Stearic Acid in Masterbatch and PVC Compounding — Grade Selection and Buying Guide
Direct answer
In Masterbatch and PVC compounding, stearic acid is not an additive you choose by name — you choose it by iodine value, titre and origin. A triple-pressed vegetable stearic acid with an iodine value below 1 and titre around 54–56 °C is the safe default for White and Colour Masterbatch: it disperses pigment, lubricates the screw, lowers torque, and does not yellow or smell at 200–230 °C. A single-pressed or tallow-based grade with a higher iodine value will oxidise, discolour the pellet and put an odour into the customer's film.
Typical loading is 0.2–1.0% of the total Masterbatch formulation as an external/internal lubricant and pigment wetting aid. In rigid PVC it functions as an external lubricant and co-stabiliser at roughly 0.2–0.8 phr.
What the specification numbers actually control
| Parameter | Typical value | What it controls in production |
|---|---|---|
| Iodine value | < 1 (triple pressed) | Oxidation resistance — high IV means yellowing and odour at extrusion temperature |
| Titre (solidification point) | 54–56 °C | Melt behaviour in the extruder; too low means tackiness and feeding problems |
| Acid value | 205–210 mg KOH/g | Free acid content; affects reaction with metal stearates and stabilisers |
| Saponification value | 206–211 mg KOH/g | Purity indicator against unhydrolysed fat |
| Colour (Lovibond / Hazen) | Lowest available | Directly limits how white a White Masterbatch can be |
| Moisture | < 0.2% | Voids and screw slip; critical for film-grade carriers |
| Origin | Vegetable (palm) vs tallow | Halal / kosher acceptance and regulatory acceptance in food-contact film |
Where it is used, and why
- White Masterbatch — wets the TiO₂ surface so the pigment disperses at high loading instead of agglomerating into specks on the film.
- Colour Masterbatch — reduces melt viscosity, improves pigment let-down and lowers extruder amperage.
- Calcium carbonate filler Masterbatch — normally paired with coated CaCO₃; the coating is often stearate-based, so the free stearic acid dose must be reduced or you get plate-out.
- Rigid PVC (pipe, profile, fittings) — external lubricant that controls metal release and fusion time alongside calcium/zinc or organotin stabilisers.
- Rubber compounding — activator for the zinc oxide/sulphur cure system.
When stearic acid is the wrong answer
If the fault on the line is **plate-out on the die or a hazy bloom on the surface, adding more stearic acid usually makes it worse, not better — you are already over-lubricated, or the CaCO₃ coating is contributing stearate you did not count. If the problem is **poor pigment dispersion at high loading, a dedicated dispersant or wax may be the correct tool rather than more fatty acid. And in a compound stabilised with certain organotins, excess free fatty acid can interfere with the stabiliser rather than help it.
For a filler-related dispersion issue, look first at the **calcium carbonate coating level; for a colour-strength issue, look first at the **pigment loading and carrier resin match.
The specification a supplier needs before quoting
- Grade — triple pressed / double pressed / single pressed
- Iodine value and titre targets
- Origin — vegetable (palm-based) or tallow, plus RSPO / halal / kosher if required
- Physical form — flake, bead/prill, powder or molten bulk
- Colour limit (Lovibond or Hazen) if used in white or light shades
- End application — Masterbatch, rigid PVC, rubber, candle, coating
- Annual volume (MT) and packaging — 25 kg bag, big bag or FCL
- Delivery point in Canada
Canadian buying reality
Stearic acid reaching Canadian compounders is almost entirely imported, predominantly palm-derived from Southeast Asia. Two things decide whether a quotation is genuinely comparable: origin (vegetable vs tallow, which is a hard requirement for many food-contact and halal-certified customers) and pressing grade, which is where a low headline price usually hides. A single-pressed grade quoted against a triple-pressed specification will look 10–15% cheaper and cost far more in rejected film.
Related content
- White Masterbatch — where stearic acid does its most visible work
- Colour Masterbatch — dispersion and let-down behaviour
- Calcium carbonate filler — the coating interaction described above
- PVC additives — lubricant and stabiliser system context
Next step
Send an RFQ with grade, iodine value, titre, origin and form. Requests stating only "stearic acid" cannot be priced against your actual process window.