Types and functions of commonly used inorganic powder materials

At present, at least 4 million tons of inorganic powder materials are used in China as part of raw materials for the production of plastic products. Replacing some petroleum products with inorganic powder materials can save millions of tons of oil every year. On the other hand, for the plastic products produced, it not only helps to reduce the cost of raw materials, but also can fill a plastic material. These properties are improved in a predetermined direction to increase the market competitiveness of plastic products.

Types and functions of commonly used inorganic powder materials

According to statistics, half of the approximately 5 million tons of products produced by more than 500 calcium carbonate manufacturers in China are sold to the plastics industry. In addition, a variety of inorganic powder materials such as talc powder, calcined kaolin, wollastonite powder, etc. are also widely used, and some even become an indispensable component of functional plastic materials.

Calcium carbonate

Calcium carbonate is the most widely used inorganic powder filler in plastic processing. According to the statistics of the Calcium and Magnesium Branch of the China Inorganic Salt Industry Association, the total amount of calcium carbonate used for plastic filling is more than 2 million tons per year, which is the largest share of various uses, about 50%.

Calcium carbonate is classified into two types, light and heavy, depending on the processing method. Light calcium carbonate (referred to as light calcium) is formed by calcining, digesting and carbonizing limestone, which undergoes a chemical reaction, while heavy calcium carbonate is ground (dry or wet), only the particle size The size changes without a chemical reaction process. At present, the calcium carbonate used in the plastic film is 1250-mesh heavy calcium carbonate, which has been widely used in the production of PE packaging bags, and is affected by light transmission in the agricultural mulch film, although it can be used, but the addition amount is small.

1) The effect of fineness of heavy calcium on the mechanical properties of PE film is very obvious, see Table 1.

Table 1 Effect of heavy fineness on mechanical properties of PE film

2) The dispersion of calcium carbonate particles plays a decisive role in the performance of PE films

PE film manufacturers are very concerned about the amount of heavy calcium added. It is hoped that the more added, the better, but at the same time, the mechanical properties, aging resistance and light transmission should not be greatly affected. In particular, how much calcium carbonate can be used in agricultural mulch is a problem worthy of discussion. Baoji Yunpeng Plastics Technology Co., Ltd. made a useful exploration and achieved gratifying results. Table 2 lists the mechanical properties of pure LLDPE mulch and LLDPE mulch film with 10%, 15%, 20%, and 33% nano-modified plastic composites produced by Yunpeng Company.

It can be seen from the data listed in Table 2 that the LLDPE mulch film with more than 10% to 33% nano-modified plastic composite material has little difference in mechanical properties compared with the pure LLDPE mulch film. When the addition amount reaches 33%, that is, when the CaCO3 component in the LLDPE film accounts for 26.4%, the mechanical properties can still maintain a high level, especially the right angle tear strength is higher than the pure LLDPE, which not only fully exceeds the GB 13735-92 film. International technical indicators of superior products, and it is impossible to do any CaCO3 filled masterbatch in the past.

Table 2 Performance test results of Yunpeng brand nano-modified plastic composite filled with LLDPE film

3) Environmental effects of calcium carbonate - environmentally acceptable

“Environmentally-dissipating plastics” refers to plastics that can be used in a comprehensive manner to adapt to the treatment of waste, which can be degraded in the environment, composted or beneficial to incineration and less harmful.

Calcium carbonate is an important component in the manufacture of environmentally acceptable plastics. The results show that the photodegradable film containing 30% heavy calcium has a carbonyl index (an indicator for the degree of photodegradation) reaching a certain value when exposed to a certain value, several days earlier than the photodegradable film containing no calcium carbonate. Good. At the same time, the presence of calcium carbonate continues to degrade after the PE film is buried, or it reduces the generation of harmful substances during incineration, and it is very advantageous to protect the incinerator. Compared with the starch-degrading film, the chemical oxygen demand CODcr value (used to measure the degree of reducing substances in water) is 0 when calcium carbonate returns to nature, while the polyethylene film containing 15% starch has a CODcr value of 96. .

talcum powder

Talc is the second only filler for plastics in calcium carbonate. It is used in plastics every year for more than 200,000 tons. With the further understanding of some physicochemical properties of talc, its application range and The number is increasing dramatically.

