Effect of steam explosion and silanization of hemp fibers on polylactic acid biocomposites;analysis of mechanical-thermal properties and fungal biodegradation

Abstract

Alkaline pretreated hemp fibers were modified by steam explosion and/or silanization before being compounded with polylactic acid (PLA). The fungal biodegradation of the PLA/hemp fibers composite was investigated using Aspergillus niger TISTR 3153 in an aqueous medium for 28 days, following the ISO 846: 1997 standard method. The influence of the different physicochemical modifications of hemp fibers on the fungal biodegradation of the composite was evaluated in terms of molecular weight, chemical structure, mechanical properties, thermal properties, and hygroscopic properties. The results showed that the modulus of all composites were averagely increased by 109%, compared to neat PLA. All the PLA/hemp fiber composites better retained their properties after fungal biodegradation than neat PLA. Therefore, the physicochemical treatments of fibers after alkaline pretreatment promoted the resistance of the composite to fungal biodegradation. The treatment of hemp fibers in the present work was alkalization followed by silanization, which resulted in a PLA-based composite that was suitable for outdoor applications.

Keywords

Polylactic acid hemp fiber composite fungal biodegradation

Introduction

The growth of the worldwide green economy has driven demand for biodegradable plastics to replace petroleum-based plastics.^1,2 Biodegradable aliphatic polyesters are considered to be the most promising biodegradable and environmentally friendly plastics due to their hydro-degradability, thermoplasticity, and favorable physico-mechanical properties.³ Polylactic acid (PLA) is a biodegradable aliphatic polyester produced from corn, beet, wheat, and other renewable starch sources.⁴ Since the mechanical properties of PLA are similar or superior to those of conventional polymers, it is one of the most widely used biodegradable plastics.^5–7 Although PLA presents some drawbacks such as brittleness, low impact strength and poor resistance to thermal deformation,⁸ its properties can be improved if the polymer is used in a composite. PLA composites that included natural fibers such as kenaf,^9,10 hemp,^11,12 flax¹³ and sisal fibers^14,15 have been used in automotive interiors,¹⁶ construction materials,¹⁷ and packaging materials.¹⁸

Some of these applications require durable products, particularly outdoor applications in humid environments. For these products, the microbial degradation of the composite must be carefully considered to ensure satisfactory long-term performances.¹⁹ Over the past two decades, the biodegradability of PLA has been investigated using aerobic microorganisms such as actinomycetes, bacteria, and fungi in laboratory settings.²⁰ Aspergillus spp. are common molds found in mesophilic environments. They have shown the ability to metabolize complex organic matter by producing hydrolytic and oxidative enzymes.^3,21,22 Aspergillus spp. degraded polystyrene (PS)/PLA/surface modified organic montmorillonite (OMMT) nanocomposites,²³ PLA/jute fiber composite³ and PLA/wheat straw composite²⁴ but few reports have addressed the fungal biodegradation of PLA/hemp composite in aqueous media and the correlation between the characteristics of the composite and the degree of biodegradation.

The present work aims to investigate the properties of a PLA/hemp fiber composite after fungal degradation by Aspergillus niger TISTR 3153 for 28 days in an aqueous medium. The ISO 846: 1997 standard method was applied.²⁵ The hemp fiber was pretreated by alkalinization and then modified by steam explosion and/or silanization. The effects of the different treatments were investigated on the molecular weight, chemical structure, thermal properties, and hygroscopic properties of the PLA/hemp composite.

Materials and method

Materials and chemicals

PLA 3052D was supplied by NatureWorks Co. Ltd. (Plymouth, MN, USA). Hemp (Cannabis sativa) fibers were provided by Hemp Thai Co., Ltd. (Bangkok, Thailand). Sodium hydroxide was from Ajax Finechem (Auckland, New Zealand). 3-(Trimethoxysilyl)propyl methacrylate (MPS, 98%) was from Sigma-Aldrich. Commercial grade acetic acid and ethanol were also used.

Aspergillus niger TISTR 3153 was received from the culture collection of the Thailand Institute of Scientific and Technological Research, Pathum Thani, Thailand. Tryptic soy broth was purchased from Difco (Franklin Lakes, NJ, USA).

