Viscosity foaming process

Demonstrations are given of the importance of extensional or elongational viscosity in the foam process. New polypropylenes are compared in extensional flow and it is shown how rheological differences allow the prodnction of low density foam on tandem extrnsion equipment. 6 refs. 

The material in the molding die has still a low viscosity permitting a foaming process. Finally, the form is cooled for a predetermined time period without pressure. 

The first step of the SRIM process is to place a fiber mat in a closed mold. Then, a low-viscosity foaming mixture is injected into the mold. The mixture penetrates into the mat and then expands to form a foamed composite. (SRIM is also called Mat-Molding RIM or MMRIM). SRIM is preferably used for making thin, large panels, such as door-trim panels, spare-tire covers, etc. 

Active ultrasound uses a source of sound radiation, which is appHed to a process sample, with a detector placed such that modification to the signal can be detected and related to changes in the sample. Signal attenuation, velocity measurements and wavelength selective absorption provide the means of probing the sample. This approach promises to provide both chemical and physical information but as yet has not been used extensively. A number of on-line polymer-related studies have been reported in which polymer flow behaviour, viscosity, blend characterisation, and foaming-process monitoring have been examined [19]. 

Reasons for use improve compatibility with other additives and polymers, improve flame resistance, improve low temperature performance, increase chain mobility, increase filler loading, increase flexibility and elongation, inflnence blood compatibility, lower glass transition temperature, lower viscosity, lower processing, gelation and fusion temperatures, modify foaming rate and microcellular stractirre... 

Warm mix asphalts (WMAs) are asphalts produced, handled and compacted at temperatures lower than those of conventional hot asphalts. This is achieved by lowering the viscosity of the binder using additives or applying foaming processes, both known as WMA systems. The aggregate mix gradation is the same as for the hot mix variety. 

The development of foams for medical purposes was described by Radusch [193], To eliminate thermal degradation, as well as to avoid the problems associated with the low melt viscosity, a process was developed based on solvent casting and subsequent particle leaching. Well-defined porous structures can be formed in such a way [197-200], for example, PHB was dissolved in chloroform, the ensuing solution was mixed with water-soluble particles like NaCl, films were then prepared by solvent casting and evaporation and, subsequently, the salt particles were washed out with water. Various foamed structures were thus prepared, depending on the particle size and amount, as well as other parameters, such as the PHB type and its concentration in the chloroform solution. 

Cross-finking increases melt viscosity, which improves the foaming process. It is used to produce heat-shrink film and tubing. It increases environmental stress-crack resistance, and it retains hot strength beyond the melting point of the crystalline phase. These improvements are particularly useful in wire and cable, pipe insulation, gasketing, and sports and orthopedic equipment. 

The reason for the use of partly cross-linked PE resins is the broader foaming window in the melt on the one hand, and the improvement of rigidity and strength of the foamed material on the other hand. Figure 1 demonstrates the influence of crosslmking of PE on the optimum melt viscosity for the foaming process [1]. 

Processing temperature is one of the most important variables in the extrusion foaming process because it affects both the melt viscosity and the amount of gas given off upon the thermal decomposition of CFA [81-83]. However, high processing temperatures should be avoided due to the thermal instability of PLA. Similarly, lower melt temperature should not be used to prevent partial decomposition of the CFA. The extruder barrel temperature profile should be set up based on the melting temperature of the polymer and the decomposition temperature of CFA [86]. 

Foaming by discharging the melt into the atmosphere, on the other hand, produced closed cells with a mean cell diameter of ca. 500 LLm. The foam cells were polyhedral in shape and quite uniform in size, which is comparable to a commercial foam with a mean cell size of ca. 1.0 mm. But, the DDC foams contained slightly collapsed cellular structures. Seemingly, this foaming process created a more rapid and uniform cooling of the melt, causing a rapid build-up of viscosity, which substantially stabilized bubbles. 

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