Bridge Bearing

Natural Rubber bearing generally consists or interleaving layers of rubber bonded sandwich fashion to steel plates. The whole unit is covered with rubber to afford weather protection.

A bridge basically consists of bridge deck supported by piers. In order to avoid over stressing and damage by movements of vehicle and loading to the piers, bridge bearings are used to accommodate these movements so as to reduce the reaction forces and bending movement to within the safety limits of structure. Natural Rubber is an ideal engineering material for bridge bearings as it is highly elastic and sufficiently soft to accommodate these movements without transmitting harmful stress and also it can absorb and isolate energies from impacts and vibrations.

Types of bearings

• Laminated bearings – an elastomeric bearing containing steel laminates bonded to rubber
• Plain pad bearing – a plain rubber pad containing no steel plates
• Strip bearing – a plain pad bearing for which the length is much greater than the width ( > 10 times)
• Pot bearing – a bearing consisting essentially of a block of solid rubber enclosed between a metal piston and metal cylinder

Manufacturing Process

• Preparation of rubber compound according to specification required
• Preparation of steel plates
• Application of bonding agent to steel plates
• Assembly of compound and plates
• Compression moulding
• Performance Test
• Ready for installation

Sleeping Policeman (Road humps / Speed bumps)

Sleeping Policeman (Road humps / Speed bumps)

The most effective measures to lower vehicle speed is by using sleeping policeman or known as road hump. The road humps are designed to have different sizes and shape to suit the requirements and place of services. Some are narrow as to deliver a sharp jolt to vehicle suspension and to gives discomfort when crossing at high speed. The wide humps are mainly for further reducing the vehicle speed due to longer crossing time. The road hump is normally extended from curb to curb across the full road with a drainage gap.

They are made of a hard, high impact rubber that lasts longer than plastic, asphalt or concrete speed bumps. They are moveable can be easily relocated. Road hump is grooved to provide traction in rain or snow and easy drainage of water. Built-in cats’ eyes provide necessary night visibility, alerting drivers to slow down.

The road hump is made from NR and SR and its blend. The black slab is made from black rubber compound base on 100% NR. The rubber is formulated with suitable hardness to with stand heavy load, forces and compression. The tear and tensile strength is immaculately high so that it resistant to tearing and chunking. The vulcanisate is also designed to be weather resistance by adding antiozonant and antioxidant.

For yellow compound similar properties were assured but with extra protection on weathering. Since coloured compound more prone to weathering it is formulated base on blending with weather resistant rubber. The colour pigment used is also of higher grade with weather fastness above approximately 5. Extra protection agents were added into the formulation such as UV absorber and light stabilizer. With these extra protections the yellow slab or portion of the hump should be more resistance to fading and weathering.

The road hump is also designed to have built in reflector. The reflector used is of high intensity grade which is bright, durable, retroreflective material designed to have similar appearance when viewed in the daylight or by retroreflected light at night. The reflective sheeting consists of optical lens elements adhered to a synthetic resin and encapsulated by a flexible, transparent plastic with a smooth outer surface. The reflector sheeting featured a pressure-sensitive adhesive for ease of application. The main properties of it performance are 3 times brighter than Engineer Grade reflective sheeting and retain good reflective to ensure safety condition.

Manufacturing Process

• Mixing of compound
  
  There are two compounds used in the manufacturing of road hump, one black and one yellow. The compounds are mixed separately either using internal mixer or two-roll mill. If internal mixer is to be used, master batch is mixed in the internal mixer first under controlled temperature. The master batch is then added with curatives using two-roll mill. If the mixing is to be carried out on two-roll mill, the mixing is to begin with mastication of the rubber and followed by addition of ingredients such as activators, filler and oil. The curatives are to be added last.
  
  
  
• Curing of components
  
  There are three separate components for making road hump, they are black and yellow intermediate slabs and the end slab. The intermediate slabs using the same mould for black and yellow. However the mould must be cleaned before changing to new compound.
  
  The curing can be carried out using the same press for each individual component. The economical moulding temperature is 170°C. The cure time is approximately 30 minutes for intermediate slab and approximately 20 minutes for end slab. The moulds are designed to facilitate easy fixing to the platens.
  
