Introduction

Some unique features of fibre-reinforced polymer-based composite materials discovered in the past decades have led to a shift in research, engineering, and industry interest from monolithic materials to fibre-reinforced polymer-based composites [1, 2]. Some of these unique features are high strength/weight ratio, high chemical resistance, non-corrosive properties, good insulating properties, and high fracture toughness. These composite materials consist of high strength fibres such as glass or carbon and low strength polymeric matrix, but they do not have renewability, recyclability, and biodegradability properties. In addition to that, they carry health risks if inhaled. Although fibre-reinforced polymeric composites have been widely used over the years in the aerospace, entertainment, automotive, construction, and sports industries to provide solutions to many structural problems due to their low cost and moderate strength advantages, the use of these materials may cause a serious environmental problem in the following years [2,3,4,5,6].

The consumer’s developing sensitivity to consider the environmental impact at all stages of the life cycle, including processes such as final disposal and recycling, and sanctions on environmental legislation increase the pressure on material manufacturers. Due to the increased environmental awareness on a worldwide scale, there has been an increasing trend of interest in the production of recyclable and environmentally sustainable composite materials [7,8,9,10]. The use of new generation biocomposite materials consisting of natural fibres and biodegradable matrix as substitutes for glass or carbon fibre reinforced polymer composites can help eliminating the environmental problems mentioned above; therefore, it can provide better living conditions [11, 12].

Biocomposite materials, which are partly eco-friendly or green composites, are classified according to their structure of the fibres and the matrix. The components of green composites are derived from renewable sources; hence, green composite materials reduce the dependence on petroleum-based fibre and matrix. Partly eco-friendly composites can be formed by two different methods. The first is made up of a combination of man-made or synthetic fibres with a biopolymer matrix. The other is formed by the combination of natural fibres and a petroleum-based non-degradable matrix [5, 13,14,15].

Poly(lactic acid) (PLA) is a significant biopolymer that is recyclable and completely degradable after its service life. The production process of the PLA begins with the transformation of starch derived from starch-rich crops such as corn. It is followed by the process of effective preparation of lactic, and finally, it ends with the polymerization of lactic. In terms of a life cycle, the use of biopolymeric matrix PLA has been found to reduce greenhouse gas emissions by up to 40% and also to reduce non-renewable energy use by up to 25% compared to petrochemical-based polymers like as polyethylene or polyethylene terephthalate [16,17,18,19,20,21,22].

Natural fibres have distinct superior properties such as availability, low density, low cost (on a volumetric basis), flexibility during processing, highly specific stiffness, high acoustic attenuation, low carbon footprint, low production energy consumption, and biodegradability compared to synthetic fibres [23,24,25]. Natural fibres can be classified in three different ways: animal, plant, and mineral based [23]. Examples of natural plant-based fibres include ramie [26], kenaf [27], abaca [28], banana [29], sisal [30], cotton [31], kapok [32], wheat [33], bamboo [34], corn [35], loofah [36], soya [37], poplar [38], pineapple [39], henequen [40], flax [41], hemp [42], jute [43], coir [44], oil palm [45], rice husk [46], and totora [47]. Mineral-based natural fibres include amosite [48], crocidolite [49], actinolite [50], chrysotile [51], tremolite [52], and anthophyllite [50]. Some natural animal-based fibres are horsehair [53], human hair [54], alpaca hair [55], sheep wool [56], and chicken feather [57].

