What are the characteristics of modified anionic waterborne polyurethane?

Traditional solvent-based polyurethanes contain a large amount of volatile organic solvents, which are flammable, odorous, pollute the environment, and harmful to humans. Water-based polyurethane uses water as a dispersion medium, and a hydrophilic group is introduced into the polyurethane molecular chain to form a stable emulsion in water. The cost is low, does not burn, does not pollute the environment, and the solvent usage is small and the production safety is high. At present, although there are more and more domestic waterborne polyurethane products, high-end products still need to be imported. Independent research and development of high-performance waterborne polyurethane has become one of the hot trends in domestic waterborne polyurethane synthesis. There are many types of waterborne polyurethane synthetic raw materials. Polyols and polyesters are commonly used in the synthesis of polyurethane soft segments. Polyester waterborne polyurethane films have high strength and good heat resistance, but poor water resistance; The polyurethane film has good elasticity and elongation, but the adhesiveness is not as good as the polyester type.

In this paper, polypropylene carbonate diol is used as the soft segment and aliphatic isocyanate IPDI for prepolymerization. Polyurethane synthesized from polypropylene carbonate diol has good strength and hardness of polyester polyurethane. Heat resistance, and better elongation and flexibility of polyether polyurethane. The self-made aminosilicone was used to modify the waterborne polyurethane to improve the mechanical properties of the waterborne polyurethane film and improve its water resistance. A new waterborne polyurethane emulsion with the advantages of waterborne polyurethane and aminosilicone was synthesized.

1. Synthesis of anionic waterborne polyurethane prepolymer

(1) Determination of prepolymerization temperature and time Prepolymerization is the most important and basic step in the synthesis of waterborne polyurethane. The prepolymerization temperature directly affects the change of NCO content in the system, which is the intrinsic power of the reaction. The prepolymerization reaction rate increases with the increase of temperature, and the degree of reaction is more complete with time, but the occurrence of side reactions also increases, so the control of prepolymerization reaction is very important. The prepolymerization reaction rate can be determined by measuring the NCO group content in the system with the di-n-butylamine method, which provides a theoretical basis for the design of a reasonable process.

In the experiment, the NCO/OH molar ratio is 1.7, the DMPA content is 5% (w/w), a series of prepolymers are synthesized at different temperatures, and the free-NCO group in the prepolymer is titrated by di-n-butylamine method. Content, experimental results are shown in Figure 1.

Effect of temperature and time response

At the beginning of the prepolymerization reaction, the concentration of NCO group and -OH group are very large, and most of the monomers are quickly polymerized into oligomers such as diprepolymer, triprepolymer and tetraprepolymer. It can be seen from Figure 1 that the free -NCO group content in the initial stage of the reaction decreases rapidly, and the conversion rate is very high in a short time. As time goes on, the reaction between the oligomers continues, and the molecular weight gradually increases. When the reaction reaches a certain time, the content of free -NCO groups does not change much.

During the same reaction time, the content of free-NCO groups decreased with the increase of reaction temperature. When the prepolymerization reaction 70 is carried out, the reaction temperature is lower, the viscosity of the system increases less in a long time, the prepolymerization reaction time required is long, and the efficiency is low; when the prepolymerization temperature is 80, the content of the NCO group is When the reaction is 120 minutes, it is reduced to 7.35%, which is almost the same as the theoretical value of residual NCO content of 7.34% at the end of the prepolymerization reaction. When the reaction is 90, the content of NCO group decreases rapidly, and at the same time, at high temperature. Under the condition, the viscosity of the system increases rapidly and the reaction is too strong. It can be predicted that the molecular weight distribution range of the prepolymerization reaction is too large. According to the experimental results, the prepolymerization temperature was chosen to be 80 and the prepolymerization time was about 2h.

