194 Chapter 8
lubricant may act as a plasticiser once incorporated into a formulation. The
major requirements for additives in plastics include:
●High stability under processing condition
●High stability when plastic is in use
●Ability to remain in the formulation throughout the intended useful life-
time of the plastic and not to migrate out or evaporate. The intended use-
ful lifetime of most plastics is between one (polyethylene carrier bag)
and 30 (unplasticised PVC window frames) years; museums aim to keep
them for longer
●Low-toxicity and inert odour or taste, especially in plastics for use with
food or medicines
●Low cost.2.3 Shaping Plastics
Shaping plastics is also known as converting because the aim of the process
is to change molten, flowing plastic into a pre-determined shape. Plastics
Table 3Types and functions of frequently used additives
Additives Major functions How achieved Examples
Plasticiser Soften polymer, Separates polymer Phthalate esters, aliphatic
reduce Tg chains from each other diesters, epoxidised oils,
phosphate esters,
polyesters
Lubricant Prevent adhesion of Sweats out due to low Calcium stearate, normal
polymer to processing compatibility with and dibasic lead stearate
equipment polymer
Impact Reduce brittleness Adds bulk to Styrene-acrylonitrile,
modifier compound ethylene-vinyl acetate
copolymer
Processing Ensure uniform flow Migrates to surface to As for impact modifier
aid and good surface improve flow during
finish manufacture
Filler Opacify compound, Changes refractive Calcium carbonate,
increase hardness, index and reflective magnesium carbonate,
reduce cost properties, adds bulk barium sulfate, carbon or
to compound glass fibres
Stabilizer or Minimise or eliminate Interrupts free-radical Tin mercaptides, barium–
anti-ageing degrading effects of reaction sequence by cadmium salts of fatty
heat, light or oxygen binding oxygen or acids, lead salts,
on polymer degradation product alkyl benzenes,
so it is unavailable for epoxidised soya bean oil
further reaction
Colouring Decoration, protection Add colour and opacity Carbon particles,
agents from radiation to surface or to bulk dyestuffs