Rubber to Metal Bonding - Substrate Preparation questions 1

[AlexEss]

Bonding rubber to metal washers (Mild Steel, Stainless Steel, Brass & Aluminum)
What is the correct sequence - Degrease/Shot-blast/Air rinse or Shot-blast/degrease/air dry ?

Thanks in advance for comments and advice.

Alexess

 

[TugboatEng]

Shot isn't ideal for profiling, a grit blast would provide better profiling.

Degreasing should be the first step, you don't want to hammer contaminants into your profile.

A second wash should be done to remove debris from the blasting.

 

[AlexEss]

Hi TugBoatEng

Thanks for your reply - I used the term 'shot-blast' in fact we grit blast with an Aluminum Oxide grit. So Degrease / Grit Blast / Rinse is the way to go.

 

[blakmax]

Hi AlexEss

There are many misconceptions about adhesive bonding. So, everyone knows the surface must be clean. However, many people believe that you need to abrade to "rough the surface" so that the adhesive keys into the roughness. So the common perception id that you clean the surface, grit blast and then clean the debris off. Just as you stated.

Let me provide the correct advice. Firstly, adhesive bonding is a CHEMICAL process that relies on having a clean, chemically active surface to enable the adhesive to react with the oxides and hydroxides on the surface of the metal to generate bond strength. But that is not the full story and I will discuss later. So, clean, blast and clean again is the usual method, but remember that the surface must be reactive, and if you clean it a second time to remove the abrasion products, you will leave a layer of solvent contamination and that will inhibit chemical reaction. So, clean first, grit blast and then BLOW the dust off with clean gas such as dry nitrogen. Solvent cleaning will only contaminate the surface.

Now for the critical step. Please realise that one of the primary causes of bond failure in later service is failure of the interface and that is usually caused by hydration of the metal surface oxides over time. Let us be clear: a failure of the adhesive by fracture is a design issue, but a failure at or near the interface is a processing issue.

Atmospheric moisture is absorbed by the adhesives which are usually polar molecules that attract water, and that water migrates along the interface such that hydrated oxides are formed on the surface of the metal. For example Al2O3 will hydrate to form AL2O3.2H2O. For that reaction to occur, the weak chemical bonds between the adhesive and the oxides formed immediately after abrasion must dissociate and that results in the failure of the interfacial layer, causing adhesion disbonding which is characterised by the absence of fracture of the adhesive with the failure propagation mainly at the surface of the metal.

So, how do we stop this? We treat the metal surface IMMEDIATELY after grit blasting with chemicals that prevent the hydration of the oxide layer in later service. When we implemented this process for aircraft repairs at one aircraft base, we reduced the bond failure rate from 40% to 0.06% over a study period of 17 years, and even then by using adhesive bond failure forensics, we could tell where the technicians took short cuts that caused these failures. There are paers on this at my web site.

Happy to talk further.

Max

Adhesion Associates
Australia

 

[AlexEss]

Max Good Morning
Thank you for such a concise and highly informative reply, and also for taking the time to respond.

Regards

Alex

 

[GrahamBennett]

Alex is correct. Your response, Max, was highly informative and I learned a lot from it. For that reason, I awarded you a Little Pink Star.

http://www.over50schat.com

 

[blakmax]

I forgot to mention that when it comes to "cleaning" a surface, please do not use detergents. Detergents function by wetting the surface better than the contamination. Unfortunately then it is almost impossible to remove the detergent residue and that inhibits chemical reactions with the surface.

Regards

Blakmax

 

[AlexEss]

Max Hi

Thanks once again for the information.

Our components are comparatively small ( 10mm - 40mm diameter )
Brass and Aluminium are the most unpredictable, sometimes achieving good bond - then sometimes not. A further factor is that the local climate is hot and humid - c. 40oC in summer, and is a maritime environment.

Parts that have failed to bond are soaked in MEK to remove the previously applied bonding agent, and then re-coated...again, this can have an unpredictable outcome.

I would appreciate any thoughts.

Thanks and regards

Alex

 

[btrueblood]

Suggest you take blakmax up on his offer to talk further (google adhesion associates australia). You could also click on his user handle and find earlier posts by this user, in which more detailed information is given for specific cases - i.e. how to find suitable bonding primers for aluminum. Which may or may not apply to brass and stainless, dunno.

 

[TugboatEng]

Have a look into silanes as an adhesion promoter or coupling agent. These look like they are effective at preventing the hydrate formation blackmax mentioned.

 

[AlexEss]

Thanks to all for your help and comments

Alex