  
  
• Trimming process
  
  
• Bolting to road surfaces

Microcellular Rubber

When rubber compound is added with blowing agent, fine cellstructure is formed on the vulcanisate after curing, this rubber vulcanisate is called microcellular. The cell structure of microcellular is normally closed with gas trap within the cell. Since the blowing can be controlled the final products density varies, the bigger the cell size the lower the density and the softer the material. If the cell size is small the density is higher and the harder the product. The properties of microcellular rubber is normally inferior compared to solid vulcanisate, this is because the cell initiates failure. Some of the properties deteriorated are tensile properties, hardness, tear strength, abrasion and compression set. The most preferable properties of microcellular are low density, lightweight, high impact absorption and soft that assured comfort.
The application of microcellular rubber is very wide from toy to important engineering applications. Some of the applications are sound absorption, vibration isolation, insulation, padding, floor backing, wall lining, footwear components cushioning and many others. For applications where physical properties is critical and light weight is required then low density is normally used and for applications where some engineering requirements are needed then higher density is used. In footwear application abrasion is important, the microcellular used is normally medium density and of harder vulcanisates.
Since microcellular is a close cell it does not allow liquid to penetrate. These advantages have been explored to be used in floatation applications. Some of the applications are life jacket, buoyancy, fishing float and many other applications as float.

Manufacturing Process

Formulation
The formulation of microcellular is just like any solid rubber with an exception that the blowing agent is added. The blowing agents are inorganic and organic compounds that are stable at room temperature and decompose and liberate gas at suitable higher temperature. The blowing agents for rubber decompose either just before or during the cure. This gives the material a porous or cellular structure and a low specific gravity.
An idle blowing agent would be a product that has the following characteristic.

* Disperse well in rubber
* Has no effect on the compound's curing characteristic
* Has a suitable decomposition temperature range, not too high and not too low
* Liberates a large volume of gas per unit weight of the chemical
* Decomposition products are not toxic and unpleasant smell
* Causes no discolouration of the vulcanisate

There are basically two types of blowing agent, decompose at higher temperature and decompose at lower temperature. Azodicarbonamide groups are mainly decomposed at higher temperature.

Porofor ADC/R Decomposition temperature = 210°C
Porofor ADC/K Decomposition temperature = 165°C

Another group is sulphohydrazide this chemical decompose at lower temperature.
Porofor TSH Decomposition temperature + 105° C
Blowing agent decomposes at higher temperature are meant for rubber vulcanisate cured at higher temperature, such as EPDM and EVM. However ADC also suitable for NR for producing fine cell structure microcellular. The Porofor TSH is normally used in NR for producing large cell structure microcellular.
Apart from blowing agent other ingredients are also important to determine the final properties of the microcellular. The microcellular can be black and coloured vulcanisate

Low Protein NR Latices (LOPROL)

Natural rubber latex (NRL) is one of nature's most versatile materials and the polymer of choice for medical devices. It is a cost-effective material that provides good barrier properties, durability and comfort. However, the emergence of latex protein allergy has generated adverse publicity on the acceptability of NRL in the health care sector by anti-latex lobby. This has prompted extensive R & D efforts to reduce the incidence of allergy and ensure that NRL medical gloves are of minimum health risk.

One of the solutions to address the latex protein allergy is to use low protein NR latex (LOPROL). This type of latex can be prepared by treating latex with proteolytic enzyme or a suitable surfactant and purification process by dilution, centrifugation or membrane separation. The reaction mechanism for breakdown of polypeptide chain by enzyme is shown below.

Manufacturing Process

A. Enzyme Treatment

The MRB has developed several processes of producing low protein NR latices using proteolytic enzymes such as Alcalase or Savinase. The flowchart below shows one of the processes of producing low protein NR latex.

Enzyme Reaction on Protein Molecule

B. Non-enzymatic process
The MRB has also developed several processes of producing low protein NR lattices using non-enzymatic process. The flowchart below shows a typical non-enzymatic processes of producing low protein NR latex.


Properties

The nitrogen content of low protein latex prepared using enzymatic and non-enzymatic process is in the range 0.05% - 0.09% and 0.02% - 0.06% respectively. The extractable protein content of wet-gel leached plus post dry-leached films prepared from these two type of latices is less than 100 g/g. The physical properties of sulphur vulcanised films prepared from these latices are comparable to those prepared from normal NR latex concentrate.

Deproteinised Natural Rubber (DPNR)

Deproteinised Natural Rubber (DPNR) is a purified form of natural rubber (NR) from which most of the ash and protein components have been removed. The rubber is produced under very closely controlled conditions at the Rubber Research Institute Experimental Station, Sungai Buloh, Selangor, Malaysia. It contains about 96% rubber hydrocarbons compared to about 93% for normal natural rubber grades. The removal of these non-rubber components confers special attributes to the rubber which enhance its value in certain specialized applications.

