Epoxy Adhesion Mechanism 8

[littlekix]

Hi all,

First post, so forgive me if I miss a few things... I'll be happy to add them if you let me know what information you would like.

I have run into a question where I was asked how an epoxy could effect a substrate material. My previous knowledge of epoxy was "it glues things together" which is no longer adequate. After viewing different forums etc. I didn't see any specific references to the mechanism an epoxy utilizes to adhere to a substrate.

My question is how does an epoxy bond with the surface? (I know there are myriad of epoxies, polyurethanes etc but for starters I am trying to keep it simple) Everything I have read has been especially vague on the topic.

If it is a chemical bond, what are the typical reactions?
If it is a mechanical bond, is this why surface prep is so vital to provide crevices and surface areas to attach/grip to?

I was in discussion with another member on the forum (Thanks Tmoose for showing me Eng-Tips btw) who suggested van der walls as being a predominant force. The conversation quickly took a turn for a previous pet gecko and the force required to remove it from surfaces, but I digress.... Is the van der walls force a significant proponent of the adhesive bond?

The background :

Epoxy was used to cover an area of corrosion. The substrate (Duplex Steel) was cleaned and a coating of epoxy was layered on top. I worry that if the epoxy bond is chemical it could potentially increase corrosion instead of retarding if done improperly or with the wrong epoxy.

Regards,

 

[MiketheEngineer]

You might want to get with some mfg's of epoxy and epoxy paints.

If they are really helpful and feel some business coming their way - they can tell you more than you could ever imagine or possibly understand...

 

[moltenmetal]

This is by no means an area where I claim any specific knowledge, but you should not forget about the effect of mechanical bonding in adhesion. A liquid or gel which penetrates a "dovetail" cavity or surface irregularity and then hardens, will be retained by this means. If I understand correctly, blasting and other surface profile processes increase adhesion not only by cleaning and increasing the effective surface area for bonding, but also by generating a profile that the material can "bite" into in this way.

As to chemical interaction with the substrate, I doubt there are many useful chemical reactions at play between epoxy and a metal surface- certainly not ones I'd be worried about with respect to inducing corrosion. There will be hydrogen bonds and polar attractions beyond mere van der walls forces at play. Embrittlement is another issue entirely, and whether the coating will induce embrittlement depends on the service, temperature, what's in the coating and what's in the corrosive environment.

What caused the corrosion of the duplex (stainless) steel in the 1st place?

 

[littlekix]

Mike - Good idea, I will be calling the epoxy manufacturer later today. Should have already done that...

Molten - The reason I was curious about chemical reactions was due to changing chemistries around the epoxy "patch". I am not a chemist by any means, but I was wondering if people had seen issues where the chemical bond that the epoxy makes with the substrate is more susceptible to corrosive attack?

The duplex steel was under corrosive attack by crevice corrosion. Sediment drops out of suspension trapping slurry against the substrate either on the base metal or around weld toes. The slurry trapped beneath the sediment is then separated from the general chemistry and can corrode quickly (Same mechanism/situation as crevice corrosion, hence the mention above).

I read also that at times epoxy has pitting issues in itself and that care must be taken when applying it it. If not and the epoxy has a pit allowing slurry to penetrate underneath the patch then the epoxy no longer prevents corrosion but accelerates it due to acting like the aforementioned crevice. Has anyone seen this type of phenomena where epoxy has pits after application?

 

[Compositepro]

This is really a very complex topic and is not fully understood, even by experts. Van der Waals forces are a major part of adhesion. These are the forces of attraction between molecules that don't actually chemically react with each other. These forces are what cause the liquid adhesive to "wet" a solid surface. When the adhesive solidifies by reacting internally these forces still exist. But mechanical interlocking is also almost always a factor as well as some degree of chemical reaction with a surface. In many cases it is impossible to isolate the different mechanisms and little reason to.

It is possible for a component in an adhesive to react with a metal surface and create a corrosion product. That would mean that the adhesive is unsuitable for the application. There are a whole bunch of different types of adhesives out there. You have to know how an adhesive works to use it correctly or get someone to tell you which one to use. For example do not use Elmers glue to bond smooth metal surfaces together.

 

[moltenmetal]

Yes, coatings can act as crevice or pitting corrosion initiation sites, but the reason is physical rather than chemical as far as I know. As far as I know, they behave the same way as any other deposit in that regard, setting up an oxygen concentration cell or the like. If the entire surface is coated except for a few pits, you can get quite accelerated corrosion under the right (wrong) circumstances).

Inspecting (non-conductive) coatings for pinholes is a relatively well established procedure.

 

[blakmax]

The most believable theory of adhesion is the adsorption theory (Kinlock, A.J., Adhesion and Adhesives,Chapman and Hall, 1987) whereby there is a combination of chemical bonds (ionic, covalent) and electrostatic attractive forces. The reactions are usually with oxides and hydroxides on the surface of the adherend. This theory also can be used to explain in-service disbonds, where the surface oxides hydrate over time, leading to dissociation of the chemical bonds between the adhesive and the surface oxides. (See attached link

http://www.adhesionassociates.com/p...es - Mixed-Mode Bond Failures Explained.pdf).

For aluminium, Al2O3 hydrates to Al2O3.2H20.

