The development of biodegradable polymers has advanced in recent years, mainly due to the overwhelming need to reduce the impact of polymeric materials when improperly discarded in nature. However, there are major challenges to be surpassed, especially regarding characteristics such as mechanical strength and cost of such materials, when compared to commodity polymers widely used today. Motivated by this goal, this study focused on the development of a composite material consisting of a biodegradable polymer the poly(hydroxybutyrate-co-valerate) (PHBV) reinforced with cellulose fibers, which are cheap and abundant. Mechanical and thermal properties of the polymer reinforced with different fiber contents were analyzed and compared with the neat material. Also, a burial-soil test was carried out to evaluate the environmentally degradable characteristics of the compositions studied. Results showed that the incorporation of cellulose fibers into the biodegradable matrix can both optimize the mechanical properties in use, and accelerating the environmental degradation of the polymeric material after use, when properly disposed, showing that biodegradable composites are environmentally friendly.

Keywords:PHBV, cellulose fibers, mechanical properties, biodegradation.

Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) is a biodegradable copolyester consisting of hydroxybutyrate (HB) and hydroxyvalerate (HV). PHBV belongs to the family of polyhydroxyalkanoates (PHA), synthesized by a wide variety of bacteria as an intracellular reserve of carbon and energy.^1,2 PHBV has gained a lot of attention in the recent years as an eco-friendly material, produced by renewable sources, biodegradable and biocompatible polymer, and can be processed by usual techniques applied to common polymers.³ The similarity of some properties to the polypropylene (PP) indicates that it may be a future substitute for the polyolefins, finding application in products with short life cycle, including toys, shavers, agricultural and cosmetic packaging, napkins, cups and plastic cutlery. PHBV has been also used in the medical field in bone grafts, soft gels, pins, sutures and in neural tissue engineering due to its biocompatibility.^3,4 The use of polymers with biodegradable characteristics, such as PHBV, mixed with natural fibers, which also have biodegradable characteristics and are abundant in nature, may have the potential to produce materials with desirable mechanical characteristics, inexpensive, and at the same time environmentally friendly as it can degrade spontaneously when disposed in nature. However, it is necessary to understand how biodegradable polymers and natural fibers interact with each other, focusing on determining the mixing conditions that provide optimal performance conditions for use of the material and improved biodegradation characteristics after discarded. The present study describes the influence of untreated cellulose fibers in the PHBV matrix on the thermal, mechanical and biodegradation properties of the resulting composite. The biodegradation properties were evaluated by soil-burial test according to standard ASTM G160.

Materials used for this study were: poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) containing 3.34 mol% of hydroxyvalerate, supplied by PHB Industrial S.A. (Serrana, Brazil) and cellulose fibers from the pine PinusTaeda, supplied by Cambará S.A. (Cambará do Sul, Brazil). 

Composites containing 20 wt% and 40 wt% of cellulose fibers were prepared. The PHBV and cellulose fibers were dried in an oven with air circulation at 70°C for a period of 72 hours. After drying, the composites were prepared by a molten mixture process in a co-rotating twin screw extruder with five heating zones and the following temperature profile: 150/155/160/170/165°C (barrel to matrix, respectively) and 200rpm rotation. The compositions used in the composites and neat PHVB are shown in Table 1. After processing in the extruder, the mix was transferred to a preheated injection molding machine for specimen fabrication. Specimens, with dimensions 127x12.7x3.2mm, according to ASTM D790 for flexural testing, were molded at temperature of 160°C and the injection pressure was 655bar. The injection tool was kept at 6°C. 

The environmental degradation test for PHBV and composites was carried out using a burial-soil test, according to ASTM G160. The detailed composition was as follows: 5kg of coarse sand (10 mesh), 5kg of fertile topsoil and 5 kg of well-rotted horse manure. The soil composition was prepared by simple mixing and sifted through 1/4 in. mesh screen. After mixing, the mixture was aged for three months. At the end of the aging time, the mixture was divided into sixty 500ml polypropylene cups, and each cup received one specimen. There were three exposure times for each composition: 30, 60 and 90 days.

