Introduction To Hot Melt Adhesives

An adhesive is a material which is used to bond two substrate surfaces together, either as a permanent or non-permanent bond.

Hot-melt adhesives are basically a form of solvent free thermoplastics, applied in a molten state.

Hot melt adhesives are generally in a solid stable state at room temperature. At temperatures above approximately 60 degrees centigrade they will become a viscous fluid. In the temperature range 60 – 200 degrees centigrade hot melt adhesives are suitable for bonding. Upon cooling, the adhesive sets rapidly to create a bond. Most hot melt adhesives can be re-heated then bonded again if required.

The correctly selected hot melt adhesive can provide an effective, lasting and permanent bond.

Moving industry forward
Innovation, time and cost savings are paramount to industry, which is why hot melt adhesives are the technology of the future. With the advent of more specialised adhesives all industries are benefiting from hot melt adhesives

Hot melt vs. cold glue
Water and solvent based adhesives have a cure or dry time; solvent based adhesives may also require costly extraction systems. The rapid cooling and bonding strength of hot melt adhesives at room temperatures eliminates associated equipment costs. With hot melts areas previously given over to drying or storage curing are not required – saving valuable shop floor space.

It could be argued that the heating of hot melt adhesives is costly, but this is totally outweighed by the cost savings of drying ovens and bond speed from production to customer.

With the technological advances of hot melt adhesives and application systems in recent year’s production systems can now be upgraded to take advantage of cost effective solutions, making industry far more efficient and cost effective.

Hot melt application processes
Hot melt adhesive systems can be as small as a hand held gun using sticks, through a variety of reservoir sizes to bulk feeding drum unloading systems. All systems can be matched to the production requirements. When specifying a system it is paramount to consider melt and pump rates to ensure a continual supply of adhesive. Once melted to a liquid state the adhesive is pumped through heated hoses to an application head where the molten adhesive is applied to the substrate. Stable temperature control throughout the complete system is vital to achieve the desired result.

Slot die heads are a common applicator and take many forms. The principle with all slot dies is to apply a layer of controlled thickness adhesive onto a substrate. The adhesive is effectively extruded through a narrow aperture resembling a very small letterbox. Self adhesive tapes and labels are typically produced with slot dies. Advances in slot die technology have now enabled high speed intermittent clean patching, the main benefits of which are faster production speeds and the application of adhesive only where required leading to significant cost savings.

Jetting applicators are used mainly in packaging and assembly procedures. Jetting applicators also known as guns come in a diverse range of sizes and designs catering for a wide range of industries. Jetting applicators apply a continuous or intermittent bead of hot melt – an intermittent bead can be applied at high speed to create a stitching effect often replacing staples. The bead size is commonly controlled by pump speed/pressure and nozzle size. With the wide variety of hot melt adhesives available the open time (setting or cooling time) can assist with assembly projects, such as the spacing of filter pleats where the hot melt adhesive acts as both spacer and bonding material.

Spray application has many benefits such as cost saving of adhesive usage to less complicated mounting equipment. Swirl spray applicators use a specially designed nozzle to spin a bead of adhesive thus covering a greater area than a bead applicator but still creating an effective bond strength. Fiberisation is a process that appears to atomise the adhesive, but is using heated air to break fine beads of jetted adhesive to produce a random fibre pattern. The adhesive and air do not mix in the applicator – the process is achieved by special nozzle design, creating a wall of fine adhesive fibre streamed onto the substrate. Many high profile diaper or medical companies use the fiberisation process to achieve high speed, low adhesive production. It is often possible to create breathable products. Recent advances have led to high speed intermittent fiberisation with clean results, pushing production techniques forward in the mailing industry.

There are many ways to apply hot melt adhesives the above is a basic outline of a few common application methods. Here at Universal Adhesive Systems we strive to help our customers from project conception to production. Our laboratory facility is designed to find the correct and most efficient answer to your adhesive application methods.

The adhesive system industry is often forgotten but without ongoing designs and applications many everyday products would not exist, as nearly all packaging or assembly procedures rely on hot melt adhesives. Next time you unwrap or open some packaging or maybe use an assembled product, you could be experiencing something that hot melt adhesives have made possible.

Author: Tim Sharp