The Complete Book on Rubber Processing and Compounding Technology (with Machinery Details) 2nd Revised Edition

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The Complete Book on Rubber Processing and Compounding Technology  (with Machinery Details) 2nd Revised Edition

Author: NIIR Board of Consultants and Engineers
Format: Paperback
ISBN: 9788178331621
Code: NI174
Pages: 680
Price: Rs. 1,875.00   US$ 150.00

Published: 2016
Publisher: Asia Pacific Business Press Inc.
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The production of rubber and rubber products is a large and diverse industry. The rubber product manufacturing industry is basically divided into two major sectors: tyre and non-tyre. The tyre sector produces all types of automotive and nonautomotive tyres whereas the non-tyre sector produces high technology and sophisticated products like conveyor belts , rubber seals etc. The wide range of rubber products manufactured by the rubber industry comprises all types of heavy duty earth moving tyres, auto tyres, tubes, automobile parts, footwear, beltings etc.

The rubber industry has been growing tremendously over the years. The future of the rubber industry is tied to the global economy. Rapidly growing automotive sector in developing economies and increased demand for high-performance tyres are expected to contribute to the growth of the global industrial rubber market. The current scenario reveals that there is a tremendous scope for the development of rubber processing industries. The global market for industrial rubber products is projected to increase 5.8 % per year. Investment in rubber industry is expected to offer significant opportunities in the near future and realizing returns to investors willing to explore this sector.

This book deals with all aspects of rubber processing; mixing, milling, extrusion and molding, reclaiming and manufacturing process of rubber products. The major contents of the book are rubbers materials and processing, mixing technology of rubber, techniques of vulcanization, rubber vulcanization, rubber compounding, rubber reclaiming, manufacture of rubber products, latex and foam rubber, silicone rubber, polybutadiene and polyisoprene, styrene butadiene rubber, rubber natural etc. The book contains addresses of plant & machinery suppliers with their Photographs.

It will be a standard reference book for professionals, entrepreneurs, those studying and researching in this important area and others interested in the field of rubber processing technology.

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Contents

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1 RUBBERS: MATERIALS AND PROCESSING TECHNOLOGY
Natural Rubber Plantation
Tapping of Rubber Latex
Preservation and Coagulation of Latex
Chemical Nature of Natural Rubber Hydrobcarbon
Hydrogenated Rubber
Cyclized Rubber
Chlorinated Rubber
Rubbers from Stereo-regular Polymerization of Isoprene and Butadiene
Styrene-Butadiene Rubber (SBR)
Polychloroprene Rubber (CR)
Nitrile Rubber (NBR)
Butyl Rubber (IIR)
Ethylene-Propylene-Diene Terpolymer (EPDM)
Polysulphide Rubber (PSR)
Polyacrylic Rubber or Acrylate Rubber (ACR)
Fluorocarbon Rubber (FKM)
Introduction
Mastication and Mixing
Open Mill
Internal Mixers
Reclaimed Rubber
Fillers
Antidegradants
Accelerators
Retarders
Activators
Tyres
Belting and Hoses
Cellular Rubber Products
Miscellaneous Applications of Rubber
Passenger Tyre
Tube Compound for Car tyres
Conveyor Belts
Insulation Compound for Cables
Shoe Soles
2 MIXING TECHNOLOGY OF RUBBER
Two-roll Mills
Internal Batch Mixers
Continuous Mixers
Advantages of continuous mixing
Disadvantages of continuous mixing
Development of the Banbury Mixer
Operating Variables
Ram Pressure
Rotor Speed
Batch Size
Coolant Temperature
Unit Operations in Mixing
Single-Pass Versus Multiple-Pass Mixing
Types of Mix Cycle
Late Oil Addition
Upside-down Mixing
Sandwich Mixes
Analysis of Changes to the Mix Procedure
Acceleration of First-pass Compound
Mill Mixing of Speciality Compounds
Acceleration in Line with Internal Mixing
Testing of Raw Materials
Elastomers as Raw Materials
Fillers
Plasticisers and Process Oils
Small Ingredients
Control of Composition
Tracking the Mix Cycle
Compound Testing
Basic SPC Charting
Rheometer Data and its Meaning
Mixing Control Software
Peptisers in Natural Rubber
Effects of Temperature
Effects of Time
Effects of Use Level
Effects of Other Additives
Peptisers in SBR
Peptisers in Sulphur-containing Polymers
Additives to Increase Viscosity
Preventing Unwanted Chemical Reactions
Filler Treatments
Bin Storage Problems
Inspection of Banbury Mixers
Inspection at the Mezzanine Level
Side Cooling
Rotor Cooling
Rotors and Bearings
Rotor Bearing Lubrication
Dust Stops
Drop Door and Latch
Hydraulic System
Grease System
Dust Stop Lubrication
Drive Gears
Couplings
Inspection of the Banbury Platform
Ram and Cylinder
Heating Weight
Piston Rod
Weight Pin Assembly
Hopper Door
Air Line Filter
Hopper Operation
Mixer Maintenance and Lubrication
Each time the mixer is started
Once per shift
Once per day
Once per week
Once per month
Every six months
Anticipating Required Service
Dust Stop Maintenance
SSA Dust Stops
Assembly
Lapping
Running
Banbury Mixer — Hydraulic Dust Stops
Assembly
Run-in
Lapping
Production
Flushing
EPDM Expansion Joint Cover
Expansion Joint Intermediate Layer
Traffic Counter Treadle Cover
SBR/IR Belt Cover
EPDM Low Voltage Electrical Connector
Peroxide-cured Black-filled EPDM Compounds
EPDM Concrete Pipe Gasket
Injection-moulded NBR Gasket
CR/SBR Blend
Low Durometer CR/SBR Blend
Non-black CR for Injection Moulding
Hard Rubber Industrial Wheel
High Durometer NBR Masterbatch
NBR/PVC Cable Jacket
NBR/PVC/SBR Blend
Butyl Masterbatch
Butyl Masterbatch, Heat Interacted
Chlorobutyl/NR Blend
CSM CORD Jacket
Non-black Millable Urethane
Some Major Changes
Tempered Water
Power-controlled Mixing
Energy Conservation
Composition of EPDM Elastomers
Variables in EPM and EPDM Elastomers
Average Molecular Weight
Molecular Weight Distribution
Ethylene/Propylene Ratio
Type of Diene
Diene Level
How Processing Relates to Structure and Rheology
Practical Guidelines for Mixing EP Elastomers
Using Internal Mixers
Polymer Composition and Form
Filler/Oil Levels and Types
Cure Systems
Processing Aids
Mixing Process
Mixing Instructions
Fill Factor
Mixing Temperature
Machine Parameters
Ram Pressure
Coolant Temperature
Automation
Machine Condition
Downstream Processing Equipment
Using Two-roll Mills
Summary
Rework
Phase Mixing
Natural Rubber Viscosity Reduction
Measurement of Mixing Efficiency
Special Considerations
Raw Materials
Typical Formulations
Internal Mixing
Mill Mixing
Summary
Accounting Methods
Farrel Continuous Mixer
Operating Principles of the FCM
Commercial Applications for the FCM
Farrel Mixing Venting Extruder (MVX)
Designing the Rotor
Analysis of Dispersive Mixing
3 TECHNIQUES OF VULCANIZATION
Pressureless Vulcanization
Rubber Moulding
Factors of Mouding
Mouldin
Compression Moulding
Transfer Moulding
Injection Moulding
Helicure
Buffed Tread Crumb
Incineration and Pyrolysis of Tyres
Reclaimed Rubber
4 RUBBER VULCANIZATION
Physical Property Tests
Free Sulphur Determination
Solvent-swell Method
Mooney-Rivlin Equilibrium Modulus
Differential Scanning Calorimetry
Determination of Spring Constant
Sulphur Vulcanization
Peroxide Crosslinking
Resin Vulcanization
Electron Beam Vulcanization
Nitroso Compounds
Metal Oxides
5 RUBBER COMPOUNDING
General Compounding Principles
Tensile Strength
Tear Resistance
The Crescent Tear Test
The Hardness of Rubber
Set
Abrasion Resistance
Flex Cracking Resistance
Resilience
Heat Build-up
Temperature Resistance
Tyres
Retreading Materials
Conveyor Belting, Transmission Belting and Hose
Footwear
Rubber Roller
Medical Applications
‘O’ rings and Seals
Rubber Blends
Master Batches
Choice of Rubber
Fillers
Vulcanizing Agents
Peptizers
Accelerators
Activators
Anti-oxidants
Retarders
Softeners and Plasticizers
Rubber Crumb
Factice
Processing Aids
Special Purpose Additives
Unvulcanized compound properties
Vulcanized compound properties
6 RUBBER RECLAIMING
7 MANUFACTURE OF RUBBER PRODUCTS
Classification
Components
Tyre Building
Parts of a Conveyor Belt
Cover rubber
Manufacturing Process
Finished belt testing
PVC Belting
Steel Cord Belting
Design of Hoses
Hose Manufacture
Braided/spiralled hoses
Testing of Hose
Constructions
V-Belt Manufacture
Main Types of Power Transmission Belts
Preparation of Ingredients
Stability of Latex Compounds
Manufacture of Latex Products
Foaming and Gelling
Vulcanization
Classification and Terminology
Fabric Lined Water-proof Shoes
Canvas Shoes
Micro-cellular Soling
Manufacturing procedure
Types of Mountings
8 LATEX AND FOAM RUBBER
Selection of Raw Materials
Preparation of Raw Materials
Compounding and Design
Maturation
Processing and shaping
Dipped Goods
Latex Thread
Vulcanisation
Hot Air Cure
Hot Water Vulcanisation
Autoclave Vulcanisation
Radiation Vulcanisation
Ultrasonic Wave Curing
Testing of Rubber Products
Packing and Marketing
Conclusions and Recommendations

