Effect of chemical modification of wood flour on long-term hygroscopic behavior of polypropylene composites

Abstract

This experimental work is aimed at studying the effect of fiber surface modification including alkaline, acetic acid, and benzyl chloride on the long-term water absorption and thickness swelling (TS) of wood flour (WF)-reinforced polypropylene composites. The samples were made by melt compounding and then injection molding. Hygroscopic rates of the samples were evaluated by immersing them in water at room temperature and monitoring moisture sorption and thickness changes for several weeks. The results indicated that chemical modification has a significant effect on the water absorption of composites. The mechanism of water absorption of all samples followed the kinetics of a Fickian diffusion process. The composites modified with acetylation-treated fiber showed the lowest and the unmodified composites exhibited the highest water absorption coefficient. TS of the composites was clearly dependent upon the WF chemical modification. The swelling rate parameter of treated samples is less than those of untreated ones.

Keywords

Polypropylene fiber surface modification water diffusion coefficient swelling rate parameter composites

Introduction

The growing production of plastics worldwide is a concern as they rely on fossil fuels that are an increasingly limited resource. As conventional petrochemical-based plastics accumulate in the environment and their resources are limited, there has been an increasing need to find alternative biodegradable materials from renewable resources.¹ In the recent years, there has been a dramatic increase in the use of natural fibers for making new types of environmentally friendly composites. Natural fiber–reinforced thermoplastic composites are the combination of cellulosic fibers and a thermoplastic such as polyethylene, polypropylene (PP), polyvinyl chloride, and polystyrene. The primary advantages of using natural fibers are renewability, low density, low cost, recyclability, and biodegradability. However, major problems associated with using natural fibers in composites are lower processing temperature and incompatibility between the hydrophilic natural fibers.^2,3 This problem can be alleviated by incorporation of coupling agent into the compound and modification of the fibers.³

Chemical modification of natural fibers is employed to improve the properties of the composites and to promote better adhesion between the natural reinforcement and the polymeric matrix. Different surface treatment methods such as mercerization, isocyanate treatment, acrylation, benzoylation, permanganate treatment, acetylation, silane treatment, and peroxide treatment have been applied on the fiber.^4,5

Water absorption and the consequent thickness swelling (TS) are the most important physical characteristics of wood–plastic composites exposed to environmental conditions and thus affecting their end-use applications. Water absorption can deteriorate both mechanical properties and dimensional stability in such composites. Therefore, hygroscopic characteristics have to be taken into account as limiting parameters in the design with regard to their final applications.^6

–12

Although there has been considerable research conducted to examine the effect of chemical modification of fiber on the characteristics of plastic composites,^{13

–20} there is no enough information regarding the influence of chemical fiber’s surface treatment on the long-term hygroscopic behavior of natural fiber plastic composites is available. The present article is a comprehensive investigation on the effect of chemical modification of wood flour (WF) on the long-term water absorption and TS behavior of composite-based PP.

Experimental

Materials

PP, with trade name of V 30 S, an injection molding grade was supplied by Arak Petrochemical Co. (Iran). Some important physical and mechanical properties of the used polymer are presented in Table 1. WF, which was used as the reinforcing material, was obtained from Cellulose Aria Co (Iran); the average size of WF particles was about 425 µm. Maleic anhydride-grafted PP (PP-g-MA) was provided by Solvay with trade name of Priex 20070 (melt flow index = 64 g/10 min, grafted maleic anhydride = 1 wt%) was used as coupling agent.

Table 1.

Physical and mechanical properties of used PP. PP: polypropylene.

Properties	Test method	Unit	Value
MFI	ASTM D1238	g/10 min	18
Density	ASTM D1505	g cm⁻³	0.91
Flexural modulus	ASTM D790	MPa	1550
Notched Izod impact	ASTM D256	J m⁻¹	30
Tensile strength	ASTM D638	MPa	33
Elongation at break	ASTM D638	%	12
Vicat softening point	ASTM D1525	°C	154
HDT	ASTM D648	°C	95
Rockwell hardness	ASTM D785	R scale	102

MFI: melt flow index.

Method

Chemical modification

Mercerization

The wood fibers were soaked in 5 wt% sodium hydroxide (NaOH) solution at ambient temperature, maintaining a solution:fiber ratio of 20:1 (by weight). The fibers were immersed in the solution for 30 min. After treatment, fibers were copiously washed with distilled water to remove any traces of alkali on the fiber surface and subsequently neutralized with 1 wt% acetic acid solution. The treated fibers were then dried in an oven at 80°C for 24 h.

