What Resin Am I Using? Let the Truth Be Known

When I sat down to write this article on laminating using epoxy resins, I began reflecting on how many phones calls I field each week from customers who think they are using an epoxy when, in fact, they aren't. Many technicians and clinicians in our industry do not understand what type of lamination resin they are actually working with, so I decided to clear up some common misconceptions about the different types of resins, the chemicals they are made of, and why epoxy might just be a good choice for your lamination work. Definitions What is an epoxy? An epoxy is an organic molecule containing oxirane groups. An oxirane group is an oxygen atom bonded to two adjacent carbon atoms that, in turn, are bonded to each other in roughly a triangular form. This oxygen is highly reactive to certain other organic groups. Oxirane groups in epoxies are commonly called glycidol ethers since they attach to the rest of the organic molecule through an ether linkage. An epoxy resin is a polymer formed by reacting to an organic molecule containing two or more oxirane groups with another organic molecule containing two or more sites that are reactive to oxirane groups. Most commonly, the reactive molecule contains amine groups. What is bisphenyl A? Bisphenyl A is made by reacting phenol and acetone under the right conditions to form a di or bis (meaning two) phenol. The "A" stands for acetone. Another term you may have come across in relation to bisphenyl A is diglycidol ether of bisphenyl A (DGEBPA). This is the basic, common (but by no means exclusive) epoxy monomer used as a reactant to make epoxy resins. [caption id="" align="alignnone" width="450"] Scott Wimberley laminates with epoxy resin. Photographs courtesy of Greg Mattson.[/caption] Now that I've established what epoxy is, I wanted to define some other resin terms for comparison. Polyester resin is a resinous substance made by reacting anhydride mixtures (usually phthalic and maleic) with molecules containing two or more alcohol (OH) groups. Polyester resins, which are most commonly used for boatbuilding, contain about 60 percent polyester and 40 percent styrene (vinyl benzene). Organometalic promoters are added to the styrene/polyester blend. The end user then adds organic peroxide such as methyl ethyl ketone peroxide (MEKP), and the vinyl groups on the styrene polymerize with similar sites on the maleic portion of the polyester to cure the material. A vinyl ester resin is entirely analogous to a polyester resin except that acrylic and methacrylic acids are chemically reacted with the DGEBPA oxirane groups. The resultant product is then cut with styrene and finished exactly like a polyester resin. One particularly confusing term seems to be the nomenclature of "acrylic modified epoxy." This term is used incorrectly to describe a vinyl ester resin. Any resemblance to an epoxy is lost once the acrylic acid reacts with DGEBPA oxirane groups. The term "acrylic modified epoxy" implies that the vinyl ester resin is really an epoxy resin that has somehow been changed or modified with an acrylic. The implication is that this change somehow makes the epoxy resin better than it would be otherwise. Finally, I wanted to clarify some possible confusion regarding ester resins that are well known and used often in the O&P industry. The AME product name commonly used was incorrectly derived from "acrylic modified epoxy"; however, the resins are actually vinyl ester/ polyester blends, not epoxy. Cautions When Using Ester Resins Before I proceed about specific cautions of using ester resins, I would like to remind you that all labs should have protocols and practices in place for handling and managing the use of hazardous chemicals, and they should be adhered to. This can include the use of personal protective equipment (eye and face protection, respiratory protection, hand protection) and proper ventilation, among other things. [caption id="" align="alignnone" width="250"] Using a digital scale ensures the epoxy resin mixture is accurate.[/caption] The use of styrene in polyester and vinyl ester resins raises some issues. First, polyester and vinyl ester resins retain styrene's characteristic pungent odor. Since styrene's odor is so pungent and detectable in trace amounts, you might run the risk of adjacent businesses complaining to authorities about noxious fumes. Both the Occupational Safety and Health Administration (OSHA) and the U.S. Environmental Protection Agency (EPA) consider styrene to be a dangerous chemical and recommend limiting workplace exposure. It has also been listed as a suspected carcinogen by the International Agency for Cancer Research (IARC). Another danger in using styrene is that it is highly flammable, which makes the ester resins in which it is used flammable. Styrene also poses an explosive risk if exposed to certain atmospheric conditions. Another chemical category used with ester resins is peroxide catalysts, which also present hazards that must be managed through strict adherence to use of safety equipment and protocols in the lab. A representative example of this category is MEKP, which can quickly cause permanent blindness if it enters the eyes. Epoxy and Ester Resins, a Comparison Epoxy resins are less hazardous than the other resins discussed thus far. They do not contain styrene, they are cured with polyamines and do not require the use of peroxides, they are not flammable, and they generally have little or no odor. This makes epoxy resins "good neighbors." No one in the next room will know you are using an epoxy resin. The chief hazard when working with epoxy resins is skin sensitization from prolonged and repeated