Sustainable rubber formulations: Investigating lignin and additive synergies

Abstract

The aim of this thesis was to estimate the viability and enhance the usability of lignin as a filler in rubbers. While lignin holds promise for use as a cheap sustainable filler, rubbers’ mechanical properties and vulcanization are adversely affected by its usage. This was sought to be remediated via experimental additives. Hexamethylenetetramine (HMTA) was used to in-situ crosslink lignin molecules for better filler-filler interaction and to expedite vulcanization. TESPT, a bifunctional silane, was used to couple lignin and elastomer molecules together for better compatibility, sharing of stresses, and polymer-filler interaction. The least detrimental vulcanization system to be used with carbon black and lignin-filled systems was also determined.

The literature review section covers the backgrounds of rubber compounding, and lignin as both a general material and a rubber ingredient. Theoretical backgrounds and results of previous studies about promising candidates to combat the problems of lignin usage in rubbers are then presented. The findings were investigated via experimental test series. In the first series HMTA and TESPT were incorporated into natural rubber recipes with the help of CCD, and their effects along with possible optimum concentrations were investigated through Design of experiments (DoE) analysis software. In the 2nd series the degrees of property changes caused by lignin addition were determined for conventional, semi-efficient, and efficient vulcanization systems to conclude which of them would be the least detrimental.

It was concluded that through exothermic reactions with lignin, HMTA was able to negate the increases in t10/t90 and decreases in MH/ΔM caused by lignin. Enhanced polymer-filler and filler-filler interactions by TESPT and HMTA usage increased moduli and significantly reduced the abrasive wear experienced by the rubbers (both alone and in combination). Tensile strength and tear strength showed no clear changes or correlations with the additives used, which was hypothesized to be due to the observed poor dispersion and re-agglomeration of lignin, with Payne effect, insufficient silanization or reactions between the additives possibly also being of influence. Through empirical observations it was deduced, that the maximum amount of additives that can react with the phenolic hydroxyl groups of lignin is slightly below 1/3 of the amount of lignin.

Lignin is known to interfere with vulcanization via several phenomena, thus adversely affecting rubbers’ mechanical properties. After recording the percentual changes in rubber properties caused by replacing 30 % of carbon black with lignin, it was concluded that the least detrimental vulcanization system regarding general property retention was the conventional one. Efficient system resisted increases in t90, elongation at break and moduli the best, while the semi-efficient system did not perform the best in any property retention. It was suggested that while efficient systems may, according to a previous study, resist losses in the amount of polysulfidic crosslinks the best, that in our compounds the significant presence of carbon black may overwhelm the importance of the phenomenon due to its high polymer-filler compatibility. The high crosslink density of our conventional compounds may have affected the results, as well as using a sulfur donor.

It was concluded that while HMTA and TESPT improved many properties, the viability of powder form lignin to replace traditional fillers in rubbers still require advances in dispersion, and additional research concerning HMTA-crosslinked lignin and the usage of silanes.