Animal-based natural fibres such as chicken feather fibres (CFF), have recently attracted great attention in engineering industries and innovative product design. Therefore, the use of CFF as a reinforcement for polymer-based biodegradable materials has slowly increased. Because of its superior characteristic properties such as recyclability and renewability, the CFF has been recognized as a new reinforcement element for polymer-based composites [2, 57,58,59]. One of the studies in which CFF is used as reinforcement in polymeric materials was done by Bessa et al. [60]. In the study, thermal and acoustic insulation properties of a composite material formed by using thermoset epoxy resin reinforced by CFF were investigated. According to thermal resistance and acoustic insulation test results, CFF has high potential to be used as reinforcements in composite materials. Bessa et al. [60] has also found that the thermal resistance of CFF/epoxy composite materials is positively dependent on the mass fraction of CFF (80:20, respectively, CFF and epoxy). Another study was conducted by Zhan et al. [61], and in this study, the electrical resistances of CFF/epoxy and E-glass/epoxy composite materials were investigated. The electrical resistance of CFF/epoxy composites has been found to be greater than that of E-glass/epoxy composites. One of the studies investigating the properties of biocomposite material, which was produced by the use of PLA as matrix and CFF as reinforcement, was conducted by Özmen et al. [62]. In this study, the change of thermal characteristic properties of CFF/PLA was investigated, and according to thermogravimetric analysis (TGA) results, it was found that CFF/PLA biocomposite material has higher thermal stability than pure PLA. In another study, Cheng et al. [63] investigated the thermal and mechanical properties of CFF/PLA biocomposite materials and found that the addition of CFF enhances the thermal stability of the green composites compared to pure PLA.

In order to determine the long-term durability of a new product such as a biocomposite and guarantee its reliability of use under certain conditions, it is important to understand the ageing process of the product. Studies on the ageing of biocomposites can support existing knowledge and help to promote their use in real-life applications. Accelerated ageing studies carried out under the conditions of artificial environment, which are prepared to investigate the effects of temperature or different parameters, are aimed to obtain faster results [64, 65]. Isadounene et al. [66] identified changes in physico-mechanical properties by accelerated ageing of PLA biocomposites reinforced with alkali-treated olive husk flour. The results show that when the ageing process was prolonged, the mechanical properties of the biocomposite reduced as a result of the plasticization of PLA and the swelling effect. Lila et al. [67] studied the accelerated thermal ageing behaviour of PLA based biocomposites reinforced with bagasse fibres. According to the results obtained from X-ray diffraction and dynamic mechanical analysis, they concluded that a significant change in crystallinity and glassy transition behaviour occurred during the ageing period. Gil-Castell et al. [68] investigated the effect of accelerated hydrothermal ageing on the thermal stability and morphological properties of sisal/PLA biocomposites. The results showed that the increased crystallinity decreases thermal stability values. Under long-term service conditions, polymeric materials are known to exhibit susceptibility to degradation, and they are subjected to thermal, photochemical, oxidative or even hydrolytic degradation, either individually or simultaneously [69, 70]. In this context, it is necessary to determine the performance of polymer-based composites under real service conditions for use in specialized applications [71]. Le Duigou et al. [72] subjected the injection-moulded flax/PLA biocomposites to natural seawater long-term ageing for two years and determined the changes in their properties. After the immersion, breakage of the fibres and reduced ability to bond between PLA and the fibres were observed.

Under atmospheric conditions, PLA slowly degrades, and the degradation process may last for 3–5 years. Due to the inherent complex morphology of polymeric composite materials, it is difficult to estimate their performance under service conditions. The parameters used in the production process, such as the production temperature and the production method, affect the thermomechanical properties of the composites; thus, the composites can exhibit anisotropic and non-linear behaviour under service conditions [73, 74]. Long-term ageing under room conditions has serious effects on the performance of composite materials, and it can alter the physical properties of the amorphous phase of glassy polymers [75]. Prolonged ageing of PLA-based biocomposites under room conditions has an important role in determining the shelf-life of these materials, and no studies have been reported in this area.

