Functions and Infl uencing Factors of UV-curable Water-borne Wood Coatings

Liu Guojie
(Science and Technology Development Department, China National Coatings Industry Association, Beijing 100079)

Abstract:This paper introduces the superiorities of UV-curable waterborne Wood coatings to the traditional UV-curable coatings and water-borne Wood coatings; discusses the influencing factors of UV-curing dynamics of PU-acrylate water-borne dispersion, deducing the feasible methods to improve the UV-curing speed and reaction degree.

1 Significance of UV-curable waterborne wood coatings
1.1 Advantages of UV-curable coatings 

UV-curable coatings have advantages such as fast curing, low pollution and energy saving in wood, ink, plastics, adhesives, printed art, printed circuit boards, electronic component packaging materials , Canned printed iron, metal, stone, cement products, fabrics, leather, glass and other fields have been applied. Since its introduction in the 1960s, there have been many reports of progress, and domestic monographs have been published. UV-curable wood coatings account for nearly 1/4 of UV-curable wood coating products . The advantages of UV-curable coatings are shown in Table 1. It can be seen from Table 1 that although the UV-curable coating is somewhat inadequate, as mentioned above, its rapid curing, high-efficiency assembly line production, low VOC, and energy saving make it have rapid development vitality. For nearly 20 years in China, UV-curable coatings have been growing in double digits.

Advantages and disadvantages of UV curing coatings

1.2 Comparative Advantages of UV Curable Waterborne Wood Coatings

Both UV curable wood coatings and waterborne wood coatings are low-pollution varieties. Why should we promote the development of UV curable waterborne wood coatings? Because UV-curable waterborne wood coatings have their advantages over them (Table 2).

Comparative advantages of UV-curable waterborne wood coatings

It can be seen from Table 2 that UV-curable waterborne wood coatings have obvious advantages compared with traditional UV-curable coatings. Compared with water-based wood coatings, the main advantages are high productivity and suitable for assembly line production, especially the resulting coating film has excellent performance and can reach the same level. The performance level of solvent-based wood coatings, some properties such as hardness are still exceeded. Waterborne coatings for woodware is a development direction. Waterborne woodware coatings account for more than 20% in Europe. At present, domestic waterborne woodware coatings account for less than 2% [7]. The important reason is that certain properties have not reached the level of similar solvent-based woodware coatings. The development of UV-curable water-based wood coatings can make up for these shortcomings, and it is of great significance to increase the proportion of water-based coatings in the entire wood coatings.

1.3 Curing characteristics of UV-curable waterborne wood coatings
The traditional UV curing coating process is to obtain a certain thickness of wet coating film on the coating coating substrate, and then cure under a certain amount of UV irradiation. The feature is that it quickly changes from a liquid wet film to a solid state under UV irradiation. Coating film, that is, a polymer that is crosslinked into a bulk network structure. Because the polymer is cured in a liquid state, the molecular movement and chain movement of the polymer in the coating are less subject to resistance, so the polymerization reaction in the coating film is fast, it is easy to expand to the deep layer, and the reaction degree is also high. The process of UV-curing waterborne wood coatings has two steps: a certain thickness of wet coating film is obtained after the coating is coated on the bottom plate, and it is first dried in a drying oven at 80 ℃ for 3 to 5 minutes. Generally, the moisture in the wet film is removed and dried. Non-sticky dry coating film; then, under a certain intensity of UV radiation, a polymerization reaction is generated to solidify the polymer into a bulk network structure. It is different from the traditional UV curing process in that the solid dry film is UV cured. The molecular movement and chain movement of the polymer in the dry film are greatly affected by the resistance, and the polymerization reaction expands slowly. Under the rated time and humidity, and liquid Compared with film curing, the reaction degree is low and the coating film performance is poor. UV curing waterborne wood coatings need to overcome this shortcoming to achieve the required degree of curing.