Manufacture of Latex Foam
Dunlop Process
Mechanism of Gelling
Compounding
Foaming and Gelling
Construction of Moulds
Curing
Washing
Drying
Finishing
Common Defects in Foam Making
Shrinkage
Foam Collapse
Setting
Complete Distortion of the Foam
Protein estimation protocol
Conlusion
9 SILICONE RUBBER
Electronics and Electrical Industries
Silicone Rubbers to Mimic Flesh
Silicone Polymers
Silicone Rubber Elastomers
Reinforcing Fillers
Semireinforcing or Extending Fillers
Additives
Curing Agents
Mixing
Freshening
Moulding
Extrusion
Calendering
Dispersion Coating of Fabric
Heavy-duty Hose
Bonding
Bonding Unvulcanised Silicone Rubber
Bonding Vulcanised Silicone Rubber
Post-baking
Condensation Cure—One-component
Condensation Cure—Two-component
Addition Cure
10 POLYBUTADIENE AND POLYISOPRENE
Polyisoprene
Cyclopolyisoprene
Gel and Branching
Polybutadiene
Isoprene
Butadiene
11 STYRENE BUTADIENE RUBBER (SBR)
Raw Materials
Production of Hydrocarbon Rubber
Manufacture of Emulsion SBR
Vinyl Content and Blockiness
Molecular Weight and Branching
Manufacture of Solution SBR
Property Control
Branching
Blending
Properties
Tg   Measurement
Molecular-weight Measurement
Dynamic Mechanical Measurements
Applications of SBR
12 RECLAIMED RUBBER
Whole Tyre Reclaim

Drab and Coloured Reclaims
Butyl Reclaim
Scrap-rubber Preparation
Reclaimed Rubber
Digester Process
Reclaimator Process
Pan Process
Engelke Process
Testing and Evaluations of Reclaimed Rubber
Millroom Operations
Special Strengths Through Reclaiming
Further Advantages of Reclaiming - Applications
Major Uses of Reclaimed Rubber
Automobile floor mat
Semi-pneumatic tyre
Butyl inner tube
Innerliner
Carcass
Applications
Process
Characterisation of Reclaimed Waste Latex Rubber (WLR)
13 NITRILE AND POLYACRYLIC RUBBER
Uses of Nitrile Rubber
Mixing and Processing
Latest Developments
Composition
Raw Polymer Characteristics
Physical Characteristics
Heat, Fluid, Low-temperature Resistance
Applications
Cure Systems
Reinforcing Agents
Plasticiscrs
Process Aids
Antioxidants
Mixing
Extrusion/Calendering
Compound Storage Stability
Vulcanisation
Bonding Characteristics
Solution Characteristics
Blends
Future Developments
14 RUBBER NATURAL
Agriculture
Exploitation
Latex Composition
Types and Grades
Production
Latex Concentrate
Processing
Chemistry
Physical Properties
Economic Aspects
Applications
15. Addresses of Plant & Machinery Suppliers
16. Plant & Machinery Photographs

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Sample Chapters


(Following is an extract of the content from the book)
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 Rubbers: Materials and Processing Technology

RUBBERS MATERIALS: INTRODUCTION

The technology of rubber began with the natural product known as natural

rubber (NR). Historically, rubber (NR) as a material was known to

and used by man as early as the sixth century, as excavations subsequent

to the discovery of America have revealed. The early reported uses of

NR were limited to such items as playing balls and waterproof fabrics or

garments, People of Europe became familiar with this natural product

and its properties by the end of the eighteenth century. From its popular

application as eraser of pencil and ink marks developed in Europe in the

middle of the eighteenth century, the name “rubber” was coined to it.

