Challenges with Using Ethyl Silicate Inorganic Zinc-Rich Primers Presented by: Rick A. Huntley, PCS Senior Coatings Consultant KTA-Tator, Inc.

Ethyl Silicate Zinc-rich Primer Challenges - Overview Ethyl silicate zinc primer curing mechanism Physical properties of inorganic zinc rich primers Application challenges Mixing Dry spray Mud cracking Top coating challenges Pinholing Splitting Determination of cure

Learning Outcomes Completion of this webinar will enable the participant to: Describe the basic curing mechanism of ethyl silicate zinc rich primer Describe the typical application challenges when using inorganic zinc rich primer. Identify appropriate methods to overcome the application challenges Identify 3 common methods to determine the cure of inorganic zinc rich primer

Inorganic Zinc Binder Curing Mechanisms There are two general categories of zinc rich primers: organic and inorganic. Both types of primers incorporate zinc powder as a pigment in order to provide galvanic protection to steel surfaces. Organic zinc rich primers use an organic resin, most often epoxy, as a binder to hold the zinc particles in the primer film. Inorganic zinc rich primers, including ethyl silicates, rely on a more complex chemical reaction to cure that involves zinc, oxygen, and silicon.

Inorganic Zinc Binder Curing Mechanism There are several types of inorganic zinc rich primer resins, all of them based on silicate chemistry. Inorganic zinc rich primers are typically classified as alkali silicates or alkyl silicates. Alkali silicates include post cured or high-temperature cured resins based on sodium silicate, and self-cured resins usually based on potassium or lithium silicate. Waterborne inorganic zinc is based on water soluble alkali silicates. These binders initially form a film simply by water evaporation. The film then hydrolyzes by reaction with metallic pigment and the steel substrate to form an insoluble film.

Waterborne Inorganic Zinc Challenges Waterborne inorganic zinc primers have been used successfully for years but have presented their own unique challenges. Since the alkali silicate binder is initially water-soluble, the initial film is water-soluble. In some cases, it has been found that the hydrolysis reaction that renders the binder insoluble was not 100% complete after a few weeks. As a result, exposure to condensing humidity dissolved a small amount of the resin. When the condensing humidity evaporated, the resin became concentrated in small craters and on the top of bottom flanges of I-beams. The concentration of resin in these areas encapsulated the zinc particles and prevented them from providing cathodic protection, resulting in rust spots.

Inorganic Zinc Binder Curing Mechanism Alkyl silicates, although considered inorganic binders, are at least initially partially organic. During the reaction with moisture after application, alkoxy groups on the alkyl molecule are replaced by hydroxyl groups. Some of the molecules condense together to create a silicone based polymer. During the reaction, ethyl alcohol is released, which in effect removes the organic constituent from the molecule, rendering it inorganic. This reaction takes a long time to complete, so it is likely that most ethyl silicate zinc rich primers are partially organic.

Curing Mechanism - Initial

Curing Mechanism – Cross-linking

Inorganic Zinc-Rich Primer Inorganic zinc rich primers protect the steel surface primarily by providing galvanic protection. The coating film is comprised of spherical metallic zinc particles bound together by an inorganic resin. In order for the coating to provide effective galvanic protection, the zinc particles must be tightly packed in the film so that they are in electrical contact with each other throughout most of the film. Because of the chemistry of the inorganic resin, the resin actually reacts with the zinc particles to provide a chemical bond. As a result, not as much resin is required to hold the film together as with conventional coatings.

With traditional coatings, there has to be at least enough resin to completely fill the void space between the pigment particles. The pigment must be totally encapsulated by the resin.

If not mostly encapsulated by the resin, the pigment particles would be held weakly in place and the coating would have poor physical properties and be somewhat crumbly. Additionally, water would transfer through the coating freely.

Use of Ethyl Silicate Inorganic Zinc-rich Primer Ethyl silicate zinc rich primers have been widely used to protect steel surfaces for decades. Because the high zinc loading of ethyl silicate zinc rich primers leads to excellent galvanic protective properties and the chemistry of the silicate binder creates a hard tough film, it is considered one of the most effective primers in most exterior exposures. The composition and chemistry of the primer creates various conditions that make successful application somewhat challenging. As a result, ethyl silicate zinc rich primers are often applied to new steel in a shop environment where application conditions can be more closely controlled.

Application Challenges Many of the challenges with applying ethyl silicate zinc rich primer are the result of either the extremely high pigment to resin ratio in the film or the need for atmospheric water during the curing process. Some of the more common application problems include: Pinhole formation in the topcoat due to outgassing Dry spray Mud cracking of the applied film Primer cohesive failure after top coating Mixing anomalies

Mixing The active pigment in zinc rich primer is a zinc dust, which is very dense. For comparison, the density of zinc is compared to other common pigments: Talc 2.75 gm/cc Titanium Dioxide 4.23 gm/cc Zinc 7.13 gm/cc Because zinc is so much denser than typical pigments it can settle out from the wet coating quickly. Most manufacturers of ethyl silicate zinc rich primer recommend that the coating be agitated continuously during the application process to prevent settling

Mud Cracking Mud cracking of the applied ethyl silicate zinc rich primer is a common problem when the primer is applied above the recommended dry film thickness. It is not uncommon for inorganic zinc rich primers to have a maximum recommended dry film thickness of around 3 mils. With some primers application above 5 or 6 mils leads to the formation of a series of cracks in the surface of the primer.