2 The effect of water-based resin types on UV curing performance
Water-based wood coatings used for UV curing are water-dispersed coatings, but they are subdivided into water-latex resin types (referred to as emulsion types) and resin water-dispersed types (referred to as dispersion types). The former is a acrylate monomer and its comonomer in the presence of an emulsifier to synthesize a latex resin and dispersed in water; the latter is an acrylate monomer and its comonomer to synthesize oligomers containing carboxylic acid groups, using alkaline The compound is neutralized to form a hydrophilic carboxylate group and then dispersed in water. It is a water-dilutable dispersion, but not an aqueous solution. Emulsion UV curing water-based coatings were developed earlier, with the addition of emulsifiers, supplemented by high shear forces, emulsifying and dispersing traditional UV-curing resins. This can be regarded as the first generation of UV water-based coatings. Water resistance decreases. Later, an internal emulsification type was developed, that is, hydrophilic segments such as polyethylene glycol segments were introduced into the resin, which can be dispersed in water moderately. Some literatures call it the second-generation UV-curable water-based coatings, but the hydrophilic segments in the resin are still Reduce the water and chemical resistance of cured coatings . The new generation of UV-curable water-based coatings is a “dispersion type”, which is stably dispersed in water to form smaller particles, has a long storage time, and has good cured coating performance.
Emulsified polyester-acrylate and dispersion-type polyurethane-acrylate water-based coatings were cured using the same photosensitizer under the same drying conditions and UV radiation intensity, and the infrared bond spectrometer was used to monitor the conversion rate of double bonds on acryloxy detecting film hardness and other properties
can. Dispersion type aliphatic polyurethane-acrylate cured coating film has a Persoz hardness of 300 s, increased the curing temperature to 80 ℃, and increased the hardness to 350 s, which is almost close to the hardness of inorganic glass, but its elastic modulus is low (E = 1 565 MPa, Tg = 96 ℃), the film flexibility is better. Emulsified polyester-acrylate cured film has low hardness (Persoz hardness about 70 s), UV curing temperature increased to 80 ℃, and Persoz hardness is only 200 s.
The coatings of the five UV cured samples were accelerated in QUV-A: UV irradiated at 70 ℃ for 8 h, then at 100% relative humidity, and protected from light for 4 h at 50 ℃. The two were alternated. The transparency (loss of light) and yellowness of the coating film were examined with the absorbance of light wave at 420 nm. The aliphatic polyurethane-acrylate dispersion coating film was accelerated aging after 3 000 h, and the light transmittance was reduced from 98% to 90%. , The light loss rate is very low. Other control coating samples (containing aryl structures) rapidly yellowed and degraded after accelerated aging for 500 h, leading to peeling [8]. According to the comparative test results, it was confirmed that the dispersion type aliphatic polyurethane-acrylate is more suitable for UV-curable waterborne wood coatings.

3 Factors Affecting the UV Curing Kinetics of Waterborne Wood Coatings
3.1 Preparation of Aqueous Dispersions and UV Curing
Focus on the factors affecting the UV curing of polyurethane-acrylate dispersions. Polyurethane-acrylate oligomer (PUA) is a binary oligomer (polycaprolactone diol) with an aliphatic diisoester (isophorone dicyanate IPDI) It is prepared by hydroxyethyl acrylate and dimethylolpropionic acid (refer to US 6444721 in 2002). The formula is designed according to stoichiometry. Dimethylolpropionic acid is added at an acid value of 10 mg23 mgKOH / g and monitored by infrared spectrometer (IP). All isocyanate groups and hydroxyl groups have been completely reacted. Then, the carboxylic acid group was neutralized with NaOH or amine, and transformed into a stable aqueous dispersion PUA-1 with a solid content of 35% and a viscosity of 40 mPa · s. For comparative experimental purposes, PUA-2 and PUA-3 were synthesized. PUA-2 and PUA-1 have the same molecular structure, but are dissolved in butyl acetate; PUA-3 and PUA-1 have the same molecular structure, but without dimethylolpropionic acid, they are also dissolved in butyl ester in. The photosensitizer is water-soluble Irgacure 2959 (ciba chemistry), which is directly added to the dispersion in an amount of 0.3% – 1% (mass). The samples were dried in a drying oven at 80 ° C for 5 minutes and then UV cured. The representative process of the photo-initiated crosslinking polymerization process is as follows:

