Zhang Guang-qiu(张广秋) Ge Yuan-jing(葛袁静) Zhao Zhi-fa(赵志发),
　　Qiao Zhi-shan(乔治山) Liu Yi-min(刘益民)
　　Laboratory of Application Physics of Beijing Institute Printing
Daxing,Beinjing,102600, China
E-mail Address: Yjgebip@public3.bta.net.cn
Abstract:

　　Some investigation on application of plasma technology in printing industry will be reported. These results showed that plasma technology has a good application future in printing area.

　　Key words: plasma surface modification, plasma application

Introduction:

　　During the past years, some investigation on application of plasma technology in printing industry has been done and many useful results were obtained in our laboratory. Some of them have been used in industry already. The main subject include: (1) Surface modification of carbon black、paper and plastic. (2) Preparation of new printing materials. (3) Elimination of environment pollution in printing factory.

　　In this paper some of them will be reported briefly.

Application

1. Modification of carbon black

　　Carbon black is a special material with fine size, deep black and conductivity .It is an important raw material to make ink and rubber. In order to get a product with high quality, the carbon black must be adhesive with binder and could be easily dispersed in it. However the raw powder is lack of these behaviors .It must be treated before used in manufacture of ink and rubber .For this reason, the plasma surface modification of carbon black in plasma was investigated .The experiment was carried out with a new type device named Induced Dielectric Barrier Discharge Device [1]. By using this device, stable uniform plasma was obtained at atmospheric pressure. The investigated result are as following:

　　(1)、After treatment in plasma, the size of carbon black became smaller than that before treatment. (Fig.1)

　　(2)、After treatment, the PH value of carbon black became much lower than that before treatment (See table.1). It means that a lot of oxy-groups appeared on the surface of carbon black. IR spectrum analysis showed that these groups are –OH, -COOH mainly.

　　When the treated carbon black used in manufacture of printing ink, the carbon black became easy to be ground (Fig.2) and the quality of ink is better than that of the ink made by original carbon black. (Table.2)

Table 1: Variation of pH value of different

Kind of carbon powder before and after treating in plasma

Sample1 Sample2 Sample3

Untreated 5.67 8.43 6.0

Treated 3.22 4.44 2.51

Table 2: Variation of parameters of ink made by treated and untreated carbon black.

Parameters of ink Fluidity(arbitrary) Adhesion (arbitrary) Brilliance(arbitrary) Density (blackness) (arbitrary) Coloration(arbitrary)

Untreated carbon black. 33.5 11.1 48.65 1.74 100

Treated carbon black. 35.0 10.6 56 1.87 110

Variation rate (%) 4.5 -4.5 15 7.5 10

2. Improvement of printability of paper

　　In our country there is lack of enough wood (forest) to be used in paper manufacture industry. Therefore we have to use some herbaceous plant to make paper .In this situation the surface strength and printability of paper is low, for this reason it is difficult to obtain print with high quality .In order to solve this problem, the plasma surface modification technique was used .The plasma device used to treat paper is a special Dielectric Barrier Discharge Device with high transfer efficiency [2]. The treatment effects were tested by using SEM, IR spectrum and contact angle of paper with water and CH2I2 respectively. The tested results are as following:

　　(1)、The surface of paper became more rough, a lot of holes with diameter 0.2-0.4μm appeared on the treated paper surface. (Fig.3)

　　(2)、The chemical constitution of paper surface was changed, a lot of active groups, such as COOH-, OH-, appeared on the treated surface. This conclusion was obtained from IR spectrum. (Fig.4)

　　(3)、The surface energy of treated paper by plasma increased obviously. This conclusion can be obtained from measured results of contact angle of paper with water (Fig.5) and Yang equation.

　　All of these physical and chemical variation appeared on treated paper surface are useful for improvement of printingability of paper and quality of print. This technique has been used in stamp printing process. The application results showed that for treated paper the absorbability to ink improved obviously: the dry time of ink on treated paper is shorter than that on untreated paper; the printing density increased obviously, especially for high bright grade, the increasement is about 60% (Table 3); the printed mark on treated stamp paper is better than that on untreated stamp paper (Fig.6) .All of these are very important for printing industry. By this way a lot of paper used to be not suitable for printing became suitable for printing.

