Minerals realise pipe dream

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Published: Monday, 26 October 2009

Liu Yingjun reviews the growing consumption of minerals in plastics and how they are playing a key role in reducing the consumption of oil

There are many factors contributing to the fast and sustained growth of plastic products and materials, which soared from 1m. tonnes in the early 1980s to over 70m. tpa at present Ð a rate which is exhibiting a steady growth rate every year.

Minerals have made exceptional contributions to this growth and are key to reducing the use of oil resources and other energies.

At present, the volume of imported oil in China is over 150m. tpa. For the most part, it has performed against the backdrop of soaring oil prices, resulting in a much higher price of synthetic resin for which oil is a raw material.

Compared with energy consumption of synthetic resin from the exploration, production, transportation, cracking and synthesis of petroleum, the manufacturing of mineral powder is much easier, as the amount of investment and the energy consumption of one tonne of mineral powder is only a tenth of that of synthetic resin.

Consequently, using large amounts of mineral powder in plastics in a rational way bears a significant impact on building an energy efficient society and realising sustainable development.

Reducing raw material costs

The price of nearly all plastic mineral fillers is lower than that of synthetic resin. For example, heavy calcium carbonate (HCC) with the general granularity of 400 mesh - the cheapest powder material available - is priced at over 100 RMB ($14), only a small percentage of the price of synthetic resin.

Materials other than HCC cost no more than 1,000 RMB ($140) either. It goes without saying that using powder material can significantly reduce the cost of raw materials providing it meets the performance of filled plastic products.

The costs of raw materials usually account for 50-70% of the total cost of plastics products. So cost reduction forecasts are seeing decreasing overall costs and more competitive edges in the market. This has sparked a rash of employing mineral powders in plastics processing plants in recent years.

Tailoring raw materials

With breakthroughs in modification technology of plastics by filling powder materials, calcium carbonate and talc are found to be not only low cost, thus increasing productivity and reducing overall costs, but also have obvious modification effects on the filled matrix plastics.

Although the performance of the filled plastics will be significantly changed, alternatives can be made regarding rigidity, toughness and intensity by strengthening and improving provided the overall performance of the filled plastics can be guaranteed.

Studies have shown that talc, kaolin, and mica powder can help plastic films better block the infrared rays, which is crucial to improve the heat preservation of greenhouse films. Besides, precipitated barium sulphate can add excellent gloss to polypropylene (PP) plastics; wollastonite in powder is conducive to enhancing rigidity of the surface of plastic material; brucite or magnesium hydroxide generated from the chemical reaction are capable of filling, retarding inflame, and abating smoke, playing weighty roles in the research and development of low-smoke non-halogen flame retardant plastics, and the like.



Talc is the major filler used in the production of
car bumpers. Key properties include rigidity while
resting impact in high and low temperatures.

Environmental benefits

In spite of the benefits brought about by the accelerated development of the plastics industry, plastics that are discarded carelessly after use will jeopardise the environment and ecology. Studies in recent years have revealed that inorganic powder material, such as calcium carbonate, is playing important role in producing environment-friendly plastic materials.

Using such powder materials in a scientific and rational way can help us mitigate white pollution (pollution related to plastic products) by retracting the plastic bags, films, etc., which may not be easily recycled.

Extensive application of powder material would be within the reach if we shore up the publicity and industrialisation of it. Mineral powder material, whether in multiplying the output of plastic products or extending its application to new fields, is a promising prospect. It will also play a crucial and irreplaceable role in the development of plastics industry as well as building an innovative and conservation-minded society in order to realise the sustainable economic growth in China.

Example mineral powders used in plastics

Plastic Products Minerals used Parts per hundred resin* (PHR) Functions
Polypropylene (PP) flat wire Calcium carbonate 10 to 20 Increasing output, whitening, better printability
PP strapping Calcium carbonate 50 to 150 Increasing output & friction coefficient
Ploythene (PE) film Calcium carbonate 40 to 50 Increasing output & environmental protection
PE pipe Calcium carbonate 20 to 40 Increasing output
PE bobbin & bellow Talc 20 to 40 Increasing rigidity
PP injection moulded item Calcium carbonate, talc 40 to 50 Reducing costs by replacing ABS
PE greenhouse film Talc, kaolin 5 to 10 Improving heat insulating properties
PE rubbish bag Calcium carbonate 40 to 50 Easier to incinerate 
PP tray Calcium carbonate 200 Reducing costs, increasing stability of performance
Automobile bumper Talc 20 to 30 Maitaining rigidity, resisting impact in high and low temperatures
Auto parts & home appliances Talc 30 to 50 Improving heat resistance
Glazed PP Precipitated barium sulphate 40 to 50 Maintaining glaze of plastic products
Electric cable casing (low smoke, non halogen) Aluminium hydroxide, magnesium hydroxide 150 Flame retardant, abating smoke 
Auto motor fan Mica 40 to 50 Improving heat resistance

