Extrusion Degradation

Mechanochemical degradation creates free macroradicals in pairs, practically without any side reactions, and most potential applications of this technique are centered around the formation and subsequent reactions of these reactive species. Elongational flow-induced degradation breaks polymer chains exactly at their center [42, 46]. This remarkable propensity is being explored in the author s laboratory as a simple means of obtaining well-defined block copolymers. Polymerization and 

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Medical extrusion can present special challenges in terms of product size, dimensional control, physical properties, and others. Automation of the extrusion line is critical to achieve high levels of process stability and reproducibility. Polymer degradation can be a significant concern in medical extrusion. Degradation is affected by the stresses and temperatures that occur in extrusion both depend strongly on the screw geometry. 

Polymer degradation is a frequent problem in extrusion. Degradation usually manifests itself as discoloration, loss of volatile components (smoking), or loss of mechanical properties. According to the mode of initiation, the following types of degradation can be distinguished ... 

In PP, peroxides promote extrusion degradation. The advantages of the controlled rheology resins lay in their high MFI and narrower MWD, preventing more output and higher orientation in some fine denier fiber operations There are some interactions between peroxides as a radical source and stabilizers, the performance of each of them being adversely affected. 

Extrusion Resins. Extmsion of VDC—VC copolymers is the main fabrication technique for filaments, films, rods, and tubing or pipe, and involves the same concerns for thermal degradation, streamlined flow, and noncatalytic materials of constmction as described for injection-molding resins (84,122). The plastic leaves the extmsion die in a completely amorphous condition and is maintained in this state by quenching in a water bath to about 10°C, thereby inhibiting recrystallization. In this state, the plastic is soft, weak, and pHable. If it is allowed to remain at room temperature, it hardens gradually and recrystallizes partially at a slow rate with a random crystal arrangement. Heat treatment can be used to recrystallize at controlled rates. 

Another event which may occur is hydrolysis. This is a chemical reaction between the plastic and water. It occurs extremely slowly at room temperature but can be significant at moulding temperatures. Hydrolysis causes degradation, reduction in properties (such as impact strength) and it is irreversible. Table 4.3 indicates the sensitivity of plastics to moisture. Note that generally extrusion requires a lower moisture content than injection moulding to produce good quality products. 

Polyurethane materials are extremely versatile in that it is possible to produce a large variety of structures which range in properties from linear and flexible to crosslinked and rigid. The crosslinked PURs are thermosets, which are insoluble and infusible and therefore cannot be reprocessed by extrusion without suffering extensive thermal degradation. At present, the main sources of recyclable waste are flexible PUR foams and automobile waste. Waste and scraps of these materials may consist of 15-25% by weight of total PUR foam production. 

Post-consumer PMMA plastics can be depolymerised back into their starting components. Degradative extrusion in twin-screw extruders can be used for this process. 10 refs. Translation of Kunststoffe, 87, No.2, Feb.1997, p.183-8... 

Ballast mats, raw material of degrading regenerated rubber The raw material of regenerated rubber Products of molding and extrusion, rubber mats, soft pipes for irrigating, vases, modihed asphalt for paving Used in renewal of tire (under 20 p.m, 30 phr)... 

Elucidation of degradation kinetics for the reactive extrusion of polypropylene is constrained by the lack of kinetic data at times less than the minimum residence time in the extruder. The objectives of this work were to develop an experimental technique which could provide samples for short reaction times and to further develop a previously published kinetic model. Two experimental methods were examined the classical "ampoule technique" used for polymerization kinetics and a new method based upon reaction in a static mixer attached to a single screw extruder. The "ampoule technique was found to have too many practical limitations. The "static mixer method" also has some difficult aspects but did provide samples at a reaction time of 18.6 s and is potentially capable of supplying samples at lower times with high reproducibility. Kinetic model improvements were implemented to remove an artificial high molecular weight tail which appeared at high initiator concentrations and to reduce step size sensitivity. 

Reactive extrusion is the chemical modification of polymer while it is being transported in an extruder. In this work, polypropylene is intentionally degraded by the addition of a free radical initiator (a peroxide) during extrusion. The product has improved flow properties because of the removal of the high molecular weight tail and the narrowing of the molecular weight distribution. 

The products formed after heating dried P-carotene at 180°C for 2 hr in a sealed ampoule (SI) with air circulation (S2) stirring with starch and water (S3) and during extrusion process (S4) were isolated. - In all systems, 5,6-epoxy-P-carotene (trans and two cis isomers), 5,8-epoxy-P-carotene (trans and four cis isomers), and 5,6,5,6-diepoxy-P-carotene were identified, along with 5,6,5,8-diepoxy-P-carotene in systems S3 and S4. Later on, along with the epoxides previously found, 5 P-apocarotenals with 20 to 30 carbons, P-caroten-4-one, and 6 different P-carotene cis isomers were isolated in systems S3 and S4, whereas lower numbers of degradation products were found in the other systems. ... 

Marty, C. and Berset, C., Degradation of rra - 3-carotene during heating in sealed glass tubes and extrusion cooking, J. Food ScL, 51, 698, 1986. 

Extrusion-cooking of cell-wall rich products (e.g. wheat bran, apple pomace, citrus peels, sugar-beet pulp, pea hulls.) led to an important solubilisation of polysaccharides of various types without extensive degradation of the polymeric structure. The possibility of obtaining gelled systems directly with the extruded pectin-rich materials was demonstrated. 

When comparing this result with that obtained with the acid-extracted pectins, the same two populations were separated but the amount of "hairy" regions is lower for the acid-extracted pectins (less than 10 % of the pectins) than for the pectins (=15 %). This result confirmed that extrusion-cooking is less degradative towards the side-chains that the acid treatment it also shows that both pectins have large "smooth" regions. 

Table 3.10 shows the recovery from PP of Irgafos 168 and its oxidised and hydrolysed by-products by various extraction procedures. As may be observed, One-Step Microwave-Assisted Extraction (OSM) and US lead both to negligible hydrolytic additive degradation. The measured additive decay (by oxidation) is essentially due to the antioxidant activity during the processing (extrusion) step of the polymer and not to the US or microwave heating treatment. 

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