Additives for improved performance in WPC extrusion

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  • The importance of lubricants & coupling agents
  • Agglomerate versus powder
  • Based on “Not all WPC are created equal” by Polymera

The current North American Wood Plastic Composite (WPC) market is estimated at around €888 million (US$1.1 billion) for building and construction, with an annual growth rate of approximately 16% by 2018. The economic production of high-quality WPC products faces some challenges from application-dependent formulation, fire and environmental hazard considerations and last but not least costs. A wide range of additives can be employed to enhance the properties and reduce costs in most cases.

When it comes to WPC in construction applications, the most relevant properties are strength, stiffness, dimensional and thermal stability, time-dependent behaviour and weight. The different major components of a WPC compound have different functions: the polymer matrix protects the fibre particles (encapsulated), transfers the stress between the fibre particles and acts as a processing carrier. The fibre particles themselves support the structure. Besides the main compound and the natural fibre, the formulations may consist of colour/UV protection, biocide, mineral filler, acid scavengers, antioxidants, lubricants, heat and co-stabilisers and coupling agents.

Wood flour mesh size mattters

The use of mineral fillers, such as CaCO3 or talc, for instance, increases the stiffness and moisture resistance,
while the CTE (Coefficient of Thermal Expansion) goes down. In comparison, certain species wood flour, such as oak, maple, or ponderosa pine, can slightly change the flexural and impact properties. Depending on the wood flour mesh size, by comparing with PP, the flexural properties of PP based WPC, for instance, can be increased about three times on modulus and approximately 25% on strength when the optimal mesh size of 60 mesh is used. In contrast, the flexural properties of WPC experience certain degree of deterioration when either a smaller or bigger wood flour particle is used. The impact strength of WPC has the same trend. Another factor to impact the properties of WPC is the wood flour loading. By balancing the flexural strength and modulus, approximately 50% of wood flour is recommended.

Lubricants & coupling agents

Two kinds of lubricants are often used in WPC and in extrusion in general: external and internal lubricants. The external lubricants improve the surface quality and lead to higher output rates as friction goes down during the extrusion process. With the use of internal lubricants the shear degradation decreases and a better dispersion can be achieved.

The function of coupling agents, such as MAPE/PP (maleated polyethylene/polypropylene), silane or isocyanate,
is to better disperse the material and increase the interfacial bonding. This can lead to increased mechanical and physical properties of 30 to 50%. Other common additives are modifiers and stabilisers: antioxidants (AO), UV and biocide stabilisers bring down thermal degradation, polymer chain scission and bio deterioration respectively.

Agglomerate versus powder

The use of agglomerate (pre-densified) instead of dry blend increases the bulk density by about 50%. In addition the agglomerate is easier to handle (less dust/fire/health hazard), and can achieve a higher extrusion output of about 15% with less energy input (J/kg). As the process temperature is below 120°C, the material experiences almost no heat history and less energy is consumed.

Summary: WPC formulations have to be based on the end product and application. The engineering of WPC helps
market penetration in regard to higher performance and cost effectiveness. And processed from agglomerate form it can solve issues of material handling and hazard and leading to higher output.

This article is based on the presentation “Not all WPC are created equal” by Derek Tsai, PhD, Polymera

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