It can be seen from the literature review, there are very few studies investigating the properties of CFF-reinforced composites, and in addition, no research has yet been made on how long-term atmospheric ageing affects the properties of the biocomposites. In general, studies on ageing are accelerated under artificial conditions due to time constraints. While it is possible to obtain useful information on the long-term performance of biocomposites from accelerated ageing studies, there is a great need for real-time studies. Because only in this way, it will be possible to determine the shelf life of composites and how long they can work under real service conditions. The aim of this study is to find out how the thermal properties of biocomposite materials produced by adding different amounts of CFF to PLA thermoplastic matrices are affected by long-term atmospheric environmental ageing condition. The thermal properties of pure PLA and CFF/PLA biocomposite samples were already determined before the ageing process, and after the ageing process applied, the thermal properties were examined with the same thermal characterisation test equipment and parameters. In this context, mass loss-temperature curves by applying TGA, derivative mass loss-temperature curves by applying derivative thermogravimetry (DTG) analysis, decomposition temperatures by overlapping TGA-differential thermal analysis (DTA) curves, some characteristic temperatures such as glass transition (Tg), enthalpy, and crystallinity values by applying differential scanning calorimetry (DSC) analysis were determined for both aged and non-aged samples. Furthermore, the analysis was deepened by the addition of scanning electron microscopy (SEM) micrographs of the fracture surfaces. CFF/PLA biocomposites produced with variable CFF composition ratios, long-term ageing duration, controlled experimental environment, and material design are critical outputs of this study. The data obtained from this long-term ageing study under real-time exposure allow us to compare the data obtained as a result of accelerated ageing studies which are carried out under the conditions of artificial environment.

Experimental procedures

Material production, specimen preparation, and long-term ageing conditions

Prior to the production process of the biocomposite material, white CFF, which were supplied in a raw condition from a local company in Manisa/Turkey, were subjected to several preparation processes. The raw fibres were in the range of 20–40 μm in diameter and about 20 mm in length. Rachises of the CF were manually cut into barbs to obtain CFF from them. A detailed illustration of a typical white chicken feather is given in Fig. 1.

Fig. 1
figure 1

The detailed view of a chicken feather

Full size image

In order to determine the effect of the CFF mass content on the thermal properties of the biocomposites, CFF mass ratios of 2, 5, and 10% were used. The WiseStir HT-50AX mixer was used to mix PLA with CFF at predetermined mass ratios. CFF/PLA biocomposite materials were produced by injection moulding with the aid of a twin-screw extruder at a speed of 150 rpm and a final extruder temperature of 205 °C. The biocomposite material variables used in this study are shown schematically in Fig. 2. In addition, detailed information on CFF/PLA biocomposite material production processes can be found in the study of Özmen et al. [62] and Baba et al. [76].

Fig. 2
figure 2

The components of CFF/PLA biocomposite material

Full size image

Environmental atmospheric long-term ageing of biocomposite materials was carried out by storing specimens in a special container for 5 years (43,800 h) in a laboratory environment at a temperature of 23 ± 2 °C and average humidity of 50 ± 5%. Laboratory temperature and humidification were provided by air conditioning during the ageing process in order to avoid unstable conditions. Samples were kept under ideal living conditions without being affected by direct sunlight and airflow. Production procedure of this study is illustrated in Fig. 3.

Fig. 3
figure 3

Detailed production procedure

Full size image

Characterisation methods

Thermal characterisation

TGA, DTG, DTA, and DSC thermal characterisation tests were performed to determine how the ageing process affects the thermal stability and thermal properties of CFF, pure PLA, and CFF/PLA biocomposite material. TGA, DTG, DTA tests were performed by heating the samples having a mass of 10–20 mg from 30 to 500 °C using a TA TGASDT Q600 thermal analysis instrument under a nitrogen atmosphere of 50 ml/min at a heating rate of 10 °C/min. In order to determine Tg values, crystallization, and melting characteristics of pure PLA and CFF/PLA samples, DSC analysis technique was used. DSC analyses of both aged and non-aged samples were performed with DSC-TA Q10 instrument at a temperature increasing rate of 10 °C per minute from 28 to 300 °C.

