Abstract: An additive for masticating natural rubber comprising at least one compound selected from the group consisting of benzoic acids, naphthoic acids, dicarboxylic acids and compounds represented by Formula (IV): ##STR1## wherein m and n each represent an integer of 1 to 3; p represents an integer of 1 to 4; m+n+p=6; X is O, NR.sup.8 or (R.sup.9 O).sub.q ; R.sup.6 is a C.sub.1 .about.C.sub.24 alkyl, a C.sub.2 .about.C.sub.24 alkenyl, or a C.sub.6 .about.C.sub.24 aryl group; and R.sup.7 is H, OH, an alkyl, alkenyl or aryl group; R.sup.8 is H or a C.sub.1 .about.C.sub.24 alkyl group; R.sup.9 is a C.sub.1 .about.C.sub.4 alkyl group; q is an integer of 1 to 5, a mastication method and a natural rubber composition each of which uses the additive, and a pneumatic tire using the natural rubber composition.

Abstract: Disclosed are a rubber composition having an improved charging property which comprises 100 parts by weight of rubber component containing diene rubber of 90 parts by weight or more, and a filler of 30 to 120 parts by weight and a nonionic surfactant or phosphoric ester of 0.2 to 8 parts by weight, and a pneumatic tire characterized by that the above rubber composition having an improved charging property is used for the tread parts of the pneumatic tire.The typical examples of the nonionic surfactant and phosphoric ester are an adduct of ethylene oxide of 2 to 30 moles to linear, cyclic or branched, and saturated or unsaturated, and monohydric or polyhydric aliphatic alcohol having carbon atoms of 6 to 25, or an adduct of ethylene oxide of 2 to 30 moles to alkylphenol, and the compounds represented by the following formula (V): ##STR1## wherein R.sup.