Rubber bonding – but the type of rubber is unknown…
Often people ask me what I do for a living. When I tell them I work for an engineering adhesives company, more times than not they stare at me with a vacant look and say “what’s that?, to which I respond “you know, glue, sticks stuff together…” “oh yes, glue…” then they quickly change the conversation. So when someone needs advice on rubber bonding, given the general lack of knowledge, excitement, and interest in glue, they don’t know where to start with adhesive selection. Then throw into the mix that they are not a rubber / materials expert either and they are really stymied.
Types of rubber
To be honest, I am not a rubber expert and would struggle to identify one type of rubber from another as well, but here are some basic types:
NITRILE RUBBER – this is very common and can often be found in applications such as hoses, o-rings, gaskets, conveyor belts, cable jacketing, and print rollers.
BUTYL RUBBER – this is very flexible and is used for things like linings, inner tubes, seals and stoppers, and valve seating.
POLYURETHANE RUBBER – often used for things like molds and modeling.
NATURAL RUBBER – often used for mountings, carpet backing, gaskets, and seals.
SILICONE RUBBER – this material has excellent high-temperature resistance so is a popular choice for o-rings, gaskets, cookware, ovenware, medical devices, and prosthetics.
EPDM RUBBER – Automotive hoses, seals, etc.
There are a whole heap of other rubbers, such as SBR, latex, chloroprene, etc… too many to mention.
Before attempting rubber bonding, it is advisable to carry out a solvent degrease. Surfaces may have mold release, slip additives, or other processing lubricants on them, so a quick wipe with isopropanol is a good idea. Acetone may be too aggressive as certain types of rubber are vulnerable to attack. Even once you’ve wiped clean the surface, rubbers can contain plasticizers which can work back to the surface again over time and can cause debonding at a later date (although not all adhesives are affected by this so don’t panic!).
Cyanoacrylate instant adhesive is generally your best bet for rubber bonding; epoxies are not usually recommended – rubber is easily peeled off. Cyanoacrylate adhesive cures in seconds so you can find out pretty quickly whether it is going to work or not! With cyanoacrylate, less is more so only use a very tiny drop and make sure components are squeezed together tightly. If the joint falls apart after ample curing time, it indicates you may have a more difficult to bond rubber (such as EPDM or natural rubber) or could be dealing with silicone rubber.
For EPDM and natural rubber bonding, an easy fix is to just use a specialist cyanoacrylate for difficult rubbers such as Permabond 105. This negates the need to use a primer. For silicone, it would be necessary to use a primer such as Permabond POP in combination with 105 or, if something more flexible is desired, use POP and Permabond 2050 cyanoacrylate. 2050 is very useful, especially for bonding soft o-rings and gaskets, as it maintains its flexibility in the joint so it is hard to detect the joint (normally the adhesive can feel a little bit “crispy” in the joint, but the 2050 keeps the seal uniform).
It can be very restrictive to consider cyanoacrylate adhesives as your only option for rubber bonding; this isn’t necessarily the case. The pros are obviously the good adhesion (typically to the point that if you try to pull the bond apart the rubber will tear) and the fast cure speed make cyanoacrylates ideal for rapid assembly processes on production lines, but there are also drawbacks. Cyanoacrylates do not allow time for re-aligning joints due to curing in seconds. Also, they offer limited gap fill (maximum 0.5mm) and can’t be spread over large areas (they will bond the spreader or roller). Their pungent smell can be off-putting. Contact adhesives can be used instead, especially offering a practical solution for large areas (but again could be a bit whiffy!), but they can be difficult to re-align too. Solvent based rubber bonders can also be considered and offer a low-cost method of joining.
Silicone-based adhesives can be used to bond silicone rubber, but some users find them messy and slow to cure so not practical for small item assembly on a rapid production line.
Permabond TA4605 and TA4610 are two part structural acrylics for bonding difficult plastics such as polypropylene, polyethylene, and PTFE that have also shown to have good adhesion to certain rubbers such as EPDM. EPDM rubber contains polypropylene, so it is quite understandable that these adhesives can be used to bond this rubber. Structural acrylic adhesives offer a slower cure than cyanoacrylates (so accurate alignment and spreading is possible). The odour is less pungent than many adhesives, and they are not solvent based. TA4605 and TA4610 have excellent environmental durability so can be used in applications which are submerged in water.
For further help and advice, please contact Permabond.