Alkaline pretreatment of hemp fibers

Hemp fibers were pretreated with a 6% (w/v) aqueous solution of sodium hydroxide at room temperature for 48 h. After treatment, the fibers were washed with water several times until the pH of the water was neutral (pH = 7). The fibers were then dried at room temperature and cut into 0.2–0.5 cm lengths.

physicochemical modifications of alkaline pretreated fibers

After alkaline pretreatment, the hemp fibers were modified by using three different physicochemical treatments: steam explosion, silanization, and steam explosion followed by silanization (Figure 1). Steam explosion was performed at ∼210°C at a pressure of 18 kg/cm² for 5 min. The codes of samples are shown in Table 1.

Figure 1.

Flow chart of the PLA/Hemp fibers composites.

Table 1.

Abbreviation and composition of the PLA/hemp composites.

Samples codes	Pretreatment and physio-chemical modifications of hemp fibers	Fiber contents (wt%)
PLA	—	0
PLA/A	Alkaline pretreatment	30
PLA/A-S	Alkaline pretreatment and silanization	30
PLA/A-SE	Alkaline pretreatment and steam explosion	30
PLA/A-SE-S	Alkaline pretreatment, steam explosion, and silanization	30

Before silanization, the alkaline-pretreated fibers were soaked in 5% (w/v) sodium hydroxide at a ratio of 1:10 (w/v) at room temperature for 1 h and washed with distilled water until neutral pH was obtained. The silanization of fibers was then performed according to the following reported method.¹⁹ A silane coupling agent was prepared as follows: 3% (w/v) MPS was dissolved in 60% (v/v) ethanol, and the solution was adjusted to pH ∼ 4 by adding 5% (v/v) of acetic acid. Hemp fibers (2.5% (w/v)) were immersed in the silane coupling agent at room temperature for 6 h. The silane-treated fibers were collected and dried in an oven (Memmert UN110, Schwabach, Germany) at 120°C for 2 h.

Preparation of PLA/Hemp fiber composites

PLA pellets were dried at 80°C to remove excess moisture. The PLA pellets were compounded for 5 min with modified hemp fibers (30% by weight of PLA) using an internal mixer (Barbender, Germany) at 180°C and 60 rpm. The obtained composites were cut into small pieces and test specimens 2 mm thick were prepared in a compression molding machine at 180°C, applying pressure in three stages: 0 psi for 3 min; 1,000 psi for 2 min and then, after releasing the pressure, 1,500 psi for 1 min.

Fungal biodegradation testing

The fungal biodegradation by A. niger TISTR 3153 of neat PLA and PLA/hemp fiber composites was evaluated in an aqueous medium, following the standard method of ISO 846-1997. Each composite sample (2 × 3 × 0.5 cm³) was sterilized in 70% ethanol and then placed into a 250 mL flask containing 100 mL of tryptic soy broth (pH 7.0 ± 0.2). A spore suspension of A. niger TISTR 3153 (5.5 × 10⁶ spore/mL) was inoculated into the flask, and incubated at 28°C for 28 days with shaking at 120 rpm. A flask containing only a composite sample and culture medium was used as a negative control. All treatments were tested in three replicates.

Characterization of PLA/Hemp fibers composites before and after fungal biodegradation

Mechanical properties

The fiber composite samples for the flexural test were investigated according to ISO 178 (three-pointed bending mode) using a Universal Test Machine (Instron Model 3365) at room temperature. The maximum load capacity of this equipment is 1 kN, and the crosshead speed was set at 2 mm/min. In terms of the impact strength, the fiber composite samples were tested according to ISO 179 using a Zwick/Roell 5102 Pendulum Impact Tester operating with a pendulum energy of 1 J. At least five specimens were tested to determine the average values including their standard deviations of the results.

Scanning electron microscope (SEM)

The morphology of fractured surfaces of samples after impact testing was observed by a scanning electron microscope (JSM-5800 LV, JEOL, Japan) with an accelerating voltage of 20 kV. The fractured surfaces were sputtered and coated with a thin layer of gold prior to analysis.

