Here's some information from "the pro's" if "someone"
...wants to brush up on their knowledge of Epoxy and "new" Polyesters...
Comparing epoxy and polyester
Polyester resin and epoxy resins differ in a number of ways. Polyester resin (often referred to as boat resin) is still used today to build most production
fiberglass boats because it is the lowest cost option for new construction when combined with
fiberglass reinforcements. Polyester resin can be quite brittle so randomly oriented chopped strand fiberglass is typically used between layers of woven or stitched fabrics so loads can be effectively transferred between the structural fiberglass plies. Without the chopped strand, micro cracks can develop over time in the resin between structural fiberglass layers.
Polyester resin is normally air-inhibited, which means it typically remains uncured (sticky or tacky) on the surface when exposed to air. To cure reliably, general-purpose polyester resin needs to be applied in at least 15 to 25 mil (.015" to .025") thickness. It is not very effective as an
adhesive (partially because it will not cure thoroughly in thin films) and is only marginally effective as a repair resin for repairing well-cured polyester/fiberglass laminates. Polyester resin is not recommended for repairing vinylester or epoxy laminates.
Epoxy is used very effectively as an
adhesive, as a laminating resin for wetting out structural fabrics, and as a coating. It has excellent thin film cure characteristics and resists micro- cracking better than polyester resin. WEST SYSTEM epoxy offers 3.5% to 4.5% tensile elongation at failure compared to 1% to 2% for typical polyester resins. It offers excellent moisture barrier qualities when used as a coating. It adheres to many different substrates including
wood, metals, cured polyester laminates, vinylester laminates and epoxy laminates. It binds extremely well to graphite fibers, which is why it is often used for making high-strength graphite fiber composites.
Epoxy is more versatile
Polyester resin manufacturers recommend that their products be cured in temperatures above 60°F. Repair yards often have to
work in much cooler conditions. Extra MEKP (methyl ethyl keytone peroxide) catalyst can be added to assist the cure in cool temperatures, but cured physical properties suffer because of it. Shelf life of general-purpose polyester resin is six months to a year from the time it is manufactured.
WEST SYSTEM epoxy is routinely used to repair structural damage to fiberglass boats over a wide temperature range. Hardeners are available for curing it from 40°F on up. Shelf life of WEST SYSTEM epoxy is several years.
Epoxy has fewer fumes
Fumes from polyester resin are quite strong and flammable. Fumes from WEST SYSTEM epoxy are slight in comparison and not a fire hazard. However, both polyester resin and epoxy are industrial chemicals and should be used with care.
Epoxy is stronger
We've compared the repair effectiveness of WEST SYSTEM™ epoxy and a DCPD (dicyclopentadiene) blend polyester resin on well-cured DCPD blend polyester/fiberglass laminate. Made of multiple layers of stitched 2315 triaxial fiberglass with mat, this laminate represents what you would expect to see in one side of a cored-composite, high-quality fiberglass boat. The cured fiberglass
panels were prepared and repaired using methods described in 002-550 Fiberglass Boat Repair &
Maintenance. A precise 12:1 bevel was ground along one edge of the laminates and the repair area was final sanded with fresh 80-grit sandpaper. (12:1 is the minimum bevel angle recommended for repairing cured composites.)
Different repair resins were used to apply multiple layers of fiberglass (the largest ply first, each ply separately laid and wet out with resin) to gradually fill the beveled sections to achieve the original thickness (see sketch). The repair was allowed to cure for a few days and then the repair zone was sanded perfectly smooth. G-10 fiberglass laminate tabs were added to both ends of the tensile specimens, providing a grip area for the test machine. The samples were allowed to cure for two weeks before being cut into 1" wide samples for testing.
To measure repair effectiveness, we first needed to determine the strength of undamaged DCPD blend specimens. The average tensile strength of the undamaged control laminate was 26,198 PSI.
The breaking strength of this same laminate after being repaired with polyester resin and the same fiberglass fabrics averaged 18,460 PSI or 70.5% of the original strength. In this case, the same resin used to fabricate the DCPD laminate was used in the repair. If a different polyester resin were used for the repair, it would likely not have performed as well.
The breaking strength of the control laminate after being repaired with WEST SYSTEM epoxy and the same fiberglass
fabric averaged 21,404 PSI or 81.7% of the original strength.
Both repairs were done using the minimum recommended 12:1 bevel angle. Longer bevel angles at 15:1 or 20:1 will yield even higher repair strengths.
Epoxy shrinks less
These were fairly small repair specimens compared to repairs often undertaken in the field. Some experts believe that the larger the repair, the more important it is to use epoxy. Their concern is related to the shrinkage that occurs in polyester resin. In a small repair, shrinkage is fairly insignificant. As the size of the repair increases, especially when a large section of laminate is being replaced, the shrinkage causes the repair to be stressed before the structure sees any working loads.
