A NEW FAMILY OF STYRENIC BLOCK COPOLYMERS FOR ELASTOMERICFILMS IN PERSONAL CARE APPLICATIONS
　　苯乙烯弹性体新成员SIBS在个人护理用薄膜的应用
　　Andre J. Uzee, Michael O. Myers & Brian Witt Dexco Polymers LP, a Dow/ ExxonMobil Venture Houston, Texas, U.S.A
Abstract
　　Styrenic block copolymers are increasingly being used in the production of elastomeric films for personal care and hygiene applications. In recent years, with severe fluctuation on the supply side regarding availability and price for isoprene, butadiene, and styrene monomers, manufacturers and end-users of block copolymers have searched for ways to reduce their costs and maintain high levels of elastic performance. This paper introduces a new family of styrene-isoprene-butadiene-styrene (SIBS) quador tetra-block polymers to produce lower cost high performance products when compared to styreneisoprene–styrene (SIS) copolymers. These new SIBS quad-block polymers are compared to triblock polymers with mixed isoprene-butadiene S(I/B)S structures.
Introduction
　　In 1990, The Dow Chemical Co., a major resin producer and ExxonMobil Chemical, a major player in the adhesives industry, joined forces and formed Dexco Polymers LP, a Dow/ ExxonMobil Venture and introduced the first commercially available “sequentially” polymerized “new to the world” pure styrenic block copolymers (SBCs). Their products include styreneisoprene and styrene-butadiene block copolymers. Unlike other SBCs offered in the market place which are produced via traditional coupling technology, SBCs produced via “sequential” polymerization are pure block copolymers (e.g., contain less than one percent diblock). These pure triblock copolymers have found widespread use in adhesives and elastic films in personal care and hygiene applications.
　　In the past five years, the use of elastic films (e.g., less than 0.038 millimeter thick) in personal care and hygiene applications has grown significantly. In baby diapers and adult incontinent wear, it is estimated that well over thirty-five million pounds of styrenic block copolymers are consumed annually. With the increased use of elastic films in diapers, especially by the private label diaper manufacturers, the market for SBCs in diapers has grown considerably. There are many different types of SBCs used in the production of elastic films. The most commonly used are the non-hydrogenated SIS (styrene-isoprene-styrene) and SBS (styrene-butadiene-styrene) copolymers and the hydrogenated copolymers SEBS (styrene-ethylenebutylene-styrene) and SEPS (styrene-ethylene-propylenestyrene). Film manufacturers cite different reasons for their choice of SBC in elastic films. Some choose a product because it is the only one that is capable of meeting the performance needs for their specific application. In many cases however, more than one type of SBC can be used to meet the needs for a particular application.
　　In many cases, the equipment used by the film manufacturer or their past experiences with a specific family or type of SBC product, whether good or bad,
serves to guide them in selecting a particular SBC for a specific application. Other factors such as ease of processing, recyclability, handling, compounding requirements, etc. are often factored into the decision process. In most cases film manufacturers are driven by the overall cost/ performance model, or cost per square meter of the film. Any of these many factors can come into play when determining or selecting the best SBC for the application.
　　Unlike polyethylene, which is typically used directly out of the bag, SBCs are typically formulated/ blended with a thermoplastic polymer for ease of processing. The SBC or SBCs are typically the major component and account for 50 to 90 weight percent of the blend. The other major component is typically a much lower cost thermoplastic polymer such as polystyrene, polyethylene, polypropylene, or ethylene vinyl acetate. It is used at 10 to 40 weight percent. Optionally, a third component which can be added to such blends is a white mineral oil. It is typically lower in cost than the SBC and used at a level of 10 to 30 weight percent based on the SBC. With the addition of a thermoplastic and mineral oil, the overall cost of the compound is generally reduced. Other ingredients such as pigments, and slip aids are optionally added to the formulation and can affect the overall cost.
Conclusions
　　The multitude of test results show that the pure SIBS quad-block polymer outperforms mixed diene S(I/B)S triblock polymer and S(I/B)S triblock/ S(I/B) diblock polymer blends. In model elastomeric film formulations, blends with thermoplastics including the thermoplastics PS and LDPE and with mineral oil, the pure SIBS quadblock polymer gave the best overall balance of properties.SIBS quad-block polymers are lower cost polymers compared to SIS copolymers due to the incorporation of butadiene.