Over the last 40 years there has been a major change in soling materials used in the Footwear trade. Leather has been the traditional soling materials or Footwear. But in 1980 only about 5% of shoes have leather soles. Nowadays most soles are made of rubber or plastics which are classed as synthetic soling materials. So new types of soling needs new types of adhesion method or technique with different types of upper materials.
The scientific study of adhesion is quite new and its name, Zygology, is scarcely known. Zygology is the science of joining things which includes the study of adhesion and of welding, riveting and all forms of mechanical jointing. It includes the principle of-
Specific adhesion: It gives a chemical bond between the adhesive and the surfaces being joined. The adhesive, therefore, does not have to penetrate the material but is bonded to the material by chemical action.
Mechanical Adhesion: It gives a physical bond which the adhesive “keys” in to the fibers or structures of the material to be bonded.
In English, the term “shoe” dates back to many centuries beginning with the Anglo – Saxon. “Sceo” means a foot covering and evolving into “Schewis”, then “Schooys” and finally “shoe”. The German “Schuh” has the name origin. Through the centuries the word “shoe” has evolved with at least 17 different spelling and some 36 variations of the plural.
Any foot covering in tae form of shoes, boots, slippers or hose used for utility and dress wear.
Any form of footwear made of various kinds of materials or combination of materials like leather, canvas, rubber, textiles, wood and synthetics to protect the foot from cold, heat, thorns, hazards etc. and to serve as a costume in the form of sandal, shoe or boot. These shall include walking shoes, dress shoes, occasional footwear, sports footwear, occupational footwear, orthopedic and surgical footwear meant for the use of babies, children, ladies or gents.
PURPOSE OF FOOTWEAR:
The shoe has two primary functions to perform and has acquired through the years other lesser ones. The primary functions are:
a) To protect the sole of the foot: To protect the sole of the foot from the heat, cold, dampness, dirt or roughness of the ground in walking and standing. In its simplest form this is achieved in the primitive “sandal” which is nothing more less than a piece of leather, wood or other material, fixed under the foot by a strap or other means.
b) To protect the top surface of the foot: The upper part of tae foot, and if required, the lag, from cold, rain, thorns, heat and insect or other bites. In its simplest form this is a bag of leather or material wrapped round the foot and is hare given the generic name of “Moccasin”. It is essentially that worn by a hunter, as it is flexible and specially suited to forest conditions.
c) To assist the foot to perform some abnormal task: This includes the various sports such as football, cricket, hockey, running, fishing, riding, mountaineering, dancing, etc. all of which today have their own special footwear. So, also, have many trades, such as mining, deep-sea diving, munitions manufacture and fire-fighting, and the armed services. It should be noted here that more primitive peoples with much stronger and tougher feet can tackle many of these sports, pastimes or occupations without special shoes, or, in fact, without shoes of any kind.
d) To complete a costume: This is particularly important today when costume designing includes not only the dress but also the hat, shoes. Gloves and handbag. In fact, the main purpose of the shoe may be to complete or enhance the remainder of the costume, the primary functions of foot-covering and sole-protection being subordinate to this.
e) To indicate rank or office: This is not applicable in western civilizations, unless we include the notorious “jackboot in this category, but formerly it was important, certain types of shoe being restricted to certain classes; there is obviously a very close link here with the purpose stated in above Para.
f) To overcome abnormalities: In the foot itself, the surgical boot being the extreme example of this, while many shoes incorporate corrective devices, some more corrective in name than in fact.
HISTORY OF FOOTWEAR:
Without footwear a human being cannot live. Even the primitive human being might have covered his feet for warm the production.
In English, the term Shoe dates back to many continues beginning with the Anglo-Saxon SCEO Meaning a Foot covering evolving in to Schewis” then “schooys” finally Shoe.
At first might well have been pieces rush, bark or hide tied around the feet to project the shoe. The Egyptians covered their feet by woven palm and papyrus leaves.
Men have been wearing hoses possibly science 10.000 BC. It was not until about 1850 that the were first made as “Right and lefts” prior to that, all shoe wear “straights”
In the 13th country stylish people started wearing shoe with long, pointed toes. Narrower, Heeled shoe for woman wear trend during the region for Queen Elizabeth. The end the seven countries brought renewed popularity to low shoes, with red heels, square toes, and enormous butterfly bows that were eventually replaced by huge buckles.
The 18th century bore the influence of French fashion.
Decorated leather shoes or silk shoes on high heels set under the arch of the footwear in fashion.
Modem shoes, as we knows them today, probably have there roots in Tudor and Elizabethan, England shoes were made with bottoms, Lace, Eyelet, and in general decoration of all finds .
The application of computer in shoe production took place towards middle of the 20th century. Today computer is used for last & shoe design, cutting, stitching, lasting & finishing.
Anatomy of the shoe
According to McPhoil (1988) the anatomy of a shoe can be divided in an upper and lower (or bottom part). Sections of the upper include vamp, quarter, toebox, throat, insole board, and topline. The sections of the lower shoe consist of an outsole, shank and heel.
The Upper of the Shoe
All parts or sections of the shoe above the sole that are stitched or otherwise joined together to become a unit then attached to the insole and outsole. The upper of the shoe consists of the vamp or front of the shoe, the quarter i.e. the sides and back of the shoe, and the linings.
The vamp covers the dorsum of the foot (includes the tongue piece) and superior aspects over the toes. This section i.e. the toe puff is reinforced which serves to give the shoe its shape as well as protect the toes.
The complete upper part of the shoe behind the vamp line covering the sides and back art. The top edge of the sides and back of the quarter describes the top line of the shoe. In athletic shoes the top line is often padded and referred to as a collar. The medial and lateral sections join in a seam at the posterior end of the shoe.
Many shoes incorporate a toecap into the upper of the shoe. Toecaps are either stitched over or completely replace the distal superior aspect of the vamp and can be made into a decorative features referred to as toe tips. The toe box refers to the roofed area over and around the part of the shoe that covers the toes.
In quality shoes the quarters and vamps are lined to enhance comfort and durability. Linings may consist of various materials i.e. leathers, fabrics, and manmade synthetics.
The central part of the vamp is just proximal to the toe box. The throat is formed by the seam joining the vamp to the quarter i.e. throat line. The position of the throat line depends on the construction of the shoe, for example a shorter vamp and longer quarters define a lower throat line.
The Sole of the Shoe
The term solo derives from ‘solea’ a Latin word meaning soil or ground.
Insole (inner sold)
A layer of material shaped to the bottom of the last and sandwiched between the outsole (or misdeed) and the solo of the foot inside the shoe. The insole covers the join between the upper and the solo in most methods of construction and provides attachment for the upper,
toe box linings and welting.
This is the outer most solo of the shoe, which is directly exposed to abrasion and wear. Traditionally made from a variety of materials, the outsole is constructed in different thickness and degrees of flexibility. Ideal soling materials must be waterproof, durable and possess a coefficient of friction high enough to prevent slipping.
The shank bridges between the heel breast and the ball trod. The shankpiece or shank spring can be made from wood, metal, fiber glass or plastic and consists of a piece approximately 10cm long and 1.5 cm wide. The shank spring lids within the bridge or waist of the shoe, i.e. between heel and ball corresponding to the medial and lateral arches.
The heel is the raised component under the rear of the shoe. Heels consist of a variety of shapes, heights, and materials and are made of a series of raised platforms or a hollowed section. The part of the heel next to solo is usually shaped to fit the heel, this is called the heel seat or heel base. The heel breast describes front face of the heel.
The welt is the strip of material which joins the upper with the sole. Most shoes will be bonded by rear-welted construction. Some shoes use an imitation welt stitched around the top flat edge of the sole for decorative purposes, but it is not a functional part of the.
“The close relationship between a man and his shoe maker was based on the shared of the client’s measurements. The statistics of clients were never disclosed.”
Traditionally before mass production, the original shoemaker started the process by a footprint outline of the sole. He whittled or chiseled a wooden last from the print. A last (‘laest’, old English meaning footprint) was traditionally made from nod but are now available now in metal or plastic.
DIFFERENT RAW MATERIALS USED IN FOOTWEAR MANUFACTURE:
We know that footwear has different components such as sole, insole, shank, upper, lace, eyelet, toe-puff, stiffener etc. So we will have to mention the name of Different materials used for different components.
