Hot Melt Adhesives with Multiple Reaction Mechanisms

Tag : acrylate adhesive

Today's editorial takes a look at advanced hot melt adhesivesystems. These adhesives supply value by reacting in different -and multiple - ways.
One future trend that can be foreseen with hot melt adhesives isdual reaction mechanisms. These adhesives go a step beyondconventional hot melt adhesives. In addition to melting, cooling,and bonding, they provide added value to the end-user through theuse of piggybacking reactions.
The earliest and most commercial forms are reactive (moistursecuring) hot melt adhesives. A reactive hot melt adhesive is anythermoplastic sealant that can be applied at elevated temperaturesas a liquid melt, cools to become a solid at room temperature, andthen subsequently reacts to become a thermosetting polymer withenhanced physical properties.
For certain applications requiring unusually high bond strength andfast set time, reactive polyurethane hot melt sealants have beendeveloped. , More recently reactive silicone hot melt adhesives andsealants have also been introduced. These systems are applied likea hot melt, and the parts can be rapidly assembled within seconds.As a hot melt, this applied formulation has a high degree ofinitial handling strength. The unique feature for these materials,however, is that they will continue to cure and crosslink byreacting with the moisture in the air so that when completely curedthey have the strength and durability typical of polyurethane orsilicone thermosetting sealants.
Another advantage of these moisture reactive hot melts overconventional hot melts is that the melt temperature is much lower.This allows application of the molten sealant to heat sensitivesubstrates. Melt temperature of conventional ethylene vinyl acetate(EVA), polyamide, or polyester hot melt assembly adhesives isgenerally well over 175°C; whereas, the melt temperature forpolyurethane or silicone reactive hot melt systems is no greaterthan 120-150°C.
Other novel dual cure reaction mechanisms have been commercialized.UV curable hot melt acrylate adhesives are already being used. ,These systems are generally used for pressure sensitive tape andlabel production. This adhesive includes free radical reactivedouble bonds and a photoinitiator in the adhesive formulation. TheUV curable acrylic resin is processed with standard hot meltequipment that can be used for SIS/SBS hot melts; the only specialequipment necessary is the UV lamps for crosslinking. To coat thehot melt on a substrate, the adhesive has to be heated to100-135°C. In this temperature range the adhesive becomesliquid enough to be coatable with common systems. Standard mercury,medium pressure UV bulbs, common to other UV applications, aretypically used as the energy source.
Another recent development is water-swellable hot melts. When theycome into contact with humidity, the hot melt swells rapidly andprovides an expanding sealant. These hot melt adhesives are usedprimarily for the construction of communication cables. Besidestheir bonding function, they also seal the cables and protect themfrom water penetration.
Today, hot melt adhesive technology certainly has reached a hightechnical level. However, the future appears to be even brighterand more exotic. By using multiple reaction mechanisms, "tandemadhesives" will provide properties unrealized by today's end-users.
Almost any fast curing adhesive can be used for wire tacking.However, UV curable adhesives and hot melts have achieved thegreatest commercial success. Both types are one component, offerfast set, and provide the capability of being applied accurately todiscrete areas of the board. Temperature and solvent resistance ofthe hot melt adhesives may be a concern in certain applicationsunless they are crosslinked via UV or additional moisture curechemistry.
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