Acetylation

The wood fibers pretreated with NaOH solution were soaked in 55% solution of glacial acetic acid for 1 h at 30°C; it was decanted and soaked in acetic anhydride containing one drop of concentrated sulfuric acid for 5 min. Then the fibers were placed in a preheated oven at 120°C for 5 h with the aim of encouraging esterification. Lastly, treated fibers were thoroughly washed with distilled water to remove residual chemicals until a pH of 7 was obtained, and then dried in an oven at 80°C for 48 h.

Benzoylation

The wood fibers pretreated with NaOH solution were suspended in 10 wt% NaOH solution and mixed well with 3 wt% (weight percent compared to the fiber) benzyl chloride for 30 min. Excess amount of distilled water was used to wash out NaOH solution, and the benzylated fibers were immersed in ethanol solution for 1 h to remove unreacted benzyl chloride. Finally, the fibers were washed out with distilled water and dried in an oven at 80°C for 24 h.

Composite preparation

PP, WF, and coupling agent were weighed and bagged according to Table 2. The materials were melt blended at 180°C for 8 min at 60 r min⁻¹ using a Haake internal mixer (SYS 9000; Dearborn, Michigan, USA). The compounded materials were ground using a pilot scale grinder (Wieser, WGLS 200/200 Model, Massachusetts, USA). The resulting granules were dried at 105°C for 4 h. Test specimens were injection molded (Imen Machine Co, Iran) at 190°C to produce standard ASTM specimens. Molding conditions were as follows: press temperature 190°C and pressure during heating 10 MPa. The specimens were stored under controlled conditions (50% relative humidity and 23°C) for at least 40 h prior to testing.

Table 2.

Compositions of the studied formulations.

Sample	PP (wt%)	WF (wt%)	PP-g-MA (phr)
Untreated	50	50	2
Treated with alkaline	50	50	2
Treated with acetic	50	50	2
Treated with benzyl	50	50	2

phr: parts by weight per hundred parts of resin; PP: polypropylene; PP-g-MA: maleic anhydride-grafted polypropylene; WF: wood flour.

Characterization

Water absorption and TS tests

Specimens with a dimension of 20 × 20 × 20 mm³ were cut for the water uptake test in accordance with ASTM D 7031 standard. Five specimens of each sample were dried in an oven for 24 h at 102 ± 3°C. The weight and thickness of dried specimens was measured to a precision of 0.001 g and 0.001 mm, respectively. Then they were placed in distilled water and kept at room temperature for 1008 h. At predetermined time intervals, the specimens were removed from the water and the surface water was wiped off using blotting paper and their wet mass values and thickness were measured. The values of water absorption and TS were calculated using equations. (1) and (2), respectively

M = \frac{m_{t} - m_{o}}{m_{o}} \times 100,

where, m_o and m_t denote the oven-dry weight and the weight after time t, respectively.

T S = \frac{T_{t} - T_{o}}{T_{o}} \times 100,

where, T_o and T_t denote the oven-dry thickness and thickness after time t, respectively.

Fourier transform infrared tests

Fourier transform infrared (FTIR) spectroscopy measurements were performed using a Digilab FTIR spectrometer (model SP-1100; Marlborough, Massachusetts, USA) equipped with a deuterated triglycine sulfate detector. The potassium bromide (KBr) pellet technique was applied for monitoring changes in the range of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹. Approximately 2 mg of the powdered components was mixed with 150 mg KBr and pressed into pellets and used for further characterization. Vibration transition frequencies were reported in wave number (cm⁻¹). A total of five scans were taken for each sample.

Result and discussion

Figure 1(a) shows FTIR spectra of the untreated WF. The strong peak at 3444.2 cm⁻¹ is characteristic of the hydrogen-bonded hydroxyl group (OH) stretching vibration. The peak at 3008.4 cm⁻¹ was considered due to C–H stretching vibration of all hydrocarbon constituents of the fibers and the shoulder band observed at 1741.4 cm⁻¹ due to the absorption of carbonyl (C=O) stretching of hemicellulose present in the fibers. The peak at 1658.5 cm⁻¹ was because CH is related to bound water and the peak at 1511.9 cm⁻¹ was considered due to the benzene ring. The peak appearing at 1461.8 cm⁻¹ was due to CH₂ bending in lignin and the peak at 1322 cm⁻¹ was due to O–H in plane bending. The band at 1249.6 cm⁻¹ was considered due to C–O stretching of acetyl in lignin. A small sharp band at 892 cm⁻¹ appears to have arisen from β-glucosidic linkages between the sugar units in hemicellulose and cellulose.^21,22

Figure 1.

