Footwear Manufacturing

Form the middle of the 19^th century, out sole of footwear has been changed in materials. Leather has been the traditional soling material, after 1980 only about 5% of shoes have leather soles. These days, most of the soles are made of rubber or plastics, which are classed as synthetics. So new types of soling needs new adhesion method or technique with different types of upper materials also they are different in bonding strength. The Polyurethane is one of them that become a most popular soling.

The unique versatility of Polyurethanes as a soling material stems from the almost limitless chemical formulation combinations that give designers and manufacturers the freedom to create innovative designs that are in step with fashion and technological change. Polyurethanes can be made light, tough, comfortable, flexible, insulating, waterproof, slip-resistant, hard wearing and shock absorbent as required, simply by varying the formulation. They can have an almost endless variety of shapes, surface textures and colors and incorporate air bags, inserts or gels for extra comfort and support. Furthermore, PU bonds well to many types of shoe upper (leather, textile).

In recent times, footwear design has become something of a science, with specialist shoes being developed for a wide range of work and leisure applications. Using Polyurethane, the manufacturer is well placed to address special needs such as energy management, anti-static properties, improved abrasion resistance, low temperature flexibility and resistance to hydrolysis, microbial attack and premature ultraviolet yellowing. Thermoplastic (TPU), rather than reaction molded Polyurethanes, offer footwear manufacturers a material even more flexible and abrasion resistant. Available in a full range of different hardness’s, TPUs are especially suitable for applications such as safety boot outsoles, heels and top pieces for fashion shoes.

There are different types of soling materials available in market for footwear manufacturing. It is the 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.

PROPERTIES OF IDEAL SOLING MATERIALS

PROCESS 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 shock absorption (to protect wearer from injury).
• Good ageing resistance.

STATISTICS OF USE

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.

 Newer materials have steadily replaced leather, the traditional soling, which cost less and last longer. For example, the proportion of UK, leather-uppered footwear with non-leather soles grew to about 90 percent in 1990. Nevertheless, leather is still sought after for its aesthetic appeal and this is likely to continue for high-class footwear. Specialty leather soling also meets the needs of some wear environments.

SOLING SELECTION
Soling is supplied in several forms, depending on the type of footwear being manufactured. They may also vary according to the manufacturing process, such as rubber vulcanization, and the nature of the soling material, for example its thermo plasticity. The common forms are:

• Sheet
• Caster shapes
• Built units
• Moulded units
• Moulded-on.

One question, which is nearly always asked in modern 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.

DIFFERENT CRITERIA OF SOLE

The performance of footwear depends on the overall design and construction. i.e. the upper, lining, insole, mid-sole 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 –

1) Provide the wearer with a stable base.

2) Reduce risk of injury through shock when running, jumping etc.

3) Provide traction and reduce risk of injury through slipping.

4) Give insulation from sharp projections on the ground.

5) Give heat insulation.

6) Provide an acceptable wear life under specified conditions.

 IDENTIFICATION OF SOLING

LEATHER (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 a smooth, slick lightly glossed surface finish.
• Magnification reveals tiny pinprick like hair shall 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 specialist industrial footwear.
• Light grey leather often with a suede surface, is probably chrome tanned and used in dance and 10 pin bowling shoes.

VULCANI/ED RUBBER (VR) (SOLID):

• 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 mid-sole, soles that are VR only are relatively heavy.
• On industrial footwear may be marked ‘heat resistant’

THERMOPLASTIC RUBBER (TR):

• 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.
• Shenned 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 mid-soles in other materials but sometimes two color mouldings can mimic an outsole/ mid-sole assembly.

 POLYURAHHANE (PU) (Reaction Moulded):

• 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 micro cellular 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
• lo mould) along the edges and at the corners of moulded detail such as tread cleats and lettering.
• All colors possible.
• Natural Crepe rubber.

 NATURAL CREPE RUBBER

• A translucent pale or honey colored sheet material with a raw appearance.
• Wrinkled, textured or corrugated surface but no moulded 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 it self under pressure.
• Sometimes found in black or dark brown and sometimes pale crepe may have blacked or browned edges.
• Mostly used on comfort casuals.

RESIN RUBBER:

• 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.
• Color mainly black or tan to simulate leather, not translucent.
• Has good visual quality.
• Always used with separate heel components.

   POLYVINYLCHLORIDE (PVC) :

• ‘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 wear very smooth.
• Not found with mid-soles in other materials but sometimes combined with a leather forepart in dress shoes.
• All moulded 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 speck-led mall surface.

 Ethyl vinyl acetate (EVA):

• Characteristic non-rubbery smell when new but this fades.
• An extremely lightweight material.
• Usually soil enough to be impressionable with a finger.
• May haven slack (moulded) surface with sheen or else a matt velvety surface which is a split through the micro cellular 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 mid-sole (shock absorbing) layer in sports shoes.

 THERMOPLASTIC POLYURETHANE (TPU):

• 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, micro cellular materials are more rubber like.
• Mouldings have good definition, often with a matt finish.

MICRO CELLULAR RUBBER (MCR ):

• An extremely lightweight material.
• It produces nitrogen gas when it is compounding.
• It has a close cell structure.
• Tire like smell.
• May have bevel edges.
• On industrial footwear may be mark “heat shrink”.
• It has low density.
• It found as a sheet.
• It used mainly in sandals & sponge.
• A similar compound to solid rubber but with a blowing agent added.
• Have good cushioning effect.

 ABOUT POLY VINYL CHLORIDE

Thought the shoe industry, PVC is used in its thermoplastic form. It is made by polymerizing vinyl chloride monomer gas, which can be derived from ethyl coal or petroleum. The polymer is a hard, resinous colorless solid, melting at 170-180 ⁰ c. to become the flexible heat-stable material familiar to the shoe industry, it must be compounded with other materials.

CHEMICAL STRUCTURE:

Structurally, PVC is a vinyl polymer. It is similar to polyethylene, but on every other carbon in the backbone chain, one of the hydrogen atoms is replaced with a chlorine atom. It is produced by the free radical polymerization of vinyl chloride.

