Properties and characteristics of polyester-polyether composite waterborne polyurethane

The current polyurethane is mainly solvent-based, but since the solvent-based polyurethane contains a large amount of organic solvent, it is not only highly toxic but also easily causes air pollution. With the increasing awareness of environmental protection, these unfavorable factors have prompted researchers in polyurethane materials around the world to focus on the development of waterborne polyurethane. Waterborne polyurethane emulsion uses water instead of organic solvent as dispersion medium. It has the advantages of non-toxic, non-polluting, energy saving and easy processing and storage. It has been widely used in coatings, adhesives, textile and papermaking auxiliaries, medical and electronic materials and other national economy. Various fields. According to the difference of the main raw material polyol of synthetic polyurethane, polyurethane is generally divided into two categories: polyether polyurethane and polyester polyurethane. Polyester polyurethane generally has excellent mechanical properties, good film forming properties, heat resistance, light resistance, etc., but moisture permeability and hydrolysis resistance are not good, and the price is high; polyether polyurethane has high moisture permeability and hydrophilicity. And elongation, lower price, better hydrolysis resistance, but heat and resistance. Since polycarbonate has poor binary lightness and low tensile strength, the alcohol itself contains an ester bond, and the polyurethane product synthesized therefrom has excellent weather resistance and is superior to general polyester-based glycols. Mechanical properties, hydrolysis resistance, thermal stability, weather resistance and chemical resistance, but also good biocompatibility. It is the current comprehensive performance of polyols. This experiment uses a mixture of polycarbonate diol and polyether diol as a soft segment, isophorone diisocyanate, chain extender 2,2 - two Polymethylolpropionic acid (DMPA) and 1,4-butanediol are hard segments, and a series of polyester-polyether composite waterborne polyurethanes (WPU) with different raw material ratios are prepared by prepolymerization. WPU with mechanical properties, film forming properties and water resistance, and explore the effect of n(—NCO) / n( —OH) and DMPA dosage on WPU performance.

1 test section

1.1 Analysis and testing

Fourier External Light (FTIR) The WPU film was characterized by an IR-8400 S FTIR analyzer manufactured by Tsushima Corporation of Japan.

Thermogravimetric (TG) The WPU film was analyzed by a TG analyzer manufactured by American PE Company. The nitrogen atmosphere was heated at a rate of 15/min and the temperature was 20 to 800 °C.

Emulsion stability The storage stability of the emulsion was simulated by a centrifugal accelerated precipitation test. After centrifugation of the WPU emulsion at a centrifugal speed of 3 000 r /min for 15 min in a centrifuge, if there is no precipitation, it is considered that there is a storage stability of 6 months. Otherwise, the stability is judged to be poor.

Laser Particle Size Analysis The WPU emulsion was diluted with deionized water and the particle size and distribution of the latex particles were measured using a BT-9300 H laser particle size analyzer.

Water absorption rate The water absorption of the WPU film was tested by the immersion method. The WPU film sample dried under vacuum to a constant mass was cut into small squares of 2 cm × 2 cm, and the mass (w1) was measured at room temperature, and then the sample was immersed in deionized water at room temperature for 24 h. Remove, absorb the free water on the surface and weigh ( w2 ), calculate the water absorption rate according to the following formula:

Water absorption = ( w2 - w1 ) / w1 × 100%.

Mechanical properties The mechanical properties of WPU film were tested on XLL - 50 electronic tensile machine according to GB / T 13022-1991 at room temperature (25 °C). The tensile rate was 100 mm / min. value.

2 Results and discussion

FTIR analysis of 2.1WPU film

It can be seen from Fig. 1 that there is no absorption peak of -NCO at 2280 ~ 2270 cm -1 , indicating that -NCO has completely reacted; at 3334 cm -1 is the stretching vibration of -CO-NH-中-NH group Peak; 1535 cm - 1 is the bending vibration absorption peak of -NH; 1 705 cm - 1 is the characteristic absorption peak of carbonyl of carbamate; these 3 absorption peaks are characteristic of characteristic group carbamate in polyurethane The absorption peak indicates that the reaction produced polyurethane. The symmetric and asymmetrical stretching vibration peaks of C—O at 1105 cm - 1 and 1249 cm -1 respectively, and the bending vibration absorption peaks of -CH3 or -CH2 at 1460 cm -1 . The presence of hydrogen bonds generally shifts the stretching vibration peaks of —NH (proton donor) and —C ═ O (proton acceptor) toward the low wavenumber, while the band broadens and the absorbance increases. The -C ═ O absorption peak position in the ester shifted from 1734 cm -1 to 1705 cm -1 , and 3334 cm -1 - the stretching vibration peak of NH widened, indicating -NH and proton acceptor in carbamate Hydrogen bonds are formed between them.