Molecular weight by gel permeation chromatography (GPC)

To determine the degree of fungal biodegradation, specimens were analyzed before and after incubation by gel permeation chromatography (GPC) with refractive index detection (Shimadzu Prominence, Tokyo, Japan). Samples were dissolved in tetrahydrofuran (THF) to a final concentration of 0.1% (w/v). The obtained solution was filtered through a 0.45 µm nylon membrane. The filtered solution (20 µL) was injected into the mobile phase of THF at 40°C at a flow rate of 1 mL/min. Polystyrene of known molecular weight was used as the calibration standard.

Chemical structure by fourier transform infrared spectroscopy

Changes in the chemical structures of samples caused by fungal biodegradation were investigated by using Fourier transform infrared spectroscopy (FTIR) in attenuated total reflectance mode (Bruker Tensor 27 FTIR Spectrometer, Billerica, MA, United States). Spectra were recorded from 32 scans from 4000 to 600 cm⁻¹ at a resolution of 4 cm⁻¹. The obtained spectra were pre-processed by baseline correction and vector normalization using the OPUS 7.5 software (Bruker Optics Ltd., Ettlingen, Germany). The semi-quantitative degree of biodegradation was evaluated from the carbonyl index, which was calculated from the ratio of the band intensities of carbonyl ester groups (C = O) (I₁₇₅₀) and CH₃ bending (I₁₄₅₂),¹² using equation (1):

Carbonyl index = I_{1750} / I_{1452}

(1)

Thermal properties by differential scanning calorimetry

The melting behavior and crystallization of samples before and after fungal biodegradation were examined under a nitrogen atmosphere by differential scanning calorimetry (DSC) (Perkin-Elmer DSC7, Waltham, MA, USA). The temperature was raised from 30 to 190°C at a rate of 10°C/min and then held constant at 190°C for 1 min. Samples were then cooled to 30°C at 10°C/min before being heated again in the same condition as the first cycle. The enthalpy values derived from DSC profiles were used to calculate the degree of crystallinity (χ_c) using the following equation (2):

χ_{c} (%) = \frac{∆ H_{m}}{{∆ H}_{P L A}} \times 100

(2)

where ∆H_m is the enthalpy of melting crystal (the area under the peak) and ∆H_PLA is the enthalpy of the pure crystalline PLA reference (∆H_PLA = 93.1 J/g).

Thermal degradation by thermogravimetric analysis

The thermal and degradation stability of samples were investigated by thermogravimetric analysis (TGA) (Perkin-Elmer TGA 8000, Waltham, MA, USA). Samples were heated from 25°C to 600°C at a rate of 10°C/min under a nitrogen gas flow.

Surface free energy (SFE) analysis

The hygroscopic properties of samples before and after fungal biodegradation were evaluated by contact angle measurement (Data Physics OCA 15EC, Filderstadt, Germany). A single 5 μL drop of distilled water, formamide or ethylene glycol was deposited from a syringe onto the sample surface. Images were captured by video camera, and analysed to calculate the contact angle (θ). The surface energy of samples was calculated using the Owens and Wendt approach,²⁶ dividing the surface energy of samples into dispersive and polar components. The resulting contact angle measurements in the case of each of the nanocomposite films were used to calculate their SFE.

Statistical analysis

All experimental data were expressed as means ± SD. Analysis of variance (ANOVA) was performed by Duncan’s multiple-range test (DMRT) using the SPSS software (SPSS for Windows, SPSS Inc., Chicago, IL, USA) at p ≤ .05.

Results and discussion

Mechanical properties

Flexural properties and impact resistance of neat PLA and PLA/hemp fiber composites are presented in Table 2. Incorporation of 30% hemp fiber of all types into the PLA matrix resulted in considerable escalation of flexural modulus (approximately 90%–120%) as compared with neat PLA. The strengths of all composites were averagely increased by 59% as well, except for PLA/A-S, which increased slightly by 15%. The alkaline-treated fiber composite (PLA/A) exhibited the highest flexural strength among all composites. It should be noted that the alkaline pretreated-silanized fiber (PLA/A-S) was explicitly reduced. A similar trend was also observed for PLA/A-SE-S. By hypothesis, the silanization of cellulose fiber always increases interfacial adhesion between the fiber and matrix phase.²⁷ In consequence, the composite modulus would be increased as observed in PLA/A-S. However, the decrease in strength of PLA/hemp composites consisting of silanized hemp fiber, both without and with steam explosion prior to silanization (PLA/A-S and PLA/A-SE-S) indicated a decline in reinforcement ability due to the fiber weakness. It is possible that the hemp fiber was partially hydrolyzed during the adjustment of the acidic condition by acetic acid while silanizing. Similarly, the impact resistances of silanized fiber composites (PLA/A-S and PLA/A-SE-S) were reduced as compared with unsilanized fiber composites (PLA/A and PLA/A-SE). Commonly, silanization of fiber enabled the improvement of interfacial adhesion between fiber and matrix due to the substitution of a hydroxyl group with a methacrylate group in the cellulose structure. The improved compatibility was attributed to the dipole-dipole interaction, as shown in the following scheme (Figure 2).

