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Footwear manufacturing is one of the world’s oldest and most globalised industries. Traditionally, shoes were made from leather, wood or canvas, but increasingly they also can be made from rubber, plastics, and other petrochemical-derived materials. The manufacture of footwear involves many different steps and a basic process is shown in Figure 1. For various types of shoes, such as Oxfords, derby, high-heels, sneakers, sandals and athletic shoes etc., different materials and processes may apply.
 
 
Figure 1. Basic Shoe Manufacturing Process

Basic footwear components can be made from leather, textile, plastics, metal, rubber and other synthetic compounds. A diverse range of chemicals are used in various manufacturing steps such as leather tanning, dyeing, finishing, sole manufacturing, leather finishing and footwear manufacturing. As global consumers become increasingly conscious of health and the environment, growing attention has been paid to hazardous chemicals and restricted substances used in the footwear production process.
 
In the Rapid Alert System for non-food dangerous products (RAPEX) in the European Union (EU) for example, in 2013, footwear and leather accounted for more than 70 recalls due to restricted substance (RS) non-compliance.  More than 90% of the cases were due to chromium (Cr) VI, which was followed by phthalates, dimethyl fumarate (DMFu), nickel release, pentachlorophenol (PCP), banned azo dyes (carcinogenic aromatic amines), polycyclic aromatic hydrocarbon (PAH), etc.
 
In addition to product recalls, reports also have been published concerning worker intoxication involving Volatile Organic Chemicals (VOC) generated from latex, neoprene and adhesives used in footwear manufacturing. Toluene and formaldehyde are also popular VOCs.

LEATHER FOOTWEAR

Chromium VI is considered to be a high risk parameter for leather footwear, because more than 85% of all leathers produced globally use chrome-based tanning technology. Tanning can help to improve leather’s dimensional stability and its resistance to mechanical action and heat, but it can also bring in Cr VI as a major risk., Although Cr VI is not intentionally added during the tanning process, it may be generated in situ in leather through the oxidation of Cr III compounds, which are added to crosslink collagen subunits in the raw hide. To minimize the risk, proper tanning recipes and processes should be planned ahead, and the necessary amount of antioxidant auxiliaries such as magnesium chloride also should be considered and used. Proper pH control should be applied in neutralization steps in tanning in order to control the formation of Cr VI.
 
Besides in the manufacturing process, formation of Cr VI from Cr III in leather has been reported during shipment and storage. Temperature and humidity must be well controlled during the manufacturing process, shipment and storage, as high temperature and low humidity favour chromium VI formation. Besides Cr VI, the use of PCP and other  chlorophenols should be of concern as they are sometimes used as preservatives and biocides for leather.

PLASTIC & RUBBER COMPONENTS IN FOOTWEAR

Phthalates are important concerns for plastic and rubber footwear. Phthalates or phthalate esters are a group of chemical compounds that are mainly used as plasticizers to increase the flexibility of plastic. For example, polyvinyl chloride (PVC) can be turned from a hard plastic into a flexible plastic by adding plasticizers. Substitutes for phthalates may include acetyl tributyl citrate (ATBC), alkylsulfonic phenylester (ASE), diisononyl adipate (DINA) etc.  However, appropriate choices can only be made after careful selection and product development.
 
Besides phthalates, special attention should be paid to nitrosamines and the vulcanization process in rubber footwear. Nitrosamines are often used in the manufacture of rubber products and are carcinogenic. Although no footwear has been recalled because of nitrosamines, relevant recall cases in cosmetics and toys have been reported by RAPEX; Vulcanization is a chemical transformation which converts rubber or related polymers into more durable materials with superior mechanical properties via the addition of sulfur or other equivalent curatives in a heating process in an oven. Vulcanization is generally not a risky process, but some conventionally used accelerators are problematic. For example, ethylene thiourea (ETU) as an important accelerator for neoprene rubber is reprotoxic. The European rubber industry is in the process to develop a safer alternative to ETU.
 
For both leather and plastics/rubber components, desiccants are used during transport and storage to prevent mould growth. Some desiccants may contain DMFu that can cause a skin allergy. DMFu has been a major recall item for footwear before 2012 and involves hundreds of cases. In 2013, however, only 1 case was reported in RAPEX. This may due to increasing awareness by brands and suppliers, and the use of alternative anti-mould agents.
 
For metal parts and accessories on footwear, nickel release should be strictly monitored and it is better to only use qualified metal parts after proper processing and testing. For VOCs which are present in adhesives and solid materials such as EVA, proper ventilation is necessary and appropriate personal protective equipment (PPE) such as gloves or masks should be worn by workers to avoid the health risk.

In summary, challenges coming from the complex chemicals used in the footwear supply chain are now faced by every buyer and supplier. To have a sound management of hazardous chemicals, testing is an indispensable measure, but control from the upstream is also a choice which cannot be ignored.
 
Find more information on SGS Services for the Textile and Footwear Industry.

Karen E. Kyllo, Ph.D.
Deputy Vice President, Global Softlines
SGS North America Inc.
t: +1 973 461 7934
 
Jane Jiang, PhD
Softlines Technical Director of Asia
Pacific, Global Softlines
SGS-CSTC Standard Technical Services Co. Ltd.
t: +86 21 61402666

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