The stone industry has been using adhesives for many years for a large variety of applications: building construction, landscaping elements, and monuments and headstones, fabrication of marble and granite countertops, and myriad restoration and repair tasks for the aforementioned. This article will investigate the various adhesive chemistries used, and comment on each one's strengths and weaknesses.
A large number of materials are used for these types of jobs, but the vast majority of work utilizes only two common polymers: epoxies and polyesters. There are two other adhesive systems worthy of note that are also sometimes used: acrylics, and hybrids (sometimes called "poly-poxys").
Let's compare the relative properties of these systems.
Each of the product types excels in certain areas, and has performance deficiencies in others:
- Epoxies generally excel where high strength and adhesion are needed, especially where long term retention of these properties is mandatory.
- Polyesters' forte is generally where a fast curing, less expensive system, with lower performance requirements, is demanded.
- Acrylics and hybrids can both be considered "souped-up" versions of polyesters. Acrylics also excel in light stability outdoors (they will not chalk and yellow like the other polymers).
Now, let's discuss the various properties individually:
Cure Speed & Pot Life: Polyesters, acrylics, and hybrids have a very fast cure speed (30-60 minutes), resulting in a very short pot life (5-10 minutes). Epoxies, on the other hand, typically have pot lives on the order of 15-30 minutes, with a cure time of an hour or more.
Tensile, Flexural, and Compressive Strengths: These important physical properties are excellent for all polymers. The only potential problem is that the continued post-cure and shrinkage of the polyesters, acrylics, and hybrids usually result in eventual brittleness and potential joint failure, especially if used outdoors.
Adhesion and Shrinkage: One of the primary reasons epoxies have such outstanding adhesion compared to the other polymers is the relatively minor amount of shrinkage that occurs upon curing. Epoxies typically only shrink 1% or less, whereas the other systems typically shrink about 5-15%. This high shrinkage results in the polymer actually "pulling away" from the substrate as cure progresses, causing high stresses on the bond line, and can result in failure. The outstanding adhesion of epoxies can also be attributed to something called the "polarity" of the epoxy molecule, which, in very simple terms, is a measure of its affinity for other materials. Polyesters are relatively non-polar, but the acrylics and hybrids are slightly more polar- this helps explain their improved adhesion compared to polyesters.
Odor, Solvents, and Flammability: While epoxies contain no solvents, greatly simplifying shipping and storage issues, almost all polyesters and hybrids contain styrene, a volatile and flammable co-reactant. Styrene is needed for three reasons: to reduce viscosity, reduce cost, and to facilitate in the cross-linking reaction (without a compound such as styrene, cure would be intolerably slow). Styrene does have its drawbacks, however: it is odorous, flammable, and can evaporate if the can of polyester is left open. Acrylics have a similar problem--low molecular weight acrylic or methacrylic monomers are usually necessary in these formulations to perform the same functions as styrene in the polyesters.
Monomers are sometimes flammable, sometimes health hazards, and always odoriferous. Epoxies, being virtually solvent-free, have little odor and are free from fire dangers. Shipping costs are also affected by the flammability issue: transportation firms restrict the shipment of flammable materials, frequently charging more for their transport and/or specifying more expensive packaging. (Note: the catalyst used with polyesters, hybrids, and acrylics is classified as an "oxidizing agent" and as such is also regulated for shipping purposes. So, with polyesters, acrylics, and hybrids there is a double shipping "whammy.")
Weatherability and Outdoor Use: All polymer types are prone to a phenomenon termed "chalking." This is a slight surface degradation (evidenced by the formation of a powdery or hazy residue resembling chalk) produced by the combined effects of sunlight, wind, and rain. This is strictly an exposed surface phenomenon, and really does not affect the strength of the polymer nor the adhesive bond.
Another common effect is yellowing of the polymer, again caused by sunshine, and again does not, of itself, cause problems. Both yellowing and chalking, however, point out the fact that the material has been exposed to weathering effects.
