Plasticizers cross-linking

Polyolefins. Interest has been shown in the plasticization of polyolefins (5) but plasticizer use generally results in a reduction of physical properties (12), and compatibiHty can be achieved only up to 2 wt %. Most polyolefins give adequate physical properties without plasticization. There has been use of plasticizers with polypropylene to improve its elongation at break (7) although the addition of plasticizer can lower T, room temperature strength, and flow temperature. This can be overcome by simultaneous plasticization (ca 15 wt % level) and cross-linking. Plasticizers used include DOA. 

Isophthahc (y -phthahc) acid [121 -91 -5] (IPA) is selected to enhance thermal endurance as well as to produce stronger, more resiUent cross-linked plastics that demonstrate improved resistance to chemical attack. TerephthaUc (p-phthaUc) acid [100-21-0] (TA) provides somewhat similar properties as isophthahc acid but is only used in selective formulations due to the limited solubiUty of these polyester polymers in styrene [100-42-5] (see Phthalic acid AND OTHERBENZENEPOLYCARBOXYLIC ACIDS). 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

PUR are a broad class of highly cross-linked plastics prepared by multiple additions of poly-functional hydroxyl or amino compounds. Typical reactants are polyisocyanates [toluene diisocyanate (TDI)] and polyhydroxyl molecules such as polyols, glycols, polyesters, and polyethers. The cyanate group can also combine with water this reaction is the basis for hardening of the one-part foam formulations. 

A thermosetting resin converts to an infusible, cross-linked plastic which is insoluble in any solvent after curing. Because of this irreversible cross-linking reaction, excellent physical properties such as heat resistance, creep resistance, mechanical strength, etc., are obtainable through design of the polymer structure. 

It is important to differentiate between the older random thermosets and these newer systems. Since control and design of structure are the key to modem prepolymer technology, we would like to introduce the new designation structo-set polymers for those cross-linked plastics which are obtained from well-defined, nonrandom prepolymer units. 

RIDER AND HARDY Prepolymers for Cross-linked Plastics... 

Each of these groups includes both linear, i.e., thermoplastic, and cross-linked plastics. Depending on the degree of cross-linking, either... 

Copolymerization is by no means restricted to two monomeric components. Copolymerization is also the term used to describe the uniting of linear polymers or polycondensates that still possess a reactive component (trifunctional monomers) with polymerizable (bifunctional) monomers. The result is a cross-linked plastic, for example unsaturated polyester -i- styrene cross-linked polyester resin. 

A multistep reaction, polyaddition, is a polyreaction of at least two bifunctional or higher functional compounds. Polyaddition can result in either linear polymers (thermoplastics) or cross-linked plastics (duroplastics), depending on the specific functionality. Cross-linked products are obtained by means of a reaction of a bifunctional reactant with a trifunctional one. The more polyfunctional the reactant, the more closely meshed the cross-linking will be. That is why the polyols in polyurethane or epoxy resin production are frequently replaced by polyesters and polyethers containing large numbers of OH groups. Polyaddition, like polycondensation, is a multistep reaction. Fig. 4. Important polyadducts include linear and cross-linked polyurethanes as well as epoxy resins (see [2]). 

Table 1 summarizes important influential factors, listed separately for uncross-linked plastics (thermoplastics) and cross-linked plastics (elastomers/duroplastics). 

The datum mean molar mass makes little sense for cross-linking plastics, since the macromolecules are combined to make single molecules with a very high molar mass. It therefore makes more sense in the case of duroplastics and elastomers to consider the mean cross-link density, the mesh width so-to-speak, or the content level of cross-linking agents (e.g., sulfur content of rubber). 

All thermoplastics flow at room temperature under load stresses far below the yield point, cross-linked plastics at higher temperatures. This clearly demonstrates why it is not permissible to use tensile and breaking strength or the tensile, bending. 

Densely cross-linked plastics (duroplastics) show little or no swelling, depending on their cross-linking density. 

Figure 4.11 EfTect of the cross-linking/plasticizing agent on the internal stress concentration (K is the concentration coefiicient) at the edges of the adhesive-bonded joints...

Thermoplastic vulcanizates (TPVs) typically contain 60-70 % EPDM and 30-40 % impact polypropylene. These products contain a low level of crosslinks, but they are true thermoplastic materials. TPVs have superior strength, high-temperature mechanical properties, hot oil and solvent resistance, and better compression set than partially cured material. These materials are almost always dynamically cured, which refers to the process whereby the rubber phase is vulcanized during melt mixing with the molten non-cross-linked plastic. 

First, a bit of a review of the basic categories of plastics materials. In general, they fall into one of two categories thermosets and thermoplastics. Thermosets undergo a chemical reaction when heated and cannot return to their original state. Consequently, they are chemical resistant and do not bum. Cross-linked plastics are an example of thermosets. Thermoplastics constitute the bulk of the polymers available. Although some degradation does occur, they can be remelted. Most are readily attacked by chemicals and they bum readily. 

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