Table 2.

Mechanical properties of PLA/hemp fiber composites.

Sample	Flexural strength, MPa	Flexural modulus, GPa	Flexural strain, %	Impact resistance, kJ/m²
PLA*	28.9 ± 4.0^c	2.2 ± 0.3^c	1.3 ± 0.2^a	17.5 ± 1.9^a
PLA/A	49.6 ± 2.5^a	4.7 ± 0.3^a	1.1 ± 0.1^b	5.6 ± 1.4b^c
PLA/A-S	33.5 ± 5.8^c	4.9 ± 0.1^a	0.6 ± 0.0^c	4.1 ± 1.5^c
PLA/A-SE	47.5 ± 1.9^a	4.6 ± 0.2^a	1.1 ± 0.1^ab	6.0 ± 0.5^b
PLA/A-SE-S	42.0 ± 0.3^b	4.2 ± 0.1^b	1.1 ± 0.1^b	5.3 ± 0.8^bc

^*neat PLA The different superscript letters (a, b, c, d and f) for mechanical properties indicated a statistically significant difference at p ≤ .05 according to Duncan's multiple range test. ns = non-significantly difference between all samples.

Figure 2.

The interaction between the silanized fiber and PLA matrix.

Furthermore, enhanced hydrophobicity after salinization could prevent water absorption in composites. Consequentially, the service lifetime of composites could be prolonged as the mechanisms of hydrolytic and enzymatic degradation were obstructed.^15,28 Nevertheless, SEM images in Figure 3(b) (PLA/A-S) and 3(d) (PLA/A-SE-S) revealed that the silanized and steam explosion-silanized fibers were more damaged than unsilanized fiber when subjected to an impact test. As described previously, during silanization, the fibers might be partially decomposed to weaken. Therefore, these caused it to lose the capability of instant load receiving.

Figure 3.

SEM images at cross section after impact testing; (a) PLA/A, (b) PLA/A-S, (c) PLA/A-SE and (b) PLA/A-SE-S.

The silanization of hemp fiber in PLA/A-S and PLA/A-SE-S composites as seen in Figures 3(b) and 3(d), revealed a rougher surface after impact testing. Jaw fish shape (rougher surface) in cross section was crucial evidence to confirm that the composites require more energy to destroy. This confirmed the improved interfacial adhesion between the silanized fiber and matrix of both composites. In addition, embedded fiber, as seen in PLA/A-SE-S (Figure 3(d)), could strongly confirmed the better compatibility of hemp fiber with the PLA matrix over PLA/A-SE (Figure 3(c)). However, based on the results of mechanical properties in Table 2, it was found that the composites with silanization of fiber (PLA/A-S and PLA/A-SE-S) had a lower impact resistance than those without silanization (PLA/A and PLA/A-SE). The results indicated that there might be other factors involved. Principally, the strength of composites is mainly dependent Even though the compatibility between fiber and matrix has improved, leading to better load transfer from matrix to fiber, the insufficient strength of the matrix and fiber may lead to the lower capability of composites to withhold the external load.

The silanization of fiber under acidic conditions might damage the fiber structure, as described previously. As seen in Figures 3(b) and 3(d), the “breakage fiber” in both figures indicates defects due to their weakening structure. Subsequently, the composites were easily cracked when receiving loads.