Earlier, the natural product was known by the term “Caoutchouc” which,

however, is now reserved in the English language to denote the pure rubber

hydrocarbon.

The process technology of making waterproof objects based on rubber-

coated fabrics passed into an advanced phase with the discovery of

coal tar naphtha as a good solvent for rubber by Charles Macintosh. This

led to the development of the “sandwich” process for the so-called double

texture fabric, imparting much improvements in the life and performance

of the waterproof garments. But the inherent drawback in the susceptibility

of rubber to changes of temperature (becoming soft and sticky

in warm wheather, and hard and stiff in cold weather) still remained unsolved,

thus limiting expansion and diversification of its use. The difficulty

was finally overcome through the discovery of vulcanization of

rubber using sulphur by Charles Goodyear in 1839 in USA. The process

was also developed in London by Thomas Hancock at about the same

time who applied for the first patent on vulcanization or curing of rubber

in 1843. The discovery of vulcanization, which in effect is a cross-linking

process, literally infused a revolution in the rubber industry and a

whole range of consumer and industrial rubber products soon became

available in the market.

NATURAL RUBBER (NR)

Natural Rubber Plantation

There was a time till the middle of the nineteenth century when raw

rubbers (NR) came almost entirely from the equatorial forest in the Amazon

valley in South America. Rubber randomly tapped and collected from

natural forests is variable in quality and is commonly known as wild rubber.

Under economic, technical and other compulsions, and for a regular,

uninterrupted supply of rubber, comprehensives plans for cultivation of

rubber trees from seeds of Hevea brasiliensis in the equatorial climatic

zones of South and South-East Asia began in the later part of the nineteenth

century. Present day NR or the Hevea rubber is almost entirely

obtained from the plantation industries.

Tapping of Rubber Latex

The rubber plant produces a milk-white latex that contains the natural

rubber hydrocarbon in a fine emulsion form in an aqueous serum.

After a thin shaving of bark of the Hevea tree has been cut, the latex that

comes out is allowed to flow into a cup through a spout that is stuck into

the bark below the bottom end of the cut. A little of sodium sulphite

solution put into the empty cup before tapping helps prevent some darkening

or discolouration of the latex which may otherwise develop as a

 consequence of an enzymatic reaction in the latex involving its phenolic

constituents producing the dark coloured pigment melanin.

Chlorinated Rubber

Natural rubber can be readily halogenated. Only chlorination has

been commercially developed. Direct chlorination with chlorine results

in both addition and substitution reactions and HC1 is evolved as a

byproduct. For only additive chlorination, maximum attainable chlorine

content would be about 51%, while for commercial products of good

stability, the range of chlorine content is 60-68%, and they are resinous

in character. It is generally believed that good degree of cyclization also

takes place on chlorination. Rubber, cut into small pieces is dissolved in

carbon tetra-chloride in presence of a small amount of benzoyl peroxide

which acts as a depolymerizing agent and lowers the solution viscosity.

A relatively uniform chlorinalion is achieved by spraying the solution at

the top of a chlorinating tower in which a stream of chlorine, let in at the

bottom, is allowed to ascend. The droplets of the chlorinated product

collect at the bottom. The collected solution from the bottom is degassed

to remove excess chlorine and then sprayed into a steam chamber where

the admitted steam causes rapid volatilization of CCl4. CCl4 is then recovered

and reused; the chlorinated rubber collected as a wet mass from

the bottom is washed, dried, milled if necessary and stored. Chlorinated

rubber is resistant to many chemicals. It even resists concentrated nitric

acid. It is, however, soluble in a wide range of solvents and it is used in

the formulation of many paints, lacquors, adhesives and printing inks.

Because of high chlorine content, chlorinated rubber has prominent flame

retardant characteristics.

Rubber can also be modified into a resinous product by

hydrochlorination. The rubber hydrochloride may be prepared directly

from stabilized latex using hydrochloric acid gas. It is, however, better

obtained by hydrochlorination with gaseous HCl using rubber in benzene

solution. Films made from the solution of the hydrochloride to which

plasticizers and stabilizers have been added are used for making laminates

with paper or films of other plastics such as cellulose acetate, etc.,

for use as heat sealable packages for dry food, cosmetics, shampoo, etc.

The hydrochloride also finds use in the formulation of adhesives and

bonding agents.

Polyacrylic Rubber or Acrylate Rubber (ACR)

Poly(ethyl acrylate) is soft and rubbery in nature. Copolymers of

ethyl acrylate (95%) and 2-chloroethyl acrylate or 2-chloroethyl vinyl

ether (cure site monomer) have been commercially developed and the

products are known as polyacrylic rubber or acrylate rubber. These rubbers

are suitably cured using aliphatic linear diamines and polyamines.

Cross-linking apparently occurs by HC1 elimination and intermolecular

link up through the diamines or via ester hydrolysis and establishment of

intermolecular amide linkages. Small amount of sulphur is used as an

anti-aging additive. The cured rubber is particularly useful as hoses, seals

and gaskets. Reinforcing carbon blacks are used as fillers. For pale shades,

siliceous fillers are used. The rubber has good resistance to oils and to

ozone and it may be used over a wide temperature zone (—40 to nearly +

200°C). The uncured polyacrylic rubbers are soluble in ketones, esters

and alcohol-ester mixtures. The cured polyacrylic rubbers are better than

nitrile rubbers in heat and oil resistance. Their resistance to ozone attack

and to sunlight and weathering are good. A terpolymer (AEM) of methyl

acrylate, ethylene and a cure site monomer, known in the trade by the

name Vamac, is of more recent development.

