Thermoplastic elastomers at surfaces

Although the two terms are often used interchangeably, strictly speaking the two are not identical. The surface tension is defined as  

This section describes analytical methods that are used to characterize the atomic composition or morphology at TPE surfaces. Measurement of surface composition in TPEs can suffer from one major drawback. Since the soft phase 

Another extremely important parameter of any surface technique is the depth of penetration (d ), which is a characteristic distance from the surface that the measurement will probe. Table V gives approximate depths of penetration for the methods discussed in this section for measurement of TPE surfaces. Since Table V shows that the depth of penetration varies widely, the change in composition can be monitored from the surface almost continuously to thousands of angstroms. Alternatively, some methods discussed in this section measure the morphology near the surface. Studies of TPE surfaces are less numerous than investigations of bulk properties, which in general speaks to the relative lack of importance of the surface properties of TPEs for most applications. However, exceptions to the latter generalization definitely exist, for example in the medical device area. 

In SEM, electrons are reflected from the surface, and detection is done on the same side of the surface as the source in TEM, the electrons pass through the sample and are detected on the other side. SEM is done under UHV conditions, generally 10 - lO torr. Probably the most common use of SEM in 

TABLE V Depths of Penetration for Snrface-Sensitive Analytical Methods 

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VI. Thermoplastic Elastomers at Surfaces Vn. Rheology and Processing Vin. Applications References... 

At constant conditions, different fluids will diffuse at different rates into a particular elastomer (with their rates raised proportionally by increasing the exposed area), and each will reach the far elastomer-sample surface proportionally more rapidly with decreasing specimen thickness. Small molecules usually diffuse through an elastomer more readily than larger molecules, so that, as viscosity rises, diffusion rate decreases. One fluid is likely to diffuse at different rates through different elastomers. Permeation rates are generally fast for gases and slow for liquids (and fast for elastomers and slow for thermoplastics and thermosets). 

The use of lightly crosslinked polymers did result in hydrophilic surfaces (contact angle 50°, c-PI, 0.2 M PhTD). However, the surfaces displayed severe cracking after 5 days. Although qualitatively they appeared to remain hydrophilic, reliable contact angle measurements on these surfaces were impossible. Also, the use of a styrene-butadiene-styrene triblock copolymer thermoplastic elastomer did not show improved permanence of the hydrophilicity over other polydienes treated with PhTD. The block copolymer film was cast from toluene, and transmission electron microscopy showed that the continuous phase was the polybutadiene portion of the copolymer. Both polystyrene and polybutadiene domains are present at the surface. This would probably limit the maximum hydrophilicity obtainable since the RTD reagents are not expected to modify the polystyrene domains. 

Thermoplastic elastomers (TPE) Styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS) block copolymers represent primary materials for other hotmelt adhesives. They are characterized by long open times, high elasticity levels, good spring qualities and a lasting stickiness, so that precoated surfaces always retain some of the qualities of PSAs. The melting temperatures of these adhesives are particularly low at about 65°C, so that they can be used for thermally sensitive materials (foams, fleeces, thin films). 

More recently, blends of a partially crosslinked thermoplastic elastomer with 5-40 parts of a PO (viz. LLDPE, PP, EPR, or PB-1) were developed for low density, foamable alloys [Okada et al, 1998a]. The density was reduced at least by a factor of two. In the following patent 1-17 wt% of a long-chain branched PP was also added [Okada et al, 1998b]. The extmded foam was free of surface roughness caused by defoaming, was soft to the touch and showed excellent heat and weathering resistance. 

Thomas and co-workers [1986] studied effects of gamma-irradiation on a thermoplastic elastomer, TPE, a polyether-ester block copolymer from 1,4-butanediol, polybutylene glycol and terephthalic acid (Hytrel D40), TPE, and its blends with PVC, in air, at doses of 1, 10, 100 and 500 kGy. Visual inspection of the samples showed no color change up to 500 kGy, but the samples irradiated at 500 kGy showed surface cracks. 

Thermoplastic elastomers (TPEs) are an extremely fast growing segment of polymer manufacturing. A rate of 5% growth per year is expected until 2007, at which time the total U.S. demand for these materials will reach 1.5 billion lb at a total annual sales of approximately 3 billion dollars per year [1]. The majority of this growth comes in the form of replacements for other types of materials, and the growth of so-called soft-touch surfaces. In the approximately 10 years since the second edition of this book appeared, there has been an important technological advancement in this area the vastly increased production of thermoplastic polyolefin elastomers as a result of the worldwide adoption of metallocene catalysts. 

Marija Pergal, MSc, works at the Department for Polymeric Materials, Institute for Chemistry, Technology and Metallurgy since 2003 as Research Scientist. Since 2007 she is also Teaching Assistant for the course Chemistry of Macromolecules at Department of Chemistry, University of Belgrade. Her research interests are focused on synthesis and characterization of siloxane homopolymers and copolymers, especially thermoplastic elastomers based on poly(butylene terephthalate) and polyurethanes, as well as polyurethane networks based on hyperbranched polyester. In addition to physico-chemical, mechanical and surface properties of polymers, her particular interest is directed towards the study of biocompatibility of polymer materials. 

The use of a commercial Cloisite 20A organoclay to prepare SBS-based nanocomposites by melt processing was recently reported [63]. In this case, the nanocomposite morphology was characterized by a combination of intercalated and partly exfoliated clay platelets, with occasional clay aggregates present at higher clay content. For this particular thermoplastic elastomer nanocomposite system, well-dispersed nanoclays lead to enhanced stiffness and ductility, suggesting promising improvements in nanocomposite creep performance. The use of stearic acid as a surface modifier of montmorillonite clay to effectively improve the clay dispersion in the SBS matrix and the mechanical properties of the SBS-clay nanocomposites was reported [64]. 

Commercially available thermoplastic elastomers based on block copolymers of diisocyanates and polyols were used to delay sharkskin and stick-slip instabilities in the extrusion of linear low density polyethylene. When elastomer is added in a small mass fraction to LLDPE, it deposits at the die surface during extrusion and may postpone the onset of sharkskin instability to a 12-20 times higher rate of extrusion. ... 

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