What is a hot melt adhesive? | Adhesives manufacturing company

What is a hot melt adhesive?

Posted 29 Aug 2024 by Aaron Plumley, Technical Specialist Manager

At H.B. Fuller, hot melts are some of our most popular adhesives. The hot melt adhesive first surfaced in 1940 as an improvement to water-based adhesives that were failing in humid climates, but to say it has really taken off in the last few decades is an understatement. One common question we receive from our customers is 'what is a hot melt adhesive?' and this is the query we're going to address in more detail today.

What is a hot melt?

Hot melt adhesives (also known as hot glues) are generally 100% solid formulations based on thermoplastic polymers (no water or solvents are involved). Hot melts are sold in a solid state at room temperature and are 'activated' upon heating beyond their softening point which is usually between 50 and 160 degrees. Once melted (in a hot melt tank), the adhesive can then be applied onto a substrate in its liquified or molten state. The hot melt wets the substrate, penetrating the surface (and any cavities), and then it solidifies, ensuring cohesion. This setting/cooling process takes very little time. Glues that cool and remain tacky are known as pressure sensitive adhesives (PSA) - these we will cover in depth another time.

When a hot melt adhesive is being cooled back to room temperature, it rapidly builds up its internal strength. The solidified state of a hot melt is where it has structural integrity and can function as an adhesive. Hot glues can be applied onto a substrate in many ways (when they are in liquid form) such as via a roller or more commonly being jetted or sprayed from a nozzle or gun. There are also many different types of application patterns such as spiral sprays, dots and continuous lines.

Hot melt adhesives are used in both the commercial and residential sectors across many different industries for countless applications. You can find hot melts in food packaging, woodworking, product assembly, labeling, children's toys, arts and crafts and so forth. At H.B. Fuller, our adhesives bond all kinds of packaging, the spines of books, the layers of fabric in nonwovens, labels on bottles and the edging on furniture - to name just a few applications.

Hot melt adhesives require a delicate balance of formulation components but the main elements of a typical packaging hot melt are polymers (delivers strength and flexibility), resins (provides wetting and adhesion properties) and waxes (controls viscosity and open time/setting speed). With a hot melt pressure sensitive adhesive (HMPSA), there is typically the inclusion of a plasticizer which aids elasticity and controls viscosity.

There are various types of hot melt such as EVA (ethylene vinyl acetate) for general purpose bonding, polyolefin for difficult to bond plastics, and the more modern packaging hot melt - metallocene (mPO) for high strength in smaller quantities, but we will address the various hot melt formulations in a separate article.

How a hot melt differs?

Hot melt adhesives have surpassed the efficiencies of their predecessors in many ways, which is one of the reasons why they have become so popular today.

In addition to ease of use, and lower processing costs and regulatory requirements, one of the main reasons why a hot melt is the adhesive of choice in many markets is due to its unrivalled versatility.

Hot melt adhesives can be used to bond a great range of substrates from cardboard, metal, glass, leather and wood to ceramics, plastic, rubber, fabric and foam. The high viscosity of a hot melt makes it suitable for use on both non-porous and porous substrates which would otherwise be more difficult to bond with, say, a solvent-based adhesive.

Thanks to factors such as their excellent water and moisture resistance (they don't weaken when exposed to humidity), hot melt adhesives create tamper-resistant seals. Hot melts can therefore be used on hot, cold and wet surfaces and, because they're thermoplastic, they can be repeatedly heated to melt and cooled to solidify. There's no one-time usage with a hot melt, which is often the case with other adhesives.

Unlike solvent-based glues, hot melts do not lose their thickness when they solidify. Being 100% solid formulations, hot melts shrink minimally when being cooled which is why they are excellent at filling gaps. In comparison, solvent-based adhesives rely on the extraction of a carrier to set, often resulting in a 50-70% reduction of their applied weight.

Hot melts are favored by many customers because they can cool down and dry almost instantly (unlike water-based and solvent-based adhesives), providing an ultra-fast method of bonding. This gives manufacturers the option of speeding up their production lines, making the entire process more efficient - perhaps one of the biggest hot melt selling points and why they are commonly used in FMCG (fast moving consumer goods) packaging applications. Furthermore, hot melt tanks can easily be turned off and on, or left on standby, without the need to completely clean down or flush the system - another manufacturing win.

Benefits of a hot melt adhesive

Unlike other adhesives, manufacturers tend to find that hot melts are not only very efficient at bonding but dependable and consistent products. Hot melts do not taint substrates and can be used across a wide range of both slow and fast machines, during all seasons. Their dispensing is easy to automate and hot melts offer a very consistent application with a better glue line and coat weight than other adhesives.

Hot glues are considered a cost-effective choice due to factors such as better mileage, lower maintenance costs, lower downtime and lower inventory costs. They have a long shelf life and before being used are 100% solid, so are clean and easy to handle and easy to store.

Hot melts are known worldwide for their bond effectiveness, ease of use and clean-running properties (no stringing for example), but they also offer excellent thermal stability, so stay clear in the glue tank and do not char.

Hot melts do not contain solvents, so one of their main advantages is that they are considered non-hazardous and eco-friendly. Hot melt adhesives have no fumes, little smell, low toxicity levels, contain little to no VOCs (volatile organic compounds) and there is minimal waste produced in their manufacturing process.

Hot melt formulas are designed to suit a wide variety of applications and substrates therefore in circumstances where multiple adhesives are in use, it's often the case that these can be replaced with just one hot melt, reducing adhesive inventory costs.

The verdict

With every adhesive there is always challenges and hot melts too are not without their limitations. Temperature, chemical and UV resistance, along with operating parameters and the type of substrates to be bonded can all impact adhesive performance and longevity. This is why H.B. Fuller advocates finding the right adhesive for the job to maximize productivity and efficiency.

Learn more about Hot Melt Adhesive.

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