Dear Mr Sundaram,
Sole is the soul of the safety footwear.
Sole has to be looked on various parameters like:
1. Mode of attachment to the upper: Stiched, Stuck on, Goodyear welted/ Direct Injected (moulded) or a combination thereof.
2. Material of the sole which can be single material like rubber/PU/PVC/TPR/TPU or it can be a combination like Rubber+PU; TPU+PU etc
3. Density: Single or Double.
Typically in safety footwear you prefer a direct injected sole as the sole material attaches directly to leather fibre giving a very good long lasting bonding. Its important that material used in sole is also act as a good adhesive. PU is very good in this respect. In case of PU the direct injection is by two process. 1/ Direct Pouring Process which is inferior as compared to Direct Injection process where closed moulds are used & PU is injected with lot of controlled parameters.
The material to be used in sole depends upon Electrical, thermal & Chemical hazard against which footwear is to provide protection. PU is good upto 120 degree Celsius while Natural Rubber is good upto 160 degree Celsius & Nitrile upto 300 degree celsius.
Pu footwear are typically antistatic while rubber are Electrical Hazard footwear. For conductive footwear rubber sole footwear are used.
Nitrile rubber provide good chemical resistance against Acid, Alkali & Fat.
Sudhir