The synthetic resins like epoxy, polyester, acrylic, polyurethane and their similar type of resins are used widely for civil engineering applications.
Synthetic resins are employed for:
- Grouting of cracks formed in the concrete structural elements
- Repair of concrete structures that got eroded
- Emergency repair of bridges
- Aqueducts repairs
- Correction of columns and beams that are chemically eroded
The epoxy resin offers a wide range of properties when they are cured with the help of different types of hardeners.
Characteristics of Epoxy Resin
The basic characteristic of epoxy resin that makes them be widely used in the civil engineering application are:
- The property of high adhesive strength to almost all the materials
- Low possibility for shrinkage during curing time
- Higher dimensional stability
- Naturally gained gap filling properties
- Thermosetting Property- Won’t melt
- Higher resistance to chemicals and variety of environment condition
- Can cure even in wet conditions. For example Underwater construction
- Greater ease of application
The epoxy resin is a generic term that represents the name of compounds that describe a variety class of thermosetting polymers. Here the primary cross linking is occurring with the reaction of an epoxy group.
The most important raw material that is employed in the production of epoxy resin is Epichlorohydrin. Diglycidyl Ether of Bisphenol-A or DGEBA is the basic epoxy resin that is used in the building industry.
The DGEBA in its standard form is the condensation product of bisphenol-A and Epichlorohydrin.
Based on the amount of excess of Epichlorohydrin to bisphenol-A that is used in the manufacturing process, the molecular weight of the epoxy resin would range from low to high. Due to its high viscosity, the basic resin of this type is not found suitable for many applications.
Base resin modification is hence necessary to achieve the required curing rate and other mechanical and chemical properties in addition to lower viscosity.
These are curing agents which are called as hardeners. The hardeners are combined with the epoxy resin to change it from liquid to the hardened mass.
Among a variety of compounds, the most commonly employed curing agents are the aliphatic and aromatic amines and polyamides. These compounds are curing compositions active at room temperature that would facilitate the construction applications.
Some of the examples that come under this division are:
- Aromatic amines including the diamino diphenyl sulfone (DDS) and the dimethylaniline (DMA)
- Amine/phenol formaldehyde like urea formaldehyde and melamine formaldehyde
- Triethylenetetramine (TETA) and diethylenetriamine (DETA)
- Anhydride like phthalic anhydride and nadic methyl anhydride (NMA)
- Some of the catalytic agents like tertiary amines and boron trifluoride complex.
To modify certain specific physical and chemical properties of the resin, modifiers are employed. The general categories of epoxy modifiers are:
- Flame retarders
- Pigments and dyes
The rubber additives are epoxy modifiers that are employed to increase the following properties of the resin:
- Fatigue resistance
- Crack Resistance
- Energy Absorption or the toughness property
The rubber additives are polymers that can be used in either liquid, solid or as particulate components.
The diluents are modifiers used to reduce the viscosity of the epoxy resins. This will hence improve the handling characteristics of the epoxy resin. This is therefore categorized as reactive and non-reactive.
The reactive diluents have a lower molecular weight glycidyl ethers with a low viscosity. This will help in reducing the cross link density of the epoxy system.
The non reactive diluents like toluene, xylene and other types of aromatic hydrocarbons will bring significant reduction in the viscosity of the resins with low molecular weights. Dibutylepthalate used with a liquid resin is a popular non reactive diluent.
The frame retardant properties in the epoxy resin can be obtained with the help of flame retardants. The flame retardants are added to the epoxy resin as a filler.
A polymer system with more amount of carbon and hydrogen is found to be highly flammable in nature. The presence of halogens in the epoxy curative based resin will help in reducing the flammability.
Unreactive fillers are used widely and can be categorized generally as minerals, metals, carbon, fibers, glass and other miscellaneous organics. They can be used in a variety of forms physically including in the form of powders, pulps, flakes, spheres, micro-balloons, whiskers and short fibers.
Pigments and Dyes
A wide variety of pigments and dyes can be used to make the epoxy resin colored. These can be organic or inorganic in nature. The insoluble particles which are a dispersion of the resin forms pigments.
The dyes are soluble organic molecules. Due to temperature limitation, the dyes are not suitable for epoxy composite applications.