Adhesive Coating Defects Causes Remedies


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Here is our First technical Bulletin
This is an attempt to address the General Coating Defects, the causes and the Remedies.
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Adhesive Coating Defects Causes Remedies

  1. 1. No : PCP 001 Date : 02 -01-2014 Adhesive Coating Defects Introduction During film formation Adhesive should transform a liquid into a closed, solid film called coating. In order to give optimum protective and aesthetical properties the coatings should be smooth and even without any defects. Adhesive Film defects can ruin protective and aesthetical properties of coatings. A huge amount of time and money is invested in preventing and resolving defects. A lot of hustle can be prevented by using available knowledge that is simple and easy. How to resolve a defect turns out to be logical as soon as the governing phenomena of that defect are known and understood. A wide variety of coating defects occur and the same are discussed. Defects , Causes and the Remedies Adhesive Coating is a simple process There are various methods available for the same. In general following methods are used Gravure Gravure with Myer Bar Kiss Coating Floating Knife Comma Coating Or Reverse Roll coating We are all aware of these Coating Methods and we know how to use them to the best to produce an Adhesive Coated Product.
  2. 2. No : PCP 001 Date : 02 -01-2014 Adhesive Oozing / Bleeding a squeezing out of the adhesive from between layers of tape so that the sides of the roll are very tacky. Blistering A small , dome like bulge in the coated film or a small recess in a coating film. Typical blisters are caused by trapped moisture , rapid drying and the use of improper solvents. Small to large broken or unbroken bubbles Broken edge craters and small blister-like spots can be caused by oil, grease or contamination on the substrate. Trapped solvents can also show up as blisters. Blisters will occur as a result of humidity testing on test panels, indicating moisture penetration through the film.
  3. 3. No : PCP 001 Date : 02 -01-2014 Cratering Is formation of bubbles (foaming) and resulting small, round concave depressions (cratering) when bubbles break in a adhesive film, during adhesive application and drying. Craters are also called Pitting .Small , uniform Indentations in coated Film . This term refers to large blisters, sometimes deep enough to expose the substrate, appearing like a volcano. They are caused by oil and grease spots on the substrate or by unwanted particles of dirt. In this latter case, a small lump may be visible at the bottom of the crater. Fish eyes are a special category of crater often caused by silicone compounds, such as waxes or lubricants. Pinhole craters are also encountered when solvent is trapped below the film surface due to insufficient flash-off or solvents evaporating too quickly.
  4. 4. No : PCP 001 Date : 02 -01-2014 Crawling Crawling, also called retraction or pulling back, is a uniform de-wetting effect: the surface energy of the solid substrate is too low to let the liquid adhesive , having a certain surface tension, cover that substrate. The resulting effect is that the liquid adhesive does not fully wet the substrate but it retracts and forms droplets, leaving part of the substrate naked. Chalking Chalking refers to a powdery appearance and a loss of gloss. It is caused by a disintegration of the film on a molecular level. Poor weather ability of the paint resin, use of poorly-balanced thinning solvents and insufficient paint agitation, can cause chalking. Formation of fine powder on the surface of the paint film during weathering, which can cause color fading. Cracking Cracking in conformal coating is where a smooth surface fractures into sections with the cracks in the coating leaving the area below exposed to potential contaminants. Contamination Contamination (dirt) can take many forms, particle and fibers all fall into the category of dirt. However, in some studies of contamination, it was found that more than 50 per cent of the dirt occurrences were caused by tiny particles. Other sources of dirt include: Surrounding shop area
  5. 5. No : PCP 001 De Lamination Peeling from undercoat or substrate Delamination in adhesive coating is where the coating lifts from the surface , leaving the area below exposed. Dirt Pickup: Accumulation of dirt, dust particles , fiber and hair , on the adhesive film; may be a major problem Fish Eyes This coating defect is characterized by circular voids or separation in the coating. Fish-eyes can be caused by oily spots or silicone particles and/or by air-borne droplets that are deposited on the coated surface. This is a typical defect when adhesive is sprayed , or coated by pickup rollers. Separation or pulling apart of wet film to Expose underlying Finish or substrate. A coating defect characterized by a small. recessed area in a coating film with a raised area in the middle , is nothing but Fish eyes. Date : 02 -01-2014
  6. 6. No : PCP 001 Date : 02 -01-2014 Foam Foam can already be present in the adhesive before application starts. It can also be introduced during the application process. Especially during Gravure or Kiss roller coating , air can be brought into the adhesive . A coating formulator wants to develop a adhesive that loses its foam as soon as application has finished and film formation proceeds. Mottle A coated defect characterized by a ring like blotch with a light colored area in the center of the ring. Mottle typically occurs when metallic flake pigment is used in a coating. Mottling is a dull, hazy or blotchy appearance on the surface of some vinyl films or over laminates. Orange Peel Orange Peel is Uneven , rough irregularities in the surface of a coated film. Orange peel often occurs when a binder fails to flow uniformly across an object's surface. Uneven surface formation - much like that of the skin of an orange - which results from poor coalescence of atomized paint droplets. Adhesive droplets dry before they can flow out and level smoothly together.