Table3. Print density of treated and entreated stamp paper

Color grade A B C D

Untreated 1.029 0.600 0.289 0.060

Treated 1.037 0.650 0.339 0.097

Increment 0.8% 7.4% 17.3% 61.9%

　　Based on the research results mentioned above, this technique could be used to improve the surface strength of paper in papermaking industry too. The used method is as showed in Fig.7. The industrial experimental results showed that the surface strength of the paper made by process with plasma technique increased about 60-70% and the comprehensive printability became much better than that of the paper made by process without plasma treatment.

3. Preparation of digital printing plate

　　Laser ablation plate is one of digital printing plate. It is composed of substrate, imaging layer and surface layer. The imaging layer is a thin uniform film with high absorbability to ultra red laser. Based this characteristic the image with high resolution can be obtained by means of computer and ultra red laser. The surface layer is very smooth and uniform thin film with low surface energy. By this way it can be used in digital waterless offset printing.

　　In order to obtain this plate with a simple preparation process and low economical cost, we have prepared it by plasma technology. The imaging layer was prepared by using a continual plasma deposition device with width of 1.5m. The film formed by this machine is a uniform metal film with thinness of about 2000 angstrom. The surface layer was prepared by plasma polymerization. It is a thin and uniform smooth film (named D4 film) with low surface energy、 high resistance to water and high surface strength.

　　The surface energy of plate was tested by contact angle of standard liquid. The results are shown in table.4.

Table 4 surfaces energy of PET substrate and prepared printing plate (erg/cm)

γ(erg/cm) γp (erg/cm) γd (erg/cm)

Surface film 21.8 6.8 21.8

Substrate(PET) 51.2 8.5 42.7

　　In table 4,γis total surface energy, γp andγd is components of polar and dispersion energy respectively. From these data we can see that the total surface energy of surface film is only 40% of substrate. The polar component of surface energy is nearly zero. This behavior is very important condition for digital waterless offset printing.

　　The strength of surface film was tested as following: prepare a D4 film on a CaF2 glass by plasma polymerization. To rub this film with leather back and forth about 100 times under a certain pressure (This process is similar to the interaction process of plate with paper during printing process), then test the chemical constitution of surface film by IR spectrum. The result was showed in Fig.8. From Fig.8 we can see that the intensity of characteristic spectral peaks of D4 film is same before and after rub with leather. It means that the strength and resistance to wear of surface film is high enough. Fig.9 is a picture of contact of plate with water and substrate. From it we can see that this plate has high resistance to water.

　　All of the behaviors mentioned above showed that the plate made by plasma technology is suitable for digital printing.



Conclusion:

　　All of these works finished in our laboratory showed that plasma is a useful technology in printing industry, including modifying the surface of printing material, preparing new printing materials.



Reference:

[1]、 Ge Y.J. et al 《Proceeding of ISPC 12》. P1515. (1995), Minneapolis, USA

[2]、 Ge Y.J. et al Proceeding of the Third China—Japan Symposium on Plasma Chemistry.

Vol.Ⅱ.p476. (1992). Nanjing China.

Fig1: SEM photograph of carbon black before (left) and

after (right) treatment in plasma

Fig.2 Size of carbon black particle in printing ink correspondent to different grinding time

　　a: Ink made by original carbon black

　　b: Ink made by treated carbon black in plasma

Fig.3 SEM photo of untreated (left) and treated (right) paper

1100 750 400

　　Fig.4 IR spectrum of untreated (a) and treated (b) paper

Fig.5 variation of contact angle of paper to water vs. treated powder

Fig. 6 Printed marks on untreated (right) and treated (left) stamp paper

Fig.7 Process of paper-making with and without plasma treatment

(a): Process with plasma technology

(b): Process without plasma technology
1400 1100 900 cm-1

Fig.8 Variation of IR spectra before (a) and after (b) weared by leather

Fig.9 contact angle of prepared printing plate (left) and PET

substance (right) to water