WO2017131536A1

WO2017131536A1 - Blocked polyisocyanates, process for preparing blocked polyisocyanates, and their use as crosslinking agents for polyurethane powder clear coatings

Info

Publication number: WO2017131536A1
Application number: PCT/PL2016/000014
Authority: WO
Inventors: Barbara PILCH-PITERA; Piotr KRÓL; Łukasz BYCZYŃSKI; Joanna WOJTURSKA
Original assignee: Politechnika Rzeszowska im. Ignacego Łukasiewicza
Priority date: 2016-01-26
Filing date: 2016-02-11
Publication date: 2017-08-03
Also published as: PL415905A1; PL238515B1

Abstract

The present invention relates to blocked polyisocyanates and a process for the synthesis of the blocked polyisocyanates as crosslinking agents for polyurethane powder coatings. Blocked polyisocyanates of the present invention after crosslinking with resins used for powder systems produce polyurethane coatings showing increased hydrophobicity, improved abrasion and scratch resistance than conventional polyisocyanates used for polyurethane powder clear coatings.

Description

Blocked polyisocyanates, process for preparing blocked polyisocyanates, and their use as Crosslin king agents for polyurethane powder clear coatings

The present invention relates to blocked polyisocyanates, the process for preparing the blocked polyisocyanates and their use for the preparation of polyurethane powder clear coatings. The invention relates to the field of powder clear coating for painting car bodies and car parts, bicycle and motorcycle frames, household appliances, metal earmarked for playgrounds, metal structures, agricultural machinery, metal nets, sheets, tubes, rods, cans, cameras such us cameras for medical devices, and others.

Typical polyurethane powder coating systems contain a solid polyisocyanate component with blocked isocyanate groups (PIC), the solid polyester or a polyacrylate resin terminated with hydroxy! groups, a catalyst, auxiliaries (charging additives, degassing agents, flow control agents), and various additives depending on the application (pigments, fillers, corrosion inhibitors, biocides, flame retardants, antioxidants).

Of great practical significance are hydrophobic coatings having tow surface free energy that are easy to clean or provide self-cleaning effect, and have an increased resistance to mechanical, chemical, thermal and aging effects, regardless of the area of their potential application. For example, the use of the aromatic diisocyanates for coatings is limited because of the lower resistance of the produced coatings to atmospheric conditions hence they can be used only in coatings for indoor use.

The coatings prepared from polyurethane systems having increased hydrophobicity are obtained in a reaction of polyisocyanates with resins with embedded siloxane or fluorineaikyl segments or by crosslinking of specially selected resins with polyisocyanates substituted with fluorineaikyl or siloxane (W. Wu, Q. Zhu, F. Qing, Han CC: Langmuir 2009, 25, 17. Wang L.-F .: Polymer 2007, 48, 894). The incorporation of siloxane groups or fluorine into polyisocyanates via ailophanate groups in order to reduce the surface tension of the polyisocyanates and the surface free energy of the resulting polyurethane coatings is known from US5541281 , US5574122, US5576411 , US5646227, US5691439, and US5747629. A disadvantage of the polyisocyanates disclosed in these patents is that they are obtained by reacting an excess of monomeric diisocyanates with the compounds containing either fluorine or siloxane groups. After the reaction is terminated the unreacted monomeric diisocyanates must be removed by an expensive thin film distillation process.

PL/EP1919974 discloses a process for incorporation of poly(dimethyisiloxane) alcohol (Silaplane FM 0411) in an amount of 1 % by weight into polyisocyanates produced with isophorone diisocyanate (IPDI), containing urethane groups and ailophanate groups. As a result of modification surface free energy (SFE) of polyisocyanates decreased from 0.047 J/m² to 0.024 J/m². PiC used for one- component moisture-cured coatings and two-component coatings contributed to the reduction of the SFE from 0.033 J/m² to 0.031 J/m² at 0.34% -Si-O- group content. The authors also describe the possibility of employing polyisocyanates having low SFE as concentrates for diluting unmodified polyisocyanates.