Fluorocarbon Rubber (FKM)

Fluorocarbon rubbers or elastomers are copolymers of vinylidene

fluoride and chlorotrifluoroethylene (50 : 50 or 30 : 70 ratio). Better products

are obtained by copolymerization of vinylidene fluoride and

bexafluoropropylene (“Viton” elastomers from Du Pont). They are usually

cured with amine type curatives in presence of a metallic oxide (litharge

or calcined magnesia). Curing is apparently effected by the elimination

of hydrogen fluoride.

The fluorocarbon (copolymer) elastomers are prepared by batch or

continuous process following the emulsion polymerization technique. The

latex-obtained is coagulated by hydrochloric acid and the polymer is

washed and dried. The Vitons are normally soluble in lower ketones and

the doughs formed are suitable for spreading over glass cloth to produce

coated fabrics useful as oil seals and gaskets with a long service life at

high temperatures (  100 h at nearly 400°C, > 5000 h at 200°C).

The fluorocarbon rubbers exhibit excellent resistance to oils, lubricants,

hydrocarbon solvents, mineral acids and chemicals and to heat; in

these respects, they are superior to almost all other commercial rubbers.

The fluorocarbon rubbers can be suitably compounded to give vulcanizates

of tensile strength of about 200 kg/cm2 and elongation at break in the

range of 200-300%. They are flame resistant and they exhibit outstanding

resistance to oxygen and ozone attack. Their good low temperature

flexibility makes them advantageously useful at low temperatures up to

— 30°C. The application of fluorocarbon rubbers is limited to only special

or unusual service conditions where other rubbers are altogether unsuitable

and where their high cost is not a hindrance. The applications

include seals, gaskets and diaphragms, fire-resistant and protective clothing

from coated fabrics, and wire and cable insulation.

THERMOPLASTIC ELASTOMERS (TPE)

Development of thermoplastic elastomers (TPE) has narrowed the

basic difference between the processing of thermoplastics and elastomers

or rubbers for many non-tyre products. The TPEs make useful products

for which the tensile and set properties are not much critical, such as in

many automotive parts, footwear, cables, sealants and adhesives, hoses,

coated fabrics, tubings and sheetings. Their light weight, flexibility, impact

resistance and weathering resistance make them useful in many of

these applications.

The thermoplastic elastomers are processed like reusable and

reprocessable thermoplastics and under service conditions, they behave

like vulcanized rubbers. The useful mechanical and elastic properties of

conventional vulcanized rubbers are attributed to the chemical cross-links

established between the rubber chain molecules during vulcanization to

produce a space network structure. In the TPEs, the network is basically

formed through thermally labile physical cohesive forces between specific

segments of different polymer chains but not really through intermolecular

chemical linkages. At elevated and processing temperatures,

the thermolabile physical bonds weaken and finally break up, permitting

flow under shear and thus enabling them to be moulded or formed like a

conventional thermoplastic material. At and around ambient temperatures,

the TPEs exist in two phases; the soft rubbery phase forms the

continuous matrix in which the hard resinous phase remains dispersed in

discrete domains. The volume fraction of the rubbery matrix is usually

higher than that of the hard thermoplastic domains which materially act

as cross-links and as stiffening fillers or points of reinforcements.

RUBBER COMPOUNDING AND PROCESSING TECHNOLOGY

Introduction

No rubber is considered technically useful if its molecules are not

cross-linked by a process known as curing or vulcanization. The process

of vulcanization is usually associated with two chemical processes taking

place simultaneously, cross-linking and chain degradation, though at

widely different rates. For natural rubber and many synthetic rubbers,

particularly the diene rubbers, the curing agent most commonly used is

sulphur. But sulphur curing takes place at technically viable rates only at

a high temperature (> 140°C), and heating with sulphur alone leads to

optimum curing after nearly 8 h at 140°C using a fairly high dose of

sulphur (8-10 phr). Use of metal oxides, such as those of zinc, calcium,

magnesium, lead, etc., brings about some advantages with respect to time

of curing and improvements in physical properties without much reduction

in the sulphur dose. Aniline is considered as the first organic accelerator

tried in an attempt to quicken the curing process. Some of the more

efficient and less toxic modern organic accelerators of rubber vulcanization

are aniline derivatives. Sulphur dose has been substantially lowered

with the advent of organic accelerators.

Extensive studies and experimentation have revealed that neither

sulphur nor application of heat is indispensable for effecting cross-linking

and associated changes in physical properties of rubbers. Peroxides,

metal oxides, amines, amine derivatives and oximes have been found to

bring about curing of selected rubbers quite effectively. High-energy

radiations can bring about effective curing; but high-energy radiation

curing has not been developed into a commercial process of even limited

acceptability. Selenium and tellurium can substitute sulphur either totally

or partly to effect satisfactory curing of diene rubbers. Sulphur

monochloride can bring about room temperature or cold curing of diene

rubbers.

Improvements in physical and mechanical properties that can be

achieved through vulcanization using sulphur/accelerator systems only

are rather limited. The needs of imparting colour, stability, resistance to

tearing and abrasion, flexibility etc., and of improving processibility and

mechanical properties necessitate incorporation of a host of additives or

compounding ingredients in the rubber by what is commonly known as

the mixing or compounding process.

Rubber Vulcanization

VULCANIZATION AND ITS EFFECTS

Chemical crosslinks between macromolecules may occur in polymerization

or in fabricating articles, resulting in polymers with network structures.

The reaction which occurs during fabrication is known as “vulcanization”

in the rubber industry, and “curing” or “hardening” in the

plastics industry. Crosslinks in rubber are formed by the reaction with a

suitable “vulcanizing agent”, usually sulphur.

A typical crosslinking reaction is the formation of short chains of

sulphur atoms linking linear molecules in rubbers during vulcanization.

Lightly-vulcanized rubber shows good elastic properties, whereas vulcanization

to the maximum extent possible will lead to a full network

structure, and a hard and rigid material. Thermosetting resins may be

crosslinked by the addition of a curing agent, such as hexamethylene

tetramine and/or heat. Typical examples are epoxy resins, polyester resin

and poly-urethane foams. Owing to the greater restriction on the mobility

of the macro-molecular chains as a result of crosslinking, the modulus

and glass transition temperature increases in both hardened resins and

sulphur crosslinked rubber.

Charles Goodyear discovered sulphur vulcanization of rubber in 1839

and developed many new applications for rubber in industry.

Crosslinking of rubber occurs by a chemical process which is initiated

at favourable reaction sites by means of some form of energy input.

