The Effect of Natural Plant and Animal Fibres on PLA Composites Degradation Process

Abstract

One of the methods to reduce long-term excessive plastic waste is the development and use of composite materials based on biodegradable polymers and natural fibres. Composites with natural fibres can exhibit very good mechanical properties, and the presence of natural fibres can significantly accelerate the degradation of the material. This study aimed to manufacture and analyse the biodegradation process of composites based on biodegradable polylactide (PLA) filled with flax and sheep wool fibres. The effect of flax and wool fibres and their content on the degradation rate compared to that of pure PLA was investigated. The degradation progress and properties of the composites were studied using an optical microscope, SEM, measurement of surface roughness, and contact angle. Additionally, flexural strength tests, a dynamic mechanical analysis (DMA), and a thermogravimetric analysis (TGA) were conducted. The effect of natural fibres on the phase transition and degree of crystallinity was analysed using differential scanning calorimetry (DSC). The results showed that PLA degrades only under UV light, but not in the composter simulating the natural environment. However, the incorporation of both types of fibres accelerated degradation of PLA/fibres composites in soil. Flax fibre composites exhibited better mechanical properties than pure PLA. For composites with wool fibres, although they showed a significant acceleration of the degradation process in the soil, their large content in the composite caused a reduction of mechanical properties. This research showed the positive effect of the addition of natural fibres on the biodegradation of PLA.

1. Introduction

In recent decades, there has been a very rapid development of polymeric materials, but more attention has been paid to the problem of storing and processing plastic waste. Conventional polymers commonly used lose some of their properties during reprocessing, and this significantly limits the possibilities of their reuse. In addition, the degradation time of the most commonly used polymers, such as polyethylene (PE), polypropylene (PP), and polystyrene (PS), is very long because their chain structure consists of only carbon linkages. Unfortunately, these polymers are mainly used to produce packaging, which has a very short lifetime and quickly becomes a waste. The packing industry generates the greatest demand for plastics, which is around 45% of all polymers produced [1]. Another serious problem is the huge amount of waste dumped in landfills that sometimes, due to poor management, can enter the seas and oceans, where it causes significant pollution of the ecosystem. Eight million tonnes of plastic is estimated to enter the oceans every year [2]. One possible solution to this difficult issue is to return to natural materials and, at the same time, reduce the use of conventional polymers.

In biocomposites, at least one of the components is of natural origin or is biodegradable. The role of the matrix is to transfer tension and protect against mechanical damage, while the most commonly used fillers (natural fibres) serve to improve mechanical properties, reduce density, and accelerate biodegradation [3]. The most popular biocomposites consist of polymer matrix and natural fibre filler.

We can divide natural fibres, according to their origin, into animal, vegetable, and mineral fibres [4]. An important advantage of natural fibres is that they reduce the density of the composite made with them. On the other hand, attention should be paid to the low thermal stability of these fibres and the difficulty of obtaining a homogeneous dispersion in the polymer [5].

One of the most studied plant fibres is flax. Its advantages are its low price and low density, as well as its very promising mechanical properties and the fact that it does not release harmful residues when burnt [6]. The properties of flax, such as its low density (1.5 g/cm³), tensile strength between 345 and 1500 MPa, and Young’s modulus between 30 and 80 GPa, are good enough for it to compete with or be used as a replacement for glass fibres [7]. Studies [8] show that the combination of natural fibres and biodegradable polymers can produce promising results. Plant fibres can significantly increase tensile strength. It is important to find the right amount of fibre addition, as too high a value may contribute to a gradual deterioration of properties, mainly due to the difficulty in maintaining a good degree of bonding between the composite components.