The chemical formula for PVC (polyvinylchloride) is [-CH2-CHCl-] n, where n are equal to ~60,000 to 140,000. Actual useful formulations of PVC also include a small percentage of various stabilizers to enhance the performance and prevent discoloration from light and heat.

PVC COMPOUND INGREDIENT

PROPERTIES OF PVC:

Fire retarding properties:

PVC has inherently superior fire retarding properties due to its chlorine content, even in the absence of fire retardants. For example, the ignition temperature of PVC is as high as 455°C, and is a material with less risk for fire incidents since it is not ignited easily

 Furthermore, the heat released in burning is considerably lower with PVC, when compared with those for PE and PP. PVC therefore contributes much less to spreading fire to nearby materials even while burning .Therefore, PVC is very suitable for safety reasons in products close to people’s daily lives.

Durability

Under normal conditions of use, the factor most strongly influencing the durability of a material is resistance to oxidation by atmospheric oxygen. PVC, having the molecular structure where the chlorine atom is bound to every other carbon chain, is highly resistant to oxidative reactions, and maintains its performance for a long time.

Oil/Chemical resistance:

PVC is resistant to acid, alkali and almost all inorganic chemicals. Although PVC swells or dissolves in aromatic hydrocarbons, ketones, and cyclic ethers, PVC is hard to dissolve in other organic solvents.

  OTHERPROPERTIES:

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   burn   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 moulding equipment. Heat establishers help to reduce the problem but moulding 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 moulding 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)  Oil and solvent resistance is poor, but again greatly improved by blending with suitable modifiers. Excessive exposure to solvents leaches out the plasticizers. Oils soften and weaken the compound.

10) PVC  units may become stained by transfer of colour 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, 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. 3g/cm  or the compound will be too weak.

PVC Injection Moulding

Bottom Preparation:

1.  Scour and rough:

Roughing for direct moulding 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 moulding machines in current use,many of which do not mould directly onto the shoe bottom, but produce moulded units for the stuck on process.Most moulding machines for direct moulding 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 refridgerated 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. Once 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 severalminutes, meanwhile the machine feeds the next station up to the moulding area and the process begins over, by the time the first station returns to the moulding 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.

 CHARACTERISTICS

Figure 9 shows the characteristics of PVC materials. Where comparison is shown with “Stiffness” verses “Density”

FIGURE 9: Sole weight versus performance.

PICTURE OF PVC SOLE

FIGURE 11: Some PVC Sole available in Bangladeshi market.

 ADHESIVES

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:

a) Viscosity:

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 he 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.

j) Coalescence:

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.

 l) Time:

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) Co-hesive Strength:

This is the bond strength between one cement film and another.

p) 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 co-hesive 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

 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)
3. Polyurathane

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, latex as tapped from the rubber tree. But usually compounded with resins to improve tack. Contains ammonia for stability.

• PROPERTIES:

1. Non flammable
2. Good initial grab
3. Clean in use
4. Spray able
5. Low plasticizer resistance
6. Versatile bonding method
7. Poor heat resistance.

• USES:

1. Fitting, Laminating, Folding
2. Toe puff attaching
3. Stiffener attaching
4. Insole binding
5. Socking
6. Heel covering

b) SYNTHETICS LATICES

It is obtained from emulsion or dispersion of synthetics polymer such as polyvinyle acetate (PVA), acrylate, polycholoprene or polyurethane (PU) in water.

• PROPERTIES:

1. Non flammable
2. Relatively poor wet “grab”
3. Superior plasticizer resistance.

• USES:

1. Socking
2. Toe puff attaching
3. Stiffener attaching
4. Heel covering

c) VEGETABLE PASTE

It is made from starch that is a derivative of starch.

• PROPERTIES:

1. Non flammable
2. Low bond strength
3. Soften by water

• USES:

                           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.

• PROPERTIES:

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

• USES:

                          1. Upper to lining attachment

                          2. Sock lining to insole attachment

                          3. Temporary bonding for edge folding

b) NEOPRENE       

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.

• PROPERTIES:

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

• USES:

                          1.Fitting, laminating

                           2.Insole laminating

                           3.Insole rib attaching

                          4. Rubber and leather sole laying

                          5. Lasting

                          6.Heel covering

c) POLYURETHANE

Polyurethane is produced when a di-isocyanate having two isocyanate groups is reacted with a diol havig two hydroxyl groups.

• PROPERTIES:

1. Flammable
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

• USES:

1. Sock attaching
2. Sole attaching
3. Hand lasting

 ADHESIVES WIDELY USED IN FOOTWEAR INDUSTRY

1. Natural Rubber Latex.
2. Synthetic Latices .
3. Rubber Solutions.
4. Polychloroprene (Neoprene) Solutions.
5. Polyurethane (PU) Solutions.
6. Hot Melt Adhesive.
   1. Polyamide.
   2. Polyester.
   3. Ethylene vinyl acetate (EVA),
   4. Thermoplastic Rubber.
   5. Heat Fusible Coatings.
   6. Pressure Sensitive

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 ADHESIVE:

These are   formulated from   linear thermoplastic polyester polyurethane polymers dissolved in solvents, such as methyl ethyl ketone (MEK), acetone and ethyl acetate to give a solid content of around 20% by mass, Chlorinated rubber and resins may be added to improve heat resistance or tack. Adhesives may be one part; having no free isocyanate present. Pre-reacted or premised, containing an isocyanate which reacts very slowly prior to application, allowing a usable shelf life of a few months.

Two parts; in which a small proportion of an isocyanatc-curing agent is mixed in before use. These have a pot life of just a few hours and require a short open time. Free isoeyanate, when present, is available to read; with and crosslink the applied adhesive film. It gives good heat resistance and may improve adhesion.

POLYCHLOROPRENE ADHESIVES:

These are formulated from polychloroprene rubbers (such as ‘Neoprene’ and ‘Bayprene’)-

The rubbery polymers arc dissolved in solvents such as toluene, petroleum and MEK to Give a solid content of around 25%.

