Key takeaways
- Raffia-grade PP is overwhelmingly homopolymer with a melt flow index of roughly 2–4 g/10 min (ASTM D1238 / ISO 1133, 230°C / 2.16 kg) — high enough to extrude a stable cast film, low enough to draw tape to high orientation without breaks.
- Tape strength is set by the draw ratio (typically 5:1 to 8:1), not just the resin; well-drawn homopolymer tape reaches tenacity in the region of 5–7 cN/dtex, and FIBC fabric is specified by tape denier, fabric weight (g/m2) and resulting tensile rather than by resin grade alone.
- UV stabilisation is a hard requirement for FIBC and any outdoor-stored sack — virgin PP without a HALS/UV package can lose much of its tensile strength after only a few months of direct sun, so buyers must specify a retained-strength threshold against a kLy dose backed by a masterbatch let-down.
- The most common raffia defects — fibrillation, tape breaks at the godet, fish-eyes and weak welds — trace back to wrong MFI, contamination or gels, moisture, and poor UV/additive dispersion, all of which are checkable on the CoA and a tape-tenacity test before a container ships.
Raffia is the highest-volume single application of polypropylene, and it is unforgiving. The product is a slit, drawn flat tape — extruded as a film, cut into ribbons, stretched five to eight times its length, then woven into fabric for woven sacks, FIBC (flexible intermediate bulk containers, the one-tonne bulk bags), tarpaulins and groundcovers. Every link in that chain depends on getting the resin specification right, because once the tape is drawn there is no recovering lost strength. A homopolymer at the wrong MFI, a missing UV package, or a few gels per kilo will not show up on the bale label — it shows up when a filled FIBC tears on a forklift.
This piece sets out what a buyer should actually put on the purchase order: the homopolymer grade and melt-flow window, how draw ratio and denier convert resin into tape strength, the UV stabilisation FIBC genuinely needs, and the defects that should fail a lot at inspection. For the broader resin-family context, our homopolymer vs copolymer PP primer is the companion read.
Raffia tape is bought for stiffness and tensile strength per unit weight, and those come from crystallinity. Polypropylene homopolymer — high isotacticity, no ethylene comonomer interrupting the chain — crystallises to a higher degree than random or impact copolymer, giving a stiffer, stronger, more drawable tape. Copolymers earn their place in film, injection moulding and pipe where impact, clarity or weldability matter, but the ethylene units that buy those properties also soften the matrix and limit how far the tape can be oriented. For raffia, that is the wrong trade. Specify homopolymer unless you have a deliberate reason — for example a sacrificial sealing layer or a speciality breathable fabric — not to.
The melt flow index is the next decision and the one buyers most often get loose about. A raffia homopolymer typically runs MFI 2–4 g/10 min (ASTM D1238 / ISO 1133, 230°C / 2.16 kg). The logic is mechanical: the resin has to form a stable, even cast or chill-roll film with enough melt strength to avoid neck-in and thickness variation, then survive high-ratio drawing in the oven without snapping. Push MFI above 4 and extrusion gets easier and faster, but melt strength and ultimate tape tenacity drop. Drop it well below 2 and you load the extruder, raise melt temperature and risk degradation. The 2–4 window is where most commercial raffia grades sit for good reason. If a supplier offers a 6–8 MFI 'raffia' grade, ask what tenacity it actually delivers at your draw ratio — see our melt flow index explainer for why MFI alone never tells the whole molecular-weight story.
Draw ratio builds the strength; the resin only decides how far you can draw before the tape breaks.
The single biggest lever on tape strength is the draw ratio — the factor by which the tape is stretched in the orientation oven, usually between 5:1 and 8:1. Drawing aligns the polymer chains along the tape axis, converting a weak, isotropic film into a highly oriented, high-tenacity ribbon. The resin sets the ceiling: a 2–4 MFI homopolymer can be drawn hard and stay intact, where a high-MFI grade fibrillates or breaks before it reaches useful orientation. So a buyer who specifies only 'PP raffia grade' has specified perhaps half of what determines the finished sack.
