Preparation and Performance Study of UV Curable Polyurethane Acrylate Peelable Coatings

Coating Industry

Strippable coating is a temporary protective coating that can be used for temporary protection of devices. During device storage, transportation, and use, it protects devices from dust, oil, scratches, or corrosion. After the protective effect of the peelable coating is completed, it can be completely peeled off from the surface of the device, without causing any damage to the surface of the substrate, and without leaving any substances that affect the performance of the substrate. Therefore, peelable coatings have been widely used in industries such as transportation, electronics, construction, national defense, automobiles, and ships.

UV curable peelable coatings have the advantages of low curing shrinkage, fast curing rate, and are not affected by environmental temperature and humidity. They can be applied in specific occasions or fields with specific requirements. Zhang Yingqiang et al. prepared a cationic polymer using epoxy resin, photoinitiator, and functional additives

The sub type UV curable peelable coating has good salt water resistance and peeling performance, and can be used in fields such as instruments, automobiles, electronic appliances, etc. Sun et al. prepared a UV curable peelable coating with good comprehensive performance using TPGDA as the active diluent and BAPO as the photoinitiator. Drain et al. prepared a UV curable peel coating using polyester polyurethane acrylate (PUA), acrylate monomers, vinyl ether monomers, photoinitiators, silica, etc., which can be used for temporary protection of electronics and electronic products. However, the above studies mostly focus on applications, and there are few reports on the factors that affect peel performance.

Based on the above research background, two polyurethane acrylates (P2000-HMDI-HEA and P2000-HMDI-PETA) were synthesized from hydroxyl terminated hydrogenated polybutadiene (P2000), 4,4 '- Hydrogenated MDI, Hydroxyethyl acrylate (HEA) and Pentaerythritol triacrylate (PETA). A series of UV curable peelable coatings were prepared using two types of polyurethane acrylate as the main film-forming agents, with the addition of different photoinitiators and active diluents, and their peel properties were studied.

1 Experimental part

1.1 Experimental Materials and Instruments

Hydroxyl terminated hydrogenated polybutadiene (P2000): industrial grade, Wuxi Liteng Chemical Co., Ltd; Hydroxyethyl acrylate (HEA): industrial grade, Shanghai Titan Technology Co., Ltd; Pentaerythritol Triacrylate (PETA): industrial grade, Jiangsu Kailin Ruiyang Chemical Co., Ltd; 4,4 '- Hydrogenated MDI (HMDI): chemically pure, Shanghai Vita Chemical Co., Ltd; Dibutyltin Lauric acid (DBTDL): analytically pure, Maclean's Biochemical Technology Co., Ltd; Isoborneol acrylate (IBOA): industrial grade, Akeli Technology Co., Ltd; 2-hydroxy-2-methyl-1-phenyl-1-acetone (1173), 1-hydroxycyclohexylphenyl ketone (184), 2,4,6-trimethyl Benzoyl group Diphenylphosphine oxide (TPO): industrial grade, Changzhou Qiangli Electronic New Materials Co., Ltd; Tetrahydrofuran (THF): Chromatographic purity, China National Pharmaceutical Group Chemical Reagent Co., Ltd. Track type UV curing machine: F300S, Fusion; Total reflection Fourier transform infrared spectrometer: Nicolet 6700, Thermo Fisher Scientific Co., Ltd; Nuclear magnetic resonance spectrometer: AVANCE III, BRUKER; Tensile testing machine: E43. 104, Meters Industrial Systems (China) Co., Ltd; Gel permeation chromatography (GPC): HLC-8320GPC EcoSEC, Toshio Co., Ltd.

1.2 Synthesis of polyurethane acrylate

The synthesis of polyurethane acrylate (PUA) takes the experimental steps of P2000-HMDIHEA as an example. Weigh 80 g of P2000 (0.04 mol) and 20.96 g of HMDI (0.08 mol), and slowly drop 1% (based on the total mass of P2000 and HMDI) of DBTDL into a three necked flask. Stir at 80 ℃ for 2 hours to obtain an intermediate product containing isocyanate (P2000-HMDI), and determine the content of NCO. Finally, HEA with a mass of - NCO was added and reacted for 2-4 hours. After infrared testing, there was no absorption peak of - NCO (2260-2280 cm-1), and the reaction was completed to obtain P2000-HMDI-HEA. Repeat the above operation and change HEA to PETA to obtain P2000-HMDI-PETA.

