Bulletin #17

Revised February 5, 2004

Multi-Purpose Mortars

Since 1980, when the first spray-dried polymer modified thin-set mortar was introduced to the ceramic tile industry, we have seen a variety of such materials become available from every major manufacturer of setting and grouting materials. There are many factors that have influenced the formulation and marketing of these materials such as changing building construction, changes in the tiles in the marketplace and marketing dynamics. This bulletin examines the evolution of these polymer modified setting materials and hopes to impart more knowledge of their composition, capabilities and limitations.

First we need to get our terminology correct. Most people in the industry are familiar with the term latex. Liquid latex additives have been used in our industry for years. In fact, the ANSI specification for these materials has the word latex in it; ANSI A118.4 Latex Portland Cement Mortars. The definition of “latex” is: “a water emulsion of a synthetic rubber or plastic obtained by polymerization”. So, what is polymerization? Polymerization occurs when monomers link together to form a repeating molecular chain. The term polymer is inclusive of every type of latex or spray-dried polymer available. As was stated, latex consists of polymer suspended in water. The manufacturing of polymers first results in the production of some type of latex. Spray-dried polymers are produced when the latex which resulted from the initial manufacturing process is sprayed through a dryer that removes the water. During this process, the polymer is dried with a small amount of material which can be dissolved in water. This gives the polymer the ability to go back into an emulsion when the finished product is mixed with water in the field. The usage of spray-dried polymers in setting and grouting materials in the ceramic tile industry spawned the terms “polymer modified” or “polymer fortified” commonly used in the marketing of these materials.

The first spray-dried polymer available was a PVA (polyvinyl acetate) homopolymer. You have been familiar with this polymer for years…Elmer’s Glue. The problem with straight PVA polymer is its sensitivity to water and instability in the highly alkaline environment of hydrating cement. By itself, even after the PVA polymer has coalesced (dried forming a continuous film), exposure to water or sufficient moisture can re-emulsify the polymer resulting in loss of bond. Also, in the very alkaline, high pH environment of hydrating cement, the PVA polymer experiences hydrolysis which breaks down the polymer into the original monomers. Hardly any manufacturers use straight PVA polymers in their products today. The second spray-dried polymer available was actually a copolymer (2). It consisted of PVA (polyvinyl acetate) with the addition of an ethylene monomer. This produced a copolymer that had two very good attributes. One, the PVA portion of the copolymer had very strong bonding characteristics. These bonding characteristics include an affinity to bond to cellulose (wood). Second, the addition of ethylene into the copolymer made it much more stable under the highly alkaline environment of hydrating cement. This product has become known in the industry as an EVA (Ethylene Vinyl Acetate) polymer. Since then, chemists were able to modify the performance characteristics of these polymers even more by adding additional monomers to the chain. Monomers such as vinyl versatate and vinyl laurate have been added to substantially increased the polymer’s resistance to water. All of these variations generate a wide assortment of polymers from which the setting materials manufacturers may choose. A polymer with more ethylene will impart more flexibility to the finished product than one with less ethylene if the polymers are blended with the same cement/polymer ratio. The manufacturer must determine what the desired performance characteristics of a product should be and then chose the proper polymer and cement/polymer ratio to put in the finished product to achieve their goals.

Acrylic spray-dried polymers were being produced at about the same time the EVA’s became the standard, but they could not achieve the same performance characteristics of an EVA in setting materials. As of the writing of this bulletin, however, there are whole new breeds of polymers hitting the market with widely varying performance characteristics; based not only vinyl acetate and acrylic, but other chemistries as well. These new polymers have made tremendous strides in technology and most manufacturers are experimenting and formulating with them for future products. Setting materials formulated with EVA copolymers became known as “multi-purpose mortars” because they were the first cement-based setting materials that could bond to plywood as well as concrete or cementitious substrates. In certain parts of the country, they are still used for bonding ceramic tile directly to exterior glue plywood.

