Micron size is the single most-asked specification when industrial buyers evaluate a mineral filler. But a single number on the technical data sheet — say “10 µm” — never tells the whole story. The full picture comes from the particle-size distribution (PSD): the spread of particle sizes within a single batch, summarised through d10, d50, d90 and d97 values.
This guide explains what a micron is, what each d-value means, how distribution width affects compound performance, how labs measure PSD, and what to look for when comparing technical data sheets from different suppliers. The content is written for procurement teams, R&D engineers and quality leads in plastics, paint, paper, cable, masterbatch and construction-chemicals industries.
Quick definition: Micron (µm) is a length unit equal to one millionth of a metre (0.001 mm). In industrial mineral fillers such as calcium carbonate (CaCO₃), micron size and particle-size distribution (PSD) define how well the filler will disperse, the surface finish it delivers, and the mechanical and optical performance of the finished product. Always read d10, d50, d90 and d97 together — never trust a single number.
What Is a Micron? Basic Definition
A micron (symbol: µm) is a unit of length defined as one millionth of a metre, or one thousandth of a millimetre.
- 1 µm = 0.001 mm
- 1 mm = 1,000 µm
- 1 cm = 10,000 µm
For visual scale: a human hair is roughly 70 µm thick, fine table salt is about 100 µm, a red blood cell is around 7 µm. Industrial micronised calcium carbonate typically falls between 1 µm and 50 µm.
Why a Single Number Is Not Enough: The PSD Concept
An industrial filler is not made of identically-sized particles. Inside a single batch you find a distribution of sizes — some particles smaller, some larger, with a population peak somewhere in the middle.
Imagine 10,000 particles laid out on a histogram. A “10 µm filler” might have:
- 10 % of the population finer than 3 µm
- 50 % finer than 10 µm (the median)
- 90 % finer than 25 µm
- 97 % finer than 35 µm
Two products both marketed as “10 µm” can have completely different histograms — and therefore very different behaviour in your formulation.
What Do d10, d50, d90 and d97 Mean?
The d-values are cumulative percentile points on the particle-size distribution curve:
| Symbol | Meaning | What It Tells You |
|---|---|---|
| d10 | 10 % of particles are below this size | Defines the “fine tail” of the distribution |
| d50 | 50 % below this size (the median) | The “headline” particle size |
| d90 | 90 % below this size | Defines the “coarse tail” — important for surface finish |
| d97 | 97 % below this size | The “top cut” — the largest particles in the batch |
A well-controlled industrial filler is reported with at least d50 and d97. Premium grades also report d10 and d90. The ratio d97/d50 is a quick measure of distribution width — values close to 3 are narrow, values above 5 are wide.
Narrow vs Wide Distribution: Which Is Better?
Distribution width directly drives compound performance:
- Narrow PSD — predictable mechanical, optical and rheological performance. Higher cost. Standard for premium plastics, paint, masterbatch and cable.
- Wide PSD — variable performance: very fine particles raise viscosity unexpectedly, large particles compromise surface finish. Lower cost. Acceptable in construction chemicals, agricultural liming and bulk uses.
Modern classifier-mill technology — such as the Hosokawa Alpine systems used at Aydın Madencilik — delivers narrow PSD by separating oversize particles after grinding and recirculating them through the mill.
Micron Range by Industry
| Industry / Application | Typical d50 (µm) | Reason |
|---|---|---|
| Premium coated paper (top-coat) | 1 – 3 | Brightness, smooth coating film |
| Writing / printing paper (filler) | 3 – 5 | Retention, opacity |
| Paint (decorative, premium) | 3 – 8 | Hiding power, gloss, rheology |
| Paint (industrial, primer) | 5 – 15 | Filler bulk, cost |
| Plastics (PP/PE high loading) | 5 – 15 | Mechanical strength + dispersion |
| HFFR cable compounds | 8 – 15 | Polar-system dispersion, mechanical |
| Masterbatch (filler + colour) | 5 – 12 | Extrusion homogeneity |
| Rubber compounds (NR, NBR) | 10 – 20 | Reinforcement, processing |
| Construction chemicals (adhesives, grouts) | 15 – 40 | Bulk volume, cost |
| Agricultural liming | 50 – 500 | Soil reactivity, cost |
How Is Particle Size Measured?
