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What are Glass Reinforced Plastic (GRP) Composites? (A Complete Guide) – TWI


Glass reinforced plastic (GRP) – also known as glass reinforced polymer, fibreglass (‘fiberglass’ in the United States), glass fibre composite, or composite plastic - is a highly versatile, light and strong material that has a range of applications across different industries.

GRP belongs to a larger family of products known as fibre reinforced plastics (FRP), yet GRP behaves differently to many conventional thermoplastics found in numerous common everyday items.


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Properties and Benefits

GRP is a versatile composite material which, being made from a combination of glass fibres and polymers or plastic, offers a number of significant advantages over materials such as metal or concrete. In addition, GRP is more adaptable than other, non-reinforced plastics, such as acrylonitrile butadiene styrene (ABS), nylon or polypropylene.

GRP offers a variety of desirable properties, including:

  • Chemical Resistance: GRP materials a resistant to a number of different chemicals
  • Corrosion Resistant: GRP is suitable for use in harsh conditions as it is capable of withstanding aggressive environments, including resisting chloride ion attacks. This makes GRP a good alternative to aluminium, steel or timber, providing a durable, long-term solution that avoids the corrosion challenges associated with many other materials
  • Easy to Fabricate: GRP is easy to fabricate as it can be cut onsite to precise specifications or complex layouts using only standard hand tools and without compromising any of it’s advantageous properties
  • Flame Retardant: Some GRP materials are available in flame-retardant variants
  • High Strength: GRP offers tensile strengths equal or greater than those of equivalent steels. Also, being light, these materials deliver impressive strength-to-weight load-bearing capabilities
  • Highly Impact Resistant: GRP’s resistance to sudden and severe loading means it can avoid permanent distortion and will return to its original shape following an impact, all without the need for costly repair or replacement
  • Inert, Non-Conductive and Non-Sparking: GRP is suitable for use on sites where electrical hazards could be an issue as it is inert, non-conductive and non-sparking. The electrical transparency of GRP means it is immune to electromagnetic fields or radio waves and is thereby good for electromagnetic or thermal insulation.The non-sparking properties, meanwhile, mean it can be used in the presence ofcombustible gases
  • Lightweight: The light weight of GRP compared to steels mean that it is easy to transport, cut or manoeuvre, with a reduced risk of manual handling injuries as well as lower transport costs.
  • Long Life Cycle: The expected lifecycle of GRP materials under normal operating conditions is in excess of 20 years, although the materials may experience some discolouration in this time span.
  • Low Maintenance: Being a durable material, GRP requires less money to be spent on maintenance, refurbishment or renovation
  • Radar Transparent: GRP materials will not interfere with radar systems

GRP materials also offer a number of other key benefits, including being warm to the touch, having a wide range of operating temperatures, and possessing resistance to insect infestation (unlike timber, for example).

The benefits of GRP materials also extend to their manufacture, as they can be produced using low energy manufacturing processes and can also be reprocessed and recycled to provide energy and raw materials for cement production.

Manufacture of GRP

GRP products are manufactured from glass fibre reinforced polymers, often with either a polyester or vinylester thermoset resin matrix. A catalyst is used to initiate a chemical reaction that irreversibly hardens the resin. The resin is also reinforced with the introduction of glass fibres during the production process. These glass fibres can be long or chopped strands or comprise of woven mats of fibre.

The production methods for GRP can be manual or automated with robots – as shown below. Each method offers advantages for different shapes, properties and uses for the final products.

Common examples of production processes for GRP include:

1. GRP Moulding:

GRP moulding is a largely manual process whereby an open mould is created with the glass fibre strands being woven across the mould with a comb-like tool to create a skeleton of the finished structure.Then, a catalysed resin with the required colour and grade is poured into the heated mould and left to cure and harden. A coating of aluminium oxide grit is then applied to the top surface of the part to finish the process.

2. GRP Pultrusion:

This highly-mechanised process is used to manufacture continuous lengths of GRP composite that are typically used for load-bearing applications such as channels or beams. This technique has been used for over fifty years to create profiles with accurate and consistent cross sections with predictable mechanical properties. The method takes glass fibre rovings that are fed from multiple bobbins. Reinforcements, fibreglass mats and a ‘surface veil’ are then pulled through a bath of resin and into a high temperature / high pressure mould which forms and cures the resin ready to be cut to the required length. The finished products are lightweight, strong and suitable for a wide range of applications traditionally accomplished with metals and woods. Corrosion-resistant and maintenance-free, these pultruded products offer long-term and cost effective solutions.

