CPD: PULTRUDED FIBREGLASS

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SPORTS AFICIONADOS will easily recognise the terms ‘graphite tennis rackets’ or ‘carbon fibre golf clubs’. However, probably only the most technically minded will appreciate that these are the product of the development of a type of materials called composites. This article will explain the concepts of composite materials, in particular fibreglass, and explore their use for the manufacture of windows and doors.

What are composites?
Composite materials (composites for short) are engineering materials made up of two or more components which, although remaining separate and distinct, act in unison, each overcoming the deficiencies of the other. Combining the advantages of each element results in a material with broader and more attractive properties than its individual components. One of the two constituents acts as reinforcement and it is surrounded by the other, a matrix which transfers loads to it.

A very early example of composites is the use of mud bricks, with straw acting as reinforcement.

Reinforced concrete is a more modern example, where the poor compressive strength of steel is made up by the good performance of the concrete, with the opposite being true for tensile strength. Reinforcement materials used can be present in the form of particulates, discontinuous fibres (short fibres) or continuous fibres. Matrix products include metals, ceramics and plastics.

This overview of composites applied to windows will focus on glass fibre reinforced plastics (FRP), where the matrix is a polymer and the reinforcement is carbon, glass or aramid fibres. Aramid is a synthetic long chain polyamide best known by its trade name Kevlar, used in the construction of bullet-proof vests. These reinforcement materials have very high tensile and compressive strengths, but in their natural form fail at levels lower than their theoretical limits because of surface flaws that cause them to crack. When the material is used in fibre form, these surface flaws are limited to a small number of fibres, whereas the remainder still behave to their theoretical limits.

The function of the matrix is to spread the load to the individual fibres, and also to protect them from damage resulting from abrasion and impact. Materials used for the matrix can be categorised as:
> Thermoset polymers, which are plastic resins that cure by chemical reaction when heated and, once cured, cannot be resoftened by heating. Their greatest advantage is that, because of their low viscosity, fibre impregnation can be carried out at low pressure. More than 90% of the polymers currently being used in composites are thermoset.

> Thermoplastic polymers, which are plastics capable of being repeatedly softened by increases in temperature and hardened by decreases in temperature. They are also tougher and less friable than thermosets, but are more expensive to process as this must be done at a much higher pressure. However, they are much more readily recyclable than thermosets and, in the future, this may swing the choice their way in spite of the additional costs. A recent innovation has been the development of a proprietary patented product consisting of fibreglass reinforcement fibres in a PVC-U matrix, which is a thermoplastic polymer.

Various polymers are currently used as the matrix constituent, including: polyester resin; vinyl ester resin; epoxy; polyimide; polypropylene, etc.

The properties of a composite are determined by:

> the properties of the reinforcement

> the properties of the matrix

> the ratio of fibre to matrix (called fibre volume fraction). This is adjusted accurately during the impregnation process to suit the intended product use and ensure an even distribution. Although, in theory, a very high fibre volume fraction would result in higher mechanical properties, there is a practical limit because all fibres need to be fully impregnated.

> the geometry and orientation of the fibre in the composite. The diameter of the fibre is of great importance, as finer fibres have proportionally a larger surface area, and therefore the loads transferred by the matrix are spread more efficiently. Composites have anisotropic properties, ie, properties that are direction-specific as a result of the orientation of the fibres.

In the construction industry, pre-impregnated materials, ie, materials where the reinforcement has been pre-impregnated with resin before forming are the most commonly used ones, as opposed to those where the reinforcement is added to the matrix at the moulding stage. There are two basic types of pre-impregnated FRPs:

> unidirectional, where the impregnated fibres are aligned in one direction only and

> woven, which is a resin-impregnated fabric.

The composite must then be formed in the required shape/profile, generally by moulding. Until recently, this has necessitated high temperature and/or pressure, with associated high costs. However, the advent of low temperature moulding materials has simplified the process and lowered costs allowing possible on-site processing without the need for autoclaves and manufacturing shops.

The very high strength-to-weight ratio of composite materials makes them very suitable for structural long span applications, and their flexibility allows complex shapes to be formed, for example, lightweight cladding panels. Durability is also high, and life cycle and maintenance costs are low.

Fibreglass
Since its invention in 1938, fibreglass reinforced plastic (FRP) also known as GRP (glass reinforced plastic) has grown in popularity and now accounts for approximately 65% of composite production.

Fibreglass fibres are extruded molten glass (usually silica based, but not exclusively so) fibres, braided into bundles to form a continuous rope. FRP is produced by combining a polymer matrix with fibreglass reinforcements, either cut into short strands or woven into a cloth. The ratio between matrix and reinforcement is generally 65-85% resin and 15-35% reinforcement. All FRP products are thermosets.

Different grades of fibreglass can be produced by varying the composition of the glass used for the fibres, for example:

> E-Glass, which has good electrical properties

> C-Glass, offering the best resistance to chemical attack, etc

> R-, S- or T-Glass are not grades, they simply refer to proprietary trade names from various manufacturers.

When the FRP has been woven into a cloth, it is then moulded into shape. There are various moulding techniques, including open moulding, which produces only one good, finished surface and a rough one, and autoclave and vacuum moulding, which produce two finished surfaces. However, FRP can also be formed into continuous profiles by means of a process called pultrusion.

