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Composite Materials Marine Composites Resins Polyester The percent of manufacturers using various resin systems is represented in Figure 2-6. Polyester resins are the simplest, most economical resin systems that are easiest to use and show good chemical resistance. Almost one half million tons of this material is used annually in the United States. Unsaturated polyesters consist of unsaturated material, such as maleic anhydride or fumaric acid, that is dissolved in a reactive monomer, such as styrene. Polyester resins have long been considered the least toxic thermoset to personnel, although recent scrutiny of styrene emissions in the workplace has led to the development of alternate formulations (see Chapter Five). Most polyesters are air inhibited and will not cure when exposed to air. Typically, paraffin is added to the resin formulation, which has the effect of sealing the surface during the cure process. However, the wax film on the surface presents a problem for secondary bonding or finishing and must be physically removed. Non-air inhibited resins do not present this problem and are therefore, more widely accepted in the marine industry. The two basic polyester resins used in the marine industry are orthophthalic and isophthalic. The ortho resins were the original group of polyesters developed and are still in widespread use. They have somewhat limited thermal stability, chemical resistance, and processability characteristics. The iso resins generally have better mechanical properties and show better chemical resistance. Their increased resistance to water permeation has prompted many builders to use this resin as a gel coat or barrier coat in marine laminates. The rigidity of polyester resins can be lessened by increasing the ratio of saturated to unsaturated acids. Flexible resins may be advantageous for increased impact resistance, however, this comes at the expense of overall hull girder stiffness. Nonstructural laminate plies, such as gel coats and barrier veils, are sometimes formulated with more flexible resins to resist local cracking. On the other end of the spectrum are the low-profile resins that are designed to minimize reinforcement print-through. Typically, ultimate elongation values are reduced for these types of resins, which are represented by DCPD in Table 2-7. Curing of polyester without the addition of heat is accomplished by adding accelerator along with the catalyst. Gel times can be carefully controlled by modifying formulations to match ambient temperature conditions and laminate thickness. The following combinations of curing additives are most common for use with polyesters: Table 2-6 Polyester Resin Catalyst and Accelerator Combinations [Scott, Fiberglass Boat Construction ] Other resin additives can modify the viscosity of the resin if vertical or overhead surfaces are being laminated. This effect is achieved through the addition of silicon dioxide, in which case the resin is called thixotropic. Various other fillers are used to reduce resin shrinkage upon cure, a useful feature for gel coats. Catalyst Accelerator Methyl Ethyl Keytone Peroxide (MEKP) Cobalt Napthanate Cuemene Hydroperoxide Manganese Napthanate 70PDF Image | Marine Componsites
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