Tape is then characterised by linear density and tenacity. Denier is grams per 9,000 m of tape; dtex is grams per 10,000 m; the difference trips up a lot of procurement specs. Tenacity is breaking force per unit linear density — cN/dtex or g/denier — and it is the honest measure of tape quality because it normalises for tape weight. Well-drawn homopolymer raffia tape lands in the region of 5–7 cN/dtex. Fabric strength then comes from tape denier multiplied by the construction (ends and picks per inch) and is reported as fabric weight in g/m2 plus a strip tensile. The table below shows how these specs typically stack up across the main raffia products — treat the figures as indicative working ranges, not a quoted standard.
| Application | Resin | MFI (g/10 min) | Tape denier | Tape tenacity | Fabric weight | UV requirement |
|---|---|---|---|---|---|---|
| Woven sacks (cement, food, feed) | PP homopolymer | 2–4 | ~600–1000 | 5–6.5 cN/dtex | 60–110 g/m2 | Optional / low (indoor) |
| FIBC body fabric | PP homopolymer | 2–3.5 | ~900–1500 | 6–7 cN/dtex | 120–200 g/m2 | Required, stated in kLy |
| FIBC lifting loops | PP homopolymer / multifilament | 2–3 | high denier webbing | high — safety critical | webbing spec | Required |
| Tarpaulin / leno mesh | PP homopolymer | 3–4 | ~700–1200 | 5–6 cN/dtex | varies | Required (outdoor) |
Note that lifting loops on an FIBC are a separate, safety-critical conversation: they carry the bag's safe working load with a safety factor (commonly 5:1 for single-trip and 6:1 for multi-trip designs), and their webbing and stitching are governed by the bag standard rather than the body-fabric resin spec. Do not let a strong body fabric distract from weak loops.
Unstabilised polypropylene is poor in sunlight. UV radiation drives chain scission and the tape embrittles — virgin PP without a UV package can shed a large fraction of its tensile strength after only a few months of direct exposure, and FIBC are routinely stored and used outdoors. The fix is a UV/HALS (hindered amine light stabiliser) masterbatch let down into the raffia at a defined loading. What the buyer must specify is not a vague 'UV stabilised' but a performance figure: the retained tensile strength after a stated cumulative dose, expressed in kilolangley (kLy).
- State a retained-strength threshold and dose — for example 'retain at least 50% of original tensile after a defined kLy exposure'. The required kLy figure rises with climate severity and service life.
- Require the UV package on the CoA or a separate UV-resistance test report; UV performance is invisible on inspection of the bale.
- Recognise that high regrind or filler content and certain pigment colours can interact with — and sometimes undermine — the UV system, so changes in recipe should re-trigger UV verification.
- For food, pharma or sensitive cargo, confirm the masterbatch and any additives are compatible with the relevant food-contact rules before locking the recipe.
kLy is a measure of cumulative solar radiation, so a bag rated to a given dose in a high-insolation climate (the Gulf, North Africa, parts of South Asia) reaches that dose far faster than the same bag in northern Europe. Match the rating to where the bag will actually live, not to a generic number.
Most raffia problems are diagnosable on paper and with a simple bench test. The recurring culprits are an MFI mismatch, gels and contamination, moisture, and poor additive dispersion. The table maps the visible defect to its usual root cause and the check that catches it — build these into your incoming-acceptance protocol and your container loading and packaging routine.
