Advancements in calcium carbonate processing technologies have enabled it to evolve from a traditional filler into a modifier. This evolution allows for cost reductions in products while simultaneously enhancing their properties. Some of them are unique to calcium carbonate. With the increasing maturity of micro-foaming technology and the successful development of hollow calcium carbonate, the weight reduction capabilities of calcium carbonate composite materials are poised for industrial production.
We can confidently predict that in the future, plastic calcium carbonate composite materials will redefine the traditional framework of “two reductions and one improvement”—namely, reducing costs and density while enhancing performance. Calcium carbonate will transition from a mere filler to a revolutionary modifier.
Traditional plastic calcium carbonate composite materials do not solely result in a reduction of all material properties. Instead, they can also enhance various attributes while causing some performance degradations. This article will specifically explore both the positive and negative effects of calcium carbonate as a modifier. It guides us in learning the development in calcium carbonate modification of future research.
Positive Modification Effects of Calcium Carbonate
1 Environmental Benefits of Calcium Carbonate
1.1 Conservation of Petroleum Resources
Calculated Impact of Calcium Carbonate in Plastic Packaging
Based on a 30% calcium carbonate content in polyethylene (PE), the annual production of 3 million tons of plastic packaging bags could save approximately 900,000 tons of petroleum-based resin and 2.7 million tons of oil.
1.2 Environmentally Friendly Performance
Incorporating calcium carbonate into plastic garbage bags intended for incineration can enhance combustion efficiency and significantly reduce incineration time. When burned, calcium carbonate expands within the plastic film, creating numerous tiny holes that increase the surface area available for combustion. This phenomenon accelerates the burning process. For instance, the incineration time for polyethylene plastic film containing 30% calcium carbonate is reduced from 12 seconds (for pure plastic) to just 4 seconds.
Furthermore, calcium carbonate-filled plastic films promote more complete combustion, which minimizes the release of black smoke due to the wick effect of calcium carbonate. The alkalinity of calcium carbonate also helps absorb a significant amount of dispersed acidic gases, thereby reducing the emission of toxic smoke and decreasing the likelihood of acid rain.
In Japan, regulations stipulate that plastic garbage bags for incineration must contain at least 30% calcium carbonate. Beyond the enhanced burning speed, calcium carbonate-filled bags generate less heat, do not produce drips or black smoke, mitigate secondary pollution, and are non-damaging to incinerators.
2. Common Modification Effects of Calcium Carbonate
2.1 Improved Rigidity of Composite Materials
Calcium carbonate enhances the bending strength, bending modulus, hardness, and wear resistance of composite materials. In plastic films, the increased rigidity significantly improves stiffness, facilitating flat curling and overall structural integrity.
2.2 Enhanced Dimensional Stability of Composite Materials
Calcium carbonate contributes to improved dimensional stability by reducing shrinkage and warping, lowering the linear expansion coefficient, minimizing creep, and promoting isotropy. The inclusion of calcium carbonate in composites significantly enhances dimensional stability.
2.3 Improvement of Heat Resistance in Composite Materials
Calcium carbonate enhances the thermal stability of composite materials by absorbing substances that promote decomposition. For example, PBAT/calcium carbonate composites exhibit significantly greater thermal stability compared to pure PBAT. Additionally, incorporating light calcium carbonate into PVC products effectively absorbs hydrogen chloride produced during decomposition, greatly enhancing the processing thermal stability of PVC.
2.4 Enhanced Tear Resistance of Films
Typical plastic films often have high longitudinal strength but low transverse strength, particularly in materials like PBS, PLA, and PHA aliphatic polyester films. The addition of calcium carbonate can improve the isotropy of these composite materials, leading to significantly enhanced tear resistance.
3. Special Modified Properties of Calcium Carbonate
3.1 Effects on Tensile and Impact Properties
The impact of calcium carbonate on tensile strength and impact strength in plastic films is not universal; it is influenced by factors such as particle size and surface treatment.
Effect of Particle Size: Different particle sizes of calcium carbonate yield varying modification effects on plastics, as illustrated in Table 1. Generally, particle sizes below 1000 mesh are used for incremental modification. Particle sizes between 1000 and 3000 mesh, with an addition amount below 10%, can achieve some modification effects. In contrast, calcium carbonate with particle sizes above 5000 mesh, classified as functional calcium carbonate, demonstrates significant modification effects and can improve both tensile strength and impact strength. Although nano-scale calcium carbonate has a finer particle size, its current difficulty in dispersion limits its effectiveness, restricting it to similar modification results as 8000 mesh calcium carbonate.
