TPR is a man-made product derived from petroleum
Thermoplastic rubber (TR or TPR), one of the most common materials for making footwear out soles. TPR is a form of synthetic rubber which is thermoplastic – meaning it melts into liquid form when heated and becomes solid when cooled.
TPR is a man-made product derived from petroleum, and not to be confused with natural rubber which comes from latex extracted from the rubber tree. When discussing TPR as used in the footwear industry we are generally referring to a block copolymer called SBS, which stands for Styrene - Butadiene - Styrene. “ Block copolymer ” means that there are blocks of each monomer (Styrene and Butadiene ) formed within the polymer instead of a random distribution. Styrene is a cyclic hydrocarbon with the formula C6H5CH=CH2 and is also known as vinyl benzene. The butadiene monomer has the formula C4H6.
Although it is not suited to every type of footwear, TPR outsole material is widely used in a variety of different types from everyday fashion or casual shoes to slippers and sneakers. In the sneak, it is often seen in the form of a thin outsole bonded to a lightweight EVA midsole for cushioning. There are a lot of reasons for its popularity as a soling material.
As a thermoplastic it is readily injection and molded to form out sole units, typically at a temperature of around 180 °C. Just a couple of minutes later the TPR will have solidified and the outsole can be removed from the mold. So it is easy processing.
Other advantages include:
1. Slip resistance of TPR outsole which is superior to almost every other common sole material;
2. Flexing resistance is also very good, performing particularly well at very cold temperatures;
3. It can be made in a wide variety of colors, even translucent;
4. good adhesion strength to upper/midsole materials can usually be achieved, if the bonding surface is properly pre-treated using halogenation - a process which adds chlorine to the polymer structure.
On the negative side,
1. Wear durability is not as good as some other materials.
2. TPR usually performs relatively poorly in abrasion resistance testing. This makes it unsuitable for work boots or other applications where a long wear life is desirable.
3. TPR is available in a range of hardness’s and this will affect the above properties to some extent. For example a softer grade would be likely to have excellent slip resistance but poor abrasion resistance.