Zinc stearate

Cadmium (II) acetate (99.995%), zinc acetate (99.99%), zinc stearate (technical grade), elemental selenium (powder), elemental sulfur (powder), trioctylphosphine (90%), 1-octadecene (90%), oleic acid (90%), MPA and TGA were purchased from Merck. All other chemicals and solvents were of analytical grade and used without additional purification. In this work, we used Sindroxocin (50 mg, Actavis, Iceland), an antitumor antibiotic whose active ingredient is doxorubicin hydrochloride (50 mg). It also contains the following excipients: methyl parahydroxybenzoate (5 mg) and lactose monohydrate (263.15 mg). Before analysis, 3 mg (corresponding to 0.5 mg of DOX) of the lyophilizate was dissolved in 1 mL of Milli-Q water. From the resulting solution (500 µg/mL), solutions with concentrations of 50, 5 and 1 µg/mL were obtained.

Carboxylated nitrile rubber (XNBR, KRYNAC X 750) with 27 wt. % of acrylonitrile and 7 wt. % of carboxylic groups was kindly supplied by Arlanxeo (Maastricht, The Netherlands) and used as a rubber matrix. Zinc oxide ( ZnO), sulfur (S), and zinc stearate (ZnSt) were acquired from Merck (Darmstadt, Germany), stearic acid (SA) from Alfa Aesar (Kandel, Germany) and n-cyclohexyl-2-benzothiazole sulfenamide (CBS) from Biosynth Carbosynth (Berkshire, UK). Toluene (ACS reagent, ≥99.5%) from Merck (Darmstadt, Germany), gasoline 95 and motor oil from Repsol (Madrid, Spain) were used to evaluate the chemical resistance. All products were used as received. The prepared formulations are summarized in Table 1.
All ingredients were mixed in a two-roll mill (MGN-300S, from Comerio Ercole S.P.A., Busto Arsizio, Italy) during 25 min. The mixing protocol is summarized in Table 2. Each compound was vulcanized by compression molding in a hydraulic press (TP300, from Gumix, Barcelona, Spain) according to its curing time at 90% of the maximum torque (t₉₀) obtained from the curing curves. At least 24 h after molding, the vulcanized samples were cut with the geometries required for each characterization technique.

Copper (I) iodide (CuI, 99.99%), indium acetate (In (AC)₃, 99.99%), zinc acetate (Zn (ac)₂, 99.99%), 1-octadecence (ODE, 90%), 1-octanethiol (OT, 98%), zinc stearate (Zn-St, 99.99%), oleic acid (OA, 90%) and 2-Ethylhexanoic acid (EHA, 99%) were purchased from Sigma-Aldrich. Octane, ethanol and acetone were purchased from Aladdin. All the chemicals were used without further purification.