FTIR spectra of the chemical-treated WFs: (a) untreated, (b) mercerization, (c) acetylation, and (d) benzoylation. FTIR: Fourier transform infrared; WF: wood flour.

FTIR spectra of alkali treatment in Figure 1(b) show that the peak at 1741.4 cm⁻¹ was disappeared; this appears to be due to the removal of noncelluloses including fats, wax, and pectin and hemicellulose present in the fibers.^15,22 Also, as can be seen in Figure 1(b) the O–H bond at 3444.2 cm⁻¹ is affected by alkali treatment and its intensity is decreased. The main reason for this behavior is that by alkali treatment an amorphous region is developed in cellulose by removal of OH groups from its network.^15,22 Figure 1(c) shows that the acetylation of fibers with acetic anhydride led to the appearance of a new peak at 1631.5 cm⁻¹ attributed to the C–O stretching of the ester carboxyl group. The lowering in intensity of the OH stretching band at 3334.3 cm⁻¹ in the spectra also provided further evidence of successful acetylation.²² Figure 1(d) shows FTIR spectra of benzoylation of fibers. The intensity of the bond at 1744.1 cm⁻¹ is decreased due to formation of ester bonds from the reaction between O–H groups and benzyl chloride (Figure 1(d)), while the OH band at 3444.2 cm⁻¹ intensity tends to decrease due to its consumption in this reaction.^15,22

Long-term water absorption

Figure 2 depicts the percentages of the water uptake for the PP/WF composites at different periods of immersion, which vary depending upon the chemical fiber surface modification. As it is clearly seen, generally water absorption increased with immersion time, reaching a certain value at saturation point, beyond which no more water was absorbed and the water content of composites remained constant. The time to reach the saturation point was not the same for all formulations. Figure 2 also shows that those composites, which were treated with chemicals reagents, absorbed less water compared to the untreated samples. The order of water absorption is as follows: untreated > mercerization > benzyolation > acetylation. Considering the fact that the WF fraction was constant in all composites (50% by weight), the different water absorption observed between various composites can be attributed to the chemical modification of fiber.

Figure 2.

Long-term water absorption of untreated and treated PP/WF composites. PP: polypropylene; WF: wood flour.

Water diffusion behavior in the polymer matrix composites can be Fickian diffusion, relaxation controlled, and non-Fickian or anomalous.⁸ These three cases can be distinguished theoretically by the shape of the sorption curve represented by equation (3).

\log (\frac{M_{t}}{M_{\infty}}) = \log (k) + n \log (t),

where, M_t is the moisture content at specific time (t) and M_∞ is the moisture content at the saturation point. In equation (3), n and k give an idea about the mechanism of diffusion that take place inside the composite. The value of coefficient shows different behavior between the three cases of diffusion: Fickian diffusion (n = 0.5), relaxation (n ≥ 1), and anomalous transport (0.5 < n < 1). The coefficients (n and k) are calculated from slope and intercept of the log plot of M_t/M_∞ versus time, which can be drawn from experimental data. An example of the fitting of the experimental data for different chemical treatments is given in Figure 3.

Figure 3.

Diffusion case fitting for untreated and treated samples.

Table 3 shows diffusion parameters for all formulations. The values of n show the absorption of water in treated and untreated samples approaches toward the Fickian diffusion. The water diffusion coefficient is the most important parameter of Fick’s model and shows the ability of water molecules to penetrate inside the composite structures. At early stages and small times (typically M_t/M_∞ ≤ 0.5), the diffusion process is presented as follows⁸:

Table 3.

Maximum water absorption and water diffusion coefficient for all samples.

Treatments	Maximum water absorption (%)	N	K (h²)	Water diffusion coefficient (× 10⁻⁹ m² s⁻¹)
Untreated	19.23	0.4781	0.0579	7.0374
Mercerization	17.16	0.4513	0.0516	6.4667
Benzoylation	16.31	0.4495	0.0483	5.3574
Acetylation	12.19	0.4264	0.0415	4.8933

\frac{M_{t}}{M_{\infty}} = \frac{4 \sqrt{t}}{\sqrt{\frac{D t}{n L^{2}}}},

where, L is the thickness of the specimen and D is the diffusion coefficient.

