Synthetic Resin Mortars – Part 2

Synthetic Resin Mortar Inputs

Fillers: Inorganic fillers are one of the important factors that substantially assist in determining the strength properties of synthetic resin concrete. Consequently, the high strength of synthetic resin concrete can only be achieved with strong fillers. The effect of particle size composition on the strength properties of synthetic resin concrete is governed by the same rules as inorganic mortars. For this reason, a particle size distribution approaching the Fuller curve can be recommended. The function of medium and fine particles is to fill larger and smaller voids. The following well-known particle fractions provide the desired particle size distribution suitable for epoxy resin mortars. When particle size composition differs from the recommended one, particularly when it contains no quartz powder, the following two results may arise. a) With the same binder: There is insufficient resin to fill all voids with the filling rate. A porous material with air gaps is created. b) When a larger pore volume is filled with more binder, that is, when the sand-to-resin ratio is changed, strength properties are compromised, and impact resistance is lower. Even higher compressive strength can be achieved with other high-quality fillers such as silicon carbide and corundum. The natural strength of ordinary aggregates customary for synthetic resin concrete is less than the concreting force of the binder. In comparative compression tests performed between synthetic resin concrete and cement concrete when fractured, the fracture in normal cement concrete always occurs in the binder, whereas in synthetic resin concrete the sand particles fracture. For this reason, it is recommended that only high-quality fillers be used in synthetic resin concrete. By using graphite combined with quartz or silicon carbide fillers, in accordance with the guidelines of the German chemical industry, electrically conductive mortars can be formulated with a leakage current resistance lower than 106 ohms. The addition of graphite to the system does not impair the good mechanical strength of heavily filled mortars. In some cases, the dark color caused by graphite may be unsightly, but this can be mitigated by adding titanium dioxide. Epoxy resin mortars can also be modified with liquid thinners such as tar, coumarone-indene resins, and other plasticizers. Sometimes these additives provide advantages in solvent-based and solvent-free paints, but they can provide disadvantages such as low reactivity and low strength in highly filled coatings. When given at a small amount such as 20% by weight of the binder with coumarone resin, curing is greatly delayed and mechanical strength is approximately 40% lower than in mortar without the additive.

Pigments:

The coloration of sand-filled putties varies depending on the color of the sand. Normally, it is sufficient to add 1 to 5% pigment to light-colored quartz sands in total to the mixture. Inorganic pigments are best. Pigments must be quite compatible with synthetic resin concrete and must also show resistance to weathering and aromatic solvents. Special decorative effects can be obtained by mixing crushed marble, colored sand, and pigments. All fillers must be dry; only sands free of organic compounds should be used.

Binders:

Binder percentage is very important for the application properties of a synthetic resin mortar. Mortars with 20% or more binder and a certain amount of fine filler (0.06 mm) are self-leveling mortars. To achieve this condition with minimum binder, approximately 20% of the fillers should contain fine filler. The remaining 80% of filler material should comply with the Fuller curve. Due to the high binder content and the presence of coarse particles up to 2 mm, the mixture may settle at a certain point, forming a coating with a non-homogeneous structure. For this reason, the impact resistance of self-leveling synthetic resin mortars cannot be expected to be at the same standard as concrete. Certain improvement in impact resistance may be possible by increasing the amount of fine filler and excluding coarse particles to reduce settling. Of course, more binder is needed accordingly to wet a larger filler surface. In some cases, when greater impact resistance is required, this can be achieved by adding glass fiber. At a rate as small as 0.5%, glass fiber significantly increases impact resistance. Slightly more binder should be used to provide application properties and adequately wet the glass fibers. The additional felt caused by the fibers allows the concrete to be applied to vertical surfaces. Self-leveling patches containing very coarse particles such as 7-10 mm can be achieved with a low binder rate when fine fillers are added in addition to fillers. The thickness of a coating should not be less than three times the largest particle size, as these mortars cannot be used for every purpose. Actual coating can be used to make fairly deep holes before application. Application of synthetic resin mortars with low binder content is more labor-intensive, but its properties develop even better. Mortar with one part binder and six parts filler composition should be applied with a trowel. Such mortars are quite resistant to wear and have very high compressive strength.

Reactivity:

To characterize reaction rate, representative values for the pot life of 2 kg mortar mixture containing a 14.3% binder and hardener mixture are provided. Unmodified epoxy resin with epoxy equivalent weight of 182-192 and viscosity of 9-13 Pa.s was used as the epoxy resin component. When modified epoxy resin with epoxy equivalent weight of 182-192 is used, pot lives are approximately 20% longer.

Filler Pastes:

Similar filler pastes can be formulated according to differences in viscosity of various liquid epoxy resins and hardeners. The hardener types mentioned above are recommended as possible hardener components for all liquid epoxy resins. Their advantages are excellent adhesion to a wide variety of substrates (for example, concrete, metal, wood, plastic), good flexibility, and very low shrinkage. Inorganic fillers can constitute 30-70% of the mixture. Depending on the purpose, mineral or metallic fillers are used. Mixtures of siliceous and non-siliceous fillers are very suitable. Hardened filler pastes wear easily and can be painted with all standard paints. Inorganic fillers for filler pastes: When modified epoxy resins are used in filler pastes, the epoxy resin component and filler may react in long-term storage. The mixture may be either too soft or too hard and may emit an unpleasant odor. Most of these reactions are prevented by using unmodified epoxy resins. In case of doubt, an accelerated test can provide information on whether the use of inorganic filler is appropriate.

