Why don’t paper cups absorb water?

Paper absorbs water naturally. The solution is a non-absorbent barrier layer. Paper cups do not absorb water because their surfaces are lined with a thin, continuous film of PE or PLA plastic.

From my hands-on experience in manufacturing, raw paperboard soaks up liquids within seconds. This is why uncoated cardboard cannot hold water. In cup production, we use a paperboard that is pre-laminated with PE or PLA sheets as thin as 15–20 microns. This plastic barrier blocks water molecules from entering the fiber structure. The coating fuses to the paper under heat and pressure, creating a smooth, sealed wall that liquid cannot penetrate. This is why the inside of your coffee cup looks slightly glossy compared to the outside — that’s the waterproof lining.

What makes paper cups waterproof?

It’s not the paper itself, but what covers it. Paper cups are waterproof because they are coated with PE or PLA through a lamination process that bonds a continuous waterproof layer to the paper surface.

In production lines, large rolls of paperboard pass through heated laminators that bond the plastic layer tightly to one or both sides of the paper. This layer is only a fraction of a millimeter thick, so it doesn’t add much weight, but it’s completely sealed. PE (polyethylene) is the standard because of its durability, low cost, and ability to withstand hot drinks. PLA (polylactic acid), made from renewable plants, offers the same performance for most applications but is compostable under industrial conditions. Without these coatings, a paper cup would collapse once filled with coffee, tea, or cold drinks with ice.

Why do paper cups not leak?

The secret is not just the coating, but also the sealing methods. Paper cups do not leak because the coated paper is heat-sealed at the side seam and bottom, making the entire interior surface continuous and gap-free.

When I watch a cup forming machine in action, it’s clear that two factors stop leaks. First, the waterproof coating covers the entire inside of the cup. Second, the edges where the paper meets — the vertical side seam and the bottom rim — are bonded under high heat so that the coating melts together. This creates a continuous plastic seal. Even tiny pinholes are avoided by using precise heat and pressure settings. A poor-quality seal can cause slow leaks, so professional manufacturers always perform water-holding tests before shipment.

What makes cardboard waterproof?

Turning plain cardboard into waterproof board uses similar principles. Cardboard becomes waterproof when it is coated, laminated, or treated with a continuous hydrophobic layer such as wax, PE, PLA, or waterproof resins.

From my experience sourcing materials, cardboard is just thick paper, and it absorbs moisture easily. To use it for packaging liquids or wet foods, we coat it with PE or PLA just like we do for cups. In some heavy-duty packaging, a wax layer or special resins are applied instead. The goal is always the same — close the pores between paper fibers so water cannot enter. For long-term exposure to moisture, multiple coatings may be added. Eco-friendly businesses are moving toward plant-based waterproof coatings, which protect the board but allow composting or easier recycling.

Conclusion

Paper cups are waterproof because of their PE or PLA coating, applied during lamination, combined with heat-sealed seams that keep liquids inside without leaks.