Curing system resin

Resin additives Resin cements Resin component Resin cure systems Resin curing agents Resin formation Resin-in-pulp Resinoid Resinols Resins... 

Hot pressing with a smooth plate has an advantage in smoothing the grain, and the heat can be used to cure the resin of the finish. The hot pressing is anticipated in the design of the finish system and in the choice of the resins by the finish manufacturer. 

Foam. PhenoHc resin foam is a cured system composed of open and closed ceUs with an overall density of 16—800 g/cm. Principal appHcations are in the areas of insulation and sponge-like floral foam. The resins are aqueous resoles cataly2ed by NaOH at a formaldehyde phenol ratio of ca 2 1. Free phenol and formaldehyde content should be low, although urea may be used as a formaldehyde scavenger. 

Catalyst Selection. The low resin viscosity and ambient temperature cure systems developed from peroxides have faciUtated the expansion of polyester resins on a commercial scale, using relatively simple fabrication techniques in open molds at ambient temperatures. The dominant catalyst systems used for ambient fabrication processes are based on metal (redox) promoters used in combination with hydroperoxides and peroxides commonly found in commercial MEKP and related perketones (13). Promoters such as styrene-soluble cobalt octoate undergo controlled reduction—oxidation (redox) reactions with MEKP that generate peroxy free radicals to initiate a controlled cross-linking reaction. 

Composite resins can be cured using a variety of methods. Intraoral curing can be done by chemical means, where amine—peroxide initiators are blended in the material to start the free-radical reaction. Visible light in the blue (470—490 nm) spectmm is used to intraoraHy cure systems containing amine—quin one initiators (247). Ultraviolet systems were used in some early materials but are no longer available (248). Laboratory curing of indirect restorations can be done by the above methods as well as the additional appHcation of heat and pressure (249,250). 

Resin Cure. Resin cure systems yield carbon—carbon cross-links and, consequendy, thermally stable materials. Butyl mbber vulcanised with resins are used as tire-curing bladders, and have a life of 300—700 curing cycles at steam temperature of 175°C at about 20 m/cycle. 

Silicon—Ca.rbon Thermoset. The Sycar resins of Hercules are sihcon—carbon thermosets cured through the hydrosilation of sihcon hydride and sihcon vinyl groups with a trace amount of platinum catalyst. The material is a fast-cure system (<15 min at 180°C) and shows low moisture absorption that outperforms conventional thermosets such as polyimides and epoxies. Furthermore, the Sycar material provides excellent mechanical and physical properties used in printed wiring board (PWB) laminates and encapsulants such as flow coatable or glob-top coating of chip-on-board type apphcations. 

The multiepoxy functionality of the epoxy novolaks (2.2 to >5 epoxy groups per molecule) (3) produce more tightly cross-linked cured systems having improved elevated temperature performance and chemical resistance than the difunctional bisphenol A-based resins. 

Ambient-cure systems are often based on lower molecular-weight soHd epoxy resins cured with aUphatic polyamines or polyamides. Curing normally occurs at ambient temperatures with a working life (pot life) of 8—24 h, depending on the formulation. Epoxy—poly amine systems are typically used for maintenance coatings in oil refineries, petrochemical plants, and in many marine appHcations. Such coverings are appHed by spray or bmsh. These are used widely where water immersion is encountered, particularly in marine appHcations (see COATINGS, MARINE). 

Adhesives. Because of exceUent adhesion to many substrates, epoxy resins are extensively used for high performance adhesives. These can be categorized into high temperature curing systems (soHds and Hquids) and room temperature curing systems (Hquids). 

Whilst the properties of the cross-linked resins depend very greatly on the curing system used and on the type of resin, the most characteristic properties of commercial materials are their toughness, low shrinkage on cure, high adhesion to many substrates, good alkali resistance and versatility in formulation. 

The methylated maleic acid adduct of phthalic anhydride, known as methyl nadic anhydride VI, is somewhat more useful. Heat distortion temperatures as high as 202°C have been quoted whilst cured systems, with bis-phenol epoxides, have very good heat stability as measured by weight loss over a period of time at elevated temperatures. The other advantage of this hardener is that it is a liquid easily incorporated into the resin. About 80 phr are used but curing cycles are rather long. A typical schedule is 16 hours at 120°C and 1 hour at 180°C. 

When cured with room temperature curing system these resins have similar thermal stability to ordinary bis-phenol A type epoxides. However, when they are cured with high-temperature hardeners such as methyl nadic anhydride both thermal degradation stability and heat deflection temperatures are considerably improved. Chemical resistance is also markedly improved. Perhaps the most serious limitation of these materials is their high viscosity. 

The resins are hardened in situ by mixing with an acidic substance just before application. A typical curing system would be four parts of toluene-p-sulphonic acid per 100 parts resin. The curing may take place at room temperature if the resin is in a bulk form but elevated temperature cures will often be necessary when the material is being used in thin films or coatings. 

Sulfur cross-links have limited stability at elevated temperatures and can rearrange to form new cross-links. These results in poor permanent set and creep for vulcanizates when exposed for long periods of time at high temperatures. Resin cure systems provide C-C cross-links and heat stability. Alkyl phenol-formaldehyde derivatives are usually employed for tire bladder application. Typical vulcanization system is shown in Table 14.24. The properties are summarized in Tables 14.25 and 14.26. 

Consist of a range of chemicals which promote cross-linking can initiate cure by catalysing ( catalysts , hardeners, initiators), speed up and control cure (activators, promoters) or perform the opposite function (inhibitors) producing thermosetting compounds and specialised thermoplastics (e.g. peroxides in polyesters, or amines in epoxy formulations). The right choice of a cure system is dependent on process, process temperature, application and type of resin. 

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