Table 3 shows that water diffusion coefficient of treated samples is less than that of untreated ones. This can be attributed to the fact that the chemical modification promoted good wettability and better matrix and fiber interface adhesion, allowing the moisture diffusion processes to be much slower because there are fewer gaps in the interfacial region.^14

–20 As a result, it is important to apply chemical modification to reduce the gaps and block the hydrophilic groups by improving the quality of adhesion at interfacial region.^23
–25

Long-term TS

The effect of chemical modification of WF on the TS behavior of PP/WF composites is presented in Figure 4. As can be seen, the TS of PP/WF composites reduced by applying chemical modification. Variations in TS of the composites are similar to variations in water absorption. The increase of the TS in the studied composites can be attributed to the same reasons as discussed concerning water absorption. The composite made with acetylation-treated and untreated fiber had the minimum (4.56%) and maximum (8.63%) TS, respectively.

Figure 4.

Thickness swelling of untreated and treated PP/WF composites. PP: polypropylene; WF: wood flour.

For more convenient comparisons, the TS rates of composites were quantified by the model described and developed by Shi and Gardner.²⁶ In this model, a swelling rate parameter (K_SR), as determined using the test data, can be used to quantify the swelling rate. The swelling model is expressed by equation (5).

TS = (\frac{T_{\max}}{T_{0} + (T_{\max} - T_{0}) e^{- K_{S R} t}} - 1) \times 100,

where, TS is the thickness swelling at time t, and T_o and T_max are initial and equilibrium thickness, respectively. K_SR is a constant referred to as the initial (or intrinsic) relative swelling rate.

The nonlinear curve fitting was used to find the constants that provide the best fit between equation (5) and the experimental data. This algorithm seeks the constants that minimize the sum of the squared differences between the observed and predicted values of the dependent variable, as seen in equation (6),

SS = \sum_{i = 1}^{n} {(y_{i} - {\hat{y}}_{i})}^{2},

where, SS is the sum of squared difference and y_i and ${\hat{y}}_{a}$ are the observed and predicted values of the dependent variable, respectively. It was found that the swelling model fits the experimental data well for all samples (the R² values for all the model fits were above 0.9).

The K_SR value of composites is given in Table 4. It can be seen that the untreated samples have higher K_SR value than those of treated ones. It is important to note that in the swelling model, K_SR was obtained considering the whole thickness process until it was equilibrated; that is, it is dependent not only on the initial rate of swell but also on the equilibrium TS of the composites.²⁶ For unmodified fiber composites with higher equilibrium TS, there is less time to reach the equilibrium thickness, as well. This can explain the very high K_SR value determined in this case. The minimum K_SR values were observed in composites made with acetylation-treated fibers.

Table 4.

Thickness swelling values and swelling rate parameters for untreated and treated samples.

Treatments	T_o (mm)	T_max (mm)	TS (%)	K_SR (×10⁻³ h⁻¹)
Untreated	9.54	10.36	8.63	0.0068
Mercerization	9.37	10.09	7.65	0.0053
Benzoylation	9.99	10.57	5.72	0.0049
Acetylation	9.83	10.28	4.56	0.0044

TS: thickness swelling; T_o: oven-dry thickness; T_max: equilibrium thickness; K_SR: swelling rate parameter.

Conclusions

In this study, the effect chemical modification of WF of the long-term hygroscopic behavior of WF/PP composites was investigated. The results showed that generally the chemical modification enhanced water resistance and dimensional stability in the studied formulations. This is mainly due to better interface quality and increased hydrophobicity in the modified composites. Water absorption of all the composites followed the kinetics of a Fickian diffusion process. Water diffusion coefficient was highest in acetylation-treated fiber composites and the lowest water diffusion coefficient was found in untreated ones. The highest TS K_SR took place in the untreated composites. FTIR spectra of treated fibers show that the intensity of the bonds at 3400 and 1740 cm⁻¹ change due to reaction of O–H groups and formation of ester groups in samples. This project has shown that the composites with treated fiber will be desirable as building materials due to their improved stability.

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) received no financial support for the research, authorship, and/or publication of this article.

References

Ashori

. Wood-plastic composites as promising green-composites for automotive industries. Bioresour Technol 2008; 99: 4661–4667.

Leao

Teixeira

Ferrao

. Production of reinforced composites with natural fibers for industrial applications – extrusion and injection WPC. Mol Cryst Liq Cryst 2008; 484: 157–166.

