Carboxymethyl cellulose (Carboxy Methyl Cellulose, CMC) is an ether derivative of natural cellulose. It is white or slightly yellow powder. It is a water-soluble anionic surfactant. It is odorless, tasteless, non-toxic, and has excellent water solubility. , Viscosity, emulsification, diffusion, enzyme resistance, stability and environmental friendliness, CMC is widely used in papermaking, textile, printing and dyeing, petroleum, green agriculture and polymer fields. In the paper industry, CMC has been widely used in surface sizing agents and coating adhesives for many years, but it has not been well developed and applied as a papermaking wet-end strengthening agent.
The surface of cellulose is negatively charged, therefore, anionic polyelectrolytes generally do not adsorb it. However, studies have shown that CMC can be attached to the surface of elemental chlorine-free bleaching (ECF) pulp, which can increase the strength of the paper; in addition, CMC is also a dispersant, which can improve the dispersion of fibers in the suspension, thereby bringing paper evenness. The improvement of the degree also increases the physical strength of the paper; moreover, the carboxyl group on the CMC will form a hydrogen bond with the hydroxyl group of the cellulose on the fiber to increase the strength of the paper. The strength of the reinforced paper is related to the degree and distribution of CMC adsorption on the fiber surface, and the strength and distribution of CMC adsorption on the fiber surface are related to the degree of substitution (DS) and degree of polymerization (DP) of CMC; the charge, beating degree, and pH of the fiber , the ionic strength of the medium, etc. will all affect the adsorption amount of CMC on the fiber surface, thus affecting the strength of the paper.
This paper focuses on the influence of CMC wet-end addition process and its characteristics on paper strength enhancement, in order to evaluate the application potential of CMC as a papermaking wet-end strengthening agent, and provide a basis for the application and synthesis of CMC in the papermaking wet-end.
1. Preparation of CMC solution
Accurately weigh 5.0 g of CMC (absolutely dry, converted to pure CMC), slowly add it to 600ml (50°C) distilled water under stirring (500r/min), continue stirring (20min) until the solution is clear, and let it cool down to room temperature , use a 1L volumetric flask to constant volume to prepare a CMC aqueous solution with a concentration of 5.0g/L, and let it stand in a cool place at room temperature for 24 hours for later use.
Considering the actual industrial application (neutral papermaking) and CMC enhancement effect, when the pH is 7.5, the tensile index, burst index, tear index and folding endurance of the paper sheet are respectively increased by 16.4 relative to the corresponding strength of the blank control sample. %, 21.0%, 13.2% and 75%, with obvious paper enhancement effect. Select pH 7.5 as the pH value for subsequent CMC addition.
2. Effect of CMC dosage on paper sheet enhancement
Add NX-800AT carboxymethyl cellulose, the dosage is 0.12%, 0.20%, 0.28%, 0.36%, 0.44% (for absolute dry pulp). Under the same other conditions, the blank without adding CMC was used as the control sample.
When the CMC content is 0.12%, the results show that the tensile index, burst index, tear index and folding strength of the paper sheet are increased by 15.2%, 25.9%, 10.6% and 62.5% respectively compared with the blank sample. It can be seen that considering the industrial reality, the ideal enhancement effect can still be obtained when the low dosage of CMC (0.12%) is selected.
3. The effect of CMC molecular weight on paper sheet strengthening
Under certain conditions, the viscosity of CMC relatively represents the size of its molecular weight, that is, the degree of polymerization. Adding CMC to the paper stock suspension, the viscosity of CMC has a significant impact on the use effect.
Add 0.2% NX-50AT, NX-400AT, NX-800AT carboxymethyl cellulose test results respectively, the viscosity is 0 means blank sample.
When the viscosity of CMC is 400～600mPa•s, the addition of CMC can achieve a good reinforcing effect.
4. Effect of degree of substitution on the strength of CMC-enhanced paper
The degree of substitution of CMC added to the wet end is controlled between 0.40 and 0.90. The higher the degree of substitution, the better the substitution uniformity and solubility, and the more uniform the interaction with the fiber, but the negative charge also increases accordingly, which will affect the combination between CMC and the fiber [11]. Add 0.2% of NX-800 and NX-800AT carboxymethyl cellulose with the same viscosity respectively, the results are shown in Figure 4.
Bursting strength, tear strength, and folding strength all decrease with the increase of CMC substitution degree, and reach the maximum when the substitution degree is 0.6, which are respectively increased by 21.0%, 13.2%, and 75% compared with the blank sample. In comparison, CMC with a degree of substitution of 0.6 is more conducive to the enhancement of paper strength.
5 Conclusion
5.1 The pH of the slurry wet end system has an important influence on the strength of the CMC-enhanced paper sheet. When the pH is in the range of 6.5 to 8.5, the addition of CMC can exert a good strengthening effect, and CMC strengthening is suitable for neutral papermaking.
5.2 The amount of CMC has a great influence on the strengthening of CMC paper. With the increase of CMC content, the tensile strength, bursting resistance and tear strength of the paper sheet increased first and then tended to be relatively stable, while the folding endurance showed a trend of increasing first and then decreasing. When the dosage is 0.12%, the obvious paper strengthening effect can be obtained.
The molecular weight of 5.3CMC also has a significant impact on the strengthening effect of the paper. CMC with a viscosity of 400-600mPa·s can achieve good sheet strengthening.
5.4 The degree of CMC substitution has an impact on the strengthening effect of the paper. The CMC with the substitution degree of 0.6 and 0.9 can obviously improve the paper strength performance. The enhancement effect of CMC with a substitution degree of 0.6 is better than that of CMC with a substitution degree of 0.9.