Wen Feng, Chen Jun, Liao Li, Wang Hui
(Water-borne Coatings Laboratory, Technical Center of Wuhan Linuo Chemical Group, Wuhan 430035, Hubei Province)
Abstract:This paper studies the selection of water-borne PU in the market, and investigates the relation between coatings properties and the sorts and amounts of water-borne PU resin and additives, focusing on the relation between additives and application properties. It is showed that the water-borne Wood varnish is of superior comprehensive properties and can be used widely in house decoration.
water-based coatings originated in Europe, and gradually the whole The development and promotion of the world, from the first preparation of water-based cationic polyurethane to the two-component water-based polyurethane used in wood coatings, has experienced a development process of more than 60 years . With the restrictions on VOCs and environmental protection in various countries, solvent-based wood coatings are facing unprecedented challenges, and the development of water-based wood coatings is inevitable . Water-based polyurethane coatings are widely used due to their outstanding mechanical properties such as abrasion resistance, gloss, and adhesion, and they are becoming more and more popular. Compared with high solids and solvent-free products, water-based polyurethane has attracted attention from Europe, America and Japan [3-4]. In this test, the relationship between the types and amounts of substrate wetting agents, film-forming aids, defoamers, and thickeners and varnish performance was systematically investigated through screening of raw materials. The effects of coating additives on coating construction performance were revealed.
1 Test section
1.1 Test materials
Water-based polyurethane samples, substrate wetting agents, film-forming aids, defoamers, thickeners, wax emulsions, deionized water, etc.
1.2 Testing methods
Pencil hardness is tested according to GB / T 6739; gloss is tested according to GB / T 9754; water resistance, alcohol resistance, and storage stability are tested according to GB / T 23999-2009; drying time is according to GB / T1728-1979 carry out testing.
2 Results and discussion
2.1 Screening of water-based polyurethane resins and performance test of varnishes
This test selected the more common water-based polyurethanes on the market and tested its basic properties according to GB / T 23999-2009. The results are shown in Table 1.
As can be seen from Table 1, the resins A, C, and D have excellent water resistance, ethanol and other properties, while the pencil hardness, adhesion and other properties are outstanding, the coating film has a high degree of fullness and a good feel. This test is based on the above performance comparison, after comprehensive consideration of price and performance, the final screening resin D is used as the test resin.
2.2 Selection of substrate wetting agent
Both the surface of the substrate and the coating formulation will affect wetting. In principle, if the surface tension of the liquid is less than the free energy of the substrate surface, the solid surface can always be well wetted. Water-based wood coatings contain a large amount of water, and the surface tension of water is 72 mN / m, so we need to use a substrate wetting agent to reduce the surface tension of the coating so that it can spread well on the substrate. These have surface activity The substance will preferentially adhere to the phase boundary, thereby reducing the surface tension. The substrate wetting agent molecule contains both hydrophobic and hydrophilic groups. The non-polar part of the molecule stays in the direction of the liquid facing the air, and the polar part stays in the water phase. The types of substrate wetting agents include: anionic surfactants, non-ionic surfactants, polyether-modified polysiloxanes, and the like. This test compares several common substrate wetting agents on the market, the amount of which is 0.5% (mass) of the coating amount, and examines the effect of the type of wetting agent and coating performance. The results are shown in Table 2.
It can be seen from Table 2 that substrate wetting agents B and C have excellent ability to reduce surface tension, meanwhile, spreading performance and foam stabilization performance are good, and they can be well compatible with the system, but substrate wetting agent B has The obvious price advantage, so in this test B is used as a substrate wetting agent, the amount of which is 0.5% (mass) of the total formulation.