In recent years, animal fibres have not attracted as much attention compared to plant fibres. The development of materials based on sheep’s wool fibres and their use in industry is one of the leading methods to the revival of declining pastoralism in mountainous regions (e.g., in the Tatra Mountains in Poland and the southern part of the country) [9], as well as in places where the inefficiency of using large amounts of wool causes significant economic losses [10]. Wool fibres do not have as high strength properties as plant fibres, but they are often considered for their potential use as a flame retardant [9]. Wool soaks up water, which can greatly accelerate the degradation of a composite made with it, but sometimes the degradation rate can be too high [11]. Wool fibres also have a high capacity for sound and thermal insulation. All of these characteristics have led to the use of wool in the textile industry for hundreds of years, but its use in other industries is constantly being developed [12,13,14]. Additionally, low-quality animal fibre waste amounts to approximately 3 million tons per year. The problem of disposal of this amount of fibrous waste is increasingly being discussed from an environmental-protection point of view [15].

Of the many biodegradable polymers, polylactide (PLA) has secured its status as the most popular on the market. This is supported by its easy availability and attractive value for money. PLA can be moulded as other thermoplastic polymers using melt processing methods, such as extrusion, blow moulding, or injection moulding. Moreover, the thermal stability of PLA, although slightly lower than that of conventional thermoplastics, is higher than that of other biodegradable polymers when subjected to melt processing and forming [16].

The combination of PLA with natural fibre is a very promising research direction. The use of both components makes it possible to reduce the disadvantages of the individual components and offers the possibility of creating a material alternative to the current solutions on the market.

Hence, there has been a great amount of research conducted on natural fibre composites. A vast majority of them are related to plant fibres because of their good mechanical properties. Pawłowska et al. [17] used the addition of modified and non-modified flax fibres in injection-moulded composites to improve their mechanical properties. The use of flax fibres resulted in an improved Young’s modulus compared to that of pure PLA. Sit et al. [18] investigated the effect of weather conditions on composites with flax fibres compared to PVC. The initial samples showed properties better than PVC, and, despite ageing, their properties were still comparable. During testing in high humidity, they noticed swelling of the fibres, which caused cracking of the matrix. Pantaloni et al. [19] compared flax composites with biodegradable and PP matrixes. They confirmed in their study the possibility of designing materials with a mechanical performance suitable for industry and are additionally able to offer faster biodegradation. In another study, Pantaloni et al. [20] investigated the effect of humidity on flax composites, where only at high humidity values (98%RH) did the mechanical properties decrease significantly. Nassiopoulos and Njuguna [21] tested flax/PLA composites against flax/epoxy for their thermo-mechanical properties. The use of epoxy resin has advantages in terms of thermal stability, but the high Young’s modulus obtained for PLA suggests that its composites have a good chance of being used in automotive applications.

Much fewer studies address the use of wool fibres in composites. Sharma et al. [22] studied the mechanical and thermal properties of wool/epoxy composites. They obtained composites with good interfacial adhesion and improved tensile strength. Abdellah et al. [23] conducted a study in which they created hybrid composites with date palm fibre/sheep wool in a polyester matrix. The compound containing 20 wt.% of the date palm fibre and wool with a mass ratio 50/50 showed the best properties in mechanical tensile and impact tests. Its properties were good enough for use in materials of daily use, while keeping the material very lightweight, which is one of the greatest advantages of using sheep wool. Haque et al. [24] made sheep wool/PCL (polycaprolactone) composites suitable for 3D printing. The composite with the addition of 10% wool showed increased biodegradability and higher yield strength.

The main objective of this research was to determine and compare the properties of biocomposites based on PLA and wool or flax fibres before and after the degradation process and to compare the effects of different natural fibres on the degradation process of the polymer matrix.

2. Materials and Methods

2.1. Materials

The PLA, in the form of granulates, that was used during this study was NatureWorks’ Ingeo 3260 HP. According to the manufacturer data sheet, the density of the material is 1.24 g/cm³, the tensile strength is 65 MPa, the elongation at break is 3.4%, and the melting point is 200 °C. The flax fibres were obtained from Safilin’s Flax Low Twist Roving Tex 2000. According to the manufacturer, the fibre density is 1.44 g/cm³, the tensile strength is 212 MPa, the elongation at break is 1.9% and the Young’s modulus is 12 GPa. The wool fibres used were obtained from the breeding of mountain sheep carried out at the Experimental Station in Bielany by the University of Agriculture in Krakow. Polish mountain sheep are a typical species found in Southern Poland. As a solvent for PLA, dichloromethane (DCM) was used as received.