Measurement of durability of PLA/hemp fiber composites via fungal growth test

Molecular weight change

The number and weight average molecular weights (Mn and Mw), and polydispersity index (PDI, Mw/Mn) of the PLA/hemp fiber composites were determined by GPC before and after fungal biodegradation (Table 3). The results indicated that fungal biodegradation caused a significant reduction in both Mn and Mw. PDI was significantly increased for all samples after fungal biodegradation (p ≤ .05), indicating molecular weight loss with increased molecular size distribution. These findings were in agreement with the findings of Stoleru, Hitruc²⁹ that the molecular weight of PLA/chitosan stratified composite was reduced, while the PDI was increased after fungal biodegradation. The increment of PDI might be due to heterogenous degradation induced by biodegradation at the polymer bonds.²⁹ The lower molecular weight of PLA/hemp fiber composites compared to neat PLA suggested that hemp fibers promoted the fungal biodegradation of PLA. It is possible that the hemp fibers might induce the fungus to produce more lignocellulolytic enzymes, and the biodegradation of PLA was then accelerated by promiscuous enzymatic activity.^3,30 According Table 3, the Mn reduction of PLA was not different among composites containing treated and non-treated hemp fibers, while the reduction of Mw was more pronounced in the PLA/A-S, PLA/A-SE and PLA/A-SE-S composites. Moreover, PLA/A-S exhibited the smallest reduction in the Mw of PLA. This result indicated that silanized hemp fibers could increase resistance to the enzymatic hydrolysis of PLA during fungal biodegradation. The greater resistance could be due to improved interfacial bonding between treated hemp fiber and the PLA matrix.¹² It should be pointed out that the chemical treatment of hemp fibers by silanization was the best approach to promote fungal biodegradable resistance of PLA in the present work.

Table 3.

The weight and number -average molecular weight (M_w and M_n) and polydispersity index (PDI, Mw/Mn) of the PLA/hemp composite before and after fungal biodegradation.

Samples	Mn (× 10⁴ g/mol)		Mn loss (%)	Mw (× 10⁴ g/mol)		Mw loss (%)	PDI (Mw/Mn)
Samples	Before	After	Mn loss (%)	Before	After	Mw loss (%)	Before	After
PLA	8.93 ± 0.16^a	6.24 ± 0.07^b	30.06 ± 1.99^ns	11.97 ± 0.04^a	10.19 ± 0.03^b	14.87 ± 0.49^b	1.34 ± 0.01^e	1.63 ± 0.01^d
PLA/A	4.75 ± 0.27^d	3.72 ± 0.01^f	21.65 ± 4.58^ns	8.12 ± 0.18^e	6.94 ± 0.02^f	14.54 ± 1.64^b	1.71 ± 0.06^c	1.87 ± 0.01^ab
PLA/A-S	5.76 ± 0.10^bc	4.23 ± 0.17^e	26.54 ± 4.20^ns	8.88 ± 0.07^cd	8.32 ± 0.14^e	6.30 ± 0.78^d	1.54 ± 0.01^d	1.97 ± 0.11^a
PLA/A-SE	5.68 ± 0.25^c	4.36 ± 0.25^de	23.05 ± 7.87^ns	8.71 ± 0.20^d	7.10 ± 0.02^f	18.45 ± 1.58^a	1.54 ± 0.04^d	1.63 ± 0.10^cd
PLA/A-SE-S	5.90 ± 0.28^bc	4.60 ± 0.35^de	22.08 ± 2.33^ns	9.11 ± 0.12^c	8.10 ± 0.26^e	11.09 ± 1.62^c	1.55 ± 0.05^d	1.76 ± 0.08^bc

The different superscript letters (a, b, c, d and f) for both before and after biodegraded samples indicated a statistically significant difference at p ≤ .05 according to Duncan's multiple range test. ns = non-significantly difference of Mn loss between all samples.

Chemical structure changes

Changes in the chemical structures of the composites after fungal biodegradation were investigated by FTIR spectroscopy. All FTIR spectra exhibited the typical bands of PLA at 1750 cm⁻¹, 1452 cm⁻¹ and 1082 cm⁻¹, which correspond to the C = O stretching of ester groups,³¹ the asymmetric bending vibration of the CH₃ groups,³² and C-O-C of the ester groups,³³ respectively. To determine the semi-quantitative degree of biodegradation of the composites, carbonyl indices were calculated (Table 4). The PLA composites incorporated with treated hemp fibers showed significantly increased carbonyl indices due to the carbonyl groups present in the fibers.³⁴ The carbonyl indices of almost all samples were higher after fungal biodegradation than before (p ≤ .05); perhaps due to the formation of carboxylic acid groups during biodegradation.^35,36 Exceptionally, the carbonyl index of the PLA/A-SE-S sample did not change after biodegradation, suggesting that treatment by steam-explosion and silanization might enhance resistance to biodegradation by A. niger TISTR 3153. Figure 4

Table 4.