Terms: PP = Polypropylene PE = Polythene PVC = Polyvinyl chloride

* 5 PHR means that 5 pounds of an ingredient would be added to 100 pounds of resin.


Despite the fact the price of mineral powder material may climb a bit in the long run, it is still acceptable by downstream customers. New mineral powder materials are expected to be launched into market, like the powder materials with little influence on the light transmittance of plastic films and no influence at all on the extrusion of fibres of non-woven fabrics.

For powder materials whose functional characteristics are already known to us, the main task is to learn how to make better use of them (eg. wollastonite powder).

Calcium carbonate

Calcium carbonate is the most important mineral used in plastic products. It accounts for 70% of the 10m. tonnes of minerals consumed in plastics every year.

HCC is a powder derived from the grinding of the ore, while light calcium carbonate (LCC) is generated from a chemical reaction, also known as precipitated calcium carbonate (PCC).

Calcium carbonate is advantageous as a filling material over others due to the following aspects:

  • low price, non-toxic, flavourless, and non-irritating
  • high-whiteness, easy colouring, and little interference into other colours
  • low rigidity, thus little friction against processing devices and moulds
  • good chemical stability, thus no chemical reaction with other matrix plastics
  • macromolecules of matrix plastics degrade or cross linked
  • good thermal stability, and the temperature of thermal decomposition is over 800¡C
  • easily dried, no crystal water, and convenience in dehydration

Talc

Talc is widely used in plastics, only second to calcium carbonate.

With millions of tonnes of talc used every year and large amounts exported to Japan, and South Korea, a considerable amount of talc is used to modify plastics.

The lamellar structure of talc can improve the rigidity and heat resistance of the matrix plastics, talc is sometimes viewed as reinforced filling. This requires a relatively large diameter-thickness ratio of talc in addition to a small particle diameter.

Mineral powders containing silica sand, like talc, mica powder, and kaolin, can be used for heat preservation in greenhouse films due to its resistance to infrared light. Although talc is weakest in preventing infrared light among them, it is the cheapest and most convenient for use. For talc used for this purpose, large length-diameter ratio is not the concern, but a small particle diameter and ideal gloss are preferred.

In recent years, talc has been widely used in PE films, because the refractive index of talc is very close to that of PE matrix plastics. Compared with HCC and other fillers, talc may allow better light transmission of PE films.



Aluminium hydroxide and magnesium hydroxide
are the key fire/heat resistant fillers used to produce
electric cable casing.

Kaolin

Kaolin used in the plastics industry in China can be divided into two categories. One is the so called washed kaolin, belonging to clay. The other is coal kaolin, a typical hard kaolinite in

China, which is in brown or black lumps associated with coal.

Owing to its high price and inconvenience in use, kaolin clay is only used to improve the insulation strength of plastics after calcination. However, recent studies have shed light on its outstanding performance as a barrier to infrared rays, which is of great help in retaining heat for plastic greenhouse films.

In filling plastics, kaolin clay could elevate the tensile strength and modulus of thermoplastics with relatively low glass temperature without significantly diminishing its elongation rate and impact strength.

Moreover, kaolin could function as nucleator after added to PP, thus increasing the number of PP crystals and curtailing the size of crystal so as to enhance the rigidity and strength of PP.

Mica

Mica powder is used mainly to improve the rigidity and heat resistance of plastic products, provided the high diameter-thickness ratio of its slice is preserved properly during processing.

Owing to its openness which lets light pass, it is possible to apply mica powder to agricultural plastic films. Adding mica film to inorganic filling that is also capable of scattering light and preventing infrared rays, the astigmatism ratio would increase considerably in spite of a slight lessening of light transmittance ratio. At the same time, the obstruction of infrared rays with wave length between 7 -25mm is most effective.