Double bonds in diene polymers or reaction sites left by abstraction of

hydrogen or halogen atoms are favourable reaction sites. These are coupled

by carbon-carbon crosslinks or by bridges formed with curatives

such as sulphur, acrylates, phenolics or triazines. Crosslinking through

sulphur atoms is quite common in biological molecules, e.g. disulphide

(cystine) bridges in polypeptides.

There are four principal changes which are brought about by vulcanization.

1. Rubber is converted from essentially a plastic substance of

very low strength to an elastic material of considerable strength

and resilience.

2. The physical properties, such as tensile strength and other

properties shown in Fig. 1 undergo a profound change as

vulcanization progresses.

3. The physical properties of vulcanizates are maintained over a

much wider temperature range than in the case of unvulcanized

rubber.

4. The crosslinked polymer (vulcanizate) only swells in liquids

which normally dissolve the uncrosslinked polymer.

Rubber Reclaiming

In the mid-1800s Charles Goodyear developed a technique for vulcanizing

rubber. He blended natural rubber with sulfur and placed it on his

wood stove, where the rubber cured into a water-impermeable sheet.

Because of the shortage of natural rubber at that time, a rubber-reclaiming

process, based on steam pressure to devulcanize used rubber, was

developed in 1858. The cured ground rubber was subjected to steam pressure

for 48 hours. Today, high cost silicone rubber polymers are reclaimed

in much the same way, although most synthetic polymers require more

complicated techniques. Rubber recycling has been extended to the recovery

of rubber in asphalt, scrap rubber as fuel, rubber pyrolysis, tire

splitting, and others. However, the discovery of plastics and oil-extended

rubbers and the drop in crude oil prices have led to a reduction in rubber

recycling except for expensive polymers, such as silicones (qv) and

fluorocarbons. Pyrolysis of scrap rubber is expensive and markets are

limited. It is more expensive to prepare and burn scrap rubber for fuel

than to burn natural gas, fuel oil, or coal. High fuel costs and petroleum

scarcity in Europe and other parts of the eastern hemisphere have resulted

in the use of recycled rubber as fuel and made it more economical

than in the United States. However, with today’s technological advances,

recycled rubber will be used more and more in the United States, but with

regulations for handling and disposing of worn tires. Crude oil costs have

dropped from about $189/m3 ($30/bbl) to $88/m3 ($14/bbl) over the last

few years, making it seemingly unprofitable to recycle rubber, especially

tires. However, tires discarded in landfills tend to float on top; mosquito

breeding and illegal tire disposal are causing problems which could be

alleviated by recycling. Approximately 70% of the scrap rubber, primarily

as tires, is discarded in landfills. Private landfills may charge up to S3

per tire, and disposal costs at municipal landfills are ca $0.30-0.60 per

tire. These costs encourage illegal disposal of tires. As the tire piles grow,

rubber recovery should become more economical. Companies entering

into multimillion dollar energy recovery projects will be encouraged by

an adequate supply of old tires.

Some states have restricted the disposal of tires in landfills, e.g., Washington,

Minnesota, and Oregon. Other states are studying legislation and

New Jersey is considering a surcharge. Iowa, Massachusetts, and Michigan

are studying tire-disposal problems, and Minnesota and Washington

have imposed a surcharge. Some communities refuse old tires while others

require tire splitting or shredding to prevent floating problems in

landfills. Fires resulting from the storage of tires have led to regulations

governing stockpiles. Mosquito infestation caused by illegal disposal of

tires has prompted Saginaw, Mich., to purchase a shredder for landfill

disposal.

Advances in the technology of shredding tires, reclaiming rubber,

retread equipment, energy-related projects, and pyrolysis are helping to

solve the disposal problem. Technical and marketing professionals associated

with the recycling and disposal industries are paying more attention

to tire-recycling and energy-generating projects.

SCRAP RUBBER AS FUEL SOURCE

The use of scrap rubber for fuel offers the best alternative for reusing

rubber, as fuel costs increase and tire disposal problems become more

serious. Tires in the form of 2.50-cm chips are most economical because

shredding is not expensive. Tires contain more than 90% organic materials

and have a heat value of ca 32.6 MJ/kg (ca 14,000 Btu/lb), compared

with coal values of 18.6-27.9 MJ/kg (ca 8,000-12,000 Btu lb). A cyclonic,

rotary-hearth boiler fired with whole tires was operated by the

Goodyear Tire and Rubber Co. from 1975 to 1977. It was designed to

burn 1400 kg/h and generated 11,300 kg of steam per hour.

In the Lucas tire-burning furnace, tires are conveyed into an airtight

chamber and then onto the outer rim of a rotating hearth. The chamber

prevents flashback fires and limits air leaks. An air-velocity head of 5.1

cm provides the turbulence necessary for combustion. Residues from the

burning tires form a char that increases combustion heat loss and tends to

clog furnace grates, but the carbon black content reduces slagging problems.

Improper combustion at the ash-removal area of the furnace may

prevent the burning of the carbon black. The Lucas-Goodyear furnace

was shut down because of mechanical problems and failure to comply

with Michigan’s air pollution emission standards.

Shredded tire chips have been burned in stoker-fired boilers. Uniroyal

fired a 15% mixture of tire chips with coal and both General Motors and

B. F. Goodrich have burned a 10% tire-chip mixture with coal. Tiregrinding

size-reduction problems and delivery costs have stymied projects

based on combined tire and coal fuel. The Lucas furnace was developed

to burn tires without size reduction. Transportation of tire scrap can cost

$0.05/kg, exclusive of grinding costs. Thus tire-fired boilers are limited

to areas with ample scrap-tire supplies, e.g., large cities or tire manufacturers.

The cost of burning one metric ton of tires per hour in an incinerator

was ca $0.20—0.40 per tire in 1974, which increased to $0.35-0.70

per tire in 1987.

The Oxford Energy Company uses a technology developed by

Gummi-Mayer Company (FRG) to incinerate tires and produce electricity.

The technology will be used in the Modesto Project near Westley,

Calif. The facility generates 14.4 MW of electricity and cost $38 × 106.

Construction began in December 1985 and was completed in August 1987.

The project is designed to generate electricity by incinerating whole waste

tires on reciprocating stoker grates located beneath two water wall boilers.

