Curing, color, stability

Resins are also used for permanent tooth-colored veneers on fixed prostheses, ie, crown and bridges. Compositions for this application include acryflcs, vinyl—acryflcs, and dimethacrylates, as well as silica- or quartz-microfilled composites. The resins are placed on the metallic substrates of the prostheses and cured by heat or light. These resins are inexpensive, easy to fabricate, and can be matched to the color of tooth stmcture. Acrylic facings do not chemically adhere to the metals and are retained only by curing the resin into mechanical undercuts designed into the metal substrate. They have relatively low mechanical strength and color stability, and poor abrasion and strain resistance they also deform more under the stress of mastication than porcelain veneers or facings. 

Polyester resins possess premium performance properties such as exterior durability, gloss, flexibility hardness, color stability, and versatility of cure. Polyesters are used in product finishes for household appliances, food and beverage containers, aircraft and equipment, automotive primers and bake coats, metal furniture, and fixtures. For example, water-soluble saturated polyesters are used in industrial baking paints, and in combination with melamine resin. Polyesters can be formulated in high solids and waterborne formulations to meet the requirements for the low VOC coatings being mandated by the EPA. 

The storage stability of the components of composite formulations is mainly limited by the poor shelf-life of the BP ingredient ( ). Paste formulations, containing BP, but free of accelerator prematurely harden when stored at 60°C. Formulations using powder-liquid constituents are more stable at these elevated temperatures. After extended storage at room temperature for two years, composite mixes showed delayed setting and decreased mechanical properties in the cured material. The purity of the BP and the amine selected greatly influences parameters such as reaction rate, color stability and biocompatibility (18). 

With multifunctional thiols such as pentaerythritol tetra(3-mercaptopropionate), added in concentrations below 10 percent to typical peroxide-amine cured resins, composites with significantly reduced setting time and excellent color stability have been obtained ( ). Dodecyl mercaptan yields comparable results (ll). The free radical addition of the thiol to the double bond of the monomer probably controls the molecular weight of the polymer. 

Doverphos. [Dover] Phosphites heat and color stabilizers, antioxidants, chelating agents for plastics, rubber lubricant additives aids curing and hardening in epoxies. 

The systems are color-stable, weather resistant, flexible, and also adhere well to tin-free and low-tin sheet metals, as used in the manufacture of beverage and aerosol cans. Although arene-ferrocenium complexes even allow the cure of bisphenol A epoxy resins, provided postcuring takes place, these systems have not yet become important in surface protection applications. The reasons for this are inadequate color stability and weather resistance as well as the intense color of these films. 

Aliphatic primary amines. Common examples are diethylene triamine (DETA), tetraethylene pentamine (TEPA), n-aminoethyl piperazine, and isophorone diamine. They give good room-temperature cure at stoichiometric ratios, but have poor HDT, inconvenient mix ratios, high peak exotherm, and are strongly irritant. Isophorone diamine produces very light colored mixes with good color stability. 

Aromatic primary amines. These ofier improved heat and chemical resistance and longer pot life with reduced exotherm, but poor color stability and sluggish cure. They are generally solids and require some formulating to produce easily handleable products. Reactions proceed best at elevated temperatures, where their irritancy can be a problem. For room-temperature cures they should be used with catalysts, of which phenols, BF3 complexes, and anhydrides are the best. m-Phenylene diamine (MPDA) and methylene dianiline (MDA) are the best examples. 

Chem. Descrip. Triphenyl phosphite CAS 101-02-0 EINECS/ELINCS 202-908-4 Uses Stabilizer for styrenics, engineering thennoplastics, polyesters (to regulate vise., improve color stability), polyolefins (as catalyst adjuvant), PU (to prevent scorching during curing and improve color stability), epoxies, PVC, coatings, adhesives antioxidant for epoxies Properties APHA 50 max. clear liq. m.w. 310 sp.gr. 1.180-1.186 (25/... 

