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Q-talk 98 - Resins for Homebuilders

There has been some question of the best resin to use for the construction of homebuilt aircraft so I considered it helpful to provide a discussion of the nature of epoxy resins w ithout getting too heavy into the chemistry. Even so. my background is not polymer chemistry, but in the engineering application of composites for the US Navy. This discussion should be a benefit to the users of epoxies to provide a bit better understanding of these materials. I will approach the discussion with an overview of the different types of epoxies and then get down to where we are in our application to homebuilding.

First, let us look at the terms that are commonly used to characterize materials. The in-service performance characteristics are the most common way that the technical community would speak to materials. This would include w oods, metals, composites or fiber reinforced plastics (FRP). and even sandwich core materials. The difference lies in the nature of the material in terms of its structural makeup. The structural makeup falls into two general categories: orthotropic and isotropic materials. Orthotropic materials are those that have oriented properties, or differing properties, in the three major axes (x, y. z). Isotropic materials have properties that are the same in all three dimensions, such as metals. A word of caution. because orthotropic materials have directional properties. the orthotropic materials, in general, cannot be directly substituted for isotropic materials w ithout further investigation and analysis of the design to verify that material allowables are not exceeded.

Physicals - The terms are basically the same for orthotropic and isotropic materials. They are tension strength compression strength, shear strength, modulus (or stiffness) and heat distortion temperature (HDT). Another common property for polymer materials is the glass transition temperature (Tg) and this is generally ten to tw enty degrees higher than the HDT. Tg is the point where the material transitions from its solid structural state into plastic deformation owing to elevated temperatures that are higher than the cure or post cure temperature. (Ed Note: My mnemonic now for Tg is the temperature it Turns to Goo) It goes then that the material properties take a significant nosedive at this point. Further, some material properties can start to degrade long before the HDT or Tg point. One can almost dictate the Tg by the nature of the polymer chemistry and so this is how a 'compositeer' selects a resin for an application. The rule of thumb is that the liigher the cure temperature, or the post cure temperature, the liigher the in-service temperature performance.

Fibers - I will only mention them briefly here and save a discussion of fibers for another article. The fibers that are available are very familiar to most that are using composites

in their construction. The problem that most have comes to the source of the fabrics as one is compelled to purchase from a mail order house that caters to the homebuilt community. Therefore your choice of fabric styles is limited. There are an abundance of suppliers across the country and around the world, but the WWW makes it easier to find them. A word of caution: carbon fiber is thought to be the high performance fiber or fabric. Don't just randomly apply it and substitute glass fibers unless you liave professional guidance on the application. Carbon fiber has a higher modulus and may have higher strength, but this may work against you. Example: in times past there were some that were looking to liave higher strength in the empennage on tail dragger composite planes because of local buckling or tail spring failure. Carbon fiber w as applied, with its higher strength and stiffness, but then the failure was moved into the fuselage ahead of the tail feathers because the area was then too stiff. Aramid (KevlarS) is good for abrasion resistance but has very poor compression strength. A good application is a skip pad for the nose of an EZ.

Epoxy Resins - An epoxy resin system consists of three basic components: resin curing agent and a diluent. There are three basic epoxy chemistries. Bisphenol-A. dianiline. and novalac based. The later tw o are higher temperature cured, and so higher Tg's. but they are more brittle because of a higher polymer chain cross-link density The epoxy resins that are available for the homebuilt and marine industry are almost exclusively bisphenol-A because they are suitable for ambient or room temperature (RT) curing. Epoxy resins are ranked in terms of their epoxy equivalent weight.

Curing Agents - These are for the purpose of initiating and reacting an epoxy molecule to cause cross-linking or. in fancy terms, polymerization. They fall into three basic categories: amines: aromatic or aliphatic, anhydrides, and catalytic. Curing agents liave reactive groups on each end of the molecule that reac.t with the epoxy molecule to cause the cross-linking structural phenomenon. These are ranked in terms of amine hydrogen equivalent. Several curing agents may be combined to create wide and varied properties.

Diluents - These are for the purpose of diluting or reducing the viscosity of the epoxy mixture for the ease of processing and handling. They fall into two basic categories: mono-functional and difunctional. They are formulated to become part of the polymer molecule w ith the completion of the polymer reaction. However, there have been alternate monomers, such as styrene in rare cases, which have been used as diluents. They make for ease of handling during lamination, but because they do not get consumed in the reaction and so they act as a plasticizer and hinder achieving good material properties compared to similar epoxy sy stems that use reactive diluents. This was especially true for Saf-T-Poxy. Critical properties like compression really suffered w hen subjected to elevated temperatures.

Epoxy formulations differ by the combinations of these three basic constituents and other additives. Wide selections of material properties are possible w ith epoxy based resin systems as a result. Propertied such as: mechanical propertie(tensile, compression, shear, peel strength, etc.) cure time, pot life. Tg. toughness, chemical resistance, and handling (viscosity, wet-out. drain-out).

Having said all this, then what should we look for as far as homebuilt aircraft construction is concerned'1 I'm sure right at the top of the list is handling and pot life during lamination process. Sure, this is important, but not to the point of sacrificing stnictural properties in the finished product. Peel strength is important as we have a significant number of secondary bonds in the way of joining multiple parts together forming the whole, and this may be over a period of years (decades in my case) of constaiction. And couldn't we really say odor and skin sensitivity. But when all the constaiction is done and one is buzzing along at 170 plus miles per hour, all of a sudden the structural properties play a major role in keeping the pilot cool just as much as the big fan turning at the front of the airplane.

