Coatings play an important role in improving casting accuracy and quality, conserving resources and protecting the environment. The rapid development of lost-foam casting, V-process molding and other new foundry processes has introduced new performance requirements. Alcohol-based coatings are widely used in foundries for their fast drying, good strength and high efficiency.

1 Research Status of Alcohol-Based Coatings
Alcohol-based coatings are mainly used in resin-bonded sand and sodium silicate sand casting, usually applied by brushing or spraying, particularly for small and medium-sized castings. They require no oven drying after application, helping improve production efficiency and save energy. Over the past decade or more, many Chinese studies have examined these coatings, and their development for specific conditions has become relatively mature. As casting processes advance, higher requirements are being placed on mold coatings, leading to the emergence of dedicated alcohol-based coatings for special processes.
1.1 Alcohol-Based Coatings for Lost-Foam Casting
Lost-foam casting has been described as the casting technology of the 21st century and a green revolution in the foundry industry, with rapid development in China and abroad. Lost-foam coatings provide isolation, reinforcement and a path for decomposition products to escape, helping prevent mold collapse, pattern distortion, gas porosity and surface carburization. Although many alcohol-based coatings exist, relatively few are specifically suitable for lost-foam casting.
Li Huanchen and colleagues found that an alcohol-based coating for high-manganese steel using high-chromium corundum as the refractory aggregate outperformed a coating using magnesia powder in every measured property. This was because the high-chromium corundum had been crushed and ground into fine powder, improving suspension stability in the carrier liquid and coating permeability. Its high refractoriness also ensured high-temperature coating strength. Xu Feng and colleagues developed an isopropanol-based coating for lost-foam steel casting that produced castings with smooth surfaces, high dimensional accuracy and no sand adhesion, while also keeping costs low. Yang Mingbo and colleagues developed an alcohol-based coating for lost-foam iron casting. A small amount of Fe₂O₃ was added to prevent sand adhesion and facilitate coating removal. Analysis of the effect of each component's addition rate on coating properties established the optimum mass-fraction formulation: refractory aggregate (35% zircon flour + 60% bauxite + 5% graphite) + 3.5% PVB binder + 1.5% bentonite suspending agent. Tan Yuhua and colleagues developed an alcohol-based coating comprising 30%–35% flake graphite by mass, 30%–35% amorphous graphite by mass, 1.0%–1.5% phenolic resin by mass, 2.5%–3.0% polyvinyl butyral (PVB) by mass, 30%–35% ethanol containing some methanol by mass, and rosin, suitable for lost-foam iron castings. Gao Qimei and colleagues developed an alcohol-based coating for aluminum alloys using talc as the principal refractory aggregate. Talc has excellent thermal insulation properties, allowing control of the casting solidification rate and improving molten-metal fluidity, thereby meeting the coating performance requirements for aluminum-alloy castings. Production validation showed that this coating was also suitable for small copper-alloy castings.
1.2 Alcohol-Based Coatings for V-Process Molding
Vacuum-sealed molding, abbreviated vacuum molding and also known as vacuum-film molding, reduced-pressure molding or V-process molding, is a new generation of casting methods. Its basic principle is to cover the pattern with plastic film and seal the flask. A vacuum pump removes air from the mold, creating a pressure difference between the inside and outside of the flask that compacts the dry sand and forms the required cavity. The pattern is then withdrawn, the mold is closed, and molten metal is poured while the vacuum is maintained. Coating properties directly affect the gasification of EVA film and the movement of gases, and also have a major influence on casting surface quality.