3. GRP Hand/Spray Lay-Up Moulding:

The hand lay-up process is a manual method of production that uses very little tooling. It is typically used for low-volume production and for bespoke products. The quality of the GRP depends upon the skills of the operative creating it. The glass fibre reinforcement comprises a chopped or woven strand mat and may also include an inner coremat that forms a sandwich between the layers of glass, providing extra strength. These laminate materials are cut to the required size and shape before being laid out on the surface of an open mould that has been coated with wax, which acts as a release agent. Resin is then applied with a brush or a roller until the laminate is saturated. The resin and laminate is left to cure in a warm and ventilated drying area until it solidifies. This can take several hours, after which the part can be removed from the mould and trimmed, coated or polished as required. An initial gelcoat layer can be applied to the mould before the reinforcing laminate is placed down, which can improve surface finishes. To achieve this, the final part’s top or visible surface needs to be the one in contact with the mould. The spray-up method is similar to the manual lay-up technique, except that it is semi-automated. This computer-controlled process uses a pressurised spray gun with an incorporated glass fibre ‘chopping’ head unit. The laminate is applied to the mould in a continuous process as the chopping head cuts it into rovings while the spray gun provides a stream of resin into the mould.

4. GRP Resin Transfer Moulding:

Used to create smaller parts in production volumes with a consistently high quality, resin transfer moulding involves injecting a mixed resin into a mould under pressure.

5. GRP Compression Moulding:

This process uses an open mould cavity into which a preheated polymer is placed. A top plug is then applied to close the mould so that the material is in contact with all areas of the mould before it is compressed. It is then left to cure so that it can harden into the specified shape.

6. GRP Long Fibre Injection Moulding:

This highly automated process is typically used for larger parts that need to be created with precision and with high strength/stiffness and a low weight. A polyurethane polymer matrix is used alongside long glass fibres that are applied into the mould by a robot, before it is compressed, cured and hardens to the required shape.


Glass reinforced composite products can be found in a wide range of industries, from construction to consumer items and electrical to transport. The light weight and corrosion resistant properties, coupled with engineering verification and creative designs mean that a large variety of products are manufactured entirely or partially from GRP materials.

Example products and items include gutters, greenhouse structures, lamp posts, signs, bus bar supports, ladders, tent poles, radio antennae, tool handles, sporting items, fence posts, umbrella shafts, non-slip floor coverings, handrails, pipes, structural supports, stairs, kick plates, luggage racks, seating for public transport, leaf springs, and more.

Creep Behaviour of GRP Composites

As with metals and alloys, GRP composites can suffer from creep damage. Creep phenomenon in GRP composites occurs under prolonged mechanical stress and can result in permanent deformation of a part.


Easy to manufacture, using a variety of different methods, GRP composite materials can be found in a wide array of applications across many different industries. Commonly found in consumer goods and many other everyday items these materials offer a range of benefits, including chemical, corrosion and impact resistance. Being lightweight, flame retardant and having a high strength, these materials are also inert, non-conductive, non-sparking and radar transparent.

GRP composites can suffer from creep after prolonged exposure to stresses, but the low maintenance and long lifecycles associated with these materials make them a desirable material for a range of different products.


What are GRP composites made of?

GRP composites comprise of a mixture of plastic and glass fibres. By joining the properties of these two materials you get a robust, reinforced material that can be used for a wide range of items.

What is the difference between GRP and fibreglass?

GRP is a type of fibreglass, made of plastic reinforced with fine glass fibres. It behaves differently to conventional thermoplastics and is also known as composite plastic or fibre reinforced plastic (FRP).

Are GRP composites strong?

GRP composites are strong, light and versatile. It is stronger than many comparable materials used in load-bearing structures as well as being corrosion resistant, lightweight and cost-effective.

Are GRP composites lightweight?

GRP composites are lightweight – often lighter than many other materials with comparable strengths.

Can GRP composites resist corrosion?

GRP composites can resist corrosion, which offers them applications in aggressive environments. A total resistance to chloride ion attack is coupled with resistance to chemicals and severe temperatures, making GRP materials ideal for a wide range of applications across industry.

Can GRP composites be moulded into complex shapes?

GRP can be moulded into a range of different shapes to meet different demands. If a designer can create a mould, then it is possible to manufacture GRP into that shape.

Are there any environmental considerations with GRP composites?

GRP releases minimal harmful products into the air during manufacture, particularly when using pultrusion (see above). The CO2 equivalent of GRP is less than half of that of a concrete bridge and a third of one made from steel. This means GRP has a relatively low carbon footprint. Added to this, it is possible to recycle GRP into high-quality substitute fuels and reclaimed glass fibres. GRP waste can also be used by the cement industry as a fuel and mineral raw material (SiO2).

Plus, the lightweight properties of composites help improve fuel emissions and economies, while the long lifecycle of GRP products means less need for replacements being manufactured.

What is the future outlook for GRP composites?

The future of GRP composites looks promising as the aerospace and automotive sectors in particular drive forward innovation. The estimated GRP composites market is predicted to be worth US$ 18.54 billion in 2023, rising to US$ 41.53 billion by 2033 as demand increases.

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