Manufacture
of Rubber Products
PVC
Belting
These
are used in flat running belts and in underground mining belts. They are of the
following types: (a) monoply or solid woven; (b) duo-ply.
The ply
belts have doubled cotton/nylon or doubled viscose/nylon as a-reinforcing
fabric. The reinforcing component should meet the flame test requirements,
create sufficient mechanical fastening, and increase the belt modulus.
For
high strength belts, solid-woven fabrics are used. These lead to improved
roughing characteristics, less ply separations, and easier splicing. In a
solid-woven fabric, two or more weft layers are interlaced by warp threads.
Solid-woven belting always means using PVC as a matrix material. Only a low
viscosity PVC paste can penetrate into the thick solid woven fabrics. To get
sufficient adhesion, the synthetic filament yarns in warp and weft often are
twisted together with cotton.
Steel
Cord Belting
The
longitudinal steel cord reinforcement gives a belt with a high tensile strength,
Thus, very long centres and high lifts or falls can be used. The high strength
toweight ratio enables the steel cord belting to carry much more load for a
given drive at a given rate of power.
A
comparison of the different belting types is given in Table 17.8.
Comparison of different of conveyor
belting
Sl. No. Characteristic
Cotton Nylon
Polyester/Nylon Steel
1.
Longitudinal 150
200 200 500
Strength to
to to to
(kN/m) 700
2000 2000 7000
2.
Belt weight 7.5
6.5 8.0 12.5
(kN/m) to to
to to
22.5 21.5
25.0 60.0
3.
Extensibility
At working load
(%) 3
3 2
0.5
4.
Maximum haul length
On single endless
belt (m) 700 4000
5000 10,000
5. Typical belt speed (m/s)
1.5 3
3 9
The
strength member-steel cord-is a twisted composite of several strands of
high-carbon zinc-coated steel wires. The construction has to be varied for
different strength ratings. In order to ensure long-term corrosion protection
during end-use, it is important that rubber penetrates fully into the
interstices of the cords, thereby preventing all chances of capillary wicking.
A specially formulated “bonder” compound, in addition to its high adhesion
characteristic, which can also flow into the interstices of the cord during the
vulcanizing process is used. Cover rubbers of a variety of grades to suit
service requirements are applied. During manufacturing, special care is taken to
ensure cord-planarity and tension controls to avoid crooked running. Special
splicing techniques are also used for steel cord belting.
The
advantage of steel cord belting.
(a)
Higher
tension rating, larger span, and higher throughput compared to textile belting;
(b)
Low
elongation and little take-up space;
(c)
Good
troughability;
(d)
Good
dynamic performance and longer life; and
(e)
Good
splicing.
HOSES
Hoses
are reinforced polymeric pipes, mainly used for transferring materials (liquids,
gases and solids up to a limited particle size) under pressure. Unlike metal
pipes, polymeric hoses are flexible, they absorb vibration, handle corrosive
fluids, dampen sound, store easily, and are available in a large range of
sizes.
Hoses
can be broadly divided into end-use, specific working pressure, and diameter
categories. End-uses may be domestic (e.g. garden hose), automotive, hydraulic
and pneumatic, oil and solvent, mining, or general industrial applications.
Hoses
may be divided into low pressure (below 20 bar) medium pressure (20-70 bar) or
high pressure (above 70 bar), based on specific working pressure. A particular
hose should be designed taking into account the specific material to be carried
(solvent, steam, etc.) application. Specific working pressure and temperature,
carrying rate, and life expectancy.
Design
of Hoses:
A
rubber hose consists of three main components:
(a)
a
lining or rubber tubing;
(b)
reinforcement (which may be braided, spiralled, circular woven, knitted, or
wrapped with a range of reinforcing textiles (cotton, rayon, nylon, polyester,
polypropylene, steel wire, glass fibre, or aramid); and
(c)
a
cover for outer protection (Fig. 17.9).