Essential additives are acid acceptors, such as  zinc or magnesium oxide and resins to impart tack; the metal oxide and tackifying resin are usually reacted together before addition. Polychloroprene adhesives can be used in two-part form, with added isocyanate improving heat and grease resistance.

It is now possible to synthesize or disperse polymers to form a latex or emulsion, defined as ‘a stable colloidal dispersion of polymer particles in an aqueous medium’. Particles each comprising thousands of individual molecules but still very small (about one ten thousandth of a mm in diameter), are kept, in suspension by the action of stabilizers. Because the polymer particles reflect light- the dispersion usually appears milk white. After application as a coating, dispersions dry by losing water, the polymer particles being forced closer together until they coalesce into a film. Chemicals, or traces of solvent, are often included in the formulation to act as coalescing aids. Water-based adhesives have the advantage that they require a similar bonding process to solvent-based adhesives, so can be handled by familiar procedures. This has led to the steady growth in their use. Polyurethane adhesives dispersions of polyurethane suitable for formulating adhesives have been available for more than 20 years. Because dispersion has molecules agglomerated as particles in the water, rather than individual molecules in solvent, viscosity of a water-based adhesive is inherently low, although thickeners can increase it. On the other hand, the solids content can be higher than with solvent-based adhesives at about 40%.

A recent development is the production of isocyanate curing agents compatible with water-based polyurethane adhesives, enabling two-part systems to be offered. These are hard to conceive, as it is well known that isoeyanates react readily with water. Again, chemistry has come into play, developing isocyanates that ‘like¹ the PU polymer more than water and react preferentially with it.

Polyditoroprene and Polychoroprene dispersions are also well established, enabling adhesives for many purposes to be formulated. Like their solvent-based counterparts these usually require the incorporation of a compatible tackifying resin. They are used in shoe manufacture for temporary sole attachment in welted footwear and other ancillary operations.

Because aqueous synthetic polymer dispersions exist in an ‘unnatural’ state they are sensitive to various influences which can cause instability and irreversible coagulation of the polymer as rubbery lumps. This means that water-based adhesives should always be stored as recommended by the supplier, usually above 5C (five degrees) to avoid freezing.

Additions of chemicals, including water, should never be made unless these are specifically permitted. Critical parts of application machinery must be of non-ferrous materials and flow paths free of severe constrictions.

The main limitation of conventional hot melt adhesives (as used for lasting) is the relatively high viscosity, leading to poor wetting and penetration of leather and other fibrous materials, it is not feasible to use conventional polyurethane adhesive polymers in hot melt form.

Recent developments in reactive polyurethane hot melts have overcome the problem by applying a partially cured polymer as a low viscosity melts. The adhesive then further cures by moisture (or heat) before either direct bonding or heat reactivation. Final cure takes place after bonding.

Attention has to be given to producing the optimum balance of welling, tack, green strength and final bond strength.

Hot melts have the advantage of 100% solids content, making adequate adhesive coatings easy to achieve. However, they require special application equipment and this is perhaps the main reason why commercial usage to date has been limited.

1. Good adhesion can only be obtained if the roughing & sole preparation is correctly done.
2. Oil Pull up Leather

i)                   After roughing wipe with MEK or a primer suggested by the adhesive manufacturer.

ii)                Wait for 10 min.

iii)              Use PU adhesive & hardener (the percentage of hardener varies from 3-7% depending on humidity. The greater the humidity greater the percentage of hardener.)

1. PVC Soles

Wipe with MEK Use PU adhesive.

1. PU Soles

Rough sole especially in the lasting margin wipe with MEK use PU adhesive (2 coats on upper & 1 coat on sole).

1.  TPR Soles

Halogenate soles with a primer at least 1 hr. before use. The adhesive will not work without halogenation. Do not flood cavity. There should be no contract with metal during halogenation. Use PU adhesive.

1. Leather Soles

Rough soles & use neoprene / PU adhesive.

1. Crepe Soles

Use petrol to wipe sole & then neoprene adhesive.

1. Resin Rubber Sole:  MEK wipe & neoprene adhesive.

    9.   EVA Soles

Rough soles & use an EVA primer. Use PU adhesive.

1. Welts:

If we use neoprene/PU adhesive the welts tends to shift after reactivation. In Italy they use neoprene adhesive with a high reactivation temperature is use to that first bond between welt & sole is not disturbed. The same principle is followed in sports shoes where we have bonds between upper & mid-sole to sole.

 REACTIVATION ‘TEMPERATURE

Neoprene       75 to 80 “C

PU                    85 to 90 “C

EVA                50 to 55 ‘C

SOLE PRESS

Time               10 to 12 sec.

Pressure         25 to 50 kg depending on the rigidity of the sole.

For EVA ultra violet light is use instead of roughing.

5.3. ADHESIVE FOR LASTING MACHINE

1.   Polyester adhesive in capable form is used. (Usually white in color). This is fast setting & has the rigidity to rigidity to hold upper at toe melting temperature is about 200 to 270°C.

2.   Polyamide adhesive in cable form is use for side & seat lasting. Melting temperature is about 160 to 170°C.

•    We need about 8 to 10 gm/ pair of adhesive for lasting forepart & side / seat.

•    For sports shoes or where white uppers or soles are used. We should not use a colored hardener. Ask for a colorless hardener.

 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.

b) Specific Adhesion.

a) MECHANICAL ADHESION:

FIGURE  12: mechanical adhesion between material and adhesive.

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, or have a fibrous structure which is porous close examination shows that even the smoothest surfaces contain microscopic pores. When adhesive is applied in liquid form to the surface, some of it flows into these pores. After drying, 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 fibres. In most bonds there is some degree of mechanical adhesion present. Examples are PU, polychloroprene or latex on leather, fabrics and rubber soling.

 B) SPECIFIC ADHESION:

FIGURE 12: Specific adhesion between material and adhesive.

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

FIGURE 13: 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 A 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.