3.2 Factors that accelerate UV curing of waterborne wood coatings [9]
3.2.1 Influence of UV curing temperature The
PUA coating is dried into a solid coating film in a drying oven before UV curing, with large molecular movement resistance and the formation of hydrogen bonds between polyurethane chains It makes it more difficult for the molecules to move in the dry coating film, and the photopolymerization reaction speed is very slow. Simultaneously with UV curing, the double bond conversion rate (infrared spectral characteristic peak 1 410 cm-1) of the acryloyloxy group in PUA was monitored simultaneously to characterize the degree of photopolymerization. When exposed to UV radiation (light intensity 100 mW · cm-2) at 21 ° C for 20 s, the double bond conversion rate is only 22%. When the UV curing temperature is increased to 100 ° C, the conversion rate at 20 s is 80% (Figure 1). Compared with the curing speed at 21 ℃, it is improved by dozens of times. The samples are directly cured from the hot samples dried in a drying furnace by UV irradiation, and the light intensity is increased to 600 mW · cm-2 (typical UV intensity on the general industrial UV curing line). Setting 80 ℃ can achieve more complete Curing.

Figure 1 UV-curable water-based PUA coating

3.2.2 Influence of reactive plasticizers

Add reactive plasticizers, such as hexanediol diacrylate (HDDA), with long carbon chain, high reactivity, can be partially miscible with water, and can be compatible with the main resin Copolymerization reaction. The amount of HDDA is 10%, and in the aqueous solution, it will not produce irritating odor to the coating. When UV curing at 25 ℃, the polymerization conversion rate of pure PUA is only 30%; but the conversion rate of PUA + 10% HDDA has reached 70%; while UV curing at 80 ℃, the conversion rate exceeds 80% (Figure 2) It can be seen that the reactive plasticizer has a significant effect on the polymerization reaction kinetics of UV curing, and can reduce the hydrophilicity of the coating film. For example, the contact angle of PUA coating film with water is 50 °, and the contact angle of cured coating film with 10% HDDA increases to 57 °. If a small amount of fluorinated acrylate is added to the resin before the PUA is dispersed in water, the hydrophobicity of the coating film can be significantly improved.

Figure 2 Effect of HDDA addition on UV curing of water-based PUA coatings in dry air at 25 ℃ and 80 ℃, respectively

3.2.3 Effect of humidity on UV curing
Another convenient way to increase the molecular movement in the PUA dry film and thereby accelerate UV curing is to place the sample in humid air before UV curing, and absorb the moisture in the wet air through the hydrophilicity of the uncured PUA to change the coating film. Soft, reduce the resistance of molecular movement, and significantly increase the UV curing speed and polymerization conversion rate. For example, at the same UV dose (2 J · cm-2) at 25 ° C, UV curing, relative humidity RH = 26%, the polymerization conversion rate is only about 50%; and when RH = 100%, the conversion rate exceeds 80% (Figure 3) ), Humidity significantly increases the rate of photopolymerization and the degree of reaction.

Figure 3 Effect of humidity on UV curing of water-based PUA

3.3 Oxygen Inhibition Problem

The traditional free-radical photocuring reaction is strongly inhibited by oxygen, which is an obvious disadvantage. In practice, different methods must be adopted to overcome it. It is mainly the free radical R formed by oxygen and photoinitiator rapidly. The polymer free radical Pn · reacts to generate inactive peroxide radicals, which inhibits free radical chain reactions.

This inhibitory effect is particularly pronounced when the thin liquid film is UV cured, because oxygen diffuses easily into the thin liquid film. Water-based coatings are UV-cured on dry films, and it is much more difficult for oxygen to diffuse into solid dry films. Therefore, oxygen does not inhibit UV curing of water-based coatings. Not only does oxygen have no inhibitory effect, it has also been found that UV curing is stronger in the presence of air than in a pure nitrogen atmosphere (Figure 4). This anomalous positive effect of atmospheric oxygen on the UV curing of water-based PUA coatings has been observed by some scholars , possibly due to the formation of new, additional free radicals by the decomposition of peroxides:

