Polymeric materials

A preform for a container comprising a polymer composition and particles, the polymer composition consisting essentially of polyethylene terephthalate and the particles consisting of tungsten oxide particles of general formula WOx where 2.65<=x<=2.95, wherein said polymer composition includes 5 ppm to 150 ppm tungsten oxide particles, wherein at least 99 wt % of said preform is made up of said polymer composition, wherein said polymer composition includes at least 99 wt % of a polyester polymer, wherein said preform has an L* of at least 63, wherein at least 99 wt % of said tungsten oxide particles have size of less than 20 μm, wherein said tungsten oxide particles have a span (S) from 0 to 5 where S is calculated by the following equation:

• S=(d90-d10)/d50
• where d90 represents a particle size in which 90% of the volume is composed of particles having a smaller diameter than the stated d90;
• d10 represents a particle size in which 10% of the volume is composed of particles having a diameter smaller than the stated d10;
• and d50 represents a particle size in which 50% of the volume is composed of particles having a diameter larger than the stated d50 value and 50% of the volume is composed of particles having a diameter smaller than the stated d50 value.