(A). UPPER MATERIALS:
Leather: The properties which made leather unparalleled to be used as upper materials are the following:
ii) Strength and stretch
(iii)Elasticity and plasticity
(iv) Flexing resistance
v) Foot comfort
(vi) Heat resistance
(vii) Water resistance
(viii) Surface performance
(ix)Ease of work
(x) Color fastness
(xi) Abrasion resistance
Types of leather.
Calf leathers Side leathers
- Full grain leathers Corrected grain leathers
- Printed side leathers Patent leather Suede leather Split leathers Kid leathers
- Sheep leathers
- Goat leathers Snake leathers
- Lizard leathers
- Kangaroo leathers ostrich leathers Horse leathers Buffalo leathers
(c) Swans down
(e) Combined linings
(g) Acme backer
(n) Nylon mesh
(a) Cow split,
(b) PU coated foam,
(c) Flame laminated foam
(d) PU film,
(e) Fabric, Cow full grain leather, etc
The following are used as insole material:- Leather
– Leather board – Fiber board
– Cellulose board
There are different types of soling materials. They are:
2) Leather board
3) Resin rubber
4) Vulcanized rubber-solid -cellular
5) Crepe rubber
6) Thermoplastic Rubber (TR)-l. 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 (PU)
15) Polyester, solid, units
17) Hytrel (EEC thermoplastic elastomer)
TOE PUFF MATERIALS:
- Toe puff materials are as follow:
- Vegetable tanned leather
- Nitro cellulose impregnated fabric
- Poly styrene impregnated fabric
- Thermoplastic toe puffs
- Print-on, Paint-on, etc
Stiffener materials are as follow:
- Vegetable tanned leather
- Leather board
- Fiber board
- Solvent activated plastics
- Thermo plastic counters
Shank materials are as follow:
- High grade carbon steel
- Mill board
- Fiber board
Fasteners materials are as follow:
- Buckles and straps
Adhesives materials are as follow:
- Natural Rubber/latex
- Polychloroprene adhesive
- PU adhesive
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, than 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, skilling, 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 molded sole units, but is also applicable to direct molded soles and to sheet soling.
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:
- Low cost raw materials.
- Cheap to process.
- Easy to mould with good definition.
- Form strong bonds with conventional adhesives
- Can be recycled
- 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)
- Good shock absorption (to protect wearer from injury)
- Good ageing resistance.
- Good appearance
TYPES OF SOLING MATERIALS
There are different types of soling, materials. They are:
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 (PE)
15. Polyester, solid, units
17. Hytrel (EEC thermoplastic Elastomer)
IDENTIFICATION OF SOLING MATERIALS
Lather (vegetable tanned)
- Unmistakable smell.
- Soles often labeled leather with the hide mark.
- A fairly hard material.
- May be lightly embossed but no tread, usually has smooth, slick lightly glossed surface finish.
- Magnification reveals tiny pin-prick like hair shaft openings. A cut edge will reveal the fiber structure of leather.
- Generally tan colored but sometimes has a colored finish.
- Mostly found on formal and dress footwear_ Some limited use in footwear in specialist industrial footwear.
- Light grey leather, often with a sueded surface, is probably chrome tanned and used in dance and 10 pin bowling shoes.
Vulcanized rubber (VR) (solid)
- Rubbery smell familiar to most people.
- Rubbery feel (grippy).
- 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’.
TR (thermoplastic ‘rubber’)
- Superficially similar to vulcanized rubber
- Carries a distinctive synthetic smell which is quite different from VR t
- Very rubbery feel (grippy).
- Quite soft as judged by easy indentation, but surprisingly stiff.
- Sheened surface finish.
- Available in all colors.
- A less even quality of appearance than VR with some hairline molding
- ‘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.
PU (polyurethane) (reaction-molded)
- Polyester type – no particular smell.
- Three type Polymer – distinctive quite strong non rubbery smell
- Not a heavy material – lighter than solid rubber or PVC.
- May feel rubbery but often quite slick with a sheen to the surface.
- Has an expanded or microcellular structure, but this may not be evident as molding produces a thin solid skin and usually there are no cut edges.
- Soles may be molded 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 molded detail such as tread cleats and lettering.
- All colors possible.
- soften used in thicker soled footwear where it may be molded to resemble cork or wood,
- Sometimes includes a softer, lighter PU midsole layer for extra cushioning and is then known as a dual density.
Natural Crepe rubber
A translucent pale or honey coloured sheet material with a raw appearance. Wrinkled, textured or corrugated surface but no molded sole profile or tread cleats. Quite soft and very rubbery with a distinctive slightly sweet smell unlike vulcanized or thermoplastic rubber.
- Cut edges show a layered effect at about 1 mm intervals.
- Clean fresh crepe will adhere to itself under pressure.
- Sometimes found in black or dark brown and sometimes pale crepe may have blacked or browned edges,
- Mostly used on comfort casuals.
- A firm material, superficially leather-like, generally found as a thin sole on women’s court shoes and fashion boots.
- No tread but winter boots may have a serrated forepart wear area.
- Usually has a glossy low friction (slick) surface finish but worn areas feel rubbery.
- Colour mainly black or tan to simulate leather, not translucent. Has good visual quality.
- Plastic’ vinyl smell familiar to most people.
- A relatively heavy material in solid form.
- Soft grades can feel rubbery, harder grades not.
- Fairly glossy, shiny surface.
- Tends to wear very smooth.
- Not found with midsoles in other materials but sometimes combined with
a leather forepart in dress shoes.
- All-molded boots such as Wellingtons are very often PVC.
- Cellular versions have reduced weight but same smell. They may have a solid skin with an irregular cell structure, or a uniform cell Structure with a finely speckled mall surface.
- 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 (molded) surface- with a sheen or else a matt velvety surface which is a split through the microcellular structure. May have cut or molded edge.
- Tread pattern often shallow, with better definition than PU (no small voids).
- Available in all colors, but not translucent.
- Generally good appearance.
- soften found as a midsole (shock absorbing) layer in sports shoes.
- (Micro VR generally similar but smells rubbery and not quite so lightweight).
TPU (thermoplastic polyurethane)
- Difficult to distinguish from PVC, but has no particular smell and the characteristic PVC smell will be absent.
- Solid material feels smooth and not rubbery, microcellular materials are more rubber-like.
- Mouldings have good definition, often with a matt finish.
A Fresh Look at Soling materials
Soling materials are available in a range of densities, weights, and chemical and physical properties They may be cut from sheet, fabricated, or available in complex mouldings containing several colors and types of material.
The choice of soling for a particular shoe will depend on its overall design, color, performance required and price.
Certain styles of footwear have traditionally used particular types of soling, and perhaps footwear manufacturers and bottom material suppliers could benefit from taking a fresh look at these established uses – looking at newer areas for traditional materials and vice versa.
Soling material usage
Figure 1 shows an estimated breakdown of solings used worldwide in 1991 in a total footwear production of some 10 billion pairs. Statistics like these are by their very nature quite broad and a few lines of qualification might be helped materials which cost less and last longer. For example, the proportion of UK leather-uppered footwear with non-leather soles gravy to about 90 per cent in 1990. Nevertheless, leather is still sought after for its aesthetic appeal and this is likely to continue for high class footwear. Specialty leather solings also meet the needs of some wear environments.
DEM PVC and Blends
The use of DV rubber, that is directly molded-on, ranges from soles for high quality safety boots to soles for less expensive and less durable canvas shoes for summer wear. PVC soles are roughly divided into injection molded-on and unit types, although both include straight PVC compounds and PVC blends – the most, common being PVC/Ntrile rubber.
Micro EVA and micro rubber form a substantial share but it shouldbe noted that EVA used for midsoles in training shoes is also included in Figure 1.
The PU category is mainly micro-cellular polyurethane, although some thermoplastic PU is included. All the major suppliers of polyurethane soling compounds – Bayer, Dow, Elastogran and ICI – have made strenuous efforts to replace CFCs (Chloro Fluoro Carbons) as the blowing agent, without affecting the Performance of the soling.
