Weathering properties

The combined effect of the factors mentioned above may be much more severe than the effect of any single factor, and the degradation processes are accelerated many times. Many test results do not include these synergistic effects, which almost always exist in real-life situations. 

Handbook of Plastics Testing and Failure Analysis, Third Edition, by Vishu Shah Copyright 2007 by John Wiley Sons, Inc. 

Ultraviolet stabilizers, unlike UV absorbers, inhibit the bond rupture by chemical means or dissipate the energy to lower levels that do not attack the bonds. The effectiveness of such additives when incorporated with the polymer can be determined by various test methods which are discussed in this chapter. 

It is necessary to evaluate the effectiveness of various antimicrobial agents both on a laboratory scale and in actual outdoor exposure. Many methods have been devised to perform these tests. The test methods and their serious limitations are also discussed in this chapter. 

Oxygen, Moisture, Thermal Energy, and Other Environmental Factors 

---

In addition to the paving grades of emulsions, a variety of products are available that are suitable for industrial uses, for paper sidings, and for use in the roofing appHcation field. Asphalts emulsified with colloidal clays are especially suitable for outdoor appHcations because of their exceUent weathering properties. Some of these products have been adapted for use with roofing systems, either with bmsh or spray appHcation. 

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. 

The hydrogenated materials have a higher maximum service temperature than the conventional SBS materials and, because of the absence of double bonds in the chain, better weathering properties. Known as SEBs these materials now find use in adhesives, sealants and roofing membrane compounds. These also find use in highly filled automotive acoustic barriers and in medical, sports and leisure applications. 

The important features of rigidity and transparency make the material competitive with polystyrene, cellulose acetate and poly(methyl methacrylate) for a number of applications. In general the copolymer is cheaper than poly(methyl methacrylate) and cellulose acetate, tougher than poly(methyl methacrylate) and polystyrene and superior in chemical and most physical properties to polystyrene and cellulose acetate. It does not have such a high transparency or such food weathering properties as poly(methyl methacrylate). As a result of these considerations the styrene-acrylonitrile copolymers have found applications for dials, knobs and covers for domestic appliances, electrical equipment and car equipment, for picnic ware and housewares, and a number of other industrial and domestic applications with requirements somewhat more stringent than can be met by polystyrene. 

ACS polymers, developed primarily in Japan, are grafts of acrylonitrile and styrene onto elastomeric chlorinated polyethylene. Although the polymer has good weathering properties it is somewhat susceptible to thermal degradation during processing and to date these polymers have been of limited interest. 

One such system involved grafting 70 parts of methyl methacrylate on to 30 parts of an 81-19 2-ethylhexyl acrylate-styrene copolymer. Such a grafted material was claimed to have very good weathering properties as well as exhibiting high optical transmission. 

Where transparency is required, a range of polymers is available. Polystyrene is the least expensive but polymethylmethacrylate has an outstanding high light transmission combined with excellent weathering properties. Also to be considered are the polycarbonates, glass-clear polyamides, SAN, butadiene-styrene block copolymers, MBS polymers, plasticised PVC, ionomers and cellulose esters such as cellulose acetate. 

P.V.C. is used extensively in the building industry for external cladding and internal partitions. It is used because it has excellent weathering properties and will protect the substrate against corrosion for periods in excess of 10 years. When it is applied at a thicknesses of about 200 /tm it can withstand the hard handling techniques often associated with building sites. 

Kaufman, F. S., Jr., "A New Technique for Evaluating Outdoor Weathering Properties of High Density Polyethylene", in ref. 

Yusuf, S., Takahashi, M. and Imamura, Y. (1989). Particleboard from acetylated albizzia particles. III. Enhancement of decay resistance, termite resistance, and weathering properties through acetylation. Mokuzai Gakkaishi, 35(7), 633-639. 

The effect of make-up of roofing asphalts on weathering properties, in terms of the fractions asphaltenes, resins, and oils, has been studied by Thurston (120). Increase of either asphaltenes or oils reduced resistance to weathering, while apparently an optimum content of resins aided permanence under exposure. Weathering properties were dependent not only on the quantities but also on the sources of these fractions, but the effect of source was not sufficiently clarified. 

Daling, P.S. Brandvik, J. Characterization and Prediction of the Weathering Properties of Oils at Sea, A Manual for the Oils Investigated in the DIWO Project, IKU SINTEF Group Trondheim, Norway, 1991. 

Douglas fir plywood with acetylated faces, when exposed to the weather on a test fence for two years without a surface finish developed only a slight roughening and checking whereas the controls weathered and checked badly (76). The weathering of exterior paints on panels with acetylatecT faces were considerably better than on the controls. Presurface acetylation also seemed to improve the weathering properties of painted wood (36, 37). 

Ultramarine blues, reds and violets are acid sensitive. As such, their thermal and weathering properties are adversely affected in acidic media. Silica-encapsulated versions offer marked improvement in thermal stability and some improvement in weathering properties. The treated versions allow broad polymer usage, even in engineering plastics such as polycarbonate, and provide sufficient durability for automotive interior applications. Weatherability is highly dependent upon the degree of exposure to acid or alkaline conditions. 

Perhaps the most extensive area of research in simple pretreatment systems to enhance the weathering properties of wood has involved application of inorganic salt solutions. Much of the early work in this field was undertaken at the U.S. Forest Products Laboratory in Madison, Wisconsin and was aimed at improving the performance of transparent finishes. Black [25] described an experimental chromate-based wood finish and Black and Mraz [26] discovered that both acid and ammoniacal-copper chromate treatments significantly improved the performance of clear finishes on western red cedar, redwood (Sequoia sempervirens), and Douglas fir plywood. Later research [27] demon-... 

Nakagawa [5] prepared poly(styrene-b-isobutylene) through atom transfer radical polymerization that had good weatherability properties. 

---