The biggest difference between waterproof backpacks and ordinary backpacks is the fabric.
The fabric used in ordinary backpacks is easy to get wet when exposed to water. Some fabrics are water-repellent, which is a coating that plate to the surface to prevent part of the water from entering the backpack. However, it is useless in heavy rain, besides, the coating has a certain service life, thus the water-repellent function will gradually fade over time.
Most of the fabrics used in waterproof backpacks are air-tight or air-impermeable, with a thicker coating, and even a film on the other side for better waterproof performance.
Nowadays, waterproof backpacks in the market are basically made of TPU fabrics. TPU is the abbreviation of thermoplastics urethane, which is a polymer material made by the reaction of diphenylmethane diisocyanate or toluene diisocyanate, diisocyanate molecules and macromolecular polyols and low molecular polyols. TPU fabric has many excellent properties such as super waterproof, antibacterial, mildew resistance, wear resistance, cold resistance, oil resistance, water-resistance and aging resistance.