A REPORT ON
SOLING MATERIAL USED IN LEATHER FACTORY OF
BATA SHOE COMPANY (BANGLADESH) LTD.
What is Sole?
The sole is the bottom part of the shoe. Also referred to as the “outsole” of the shoe, this is the part that comes in direct contact with the ground.
Outsoles can be made of a variety of materials, including leather and rubber. Certain types of soles provide more traction than others, by using specific materials or designs. A ballroom dancer might prefer a shoe with a smooth leather outsole, as it’s easier to turn and glide in, while a hiker needs an sole that is thick, durable and waterproof.
The sole of the shoe is often the part that will wear out first, but some shoes can be resoled by a shoe repair shop.
The sole is the most basic part of a shoe. The sole is a protective layer of material between the foot and the ground. The word itself comes from the Latin word solum, which means soil. Soles have a wide variety of functional properties such as wear, flexibility, traction, insulation to mention a few. These properties vary in accord with the type of footwear. Most shoes today use a “unit sole” which means the sole and heel are combined into one unit. The unit sole is applied in one operation and reduces the cost of production.
Soling material:
The sole is a critical component of any item of footwear. It protects the foot from the ground, and contributes substantially to the structural integrity of the shoe. The user must ensure that footwear chosen protects against the risks involved in the work place, that the styles and materials are compatible with the working conditions in terms of both withstanding the effects and protecting the wearer against them.
The performance of footwear depends on the overall design and construction. i.e. the upper, lining, insole, midsole and outsole all contribute. The sole of a boot or shoe is important because it is in direct contact with the ground. It should help to:
- Provide the wearer with a stable base.
- Reduce risk of injury through shock when running, jumping etc.
- Provide traction and reduce risk of injury through slipping.
- Give insulation from sharp projections on the ground,
- Give heat insulation.
- Provide an acceptable wear life under specified conditions.
The relative importance of these factors depends on the purpose of the footwear, e.g. Indoor slippers, town shoes; industrial safety footwear, army boots and sports shoes all have very different requirements. Even the design of a sports shoe sole will depend on the type of sport. Rock climbing, fell walking, skiing, jogging, squash, football etc. all involve different types of movement on different types of ground surface.
In spite of the diversity of applications, it is possible to give some general design guidelines. The following applies mainly to moulded sole units, but is also applicable to direct moulded soles and to sheet soling.
Soling Thickness:
The thickness of a sole must be sufficient to provide an acceptable level of ground insulation, and an acceptable wear life. A minimum of 2mm in, any type of sole is needed to give a stable base for the tread.
Properties of an ideal soling material:
Processing needs:
· Low cost raw materials.
· Cheap to process.
· Easy to mould with good definition.
· Form strong bonds with conventional adhesives
· Can be recycled
Wearing needs:
· Low density (light weight for comfort)
· High elasticity (Soles must not spread especially in hot conditions)
· High resilience (for energy return)
· High flexibility (for comfort in walking)
· Good flex crack resistance, especially in cold condition
· Good wet and dry slip resistance (for safety)
· Good abrasion (wear) resistance.
· Good water resistance (for comfort)
· Good rest resistance.
· Good cushioning ability (for comfort)
Ancient leather sole
· Good shock absorption (to protect wearer from injury)
· Good ageing resistance.
· Good appearance.
TYPES OF SOLING MATERIALS:
Leather is the traditional soling materials for footwear. But now days things have been changed, most soles are made of rubber or pastic. This kind of soling esed depends on such factor –
· Fashion
· Case of manufacture
· Price
· Durability
· Satisfactory source of supply
The effect of these developments 1st felt in USA in about 1948. For these USA used as 83% non leather soling material which known as synthetic soling materials. Although 1940-50 is the turning point. In this period of time various kinds of soling materials are produce. Like composition rubber soles, crape sole, resin rubber, neolite, auonite etc.
After that in 1952, cellular soling was performed in USA names such as Crush-N-Crepe & Naron crepe [ref: American trade journals]
Then the term microcellular resin rubber was widely adopted throughout the world, whice is the composition of those din not differ from normal resin rubber.
There is no one type of outsole that is regarded “best” for all footwear.
There are different types of soling. materials. They are:
1. Leather
2. Leather board
3. Resin rubber
4. Vulcanized rubber-solid -cellular
5. Crepe rubber
6. Thermoplastic Rubber (TR)-1.Solid , 2.Cellular
7. PVC (Solid)
8. PVC (Cellular)
9. PVC (Blends)
10. PU reaction moulded-1 Cellular, 2. Solid
11. Thermoplastic PU-1.Solid, 2.Cellular
12. EVA (Cross-linked)
13. EVA (Thermoplastic)
14. Nylon (PA)
15. Polyester, solid, units
16. Polycarbonate.
17. Hytrel (EEC thermoplastic Elastomer)
Soling material used in leather factory of BSO:
1. PVC
2. PU
3. TPR
4. RUBBER
5. EVA
PVC (Poly vinyl chloride):
Chemical structure:
Physical properties:
1. ‘Plastic’ vinyl smells familiar to most people.
