Molten resin

Melt Viscosity. Viscosities of resins at standard temperatures yield information about molecular weight and molecular weight distribution, as weU as valuable information with respect to appHcation logistics. Some customers prefer to receive resins in molten form. Melt viscosities help to determine the required temperature for a resin to be pumpable. Temperature—viscosity profiles are routinely suppHed to customers by resin manufacturers. In general, a molten viscosity of 1—1.1 Pa-s (1000—1100 cP) or less at process temperatures is convenient for the pumping and handling of molten resin. 

Reduction of the aromatic nuclei contained in catalytic C-9 resins has also been accomplished in the molten state (66). Continuous downward concurrent feeding of molten resin (120°C softening point) and hydrogen to a fixed bed of an alumina supported platinum—mthenium (1.75% Pt—0.25% Ru) catalyst has been shown to reduce approximately 100% of the aromatic nuclei present in the resin. The temperature and pressure required for this process are 295—300°C and 9.8 MPa (lOO kg/cni2), respectively. The extent of hydrogenation was monitored by the percent reduction in the uv absorbance at 274.5 nm. 

Oxidation of LLDPE starts at temperatures above 150°C. This reaction produces hydroxyl and carboxyl groups in polymer molecules as well as low molecular weight compounds such as water, aldehydes, ketones, and alcohols. Oxidation reactions can occur during LLDPE pelletization and processing to protect molten resins from oxygen attack during these operations, antioxidants (radical inhibitors) must be used. These antioxidants (qv) are added to LLDPE resins in concentrations of 0.1—0.5 wt %, and maybe naphthyl amines or phenylenediamines, substituted phenols, quinones, and alkyl phosphites (4), although inhibitors based on hindered phenols are preferred. 

The metal parts of the injection molder, ie, the liner, torpedo, and nozzle, that contact the hot molten resin must be of the noncatalytic type to prevent accelerated decomposition of the polymer. In addition, they must be resistant to corrosion by HCl. Iron, copper, and zinc are catalytic to the decomposition and caimot be used, even as components of alloys. Magnesium is noncatalytic but is subject to corrosive attack, as is chromium when used as plating. Nickel alloys such as Duranickel, HasteUoy B, and HasteUoy C are recommended as constmction materials for injection-molding metal parts. These and pure nickel are noncatalytic and corrosion-resistant however, pure nickel is rather soft and is not recommended. 

Latex Compounding. Latex compounding must take into account the stability of the latex both before and after compounding. Where consideration of soHds concentration permits, the additives are best predispersed in a compatible aqueous surfactant before addition to the latex. The volume of additives, especially if clay fillers are involved, may easily be enough to starve the system for soaps and flocculate the compound. On the other hand, dry powders or molten resins may often be added directly to the nonionic latex. 

Advancement Process. In the advancement process, sometimes referred to as the fusion method, Hquid epoxy resin (cmde diglycidyl ether of bisphenol A) is chain-extended with bisphenol A in the presence of a catalyst to yield higher polymerized products. The advancement reaction is conducted at elevated temperatures (175—200°C) and is monitored for epoxy value and viscosity specifications. The finished product is isolated by cooling and cmshing or flaking the molten resin or by allowing it to soHdify in containers. 

The foam effect is achieved by the dispersion of inert gas throughout the molten resin directly before moulding. Introduction of the gas is usually carried out by pre-blending the resin with a chemical blowing agent which releases gas when heated, or by direct injection of the gas (usually nitrogen). When the compressed gas/resin mixture is rapidly injected into the mould cavity, the gas expands explosively and forces the material into all parts of the mould. An internal cellular structure is thus formed within a solid skin. 

The terms embalming and mummification are often employed as synonyms to refer to the deliberate preservation of corpses so that they keep, as much as possible, their lifelike appearance. To embalm or mummify a dead body is to preserve it by artificial, chemical means. Dehydratation, the removal of water, for example, provides suitable conditions for the preservation of organic mater in general and of corpses in particular many ancient corpses have been mummified by dehydration. In some ancient societies, after the corpse was dry it was impregnated or filled with aromatic substances, usually known as balms, such as molten resin, pitch, or tar, preventing it from becoming unsightly. 

The pigments are incorporated into the resins before crosslinking to ensure homogeneous coloration. This can be done in the molten resin, for example in a kneader at about 90°C, or in dissolved or liquid resins by the liquid resin method. Ball mills are normally used for coloring prewetted powder molding compounds that have not yet been cured. 

Figure 6.1 Schematic for a barrier melting section (courtesy of Jeff A. Myers of Robert Barr, Inc.). The barrier flight is undercut from the main flight to allow molten resin to transfer from the solids channel to the melt channel...

Figure 6.11 Schematics of the solid bed just prior to complete melting (a) the solid bed is pushed to the trailing flight with the Tadmor melting model and barrel rotation physics, and (b) the solid bed is a thin plate and positioned as in the diagram (screw rotation and observation). The cream color represents molten resin...

Figure 6.12 Schematic for the zones of the new melting concept Zone A is the solid bed. Zone B is the melt pool, Zone C is the melt film located between the solid bed and the barrel wall, Zone D is the melt film between the solid bed and the screw root, and Zone E is the melt film between the solid bed and the trailing flight. The cream color represents molten resin...

Figure 6.14 Qualitative shape of A and Kbed dimensions and melt film thickness for melting in a conventional transition section a) top view, and b) side view. The cream color represents molten resin...

At the start of the melting process, the pressure in the channel is relatively low and the solid bed may not be fully compacted. In this case, molten resin from all films has the ability to flow into the voids between the individual pellets. This process is often referred to as melt infiltration. A photograph of a cross section of a Maddock solidification experiment at the start of the melting process is shown by Fig. 6.21. For this figure, the molten material prior to the solidification was black. Melt infiltration is shown by the black resin that has flowed from the films and in between the pellets. The flow of resin into the solid bed will likely cause the pressure in the films to decrease. 

Barrier melting sections are constructed by positioning a second flight (or barrier flight) in the transition section such that the solids are maintained on the trailing side and the molten resin on the pushing side. A schematic of a cross section of a barrier melting section is shown in Fig. 6.22. The resin is melted as discussed in Section 6.3.1 in the solids channel of the device. The resin that is melted near the... 

The axial pressure and temperature distributions for the molten resin in the melt-conveying channel are calculated using the control volume method outlined in Section 7.7.5. For this method, the change in pressure and temperature are calculated using the local channel dimensions, HJ z) and FK (z), and the mass flow rate in the channel using Eq. 7.54 for flow and the methods in Section 7.7.5.1 for energy dissipation and temperature. The amount of mass added to the melt chan-... 

Recalling the discussion earlier in this chapter, in most cases melting in the channel typically occurs at all four edges of the solid bed, with the majority of the melting occurring at the solid bed-melt film interface located between the solid bed and the barrel wall, as shown in Eig. 6.2. The newly molten resin from this location is then conveyed by the motion of the screw to a melt pool located at the pushing side of the channel. Eor very special and sometimes unpredictable conditions, the melting process can occur by a different mechanism. In these cases, the... 

Figure 6.29 Conceptual melting physics for one-dimensional melting. The cream color represents molten resin. The dark color is a mixture of solid polymer particles with molten resin in the void regions...

Qmeit,k volumetric rate for molten resin at the entry to increment k R compression rate of the transition section... 

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