The boilers produce 55,000 kg of steam per hour and power a single

General Electric turbine generator. The facility is being constructed next

to the Filbin Tire Collection Agency, Inc., which administers one of the

largest stockpiles of waste tires in the United States (ca 300 × 103 t); the

stockpile increases annually by 27 t. Oxford Energy is also involved in

the construction of a New Hampshire tire incineration project, generating

15 MW of electricity; facility is expected to be completed in late

1988 at a cost of $35 × 106. The company has received air quality and

waste management permits from the State of New Hampshire. The firm

also plans to build a 22-MW electricity-generating facility in northeastern

Connecticut; construction is expected to be completed in 1989 at a

cost of $50 × 106. The company has projects to incorporate waste produced

from manufacturing facilities along with waste tires, thus providing

energy to manufacturing facilities as well as local utilities. Approximately

20 × 106 tires are stockpiled at two sites in New England.

These sites, together with the Filbin stockpile, contain at least 480 × 103

t of tires (53 × 106 tires).

CRYOGENIC PULVERIZING AND MECHANICAL TIRE

SHREDDING

Tires must be pulverized or shredded before they can be reclaimed

by devulcanization or used in asphalt and other recycling processes. The

tires are mechanically ground, sometimes using cryogenic freezing or

solvent-swelling techniques to enhance grinding efficiency. In one process,

a polar solvent is used to swell the rubber, followed by shearing to

reduce particle size. Ground tire-crumb rubber is commonly referred to

as rubber reclaim, even though the rubber has not been devulcanized.

Cryogenics in conjunction with mechanical action has been used to

make crumb rubber. Nitrogen cools the rubber below the glass-transition

temperature, and the brittle rubber is pulverized in a grinding mill. A

small cryogenic system can be installed at the site to integrate the scraprubber

crumb into the compound mixing process. Cryogenic-grinding

costs are $0.20-0.40/kg, depending on the desired particle size and the

type of rubber; harder rubber is easier to grind. Ground-rubber scrap can

be devulcanized, pyrolyzed, or recycled directly into the rubber compound.

Ground rubber is also added to plastics.

Air Products and Chemicals, Inc., Allentown, Pa., developed a cryogenic

process for grinding scrap tires in the mid-1960s. Liquid nitrogen

freezes the rubber to facilitate shredding. Midwest Elastomers installed a

cryogenic system in 1980 to powder tire peels in its Wapakoneta, Ohio,

plant. The equipment and materials for cryogenic processing are expensive

and have slowed expansion. Cryogenic grinding requires recycling

close to an air-processing facility to make the use of liquid nitrogen economical.

Air Products has a marketing and development relationship with

Brown & Ferris Industries to develop waste-to-energy and alternative

fuel projects.

The tires are mechanically ground with a two-roll, grooved-rubber

mill. The two-mill rolls turn at a ratio of ca 1:3, providing the shearing

action necessary to rip the tire apart. The rubber chunks are screened and

the larger material is recycled until the desired size is reached. Bead wire

is removed by hand or with magnets. For most applications, e.g.,

devulcanization or pyrolysis, crumb-rubber particles smaller than 1.19

mm (16 mesh) are desired, and several milling steps are required. Tire

fiber is removed in intermediate operations with hammer mills, reel beaters,

and air tables that blow a steady stream of air across the rubber

Latex and Foam Rubber

INTRODUCTION

Rubber latex is a term used to cover a range of colloids having macro

molecular substances as the dispersed phase and water as the dispersion

medium. Most important latex used in the rubber industry is natural rubber

(NR) latex. This is an aqueous dispersion of cis 1,4 polyisoprene in

water containing dissolved serum substances. NR latex is harvested from

Hevea brasilienesis trees by inflicting controlled wounds on the bark of

the trunk of the tree.

Under inorganic salts seven different types of ions are identified.

Similarly, under each of the organic ingredients a number of compounds

belonging to that group are characterised. In synthetic lattices also, a

number of ingredients other than the polymer and water are present.

PRODUCTS FROM LATEX

There are eight important steps in the manufacture of products from

latex –

1. Selection and quality control of raw materials

2. Preparation of raw materials

3. Compounding and design of the mix

4. Maturation

5. Processing/shaping

6. Vulcanisation

7. Testing for grading

8. Packing and marketing.

There are different types of products manufactured from latex. Some

of the steps are common in all products but differences exist in others.

The similarities and differences in the various steps for different types of

products are discussed below briefly:

Selection of Raw Materials

Following are the raw materials used in the latex industry —

Latex containing the polymer

Stabilisers and viscosity modifiers

Activator

Accelerator

Curing agent

Antioxidant

Special additives, if any

Gelling/coagulation system.

 

The raw material selected for product manufacture must be of good

quality and conforming to specifications prescribed for this by reputed

institutions like the BIS (ISI). In the case of natural rubber latex, there are

few processors who can supply high ammonia-preserved concentrated

natural rubber latex conforming to IS 5430. It may be noted that the specifications

for this HA latex have been formulated by the combined effort

of experts from rubber plantations and the goods manufacturing industry.

It is desirable for the industry to insist that the suppliers should deliver

of HA latices conforming to IS 5430.

In foam rubber there are two important processes in vogue commercially.

One is called the Dunlop process and the other called Tulalay process.

In India, most factories use the Dunlop process. In Dunlop process,

foaming is carried out in two stages. In the first stage latex is whipped up

with foam promoters and stabilisers till the desired expansion is obtained.

Thereafter the whipping rate is reduced and the foam is clarified or refined.

Gelling agents are added at the end of the foam clarifying stage.

The density of foam rubber produced depends on froth height. The level

in which latex is whipped should be calibrated properly for specific gravity

for various froath heights.

The materials selected for mould construction for foam production

must be light, cheap, should have high thermal capacity and good thermal

conductivity: A mould designed from copper free cast aluminum

meets these requirements. It is generally reported that in the cost of production

of foam rubber around 5% of the cost is on account of contribution

from mould cost. The tensile and cure properties of the hot wet gel

are appreciably lower than those of the dry finished products. So if care is

not exercised in stripping the foam from mould, it may as well be damaged.

For enabling easy and quick stripping, the mould surface should be

kept clean and polished. Mould release agent has to be applied on surface

of mould after each operation. Examples of mould release agents are

aqueous or alcoholic solutions of polyethylene glycol of fairly high molecular

weight such as carbo wax-4000, carboxymethyl cellulose or silicone

fluids.

In Tullalay process, the expansion of latex compound is achieved by

liberation of gases like oxygen liberated by catalytic decomposition of

hydrogen peroxide.

