Resistivities humidity effects

In terms of trend the company is looking at a number of materials with interest. Photo manufacturers are studying sindiotac-tic polystyrene because it has very low moisture take-up. All its products need dimensional stability so materials which resist the effects of temperature and humidity changes are highly desirable. 

Uses Used in any type of coating (varnish, stain, solv.- or water-based, bake or air dry enamels) food pkg. coatings Features Exc. weather durability, water repellency, improved salt spray and humidity resist., improved slip and mar resist. min. effect on dirt pick-up, gloss, and clarity preserves integrity of thermoplastic coatings Reguiatory FDA 21CFR 175.300... 

Use of cyclic humidity, cyclic temperature, and cyclic bias are common to introduce conditions of condensation, distortions and electrolysis, and determine coating adhesion properties. The combination of physical distortion due to cyclic temperature, humidity producing condensation, and potential bias producing migration is important in determining the resistance and effect of moisture ingress in electronics. 

Effect of Conditioning Humidity on the Electrical Resistance of Rayon Yams, British Cotton Industry Research Association, London, 1945. 

In neutral and alkaline environments, the magnesium hydroxide product can form a surface film which offers considerable protection to the pure metal or its common alloys. Electron diffraction studies of the film formed ia humid air iadicate that it is amorphous, with the oxidation rate reported to be less than 0.01 /rni/yr. If the humidity level is sufficiently high, so that condensation occurs on the surface of the sample, the amorphous film is found to contain at least some crystalline magnesium hydroxide (bmcite). The crystalline magnesium hydroxide is also protective ia deionized water at room temperature. The aeration of the water has Httie or no measurable effect on the corrosion resistance. However, as the water temperature is iacreased to 100°C, the protective capacity of the film begias to erode, particularly ia the presence of certain cathodic contaminants ia either the metal or the water (121,122). 

Schmidt and Anderson (op. cit.) and Anderson [Physics, 3, 23 (July 1932)] claim that resistivity of the collected dust may be a controlling factor which is very sensitive to moisture. They state that an increase in relative humidity of 5 percent may double the precipitation rate because of its effect on the conductivity of the collected dust layer. 

The greater the degree of crystallinity the less the water absorption and hence the less will be the effect of humidity on the properties of the polymer. The degree of crystallinity also has an effect on electrical and mechanical properties. In particular high crystallinity leads to high abrasion resistance. 

The nylons are reasonably good electrical insulators at low temperatures and under conditions of low humidity but the insulation properties deteriorate as humidity and temperature increase. The effects of the amount of absorbed water on the volume resistivity of nylon 66 is shown in Figure 18.15. This effect is even greater with nylon 6 but markedly less with nylon 11. Some typical electrical properties of the nylons are given in Table 18.5. 

With tin coatings on brass, the interdiffusion of coating and substrate brings zinc to the surface of the tin the action can be rapid even with electrodeposited coatings. The effect of zinc in the surface layers is to reduce the resistance of the coating to dulling in humid atmospheres, and the layer of zinc corrosion product formed makes soldering more difficult. An intermediate layer of copper or nickel between brass and tin restrains this interdiffusion . 

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