EP0848024 discloses a method of incorporation of poly(dimethylsiloxane) alcohol into polyisocyanates prepared from 1 ,6-diisocyanatohexane (HDI) or isophorone diisocyanate (IPDI), comprising preparation of urethane in first step by reacting a diisocyanate with an alcohol and subsequently ailophanate groups. SFE of two- component solvent coatings obtained from synthesized PIC, having a siloxane group content of 0.06%, decreased from the value of 0.0315 J/m² to 0.0217 J/m².

For coatings described in the patent US5574122 obtained from crosslinking of a mixture of polyacrylic and polyester resins with potyisocyanate containing ailophanate groups with incorporated poly(dimethylsiloxane) alcohol having a molecular weight of 1000, SFE was reduced from the value of 0.0437 J/m² to 0.025 J/m². Similar values of SFE were obtained in case of PIC-curable coatings with integrated poly(dimethylsiloxane) and poly(oxyethylene) copolymer of molecular weight 1200 comprising two hydroxyl groups, as disclosed in patent US5646227. The technical problem to be solved by the present invention is to propose such a modified polyisocyanate and its synthesis method that could be used in the manufacture of improved powder systems, wherein after the crosslinking of conventional resins used for powder systems with this modified polyisocyanate it will allow to obtain polyurethane coatings of high hydrophobicity, low surface free energy and other valuable properties, including improved resistance to abrasion and scratching than conventional crosslinked products of polyisocyanates used for powder coatings. It is also desirable that produced polyurethane based powder system is compatible with common processing and coating methods, in particular by Tribo and Corona. Surprisingly, these technical problems have been solved by the present invention.

The present invention relates to blocked polyisocyanates comprising 4-25 wt%, polysiloxane, 49.5-63.5 wt% an isocyanate component and 25.5-32.5 wt% blocking agent.

Preferably, the blocked polyisocyanates comprise 19 wt% polysiloxane, 53.6 wt% an isocyanate component and 27.6 wt% blocking agent.

Preferably, the polysiloxane is a linear polysiloxane moiety having at both ends hydroxyalkyl or hydroxyalkoxy groups or terminated at one end with two hydroxyl groups, and a molecular weight in the range from 500 to 10000 or from 800 to 5000.

Preferably, the isocyanate component is selected from the group consisting of isophorone diisocyanate (IPDI), 1 ,6-hexamethylene diisocyanate (HDI), 2,2,4- and 2,4,4-trimethyl-l ,6-hexamethylene diisocyanate (TMDI) 4,4'-dicyclohexylmethane diisocyanate (H12MDI), 1 _t4-bis(isocyanatomethyi)cyclohexane, 1 ,3,5-tris (isocyanatomethyl)cyclohexane, 2,4- and 2,6-toluene diisocyanate and 3,5-tris (isocyanatomethyl)benzene, m- and p-tetramethylxylylene diisocyanate (T XDI).

Preferably, the blocking agent is selected from the group consisting of ε- caprolactam, diethyl malonate, oximes or 3,5-dimethylpyrazole.

Another object of the invention is a process for the preparation of blocked polyisocyanate for polyurethane powder coatings having following steps: a) reacting isocyanate component monomers in the presence of a biuretizing agent to form ureapolyisocyanate b) performing biuretization of ureapolyisocyanate obtained in step a) c) performing addition of the polysiloxane having at both ends hydroxya!kyl or hydroxyalkoxy groups or terminated at one end with two hydroxyl groups d) blocking the free isocyanate groups of the poiyisocyanate with a blocking agent.

In a preferred embodiment step a) is carried out by placing the isocyanate component and at least one catalyst in an amount of 0.10 wt% - 0.15 wt% in the reactor, heating to a temperature below 65°C, said biuretizing agent is dosed in an amount from 0.1 mol to 0.4 mol per 1 mol of the isocyanate component, preferably in an amount from 0.2 mol to 0.3 mol per 1 mol of the isocyanate component, and then in step b) heating the mixture to a temperature below 140°C and maintaining until completion of biuretization of isocyanate component.

Preferably, in the step c) polysiloxane is added in an amount from 1 wt% to 40 wt%, preferably from 3 wt% to 25 wt%.