Yang Lijuan and colleagues developed an alcohol-based coating for V-process casting of high-manganese steel parts. Phosphate was added as an auxiliary material to promote accelerated setting and self-hardening on plastic surfaces and to improve the coating's resistance to sagging. Performance tests on the optimum coating formulation identified through orthogonal experiments showed that its coating ability, thixotropy and permeability all met V-process casting requirements. Zhao Rong and colleagues developed an alcohol-based powder coating for V-process casting of steel and iron. This powder coating overcame the transport and storage drawbacks of slurry and paste coatings. Sodium fluorosilicate was added as a high-temperature sintering aid, causing partial liquid-phase sintering at high temperature and thereby improving the coating's resistance to erosion and penetration by molten metal and reducing defects such as sand adhesion. Coatings with different refractory aggregates were validated in production on 35# carbon-steel axle housings, 40Mn alloy-steel axle housings and ductile-iron reducer housings, fully meeting production requirements. Shen Jianhua and colleagues developed a white alcohol-based V-process coating using forsterite, talc and chlorite in place of graphite as the refractory aggregate. Comparison with foreign coatings showed comparable density, pH and suspension properties, but relatively lower thixotropy, making the coating suitable for spraying. Production applications showed that it improved casting surface quality, achieving a surface roughness of 6.3 μm. Zhu Zhiling and colleagues developed a fast-drying white coating for vacuum molding using forsterite and zircon flour as refractory aggregates. Tests yielded a density of 1.40 g/cm³, an 8-hour suspension rate of 95%, a viscosity of 11 s, gas evolution of 20 mL/g and no cracking at high temperature. Production validation showed that, for casting weights ranging from 74 kg to 13,460 kg, there was no sand adhesion and the coating peeled away easily.
1.3 Alcohol-Based Coatings for Green Sand
Green sand plays a very important role in molding and coremaking. In recent years, many green-sand production lines have added spraying stations, generally using alcohol-based coatings, to prevent sand adhesion, gas holes and sand inclusions and to improve casting surface quality.
Song Changyun and colleagues, and Zhuo Rongming, studied alcohol-based coatings for green-sand iron casting. Both selected the same refractory aggregates: amorphous graphite powder and flake graphite powder. The difference was that one study used phenolic resin as the low-temperature binder, PVB as a suspending agent, and lithium clay as both a suspending agent and a high-temperature binder, with talc selected as a sintering agent to improve the coating's self-peeling properties. The other used dextrin as the low-temperature binder and a newly developed organic–inorganic composite, SN-1, as both the suspending agent and high-temperature binder. Experimental results showed that both coatings offered high suspension stability and permeability, low gas evolution and good resistance to high-temperature cracking. After pouring, castings were free of sand adhesion, the coatings were easy to remove, and casting surface roughness was significantly improved. A study developed a new alcohol-based composite dispersing and suspending agent, noting that the combined effects of electrostatic dispersion stabilization, steric dispersion stabilization and viscosity enhancement gave the coating good suspension stability and thixotropy. This agent was combined with other materials to produce a light-colored composite alcohol-based graphite coating for clay-bonded green-sand iron casting and an alcohol-based fused-magnesia coating for clay-bonded green-sand steel casting. Production applications yielded smooth castings without sand adhesion or similar defects. Xu Zili and colleagues used zircon flour, white corundum and brown corundum as refractory aggregates; organobentonite and PVB as suspending agents; organic and inorganic binders; and a mixed-alcohol solvent carrier to produce an alcohol-based spray coating for steel casting. After air drying, the coating did not peel or crack and produced smooth casting surfaces.