Lining
carries the material being handled, transmits the pressure to the reinforcement,
and is made of suitable compounded rubbers which are impervious to and
unaffected by the material being handled. For braided hoses, the lining
compound should have good extrudability and flow readily around the braid. The
compound should have sufficient stiffness to withstand the tension during
braiding and vulcanize to a smooth and seamless finish so that it offers minimum
resistance to the flow of material in contact with it.
Reinforcement: The reinforcement of hoses may be done in five different ways
braiding, spiralling, weaving, knitting, and wrapping. Braiding is a simple
process which gives a flexible hose and is more resistant to misuse (better
impulse and flex performance) than spiralling. Spiralling can be done at higher
speeds (10 times) than braiding, giving high bursting pressures and good impulse
resistance.
Circular woven hoses are foldable and are useful in fire hoses. Vertical
knitting can be done at four4 times the production rate of horizontal braiding
machines and the hoses can have good flexibility and range of diameters, e.g.
automotive coolant hoses.
Wrapping is suitable for large diameter hoses, e.g. marine hose.
The
last three processes are not suited for high burst pressure applications. When
more than one layer of reinforcement is used, an additional insulation rubber is
generally applied between the layers.
Cover:
To prevent damage to the reinforcement member in service, a suitable rubber
compound is used as cover. The cover finish may be smooth or fluted. The burst
pressure is generally determined to be four times the operating pressure. Burst
pressure of a hose is calculated by the formulae below:
(a)
Braided or spiralled hose:
2 N R Sinθ
__________
P = DL
P = burst pressure,
Where N
= no. of yarns or cords, R = breaking strength of single yarn/cord, D =
diameter, L = pitch length, and θ = braid angle.
(b)
Wrapped hose:
2
V F
2 (2S R Sin θ) N
P = DL
= DL
Where P
= burst pressure, V = hoop force, D = mean ply diameter, L = length of fabric
unit, S = fabric strength, N = no. of plies, F = efficiency factor, and θ =
braid angle.
The
angle of application of the reinforcing material around a hose lining is known
as braid angle. Braiding is normally done at 54° 44’ which is mathematically
derived as the natural angle. At this angle, the length and diameter change
under pressure will be the minimum.
Testing
procedures and requirements for hoses are specified in various standards such as
BIS, BS, ISO, DIN, and SAE.
Hose
Manufacture
Although there are many different processes and machinery used for making hoses
these can be divided into the following broad categories by diameter, pressure,
and production methods (plate 16, Exhibit 17.9). a classification of different
hose types is presented in Fig. 17.10.
Hand-made hoses: These cover a range of sizes from fractional bore vacuum hoses
to 40” diameter sleeves. This process is most suited for large diameter hoses
for oil suction and discharge (up to 600-mm diameters), dreading, etc. These
hoses are generally hand-built on mandrels using wrapping and are cured in
vulcanizers. Calendered inner liners is applied on mandrels, then rubberize
woven fabric, and lastly, Calendered cover rubber, all to the requisite gauge
and width. Many layers of wrappers are applied and the mass cured in
vulcanizers. A common feature of many hand-built hose constructions is
inclusion of several types of wire, braided wire, or woven fabric to improve the
strength or collapse resistance. For example, for oil suction and discharge
hoses, a helix of heavy metal strip or a thick wire is built onto the carcass to
provide rigidity. Special end fittings r couplings may be built in to provide
for special
Manufacture
High |
Spiralled or
Braided,
Lead press/
Lead extruder
vulcanizer |
Hand built on mandrels, reinforced with
Several types of wire, braided wire,
Woven fabric, wrapped and cured in
Vulcanizer,
Not suitable for automated or continuous
manufacture. |
|
20,000 psi
to
11,000 psi
Pressure
Psi
Medium
3000 psi
Low
20 psi
|
Hydraulic
|
Rotary oil drilling
|
|
|
Sand blast
|
Oil suction
and
Discharge
Fire hoses |
Off shore
Oil transportation,
Aircraft refuelling
marine
|
Food
Welding
Fuel & emission
Automotive
-hydraulic brake
-radiator
-air brake
-power steering
water
gas/air
LPG
|
Oil dock
Water |
Chemical
Hose
(XLPE
cover) |
|
0 |
38
150
Hose bore (mm)→ |
The
process is not suited for continuous vulcanization methods, but development
efforts are going on to make large bore hoses with semi-automatic or automatic
devices.
