Compatibility between calcium carbonate (CaCO₃) and polymer matrices refers to wettability, dispersion uniformity, interfacial adhesion, and melt processing matching. Poor compatibility causes CaCO₃ agglomeration, deteriorated mechanical strength, rough product surface, unstable melt flow, and reduced thermal performance.
This article summarizes standard physical, morphological, rheological, mechanical and chemical testing methods to evaluate CaCO₃ compatibility with plastics like PE, PP, PVC, PA, PET, epoxy resins.
1. Fundamental Evaluation Dimensions
Compatibility testing focuses on 5 core indicators:
- Surface wettability and surface energy matching
- Particle dispersion uniformity in polymer
- Interfacial bonding strength between CaCO₃ and polymer
- Melt rheological & processing compatibility
- Retention rate of mechanical and thermal properties
2. Main Testing Methods & Operation Guide
2.1 Contact Angle & Surface Energy Test (Wettability Evaluation)
Purpose
Judge whether the polymer melt can wet CaCO₃ surface; the basic premise of compatibility.
Operation
- Press pure CaCO₃ powder into a smooth compact tablet.
- Test contact angle of polymer melt droplet on CaCO₃ surface by contact angle goniometer.
- Calculate surface free energy of CaCO₃ and polymer respectively.
Judgment Standard
- Contact angle ＜90°: Good wettability, favorable compatibility.
- Contact angle ＞90°: Poor wettability, easy agglomeration.
- The closer the surface energy of CaCO₃ is to polymer, the better the compatibility.
2.2 Microscopic Morphology Observation (Dispersion Compatibility)
SEM (Scanning Electron Microscopy)
Method
Prepare fractured cross-section of CaCO₃/polymer composite sample, observe under SEM.
Compatibility Judgment
- Good compatibility: CaCO₃ particles uniformly dispersed, no large agglomerates, tightly combined with resin matrix, no obvious gaps at interface.
- Poor compatibility: Serious particle agglomeration, obvious voids and peeling at interface, particles exposed separately.
OM Optical Microscope / Particle Size Analysis
Observe thin slices of composite; count agglomeration size and distribution uniformity.
2.3 Melt Rheology Test (Processing Compatibility)
MFI Melt Flow Index Test
Steps
- Prepare composites with different CaCO₃ loading (5%,10%,20%,30%).
- Test MFI under standard temperature and load.
Judgment
- Small change of MFI with CaCO₃ addition: Good compatibility, stable processability.
- Sharp drop/rise of MFI: Poor compatibility, difficult extrusion/injection molding.
Rotational Rheometer
Test viscosity, storage modulus (G′) and loss modulus (G″) of composite melt.
- Well-compatible system: Smooth viscosity curve, no abnormal fluctuation.
- Incompatible system: Viscosity increases sharply, obvious shear thickening or poor fluidity.
2.4 Mechanical Property Test (Macro Comprehensive Compatibility)
Prepare standard dumbbell and impact specimens by blending, granulation and injection molding; test:
- Tensile strength & elongation at break
- Flexural strength & flexural modulus
- Notched / unnotched impact strength
Judgment Rule
- Good compatibility: Mechanical properties decrease slightly or remain stable with CaCO₃ filling.
- Poor compatibility: Sharp decline in toughness and elongation, brittle fracture, obvious strength attenuation.
2.5 Thermal Analysis Test
TGA Thermogravimetric Analysis
Compare thermal decomposition temperature and residual carbon rate of pure polymer and CaCO₃ composite.
- Good compatibility: Thermal stability slightly improved or maintained.
- Poor compatibility: Early thermal decomposition, reduced heat resistance.
DSC Differential Scanning Calorimetry
Test melting point, crystallization temperature and crystallinity.
Compatible CaCO₃ can act as nucleating agent; incompatible filler causes irregular crystallization.
2.6 Interface Chemical Characterization
FTIR Fourier Transform Infrared Spectroscopy
Detect chemical bond changes before and after CaCO₃ compounding with polymer.
- New characteristic absorption peaks = chemical interaction at interface → good compatibility.
- No peak shift = only physical mixing → weak interfacial bonding.
XPS / BET
Analyze surface element combination and specific surface area change to verify interfacial interaction.
3. Standard Test Procedure Step by Step
- Sample preparationUse original/unmodified and surface-treated CaCO₃ (stearic acid, silane, titanate).
- Formula designSet gradient filling ratio: 0%, 10%, 20%, 30% CaCO₃ in target polymer.
- Blending & processingHigh-speed mixing → twin-screw granulation → injection molding / hot pressing sheet.
- Performance testingWettability → microstructure → rheology → mechanics → thermal & FTIR.
- Data comparisonCompare with pure polymer to judge compatibility level.
4. Compatibility Grade Judgment Standard
| Grade | Performance Manifestation | Compatibility Level |
|---|---|---|
| A | Uniform dispersion, stable MFI, mechanical retention＞90% | Excellent |
| B | Slight agglomeration, minor property decline | Good |
| C | Serious agglomeration, obvious interface voids, sharp performance drop | Poor |
5. Compatibility Matching for Common Polymers
- PE / PP PolyolefinsPrefer stearic acid coated CaCO₃; uncoated hydrophilic CaCO₃ has very poor compatibility.
- PVCGood natural compatibility with GCC/PCC; CaCO₃ also acts as smoke suppressant.
- PA NylonChoose silane or titanate modified nano-CaCO₃ to improve interfacial adhesion.
- PET / PBT PolyesterAragonite PCC with titanate treatment shows best compatibility.
- Epoxy ResinSilane-modified CaCO₃ significantly enhances interface bonding.
6. Common Causes of Poor Compatibility & Solutions
- Hydrophilic CaCO₃ + non-polar polymer → Use fatty acid/silane surface modification.
- Excessive particle size and wide distribution → Switch to ultrafine PCC or nano-CaCO₃.
- Over high filling loading → Control dosage within 5–25 wt%.
- Insufficient mixing processing → Optimize twin-screw speed and temperature.