contact with uncured resin. Sensitization is an allergic reaction that causes you to break out in a rash whenever you come in contact with epoxy resins. This particular hazard can be avoided by observing good workplace safety practices, which, as mentioned previously, include wearing gloves. If you keep the material off your skin, the odds of becoming sensitized are extremely low. If you are considering switching to using an epoxy resin rather than an ester resin, you may be wondering how it will compare in terms of strength. Both are equally strong at the onset. Epoxy resins, however, are considerably tougher in the long run. Toughness is a measurable physical property and has to do with fracture behavior. Tough materials generally have higher impact resistance and more resistance to fatigue. While materials with poor fatigue resistance may have high initial strength, when they are load cycled many times, the strength drops off and their strength at failure will be a mere fraction of the original strength. Fabrications that at first tested well may fail catastrophically after a few months of service due to fatigue. Thus, fracture toughness can be thought of as the physical property to measure as a safeguard against a product with mechanical properties that would initially test adequately in terms of strength. There is an easy way to determine if you are using a tough resin. If you make a laminate using several layers of reinforcing material, allow it to cure, and then bend it while holding it close to your ear, you may hear a faint popping and crackling sound. This is the sound of the matrix resin delaminating from the reinforcing fibers. Such a laminate will never test as strong as when it was first made. For example, ester resins will produce a great deal of noise when bent. On the other hand, tough resins such as epoxies make no sound when bent, as this "test" will not cause delamination. Some technicians may resist using an epoxy resin because of its lengthy curing time. I've been asked whether it's possible to make it cure faster by adding a hardener similar to adding more catalyst to the ester resin. The answer to that question is, "No." This actually produces a weaker cured epoxy. Always follow the manufacturer's instructions with respect to ratios of the resin and hardener. You can, however, greatly speed up the curing time of an epoxy by applying heat. Each 10 degree C (18 degree F) increase in temperature will cut the cure time in half. For example, if a socket made with an epoxy resin takes six hours at room temperature (72 degrees F), it will only take three hours at (90 degrees F), 90 minutes at 108 degrees F, and 45 minutes at 126 degrees F. While you can speed up an ester resin cure by adding more promoter and more catalyst, the effect will not be as dramatic and, if too much is added, it will create a polymer with different mechanical properties than what was intended. Using heat with an ester resin cure will result in the evaporation of more styrene, since styrene is evaporated from styrene-based resins during the curing process. Another factor that I have been asked about is the cost of epoxy versus ester resins. The ester resin product itself may cost less than epoxy resins, but you must take into consideration other associated, sometimes hidden, costs. There may be hazardous shipping charges with both the ester resin and the peroxide catalyst. The catalyst may have to be shipped separately, incurring additional shipping charges. There might be local regulations that require you to buy a flammable storage locker to store the ester resins. Your fire insurance may be higher because you use and store flammable materials. Perhaps you've had a skilled employee quit because of the styrene odor. Maybe you've had to deal with irate neighbors because of the odor. In my experience, there are far fewer hidden costs with using an epoxy resin so the final costs are more equal. For example, with epoxy, the resin portion (Part A) is nonhazardous. Generally, the hardener portion (Part B) is hazardous or corrosive when shipped in containers exceeding one gallon. Quantities of one gallon or less ship as "other regulated materials for domestic transport" (ORM-D) and no extra fees are charged, so you may not have the additional hazardous shipping charges you might with ester resins. Why I Use Epoxy In, short-epoxy resins are cool! If you are willing to invest some time in learning a few new processes, you will be rewarded. Here are some features and benefits of epoxy resins: Cure time: You can address cure times by slowing it down or speeding it up. Just add heat. This is a great benefit if you laminate complex orthotics. Viscosity: You can change viscosity by warming the resin or by cooling it down. Again, depending on what you are laminating, this brings in an element of control other resins cannot deliver. Infinite shelf life: Epoxy is not pre-promoted, so it will never go bad in the bottle. If you laminate infrequently then epoxy is a great choice to keep on your shelf. Odor: Epoxy emits low to no odor. The office staff, your patients, and neighbors will appreciate it. Greg Mattson, CPA, CPTO, is president and CEO of Fabtech Systems, Everett, Washington, and has been a technical lecturer at numerous O&P industry conferences and seminars.

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Related posts:

Fab Lab Lingo

Kenneth Cook Sr.: Testing the Limit

“Smart Skin” Could Provide Sense of Touch to Prostheses

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Certified Prosthetist / Prosthetist-Orthotist (CP/CPO) Gates Prosthetics + Mobility Clinic — Mountain Home, AR

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