The main difference is claimed to be in the shrinkage characteristics. With CFC blown systems, there is about 1.5 per cent shrinkage compared with 0.5 percent for the water blown systems, which do not have a surface. For direct molded footwear, this should not present problems but fitting tolerances will be affected in unit sole production. It is important to check the fit of sole units, produced in the same moulds, before switching to a CFC-free system. Where this presents problems, some suppliers have soft CFC systems available with much reduced ozone depletion potential- that have similar shrinkage characteristics to the standard CFC blown compounds.
Solings are supplied in several forms, depending on the type of footwear being manufactured. They may also vary according to the manufacturing process, such as rubber vulcanisation, and the nature of the soling material, for example its thermo plasticity. The common forms are:
2. Caster shapes
3. Built units
4. Molded units
One question which is nearly always asked in modem footwear manufacturing is how heavy the shoe will be. The trend is towards lightness-for comfort and performance- and so the term lightweight material’ is frequently heard. Of course, this can mean low density, but it can also mean light in wear because of low soling substance.
It is probably better for the shoe manufacturer to be aware of the densities of the various soling materials on offer and then the weight of the sole will be thickness-dependent Figure 2 explains graphically the range of densities available for most of the common soling materials.
Rubber-faced composite soles
A growth area is infills for dual density PU or rubber-faced composite soles, and infill density as low as 0.25g/cm 3 has been suggested for use with a mud guarded facer. Clear, translucent solid PU facers are of interest and can be produced by careful elimination of blowing agents such as water and compression of the reaction mixture in the mould.
In the Gusbi ‘Newflex’ process, a solid molding with excellent surface definition, free from bubbles, is produced by applying high pressure after mould
A recent development in microcellular rubber solings is the production of oil resistance by incorporating nitrile rubber.
A major machinery development with microcellular EVA is the heat embossing of sheet material to produce patterned units.
Machines are available that produce units from sheet microcellular EVA by first softening with heat, then by forming between male and female moulds and finally cooling by refrigeration. BDF’s approach is to soften the sheet, press and multi-cut.
Leather was a major constituent of a shoe i.e. upper, lining, soling etc. were all made up of leather. Footwear industry started looking to other substitute material due to
- Shortage of skin and hides and its consequent high price.
- Substitute materials offered better properties in manufacture and wear which leather could not offer, e.g.. synthetics are relatively uniform in thickness, surface and physical properties and are available in continuous roll from:’
- Substitute materials are cheaper to produce.
Substitute Soling Materials
- A. Natural Rubber: Natural rubber was one of the first materials to be used in shoe soling application. Vulcanization of the Rubber was discovered by Goodyear in 183940, followed and developed by Dunlop.
Vulcanized rubber is still used in casual & canvas shoes. Recently TPNR Thermoplastic natural rubber was developed in Malaysia to avoid vulcanization process associated with Natural Rubber. TPNR is manufactured by blending of dynamically cross linked NR with PP. But commercially it did not succeed as it could not produce softer soling material below hardness of 70 shores, and also escalating natural rubber prices made it unattractive.
B. Synthetic Rubber: It was developed after researching on different synthetic materials and oils in 1920. By the end of World-11 in 1945 SBR rubber was finally taken-up as a substitute for natural rubber.
In 1920 Bata started canvas upper with rubber soles in its FECTORY AT ZLIN (Czechoslovakia). Bata made canvas upper with molded sole shoes in CENEL PRESS in 1935-45. Bata also developed rubber in action molding in 1955.
Microcellular rubber soles were developed in 1950 which was later called HAWAI in 1958. Bata developed BEIKUTE sole sheets from rubber which looked like leather sole sheet.
1) PVC- Polyvinyl chloride is a wonderful material which can be compounded to produce soles in various harnesses and can be compact or microcellular. But its higher weight maker it least preferred choice in high value high performance shoes. With the advent of SUPER- EXPENDED PVC now it is possible to reach a specific weight of 0.5, but this presents a poor resistance or abrasion and hence it is used only for seasonal articles such as beach sandals and slippers. The soles are produced on air blown machines using a foamed PVC compound.
2) TPR- Thermoplastic rubber is manufactured from oil bound S.tyrene-butadine-Styrene block copolymer. Similar to PVC this material can also be SUPER-EXPENDED to produce light weight soles. This material can also reach a specific weight of 0.5 but has a fairly good resistance to abrasion and other characteristics appreciated by the sole market. Moreover, it lends itself very well to the two-material using a semi-compact TR for the outsole and a super expanded TR as midsole.
3) EVA- Traditional System of Sheet Vulcanization.
Rubber Vs Polymers for Soling
There is a big market out there for soling materials. Global footwear production last year was around 11.5 billion pairs and is estimated that it will be 12.7 billion pairs by the year 2005. Looked at another way, the soling market consumed some five million tons of materials through the late 1990s but is expected to expand to over six million tons by 2004, an increase of nearly 30 percent in the ten years 1994-2004.
Leather was the first shoemaking material of note because it combined flexibility and moisture absorption with a degree of durability and water resistance. It could also be stitched and repaired and coloured.
Down the ages leather has main-tamed its hold on the shoe upper market but in the last fifty years or so it has relinquished its grip in solings, today being confined to niche sectors and the highest quality footwear where price and durability are not prime considerations. Interestingly, with the exception of resin rubber, there appears to have been no attempt to simulate a leather soling material. Rather, new soling materials have stood on their own merits, being accepted for the aesthetic, performance and processing benefits they themselves offer.
The first real challenge to leather came with the availability of natural rubber in the 1930s. This offered good flexibility and greater durability than leather but was itself soon usurped by vulcanized rubber, introduced around 1940, which additionally offered considerable advantages in shoemaking.
Vulcanised rubber is still top
Its use provided, in fact, a genuine revolution in shoemaking. At a stroke, all the multiple sole attaching and finishing operations relating to leather soles, for example, were reduced to just one operation. Shoes made on the vulcanized process became the favoured method of production for men’s work boots and shoes, uniform shoes and military footwear, safety footwear and boys’ and girls’ school shoes.
The productivity gains using vulcanisation to attach the sole were very significant. The boots and shoes were also superior in comfort and durability to the heavy, riveted, screwed and stitched work boots of those days. Additionally, non-slip and resistance to chemical attack are two other valuable features.
Interestingly, development in the vulcanization process is ongoing as seen in the vulcanizing adhesive introduced last November by Caswell. Their new product is very stable and opens up new exporting possibilities because it will not quickly degrade during transportation or storage, even under adverse conditions.
The popularity and suitability of the vulcanizing process to produce hardwearing footwear for service, military and safety footwear continues to this day. Indeed, vulcanized rubber is still the most used soling material with an estimated 27.5 percent global market share by pairage.
This spectacular success has been achieved and maintained in spite of a somewhat longwinded and inflexible process which is still fundamentally unchanged from when it way first developed. But, equally significant, the vulcanizing process, where the sole is attached and finished one operation, opened up the route for later technologies, such as injection molding, which are far more versatile.
PVC was the first injection mould-able soling material on the market, wearing material but in solid form it is somewhat heavy and its slip resistance tends to be low, particularly in harder grades. It also suffers from a utility image and its traditional shiny appearance somewhat restricted its range of design possibilities. Nevertheless, PVC is used in over 25 per cent of all shoe soles and it remains the second most popular soling material after vulcanized rubber, commanding virtually 20 per cent of the global market.
Whether this will remain the situation in the longer term remains to be soon, from a recent presentation, SATTRA said that PVC was chlorine-bearing and as perceived as being non-green, especially during production and disposal. It needed the addition of plasticizers to make it soft and flexible for soling applications-and these were usually of the phthalate type which are now thought to present health risks. Possibilities to overcome these problems include using alternative plasticizers, PVC blends or new polymers developed from biotechnology. All approaches were feasible but PVC would lose its cost advantage.
Range of soling materials doubles The 1970s saw a doubling of the solingmaterials available to the shoe manufacturer with the introduction of polyurethane (PU), thermoplastic rubber (TR) and thermoplastic poly-urethane (TPU). Early PU soles experienced severe and widespread flex cracking which might, have led to the material being rejected by the shoe industry. But once it was realized that processing conditions were critical and the shoe manufacturer did not enjoy the latitude available in other soling systems, PU soon gained acceptance as a gravity soling material with good performance characteristic and extensive design possibilities.
