Selection of Mineral Oils for Hot Melt Pressure Sensitive Adhesives

Selection of an adequate mineral oil for a Styrenic Block Copolymer (SBC)-based adhesive is very important. Although most mineral oils look alike, they impart very different compatibilities to the SBC used and will greatly affect both adhesion and aging performances of Hot Melt Pressure Sensitive Adhesives (HMPSAs).

Mineral oils are a necessary component for most SBC-based HMPSAs. Below are some of the major advantages contributed by mineral oils.

  1. Significantly reduce the viscosity for easy mixing and processing.
  2. Lower the hardness of adhesive for better deformation and wetting.
  3. Decrease glass transition temperature (Tg) to improve low temperature resistance.
  4. Lower the total raw material cost of HMPSAs. Among all used ingredients in most SBC-based HMPSAs, mineral oil is normally the cheapest raw material

Mineral oils are extremely complicated mixtures including various ratios (contents) of the following major hydrocarbon species: aromatic (unsaturated rings), naphthetic (saturated cyclic rings) and paraffinic (linear) components. The content of each oil species, Ca (aromatic), Cn (naphthenic), and Cp (paraffinic) is normally stated on the technical data sheet (TDS) of mineral oils. These mineral oils are different in polarity or solubility parameter and will exhibit different compatibilities to SBCs and tackifiers.

Aromatic oil is very compatible to polystyrene domain of SBC. It will soften the physical cross-linked domain and greatly lower the cohesion and heat resistance of the formulated adhesive. Paraffinic oil is very non-polar and less compatible with both end (poly styrene) and mid block (rubber) of SBC. It tends to migrate out from adhesive matrix due to its lower compatibility to most SBCs. Naphthenic oil is a cyclic and saturated low molecular weight hydrocarbon. It offers excellent heat stability and is the most compatible oil for use in most SBCs. Naphthenic oil is normally more expensive than others due to the limited sources of this type of oil. No pure Naththenic oil is available on the earth. When the content of Naththenic (Cn) oil is more than 40% in the mixture; it is categorized as Naphthenic oil. As Cn is greater than 50%, it is considered as highly purified Naphthenic oil.

The solubility parameters,ä, of those mid blocks of SBCs are 7.9, 8.1, and 8.4 for Ethylene/Butylene (EB), Isoprene (I), and Butadiene (B) respectively. And the ä for most paraffinic and naphthenic oils range from 6.5 to 7.7. The value increases as Cn raises; and the compatibility with the mid block of SBC improves. Empirically, when the difference of ä for mid block and oil is less than 0.5, they are more compatible. On the other hand, when the difference is greater than 0.5, they are less compatible. Accordingly, it is easier to form a compatible system with SEBS, SIS and both paraffinic and naphthenic oils. Surprisingly, almost none of these commercially available mineral oils are actually compatible with Butadiene mid block. SBS and these oils are simply miscible instead of compatible. For such less or incompatible blend, the included mineral oil will eventually migrate out from rubber phase and change adhesion performance.

For more information call or email Pierce Covert,
Glue Machinery Corporation


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