2. A relatively heavy material in solid form.
3. Soft grades can feel rubbery, harder grades not.
- Fairly glossy, shiny surface.
- Tends to wear very smooth.
6. Not found with midsoles in other materials but sometimes combined with a leather forepart in dress shoes.
7. All-moulded boots such as Wellingtons are very often PVC.
8. Cellular versions have reduced weight but same smell.
9. They may have a solid skin with an irregular cell structure, or a uniform cell structure with finely speckled small surface.
Mechanical and Chemical Properties:
Vinyls are strong, with excellent resistance to damage caused by water and chemical wearing. However, they are ruined by prolonged exposure to sunlight. They are available in a wide variety of colors. Vinyls range from being hard and rigid to soft and flexible. Vinyls are self-extinguishing.
PVC sole
Cross section:
There are air bubbles in PVC. The bubbles are larger in blended PVC and less in solid PVC.
Polymer identification:
The material burns but extinguishes itself on removal from the flame.
| Polymer | Flame color | Odour | Other features |
| PVC | Yellow with green base | Acrid | Acidic fumes |
Properties of material:
| Properties | SI unit | PVC unit sole | PVC sandak ( super expanded PVC) | ||
| Min | max | Min | Max | ||
| Specific gravity | 1.20 | 1.26 | .45 | .55 | |
| Hardness | IHRD | 60 | 70 | 35 | 55 |
| Cut growth flex | K.CYCLES | 1000 | 1000 | ||
| Shrinkage | % | 2 | 2 | ||
Applications:
Vinyls are used in automobile seat covers, shower curtains, raincoats, bottles, visors, shoe soles, garden hoses, and mannequins.
PU (poly urethane):
Chemical structure:
Physical properties:
· Polyester type – no particular smell.
· Poly ether type – distinctive quite strong non rubbery smell
· Not a heavy material – lighter than solid rubber or PVC.
· May feel rubbery but often quite slick with sheen to the surface.
· Has an expanded or microcellular structure, but this may not be evident as
moulding produces a thin solid skin and usually there are no cut edges.
· Soles may be moulded with or without a tread pattern.
· A key feature is very small voids (caused by trapped air bubbles in mould)
along the edges and at the corners of moulded detail such as tread cleats and lettering
· All colors possible.
· Often used in thicker soled footwear where it may be moulded to resemble cork or wood.
· Sometimes includes a softer, lighter PU midsole layer for extra cushioning and is then known as a dual density sole.
Chemical Properties:
Polyurethanes are in the class of compounds called reaction polymers, which include epoxies, unsaturated polyesters, and phenolics. A urethane linkage is produced by reacting an isocyanate group, -N=C=O with a hydroxyl (alcohol) group, -OH. Polyurethanes are produced by the polyaddition reaction of a polyisocyanate with a polyalcohol (polyol) in the presence of a catalyst and other additives. In this case, a polyisocyanate is a molecule with two or more isocyanate functional groups, R-(N=C=O)n ? 2 and a polyol is a molecule with two or more hydroxyl functional groups, R’-(OH)n ? 2. The reaction product is a polymer containing the urethane linkage, -RNHCOOR’-. Isocyanates will react with any molecule that contains active hydrogen. Importantly, isocyanates react with water to form a urea linkage and carbon dioxide gas; they also react with polyetheramines to form polyureas. Commercially, polyurethanes are produced by reacting a liquid isocyanate with a liquid blend of polyols, catalyst, and other additives. These two components are referred to as a polyurethane system, or simply a system. The isocyanate is commonly referred to in North America as the ‘A-side’ or just the ‘iso’. The blend of polyols and other additives is commonly referred to as the ‘B-side’ or as the ‘poly’. This mixture might also be called a ‘resin’ or ‘resin blend’. In Europe the meanings for ‘A-side’ and ‘B-side’ are reversed. Resin blend additives may include chain extenders, cross linkers, surfactants, flame retardants, blowing agents, pigments, and fillers.
PU sole
Identification:
Cross section:
The PU sole is solid in its inner structure but there are air bubbles on the surface of the PU sole.
Polymer identification:
The material burns but extinguishes itself on removal from the flame.
| Polymer | Flame color | Odour | Other features |
| PU | Yellow with blue base | Acrid |
Properties of material:
| Properties | SI unit | PU sole | |
| Min | max | ||
| Specific gravity | .40 | .60 | |
| Hardness | IHRD | 55 | 70 |
| Cut growth flex | K.CYCLES | 300 | |
Applications:
PU is used as footwear sole, adhesive, finishing chemical of wood, insulating foam, sealing etc.
