**1. Introduction**

Carbon black (CB) has been the dominant filler in the tire industry for over a century [1]. However, there are important reasons why sustainable alternatives should be pursued. CB is a petroleum product, and dependence on foreign fossil fuel sources is challenging since oil prices can be very unstable due to global conflicts and other political reasons [2]. Shrinking CB's footprint in the global marketplace also improves air quality with less pollution [3].

Biochar has been a popular candidate as a sustainable source of carbon from biomass [4]. Most biochar research has been carried out with intended applications being carbon sequestration [5], catalysts [6–8], energy storage [9–11], filtration media [12,13], and sorptive media [14–16], the latter two taking advantage of the (frequently) porous nature of most biochars, although porosity does depend on many factors such as feedstock and processing conditions [17]. However, when using biochar as rubber composite filler, porosity is not as important as other characteristics such as carbon content and particle size. Current literature for using biochar as rubber composite filler is limited but slowly growing. Greenough has reviewed the relevance of biochar as rubber composite filler [18]. Current examples include Xue et al. [19] using rice husk, with its high silica content, as feedstock to make biochar that was ball-milled and used as reinforcing filler. Other sustainable feedstocks studied for biochar use as rubber composite filler include waste lignin [20] and leaf biomass [21].

Suliman and coauthors have studied biochar surface chemistry extensively; both in how feedstock and pyrolysis temperature affect it [22] as well as how oxidation by air has a role in modifying it [23]. Suliman found that for poplar wood as a biochar feedstock, greater quantities of carbonyl and other carboxyl groups were formed on biochar surfaces produced at lower temperatures of 300–400 ◦C as opposed to higher temperatures ranging up to 600 ◦C. The formation of these oxygenated functional groups would increase negative charges on the biochar. Carbon dioxide was chosen for comparison with air as a biochar gas treatment. Yi and co-workers studied the influence of CO<sup>2</sup> on cellulose biochar properties and found that it enhanced carbon content and increased the C:O ratio [24], and others [25,26] have also observed that gaseous CO<sup>2</sup> can react favorably to increase the carbon content in biochar production.

For biochar used as rubber composite filler, modifying the surface chemistry to make the biochar more hydrophobic will improve dispersion in the rubber matrix. Traditionally this has been done using stearic acid [27] (pp. 170–171). Since most biochars have highly charged surfaces [28], adding surfactants creates a 'coated' biochar that will then have an outer surface with many aliphatic long-chain hydrophobic tail groups and thus make the biochar particles themselves more hydrophobic. Navarathna et al. employed this method by saturating biochar with lauric acid (LA) to make it more hydrophobic [29]. Navarathna proposed that the polar carboxylic acid groups in LA are attracted to the oxygenated functional groups on the surface of the biochar, which results in the hydrocarbon chains creating a more hydrophobic outer surface layer on the biochar particles. We hypothesize that this same mechanism will work in dry milling LA with biochar as carried out in this manuscript.

In this work, our goal was to study these two surface chemistry modification methods as a two-step process for biochar, with the first step being gas treatment of the biochar. Using a technique similar to that of Suliman [23], separate samples of poplar biochar were treated with either air or carbon dioxide. In the secondary step, each of these three biochar samples (untreated biochar control, air-treated, or CO2-treated) were then coated with lauric acid using a method similar to that of Navarathna [29], but altered to a dry-milling method more amenable (and scalable) to the rubber compounding industry. Our hypothesis was that LA-coated biochar, being more hydrophobic than uncoated biochar, would have better dispersion in the rubber matrix during compounding and result in more strongly reinforced rubber composites that would show higher tensile strength and/or stiffness than uncoated biochar samples. We were also interested to observe any differences in the gas treatments combined with LA coating, and to determine if the gas treatments acted as favorable or unfavorable pre-treatments for LA coating in terms of final tensile properties of the composites.