2.2. Manufacturing Methods

The first step in the preparation of the composite material was to clean the wool fibres by washing them (pure soap and water 60 °C) and then cutting the fibres into short pieces of several millimetres. Composites with 10 wt.% and 20 wt.% flax or wool fibre were obtained via mechanical mixing of the polymer granules with the fibre and adding a solvent (DCM p. a. 99.7%, PureLand^®) to dissolve the polymer. Then, the material was poured onto a flat glass. After evaporation of the solvent, the material was cut (Figure 1a). Next, the granulate was used to obtain the samples using injection moulding process. A Zamak injection moulding machine was used. During the tests, the following parameters were set: injection time for pure PLA—10 s; injection time for natural fibre composites—12 s; mould temperature for PLA—50 °C; and mould temperature for composites—80 °C. Common parameters to all materials were a cylinder (chamber) temperature of 210 °C and injection force of 12 kN. Figure 1b shows specimens with 20 wt.% of wool added after the injection process.

2.3. Degradation Methods

The composites obtained were exposed to UV-A and UV-B radiation in a specially prepared ageing chamber (12 W bulb). Samples were placed on foil stands in Petri dishes, in which a small layer of water (25 mL) was poured to ensure constant humidity during the ageing process. The samples were exposed to UV radiation for 12 days. The study was conducted at room temperature.

The next portion of the composite samples was composted. Freshly cut grass, forest mulch, wet soil, and flower fertiliser soil were used to prepare the compost. All ingredients were mixed and placed in containers. The composite samples were placed 5 cm deep in such a prepared composter. The soil was watered every 5 days to ensure moisture and covered with a punctured stretch film to ensure air flow. A lamp was directed at the entire set of containers to keep the site warm. The samples remained in the composter for 16 days. The study was conducted at room temperature.

All types of samples prepared, their labels and composition and the degradation process used are summarised in

Table 1. List of all the sample types tested, their composition, and the degradation process they were subjected to.

Sample Name	PLA (% wt.)	Flax (% wt.)	Wool (% wt.)	UV Degradation Process	Soil Degradation Process
PLA	100	-	-	-	-
W10	90	-	10	-	-
W20	80	-	20	-	-
L10	90	10	-	-	-
L20	80	20	-	-	-
PLA_UV	100	-	-	+	-
W10_UV	90	-	10	+	-
W20_UV	80	-	20	+	-
L10_UV	90	10	-	+	-
L20_UV	80	20	-	+	-
PLA_CO	100	-	-	-	+
W10_CO	90	-	10	-	+
W20_CO	80	-	20	-	+
L10_CO	90	10	-	-	+
L20_CO	80	20	-	-	+

.

2.4. Research Methods

Contact angle testing was performed on a Krüss DSA 10 MK2 goniometer to determine the changes in wettability of the samples before and after degradation. Roughness measurement was performed using the Hommelwerke T1000 profilometer. The software calculated R_a (average roughness of a surface) and R_z (difference between the tallest “peak” and the deepest “valley” in the surface) to characterise the roughness. Mechanical tests were carried out on a Zwick/Roell RetroLine 1445 test machine. The method used was the three-point bending of the specimens (PN-EN ISO 178). Each type of composite tested was subjected to DMA (dynamic mechanical analysis) tests on a DMA 850 (TA Instruments). The tests were carried out at a constant frequency of 10 Hz and in the temperature range of −60 ÷ 100 °C, with a heating rate of 5 °C/min, using three-point bending mode. PLA, L10, and W10 samples (before and after degradation processes) were subjected to thermogravimetric analysis (TG). Measurements were preformed using a TGA 550 (TA Instruments) thermal analyser, in platinum crucibles, under a nitrogen atmosphere. The heating rate was 10 °C/min. The study was conducted at the temperature range 45 ÷ 600 °C. DSC analysis was performed on PLA, L10, and W10 samples (before and after degradation processes), using DSC 1 from Mettler Toledo with STARe software. The samples were subjected to a heating (10 °C/min)/cooling (30 °C/min)/heating (10 °C/min) temperature program in the temperature range of 0–210 °C, under nitrogen atmosphere, with a flow rate of 30 mL/min. The samples were placed in pierced and sealed aluminium pans. The reference sample was an empty crucible. Using data from the DSC analysis, the degree of crystallinity (X_c) was calculated using the following formula (Equation (1)):