Uses Reducing agent in redox-catalyzed polymerization color stabilizer, modifier, catalyst for polyamides antioxidant intermediate for forming metallic salts used as stabilizers accelerator for org. peroxide catalysts improver of polysiloxane resins free radical promoter in emulsion polymerization promoter in polyester resin curing mfg. of self-extinguishing fibers (reacts with polyester resin and polyolefin) color/odor inhibitor, catalyst for some thermoplastics discoloration inhibitor in PE corrosion inhibitor on thin aluminum surfs. lubricity improver in polyphenyl thioether-based lubricants Manuf./Distrib. Akzo Nobel http //www.akzonobel.com, Ferro http //WWW. ferro. com... 

Hazardous Decomp. Prods. Heated to decomp., emits acrid smoke and irritating fumes Storage Light-sensitive Uses Antioxidant, preservative in foods, brewing, pharmaceuticals, industrial applies. curing accelerator for nitrites on meat color stabilizer, flavor stabilizer in meat prods. ... 

Allylic resins Long pot life, low vapor pressure monomers, high cure temperature, low viscosity for monomers Excellent clarity, abrasion resistant,-color stability, resistant to solvents and acids Safety lenses, face shields, casting impregnation, as monomer in polyester... 

Cast allylics. The ally lie ester resins possess excellent clarity, hardness, and color stability and thus are used to cast them into optical parts. These castings can be either homo or co-polymers. The free radical addition polymerization of the allylic ester presents some casting difficulties such as exotherm control, monomer shrinkage during curing and the interaction between the exotherm, the free radical source, and the environmental heat required to decompose the peroxide and initiate the reaction. 

In conclusion, the porosity, hardness, and strength of microwave cured acrylic resins that are processed for less than 5 min exhibited no significant differences in properties when compared with conventionally polymerized resins [60-63]. However, the investigation of color stability of several commercially available heat-cured, quick heat-cured denture and microwave cured denture base resins (tested exclusively under microwave conditions) revealed that the materials demonstrated differences in color stability, but the standard heat-cured materials treated under microwave irradiation exhibited color changes that were negligible [64]. 

Silicones. These materials have some only recently been used to produce maxillofacial prostheses. Both the RT- and heat-vulcanized materials may be used. Heat-vulcanized formulations are supplied as a semisolid or putty-like material, which requires the addition of colorants. The molded material is cured under pressure at 180°C/30 min. The heat-cured materials exhibit better strength and color stability than the RT-cured materials. 

However, chemical resistance is inferior to that of aromatic amines. Becanse cycloaliphatic amines are less reactive than acyclic aliphatic amines, their nse results in a longer pot life and in the ability to cast larger masses. Unmodified cycloaliphatic amines require elevated temperature cure, but modified systems are RT-curable. Properly formulated, they can give an excellent balance of properties fast cure, low viscosity, low toxicity, good adhesion to damp concrete, and excellent color stability. They are, however, more expensive than other types of curing agents. 

Several vitamins have some desirable additional effects. Ascorbic acid is a dough improver, but can play a role similar to tocopherol as an antioxidant. Carotenoids and riboflavin are used as coloring pigments, while niacin improves the color stability of fresh and cured and pickled meat. 

Color stabilization by the addition of nitrate or nitrite (meat curing) plays an important role in meat processing. Nitrite initially oxidizes myoglobin to metmyoglobin ... 

Salting by the addition of sodium nitrite and/or nitrate (curing or pickling) produces products with highly stable color (cf. 12.3.2.2.4). Since nitrite reacts faster and less is required for color stabilization, it is widely used in place of nitrate. Salt... 

Saturated polyester resins exhibit high peel strength and are used to laminate plastic films such as polyethylene terephthalate (Mylar). They also offer excellent clarity and color stability. These polyester types, in both solution and solid form, can be chemically cross-linked with curing agents such as the isocyanates for improved thermal and chemical stability. 

Lower Movement Caulks Oil/resin-based 5% Lowest cost easily applied and tooled primerless preparation good color stability fast skinning No recovery slow curing can stain substrate moderate shrinkage for static joints... 

Butyl 7.5% Good adhesion good water resistance good color stability little surface preparation caps neoprene gaskets Slow curing high shrinkage low re-coveiy relatively soft... 

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