Resistance to tear

The second parameter, tear strength, describes the film resistance to tear propagation. It is measured with a special apparatus, the Ehnendorf Tear Tester (ASTM D1922), and defined as the weight of a loaded pendulum capable of tearing a notched piece of film. Two values are usually measured for each film sample. One determines tear propagation in the machine direction of the film, the other in the transverse direction. 

Tables 4—6 Ust ASTM methods used for the characterization of PB and PMP. A number of specialized methods were developed for testing particular articles manufactured from polyolefins several of these determine the performance of PB and PMP film, including the measurement of the film s dart impact strength and tear strength. Dart impact strength is measured by dropping a heavy dart with a round tip on a stretched film. Tear resistance, which reflects the film s resistance to tear propagation, is measured with the Ehnendorf tear tester. Two values for the tear strength are usually reported, one in the machine dkection of the film and the other in the transverse dkection. Pipes manufactured from PB are tested by pressurizing them internally with water the time-to-burst failure is determined at various temperatures (46). The standard test method for haze and luminous transmittance (ASTM D1003) is used for the measurement of PMP optical characteristics.

Film. Blown film manufactured from PB has a high tensile strength and exhibits good resistance to tear, impact, and puncture (47). Such film also exhibits hard-elastic behavior that is, it can recover its original length even after extensive stretching. Some properties of PB film are given in Table 5. 

Struktur-farbe, /. structural color, -festigkeit, /. resistance to tearing, -formel, /. structural formula. 

There are several ways to measure a material s resistance to tearing. In these tests, the applied force is not distributed over the entire specimen but is concentrated on a slit or notch and the tear strengtii is reported as the force required to propagate a tear from this point. For urethane elastomers and foams, the most... 

Molecular weight. The molecular weight influences the melt viscosity, tensile strength, the low temperature brittleness, and the resistance to tearing. 

Thermal black Soft carbon black formed by the decomposition of natural gas (e.g., MT, medium thermal black). It has little stiffening effect, but imparts toughness, resilience, good resistance to tearing, and fair abrasion resistance. 

Carboxylated nitrile rubber (XNBR), a high-performance elastomer, is remarkable for providing vulcanizates that exhibit enhanced tensile strength, elastic modulus, hardness, and improved resistance to tear, abrasion, and the deleterious action of... 

Blown film manufactured from PB has a high tensile strength and exhibits good resistance to tear, impact, and puncture. 

The two most commonly specified tests are the Die C test and the "trouser tear" (D470) test. Both of these tests measure resistance to tear propagation. Other test shapes include the crescent tear with or without end tabs. This sample shape is often nicked. The "Delft" (IS0816) sample with its internal nick is often used from samples taken from finished products. The exact size of the nick does not appear to be critical to the result. 

The application for the film helps to determine which of the two types is preferred if resistance to tearing is very important a film with constant thickness is better, but if the reverse is to be coated with adhesive it may be advantageous if it is smooth. 

The textural implications of the above characteristics of the stress-strain relationships are not always clear. When one examines a bread loaf or a roll with the fingers to evaluate its freshness, it seems obvious that the perceived mechanical stimulus is associated with the first region of the curve. Yet, in mastication, the compact s resistance to tearing probably plays a more significant role than the first and second stages of the compression. At the point where the bread crumb is tom, however, the specimen may have already been wetted by saliva so that the relationship between the stress-strain characteristics of the dry sponge and its perceived textural properties is usually obscured. 

The service performance of rubber products can be improved by the addition of fine particle size carbon blacks or silicas. The most important effects are improvements in wear resistance of tire treads and in sidewall resistance to tearing and fatigue cracking. This reinforcement varies with the particle size, surface nature, state of agglomeration and amount of the reinforcing agent and the nature of the elastomer. Carbon blacks normally are effective only with hydrocarbon rubbers. It seems likely that the reinforcement phenomenon relies on the physical adsorption of polymer chains on the solid surface and the ability of the elastomer molecules to slip over the filler surface without actual desorption or creation of voids. 

White side wall processability, cure rate, hardness, stress-strain properties, adhesion to adjoining components, resistance to tear, cut/crack propagation resistance, resistance to ozone, oTcygen and UV, retention of white color on exposure to environment, low cost... 

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