The carbonyl index of the PLA/hemp composite before and after fungal biodegradation.

Samples	Carbonyl (C = O) index
Samples	Before	After
PLA	5.19 ± 0.54^d	5.41 ± 0.42^bcd
PLA/A	5.81 ± 0.19^abc	6.30 ± 0.60^a
PLA/A-S	5.34 ± 0.22^abc	6.25 ± 0.47^ab
PLA/A-SE	5.99 ± 0.53^cd	6.25 ± 0.63^a
PLA/A-SE-S	5.48 ± 0.35^bcd	5.45 ± 0.25^bcd

The different superscript letters (a, b, c and d) for both before and after biodegraded samples indicated a statistically significant difference at p ≤ .05 according to Duncan's multiple range test.

Figure 4.

FTIR spectra of PLA/Hemp biocomposites (a) before and (b) after fungal biodegradation: (a) PLA, (b) PLA/A, (c) PLA/A-S, (d) PLA/A-SE and (e) PLA/A-SE-S.

Differential scanning calorimetry

The influence of fungal biodegradation on the thermal behavior of the studied PLA/hemp composites was investigated by DSC. The DSC thermograms of all samples were presented in Figure 5, and the data from DSC analysis were summarized in Table 5. The glass transition temperature (T_g) of neat PLA did not change, while the T_g values of the PLA/hemp composites were significantly different after exposure to A. niger TISTR 3153 for 28 days (p ≤ .05). The T_g values of most of the PLA/hemp composites were lower after biodegradation due to the lower Mn.^3,12,32 It could be that T_g was correlated with biodegradation due to PLA chain scission by enzymatic hydrolysis The temperature of crystallization (T_c)and melting temperature (T_m) could be ascribed to structural changes in PLA caused by biodegradation. The T_c values of biodegraded PLA/A-S and PLA/A-SE-S were significantly shifted to lower temperatures (p ≤ .05), suggesting that the samples could easily recrystallize owing to the shorter polymer chains of biodegraded PLA.¹² The T_m of PLA, PLA/A-S and PLA/A-SE-S was slightly reduced after biodegradation (∼0.39–0.96°C) (p ≤ .05). However, different treatment techniques did not influence the T_m of the PLA/hemp fiber composites both before and after biodegradation. The melting enthalpy (ΔH_m) of the PLA composites was significantly reduced by all treatments. The ΔH_m of biodegraded PLA was significantly lower after biodegradation, while that of other samples remained unchanged, indicating that the perfect crystals of the PLA/hemp fiber composites were not affected by biodegradation.³ Moreover, the crystallinity of the biodegraded PLA/A-S and PLA/A-SE-S samples was significantly increased compared with the crystallinity of the samples before biodegradation. The changes in crystallinity might be due to reductions in the amorphous regions caused by fungal biodegradation.²⁹

Figure 5.

Differential scanning calorimetry (DSC) thermograms of PLA/Hemp biocomposites (a) before and (b) after fungal biodegradation: (a) PLA, (b) PLA/A, (c) PLA/A-S, (d) PLA/A-SE and (e) PLA/A-SE-S.

Table 5.

Differential scanning calorimetry (DSC) results of the PLA/hemp composite before and after fungal biodegradation.