Preferably, in the step d) after adding the blocking agent the mixture is heated to a temperature below 65°C until any free isocyanate groups are present. in another preferred embodiment of the invention the isocyanate component is selected from the group consisting of isophorone diisocyanate (IPDI), 1 ,6- hexamethylene diisocyanate (HDI), 2,2,4- and 2,4,4-trimethyl-1 ,6-hexamethylene diisocyanate (T DI) 4,4^,-dicyclohexy!methane diisocyanate (H ₂MD1), 1 ,4- bis(isocyanatomethyi)cyclohexane, 1 ,3,5-tris (isocyanatomethy!)cyclohexane, 2,4- and 2,6-toluene diisocyanate and 3,5-tris (isocyanatomethyl)benzene, m- and p- tetramethylxylylene diisocyanate (TMXDI).

Preferably, the catalyst is an organic phosphorus compound, an organometailic compound or an amine compound selected from the group comprising dibutyl phosphate, dibutyltin dilaurate, tin octoate, triethylamine, triethylenediamine. Preferably, said biuretizing agent is water, formic acid or alcohol.

In a further preferred embodiment, the polysiloxane is a linear polysiioxane moiety having at both ends hydroxyaikyl or hydroxyalkoxy groups or terminated at one end with two hydroxy! groups, and a molecular weight in the range from 500 to 10000 or from 800 to 5000.

Another object of the present invention is blocked polyisocyanate obtained by the process as described above.

Another object of the present invention is polyurethane powder coating comprising blocked polyisocyanate obtained by the method described above.

Another object of the invention is the use of the blocked polyisocyanate as described above or obtained by the above method for the preparation of polyurethane powder systems.

Surprisingly, it was found that a significant increase in the hydrophobicity of the powder coatings may be achieved by use as crosslinking agents blocked pofyisocyanates containing biuret bonds that are modified with polysiloxanes containing hydroxyaikyl groups or hydroxyalkoxy groups and terminated at one end with two hydroxyl groups.

In the reaction, it is also possible to use all of the higher molecular weight polyisocyanates that are based on the above mentioned diisocyanates and have isocyanurate, uretdione, iminooxadiazinedione, oxadiazinetrione, urethane, allophanate and/or carbodiimide structures. These polyisocyanates and their modes of preparation are described for example in J. Prakt. Chem. 336 (1994) pp. 185-200.

All steps of the synthesis process of the modified PIC are carried out in the same reactor designed for conventional chemical synthesis equipped with a stirrer, reflux condenser, thermometer, heating mantle, and nitrogen blanket. In the reactor are placed diisocyanate and catalysts in an amount of 0.10-0.15%, heated to a temperature not exceeding 65°C and said biuretizing agent is slowly dispensed in an amount of 0.4-0.1 mol, preferably 0.3-0.2 mol per 1 mole of diisocyanate. As the catalysts generally known in the polyurethane industry organic compounds of phosphorus or organometallic compounds may be used (see J. Prakt. Chem. 336 (1994) pp. 185-200, EP0157088 and EP0716080).

After the introduction of a biuretizing agent progress of the reaction is monitored for example by acid base titration method determining the content of unreacted NCO groups. After the first stage, the mixture in the reactor is heated to the temperature not exceeding 140°C and maintained at this temperature until biuretization of IC is completed. Upon completion of this step, the polysiloxane is added in an amount of 1-40%, preferably 3-25%. Preferred is the use of linear polysiloxanes terminated on both sides with hydroxyalkyl or hydroxyalkoxy groups and terminated at one end by two hydroxyl groups, and of a molecular weight in the range 500-10000, preferably 800-5000. Use of a polysiloxane terminated with hydroxyalkyl moiety with two hydroxyl groups at one end of the chain is particularly prefered because this compound is incorporated into the chain of a polyisocyanate only with one end, while the other remains unbound and is non-reactive chemically, and therefore has a greater possibility of mobility and migration to the surface of the coating thus bringing properties typical for silicones. in the final stage -NCO free groups are blocked by adding a blocking agent and the mixture is heated at a temperature not exceeding 65°C until any -NCO groups are present. The final product in the highly viscous liquid form is poured from the reactor, cooled to room temperature and crushed. Synthesized polyisocyanates are used as crosslinkers for the preparation of powder clear coatings, which are subsequently used for forming coatings.

