Effect Mechanism of Urea-Formaldehyde Resin on Properties of Oxidized Starch Agent

Corrugated cartons are the most widely used containers. Factors affecting the quality of corrugated cartons In addition to corrugated cardboard itself, the quality of the adhesive used is also an important factor. In recent years, oxidized starch adhesives have been used in the production of corrugated cartons. Oxidized starch adhesives have high strength, light weight, no corrosion, no pollution, but at the same time because its own water content as high as 60% -70%, so there is a short storage period, poor water resistance, cardboard drying speed, early sticky Disadvantages such as bad relay, and then affect the overall quality of the carton. By adding a urea-formaldehyde resin to the oxidized starch binder, modification of the oxidized starch adhesive can greatly improve the adhesiveness, fluidity, and water resistance.

(I) Effect mechanism of Urea-formaldehyde resin on properties of oxidized starch adhesive

The modified oxidized starch contains hydrophilic aldehyde groups, carboxyl groups and hydroxyl groups, and the urea-formaldehyde resin contains a large amount of dimethylol urea, which has an active hydroxymethyl group. When the urea-formaldehyde resin is added to the oxidized starch adhesive, the dimethylol urea and oligomers in the resin react with hydroxyl groups in the starch molecule to generate hemiacetal and acetal structure, which is a kind of starch chain involved Crosslinked structure. It is because urea-formaldehyde resin and modified starch react with each other to form a hydrophobic network structure. Once brushed on corrugated paperboard, a layer of film is formed, which effectively prevents moisture from penetrating into the interior of the paperboard and improves the early stage of starch adhesive. Tack force, water resistance, shorten the drying time.

(b) Effect of Urea-Formaldehyde Resin on Properties of Oxidized Starch Adhesives

l, experiment
Add the specified amount of water and starch to the reactor, mix well, warm up to 50°C～60°C, add a certain amount of oxidant—hydrogen peroxide at constant temperature, react for a certain period of time; add sodium thiosulfate solution to stop the oxidation reaction : After mixing, add the prepared sodium hydroxide solution for gelatinization, add borax and polyvinyl alcohol solution, continue stirring, and lower the temperature to below 40°C to make the oxidized starch adhesive. The self-made urea-formaldehyde resin was mixed with the oxidized starch adhesive at different percentages at 40° C. to make a composite starch adhesive. Through experiments, the drying speed, water resistance, initial adhesion of the composite starch adhesive were tested, and the effect of the addition amount of urea formaldehyde resin on the performance of the oxidized starch adhesive was studied.

2. Results and Discussion

(1) Influence of the amount of urea resin added on the drying speed The drying speed was measured by a weighing method. The rate of drying of the adhesive was evaluated at the rate of water loss per gram of adhesive. The effect of different urea formaldehyde addition on drying speed is shown in Table 1 and Figure 1.

As can be seen from Figure 1, the addition of urea formaldehyde resin directly affects the drying speed of the adhesive. When the amount of urea formaldehyde resin is 10%, the drying rate is fastest, especially in the first 10 minutes. With additions of 5% and 2%, although the drying rate was faster than a single oxidized starch binder, the change between the two was not significant. When the addition amount of urea formaldehyde resin is 15%, the drying speed will decrease instead. This is due to the excessive amount of the added urea-formaldehyde resin, and the adhesive film is too thick, which affects the drying speed. Therefore, when the urea-formaldehyde resin is used to increase the drying speed of the adhesive, its addition amount is preferably between 5% and 10%.

(2) Effect of the amount of urea formaldehyde resin added on the water resistance The paperboard bonded with the composite starch adhesive was completely immersed in water and timed until the adhesive layer was naturally separated, and the water resistance of the adhesive was evaluated by the water soaking time.

As can be seen from FIG. 2 , the water resistance of the adhesive increases with the increase of the proportion of the urea formaldehyde resin. When the addition amount exceeds 10%, the effect of improving the water resistance of the film is not obvious, and the production cost of the adhesive is increased. In addition, when the addition amount of urea formaldehyde resin exceeds 15%, the thickness of the film is increased, and the drying time of the adhesive is reduced. Considering the drying speed and water resistance, the amount of urea-formaldehyde resin can be selected between 5% and 10%, which can meet the ideal moisture-proof requirements.

(3) Effect of the amount of urea formaldehyde resin on the initial adhesive force of the adhesive

Take a piece of corrugated cardboard with an area of ​​5×10mm2, evenly apply the adhesive, bond it with another piece of paper of the same size and variety, apply a certain pressure, peel off the adhesive layer after 10 minutes, check the paper surface area, The initial adhesive force was evaluated as a percentage of the total bonded area.

Table 3 compares the initial adhesion of different types of adhesives. The results showed that the oxidized starch adhesive with urea-formaldehyde resin added was superior to the other two adhesives in terms of initial adhesion and one-minute bonding area. It can make corrugated cardboard sticky in one minute. The joint area reaches 42.7%, which greatly shortens the initial bonding time of the board and can enter the next production process in a short time, thereby improving the production efficiency.

In summary, urea-formaldehyde resin as a modifier of starch binder can improve the drying speed, water resistance, initial adhesion and other properties of the adhesive, and with the different amount of addition, the impact on performance is also different. . Through experimental research, taking into account the adhesive drying speed, water resistance, initial adhesive strength and production costs and other factors, we conclude that the optimal ratio of urea-formaldehyde resin to oxidized starch is: 5% of the total mass of urea-formaldehyde resin - 10%. (Text: Wan Lili Han Yongsheng)