Calcium carbonate's application in rubber.

Origins of calcium carbonates

Calcium carbonate is one of the most abundant minerals found on the earth. Almost all calcium carbonates originated from the remains of organic skeletal material produced in a marine environment. A much smaller quantity originated from reaction of calcium salts with carbon dioxide. There were several different sub-environments within this marine environment. These were reef formations (such as coral), shallow water organisms (such as oysters and mussels found in lagoons), and the continental shelf where skeletal sediments formed the main sea bed building mechanism. Over a period of millennia, the environment of the deposit changed. Silt buried some of these deposits while others formed mountains. During these processes, the original deposits experienced a variety of forces that modified their form. Many underwent varying degrees of compression forming soft chalk or harder limestone. Others experienced extremes of pressure and heat, resulting in metamorphosis and re-crystallization into marble. Solution and re-crystallization changed others to form travertine found as stalagmites and stalactites. Compound these changes with the inevitable variation in localized impurities and we have a variety of materials that the industrial world knows simply as calcium carbonate.

Several basic crystal forms of calcium carbonate are found, the most common being calcite and aragonite. Dolomite and vaterite, which are very unstable, also occur naturally. Aragonitic materials are relatively rare and most often found as a synthesized precipitated product. Almost all material used within the polymer industry is calcite, this being the most stable form of calcium carbonate.

Classifications

There are several ways to classify calcium carbonates, each method having its own merits.

Initially these are by geology, being chalk (a soft limestone), limestone, dolomite (a calcium-magnesium carbonate) and marble. Other sources do not fit well into this scheme as they have recent origins, e.g. precipitated calcium carbonates and ground oyster shells.

Another classification used within the calcium carbonate industry is based on whiteness as the color often determines the practical application. The high whiteness class (90+) includes precipitated calcium carbonate and marble; medium to low whiteness (80-90) includes dolomite, limestone and chalk.

This discussion will characterize calcium carbonates by process route and particle size, since the rubber industry does not often show concern with the origin or color of the source rock.

Classification by production route

Five main production schemes are used to produce industrial calcium carbonates. These are: screen separation; dry milling with air classification; low solids, wet grinding; high solids, wet grinding; and precipitation. In practice, the industry uses a variety of equipment of similar design within each of these generic process routes to manufacture the same end products, but the principles behind these different designs remain the same.

As a pre-cursor to all of these processes, the ore from the mine is extracted, crushed, washed and blended as a first step toward production of a uniform product. Feed stone for grinding processes is typically 25-100 mm in size for ease of transportation.

Screen separation

In these processes roller mills or cage mills crush the ore, producing a coarse rock dust. This then passes sequentially through a range of sieves of differing sizes producing a range of coarse products of well-defined distribution. These products typically fall into the size range 40 to 2,000 micrometers mean particle size, or using U.S. mesh sizes between 4 and 200. These products are too coarse for widespread use in rubber applications.

Dry milling

The lowest cost method of producing ground calcium carbonates (GCC) is by dry milling. Dry milling is usually preceded by a simple hammer mill that reduces particle size of the feed to a few millimeters. The feed then enters another mill such as a roller mill, which contains heavy rollers that crush the coarse material into a fine dust. Alternatively, a tube mill or dry ball mill, containing a medium such as ceramic or steel balls, may be used. Air is blown through the mill's body carrying the finer fraction out by way of an air classifier. The classifier may be built into the mill or a discrete unit. A polar grinding aid such as a glycol or amine is often used to increase the powder's affinity to enter the air stream. The classifier can be operated at different speeds to control the size of the output. Different sized products can be manufactured using this process ranging from 2 to 20 microns mean diameter.

It should be noted that for these processes the output is no more than a finer particle sized version of the input. The product, therefore, will reflect any variation in the feed consistency from the point of view of color or chemical purity.

Low solids, wet grinding

In this process, rock is added directly to a large tube mill or ball mill along with water. The mill may operate autogenously, where the rock is its own grinding agent, or may contain grinding media. The discharge side of the mill is a low solids slurry, typically 20% mineral. This slurry will usually pass through a froth flotation system where a chemical foaming agent is added. The agent is selected for its affinity with the specific impurities found in the ore locally and is also hydrophobic. When air is beaten into the liquid, the hydrophobic tail attaches to the air bubble and rises to the surface, carrying the impurity with it. Removal is achieved by mechanical means. The beneficiated product will then enter a centrifugal classifier that selectively fractionates the slurry into fine and coarse products. The coarse product may re-enter the mill feed or feed another grinding system allowing it to be re-circulated through the centrifuge. After classification the product is normally screened at 325 or similar mesh and passes to a settling bowl where it sediments. The under-flow from the bowl will then go through further dewatering by vacuum or mechanical filtration and will finally enter a mill that simultaneously dries and pulverizes the product. This route typically produces material in the 0.7 to 11 micron mean diameter range.

The resultant output of this circuit is a range of materials that are free from addition of chemicals with a high order of chemical purity.

High solids, wet grinding

The high solids route is somewhat different to the low solids route in that it receives a feed from a roller or attrition mill. This feed will be mixed with water and a surfactant, usually a polyacrylate salt, at high (around 50%) solids. This mixture will then undergo froth flotation and pass to a high energy vertical mill filled with a grinding medium such as a hard silicate or zirconate. Exit from the mill is via screens, centrifugal classifiers then to a drier and pulverizer system. These products are also available as stable high solids slurries. Products from this route lie between 0.6 and 3 microns mean diameter and are characterized by the presence of the electrolytic dispersant added to form a stable and fluid high solids aqueous form.

