Heat sensitive resins

In addition to single screw extruders, there are twin and multiscrew extruders that perform essentially the same functions, but with additional benefits. Among these, the intermeshing twin screw extruders are the most important ones. They are used primarily for heat-sensitive resins (such as PVC), which are difficult to process. The intermeshing screws create a relative motion of one flight in another, such that it acts... 

The flash, after being trimmed, usually is immediately ground up and fed back into the extruder at a controlled rate, mixed with the virgin resin. The use of regrind can be problematic for heat-sensitive resins like PVC, especially if the proportion of flash is high. 

Why is extrusion blow molding preferred to injection blow molding for heat-sensitive resins ... 

When the melt is forced into the mold, the plunger action could cause the melt to flow back into the screw flights. Generally, with heat-sensitive resins such as PVC and thermosets, a plain or smearhead screw tip is used. For other resins this is not adequate, and a number of different check valves are used. These devices work in the same manner as a check valve in a hydraulic system, allowing fluid to pass only in one direction. These check valves, which are basically a sliding ring or ball check design (Fig. 2-10) are supplied by many manufacturers. Here are some comparisons (100) ... 

Decomposition, or burning, of the plastics material in the injection molding machine will cause color changes and will then result in the subsequent injection moldings being rejected. When this happens a complete shut down and clean out may be necessary. To prevent this it may be necessary to purge a heat sensitive resin with another, more heat stable, plastics material as this will withstand subsequent reheating. 

Substituted heat-reactive resins are most widely used in contact-adhesive appHcations and, to a lesser extent, in coatings (77,78) -butylphenol, cresol, and nonylphenol are most frequendy used. The alkyl group increases compatibiHty with oleoresinous varnishes and alkyds. In combination with these resins, phenoHcs reduce water sensitivity. Common appHcations include baked-on and electrical insulation varnishes, and as modifiers for baking alkyds, rosin, and ester gum systems. Substituted heat-reactive resins are not used for air-dry coatings because of theh soft, tacky nature in the uncured state substituted nonheat-reactive phenoHcs are the modifying resin of choice in this case. 

Considering their heat sensitivity, the separation of fatty acids and rosin with minimal degradation by fractional distillation under vacuum and/or in the presence of steam is surprisingly good (3). Tad od rosin (TOR) contains about 2% fatty acid and smad amounts of neutrals. Tad od fatty acid (TOFA) contains as Htde as 1.2% rosin and 1.7% neutrals. In typical U.S. TOFA, 49% of the fatty acids is oleic, 45% linoleic, and 3% palmitic, stearic, and eicosatrienoic acid. TOR and TOFA are upgraded to resins and chemicals for the manufacture of inks (qv), adhesives (qv), coatings (qv), and lubricants (see Lubrication AND lubricants). 

Heat-Sensitive or Thermoplastic Adhesive-Based Labels Activated by Heat. Two types are in use, instant tack and delayed tack. Both are based on synthetic resins. The former has to have heat and pressure applied to effect the transfer, but sets immediately after the source heat is removed. 

Liquid color is not a universal coloring system, but nothing is. It can perform in a variety of processes, in nearly all resins. Liquid is at its best in small percentages, where its distribution in the resin is excellent it does modify physical properties somewhat but enhances some properties. It is excellent for heat-sensitive additives, such as blowing agents. Look into using liquid color as a coloring system the economics may work out favorably. 

Some mortars are heat sensitive. The speed of set of furan mortars in particular, is much accelerated by heat, so that "open time" can be reduced to 10 minutes at 85° to 90°F. The effects of temperature on setting speeds of other resins are less extreme, but the contractor will have more time to use up his mix at a 60° to 70°F material temperature. Phenolic mortars must be stored prior to use at temperatures of 60°F or lower. If they are left in areas where they may be overheated, they can be ruined in a short time or so badly affected as to be unusable. There is a case on record of phenolic mortars being delivered to a site in Texas late on Friday, and left on the loading dock, resulting in materials which were totally unusable by 7 A.M. on Monday morning when the construction crew arrived. On the other hand, no serious damage will result if the components of resin mortars are subjected to temperatures below freezing. 

PVC is inherently the most heat-sensitive of the major commercial thermoplastic resins. Stabilization consists primarily of a stabilizing action on labile chlorine atoms in the PVC resin in order to prevent dehydrohalogenation and secondly in the neutralization of HCl by incorporation of bases [138]. Worldwide consumption of heat stabilizers is slightly above 300 thousands of metric tonnes per annum [139] (Table 16). 

Polymers of this nature can be polymerized either in solution or in bulk in the latter case they are normally reacted at high temperatures, e.g., 100-150 C. Since our goal was a casting resin, the formulations were reacted in bulk and at lower temperatures to protect heat sensitive electronic components furthermore, low reaction temperatures minimize side reactions that can lead to crosslinking and polymer insolubility. In this process the polyols and diisocyanates were mixed and allowed to react for about 25 minutes at 71 C to form the prepolymer formation while longer times resulted in material too viscous to cast or deaerate. After the indicated time, 1,4-butanediol was added followed by deaeration and subsequent encapsulation of a preheated (71 C) electronic device. A second deaeration of the encapsulated part is usually necessary. Pot life for such a system is about 15 minutes. Final reaction or "cure" was 24 hours at 71 C. 

Heat sensitive labels—applied after the activation of a thermoplastic coating by the use of heat. Two types of thermoplastic resin exist ... 

Exchange between product and pack can occur in both directions, e.g. certain labelling materials such as heat sensitive and self-adhesive labels when in contact with plastic materials. Both the plastic and the adhesives may contain plasticisers or migratory constituents. Most cellulosics use phthalate, sebacate, phosphate-type plasticisers (e.g. methyl phthalate (DMP) may be used in cellulose acetate). Plasticisers may also be found in poly vinyl chloride/acetate copolymers, polyvinyl acetate and polyvinyl alcohol formulations, polymethyl methacrylate, nylon and certain thermosetting resins. 

VIC . [Ashland] Pcdyester and acrylic urethane resins fw gas curing finishes for plastics, metal, heat-sensitive substrates. 

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