The permanent water-proof treatment of carton’s surface

According to the definition of JISZ1537-1989, the permanent water-proof treatment is keeping the paper impervious although for a long time in water.

In China, this method has been used for a long time for the carton’s water-proof treatment. In practical operation, it’s usually to paint the carton’s surface by hand with waterproofing agent, the surface will then keep a fine gloss and good strength, its water resistance can be upgraded as well, and the corrugation won’t collapse after paint. However, this kind of waterproofing agent is usually solvent based coating (or called oil-based as well), is especially harmful to the worker’s health and is also contaminative, moreover it is negative for recycling the cartons. Therefore, it has been replaced by water-based or emulsion waterproofing agent.

The water-based and emulsion waterproofing agent meet the requirement of keeping the paper impervious for a long time in water, but if there’s too much water in the agent while painting, it will prolong the natural drying time and will cause the corrugation to collapse or the separation between the surface paper and the corrugations. To solve all of these problems, first the NVV should be high enough, second to add appropriately some nontoxic water-soluble volatile solvent in the agent to make the water volatilize faster, third to change manual painting to painting by machine with drying process.

Corrugation board dries and forms at the temperature around 150℃, and some even up to 180℃. The thermoplastic polymer coating commonly becomes sticky at this temperature. Therefore the paper board painted with this coating will be napped or even broken when it is dried on the hot plate.

There is one waterproofing agent for corrugated board will overcome this shortcoming. This coating is a combination of polymer emulsion and wax emulsion. The polymer emulsion has good film-forming property and water resistance, its particle diameter is smaller than the one of wax emulsion, so its permeability is better than wax emulsion after painting. Because its compatibility with wax is not fine, so the wax will dissolve out from the polymer coating after filming, it floats on the surface with most of the other wax particles which are blocked by the paper fibers.

Because wax has the protection function in this way, it will reduce the sticky problem of the paper board’s surface when the corrugated board passes the hot forming plate and won’t nap the paper anymore, and then the paper board will has the permanent waterproof property. This coating has also solved the problem of low efficiency caused by manual painting. Moreover, it acts as delustrant as well because of the component of wax and it keeps the original color of the paper board. Besides, this coating is water emulsion and makes the paper dipped nicely when printing with water-based ink. The last but not least, the polymer coating’s main resin can be decomposed easily by alkaline water, and this is beneficial to paper recycle.

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