Figure 4 Effect of air on UV-curable water-based PUA coatings

3.4 Effects of hydrophilic groups
3.4.1 Effects of carboxylic acids on UV curing
To investigate the effects of carboxylate groups (hydrophilic groups) on UV-curable water-based coatings, a series of experiments were performed. As previously described, three samples of PUA-1, PUA-2 and PUA-3, each containing 3% (mass) photosensitizer Irgacure 2959, were dried at 80 ° C to remove water and organic solvents, and 20 μm were obtained. Thick dry coating film, UV curing at room temperature and 40% relative humidity. When the UV dose reaches 2 J · cm-2, the polymerization conversion rate of PUA-1 is about 50%, and the conversion rate of PUA-2 is over 60%. PUA-2 is a solvent-based formula. After drying at 80 ℃ for 5 minutes, butyl acetate remains in the coating film (generally organic solvents can be removed only at 130 ℃ / 60 minutes). The coating film is soft and it moves molecules. The resistance is small, so the photopolymerization conversion rate is higher than PUA-1. PUA-3 is solvent-based and has no carboxylic acid group. Not only does the organic solvent remain in the coating film, the coating film is soft, but because there is no carboxylic acid group, there are fewer hydrogen bonding forces between the molecules. These two reasons The molecular movement resistance in its coating film is much smaller than PUA-1 and smaller than PUA-2, so the conversion rate of PUA-3 reaches 90% at the same UV dose (2 J · cm-2). In fact, after drying in a drying furnace at 80 ° C for 5 minutes, the Persoz hardness of the PUA-1 coating film has reached 240 s, while the PUA-3 hardness is only 66 s, confirming the foregoing analysis. The conversion rates of the UV curing of the three samples are shown in Fig. 5. When the UV light intensity is increased (I = 600 mW · cm-2), the three coatings are cured at room temperature, and the conversion rates of UV curing are different at a dose of 3 J · cm-2, but the Persoz hardness of the coating film is very similar It is about 350 s (Table 3). It shows that the UV-curable water-based PUA-1 coating has the same hardness as the solvent-based PUA-3. It only requires less acryloxy double bond polymerization and can show excellent scratch resistance. They are still very damaging, and they still show better flexibility and impact resistance because of their lower crosslink density.

Figure 5 UV curing of PUA-1, PUA-2 and PUA-3
Hardness of PUA-1, PUA-2 and PUA-3 after UV curing at 25 ° C

3.4.2 Effect of Carboxylate Group on Coating Hydrophilicity
The hydrophilicity of UV-cured PUA coatings is directly related to the carboxylate group content of the neutralized carboxylic acid groups. It is characterized by the water absorption of the coating film and monitored by the characteristic peak of IR 3 400 cm-1—OH group absorption. , Correspondingly determine the water absorption of the coating film. It can be seen from Figure 6 that the water absorption of PUA cured film neutralized with NaOH is the highest, and the water absorption of PUA neutralized with amine is significantly reduced, while the solvent-based PUA-2 neutralized with amine has a lower water absorption, without carboxylic acid groups. The PUA-3 has the lowest water absorption. This shows that the carboxylate group has a significant effect on the hydrophilicity of the coating film and is related to the type of neutralizer. Analyze from the following reaction formula:
This is a reversible reaction. If the neutralizing agent is volatilized during the drying and UV curing of the aqueous PUA coating, the reaction will move to the left, that is, a reversible reaction will occur, and the carboxylate group will The carboxylic acid group reduces the hydrophilicity of the coating film. Organic amines and carboxylic acid groups are salts formed by weak acids and bases. They are easy to decompose when heated, and the organic amines are volatile, meeting the conditions for reversible reactions, while NaOH cannot be volatile. Therefore, the neutralized PUA coating film is hydrophilic. Strong. At 25 ℃, PUA + amine neutralizer, UV curing is also very slow. When the UV curing temperature is increased to 80 ℃, exposure to UV for 3 s, the conversion rate is more than 80%; compared to PUA neutralized with NaOH at 80 ℃ The lower conversion rate is much higher (Figure 7), indicating that the type of neutralizer also has an effect on the UV curing reaction.

Figure 6 Effect of neutralizer on UV-curable PUA coating
Fig. 7 Effect of neutralizer on UV curing of water-based PUA coatings at 80 ℃

3.5 Performance comparison between UV-curable water-based PUA and traditional UV-curable coatings
Comparing water-based PUA coatings and typical traditional polyurethane-acrylic coatings (Laromer LR8987), under the same conditions, UV-curable coatings have slightly hardened cured coatings. Inferior, but the abrasion resistance, scratch resistance, chemical resistance and cupping test results are better than Laromer LR8987, the comparison results are shown in Table 4.

