Weathering, polymeric materials

When exposed to outdoor weathering, polymeric materials can undergo degradation that could affect their optical, mechanical, and electrical Insulation properties. Outdoors, polymeric materials can degrade from one or more of the following weathering actions ... 

What is meant by weather resistance What environmental factors can alter the properties of polymeric materials ... 

IR spectroscopy can be used to characterise not only different rubbers, but also to understand the structural changes due to the chemical modification of the rubbers. The chemical methods normally used to modify rubbers include hydrogenation, halogenation, hydrosilylation, phosphonylation and sulfonation. The effects of oxidation, weathering and radiation on the polymer structure can be studied with the help of infrared spectroscopy. Formation of ionic polymers and ionomeric polyblends behaving as thermoplastic elastomers can be followed by this method. Infrared spectroscopy in conjunction with other techniques is an important tool to characterise polymeric materials. 

It is a fact that while research on UV stabilizers has advanced to products which significantly improve the weatherability of polymeric materials, the exact processes by which the various classes of stabilizers fulfill their functions are not entirely certain. This especially holds for the very important class of Jlindered Mine Light Stabilizers (HALS, tetramethylpiperidine derivatives, TMP). 

U.S. Pat No. 6,827,995 [53] discloses a single coextrusion process of making a WPG as a hollow profile comprising a weatherable outer layer made of a first polymeric material, a core layer made of a thermoplastic polymeric foamed composition including a wood component, and an inner layer made of a third thermoplastic material. The first and second polymeric materials are PVC or acrylonitrile-styrene-acrylic polymer or a combination thereof. The third polymeric material is PVC. 

An aging atudy has heen completed which evaluated a number of polymeric materials for potential use as 1) protective coatings for back surfaces of mirrors and 2) solar hellostat edge seals. These investigations were conducted in an artificial weathering chamber that accelerated thermal cycling. We observed the primary mirror failure mode to be silver corrosion resulting from moisture exposure. 

Many current solar hellostat and flat plate collector designs which have been field tested show a vulnerability to long-term outdoor weathering ( ) Therefore, the purpose of this investigation was to evaluate potential protective polymeric materials,... 

Commercial weathering stations measure both the usual meteorological data, such as temperature, relative humidity, and direction and speed of the wind, and the following data, which are important for the aging of polymeric materials. 

As far as the aging of polymeric materials is concerned, the duration of rain is more important than the amount. It makes a great difference if a certain amount of rain falls, say. in a matter of a few minutes in a sudden shower, or in the form of a drizzle lasting several hours. The depth of penetration into the material, and thus the influence on the weathering behavior is much greater in the second case than in the first ... 

The effects of temperature on the aging behavior of polymeric materials became especially clear when materials for the interior equipment of motor vehicles were tested. Since these are often exposed to solar radiation at high temperatures, the test conditions for artificial weathering must be selected accordingly. A specific black panel temperature is set in the weathering devices. 

A nonconcentrating solar heat collector consists of a transparent cover, an absorber, tubes or ducts for the heat transport fluid, and an insulated case that limits the thermal losses to the environment. The collector cover serves several important functions it permits the passage of the solar radiation, prevents loss of heat, and protects the absorber from mechanical and weathering damage. The cover is selected for the highest transmittance of solar radiation and the lowest transmittance of the infi ared reradiated from the absorber. Polymeric materials have a low absorptivity for solar radiation and the absorptance can be decreased, compared to glass, by using thinner sheets or films. 

Polymeric materials in outdoor applications are exposed to weather extremes that can be extremely deleterious to the material. The most harmful weather component, exposure to ultraviolet (UV) radiation, can cause embrittlement fading, surface cracking, and chalking. After exposure to direct sunlight for a period of years, most polymers exhibit reduced impact resistance, lower overall mechanical performance, and a change in appearance. 

Other factors affecting the life of an adhesive bond are humidity, water immersion, and outdoor weathering. Moisture can affect adhesive strength in two ways. Some polymeric materials, notably ester-based pol)nirethanes, will revert, i.e., lose hardness, strength, and in the worst case, turn to fluid during exposure to warm humid air. Water can also permeate the adhesive and displace the adhesive at the bond interface. Structural adhesives not susceptible to the reversion phenomenon are also likely to lose adhesive strength when exposed to moisture. 

The accelerated weathering of polymeric materials in laboratory instruments is covered by ISO Standard 4892. In the US, ASTM standards G23, G26 and G53 cover such testing, and other ASTM standards are derived from these, as shown in Fig. 6.2. Many car manufacturers have their own acceptance specifications. Reviews of these standards and of factors affecting results have been published [4, 9-11]. 

M. B. Neiman, Aging and Stabilization of Polymers, Consultants Bureau, New York, 1965. S. H. Pinner, Weathering and Degradation of Plastics, Columbine Press, Manchester, 1966. J. Voigt, Die Stabilisierung der Kunststoffe gegen Ucht und Wdrme, Springer, Berlin, 1967. D. V. Rosato and R. T. Schwartz, Environmental Effects on Polymeric Materials, Interscience, New York, 1968. 

Exposure to the outdoor environment affects not only the polymeric material itself but also other components within the matrix, such as dyes, pigments, processing additives, and stabilizers. Complex interactions of the combination of weather factors with the polymer and its components result in irreversible changes in the chemical structures and physical properties in a direction that generally changes both the appearance and mechanical properties and reduces the useful life of the material. Most organic polymeric materials require the use of one or more types of stabilizers for protection against the effects of the environment in order... 

WEATHER FACTORS AND THEIR EFFECTS ON POLYMERIC MATERIALS... 

---