Some metal ions may contribute to accelerated degradation. For polyolefins in contact with metals, such as cable insulation of copper wires, metal deactivators are added to passivate the degradation reactions.
A property of the polymer melt which indicates its ability to withstand drawing without breaking. Melt strength is improved by the presence of high molecular weight tail or long chain branches.
Medium low density polyethylene
Based on the same technology as HDPE, but the density is reduced through the incorporation of longer a-olefins as comonomers. With this, another way of adapting the products to the final applications turns up. Single-site catalysts allow an expansion of the application range, mainly by allowing higher comonomer contents and a more even distribution of these.
Melt mass flow rate or melt flow rate (MFR) gives an indication of melt processing properties of a polymer. Shear rate range is, however, narrow and much lower than in the most of processing applications. It gives thus very limited information about melt flow behaviour. The method is a kind of capillary measurement.
The equipment consists of a barrel, a piston with a place for loading masses and a die. The sample is charged into the barrel and heated up to a specified temperature. After that the piston is loaded extruding the polymer through the die.
Melt flow rate (MFR) is defined as a mass flown through the die in 10 min. The weights mostly used with polyolefins are 2.16 kg, 5 kg, 10 kg and 21.6 kg. In addition to the evaluation of flow properties MFR serves as a rough estimation of molecular weight.
Usually also Flow Rate Ratio (FRR) is determined. This ia defined as a ratio of two MFR values obtained with two different weights. FRR can be used as a rough estimation of molecular weight distribution.
Monomer forms a repeating unit in a polymer chain.
Morphology is the internal or external structure of a solid substance. Important sub-definitions include
(a)Crystal morphology of a polymer film or part (b)Phase morphology of heterophasic polymer systems (blend, copolymer) (c) Powder morphology of a polymer powder from the polymerisation reactor.
|Molecular Weight Distribution |
The relation between the number of molecules in a polymer and their individual chain length. It is often also called molar mass distribution (MMD). Technical polymers are always polydisperse as a result of the nature of the catalyst and the polymerisation conditions. The MWD is often characterised by size exclusion chromatography (SEC), but it can also be calculated approximately from rheological data for purely linear polymers.
A catalyst which contains a metallocene compound of a transition metal. Often used as a synonym for single site.
Multimodal polymer includes more than one molecular weight fraction.
|Melt tension test|
Extensional properties play an important role in some processes such as melt spinning, blow moulding and film blowing. Melt tension test is a simple method for characterisation of extensional properties. The measuring principal copies industrial spinning process. A fluid is pressed or extruded through a round hole die, the resulting thread is hauled off with increasing speed and the applied extensional force is measured. The force is recorded as a function of haul-off speed or draw-down ratio, which is the ratio of the haul-off speed to the initial speed. Melt strength is defined as a force at filament break, while drawability can be estimated qualitatively from the maximum draw-down ratio at the failure.