Comparing Resins - fairly

[LMB]

Self taught in fiberglass work, I enjoy research. I work in practically all types of resins and top coat systems, designing each repair to most appropriately fit the job. Of course, opinions will vary and I'm always open minded enough to listen.

Lately I've been looking at technical data for a few resins. I'm not a chemical engineer but assumed comparing things like tensile elongation, tensile strength, flexural modulus and so -on would give some idea on performance varitions. I think I'm starting to realize that this might not be a fair method, as it's not really an all things being equal comparison. In other words the base controls in each testing situation might vary greatly. Further, I'm questioning generalizatoins like epoxy being far superior to vinylester or even polyester. I would like to back it up with data. So far, comparing one popular vinylester with a well known epoxy showed the vinylester outperforming the epoxy in almost every catagory. Maybe it's not fair comparison, and maybe I don't really understand all the numbers. Anybody that can give some basics on interpreting technical data would be a big help? Also, what effect does reinforcing resins with additives generally have?

[marshmat]

Hi LMB,

You are most certainly right that there is more to those technical data sheets than meets the eye. Those tests can only be compared directly when done to the same test method under the same conditions. For example, if two resins were both tested under ASTM D897-08 for tensile properties of adhesive bonds, you could directly compare the results. But if one quoted a value from a test based on D897 and a different resin had been tested based on ASTM D1144 (strength development of adhesive bonds), you couldn't directly compare the numbers.

As a simpler example, look away from boats and consider concrete. Concrete compressive strengths aren't quoted as an "actual" value. They're quoted as a measured result, obtained by breaking a sample of standard dimensions in a standard testing jig. If a sample of different dimensions is used, corrections must be applied to get a value in terms of the standard test. The resulting number is meaningless without reference to the test standard.

I agree that it's not necessarily fair to generalize "epoxy always better than x-ester". The specific resins in question must be compared, with consideration given to what they will actually be used for. In wood-composite strip plank, for instance, one might choose an epoxy with lower tensile strength than a particular vinylester, because its adhesion to wood is superior. It's very hard to make blanket statements that remain valid for all cases.

[mastcolin]

Marshmat is correct. Be very careful you are comparing apples with apples. In addition a lot of the tests quoted have only limited relevence, depending on what they are testing and how the resin is evaluated. eg you wouldn't build a boat without some sort of fibre and obviously the type of fibre and the method of lay-up with also have a larger effect than the resin.

Epoxies are generally assumed to be better due to their long term performance, especially when in wet environment than either vinylesters and especially common polyesters. Of course, as you have stated many vinylesters have superb performance.

Epoxies generally degrade to much lower extent than polyesters. They are intrinsically more stable to moisture. The cure mechanism is also more robust in reality (assuming other factors equal).

The glass/resin ratios are also generally better engineered for epoxies ie less resin.

"best" is very subjective. What is best for laying up thick cloth underwater isn't alwasy best for thin skin on wood. Some people just feel happier with certain brands/types due to familiarity.

Then there is subject of cost - not only raw material pricing but also how it affects the work process. Do you need long working time? does it exotherm in large mixes too rapidly? etc etc. What is your trade off between weight and performance in new build situation?

If you are in a painting situation after resin work, you are alwasy better with epoxy. Vinylesters and polyesters should be left a minimum of 3-4weeks before painting as they release unreacted styrene. This can often cause problems with paint system. I know of people who apply thin coat of epoxy primer in mould and lay up on top of this. This would be impossible with esters. (sure you can get bonding/intermediate coatings but this just seems to be making everything more complicated. I've always been with the KISS principle)

"Some gentlemen prefer blondes", it doesn't make Halle Berry a bad option as wife/lover/mother/someone to pick you up from a bar after you've been out drinking with your mates.