The first multi-purpose mortars introduced to the industry were, quite frankly, overbuilt. Since these mortars were the first cementitious materials to be used for bonding directly to exterior glue plywood, the manufacturers formulated the products with more polymer and water retention agents in them than were actually needed. They did this to avoid having problems with these new mortars in the field. As the multi-purpose mortars gained acceptance by the contractors, competition between manufacturers forced them to remove some of the polymer and water retention agents from the original formulas and produce less expensive versions of multi-purpose mortars. This gave the contractors the option of going with the original version or a less expensive second version of multi-purpose mortar. Some manufacturers even had a third version of the polymer modified setting material designed for bonding to concrete or cementitious substrates. These products had sufficient polymer in them to function as a good latex Portland cement mortar meeting ANSI A118.4 but not enough polymer to be promoted as a multi-purpose mortar for bonding to plywood. This prompted the MMSA to develop the ANSI A118.11 EGP (Exterior Glue Plywood) Latex Portland Cement Mortar specification. This specification allows the contractor to determine if the product is a true multi-purpose mortar or not. If the product does not say it meets ANSI A118.11, do not use it to bond tile to exterior glue plywood.

For years, the manufacturers endeavored to produce multi-purpose products with the highest shear bond and strengths possible while balancing this with the economic conditions of competition. However, construction practices were changing. Post and beam construction on large commercial projects and the introduction of engineered wood joist and web joist in small commercial and residential construction became the norm. These systems introduced significantly more deflection in substrates when live loads were applied to the finished installations. With normal setting materials, pressures resulting from this increased deflection usually resulted in tenting of the tile in the middle of the floor. Manufacturers realized that they needed products that would impart more flexibility in the bonding characteristics than the strong but rigid materials they had produced in the past. Contractors were also requesting multi-purpose mortars that were capable of bonding to existing vinyl floors as well as floors that had cutback adhesive residue on the substrate. Bonding to vinyl and cutback adhesive is not difficult with a polymer modified product. However, there is a saying in the ceramic tile industry; “Your installation is only as strong as what you are bonding to”. We have always known when pressures from thermal and moisture expansion or contraction occur or substrate movement from settling occurs in sufficient amounts, the installation will fail at its weakest link. Cutback adhesive, as well as vinyl adhesives, only have shear bond strengths of 40 to 50 psi, the weakest link is not the bond of the setting material to the vinyl or cutback. The weakest link is usually within the matrix of the cutback or vinyl adhesive. Setting materials were needed that had enough flexibility to alleviate these pressures and prevent them from overcoming the relatively week shear bond strengths of the vinyl or cutback adhesives.

As if by design, the suppliers of spray-dried polymers were coming out with polymers that were softer and more flexible after coalescing than the previous materials. Not long afterward, “flex-mortars” were making their way into the marketplace. With these new polymers, products could be formulated that still had good bond strengths yet, were more flexible allowing the tile to move very small amounts in relation to the substrate. This minor movement capability would help to relieve the pressures introduced in tile installations. With the new flex-mortars and proper installation procedures, tenting and bond loss could be avoided.

As each manufacturer came out with and promoted their versions of “flex-mortars”, the term “flex” became a buzz word in the ceramic tile industry from the tile contractor all the way up to the architect. The term “flex” became predominant in the packaging and marketing materials of these new products. As with any word or phrase that becomes a buzz word, eventually the term will be misused. There are now products in the ceramic tile industry with the term “flex” in the name that have absolutely no polymer in the product. This has resulted in a great deal of confusion in the market, especially since there are no ANSI standards which govern flex-mortars at this time. So…How is a contractor or architect supposed to know if a product is truly a flex-mortar? Simple; acquire a data sheet for the product in question. Look in the “technical or performance characteristics” section for a test which indicates that this product is a true “flex” mortar. There are at least three tests which would indicate this. 1) Elastic Modulus ANSI A118.4 – This is an old test that has been deleted from the current ANSI A118.4 test methods. 2) Modulus of Elasticity In Bending. 3) Elongation Characteristics. If at least one of these tests is not in the technical section, chances are the product is not a true flex-mortar. If the test are there, compare the test results (values) to a product you know to be a real flex-mortar. The values should be comparable or greater. If these tests are not published and the product is being promoted to be a real flex-mortar, call the manufacturer’s technical department and ask for the test results. When a flex-mortar is needed, make sure you know what you are getting.