Epoxy copolymerization modification application of waterborne polyurethane

As an environmentally friendly material, waterborne polyurethane (WPU) has become one of the most important materials. It can react with polyisocyanates of different structures and polyols to change the structure of long chains, and adjust the molecular weight distribution by using different reaction conditions. , size and crosslink density, etc., can be varied in a wide range to meet different needs. However, since a hydrophilic group is introduced into the molecule, the water resistance, solvent resistance, heat resistance and gloss of the coating are poor, and it needs to be modified. At present, the most common method in the world is to post-treat WPU by various grafting copolymerization of acrylate monomers. Although this method can improve the water resistance and mechanical properties of waterborne polyurethane to some extent, The crosslink density of the emulsion is low, and the degree of crosslinking affects the stability of the emulsion. The water resistance and other mechanical properties are still inferior to the conventional solvent-based polyurethane. Epoxy resin (EP) has excellent bonding ability, high modulus, high strength, excellent adhesion, low shrinkage and chemical resistance, and can directly participate in the synthesis reaction of waterborne polyurethane, improve waterborne polyurethane coating Mechanical properties, water resistance, etc. of the film. The author explored the synthesis process of waterborne polyurethane, synthesized a dispersion of waterborne polyurethane, and changed the hydrophilic agent, introduced the internal crosslinking agent trimethylolpropane, epoxy resin E-44, etc. to achieve emulsion copolymerization to achieve waterborne polyurethane. The purpose of the cross-linking modification of the structure. The effect of EP dosage and addition method on WPU was studied. On the basis of this, the epoxy copolymer modified polyurethane emulsion was prepared by changing the hydrophilic agent, which improved the water resistance and mechanical properties of the coating.

Experimental part

1.1 Analysis test

Water resistance test: Cut the film into small pieces of 3 cm*3 cm (thickness about 4 mm), weigh it (m1), put it in tap water for 24 hours, take it out, use the filter paper to dry the surface moisture, weigh ( m 2) , Calculate the water absorption rate = [ ( m2- m1) / m1] * 100% .

Hardness test: The emulsion was uniformly coated on a dry glass plate and placed in a cool place for 48 h, and then measured by a QBY type pendulum hardness tester.

Test for tensile strength and elongation at break: The paint film was made into a dumbbell-shaped test strip, and the tensile strength and elongation were measured using a XL-500 universal testing machine. The separation speed of the jig was 50 mm/min.

Infrared spectroscopy: US P-E783 infrared spectrometer (KBr) tableting, measuring range 400~ 4 000 cm-1.

2 Results and discussion

2.1 The effect of epoxy resin on polyurethane

(1) The influence of the way of adding epoxy resin

Epoxy resin E- 44 is added in different ways, the structure of the emulsion particles is different, and the properties of the obtained coating film are also different. During the modification process, three feeding processes were used: EP and JW2044 were added to the reactor at the beginning of the reaction (Mode 1); EP and DMPA were added to the reactor together in the middle of the reaction (Mode 2); EP and TMP were in the late stage of the reaction. Add together to the reactor (Mode 3) and the results are shown in Table 1.

Using the same NCO/OH ratio and an equivalent amount of EP, the EP participates in the reaction for a longer period of time, and the obtained emulsion has a large viscosity and poor storage stability. In mode 3, EP participates in the reaction for a shorter period of time, the emulsion has less viscosity, better appearance and stable storage. However, there is competition between EP and TMP, which makes the molecular segment structural unit not regular enough, and the comprehensive performance is not as good as the method 2 . The reason is that in the EP modified PU reaction, the epoxy group and the hydroxyl group on the EP molecule participate in the reaction, and as the reaction time increases, the probability of the crosslinking reaction increases, and the crosslinking density of the resin and the stretching of the coating film are increased. Strength, which reduces the hydrophilicity of the resin and the storage stability of the emulsion. In the mode 2, since the reaction time is relatively short, the crosslinking density of the emulsion is appropriately increased, and the storage stability of the emulsion is reduced, and the comprehensive performance is better.

(2) The influence of the amount of epoxy resin added

The epoxy resin E-44 was used to modify the waterborne polyurethane. Under the same formulation conditions, the effect of the amount of epoxy resin on the properties of the emulsion and the coating film was investigated. The results are shown in Table 2.

It can be seen from Table 2 that as the amount of EP added increases, the mechanical properties of the product increase, such as the tensile strength increases from 7181 MPa to 10 MPa or more. However, the elongation at break has decreased, which is mainly caused by the increase in cross-linking of the system as the EP content increases. The hardness and water resistance of the coating film are also greatly improved by the improvement of the degree of crosslinking. However, as the EP content increases, the emulsification of the prepolymer becomes more and more difficult, and the storage stability of the aqueous dispersion becomes worse. Therefore, the amount of EP added must be controlled within a certain range (typically 4.5% ~ 6%) in order to obtain a product with excellent comprehensive performance.