Circular woven hoses: Fire hoses are mostly produced by this method. Yarns are
applied with a circular knitting machine. The pattern may be plain knit, lock
stitch, or warp knit, depending on the design of the knitter. The process is
usually limited to low burst pressure use such as garden hose and radiator hose.
Reinforcement for fire hoses consists of long lengths of fabrics woven on
circular looms. The warp is run in the longitudinal direction and weft threads
are interlaced to produce a seamless circular weave. The lining is either
extruded or hand-built from Calendered material, either unvulcanized or partly
vulcanized; this is inserted into the woven jacket.
The
ends are clamped and steam is admitted through the lining. This blows out and
presses the lining against and into the jacket during vulcanization. The woven
fabric itself acts as a cover and is usually treated to prevent mildew
formation. The main advantage of this hose is that it can be coiled flat and,
therefore, a long length can be accumulated in a small coil-size range is 37-62
mm is diameter.
Wrapped
ply hoses: In this process, low pressure hoses up to 40 m continuous length can
be made on simple, inexpensive equipment with accurate bore diameter.
The process
involves wrapping several plies of rubber impregnated square woven fabric cut at
45° around an extruded tube on a mandrel in three roll building machine.
The
mandrels are usually made of steel for smaller diameter hoses and of light alloy
nominal bore diameter of the hose, and its maximum length can be 40m. The cut
fabric is applied lengthwise in strip form through rollers. Extruded cover is
also applied lengthways and consolidated. A wrapping cloth (usually of nylon) is
applied over the built hose and the assembly is vulcanized. After
vulcanization, the cloth is removed and the cured hose extracted from the
mandrel. These hoses have poor appearance and low flexibility. For suction
applications, hoses can be built with metal helix or springs under or above the
inner lining.
Braided/spiralled hoses
Braided
reinforcement: Braiding is the most popular method of reinforcing industrial
hoses. Before braiding, a mandrel which is equal in diameter to the inner.
Tube is inserted into the tube. This is done in order to avoid the tube from
collapsing under pressure of the braiding threads. For convenience, the maximum
lengths produced at a time are from 10-15 m.
A braid
is formed by the interweaving of yarns while they are helically spiralled over
the tube. Half of the reinforcing yarns spiral in a clockwise direction while
the rest move counter-clockwise giving usually a 2 up-2 down braid pattern.
Size
range: 3-100 mm bore diameter.
A
braiding machine may have one or two decks. On double deck machines, two layers
of braiding can be applied simultaneously. There are two types of braiding
machine: vertical braider, also called planetary or “maypole” braiders and
non-planetary or horizontal braiders (Table 17.9)
Spiral
reinforcement: Spiral reinforcement is applied in separate plies which are
always in opposite bias and remain separated by a Calendered rubber sheet,
dough, or cement. Since it requires two spiral layers to replace one braid, it
is important that there is “strike-through” from tube to cover to bond the two
layers, otherwise an extra layer of rubber must be added.
Vertical and horizontal braiders-a
Comparison
Vertical
braider Horizontal braider
1.
Economical for long length moulded
hose Not economical for manufacture of long
Long continuous
length eliminates use of length moulded hose. Length is
governed
Couplings and hence
the possibility of by length of mandrels and
ancillary
Leakage from
them equipment such
as covering and
Wrapping machines and pans
2.
Variation in bore and other components May use both
solid and flexible mandrels,
Cannot be closely
controlled unless a flexible Therefore, dimensions can be
accurately
Mandrel is
used.
Controlled.
3.
