Home Workshop Supplies
Sometimes the most difficult aspects of working on fiberglass projects at home is obtaining the proper supplies. Now that we have the internet, this is greatly simplified, and many online stores exist to sell many different types of resin and reinforcement; tools and supplies. One of the problems with this is the cost of shipping the resins as hazardous materials. And actually seeing and handling the reinforcement will help in determining if it is up for your application.
Sometimes it is advantageous to buy supplies from a physical store. This saves time and money from shipping, and allows for better inspection of the purchased products. Local sources are not always easy to identify. Many times automotive bodyshop repair stores carry fiberglass resin. I have found that marine repair supply stores carry a more extensive repair supply, including epoxy, polyester, fiberglass sheeting, and even gelcoat and associated chemical additives.
When purchasing esins and gelcoats always keep in mind the shelflife. My experience has always been that polyester, vinylester, and gelcoat have about a six month shelflife from when it was manufactured. Be cautious at autobody supply houses that sell low volume amounts of resin and may have some old stock. Epoxy generally has a longer shelflife.
A good practice when obtaining new materials is to always do a test run in a small dixie cup, sometimes called a “Gel Test”. Mix the resin and hardener together and stir it up. Monitor the time it takes to start getting thick and lumpy. And time full hardness. As it gets towards full cure, there is potential to get hot enough to start a fire, so take proper precautions. This test will tell whether the resin and hardener are even compatible and what sort of working time to expect. This will make life much easier under actual fabrication conditions.
Alligatoring Causes
One of the challenges of using Gelcoat is the potential for the defect called alligatoring. This is described as a wrinkled surface on the completed part after it is removed from the mold, and results in extensive post-mold repairs to the cosmetic surface of the part.
There are four scenarios that may cause this alligatoring to occur.
Applying the gelcoat film too thin is the primary cause. This thin layer of gelcoat will lose a larger proportion of monomer to evaporation than thicker layers of gelcoat. The crosslinking process will deviate from the manufacturer’s intended formulation. This undercured gelcoat is then attacked by the laminating resin which wrinkles the gelcoat layer. Applying the manufacturer-specified mil thickness of gelcoat will prevent this condition from happening.
Another scenario of alligatoring is caused by laminating on undercured gelcoat that is the correct mil thickness. This undercure may be caused by: insufficient cure time, insufficient cure temperature, initiator problems, and compressed air contamination. Closely following the manufacturer’s specifications with regards to shop conditions, initiators, and proper equipment will minimize these problems.
The spraying process is another cause of alligatoring. Keeping a wet edge is important. This means that the new gelcoat is applied over gelcoat that is still wet, which allows for all of the gelcoat to cure at the same time. Fresh gelcoat sprayed over cured gelcoat will attack itself and can lead to alligatoring problems.
A fourth cause of alligatoring can be the result of exposure to a solvent such as MEKP or acetone before the laminate is applied. The most common cause for this is an equipment leak during laminating operations. Solvent rags placed on the gelcoated surface may also be a culprit.
These above are a few of the causes of gelcoat alligatoring. Once these conditions are under control, few problems will present themselves.
Pultrusion Processing
Pultrusion is a continuous process similar to steel or plastic extrusion for creating products with a constant cross section. Examples of these products are rod stock, structural shapes, beams, channels, pipe, tubing, fishing rods, and golf club shafts. High structural properties are created because of the extremely high fiber loading afforded with this process.
A resin bath impregnates the continuous strand fiberglass roving, mat, cloth, or surfacing veil before it is pulled through a steel die. This die is responsible for the shape, consolidating the reinforcement, and controlling the fiber/resin ratio. This die is heated to cure the resin as it passes through and out to the pulling mechanism which controls the speed of processing.
Advantages of this process include low labor costs due to the ability to automate. It also allows for cross-sectional shapes ranging from simple to very complex. Very high strengths can be achieved due to the high fiber loading.
Composites in the Military
The military has been using composite materials for decades. Mostly applied to the aerospace segment, composites are now finding more uses to the traditional armed forces.
One of the uses for military ground forces is in the segment of armored vehicles. Metal armor competes with composite armor with metal being less expensive but heavier. This weight advantage has required the use of composite armor for the “up armoring” of vehicle systems that were already near their max payload. This minimizes the amount of other weight sacrifices being made in order to add armor.
Weight and performance are becoming more important as military tactics transition towards fast and nimble. This will create many future applications for composite materials as acceptance and testing open new opportunities.
Intro to Filament Winding
Filament winding is an open molding process that is automated by using a rotating mandrel as the mold. This mandrel acts as a male mold which allows for the control surface to be on the inside, and the outside diameter size being dependent upon the laminate schedule.
The reinforcement to resin ratio can be very high with filament winding because of the automated nature of the process. Very high tensile strengths can be achieved depending upon the use of materials and their orientation.
Products that are manufactured include chemical storage tanks, pipes, stacks, pressure vessels, and rocket motors. Lightweight storage tanks have been a real area of growth as of late. The use of these high pressure vessels has been applied to air packs for firefighting as well as Liquid Natural Gas storage devices for alternative fuel vehicles.
The process works by feeding continuous strand roving through a resin bath and winding it onto a rotating mandrel. This feed mechanism traverses the length of the mandrel to create a predetermined geometric pattern. After sufficient layers have been applied, the laminate is allowed to cure on the mandrel before the molded part is stripped away to begin another.
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 cure using lower exotherm temperatures which reduces resin shrinkage. The choice for use with vinyl esters, they also work well on thick laminates to control the mass exotherm. These can be blended with MEKP for specialized purposes.
Benzoyl Peroxide (BPO) is the next most common initiator, and is available in several forms. It is commonly available as a paste and is safer for handling and health hazards.
Room temperature curing resins must have the addition of a promoter to help rapidly decompose the initiator and ensure an appropriate curing time.
Room temperature curing resins must have the addition of a promoter to help rapidly decompose the initiator and ensure an appropriate curing time. These chemicals are metal salts or amine compounds. The most common promoter used with MEKP initiator is cobalt napthenate (CoNap) or cobalt octoate. For BPO initiator, amine promoters are used such as dimethylaniline (DMA), diethyaniline (DEA) and dimethyacetoacetamide (DMAA). These amine promoters may be used in concert with cobalt promoters to produce a rapid cure folloowing gpelation.
Styrene and polyester are mixed at the factory and would polymerize without an initiator (MEKP). This is controlled with the addition of an inhibitor. When initiator is added to the system, it first reacts with the inhibitor free radicals before it moves to crosslinking the styrene and polyester resin. Inhibitors include hydroquinone(HQ), tertiary butyl catechol(TBC) and toluhydroquinone(THQ).
The bottom line is that there are lots of chemicals. They all have a purpose, they all are important, and none of them should be taken lightly.
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 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 can be eliminated with a good print blocking material, good skin coat, and good resin. Paint adhesion can be optimized with the correct gelcoat and paint preparation methods and materials.
Fiberglass composites are well utilized in the RV and specialty vehicle market and will remain there forever. They are the best solution for the many challenges faced by the folks that build these great RVs.
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