Advantages of Using FRP
R Srinivasan
C V Manian, PhD
Samir Degan, PhD
Technical Committee
NACE International Gateway India Section
Corrosion resistance is a big challenge for the chemical industry. Fibreglass Reinforced Plastics (FRP) materials are being considered as a high strength and corrosion resistant for components and materials. The article highlights numerous applications of this innovative product across various industries and explains about FRP's benefits in detail.

Fibreglass Reinforced Plastics are versatile materials used in varied applications like armoured vehicles, missiles, boats, cable-trays, water and sewage transportation, wind energy, chemical storage vessels and many other applications. The prime consideration for all these applications is the corrosion resistance properties, light weight and high strength of the material.

What is FRP or GRP?
The term FRP or GRP refers to plastic or polymers that have been reinforced with glass fibres. Although most of the thermoplastics are also reinforced with glass fibres (FRTP), this article deals with thermoset resins like unsaturated polyester resins, epoxy, furan and phenolics.

Reinforcement Materials: Various types of reinforcement materials like asbestos, fibres made out of polyester, carbon and glass are in use, but glass fibre and its hybrid form constitute 90 per cent of the composite products. Glass fibre for reinforcements are commonly available in E, ECR and C glass, where E stands for electrical, ECR is for Electrical and Chemical and C Glass type is chemical resistant. From the authors’ experience, the details of types of glass are listed in Table 1 below.

Thermoset Resins
Almost 80-90 percent of FRP components are made from unsaturated polyester resins, with differing resin chemistry based on orthophthalic, isophthalic, vinylester and bisphenol compounds to meet end user applications followed by epoxy resins which are primarily used for pipes and vessels.

Strength Characteristics of FRP
The maximum strength of glass fibre reinforcement is achieved when glass fibre is under tension. There are three basic rules to consider the structural strength properties of reinforced thermoset plastic materials are :
  • The strength is directly proportional to the glass content. The higher the glass content the higher the strength.
  • The longer or more continuous the glass filaments, the higher are the strength properties.
  • The physical properties, more specifically the tensile strength are influenced by the direction or orientation of the glass fibre.
From the authors' experience, an indicative ratio of glass to resin for each of the processes is given in Table 2.

Corrosion Resistance
Corrosion resistance for FRP equipment is a function of both the resin content and the specific resin used in the laminate. Thus when building the laminate, the surface nearest the corrosive medium is made by a layer that is 90 percent resin and 10 per cent glass (the surface-veil layer). The surface-veil layer is invariably made of C glass, which is a chemical resistant glass.

The prime consideration while dealing with corrosion resistant properties for FRP components is the appropriate selection of the resin system. A qualified fabricator or resin manufacturer will provide this information to the user. Usually test coupons are also made to justify these recommendations.

The following facts should be of use to the end user for what to expect from FRP with respect to corrosion properties when compared to steel. Neither the steel manufacturers nor fabricators provide any type of corrosion warranty. The reasons are three fold:
  • Not all of the materials in a gas or liquid stream are known beforehand. If trace solvents or chlorides are present, but have not previously been identified, this could play havoc with an otherwise satisfactory installation.
  • A process change or new material or just temperature increase will change the suitability of a given resin for specific application.
  • Selection of corrosion resistant materials is proprietary and still somewhat of an art.
Weight Advantage
The distinct advantage of FRP is its low weight to strength ratio compared to steel (1:7) and aluminium (1:5). Light weight properties are important when considering the cost and ease of installation, especially for pipes, tanks, scrubbers, ducts, etc.

Design Considerations
  • The user must define the process parameters that the FRP equipment will be handling, ie, liquid, gas, pressure, temperature, solids if any etc.
  • The user must work closely with the equipment supplier to determine the resin system that would be suitable. Any qualified fabricator/resin manufacturer normally would supply this data.
  • Based on the data, the basic objective needs to be prioritised; strength , corrosion resistance, surface finish, etc.
  • The appropriate process for fabrication is then selected. eg, (i) sewage handling FRP pipes operate in a highly corrosive environment. Hence, the inside portion of the pipe has to be made with a veil mat and a resin that is corrosion resistant, else the pipe will fail; (ii) If large quantities of electrical switch boxes, or automobile parts are required with a good finish and fire retardant properties, appropriate manufacturing process needs to be chosen.
  • Appropriate codes should be adopted for fabrication, installation and testing. Various ASTM, BS, DIN, IS standards are available in the literature and these should be consulted. (See Typical FRP/GRP service conditions in table 3)
Precautions
  • Designs should be based on realistic known properties of the resin and not based on the properties of steel and a quote solicited from a fabricator from such drawings.
  • When data available is insufficient from the resin manufacturer or fabricators about the desired parameters of operation, then the fabricator should be requested to provide test coupons for evaluation for the desired parameters.
  • Selection of a good FRP fabricator is essential for success; experience, quality and design back up must be reviewed.
  • Once the designs are accepted, physical checks and dimensional tolerances of the component is very important.
  • Where pressure and vacuum are a process must, the same should be tested at the fabricators end prior to installation.
Conclusions
FRP materials provide the advantage of substantial weight reduction together with excellent corrosion resistance in the fabrication of specialised equipment. In order to get the maximum benefit out of FRP equipment, it is essential that the equipment be designed, fabricated, installed and operated as per accepted operating procedures to provide satisfactory service life. It is advised that in critical equipment, test coupon evaluations are carried out to ensure that the desired end objectives can be achieved without compromising quality.