| Defect | Typical cause | Check / control |
|---|---|---|
| Tape breaks at godet / oven | Over-draw for the resin; MFI too high; gels | Verify MFI on CoA; tape-tenacity test; reduce draw ratio |
| Fibrillation (flat tape splitting) | Over-orientation; low molecular weight | Confirm MFI/draw match; inspect drawn tape |
| Fish-eyes / gels in film | Cross-contamination; degraded or wrong resin; poor screen | CoA + visual film check; gel count; screen-pack maintenance |
| Weak welds / seam failure | Wrong resin family; moisture; contamination | Confirm homopolymer; dry filler/regrind; seam-strength test |
| Premature UV embrittlement | Missing or under-dosed UV package | Demand kLy spec + UV test report |
| Inconsistent tape width / thickness | Melt strength too low (high MFI); unstable cast film | Specify 2–4 MFI; control die and chill-roll |
The practical takeaway: a raffia PO should never read just 'PP raffia grade'. Pin down the homopolymer, the MFI 2–4 g/10 min window, a target tape tenacity (cN/dtex) at your draw ratio, the fabric weight in g/m2, and — for any FIBC or outdoor product — an explicit kLy UV rating with a retained-strength threshold, all evidenced on the CoA. Then verify with a melt-flow check and a tape-tenacity test on the first roll before you accept the container. The desk routinely builds these clauses into raffia and FIBC contracts; if you want help drafting a spec that holds up at inspection, talk to OmniaStrata or see our polypropylene sourcing services.
Frequently asked
Questions on the desk
What MFI should I specify for PP raffia tape?
For standard woven sacks and FIBC tape, specify a polypropylene homopolymer with a melt flow index of about 2–4 g/10 min measured under ASTM D1238 / ISO 1133 at 230°C and 2.16 kg. This window gives enough melt strength to form a stable cast or chill-roll film while still allowing the tape to be stretch-oriented at high draw ratios without frequent breaks. Higher MFI (above 4) eases extrusion but tends to lower tape tenacity; much lower MFI strains the line and the drawing zone.
Why is homopolymer used for raffia instead of copolymer?
Raffia tape needs high stiffness, high tensile strength and good orientation, which homopolymer polypropylene delivers because of its higher crystallinity and isotacticity. Random and impact copolymers add ethylene to improve clarity, low-temperature impact and weldability, but they reduce the stiffness and drawability that make a strong, dimensionally stable tape. For the trade-offs between the two families see our note on homopolymer versus copolymer PP.
How much UV stabiliser does an FIBC need?
It depends on service life and climate, but FIBC fabric is normally specified by a UV-exposure rating — for example tape that retains at least 50 percent of its original tensile strength after a stated cumulative dose in kilolangley (kLy). That is delivered through a UV/HALS masterbatch let down at a defined loading, not by the base resin. Always state the retained-strength threshold and the kLy figure in the contract, and require it on the certificate of analysis or a separate UV test report.
What are tape denier and tenacity, and how do they relate to sack strength?
Denier (or dtex) is the linear mass of the drawn tape — grams per 9,000 metres (denier) or per 10,000 metres (dtex). Tenacity is breaking force divided by that linear mass, expressed in cN/dtex or g/denier, and it captures how strong the tape is for its weight. A heavier tape and a higher tenacity both raise fabric tensile, but it is the combination of tape denier, picks-and-ends count and fabric weight in g/m2 that ultimately determines the sack or FIBC safe working load.
What causes tape breaks and fibrillation on a raffia line?
Tape breaks at the godets or in the oven usually come from too high a draw ratio for the resin, gels or contamination acting as stress concentrators, moisture in filler or regrind, or an MFI that is too high for the orientation being applied. Fibrillation — the tape splitting lengthwise into fine fibres — is deliberate for some products but is a defect in flat tape, and points to over-drawing or low molecular weight. Most of these are diagnosable from the CoA, a melt-flow check and a simple tape-tenacity test before acceptance.
General market commentary from the OmniaStrata desk, provided for information only. It is not legal, financial, tax, or trading advice, and it is not an offer or a commitment to any terms. Figures such as price ranges, spreads, financing costs, and credit periods are illustrative market context, not OmniaStrata's rates or terms. Actual contract terms — including price, payment instrument, credit, insurance, and Incoterms — are agreed in writing on a per-transaction basis and at OmniaStrata's discretion. Market conditions change; figures reflect the publication date.