Maximum strength of reinforcing yarn cannot Maximum strength of yarn
can be utilized
be obtained since raw hose is not made to as raw hose is
made to finished design
finished design
4.
Calendered rubber sheet cannot be used for Calendered rubber sheet may be used
for
lining
lining
5.
Rigid mandrel cannot be used Rigid
mandrel can be used
6. Takes less floor
space Takes more floor
space
A Comparison of braiders and spiral machines is
gives in Table 17.10.
Comparison of braiders and spiral machines
Vertical
Horizontal Spiral
braider
braider machine
(24 carrier)
(24 carrier)
Deck speed (rpm) 33
100 240-600
Bobbin size (cm) 640
190 476-2300
Carrier tension (kg)
0.9-2.7 0.2-0.7 0.05-0.45
The
speed of rotation determines the output. Bobbin size and number of bobbins also
influence the output. Carrier tensions are linked with the type of
reinforcement yarns used. Higher tensions are required to give maximum burst
pressure and the minimize dilation, so for some hoses, vertical braiders are
still a necessity.
Spiralling machine operate at a very high speed.
Size range: 3-100 mm bore diameter.
Flexible and rigid mandrels: In the flexible mandrel method, lining is extruded
over flexible mandrel made of nylon or polypropylene through a cross head
extruder and then reinforced by braiding or spiralling through a horizontal
braiding or spiralling machine. After reinforcement, the hose is rubber covered
through a cross-head extruder and then lead covered by lead extruder of lead
press.
In the
rigid mandrel method, lining is blown to a rigid mandrel made of steel (for low
diameters) of light alloy (for larger diameters), treated to prevent sticking.
Since the length is restricted to 40 m only, flexible mandrels are preferred for
critical uses such as high pressure hydraulic hoses, even though the production
cost is very high. Hydraulic hoses are generally reinforced with rayon,
polyester, polyvinyl alcohol, aramid, or high-tensile brass coated steel wire,
depending on working pressure requirement.
Society of
Automotive Engineers (SAE) has formulated standards for hydraulic hose-grades R1
to R11. For example, SAE 100 R1 is a rubber hose with one layer of wire braid
reinforcement, sizes ranging from 10 mm to 50 mm and for working pressure
ranging from 50 bar to 200 bar. It is used for medium pressure applications.
Hose SAE R11 has six layers of wire spiraled in opposite directions around the
tube. Hydraulic hoses may be manufactured for very high pressure u9p to 11000
psi or 750 kg/cm². NBR based lining compound and CR cover compounds are usually
applied to meet the service conditions.
The
lining is stiffened by freezing with liquid nitrogen before steel cord
reinforcement under high tension is applied over it.
Moulding: Raw hose is now inflated up to the desired outer diameter by light air
pressure in lining and then lead covered by passing it through a lead extruder
or lead press. Lead extruders, similar to that used in lead cable industry, are
used.
Lead
(99.9% purity) is melted in an iron kettle and is poured into a water-jacketted
mould to cast billet. The billet is allowed to cool and is placed inside the
lead press (a ram type extruder cavity) by mechanical aid. The ram then moves
into position and presses the lead billet through the die assembly to extrude a
lead pipe. The hose is inflated with air and is fed into the press for lead
covering. The lead sheath may heave a smooth or a fluted inside surface which
is imprinted on rubber cover of the hose. The hose in the lead sheath is then
coiled on iron reels.
Curing:
Ends of the lead covered hoses are cut, the hoses in lead sheath are filled with
water, and the reels are transferred to a horizontal curing pan. The hose is
given as open-steam cure, maintaining a constant water pressure inside the
lead-covered hose through out the cure.
Lead
Sheath Stripping: Cured hose is passed through a stripping machine where the
lead sheath is taken off the hose, cut into small pieces, and the pieces are fed
back to lead kettle for reuse. The lead sheath imparts a smooth or corrugated
finish which is characteristic of long length hose.
