Polybutylene Terephthalate (PBT) for Exterior Parts: UV Stabilisation and the Limits of Unpainted Use

Polybutylene terephthalate (PBT) is a semicrystalline aromatic polyester produced by the polycondensation of terephthalic acid or dimethyl terephthalate with 1,4-butanediol. In exterior components, its selection is normally driven by the combination of a crystalline melting point between 223 °C and 225 °C, high flow in thin-wall sections, low moisture absorption compared with polyamide, and good dimensional stability under underbonnet heat sequences. Dry as-moulded tensile modulus for an unfilled grade is commonly reported between 2.4 GPa and 2.8 GPa; a 30 wt% short-glass compound raises tensile modulus above 9 GPa and extends the heat deflection temperature under 1.82 MPa load to approximately 200 °C. Before compounding or moulding, the resin is dried in a desiccant dryer to a residual moisture content below 0.02 wt%, typically at 120 °C for 4 h with a dew point of -40 °C. Failure to meet this moisture threshold causes hydrolytic chain scission in the melt, producing a measurable drop in intrinsic viscosity and a subsequent loss of notched impact resistance, even if the part remains visually acceptable immediately after moulding.

What Photochemical Mechanisms Control PBT Weathering Stability?

The photochemistry of unpainted PBT begins with absorption in the 290 nm to 350 nm band. The ester carbonyl and the aromatic terephthalate repeat unit absorb UV energy, and the excited states undergo Norrish Type I and Type II scission, photo-Fries rearrangement, and radical formation. At the surface, atmospheric oxygen reacts with polymer radicals to form hydroperoxides, which decompose into alkoxy and hydroxyl radicals. The resulting chain scission creates low-molecular-weight fragments and carboxylic acid end groups. This surface-restricted reduction in molecular weight causes microcracking, chalking, colour shift, and loss of specular gloss, while the underlying core can remain mechanically sound for much longer. In glass-filled PBT, resin erosion exposes fibres at the surface and creates a rough, fibrous texture that is immediately rejected on visible exterior parts.

Accelerated weathering for exterior PBT is screened under ISO 4892-2:2013, cycle 1, with a xenon-arc source, daylight filters, irradiance of 0.35 W/m² at 340 nm, and black-standard temperature between 60 °C and 65 °C. For automotive components, SAE J2527 or SAE J2412 are often substituted because the filtered spectral output better reproduces service conditions. Weathering data are not portable across additive packages: replacing a benzotriazole UV absorber with a triazine at the same nominal loading can shift post-exposure ΔE* by more than 1 unit, and the same is true when a black pigment is changed from carbon black to a mixed inorganic black.

Stabilisation of unpainted PBT typically combines a hindered amine light stabiliser, such as bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, at 0.3 wt% to 0.6 wt%, with a benzotriazole or triazine UV absorber at 0.2 wt% to 0.4 wt%. Black exterior parts use carbon black at 1.5 wt% to 2.0 wt% as the primary UV screen, which also masks yellowing. White or light-coloured parts incorporate rutile titanium dioxide at 3 phr to 5 phr; only surface-treated grades are acceptable because uncoated photocatalytic TiO₂ accelerates chalking. Because hindered amine light stabilisers are basic, a high carboxylic acid end-group concentration in hydrolytically aged surface layers can deactivate a portion of the hindered amine at the oxidation front. This is one reason why weathering performance does not improve linearly with hindered amine concentration above the grade-specific saturation point.

On a 40:1 L/D co-rotating twin-screw extruder producing a 30 wt% short-glass PBT compound, the barrel profile is maintained from 245 °C to 260 °C, with a vacuum devolatilisation zone at -0.08 MPa to -0.09 MPa after the first atmospheric vent. Glass fibre is side-fed downstream to control fibre length, and the screw uses kneading blocks in the glass incorporation zone followed by low-shear conveying elements before the die. Local melt temperatures above 265 °C accelerate thermal and hydrolytic chain scission; local melt temperatures below 240 °C increase melt viscosity and surface orientation. The practical processing window for one exterior grade is therefore 250 °C ± 5 °C. Injection moulding of exterior plaques on a 1200 kN clamp force machine is performed at a melt temperature of 250 °C to 255 °C and a mould temperature of 60 °C to 80 °C. Low mould temperatures produce a highly oriented skin that weathers to visible fibre prominence and differential gloss; textured cavities with a grain depth above 20 µm are preferred for unpainted surfaces.

