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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.52 no.1 Concepción Mar. 2007

http://dx.doi.org/10.4067/S0717-97072007000100004 

 J. Chil. Chem. Soc., 52.No. 1 (2007)

THE EFFECT OF WOOD ACETYLATION ON THERMAL BEHAVIOR OF WOOD-POLYSTYRENE COMPOSITES

 

JUSTO LISPERGUER1*, CYNTHIA DROGUETT2 , BEATRIZ RUF1 AND MARIO NUÑEZ3.

1Facultad de Ciencias. Universidad del Bío-Bío, Avda Collao 1202, Concepción, Chile. E-mail : jlisperg@ubiobio.cl

2 Dirección de Transferencia Tecnológica. Universidad del Bío-Bío, Avda Collao 1202, Concepción, Chile.

3 Centro de Materiales Compuestos. Universidad del Bío-Bío, Avda Collao 1202, Concepción, Chile.


ABSTRACT

Wood-plastic composites are probably one of the most dynamic sectors of today’s plastic industry. Although the technology is not new, there is growing interest in the new design possibilities offered by this marriage of materials.

In the world, polystyrene (PS) is the third most utilized thermoplastic after polyethylene (PE) and polypropylene (PP). The uses of PS are different from those of PE and PP because PS’s glass transition temperature (Tg) is approximately 90ºC, and PS is rigid and brittle below this temperature (environmental conditions).One method used to increase PS impact resistance is to include wood fiber, due to a number of potential advantages, as a suitable candidate in fiber-reinforced polymer composites. The principal disadvantage is a poor adhesion between wood and the polymer due to the different chemical structure and polarity.

The study’s objective is to investigate the influence of an acetylation reaction on wood flour to reduce wood’s polarity and to improve the affinity and adhesion between polystyrene and wood.

Pinus radiata wood flour was acetylated with acetic anhydride, and acetylated wood-polystyrene composites with 20, 40 and 50 % of wood content by weight were prepared. Composites prepared with unmodified wood in the same composition were used to study the influence of the acetylation reaction on thermal behavior of wood-plastic materials.

Glass transition temperatures ( Tg ) of acetylated wood-polystyrene composites were compared with pure polystyrene and unmodified wood-polystyrene composites. Dynamic mechanical analysis (DMA ) was utilized to determine Tg and Fourier transform infrared ( FTIR ) was used to study the changes in functional groups during the acetylation process.

DMA showed changes in polystyrene’s Tg when mixed with wood. The major effect was the significant increase of 8º C in 50/50 of acetylated wood- polystyrene composites, showing that the use of acetylated wood flour produces wood-plastic composites with better thermal stability than non-acetylated wood.

Keywords : Wood acetylation, wood-polystyrene composites, thermal behavior.


 

INTRODUCTION

The use of lignocellulosic natural fiber in the wood-plastic industry has been rapidly expanding in the last decade. Interest has been increased due to the success of wood-plastic materials as lumber, window profiles, door frames, furniture and other building products( 1,2,3,). One of the most common natural fibers used in the thermoplastic industry is wood flour produced from post-industrial sources such as planer shavings or scrap wood ground to specific particle-size distributions(4).

Polyethylene (PE) and polypropylene (PP) are the thermoplastics most utilized to manufacture wood-plastic materials with improved mechanical properties and enhanced thermal stability in comparison with pure solid thermoplastics(5).

Other thermoplastics such as polystyrene (PS) and polyvinyl chloride (PVC) are also used ( 6,7).

In the world, PS is the third most utilized thermoplastic after PE and PP. The applications or uses of PS differ greatly from the uses of PE and PP because the glass transition temperature ( Tg) of PS is approximately 90 ºC, and PS is rigid, brittle, and easily cracked or shattered below this temperature (environmental conditions ).

The copolymerization of PS with elastomeric materials increases its impact resistance in comparison with the pure polymer, and can beextensively used for packaging, containers, appliances and in many other disposable products.

An alternative, less expensive method that can be used to increase PS impact resistance is to incorporate wood fiber, which possesses a number of potential advantages, as a suitable candidate in fiber-reinforced polymer composites ( 8,9,10).

