Synthetic paper refers to the use of high-molecular organic compounds (such as polystyrene, polypropylene, polyethylene and other plastic fibers), artificial short synthetic fibers, wood pulp and other raw materials that are made of synthetic resin as the basic component, which are calendered or extruded Flat material.
In the forming process of synthetic paper, in order to make it have good whiteness, opacity, printability and writing, according to the raw materials used, various inorganic fillers (such as: clay, talc, calcium carbonate) are added , Iron dioxide, barium sulfate, etc.), tackifiers, stabilizers and anti-aging agents, etc. After the paper is formed, it must be treated with water, inorganic acid solution, phosphate solution, aluminum sulfate, slaked lime solvent, and then oxidized ( Hydrogen peroxide, sodium oxide, ozone, air, etc.) can only be used for oxidation treatment.
The composition structure of synthetic paper not only retains the appearance, whiteness, stiffness, opacity, printability and writing properties of natural fiber paper, but also has the lightness, moisture resistance, oxygen resistance, acid resistance, alkali resistance and alkali resistance of polyolefin plastic film. Excellent characteristics such as pollution. Therefore, synthetic paper has become a popular high-end new material for printing, packaging and decoration in the world market today.
The main requirements for synthetic paper used as printing materials are:
â‘ White and opaque appearance;
â‘¡ Must meet the suitability of the printing machine;
â‘¢ There is a good relationship with the printing ink. These three conditions are not controlled independently, but affect each other. How to make these elements cooperate is the key to their quality design.
1. Appearance
Except under special circumstances, it should have a white and opaque appearance. After the wood pulp has been treated with chemicals, the white paper is only obtained after the loss of low utilization rate and the reduction of the relative molecular weight of cellulose. In contrast, it is easy to make synthetic paper only white and not transparent. That is, after the white filler is filled or the foam layer and the porous layer are designed, not only can high whiteness be obtained, but also a white product with higher purity than natural paper. At the same time, the opacity can also be changed by changing the above processing conditions.
2. Printability
(1) When static electricity is printed on a sheet-fed printing machine, static electricity often causes malfunctions, so it is necessary to prevent electrification. In the case of Dongxiu's low humidity, ordinary paper will often have static barriers, so when using paper made of synthetic polymers that are easily charged in nature, more attention should be paid.
The method to prevent electrification is to add or apply an anti-charge agent, but if it is used excessively or the selection is wrong, the following adverse effects will occur. First of all, bleeding will occur when taking notes, and it will also cause scumming during film printing, and even become the cause of ink discoloration. In addition, the added inhibitor will lose its effect if it volatilizes on the surface, so long-term stability is also one of the key requirements.
(2) Hardness is well known. Cellulose has a very hard molecular skeleton as its basic structure. In contrast, people say that the polymers used in light synthetic paper materials are soft. Therefore, the average film has a lower hardness than natural paper of the same thickness.
If the polymers used in synthetic paper are arranged according to the Young's modulus, polystyrene and polyvinyl chloride are almost the same, followed by polypropylene, and the smallest is polyethylene. Needless to say, the first two, polypropylene and polyethylene, just made the film as it is, the hardness is not enough, and measures should be taken to improve it.
The addition of fillers also has an effect, which varies depending on the type of polymer, the type and amount of filler, for example, in the polypropylene with a bending coefficient of (1.2 × 104) kg / cm2, glass 30%, asbestos 40%, talc 40 After%, increase to (4.9, 2.8, 2.5) × 104kg / cm2 respectively.
Of course, even if a resin with a high Young's modulus is used for stretching and filler filling, the hardness of general synthetic paper is still worse than that of natural paper, but this is not related to the difficulty of conveying paper.
The workability of paper feeding is determined by the proficient program of the operator in the printing press. For example, taking the product of Japan Synthetic Paper Co .: the same thickness coating (about 120μm) and the same filler as an example, the original printing factory used 120 sheets / min speed to print without failure. On the contrary, some printing factories Although various adjustments were made, the speed was only maintained at 60 sheets / min.
(3) Dimensional stability From the perspective of the nature of the selected material, any kind of synthetic paper is much superior to natural paper in dimensional stability of humidity changes, and has almost no stretch. Not only does it not need to adjust the humidity of the printing workshop, it does not even need to dry the paper before printing.