Morphological characterisation

SEM analysis was carried out to analyse the fracture surface morphology of the PLA and CFF/PLA samples. In order to achieve micrographs from the fracture surface of the non-aged CFF/PLA composites, a COX EM-30 SEM device with an acceleration voltage of 15 kV in accordance with the E986 standard was used. Field Emission Scanning Electron Microscope Carl Zeiss 300VP was used with the same parameters to investigate the fracture surfaces of aged samples.

Results and discussion

TGA, DTG, DTA, DSC, and SEM analyses were performed to determine how thermal properties of CFF/PLA biocomposite material, which is composed of PLA matrix and CFF fibre reinforcement in mass ratios of 2, 5, and 10%, are affected by long-term ageing (43,800 h) process.

Thermogravimetric, derivative thermogravimetry, and differential thermal analyses

The thermal decomposition of non-aged and long-term aged CFF/PLA biocomposites is shown in TGA curves presented in Fig. 4. The mass loss of non-aged and aged PLA, CFF, and CFF/PLA biocomposite samples occurs in three steps [63, 77, 78]. In the first mass loss step, it is noticed that the volatile content and moisture in the sample evaporate by the effect of increasing temperature [63, 77, 79]. It is clear from the thermographs that the percentage of volatile content relative to the total mass is quite low. In the second mass loss step, pure CFF, pure PLA, and CFF/PLA biocomposite samples begin to degrade. In the third step, the material starts to decompose. It is observed that the mass losses due to decomposition increase continuously as the temperature increases, and the most serious mass losses occur at this step. The mass loss at this step means that the thermal stability is reduced due to the decomposition of the material [80, 81].

Fig. 4
figure 4

TGA curves of (a) non-aged and (b) aged samples

Full size image

Among the non-aged samples (Fig. 4 (a)), PLA has the highest total mass loss, while CFF has the lowest mass loss. Three mass loss steps for the CFF are consecutive to eliminate the adsorbed water, destruction of disulphide bonds (cystine) arising from amino acid in the keratin structure and evaporation of hydrogen sulfide, and moderate decomposition of keratin structure in CFF [82]. PLA shows a decomposition of 99.46% from the initial testing temperature of 30 °C to the end testing the temperature of 500 °C. The mass losses of non-aged CFF/PLA biocomposites decrease with the increasing mass content of CFF in the biocomposite, as evidenced by the TGA curves in Fig. 4 (a).

According to the TGA curves of the aged samples in Fig. 4 (b), the order of mass losses of the aged samples is the same as that of the non-aged ones (Fig. 4 (a)). The highest mass loss belongs to PLA with 99.37%, and the lowest mass loss belongs to CFF with 71.25%. When the mass losses of the non-aged samples and the aged ones are compared with each other, it is seen that the long-term ageing process does not affect the mass loss of PLA, but it increases the mass losses of CFF/PLA biocomposites and reduces that of CFF.

The total mass losses are higher for the aged biocomposites compared to the non-aged ones. CFF/PLA biocomposites are thermally stable until 250 °C, and above 250 °C, they begin to decompose due to the presence of CFF. Since the mass of the samples remains unchanged around 400 °C, PLA matrices are decomposed, and the remaining residue results from CFF. Due to the higher amount of keratin, the composites with higher CFF content start degrading at lower temperatures than the composites with lower CFF content [77] for both non-aged and aged samples.

Figure 5 shows derivative mass-temperature graphs of the non-aged and long-term aged samples, and it points out the temperature at which the highest decomposition rate of the CFF, PLA, and CFF/PLA biocomposite samples with the same CFF content. The decomposition temperature is indicative of materials resistance to temperature, and it can be affected by factors such as thermal ageing process [83, 84]. For the case of non-aged samples, the highest decomposition peak belongs to PLA samples with 357.70 °C, and the lowest decomposition peak belongs to CFF with 329.79 °C. It is observed that decomposition temperatures of 2, 5, and 10% CFF/PLA samples decrease with the increasing CFF content. Similarly, the aged sample with the highest decomposition temperature is PLA (348.03 °C), and the lowest one is CFF (326.53 °C). Although the decrease in decomposition temperatures due to ageing effect is more pronounced in PLA than in CFF, it can be commented that the ageing process reduces the decomposition temperatures of all the samples.

