Hot Melt Adhesives: Application Methods And Variables To Consider
Author:Jojo View:457
Spray applications usually need higher temperatures due to the heat loss as the adhesive fiber travels from the nozzle to the substrate.
Other considerations to make include:
Adhesive heat stability
Substrate heat tolerance
Equipment limitations in terms of open time or compression time
Add-On (coat weight), the amount of adhesive applied to make a bond
An application that requires a higher coat weight means the adhesive will take more time to set up. Set up is the process of the hot melt losing its heat and solidifying. This is because more heat is present when more adhesive is applied to the substrate, and the heat must be dissipated for the adhesive to set up.
The amount of adhesive affects the primary and secondary substrate bond especially in bead extrusion applications. The more adhesive applied to the primary substrate, the more area it will cover when molten and compressed. Having more adhesive enables it to stay molten for longer and wet out better to the second substrate.
The coat weight also determines the surface area of the finished bond.
Compression, bringing the two substrates into physical contact by the use of pressure
Compression mainly affects the bond to the secondary substrate. It forces the hot melt adhesive to flow into the primary and secondary substrates. Compression time must be long enough to allow the adhesive enough time to develop the cohesive strength to overcome substrate memory or stress. Insufficient compression time results in an improper or inadequate bond.
To facilitate maximum wet-out and bond surface area, compression should take place as soon after adhesive application as possible.
Open Time, the time (seconds) measured from the point of adhesive application to the point of compression with the secondary substrate
Ideally, open time should be as short as possible to achieve maximum wet-out and penetration of the secondary substrate.
However, keep in mind these other considerations:
Set speed of the adhesive
Physical space on the line
Pressure sensitivity of the adhesive
PSAs generally have a longer effective open time.