Technical Field

The present invention relates to a process developed for synthesis of zinc borate which is used as a flame formation/growth retardant and a smoke suppressant.

Background of the Invention

Progress of boron technology increases usage area of boron minerals, especially zinc borate. Zinc borate is used particularly to retard flame formation/growth and suppress the smoke for industries of pvc coating, mdf, eva products, wood, cable, dye, textile products, electrics/electronics parts, carpet coatings, internal parts of automotive/airplanes and paper. This makes the products more durable. While having the property of flame formation/growth retardant and smoke suppression, zinc borate used in polymer and wooden materials has also anticorrosive and antifungal properties. Moreover, when compared to other flame retardants (aluminium trihydrate, magnesium hydroxide, antimuan trioxide), zinc borate is a less costly flame formation/growth retardant additive.

According to particle size of the above-mentioned zinc borate, the degree of flame formation/growth retardant and smoke suppression effect of zinc borate varies. Thus, many processes have been developed for synthesis of zinc borate that has the best flame formation/growth retardant and smoke suppression effect. In TR 1999/01134, which is an example for said processes, there is disclosed a method developed for production of zinc borate containing crystalline water of 3.5 moles. According to said patent application, the reaction time takes 20-24 hours. However, such a long reaction time causes disruption of other processes.

TR 2006/02072 which is another example for said processes discloses a process for production of zinc borate containing crystalline water of 3.5 moles by using raw materials of boric acid, water and zinc oxide. Said process comprises the steps of preparing an aqueous solution of boric acid; adding zinc oxide to the solution of boric oxide; adding zinc borate to the solution prepared, the zinc borate acting as seed crystal; carrying out the reaction at 85°C; carrying out the filtering process; carrying out the washing process; and carrying out the drying process. The process provides zinc borate having a particle size of at least 4 pm.

TR 2018/03496 which is a further example for said processes discloses a method for decreasing particle size of zinc borate. The method comprises the steps of adding zinc borate to a solution containing a polymer and a polar aprotic solvent; and forming by zinc borate a colloidal mixture in said solution. Therefore, there are provided zinc borate particles which are 0.1 - 0.9 microns in size. These known methods in the art cause increase in energy costs and particle sizes.

Brief Description of the Invention

The present invention discloses a process for synthesising a crystalline zinc borate having a particle size of 4 pm or less in order to increase the degree of flame formation/expansion retardant and smoke suppressing effect of the zinc borate. The process comprises the steps of:

- mixing zinc borate (B ₆ 0nZn ₂ ) with zinc oxide (ZnO) in a mixer for a first time period to obtain zinc oxide with seed crystal;

- mixing a first amount of solid and/or aqueous solution of boric acid, a second amount of aqueous solution containing at least one dispersion agent and a third amount of zinc oxide with seed crystal obtained to obtain a mixture;

- reacting this mixture for a second time period at a first temperature value;

- transmitting the product obtained as a result of the reaction to a filtering-pressing unit in a hot manner and pressing the same therein to separate the product into two parts as a zinc borate SLUDGE and a reaction water rich in boric acid;

- transferring the reaction water rich in boric acid to a heated stock tank and keeping it in the heated stock tank for use in a subsequent reaction;

- mixing zinc borate SLUDGE by gradually transferring it to a washing unit containing water at a second temperature value, and removing the non-reacted boric acid in the zinc borate SLUDGE from the zinc borate SLUDGE; - after the washing process, transferring the mixture of zinc borate SLUDGE and the water at the washing unit to the filtering-pressing unit to press the mixture;

- after pressing, separating the mixture into two separate parts as a SLUDGE and a washing water;

- drying the separated SLUDGE in a preferably cylindrical dryer at a third temperature value until the moisture ratio decreases below 1 % and obtaining a crystalline zinc borate;

- transferring crystalline zinc borate obtained to a cyclone to separate crystalline zinc borate as those having a particle size of 4 pm or less and those having a particle size of more than 4 pm;

- transferring the grains which have a particle size of 4 pm or less to a packaging unit; and

- sending the grains which have a particle size of more than 4 pm to a grinder so as to decrease particle sizes, and transferring them to the packaging unit.

Thanks to the process provided by the present invention, a crystalline zinc borate having a particle size of 4 pm or less is obtained and the degree of flame formation/expansion retardant and smoke suppression effect thereof is increased. Production costs are decreased since the reaction water rich in boric acid and the washing water which are stored at the stock tanks are utilised repeatedly during the process. In addition, coating the crystalline zinc borate preferably with chemical components increases compatibility with polymer systems. This, in turn, minimises the decrease in mechanical properties of the polymer systems.

Object of the Invention

An object of the present invention is to provide a process for obtaining zinc borate having a particle size of less than 4 pm so that the degree of flame formation/growth retardant and smoke suppression effect thereof increases.

Another object of the present invention is to provide a process for obtaining zinc borate having a particle size of less than 4 pm within a shorter reaction time. A further object of the present invention is to provide a process for producing zinc borate with reduced costs and energy consumption by using circulated water repeatedly at the process.

Yet another object of the present invention is to provide a process for subjecting zinc borate to a hydrophobication process in order to be able to use zinc borate in polymer systems.

Description of the Invention

Zinc borate, one of the boron minerals, is used particularly for industries of pvc coating, mdf, eva products, wood, cable, dye, textile products, electrics/electronics parts, carpet coatings, internal parts of automotive/airplanes, paper, and generally for polymer industry. By means of zinc borate, flame formation/growth is retarded and the smoke formed is suppressed in these materials. This, in turn, makes said materials more resistant to fire and smoke occurrence. Moreover, zinc borate used in polymer and wooden materials makes said materials anticorossive and antifungal, as well as retarding flame formation/growth and suppressing smoke. However, in order to increase degree of said effects, particle size of zinc borate is required to be substantially small. Therefore, with the present invention there is provided a process for synthesising a crystalline zinc borate having a particle size of 4 pm or less.