To produce powder clear coating a modified PIC is mixed with conventional polyester resin having LOH in the range of 30-45, preferably 35-40 mg KOH/g, Tg = 50-65X, preferably 55-60°C, for example Rucote 102 or Sirales 6110, and degassing additives, the catalyst and flow control additives. Weighed amounts of raw materials are premixed by i.e. an electric mixer and the resulting powder is extruded. The extruded blend is crushed and fine grinding in a grinding mill, and then in order to remove oversized particles it is sieved through a sieve having a mesh in the range of 30-200 μηη, preferably 60-150 pm. Such prepared powder clear coating can be used for painting various elements intended for use outdoors e.g. by means of special charging guns Corona or Tribo or other techniques, in the case of application by Tribo gun in order to improve the charging, the tribo- charging additives shall be added. The painted elements shall be placed in an oven at 140-170°C for 10-30 minutes in order to cure the film.

The powder compositions of the invention are transparent and colorless. If necessary, coloring may be introduced by addition of conventional pigments used in the powder coating industry, e.g. titanium dioxide (rutile).

The modified po!yisocyanate according to the present invention, after crosslinking with available resins used for commercial powder systems allows to obtain polyurethane coatings that are more hydrophobic and have better resistance to abrasion and scratching than products crosslinked by conventional polyisocya nates used for powder paints and varnishes. In particular, a modified polyisocyanate and obtained on its basis powder system can be characterized by the following features: improved flow, reduced roughness, higher gloss, higher contact angle, reduced surface free energy, better abrasion resistance, higher impact resistance, higher cupping and excellent adhesion to the substrate, in particular steel.

All of these advantageous properties of the resulting products allow to use the present invention for coating metal parts for outdoor spaces in different sectors of the economy, particularly in the automotive industry (painted car body frames of bicycles and motorcycles), the industry producing household (painting refrigerators, washing machines, everyday objects), construction (painting sheets for roof coverings, radiators, fagades, metal constructions, e.g. earmarked for bridges and rails), marine (bottoms and other underwater parts of boats), machinery (painting parts of machines).

The invention is illustrated by the following examples.

Example 1 - method of synthesizing polyisocyanate (comparative) To the three-necked flask equipped with stirrer, reflux condenser, thermometer, dropping funnel and nitrogen inlet tube was introduced isophorone diisocyanate (111.10 g; 0.500 mol) and the catalysts: dibutyl phosphate and triethyiamine (0.11 g; 0.1 wt% with respect to the diisocyanate). With the use of dropping funnel to the mixture for 0.5 hours were added formic acid (5.75 g, 0.125 mol) at 60°C. Then, the reaction mixture was maintained at 60°C at vigorous stirring until the isocyanate groups content dropped to 27 wt%. Afterwards, the temperature was increased to 140°C. The reaction mixture was maintained at this temperature with vigorous stirring until the isocyanate groups content dropped to 19 wt%. In the next step, the mixture was cooled to 65°C, ε-caprolactam (49.74 g, 0.440 mol) was added and heated at 65°C with vigorous stirring until disappearance of the isocyanate groups. After cooling to room temperature, a straw yeilow, transparent solid polyisocyanate was obtained with the following characteristics:

• NCO content after unblocking = 19 wt %,

• average functionality (determined by gel permeation chromatography (GPC)) - 3.87,

• viscosity at temp. 120°C = 10 Pa s at a shear rate of 200 s^" .