Acrylics are by far the best polymer for resistance to chalking and yellowing--they are excellent in this regard. With an epoxy, chalking and yellowing effects do occur, but this is primarily an aesthetic. With a polyester glue and the hybrids, however, prolonged exposure to the forces of nature cause further crosslinking (curing) of the polymer, resulting in additional shrinkage and possibly leading to embrittlement. In addition, moisture will slowly hydrolyze the ester linkages in polyesters and hybrids, destroying chemical bonds and ultimately the mechanical strength of the polymer. For these reasons, the use of polyesters and hybrids is best restricted to interior applications only.
Shelf Life: Shelf life is defined as the length of time that a properly stored container of unused material can conveniently remain on the shelf and still retain usable properties. Epoxies can have several years' shelf life; not much happens to either the epoxy or hardener components if kept in tightly closed containers at moderate temperatures.
Polyesters, acrylics, and hybrids, on the other hand, contain highly reactive styrene or acrylic monomers, along with various promoters. Shelf life of these resins is usually limited to less than one year; the material slowly begins to self-polymerize and eventually gels to an unusable material. The gelling is greatly accelerated by storage at elevated temperatures.
Critical Mix Ratio, Speed Adjustment, and Critical Mixing: Epoxies must be correctly combined with the required amount of hardener, and the mixture must be thoroughly homogenized. These two "musts" are probably responsible for the majority of problems encountered with the use of epoxies. The epoxy and hardener are expressly designed to work with each other, and the right amount of each has to be used. Too much of either will be detrimental to proper cure and property development. It should be obvious that adding additional hardener in an attempt to speed up the cure (something frequently done with the other systems) is definitely not recommended.
Polyesters, acrylics, and hybrids, on the other hand, are much more "forgiving" in this respect. For example, if a 50:1 mix ratio of resin to catalyst is called for, a little more or less catalyst will not drastically affect final properties- primarily just the cure speed and pot life will be affected. But a lot more catalyst will drastically reduce the final strength of the cured polymer due to excessive shrinkage--so be careful.
Furthermore, even if the catalyst is not thoroughly mixed into the resin, a reasonable degree of cure and property development will still take place. These differences in "user friendliness" are due to the inherent nature of the respective polymer structures and their cure mechanisms. It is a complicated subject.
Bond to Damp Substrates: Epoxies can be specially formulated for adhesion to substrates that are not entirely free of moisture; the other systems usually give poor bond strengths to such substrates. Furthermore, as noted above, the other resins can be hydrolyzed; even though a strong initial bond may be obtained, constant exposure to water will weaken the bond. (Note: some epoxies are the so-called "1:1 clear laminating" systems--be very careful with these, as the diluents required to achieve the 1:1 mix ratio usually impart greatly increased water and heat sensitivity to the adhesive. If your assembly will be exposed to water, either indoors or outdoors, or high temperatures [on the truck bed for a few hours in Phoenix in August, for example], another epoxy system would be a better choice).
Epoxies and polyesters are both versatile polymers that find utility in numerous applications in the stone fabrication, construction and restoration industry. Each has its own relative merits and shortcomings, and each repair or construction job would best be suited by one or the other technologies. Now that you know the relative performance of each, the choice should be easy, but if not, the following general guidelines should help.
Outstanding adhesion is essential; outdoor exposure is probable; long term retention of strength and adhesion is required; damp substrates are likely; solvents/odors/flammability cannot be tolerated; a long product shelf life is necessary. In short, anywhere load-bearing forces are present, especially outdoors.
Typical applications: anchor bolts, stone laminating, countertop rodding, slab reinforcing (gluing mesh on the back of stone).
A low cost, quick curing material is desired; outdoor exposure is not likely; and long term durability is not of paramount importance.
Typical applications include marble laminating (but not granite!), countertop seaming.
Resistance to yellowing and chalking is required; a quick curing material is desired, but something stronger than polyester is desired. Typical applications: mostly used in fabrication of engineered stone; quick-curing anchor bolt adhesives.
Pretty much the same uses as a polyester, except its more expensive! Typical applications: same uses as polyester, but where a slightly-stronger bond is desired.
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