Patent ID: 6723456
Filing Date: 2004-04-20
Classification: G11B,Y10T

Abstract:
Hematite particles aggregates comprising aggregates of acicular hematite particles oriented in a major axis direction thereof, said acicular hematite particles having an average major axis diameter of 0.005 to 0.3 &mgr;m and an average minor axis diameter of 0.0005 to 0.10 &mgr;m,said hematite particles aggregates exhibiting a compressiblity of a coating film of 9.0 to 20.0%, when measured by the following method: (i) 12 g of the hematite particles aggregates are mixed with a binder resin solution (containing 30% by weight of a vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group, and 70% by weight of cyclohexanone) and cyclohexanone, thereby obtaining a mixture having solid content of 72%, and a resultant mixture is further kneaded for 30 minutes using a plast-mill to obtain a kneaded material; (ii) the obtained kneaded material together with 95 g of 1.5 mm&phgr; glass beads, an additional amount of a binder resin solution (comprising 30% by weight of a polyurethane resin having a sodium sulfonate group, and 70% by weight of a mixed solvent composed of methyl ethyl ketone and toluene at mixing ratio of 1:1), cyclohexanone, methyl ethyl ketone and toluene, are charged into a 140-ml glass bottle at the following mixing ratio; and a resultant mixture is mixed and dispersed for 6 hours using a paint shaker, thereby obtaining a non-magnetic coating material: Composition of non-magnetic coating material:Hematite particles aggregates100parts by weightVinyl chloride-vinyl acetate10parts by weightcopolyner resin having a sodiumsulfonate groupPolyurethane resin having10parts by weighta sodium sulfonate groupCyclohexanone44.6parts by weightMethyl ethyl ketone111.4parts by weightToluene66.9parts by weight;(iii) then, the obtained non-magnetic coating material is applied onto a non-magnetic base film using an applicator with a coating thickness of 55 &mgr;m and then dried, thereby forming a non-magnetic undercoat layer; and (iv) the thus obtained dried non-magnetic undercoat layer is subjected to calendering treatment at 85Â° C. under a load of 200 kg/cm four times, and the compressibility of the coating film is calculated from thicknesses t0 (&mgr;m) and t1 (&mgr;m) of the coating film before and after the calendering treatment, respectively, according to the following formula: Compressiblity of coating film (%)={(t0âˆ’t1)/t0}Ã—100 wherein t0 represents a thickness of the non-magnetic undercoat layer before the calendering treatment; and t1 represents a thickness of the non-magnetic undercoat layer after the calendering treatment.