Although epoxy systems have been commercially available since the late 1940s, only in the last decade have they found widespread use as a means to consolidate and seal ornamental stone slabs. These systems are far simpler than those used in the paint industry, for example, in thin film applications, but they are nevertheless superior to polyester resins used for many years to consolidate marble slabs. Epoxies give better chemical, thermal and mechanical resistance as well as improved adhesive strength. Furthermore, it's possible to tailor the epoxy formula to enhance desirable characteristics. Even higher versatility is provided because two constituents are involved, and the final properties will be a contribution from both the resin and the curing agent.
Because of their excellent technical properties and versatility, epoxy systems find use in an almost infinite variety of applications. To cite just a few, as protective coatings they can be used to paint marine platforms, ships, concrete floors, automobile parts, truck chassis, etc.; they serve as structural adhesives in civil engineering applications; and in the electronic industry, they are used in varnish formulations, electrical insulators and as encapsulates. Other important applications include repair of cracks and fissures in structural concrete above or underwater and as adhesives for ceramics.
This work was based on literature and tests conducted on various types of ornamental stone slabs at Granito Zucchi Ltda. in Brazil. It provides a basic insight into the chemistry of epoxy systems, and the factors which influence their coating properties after application on stone surfaces.
1. Epoxy Resin
Resins or epoxy polymers are obtained through a condensation reaction of bisphenol A with epichlorohydrin in the presence of sodium hydroxide (Figure 1, left). The bisphenol A is a product of the reaction of phenol with acetone (Figure 1, right). They are known as epoxies because of the oxygen-containing ring also called the oxirane ring. The reaction on the right of Figure I, known as the â€œTaffyâ€ process, produces low molecular weight epoxies. Resins obtained this way are commonly called â€œliquid epoxy resinsâ€ or diglycidyl ether of bisphenol A (DGEBA). DGEBA is a rigid moiety and is responsible for the high-performance profile of these systems. It is this type of resin that has proved most successful in the ornamental stone industry.
The pure monodisperse diglycidyl ether of bisphenol A is not commercially available. Instead, the low molecular weight resins are a polydisperse system with n between 0 and 1 and molecular weights between 350 and 600. The n value represents the number of times the repeating unit occurs in the polymer. If n is greater than 1, the resin is a brittle solid. The higher n values are a result of secondary reactions that occur because the active hydrogen on the hydroxyl group of another Bisphenol A molecule can add to open the oxirane ring. The relative amounts of reactants determine the value of n, and an excess of epichlorohydrin is added in the â€œTaffyâ€ process to favor the formation of a liquid. The low molecule weight resins are sold as Dow 331, Shell Epon 828, Araldite 6010 or Kudko YD128.
The â€œTaffyâ€ process is rarely used to produce higher molecular weight molecules; instead the fusion or advancement process is preferred. This involves adding bisphenol A to the liquid epoxy resin in the presence of a basic catalyst, benzyl trimethyl ammonia hydroxide.
DGEBA, in its low molecular weight form, has viscosities between 11,000 and 16,000 cps, which are much too high to be effective in stone consolidation. To reduce the viscosity reactive or non-reactive diluents are added. Reactive diluents contain epoxy groups and have viscosities between 1 and 70 cp and can be mixed in at levels of up to 20%. Bi-functional diluents produce less negative side effects than monofunctional ones, which stop the linking process between epoxy resin and the curing agent. Because the concentration of DGEBA is reduced, some loss of mechanical and chemical properties will occur. Pot life is increased, and the surface tension is reduced, which improves wetting of the substrate.
Non-reactive diluents can also be used in epoxy formulations. They will tend to increase flexibility and decrease mechanical and chemical resistance much more than was the case for reactive diluents, since they do not take part in the linking process. A popular non-reactive diluent is benzyl alcohol, which can be added at levels of up to 10% with little apparent side effects. In some systems, such as those with cycloaliphatic curing agents, common in the stone industry, levels of up to 40% are added. Benzyl alcohol will tend to disappear with time, leaving the epoxy harder, but less impact resistant.
1.2 Differences between epoxy and polyester resins
Polyester resins are cheaper than epoxies, but have disadvantages such as poor adhesion, water absorption difficulties, higher porosities and toxicities. They also show higher shrinkage. The chart below compares these two systems.