1.4 Alcohol-Based Flow Coatings
Flow coating is a new, rapid application process in which a coating is spread evenly over the sand-mold cavity or core surface under the low pressure supplied by the flow-coating machine's pump. It differs from traditional dipping, spraying and brushing, so its coating performance requirements are also different, particularly in terms of rheology. Li Tiancai and colleagues developed a low-viscosity alcohol-based flow coating. To meet the rheological requirements of flow coating, hydroxypropyl methylcellulose was added as a thickener and SiF-706 as a leveling agent. Analysis of the effects of their addition levels showed that coating leveling first improved and then declined as the amounts increased. Tests on a 1 t casting found no sand adhesion; the coating was easily removed and the casting quality was sound. One publication described a trial flow coating developed from a brush-applied coating. Its analysis of the flow-coating process indicated that the coating flow rate should be controlled at 30 L/min and the mold should preferably be inclined at 60°–75°. Tests on resin-bonded sand achieved a casting surface roughness of 25 μm, with flow-coating efficiency more than six times higher than brushing. Another publication gave a detailed account of flow-coating component selection and main performance indicators. It considered bentonite unsuitable as a suspending agent for flow coatings because it gives the coating thixotropy, while adding a small amount of emulsion polymer can improve coating surface quality. Coating density, degrees Baumé and viscosity must be strictly monitored during production use. Another publication comprehensively analyzed flow-coating process properties, equipment and technical indicators in relation to actual production, and proposed solutions to common problems. For example, increasing coating viscosity and sand-mold compactness can ensure coating thickness. Changing the aggregate composition, increasing coating refractoriness and degrees Baumé, and brushing highly refractory coatings onto hot-spot areas can prevent sand adhesion on castings.
1.5 Alcohol-Based Coatings for Heavy Steel Castings
Large, thick-section steel castings are particularly prone to sand adhesion because of their large pouring tonnage, thick walls and the ease with which molten steel oxidizes. Production practice therefore often uses special facing sands or refractory coatings on sand-mold surfaces to prevent penetration-related sand adhesion. The former method, however, is gradually being replaced by refractory coatings because it is difficult to operate. Based on an in-depth analysis of sand-adhesion mechanisms in large, thick-section steel castings, and the anti-adhesion effect of chromite facing sand, Li Sinian and colleagues developed an alcohol-based chromite sand coating to prevent penetration-related sand adhesion. Applications included steel-casting riser sections 350 mm thick and heavy steel castings with a gross weight of 29 t and hot spots of 400 mm. The castings showed no sand adhesion, and the coating peeled away well as a whole. Qu Yinhu and colleagues developed a paste-form zircon flour coating to prevent sand-adhesion defects. Its distinctive feature was to combine the paste-form zircon flour coating as a base coat with a slurry-form zircon flour coating as a top coat. Applied to large, thick-section ZG35 and ZG45 steel castings, this combination produced good surface quality and made the castings easy to clean. Chen Zhanfei and colleagues developed an alcohol-based zircon flour coating using zircon flour as the refractory aggregate together with a special suspending agent and binder. Tests showed that a thick coating could be deposited in a single application, reducing the number of coats and increasing production efficiency. The special suspending agent and binder also reduced repulsion between incompatible components and improved system stability, giving the coating excellent suspension, brushability and leveling properties.

2 Analysis of Key Properties of Alcohol-Based Coatings
2.1 Suspension Performance
Some coating components cannot mix with alcohol to form a stable suspension, causing slurry or paste coatings to separate during transport and storage and affecting on-site use. Powdered or granular alcohol-based coatings were developed to address transport and storage problems, but inadequate activation of the suspending agent can markedly reduce suspension stability during on-site preparation.
Sheng Xiaobo and colleagues treated attapulgite clay with a TM-MA composite modifier to obtain modified clay. A coating prepared by mixing it with other additives achieved a 2-hour suspension rate of 99%. XRD analysis of phase changes indicated that the improvement in suspension resulted from a polymerization reaction between the modifier's —H groups and the —OH groups on the attapulgite surface, causing a shift in the 2θ position. Zhang Xuefeng and colleagues identified a new environmentally friendly suspending agent capable of replacing organoclay: a binary mixed suspending agent. Coatings prepared with it met Grade 1 suspension requirements. Li Shiping and colleagues developed a new nanodispersant to improve coating suspension, noting that suspension was even better when it was used with suitable organic additives. They attributed the improvement to the dispersant increasing the solid particles' ε-potential and surface hydrophilicity, which strengthened repulsion between particles. Zhang Xiangjuan used alcohol and silica sol to form a new suspension system, achieving a 2-hour coating suspension rate of 99%. Sun Luhong and colleagues observed the surface of paste coatings after alcohol addition using scanning electron microscopy. They found that the nanosuspending-agent particles had no clear boundaries and accumulated on the surfaces of the refractory aggregates. They concluded that the particles could swell directly in the alcohol solvent, thereby improving the coating's suspension stability.