Table 1: Effect of Heavy Calcium Carbonate with Different Particle Sizes on the Performance of PP Composite Materials
| Coupling agent treated heavy calcium carbonate (30%) mesh size | 2000 | 1250 | 800 | 500 |
| Melt flow index (g/10min) | 4.0 | 5.0 | 5.6 | 5.5 |
| Tensile strength (MPa) | 19.3 | 18.4 | 18.7 | 18.1 |
| Elongation at break (%) | 422 | 420 | 341 | 367 |
| Flexural strength (MPa) | 28 | 28.6 | 28.2 | 28.4 |
| Flexural modulus (MPa) | 1287 | 1291 | 1303 | 1294 |
| Izod impact strength (J/m) | 113 | 89 | 86 | 78 |
As shown in Table 1, finer particle sizes of calcium carbonate lead to increased impact strength, tensile strength, and elongation at break, while the flexural strength and flexural modulus remain relatively unchanged. However, the fluidity of the composite material decreases with finer particle sizes.
Effect of Surface Treatment : Proper surface treatment of calcium carbonate with suitable particle sizes can significantly enhance the tensile and impact strengths of composite materials. Recently, advancements in organic/inorganic composite theory have transformed calcium carbonate from a simple filler into a novel functional filling material. For instance, the notched impact strength of a homopolymer polypropylene (PP)/calcium carbonate composite can more than double compared to the base plastic.
3.2 Smoke Suppression During Combustion
Calcium carbonate exhibits excellent smoke suppression capabilities. This is due to its ability to react with hydrogen halides in smoke, forming stable calcium chloride (CaCl₂). Therefore, it can be used as a smoke suppressant in any polymer that produces hydrogen halides during combustion, including vinyl chloride, chlorosulfonated polyethylene, and chloroprene rubber.
Since combustion is a solid-gas heterogeneous reaction that occurs at the surface of solid particles, the particle size of calcium carbonate plays a crucial role in its smoke suppression effectiveness. Finer particles possess a significantly larger specific surface area, which enhances the smoke suppression effect.
3.3 Anti-Adhesion Agent
Blown tubular films containing calcium carbonate demonstrate excellent opening properties and resist adhesion during curling. In this context, calcium carbonate functions effectively as an anti-adhesion agent.
3.4 Increase thermal conductivity
The addition of calcium carbonate increases the thermal conductivity of the film, so that the bubble of the blown film cools faster, improves production efficiency, and increases the output of the extruder. Taking the addition of 25% light calcium carbonate to PVC sheet as an example, it only takes 3.5 seconds to heat to 200°C, while pure PVC sheet takes 10.8 seconds, and the thermal conductivity is increased by 3 times.
3.5 Improve fluidity
Calcium carbonate can improve the fluidity of the composite system, reduce melt viscosity and extruder torque, increase extruder output, and improve production efficiency. Different types of calcium carbonate have different effects on flow. The order of the fluidity of the specific composite material is large calcite calcium carbonate> marble calcium carbonate, dolomite calcium carbonate> small calcite calcium carbonate> light calcium carbonate.
3.6 Color matching performance
Replacement of some white pigments: High whiteness calcium carbonate can replace some white pigments such as titanium dioxide, thereby saving the content of expensive titanium dioxide. Large calcite calcium carbonate is the first choice because of its high whiteness and high hiding power. The reason why calcium carbonate can be used as a white pigment is mainly because it has a certain hiding power. The hiding power of a coating refers to the minimum amount of paint required to evenly apply the paint on the surface of an object so that the base color no longer appears. It is expressed in g/㎡.
The hiding power of various colorants in coatings is shown in Table 2:
Table 2: Hiding Power of Some Inorganic and Organic Pigments
| Pigment name | Covering power(g/cm) |
| Para red (light hue) | 18.1-16.3 |
| Para red (dark hue) | 17.1-15.0 |
| Red lake c | 23.8-18.8 |
| Lithol red (Ba lake) | 33.7-21.7 |
| Lithol red (Ca lake) | 49.0-33.7 |
| Lithol ruby | 33.9 |
| Yanke scarlet lake | 88.5 |
| Rhodamine Y (tungstate precipitate) | 25.1 |
| Rhodamine B (phosphotungstate precipitate) | 16.1 |
| Toluidine chestnut red | 34.8-37.7 |
| Light-fast red BL | 12.4 |
| Titanium dioxide | 18.4 |
| (rutile type, anatase type) | 19.5 |
| Zinc oxide | 24.8 |
| Barium sulfate | 30.6 |
| Calcium carbonate | 31.4 |
| Hansa yellow G | 54.9 |
| Hansa yellow 10G | 58.8 |
| Permanent orange | 29.6 |
| Malachite green | 5.4 |
| Pigment green B | 2.7 |
| Malachite blue (phosphotungstate precipitate) | 7.7 |
| Malachite blue | 68.5 |
| Methyl violet (phosphotungstate precipitate) | 7.6 |
| Methyl violet (tannin precipitant) | 4.9 |
| Sunlight fast violet | 10.2 |
| Phthalocyanine blue | 4.5 |
| Zinc barium mortar (lead powder) | 23.6 |
| Lead mortar (basic lead sulfate) | 26.9 |
| Antimony trioxide | 22.7 |
| Talc | 32.2 |
The hiding power of a material is closely related to its refractive index. Generally, a higher refractive index results in greater hiding power and a more intense white hue. The refractive index of various white materials is detailed in Table 3.
