Dissolvers coatings

Fig. 2 Two types of dissolution-controlled, pulsed delivery systems (A) single bead-type device with alternating drug and rate-controlling layers (B) beads containing drug with differing thickness of dissolving coats.

Thermoplastic lesin dissolve flow casting film, hot-pressing film or dissolve coated tablet. 

Some salts act as acids. Salts that act as acids react with hydroxide ions. Ammonium chloride, NH4CI, is such a salt. This salt is also called sal ammoniac. As a flux added to solder used to solder copper plumbing or automobile radiators, ammonium chloride dissolves coatings of corrosion on the metal surfaces so that the solder can stick. In the presence of a base, NH4CI reacts with the hydroxide ion to produce ammonia gas and water ... 

On ageing, dammar polymerizes and some parts even become cross-linked, yellow and insoluble (Boon and van der Doelen, 1999). Dissolving coatings with solvents will therefore leave coloured insoluble material on the surface. For this reason, a mechanical method of removal should also be considered when the application and reversal of a dammar coating are being proposed. 

Cadmium is a soft metal, which forms a protective coating in air, and burns only on strong heating to give the brown oxide CdO. It dissolves in acids with evolution of hydrogen ... 

In air, the metal becomes coated with a layer of oxide. Acids dissolve the metal, but it is unaffected by alkalis. 

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. 

Corrosion. Ammonium bifluoride dissolves in aqueous solutions to yield the acidic bifluoride ion the pH of a 5% solution is 3.5. In most cases, NH4HF2 solutions react readily with surface oxide coatings on metals thus NH4HF2 is used in pickling solutions (see Metal surface treatments). Many plastics, such as polyethylene, polypropylene, unplasticized PVC, and carbon brick, are resistant to attack by ammonium bifluoride. 

Types of internal enamel for food containers include oleoresins, vinyl, acryflc, phenoHc, and epoxy—phenoHc. Historically can lacquers were based on oleoresinous products. PhenoHc resins have limited flexibiHty and high bake requirements, but are used on three-piece cans where flexibiHty is not required. Vinyl coatings are based on copolymers of vinyl chloride and vinyl acetate dissolved in ketonic solvents. These can be blended with alkyd, epoxy, and phenoHc resins to enhance performance. FlexibiHty allows them to be used for caps and closures as weU as drawn cans. Their principal disadvantage is high sensitivity to heat and retorting processes this restricts their appHcation to cans which are hot filled, and to beer and beverage products. 

Diacetone Alcohol. Diacetone alcohol (DAA) (4-hydroxy-4-methyl-2-pentanone) is a colorless, mild smelling Hquid which is completely miscible with water and most organic solvents. It is the simplest aldol condensation product of acetone, and because of its keto-alcohol functionahes it has special utility in the coatings industry where it is used to dissolve cellulose acetate to give solutions with high tolerance for water (115). 

Lead Tetroxide. Lead tetroxide (red lead minium lead orthoplumbite), Pb O, is a brilliant orange-red pigment which accounted for U.S. shipments of 17,780 t ia 1977, mainly to the ceramics and storage battery iadustries (40). U.S. shipments ia 1993 amounted to approximately 12,000 t. The decrease ia usage siace 1973 (19,000 t) is attributable to discontinued use ia the paint and coatings (qv) iadustry, and alterations ia mbber and ceramics (qv) markets. It is iasoluble ia water and alcohol, and dissolves ia acetic acid or hot hydrochloric acid. Red lead is manufactured by heating lead monoxide ia a reverberatory furnace ia the preseace of air at 450—500°C uatil the desired oxidative composition is obtaiaed. 

In order to make an efficient Y202 Eu ", it is necessary to start with weU-purifted yttrium and europium oxides or a weU-purifted coprecipitated oxide. Very small amounts of impurity ions, particularly other rare-earth ions, decrease the efficiency of this phosphor. Ce " is one of the most troublesome ions because it competes for the uv absorption and should be present at no more than about one part per million. Once purified, if not already coprecipitated, the oxides are dissolved in hydrochloric or nitric acid and then precipitated with oxaflc acid. This precipitate is then calcined, and fired at around 800°C to decompose the oxalate and form the oxide. EinaHy the oxide is fired usually in air at temperatures of 1500—1550°C in order to produce a good crystal stmcture and an efficient phosphor. This phosphor does not need to be further processed but may be milled for particle size control and/or screened to remove agglomerates which later show up as dark specks in the coating. 

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