Accelerated epoxy system

Figure 1. Isothermal Kinetic Behavior - Accelerated Epoxy System,...

Figure 6. Engineering Cure Transformation Diagram Accelerated Epoxy System.

The binder system of a plastic encapsulant consists of an epoxy resin, a hardener or curing agent, and an accelerating catalyst system. The conversion of epoxies from the Hquid (thermoplastic) state to tough, hard, thermoset soHds is accompHshed by the addition of chemically active compounds known as curing agents. Flame retardants (qv), usually in the form of halogens, are added to the epoxy resin backbone because epoxy resins are inherently flammable. 

Tertiary amine accelerated polymercaptan/ epoxy systems, 10 410 Tertiary amine-carbonate technology,... 

In essence, the durability of metal/adhesive joints is governed primarily by the combination of substrate, surface preparation, environmental exposure and choice of adhesive. As stated earlier, the choice of the two-part nitrile rubber modified epoxy system (Hughes Chem - PPG) was a fixed variable, meeting the requirement of initial joint strength and cure cycle and was not, at this time, examined as a reason for joint failure. Durability, as influenced by substrate, surface preparation, and environmental exposure were examined in this study using results obtained from accelerated exposure of single lap shear adhesive joints. 

These compounds do not readily react with epoxy resins except in the presence of water, alcohol, or some other base, called an accelerator. Tertiary amines, metallic salts, and imidazoles often act as accelerators for anhydride cured epoxy systems. The reaction between acid anhydride and epoxy resins is illustrated in Fig. 5.7. 

Polysulphides. Polysulphides are used as curing agents for epoxies in the presence of tertiary amines which accelerate the cure such cured systems exhibit good flexibility and tensile strength at ambient temperature. In a real sense they are flexibihsers for epoxy systems. Specific applications are for the crack injection system (for moving cracks) and coatings (Chapter 10). 

Accelerators for epoxy systems. Accelerators shorten the gel time and increase the curing rate. Acelerators are employed based on the curing agent. 

Adolf et al [89] focused primarily on epoxy systems. They consider the key to the success of a constitutive model to be its choice of strain measure and the inclusion of free-energy-accelerated relaxations. The model only requires linear properties (i.e., properties that may be predictable by the methods developed in this book) for materials prior to their synthesis, since nonlinear behavior arises naturally from the formalism. Thermal properties and epoxy curing are also treated by their model. The authors have also attempted to treat failure by identifying a critical hydrostatic tension consistent with glassy failure. The model has been validated with a wide variety of types of material tests. The finite element simulations are performed in three dimensions. These authors have, thus far, done only a limited amount of preliminary work with heterophasic systems, but they report that the results were encouraging. 

Dicy Accelerator and adhesion promoter for epoxy systems Dicy-100 (Suzhou Fun Chemical Co.) 2-5 phr (P-104) and 4-6 phr (P-106), fast gel time... 

Within the different types of epoxies, are found epoxy diacrylates or vinyl ester resins, used to produce specific corrosion and chemical resistant composite systems. Vinyl ester resins are produced by either reacting epoxy resins of glycidyl derivatives with methacrylic acid, or from BPA and glycidyl methacrylates, where an active monomer (usually styrene) as crosslinker, hardener (usually organic peroxides), accelerators (cobalt) are added to the system. In the thermoset epoxy systems, there are also the mould releasers , which can be either internal such as, lecithin, or stearates of zinc and calcium, certain organic phosphates that are mixed in the resin, or, external - such as, fluorocarbons, silicone oil, and certain waxes, that are directly laid on the mould. 

Even more important to the formulation is a photoinitiator for the UV curables or an accelerator for the EB curables. These do not necessarily have to be a separate ingredient, but they may be a molecular modification of a resin or monomer ingredient. A typical photoinitiator for a UV curable acrylic system is based on an aromatic keto compound, and for an epoxy system it is based on a diazonium or a sulfonium compound. 

It is a Lewis acid t e catalyst, which initiates the homopolymerization of epoxy resins that predominantly form ether linkages [25]. The complexes are normally used at 3-5 phr and when used as an accelerator in systems involving anhydrides or amines, 1 phr or less. At room temperature, it is stable when mixed into epoxies, resin soluble salt that has essentially no catalytic activity. Due to this latency, most resin systems incorporating BF3MEA can be stored at room temperature for several months without any appreciable change in viscosity. The effect of BF3MEA on the gel time of an 828/DDS resin system at different temperatures is shown in Figure 13.3. 

Cumylphenol derivative n. One of a group of polymer intermediates based on cumylphenol that offer higher performance at lower cost than nonylphenol competitors. The free phenol is an accelerator for amine hardeners of epoxy resins. Cumylphenyl acetate and the glycidyl ether are reactive in epoxy systems, giving enhanced strength. The benzoate of cumylphenol aids extrusion of PVC compounds. 

Polymercaptans are used as low-temperature curing agents i.e. they allow the curing process of the epoxy system at between -20°C and 0°C with the addition of a tertiary amine as accelerator. On the other hand, at ambient temperature, epoxy/polymercaptan systems show a short curing time (i.e. pot life between 2 minutes and 10 minutes), rapidly reaching their optimum properties in 30 minutes. 

A modified epoxy matrix for Kevlar FRP composites was produced from ep>oxy/polyphenylene oxide (PPO) blends cured with multifunctional cyanate ester resin [94]. The effects of the PPO content on the cme behavior in the cyanate ester-cmed epoxy were investigated with FTIR. The cme reaction in the ep>oxy/PPO blends was faster than that of the neat epoxy system. FTIR analysis revealed that the cyanate fimctional group reactions were accelerated by adding PPO and that several co-reactions had occurred. Thermal mechanical analysis showed that the thermal stability of the epoxy/PPO matrix is improved by adding PPO. In the respective compursites, the ISS values between Kevlar fiber and the epoxy/PPO blends are almost the same as those between Kevlar fiber and neat epoxy. The ILSS in the respective laminates increases with the PPO content, which was attributed to an increase in the composites ductility. 

Crosslinkers and Accelerators. Besides the polymers themselves, changes and improvements are taking place in curing agents, hardeners, catalysts, and polymer modifiers. For example, if epoxy adhesives with improved heat resistance are needed, the hardeners can be aromatic anhydrides and amines such as benzophenone tetracarboxylic dianhydride, py-romellitic dianhydride, diaminodiphenyl sul-fone. To enhance curing speed, epoxy systems utilize mercaptan hardeners, while acrylic monomer/polymer blends avail themselves of amine/aldehyde catalysts with benzosulfimide (saccharin) accelerators. Radiation-curing systems for today and tomorrow utilize both visible light and laser beams. 

A two-component epoxy system consists of a resin and a hardener, along with possible additives such as accelerators, reactive diluents, resin modifiers, plasticizers, and fillers. Typical hardeners include aliphatic polyamines, which cure at room temperature or at slightly elevated temperatures polyamides, which provide flexibility and are widely used aromatics, which are solid and anhydrides, which require elevated temperature cure and produce thermally stable but brittle adhesives. Low molecular weight epoxies are liquid and are usually cured by amines, carboxylic acid anhydrides, and Lewis acid and base catalyst. Higher molecular weight epoxies are cured through their hydroxyl groups. Cure of epoxies involves an exothermic reaction. 

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