1. Elastomeric Materials

2. CHRISTOPHER COLUMBUS- 15th CENTURY OBSERVED – IN SOUTH AMERICA CHILDREN PLAY WITH BALLS MADE OF NATURAL RUBBER
21 st CENTURY- RUBBER IS A
MULTIBILLION DOLLAR INDUSTRY

3. Elastomeric Materials
Common characteristics;
Large elastic elongation (i.e.200%)
Can be stretched and then immediately return to their original length when the load was released
Elastomers are sometimes called rubber or rubbery materials
The term elastomer is often used interchangeably with the term rubber
Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic (see thermoplastic elastomer).

4. All materials have some elastic elongation
“elastic elongation = elongation of any material when that material is at its yield point”
Ceramic & metal- small elastic elongation; 2%
PE, elastic elongation; 50%

5. Stress-strain diagram

6. Idealized stress-strain curves for metals, conventional plastics and elastomer

7. Diagram showing the random, natural state of elastomer when under no stress and when stressed

8. A material may be elastomeric at room temp, however rigid at lower temp (why???)
They are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible.
At ambient temperatures rubbers are thus relatively soft (E~3MPa) and deformable

9. Most elastomers are crosslink
Most elastomers are crosslink. Atoms between crosslink can still move, uncoil and coil.
The long polymer chains cross-link during curing and account for the flexible nature of the material.
Without crosslink, an elastomer may be elongated beyond elastic limit, with crosslink, max. elongation is set safely within the elastic region
Crosslink density- total number of crosslink in the system (less elongation is desired, number of crosslink can be increased)

10. Natural Rubber Rubber tree (Hevea Braziliensis)
Natural rubber is obtained by drying a latex rubber (milk in which the butter fat component is suspended in water salution)
High temperature stability – cooking the crude natural rubber with sulphur (vulcanization)
Vulcanization creates crosslinking between rubber molecules
Natural rubber is highly elastomeric (elongation 1000% for vulcanized natural rubber)
Compared to other elastomeric materials, natural rubber shows higher tensile strength, high tear strength, high resilience, resistance to wear, etc

11. Polymer repeating groups
Crude natural rubber was chiefly composed of cis-polyisoprena (a polymer chain with carbon carbon double bond with repeating unit)
Cis means that two pendent group (H and CH3) that are attached to the two carbons in the carbon carbon double bond
The alternate configuration where the two groups are located on the opposite side of the carbon carbon double bond is called trans
The presence of methyl group interfere the movement in polyisoprene polymer- restricted bending and twisting motion (increased stiffness, higher strength, and higher temperature stability

12. Polyisoprene structure
The most frequent causes of death are congenital heart defects and respiratory infections.
Cis-poliisoprena
(Hevea rubber)
Trans-poliisoprena
(Gutta percha)

13. Properties of cis and trans are quire different
Cis is highly elastomeric & sensitive to heat softening
Trans materials is called gutta percha, much harder than cis isoprena-used for golf balls
During vulcanization process, sulphur will react with carbon carbon double bond

14. Synthetic Polyisoprena or Isoprene Rubber (IR)
Disruption of supplies of natural rubber during world war I and II & increase needs for elastomeric materials- needs for synthetic rubber
Synthetic polyisoprena made in early 1900s, used for tires for lightweight vehicles
Combination of cis and trans molecular forms- mixture of properties
Ziegler-Natta catalyst system was developed in 1950s, it was found that 90% pure cis-isoprena could be produced by this catalyst system
However, natural rubber is used mre extensively because of its low cost

15. Butadiene Rubber (BR)
Synthetic rubber
Repeating units of both have a backbone of four carbon atoms including carbon carbon double bond
Polybutadiene has just two hidrogen attached to the carbon carbon double bond
Absence of methyl group in polybutadiene results in poorer strength & tear strength than would polyisoprena. Resilient is about the same. Polybutadiene has poor resistance to solvents
Information included demographic information, codes for underlying cause of death, and up to 20 disorders that are listed on death certificates.

16. Advantages of Polybutadiene; low cost, improvement in low temp
Advantages of Polybutadiene; low cost, improvement in low temp. flexibility, compatibility with many other polymeric materials, good adhesion to metal
Butadiene monomer is added to the monomer of the other plastic – copolymer is created
Butadiene monomer + polystyrene = styrene butadiene rubber (SBR)
Bulky Styrene molecules add stiffness and intermolecular interference to butadiene while butadiene adds flexibility and toughness to styrene

17. SBR
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18. BR
Information included demographic information, codes for underlying cause of death, and up to 20 disorders that are listed on death certificates.

