested for tensile properties calculations. On the other hand, for composites, a minimum of five specimens were tested at the rate of 1 mm/min, and tensile strength, modulus, and the deformation at break were calculated. 2.4.6. Scanning Electron Microscopy SEM micrographs of the fractured surface of composites were performed by JEOL JSM-6400 (Tokyo, Japan) equipment. Fractured surfaces were previously coated with gold using Q150TES metallizer (Lewes, UK). 2.4.7. Vertical Burn Tests As preliminary results and for comparison purposes, using the tensile specimens, the flammability and self-extinguishing performance of prepared systems were evaluated using vertical burn tests. Composites with 30 wt% fiber content were burned and, also, the neat PLA specimens were tested for comparison purposes. A Bunsen burner WLD-TEC GmbH (Arenshausen, Germany) was used, the spacing between the top of the burner and the lower end of the tensile specimens was set 70 mm. The flame was applied for 10 s and, after removing it, the performance of samples was observed. 3. Results Wool fiber is fibrous protein, keratin, with a high abundance of cysteine amino acid. Between protein chains, the cysteine amino acid creates disulphide linkages. These disulphide bonds can be interand intramolecular and, consequently, a compact three-dimensional structure is created that stabilizes and insolubilizes the keratin proteins [ 4 , 24 ]. FTIR spectra of different wool fiber systems are shown in Figure 1. Wool fibers showed a strong broad band at 3275 cm −1 related to the N-H and O-H stretching vibrations. The bands at 1635, 1508, and 1228 cm −1 correspond to amide I, II, and III bands, respectively, related with amino acid groups of wool. The amide I band is attributed to the vibration of C=O groups and the amide II band is related with N-H bending and C-H stretching vibrations. The amide III band is derived from a combination of C-N stretching and N-H bending with contribution from C-C stretching and C=O bending vibrations [ 25 ]. The systems treated with peroxide showed a band around 1036 cm −1 assigned to the S-O symmetric stretching vibration of cysteine-S-sulphonate or cysteine sulfonic acid [ 21 , 26 – 28 ]. In addition, a new band appeared around 1169 cm−1due to oxidation reactions. In Figure 2, the mechanism of the cleavage reaction of intramolecular disulphide bonds due to oxidation treatment with peroxide was proposed. The intensity of these bands, 1036 and 1169 cm −1 , increased as the peroxide treatment time was increased. Bhavsar et al. [ 29 ] suggested that the variations observed in the region 1000–1300 cm −1 are attributed to different sulphur-containing chemical groups of wool that comprise the oxidative disulphide intermediates and the amide III band. Materials 2024,17, 4912 5 of 17 Materials 2024, 17, x FOR PEER REVIEW 5 of 17 Figure 1. FTIR spectra of studied wool fibers. In Figure 2, the mechanism of the cleavage reaction of intramolecular disulphide bonds due to oxidation treatment with peroxide was proposed. The intensity of these bands, 1036 and 1169 cm−1, increased as the peroxide treatment time was increased. Bhavsar et al. [29] suggested that the variations observed in the region 1000–1300 cm−1 are attributed to different sulphur-containing chemical groups of wool that comprise the oxidative disulphide intermediates and the amide III band. (a) (b) Figure 2. The possible cleavage of intramolecular disulphide bonds due to oxidation treatment with peroxide: (a) Intramolecular; scission in two molecules with the surface chemically modified and (b) Intramolecular; the surface chemical modification. Figure 1. FTIR spectra of studied wool fibers. Materials 2024, 17, x FOR PEER REVIEW 5 of 17 Figure 1. FTIR spectra of studied wool fibers. In Figure 2, the mechanism of the cleavage reaction of intramolecular disulphide bonds due to oxidation treatment with peroxide was proposed. The intensity of these bands, 1036 and 1169 cm−1, increased as the peroxide treatment time was increased. Bhavsar et al. [29] suggested that the variations observed in the region 1000–1300 cm−1 are attributed to different sulphur-containing chemical groups of wool that comprise the oxidative disulphide intermediates and the amide III band. (a) (b) Figure 2. The possible cleavage of intramolecular disulphide bonds due to oxidation treatment with peroxide: (a) Intramolecular; scission in two molecules with the surface chemically modified and (b) Intramolecular; the surface chemical modification. Figure 2. The possible cleavage of intramolecular disulphide bonds due to oxidation treatment with peroxide: (a) Intramolecular; scission in two molecules with the