1) Use as a film insulation agent

Silicon-containing minerals such as mica, kaolin and talc have a barrier to infrared light. Adding an appropriate amount of this mineral powder to the agricultural greenhouse film can improve the barrier property of the plastic film to infrared rays, thereby reducing the heat in the shed to be lost to the outside of the shed in the form of infrared radiation at night, thereby improving the heat preservation property of the greenhouse.

The research results of the Light Industry Plastics Processing Application Research Institute in the early 1990s show that:

1 Mica powder, kaolin, talcum powder and light calcium carbonate have similar effects on the mechanical properties of polyethylene film when the filling amount is the same (the similarity is similar and the surface treatment), and the mechanical properties of the film filled with kaolin and mica powder are more. a little better.

2 The filler-filled LDPE film containing silicon has a significantly better barrier effect on 7-25 μm infrared light than the inorganic filler without Si—light calcium carbonate, while the mica powder, kaolin and talc have similar infrared barrier properties.

Among the three Si-containing fillers, the LDPE film filled with mica powder has the highest light transmittance, and is close to the light transmittance of pure LDPE plastic film, followed by kaolin and talc powder, but higher than the film filled with calcium carbonate.

Because talcum powder is cheap and easy to operate, its light transmittance and infrared light barrier properties are not as good as mica powder and kaolin, but it can still improve its heat preservation while maintaining good light transmission, so it has been used in agricultural plastic film. Widely used. At present, the film production plant uses 1 to 6% of ultrafine talc powder depending on the type of film (resistance film, double film, multi-layer film, etc.).

2) Use as a nucleating agent

Crystalline polymers such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), etc., during the cooling and shaping process after processing, a part of macromolecules It will be arranged in order, called crystal.

Crystallization requires not only a certain temperature and cooling rate, but also the formation of crystal nuclei, followed by crystal growth. The nucleating agent has two main functions. One is that the total crystallization rate is increased, which ensures that the molten polymer solidifies more rapidly during the cooling process, thereby shortening the injection molding cycle and improving the work efficiency; second, the average spherulite size decreases, pulling The tensile strength, heat distortion temperature and hardness are enhanced by the action of the nucleating agent, and the transparency is increased and the turbidity is lowered.

As a nucleating agent for PE or PP, talc is first required to have a small particle size, and the smaller the particle size, the larger the number of particles, which means that the crystal center is more. The more crystals are grown at the same time, the smaller the size of the crystal itself, the better the performance of the entire material. At the same time, it is also required that the dispersion of the slick powder in the molten state is as thorough as possible, and the agglomeration phenomenon is as slight as possible.

3) Transparent filling masterbatch with talcum powder as main filler

The use of calcium carbonate in a plastic film can achieve a cost reduction effect, but when the amount is large, the transparency of the film is greatly affected, causing some users to misunderstand. In view of this situation, the transparent filler masterbatch has made significant improvements in the selection and processing of the filler, and the transparency of the PE film has been greatly improved. Tables 3 and 4 show the optical and mechanical properties of the films after addition of 20% and 30% transparent masterbatch, respectively.

Table 3 Effect of transparent masterbatch on optical properties of HDPE film

Table 4 Effect of transparent masterbatch on the mechanical properties of HDPE film

Calcined kaolin

Since coal-based kaolin contains combustible carbon and various forms of water (adsorbed water, crystal water), it must be calcined before being used as an additive for plastic film to achieve dehydration and decarburization, and the process from black to white is calcined. . When calcined at high temperature, the mineral not only undergoes the process of dehydration and decarburization, but also may undergo various changes in the crystal form of the mineral. If the mullite phase content is too high, because the hardness of mullite is too high, it will cause serious wear and tear on the key components of the plastic processing machinery equipment, the screw barrel and the screw, which will bring about the negative effect of the production efficiency and the unstable product quality. . In addition, the particle size and distribution, whiteness and price of calcined kaolin must also be taken seriously.