Satisfactory adhesion of cement C to both A and B depends on a number of factors:-

a) Correct surface preparation:

i) Roughing / scouring increases the number and depth of surface pores. 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, plasticizer from PVC, mould release agents from soling materials and soaps in VR. 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. ‘satreat’ for chlorinates crepe, VR and Thermoplastic Rubber (TR) to allow some specific adhesion to PU 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 for A and B.

c) Correct viscosity and surface tension:

Surface tension should be low to give better wetting of surfaces. Viscosity 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.

 LINK 3

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 is poor cohesion between the two cement layers. To avoid problems it is best to soften on or both dry cement surfaces by heating in a re-activator to the correct temperature. (85-90⁰ 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 confirm 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 to the correct profile. An embossed aluminum foil is available from SATRA which is used to check the pressure distribution in sole bonding.

d) Correct dwell 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.

SURFACE PREPARATION

 SURFACE PREPARATION & BONDING METHODS

In order to obtain good adhesion & bond strength between the materials, a few procedures are followed before, during & after the attachment of the two bonding surfaces. These procedures in steps being

• Coating of the surfaces by adhesive
• Drying of the adhesive coating Attachment of surfaces together.
• Setting.
• Surface preparation.

Surface preparation is pretreatment carried out on the bonding surfaces before the application of an adhesive. The bonding surfaces are generally uneven contains contamination I like oil, dust & chemicals. Further, during lasting pleats are formed by the upper on the lasted margin & these are more prominent across the toe region. Non-uniform spreading of cements between the insole & the upper results an uneven surface.

During the pretreatment, depending on the surface of the material, surfaces are subjected to mechanical or chemical or both mechanical & chemical treatments.   Surface Preparation Consists of

• Scouring the surfaces using 24 to 40 grit emery paper.
•  Roughing of the surfaces using 24 to 36 swg wire brush.
• Dusting & cleaning.
• Solvent wiping.

One or more above operations are required depending on the nature of the material of substrate. During scouring or roughening, light touch of the material with, rotating wire brush or emery paper is required to prevent heat generation.

 Surface PRE-TREATMENT:

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-

• Physical treatment (e.g.-roughing)
• Chemical treatment
• A combination of both.

Chemical primers may be classified as follows:

(i) Cleaners (Solvent wipes): These are solvents which remove greases, oils, plasticizer, mould release agents, oxidation (ageing) products and other contaminants which could reduce 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, Bostic 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 Bostic 123 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. A polymer film is deposited from sole which acts as an intermediate layer, forming a strong bond to both material and adhesive. Each modifier 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 can form strong chemical bonds to the adhesive. They are much more hazardous to use 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. Halogenating Examples are SATRA formulation SDP102, Bostic 313, Bostic 300, Satreat, Super Satreat.

Chlorination solution consists of a highly reactive chlorine liberating substance dissolved in an organic solvent, by addition, substitution or any other reaction. The chlorinating agent contains about 2% of the solid chlorine releasing agent. On application of the solution nascent chlorine is liberated & reacts with the surface material, a layer of chlorinated rubber is formed.

Advantages of using chemical primers

(i)                Often easier and sometimes quicker to use than roughing

(ii)             Less chance to weaken the material.

(iii)           May be the only way with some materials

Disadvantages of using chemical primers

(i)                Hard to see whether a surface has been treated.

(ii)             Solvents may spill and damage non-bonding parts of the shoe.

(iii)           Mechanical adhesion is not greatly improved.

(iv)           Usually produce Toxic/Flammable vapors.

Leather:

Leather is a wonderful and primary material for shoe upper. Its unique properties and characteristics make it the ideal choice for many different applications. Here we will discuss some of the most useful properties of leather.

It has a high tensile strength and is resistant to tearing, flexing and puncturing. This helps leather items last for a long time while retaining their look and feel.

It is a good heat barrier and provides excellent heat insulation. Leather contains a large amount of air and air is a poor conductor of heat. This makes leather a very comfortable item for the human skin.

It is able to hold large quantities of water vapor such as human perspiration and then dissipate it later. This makes leather a comfortable item to wear or sit on.

Leather’s thermostatic properties make it warm in the winter and cool in the summer. This makes leather comfortable to wear.

It can be made to stiffen or can be made to be flexible. It can be molded into a certain shape and then remolded into another shape later.

Leather is resistant to abrasion in both wet and dry environments. This makes leather an excellent protector of human skin.

It is resistant to heat and fire. It is also resistant to fungal growth such as mildew.

It consists of many fibers that are breathable. This breath ability makes it very comfortable to wear in any climate.

Leather can be dyed many different colors that makes it attractive in the production of leather clothing, as a cover for furniture and for many other color sensitive applications.

It is can be soft and supple. Leather clothing becomes a literal second skin. It warms to your body temperature. It is not itchy and does not scratch. It is non-irritating to the skin.

Leather is a fantastic material with excellent physical properties that enables it to be used in many diverse applications from shoe upper to soling.

Textile & fabrics:

Fabric, or cloth, is a supple artificial material which is made up of a network of artificial or natural fibers (yarn or thread) formed by knitting (textiles) or weaving, or pressed into felt. The terms material and fabric are frequently used in the weaving assembly trades such as dressmaking and tailoring, and are synonyms for cloth.

Fabric is most often used in the manufacture of shoe as upper materials. Before woven cloth made its appearance, the roles of textiles had been fulfilled by leather and furs.

There are a large number of different types of fabric, each has its own unique fabric properties such as strength and degree of durability, color hue and color intensity. The thickness, one of the fabric properties, is estimated in deniers. The term “micro-fiber” denotes fibers that are made of strands with the thickness less than one denier. Here are some types of fabric followed by a short description of a few most used types: cotton, wool, silk, polyester, nylon, viscous rayon, Acrylics, jute etc.

Rayon:

Rayon is a manufactured regenerated cellulose fiber. Because it is produced from naturally occurring polymers it is neither a truly synthetic fiber nor a natural fiber; it is a semi-synthetic fiber. Rayon is known by the names viscose rayon and art silk in the textile industry. It usually has a high luster quality giving it a bright sheen.