$${\mathrm{X}}_{\mathrm{c}}=\frac{∆{\mathrm{H}}_{\mathrm{m}}}{∆{\mathrm{H}}_{\mathrm{m}}^0(1-\mathrm{a})}$$

(1)

where ΔH_m is the heat of melting of studied PLA composite, ΔH_m⁰ is the heat of melting of 100% crystalline polymer (for PLA 93.7 J/g [25]), and a is the mass fraction of the additive (fibres). The microstructure of the composites was investigated using an FEI Nova NANO SEM 200 scanning electron microscope equipped with an EDS analyser from EDAX and using a Keyence VHX-900F digital microscope.

3. Results and Discussion

The results of the contact angle tests provided information on the surface quality of the manufactured biocomposites before and after degradation. The contact angle results obtained are presented as a graph in Figure 2.

The incorporation of natural fibres of both wool and flax has a significant effect on the wettability of the PLA-based composites. It was observed that the degradation of pure PLA under UV light and composting conditions causes a decrease in the contact angle. The presence of natural fibres increases the values of contact angles on the surface of samples of nondegraded composites (immediately after their production). However, after degradation, the measured contact angles of the biocomposites are different, probably related to the uneven degradation of PLA and the presence of areas of exposed fibres. Although cellulose is strongly hydrophilic, there are studies that describe its influence on the wetting of the surfaces and thin layers. The high hydrophilicity of cellulose can be reduced with mineral additives such as TiO₂ [26]. Also, studies by Stark and Matuana [27] confirmed an increase in contact angle value when composite samples containing cellulose are subjected to UV ageing processes. Despite the use of a HDPE matrix, significant changes in the surface of the investigated HDPE with woof flour composites were observed. In our studies, it was found, that there is a tendency for the contact angle to decrease after UV degradation and increase under simulated composting degradation for PLA/natural fibre composites with 20 wt.% fibre content.

Roughness measurements were the second test performed to assess the surface quality of the composite samples. The results obtained in the study are presented in Figure 3 and Figure 4.

The evidence of strong UV degradation on the PLA surface was confirmed by the significant increase in roughness after ageing under UV radiation. The presence of natural fibres causes an increased surface roughness in the resulting composite after exposure to ultraviolet (UV) radiation; however, compared to pure PLA, flax fibres slow down degradation, while wool fibres accelerate it. Both linen and wool accelerated degradation in the composter compared to the degradation of pure PLA.

In the case of the addition of wool, an increased roughness was observed compared to pure PLA after both irradiation and composting, while linen delayed the degradation under UV light but accelerated it in the soil, as seen in Figure 4. Other scientists in their research observed similar changes in roughness after ageing processes. The presence of flax fibres in the composite significantly increases the R_a values [18], and the sisal fibre in a PLA matrix, after 15 days in compost, doubles its roughness (from a value of 1.2 µm to values ranging from 2 to 3.5 µm of the R_a index). The pure PLA matrix itself increases its roughness from a value of 0.5 µm to a value of 1 µm. In all studies, it was observed that, with an increase in the content of natural fibres in the composite, the rate of surface roughening also increases [28]. The increase in roughness can be correlated with an increase in the rate of biodegradation because the greater surface area allows easier transport of water deep into the material and the anchoring of fungi and microorganisms in the pores and defects of the composite. The right combination of fibres in the composite will allow the life of the biomaterial to be regulated.

The results of the flexural strength test are summarised in

Table 2. Young modulus and tensile strength for PLA and PLA/natural fibre composites before and after degradation.

