And it's no wonder. SBCs are synthesized via anionic polymerization, which allows a high degree of precision in engineering molecular architectures. So far, more than 600 patents in the United States and more than 3,200 patents in countries outside the United States have been issued for KRATON polymers, their manufacturing processes and their product applications. More than 80 of those are in the area of pressure sensitive adhesive (PSA) applications.
Two innovative crosslinking methods to be discussed here -- one with a chemical crosslinker, the other with radiation -- have been developed by KRATON Polymers to significantly increase the temperature resistance of PSAs, opening the door to much broader applications for PSAs including under-the-hood uses and other environments where resistance to high temperatures is crucial.
But because these crosslinks are physical rather than chemical, they are reversible. When the polymer is heated to above the glass transition temperature (Tg) of 90?C of the polystyrene end-blocks, the crosslinks are released, and the polymer will flow like a thermoplastic. When the polymer is dissolved in a solvent, it can be processed as a relatively high-solids-content solution. When the polymer cools after melt processing, or the solvent evaporates from a solvent cast film, the polystyrene end-blocks regain their integrity, and the polymer becomes strong and elastic again.
SBCs that have a hydrogenated polybutadiene (SEBS) or hydrogenated polyisoprene (SEPS) midblock, are much more difficult to crosslink than their non-hydrogenated analogs because they are much less reactive. SEBS polymers to which a small amount (typically 1%w to 5%w) of maleic anhydride (MA) has been grafted are commercially available. This acid/anhydride functionality can be used for crosslinking reactions. For curing reactions that will be accomplished in a bake system, these SEBS/MA polymers can be cured with a variety of crosslinkers such as melamine resins, epoxy resins or blocked isocyanates. But the use of AlAcAc, a metal chelate, as a crosslinker for ambient temperature cure of SEBS/MA polymers has enhanced high-temperature performance in PSAs, sealants, modified waxes, modified asphalt and oil gels.
The desired effects of crosslinking are to increase upper service temperature and to improve solvent resistance of the adhesive. Standard industry tests for various applications have been administered, with the following results:
The need for higher temperature resistance translates to a higher crosslink density of the polymer network. This can be achieved either by using a crosslinker, noted above, or radiation-initiated crosslinking. The latter has the benefit of good process control.
Initially, research focused on linear SIS polymers. Radiation exposure of polyisoprene, however, leads to two competing reaction paths: crosslinking and chain cleavage. In addition, both electron beam (EB) and ultraviolet (UV) light curing required nitrogen blanketing and higher energy doses. In the end, it was hard to find the appropriate balance between cohesive strength and tack.
The next step for radiation-curable styrenic block copolymers3 was a branched-structure polymer that compensated for the competing chain cleavage in the radiation-crosslinking step. Some chain cleavage led to increases in tack while maintaining the mechanical integrity of the polymer. As a result, a good balance of adhesive properties was achieved, but nitrogen blanketing was still required since the crosslinking was inhibited by oxygen, leading to lower productivity.
One innovative step was the move from isoprene rubber midblocks to butadiene midblock-based materials. This SBS (see figure) has the inherent benefit of a butadiene rubber. During irradiation, the crosslinking reaction dominates. Also, the adhesive properties are well-balanced, no nitrogen blanketing is necessary and productivity is high. A UV line for example can be run at 200 m/min with acceptable coating weights. In other words, productivity is not compromised by process restrictions requiring lower coating weights or longer irradiation times.
Most polymer technologies can be processed on existing commercial hot melt lines fitted with an extra irradiation unit. In most cases, conventional hot melt production can be run on the same line. So, high-performance formulations requiring irradiation may be produced on demand with minimal capital investment. The implication for the coater and formulator is striking. They can make use of existing technology and formulation know-how to penetrate market segments not available to them prior to such advancements.
Every processing technology has its merits. Waterborne PSAs are usually economical to purchase and commonly employed in packaging tape and label applications. Solventborne PSAs are based on a mature technology stemming from natural rubber systems and now include high-performance acrylates. Today, the high end of the tape market is dominated by the high-performance acrylates. Hot melt PSAs usually have low processing costs and the lowest emissions of all adhesive systems.
Generally, hot melt adhesives do not require a carrier solvent, so transportation costs are reduced by a factor of at least two. It requires less (often no more than 10 percent) plant space than required by solventborne and waterborne technologies. Investment is simply limited to a coating line. Because emissions are usually low, no further environmental investments need be made, such as water-treatment units. Because solvent removal is not required, the productivity and throughput of hot melt processing is superior to either of the other processes. Typically, hot melt units produce twice the amount of finished product with the same number of personnel normally required in solventborne or waterborne units, which doubles productivity.
Each crosslinking technology provides specific advantages to the processor. Acid/anhydride-functional SEBS polymers can be chemically crosslinked with AlAcAc, converting the polymer from a thermoplastic to a thermoset. While crosslinking SBCs using a crosslinker has potential applications in solvent-based PSA, hot melt PSA, sealants, modified waxes and other applications that lend themselves to high-temperature uses, crosslinking through radiation curing is cost-efficient and provides increased process control when curing thin films.
Additional information on SBS crosslinkable polymers is available from Kraton Polymers, PO Box 61070, Houston, TX 77208-1070; phone toll-free 800-4KRATON or 832-204-5400; fax 832-204-5460; e-mail info@KRATON.com; or visit www.KRATON.com. Or Circle No. 66.