Methods of characterization

Fourier transformed infrared spectroscopy (FTIR): FTIR analysis was performed using a model Nicolet iS10 spectrometer from Thermo Scientific. FTIR analysis were performed for all the compositions, before and after the burial-soil test, to investigate changes related to the chemical composition of the samples, as when incorporating cellulose fibers as during the burial-soil test. Each sample was milled in a cryogenic mill, cold with liquid nitrogen, for a period of 5 minutes. After milling, the powder obtained was mixed with KBr and made into a pellet, and then it was fixed to the FTIR sample plate. Spectra were taken in triplicate for each sample at 400 to 4000cm^-1.

Differential scanning calorimetry (DSC): DSC of the samples was performed using a model DSC-50 from Shimadzu Corporation. The same heat/cool method was adopted for all specimens testing, before and after burial soil test. Specimens were heated from 0°C to 200°C, at a ramp hate of 10°C.min^-1 and cooled with the same ramp. The data for melting points and enthalpy of fusion were recorded during the heating cycle. The crystallinities of neat PHBV and PHBV in the composites were obtained from the following equations:

$X{c}_1=(\Delta H_{f\text{}}/\Delta H_o)\times 100$(1)

$X{c}_2\text{}=(\Delta H_f/(\Delta H_o\times W))\times 100$(2 )

where Xc₁ and Xc₂ are the crystallinities of the neat PHBV and PHBV in the composites, respectively, ∆H_o is the thermodynamic enthalpy of fusion per gram of PHBV (109 J.g^-1), ∆H_f is the apparent enthalpy of fusion per gram of PHBV/Cellulose fiber composites and W is the fraction of mass content of PHBV in the composites (Sing et al., 2010).⁵

Thermogravimetric analysis (TGA): TGA of the samples was performed using a model TGA-50 from Shimadzu Corporation. The samples were heated from room temperature to 700°C, with a heating rate of 20 °C.min^-1, using nitrogen atmosphere.

Mechanical behavior: Flexural properties of the neat PHBV and composites were performed using an EMIC DI-3000 testing machine, according to ASTM D790.The samples were evaluated before and after soil-burial tests. The numbers of samples tested in each case were five for all the respective compositions.

Scanning electron microscopy (SEM): The surface morphology of the neat PHBV samples before and after soil-burial tests was investigated by scanning electron microscopy (Superscan S-550, Shimadzu, Japan), to check for structural changes during the burial-soil period. The samples were mounted on aluminum stubs and then coated with carbon in vacuum by evaporation in order to make the samples conducting.

Before burial-soil test

FTIR: Figure 1 illustrates FTIR spectra for PHBV, cellullose fibers and the composites. Analyzing Figure 1 it was possible to note that FTIR spectra for the composites are a combination from the PHBV and cellullose fibers spectras. When increasing the cellullose fibers content, there is a change in the region between 3700 to 3000 cm^-1. The shape of the band stays the same with its center around 3200 cm^-1, but there is an increasing at the area above the band when increasing the fiber content. This region at the FTIR spectra is related to hydroxyl stretching, possibly indicating hydrogen bonds between the hydroxyls belonging to cellulose fibers and the carbonyls belonging to PHBV. The band at 3437 cm^-1 from the PHBV spectra can be related to water content from KBr used in the specimens as well an overtone from carbonyl band at 1720 cm^-1.

Figure 1 FTIR spectra for neat PHBV, cellulose fibers and composites PHBV/CF.