Fig. 5
figure 5

DTG curves of (a) non-aged and (b) aged samples

Full size image

By overlapping the DTA and the TGA thermographs, the peak (Tpeak), onset (Tonset), and endset (Tenset) temperatures on the DTA curve corresponding to the TGA curve as mass loss values can be found (Fig. 6). As a representative example, the mass loss steps of aged 2% CFF/PLA biocomposite samples are detailed in Fig. 6. The first two steps correspond to a lower mass compared to the third mass loss phase, which can be called the complex area on the DTA thermograph includes two endothermic peaks and a complex peak. Since this area includes the Tonset, the temperature at which the decomposition begins, the thermal stability of the composite can be determined with Tonset [85,86,87]. 1.97% of the mass is lost by evaporation of adsorbed water from hydrophilic groups of CFF, and 10.33% mass is lost by the degradation of the composite between steps two and three corresponding to 290.58 (T2i) to 315.30 (Tonset) °C, respectively. In step three, Tonset, at which the material begins to decompose, is also accepted as the thermal stability, and this value is 315.30 °C. The material experiences the most serious decomposition rate at 344.44 °C, and the complex area ends at 364.71 °C. In the complex area, 2% CFF/PLA biocomposite loses 85.54% of the sample mass. Composite materials can be used up to high temperatures without degradation due to their good thermal properties such as thermal stability. These temperatures correspond to the onset temperature of the material [81, 86, 88].

Fig. 6
figure 6

TGA – DTA curve of the aged 2% CFF/PLA biocomposite sample

Full size image

Critical values obtained from TGA-DTA curves of the all samples are given in Table 1. As can be seen from Table 1, Tonset of the non-aged 2% CFF/PLA biocomposite sample is 319.60 °C, while this temperature dropped to 315.30 °C with the effect of long-term ageing. The aged 5% CFF/PLA sample has less volatile content (3.04% vs. 3.24%), while the percentage of content degraded in step two (6.64% vs. 4.97%) and the percentage of content decomposed within the complex area increase (85.19% vs. 84.56%). In addition to this, with the effect of ageing process, greater mass loss occurs in the aged 5% CFF/PLA sample (%96.73 vs. %95.78). The thermal stability of the 10% CFF/PLA biocomposite sample decreases with the effect of ageing. Tonset of the non-aged sample is 316.10 °C, while Tonset of the aged one is 315.66 °C. When the volatile content percentages are evaluated, non-aged 10% CFF/PLA sample has greater content with 4.67%. The content of the aged 10% CFF/PLA sample that degraded in step two is higher with 17.41%. The total mass loss of the aged 10% CFF/PLA sample is higher than that of the non-aged one (96.10% vs. 95.34%). When Table 1 is examined for PLA, it is observed that aged PLA has less volatile content, and the start temperature of second step (T2i), which is the starting point of the degradation, decreases sharply from 326.43 to 304.64 °C. The thermal stability of the PLA sample also decreases from 338.41 °C to 336.43 °C with the effect of ageing. When the percentages of PLA sample decomposed within the complex area is examined, they are 91.41% and 80.59% for the aged and the non-aged, respectively. In addition, when the total mass loss of PLA is evaluated, it is seen that the mass losses of the non-aged and aged sample are almost the same. The thermal stability value of the CFF decreases with the effect of ageing (232.75 °C vs. 231.57 °C). The volatile content of the non-aged CFF appears to have a greater percentage by mass (9.10% vs. 6.48%), while the percentage of aged CFF decomposed within the complex area appears to be greater (14.04% vs. 30.99%). Taking the entire table into consideration, it is noticed that the non-aged samples contain more volatile content. It is obvious that Tonset, which indicate the thermal stability in all samples, and T2i are seen to decrease with the effect of long-term ageing in all samples. Under the influence of ageing process, Tonset ,T2i, and volatile contents decrease, while the total mass loss in both CFF and CFF/PLA biocomposite samples increases.

