1. Where Water Gets Into a Boot: The Opening, Not the Walls
The material and construction of the boot resist water; water enters when it rises above the opening of the boot. It does not seep in through the walls near your ankles or through the bottom sole.
This is because the boot is shaped like a bag where the sides and bottom are connected without gaps, leaving just the top open. If this is true, we need to answer two questions: Why does the material of the wall resist water? And how is it built so that no holes appear in the walls?
2. Why Natural Rubber Repels Water
The main component of natural rubber is a long, chain-like molecule called cis-1,4-polyisoprene. This molecule is made only of carbon and hydrogen, which classifies it as a hydrocarbon [1].
Just like oil and water do not mix, hydrocarbons do not mix well with water. Because rubber does not mix well with water, this property is one reason why boots can keep water out. However, waterproofing depends not just on the material but also on how it is made and finished.
The habit of using tree sap to make clothes and shoes waterproof existed among Indigenous peoples in South America. It is said that French explorers, including La Condamine, reported this practice to Europe in the 1730s, though records vary between 1735 and 1736 [2].
3. How Heat Treatment Fixes Sticky Rubber
Rubber shoes and clothes used to be unstable materials. They became sticky in hot weather and hard in cold weather.
An American inventor named Charles Goodyear worked on solving this problem. Around 1839, he discovered that heating rubber with sulfur made it stable. He received US Patent No. 3633 for this process on June 15, 1844 [3].
This treatment is called vulcanization. By adding sulfur and applying heat, bridges form between the long polymer chains. This creates a strong material that still bounces back, known as rubber elasticity [1]. It is similar to tying loose strings together into a net so they hold their shape better.
4. How Rubber Boots Are Glued and Baked by Hand
In Otaru, Hokkaido, the company Daiichi Rubber makes rubber boots one pair at a time by hand. The process involves stretching the rubber, cutting it into shapes, gluing the pieces together, and baking them in a kiln.
This method has not changed for more than 80 years, and they produce about 100,000 pairs per year [6].
Cloth or leather shoes are often sewn with thread. The holes made by the needle can become paths for water to enter. Rubber boots, however, are joined using glue and heat. Whether a product has sewing holes depends on the specific item; some records show that sewing and waterproof testing occur after gluing and baking.
These boots use a high ratio of natural rubber, which makes them flexible and less likely to crack [6].
5. How Vinyl Boots Are Made in One Piece
The main materials for boots today are rubber and PVC (polyvinyl chloride). Some vinyl boots, called injection-molded boots, are made by pouring melted material into a metal mold [7].
Because the material hardens into one shape inside the mold, it is thought that this method reduces the need for seams from gluing or sewing.
6. The History of Boots, From Leather to Rubber
The origin of rubber boots is often traced to leather knee-high boots designed by the Duke of Wellington in the early 19th century. It is said that the first rubber Wellington boots were made in the 1850s [4]. The rubber boots came later; the earlier boots were leather.
In Japan, the rubber industry in Hokkaido began around the early Taisho era (early 20th century). It began in Otaru with rubber soles attached to tabi. Later, companies that produced and repaired rubber shoes were established [5]. Because dates vary in historical records, we simply say this happened "around the Taisho era."
7. An Experiment to Test Your Own Rubber Boot
If you have a rubber boot, try an experiment in a place where getting wet is okay, like a bathroom or garden. Use a basin or bucket.
Do not wear the boot for this test. Submerge the boot itself in water up to a level lower than the top opening. Check if the inside stays dry. Then, raise the water level above the opening and see what happens. You can observe whether water enters when it passes the top edge.
You can also place a rubber boot next to a sneaker and look for seams. Can you find the holes made by thread in the sneaker?
If you want to learn more, read about companies like Daiichi Rubber in Otaru that make rubber boots. Their materials often include photos and explanations of the gluing and baking process [6].