MELT FLOW STABILITY

In polypropylene tape applications. HP-136 is very effective when used with a classic phenolic or in a ternary blend and. compared with a traditional binary blend, data indicate that there is potential for a 50% reduction in total stabilizer concentration, with comparable melt flow stability, improvement in colour, and an improvement in water carry-over, again providing a significant margin for optimization of formulation. In polypropylene fibre applications, a combination of HP-136 with a hindered amine and a phosphite shows melt flow and colour comparable with a traditional binary system, but superior gas-fade resistance and UV stability in phenol-free blends. This improvement is seen at 40% less concentration. 

Figure 6. Melt flow stability of acid sensitive compounds loaded with neutral Eeonomer nylon-6 at 235 °C (V) and PBT at 250 °C (x). Reproduced with permission from ref. 7. Copyright 1998.

Table 2 Melt Stabilizing Efficiency of Antioxidants in PP (processed in an internal mixer at 190°C). Melt flow index (MFI) Measured at 230 C and 2.16 Kg...

This combination gives a very useful blend of mechanical properties. The PBT phase provides melt flow, solvent resistance and the ultimate heat performance of the blend (Tm). The PC phase provides reduced shrink, better dimensional stability, higher heat capability under low load (66 psi HDT) and improved impact strength. Interestingly, the PC also provides improved paint adhesion by being present as a very thin outer layer in molded parts. PBT, by virtue of its solvent... 

The polymer material contains a linear ethylenic polymer having a melt flow ratio, 11012, of 5.63 or above, a MWD, Mw/Mn, defined by the equation Mw/Mn less than or equal to (I10/I2)-4.63 and a critical shear rate at onset of surface melt fracture of at least 50% greater than that of a linear olefin polymer having about the same 12 and Mw/ Mn. The foam structures have toughness and elasticity similar to those formed from conventional LLDPE without the poor dimensional stability and foam quality associated with those structures and foam quality similar... 

Polypropylene (PP) films were first produced by extrusion casting. Polymer is extruded through a slit or tubular die and quenched by cooling on chill rolls or in a water bath. Cast films can be sealed over a wide range of temperatures and do not shrink in a steam autoclave, Polymers with melt flow rates below 5 dg/min are usually used to maintain the stability of the extra date. Higher clarity films are produced using random copolymers. 

Copolymerization. Vinyl chloride can be copolymerized with a variety of monomers. Vinyl acetate, the most important commercial comonomer, is used to reduce crystallinity, winch aids fusion and allows lower processing temperatures. Copolymers are used in flooring and coatings. This copolymer sometimes contains maleic add or vinyl alcohol (hydrolyzed from the poly(vinyl acetate ) to improve the coating s adhesion to other materials, including metals, Copolymers with vinylidene chloride are used as barrier films and coatings. Copolymers of vinyl chlonde with acrylic esters in latex from are used as film formers in paint, nonwoven fabric binders, adhesives, and coatings. Copolymers with olefins improve thermal stability and melt flow, but at some loss of heat-deflection temperature,... 

Amorphous nylons are transparent. Heat-deflection temperatures are lower than those of filled crystalline nylon resins, and melt flow is stiffer hence, they are more difficult to process. Mold shrinkage is lower and they absorb less water. Warpage is reduced and dimensional stability less of a problem than with crystalline products. Chemical and hydrolytic stability are excellent. Amorphous nylons can be made by using monomer combinations that result in highly asymmetric structures which crystallize with difficulty or by adding crystallization inhibitors to crystalline resins such as nylon-6 (61). 

On the basis of melt flow rate measurements after 1, 3 and 5 extrusions, compounds 1, 2 and 2 when used alone at 0.1 in the polypropylene base resin provide superior stabilization to BHT, 7 and 8. Under these conditions compound J2 provides the lowest Yellowness Index (Y.I.) color over other compounds evaluated. 

Bogaerds et al. (47) developed a linear flow stability analysis toolbox in conjunction with the single-mode extended pom-pom (XPP) constitutive equation (56-58). Their analysis did not show the periodic nature of the flow-front motion observed experimentally with instabilities. On the other hand, their simulations do show that the onset of the linear instability can be postponed by increasing the number of the pom-pom-bearing arms of the XPP model, which would render in the melt increased, strain-hardening behavior. 

The additives gave considerable stability to the resin as shown by the graphs. However, unstabilized resin discolored badly, in addition to experiencing a tremendous increase in melt flow (or decrease in melt... 

Some of the test methods being used to measure the processing stability of polypropylene include melt flow drift measurements at elevated temperatures using an extrusion plastometer (melt indexer), melt viscosity retention measurements using a torque rheometer, retention of melt flow after repeated extrusions, and injection molded spiral test measured by the flow in inches at various temperatures and the retention of melt flow of the injected spirals. The nine commercial resins were evaluated by these methods. 

Extrusion Stability (20). Processing stabilities were also determined by six succesive extrusions of the resins through a 1-inch extruder and measuring the change in melt flow (ASTM D 1238-65T, Condition L) after each extrusion. The data shown in Figure 7 indicate that Resins C, E, and I are least stable and G, A, and D are the most stable. 

During the extrusion of polymers different defects and flow instabilities occur at very low Reynolds numbers. The commonly known ones are sharkskin, melt fracture, slip at the wall and cork flow. These defects are of commercial importance, since they often limit the production rate in polymer processing. Many researchers have been interested in the subject, and thorough reviews on flow stability and melt fracture have been written in the last 30 years [1-4]. More recently, two review papers deahng with viscoelastic fluid mechanics and flow stability, were published by Denn [5] and Larson [6]. However, although much work has been done in the field of extrusion distortions, controversy still exists regarding the site of initiation and physical mechanisms of the instabilities. 

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