Effects of microcrystalline cellulose surface modification on the mechanical and thermal properties of polylactic acid composite films

Abstract

Microcrystalline cellulose (MCC), extracted from distiller's grains by nitric acid-ethanol hydrolysis, was prepared for PLA composite films by solution casting. MCC based on the optimum comparison was characterised by particle size analysis and scanning electron microscopy. The results show that the ultrasound combined sulphonation treatment was remarkable for the surface structure of MCC, compared with that modified by sulphonation and ultrasound methods. The MCC/PLA composite films were analysed using Fourier-transform infrared spectroscopy (FT-IR), mechanical properties and thermal resistance. The results show that the properties and performance of MCC/PLA composite film using ultrasound combined sulphonation treatment are obviously better than those of modified films by other methods. For the modified MCC/PLA composite film by sulphonation and ultrasound methods, the FT-IR analysis carried out a large number of hydroxyl groups that are hydrolysed, the tensile strength upped to 505.24 MPa and the thermal-decomposed temperature increased to 320°C from 260°C. The modified MCC would be a beneficial filling material for the properties of PLA composites.

Keywords

Microcrystalline cellulose polylactic acid biocomposite sulphonation ultrasound

Introduction

In the past a few years, the growing concern for environmental issues has motivated extensive research on new environment-friendly materials [1-3]. Polylactic acid (PLA), a substitute for traditional petroleum-based thermoplastics, has been extensively studied owing to its remarkable biodegradability, high structural properties and environmental resources [4-6]. PLA, as a biodegradable bio-material matrix, was derived from plant fibres including flax, hemp, kenaf and bamboo fibres. But the practical application is limited because of its low-softening temperature and weak ductility [7-10]. Methods of the reactive compatibility of PLA composites with cellulose fibres have been described [11, 12]. Microcrystalline cellulose (MCC), produced by acid hydrolysis of vegetable or cellulose fibres to level-off degree of polymerisation (LODP), has been widely used in medicine, cosmetics, food industry and the chemical industry due to low cost, recyclability, biodegradability and non-hazardous nature, which contribute to excellent physical and chemical properties of MCC [13-16].

Distiller grain (DG), a cast-off used in the production of alcoholic beverages, is highly rich in cellulose, hemicellulose and lignin [17-19]. China, a major country of producer and consumer of alcohol, has contributed to the accumulation of DGs and waste of resources [20, 21]. However, MCC is difficult to absolutely disperse in PLA, because a large amount of hydrophobic groups in molecular structure caused the formation of composites with uneven surface and strongly influenced the composite performance. A study of the MCC, as a reinforced material added to PLA matrix, displayed the obvious effect of mechanical properties of PLA and reduced to 5% with the concentrations of MCC drop [22]. MCC/PLA biocomposites were found to exhibit reduced fracture elongation and tensile strength with MCC obtained from palm oil as the filling material [23].

MCC was characterised by low solubility in organic (hydrophobic) solvents, however, which has been obtained by hydrolysis with nitric acid because of the presence of a large number of hydroxyl groups of the MCC surface [24, 25]. The solubility of MCC can be improved using differently modified methods, including chemical modification (sulphonation with sulphuric acid), physical modification (ultrasounds) or physicochemical modification (ultrasounds combined with sulphuric acid). Some −SO3H groups of the surface of MCC can be partially deprotonated (negatively charged) −SO3H groups by sulphonation in certain conditions, which could promote the dispersion of the modified material in solution. Therefore, the goal of this work is to examine the effects of differently modified methods of the surface of MCC on the performance of MCC/PLA biocomposites. Furthermore, the differently modified MCC was obtained from DGs. The physicochemical properties of MCC/PLA composite films were characterised by infrared spectroscopy (FT-IR), scanning electronic microscopy (SEM), mechanical analysis and thermal stability analysis.

