Polypropylene vs Nylon: UV Resistance, Weathering Tests and Grade-Level Selection

Polypropylene and nylon both fail acceptance after 2000 hours of ISO 4892-3 UVA-340 exposure when run without stabilizers; both pass the same test with 0.30 wt% HALS and 0.20 wt% benzotriazole added. That outcome shifts the engineering question from “which polymer is more UV resistant” to “which grade, which additive dose, and which test cycle match the service environment.” This comparison walks through the free-radical photodegradation mechanism, the PA6 vs PA66 vs PA12 grade split, ISO 4892-2 and ISO 4892-3 method selection, moisture conditioning for nylon, and a validation workflow tied to EPOC CRAFTER shop-floor QUV data on 2 mm specimens molded through our rapid injection molding services.

1. UV Resistance of Polypropylene vs Nylon: Why Neither Is Naturally UV-Safe

Polypropylene vs nylon in the base, unmodified form is not a contest between a UV-safe polymer and a UV-vulnerable one. Both polymers absorb UV in the 290-370 nm band that reaches the Earth’s surface, and both lose gloss, color, and elongation at break faster than most outdoor acceptance criteria allow. The field performance gap comes from the stabilizer package, color, wall thickness, and the test cycle the formulation has been validated against.

Three guidelines hold across most outdoor plastic selection decisions:

• For dark-colored, long-service parts, stabilized polypropylene with 2.0 to 2.5 wt% carbon black and a HALS package matches nylon on service life. Carbon black blocks the UV photon before it reaches the polymer rather than slowing propagation afterwards.

• For strength retention under continuous UV, nylon 6 and nylon 12 hold more backbone intact than polypropylene in base form. For color stability as ΔE, the ranking reverses: amide photolysis produces more yellow chromophores per unit absorbed energy than carbonyl formation in PP.

• For intermittent, filtered, or indoor UV, the gap between polypropylene and nylon narrows, and moisture pickup, lubricity, and cost per kg usually drive the final choice.

Service condition	Preferred base polymer	Required stabilization	Validate with
Outdoor, continuous sun, dark color acceptable	PP homopolymer	Carbon black 2.0-2.5 wt% + HALS 0.1-0.3 wt%	ISO 4892-2 Method A Type I, 2000+ h
Outdoor, continuous sun, natural / light color	PA12	HALS 0.2-0.5 wt% + benzotriazole 0.2-0.3 wt% + TiO2 1-3 wt%	ISO 4892-3 Cycle 1 UVA-340, 2000+ h
Outdoor, moisture cycling, strength critical	PA6	HALS 0.3 wt% + benzotriazole 0.2 wt%	ISO 4892-3 Cycle 1 after ISO 1110 moisture conditioning
Behind window glass or filtered UV	PP copolymer or PA6	HALS 0.1-0.2 wt%; UV absorber not usually needed	ISO 4892-2 Method B, Type I filter
Indoor, UV from fluorescent lighting only	Standard grades	Minimal package or none	500-1000 h screening

1.1 UV-Resistant vs UV-Stabilized Plastic: The Difference That Matters

The industry splits UV-stable polymers into two groups. UV-resistant plastic refers to polymers whose chemistry absorbs or deflects UV without added protection: fluoropolymers (PTFE, PVDF), acrylic (PMMA), and polycarbonate with aromatic ring absorption. UV-stabilized plastic refers to polymers that need an additive package to survive outdoor exposure. Polypropylene and nylon both sit in the second group.

A drawing callout that reads “UV resistant” without an additive code or test method is ambiguous. For a procurement specification, the correct phrasing is the stabilizer code plus the acceptance criterion, for example: “PA 66-GF30 L1, acceptance per ISO 4892-3 Cycle 1 UVA-340, 2000 h, tensile retention ≥ 80%, ΔE ≤ 6.” Without those fields, two suppliers can quote a part that passes incoming inspection and still fails in year one of outdoor service.

2. How UV Damages Plastic: Free-Radical Photodegradation Mechanism

UV degradation follows a two-stage free-radical pattern in both polypropylene and nylon. Stage one is initiation: a UV photon with enough energy breaks a vulnerable chemical bond on the polymer backbone. Stage two is propagation: the free radical reacts with atmospheric oxygen within microseconds, forming peroxides that decompose into carbonyl groups, chain breaks, and more radicals. The chain continues until a stabilizer intercepts a radical or the polymer runs out of accessible bonds.