DPNR CV Production Process

DPNR is prepared by treating field latex with a proteinase to hydrolyse the proteins in the latex which are then washed away during processing. Ammonia, a non-ionic surfactant, a proteinase and hydroxylamine Neutral Sulphate (HNS) are added into bulked field latex and allowed to react for 72 hours in a stainless steel conical bottom reaction tank. After the completion of the enzymatic hydrolysis reaction, the reacted latex is neutralized with dilute formic acid and coagulated by steam in a specially designed steam column coagulator. The resultant coagula are then processed in a continuous fashion through a series of crepers and finally chopped into small crumbs in a shredder. The crumbs are pumped through a static screen before entering the dryer boxes. The wet crumbs are then dried at about 85oC for about 6 - 8 hours in a hot air dryer. After cooling the crumbs are weighed, pressed, baled and packed to SMR standards.

DPNR – Grades

Two principal grades of DPNR are available. They are:

• DPNR CV – Viscosity stabilised at between 60 – 70 Mooney units.
• DPNR S – No viscosity stabilisation feature. The initial viscosity is between 70 – 80 Mooney units. Owing to the education on non-rubber content, the DPNR S has a very much less storage hardening than the normal NR.

Characteristics Of DPNR

• Very low protein content
  
• Very low dirt and ash contents
  
• Low volatile matter content
  
• Light colour
  
• Negligible nitrosamines
  
• Very low antigen content

Specifications Property DPNR CV DPNR S Dirt retained on 44 aperture sieve (% wt max) 0.01 0.01 Ash content (% wt max) 0.15 0.15 Nitrogen content (% wt max) 0.12 0.12 Volatile Matter content (% wt max) 0.30 0.30 Mooney viscosity [ML(1+4)@100ºC] 60-70 - AP(Max) 8 - Wallace Plasticity (min - 35 These raw rubber characteristics are reflected in the following valuable attributes of DPNR vulcanisates particularly when compounded using soluble vulcanising systems. Lower creep and stress relaxation Lower water absorption Greater consistency in modulus under conditions of variable humidity Area of Application Features Applications Low stress relaxation & creep Seals, Joint rings, Hydro-mounts Low water absorption Undersea applications Good dynamic properties Anti-vibration mountings Low protein Medical and food applications Packaging The standard packaging for DPNR is in 1.2 metric tonne wooden crates with the following specifications:- No. of bales per crate 36 Bale weight (kg) 33.3±0.5 Bale nominal dimensions 330 x 670 x 170 mm Bale wrapper Thickness: 0.03-0.05 mm Density: 0.92 g/cc Melt point: 109ºC Compatible with rubber @ 110ºC min Other forms of packaging/wrapping can be provided on request

Natural Latex Foam

Latex foam rubber consists of a network of open or interconnecting cells, which may be subjected to large, repeated deformation without damage. Latex foam is widely used in applications requiring a cool and comfortable cushioning material, such as pillows.

Manufacturing Process

Batch foaming is suitable for small production units or when miscellaneous products of varying density and/or colour are required. It is carried out in a foam mixer consisting of metal bowl and a wire whisk whips the latex to froth. Control of the volume to which the latex is foamed is essential and is achieved by marking the frothing bowl or by using a measure to check the distance of the foam from the top of the bowl when the whisk is stopped.

The initial foaming is carried out at high speed; when the foam is approaching the final volume the speed is reduced to a medium rate. Just before the required volume is reached zinc oxide dispersion (10 parts p.h.r of a 50% dispersion) is added.

Stirring is continued for 45-60 seconds and the speed of the mixer is again reduced to the slowest rate and the sodium silicoflouride dispersion (usually 4 - 5 parts p.h.r of a 20% dispersion) is slowly added

For high rates of production involving large quantities of products at a fixed density, continuous foaming equipment, using a mould conveyor system, is normally used. In a continuous foamer the latex compound and air are metered under pressure into a foaming head consisting of a rotor enclosed by two stators. Both rotor and stators have a large number of protruding lugs with clearances small enough to provide sufficient shear to foam the latex when the rotor revolves. The foam passes, via a length of hose, to a blender into which the dispersions of zinc oxide and gelling agent are metered and mixed with the foam. From the blender the fully compounded foam is discharged into moulds again by way of flexible hose.

When the foaming operation is completed the foam is immediately transferred into two piece mould designed to produce the desired shape and size of the finished unit. After gelation, the foam is vulcanised by heating for 20 - 50 minutes at 100°C. After vulcanisation the mould is opened and wet cured foam is removed and washed in running water. After the excess water has been removed, drying may be completed either bath - wise in an oven or tunnel oven. After drying, the products are trimmed and examined before packing.

