Why is calcium carbonate chosen as a filler for adhesives?
Classification of Adhesives and Filler Modification
Based on their chemical composition, adhesives can be classified into categories such as silicone, polyurethane, epoxy resin, acrylic, and synthetic rubber adhesives.
Modifying various types of adhesives through the incorporation of fillers has become a common and effective practice. By adding fillers, the volume of the adhesive can be increased, thereby reducing costs. Simultaneously, the addition of fillers helps to mitigate inherent issues with the adhesive—such as poor heat resistance, high curing shrinkage, low hardness, and viscosity instability—thereby playing a positive role in expanding the scope of adhesive applications. Common fillers include calcium carbonate, montmorillonite, kaolin, silicon dioxide, titanium dioxide, and silicon carbide.
1. Calcium Carbonate Takes the Top Spot Among Traditional Adhesive Fillers.
A comparison of material properties—varying both the type and content of fillers—revealed that in the PU/EP system, all the different fillers yielded optimal mechanical properties at a filler content of 30%. This is because there is an upper limit to the amount of filler that can be added; appropriate addition allows the filler to uniformly occupy the voids between the molecular chains, whereas excessive addition disrupts the molecular chain structure.
At equivalent filler loading ratios, calcium carbonate demonstrates the superior filling performance. The material achieves a Shore D hardness of 86 and an impact strength of 60.5 kJ/m², representing a 24.9% increase in impact strength compared to the unfilled PU/EP system. Furthermore, the shear strength reaches 24.27 MPa—a 54% improvement over the unfilled PU/EP adhesive. Consequently, it can be concluded that the PU/EP adhesive exhibits optimal mechanical properties when the calcium carbonate content is 30%. Additionally, the study indicates that the incorporation of fillers enhances the thermal stability of the system and facilitates toughening via rigid particles, thereby further reinforcing the overall performance of the PU/EP adhesive.
2. Calcium Carbonate Holds Its Own Among High-End Fillers for Adhesives
The research results indicate that the addition of silicon carbide, silicon dioxide, and calcium carbonate fillers increased the elastic modulus of the epoxy adhesive, while reducing its Poisson's ratio and coefficient of linear expansion.
As the filler content increases, the elastic modulus rises, while both the Poisson's ratio and the coefficient of linear thermal expansion decrease. Silicon carbide contributes significantly to increasing the elastic modulus of epoxy adhesives, whereas calcium carbonate has a comparatively smaller effect; however, calcium carbonate is the most effective in reducing the coefficient of linear thermal expansion of the epoxy adhesive. For adhesives based on the same matrix material, an increase in filler content leads to a higher modulus, which in turn enhances the adhesive's yield strength; a reduction in Poisson's ratio minimizes lateral strain; and a decrease in the coefficient of linear thermal expansion reduces internal curing stresses—all of which contribute favorably to improving the bonding strength.
As the filler content increases, the coefficient of linear thermal expansion of the adhesive gradually decreases; notably, adhesive formulations containing calcium carbonate filler exhibit a lower coefficient of linear thermal expansion compared to those containing silicon carbide or silicon dioxide. This indicates that modification using calcium carbonate filler is more effective in enhancing the dimensional stability and creep resistance of the adhesive.
Key Technologies for the Application of Calcium Carbonate in Adhesives
Currently, the application of nano-calcium carbonate in adhesives primarily focuses on three key aspects: first, the interaction between the filler and the adhesive matrix; second, the influence of the filler on thixotropic properties; and third, the reinforcing effect on sealants.
The factors primarily influencing the compounding performance of specialized nano-calcium carbonate within sealants include: particle size, crystal morphology, oil absorption value, specific surface area, pH value, sieve residue, and moisture content. Among these, the factors predominantly affecting the thixotropy of the adhesive matrix are particle morphology, oil absorption value, surface modification, and pH value; meanwhile, the factors primarily influencing the reinforcing effect are particle size and dispersibility.
Heavy calcium carbonate is frequently utilized as a filler in two-component polysiloxane sealants; its primary objective is to reduce the volume of sealant required, thereby lowering costs. Currently, a common practice involves incorporating a graded blend of heavy calcium carbonate and nano-calcium carbonate into construction-grade polysiloxane sealants; this approach not only yields superior tensile properties but also endows the sealant with excellent flow characteristics. Furthermore, certain grades of ultrafine heavy calcium carbonate (1–2 μm) can be added as a filler to polyurethane sealants. On one hand, this serves to increase volume—thereby reducing the quantity of sealant consumed and lowering costs—while on the other, it acts as a reinforcing agent for the sealant, enhancing its mechanical properties and reducing parameters such as the coefficient of thermal expansion and curing shrinkage rate.
Calcium carbonate, leveraged through adhesives, has successfully opened up numerous niche markets.
According to data from the Annual Conference of China's Adhesive and Adhesive Tape Industry, the development targets for China's adhesive sector during the "14th Five-Year Plan" period include an average annual growth rate of 4.2% in production volume and 4.3% in sales revenue. According to incomplete statistics from the *Outlook on the Adhesive Industry During the 14th Five-Year Plan*, domestic adhesives are currently primarily utilized in the construction, packaging, and timber industries, collectively accounting for over 50% of total applications. Looking ahead, demand for adhesives is projected to grow rapidly in mid-range application sectors—such as automotive, energy, and electronic information—as well as in high-end sectors, including aviation, aerospace, shipbuilding, and semiconductors; the combined share of these applications is expected to reach 40%.
In terms of application categories and directions, calcium carbonate has, in conjunction with adhesives, successfully penetrated numerous niche markets. Construction adhesives account for approximately 26.1% of the total adhesive product market; key application categories include silicone, epoxy, polyurethane, and acrylate adhesives—among which calcium carbonate serves as the primary filler. Polyurethane adhesives command a market share exceeding 90% within China's flexible packaging adhesive sector, representing one of the key growth frontiers for medium-to-high-end calcium carbonate. Adhesives for automotive applications utilize base materials such as polyurethane, epoxy, silicone, and acrylate, with polyurethane and silicone emerging as the dominant base materials; domestically, specialized nano-calcium carbonate products tailored for this specific application segment are already available on the market.