2. Curing Agents
2.1 What are they?
Curing agents or hardeners are chemical species that react with the epoxy resin to form a cross-linked polymer with its own unique characteristic. The most common are the amine curing agents, which react by opening the oxirane ring with amine active hydrogen. The basic amine is classified according to its functionality as: primary, two active hydrogens; secondary, one active hydrogen and tertiary, no active hydrogen. Within this group of amine curing agents, the most common are: polyamides, aliphatic amines, cycloaliphatic amines, amido-amines, aromatic amines and dicyandiamide. Each group has its own characteristics, as shown in Table II. The choice of curing agents varies with the final characteristics that one desires. One can formulate a system, for example, to be more tolerant to application conditions, such as temperature and humidity; to have high/low peel strength depending on the type of substrate - iron, galvanized steel, aluminum, concrete, stone, etc.; to have short or long cure times; or to have a high/low viscosity. Cycloaliphatic amines are more common in the ornamental stone industry, being less toxic than aliphatic or aromatic amines, and less viscous than the polyamides.
2.2 Why Modify the
Curing agents are modified for a number of reasons. Some of these are: to lower viscosity, to increase compatibility with the resins, to increase the rate of cure, to minimize migration to the surface and formation of carbamates or bicarbonates (blushing), shown in Figure 3, or to increase toughness or plasticity. Three common ways of modifying an amine are: (1) blending with phenols, primary aliphatic amines or other accelerators, (2) adducting the amine with an epoxy resin, (3) reacting the amine with phenol and formaldehyde to form Mannich bases.
The formation of a Mannich base will reduce the functionality of the amine, but the presence of the hydroxyl group on the aromatic ring will act as an acid catalyst to open the oxirane ring and accelerate the epoxy/amine reaction. With the decrease in functionality the blushing reaction, this is less likely. Mannich bases also show better adhesion to damp substrates, as well as increased compatibility with the resin.
A process called adducting increases molecular weight, producing a moiety with less mobility and less tendency to blush and corrode. These resulting products are also more compatible with the resin. These advantages are offset by an increase in viscosity. To lower viscosity, solvents are added, such as benzyl alcohol. The presence of the active hydrogen, as in the case of Mannich bases, acts as an accelerator.
3. The epoxy curing process
3.1 The reaction
Although there are various cure mechanisms, the one used most today in the stone industry is the polyaddition polymerization with primary and secondary amines. The key player in the curing process is the strained oxirane ring, which opens in the presence of amine activated hydrogen, as shown in Figure 4. The reaction occurs without the formation of by-products, and shrinkage is less than 5%. Highly reactive amine hydrogen will permit the reaction to occur at below ambient temperature. Strong adhesive properties are due to the availability of nitrogen and oxygen atoms in the polymer, which coordinate bond with the substrate. The result is a highly cross-linked, three-dimensional super molecule, schematized in Figure 5. The linkages between adjacent polymer chains produce a rigid structure with excellent mechanical and thermal stability, a characteristic common to all thermosets.
The reaction between the amine and resin is exothermic, and the cure time is directly proportional to the temperature of the slab. An approximation is that for every 10 degrees C rise in temperature, the cure time will decrease by one half of its normal rate. The resins used in the ornamental stone industry are approximately 70% to 80% cured after several days, and to be 100% cured may take months.
Besides amine curing, the epoxy polymer can self-polymerize in the presence of a catalyst, such as a tertiary amine, amine salt or Lewis acid. These reactions open the oxirane ring and produce anions, which can then react with another epoxy group or attack the active hydrogen, providing another anion capable of further reaction with other epoxy groups. Resins cured in this fashion are very rigid and hard. There are other species that are catalytic as well and are not amines, such as phenols and aliphatic alcohols.
4. Mix ratio (stoichiometry)
To optimize the properties of the cured epoxy and to ensure maximum crosslinking, the mix ratio (stoichiometry) must be controlled. This means, an amine group must satisfy each epoxy group. Working outside this ideal will affect the final characteristics. In some cases, this may be desirable. For example, in some formulated systems, increasing the level of curing agent will reduce the cross-link density, thus providing increased flexibility. At the same time, another characteristic will be weakened, such as chemical resistance. If the level of epoxy resin is increased outside the mix ratio, the system will not cure, and in this case, heat will have to be applied to harden the epoxy.
