Synthetic paper printing conditions
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Synthetic paper refers to a material such as a polymer organic compound (such as polystyrene, polypropylene, polyethylene, etc.) made of synthetic resin, synthetic staple fiber and wood pulp, which is calendered or extruded. Flat material.
In the molding process, in order to make it have good whiteness, opacity, printability and writing, various inorganic fillers (such as clay, talcum powder, calcium carbonate) are added depending on the raw materials used. , iron dioxide, barium sulfate, etc.), tackifiers, stabilizers and anti-aging agents, etc., after the paper is formed, it is also treated with water, inorganic acid solution, phosphate solution, aluminum sulfate, slaked lime solvent, and then oxidant ( Oxidation treatment of hydrogen peroxide, sodium oxide, ozone, air, etc. can be applied.
The composition of the synthetic paper not only retains the appearance, whiteness, stiffness, opacity, printability and writing property of the natural fiber paper, but also has the lightness, moisture resistance, oxygen resistance, acid resistance, alkali resistance and protection of the polyolefin plastic film. Excellent characteristics such as pollution. Therefore, synthetic paper has become a high-end new material for printing and packaging decoration popular in the world market today.
Synthetic paper used as a printing material should have the following conditions:
1 white opaque appearance;
2 must meet the operational suitability of the printing press;
There is a good correlation between 3 and printing ink. These three conditions are not controlled independently, but affect each other. How to make these elements work together is the key to their quality design.
Appearance
Unless otherwise specified, it should have a white, opaque appearance. Wood pulp is treated with a drug, and after obtaining a low utilization rate and a decrease in the relative molecular mass of cellulose, white paper is obtained. In contrast, it is easy to make the synthetic paper only white and opaque. That is, by filling or designing the foamed layer or the porous layer with a white filler, not only high whiteness but also a white product having higher purity than natural paper can be obtained. At the same time, the opacity can also be changed by changing the above processing conditions.
2. Print job suitability
(1) Static electricity When printing on a sheetfed press, static electricity often occurs and malfunctions occur, so it is necessary to prevent charging. In the case of low humidity in winter, ordinary papers often have static electricity barriers. Therefore, when using paper made of synthetic polymers that are inherently easy to charge, more attention should be paid.
The method of preventing electrification is to add or coat the antistatic agent, but if it is used excessively or incorrectly selected, the following adverse effects will occur. First, it will emboss when taking notes, and it will also cause paste on the film printing, and even cause the ink to change color. In addition, the added preventive agent loses its effect if it volatilizes on the surface, so long-term stability is also one of the required points.
(2) Hardness It is well known that cellulose is basically constructed from an extremely hard molecular skeleton. In contrast, the polymers used in the light synthetic paper raw materials are all soft. Therefore, a general film has a small hardness compared to a natural paper of the same thickness.
If the polymer used in the synthetic paper is arranged in the Young's modulus, the polystyrene and the polyvinyl chloride are almost the same, followed by the polypropylene, and the smallest is the polyethylene. Needless to say, the first two do not say that polypropylene and polyethylene are only made into a film as it is, and its hardness is not sufficient, and measures should be taken to improve it.
It is also effective to add a filler, and the effect varies depending on the kind of the polymer, the kind and amount of the filler, for example, 30% of glass, 40% of asbestos, and talc 40 are added to polypropylene having a bending coefficient of (1.2 × 10 4 kg/cm 2 ). After %, it is increased to (4.9, 2.8, 2.5) × 104 kg/cm2.
Of course, even if a resin having a high Young's modulus or a stretching and filler filling is used, the hardness of a general synthetic paper is worse than that of natural paper, but this is not related to the difficulty of paper feeding.
The workability of paper feeding is determined by the skilled program of the operator in the printing press. For example, in the case of Japan Synthetic Paper Co., Ltd.: the same thickness coating (about 120 μm), the same filler, for example, the original printing factory can print without failure at a speed of 120 sheets/min. On the contrary, some printing plants Although various adjustments were made, only a speed of 60 sheets/min was maintained.
(3) Dimensional stability Any synthetic paper, from the nature of the material selected, its dimensional stability against humidity changes is much superior to natural paper, and there is almost no expansion and contraction. Not only does it not require adjustment of the humidity in the pressroom, it does not even require paper drying prior to printing.
