Materials for shoe soles Article taken from GEM-Chem website Shared by Deny Kyriacos and 1 more in SPE section Benelux A-Introduction The present article is a short survey on the various materials which a re most widely used by the footwear industry in the manufacture of soles for different types of shoes. In essence, a sole must: a-Give the shoe an appealing form, and therefore make it attractive to the buyer. The effect of the form on the consumer's choice is nowadays even more pronounced with casual and sport shoes. b-Absorb impact. In other words soles must impart a comfortable feeling to the foot. c-Have a good resistance to abrasion. The replacement of the eroded leather or rubber heels of street shoes and the protection of soles by rubbers is a vivid example. A tennis player expects shoes with soles which do not wear easily. d-Have a good resitance to flexure. Walking and running subject the soles to continuous flexing. Consumers expect their shoes to remain intact during the exercise whatever the temperature and extent of erosion of the soles. e-Have a good resistance to skidding. In other words the extend of friction, or the coefficient of friction, between the shoe material and various types of floors must be high. f- Special shoes like safety boots, must offer a certain degree of protection to the user. Depending upon the work environment, they must be resistant to solvents, oils, hydrolysis, static electricity, wear etc. Antistatic agents, fungicides, etc are incorporated to the polymer in order to improve several of its properties.
B- Types of shoes and their soles Depending on their use shoes fall in the categories shown below. The reader, will certainly realise that the properties expected from soles of one category are not necessarily the same as the properties of another category. In short, street shoes are made for walking. As usual, price is li nked to quality and to brand name.
1-Street/fashion shoes soles
Leather
ABS
2-Casual shoes soles
Dual density polyester PU
Polyether PU
Stitched SBR or SBS sole Leather midsole, stitched NR outsole
Flexible PVC compound
TPU sole
Moulded SBS sole
Dual density polyester PU
Natural rubber sole
SBR sole
PU sole
PVC sole
TPU sole
EVA misdsole, SBS sole
3-Sandals soles
Three layer eva sole
Plasticised PVC
Injection moulded eva SBR outsole and expanded polyester PU misdsole
EVA mids ole, SBS outsole
N atural rubber
Flexibl e PVC compound Pol yether PU soles for ladies'sandals
4-Sport shoes soles
Dual density polyester PU
Injection moulded SBR
Injection moulded TPU
Natural rubber outsole
SBR /SBS outsole, EVA midsole
NR outsole, EVA midsole Solid polyester PU outsole, EVA midsole
Golf shoe, TPU outsole
Jogging shoe, PVC sole Golf shoe, SBS outsole
Injection moulded TPU
Sailing shoe, NR outsole
5-Safety boots soles
NBR sole, expanded polyester PU midsole
NBR/PVC sole
NR (vulcanised) overshoes
Neoprene overshoes
Specially formulated PVC Dual density polyester PU
PVC compd. overshoes
C-Materials used in the production of shoe soles 1-Midsoles The examples shown above indicate that the most common materials used for midsoles are EVA(Ethylene Vinyl Acetate) and Polyurethanes. EVA is also used as sole material for sandals. a-EVA: Is the most popular midsole material, because of its low cost and light weight. The solid copolymer of Ethylene and Vinyl Acetate is expanded during processing, by the i ncorporation of azo compounds or the injection of carbon dioxide. The resulting closed cells impart to the foam a pronounced cushioning effect. EVA can be injection moulded, compression moulded and extruded. Extruded sheets of several hardnesses or colours can be flame laminated to a desired thickness and cut to shape. b-Polyurethanes: Those are two component systems, which can only be processed by machinery described in the webpage on shoe sole machinery. In short, the isocyanate is a prepolymer with a polyester polyol and the polyol component, consists of a polyester polyol, a chain extender like butanediol or ethylene glycol, water and catalysts. The resulting products are open celled. They are not however as cheap and as light as EVA foams. Due to their open celled structure, their cushioning effect is not as pronounced as that of EVA foams. However, their compression set is lower than that of EVA, therefore they last much longer.
