Polypropylene is one of most versatile plastics used in our daily life. Two types of Polypropylenes are made by chemical manufacturers, i.e. iso-tactic polypropylene (IPP) and a-tactic polypropylene (APP). IPP is a tough, crystalline polymer, which is primarily used for manufacturing of film and molding. APP is a high softening point, soft, and slightly rubbery amorphous polymer. It is a minor by-product of IPP manufacture. Most APP is used as a modifier in asphalts for road pavement and roofing. It has also used in the hot melt industry as a primary polymer or a modifier for many years. Since the 1980s, the availability of this by-product has been reduced due to the high yield of produced IPP. To compensate for the shortage of APP, which has been widely consumed in hot melt applications, the tailor manufactured amorphous poly-olefins (APOs) are therefore developed and mass produced.
Various olefin copolymers are found in commercially available APOs. They are propylene homo-polymer – amorphous propylene (APP); co-polymer (two component) – amorphous propylene/ethylene (APE), amorphous propylene/butene (APB), amorphous propylene/hexene (APH); and ter-polymer (three components) – propylene/ethylene/butene. The order of hardness of various APOs is: APP > APE > APB > APH.
APOs alone provide relatively lower viscosities, 400 cps to 30,000 cps at 375°, and better low temperature adhesion than EVAs (Ethylene Vinyl-Acetate). APOs are non-polar and resistant to acid-base and organic solvent. They exhibit very high Ring and Ball softening points (ranging from 210° to 320°F) but their cohesive strengths are rather low and tend to creep at elevated temperatures.
Some APOs alone could be used as adhesives for certain applications, however, the modification of APOs with various tackifiers, plasticizers (e.g. poly-butene oil and mineral oil), and/or waxes, can greatly extend the usefulness of APOs in diverse application markets. Similar to EVA-based Hot Melt Adhesives, APO-based Hot Melt Adhesives are bonded to substrates while they are heated at elevated temperatures. These adhesives offer relatively long open time and slow set time compared to those of EVA-based HMAs.
Most chemists believe that the low polarity character of APO is the primary reason why APO-based hot melt adhesives can offer good adhesion to low surface energy materials such as PE and PP. In fact, the “like dissolve like” principle is good for chemical mixing, but is not applicable to adhesion behavior. On the contrary to the “like dissolve like” principle, materials with greater polarity differences are the key factors that create higher physical adsorption and result in better adhesion.
What is the major reason why non-polar APO-based hot melt adhesives can offer good adhesion on most substrates? APO-based hot melt adhesives exhibit relatively low cohesion or internal strength. Upon separation, when an external force is applied to the adhesive layer, it is first elongated; then, most applied forces are dissipated within molecular chains by means of disentanglements. Only minimally applied forces can actually reach the interface of the adhesive and substrates. As a result, cohesive failure is commonly observed for APO-based HMAs. The fracture energy resulted from a cohesive failure mode is often much higher than that determined from an adhesive failure mode.
In summary, APO-based HMAs are adhesives with long open time and slow set time. They are chemically inert, offer low cohesion, and can easily adhere to low surface energy materials.