19. Thermoplastic Elastomer (TPE)
These materials are not crosslinked, have some distinct processing advantages over traditional thermoset elastomers and physical properties of vulcanised elastomers
TPEs are able to be molded like thermoplastic (injection molding, extrusion, etc)
Thermoplastic elastomers are more temperature sensitive
Scrap and reject of these materials can be recycled-environmetal friendly behavior
Normal crosslinked polymers cannot be recycled because they don't melt. They don't melt because the crosslinks tie all the polymer chains together, making it impossible for the material to flow.

20. Thermoplastic Elastomer (TPE)
The capability to repeatedly process thermoplastic elastomers provides the major benefit of TPEs over elastomer.
Variable
TPE
Elastomer
Fabrication
Rapid (seconds)
Slow (minutes)
Scrap
Reusable
High Percentage waste
Curing Agents
None
Required
Machinery
Conventional Thermoplastic Equipment
Special Vulcanizing Equipment
Additives
Minimal or None
Numerous Processing Aids
Design Optimization
Unlimited
Limited
Remold Parts
Yes
Unlikely
Heat Seal No

21. Thermoplastic Elastomer (TPE)
Benefits of TPEs compared to Elastomer
Design flexibility
Lower fabrication costs
Shorter processing times
Little or no compounding required
Scrap is fully recyclable
Consistency of product
Can be blow molded
Can be thermoformed
Lower consumption of energy
Simpler processing
Better control of product quality
Broader range in product density
Lower per-piece finished part cost
More environmentally friendly

22. Silicones, or polysiloxanes
Silicones, or polysiloxanes, are inorganic-organic polymers with the chemical formula [R2SiO]n, where R = organic groups such as methyl, ethyl, and phenyl.
These materials consist of an inorganic silicon-oxygen backbone (...-Si-O-Si-O-Si-O-...) with organic side groups attached to the silicon atoms, which are four-coordinate.

23. Silicones, or polysiloxanes
In some cases organic side groups can be used to link two or more of these -Si-O- backbones together. By varying the -Si-O- chain lengths, side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions.
They can vary in consistency from liquid to gel to rubber to hard plastic. The most common type is linear polydimethylsiloxane or PDMS

24. Silicones, or polysiloxanes
Service temperature to about 260C
Good chemical resistance, low water absorption, good electrical properties, & available in flame retardant grade
In the plumbing and automotive fields, silicone grease is often used as a lubricant. In plumbing, the grease is typically applied to O-rings in faucets and valves.

25. In the automotive field, silicone grease is typically used as a lubricant for brake components since it is stable at high temperatures, is not water-soluble

26. PROCESSING OF ELASTOMER
Common machine used for rubber compounding:-
Banbury mixer
2-roll mill

27. Overall processing route for rubber
1st Stage mix
Rubber, filler, oils, etc
2st Stage mix
Active vulcanizing agent
Preform: calendering, extruder, etc
Testing; compound plasticity-viscosity testing, curing, properties-tensile, hardness, etc
Cure: Vulcanizing by heat and temperature; compression molding, oven steam cure, etc
150ºC

28. RUBBER CURING/ CROSSLINK DENSITY

29. INTRODUCTION
Common instrument used to determine the kinetics of crosslinking
Oscillating disc rheometer (ODR)
An oscillating rotor is surrounded by a test compound, which is enclosed in a heated chamber.
The torque required to oscillate the rotor is monitored as a function of time at the temperature chosen for vulcanisation.
Moving-die rheometer (MDR)
Use thinner samples and has a faster thermal response than ODR

30. Oscillating disc rheometer (ODR)
In order to predict the cure behavior of rubber compounds, “torque rheometers” or “curemeters” are used.

31. Cure Behavior
These instruments measure the torque required to oscillate a biconical disk within the rubber compound. The result is a torque-time trace:
Tmin is the minimum torque.
ts is the scorch time
Tplat, is the plateu torque at tp
The thermal stability of the network at curing temperature is indicated by the stability of the plateu torque.