1.3 Preparation of UV curable peelable coatings

Add 30% (based on the total mass of resin and diluent) of active diluent IBOA to resin P2000-HMDI-HEA or P2000-HMDI-PETA, and add photoinitiators 1173, 184, or TPO respectively. After stirring evenly, prepare a photocurable peelable coating. Complete the scraping coating on the substrate and expose it for 60 seconds in a tracked light curing machine to complete the curing of the coating, with a total radiation energy of 800 mJ/cm2.

1.4 Performance testing and characterization

Infrared characterization: Place the sample on the sample stage and use a total reflection infrared spectrometer to scan, with a scanning range of 500-4000 cm-1, a collection frequency of 16 times/s, and a spectral resolution of 4 cm-1.

Nuclear magnetic resonance characterization: Weigh 10 mg of the sample and dissolve it in 0.6 mL of deuterated reagent (CDCl3), sonicate to completely dissolve it, and use nuclear magnetic resonance spectroscopy

Perform structural analysis on the product.

Relative molecular weight and its distribution: Using GPC testing, weigh about 10 mg of the sample and dissolve it in 1.5-2 mL of tetrahydrofuran solvent. Shake it completely with ultrasound and use 22 μ M's needle filter filters.

Double bond conversion rate: Apply the coating onto KBr salt flakes and cover another salt flake before testing. The OmniCureS1000UV visible light point light source is used for irradiation. The radiation energy density measured by the irradiation meter is 6.5 mW/cm2, and the irradiation curing time is 600 s. A real-time infrared spectrometer (with a spectral range of 500-4000 cm-1, a collection frequency of 4 cm-1, and a resolution of 4 cm-1) was used to monitor the disappearance of polyurethane acrylate double bonds during the initiation polymerization process and study the polymerization kinetics. Calculate the conversion rate through equation (1).

In the formula: At - the characteristic peak area of the double bond at time t; A0- Initial peak area.

Tensile performance: After peeling off the peelable coating, standard specimens were prepared using a dumbbell shaped cutter. The tensile performance was tested using MTS E43.104 universal material testing machine at room temperature at a speed of 20 mm/min.

180 ° peel performance: Apply peelable coating evenly on 20 cm × After curing on a 5 cm steel plate, the 180 ° peel strength of the peelable coating is tested according to GB/T 2792-1998.

2 Results and Discussion

2.1 Infrared characterization of PUA

The infrared spectrum of polyurethane acrylate is shown in Figure 1.

It can be seen from Figure 1 that the P2000-HMDI has a characteristic absorption peak of - NCO near 262 cm-1, and a characteristic peak of Carbamate near 3336 cm-1, and there is no strong absorption peak of - OH near 3400 cm-1, which indicates that - OH on P2000 reacts with - NCO. The - NCO absorption peak of P2000-HMDI-HEA and P2000-HMDI-PETA disappeared at 2 262 cm-1, the Carbamate characteristic peak near 3 336 cm-1 was more obvious, and the C=C double bond characteristic absorption peak appeared near 1 636 cm-1. The above results preliminarily proved the success of the reaction.

2.2 Proton nuclear magnetic resonance of PUA

The Proton nuclear magnetic resonance of polyurethane acrylate is shown in Figure 2.

As shown in Figure 2, δ= The peak at 7.5 belongs to the hydrogen proton (c) on the Carbamate, δ= The peaks at 5.7~6.5 belong to the hydrogen proton (b) on the carbon carbon double bond, δ= 4.0 From the methylene peak (a) connected between Carbamate and Diol, δ= 0.7~1.4 are the proton peaks on the aliphatic ring and the hydrogen proton peaks of the methylene of the repeating unit in the Diol. In addition, P2000-HMDI-HEA δ = The peaks (e, f) at positions 4.3 to 4.4 and P2000-HMDI-PETA δ= The peaks (d) at 4.3 are all proton peaks on the methylene group connected to the acryloyloxy group. The above results indicate that two types of PUA have been successfully synthesized.

2.3 Relative molecular weight and distribution of PUA

Table 1 shows the GPC test results of polyurethane acrylate synthesized from hydroxyl terminated hydrogenated polybutadiene (P2000).

