Ozone, polymer weathering

Because of the inorganic nature inherent in the strings of sulfur atoms in their backbones, these polymers have excellent resistance to hydrocarbon solvents that can swell common hydrocarbon elastomers. They also have good resistance to moisture, oxygen, and ozone, good weatherability, and better-than-average low temperature properties.62... 

CPE is an ignition-resistant PE. It is also used in blends to change the ignition characteristics of other polymers. CPE has no unsaturation in the polymer backbone, giving it excellent ozone and weathering properties. The saturated backbone also results in a temperature stability that allows CPE to perform well continuously at temperatures of 150 °C. CPE can provide satisfactory resistance to most acids, bases, oils and alcohols. 

The diene ethylidiene norbornene in Vistalon EPDM allows sulfur vulcanization (see Table 3.12). 1,4-Hexadiene and dicyclopentadiene (DCPD) are also used as curing agents.The completely saturated polymer backbone precludes the need for antioxidants that can bleed to the surface (bloom) of the finished product and cause staining. Saturation provides inherent ozone and weather resistance, good thermal properties, and a low compression set. Saturation also allows a relatively high-volume addition of low-cost fillers and oils in compounds while retaining a high level of mechanical properties. The ethylene/propylene monomer ratio also affects the properties. 

Elastomeric polymers are widely used in applications snch as foamed sheets, automotive parts, durable goods, impact-modifiers in engineering plastics, and wire and cable owing to their outstanding heat, ozone, and weather resistance. 

Environmental Impact of Ambient Ozone. Ozone can be toxic to plants, animals, and fish. The lethal dose, LD q, for albino mice is 3.8 ppmv for a 4-h exposure (156) the 96-h LC q for striped bass, channel catfish, and rainbow trout is 80, 30, and 9.3 ppb, respectively. Small, natural, and anthropogenic atmospheric ozone concentrations can increase the weathering and aging of materials such as plastics, paint, textiles, and mbber. For example, mbber is degraded by reaction of ozone with carbon—carbon double bonds of the mbber polymer, requiring the addition of aromatic amines as ozone scavengers (see Antioxidants Antiozonants). An ozone decomposing polymer (noXon) has been developed that destroys ozone in air or water (157). 

Ozonc-rcsjstant elastomers which have no unsaturation are an exceUent choice when their physical properties suit the appHcation, for example, polyacrylates, polysulfides, siHcones, polyesters, and chlorosulfonated polyethylene (38). Such polymers are also used where high ozone concentrations are encountered. Elastomers with pendant, but not backbone, unsaturation are likewise ozone-resistant. Elastomers of this type are the ethylene—propylene—diene (EPDM) mbbers, which possess a weathering resistance that is not dependent on environmentally sensitive stabilizers. Other elastomers, such as butyl mbber (HR) with low double-bond content, are fairly resistant to ozone. As unsaturation increases, ozone resistance decreases. Chloroprene mbber (CR) is also quite ozone-resistant. 

EPDM vulcanizates exhibit some unique properties such as ozone, heat, light, weathering, and chemical resistance [25]. Because of this attractive combination of properties, EPDM has taken over a wide variety of applications. EPDM has relatively low unsaturation and therefore requires complex cure systems to achieve the desired properties. Nearly every conceivable combination of curing ingredients has been evaluated in various EPDM polymers over the years [26], the following systems are described as summarized in Table 14.11. 

Most polystyrene products are not homopolystyrene since the latter is relatively brittle with low impact and solvent resistance (Secs. 3-14b, 6-la). Various combinations of copolymerization and blending are used to improve the properties of polystyrene [Moore, 1989]. Copolymerization of styrene with 1,3-butadiene imparts sufficient flexibility to yield elastomeric products [styrene-1,3-butadiene rubbers (SBR)]. Most SBR rubbers (trade names Buna, GR-S, Philprene) are about 25% styrene-75% 1,3-butadiene copolymer produced by emulsion polymerization some are produced by anionic polymerization. About 2 billion pounds per year are produced in the United States. SBR is similar to natural rubber in tensile strength, has somewhat better ozone resistance and weatherability but has poorer resilience and greater heat buildup. SBR can be blended with oil (referred to as oil-extended SBR) to lower raw material costs without excessive loss of physical properties. SBR is also blended with other polymers to combine properties. The major use for SBR is in tires. Other uses include belting, hose, molded and extruded goods, flooring, shoe soles, coated fabrics, and electrical insulation. 

More than 800 million pounds of EPM and EPDM polymers were produced in the United States in 2001. Their volume ranks these materials fourth behind styrene-1,3-butadiene copolymers, poly( 1,4-butadiene), and butyl rubber as synthetic rubbers. EPM and EPDM polymers have good chemical resistance, especially toward ozone. They are very cost-effective products since physical properties are retained when blended with large amounts of fillers and oil. Applications include automobile radiator hose, weather stripping, and roofing membrane. 

Vinyl Acetate—Ethylene Copolymers. In these random copolymers, the ratio of ethylene to vinyl acetate (EVA) is varied from 30—60%. As the vinyl acetate content increases, the oil and heat resistance increases. With higher ethylene content the physical strength, tensile, and tear increases. The polymers are cured with peroxide. The main properties of these elastomers include heat resistance, moderate oil and solvent resistance, low compression set, good weather resistance, high damping, excellent ozone resistance, and they can be easily colored (see Vinyl polymers, polyvinyl acetate)). 

Viton. A trademark of the E.I. duPont de Nemours Co for a series of fluorelastomers based on the co-polymer of vinylidine fluoride and hexafluoropropylene with a repeating structure of [-CF2-CH2-CF2-CF(CF3H white, transparent solid sp grav 1.72—1.86. Resistant to corrosive liqs and chemicals up to 600°F, and useful in continuous service at 400—500°F. The material is resistant to weather, ozone, flame, oils, fuels, lubricants and many solvents According to Refs 2 3, Viton-A is used in many expl formulations developed by the Lawrence Livermore Laboratory, such as LX-04, LX-07, LX-10-0, LX-10-1 and LX-11-0 (see Vol... 

Polyethylene in solution is treated with chlorine and sulfur dioxide to introduce approximately 1.39k sulfur and 29% chlorine into the polymer. Most of the chlorine is attached directly to the carbon atoms in the backbone of the polymer, The remainder is in the form of sulfuryl chloride groups, SO CI, through which crosslinking occurs In the curing step with metal oxides. The material has good oxidation and ozone resistance and thus overall excellent weather resistance. Calendered stocks are used for lining ditches and ponds, for example. 

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