Research on polyurethane adhesive for ink industry

Abstract : Polyurethane resin and diphenylmethane diisocyanate (MDI) were used as the main raw materials to prepare polyurethane resin for ink. The appearance of the polyurethane resin was colorless and transparent. The softening point was 110-130°C and the Shore A hardness was 75. ~ 85, tensile strength t ≥ 30 MPa, elongation ≥ 500%, using methyl ethyl ketone (MEK) and cyclohexanone as the solvent, dubbed the polyurethane with a solid content of (15 ± 1)% to meet the ink For the needs of industries such as composite film and other industries, the effects of polyol molecular weight, acid value, isocyanate index, and chain extender dosage on the properties of PU adhesives were investigated.
Keywords: Polyurethane; Adhesive; Adhesion strength; Polyester polyol
Polyurethane adhesive has excellent low temperature performance, high adhesive strength, excellent flexibility and water resistance, oil resistance and other properties, and is widely used in the footwear industry, ink industry, tape industry, composite film and so on. So far, the polyurethane adhesives used in the domestic ink industry are generally imported from abroad. After being processed and used, we have conducted in-depth research on the structural characteristics and functionalities of polyurethanes, and developed a gloss that is better. Polyurethane adhesive with high cohesive strength and good heat resistance.
1 Technical route
The polyurethane adhesives used in the ink industry require good gloss, high bonding strength to the material, and require better blocking resistance at a certain temperature. Based on these performance requirements, tests are conducted on the structural characteristics, functionality, synthesis recipes, and processes of polyurethanes:
(1) Select a suitable polybasic acid and polyol to synthesize a polyester polyol with temperature resistance;
(2) Introducing polar groups with strong adhesion to materials in adhesives;
(3) Select suitable raw materials to improve the crystallinity of polyurethane adhesives;
(4) Adjust the proportion of n(NCO)/n(OH) to obtain suitable viscosity and bonding strength;
(5) Use coupling agents and other additives to improve the heat and humidity resistance of the adhesive.
2 experimental part
2.1 The main raw materials
Polyester diols, homemade; 1,4-butanediol, 1,6 hexanediol, neopentyl glycol, methyl propylene glycol, industrial grade, imported; dimethyl terephthalate, trimethylol propane , industrial grade, import; MDI (brand: MDI-l00), Yantai Wanhua Polyurethane Co., Ltd.; antioxidants, UV absorbers, industrial grade, imported.
2.2 Synthetic process of polyester polyol
The dibasic acid (or polybasic acid) and dihydric alcohol (or polyhydric alcohol) are added to a stainless steel reactor for heating and heating, and are protected by N2 gas. When the temperature reaches 140°C, the water starts to circulate, and the speed of the water and the reflux tower are controlled. The temperature of the top (the temperature of the top of the tower under atmospheric pressure should be between 100 and 102 °C), then the temperature inside the kettle should be slowly raised to 160~180 °C and kept warm for 2~3h (adjust the molecular weight distribution of polyester polyol), and then the temperature Slowly rise to 220 ~ 230 °C, heat 2h (theory basically out of the end of the water), sampling measured acid value, hydroxyl value and moisture and other indicators. Then vacuum is applied to gradually increase the degree of vacuum (a maximum vacuum of about 4 h is required). In the meantime, the acid value, hydroxyl value, and moisture were sampled at regular intervals until they were qualified (the appearance of the product was a white waxy solid or viscous liquid).
2.3 Preparation of Polyurethane Adhesives
In a four-necked flask equipped with an electric stirring device, a heating device, a thermometer, and a vacuum interface, a polyester polyol is added and heated to a temperature of 100 to 120° C., and the residual pressure is less than 0.67 kPa, and dehydration is performed for 1 to 2 hours. When the moisture content is less than 0.1%, the temperature is lowered. When the temperature is between 60 and 90 DEG C, the distilled chain-dried chain extender is added, and the mixture is uniformly mixed with the polyester. Diisocyanate was then added, rapidly stirred and vacuum degassed for 5 to 10 minutes. When the bubbles are basically pumped off, the stirring is stopped, the vacuum is released, and the plate is poured into a preheated PTFE disk and aged in an oven at 140-150° C. for 2 to 3 hours, and allowed to stand at room temperature for 1 week. Then use a solvent such as butanone to prepare a sol with a solids content of 15%.
2.4 Performance Test
Viscosity GB/T 2794-1995;
Solid content GB/T 2793-1995.
2.5 Technical Specifications
(1) Specifications of Polyurethane Rubber Particles
Softening point/°C 110～130
Appearance Transparent or light yellow transparent particles
Tensile strength/MPa ≥30
Hardness (Shore A) 75~85
Elongation/% ≥500
(2) Specifications of polyurethane glue
Appearance Colorless transparent solution
Viscosity/mPa·s 50～120
Solid content/% 14～16
Storage period/month 12
3 Results and Discussion
3.1 Effect of Molecular Weight on Polyester Polyols
The polyurethane adhesives used in the ink industry are required to have good gloss and high bonding strength. Therefore, the preferred molecular weight of polyesters is between 1 500 and 2 500, which not only provides strong polar groups. , increase the bonding strength of the material, and can reduce the crystallinity. This is because when the relative molecular weight of the polyester soft segment is low, the soft segment and the hard segment infiltrate each other to increase the compatibility of the two phases, thereby increasing the strength and the modulus. However, the relative molecular mass is too low, the cohesive strength is small, and the adhesive strength of the adhesive to the material is poor; if the molecular weight is too high, the reaction is difficult to control and the gel is easily generated. Table 1 shows the effect of different molecular weight polyester diols on the adhesive properties.

