Plastic and Decorative Coatings

Decorative coating is where surface chemistry and appearance meet. A cosmetic case, a glass bottle or a consumer electronics housing is judged on gloss, depth of colour and tactile quality, and then has to survive filling lines, transport and years of handling. This category covers the substrates that give coatings the most trouble: untreated polyolefins, engineering plastics and glass, where adhesion depends on pre-treatment and primer selection as much as on the coating itself.

Surface energy governs everything that follows. Polypropylene and polyethylene sit around 29 to 31 mN/m, and a liquid coating will not wet and anchor to a surface of lower energy than its own. Flame, corona or plasma treatment oxidises the top few nanometres and lifts wetting tension above the 38 mN/m that most decorative systems need. A chlorinated polyolefin adhesion promoter reaches the same end by a different mechanism: it does not oxidise the surface, it swells into the polyolefin and presents a polar film that the coats above can bond to, which is the route used where the geometry or the line does not allow physical treatment. Treatment decays, so the interval between treating and coating has to be fixed and held. ABS at roughly 42 mN/m and polycarbonate at roughly 46 mN/m accept coatings directly, but they bring their own constraint: aggressive solvent blends and residual moulding stress combine into environmental stress cracking, so solvent balance and flash time matter more on those substrates than adhesion does.

Soft-feel finishes fail in a specific and predictable way. Most tactile topcoats are polyester polyurethanes, and the ester link in the polyol hydrolyses in hot humid service. The film first goes tacky, then picks up dirt, then sloughs under a fingernail. Vehicles parked in the sun, tropical warehousing and bathroom or gym environments all produce it. Systems built on aliphatic polycarbonate polyols have no readily hydrolysed ester group in the backbone and hold their tactile character through far longer damp heat exposure, and an aliphatic isocyanate crosslinker keeps the film from yellowing under light. That is a formulation choice made at specification time, not a defect discovered in the field, so hot humid service should be declared before a soft-feel grade is chosen.

The remaining routes in this category are chosen by throughput and by substrate. UV-curable hardcoats crosslink in seconds under a mercury or LED source and give scratch and abrasion resistance on parts that cannot take an oven, with cure governed by delivered dose and by the match between the source spectrum and the photoinitiator, not by lamp hours. Vacuum metallising works as a pair: a base coat that seals and levels the moulding so the aluminium deposit reads as a mirror, and a topcoat that protects a metal film only tens of nanometres thick from handling and moisture. Glass decoration splits into two routes. Ceramic frit fired at 560 to 620 degrees C is permanent and dishwasher durable but energy heavy, limited in colour and confined to substrates that survive the firing temperature. Organic coatings cure at 140 to 160 degrees C or under UV, reach colours, matt and gradient effects that firing cannot, and depend on silane coupling to the silica surface, which is why alkali and condensation resistance are tested on the coating rather than assumed from the substrate.

Chlorinated polyolefin and two-component polyurethane primers that let decorative and functional coatings wet out and bond to polypropylene, polyethylene and TPO mouldings. Used after flame, corona or plasma treatment, or as the chemical adhesion route where physical treatment is not available on the line.

Two-component polyurethane topcoats giving a velvet or rubberised touch at controlled matt gloss. The subfamily separates a general purpose grade from a polycarbonate-polyol grade for hot and humid service, where polyester polyurethane soft-feel hydrolyses and turns tacky.

Radiation-cured acrylate clears that build scratch and abrasion resistance in seconds rather than in an oven. Used where line speed, immediate handling after cure or a heat sensitive substrate rules out a thermal bake.

Matched pairs for the metallising chamber. The base coat seals and levels the moulded surface so the deposited aluminium reads as a mirror, and the topcoat protects that very thin metal layer, with tinted versions producing chrome, gold, smoke and coloured chrome effects from the same deposit.

Silane-promoted organic coatings for container and flat glass, cosmetic jars, compacts and closures. They decorate at oven temperatures instead of ceramic firing temperatures, and are specified against the alkali, condensation and cosmetic contact conditions a filled pack actually meets.

The figures below are the typical operating window for this range, not a measured result for any one batch. Confirmed values are issued with the Technical Data Sheet and Certificate of Analysis by our laboratory.

Nothing in this category succeeds without the pre-treatment step being under control. On polyolefin, flame, corona or plasma treatment is verified by wetting tension before the primer gun is opened, not after a batch has failed a cross-cut. Treatment decays, so the line needs a fixed maximum interval between treating and priming, and parts that sit past it are re-treated rather than coated. Mould release is the other common cause of adhesion loss: silicone and stearate release agents survive treatment and re-deposit during handling, so release agent selection is part of the coating specification. On ABS and polycarbonate no treatment is needed, but retained moulding stress plus an aggressive solvent blend produces environmental stress cracking, which appears as fine crazing at gate and boss areas hours or days after coating, so annealing and solvent balance are set together.