At relative humidity above 60 %, open storage after drying becomes a processing risk. A 30 min exposure of dried pellets can raise surface moisture enough to cause streak defects in moulded parts. A closed hopper dryer with return-air dew point no higher than -20 °C is therefore used when the moulding shop cannot control ambient humidity.

Unpainted Surface Retention Becomes the Controlling Specification, Not Bulk Strength

In unpainted PBT exterior parts, visual failure precedes bulk mechanical failure. Accelerated weathering under ISO 4892-2:2013 therefore produces two parallel data sets: colour and gloss change on the exposed surface, and mechanical property retention measured on the same plaques under ISO 527-2:2012 and ISO 180:2023. The representative screening ranges in the table below are drawn from commercial compounder datasheets for a 30 wt% glass-filled PBT after 2000 h of cycle 1 exposure. They are not multi-lot guaranteed values; production qualification requires a statistically constrained test on at least three lots.

A weathered part can retain more than 85 % of its tensile strength and still fail an appearance specification. The 60° gloss value can drop from 90 to 50 within 1000 h of accelerated exposure if the skin orientation is high and the stabiliser package is marginal. In 30 wt% glass-filled PBT, surface resin erosion exposes glass fibres after 2000 h to 3000 h, creating a rough surface that cannot be repaired in service. Once gloss retention falls below 70 % absolute, or ΔE* exceeds 3.0, the part is generally rejected from visible exterior applications, even when bulk tensile strength retention remains above 80 %. The practical upper dosage of hindered amine stabiliser in PBT usually remains below 1.0 wt% because higher loadings can reduce weld-line strength and increase mould deposit without a corresponding linear improvement in surface retention.

When Unpainted PBT Is a Valid Replacement for Painted Surfaces

Moulded-in black based on carbon black at 1.5 wt% to 2.0 wt% is the most robust unpainted option for exterior brackets, sensor covers, lower trim, and undertray components. The carbon black absorbs UV across the 290 nm to 400 nm range and hides oxidative yellowing that would be visible in natural or white grades. The technical limit is that carbon black does not stop surface erosion; it delays the visual detection of erosion. An unpainted PBT part should therefore be specified only with a low initial gloss, a deliberate texture, and a semi-visible or non-decorative position. On a smooth polished cavity surface, unpainted PBT cannot match the colour depth or distinctness of image of a painted Class A surface, and it will show oxidation haze and micro-scratches after relatively short outdoor exposure. Published data for unpainted PBT as a direct replacement for body-colour painted exterior panels are limited, and the available field evidence indicates that it is not a viable substitution for high-gloss decorative surfaces.

The operational boundary is also climate-dependent. Field surveys on production exterior parts indicate that unpainted black PBT can remain acceptable for 3 to 5 years in temperate climates, while unpainted natural or white parts can be rejected after 12 to 24 months because of colour change alone. In desert and high-altitude UV environments, these service periods contract. For structural exterior brackets, the limiting mechanism may be hydrolysis rather than UV: hot-wet exposure reduces molecular weight in the stressed mounting area and produces brittle fracture at the gate or insert. A UV-stabilised surface package does not protect the core against hydrolytic chain scission. Long-term hot-humid use above 60 °C and 85 % relative humidity may require a separate polycarbodiimide chain-extension additive or a change to a hydrolysis-resistant polyester grade.

For exterior unpainted PBT, qualification testing pairs as-moulded reference panels with weathered panels to separate surface ageing from bulk property loss. The minimum test matrix used by tier-one suppliers typically includes the following methods and acceptance criteria for black non-aesthetic exterior PBT.

Chemical exposure from road de-icing fluids and windscreen washer solutions is a separate boundary for unpainted PBT. ISO 2812-1 immersion testing in a 50 vol% methanol-water mixture at 23 °C for 168 h can reveal stress-crazing that is not detected by standard xenon weathering. If the exterior part is located near a washer nozzle or in a direct salt-spray path, the same chemical test is added to the weathering matrix before unpainted material approval is granted. Unpainted PBT that passes colour and gloss criteria in black may still fail in white, in polished surface applications, or in load-bearing locations where hydrolysis and UV attack operate simultaneously. Those boundaries define the limit of unpainted use, not the bulk thermal or mechanical properties of the moulded compound.