Making materials for a healthier planet
Between belts and shoes, jackets and wallets, and even car seats and footballs, you probably see a lot of leather in your day to day life. Leather starts as an animal hide and has to go through a lot of processing steps to become a flexible, strong, and long-lasting material. But these steps traditionally involve intensive chemical treatments, treatments that produce environmentally harmful waste products. To address this problem, a chemist named Dr. Cecilia China is pioneering an eco-friendly method of processing leather. Instead of using toxic chemicals, her method can process leather using compounds from vegetables! In addition to reducing environmental harm, Dr. China’s work aims to boost the leather economy in her native Tanzania, because her technique is inexpensive and relies on widely available natural resources.
Table of Contents:
The chemistry of leather tanning
Tanning isn’t just what may happen to your skin if you stay in the sun too long! It’s also how humans make leather. Leather begins its life as the hide, or skin, of an animal. Leather is usually made from the hide of cows, but it can also be made from pigs, sheep, or many other animals. Rawhides used in leather production are often hides sourced from the meat and dairy industries, meaning that parts of an animal that would otherwise go to waste can be turned into long-lasting material.
Rawhides are converted into leather through a chemical process called tanning. When put in a bath of a chemical solution called tanning agent, the hide undergoes a chemical reaction that changes its chemical and physical properties. This reaction stabilizes a protein called collagen in the rawhide. Collagen provides structure to the skin and bones of most mammals. Humans actually have collagen in our skin that helps with healing and keeps our skin elastic.
When collagen is placed in the tanning agent, the atoms in both compounds form covalent bonds. A covalent bond is a type of chemical bond where two atoms share electrons. This stabilizes the molecules containing the atoms, improving the strength and flexibility of the collagen in the rawhide. Different tanning agents can enhance different properties of the rawhide. For example, certain tanning agents may lead to a more flexible leather that is ideal for a leather sofa, whereas others may create a stiff leather that is best for a pair of sturdy boots.
Bonded together
Atoms are tiny building blocks that make up everything around us. Everything from the computer you’re looking at to the chair you’re sitting on is made of atoms. Even you yourself are made of atoms! But atoms aren’t the smallest things that exist. Atoms themselves are made up of three kinds of particles: protons, neutrons, and electrons. Protons and neutrons exist in the nucleus at the center of an atom. Electrons orbit around the nucleus at specific distances, called shells.
In order to create matter, atoms need to stick to each other. This is done by forming chemical bonds. There are two main kinds of chemical bonds: ionic and covalent. Both of these kinds of bonds happen because atoms want their outermost electron shell to be completely filled.
A covalent bond, which is the kind of bond that forms between a leather tanning agent and collagen, is a bond where two atoms share some electrons. For example, the element oxygen has six electrons in its outer shell. In order to fill the shell, oxygen forms a covalent bond with two hydrogen atoms that each have one outer electron. This creates the molecule H2O, also known as water!
The other main type of bond is an ionic bond, which is formed when one atom steals an electron from the other. Because electrons are negatively charged, the atom that loses its electron becomes positively charged and the atom that takes the electron becomes negatively charged. Positive and negative charges are attracted to one another, so the two atoms stick together!
Tanning can be toxic
We talked about how the chemical bonds between the tanning agent and the hide are what make leather into leather. But the problem is that the main chemical we use for a tanning agent is a compound containing the element chromium. The form of chromium used in tanning is Cr(III), which is chromium that is missing 3 electrons. When Cr(III) comes into contact with some of the other chemicals used in tanning, it oxidizes into Cr(VI). Each atom of Cr(VI) has 3 fewer electrons than an atom of Cr(III). This form of chromium is unfortunately toxic to humans– it can irritate the skin, lungs, and eyes and even increase the risk of lung cancer. The compound isn’t just toxic to humans; it can also hurt local ecosystems when it gets into water networks.
This chromium-based tanning agent creates leather that is very temperature stable, soft, and can be easily dyed, so it’s become a popular method despite its toxicity. But the toxic chemical byproducts and shavings can’t be easily disposed of, and end up in landfills. This is especially a problem in economically developing countries like Tanzania, where Dr. China lives and works, because they don’t have access to the expensive resources needed to easily process this waste.
An eco-friendly innovation
There are a lot of alternatives to chromium-based leather tanning, but the leather they produce isn’t as high of a quality. For example, tanning agents extracted from plants have been used for centuries, but the leather created from plant-based tanning shrinks when it gets warm. Aluminum-based tanning agents face similar problems. Both these methods form covalent bonds with the collagen in rawhides just like the chromium method, but the strength of the bonds varies. Dr. China’s research focuses on a new technique, called combination tanning. With combination tanning, aluminum and plant tanning agents are used together to produce better leather than either method can produce on their own.
This method could be particularly important for leather tanneries in Tanzania. Dr. China has pioneered methods for extracting the materials needed for the tanning method from natural sources and native Tanzanian plants. While chromium tanning requires expensive waste recovery, the method that Dr. China studies can be sourced naturally and cheaply. As a result, combination tanning is more cost effective than chromium tanning. Tanzania has a large livestock population and an abundance of rawhide, but its leather industry has been limited by the cost of dealing with the toxic chromium waste. Dr. China hopes that her work will improve the leather industry in Tanzania, reducing unemployment and boosting socio-economic development.
Women supporting women in the sciences
Beyond her work on eco-friendly leather tanning, Dr. China is also passionate about encouraging women and girls to pursue science. She is part of an international team that designed lab kits for Women Supporting Women in the Sciences (WS2, https://ws2global.org/), an organization dedicated to unifying graduate and professional women in science while also providing outreach to elementary and secondary level students. The lab kits that Dr. China helped design are currently being distributed to students across eastern Africa.
Humans have been making things out of leather since prehistoric times. And with modern chemical processes, we can make leather faster and better than ever. But these chemicals can come with a huge health and environmental cost. That’s why scientists like Cecilia China are working to develop alternative methods for leather tanning, methods that could lead to a better economy and healthier ecosystem for Tanzania and the planet overall.
Written by Ashely Cavanagh
Edited by Justina Yang and Ella King
Illustrations by Lindsey Oberhelman
Primary sources and additional readings:
Tanning capacity of Tessmannia burttii extracts: the potential eco-friendly tanning agents for the leather industry by C. China et al. in Journal of Leather Science and Engineering
Alternative tanning technologies and their suitability in curbing environmental pollution from the leather industry: A comprehensive review by C. China et al. in Chemosphere
The Leather Industry: A Chemistry Insight Part I: an Overview of the Industrial Process by V. Beghetto et al.
Dive deeper into material science!
Define (10-20 minutes): Google search the term “leather tanning” then try writing a definition for it in your own words.
Learn (30-45 minutes): Pick either the aluminum or the vegetable oil tanning process and research. Then take a blank piece of paper and fold it so there are four equal sections. In the first three sections try to summarize the process you chose into three steps. You can draw a picture or describe it with words. Then in the fourth box describe the disadvantage that the method has.
Expand (20 minutes): Complete the Crossword to check how much you learned about the science of leather.
Investigate (45-90 minutes): Use the internet to start investigating the vegetable tanning process in more detail. How is the vegetable oil made for tanning leather made? Then spend some time looking up native Tanzanian plants. Find a plant that you think would be a good choice for Dr. China’s research. Follow along on the worksheet.
Challenge (1-2 hours): Covalent bonds are an important part of the tanning process. This lab will challenge students to create their own molecules held together by covalent bonds.