This article focuses on the materials mostly used in industrial shoe manufacturing.
The industrialization of shoe-making started in the second half of the 19th century (more or less at the same time the industrial dyes came around, see the article here) and was dominated by areas of New England and the Atlantic seaboard. In the beginning, the materials used were not different from traditional shoe-making, just the production became automized. It changed in the 20th century when stitching was often replaced with glue and synthetic materials came around (rubber, synthetic textiles, etc) to replace leather.
The more time passed, the more sophisticated the shoe-making process became, and with it, of course, the user requirements increased. In cold climates people now wanted the footwear to be warm, waterproof, and quick-drying. It meant new materials and techniques had to be developed further as traditional footwear-making gear and skills were simply not enough anymore.
The geography of shoe manufacturing has also changed. Nowadays China and Brazil have taken the leading position followed by Italy, Columbia, and Spain.
Leather and synthetic leather
Leather has been used in traditional shoe-making for centuries and has not disappeared after its replacements came around. Leather is known to be very durable and flexible. It breathes and regulates temperature well and is comfortable to wear. Due to its great properties, the material tends to be more expensive compared to other materials used in shoe-making (like synthetic leather). We also all know where the real leather comes from so alternative materials were needed due to both ethical concerns (especially these days) and increasing footwear demand.
Artificial leather or synthetic leather was created with the intention to replace real leather and is often made of plastic. True, more and more alternatives are made of natural materials, but plastic-based alternatives that started to dominate in the middle of the 20th century are still very much around. They are created by covering a natural or synthetic fiber (e.g. polyester) with a plastic coating. Then substances called plasticizers (phthalate) are added to make the material softer and resemble more real leather. During the manufacturing process of synthetic leather toxic byproducts are formed which are dangerous both to nature and humans.
Rubber
Shoe rubber is made from natural rubber that is heated and mixed with sulfur. The process is also called vulcanization and was created by Charles Goodyear in 1844 after years of experimenting to find the right combination of mixing, heat, and curing agents. The method is still used today, including by well-known brands like Nike, Adidas, Jordan, and others.
In the present-day rubber is mainly harvested in the form of latex from rubber trees. The latex is a sticky, milky, and white colloid that is refined into a rubber that is ready for commercial processing.
Rubber is known to be durable, with great elasticity, waterproof, and resistant to abrasion, tensile, and slip.
The main sources of natural rubber are Amazonian rubber trees, Congo rubber, and dandelions. There are of course other plants that contain latex, but the extraction process is often more complicated and time-consuming. The life cycle of rubber trees in plantations is about 32 years of which 7 years is an immature phase and 25 years a productive phase. For optimum growth, they also require a specific type of soil, temperature range, rainfall, humidity, and absence of strong winds.
Now, as the base of shoe rubber comes from nature, is the vulcanized rubber biodegradable? Well, it takes much more time to decompose than natural rubber and requires very specific conditions. It is believed it takes about 50-80 years for rubber shoe soles to decompose. Some other sources say natural rubber takes more than 100 years to decompose so the vulcanized rubber even more. I would therefore conclude that both natural and vulcanized rubber and not very biodegradable.
Cork and wood
When rubber is the dominating material for shoe soles when it comes to sneakers and sports shoes, cork and wood (as well as leather) are also well-known shoe sole materials, especially, let’s say, for classier shoes.
Wood has been used in traditional shoe-making for a long time and is still in use in modern-day shoe-making. Lighter wood that weighs less is always preferred: cherry, alder, poplar, maple, and walnut. Now, shoe-makers often put a layer of rubber on the bottom of the wooden sole to make it softer and more comfortable. So in case you were already clapping hands and happy for its eco-friendliness in the shoe business, I am sorry for this bit of information.
Cork has been used in shoes for centuries and is known to be soft, light-weighted, and flexible so it is great for shoe soles and makes shoes very comfortable. It is also very durable, water- and flame-resistant, and hypoallergenic. Cork is often used in orthopedic shoes both due to its comfort, but mainly because it absorbs vibrations during walking and running. When you thought there are too many positives of cork already, let me add that it is also moisture-wicking material that diminishes the moisture build-up in your shoes and therefore kills the bacteria. Pretty amazing, I know.
Foam
Foam is mainly used in the upper parts of your shoes (also mid-soles) and is, unfortunately, made of plastic. There are two types of foams: open-cell and closed-cell foams. The open-cell foam has tiny cells which are not completely closed and therefore have air in them. It makes the foam softer and less firm. Closed-cell foam is air-tight and therefore more rigid. However, it is great for insulation.
In shoes, different types of foams are being used. The most common one is EVA (ethyl vinyl acetate) foam which is often used in mid-soles to absorb shock when walking or running. It is a closed-cell foam and therefore lightweight and durable. It is available in a wide range of densities and can be soft and flexible, but also stiff. It is found in different shoe types and styles.
Closed-cell PU (Polyurethane) is known for its great insulation. As the name indicates it is a closed-cell foam and often found in hiking- and ski boots. PE (Polyethylene) is easily die-cut and laminated and therefore used for internal pads and tongues of the shoes. It is not resistant to compressing so it is not used underfoot. SBR (Styrene-butadiene rubber) is a very soft foam and is mainly used to make parts of shoes waterproof and is also a closed-cell foam. Open Cell PU (Polyurethane) is an open-cell foam that can be often found in sport’s shoes due to its softness.
Even though there are bacteria and fungi that are capable of biodegrading certain types of foams (e.g. polyurethane and polyethylene), the process is often slow and not complete so the material cannot be considered (very) biodegradable and often accumulates in landfills.
Glue
Most glues used in shoe-making are highly toxic. A very common glue used is contact cement that has a high adhesive capacity and is very resistant. It is made of elastic polymers (e.g. latex, neoprene, synthetic rubber) and creates a fast, flexible and permanent bond. It can be used for almost anything.
These types of glues contain volatile and flammable solvents, so it is very important to use the glue in well-ventilated rooms and wear a mask that protects the nose and mouth from vapors. Otherwise, you may experience sensations from intense euphoria to vivid hallucinations.
Textiles
The most common textiles used in shoe-making are cotton, polyester, wool, nylon, rayon, and elastane. You may get to know each fabric in this article.
The textile chosen for a specific type of shoe depends on its characteristics.
Different fabrics come also with a wide range of colors and patterns, are lightweight, strong, and flexible. They have therefore a special place in footwear design.
However, as different fabrics are often soft and shapeless, different sizing and backing treatments are being used to give the fabric toughness and body. Sizing is a liquid resin treatment applied to the fabric. The fabric is stretched, heated, and treated with sizing resin. It holds the fibers in place. There are two different backing types: PU and PVC backing. The first one is a thinner clear coating which is also cheaper, lighter, and less waterproof. The PVC coating is used to make a very sturdy waterproof fabric and you cannot see the fibers through PVC backing.
On top of sizing and backing, different plastics are also used for fabric surface treatment and lamination. The latter is a procedure where treated fabric is being covered with a thin layer of PU foam (see above).
All of us want our shoes to last long, be comfortable and serve their purpose well. No one wants to wear rain boots that are not waterproof or ski boots that are not well isolated and warm. However, as you may have understood after reading this article, it comes with a price. While it may be difficult to find eco-friendly rain- or ski boots, there are definitely great alternatives for everyday footwear. We will talk about what these shoes are made of soon, so meanwhile, just stay moderate with your footwear purchases.