The polymer family decides which bond breaks first. Polypropylene’s tertiary C-H bond on every repeat unit is weaker than the primary and secondary C-H bonds around it, so the tertiary site is the initiation point for photooxidation. Nylon’s amide bond (CO-NH) absorbs light in a narrower band but cleaves directly under UV, producing carbonyl end groups and amine radicals. In both cases, the visible result is yellowing, surface chalking, and loss of elongation at break long before the tensile strength drops enough to be obvious to the eye.

2.1 Visible Symptoms: Chalking, Yellowing, Fading and Embrittlement

Field failure of an outdoor plastic part arrives in a predictable order. Color shift is usually first: ΔE climbs above the acceptance threshold within the first 500 to 1000 lab hours of accelerated weathering on an unstabilized sample. Gloss retention drops next as surface chemistry roughens the molded skin. Chalking appears as a dusty white film when oxidized polymer fragments slough off the surface. Mechanical embrittlement arrives last: elongation at break collapses to under 10% while tensile strength may still read 50 to 60% of baseline. Cracking under thermal cycling or handling closes out the failure sequence.

The order matters for acceptance criteria. A dashboard trim part or a cosmetic enclosure fails on ΔE and chalking before mechanical criteria would catch it. A load-bearing clip or snap-fit fails on elongation at break first. The same formulation can pass one acceptance test and fail the other on the same specimen, which is why plastic yellowing and tensile retention both belong in the drawing when UV is a design factor.

3. Polypropylene UV Resistance: Spectra, Weathering and Outdoor Behavior

3.1 Polypropylene Spectra Maxima: 290-300, 330, 370 nm

Polypropylene absorbs UV with reported peaks near 290, 330, and 370 nm. The 290 nm peak overlaps the solar UV cutoff at the Earth’s surface and drives most of the field degradation. Photons at these wavelengths carry enough energy to abstract the tertiary hydrogen on the polymer backbone, forming an alkyl radical that reacts with atmospheric oxygen within microseconds. The resulting peroxide decomposes into carbonyl groups, chain breaks, and more radicals.

Industry references cite roughly 70% tensile strength loss within a few days of continuous high-UV outdoor exposure for unstabilized PP film and thin sheet. EPOC CRAFTER shop-floor QUV data in Section 8 confirms the direction at accelerated-lab timescales: an unstabilized PP 2 mm specimen lost 51% of its tensile strength and 97% of its elongation at break after 2000 hours of UVA-340 Cycle 1 exposure. Fluoropolymers sit outside this discussion because the C-F bond is one of the strongest single bonds in organic chemistry; EPOC CRAFTER’s

PTFE machining and tolerances reference covers that chemistry in depth.

3.2 Polypropylene Outdoor Use: Furniture, Enclosures and Sheet

Stabilized polypropylene handles three outdoor application families. Polypropylene outdoor furniture (stackable chairs, garden tables, planter pots) runs on carbon black 2.0 to 2.5 wt% with 0.1 wt% HALS and holds mechanical properties through 5+ summers in temperate climates. Outdoor plastic enclosures for irrigation controllers, electrical junction boxes, and valve housings use the same carbon black plus HALS package but add 0.2 to 0.3 wt% benzotriazole when wall thickness drops under 2 mm, where UV absorption length is too short for the blocker to work alone. UV resistant plastic sheet for signage, roofing, or construction protection uses either carbon black (opaque, black) or TiO2 at 2 to 3 wt% (opaque, white) depending on color requirements.

Wall thickness, gate location, and sink geometry change the effective UV path length and additive concentration gradient across the molded part. Our plastic part DFM guidelines cover rib, boss, and wall-transition rules that apply equally to PP and nylon housings intended for outdoor use.

4. Nylon UV Resistance: PA6 vs PA66 vs PA12 and Moisture Effects

Nylon degrades at the amide bond (CO-NH), which absorbs light in the 290 to 315 nm band. The photolysis cleaves the C-N bond, producing carbonyl end groups and amine radicals that give the characteristic yellow discoloration. Nylon as a material family hides a wider spread of UV behavior than most selection guides show, and the ISO 16396-1:2022 designation system encodes it directly.

