On Applications of Sodium carboxymethyl cellulose in Surface Sizing
1. Comparison of Sodium carboxymethyl cellulose with Modified Starch and PVA
Sodium carboxymethyl cellulose (CMC) is a cellulose ether made from natural cellulose by refined modification. For the Sodium carboxymethyl cellulose used as the surface sizing agent, generally it is enough if the viscosity is 30~700mpa.s and the degree of substitution 0.7~0.85 (equivalent to the Sodium carboxymethyl cellulose products of CX-30 to 700 series in our company). As the surface sizing agent, Sodium carboxymethyl cellulose has excellent film-forming property and whole film transfer performance, and can form good closure property and oil resistance on the surface of paper or paperboard. However, the modified starch is poor in the film-forming property, impossible to form the continuous and complete film on the surface of paper, can form “accumulations” on the paper surface and clog the pores, resulting in spots in printing. Although polyvinyl alcohol (PVA) also has very good film-forming performance, since it is a thermoplastic resin, very difficult for dissolution, with a lot of foam, and easy for crusting, in the sizing liquid, it may form the small gel with starch and promote the glue to form accumulations on the paper surface. When the paper is pasted onto the drying cylinder, then blocking under warming may occur and paper defects may be produced. On the contrary, Sodium carboxymethyl cellulose is a dispersing agent, and it can make the pigment particles or other colloidal particles compatibly used with it, fully dispersed, making the whole sizing or coating material system form a stable and homogeneous system, thereby obtaining good application effects.
2. Applications of Sodium carboxymethyl cellulose
In the printing process, we often find that the paper or paperboard may produce edge lifting and curling, and the errors of size may occur in the color overprinting. The main reasons for such deformation and dimensional errors can be classified into mechanical, internal structural, and moisture. If the sizing material with Sodium carboxymethyl cellulose alone or with an appropriate amount of Sodium carboxymethyl cellulose is used for surface sizing, then a layer of continuous, complete, and flexible film can be formed on the surface of paper. This layer of film has better humidity-controlling function, helps to eliminate the paper internal strain caused by the drying section, and prevents the warpage and deformation of paper or paperboard and eliminates the dimensional errors in overprinting.
In China, there is a medium-sized paper mill producing the ordinary offset paper, and the surface intensity of quality indicators always cannot reach the standard. We took the Sodium carboxymethyl cellulose produced by our company to that mill for production test. Up to the test, the sizing material formulation of this mill is 80 portions of low-viscosity phosphate starch + 20 portions of PVA of 17-99 type. Starting from the product grade and cost principle of the mill, we decide to use a small amount of Sodium carboxymethyl cellulose to completely replace PVA. The three formulations we proposed are: (1) 20 portions of starch + 1 portion of Sodium carboxymethyl cellulose; (2) 15 portions of starch + 1 portion of Sodium carboxymethyl cellulose; (3) 10 portions of starch + 1 portion of Sodium carboxymethyl cellulose. The test results of surface intensity are: 2.28m/s for formulation (1); 2.9m/s for formulation (2); over 3.0m/s for formulation (3); the maximum value in the test reaches 5.0m/s. After we have compared the costs of several formulations, calculating by the present market prices of PVA and Sodium carboxymethyl cellulose, the use of formulation (1) can not only guarantee the product quality, but also can reduce costs. In the above test, the sizing equipment is the horizontal size press.
Different paper machine sizing systems should choose different viscosity specifications of Sodium carboxymethyl cellulose. To be specific, for the horizontal size press (with a vehicle speed less than 800m/min) and the compounded use with modified starch, select from CX-100 / CX-200 / NCX-300. The principle is that the higher the concentration of the sizing material is, the lower the viscosity of selected Sodium carboxymethyl cellulose will be; the higher the compound portion of Sodium carboxymethyl cellulose is, the lower the concentration of sizing material should appropriately be, targeted at controlling the on-machine viscosity and concentration of sizing material to select the appropriate Sodium carboxymethyl cellulose viscosity specifications. If you want to use the single Sodium carboxymethyl cellulose for slight sizing, then you can even choose CX-700. According to the data published abroad, it has been confirmed that in the advanced offset paper, the surface size with the single Sodium carboxymethyl cellulose as the sizing material can reach 0.1g/㎡ each face.
If the film transfer metering size press (with a vehicle speed more than 800m/min) is used, when compound used with modified starch, it is appropriate to choose CX-20/CX-30/CX-50. When Sodium carboxymethyl cellulose is used alone as the sizing agent, choose those with higher viscosity based on the sizing amount (sizing liquid concentration). In short, for the film transfer metering size press, it is appropriate to choose Sodium carboxymethyl cellulose of lower viscosity. For some paper types with special requirements on water resistance and wet strength, such as bond paper, banknote paper, tipping paper, and trademark paper, then an appropriate amount of water-resistant agent should be blended together with the materials in the formulation such as starch and Sodium carboxymethyl cellulose to produce the crosslinking and form the water-repellent film. The addition amount thereof is generally controlled at 0.5-2.0 shares (based on 100 shares for the dry solid volume of crosslinkable sizing materials).
Although PVA and Sodium carboxymethyl cellulose has very good compatibility, we do not advocate the compound use of the two materials together. Sodium carboxymethyl cellulose has good dispersibility, can make other sizing material particles fully dispersed to form a homogeneous and stable system, and can can effectively delay the aggregation and precipitation of other colloidal particles generated by aging in a given period.
After Sodium carboxymethyl cellulose is used or Sodium carboxymethyl cellulose has replaced other additives, no big change is needed for the equipment. The original starch gum preparation system can still be used, only adding a dissolving tank and coupled with an agitator with the rotational speed of 300-600r/min. The tank volume may be determined by the production capacity of paper machine and can be 800 ~ 1200L. It will also need a storage tank of glue liquid slightly bigger than the dissolving tank in order to maintain a continuous feeding supply. For the manufacturers that have used PVA, only an agitator needs to be converted. Depending on the viscosity level of Sodium carboxymethyl cellulose used, the solution concentration of Sodium carboxymethyl cellulose should be controlled at 2% to 5%.
Some manufacturers have adopted the implementation of pigment sizing for offset paper to improve the product grade. Since it is similar to slight coating (2~3g/㎡ per face), the solid content of sizing liquid is usually between 28% and 32%; the main adhesive usually chooses the modified starch; the main pigment selects superfine heavy calcium or ultrafine precipitated calcium carbonate. In order to make this sizing (coating) material with good coatability and make the coating performance excellent, Sodium carboxymethyl cellulose has become an essential additive, because Sodium carboxymethyl cellulose also has a relatively low fixed point of coating concentration in addition to all the above advantages. The blended amount of Sodium carboxymethyl cellulose and how to choose the specifications can be applied flexibly in accordance with the above major principles. In order to take into account the wide range of functions of Sodium carboxymethyl cellulose, usually choose its viscosity of 50~120mpa•s.
The expanded experimental data of factory have shown that using the surface sizing can form a layer of flexible film on the sizing surface, and produce good sizing effects. In terms of its one indicator, oil resistance, the effect produced by the Sodium carboxymethyl cellulose aqueous solution with the concentration of 1% can be comparable to that of the starch solution with the concentration of 12%.
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