2.2 Resistance to Sand Adhesion
A coating separates molten metal from the sand mold, preventing the metal from penetrating between sand grains or reacting with mold constituents to cause sand adhesion. In industrial production, insufficient refractoriness or high-temperature strength can cause the coating to crack and lead to sand adhesion. The main prevention theories are high-refractoriness prevention and sintering-and-peeling prevention.
Based on an understanding of the types and causes of sand adhesion on large castings, Hong Yi analyzed how factors such as coating components and refractory particle-size combinations affect resistance to sand adhesion. He considered the refractory aggregate to determine the coating's anti-adhesion performance, and found that different particle-size combinations could reduce aggregate thermal expansion and system refractoriness, bringing the aggregate into a sintered state. He also noted that adding flux could promote glass-phase formation and the sintering of refractory aggregate. Based on the theory of preventing sand adhesion through sintering and peeling, Feng Shengshan and colleagues developed a light-colored, alcohol-based coating for iron castings that works through sintering and peeling. They considered that adding metal oxides such as Fe2O3 could reduce the rate of oxide consumption, promote formation of an easily removable coating and prevent sand adhesion. Liu Xulin and colleagues added a flux composed of several compounds, primarily quartz powder, to a coating and observed its sintering characteristics. They pointed out that SiO2 in the flux could promote sintering of Al2O3 in bauxite, and that aluminosilicates such as bentonite in the coating could release amorphous silica and alumina. Under the action of FeO, these could form low-melting mixtures, promoting sintering of the refractory aggregate and preventing sand-adhesion defects.

3 Performance Improvements and Development Trends
3.1 Performance Requirements and Improvement Directions
Alcohol-based coatings must meet different performance requirements depending on their application and the type of casting. Green sand has poor permeability and generates a large amount of gas, so the coating used must have good permeability and low gas evolution. In lost-foam casting, the high pouring temperatures of steel and iron castings cause the pattern decomposition products to be mainly gaseous, so the coating must have high permeability. Excessively high permeability, however, affects the way molten metal fills the mold and can cause defects such as wrinkling and carbon black, so permeability must be appropriate. Aluminum alloys are poured at much lower temperatures than steel and iron, and their decomposition products are mainly liquid. The coating must therefore have a good ability to wet the liquid decomposition products and provide some thermal insulation to prevent defects such as cold shuts and misruns. To allow smooth brushing, produce a surface without brush marks and an even coating thickness, and prevent cracking during drying and at high temperatures, the coating must also have very good coating retention, leveling and suspension stability, together with relatively high strength.
Suspension is one of the main properties of alcohol-based coatings, but it still falls far short of that of water-based coatings, and granular or powdered coatings perform even worse. Proper selection of refractory aggregate particle size and grading plays an important role in improving suspension. Aggregates of different particle sizes make full use of the interlocking effect between coarse and fine particles, allowing fine particles to fill the spaces between coarse particles and form a stable dispersion. Different aggregate gradings can reduce coating density and help improve suspension. The purpose of a suspending agent is to provide suspension, and it is the main factor in improving this property. Common suspending agents for alcohol-based coatings include organobentonite, lithium bentonite and PVB. However, organobentonite requires activation with xylene and is toxic; lithium bentonite requires activation with water, resulting in poor suspension stability; and PVB readily causes blistering during burning. Nanocomposite suspending agents have therefore been introduced into coatings. These agents provide good suspension partly because the highly active atoms on nanoparticle surfaces can form hydrogen bonds in alcohol solvents and interact with other components to form a loose network structure. Their expansion is also irreversible, so the particles do not aggregate again even when dry. In addition, most binders in alcohol-based coatings are oleophilic and hydrophobic, and repel the inorganic suspending agents, reducing suspension performance. Modifying suspending agents and developing special suspending agents and binders to avoid mutual repulsion among coating components is therefore an important research direction for improving alcohol-based coatings.
