Nylon failure

Repellents on Cloth. Each candidate repellent is appHed to a knit cotton stocking or cloth patch at 3.3 g/m cloth, usually as a 1% solution of active ingredient (AI) ia acetone. Two hours later, the stock or cloth patch is placed over an untreated nylon stocking on the arm of a subject, the hand covered, and the arm exposed to 1500 female mosquitoes for one minute. If fewer than five bites are counted, the test is repeated at 24 h, then weekly until failure, which is, by definition, five bites per minute. The standard mosquitoes used are Piedes aegppti Anopheles quadrimaculatus or M. albimanus. Candidate repellents ia cloth tests are ia one of the foUowiag classes class 1, effective 0 d class 2, 1—5 d class 3, 6—10 d class 4, 11—21 d and class 5, >21 d. 

Adhesion. Commercially available 1- or 2-coat adhesive systems produce mbber failure in bonds between ethylene—acryflc elastomer and metal (14). Adhesion to nylon, polyester, or aramid fiber cord or fabric is greatest when the cord or fabric have been treated with carboxylated nitrile mbber latex. 

Whilst the aliphatic nylons are generally classified as being impact resistant, they are affected by stress concentrators like sharp comers which may lead to brittle failures. Incorporation of mbbers which are not soluble in the nylons and hence form dispersions of rubber droplets in the polyamide matrix but which nevertheless can have some interaction between mbber and polyamide can be most effective. Materials described in the literature include the ethylene-propylene rubbers, ionomers (q.v.), polyurethanes, acrylates and methacrylates, ABS polymers and polyamides from dimer acid. 

Figure 19.7. Fatigue resistance of acetal resin compared with nylon 66 and with polyethylene. Measured as the cycles to failure at a given applied stress. (Du Pont trade literature)...

All failures were cohesive within the adhesive. Adhesive E610 nylon epoxy (Asulab SA). 

Polycarbonate is perhaps the most notoriously notch-sensitive of all thermoplastics, although nylons arc also susceptible to ductileAjrittle transitions in failure behaviour caused by notch sharpening. Other plastics such as acrylic, polystyrene and thermosets are always brittle - whatever the crack condition. 

Nimmo, S. R. Nunns, and B. W. Eddershaw, Lessons Learned from the Failure of a Computer System Controlling a Nylon Polymer Plant, Paper presented at Safety and Reliability Society Symposium, Altrincham, UK, Nov. 1987. 

Based on this analysis it is evident that materials which are biaxially oriented will have good puncture resistance. Highly polar polymers would be resistant to puncture failure because of their tendency to increase in strength when stretched. The addition of randomly dispersed fibrous filler will also add resistance to puncture loads. From some examples such as oriented polyethylene glycol terephthalate (Mylar), vulcanized fiber, and oriented nylon, it is evident that these materials meet one or more of the conditions reviewed. Products and plastics that meet with puncture loading conditions in applications can be reinforced against this type of stress by use of a surface layer of plastic with good puncture resistance. Resistance of the surface layer to puncture will protect the product from puncture loads. An example of this type of application is the addition of an oriented PS layer to foam cups to improve their performance. 

There is another type of condition that results from exposure to high humidity. The alteration in electrical properties caused by moisture absorption in nylon and phenolics is reversible. When the moisture content is decreased, the properties of the materials recover to close to the original values. In some instances the exposure to moisture and electrical fields can cause irreversible damage that can lead to failure. 

The tensile strength of compacts [30] also provides useful information. Excellent specimens of square compacts are necessary to conduct the tensile testing. For this reason, a split die [31 ] (Fig. 2) is used to make compacts that are not flawed. The split die permits triaxial decompression, which relieves the stresses in the compact more uniformly in three dimensions and minimizes cracking. These specimens are then compressed with platens 0.4 times the width of the square compacts in the tensile testing apparatus. (Fig. 3). Occasionally nylon platens and side supports are used to reduce the tendency to fail in shear rather than tension. The force necessary to cause tensile failure (tensile forces are a maximum... 

It is common for liquid nitrogen frozen protein crystals to acquire a patina of ice on the surface of the cryoprotectant. Diffraction of X-rays from even small ice crystals can mask reflections from the protein crystal. In addition, the presence of excessive amounts of ice can obscure the true position of the nylon loop, thereby resulting in the failure to place the crystal in the X-ray beam. It is, therefore, essential to remove ice crystals prior to diffraction analysis. 

Not only is low tensile shear strength noticed on moisture aging, but also the mode of failure changes from one of cohesion to adhesion. Table 7.6 shows the effect of humidity and water immersion on an epoxy-nylon adhesive compared to a nitrile-phenolic adhesive. Substrate primers have been used with epoxy-nylon adhesives to provide improved moisture... 

Certain adhesive systems are more resistant to interfacial degradation by moist environments than are other adhesives. Table 15.16 illustrates that a nitrile-phenolic adhesive does not succumb to failure through the mechanism of preferential displacement at the interface. Failures occurred cohesively within the adhesive even when tested after 24 months of immersion in water. A nylon-epoxy adhesive bond, however, degraded rapidly under the same conditioning owing to its permeability and preferential displacement by moisture. 

Type 2 is ductile crack propagation with the crack opening to form a V-notch. Finally, the crack becomes catastrophic. It is found in nylon and other melt-spun synthetics. Classical illustrations of this type are readily available (3, 4). However, variations can occur. Figures la and lb show a polyester fiber subjected to tensile stress in which, after the V-notch formed, failure continued along a plane parallel to the fiber axis before eventually crossing the fiber. Thus, a split-level transverse break had occurred. 

Figure 30 shows how the number of microvoids increased with time under load in a PVC film, though a simflar behaviour was found in oriented polycaproamide (nylon 6). The initial rate of formation of voids, dNjdt was logarithmic with tensile stress. A further important observation was that the critical concentration of cavities just before macroscopic failure appeared to be constant for specimens whose lifetimes under load varied over four decades of time (accordii to the applied load). This is shown in Fig. 31. The same critical concentration of microvoids was found close to the tip of a propagating crack.

Fig. 31. Concentration of miaovoids at failure as a function of time to failure for nylon 6 fibres (after Ref. ...

Temperature stability is important for some applications of SILMs in gas separation, such as capture of CO from coal gasification plants. Ilcovich et al. [25] analysed the stability of a SILM based on [hmim+lfNTfj ] supported on a polysulphone organic membranes in the selective separation of CO from He at high temperature. This membrane was found to be stable up to 125 C, the failure of the membranes above that temperature being attributable to support failure rather than any effect on the ionic liquid. Recently, Myers et al. [32] reported operation of [hmim ][NTfj ] supported on nylon membranes up to 300 C. It was found that permeability in this [hmim [NTfj ] membrane increased with temperature while the selectivity decreased. 

Coleman [11] has exploited the additivity of damage concept to describe the failure of nylon fibers. He extended the concept by using the statistics of extreme values to describe the distribution of failure times in different loading conditions. His analysis demands that a material which obeys linear additivity of damage should have the same distribution of failure times Independent of the type of stress history. A good measure of this Is the coefficient of variation of the distribution of... 

---