I want to preface the follow ing discussion by saying that there has not been much in the way of reports of structural failure of composite homebuilts as a result of the composite itself. The failures are more the result of craftsmanship issues. especially with respect to repairs, and a maybe a few cases related to fuel contamination from dissolved tank linings. I prefer as much factor of safety tliat I can achieve in a structure and may tolerate having to work a little more during laminating, because this is cheap life insurance. The more modifiers that are in the epoxy then the more loss of laminate performance in the composite.

When one looks at the primary properties at work, the key one for aircraft is the compression strength. Consider the aerody namic and landing loads. This is the one property that suffers the most in elevated temperature situations. What are the failures that are most commonly reported, other than the foam-eating fuel of the wing core trick? Wing skin buckling! There liav e been several reports over the years where the top of the canard wing skins have essentially failed just outboard of the fuselage. Clearly many of these hav e been from hard landings on the tail dragger birds, but it is interesting to ponder w hat w ould hav e been the result if a better resin w as used instead of Saf-T-Poxy.

The other properties of concern w ould be that of the Tg / HDT and the peel strength, again try ing to increase that factor of safety . Now . all one really needs in the HDT is simply to meet or exceed the HDT of the poly styrene foam that is popular for wing cores. The Dow Buoyancy Billet ? has a HDT of around 165?F. Saf-T-Poxy had" a HDT of around the same. Interestingly, at this temperature the compression strength drops by 50% at 145?F. We want to achiev e consistent material properties then tliat is at least 165?F or more.

By the way. Dow does not promote this foam as structural foam and they insisted that it not be used in tliis manner during a conversation that I had with Dow to locate local sources for the billets. Indeed. I have never encountered any other application of this particular product for structural foam core outside of the homebuilt community. There are much better materials and foams available for composite sandwich construction.

Which of the resins out there then offers this kind of performance? The first one that I have the most experience with is the Gougeon Pro-Set? and WEST? System. A decade ago Pro-Set? w as known as the Gougeon GLR and GLH laminating resin system. Gougeon introduced the laminating system because WEST? is not a laminating resin, it is made for wood adhesive, cold molding or epoxy repair. I had some limited experience with some of the PTM&W epoxies. but AEROPOXY was not available at that time. At the time, the only other resins that were available to the homebuilder were the ALPHA epoxy and RAF epoxy laminating systems. It was well know that each of these had a limited application, as there were reported problems in areas such as fuel tanks, and they still suffered from low Tg's / HDTs.

Ten years ago I was involved with a team of folks that engaged in building a human powered submarine for the races. We had just purchased a 5-axis filament winder for the US Navy materials laboratory and we thought it would be novel to fabricate the hull using this technology. The only bisphe-nol-A based epoxy. aliphatic amine. orRT curing, resin that w as suitable for the job at that time was the GLR /GLH epoxy. Hav ing spent the next several months working with the resin to build and fabricate various parts for the sub we came to know it very well. Any previous experience that I had with an amine epoxy was the Saf-T-Poxy or high temperature cure resins. As a result. I w as very pleased, and remain so. with the resin now marketed as Pro-Set.

Therefore. I'm not too familiar with other resins. Besides, these resin choices are from homebuilt mail-order sources that are expensive in the small quantities that they are packaged. I have never had a problem getting Pro-Set? for a reasonable price at local sources. Pro-Set? prov ides great toughness, secondary bonding, it handles veiy well, and it has great mechanical properties for a aliphatic amine, because it formulated to yield an increased crosslink for a RT cured epoxy compared to others. Further, it is easy on the "sniffer'. Gougeon provides fabulous technical support, and is esthetically pleasing. CAUTION - Pro-Set? can still cause dermal reaction.

We have a saying at the Lab. "we let the data speak for itself ". I have nev er been let dow n by the material and so w hy change or go anyw here else!? All other epoxy resins that I liave used in applications at the Lab have either been pre-preg. liigh temperature cured resins or we've mixed them in-house.

I'm not pushing Pro-Set. but simply if it does all that an epoxy is supposed to do. it is readily available, the company has been, and is going to be around for a long time, and it exceeds our requirements, then it is very suitable for the application to experimental aircraft. Further, when I see that Scaled Composites lias built the Vantage. Adam M-309. and Proteus aircraft, and I hear of and see many different high performance yachts being built with it for racing (several weeks ago I saw a 42 meter. 52 knot, mega-yacht being built), it just bolsters my confidence in the product even more. I don't know of these other RT epoxy resin that can boast this kind of a resume'.

Pro-Set? is formulated to provide the increased crosslink density without getting too brittle, which would compromise toughness, interlaminar and secondary bonding performance. If the crosslink density gets too high, it w ould have good HDT. but the rest of the needed performance is sacrificed. Balanced properties are what one is looking for in a RT cured epoxy lamination system! Other resins in the past have focused in on two or three of the desired characteristics that I've spoke to. but no others have struck such a nice balance of properties as Pro-Set?. The only problem that I've experienced with the Pro-Set? is that some resin / hardener combinations do not do well on vertical surface laminations because it tends to run out as it is curing. Also, some foaming takes place if you over w ork the resin during lamination. But. it seems that Gougeon lias addressed both of these, with the current formulations.

Next time I will provide a comparison chart and discussion of the various resins that are available to the homebuilder. The chart will have the ASTM test standard results that report the material properties. I will take each of these one at a time. Subsequent articles will cover aspects of the fibers that we work with I will deal with what we need to look for in a good lamination, how to assure a good lamination, how to tell if the fibers are too old to use. and the like. There are considerations and procedures to proper storage, and considerations on the fiber surface treaUnents that age. and this may affect the laminate properties.

That's it from Balt-mer'. where the air is now clean, but the kids are still below average. Work clean and safe!

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