Rayon is a very versatile fiber and has the same comfort properties as natural fibers. It can imitate the feel and texture of silk, wool, cotton and linen. The fibers are easily dyed in a wide range of colors. Rayon fabrics are soft, smooth, cool, comfortable, and highly absorbent, but they do not insulate body heat, making them ideal for use in hot and humid climates.

The durability and appearance retention of regular rayon are low, especially when wet; also, rayon has the lowest elastic recovery of any fiber. However, HWM rayon is much stronger and exhibits higher durability and appearance retention. Recommended care for regular rayon is dry-cleaning only. HWM rayon can be machine washed.

It is strong and durable.

It is extremely absorbent.

It is soft and comfortable.

It is breathable.

It is easily dyed in vivid colors.

It is abrasion resistant.

It resists insect damage.

It does not pill.

It drapes well and does not have a problem with static.

It wrinkles easily.

It loses 30% to 50% of its strength when wet.

 Wool:

Long and short hair wool at the SouthCentralFamilyFarmResearchCenter in Booneville, Arkansas

The term wool is usually restricted to describing the fibrous protein derived from the specialized skin cells called follicles in sheep.^[1]

Wool is taken from animals in the Caprinae family, principally sheep, but the hair of certain species of other mammals is also sometimes called “wool”, including cashmere from goats, mohair from goats, vicuña, alpaca, and camel from animals in the camel family, and angora from rabbits.

The quality of wool is determined by the following factors, fiber diameter, crimp, yield, color, and staple strength. Fiber diameter is the single most important wool characteristic determining quality and price.

Merino wool is typically 3-5 inches in length and is very fine (between 12-24 microns).^[9] The finest and most valuable wool comes from Merino hoggets. Wool taken from sheep produced for meat is typically more coarse, and has fibers that are 1.5 to 6 inches in length. Damage or breaks in the wool can occur if the sheep is stressed while it is growing its fleece, resulting in a thin spot where the fleece is likely to break.^[10]

Wool is also separated into grades based on the measurement of the wool’s diameter in microns and also its style. These grades may vary depending on the breed or purpose of the wool. For example:

<15.5 – Ultra fine Merino

15.6-18.5 – Superfine Merino

18.6-20 – Fine Merino

20.1-23 – Medium Merino

23< – Strong Merino

Comeback: 21-26 microns, white, 90-180 mm long

Fine crossbred: 27-31 microns, Creedless etc.

Medium crossbred: 32–35 microns

Downs: 23-34 microns, typically lacks luster and brightness. Examples, Aussiedown, Dorset Horn, Suffolk etc.

Coarse crossbred: 36> microns

Carpet wools: 35-45 microns

Cotton:

Cotton is a soft, fluffy, staple fiber that grows in a boll around the seeds of the cotton plant. It is a shrub native to tropical and subtropical regions around the world, including the Americas, India and Africa. The fiber most often is spun into yarn or thread and used to make a soft, breathable textile, which is the most widely used natural-fiber cloth in clothing today.

It is soft

It “breathes”

It absorbs body moisture

It is comfortable

It is strong and durable

It is versatile

It performs well

It has good color retention

It is easy to print on

It wrinkles easily

It is easy to care for, easy to wash

It is a natural resource that is fully renewable

 Silk: 

Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from cocoons made by the larvae of the mulberry silkworm Bombyx mori reared in captivity (sericulture). The shimmering appearance of silk is due to the triangular prism-like structure of the silk fiber which allows silk cloth to refract incoming light at different angles thus producing different colors.

Silks are produced by several other insects, but only the silk of moth caterpillars has been used for textile manufacture. There has been some research into other silks, which differ at the molecular level. Silks are mainly produced by the larvae of insects that complete metamorphosis, but also by some adult insects such as web spinners. Silk production is especially common in the Hymenoptera (bees, wasps, and ants), and is sometimes used in nest construction. Other types of arthropod produce silk, most notably various arachnids such as spiders (see spider silk).

Silk fibers from the Bombyx mori silkworm have a triangular cross section with rounded corners, 5-10 μm wide. The fibroin-heavy chain is composed mostly of beta-sheets, due to a 59-mer amino acid repeat sequence with some variations.^[14]

The flat surfaces of the fibrils reflect light at many angles, giving silk a natural shine. The cross-section from other silkworms can vary in shape and diameter: crescent-like for Anaphe and elongated wedge for tussah. Silkworm fibers are naturally extruded from two silkworm glands as a pair of primary filaments (brin) which are stuck together, with sericin proteins acting like glue, to form a bave. Bave diameters for tussah silk can reach 65 μm. See cited reference for cross-sectional SEM photographs.

Silk has a smooth, soft texture that is not slippery, unlike many synthetic fibers. Its denier is 4.5 g/d when dry and 2.8-4.0 g/d when moist.

Silk is one of the strongest natural fibers but loses up to 20% of its strength when wet. It has a good moisture regain of 11%. Its elasticity is moderate to poor: if elongated even a small amount, it remains stretched. It can be weakened if exposed to too much sunlight. It may also be attacked by insects, especially if left dirty.

Silk is a poor conductor of electricity and thus susceptible to static cling.

Unwashed silk chiffon may shrink up to 8% due to a relaxation of the fiber macrostructure. So silk should either be pre-washed prior to garment construction, or dry cleaned. Dry cleaning may still shrink the chiffon up to 4%. Occasionally, this shrinkage can be reversed by a gentle steaming with a press cloth. There is almost no gradual shrinkage nor shrinkage due to molecular-level deformation.

Silk is made up of the amino acids Gly-Ser-Gly-Ala and forms Beta pleated sheets. H-bonds form between chains, and side chains form above and below the plane of the H-bond network.

The high proportion (50%) of glycine, which is a small amino acid, allows tight packing and the fibers are strong and resistant to stretching. The tensile strength is due to the many interseeded hydrogen bonds. Since the protein forms a Beta sheet, when stretched the force is applied to these strong bonds and they do not break.

Silk is resistant to most mineral acids, except for sulfuric acid which dissolves it. It is yellowed by perspiration.

