Author: AKeson

Chemistry of Cure

Lets look a little bit closer at the chemistry of curing a polyester/vinylester resin. There is lots of chemistry involved, but it is broken down so it is not too complicated. The initiator is the correct technical term for what some call the catalyst or hardener. This initiator is the additive that begins or speeds up the chemical reaction and becomes part of the crosslinked polymer. Free radical polymerization is the process that occurs during the curing cycle of a polyester resin. The initiator (catalyst) decomposes into free radical molecules, which work to crosslink the polyester and styrene molecules in the resin. The rate of cure can be increased with the addition of more initiator. Selecting the appropriate initiator is important to the control of the chemical reaction. Styrene-based resins use several types of initiators. These include: ketone peroxides, cumine hydroperoxides, acetylacetone peroxides, and benzoyl peroxides. These can also be blended and the initiator package will be recommended by the resin supplier. Methyl Ethyl Ketone Peroxide(MEKP) is the most common and widely used initiator, and is most cost effective and easy to use. It comes in clear or red tinted and can be used as a fine-tuning for resin cure time with the adjustment of its percentage from 1.25% to 3.0%. Cumene Hydroperoxides (CHP) achieve All new Live Dealer games for Blackjack, Baccarat and Roulette have gone down a...

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RV and Specialty Vehicle Market

One of the industries that makes heavy use of fiberglass parts is in the RV and specialty vehicle market. Mostly centered around Elkhart, Indiana, the manufacturers use Fiberglass in many form and function areas of their vehicles and trailers. These RVs, trailers, and specialty vehicles range from small, single axle campers to trailers pulled by semis, and from vans to tandem axle viagra for sale buses. Specialty vehicles include ambulances, firetrucks, handicapped vans, and commuter vans. The advantages of fiberglass composites mesh very well with their use in various applications in this industry. The strength-to-weight ratio is very important, along with the good adhesion of automotive paint. The low tooling cost of fiberglass relative to sheetmetal is a huge advantage, allowing for inexpensive low-volume production. The long service life and resistance to corrosion is another advantage over competing materials. There are a few disadvantages for composites. Cracking can develop over time with improperly supported structures. Depending on processing, the smoothness of the surface (surface profile) can be subpar to that of sheetmetal, and is subject to worsen over the first year of its life. Paint adhesion can be a problem for all materials, and fiberglass composites have their own unique issues. With proper processing, all of the disadvantages can be overcome. A properly designed and supported structure with good workmanship will never crack and will last forever. Surface profiling...

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Styrene Monomer

Polyester and Vinylester resins and gelcoats contain a significant amount of styrene; anywhere from 25% to 50% by weight. This chemical is a monomer, and serves several purposes in the resin system. Styrene monomer is a reactive diluent for the resin system. The diluent part relates that it works to control viscosity of the resin (more styrene means lower viscosity). The reactive part relates that it is part of the crosslinking chemical reaction. Methyl Methacrylate (MMA) is also a commonly used monomer. During the crosslinking reaction, most of the styrene monomer is captured and becomes part of the solid form of the resin. Open molding processes allow styrene to evaporate before final cure, which is the characteristic smell of polyester and vinylester resins. Worker exposure and environmental impact are negative side effects of the open molding processes. Federal and state laws require special air quality permits for industrial styrene emissions. Worker exposure to styrene must be monitored and...

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The Different Polyester Resins

Polyester Resins can be further defined by their chemistry. These categories are described as Orthophthalic (Ortho), Isophthalic (Iso), Dicyclopentadiene (DCPD). Vinylesters have a unique chemistry and that shares many working properties with the polyesters. Orthophthalic (Ortho) resins are based upon orthophthalic acid and are a good basic, general-purpose, inexpensive resin. They have styrene content between 35% and 45%, and are used in applications that do not require elevated service temperatures, high corrosion resistance, or high mechanical properties. Isophthalic (Iso) resins are a step above Ortho resins, and are better suited for corrosion environments, elevated service temperatures, and have greater mechanical properties. Iso¬† resins have between 42% and 50% styrene because the higher molecular weight more solvent is required to create a workable viscosity. Iso’s better properties do bring higher cost as compared to Ortho Resins. Dicyclopentadiene (DCPD) resins are used for applications requiring high surface finish. This is due to their low volumetric shrink rate. Physical properties are similar to the Ortho resins, but toughness is sacrificed along with the ability to create strong secondary bonds. DCPD resins are on the low end of styrene content, ranging from 35% to 38%. DCPD resins are commonly blended with other resins to minimize the negative aspects and increase positive aspects of these resins. Vinylesters are used in applications requiring superior corrosion resistance or toughness properties. Vinyesters are a formulation of epoxy...

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Gelcoat Application

Making a traditional fiberglass part is accomplished by working from the outside surface into the part. A clean and waxed mold is placed in a clean room where contamination will not interfere. Then gelcoat is sprayed on as the first layer, and followed by the rest of the laminate. The gelcoat application process is much more art than science. The objective of gelcoat is to create a uniform thickness across the part, and have it be 18 to 25 mils thick when wet. As it cures, this thickness is reduced when some of the chemicals evaporate off. Spraying gelcoat onto a mold returns the best quality finish, but it may also be brushed in areas that are difficult to spray. Achieving a uniform thickness is difficult at intersecting corners, deep narrow areas, and difficult-to-reach sections of the mold. The gelcoat is typically applied in three passes and allowed to “gas off” in between coats. This allows for some of the chemical evaporation to start, and can lead to problems if not done correctly. These passes are also usually bi-directional, where the first and third pass are in one direction and the second is in another, again to help achieve uniformity. Areas of excessive gelcoat thickness can lead to cracking and surface finish problems. Areas of insufficient gelcoat thickness can “alligator” which is a surface flaw requiring extensive repair. Gelcoat...

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