TPR (thermo plastic rubber):
Chemical structure:
Physical properties:
· Superficially similar to vulcanized rubber
· Carries a distinctive synthetic smell which is quite different from VR.
· Very rubbery feel (grippe).
· Quite soft as judged by easy indentation, but surprisingly stiff.
· Shined surface finish.
· Available in all colors.
· A less even quality of appearance than VR with some hairline moulding
· `Flaws’ apparent in some locations on the surface.
· Not found with midsoles in other materials but sometimes two color
mouldings can mimic an outsole / midsole assembly.
Chemical Properties:
Thermoplastic rubbers based on styrene-butadiene-styrene block copolymers were introduced for footwear in the late 1960s, offering rubbery appearance and properties with the simplicity of thermoplastics processing. Initially used to simulate natural crepe rubber, thermoplastic rubber has proved attractive for many styles of everyday and fashion shoes, especially with thicker or platform soles.
Compounding:
Elastomers in a versatile hardness range can be produced by compounding. Extending the elastomeric matrix with processing oils improves flow during molding, softens the material and reduces its cost, generally at the expense of wear resistance. Extending the polystyrene domains with compatible polymers, such as polystyrene, hardens the base polymer and improves wear to some extent, but large amounts may inhibit adhesion. Including a third discreet mineral phase serves to harden and cheapen.
TPR Sole
Identification:
Cross section:
The TPR sole is solid in its inner structure.
Polymer identification:
The material burns but extinguishes itself on removal from the flame.
| Polymer | Flame color | Odour | Other features |
| TPR | Yellow very smoky | Styrene |
Properties of material:
| Properties | SI unit | TPR sole | |
| Min | max | ||
| Specific gravity | 1.00 | 1.10 | |
| Hardness | IHRD | 55 | 65 |
| Cut growth flex | K.CYCLES | 300 | |
Applications:
Superior in quality, the thermoplastic rubber offered by us is used in various industrial applications such as:
Footwear:
In footwear industry, thermoplastic rubber is used in the production of shoes soles, loaters sole, safety shoes sole & industrial shoes sole, sports shoes sole, ski-boot soles, kiddy shoes sole and related decorative accessories, unisole. modifier asphalt, modification modifier for SMC (sheet moulding compound) and other thermoset & thermoplastic composites.
Automotive, Sports, Leisure (General):
In automotive industry, thermoplastic rubber is used in profiles, gaskets, lip-seals, tubings, pipes, co-extrusion automotive gasket, o-ring, bushings, bellows, floor mat, protecting covers, automotive grip. food & medical, ball pen grip, tooth brush grip, umbrella handle grip, milk tubing, disposable medical product, beverages.
Electrical &. Electronics:
In electrical and electronics industry, thermoplastic rubber can be used to manufacture welding cable, jacketing, flexible cord, primary wire, fire retardant control cable.
Rubber:
Chemical structure:
Physical properties:
· Rubbery smell familiar to most people.
· Rubbery feel (grippe).
· Very good appearance with excellent definition of edges and fine detail.
· Malt surface finish or slight sheen.
· All colors possible including natural translucent.
· Very soft versions sometimes labeled ‘Latex’.
· Often found as an outsole combined with a softer PU or
· EVA midsole. Soles which are VR only are relatively heavy,
· On industrial footwear may be marked ‘heat resistant’.
Chemical Properties:
Rubber can occur naturally in the environment or be produced synthetically in a laboratory. In nature, it exists as a milky colloidal suspension that is found in the sap of certain plants, such as the Para rubber tree. Approximately 60 percent of rubber is produced synthetically by deriving it from petroleum.
Rubber is an elastomer, which is an elastic polymer that has high molecular weight compounds consisting of long chains of hydrogen and carbon molecules. This polymer is amorphous, allowing for molecular motion at room temperatures, and, thus, it remains soft.
Rubber sole
Identification:
Cross section:
There are very small air bubbles in the inner structure of rubber sole.
Polymer identification:
The material burns but extinguishes itself on removal from the flame.
| Polymer | Flame color | Odour | Other features |
| PVC | Yellow smoky | Acrid | Acidic fumes |
Properties of material:
| Properties | SI unit | Rubber sole | |
| Min | max | ||
| Specific gravity | 1.35 | ||
| Hardness | IHRD | 60 | 70 |
| Cut growth flex | K.CYCLES | 500 | |
| Abrasion | Mm3/2000cycles | 80 | |
Applications:
Compared to vulcanized rubber, uncured rubber has relatively few uses. It is used for cements; for adhesive, insulating, and friction tapes; and for crepe rubber used in insulating blankets and footwear. Vulcanized rubber, on the other hand, has numerous applications. Resistance to abrasion makes softer kinds of rubber valuable for the treads of vehicle tires and conveyor belts, and makes hard rubber valuable for pump housings and piping used in the handling of abrasive sludge.