From the data in Table 1, it can be seen that as the molecular mass increases, the viscosity and hardness of the solution gradually increase, and the gloss of the resulting film gradually becomes darker and eventually becomes white. This is because as the molecular mass increases, the polar groups of the polyester increase and the crystallinity increases.
3.2 Selection of raw materials
The adhesive strength of polyurethane adhesives depends on the interfacial adhesion strength and the cohesive strength of the adhesive layer. In the crystalline polyurethane adhesive, the volume shrinkage caused by the recrystallization of the soft segment is greater, resulting in a greater loss of adhesive strength. The amorphous segment-type rigid polyester polyol as a soft segment of the adhesive can increase its adhesion to the material. Strength, reduce the crystallinity, and the resulting film is also good gloss. Therefore, it is suitable to select a long-chain aromatic polybasic acid and an asymmetric structure polyol having pendant groups in the main chain to synthesize a polyester polyol, such as dimethyl terephthalate, neopentyl glycol and the like.
3.3 Effect of Isocyanate Index (R)
In the preparation of polyurethane adhesives, the value of R, ie, the value of n(NCO)/n(OH) has a great influence on the molecular weight, storage stability, and adhesive strength of the polyurethane adhesive. The experimental results show that when the R value is less than 0.95, the resulting colloidal particles or glue solutions have poor storage stability. This is because the polyurethane adhesive has too much hydroxyl content and activity; when the R value is 0.95 When the value of R is greater than 1.0, the glue viscosity is too large and it is not suitable for the ink industry. Table 2 shows the effect of different R values ​​on the performance of the adhesive.

From Table 2, it can be concluded that the R value has a great influence on the viscosity of the adhesive, and the viscosity increases as the R value increases. This is because the hard segment content of the polyurethane increases, the cohesive strength increases, and the material can be better Chemical bonds and physisorption occur. When R>0.995, the viscosity of the adhesive is too large to be used as a high solid content adhesive. When R>1.00, the gloss of the adhesive film is also poor.
3.4 Influence of Chain Extender
Chain extenders are commonly used functional additives for preparing polyurethane adhesives. It is characterized by a small molecular mass, short chain links, and generally contains two active hydrogen atoms. The reactive hydrogen atoms are highly affected by the side chain, and chain extenders and derivatives with different structures can be selected. To control the activity of the chain extension reaction, or to use 2 or more mixed chain extenders to achieve synergistic effect, which can effectively adjust its performance indicators. With the addition of the chain extender, the cohesive strength, softening point and adhesive strength of the adhesive can be improved. This is because the addition of the chain extender increases the content of urethane groups in the polyurethane segment, so that a physical adsorption and chemical bonding are formed between the adhesive and the material, and the temperature resistance of the adhesive is also improved. In the preparation of a good gloss adhesive, it is possible to select two or more polyols having a low molecular weight and an asymmetric structure in the main chain, which not only disrupts the regularity of the polyurethane structure but also increases the Cohesive strength, to achieve synergies.
3.5 Effect of polyester polyol acid value
The acid value of the polyester polyol not only affects the relative molecular weight of the polyol, but also has a great influence on the adhesive strength and viscosity of the adhesive. If the acid value is too large, the resulting gum has a poor glossiness and a small bonding strength; the acid value is too small, the reaction speed is too fast, and it is difficult to control. Table 3 shows the effect of different acid values ​​on the properties of the adhesive.

As can be seen from Table 3, when the acid value is 0.30 to 0.50, the viscosity and gloss of the glue are both ideal, and the reaction is easier to control. When the acid value is 0.10, the reaction time is short. Easy to gel, more difficult to operate.
3.6 Other Influencing Factors
Because the urethane group in the adhesive is easily decomposed at a high temperature and the heat resistance is poor, an oxazolidone (OX) group may be introduced into the synthetic polyurethane molecule. Can significantly improve the heat resistance of the adhesive. According to reports in the literature, in the polyurethane adhesive, adding a coupling agent can not only improve the bonding strength of the material, but also improve the moisture resistance and heat resistance of the adhesive.
4 Conclusion
The polyester polyol (relative molecular weight of about 1 500) was used as the main raw material, 1,4-butanediol and methyl propanediol as chain extenders, the isocyanate index was controlled at about 0.980, and the synthetic polyurethane adhesive was Stable performance, and has a good gloss, high transparency, low viscosity, high bonding strength and other characteristics, basically meet the needs of the composite film, ink printing industry. (Zhan Zhongxian Zhu Changchun)