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Epoxy Resin

What is Epoxy Resin

By

Composite Wind Turbine

Wind turbine blades often use epoxy resin

Todd Johnson

The term epoxy has been widely adapted for many uses beyond fiber reinforced polymer composites. Today, epoxy adhesives are sold in local hardware stores, and epoxy resin is used as the binder in counter tops or coatings for floors. The myriad of uses for epoxy continues to expand, and variates of epoxies are constantly being developed to fit the industries and products they are used in.

Epoxy resins are commonly used in:

  • General purpose adhesives
  • As the binder in cements and mortars
  • Rigid foams
  • Non-skid coatings
  • Solidifying sandy surfaces in oil drilling
  • Industrial coatings
  • Potting and encapsulating media
  • Fiber reinforced plastics

In the realm of fiber reinforced polymers (plastics), epoxy is used as the resin matrix to efficiently hold the fiber is place. It is compatible with all common reinforcing fibers including fiberglass, carbon fiber, aramid, and basalt. Common products and manufacturing methods for fiber reinforced epoxy include:

  • Filament Winding
    • Pressure vessels
    • Pipes
    • Rocket housings
    • Recreational equipment
  • Pultrusion
    • Insulator rods
    • Arrow shafts
  • Compression Molding
    • Aircraft parts
    • Skis and snowboards
    • Skateboards
    • Circuit boards
  • Prepreg and autoclave
    • Aerospace components
    • Bicycle frames
    • Hockey sticks
  • Vacuum Infusion

Obviously there are many more FRP composite products manufactured from epoxy, but mentioned were a few products that are commonly manufactured with epoxy and with a particular manufacturing process. Additionally, the same epoxy resin likely cannot be used for each of the mentioned processes. Epoxies are fine tuned for the desired application and manufacturing process. For example, pultrusion and compression molding epoxy resins are heat activated where as an infusion resin might be ambient cure and have a lower viscosity.

When compared to other traditional thermoset or thermoplastic resins, epoxy resins have distinct advantages, including:

  • Low shrink during cure
  • Excellent moisture resistance
  • Excellent chemical resistance
  • Good electrical properties
  • Increased mechanical and fatigue strength
  • Impact resistant
  • No VOCs
  • Long shelf life

Chemistry

Epoxies are thermosetting polymer resins where the resin molecule contains one or more epoxide groups. The chemistry can be adjusted to perfect the molecular weight or viscosity as required by the end use. Their are two primary types of epoxies, glycidyl epoxy and non-glycidyl. Glycidyl epoxy resins can be further defined as either glycidyl-amine, glycidyl-ester, or glycidyl-ether. Non-gylcidyl epoxy resins are either aliphatic or cyloaliphatic resins.

One of the most common glycidyl epoxy resins is created using Bisphenol-A, and is synthesized in a reaction with epicholrohydrin. The other frequently used type of epoxy is known as novolac based epoxy resin.

Epoxy resins are cured with the addition of a curing agent, which is commonly called a hardener. Perhaps the most common type of curing agent is amine based. Unlike in polyester or vinyl ester resins where the resin is catalyzed with a small (1-3%) addition of a catalyst, epoxy resins usually require the addition of the curing agent at a much higher ratio of resin to hardener, often 1:1 or 2:1.

As mentioned, the properties of epoxy can be altered and tweaked to fit the desired need. Epoxy resin can be "toughened" with the addition of thermoplastic polymers.

Prepregs

Epoxy resins can be altered and impregnated into fiber and be in what is called a B-stage. This is how prepregs are created.

With epoxy prepregs, the resin is tacky, but not cured. This allows layers of prepreg materials to be cut, stacked, and placed in a mold. Then, with the addition of heat and pressure the prepreg can be consolidated and cured. Epoxy prepregs and epoxy B-staged film must be keep at a low temperature to prevent from premature curing. Because of this, companies using prepregs must invest in refrigeration or freezer units to keep the material cool.

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