Samples	T_g (°C)		T_c (°C)		T_m (°C)		ΔH_m (J/g)		X_c (%)
Samples	Before	After	Before	After	Before	After	Before	After	Before	After
PLA	58.80 ± 0.15^cd	58.81 ± 0.13^cd	98.00 ± 0.24^b	110.45 ± 0.11^a	170.25 ± 0.82^a	169.29 ± 0.59^b	48.36 ± 0.17^a	40.85 ± 1.42^b	19.19 ± 0.64^b	11.67 ± 1.59^c
PLA/A	61.23 ± 0.88^a	59.55 ± 0.66^bc	93.25 ± 0.35^de	ND	168.42 ± 0.59^bc	167.87 ± 0.23^c	33.82 ± 0.12^c	32.56 ± 6.80^c	17.90 ± 3.12^b	ND
PLA/A-S	56.21 ± 0.04^e	58.53 ± 0.10^cd	95.42 ± 0.35^c	92.03 ± 1.41^ef	168.67 ± 0.23^bc	168.12 ± 0.12^c	32.34 ± 0.03^c	33.40 ± 0.81^c	16.22 ± 0.15^b	32.71 ± 0.91^a
PLA/A-SE	60.33 ± 0.45^ab	59.91 ± 0.25^abc	94.42 ± 0.83^cd	ND	168.75 ± 0.11^bc	167.79 ± 0.12^c	34.98 ± 0.19^c	31.53 ± 0.55^c	19.54 ± 0.01^b	ND
PLA/A-SE-S	60.73 ± 0.07^abc	57.80 ± 0.57^d	92.50 ± 0.24^ef	91.04 ± 0.23^f	168.59 ± 0.12^bc	168.20 ± 0.24^c	33.46 ± 0.42^c	32.47 ± 0.07^c	18.80 ± 1.30^b	32.85 ± 0.15^a

T_g glass transition temperature, T_c crystallization temperature, T_m melting temperature, ΔH_m enthalpy of melting, X_c percentage of fractional crystallinity, ND refers to not determined due to the absence of a crystallization peak for PLA.

Thermogravimetric analysis

The thermal stability and decomposition temperature (T_d) of samples were analyzed by TGA before and after fungal biodegradation (Figure 6). The T_d values of biodegraded samples were significantly reduced (5 °C–19°C) (p ≤ .05), indicating deteriorations in thermal stability (Table 6). It is generally acknowledged that the thermal stability of the polymer matrix can deteriorate when natural fibers are incorporated.⁹ The alkaline-treated hemp fiber reduced the thermal properties of PLA the most due to its chemical composition and fiber–matrix adhesion. By removing organic impurities, the alkaline pretreatment increased cellulose purity³⁷ and hydrophilicity. Due to the increased incompatibility between hemp fiber and PLA matrix, thermal stability was lower. Moreover, the T_d of neat PLA was lower than the T_d of the PLA composites, due to its lower molecular weight. The thermal stability of PLA/hemp fiber composites was higher when alkaline-treated fibers were modified by steam explosion and silanization. Thermally unstable chemical components of hemp fibers such as hemicellulose, residual lignin and wax were removed during steam explosion^38–40 and the coupling reaction of silanized hemp fibers embedded in the PLA matrix created strong covalent Si-O-Si bonds that enhanced fiber–matrix adhesion.³⁷ It should be noted that PLA and hemp fiber were more compatible after physical and/or chemical treatments. After biodegradation, the thermograms of all samples were slightly different, except in the case of PLA/A. The results showed that T_d was ∼8°C higher, indicating increased thermal stability. The hydrolyzation of fibers by cellulolytic enzymes of A. niger might result in more PLA remaining in the sample matrix. This result was consistent with the reports of Karimi-Avargani, Bazooyar³ and Karimi-Avargani, Bazooyar³⁰ that cellulolytic enzymes were produced during the biodegradation of PLA/jute fiber composite by A. flavus in an aqueous medium.

Figure 6.

The thermograms of thermogravimetric analysis (TGA) and derivative thermal gravimetric analysis (DTG) of PLA/Hemp biocomposites (a) before and (b) after fungal biodegradation: (a) PLA, (b) PLA/A, (c) PLA/A-S, (d) PLA/A-SE and (e) PLA/A-SE-S.

Table 6.

Thermal decomposition of the PLA/Hemp composite before and after fungal biodegradation.

Samples	Td (°C)
Samples	Before	After
PLA	361.34 ± 2.09^a	359.81 ± 0.14^a
PLA/A	342.52 ± 3.42^d	350.04 ± 1.48^c
PLA/A-S	353.20 ± 0.36^bc	353.70 ± 0.45^b
PLA/A-SE	352.48 ± 0.66^bc	354.37 ± 0.01^b
PLA/A-SE-S	354.22 ± 0.35^b	354.79 ± 1.03^b

T_d decomposition temperature.

