Epoxy casting resins, properties

Epoxy casting resins are frequently applied for encapsulation of electro-technical parts such as coils and transformers. The electrical properties of epoxies are, in general, better than those of polyesters. 

Table 3.5 Typical properties of epoxy cast resins (reference 3.3).

The thermal stabiUty of epoxy phenol—novolak resins is useful in adhesives, stmctural and electrical laminates, coatings, castings, and encapsulations for elevated temperature service (Table 3). Filament-wound pipe and storage tanks, liners for pumps and other chemical process equipment, and corrosion-resistant coatings are typical appHcations using the chemically resistant properties of epoxy novolak resins. 

In the crosslinked state, epoxy resins are highly resistant to chemicals, temperature, and solvents and are also endowed with good electrical properties. They are therefore employed, for example, as casting resins in electro- and electronic industry as well as resistant lacquers and coatings. Moreover, they possess excellent adhesive power for many plastics, wood, and metals ( reaction adhesives two-component adhesives ). 

Superior toughness and mechanical properties The strength of properly formulated epoxy resins usually surpasses other types of casting resins. The cured epoxy resins [Structure (4.35)] are seven times tougher than the cured phenolic resins [Structure (4.36)] as is evident from Figure 4.15. The relative toughness is attributed to the distance between the crosslinking sites and presence of internal aliphatic chains. 

This paper presents data generated to characterize the physical properties of tris(hydroxyphenyl)methane-based epoxy resins. Resin and cured clear casting physical properties, electrical properties, moisture resistance, formulated stability, reactivity and retention of properties at elevated temperature are covered compared to other selected resins. 

Chem. Descrip. Aluminum trihydrate CAS 21645-51-2 EINECS/ELINCS 244 92-7 Uses Flame retardant, filler for polyester, acrylic, melamine, and epoxy resins, polyurethanes (casting resins), powd. coatings Properties Powd. = 6 pm median particle size bulk dens. = 550 kg/m oil absorp. = 18 cmViOO g conduct, [efl 130 pS/cm < 0.3% moisture Martinal ON-908 [Martinswerk GmbH Lonza]... 

Chem. Desaip. Alkyl-atyl-modified methylpolysiloxane Uses Deaerating agent, flow aid, slip agent for solv.-based and solvent-less systems u in building protection (casting materials, mortars, floor coalings based on solventless and solv.-based epoxy and PU resins) Properties Sp.gr. 0.935-0.955 dens. 7.9 Ib/gal vise. 500-1000 mPa s ... 

Epoxy resins have proven very popular because of their versatility, excellent adhesion, low cure shrinkage, good electrical properties, compatibility with many other materials, resistance to weathering and chemicals, dependability, and ability to cure under adverse conditions. The most widely used epoxy resins in the casting field are the bisphenol-A and cycloaliphatic epoxies. Novolac epoxy resins have higher heat deflection temperatures, but they require high-temperature cure. The novolacs also have excellent resistance to solvents and chemicals. Epoxy castings are often used in electrical/electronic applications. (5ee also epoxy resin.)... 

Deanin and Dossi used the propylene oxide-MA reaction to prepare cured polyester resin products in a one-step procedure. In the method explored, the epoxide, MA, phthalic anhydride, styrene, lithium bromide, and benzoyl peroxide were combined, heated to a melt, and cured as castings. The properties of the cured materials were inferior to conventional systems. Others have also studied this concept.A one-step (condensation-addition) polymerization procedure has also been studied as a method for preparing coatings " and laminates with epoxy resins. 

Table 5. Mechanical Properties of High Performance Epoxy Resins in Unreinforced Resin Castings...

These transformers may be PVC taped, thermoplastic (polypropylene) moulded, fibreglass taped, polyester resin cast or epoxy resin cast depending upon the system voltage and the surroundings. HT indoor transformers, for instance, are generally polyester or epoxy resin cast, and are economical with good dielectric properties. They are resistant to humid, chemically contaminated and hazardous areas. Outdoor HT transformers, how-ever. may be epoxy... 

Epoxy resins have outstanding mechanical and electrical properties, dimensional stability, resistance to heat and chemicals, and adhesion to other materials. They are used for casting, potting, encapsulation, protective coatings, and adhesives. Epoxy glues separate the resin from the curing. igciU to be mixed just prior to use. 

Epoxies Powder, one and two-part liquids and paste Many types of resins available, providing wide spectrum of properties. Easy to compound. Low shrinkage and excellent dimensional stability. Good to excellent adhesion. May be cast or molded. Coatings, sealants, adhesives, solderless PC boards. 

The inhibition of composite propellants is somewhat easier than that of DB propellants. The binders used for composite propellants (with or without fillers) have been reported for inhibition of composite propellants. Such inhibition systems possess stronger bonds with composite propellants and prove to be more compatible coupled with better shelf-life of the inhibited propellants. However, epoxy or novolac epoxy resin with or without inert fillers is generally preferred for the inhibition of composite propellants due to a combination of properties possessed by them. The inhibition is usually done by casting technique and inhibition thickness is usually required on higher side in order to make the missions successful. In India, thread winding technique or inhibitor sleeve technique is preferred where 2.5-3.0mm inhibition thickness is sufficient as against 3.5-4.0 mm in case of inhibition by casting technique . 

Cycloaliphatic and heterocyclic epoxy have better weather resistance and less tendency to yellow and chalk than do aromatic epoxy resins. These resins possess excellent electrical properties and are often used in electrical and electronic applications. They are generally formulated into casting and filament winding compounds. 

Table 13.6 shows the characteristics and properties of supported and nonsupported high-molecular-weight epoxy resins cast from solvent solution. A crosslinked and uncrosslinked formulation using the same high-molecular-weight epoxy is shown for comparison. 

---