Styrene-butadiene Block-copolymer Solutions for Protective Coating of Metals in the Chemical Milling Process

This paper presents an experimental study aiming to obtain a new polymer composition by using styrene- butadiene radial block-copolymer with styrene-butadiene diblock-copolymer blend, in order to manufacture masking solution for metal protective coating in the chemical milling process. Table 1 PROPERTIES OF RADIAL BRANCHED STYRENE-BUTADIENE BLOCK-COPOLIMERS The chemical milling process is used as a rule to chemically remove layers from a metal surface during manufacturing, a portion of the metal piece needing to be protected from corosive action of an etchant solution by etching resist masking material. The protective coverage surface of the metal is usually obtained by immersing the metal piece in a masking solution, and after that follows the solvent evaporation. For most applications, the masking solution contains mainly an elastomer and an organic or more often an anorganic filler and anti-setting agents, all materials being solved-dispersed in a non-inflammable solvent (1,2). The masking formulation contains an usual elastomer (polybutadiene, styrene-butadiene rubber, nitryle rubber, natural rubber) the final coat of mask being tack free. The film need to be oven cured for 30 to 60 min at elevated temperature(80-140 o C). This paper investigates the case of radial branched styrene-butadiene block- copolimers (SBS) using as elastomer component part and styrene-butadiene diblock-copolimers (RS) as fillers in masking formulation. The radial branched styrene- butadiene block-copolimers exhibit at ambient temperature many of the properties of a vulcanized rubber as a consequence of the polystyrene- polybutadiene incompatibility. Due to the thermodynamic incompatibility of the two kinds of blocks, a phase separation occurs, resulting in the formation of a glassy domain in which polystyrene blocks belong to different molecules aggregate. This virtual crosslinking is reversible by dissolvation. The elimination of the chemical cure, a notoriously energy - consuming operation, offers large energy and time savings providing a new masking solution based on SBS. Experimental part In the frame work of the performed experimental research study were synthesized three radial branched styrene-butadiene block-copolimers and three styrene- butadiene diblock-copolimers by means of sequential anionic polymerization of monomers. Only after the full consumption of the previous monomer the next one was added , in cyclohexane solution, initiated with butyllithium. The active diblock-copolimer polystyren-polybutadien- lithium—was finally coupled with Si Cl 4 to obtain radial branched styrene-butadiene block-copolimers (3-6). The optimal polymerization periods for ever y step were computed by using previously established kinetic relations (7-8). The molecular weights of polymers were measured by gel permeation chromatography (GPC) on a Waters - Millipore 244 apparatus, in tetrahydrofurane (flow rate 1mL/ min.). The GPC was calibrated with polystyrene standards. The polystyrene content was determined by means of density-refraction index method (9).The tensile properties of synthesized SBS were obtained on films resulting by centrifugal casting from toluene solution on standard dumbbel specimens, at a jaw separation speed of 0.5 m/ min., using a Zwick 1454 tensile tester, the retractive force being recorded during extension. Hardness was measured with a dead load Zwick hardness meter in the Shore A scale.