Three measurement techniques dominate industrial particle-size analysis:
1. Laser Diffraction
The industrial standard for the 0.1 – 3,000 µm range. A laser beam passes through a dispersed particle sample; the diffraction pattern is converted into a PSD curve. Instruments: Malvern Mastersizer, Beckman Coulter LS, Microtrac. ISO 13320 defines the method. Reproducibility: typically ±2 % on d50.
2. Sieve Analysis
Suitable for coarser materials (above 38 µm). A stack of standardised mesh sieves separates the sample into size fractions. Common in cement, aggregates and agricultural fillers. Defined under ISO 3310 and ASTM E11. Less accurate for fine micronised CaCO₃.
3. Sedimentation
Uses Stokes’s law to infer size from sedimentation velocity in a liquid. Useful for fine sub-micron particles. Less common in modern QC because laser diffraction has displaced it for routine work.
What to Watch When Reading a Technical Data Sheet
- Is the d50 stated alongside d97? If only d50 appears, the distribution shape is hidden.
- Which measurement method is reported? Laser diffraction values are not directly comparable to sieve-based values. Always compare like with like.
- Is the measurement reported in volume or number? Industrial standard is volume-based (Q3). Number-based distributions exaggerate the fine tail.
- Batch-to-batch variation — a serious supplier reports historical statistical limits (e.g. d50 = 8.0 ± 0.5 µm).
- D97 versus d99 — d97 is the conservative top-cut value. Some suppliers report d99 to flatter the appearance; ask for d97 explicitly.
Aydın Madencilik Product Codes and Micron Ranges
Aydın Madencilik’s product family is engineered with narrow PSD using Hosokawa Alpine classifier mills. Typical specifications:
| Grade | d50 (µm) | d97 (µm) | Recommended Use |
|---|---|---|---|
| AY 103 | 3 | 10 | Paper top-coat, premium paint, fine polymers |
| AY 106 | 6 | 20 | Paper wet-end filler, plastics, masterbatch |
| AY 108 | 8 | 25 | HFFR cable, plastics, premium masterbatch |
| AY 210 | 10 | 35 | General plastics, rubber, paint |
| AY 215 | 15 | 45 | Economical HFFR, high-loading compounds |
Each grade ships with a Certificate of Analysis reporting d10, d50, d90 and d97 — measured by laser diffraction in volume mode. See our full calcium carbonate product range.
Frequently Asked Questions
What is the difference between a 5 µm and a 10 µm calcite?
The headline number (d50) doubles, but the wider implication is dispersion behaviour. A 5 µm calcite gives better optical and surface performance but increases compound viscosity and cost. A 10 µm calcite is easier to process and more economical but compromises surface finish. The choice depends on the application.
Why is d97 critical?
d97 represents the largest particles in the batch — the ones that show up as surface defects in extruded film, paint roughness or weak spots in cable insulation. Two fillers with identical d50 can have very different d97; the one with lower d97 always performs better.
What is the relationship between micron size and ISO whiteness?
Finer particles scatter visible light more efficiently — so finer micronised calcite typically delivers higher ISO brightness. This is why coated paper top-coats use d50 = 1–3 µm grades. The trade-off is higher cost and processing difficulty.
Does narrow PSD always mean higher cost?
Yes, slightly. Narrow PSD requires more aggressive classification — separating oversize particles and recirculating them, which lowers throughput. The cost premium is typically 5–10 % over wide PSD grades, but is repaid by extrusion stability, mechanical performance and reduced reject rates.
How often should a paper or plastics plant request PSD verification?
Every batch ships with a Certificate of Analysis. Premium customers verify in-house every 5–10 batches with their own laser diffraction equipment. Any shift in d50 or d97 beyond agreed tolerances should trigger a supplier conversation.
Are micron and PSD specifications standardised internationally?
Yes. The most relevant standards are ISO 13320 (laser diffraction), ISO 3310 (test sieves), ISO 9276-2 (representation of PSD data) and ASTM E11. Aydın Madencilik labs use ISO 13320 as the standard reporting method.
Read the Whole Distribution, Not One Number
Micron size is a starting point, not an answer. Compound performance is driven by the full particle-size distribution — d10, d50, d90, d97 — and by the consistency of those values from batch to batch. A serious B2B supplier publishes them all.
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