TESTING OF HOSE:
Common
tests which are used for hose are:
(a)
visual
check on appearance and finish;
(b)
dimensional check;
(c)
braid
angle and adhesion between the components;
(d)
physical properties of rubber components (initial and aged); and
(e)
proof
test and burst pressure test.
Hoses
cannot be used at high operating pressures in hydraulic systems without end
fittings. Couplings or end fittings must also be made to close dimensional
tolerances to resist high fluid pressures. The proof pressure test is conducted
at twice the working pressure. In the case of rotary oil drilling hose, for
example, the proof test pressure is 10,000 psi and 5000 psi in actual service,
while swirling and twisting in an oil rig. With couplings applied, a length of
rotary petroleum drilling hose can weigh 500 kg or more.
Couplings may be permanent (one piece) or re-usable (with connecting ends). Toi
basic components make up a hose connection-the hose attachement which holds on
to the hose end and the fitting connection which holds the hose assembly to the
point of transfer. Hose can be purchased in relatively long lengths or in
complete assemblies.
Hoses
and assembly life is influenced by the following factors:
(a)
the
design of the hose;
(b)
the
design of the fitting/coupling;
(c)
the
technique of fitting;
(d)
the
technique of mounting the hose assembly onto a machine;
(e)
the
working conditions; and
(f)
environmental conditions.
The tests
usually specified in hydraulic hose specifications are the leakage test, burst
test, and impulse test.
Leakage
test: This is a destructive test which is used for sampling only. Here, a hose
with end fitting is subjected to 70% of the minimum burst pressure for 5 min.,
then the pressure is reduced to zero after which the initial pressure is
reapplied for 5min. These should be absolutely no sign of leakage or any
evidence of failure in hydraulic hoses.
Brust test
: This is a destructive test done on hose assemblies which are gradually
pressurized up to the burst pressure to check if any leaks or indication of
failure occurs below the burst pressure.
Impulse
Test: In this test, hose assemblies are impulse on suitable test rigs with the
hose bent to its minimum recommended bend radius. Hoses less than 25mm nominal
size are bent within 90° or 180° and hoses of 25mm and above are bent 90°. The
test fluid is circulated at the specified temperature at the impulse rate of
30-100 cpm. the pressure wave applied may be in square or secant or half-omega
form. Specifications indicate the minimum number of impulse cycles to failure
(Fig. 17.11)
Other type
tests include swelling, steam ageing, ozone and low temperature resistance,
concentricity, whip, pull-out, and salt-spray tests.
FAN AND V-BELTS
Fan belts
are used in automobiles for driving the fan pulley and the dynamo pulley, and
are designed to suit the layout of the automobile. V –belts are used for
transmitting power between V-Shaped pulley (sheaves) of short-centre distances.
The V-belt is chosen in preference to the flat belt since the wedging action
between V-belt and pulley grooves enables higher power transmittance.
Transmission systems may be standard, synchronous, variable speed, or
free-clutch systems.
The main
types of V-belts used for power transmission may be classified into wrapped, raw
edge (cogged or uncogged), V-ribbed (serpentine), or self-tensioning (maintenancefree)
categories (Fig. 17.12). The traditional V-belt is jacketted with woven fabric,
raw V-ribbed belt replaces sets of V-belts by a single belt tracked in a
serpentine fashion through the accessory sheaves, the tension being
automatically maintained by one single take up. The maintenance free V-belt is
self-tensioning during operation by means of frictional heat which causes the
reinforcing polyester cords to shrink.
Construction:
Generally
fan belts and V-belt have the following components:
(a)
base
rubber
(b)
cord;
(c)
top
rubber; and
(d)
rubberized fabric jacket.
The
cord is held in place or supported by rubber in the finished belt, which also
transfers stress from one cord to another, of from the pulley to the cord. In
some belt section, it is usually to place the cord layer in neutral axis so that
the cord is less fatigued during run on drive.
