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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

            Process                                                           Typical product

            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.

            Bonding                                  Adhesives

 

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

 

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