Bonding strength of epoxy, vinylester and polyester.

We have been led to believe for years that epoxy has the best bond strength followed by VE and PE but i have never seen any test results to verify this. Most of the different resin systems that i use have Technical Data Sheets available which i can print out, but none of them show a test specifically for adhesion. I feel like the ASTM638 test for tensile strength may show bond strength, at least in shear, being that i believe they use a coupon made to a specific dog bone shape and dimensions bonded to an aluminum fixture which is held in the test machines jaws and then pressure is applied. What do you all think? or maybe there is comparative data available somewhere that i havn't found yet.

 

I do this type of testing.

There are so many variables involved that it's very difficult to make broad statements about bond strength. You can test individual products on specific substrates with a precise surface prep procedure, but change any aspect of this and the results can change significantly.

 

It greatly depends on bonding what materials. For example, PVC glue is better for PVC pipe than epoxy. The coupon test have to take into consideration the length of the tested material. Even with chopped strand, the test would be almost continuous fibers. Also, test coupons often give higher values because they don't have the discontinuities of a large part.

 

I understand that there are variables as there are in all of the tests on the TDS but i presume that if all the samples are tested by the same ASTM test such as ASTM D638 then the results would be useful as long as we are comparing like samples, ie, samples either with or without fiber. Please correct me if i'm wrong but my understanding is that the coupon is bonded to aluminum which is then held in a fixture on the testing machine. Most of the TDS results for tensile strength are without reinforcing fiber so they are testing the resin only although some do have reinforcements but they are rarely the same so, less useful results.

 

I misunderstood that it was the testing of laminates (composites). Are you refering to where there are paralel strips of glued metal and are tested in shear?

 

You need to test it exactly how you plan to use it, and for best performance it would be a laminate schedule designed for a good bond and avoid a peel situation.

On one test I made samples and bonded two laminates together with a specialty bonding putty for a customer. I tested different catalyst ratios because that was something we needed to understand to help them with production. I also compared it with some other putties I had on hand that on paper should do a bit better. None of the samples really stood out as being much better.

The failures were so commonly in the actual laminate (substrate) that even the stronger better (on paper) putties had similar results.

So even if an astm test method indicated the bond of some resin types were better, it would be for that exact test, not real world.

This is where some of the "you must always use epoxy for a repair" group get it wrong. Yes the repair laminate itself will be strong, but the surrounding area will have very different physical properties that will still fail at a lower level of stress, you just moved the failure point over a little. Or the super strong repair material just rips off taking part of the old substrate with it.

I relate it to lug nuts on a car wheel. An old honda civic had four lug nuts, a one ton truck may have eight. You could put 8 lug nuts on the civic and it would be stronger, but it didn't fail with four, so you didn't gain anything by going to eight.

 

Even if there were only one test on a TDS it would be better than none. For half a century we have been marketed to by epoxy manufacturers that epoxy is superior in every way but without any actual data to back up the claims. When we actually look at the TDS which apparently few do, we get a different picture. I personally have been using epoxies for over 5 decades but use it selectively, mainly for wood preferring VE for laminating unless smell is an issue. Last winter i built 6 new elliptical rudders for Olson 30 sailboats but we cut up the old rudders and salvaged the 2" pultruded shafts for re use (not my choice) My feeling is that pultruded was not ideal for a rudder shaft but its hard to argue with 40 odd years of success. Anyway, the point is that the bond strength of the polyester skins of the old rudders to the foam cores (about 16 lb/ft3) was impressive, it could not have been better, i literally had to cut the skins into 2" strips the get them off and this is peel strength which is the weakest bond.

 

There is a significant amount of data from many manufacturers. I think you should do at least a google search before slandering the manufacturers. These are two websites with hundreds of data sheets:
https://www.farnell.com/datasheets/841920.pdf
Epoxy and 2 Component Resin Systems Technical Data Sheets https://epoxy.com/techdata/Default.aspx

 

Gonzo,
Of course i have searched for data pertaining specifically to adhesive strength, i collect the TDS for any resins i use and have yet to find any. I have not searched for this data for materials i do not use such as those in the links you provided but while there is a lot of data there (as there is on the TDS of the products i do use) i don't see this data there either. Perhaps you can lead me to such data, the closest i can find i think would be the test for tensile strength which would show the bond to properly prepared aluminum, in shear. Actually a simple lap shear test would suffice.

 

That only tells you how it bonds to aluminum, unless you're bonding to aluminum it's almost useless information.

 

You claimed there was no data at all. There is no such thing as adhesive strength without specifying materials, surface roughness, etc. That is your responsibility for the specific application. If you need precise data, either do it yourself or pay someone equipped and knowleadgeable to do it.

That depends on the test protocol. We made many tests for resins and composites that were held by jaws on an Instron tester. There was no aluminum involved.