Grade	Repeat unit length	Moisture at 23 °C / 50% RH (wt%)	Relative UV tolerance (base form)	Typical outdoor use with stabilization
PA6	6 carbons	2.7 – 3.0	Moderate	Non-structural covers, cable ties, electrical housings
PA66	6 + 6 carbons	2.5 – 2.8	Lower than PA6	High-temperature parts behind heat barriers, under-hood clips
PA12	12 carbons	0.7 – 1.1	Highest of the three	Automotive tubing, fuel line clips, pneumatic connectors
PA6/66 copolymer	mixed	2.0 – 2.5	Between PA6 and PA66	Mid-range industrial parts
PA MXD6 (aromatic)	mixed	0.3 – 0.5	Higher than PA6 but discolors fast	Barrier applications, rare outdoors

4.1 Nylon 6, Nylon 66 and Nylon 12 Grade Differences Under UV

Nylon 6 keeps more backbone flexibility after exposure than nylon 66 because the lower density of hydrogen bonds between chains lets the matrix absorb a crack-opening event without full embrittlement. Nylon 12 keeps the most backbone of the three because the longer aliphatic segment between amide groups means fewer initiation sites per unit volume, and the lower moisture pickup (0.7 to 1.1 wt%) means the glass transition does not shift as much under humid-UV cycling.

ISO 16396-1:2022 encodes the stabilizer status in the data block. A designation PA 66-GF37 L1 means PA66 base, 37% glass fiber, UV stabilized only. L2 adds heat and light stabilization. W adds hydrolysis stabilization. H adds heat aging. Reading the material code on an incoming TDS or acceptance report confirms whether the material has UV protection at all, and if so, what else it has been designed against. A PA 6-GF30 L and a PA 6-GF30 H look identical on the pellet bag; the field performance under combined UV and heat differs by 2 to 4 times.

4.2 Nylon Yellowing and Color Fade Under Sunlight

When color change is the acceptance criterion, unstabilized nylon 6 degrades faster than unstabilized polypropylene on ΔE. EPOC CRAFTER shop-floor data in Section 8 shows ΔE 17.0 for unstabilized nylon 6 against ΔE 13.2 for unstabilized PP at the same 2000 h ISO 4892-3 UVA-340 exposure. The common claim that “nylon is more UV resistant than PP” refers to mechanical property retention, not optical property retention. On tensile strength the ranking inverts in the same test: nylon 6 held 56% versus polypropylene’s 49%.

The practical rule for drawings that specify light-color nylon outdoors: use PA12 with TiO2 1 to 3 wt% plus HALS 0.3 wt% plus benzotriazole 0.3 wt%, and verify ΔE at 2000 h on the actual molded wall thickness, not on a 4 mm test bar. The engineering material properties guide provides a broader reference for comparing polymer properties, while grade-specific moisture absorption and thermal data should be verified against the selected nylon supplier’s technical data sheet.

4.3 Moisture Conditioning: The Step That Makes Nylon UV Data Trustworthy

Nylon test coupons straight from a drying oven show tensile strength and modulus 20 to 40% above the same part at equilibrium moisture content. For UV testing, the failure mode you measure depends on which moisture state the coupon was in. Running UV on dry nylon measures the polymer response in a condition the field part will never see; running UV on moisture-conditioned nylon measures what the service environment will actually produce.

Three conditioning routes are standard for nylon QUV baseline:

• Dry as molded. Specimen comes out of injection, is bagged, and tested without conditioning. Gives the highest tensile numbers. Use for QC batch comparison only, not for outdoor life data.

• ISO 291 standard atmosphere (23 °C / 50% RH for 168 hours). Brings most nylon grades to roughly 1.5% moisture content for PA6 and PA66 and 0.5% for PA12. The default for engineering data and most QUV baseline conditioning.

• ISO 1110 accelerated conditioning (immersion or 72 °C / 62% RH exposure). Pushes PA6 and PA66 to 2.5 to 3.0% moisture (near equilibrium at 50% RH) in days rather than weeks. The right choice when the service environment is humid and the acceptance criterion sits at equilibrium-state properties.

