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4 Characteristics of natural rubber (NR) for vibration isolation and earthquake protection

4 Characteristics of natural rubber (NR) for vibration isolation and earthquake protection

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376



Chemistry, Manufacture and Applications of Natural Rubber



modulus of elasticity. The great advantage of natural rubber for the system

of vibration isolation is its linear elastic characteristics at static and dynamic

loadings, i.e. reversible deformation, as shown in Fig. 14.5. This advantage

also depends on the stable stiffness over the range of service temperatures,

which result from the fact that low temperature stiffening due to crystallization

occurs less readily with natural rubber.6

Creep in rubber is in general considered to occur due to both physical

(viscoelastic) effects and chemical effects such as oxidation. As a rule,

chemical effects greatly increase the total amount of creep over time. 7 In

the case of rubber bearings, however, the surface of rubber is protected from

oxygen by bonded metals, and thus the creep level of the bearing is expected

to be kept a low level during its service life. Figure 14.6 shows the value

of compression creep against logarithmic time for a natural rubber-metal

laminated bearing, indicating the initial low creep at short times due to

physical creep. In addition, however, the secondary rapid increase in creep

appears over longer periods.

A key question is why secondary creep is generated over longer periods

in rubber-metal laminated bearings under compression. Fukahori and coworkers5,8–9 showed that the materials such as gases, liquids, fillers and

organic materials induced in compounding or vulcanization processes are

transported from the center to the outer free surfaces of the bearing, resulting

in the reduction of the height of rubber layer sandwiched between metals.

This transportation occurs within the rubber at a slower rate than general

physical creep and appears as secondary creep over longer periods.

An internal hydrostatic pressure gradient generated between both metals

under compression (Fig. 14.7) is the driving force to transport the materials



Shear force (ton)



1



0



–1

–6



0

Shear displacement (cm)



6



14.5 Linear load–displacement relation of NR-metal laminated

bearing.5



Use of NR for vibration isolation and earthquake protection



377



2.5



2.0



100°C



e (%)



83°C

50°C



1.5



30°C

1.0



0.5

100



101



102



103

Time (min)



104



105



106



14.6 Compression creep e vs. log time at various temperatures.8



0

1.4



1.3



1.2



1.1



1.0



0.8



0.6



0.4 0.2



1.5



14.7 Hydrostatic pressure gradient within a rubber layer sandwiched

between metals under compression represented by the normalized

pressure (P/P0).8



from the center (highest pressure) to the free surface (no pressure). Figure 14.8

re-plots Fig. 14.6 using the relation of log creep rate vs. log time, where the

physical creep rate is given by the initial gradual increase and the secondary

creep rate is represented by a linear relation. Using the linear relation, the



378



Chemistry, Manufacture and Applications of Natural Rubber

101



100



D e (%)



100°C



10



83°C



50°C

30°C



–1



10–2



10–3

100



101



102



103

Time (min)



104



105



106



14.8 Re-plots of Fig. 14.6; log compression creep rate D e vs. log

time.8



total amount of creep of natural rubber bearing under compression can

be estimated as about 5% during the designed life time (60 years), based

on Arrhenius plots and the concept of activation energy derived from the

temperature dependence of creep rate given in Fig. 14.8. This low level of

creep rate is sufficient for the superstructure.

The fatigue life is the number of cycles required to break a specimen into

two pieces at the designed stress or strain. In mechanics of materials, the

fatigue characteristics are determined by an S-N curve, where S denotes the

applied dynamic stress or strain and N is the number of cycles to failure.

Thus, the curve gives the direct information to the engineering designer to

judge whether the material used is available or not for the designed stress

or strain level. Figure 14.9 shows S-N curves of natural rubber used for a

rubber-metal laminated bearing for earthquake protection. Simple shear

test pieces are used for testing, where the filled and open circles denote the

cycles for a crack to initiate at the corner and the center of the test piece,

respectively.

These data relate to the virgin sample whilst the other square marks

correspond to the aged state of material after 70 years’ degradation. All

results before and after the degradation indicate that the material used might

have sufficient fatigue life for earthquake protection during the designed

lifetime (60 years). Thus, it might be concluded that the excellent fatigue

and durability of rubber-metal bearing used for earthquake protection is

undoubtedly owed to the mechanical characteristics such as high strength

and crack-propagation resistance of natural rubber.