Experimental

Materials and instrumentation

MCC was obtained from DGs supplied by Gansu Jinhui Wine Industry (Gansu, China). The DGs were dried and ground down a hammer mill to pass through a 0.42 mm screen in order to obtain a homogeneous particle size. PLA was purchased from Nature Works (USA). All chemicals were purchased from FENLIDA Co., Ltd (China). FT-IR spectra were obtained using an IFS 66v/s FT-IR spectrometer (Bruker Ltd., Germany). Scanning electron microscopy (SEM) images were obtained using a JSM-5600LV microscope operating in the range 0.5–30 kV. UV-visible light transmission properties were tested using a U-3010 Ultraviolet visible spectrometer (Hitachi Limited, Japan). MCC particle size analysis was carried out by Mastersizer 2000 laser particle size analyser (Japan). Tensile strength and fracture tensile ratio were tested using a Multi Test 2.5-i texture analyser (FTC, United Kingdom).

Methods

Preparation of MCC by hydrolysis of DGs with nitric acid-ethanol

The preliminary analytic results indicated that MCC was about 50.18% in DG powder. Precisely weighed 20 g of the dried DG powder was treated with 3% NaOH (m/v) solution to 90 min under the solid–liquid ratio of 1:30 (g/mL) at 60°C and centrifuged at 3500 r/min for 10 min. The obtained solid residue was pulverised in a mixture of nitric acid-ethanol (volume ratio of 1:3) under the solid–liquid ratio 1:43 (g/mL) and refluxed for 92.3 min at 70.1°C and centrifuged at 3500 r/min for 10 min. The pulverised solid residue was treated with a 6% NaClO (v/v) solution to 60 min under the solid–liquid ratio 1:20 (g/mL) at 60°C. The pretreated suspension was filtered and washed until pH reached until neutrality. Finally, the wet samples were dried in the oven for 4 h at 105°C and MCC powder was obtained.

Modification of MCC by sulphonation, ultrasounds and ultrasounds combined with sulphonation

The MCC obtained from DGs (defined as MCC) was extracted by nitric acid-ethanol hydrolysis. DGMCC was modified by sulphuric acid and ultrasound methods adopted in literatures [26]. First group was modified by sulphonation of sulphuric acid, MCC samples were treated separately in sulphuric acid solution (concentration of 40, 43, 46, 49, 52%) for 30 min under the solid–liquid ratio of 1:40 (g/mL) at 45°C. Second group was modified by ultrasound, MCC samples were separately treated with deionised water for 30 min at 45°C under ultrasonic power (60, 70, 80, 90, 100 W). Third group was modified by sulphonation combined ultrasound, MCC samples were treated in sulphuric acid solution (concentration of 40, 43, 46, 49, 52%) for 30 min under ultrasonic power at 90 W and 45°C. The suspension was washed by deionised water until neutrality and dried to achieve constant weight.

Optimisation experimental conditions of the modified MCC, according to the preliminary single-factor experiment, were determined by 46% of the sulphuric acid solution and at 90 W of the ultrasonic power. Based on these factors, MCC was modified in accordance with different methods. The modified MCCs were separately defined as S-MCC by modified sulphonation, U-MCC by modified ultrasound and SU-MCC by modified sulphonation combined ultrasound and modified MCC powder was obtained.

MCC/PLA composite film preparation

MCC/PLA composite films were prepared by the solvent casting method [27]. MCC, S-MCC, U-MCC, SU-MCC and PLA samples were dried by drum wind drying machine for 5 h at 105°C and it was ensured the moisture content below 1%. The MCC samples were added to a beaker with dichloromethane in the ratios indicated in Table 1 and stirred at 500 r/min for 25 min. Then, the MCC was mixed into PLA matrix, sealed in a plastic wrap to prevent volatilisation of dichloromethane and homogenised at 35°C for 2 h, until PLA was completely dissolved and homogenised for 2 h further. Finally, the suspensions were cast in panels and films of 0.35 mm thickness by a JFA-II automatic coating machine and dried in the oven for 4 h at 105°C and MCC/PLA composite films were obtained.

Table 1

Weight ratios of MCC/PLA composite films.

Samples	MCC (g)	PLA (g)	Dichloromethane (g)
PLA	0	10	100
PLA-3	0.3	9.7	100
PLA-6	0.6	9.4	100
PLA-9	0.9	9.1	100
PLA-12	1.2	8.8	100

Results and discussion

Morphological analysis of modified MCC powder

The particle size analysis was done by Mastersizer 2000 laser particle size analyser. In contrast to unmodified MCC, S-MCC, U-MCC and US-MCC powder, the use of a laser particle size analyser is shown in Figure 1. Different modified methods lead to unequal particle sizes, and the 90% size distribution concentrates at 95–519 µm. The particle sizes, however, are distributed among different ranges.

