Greengrip
Development of materials for climbing holds
Safety first!
For information and advice on the safe handling and storage of chemicals/products, users must refer to the current Material Safety Data Sheet/Technical Data Sheet containing physical, ecological, toxicological and other safety related data.
Epoxy Resins
Epoxy resins are products of reaction of polyfunctional hydroxy compounds with 1-chloro-2,3-epoxypropane (epichlorohydrin).
Thermosetting epoxide polymer cures (polymerizes and crosslinks) when mixed with a catalyzing agent or "hardener". Most common epoxy resins are produced from a reaction between epichlorohydrin and bisphenol-A.
Epoxies represent some of the most versatile resins available to the composite manufacturer.
Types of Epoxy Resins:
There are two main categories of epoxy resins - the glycidyl epoxy, and non-glycidyl epoxy resins. The glycidyl epoxies are further classified as glycidyl-ether, glycidyl-ester and glycidyl-amine. The non-glycidyl epoxies are either aliphatic or cycloaliphatic epoxy resins.
Glycidyl-ether epoxies such as diglycidyl ether of bisphenol-A (DGEBA) and novolac epoxy resins are most commonly used epoxies.
Structure of Epoxy group, DGEBA, and Novolac epoxy resin.
Diglycidyl Ether of Bisphenol-A (DGEBA):
Diglycidyl ether of bisphenol-A (DGEBA) is a typical commercial epoxy resin and is synthesised by reacting bisphenol-A with epichlorohydrin in presence of a basic catalyst.
The properties of the DGEBA resins depend on the value of n, which is the number of repeating units commonly known as degree of polymerisation The number of repeating units
Novolac Epoxy Resins:
Novolac epoxy resins are glycidyl ethers of phenolic novolac resins. Phenols are reacted in excess, with formaldehyde in presence of acidic catalyst to produce phenolic novolac resin. Novolac epoxy resins are synthesised by reacting phenolic novolac resin with epichlorohydrin in presence of sodium hydroxide as a catalyst.
Novolac epoxy resins generally contain multiple epoxide groups. The number of epoxide groups per molecule depends upon the number of phenolic hydroxyl groups in the starting phenolic novolac resin, the extent to which they reacted and the degree of low molecular species being polymerised during synthesis. The multiple epoxide groups allow these resins to achieve high cross-link density resulting in excellent temperature, chemical and solvent resistance.
Curing of Epoxy Resins
In order to convert epoxy resins into a hard, rigid material, it is necessary to cure the resin with hardener. The curing process is a chemical reaction in which the epoxide groups in epoxy resin reacts with a curing agent (hardener) to form a highly crosslinked, three-dimensional network. Epoxy resins cure quickly and easily at practically any temperature from 5-150oC depending on the choice of curing agent.
A wide variety of curing agent for epoxy resins is available depending on the process and properties required. The commonly used curing agents for epoxies include amines, polyamides, phenolic resins, anhydrides, isocyanates and polymercaptans. The cure kinetics and the Tg of cured system are dependent on the molecular structure of the hardener. The choice of resin and hardeners depends on the application, the process selected, and the properties desired. The stoichiometry of the epoxy-hardener system also affects the properties
Many epoxy resin systems and curing agents are commercially available. Naturally, different systems give final materials of different mechanical properties.
Formation of one well-known commercial epoxy:Curing of epoxy resin:
Curing of diepoxy with a diamine.
Epoxy resin toughening
Epoxy resins have many advantages comparing to other thermosetting resins (polyester, polyurethane), but many epoxy resins shows poor fracture toughness, and are much more expensive than polyester or polyurethane. Fortunately, there is possibility for epoxy resin toughening, by which, some standard (lower cost) epoxy resins can be converted to epoxy of hardness and stiffness of much more expensive, highly crosslinked epoxies, along with enormous toughness.
There are many types of toughening agents available. We are going to take more detailed look to a few.
Inorganic Particles
A various of inorganic particles have been considered as modifiers for epoxy materials - glass beads, rubber particles, silica, flyash, metal particles, specialy made particles, and so on. The degree of toughness was found to depend upon particle size, shape, dispersion, and filler-resin adhesion strength.
Disperson of the particles into the resin is of a big importance. In the absence of a uniform dispersion, particles form agglomerates and cause particle-to-particle rather than the intended particle-to-resin interaction and result in the degradation of final material properties.
For better adhesion strength, inorganic particles can be specailly treated before toughening process. For example-treatment with organofunctional silane improves the resin-particle bonding which leads to improvement in fracture toughness.
Hydroxyl Terminated Polyesters
One way to improve the toughness of epoxy resins is the addition of a thermoplastics. One example is hydroxyl terminated polyester, a polyol. Polyol molecular weigth and concentration are parameters that significantly affect the mechanical properties of final material. By proper choice of the same parameters, same results can be obtained by using linear aliphatic diols - long diols rapidly forms network with low density, while short diols takes longer times but yields materials with a higher density.
Polyurethane Resins
Polyurethane is any polymer consisting of a chain of organic units joined by urethane links (NH-COO). Crosslinking occurs between isocyanate groups (-N=C=O) and the polyol's hydroxyl end groups (-OH). Each isocyanate and polyol will give different properties to the end product.
Polyurethane = Isocyanate + Polyol.
An important property of an isocyanate is its functionality (the number of -NCO groups per molecule) - average functionality is little over two, higher functionality of are used for special applications. The common isocyanate used to make polyurethanes are MDI (diphenylmethane 4,4-diisocyanate), NDI (naphtalene 1,5-diisocyanate), TDI (toluene diisocyanate), HDI (hexamethylene disiocyanate).
There are two main type of polyols used in the polyurethane industry - polyethers and polyesters. More widely used polyethers are manufactured from propylene oxide (major constituent of polyol) and ethylene oxide (modify the properties of the polyol).
One of the biggest polyurethane advantage is high strength to weight ratios.
Polyester Resins
Polyesters are macromolecules whose monomeric units are joined through an ester group.
Polyesters contain an unsaturated polyester or hybridized vinylester backbone which is catalyzed with a peroxide to condense into cross-linked solid resin.
Monomers are required (e.g . Styrene in polyester systems) to react with the unsaturations in the polyester molecules to form solid polymer .
Polyesters have the advantage of being extremely inexpensive when compared with other thermoset resins i.e. vinylesters and epoxies. If the upside is cheap pricing, the down side includes poor adhesions, high water absorption, high shrinkage, and high VOC's.
Polyester resins are the most commonly used matrix in the marine and composite industry. These resins are styrene-based (20-60%), flammable and catalyzed when combined with Methyl Ethyl Ketone Peroxide(MEKP).
Colors
All colors used have been food colors, which are safer to use than normal dyes and pigments. Food colors are tested for presence of toxic chemicals, so this fact should be used. In all our materials, independently on resin types, we used food colors coded by E-numbers (used in EU). Complete list of E-number coded food colors can be seen here.
