Footwear materials: Thermoplastic rubber
June 2010
Thermoplastic rubber (TR or TPR), one of the most common materials for making
footwear outsoles, has been in use in the footwear industry since the 1960s. TPR is a
form of synthetic rubber which is thermoplastic – meaning it melts into liquid form when
heated and becomes solid when cooled.
Not to be confused with natural rubber which comes from latex extracted from the rubber
tree, TPR is a man-made product derived from petroleum.
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, outsole material TPR is widely used
in a variety of different types from everyday fashion/casual shoes to slippers and
sneakers. In the latter, application it is often seen in the form of a thin outsole bonded to a
lightweight EVA midsole for cushioning.
There are a number of reasons for its popularity as a soling material, not the least of
which is its ease of molding. As a thermoplastic it is readily injection and molded to form
outsole 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. Other
advantages include: slip resistance of TPR solings which is superior to almost every other
common soling material; flexing resistance is also very good, performing particularly
well at very cold temperatures; it can be made in a wide variety of colors, even
translucent; 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, wear durability is not as good as some other materials and 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.
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
very poor abrasion resistance.
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