The light from the sun received on the earth has a wavelength of 98% concentrated in the range of 0.3-3.0μm, and is divided into three parts: ultraviolet light (0.3-0.4μm), visible light (0.4-0.7μm) and infrared light (0.7μm or more). In part, the visible light for photosynthesis during the day is the main form of sunlight converted into heat on the earth. The main form of heat that radiates heat from the Earth's surface to the atmosphere at night, that is, 90% of the energy is radiated by infrared light of 7-25 μm, and its peak value is 11-13 μm. The greenhouses made of agricultural plastic film are mainly used to make the temperature inside the shed far higher than that outside the shed, so that the crops can germinate early and mature early, and the crops in the shed can grow in the cold season. Ordinary non-infrared light barrier polyethylene film has a poor barrier to infrared light, less than 25%, so although sunlight passes through the shed film during the day, the energy is left in the shed and converted into heat, so that the temperature inside the shed Raised, but at night due to poor barrier to infrared light, most of the heat will be lost to the outside of the shed. For this reason, only the thickness of the film can be increased, and this increase not only increases the barrier property, but also the film can be thickened without limitation. The only way is to add a substance that blocks the infrared light to the plastic film so that the infrared light is radiated onto the shed film, and it cannot penetrate the past and re-reflect it back into the shed to achieve the effect of plastic greenhouse insulation.

The pure polyethylene film (thickness 0.08-0.1mm) has an infrared light transmittance of 70%-80% in the wavelength range of 7-25 μm, and the infrared light transmittance can be reduced to 50% or less by adding an infrared light blocking agent. Germany CONSTAB Polymer-Chemical Co., Ltd. can reduce the infrared light transmittance to less than 25%; 0.05mm thick inorganic filler-filled polyethylene film developed by Beijing Institute of Plastics, infrared light transmission in the wavelength range of 7-11μm The rate can be reduced to 36%; the 0.05mm thick inorganic filler-filled polyethylene film produced by the first plastics factory in Jiaozuo, Henan Province can reduce the infrared light transmittance in the wavelength range of 7-14μm to 39%. Germany CONSTAB Polymer-Chemical Co., Ltd. uses the clay produced in China as the raw material for the infrared light barrier function masterbatch of Co nstab IR 0404 Id. Add 7% to the polyethylene film to make the infrared in the range of 7-14μm. The light transmittance is reduced to less than 25%.

After the calcined coal-based kaolin is added to the polyethylene plastic, the mechanical properties of the filling system are better than those of the talc or calcium carbonate filling system under the same conditions, as shown in Table 5.

Table 5 Mechanical properties of polyethylene filled plastic film filled with different fillers

Through the shed experiment, it is shown that the agricultural film filled with talc powder and calcium carbonate has good transparency of the agricultural film using kaolin, and the heat preservation effect in the shed is good, which is beneficial to the increase of crop yield.

Common problem in application

The application of inorganic powder materials in plastics has been widely recognized, playing three important roles of increment, modification and environmental protection effects, but at the same time there are some problems that restrict the application of inorganic powder materials in some aspects. These problems also exist on plastic films, which need to be highly valued by us and solved by technological innovation.

Weight gain problem

The density of non-metallic minerals is much larger than that of synthetic resins, usually two or three times larger, and some such as barite powder is about five times more dense than polyethylene or polypropylene. Although the mineral filler replaces the base plastic one-to-one in quality, it occupies only a fraction of the volume of the same quality base plastic. If the particles of the mineral filler are in intimate contact with the matrix resin and there are no voids, then this difference in volume will directly affect the amount of plastic material and article measured in area or length, such as the length of the pipe and profile or the area of ​​the artificial leather. It also directly affects the number of injection molded products, because the volume of the injection molding cavity is constant, and if the same quality of the melt is different in volume, the number of injection molded articles that can be formed is reduced, and the result is reduced in the use of filler. At the same time as the cost of products and the increase of economic benefits, there are negative effects due to the decrease in length, area and number of products.