Mechanical interlocking only gives short-term strength until the oxides hydrate. I have seen many examples of surfaces which were roughed up to the extent that if the scratches were on the non-bonding surface, they would need to be blended out for fatigue reasons, and yet the surface totally disbonded.

The secret to successful adhesive bonding is to treat the surface prior to bonding such that hydration of the oxides is prevented.

 

[unclesyd]

Here are three articles that add to the information given above. NASA has done a lot of work with Epoxy adhesion with the paper having some good references. When looking at adhesion of epoxy you will find that a lot studies concern only one particular epoxy metal combination. The NSC paper defines the adhesion of epoxy and cold rolled steel.

http://scholar.lib.vt.edu/theses/available/etd-110498-122417/unrestricted/etd.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080040188_2008039121.pdf

http://www.nsc.co.jp/en/tech/report/pdf/6303.pdf

 

[blakmax]

Thanks for those references UncleSyd. These articles really support a theory of adhesive bond formation and degradatrion which I had held for some considerable time, but did not have the resources to follow up.

My concern is that the aviation regulations (FARs) in their current form simply fail to address adhesive bond degradation by hydration. They only require demonstration of static strength and fatigue performance, both of which can be easily satisfied before the interface has had time to hydrate. Some recent amendments to the Advisory Circular (AC 20-107) have made some tentative steps towards addressing the issue but fall far short of actually clearly spelling out the consequences of poor bond preparation processes. I think thay are still stuck on the mantra of "cleaning" the surface. A clean surface is a necessary but not sufficient condition for bond durability.

All the best

Blakmax

 

[unclesyd]

These two papers might shed a little more light on your quest.

http://www.engineering.schneider-electric.dk/Attachments/ed/ct/Insulation_dielectric_ICSD_2007.pdf

https://www.corrdefense.org/Academia Government and Industry/SESSION A - TAYLOR FINAL.pdf

 

[littlekix]

All,

Thanks for some great information. I am always amazed at high technical level of information found on this forum.

Regards,

Sean

 

[dik]

I don't know if they are still around, but 'epoxybot' and 'adhesiveman' handles had a lot of useful information about epoxies wrt the 'Big Dig' failure in Boston a few years ago...

Dik

 

[blakmax]

Littlekix, Your original comment/question was:

"Epoxy was used to cover an area of corrosion. The substrate (Duplex Steel) was cleaned and a coating of epoxy was layered on top. I worry that if the epoxy bond is chemical it could potentially increase corrosion instead of retarding if done improperly or with the wrong epoxy."

The answer is clear. Unless something was done to
a. completely remove the corrosion products and
b. to generate a chemically active surface and
c. treat the steel surface with a chemical to prevent subsequent hydration,
then it would not matter what type of epoxy was used, the poor result will always be the same. Change epoxies, and all you change is the colour of the disbond prior to re-emergence of the corrosion.

I would suggest 1. solvent degrease to remove surface contamination, 2. grit blast to remove the corrosion products and expose a fresh, chemically active surface, 3. treat the surface with an organo-funstional epoxy-silane/water solution (look up Bogel or AC120) 4. possible apply an adhesive bonding primer and then bond to that surface (although our experience is that the primer is an overkill on aluminium alloys).

We have 18 years service experience with that process and we reduced the bond failure rate for aircraft repairs from 43% in 1992 to less than 0.02% (and those failures can be attributed ro technician short-cuts or errors). This process works on aluminium alloys, titanium, stainess steels and D6Ac high strength alloy steel. I am really pleased that the references from UncleSyd support the findings we developed from practical experience. If you look at the papers on the web site in my previous posting, these give examples of what we have found.

Regards

Blakmax

 

[StrykerTECH]

When it comes to epoxy, always trust the pro's. The manufacturer and producer generally have done vigorous levels of testing to flush out different processes for different substrates. I highly recommend contacting the manufacturer as have others.

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[blakmax]

StrykerTech

In my experience, approaching the manufacturer for advice can be fraught with problems, and that is not necessarily a reflection of the competency or integrity of the company.

Many manufacturers are driven by compliance with standards such as MMM 312A. The crux of the issue is that standards such as MMM 132A address mainly strength tests which are short term. It is possible to meet most of these test requirements using relatively simple preparation processes which give adequate short term strength because the interface has not had time to hydrate. Even the tests which are used to determine environmental resistance are strength based and only require a relatively short conditioning period.

Hence, if the manufacturer meets all of these requirements, then he is confident that he has a good process. This may or may not be the case.

We have found that the most reliable indicator of long term bond durability is the wedge test ASTM D3762, but even that has problems. That standard states that acceptable results give an average crack extension of less than 0.5 inches and no more than 0.75 inches after one hour of exposure to a hot humid environment. That results is a catastrophic failure. We use no more than 0.20 inches growth after 24 hrs and no more than 0.25 inches after 48 hrs AND no more than 10% interfacial failure in the test zone. With processes that meet these criteria, and which are correctly implemented, we have never encountered bond failures, even after 18 years of service.

These requirements exclude a number of commonly used processes such as Pasajel etching, the P2 etch and alodine.

In essence, the standards need to be changed and then if the manufacturers can demonstrate compliance, their advice will be of value. But as long as they only comply with current standards, I would not always be confident in accepting that advice.

Regards

Blakmax