Nylon: 

Nylon is a generic designation for a family of synthetic polymers known generically as polyamides and first produced on February 28, 1935 by Wallace Caruthers at DuPont. Nylon is one of the most commonly used polymers.

Variation of luster: nylon has the ability to be very lustrous, semi lustrous or dull.

Durability: its high tenacity fibers are used for seatbelts, tire cords, ballistic cloth and other uses.

High elongation

Excellent abrasion resistance

Highly resilient (nylon fabrics are heat-set)

Paved the way for easy-care garments

High resistance to insects, fungi, animals, as well as molds, mildew, rot and many chemicals

Used in carpets and nylon stockings

Melts instead of burning

Used in many military applications

Good specific strength

Transparent under infrared light (-12dB) ^[2]

Above their melting temperatures, T_m, thermoplastics like nylon are amorphous solids or viscous fluids in which the chains approximate random coils. Below T_m, amorphous regions alternate with regions which are lamellar crystals. The amorphous regions contribute elasticity and the crystalline regions contribute strength and rigidity. The planar amide (-CO-NH-) groups are very polar, so nylon forms multiple hydrogen bonds among adjacent strands. Because the nylon backbone is so regular and symmetrical, especially if all the amide bonds are in the trans configuration, nylons often have high crystallinity and make excellent fibers. The amount of crystallinity depends on the details of formation, as well as on the kind of nylon. Apparently it can never be quenched from a melt as a completely amorphous solid.

Nylon 6,6 can have multiple parallel strands aligned with their neighboring peptide bonds at coordinated separations of exactly 6 and 4 carbons for considerable lengths, so the carbonyl oxygen and amide hydrogen’s can line up to form interchain hydrogen bonds repeatedly, without interruption. Nylon 5,10 can have coordinated runs of 5 and 8 carbons. Thus parallel (but not ant parallel) strands can participate in extended, unbroken, multi-chain β-pleated sheets, a strong and tough super molecular structure similar to that found in natural silk fibroin and the β-keratins in feathers. (Proteins have only an amino acid α-carbon separating sequential -CO-NH- groups.) Nylon 6 will form uninterrupted H-bonded sheets with mixed directionalities, but the β-sheet wrinkling is somewhat different. The three-dimensional disposition of each alkenes hydrocarbon chain depends on rotations about the 109.47° tetrahedral bonds of singly-bonded carbon atoms.

Block nylon tends to be less crystalline, except near the surfaces due to shearing stresses during formation. Nylon is clear and colorless, or milky, but is easily dyed. Multithreaded nylon cord and rope is slippery and tends to unravel. The ends can be melted and fused with a heat source such as a flame or electrode to prevent this.

1) It is strong and elastic.
2) It is easy to launder.
3) It dries quickly.
4) It retains its shape.
5) It is resilient and responsive

Polyester: 

Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many types of polyester, the term “polyester” as a specific material most commonly refers to polyethylene terephthalate (PET). Polyesters include naturally-occurring chemicals, such as in the cetin of plant cuticles, as well as synthetics through step-growth polymerization such as polycarbonate and polybutyrate. Natural polyesters and a few synthetic ones are biodegradable, but most synthetic polyesters are not.

Depending on the chemical structure polyester can be a thermoplastic or thermo set, however the most common polyesters are thermoplastics.

Fabrics woven from polyester thread or yarn are used extensively in apparel and home furnishings, from shirts and pants to jackets and hats, bed sheets, blankets and upholstered furniture. Industrial polyester fibers, yarns and ropes are used in tyre reinforcements, fabrics for conveyor belts, safety belts, coated fabrics and plastic reinforcements with high-energy absorption. Polyester fiber is used as cushioning and insulating material in pillows, comforters and upholstery padding.

While synthetic clothing in general is perceived by some as having a less-natural feel compared to fabrics woven from natural fibers (such as cotton and wool), polyester fabrics can provide specific advantages over natural fabrics, such as improved wrinkle resistance. As a result, polyester fibers are sometimes spun together with natural fibers to produce a cloth with blended properties. Synthetic fibers also can create materials with superior water, wind and environmental resistance compared to plant-derived fibers.

Polyesters are also used to make “plastic” bottles, films, tarpaulin, canoes, liquid crystal displays, holograms, filters, dielectric film for capacitors, film insulation for wire and insulating tapes.

Liquid crystalline polyesters are among the first industrially-used liquid crystalline polymers. They are used for their mechanical properties and heat-resistance. These traits are also important in their application as an abatable seal in jet engines.

Polyesters are widely used as a finish on high-quality wood products such as guitars, pianos and vehicle / yacht interiors. Burns Guitars, Rolls Royce and Sun seeker are a few companies that use polyesters to finish their products. Thixotropic properties of spray-applicable polyesters make them ideal for use on open-grain timbers, as they can quickly fill wood grain, with a high-build film thickness per coat. Cured polyesters can be sanded and polished to a high-gloss, durable finish.

The properties of polyester fabrics include: inexpensive cost; superior strength and resilience; lightweight; hydrophobic (it feels dry or moves moisture effects away from touch); it has an unusually high melting point; is resistant to dyes, solvents and most chemicals; stain resistant; resists stretching and shrinking; quick drying; wrinkle, mildew and abrasion resistant; retains heat-set pleats and creases and is easy to launder.

The fabric can also develop small fuzz balls or pills, which may be related to friction, abrasion resistance, and stiffness and breaking strength, according to a University of Tennessee, Knoxville paper. Polyester is sensitive to alkalizes and resistant to most conventional textile bleaches. It is oleophilic, which means that it is difficult to remove oil stains from the fabric. It exhibits static cling tendencies and it is frequently used in fabrics that give the appearance of being bright and shiny. Newer micro fibers offer softer appearance potentials and are texturally more similar to the luster and feel of silk. It has good fade resistance, particularly when protected from UV radiation and it is noted to retain its shape. Not all polyesters have the same properties and characteristics but they will share most of them.

1) It is resists wrinkling.
2) It is easy to launder.
3) It dries quickly.
4) It is resistant to stretching and shrinking.