Sample	E (GPa)	Tensile Strength (MPa)
PLA	2.39 ± 0.08	47.61 ± 0.08
L10	3.22 ± 0.09	60.57 ± 9.73
L20	3.73 ± 0.37	63.59 ± 8.76
W10	2.20 ± 0.28	33.82 ± 6.77
W20	2.04 ± 0.05	23.86 ± 0.82
PLA_UV	1.09 ± 0.12	3.13 ± 1.13
L10_UV	2.68 ± 0.21	28.22 ± 5.94
L20_UV	3.37 ± 0.50	19.71 ± 9.23
W10_UV	1.24 ± 0.17	7.64 ± 3.59
W20_UV	1.08 ± 0.01	13.35 ± 3.35
PLA_CO	2.59 ± 0.07	51.28 ± 5.15
L10_CO	2.46 ± 0.33	39.56 ± 5.70
L20_CO	2.30 ± 0.29	28.05 ± 8.40
W10_CO	1.36 ± 0.52	13.66 ± 10.46
W20_CO	1.06 ± 0.54	13.98 ± 11.12

, and the selected flexural curves are shown in Figure 5. In Figure 5a, it can be seen that the mechanical properties of PLA after composting are slightly altered, while a significantly broken and degraded structure of the material with very poor properties can be seen after UV irradiation. In Figure 5b, flax is observed to have slowed the loss of mechanical properties during UV degradation of the sample, while it significantly accelerated the composting process. For the addition of wool (Figure 5c), there are also similar relationships to those for the addition of flax.

PLA has poor resistance to UV radiation (PLA_UV—a 92% drop in transmitted force during testing). Radiative degradation leads to the almost-complete destruction of PLA samples. PLA in damp soil, on the other hand, does not change as much (PLA_CO—a 10% drop in transmitted force during testing). The use of flax as a filler in the PLA matrix reduced the destructive impact of UV radiation on the PLA matrix in the investigated biocomposites. However, it was observed that, in this case, degradation in moist soil (L20_CO) has a more significant effect. However, this degradation is at a level of 63% compared to a sample containing flax and not exposed to degradative factors. Wool as an additive, similar to the addition of flax, slows UV degradation. However, the addition of wool fibre results in a significant decrease in the transmitted force values when exposed to moist soil (compared to a similar sample with flax). PLA undergoes hydrolytic degradation that involves several steps, including the diffusion of water into the amorphous phase of PLA, macromolecules chain scission by hydrolysis of the ester bonds, and the formation of the oligomers and monomers; and, in the next step, we see partial hydrolysis of the crystalline fractions [29]. Since washed wool fibre is hygroscopic and hydrophilic, it facilitates the transport of water deep into the material, leading to the hydrolysis of the PLA matrix and its degradation, thus accelerating the degradation of the entire material. UV degradation was slowed down by each type of fibre, and more force had to be applied to destroy the composite. The more fibres, the greater the difference in destructive forces, and the addition of flax slowed UV degradation significantly more than the addition of wool. For the degradation caused by composting, an acceleration of the degradation rate can be seen under the influence of fibre addition, since the force used to destroy the composite is, in each case, lower than that used with pure PLA. For L10 samples, the difference is very small compared to PLA. In this case, the greater the addition, the faster the degradation, and the wool fibres accelerated the degradation more.

It can be seen (Table 2) that the mechanical properties of the composite increased after the addition of flax fibre in both cases compared to pure PLA. For small amounts of wool fibre, the properties are comparable to those of PLA, but with the large addition of these animal fibres, the mechanical properties decreased. Few studies have focused on the influence of sheep wool fibres on the mechanical properties of polymer matrix composite materials. Wool fibre is characterised by relatively low tensile strength compared to even natural fibres from plants (around 120 MPa). Sharma et al. [22] tested a modified composite with sheep wool fibres in an epoxy matrix and obtained a composite with improved mechanical properties of 45 MPa. Such strength values are typical for pure plastics and even some thermoplastics. Hence, the conclusion was drawn that sheep wool fibres are not suitable as a reinforcing modifier for composites, but rather as a filler or functional additive. The ageing process (degradation of the biocomposite in water) occurs quite rapidly. In other studies, the strength values decreased to a level of 30% (L20 samples experienced a 50% decrease after exposure to a humid environment) [30,31]. Modification with cut, short natural fibres in polymer matrices results in an improvement in mechanical properties by up to 30%. The continued increase in the proportion of natural fibres in composites is associated with a slight increase in conjunction with deterioration of technological conditions (flow decrease and increased porosity of the biocomposite) [32].

