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 minimized.
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 resin and methacrylic acid, resulting in a polymer that has characteristics of polyester and epoxy.
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 Application is a tricky part of the process and the manufacturer of the application equipment and the raw material is the best resource for best practices. Experience is the best formula for great results.
Decorative Carbon Fiber
The automotive aftermarket makes use of decorative carbon fiber as an aesthetic means to differentiate a vehicle. Carbon fiber hoods, spoilers, and interior pieces add a cool aspect to many of the “tuner” vehicles that are specialized. A sample of this is would look appear to have a black woven pattern underneath a clear topcoat. True carbon fiber panels can be much lighter and stronger than a comparative sheet metal piece.
These parts and panels can be made using a fiberglass mold that has been made in the desired shape. The mold is waxed and then sprayed with a nice layer of clear gelcoat. It is very important to have a clear layer on top of the carbon fiber to distance it from the surface finish. Then a good polyester or vinylester resin is mixed with clear catalyst before wetting out the carbon fiber and laying it in the mold. Extreme care must be taken to orient the pattern of the carbon fiber so that it has good presentation, as the topside of this first layer will be seen through the clearcoat. The laminate can be backed with additional carbon fiber, fiberglass, or coring to achieve sufficient structure for the part being made.
One of the projects I have worked with in the past was a complex carbon fiber part where orientation was tricky. This required the mold to be made to be transparent. This was done using clear gelcoat, fiberglass reinforcement, and clear catalyst. This allowed for viewing the completed surface through the mold to ensure good cosmetics for the orientation of the weave on the finished side of the part.
One of the recent developments over the last ten years is dyed and woven fiberglass that appears to be real carbon fiber. The big advantage is cost; as it is does not have the same weight savings or strength properties of carbon fiber. Offerings also include red, yellow, and combinations of these colors in the same weave to achieve interesting decorative surfaces.
Curing and Shrinkage
A very important aspect of thermoset resins is their cure cycle. Unsaturated polyester and vinylester, along with epoxy, require time and temperature in order to achieve what we call “Crosslinking.” This is the the “set” part of thermoset, and is the permanent and irreversible chemical bonds in the resin. The amount of time and temperature is dependent upon the formulation of the resin, the ratio of resin-to-hardener, and the presence of additional chemicals used to modify the properties.
Outside of the chemistry, the control of the time and temperature is important to the curing of the resin. If the actual temperature is outside the range of the intended formulated temperature, it will affect the curing reaction. If the part is demolded too early, the resin will continue to cure, but the final shape of the part may not match the mold. The manufacturer of the resin is the very best source for information on the recommended cure time and temperature.
As these resins change from liquid to solid states, there is a certain amount of shrinkage involved. A part made on a female mold will shrink towards the center, and a part made on a male mold will tighten around that mold. This shrinkage factor depends upon the resin chemistry and its additives, but is generally less than 3% by volume. This is why male molds more difficult to demold, and the design of the mold needs to account for part shrinkage and part removal.
Chopper Guns
The method of fiberglass sprayup technology requires the use of equipment specialized for this work. Commonly called a “Chopper Gun,” this equipment meters the ratio of catalyst to resin, and chops the glass roving as the whole mixture is sprayed into the mold.
There are several techniques and styles of this equipment, but the normal components of the equipment are as follows. A center of the process is a resin pump which draws resin from buckets, drums, or tanks and delivers it out of a long hose to the operator’s gunhead. There is also a catalyst slave pump that is driven along with the resin pump and also has a hose out to the gunhead. An single air motor is mechanical force to operate these items. There is also sometimes a solvent tank at the equipment which has a line out to the gunhead for cleaning.
At the gunhead, the resin and catalyst meet one of two ways. There are internal and external mix gunheads, which each have pros and cons. The internal mix units have the solvent flush lines attached. There is also an additional airline or two feeding the gunhead for creating the resin spray fan. The gunhead has a glass chopper motor on the topside which turns continuous roving into chopped roving at a high rate.
So the gunhead has a bunch of hoselines running into it to result in a mixture of glass, resin, and catalyst coming out. Normally this equipment is completely run from a compressed air source. The only electricity required may be an electrical resin heater, though they are not really that common and can cause problems.
The adjustability of the equipment is one of its advantages. The catalyst-to-resin ratio can be set with on a sliding scale to hold the ratio from between .75% to 3%. There is an air manifold with regulators to control resin pump speed and resin pressure, as well as the refining air at the gunhead to develop a fan.
Safety is important with this equipment. There is not a lot of mass to the equipment, but there are a lot of pressurized hoses and chemicals. The movement of materials through hoses can create static electricity, so the equipment must be grounded at all times. It must be inspected for worn or bulging hoses as well as leaking fittings.
The equipment is relatively straight forward and can be inexpensive, but does require a lot of understanding in how it works in order to achieve good results.
Publicly Traded Composite Suppliers
Today I am going to discuss some of the companies involved with supplying the raw materials to the industry. There are literally thousands of companies involved and for this post I am only going to highlight some of the publicly traded ones. These are most of the major ones that I have come up with so far. It is important to note that very few of them are a pure composites supply company. Most of them have businesses that are sometimes complimentary, and sometimes not.
I have included the company name, ticker symbol, website, and primary composites businesses.
Dow Chemical (DOW) www.dow.com Epoxies, Urethanes, Chemicals (Styrene etc.)
Ashland Chemical (ASH) www.ashland.com Unsaturated Resins, gelcoat, distribution
Owens Corning (OC) www.owenscorning.com Glass fiber reinforcements
PPG (PPG) www.ppg.com Glass fiber reinforcements
Total (TOT) www.total.com Unsaturated Resins
Rio Tinto (RTP) www.riotinto.com Endgrain Balsa
Hexcel (HXL) www.hexcel.com Carbon Fiber
Cytec Industries (CYT) www.cytek.com Carbon Fiber, Epoxy Prepregs
Dupont (DD) www.dupont.com Kevlar Reinforcement
Zoltek (ZOLT) www.zoltek.com Carbon Fibers
Sponsor
