Quantitative Analysis and Application of Unbound and Bound AKD in Wet and Dry Paper

Method for determining unbound AKD (AKD that has not reacted with the solids of the paper stock) and bound AKD (AKD that has reacted with the solids of the paper stock). The main principle is to first perform appropriate extraction on the wet paper sheet before drying (paper sheet after wet pressing, with a moisture content of about 55%) or the finished paper after drying (paper sample on the paper reel, with a moisture content of about 6%), and then perform quantitative analysis of the AKD-related extraction products by gas chromatography. First, use dichloromethane as the solvent and use a Soxhlet extractor to extract the paper sample for 24 hours at 60-70°C to remove the unreacted sizing agent, i.e., unbound AKD (UAKD). Then, these paper samples are subjected to a second step of extraction, in which a mixture of 22.4 g KOH sample dissolved in 1L methanol is used as the extracting liquid, and the paper sample is stored in it for 3 hours (under the condition of 40-50°C) [Comment: The purpose of this is to hydrolyze the ester bond formed by the reaction of AKD with the paper material], and then acidify with hydrochloric acid and extract with toluene. An appropriate amount of anhydrous magnesium sulfate was added to the collected toluene layer and filtered. The two types of extract samples (referring to the samples corresponding to the wet paper sheets and the dry paper sheets) were evaporated, mixed with the internal standard (n-hexadecane) solution, and analyzed by gas chromatography (GC).

The chromatograph was a Perkin Elmer 1022 GC Plus GC, equipped with a flame ionization detector (FID), a 25m non-polar capillary column, and the inner wall stationary phase of the column was 5% phenyl- and 95% dimethyl-polysiloxane. The typical AKD chromatogram obtained from the instrument is shown below.

After the above method is established, the UAKD and BAKD (Bound AKD, i.e. AKD that reacts with the solids of the paper material, generally refers to those AKD that form ester bonds) in wet and dry paper sheets with and without fillers can be quantitatively analyzed, thereby obtaining some research results that can provide insight into the AKD sizing mechanism, such as:

1. Analysis of UAKD and BAKD in PCC unfilled paper and PCC filled paper wet paper sheets. The results show that the content of UAKD and BAKD in filled paper is higher than that in unfilled paper. [Comment: This is easy to understand, because PCC has a large area and can absorb more AKD. As long as there is no major problem with the retention of PCC during papermaking, this phenomenon will inevitably occur]

2. Compared with wet and dry paper sheets, the drying process can greatly increase the amount of BAKD (PCC unfilled paper increases by 2-3 times, and filled paper increases by 4 times), indicating that the drying process can indeed promote the chemical reaction between AKD and solids such as fibers or fillers.

3. In either case, the amount of BAKD is much smaller than that of UAKD; taking the highest AKD addition (5 mg AKD/g absolute dry pulp) as an example, only 21% of all AKD remaining in the paper is BAKD, or in other words, only 15% of the total AKD addition becomes BAKD.

4. PCC addition obviously leads to a larger amount of BAKD, so it is necessary to examine the reaction between PCC and AKD in detail.

5. After the sizing degree of paper reaches a certain value, the amount of UAKD continues to increase, and the sizing degree is no longer increased.

6. Although PCC addition leads to a higher BAKD, the sizing degree of the paper is not as good as that of the unfilled paper.

In summary, the proportion of AKD that reacts with paper stock (i.e. BAKD) in the total AKD retention is very low, but its contribution to paper sizing is large; while UAKD and AKD adsorbed by fillers (including reacting with fillers) may be the main reasons for sizing reversal and paper slippage. Therefore, when applying AKD glue, in addition to the appropriate dosage, a method for its rapid reaction with paper stock should also be developed.