2-Outsoles From paragraph B, above, sole materials consist, essentially of leather, thermoplastics, vulcanised rubbers, thermoplastic elastomers, polyether polyurethanes. a-Leather: Despite its vulnerability it is the preferred soling material for fashion shoes. Cattle skins are usually over a quarter of an inch thick across the shoulder and along the backbone. These areas also have the most compact and coarsest fibre structure. When tanned in their full thickness cattleskins are used mostly to make sole leather. The leather is treated with a tanning agent to stabilise the collagen fibres so that they are no longer biodegradable. Vegetable tanning (vegetable tanages use wood, bark or leaf extracts containing either derivatives of the esters of phenolic carboxylic acids or derivatives of hydroxybenzophenones) is used in the manufacture of leather of shoe soles. Sole leather should be compact and resistant to abrasion. Compactness is a function of the degree of filling with the tanning agent. Vegetable tanning materials, which have a strong affinity for collagen, can act as cementing agents and bind the fibrils together. Slowly tanned leather contains 80-90 parts by weight of bound tannins per 100 parts by weight of collagen, more rapidly tanned leather about 60 - 70 parts by weight. Rolling is also used to compress the fibre structure. Sole leather should al so be water-resistant and is sometimes impregnated with pore sealers and water repellents. b-Thermoplastics: Compounded PVC is the most widely, used, low cost material for solid as well as expanded soles for street shoes, casual shoes, sandals and slippers. Densities vary from 0.75 to 1.30, and hardnesses from 45 shore A to 95 shore A. When compounded with thermoplastic polyurethanes (TPU), PVC acquires flexibility and resistance to abrasion. PVC provides good protection against animal fats, many acids, alcohols, alkalies, bases, oils and petroleum hydrocarbons. PVC is not recommended for use in ketones, aldehydes and many solvents. Dry blending with Nitrile Rubber powder(NBR) imparts to the PVC polymer additional oil resistance, and, the compound, can therefore used in the moulding of soles for safety boots. In addition to being used in the production of heels for ladies' fashion shoes, ABS is also used in the manufacture of the whole sole and heel . Such soles are subject to limited flexing. c-Vulcanised rubbers: -Natural rubber (NR) The structure of NR is cis-1,4, polyisoprene. Its synthetic counterpart is called Isoprene rubber (IR). Its outstanding stretch characteristics and excellent low temperature properties allow the vulcanised material to stay supple in cold temperatures. It has superior slip, puncture and cut resistance compared to PVC. Rubber resists bases, acids, alcohols and diluted water solutions of most chemicals that are water soluble. It is a hydrocarbon and does not withstand constant contact with petroleum and oil based solvents. NR safety boots are used in food processing, transportation, utilities, construction, general industry, municipalities, agriculture. -Styrene butadiene rubber (SBR) Styrene-butadiene rubber is the most common and cheapest synthetic rubber. It is very weak unless reinforcing fillers are incorporated. With suitable fillers it i s a strong rubber although not approaching natural rubber or polychloroprene. Otherwise it has similar chemical and physical properties to natural rubber, with generally better abrasion resistance but poorer fatigue resistance. It is compounded, vulcanised and processed as rubber. -Nitrile rubber (NBR) NBR or nitrile elastomers are copolymers of butadiene and acrylonitrile, used primarily for application requiring excellent resistance to petroleum oils and gasoline. Resistance to aromatic hyrocarbons is better than Neoprene. NBR has excellent resistance to mineral a nd vegetable oils, but relatively poor resistance to the swelli ng action of oxygenated solvents such as acetone, methylethyl ketone and other ketones. It has good resistance to acids and bases with the exception of those having strong oxidizing effects. Resistance to heat aging is good, often a key advantage over natural rubber. With higher acrylonitrile contents, the solvent resistance is increased but lowtemperature flexibility is decreased. Low-temperature resistance is inferior to natural rubber, and alt hough NBR can be compounded to give improved performance in this area, the gai n is normally at the expense of oil and solvent resistance. NBR is used instead of natural rubber where added resistance to petroleum oils, gasoline or aromatic hydrocarbons is required. NBR compounds can be injection moulded and vulcanised with a S/accelerator system. NR safety boots are used in food processing, transportation, utilities, construction, general industry, municipalities, agriculture -Neoprene (Chloroprene, CR) rubber Neoprenes are a large family of rubbers that have a property profile approaching that of natural rubber, and with