Oksman

Sain

. Wood-polymer composites. Great Abington, Cambridge: Woodhead Publishing Ltd, 2008, p. 366.

Tabil

Panigrahi

. Chemical treatments of natural fiber for use in natural fiber-reinforced composites: a review. J Polym Environ 2007; 15: 25–33.

Susheel

Kaith

Inderjeet

. Pretreatments of natural fibers and their application as reinforcing material in polymer composites: a review. Polym Eng Sci 2009; 49: 1253–1272.

Khan

Islam

Alam

. Study of water absorption behavior in wood plastic composites by using neutron radiography techniques. Polym Plast Technol Eng 2003; 42: 925–934.

Stark

. Influence of moisture absorption on mechanical properties of wood flour-polypropylene composites. J Thermo Compos Mater 2001; 14(5): 421–432.

Adhikary

Pang

Staiger

. Long-term moisture absorption and thickness swelling behavior of recycled thermoplastics reinforced with Pinus radiata sawdust. Chem Engi J 2008; 142: 190–198.

Ghasemi

Kord

. Long-term water absorption behavior of polypropylene/wood flour/organoclay hybrid nanocomposite. Iran Polym J 2009; 18(9): 683–691.

10.

Kord

Kiaeifar

. Hygroscopic thickness swelling rate of wood polymer nanocomposite. J Rein Plast Compos 2010; 29(23): 3480–3485.

11.

Zabihzadeh

. Effect of lignocellulosic type on long-term hygroscopic behavior of natural filler/HDPE composites. J Polym Environ 2011; 19: 133–136.

12.

Homkhiew

Ratanawilai

Thongruang

. Long-term water absorption and dimensional stability of composites from recycled polypropylene and rubberwood flour. J Thermo Compos Mater. Epub ahead of before print 7 1 2014; DOI: 10.1177/0892705713518789.

13.

Jacob

Anandjiwala

. Chemical modification of flax reinforced polypropylene composites. Compos A 2009; 40: 442–448.

14.

Dányádi

Móczó

Pukánszky

. Effect of various surface modifications of wood flour on the properties of PP/wood composites. Compos A 2010; 41: 199–206.

15.

Ghasemi

Farsi

. Interfacial behavior of wood plastic composite: effect of chemical treatment on wood fibers. Iran Polym J 2010; 19(10): 811–818.

16.

Raj

Kokta

. Effect of chemical treatment of fiber composites. J Adh Sci Technol 1989; 3(1): 55–64.

17.

Matuana

Balatinec

Park

. Surface characteristics of esterified cellulosic fibers by XPS and spectroscopy. Wood Sci Technol 2001; 35: 191–201.

18.

Liu

Mohanty

Askeland

. Influence of fiber surface treatment on properties of Indian grass fiber reinforced soy protean based biocomposites. J Polym Sci 2004; 45(22): 7589–7596.

19.

Kim

Chun

Lee

. Effect of plasma treatment on the wood flour for wood flour/PP composites. J Thermo Compos Mater 2009; 22(1): 21–28.

20.

Farsi

. Wood–plastic composites: influence of wood flour chemical modification on the mechanical performance. J Reinf Plas Compos 2010; 29(24): 3587–3592.

21.

Pavia

Lampman

Kriz

. Introduction to spectroscopy. 4th ed. Belmont: Brooks Cole, 2008, p. 656.

22.

Sawpana

Pickering

Fernyhoughc

. Effect of various chemical treatments on the fiber structure and tensile properties of industrial hemp fibers. Compos A 2011; 42: 888–895.

23.

Kord

. Evaluation on the effect of wood flour and coupling agent content on the hygroscopic thickness swelling rate of polypropylene composites. Bioresour 2011; 6(3): 3055–3065.

24.

Poletto

Zeni

Zattera

. Effects of wood flour addition and coupling agent content on mechanical properties of recycled polystyrene/wood flour composites. J Thermo Compos Mater 2012; 25(7): 821–833.

25.

Zhang

Yang

Lin

. Polypropylene hybrid composites filled by wood flour and short glass fiber: effect of compatibilizer on structure and properties. J Thermo Compos Mater 2013; 26(1): 16–29.

26.

Shi

Gardner

. Hygroscopic thickness swelling rate of compression molded wood fiber and wood fiber/polymer composite. Compos A 2006; 37: 1276–1285.