A nylon part that passes QUV in dry-as-molded condition but fails in service after one humid season is a predictable failure mode. For UV acceptance testing on PA parts destined for outdoor service in humid regions, pre-condition per ISO 1110 before QUV exposure.

5. UV Stabilizers, Absorbers and Blockers for PP and Nylon

A UV additive package for PP or nylon is almost never a single ingredient. Three functions handle the degradation sequence: blocking the photon before it hits the polymer, absorbing the photon if it does, and quenching the radical if the photon still breaks a bond. Three different chemistries cover those three functions, each with its own dose window. Mixing them is cheaper and more effective than loading one type to compensate for the absence of the other two.

5.1 HALS Hindered Amine Light Stabilizers

A hindered amine light stabilizer does not absorb UV. It works in the propagation phase by quenching the alkyl and peroxy radicals produced after a bond has already broken. The nitroxyl species regenerates after it reacts, which is why 0.1 to 0.5 wt% is enough for several thousand hours of QUV exposure. Dose windows: PP 0.1 to 0.3 wt% for most outdoor applications; nylon 0.2 to 0.5 wt% because the amide radical environment regenerates fewer stabilizing cycles than the polyolefin chain. Low-molecular-weight HALS migrates out of thin walls over time, so for parts under 1.5 mm wall thickness, pick a high-molecular-weight HALS or an oligomeric HALS.

5.2 UV Absorbers: Benzotriazoles and Benzophenones

Benzotriazoles and benzophenones absorb the UV photon and dissipate the energy as heat before the polymer sees it. The typical dose window is 0.2 to 0.5 wt% for both chemistries. Benzotriazoles cover a broader absorption band (280 to 385 nm) than benzophenones (280 to 340 nm), which is why benzotriazoles are the default for outdoor use. Both have one application rule: they need enough path length to work. In a part under 0.5 mm wall thickness, the UV absorber lacks the polymer thickness to absorb the photon, and HALS plus a pigment is a better choice.

5.3 UV Blockers: Carbon Black and Titanium Dioxide

Carbon black at 2.0 to 2.5 wt% is the most cost-effective UV blocker for polyolefins and nylon alike. It absorbs essentially all UV before the photon reaches the polymer, which is why a well-dispersed carbon black package gives 20+ year outdoor life to a pipe or geotextile without any HALS added. Two constraints apply: the part has to be black, and the carbon has to be well-dispersed (channel grade or fine furnace black, not the first grade on the price list). Titanium dioxide at 1 to 3 wt% is the equivalent for white and light-colored parts, with the surface-treated rutile grade preferred over sulfate anatase because anatase itself is a photocatalyst that accelerates degradation around it.

Target outdoor life	Color	Recommended package	Added cost vs base resin
5+ years outdoor	Black	Carbon black 2.0-2.5 wt% + HALS 0.1 wt%	3-4%
5+ years outdoor	Light / natural	TiO2 1-3 wt% + HALS 0.3-0.5 wt% + benzotriazole 0.3 wt%	6-10%
2-5 years outdoor	Any	HALS 0.2 wt% + benzotriazole 0.2 wt% + optional pigment	3-6%
Under 2 years, intermittent sun	Any	HALS 0.1-0.2 wt% + pigment	1-3%
Behind window glass	Any	HALS 0.1 wt%; UV absorber usually not needed	1-2%

Picking a stabilizer package is a specification activity, not a procurement one. A molder runs any resin they are given; the acceptance ΔE at 2000 hours or the elongation retention at 5 years was decided when the compounder measured HALS wt%. If outdoor life is specified on the drawing, the stabilizer code and additive content should be documented alongside the applicable acceptance requirements, with dimensional controls defined using relevant engineering tolerances and standards.

6. Weathering Test Standards for PP vs Nylon UV Verification

An accelerated weathering test gives a conservative, repeatable estimate of how a part behaves in a service environment that would take years to replicate outdoors. UV source choice, filter choice, chamber conditions, and condensation cycle each shift the acceleration factor independently. The two dominant international standards are ISO 4892-2 (xenon arc test) and ISO 4892-3 (fluorescent UV, usually called QUV test after the chamber brand name). The US equivalents are ASTM G155 and ASTM G154 respectively. The older term weatherometer still appears on drawings and refers to the xenon arc test on ISO 4892-2 Method A conditions.

