1 introduction

Powder coating began in the early 1940 s, in powder particle morphology of coating and coating. Powder coating in the process of production and coating without solvent release, less pollution, conforms to the requirements of environmental protection, coating process of eruption of powder can be recycled, is beneficial to save resources, powder coating, easy to implement automated production can improve the production efficiency, powder coating and coating of coating has become a fast-growing new products and new technology, has now become a recognized in 4 e coating product.

In recent years, along with the rapid development of aluminum industry, heat transfer powder coating (commonly known as wood grain powder coating) also get fast development, the wood powder on the market in China mainly polyester/TGIC type, the researchers also has a lot of powder on the a lot of research and a large number of applications on the aluminum extrusions, technology is mature. But as people demand increasing, polyester/TGIC ink powder has been gradually cannot satisfy the needs of customers, so it need more high-grade polyurethane wood grain powder coating instead. Relative to the polyurethane wood grain powder coating polyester/TGIC wood grain powder coatings, has good flow better, fullness, transfer printing texture clear more lifelike and non-toxic etc., so very good to satisfy the customers on the aspects of aesthetics, performance and health needs.

2 polyurethane wood grain powder coating the current application situation and characteristics

As a broad categories of powder coatings, polyurethane powder coating has a full coating surface smooth, good leveling, high hardness, good weathering resistance, chemical resistance and good anticorrosive and many other excellent performance. At present is mainly used in high-grade products such as refrigerators, washing machines and cars. In terms of thermal transfer, polyurethane coating rely mainly on the coating hardness is high, the crosslinking density, good film flow leveling and transfer printing design more clearly and application is obtained. In aluminum industry, mainly used for the production of high-grade polyurethane powder coating product, this big reason is due to the current domestic polyurethane curing agent mainly rely on import, the price is expensive, application in polyurethane wood grain powder can only small batch, and polyurethane wood grain powder mainly used more in terms of low gloss. For specular polyurethane wood grain powder on aluminium profile: the main reason for the less used in a high gloss on the surface microscopic more level off, can cause tearing more difficult. And with the polyester/TGIC highlights the application wood grain powder on aluminium profile, because such powder whole upper flat is not very good, feel is rough, so customer need a higher grade of powder coating to meet the needs of the consumers. This paper mainly discusses a specular polyurethane wood grain powder coatings applied to aluminum.

3 aluminum with high polyurethane wood grain powder coating formulation

This article "highlights" refers to using the gloss meter in 60 DHS gloss of Angle measurement unit is in 80% above. Aluminum heat transfer way at present is mainly through vacuum heat transfer method, for heat transfer powder, you must ensure that its not sticker, you need the flow better, good mechanical properties and water resistance is excellent to suitable for application in aluminium profile. Polyurethane wood grain powder is mainly composed of hydroxyl resin and closed many isocyanate, and the Chinese market closed many isocyanate mainly imported degussa B1530 mainly produced by our company, there are kinds of hydroxyl resin at home and abroad, roughly divided into three categories, namely low hydroxyl value resin (general hydroxyl value in the 30-50 mgkoh/g between mgkoh/g), hydroxyl value of resin (hydroxyl value in commonly 90 mgkoh/g - 110 mgkoh/g) and high hydroxyl value resin (general hydroxyl 280 mgkoh/g - 320 mgkoh/g). So I need to study the effect of different hydroxyl value resin on the coating.

From table 1, hydroxyl value of hydroxyl resin size on the influence of the coating is very large, as a whole are good flowing property of polyurethane powder, but with the increase of hydroxyl value, its mechanical performance worse and worse, hardness is higher and higher, the heat transfer effect is gradually getting better, boiling water resistance variation. Formula 2 # and 3 # and comparison, the hydroxyl value crosslinking density and hardness of the resin has been basically can guarantee thermal transfer effect is good, and high hydroxyl value resin has poor mechanical properties and low luster and the characteristics of the boiling water resistance is poor, so don't consider the preparation of specular polyurethane wood grain powder resin with high hydroxyl value. So for formula 1 #, just solve the problem of the transfer printing effect is not very good, can be designed to highlight the wood grain powder coating. For formula 2 #, as long as the solution to the problem of the boiling water resistance is bad also can highlight wood grain powder is prepared. For formula 1 #, we can through the screening of different manufacturer's low hydroxyl value resin coating effect if there is a difference, also can consider to increase hard wax powder was added to improve, or join organic tin promoter to improve curing degree. The exploring experiment, such as table 2 and table 3.