surface chemically modified and (b) Intramolecular; the surface chemical modification. The outermost layer of a woolen fiber, epicuticle, is made of overlapping scales. These overlapped scales act as liquid water repellent, as can be observed in contact angle photographs of pressed wool fiber disc with a droplet of water (Figure 3). Even though the roughness of the prepared surfaces makes it difficult to determine the contact angle accurately, both systems showed contact angle values between 110 and 120 ◦ . Theoretically, the surface for the contact angle should be smooth, but in practice this assumption does Materials 2024,17, 4912 6 of 17 not pertain. The lack of smoothness is more evident in the peroxide-treated sample where some fibers can be observed inside the water drop. Gama et al. [ 7 ] reported a contact angle of 133◦when a drop of water was deposited on the surface of wool. Materials 2024, 17, x FOR PEER REVIEW 6 of 17 The outermost layer of a woolen fiber, epicuticle, is made of overlapping scales. These overlapped scales act as liquid water repellent, as can be observed in contact angle photographs of pressed wool fiber disc with a droplet of water (Figure 3). Even though the roughness of the prepared surfaces makes it difficult to determine the contact angle accurately, both systems showed contact angle values between 110 and 120°. Theoretically, the surface for the contact angle should be smooth, but in practice this assumption does not pertain. The lack of smoothness is more evident in the peroxide-treated sample where some fibers can be observed inside the water drop. Gama et al. [7] reported a contact angle of 133° when a drop of water was deposited on the surface of wool. (a) (b) Figure 3. Photographs used for contact angle values measurements: (a) soap-cleaned wool fibers and (b) peroxide-treated fibers for 24 h. Even though the wool surface acts as liquid water repellent, there are small spaces between the scales from which water vapor can slowly enter the fiber. The TGA curves (Figure 4) showed that all fibers showed around 100 °C, a weight loss related to the water evaporation in agreement with other works [4,21,30]. The weight loss due to water evaporation in peroxide-treated wool systems is higher than for the soap-cleaned counterpart, shown in Table 1. The results suggested that the epicuticle of wool fiber seemed to be damaged by the peroxide treatment and would facilitate the diffusion of water inside of wool fiber. Furthermore, after peroxide treatment cysteine sulfonic acid was created, as observed in FTIR spectra, which led to more possibilities for the creation of hydrogen bonds with water molecules compared to the untreated counterpart, and consequently the water absorption capacity of the treated wool fibers was increased. (a) (b) Figure 4. (a) Thermogravimetric analysis and (b) derivative thermogravimetry curves of wool fibers. 200 400 600 800 0 20 40 60 80 100 Weight (%) Temperature (ºC) Soap cleaned Soap + 30 min H2O2 Soap + 24 h H2O2 Figure 3. Photographs used for contact angle values measurements: (a) soap-cleaned wool fibers and (b) peroxide-treated fibers for 24 h. Even though the wool surface acts as liquid water repellent, there are small spaces between the scales from which water vapor can slowly enter the fiber. The TGA curves (Figure 4) showed that all fibers showed around 100 ◦C, a weight loss related to the water evaporation in agreement with other works [ 4 , 21 , 30 ]. The weight loss due to water evaporation in peroxide-treated wool systems is higher than for the soap-cleaned counterpart, shown in Table 1. The results suggested that the epicuticle of wool fiber seemed to be damaged by the peroxide treatment and would facilitate the diffusion of water inside of wool fiber. Furthermore, after peroxide treatment cysteine sulfonic acid was created, as observed in FTIR spectra, which led to more possibilities for the creation of hydrogen bonds with water molecules compared to the untreated counterpart, and consequently the water absorption capacity of the treated wool fibers was increased. Materials 2024, 17, x FOR PEER REVIEW 6 of 17 The outermost layer of a woolen fiber, epicuticle, is made of overlapping scales. These overlapped scales act as liquid water repellent, as can be observed in contact angle photographs of pressed wool fiber disc with a droplet of water (Figure 3). Even though the roughness of the prepared surfaces makes it difficult to determine the contact angle accurately, both systems showed contact angle values between 110 and 120°. Theoretically, the surface for the contact angle should be smooth, but in practice this assumption does not pertain. The