Molecular weight blow moulding

Fligh molecular weight, hroad MWD polymer—used primarily for blow moulding and pipe. 

High molecular weight HDPE (HMW-HDPE) is used for blown film and for demanding moulding and structural uses. Examples are blow moulded drums for packing dangerous chemicals and pressure piping. 

For extrusion and blow moulding the polysulphones used are of higher molecular weight. Melt temperatures for blow moulding are of the order of 300-360°C with mould temperatures about 70-95°C. 

It will be seen that a polymer of molecular weight A/, may be processed either as a liquid (by injection moulding, extruding, etc.) or as a rubber (by vacuum forming, sheet blowing, warm forging). In the case of the polymer... 

Another serious effect occurs with liquids which are not in themselves solvents but which may wet the polymer surfaces. These facilitate relief of frozen-in stresses by surface cracking which can be a severe problem in using many injection and blow mouldings with specific chemicals. Examples of this are white spirit with polystyrene, carbon tetrachloride with polycarbonates and soaps and silicone oils with low molecular weight polyethylenes. 

PET is a material that finds widespread use for soft drink and beverage bottle applications. For injection or blow moulding applications, high-molecular-weight... 

In this context plasma is a gas containing free electrons, ions, and neutral particles and may be created by applying external excitation to the gas. When used to polymerize ethylene a plasma can give coatings of higher molecular weight, cross-linked and without pores—and provide relatively impermeable barrier layers on blow mouldings. 

PET is a material that finds widespread use for soft drink and beverage bottle applications. For injection or blow moulding applications, high-molecular-weight PET Mn > 30000 is required. While PET with of 15 000 — 25 000 can be achieved by a standard melt-polymerization process, the high-molecular-weight PET grades require a solid-state polymerization process. Commercial solid-state polymerization process systems are usually composed of a crystallizer and a polymerization reactor. First, melt-polymerized chips are fed into the crystallizer unit and crystallized to the extent of about 40 %. In a second step, tlie crystallized chips are fed into a polymerization reactor vessel and then polymerized in the solid state at a temperature of around 220 °C. 

Melt degradation is used commercially to narrow the molecular weight distribution, from a starting value of Mw/Mn = 5, to make it more suitable for fibre spinning and blow moulding. Random chain scission at tertiary C—H bonds eventually produces a most probable molecular weight distribution with M /Mn = 2. In practice, the distribution, produced by deliberately degrading the polypropylene with added peroxides, is broader, but... 

In PVC technology, acrylic-processing aids made from high molecular weight PMMA are used to improve the siuface finish in extruded profiles, film, sheet, blow mouldings and injection mouldings. The dose is typically between one and two phr. 

Expansion tanks, instrument panel sections and heater ducting are long-established blow moulding applications (in PP), but by far the most important application is the fuel tank, in high molecular weight forms of HOPE. Since its first volume application in the VW Passat of 1973, the concept has spread very widely, into some very strange custom-built shapes, literally designed to fit the space available. 

Over the last forty years or so there has been a steady increase in the variety and scope of underbonnet fluid containers made in plastics. Washer fluid reservoirs in LDPE were the first much more demanding applications have followed, with HOPE and PP as cooling system expansion tanks and nylon for hot oil containers. A spectacular recent arrival from BASF is a 50-litre hydraulic oil reservoir for trucks, blow moulded in nylon 6 and with very high molecular weight. 

With the advent of very high molecular weight HDPE for applications such as large rigid blow-moulded containers, there was an impetus to improve the forwarding efficiency of the extruder feed zone as the feed material was a low coefficient of friction powder/crumb, with a bulk density less than other polyolefines. 

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