Melting strength

Thermoformability is a property required by the many sheet materials used in the thermoforming industry. These properties are unique for the specific forming methods used, and are best determined by actual thermoforming tests on smaU-scale equipment. The softening or drape temperature of the material, residual stress in the sheet from its manufacture, and its melt strength and viscosity are important parameters relating to this use. 

Very high melt strength is also observed in situations where the molten polymer contacts a sharp object. Skin packaging trials have successfliUy packaged objects such as arrowheads and fishhooks in ionomer films. 

Hot tack strength is the abiUty of a heat-seal layer to hold together while molten, before the seal cools and sets up. This is a technically important property which is difficult to measure reproducibly in the laboratory. Owing to the reinforcing effect of ionic bonding on melt strength, ionomer sealing layers provide superior performance in a wide spectmm of appHcations. 

Extensional Viscosity. AH three types of extensional viscosity can be measured (101,103) uniaxial, biaxial, and pure shear. Only a few commercial instmments are available, however, and most measurements are made with improvised equipment. Extensional viscosity of polymer melts can be estimated from converging flow (entrance pressure) or from a melt strength drawdown test (208). 

The high melt strength, facilitating thermoforming and blow moulding techniques. 

Narrow molecular weight distribution, which is characteristic of metallocene-based polyethylene (Fig. 7), causes processing difficulty in certain applications due to increased melt pressure, reduced melt strength, and melt fracture [14,15]. This problem can be overcome by blending the metallocene polymer with other prod-... 

Narrow molecular weight distribution Difficult extrusion except where NMWD resins are used. Low melt strength, good melt drawability Fiber/nonwovens Fiber/nonwovens... 

Tolerance The penalty for having an unbalanced wall is the reduction of tolerance control. Tolerance limits are usually at least doubled. Also, with certain plastics it is more difficult to process them, such as those with low melt strength. Although the balanced wall is the ideal, having it is not always possible. Recognize that the unbalanced wall can be extruded with proper die design and control of the extruder line from upstream to downstream equipment. 

The presence of long chain branches in low density polyethylene (LDPE) accounts for the difference in properties e.g. higher melt strength, greater toughness for the same average molecular weight) between LDPE and linear low density polyethylene (LLDPE, made by coordination polymerization). 

The coating of polymer that we put on paperboard serves two purposes when we convert it into juice and milk cartons. It prevents the contents from leaking out and it acts as an adhesive so that we can heat-seal the coated paperboard to itself to form a container. Low density polyethylene is the polymer of choice for this application it has sufficient melt strength to... 

The polymers used in blow molding must have a high melt strength. Why is this ... 

PBT-PC blends show increased melt strength allowing them to be easily processed by blow molding and profile extrusion. The PBT-PC blends have been extruded into sheet and thermoformed into parts. Enhanced melt strength allows PBT-PC blends to be foamed. Structural foam grades for injection molding (10-30% density reduction) are commercially available. 

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