CPD Module 3: Composite materials & pultruded fibreglass

*** This module is open until 22 September 2010 ***

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How to use this module: BD Reviews’ free continuing professional development distance learning programme is open to everyone who wants to develop and improve their professional knowledge and skills. These modules can contribute to your annual programme of CPD activity to help you maintain membership of professional institutions and bodies. All you have to do is read this module then take the questions via the link below or at the bottom of the page.

Introduction to composite materials

Composites are formed by combining two or more materials that have different properties. Individual materials are easily distinguishable within the composite – they do not dissolve or blend into each other. The materials work together as a composite to provide unique properties that are typically superior to the individual materials in isolation.

For example, materials such as glass or carbon have high tensile and compressive strength but in solid form have many random surface flaws that can cause the material to crack and fail. Tensile strength is defined as the maximum stress the material will bear when subjected to a stretching load. Compressive strength is defined as the maximum stress that a material will bear when subjected to a load that pushes it together. By producing the material in fibre form, the flaws are reduced to a smaller number at any one point. But while these fibres are extremely strong, they are still susceptible to damage. To counteract this, a composite consists of a second material which has distinct properties that overcome these flaws. This complementary material tends to be relatively weak, but very flexible and tough. Together, the two materials combine to produce a composite with unique and superior performance.

The individual materials that make up the composite are called constituents.

Most composites have two constituents: reinforcement and matrix materials. Reinforcement materials come in three forms:

• Particulate - these are small in size, with roughly equal dimensions in all directions.
• Discontinuous fibres - reinforcement materials become fibres when one dimension becomes long compared to others. Discontinuous fibres vary in length and typically are relatively short.
• Continuous fibres - these have fewer breaks and so produce higher performance properties. Continuous fibres are used in most high performance products, such as aerospace structures and sporting goods. Composite materials are usually strongest in the direction of the fibres.

The most common form of matrix material is plastic. The matrix material, or binder, holds the reinforcement together, and also transfers the load across the reinforcement. Plastic matrix materials are usually in the form of a resin.

The use of composite materials dates back thousands of years to the mud bricks used to build early homes. Dried mud can make a strong wall but it is easy to break by bending. A piece of straw seems strong when you try to stretch it, but not when you twist and bend it. Mud bricks are a composite that uses the straw as a reinforcement material and the block of mud as the matrix material. When dried, the mud brick became strong and resisted tearing and squeezing, creating an excellent building material. In the modern construction industry, reinforced concrete is a common composite material.

Advantages of using composite materials

Composite materials can be designed to produce specific attributes, and performance can be predicted and controlled to meet the requirements of specific applications.

For example, in aeronautics, composite parts are designed to withstand specific pressures while meeting lightweight requirements.

Composite materials provide superior performance characteristics over most other materials. In addition to performance predictability, composites resist fatigue, so can last much longer than other materials. Composites offer strong benefits in two key areas:

• Strength - they provide incredible tensile and compressive strength, and high-impact strength.
• Dimensional stability - they are extremely stiff, and there is less material movement.

Despite this, composites are still relatively light compared with other materials.

Fibreglass is a composite material made of fine glass fibres woven into a cloth and bonded together with a plastic or resin. It is considered the first modern composite, and has been in common use since the 1950s. It is also the most common composite material, and is used in 95% of boats; the bodies and bumpers of cars as well as in heavy duty construction and commercial vehicles; sporting equipment including golf clubs, hockey sticks, skis and fishing rods; and increasingly in the construction of bridge spans, guard rails and other construction components.

Fibreglass has extremely high strength-to-weight ratios. The continuous glass fibres provide high tensile strength, while the resin matrix provides high compressive and impact strength. Fibreglass also has a low coefficient of thermal expansion, which means the material demonstrates negligible contraction and expansion in extremes of temperature, and so can be used extensively in freezers or hot tubs, windows and doors, and fibre optics, for example. It is also durable, and able to resist moisture and corrosion, chemicals and environmental factors, which makes it ideal for use in cars, boats, underground storage tanks and structures exposed to environmental and chemical factors over time such as bridges.

Fibreglass is low maintenance, a significant factor in the decision to use it for construction and building materials.

