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.
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.
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.
Curing Mechanisms
Temperature plays an important role in the curing process of the resins used in composites. Many of the resins are setup for room-temperature curing. This requires that the ambient room temperature is ideally set between 65 and 75 degrees. And that the resin itself is near this temperature. The old rule of thumb is that a drum of resin takes about 24 hours to get to room temp when moved in from shipping or storage. These room temperature cured resins have windows of open working time before the curing cycle begins to happen. Elevated temperatures in the summer can cause havoc, but can be managed with special mixtures and ingredients.
Some resins cure with time and elevated temperatures, which are achieved with the use of ovens. These allow for nearly unlimited open working time before cure. When things are satisfactorily placed, the temperatures are elevated to start the cure process.
UV Light is another curing mechanism that has special applications and takes the temperature consideration away. This has a big use with the infrastructure restoration industries working onsite and underground. It is much more of a specialized niche application.
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