  7. 7. No : PCP 001 Date : 02 -01-2014 An orange peel effect is generally caused by the Adhesive viscosity that’s too high. However, poor operator techniques, or too much adhesive coating thickness , will also produce a rough finish. Pin holing Pin holing is one of the most common surface issues especially in sprayed application. This defect is characterized by pore-like holes of about the size of a pinpoint. It frequently extends entirely through the coating film. Tiny deep holes exposing substrate A small , sunken area that forms on a coating film after a bubble ruptures. Pinholes are typically larger and less abundant across a coating's surface than solvent pops. Peeling Peeling: Loss of bond due to poor adhesion. Where there is a primer and top coat of adhesive, peeling may involve some or all coats.
  8. 8. No : PCP 001 Date : 02 -01-2014 Wrinkling , Rough Surface Wrinkling: A rough, crinkled adhesive surface, which occurs when uncured adhesive forms a "skin.” Defects Related to Substrate Surface Quality It is well known that defects and imperfections pre-existing on the substrate surface can be easily duplicated and magnified in the hot-dipping stage, resulting in defective coatings. Rough Surface A rough substrate surface containing iron fines and slivers is prone to the development of outbursts in the coating. The resultant coating is rough with numerous areas of localized thick coatings. After being flattened in temper rolling, these areas reflect light differently, appearing as either dark or bright spots depending on the lighting condition. A typical sample is shown in Figure 1. Studies indicate that the density of this kind of
  9. 9. No : PCP 001 Date : 02 -01-2014 defect decreases with increasing coating weight,(4) suggesting that such a defect can be masked by a thick coating. The Causes Adhesive Oozing / Bleeding Blistering Possible Causes: • Applying highly plasticizer loaded or softer adhesive . • Exposure of the Adhesive film to high temperature , humidity or moisture shortly after adhesive is coated , especially if there was inadequate exhaust.
  10. 10. No : PCP 001 Date : 02 -01-2014 Cratering Crawling Possible Causes: Use of low quality adhesive or very old adhesive. Applying (especially rolling) paint too rapidly. Wetting becomes more critical when the surface energy of the substrate is low and/or when the surface tension of the liquid Adhesive is high. A substrate having low surface energy is called hydrophobic, implying that the surface of the substrate is not able interacting strongly with a material brought on top of it. Unmodified plastics, for example, are a group of materials having low surface energy. Even when the surface energy of the substrate is
  11. 11. No : PCP 001 Date : 02 -01-2014 relatively high, wetting may become a critical factor if highly dynamic application processes are involved. On fast-running printing presses or roller-coater application, for example, wetting must be very fast to obtain good results. If the substrate wetting is too slow, uneven ink-lay, print defects or even poor ink transfer may result. Under static conditions, the contact angle Q of a liquid drop placed on a given substrate determines whether wetting is good or poor Cracking Cracking in conformal coating is where a smooth surface fractures into sections with the cracks in the coating leaving the area below exposed to potential contaminants.     Cure temperature too high Conformal coating heat cured too quickly without allowing enough time for room temperature (RT) drying Film thickness too great causing coefficient of thermal expansion (CTE) mismatch and cracks occurring in the coating Operating temperature too high or too low causing the conformal coating to flex too much & crack. Chalking • Use of a low-grade, highly pigmented paint. • Use of an interior paint for an outdoor application. Contamination Dirt Pickup:
  12. 12. No : PCP 001 Date : 02 -01-2014 Electrostatic charges develop on surfaces of film webs as they are transported through various converting processes such as Coating, Laminating , rewinding, slitting. These charges often cause attraction of Dirt , Dust , fibre and hair on to the web material and defects in coating uniformity resulting in quality problems and customer dissatisfaction. Static electricity is the cause of many problems encountered in the transporting and converting of film webs in today’s modem machinery. As progressing technology allows faster production speeds, static related problems are also increasing, hampering production and affecting product quality. Static charges are generated on the surface of the film as it unwinds from the roll and as it contacts and separates from surfaces such as idler rolls, nip rolls and printing or coating rolls. Charges exist in two polarities: positive and negative. Surfaces charged to the same polarity will repel each other. Opposite polarities will attract. A charged film of either polarity will attract to uncharged surfaces of insulators or conductors. These occurrences become especially evident in converting operations such as sheeting, bag making and die-cutting where the film is no longer under the control of the mechanical structure of the web and its transport system. Film webs tend to develop high charges; 30 kV to 40 kV is common. The strong electrostatic fields associated with these charges attract dust particles, fibers, bugs and hair resulting in surface contamination. This causes obvious quality problems in printing, coating and laminating and cleanliness problems with food, medical and pharmaceutical packaging films. Uneven coatings and “wicking” of adhesive are often the result of static charges. De Lamination Factors that influence delamination include:
