A PP COA might list “1500 ppm phenolic AO + 1000 ppm phosphite + 500 ppm slip” and look like a complete additive package. Whether those numbers are correct depends on whether the grade is heading into a hot-water pipe, a UV-exposed woven bag, or a clear yogurt cup.
PP is chemically vulnerable in a way most thermoplastics are not. The tertiary carbon on every repeat unit is the preferred site for radical oxidation. A stripped-down PP resin embrittles within months of melt processing and faster outdoors.
Why PP Needs Additives in the First Place
Polypropylene without stabilizers loses molecular weight in hours under processing-temperature shear. The tertiary C-H bond on each propylene repeat unit dissociates roughly 80 kJ/mol weaker than a polyethylene secondary C-H. PP forms hydroperoxides much faster during extrusion and injection molding.
Chain scission follows, MFI drifts upward, and the part either cracks in service or fails an Izod requalification six months in.
Three vulnerabilities drive the package design. Thermal-oxidative attack during melt processing (220-280 °C) is handled by an antioxidant pair.
UV photo-oxidation in outdoor service is handled by HALS or UV absorbers. Surface and processing behavior — slip, antistat, clarity — is handled by smaller-volume functional additives.
Antioxidant Additives: Phenolic Primary plus Phosphite Secondary
The default PP antioxidant system is a phenolic primary plus a phosphite secondary, run at 1000-2500 ppm total (0.1-0.25 wt%) for general-purpose grades. Irganox 1010 is the most common phenolic. Irgafos 168 is the most common phosphite.
The two work at different points in the oxidation chain. Irgafos 168 preferentially decomposes hydroperoxides during processing. Irganox 1010 captures free radicals long-term in service and governs how the resin ages in storage.
Real-world ratios skew much further toward phosphite than textbook examples suggest. A measured PP film sample analyzed in ACS Omega in 2020 showed Irganox 1010 at 220 wppm and Irgafos 168 at 1580 wppm.
That roughly 7× phosphite-to-phenolic ratio reflects how much processing-protection load Irgafos carries on high-shear extrusion lines. For long-term thermal stability alone, Irganox 1010 in the 500-1000 ppm window is typical.
UV Stabilizers and HALS for Outdoor Service
PP that sees sunlight — woven bags, agricultural film, garden furniture, automotive exterior trim — needs a hindered amine light stabilizer (HALS) at 0.1-0.5 wt% (1000-5000 ppm). Chimassorb 944 is the most common HALS for general outdoor PP.
Tinuvin 622 at 0.3% is a common alternative in fiber. Severe UV exposure pushes loading to 2.0 wt%.
HALS works differently from the antioxidants above. A UV absorber dissipates incident UV as heat and depletes as it works.
HALS traps free radicals through a nitroxyl-radical regeneration cycle, so the stabilizer cycles rather than being consumed. One-time shield vs self-healing armor.
USPTO patent 8721946 documents automotive interior PP at 1000-2500 ppm HALS on top of 400-2400 ppm phenolic AO plus 500-1500 ppm phosphorus AO. That HALS top-up over the producer baseline is what most outdoor and automotive grades look like.
Slip and Antistat Agents
Two surface-acting families share the same bloom mechanism but different jobs — slip controls film friction; antistat dissipates charge on molded parts.
Slip agents
Slip agents lower the coefficient of friction on PP film so wound rolls unwind cleanly. Erucamide and oleamide are the standard fatty amides at 500-2000 ppm.
The trade-off is bloom-rate control. Too low and the COF stays high. Too high and you get plate-out on the chill roll.
Erucamide is also one of the fastest-degrading additives in PP processing, alongside Irgafos 168. Akoueson et al. in Science of the Total Environment (2023) and ACS Omega (2020) both measured these two as the highest-degradation-rate species under typical extrusion conditions.
Push the extruder 20 °C hotter than the data-sheet target and the in-part erucamide concentration after processing can already sit well below the nominal. The 1500 ppm spec assumes a controlled process window, not infinite headroom.
Antistat agents
Antistat agents — typically glycerol monostearate (GMS) or ethoxylated amines at 500-2000 ppm — manage static charge on injection-molded parts and fibers. The mechanism is similar to slip in that the additive blooms to the surface. Antistat is omitted in food-contact PP where migration is regulated.
Clarifier and Nucleator Additives for Clear PP
For clear PP cups, jars, and thin-wall packaging, sorbitol-based clarifiers like Millad NX 8000 and Millad 3988 run at 0.2-0.25 wt% (2000-2500 ppm). Organophosphate nucleators like Milliken HPN-68 and NJStar NU-100 run lower at 0.1-0.15 wt%.
Nucleators do a different job. They raise crystallization temperature without targeting transparency, which speeds injection cycle time.
New-generation nucleators are roughly 3× more efficient than legacy sodium benzoate. Milliken documented a 39.8% cooling-time reduction with HPN-68 in injection molding.
Less-Common PP Additives: Flame Retardants, CR Peroxide, Color Masterbatch
Three families round out the typical PP additive package, with much wider loading ranges.
- Flame retardants — Brominated systems like BDDP with antimony trioxide synergist run at 5-10 wt% to hit UL 94 V-0 or V-2. Halogen-free mineral systems (ATH, Mg(OH)₂) need 15-30 wt% for the same rating, and at those loadings mechanical properties change measurably.
- Controlled-rheology peroxide — DHBP (Luperox 101) at 200-600 ppm at the extruder visbreaks the resin to raise MFI and narrow molecular-weight distribution. Producer-side chemistry, generating the “CR” grade variants in supplier catalogs.
- Color masterbatch — Letdown ratio 1-4 wt% depending on pigment loading in the carrier. Color arrives as a separate pellet stream fed at the throat; the base-resin COA additive note does not include it.
The Producer Baseline: What Additives Already Ship in PP Pellets
The most useful mental model for reading a PP additive note on a COA is the two-layer one: producer baseline plus compounder top-up. State-owned Chinese producers — Sinopec, PetroChina (including the Dushanzi complex), and CNOOC — ship general-purpose PP grades with a baseline antioxidant and slip package added at the polymerization plant.
A typical baseline for a homopolymer like PetroChina T30S or a random copolymer like PetroChina Dushanzi T4401 lands at 800-1500 ppm phenolic AO, 800-1500 ppm phosphite, and 500-1500 ppm slip. The compounder layers more on top depending on end-use.
For outdoor: 1000-5000 ppm of HALS, often 2-5× the baseline stabilizer total. For clear injection: 2000-2500 ppm of sorbitol clarifier. For automotive interior: the HALS + slip package above.
The COA additive note describes the producer baseline. The TDS for a compounded grade describes the full package.
What COA Readers Most Often Misjudge
The mistake I see most often is treating the producer’s baseline phenolic + phosphite + slip note as the full additive package, then specifying an outdoor or automotive part against it. That baseline is correct for a generic injection-molded indoor part. It is not correct for anything that sees UV, sustained heat, or a slip-critical film line.
Read the COA additive note for what it is — the producer baseline. Ask the compounder for the full TDS, including the HALS, UV absorber, clarifier, or specialty top-up that brings the package to the end-use spec. A loading that is normal for an injection cup is wrong for a woven bag in Riyadh sunlight.