Effects of Particle Size on Wood-Plastic Composites

By Susha Cheriyedath, M.Sc.Apr 19 2022

Wood-plastic composites (WPC) have a range of applications in varying industries, and their quality and properties are influenced by the particle size and distribution of filler materials. The quality of WPC can be considered and estimated by means of various particle analysis methods. 

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Wood-Plastic Composites and Their Applications

Wood-plastic composites (WPC) have a wide range of applications in outdoor decking, automotive interior substrates, packaging, furniture, housing, and cladding, and their current worldwide production is over 1.5 million tons. According to the European Committee for Standardization, WPC’s composition contains one or more thermoplastic resins like polyvinyl chloride (PVC), polyethylene (PE), or polypropylene (PP) as matrix materials and lignocellulosic wood fiber or wood flour as fillers.

The reinforced plastic with fillers enhances the mechanical properties of WPC, and the use of wood-based fillers reduces the overall cost. While the wood-based filler’s quality depends on the surface’s chemical nature, particle distribution, impurities, shape, and porosity, the strength of WPC depends on the size and configuration (shape) of the particle.

Advantages of Analyzing Particle Size/Distribution

Previous literature shows that a bigger particle size of the filler causes an inhomogeneous distribution of particles in the matrix, while smaller particles can homogenously distribute in the matrix and enhance the mechanical strength of WPCs. However, fine particles can form agglomerates on inadequate distribution, leading to failure due to stress concentration effects. Consequently, wood fiber is a preferred filler over wood flour.

Pre-experimental analysis of filler particle size and shape before compounding them to design WPCs is a common practice adopted by researchers to study the variation in WPC’s mechanical strength based on the initial morphology of the filler. However, these wood fibers can change their morphology during processing due to various interactions among particles or between particle-polymer and equipment-particle. Thus, it is possible to analyze the filler after processing to relate its effect on WPC’s mechanical properties.

Apart from particle size, few authors report that the aspect ratio (ratio of length to diameter) is the main parameter to consider when using wood fiber as a filler. Another aspect to be considered while analyzing particle size is particle distribution, calculated in terms of average particle length. In this context, while the arithmetic mean is representative of the symmetrical distribution, the median is representative of the asymmetrical distribution.

A lot of information is lost while restricting the size distribution to only one parameter. To this end, Inceoglu et al., in their study on glass fibers, observed that there was a decrease in maximum fiber length, without much effect on mean and minimum length. This decrease is attributed to the degradation of long fibers during processing.

Methods for Analyzing Particle Size

While there are various methods to determine particle size distribution, each is specific to industrial application. Sieve fractionation is convenient for the wood industry, while other methods like X-ray tomography, light scattering, and light polarization are adopted at the lab research level to characterize particles or fibers.

Manual or automated static techniques involving scanning and microscopy can help obtain quality results. However, these methods need good sample preparation with good particle distribution and no agglomeration. In addition, the manual static technique is time-consuming, and the results are user-dependent.

Recent Studies

In a recent article published in the journal BioResources, researchers from Turkey manufactured PP-based WPC using 40% mahogany wood flour and studied the effect of particle size on selected properties. The results showed that a decrease in particle size increased the density, flexural modulus, tensile modulus, and impact strength values.

In another study published in the Journal of Tropical Resources and Sustainable Science, the researchers reported that wood fiber of 75-µm size, with the composition ratio of 30:70 wood fiber to PP, exhibited increased mechanical strength over 250-µm size wood fiber. The authors reported that the increased wood fiber size decreased the strength and stiffness of the thermoplastic composite.

In a study published by Marek et.al. in the European Journal of Wood Production, the authors manufactured three-layered particleboards prepared using industrial wood particles and PVC. This study revealed contrasting results, where the mechanical properties like tensile and flexural strength increased with increasing particle size. This trend is due to the greater aspect ratio of larger wood particles.

Conclusion

To conclude, although previous literature demonstrates the influence of particle size or distribution on the mechanical strength of WPC, the increasing or decreasing trend is still unclear.  There are also other aspects to consider, like the addition of coupling agents and the nature of plastic while studying the mechanical strength of WPC.

Compared to wood flour, wood fibers are better for lignocellulosic fillers to design WPC since wood flour has a minute particle size and might not result in the symmetric distribution in the polymer matrix, hampering its mechanical properties.

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References and Further Reading

Teuber, L., Militz, H., & Krause, A. (2016). Dynamic particle analysis for the evaluation of particle degradation during compounding of wood plastic composites. Composites Part A: Applied Science and Manufacturing, 84, 464-471. https://www.sciencedirect.com/science/article/pii/S1359835X16300021?via%3Dihub

Delviawan, A., Kojima, Y., Kobori, H., Suzuki, S., Aoki, K., & Ogoe, S. (2019). The effect of wood particle size distribution on the mechanical properties of wood–plastic composite. Journal of Wood Science, 65(1), 1-11. https://jwoodscience.springeropen.com/articles/10.1186/s10086-019-1846-9

Cavus, V., & Mengeloğlu, F. (2020). Effect of wood particle size on selected properties of neat and recycled wood polypropylene composites. BioResources, 15(2), 3427-3442. https://bioresources.cnr.ncsu.edu/resources/effect-of-wood-particle-size-on-selected-properties-of-neat-and-recycled-wood-polypropylene-composites/

Kociszewski, M., Gozdecki, C., Wilczyński, A., Zajchowski, S., & Mirowski, J. (2012). Effect of industrial wood particle size on mechanical properties of wood-polyvinyl chloride composites. European Journal of Wood and Wood Products, 70 (1), 113-118. https://link.springer.com/article/10.1007/s00107-011-0531-5

Inceoglu F, Ville J, Ghamri N, Pradel JL, Durin A, Valette R, et al. Correlation between processing conditions and fiber breakage during compounding of glass fiber-reinforced polyamide. Polym Compos 2011;32(11):1842–50. https://onlinelibrary.wiley.com/doi/10.1002/pc.21217

Rasat, M. S. M., Wahab, R., Shafie, A., AG, A. M. Y., Yusoff, M., Ramle, S. F. M., & ZulhisYam, A. K. (2013). Effect of Wood-Fiber Geometry Size on Mechanical Properties of Wood-Fiber from Neolamarckia Cadamba Species Reinforced Polypropylene Composites. Journal of Tropical Resources and Sustainable Science (JTRSS), 1(1), 42-50 http://journal.umk.edu.my/index.php/jtrss/article/view/669

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