lack of smoothness is more evident in the peroxide-treated sample where some fibers can be observed inside the water drop. Gama et al. [7] reported a contact angle of 133° when a drop of water was deposited on the surface of wool. (a) (b) Figure 3. Photographs used for contact angle values measurements: (a) soap-cleaned wool fibers and (b) peroxide-treated fibers for 24 h. Even though the wool surface acts as liquid water repellent, there are small spaces between the scales from which water vapor can slowly enter the fiber. The TGA curves (Figure 4) showed that all fibers showed around 100 °C, a weight loss related to the water evaporation in agreement with other works [4,21,30]. The weight loss due to water evaporation in peroxide-treated wool systems is higher than for the soap-cleaned counterpart, shown in Table 1. The results suggested that the epicuticle of wool fiber seemed to be damaged by the peroxide treatment and would facilitate the diffusion of water inside of wool fiber. Furthermore, after peroxide treatment cysteine sulfonic acid was created, as observed in FTIR spectra, which led to more possibilities for the creation of hydrogen bonds with water molecules compared to the untreated counterpart, and consequently the water absorption capacity of the treated wool fibers was increased. (a) (b) Figure 4. (a) Thermogravimetric analysis and (b) derivative thermogravimetry curves of wool fibers. 200 400 600 800 0 20 40 60 80 100 Weight (%) Temperature (ºC) Soap cleaned Soap + 30 min H2O2 Soap + 24 h H2O2 Figure 4. (a) Thermogravimetric analysis and (b) derivative thermogravimetry curves of wool fibers. Table 1. The first mass loss percentages, the onset and maximum degradation temperatures of second mass loss and the char percentages for wool fibers. Wool Fiber 1st Weigth Loss 2nd Weigth Loss Char at 800 ◦C (%) Tonset (◦C) Tmax (◦C) (%) Soap cleaned 2.5 195.7 272.7 30.5 Soap + H2O230 min 5.9 203.5 275.6 25.1 Soap + H2O224 h 6.9 209.3 275.2 24.4 The second weight loss is related to the thermal degradation of the wool fibers. The degradation curve of the soap-cleaned wool fiber is slightly different compared to peroxidetreated ones. In the soap-cleaned wool fiber, different small shoulders can be observed at Materials 2024,17, 4912 7 of 17 the beginning of this degradation step. Those shoulders could be related with the thermal degradation of low molecular weight compound such as lanolin. In the literature [ 31 , 32 ] it was observed that the lanolin started to degrade around 200 ◦ C and showed a multi-step degradation. Due to that shoulder, the soap-cleaned wool fibers showed a slightly lower onset degradation temperature than the peroxide-treated ones, as shown in Table 1. Wool fiber treated with peroxide for 30 min seemed to show a small shoulder; however, after 24 h treatment this shoulder was missing in the thermogram. The TGA results suggested that after soap cleaning of the fibers, some lanolin is present in the wool fibers, but the peroxide treatment seemed to be effective to remove this residual lanolin. In agreement with the TGA results, the intensity of the absorption band observed by FTIR technique at 2926 cm −1 , attributed to –CH 2 stretching [ 29 ], seemed to diminish after peroxide treatments (Figure 1). This band reduction could be related with lanolin removal from wool fibers since chemically, lanolin consists of a mixture of several sterols, fatty acids, and their esters [ 33 ]. The onset temperatures of the peroxide-treated fibers were superior compared to the soap-cleaned fiber ones, indicating that the peroxide treatment improved the thermal stability of wool fibers. Kim et al. [ 21 ] observed that wool fiber started to degrade at around 250 ◦ C due to ruptures of the helical structure, and afterwards cystine disulphide bonds were broken at around 320 ◦ C. At high temperatures, all wool systems showed a significative residue amount, being higher for wool fibers without peroxide treatment, as shown in Table 1. In the literature, the charring ability of wool was observed previously [ 21 ]. In Table 2, the tensile properties of wool fibers are reported and compared with literature data of vegetable fibers. The soap-cleaned wool fiber showed strength, modulus, and deformation at the break values of 163 MPa, 6.2 GPa and 16.1%, respectively. It must highlighted that the standard deviation values were high, indicating a high variability in the tensile properties. Mechanical properties differed from one wool fiber to another due to several factors [ 24 ]. We noticed that the average diameter values varied after peroxide treatments. The soap-cleaned fiber was around 63 µ m, but after the oxidation