MANUFACTURING TECHNOLOGY OF RUBBER FOORTWEAR
INTRODUCTION
A shoe is an item of footwear intended to protect and comfort the human foot while doing various activities. Modern footwear varies widely in style, complexity and cost.All shoes have a sole which is the bottom part of the shoe in contact with the ground.
Rubber and plastics are common use these days as rubber sole. Modern shoes have soles made from Natural rubber, Styrene Butadiene rubber , Thermoplastic Rubber(SBS), Polyurethane rubber, Halobutyl rubber and PVC Compounds or a blend of PVC or NBR/BIIR etc.
Materials for Footwear:
Though leather was used as original footwear but gradually man also learnt to use fabric along with leather for comfort and for slipper or sandal use, wood become dominant as men learn wood crafts. Surprisingly all the above three components are still being used in the shoe industry today. following materials are commonly used in a shoe industry today Leather, Plastic, Rubber, Textiles, wood, Jute, Metal.
Because of traction, comfort and durability rubberized sole is still popular in any formal or special application of footwear today.
Soles can be simple a single material in a single layer or they can be complex with multiple in a single layer or they can be complex with multiple structures or layers and materials. When various layers are used soles may consist of an insole, midsole, and an outsole.
The following materials are used in making modern shoes.
Adhesives Buckle, Counterfort, Eyelet, Heel, Hook, Insole, Laces, Shank, Sole, Upper, Tack,Tread, Welt.
SHOE SOLE:
It has two parts, the insole and the outsole. The insole is the interior bottom of a shoe which sits direct beneath the foot under the foot bed.The purpose of insole is to attach to the lasting margin of the upper which is wrapped around the last during the closing of the shoe during the lasting operation.Insoles are usually made of cellulosic paper board or synthetic non woven insole board.
Many shoes have removable and replaceable foot beds. Extra cushioning is often added for comfort(to controlt he shape,moisture or smell of the shoe)
The out sole is the layer in direct contact with the ground.Dress shoes often have leather or resin rubber out-soles. casual or work oriented shoes have out sole made of Natural rubber or a synthetic rubbers. The out sole may comprise a single piece or may be an assembly of separate pieces often of different materials.Forexample in tennis shoe, the outer sole should be at least in two layers and the part that touches the ground will be made with halobutyl rubber to prevent skidding.
On some shoes the heel of the sole has a rubber plate for durability and traction while the front is leather for style. Specialized shoes will often have modification on this design.Athletic shoes like soccer, rugby, baseball and golf shoes have spikes embedded in the out-sole to improve grip
The midsole is the layer in between the outside and the insole typically there for shock absorption.Some types of shoes like running shoes have additional material for shock absorption, usually beneath the heel of the foot.Some shoes may not have a midsole at all.The heel is the bottom rear part of the foot. They are often made of the same material as a sole of the shoe.
UPPER SHOE:
The upper helps hold the shoe onto the foot.In the simplest cases, Such as sandals or flip slops this may be nothing more than few straps for holding the sole in place. Closed footwear such as boots,Trainers and most men shoes will have a more complex upper.This part is often decorated or is made in a certain style to look attractive.
TYPES OF SHOES
There are a wide variety of different types of shoes.Most types of shoes are designed for specific activities. For Example boots are typically designed for work or heavy outdoor use.Athletic shoes are designed for particular sports such as running, walking.
Sandals , High heel seals, dress and formal shoes, work shoes, Athletic Shoes,Dance shoes, Boots, rain boots winter boots.
Sandals:
They are a type of footwear that consist of a covering for the sole of the feet and straps or ties which hold the scandal onto the foot.In many countries scandals are also called as slippers.
Plastic moulded EVA products generally injection moulded were available in attractive colour and was very popular in end 1990 s and they can be used in rain or water as well.
Use of Rubber In Footwear Industry:
NR, SBR, High styrene SBR, BR, NBR, Polyuethene, BIIR, SBS etc have been widely used as rubber sole. Thermoplastic rubber soles are made by injection moulding and compounding rubber granules.
Manufacture of Rubberized shoe sole:
Rubberized shoe soles can be made out of a variety of products, depending on the type of shoe and its purpose.
Rubber compounds :
1. Type of rubber (NR grades, BR grades, SBR, NBR, Polyurethane, SBS etc)
2. Blend of rubber ( Combined properties of rubber in desired ratio)
3. Carbon black
4. Type of process oil/ processing aids
5. Type of Antioxidants
6.Type of Cure
7. Cure level
Canvas Shoe:
When upper materials is made with rubberized fabric they are generally called canvas shoe. Fabric may be of cotton, rayon, nylon, polyester etc. Shoes have traditionally been made from leather, wood or canvas but are increasingly made from rubber, plastics and other petrochemical derived materials.