2.2 Comparison of performance between DMBA and DMPA in epoxy copolymer modified polyurethane emulsion

At present, the hydrophilic agent which is widely researched and applied in China is DMPA, but there are few DMBA applications with different structures. Compared to DMPA, DMBA can easily methylate or esterify carbamate by the action of two primary hydroxyl groups without protecting the carboxyl group. In terms of solubility, DMBA has its own unique advantages, and its excellent solubility can greatly improve work efficiency. And DMBA has a low melting point, generally about 108~114 °C, and can be dissolved in acetone to facilitate homogeneous reaction. To this end, under the same conditions, the same number of moles of DMBA and DMPA were added to synthesize the modified polyurethane, and the properties of the emulsion and the coating film were examined. The results are shown in Table 3.

It can be seen from Table 1 that whether the hydrophilic group is DMPA or DMBA, when the carboxyl group content increases, the particle size of the polyurethane emulsion gradually becomes smaller due to the increase in the amount of the hydrophilic monomer, and the emulsion changes from milky white to the appearance of the emulsion. Blue, the viscosity of the emulsion increases. This is mainly due to the smaller particle size and the electrical condensing effect of the electric double layer. In addition, as the amount of the hydrophilic group increases, the water resistance of the PU film decreases, and the water absorption rate of the film increases. This is because the hydrophilic group in the PU molecule increases, and water is easily adsorbed by the hydrophilic group on the molecular chain.

In addition, when the carboxyl group content is the same, the emulsion obtained by DMBA is obviously superior to the emulsion obtained by DMPA, and the water absorption rate is low and the hardness is high. The mechanical properties of polyurethane are determined by the ratio between the hard segment composed of the urethane bond in the molecular chain and the soft segment composed of the polymer polyol. The hydrophilic monomer is a small molecule and is equivalent to a hard one in the molecular chain. The role of the segment, the molecule of DMBA has a large side chain) CH2COO), which acts as a hard segment in the molecular chain, resulting in an increase in the hardness of the molecular chain and a decrease in elongation at break.

The above proves that when the carboxyl group content is the same, the performance of the emulsion obtained by DMBA is significantly better than that of the emulsion obtained by using DMPA. Therefore, the hydrophilic chain extender is DMBA, and the carboxyl group content is preferably 3% or less.

2.3 Infrared spectroscopy

For the copolymerization modification of epoxy resin and the influence of its addition amount, see infrared spectrum 1 and Figure 2.

Figure 1 is an infrared spectrum of an epoxy resin, an unmodified aqueous polyurethane, and an epoxy-modified waterborne polyurethane. In Figure 1, line 1 is epoxy resin E-44, 914 cm-1 is a characteristic peak of epoxy group, 3 490 cm-1 is a characteristic peak of hydroxyl group; line 2 is spectrum of unmodified waterborne polyurethane, 3334 cm -1 is N) H characteristic peak; line 3 is an epoxy-modified waterborne polyurethane spectrum, and the characteristic peak of the epoxy group (914 cm-1) and the characteristic peak of the hydroxyl group (3490 cm-1) disappear, and the spectrum disappears. In the figure, there is no NCO base peak at 2 280~ 2 270 cm-1, which proves that EP reacts with PU.

Figure 2 shows the infrared spectrum of different EPs added (from bottom to top). It can be seen that with the increase of EP addition, the peak area of NH stretching vibration at 3334 cm-1 is obviously increased. This is PU. The NCO group in the prepolymer reacts with the OH in the EP to form NHCOO).

This demonstrates the series of reactions that occur during EP modification, ie, the EP through the NCO reaction in OH and PU, which produces a chemical bond between the two; the epoxy group in EP occurs under TEA catalysis The ring opening reaction initiates cross-linking between EP/PU. It is this series of reactions that form the local interpenetrating polymer network structure of the EP modified WPU system.

3. Conclusion

The WPU emulsion prepared by modification with E-44, with the increase of the addition amount, the viscosity of the emulsion increases, the hardness and mechanical properties of the coating film are enhanced, and the water resistance is improved, but the excessive amount has an effect on the stability of the WPU emulsion. , EP mass fraction in the resin is preferably less than 6%. Infrared spectroscopy indicated that the epoxy group formed a cross-linking reaction in the copolymerization process, and formed a local interpenetrating polymer network structure with PU. Compared with DMPA, DMBA is used to prepare epoxy modified WPU emulsion, which has the characteristics of simple synthesis process, easy operation and excellent product performance. Therefore, DMBA is a hydrophilic agent superior to DMPA.