From Table 1, it can be seen that the Mn of P2000-HMDI-HEA is 8560, and the Mn of P2000-HMDI-PETA is 10860. In addition, the relative molecular weight distribution of PETA terminated polyurethane acrylate is 1.65, which is wider than that of P2000-HMDI-HEA. This may be due to the lower purity of PETA compared to HEA.

2.4 Double bond conversion rate of UV curable peelable coatings

Figure 3 shows the double bond conversion rates of a UV curable peelable coating prepared by adding 30% IBOA to two resins, P2000-HMDI-HEA (a1, a2, a3) and P2000-HMDI-PETA (b1, b2, b3), at different amounts of photoinitiators 184, 1173, and TPO.

Figure 3 Double bond conversion rates of two peelable coatings at different amounts of photoinitiator 184, 1173, and TPO

Fig. 3 Double bond conversion of two striped coatings in different amounts of photo initiator 1841173 and TPO

From Figure 3, it can be seen that the double bond conversion rate of P2000-HMDI-HEA-30% IBOA is over 90% when cured with photoinitiator 1173. When 184 and TPO are used, the conversion rate of double bond is lower than 90%, which may be due to the high activity of initiator, which leads to the early arrival of the gel point of the coating and affects the further photocuring reaction of double bond. When the coating P2000-HMDI-PETA-30% IBOA was cured with initiators 184, 1173, and TPO, the double bond conversion rate was around 80%. This may be due to the larger relative molecular weight and higher viscosity of the resin, which makes it difficult to move the molecular chain segments and reduces the chance of collision between acrylic double bonds between molecules. In addition, with the increase of initiator content, the double bond conversion rate of both coatings basically increases first and then decreases. Based on the above test results, the optimal dosage for 1173, 184, and TPO is selected as 3%, 3%, and 1%, respectively.

2.5 Tensile properties

The Stress–strain curve of the UV curable strippable coating is shown in Figure 4.

From Figure 4, it can be seen that coatings prepared with different resins and photoinitiators exhibit significant differences in tensile stress and elongation at break. The coating prepared with PETA capped PUA can increase its tensile strength from 20 MPa to 27 MPa after adjusting the type of initiator, while the coating prepared with HEA capped PUA can increase its tensile strength from 13 MPa to 23 MPa, but its maximum elongation at break can reach 190%. The change in mechanical properties is closely related to the degree of crosslinking of the coating. PUA capped with PETA has more double bond content and a greater degree of crosslinking. The above results indicate that the coating prepared by HEA capped PUA has better comprehensive performance.

2.6 180 ° peel strength test

Figure 5 shows the 180 ° peel strength test results of two types of PUA photocurable coatings prepared with different initiators.

From Figure 5, it can be seen that the 180 ° peel strength of the PUA coating with PETA sealing is significantly lower than that of the HEA sealing coating, both of which are lower than 1 N/cm. This may be because the crosslinking density of the coating after HEA sealing is low, and the adhesion of the peelable coating is good. However, the coating prepared by PETA sealing PUA has a high degree of crosslinking, and the curing film is brittle, resulting in a decrease in peel strength. The coatings prepared by two types of PUA have the highest peel strength when TPO is used as the initiator. From the above results, it can be concluded that the UV curable peelable coating prepared by using HEA capped PUA and photoinitiator TPO has the best comprehensive performance.

3 Conclusion

This study successfully synthesized two types of polyurethane acrylate using hydroxyl terminated hydrogenated polybutadiene as raw material. After adding diluents and three different initiators, 1173, 184, and TPO, a UV curable peelable coating was prepared and its basic properties were characterized. The effects of the types of blocking agents and initiators on the peelable coating were investigated. The results showed that the breaking elongation of HEA capped PUA peelable coating was significantly better than that of PETA capped PUA, while the tensile strength was lower than that of PETA capped PUA; The 180 ° peel strength of PETA capped PUA peelable coating is significantly lower than that of HEA capped PUA peelable coating; After using the initiator TPO, the 180 ° peel strength of the peelable coating is significantly higher than that of the initiators 1173 and 184. Good tensile performance can ensure that the coating does not break during peeling, and a moderate 180 ° peeling strength can provide a certain adhesion between the coating and the substrate, providing temporary protection. The prepared UV curable peelable coating has broad application prospects in the field of temporary protection.