The 1980s saw the introduction of EVA which initially made its impact as a foam wedge and soon became a preferred midsole material because it was a lightweight material offering equivalent cushioning to heavier materials. Earlier processing methods were somewhat complicated but it can now be injection molded.
The newest soling material is POE-polyole of elastomer-a thermoplastic material which can be processed on conventional injection molding.
equipment and is expected to take market, share from EVA, PVC, and possibly PU.
Steve Lee, in a paper Footwear Market overview, says the real growth in soling materials has been in thermoplastic materials such as PVC. TPU, TR, EVA and Pose which can be molded on conventional injection machinery. Their, other major advantage is the fact that any scrap material can be recycled which is not the case with thermoset materials such as vulcanized rubbers and polyurethane,
Mr. Lee also makes the point that the actual choice of soling materials available is, in fact, much, greater since these primary materials can also be compounded together. For instance, TRs normally contain EVA, and TPU and PVC are commonly blended together. Also it is becoming very common for several materials to be used to build up a complete sole. Examples are VR/PU, VR/EVA, TPU/PU, and TR/TR where the combinations refer to the outsole/midsole. In all cases, the midsole are a lower density than the outsole. Sane ink resting combination, made possible by technical developments in the 1990s, is rubber polyurethane direct, soling produced in one operation on an injection molding carousel. First, rubber is injected into an open mould and is compressed. A lightweight PU midsole is then injected conventionally.
The most, recent trend is towards blowing thermoplastic materials so that TK/TR, TPU/TPU, PVC/PVC sole combinations can be produced using; like materials with the midsole being low density.
VR and PVC will continue on top Looking to the future, SATRA anticipates that the overall usage of leather, natural crepe and resin rubber are all likely to decline slightly in percentage terms though not necessarily in tonnage terms. In contrast, vulcanized rubber, PU, TPU and TK should while PVC will stay the same.
ABOUT POLY VINYL CHLORIDE(PVC)
PVC Injection Molding
1. Scour and rough
Roughing for direct molding must always be slightly over the edge of the insole because the sole actually has a small ‘wall’ around the feather edge.
2. Attach shank and filler block.
Special lightweight shank and heel fillers are stapled onto the bottom of the insole to reduce the weight of the sole and the amount of PVC needed.
3. Bottom Cementing
A special polyurethane adhesive is used, and at the same time a piece of paper is often stuck onto the bottom of the shoe to prevent any molten PVC from creeping into the inside of the shoe through the lasting allowance, this is particularly important in the case of tack lasted work.
PVC Injection mould
There are many types of injection molding machines in current use, many of which do not mould directly onto the shoe bottom, but produce molded units for the stuck on process. Most molding machines for direct molding have between 24-36 ‘stations’ each station holds one set of moulds. Because of the amount of stations, by the time a shoe gets back to the injection area the sole which is on it has cooled and is ready for removal. Smaller machines have refrigerated moulds to assist cooling.
A set of moulds consist of a left or right sole mould, with the attendant side moulds and last mould, the moulds are made from metal alloy.
The shoe is slipped onto the metal foot or last, which usually has a device for shortening its length to allow easy entry and exit of the shoe, the last then turns and presents the shoe into the sole mould as the side moulds close in and grip the shoe around the edge of the insole, since the moulds are locked under low pressure, the heated injection screw (165°-175″c) forces the molten PVC into the mould. When the mould is full, a small plunger fitted to the mould surface is forced upwards cutting off the molten PVC supply, alternatively an air valve is fitted which cuts out once the mould is full.
The sole is then left to cool, still inside the moulds for several minutes, meanwhile the machine feeds the next station up to the molding area and the process begins over, by the time the first station returns to the molding area the moulds have opened automatically and the shoe can be slipped off.
After slipping, a strip of plastic known as the ‘sprue’ is left protruding from the sole, this was where the PVC was injected into the mould, this is now removed by a rotating cutting blade or by hand knife, the shoe is then ready for shoe rooming.
1) It has the lowest unit cost of all man-made soling materials currently available. This is because the race materials are cheap and because a low cost, high speed screw-injection process can be used. In addition compounds can contain up to 10% of recycled scrap without serious loss of properties.
2) It is very versatile because it can be blended with a wide range of plasticizers and other plastic materials in a wide range of proportions. This allows considerable control over the properties.
3) Wear resistance is satisfactory provided the compound is not too hard, and can be quite good if blended with modifiers like Thermoplastic PU or Nitrite rubber.
4) Good mould definition. Smooth surfaces are easy to obtain, although sometimes a little too shiny.
5) PVC has flame-retardant properties. It will bum but the flames are self-extinguishing once the source of ignition is removed.
6) Sensitive to heat, giving off toxic corrosive fumes, as well as being a health hazard, the fumes damage molding equipment. Heat establishers help to reduce the problem but molding temperatures must not be allowed to exceed the recommended values. Maintaining the compound in a molten state for too long when not required should also be avoided. Adequate fume extraction should be provided for molding machine operators.
7) Slip resistance can be poor, especially in wet conditions and with the harder compound grades. Soles tend to wear smooth and harden with age.
8) PVC hardens as the temperature falls so that flexibility and flex crack resistance can be quite poor in cold conditions. This problem is reduced by blending with certain modifiers.
9) Oill and solvent resistance is poor, but again greatly improved by blending with suitable modifiers. Excessive exposure to solvents leaches out the plasticizers. Oills soften and weaken the compound.
10) PVC units may become stained by transfer of color from upper components. Colour is absorbed and carried by plasticizers and other liquid components in the compound. The more easily the plasticizer migrates, the quicker and more extensive is the staining, and stains cannot easily be removed. Uppers may themselves become stained by contact with coloured PVC soles.
11) Sole adhesion problems can arise of plasticizers are used which migrate too easily, Polychloroprene (Neoprene) adhesive is badly affected and should not be used Polyurethane cements are more resistant to the softening effect of plasticizers and will usually form good bonds, provided the PVC surface is properly solvent wiped and dried before application.
12) Density is relatively high (1.16-1.23 g/cm) so that solid sole units are heavier when made from PVC than when made from TR, PVC can be expanded a little to reduce density, but it should not be taken much below 0.8 g/cm3 or the compound will be too weak.
Adhesive is a sticky substance used to bond two more pieces of material. In the shoe trade it also known as cements and used to bond the outsole or linings to the upper. To sole bonding system it plays a vital role. There are some characteristics that influence its property. They are:
This is a measure of the ease with which a liquid adhesive will flow. The higher the viscosity the less easily it flows. Viscosity generally decreases with rise in temperature unless the adhesive is of the hear-curing type, in which case the reverse is true. Viscosity can also be decreased by addition of solvent thinners, or increased by evaporation of solvent. Together with its surface tension, viscosity controls the spreading and penetrating ability of an adhesive.
b) Total Solid Content:
This is the percent by weight of solid matter in an adhesive. Solid matter comprises the polymer together with any other material which may be added to increase tackiness of reduce chemical degradation. The higher the solids content the more thinly need cement be spread to obtain a good bond, and the more viscous it will be.
c) Shelf Life:
This is the expected storage life in unopened containers. It is determined by the stability of the adhesive at the storage temperature to chemical and physical changes.
d) Pot Life:
With 1-part cements it refers to the maximum usable life after opening the container. With solvent cements it is almost impossible to re-seal containers efficiently and solvent will be lost by evaporation more or less slowly depending on volatility. It can of course be replaced, but if this is not done the liquid becomes too thick and lumpy to reconstitute effectively. Rate of solvent loss when a container is in use depends on its dimensions. In general the pot life will be longer if the container is narrow and deep rather than wide and shallow. There are applicators available designed to reduce evaporation to a minimum. Some adhesives are affected by exposure to atmospheric moisture. With 2-part cements, pot life refers to the maximum usable life after mixing the two components. The mixing initiates a chemical reaction, which results in the adhesive eventually becoming solid Rate of reaction depends on the chemical nature and amount of ‘hardener’ used, temperature, and total amount mixed. In general the greater the proportion of hardener and the higher the temperature the faster is the reaction and the shorter the pot life. The reaction produces heat. The greater the amount mixed the harder it is for the heat to escape and the higher the temperature rises. Consequently the faster the reaction proceeds and the shorter the pot life.
e) Drying Time:
This is the time required for a spread cement film to lose sufficient water or solvent for a successful bond to be made. It applies mainly to water or solvent-based cements using a two-way dry stick method. The times are normally given for natural drying (at room temperature) but can be reduced considerably by parsing coated components through hot air tunnels. The higher the temperature and the better the air circulation the faster the drying rate. Drying limes are shorter for films on porous materials like leather and fabrics than on non-porous materials like PVC. VR, TPR etc. The thicker the film the longer the lime Failure to allow sufficient drying time will result in bonds with poor green strength and likely to ‘spring back’. With hot melt cements; there is no drying time because there is no solvent. For liquid curing adhesives, drying time refers to the time taken for the film in develop sufficient tackiness (spotting tack) for efficient bonding using a minimum press time. As a general rule the shorter the drying time the shorter is the pot life but the more cost -effective is the bonding process.
f) Sensitive Life:
This is the time after drying that a cement film retains enough tackiness for a bond to be made by applying pressure alone to the components. Dry tack of many adhesives can be increased by the addition of various resins.
