Calender swell

Such elastic effects are of great importance in polymer processing. They are dominant in determining die swell and calender swell via the phenomenon often... 

When polymers are extruded or calendered the cross-sectional area of the extrudate or the calendered sheet is usually greater than the cross-sectional area of the extruder die or the space between the calender rolls. Known as die swell and calender swell these effects are due to relaxation of molecules that had been oriented during shear. As a general rule die swell is found, like viscosity, to increase with molecular weight and decrease with a rise in temperature. 

Plasticisers are compatible and miscible with their host polymer causing them to swell and allowing them to be used for coating, moulding, spraying (when used as a suspension in a liquid solvent) or calendering, extrusion and injection moulding (when used as a polymer melt). 

A process for making hollow mbber articles from a single sheet of calendered unvulcanised mbber. The surface of the sheet is semi-vulcanised and the application of a swelling agent causes the sheet to split thus forming the hollow article, the vulcanisation of which is completed thereafter. Homo polymer... 

The degree to which a raw elastomer or unvulcanised compound returns to its original dimensions on the removal of an applied stress which has caused deformation. Die swell in extrusions and increase of thickness of calendered sheet are examples of recovery. 

The principal benefit of using reclaim is its lower cost compared to virgin mbbers. Reclaim typically sells for 20—30% of the cost of its none-reclaimed counterparts. Reclaimed mbber also imparts some desirable improvements in processing it has much lower nerve than virgin polymers. As a result, compounds containing reclaim have much lower die swell and extrusion rates. It also increases calender rates and improves flow and mold filling. 

The KAURIT types improve the crease-recovery and dimensional stability of cellulosic textiles. They reduce the swelling capacity of these textiles and produce permanent calender and plisse effects. 

Conditions recommended by the manufacturers for extrusion of elastomer are listed blend in Table 10.23. Articles made of NBRIPVC blends (Carloy , Geon/Hycar , Denka LCS, JSR NV, Krynac NV, Nipol , Oxyblend , Paracril OZO, Vynite ) can be manufactured by dispersing powdered nitrile elastomers in a continuous mixing operation, which enables them to be used directly in extrusion. Easy and smooth extrusion and calendering are usual. Plasticizers and stabilizers should be pre-dispersed in PVC otherwise they may swell the elastomer and cause formation of lumps. 

Addition of impact modifier also frequently affects other properties than impact strength. It was already reported that filler intake can be increased and the rate of flow is increased. Addition of impact modifier lowers flame resistance of PVC (with the exception of when CPE is used) because impact modifiers are more flammable than PVC. Impact modifiers aid in dispersion of additives, modify the elasticity of melt, and enhance the processing window.Some impact modifiers help to improve the surface of calendered film and preserve the shape of molded and extruded products. At the same time, impact modifiers may increase die swell (e.g., Metablen P). 

Sol-Gel blends Improved rheology Low nerve Fast extrusion Low die swell Smooth extrudate surface Collapse resistance Better calendering Low tensile strength... 

Tougher and more resilient elastomeric materials were obtained after vulcanization (hot-press) of the elastomer with dicumyl peroxide, which was previously incorporated by calendering. In this case, semi-interpenetrating networks (semi-lPNs) were obtained by swelling the vulcanized rubber with a tetrahydrofiiran solution of FeCla, followed by exposure to pyrrole vapors [113]. A significant variation was observed when comparing the modulus, tenacity, and composi-... 

At high shear in deformation of the bulk specimen, extensional elasticity or viscosity is important. The above effect concerns processing of polymeric materials, such as die swell by extrusion, extension of sheet by calendering, expansion of film by inflation, and so forth. 

Still further modifications of natural rubber are the superior processing or SP rubbers. Here a portion of the latex is vulcanized before coagulation. In a sense, the vulcanized globules serve as small particle reinforcement of the rubber. Such rubbers help keep close dimensional tolerances or smooth finish requirements. Extrusion and/or calender output is improved, and die swell is lessened in lightly loaded compounds. In the interest of economical overseas shipments from the Far East, masterbatches of these rubbers, called Processing Aids (PA), are used. For example, PASO is a rubber that has 80 parts of vulcanized rubber and 20 parts of raw rubber. A softer material called PA57 has 57 parts of vulcanized rubber, 14 parts of raw rubber, and 29 parts of oil. 

SP rubbers find particular advantage when used in lightly loaded extruded or calendered products to the extent that even pure gum formulations can be processed as easily as heavily filled formulations. Particular benefits in extrusion are reduced die swell and faster throughput (Table 12). The effect of SP rubbers on vulcanizate properties is negligible. 

N624B 33 38-50 0.98 c NS Medium viscosity/medium resistance to fuels and oils. Specialty grade for extiuded/calendered flat goods, sponge, hose, and belting. Well-suited for mill mixing. Low water swell... 

Processing aids or additives are a class of chemicals which can be added to an elastomeric compound for the purpose of resolving processing problems. Those problems include poor release from the calender mill and mixer slow extrasion rates and high die swell poor mold flow, mold release, and mold fouling. 

Gel content in NBR is usefiil in limiting die swell and shrinkage in extrusion. With substantial NBR content, compounds are extraded like mbber— more or less on size and not drawn down. Minor gel content may also assist calendering and molding, but is not useful in coating applications. PVC gels, on the other hand, arc not useful in PVC/NBR blends they inhibit poor dispersion, and should be minimized. In dry-blend operations, particle size less than 1 mm is recommended for uniform distribution, particularly in low-shear operations. With compounds fluxed in an internal mixer, particle size is less significant. 

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