Additives in starch adhesives

Starch adhesives are natural polymeric binders. It is widely used because of its low price, non-toxic and tasteless, and no pollution to the environment. At present, starch adhesives are mainly used in the production of paper, labels and corrugated cardboard. Its main ingredients are starch and additives.

Starch is a granular carbohydrate that is the main component of cereals and stems. These particles are usually bonded to the protein matrix and surrounded by cellulose molecules. In order to separate starch from protein and cellulose, starch and raw materials are usually ground and then suspended in water. The fibers are sieved to remove the fibers. The proteins are then removed by centrifugation to obtain a starch slurry, and the starch slurry is concentrated to remove water. Dry to give a solid starch. The water content of solid starch is generally 12-13%.

The uniqueness of the starch granules is manifested when the starch granules swell and eventually burst when soda or heat is added to the starch suspension. This feature is called gelation. Gelation is accompanied by a huge increase in viscosity, and this sudden increase in viscosity can be achieved in an instant. That is, during the corrugating process, the zero point of heating is completed within a few seconds. Adhesion of corrugated cardboard with starch adhesives is based on this principle.

In the process of influencing gelation, the effects of heating and alkali soda are mutually compensating. Therefore, by adjusting the amount of soda to be added, the gelatinization temperature of the starch can be adjusted as desired. For example, the more soda is added, the lower the gelation temperature.

The starch should be stored in a dry, rain-proof place. If the bag is damp, the adhesive mixture in the bag is prone to agglomeration. If the content of the bag is poor after it is opened, it generally indicates that the bag has been wet.

The adhesive used to process corrugated boxes is a mixture of starch and a colloid. The function of the colloid is to increase the water content of the binder. The mixed starch binder after adding the colloid has a higher water content than the pure starch mixture.

Colloidal additives are the following:

▲ uniform colloidal concentrated starch slurry: colloidal binder added to facilitate the carton processing companies, but also prevents the sinking of starch particles without stirring. The slurry is alkaline and can be stored in mild steel steel tanks. It must also be noted that concentrated starch slurries are susceptible to bacterial attack. Despite the strict storage requirements, the possibility of bacterial contamination is readily present, especially under humid and warm conditions. Therefore, it is absolutely necessary to carefully care for the storage tank. At least once a month, the “rubber” storage tank should be cleaned and disinfected once. In wet, high-temperature seasons, the number of cleanings should increase. Starch scaling in the tank should always be removed. Tanks should be agitated at low speeds at all times to prevent the formation of a preservative-free layer on the surface of the slurry and cause bacterial growth.

â–² NaOH: Almost all sodium hydroxide was previously in flakes, which was then resolubilized and then dissolved in water. The use of such flake sodium hydroxide is inconvenient, and when dissolved, a large amount of heat is emitted to form smoke, which can easily burn the skin. Currently, the use of soda in liquids tends to occur. The concentration of liquid soda is generally between 40% and 50%, which varies from company to company. The structure of the large storage tank and the cleaning water system are provided by the supplier. The advantage of liquid alkali is that it is easy to use, economical and safe. The only problem is that crystallization occurs on the conveyor line at low temperatures, so long-distance transport should be avoided. Once the crystallization occurs, the crystals on the conveyor line are generally removed by spray heating.

â–² Boric acid: Boric acid is now used in most plants to replace borax, with particular emphasis on the use of granular boric acid. Powdery boric acid tends to agglomerate, float on the surface of the adhesive for a long time, and does not dissolve easily. The amount of soda must be adjusted at the same time because borax is alkaline.

Boric acid and borax are used as buffering agents for starch adhesion, and the use of boric acid and borax is very precise. There are only a few manufacturers supplying borax and boric acid in the world, and these chemicals are often in short supply. Therefore, the corrugated box factory should have a certain amount of reserves for this chemical.

â–² Aluminum Sulfate: This compound is commonly used to control the gelation temperature of adhesives for single-sided single-layer corrugations. When the compound is dissolved, the gelation temperature can be increased. When a gel appears on the adhesive, the compound can be added at any time for adjustment. Aluminum sulfate compounds are inexpensive and easy to store.

â–² Sodium silicate: This compound is a neutralizing temperature difference that plays a role in wetting and is used in many starch adhesive formulations.

â–² Defoamer: High-speed machines often exhibit adhesive blistering. Adding a defoamer to the adhesive formulation can solve the problem of foaming, and there are various defoamers on the market.

â–² Preservatives: There is a preservative that acts as the most effective anticorrosive agent for short-term storage of starch adhesives. It is called formalin. The chemical reagents can also make the adhesive water-resistant, and the added amount is generally 200 gallons. The mixture plus 1 pint of formalin, but its anti-corrosion is temporary. Because in formal conditions, formalin slowly reacts with the residue of amylaceous protein and does not completely stop the erosion of the bacteria, it should be very careful when stored.

â–² Water repellent: The main chemicals available are as follows:

1 Urine formaldehyde resin - is the oldest water repellent agent for starch adhesives, and it is still widely used in many countries. It can use atomized dry form or 50%-60% aqueous solution (solid content) . The urea-formaldehyde resin has advantages that it is easy to use, has no color, and has little effect on the viscosity of the starch adhesive, and the waterproof starch after adding the urea-aldehyde resin has a relatively long pot life. The disadvantage is that the waterproof effect is poor when the single-faced single-layer corrugation is bonded. In addition, formaldehyde gas is released when heated and pollutes the working environment.

Hydroquinone and resorcinol resins - Resorcinol and its compounds are very expensive, but if used correctly, they can still receive good water resistance. The problem of short lifetime of resorcinol waterproof starch adhesive cans has been solved. The only problem with using resorcinol is its price and the problem of staining starch.

3 Aldehyde and ketone resin - This resin has a great influence on the viscosity of starch and has not been used recently. Japan uses a Tamanori KNI41 resin, which is quite expensive.

4 Cyanide is still used in a few factories in the United States, but it is no longer used in Australia because it is more irritating to the skin. The water repellent effect of this compound is quite good.

5 Tannin—The tannins with good waterproof effect have only one type of product, which is derived from a tannin derivative contained in willows. The starch binder containing boric acid has poor fluidity when used as a waterproofing agent for tannins. However, adding a small amount of urea formaldehyde resin to the willow bark extract can greatly improve its fluidity.

Although tannins are not as good as resorcinol in water repellency, they are cheaper than resorcinol and are easily accepted by users. The long life of the tannins in the cans, the disadvantage of using tannins waterproofing agent is that the adhesive is black color, easy to adhere to the tank wall, causing difficulty in the cleaning of the tank.

6F-4 Waterproof Compound - This compound is modified based on a resorcinol waterproofing agent. The compound is easily added to the prepared starch mixture, and the effect of the compound on viscosity and gel point is mild due to partial reaction and buffering.

(Author/Shen Jia)
-- Shanghai Packaging