Wollastonite

Acicular wollastonite as powder, scattered within matrix plastics, is conducive to enhancing the tensile strength and cross-breaking strength of matrix plastics. In addition, wollastonite can allow filled plastics to better resist water.

This characteristic, when applied to nylon, will improve the hygroscopicity of nylon products under damp circumstances, since the rigidity and modulus of nylon will decrease after absorbing water.

Al hydroxide & Mg hydroxide

Adding aluminium hydroxide and magnesium hydroxide into plastics can realise triple functions of filling, retarding flame, and abating smoke. They are the ideal choices of low-smoke non-halogen flame retardant materials.

Montmorillonite

Montmorillonite, a layer-silicate, can be used to manufacture nanoplastics and has commanded more attention from different fields in recent years.

The tensile strength of nylon6 containing 4.2% montmorillonite can increase by 50% compared with pure nylon6. Likewise, modulus increases by 100%, and heat distortion temperature by 90¡C. While the transparency heightens and hygroscopicity decreases (weakening the rigidity of nylon), the shock resistance of the material remains almost unchanged.

The reason for this is that the lamella of montmorillonite is evenly dispersed in nano scale within nylon6, thus forming exfoliated composite material-nanoplastics in real sense.

Mineral concerns and requirements

Minerals used in plastics tend to follow the same general requirements: reasonable price; minor impurities; low rigidity; and easy handling and using.

But there are some specific requirements that the mineral must meet in order to be fully suitable to use as a plastic additive.

Geometrical features

Powder material is utilised used as a filler in uneven granular form. However, as far as the performance of plastics is concerned, the geometrical shape of filling is of great significance to the physical and mechanical properties of the filled system. In this regard, first concern should be given to the granular shapes of powder materials before using them.

Particle diameter

The knack of modification technology by filling powder materials is to spread the powders as evenly as possible to the matrix plastics, which resembles the islands of various sizes in the sea, this is also known as the “island structure”.

Generally speaking, if distributed evenly, the smaller the diameter of the filling is, the better the mechanical properties of the filled system.

However, the cost of processing powders is inversely proportional to the particle diameter, and it will be more difficult to evenly distribute powder material of smaller diameter. Consequently, making the right choices based on the knowledge of the particle diameter and its distribution is all the more important.

Specific surface area

Specific surface area indicates the surface area of filling by a unit mass, the size of which is directly linked to the compatibility between filling and resin, the activation of surface and production costs.

Surface free energy

The power of surface free energy of filling particles is related to their distribution in the matrix resin. With the same specific surface area, the larger the energy, the easier for agglomeration of particles and more difficult for dispersal. In the surface treatment of filling, one of the major aims is to reduce the surface free energy.

Density

The apparent density of filling is linked with the arranging of filling particles. In the field of plastics modification by filling powder material, the factor truly contributing to the overall density of the filled system is no other but the density of individual filling particles and the forms of their existence in matrix plastics. In other words, whether they are agglomerated, and whether there is space between matrix plastics molecules or not.

Hardness

The hardness of filler particles is a double-edged sword. On the one hand, fillers of good hardness can allow better abrasion resistance for plastic filling materials, but on the other hand, using this kind of filler may lead to serious wear and tear of the processing devices and moulds during the processing of filled system.

Refractive index

When the refractive index of the powder filler is close to or the same as that of the matrix plastics, the influence on covering light after blending the powders into the matrix will become relatively insignificant, or vice versa.

Absorption & reflection of light

Ultraviolet rays can prompt degradation of the macromolecules of polymers. The wave length of infrared rays is above 0.7mm. Light waves within this category can be absorbed or reflected by some fillers. Employing minerals such as mica, kaolin, and talc can effectively reduce the light transmittance of infrared rays, thus significantly enhancing the performance of preserving heat in agricultural greenhouses.

Thermal performance

The processing of filled plastics integrates heating, fusion, cooling, moulding etc. The original thermal performance of the filler and the discrepancies between that performance of matrix plastics and the filler itself will influence the processing.

Electrical performance

Since metal conducts well, metal powders used as a filler will affect the electrical performance of filled plastics.