The
cord is usually polyester or rayon, with some usage of steel or glass fibre for
special applications. Polyester and rayon have good dimensional stability,
fatigue resistance, and flexibility. A single layer of cord is used in smaller
cross-sections and multi-layers in larger sizes. The jacket helps in achieving
positive traction, flexibility wear resistance, and is resistant to oils.
V-Belt
Manufacture
Base
and cushion (top) compounds are calendered into thin sheets of specified
thickness and batched on liners. These are not extruded, but built up from
sheet, in order to avoid a transverse joint.
For
jacket, cotton or blended cotton canvas is frictioned or spread with a suitable
rubber compound, and then cut into specified width and bias angle, and batched
on liner. Carcass building is done on single-drum or two-drum building
machines.
The
single-drum building machine essentially consists of an expandable metal drum,
over which a rubber sleeve is fitted. The drum is rotated, and a cylinder
composed of the components of V-belt is built over it. The calendered base
rubber is first built up to the required thickness. One layer of calendered
cushion rubber is applied over it and then the treated cord is spirally wound on
the drum at the specified pitch under tension. Thereafter, further layers of
cushion and filler rubber are built over it. Then the built up cylinder is slit
into rectangular sections of desired width with circular rotating knives and
taken out from the drum after collapsing the drum. The cut sections are skived
to trapezoidal shape with the help of two circular angularly placed rotating
knives. The raw belts are jacketted with rubberized fabric in a jacketing
machine.
Long
belts are usually built on a two-drum machine, with the distance between the
drum axes being adjustable, both for setting the required length and also for
removal of the belt. Equal cord tensions are an important factor in achieving
minimum spread of belt lengths in the finished product. This is necessary in
industrial drives where several belts are fitted in parallel on multi-groove
pulleys. The cord elongation must be minimal. Predictable, and uniform.
Vulcanization: The processes usually employed for vulcanizing V-belts use ring
moulding, press moulding, or rotary vulcanizing method (Plate 16, Exhibit 17.10;
Fig. 17.13).
Ring
Moulding: Jacketed raw belts are placed between two rings of a multi-cavity
circular mould. The moulds with the belts are then bolted up axially and wrapped
with wet wrapping (impression fabric) cloth under tension so that it can exert
pressure on the top of the belt during cure. The wrapped mould is then placed
in an autoclave for vulcanization in open steam. After cure, the belts are
taken out from mould cavities. A separate set of rings has to be available for
each size-and cross-section of belt required.
Double-deck daylight: Presses may be used with V-grooved moulds for vulcanizing
long V-belts. The load, consisting of a number of belts, will be supported by
drums or pulley sets mounted so that tension can be maintained in the belts
during vulcanization. The mould ends are water cooled so that pressure can be
maintained in the hot rubber as it passes through a low viscosity stage before
vulcanization commences. The belts are rotated through the machine between
vulcanization steps so that sections are treated successively.
Rotary
Vulcanizers: Consist of a rotatable grooved drum maintained at a high
temperature. The load of belt carcasses in hung over the drum, one belt to each
groove, and an apron covering about half the circumference of the drum is
applied at a high tension. The drum and the back of the apron are heated. The
drum speed is controlled so that the belt is vulcanized in half a revolution.
An auxiliary cold grooved roller maintains the tension or length of the belts
during the process with the rotary vulcanizer, one machine can be used for a
wide range of belt lengths (Fig 17.13). In the case of raw-edge belts, the
individual belts are cut from vulcanize sleeves.
MAIN TYPES OF POWER TRANSMMISION
BELTS
Conventional V-belts are of standard sections (namely A, B, C, D, E, and Y, Z)
for specified top width and depth as given in Table 17.11
V-belt standard section
Table 17.11
Section Top
width (mm) Depth (mm)
A
13.0 8
B
17.0 11
C
22.0 14
D
32.0 19
E
38.0 23
Y
6.5 4
Z
10.0 6
Wedge
section belts SPZ, SPA, and SPC have similar top widths to the sections Z, A, B,
C but with a lower top width to height ratio (1.2). With a narrower
cross-section, belts are smaller and weigh less; the sheaves are also smaller
and weigh less. The belts transmit more power at the same pulley diameter or it
is possible to achieve a required horsepower rating with fewer wedgebelts, or to
downsize drive motors, or to increase drive efficiency.