Table 3: Refractive Index of Various White Materials
| White materials | Colorant index number | Refractive Index |
| Titanium dioxide (rutile type) | Pigment mortar 6 | 2.70 |
| Titanium powder (anatase type) | Pigment mortar 6 | 2.55 |
| Zirconium oxide | Pigment mortar 12 | 2.40 |
| Zinc sulfide | 2.37 | |
| Antimony trioxide | Pigment mortar 11 | 2.19 |
| Zinc oxide | Pigment mortar 4 | 2.00 |
| Lithopone (zinc-barium powder) | Color mortar 21 | 2.10 |
| Barium sulfate | Pigment mortar 18 | 1.64 |
| Calcium carbonate | Pigment mortar 27 | 1.58 |
| Talc | Colorant index number | 1.54 |
Impact on Coloring Calcium carbonate’s natural white color influences its ability to match bright colors, making it challenging to achieve bright color combinations. Additionally, it can complicate the matching of special blacks.
Impact on Color Light Beyond its natural white color, calcium carbonate can exhibit different color lights, affecting color purity. Color light refers to the additional hues that an object displays alongside its main color. For instance, complementary colors are found at opposite ends of the color spectrum; blue, for example, is complemented by yellow. Mixing these can produce white light, an effective method for neutralizing color light.
The base color emitted by calcium carbonate varies by origin. For example:
- Calcium carbonate from Sichuan has a blue base color.
- Calcium carbonate from Guangxi has a red base color.
- Calcium carbonate from Jiangxi also has a blue base color.
When matching colors, the color light of calcium carbonate should align with the primary coloring hue. For instance, calcium carbonate with a blue tint can counteract the coloring power of yellow pigments. It is also commonly used to neutralize yellow color light in products.
Improving Astigmatism in Plastic Products: While the addition of calcium carbonate does not enhance the gloss of plastic products, it effectively reduces gloss, providing a matting effect.
3.7 Increasing Breathability
Plastic films filled with calcium carbonate create tiny pores during stretching, allowing water vapor to pass through while preventing liquid water infiltration. This characteristic makes them suitable for producing breathable plastic products. For optimal results, only calcium carbonate with a particle size of 3000 mesh or finer should be used, with a narrow particle size distribution.
3.8 Promoting Degradation Performance of Products
When polyethylene plastic bags containing calcium carbonate are buried, the calcium carbonate may react with carbon dioxide and water to form water-soluble calcium bicarbonate (Ca(HCO₃)₂), which can leave the film. This process creates tiny holes in the film, increasing the surface area in contact with air and microorganisms, thereby facilitating the degradation of the product.
3.9 Nucleation Role of Calcium Carbonate
Nano-calcium carbonate (CaCO₃) plays a crucial role in the crystallization nucleation of polypropylene, increasing the β-crystal content and thereby enhancing the impact toughness of polypropylene.
3.10 Reduction of Water Absorption in PA Plastics
The water absorption of polyamide (PA)/calcium carbonate composites is significantly lower than that of pure PA resin. For instance, incorporating 25% calcium carbonate into PA6 can reduce the water absorption rate of the composite material by 56%.
3.11 Improvement of Surface Properties
Calcium carbonate can enhance the surface tension of composite materials and exhibits excellent adsorption properties, which improve the electroplating, coating, and printing characteristics of these materials.
3.12 Effects of Calcium Carbonate on Foaming
The influence of calcium carbonate on the foaming performance of plastic materials is complex and depends on both the particle size and the amount used:
Calcium Carbonate Size: When the particle size of calcium carbonate aligns with that of the foaming agent, it can act as a nucleating agent, positively influencing foaming. The ideal particle size is less than 5 μm and should avoid agglomeration. If the particle size exceeds 10 μm or is too fine and agglomerates, it can negatively impact foaming. It is recommended to use 3000 mesh (approximately 4 μm) calcium carbonate to ensure a size under 5 μm without agglomeration.
