Commercial products are available where "n" varies from 2 to 7.
#4. Waterborne Epoxy Resins
Epoxy resins are hydrophobic in nature and consequently are not, by themselves, dispersible in water. However, water dispersibility can be conveyed to epoxy resins by two general methods:
- "Chemical modification" of the epoxy resin, or
- Through the process of emulsification
Chemical modification of the epoxy resin generally includes either attaching hydrophilic groups to the epoxy resin or attaching the epoxy resin to hydrophilic polymers. The emulsification method is primarily used for
waterborne epoxy resin adhesive systems.
The epoxy resin is made water-dispersible by partitioning the epoxy resin within a micelle, effectively separating the resin from the water. Emulsification can be achieved by a suitable
surfactant. The
choice of surfactant and processing parameters determines the long-term mechanical and chemical stability of the dispersion.
Epoxy resin emulsions are commercially available from several sources. As a group,
the typical particle size of the dispersion is in the 0.5-3.0 micron range. Solids typically range from 50-65% and viscosity from 10,000-12,000 cps. Characteristics of these epoxy dispersions are summarized in the table below.
Property
|
Bisphenol – A
|
Polyfunctional
|
Modified
|
WPE (Weight per epoxy based on resin solids)
|
195 – 2200
|
Viscosity, cps
|
500 – 20,000
|
Functionality, epoxy groups per molecule
|
2
|
3-8
|
Depends on base resins
|
Other characteristics
|
Higher MW provides greater flexibility
Lower MW provides greater crosslinking density
|
Higher Tg and crosslinking density than bis-A systems
|
Modifications are generally for greater toughness, flexibility, and adhesion to substrates such as vinyl
|
Characteristics of Major Classes of Epoxy Dispersions Used in Adhesive Applications
#5. Epoxy Acrylate Resins
There are basically two types of epoxy acrylate resins used in formulating adhesive systems.
- One is a vinyl ester resin that is used in two-component adhesive formulations much like a DGEBA epoxy resin.
- The other is a special type of resin that is used in radiation cure processes.
This latter type of epoxy acrylate does not have any free epoxy groups, but reacts through its unsaturation.
Epoxy Acrylate Resins Production
Epoxy acrylate resins or "vinyl esters" are made from the esterification of epoxy resin. The resultant polymer is typically dissolved in a reactive monomer such as styrene. These epoxy acrylate resins act more like polyester resins than they do epoxy resins. They are easily processed, have fast cure rates at room temperature, and can be cured with peroxides.
Once crosslinked vinyl esters have excellent chemical resistance and mechanical properties at both room and elevated temperatures. In adhesive systems, epoxy acrylate resins impart low viscosity, flexibility, and superior wetting characteristics compared to DGEBA type epoxy resins. However, their shrinkage is greater than any conventional epoxy resins and often the formulator will have to counteract this.
Epoxy acrylate resins are also commonly used as oligomers in radiation curing adhesives. However, their name often leads to confusion. In most cases, these epoxy acrylates have no free epoxy groups left but react through their unsaturation.
Epoxy acrylate resins are formulated with photoinitiators to cure via ultraviolet (UV) or electron beam (EB) radiation. The reaction mechanism is generally initiated by free radicals or by cations in a cationic photoinitiated system. Epoxy acrylate oligomers that are used in UV/EB curing are very low viscosity systems with high vapor pressures. Within this group of oligomers, there are several major sub-classifications.
#6. Other Epoxy Resins
Bisphenol F
Bisphenol F epoxy resins are produced by condensing phenol with formaldehyde. Bisphenol F epoxy resins have
a lower viscosity than DGEBA for the same molecular weight. Cured bisphenol F epoxy resins also have increased solvent resistance. Bisphenol F resins are often mixed with conventional DGEBA epoxy resins because of the relatively high cost of the bisphenol F product. When mixed with bisphenol-A resins, the two-form crystallization-free resins of moderate viscosity.
Tetraglycidyl ether of tetraphenolethane
Tetraglycidyl ether of tetraphenolethane is an epoxy resin noted for high temperature and high humidity resistance. It has a functionality of 3.5 and, thus, exhibits a very dense crosslink structure. The resin is commercially available as a solid. It can be crosslinked with an aromatic amine or a catalytic curing agent to induce epoxy-to-epoxy homopolymerization. High temperatures are required for these reactions to occur.
Diglycidyl ether of resorcinol
Diglycidyl ether of resorcinol-based epoxy resins provides the highest functionality in an aromatic diepoxide. It is one of the most fluid of epoxy resins, with a viscosity of 300-500 cps at 25°C. Because of its high functionality, it is a very reactive resin and cures more rapidly than DGEBA epoxies with most conventional curing agents.
Cycloaliphatic epoxy resins
Cycloaliphatic epoxy resins have better weather resistance and fewer tendencies to yellow and chalk than aromatic epoxy resins. These resins possess excellent electrical properties and are often used in electrical/electronic applications. Their use in adhesive systems is limited because they are relatively brittle and higher in cost than aromatic resins. However cycloaliphatic epoxy resins are used in cationically cured (UV and EB) epoxy resin adhesive formulations.
Glycidyl amine
Glycidyl amine epoxy resins are reaction products of aromatic amines and epichlorohydrin. They have high modulus and high glass transition temperature. These resins find use in specialty aerospace composites and high temperature adhesive formulations.
Biobased epoxy resins
Biobased epoxy resins are based on agricultural sources such as plant-derived glycol used in the manufacture of epichlorohydrin. Otherwise, the characteristics are similar to petroleum synthesized epoxy resins.