**1. Introduction**

The leather industry is one of the most polluting sectors of the economy [1]. Wastes from the leather industry represent 5% by weight of the raw materials [2]. The processing of leather, in particular tanning, also involves toxic chemicals, which can escape into the environment. The most widely used tanning method is chromium tanning (85–90% of world production), which is a type of chemical modification [3–5]. It uses chromium (III) salts in the form of chromium (III) sulfate, in combination with sodium sulfate. The tanning process imparts the skin with desirable properties, such as strength, thinness, and hydrothermal resistance [3,6], while also simplifying further processing. To ensure the durability and elasticity of the skin after tanning, it is subjected to a buffing process. A by-product of that process is buffing dust collagen (BDC). Buffing dust collagen is formed in the process of fat liquoring leather. According to the literature [6], chromium can attach to the active sites of collagen. Molecular modeling and IR analysis confirm that chromium can react with amino as well as carboxylate groups. Each ton of raw material generates ~0.6% buffing dust. If this type of waste goes to landfill, it may be hazardous for the environment, because the oxidation state of chromium salts changes from toxic III to VI, resulting in dangerous chromium salt products [4,7]. Another way to deal with such waste is incineration. Leather wastes have high calorific value (12–14 MJ/kg). However, sulfides are formed during their combustion, Volatile Organic Compounds (VOC) and greenhouse gases may be

emitted to the atmosphere [5,8,9]. A final way of dealing with BDC waste is collagen extraction [2,10,11]. Due to the environmental impact of bu ffing dust and chrome shavings, tanning waste has become the object of intense research.

Studies have shown that BDC and chrome shavings collagen (CSC) can be successfully used as fillers in rubbers [12–17]. The dust form should simplify its dispersion in an elastomeric matrix [11]. Bu ffing dust can be introduced into natural latex rubbers and used as a filler in dust systems. Chromium strings have been applied in acrylonitrile butadiene (NBR) and styrene butadiene (SBR) rubber matrices [18,19]. Kowalska et al. [20] subjected leather waste from pork skins to alkaline reagents, which increased their polymer bonding e fficiency and resulted in improved stabilization of interfacial interactions, thereby reducing the evolution of chromium. This led to improved mechanical parameters, which in turn increased the collagen added to PVC (polyvinyl chloride)-produced materials. Adding particles of bu ffing dust to poly(vinyl chloride) increased its Young modulus, the value of melt flow index (MFI), and susceptibility to biodegradation. Chro ´nska-Olszewska and Przepiórkowska [13] report that when applied in the form of a leather shavings/dust mixture to NBR and *X*NBR, the filler produced biodegradable collagen–elastomer materials with improved mechanical properties and hardness. The leather shavings/dust mixture was an active filler for NBR and *X*NBR.

Residues after tanning are not of particular value, and the Cr (III) salts may naturally turn into toxic Cr (VI) waste. Zhou et al. [21] used chromium-heated leather with active zirconium particles as a material for removing fluoride ions from groundwater. Research is also being carried out to transform tanning waste into carbon adsorbents at low temperatures, below 600 ◦C, e.g., using ZnCl2 as the activating agen<sup>t</sup> [22]. Leather waste has been used as a filling and stabilizing additive for bituminous and asphalt masses (Stone Matrix Asphalt (SMA)), improving their mechanical parameters, creep resistance, hardness, and humidity [23]. Ma et al. [24] developed a mesoporous material by high-temperature carbonization of chrome-tanned leather waste, which was then used for the electrodes in supercapacitors. The material was characterized by a high specific surface, low resistivity, and a high concentration of functional groups containing oxygen and nitrogen atoms. The admixture of leather waste with gravity substitutes for natural rubbers, acrylonitrile butadiene, or polyvinyl alcohol has led to interesting results [25,26]. The creation of hydrogen bonds is promoted, as well as chelation at interfaces, for example, between PVA and leather shavings, leading to greater compatibility of the tested centers. The elastomers of the tested rubbers also showed a significant increase in tear strength, due to the influence of skin particles.

The aim of the present study was to research the e ffect of applying bu ffing dust as a filler to styrene–butadiene rubber. There have been no previous reports of using BDC sanding dust from chrome tanning processes as a filler for SBR rubber in combination with a conventional seeding unit. Before being introduced into the elastomer matrix, the BDC was characterized by FTIR, SEM analysis, elemental analysis (including determination of chromium content), dynamic light scattering (DLS), and the DBP test. The crushed dust collagen reacted with the elastomer matrix and other components in the mixtures, as was confirmed by FTIR and mechanical studies.