Dunlop Process
The process for making latex foam by the Dunlop process can be summarized as follow:

* Preparation of latex compound
* Foaming of latex - mechanical incorporation of air into a suitable latex compound
* Addition of a delayed gelling agent
* Time to shape into mould before it gels (depends on the gelling agent)
* Gelling process in mould
* Vulcanisation on of wet gel
* De-moulding the foam
* Washing and Drying
* Finishing

Latex Tubing

Natural Latex tubing has been used extensively in the health care industry. Known for its extreme flexibility, the applications has been expand to the other applications such as Drain tubing, Model glider launching, Exercise devices, Elastic band, Slingshots, Tourniquet bands, Non-aerol can lines etc.

Manufacturing Process (using Heat-sensitised latex compound)

• Compounding of latex Latex is first de-ammoniated using formaldehyde to provide a base compound ready for heat.

• Addition of heat-sensitising agent To prepare heat-sensitised compound, heat-sensitise agent is added to base compound. Blend thoroughly, then allow to stand for removal of bubbles. The heat-sensitised compound is stored in a storage tank (storage temperature should be kept at 20°C or slightly lower).
  

• Extrusion
  
  Compound from tank T1 is allowed to flow through the glass extrusion tube (B) over a fixed glass core (C). The extrusion tube is surrounded by a water jacket (D) through which water at 50°C - 60°C is circulated to gel the tubing. An adjustable support (E) is fitted so that the height and consequently the gravity head of compound, can be adjusted slightly to maintain even flow. The tube emerges from the outlet (F) is in a firm gelled state.

• Washing
  
  The gelled tubing is led directly into a perforated tray immersed in a tank of cold running water. The tubing may be cut off from the extrusion outlet and the tray is removed to another washing tank. Production may continue collecting more tubing in another tray. Time required for washing depends upon the thickness of the tubing, but is typically about 4 hours.
  
  
  
• Chlorinating process
  
  Chlorinating helps to reduce surface tack and preserve the general appearance of the tubing. The tubing should be immersed in the chlorinating solution for about 8 minutes. Provision should also be made for the chlorinating solution to circulate through the tubing when long lengths of tubing being processed. After chlorination, the tubing should be thoroughly rinsed in clean water.
  
  
  
• Drying
  
  After chlorinating process and rinsed, the tubing should be dried in an oven or heated room at approximately 50-60°C. The time required will depend upon the wall thickness and bore of the tube, quantity of tubing being dried, oven efficiency etc.

Electrician Gloves

Rubber electrical gloves are used to protect workers against electrical shock while working around energized systems.

Electrician gloves shall be made from good quality Natural or Synthetic rubber or from a mixture of these, in conjunction with suitable compounding ingredients. Electrician gloves may be dipped using the coagulant technique or using the heat sensitive method.

Production of Electrical Gloves by the Coagulant Process
This is the most generally used process in production of Electrician gloves. Gloves thickness must comply with standard specifications for various rates voltages. The required thickness can be achieved by a single dip or by multidip process.

• Reception and preparation of latex compound
• Heating of formers (40 to 50°C)
• Coagulant dipping (50% calcium nitrate solution in alcohol/water mixture)
• Dry at 70°C
• Dwell in latex compound (about 4-6 minutes, depending on the thickness)
• Set gel (Room temperature)
• Coagulant dipping
• Dry at 70°C
• Second latex compound dwell (about 3-5 minutes, depending on the thickness)
• Set gel at 70°C
• Leach in water
• Vulcanisation in air ovens
• Dry, strip and then leach in cold water for at leat 24 hours
• Electrical testing of glove according to specification

Production of Electrical Gloves by the Heat Sensitive Method

• Reception and preparation of latex compound
• Heating of formers (by immersion in a warm water bath at 50-70°C)
• Formers are allowed to dry in temperature controlled oven
• Dipping of former in heat sensitive latex compound. Precise control of the former temperature, the temperature of the latex batch and time of immersion is essential.
• Leach in cold water for 10-15 minutes
• Vulcanisation in air ovens
• After drying and stripping the gloves must be further leached in water for at least 24 hours. The best way to remove all the non-rubber materials and so achieve high electrical resistance is to tumble the gloves in water which is replenished several times during leaching
• Chlorinated and dried
• Electrical testing of glove according to specification

Tube Extrusion

There are two important natural rubber latex extrusion processes, namely latex thread extrusion for the production of latex thread in small diameters and latex tube extrusion for the production of inner tubes. The latter method also is used for the manufacturing of tubes for medical devices. In this article the extrusion process for the production of inner tubes for sporting bicycles such as racing-bicycles and mountain bikes is described. Most inner tubes are manufactured from butyl rubber that provides low gas permeability. For sporting bicycles such as racing-bicycles and mountain bikes natural rubber mostly is used because such inner tubes can have thin walls, have half the weight of butyl rubber tubes and still are more puncture resistant. Gas permeability is a less important issue for (professional) racing.