One of disadvantages of wood-plastic composites is wood’s water adsorption, which contrasts with solid thermoplastics where this phenomenon is virtually nonexistent. Moreover, the chemical structures of wood and thermoplastic polymers such as PS are quite different, producing a different polarity and a poor adhesion between cellulose and the polymer(11, 12) . On the other hand, itis also observed that the degree of adhesion/interfacial bonding depends on other factors as composition, ratio of the fiber or wood particle size, as well as thetreatmentof the polymer or fiber with chemicals, plasticizers, coupling agents, etc.(13, 14 15 )

Many reagents have been used to modify the cell-wall components to improve adhesion with varying degrees of success, including acid chloride, isocyanates, aldehydes, alkyl halides, anhydrides, lactones, nitriles, etc(16).

Among these, chemical modification of lignocellulosic using acetic anhydride is perhaps the simplest, safest and cheapest method for improving the strength, dimensional stability and rot resistance of wood or bio-composites.

Wood -OH + CH3 - CO - O -CO - CH3
Wood -O - CO - CH3 +
CH3 - COOH

Hydroxyl groups are the most abundant and reactive sites for polymers on the cell-wall of a lignocellulosic material. The replacement of some of the hydroxyl groups with acetyl groups reduces the hydrophilic property of the cell-wall components, decreasing wood’s polarity (17).

The effect of wood acetylation in order to promote the chemical affinity between wood fiber and PS has not been well studied and the majority of work with PS has been oriented to increasing the polarity in thermoplastic(18), or researching several types of coupling agents to increase the interfacial bonding in wood-fiber/polystyrene composites.(19, 20) Moreover, we can considerthat immiscible polymer blends, such as wood and polystyrene, are very complex phenomena due to their different morphologies and the subsequent interaction between the components phases is a critical feature of immiscible blends, which has the ability to transform properties such as the melting temperature ( Tm ) and glass transition ( Tg ). The thermal behavior of polystyrene – wood flour composites using acetylated wood have not been studied, although there is some information related to the mechanical properties of polystyrene-wood flour boards, wherethe results show that the strength and water resistance of these composites are improved and it is possible to produce boards with a good performance.(21,22,23 ) However, it is not clear how the acetylation process influences the thermal behavior of resultant wood-polymer composites .

Dynamic mechanical analysis ( DMA ) is an extremely versatile analysis method, and no other single test method provides more information about a sample’s physical properties in a single test. ( 24,25 )

The calculation method for Tg from DMA data is important. Some researchers use the onset of change in the slope of the storage modulus ( E` ), while others use the maximum loss modulus (E”) or the maximum loss tangent(tan d).We have found that for more accurate determination when comparing several samples, the maximum of E” is adequate and more clearly shows the different values of Tg. The change in storage modulus is also used to study the curing reactions of liquid resin adhesives.( 26 )


he objective of this study is to investigate the effects of wood acetylation on thermal properties of wood-polystyrene composites. The thermal property studied was the glass transition temperature ( Tg )of the composites with different content of acetylated and non-acetylated wood in comparison with the Tg for polystyrene alone. Viscoelastic materials properties and their applications depend onTg.

Dynamic mechanical analysis ( DMA ) was used to determine Tg and Fourier transform infrared (FTIR) analysis was performed to investigate the acetylation reaction of wood flour, following the method published by Xiao Feng Sun et al.(12 )

EXPERIMENTAL

Wood flour Acetylation

The acetylation reaction of wood, without catalyst, was carried out by the method described in literature with a weight percent gain of wood flour of 10.7 % for 1 hour of reaction time ( Xiao Feng Wood and f Sc.).

A quantity of 15 g of Pinus radiata wood flour with a mesh size of 100 was oven-dried at 60º C for 16 hours. A quantity of 10 gof wood flour was placed in a 500-mL flat bottom flask. Then, 300 mL of acetic anhydride was added. The flask was dipped into an oil bath set at 120º C and fitted with a reflux condenser. Once the reaction time of 1 hour was completed, the flask was removed from the oil bath and the reagents were filtered off. The wood flour was then washed with ethanol to remove unreacted anhydride and acetic acid by-products. The acetylated wood flour was oven-dried at 60º C for 16 hours and weighed. The weight percent gain due to acetylation was calculated based on the weight of oven-dried unreacted wood flour.

Wood-polystyrene composites preparation

Commercially available, high impact polystyrene with a melt flow index of 0.7 g/ 10 min ( at 220º C )was obtained from Interquímica S.A.

Mixtures of polystyrene with acetylated and non-acetylated wood flour were prepared from 20 to 50 %of wood content by weight,via melt processing in a Haake Rheomix R 600 with amixing roller rotor at 60 rpm of speed for 4 min at 220º C. The table 1 shows the composition of samples.


