Honeycomb paperboard has received great attention since its inception due to its wide application and good performance. The adhesive used to bond core paper and face paper can be said to have certain decisive effects on the performance of honeycomb paperboard. Adhesives with excellent properties are of great value. Currently widely-regarded corn starch adhesives are used as a good, non-toxic and inexpensive natural adhesive in the paper packaging industry. Starch gum replaces silicates, white latex and PVA. Has become a trend. However, in actual production, the problems of low solid content of starch binder, slow drying speed, and low initial tack force have brought obstacles to the further promotion of starch adhesive, and have also brought a lot of problems to the production of honeycomb paperboard. In response to this problem, I conducted a lot of experimental research, and finally developed a solid content of starch binders can reach 35%, significantly improve the drying speed and improve the initial viscosity, reducing costs.
First, process flow and steps
(a) Oxidation stage
Weigh a certain amount of starch, add a certain amount of water (starch: water 1:3) and the temperature is room temperature. Stir with a stirrer, add a certain amount of NaOH solution, adjust the PH value to about 10. Finally, add CuSO4 solution, H2O2 solution and an appropriate amount of defoamer, and stir for 1 hour.
(B) The gelatinization phase
An appropriate amount of the filler is added to the above solution, and then the NaOH solution is added so that the gelatinization reaction takes place and the reaction is fully performed. The reaction time is controlled at about 20min.
(III) Phase of suspension
Stabilizer was added to stop the reaction and stirred for about 5 minutes.
(d) Network and phase
Add a solution of borax and slowly add to the liquid, and stir for 5 minutes.
Second, the experimental analysis
(a) Oxidation stage
1, oxidants
In order to increase the solubility and fluidity of the starch and prevent the gelling, the oxidative degradation method is often used in the preparation of the corn starch adhesive. Considering the difficulty of controlling the oxidation conditions, raw material cost, and the color requirements of the product, the experiment used a starch binder made by the H2O2 oxidation process, and in order to shorten the oxidation time, a catalytic oxidation method was used. The catalyst used in this study was CuSO4.
When oxidizing corn starch with 30% H2O2, carboxyl, carbonyl and aldehyde functional groups are formed in the starch. The formation ratio of the three functional groups is related to the pH of the reaction system. When the pH is in the range of 9 to 10, the amount of carboxyl groups generated is relatively high. When the pH is around 7, the amount of carbonyl groups and aldehyde groups is relatively high, due to the strong adhesion of carboxyl groups to fibrous substances. The carboxyl group is anionic under alkaline conditions, which can reduce the association between starch molecules and improve the storage stability of the starch binder. Therefore, the pH of the reaction system should be controlled at about 10 to facilitate the formation of more carboxyl groups. Therefore, the reason for adding the appropriate NaOH solution at the beginning is this. The amount of oxidant used in this experiment was controlled at about 4%.
2, catalyst
Copper ions have a significant catalytic effect on the oxidation of starch with hydrogen peroxide. When the catalyst is not used, the carboxyl content of the oxidized starch is rather low. Once the catalyst is added, the carboxyl content rapidly increases, and as the amount of catalyst increases, the carboxyl content increases. This is mainly due to the addition of a catalyst that breaks the granule structure of the starch, increases the chance of contact between the oxidizing agent and the starch molecules, improves the oxidation effect, and increases the carboxyl content.
3, defoamer
During the production and use, due to the large amount of bubbles generated in the oxidation stage of starch, affecting the normal sizing of adhesives, especially using hydrogen peroxide as an oxidant, it is easier to produce foam, so an appropriate amount of antifoaming agent must be added for defoaming, and antifoaming agent can be added. Silicone oil or tributyl phosphate, n-octanol, etc. were selected. Tributyl phosphate was selected for this study. Special attention should be paid to the amount of defoamer should not be too much, excessive will cause the adhesive surface tension to reduce, lose the viscosity, the adhesive strength can not meet the requirements, the best dosage is 0.3 ~ 0.5% of the amount of starch.
(B) The gelatinization phase
1, filler
Domestic honeycomb cartons are mainly produced from straw pulp paper, which has a rough surface and high water absorption. If the binder itself has a large amount of moisture, most of the water is absorbed by the straw pulp after sizing, and the natural drying speed will be significantly slowed down. Adding a certain amount of bentonite as a filler in the cornstarch binder can increase the solid content and reduce the water content. It can also effectively block the pores of the surface of the corrugated fiber and prevent water from permeating into the paper, so as to achieve rapid drying.
2, pasting agent
NaOH acts as a pasting agent, which binds to the unoxidized hydroxyl groups in oxidized starch, breaks hydrogen bonds, weakens the interaction between starch molecules, renders it soluble in gelatinization, and reacts with the functional groups of oxidized starch to form sodium salts and increases The hydrophilicity and solubility of the starch increase the viscosity and hardness of the binder. At the same time, it improves the freeze-thaw resistance of the starch gum, increases the fluidity of the starch gum, and makes it easy to store. If the amount is too much, although the adhesive has good transparency and good fluidity, the viscosity of the adhesive will decrease and affect the stickiness, and the foam will be easily generated, resulting in the phenomenon of degumming, and the alkalinity is too strong and has a certain corrosive effect on the paper fiber. Easy yellowing on paper. If the amount is too small, starch gelatinization may be insufficient, the binding force is small, and the flowability is poor. Experiments show that the amount of alkali is controlled at 8% to 10% of starch.
3, suspension phase
Stabilizer is added to neutralize the unreacted excess NaOH solution to prevent the gelatinization reaction from continuing. Otherwise, the viscosity of the adhesive is reduced, which will affect the stability of the product.
4, complexation phase
In order to increase the degree of crosslinking and initial viscosity of the binder, borax was added as a cross-linking agent. The crosslinking agent is used to cross-link the linear structure macromolecules or the small-molecule starches with branched linear structures, and the linear structure is converted into a network structure. Therefore, the cross-linking agent used is a borax binder. Add borax after gelatinization, it can make short-chain oxidized starch with its hydroxyl or hydroxyl and boron atoms to form a complex, through these irregular cross-linking to form a network structure, with cross-linked thickening effect, is conducive to improve Initial tack and accelerated drying, in addition to anti-corrosion, anti-seepage and termination reaction. However, the amount is not as good as possible. Too much will cause the binder to have poor fluidity and be jelly-like, making it difficult to sizing and the brittleness of the gum. Too little complexation is not enough, poor adhesion. It was determined that the amount of borax should be controlled at about 0.5% of the starch content.
Conclusion
The solid content of starch binder produced by this method can reach 35%, which effectively solves the problems of slow drying speed, low initial viscosity, etc., and can be preserved under conditions of high temperature and high humidity (temperature 30°C, humidity 90%). About 25 days, the production process is simple and the quality is high, which greatly ensures the production of high-quality honeycomb paperboard, and has great market value.
