## 1 Introduction  
The global production of WEEE (waste electrical and electronic equipment) is increasing constantly. The continuous expansion in the consumption and the short life-span of electric and electronic devices have led to the increasing accumulation of WEEE [1]. WEEE plastics include mainly polystyrene (PS), acrylonitrile-butadiene-styrene (ABS) and polypropylene (PP), comprising about 84.74 wt% of total WEEE plastics [2]. These plastics are characterized by high recycling potential, but their processing is difficult due to minute differences in their densities. Moreover, the molecular structure of ABS and HIPS is similar to that of the monomer styrene, thereby leading to similar surface characteristics: a dielectric constant and hydrophilicity [3]. Therefore, separating plastic mixtures into individual components is considered to be a challenge in solid waste management [4]. There are many methods for separating plastic mixtures, such as the following:  
* Magnetic density separation [5];
* Triboelectrification [6];
* Hydrocyclone separation [7];
* Selective flotation [8]. Selective flotation separation is based upon the selective attachment of air bubbles to the plastic surface, resulting in the floating or sinking of proper plastics. This is an effective replacement for other methods because it is characterized by high efficiency, low cost and great accuracy. As most plastics have a naturally hydrophobic surface, the selective enhancement of surface hydrophilicity for one or more components in mixtures becomes necessary. Air bubbles generated in the flotation process are more likely to attach to the hydrophobic surface [9]. The selectivity of flotation separation depends on the method of surface modification. Some methods can make plastic surfaces hydrophilic, such as the following:  
* Boiling treatment [8];
* Microwave treatment [10];
* Chemical oxidation [11]. What is more, some wetting agents are effective for changing the surface characteristics of some plastics (e.g., quebracho, methyl cellulose, lignosulfonates or tannic acid) [12]. It is necessary to improve the flotation separation of acrylonitrile-butadiene-styrene (ABS) and polystyrene (PS) plastics because, as already mentioned, ABS and HIPS are common plastics coexisting in waste electrical and electronic equipment (WEEE) [13]. Moreover, HIPS and ABS are not compatible plastics and, as such, the recyclate will be characterized by reduced properties and difficult processing [14]. Restoring plastics with properties as close as possible to virgin properties will allow such plastics to be recycled multiple times. The purer the composition of the regranulate made from recycled plastics, the better the properties of the elements made of such regranulate. In the study [15], the substance used to change the surface characteristics of PS and ABS was chlorine oxide [IV] in an amount of 0.5 g/L. The flotation of plastics was performed in distilled water. A temperature of 70 \({}^{\circ}\)C and a 60 min treatment time were applied. The difference in the contact angles after the application of CIO\({}_{2}\) for PS was 3.10\({}^{\circ}\), and for ABS it was 12.10\({}^{\circ}\). In the paper [16], in order to change the surface characteristics of PS and ABS, a solution of NaOCl in distilled water was used. A concentration of 0.05 M/L sodium hypochlorite, a temperature of 67.5\({}^{\circ}\)C and a 59.5 min treatment time were used. The difference in the contact angles for PS was 0.15\({}^{\circ}\), and for ABS, 13.64\({}^{\circ}\). In the paper [17], it was stated that a solution of potassium manganate in deionized water at a concentration of 50 mM/L was used to change the nature of the surface. Applying a temperature of 60 \({}^{\circ}\)C and a 30 min treatment time, differences in the contact angles of 1.16\({}^{\circ}\) for PS and 13.64\({}^{\circ}\) for ABS were obtained.