(2) Determination of temperature and time of chain extension reaction: dimethylolpropionic acid (DMPA) is used as hydrophilic chain extender, DMPA is an anionic hydrophilic chain extender, and DMPA can be added in two ways: (1) Solution method: DMPA is dissolved in NMP (N-methyl-2-pyrrolidone) solvent and then added to the reaction system for reaction; (2) Direct method: DMPA is directly added to the reaction system in powder form to participate in the reaction. The reaction of DMPA added by direct method belongs to the reaction between two phases. The reaction is slow and the cycle is long. The reaction of DMPA added by solution method belongs to homogeneous reaction. The homogeneous reaction is beneficial to mass transfer and heat transfer, which accelerates the reaction rate and shortens the synthesis reaction. time. Therefore, the method of solution addition is used for chain extension. In the experiment, the molar ratio of NCO/OH was 1.7, the content of DMPA was 5% (w/w), and the prepolymerization reaction was 80 × 2 h. The DMPA was extended by the method of solution addition. The experimental results are shown in Fig. 2.

It can be seen from Fig. 2 that as the temperature of the chain extension increases, the time taken for the content of the NCO group to reach the theoretical value of the experimental formula design is gradually shortened. When the chain extension reaction is carried out at 60, the free-NCO group content in the system is 4.2% after 210 minutes of reaction, and the reaction rate is slow, which is not conducive to the progress of the polyurethane chain extension reaction and the molecular weight increase; when the chain extension reaction is 70, the reaction After 180 min, the NCO group content decreased to 4.11%, which is close to the theoretical value of residual NCO group content of 4.2% after the end of the chain extension reaction; when the chain extension reaction temperature is 80, the viscosity of the system increases sharply. The chain extension reaction is intensified, and the probability of side reactions of hydrogen atoms on NCO and -NHCOO increases, which is difficult to control and ultimately disperse. In summary, the chain extension conditions of this experiment are set to 70 × 3h. Under this condition, the content of NCO group is moderately changed, the reaction is relatively gentle, the viscosity of the system is moderate, and the final dispersion of the emulsion is better.

2. Synthesis of amino silicone modified waterborne polyurethane

The D4 and silane coupling agent are used to prepare the active group aminosilicone under the action of the base catalyst, and the self-made aminosilicone modified waterborne polyurethane is used. The NCO group and the -NH2 group are relative to the -NCO group and the -OH group. The reactivity is much higher. The initial NCO content in the system is high. The reaction between NCO and NH2 groups makes the viscosity of the system increase rapidly and it is prone to gelation.

(1) Effect of aminosilicone modification method on waterborne polyurethane The aminosilicone modified feeding method includes three methods: prepolymerization process addition method, chain extension process addition method and emulsification process addition method. The prepolymerization process is to dehydrate the polypropylene carbonate diol and the aminosilicone together, and participate in the prepolymerization reaction, and then use DMPA to extend the chain, and finally neutralize the emulsification; the chain extension process is to add the polypropylene carbonate. The diol and IPDI are prepolymerized first, then the aminosilicone reaction is added during the DMPA chain extension, and then the emulsification is carried out. The emulsification process refers to the addition of the aminosilicone during the emulsification process to obtain the modified waterborne polyurethane. The first two methods are much higher than the -NH2 group and the -NCO group. The NH2 group and the -NCO group are preferentially reacted rapidly. The molecular structure of the product is not ideal, and the process is difficult to control.

In this experiment, the emulsification process was added to add the aminosilicone to the modified waterborne polyurethane during the emulsification process. This method firstly reacted with the polypropylene carbonate diol and then with the anionic hydrophilic chain extender DMPA. Chain reaction, and finally react with aminosilicone, the method is easy to connect -COOH group and aminosilicone to the polymer macromolecular chain, the process is easier to control, and the product stability is good. In addition, the modification method can make the molecular chain of various polymers disperse in water to a reasonable extent, and can distribute the aminosilicone molecules more uniformly in the prepolymerized molecular chain.

