Light-Stabilizer Solutions for Automotive Exterior Parts

Automotive exterior parts are where aging is most visible and appearance demands are highest of components. They carry none of the interior's VOC and fogging pressure, yet must face year-round unobstructed UV radiation, midday heat, gravel impact and rain, snow and salt spray. A grayed bumper or a brittle door-sill guard is usually judged a "quality problem" the moment the customer test-drives the car - the weather resistance of exterior parts is, in essence, brand image.

Automotive exterior plastic aging comparison: glossy new part vs. powdered and cracked aged part — Typical signs of exterior-part aging: surface powdering and graying, gloss loss, stress cracking, reduced impact strength

1. Exterior-part materials and failure modes

Part	Common materials	Main aging issues
Front/rear bumpers, lower sill protectors	PP, PP+EPDM (TPO)	UV causes powdering, gloss loss, color fade; brittle at low-temperature impact; gravel damage
Front grille, air intake	PP, ABS, PC/ABS	the above plus salt spray, asphalt and exhaust pollution; often paint-free black parts
Exterior mirror housings, wheel arches	PP, TPO	UV powdering; often black or paint-free parts
Roof racks, spoilers	PC/ABS, PPA, PPS	High roof-surface temperatures in summer, UV + high temperature + vibrationcompound
Spare-tire cover	PP, EPDM	UV + ozone (EPDM systems additionally need an antiozonant)
Body weatherstrips	EPDM, TPV, SEBS	Ozone + UV + thermal cycling, leading to cracking and compression set

Why PP is the hardest exterior-part material

The PP main chain contains tertiary C-H bonds, which readily lose hydrogen under UV or thermal excitation, triggering a self-catalyzing free-radical chain reaction. Once the chain reaction starts, the material jumps from slight discoloration to powdering + a sharp drop in impact strength within a short time. Unstabilized PP in strong sunlight can lose up to 70% of its strength within days, while a complete stabilization system extends this to years. The difference lies not in "which one was added", but in whether the system is formulated as a matched set.

2. Tiangang's recommended solutions

2.1 Staged interception: polymeric base, low-molecular-weight surface touch-up

The proven approach for exterior parts is a four-layer combination of "polymeric HALS + low-molecular-weight HALS + UV absorber + antioxidant":HS-944 / HS-950 Polymeric products of this type have high molecular weight, resist extraction and resist blooming, providing the base weather resistance over long cycles; HS-770 and other low-molecular-weight products migrate faster, building effective concentration at the part surface - particularly suited to thin-wall parts, fibers and tapes; the two combined are what the industry calls "staged protection".

Protection layer	Recommended product	Function	Typical loading
Polymeric base	HS-944 / HS-950	Long-cycle base weather resistance, low volatility and migration	0.2–0.5%
Low-molecular-weight surface protection	HS-770 / HS-622	Surface-migration protection, marked effect on thin-wall parts	0.1–0.3%
UV front-end interception	UV-326 / UV-328 / UV-360	Preferentially absorbs 290-400 nm UV	0.2–0.4%
Thermo-oxidative protection	AO-1010 + AO-168 or AO-B215	Protects the process and long-term thermal stability	0.1–0.3%
Paint-free black parts	Carbon black / carbon-black masterbatch (used with the above system)	The dark part itself resists UV; HALS suppresses carbon-black bloom	Carbon black about 2%

Special note on paint-free black parts

Carbon black significantly improves the UV resistance of black PP, but carbon black readily blooms at the surface ("black specks", "whitening"), requiring a polymeric HALS base to encapsulate and suppress the bloom. Tiangang's MB0607-1 / MB0607-2 light-stabilizer functional masterbatches serve as a matched solution at the masterbatch end for black parts, reducing the dust and dispersion differences of downstream direct powder addition.

2.3 A few exterior parts also have flame-retardancy requirements

Conventional exterior parts are not required to be flame retardant, but the following scenarios may be involved: Roof-load-bearing parts such as roof racks and spoilers (some OEMs manage these to interior standards), Spare-tire covers and battery-pack outer guards (underbody guards on NEV models often must meet a flame-retardancy rating), and Commercial and special-duty vehiclesdecorative parts.

Where both weather resistance and flame retardancy are needed here, the FR-810A polymeric halogen-free flame retardant combines flame retardancy, light stabilization and long-term thermal stability in one product, reaching flame-retardancy ratings at low loadings (0.2-1.2%) and avoiding the dilemma of "acidic flame-retardant by-products neutralizing conventional HALS and causing premature weather-resistance failure". More on flame retardancy: ③ Wiring, Electrical & Flame Retardancy.

2.2 Ready to use out of the bag: the T-78 automotive-modified dedicated light-stabilizer package

If a customer prefers not to compound the four additives themselves, Tiangang offers the T-78 automotive-modified dedicated light-stabilizer package. Its design logic: light stabilization and antioxidation in synergy, staged interception of aging, dual protection in processing and use, all pre-blended. Compared with customers compounding in-house, the package approach reduces batch deviation in compounding ratios, reduces incoming-inspection items and lowers compounding dust - of practical significance in an automotive supply chain that demands batch consistency and 4M change management.

3. Validation recommendations

For aging validation of exterior parts, the industry commonly uses the following combination (specifics per the OEM's corporate standard):

Validation item	Standard	Metrics of interest
Exterior xenon-arc aging	SAE J2527 (formerly SAE J1960)	Includes front/back spray, black-panel temperature about 70°C; focus on shade difference ΔE and impact-strength retention
General xenon-arc	ISO 4892-2 / GB/T 16422.2	Method A outdoor conditions; for cross-material comparison screening
Natural-exposure control	ISO 877 / Florida outdoor exposure	Serves as the baseline reference for accelerated tests
Post-aging assessment	OEM corporate standard	Shade difference ΔE ≤2 (some ≤3); 50% elongation-at-break retention ≥50%; impact-strength retention ≥70%; gloss retention ≥80%

Tiangang can assist with formulation screening and aging testing - please contact our technical team. To look up formulations quickly by part, see ⑤ Selection Guide & Product Matrix.

Last Updated: 2026-10-20

Certifications

National High-Tech Enterprise
Vice Chairman Unit, China Plastics Processing Industry Association (CPPIA)
Vice Chairman Unit, Agricultural Plastics Branch, China Agricultural Plastics Application Technology Society
Member, Ageing Methods Subcommittee, National Technical Committee on Plastics (SAC/TC15/CS5)
Member, Society of the Plastics Industry (SPI), USA

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