Starch gelatinization process, as shown in Figure 5 starch is composed of 100-10000 glucose combined polymer compounds, as shown in the following figure of starch granules, its size is about 2-100u. If observed under the microscope, the gelatinization phenomenon can be found, a starch grain (Figure 6)
During the gelatinization process, it swells due to the absorption of moisture from the surroundings.
From several times expansion to dozens of times its state is equivalent to the point A to point B shown in the figure above. When reaching point B, the maximum temperature is reached. Generally speaking, the temperature at this time is the “temperature of gelatinization” and the gelatinization of different starches. There may be some differences in temperature.
This characteristic is that the viscosity of starch paste has increased dramatically to tens of thousands of times.
As the temperature continues to rise, the starch begins to collapse and gradually disintegrates into small pieces that, when soaked into the inside of the tissue and the paper, begin to bond. As the temperature rises, the moisture contained in the starch granules also evaporates, and some of them are absorbed by the tissue paper and wadding paper, so that the initial bonding speed is continued.
In corrugated board bonding, this phenomenon occurs in a very short time, and the bonding conditions on one side and on both sides are significantly different.
In the case of single-sided corrugated cardboard, since the starch adhesive coated on the crest of the wavy paper is bonded at a temperature of about 160°C and a line pressure of about 40 kPa, the adhesive will press out the tissue and tile. The inside of the paper, but the residual adhesive of the wave peaks tends to remain in the state of the formed corrugation peaks and is gelatinized. This phenomenon is usually called "shoulder shoulder".
(V) The most basic conditions for corrugated board adhesives
It is self-evident that the most basic condition for corrugated paperboard adhesives is adhesiveness. Let us focus on the several necessary conditions for the starch paste to fully exert its adhesiveness.
1, water rate (concentration)
The so-called double water rate refers to the starch concentration in the paste, that is, the weight ratio of starch to water used. For example, 3 times water or 5 times water.
In general, in order to increase the adhesion speed of corrugated board, the water doubled rate should be reduced to reduce the amount of water evaporation in the gelatinization process.
In addition, in order to reduce the warpage phenomenon, it is also necessary to minimize the moisture transferred to the paper and the paper. It can be seen that when the corrugated paper board is manufactured, the doubling rate of the used paste has an impact on the yield and quality of the paper, and it must be cautiously Selected.
2, viscosity (usually called flow measured in seconds)
The viscosity of the paste is one of the most important factors for bonding corrugated cardboard. This is a point to be noted that sometimes it is often erroneously assumed that the higher the viscosity is, the higher the concentration is, whereas the lower the viscosity is, the lower the concentration is.
As mentioned above, the concentration is determined by the water doubled rate, and the viscosity should be determined according to the optimum conditions of various corrugated board machines. It is also important to maintain the specified viscosity after the maximum viscosity is determined. The viscosity of the paste remains stable, so that the paste can be constantly supplied to the formed tile peaks in a stable manner, so that the production of the corrugated cardboard can be performed under a stable adhesion state. Therefore, high-speed corrugated cardboard manufacturing machines generally use a stable low-viscosity paste. .
3. Drawing The drawing property is a manifestation of the viscosity of the paste and is a state of the transfer of the paste from the paste roller to the tile peak. The strong drawing of the paste makes it difficult for the paste to be transferred from the paste roller to the peak of the paper, and even stays on the paste roller completely. The stronger the wire drawing, the more difficult the use of the paste is, and the less it is used. stable.
(6) Inspection of the Bonding Quality of Corrugated Cardboard
Gelation (gelatinization temperature)
Gelation is usually related to the gelatinization temperature of the starch. Because when the starch heated to a certain temperature cools, it becomes gelatinous. In bonding corrugated cardboard, the starch used is fully gelled. If it is not completely gelated, it goes without saying that the bonding temperature is significantly decreased, and there is a place where the adhesive is applied, and the white starch is also visible to the naked eye. If you use a polarized light microscope, you can easily see the un-gelatinized starch crystals. There are many reasons for this phenomenon. For example, the heat of corrugated paperboard makers is not enough, and the compatibility of starch pastes and manufacturing processes are problems that require full attention.
2, corrugated board temperature test
In order to ensure that the corrugated board has a good structural shape, its adhesive quality must be guaranteed. The quality of the corrugated board adhesive is ultimately determined by the determination of the bond strength. This test item is also very important in the process management.
The commonly used method for checking the adhesion quality is to peel off the corrugated board immediately after it is cut, and judge based on the degree of destruction of the paper and the paper.
In fact, the binding process of the starch paste does not completely end when the corrugated board is cut by the cutter. At this time, the temperature of the corrugated board is maintained at about 60°. In this state, the bonding process of the corrugated board is continued until all the corrugated paper reaches the normal temperature state, and the bonding is completely terminated. (to be continued)
