Key takeaways
- K-value is a viscosity-derived index (Fikentscher equation, per DIN EN ISO 1628-2) that acts as a practical proxy for the average molecular weight of suspension PVC — higher K means longer chains, higher melt viscosity, and tougher but harder-to-process resin.
- Commercial S-PVC runs roughly K57 to K70: low-K grades (K57–K60) suit injection moulding and rigid fittings where flow matters; mid-K (K64–K67) covers pipe and rigid profile; high-K (K67–K70) goes into flexible cable, hose, and calendered film where mechanical strength dominates.
- K-value is not melt flow index — MFI is a direct flow measurement on a molten polymer under load (ASTM D1238 / ISO 1133), while K-value is measured cold in dilute solution, because PVC degrades before it forms a stable melt to measure.
- Choosing K-value is a trade-off, not an optimisation: every step up in K raises impact strength and chemical resistance but demands more plasticiser, lubricant, heat stabiliser, and processing energy — get it wrong and you either scorch the resin or ship a brittle part.
Ask a PVC compounder which single spec they check first on an incoming resin and the answer is almost always the same: K-value. It is the number that decides whether a batch runs cleanly on the line or scorches in the barrel, whether a moulded fitting fills the cavity or short-shots, and whether a finished part survives a drop test or cracks. Get the K-value right and most other problems become tractable. Get it wrong and no amount of additive tinkering fully recovers.
K-value is shorthand for molecular weight — and molecular weight is the master variable behind almost every mechanical and processing property of PVC. This guide explains what the number actually means, how the commercial K57–K70 range maps onto real processes, and why PVC is specified by K-value when polyethylene and polypropylene are specified by melt flow index.
K-value is not measured on molten polymer at all. A small mass of PVC is dissolved in a solvent — usually cyclohexanone — at a defined concentration, and the relative viscosity of that dilute solution is measured against the pure solvent. The result is fed into the Fikentscher equation, which converts relative viscosity into a single dimensionless index: the K-value. The governing method today is DIN EN ISO 1628-2; older data sheets may cite ASTM D1243, which uses a different concentration, so the figures are close but not identical.
Because solution viscosity rises with chain length, K-value is a reliable proxy for the average molecular weight of the resin. A K57 suspension grade carries a lower viscosity-average molecular weight; a K70 grade is meaningfully higher. The relationship is monotonic and well understood, which is why the trade treats K-value as a direct stand-in for molecular weight without quoting g/mol on the label.
K-value is the only PVC property measured cold, in solution — because the polymer degrades before it ever forms a stable melt to measure.
That last point is the whole reason K-value exists. PVC has a narrow processing window: its degradation temperature sits uncomfortably close to the temperature needed to make it flow, and without heat stabilisers it begins shedding HCl and discolouring almost as soon as it melts. You cannot run a clean, repeatable melt-flow test on neat PVC the way you can on a polyolefin. A solution method sidesteps thermal history entirely — which is precisely why K-value, not MFI, became the industry's molecular-weight currency for vinyl.
Suspension PVC — the S-PVC that dominates rigid and most flexible applications — is sold across a band that runs roughly K57 at the low end to K70 at the high end, with the bulk of tonnage concentrated between K65 and K67. Moving up the ladder buys you toughness, impact strength, and chemical resistance; it costs you flow, and therefore demands higher temperatures, more lubricant, and (for flexible compounds) more plasticiser.
| K-value | Relative molecular weight | Melt viscosity / flow | Typical applications |
|---|---|---|---|
| K57–K60 | Lowest | Highest flow, easiest fill | Injection-moulded fittings, complex thin-wall parts, rigid foam, transparent bottles |
| K60–K64 | Low–mid | Good flow | Blow-moulded bottles, clear sheet, easy-processing rigid compounds |
| K64–K67 | Mid | Balanced | Pressure and drainage pipe, rigid window and door profile, siding |
| K67–K70 | Highest | Lowest flow, needs most heat/lube | Flexible cable and wire insulation, hose, calendered film, footwear, gaskets |
The logic of the ladder is consistent across the range. Low-K resin flows easily, so it goes where geometry is demanding and melt strength matters less — injection moulding of fittings, intricate thin-wall parts, and clear rigid packaging. High-K resin is mechanically superior, so it goes where the part must flex, stretch, or survive abuse — cable insulation that gets pulled through conduit, hose that bends under pressure, calendered film that must resist tearing.
