US patent 5874143 teaches a method to make PSAs for use on low energy surfaces.
The PSA adhesives includes: acrylic copolymer, tackifier, plasticizer, cross-linker, and other suitable additives. Acrylic copolymers include acrylates with long alkyl chain and acrylates with polar groups, e.g. acid, hydroxyl group. The first group acrylates have low Tg, low polarity with good adhesion with low energy surfaces. The polar acrylate segments deliver stronger cohesion and shear strength. The following figure gives examples for the acrylate copolymers.
Incorporation of compatible tackifiers into the PSA system can reduce the storage modulus and improve the bonding strength to low energy surfaces, e.g. polyolefin substrates. The tackifiers are rosin esters, terpene phenolic resin or hydrocarbon resin. Although the use of a tackifier dramatically improves bond strength as measured by peel force at low speeds ,inclusion of these tackifiers raises the glass transition temperature (Tg) of the PSA which results in reduced low temperature performances and also causes a’shock’ or ‘Zippy’ peel characteristic at faster peel rates. Increasing the amount of tackifiers may result in loss of shear strength or cohesive force.
Examples of tackifiers are shown the following table.
Incorporation of compatible plasticizers also improves the bonding to low energy surfaces, through improving the wet-out or “quick Stick” to the surfaces. The addition of plasticizers reduces the PSA’s glass transition temperature, Tg, and offsets the effect caused by the tackifier. Typically incorporated at a level of less than about 10 parts plasticizer per 100 parts adhesive, the presence of the plasticizer has negligible effect on shear performances or cohesive strength of the adhesive. The examples of plasticizers are:
The patent also teaches a guidance to develop PSA for low energy surfaces:
- about 100 parts by weight of an acrylic copolymer, said acrylic copolymer comprising from about 70-98% by weight of one or more mono functional acrylates having non-tertiary alkyl groups with between 1 and 14 carbon atoms and from about 30-2% by weight of a polar monomer
- about 10-40 parts by weight of a tackifier;
- about 3–10 parts by weight of a plasticizer
- and optional a crosslinker