Latex Thread

In the production of latex rubber thread the matured latex has to be

carefully processed to avoid defects. The compounded matured latex is

de-aerated and filtered before it is allowed to pass through capillaries for

injection into an acid bath. The nozzle of the capillary is immersed in

coagulant. The nozzle depth in coagulant bath has direct relation to the

thread diameter. The rate of extrusion is 30-40 feet per minute. The orifice

of the nozzle is usually round and the size of the bore varies from 0.5

to 1 mm. Coagulant normally used is 15-40% acetic acid. Generally,

weaker solutions of acid are used for large diameter thread and strong

solutions for higher count threads. For the production of high count thread

alcohol is the most suitable coagulant. Between extruding nozzle and

winding, the thread is usually subjected to some degree of stretching which

may be upto 200%. Stretching is necessary to facilitate washing, to reduce

permanent set and to increase modulus of the thread.

Vulcanisation

Latex products are generally vulcanised at low temperatures and pressure

when compared to dry rubber products. It is desirable to wash, leach

and dry the latex products before subjecting them to vulcanisation temperature.

Vulcanisation of some products is carried out in hot air. Some

others are vulcanised in open steam in autoclaves.

Construction of Moulds

The moulds for foam rubber are usually made from cast aluminium

because of the lightness, durability and good heat conductivity of the

metal. When cushions and mattresses are made, cylindrical cores are attached

to the lids which give the characteristic cavity structure of the

back side of the foam. These cavities help to minimise the actual thickness

of the product to about 1 inch to ensure uniform curing throughout

the mass of the product. The cavities also assist in great saving of the

latex compound (of the order of 40%) without affecting the comfort. In

designing moulds, allowance should be given to shrinkage of the product

which amounts to roughly 8-10% linear.

Mould lubricants like polyethylene glycol and alkyl sulphate solutions

are applied to the mould surface to facilitate easy stripping of the

product after vulcanisation.

Curing

Mould foam products are normally cured by open steam (at 100°C)

which is more efficient and quicker than hot air curing. Thin sections of

foam, e.g., as used in carpet underlays, can be suitably cured by

hot air.

 

Polybutadiene and Polyisoprene

INTRODUCTION

Polyisoprene is a major component of manufacture rubber. It is also made

synthetically and forms are stereospecific cis 1-4, and trans-1-4-

polyisoprene. Both can be produced synthetically by the effect of heat

and pressure on isoprene in the presence of stereospecific catalyst. Natural

rubber is cis-1-4. Synthetic cis-1-4 is sometimes called synthetic natural

rubber. Trans-l, 4-polyisoprene resembles gutta percha. Polyisoprene

is the thermoplastic until mixed with sulphur and vulcanised. It supports

combustion and is non toxic

Polybutadiene is a synthetic thermoplastic polymer made by polymerising

1,3 butadiene with a stereospecific organometallic catalyst (butyl

lithium) though other catalysts such as titanium tetrachloride and aluminium

iodide may be used. The cis-isomer, which is similar to natural

rubber, is used in the tread due to its abrasion and crack resistance and

low heat built up. Large quantities are also used as blend in SBR rubber.

The trans-isomer resembles gutta percha and has limited utility, liquid

polybutadiene, which is sodium catalysed has speciality uses as a coating

resin. It is cured with organic peroxides. Combustible liquid form is probably

toxic by ingestion and inhalation, as well as skin irritant. The different

polymer structures of polybutadiene are given in Fig. 6.

POLYMER STRUCTURE

Polyisoprene

The polymerisation of isoprene and butadiene are examples of addition

polymerisation in which the repeating structural unit within the poly

mer backbone has the same molecular weight as the entering monomer

unit.

With isoprene, the building of this polymer backbone can occur in

several ways, depending upon where the addition occurs. The polymerisation

addition process can be a reaction involving the 1, 2-, 3, 4- or 1, 4-

positions of isoprene to give the structures shown in Figure 1.

In case of 1,2- and 3, 4- addition, an asymmetric carbon is formed

that can have either an R or an S (“d” or “1”) configuration. In general,

equal numbers of R and S configurations are produced during an addition

polymerisation, which results in no net optical activity for the polymer.

The disposition of the R and S configurations along the polymer

backbone, however, results in diastereomeric isomerism. Although many

combinations of sequences are possible, only three arrangements are commonly

considered in polymers. These diastereomeric isomers are referred

to as isotactic, syndiotactic, and atactic.

Gel and Branching

The structure and behaviour of polyisoprene is further complicated

by the possibility of branching and gel. The degree of gel and branching

is dependent upon the source of the cis-1, 4-polyisoprene. For Al-Ti catalysed

cis, 4-polyisoprene, the gel content is 5-25% most of which is termed

“loose” gel since it breaks down readily upon mastication. The gel content

in natural rubber is usually greater than that in the Al-Ti catalysed

polymer. This gel is also a “loose” gel. Lithium polymerised cis-l, 4-

polyisoprene usually contains no gel. Both natural rubber and Al-Ti-catalysed

cis-1,4-polyisoprene also contain large quantities of microgel.

The determination of the nature and degree of branching in natural

rubber and Al-Ti catalysed cis-1, 4-polyisoprene are complicated by the

presence of the gel and microgel. Removal of the gel/microgel fraction

from the soluble portion can have a marked effect on the resulting physical

properties such as viscosity, molecular weight, and branching.

Polybutadiene

The configurations of polybutadiene are cIS-, trans-, and vinyl. Since

either or both of the double bonds in butadiene can be involved in the

polymerisation mechanism, the resulting polymer may have a variety of

configurations. These result from the fact that the spatial arrangement of

the methylene groups in the polybutadiene backbone allow for geometric

isomerism to occur along the polymer chain. The different polymer structures

of polybutadiene are given in Figure 6.

Participation of both double bonds in the polymerisation process gives

rise to a 1, 4-addition, which can be either cis-1,4- or trans-l,4-, depending

upon the disposition of groups about the polymer double bond. Participation

of only one double bond results in a vinyl, or 1,2-addition, that

can have three possible structures just as 1,2- and 3,4- polyisoprene have

isotactic, syndiotactic, and atactic.

Reclaimed Rubber

INTRODUCTION

Reclaimed rubber is the product resulting when waste vulcanised scrap

rubber is treated to produce a plastic material which can be easily processed,

compounded and vulcanised with or without the addition of either

natural or synthetic rubbers. It is recognised that the vulcanisation process

is not truly reversible; however, an accepted definition for

devulcanisation is that it is a change in vulcanised rubber which results in

a decreased resistance to deformation at ordinary temperatures.

