8.3 Self-Reinforcement Behavior of NR
159
uniquely named ‘elasotocaloric’ phenomena including shape-memory effect [119–
121]. Among their functionalities, ‘elastocaloric refrigeration’ is uniquely included
[121]. These examples are suggesting more versatile possibility of NR vulcanizates
associated with reversible template crystallization process. The details of these functions are still to be elucidated, and at the same time, shedding more light on the elementary processes of template crystallization is urgently requested for the systematic
and organized progress of these functional developments of NR vulcanizates.
In this chapter, unique self-reinforcement effect of NR, in particular of the NR
vulcanizates, is explained in terms of template crystallization upon stretching and
elastocaloric melting of the crystallites upon contracting. Instantaneous and spontaneous characters of them enable one of the highest speed reversibility of the cycles,
which have given rise to a good possibility of various highly functional devices as
well as the already established application to top tread tire rubbers of airplanes and
automobiles.
For rubber researchers and engineers, however, it is more urgent to realize the
historical significance of vulcanization of rubber and pay more attention to the difference between SIC and LTC of NR, before going to the details of them [1, 39, 40,
113]. Particularly, the differentiation of SIC (template crystallization) from LTC on
the NR vulcanizates has been unnoticed among rubber people. For instance, reviews
on rubber crystallization published in Rubber Chemistry and Technology [74, 122]
did not pay any attention on the difference. Based on these cognitions, elucidation of
template crystallization mechanism is further to be investigated, in order to accelerate the studies on elastocaloric contractile process, in particular, both in theoretically
and experimentally. Such efforts may be of use in the designing NR products to make
them much more versatile in rubber industries.
Prefatory Remark
1 : A Retrospect on the Complications Relevant to Natural
Rubber Crystallization
Crystallization of natural rubber (NR) has been uniquely complicated, and one of the
reasons may be the failure in crystallizing the difference between low-temperature
crystallization (LTC) and strain-induced crystallization (SIC) [1, 39, 40]. Majority
of the past studies has adhered to one mechanism, i.e., nucleation is the initiation
step of NR crystallization by SIC as well as by LTC. This failure has some historical
bases, which are discussed in this appendix. It is notable first that nucleation has
predominantly been recognized the case for polymer crystallizations in general [26].
It is well known that Hancock (1786–1865) in England, Goodyear (1800–1860)
in the USA, and many others had struggled for many years to find out a way to make
full use of NR. Then, a tough problem in NR usages was the temperature effect on NR
products: In winter, they turned too rigid just like frozen (due to LTC), and on very
hot days in summer, they turned too soft (the rubbery state without cross-links). The
invention of vulcanization by Goodyear in 1839 afforded the almost perfect solution
1 This remark was originally published as Sect. I in the article appeared in KGK, October issue,
2017, pp. 38–39 (see, www.kgk-rubberpoint.de).
159
uniquely named ‘elasotocaloric’ phenomena including shape-memory effect [119–
121]. Among their functionalities, ‘elastocaloric refrigeration’ is uniquely included
[121]. These examples are suggesting more versatile possibility of NR vulcanizates
associated with reversible template crystallization process. The details of these functions are still to be elucidated, and at the same time, shedding more light on the elementary processes of template crystallization is urgently requested for the systematic
and organized progress of these functional developments of NR vulcanizates.
In this chapter, unique self-reinforcement effect of NR, in particular of the NR
vulcanizates, is explained in terms of template crystallization upon stretching and
elastocaloric melting of the crystallites upon contracting. Instantaneous and spontaneous characters of them enable one of the highest speed reversibility of the cycles,
which have given rise to a good possibility of various highly functional devices as
well as the already established application to top tread tire rubbers of airplanes and
automobiles.
For rubber researchers and engineers, however, it is more urgent to realize the
historical significance of vulcanization of rubber and pay more attention to the difference between SIC and LTC of NR, before going to the details of them [1, 39, 40,
113]. Particularly, the differentiation of SIC (template crystallization) from LTC on
the NR vulcanizates has been unnoticed among rubber people. For instance, reviews
on rubber crystallization published in Rubber Chemistry and Technology [74, 122]
did not pay any attention on the difference. Based on these cognitions, elucidation of
template crystallization mechanism is further to be investigated, in order to accelerate the studies on elastocaloric contractile process, in particular, both in theoretically
and experimentally. Such efforts may be of use in the designing NR products to make
them much more versatile in rubber industries.
Prefatory Remark
1 : A Retrospect on the Complications Relevant to Natural
Rubber Crystallization
Crystallization of natural rubber (NR) has been uniquely complicated, and one of the
reasons may be the failure in crystallizing the difference between low-temperature
crystallization (LTC) and strain-induced crystallization (SIC) [1, 39, 40]. Majority
of the past studies has adhered to one mechanism, i.e., nucleation is the initiation
step of NR crystallization by SIC as well as by LTC. This failure has some historical
bases, which are discussed in this appendix. It is notable first that nucleation has
predominantly been recognized the case for polymer crystallizations in general [26].
It is well known that Hancock (1786–1865) in England, Goodyear (1800–1860)
in the USA, and many others had struggled for many years to find out a way to make
full use of NR. Then, a tough problem in NR usages was the temperature effect on NR
products: In winter, they turned too rigid just like frozen (due to LTC), and on very
hot days in summer, they turned too soft (the rubbery state without cross-links). The
invention of vulcanization by Goodyear in 1839 afforded the almost perfect solution
1 This remark was originally published as Sect. I in the article appeared in KGK, October issue,
2017, pp. 38–39 (see, www.kgk-rubberpoint.de).