Table 1 Product specification
High mechanical strength
High puncture resistance
Low gas permeability
Low modulus/high flexibility
Low weight/thin walls Smooth surface/possibility of different colors
Low cost/easy processing

Table 2 Dimensions and weight of latex tubes

Type Size Weight
Road
Ultra light 700 . 18/19 mm 55 gr
Racing 700 . 19/23 mm 65 gr
Standard 700 . 25/28 mm 68 gr
MTB
Racing 26 . 1.5/1.9 inch 85 gr
Standard 26 . 1.9/2.1 inch 95 gr

In table 1 the most important product requirements are summarized and in table 2 dimensions are qualified. For inner tubes, in principle various materials are interesting, such as polyurethane, polychloroprene and natural rubber. However, most synthetic polymer latices
are very expensive, sometimes up to 6 times the price of natural rubber latex. Because of costs, low modulus, high mechanical strength and ease of processing,

Table 4 General formulation for a heat-sensitive inner tube compound

Compound ingredients PVME - system (phr)

60% Natural Rubber latex, HA 167.0
50% Sulfur dispersion 2.5
50% ZDEC dispersion 2.0
50% Zincoxide dispersion 2.0
50% Titaniumdioxide dispersion 2.0
50% Color 0.2
30% Formaldehyde (pH 7.5-8) 2.0
Demi water 30.0
25% Stabilizer solution 1.2
10% PVME solution 20.0

Preparation of heat sensitive compound

The heat-sensitive latex is prepared by incorporating a heat-sensitizing agent in the compounded or pre-vulcanized latex. A general method for preparing dispersions is to grind the ingredients. Then a slurry is made of the powders (including the various additives, dispersing agents and stabilizers) with water. The slurry is grinded in a ball mill. Modern grinding methods to prepare dispersions are colloid mills, attrition mills or ultrasonic mills. As a function of time (depending on the type of mill used) the particle size declines. In the preparation of latex compounds for tube extrusion it is essential that the dispersion is finely ground to particle sizes
less then 5 microns. In table 3 a general formulation for an (inner) tube compound with heat gelling agent PVME is given.

After preparing the various dispersions such as the vulcanization dispersion, including anti-oxidants, the activation dispersion including colorants and the heat-sensitive (PVME) solution, the ingredients have to be compounded. In figure 2 the process is schematically drawn.
The vulcanization dispersion is mixed into the high ammonia (HA) latex and left over night. A pre-cure during 4 hours at a temperature of about 70 .C is performed and subsequently the activation dispersion is added followed by the heat-sensitive solution the next day. After preparation, the latex compound is stored in a reservoir and maintained at a constant temperature of 15 - 20 .C to avoid premature gelling due to the PVME. It is also possible to prepare the compound from pre-vulcanized latex usinga similar formulation, omitting the curatives, and ensuring a pH between 7.5 and 8.0 by adjustment of the level of formaldehyde.

Latex tube extrusion

A heat-sensitive natural latex compound flows from a constant level reservoir (A) to the extruder (figure 3). The extruder is made of concentric polished glass tubes fitted with a cold water jacket (B) at the top and a hot water jacket (C) below. When the latex compound enters the heated
zone around the hot jacket, it gels in the annulus between the concentric tubes and is slowly extruded from the bottom of the apparatus. The cold jacket is normally kept at a temperature of about 20 .C, and the hot jacket is normally kept at a temperature between 50 and 70 .C depending on the required cross-section of the tubing. The hydrostatic pressure of the latex in the reservoir above the extruder controls the extrusion rate. The tube is passing the jacket with a rate of 200 to 300 mm/min as a wet-gel and then goes through a spraying jet (D) to coat the tube with a butyl layer of about 0.1 mm. The tube passes a waterbath for leaching and a detackifying bath containing talc slurry. The tube is dried and vulcanized in a hot-air oven at a temperature of about 100.C. After vulcanization the tube is cut to length and turned in side out for further finishing. The ends are bonded together, punched and the valve is integrated into the tube. After assembling the tubes are ready for quality control.

Sources

Dipping with natural rubber latex,
NR technical bulletin, MRPRA, 1980.
Polymer latices and their Applications:
K.O. Calvet, Applied Science Publishers,
London, 1982.
Patent DE 2932177 C2, 1984.
TNO Industrial Technology, internal report.
Ben van Baarle LPRI

Natural Rubber for Fashion

The use of natural rubber for clothes and footwear is certainly not one of today’s whims. Long ago Indians already made sheets, bags and shoes and later on Macintosh manufactured waterproof clothing. But it lasted till the 20th century before fashion designers discovered NR as a material to create with all sorts of wearable (e.g. clothing, jewelry, accessories) and non-wearable (e.g. upholstery) products. This attention was coupled with a fundamentally other view on the material. No longer NR was only an expedient to give clothing functional properties like elasticity and water-resistance, now it became a material to design in too. Even creations appeared that by most people were felt as extreme ones. But it was proper technology that put NR at the disposal of fashion by delivering thin and strong sheet material.

