P09221: Innovative Composite Parts for a Formula SAE Racecar Vehicle

Resin Selection

Resin Selection

Resin Comparison Table

Selection Criteria
Criteria Explanation
Adhesive Properties Governed by resin's chemistry. Determines the compatibility between a resin and fiber or between a resin and a fiber's surface treatment. Poor adhesion leads to poor fatigue properties and premature failure.
Ductility/Toughness/Elasticity Determines ratio between first microcracks in matrix and ultimate failure of laminate. (I.e. Polyester/glass woven roving begins cracking at .2% elongation, but fails at 2%. This means the designer can only use 10% of the ultimate strength of the laminate.) Resin microcracks lead to shorter lifetime by accelerating delamination and exposing fabric to the environment.
Strength N/A
Shrinkage While Curing Affects dimensional stability of a component. Also dictates level of residual stress left in a part due to manufacturing (higher % reduction in volume results in higher internal compressive stresses).
Water Degredation Some matrices experience dramatic changes in material properties as a result of hydrolysis. I.e. Polyester when soaked in water for 1 year exhibited only 65% of its original strength.
Chemical Resistance Matrix should withstand exposure to harsh chemicals like gasoline, lubricants, and various cleaning solutions while retaining almost all of its original properties.
Viscosity Affects how easy it is to "wet out" a material. Lower viscosity resins tend to be easier to work with.
Cost N/A
Discussion of Researched Resin Types
Resin Category Assessment Impacts if used on Racecar Idealized Molecule
Polyester Polyesters are low viscosity resins based on unsaturated polyesters dissolved in a reactive monomer, generally styrene. They generally cure in the presence of heat and a catalyst through a cross linking process. They typically have "tailorable" properties (i.e. excellent UV resistance when used with a styrene-MMA blended monomer, effective flame retardancy when incorporating halogens, etc...). As a general rule, they are lacking in dimensional stability while curing (shrinkage of up to 8%) and have low mechanical properties (strength of adhesion, bulk material strength, elastic modulus, etc...). In addition, polyesters exhibit decreased strength in the presence of water due to the high tenancy of of ester groups to be hydrolyzed. Nonetheless, polyester is a very popular resin due to its low cost, ease of manufacturing (low viscosity makes lay-up easier), and ability to have custom tailored properties. Positive impacts on racecar would be lower cost and easier manufacturing procedures. Almost all other facets of performance would be less than desirable. Poor adhesive properties and less than ideal behavior in wet environments would result in decreased part life and possible unsafe behavior in the very likely event of a high-speed cone impact. Idealized Polyester Molecule (Isophthalic)
Vinylester Vinyl ester is similar chemically to polyester, but has reactive sites primarily at the ends of molecular chains (like epoxy). This difference increases the toughness/resiliency of vinylester as the whole length of the molecular chain is used to disperse shock loadings. Also, vinyl has fewer ester groups, making it less susceptible to degredation by water than polyester. Unfortunately, vinylesters usually require elevated temperature post-cure to obtain full mechanical properties. Like polyester, dimensional instability while curing is still an issue (shrinkage of up to 8%). Regardless, vinylester is an excellent intermediate step in between polyester and epoxy in terms of mechanical properties, ease of application, and cost. Vinylester would yield slightly reduced performance relative to epoxy in fatigue, damage tolerance, and stability in varying environmental conditions, but would result in lower overall system cost. Idealized Vinylester Molecule
Epoxy Epoxy has very good adhesive properties, strength, toughness, and resistance to environmental degredation. Ring groups at the centers of molecules enable epoxy to endure both mechanical and thermal stresses better than resins that use linear groups. Toughness is increased by locating reactive sites at the end of molecular chains, like vinylester, but Ethylene Oxide groups are used instead of esters. This also contributes greatly to its resistance to water. Epoxy also exhibits low shrinkage during curing (1.2-4% by volume). However, since curing of epoxy is an additive process (one in which hardener is attached, or "added" to the ends of epoxy molecules), it requires that extra attention be paid to the mixture ratio. Any "unused" amine or epoxy molecules will never cure. This is in contrast to ester based resins which undergo a "catalyzed" cross-linking in order to cure. Finally, epoxy's relatively high viscosity also increases the difficulty in manufacturing. Racecar components would see increased fatigue life, toughness, strength, stiffness, and resistance to creep if epoxy were used as a matrix. In addition, components would be rugged enough for use in the presence of water, gasoline, oil, etc.. Primary downsides would be increased manufacturing effort and cost. Idealized Epoxy Molecule (Bisphenol A) Idealized Epoxy Molecule (1,2 Epoxy; Ethylene Oxide))

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