The pressure-sensitive life of cement films is reduced by the presence of dust in the air. Many types of cement have no pressure-sensitivity, only dry.
g) Tack Retention Time (Reactivation Life or Tack Life)
This refers to the maximum time after drying that a cement film can be successfully reactivated by heat for good bonding. For some time after the pressure-sensitive life of an open film has expired, tacking can be recovered (or generated) by heating the surface of the film to the correct activation temperature. Tack life is reduced by dust and ageing, and can sometimes be extended wiping the film.
h) Open Time:
This term is ‘used by some people to mean tack life. Here is refers to the actual time, which elapses between applying the adhesive, and making the bond in the shoe factory. It depends on the work organization. Maximum open time should not exceed the tack life of the cement.
i) Spotting Tack:
This refers to bond strength developed when two cement films are lightly pressed together. It should be sufficient for the components to remain in place while they are transferred to the press, but not so high that the components cannot easily be repositioned if necessary before applying full pressure.
This refers to the joining of two cement films. If coalescence is not efficient (due to expiry of tack life or failure to beat activate properly) the cohesive strength of the bond will be poor.
k) Green Strength:
This is the strength of a bond immediately after leaving the press. Full strength rarely achieved at the time of bonding owing to the presence of truces of solvent, which have a plasticizing affect. It would be uneconomic to wait until ail solvent absorbed by porous materials has evaporated before making the bond (and may cause the tack life to be exceeded). With 2-part (curing) cements the chemical reactions slow down as they approach completion, and only when they are complete will full bond strength be reached. Green strength should be at least 80% of full strength to allow the shoes to continue without delay through the factory. Even under these conditions, it may take from 2 to 7 days for the full strength to develop.
This is the minimum time for which the components must be under pressure in the bonding press. It is determined by the rate of set-up.
m) Bonding Pressure:
This is the recommended pressure to be applied to the particular combination of materials and adhesive in the press to achieve the best results. It controlled by the crescent properties of the cement and the -compressive properties of the materials, e.g. Expanded soling should not he subjected to as high a pressure as harder solid soling because of the risk of permanent deformation. Up to a point, the higher the pressure the stronger the bond.
n) Rate of Set Up:
This is the speed with which green strength develops in the press. It is influenced mainly by the chemical and physical properties of the adhesive system. For dry stick methods the set-up should be rapid to reduce “press time. With wet stick ‘methods the set-up needs to be slow to allow components to be positioned correctly after making the bone (e.g.-stiffener attachment and socking).
o) Cohesive Strength:
This is the bond strength between one cement film and another adhesive Strength:
This is the bond strength between an adhesive film and a material,
q) Bond Strength:
This refers to the overall strength of a material / cement / material combination. It requires that cohesive strength, adhesive strength and the materials strength should he satisfactory. A bond is only as strong as its weakest link.
IMPORTANT FACTORS AFFECTING SELECTION AN ADHESIVE
Safety : Whether flammable whether vapor is dangerous to inhale
Application : Hand (brush); roller; machine; spray gun
Drying time : Quick; slow
Tack retention time : Time available to make bond after drying (Open time)
Final bond strength : Permanent or temporary; resistance to heat, moisture, &
Compatibility : With materials to be bonded.
Type of stick : Dry; wet; self adhesive; heat & pressure
Pot life : Length of time it can be used after opening the container
Self life : Length of time it can be store in a sealed container
Storage : Temperature conditions e.g. lattices must not be allowed to
Cost : Petroleum liquid storage regulations the initial cost is not only cost that goes into the establishment of the real cost.
CLASSIFICATION OF ADHESIVE:
There are mainly two types of adhesive
a. Water based adhesive
b. Solvent based adhesive
a. Water based adhesive are:
1. Natural rubber latex
2. Synthetics lattices
3. Vegetable paste
b. Solvent based adhesive are:
1. Rubber solution
2. Polychloprene solution (Neoprene)
WATER BASED ADHESIVE
a) NATURAL RUBBER LATEX
Natural rubber is a product existing as a milky substance known as latex. It is obtained from “Heveabrasiliensis”. Latex is a colloidal dispersion of rubber particle in water.
Basically, latexes tapped from the rubber tree. But usually compounded with resins to improve tack. Contains ammonia for stability.
1. Non flammable
2. Good initial grab
3. Clean in use
5. Low plasticizer resistance
6. Versatile bonding method
7. Poor heat resistance.
1. Fitting, Laminating, Folding
2. Toe puff attaching
3. Stiffener attaching
4. Insole binding
6. Heel covering
b) SYNTHETICS LATICES
It is obtained from emulsion or dispersion of synthetics polymer such as polyvinyl acetate (PVA), acrylate, polycholoprene or polyurethane (PU) in water.
1. Non flammable
2. Relatively poor wet “grab”
3. Superior plasticizer resistance.
2. Toe puff attaching
3. Stiffener attaching
4. Heel covering
c) VEGETABLE PASTE
It is made from starch that is a derivative of starch.
1. Non flammable
2. Low bond strength
3. Soften by water
1. Box leveling
SOLVENT BASED ADHESIVE
a) RUBBER SOLUTION
It is prepared by milling latex together with the compounded rubber and dissolving the same in a solvent such as benzene or gasoline. A resin tackifier is also used.
1. Generally flammable
2. Good tack but not high strength
3. Limited plasticizer resistance
4. Moderate heat resistance
5. Sensitive to oil and organic solvents
6. Petroleum solvent based solution unlikely to damage material
1. Upper to lining attachment
2. Sock lining to insole attachment
3. Temporary bonding for edge folding
Polychloroprene is a synthetic elastomer with many of the properties of natural rubber. These are prepared in number of grades depending on the crystallization rate.
1. High bond strength
2. Good grab
3. Limited plasticizer resistance
4. Easy handing
5. Can use with brush or spray
6. Difficult to remove if materials are contaminated
7. Long tack life
1. Fitting, laminating
2. Insole laminating
3. Insole rib attaching
4. Rubber and leather sole laying
6. Heel covering
Polyurethane is produced when a di-isocyanate having two isocyanate groups is reacted with a diol having two hydroxyl groups.
2. Strong bond with most martial
3. Superior plasticizer resistance
4. Grease and oil resistance
5. High green strength but at least 48 hours needed to reach full strength
6. Difficult to remove if materials are contaminated
1. Sock attaching
2. Sole attaching
3. Hand lasting
ADHESIVES WIDELY USED IN FOOTWEAR INDUSTRY
1. Natural Rubber Latex.
2. Synthetic Lattices.
3. Rubber Solutions.
4. Polychloroprene (Neoprene) Solutions.
5. Polyurethane (PU) Solutions.
6. Hot Melt Adhesive,
c. Ethylene vinyl acetate (EVA),
d. Thermoplastic Rubber.
7. Heat Fusible Coatings.
8. Pressure Sensitive
HOW ADHESIVE WORK
In simple terms, materials are held together by attractions (‘magnetic forces’) between the atoms and molecules from which they are built up. The same forces might reasonably be expected to cause materials brought into contact to stick together.
