Expandable styrene resin particles, expandable beads, and foamed article

Abstract

A process of producing expandable styrene resin particles, wherein in suspension polymerization of styrene monomers, the concentration of oxygen in a reaction vessel is kept low at least in a late stage of the polymerization, and the resulting styrene resin particles are impregnated with an expanding agent before or after completion of the polymerization. Preferably, at a polymerization rate of 60% or higher, the concentration of oxygen in the reaction vessel is kept at 7 vol % or lower. When additional styrene monomers are added during the polymerization, they are added and adsorbed to styrene resin particles in the course of polymerization while the concentration of oxygen is kept low. By this method, the particles whose internal portion has a low molecular weight and surface portion has a high molecular weight are obtained. A foamed article produced from the particles has high strength and a good appearance.

1 . An expandable styrene resin particle, wherein the inclination of a correlation expression of a logarithm (R.M.S radius) and a logarithm (MW), measured by a GPC/MALLS method, of a surface portion from the surface of the particle to a depth of ⅕ of the radius toward the center is not larger than 0.53.  
   
   
       2 . The expandable styrene resin particle according to  claim 1 , wherein the weight average molecular weight of a surface portion from the surface of the particle to a depth of ⅕ of its radius toward the center is higher than that of a central portion from the center to a distance of ⅕ of the radius toward the surface, and a chart of gel permeation chromatography of the surface portion has two crests or a shoulder.  
   
   
       3 . The expandable styrene resin particle according to  claim 1 , wherein, when a surface portion from the surface of the particle to a depth of ⅕ of the radius toward the center is further divided into 6 equal portions from the surface toward the center of the particle, a ratio (B)/(A)×100(%) is at least 130 wherein (B) is the weight average molecular weight of the outermost portion out of the 6 equal portions, and (A) is the weight average molecular weight of the whole particle.  
   
   
       4 . The expandable styrene resin particle according to  claim 1 , wherein a ratio (B)/(A)×100(%) is larger than 130 but not larger than 200, wherein (B) is the weight average molecular weight of a resin component forming up to 10 wt % from the surface toward the center of the particle, and (A) is the weight average molecular weight of the whole particle.  
   
   
       5 . The expandable styrene resin particle according to  claim 1 , wherein the weight average molecular weight of a surface portion from the surface of the particle to a depth of ⅕ of its radius toward the center is 300,000 to 450,000, the weight average molecular weight of a central portion from the center to a distance of ⅕ of the radius toward the surface is 200,000 to 300,000, and the weight average molecular weight of the surface portion is at least 1.2 times as large as that of the central portion.