Processing, thermoplastics voids

LARC-TPI is a linear thermoplastic PI which can be processed ia the imide form to produce large-area, void-free adhesive bonds. Mitsui Toatsu Chemicals, Inc., has obtained Hcense to produce this product commercially for appHcations such as adhesives, films, mol ding compounds, etc. These are thermooxidatively stable and show essentially no loss ia weight at 300°C ia air. Weight loss does not exceed 2—3% after isothermal aging ia air at 300°C for 550 h. 

Differences between the hydrophilic wood and the hydrophobic thermoplastic during processing can limit bond development, resulting in poor adhesion between wood fibers and the plastic polymer. Without chemical or physical bonding, failures on the wood-polymer interface and interfacial voids can develop because of poor processing or as a result of external factors, such as moisture uptake or UV degradation. These failures in the wood-polymer interface... 

Calendering n. The process of forming thermoplastics sheeting and films by passing the material through a series of rigid, heated rolls and rolling to press out voids. Four rolls are typical. The gap between the last pair of heated rolls determines the thickness of the sheet. Subsequent chilled rolls cool the sheet. The plastic compound is usually premixed and plasticated on separate equipment, then fed continuously into the nip of the first pair of calender rolls. 

In RIM, as in injection moulding, whether thermoplastic or thermoset, there is a transition zone between the nozzle (mix head) and the mould. In injection moulding, this transition zone includes the sprue/runner/gate system, and, in special cases, can also include a static mixer. The analogy to RIM is very apt. The transition zone for the RIM process also includes a sprue/runner/gate system, as well as an after mixer. One function of the RIM transition zone is to convert the mixing head liquid stream from a turbulent state to one which is in a laminar flow mode. If this does not occur, air traps (i.e. large subsurface voids in the finished part) can become a problem. 

It is now believed that the process of particle cavitation relieves the local buildup hydrostatic tension caused by constant volume shear banding. This allows additional enhancement of shear yielding in both thermosets and thermoplastics (16). Hence, soon after development of some initial shear yielding, the local triaxial constrains are relieved by cavitation even in relatively thick specimens. Similar explanations were proposed for PC/PE, PC/MBS (17), and rubber-modified epoxies (18-23). In polymers or under test conditions favoring crazing as a major deformation mechanism, cavitation and voiding of the rubber particles leads to a premature craze breakdown and it is damaging to the polymer. 

Starch can be nsed as a natural filler in traditional plastics (11,23-33) and par-ticnlarly in polyolefins. When blended with starch beads, polyethylene films (34) biodeteriorate on exposure to a soil environment. The microbial consumption of the starch component, in fact, leads to increased porosity, void formation, and the loss of integrity of the plastic matrix. Generally (32,35-38), starch is added at fairly low concentrations (6-15%) the overall disintegration of these materials is achieved by the use of transition-metal compounds, soluble in the thermoplastic matrix, as pro-oxidant additives which catalyze the photo- and thermooxidative process (39-44). 

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