DSC: Figure 2 illustrates DSC melting curves of neat PHBV and composites. The details are given in Table 2. Adding cellulose fibers in the PHBV matrix did not affect its melting point which remains at 161°C for both compositions. As shows Table 2, the addition of cellulose fibers increased the crystallinity of PHBV. This behavior indicates that the cellulose fiber facilitates the crystallization of PHBV, which can be correlated with the easier processing and higher dimensional stability of the composites specimens if compared to neat PHBV. Neat PHBV showed no crystallization peak during cooling, Figure 3, and showed a peak in the second heating. This behavior indicates that the polymer has slow crystallization kinetics, not crystallizing for the cooling rate used in the analysis. However, when adding cellulose fibers in the polymer matrix, it showed crystallization peak during cooling, reinforcing the idea that cellulose fibers facilitate the crystallization of PHBV. Figure 3 illustrates crystallization peaks for neat PHBV and composites at the cooling stage. The maximum temperature of crystallization occurs at 67°C and 71°C, for 20 wt.% and 40 wt.% of cellulose fiber in the composites, respectively.

Figure 2 Melting curves for neat PHBV and composites PHBV/CF.

Figure 3 Crystallization curves of PHBV and composites PHBV/CF.

TGA: Figure 4 illustrates the weight loss curves of neat PHBV, composites and cellulose fibers. PHBV begins to thermally degrade at 260°C and degrades completely at 350°C, reaching the maximum degradation at 310°C. Cellulose fibers have a first weight loss from 40°C to 120°C, related to water loss, and a second weight loss related to thermal degradation, beginning at 250°Cuntil around 400°C, reaching the maximum degradation at 360°C. The PHBV lost weight gradually with the increasing temperature. The PHBV degradation process involves chain scission and hydrolysis which leads to reduction in molecular weight and formation of crotonic acid (Singh et al., 2008). As illustrates Figure 4, the thermal degradation behavior of composites is a cumulative phenomenon of thermal degradation of neat PHBV and cellulose fiber. Degradation of both composites starts at 260°C and ends at 390°C.The TGA of composites also showed the fibers to be the final constituent to undergo degradation in composites. Figure 5 illustrates the derivative weight loss of PHBV and composites. It is noted that the addition of 40 wt% cellulose fiber gently moves the peak of the TGA curve to the left, indicating that the addition of cellulose content decreases slightly the thermal stability of PHBV, and this behavior may be due to degradation of hydroxyl groups in cellulose.⁵ Table 3 shows thermal events and corresponding temperatures observed through the TGA curves.

Figure 4 Weight loss of PHBV, cellulose fiber and their composites.

Figure 5 Derivative weight loss of PHBV and composites PHBV/CF.

Figure 6 FTIR spectra for neat PHBV before and after burial-soil test.

Figure 7& Figure 8 illustrate FTIR spectra for composites PHBV/FC 80/20 and 60/40 respectively. Possibly physical disintegration observed in samples can be facilitating both access of microorganisms into the polymer matrix and the diffusion of water, enhancing the process of hydrolysis and material degradation. Moreover, Figure 8 illustrates the appearance of a band in the region around 1650 cm^-1 (indicated by arrows). This region of transmittance is indicative of amide I, which may result from proteins produced by microorganisms. However, this same region, along with a slight increase in intensity observed near 1425 cm^-1 corresponds to the carboxylate anion.¹⁰ No significant change was observed on the carbonyl band at 1720 cm^-1. Analyzing Figure 7 & Figure 8 it is possible to observe an increase in transmittance in the band at about1050 cm^-1. This band can be related to the primary alcohol, which its characteristic bands are between 1085 to 1050 cm^-1.

Figure 7 FTIR spectra for PHBV/FC 80/20 before and after burial-soil test.

Figure 8 FTIR spectra for PHBV/FC 60/40 before and after burial-soil test.