treatment with peroxide the diameter was reduced to around 50 µ m. Based on the proposed cleavage mechanism of Figure 2a, this reaction could be the reason for reducing the fiber diameter. The strength value reported in the current study is in the range of the values reported in the literature [ 24 , 34 , 35 ]. Zhang et al. [ 34 ] reported for merino wool fibers a strength of 151 MPa and an elongation at the break value of 43.5%; unfortunately, they did not report the modulus value. Kim et al. [ 35 ] characterized the tensile properties of wool fibers as the average strength, modulus, and strain at the break values of 160.9 MPa, 4.8 GPa, and 27.7%, respectively. Bouagga et al. [ 24 ] studied the physico-chemical, thermal, and mechanical properties of Tunisian wool. They reported that wool fibers showed an average tensile breaking force of 16.89 cN, with the average diameter of 28.33 µ m. Based on these data, the estimated tensile strength for Tunisian wool is of 268 MPa. The strength data reported in the current study are lower than those estimated for Tunisian wool. Regarding the elongation value, Bouagga et al. [ 24 ] reported an average elongation value of 32.5%, being the value reported in the current study lower. However, the young modulus reported by them, 907 MPa, is significantly lower compared to what we determined in the current work. Table 2. Tensile strength, modulus, and deformation at break values of different natural fibers. Natural Fibers Strength (MPa) Modulus (GPa) Deformation at Break (%) Reference Soap cleaned 163 ±23 6.2 ±2.0 16.1 ±7.1 Current work Soap + H2O2 30 min 160 ±33 6.7 ±2.3 10.0 ±7.5 Current work Soap + H2O2 24 h 170 ±33 8.2 ±3.6 19.0 ±12.0 Current work Flax fiber 802 46.9 1.5 [36] Sisal fiber 366 9.5 ±3.4 3.9 ±1.3 [37] Materials 2024,17, 4912 8 of 17 After the peroxide treatment, the strength value hardly changed. Even though the oxidation reaction with peroxide resulted in chemical modifications of the wool fibers, as observed by FTIR, these modifications did not alter the tensile strength significantly. While the oxidation treatment with peroxide introduced a cleavage of the covalent intramolecular disulphide bonds, it seems that these bonds were not mainly responsible for the strength of the wool fibers [ 38 , 39 ]. Zahn and Blankenburg [ 40 ] suggested that the hydrophobic interactions between the chains were responsible for retaining the strength of wool even at high moisture regains. It is probable that the peroxide treatment did not alter hardly the hydrophobic interactions between chains, and consequently the mechanical properties did not alter significantly. Regarding the modulus, the wool fiber treated with peroxide showed higher modulus values than untreated one. Furthermore, as the treatment time was increased, the modulus value was increased. One possible explanation of this increase would be the removal of lanolin from wool fibers that can act as plasticizer. On the other hand, the modulus increment could be also due to the creation of new H-bonds among different peptide chains thanks to cysteine sulfonic acid groups created in peroxide treatment. As the treatment time was increased, the intensity of FTIR bands related with cysteine sulfonic acid groups was increased, suggesting that the number of cysteine sulfonic acid groups were increased. It should be indicated that the increment observed should be taken with caution due to the high standard deviations. Regarding the comparison with vegetable fibers, the wool fibers showed higher deformation at the break value compared to the vegetable counterparts. However, the strength and modulus values of vegetable fibers were higher than wool fiber ones, suggesting that the reinforcing effect of wool fibers was, theoretically, lower than the vegetable fibers. The lack of crystallinity of wool fibers [ 7 , 21 , 30 ], among other reasons, led to a material with lower mechanical properties compared to the vegetable fiber ones. Usually, highly crystalline fibers show higher strength and can increase the stiffness of composites [7]. Figure 5shows the injection molded specimens of PLA/wool fiber with different fiber loading. Materials 2024, 17, x FOR PEER REVIEW 9 of 17 Figure 5. The fiber loading increased from 5 wt.% (left) up to 30 wt.