Tear strength

Tear strength is important where there is any tension and the potential for nicking of the surface. The structure and internal bonding in the polyurethane give the tear properties. The high toughness of the esters makes them most suited, whereas materials made from the low-cost ethers have the worst resistance. 

Low-temperature applications are controlled by the stiffening of the polyurethane as the temperature goes from 20°C down to -20°C. In these applications, MDI ethers are the most suited, whereas ester-based materials become very stiff. At very low temperatures, esters have a lower brittle point than ethers but they must be kept from crystallizing. 

Small additions of non-interacting fillers cause only small changes in product hardness. Usually, such additions produce a 10-20% increase in the hardness compared to unfilled material.  

Tear strength data are very limited.Tearing energy is given by the following equation  

The quantities of multifunctional additives used must be selected with care as each type and grade of carbon black requires a specific but different amount to achieve optimum performance. To achieve optimum adhesion, the concentration of additive should coat the surface of carbon black with a monomolecular layer. Building additional layers on the carbon black surface reduces adhesion which also reduces tear strength. 

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Material Tensile strength, MPa Elongation, % Impact strength, (kN-m)/m Tear strength N/mm" Burst strength (Mullen)... 

CeUulose phosphate esters are also produced by treatment with sodium hexametaphosphate [14550-21-1] by the pad-dry-cure technique. These treated fabrics have high retention of breakiag and tearing strength (61). The reaction products contain more than 1.6% phosphoms and are iasoluble ia cupriethylenediamine [15243-01 -3] iadicating that some ceUulose cross-linking occurs. However, siace durable-press (DP) levels and wrinkle recovery values are low, it seems reasonable that only limited cross-linking takes place. 

Combination Flame Retardant—Durable Press Performance. Systems using THPC, urea, and TMM can be formulated to give fabrics which combine both flame-retardant performance and increased wrinkle recovery values (80). Another system employs dimethylol cyanoguanidine with THPC under acidic conditions (115). Both of these systems lead to substantial losses in fabric tensile and tearing strength. 

Tear Strength. A relation for the tearkig stress of flexible foams that predicts linear kicrease ki the tearkig energy with density and kicreased tearkig energy with cell size has been developed (177). Both relationships are verified to a limited extent by experimental data. 

Other reinforcements that may be used in the substrate layers of decorative laminates and throughout the stmcture of industrial laminates are woven fabrics of glass or canvas and nonwoven fabrics of various polymeric monofilaments such as polyester, nylon, or carbon fibers. Woven and nonwoven fabrics tend to be much stronger than paper and have much more uniform strength throughout the x—y plane. They greatly enhance properties of laminates such as impact and tear strength. 

The original expanded film membranes were sold ia roUs as flat sheets. These membranes had relatively poor tear strength along the original direction of orientation and were not widely used as microfiltration membranes. They did, however, find use as porous inert separating barriers ia batteries and some medical devices. More recentiy, the technology has been developed to produce these membranes as hoUow fibers, which are used as membrane contactors (12,13). 

Product Basis weight, g/m Thickness, mm a b Tensile strength, N a b Tear strength, N Mullen burst, kPa Bonding method... 

The tailoring of PE properties in commercial processes is achieved mostiy by controlling the density, molecular weight, MWD, or by cross-linking. Successful control of all reaction parameters enables the manufacture of a large family of PE products with considerable differences in physical properties, such as the softening temperatures, stiffness, hardness, clarity, impact, and tear strength. 

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. 

Blending with LLDPE is used to upgrade the properties and improve the processing of conventional LDPE. For example, by adding 25% of ethylene—1-butene LLDPE resin with I2 of 0.5 to conventional LDPE resin, the dart impact strength of 75 p.m film is increased from 490 to 560 g, the puncture strength from 41 to 49 J /mm (770-920 ft-lbf/in.), and the tear strength from 43 to 63 N /mm (246—360 ppi). CompositionaHy uniform VLDPE resins are used in blends with HDPE, commodity LLDPE, and polypropylene (PP) (70,71,89). 

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.

This reduces pulp tensile strength but improves tear strength not because of fiber degradation but because the hemiceUuloses bond chemically to give added tensile strength in the final paper product. To overcome this loss in hemiceUulose polymers, starch is added on the wet end of the paper machine as a dry strength additive. 

SiHcone elastomers possess outstanding resistance to heat aging. The Si—O—Si backbone imparts resistance to oxygen, o2one, uv, and to some polar fluids. However, the strength of these elastomers is usually just adequate. They have low abrasion resistance and tear strength (see Silicon compounds, silicones). 

Fig. 3. Effect of cross-link density where A represents tear strength, fatigue life, and toughness B, elastic recovery and stiffness C, strength and D,...

Conventional cure systems use relatively high levels (2.5 + phr) of sulfur combkied with lower levels of accelerator(s). These typically provide high initial physical properties, tensile and tear strengths, and good initial fatigue life, but with a greater tendency to lose these properties after heat aging. 

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