A canvas shoe is stiff soled , protective foot wear that encloses the whole foot.The upper part is made up to a closely woven, plain woven fabric made of natural or synthetic fibers and the sole part is generally a plastic material but can also be made with rubber sole as well.
Polymers;
Natural Rubber:
Natural rubber, Hevea brasiliensis,
is approximately 96% cis-1,4 polyisoprene with proteins and lipids making
up the remaining 4% [4]. The high linearity of chains results in the unique
property of strain crystallisation. Due to its highly regular structure, it
rapidly crystallises upon stretching, resulting in high tensile strength and
tear resistance even in a pure gum or an unreinforced vulcanisate.
It is used in many applications
where high tear strength is needed. Natural rubber also has good resistance to
flexing, fatigue, and heat generation, which makes it useful in an outsole
compound that is flexed thousands
of times per day. Abrasion resistance is high. Natural rubber is
tough and nervy due to
its high molecular weight (MW). Before blending with other
polymers, it is often
pre-masticated in an internal mixer (above 120 °C) or on the mill
(below 80 °C) to ‘break
down’ or lower the molecular weight of the chains [3].
Chemical peptisers such
as pentachlorothophenol are used to lower the temperature needed.
Silica
There are two types
of silica fillers: precipitated and fumed.
Fumed, pyrogenic or
anhydrous silica is produced by treating silicone tetrachloride with hydrogenand
oxygen at high temperatures.
A fine particle size silica without any
absorbed water is produced. The extra cost is justified only for silicone
rubbers and transparent compounds. Precipitated grades are produced by the
controlled precipitation of sodium silicate with an acid. Process conditions
can be varied to produce grades from semi- to highly reinforcing.
Silica grades are characterized in the same way as
carbon black; by particle size, surface area (CT AB absorpt ion) and structure (DBP
absorption).Silica particles have highly reactive hydroxyl groups on the
surface that will react with accelerators to slow
the cure rate of most compounds. Coating the particle surface with polyethylene
glycol (PEG), diethylene glycol (DEG),or another polyol will reduce the number
of sites for the accelerator to bond.
Coupling agents are
used with silicas to enhance reinforcement, abrasion resistance, and
dynamic properties.
Silane coupling agents have amino, mercapto, or polysulphidic
linkages. Commonly
used coupling agents are Degussa’s Si69, and OSI’s A1289.
Other white fillers
Other white
fillers, range from inert fillers such as ground calcium carbonate to
semireinforcing Aluminium silicate.Clays are naturally occurring aluminium
silicates and are the most widely used non-black filler. They are low cost and
provide low to moderate levels of reinforcement.
Clays are dried,
pulverised, and are separated by either a water or air process. Water washed
clays have far fewer impurities such as mica or quartz. Both water washed and
air floated clays are further classified as hard or soft, depending on the
hardness levels produced in rubber.
Clays can be
calcined (bound water is removed) or modified with silane coupling agents
tofurther improve reinforcement. Clays have a retarding effect on the cure rate
and also need to be treated with diethylene glycol (DEG), PEG or other polyols.
Calcium carbonate
can be produced naturally by grinding limestone, chalk or other
naturally occurring
sources or synthetically by precipitation. Ground calcium carbonate
(GCC) is often used
just as an extender, at levels up to 250 phr. The particle size and
surface area of
precipitated calcium carbonates (PCC) can be controlled well; PCC grades
are
semi-reinforcing. Calcium carbonates are basic in nature and do not affect
compound
cure rate
Synthetic silicates
such as aluminium silicates, sodium aluminium silicates, magnesium
aluminium
silicates, and calcium silicates are comparable to low structure,
semi-reinforcing silicas.
Titanium dioxide
Titanium dioxide,
TiO2 is the most commonly used white pigment; it exists in
two
forms: rutile and
anatase. In the naturally occurring forms, anatase TiO2
is a blue-white colour, compared to the more
yellow colour of the rutile grade. Modified rutile grades have the same
blue-white colour but have 20–25% better colour coverage, and the compound has
better mechanical properties after weathering . A blue-white colouris generally
preferred for making a brighter white product.
Methods
of manufacture
Outsoles are commonly
made by compression moulding a preform of one or more rubbersin a mould that is
cured in a hydraulic press under high temperature and pressure. Presses may be heated by hot oil, steam or electricity. During
the cure cycle moulds are usually ‘bumped,’
the press is opened to allow air to escape.
Outsoles can also
be produced by injection moulding, although the number of different
colours or
different rubbers is limited to two in a single moulding operation at the
present
time. The design is also more limited
than in compression moulding.
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