Surface tension

Polar and dispersive surface free energies were estimated by contact angle measurement (Table 7). Before biodegradation, the polar surface tension of PLA was significantly increased by the incorporation of alkaline-treated hemp fibers (p ≤ .05) due to increased hydrophilicity and weak fiber–matrix interfacial bonding.¹⁹ After biodegradation, the polarity of PLA/A was significantly reduced (p ≤ .05). The alkaline-treated fiber might be hydrolyzed more easily than PLA, resulting in an increased content of hydrophobic PLA remaining in the biodegraded composites. The unchanged polarity of pure PLA and PLA/A-SE-S samples after biodegradation was perhaps due to the hydrophobicity of pure PLA and PLA/A-SE-S. The former could be explained by the nature of PLA, while the latter by the strong interfacial adhesion of well dispersed silane-treated fibers in the PLA matrix.

Table 7.

The total, polar and dispersive surface tensions of PLA/hemp biocomposite before and after fungal biodegradation.

Samples	Surface tensions (mN/m)
	Total		Polar		Dispersive
	Before	After	Before	After	Before	After
PLA	22.95 ± 2.14^d	31.73 ± 1.46^b	4.56 ± 0.13^b	5.48 ± 0.97^b	18.38 ± 2.02^de	26.26 ± 2.24^ab
PLA/A	28.37 ± 1.73^c	29.15 ± 2.00^c	11.60 ± 3.14^a	1.84 ± 0.96^c	16.79 ± 2.76^e	27.32 ± 2.89^b
PLA/A-S	27.38 ± 0.98^c	33.75 ± 1.61^a	6.36 ± 0.85^b	12.65 ± 1.96^a	21.02 ± 1.64^cd	21.10 ± 3.06^cd
PLA/A-SE	29.10 ± 1.46^c	29.45 ± 0.44^c	5.23 ± 0.50^b	10.71 ± 1.99^a	23.88 ± 1.96^bc	18.73 ± 2.17^de
PLA/A-SE-S	35.12 ± 1.25^a	29.05 ± 1.60^c	6.52 ± 0.54^b	6.07 ± 0.70^b	28.66 ± 1.79^a	22.98 ± 2.27^bc

The different superscript letters (a, b, c, and d) for both before and after biodegraded samples indicated a statistically significant difference at p ≤ .05 according to Duncan's multiple range test.

It was noticeable that steam explosion and/or silanization significantly increased the dispersive non-polar surface tension of PLA/A-S, PLA/A-SE and PLA/A-SE-S samples (p ≤ .05). It might be that good fiber–matrix compatibility increased the hydrophobicity of the composite.⁴¹ When silane-treated fibers (A-S) were present in the composite, the dispersive surface tension component was still present after biodegradation, suggesting some resistance to the enzymatic activity of the fungi.

Many previous studies reported that the improved hydrophobicity of lignocellulosic fiber composites, influenced the delay of biodegradation due to inappropriate conditions for microbial growth.⁴² The moisture or water absorbed in composites could facilitate inducing fungal or bacterial invasion.⁴³ The enhancement of hydrophobicity on PLA/hemp fiber composites led to their decreased moisture or water absorption. This was subsequently followed by less hydrolytic and enzymatic degradation, respectively. Therefore, it is considered a sustainable approach for expanding the service lifetime without any antimicrobial agent incorporated.

Conclusion

The durability of PLA and PLA/hemp fibers composites in outdoor applications was investigated by simulated fungal biodegradation for 28 days, following the ISO 846: 1997 standard method involving an aqueous medium containing A. niger TISTR 3153. The study confirmed that physicochemical modification of fibers after alkalization, especially after silanization, promoted the resistance of the PLA/hemp fiber composites to fungal biodegradation. Gel permeation chromatography, surface energy analysis and SEM photographs showed that the best resistance to the fungal biodegradation of PLA was exhibited by a composite of PLA with alkalized and silanized hemp fibers. These modified treatments applied to hemp fibers present a sustainable method to extend the service life of hemp fiber/PLA composites without incorporating any antimicrobial agents. These treatments are specifically designed to improve the interfacial adhesion of the fiber within the PLA matrix.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was financially supported by the National Research Council of Thailand (NRCT), under grant no. SCI600609S.

ORCID iDs

Rungsima Chollakup

Thorsak Kittikorn

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