Material properties tear strength

The tear strength is considered as the clinically most important physical property of the material. The tear strength of the PDMS is extremely important particularly at the thin margins surrounding the finishing borders of the prosthe-ses. This thin margin helps to camouflage the presence of... 

Cured sihcone LIM mbber can be fabricated with physical properties equivalent to heat-cured mbber (385). Shore A hardness can range from 30 to 70, depending on formulations. Typical physical properties include tensile strengths as high as 9.7 MPa (1400 psi), 500—775% elongation at break, and tear strength of >30 N/mm (180 Ib/in.). Compression sets of less than 10% can be achieved if the material is baked after processing. 

Although a variety of test methods, eg, Dk, modulus, and tear strength, exist to determine key properties of potential contact lens materials, a number of properties, eg, wettabihty and deposition, have no predictive methodology short of actual clinical experience. 

To improve the rheological properties and extend the very short working time, a simple polyester is kicluded as thinner. Mixing is easy, and dimensional change ki ak is less than 0.1% over several hours. Elastic recovery and reproduction of detail are exceUent. The elastomeric cycHc imine impression materials have a higher modulus of elasticity than the condensation siHcone or polysulfide mbbers, and are more difficult to remove from the mouth. The materials have relatively low tear strength and an equUibrium water sorption of 14% thus, polyether impression materials tear readily. Because of thek poor dimensional stabUity ki water, they should be stored ki a dry environment. 

Tear strength is only applicable to flexible materials and is very little used to monitor ageing simply because tensile strength will serve perfectly well. There are circumstances where compression stress-strain properties would be relevant but the relatively bulky test pieces will be subject to the limitation of oxygen diffusion in any accelerated tests and changes can probably be estimated from tensile measurements. Similarly, shear stress-strain is very rarely used for monitoring ageing. 

It is not surprising that, given the importance of tearing and the different levels of result obtained from different geometries, a considerable number of tear tests have been devised which, in part, reflect the different stress concentrations found in various products. The arbitrary nature of the geometries means that, in general, the measured tear strength is not an intrinsic property of the material and it is difficult to directly correlate the results of laboratory tests with the performance of products in service. 

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. 

The manufacturing process and properties of polybutene-1 are comparable to PP. Compared to the aliphatic hydrocarbons, this material is not as inert as PE and PP. Its high burst strength and tear strength are very advantageous for the manufacture of hot water pipes (resistant up to 95 °C). 

These plastics are quite inert thus, compatibility with other substrates does not pose major problems. The terephthalates have high tensile and tear strengths, excellent chemical resistance, good electrical properties, and an operating temperature range from —55 to 200°C. These materials are generally joined with adhesives, and surface treatments are used to enhance adhesion, if required. 

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