However, in general, plastics have a larger thermal expansion rate than other materials, so perhaps it should be considered that the dimensional change due to temperature changes, the linear expansion rate of the polymer used as a raw material for synthetic paper after heating, such as polystyrene It is (6-8) × 10-5, high-density polyethylene is (10-16) × 10-5, etc., which is roughly in the range of (5-30) × 10-5. In a printing shop without temperature adjustment, if there is a temperature change of 10 ° C, the expansion and contraction rate can be counted as 0.05% -0.3%. From the standard state of about 60% relative humidity to the expansion and contraction of natural paper caused by humidity fluctuations of about ± 0%, the coated paper is 0.5% -0.9%, and the glass fiber mixed paper is 0.15% -0.3%. Comparing with this value, it can be seen that the dimensional change of synthetic paper due to temperature change is within the allowable range.
In terms of dimensional stability, it should be noted that its resistance to dynamic tension during printing is extremely small compared to natural paper and easy to extend. Nowadays, almost all synthetic papers improve their dimensional stability through the above-mentioned methods for improving stiffness and filling fillers. However, under normal printing conditions, even if it is roughly satisfied, stricter requirements in terms of tension or printing processes that require higher precision, such as three-dimensional printing, will also bring unsatisfactory results.
(4) Flat natural paper and synthetic paper are no exception. If the paper surface is corrugated and deformed seriously, overprinting will occur when passing through the printing cylinder, and even wrinkles will occur and become a major failure.
The corrugated state of natural paper can be restored to normal by drying the paper, but the synthetic paper will expand and contract due to changes in humidity, which cannot be corrected by this method. The corrugation of synthetic paper is mainly due to material creep caused by uneven tension during rewinding, so as long as the rewinding method is paid attention to, this phenomenon can be prevented. That is: reduce the thickness error in the width direction when manufacturing the film, use the rotary table to spread the difference in the width direction, slowly rewind, etc., and take different measures according to the manufacturing method.
In addition, the synthetic paper used for printing food packaging should also meet the following conditions: a. The synthetic paper used must fully comply with food packaging hygiene standards. That is, no migration of toxic substances occurs, and it is non-toxic and odor-free. b. It has the appearance similar to natural fiber paper with smooth, white and opaque surface. c. Has good moisture resistance, waterproof, heat resistance, cold resistance, corrosion resistance, moth resistance and UV penetration resistance.
3. Interrelation with printing ink
The surface of natural paper has the property of absorbing printing ink. Offset or letterpress ink has a certain degree of penetration and solidification on the surface of the paper immediately after printing, followed by polymerization and solidification. Synthetic paper is only required to have the adaptability of sheet-fed offset printing and letterpress printing, and it is expected that its surface will have properties similar to natural paper. That is, the most important point of the so-called paper processing target is to make it have such a surface that the ink can penetrate.
In order to achieve this goal, there are several methods, which are only briefly described here. However, it is not that these methods have all their own shortcomings. The adjustment of their advantages and disadvantages requires the manufacturer to master.
(1) Filling fillers such as iron dioxide, calcium carbonate and other fillers themselves are very tolerant to printing inks, but it must be noted that only 20% -30% of such fillers are mixed, and the permeability cannot be obtained because At this time, the polymer covers the filler, and there is no necessary capillary tube. If the effect is obtained only by filling the filler, either the content must be increased or the surface layer must be washed off with a solvent to expose the filler.
(2) When the filler and the extension are used in combination to extend the polyolefin film containing a large amount of filler, fine cracks will occur. By adjusting the occurrence of such cracks, it helps penetration.
(3) Pigment coating This is a method of coating clay, titanium dioxide, calcium carbonate, etc. with a binder on the surface. The principle is the same as coated paper and coated paper in natural paper. With this method, not only can you get Natural paper has the same permeability and excellent smoothness, so it can obtain ink tolerance and printing effect that exceeds that of natural paper.
In addition, when the film is extruded and its surface has not been completely cured, the pigment can be evenly dispersed in it to obtain the coating effect.
(4) Drug treatment There are methods for treating the surface of the film with a solvent-non-solvent mixture; methods for curing it with a swelling agent or solvent; methods for impregnating the surface layer with a foaming agent to make it foam, etc. . In either case, the permeability is adjusted by controlling the processing conditions.
(5) The destruction of the foaming unit can also be treated with the above-mentioned chemicals. By treating the surface of the independent bubble membrane through the chemical treatment or destroying the foaming unit by an extension method, a continuous or semi-continuous bubble membrane can be made to obtain permeability.
(6) After extracting and mixing soluble materials into a film, and then removing them, a porous film can be obtained. The capillary size can also be adjusted by selecting the added substance.
(7) The porous film obtained by powder sintering and sintering fine polymer powder has ink permeability.
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