A spunbond line runs a fibre-grade polypropylene homopolymer resin whose melt flow rate sits between 25 and 40 g/10 min at 230 °C and 2.16 kg. Where in that band you buy follows from line speed and fabric weight: the top end spins finer and runs faster, the bottom end gives the stronger filament.
Other polymers are spun on the same principle, but the standard hygiene and medical line is built for PP, and the choice is among the fibre-grade homopolymers sold into it.
What Melt Flow Rate a Spunbond Line Needs
The band a standard spunbond line is built around is 25 to 40 g/10 min at 230 °C and 2.16 kg, and named grades land inside it. The Dushanzi fibre grade we quote for spunbond reads 37 g/10 min on the producer’s in-house method.
A faster line and a lighter fabric take the top of the band; a fabric bought for tensile strength takes the bottom.
The strength given up is measurable, and smaller than the effect of basis weight. A published trial ran 25 and 35 MFI resin on one spunbond line at four fabric weights. At 70 g/m² the higher-flow resin lost about 10 % of machine-direction tensile strength and about 19 % of the cross-direction figure, and fabric weight moved strength more than MFI did at every weight tested.
The band describes the standard line and not a ceiling. One producer now sells a hygiene spunbond grade at 58 g/10 min, offered against a 36 MFR reference for finer fibre and lighter fabric on high-speed machinery.
Low-flow resin can be spun on a line built for it. On a line already built, specify inside the band.
Homopolymer, Random Copolymer or Metallocene PP for Spunbond
Homopolymer is the spunbond default.
Metallocene homopolymer is bought for its melting point as much as for its uniformity. Two spunbond grades from one producer at almost the same flow read 160 to 165 °C and 151 °C to the same method, and the calender bonding window moves with them.
Do not assume the metallocene grade also carries a narrower molecular weight distribution.
Where softness or a lower bonding temperature is the target, the producers answer with a blend into the homopolymer — a propylene-based elastomer or a butene-1 copolymer let down into it. A random copolymer spunbond grade exists, while impact copolymer is not offered for the process at all.
Which Other Datasheet Figures Qualify a Spunbond Grade
Ash, isotacticity, molecular weight distribution, volatiles and the stabiliser package are the other lines that qualify a grade. MFR alone does not make one spinnable.
Most of them are not on the datasheet. Ask the producer for them in writing, or read them off the lot’s certificate.
| Figure | What a fibre grade holds | What it protects on the line |
|---|---|---|
| Ash | 0.014 to 0.024 % where grades in fibre service have been measured; no row on most producer sheets | Pack life — residual catalyst builds pressure in the spinneret and forces pack changes |
| Isotacticity | Published as a word; a number is readable only with its method | Filament stiffness and drawability |
| Molecular weight distribution | “Narrow”; PDI 2.4 to 2.9 where commercial fibre grades have been measured | Even spinning and fewer filament breaks |
| Volatiles and odour | Not published; producers state low VOC or odour-free technology in prose | Smoke at the die, odour in hygiene fabric |
| Gas-fading stabilisation | Stated as a feature on every spunbond grade in producer catalogues | Fibre colour through storage |
Recycled or powder feed is where ash and filter life go first, and neither arrives with a producer’s ash figure behind it. Qualify that material on a trial run before it goes on a purchase order.
Controlled Rheology or Reactor Grade
A controlled-rheology grade is a reactor resin cracked to its final flow with a peroxide during extrusion, which raises MFR and narrows the molecular weight distribution at once. Peer-reviewed pilot work shows the distribution tightening as peroxide content rises, and with it a wider stable spinning window before draw resonance sets in.
The datasheet states the route in words, because there is no number to state. One producer lists “Controlled Rheology” and “Narrow Molecular Weight Distribution” among a spunbond grade’s features. Producers advertise the reverse where it applies: a metallocene meltblown homopolymer is sold on containing no peroxide or peroxide decomposition by-products.
The route has a cost. The same work found xylene solubles rising and mechanical properties slightly degraded as peroxide content went up. Read “no peroxide by-products” as a producer’s differentiator and let it disqualify nothing.
Which Grade the Meltblown Beam of an SMS Line Needs
The M beams of an SMS or SSMMS line need a meltblown grade of far higher flow than the spunbond band: peer-reviewed work puts the requirement around a thousand g/10 min. The S beams run the spunbond band above, and producers sell the two as a pair for one structure.
The meltblown grade is traded in much smaller volumes than standard grades and priced well above them, and it is a separate purchase with its own qualification.
How to Verify a Quoted Spunbond Grade Before Ordering
A quoted grade is a datasheet plus the lot’s certificate, and the check is three lines read against what the line needs — MFR with its test condition and its lot-to-lot spread, ash, and isotacticity with its method — plus two things neither document carries.
Read the datasheet as what the producer says it is. Spunbond-grade sheets carry the line “typical properties: these are not to be construed as specifications”. Whatever you need held, put it in the purchase order.
The MFR line is narrower in meaning than it looks. ASTM D1238 states that the flow rate indicates the uniformity of the polymer as made by one process and is not to be used as an indication of uniformity of other properties, and that it is not a fundamental polymer property and need not correlate with processing behaviour.
Substituting One Producer’s Grade for Another
Two producers’ fibre grades at the same nominal MFR still differ in rheology route, ash level, additive package and lot-to-lot spread, and none of that sits in the number you matched.
The same number can also be two different measurements. Producers print one value against both ISO 1133 and ASTM D1238, and Chinese-origin grades often cite an in-house method code instead of either.
An application paragraph is a claim to check, including on our own pages. One of ours says a fibre grade drops into a line running two named foreign grades with no change to parameters; the cross-reference table on the same page shows 22.4 g/10 min against 25 and 25, and carries no isotacticity row for either grade.
A substitution is qualified on the line. Run a trial lot before the container order.
What the COA Confirms and What It Misses
The certificate confirms, for one lot, the MFR, ash, isotactic index and pellet appearance actually measured, each against the method named beside it. It does not show molecular weight distribution, the additive package or whether the resin spins.
Two things on the certificate need checking before the figures do. The MFR row has to carry its temperature and load, because a figure measured at another condition is a different number. The edition of each cited method has to be current as well.
What is left comes from the datasheet and from your own line. Reading the certificate line by line tells you what arrived in the container; only a trial tells you it spins.