Test Method for Fillers:

Mix the epoxy resin to be tested with the necessary filler until it reaches a putty consistency. Place approximately 25-30 ml of this mixture into a covered container of approximately 50 ml. Keep this closed container in an oven at 140°C for four hours. After cooling, any changes that may occur are easily detected when compared with the unheated sample. This test method shows the following effects: • The consistency of the filler paste remains the same when various barium sulfate types and blanc fixe, Lithopone, calcium carbonate and other calcite types, Microdol, zinc oxide, quartz powder, aluminum and iron powder (reduced iron) are used. • The filler paste noticeably hardens when talc and slate powder are used. • The filler paste becomes solid when kaolin, graphite, Neuburg siliceous chalk are used.

Surface Preparation:

For the coating to fully meet its purpose of protecting the substrate against damage, the two surfaces must adhere as tightly as possible. Perfect surface preparation is a necessary condition for good adhesion of synthetic resin concrete. A poorly prepared surface can jeopardize good performance from the outset, regardless of the binder system. Metal surfaces must be cleaned with appropriate mechanical methods. Steel surfaces are best cleaned of rust and similar residues by sandblasting. Where sandblasting is not possible, at least manual rust removal must be performed. The substrate must be freed of grease, oil, and other foreign matter to ensure proper wetting as required by the base layer. Any coated concrete can also be prepared. Sometimes a coating residue prepared by sprinkling cement has only very little strength. Another situation is that the binder primer only penetrates this sealed surface with great difficulty, and therefore such coatings must be removed before application. The same applies to very weak concrete described as "thirsty for water." Flame cleaning and sandblasting are suitable preparation methods. Old concrete should be checked for mechanical strength. Contaminated patches (oil, grease, chemicals, etc.) are most reliably cleaned by burning with a flame. Cleaning with solvent alone is not sufficient. It is recommended that the concrete be sealed with a sealant after cleaning. The purpose is to seal the pores and provide a better surface for the next coat. Thus, the internal strength of the crumbling concrete in the impregnated area is also increased. A sealant is particularly recommended when concrete is very dense and smooth or when water pressure from behind is expected (groundwater). Both liquid and solid resin systems can be used as suitable sealants, but liquid resin is preferred. These spread better on the substrate and, as they contain more solids, strengthen the adhesion area more. Liquid resin systems to be used for this purpose must be used with moisture-insensitive hardeners to prevent hydration that would damage the adhesion of the next coat.

Primer (Binder Agent):

If a surface is to be coated with a high-filler synthetic mortar (e.g., 10% binder, 90% fillers), a primer is definitely required, because the mortar does not contain enough binder to completely wet the surface. In practice, a primer normally consisting of the binder components of the mortar is applied to the surface before being coated with the actual synthetic resin mortar. The primer has two purposes: 1. It serves as a binding agent between the substrate and the applied synthetic resin concrete. 2. It greatly improves the coating work by creating a tacky surface that facilitates the spreading of the mortar with a trowel. It is recommended that the primer be applied immediately before coating with synthetic resin concrete so that the primed area is exposed to atmospheric moisture for only a short time. This point must be strictly observed when the primer contains moisture-sensitive hardeners. Since the mortar spreading process is continuous and this work is always interrupted by the primer, in practice it would be appropriate to pre-prime the entire area expected to be coated for a considerable period (likely an entire working day). For this reason, in the worst case, the primer will be exposed to atmospheric moisture for eight hours. To prevent hydration of the synthetic resin mortar and poor adhesion, absolutely moisture-insensitive hardeners must be used. To ensure the desired bond between primer and mortar, the final coat must be applied before the primer reaches gel state.

Thixotropic Primers:

On rough surfaces, large amounts of primer in surface depressions risk the primer rising toward the coating surface, causing uneven spreading, oily appearance, or adhesion problems. These disadvantages can be prevented by adding approximately 3% Aerosil (fumed silica) to the primer. To prevent the mortar from sliding on vertical surfaces when applying one coat of synthetic resin mortar, a thixotropic primer should be used. For this purpose, the following primer composition provides easy application:

Bonding Agent Between Old and Fresh Concrete:

Fresh cement concrete adheres very weakly to old concrete. This type of adhesion problem can be solved with an epoxy resin-based bonding agent. In practice, a self-leveling synthetic resin mortar based on modified aminopolyimidazoline type hardener / epoxy resin with amine value 230-260 and AHEW 186, or aminopolyimidazoline type hardener / epoxy resin with amine value 370-410 and AHEW 93 is applied. Bonding agents containing modified aminopolyimidazoline type hardener with amine value 230-260 and AHEW 186 are suitable for all types of fresh concrete (wet, plastic, pumpable concrete). Bonding agents containing aminopolyimidazoline type hardener with amine value 370-410 and AHEW 93 should only be used on naturally moist or consolidated concrete (maximum water/cement ratio = 0.5). Cement concrete is poured over fresh synthetic resin mortar before it gels. After hardening, the bond between the substrate and synthetic resin mortar, and between the latter and the new concrete layer is perfect. When the bond is tested, fracture occurs in the concrete body rather than in the joints. The bonding agent must contain at least 75% filler. Fillers also containing coarse particles prevent the adhesive from being pushed away where cement concrete is poured.   M. Namık Kayaalp Chemical Engineer Ecelak Boya Kimya Ltd. Co.      

References: 1. Schering Industrie-Chemikalien. 2. H.Lee, K.Neville handbook of Epoxy Resins. 3. Huntsman Advanced Materials (Switzerland) GmbHKlybeckstrasse 200 P.O. Box 4002 Basel Switzerland. 4. Epoxy Polymers, Edited by Jean-Pierre Pascault and Roberto J. J. Williams, WILEY-VLH Verlag umbH. 5. Paint and Coating Testing Manual Fifteenth Edition of the Gardner-Sward Handbook, Joseph V. Koleske.