Resistance to Chemical Environments

In general, amorphous plastics are susceptible to cracking by particular chemical environments when under stress. This behavior is called 

TABLE 13.7 Electrical Properties of Polysulfone, Polyethersulfone, and Polyphenylsulfone 

The exact mechanism of environmental stress cracking is not fully understood, but it involves a weakening of secondary bonds between neighboring polymer chains due to solubility and diffusion of a chemictd 

TABLE 13.8 Resistance of Natural (Unreinforced) Polysulfone, Polyethersulfone, and Polyphenylsulfone Resins to Various Chemical Environments  

The above data are for comparative purposes only actual resistance depends on many factors including stress, temperature, concentration, and exposure duration. 

The most important aspect of titanium is its high resistance to oxidizability. Generally, titanium is resistant to common oxidizing solutions. The metal is attacked by dry chlorine gas and introduction of 1 % of water into chlorine medium results in preventing the attack of the metal. Features relevant to particular environments are as follows  

Moderate resistance general corrosion rate varies with the type of acid, concentration and temperature High corrosion rates, used for pickling, etching Pd, Ru, Mo, Ni render the titanium to be more resistant to corrosion 

Low corrosion rates in NaOH, KOH and NH4OH solutions over wide concentration and temperature ranges anodic dissolution in 2CM-0 wt % NaOH results in hydrogen absorption leading to hydrogen embrittlement hydrogen absorption can be inhibited by adding nitrate or chlorate 

Hydrogen peroxide Corrosion of radioactive waste container at 10 4 M H202  

Organic compounds Resistant to terephthahc acid, adipic acid, alcohols, aldehydes, 

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Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undergo ESC depends on several factors, the most important of which are appHed stress, temperature, and the concentration of the aggressive species. 

P(l) Surfacing veils should be considered where their inclusion is beneficial to improving the appearance and properties of surface layers and coatings. They should be used where high resistance to chemical environments is required. C-glass materials are commonly used. 

The Ryton polyphenylene sulphide compounds are formulated for the encapsulation of semiconductors and discrete components and are particularly suited to this application because of their excellent thermal stability, resistance to chemical environments, flame retardancy and good electrical properties over a wide temperature range. In order to be suitable for the encapsulation of microcircuits and transistors the material must have good flow during moulding, good dimensional stability, low levels of impurities and low moisture absorption characteristics. 

Elastomeric materials, which provide relatively low practical static deflections and have relatively high natural frequencies, are used only to isolate higher frequencies. The volume compressibiUty of elastomeric materials is relatively low, therefore the shape of the elastomeric isolator must be taken into account, and space must be provided for lateral expansion. Because of their inherent resistance to chemical and environmental deterioration, neoprene and other synthetic materials often can be used in severe environments where natural materials would deteriorate. 

Many engineering thermoplastics (e.g., polysulfone, polycarbonate, etc.) have limited utility in applications that require exposure to chemical environments. Environmental stress cracking [13] occurs when a stressed polymer is exposed to solvents. Poly(aryl ether phenylquin-oxalines) [27] and poly(aryl ether benzoxazoles) [60] show poor resistance to environmental stress cracking in the presence of acetone, chloroform, etc. This is expected because these structures are amorphous, and there is no crystallinity or liquid crystalline type structure to give solvent resistance. Thus, these materials may have limited utility in processes or applications that require multiple solvent coatings or exposures, whereas acetylene terminated polyaryl ethers [13] exhibit excellent processability, high adhesive properties, and good resistance to hydraulic fluid. 

Air Products and Chemicals pioneered the use of these potassium carbonate promoted hydrotalcite-based materials (K-HTC) for sorption-enhanced reforming of methane [26]. Mixing the K-HTC with a SMR catalyst in a 2 1 ratio gave high (90+%) conversions of methane at temperatures as low as 400 °C. In the first instance Ni-based catalysts were used, but they were not resistant to the environ-... 

Recent studies on monitoring of a carrier environment( 3 ) have shown that its corrosion severity parallels to a laboratory cyclic environment of 5% NaCl spray (fog) with 30ppro of SO2. The development of a new chemical conversion coating which is also resistant to this environment would be of great inportance. 

Metals develop a natural corrosion-resistant film when exposed to the environment. Examples include the rusting of iron, tarnishing of silver, and the formation of the patina on copper. These passive films help prevent further corrosion. However, films do not provide complete resistance to chemical attack and are destroyed by various corrosive agents. 

Fiber-reinforced plastics have varying degrees of resistance to adverse environments such as moisture, alkali, acid, and other chemicals. The degree of resistance depends on the fiber-resin system. Moisture absorption and chemical infiltration will be different for different fiber-resin systems. The degradation of composite materials may result from several factors ... 

In general these factors determine cost, mechanical and visco-elastic properties, resistance to degrading environments and influences, processing characteristics of the compound, and special requirements such as flame resistance, oil resistance, low temperature flexibility, chemical resistance wear and abrasion resistance and non-toxicity requirements for application in contact with foodstuff, etc. 

Our ENVIRONMENT exposes us daily to a wide variety of xenobiotics in our food, in the air we breathe, or as a result of industrial exposure and toxic wastes. However, despite this exposure, most of us are living long, healthy lives. Certainly individual variation could account for some of the variability in resistance to disease, but other factors are undoubtedly involved. According to a growing body of evidence, diet may be extremely important in increasing resistance to chronic disease. One is tempted to speculate, or hope, that improved dietary habits could improve individual resistance to chemically induced chronic disease. 

The main difference between primers and adhesion promoters is that primers are liquids that are applied to the substrate as a relatively heavy surface coating prior to application of the adhesive. Adhesion promoters, on the other hand, are liquids that form a very thin (usually monomolecular) layer between the substrate and the adhesive. Usually chemical bonds are formed (1) between the adhesion promoter and the adhesive and (2) between the adhesion promoter and the substrate surface. These bonds are stronger than the internal chemical bonds within the adhesive. These new bonds also provide an interface region that is more resistant to chemical attack from the environment. Adhesion promoters are also sometimes referred to as coupling agents. 

There is no universally best adhesive for all chemical environments. As an example, maximum resistance to bases almost axiomatically means poor resistance to acids. It is relatively easy to find an adhesive that is resistant to one particular chemical environment. It becomes much more difficult to find an adhesive that will not degrade in two widely differing chemical environments. Generally, adhesives that are most resistant to high temperatures have good resistance to chemicals and solvents because of their dense, crosslinked molecular structure. 

Many fluorinated polymers have been prepared using free-radical processes and the intensity of interest in this field stems from the number of rmique properties that are bestowed on the polymer by the presence of the carbon-fluorine bonds in a system. For example, excellent resistance to chemically aggressive environments, high thermal stability, low dielectric constant, low flammability and very low surface energies are just some of the properties of fluorinated materials that have been exploited. Uses range from... 

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