Material Futures - Part 3




The New Composites (1)

Having set the scene we can now look at the contenders for the label of 'the new PVC-U'. As a point of pure science (or pedantry) it is interesting to note that the term is “PVC-U” and never 'PVC-u' or 'PVCu'. If you see either of these terms used you will know that the person or organisation that wrote or used them doesn’t actually know what they are talking about - would you buy a car from a person who advertised it a 'Frod' on all their literature!

Pultrusion - Resins and glass fibres

One of the most promising of the newer materials processes is pultrusion. The process has been investigated many times over the past 20 years but it is yet to achieve a strong market commercialisation in window frames. The extremely high dimensional stability, low thermal conductivity, high corrosion resistance, high modulus and colour possibilities make pultrusion an excellent candidate for some window applications.

The main disadvantages are high cost, low workability and historical difficulties in producing the fine surface details needed although there are significant improvements being made in surface quality to reduce this concern. These disadvantages have not stopped the development of window and patio door systems in both the UK and USA and products from this material will continue to be developed for products where the advantages offered far exceed the disadvantages. The cost of the pultruded profile itself may be higher than an equivalent length of PVC-U but the high stiffness means that no reinforcement is needed and this effectively reduces both the raw materials and labour costs for the final product.

An alternative to the standard pultrusion is the Owens-Corning process of CLM (continuous lineal moulding), which is pultrusion over an insulating core of dense glass fibre wool. This process greatly improves the thermal response of the product whilst maintaining all the other advantages of pultrusion.

Potential applications are for windows and similar products where high stiffness properties, high load bearing properties and extreme chemical or thermal inertness are required.




  • High modulus
  • Extremely high dimensional stability
  • Low thermal conductivity
  • High corrosion resistance
  • Colour possibilities
  • High perceived cost
  • Low workability
  • Some difficulty with fine surface details
  • Surface finish

In terms of the ideal window material, pultrusion has great potential for window and door applications where the high stiffness greatly improves the product characteristics and provides substantial benefits.

Thermoplastic pultrusion - PVC-U and glass fibres

A recent PERA Craft Project involving a consortium of UK, German and Italian companies resulted in the development of a “PVC-U extrusion locally stiffened by continuous glass fibre” and patents have been applied for the resulting process and products. This material uses a localised glass fibre reinforcing encapsulated via co-extrusion into a PVC-U matrix. The localised glass fibres allow precise placement of the fibres to give a high modulus composite with the glass fibres adding the strength and the PVC-U providing the matrix. The PVC-U enables details and surface finishes to be formed similar to conventional extrusions and the technology is essentially that of extrusion of PVC-U.

The product cannot be structurally welded as with standard PVC-U because of the presence of the glass fibres but a cosmetic weld is said to possible. Mechanical strength requires additional fastening.

Potential applications include conservatory roof bars and similar products that do not require welding to form a frame structure.

Thermoplastic pultrusion - PVC-U and glass fibres



  • High modulus
  • Conventional PVC-U surface details possible
  • Conventional process technology
  • Other properties as per PVC-U
  • Cannot be structurally welded
  • Development project only, no current commercialisation

This is a process with great potential for specific projects such as conservatory roofs and similar products.

Polystyrene based materials

Extruded polystyrene structural foams (sold under a variety of trade names) have frequently been proposed as possible window frame materials. These materials have a good “wood like” finish and have advantages such as workability similar to wood, low cost and the ability to be produced on conventional single screw extrusion lines from a variety of recycled and virgin materials.

The greatest disadvantage is the fire response of the product, adding fire retardant to the product improves the fire response and the material can achieve Class 2 of BS 476: Part 7: 1987. As with any polystyrene based material, the products “drip and burn” when subjected to real flame testing and this may restrict the potential for window frame applications.

Polystyrene based materials



  • Good workability
  • Low cost tooling and processing
  • Wood-like finish
  • Some concerns with fire response
  • No successful commercial window applications known to date

A process that has been in existence for some years and current products suffer from the experience of poor performance with earlier similar products.

ABS/ASA materials

In the USA, GE Plastics have released and promoted the use of ABS/ASA as a window frame material, this composite uses ABS as an inner layer and an ASA co-extruded outer layer to improve the weather resistance to usable levels. Despite the current price difference between PVC-U and ABS/ASA, it is claimed that the ABS/ASA composites can use much the same production machinery but products can be thinner walled, run 30-40% faster and have better physical properties than the equivalent PVC-U products.

The process has been developed and tested with trial products in the USA but little progress has yet been made in Europe with these materials.

ABS/ASA materials


  • Lighter weight than PVC-U for same stiffness
  • Conventional process technology but runs faster than PVC-U
  • Good surface finish
  • Higher cost than PVC-U
  • Needs ASA layer for UV and weather resistance but exceptional performance is achieved
  • Cannot be structurally welded

A conventional process using a newly developed material and technology in an attempt to enter the window market. Probably matched in all benefits by other materials that are better established and cheaper.


Many of the new composites suffer from cost disadvantages when compared to PVC-U but specific markets will begin to use other plastics as their advantages in terms of stiffness become more important.

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) (This article)

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


Last edited: 11/03/10

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