Material Futures - Part 4




The New Composites (2)

Cellular PVC-U products

Cellular PVC (PVC-UE) has long been used in the UK for many types of trim and finishing applications but is now being used in the USA for main frame window products. The products can be treated like wood (nailed, planed and screwed) but have many of the properties of PVC-U with regard to corrosion and fungus resistance.

The cost is low, the mechanical properties good and the technology is common with the existing extrusion technology.

Cellular PVC-U products


  • High thermal efficiency.
  • Can be nailed, screwed and treated as wood.
  • Conventional process technology.
  • Other properties as per PVC-U
  • Cannot be structurally welded.
  • Lack of detailed surface features.
  • Price may be a disadvantage

 A process already used for 'roofline' and trim products but not yet used in the UK for full window and door systems. Good possibilities for the future if the market wants to move in this direction.

Plastics and cellulose material composites

See also: Wood Plastics Composites - A Review Paper

Plastics and cellulose materials represent a new area and the range of materials being developed in this area is wide and exciting. The new materials cover a wide range of polymer matrix types as well as a wide range of cellulosic fillers and stiffeners. The plastics being used include PP, PE and PVC and the fillers used include wood flour, flax, jute and other cellulose based fibre fillers. This gives a wide range of properties and only a summary can be attempted in this article. The high wood flour content of some products (up to 70%) may lead to some confusion in the market - is it wood or is it plastic? The wood and plastics window industries may not only reach an accord but also come together in a more intimate sense as both the wood and plastics processors transform themselves into composites companies. Application of these composites is not restricted to window products and several car companies are investigating similar materials for use in body panels and other parts such as parcel shelves. Cellulose based composites may well be the material of the future for more than just the window industry!

The majority of the current work on windows is being carried out in the USA and significant advances are being made. In fact, finished products are already being released onto the market. As yet there is little commercial activity in the UK but this will come as the benefits of the materials are recognised. Reliable information on the exact technology is difficult to obtain because of both commercial sensitivity and some ongoing patent disputes.

In some cases the producers are existing timber window manufacturers who have access to large quantities of cellulosic materials (sawdust and scrap wood products) that are suitable for treatment and inclusion in this type of product. This means that no wood resources are depleted in producing the products and waste products that used to cost £25 per tonne to dispose of are now a valuable resource - recycling can become both profitable and ethical. The sources of the base plastic vary depending on the process but range from recycled PE bags and PP battery case materials to the use of virgin polymer. The recycling ethos is to use materials from short life cycle applications in long life cycle applications.

One of the major barriers to development is the difficulty of combining a hydrophobic material (most plastics) with a hygroscopic and hydrophilic materials (most cellulose based fibre products). This results in difficulties in compounding of the materials and poor stress transfer characteristics of the combined products. The general technique is to use a “compatibiliser” or “coupling agent” to improve the blending of the products and the interfacial interaction of the two phases. A typical compatibiliser is maleic anhydride modified polypropylene (MAAP) that is used to treat the plastics and cellulose products. This gives improvements in the processability and mechanical strength of the final product. The exact content of most compatibilisers is confidential and MAAP is generally not the only agent used in the compound.

PE based products are cheaper, have a higher heat distortion temperature and are less stiff than PVC based products but the PVC products do not suffer form the low surface energy of the PE products which makes painting and post-treatment difficult. In both cases the cellulose products improve extrusion speeds because of the higher heat transfer rates from the cellulose materials.

These composite products can generally be nailed, painted and otherwise treated as wood whilst retaining many of the benefits of plastics in the areas of fungus and corrosion resistance.

Cellulose based materials reduce costs, increase production rates and offer a host of benefits to the profile production. One of the virtuous areas is that the more cellulose material that is added, the lower the price of the raw material and also the higher the stiffness of the raw material. Lowering the cost actually improves the performance!

Typical of the products that are being used in the USA are:

Fibrex - Andersen Corp.

Andersen launched their first wood-fibre reinforced PVC profiles in 1993 under the name of Fibrex. This is a base of wood filled plastic with a virgin PVC outer layer co-extruded onto the core. The approximate ratio is 60% PVC-U and 40% wood fibres generated by the company from other operations.

Comptrusion Corp.

Produces products based on the Strandex licence and makes window and door profiles using hardwood and softwood flour in PE and PVC. The ratio of cellulose filler used is thought to be higher than that used in the Andersen product. The final profiles can have a co-extruded outer layer of PVC for cosmetic purposes.

Plastics and cellulose material composites



  • Low cost and plentiful raw materials.
  • Recycled raw materials.
  • Hybrid materials - combine the best properties of both the biological materials and polymers.
  • Good stiffness.
  • Compatibilisers raise costs.
  • Affects all current technology investments.
  • Need to learn how to handle new materials (powder and dust).

These stand the best chance of being the “new PVC-U” that the windows market has been waiting for. The materials have lower and more stable materials costs, are environmentally friendly and have a wide range of features and benefits for all sectors of the market. The timber window may well come back in another form to compete with (and possibly defeat) PVC-U.

What now?

Low cost materials will revitalise, threaten and change the industry and give a huge advantage to the first companies to develop the products. Many companies in the UK are not developing products purely because of their current investment in existing technology. This will change in the future as low cost materials enter the market and compete against the existing materials. Staying in a particular technology simply because of your investment in the technology is not a recipe for success in any fast moving market and the window market is no exception.


The way ahead for plastic and composite windows is not a simple one. Market forces of security, standards and increased environmental pressures will meet an industry struggling with low margins, consolidation, new materials and a decreasing market. It may not be a rosy future but it will certainly be interesting.

The Material Futures series is designed to look at some of the possible materials that could be used for window frames in the future. The series is:

Part 1: Material Futures - The Way Behind

Part 2: Material Futures - The Way Ahead

Part 3: Material Futures - The New Composites (1)

Part 4: Material Futures - The New Composites (2) (This article)


Last edited: 11/03/10

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