Degradable plastic PLA polylactic acid crystallization drying and dehumidification，Degradable plastic，dehumidification，PLA polylactic

**Crystallization, Drying, and Dehumidification Equipment for Degradable PLA (Polylactic Acid)**  

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### **I. PLA Characteristics and Processing Challenges**  

1. **High Hygroscopicity**  

  - PLA readily absorbs moisture from the air (hygroscopicity: 0.5%–1%). Insufficient drying leads to hydrolytic degradation during melting, reducing molecular weight and compromising mechanical properties and transparency.  

2. **Thermal Sensitivity**  

  - Narrow processing temperature window (160–190°C). Prolonged exposure to high temperatures causes thermal decomposition.  

3. **Crystallization Behavior**  

  - PLA is a semi-crystalline polymer, with crystallinity typically ranging from 10% to 40%, directly affecting heat resistance (e.g., HDT), shrinkage, and mechanical strength.  

  - **Key Challenges in Crystallization Control**:  

    - Rapid cooling results in amorphous structures (transparent but poor heat resistance).  

    - Slow cooling or annealing enhances crystallinity (improves heat resistance but may reduce transparency).  

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### **II. Specialized Drying and Dehumidification Equipment for PLA**  

To meet PLA’s stringent drying requirements, **low-temperature deep dehumidification systems** are essential to avoid degradation and residual moisture.  

#### **1. Types of Drying Equipment**  

| **Equipment Type**               | **Principle & Features**                                                                 | **Applications**                  |  

|-----------------------------------|-----------------------------------------------------------------------------------------|-----------------------------------|  

| **Molecular Sieve Dehumidifying Dryers** | Uses molecular sieves for moisture adsorption (dew point: -40°C), precise temperature control (50–80°C). Ideal for hygroscopic materials. | Continuous production, high-precision drying. |  

| **Vacuum Drying Ovens**           | Low-pressure environment lowers water’s boiling point, enabling low-temperature drying (60–80°C) to prevent oxidation and thermal degradation. | Small-batch, high-purity PLA processing. |  

| **Twin-Tower Dehumidification Dryers** | Dual towers alternate adsorption/regeneration cycles, ensuring stable dew point (-20°C to -40°C) with lower energy consumption. | Large-scale continuous production lines. |  

| **Hot-Air Circulation Dryers**    | Traditional hot-air drying paired with low-dew-point dehumidification (dew point ≤-20°C). Cost-effective but energy-intensive. | Small-scale production with budget constraints. |  

#### **2. Critical Parameters**  

- **Drying Temperature**: 50–80°C (avoid exceeding 80°C to prevent pre-crystallization or degradation).  

- **Drying Time**: 2–4 hours (adjust based on pellet size and initial moisture content).  

- **Target Moisture Content**: ≤0.025% (typically ≤250 ppm).  

- **Dew Point Control**: ≤-20°C (molecular sieve systems achieve -40°C).  

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### **III. Crystallization Process and Equipment Integration**  

PLA crystallinity can be optimized through **synergistic control of drying and post-processing**:  

1. **Pre-Crystallization During Drying**  

  - Apply a stepwise heating profile (e.g., 60°C → 80°C) in dryers to promote molecular chain alignment and nucleation.  

  - Precise temperature/time control prevents pellet agglomeration.  

2. **Post-Molding Crystallization**  

  - **Mold Temperature Control**: Higher mold temperatures (80–110°C) slow cooling to enhance crystallinity (balance transparency vs. heat resistance).  

  - **Annealing Equipment**: Post-process parts in thermal chambers (90–120°C for 1–2 hours) to significantly boost crystallinity and heat resistance.  

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### **IV. Equipment Selection Guidelines**  

1. **Small-to-Medium Facilities**  

  - Recommend **Molecular Sieve Dryers + Vacuum Conveying Systems** for precision and cost efficiency.  

  - Example: 50–200 kg/h capacity, -40°C dew point, PLC-controlled.  

2. **Large-Scale Continuous Production**  

  - Opt for **Twin-Tower Dryers + Centralized Feeding Systems** to supply multiple injection molding machines.  

  - Integrate moisture sensors (e.g., NIR) for real-time monitoring.  

3. **High-Transparency Product Requirements**  

  - Prioritize **Vacuum Dryers** to minimize thermal oxidation and yellowing.  

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### **V. Operational Considerations**  

1. **Moisture Prevention**  

  - Store dried PLA in sealed containers with nitrogen purging; exposure time <30 minutes.  

2. **Maintenance**  

  - Replace molecular sieves (~2000 hours) and filters regularly to maintain efficiency.  

3. **Process Validation**  

  - Use DSC (Differential Scanning Calorimetry) to measure crystallinity and adjust drying/annealing parameters.  

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### **VI. Emerging Technologies**  

1. **Integrated Smart Drying Systems**  

  - Combine drying, crystallinity control, and auto-feeding with AI-driven parameter optimization.  

2. **Energy-Efficient Designs**  

  - Waste heat recovery (e.g., using injection molding coolant to preheat drying air) reduces energy use by ≥30%.  

3. **In-Line Monitoring**  

  - Fiber-optic sensors track PLA melt moisture and crystallinity for closed-loop process control.  

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### **Conclusion**  

Processing PLA requires strict control of drying and crystallization. Selecting appropriate dehumidification equipment (e.g., molecular sieve or vacuum dryers) and integrating annealing processes can significantly enhance product performance (heat resistance, dimensional stability). Prioritize smart, energy-efficient systems to address future challenges in biodegradable plastic manufacturing.