Unfortunately, just as a toy magnet will only attract a pin at close range, intermolecular forces only operate over very short distances (one millionth of a mm), so the necessary close contact is never achieved even with highly-polished
Adhesive must wet surfaces
An adhesive overcomes the problem by making intimate contact with the surface; it does so by being applied as a liquid which will flow and wet the material to be joined.
How a liquid can create adhesion is easily demonstrated-a little spilt drink between glass and drink mat may cause the mat to cling when the glass is raised. However, very little force is needed to make the mat drop off again.
This brings us to the second fundamental requirement of an adhesive-having wet the surface it must develop strength to give a useful bonded joint. Strength comes from the setting or curing process during which the adhesive changes from liquid to solid.
Wetting followed by setting
Wetting followed by setting, is common to all adhesive types. In addition, an adhesive must bridge the gap between the adherents to be joined and have an affinity for them.
The attraction of the adhesive molecules for the molecules at the material surface must be at least as great as the attraction of the adhesive molecules for each other. This compatibility of adhesive and surface depends on the adhesive type and may be inherent, or achieved by pretreatment of the adhered surfaces.
Bond must De-durable
For satisfactory service performance the finished adhesive bond must have adequate durability. For the service life of the product it must resist cyclic mechanical stresses, as in the flexing of shoes and heat, moisture and other media in the service environment.
Application may be as a solvent-based (solvent-borne) or water based (waterborne) solution; an emulsion or latex, usually water-based; a liquid chemical or a molten solid (hot melt).
Several methods of setting and bond formation
Methods of setting or curing and bond formation include:
• Solvent/water loss after wet stick (wallpaper paste).
• Solvent/water loss followed by cold bonding (contact adhesive).
• Solvent/water loss followed by heat reactivation (most sole attaching adhesives).
• Chemical curing (epode, cyano-acryl ate ‘superglue1). Cooling of melt (hot melt).
Sole bonds require adequate ‘green’ strength
Shoe sole bonds are usually under stress immediately after bonding and long clamping times would be inconvenient in production.
Therefore heat is commonly used to reactivate one or both of the essentially dry adhesive layers before combining them to produce a bond which, after a short pressing time, will have satisfactory ‘green’ strength and resistance to any tendency of the sole springing away. The adhesives for this process require the ability to coalesce, so that the two films brought into contact after reactivation will form one by diffusion of the molecules across the interface between them.
DEVELOPMENT OF SOLE BONDING ADHESIVES
The development and use of advent-based adhesives for sole attaching is linked with development in adhesive polymers and footwear materials.
Polychloroprene adhesive capable of heat reactivation adhesives based on nitrile or polychloroprene rubbers were in use by the late 1940s with polychloroprene favored because of their excellent coalescing properties.
The period 1950-65 saw the main growth of stuck-on sole attachment, as Polychloroprene adhesives proved capable of heal reactivation days or even weeks after application, before bonding with short press times. This gives the stuck-on process an attractive degree of flexibility.
POLYURETHANE (PU) SOLUTIONS:
Forming Solution of PU polymer in ketone and other solvents.
- Superior plasticizer and grease resistance
- Limited cold tack life
- Strong bonds with most materials
- Difficult to remove if materials are contaminated separations in which PU solutions are used are listed, together with any associated problems are given below:
|Hard||The adhesive is usually hand||Poor bonds||Increase heat|
|Lasting||brushed onto the upper and||caused by||activation of the|
|e.g. with||Insole||inadequate||adhesive coated|
|PVC||margin; after drying the bonds||tack.||surfaces.|
|made by heat activation of one|
|or both coated surfaces,|
|Rand||The rending is roller coated||Poor adhesion||Check that correct|
|attachment||with adhesive, generally just||to sole or||preparation of|
|To soles||prior to attaching, and the sole||rand material.||material has been|
|PVC rands||also roller coated either overall||Poor||carried out prior to|
|In||or just around the margin.||coalescence||cementing.|
|particular)||Efter drying, the rand is laid||between||Check machine|
|on the sole.||adhesive||setting for application|
|films.||of adequate heat and|
Pre Treatment of Soling (Primers):
To obtain satisfactory bonding between two materials using an adhesive normally requires some form of surface preparation to improve the adhesive between the cement and the material surfaces. This may involve
(i) Physical treatment (e.g.-roughing)
(ii) Chemical treatment
(iii) A combination of both.
Chemical primers may be classified as follows:
(i) Cleaners (Solvent wipes): These are solvents which remove greases, oils, plasticizes, mould release agents, oxidation (ageing) products and other contaminants which could seduce adhesion or weaken the adhesive. They will also make the surface more easily wetted by the adhesive. Most cleaning agent are highly flammable and may be toxic, Examples are Lacsol, Boston 6453 and 6457.
(ii) Fillers: These types are used to prevent bond starvation caused by excessive absorption of adhesive into the porous surfaces like leather and fabrics. Examples are Bostocl23 and 194.
(iii) Modifiers: This type is mainly used on rubber and plastic materials. These tend to be less compatible with adhesive than fibrous surfaces. E polymer film is deposited from sole which acts as an intermediate layer, forming a strong bond to both material and adhesive. Each modifies in designed to be used with a particular adhesive, Examples- Bostic M927 & 170.
(iv) Activators: This type of primers activates a surface so that it conform strong chemical bonds to the adhesive. They are much more hazards us to than other primers. In most cases their activity is destroyed by moisture, so containers should be kept sealed and adhesive applied to the primed surface within a specified time limit. Examples are SATRA formulation SDP102, Bostic313.
PREPARETION OF PVC SOLE:
Poly Vinyl Chloride (PVC):
The PVC polymer, made by polymerizing vinyl chloride monomer gas, in a hard resinous colorless solid that must be compounded with other materials to be suitable for shoe solings.
1) It has the lowest unit cost of all man-made soling materials currently available. This is because the race materials are cheap and because a low cost, high speed screage-injection process can be used. In addition compounds can contain up to 10% of recycled scrap without serious loss of properties.
2) It is very versatile because it can be blended with a wide range of plasticizers and other plastic materials in a wide range of proportions. This allows considerable control over the properties.
3) Wear resistance is satisfactory provided the compound is not too hard, and can be quite good if blended with modifiers like thermoplastic PU or nitrite rubber.
4) Good mould definition. Smooth surfaces are easy to obtain, although sometimes a little too shiny.
5) PVC has flame-retardant properties. It will burn but the flames are self-extinguishing once the source of ignition is removed.
1) Sensitive to heat, giving off toxic corrosive fumes, as well as being a health hazard, the fumes damage molding equipment. Heat establishers help to reduce the problem but molding temperatures must not be allowed to exceed the recommended values maintaining the compound in a molten state for too long when not required should also be avoided. Adequate fume extraction should be provided for molding machine operators.
2) Slip resistance can be poor, especially in wet conditions and with the harder compound grades. Soles tend to wear smooth and harden with age.
3) PVC hardness as the temperature falls so that flexibility and flex crack resistance can be quite poor in cold conditions. This problem is reduced by blending with certain modifiers.
4) Oil and solvent resistance is poor, but again greatly improved by blending with suitable modifiers. Excessive exposure to solvents teacher out the plasticizers. oils soften and weaken the compound.
5) PVC units may become stained by transfer of color from upper components. Color is absorbed and carried by plasticizers and other liquid components in the compound. The more easily the plasticizer migrates, the quicker and more extensive is the staining, stains cannot easily be removed. Uppers may themselves become stained by contact with colored PVC soles.
6) Sole adhesion problems can arise of plasticizers are used which migrate too easily, polychloroprene (Neoprene) adhesive is badly affected and should not be used polyurethane cements are more resistant to the softening effect of plasticizers and will usually form good bonds, provided the PVC surface is properly solvent wiped and dried before application.
7) Density is relatively high (1.16-1.23 g/cm3) so that solid sole units are heavier when made from PVC than when made from TR, PVC can be expanded a little to reduce density, but it should not be taken much below 0.8 g/cm3 or the compound will be too weak.
Compounding and Processing: PVC compound
( Tarts by weight)
PigmentsThe main ingredient that makes PVC sole.