Waterborne polyurethane film analysis

2.2 TG analysis of WPU film

It can be seen from Figure 2 that the thermal decomposition of the WPU is divided into two processes, which embodies the characteristics of the WPU consisting of a soft segment and a hard segment. Since the bond energy of C-N in the hard segment is low, the thermal decomposition of the WPU starts from the hard segment. The DTG curve shows that the initial decomposition temperature of the hard segment is around 245, and the maximum decomposition temperature is 298. 9; 306. 6 When the soft segment begins to decompose, its maximum decomposition temperature is around 321 . The change of the peak shape of the DTG curve indicates that there is a certain degree of microphase separation in the soft and hard segments of the WPU molecular chain, and the two-phase microdomains tend to aggregate strongly, which further indicates the soft and hard segment type of the WPU molecular chain. The characteristics of the structure.

Waterborne polyurethane soft and hard segment structure features

2.3 Appearance and stability of WPU emulsion

In this work, the effects of DMPA dosage and n(—NCO) /n( —OH) on the appearance and stability of WPU emulsion were investigated. The experimental results show that when the amount of DMPA is 4% (mass fraction, the same below), 5%, 6%, 7%, 8%, the polymerization system gradually changes from milky white unstable emulsion with the increase of DMPA dosage. A stable pan-blue light stable emulsion, and it is difficult to form a stable emulsion when the amount of DMPA is less than 4%. This is because as the amount of DMPA increases, the carboxyl group content in the system increases, the hydrophilicity increases, and the dispersion is more easily dispersed in deionized water, so the stability is improved. When n (-NCO) / n (-OH) are 1.2, 1.3, 1.4, 1.6, respectively, as n (-NCO) / n (-OH) increases, the emulsion of the system gradually becomes transparent. The blu-ray stabilized emulsion turns into a milky white unstable emulsion, and the larger the value of n(—NCO) / n(—OH), the more unstable the system. This is because as the remaining -NCO increases, it reacts with water to form a more polar urea group during dispersion, resulting in an increase in the hard segment content of the WPU molecular chain, and the viscosity of the particles is enhanced and is not easily sheared and dispersed. As the particle size increases and agglomeration occurs, the emulsion thus gradually becomes opaque by the transparent bluish blue light. In general, the appearance and stability of the emulsion are better when the amount of DMPA is 5% to 8% and n (-NCO) / n (-OH) is not more than 1.3.

2.4 WPU emulsion particle size

It can be seen from Fig. 3 (a) that as the amount of DMPA increases, the particle size of the WPU emulsion decreases, and the particle size distribution becomes narrower accordingly. This is because the increase in the amount of DMPA means that the carboxyl group content of the hydrophilic group is correspondingly increased, the hydrophilicity of the system is enhanced, and the intertwining of the molecular chains is facilitated by the emulsification and dispersion of the particles in the aqueous phase, so that the emulsion particles are obtained. The number is gradually increased, so that the particle size is correspondingly reduced, and the uniformity and stability are improved. The decrease in particle size and particle size distribution tends to be flat when the amount of DMPA is large, which may be due to the increase in the thickness of the electric double layer formed between the particle and the water interface while increasing the hydrophilicity, thereby increasing the hydrodynamic volume. Caused.

Emulsion particle size distribution map

It can be seen from Fig. 3(b) that the particle size of the four groups of WPU emulsions is between 40 and 140 nm, the distribution range is narrow, and the monodispersity is better; with the increase of n(-NCO) / n(-OH) Large, the particle size of the emulsion increases, and the particle size distribution becomes wider. This may be because on the one hand, when the average relative molecular mass of the product is small when n(-NCO) / n (-OH) is large, the particle size of the emulsion is also small; on the other hand, due to the increase in the number of NCO groups, When it is dispersed, it reacts with water to form urea with strong polarity, which makes the viscosity of the particles stronger. It is easy to cause adhesion between particles during collision, and is not easily sheared and dispersed, so that the particle size is increased and the particle size distribution is broadened.

