Calcium carbonate is one of the most important inorganic powder additives used in plastic processing. This article expresses views on several issues that are of great concern to both the calcium carbonate and plastic processing industries, aiming to clarify and promote the correct use of calcium carbonate and expand its applications.

1. The important role of calcium carbonate in plastics

China has become a major producer and consumer of plastic products, with the output of plastic products exceeding 40 million tons in 2006, ranking second in the world, and maintaining a double-digit annual growth rate for more than twenty years. It is foreseeable that with the growth of domestic consumption demand and the increase in export quantities as a global manufacturing base supplying the international market, the annual output of plastic products in China will continue to grow rapidly for a considerable period of time.

In the plastic processing process, in addition to synthetic resin as the basic raw material, the scientific, correct, and reasonable use of various additives and auxiliaries is indisputable, among which inorganic mineral powder materials are one of the most important additives. As everyone knows, adding inorganic mineral powder materials to plastics can play an important role in reducing raw material costs, improving performance, and imparting new functions. In recent years, it has been further discovered that the use of inorganic mineral powder materials has environmental protection effects in reducing white pollution and protecting the environment. In today's trend of emphasizing the implementation of a circular economy and building a resource and energy-saving, environmentally friendly society, the significant importance of inorganic mineral powder materials in plastic applications is further highlighted.

Not all plastic materials and products need to add inorganic mineral powder materials, nor do they all add the same quantity. When calculating the amount of inorganic mineral powder materials used in plastics, it is usually based on 10% of the total output of plastic materials and products, which means that the inorganic powder materials used in China's plastic processing industry each year are at least over 4 million tons. Calcium carbonate (including heavy calcium and light calcium) is the most widely used and has the largest consumption among inorganic mineral powder materials, accounting for over 70% of the total amount of inorganic mineral powder materials used. This is not only because calcium carbonate is abundant and inexpensive, but also due to its good stability, pure color, low wear, easy drying, easy processing, and non-toxicity, which are important reasons for its widespread use.

Looking at the history of calcium carbonate use in the plastic industry, it can be said that since the beginning of China's plastic industry in the 1960s, the application of calcium carbonate has always developed alongside the plastic processing industry. Initially, the idea of using calcium carbonate in plastics benefited from the application experience of light calcium carbonate in rubber materials and products, which is why the use of light calcium preceded that of heavy calcium and has continued to this day. Starting in the 1980s, with the gradual deepening of China's reform and opening up, the processing and application of heavy calcium carbonate, characterized by resource economy, rapidly emerged, marking the arrival of a historical period of large-scale use of heavy calcium carbonate represented by filling masterbatches. By the mid-1990s, heavy calcium processing experienced a leap, with ultrafine heavy calcium with a particle size below 10μm entering the market, marking the beginning of the third stage where heavy calcium and light calcium were used together, and different particle size ranges of heavy calcium coexisted. By the early 21st century, the industrialization of nano-calcium carbonate opened up a new realm for the application of calcium carbonate in plastics.

Tables 1 and 2 list the basic situation of the plastic products industry and product output, while Table 3 lists the use of inorganic mineral powder materials such as calcium carbonate, talc, and kaolin in major plastic products.

2. Basic requirements of plastic processing enterprises for the calcium carbonate used

Calcium carbonate, as a commonly used powder material in plastics, has many advantages that other powder materials do not possess, such as high whiteness, ease of surface organic treatment, light wear on processing equipment and molds, and good flowability in molding processing. Coupled with its abundant resources and low price, it has become the preferred inorganic mineral powder material in the plastic processing industry. Based on the current thoughts and experiences of plastic processing enterprises, the factors considered when selecting calcium carbonate constitute the basic requirements for calcium carbonate.

(1) Low price

Although the price of calcium carbonate is already very low compared to synthetic resins, and it is also considered cost-effective compared to any other powder material, due to the large number of suppliers, enterprises remain sensitive to price, preferring the cheaper the better as long as it can be used.

(2) The higher the whiteness, the better

Calcium carbonate itself is whiter than other inorganic mineral powder materials, and specifically, a whiteness of 90 degrees is sufficient. The impact of whiteness on the performance of filled plastics is not significant, but plastic processing enterprises still hope for higher whiteness based on

the following reasons:

① The higher the whiteness, the higher the purity, and the lower the content of other mineral components that are not calcium carbonate.

Gray calcium powder may indicate the presence of free carbon, yellowing may indicate a higher content of iron compounds, and darkening may indicate a high silicon content. High impurity content can sometimes affect the color, appearance of filled plastic products, and cause significant wear on processing equipment and molds during processing.

② The higher the whiteness, the better the visual impression, and the higher the perceived quality.

③ The higher the whiteness, when paired with other pigments, the final color will be more accurate, stable, and uniform.

(3) Appropriate particle size and distribution

As a powder material, one must consider both the particle size of the particles themselves under testing conditions and the proportion of particles within a certain size range, and also consider the state of these particles in practical applications, whether they are agglomerated together, and whether the agglomerates can disperse during the processing of the filling system, like islands of various sizes dispersed in the "ocean" of the base plastic?

① From a cost perspective, as long as it meets the usage requirements, it is better to use coarser particles rather than finer ones. For example, for the heavy calcium used in producing polypropylene woven bags (cloth), a particle size of 400 mesh is sufficient. If finer calcium powder is used, not only will the price itself increase significantly, but also more additives and carrier resins will be consumed in making the filling masterbatch, which is not cost-effective.

② From the perspective of the performance of filled plastics, under the premise of being able to appropriately increase costs, using finer particle sizes will yield better results. Table 4 lists the effects of heavy calcium of different particle sizes on the performance of polyethylene (PE) plastic films under the same filling conditions.

③ From the perspective of processing flowability, under the premise of controlling the maximum particle size, a larger average particle size is preferable.

For calcium carbonate products of the same particle size, the particle size distribution can vary greatly, which can be reflected in the average particle size. For plastic products that require good processing flowability (such as injection molded products, cast PE films, etc.), it is essential not to use calcium powder with an average particle size that is too small. If possible, it is best to separate out the small-sized particles while controlling the maximum particle size. This is because the more small-sized particles there are, the worse the processing flowability of the filling system.