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How to Prevent Additive Segregation in PVC Dry-Blend Mixing
release time:
2026-10-13
Why PVC dry blends segregate, the five segregation mechanisms, the correct additive addition sequence and temperatures, and how to select and run a mixer that holds uniformity.
PVC dry blending is where about 100 parts of suspension resin meet a handful of additives measured in single-digit parts per hundred — heat stabiliser, internal and external lubricants, processing aid, filler and pigment. Nothing melts; the job is to coat every resin particle evenly before the blend ever reaches an extruder. The catch is that a blend which leaves the mixer perfectly uniform can come apart again long before it is fed — in the transfer line, the storage bin or the feeder. This guide explains why PVC dry blends segregate, where it happens, and how the mixer, the addition sequence and the handling after the mixer all decide whether the uniformity you paid for survives to the die. For the wider picture see our plastic and PVC mixing equipment and dry building materials mixing pages.
Segregation is not the same as poor mixing
Three different failures get blamed on one word, and each needs a different fix. Dispersion is about breaking agglomerates down; distribution is about spreading particles evenly; segregation (demixing) is what happens when a blend that was already uniform separates again. A mixer can deliver a good blend and the plant can still see off-spec product, because most segregation happens after the mixer, not inside it.
| Defect | What actually failed | Where it shows up | First fix |
|---|---|---|---|
| Poor dispersion | Agglomerates were never broken down | Specks, gel, weak colour strength | Raise the shear energy and lengthen the dispersive cycle |
| Poor distribution | The blend never became even | Streaks, colour drift, high and low spots | Improve convection; correct the fill level and time |
| Segregation (demixing) | A uniform blend separated again | Top-to-bottom or start-to-end variation; “difficult” first and last bags | Fix handling: bin design, drop height, transfer steps |
The rest of this guide is about that third row — the failure that stops a perfectly good blend from behaving like one.
Why PVC dry blend segregates so easily
A PVC dry blend is almost purpose-built to segregate. Its components differ in both size and density, and the resin is free-flowing enough that the finer additives can move through it.
| Component | Typical particle size | True density (g/cm³) | Segregation tendency |
|---|---|---|---|
| PVC suspension resin | D50 ≈ 100–160 µm | ≈ 1.4 | Reference |
| Calcium carbonate (filler) | ≈ 1–10 µm | ≈ 2.7 | Sinks — heavier and finer |
| Titanium dioxide (opacifier) | ≈ 0.2–0.5 µm | ≈ 4.2 | Strong — very fine and heavy |
| Heat stabiliser (one-pack) | ≈ 5–40 µm | ≈ 1.2–2.0 | Moderate |
| Wax / external lubricant | flakes or powder | ≈ 0.9–1.0 | Floats; can coat the fines |
| Impact modifier (CPE/ACR) | ≈ 100–300 µm | ≈ 1.1–1.2 | Low to moderate |
Figures above are typical values for illustration; confirm the properties of your own grades. Two rules of thumb matter. Segregation becomes a problem once the particle-size ratio between components passes roughly 1.3, and gets worse as the ratio grows; density contrast then adds a second, independent driver. Because the calcium carbonate and titanium dioxide fractions are both finer and heavier than the resin, they respond differently to vibration, air and impact — and the wider the differences, the harder it is to hold a uniform blend, and the higher the coefficient of variation (CV) a given mixer can reach.
Because the resin is free-flowing, percolation is easy: under vibration or repeated start-stop conveying, the fines work their way down through the voids between the coarser resin particles.
The five mechanisms that pull a PVC blend apart
Segregation is rarely a single mechanism. In a PVC line these five normally act together, and each leaves a different fingerprint in the plant.
| Mechanism | Pattern in the plant | Main driver | First-line countermeasure |
|---|---|---|---|
| Percolation (sifting) | Fines concentrate low, coarse at the top | Size ratio, voids, vibration | Narrow the size spread; cut drop height; avoid vibration |
| Density segregation | Heavy CaCO3 and TiO2 settle out first | Density contrast | Mass-flow bin; slightly more cohesive blend; fewer transfers |
| Trajectory segregation | Heavy particles land farthest from the fill point | Drop geometry and velocity | Fill pipe or deflector; fill from below; shorter drop |
| Fluidisation and air entrainment | Fines rise; dust in vents; level-dependent composition | Entrained air and permeability | Vent and deaerate; slow the fill; avoid dumping |
| Coning and rolling | Coarse to the centre, fines to the rim | Heap formation and free surface | Rotating or central fill; no tall heaps; no repeated collapse |
One counter-intuitive consequence: over-mixing can make things worse. In a tumbling or diffusive mixer, once the blend is uniform, extra turning simply gives the segregation mechanisms more time to act. Mix to a measured endpoint, not to a stopwatch.
