Crosslinking thermoplastic rubbers

Improved tensile strength Rubber crosslinker Thermoplastic rubbers Vulcanizable rubbers Rubber composites with improved impact... 

Recent work has focused on a variety of thermoplastic elastomers and modified thermoplastic polyimides based on the aminopropyl end functionality present in suitably equilibrated polydimethylsiloxanes. Characteristic of these are the urea linked materials described in references 22-25. The chemistry is summarized in Scheme 7. A characteristic stress-strain curve and dynamic mechanical behavior for the urea linked systems in provided in Figures 3 and 4. It was of interest to note that the ultimate properties of the soluble, processible, urea linked copolymers were equivalent to some of the best silica reinforced, chemically crosslinked, silicone rubber... 

Although styrene-diene diblock copolymers are used in some applications, particularly in the area of viscosity index improvement (VII) additives for motor oil, styrenic block copolymers are most often used as thermoplastic elastomers. In these applications the styrene blocks phase separate, crosslinking the rubber blocks in a thermally reversible fashion. The simplest structure capable of exhibiting this behavior is a linear styrene-diene-styrene triblock. The most obvious way to produce such a molecule is by sequential polymeriza-... 

Although the dynamic mechanical properties and the stress-strain behavior iV of block copolymers have been studied extensively, very little creep data are available on these materials (1-17). A number of block copolymers are now commercially available as thermoplastic elastomers to replace crosslinked rubber formulations and other plastics (16). For applications in which the finished object must bear loads for extended periods of time, it is important to know how these new materials compare with conventional crosslinked rubbers and more rigid plastics in dimensional stability or creep behavior. The creep of five commercial block polymers was measured as a function of temperature and molding conditions. Four of the polymers had crystalline hard blocks, and one had a glassy polystyrene hard block. The soft blocks were various kinds of elastomeric materials. The creep of the block polymers was also compared with that of a normal, crosslinked natural rubber and crystalline poly(tetra-methylene terephthalate) (PTMT). 

Since the ions in ionic polymers are held by chemical bonds within a low dielectric medium consisting of a covalent polymer backbone material with which they are incompatible, the polymer backbone is forced into conformations that allow the ions to associate with each other. Because these ionic associations involve ions from different chains they behave as crosslinks, but because they are thermally labile they reversibly break down on heating. lonomers therefore behave as cross-Unked, yet melt-processable, thermoplastic materials, or if the backbone is elastomeric, as thermoplastic rubbers. It should be noted that it is with the slightly ionic polymers, the ionomers, where the effect of ion aggregation is exploited to produce meltprocessable, specialist thermoplastic materials. With highly ionic polymers, the polyelectrolytes, the ionic cross-linking is so extreme that the polymers decompose on melting or are too viscous for use as thermoplastics. 

A very important thermoplastic elastomer is comprised of a blend of polypropylene (PP) with an ethylene-propylene-diene (EPDM) terpolymer. This latter material is, of course, a crosslinkable thermoset rubber ... 

Block copolymers display the unique behavior of being able to function as conventionally crosslinked elastomers over a certain temperature range, but they soften reversibly at high temperatures. As such, they can be processed as a thermoplastic. These polymers are macromolecules comprised of chemically dissimilar, terminally connected segments. According to the components that make up the segments, the thermoplastic rubbers can be classified according to Table 1. 

With very densely crosslinked polymers, rubbers are ineffective as toughening agents. Thermoplastic additives are used instead, such as polyethersulphones, polyamides and polyetherimides. Like the butadiene-acrylonitrile copolymers mentioned above, the sulphone additives can be fimctionalised with hydroxyl or amine end-groups. 

This physically crosslinked matrix exhibits typical physical properties shown in Table 1. The data represent various neat S-B-S and S-I-S thermoplastic rubber samples cast from toluene solutions. 

The temperature range over which thermoplastic rubber compositions can be used as elastomeric solids depends on the glass transition temperatures (Tg) for the two polymer phases. As illustrated in Fig. 4, this useful range lies between the Tg of the rubber phase and the Tg of the endblock phase. Below the Tg of the rubber phase, the midblocks become hard and brittle. Above the Tg of the plastic phase, the domains soften and cease to crosslink the rubber midblocks. 

The addition of a thermoplastic rubber at 10-30 %w produces a truly thermoplastic product with elasticity, resilience, and high adhesive strength. Such mixtures can form the basis for a variety of sealants as discussed in the section on permanent crosslinking. Although the selection of the asphalt and its modifications with aromatic or paraffinic oils is not simple, a balance between resistance to phase separation at 300 F and the formation of a coherent network at ambient temperatures can be obtained by empirical tests. 

Solvating the plastic endblock domains serves to unlock the physically crosslinked rubber network. Consequently, solution blending of thermoplastic rubber with resins, plasticizers, fillers, etc., requires a solvent which will dissolve the endblocks as well as the rubber midblocks. The selection of solvents was discussed previously. 

Another approach to improve the high temperature cohesive strength of adhesives based on thermoplastic rubber is by establishment of a thermoset network extending throughout the rubber phase. This can be accomplished by the use of reactive phenolic resins in combination with a metal catalyst. An example of a formulation which is claimed to be effective is given in Table 21. This type of crosslinking system can be mixed directly into the adhesive solution and crosslinking is initiated thermally in the solvent evaporation ovens. 

THERMOPLASTIC RUBBER (A-B-A BLOCK COPOLYMERS) IN Table 24. Effect of Resins on Electron Beam Crosslinking of (S-l) Rubber Pressure Sensitive Adhesives. ADHESIVES 267 Thermoplastic... 

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