Table 1 Degradation and decomposition temperatures of PLA, CFF, and CFF/PLA biocomposites
Full size table

In this study, the degradation rate, which means the decomposition of the material with the effect of temperature, is obtained by summing up the mass losses of material occurring in the 2nd and 3rd (complex area) steps. The degradation rates of all samples are given in Fig. 7. Accordingly, when the aged and non-aged of the CFF, PLA, and CFF / PLA biocomposites are evaluated among themselves, an increase in the degradation rates is observed in all samples with the effect of the ageing process. At the same time, the degradation rates of the samples, whose thermal stability decreases with the effect of ageing, increase.

Fig. 7
figure 7

Degradation rates of the samples according to TGA-DTA curves

Full size image

Differential scanning calorimetry analyses

DSC thermograms are recorded to observe the effects of the long-term ageing process on the crystallization and melting behaviour of PLA and CFF/PLA biocomposite samples. Figure 8 illustrates the DSC curves of the samples while Table 2 shows the DSC parameters of the non-aged and aged PLA and CFF/PLA biocomposite samples. In general, DSC measures the flow of heat during an endothermic or exothermic process that develops in a controlled environment due to time and temperature function. In the DSC plot, the upper transition peaks indicate the heat requiring endothermic zone, while the down transition peaks reveal heat-releasing exothermic zone. Tg and decomposition temperatures (Tm) are obtained from the endothermic transition peaks, while the crystallization temperature (Tc) is obtained from the exothermic peak. In addition, cold-crystallization enthalpy (ΔHc) and melting enthalpy (ΔHm) can be obtained from DSC graphs while the degree of crystallization (Xc) can be calculated by the following Eq. (1) [89]:

$$ {X}_c=\frac{\Delta {H}_m-\Delta {H}_c}{w\Delta {H}_m^0}\times 100\% $$
(1)
Fig. 8
figure 8

DSC curves of (a) non-aged and (b) aged samples

Full size image
Table 2 DSC results of non-aged and aged PLA - CFF/PLA biocomposites
Full size table

w represents the mass fraction of PLA, and the melting enthalpy of all crystalline PLA (\( \Delta {H}_m^0 \)) is taken as 93 J/g in the literature [62, 87, 90].

The DSC thermogram generated for non-aged PLA and CFF/PLA biocomposites is presented in Fig. 8 (a). All exothermic and endothermic peaks are clearly visible in Fig. 8. Accordingly, the first endothermic peak between 57.65–62.85 °C indicates the Tg. PLA has the highest Tg with 62.85 °C, the Tg of 2% CFF/PLA is 58.65 °C, the Tg of 5% CFF/PLA is 57.65 °C, and the Tg of 10% CFF/PLA is 60.45 °C. All CFF/PLA biocomposite samples have lower Tg values than pure PLA. As can be seen in the graph, the Tg of the 10% CFF/PLA biocomposite sample, which has the highest CFF mass, is greater than the 2% and 5% CFF/PLA samples. The reason for the lack of a trend may result not only from the non-homogeneous mixture of PLA at higher concentration of CFF (10%) but also from the heterogeneity of the different parts of the CFF [64]. Exothermic transition peaks between 106.35 °C and 124.85 °C refer to the crystallization temperatures. The highest Tc belongs to PLA with 124.85 °C, and the lowest Tc belongs to 5% CFF/PLA biocomposite sample with 106.35 °C. Considering the energies released during crystallization, the amount of energy released increases with the increasing CFF content. The lowest ΔHc belongs to PLA with 2.89 J/g, while the highest ΔHc belongs to 10% CFF/PLA biocomposite sample with 23.20 J/g. Declines in the ΔHm and Xc values of non-aged samples are observed with the increasing CFF content. The ΔHm value for pure PLA is 30.10 J/g, while it is 25.31 J/g for the 10% CFF/PLA biocomposite sample, resulting in a loss of 15.91%.

