Most hot melt pressure sensitive adhesives (HMPSAs) are based on Styrenic block copolymers (SBCs). SBCs consist of two consecutive domains: a plastic phase (styrene, S) and a rubber phase (di-ene). Typical di-ene phases are Isoprene (I), Butadiene (B), and their hydrogenated forms, Ethylene-Butylene (EB), and Ethylene-Propylene (EP).
Besides those differences in di-ene phases, SBCs are synthesized purposely into various ratios of styrene/di-ene (S/D) and di-blocks/tri-blocks; and melt flow rate (MFR, a test used to measure the flow properties or viscosity of thermoplastic high molecular weight polymers.) or melt index (MI). The typical ratio for S/D is in the range of 15/85 to 45/55. The ratio of di-block/tri-block is between 0/100 (a pure tri-block) and 80/20. The MFR varies from 3.0 to 100.
When SIS and SBS were initially developed in the early 1970, scientists tried to match their tensile strengths and elongations as those of Isoprene (IR) and Styrene Butadiene (SBR) rubbers. Those early-developed SBCs, e.g. 15% styrene and 20% di-block of a typical SIS, remain the most versatile and popular thermoplastic elastomers for use in the hot melt industry to date.
After SBCs were introduced to the hot melt industry – new applications were discovered and developed. Several new SBC grades with various combinations of S/D and di-block/tri-block are gradually developed in order to satisfy diverse market needs.
Tough SBC suppliers provide lots of technical data tables including chemical structures, physical properties, and typical adhesion performances based on reference formulations for their produced SBCs, very few of them can clearly explain why each individual grade of SBC offers its own unique adhesion performance. Two major questions are frequently asked by many adhesive formulators.
- What is the correlation between these molecular structures and adhesion performance?
- What molecular structure can affect processibility?
To address these two questions, again, one should have some background in viscoelasticity or rheology. We will discuss the correlation between molecular structures and adhesion performances more thoroughly in later articles. However, three important findings that answer the above questions are summarized as follow.
- S/D ratio: the higher the %S – the stronger, harder and greater cohesion of adhesive blend that can be observed by a higher storage modulus (G’) at a rubbery plateau zone.
- Di-block/tri-block ratio: the higher the di-block ratio – the better wetting of adhesive blend that is presented by higher Tan delta values at a rubbery plateau zone.
- MFR value: the smaller the MFR value – the higher the molecular weight and heat resistance of adhesive blend, but the higher the melt viscosity (which may affect processability). The magnitude of MFR can be easily distinguished by a flow point, where Tan delta equals one.
More tips related to the viscoelasticity of SBCs and various tackifiers blends, and their correlations to adhesion performances and processability, will be discussed in future articles.
For more information call or email Pierce Covert,
Glue Machinery Corporation