Heat Cure

Textiles are finished with amino resias ia four steps. The fabric is ( /) passed through a solution containing the chemicals, (f) through squeeze roUs (padding) to remove excess solution, (J) dried, and (4) heated (cured) to bond the chemicals with the cellulose or to polymerize them on the fabric surface. 

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

The processing methods for siHcone mbber are similar to those used in the natural mbber industry (59,369—371). Polymer gum stock and fillers are compounded in a dough or Banbury-type mixer. Catalysts are added and additional compounding is completed on water-cooled roU mills. For small batches, the entire process can be carried out on a two-roU mill. Heat-cured siHcone mbber is commercially available as gum stock, reinforced gum, partially filled gum, uncatalyzed compounds, dispersions, and catalyzed compounds. The latter is ready for use without additional processing. Before being used, sihcone mbber is often freshened, ie, the compound is freshly worked on a mbber mill until it is a smooth continuous sheet. The freshening process eliminates the stmcturing problems associated with polymer—filler interactions. 

The compression set of sihcone mbber is similar to organic types of mbber at low (0—50°C) temperatures, ranging from 5 to 15% (380). Above 50°C, sihcone mbber is superior, but compression set increases with time and temperature. Sihcone mbber is more tear-sensitive than butyl mbber, and the degree of sensitivity is a function of filler size and dispersion, cross-link density, and curing conditions. The electrical properties of sihcone mbber are generally superior to organic mbbers and are retained over a temperature range from —50 to 250°C (51). Typical electrical values for a heat-cured sihcone mbber are shown in Table 9. 

Cured sihcone LIM mbber can be fabricated with physical properties equivalent to heat-cured mbber (385). Shore A hardness can range from 30 to 70, depending on formulations. Typical physical properties include tensile strengths as high as 9.7 MPa (1400 psi), 500—775% elongation at break, and tear strength of >30 N/mm (180 Ib/in.). Compression sets of less than 10% can be achieved if the material is baked after processing. 

Quahty control testing of siUcones utilizes a combination of physical and chemical measurements to ensure satisfactory product performance and processibihty. Eor example, in addition to the usual physical properties of cured elastomers, the plasticity of heat-cured mbber and the extmsion rate of TVR elastomers under standard conditions are important to the customer. Where the siUcone appHcation involves surface activity, a use test is frequently the only rehable indicator of performance. Eor example, the performance of an antifoaming agent can be tested by measuring the foam reduction when the sihcone emulsion is added to an agitated standard detergent solution. The product data sheets and technical bulletins from commercial siUcone producers can be consulted for more information. 

Eor instance, exhaust appHcation is possible with cationic finishes which have an affinity for the anionic groups in polymeric materials. After appHcation, the textile is dried. Durable antistatic finishes require cross-linking of the resin. Cross-linking is usually achieved by subjecting the treated, dried material to heat curing. A catalyst is often incorporated to accelerate insolubilization. 

Fig. 1. Gel time as a function of temperature for A, residual heat curing B, fast curing C, medium-fast curing and D, slow curing powders.

Soft reliners can weaken the strength of the heat-cured resin, because they reduce the thickness of the denture base and allow the diffusion of the monomer or solvent from the reliner into the base. Reflned dentures stain readily and are difficult to clean. A polyphosphazine fluoroelastomer has also been developed in an attempt to overcome the deficiencies of available liners (218). 

For solvent-based ambient cure systems polyamides are often the hardeners of choice. For heat-cured systems, anhydrides are used to provide higher heat-resistance systems but at the expense of flexibiHty. 

Parallam, or laminated strand lumber (LSL) is a beam made by a continuous manufacturing process composed of bigger-size wood needles (very elongated wood particles) reassembled with a structural exterior grade adhesive, the favorite adhesive being isocyanates (pMDI) when heat-curing and PRFs when cold-curing. 

Operating conditions are important determinants of the choice of filter media and sealant used in the cartridges. Some filter media, such as cellulose paper filters, are useful only at relatively low temperatures of 95 to 150"C (200 to 300°F). For high-temperature flue gas streams, more thermally stable filter media, such as nonwoven polyester, polypropylene, or Nomex, must be used. A variety of commercially available sealants such as polyurethane plastic and epoxy will allow fabric operating temperatures up tol50°C (300°F). Selected sealants such as heat cured Plasitcol will withstand operating temperatures up to 200°C (400°F). 

Pouring the Resin. Controlled heat-curing (no open flame) at a temperature range of 250 to 300°F (121 to 149°C) is recommended and is required if ambient temperatures are less than 60°F (16°C) (which may vary with different resins). When controlled heat curing is not available and ambient temperatures are not less than 60°F (16°C), the attachment should not be disturbed and tension should not be applied to the socketed assembly for at least 24 hr. 

Insulation formed by slurry casting or heat curing under pressure in molds in a number of insulation types. Most common moldings are preformed bends, valve boxes and flange covers. 

The advantage of sol-gel technology is the ability to produce a highly pure y-alumina and zirconia membrane at medium temperatures, about 700 °C, with a uniform pore size distribution in a thin film. However, the membrane is sensitive to heat treatment, resulting in cracking on the film layer. A successful crack-free product was produced, but it needed special care and time for suitable heat curing. Only y-alumina membrane have the disadvantage of poor chemical and thermal stability. 

Polyphenyls Polyphenylene oxide Decompose at 530°C (986°F) infusible, insoluble polymers. Decomposes close to 500°C (932°F) heat cures above 150° C (302°F) to elastomer usable heat range —135-185 C (—211-365°F). 

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