The thermogram with DSC curves of aged pure PLA and CFF/PLA biocomposite samples is given in Fig. 8 (b). According to this thermogram, the Tg values ranged from 66.80 °C to 67.96 °C, so it can be stated that there is no significant change. When Tc values are considered, the highest Tc belongs to PLA with 117.10 °C, whereas for biocomposite samples these values range from 108.75 °C to 109.78 °C. The ΔHc value of PLA is 25.18 J/g. The ΔHc value is 20.84, 25.25 and 21.94 J/g for 2, 5, and 10% CFF/PLA biocomposites, respectively. When Tm values are taken into consideration, values ​​of CFF/PLA biocomposites are higher than pure PLA. As CFF content increases, Tm value increases. The lowest Tc belongs to PLA with 150.00 °C, while the highest Tc value belongs to 10% CFF/PLA with 155.66 °C. In addition, the highest ΔHm value belongs to 5% CFF/PLA biocomposite samples with 29.65 J/g.

Tg refers to the temperature region where the polymer changes from a rigid, glassy structure to a soft, rubbery structure. The Tg value can be affected by parameters such as ageing, thermal applications, polymer architecture, and degree of crystallinity [91, 92]. Ageing mechanism and ageing products can cause a change in the molecular chain structure of PLA [91]. According to Table 2, it is observed that all Tg values of aged samples increase compared to those of the non-aged samples with the same CFF mass content. This increase in Tg is attributed to the steric hindrance effect in biocomposite samples. This finding suggests that ageing mechanism may inhibit the mobility of the chains, possibly due to recrystallization phenomena occurred in the long-term ageing period [35, 84, 93].

When the ΔHm and Xcvalues of CFF/PLA biocomposites are compared, it is observed that these values ​​are higher in the aged ones. This finding may be explained by the phenomenon of lamella thickening. The phenomenon of lamella thickening results from chain scissions of CFF/PLA biocomposite sample chains during the long-term ageing process. Chain scission results in the formation of shorter molecules with higher mobility. These modified chains recrystallize during the long-term ageing process; thus, they increase the total degree of crystallinity [35, 84, 94, 95]. The increase in crystallinity justifies the decrease in thermal stability. The small shoulders seen in the DSC curve of aged 10% CFF/PLA sample (Fig. 9) demonstrates the presence of new crystallites produced by the chain scission process, which leads to shorter molecular chains [94, 96]. It is also expected that yield and fracture stresses, and elastic and storage modules of CFF/PLA biocomposite samples to decrease with increasing degree of crystallinity [94, 97, 98]. In addition, it is obvious from the DSC curves that all the non-aged CFF/PLA biocomposite samples show a double endothermic melt peak. This behaviour has been associated with the melt recrystallization mechanism; the less perfect crystals in this region show melting at lower temperatures, and they reorganize as the heating process continues in stable crystals [94, 99].

Fig. 9
figure 9

DSC curves for melting heat of 10% CFF/PLA biocomposite samples

Full size image

Morphological analyses

The thermal characterisation results reveal that the crystallization behaviour is not similar for PLA and CFF/PLA composites. Comparing the ΔHm and Xc values of the non-aged and aged PLA, it is seen that ΔHm and Xc values decrease with the effect of the long-term ageing process. This decrease may be due to an application that can affect the polymeric structure of the material, such as the ageing process, which prevents polymeric molecular chains to diffuse and migrate to the surface of the growing face of the polymer [35, 79, 84, 93, 100,101,102]. The reason that the ageing process can prevent the diffusion and migration of the polymeric molecular chains and prevent the rearrangement of the polymeric molecular chains is the impurities formed on the PLA surface [35, 100]. These impurities are formed by the ageing effect on the PLA surface as can be seen in Fig. 10. The aged PLA morphology demonstrates more broken and a rougher surface than the non-aged PLA.