  13. 13. No : PCP 001 Date : 02 -01-2014 1. Cleanliness of the surface for adhesion 2. Compatibility between the coating and the surface adhered to due to surface energies mismatch 3. Permeability to moisture 4. Degree of coating cure Fish Eyes "Fish-eyes” are areas in a freshly applied surface coat that tends to thin or open up on their own accord. This problem is caused by the high surface tension of the adhesive applied on a polymer surface, such as a plastic films. Foam Two aspects strongly dominate the formation, stabilization and release of foam. First, foam bubbles can be stabilized by additives having a surfactant (soap) structure. This problem is experienced most strongly in water based adhesives. The second important aspect is the rheology of the Adhesive. Despite the viscosity being high the foam bubbles might be able to move to the surface of the film where they can break. However, the resulting hole possibly can not level out when the viscosity of the film is too high, Mottle The plasticizers that make films soft, also make them very impressionable. So much so that when vinyl films and overlaminates are rolled up on a roll, the films can pick the impression of backside of the release liner.
  14. 14. No : PCP 001 Date : 02 -01-2014 Orange Peel Adhesive might be too viscous ( high viscosity ) Or The coating nip is too tight solvent evaporation too fast Origin and Potential Causes: Improper gun adjustment and techniques. Too little air pressure, wide fan patterns or spraying at excessive nozzle distances causes droplets to become too dry during their travel time to the work surface and they remain as formed by gun nozzle. Extreme shop floor temperature. When air temperature is too high, droplets lose more solvent and dry out before they can flow and level properly. Improper dry. Factors that can cause orange peel effect include: 1. The improper application of the conformal coating. This manifests in aerosol or spray gun application where the coating is applied too far away from the substrate and the coating is too “dry” on the surface to level out. 2. The incorrect thickness of the coating where not enough coating is deposited so that there is insufficient conformal coating to self level. 3. The incorrect cure profile when using an accelerated curing mechanism. This could be that the flash off zone before heat curing is inadequate or the oven profile is incorrect. Improper flash or recoat time between coats. If first coats of enamel are allowed to become too dry, solvent in the paint droplets of following coats will be absorbed into the first coat before proper flow is achieved. Wrong solvents or reducer. Under-diluted adhesive or adhesive thinned with fast evaporating solvents or reducers causes the atomized droplets to become too dry before reaching the surface. Too high viscosity. Low shop floor temperature. Too little solvent or reducer. Materials not uniformly mixed. Many finishes are formulated with components that aid coalescence. If these are not properly mixed, orange peel will result.
  15. 15. No : PCP 001 Date : 02 -01-2014 Substrate not treated thoroughly Peeling Pin holing Pin holing is the result of trapped solvents, air or moisture. Factors that can cause these effects include: 1. When applying the conformal coating the wet film surface can skin over, entrapping solvents under the surface which can bubble or burst out. 2. If the coating is applied too thick or too viscous any bubbles which are created in the process of application can become entrapped in the coating before they settle out. 3. Air entrapped under components by the coating process can be forced out during the drying / curing process and cause bubbles. 4. Pressure pots with conformal coating inside can absorb air which can manifest as champagne bubbles when applied. 5. Brush coating with too viscous a material or working the coating can cause bubbles. 6. Incorrect spray equipment or pressures can cause excessive bubbles & foam. Wrinkling , Rough Surface Possible Causes: • Adhesive is applied too thick. • Adhesive coated during extremely hot weather or cool damp weather, which causes the adhesive film to dry faster on top than on the bottom. • Exposure of uncured adhesive to high humidity levels. • coating of top coat of adhesive to insufficiently cured primer. • Coating over contaminated surface (e.g., dirt or wax)
  16. 16. No : PCP 001 Date : 02 -01-2014 The Remedies Adhesive Oozing / Bleeding Blistering Setting appropriate drying temp is a key to get raid of Blisters. The drying Air temp and the velocity is critical. Adhesive coated film should never be exposed to very high temp in the first zone. As a matter of fact a temp gradient setting is essential. The most ideal temps for drying zone wise beginning from First to last zone should be set. This allows the entire thickness of the wet adhesive film warm up uniformly. Thus the sudden drying of the top layer , and entrapment of solvent in the layer below is avoided.