Process choice and K-value are coupled — you cannot pick one without the other. The table below is the working logic most converters apply when specifying resin for a given line.
| Process | Typical K-value | Why |
|---|---|---|
| Injection moulding (fittings, accessories) | K57–K60 | Low melt viscosity fills cavities and detail at workable pressure; melt strength is a secondary concern |
| Pressure / drainage pipe extrusion | K65–K67 | Needs melt strength to hold the die profile plus impact resistance for buried service; the industry default |
| Rigid profile (window, door, siding) | K64–K67 | Dimensional stability and impact balance; weatherable rigid compounds |
| Flexible cable & wire | K67–K70 | Maximum tensile and abrasion resistance; high plasticiser loading is expected |
| Calendered film & sheet | K65–K70 | High K gives the melt strength and toughness calendering demands |
| Blow moulding (bottles, containers) | K58–K64 | Needs enough melt strength to hold a parison but enough flow to blow cleanly |
Two practical rules follow. First, a fitting should be moulded from lower-K resin than the pipe it joins — the pipe wants melt strength (K65–K67), the fitting wants flow (K57–K60). Second, the higher you go in K, the more your formulation cost rises: flexible high-K cable compound needs heavier plasticiser, lubricant, and heat-stabiliser packages than a rigid low-K part. When you price a resin, price the additive package alongside it — the cheapest K-value on a spot sheet is not always the cheapest finished compound.
Buyers crossing over from polyolefins routinely ask for the "MFI of this PVC". There isn't one in any useful sense. The two indices answer related questions by opposite methods, and they are not interconvertible.
- Method: K-value is measured cold, in dilute solution (ISO 1628-2). MFI is measured on a molten polymer forced through a die under a fixed load and temperature (ASTM D1238 / ISO 1133).
- What it captures: K-value is a clean molecular-weight proxy untouched by additives or thermal history. MFI captures the flow of the whole compound — molecular weight, additives, and any thermal damage all show up in the number.
- Direction: higher K means lower flow, so the two move in opposite directions — but there is no conversion factor, because MFI also depends on the full formulation.
- Why PVC uses K-value: PVC degrades near its processing temperature, so a hot melt-flow test on neat resin is unreliable. Polyolefins are thermally stable enough to make MFI the natural choice.
If a supplier offers a melt-flow figure for a PVC compound, treat it as a property of that specific compounded formulation, not of the base resin — and always confirm the K-value separately. For the wider context of how flow indices work and where each is used, see our melt flow index explainer.
K-value appears on both the technical data sheet and the batch Certificate of Analysis. Three checks protect you. Confirm the standard cited — ISO 1628-2 versus the older ASTM method changes the absolute figure slightly, so compare like with like. Check whether the value is quoted as a K-number or as a viscosity number in mL/g; some European sheets give both. And confirm the batch CoA value matches the data sheet nominal, not just the grade name — molecular weight drifts batch to batch, and a half-point of K can shift your line settings. Our walkthrough on reading a polymer CoA covers the full document.
The discipline is simple: decide the process first, let the process set the K-value band, then negotiate grade, additive package, and price within that band — never the other way round. A K-value chosen to fit a spot deal rather than the line is the most expensive saving in PVC. When you are scoping a programme and want the band pinned down against your tooling and throughput before you commit, the OmniaStrata PVC desk will spec it with you, or start a conversation through contact.
Frequently asked
Questions on the desk
What does the K-value of PVC actually measure?
K-value is an index calculated from the relative viscosity of PVC dissolved in a solvent (typically cyclohexanone), using the Fikentscher equation under DIN EN ISO 1628-2. It is a proxy for average molecular weight — higher K means longer polymer chains. It tells you nothing directly about colour, particle size, or stabiliser package; it is purely a molecular-weight signal.
What K-value do I need for PVC pipe versus injection-moulded fittings?
Pressure and drainage pipe is typically extruded from K65–K67 resin, which balances melt strength and impact performance. Injection-moulded fittings and complex thin-wall parts usually need lower K57–K60 resin, because the shorter chains flow more easily into the mould and fill detail without excessive injection pressure. Matching them keeps a fitting compatible with the pipe it joins.
Is K-value the same as melt flow index (MFI)?
No. MFI (ASTM D1238 / ISO 1133) measures how fast a molten polymer flows through a die under a fixed load and is used for polyolefins like PE and PP. PVC degrades thermally before it forms a clean, stable melt, so a solution-viscosity method — K-value — is used instead. As a rough rule, higher K corresponds to lower flow, but the two numbers are not interconvertible.
Why does a higher K-value need more additives?
Longer chains in high-K resin raise melt viscosity and processing temperature, so they need more lubricant and heat stabiliser to move through the machine without scorching, and more plasticiser to reach a target flexibility. That is why flexible cable and hose compounds — often K67–K70 — carry heavier formulations than rigid low-K parts. Budget for the additive cost, not just the resin price.
Where do I find the K-value on a supplier's paperwork?
It appears on the resin technical data sheet and the batch Certificate of Analysis, usually expressed as a number (e.g. "K67") or sometimes as a viscosity number in mL/g. Always confirm the measurement standard cited (ISO 1628-2 versus the older ASTM D1243 method), because solvents and concentrations differ slightly. See our guide to reading a polymer CoA for the full checklist.
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.