Different plastics present different painting problems, but on the whole, once clean they do not often present problems of paint adhesion. Exceptions are the polyolefins, e.g. polyethylene and polypropylene. Because these polymers lack polar groups, they have low surface energy which are difficult to wet and are not penetrated readily by solvents.

                                                            H         H         H         H

                                                            |          |          |          |         

                                                  ____  C____ C____ C____ C____      etc. Polyethylene

                                                            |          |          |          |

                                                            H         H         H         H

In addition, the surface of the plastic. a few nanometres thick, is often different to the bulk, being lower in molecular weight and weak. Paint adhering to this layer will pull the layer away and peel off, thus appearing to have poor adhesion.

Plastics made by injection moulding processes will have plasticisers and release agents added to the plastic to achieve the correct degree of flexibility and prevent the plastic sticking into the cold mould. The plastic is heated in a cylinder to a temperature sufficiently high to allow the plastic to be squeezed into cold mould, take on the mould shape and then be removed.

Plasticisers are often liquids which boil or evaporate at temperatures above that needed for moulding (often above 200°C) and remain as liquids within the plastic structure. These can migrate into the paint and cause discolouration or can migrate to the surface of the plastic and cause loss of adhesion.

Release agents, as their name implies, release the plastic from the mould. They are normally soaps which, under heat and pressure, melt and squeeze out of the plastic structure, cool on the mould face and solidify preventing the plastic from sticking. Paint, likewise, will not stick to the release agents and at increased temperatures release agents can sweat out of plastics.

To get good adhesion on polyolefins it is necessary to modify the surface of the plastic, to crosslink the weaker layer into the plastic below and tooxidise it, thus creating polar groups and a higher energy surface. This can be achieved by Crona discharge for plastic films and flame treatment for moulded articles.

Corona discharges involves passing the plastic sheet under electrodes with high voltage and high frequency, the discharge produces a plasma, in which gas molecules are broken down and they react with the plastic surface. This causes a rise in surface tension allowing the paint to wet up and improve adhesion because surface layers are no longer weak and because of strong attractions between polar groups in the paint and plastic.

Flame treatment, the oxidising portion of a gas flame (6-1omm from the blue cone tip) is brought into contact with the plastic surface for a brief period (0.02-0.1 second). The flame is a plasma which oxidises the plastic surface by recombination of free radicals created.

The highly reactive particles produced from a corona discharged or flame impact on the plastic surface, oxidising and disrupting 0-H and 0-0 bonds. Subsequent reconbinations of free radicals will contain polar groups C = 0, C - OH. C - OOH, C - ONO and C – ONO₂ . These changes are reflected in a rise in critical surface tension of the plastic e.g. from 31-47 dynes cm^-1

Wetting and adhesion of the paint are thus improved, this is a popular technique for improving the adhesion of printing inks to the surface of mould polyolefin containers.

The action of specific plastic adhesion paints is to have chemical species which will give specific polar groups the opportunity to bond to the plastic and other polar groups to bond to the paint.