  17. 17. No : PCP 001 Date : 02 -01-2014 Cratering • Use high quality adhesive which will allow good flow and appearance • coat an appropriate primer before using such Adhesive over the surface. Crawling Crawling can be prevented in a number of ways. A pre-condition that has to be met in order to obtain complete wetting is that the substrate is clean. In some cases the substrate must be modified in such a way that the surface energy of the substrate is raised, i.e. the substrate is changed from hydrophobic to hydrophilic. Substrate treatment is often applied to plastics prior to coating. Improved Wetting There are two ways to improve wetting characteristics:   Raising the surface energy of the substrate through cleaning - removal of oils and/or other contaminants - or through surface treatment (corona pre-treatment, flaming, acid or caustic wash). Lowering the surface tension of the coating by using additives made specifically for this purpose. Corona treatment helps to a great extent in improving the wettability of the substrate. Another approach being most often used to prevent crawling is lowering the surface tension of the liquid adhesive by adding wetting agent. However, the use of wetting agents can worsen key properties of the system, like hardness, recoat ability and foam sensitivity.
  18. 18. No : PCP 001 Date : 02 -01-2014 Cracking It is possible to minimise conformal coating cracking effects by      Lowering the initial cure temperature Allowing an initial drying time at room temperature before exposure to elevated temperature Reducing the coating thickness to avoid CTE mismatch issues Select a coating with a wider temperature range performance. Select a more flexible coating Contamination Dirt Pickup: NEUTRALIZING CHARGES ON THE FILM WEB If static charges on the film web can be controlled to a low level, most electrostatic related problems can be eliminated. A neutral material will have no electric field so induction charging, particle attraction and most ESD events will not occur. Unlike conductors, charges on insulative materials cannot be neutralized by simply grounding. Methods such as modifying the surface of the material to give it some degree of conductivity or the use of air ionization must be employed. Humidity Raising relative humidity is sometimes used to increase surface conductivity of some normally insulate materials in an effort to reduce static charging.
  19. 19. No : PCP 001 Date : 02 -01-2014 Ionization The use of air ionizers is the most common, cost effective method of controlling static charges on the film web. Ionizers emit vast quantities of both negative and positive charged gas molecules called air ions. The ions are made available to the charged web where the electric field attracts the necessary ions of opposite polarity to its surface, thus balancing the charge on the web. The most common ionizers used in film transport systems are static bars and ionized air blowers. They may be electrically operated, passive, or may use radioactive materials as the ion generation source.
  20. 20. No : PCP 001 Date : 02 -01-2014 De Lamination Fish Eyes Reduction of the surface tension is required to avoid fish-eyes. This can be achieved thanks to surface modifying agents such as surfactants. Incorporation of surfactants in the formulation will help you to improve the coating appearance. Foam The prevention of foam in films can be arranged in a number of ways. At first, the use of surfactants should be minimized. Secondly, introduction of air in the adhesive during production, handling and application should be prevented as much as possible. Finally, the rheology of the system should be such that the air that was introduced can escape in a short period of time during the first stage of film formation. Be aware that the use of defoamer, being an approach that cannot always be prevented, increases the risk of
  21. 21. No : PCP 001 Date : 02 -01-2014 cratering , as well as reduction in Peel adhesion values. However addition of around 0.5 to 1.0 % of Defoam / Anti Foam is generally followed . Mottle Orange Peel Adjust the Adhesive viscosity or the nip replace the solvent Peeling Pin holing Surface modifying additives, especially surfactants, will reduce the surface tension, and help entrapped bubbles to move to the surface. The use of surfactants in adhesive formulation will avoid pinholes to appear on the dry paint film. Wrinkling , Rough Surface • Scrape or sand substrate to remove wrinkled coating. If using a primer, allow it to dry completely before applying top coat. Recoat , (avoiding temperature/humidity extremes) . Dear Members , I have made an attempt to highlight the issue’s related to coating defects in general. Please feel to write to me for any specific problem you might be facing . It’s a promise from my side to guide you , in terms of selection of a suitable additive or a process.