Compression, thermal degradation

Discoloration. When PVC, free of plasticizer or stabilizer, is compression molded in air at 200°C under adequate pressure (c.g., 500-6000 psig), thermal degradation results in discoloration to a pink to brown colored film. When the modified PVC containing as little as 3% of grafted cis-1,4-polybutadiene was pressed under the same conditions, the resultant film was essentially colorless or no more than faintly discolored. 

The compression-molded part, by definition, does not have flow-induced orientation. Comparison of compression-molded part properties with those of an injection-molded part can show the effect of melt temperature on properties. In the compression-molded article without flow-induced orientation, the impact strength remains constant until a certain melt temperature is surpassed and then decreases. This thermal degradation effect can be attributed to the polybutadiene component, which acts as an initiation site for oxidative degradation of the matrices. 

A supercritical fluid (SCF) is any compound above its critical point, which is the maximum in both temperature and pressure at which a gas and liquid can coexist. Above the critical point, isothermic compression yields a continuous increase in density without condensation to a liquid state. All substances theoretically have a critical point, but many experience thermal degradation well before reaching it. 

ABS/PVC blends (e.g., Triax CBE, Enplex tm) are formulated to provide a broad window of molding conditions. Because of the presence of PVC, shear or thermal degradation must be minimized. Shot size should be about 50% of machine capacity — less than 35% will result in a long residence time, whereas in excess of 65% will increase plasticating time, both of which create conditions for resin degradation. High compression screws should be avoided since they generate excessive shear heat — a linear taper screw with a compression ratio between 1.5 and... 

A low shear heat screw is recommended, with L/D = 18 to 22 and a compression ratio from 2.0 to 2.6. The screw should be protected against corrosion and wear, by either plating or nitrating. If any scratch is observed on the surface of the barrel, screw, or nozzle, it must be repaired immediately, otherwise owing to resin retention, it may cause burn marks or black spots on the moldings. The resin temperature may be checked with a few air shots. Due to thermal degradation, temperatures above 210°C may cause burn marks, black spots or mold-rust. 

The compression set data, which we had obtained, reflect far poorer performances than we had anticipated on the basis of conversations with manufacturers of solar collector units and suppliers of sealants. A summary of thse data are presented in Table VIII. All three silicones, G, I, and J, tested exhibited compression sets of the order of 100% after only one day of aging at either 250°C or 225°C. (Compression set values greater than 100% are attributed to essentially complete loss of resilience by the specimen combined with an additional reduction in sample thickness due to shrinkage associated with thermal degradation). For example, the best of the three silicones, J, showed compression set values of 94% after one day at 225 C and 89% after 28 days at 175°C. We feel that a sealant should not exceed 50% in compression set if it is to retain good sealing ability. 

By exploiting the good dissolution properties of compressed gases in the supercritical state in a flow apparatus, thermal degradation of cellulose has been achieved without the secondary reactions and strong carbonization usual in conventional pyrolysis methods. Cellulose could be degraded to the extent of 98%. 

Chitin. — The primary product of the thermal degradation of chitin, namely 2-acetamido-l,6-anhydro-2-deoxy- 3-D-glucopyranose, has been detected for the first time and isolated on a preparative scale. Thermal degradation was achieved with few secondary reactions by using compressed gases in a supercritical state in a flow apparatus. 

These differences between specimens presumably arise from varying amounts of compressive prestress, which the bronze matrix exerts on the NbaSn reaction layer (because of thermal contraction after the fabrication heat treatment). The compressive strain degrades the initial /c, so that when tension is applied to the composite, the first effect is to relieve the compressive strain on the NbsSn and increase Ic- Eventually, when enough tensile stress is applied to the composite, the... 

Polymeric materials, in application, may be exposed to a wide range of environmental stresses, including solvent attack, oxidation, photochemical damage, mechanical abrasion, flexion, compression and tensile stress, impact, and thermal degradation. These stresses all have the potential to produce irreversible damage to the material s structural characteristics. It has been proposed that such damage starts at the microscopic level with the formation of microvoids, which then expand to generate microcracks, and ultimately lead to macroscopic failure of the material. 

This class of solids is an extension of the sample types already discussed, and many of the procedures already highlighted may be used here. If the material dissolves in a suitable solvent, then a cast film may be prepared on an IR transmitting window or on the surface of an ATR element. Moldable materials, such as polymer pellets, may be prepared as hot-pressed films, with care taken to ensure that material does not thermally degrade. Grind-able materials can be handled as previously discussed for powders using the compressed halide pellet, mineral oil mull, or diffuse reflectance methods to acquire the spectrum. 

Extrusion grade SVA is Suprel 9806. It should be extruded at 180-185°C melt temperature, measured with a hand-held p5rrometer. Screws should be 1.5-2.0 compression ratio with no mixing section. Tooling should be stainless steel or plated with chrome or nickel. Resin drying is not required. Overheating the melt will cause thermal degradation. 

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