Low viscosity molding compound

ALLYL ESTER RESINS. The allyl radical (CH2CH=CH2) is the basis of the allyl family of resins. Allyl esters arc based on monobasic and dibasic acids and are available as low-viscosity monomers and thermoplastic prepolymers. They are used as crosslinking agents for unsaluraied polyester resins and in the preparation of reinforced thermoset molding compounds and high-performance transparent articles. All modem thermoset techniques may be used for processing allyl resins. 

PVC has a unique ability to be compounded with a wide variety of additives, making it possible to produce materials that range from flexible elastomers to rigid compounds, that are virtually unbreakable. Compounds arc also made that have stiff melts for profile extrusion or low viscosity melts fur thin-walled injection molding. 

Properly compounded PTFE dispersions are suitable for impregnation because of their low viscosity, very small particles, and ability to wet the surfaces. The surfactant aids the capillary action and wetting interstices in a porous material. After the substrate is dipped and dried, it may or may not be sintered. This depends on the intended application. In fact, the unsintered coating exhibits sufficiently high chemical resistance and antistick property. If required, the coated substrate may be heated to about 290°C (555°F) for several minutes to remove the surfactant. Lower temperatures and longer times are used if the substrate cannot tolerate such a high temperature. In some cases, the impregnated material is calendered or compressed in a mold to compact the PTFE resin and to hold it in place. 

As the viscosity of the rubber compound increases with the stage of cure, it is important to know exactly the temperature at which the cure takes place, as heat increases viscosity, so as to inject into the mold the rubber with an appropriately low viscosity. 

Other types of low viscosity resins suitable for processing by resin transfer molding and resin infusion are based on 4,4 -BPDA, 2,2 -bis(4-(4-amino-phenoxy)phenyl)propane and CA. 1 mole of 4,4 -BPDA is used the other compounds are fed in amounts of 2 mole. 

Low-pressure SMC Low-pressure sheet molding compounds can present problems due to difficulty in controlling the chemical thickening process at low viscosity levels, and achieving a dry, tack-free easily handled compound without any resin separation. New technology from Union Carbide is claimed to overcome this, offering wide process latitude for thickener control, with dry easily handled compounds of stable viscosity. 

The viscosity of the curable molding compounds (before curing is completed) is usually very low (at 70-120 °C) - around 1-50 Pa s. Thermoplastic melts are between 200 Pa s and 800 Pa s (see Table 10, Sect. Processing of thermoplastic elastomers ). In contrast to injection molding of thermoplastics, duroplastics form flashes and skins the subsequent removal of which is expensive. 

When processing (CM, injection molding, extrusion, ICM, etc.) from uncrosslinked (A state) or crosslinked (B state) duroplastic molding compounds, the shearing forces applied upon injection through dies impose orientations upon the macromolecules and, if any are present, upon the reinforcing fibers as well. Because of the low viscosities of uncrosslinked masses such as phenolic, melamine, UP and epoxy resins and the hot mold wall, relaxation of the molecular orientations sets in quickly. As a rule, duroplastics show little or no orientation. Crosslinking fixes this state. 

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