Application of Adhesion Model for Developing Hot Melt Adhesives Bonded to Polyolefin Surfaces

We have found that the adhesive strength, P, of hot melt adhesives (HMAs) based on semi-crystalline polymers can be described by the following equation: 1 P = P 0 BD previously derived for tack of pressure sensitive adhesives (PSAs). 2,3 In this equation, P 0 is the interfacial adhesion between the adhesive and the substrate, B is the bonding term, and D is the debonding term. This adhesion model is then applied to the development of HMA formulations based on Exxon Chemical's olefinic polymers and Escorez® tackifiers, bonded to Escorene® polypropylene and polyethylene substrate surfaces. Two olefinic polymers, Escorene® EVA polymer (ethylene/vinyl acetate copolymer) and Exact TM polymer, will be used. EVA polymer is the industrial leader in HMA applications, whereas Exact TM polymer is a new family of linear ethylene copolymer products made using a new class of proprietary catalysts. Therefore, performance of HMAs based on EVA polymer and Exact TM polymer is compared in the context of the P 0 , B, and D terms of our adhesion model. The interfacial adhesion P 0 term of these HMAs is generally governed by the surface tensions of the three HMA components: polymer, tackifier and wax. The bonding term, B, depends on the bonding temperature exponentially, and increases with the bonding time to a power of 1/6-1/2 at short bonding time. During the HMA bonding process, the low melt viscosity of the HMA facilitates sufficient spreading on a microscopically rough substrate surface. Therefore, the bonding term B is independent of the bonding pressure. The debonding term, D, depends on adhesive bulk properties. It is related to viscoelastic loss tangent in the industrial peel frequency range when the HMA/substrate assembly exhibits an apparent interfacial failure (AIF) mode, but to the tensile draw ratio at break of the bulk adhesive when the HMA/substrate assembly exhibits a mixed cohesive/apparent interfacial failure (CF/AIF) mode. Overall, compared with HMAs based on EVA polymers, HMAs based on Exact TM polymers show higher viscosity and inferior tensile strength, but better bond strength to polyolefin surfaces, higher strain at break, and lower yield stress.