Watch the Water!

Mind the Water! Calcium Carbonate in the Adhesives and Sealants Industry: A Discussion

1. Summary

This article is the first of three papers prepared on the science of Calcium Carbonate in adhesives and sealants, focusing on the role water plays in moisture-sensitive formulations. The subsequent articles will address the fundamentals of rheological modification and mechanical reinforcement. Calcium Carbonate is the most commonly used mineral filler in adhesives and sealants. Fine-grained and processed products, unlike coarse-grained products (fillers) originally used to reduce formula costs, offer additional functions such as mechanical reinforcement and rheological modification. In moisture-cured and moisture-sensitive systems, particularly when using micrometric and sub-micrometric products, the amount of water carried by Calcium Carbonate into the formulation must be considered (Figure 1). Essentially, to prevent unwanted variability in the quality and performance of adhesives and sealants, time-consuming and costly steps are added to the production process, such as pre-drying of fillers, extended mixing under vacuum, heating/cooling, or chemical drying. From economic and operational perspectives, it is important for formula developers and production engineers to understand certain fundamentals regarding moisture in Calcium Carbonate-based fillers. This article will focus on moisture uptake mechanisms and technologies used to minimize moisture content in Calcium Carbonate-based fillers. Based on existing models in the literature, it will begin with a discussion of moisture uptake mechanisms and will emphasize findings and information applicable to real-world production conditions. This article also covers the fundamentals of the surface treatment process of Calcium Carbonate-based fillers, the most commonly used technology to minimize moisture uptake. Finally, advanced processing and materials engineering solutions will be summarized, and the industrial importance of this topic will be explained and supported through case examples.

2. Water in Moisture-Sensitive Adhesive and Sealant Production

The presence of water in the moisture-curing process of moisture-sensitive adhesives and sealants is an undesirable condition, as it negatively affects both product quality and the production process. Most notably among these is the reduction in shelf life of the final product. Removing water from component materials and formulations requires long-duration and high-cost steps such as pre-drying fillers or extended mixing and heating under vacuum. When controlled water content is achieved, formulations can be stabilized with chemical moisture scavengers. Although the importance of water content in Calcium Carbonate-based fillers is generally accepted among formula developers working with moisture-cured systems, there are differing views on the source of water content and how it enters the aforementioned systems.

3. Calcite and Water

Calcite, the fundamental building block of marble, limestone, and chalk, is the most commonly observed crystal form of Calcium Carbonate. The dominant surface plane [1014] of this rhombohedral crystal structure is the most frequently studied surface for scientific purposes. In nature, the morphology of calcite is heterogeneous; its surface contains numerous protrusions and micro-cracks, as well as flat areas. The characteristic surface morphologies of calcite are shown in Figure 2 [1]. Despite being millions of years old, Calcium Carbonate exhibits an extremely dynamic system. The CaCO₃ surface reacts and transforms remarkably rapidly even in ultra-high vacuum [3]. During industrial processing, new surfaces are created by fracturing crystals and disrupting the calcite structure. However, Ca²⁺ and CO₃²⁻ ions rapidly reorganize and begin interacting with the environment, significantly altering the material's interaction with water. High-temperature industrial conditions accelerate this effect [8]. Wojas and colleagues [1] used atomic force microscopy studies to describe the mechanism underlying the reorganization of the CaCO₃ surface (Figure 1): 1. With increasing humidity and time, Ca²⁺ and CO₃²⁻ ions dissolve in the adsorbed water layer. 2. Ions diffuse to energetically preferred areas and recrystallize on the surface. 3. Recrystallization creates nano-sized cracks and bump-like areas containing CaCO₃ hydrated salts. 4. When exposed to dry environments, these surface areas undergo dehydration. 5. However, during dehydration, ions continue to distribute and crystallize at a low rate, resulting in a residual water layer remaining on the surface. In summary, calcite surface degradation irreversibly promotes the new surface's interaction with water. The consequences of this model are limited to interaction with water. However, it should not be forgotten that Calcium Carbonate is also highly reactive with other molecules. When organic contamination occurs, foreign substances are rapidly adsorbed by the calcite surface. Calcite areas blocked by foreign matter cannot react with fatty acids, which limits the effectiveness of hydrophobization processes (see section 5).

4. Calcium Carbonate in Humid Environments

After explaining the dynamic reorganization of the calcite surface exposed to moisture, we focus on the interaction of Calcium Carbonate particles with environmental moisture. According to scientific publications [1,4,5], the water uptake curve shown in Figure 2 is governed by three different water adsorption mechanisms depending on environmental moisture levels: • Below 50% relative humidity, water forms on hydrated CaCO₃ areas, creating a monomolecular water layer on the existing Calcium Carbonate surface. • At approximately 50% relative humidity, a three-dimensional water surface begins to form. • At relative humidity levels above 70%, capillary forces become dominant and water adsorption increases rapidly.

5. Fatty Acid Treatment

Most scientific research on the interaction of organic compounds with Calcium Carbonate focuses on the scale of the petroleum industry. The motivation behind these studies lies in the fact that the petroleum industry has developed an extensive knowledge base on Calcium Carbonate adsorption, hydrophobization, and interactions, as well as advanced techniques used in extracting petroleum from limestone reservoirs. The most commonly used technology in Calcium Carbonate hydrophobization is "drying" with fatty acids. This technology is based on the chemical bonding of RCOO⁻ carboxyl groups to reactive Ca⁺ ions present on the calcite surface. Hydrophobization is extremely effective in reducing Calcium Carbonate's water uptake, particularly when relative humidity exceeds 50% (gray line in Figure 3). There are two main reasons that make fatty acids an attractive compound for Calcium Carbonate hydrophobization. Calcium sites are distributed at regular intervals on crystalline calcite surfaces and occupy an area of 20.8 Ų. This is close to an area of 20.5 Ų occupied by a carboxyl group of regularly spaced fatty acid [9]. Second, the long linear tail of fatty acids provides structural stability. These tails can be positioned perpendicular to the surface in an extremely effective, closely-spaced arrangement [10]. This two-factor combination—the "head" that binds well to calcium sites on the calcite surface and the "tail" that clusters alongside it—gives fatty acids a thermodynamically extremely stable structure, making them particularly well-suited for effective Calcium Carbonate hydrophobization. It should be noted that clean surfaces with accessible calcium sites are required to effectively ensure chemical bonding of fatty acids.

6. Conclusions

Freshly fractured Calcium Carbonate surfaces are remarkably reactive. To reduce water content and water uptake: 1. The surface must be clean and free of contamination; raw materials should be selected from high-quality marble to enable the surface to interact with fatty acids. 2. The hydrophobization reaction should be carried out immediately after fracturing to prevent new fractured surfaces from becoming contaminated and degraded. 3. Fatty acid hydrophobization is one of the best chemical reactions for ensuring effective surface modification and maintaining water uptake at low levels. When all the above criteria are met, Calcium Carbonate water content can be maintained at low levels across all environmental conditions (light blue line in Figure 3), which provides significant economic benefits to manufacturers of moisture-cured systems in the adhesives and sealants industry. Understanding the mechanism by which water interacts with Calcium Carbonate enables formula developers and process engineers to better control Calcium Carbonate and water-related issues. When this information is considered during raw material selection, formula developers and process engineers gain better understanding and control of water-related issues in the production of moisture-cured adhesives and sealants.

References

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