To make flexible
To make flexible & cheaper
To prevent heat degradation
To help processing, compounding and molding
To cheapen & sometimes to help
THE ADHESIVE BOND:
Mechanisms of Adhesion:
There are two main mechanisms by which an adhesive (cement) sticks to a material. These are referred to as Mechanical adhesion and Specific adhesion.
a). Mechanical Adhesion:
This is the more common of the two, being effective to some extent in almost all examples of bonding’ Many materials have visibly rough surfaces, smoothest surfaces contain microscopic pores. When adhesive is applied in liquid form to the surface, some of it flows into these pores. After drying,1 the adhesive layer will be ‘keyed’ to the material surface rather like two pieces of a jig-saw puzzle are joined, together.
The effectiveness of the bond will depend on the strength of the material, and on the size, depth and shape of the pores. Deep and under-cut pores will lead to stronger bonds than shallow indentations.
Good bonds can be formed to fibrous surface because adhesive can surround the fibers. In most bonds there is some degree of mechanical adhesion present. Examples are PU, polychloroprene or latex on leather, fabrics and rubber soling.
On a molecular scale, bonding occurs. When adhesive molecules diffuse into and become intertwined with the molecules in the material surface. For this to happen the attractive forces between adhesive molecules and
Material molecules must be at least as strong as the attraction of adhesive molecules for each other. I.e. the adhesive is more specific in what it will bond to.
The diffusion process is helped by the presence of solvents in the adhesive which swell or partially dissolve the material surface. Heat performs a similar function when using hot melt cements, and when causing one cement film to coalesce with another after reactivation.
In another type of specific adhesion the molecules in the cement become bound by strong chemical bonds to molecules in the material. For this to happen there must be specific chemical structures present in the two bonding surfaces. Frequently the material surface is made chemically reactive to the cement just before spreading by applying a special primer. An example is the use of a halogenating agent on thermoplastic rubber before applying PU cement.
LINKS OF ADHESION
It is possible to identify 5 areas (links) within an adhesive bond which contribute to its strength. The bond will only be as strong as the weakest of these links. To ensure that each link is as. Strong as possible, the correct bonding procedures should be followed. The account below as particularly relevant to sole bonding, but is applicable to any type of bond.
Links 1 and 5
Weak layers within materials E and B must be removed by roughing or scouring, e.g. The grain layer from leather, the PU layer from PU coated fabrics
(PUCF’s) and sometimes the surface layers from vulcanized rubber (VR) soling materials.
Links 2 and 4
Satisfactory adhesion of cement C to both E and B depends on a number of factors:
a) Correct surface preparation:
i) Roughing / scouring increases the number and depth of surface poses. This gives better mechanical adhesion to most materials but care must be taken not to overdo it, and to remove all loosened material before applying the cement.
ii) Solvent wiping removes surface contamination such as grease from leather plasticizes from PVC, mould release agents from soling materials and soaps in VTR. It also improves surface wetting by the cement.. With rubbers and plastics, suitable solvents can soften and swell surface layers to facilitate infusion of cement.
iii) Chemical printing makes a surface more compatible with certain cements by chemically modifying it e.g. ‘starlet’ for chlorinates crepe, VTR and Thermoplastic :Rubber (TR.) to allow some specific adhesion to UP cement. Isocyanate primers improve specific adhesion to Nylon, Polyester and Ethylene Vinyl Acetate.
(EVA). A special primer for EVA deposits a polymer film. Dilute cement solutions are used as primers on very porous surfaces to provide a better foundation for the main cement layer.
b) Correct selection of adhesive:
The adhesive must be compatible with both surfaces to be bonded. (Although not an absolute rule, it is normal to choose the same adhesive forA and B.)
c) Correct viscosity and surface tension:
Surface tension should be low to give better wetting of surfaces. V is costly should not be too high or penetration into small surface pores may be prevented. Too low a viscosity generally means that two or more coats will have to be applied on porous materials. The viscosity of water and solvent-based cements is determined by solids content, and of hot melt cement by its temperature.
d) Correct amount and distribution:
There must be enough adhesive left on the surface after drying, and it should be uniformly spread over the whole bonding area. e.g.. not too thin at the toe in a sole bond.
e) Correct drying time:
For full strength to develop, it is important that all traces of solvents from cements and primers are removed from the bond. This happens more quickly if the bond is left open to dry for sufficient time. Drying times depend on the type of solvent, the porosity of the material, the temperature and the efficiency of air flow.
With hot melt cements ‘drying time’ means the time it takes to cool and solidify. Since this is usually rapid, the ‘drying’ time should not be too long.
Open times should not be too long or the cement may become contaminated by dust, ageing etc.
As well as correct preparation of surfaces and correct selection and application of cement, it is equally important that the bond is closed properly.
a) Correct reactivation temperature:
A weak bond will result if there are poor cohesion between me two cement layers. To avoid problems it is best to soften on or both dry cement surfaces by heating in a reactivator to the correct temperature. (85-90° for PU and Polychloroprene cements). N.B. the bond must be closed immediately after heating.
b) Correct bonding pressure:
The pressure applied to the bond should be adjusted according to the softness of materials being bonded, Softer materials will need less pressure than harder materials, e.g.. too much pressure may cause permanent deformation of cellular soling materials. Too little pressure and harder soles will not confine closely enough to the upper.
N.B. In a press, it is the oil pressure fed to the hydraulic cylinder which is controlled. The cylinder provides the bonding force. The pressure created in the bond depends on the size of this force and on the bonding area. e.g. larger soles require larger forces to create the same bonding pressure, therefore greater oil pressures will be used on men’s shoes than on ladies,oil pressures for sole attaching vary from 2-7 Bar.
c) Correct pressure distribution:
Where the bond is not flat, as in most sole bonds, it is important to support the underside of the bond in such a way that the pressure is evenly distributed Sometimes toe spring causes lower pressure and so poorer bonding at the toe. Most modern sole attaching machines have a means of blocking up the sole too the correct profile. An embossed aluminum foil is available from SATRA which is used to check the pressure distribution in sole bonding.
d) Corrected well time:
When pressure is applied to a bond the materials do not react instantly. Time is required for them to conform to each other. Dwell times depend on the elasticity and plasticity of the materials, but are around 9-15 seconds for sole attaching.
Effective surface pre-treatment, is essential for good bonding. Pre-treatments may be either mechanical, such as roughing or scouring, or chemical, such as solvent wiping and priming.
Both types may remove surface contaminants, remove or consolidate weak surface layers, or enable the material surface to be more easily wetted by the adhesive. In addition, chemical treatments may chemically modify the surface to give it greater affinity for the adhesive.
The main recent development in roughing of uppers is the introduction of computer-controlled automatic machines which are programmmed to follow the lasted margin of each shoe style.
With the pressures to reduce solvent usages cleaners based on detergent solutions or citrus oils have been introduced and water-based primers are under development.
ADHESIVE APPLICATION AND DRYING
Adhesive is applied by hand brush, pressure-fed brush and roller or extruder machines. Modifications to suit water-based adhesives are available; for example the substitution of foam rubber rollers for gelatin rollers, and use of stainless steel or plastics for parts in contact with the adhesive.
Use of computer-controlled application machinery is growing and applicator heads for water-based or hot melt adhesives have been developed.
Water-based adhesives are more difficult to dry than solvent-based adhesives, leading to developments in drying equipment with increased air movement and/or higher temperatures.
THE BONDING PROCESS
In much of the world, heat reactivation, before immediately spotting and pressing, continues to be the favored method of sole bonding. Some modem equipment combines drying and reactivation with the warm dry components emerging from the dryer being healed immediately to the reactivation temperature. This facilitates two-way reactivation of both soles and uppers which are favoured with many water based adhesives.
There is also a move back to using longer wavelength infra-red energy for reactivation; the advantage being low sensitivity to surface color, rather than the ‘flash1 reactivation which has been almost universal for many years.
In Asia the adhesive on soles and uppers is usually dried in successive warm air cabinets, followed by a reactivation stage giving a temperature sufficient for coalescence. The relative high amount of heat absorbed by the materials allows for a relatively long delay between heating and pressing.
Indeed, the soles of sports and casual footwear, after spotting, are often pressed in successive operations dealing with sidewalk, toe/seat and sole bottom, using simple presses in each case.
