Strength, impact

Impact strength, or ability of a part to absorb energy, translates into its ability to develop an internal force multiplied by the deformation of the part without failure. This is difficult to predict because the design characteristics of a part, such as a metal spring as opposed to a flat metal plate, have a major effect on its ability to absorb impact. 

Designing flexibility into the part to increase the length over which energy is distributed reduces the internal force required to resist impact. 

Generally, fluoropolymers have excellent impact strength in a broad temperature range. They can withstand significant impact force at cryogenic temperatures. Tables 3. 49 and 3. 50 present examples of impact strength of different fluoroplastics. 

Molecuiar Weight (MW) MFR, g/10 min Flex Life, cycles to failure (0.19 mm thickness specimen) 

To obtain the impact strength of nanocomposites, an Izod impact machine tester with digital display (JBL-22, Shenzhen KQLTesting Instruments Co., Ltd) was used according to GB/T 1843-1996. Unnotched specimens with rectangular dimensions of 80 X 10 X 4 mm were fractured by an impact energy of 5.5 J with an impact speed of 3.5 m/s at room temperature. The distance between the impact point and fixed point was set to be 22 mm. 

ASTM D950-03 Standard test method for impact strength of adhesive bonds. 

Other properties must be considered for design purposes. Two important mechanical properties that remain to be discussed are the impact strength and fracture toughness. These are considered in depth in this chapter. 

One measure of the impact strength is the area under the stress (strain) curve for a tensile test conducted at a high strain rate. This measure is called the modulus of toughness. Obviously, a thermoplastic-type polymer with a relatively high elongation (ductility) will have a higher modulus of toughness than does a more brittle thermoset. 

FIGURE 5.1 Charpy and Izod impact specimens, (a) Impact pendulum device, (b) loading of Charpy specimen, (c) loading of Izod specimen, and (d) dimensions of Izod specimen. (Adapted from Tensile Impact Energy to Break Plastics and Electrical Insulating Materials, ASTM Standard DI822, West Conshohocken, PA, 1999.) 

Sources Adapted from Guide to Plastics, Property and Specification Charts, McGraw-Hill, New York, NY, 1985 Engineering Plastics, Engineering Materials Handbook, Vol. 2, ASM, Metals Park, OH, 1988. 

Polymers will exhibit a ductile/brittle transition temperature in impact tests similar to that observed in metals. In polymers, the transition temperature T, is close to the glass-transition temperature for large-width specimens (plane strain condition). 

The valuable characteristics of polyblends, two-phase mixtures of polymers in different states of aggregation, were also discussed in the previous chapter. This technique has been widely used to improve the toughness of rigid amorphous polymers such as PVC, polystyrene, and styrene-acrylonitrile copolymers. 

BRYDSON, J. A., Chapter entitled Glass Transition, Melting Point and Structure in Polymer Science (Ed. JENKINS, A. D.), North-Holland, Amsterdam (1972) 

BiLLMEYER, F. w., Tcxthook of Polymer Science, Interscience, New York (1962) 

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Tetralluoroethylene polymer has the lowest coefficient of friction of any solid. It has remarkable chemical resistance and a very low brittleness temperature ( — 100°C). Its dielectric constant and loss factor are low and stable across a broad temperature and frequency range. Its impact strength is high. 

The isopropylidene linkage imparts chemical resistance, the ether linkage imparts temperature resistance, and the sulfone linkage imparts impact strength. The brittleness temperature of polysulfones is — 100°C. Polysulfones are clear, strong, nontoxic, and virtually unbreakable. They do not hydrolyze during autoclaving and are resistant to acids, bases, aqueous solutions, aliphatic hydrocarbons, and alcohols. 

Hardness, Rockwell (or Shore) Impact strength (Izod) at 23°C, M94 M78 M90 M79 M78... 

Testing. Chemical analyses are done on all manufactured abrasives, as well as physical tests such as sieve analyses, specific gravity, impact strength, and loose poured density (a rough measure of particle shape). Special abrasives such as sintered sol—gel aluminas require more sophisticated tests such as electron microscope measurement of a-alumina crystal si2e, and indentation microhardness. 

Development of toughened acetals has recendy dourished. One such grade, Delrin lOOST, boasts a notched-impact strength of 900 J/m (16.86 ft-lb/in.) at 23°C, substantially higher than that of nonmodified, general purpose grades (Table 2). 

Acryhc modifiers for cement impact strength and adhesion to substrates are discussed in reference 211. Both water-soluble acryhc and acryhc emulsion polymers are used in the ceramic industry as temporary binders, deflocculants, and additive components in ceramic bodies and glazes (212) (see Ceramcs). 

The routine compositional and functional testing done on the adhesives includes gas chromatographic testing for purity, potentiometric titrations for acid stabilizer concentrations, accelerated thermal stabiUty tests for shelf life, fixture time cure speed tests, and assorted ASTM tests for tensile shear strengths, peel and impact strengths, and hot strengths. 

AN, wt % Tensile strength, MPa Elongation, % Impact strength, J/m notch Heat distortion, temp., °C Solution viscosity, mPa-s (=cP)... 

Thermal Properties. ABS is also used as a base polymer in high performance alloys. Most common are ABS—polycarbonate alloys which extend the property balance achievable with ABS to offer even higher impact strength and heat resistance (2). 

Some cast (unoriented) polypropylene film is produced. Its clarity and heat sealabiUty make it ideal for textile packaging and overwrap. The use of copolymers with ethylene improves low temperature impact, which is the primary problem with unoriented PP film. Orientation improves the clarity and stiffness of polypropylene film, and dramatically increases low temperature impact strength. BOPP film, however, is not readily heat-sealed and so is coextmded or coated with resins with lower melting points than the polypropylene shrinkage temperature. These layers may also provide improved barrier properties. 

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