Polypropylene degrades in sunlight considerably faster than polyethylene. That is not a quality problem – it is the chemistry of the polymer, and if you make raffia tape, woven sacks or FIBCs you are working against it every day.
Why PP is the harder polymer to stabilise
Polypropylene has a methyl group on every alternate carbon, which creates a tertiary carbon atom along the backbone. The hydrogen attached to a tertiary carbon is easier to abstract than the hydrogens on polyethylene’s simpler chain.
UV light abstracts that hydrogen, a free radical forms, oxygen attacks it, and the chain scission cascade begins. Because the vulnerable site repeats all along the molecule, PP oxidises faster and embrittles sooner than PE under the same exposure.
Two things make it worse in raffia specifically:
Tape is thin and highly oriented. A stretched tape has its molecules aligned along the length. Chain scission in an oriented tape translates almost directly into lost tensile strength, because the load path is the molecular chain. A moulded part can lose some molecular weight and still function; a tape cannot.
Surface area is enormous. A woven fabric presents far more surface per kilogram than a solid section. Photo-oxidation is a surface-driven process. More surface, faster degradation.
Which is why an unstabilised PP sack left outdoors can lose most of its strength in a matter of weeks, while it still looks perfectly serviceable.
The specification that actually protects you
The failure mode for a sack is loss of tensile strength, not appearance. A bag that looks fine and tears when lifted is a worse commercial outcome than one that visibly chalked.
So specify against retained strength:
“Retains not less than 50% of original tape tensile strength after X kLy of exposure”
Some buyers use 6 months or 12 months outdoor. That is workable only if the geography is also stated, because radiation load varies widely across India. kLy is the honest unit.
For FIBCs there is an extra layer: bulk bags are rated with a safety factor – commonly 5:1 for single-trip and 6:1 for multi-trip – against safe working load. That safety factor has to survive the UV exposure too. A bag that met 6:1 when it left your factory and sits on a dockside for four months may not meet it when it is filled. If your customer stores bags outdoors, the UV specification is a safety specification, not a cosmetic one.
Typical dosage
UV masterbatch addition for raffia commonly falls between 1% and 3%, but as always the number that matters is the active stabiliser concentration, not the masterbatch percentage.
| Application | Typical addition | Notes |
| Woven sacks, indoor storage, short life | 0.5 – 1% | Often only a processing stabiliser is needed |
| Woven sacks, 6 months outdoor | 1 – 2% | HALS-based |
| FIBC / jumbo bags, extended outdoor | 2 – 3% | HALS-based, retained-strength criterion |
| Shade net, agro-textile, long life | 3 – 5%+ | Often NOR-HALS |
Suppliers quote in different ways – percent masterbatch, percent active, or ppm of stabiliser. Convert everything to one basis before you compare. Our let-down ratio calculator will do the conversion.
The titanium dioxide trap in white sacks
This one catches a lot of people, and it is entirely avoidable.
Titanium dioxide comes in two crystal forms. Rutile is the stable one. Anatase is photocatalytic – under UV it generates free radicals at the pigment surface, which is precisely the reaction you are paying HALS to suppress.
Put anatase TiO₂ into a PP sack and you have added an accelerant. The sack degrades faster than the natural, unpigmented version, and the customer quite reasonably concludes your white grade is defective.
Always specify rutile grade titanium dioxide, surface-treated with alumina and silica, for any product with outdoor exposure. Anatase is cheaper. That is why it appears.
There is a second, subtler version of the same problem: even rutile TiO₂ at high loadings interacts with some stabiliser systems and adsorbs stabiliser onto the pigment surface. White outdoor products generally need a somewhat higher UV package than natural ones at the same life, not the same.
We cover this in more detail in titanium dioxide in plastics: rutile versus anatase.
Filler, and what it does to your UV performance
Calcium carbonate filler is standard practice in raffia – it reduces cost and can improve tape stiffness and printability.
It also dilutes your stabiliser. Add 15% filler and, unless you compensate, you have 15% less polymer carrying the same UV package across the same surface area. On top of that, some filler grades carry moisture and trace metal impurities, and transition metal ions – iron and copper in particular – catalyse polymer oxidation.
If you increase filler loading, revisit the UV dosage in the same conversation. Changing one and not the other is a common route to an unexplained field failure.
What to send your masterbatch supplier
- Polymer grade and MFI
- Tape denier, thickness and stretch ratio
- Required life, stated in kLy with the region, or in months with the region
- The retained-property criterion – normally ≥50% tensile
- Filler type and loading
- Pigment, and confirmation of rutile if white
- Whether the product is an FIBC, and its safety factor
- Any chemical exposure – fertiliser, sulphur, cement
Then run an accelerated weathering test on the actual tape, not on a moulded plaque. A plaque tells you very little about how an oriented tape will behave.
FAQs
Why does polypropylene degrade faster than polyethylene in sunlight? PP has a tertiary carbon on every alternate backbone atom, and the hydrogen on a tertiary carbon is easier for UV to abstract. That starts the oxidation cascade sooner. Thin, highly oriented tape and high fabric surface area make it worse.
How much UV masterbatch do PP woven sacks need? Commonly 1% to 3%, depending on required life, filler loading and pigmentation. FIBCs with extended outdoor storage sit at the top of that range or above it.
Why do my white sacks fail faster than natural ones? Almost always anatase titanium dioxide. Anatase is photocatalytic and actively generates the radicals that break the polymer. Specify rutile grade, surface treated, for anything with outdoor exposure.
Does calcium carbonate filler affect UV life? Yes. Filler dilutes the stabiliser package and some grades introduce moisture and transition metal impurities that catalyse oxidation. Increase the UV dosage when you increase filler.
How should a UV guarantee for woven sacks be written? As retained tensile strength after a stated radiation exposure – for example, not less than 50% of original tape tensile after a given kLy. A guarantee in months without a location is not measurable.
Do FIBC safety factors need to account for UV? They should. A 6:1 bag that has sat in the sun for months may no longer meet 6:1 when filled. If bags are stored outdoors, treat the UV specification as part of the safety case.