6.1 ISO 4892-2 Xenon-Arc vs ASTM G155

Xenon-arc testing uses a xenon lamp with a filter stack to shape the spectrum close to solar radiation at the Earth’s surface. ISO 4892-2:2013 plus Amendment 1:2021 distinguishes two methods: Method A uses a daylight filter to simulate direct outdoor sun; Method B uses a window-glass filter to simulate indirect, window-filtered UV. Amendment 1 added a split within Method A between Type I and Type II daylight filters based on the 20 nm UV cutoff band, with Type I representing the more aggressive summer-sun spectrum and Type II a milder average spectrum. Picking Method A Type I when the part sees direct equatorial sun gives a shorter test; picking Method A Type II for temperate-latitude direct sun gives data that correlates better. ASTM G155 is the US equivalent and uses the same xenon-arc approach with overlapping but not identical filter definitions.

Cycle 2 (dry) matches arid outdoor exposure; cycles 4 to 6 add water spray for humid outdoor or marine exposure. The black panel temperature (BPT) and black standard temperature (BST) are not interchangeable; a BPT 65 °C setpoint sits roughly 10 °C below the equivalent BST setpoint because of thermal mass differences, so check which the test report quotes.

6.2 ISO 4892-3 Fluorescent UV vs ASTM G154 and QUV

Fluorescent UV testing uses one of three lamp types defined in ISO 4892-3:2024: UVA-340 with peak at 340 nm, UVA-351 with peak at 351 nm, and UVB-313 with peak at 313 nm. UVA-340 is the standard choice for outdoor service correlation because its spectrum from 300 to 360 nm closely matches the solar UV cutoff. UVA-351 shifts the peak into the near-UV and gives less aggressive acceleration. UVB-313 is more aggressive than natural sunlight (it contains wavelengths shorter than those that reach Earth’s surface) and should stay reserved for QC screening or short-timescale degradation studies, not for outdoor service life prediction.

The three lamp types are not interchangeable; a 2000 h UVA-340 test and a 2000 h UVB-313 test on the same specimen give materially different ΔE and strength retention. ISO 4892-3 Table 4 lists seven cycle options combining light and condensation phases; Cycle 1 (8 h at 60 °C UV + 4 h at 50 °C condensation) is the most commonly cited for outdoor correlation. ASTM G154 is the US equivalent and uses the same fluorescent UV lamp families. The QUV chamber brand name is often used as shorthand for both standards; a QUV test report should still name the specific lamp and cycle.

6.3 Lab-Hours Do Not Equal Outdoor-Years

The conversion between lab QUV hours and outdoor service years is site-specific, geometry-specific, and failure-mode-specific. Industry rules of thumb like “1000 QUV hours ≈ 1 year outdoor” or “2000 hours ≈ 2 to 3 years in Arizona” are starting points for triage, not design acceptance. For a Phoenix rooftop part with 7 kWh/m²/day summer average and no shade, a 2000 h UVA-340 Cycle 1 exposure under-estimates the first-year outdoor dose. For a Northern European part on a building north face, the same 2000 h test over-estimates by a factor of 3 or more. The reliable use of lab data is to compare materials or stabilizer packages against each other on the same test setup, not to predict calendar life in isolation.

7. Choosing UV-Resistant Plastics for Outdoor Applications

The best plastic for outdoor use is not a single polymer; it is the base polymer plus the stabilizer code plus the test cycle it has been validated under. A dark polypropylene part with the right carbon black and HALS loading outlasts a nylon part with no stabilizer code on the drawing. A nylon 12 part with the right UV absorber package outlasts an un-coded PP in a light-colored application.

7.1 Outdoor Plastic Enclosures, Parts and Sheet

Three application families cover most outdoor plastic selection decisions. Outdoor plastic enclosures for electrical junction boxes, irrigation controllers, and small instrument housings carry the UV load across a 1.5 to 3 mm wall; the stabilizer package has to work in that path-length range. Outdoor plastic parts for mounting brackets, snap clips, and fasteners carry the load across section thicknesses as low as 0.5 mm, where UV absorbers lose effectiveness and HALS plus pigment is the only working route. UV resistant plastic sheet for signage, roofing, and barrier applications carries the load across 2 to 6 mm bulk thickness, where carbon black or TiO2 blocking is cost-effective and HALS can be reduced.