Fillers made from non-metallic minerals do not conduct, and that is to say no influence will be imposed on the electrical performance of matrix plastics. However, the valence bond of metal powders may be broken during cutting and grinding, and a static charge may occur. As a result, mutually attractive aggregates will take the shape, which is more likely to take place in production of minute filler.

Thermochemical effect

High molecular polymer is inflammable, but most filler materials are non-inflammable.

This fact can help to reduce combustible concentration and retard the combustion of the matrix if those non-inflammable materials are added to polymers.

Innovations and trends

The benefits of applying mineral powder material to plastics are evident, whether in terms of using scope, modification effects or the economic and social benefits. Years of practice have manifested that innovative application is the key to increasing use, and is also a must in continued expansion of mineral usage in plastics.

Innovations in surface treating technology and treating chemicals can raise the price performance ratio of filled plastic material. The variety of the chemical compositions of powder particles and the complexity of surface nature means that not all minerals can be subjected to surface treating technology and auxiliaries. The experiences of some companies have told us that it is all the more necessary and possible to use tailored surface treating chemicals with higher price-performance ratio.

To use relatively cheap inorganic powder materials as the core, and to deposit the functional auxiliary materials on the surface of powders in the form of molecules is not only conducive to the distribution and dispersal of auxiliaries, but also does not affect performance of the powder materials. Thus, a higher price performance ratio is achievable, so is a better synergistic effect based on the toughening theory of rigid particles.

With the same composites in PVC sectional material, better or at least the same physical and mechanical properties can be reached by replacing ACR with ATM-310B. But owing to the core-shell structure of ATM, an obvious competitive edge in the costs of using is self-evident.

Most of the plastic materials concurrently feature high surface resistivity and low dielectric constant, and that is why electric charges, ie. static, are easily accumulated on them. Problems triggered by static can range from small matters, like attracting dusts or mutually repelling with the like charges, to disasters, such as fires or explosions.

A new approach in improving the anti-static performance of plastics lies in the use of tint conducting material, which is made by depositing a layer of conductible material on the surface of non-metallic mineral powders that do not conduct.

Changing plastics consumption

The true density of non-metallic minerals is usually two or three times larger than synthetic resin. If we can, during the processing of injection-moulded items like pipes or sectional materials, use fillers without enlarging the density of the items or the increment is within reasonable scope, to add mineral fillers into plastics will become more likely.

In the past, pursuing perfect performance was the main concern of plastic material, with little or even no consideration of whether they could be absorbed by nature or recyclable after use.

The discarded plastics in every corner of our society have not only polluted, but have also led to a huge waste of the invaluable oil resources.

Environment-friendly material adheres to green production and green consumption, and it also emphasises the coordination between the environment and the phases of production, use, and post-use of products.

Using calcium carbonate is expected to significantly reduce the use of oil resources and other energies. Over 25% of resin could be saved when calcium carbonate accounts for 30% in PE plastic bags, compared with those containing no calcium carbonate.

Non-metallic minerals will encourage degradation of plastic materials while calcium carbonate is conducive to the burning of PE plastics. Burning of PE plastics is still feasible since not all plastics have recyclable values or worth recycling, and landfill requires extensive land.

Experiments have shown that 100g PE films containing 30% calcium carbonate and 1% burning thermal-oxidative degradant additive finishes complete combustion in four seconds, compared with 12 seconds of films with the same weight.

Additionally, calcium carbonate poses no negative influences on groundwater after landfill and does not contain any detrimental heavy metals.

Conclusion

The history of applying mineral powder material to plastics shows that it has already become an indispensable material in the plastics processing industry. Moreover, it plays a more and more important role in improving the price-performance ratio and expanding functional application of plastic material, conserving resources, and protecting the environment.

To better and more efficiently utilise the mineral powder material by bringing its strengths into full play shall not only reduce costs, but also aim to build an environment-friendly society by achieving sustainable development. To scale the application of mineral powder material in plastics to a new height, we should further facilitate research and development, accelerate the transformation of research achievements, and shore up cross-industrial exchanges and cooperation.

Contributor: Liu Yingjun, senior engineer, vice chairman and secretary general of the Committee of Modified Plastics of China Plastics Processing Industry Association

Adapted from a presentation at the 8th Chinese Industrial Minerals Conference, 7-9 September 2009, Qingdao, China.