2.3 Effect of the types and dosages of film-forming additives on coating properties
Film-forming aids are used to reduce the glass transition temperature of polymers. After the film formation process of the latex particles is completed, the film-forming aid can be volatilized from the coating film at a relatively fast rate, so that the polymer glass transition temperature value is restored to the initial value to maintain the mechanical strength and hardness that the coating film should have. If the coating layer is very thick or the amount of the film-forming aid is too large, it will be difficult for the film-forming aid to completely volatilize. The film-forming aid remaining in the coating will act as a permanent plasticizer of the coating film, which will change the coating film. soft. According to the hydrophobicity and hydrophilicity of the film-forming auxiliary agent, it can be divided into three categories: hydrophobic hydrocarbon-based film-forming auxiliary agent, hydrophilicity stronger than pure hydrocarbon-based film-forming auxiliary agent, and hydrophilic film-forming auxiliary agent. In this test, the effects of four film-forming additives, namely propylene glycol, dipropylene glycol methyl ether, dipropylene glycol butyl ether, and ethylene glycol monobutyl ether, on the performance of the coating film were investigated. The test results show that a mixture of hydrophilic ethylene glycol monobutyl ether (BCS) and lipophilic dipropylene glycol butyl ether (DPnB) is used. When n (BCS): n (DPnB) = 2: 1, the coating film is used. The best film-forming performance (It is worth noting that in the future, the film-forming additives of the glycol ether series will be limited). We used Resin D as the base resin and investigated the effect of the amount of mixed film-forming aids on the coating film performance. The results are shown in Table 3.
It can be seen from Table 3 that with the gradual increase in the amount of film-forming aids, the coating can not form a film until the end can form a better performance coating film; but when the amount of film-forming aid is too large, the drying of the coating film is seriously affected time. After comprehensive consideration, the amount of mixed film-forming aids is set to 5% of the total formula.
2.4 Effect of defoamer on coating performance
In water-based formulations, the role of the defoamer is to destroy large bubbles on the surface and prevent a large amount of air from stagnating, and to remove air dispersed in the coating film as soon as possible during construction. Therefore, the choice of defoaming agent should take into account the two functions of defoaming and defoaming . In this test, the type and amount of the antifoaming agent were examined. The results are shown in Table 4.
It can be seen from Table 4 that the mineral oil defoamer is incompatible with the waterborne wood coating system, and at the same time, it has a greater impact on the gloss and transparency of the varnish, and is not suitable for use. The polyether siloxane copolymer emulsion should be selected as the defoamer in waterborne wood coatings. At the same time, the defoamer D with better defoaming performance can be used to achieve better appearance and construction effect. The performance of the defoamer C is 0.5% and the defoamer D is 0.3%.
2.5 Selection of
thickener Thickener can adjust the viscosity and rheology of water-based coatings, and can also improve the thixotropy of the coating, making it easy to apply and reducing dripping and sagging. Common thickeners for water-based coatings are cellulose ethers, polyacrylates, associative thickeners, and inorganic thickeners. There are two types of thickening mechanisms for water-based coatings: (1) associative thickening, thickening through the non-specific interaction between the hydrophobic end groups of the thickener molecule and itself and other components in the coating. The effect of thickening is It is produced by the interaction of hydrophobic end groups with other components in the formula; (2) Non-associative thickening, which is thickened by the winding of water-soluble high molecular weight polymer chains. The effect of thickening depends mainly on the polymer Molecular weight. In this test, the non-ionic polyurethane thickener tego-3000 was used, and the influence of its amount on storage stability and construction performance was investigated. The results are shown in Table 5.
It can be known from Table 5 that when the amount of thickener is low, the storage stability of the coating is not good; as the amount of thickener is increased, the storage stability of the coating becomes better, but the workability and coating leveling properties of the coating become Poor, so the amount of thickener is 0.4% to 0.6% is the best.
2.6 Varnish performance test
2.6.1 Basic formula of
varnish See Table 6 for basic formula of varnish.
2.6.2 Production process
Add 1 to 4 components to the high-speed disperser, and stir evenly for 15 minutes at a speed of 900 r / min; then slowly add 5 to 6 components, stir for 25 minutes, and finally add 7 to 9 components in sequence. Stir and filter Out of the material, water-based polyurethane wood coatings are obtained.
2.6.3 Product performance test The
basic performance of high-gloss varnish is shown in Table 7.
This test prepared a single-component high-gloss wood varnish. The relationship between water-based resins, water-based additives, and other factors and the performance of the varnish was systematically investigated. The performance of wood varnish was tested. The test results show that resin D, substrate wetting agent B, defoaming agents C and D can be used to prepare waterborne wood varnishes with excellent performance. The varnish has excellent comprehensive properties, especially gloss, water resistance, and alcohol. And has a higher hardness. With the continuous updating of waterborne technology at home and abroad, waterborne wood coatings have also ushered in a period of rapid development. The waterborne wood coatings can replace solvent-based products and are widely used in the field of home improvement.