better resistance to oils, ozone, oxidation, and fire. They age better and do not soften on heat exposure, although high-temperature tensile strength may be lower than that of NR. These materials, like NR, can be used to make soft, high-strength compounds. Its high density contributes even more to its high price. Neoprenes do not have the low-temperature flexibility of natural rubber, which detracts from their use in low-temperature shock or impact applications. Neoprene is resistant to a broad range of animal fats and blood, oils, certain acids, alcohols, alkalies, caustics and certain solvents. It is less resistant to cuts, punctures and abrasion that rubber. Applications of safety boots with neoprene outsoles are used in food processing, vegetable oil refining, chemical production and hazardous waste cleanup. d-Thermoplastic elastomers (TPE): Whereas rubber vulcanisates acquire their structural strength from sulphur chemical crosslinks connecting chains at the double bond sites, the strength of thermoplastic elastomers results from crystallites which act as physical crosslinks between chains. In thermoplastic polyurethanes (TPU), the crystallites consist of MDI/chain extender agglomerates, whereas in thermoplastic rubbers(TPR), the crystallites result from the precipitation of one component (Polystyrene) from the polymer (Styrene butadiene) matrix. Thermoplastic vulcanisates (TPV) where double bonds crosslink under the effect of processing temperatures, will not be discussed here since t heir use as sole materials is not so wi despread. -Thermoplastic polyurethanes Poly(caprolactone), polyester based and butane diol extended MDI polyurethanes result in injection moulded elastomers with superior abrasion resistance. They constitute the preferred material for tennis, running, soccer, etc. shoe soles. On the other hand, polytetramethylene ether glycol based thermoplastic polyurethanes have a better resistance to hydrolytic degradation but their abrasion resistance is relatively low. -Styrene Butadiene Styrene SBS synthetic rubbers (styrene-butadiene-styrene) are sequenced styrene and butadiene polymers. They are formed of polystyrene (PS) and polybutadiene blocks (PBu). The PS phase provides hardness, whereas the PBu phase provides adherence. The main descriptive parameter of SBS is the styrene/butadiene ratio. The styrene content is between 29 and 55%. Low content styrene SBS's are called " soft grade " SBS's. High styrene content TPR's result in hard soles. e-Liquid polyurethane systems -Polyester systems Polyester polyol, and butane diol extended systems react with an polyester MDI prepolymer, in the absence of water to give solid outsoles with excellent properties, which are only second to TPU's. Such systems can be processed by open pouring as well as by l iquid screw injection machines. -Polyether systems Polyether polyol, butane diol extended systems, blown with either water or a a blowing agent react, in a closed mould, with a polyether MDI prepolymer, in the presence of catalysts and surfactants, to give soles with a solid skin and a microcellular core. They combine, good flow, light weight and good properties. f-Properties All materials and compounds must result in shoe soles with excellent flex fatigue characteristics. The table below shows some properties ot the elastomers described above.
For information and comments, contact Dr Demosthenes Kyriacos, President, GEM-Chem, E-mail:
[email protected] ; or phone +32-2-7710649 D.Kyriacos has worked at Upjohn, GE and I CI in international TS, Sales and Marketing. He holds a B.Sc.(Distinction, Honours, Univ. award of Chemistry) from Alexandria, a M.Sc., course,(ICI scholarship award) in Polymer Technology and, a Ph.D. from Loughborough (UK).
D. Kyriacos is the founder of DK Business Group and GEM-Chem. Deny Kyriacos: LinkedIn profile