Jute:

Jute is a long, soft, shiny vegetable fiber that can be spun into coarse, strong threads. It is reduced from plants in the genus Corchorus, family Tiliaceae.

Jute is one of the most affordable natural fibers and is second only to cotton in amount produced and variety of uses. Jute fibres are composed primarily of the plant materials cellulose (major component of plant fiber) and lignin (major components of wood fiber). It is thus a lignocelluloses fiber that is partially a textile fiber and partially wood. It falls into the best fiber category (fiber collected from bast or skin of the plant) along with knave, industrial hemp, flax (linen), ramie, etc. The industrial term for jute fiber is raw jute. The fibers are off-white to brown, and 1–4 meters (3–12 feet) long.

Jute fiber is often called Hessian; jute fabrics are also called Hessian cloth and jute sacks are called gunny bags in some European countries. The fabric made from jute is popularly known as burlap in North America.

Jute fiber is 100% bio-degradable and recyclable and thus environmentally friendly.

It is a natural fiber with golden and silky shine and hence called The Golden Fiber.

It is the cheapest vegetable fiber procured from the best or skin of the plant’s stem.

It is the second most important vegetable fiber after cotton, in terms of usage, global consumption, production, and availability.

It has high tensile strength, low extensibility, and ensures better breath ability of fabrics. Therefore, jute is very suitable in agricultural commodity bulk packaging.

It helps to make best quality industrial yarn, fabric, net, and sacks. It is one of the most versatile natural fibers that have been used in raw materials for packaging, textiles, non-textile, construction, and agricultural sectors. Bulking of yarn results in a reduced breaking tenacity and an increased breaking extensibility when blended as a ternary blend.

Unlike the hemp fiber, jute is not a form of cannabis.

Advantages of jute include good insulating and antistatic properties, as well as having low thermal conductivity and a moderate moisture regain. Other advantages of jute include acoustic insulating properties and manufacture with no skin irritations.

Jute has the ability to be blended with other fibers, both synthetic and natural, and accepts cellulose dye classes such as natural, basic, vat, sulfur, reactive, and pigment dyes. As the demand for natural comfort fibers increases, the demand for jute and other natural fibers that can be blended with cotton will increase. To meet this demand, it has been suggested that the natural fiber industry adopt the Reiter’s Elite system, in order to modernize processing. The resulting jute/cotton yarns will produce fabrics with a reduced cost of wet processing treatments. Jute can also be blended with wool. By treating jute with caustic soda, crimp, softness, pliability, and appearance is improved, aiding in its ability to be spun with wool. Liquid ammonia has a similar effect on jute, as well as the added characteristic of improving flame resistance when treated with flame proofing agents.

Some noted disadvantages include poor derivability and crease resistance, brittleness, fiber shedding, and yellowing in sunlight. However, preparation of fabrics with castor oil lubricants result in less yellowing and less fabric weight loss, as well as increased dyeing brilliance. Jute has a decreased strength when wet, and also becomes subject to microbial attack in humid climates. Jute can be processed with an enzyme in order to reduce some of its brittleness and stiffness. Once treated with an enzyme, jute shows an affinity to readily accept natural dyes, which can be made from marigold flower extract. In one attempt to dye jute fabric with this extract, bleached fabric was mordent with ferrous soleplate, increasing the fabric’s dye uptake value. Jute also responds well to reactive dyeing.

 Acrylic fibre:

Acrylic fibers are synthetic fibers made from a polymer (Polyacrylonitrile) with an average molecular weight of ~100,000, about 1900 monomer units. To be called acrylic in the U.S, the polymer must contain at least 85% acrylonitrile monomer. Typical co monomers are vinyl acetate or methyl acryl ate. The DuPont Corporation created the first acrylic fibers in 1941 and trademarked them under the name “Orlon”.

Acrylic is lightweight, soft, and warm, with a wool-like feel. Acrylic is colored before it is turned into a fiber as it does not dye very well but has excellent colorfastness. Its fibers aren’t very resilient, and wrinkle easily, but most acrylic fabrics have good wrinkle resistance. Acrylic has recently been used in clothing as a less expensive alternative to cashmere, due to the similar feeling of the materials. The disadvantages of acrylic are that it tends to fuzz or pill easily and that it does not insulate the wearer as well as cashmere. Many products like fake pashmina or cash mina use this fiber to create the illusion of cashmere.

Acrylic is resistant to moths, oils, chemicals, and is very resistant to deterioration from sunlight exposure. However, static and pilling can be a problem.

Acrylic has a bad reputation amongst many knitters – however cheap the yarn is, its performance does not come near natural fibers. Also, some knitters complain that the fiber “squeaks” when knitted.

Acrylic can irritate the skin of people with eczema.

Others:

A small extension of Polyethylene, Polyvinyl acetate, Polyvinyl chloride(PVC), Polyurethane(PU), Thermoplastic Rubber(TPR), Cellulose Rubber etc are used in footwear industry as upper materials. However, they are highly used as the soling materials in footwear industry

PHYSICAL TEST FOR SOLE BONDING

 BONDING TEST:

Sole bonding system of PVC with different upper materials

 Soling materials:

PVC preparation:

• Only roughing
• Roughing and solvent wiping

Adhesive: PU

 REFERENCES

 BS 1610 Part 1: I 992 – Specification for the grading of forces applied by materials testing machines.

SATRA test method AM2: 1992 – Preparation of water borne and solvent borne, bonded assemblies for peel tests.

SATRA test method AM 34: 1992 -Preparation of hot molt bonded assemblies for peel tests.

SCOPE

This method is intended to determine the tensile strength .The method is applicable to all types of bonded joint.

PRINCIPLE

Test specimens are cut from a bonded assembly which has been previously prepared, typically using the procedure described in:

SATRA Test Method AM2-   Solvent borne or water borne adhesive.

SATRA Test Method AM M – 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.

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 15 N, to an accuracy of 2% as specified by ‘Grade 2″ in BS 1610 Part 1: 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. A steel rule or vernier calipers 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 25 mm and length 100± 25 mm. For each bonded assembly, see Figure use the knife.