The particle size analysis shown in Table 2 indicates differently modified methods of the particle size of MCC. The d(0.9) distribution of particle size of unmodified MCC, S-MCC, U-MCC and SU-MCC is separately distributed at 519, 405, 410 and 95 μm, respectively. The results show that the minimal amount is the SU-MCC powder. The physical structure of MCC is transformed apparently by the modified sulphonation combined ultrasound method.

Table 2

Distribution of different modified particle sizes.

Modified methods	d(0.9)	d(0.5)	d(0.1)
MCC	519.656	145.365	24.014
S-MCC	405.304.	113.612	40.818
U-MCC	410.039	108.894	38.980
SU-MCC	95.084	55.437	37.654

d(0.9), d(0.5), d(0.1) represent the percentage of granularity reached at this size.

SEM analysis of MCC powder

MCC, as a powdery and white particle, was extracted from DGs by pretreatment with sodium hydroxide and nitric acid-ethanol hydrolysis, which dissolved lignin and hemicellulose. The dissolution of any residual traces of lignin and hemicellulose was done by bleaching agents with sodium hypochlorite. The scanning electron microscope (SEM) analysis is shown in Figure 2. It displayed different shapes of surface according to different modified methods. As seen in Figure 2(a), the unmodified MCC powder is composed of bulky and rod shape, and the surface of the structure is rough and tumble. The similar shape and surface structures are seen in Figure 2(a,b), however, partially hydrolysed region on the surface of U-MCC and U-MCC powders. Its regular crystalline structures are attributed to disposal on the surface of MCC. The evident changes on the surface of SU-MCC powder are seen in Figure 2(d). The sulphonation combined ultrasound-modified stalks displayed many cracks, as well as broken layers, which lead to a large number of regular crystallisation areas on the surface of MCC. Regular crystallisation areas are exposed on the surface of cellulose, because many hydroxyl groups on the surface have become sulphonic group by treated with sulphuric acid sulphonation. More crystallisation areas are displayed with the help of ultrasound, which is beneficial to the mixing of MCC into PLA.

MCC/PLA composite film FT-IR analysis

FT-IR analysis is a common method to identify the interaction and phase behaviour of polymer composites. The FT-IR spectra of the chemical compositions of pure PLA, MCC/PLA, S-MCC/PLA, U-MCC/PLA and US-MCC/PLA composite films are shown in Figure 3. The samples contain ester and hydroxyl groups from the alkyl structure of PLA, a wide band represents O–H groups at 3405.22 cm⁻¹, and at 2891.58 cm⁻¹ there appears a characteristic band related to the C–H stretching mode. The vibration peak at 2815.33 cm⁻¹ is attributed to the –CH₂ stretching of methylene, contains terminal carboxyl groups, (which are acidic) and hydroxyl groups. A strong band is shown at 1752 cm⁻¹, which corresponds to –C=O stretching vibration of the carboxyl group. The peak at 1492 cm⁻¹ is attributed to the C–H deformation mode. The peaks at 1382 and 1366.51 cm⁻¹ are characteristics of the –CH₃ group. The strong bands between 1220 and 1080 cm⁻¹ correspond to –C=C– stretching vibrations, and the same situation appears between 810 and 750 cm⁻¹.

Evident stretching band was changed into a lower wave number and somewhat broadened as a result of the 9 wt-% MCC into PLA composites. The peaks at 3580 and 3405 cm⁻¹ correspond to bending modes, which are characteristic vibrational features of hydrogen-bonded hydroxyl group in cellulose. The absorption peaks at 3405.22 cm⁻¹ are significantly changed in width at the O–H tensile vibrations. The stretching vibrations at 810 and 750 cm⁻¹ are obviously wider in MCC/PLA composite films. The appearance of these peaks at spectra indicates that the chemical structure of cellulose is preserved. A mass of hydroxyl group of the surface of MCC was hydrolysed, which enhanced the affinity with hydrogen bond between the MCC and PLA matrix.

