At present, the main types of waterproof coatings in China are oil-soluble, alcohol-soluble and water-soluble waterproof coatings. Among these three types of waterproof coatings, water-soluble waterproof coatings have better gloss and moisture-proof performance than oil-soluble and alcohol-soluble waterproof coatings. Water-soluble waterproof coating for cartons is usually a water-based emulsion formed by combining paraffin and oxidized polyethylene as the main film-forming substances with various additives such as dispersants, stabilizers, thickeners, brighteners, regulators, and water. Its outstanding advantages are: non-toxic and odorless, good resistance to water penetration, not easy to burn, good stability, and good adaptability to cold and hot temperature differences. It features safety in use, low cost, excellent waterproof and moisture-proof performance, and ease of use (can be sprayed, coated or brushed).

Corrugated cartons, as outer packaging containers for goods, not only ensure the safety of transportation but also play a role in beautifying the products, increasing added value and promoting sales. Especially in recent years, various new moisture-proof and glossy cartons with exquisite printing have emerged in the market, which have greatly improved the product quality of packaging cartons and fundamentally solved the problems of easy moisture, mold and damage of carton packaging. At present, the demand for waterproof cartons is constantly increasing, especially in the packaging of vegetables, food, household appliances and industrial products, where their application is gradually increasing. In particular, the usage of water-resistant cartons for export products has grown rapidly, which has greatly promoted the development of the corrugated carton manufacturing industry. Introduction to the Functions of Waterproof Cardboard and Waterproof Coatings: Waterproof cardboard refers to the general term for cardboard that can resist water from damaging the strength of cartons. It is generally made by brushing a layer of waterproof coating on corrugated cardboard and then drying it. After being applied to the carton, the waterproof coating dries quickly to form a film. The film layer has high transparency, good gloss, is smooth and does not stick together, and has no effect on the printing ink of the carton. After being coated with waterproof paint, the surface wear resistance of ordinary corrugated cartons can be enhanced, their scratch resistance can be increased, and they have high strength. Moreover, they have strong water resistance and good moisture-proof performance, which can beautify the cartons and provide long-term protection.

This is because when the corrugated cartons are taken out of the freezer, the packaging is cooler than the outside. Just as water droplets form on the surface of a cup when cold water is poured into it in summer, this is the dew point phenomenon. Generally, the dew on the surface of corrugated cardboard can be absorbed by the corrugated cardboard in a short time, increasing the moisture content of the carton and reducing its strength. Using hydrophobic cardboard can turn dew into water droplets and prevent it from seeping into the interior without reducing the strength of the cardboard. However, if it is left outside for a long time or repeatedly taken in and out of the freezer, because the hydrophobic treatment only processes the surface of the cardboard, the hydrophobic effect gradually decreases and the moisture content increases. After a certain period of time, its strength will be consistent with that of ordinary corrugated cardboard. On the other hand, due to the hydrophobic processing of the cardboard surface, the surface wear resistance of the faced cardboard has been significantly enhanced, which can prevent multiple frictions between the cartons during the circulation process, as well as the peeling of the cardboard surface and the generation of paper dust. Water-resistant corrugated board (water-blocking corrugated board) Water-resistant corrugated board is also known as water-blocking corrugated board. This type of board is in prolonged contact with water and basically does not allow water to pass through. From another perspective, although it can prevent water from seeping in, once water enters, the water resistance of corrugated cardboard will completely disappear.

During the bonding process of corrugated cardboard, water-resistant waterproof coatings are applied to the surface of the paper by spraying or roller coating to endow the cardboard with excellent water resistance. If both sides of each sheet of paper are coated, the water resistance of the cardboard will be better. The representative method for manufacturing this kind of corrugated cardboard is the curtain coating method. As shown in Figure 1, the so-called curtain method is a technique that involves spraying molten waterproof coating in the form of a film curtain downward, allowing corrugated cardboard (or already printed corrugated cardboard boxes) to pass through and form a coating film on the surface of the corrugated cardboard (either by passing through once on each side or by installing two curtain film devices for processing on both sides).

Corrugated cardboard made by this method can fully prevent water from seeping in from the surface and the processing cost is relatively low, but it cannot prevent water from seeping in from the cross-section of the corrugated cardboard. This corrugated cardboard is generally used for packaging aquatic processed products. Besides being used for waterproofing, due to the soft waterproof layer on the surface of the corrugated cardboard, it is sometimes also used for packaging items that are prone to damage, such as furniture, pianos, and precision machinery. Corrugated board with strong water resistance is called corrugated board whose strength does not decrease significantly after being processed and soaked in water for a long time. In other words, corrugated cardboard with strong Waterresistance is a type of paper that has the drawback of weakening when exposed to water, but after processing, it still has strong water resistance when exposed to moisture. There are two manufacturing methods. One is the one-time processing method: that is, endowing the base paper with water resistance during the papermaking stage. There is also the secondary processing method: that is, the processing method of endowing corrugated cardboard with water resistance during the manufacturing stage.