Cogged
construction provides high flexibility for small sheaves or for drives with
relatively short centre distances. The cogs also provide a larger surface area
to dissipate heat and prolong belt life.
Variable speed belts have to fit between the flanges of the variable speed
pulleys, consequently they are much wider than the conventional V-belt. They
can be used on small pulley drives where the belt stresses are the greatest.
Hex
belts (also know as double V-belts) are designed for use on drives with one or
more reverse bends. They usually transmit power from both sides of the belt.
Sometimes V-belts in a drive can resonate into pronounced vibration, turn inside
out, and rapidly break up. To avoid this, a set of belts can be built with
multiple belts joined by a backing fabric that regulates belt travel so that all
ribs pull together as a single, perfectly matched group.
Mismatching of individual belts in a drive, misalignment, shock loads,
pulsation, and vibration can reduce drive efficiency. Open and V-belting is
used in applications where endless V-belts are difficult or impossible to
install. They are fastened and are usually employed as an emergency replacement
when the exact length to endless belt is not readily available.
Poly
V-belts are used on automobiles to drive the accessory equipment. One belt
drives all of the accessories and a spring loaded tensioning device eliminates
need for belt adjustment. The belt design incorporates the power transmission
capabilities of V-belt with flexibility of flat belts.
LATEX PRODUCTS
There
are several advantages in using a latex for producing rubber articles.
(1)
It can
be shaped and worked easily and thus find certain application where dry rubber
or a rubber solution cannot be used. (2). It does not require any heavy or
expensive machinery. (3) Power requirement is less. (4) Certain latex processes
are amendable to continuous operation. (5) The physical properties of latex
articles are usually good. (6) Latex is safer to handle than rubber solutions.
The
disadvantages are: (a) Latex processing requires strict controls and
batch-to-batch adjustments due to variability in latex properties. (b) Latex
Processes are not suitable for making rubber goods of heavy or bulky nature. (c)
Fillers do not reinforce latex, they act as diluents reducing the product
properties. (d) Latex is usually more expensive then dry rubber of comparable
rubber contents.
PREPARATION OF INGREDIENTS
Like
dry rubber compounding, the latex should be mixed with vulcanizing ingredients
in order to cure the dried film. Other additives like anti-oxidant, softeners,
and fillers are also added to latex to impart the desired properties in the
finished articles. In compounding latex, the fillers and vulcanizing
ingredients must be in the form of small particles completely wetted.
Otherwise, coagulation or quick settling out of the powders from latex will
occur. Both grinding and wetting are carried out by means of pebbles and is
mechanically rotated. Grinding of the powders takes place between the pletely
wet. The efficiency of ball milling depends on the size of the pebbles, amount
of pebbles and water used, and speed of rotation. At least 24 hours is usually
necessary to prepare a satisfactory dispersion by ball milling.
To
ensure uniform mixing of compounding ingredients and to avoid subsequent
settling out, water-soluble ingredients are added to latex as aqueous solutions
and water insoluble ingredients (except for fillers) as aqueous dispersions or
emulsions.
Sulphur,
zinc oxide, accelerator and solid anti-oxidant dispersions are prepared in a
ball mill, vibro-energy mill, or attritor. A suitable general purpose
formulation for variety of compounding ingredients is given below:
Parts by
weight
|
|
Additive
Dispersant
(sulphonate
type)
Soft water |
50.0
2.0
48.0
100.0 |
Ball
mill for 24-48 hours. The dispersion is checked once or twice during milling to
deal with any problems that may arise, such as, excessive foaming, which will
reduce dispersion efficiency. Certain anti-oxidants, such as substituted
phenols may require a different formulation such as
Parts by
weight
|
|
Anti-oxidant
Dispersant
Bentonite clay
Polyvinyl pyrollidone
Sodium dodecyl sulphate
Water |
50.0
2.0
0.5
1.0
0.1
46.4
100.0 |
Anti-oxidant dispersions frequently became viscous on milling due to foaming or
wetting effects. Dilution to 40% of active ingredient or use small amounts of
antifoaming or wetting agents will overcome the problem.
