Polymeric compositions

A thickened organic polymeric molding composition from which is absent an inorganic thickener selected from metal oxides and hydroxides and which said composition comprises a cooled mixture of:

• a cross-linkable base resin dissolved in an unsaturated monomer;
• and
• as a thickener and shrinkage control agent, an additive resin having a minimum of 8 and a maximum of 40 repeating units and selected from saturated polyesters and saturated amide waxes;
• said additive resin being crystalline at ambient temperature and having a melting point (T.sub.m) below a temperature (T.sub.c) at which the base resin cross-linking reaction proceeds at a significant rate;
• said additive resin and said base resin having a solubility parameter difference therebetween in the range 1 to 3.5 MPa.sup.1/2, whereby said additive resin is only partially compatible with the base resin;
• said partial degree of compatibility allowing crystallization of said additive in said mixture upon cooling from a temperature between T.sub.m and T.sub.c to a temperature between T.sub.m and ambient;
• whereby,
• (a) due to said cooling of said mixture from a temperature between T.sub.m and T.sub.c to a temperature below T.sub.m to produce said cooled mixture, said additive resin is in the form of crystals thereof distributed throughout the mixture, said crystallization thereby rendering the mixture thickened in the absence of a thickener selected from metal oxides and hydroxides;
• and
• (b) when the mixture is heated to a temperature above T.sub.m, said mixture is capable of melting and reversibly breaking down to enable said additive resin, in a molten state, to render the composition capable of shrinkage control during molding.