With humid weather on the way, how can we enhance the moisture-resistant performance of corrugated cardboard?

When the relative humidity in the air is too high, cardboard that comes into direct contact with the air absorbs moisture from the environment. As a result, even the finished corrugated cardboard produced by paper mills becomes soft and flimsy—easily soaked, losing its strength, and ultimately compromising the quality and performance of the material. For corrugated cardboard, it’s essential to take proper precautions to address this issue. Packaging material testing.

Business owners in the cardboard industry face a recurring challenge every year: customers relentlessly asking, "Why is it so soft?" During this period, many cardboard factories receive wave after wave of customer complaint calls. Faced with unavoidable grievances—ranging from product returns and fines to costly compensation claims—addressing the issue of cardboard softening has become the top priority for these manufacturers.

01 Why does this phenomenon occur?

From the perspective of packaging material testing, this is primarily related to the structural properties of paper. Paper is mainly composed of plant fibers and additives, resulting in a porous structure with strong hydrophilicity. When the relative humidity in the air is too high—and if there’s a mismatch between ambient humidity and the moisture content of the paper—paper comes into contact with air and begins to absorb moisture. As the paper absorbs moisture, it may undergo localized or widespread deformation, leading to phenomena such as curled edges or wrinkled surfaces.

02 How to effectively waterproof and prevent moisture damage to paper

Physically waterproof and moisture-proof

1. Paper Products and Packaging

1. In response to the rainy season, packaging material testing should be used as a basis to raise the control standards for original paper's water absorption. Additionally, inspections of the original paper's moisture content and water absorption capabilities must be intensified, along with enhancing the sizing degree of the paper.

2. Roll paper is protected by wrapping it in stretch film, while the outer packaging uses laminated paper. Flat paper is sealed with heat-shrink film, which effectively minimizes weather-related softening issues.

2. Storage Environment

1. The storage environment for paper should be well-ventilated, clean, and dry.

2. Paper should not be stacked directly against the walls or placed directly on the floor—instead, keep it at least 10 centimeters (about 4 inches) above the ground. As a general recommendation, use wooden pallets to help absorb some of the moisture from the floor.

3. When storing, ensure there is adequate space between stacks of paper to promote ventilation and heat dissipation.

4. Where conditions permit, the relative humidity in paper warehouses and production workshops should be maintained between 50% and 65% whenever possible.

5. Wrapping quicklime in cloth or burlap and placing it throughout the paper warehouse helps keep the air inside dry.

6. Ensure proper rain protection facilities—are always equipped with a canopy in the warehouse.

Chemically waterproof and moisture-resistant

According to the information on packaging material testing, waterproof and moisture-resistant measures for corrugated cardboard can also be implemented from a chemical perspective. This is typically achieved by either adding water-repellent components directly into the pulp or applying a coating process after production to create a waterproof barrier. The treated cardboard is then subjected to continuous water-resistance leakage tests to verify its effectiveness. Specifically, cardboard with a basis weight below 80 g/m² should withstand water penetration for at least 72 hours, while boards weighing 300 g/m² or more must resist leakage for no less than 200 hours. Importantly, the waterproofing treatment has minimal impact on subsequent printing processes.

Waterproof and moisture-resistant paper products can widely replace plastic items, and their waste materials are also recyclable. Packaging material testing provides analyses that meet the demands of today’s low-carbon economy and sustainable development goals.

03 Transportation Environment

1. During transportation, open-top trucks and box trucks are commonly used. The latter provides better protection for paper products, helping to prevent and minimize softening.

2. When faced with rainy or overcast weather, logistics operations prioritize using trucks equipped with fly-wing doors.

3. For ordinary cargo trucks, the following three protective measures are typically implemented to prevent paper products from getting damp or wet:

1 ) The truck bed is lined with wooden planks, and a layer of tarpaulin is placed on top to prevent rainwater from splashing into the vehicle and soaking the paper products.

2 ) Using corrugated cardboard for full-vehicle protection, the material not only effectively prevents paper products from getting damp or wet but also safeguards them against impacts and damage.

3 ) During rainy or overcast weather, carrier vehicles are covered with three layers of tarpaulins to prevent rainwater from seeping in and causing dampness.

04 Paper Box Manufacturing Process

After exiting the cardboard production line, corrugated cardboard should be moved to the next process as soon as possible. The longer the delay, the more likely the cardboard is to absorb moisture from the air and become soft again.

Additionally, during printing—especially in the case of multi-color or large solid-area printing—pressure from the printing plate can easily cause cardboard boxes to become softened. To minimize this issue, try to reduce the time cardboard spends in the production and processing stages, and avoid prolonged exposure to humid air, which could lead to re-softening. Furthermore, the workshop should be moderately isolated from outdoor conditions to prevent moist air from directly entering the facility.

Packaging material testing data indicates that, prior to printing, it’s crucial to ensure the corrugated cardboard maintains optimal stiffness, and during printing, excessive pressure should be avoided. Additionally, when it comes to die-cutting processes, efforts should be made to steer clear of round-to-round die-cutting methods, as these can cause significant damage to the flat surface of the cardboard.

In short, the issue of cardboard boxes becoming soft ultimately boils down to moisture. Experienced packaging material testers advise that the key to ensuring cardboard remains rigid lies in doing everything possible—during raw paper storage, corrugated board processing, box manufacturing, warehousing, and transportation—to prevent the boxes from absorbing moisture.