32. 3 Stages of curing/vulcanization characteristics
The vulcanization process can be described by rheometer curve.
The curve has three stages:
Induction and scorch
Curing : This region can sub-divided into
Under vulcanization / undercured
Optimum vulcanization
Overcure : this region may show
Plateau cured
Reversion cured
Marching cured

33. Initially  a sudden increase in torque as the chamber closed.
INDUCTION / SCORCH
Initially  a sudden increase in torque as the chamber closed.
As the rubber is heated  its viscosity decreases, resulting in a net decrease in torque.
CURING
Eventually, the rubber compound begins to vulcanise and transform into an elastic solid (known as an elastomer)  the torque rises.
Molecular chain scission may also be occurring  an increasing torque indicates that an increase in crosslinks is dominant.
OVER CURED
If the torque reaches a plateau  this indicates a completion of curing and the formation of a stable network.
If chain scission and/or crosslink breakage become dominant during prolonged heating  the torque passes through a maximum and then decreases, a phenomenon termed reversion.
Some NR compounds, particularly at high curing temperatures, exhibit reversion.
Some compounds show a slowly increasing torque at long cure times  creeping/marching cure.
This behaviour often occurs in compounds that initially form many polysulphidic linkages.
With extended cure times, these linkages may break down and re-form into linkages of lower sulphur rank, thereby increasing the total number of crosslinks.

34. A typical torque-time curve along with characteristic terms to describe the different behaviours.

35. Creeping
Tmax
Plateau
Reversion
0.9(Tmax – Tmin)
The scorch time, ts1 is the time at which the torque is 0.1 Nm above minimum torque gives an indication of the safe period before the mix becomes un-processable due to the formation of crosslinks

36. Kinetics of Crosslinking
To avoid premature crosslinking (“scorch”), the mold cavity during processing should be filled during the induction period, therefore the determination of the induction time, ti, is important.

37. Important Definitions
Scorch: Scorch is premature vulcanization, in which the “stock” becomes partly vulcanized before the product is in its final form and ready for vulcanization.
Rate of cure: Is the rate at which crosslinking and the development of the stiffness (modulus) of the compound occur after the scorch point.
State of cure: A term to indicate the development of a property of the rubber as cure progresses.
Cure time: is the time required during the vulcanization step for the compounded rubber to reach the desired state of cure
Overcure: A cure which is longer than the optimum.

38. Determination of crosslink density after curing/ vulcanization
The crosslink density of an elastomer can be determined from swelling or mechanical measurements.
The determination of crosslink density by equilibrium swelling measurement is a practical method for
estimating the state of cure of a rubber product, and also
for monitoring the degradation process including scission and crosslinking reactions.
In a crosslinked elastomer the presence of an interconnected network precludes the possibility of solution but immersions in a solvent results in swelling.
Swelling will continue until the retractive forces in the extended chains balance the forces tending to swell the network.
This equilibrium swelling in a solvent like toluene or n-decane can be used to determine the crosslink density.

39. The mass uptake of the solvent is plotted against the square root of time until saturation.

40. Swelling measurements
Weigh the initial mass of each 2-3 mm cube test piece using a laboratory balance capable of reading to ± gm.
Then immersed the test pieces in toluene at room temperature at least 6 hours until a state of equilibrium swelling is obtained.
On attainment of equilibrium, take out the test pieces from swelling solvent, wipes the surfaces and weigh.

41. Flory-Rehner equation
One expression widely used to relate the amount of swelling to the crosslink density is the
where
Vo is defined as molecular volume of the swelling liquid,
vr is the volume fraction of the original rubber network in the swollen gel and
 is the polymer- solvent interaction parameter (often depending on vr for a particular rubber-solvent system and normally given as 4.2 for n-decane)
Mc is crosslink density..

42. Mooney-Rivlin equation
Another way of to determine crosslink density are from equilibrium stress-strain measurement using equation :
where
σ is engineering stress (force per unit original cross sectional area),
λ is an extension ratio and
C1 , C2 is a constant.

43. According to equation, a graph of reduced stress, versus should give
a slope of C2 and an intercept of C1 on the ordinate.
It is known that the C1 value relates to the number of chemical and physical crosslinks which gives the stiffness of the rubber material and
the C2 value is believed to relate to the number of chain entanglements

45. Stress-Strain measurements
Stress-strain measurements are determined by stretching strip test pieces at a constant rate using tensile machine (Instron).
The test pieces are cut from molded sheets.
The measurements are similar as tensile strength measurements except that it should be carried out at a lower strain rate.
Normally it was carried out at speed 30mm/min

46. Mooney viscosity
For a raw polymer type, choice of correct viscosity level is important to ensure acceptable mixing and processing characteristics.
Viscosity levels for polymers are commonly expressed in terms of Mooney viscometer readings.
Mooney viscosity is the most commonly used for measurement of mix viscosity.
Viscosity of rubbers is measured using the shearing disk viscometer. The torque of the rotor is taken after 1 minute pre-heating the rotor plus 4 minute after that.
The result for viscosity measurements are reported in the form 50 ML, 1+4 (100°C)
where 50 M is the Mooney viscosity number,
L indicates the use of the large 1.5 in. rotor (S would indicate the small 1.25 in. rotor),
1 is the time in minutes that the specimen was permitted to warm in the machine before starting the motor at which the reading is taken,
100°C is the temperature of the test.