Mud Cracking Mud cracking occurs shortly after application of the coating. The cracking is primarily caused by a rapid drying of the coating before the binder has had a chance to cure. The problem is somewhat worse in warmer temperatures since drying of the coating is accelerated. The cracking problem is more pronounced with ethyl silicate zinc rich primers because of the extremely high pigment to binder ratio. During the initial drying of the film, some shrinkage occurs. The small amount binder has not sufficiently cured to hold the pigment particles together, and cracking occurs to relieve the shrinkage stress

Mud Cracking

Ethyl Silicate Zinc-rich Primer Mud Cracking

Mud Cracking Prevention The prevention of mud cracking is primarily in the application. Care should be taken not to apply the ethyl silicate zinc rich primer at thicknesses significantly above the recommended maximum thickness. The chances of mud cracking can be reduced during the formulation phase of the coating. Variation in the evaporation rate of the solvents and the pigment content and size can affect the propensity for the coating to mud crack. Addition of fibers to the coating can also lessen the chance of mud cracking.

Mud Cracking Repair Once mud cracking has occurred it is difficult to repair without completely removing the primer from affected areas. Often the affected areas are on inside corners where spray passes of the coating have overlapped creating greater thickness. The cracking extends from the top surface down to the substrate and cannot be removed effectively by sanding.

Dry Spray Dry spray is a condition where the surface of an applied coating has a rough grainy appearance. Almost all liquid applied coatings can be applied in a manner that could lead to dry spray but some coatings are more prone to the condition. Dry spray is created when the small globules of coating that travel from a spray gun to the painted surface do not flow into the body of the coating but instead loosely attach to the surface.

Dry Spray Causes Dry spray is typically caused by: The spray gun being held too far from the surface. With conventional spray, the atomization air at too high of a pressure or fluid pressure too low. Too much air movement caused by wind or too high of an air velocity in a spray booth. All of theses conditions result in rapid evaporation of the solvent from the spray particles between the time they leave the spray gun and the time they impact the surface, leaving insufficient solvent to allow the particles to flow into the coating.

Dry Spray Ethyl silicate zinc rich primers are extremely prone to dry spray. When solvent evaporates too quickly from the film the low resin-to- pigment ratio causes the globules of coating to become too viscous to flow into the film. The causes are the same as any coating with solvent in the formulation but are intensified by the composition. Application on exterior surfaces can be challenging when wind is present because dry particles of the primer deposits dry spray on adjacent surfaces.

Dry Spray Avoidance Dry spray can be prevented when applying ethyl silicate zinc-rich primers by avoiding the application parameters that lead to the condition. The spray gun should be held perpendicular to the surface that is being coated and at a distance no further than recommended. Good spraying technique is critical. Avoid spraying inorganic zinc-rich primer when there is wind or high air movement past the surface to be coated. If dry spray occurs, it can often be removed to some degree by rubbing a screen over the surface to remove the loose particles, or by sanding.

Top Coating Problems Ethyl silicate zinc rich primer is capable of protecting a steel surface for years with a single coat. Regardless, for various reasons, it is usually top coated. Top coating of ethyl silicate zinc rich primer has its own challenges. These top coating challenges are the result of the porous nature of the film and the need for moisture to cure the film. Common top coating problems include: Pinholing (outgassing) Cohesive splitting of the film

Pinhole formation Pinholing is the formation numerous small holes in a coating. Pinholing can occur for several reasons, but the primary reason is the escape of air or solvent vapor out of a coating film before the coating has dried. Pinhole formation often occurs when a liquid applied coating is applied over a porous substrate. Concrete is well known for causing pinholing due to the many voids in the substrate. During application, the coating bridges over small holes in the substrate or underlying coating layer. Sometime shortly after application, air or solvent vapor that is trapped in the void space on the surface expands blowing small holes through the coating.

Zinc-rich Primer Topcoat Pinholing

Pinholing in Coatings over Zinc Rich Primers The dried ethyl silicate zinc-rich primer film is porous because there is not sufficient binder to completely fill the void space between the particles of zinc metal. The coating applied directly over the primer does not typically penetrate into the zinc rich primer enough to fill in the void space in the primer film. Instead, the void space is filled with air and solvent vapor. Just like on any porous surface, the entrapped air can escape through the overlying coat while it is still wet, creating pinholes. The holes that are created in the coating, often an intermediate coat, allow free penetration of moisture and oxygen though the coat and into the primer.

