Processing
Wood & plastic are not the only components in WPCs. These composites also contain materials that are added in small amounts to affect processing & performance. Although formulations are highly proprietary, additives such as coupling agents, light stabilizers, pigments, lubricants, fungicides & foaming agents are all used to some extent. Some additive suppliers are specifically targeting the WPC industry.

The manufacture of thermoplastic composites is often a two-stage process. The raw materials are first mixed together in a process called compounding & the compounded material is then formed into a product. Many options are available for compounding; using either batch or continuous mixers. The compounded material can be immediately pressed or shaped or formed into pellets for future processing. Some product manufacturing options for WPCs force material through a die

(Sheet or profile extrusion), into cold mould (injection molding) , between calendars(calendaring) or between mold halves(thermoforming & compression molding). Combining the compounding & product manufacturing steps is called In-Line Processing.

The majority of WPCs are manufactured by profile extrusion in which molten composite material is forced through a die to make a continuous profile of the desired shape. Extrusion lends itself to processing the high viscosity of the molten WPC blends & to shaping the long continuous profiles common to building materials. These profiles can be a simple solid shape, or highly engineered & hollow. Outputs up to 3 mtrs / min are currently possible.

Although extrusion is by far the most common processing method for WPCs, the processors use a variety of extruder types & processing strategies. Some processors run compounded pellets through single – screw extruder to form the final shape. Others compound & extrude final shapes in one step-using twin – screw extruders. Some processors use two extruders in tandem, one for compounding & the other for profiling. Moisture can be removed from the wood component before processing, during a separate compounding step (or in the first extruder in a tandem process), or by using the first part of an extruder as a dryer in some in –line processes. Equipment has been developed for many aspects of WPC processing, including material handling, drying & feeding systems, extruder design, die design, and downstream equipments such as cooling tanks, pullers, & cut-off saws. Equipment manufacturers have set up partnerships to develop complete processing lines for WPCs. Some manufacturers are licensing extrusion technologies that are very different from conventional extrusion processing.

Compounders specializing in wood & other natural fibers mixed with thermoplastics have fuelled growth in several markets. These compounders supply preblended, free-flowing pellets that can be reheated & formed into products by a variety of processing methods. These pellets are a boon to manufacturers who donot wish to compound in- line.

Other processing technologies such as injection molding & compression molding are producing WPCs, but the tonnage is less. These alternative processing methods have advantages when processing of a continuous piece is not desired or a more complicated shape is needed. Composite formulations must be adjusted to meet processing requirements (e.g.; the low viscosity needed for injection molding can limit the wood content).

New Processing Technologies: “Plastic Steel From Wood Filled PP”
Synthetic Hardwood Technologies (SHW); USA has developed a new technology of applying low temperature die drawing to take advantage of a new raw material; expanded, oriented, wood filled polypropylene (EOW PP). It looks and feels almost like natural wood, but is 150 % stronger. It is also 300 times stronger than PP & close enough to the strength of steel. Hence the term “Plastic Steel” is no exaggeration.

This unusual new material is made of a combination of two metalwork processes: solid –state extrusion & die drawing. SHW's new low density, wood filled PP composites profiles are targeted mainly for four application areas, indoor construction (flooring), automotive, outdoor construction & defence applications. Market opportunities are promising for outdoor construction.

Die Drawn OPP Profiles With & Without 30 % wood

Product	Flexural Strength	Flexural Modulus
	MPa	MPa
PP	48	1862
Wood	96	8694
O PP	276	7585
EOW PP	138	7585

Additives Developments Aid Growth In Wood-Plastic Composites
Additives for wood – plastic composites in the USA are expected to grow at a rate of 16 % AAGR from 19 Million Kgs in 2001 to 40 Million Kgs in 2006, according to a survey By Principia Partners. While the use of colorants and lubricants is well established, use of other additives such as UV stabilizers, antimicrobials & coupling agents is growing, as product lifetime & strength requirements become more critical. The use of lubricants is expected to grow from 5.4 Million kgs.in 2001 to 11.3 million kgs in 2006. Lubricants are expected to increase output in extruded applications such as decking. Struktol Company of America, introduced two new lubricants in 2002 for PVC & Polyolefin wood composites as a substitute for the commonly used Zinc stearate / ethylene – bis – stearamide package. These new lubricants are supposed to improve output of polyolefin wood composites by 25 %.

Stearate lubricants & maleated coupling agents have an antagonistic effect when used together, resulting in a drop in output without significantly improved properties. Honeywell's Optipak 300 combines proprietary lubricant packages & coupling agents for PE & PP composites. . The trend is to move towards non-stearate based lubricant & coupling agent packages.

Special HALS UV stabilizers & anti microbial additives are also being offered for WPC's.

Although foamed WPC's are relatively small portion of the market currently, there is great interest in foaming. Clariant's new Hydrocerol foaming agents, which are mixtures of endo & exothermic chemical foaming agents are claimed to produce more than 30 % weight reduction. Higher levels of foaming are a development target.

A niche area is additive blends to enhance the performance of natural fibers such as flax, hemp, kenaf and sisal in semi structural applications in the automotive sector. Higher loadings of these fibers can yield substantial improvement in performance over WPCs.