MCC/PLA composite films’ mechanical property analysis

The mechanical properties of MCC/PLA composite films have been studied using procedures described in the literature [28, 29]. Mechanical performances of MCC/PLA composite films for the tensile strength and elongation at break were tested using the multi-test texture analyser, and different changes were displayed according to differently modified methods, as shown in Figure 4. The unmodified MCC/PLA and U-MCC/PLA composite films exhibited poor tensile properties, which declined with increasing MCC mass fraction, as seen in Figure 4(a), and the tensile strength of MCC/PLA and U-MCC/PLA films is always lower than that of pure PLA film. In contrast, the tensile strength of S-MCC/PLA and SU-MCC/PLA composite films is seen a trend of up and afterward down with the increase of MCC mass. The tensile strength of SU-MCC/PLA and S-MCC/PLA composite films with MCC mass at 9 wt-% is 1686.54 and 1671.04 MPa, respectively, which is far larger than MCC/PLA of 1181.3 MPa. The result indicated a significant effect of MCC modified by sulphuric acid treatments. MCC exhibits the best molecular compatibility of the PLA matrix because a mass of hydroxyl groups of the MCC surface was hydrolysed. The elongation at break is seen in different composite films as in Figure 4(b), compared with pure PLA, the other three modified composite films show a downward trend with increasing MCC mass fraction. It shows that the addition of MCC has obvious changes in the elongation at break of the composite films; however, the trend is far less marked than unmodified MCC/PLA film. Consequently, MCC modified by different methods would display different mechanical performance.

MCC/PLA composite films’ thermal stability analysis

As shown in Figure 5(a), the thermo gravimetric analysis (TGA) of MCC/PLA composite films was carried out according to the literature prescriptions [30]. Obvious changes of thermally resistant stability of MCC/PLA composite films are better displayed than pure PLA. The TGA curves of pure PLA and MCC/PLA composite films have a similar weight pattern, corresponding to one-step degradation process represented by a single peak. The weight loss, upped to 90% from 280 to 360°C, is observed, and no further weight loss above 360°C. The effect is significant in the thermal degradation behaviour of the MCC-modified method. The degradation temperature of the MCC/PLA composite films is increased from different modification of MCC. Pure PLA and unmodified MCC/PLA composite film are thermal-decomposed sharply from 260°C to 280°C. However, the thermal-decomposition of the SU-MCC/PLA composite films is sharply decreased at 300°C. As seen in Figure 5(b), the differential scanning calorimeter (DSC) indicated some disparity in thermal-decomposed behaviour of different composite films. The temperature is enhanced from 330C to 360°C when the weight loss of composite films is below 10%. The peak value of rheological deformation is raised to 1.8 W/g from 0.2 W/g, compared with SU-MCC/PLA and MCC/PLA composite films, which resulted in a crystal transformation. The results suggested that the SU-MCC/PLA composite film presents an exceptionally high thermal resistance.

Conclusions

MCC has been modified using chemical sulphonation treatment, physical ultrasound treatment and physicochemical sulphonation combined ultrasound treatment, which are incorporated successfully into a PLA polymer matrix through melt blending. The best-dispersed composite film was obtained with an MCC 9 wt-% mass fraction and by sulphonation combined ultrasound treatment. The chemical structure of composite film was successfully changed by FT-IR, thermal resistance and mechanical pressure resistance analysis. At 3405.22 cm⁻¹, 810 and 750 cm⁻¹, there is significant transformation because a large number of hydroxyl groups of the surface of MCC are hydrolysed. The results of tensile strength and elongation at break analysis indicated a noteworthy correlation between the mechanical properties of the composite films and the modified method of MCC. Under the condition of an MCC 9 wt-% mass fraction and by sulphonation combined ultrasound treatment, the tensile strength of SU-MCC/PLA composite film upped to 505.24 MPa. Furthermore, the TGA and TSC curve analysis results show that the thermal stability improved a certain temperature range, the thermal-decomposed temperature increased 20∼40°C. The thermal-decomposed temperature is increased to 300°C from 260 °C of SU-MCC/PLA composite film compared to that of pure PLA. It can be concluded from these results that the MCC is well modified by physicochemical sulphonation combined ultrasound treatment, which shows great prospect of the applications of biomass materials.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the author(s).

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