Introduction

The use of epoxy resin systems by industry is now an accepted practice, however, reports of their use and performance vary. The problems have been mainly, which epoxy resin system to use and where and how to use the chosen epoxy resin system correctly. The "Handbook of Epoxy Resins" by Lee and Neville lists 63 types of epoxy resins and 62 chemical groups which can be used to cure these resins. That adds up to 3,906 possible combinations as a start, but within each of the 63 types of epoxy resins there are many different resins of various molecular weights. There are many chemicals which can contain each of the 62 curing groups.

The possibilities are broadened further by the fact that these systems can be modified with reactive flexibilizers, diluents, accelerators and fillers.

There are about 30 variations of epoxy, depending on the raw chemicals used. There are about one dozen commercial producers of epoxy worldwide. About 2,000 different curing agents (hardeners, catalyst, promoters) have been developed since commercial use of epoxy began. In America alone, there are in excess of 1,000 commercial formulators. Considering that perhaps 1,000,000 formulations are possible, it is realistic to only summarize the range (from minimum to maximum) in values of the physical characteristics of cured epoxy resin.

Why epoxy resins? The most widely recognized property of the epoxy resin family is their excellent adhesion to a very broad range of substrates and rapid room temperature cure. One contributing factor to this end property is the low shrinkage exhibited by the systems during cure which results in lower stress levels in the finished piece that is found in other polymer systems with higher values of shrinkage. Furthermore, it is evident that the already low shrinkage can be made even lower by judicious use of fillers.

Epoxy resins are truly a family of products. They range in form from low viscosity liquids of 5 to 10 poise to friable solids (easily crumble or pulverized) with molecular weights from less than 500 to over 3,000 respectively. They thus provide broad latitude in formulating. As with the epoxy resin family, members of these curing agent families cover a range from low viscosity liquids to high melting solids and for each family, accelerators are available to increase the cure rate of the system, if that is desired. Consequently there is a tremendous versatility of the total resin/curing agent system. Besides being able to choose or adjust cure rate from slow (hours) to fast (seconds), the final product can be tailored to what is needed from a soft and flexible to a hard material.

Epoxy Resin

General

Epoxy, when mixed with a curing (hardening) agent, becomes epoxy resin which is a thermosetting plastic that rapidly develops adhesive strength which is maintained under moist conditions. This synthetic organic compound is stable chemically and physically; it is durable, crack resistant, and undergoes little reduction in volume (2 to 3%) as result of curing. Adhesives of this type become set (irreversible) as a result of exothermic chemical changes initiated by the chemical curing agent. Thermosetting plastics require heat to cure in order to do so, and are non-meltable and insoluble after becoming hardened. Epoxy resins may be formulated to have specific values of chemical and physical characteristics; this is accomplished by means of various hardeners, fillers, flexibilizers and plasticizers.

Terminology

Certain words and phrases relating to the technology of epoxy resin are defined here, rather than in an appended glossary, for the convenience of the reader and to insure continuity in the paper.

Polymer: A substance consisting of molecules characterized by the repetition of one or more types of chemical compounds.

Resin: (The A part of the A,B,C's of Epoxy Grout.) Any of a class of either liquid or solid organic compounds that are either natural or artificial; most resins are polymers, but not all polymers are resin.

Synthetic Resin: A substance that is a product by chemical treatment of natural hydrocarbons (derived from by-products of the oil refining industry) and in which a particular group of molecules repeats itself many times; also known generally as a polymer.

Polymerization: A chemical reaction in which two or more molecules of synthetic organic compounds are combined to form larger molecules.

Exothermic Reaction: A chemical reaction in which heat is evolved; the reaction is irreversible.

Epoxy: A molecular group of oxygen, carbon and hydrogen atoms. The simplest group may be envisioned as a three-member ring wherein one atom of oxygen is bonded to two carbon atoms each of which is previously combined with hydrogen atoms. A good example is ethylene oxide which can be chemically symbolized as:

The terms "epoxide" and "epoxy" are synonymous, and denote this particular class of bonding systems. The group may be either in the fluid or solid state.

Thermoset: A resin that either undergoes reaction with a curing agent (hardener) to produce an infusible plastic that is noncrystalline, struted as a three-dimensional network, and either flexible or rigid.

Curing Agent: (Hardener) The "B" of the A,B,C's of epoxy grouting. A viscous liquid at ordinary ambient temperatures and composed primarily of complex derivatives (phenols, inorganic acids and organic acids); used for chemically reacting with epoxy so that the mixture becomes a rigid thermosetting solid. Most hardeners are based on polyamide resins, polyamine resins or anilene formaldehyde resins. It formulated with polyamide resin, the lattice structure of the epoxy resin is appreciably improved and the cured epoxy resin exhibits improved resiliency, toughness, impact strength and abrasive resistance.

Cross-link: A three-dimensional and inseparable connection between adjacent molecular chains of a polymer; in most thermosets. An extensive degree of such chemicals connections produces a coherent mass (i.e., one infusible supermolecular of all the chains).

