What is resin?
A resin is a liquid that becomes a solid due to a chemical reaction, usually induced by mixing it with a 'hardener'. Resins are solid or semi-solid organic compounds with high molecular weight and no defined melting point. Resins can be natural or man-made.

What role does resin play in a composite?
Resin has two major functions in a composite. It bonds to reinforcing fibers binding them together, so adhesive strength is very important. Resin orientates reinforcing fibers, holding them relative to each other within a rigid matrix. A resin's mechanical properties affect how well it does this, and how effectively it distributes load to fiber reinforcements throughout a composite panel. The mechanical properties of resin are measured by tensile strength and tensile modulus (stiffness).

What is a 'matrix' ?
'Matrix' is the name given to the homogenous material in which reinforcing fibers are embedded.

   • Click here to see diagram of a matrix

How are thermoplastic and thermosetting resins different?
There are two groups of resins (or plastics), thermoplastic and thermosetting. Thermoplastic resins become soft when heated and can be molded when in a semi-liquid form. They become a solid when cooled, and can be re-heated and re-molded. Thermosetting resins are initially liquids or low melting point solids that are cured to a solid state by a catalyst, heat, or a combination of both. Thermosetting resins cannot be re-heated and melted. Resins used in FRP composites are of the thermosetting type.

What kind of thermosetting resins are there?
There are numerous types of thermosetting resins including unsaturated polyester, epoxy, vinyl ester, phenolic, cyanate ester, silicone, polyurethane, bismaleimide, and polyimide that all have different qualities suited for different applications. For example, phenolic resin can withstand high temperatures and vinyl ester has high chemical resistance.

How are different resins compared?
Resins can be compared in terms of adhesion, mechanical properties, micro-cracking resistance, fatigue resistance, and degradation from water absorption. The two most common resins are polyester and epoxy.

What should I know about polyester resin?
General-purpose polyester resins are the most commonly used because they're relatively inexpensive. They are pale, most often blue or red coloured liquids. There are two types of polyester resin, 'orthophthalic' and 'isophthalic', the latter being preferred due to its superior water resistance. Polyester resins are usually pre-mixed with up to 50% styrene to lower their viscosity, making them easier to use. The strong chemical smell associated with glass fiber products is primarily due to styrene emissions. The polymerization process that converts the liquid resin into a solid is activated by the addition of a catalyst. Water and ester are by-products of this chemical process, which can lead to voids in the resin matrix.

How well do polyester resins rate?
Polyester resins have significantly lower adhesive properties than epoxies and this affects the strength of bonding to reinforcement fibers and the ability to transfer loads throughout a composite. The mechanical properties of resins are measured by tensile strength and tensile modulus (stiffness), which are significantly lower in polyester than epoxy resin. Polyester also exhibits a relatively high rate of shrinkage of up to 8%, which contributes to built-in stresses that can weaken a component and lead to 'print through' of the reinforcing fibers into the surface finish.

What are the potential problems using polyester resin?
Polyester's weaker mechanical properties make it more prone to micro-cracking, which refers to the stress level where resin begins to break away from the fiber reinforcement and transverse micro cracks begin to spread throughout the resin matrix. This happens before catastrophic breakdown when the fiber reinforcements themselves break, however micro-cracking signals the process has started. Fatigue resistance is also directly affected by a resin's mechanical properties and polyester's lower strength and stiffness make it more prone to fatigue. Small amounts of damage accumulated over time from tensile, compressive, shear and flexural loads contribute to fatigue. To varying degrees, all resins are prone to absorbing small amounts of water, however more important is how water affects the resin and the strength of its bond to reinforcing fibers. Compared to epoxy, polyester resin is prone to water degradation due to its different molecular structure, leading to a gradual long-term loss of mechanical properties.

What should I know about epoxy resin?
Epoxy provides superior all-around benefits compared to polyester resin. It is considerably more expensive and is used exclusively in fabrication of high performance composites. Epoxy resin is usually light brown or amber colored, and its low viscosity eliminates the need to mix it with additives to make it easier to handle. Unlike polyester resins, epoxies are cured by the addition of a hardener rather than a catalyst. Epoxy has a different molecular structure than polyester that on curing does not produce by-products such as water and ester. Its molecular structure also results in higher stiffness, toughness, thermal, and water resistance compared to general purpose polyester resin.

