Effect of Ca2+ Content on Properties of Waterborne Polyurethane Emulsion and Its Film

Waterborne Polyurethane (WPU) is an environmentally friendly product with low VOC (Volatile Organic Compound) and HAP (Hazardous Air Contaminant) values in adhesives, leather finishes, coatings, textiles and paper. Widely used. However, when WPU is prepared by internal emulsification, the hydrophilic group contained in the structure may cause the water resistance of WPU to deteriorate, the thermal stability of the film is not satisfactory, and the bonding strength is lower than that of solvent-based PU. (Polyurethane), so the application field of waterborne polyurethane emulsion is greatly limited. To eliminate the above drawbacks of WPU, crosslinkers are usually added during the use of WPU.

The carboxyl group (-COOH) in the surface layer of the WPU emulsion is in an ionized state. Therefore, as the small molecule is volatilized and the metal crosslinker is added during the film formation, a cross-linking reaction occurs between the WPU molecular chains. A thermodynamically stable coordination complex can be formed between the metal cation and -COO-, and the cross-linking reaction can multiply the Mr (relative molecular mass) of the original linear polymer, so the bonding property of the WPU film , thermal performance, water resistance and stain resistance are significantly improved.

Therefore, in this study, calcium hydroxide was used as a modifier of WPU, and calcium hydroxide was introduced into the WPU system according to different ratios of n(Ca2+):n(-COOH), and the WPU emulsion was determined. The properties of the film are preferably a suitable n(Ca2+):n(-COOH) ratio for the preparation of the modified WPU.

1. Testing and characterization

(1) Storage stability: The measurement is carried out in accordance with GB/T 6 753.3-1986.

(2) Particle size: The measurement was carried out by a laser particle size analyzer according to a dynamic light scattering method (test temperature was 25).

(3) Viscosity: measured by a rotary viscometer (test temperature is 25, 1# rotor, speed is 75 r/min).

(4) Thermal properties: Characterized by the thermogravimetric analysis (TGA) method (temperature up rate of 10 K/min, N2 atmosphere).

(5) Water resistance: a film with a size of 2 cm × 2 cm (mass W0), soaked in water for 24 h, taken out, wiped the surface moisture and weighed (W1), then the water absorption rate = (W1- W0)/W0.

(6) T-type peel strength: in accordance with GB/T 2 791-1995 standard,

The measurement was carried out using a universal testing machine (based on leather and rubber). (7) Tensile properties: The test was carried out according to the GB/T 1 040-1992 standard using a universal testing machine (stretching rate of 100 mm/min).

2. Results and discussion

2.1 Effect of n(Ca2+):n(-COOH) ratio on emulsion properties

The effects of different ratios of n(Ca2+):n(-COOH) on the particle size, viscosity, appearance and storage stability of the emulsion are shown in Table 1 and Table 1. It can be seen from Fig. 1 that when n(Ca2+):n(-COOH)<0.6:1, the particle size and viscosity of the emulsion increase significantly with the increase of the ratio of n(Ca2+):n(-COOH); when n(Ca2+) When n(-COOH)>0.6:1, the particle size and viscosity increase of the emulsion slowed down. It can be seen from Table 1 that as the proportion of n(Ca2+):n(-COOH) increases, the appearance of the emulsion gradually changes from micro-transparent (pan-blue light) to milky white opacity, and the storage stability of the emulsion gradually deteriorates.

Waterborne polyurethane emulsion stability

This is due to the introduction of Ca2+, which causes the cross-linking reaction of the polymer macromolecules, so the particle size of the emulsion increases (the larger the particle size, the more opaque the emulsion is); when n(Ca2+):n(-COOH)<0.6: 1 (ie, the Ca2+ content in the emulsion system is low), the Ca2+ concentration is the main factor affecting the degree of crosslinking; when n(Ca2+):n(-COOH)>0.6:1, the higher Ca2+ content has been made in the emulsion. The degree of crosslinking of the polymer is saturated, so the particle size of the emulsion increases with the increase of Ca2+ content.