Epoxy Resin: A synthetic resin (a) comprising a molecular group that contains two or more epoxide groups which have been cross-linked, by reaction with a curing agent (B hardener), to produce what is commonly known as a thermoset (A + B). If the distance between cross-links are decrease, the compressive and tensile strengths of the epoxy resin are increased.

Cure: To change a polymer into a stable and usable condition by means of an irreversible exothermic reaction which promotes extensive cross-links.

Adhesives: A substance that is applied as an intermediate between two solid materials and capable of holding the solid material (adherends) together by surface attachment (sticking).

Adherend: The surface of a solid material onto which an adhesive holds fast.

Set: To convert an adhesive (i.e., a resin) into a solid state by curing.

Flexibilizer: An additive that makes a cured resin less stiff so that it can be deformed without breaking.

Plasticizer: A substance that softens (plasticizes) another substance through its solvent action.

Elastomer: A substance that resists deformation by stretching; it stretches to at least twice its original length, and after having been stretched and the tension removed, returns to nearly its original length in a very short time (i.e., natural rubber). NOTE: An epoxy resin can be formulated to be flexible; it cannot be formulated to be elastomeric.

Pot Life: The time available for working the epoxy resin (or epoxy resin grout) after the resin, filler (in the case of epoxy resin grout) and hardener have been mixed; the available time may be varied from a few seconds to several hours or days depending on various factors, but in practice this period is usually within the range of 1 to 3 hours.

Filler: A carefully graded inert mineral aggregate which is added to the epoxy before the latter is intermixed with the hardener; it not only reduces the pot life but ensures less volume of epoxy, thus reducing the cost of epoxy resin grout. In grouting, the aggregate is added after the A and B components are mixed together.

A,B,C of EPOXY GROUT

The national standard for epoxy resins and hardeners has been designated as A for the resin and B for the hardener.

Component A Resin. (Figure 1) As previously mentioned the epoxy resin A may be in the fluid or solid state. Viscosity of epoxy resins can be varied by adding reactive diluents or non-reactive diluents and is generally regarded as any material whose prime function is to reduce viscosity.

Commercial types of epoxy resin are amber colored and have the viscosity (before curing) of heavy motor oil or honey. The synthesis of epoxy was first accomplished by P. Castan in 1936 in Switzerland. Major areas for consumption of epoxies are: protective coatings, bonding and adhesives, reinforced plastics, electrical and electronic application, concrete repair and epoxy grouts.

Normally the commercial resin purchased is too visous to be a workable product and can be made less visous by the addition of diluents. Therefore, let's briefly discuss the reactive diluent and non-reactive diluent. Most reactive diluents are glycidyl ethers. The reactive diluent is mostly monofunctional providing slightly higher DT (deflection temperature) and better performance for equal viscosity. Reactive diluents do not hurt the heat deflection, chemical resistance of physical properties as much as non-reactive diluents. Non-reactive diluents are mostly aliphatic or aromatic solvents. Also asphaltic compounds such as coal tar are used to impart a degree of flexibility to the cured system. The biggest problem from adding non-reactive diluents to an epoxy system is the reduction of heat resistance or compressive strength.

Component B Hardener.The hardener is the component that makes the epoxy perform to prescribe physical properties. Hardeners or curing agents determine whether the epoxy will have heat resistant, set under water, have short or long pot life, low or high exotherm, brittle or flexible and determine coefficient of expansion. Curing agents must be carefully selected for each type of job to attain the best repair. Hardeners are usually the cause of most dermatitis or skin sensitivity. Curing agents may require special handling. Some are quite sensitive to atmospheric moisture and should be stored tightly closed; others may crystallize at lower temperatures and require reheating; other may require storage in glass or other special containers.

Component C Filler.Epoxy resin can be modified by adding an inert inorganic filler. Fillers are used for a variety of reasons, one of which is cost reduction. The more common fillers in use are silicas, talcs, chopped fiber, calcium carbonates, calcium silicates, micas and clays. Specific fillers may be used to provide specific performance characteristics. In general, the effects provided by fillers are more dependent on the amount incorporated rather than the type. One of the most important effects is a reduction of peak exotherm temperature. Equally important is the fact that fillers reduce the over-all shrinkage during cure. Since the fillers themselves do not shrink and because they occupy a large volume of the total system, they provide numerous benefits. Not to be minimized is the fact that fillers will reduce the coefficient of thermal expansion. This greatly aids thermal stability and enhances thermal shock properties where adhesion to metallic surfaces is needed. The advantage of adding fillers is reduction in shrinkage of the cured epoxy resin. Hardness is increase, cure shrinkage of a standard epoxy-room temperature curing agent is cut in half (0.002 to 0.003 in per. in.) and compressive strength increase with such fillers. Impact, flexural, and tensile strength may suffer....Most mineral fillers increase the thermal conductivity of a clear epoxy coating by about two to four-fold.