47. Mooney viscosity
For a raw polymer type, choice of correct viscosity level is important to ensure acceptable mixing and processing characteristics.
Viscosity levels for polymers are commonly expressed in terms of Mooney viscometer readings.
Mooney viscosity is the most commonly used for measurement of mix viscosity.
Viscosity of rubbers is measured using the shearing disk viscometer. The torque of the rotor is taken after 1 minute pre-heating the rotor plus 4 minute after that.
The result for viscosity measurements are reported in the form 50 ML, 1+4 (100°C)
where 50 M is the Mooney viscosity number,
L indicates the use of the large 1.5 in. rotor (S would indicate the small 1.25 in. rotor),
1 is the time in minutes that the specimen was permitted to warm in the machine before starting the motor at which the reading is taken,
100°C is the temperature of the test.

48. Mooney schorch
For a raw polymer type, choice of correct viscosity level is important to ensure acceptable mixing and processing characteristics.
Viscosity levels for polymers are commonly expressed in terms of Mooney viscometer readings.
Mooney viscosity is the most commonly used for measurement of mix viscosity.
Viscosity of rubbers is measured using the shearing disk viscometer. The torque of the rotor is taken after 1 minute pre-heating the rotor plus 4 minute after that.
The result for viscosity measurements are reported in the form 50 ML, 1+4 (100°C)
where 50 M is the Mooney viscosity number,
L indicates the use of the large 1.5 in. rotor (S would indicate the small 1.25 in. rotor),
1 is the time in minutes that the specimen was permitted to warm in the machine before starting the motor at which the reading is taken,
100°C is the temperature of the test.

49. Compound costing & Specific gravity calculations
In the rubber industries  it is customary to buy by weight and sell by volume
The density of the mix thus plays a vital part in material costing calculations.
Only a volume vs volume cost comparison is meaningful when costing a product
For a raw polymer type, choice of correct viscosity level is important to ensure

50. Density of the Mixture & Specific gravity
The normal units are g/ml3
2. Specific gravity (S.G)
Specific gravity = weight of a given volume of material

51. APPLICATION OF ELASTOMER
Bearings - structural joints that are installed between a structure and its foundation.
The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.
1.0 Introduction

52. Figure: Base-Isolated and Fixed-Base Buildings
HOW THE BEARING WORKS
Figure: Base-Isolated and Fixed-Base Buildings
A base isolated structure is supported by a series of bearing pads which are placed between the building and the building's foundation

53. Each building responds with movement which tends toward the right.
2.0 How The Bearing work?
As a result of an earthquake, the ground beneath each building begins to move.
Each building responds with movement which tends toward the right.
The building's displacement in the direction opposite the ground motion is actually due to inertia.

54. 2.0 How The Bearing work?
In addition to displacing toward the right, the un-isolated building is also shown to be changing its shape-from a rectangle to a parallelogram. –deforming
The primary cause of earthquake damage to buildings is the deformation which the building undergoes as a result of the inertial forces acting upon it.

55. The base-isolated building retains its original, rectangular shape.
2.0 How The Bearing work?
The base-isolated building retains its original, rectangular shape.
It is the elastomeric bearings supporting the building that are deformed.
It implies the inertial forces acting on the base-isolated building have been reduced.

56. ELASTOMERIC BEARINGS
Fig: Basic structure of rubber bearing
3.0 Elastomeric Bearings
Consist of thin rubber sheets bonded onto thin steel plates and combined with an energy dissipation mechanism.
The rubber sheets are vulcanized and bonded to the thin steel plates under pressure and heat.
it is designed in such a way that bearing is very stiff and strong in vertical direction, but flexible in horizontal direction.
Thick mounting steel plates are bonded to the bottom and top surfaces allowing the isolator to be firmly connected to the foundation below and the superstructure above.

57. Processing Flow Chart - Seismic Rubber Bearings