Hot Melt Adhesive Model: Interfacial Adhesion and Polymer/Tackifier/Wax Interactions

This work continues our study of the hot melt adhesive (HMA) model published earlier 1]. This HMA model was developed based on the pressure sensitive adhesive (PSA) tack model established previously 2]:

P = P₀BD (1)

where P is the adhesive bond strength, P₀ is the interfacial (intrinsic) adhesion term, B is the bonding term and D is the debonding term. The previous paper 1] describes the B and D terms in detail. However, only a brief discussion of the P₀ term was given. The present paper will provide a more in-depth but still rather qualitative study of the P₀ term within the framework of the adhesion model described in Eq. (1). HMAs studied are ethylene/vinyl acetate copolymer (EVA)/tackifier/wax blends. Substrates studied are untreated and corona-discharge-treated polyolefins such as polypropylene (PP) and polyethylene (PE). First, it has been found that the tackifier surface tension could be roughly correlated with one of its thermodynamic parameters: the solubility parameter dispersion component. Secondly, except for EVA/tackifier binary blends, the compatibility of any two of these three components, the EVA polymer, the tackifier and the wax, in a HMA can be estimated from surface tension and X-ray photoelectron spectroscopy (XPS) measurements. Thirdly, based on the study of the EVA/mixed aliphatic-aromatic tackifier/wax model HMA system, it has been observed that the HMA/polyolefin substrate interfacial composition depends on the wax/substrate compatibility. The cause of an inferior peel strength of a HMA containing a high wax content to a polyolefin substrate is possibly due to the formation of a weak boundary layer (WBL) of wax at the interface and/or low dissipative properties of the HMA.

Also, the relationship between EVA/tackifier/wax interactions and HMA peel strength will be discussed. A correlation between the EVA/tackifier compatibility measured by cloud point and viscoelastic experiments to the debonding term, D, in Eq. (1) has been found.