The second test used to evaluate the mechanical properties and how they change as a consequence of the conditions and degree of degradation was a dynamic mechanical analysis (DMA), for which some of the curves are shown (Figure 6). The range to which particular attention was paid includes temperatures from −30 °C to 50 °C, as within this range, the material would be mainly used.

From the graphs shown in Figure 6, it can be seen that a very high storage modulus was obtained for the materials immediately after manufacture. As confirmed by other tests, PLA samples subjected to composting were not significantly different from PLA samples not subjected to degradation. Therefore, the values of the storage modulus between these samples do not have significant differences, and the slightly lower modulus for the PLA sample without degradation may be due to slow degradation and physical ageing processes in the composted sample. As a result, composted PLA is characterised by higher degree of crystallinity and stiffness, and the storage modulus is higher. For each fibre, with the exception of W10, the value of storage modulus improved compared to pure PLA. It can be seen that a large content of wool fibres destabilises the material properties more quickly. In addition, the low content of fibres significantly reduces the difference in the loss modulus responsible for stress dissipation. From the available studies, it can be observed that the addition of flax fibres does not affect the glass transition temperature, but it does alter the ratio of the storage modulus to the loss modulus. Short flax fibres stiffen the composite and decrease the PLA macromolecules’ mobility [21,33,34]. Composites with a PLA matrix containing flax fibres increase the value of the storage modulus almost twice [35]. The UV-exposed PLA sample was the only one to deteriorate before reaching 100 °C. When composted samples were tested for pure PLA and the L10 composite, no deterioration in properties was obtained. For the L20_CO sample, both the storage modulus and the loss modulus decreased steadily and dropped by 15% compared to the nondegraded L20. A very high decrease was recorded for the wool composites, indicating a very high degradation of these composites in the ground. For the composted samples, there is a clear difference between the flax and wool composites, which have significantly lower properties.

The TG curves and the corresponding DTG curves are presented in Figure 7. A summary of the most important parameters determined from the TG curves to assess the dynamics of material degradation is included in

Table 3. Results of TG analysis *.

Sample	T1% (°C)	T3% (°C)	T5% (°C)	T20% (°C)	T50% (°C)	TDTGmax (°C)	Wr (%)
PLA	288	309	318	342	359	367	0.53
L10	266	301	310	334	351	358	2.17
W10	239	292	305	330	348	357	4.16
PLA_UV	267	307	317	343	360	367	0.57
L10_UV	155	271	296	329	348	355	2.46
W10_UV	235	289	305	331	349	358	3.58
PLA_CO	290	311	318	338	353	358	0.99
L10_CO	285	311	319	340	356	361	2.31
W10_CO	237	275	287	315	334	341	5.11

* T_{1(3,5,20,50)%—}temperature at 1, 3, 5, 20, and 50% mass loss; T_DTGmax—degradation temperature; W_r—char residue at 600 °C.

.

From Figure 7, it can be seen that the manufactured composites show high thermal stability. The greatest difference can be seen for sample W10_CO, where degradation occurs significantly earlier than for the other samples (Figure 7e). In Table 3, it can be seen that much higher residue values at 600 °C were obtained for the samples with natural fibres. The degradation under UV light accelerated the weight loss in the samples with flax; this is due to the strong destruction of the matrix, which accounted for 90% of the weight of the composites. For wool fibres in the case of composting, no differences can be seen at first; they only appear around 350 °C, where the fibres play a significant role in the acceleration of degradation.

