Melt Strength Improvers

In extrusion - blow moulding operations, molten polymer passes through a stage where it is processed with little external support. In these circumstances the moulding is likely to distort. The incorporation of a melt strength additive reduces the possibility of this occuring. Frequently, the materials used to improve melt strength are the same as those used for internal lubrication or plasticisation. 

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Melt strength improvers Mould release agents Plasticisers... 

One of the main commercial uses of PMDA is to improve the melt strength of PET to allow it to be foamed. Unmodified PET cannot be foamed properly because its inherently low melt strength causes the cells to collapse and coalesce. 

Heitz et al. [35] (BASF) describe the use of polycarbodiimide as an additive to produce hydrolysis-resistant PBT. Further patents describe glass-fiber-reinforced mixtures of PBT with an aromatic polycarbodiimide having improved impact strength [36], and mixtures of PBT with an aromatic polycarbodiimide having improved melt strength and intrinsic viscosity [37],... 

Polypropylene s low elongational viscosity and poor melt strength have prevented its widespread use in extruded, low-density foams. However, significant improvements in PP technology have made possible the production of low-density foam on conventional equipment. This article demonstrates the importance of extensional or elongational viscosity in the foam process, compares new and conventional PPs in extensional flow, and shows how these rheological differences allow production of low-density foam on tandem extrusion equipment. 

Long-chain branching (LCB), generally less than 0.1 branch per 1000 carbons, has been observed in some metallocene polymerizations of ethylene and propene [Nele and Soares, 2002 Soares, 2002 Weng et al., 2002]. The presence of even small amounts of LCB improves melt strength and melt processability of narrow PDI polymers. Thus, it is often useful to choose conditions, such as the metallocene, temperature, and other reaction conditions, that deliberately introduce long chain branching. 

The narrow molecular-weight distribution and the uniform incorporation of co-monomers lead to improved product properties such as high impact strength, transparency, and heat seal, together with less stickiness and blocking of films. Furthermore, mPE exhibits a smaller fraction of extractables. On the other hand, mPE suffers from a more difficult processibility, low melt strength, and higher melt fracture. A main deficiency is the inability of metallocene... 

As a route for improving the melt-elongational properties of semicrystalline polymers, Siripurapu et al. [7] proposed the blending of amorphous and semicrystalline blends of PS and PVDF nevertheless, their approach showed only limited success. In contrast, Reichelt et al. [29] successfully developed blends of HMS-PP and PP-fe-PE block copolymers. As could be shown, the melt strength increases with the HMS-PP content, while blends rich in HMS-PP also show the lowest densities. 

Starch can be destructured in the presence of more hydrophobic polymers such as aliphatic polyesters. Aliphatic polyesters with low melting points are difficult to process by conventional techniques such as film blowing and blow moulding. Films such as polycaprolactones (PCL) are tacky as extruded and have a low melt strength (over 130 °C). Also, the slow crystallisation of the polymer causes the properties to change with time. Blending starch with aliphatic polyesters improves processability and biodegradability. 

ABS resins can be added to semi-rigid PVC formulations to improve hot melt strength, elongation and low temperature flexibility. Particular use is in deep draw forming applications. 

Let us consider these two approaches in some detail. Recall that stretching polymers at temperatures below their melting point improves their stiffness and strength. Techniques involving some stretching are used commercially for nylon, polyester, and polypropylene fibers (as well as some polymer films) (see Section... 

MYKON SEW can be readily made into a water based emulsion under atmospheric conditions. It does not require the use of a hot melt tank. The emulsified product provides excellent sewability and tear strength improvement to cellulosic fabrics. 

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