Since their introduction in the late 1960s Styrenic Block Co-polymers (SBCs) have had a significant impact on many articles from Adhesives & Asphalt to Rubber & Plastics. The wide range of characteristics that are available in SBCs are very appealing; Flexibility & strength, chemical resistance and moldability are just a few prized properties.
Frequently one needs to modify the properties of the SBCs to achieve a desired level of performance and for that Hydrocarbon Resins are well-known for their ability to affect adhesion, tensile strength and other properties. Hydrocarbon resins are available in a range of chemical compositions that are completely aliphatic to completely aromatic and everything in between.
The uniqueness of SBCs is their dual-phase nature consisting of aromatic Styrenic Domains (End-blocks) that function as physical crosslinks that dramatically increase the effective molecular weight of the polymer, and the Rubbery Phase (Mid-block) which can provide adhesion, tack or flexibility. In order to change or enhance these properties with hydrocarbon resin additives it is extremely important to understand the chemistry of the resin and the domain that you want to affect.
Hydrocarbon resins that are either completely aliphatic (C5) (like Wingtack 98) or near-completely aliphatic (C5/C9) (like Wingtack ET) are the best choice for compatibility with the mid-block portion of a SBC. The use of aliphatic resins at low levels can dramatically increase the % Elongation of a SBC while high levels of aliphatic resin can cause the SBC to be pressure sensitive, that-is, have tack and provide auto-adhesion by just pressing on the adhesive. As one moves to higher aromatic-containing C5 resins the resins do have an impact on the styrene domains and can reduce the effectiveness of the physical crosslinking of the Styrenic domains.
To enhance the performance of the Styrenic Domains of a SBC use a highly Aromatic (C9) resins. A pure Aromatic Monomer resin with a Tg lower than 100 °C (like Cleartack) will soften the end-blocks slightly; this will also act as a processing aid and reduce the viscosity of the polymer. Choosing an aromatic resin with a Tg close to or above the Tg of the Styrenic Domains (like Cleartack W140) will reinforce the domains resulting in significantly increased Tensile Strength.