Calcium Carbonate for White and BOPP Film: Choosing a Grade by Top Cut

In short: For film, the parameter that decides everything is the D97 top cut — the coarse tail of the particle-size distribution, not the average. In a thin gauge, a single oversize particle becomes a gel, speck, pinhole, or tear, so film needs the finest grade with the tightest top cut (KC-4 for thin, high-gloss, and BOPP; KC-6 for fine and breathable film), and it needs to be coated so it disperses without agglomerating. For white and opaque film, high whiteness adds a second benefit — it lets you extend the titanium dioxide. This guide matches the grade to the film type.

Every filler decision in film comes back to one physical fact: the film is thinner than the defect a coarse particle causes. A blown or cast film may run 20–50 microns; a stretched BOPP film runs thinner. A calcium carbonate particle of 20 or 30 microns in that gauge is not a filler — it is a hole. So the first and most important number is not the average particle size, it is the top cut.

• D97 (top cut) — the size below which 97% of the particles fall. This is the number that predicts film defects, because the defects come from the small number of oversize particles in the coarse tail, not the median particle. Two grades with the same D50 (median) can perform completely differently if one carries a longer coarse tail.
• Coating — a stearic-acid coating keeps fine particles from clumping together into agglomerates, which act like coarse particles even when the powder's top cut is fine. In film, an uncoated or poorly dispersed fine grade is a common hidden cause of specks and pinholes.
• Whiteness — for white and opaque film, it sets how much titanium dioxide you can extend (below). For clear or lightly-tinted film it matters less.

Match the D97 to your gauge, choose a coated grade, and most film filler problems never appear.

Film is not one application. Three film types use calcium carbonate differently:

White and opaque film (packaging, labels, bags). The filler carries whiteness and lets you extend the titanium dioxide. You need a fine top cut for the gauge and high whiteness. In the KC Series this is KC-4 (D97 ~5.5 µm) for thin and high-gloss film, or KC-6 (~6 µm) for general film.

BOPP and high-gloss film. Stretched thin and judged on surface, so the tolerance for a coarse particle is the smallest of all. Use the finest, tightest grade — KC-4 — and confirm the top cut and gloss on your own line.

Breathable (microporous) film — hygiene backsheets, medical, some packaging. Here calcium carbonate does something different: loaded high and then stretched, each particle makes a micropore that passes water vapour but not liquid. This depends on a fine, uniform, coated grade — a coarse particle makes an oversize hole or a tear instead of a clean pore, so top-cut consistency matters even more than in ordinary film. KC-4 or KC-6, coated, with a consistent top cut lot to lot.

(For non-white cost-down film and the loading arithmetic, the filler-masterbatch view is in filler masterbatch / calpet; the fuller grade ladder is in which grade for your application.)

It is worth being precise about this, because it is the single most useful idea in film filler selection. When a film maker gets specks, gels, or pinholes and checks the filler's D50, the median often looks fine — because the median is not the problem. A grade can have a good average and still carry a few oversize particles in its coarse tail, and in a thin gauge those few particles are the defects.

So two rules follow:

• Read the D97, and match it to your gauge. The thinner the film, the tighter the top cut it needs. Ask for the D97 on the certificate of analysis, not just the D50.
• A "fine" grade that is uncoated or poorly dispersed behaves coarse. Agglomerates act like large particles. This is why coating is part of the top-cut question, not separate from it.

Film runs at the edge of what dispersion allows, so a stearic-acid coating is close to mandatory. It makes the calcium carbonate organophilic, so it wets into the PP or PE cleanly, resists moisture pickup, and does not agglomerate. Fine grades benefit most, because the finer the particle the more it wants to clump without a coating. An uncoated fine grade often disperses worse than a coated coarser one — the opposite of what buyers expect. (When uncoated is acceptable, and why: coated vs. uncoated calcium carbonate.)

For white and opaque film, high whiteness lowers cost. Titanium dioxide is the expensive ingredient that makes film opaque, and a high-whiteness calcium carbonate lets you extend it — remove part of the TiO₂ while the film holds its whiteness and opacity. Calcium carbonate does not replace TiO₂'s hiding power (its refractive index is too close to the polymer's), but a clean ≥98-whiteness grade lets you remove the pigment that is doing whiteness rather than hiding work.

Application work puts white film around 10–30% of the TiO₂ loading — the low end with an ordinary 90–96 whiteness filler, the high end with a ≥98 grade. It is your lab's number, confirmed with a step-down trial, and the pigment removed is usually worth far more than the difference in filler price. (The mechanism, the honest limits, and a worked example: how much titanium dioxide calcium carbonate can replace and the filler economics calculator.)

Check the per-lot certificate of analysis against the technical data sheet, with these first:

• The D97 top cut, on a named laser method — the film-defect predictor. Match it to your gauge.
• Coating level and moisture — dispersion in a thin gauge depends on them.
• Whiteness and ISO brightness, method stated — for white film, the extension ceiling. (Compare like with like: whiteness vs. ISO brightness.)
• Iron (Fe₂O₃) — for white film, it predicts whether the whiteness holds lot to lot.

The full paper-then-line method — reading the COA, then a sample, then one trial container — is in how to qualify a new calcium carbonate supplier.

Film is where a grade proves itself or fails, and it does so quickly — a coarse tail shows up as defects on the first roll. Send your film type, gauge, and — for white film — your whiteness spec and TiO₂ loading, and we will recommend a KC grade and send a free sample (under a tonne) with its per-lot COA to run on your own line before you commit. Start from the product page.

One honesty note: the KC Series COA today reports whiteness, brightness, and particle size (on a Malvern analyser) to our current spec; a heavy-metals panel at an accredited laboratory is in progress and will be supplied with the COA once complete, which matters if your film is for food-contact use.

Which calcium carbonate grade is best for thin or BOPP film?

The finest grade with the tightest top cut — KC-4 (D97 ~5.5 µm). Film is the least forgiving application for particle size: one oversize particle in a thin gauge becomes a gel, speck, pinhole, or tear, and BOPP is stretched thinner still. Match the D97, not the average, to your gauge.

Why does calcium carbonate cause specks or pinholes in film?

Defects come from the coarse tail of the distribution, not the average. Two grades can share a D50 yet differ if one has a longer coarse tail — the few oversize particles puncture a thin gauge. That is why the D97 top cut predicts film quality, and why an uncoated or poorly dispersed fine grade agglomerates into the same problem.

What calcium carbonate is used in breathable film?

A fine, coated, consistent grade at high loading. Breathable film is made by loading calcium carbonate and stretching it so the polymer forms micropores around each particle. A coarse particle makes an oversize hole or tear instead of a clean pore, so top-cut consistency matters even more than in ordinary film.

How much titanium dioxide can calcium carbonate save in white film?

A high-whiteness grade lets you extend the TiO₂ — around 10–30% of the loading for white film, depending on gauge, whiteness spec, and the filler's whiteness. It is your lab's number, confirmed with a step-down trial; the pigment removed is usually worth far more than the filler.

Does calcium carbonate for film need to be coated?

Effectively yes. Film runs thin gauges where dispersion has no margin, and a stearic-acid coating makes the calcium carbonate disperse cleanly and agglomerate less — agglomerates being what become specks and pinholes. Fine grades especially need coating; uncoated fine grades are a common hidden cause of film defects.