Example 2 - method of synthesis of the polysiloxane-modified polyisocyanate

To the three-necked flask equipped with stirrer, reflux condenser, thermometer, dropping funnel and nitrogen inlet tube was introduced isophorone diisocyanate (11 .10 g; 0.500 mol) and the catalysts: dibutyl phosphate and triethyiamine (0.11 g; 0.1 wt% based on diisocyanate). With the use of dropping funnel to the mixture for 0.5 hours were added formic acid (5.75 g, 0.125 mol) at 60°C. Then, the reaction mixture was maintained at 60°C at vigorous stiring until the isocyanate groups content dropped to 27 wt%. Afterwards, the temperature was increased to 140°C. The reaction mixture was maintained at this temperature with vigorous stirring until the isocyanate groups content dropped to 17 wt%. In a next step the reaction mixture was cooled to temp. 65°C and poiysiloxane X-22-160AS (Shin Etsu) (30.6 g) was added. The reaction mixture was maintained at this temperature with vigorous stirring until the isocyanate groups content decreased to 13.5%. After cooling the mixture to 65°C, ε-caprolactam (46.47 g, 0.411 moi) was added and heated at 65°C with vigorous stirring until disappearance of the isocyanate groups. After cooling to room temperature, a straw yellow, transparent solid polyisocyanate was obtained with the following characteristics:

• NCO content - 13.5 wt%

• average functionality (GPC) - 3.38,

• viscosity at temp. 120°C - 3 Pa · s at a shear rate of 200 s^"1

• the contents of the polysiloxane - 19 wt%.

An example of the PIC content: 19 wt% polysiloxane, 53.6 wt% diisocyanate 27.6 wt% ε-caprolactam. Using this PIC as a cross!inking agent we obtain a coating with a content of 4.5 wt% polysiloxane.

Example 3 - a method of producing powder clear coating system

Powder coating is made by mixing produced powder polyisocyanate (136.30 g) with a polyester resin Sirales 6110 (552.00 g), catalyst (WorleeAdd ST-70 (0.5%)), a flow control agent (Resiflow PH-240 (3%)) and degassing agents (WorleeAdd 902 (1.5%) and benzoin (1%)). The mixed components are premixed, and then extruded through a co-rotating twin-screw extruder at 125°C at a screw rotation speed of 150 rpm, and then fine grinding in a pin disc mill at a rate equal to the rotor speed 11000 rpm. The milled powder was screened using a vibratory screening machine on a sieve with a mesh diameter of 100 m.

Example 4 - way of making cured coatings

Coatings were made by applying a sieved fraction of the powder by electrostatic method using a manual gun PEM X-1 with integrated electrode, and the controller EPG Sprint X into standardized steel panels and curing them at 170°C for 20 min. The content of polysiloxane in the coating was 4.5 wt%. Obtained in the described embodiments samples of powder paints and coatings shown properties presented in Table 1 that are listed with a comparative sample. This statement confirms that better properties are obtained for coatings made from the poiyisocyanate modified with polysiloxane in comparison with coatings obtained using classical poiyisocyanate.

Table 1 comparative

a measured parameter 4,5% polysiloxane X-22-160AS sample

Flow (levelling), mm 7,0 27,7

roughness (R R₂), μηι 1 ,16/3,18 0,46/1 ,87

gloss, 60°, % 61 ,5 79,0

contact angle, deg 90,4 102,5

surface free energy mJ/m² 38,77 18,43

abrasion resistance kg/μηη 0,49 0,73

impact resistance, cm 30 50

cupping, mm 7,0 11 ,4

adhesion to steel substrate,

range 0-5 0 0

(0-best, 5-worse)

Claims

1. Blocked polyisocyanates containing 4-25 wt%, polysiloxane, 49.5-63.5% an isocyanate component and 25.5-32.5 wt% blocking agent.

2. Blocked polyisocyanates according to claim 1 characterized in that it comprises 19 wt% polysiloxane, 53.6 wt% an isocyanate component and 27.6 wt% blocking agent.

3. Blocked polyisocyanates according to claim 1 , characterized in that the polysiloxane is a linear polysiloxane moiety having at both ends hydroxyalkyi or hydroxyalkoxy groups or terminated at one end with two hydroxy! groups, and a molecular weight in the range from 500 to 10000 or from 800 to 5000.