Spectroscopic and thermal characterization

The FTIR spectra of unmodified and acetylated wood were recorded on a Nicolet model Nexus spectrophotometer, using KBr pellets containing 1% finely ground samples.

A mechanical dynamic analyzer ( DMA, Perkin Elmer 7 e ) was used to study the thermal behavior of the polystyrene and wood-polystyrene composites. The samples dimensions were 4 x 2 x 20 mm. The samples were tested in bending of three points with a light of 15 mm. The test was dynamic with a temperature range of 20º to 100º C at heating rate of 5º C / min. A load frequency of 1 Hertz was used.

RESULTS AND DISCUSSION

FTIR Spectra

The IR spectral characterization was performed to determine whether a chemical reaction was taking place between the wood flour and acetic anhydride.


Figure 2 illustrates the FTIR spectra of unmodified (b) and acetylated wood (a) prepared at 120º C for 1 hour. The most important features of the spectrum of acetylated wood flour are: the increasing of three ester bands at 1746 ( C=O ), 1383 ( - C - CH3 ) andthe - C -O- stretching band at 1234 cm -1 . Furthermore, the esterification reaction of wood flour decreased the peak area at 3434 cm -1 due to stretching vibrations of OH, indicating a partial acetylation. The other bands in the spectra do not show significant differences between acetylated and unmodified wood flour.

The weight percent gain of this acetylation reaction was of 12,8 %.

Dynamic mechanical analysis

 DMA supplies an oscillating force, applying a sinusoidal stress to the sample, generating a sinusoidal strain. By measuring the deformation magnitude at the peak of the sin wave and the lag between the stress and strain waves, properties such as modulus, viscosity and damping can be determined. When this is performed for a range of temperatures, the changes in the flexural modulus of polymers as a function of temperature can be monitored, allowing the identification of Tg and melting point and the determination of the effects of these changes on load-bearing characteristics.

Figure 3 shows the loss modulus for the DMA curves of polystyrene alone( PS)and unmodified wood-polystyrene composites with 20 , 40 and 50 % of wood content. The Tg values obtained from the maximum of loss modulus ( E” ) shows an increase of polystyrene Tg from 83.05º Cfor pure PS to 90.18º C for 50/50wood-PS blend. The overall magnitude of Tg shift is significant, about 7º C for the highest PS replacement. For the other two samples (20 and 40 % of PS replacement), the shift is the 5º C approximately.


Figure 4 shows the Tg values of pure PS and acetylated wood-polystyrene composites, where a clearer difference between the Tg for pure PS and for the acetylated wood composites can be observed. With higher wood content ( 49 and 50 % ) replacing PS, the increase of Tg is above 8º C.


Although wood and polystyrene can be considered as immiscible polymer blends, the Tg of wood-polystyrene composites shows a clear increase in comparison with polystyrene alone. The presence of amorphous polymers in the PS matrix, such as hemicelluloses and lignin, and partially crystalline cellulose with Tg over 200º C, as reported in literature( 27 ), produce a higher Tgin comparison with pure polymer.

The increase of Tg for polystyrene with 50 % of unmodified wood content (Fig. 3) is similar to the increasing when 20 % of acetylated wood is utilized ( Fig. 4 ). This is a demonstration that the acetylation-wood reaction produces a better chemical affinity between wood and polystyrene and probably has a positive effect on the mechanical properties of the wood-polystyrene composites.

CONCLUSIONS

Acetylation of wood significantly influenced thermal behavior of wood-polystyrene composites. The Tg of polystyrene calculated by the maximum of loss modulus ( E” )is 83º C and this value is increased to 90º C with 50 % of unmodified wood replacement. A higher increase of Tg for wood-polystyrene composites is obtained when acetylated wood is used. For 50/50 of acetylated wood – polystyrene composites, the Tg is increased to 91.5º C. This shift of Tg at higher temperatures is probably due to a better chemical affinity between acetylated wood and polystyrene.

The dynamic mechanical study revealed that the use of acetylated wood flour produces wood-plastic composites with better thermal stability than non-acetylated wood.

ACKNOWLEDGEMENTS

This research was supported by the Dirección de Investigación of the Universidad del Bío- Bío ( Grant Nº 0525063/R ).

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(Received: August 31, 2006 - Accepted: October 10, 2006)