In terms of tensile strength the strongest adhesives are epoxy adhesives, specifically single component heat curable epoxy adhesives. Let me clarify – single component epoxy adhesives have the highest tensile strength often 35-41 N/mm² (5100– 6000 psi).
Tensile strength of the adhesive is simply how strong the cured adhesive is. It doesn’t say anything about how well they bond to various substrates. In order of tensile strength let’s review a few of the strongest adhesive types. As it turns out, these same adhesives listed below have the highest shear strength of metal to metals. Not bonding metals??…See below for the best adhesive for bonding plastic, and the best adhesive for bonding glass and the best adhesive for bonding composites or click here if you are looking to bond dissimilar materials.
Single component epoxies bond well to metals and offer a good alternative to welding or brazing. When bonding metal they offer high shear, impact and peel strength. And provide excellent resistance to chemicals and temperatures. Although specialty grades are available with lower temperature curing for heat vulnerable parts – they do not provide the best adhesion to many plastics and other materials. Shear strength on steel can reach 41 N/mm² (6000 psi). Note the surface treatment of the steel and gap can affect the ultimate strength.
Because they are so resistant to chemicals and heat, single component epoxy adhesives are difficult to de-bond. To debond, heat above 250°C to soften, and pry part apart while the adhesive is still hot.
Acrylic and methyl methacrylate structural adhesives also have very high tensile strength and also bond well to metals and plastics – even some of the very difficult to bond, low surface energy plastics. They are also very resistant to chemicals and temperature. To de-bond the cured adhesive, heat above 200°C to soften, and parts pry apart while the adhesive is still hot. Tensile strength varies by type and grade but is often over 28N/mm² (4000psi).
Two component epoxies are about the same tensile strength as many structural acrylic adhesives. Added advantages of two component epoxies are that they adhere well to many more substrates and cure at room temperature. They do bond many plastics and specialty grades perform exceedingly well on composites. To de-bond two component epoxies, heat parts to above 120°C to soften and pry apart while the adhesive is still hot. Tensile strength is approximately 20-30 N/mm² (2900-4400psi).
On all three types listed above, 1 and 2 component epoxies and acrylic structural adhesives, methylene chloride will have some effect de-bonding but will work on the exposed edges of the bond slowly.
Cyanoacrylate adhesives, especially pure methyl grades like Permabond 910 and impact modified grades like Permabond 731 have exceptional strength on steel 30 N/mm² (4400psi). They do not have the same chemical and temperature resistance. The epoxy adhesives and acrylic adhesives both provide better impact resistance than cyanoacrylates. The upside is cyanoacrylate may be easier to remove. To de-bond from skin – warm soapy water with a slight rolling motion of the bonded area works well. De-bonded parts in hot water or in most polar solvents. Or, use heat if the components can take it – to weaken a cyanoacrylate with heat, heat parts to 100 to 120°C.
Anaerobic adhesives and sealants are also ideal for metal (they are designed only for metal). Some might question why I didn’t list them first? They are strong, ideal for metal so in theory they could be listed higher – but for this article I’m talking about bonding. Although anaerobic adhesives do bond, they are generally used as threadlockers, retaining compounds, thread sealants and form in place gaskets. They come in a variety of strengths see (link to AA color blog) for how to debond.
That is a quick look at some of the best metal bonding adhesives. Contact Permabond for information on which product is best for your application.
To truly say which is the best adhesive for any application we need to know all the particulars. Including what is the environment, surface area, stress etc… But in general terms most glass to glass applications are very well met with UV curable adhesive. Providing the glass is somewhat UV transparent and you aren’t going to sink it to the bottom of the ocean for years or try to light it on fire – as I say, generally speaking. So why are UV curable adhesives so good for glass? Several reasons including stress absorption within the joint, invisible bonds make for pleasing aesthetic appearance.
De-bonding UV Curable adhesives is generally best done by allowing them to absorb water – hot water will work faster but check the technical data sheet for water absorption rates as some grades are quite water resistant. If you are able to get the adhesive to soften in water, de-bond before it dries out as strength will return.
Epoxies and structural acrylics also bond well to glass but the aesthetics are not as good and the process is not as fast. UV curable adhesives allow for cure on demand which is desirable in applications where proper alignment before curing is desired. Flexible adhesives including MS Polymers and modified epoxies can also be considered for shock absorbing applications or applications that require long term severe environmental resistance.
Plastic is a big word! It means so many different things. Although there are many adhesives that bond various plastics, looking at which specific plastic is used is generally needed to dial in the ideal adhesive. But let me try to answer…
UV Curable adhesives are again often desired for aesthetics and ideal process. They have excellent adhesion to many plastics. Keep in mind that many plastics are UV stabilized to prevent yellowing but plastic bonding UV adhesives often cure within the visible light range as well. At least one of the plastics must be light transmitting.
Cyanoacrylates bond most plastics, a primer may be needed for bonding polyolefin and silicone but cyanoacrylates have very high shear strength on most plastics. These instant adhesives offer quick process and good resistance to non polar solvents. Resistance to impact and polar solvents is not as good as UV curables.
Structural acrylics, especially methyl methacrylates provide very high strength on plastics. Substrate failure occurs on PPO, PVC, SMC, and HIPS. Specialty products for individual plastics such as polyolefins are best matched with specialty adhesives for those plastics like Permabond TA4610 and TA4605 both polyolefin bonders.
Methyl methacrylates, polyurethanes and epoxies all offer composite bonders with unique benefits. Permabond 2 component epoxies ET5428 and ET5429 offer extremely high on carbon fiber (up to 38 N/mm2 (5500psi). Permabond polyurethanes provide a faster set time and good color match to carbon fiber for applications like interior auto trim.
Well, that depends. Each substrate class listed above contains a variety of specific substrates.
Product recommendations for the strongest adhesive for stainless steel can be different from the strongest adhesive for aluminum. Similarly, surface preparation for stainless steel and aluminum are different. For information on each substrate, please get in touch with Permabond or visit materials bonded and select the substrate class (ex. metals, plastics, etc.) and scroll down to specific substrates (ex, Aluminum, Brass, etc.)
For further help with selecting the strongest adhesive, please contact Permabond.