Since the polyethylene film is stretched in both the machine direction and the radial direction, the effect of the inorganic powder material on the density of the PE film is slightly slight. For example, the density of PE film with 30% heavy calcium is not more than 1.1 g/cm3, although it is about 15% higher than that of pure PE film, but the increase is much smaller than that of injection molded products. This is because if the polymer material is stretched during the cooling process in which the viscous flow state is changed to the fixed state, the void between the macromolecules is inevitably increased. Calcium carbonate particles themselves are not deformed, but the PE macromolecules around them can be deformed. 100% pure PE molten material and 70% PE molten material except calcium carbonate are subjected to the same longitudinal stretching and diameter. To the inflated, deformable material still achieving a predetermined longitudinal elongation (depending on the rotational speed) and radial elongation (depending on the diameter of the bubble), more and larger voids must be present. The increase and increase of the void means a decrease in the overall density of the material.

When the PE film is added in a large amount in the inorganic powder material, the density will increase, and it is necessary to calculate whether the cost is reduced by the area by the addition of the inorganic powder material. Since the price of inorganic powder materials is generally much lower than that of polymer resins, in view of the above factors, the addition of inorganic powder materials, especially calcium carbonate, can reduce the raw material cost of PE films.

Aging problem

The polyethylene film to which the inorganic powder material is added has a certain problem of aging, and it is advantageous from the viewpoint of environmentally acceptable consumption, but it is disadvantageous from the viewpoint of agricultural film or film.

Table 6 lists the research results of the College of Chemistry and Materials of Fujian Normal University. When they developed environmentally acceptable plastics, they found that under the premise of adding photodegrading agents, PE film with calcium carbonate or talc powder, the carbonyl index reached a certain value in the aging test less than no carbonation. PE photodegradable film of calcium or talc.

Table 6 Effect of inorganic powder materials on aging properties of PE film Unit: d (days)

Transmittance

Everyone thinks that the inorganic powder material is added to the polyethylene, and its transparency is greatly affected. Even if the material properties can meet the requirements of use, for the agricultural film and the mulch film, the good transmittance is still a large amount. Prerequisites for the use of inorganic powder materials.

The light transmittance of the filled PE film mainly depends on the particle size of the filler and the difference in the refractive index of the inorganic powder material and the matrix plastic. Unlike titanium dioxide, lead white (lead oxide), and zinc white (zinc oxide) (refractive index of 2.52, 2.01, and 1.79, respectively), the two refractive indices of calcite are 1.658 and 1.486, respectively. The refractive index of plastic is about 1.55, so it is natural to add a small amount of calcium carbonate to show opacity. It is also impossible to make a large amount of calcium carbonate particles exhibit nanometer-scale dispersion. The refractive index of talc is closer to that of PE plastic, so if it can be dispersed, it can be filled with PE film to have better light transmittance than calcium carbonate. Recently, a mineral has been found. The filled PE film has a high transparency, almost the same as pure LDPE, and its price is not very high, which brings hope to a large amount of use in agricultural film.

Table 7 Effect of Several Inorganic Powder Materials on Transmittance of LDPE Films

The use of inorganic powder materials in plastic films has achieved remarkable results. Because agricultural plastic film, whether it is mulch film or shed film, most of them use polyethylene resin, so it is very important to study the influence of various inorganic powder materials on the performance of PE plastics. I don't expect the agricultural film to use a large amount of inorganic powder materials, but I sincerely hope that from the strategic height of sustainable development, from the perspective of coordinated development of plastic industry and environmental ecological protection, from the perspective of enterprises to improve product market competitiveness through technological innovation, everyone Work together to open up a new world of inorganic powder materials for use in agricultural film.

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