Prevention starts with the addition sequence
Order and temperature are not housekeeping detail; they decide whether each additive ends up coated on the resin or left loose in the blend. Because PVC begins to degrade from around 140 °C, the stabiliser has to go in early enough to protect the polymer through the whole heating stage.
| Step | Target temperature | Add | Why it goes here |
|---|---|---|---|
| 1 — charge | ambient, low speed | PVC resin (S-PVC) | Preheats and dries the resin; opens the particle surface |
| 2 | ≈ 60 °C | Heat stabiliser (Ca/Zn, lead or OBS system) | Protects the polymer from the start of its heat history |
| 3 | 70–80 °C | Internal lubricant | Reduces inter-particle and inter-chain friction |
| 4 | 80–90 °C | Processing aid (acrylic) and impact modifier (CPE/ACR) | Promotes gelation and impact strength |
| 5 | 90–100 °C | Filler (CaCO3) and pigment | Dispersed while the particle surface is still open |
| 6 | 110–120 °C | External lubricant (PE wax, zinc stearate) | Forms the surface slip layer last, so it is not buried |
| Hot-mix endpoint | 110–120 °C | — | Full coating of every resin particle |
| Cooling | discharge below 40–45 °C | — | Locks the blend; stops agglomeration and further reaction |
The logic is not arbitrary: liquid or low-melting additives go in before the blend is sealed, fine solids while the pores are still open, and the external lubricant last so it stays on the outside where it belongs. A one-pack stabiliser changes the exact timing, but not the principle.
Typical starting loadings, in parts per hundred resin (phr), look like this. Treat them as a starting point and confirm against your own formulation and the relevant standard.
| Component | Typical loading (phr) | Note |
|---|---|---|
| PVC resin (S-PVC) | 100 | The reference |
| Heat stabiliser | 2.5–5 | Mandatory; Ca/Zn or OBS systems replacing lead |
| Processing aid (acrylic) | 1–3 | Gelation |
| Impact modifier (CPE/ACR) | 4–8 | Ductility and point-load resistance |
| Calcium carbonate filler | 5–20 | Keep low for pressure pipe |
| Pigment / TiO2 | 1–5 | Colour and opacity |
| Lubricants (internal + external) | 0.5–1.5 | Balance the internal/external split |
Note the filler row. In pressurised pipe, filler is deliberately capped to protect strength and impact; heavier filler use belongs in non-pressure and structured-wall products where stiffness is the goal. Adding filler without a matching impact modifier is one of the most common causes of brittle product.
Operating rules that hold a blend together
Once the recipe and the sequence are right, how the mixer is run decides how uniform the discharge is.
| Parameter | Target | Why it matters |
|---|---|---|
| Fill level | About 40–70% of working volume | Too little gives poor particle contact; too full leaves dead zones and slows circulation |
| Mixing time | Stop at the measured endpoint | Extra time in a tumbling mixer lets segregation begin again |
| Speed | Enough for fast convection, below the point where fines fluidise and dust | High speed lifts the lightest fractions first |
| Discharge | Bottom discharge, emptied completely | A stagnant heel and the first and last fractions off a funnel-flow bin are the most off-spec |
| Addition timing | Follow the temperature steps above | The wrong order seals additives inside the resin or leaves them loose |
Choosing a mixer for PVC dry blend
The mixer type decides whether you can even hold uniformity. Convective, forced mixers that keep the whole bed moving resist percolation; tumbling or diffusive mixers are gentler but more prone to re-segregating a wide particle-size blend, so they suit powders whose components are already similar in size and density.
| Mixer | Flow type | Best for | Segregation behaviour |
|---|---|---|---|
| DLH ribbon mixer | Convective — inner and outer ribbons move material in opposite directions | Batch dry blends, repeatable batches, frequent colour changes | Low when run below maximum fill |
| DJH double-shaft paddle mixer | Fluidised convection, higher shear | Fast premix, fines-rich blends, solid-liquid additions | Low — forced circulation keeps fines in the flow |
| DPH paddle-ribbon mixer | Combined convection and shear | Wide particle-size blends, filled and viscous mixes | Low |
| DJHX continuous twin-shaft paddle mixer | Continuous fluidised zone | High-throughput inline blending of powders and granules | Uniform residence time; little batch-to-batch spread |
| Plow mixer | High-intensity mechanical fluidisation, optional choppers | Dry powders, fibres and small lumps; dry blend with pigment | Very low — the high-intensity zone keeps particles in motion |
| DZH double-cone mixer | Diffusive (tumbling) | Gentle, low-residue blending of free-flowing powders | Higher risk on a wide size or density spread — match the components |
For most PVC dry-blend duties the answer is a convective batch mixer with a bottom discharge and full emptying, sized so the working fill sits comfortably inside the vessel. Where the line runs continuously, a continuous twin-shaft paddle mixer holds the composition steady without a batch cycle. See the full range on our PVC and plastics mixing equipment page.