Hardness is an index of the cured epoxy resin's resistance to either temporary or permanent indentation (of a relatively small area of the resin) due to application of concentrated load. Thus, the hardness of cured epoxy resin is a good index of the resin's projected behavior in compression, shear, tension and tortion.

For a solid, the coefficient of linear thermal expansion is the ratio of the length change (in.) per degree F of temperature change to length (in.) at 32 deg. F. The value of this coefficient varies with the temperature (i.e., the rate of thermal expansion varies with the temperature). For a solid, the coefficient of volumetric thermal expansion approximates three times the coefficient of linear thermal expansion. Coefficient of expansion for testing epoxy grout is ASTM C-531 and is run at 72 deg. - 210 deg. F. The lower the coefficient of expansion of an epoxy grout, the more compatible it will be with steel and concrete. Coefficient of steel is approximately 6.5 - and concrete is 5.5 -. Although steel and concrete have similar coefficients they move at different rates because of their difference in heat absorption or insulating properties.

USE OF EPOXY GROUT

Today most engines or compressors are grouted or regrouted with epoxy grout. Compressor foundations are rebuilt using epoxy grout and cracks are injected to stop oil migration. Epoxy has been used to repair holes in oil pans caused by chipping hammers. On new installations of large equipment, sole plates or rails are pregrouted without the equipment in place or on the job site. Concrete or cementious grouts cannot hold the steel sole plate in alignment because of shrinkage and inadequate bond strength. So, epoxies have resolved that problem,

SELECTION OF AN EPOXY GROUT

From our experience, the most important factor determining selection of an epoxy grout is its heat resistance at operating temperatures (140 deg. to 180 deg. F). Some epoxy grouts will soften when exposed to temperatures in excess of 120 deg. F. One test for the above is to crush casted specimens of grout after having been conditioned at 180 deg. F for two hours. This test will provide their compressive yield at that temperature. Two hours at operating temperatures seems to be the time limit that epoxy grouts are at their lowest compressive strength and will gain strength after two hours but never attain the ambient compressive strength. The epoxy grout will continue this cycle forever. When it cools down it gets hard again and when reheated softens. A good test for your epoxy grout while in service is sticking it near the edge of the equipment with a knife while the equipment is operating. If the knife sinks into the epoxy, your grout is losing compressive strength at operating temperature. Any solvents added to an epoxy grout tend to reduce heat resistance. Solvents break up the molecular structure of the epoxy system. When grouting sole plates or rails, one must have a high heat resistant grout system. Too high of a heat deflection causes shrinkage and reduced bond strength. The heat and dynamic forces will wallow out a sole plate within a year's time. A grout has to be able to be poured at least three inches thick without having the thermal runaway or over heating. The more an epoxy grout exotherms the more shrinkage and chances for cracking. Shrinkage in a 100% epoxy grout (without solvents) volume is due to condensation of molecules, not evaporation of any additives or chemicals.

STEEL CHOCKS VERSUS EPOXY CHOCKS

There has been a great push in the industry to put engines on epoxy chocks instead of steel sole plates and chocks. Epoxy is not as strong as steel and eventually may wallow out or wear. This is not a new concept. The Europeans have been doing this for 20 years. In some cases the epoxy chock is necessary to remedy an alignment problem without regrouting or being unable to adjust a misaligned soled plate. For a permanent installation we recommend a good heat resistant epoxy grout and steel sole plate and chocks. One of the major gas pipeline companies regrouted in excess of 80 engines using 3,200 sole plates prior to 1975. As of this date, not one sole plate has had to be removed and are all holding alignment. Now that may be because of the grout representative (the author) that sold and supervised all the installations for the five year regrout program.

CONCLUSION

There is more to epoxy grout than one would imagine. Epoxy grout has done wonders for the compressor and machinery industry facilitating alignment and protecting foundations. Selection of an epoxy grout should be done very carefully. Also, there are more ways to mix the material wrong and only one way to mix it right. Always Contact your grout representative if there are any questions prior to mixing. If one waits until the job is over, it is like unrining a bell or trying to stop a sunrise....it could be too late. Also, the grout installers should know all the characteristics of the grout they are using. Don't for one minute think all grouts are alike and can be installed the same way. Each grout is different and has its own idiosyncrasies, and each grout job is different. Be sure the heat deflection temperature is known and independent compressive tests have been conducted at 180 deg. F for two hours. Check your existing grout in service using the pen knife test. If the point is able to penetrate the grout, the operating temperature has exceeded the epoxy grout's heat resistance. It is always advisable to have a knowledgeable, experienced crew or grout representative on each grout job.

Epoxy grouting is an art as well as a science. Mr. Ed Clements of Natural Gas Pipeline Company of America said, "Epoxy is a man-made product from various chemicals, and the chemicals are not what they want to be and are desperately trying to get back to their original state". Mr. Clement's statement almost says it all about epoxies.

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