Reclaimed rubber is manufactured by suitable treatment to old and

worn out tyres, tubes and other used rubber articles with certain chemical

agents. A substantial devulcanisation or regeneration is effected to the

rubber compound in this process whereby its original plasticity is regained.

In short it may be stated that reclaiming is essentially a

depolymerisation process where the combined sulphur is not removed.

The reclaimed rubber is used in the manufacture of rubber goods, with or

without admixture of natural or synthetic rubber.

Recycled rubber can be more generally described as any sort of rubber

waste that has been converted into an economically useful form such

as reclaimed rubber, ground rubber, reprocessed synthetic rubber, and

die-cut punched parts.

One method of recycling some scrap rubber is to grind it as fine as

possible and work it into new rubber as an elastomeric filler. This was the

first method of reclaiming and is suitable only for compounding carriage

springs, which were fairly large barrel-shaped molded articles but not

suitable for footwear products.

In order to make a high-quality reclaim, the fibre must be removed

from the rubber scrap by soaking the rubber in water, and then taking a

small knife and starting the rubber from the cloth and stripping it off. But

this was not an effective way to produce large quantities of products.

TYPES OF RECLAIM

A variety of grades of reclaimed rubber is offered today, but mention

is made here of only the important ones.

Whole Tyre Reclaim

Whole tyre reclaim is the one produced in the largest quantity. Firstquality

reclaim made from whole tyres contains about 45% rubber hydrocarbon

by weight. The remaining 55% consists of valuable carbon

black, a little mineral filler, and softeners, all of which are substantially

unchanged by the reclaim manufacturing operation, and may be considered

to function as virgin materials. The manufacturing Minimum Staining Reclaim

Minimum staining reclaim can replace the conventional whole tyre

material when occasion demands. As implied, it has a much lower tendency

to stain, by either migration or contact, than conventional reclaim.

The reduction in staining characteristics is achieved by the use of activated-

carbon non-staining oils and by selecting tyres containing a higher

proportion of natural to synthetic rubber.

Drab and Coloured Reclaims

As the names imply, drab and coloured reclaims are made from nonblack

scrap. The digester process is usually employed and, when fabric is

present, a small addition of caustic is made in order to destroy it. The

period of heat treatment is usually several hours at 195°C.

Butyl Reclaim

Whereas reclaimed rubbers have been successfully produced from

scrap CR, NBR,SI, and other speciality rubbers, the only one of substantial

commercial importance is butyl reclaim. The starting material for this

is butyl inner tubes. A modified digester process is adopted, every precaution

being taken to avoid contamination by NR or SBR, because of

their adverse effect on the curing characteristics of the butyl. Extensive

control tests are necessary to ensure that the curing properties are satisfactory.

The nerve of butyl reclaim is much reduced compared with that

of the original polymer. Because of this, compounds containing butyl

reclaim will mix, calender, and extrude faster and more smoothly than

similar compounds based on virgin rubber.

Reclaimed Rubber

By the application of heat and chemical agents to ground vulcanised

waste rubber, a substantial regeneration of the rubber compound to its

original plastic state is effected, yielding a product known as ‘reclaim’ or

reclaimed rubber, capable of being processed, compounded, and

revulcanised. The process is essentially one of depolymerisation. Reclaimed

rubber has become widely accepted as a raw material which possesses

processing and economic characteristics that are of great value in

the compounding of natural and synthetic rubber stocks.

There are four principal reclaiming processes in use today, of which

the digester and reclaimator processes are important.

The raw material for reclaiming is scrap rubber in a wide variety of

forms, but tyres, as is to be expected, form the major quantity.

The first stage, in all processes, are the cracking and grinding of the

scrap rubber to reduce it to a crumb passing through a 20 to 30 mesh

screen.

Digester Process

At one time, most reclaim was made using the digester process. A

digester is essentially a steam-jacketed, agitator-equipped autoclave,

mounted either horizontally or vertically. This is a wet process in which

the coarsely ground scrap is submerged in a solution of water and reclaiming

agents. These agents may include many types of light and/or

heavy oils, naval stores, pine tar and coal tar pitches, and chemical

peptisers.

Until the advent of synthetic rubber, the digesting solution also included

caustic soda to remove free sulphur and to act as a defibring agent.

In fact, the process was generally referred to as the alkali digester

method.

The ground waste is loaded into a digester along with water, reclaiming

oils, and other additives, such as activated black (for minimum stain

ing grades). The digester is a cylindrical jacketed pressure vessel fitted

with a horizontal agitator, and steam can be supplied to both interior and

jacket, thus enabling a uniform temperature to be maintained throughout

the mass. The contents of the digester are then heated to about 190°C and

maintained at this temperature for some 4-10 hours with continuous agitation.

The digester is then ‘blown down’, and the contents deposited on

to a conveyor. Any necessary adjustments to the specific gravity and plasticity

by addition of plasticiser, carbon black, or fillers are carried out in

a ribbon blender, and the stock is then automatically conveyed to extruders

for straining, refining, and leafing on to a drum from which it is removed

in slabs.

Reclaimator Process

The reclaimator process is the only commercially successful continuous

technique for devulcanising tire scrap; all the others are batch

processes. Tyres are ground, the metal and fibre are mechanically separated,

then the rubber is further ground to a fine particle size. This fineground

rubber and the various reclaiming agents are all metered into a

blending system and conveyed to the reclaimator.

The reclaimator is a special type of screw-extrusion machine. It is

jacketed to provide for several zones of controlled temperature using either

hot oil or cooling water; in addition, the clearances between the screw

and the chamber wall are close and adjustable. The object is to subject

the rubber to a controlled amount of high heat and pressure in a continuously

moving environment. The residence time of the rubber in the machine

is less than 5 minutes. During this period, the rubber undergoes

devulcanisation. After the softened rubber is discharged from the head of

the machine, it is cooled and further processed in refining mills just as is

done in other reclaiming methods.

It can be shown that ground vulcanised rubber heated in a temperature

range of 120-200°C undergoes a rapid initial increase in plasticity,

and, on continued heating, passes through an inversion point and rehardens

unit, after prolonged heating, a further but slower increase in plasticity is

attained. It follows, therefore, that three points of equal plasticity occur

in this cycle.

 


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