Application is predominantly spray, on rotating spindles or reciprocators, with dip and flow coat used for lenses and glazing and curtain coat for flat glass. Film build is controlled by gun setting and pass count, and it is checked as dry film thickness rather than inferred from wet appearance. Every two-component grade has a pot life. Viscosity climbs as the reaction proceeds, and material used near or past the end of pot life can still spray and flow acceptably while building a film of lower crosslink density and weaker chemical and humidity resistance, so mixed material is timed and discarded rather than stretched. Flash time between coats and before bake is what lets solvent leave; shortening it to gain line speed traps solvent, which shows up as pinholing on bake, as poor humidity resistance, and in vacuum metallising as a slow pump-down and a cloudy deposit.

Cure is governed differently in each route. Thermal grades are specified as part temperature held for a stated time, not as oven air setting, and the oven profile is proven with a datalogger through the rack rather than assumed from the controller. Bake temperature is capped by the heat distortion temperature of the moulding, which is why polycarbonate, ABS and PP each run a different window. UV grades are governed by delivered dose and by the spectral match between the source and the photoinitiator package, so a line that changes from mercury to LED must have the cure re-qualified rather than the belt speed copied across. Dose is verified with a radiometer travelling the belt at production speed, at intervals set by the line, because output falls long before a lamp visibly fails. Surface cure and through-cure fail for different reasons and are checked separately. The oxygen inhibited layer sits at the film surface and is roughly a fixed depth, so it dominates on thin films, on matt finishes and on low intensity LED sources, and those are the cases that need a nitrogen or reduced oxygen zone. Thick or pigmented films cure at the surface and fail underneath, where the limit is delivered dose and long wavelength output, checked by a solvent rub and by adhesion rather than by surface hardness.

Two sequences carry their own discipline. Vacuum metallising is a pair, not two independent coats: base coat outgassing determines chamber cycle time and deposit clarity, and the topcoat must go on soon after the chamber because a bare aluminium film oxidises and finger salts etch it. Glass decoration depends on the surface it meets. The cold-end coating, the organic wax lubricant applied after the annealing lehr, must be removed from the decorated zone by washing or burn-off. The hot-end tin oxide layer is permanent and is allowed for in silane selection rather than removed. The glass is brought above dew point before coating, because condensed water disrupts wetting and film formation, while the trace moisture on a clean glass surface is what the alkoxysilane needs in order to hydrolyse and condense with the surface silanols and give long term alkali resistance at the specified cure. The windows quoted in this category are industry-typical process windows for these product types, and the values that apply to a supplied grade are those on its technical data sheet and in the protocol agreed for the part. A good result on this line is one where wetting tension, film build, flash, cure dose or part temperature, and a cross-cut on a production part are each recorded per batch, so that a field failure can be traced to a process variable instead of being attributed to the coating.

Arctic specifies from the part and the process, not from a catalogue number. The following answers are what our technical centre needs before recommending a grade.

• Which polymer, and in what grade? Polypropylene, TPO, ABS, ABS/PC, polycarbonate, PMMA, PBT or glass. Filler and glass fibre loading, any regrind content, and the mould release agent in use.
• What pre-treatment is available on the line? Flame, corona, plasma, none, or a chemical adhesion promoter only. If treatment exists, what wetting tension it reaches and what interval passes before coating.
• What is the service environment? Interior, exterior, or hot and humid. Declare tropical, vehicle interior or bathroom and gym service explicitly, because it decides whether a soft-feel finish must be polycarbonate-polyol based.
• What does the surface have to survive? Sun cream, insect repellent, hand sanitiser, perfume, cosmetic oil, alkaline wash, dishwasher cycles, or abrasion. Name the liquids and the contact time and temperature.
• What appearance is being matched? Colour standard and tolerance, gloss with the measurement geometry stated, matt and tactile reference, and whether a metallised effect must read as chrome, gold, smoke or coloured chrome.
• What cure is available? Maximum part temperature and dwell in the oven, or UV source type, spectral output and belt speed. Whether an inert or reduced oxygen zone exists.
• Are heat sensitive components decorated in the same set? Plastic closures, caps, pumps and gaskets running alongside glass need their own low temperature cycle, so their maximum part temperature has to be stated with the glass cure.
• What is the application method and target film build? Spray on spindle or reciprocator, dip, flow coat, curtain coat, and the dry film thickness the part drawing calls for.
• For metallising, what is the chamber cycle? Pump-down time available, base coat cure route, and the interval between deposition and topcoat.
• For glass, which decoration route? Organic coating at 140 to 160 degrees C, or ceramic firing. If organic, the cold-end coating condition of the decorated zone, whether it is washed or burnt off, and the alkali exposure the filled pack will see.
• What test protocol will the part be judged against? The adhesion, humidity, chemical and weathering methods, their severities and the pass criteria, so that the specification is written to the test rather than to a description.

Compliance documentation and a declaration for the supplied grade are issued on request.

The Technical Data Sheet, Safety Data Sheet and regulatory declaration for any grade in this range are issued by our laboratory against the substrate and process you are running. Tell us the job and we will specify against it.