Properties of UV-curable polyurethane-acrylate coatings

Note: ①Measure the extinction rate after rubbing back and forth 50 times under “Skoch” wool rubbing load 750 g.
② Chemical resistance is characterized by the average print marks of 10 representative household chemicals (such as mustard oil, red wine, ink, etc.) after wetting the coating film. 5 = Coating film is severely damaged, 0 = No change in coating film.
③ Ko nig hardness of the coating film.
④ Cup protrusion test, large numbers reflect better flexibility of the coating film.

4 Conclusions
4.1 Advantages of UV-curable waterborne wood coatings
UV-curable coatings have been widely used for their advantages such as fast curing, low pollution and energy saving. Compared with traditional UV-curable coatings, UV-curable water-based coatings have the outstanding advantages that VOC can be zero, and the viscosity adjustment is easy without reactive diluents. Not only does it reduce the odor of reactive diluents and irritating to the human body, but also can obtain high hardness and The flexible coating is not affected by oxygen inhibition. The disadvantage is that drying furnace equipment is required, which increases the one-time investment and increases the energy consumption of curing, but the overall comparison is still better than the traditional UV curing process. Compared with water-based wood coatings, UV-curable water-based wood coatings have the advantages of lower VOC and fast curing, and can obtain a coating film with high hardness and flexibility. It has advantages in chemical resistance and scratch resistance, and can overcome the current water-based coatings. The shortcomings of wood coatings are a development direction of wood coatings.

4.2 Polyurethane-acrylate aqueous dispersion is the first choice for UV-curable waterborne wood coatings.

The resin-emulsion dispersion has fast UV curing speed, but the coating film is soft. Whether it is emulsified with external surfactant or internal emulsified, the resulting cured coating film is water-resistant. The difference is poor and is no longer used. Aqueous dispersion (also known as self-emulsifying) polyurethane-acrylate (PUA) aqueous dispersion has significantly better performance than emulsified dispersions, and also better than acrylate, polyester-acrylate dispersions. It can be UV cured to form high hardness Good coating film with good flexibility, chemical resistance and abrasion resistance. Although the UV curing speed is slow, there are many ways to accelerate the curing. Although there are hydrophilic group carboxylate groups, there are also feasible methods for improvement. Tests have confirmed that polyurethane-acrylate aqueous dispersions are the first choice for UV-curable waterborne wood coatings.

4.3 Ways to Improve UV Curing Speed ​​of Waterborne PUA
The process of UV curing waterborne wood coatings is divided into two steps: drying, dewatering and UV curing. UV curing of dry film is different from traditional UV curing of liquid coating film. The molecular movement resistance in solid film is large, UV curing speed is slow and curing degree (Aggregation conversion rate) is low. The effective method to increase the UV curing speed of water-based PUA is to increase the UV curing temperature, humidity, and add a reactive plasticizer. The purpose is to soften the dry film entering the UV curing and reduce the resistance of molecular movement in the film, thereby increasing the curing speed. After drying the water-based PUA coating film in a drying oven at 80 ℃ for 5 minutes, the hot sample enters the UV curing process. Infrared spectroscopy was used to simultaneously monitor the conversion of double bonds in the molecule to characterize the degree of UV curing reaction. The UV curing temperature was increased from 21 ℃ to 80 ℃, and the polymerization conversion rate was increased from 22% to 80%. Similar effects were obtained by placing in humid air or adding a small amount of reactive plasticizer hexanediol diacrylate before UV curing. If more than one improved method is used at the same time, the effect of complete conversion can be achieved.

4.4 The effect of hydrophilic groups and neutralizing agents is to stably disperse in water, introducing carboxylate groups into PUA molecules. Although the content is not high, it increases the hydrophilicity of the coating film. Using amine as a neutralizing agent, Volatilization during the drying process can change some hydrophilic carboxylate groups into weaker carboxylic acid groups, reducing the hydrophilicity of the coating film.

4.5 UV-curable waterborne wood coatings are not inhibited by oxygen.
UV-curable waterborne wood coatings are not inhibited by oxygen, and they cure faster in the air than in a pure nitrogen atmosphere. It can form new free radicals and accelerate the initiation reaction. This is superior to One of the main advantages of traditional UV curing, and the scratch resistance, chemical resistance and flexibility of the obtained coating film are also better than traditional UV curing coatings.

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