Are hot melt adhesives the best option for bonding materials?
While you might not have heard of the term ‘hot melt adhesives’ before, you will have certainly encountered them thousands of times every single day. This high versatile method of bonding two materials together is used in almost every aspect of day to day life. Here we try to explain a little more.
What are hot melt adhesives?
Also known as hot glue, hot melt adhesives are essentially a type of thermoplastic adhesive, where the glue is heated to just the right temperature to allow the bonding of two similar or different materials. The time it takes to make the bond, is very quick due to the rapid cooling of the adhesive, which can save a lot of time during a manufacturing process.
There is a huge range of types of hot melt adhesive, and different types can be used for different materials. They don’t contain solvents or water, which has huge benefits particularly in the speed at which they set. There is less need for a lengthy drying or curing time and they also do not lose and thickness during the solidifying process like solvent based adhesives.
They are very strong when applied correctly, and due to the solid state they are supplied in, can be easily handled and stored, whether for private or industrial use. They can be applied by extruding, rolling or spraying and the high viscosity makes them perfect for porous or permeable substrates and can be used on rubbers, ceramics, metals, plastic, glass and wood. They are one of the best options on the market for joining pieces or substrates that require the best resistance.
So how do they work?
how do hot melt adhesives work? The easy answer is – good science. By ensuring the adhesive is at just the right temperature, and the right liquidity, when a second material is applied the rapid cooling of the polymers during the compression of the two materials together gives the bond strength and also flexibility.
Hot glues will also contain resins, which gives the immediate hot-tack of the glue, and wax thins the adhesive so that application is easier. Finally anti-oxidants help to preserve the adhesive at high temperatures.
Getting the balance right for the materials being bonded together is quite precise, as is ensure the exact right temperature. If the glue is too hot or too cold, the bond will not form properly and could have weak spots that will fail later.
Some types of hot melt adhesive should only be used indoors, for example the common types of ethylene-vinyl acetate (EVA), acrylic, polyamide and polyolefin. Polyurethane however, can be used in external environments.
They have a huge range of uses
There are a wide variety of industrial processes taking advantage of the benefits of hot melt adhesives. While we will all have seen the use in crafts of the glue gun but it is not just limited to that.
For example hot melt adhesives are used extensively in printing presses for the production of books and magazines. Not only that, those freebies and samples that are glued in between the pages with a glue dot – also hot melt adhesive. It is very common for it to be used in display mounting, the creation of photograph albums and the manufacture of paper bags.
In fact companies dealing with all types of packaging, from farm product crates to corrugated packaging will use some form of hot glue in the manufacturing. Hot melt adhesives are non-toxic, do not degrade and don’t require special disposal and destruction, which makes them perfect in all types of food packaging and labeling of glass and plastics.
That they can be applied to all types of surfaces, and are resistant to environmental conditions such and moisture and temperature is vital to all these industries. To give an example in relation to labelling, starch based glue can be useful in the labelling of warm and hot products and will help prevent bubbles and blisters in the label. It is typically used for labelling glass. Where as casein based labelling and synthetic based labelling are more resistant to cold and moisture, and will likely have a colour such as beige or off-white.
Not just for bonding paper products
Hot melt adhesives are also widely used in the furniture and mattress industry, producing excellent results in the manufacture of plywood and laminates, the coating of frames and the mounting of furniture.
It is also great for the sticking of foams and coatings, and used in mattresses, bringing together the joints of the centre inside the springs and giving strength to the base, the frame and each layer of the upholstery.
And that is not all. Because the softness and viscosity is so variable, they can be easily used for temporary sticking materials, such as adhesive tape. EVA hot melt adhesives, due to their chemical composition give extraordinary grip and are removed easily to avoid unwanted residue, give good results when in contact with moisture, and can have an invisible finish. Great for wrapping those birthday presents.
Also great for other types of products that need sticking together, such as disposable nappies, sanitary towels and plasters.
When you think about all the things children get up to while wearing a nappy, running, sleeping crawling, it is incredibly important that the materials used keep them dry and comfortable.
It also needs to be non-toxic and odorless. Hot melt adhesives are perfect for this. And the hot melt adhesive isn’t just used for the side tapes, it is used in every piece, gluing the absorbent pad and all the elastics.
The outer part is usually covered with a polyethylene film to prevent water escaping, and laminated on another layer, all stuck down with hot melt adhesive. The side tapes also then need to be able to be reapplied several times to the front tape without losing stickiness or tearing the plastic film. It is quite a feat to bring all those elements and layers together in such a way that maintains both strength and flexibility.