Moisture-proof treatment of corrugated cardboard boxes is primarily aimed at preventing water from penetrating the box itself, in addition to protecting the packaged goods from water damage. Once a corrugated box becomes wet, its compressive strength and burst resistance are significantly reduced, which can severely compromise the quality of the packaging. Moisture-proof treatment is typically achieved by applying a waterproof coating. Accordingly, based on the nature of the waterproofing agent’s action, moisture-proof corrugated boxes can be manufactured using either temporary or permanent waterproofing methods.
Temporary waterproofing agent
Corrugated cardboard that has been treated with a temporary water-repellent agent during the manufacturing process exhibits temporary water resistance. According to JIS Z 1537, when water briefly contacts the surface, it does not soak in but instead beads up, thereby preventing water from penetrating the cardboard. Over the past decade, the process of treating cardboard with a temporary water-repellent agent has been widely adopted by major carton manufacturers throughout China. This water-repellent agent was originally introduced from Japan, where it is literally referred to as “water-repelling agent.”
This type of coating was initially applied to textile fibers and only later adopted for treating cardboard. Consequently, most corrugated-box manufacturers refer to it as a “water-repellent agent.” A water-repellent agent is a wax emulsion with a solid content typically ranging from 48% to 51%. Its most distinctive feature is that it can be diluted with about 30 parts of water before being applied to the surface of corrugated board during the forming process. After drying at around 150°C, the paper fibers on the surface become coated with wax, resulting in low surface tension and creating a water-repellent outer layer. When exposed to water, droplets bead up and roll off, thereby achieving a waterproof effect. However, wax has poor adhesion to paper fibers; especially after the water-repellent agent is diluted 30-fold, its solid content drops to only about 1.5%, leaving only a very thin wax coating on the fiber surface. In addition, because the particle size of wax in wax emulsions is generally larger than that in conventional polymer emulsions, it is difficult for the wax to penetrate deeply into the paper fibers. Moreover, water-repellent agents are often sensitive to chemicals such as alum, which can cause rapid demulsification and precipitation on the paper surface. As a result, the vast majority of wax particles in the water-repellent agent remain confined to a very thin surface layer. Under external friction, the wax coating is easily worn away, exposing the underlying fibers and significantly reducing the waterproof performance. To enhance the adhesion of wax to paper fibers, water-repellent agents often contain rosin derivatives or carboxylated polymer emulsions, which exhibit strong adhesion to paper fibers and good compatibility with wax. Their addition promotes better wax adhesion to the fibers; similarly, modifying the wax—such as oxidizing or functionalizing it—can also improve its adhesion to paper fibers. Nevertheless, it must be emphasized that even with these enhancements, the coating formed by the water-repellent agent at such concentrations still does not constitute a true waterproof barrier. Consequently, when subjected to friction—for example, during handling or stacking—the waterproof surface can easily lose its effectiveness. This is why boxes treated with water-repellent agents are classified as “temporarily waterproof” cartons. At present, water-repellent agents are widely used in many corrugated-box factories and have become a standard processing step, largely due to four key factors: their cost-effectiveness and suitability for machine application; their ability to preserve the natural color of the paper, aligning with the current trend toward natural materials; their non-adverse impact on the recyclability of the boxes; and their adequate waterproof performance for typical box applications. However, it should be noted that for boxes used in special environments—such as cold-storage boxes, boxes intended for high-humidity storage, or boxes destined for maritime transport—caution is warranted when using water-repellent agents. In practice, there have been cases where boxes coated with water-repellent agents deform upon removal from cold storage, leading to economic disputes. Therefore, when recommending a specific waterproof treatment to customers, it is essential to take full account of the actual service environment of their boxes. Furthermore, different brands of water-repellent agents often differ in raw materials and manufacturing processes, resulting in variations in the water-to-agent ratio. It is crucial to maintain the appropriate water-to-agent ratio during application: if the water ratio is too high, the water-repellent effect will decline markedly, rendering the treatment ineffective; if the water ratio is too low, dark wax-like oil spots may form on the cardboard surface, compromising its appearance.