4. Blocked polyisocyanates according to claim 1 , characterized in that the isocyanate component is selected from the group consisting of isophorone diisocyanate (IPDl), 1 ,6-hexamethylene diisocyanate (HDI), 2,2,4- and 2,4,4- trimethyl-1 ,6-hexamethylene diisocyanate (TMDI) 4,4 -dicyclohexylmethane diisocyanate (H12MDI), 1 ,4-bis(isocyanatomethyl)cyclohexane, 1,3,5-tris (isocyanatomethyl)cyclohexane, 2,4- and 2,6-toluene diisocyanate and 3,5-tris (isocyanatomethyl)benzene, m- and p-tetramethylxyiylene diisocyanate (TMXDI).

5. Blocked polyisocyanates according to claim 1 , characterized in that the blocking agent is selected from the group consisting of ε-caprolactam, diethyl malonate, oximes or 3,5-dimethylpyrazole.

6. A process for the preparation of blocked polyisocyanate for polyurethane powder coatings with following steps: a) reacting isocyanate component monomers in the presence of a biuretizing agent to form ureapolyisocyanate b) performing biuretization of ureapolyisocyanate obtained in step a) c) performing addition of the polysiloxane polysiloxane having at both ends hydroxyalkyl or hydroxyalkoxy groups or terminated at one end with two hydroxyl groups d) blocking the free isocyanate groups of the polyisocyanate with a blocking agent.

7. The method of claim 6, characterized in that step a) is carried out by placing the isocyanate component and at least one catalyst in an amount of 0.10 wt% - 0.15 wt% in the reactor, heating to a temperature below 65°C, said biuretizing agent is dosed in an amount from 0.1 mol to 0.4 mol per 1 mol of the isocyanate component, preferably in an amount from 0.2 mol to 0.3 mol per 1 mol of the isocyanate component, and then in step b) heating the mixture to a temperature below 140°C and maintaining until completion of biuretization of isocyanate component.

8. The method of claim 6, characterized in that in step b) heating the mixture to a temperature below 140°C and maintained until the end biuretization isocyanate component.

9. The method of claim 6, characterized in that in step c) polysiloxane is added in an amount from 1 wt% to 40 wt%, preferably from 3 wt% to 25 wt%.

10. The method of claim 6, characterized in that in the step d) after adding the blocking agent the mixture is heated to a temperature not exceeding 65°C until any free isocyanate groups are present.

1. The method of claim 6, characterized in that the isocyanate component is selected from the group consisting of isophorone diisocyanate (IPDI), 1 ,6- hexamethylene diisocyanate (HDI), 2,2,4- and 2,4,4-trimethyl-1 ,6-hexamethylene diisocyanate (TMDI) 4,4'-dicyclohexylmethane diisocyanate (H₁₂MDI), 1 ,4- bis(isocyanatomethyl)cyclohexane, 1 ,3,5-tris (isocyanatomethyl)cyclohexane, 2,4- and 2,6-toluene diisocyanate and 3,5-tris (isocyanatomethyl)benzene, m- and p- tetramethylxylylene diisocyanate (T XDI).

12. The method of claim 6, characterized in that the catalyst is an organic phosphorus compound, an organometallic compound or an amine compound selected from the group comprising dibutyl phosphate, dibutyltin dilaurate, tin octoate, triethylamine, triethylenediamine.

13. The method of claim 6, characterized in that said biuretizing agent is water, formic acid or alcohol.

14. The method of claim 6, characterized in that the polysiloxane is a linear polysiloxane moiety having at both ends hydroxyalkyi or hydroxyalkoxy groups or terminated at one end with two hydroxyl groups, and a molecular weight in the range from 500 to 10000 or from 800 to 5000.

15. The method of claim 6, characterized in that the blocking agent is selected from the group consisting of ε-caprolactam, diethyl malonate, oximes or 3,5- dimethylpyrazole.

16. Blocked polyisocyanates obtained by the process as defined in any of claims. 6 to 15.

17. Polyurethane powder coatings comprising blocked polyisocyanates as defined in claims 1-6 or 16.

18. Use of the blocked poiyisocyanates as defined in claims 1-6 or 16 for the preparation of polyurethane powder clear coating systems.