Differential scanning calorimetry (DSC) for neat PHBV and composites after burial-soil test: Table 5 show the thermal characteristics and crystallinity for neat PHBV and composites before and after burial-soil test. Table 5 shows that there were no major changes in both melting point and crystallinity of the neat PHBV, comparing the different exposure times to burial-soil test. However, it is possible to observe a slight decrease in crystallinity with 90 days of exposure to burial-soil test. According to Table 5 for PHVB/CF/80/20 PHVB/CF/60/420, composites showed a large decrease in crystallinity of the polymer with increasing exposure time to burial-soil test. The degree of crystallinity can be defined as an index which evaluates the microstructural ordering of atomic or molecular groups present in The degree of crystallinity is directly related to the composition of material, chemical structure, molecular weight and processing conditions in which the material is submitted. In general, as higher is the crystallinity, as greater is the resistance of the material to hydrolysis and oxidation.^4,11,12 Amorphous polymers with lower chain packing tend to present higher rates of degradation. In the case of PHBV, in which the crystalline phase corresponds to the hydroxybutyrate (HB) and amorphous phase corresponding to hidroxivalerato (HV), it leads to deduce that the degradation process occurs preferentially in the amorphous phase, increasing material crystallinity. For exposure times of up to 60 days, this behavior was not observed. In composites, cellulose can absorb soil moisture and swell. In this way, the reinforcemegoogle tradutornt can make the polymer matrix more susceptible to microorganisms. The loss of physical integrity of the composite, PHBV becomes susceptible to degradation for hydrolysis, causing chain scission and loss of molecular weight, leading to a decrease in crystallinity. For samples exposed for 90 days to the burial-soil test, an increase of crystallinity was observed, which may be related to a more severe degradation of the amorphous phase in the PHVB¹⁰ and de cellulose fibers. The results with 90 days of exposure are similar to those of neat PHBV.

Figure 9 TGA curves for PHVB and composites before and after burial-soil test.

Mechanical behaviour for neat PHBV and composites after burial-soil test: Results show a loss of mechanical properties for all samples after burial-soil test (Table 6). After 30 days of exposure to burial-soil test, PHBV practically did not show changes in flexural strength, but showed a decrease of around 3.5% in Young´s modulus. On the other hand, composite PHBV/FC 80/20 showed a decrease in flexural strength of around 22% and a decrease in Young´s modulus of around 13%. PHBV/FC 60/40 showed the highest decrease in mechanical properties after 30 days of exposure to burial-soil test. Flexural strength decreased around 64% and Young´s modulus decreased around 45% comparing to the values before the burial-soil test. After 60 days of exposure to burial-soil test all the samples showed decrease in mechanical properties. For neat PHBV flexural strength decreased 10% and Young´s modulus decreased around 6%. For PHBV/FC 80/20, flexural strength decreased around 51% and Young´s modulus decreased around 58%. PHBV/FC 60/40 showed a decrease in flexural strength around 94% and Young´s modulus decreased around 95%. After 90 days of exposure to burial-soil test, it was possible to observe a continuing loss of mechanical properties for neat PHBV. On the other hand, both composites showed a slight increase in flexural strength and Young´s modulus which agrees with the increasing crystallinity and better thermal resistance if compared to the samples with 60 days of exposure to burial-soil test.

Figure 10 SEM micrographs for neat PHBV with different exposure times to burial-soil test.

Figure 11 SEM micrographs for neat PHBV after 60 days of exposure time to burial-soil test.

Figure 12 SEM micrographs for neat PHBV after 90 days of exposure time to burial-soil test.

Results showed that the addition of cellulose fibers improves the mechanical properties of PHBV with no appreciable changes to its thermal properties. Regarding to biodegradation, results showed that all samples exposed to burial-soil test experienced a decreasing on their thermal and mechanical properties. Composites with 40 wt% cellulose fibers showed higher degradation with exposure to burial-soil test probably due to the cellulose susceptibility to moisture and, consequently a microbial degradation. The degradation of cellulose facilitates the access of microorganisms to PHBV. Thus, after use, the composites can have their degradation process favored.

Authors would like to acknowledge the companies Cambara S.A. and PHB Industrial S.A. for donating the materials used in this research. The authors would like to express their gratitude to CAPES-PROSUP and the support of the National Council for Scientific and Technological Development (CNPQ) for the Productivity Research PQ2 scholarship.

The authors declare that there is no conflict of interest.

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