Highly transparent polyurethane adhesives were synthesized from dicyclohexylmethane diisocyanate (HMDI) and isophorone diisocyanate (IPDI) with polycaprolactone polyol (PCL), polytetrahydrofuran ether diol (PTME), and polyether diol, and the performance of the adhesives was tested. The high transparent polyurethane adhesive was used to bond high transparent polyurethane film on Plexiglas to make a protective layer, and the protective layer was tested for heat resistance, UV aging and abrasion resistance. The results show that the adhesive has good adhesion and optical properties, and also has good heat resistance, UV aging resistance and good water resistance. The protective layer can effectively reduce the abrasion caused by wind sand on the surface of Plexiglas and has excellent protection effect against wind sand.
Keywords
High transparency; polyurethane; adhesive; abrasion resistance
Introduction
In this paper, a high transparent polyurethane resin adhesive is used to apply a high transparent polyurethane film to the front windshield of an aircraft for wind sand protection.
1 Experiment
1.1 Experimental raw materials
The experimental raw materials are shown in Table 1.
1.2 Preparation of adhesive
Add measured polyether type polyol to a three-neck flask equipped with stirrer, thermometer and condensation tube, heat and dehydrate under vacuum, then cool down to 90°C and slowly add diisocyanate monomer and catalyst and stir the reaction to the specified time and discharge to make prepolymer component A. Add polyol, chain extender, UV stabilizer and antioxidant to the three-neck flask in certain proportion, pass nitrogen and heat under vacuum. After dehydration, stir for 2h at 80℃ to make component B. Preparation of adhesive: component A and component B are added to the container in proportion, mixed with a three-dimensional high-speed mixer for 30s, cured at room temperature for 24h and 80°C for 12h.
1.3 Performance testing and characterization
Viscosity test: The viscosity meter of vertebral plate of American Bolerfly CAP2000+ is used, and the test is conducted at 25℃ at constant temperature. Light transmittance and haze test: Lambda850 UV spectrometer from PE, USA, using integrating sphere measurement system, scanning range 380~780nm, slit width 5.00nm. test standard refer to GB/T 2410-2008. mechanical property test: INSTRON-4467 type universal Material testing machine measurement, according to the standard GB/T 528-2009, using the mold to make 1 type dumbbell sheet sample strip, using tensile mode to measure the sample tensile strength and elongation at break. The thermal weight loss analysis was measured using a Q50 thermogravimetric analyzer from TA, USA, under a nitrogen (N2) atmosphere, and the test temperature range was from room temperature to 600°C, with a heating rate of 10°C/min. The surface abrasion resistance test was conducted using a sand-blowing test, and the experimental conditions were 40 mesh mixed sand, wind speed of 550 m/s, sand content of 5 g/m3, distance between sample and outlet of 15 The distance between the sample and the outlet was 15 cm, and the angle of sand blasting was 45°. Figure 1 shows the schematic diagram of the sand-blowing device.
2 Results and discussion
2.1 Synthesis of high transparent polyurethane adhesives
Conventional transparent polyurethane mostly uses aromatic polyisocyanate, which has the advantages of high reactivity and low price, but aromatic polyisocyanate is highly toxic, easily oxidized and easily yellowed. Aliphatic polyisocyanates do not contain benzene rings, are not easy to age and yellow, and have good weather resistance. The aliphatic polyisocyanate, dibutyltin laurate as catalyst, polytetrahydrofuran ether diol, polyether diol, polycaprolactone triol and small molecule chain extender were used to synthesize high transparent polyurethane adhesives. The formulation is shown in Table 2.
2.2 Properties of Highly Transparent Polyurethane Adhesives
2.2.1 Adhesive properties
The properties of polyurethane adhesives are shown in Table 3. 52Y72 resin has excellent mechanical properties and light transmission, and good adhesion. 2.2.2 Trial period of adhesives
The viscosity-time curve of the highly transparent polyurethane adhesive is shown in Figure 2. It can be seen from the graph that the initial viscosity of the polyurethane adhesive is very low. The viscosity of the initial reaction changes very slowly, and after 2h the viscosity starts to increase significantly, and the period of application of the adhesive is determined to be 2h.
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Table 4 shows the properties of the adhesive resin during the applicable period. The differences in tensile strength, elongation at break and adhesion measured within 2h were not significant after the components of the adhesive were fully mixed at room temperature. It means that the adhesive has stable mechanical properties and adhesion during the applicable period.
