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7 Rubber Reinforcement with Lignin
Lots of papers on these topics have already published. For example, see Refs. [3–15,
19, 20, 21, 31, 32].
In the arena of rubbery materials, the use of lignin as a reinforcing filler was
highlighted as early as the middle of the twentieth century [7–12], and its potential
has been recognized since then. Among rubber engineers, the most difficult issue
associated with lignin has been recognized its processability: For any additives onto
rubber, good miscibility with rubber during the mixing is of utmost importance. This
difficulty is still to be solved for the utilization of lignin in rubber industries. Since
lignin is a sustainable biomass, industrial usages of it would remain to be crucial
toward the end of this century. In the next two sections, our recent trials of using it
as a reinforcing filler for rubber are to be explained.
7.2 Mixing of Lignin into Rubber by a Soft Processing
In 1947, Keilen and Pollak proposed to use lignin as a reinforcing filler for rubber
[7]. This study was one of the reports from a project started during the World War
II, the aim of which was developing a substitute for carbon black in manufacturing
tires. The difficulty of lignin in rubber processing was first found at this project, and
it remains to be solved until now. Namely, how to mix lignin into rubber has to be
elucidated before discussing reinforcing effect on rubber.
Since lignin is known soluble in an alkaline aqueous solution, its mixing with
natural rubber (NR) latex [33–37] has been conducted. NR latex is preserved in
ammoniac state and is very conveniently mixed with the lignin aqueous solution.
Sagajllo reported this procedure in 1957 [38]. In this paper, a pilot plant preparation
of lignin/NR mixture was mentioned, too. In 1978, Kumaran et al. disclosed their
study on lignosulfonic acid (containing approximately 33% water), which was well
mixed into solid NR [39]. This paper much stimulated trials of mixing by conventional mechanical mixers, because a paste of the lignin/water (weight ratio, 2/1) was
reported to disperse in NR, while the use of lignosulfonic powders did not work. The
presence of water was assumed to decrease the hydrogen bonding inside the lignin
to result in the better dispersion of lignin into rubber (than that of lignin powder).
In these studies, it is reported that tensile strength, rebound elasticity, heat buildup,
and compression set are not improved, but the betterment of thermal stability, tear
resistance, and abrasion resistance is found. In fact, these results promoted a trial of
mixing lignin with rubber by processing using a calender roll [40], and such trials
using conventional rubber processing machines seem to be conducted still now. On
the other hand, the usage of polyion complexes of lignosulfonic acid for rubber
mixing was reported [41]. However, these two trials, i.e., modification of lignin
itself and utilization of conventional mechanical method, the aim of which are to be
fitted for the conventional rubber processing technics, seem still to overcome lots
of difficulties, and some other approaches are now to be considered. Possibly, one
candidate of them is presented next.
7 Rubber Reinforcement with Lignin
Lots of papers on these topics have already published. For example, see Refs. [3–15,
19, 20, 21, 31, 32].
In the arena of rubbery materials, the use of lignin as a reinforcing filler was
highlighted as early as the middle of the twentieth century [7–12], and its potential
has been recognized since then. Among rubber engineers, the most difficult issue
associated with lignin has been recognized its processability: For any additives onto
rubber, good miscibility with rubber during the mixing is of utmost importance. This
difficulty is still to be solved for the utilization of lignin in rubber industries. Since
lignin is a sustainable biomass, industrial usages of it would remain to be crucial
toward the end of this century. In the next two sections, our recent trials of using it
as a reinforcing filler for rubber are to be explained.
7.2 Mixing of Lignin into Rubber by a Soft Processing
In 1947, Keilen and Pollak proposed to use lignin as a reinforcing filler for rubber
[7]. This study was one of the reports from a project started during the World War
II, the aim of which was developing a substitute for carbon black in manufacturing
tires. The difficulty of lignin in rubber processing was first found at this project, and
it remains to be solved until now. Namely, how to mix lignin into rubber has to be
elucidated before discussing reinforcing effect on rubber.
Since lignin is known soluble in an alkaline aqueous solution, its mixing with
natural rubber (NR) latex [33–37] has been conducted. NR latex is preserved in
ammoniac state and is very conveniently mixed with the lignin aqueous solution.
Sagajllo reported this procedure in 1957 [38]. In this paper, a pilot plant preparation
of lignin/NR mixture was mentioned, too. In 1978, Kumaran et al. disclosed their
study on lignosulfonic acid (containing approximately 33% water), which was well
mixed into solid NR [39]. This paper much stimulated trials of mixing by conventional mechanical mixers, because a paste of the lignin/water (weight ratio, 2/1) was
reported to disperse in NR, while the use of lignosulfonic powders did not work. The
presence of water was assumed to decrease the hydrogen bonding inside the lignin
to result in the better dispersion of lignin into rubber (than that of lignin powder).
In these studies, it is reported that tensile strength, rebound elasticity, heat buildup,
and compression set are not improved, but the betterment of thermal stability, tear
resistance, and abrasion resistance is found. In fact, these results promoted a trial of
mixing lignin with rubber by processing using a calender roll [40], and such trials
using conventional rubber processing machines seem to be conducted still now. On
the other hand, the usage of polyion complexes of lignosulfonic acid for rubber
mixing was reported [41]. However, these two trials, i.e., modification of lignin
itself and utilization of conventional mechanical method, the aim of which are to be
fitted for the conventional rubber processing technics, seem still to overcome lots
of difficulties, and some other approaches are now to be considered. Possibly, one
candidate of them is presented next.