Colouring Engineering Plastics: ABS, PC, Nylon and POM

Engineering plastics are processed 60 to 120°C hotter than polyolefins, and that single fact invalidates most of the pigment set used in commodity plastics. Add moisture sensitivity, chemical stress cracking and applications where the part is structural, and colouring an engineering polymer becomes a materials engineering task rather than a shade-matching one.

This covers what changes for ABS, PC, PA and POM, and what to specify.

Why heat is the central constraint

Polymer	Typical melt temperature range
PE / PP	180 – 250°C
ABS	220 – 260°C
PA 6	230 – 280°C
POM	190 – 230°C, with a narrow window
PC	280 – 320°C
PC/ABS	250 – 290°C

Every pigment has a temperature above which it degrades, changes shade or decomposes. A pigment perfectly stable in polypropylene at 230°C may shift or char in polycarbonate at 310°C, and residence time compounds the effect – a pigment that survives a short residence in a fast-cycling tool may not survive a slow one.

Heat stability must therefore be specified at your actual processing temperature and residence time, not as a general property. This is the single most common cause of an engineering plastic colour failing after it passed lab approval on a lower-temperature plaque.

Carrier selection

In polyolefins the carrier is chosen for MFI compatibility. In engineering plastics the constraint is chemical.

A polyolefin carrier in an engineering polymer is a foreign, immiscible phase. In PC it hazes and creates weak boundaries. In PA it separates. In any structural part, an immiscible phase is a mechanical defect – a site where a crack begins.

The usual options:

Matched carrier. A PC-carrier concentrate in PC, a PA carrier in PA. Clean, compatible, and it must be dried like the resin.

Universal carriers designed for broad compatibility across engineering polymers. Convenient for a converter running several materials, and worth checking against your specific grade rather than trusting the label.

Carrier-free or low-carrier systems. Pigment with dispersing aid and minimal polymer. Minimises foreign material – important for optical parts and for anything structural.

Liquid colour, dosed at the throat, avoiding a solid carrier entirely.

For engineering polymer masterbatch applications we specify the carrier against the host polymer and grade, because the generic answer is wrong often enough to matter.

Moisture

PA, PC, PET, PBT and ABS are all hygroscopic, and moisture at melt temperature causes hydrolytic chain scission – permanent molecular weight loss, not a cosmetic defect.

Nylon is the extreme case, absorbing moisture readily from Indian ambient humidity within hours of opening a bag.

The masterbatch must be dried to the same standard as the resin if its carrier is hygroscopic. Undried colour concentrate is a common and under-diagnosed cause of property loss, because the investigation focuses on the resin dryer.

Symptoms of moisture damage: splay and silver streaking on the surface, brittleness, and measurable loss of impact strength and tensile properties in a part that looks acceptable.

Material by material

### ABS

The most forgiving of the group and the most widely coloured. Processed at moderate temperature, takes colour well, and gives good opacity.

The real weakness is ultraviolet. ABS contains polybutadiene rubber, and the unsaturated bonds in that rubber are readily attacked by UV. Unstabilised ABS yellows, chalks and embrittles outdoors, and this happens faster than most people expect.

An outdoor ABS part needs a proper UV package, and the pigment matters as much as the stabiliser: carbon black gives excellent protection, while light and white shades give none and depend entirely on the additive system. See HALS vs UV absorbers.

ABS also yellows on thermal ageing, so heat stability of the whole system matters, not only of the pigment.

### Polycarbonate

The most demanding.

Processing temperature is very high – up to 320°C – which excludes a large part of the pigment set.

Transparency is usually the point. PC is chosen for optical clarity, and a transparent tint requires soluble dyes rather than pigments, since a pigment particle scatters light by definition. Dyes have their own limitation: lower lightfastness than pigments, so a transparent tinted PC part outdoors is a genuine engineering compromise.

Stress cracking is the trap. PC is susceptible to environmental stress cracking, and certain chemicals – including some additives and processing aids – initiate it in a moulded-in stressed part. A colourant carrying an incompatible dispersing agent can cause field failures that look structural and are chemical. This is a specific reason not to use a generic concentrate in PC.

Notch sensitivity. PC is notch sensitive, and a pigment agglomerate is a notch. Dispersion quality has a direct mechanical consequence here, not merely a cosmetic one.

### Polyamide (nylon)

Processing temperature is high and PA 66 higher than PA 6.

Moisture is the dominant handling issue, as above.