Liquid
compounding ingredients which are not soluble in water, such as certain
accelerators, anti-oxidants, and plasticizers, are normally added to latex as
oil-in-water emulsions. Emulsions are prepared using a high-speed stirrer or
homogenizer to a general formulation of the type given as follows.
Parts by
weight
|
|
Liquid compounding ingredient
30% Potassium oleate solution
Soft Water |
50.0
7.5
42.5
100.0 |
The
liquid and surfactant may be added to water in a slow stream until a
satisfactory particle size has been obtained. Alternatively, in the case of
viscous liquid ingredients, the water may be added to the oil phase gradually to
give first a water-in-oil emulsion and then an oil-in-water emulsion.
Fillers
are usually added to the latex compound as aqueous slurries. They essentially
act as diluents and have the effect of reducing the strength properties of the
vulcanizate, unlike dry rubber compounding where “reinforcing” fillers are
used.
STABILITY OF LATEX COMPOUNDS
Prior
to further concentration, the addition of fillers and other compounding
ingredients, the raw latex has to be stabilized agaist conagulation. This is
done with 0.25% caustic soda made up as a 5% solution in water. The addition of
zinc oxide to the latex is generally left until the last, as the oxide a
thickening and destabilizing effect.
When
added a latex in the presence of ammonia, a small amount of zinc oxide dissolves
in accordance with the equations.
ZnO + 2NH3 + 2NH4
ÕZn (NH3)4
+ H2O
Zn(NH3)4
Õ(NH3)x
+ (4-x) NH3,
Where x
= 0 to 3.
The lower zinc ammine ions, such as Zn (NH3)2
from uncharged compounds (zinc ammine soaps) of low solubility by reaction with
the fatty acid soaps at the surface of the rubber particles. Typically, this
leads to a slow increase in the viscosity of the latex (zinc oxide thickening)
accompanied by a loss in mechanical stability. Therefore, further addition of a
stabilizer such as anionic or non-ionic surfactant is usually necessary.
In many
important latex manufacturing processes, and e.g. production of dipped gloves,
moulded foam, and latex manufacturing processes, and e.g. production of dipped
product. In the course of gelation, the rubber particles aggregate into a
three-dimensional network without separation of the aqueous phase, which is
retained in the interstices of the network. There are three basic methods of
effecting gelation involving (a) use of acids or acid-producing substances; (b)
use of calcium salts; and (c) heat latex thread, and in the Dunlop process for
moulded foam rubber. Calcium salts are polyed in dipping compounds and foam
manufacture. However, certain processes, such as adhesives, do not require a
gelation step.
Manufacture
of Latex Products
The
preparation of rubber articles from latex is briefly described in Table 17012
Latex Processes
Table 17.12
Dipping Balloons, gives, nipples,
fountain pen sacks, etc.
Casting
(i) Heat sensitization Foamed latex sponge,
(ii) Plaster casting Moulded toys, etc.
Spreading, doubling, Toys
Impregration Coated fabrics, paper, etc.
Manufacture of latex foam The manufacturing process for latex foam essentially
consists of: (I) Compounding of latex; (ii) expanding (foaming) to a desired
volume; (iii) setting (gelling) the rubber particles; and (iv) vulcanizing.
There
are mainly two methods of making latex foam, namely, (a) Dunlop process and (b)
Talalay process, which differ mainly in foaming and gelling method.
Dunlop
process Table 17.13 gives the typical formulations for the Dunlop Process for
making foam.
The Dunlop process for
making latex foam
Table 17.13
A typical formulation
Compounding Parts by weight