For flax fibres with PLA, rapid changes occur from about 300 °C, where the degradation of the polymer itself begins, and the presence of fibres shifts the temperature of maximum degradation towards higher values [17]. PLA samples reinforced with other plant fibres (coir, pineapple leaves, and bamboo) also showed stability in the same temperature range, and from about 350 °C, they degraded in one step [36,37]. The TG studies showed that the presence of both flax and wool fibres increased the degradation under UV light and during composting.

A DSC study was carried out on PLA, L10, and W10 samples to assess the effect of the presence of the fibre on the processing and degradation properties of the polymer. Plots of the first- and second-cycle heating curves are shown in Figure 8, and a summary of the most important parameters analysed is given in

Table 4. Results * of T_g from DMA, DSC analysis and crystallinity in 1st and 2nd heating cycle.

Sample	Tg,DMA (°C)	Tg,DSC1 (°C)	Xc1	Tm1 (°C)	Tg,DSC2 (°C)	Xc2	Tm2 (°C)
PLA	74	67	55%	177	60	54%	175
L10	75	64	48%	177	60	49%	174
W10	72	65	50%	176	59	49%	173
PLA_UV	-	64	52%	172	58	55%	172
L10_UV	76	64	31%	175	54	41%	172
W10_UV	80	68	46%	175	59	46%	172
PLA_CO	74	65	57%	176	60	56%	174
L10_CO	73	67	51%	177	59	49%	174
W10_CO	78	67	49%	175	59	48%	173

* T_g,DMA—glass transition temperature obtained from DMA; T_g,DSC1(2)—glass transition temperature obtained from DSC (1: 1st heating cycle; 2: 2nd heating cycle); X_c1(2)—crystallinity degree (1: 1st heating cycle; 2: 2nd heating cycle); T_m1(2)—melting temperature obtained from DSC (1: 1st heating cycle; 2: 2nd heating cycle).

. Additionally, the glass transition temperature value from the analysis of the DMA curves (Figure 6), calculated as a peak in the tan(δ) plot, is added to the table for comparison.

In the first heating cycle (Figure 8a), strong relaxation of the polymer can be seen after the injection process in each case. Performing the degradation by UV irradiation led to the destruction of the polymer structure, as can be seen when comparing the graphs for PLA and PLA_UV. During the second heating cycle (Figure 8b), a significant reduction in processing-induced relaxation and significant exothermic peaks can be observed from cold crystallisation. These are more pronounced for samples with added natural fibres, which can thus be considered to be the centres of crystallisation.

Due to the different characteristics of the methods, the glass transition temperatures obtained from DMA are higher than the corresponding temperatures from DSC (the PLA_UV sample in DMA was destroyed before the result could be obtained). In the second heating cycle, the glass transition occurred at lower temperatures, and the lowest values were obtained for the samples after UV degradation. The calculated degree of crystallinity of the pure PLA was 54%, and the presence of fibres reduced its value. The melting temperatures are very close to each other, but lower values were obtained for samples with the addition of fibres, especially wool.

A reduction in T_g values due to the addition of natural fibres has also been noted in other studies. Furthermore, the authors observed that the addition of flax increases the enthalpy of the cold crystallisation of PLA [17]. In the study, the addition of kenaf fibres lowered the melting temperatures of PLA-based composites and accelerated the recrystallisation process [38]. For various plant-based natural fibres, e.g., coir and bamboo, an increase in T_g values can be seen in relation to pure PLA [36,37]. The presence of natural fibres in PLA composites mainly affects the degree of crystallinity of the polymer and its properties during processing; furthermore, a significant reduction in the parameters studied after degradation processes confirms its successful course.

Selected SEM microphotographs are shown in Figure 9. The SEM observations for pure PLA show very clear cracks for the exposed composite. The appearance of the sample after composting is not significantly different from that of the sample immediately after fabrication. Figure 9d,g provide a very good comparison of the size and surface area of plant and animal fibres. It can be seen that PLA homogenised much better with the flax fibres, covered them, and formed a physical bond over a significant area of the fibre.