The three families map to different acceptance criteria. An enclosure fails on cosmetic ΔE and gloss first; a load-bearing clip fails on elongation at break; a sheet fails on surface chalking and dimensional stability. Consumer electronics plastic housings that see window-filtered UV through a storefront or vehicle windshield need ISO 4892-2 Method B and a lighter stabilizer package. Automotive exterior parts need ISO 4892-2 Method A Type I and the full stabilizer package; interior parts with UV plus heat aging need an H designator and HDT verification at the service temperature.

7.2 UV-Resistant Injection Molding: Grade Selection and Mold Design

UV-resistant injection molding adds four decisions to the normal molding workflow. First, grade selection locks the stabilizer code, which the molder cannot change after the RFQ. Second, pigment loading changes melt viscosity by 2 to 5% per weight percent of carbon black or TiO2 and shifts the fill pattern on thin-wall parts; the molding window has to be re-qualified after a color change. Third, mold design should avoid gate marks on A-faces because the gate vestige concentrates stress under thermal cycling and often becomes a yellowing or cracking initiation site after UV exposure. Fourth, wall thickness transitions above 0.5× the nominal wall concentrate pigment distribution and create visible ΔE differentials even in the same molded part.

For long-service outdoor parts, specifying the resin grade and the UV test cycle before the first prototype is cheaper than qualifying a stabilizer package after T1 shots. For automotive plastic parts that combine UV and heat aging on the same component, the stabilizer code must address both failure modes, not just the sunlight exposure.

8. EPOC CRAFTER Shop-Floor QUV Validation Data

Four 2 mm ISO 527-2 Type 5A specimens: Moplen HP500N (PP homopolymer) and Ultramid B3S (PA6), each in an unstabilized pair and a stabilized pair (HALS 0.30 wt% + benzotriazole 0.20 wt%). All ran through ISO 4892-3:2024 UVA-340 Cycle 1 at 0.76 W·m⁻²·nm⁻¹ at 340 nm, black panel 60 °C, 4 h condensation at 50 °C, 2000 h total exposure. Acceptance criteria for the application (outdoor non-structural covers): ΔE ≤ 6, gloss retention ≥ 70%, tensile retention ≥ 80%, no visible cracks, warp ≤ 1.0 mm.

Sample	Material + stabilizer	ΔE	Gloss ret.	Tensile ret.	Elong. ret.	Chalk / crack	Warp	Acceptance
S-01	PP, no stabilizer	13.2	21.8%	48.7%	3.3%	3 / 2	0.8 mm	FAIL
S-02	PP + HALS 0.30 + benzotriazole 0.20	3.4	82.3%	89.7%	65.4%	1 / 0	0.3 mm	PASS
S-03	PA6, no stabilizer	17.0	26.8%	55.7%	7.4%	3 / 2	1.1 mm	FAIL
S-04	PA6 + HALS 0.30 + benzotriazole 0.20	5.2	80.6%	91.0%	65.3%	1 / 0	0.4 mm	PASS

Intermediate readings at 500, 1000, and 1500 h on the unstabilized pair show the knee of the degradation curve between 1000 and 1500 h; the stabilized pair stays close to baseline across all intervals. The stabilized nylon 6 edges the stabilized polypropylene on tensile retention (91% vs 90%) and loses the ΔE comparison (5.2 vs 3.4). Both stabilized formulations clear every acceptance criterion.

Three observations worth repeating from this run:

• Both unstabilized materials failed every acceptance criterion at 2000 h. Specifying “PP” or “nylon” without a stabilizer code is not a UV-ready specification.

• Both stabilized materials passed every criterion at 2000 h. The 0.30 wt% HALS + 0.20 wt% benzotriazole package is a working baseline for 2 mm non-structural outdoor covers in both polymers.

• The ΔE ranking in the unstabilized state (PA6 17.0 vs PP 13.2) ran opposite to the tensile retention ranking (PA6 56% vs PP 49%). This is why the question “which polymer is more UV resistant” needs a failure-mode qualifier before it has an answer.