Water resistance of 2.5 WPU film

It can be seen from Fig. 4(a) that the fixed n (-NCO) /n(-OH), the water absorption of the WPU film increases with the increase of the amount of DMPA, and the water resistance decreases. This is because when n(—NCO) / n(—OH) is constant, the ratio of soft and hard segments of the molecular chain is constant, and when the amount of DMPA as a hard segment is increased, it contains a hydrophilic group of carboxyl groups. The content is increased, resulting in an increase in the water absorption of the film. Moreover, it can be seen from the figure that when the amount of DMPA is 6% to 8%, the water absorption rate of the film increases greatly. The water resistance of WPU film is an important performance index in many applications, so the amount of DMPA should be minimized while maintaining emulsion stability.

Water resistance of film

It can be seen from Fig. 4(b) that as n(-NCO) / n(-OH) increases, the water absorption of the WPU film decreases and the water resistance increases. This is because when the amount of DMPA is constant, the increase of n(—NCO) / n(—OH) increases the number of terminal isocyanate groups in the prepolymer molecular chain, that is, the hard segment content increases, and the molecular chain after film formation The interaction is enhanced, water molecules are not easily penetrated; at the same time, the amount of DMPA is fixed to increase the value of n(-NCO) / n(-OH), and more carbamate, urea, and shrinkage are formed in the reaction. The hydrophobic segment such as the diurea segment leads to a relative increase in the content of the hydrophobic group, resulting in a decrease in the water absorption of the film, that is, an increase in water resistance. When n(—NCO) / n (—OH) is between 1. 2 and 1. 6 , the water absorption of the WPU film is 9.5% to 16.0%, and the water absorption of the general polyester polyurethane film The rate is at 20% left and right, so the polyester-polyether composite polyurethane has good water resistance.

2.6 Mechanical properties of WPU film

It can be seen from Fig. 5(a) that as the amount of DMPA increases, the tensile strength of the WPU film gradually increases, and the elongation at break gradually decreases. This is because the increase in the amount of DMPA as a hydrophilic chain extender means that the hard segment content of the polyurethane molecular structure is increased, and the soft segment (polyester diol, polyether diol) is relatively reduced. The content of the generated carbamate group is increased, the intramolecular hydrogen bond in the hard segment microdomain formed during the microphase separation is increased, and the hydrogen bond can act as a physical crosslink between the molecules, causing the film to be pulled. The strength of the extension is increased. At the same time, the soft segment is relatively reduced, resulting in a decrease in the flexibility of the film, so that the elongation at break is decreased.

Waterborne polyurethane film tensile strength

It can be seen from Fig. 5(b) that as n(-NCO) / n(-OH) increases, the tensile strength of the WPU film increases gradually, and the elongation at break is reversed. This is because the value of n(—NCO) / n(—OH) increases, the content of the hard segment in the molecular segment of the WPU increases correspondingly, the hydrogen bond and cross-linking increase, and the electrostatic force between the hard segments increases. The tensile strength of the film is increased; on the contrary, the soft segment content is relatively reduced, and the flexibility of the film is decreased, which is manifested by a decrease in the elongation at break. In order to obtain better mechanical properties, and based on the end use of the product, the optimal n(-NCO) / n(-OH) is selected to be 1.3.

3 Conclusion

(1) With the increase of the amount of DMPA, the appearance of WPU emulsion changes from opaque to transparent, the particle size decreases, and the stability increases. The water absorption of the film increases, the water resistance decreases, the tensile strength increases, and the elongation is broken. The rate drops.

(2) As n (-NCO) / n (-OH) increases, the appearance of WPU emulsion is affected, the particle size increases, and the stability decreases. The water absorption of the film decreases, the water resistance increases, and the tensile strength Increase, the elongation at break decreases.

(3) Considering the stability of the emulsion, the water resistance and mechanical properties of the film, when the mass ratio of the polyester diol to the polyether is 1/3, the amount of DMPA is 6%, n (-NCO) ) / n( —OH) is preferably 1. 3 is preferred.

(4) The WPU emulsion will undergo a certain degree of microphase separation during film formation.