Oxygen in atmosphere is not a serious problem for rubber-metal laminated



Use of NR for vibration isolation and earthquake protection



379



70 years aged Virgin

700



Shear strain (%)



600

500

400

300

200

10



102



103

104

Fatigue life (cycles)



105



14.9 Shear stress vs. fatigue life of NR for virgin sample and a 70

years-aged sample.8

106



Crack initiation time (hr)



105



104

EPDM

e = 50%



103



102



101



NR

e = 20%



100



10–1



40%

100%

1



102

101

Ozone concentration (pphm)



103



14.10 Crack initiation time as a function of ozone concentration for

NR and EPDM.5



bearings, because most rubber surfaces are protected from oxygen by bonded

metals, whereas ozone attack seriously damages the free surface of rubber.

Ozone reacts very rapidly with carbon–carbon double bonds and causes a

very fast crack initiation and propagation even at small strain levels. Figure

14.10 shows the effect of the ozone concentration on the crack initiation



380



Chemistry, Manufacture and Applications of Natural Rubber



time as a function of external strain e, where it is shown that the increase of

strain shortens the initiation time remarkably. Figure 14.10 also shows the

great ozone-resistant characteristic of EPDM, and thus in reality, the bearing

is protected from ozone by covering with a thin layer of EPDM just 5–10

mm thick. It is therefore essential to protect rubber from ozone.

As shown in Eq. [14.1], the transmissibility is a function of tan d as well

as the stiffness of the material. An increase in the magnitude of the tan d

lowers the peak value of the transmissibility significantly as shown in Fig.

14.11. Higher energy dissipation is also desirable to strengthen the material,

and thus filling with carbon black is quite important for natural rubber. The

suitable balance between the linear elasticity and non-linear viscoelesticity

may be required to make the best use of natural rubber for engineering

applications.



14.5



Conclusion



Natural rubber has a great advantage as an engineering material, due to its

linear elasticity, high strength and fatigue life and excellent adhesion to

metals. These characteristics are well suited for the vibration isolation and

seismic isolation of structures. Natural rubber-metal laminated bearings have

been widely used for vibration isolation and earthquake protection of heavy

tan

tan

tan

tan

tan



40

30

20

10



d1Gw = 0.023

DPw = 0.091

DPw = 0.142

DPw = 0.182

d2w = 0.515



Transmissibility (dB)



0

Natural rubber mounting

Thiokol rd mounting



–10

–20

–30

–40



a = 0.1

a = 0.2

a = 0.3



tan d2Gw = 1.010



–50



tan DPw = 0.505

tan DPw = 0.407



–60



tan DPw = 0.262



–70



tan D1Gw = 0.024



–80

–90

–100

1



2 3 4 5



10



tan d2Gw = 1.775

tan DPw = 1.312

tan DPw = 1.161

tan DPw = 0.867

tan d4Gw = 0.0388



20 40 50 100 200300 500 1000

Frequency (CPS)



14.11 Frequency dependence of the transmissibility as a function of

tan d.10



Use of NR for vibration isolation and earthquake protection



381



superstructures. Base isolation systems have been shown to protect not only

the structure itself but occupants and internal equipment from earthquake

damage, compared with a conventionally strengthened structure.



14.6



References



1. P.B. Lindley and A.R. Payne: Use of Rubber in Engineering, 1966, ed. by P.W.

Allen, P.B. Lindley and A.R. Payne, Maclaren and Sons, London.

2. P.K. Freakley and A.R. Payne: Theory and Practice of Engineering with Rubber,

1978, Applied Science, London.

3. J. Takeda: Kozobutsu no mensin, bosin, seisin, 1988, Gihodo Press, Tokyo.

4.A.N. Gent and P.B. Lindley: Proc. Instn. Mech. Engrs., 1959, 173, 111.

5. Y. Fukahori: Kobunshinorikigaku, 2000, Gihodo Press, Tokyo.

6.A.D. Roberts (ed.): Natural Rubber Science and Technology, 1988, Oxford University

Press, Oxford.

