This is an interesting not-too-technical article I glommed onto quite some time ago and just re-read. It contains some useful points relating to polyester resin, catalyst use and curing. This is a trial post, I don’t know if it will all fit. If not, there will be a part 2. If anyone is concerned about copyright, I figure we’re all artists, and replicating material for private, non-commercial or artistic use is not prohibited. The article source is also mentioned. I make no claims as to the content or it’s effects, YMMV, and so on.
Not his comments about improper catalyst ratios… if it ain’t right the first time, it’ll never be. This has come up from time to time.
**Deliquent Molecules And Free Radicals **by Bob Lacovara, CFA Technical Director
Curing is a rather important part of the composites fabrication process. When we look at all the possible problems that can arise in composites production, the focus tends to be drawn to resin and gel coat curing. When difficulties occur, troubleshooting may directly or indirectly involve a curing problem as the root cause.
There is no doubt the thermoset crosslinking reaction is the single most important aspect of producing composites products. The act of converting a liquid polyester resin into a solid is called curing. The mechanism by which curing occurs is termed crosslinking or addition polymerization. Having a grasp of how the reaction works makes a significant difference in how things are done in the shop and the quality of your products.
Polyester resins consist of a polymer (a long chain molecule) and a crosslinking agent, usually styrene. Styrene not only works in the crosslinking role, but lowers the viscosity of the polyester polymer to provide a workable product. Picture a series of tennis balls connected in a string. These chain-like molecules are floating around and possibly mechanically intertwined but other wise unconnected; this is resin in the liquid state.
When we are ready to cure the resin we add a catalyst - or at least it is sometimes erroneously called a catalyst. Actually the MEKP, BPO or the other “curing agents” we use are initiators. Technically a catalyst starts a reaction but is not consumed in the process. An initiator enters into the reaction and is consumed, therefore MEKP and BPO are actually initiators and not catalysts as commonly called.
The initiator, usually an organic peroxide, produces a set of free radical molecules. These free radicals open the bonding sites on the polyester chain molecule. In other words, imagine the initiator causes a tennis ball in the molecular chain to be covered with velcro. The bonding sites on the styrene molecules are also opened, so we also have a styrene “tennis ball” covered with velcro. The “bridging step” in the reaction takes place when the velcro tennis balls get together to form a bond. The bond between the polyester molecule and the styrene molecule actually produces a new free radical which is available to open a new bonding site and keep the reaction going.
The result of the molecular bridging or bonding is a network of interconnected polyester polymers with the styrene molecules serving as the links between the chains. Resin converting from a liquid to a solid is the result of this crosslinking or addition polymerization.
The crosslinking reaction is at maximum activity from the time the initiator is introduced to the time of peak exotherm. While the molecular matrix proceeds to interconnect, the crosslinking activity slows down as the conversion proceeds to a solid. In the beginning of the reaction there are lots of molecules available for bonding and they are very mobile in the liquid state. However as the reaction progresses, and the crosslinking density increases, there are less available bonding sites and the molecules are less mobile.
What does this means in the practical terms of the real world? A vast majority of crosslinking takes place in the early stages of the curing process from catalyzation (wrong word) to peak exotherm. As Barcol hardness builds, a geometrically smaller number of reactions take place. Molecules just are not migrating through this solid material to find mates. A small amount of “bonding of proximity” may take place among molecules close together, but this is a very small percentage of the overall crosslinking process.
It is important to get a good crosslinking reaction up front during the early stages of the curing process. If you get a poor reaction in the initial stages of cure, it is ineffective to try to remedy the problem at a later time.
What causes a poor initial crosslinking reaction? Typically the common items associated with a slow or poor cure. Low catalyst (Initiator - I know, it’s just habit) levels or the wrong catalyst are always suspect. Low shop temperature, anything below 60 degrees F is a definite problem, because the crosslinking reaction is very dependent on temperature. A thin laminate or gel coat may contribute to a sluggish cure. And other common problems such as poor catalyst mixing or water in the air lines are contributors to improper curing.
In many cases, fabricators attempt to compensate for a poor “up front” cure with dubious methods. One is the mistaken idea that given enough time laminates will fully cure. This has bred the fiberglass folklore that laminates never stop curing, or it takes two months, two years or whatever to get a complete cure. Wrong. If curing is defined as crosslinking, very little takes place in a solid resin matrix. What many people think is a continuing cure, over a period of time, is usually unreacted styrene making its way out of the laminate. Eliminating these “loose” styrene molecules will cause some shrinkage in the laminate and sometimes can be identified by styrene odor in a confined area, but additional “curing” is not taking place.
The same holds true for post curing at elevated temperatures. If two unreacted bonding sites are in intimate contact a crosslink may occur if the temperature is elevated. However, for the most part, “post curing” is not further curing, but eliminating unreacted styrene from the resin matrix.
So what does all this mean? The significance of this discussion is that it is important to get the proper cure up front, and that you really can’t compensate for a poor cure with either “time” or “post cure”. Many composite quality problems are a direct result of a faulty cure and can be solved by implementing solid production procedures. For example, use the right catalyst in the right ratio. Keep shop temperatures within the operating range of the resins being processed. Pay attention to details like catalyst mixture, compressed air quality and application techniques.
Chemistry is going to happen whether you understand it or not. Understanding the concept of getting a high crosslinking density in the initial stages of curing will contribute to quality laminates. Make free radicals work for you. The molecule you crosslink today will be one less molecular delinquent to deal with tomorrow.