Nylon has a natural yellowish cast which deepens with thermal history, so light and white shades need correction and a whitening or toning approach – related to the mechanism in optical brighteners in plastics.

Glass-filled grades dominate in engineering applications, and glass changes everything about appearance: fibres come to the surface, colour looks different from an unfilled plaque, and the surface has an inherent texture. Always match colour on a plaque moulded in the actual filled grade. A match approved on unfilled PA will not be right in 30% glass-filled PA.

### POM (acetal)

The narrowest processing window of the group. POM depolymerises above a fairly low threshold, releasing formaldehyde. The degradation is autocatalytic once started, and the process window is narrow enough that residence time matters as much as temperature.

Acidic and basic pigment residues catalyse depolymerisation. Pigments for POM must be chemically neutral and specifically qualified for it. This is not a preference – the wrong pigment produces gassing, splay, mould deposit and a genuine safety concern from formaldehyde release.

POM is also difficult to bond and print, and colour is often the only decoration available, which raises the stakes on getting it right.

Additional considerations

Flame retardancy. Many engineering applications require a UL 94 rating. Flame retardant packages interact with colourants – pigment loading can affect rating, and some pigments interfere with the FR system. If the part is rated, requalify the rating in the production colour. A rating obtained in natural does not automatically transfer.

Chemical resistance. Automotive and appliance parts see fuels, oils and cleaning agents. Some pigments extract or bleed.

Regulatory. Engineering plastics often go into electrical and automotive assemblies, so RoHS and REACH and customer-specific restricted substance lists apply.

Dimensional stability. Filled grades shrink anisotropically, and pigments can nucleate in semi-crystalline PA and POM – see plastic shrinkage and dimensional stability.

Specification checklist

– Polymer, grade and filler content – and match colour on the filled grade – Melt temperature and residence time, for heat stability qualification – Carrier requirement, or carrier-free – Moisture specification for the concentrate – Colour standard with illuminant and tolerance, on a plaque of your actual material and thickness – UV requirement if outdoors, especially for ABS – Flame rating, if any, and confirmation it will be requalified in colour – Chemical exposure in service – Regulatory requirements

Talk to us

Tell us your polymer, grade, filler content, processing temperature and colour standard and we will formulate for that combination rather than adapt a commodity product to it.

Related: automotive, electrical, household, colour masterbatches, delta E and colour tolerance.

FAQs

Why do engineering plastics need different masterbatch? Because they are processed 60 to 120 degrees hotter than polyolefins, which excludes many pigments; because polyolefin carriers are immiscible foreign phases in them; and because they are used in structural and optical parts where an inclusion is a mechanical or visual defect rather than a cosmetic one.

Can I use a normal polyolefin masterbatch in ABS or PC? It is a poor idea. A polyolefin carrier is immiscible in these polymers, causing haze in PC and weak boundaries in any structural part. Use a matched carrier, a qualified universal carrier, a carrier-free system or liquid colour.

Why does ABS yellow outdoors? Because ABS contains polybutadiene rubber whose unsaturated bonds are readily attacked by ultraviolet light. Unstabilised ABS yellows, chalks and embrittles outdoors relatively quickly, so an outdoor ABS part needs a deliberate UV package.

What makes POM difficult to colour? POM depolymerises above a fairly low temperature and the reaction is autocatalytic, so the process window is narrow. Acidic or basic pigment residues catalyse that depolymerisation, producing gassing, mould deposit and formaldehyde release, so pigments must be chemically neutral and qualified for POM.

Why must nylon colour be matched on the filled grade? Because glass fibres come to the surface and change both the appearance and the surface texture. A colour approved on unfilled nylon will not look the same in a 30 percent glass-filled grade, so the match must be made on a plaque of the actual material.

Does masterbatch need drying for engineering plastics? Yes, if the carrier is hygroscopic, and to the same standard as the resin. PA, PC, PET, PBT and ABS all suffer hydrolytic chain scission from moisture at melt temperature, and undried colour concentrate is a common hidden cause of property loss.

Can pigment affect a UL 94 flame rating? Yes. Pigment loading can influence the rating and some pigments interfere with the flame retardant system. A rating established in natural material does not automatically transfer to a coloured version, so requalify in the production colour.

Why is dispersion quality more critical in polycarbonate? Because PC is notch sensitive, so a pigment agglomerate acts as a stress concentration with a direct mechanical consequence. In an opaque commodity part the same agglomerate would be a cosmetic issue at worst.