Tieteestä & Tekniikasta
The dispersion of lignin in rubbers Text and figures: Jukka Koskinen and Nazanin Pournoori, Tampere University T he global demand for rubbers is increasing every year. Most of the rubbers are used in demanding conditions and rein- forcing fillers are applied to achieve the required proper-
ceed 100 µm in diameter. To address this issue, various chemical modifications can be applied to lignin to improve its compatibility with rubber, but no industrial scale solution has yet been devel- oped. With traditional fillers, like carbon black and silica, the pro- cessing conditions play a critical role in ensuring proper dispersion during mixing. With lignin filler, optimal mixing parameters have not been studied, although some research results suggest that high mixing temperatures can affect the dispersion. Analyzing the dispersion While industrial equipment exists to measure the macro disper- sion of carbon black, those are not feasible for measurements with lignin particles. Thus, a new method was developed to measure the lignin dispersion. In this method, the cured sheet is first cut with razor blade and then the cut section is polished with argon using a cross-section polisher. This polishing removes the topography
ties. Typically, 20–40 % of a rubber compound consists of these fillers. The most commonly used reinforcing fillers, carbon black and silica, are not sustainable. Their production generates signifi- cant CO2 emissions, and neither material is renewable. Therefore, one of our scientific focuses at Tampere University (TAU) in the Plastics and Elastomer Technology research group is to find the sus- tainable alternatives, with lignin emerging as a promising option. Lignin Lignin, the second most abundant organic polymer after cellulose, constitutes over 15 % of the weight of plants. It is a byproduct of the pulp and paper industry, making it widely available. While lignin has potential for various applications, its industrial use
is still limited, and it is primari- ly burned as a source of energy in industrial processes. Its highly branched three-dimensional poly- meric structure including hydrox- yl, carboxyl and methoxy groups offers potential for various uses, such as biofuels, adhesives, resins or electrochemical materials, or a reinforcing filler in rubbers. It is important to note that the struc- ture of the lignin is based on the source material (hardwood, soft- wood, grass) and the extraction process (kraft, organosolv, soda etc.), meaning there is not a sin- gle type of lignin. Lignin as a filler in rubbers Despite its potential, incorporating lignin into rubber poses several challenges. One major issue is the tendency of lignin particles to ag- gregate, which makes it difficult to achieve a uniform dispersion with- in the rubber matrix. Typical par- ticle diameter for lignin is around 150 nm, but aggregates can ex-
Rubber
Lignin
Sustainable Rubber products
Higher mixing temperature
160 °C
120 °C
190 °C
Fig. 1. The dispersion of lignin in rubbers with different mixing temperature.
16 MUOVIPLAST 5/2024
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