The mechanisms by which calcium carbonate promotes foaming include:
Acting as a nucleating agent by absorbing foaming gas to create bubble nuclei, thus controlling the number of pores and refining their size.
Providing rigidity that slows down the deformation and mobility of the melt, which helps inhibit rapid pore expansion and allows for finer pore sizes. Nano-calcium carbonate can even generate microporous foam plastics due to the small size of the nucleating agent.
Amount of Calcium Carbonate Added: The optimal filling amount for calcium carbonate to enhance foaming quality typically ranges from 10% to 30%. If too little is added, there won’t be enough nucleation points, leading to a low foaming ratio. Conversely, if too much is used, while more nucleation points are created, the melt strength may decrease excessively, resulting in numerous broken bubbles and a reduced foaming ratio.
Dispersibility of Calcium Carbonate: Even dispersion of calcium carbonate is essential for promoting foaming quality. Uniformly distributed calcium carbonate ensures no agglomeration. If the particle size is within 5 μm, it will effectively function as a nucleating agent without adversely affecting foaming.
Water Content of Calcium Carbonate: If the water content of the inorganic powder is below 0.5%, it will have minimal impact on foaming.
Other Properties: Calcium carbonate also contributes to improved wear resistance and hardness in composite materials.
Negative Modifications of Fillers
1. Increased Density of Composite Materials
The addition of calcium carbonate to resin results in a rapid increase in the density of the composite material. For products sold by weight, length, or area, this increased density can offset some cost advantages. The extent of weight gain varies among different types of calcium carbonate, with the specific density order as follows:
Light calcium carbonate < Large calcite calcium carbonate < Marble calcium carbonate < Dolomite calcium carbonate < Small calcite calcium carbonate.
How to Reduce the Density of Calcium Carbonate Composite Plastics:
1.1 Product Stretching for Weight Reduction:
Stretching creates deformation gaps between the plastic and calcium carbonate, slightly reducing the overall density. For example, a stretched polyethylene film filled with 30% calcium carbonate has a density of 1.1 g/cm³, compared to 1.2 g/cm³ for the unstretched version. This technique is applicable to various plastic products such as flat wire, blown film, strapping tape, and tear film.
1.2 Product Micro-Foaming for Weight Reduction:
Utilizing the moisture absorbed by the filler for micro-foaming can significantly decrease density without compromising performance. For instance, our 50% lightweight calcium carbonate composite material can achieve a minimum density of 0.7 g/cm³ when used to produce films, representing a 45% reduction.
1.3 Hollow Filling for Weight Reduction:
Employing simple and cost-effective inorganic powder hollowing technology allows for the production of hollow calcium carbonate products, which greatly reduces density. The density of these hollow products can be reduced to approximately 0.7 g/cm³.
2. Reduction of Gloss in Composite Materials
The impact on the surface gloss of composite products is influenced by the processing method and the specific type of calcium carbonate used. The order of gloss for different composite materials is as follows:
- Wet process > Dry process
- Light calcium carbonate > Large calcite calcium carbonate > Marble calcium carbonate > Small calcite calcium carbonate > Dolomite calcium carbonate.
3. Reduction of Transparency in Composite Materials
Calcium carbonate has a refractive index that significantly differs from that of common resins like polyethylene and polypropylene. As a result, conventional-sized calcium carbonate fillers can negatively impact the transparency of films. In contrast, only nano-calcium carbonate with a particle size below 200 nanometers can maintain the transparency of the composite product, as light waves can effectively bypass such small particles.
4. Reduction of Elongation at Break in Composite Materials
The high rigidity of calcium carbonate can diminish the original ductility of the composite material. This increased rigidity reduces the mobility of the macromolecular chains, resulting in a decreased elongation at break for the final product.
5. Decrease in Tensile Strength and Impact Strength
In many cases, the addition of calcium carbonate can lead to reduced tensile strength and impact strength in the composite material. This is particularly true if the calcium carbonate particles are too large or if the surface treatment of the calcium carbonate is inadequate. The most notable decline is often seen in tensile strength.
6. Increased Stress Whitening Phenomenon
When a substantial amount of calcium carbonate is incorporated into the resin, stretching the product can create numerous gaps and silver streaks, which exacerbates the stress whitening phenomenon in the resin.
7. Acceleration of Product Aging
All inorganic powder materials, including calcium carbonate, can accelerate the aging of composite materials, leading to a reduction in the longevity and performance of the products.
8. Reduced Bonding Strength Between Materials
The use of calcium carbonate can lower the bonding strength of films, such as reducing the heat sealing strength, and can also diminish the welding strength of pipes.