Three Major Causes of Blooming in PVC Products and Practical Solutions

Have you ever encountered a frustrating situation where your carefully manufactured PVC film suddenly shows a strange “white frost” on the surface, or where newly produced PVC pipes develop an oily layer just days after production? This seemingly trivial issue of “blooming” can significantly degrade the appearance of your product and even lead to customer complaints or returns.

From hidden formulation pitfalls to temperature traps on the production line, and from warehouse storage risks to the delicate balance of additive ratios, this article explores the root causes behind blooming and provides comprehensive solutions from raw materials to process control.

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What is Blooming?

Blooming refers to the migration and phase separation of additives from within a polymer to its surface.

Typical causes include:

A. Solubility Change with Temperature:

Additives soluble in the polymer at high processing temperatures may become less soluble at room temperature, resulting in a supersaturated state that leads to migration.

B. Molecular Weight and Diffusivity:

Low-molecular-weight additives have higher mobility and diffuse more easily to the surface, especially in non-polar matrices.

C. Metastable Distribution:

Additives in a metastable dispersed state may migrate during aging, storage, or post-processing heating.

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1. Causes of Additive Blooming

1.1 Thermodynamic Factors

- Key Parameter: Additive solubility in the polymer

- Note: Lower solubility leads to higher blooming risk; affected by molecular structure, glass transition temperature, and polymer crystallinity.

1.2 Kinetic Factors

- Key Parameter: Diffusion coefficient

- Note: Smaller molecules and polymers with active chain segments below Tg promote faster diffusion.

1.3 Polymer Structure Factors

- Key Parameter: Crystallinity

- Note: High crystallinity pushes additives into amorphous or surface zones.

1.4 Processing Parameters

- Key Parameters: Cooling rate, temperature gradient

- Note: Slow cooling and large temperature gradients promote additive migration.

1.5 Environmental Conditions

- Key Parameters: High temperature, humidity, mechanical stress

- Note: All accelerate blooming.

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2. Blooming in PVC Products: Forms, Causes, and Prevention

2.1 Manifestations of Blooming

Blooming appears as a white or light-colored powdery, oily, or waxy substance on the surface of PVC products such as films, sheets, and pipes.

- Powdery Blooming: Fine white particles with rough touch, easily wiped off but may recur.

- Oily/Waxy Blooming: Shiny or waxy surface with visible droplets in severe cases, often in high-plasticizer soft PVC.

2.2 Core Causes and Analysis

2.2.1 Formulation Defects

- Poor Compatibility of Raw Materials:

  Stabilizers (e.g., lead salts, Ca-Zn stabilizers), lubricants (stearates), and plasticizers (DOP, DBP) with low compatibility with PVC can migrate in low temperatures or long storage.

  Excess or improperly treated fillers (e.g., CaCO₃, talc) disrupt system homogeneity.

- Overuse of Additives:

  Excessive lubricants (e.g., zinc stearate) can form rich lubricant phases that bloom at low temps.

  Plasticizer levels beyond PVC absorption capacity tend to migrate.

2.2.2 Improper Processing

- Insufficient Plasticization:

  Low temperatures (e.g., <160°C) during extrusion/injection prevent proper additive dispersion.

- Uncontrolled Cooling:

  Rapid cooling (e.g., water quenching) causes internal stress and supersaturation.

  Slow cooling allows prolonged chain relaxation, facilitating additive migration.

2.2.3 Environmental Inducement

- Temperature and Humidity Fluctuations:

  Low temps (<10°C) reduce additive solubility; high humidity accelerates plasticizer hydrolysis.

- Storage Conditions:

  Compression, contact with metal or solvents (e.g., gasoline, alcohol) may catalyze blooming.

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2.3 Solutions to Prevent Blooming

2.3.1 Formulation Optimization

- Selection and Dosage Adjustment:

  Use high-MW, high-compatibility plasticizers (e.g., polyester-based) to replace part of DOP.

  Prefer organotin or high-efficiency Ca-Zn stabilizers over lead salts.

  Use lubricant blends (e.g., Ca stearate + stearyl ester) within 0.5%–1.5% total dosage.

- Add Compatibilizers and Anti-migration Aids:

  Introduce ACR or CPE to improve compatibility, or grafted rubber compatibilizers.

  Add anti-migration agents (e.g., paraffin-type inhibitors).

2.3.2 Precise Process Control

- Optimize Plasticization and Cooling:

  Set extrusion zones: Feed 140–160°C, Melt 170–190°C, Die 180–190°C.

  For soft PVC, use combined air + slow cooling to avoid thermal shock.

- Adjust Pressure and Timing:

  Injection molding: Hold pressure 80–120 MPa, hold time 10–20s.

  Calendering: Keep roller temperature difference ≤5°C.

2.3.3 Storage and Environment Management

- Control Conditions:

  Workshop: 20–25°C, humidity ≤60%.

  Storage: Avoid metal contact; use moisture-proof pallets, stack ≤1.5 m.

- Post-processing Treatment:

  Perform annealing at 60–80°C hot air for 2–4 hours.

  Apply anti-bloom coatings (e.g., silicone-based films).

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3. Case Study

- Rigid PVC Pipe Blooming:

  Problem: CaCO₃ loading >40%, untreated filler, extrusion temp only 150°C → filler agglomeration and blooming.

  Solution: Reduce CaCO₃ to 30%, treat with titanate coupling agent, raise temp to 180°C → issue resolved.

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In the End

By harmonizing formulation, processing, and storage parameters, the risk of blooming in PVC products can be significantly reduced. Practical adjustments should consider specific application needs (e.g., solvent exposure, weather resistance), and compatibility tests are recommended during development.