Fig. 10
figure 10

SEM micrographs of (a) non-aged and (b) aged PLA samples

Full size image

SEM micrographs of fracture surface of non-aged and aged CFF/PLA biocomposite samples are given in Fig. 11. When micrographs of samples with the same CFF mass ratio are compared to each other, it is seen that the aged samples have more microvoids, cavities and gaps between fibre and matrix. As a result of the long-term ageing process, it is seen that cavities and microvoids on the fracture surfaces of CFF/PLA biocomposite samples increase, and the interfacial bonding success between the CFF and matrix material PLA decreases. One of the parameters that express the quality of the interfacial bonding between the fibres and the matrix is the gaps formed between the fibre and the matrix. As the numbers of gaps increases, the quality of the interfacial bonding between the fibres and the matrix decreases [103]. In this context, when micrographs of aged and non-aged samples are examined, more gaps are observed between the fibres and matrix in the aged samples. This indicates that the interfacial bonding between the fibres and the matrix of aged samples is weaker.

The strong interfacial bonding between the matrix and the reinforcing element, and the morphological stability are considered among the factors that cause the high thermal stability of the composite [89, 104,105,106,107]. As can be seen in the Fig. 11, the ageing process has an impact on the surface morphology of the biocomposite sample; thus, the presence of microvoids and cavities on the surface led to morphological instability. In addition, the interfacial bonding failure between PLA and CFF with the effect of the ageing process is one of the reasons that cause thermal stability to decline.

Fig. 11
figure 11

Fracture morphology of non-aged and aged CFF/PLA biocomposite samples (a and b) 2%, (c and d) 5%, (e and f) 10%. The left panel shows the non-aged biocomposite samples and the right panel shows the aged biocomposite samples

Full size image

Conclusions

In this study, the effect of long term natural atmospheric ageing process on thermal properties of CFF/PLA biocomposite samples having different CFF mass concentration ratios were investigated. With the findings obtained from the experimental study, the following conclusions can be drawn;

  • According to the results of TGA, the residual mass ratio of PLA does not change with the effect of ageing, whereas the ageing process increases the mass loss of CFF/PLA samples as the CFF mass ratio decreases. Accordingly, the reduction in residual mass ratios is 2.79% for 2% CFF/PLA, 0.95% for 5% CFF/PLA, and 0.76% for 10% CFF/PLA.

  • According to DTG curves, decreases in decomposition peaks of CFF/PLA biocomposite samples are observed with the effect of ageing process. The decomposition peak of PLA decreases by 2.70% to 348.03 °C, that of the CFF decreases by 0.99% to 326.53 °C, that of 2% CFF/PLA decreases by 3.79% to 338.07 °C, that of 5% CFF/PLA decreases by 3.77% to 335.48 °C, and that of 10% CFF/PLA decreases by 1.92% to 339.05 °C.

  • By overlapping the DTA and TGA curves, it is seen that the thermal stability of the samples decreases with the effect of the ageing process. Accordingly, Tonset of the CFF decreases by 0.51% to 231.57 °C, that of PLA decreases by 0.59% to 336.43 °C, that of 2% CFF/PLA decreases by 1.35% to 315.30 °C, that of 5% CFF/PLA decreases by 2.31% to 308.40 °C and that of 10% CFF/PLA decreases by 0.14% to 315.66 °C.

  • According to the DSC analysis results, increases in the Xc of the aged CFF/PLA biocomposite samples are noticed. This increase, which associated with the lamella thickening phenomenon, is 6.79% for 2% CFF/PLA, 4.98% for 5% CFF/PLA and 6.11% for 10% CFF/PLA.

The results contained herein may be useful in understanding the ageing mechanism of biocomposite materials. Moreover, the results can be used to determine the critical parameters that should be considered to ensure long-term performance of these composites.