SATRA test method AMI4 Method 1 (pre-coated materials)

The procedure in AM 14 Method 1 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.

PROCEDURE

• 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.

Optional Pre-treatments

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 a/located to the same pro-treatment nor both tested without a pre-treatment.

The four main types of pre-treatment are:

Room temperature storage

Store the test specimens, not in contact with each other, at 20 ± 2°C for a minimum, of 48 hours, At least the fast 48 hours of this storage should be in a standard controlled humidity of 65± 2% rh, This treatment can be carried nut on uncut sheet materials if this is more convenient.

 Wetting and drying

Immerse the test specimens in distilled water at a temperature of 20 ± 2°C for 6.0 ± 0.5 hours. The Initial wetting of the adherents can be assisted by placing the distilled water and test specimens in a vacuum vessel and reducing the pressure to 5kPa, for about three minutes. Feel half the test specimens immediately after removal from the water. The other specimens should be stored in a standard controlled environment of 20 ± 2°C and 65 ± 2% rh for 48 ± 1 hour to allow them to dry before being peeled.

 Heat ageing

Store the test specimens, not in contact with each other, for 14 days at a temperature of 50 ± 2°C. Then re-condition them in a standard controlled environment of 20 ± 2″C and 65 ± 2% rh for a minimum of 48 hours,

 High humidity

Store the test specimens, not in contact with each other, for 28 days at 40 ± 2°C at a humidity of 97 ± 2% rh. Peel half the test specimens immediately after removal from the high humidity atmosphere. The other specimens should be stored in a standard controlled environment of 20 ± 2°C and 65 ± 2% rh for 48 ± I hours before being peeled.

 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.

 On all separated lest specimen; estimate, where possible to the nearest 5%. 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 Newton’s, see section 8.1. If failure is by tearing through either adherent with no subsequent peeling, measure the peak force.

For each test specimen divide the average peeling force (6.8) by the width of the specimen in 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 AM H Method 1 (pre-coated materials) calculate the arithmetic mean peel strength for the two test specimens 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 in N/mm.

Test Report:

For PVC Sole

Additional Notes:

Estimating the average peeling force on a force versus extension graph

The average peeling force can be estimated by visually comparing areas. When a horizontal line is drawn at the average peeling force (line XY in Figure) the area bounded by the line and the curve above the line is equal to the area bounded by the line and curve below the line.

 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 traces of solvent which have a plasticizing effect. It would be uneconomic to wait until all 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.

 GREEN STRENGTH TEST

Sole to upper adhesion of s shoe at the toe, sides and heel portion could be found out using “SATRA” sole adhesion tester. The equipment consists of a load measuring device in contact with a contact with a cantilever beam. Toe or heel pieces confirming to the curvature of the toe or heel are fixed to the free end of the beam.

The lasted shoe is supported on the anvil. The curvature of separation is held against toe piece between the sole and the toe. The shoe is pressed on the anvil until the toe is separated from the sole. The reading on the load measuring gauge is noted. Similarly, the load required separating at sole to heel and sole to the sides of the shoe are found out.

Guidelines: Upper to PVC Sole Adhesion Test

For Men’s Shoe

Additional Notes:

Estimating the average peeling force on a force versus extension graph.The average peeling force can be estimated by visually comparing areas.When a horizontal line is drawn at the average peeling force (line XY in figure)The area bounded by the line and the curve above the line is equal to the area bounded by the line and curve below the line.

RECOMMENDATION & CONCLUSION

PVC sole bonding to upper is mainly depending on –

i)                   Sole Priming or halogenation

ii)                Upper surface preparation

iii)              Drying time of the adhesive

iv)               Properly application of adhesive i.e. right adhesive for the right sole & upper materials

v)                 The coat of adhesive must be even,

vi)              Adhesive reactivation temperature& time

vii)            Sole attaching pressure.

Considering the above condition, I suggested the following machine can be used for –

i)                   improving sole Bonding

ii)                increasing Productivity

iii)              decreasing Rejection

iv)              better Quality achievement

v)                 skill up manpower

Machine Requirement:

• Automatic shoe bottom roughing machine
• Automatic Shoe Bottom Cementing
• Combine Roughing & Cementing Machine

Conclusion

The shoes usually have a positive sexual identity with distinct differences in design to establish the “separation of the sexes”. Our psychosexual personality & general personal image is mirrored in the shoes we wear. Shoes are needed for foot protection against hard pavements, or as protection against rocky terrain or infection. Thus sole plays a vital role in a shoe. So the adhesion of sole is very important in manufacturing shoes.

According to the peel bond strength test I found average better result by doing Physical treatment (roughing). Roughing operation can help better bonding between PVC with Neoprene adhesive. I found the highest bond strength using Dendrite PU adhesive in corrected grain leather sample. The strength is 3N/mm for PVC sole. There is some deviation from standard value in my bonding system for some limitation. They are –

• The bonding pressure is applied by downing Hydraulic Clicking Press Machine’s head.
•  The pressure is applied for 30 min.
•  There is bonding problem for having some cleats in some of my soling samples.

I think I will find better result if I can follow the standard method properly and using better quality adhesive, primer, sole and upper materials. As I found good result in Green test the peel bond strength test results will be good when the standard bonding method is followed.

BIBLIOGRAPHY

1. World Footwear, vol. 07,No. 1, Jan/Feb, 1993
2. World Footwear, vol. 13,No.4, Jul/Aug, 1999
3. World Footwear,vol.14, No.5, Sep/Oct, 2000
4. World Footwear, vol. I 5,No. 1, Jan/Feb, 2001
5. Footwear Wikipedia
6. FDDI Footwear Digest,Issue.24, Jan,2000
7.  FDDI Lasting Handouts, vol. I
8.  SATRA Bulletin, Jan,1995
9. SATRA Bulletin,Jun.1999
10.  SATRA Bulletin,Dec,1999
11. Lecture Notes Provided by teachers.
12. Manual of Shoe Making by the Training Department CLARKS Ltd.
13. Adhesive In Shoe Manufacture by B. Venkatappaiah