Effect of Pasting Temperature of Starch Glue on Quality of Corrugated Paperboard

Pasting temperature is one of the important technical indicators to measure the quality of starch glue. The standard pasting temperature is 58±2°C. In general, summer starch glue temperature is high, paper and mechanical heat are enough, easier to paste, high gelatinization temperature (57 ~ 59 °C), the viscosity is relatively stable; winter weather is cold, starch glue temperature is low, for To reduce the heat required for starch gelatinization, the gelatinization temperature (55 to 57°C) must be reduced to facilitate the bonding.

First, the gelatinization process of starch glue

The starch granules are fully stirred under the action of a certain temperature and water, and they gradually expand under the action of the gelatinizer to reach the gelatinization temperature. As the temperature rises, the starch granules begin to collapse and gradually disintegrate into small pieces. After soaking into the inside of the tissue paper and the core paper, they begin to adhere and require a certain initial viscosity (mainly relying on NaOH solution and borax solution). As the temperature rises, the moisture content of the starch granules evaporates, and is partially absorbed by the facial paper and the core paper, so that the initial viscosity is rapidly performed. The starch glue connects the paper, the core paper, the corrugated medium, and the facial paper into a whole and achieves the purpose of adhesion. .

Second, factors that affect the gelatinization temperature of starch

Starch gum begins to thicken and the temperature at which the adhesive properties appear due to the gradual gelatinization of raw starch in the starch gum raw material is the gelatinization temperature. There are many factors that affect the gelatinization temperature, mainly in the following aspects.

1, the amount of gelatinizer NaOH

(1) The effect of pasting agent

The starch granule itself has a strong cohesive force, water molecules are not easily penetrated, and water molecules must be brought into the starch granules with the aid of the strength of the gelatinizer, so that the interaction between starch macromolecules is weakened and the gelatinization temperature is lowered. Second, when the gelatinizer dissolves in the starch liquid, it will release a certain amount of heat, so that the starch granules will swell and become viscous, and the starch glue will also have good fluidity and will not be easily deformed.

In the carton industry, sodium hydroxide (NaOH) is generally used as a gelatinizer, and the amount is generally controlled to be less than 1% of the mass of the starch glue, and is added to the oxidized starch liquid, and it is appropriate to stir the starch liquid for a translucent paste for 20 min.

(2) The amount of gelatinizer NaOH has a great influence on the gelatinization temperature of starch glue

The more NaOH is used, the lower the gelatinization temperature of the starch gel and the better the penetration. However, if the weather is too hot, the temperature of the starch glue at the site will rise, and the NaOH activity will increase, which will easily increase the viscosity. Therefore, with different climates, the amount of NaOH used must be slightly adjusted to achieve a proper gelatinization temperature, which is usually adjusted with the second amount of NaOH.

The large amount of NaOH will cause the starch molecules to swell, gelatinize, make the starch glue thicken, increase the viscosity (more than 80s), is not conducive to the proliferation of oxidants, not easy to glue, or appear gum. Need to reduce the amount of NaOH according to the situation, while taking measures to give dilution. If the amount is too large and exceeds the amount required for the entire gelatinization process, the flowability of the glue liquid becomes large, so that the viscosity of the starch gum is lowered, the pH value is increased, and the corrugated cartons produced are liable to return yellow, resulting in discoloration of the ink on the surface of the corrugated cardboard box.

The amount of NaOH is too small, white or milky paste is added after 20 minutes, opaque and not sticky, which is not conducive to the oxidation of starch, incomplete gelatinization, muddy liquid, poor fluidity, difficult to store; if the amount is too small, the amount of heat released Less, insufficient gelatinization, low viscosity (viscosity lower than 20s), poor adhesion, and the amount of NaOH to be added according to the situation (to find out the net amount of NaOH used). When adding NaOH, a certain amount of NaOH can be opened with 5 to 10 times cold water, slowly added to the container, and then continue stirring.

In addition, the NaOH concentration should not be too high. The concentration is too high, not only when the gelatinization reaction is violent, but also it is easily saccharified, which causes the squeegee to squeezing the glue, and the appearance of the lacquer phenomenon on the corrugation peak, causing the cardboard to be opened and difficult to dry, and also destroying the paper fibers. When the air humidity is high , easy to absorb moisture yellowing, generally should be controlled at 10% to 13% is appropriate.