Pinhole Prevention and Repair Once pinholes develop in a coating, they can be difficult to repair. Simply applying an additional coat often results in the redevelopment of the pinholes in the same location. Air is still trapped in the ethyl silicate zinc rich primer and can now escape through the pinholes in the first intermediate coat through the subsequently applied coats. Fortunately, the pinholing problem occurs very shortly after application of the first coat over the zinc rich primer. Once it is determined that pinholing is occuring, steps can be taken to avoid further pinholing. Pinholes can be repaired by forcing a thinned coating into the pinholes, usually with a brush, then applying an additional coat.

Pinhole Prevention Pinholes are caused by the porosity the zinc rich primer. To prevent the pinholes, the porosity has to be sealed. One effective method is to apply a mist coat. A mist coat is a layer of the 2nd coat applied so thinly that it is semi-transparent. Optimally the mist coat is thinned with the maximum amount of solvent allowed. The thin mist coat (semi-transparent) seals some of the porosity in the inorganic zinc rich coating. It is allowed to dry to a “tacky condition” then the full 2nd coat is applied.

Zinc Splitting Zinc splitting is a cohesive failure within the zinc rich primer layer that usually occurs well after the intermediate and topcoat have been applied. When the failure occurs, generally large chips of coating separate from the surface. The delaminated chips have a continuous layer of zinc rich primer on the back and there is also a continuous layer of primer left on the steel substrate.

Zinc Splitting The cohesive break in the primer occurs when the primer is over coated before it has sufficiently cured. Generally, the failure does not occur immediately. As the intermediate and/or topcoat dry and cure, they developed shrinkage and contractive curing stresses. Once the stresses become greater than the cohesive strength of the zinc rich primer, the primer splits and the coating system delaminates. The problem occurs most often when the entire coating system is applied in a shop, especially during winter months in colder environments, when the relative humidity is low.

Zinc Splitting In order for an ethyl silicate zinc rich primer to adequately cure, it needs 3 things. Moisture Adequate temperature Time When steel is primed in a shop and top coated in the field, there is generally no problem. When an ethyl silicate zinc rich primer and the appropriate topcoat is applied in a shop environment, the temperature is generally not a problem, but time and moisture may be in short supply.

Zinc Splitting - Time Time it is often an issue when coatings are applied in a fabrication shop. When applying 3 coat systems, the primer must be applied and allowed to cure, followed by the intermediate coat and the topcoat. This process may take considerable time. During winter months, outside storage may not be possible and indoor space may be limited. There is pressure to apply the coating system as quickly as possible so that production is not delayed.

Zinc Splitting - Moisture Ethyl silicate zinc rich primer requires moisture to cure. The relative humidity must be high enough to supply adequate water for the reaction. If the relative humidity is too low, the reaction proceeds slowly. Most ethyl silicate zinc rich primers cure very slowly if the relative humidity is below 50%. In winter months, the exterior air in northern climates can be extremely cold. When that air is heated in the shop, the resultant relative humidity of the air can be extremely low. To compensate for the low humidity, a mist of water can be continuously applied to the steel a couple of hours after the primer has been applied.

Zinc Splitting - Cure To avoid zinc splitting, care must be taken to assure that the primer has adequately cured before application of the 2nd coat. Three methods to determine the cure are: Solvent rub Pencil hardness Coin Rub

Solvent Rub ASTM D 4752 – Measuring Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub Ethyl silicate zinc rich primers develop excellent resistance to methyl ethyl ketone (MEK) when they cure. The surfaces are first cleaned with tap water or a dry cloth to remove loose material. A cloth saturated with MEK is rubbed over the surface until the metal substrate is exposed or until 50 double rubs have been completed. No residue or only a trace of residue should be transferred to the cloth from a well-cured ethyl silicate zinc rich primer.

Pencil Hardness ASTM D3363 – Standard Test Method for Film Hardness by Pencil Test The silicate resin develops considerable hardness when it is substantially cured. Pencils of varying hardness are held against the film had a 45° angle and pushed away from the operator. The gouge hardness is the hardest pencil that will not gouge the primer. Consult the coating manufacturer for the minimum acceptable hardness. The hardness scale: 6B-5B-4B-3B-2B-B-HB-F-H-2H-3H-4H-5H-6H Softer Harder

Coin Rub The coin rub can be used on some but not all the zinc rich primers. A edge of a nickel (since it is non-serrated) is rubbed repeatedly across the surface of the zinc rich primer. If the coin causes the zinc rich primer to crumble or powder, or the primer is otherwise removed by the edge of the coin, the primer is considered to be in sufficiently cured. A sufficiently cured ethyl silicate zinc rich primer should burnish when rubbed with the edge of the coin creating a somewhat shiny surface. The manufacturer of the primer should be consulted to determine whether the coin rub test is an indicator of cure.

Summary Described the basic curing mechanism ethyl silicate zinc rich primers Described the typical application challenges when using inorganic zinc rich primers Identified appropriate methods to overcome the application challenges Identified 3 common methods to determine the cure of inorganic zinc rich primers

Challenges with Using Ethyl Silicate Inorganic Zinc-Rich Primers Questions