TMM4175 Polymer Composites

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Polymer matrix materials¶

The matrix phase of a composite is the phase having a continuous character, and it is usually the more ductile and compliant phase of a composite material. Most commercially produced composites use a polymer matrix material and there are many different polymers available for different applications and solutions. Common thermosets include Epoxies, Polyesters, Vinyl esters, Phenolics and Polyurethanes while examples of commercially important thermoplastics for composites are Polyamides, Polypropylene, Polyethylene, Polycarbonate and PEEK.

The chief advantages of polymers as matrix are low cost and efficient manufacturing, chemical resistance and low density. On the other hand, low strength, low modulus, and low operating temperatures limit their use

Basic knowledge on polymers as provided in introductory material science and technology courses is presumed. You should be able to define, describe and apply the following terms and concepts:

Polymer, repeat unit, monomer
Addition polymerization, condensation polymerization, degree of polymerization and molecular weight
Unsaturated, isomers, linear and side side branched and thermoplastic polymers
Cross-linked polymers, network polymers, thermosets, curing and cure shrinkage
Copolymers of types: random, alternating, block, graft.
Crystalline polymers and degree of crystallinity
Melt temperature, glass transition temperature and heat deflection temperature
Important/common chemical groups involved in polymer synthesis
Examples of thermoplastics and thermosets
Amount and mass

Thermosets¶

Thermosets are by far the most used polymer matrix in polymer composites. Thermosetting polymers are characterised by having cross links between the polymer chains which prevents them from melting.

Epoxies¶

Epoxy resins consist of monomers with two or more epoxy groups. Through a curing process, the resin becomes cross linked and an amorphous thermoset is formed. The mechanical properties of the cured resin depends on the degree of polymerisation and choice of curing agents and other constituents. Depending on the constituents, a cured epoxy resin may achieve a $T_g$ ranging between 60$^\circ$C and 250$^\circ$C with a tensile strength exceeding 80 MPa. The strength of cured epoxies is among the highest achievable for thermosets. The most common epoxy resin uses the constituent diglycidyl ether of bisphenol A (DGEBA). Epoxies are widely used as the polymer matrix of a polymer composite structure. This is due to their excellent strength, temperature, water and chemical resistance, low cure shrinkage and their adhesive properties. Epoxy is, however, more expensive than other thermoset polymer matrix candidates and should be chosen as a polymer matrix if the need for a high strength matrix justifies the higher price.

Polyesters¶

A polyester is a polymer with ester groups (R-COO-R') in its main chain. In the context of composite structures, polyester usually refers to unsaturated polyester resins (UPEs). Two types of UPE are widely used: Orthophthalic and isophthalic where the orthophthalic is the most costeffective and therefore most used. The isophthalic have better material properties, but is more expensive.

UPE is made up of oligomers containing ester groups and unsaturated carbon groups (-C=C-). The liquid resin is made solid through free radical addition polymerisation, commonly with styrene as a reactive dilutant. The addition of styrene reduces the viscosity of the polyester, making it easy to handle. A drawback of using styrene as a dilutant, however, is that it poses a health risk to humans involved in the production. Cured polyester resins experience shrinking on curing and have relatively low tensile strength, but they are in turn the cheapest thermoset resins available. The main application for UPEs is as the matrix material in polymer composites where high strength fibres carry tensile loads.

Vinyl-esters¶

Vinylester resins can be considered as a hybrid of epoxy and polyester resins. They are manufactured by using styrene as a cross linking agent in an epoxy resin. The presence of styrene enables vinylesters to have the low viscosity and ease of usage of polyesters while maintaining the chemical resistance of epoxy. As with polyester, a drawback of using styrene in the curing process is the health hazard it poses to persons involved. The mechanical properties, such as strength and stiffness, of vinylester usually lie between polyester and epoxy. One interesting property of vinylester is that the pressure sensitivity of the yield stress is low, meaning that the tensile and compressive strengths are similar. The cost of vinylester resins is higher than polyester and lower than epoxy. One application for vinylesters is as a protective coating on polyester composites submerged in water. The vinylester "skin", with its improved water resistance, protects the polyester matrix from water absorption.

Comparing thermosets¶

It is observed that epoxy is the superior resin when mechanical properties such as strength and ductility is considered, followed by vinylester and polyester. The cost of the resins is proportional to their mechanical strength, with epoxy being most expensive and polyester being the least expensive. The costs lie in the range of 30-1500 NOK/kg for epoxy, 20-40 NOK/kg for vinylester and 10-20 NOK/kg for polyester resins.

The manufacturing process differs between the resins. In the curing process, epoxies are cured by a hardener while polyester resins are cured by a catalyst. The curing of a resin is an exothermic process and polyester resins generally produce more heat during curing than epoxies, which results in a shorter curing time.

Polyesters and vinylesters experience shrinkage of up to 8% during curing which must be accounted for when manufacturing a component. Due to the different curing mechanism in epoxy resins, they only experience shrinkage of up to 2% during curing. The low cure shrinkage of epoxy is a part of the reason of why it renders stronger than the ester resins. This is because shrinkage introduces residual stresses in the resins, which will influence the strength negatively.

The ester groups present in polyester and vinylester chains are susceptible to hydrolysis which make the resins prone to water ingress. This is not the case for epoxy, and as such, epoxy is more water resistant than the ester resins. Vinylester, however, contains fewer ester groups than polyester and has greater water resistance than polyester.

Thermoplastics¶

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