Can be seen from table 2, by comparing the formula 4 #, 5 #, 6 #, import A resin coating hardness is A little high; Low hydroxyl value preparation of polyurethane powder resin transfer printing effect is defective; There was a slight difference in the quality of different manufacturer of transfer paper (in this little discussion); Relative imports more conducive to the preparation of A resin highlights polyurethane wood grain powder; Boiling water resistance of low hydroxyl value resin are all well. So selected 5 # formula based on the formula, and then through some additives to further improve the coating hardness and crosslink density. On one hand we increased by adding a hard wax powder coating to improve the surface hardness, on the other hand, as a result of the powder coating curing degree can't completely reached 100%, so join the organotin catalyst can increase the speed of polymerization, then cured at a certain time, equivalent to improve the degree of cure, bring it closer to 100%, thus increase the crosslinking density of the coating. The exploring experiment in the following table 3.

Can be seen from table 3, choose the appropriate increase hard wax powder can improve the coating hardness, organic tin promoter on the improvement of the film heat transfer effects play a role. Formula # 8 and formula of performance in 9 # meet the requirements of aluminum with high polyurethane wood grain. So the improvement of formula 1 # has reached the expected effect, to focus on the improvement of the formula 2 # below. In the hydroxyl value of resin system highlights polyurethane wood grain system, because of its low hydroxyl value significantly greater than hydroxyl value resin, so according to the above experiment is known by its crosslinking density and coating hardness has reached the requirement of transfer printing powder, can guarantee the transfer printing effect is excellent. According to the results of the experiment formula 2 #, defects of this formula is boiling water resistance is not good. Hydroxyl value in our analysis may be due to the resin of hydroxy, relative to the low hydroxyl value, hydroxyl value of the resin, if the two curing degree at the same time, relative residual hydroxyl value resin in the hydroxyl in the coating is more, and hydroxyl is hydrophilic group, so the hydroxyl value resin in the boiling water resistance is not good. So in order to solve the problem of hydroxyl value polyurethane wood grain powder boiled in coating must be reduced the residual hydroxyl content after curing, means to improve the coating of curing degree. So we can through comparing different manufacturer of resin on the coating, influence of adding catalysts to increase the degree of curing and resin compound with low hydroxyl value to reduce the end of the hydroxyl content in the coating. The exploring experiment, such as table 4.

Can be seen from table 4, different manufacturers in the boiling water resistance of hydroxyl value resin were similar, means that the curing degree difference degree still can't pass to distinguish the results of the boiling water resistance. According to table 3 experiments, adding organic tin catalyst in a certain extent, can increase the crosslink density, but with 11 # and # 13 two groups of the experimental results show that the increase of crosslinking density is not enough, still remain in the film boiling water resistance caused a certain amount of hydroxyl is not qualified. Then we by compound with low hydroxyl value resin coating can decrease the hydroxyl content to improve the water resistance, compound with hydroxyl value of the low hydroxyl value of resin selection on the high side some resin as far as possible, so that can ensure that do not produce larger reaction rate is poor and can not reduce the crosslinking density of too many, so as to meet the requirements of high gloss and transfer printing with good effect. With the increase of resin content, low hydroxyl value, the surface hardness is slightly lower, flow leveling basically no change, transfer printing effect is not obvious change and are better, boiling water resistance improved. So considering the boiling water resistance and heat transfer effect, choose low hydroxyl value resin can be prepared with hydroxyl value in resin 1:1 to meet the demand of polyurethane powder. According to the above experiments and analysis, we concluded that formula 8 #, 9 # and 15 # with high gloss, resistance to boiling water can be prepared good performance, mechanical performance is excellent and transfer the polyurethane wood grain powder coatings with good effect. The three groups of 8 # and 9 # in the recipe is close, and 15 # some of the differences in cost and transfer printing effect. 8 # and 9 # formulation has the advantage of relative to 15 #, its cost is low, a more excellent boiling water resistance; 15 # formulation has the advantage of a slightly higher surface hardness of the coating, crosslink density is larger, slightly better heat transfer effect. So in practical application, need according to the spray process and the quality of the transfer paper used to design the formula, if transfer paper quality is better, curing time is enough, can choose # # 8 or 9 formula production, and is beneficial to reduce the cost, also can help improve the water resistance of aluminum finished products; If there are any transfer paper quality a little poor, would need to refer to 15 # formula structure for debugging.

4. To summarize

Above all, for pu heat transfer powder formula design need to pay attention to the following:

1. The need to design reasonable formula to make coating can fully cured;

2. Increase the crosslink density and the coating hardness when should pay attention to its flat, luster of convection and the influence of the boiling water resistance;

3. The design formula through the quality of the transfer paper and transfer printing technology and reasonable debugging formula, make its cost, the heat transfer effect and the coating appearance to achieve the best balance.

Polyurethane wood grain powder coating for its excellent coating appearance, smooth feel and non-toxic overall excellent performance is knowing and accepting by people gradually. In the domestic demand is also more and more big, can foresee the development prospects are very broad. At present the main factor of restricting its development is its curing agent mainly rely on import, its expensive price, not a wide range of alternative type polyester/TGIC powder coatings. So now you need to relevant personage to develop domestic alternative to expensive imported polyurethane curing agent, polyurethane powder get better development.