In conventional shoemaking the requirement to press the whole of the sole evenly in one operation has led to flat bed presses with self leveling pad boxes and diaphragm presses which envelope the whole sole.
Most soles are attached to uppers by adhesive bonding, and it is inconceivable that this will not continue. The evolution of sole attaching adhesives to dale has, fallen into three phases: introduction of solvent-based adhesives; adapting adhesives and pre-treatments to suit a wider range of materials; and development of solvent fine systems to reduce solvent usage.
The next phase is likely to see developments in reactive liquid systems with the aim of rapid, simplified bonding. Adhesive costs may be relatively high, but offset by savings in processing.
MY LABORATORY WORK
Sole Bonding System of PVC with Different Types of Upper
|Semi Aniline Finish Leather||Rough or Scour||PU|
|Silk Grain Leather||Rough or Scour||PU|
|Suede Leather||Rough or Scour||PU|
|Corrected Grain leather||Rough or Scour||PU|
|Silk Grain Leather||Rough or Scour||PU|
|Pigment Finish Leather||Rough or Scour||PU|
|Garments Leather||Rough or Scour||PU|
|PVC coated fabrics||Solvent wipe||PU|
PVC (Poly Vinyl Chloride) Preparation
• Only Roughing
• Roughing and Solvent wiping Adhesive: PU
This method is intended to determine the peel strength of an adhesive bond. The method is applicable to all types of bonded joint where at least one of the adherents is flexible.
Test specimens are cut from a bonded assembly which has been previously prepared, typically using the procedure described in:
• SATRA Test Method EM2- Solvent borne or water borne adhesive.
• SATRA Test Method AM 14 – Hot melt adhesives. or other procedures such as factory prepared test specimens.
The test specimen is then peeled using a tensile testing machine while the forces required to separate the two adherents is measured and the type of bond failure is assessed.
BS 1610 Part 1: 1992 – Specification for the grading offered applied by materials testing machines.
SATRA test method AM2: 1992 – Preparation of water borne and solvent borne, bonded assemblies for peel tests.
APPARATUS AND MATERIALS
A low inertia tensile testing machine with:
• A means of continuously recording the force throughout the test
• A jaw separation rate of 100 ± 20 mm/mm.
• The capability of measuring forces up to 500 N, to an accuracy of 2% as specified by’ Grade 2″ in BS 1610 Part l: 1992, preferably with several lower force ranges for more sensitive measurements. For test specimens cut from direct vulcanized bonds, forces above 500 N may be necessary to peel the bond.
Suitable machines are available from SATRA reference numbers STM 161 and STM 466, together with a quick release type jaw. reference number STD 160 OR.
• A cutting device such as a sharp knife or rotary disc cutler for cutting the test specimens from the bonded assemblies. This shall neither unduly compress nor force apart the layers of the test specimen at the edges during cutting and therefore a press knife is unsuitable.
• A device for measuring lengths up to 70 mm to an accuracy of 0.5 mm. E steel rule or vernier calliper is suitable.
CUTTING OF TEST SPECIMENS
Between bonding and cutting the test specimens they should be stored in a standard controlled environment of 20 ± 2°C and 65 ± 2% rh for a minimum time of;
- 24 hours – hot melt bonded assemblies.
- 48 hours all other types of bonded assemblies.
SATRA test method AM2 Method I (solvent borne and water borne adhesive)
The procedure in AM2 produces two bonded assemblies of width 70mm and length 50 mm. For each bonded assembly, see Figure use the knife or circular cutter.
• Cut an equal amount from the shorter edges of the assembly to produce an assembly of width 60 ± I mm.
• Cut the remaining central portion in half to produce two test specimens of width 30.0 ± 0.5 mm and length approximately 50 mm. The circular cutter may be capable of tarrying out both operations in one pass.
• SATRA test method AM4 Method 1 (pre-coated materials)
The procedure in AM 14 Method I produces four bonded assemblies of width 70 mm and length 50 mm. For each bonded assembly use the knife or circular cutter to follow the procedure in sections 5.1.1 and 5.1.2.
• SATRA test method AM 14 Method 2 (direct bonding)
The procedure in AM 14 Method 2 produces four bonded assemblies of width 20 mm and length 100 mm. These bonded assemblies require no further cutting. However, if the maximum jaw separation of tensile testing machine is less than. Approximately 150 mm it may be necessary to shorten the unbounded tabs.
• Non-standard sized bonded assemblies
If the bonded assemblies are less than the standard size use the knife or circular cutter to cut two individual test specimens of equal width, discarding marginal strips, approximately 5 mm wide on the 50 mm sides of the assembly.
If the test specimens were cut with a calibrated rotary disc cutter use the device to measure the width of each test specimen in millimeters, and record this value to the nearest 0.5 mm.
If required, conduct any pro-treatments at this stage. In the case of test specimens prepared by the procedure In SATRA test method AM 14 Method / 1 (pre-coated materials) ensure that the two test specimens cut from each assembly are subjected to the same pre-treatments before peeling. In all other cases where the toil specimen is to be subjected to a pro-treatment before being peeled, ensure that the two test specimens from an assembly are not allocated to the same pro-treatment nor both tested without a pre-treatment.
Adjust the tensile testing machine to an appropriate force range.
Firmly clamp one of the free ends of the test specimen into each of the jaws of the tensile testing machine.
Activate the continuous recording system and operate the tensile testing machine with a jaw separation rate of 100 ± 20 mm/min until either a bonded length of 30 mm has been peeled or one of the adherents tears through.
As the jaws separate’ observe the type (s) of bond failure, see Figure 2 and Table I. This is particularly important in the case of cohesive failure as this can only be identifier during peeling. So all separated lest specimen; estimate, where possible to the nearest the percentage of the bonded area presented by each type of failure exhibited.
Study each force versus extension graph produced by the continuous recording system in section 6.6. If the force shows rapid fluctuations but the average appears to remain constant throughout the test, excluding any significant initial peak, see section 6.11, then for each graph estimate the average peeling force, in newtons, see section-8.1 If failure is by tearing through either adhered with no subsequent peeling, measure the peak force.
For each test specimen divide the average peeling force (6.8) by the width of tire specimen hi millimeters, as measured in section 6.1, to give the average peel strength of each bond in N/mm, to the nearest 0.1 N/mm.
For test specimens cut from bonded assemblies prepared by the procedure in SATRA test method EM H Method 1 (pre-coated materials) calculate the arithmetic mean peel strength for the two test specimen cut from each of the four assemblies.
Calculate the arithmetic mean peel strength for those specimens or parts of specimens which show the same type of failure. Record the number of test specimens from which each result is derived, and the type of failure.
If there is a significant initial peak force on the graph, as shown in Figure, for example due to breaking of a surface layer, measure this value in newtons and divide it by the width of the test specimen in millimeters to give the initial peel strength of the bond hi N/mm.
According to the test I have found good result from the resin finish and opaque finish (2 specimens). The standard test result for PVC sole bonding is 5N/cm. These(2 specimens) are nearer to this standard value. So it is clear to me that this type of bonding method for (2 specimens) upper material is good for PVC sole bonding. It should carry in mind that this type of good result only achievable when the standard bonding method is followed.
• World Footwear, vol. 07,No. 1, Jan/Feb, 1993
• World Footwear, vol. 13,No.4, Jul/Aug, 1999
• World Footwear,vol.l4, No.5, Sep/sct, 2000
• World Footwear, vol. 15,No.l, Jan/Feb, 2001
• World Footwear, vol. 15,No.5, sep/sct, 2001
• FDDI Footwear Digest,Issue.24, Jan,2000
• FDDI Lasting Handouts, vol. I
• SATRA Bulletin, Jan, 1995
• SATRA Bulletin,Jun.l999
• SATRA Bulletin,Dec,1999
• Class Lecture by Amal Kanti Deb sir.
• Class Lecture by Noor Muhammad sir.
• Class Lecture by Fatema-tuj-zohra madam.
• Manual of Shoe Making by the Training Department CLARKS Limited.
• Adhesive In Shoe Manufacture by B. Venkatappaiah
• Footwear Materials And Process Technology by A.J. Harvey