The flexibility of rubber is often used in hose, tires, and rollers for a wide variety of devices ranging from domestic clothes wringers to printing presses; its elasticity makes it suitable for various kinds of shock absorbers and for specialized machinery mountings designed to reduce vibration. Being relatively impermeable to gases, rubber is useful in the manufacture of articles such as air hoses, balloons, balls, and cushions. The resistance of rubber to water and to the action of most fluid chemicals has led to its use in rainwear, diving gear, and chemical and medicinal tubing, and as a lining for storage tanks, processing equipment, and railroad tank cars. Because of their electrical resistance, soft rubber goods are used as insulation and for protective gloves, shoes, and blankets; hard rubber is used for articles such as telephone housings, parts for radio sets, meters, and other electrical instruments. The coefficient of friction of rubber, which is high on dry surfaces and low on wet surfaces, leads to the use of rubber both for power-transmission belting and for water-lubricated bearings in deep-well pumps.
EVA (Ethyl vinyl acetate):
Chemical structure:
Physical properties:
· Characteristic non-rubbery smell when new but this fades.
· An extremely lightweight material,
· Usually soft enough to be impressionable with a finger.
· May have a slick (moulded) surface- with a sheen or else a matt velvety surface which is a split through the microcellular structure. May have cut or moulded edge.
· Tread pattern often shallow, with better definition than PU (no small voids).
· Available in all colors, but not translucent.
· Generally good appearance,
· Often found as a midsole (shock absorbing) layer in sports shots,
· (Micro VR generally similar but smells rubbery and not quite so lightweight).
Chemical Properties:
Despite the fact that copolymerisation with vinyl esters, such as vinyl acetate, was studied in the very early days of polyethylene production, it is only comparatively recently that these materials have made an impact in injection moulding. Copolymerising vinyl acetate (VA) with ethylene can disrupt the crystal structures that are present in polyethylene (PE) and will eventually give an amorphous, thermoplastic material. By varying the percentage of VA in the composition, polymers with significantly different properties are produced. As the percentage of VA is increased the transparency and flexibility of the copolymers increase. Injection moulding grades of EVA contain from 4 to 30 per cent of VA and such materials are tough, semi-opaque thermoplastics with a comparatively low upper working temperature, for example, 65 degC. EVA is softer, clearer and more permeable than LDPE; has better environmental stress cracking resistance; is tougher; and will accept fillers more readily. Some grades of EVA retain their flexibility at temperatures as low as -70 degC.
EVA can be cross linked by peroxides and may also be chemically blown to give cellular products; the density of cross linked, foamed EVA can be much lower than is possible without cross linking. Such cross linked, foamed EVA can have a very fine structure, similar to that of micro-cellular rubber. The inherent flexibility, good processing characteristics, absence of plasticizers’ to migrate, and low odor, make this material an attractive alternative to PPVC, and to natural and synthetic rubbers.
EVA sole
Identification:
Cross section:
The inner structure of EVA sole is like sponge.
Polymer identification:
The material burns but extinguishes itself on removal from the flame.
| Polymer | Flame color | Odour | Other features |
| PVC | Yellow smoky | Vinyl acetate | Black residue |
Properties of material:
| Properties | SI unit | Rubber sole | |
| Min | max | ||
| Specific gravity | .28 | .35 | |
| Hardness | IHRD | 45 | 55 |
| Cut growth flex | K.CYCLES | 300 | |
| Abrasion | Mm3/2000cycles | 2 | |
| Shrinkage | % | 3 | |
Applications:
Footwear sole, hot melt adhesives, hot glue sticks, top of the line soccer cleats, are usually made from EVA, usually with additives like wax and resin. EVA is also used as a clinginess-enhancing additive in plastic wraps.
EVA is one of the materials popularly known as expanded rubber or foam rubber. EVA foam is used as padding in equipment for various sports such as ski boots, hockey, boxing, mixed martial arts, wakeboard boots, waterski boots, fishing rods and fishing reel handles. EVA is also used in biomedical engineering applications as a drug delivery device. The polymer is dissolved in an organic solvent (e.g., methylene chloride).
CONCLUSION:
Sole is a very important part of footwear. Durability and comfort of footwear largely depend on sole. So, it is really very important to choose the right soling material. Sound knowledge about soling material is very necessary for shoe manufacturing.
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