9. FAQ

9.1 Is Nylon UV Resistant?

Not in base form. Unmodified nylon absorbs UV at the amide bond in the 290 to 315 nm band and loses tensile strength and color within months of continuous direct sunlight. With a HALS + UV absorber package (typical dose 0.3 wt% HALS + 0.2 wt% benzotriazole) and the right PA grade (PA12 for best color stability, PA6 for strength retention), nylon reaches 5+ year outdoor service life. The drawing callout must specify the stabilizer code (ISO 16396-1 L1, L2, or H) and the acceptance test cycle, not just the polymer family.

9.2 Is Polypropylene UV Resistant?

Not in base form. Unstabilized polypropylene can lose roughly 70% of tensile strength within days of continuous high-UV outdoor exposure in thin-section form. With 2.0 to 2.5 wt% carbon black and 0.1 to 0.3 wt% HALS, polypropylene reaches 20+ year outdoor service life for black parts. For light-colored or natural polypropylene outdoor use, TiO2 1 to 3 wt% plus HALS 0.3 to 0.5 wt% plus benzotriazole 0.3 wt% is the baseline, with shorter service life than the black carbon-black version.

9.3 How Long Does Nylon or Polypropylene Last Outside?

Service life depends on color, wall thickness, stabilizer package, and climate. Unstabilized natural nylon or polypropylene cable ties fail (become brittle and crack under handling) within weeks to months of direct sun exposure. UV-stabilized black nylon or polypropylene with 2+ wt% carbon black lasts 5 to 20+ years in the same environment. Light-colored stabilized grades sit between those two, usually 2 to 7 years depending on the HALS and UV absorber dose. Verification needs ISO 4892 testing on the actual molded wall thickness and color.

9.4 How Can I Make Plastic UV Resistant?

Add a HALS stabilizer, a UV absorber, or a UV blocker to the formulation at the compounding stage, before pellets reach the molder. The three chemistries work on different parts of the degradation pathway: HALS quenches free radicals after a bond breaks, UV absorbers (benzotriazoles, benzophenones) absorb the photon before it hits the polymer, and UV blockers (carbon black, TiO2) prevent the photon from penetrating in the first place. A combined package covers all three fronts. Thin walls under 0.5 mm need HALS + pigment rather than UV absorbers because the path length is too short for absorber chemistry to work.

9.5 What Plastic Is Best for Outdoor Use?

For continuous direct sun on a dark part at 5+ year life, polypropylene with 2.0 to 2.5 wt% carbon black and HALS is the lower-cost choice. For a light-colored or natural part at the same life, PA12 with TiO2 and a full HALS + benzotriazole package is more reliable than stabilized PP. For parts combining UV with heat above 60 °C, nylon with an H designator (heat-stabilized) or a specialty grade is the right path. Fluoropolymers (PTFE, PVDF) and acrylic (PMMA) are naturally UV-resistant and skip the additive package, but material cost is 3 to 10 times higher than stabilized PP or nylon.

9.6 Does Carbon Black Actually Protect Polypropylene from UV, or Does It Just Hide the Yellowing?

Carbon black does both. The 2.0 to 2.5 wt% dose standard for outdoor polypropylene absorbs essentially all UV photons before they reach the polymer backbone, which is why black polyolefin pipe and geotextile fabric hold mechanical properties for 20+ years outdoors with no HALS added. Yellowing is irrelevant for a part that was already black, but the gloss retention, tensile retention, and chalking resistance of carbon-black polypropylene are all improved on top of the color masking. The constraint is dispersion. Poorly dispersed carbon black gives streaky parts and leaves local areas of unprotected polymer that fail years before the bulk.

Lock Grade, Stabilizer Code and Test Cycle Before Release

The polypropylene vs nylon question under UV is less about picking a winner and more about writing a specification that locks the grade, the stabilizer code, the conditioning, and the test cycle together. Put the ISO 16396-1 stabilizer code (L1 / L2 / H) and the ISO 4892 cycle on the drawing before release for quote. For prototyping and short runs, 500 to 1000 h QUV screening on three stabilizer variants costs less than a tool steel change at the mold, and the data decides the formulation before cutting any production tooling.