Microspheres comprising titania and bismuth oxide

Abstract

The present invention relates to microspheres (i.e., beads) that comprise titania and bismuth oxide. The glass microspheres further comprise zirconia. The invention also relates to retroreflective articles, and in particular pavement markings, comprising such microspheres.

1 . (canceled)  
   
   
       2 . The method of  claim 21  wherein the microspheres are transparent.  
   
   
       3 . The method of  claim 21  wherein the amount of bismuth oxide ranges up to 30 wt-%.  
   
   
       4 . The method of  claim 21  wherein the amount of titania ranges up to about 92 wt-%.  
   
   
       5 . The method spheres of  claim 21  wherein the microspheres further comprise at least 10 wt-% alkaline earth metal oxides.  
   
   
       6 . (canceled)  
   
   
       7 . The method of  claim 5  wherein the at least 10 wt-% alkaline earth metal oxides comprise at least 5 wt-% CaO, at least 5 wt-% BaO, and optionally at least 5 wt-% SrO.  
   
   
       8 . (canceled)  
   
   
       9 . The method of  claim 21  further comprising up to about 20 wt-% zinc oxide.  
   
   
       10 . The method of  claim 21  wherein the microspheres are fused.  
   
   
       11 . The method of  claim 21  wherein the microspheres have an index of refraction of greater than 2.2.  
   
   
       12 - 20 . (canceled)  
   
   
       21 . A method of producing microspheres comprising: 
 a) providing a starting composition comprising at least 5 wt-% bismuth oxide and at least 50 wt-% titania;    b) melting that starting compostion to form molten droplets;    c) cooling the molten droplet to form quenched fused microspheres; and    d) heating the quenched fused microspheres such that microspheres having a glass-ceramic structure are formed.    
   
   
       22 . The method of  claim 21  comprising at least 50 wt-% titania, at least 5 wt-% bismuth oxide and at least 5 wt-% zirconia, based on the total weight of the microspheres.  
   
   
       23 - 26 . (canceled)  
   
   
       27 . The method of  claim 22  wherein the amount of titania ranges up to about 84 wt-%.  
   
   
       28 - 35 . (canceled)