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There is a large array of adhesives available for whatever needs to be held together and many ways to make it happen. Considering adhesives are used in all aspects of life, from baby wipes to office supplies to medical tools, it makes sense that there are so many forms of adhesion across the board. According to Adhesives.org, an adhesive is defined as a non-metallic binder that acts via adhesion and cohesion. To classify adhesives, it can depend on their form (paste, liquid, etc.), their chemistries (polyimides, epoxies, etc.), their type (hot melt, contact, etc.) and their load carrying capability (structural, non-structural).
The first type of adhesive to cover is structural adhesives. These are strong adhesives, otherwise known as load-bearing and are useful for engineering applications. Consisting of epoxies, cyanoacrylates, and some acrylic adhesives — the product can undergo shock, vibration, and other destructive agents and remain bonded.
Next, there are pressure sensitive adhesives. These are normally purchased as tapes or labels and do not solidify to form a solid material. Bonds are formed by bringing the adhesive film in contact with the substrate and applying pressure. Pressure sensitive adhesives are typically used to temporarily hold objects in place during assembly.
There are thermosetting structural adhesives that are typically available in two-part forms and are mixed carefully with a limited time window. Thermosetting structural adhesives normally have a “pot life,” which is the amount of time from mixture that the adhesive is workable and able to make a satisfactory bond.
Reactive adhesives can be characterized by the formation of permanent bonds between substrates that create resistance against chemicals, moisture, and heat — think structural adhesives. Typically, a reactive adhesive is made of a monomer (resin) and an initiator. Due to the chemical bond, reactive adhesives have long-term durability and high bond strength. When choosing an adhesive, it is important to consider what the product will need to endure. Reactive adhesives cure to a material, as opposed to hot melt or solvent adhesives, which means they are more durable to the effects of extreme temperature and general environmental resistance. Epoxies are the most commonly used of all of the structural adhesives, says Composites World. Epoxies are common due to their range of forms, their high tensile strength, dimensional stability and their ability to adhere to many materials. They can also be room-temperature cured, or with heat (depending on the additives). Advances in additives and formulation have made epoxies more resistant to chemicals.
A reactive two-part adhesive works as follows: base resin + hardener/curing agent → plastic or rubber. It transforms into a thermoset polymer via a cross-linking process, says the MIT D-Lab. For a reactive one-part adhesive, it needs UV light, heat or moisture (it is a pre-mixed, two-part adhesive but the reaction needs an application of light, heat or moisture to begin).
Non-reactive adhesives do not have a permanent chemical bond, and instead bond through a physical change. Examples include: polyvinyl acetate, construction adhesive, hot glue and contact adhesive
An emulsion adhesive works as follows: adhesive + evaporative solvent → solvent evaporates and leaves the adhesive behind. As the solvent evaporates, it leaves the adhesive behind.
Another categorization of the many forms of adhesives is if they’re natural or synthetic. Historically, adhesives were made out of naturally-occurring components, but now it is common to find a combination of natural compounds and synthetic compounds when looking at adhesive formulations, says Thomas Net.
Natural adhesives are made from naturally occurring materials from the planet, like animal and plant matter, and occasionally, minerals. Animal-based adhesives are made from albumin, beeswax, casein and gelatin. Plant-based adhesives are made from components like dextrin, natural resins (gum arabic, balsam), oils, waxes, soybean protein and starch. Mineral-based adhesives can come from components of amber, paraffin, silicates and sulfur.
While these types of adhesives are generally affordable to manufacture and are still used for wood, paper, film and foil materials, most adhesives employed today belong to the newer synthetics category. Synthetic adhesives are developed from human-made polymers. While more expensive than natural adhesives, synthetic alternatives are more customizable and feature greater bond strengths.
Starting with the use of reactive adhesives, you can see a wide variety of applications of these formulations. Manufacturers, when deciding on an adhesive, consider tensile strength, tensile shear strength, dimensional stability, temperature resistance and the compatibility of materials.
For example, as previously discussed, epoxy is a common choice for reactive adhesives as it is the strongest structural adhesive. This makes it useful for an expansive stretch of industries, like aerospace, industrial, marine, electronics, transportation and many others. Meanwhile, urethanes and urethane acrylates have a highly elastic bond that lends itself to high elongation. So while it is not as resistant to temperatures as epoxy, it is often utilized in automobile and transportation applications. The elongation is useful in the manufacturing of car body parts because they are often made of various materials and move frequently in relation to each other. A highlight of acrylics is that they can bond to most surfaces with little to no preparation (they also have greater flexibility than epoxies). Acrylics are often used in marine and metal-bonding applications and are especially useful because they bond to tricky plastics and oily metals. A drawback, though, is that acrylics release a non-hazardous but potentially annoying sweet odor.
Non-reactive adhesives are typically used non-structurally, due to the lack of chemical bond, and include more household-friendly names like — polyvinyl acetate (white glue), hot glue and contact adhesive.
It is important to note that beyond the use of adhesives in a realm of industrial and electronics, they also play a vital role in the creation of consumer goods like paper products, furniture, clothing, ceramics and woodworking. Across the board, the type of adhesives being used in a project is a necessary consideration.
However, custom formulations of adhesives exist in the case that there is no off-the-shelf adhesive that perfectly matches your application requirements. If your go-to adhesive has been discontinued or no longer meets industry requirements, you can formulate an equivalent that functions even better than your previous adhesive.
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