7.A.N. Gent: J. Appl. Polym. Sci., 1962, 6, 442.

8. Y. Fukahori, W. Seki and T. Kubo: Rubber Chem. Technol., 1996, 69, 752.

9. W. Seki, F. Fukahori, Y. Iseda and T. Matsunaga: Rubber Chem. Technol., 1987,

60, 856.

10. J.C. Snowdon: J. Acoust. Soc. Am., 1963, 34, 54.



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Part III

Environmental and safety issues



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15



Improving the sustainable development of

natural rubber (NR)



S. K O H J I Y A, Kyoto University, Japan

DOI: 10.1533/9780857096913.3.385

Abstract: The sustainable aspects of natural rubber in the twenty-first

century are discussed. Consideration is given to the increasing demand

for natural rubber and to its sustainable supply. Sustainable development

may be improved by the expansion of biodiversity which may require

advanced biotechnology techniques. South American leaf blight has made

improvements in the biosafety of natural rubber of the greatest importance,

and future policy in this area is discussed from the point of view of

sustainable development.

Key words: sustainable development, natural rubber (NR), biotechnology,

biosafety, South American leaf blight.



15.1



Introduction



This chapter reviews the prospects for natural rubber up to the end of the

twenty-first century. The limitation is partly a matter of convenience but

also reflects the fact that the world of the twenty-second century cannot be

accurately forseen. This chapter suggests possible future trends in natural

rubber over this period.

The view is generally held that sustainability will remain important in the

future. Therefore, all current developments must be sustainable. According to

the World Commission on Environment and Development (WCED) of 1987,

a United Nations organisation also known as the Brundtland Commission,

the definition of ‘sustainable development’ is:

Development that meets the needs of the present without compromising

the ability of future generations to meet their own needs.1

In economic terms, it may be translated thus:

An increase in well-being today should not have as its consequences a

reduction in well-being tomorrow.2

The most important issue in sustainable development is the environment.

Although the future is the main concern, recognition of past events is essential.

Figure 15.1 shows the environment over the past 13.6 billion years and into

the foreseeable future.

385

© 2014 Woodhead Publishing Limited



386



Chemistry, Manufacture and Applications of Natural Rubber

The Universe

13.6 billion years ago



Big Bang, the origin of the Cosmos



10 billion years ago



Formations of galactic nebulae, including The

Milky Way



The Solar System

5 billion years ago



The solar system was formed in The Milky Way



The Earth

4.5 billion years ago



The Earth was formed



Life

3.9 billion years ago

Appearance of life

600 million years ago Appearance of plants and animals on land

250 million years ago Appearance of mammals

The Human Race

7 million years ago

200,000 years ago

10,000 years ago

300 years ago



Humanoids appeared

The present human race appeared

Agricultural Revolution

Industrial Revolution



Where are we heading in the 21st century?



15.1 13.6 billion years of the events and the environments

surrounding us.



On the left-hand side of Fig. 15.1, the environments or spheres are listed

from the upper to the lower in chronological order followed by approximate

past years. On the right-hand side, historical events starting with the Big Bang

are listed. The biosphere on the earth (Life) became increasingly important

for the human race and its social and natural environments (geosphere,

hydrosphere and atmosphere). However, in considering sustainability, it is

necessary to think of the wider range of surrounding environments (The Earth

and The Solar System) as the most concerned issue. When human activity

is extended beyond the solar system, it may become necessary in the future

to add the sphere of the Milky Way.

At present, and for the near future, anxiety is high following the failures

of nuclear power plants at Chernobyl (Russia) in 1986 and Fukushima

(Japan) in 2011. Sustainability has come to be seen as of great importance

for future generations. The sustainable development of natural rubber is

therefore of great value for scientists, technologists, agriculturalists, plant

pathologists, farmers and growers. Natural rubber itself is a sustainable

material as described in the Introduction.

Among issues relevant to the sustainable development of natural rubber,

the recycling and reuse of natural rubber products and allergies caused by

natural rubber latex are important topics for discussion. Chapters 16–18

complement the discussions in this chapter.



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