How well does epoxy resin rate?
Epoxy exhibits strong adhesive qualities that maximize bonding to reinforcing fibers. The resin's higher tensile strength and stiffness are mechanical properties that carry loads more effectively throughout a composite. Epoxy's superior mechanical properties also reduce micro-cracking and offer superior fatigue resistance. Due to its different molecular structure epoxy exhibits marginal shrinkage of around 2%, is less susceptible to water degradation and features a higher 'Tg' value. Epoxy resin also exhibits lower exotherm temperature as it cures compared to polyester resin.

What is curing?
Curing is the process whereby a resin transforms from a liquid to a solid.

What is 'exothermic heat' ?
When a resin cures from a liquid to a solid, a by-product of the chemical process is heat or 'exotherm'. Exothermic reaction can be dangerous if high temperatures are reached. This can be caused by large quantities of curing resin in a multi-layer laminate or by resin that has been mixed with too much catalyst or hardener. Under controlled conditions exothermic heat speeds up the curing process.

What is 'gel point' ?
During the curing process the 'gel point' is when the viscosity of the resin increases to a point where it can no longer flow as a liquid.

What is the benefit of curing at elevated temperatures?
For both polyester and epoxy resins the application of heat will accelerate curing, and the higher the temperature the faster the cure. There are other important benefits to curing at elevated temperatures as this increases the mechanical properties of a resin.

Why is post-curing important?
Many resins will not achieve their maximum mechanical properties unless given a 'post-cure.' After curing at room temperature this involves increasing the laminate temperature, which in turn increases the amount of molecular cross-linking in the resin. Post-curing will naturally occur due to warm ambient temperatures, however elevated temperatures will achieve ultimate mechanical properties in a shorter period of time. For example, a higher Glass Transition Temperature (Tg) will be achieved with higher post-cure temperatures although this is ultimately limited.

   • Click here to see comparison of mechanical properties of cured and post-cured resins
   • Click here to see comparison of stress/strain of post-cured resins
   • Click here to see comparison of tensile strength of post-cured resins
   • Click here to see comparison of tensile stiffness of post-cured resins

What is a 'Tg' value?
The 'Tg' or 'Glass Transition Temperature' refers to the value after which resin properties begin to change. 'Tg' values can vary depending on the type of resin, if it was mixed correctly, and the degree of cure. At temperatures higher than the 'Tg' value the molecular structure of a thermoset resin begins to loose stiffness, compressive and shear strength, water resistance and color stability. However when cooled below the 'Tg' value the changes are reversible.

Why is the bond between fiber reinforcement and resin important?
Strong adhesive bonding increases the strength of a composite because it provides higher shear between the resin and fiber reinforcement. The composite is stronger because it takes a greater load to separate the reinforcing fibers from the resin.

Why is low shrinkage important?
Shrinkage affects composite panels in several ways. It can generate internal stresses in the laminate that contribute to micro-cracking and 'print through' of reinforcing fibers into the surface finish. 'Print through' occurs over time, so when a composite is new it may not be apparent. However in time, weaker resin systems like polyester can allow the texture of reinforcing fibers to 'print through' to the surface. This textured finish is extremely difficult to eliminate, and in automotive applications repainting a vehicle will rarely solve the problem permanently. Shrinkage can also affect the quality of fit of composite panels. For example, if a steel automotive body panel is used to make a polyester resin mold and a replica body panel is produced from this using polyester resin, the component may not exhibit a high tolerance fit. It's common practice for polyester/glass fiber automobile panels to be copied or 'flopped' numerous times, and with each duplication involving up to 8% shrinkage the dimensions of the original panel can change significantly - taking with it the ability for a close tolerance fit.

What is gel coat?

Gel coat is a resin-based material first applied to a mold and backed by layers of reinforcing fiber and resin. When a composite part is removed from its mold, gel coat provides a high quality cosmetic surface finish that protects the substrates behind it. Gel coat can be clear or colored, and commonly exhibits a high gloss. While gel coats are durable, the higher the gloss the more brittle the gel coat becomes, ultimately making it susceptible to micro-cracking. Gel coat is a weatherproof finish that's not designed to be painted. Automotive composite panels that will be painted should feature an in-mould primer or surfacing film that's specially formulated to be compatible with automotive paint systems..........read more

 

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