The amount of NaOH should be based on the actual situation, generally about 12% of starch is more appropriate.

2, the type and quality of starch

Starch is the main adhesive substance. It is insoluble in water. It expands and gelatinizes as the water temperature rises. When it reacts with NaOH solution, it is fully expanded and the viscosity increases.

Commonly used starches mainly include corn starch, wheat starch, tapioca starch, and potato starch. Due to differences in the origin and production climate, the brand or quality of starch may cause different gelatinization temperatures. The gelatinization temperature of different starches is referred to in Table 1. Therefore, in actual operation, combined with the actual situation in different places, the starch with lower price is used to produce starch glue. Mainly by changing the amount of sodium hydroxide, to adjust the level of gelatinization temperature: an appropriate reduction in the amount of sodium hydroxide can increase the gelatinization temperature, and vice versa. However, it is important not to add sodium hydroxide to the prepared starch glue. Generally, the gelatinization temperature of the starch glue used for bonding double corrugated paperboard is lower than that of the single corrugated paper. This is because the heat transfer of the double-sided machine is not as good as the single-side machine, but the too low gelatinization temperature often causes the adhesive layer to become brittle and form a surface layer. Adhesion

At present, there are also methods of using puffed gelatin to prepare various starch glues. The extruded corn starch has a loose structure and is sponge-like and soluble in water. Infiltration and expansion in water can increase the viscosity, increase the initial viscosity, and increase the bonding strength. This allows the extruded corn starch to be used directly as a starch glue.

3, temperature

The higher the water temperature, the stronger the NaOH activity, the faster the viscosity of the starch gum rises, and the lower the gelatinization temperature, the lower the secondary NaOH dosage to achieve a stable gelatinization temperature. The lower the water temperature is, the lower the temperature is.

4. Preparation process After the first NaOH addition, it needs to be stirred for about 25 minutes so that there will be sufficient time for the starch granules to expand evenly to obtain the correct gelatinization temperature. If the gelatinization temperature is too low, some starch granules will swell excessively, some will not swell, and the viscosity will change greatly, and the amount of NaOH will have to be reduced. If the gelatinization temperature is too high, the viscosity will be reduced by shearing during use, and the amount of NaOH needs to be increased. .

Third, the gelatinization temperature of starch gelatin storage performance

The biggest drawback of starch glue is that it is prone to spoilage, its storage period is short, and its use performance is unstable. The reason is related to the gelatinization temperature.

The low gelatinization temperature easily leads to insufficient gelatinization reaction, and has a significant influence on the storage period of starch gelatin. The more complete the gelatinization reaction, the longer the storage time of the gel. If the starch glue deteriorates during the storage period from 3 days to 6 months, it indicates that the starch gelatinization reaction is insufficient because the non-gelatinized starch molecules are more likely to be attacked by bacteria than the gelatinized starch molecules.

Fourth, the gelatinization temperature on the quality of corrugated board

The gelatinization temperature is too high, the starch gum is not ripened, and the glutinous tip is pasted into a white powder, which can cause poor adhesion of the noodle paper, or looseness of the noodle paper, preliminary bonding of the noodle paper and the core paper, and separation; when the single-side machine runs at a high speed, In the center of a single-sided web or across a substantial portion of the web, a continuous, usually spherical, unbonded area appears, causing the paperboard to foam.

The gelatinization temperature is too low, so that the cardboard is not gelatinized before entering the hot plate, and the tip starch is glued into a black line and shiny, resulting in a bad face paper bonding.

The gelatinization temperature is too high or too low, resulting in poor lamination, resulting in poor A, B lamination, loosening of single-sided paperboard with slit edges, and looseness of single-sided paper edge.

Adhesion is one of the most critical processes in the manufacture of corrugated paperboard. It has the greatest influence on the quality of the paperboard. The quality of the glue depends on the quality of the starch glue. Gelatinization temperature is one of the important indexes of starch glue, which directly affects the adhesive properties of corrugated paperboard. Only the gelatinization temperature is stable, in order to ensure the stable bonding quality of corrugated cardboard.

Shaanxi University of Science and Technology Liu Xiaoxia Gong Guifen Information Source: "Guangdong Packaging" 2005/6