(2) Effect of modification temperature on waterborne polyurethane After the end of DMPA chain extension, a prepolymer containing terminal-NCO group was synthesized, and then the temperature was lowered to modify the aminosilicone. The NCO group and the NH2 group were highly reactive. Therefore, compared with the chain extension temperature, the modification temperature is generally controlled to be low. The experiment selects three temperature ranges of 10, 25, 45 and so on to modify the physical properties of the modified waterborne polyurethane emulsion, and finally determines the modification temperature.

According to 1.2, the molar ratio of NCO/OH is 1.7, DMPA is 5% (w/w), pre-polymerization and chain extension temperature and time control are obtained according to the experiment in Section 2.1, and the amount of aminosilicone is used. At 10% (w/w), the modification temperature is 10, 25, 45, and the temperature is 50 min. Finally, the neutralization and water addition are emulsified to obtain a series of products. The emulsion properties of the synthetic products are investigated. The experimental results are shown in Table 1.

Effect of modification temperature on properties of waterborne polyurethane emulsion

It can be seen from Table 1 that the transparency of the modified aqueous polyurethane product is reduced, the water absorption rate of the film is obviously improved, the particle size is increased, and the physical properties such as the appearance of the product modified by different temperatures and the water absorption rate of the film are different. This is mainly because the modification of amino silicone changes the oily group in the macromolecular chain of the aqueous polyurethane, which reduces the hydrophilicity of the product, so that the water absorption of the modified polymer film is reduced, and the molecular chain of the product is modified. Prolonged, increased particle size, ultimately affecting the transparency and stability of the emulsion.

The aminosilicone synthesized in this experiment contains a primary amino group, and the reaction with the -NCO group has a high reactivity with the -NH2 group than the -OH group, so the modification temperature is selected to be low. When the modification temperature is set to 10, the reaction speed of aminosilicone and polyurethane is relatively stable, no gelation occurs, and the molecular chain is continuously and uniformly grafted onto the polyurethane macromolecular chain. The modified product has good stability and small particle size. At 280nm, the appearance of milky white is slightly transparent and bluish blue, and the water absorption rate of the film is 32.3%. When the modification temperature is raised to 25, the appearance of the emulsion becomes milky white, and the particle size increases to 330 nm. Centrifugal stability of the emulsion; when the modification temperature is 45, the amino group and the -NCO group react rapidly in a short time, and the aminosilicone cannot be uniformly distributed on each molecular chain, and a gel floc appears, resulting in an emulsion. The stability is lowered, the particle size is as large as 1200 nm, the water absorption of the film is also increased, and the water resistance is lowered. Therefore, the modification temperature is determined to be 10 .

3. Effect of modification time on waterborne polyurethane

The control of modification time is also an important condition in the synthesis of amino-silicone modified waterborne polyurethane. The modification time is too long to affect the synthesis efficiency. If the reaction time is too short, the reaction is insufficient and the product performance is affected. Therefore, the experiment has studied different modification times. The physical properties of the synthetic product to determine the modification time.

Refer to 1.2 for the experimental procedure, taking NCO/OH molar ratio of 1.7, DMPA of 5% (w/w), prepolymerization and chain extension temperature and time control. Refer to the experimental results in Section 2.1, Aminosilicone dosage. At 10% (w/w), the modification temperature was 10, and the reaction time was 25 min, 50 min, and 65 min, respectively. The emulsion properties of the synthesized product were examined. The experimental results are shown in Table 2.

It can be seen from the results in Table 2 that the modified waterborne polyurethanes synthesized by different modification times have different properties. The product stability is poor when the modification time is 25 min. The possible reason is that the reaction time is too short and the reaction is insufficient. When the emulsification time is 50 min, when the modification time is 50 min, the appearance of the modified water-based polyurethane is slightly transparent, bluish blue, and the centrifugal stability is good, and the water absorption rate of the film is reduced from 48.6% of the unmodified. About 15.7%, the smaller particle size is 280nm. When the modification time is 65min, the reaction is sufficient, the product stability is good, and the particle size is small. However, the reaction time is too long and the side reaction is easy to occur, which affects the product performance and finishing effect, so the modification time is determined to be 50 min.