2.2.3 TG curve of the adhesive
The thermal weight loss curve of 52Y72 resin adhesive is shown in Fig. 3. The adhesive starts to decompose at about 250℃, loses 3% of weight at 270℃, 50% at 345℃ and 90% at 408℃, with good heat resistance.
2.3 Application of high transparent polyurethane adhesive
2.3.1 Protective layer preparation
Add the adhesive raw materials to the container in proportion, mix for 30s using 3D high-speed mixer, pour on the surface of No.11 aviation glass, coat the adhesive evenly with the coating roller, then cover with homemade polyurethane film and roll evenly again. Curing at room temperature for 24h, curing at 80 ℃ for 12h, measuring the properties, as shown in Table 6.
2.3.2.1 UV Aging
The samples were put into the UV aging instrument for testing. The main wavelength was 310 nm, and the ambient temperature was 65°C. The samples were irradiated on one side for 500 h. Table 7 shows the comparison of the optical properties of the samples before and after UV aging.
Compared with Plexiglas, the light transmittance of the protective layer samples with adhesive film decreased by 0.28%, and the haze and yellowing index increased by 0.11% and 0.54%, respectively; after UV aging, the light transmittance of the protective layer samples decreased by 1.25%, and the haze and yellowing index increased by 0.03% and 1.95%, respectively. In summary, the samples still have excellent optical properties after UV aging.
2.3.2.2 Abrasion resistance
The abrasion resistance was evaluated by sand-blowing test, and the enlarged pictures of the samples before and after the sand-blowing test are shown below. The effect of the protective layer before and after the sand-blowing test is shown in Figure 5. Figure a is an enlarged picture of the surface of the Plexiglas, which has a light transmission rate of 92.1% and a small amount of scratches on the surface. This is because the surface hardness of plexiglass is small, only equivalent to the hardness of aluminum, wear resistance is poor, the surface is easy to be scratched. Figure b is a magnified view of the surface of the plexiglass after the sand-blown test, which shows that the surface of the plexiglass is very severely worn, the surface is uneven, and the transmittance is reduced to 32.4%. Figure c is the magnified view of the surface of the protective layer sample, the transmittance is 91.82%, and the surface is smooth and without scratches. Figure d is the magnification of the surface of the protective layer sample after sand-blowing experiment, it can be seen that the surface of the protective layer after sand-blowing is rougher than that before sand-blowing, but it is obviously better than that of Plexiglas, and the light transmittance of the protective layer after sand-blowing can still reach 79.2%. In summary, the use of high transparent polyurethane adhesive paste transparent polyurethane film can effectively reduce the abrasion caused by wind sand on the surface of Plexiglas, and the wind sand resistant protection effect is very good.
2.3.2.3 Damp heat aging
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The performance of the protective layer after damp heat aging is shown in Table 8. After 5 cycles of damp heat aging test, there is no change in appearance, no obvious change in light transmission and haze, and no decrease in adhesion. It shows that the adhesive has good hygrothermal aging resistance.
2.3.2.4 Water resistance
Using the water immersion test method, the sample was sealed with paraffin wax before the test. The samples were soaked in distilled water for 5 d. At the end of the experiment, the samples were taken out of the tank, blotted with filter paper and checked visually. The samples did not show any loss of light, discoloration, blistering, wrinkling, shedding, etc., and there was no visible change in sample transmittance and haze, indicating that the samples have good water resistance.
2.3.2.5 High and low temperature test
After 100h of high temperature test at 80℃, the appearance is smooth and flat, and the mechanical properties, optical properties and adhesion are not affected. After 100h of -18℃ low temperature test, the appearance is smooth and flat, and there is no decrease in light transmission and haze increase in visual inspection. And mechanical properties and adhesion are not reduced. After -55℃ low temperature test, the appearance of the adhesive resin is smooth and flat, and the tensile strength is 7MPa, and there is no significant change in light transmission and haze.
3 Conclusion
(1) The 52Y72 high transparent polyurethane adhesive was synthesized from aliphatic polyisocyanate and polyol with excellent optical properties and adhesion.
(2) After sand-blowing test, the transparent polyurethane film can effectively reduce the abrasion caused by wind sand on the surface of Plexiglas by using high transparent polyurethane adhesive.
(3) The adhesive has good UV aging resistance and water resistance, and good humidity and heat aging resistance.
