Rubber modified impact grades

The hard (low natural mbber content) grades are best regarded as rubber-modified polypropylenes. Their main attribute is impact resistance, especially at low temperatures, for any given stiffness, as shown in Table 6. 

Craze formation is a dominant mechanism in the toughening of glassy polymers by elastomers in polyblends. Examples are high-impact polystyrene (HIPS), impact poly(vinyl chloride), and ABS (acrylonitrile-butadiene-styrene) polymers. Polystyrene and styrene-acrylonitrile (SAN) copolymers fracture at strains of 10 , whereas rubber-modified grades of these polymers (e.g., HIPS and ABS) form many crazes before breaking at strains around 0.5. Rubbery particles in... 

While bulk or emulsion polymerization can also be used for the purpose, the commercial manufacture of polystyrene is mostly carried out in a solution process using a free-radical initiator. The solvent, typically ethylbenzene, used at a level of 2-30%, controls the viscosity of the solution. High-impact-grade polymer used in injection-molding and extrusion is modified with butadiene rubber incorporated during polymerization. The solvent and residual monomer in the crude resin is removed by flash evaporation or in a devolatilizing extruder (at about 225°C). Figure 2.9 is a schematic of the polymerization process. 

The addition of rubber to polypropylene can lead to improvements in impact resistance. One of the most commonly added elastomers is ethylene-propylene rubber. The elastomer is blended with polypropylene, forming a separate elastomer phase. Rubber can be added in excess of 50 percent to give elastomeric compositions. Compounds with less than 50 percent added rubber are of considerable interest as modified thermoplastics. Impact grades of PP can be formed into films with good puncture resistance. 

Test and service data with PVC both rate low in notched Izod impact tests and performs well in normal service applications that involve impact loading. Another example is with some grades of rubber-modified high impact PSs that show up well in the Izod test fail on impact under field test conditions. These results have led to continual reexamination of the tests used to determine the toughness of plastics. 

Examination of the THF-insoluble fraction. This fraction consists of inorganic pigments and fillers such as chalk, titanium dioxide, alumina trihydrate, clay, etc. fire retardants such as antimony trioxide lead-based stabilisers such as TBLS or DBLP polymeric impact modifiers that are totally insoluble in THF, such as MBS and various acrylic rubbers and polymeric impact modifiers that are partially soluble in THF, such as nitrile rubber and some grades of ABS which, while mainly insoluble in THF, will yield a fraction consisting of SAN which is soluble in THF and will be found with the precipitated polymer. 

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