Low cost marking of hot-melt road coating (bubble section)

As the temperature rises, the problems that are prone to occur during marking construction also increase; Bubbles appearing after the application of hot-melt marking paint are a common quality issue, which can seriously affect the appearance, durability, and reflective performance of the marking. Analysis of the moisture content of calcium powder, solvent content of petroleum resin, low molecular weight of wax, boiling point of plasticizing oil, melt viscosity, etc.: High moisture content of calcium powder (filler): Reason analysis: This is one of the most common and direct reasons. Calcium powder (usually heavy calcium carbonate) is used as the main filler. If stored improperly or the production process is not strictly controlled, resulting in high moisture content, during coating melting (usually 190-220 ° C) and construction, the water in it will rapidly vaporize, producing a large amount of water vapor.
Bubble formation: These water vapor are wrapped in a molten viscous coating and cannot escape in time. After cooling and solidification, bubbles are formed, usually manifested as dense small bubbles or pinholes. Key point: Moisture content is an absolute taboo. Even a small amount of moisture can generate a large amount of steam at high temperatures. Petroleum resin contains residual solvents or low molecular weight components: Reason analysis: Petroleum resin is the main film-forming substance and binder of hot melt coatings. If the solvent is not completely removed after resin synthesis, or if the resin itself contains too many low molecular weight volatile components (equivalent to “impurities”), these solvents or low molecular weight substances will quickly evaporate at high temperatures during coating melting and application. Bubble formation: The evaporated steam will also be wrapped in the molten coating to form bubbles (the shape of the bubbles may be similar to that formed by water vapor, but sometimes they may be larger or more irregular). In addition, residual solvents can also affect the bonding performance of resins and the durability of coatings. Key points: It is crucial to choose petroleum resins with stable quality, thorough solvent removal, and appropriate molecular weight distribution.

Low molecular weight of wax (melting point too low): premature volatilization/decomposition: Low molecular weight wax may evaporate or thermally decompose prematurely at high temperatures (close to or exceeding its decomposition temperature) in the melting kettle (especially during long-term insulation) or the line drawing hopper, producing gas. Viscosity effect: Low molecular weight wax usually has a more significant effect on reducing the viscosity of the system. Low viscosity makes it easier for the generated bubbles to escape and rupture on the surface, but if the volatile gas generation rate is too fast (especially inside thick coatings or coatings), it may still be wrapped and form bubbles. More importantly, premature failure of wax can affect the overall performance of the coating. Reason analysis: Wax mainly plays a role in leveling, anti settling, hardening, and anti tire adhesion in hot melt coatings. If the molecular weight of the selected wax is too low, its melting point will also decrease accordingly (for example, much lower than the construction temperature). Bubble formation: Key points: Wax varieties with moderate molecular weight (such as wax molecular weight usually in the range of 1000-3000), suitable melting point, and good thermal stability should be selected. The boiling point of plasticizers/plasticizers is too low: Reason analysis: Plasticizers (such as cyclic oils, aromatic oils, etc.) are used to adjust the flexibility, low-temperature crack resistance, and viscosity of coatings. If the selected plasticizing oil has a low boiling point (or initial boiling point), its lightweight components are prone to volatilization at high temperatures during coating melting and application. Bubble formation: Volatile oil and gas are wrapped in the coating to form bubbles. In addition, the volatilization of plasticizing oil can cause the coating to become hard and brittle, affecting long-term flexibility and adhesion. Key point: Plasticizing oil with high boiling point, low volatility, and good thermal stability must be selected. Its flash point and distillation range (especially the initial boiling point) should be much higher than the coating application and melting temperature (usually requiring an initial boiling point>250 ° C and a flash point>220 ° C). Low melting viscosity of the coating: low resistance to gas escape: low viscosity coatings have good fluidity, and the gases generated inside (whether it is water vapor, solvent gas, or decomposition gas) are more likely to migrate upward and escape from the surface. This sounds like a good thing, but in actual construction: surface bubbles lead to depressions: bubbles break when they reach the surface, and if the viscosity of the coating is too low and the surface tension is not appropriate, the ruptured bubble mouth cannot be timely leveled and covered by the surrounding coating, forming depressions or volcanic craters, which is also a generalized “bubble” defect. Risk of internal large bubbles: In thick coating areas or at the bottom of the coating, if the amount of gas generated is large, low viscosity may not be sufficient to prevent the formation and floating of large bubbles, ultimately leading to the formation of large bubbles on the surface. Increased risk of air entrapment: During construction processes such as mixing, pumping, and scraping, low viscosity coatings are more prone to air entrapment, forming bubbles.

Other important reasons: construction temperature is too high: far above the recommended temperature. Construction will intensify the volatilization of all volatile substances (moisture, solvents, low boiling point oils, low molecular wax) and the thermal decomposition of resins/additives, producing a large amount of gas. It also leads to lower viscosity. Wet or unclean substrate: Road surfaces (especially cement surfaces) have high moisture content, dew, oil stains, dust, etc. When in contact with high-temperature coatings, moisture and pollutants will quickly vaporize or produce gas, which will be covered by the coating and form bubbles (often at the bottom or edge of the marking). Improper mixing method: Excessive or poorly designed mixing in the melting kettle or marking truck hopper can draw air into the molten coating. Excessive filler ratio/low oil absorption: Excessive filler content will reduce the resin matrix’s ability to wrap around the filler. If the filler has low oil absorption (such as certain gradations of calcium powder) and large particle gaps, it is more likely to retain air and volatile gases. Pigments (such as titanium dioxide) contain water or volatile components: Similar to calcium powder, pigments containing water or volatile impurities can also cause bubbles.

Summary and Solution: The bubble problem in hot-melt marking coatings is mainly caused by two core factors: the generation of gas at high temperatures and the insufficient viscosity of the molten coating to allow the gas to escape in a timely manner. The five aspects you listed accurately cover the main sources of gas production and the key physical properties (viscosity) that affect escape. Strictly control the quality of raw materials: ensure that the moisture content of fillers such as calcium powder is extremely low (usually required to be ≤ 0.1%), petroleum resin has no residual solvents and stable molecular weight, and use high boiling point plasticizing oil and wax with appropriate molecular weight and melting point. Strictly inspect raw materials (especially fillers and pigments) upon arrival at the factory. Optimize formula design: Balance the proportion of resin, plasticizer, wax, and filler to ensure appropriate melt viscosity at construction temperature (ensuring both leveling and workability, while effectively blocking the escape of bubbles or at least allowing them to slowly escape without breaking into pits). Avoid using raw materials that contain volatile components or have poor thermal stability. Consider adding a small amount of defoamer (choose a variety that is resistant to high temperatures and compatible with the system, the effect is limited, and it only treats the symptoms but not the root cause). Strict construction process control: Control construction temperature: Strictly follow the recommended melting temperature and construction temperature range of the coating supplier to avoid excessive temperature. Fully preheat the road surface: Especially in low temperature and humid environments, preheating the road surface can effectively remove surface moisture and dampness. Ensure that the road surface is dry and clean: thoroughly clean the road surface before construction, free of moisture, oil stains, and dust. Optimize mixing: The melting and insulation mixing should be uniform but avoid excessive agitation that may cause air to be drawn in. Use a suitable mixing blade. Control coating thickness: Overly thick coatings can increase the length of gas escape paths and the risk of retention. Solving the bubble problem requires systematic investigation and control from multiple aspects such as raw material quality control, formula optimization, production control, and standardized construction. Prioritizing checking the moisture content of fillers and the quality of petroleum resins is usually the most efficient entry point, and all raw materials should be traceable (knowing what they are).