% (right). Figure 6 shows the strength values of neat PLA and PLA/wool fiber composites with different fiber loadings. The strength values of the composites were lower than the neat PLA one, indicating that the wool fibers were not reinforcing the polymer matrix. In general, as the content of fiber was increased in the composites, the strength value was decreased. As the strength value of fiber was considerably superior to neat PLA, the results indicated that the wool fiber/PLA adhesion was poor and there was a deficiency of stress transfer from the matrix to the fiber. (a) (b) (c) Figure 6. Tensile properties as a function of wool fiber loading and fiber treatment: (a) strength; (b) modulus and (c) deformation at break. The composites that were reinforced with wool fiber washed with soap showed lower strength values than the composites reinforced with the peroxide-treated ones. Probably, the presence of lanolin could, in addition to hampering the fiber/matrix adhesion, reduce the fiber friction during the fiber pull-out. Even though the composites containing the 510 15 20 25 30 0 20 40 60 Soap cleaned Soap + 30 min H2O2 Soap + 24 h H2O2 Strength (MPa) Fiber content (wt%) PLA 510 15 20 25 30 0 2000 4000 6000 PLA Soap Soap + 30 min H2O2 Soap + 24 h H2O2 Modulus (MPa) Fiber content (wt%) 510 15 20 25 30 0 1 2 3 PLA Soap Soap + 30 min H2O2 Soap + 24 h H2O2 Deformation at break (%) Fiber content (wt%) Figure 5. The fiber loading increased from 5 wt.% (left) up to 30 wt.% (right). Figure 6shows the strength values of neat PLA and PLA/wool fiber composites with different fiber loadings. The strength values of the composites were lower than the neat PLA one, indicating that the wool fibers were not reinforcing the polymer matrix. In general, as the content of fiber was increased in the composites, the strength value was decreased. As the strength value of fiber was considerably superior to neat PLA, the results indicated that the wool fiber/PLA adhesion was poor and there was a deficiency of stress transfer from the matrix to the fiber. Materials 2024,17, 4912 9 of 17 Materials 2024, 17, x FOR PEER REVIEW 9 of 17 Figure 5. The fiber loading increased from 5 wt.% (left) up to 30 wt.% (right). Figure 6 shows the strength values of neat PLA and PLA/wool fiber composites with different fiber loadings. The strength values of the composites were lower than the neat PLA one, indicating that the wool fibers were not reinforcing the polymer matrix. In general, as the content of fiber was increased in the composites, the strength value was decreased. As the strength value of fiber was considerably superior to neat PLA, the results indicated that the wool fiber/PLA adhesion was poor and there was a deficiency of stress transfer from the matrix to the fiber. (a) (b) (c) Figure 6. Tensile properties as a function of wool fiber loading and fiber treatment: (a) strength; (b) modulus and (c) deformation at break. The composites that were reinforced with wool fiber washed with soap showed lower strength values than the composites reinforced with the peroxide-treated ones. Probably, the presence of lanolin could, in addition to hampering the fiber/matrix adhesion, reduce the fiber friction during the fiber pull-out. Even though the composites containing the 510 15 20 25 30 0 20 40 60 Soap cleaned Soap + 30 min H2O2 Soap + 24 h H2O2 Strength (MPa) Fiber content (wt%) PLA 510 15 20 25 30 0 2000 4000 6000 PLA Soap Soap + 30 min H2O2 Soap + 24 h H2O2 Modulus (MPa) Fiber content (wt%) 510 15 20 25 30 0 1 2 3 PLA Soap Soap + 30 min H2O2 Soap + 24 h H2O2 Deformation at break (%) Fiber content (wt%) Figure 6. Tensile properties as a function of wool fiber loading and fiber treatment: (a) strength; (b) modulus and (c) deformation at break. The composites that were reinforced with wool fiber washed with soap showed lower strength values than the composites reinforced with the peroxide-treated ones. Probably, the presence of lanolin could, in addition to hampering the fiber/matrix adhesion, reduce the fiber friction during the fiber pull-out. Even though the composites containing the wool fiber treated with peroxide showed superior strength compared to the composites reinforced with wool fiber only washed with soap, the strength values achieved were lower than the neat PLA one. The aspect ratio of the reinforced fiber and the interfacial adhesion ultimately determined the tensile strength of composites. The obtained results indicated that, irrespective to the treatment, the wool fibers were not able to improve the strength in composite due to the poor fiber/matrix adhesion. However, it is clear that the peroxide treatment improved to some extent the fiber/matrix adhesion, but it was not enough. Among the prepared composite systems, the highest strength values were observed in systems treated with peroxide for 24 h. A possible explanation of the strength improvement could be the total removal of lanolin after 24 h in peroxide solution, which is in agreement with TGA results obtained. 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