Weathering particle size effects

The weathering properties of polyethylene are improved by the incorporation of carbon blacks. Maximum protection is obtained using blacks with a particle size of 25 p,m and below. In practice finely divided channel or furnace blacks are used at 2-3% concentration and to be effective they must be very well dispersed into the polymer. The use of more than 3% black leads to little improvement in weathering resistance and may adversely affect other properties. 

A study of the effect on insects of varying the particle size of deposits on different surfaces indicated that the residues from sprays of flne particle size are most effective soon after application, but that residues of the large particles will weather best and be least absorbed in the material. Large particles, therefore, wUl give more effective residue over a longer period than the fine particles. 

Particle size is important and, for some applications requiring good weathering and impact performance (window profile), the ultrafine milled, high whiteness, natural version is normally used. To ease dispersion, the filler is usually coated with stearic acid. Coated ultrafine and precipitated calcium carbonates are claimed also to have a positive effect on impact properties in impact modified formulations (52, 294, 462). The abrasive wear of calcium carbonate, on melt processing equipment, is not significant but increases with increasing levels (177). 

It is obvious from the data of Tables ll-VIII to I l-XII that the emanation power varies widely. As a rule, the finer the particle size the greater the emanation power. However, when samples are severely weathered or damaged, as in the case of radioactive ores, further comminution of a specimen has little effect on the emanation power. In both cases it is believed that Ra has migrated from the original site of its parent in the crystal lattice and has adsorbed on the surfaces of microfractures facilitating Rn escape. When certain samples are pulverised in the laboratory they may even emit less Rn than the whole lump. This can happen when U and its decay products are associated with minerals that do not pulverise as easily, i.e., are less friable. Starik and Melikova (1957) found that a whole lump of ore had an emanation power of 46%... 

The addition of pigments to polymers is a very effective practical method for increasing the resistance to photodegradation. Carbon black is by far the most effective. It has been used for years as a filler in rubber vulcanites, but in thermoplastic materials such as polyolefins it also has a protective effect against photodegradation. This was shown for the first time in 1950 by Wallder et al. [126] they demonstrated that the weathering properties of polyethylene are improved by the addition of about 2% carbon black. The effect is dependent on the degree of dispersion and on the size of the particles. Channel Black (particle size about 30 nm), for instance, is more efficient than Furnace Black (particle size 80 nm). The role of carbon black is not only to prevent the absorp-... 

Figure 7.2 summarizes the relative weathering rates of major minerals in igneous and metamorphic rocks. Actual weathering rates depend also on soil temperature and moisture, particle size, and planes of physical weakness (cleavage) in the crystal. The effect of moisture includes both the flow rate of soil solution past mineral surfaces and the composition of the solution. Solids dissolve more slowly if the solution already contains their constituent ions. High electrolyte concentrations, on the other hand, can maintain higher ion concentrations at equilibrium because of lower activity coefficients and because of complex-ion and ion-pair formation. 

Smaller particles weather more rapidly, but the size effect is great only when the particles are less than several micrometers in size. Cleavage planes allow particles to be more easily broken apart. Feldspars and micas, for example, have clearly defined cleavage planes. Particularly in the case of feldspars, the cleavage planes hasten the rate of mineral breakdown. 

Much of the use of karaya is dependent upon its viscosity and capacity for water absorption. The viscosity of the crude gum is influenced to a large extent by the weather conditions and season during which the gum is collected. After collection, karaya loses viscosity. Particle size, heat, and moisture are important factors contributing to this deterioration. In the crude gum, where the size of the tear is relatively large, the loss in viscosity occurs at a slow rate. When the gum is powdered, the rate of loss of viscosity is appreciably increased. It is not the fine division of the gum, in itself, that is responsible for the increased deterioration, but rather the increased effect of heat and moisture on such material. The conditions described that increase the rate of acetic acid liberation are similar to those that cause viscosity deterioration. A loose correlation is, therefore, suggested between the loss of acetic acid and the decrease in viscosity. 

If the size limits of the analyzed fractions are close, two or three fractions should be studied, and the results set out to show differences in the particle-size distributions of minerals between successive horizons of the soil profile. Such differences can be attributed to the effects of pedological weathering. 

---