Demands

Clothing manufactured from rubber has to meet demands that are required by the application as well as by designers who, for instance, are attracted by the possibility of creating clothing that fits onto the body like

Table 1. Typical properties of vulcanized NR latex sheet

Mechanical properties Typical values
Tensile strength 25 - 30 MPa
Modulus 100% 0.1 - 0.5 MPa
Modulus 500% 2.5 - 3.5 MPa
Elongation 800 -1000%
Tear strength 100 - 150 N
Hardness 30-35 micro IRHD
Specific gravity 0.94 -0.95 g/m3

a second skin. Therefore the material has to be very stretchable, should not tear, must have a very low permanent set, must be puncture resistant to allow stitching and has to be colourable and bondable. NR’s unique characteristics meet all these demands.

Properties of NR latex sheet

Without fillers, almost all synthetic rubbers have low strength properties. This is due to the fact that those rubbers don’t crystallise under elongation. NR (latex) crystallises at an elongation of about 500%, even without fillers. Latex sheet is very strong at the crystallisation point. At the same time, the material is very flexible (low modulus of elasticity), has an excellent resistance to puncture (high tear strength), returns almost completely to its original shape after deformation (very low permanent set), can be bonded to itself or to other materials rather easily and can be produced in many colours. All this makes latex sheet a very suitable material for clothing. Table 1 shows some typical mechanical properties for NR sheet. The only type of synthetic rubber showing more or less similar properties is polychloroprene rubber and indeed this material is also used for clothing. However, compared to NR it has at least one significantly higher modulus of elasticity as a real disadvantage.

Table 2. Typical formulation

Formulation Parts

60% Pre-vulcanized latex 167
20% Ethylene oxide condensate 0.5
50% Zinc oxide 1.6
50% Antioxidant 1
50% Titanium dioxide 10
50% Color as required
25% Non-ionic surfactant 5
25% Polypropylene glycol 5

This reduces the ‘second skin’-experience dramatically and therefore most polychloroprene rubber clothing is loose fit types.

The production of NR latex sheet: Compounding and sheeting

Most commonly used are latices concentrated to 60% dry rubber content by centrifuging. High-ammonia (HA) and low-ammonia (LA-TZ) types are particularly recommended, although also pre-vulcanized types are used. To ensure uniform mixing of compounding ingredients, water-soluble ingredients are added to the latex as aqueous solutions, water-insoluble ingredients as aqueous dispersions or emulsions. Aqueous dispersions from sulfur, zinc oxide, titanium oxide accelerators and antioxidants are prepared in a ball mill or attritor mill. Water-soluble liquid compounding ingredients areemulsified using a high-speed stirrer or homogeniser. The colloidal stability of the latex has to be maintained at all times during the addition of the various ingredients and the handling thereafter. With the exception of the heatsensitizing solution and the (water-based) coloring agents, all ingredients are added to the latex under slowly stirring in order to prepare a base compound. As a heat stabilizer, poly vinyl methyl ether (PVME) has been used for a long time, but this required the use of formaldehyde (now discouraged on health grounds), whereas also the pH of the compound had to be adjusted. Therefore one has changed to a formaldehyde free sensitized pre-vulcanized formula, requiring also far less attention to the need for positive chilling of the compound. Special effects are possible by adding for instance phosphorescent pigments. Finally, the coloring agent(s), odorants and the heat-sensitizing system are added. In table 2, a typical formulation for pre-vulcanized latex is given. The latex compound then is transferred to a heated roll mill. The carrying belt may be either stainless steel or textile impregnated and coated with a suitable polymer. A good choice of the latter is necessary because the compounds are alkaline and also water ingres from the latex has to be prevented. The compound also will deposit traces of proteins etc. on the belt, which can adversely affect the sheet quality. These deposits have to be removed periodically. This may be circumvented by selection of a polymer with a high surface tension nature, causing the proteins to travel preferentially with the compound. The gelation is carried out in an infrared heating oven prior to the main drying. Then, the resulting gelatinous layer continues into another oven with a steadily increasing temperature over the length. In this way, the surface does not dry too quickly. Sheets are
produced on a continuous basis with widths up to 4 meters and a thickness between 0.2 and 3.0 mm. The nominal thickness can be controlled with a tolerance of . 10%. Prior to roll up the sheet an (anti-tack) agent is applied to one surface.

