Discoloration Color

The performance of stabilizers is not only determined by its effectiveness to protect the polymer against degradation, but also by its physical properties, color, discoloration, interactions with ofeer additives, chemical resistance, dosability, toxicity, food approval, and price. 

Acryhc fibers discolor and decompose rather than melting when heated, but they have very good color and heat stabihty at temperatures less than I20°C. In a study by American Cyanamid (using Federal Test Specification TT-P-I4Ia. Method 425.2) the yeUowness of acryhc fiber was measured as a function of temperature. Compared to a value of 0.0 for a pure white body, the original fiber had a yeUowness index of 0.04—0.10. After 30 minutes of exposure at II5°C the yeUowness increased only slightly to 0.II—0.17. After 6 h at I30°C, however, the yeUowness increased to 0.38—0.41. 

A varnish is often appHed on top of the paint layers. A varnish serves two purposes as a protective coating and also for an optical effect that enriches the colors of the painting. A traditional varnish consists of a natural plant resin dissolved or fused in a Hquid for appHcation to the surface (see Resins, natural). There are two types of varnish resins hard ones, the most important of which is copal, and soft ones, notably dammar and mastic. The hard resins are fossil, and to convert these to a fluid state, they are fused in oil at high temperature. The soft resins dissolve in organic solvents, eg, turpentine. The natural resin varnishes discolor over time and also become less soluble, making removal in case of failure more difficult (see Paint and FINNISH removers). Thus the use of more stable synthetic resins, such as certain methacrylates and cycHc ketone resins, has become quite common, especially in conservation practice. 

In unalloyed steel containers formamide discolors slowly during shipment and storage. Both copper and brass are also subject to corrosion, particularly in the presence of water. Lead is less readily attacked. Aluminum and stainless steel are resistant to attack by formamide and should be used for shipping and storage containers where the color of the product is important or when metallic impurities must be minimized. Formamide attacks natural mbber but not neoprene. As a result of the solvent action of formamide, most protective paints and finishes are unsatisfactory when in contact with formamide. Therefore, formamide is best shipped in containers made of stainless steel or in dmms made of, or coated with, polyethylene. Formamide supphed by BASF is packed in Lupolen dmms (230 kg) or Lupolen canisters (60 kg) both in continental Europe and overseas. 

Stability. Diesel fuel can undergo unwanted oxidation reactions leading to insoluble gums and also to highly colored by-products. Discoloration is beheved to be caused by oxidation of pyrroles, phenols, and thiophenols to form quiaoid stmctures (75). Eventually, these colored bodies may increase in molecular weight to form insoluble sludge. 

Isophorone. Isophorone (3,5,5-trimethyl-2-cyclohexen-l-one) is a cycHc a,P-unsaturated ketone derived from the trimeri2ation of acetone. It has a light yellow color and a disagreeable camphoraceous odor. It has the tendency to discolor and form residues on prolonged storage. Isophorone is completely miscible with organic solvents, and other physical properties are Hsted ia Table 1. 

Specifications and Standards. Typical specifications for phthahc anhydride are given in Table 10. AH specifications are measures of purity. Sohdification point is a sensitive indicator of absolute purity, and is a key specification. Another key specification is molten color stabiUty, which is the color after being held at 250°C for two hours. This test ensures acceptable color after shipment in molten form and detects the presence of impurities that can cause discoloration at elevated temperatures. Phthahc acid level is a monitor of how well moisture has been excluded during storage and shipment. 

Storage tanks, lines, and pumps should be heat traced and insulated to enable product handling. Temperature control is required to prevent product degradation because of color alkan olamines have poor heat transfer properties. Exposure to air will also cause product discoloration. Storage tanks should be nitrogen-padded if low color product is required. 

Use of a good antioxidant is recommended for almost all neoprene compounds where color is not of importance, a staining antioxidant can be used. The substituted or hindered phenols, eg, Naugawhite (Uniroyal) and Antioxidant 2246 (American Cyanamid), are used where a minimum of product discoloration or staining is requited. 

Elemental sulfur in either its ore or its refined state can generaUy be recognized by its characteristic yeUow color or by the generation of sulfur dioxide when it is burned in air. Its presence in an elemental state or in a compound can be detected by heating the material with sodium carbonate and mbbing the fused product on a wet piece of silver metal. A black discoloration of the silver indicates the presence of sulfur. The test is quite sensitive. Several other methods for detecting smaU amounts of elemental sulfur have also been developed (34). 

Thiols iateract readily with many mbber-containing materials. For this reason, care should be taken ia the selection of gasket and hose materials. Teflon, Kel-F, Viton, or other suitable fluoroelastomers function as gasket materials. Viton is suitable for hoses. Carbon steel is useful for many thiols, although some thiols become very discolored when carbon steel is utilized. In these cases, the use of stainless steel is very desirable. Isolation from air and water also minimizes color formation. 2-Mercaptoethanol and 1,2-ethanedithiol should be stored ia stainless steel (61). 

The intense reddish-brown color of the acetylacetone titanium complexes impart a yellow discoloration to white inks. This discoloration is accentuated when the inks are used to print substrates that contain phenol-based antioxidants. The phenoHc compounds react with the organic titanate to form a highly colored titanium phenolate. Replacement of 0.25 to 0.75 moles of acetylacetone with a malonic acid dialkyl ester, such as diethyl malonate, gives a titanium complex that maintains the performance advantages of the acetyl acetone titanium complexes, but which is only slightly yellow in color (505). These complexes still form highly colored titanium phenolates. 

Mixtures of a titanium complex of saturated diols, such as TYZOR OGT, and a titanium acylate, such as bis- -butyl-bis-caproic acid titanate, do not have a yellowing or discoloring effect on white inks used to print polyolefin surfaces (506). The complexes formed by the reaction of one or two moles of diethyl citrate with TYZOR TPT have an insignificant color on their own and do not generate color with phenol-based antioxidants (507). The complexes formed by the addition of a mixture of mono- and dialkyl phosphate esters to TYZOR TBT are also low color-generating, adhesion-promoting additives for use in printing polyolefin films (508). 

Stilbenequiaones such as (5) absorb visible light and cause some discoloration. However, upon oxidation phenolic antioxidants impart much less color than aromatic amine antioxidants and ate considered to be nondiscoloring and nonstaining. 

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