In the extrusion process, the material to be formed is pushed through a die, but fibre-reinforced composites need to be pulled instead. This process is called pultrusion (the name is a portmanteau word derived from the words ‘pull’ and ‘extrusion’).

Figure 1 explains the pultrusion process in schematic form: the reinforcement fibres are pulled through a resin bath, where they are impregnated, through a forming plate that confers the composite its profile section and then finally into a heated die where the resin is polymerised. As it cools down, the profile solidifies and is eventually cut into the required lengths.

The process of pultrusion has several advantages:

> it is a fast, economic way of impregnating and curing materials

> volatile emissions can be limited because resin impregnation takes place in enclosed surroundings

> resin and fibre content can be accurately controlled

> structural properties of laminates can be good since the profiles have very straight fibres and high fibre volume fractions can be obtained.

An even more recent development is the process of co-pultrusion, where two or more materials pass through a single die, and the resulting product is a laminate profile, where each ply of the laminate confers to it a desired property (for example, resistance to some specific environment, stiffness, etc).

Pultruded fibreglass products are very strong, dimensionally stable and durable, and can be formed accurately into sophisticated profiles. It is therefore not surprising that they are now increasingly being used in the manufacture of windows and doors. Table 1 compares the mechanical and physical properties of pultruded profiles with those of other materials.

Pultruded fibreglass technology in fenestration
Fibreglass windows were first produced in Canada in 1984, were subsequently introduced to the USA and European markets and are becoming increasingly popular. Market studies conducted on behalf of North American manufacturers predict a tenfold increase in the next few years. In the UK, pultruded fibreglass windows have been available only since the early 2000s.

Pultruded fibreglass windows offer a number of advantages over other types of window systems, as follows:

> Thermal performance. Pultruded fibreglass has a low coefficient of thermal conductivity that compares favourably with other low conductivity products like PVC or wood. Because of its great strength, profiles can be very thin, thus limiting the potential for cold bridging. Therefore, the thermal performance of the manufactured product is very good.

> Strength. As shown in Table 1, pultruded FRP profiles have greater flexural and tensile strengths than other materials used in fenestration. Therefore, they are suitable for large openings without the need for metal reinforcements.

> Dimensional stability. Pultruded fibreglass has a low coefficient of linear expansion which is very similar to that of glass. Other window materials have much higher coefficients – aluminium’s is double that of glass and PVC’s is seven times greater. As a result, pultruded FRP frames do not distort due to thermal variations.

> Resistance to moisture. Pultruded fibreglass is virtually impervious to moisture, and therefore does not rot, warp, crack or twist.

> Chemical resistance. Pultruded fibreglass is unaffected by chemicals or salt air, and is therefore suitable for coastal locations.

> Appearance. As pultruded FRP profiles are dimensionally and hydroscopically stable, they are a good base for sophisticated finishing systems.

> Cost. Initial capital expenditure is higher than for PVC, aluminium or timber windows. However, a whole life cost study conducted by the Building Research Establishment concluded that over a 30-year period, pultruded FRP was more economical than PVC.

A relatively minor limitation of the product is that FRP profiles cannot be welded, and therefore joints must be formed using adhesives.

Environmental considerations
There are a number of factors to be considered when assessing the environmental impact of pultruded FRP windows, among others:

> Resource depletion. Glass, which is silica based, accounts for approximately 65-85% of the components of FRP profiles. For all intents and purposes, sand can be considered an inexhaustible material. The polymer-based matrix is, of course, subject to the availability of oil for its production.

> Energy during manufacture. Because the main component is silica sand, this is low for FRP profiles.

> Gas release during manufacture. Unlike that of PVC, the manufacture of FRP profiles is a sealed operation, and the release of gases into the atmosphere can be closely controlled.

> Energy used during life. This is generally low, because of the good thermal performance of FRP.

> Disposal. As previously explained, the thermoset resins used in FRP are not easily recyclable, and may, in due course, steer the industry towards the use of thermoplastics.

Pultruded FRP has achieved an A rating in the latest BRE ‘Green Guide to Composites’, which is an environmental profiling system for composite materials and products published by BRE (‘A’ is the highest grade, ‘E’ the lowest). Table 2 compares the environmental impact of pultruded FRP windows with that of two alternatives.

Summary
The use of composites has been embraced wholeheartedly by the sports, aeronautic, car and maritime industries among others, but acceptance by the construction industry has been much more muted. In the UK, the BRE has created a specific forum called the Network Group for Composites in Construction with the aim of disseminating the advantages of composites throughout the building industry. Pultruded fibreglass window profiles, in particular, appear to offer the designer the opportunity of using very slim but strong and durable profiles suitable for large openings without compromising the ‘green’ credentials of the design.

Download the CPD test paper by clicking on the paper icon underneath the main image. Complete the form and send it back to us by 30th September, 2009. In return you will receive confirmation from Marvin Architectural, in accordance with RIBA CPD Providers Network, validated proof of study. Correct answers to this module will be published in the October issue.

Further reading
www.marvin-architectural.co.uk
For more information contact Isabel Blanco on 020 8569 8222 Atul Mittal & Soumitra Biswas Pultrusion of Composites – An Overview TIFAC
www.tifac.org.in
Guide to Composites NETCOMPOSITES www.netcomposites.com/education.asp?sequence=2

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