Technical parameters
Representative DLH ribbon mixer models. Working volume is quoted for powder with a bulk density of 1; final selection must be calculated from your own material and process data.
| Model | Working volume (L) | Total power (kW) | Speed (r/min) | Overall size (mm) | Weight (kg) |
|---|---|---|---|---|---|
| DLH-500 | 180–300 | 7.5 | 49 | 2800×920×1320 | 680 |
| DLH-1000 | 350–600 | 11 | 42 | 3180×1020×1550 | 1200 |
| DLH-2000 | 700–1000 | 18.5 | 42 | 3750×1290×1820 | 2200 |
| DLH-5000 | 2000–3000 | 30 | 31 | 4700×1610×2260 | 4300 |
| DLH-10000 | 3200–5000 | 55 | 27 | 5720×2520×2840 | 7900 |
Representative DJH double-shaft paddle mixer models.
| Model | Working volume (L) | Motor power (kW) | Speed (r/min) | Overall size (mm) | Weight (kg) |
|---|---|---|---|---|---|
| DJH-500 | 200–300 | 7.5 | 37 | 1450×1500×950 | 800 |
| DJH-1000 | 400–600 | 11 | 33 | 1800×1800×1350 | 1150 |
| DJH-2000 | 800–1200 | 18.5 | 31 | 2200×2400×1600 | 2800 |
| DJH-5000 | 2000–2500 | 37 | 23 | 2950×3050×2040 | 6000 |
| DJH-10000 | 4000–5500 | 55 | 21 | 3550×3350×2400 | 9500 |
After the mixer: storage, transfer and feeding
Most segregation is created by handling, not mixing. These are the levers that matter most after discharge.
Do
- Cool the blend below 40–45 °C before it goes to storage — a cooled dry blend keeps for well over 24 hours, while a hot one keeps reacting and can agglomerate.
- Store and discharge from a mass-flow bin rather than a funnel-flow bin, so the whole cross-section moves and no stagnant zone is left to release off-spec material.
- Fill with a fill pipe or deflector and keep the heap low; avoid repeated heap collapse, which rolls fines to the rim.
- Keep transfer steps and drop heights to a minimum, and keep the blend sealed and dry — a rise of about 0.5% in the moisture content of the filler is enough to cause visible bubbles.
Avoid
- Free-falling the blend into a tall bin, or tipping an IBC in a way that makes the heap slide.
- Start-stop conveying and unnecessary vibration, which repeatedly dilate the bed and let fines percolate.
- Pneumatic transfer at high velocity, which can air-classify fines into the dust collector and quietly change the composition.
Proving uniformity: sampling and CV
Uniformity is a measured number, not a feeling. The coefficient of variation (CV) of the most segregation-prone additive — usually the filler or the pigment — is the figure to track.
| Check | Typical target | How to sample |
|---|---|---|
| CV of the critical additive | ≤ 5% is a common dry-blend target | At least five increments from top, middle and bottom, and from the first, middle and last discharge fractions |
| Moisture | Keep dry; watch filler moisture | Dry before blending; store sealed |
| Thermal stability | Per your product specification | Static heat test (for example the Congo red test) on the finished blend |
Sample position matters more than sample count. A single composite sample can hide a top-to-bottom gradient, which is exactly the pattern percolation leaves behind; sampling top, middle and bottom separately is what reveals it.
FAQ
- Is segregation the same as poor mixing? No. Poor mixing fails to create uniformity; segregation destroys uniformity that already existed, usually after the mixer. Diagnose which one you have before changing the recipe.
- Can I fix segregation by mixing longer? Usually not, and in a tumbling mixer extra time can make it worse once the blend is already uniform. Mix to a measured endpoint, then protect the blend during handling.
- Which mixer type is best for PVC dry blend? A convective, forced mixer with a bottom discharge — a ribbon, paddle or plow type — rather than a purely tumbling one, because a wide particle-size and density spread segregates easily in diffusive mixers.
- How much filler can I use? It depends on the product. Pressurised pipe caps filler to protect strength and impact under standards such as ASTM D1784; non-pressure and structured-wall products can carry more because stiffness is what matters there.
- Why does my pipe or profile show black specks or burned particles? Often a lubricant balance or stabiliser problem rather than segregation — too little external lubricant causes local burning at high output, while too much causes plate-out. Check the lubricant system before blaming the blend.
- Do I need both a hot and a cold mixer? A high-speed heater followed by a cooling mixer is the conventional two-stage line, and the cooling stage is not optional because a hot dry blend keeps reacting and segregating. Where the mixed blend is finished in a single convective batch blender, a cooled, mass-flow handling system still has to protect it afterwards.
Stopping segregation is cheaper than screening it out. Get the size and density spread under control, add the components in the right order at the right temperatures, run a convective mixer to a measured endpoint, and handle the blend gently after discharge — and the uniformity you make at the mixer is the uniformity the extruder gets. Send us your formulation, batch size and throughput and our engineers will recommend a mixer and a handling setup for it. For the full range, start from our PVC and plastics mixing equipment page.
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