Manufacturing and assembling

Most processes in the production of latex clothing manufacture are similar to those for textile clothing and will not be discussed in detail here. Like textile, NR latex sheet is available in various designs. The most common printing techniques are silkscreen printing and flat printing. For the flat printing technique the sheet is stretched over a table of 40 to 80 meters after which with the use of stencils and various coloured inks a design can be printed. For each new colour a separate stencil is used. Afterwards, the sheets have to dry overnight to ensure good adhesion. A difference with textile clothing is that for latex clothing more often bonding is used for putting the pieces together. The glues for bonding are mostly latex based adhesives. Before bringing together the surfaces to bond these have to be cleaned thoroughly and roughened in order to ensure proper bonding. However, also the standard stitching techniques are used. The puncture resistance of a good quality NR sheet is sufficient to prevent tearing starting from the point of puncture even when the material is stretched in wearing.

Recommendations

Store latex clothing below 25.C; keep it awayfrom heaters and from natural light. The best way is to store clothes in a black plastic bag,when not in wear.Avoid contact with copper and copper alloysand with oils, solvents and greases.Wash clothing in clean warm water without soap or detergent. Wipe off excess water with care. Do not tumbledry, dry clean or iron under any circumstances.

Conclusion

In fashion and, although not discussed here, in upholstery NR sheet offers unexpected
possibilities to designers for realising their ideas, even when it are extreme ones. However, the designer as well as the end user mostly does not realise that this is only possible due to the ease of manufacturing sheets from NR latex and the unique mechanical properties of NR.

Sources

1) “Rubber in Fashion”, Rubber Developments, MRPRA, 1984
2) “Latex – The ultimate designer’s material”, Rubber Developments, MRPRA, 1992
3) “Student produces novel textures using latex”, Rubber Developments, MPRPA, 1993

4) “Rubber in Fashion”, Institute for Fashion Management and Design Koetsier/Montagne,

Amsterdam, 1996
5) “Sheeting”, R. Scott, Natuurrubber/Natural Rubber 17, Special on NR Latex, 1st quarter
2000

Natural Rubber in Engineering High-speed Tyres

(Paul Bremmer)

Introduction

The construction of passenger car tyres depends on the demands that can be stated. The car dynamics, its top speed and the desires of the consumer have to be translated to the functional demands on the tyre. This article illustrates the development of Vredestein Ultrac high-speed tyres, high performance tyres used for top speeds until 300 km/h. The high-speed tyres are designed for cars like the BMW M3. The M3 is the fastest and sportiest type of the BMW 3-series, with suspension and tyres optimised for handling. The engine performance numbers are impressive: the maximum engine power is 252 kW (343 bhp); the maximum engine torque is 365 Nm. The top speed is limited to 250 km/h by the manufacturer, without the electronic limitation the top speed will exceed 250 km/h by far. The powerful brakes and the tyre grip ensure high braking performance. The tyre sizes are 225/45 ZR 18 at the front, and 255/40 ZR 18 at the rear wheels.

Demands on high-speed tyres

On high speed tyres the demands arefocused on handling, grip, quick vehicle responses and safety at high speeds. Characteristics as comfortable ride and low wear rate are of less importance. The tyre must be constructed to perform well on:

• Braking performance
• Handling
• Aquaplaning resistance
• High speed behaviour
• Tearing resistance

Braking performance

The braking performance is determined by the grip of the tread compound on the road surface. The grip results from the total friction force of the tyre on the road surface. Therefore the dynamic footprint (the pressure distribution and the shape) and the friction coefficient of the tread rubber are crucial. The friction coefficient varies with normal stress and is directly related to the hysteresis properties of the tread rubber. For good braking performance, high-energy absorption as a result of hysteresis is required. The hysteresis can be attained by using the right elastomers with relatively high glass transition temperatures (Tg), and by using the right
amounts and types of fillers. SBR (Styrene Butadiene Rubber) type elastomers with relatively high amounts of styrene are normally used.

Figure 1. Components of a high speed tire

Because of their low Tg’s NR (Natural Rubber) and BR (Butadiene Rubber) are hardly used when ultra high grip is desired. The braking performance both on wet and dry roads can be improved by lowering the air ratio. The air ratio is defined by the ratio of groove area and tread area in the contact patch. For dry roads a slick (a tyre without profile, thus with air ratio zero) would give the best braking performance, but for the resistance to aquaplaning – as discussed next-grooves in the thread are essential.

