Amorphous polyolefin hot melts (APO/APAO) are one of the most popular hot melt adhesives. APOs bond well to nonpolar substrates like polyethylene and polypropylene but are usually not recommended for polar surfaces. They also have good barrier properties, i.e. low moisture and water vapor permeability, and excellent chemical resistance against polar solvents and solutions including acids, bases, esters, and alcohols but only moderate heat resistance and poor chemical resistance against nonpolar solvents like alkanes, ethers, and oils.

APO’s can be formulated with a range of melt viscosities, hardness, softening points, surface tackiness, and open times.

When compared with EVA and polyamide hot melt adhesives, polyolefins have extended open times for positioning of parts. They also have lower melt viscosity, and slower set times than comparable EVAs. Some APOs can be used without any additives, but often they are compounded with tackifiers, waxes, and plasticizers (mineral oil, poly-butene oil). They are compatible with many nonpolar solvents, and hot mold additives. Common APOs include amorphous (atactic) propylene (APP), amorphous propylene-ethylene (APE), amorphous propylene-butylene (APB), amorphous propylene-hexylene (APH), amorphous propylene-ethylene-butylene. These APOs have different hardness and softening points, which decrease in the following order: APP > APE > APB > APH, in accordance with decreasing crystallinity. All APO’s have low energies of cohesion and low entanglement weights. The polymer chains are rather flexible which provides good interdiffusion and entanglement across the interface between the APOs and the low surface energy substrates. Under mechanical load, most of the strain is dissipated by deformation and disentanglement of the polymer chains. Cohesive failure with high peel energies is therefore the typical failure mode of APOs.

The most common APO is polypropylene. It has a service temperatures from -30°C to 110°C (-20 to 230°F).