Generally, the particle size of the emulsion and the viscosity of the emulsion are inversely proportional. That is, the larger the particle size of the emulsion, the smaller the total surface area of the particle, and the less the hydration layer of the outer layer of the polymer droplet, the lower the viscosity of the system; however, Ca2+ is added to The process in the emulsion is an exothermic process, and the heat released will accelerate the evaporation rate of water in the emulsion. Therefore, the more Ca2+ content, the higher the solid content of the emulsion, the greater the viscosity, and the poor storage stability of the emulsion. .

2.2 Thermal properties of WPU film

Under the premise of other conditions, the effect of n(Ca2+):n(-COOH) ratio on the TGA curve of WPU film is shown in Fig. 2. It can be seen from Fig. 2 that the initial decomposition temperature of the film increases with the increase of Ca2+ content, indicating that the introduction of Ca2+ is beneficial to improve its thermal stability. The carbon residue rate of the film at 500 is related to n(Ca2+):n(-COOH). The proportional increase is first high and then low, and reaches the highest value when n(Ca2+):n(-COOH)=0.6:1.

From the chemical structure point of view, the thermal stability of the polymer depends mainly on its chemical composition, Mr size and degree of crosslinking. Generally, the higher the degree of crosslinking of the polymer, the larger the Mr, and the higher the thermal decomposition temperature. Ca2+ has crosslinking and chain extension in the emulsion system, which is beneficial to improve the crosslinking degree and Mr of WPU. Therefore, the proper amount of Ca2+ can improve the thermal stability of WPU film.

Waterborne polyurethane emulsion film GTA curve

2.3 Water resistance of WPU film

The effect of n(Ca2+):n(-COOH) ratio on the water resistance of WPU film is shown in Figure 3 under the same conditions. It can be seen from Fig. 3 that the water absorption of the WPU film gradually decreases as the ratio of n(Ca2+):n(-COOH) increases, indicating that the water resistance is gradually increased.

This is because the cross-linking of Ca2+ and -COOH (hydrophilic groups) causes the WPU molecular chains to entangle each other, thereby effectively increasing the crosslink density of WPU, so the penetration of water into the WPU film is significantly hindered; however, the Ca2+ content is excessive. When high, the hydrophilicity of WPU is too low, and full emulsification becomes relatively difficult, which is manifested by coarse particles and unstable emulsion. Considering that n(Ca2+):n(-COOH)=(0.4~0.6):1 is more suitable.

2.4 Mechanical properties of WPU film

The effect of n(Ca2+):n(-COOH) ratio on the mechanical properties of WPU film is shown in Table 2 under the same conditions. It can be seen from Table 2 that as the ratio of n(Ca2+):n(-COOH) increases, the peel strength and tensile strength of the WPU film gradually increase, but the elongation at break gradually decreases.

The peel strength of the adhesive is closely related to the cohesive strength. The higher the Ca2+ content, the higher the crosslink density of WPU, and the cohesive strength increases with the increase of crosslink density, so the peel strength of WPU adhesive increases gradually. At the same time, the increase of crosslink density makes the crosslinking distance of molecular chains change. Short, the molecular deformability is lowered, so the tensile strength of the WPU film is increased, and the elongation at break is lowered (the flexibility of the film is lowered). Under the premise of maintaining emulsion stability, it is more suitable to choose n(Ca2+):n(-COOH)=0.4:1. At this time, the peeling strength of WPU adhesive is larger, the bonding effect is better and the film flexibility is better. (Elongation at break is 454.5%).

3. Conclusion

(1) The degree of crosslinking of WPU increases with the increase of Ca2+ content, and the greater the degree of crosslinking, the better the adhesion strength, water resistance and heat resistance of WPU adhesive; however, when the content of Ca2+ is too much, WPU The crosslink density is too large and the emulsion stability is deteriorated.

(2) The larger the ratio of n(Ca2+):n(-COOH), the higher the peel strength and tensile strength of WPU, but the smaller the elongation at break (the lower the flexibility). When n(Ca2+):n(-COOH)=0.4:1, the peel strength (4.5 N/mm), tensile strength (14.5 MPa) and elongation at break (454.5%) of WPU are relatively high. Waterborne polyurethane adhesives have excellent mechanical properties and bonding properties, as well as good flexibility.