However, in general, plastic has a large thermal expansion rate compared with other materials, so it may be considered that it changes in dimensional change due to temperature changes, and the coefficient of linear expansion of a polymer used as a raw material for synthetic paper, such as polystyrene. It is (6-8)×10-5, and the high-density polyethylene is (10-16)×10-5, etc., and is approximately (5-30)×10-5. In a printing shop without temperature regulation, if there is a temperature change of 10 ° C, the expansion ratio can be calculated as 0.05% - 0.3%. From the standard state of about 60% relative humidity to the humidity fluctuation of about ±0% of the field, the stretch of the natural paper is 0.5%-0.9% for the coated paper and 0.15%-0.3% for the glass fiber mixed paper. Comparing with this value, it can be seen that the dimensional change caused by the temperature change of the synthetic paper is within the allowable range.
What should be noted in terms of dimensional stability is that it has no resistance to dynamic tension during printing and is less than natural paper and easy to extend. Today's synthetic papers are almost always improved in dimensional stability by the above-described method of improving stiffness and filling filler. However, under normal printing conditions, even if it is substantially satisfied, more stringent conditions are required for tension or a printing process requiring higher precision such as embossing, which also causes unsatisfactory results.
(4) Planarity Natural paper and synthetic paper are all without exception. If the surface of the paper is corrugated, the overprinting will be inaccurate and even wrinkles will become a major failure.
The corrugated state of natural paper can be restored to normal by airing paper, but the synthetic paper will be stretched due to humidity changes and cannot be corrected by this method. The corrugation of synthetic paper mainly depends on the material creep caused by uneven tension during rewinding, so it is possible to prevent this phenomenon by paying attention to the rewinding method. That is, the thickness error in the width direction when the film is produced is reduced, the difference is dispersed in the width direction by the turntable, and the film is slowly wound up, and different measures are taken 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 the food packaging hygiene standards. That is, there is no migration of toxic substances, no toxicity, no odor. b. It has the appearance of natural fiber paper, and the surface is smooth, white and opaque. c. It has good moisture-proof, waterproof, heat-resistant, cold-resistant, corrosion-resistant, anti-mite and anti-UV penetration properties.
3. Interrelationship with printing ink
The surface of the natural paper has the property of absorbing printing ink, and the offset or relief ink has a certain degree of penetration and solidification on the surface of the paper immediately after printing, followed by polymerization curing. Synthetic paper is only required to have the suitability of a single offset and relief printing, which is expected to have a surface similar to natural paper. The most important point of the so-called paper processing target is to make it have a surface that the ink can penetrate.
There are several ways to do this, and here is a brief explanation. But it does not mean that these methods have their own shortcomings, and the adjustment of their advantages and disadvantages requires the master to master.
(1) Filler filled with fillers such as iron oxide and calcium carbonate is very resistant to printing inks, but it must be noted that only 20%-30% of such fillers are mixed, and permeability cannot be obtained because At this time, the polymer is covered around the filler, there is no necessary capillary, and if the effect is only filled by the filler, the content must be increased, or the surface layer must be washed away with a solvent to expose the filler.
(2) Use of a filler and an extension of the filler. When a polyolefin film containing a large amount of filler is stretched, fine cracks occur. It helps to penetrate by adjusting the occurrence of such cracks.
(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 that of coated paper and coated paper in natural paper. In this way, not only can Natural paper has the same permeability and excellent smoothness, so it can obtain ink acceptance and printing effects over natural paper.
Further, in the case where the extruded film and the surface thereof are not completely cured, the pigment is uniformly dispersed therein, and a coating effect can also be obtained.
(4) Drug treatment Here, a method of treating a surface of a film with a solvent-non-solvent mixture; a method of solidifying with a swelling agent or a solvent; and a method of impregnating the surface layer with a foaming agent to foam it . Either way, the permeability is adjusted by controlling the processing conditions.
(5) Destruction of the foaming unit It is also possible to treat the foaming unit by treating the surface of the closed cell membrane with a drug or by a method of stretching to form a continuous or semi-continuous bubble film, and to obtain permeability.
(6) Extraction After the soluble substances are mixed into a film, they are extracted and removed to obtain a porous film. The size of the capillary can also be adjusted by selecting the added substance.
(7) Powder sintering A porous film obtained by sintering a fine powder of a polymer has ink permeability.