# Particle size, aspect ratio and pith: why filler geometry matters

Canonical: https://woodplasticcomposite.net/guides/wpc-particle-size-aspect-ratio-pith/
Language: en
Updated: 2026-10-16
Version: 1
Review: AI-assisted English article. Human technical review has not been recorded.

## Summary

Mesh size describes a screening operation, while aspect ratio describes particle shape. Neither number alone predicts composite strength.

## Read a mesh specification correctly

The phrase 20–60 mesh is useful only when the sieve convention and retained/passing fractions are defined. It is not a universal optimum. Two fillers with the same nominal mesh label can contain different mixtures of fines and elongated particles.

Stark and Rowlands compared several wood-flour sizes in polypropylene. Their results included different trends for notched and unnotched impact, illustrating why a larger particle cannot simply be labelled better. A separate investigation of wood flour and polyethylene considered size, aspect ratio and moulding temperature together. [R20,R21]

https://woodplasticcomposite.net/guides/wpc-particle-size-aspect-ratio-pith/#read-a-mesh-specification-correctly

## Look beyond the particle width

Aspect ratio is length relative to width or diameter under a stated measurement method. Specify whether it was measured before compounding or in the finished composite. The distinction matters when a process changes the reinforcement geometry.

Canola straw with pith and depithed straw is a sensible comparison to investigate, but this collection does not claim a verified numerical advantage for either. A study of wood-flour geometry provides a more general basis for keeping particle form and processing history visible in the comparison.

https://woodplasticcomposite.net/guides/wpc-particle-size-aspect-ratio-pith/#look-beyond-the-particle-width

## A practical supplier comparison

Request the sieve distribution, microscopy method, moisture result and batch identity. Compare fillers using the same polymer, equipment and test conditions before changing production specifications. Add the handling question: can the filler be fed consistently at the required output?

For specifiers, the consequence is simple. A supplier's particle-size claim may explain part of its process, but the finished product still needs the relevant performance evidence. Mesh numbers do not replace a board test.

https://woodplasticcomposite.net/guides/wpc-particle-size-aspect-ratio-pith/#a-practical-supplier-comparison

## Evidence and source scope

[1] Experimental study; official abstract.

[2] Experimental study.

[3] Experimental study; geometry, not proof of a canola-pith result.

https://woodplasticcomposite.net/guides/wpc-particle-size-aspect-ratio-pith/#evidence-scope

## Key points



## Comparison





## Sources

1. Effects of wood fiber characteristics on mechanical properties of wood/polypropylene composites; Stark and Rowlands — https://research.fs.usda.gov/treesearch/8558
2. Investigation of Wood Flour Size, Aspect Ratios, and Injection Molding Temperature on Mechanical Properties of Wood Flour/Polyethylene Composites — https://pmc.ncbi.nlm.nih.gov/articles/PMC8235768/
3. Study on the effects of wood flour geometry on physical and mechanical properties of wood-plastic composites — https://www.scielo.cl/scielo.php?pid=S0718-221X2015000300009&script=sci_arttext