Choosing the Right Resin
|Sharon Dolly prepares a lamination at PEL Supply Company. Photograph courtesy of PEL Supply Company|
No matter how long you've been fabricating O&P devices, weighing the advantages of one lamination resin over another can be complex. Some resins seem especially suited for particular devices or fabrics, while others seem versatile enough to handle anything. Perhaps cost is a major determining factor. Or, perhaps safety and sustainability are your bottom- line priorities.
Al Gracon, director of logisitics for PEL Supply Company, Cleveland, Ohio, says, "This industry is pretty conservative, and once a person starts using one resin, they tend to stick with that." However, just as you regularly check the fit of a patient's device, you may find that reevaluating your resin choice could improve your work experience, your costs, and not only your health, but that of your family for generations to come.
The O&P EDGE talked with a cross section of experts to bring you their perspectives on resins. Because space does not allow for this article to provide a comprehensive overview of all resins on the market today, what follows is a representative sample of available products and a selection of opinions from several fabrication experts.
A Brief History of Lamination Resins
Ronnie Graves CO, LPO, RTP, BOCPO, has been a technician for 30 years, as well as a clinician and the owner of Prosthetics Research Specialists (PRS), Bushnell, Florida, since 1989. He gave The O&P EDGE a history of lamination resins, the materials that sank the wooden socket.
"In the beginning," Graves says, "we were using polyester resins...with promoters and benzoyl peroxide catalysts, so you were dealing with both a poisonous material and a hazardous material, plus the resin, which was loaded with styrene monomers." These polyesters are still sold in inexpensive original, pre-promoted, rigid, and flexible versions.
"Acrylic resins came next," Graves says. He describes them as "expensive, but very fast-setting." Graves says that their strength makes acrylics suitable for ultra-lightweight devices, but that "they're really brittle."
Jim Williams has worked as a technician and customer advisor for SPS, Alpharetta, Georgia, for a total of 18 years, plus a break in between as co-owner of Peaster & Williams Central Fab, Cumming, Georgia. He says that acrylics have a horrible smell and get so hot that they can boil while curing. However, they also have low viscosity, for a quick wet-out, and are available in flexible formulas.
The next resins-vinyl esters-came from the world of racing boats, Graves says. He calls vinyl esters "a good resin-light, strong, flexible," but says, "they still have styrene monomers."
Epoxies are the latest material to come into vogue in O&P fabrication. When epoxy lamination resins came out, Graves recalls, "You had to laminate the thing and leave it on vacuum for 24 hours-they were a hassle to work with and didn't really catch on. Now, you've got epoxy resins that set in 15 minutes on the vacuum [plus cure time]." Epoxies are extremely strong, Graves says, and while their toxicity varies among brands, none harbor styrene monomers. At least one formulation is waterproof during working, allowing fabricators to work unsealed casts.
Epoxies and acrylics are sold in a variety of mixtures. However, it's important to look at the Material Safety Data Sheet (MSDS) for each product to know what you're really getting. "You can name your product anything you want," Graves says, recalling at least one company that labeled its vinyl ester as an epoxy-acrylic.
What's the Application?
Is there a "best" resin for any particular application? Maybe not. Keith Crownover, CPO, director of the O&P programs at Oklahoma State University Institute of Technology (OSU), Okmulgee, says, "In the end, all of the various resins can be made to do most things you want if they're used the way they were intended-with a good technique and good vacuum system, proper layups, and proper working of the actual lamination."
|Sharon Dolly completes her lamination.|
However, it's worth asking your distributor about the particular characteristics you're looking for. For example, Williams says that if speed is of the essence, some formulas cure much faster than others, and others are heat-moldable after curing.
Which Fabric Are You Using?
When matching resins to fabrics, expert opinions differ. "There's no preferred resin for each fabric," contends Graves, but Williams says that higher-tech fibers demand particular resins: polyesters' honey-like viscosity works fine for nylon and Nyglas layups but doesn't adequately bond to fiberglass or carbon fiber. "Acrylics give good bonding to the material," he asserts, "and make a chemical bond to itself so you don't have to rough it up between laminations, like you do with polyesters and epoxies. With the newer fabrics-Aralon, Spectralon, Kevlar-acrylics and epoxies are about all that really work."
What's the Cost?
Cost is king for many practitioners, but price per gallon isn't. Per gallon, polyester resins are the least expensive on the market, epoxies cost approximately 20 percent more, and acrylics are twice as expensive. However, Williams says that the top question his customers ask is whether a product incurs hazardous-material (hazmat) shipping fees. Such fees can increase the shipping cost as much as 50 percent. A polyester, for instance, with three separate hazardous components, requires three separate hazmat fees. It's also worth considering that lighter-weight layups require less resin overall.
How Safe Is It?
When we think of value in O&P, many people mean near-term financial value. However, as our awareness of chemicals' long-term effects become clearer, the bottom line of the future may become safety.
Rob Kistenberg, MPH, CP, FAAOP, has compiled a paper on the known dangers of the individual chemicals used in O&P compounds. Polyester resins contain styrene monomers, he writes, which are considered by the Environmental Protection Agency (EPA) to be "known or suspected" carcinogens. Its promoter contains n,n-diethylaniline—a chemical so dangerous that its MSDS recommends that it be handled only under a hood. Its catalyst contains tricresyl, which the National Toxicology Program (NTP) calls "toxic by inhalation, ingestion, or absorption," and benzoyl peroxide, a potential tumor promoter and mutagen. (Mutagens are chemicals that can mutate DNA in both men's and women's germ cells, programming in birth defects even before conception.) Acrylic resins generally contain methylmethacrylate, whose effects include irritation or damage to the eyes, respiratory tract, and central nervous system. The hardener for epoxy resins is typically benzoyl peroxide, which is described above.
Even products marketed as "safe" may just be "safer." Some contain fewer volatile substances, but if you glance at the MSDS sheets of some such resins, you'll see chemicals listed as untested by regulators but "expected to be" hazardous. However, of resins our experts noted for safety, epoxies were favorites, while polyesters were on the no-buy list. (Editor's note: For more information, read "Working in O&P: Protecting Yourself from Environmental Hazards," The O&P EDGE, April 2009.)
No matter what you choose, you can make your resin effectively less toxic by keeping it out of your body. Kistenberg's students at the Georgia Institute of Technology (Georgia Tech), Atlanta, enjoy ventilation that overturns their lab's entire volume of air approximately every four minutes. Hoods and vents draw away chemicals that rise and those heavier than air that settle on the bench. Students wear lab coats, eye protection, gloves, and respirators, Kistenberg says. "A dust mask is really not going to help. Those particles are smaller than the filter capability of a dust mask."
Crownover concluded with a sentiment echoed by several of the fabricators we surveyed: "There are so many choices today, and if it falls into the 'greener' category, I'm all for it.... [However,] any resin can be effectively utilized-proper technique is really foremost, no matter what you choose."
Morgan Stanfield can be reached at