What if paper were waterproof?

Leave a paper drawing out in the rain. It gets soft and floppy.

It might even fall apart! What if paper never got wet?

Spill a glass of water on your homework and the paper soaks it up, wrinkles and tears easily. A paper towel does the same thing on purpose, drinking up spills in seconds.

So what if paper were waterproof? This lab is a thought experiment. Some paper already keeps water out, like the paper cups that hold your drinks. Following the clues takes us from plant fibers to the recycling bin.

Britannica defines paper as a matted or felted sheet, usually of cellulose fibers, formed on a wire screen from a water suspension. Water is thus central to how paper is made, how it fails when wet, and how it is recycled.

This thought experiment asks what would change if paper were inherently waterproof. It uses the question to explore why fibers attract water, what holds them together, and what happens to paper products that are made water-resistant with coatings.

Paper is made from plants, like wood. The plant fibers are mixed with water into a mush.

The mush is spread on a screen, pressed and dried. Then it's paper!

Paper is made from cellulose, the tough material in plant walls. Wood is about half cellulose, and cotton is about 90 percent. First the fibers are separated and soaked in water to make a soupy pulp.

The pulp is poured onto a screen, which lets the water drain away and leaves a mat of fibers. The mat is pressed to squeeze out more water and then dried. Papermakers sometimes add a substance called sizing, which slows down ink soaking in, or coat the sheet afterward.

Fibers are separated and wetted to produce pulp, which is filtered on a woven screen to form a sheet, pressed to squeeze out most of the water and dried by evaporation. The dry sheet may then be compressed, coated or impregnated. Additives include bleach, coloring and sizing, which retards penetration by ink.

The oldest known paper fragment, a map from a tomb at Fangmatan in Gansu Province, dates to the 2nd century BCE, and the first records of a formal papermaking process appeared in China about AD 105 (Britannica).

Pretend paper never gets wet. Your drawings would survive the rain!

But paper towels couldn't soak up spills. And old paper might be hard to recycle.

Waterproof paper sounds great at first. Books, maps and drawings might survive the rain, and paper bags might not split when wet.

But paper towels and tissues work because paper soaks up water, so they might stop working. And recycling paper depends on soaking it in water until the fibers come apart. If water couldn't get in, recycling might get much harder. These are ideas, worked out from how paper behaves today.

Inherently waterproof paper would keep its strength when wet and protect what it holds. But absorbent products depend on water wicking into fibers, and recycling depends on water breaking the bonds between fibers so they can be repulped.

Coated paperboard hints at the trade-off. Polyethylene-coated board for cups and juice cartons is difficult to recycle, because a significant share of the fiber stays embedded in the plastic film. These consequences of fully waterproof paper are reasoned speculation.

Paper is full of tiny fibers with tiny spaces between them.

Water sticks to the fibers. It climbs up into the spaces, all by itself!

Water is sticky. Its molecules cling to each other and to many other things, including the fibers in paper. Paper is made of tiny fibers with air spaces between them, so water creeps into those spaces and climbs, even against gravity. This is called capillary action. Dip the corner of a paper towel in water and you can watch it happen.

The fibers in dry paper hold onto each other with links called hydrogen bonds. When water soaks in, it grabs onto the fibers instead, and the paper goes soft.

Capillary action moves water through porous materials by adhesion, cohesion and surface tension; water adheres to paper fibers and climbs until gravity balances it (USGS). Paper is fibers and air spaces, so it wicks readily.

Hydrogen bonding plays a central role in holding cellulose fibers together: as the last water evaporates between fibers in papermaking, hydroxyl groups on adjacent fibers bond. Because hydrogen bonding is reversible, rewetting lets fiber-fiber bonds be quickly replaced by fiber-water bonds, explaining both paper's sensitivity to moisture and its ease of recycling (Hubbe, 2014).

Paper cups and juice boxes have a thin layer of plastic. It keeps the drink inside.

But that plastic makes them harder to recycle.

We already make some paper water-resistant by coating it. Paper cups, juice cartons and some food packages have a thin layer of plastic called polyethylene on the paper.

The coating works well, but it causes a problem at the recycling plant. Paper fibers get stuck in the plastic film and can't be separated. Scientists are testing new coatings that let go of the paper more easily when wet, so the fibers can be saved.

Polyethylene-coated paperboard is a conventional material for liquid packaging such as paper cups, aseptic packaging and juice cartons. It is difficult to recycle because a significant percentage of the fiber becomes embedded in the PE film and is not recovered at repulping.

Al-Gharrawi, Wang and Bousfield (BioResources, 2021) tested a nanofibrillated cellulose layer designed to weaken when wet: in laboratory trials, samples with it showed much improved fiber recovery. The authors note that actual recycling systems may differ.