Stoichiometric mix ratios are calculated using the epoxy equivalent weight (the weight of epoxy resin containing one epoxy group) and the amine hydrogen equivalent weight (the weight of amine containing one active hydrogen). This implies the following:
1. Epoxy Equivalent Weight (EEW) = Active Amine Hydrogen Equivalent Weight (AHEW)
2. phr (parts of hardener per 100 parts of resin) = AHEW/EEW x 100
A higher amount of AHEW means the end product will not be forgiving to mixing ratio errors, and from a practical perspective, is better to have a high AHEW or non-critical stoichiometric mix ratio. This can also be advantageous for reducing the epoxy resin viscosity, which is an important factor for determining penetration velocity.
5. Considerations when formulating
Epoxy systems are formulated in order to enhance certain characteristics, adhesion, hardness, flexibility, heat and chemical resistance, cure rate, etc. If one only considers the principal properties that are important for ornamental stone consolidation and sealing, then one would want systems that:
A. Penetrate profoundly to strengthen the stone and seal the surface to give protection and enhance the natural color of the stone.
B. The end product should have sufficient hardness that during the polishing process, the epoxy resin cannot be mechanically hooked by the abrasives and torn away.
C. The cure rate has to be sufficiently fast that it develops the desired degree of hardness within the time constraints of the production process.
D. The curing agent and other additional components should exhibit a molecular structure that is compatible for the formation of strong adhesive bonds with the substrate.
E. It should exhibit sufficient flexibility to support stress during movement without allowing the substrate to fracture, for example in the epoxy-filled micro cracks.
F. The curing agent and other additional components should be compatible to accept low moisture conditions.
G. It should have a working time during the application process that permits sufficient penetration.
H. It should not present severe health risks to the worker.
5.1 Epoxy Penetration and Wetting
Whether an epoxy system penetrates effectively a porous substrate depends on its viscosity and those characteristics that enable it to wet the substrate. The affinity for the solid will depend on the surface tension existing between the phases of gas, liquid and solid.
Surface tension (or energy) is defined at the boundary between two phases - usually liquid and air - and is a measure of the cohesive forces active at the interface. In the bulk of a liquid, a molecule will experience a balance of forces, whereas at the surface the intermolecular interactions are attractive and unbalanced. A drop of water on a surface is curved because there is an attractive force towards the bulk of the liquid. Polar liquids like water have high intermolecular interactions; and consequently high surface tensions. If one were to bring a molecule from the interior of the liquid to the surface, it would require energy. In the case of water, this energy would be 7.3 x 10-2 joules per square meter. That is, this amount of energy is necessary to expand the surface by one square meter. If this amount of energy were very low, the surface area would increase to the point that the two phases would mix. Anything that reduces the intermolecular forces at the surface will reduce the interface tension. An increase in temperature, for example, or molecules absorbed at this interface will lower the surface tension. The liquid/gas as well as the liquid/solid interfacial tension are critical factors when determining the rate of migration of a liquid in a porous medium.
Solid liquid gas boundaries are characterized by the contact angle shown in Figure 6, as measured through the liquid phase. The contact angle specifies the degree of affinity the fluid has for the substrate. If the substrate has a surface energy greater than the liquid, the adhesive forces of the liquid will overcome the cohesive forces, and the liquid will â€œwet outâ€ or spread over the substrate. This situation gives a low contact angle - less than 90 degrees - for something such as water on glass. A non-wetting situation would be water on certain plastics or mercury on glass. The glass surface has a lower surface energy than the mercury - or one can say the cohesive forces of mercury are greater than the adhesive forces between mercury and glass - and the contact angle will be greater than 90 degrees.
Consider the idealized case in Figure 7, in which a liquid enters an open-ended pore. The pore is horizontal, so the effects of gravity are not considered. The adhesive forces between the substrate and liquid are sufficiently strong to overcome the cohesive forces between the liquid molecules, and a convex surface is formed. The liquid wets the solid. The balance of forces, cohesive, adhesive and surface tension can be described as capillary pressure. Capillary pressure is defined as the difference in pressure between the non-wetting phase and the wetting phase.
The penetration time will be lower for the following: high liquid gas interfacial tension, low contact angle, large pores, and low viscosity. Viscosity is time and temperature dependent, increasing as the cure rate progresses and initially decreasing as the temperature increases. The penetration time or velocity will also slow as length increases because of viscous drag, since more and more fluid has to be dragged along as time passes.
