Georges N. Altounian
Hercules Incorporated
In conventional hot melt adhesives, the customer buys the polymer, the tackifier, the wax and a selection of additives (antioxidants, etc.), mixes the ingredients in-house and produces the final adhesive. In polyurethane reactive hot melt adhesives (RHMA), the customer produces their own polymer backbone by prepolymerizing crystalline bifunctional polyols (3000 to 5000 molecular weight) with diisocyanates. This allows the customer to produce a more suitable adhesive for a specific application. Hence, there is almost no polymer supplier for this particular technology. The major suppliers are polyol and isocyanate manufacturers.
The market expects that Polyurethane RHMAs provide the following characteristics:
High temperature performance,
Adhesion and cohesion strength on many substrates (leather, synthetic leather, PVC, PE, metal, wood, polyester, textile, paper, EPDM etc.),
Compatibility within the reactive polyol families of similar molecular weights,
Versatility in green strength build up,
Low application viscosity @ 121 C,
Good chemical resistance
Good shelf life stability, and
Versatility in open time to allow the adequate time for processing complex and large products.
To meet the above criteria, we have modified conventional polyurethane reactive hot melt adhesive with high Tg, semi-crystalline ester polyfunctional alcohol modifiers. Those modifiers become an integral part of the polyurethane backbone. They either maintain or improve the original adhesion characteristics of the polyurethane reactive hot melt. The mechanical properties of the hot melt are improved also, as well as its open time and green strength properties. The semi-crystalline structure of the modifier avoids any considerable loss in green strength, because it maintains the overall crystallinity of the adhesive.
Influence of the Resin and Polymer Structures on the Performance and Handling of Polyurethane Reacti