Wood flour is a great organic filler for plastics because it has several advantages over other inorganic fillers, including a variety of sources, low cost, low density, effective insulation, and less wear and tear on processing machinery.

Due to its low compatibility with matrix resin, poor dispersion effect in molten thermoplastics, poor fluidity, and difficulties in extrusion molding, it has not, however, been utilized as widely as inorganic fillers.

While most thermoplastics are non-polar and hydrophobic, which makes them incompatible with one another and reduces the bonding force at the interface, cellulose, the primary component of wood flour, contains a large number of hydroxyl groups. These hydroxyl groups can form intramolecular or intermolecular hydrogen bonds, making wood flour water-absorbent with a moisture absorption rate of 8% to 12%.

Consequently, it is essential to find plastic-wood composite materials that function and behave superbly.
The issue of material compatibility must be resolved first. The major method for resolving the compatibility issue is by applying different modifications.
Why does the composite material perform better after employing different modifiers? What alterations have been brought about by the application of modifiers at the interface between the polar wood flour and the non-polar matrix resin?

By examining images taken by a scanning electron microscope (SEM) of the impact fracture of the composite material, we attempt to find a solution to this query. Six parts of the PE-g-MAH and EVA-g-MAH modified composites were subjected to impact fracture morphology analysis, as were the composite materials produced by directly combining wood flour (60 mesh) with HDPE without the use of any modifier and the composite materials produced by treating wood flour with 1.5% silane coupling agent.
The image depicts the impact fracture morphology of HDPE/wood powder composite material at room temperature.

(Note: modification types are as follows: unmodified (a), silane coupling agent (b), PE-g-MAH modification (c), and EVA-g-MAH modification (d).)

According to the figure: The interface between the composite material HDPE and wood flour without any modifier is very smooth and tidy after peeling off, as can be seen in photographs a-1 and a-2, demonstrating that the interface between the two phases of wood flour and HDPE is two-phase. The two have extremely little contact adhesion and very low compatibility. The wood powder and matrix resin are so readily ripped off when the material is exposed to an external force because the interface layer is unable to produce an efficient force transfer.

The composite material’s impact fracture morphology has significantly altered after being treated with 1.5% silane coupling agent, as can be observed from photographs b-1 and b-2, and the surface is no longer as it was previously without modification. There are several fiber materials adhered to the surface since it is so smooth. This is a result of the matrix resin deforming during the fracture process, which shows that an efficient interfacial layer is generated between the HDPE matrix and the wood powder modified by the coupling agent, improving the interfacial bonding between the two materials. To disrupt the contact between wood flour and HDPE, a powerful force is needed.

The surface of the wood-plastic composites prepared by adding 6 parts of PE-g-MAH is not as smooth and flat as it was before the addition, indicating that the use of PE-g-MAH also improved the compatibility of the composite interface and improved the affinity between HDPE and the surface of the wood flour in photos c-1 and c-2.

The impact fracture morphology of composite materials manufactured by adding 6 parts of EVA-g-MAH as a compatibilizer is shown in photos d-1 and d-2. After fracture, certain elongated fibrous materials, such as wood, can be observed appearing on the fracture surface. There is evident distortion in the cavity created by the powder and matrix resin peeling. This is a result of the composites’ improved interfacial compatibility being enhanced by the usage of EVA-g-MAH. Under the influence of an external force, the matrix resin deforms as the wood powder and matrix are peeled off.

Вышеупомянутый анализ и наблюдения позволяют сделать вывод, что использование компатибилизатора или обработка древесного порошка связующим веществом может значительно повысить совместимость между древесным порошком и границей раздела ПЭВП, увеличить адгезию границы раздела и, следовательно, повысить эксплуатационные характеристики композиционного материала.