4. Effect of aminosilicone content on emulsion properties

Amino silicone has the common characteristics of silicone, low surface energy, low temperature resistance, aging resistance, organic solvent resistance and hydrophobicity. At the same time, amino silicone can also give the fabric outstanding softness and smooth silk feel. The amino-organic silicone self-made in this experiment avoids the problem of yellowing of the general silicone softener, and the use of the amino-functional silicone-modified water-based polyurethane can not only improve the mechanical properties of the aqueous polyurethane film, but also improve the water resistance of the film. Moreover, it can also improve the softness and wet friction resistance of the film.

Refer to 1.2 for the experimental procedure, taking NCO/OH molar ratio of 1.7, DMPA of 5% (w/w), pre-polymerization and chain extension temperature, time control as referred to in Section 2.1, and modification temperature of 10 At 50 min, a series of products were synthesized by selecting different amino-organosilicon modifiers. The test method is referred to Section 1.3, and the experimental results are shown in Table 3.

Effect of Amino Silicon on the Properties of Modified Waterborne Polyurethane

It can be seen from Table 3 that the modification of aminosilicone has a great influence on the appearance state and stability of the aqueous polyurethane emulsion. As the content of aminosilicone increases, the emulsion changes from translucent and blu-ray to milky white. When the silicone content reaches 35% (w/w), the emulsion stability of the emulsion is significantly reduced. At the same time, as the aminosilicone content increases, the particle size and viscosity of the emulsion increase, and the water absorption of the film decreases.

The main reasons are as follows: First, aminosilicone is a hydrophobic substance. With the increase of the amount of aminosilicone introduced, the hydrophilic group content in the polyurethane molecular chain is relatively reduced, and the hydrophobicity is enhanced, which makes the emulsion stability. Decreased, while the film-forming water absorption rate of the polymer decreases; secondly, the content of amino-organosilicon in the aqueous polyurethane increases, the molecular weight of the polymer increases, and the segment increases, then the hydrophobicity of the polymer increases, and the particle size and viscosity become larger due to The molecular weight of the polymer increases and the molecular structure is more complicated. The water infiltration process of the film is slowed down, and it is difficult to associate with water, so the water absorption rate of the film decreases. Third, the introduction of the aminosilicone segment makes the molecular chain The growth is intertwined, which has a certain influence on the stability of the emulsion. At the same time, the intertwining of the molecular segments with complex structure also increases the viscosity of the modified anionic waterborne polyurethane.

The increase of aminosilicone content can improve the mechanical properties of waterborne polyurethane film, improve the softness and wet friction resistance of the film, reduce the water absorption of the film and improve the water resistance of the film, but at the same time affect the stability of the emulsion and increase the emulsion Diameter and viscosity. According to the data in Table 3, considering the mechanical properties of the waterborne polyurethane film to the greatest extent on the basis of ensuring the good stability of the synthesized product, the aminosilicone content is selected to be 30% (w/w).

5. Characterization of the synthesized product by infrared spectroscopy (FTIR)

In this experiment, the infrared spectroscopy analysis of the products synthesized by the best process was carried out by means of modern instrumental analysis. The specific results are as follows:—The NCO group has asymmetric stretching vibration at 2273 cm-1. It can be seen from Fig. 3 that the characteristic peak disappears at 2273 cm-1, indicating that the NCO group in the modified aqueous polyurethane is completely involved in the reaction. 1692cm-1 is the stretching vibration peak of CO in CO-NH, and 1515cm-1 is the deformation vibration peak of NH in CO-NH, indicating that the synthetic substance has carbamate formation. The coupled -NH-CO-NH- structural unit in Figure 3 has an absorption peak at about 1600 cm-1, 3309 cm-1 is the NH-bond stretching vibration peak, and the CH stretching vibration absorption peak is enhanced at about 2900 cm-1. It shows that the primary amino group reacts with the -NCO group to form a urea group. It has a broad strong absorption peak at 1000～1130cm-1, indicating that the composite macromolecular chain contains Si—O, Si.