Research into sand-adhesion-resistant alcohol-based coatings has already achieved some success. However, large fluctuations in raw-material quality or the large mass of castings can reduce coating stability. Material quality must therefore be strictly controlled during selection, and casting dimensions must also be considered. Adding several fluxes to a coating is more effective than using a single flux, and the addition level can be reduced to a very small amount, even to a few parts per thousand. In addition, changing the binder to increase high-temperature strength and prevent coating cracks can improve resistance to sand adhesion. In certain regions, climatic and other local factors affecting coating quality should also be considered.
3.2 Development Trends for Alcohol-Based Coatings
Green, intensive foundry production and near-net-shape forming are inevitable trends in the development of casting technology, and they also determine the direction of foundry coating development. Transfer coatings have emerged to ensure casting dimensional accuracy and achieve little or no machining. These coatings are initially applied to the core-box wall but transfer to the sand-core surface when the core is withdrawn. Because they do not occupy effective cavity space, they are also known as non-space-occupying coatings. They effectively address the complex processes, high costs and long cycles of traditional precision casting, meet the requirements of near-net-shape forming, and represent a future direction for alcohol-based coatings. Iron castings account for a large proportion of foundry production. Graphite coatings are used for most iron castings, but shakeout and cleaning generate large quantities of dust that pollute the environment and damage human health. Research into light-colored alcohol-based coatings can therefore both improve the cleaning environment and enhance casting appearance. Although research into light-colored coatings has been reported, their use is restricted by an inability to meet the production requirements of different casting sizes or accommodate different pouring temperatures, so further in-depth research is needed. Powdered or granular alcohol-based coatings can effectively address transportation safety and suspension problems and are therefore another direction for future coating development. Functional coatings have also developed considerably as casting technology advances. Adding substances that give coatings particular functions is an important trend in alcohol-based coating development. In recent years, the growing use of ductile iron for cast pipes, fittings and automotive castings has made the problem of graphite nodule degeneration into vermicular or flake graphite, caused by sulfur gas absorption at the casting surface, increasingly prominent. The most effective and economical measure is to use sulfur-penetration-resistant coatings. Some alcohol-based sulfur-barrier coatings have already been developed, improving sulfur penetration in castings. Magnesium and titanium alloys have very broad prospects in aerospace applications. Few related coatings are reported abroad because of technology restrictions. In China, only a small number of patents have been reported for magnesium alloy coatings and alcohol-based coatings for titanium alloy sand casting, and few publications have studied in depth how coating components affect performance. Developing flame-retardant alcohol-based coatings for magnesium alloys and alcohol-based coatings for titanium alloy sand casting can therefore reduce casting defects, lower costs and shorten production cycles, while accelerating China's progress toward becoming a strong foundry nation and making a significant contribution to national defense.

4 Conclusion
Alcohol-based coatings continue to play a major role in foundry production because of their advantages in rapid production cycles and good performance in use. Nanocomposite and modified suspending agents have significantly improved their suspension properties. However, mutual repulsion between certain incompatible components reduces coating stability, creating a need for special suspending agents and binders that are soluble in both water and alcohol phases. To meet the sand-adhesion resistance requirements of large steel castings, the high-temperature strength and crack resistance of coatings should also be studied in depth. New materials should be sought to develop high-strength coatings suitable for preventing sand adhesion on steel castings, thereby resolving sand-adhesion defects caused by coating problems. In addition, coating performance testing equipment has not become specialized, test methods vary, and there are no formally established coating assessment standards. As a result, the same coating can perform very differently at different manufacturers. Relevant universities, research institutes and large enterprises should therefore accelerate the establishment of coating performance assessment standards, promoting alcohol-based coatings with more consistent quality and better performance.