Hot Melt Basics
Hot Melts are primarily used to bond substrates. They are applied when molten and, after cooling, a bond is formed. Hot melt has many alternative applications, such as sound deadening, oxidation protection, and even conveyor belt repair. Clearly it is a versatile product but let’s “stick” with bonding for now.
Chemistry of a Hot Melt Adhesive
Hot Melt adhesives are composed of a base polymer blended with resins, plasticizers, fillers, antioxidants, and UV inhibitors. The chemistry of the hot melt; the application method; and the substrate qualities determine the type of bond created.
- Base Polymer -Sets the parameters of the hot melt. It provides strength, chemical resistance, and application qualities. Most importantly, the base polymer also defines the viscosity range, the application temperature range, and the melt rate of the adhesive.
- Tackifier (Resins) – Tackifiers, a natural petroleum product, adjust the stickiness or tack of the surface of the adhesive. Up to 40% of the adhesive formulation can be made up of tackifiers and is usually the highest cost component of hot melt.
- Plasticizer (Processing Oils and Waxes) – Used to adjust viscosity and set-up times.
- Antioxidants – Used to provide extended pot life and reduced degradation by adding oxidation resistance
- Fillers – Used to further enhance viscosity, set time, shrinkage, thermal expansion, etc.
- UV inhibitors – Used to prevent hot melt breakdown in the presence of light.
How will the hot melt be loaded and how quickly must it melt are the two questions to consider when choosing a hot melt form. The more surface area, the faster hot melt will reach a molten state.
- Chubs (Bigger Pillows)
Common Application Methods
- Swirl Spray
- Rolling Coating
Common Hot Melt Terms
Hot Melt Adhesives – Solvent and VOC free formulations that are heated to a molten state and applied to join various substrates. Bonds are formed as the adhesive cools.
Recommended Application Temperature: The temperature range in which the adhesive will provide the best bonding performance and pot stability
Viscosity – How thick the adhesive is when melted at the recommended application temperature. The higher the viscosity, the thicker the adhesive. The thicker the adhesive the slower the melt rate and the more difficult it is to pump.
Open Time –How quick or slow the adhesive sets-up after being dispensed onto the substrate.
Wetting – Ability to form intermolecular bonds between the hot melt and the substrate. Applying a hot melt to a cold substrate will result in poor “wetting” and a poor bond.
Set time – Time required for the adhesive to cool and the bond to form between substrates
Green Strength – the strength of the bond while the adhesive is setting up.
Pot Life – How long the hot melt can remain melted in the melt tank before the adhesive begins to darken and degrade.
Char – The ultimate sign of adhesive degradation – the result of prolonged heat exposure and/or overheating of hot melt. How quickly a hot melt degrades and char begins to form, is based on the chemistry (pot life) and the care taken during the application process.
Surface Energy – Technical. Influences how well substrates accept different hot melt polymers