Handling

Handling of the car is the way a car feels and behaves during driving manoeuvres. A car with a good handling has predictable behaviour and a reasonably quick dynamic response to the steering input. Furthermore it is easily controllable during extreme manoeuvres, and it offers a safe feeling to the driver. Additionally the car must be able to achieve high cornering speeds.

Handling of the car is determined by a series of tyre and car characteristics. Three important tyre characteristics for handling are the grip during cornering, the feedback from the tyre forces and moments to the steering wheel, and the dynamic tyre response to steering. These characteristics are influenced by the tyre components as follows (see also figure 1)

Tread

A stiff tread compound is beneficial for the grip during cornering as the stiff profile blocks show little deflection. A very stiff undertread compound is applied for raising the total tread stiffness, which refines the quick tyre response. The total tread height is minimized for improved stiffness.

Belt layers

Wide and stiff steel belt layers reduce the deflection of the tyre construction during cornering, resulting in a quicker response and more grip. For minimizing belt edge separation, a possible defect of a tyre after many million cycles, natural rubber is the basis for the belt compound because of its high resistance to tearing.

Apex and rim cushion

These components are manufactured from a stiff compound, normally consisting of NR (Natural Rubber) and BR. The stiff construction results in a short reaction time of the lateral tyre force on the steering input.
The dynamic behaviour of the tyres with regard to handling is tested on the N.rburgring, a racetrack that is frequently used by car manufacturers to make the final adjustments on the car suspension in order to get the desired handling. On this 20-km longtrack with various curves several tyres with varying constructions and compounds are subjectively tested to achieve the best handling performance.

Resistance to aquaplaning

When a tyre rolls through a water film with increasing speed, at a certain moment the tyre will loose contact and starts floating. This phenomenon is called aquaplaning. To prevent aquaplaning, water has to be transported from the contact patch through the grooves in the tread. For this reason a slick will perform worse on aquaplaning. The resistance to aquaplaning of a tyre can be affected mainly by the combination of design and dimension of the grooves in the tyre on one hand, and the shape of the dynamic footprint on the other hand.

Effects of high rotational speed

Centrifugal force

The contour of the tyre is influenced by rotational speed. Centrifugal forces act on the tyre construction and cause growth of the contour. The rolling radius increases, but moreover the contour could get an undesired shape by non-uniform growth. To control the grow shape and to strengthen the tyre construction in order to cope with high rotational speeds, a cap ply is applied. The cap ply (usually embedded nylon) is applied with a controlled amount of pre-stress at the desired locations. The cap-ply compound partly consists of NR, because of its low heat build up properties.

Heat production

A tyre deforms by the longitudinal, axial and vertical force acting on it. Rotating the tyre at a high speed causes fast changes in global and local deformation of the tyre. As rubber mixtures have hysteresis, cyclic deformation of the rubber in tyres results in heat production. Therefore the heat production increases with rotational speed, causing large thermal loads on the rubber compounds. Too high thermal loads cause a degradation of rubber mixtures. To reduce the heat production and

to allow better cooling, the amount of rubber material must be minimized, and the compound must be tuned for less heat production, for example by altering the stiffness, increasing the cross link density and by using less heat producing elastomers. Side walls, for instance, are made from NR/BR blends. As for natural rubber, BR has low heat build up properties in comparison to the common SBR type elastomers. To improve the cooling of the material, the heat transport can be increased by improving the heat conductivity and capacity of the several compounds. The heat stability of rubber compounds can be influenced by the use of anti-degradants and the type of vulcanisation system.

Uniformity

The uniformity of the tyre gets more important when the rotational velocity increases. A too large non-uniformity of the tyre has negative effects (vibrations, pulling, local heat build up) that will increase at high speeds. As a consequence this results in joints with some overlap. For high performance tyres non-uniformity is avoided by applying the nylon overhead as continuously wound jointless strip (cap strip). Advantage of this method is the possibility of changing the pre-stress and strip distribution during cap strip winding, offering further control of the tyre contour both at low and at high speeds.

Resistance to tearing

The friction forces at road level are transported through the tyre construction to the rim. During extreme cornering or braking the shear stresses in the several rubber compounds are considerable. The adhesion between the several compounds and materials, and the cohesion of the separate materials must resist the shear stresses. To investigate the resistance to tearing, driving tests are performed under extreme conditions at the N.rburgring. The tyres are subjected to much higher loads than in typical road use, providing a valuable means of testing tearing resistance in extreme situations. In this way the resistance to tearing during road use is guaranteed. Besides the handling and tearing tests at the racetrack several indoor and outdoor tests are performed to investigate safety and durability of the tyre, like endurance tests, aquaplaning tests and high-speed tests. In this way maximum effort is put in assuring the performance of the high-speed tyre, a crucial part of high performance cars.