WHAT IS WOOD PLASTIC COMPOSITE MATERIAL?
As its name might suggest, wood plastic composite (WPC) is a material created from a unique blend of natural wood and plastic fibres.
The two materials, let’s call them “wood” and “plastic”, are mixed together to create a relatively thick and homogenous mix. At this stage, some additives are also combined, such as colorants, reinforcing agents, potentially fire retardants and lubricants to achieve manufacturers specification and a high level of specification for the intended product’s use.
Basically, these combined products are heated and then pushed through relatively low technology plastic extrusion moulds to produce a continuous length of hot moulded product. The product is cooled (with water) post extrusion and eventually cut into lengths, which can then be stacked and packaged for sale.
Developments in recent years has led to two additional enhancements upon the basis 2nd generation wood plastic composite extruded decking or wood substitute product.
Firstly; the WPC extruded product has been combined with an outer layer in the extrusion process. This is called “co-extrusion” and bonds a polymer layer to the inner WPC core during the hot extrusion process. The coating, essentially a plastic, protects the wood plastic combined product from colour fading and staining, whilst improving the scratch resistance of the finished product. Whilst clearly more expensive, and in the early days difficult to perfect the bonding of the layers, this is what is called 3rd generation WPC or Co-Extruded decking.
Secondly, the wood-grain pattern or effect has now been incorporated into the “hot” part of the co-extrusion process, rather than added by “embossing: or stamping cold. This means the woodgrain contours are permanent and do not” fade away”. Looks like wood with natural contours forever!
So, we now have three WPC product types of decking (and other timber substitute products):
- 2nd Generation WPC throughout with grain effect stamped on, and fading and staining
- 3rd Generation WPC “coated” to eliminate fading and staining; and
- 3rd Generation WPC “coated” plus permanent wood grain profile.
Clearly, cost increases from 1 to 3, but ask any composite decking or timber substitute product supplier, which category their product falls into.
WHAT IS WOOD IN WPC MATERIAL?
Unfortunately, as with most things, WPC is not a standard product – A major factor is the actual WOOD content. Manufacturers classify as “wood” all types of wood flour and even plant materials like bamboo and rice husks or hulls.
Why does this matter? Well, intrinsically it should not but the properties of the “lignocelluloses fillers” does make a big difference in terms of performance characteristics. “Wood Fibre” used is broadly as follows:
- Wood Flour
- Sawdust (courser grain wood flour)
- Rice Hulls (Husks); and
- Long natural fibres like Hemp, Linseed and Flax
Generally, hardwood timber fibres tend to be most expensive and produce denser and stronger products. Softwoods, bamboo and rice hulls seem to produce lighter and less strong WPC products, but this can be obscured by the amount of wood fibre used in the wood plastic composing process (see more below). Strength tends to increase the greater the “wood” content of the WPC mix.
WHAT IS PLASTIC IN WPC
Almost universally the plastic component is Polyethylene. There are a few suppliers who use Polypropylene (“PP”) and even fewer Polyvinyl Chloride (“PVC”).
Polyethylene is rather soft, making PE-based composite boards easier to nail, screw, cut and saw. Polyethylene, as well as polypropylene show near zero moisture absorption also.
Most manufacturer use High Density Polyethylene (“HDPE”) rather than low density (“LDPE”) or medium density (“MDPE”), which is typically a mixture of LDPE and HDPE. Importantly, HDPE is stronger and stiffer than LDPE. LDPE is also more easily scratched.
Density of HDPE is generally around 0.95 g/cm3.
Much is made by wood plastic composite manufacturers of the recycled nature of the plastic used; the most common reference being to recycled milk bottles. Indeed, most WPC manufacturers do use recycled HDPE but we suspect there are some who purchase HDPE pellets directly from plastic manufacturers instead or as well to ensure a consistency of plastic material
HOW IS WOOD AND PLASTIC COMBINED TO PRODUCE WPC PRODUCTS
Once the wood and plastic content is selected, the next key issue is the proportions of both, when combined with filler, colourants, binders, etc. to establish the final product physical properties and eventual cost.
Generally, WPC materials contain at least 50%” wood” and up to 45% “plastic” and the numbers always add up to less than 95% when fillers, colorants, lubricants are added.
Think of wood % as density and strength and plastic % as finish and expansion. There is no right answer, so variations across quality manufacturers cannot be interpolated without additional strength and other quality measurement data.
What we at DECKER believe, however, is as follows:
- The quality of the wood fibre is important; hardwood fibre is best;
- 60%+ wood fibre is optimal; and
- Density of the WPC is the best measure of strength and longevity (in all Australian conditions).
DECKER DESIGN and DECKER PROTECT are WPC products made from 60% hardwood fibres, 30% HDPE and approximately 10% other additives including colourants.
In our next technical review, we will state what Australian WPC suppliers say about their WPC board composition, which is surprisingly little!!