Sustainability

This graph (see Fig 1) illustrates the estimated lifecycle costs of three different window types: timber, PVC and fibreglass. Though timber windows can last a lifetime if properly

maintained, the cost of maintenance adds to the overall cost of the window. PVC windows do not need to be maintained but may suffer discolouration or fading and chalking after 30 years and are often replaced. Although the initial costs of fibre-glass windows are typically higher, over a 60-year period considerable savings may be made as they are more resilient and require little maintenance.

Pultruded fibreglass is made primarily from silica sand, which is an essentially unlimited natural resource and an inert material that does not contribute to environmental degradation.

It requires significantly less energy to manufacture a finished fibreglass product than PVC or aluminium.

Thermal efficiency

Three factors are important when examining the thermal efficiency of windows (see Fig 2). The material which makes up the frame and sash of the window, the thermal efficiency of the warm-edge spacer bars, and the thermal efficiency of the glass itself. Pultruded fibreglass is far less conductive than aluminium and similar to wood and PVC.

• Ultrex = 0.30 W/mK (watts per meter square per degree Kelvin)
• PVC = 0.29 W/mK
• Aluminium = 115 to 231 W/mK

Combining pultruded fibreglass with low-E glass and argon gas produces some of the most energy-efficient windows and doors, and most products with this material and glass combination will exceed Part L requirements. These products can reduce a homeowner’s energy costs by 30-40% compared with a standard double-glazed unit.

The pultrusion process

Pultrusion is the manufacturing process for producing continuous lengths of reinforced fibreglass shapes (see Fig 3). It is a continuous process, and so can be highly automated, which creates consistency and efficiency and means little waste material is produced.

Pultrusion also produces consistent cross-sectional shapes, which is important for predictable, consistent and reliable performance, and contributes to the superior strengths that are characteristic of pultruded fibreglass. Because of its high strength-to-weight ratios, pultruded fibreglass can be produced as a very thin-walled profile while meeting maximum structural performance requirements. This allows for cost efficiencies, and also the ability to handle complex designs.

Pultrusion is a cost-effective and high-volume manufacturing process.

The steps of the pultrusion process are as follows:

• A fibre reinforcement is pulled through a resin bath (hence the name “pultrusion”), and is thoroughly coated or impregnated with a liquid thermosetting resin.
• The resin-coated fibre reinforcement is pulled through a heated die to cure the resin. Inside the die, the heat is set at a precise temperature to activate the curing of the resin, changing it from a liquid to a solid. This is where dimensions and shape of the final part are defined.
• A solid composite emerges from the die and is continuously pulled through the pultrusion machine. At this point, the composite solidifies as it cools in the exact shape of the die.
• The final step is to cut the pultruded piece to the desired length.

There are numerous process variables such as pull speed, die temperature, quality of the fibre and resin, and fibre volume which can all affect the quality of the final product.

The benefits of using pultruded fibreglass products

Fibreglass products are thermoset. This is because the heat used in the pultrusion process causes a chemical reaction which defines the shape and dimension of the part permanently. The cooling process produces a part that cannot be reformed – the process is irreversible.

In a thermoplastic process, heat is used to soften or melt the material during the forming process and the shape is retained after the part is cooled below the softening point.

The shape is not permanent since heating will soften the material and allow it to reshape. PVC is a thermoplastic material.

Pultruded fibreglass is significantly stronger than PVC, and in some characteristics stronger than steel, pound for pound. For example:

• Pultruded fibreglass = 414 MPa tensile strength
• Steel = 228 MPa tensile strength
• PVC = 45 MPa tensile strength

Pultruded fibreglass is also very stiff, so it is less likely to twist or warp. Flexural is the force needed to create movement in material.

• Marvin Architectural Ultrex windows = 20693 MPa flexural
• PVC = 2414 MPa flexural

Pultruded fibreglass has an extremely low thermal expansion rate, and because of the high concentration of glass in pultruded fibreglass, it is the same as that of plate glass (see Fig 4). This means it will not expand and contract in temperature extremes and creates less opportunity for stress cracks and seal failures. Pultruded fibreglass provides resistance to distortion at temperatures up to 177°C.

The change in a 4.8m stile from -34°C to 21°C is less than 0.8mm. An equivalent piece of PVC would change by around 4mm.

Click here to take this CPD Module

*** This module is open until 22 September 2010 ***