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Resins are of utmost importance within the composites markets as they bind the fibers together and help create the material’s strength and stiffness characteristics. There are two types of resin systems: thermoset and thermoplastic. In a thermoset, the resin molecules are locked together in a irreversible way after a thermal cure; it is a one-way cure that cannot be undone. In a thermoplastic resin the chemical link which is bonding the molecules together can be broken again and again by increasing the temperature steadily which causes the matrix to go from solid to liquid. In the same way, when cooled down the thermoplastic matrix solidifies.
We will now concentrate on the Thermoset resins that are playing a key role in the composite industry: Polyester, Vinyl Ester, Epoxy, Phenolic Resins and Cyanate Ester and Bismaleide.
Polyester resins are one of the most common resin used in the composite industry. The success of Polyester is mainly based on the low cost that makes this it more attractive than some higher performing resins like epoxies (below).
Polyester resins require a catalyzer, which functions as an initiator of the chemical reaction, to solidify. The quantity of catalyzer has an effect on the speed of the reaction and is responsible for the solidification. During the cure the polyester resins shrink significantly.
Polyester resins have numerous advantages. The most well known benefit of polyester is the price; the resin is relatively cheap in comparison to other types of resin. Furthermore, polyester has the ability to accept a broad variety of fillers which makes them applicable to a wide range of projects. Another benefit of polyester is that once demolded, the part can be sanded and finished resulting in an optically clear surface. Finally, Polyester is not impacted by UV radiation like other resins which is essentials for applications exposed to the sun, for instance, surfboards.
The most striking disadvantage are the mechanical properties which are not as good as, for example, epoxies. Furthermore, polyester resin has high styrene emission in open mould which is perceived as a disadvantage and that requires special precautions when processing. The high curing shrinkage is another major drawback. Moreover, the limited range of working times minimizes the usability for the polyester resin.
Polyester will be still used in the future for large components where mechanical properties and weight are not a big issue.
Summarizing: polyester resin is one of the most common resins used, because of its
- Low costs aspect
- UV resistance
Although it is more expensive than other resin systems, the advantages outrun the disadvantages. Epoxy has much better physical and adhesion properties compared to polyester and/ or vinyl ester. Based on its properties, this resin system is used in various industries including the aerospace, automotive and marine industries.
The mechanical properties and its resistance or environmental degradation which makes the resin system especially attractive to the aircraft industry. Furthermore, epoxy is water resistance and therefore used heavily within the marine industry. The adhesive properties and the low shrinkage are further benefits of epoxies. Finally, epoxies cure easily and quickly making them beneficial for numerous projects.
However, the epoxies systems also has some drawback that limit it to a certain degree. First of all the material itself is very expensive and the cost factor can determine the decision significantly. Moreover, the workers have to be skilled and know the exact mixing procedure and amounts to perform a satisfactory end result; the mixing is critical. Epoxies need a hardener that has to be mixed with the resin in order to start the curing process. The mixture of both components to get the required end result is critical; the correct ratio is needed to ensure optimal cross linking of the molecules during the cure. Skilled and experienced workers are more expensive and thus, influence the overall cost tremendously.
Epoxies are the main choice for a lot of applications within the motorsport, marine and aerospace industry. As understanding of the manufacturing process becomes more widespread they should see a cost reduction.
Summarizing: epoxy resin is one of the most common resins used, because of its
- Positive mechanical properties
- Resistance to environmental degradations
- Resistance to water
- Adhesive properties
- Low shrinkage
- Easy and quick curing process
Vinyl Ester resins have similar properties to polyester and epoxies, but are slightly more preferred than polyester. The reason for its properties is its epoxy backbone with vinyl groups at the end of each molecular chain which are connected by ester linkages. Vinyl Ester is sometimes referred to as ‘epoxy vinyl esters’. On a rating scale Vinyl Ester would be ranked between polyester and epoxy resins.
Although similar to polyester, vinyl ester or epoxy vinyl esters have some tremendous benefits preferred for specific projects and industries. The resistance to water and other chemical is advantageous for certain projects and area of usage, for instance, pipeline or chemical storage tanks. Moreover, the resin system has better mechanical properties than polyester as well as strength and fiber adhesion. Another advantage is the vinyl ester’s lower shrinkage compared to polyester; however, the shrinkage is still higher than epoxy.
Based on the fact that vinyl ester and its properties are in-between polyester and epoxy, the disadvantages are benefits compared to polyester but drawback compared to epoxy. Generally vinyl esters require a post cure for high performance properties. Furthermore, they cost more (than polyester), have a higher styrene content and a higher cure shrinkage (than polyester).
Vinyl Esters will remain a good solution if cost is an issue as they can be used as a skin coat for polyesters if epoxies are too costly. While epoxies remain highly priced, Vinyl Esters continue to be a useful alternative.
Summarizing: vinyl ester has slightly better properties than polyester; however, is not comparable to epoxies.
- better mechanical properties [than polyester]
- Resistance to water
- Resistance to other chemicals
- Fiber adhesion
- Lower shrinkage [than polyester]
Phenolic Resins are reacted phenols and aldehydes and are especially useful for high temperature projects. Caterham Composites will commonly use the resin in aircraft interior projects.
One major benefit from Phenolic Resins is its fire resistance; phenolic systems offer good flame resistance and produces low toxic fumes when burned. Furthermore, its environmental and chemical properties are the main reason for choosing phenolic resins over others. Phenolic resins have excellent static-dissipative properties which is the major advantage. The resins’ ablative properties are also considered a benefit.
Phenolic Resin also has disadvantages which are mainly characterized by reasonable to low mechanical properties. The reaction of the phenol and aldehyde creates the danger of free formaldehyde (during production).
Phenolic resins are approximately half the price of vinyl ester making them an ongoing attractive alternative. Their excellent fire, smoke, toxicity properties give them wide field of applications in the transportation sector.
Phenolic resins are used primarily in the transport industry because they have:
- High flame resistance
- Low toxin levels
- Half the price of vinyl ester
Cyanate Ester is a resin system belonging to the family of high-temperature thermosetting resins. They can achieve high Glas-Transition temperatures up to 250°C. It is used due to excellent mechanical performance and toughness and as such is often used within the defense market.
The major advantage of cyanate ester is its good balance of high mechanical performance and toughness. Furthermore, the resin system has a high temperature service capabilities after post-curing and very good performance in hot/wet conditions. The low moisture absorption after curing is another benefit of cyanate ester. Further advantages are characterized by minimal microcracking and good dielectric properties.
However, cyanate ester also has some disadvantages. The most critical aspect is the poor hydrolytic decomposition resistance.
In summary, Cyanate Esters have:
- Good balance of performance and toughness
- Good performance in hot/wet conditions
- Minimal microcracking
- Poor hydrolytic decomposition resistance
Bismaleimide resins or BMI are mainly used in high performance structural composites requiring increased toughness as well as high temperature resistance. Caterham Composites commonly uses the resin system for F1 applications such as rear suspension which is exposed to the exhaust.
The major advantage of using bismaleimide is the low viscosity that is an important parameter for infusion processes as well as its high temperature resistance; applications with temperatures of 300°C can be reached with this resin system. Moreover, Bismaleimide are water resistant.
Nonetheless, there are some disadvantages with Bismaleimide, which are mainly characterized by its high cost and the fact that can be brittle.
BMI resin systems are mainly used in motorsport, aerospace and numerous markets where heat sources are in close proximity to composite structures.
Bismaleimide is used because of its low viscosity and its high temperature as well as water resistance and is mainly used in motorsport, aerospace and defense.