Stearate treated grades

Products manufactured from the slurry route processes are often available in stearate treated form. Treatment occurs either during the drying process or immediately after drying by reaction with stearic acid or one of its salts. Soluble ammonium or sodium stearate may also be added to the slurry products just before de-watering and drying.

Precipitated calcium carbonate

Precipitated products are produced by controlled introduction of carbon dioxide into a dilute solution of calcium hydroxide. Some producers produce their own calcium hydroxide from calcium carbonate while others purchase the feed on the open market. Control of reaction concentrations, time and temperature leads to the formation of different crystal forms, of either calcite or aragonite, plus a range of different crystal shapes. For use in rubber, usually only the finer products below 0.1 microns are considered. The low solids (below 20%) slurry is often conditioned for a period of time After conditioning, the slurry is screened then passed to a reactor where reaction with stearic acid or one of its salts occurs, followed by de-watering and drying plus pulverizing. The resultant stearate treated product, often referred to as activated calcium carbonate, is a semi-reinforcing filler.

The basic calcium carbonate types are shown in table 1.

Table 1 - characteristics of callcium carbonates

              Size range     Stearate      Relative      Comment
              (microns)      coating     Price (1 = low
                                          10 = high)

Screened grades  40-2,000      No              3       Essentially
                                                    chemical free.

Dry ground         12-800      No              1    Small quantities
                    6-11       No              3       of grinding
                    2-5        No              4       agent often
                    2-5        Yes             6    associated with
                                                      these grades.

Wet, low solids     6-11       No              6    Essentially free
products            3-5        No              5      from added
                    3-5        Yes             7    process chemicals.
                  0.7-2        No              7    High
                  0.7-2        Yes             8    chemical purity.

                                                    Water soluble
Wet, high solids  1.5-3        No              6    dispersant
products          1.5-3        Yes             8    associated with
                0.6-1.0        No              7    these grades
                0.6-1.0        Yes             9    (0.15-0.8%).
                                                    High chemical
                                                     purity.
Precipitated
calcium         0.5-2.0        No              8     High pH residules
carbonates     0.5-0.15        Yes             10    often associated with
                                                     PCCs.

Characteristics of calcium carbonates in elastomers

Calcium carbonates, like all fillers used by the rubber industry, have their own particular characteristics or fingerprints. For many processors, the primary reason for addition of this mineral is purely cost reduction. The mineral has minimal impact on formulation hardness allowing relatively large quantities to be added to many formulations for this purpose alone. Calcium carbonates do, however, provide other characteristics that are of benefit to elastomers (table 2). The overall industry need is for an economical product which is low in moisture and free from coarse particles that may damage equipment or the finished product. The industry invariably uses uncoated products in the 2 to 7 micron mean diameter range as its standard workhorse and uses products outside this range only for specific reasons of cost or technical performance.

Table 2. Characteristics of calcium carbonates in elastomers

Characteristics in elastomers which are influenced
by particle size

Property                 Coarser<-------------------------->Finer
Cost                     Very low<------------------->Moderate
Moisture content         Very low<-------------------------->Low
Color                    Off-white<------------------------>White
Reinforcement            None<-------------->Semi-reinforcing
Flex fatigue resistance  Very poor<----------------->Very good

Characteristics influenced by minerology and process route

Property                                 Minerology  Process route
Cost                                                   Very low to
                                                          moderate
Moisture content                                Low  Low to v. low
                                                          (coated)
Specific gravity (calcite)                     2.71
Hardness                     3.0 Mohs (fairly soft)
Dielectric properties         Moderate to excellent  Some influence
                                                        by chemical
                                                          additives
Thermal conductivity    High (relative to polymers)
Coefficient of thermal   Low (relative to polymers)
  expansion
pH                                             9-10
Modulus and hardness                            Low
Dispersibility            Easy to very easy (coated)   Very easy to
                                                               easy
Chemical resistance (acids)                    Poor
Permanent set                                   Low
Die swell                                      High
Compound viscosity                              Low
Green strength                                  Low
Building tack                                  High

Applications

One of the prominent deficiencies of calcium carbonates is the lack of reinforcement. It is normal, therefore, for calcium carbonates to be used in combination with other reinforcing or semi-reinforcing fillers in order to obtain the most cost-effective overall performance. Almost every sector of the rubber industry utilizes the cost effectiveness of this mineral.

There are a few instances where calcium carbonate may be used alone, usually in those elastomers that exhibit self-reinforcement such as natural rubber. In these instances, the mineral is usually used for its own unique qualities such as hot tear strength (precipitated grades), low modulus, whiteness or electrical insulation characteristics.

Formulations illustrating the use of calcium carbonate in a variety of rubber applications with comments on specific areas of each recipe follow the text of this article. The type of calcium carbonate chosen is dependent on economics and physical and processing requirements of each specific compound. In these formulations, the precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC) are identified by size and process route from table 1.

Conclusion

Calcium carbonate is regarded by most within the industry as the key means of reducing cost. It is available as a range of products derived from a variety of sources. Its basic behavior, and hence reason for selection, in elastomers is primarily influenced by its source (color), particle size and manufacturing route. When these are taken into consideration by the formulator, the mineral is able to provide a large number of significant technical benefits to the industry in addition to its conventional role of cost reduction. These benefits include low moisture, high thermal conductivity, low hardness and modulus, low permanent set and easy processing. These make calcium carbonate an essential ingredient to the financial viability of the rubber industry.