Forbes Aird in his book Fiberglass and Composite Materials provides a good description of what can happen in a bond line when polyester is used as an adhesive. He says that polyester has a volumetric shrinkage of about 7% during cure. Because of this, a bond line is subjected to significant stress which will occupy or use up a substantial fraction of the resin's bond strength even before subjected to any working loads. So, the 70.5% repair effectiveness for polyester identified in the testing becomes something less when used for large-scale repairs.
Epoxy vs. Vinylester vs. Polyester Resins
There are three main types of Resins used today for use with Carbon Fiber, Fiberglass, and Aramid (Kevlar). These are Epoxy, Vinylester, Polyester Resins. Each has different characteristics and associated costs. Below we briefly discuss each of these resins.
Please be aware that all resins & hardeners have their
safety related issues. Please
research the products you intend to use and fully read the manufacturer’s
safety information and follow their recommendations.
II. Epoxy resins
These are the most expensive of the three resin types, but well worth the cost.
Epoxy resins are typically about three times stronger than the next strongest resin type. Epoxy adheres to Carbon Fiber, Fiberglass, and Aramid (Kevlar), very well and forms a virtually leak- proof barrier. Epoxy also adheres to older epoxy and most materials quite well.
Most epoxies do have a tendency of yellowing when exposed to
water. When
purchasing epoxy for applications that have extreme temperature changes or are exposed to
water, make sure you
purchase an “all weather” epoxy hardener. One example of such a hardener is West System’s 207 Hardener.
Note that most epoxies are slightly amber in color. When you apply the typical amount of resin to a composite, that is just enough to wet-out the composite, the epoxy is clear. The exception to this is when you wet-out yellow Kevlar or white colored fiberglass. In these cases you will see the yellow kevlar darken slightly and you will most likely see a very slight yellow tint to a white colored fiberglass.
You can buy perfectly clear epoxy resins. West System's 207 hardener is now perfectly clear and has UV protection. It is the only epoxy hardene rwe are aware of that has both of these characteristics.
Don't confuse the "yellowing" of epoxies over time with the slight amber initial color of most epoxies. There are only about 4 epoxies in the market that will not yellow over time. The West System 207 is the best of them all (that is the reason we sell West System's Epoxy). Even for indoor applications, UV will eventually yellow your epoxy. So if you don't plan on painting you piece/application and you want your piece to look good and last as long as possible, plan either protecting it with UV coating (such as a UV urethane), and/or use the 207 hardener.
III. Vinylester Resins
These resins typically have about one third the strength of Epoxy resins. They adhere poorly to Carbon Fiber and Aramid (Kevlar), but can be used for aesthetic applications for these fibers. Vinylester resins are primarily used with fiberglass, but are also commonly used with carbon mostly for cosmetic applications when a polyester clear coat or polyester based
gelcoat is needed. This type of resin should not be used with carbon or aramid
fabric if strength is a primary requirement. Note that urethane based clear coats can be used with epoxies.
IV. Polyester Resins
These are the cheapest of all the resins. They have poor bonding capability and should never be used for any structural carbon or aramid work. They typically work well only on fiberglass. One should generally never consider using this resin with structural applications with Carbon Fiber or Aramid.
Polyester or Epoxy Resin?
By Don Casey
Revised by BoatUS editors in April 2012
What kind of resin you should select depends on the job you are doing. Polyester resin is excellent for fiberglass lay-up--building a boat, for example. Polyester laminating resin doesn't fully cure while exposed to air, so when it is used for lay-up, every application joins to the previous one on a molecular level. You end up with what is essentially a
single substance encapsulating multiple layers of glass fabric.
However, when you are doing a repair, you need for the resin to also function as an adhesive--gluing the patch to the surrounding surface. Polyester is an adequate adhesive but not as good as epoxy. As a general
rule, the tensile strength of a polyester bond will be around 20 percent weaker than the same bond made with epoxy. That makes epoxy resin usually the best choice for fiberglass repair work.
There are a couple of exceptions. When the repair will be finished with
gelcoat, the laminating or filling needs to be done with polyester. While epoxy adheres tenaciously to cured polyester, the reverse is not true, so the bond between an epoxy repair and an overcoat of polyester gelcoat will not be strong. Use polyester for repairs that will be gelcoated.
Cost can be the second consideration. Epoxy is three to five times more expensive than polyester. This is relatively insignificant for a small repair, but where the damaged area is large, using polyester resin significantly lowers the cost of the repair.
What about vinylester resin? Something of a cross between polyester and epoxy, vinylester is typical substituted for polyester when improved resistance to moisture permeation is the objective. Vinylester resin is often used when resurfacing a badly blistered
hull, but for most other do-it-yourself repairs, you should probably choose the economy of polyester or the strength (and impermeability) of epoxy.
Grinding (sanding) is essential regardless of which resin you choose, but if, for reasons of cost, you elect to make your repair with polyester resin, it is imperative to grind the repair area especially heavily. First wipe the surface thoroughly with a dewaxing solvent, then use a 36-grit disk to grind everywhere you want the resin to adhere. Heavy grinding should somewhat offset polyester's weaker adhesion.
I hope this helps...made me feel good...
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