Based on this equation, one can try to increase penetration by reducing the viscosity through dilution of the resin, as already discussed, or working at a higher application temperature. Another option is to choose a curing agent with low viscosity and high AHEW, as mentioned in section 4. Viscosity decreases as the temperature increases, but the working time or gelling time also falls as well as the cure time. Choosing a temperature too high can actually decrease penetration, and it could cause the curing agent to vaporize, depleting the resin of hardener and compromising the end product's physical and chemical properties. Since many different types of stone are processed with varying permeability and porosity, one usually works with a slab temperature between 40 and 50 degrees C.
The time dependence of the viscosity can also be controlled by choosing a fast or slow hardener, or by choosing to add accelerators to the mixture. The proper choice can only be decided after experimenting and examining the results.
Since the pore diameters and length of penetration are independent variables, we can try to increase penetration by modifying the contact angle. Lowering the contact angle can be easily achieved at low cost with significant results. There are a variety of wetting agents that can be added at very low levels to provide a more efficient angle. One part of these agents binds to the substrate and the other part remains in the liquid. Absorption at the interfacial boundary reduces the liquid solid surface tension. As the fluid moves through the pores, the wetting agent is depleted, and the rate at which they can diffuse up to the advancing liquid front determines the penetration velocity.
As the resin penetrates a dead end pore, it will stop if the trapped air is compressed until its pressure equals the maximum surface tension that the epoxy mixture can apply. One can reduce the surface tension of the air liquid boundary by adding chemicals such as foam breakers, which would permit the trapped air to pass through the liquid, reducing this pressure.
5.2 Formulating for Physical and Chemical Properties
The degree of hardness, adhesion, flexibility and application under humid conditions are mainly controlled by proper selection of the curing agent, diluent and the choice of accelerator.
In the case of adhesion, the key bonding components involved are: mechanical, chemical and polar. For these forces to act, it is necessary that the epoxy wet the substrate, and this can be improved by the judicious selection of a wetting agent.
Mechanical adhesion is the gripping force that results from the roughness of the substrate, (i.e. peaks and valleys). Changing from a round to angular surface profile and increasing the depth of the valleys can improve this type of adhesion. Ornamental stone slab surfaces are normally honed to abrasive grain size 120 and therefore should already have an adequate surface profile.
Polar adhesion is the total hydrogen bonding which occurs between the substrate and epoxy coating. Curing agents, diluents and accelerators that have strong polar groups and low steric hindrance will increase the adhesive force.
Chemical bonds are those that form through electron sharing by groups on the substrate and epoxy resin. These are by far the strongest and contribute most to adhesion. Groups such as nitrogen and oxygen can bond with silica and iron in the slab through this process.
Impact resistance or hardness is a function of the cross-link density with a higher density favoring increased hardness. Higher densities can be achieved using low molecular weight curing agents that show little steric crowding and can form tightly cross-linked structures. Adding non-reactive diluents can interfere with this structure, leaving the end product with some flexibility and toughness. By blending various types of amine groups, one can tailor the system and impart the degree of hardness, flexibility, etc. that is desired in the end product.
6. Epoxy Coating Failures on Ornamental Stone Slabs
The reasons for coating failures on ornamental stone slabs are in large part due to: (1) surface preparation, including factors such as excessive slab humidity or loose particles, both of which interfere with adhesion; (2) too high an application temperature which, can deplete the curing agent from the mixture, changing the mix ratio and accelerating the cure process, which will increase the viscosity and reduce the penetration; and finally; (3) not respecting the limits of the mix ratios.
When applying a resin to a stone slab, the best choice is the one that gives superior penetration. This will not only improve the surface gloss and strengthen the slab, but during polishing it is less likely that small or micro imperfections on the slab surface will occur.
The search for new and exotic stone continues. Some of this material is too fragile to endure the transportation or polishing process and without the application of epoxies, this material would have been of little value. The use of epoxy systems in the ornamental stone industry continues to steadily grow. The additional costs are more than offset by improvement in the quality of the processed material. Benefits include increased slab gloss, color enhancement and chemical and mechanical resistance. The tendency is for a growing number of stone slabs to be coated with resins before being exported.
As this trend continues, resins with specific characteristics will also become commercially available. For example, ultraviolet cured resins, also known as radcure, which harden in seconds, could be used as a protective sealer at the end of a high-performance polishing line or applied to correct small defects after polishing. Resins tailored for application on cold and damp surfaces already exist for concrete and could find applications in the stone industry, eliminating the need for costly drying ovens.
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