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What are composites made of?

What is composite?

The word composite means a combination of two or more things.  More recently the term composite is used to describe fiber reinforced plastic or FRP.

The very first question an industrial adhesive specialists ask when an engineer is looking for an industrial adhesive is “What substrate are you bonding?”  When the engineer answers “Composite” it leaves the adhesive specialist with quite a few more questions to ask.

Often, due to the nature of what the engineer was producing, adhesive specialists have been able to guess with a high degree of certainty what the composite was comprised of.  Because of the design flexibility and strength to weight ration of composites, we are finding them in new industries and applications and increasingly new composite compositions.  It is no longer practical to surmise what the composite is made from.

Simply put, composites are fiber reinforced plastics.  Various fibers and resins are combined with fillers to dial-in the properties required in the final product.  Knowing what the fiber is, and what the plastic is, help to determine the best adhesive for the application.

Why do industrial adhesive specialists ask so many questions?

One of the most common composites we encounter is carbon fiber reinforced polymer.  Also known as CFRP, CRP, graphite reinforced polymer or simply carbon fiber.  Many different manufacturing processes can be used to create CFRP and many different materials.  Although all are called CFRP the end product is a result of the materials and processes used to create it and the best adhesive for each different type can vary.  Following is a look at some materials used to make composites.

FIBER

The type and amount of fiber alters the properties of the composite and is helpful in determining the stress and potential differential thermal expansion.

Glass fibers are strong and stiff, when they are combined with resin the load is distributed throughout the composite.  The result is a high tensile strength, material with excellent thermal and impact resistance.

Carbon, boron and aramid are more expensive than glass fibers and are often used in high tech applications as they result in a higher tensile strength lower weight.

Hybrid fibers of glass, boron and other materials are used to balance cost with performance features required.  Variation in fiber type and quantity alter the thermal and insulating properties of the composite.

Although adhesion is not affected by fiber type, the stress mode can be.

RESIN

Polymer matrix resins are either thermoset or thermoplastic.  I find the easiest way to remember the difference is to remember that thermoset resins are ‘set’ into their final form.  It is a cure process that is a one way street.  Once done they cannot be re-melted.  Thermoplastics can be reheated and reshaped.

Thermosets

Common thermosets include; polyester, vinyl ester, epoxy, phenolic, thermoset polyurethane and thermoset polyamide.

Thermoplastics

Thermoplastics are less common and more expensive.  They are used where the benefits outweigh the additional costs.  The benefits include; higher temperature performance than most thermosets, vibration damping and viscoelasticity as well as low coefficient of thermal expansion.

Common thermoplastics include; Polyethylene, polystyrene, polypropylene, polyethylene terephthalate(PET), polybutylene terephthalate (PBT) which are characteristically easy to process.

Engineering thermoplastics include acrylonitrile butadiene styrene (ABS), acetal, nylon, polycarbonate (PC), polyvinyl chloride (PVC), and polysulfone resins.

To discuss your composite bonding requirements, please contact Permabond today!