Written By: Jason Gillen, Technical Director
Common modified asphalt pressure sensitive adhesive (PSA) tapes employed in building envelope and construction applications are comprised of polymer modified asphalts and typically incorporate either a random styrene butadiene polymer (SBR) or a styrene-butadiene-styrene (SBS) block co- polymer chemistry. While these polymers can be effective at modifying the base asphalt to impart the necessary physical properties for the composition to function as an adhesive they do have their limitations and weaknesses. Aging and weathering performance is one of these weaknesses when compared to other available co-polymer and butyl based PSA tapes. Asphalt based adhesive tapes are particularly prone to property loss through this type of degradation due to the fact that both the asphalt base as well as the unsaturated polymer used to modify it will contribute to the formation of oxidation products and the coupling of polymeric radicals.
SBR and SBS polymers owe their strength and elasticity to their block copolymer structure and design. The hard, glassy styrene end-blocks and their soft, rubbery butadiene mid-blocks have solubility parameters that are different enough to create a phase separation that drives much of their physical properties. In the polymerization processes used to manufacture SBS block co-polymers the polymer chains are left with vulnerable C=C bonds as they are unsaturated polymers. These C=C bonds commonly referred to as unsaturation in the molecule, make these polymers susceptible to various environmental agents such as ozone, UV light and other thermo-oxidative agents that can compromise their physical properties as they naturally age.
Typical manifestations of polymer degradation in adhesives based on asphalt that has been modified with a SB or SBR or SBS block copolymer are increased hardness or a resulting decrease in the penetration. This increase in hardness is actually an increase in modulus of the system and is typically accompanied by a decrease in tack aggression. The predominant degradation kinetic at work in SBS polymers is a cross-linking of the butadiene segments. This diene cross-linking can create an insoluble gel content that severely diminishes the adhesive properties and cause failure in the bond line. This cross-linking can also increase the softening point of the adhesive as well as increase the high molecular weight species of the adhesives. The elongation and elastic recovery of these degraded systems is negatively affected making the adhesive system more prone to failure under cyclical dimensional changes in the substrate from repeated thermal swings. The increase in hardness or modulus can create poor “wetting” out of the tape to the intended substrate. These effects taken as a whole typically equate to a compromised adhesive that has lower room temperature tack, is physically harder and has substantially decreased cold temperature performance.
Edge Adhesives has long been aware of the poor ageing and weathering of the SBS chemistries and found the hydrogenation process that converts the butadiene mid-block to ethylene-butylene, substantially improves upon these particular performance attributes creating an SEBS polymer. Our SEBS (styrene-ethylene/butylene-styrene) based tapes have essentially none of the vulnerable double bonds remaining with residual unsaturation levels below 3%. The hydrogenation process not only creates a much more thermally stable polymer it also tends to yield increased mechanical properties of our PSA tapes and increase the ageing and weathering properties as well when used in building envelope.
Look for Edge Adhesives and Plioseal to publish ad expanded case study on aging and its effects on the performance of Window and Door flashing tapes coming Fall 2017. Ask your sales associate for more information or call 1-866-232-2026 to find your local representative.