Moisture weatherability

Polyvinyl chloride (pol-ee-VYE-nul KLOR-ide) is also known as PVC, vinyl, chlorethylene homopolymer, and chlor-ethene homopolymer. It is the third most commonly produced plastic in the United States, exceeded only hy polyethylene and polypropylene. It is offered commercially in a variety of formulations, usually as a white powder or colorless granules. The compound is resistant to moisture, weathering, most acids, fats and oils, many organic solvents, and attack by fungi. It is easily colored and manufactured in a variety of forms, including sheets, films, fibers, and foam. 

Chem. Descrip. Silicone polymer sol n. in toluene and IPA Uses Film-former, conformal coating, encapsulant tor electronic components, for elec, insulation and protection against moisture Features Produces clear elastomeric tilm with exc. heat stability and flexibility, providing protection from moisture, weathering, and vibration ready-to-use prod. flexible over wide temp, range good release props. heat curable... 

Moisture, weathering resistant resins (vanishes) Chemical-resistant resins (lining materials, etc.) Abrasion-resistant resins (coating materials) Insulating resins (sealing compounds) Fire-resistant resins (vanishes) Pressure-sensitive adhesive resins Spherical powders (improvers to blocking and sliding properties for films)... 

Improved Hot—Wet Properties. Acryhc fibers tend to lose modulus under hot—wet conditions. Knits and woven fabrics tend to lose their bulk and shape in dyeing and, to a more limited extent, in washing and drying cycles as well as in high humidity weather. Moisture lowers the glass-transition temperature T of acrylonitrile copolymers and, therefore, crimp is lost when the yam is exposed to conditions requited for dyeing and laundering. 

Electrical Properties. Poly(methyl methacrylate) has specific electrical properties that make it unique (Table 4). The surface resistivity of poly(methyl methacrylate) is higher than that of most plastic materials. Weathering and moisture affect poly(methyl methacrylate) only to a minor degree. High resistance and nontracking characteristics have resulted in its use in high voltage appHcations, and its excellent weather resistance has promoted the use of poly(methyl methacrylates) for outdoor electrical appHcations (22). 

Xanthate dmms should be kept as cool and dry as possible. Protection from moisture is the most important factor. A combination of moisture and hot weather causes sodium ethyl xanthate to ignite spontaneously (14). 

Adhesives (qv) used to make plywood are classified as either the exterior adhesive or the lesser quaHty interior adhesive. The terms relate to the abUity of the adhesive to survive exposure to moisture and weather. PhenoHc resins (qv) are commonly used as adhesives to make plywood. The difference between interior and exterior phenoHc resin adhesive is the filler level. Exterior rated plywood uses higher resin content adhesives. Interior rated plywood uses either highly extended (below 24% resin soHds) or protein-based adhesives. The adhesive is appHed to the veneer by roU coating, spraying, curtain coating, or foam extmsion. 

Totally enclosed motors offer the greatest protection against moisture, corrosive vapors, dust, and dirt. Totally enclosed fan-cooled (TEFC) motors are the obvious choice rather than weather-protected below 186.5 kW (250 hp). Their internal and external ventilating air are kept separate external air never gets inside except for the small amount that enters by breathing. 

Special insulation and coating of stator windings and overhangs are sometimes essential to ensure protection against tropical weather, fungus growth, moisture, oil abrasives, and acid and alkali fumes. Powerhouse treatment is one insulating process that ctin meet all these requirements (see Section 9..T). 

Sealants for insulated glass windows are based on butyl polymers. These sealants have excellent weathering and ageing, low-moisture vapour transmission and are non-fogging [66]. 

Nonmetallic materials should have the following desirable characteristics low moisture absorption, resistance to microorganisms, stability through temperature range, resistance to flame and arc, freedom from out-gassing, resistance to weathering, and compatibility with other materials. 

Electrical equipment can be mounted in various types of enclosures. A weather-tight enclosure normally has a gasket and does not allow air (and the moisture contained in the air) to enter the enclosure. Offshore, such an enclosure, if properly closed, will help protect the enclosed electrical equipment from, corrosion due to salt water spray. These types of enclosures can be used in Division 2 areas provided they do not enclose arcing, sparking or high temperature devices. 

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