In Figure 9e,h, it can also be seen that both the fibres and the matrix have been degraded, but in Figure 9f,i, it can be seen that the main degrading component are the fibres. Broken fibres and pulling out of fibres from the matrix (pull-out) have also been observed by other scientists studying similar biocomposites [18,20]. They have reached similar conclusions, noting that the higher the content of natural fibres in the biocomposite, the worse the degradation that occurs [24].

Additionally, EDS studies were performed on the samples, which confirmed the qualitative composition of the composites tested. For the pure PLA and L20 samples, peaks were obtained for carbon and oxygen. For the W20 composites, peaks were obtained for carbon, oxygen, nitrogen, and sulphur.

Surface microscopic observations of the samples after and before degradation were carried out to assess the visual effects of the degree of degradation—Figure 10.

When comparing samples made from PLA alone (Figure 10a–c), clear cracks can be seen for the samples degraded under UV radiation. They have also lost their transparency. The composted PLA sample shows slight wrinkling on the surface, which may indicate the beginning of surface degradation, but apart from this, there is little difference from the non-degraded PLA.

For the samples in group W10 (Figure 10d–f), a clear difference can be seen between the degraded samples and the samples after generation. The degraded samples are much lighter in colour, with uneven surfaces and protruding fibre fragments visible. It is caused by the appearance of microcracks in the volume of the PLA sample, on which the passing light disperses, giving the sample a milky discolouration effect. The composted W10 sample shows deep cracks and significant distortion of the whole sample. The brightest (milky) fragments on the edges and around the fibres signal that biodegradation is taking place mainly on the surface and in the adjacent fibres due to their high hygroscopicity. The degraded W20 samples, like the W10 samples, differed significantly in colour from the undegraded sample. Degradation progresses along the fibres. This is likely due to the easy absorption of water in the volume of wool fibres, and in a humid environment, it is easier to degrade the PLA matrix.

The L10 samples (Figure 10g–i), as the wool fibre samples, changed colour after degradation to a lighter colour. In the literature, similar observations can be found; the presence of moisture in the environment accelerates the degradation process of biocomposites. The fibres transport moisture into the interior of the sample, causing them to swell. This phenomenon speeds up the degradation [18,19]. A difference can be seen between the type of degradation of the two samples (Figure 10h,i): the composted sample shows white traces arranged mainly along the fibres and smooth brown areas of the matrix, while on the UV-illuminated sample, the degradation is more uniform throughout the composite. In the L20 samples, the colour change in the sample during the degradation test was comparable to the samples in the L10 category.

4. Conclusions

The development of composite technology based on natural and biodegradable materials may be one of the answers to the ever-increasing problem of plastic waste disposal. A major challenge is to develop materials with properties similar to those of conventional polymers at a competitive price.

In this work, two types of composites based on a PLA with different contents of flax or wool fibres were obtained. The effect of the type of fibre on PLA degradation was investigated. For samples with flax added, composites with better properties than pure PLA were obtained, the better the greater the fibre addition. Flax fibres accelerated the simulated degradation in the environment and slowed the fast degradation of pure PLA under UV exposure. Wool fibres in small amounts did not significantly change the properties of PLA, and high filler addition resulted in lower mechanical properties. Wool fibres significantly accelerated degradation by composting, mainly due to their high hygroscopicity, which allowed water transport and hydrolysis of the polymer.

Given the latest trends associated with a circular economy, more and more attention is paid to materials that are of natural origin and do not burden the environment. Sheep’s wool fibres have been present in our civilisation for thousands of years and are characterised by unique functional properties. The amount of waste, mainly packaging, is constantly increasing and the main problem is the decomposition time, which is inadequate for the duration of use. Wool accelerates the degradation of polymer matrix materials and can serve as a factor in controlling and determining the lifetime of a biocomposite that contains sheep’s wool. Due to the hygroscopic nature of washed wool, water can be transported deep into the material, and, thus, hydrolysis of the matrix occurs throughout the composite, accelerating the degradation of the entire material. Natural fibres, through their structure and properties, improved PLA degradation. The use of low-cost natural fibres or even a waste product such as sheep wool further reduces the price and increases the attractiveness and competitiveness of such a material on the market.