The process according to the present invention comprises the steps of:

- mixing (preferably at a high speed) zinc borate (B ₆ 0nZn ₂ ) with zinc oxide (ZnO) preferably in a mechanical mixer for a first time period (preferably within the range of 30-60 minutes) to obtain zinc oxide with seed crystal;

- mixing a first amount of solid and/or aqueous solution of boric acid (preferably with a ratio from 0.1 % to 5% by weight), a second amount of aqueous solution containing at least one dispersion agent and a third amount of zinc oxide with seed crystal obtained (preferably with a ratio from 0.1% to 0.5% by weight) to obtain a mixture;

- reacting this mixture for a second time period (preferably 4-6 hours) at a first temperature value (preferably 95-100°C to shorten maturation period of crystal) ; - transmitting the product obtained as a result of the reaction to a filtering-pressing unit in a hot manner and pressing the same therein to separate the product into two parts as a zinc borate SLUDGE (more solid component remained after the pressing process) and a reaction water rich in boric acid;

- transferring the reaction water rich in boric acid to a heated stock tank and keeping it in the heated stock tank for use in a subsequent reaction;

- mixing zinc borate SLUDGE by gradually transferring it to a washing unit containing water at a second temperature value (preferably 70-90°C), and removing the non-reacted boric acid in the zinc borate SLUDGE from the zinc borate SLUDGE;

- after the washing process, transferring the mixture of zinc borate SLUDGE and the water at the washing unit to the filtering-pressing unit to press the mixture;

- after pressing, separating the mixture into to two separate parts as a SLUDGE and a washing water;

- drying the separated SLUDGE in a preferably cylindrical dryer at a third temperature value (preferably 100-130°C) until the moisture ratio decreases below 1% and obtaining a crystalline zinc borate;

- transferring crystalline zinc borate obtained to a cyclone to separate crystalline zinc borate as those having a particle size of 4 pm or less and those having a particle size of more than 4 pm;

- transferring the grains which have a particle size of 4 pm or less to a packaging unit; and

- sending the grains which have a particle size of more than 4 pm to a grinder so as to decrease particle sizes, and transferring them to the packaging unit.

At least one dispersion agent is used for reaction phase of the abovementioned process. Thanks to the dispersion agent and the grinding process carried out for the particles which are more than 4 pm in size, particle size of crystalline zinc borate obtained is decreased, and particle sizes of 4 pm or less are obtained. The fact that the grinding process is a mechanical process provides lower costs while making the method more practical. In the process according to the invention, the component having more moist content and separated after the pressing process is called SLUDGE. As well as SLUDGE, circulated water is provided during this pressing process and it may be used again. This improves compliance of the product, which is obtained by the process, inside the polymer and also decreases costs and energy consumption. Both achieving particles which are smaller in size and modifying the product increase property of flame retardant and smoke suppression. Moreover, since the distribution and compliance of the product in the polymer are much more by modification of the product, mechanical properties of the polymer are not decreased.

In the first step of the process according to the invention, zinc borate and zinc oxide (ZnO) is mixed preferably in a mechanical mixer, preferably in a high speed mechanical mixer, for preferably 30-60 minutes. Thanks to using a high speed mixer for mixing process, a homogenous mixture is achieved, which in turn increases yield of the zinc oxide with seed crystal.

In a preferred embodiment of the invention, the dispersion agent is a water soluble salt of poly(acrylic acid) (preferably a water soluble alkali metal salt of poly(acrylic acid) or an ammonium salt of poly(acrylic acid)) or a water soluble phosphate salt of an alkali metal (preferably Sodium hexametaphosphate, sodium pyrophosphate or sodium tripolyphosphate) having an average molecular weight of 1000 to 10000.

In the process according to the invention, solid and/or aqueous solution of boric acid, aqueous solution containing the dispersion agent, and zinc oxide with seed crystal can be mixed with any order. However, in a preferred embodiment of the invention, this step comprises mixing a first amount of solid and/or aqueous solution of boric acid with a second amount of aqueous solution containing the dispersion agent at first, and then adding a third amount of zinc oxide with seed crystal obtained to obtain the mixture. Yet in another preferred embodiment of the invention, this step comprises mixing a third amount of zinc oxide with seed crystal obtained with a second amount of aqueous solution containing the dispersion agent at first, and then adding a first amount of solid and/or aqueous solution of boric acid to said mixture to obtain the mixture. These step orders are varied and determined according to the application/process ease.

In a preferred embodiment of the invention, the process comprises the step of stocking the washing water in a different stock tank for a subsequent washing process since the washing water comprises boric acid. This enables washing water to be used again for later productions. This, in turn, enables production costs to be decreased.

Reaction equation of zinc borate having a particle size of less than 4 p and obtained as a result of abovementioned process is as follows:

6B(OH) ₃ (solution) + 2ZnO (solid) ® 2Zh0·3B ₂ 0 ₃ ·3.5H ₂ 0 (solid) + 5.5H ₂ 0 (liquid)

As a result of said reaction, 2Zh0·3B ₂ 0 ₃ ·3.5H ₂ 0, a crystalline zinc borate, is formed. The main reason for choosing this reaction is that zinc borate releases its water when the crystal medium reaches to an average temperature of 290°C, due to crystal structure (pattern) of zinc borate formed. Since this temperature is higher than a treating temperature of known common polymers (Polyolefin, PVB etc.) by extrusion, the possibility of losing crystal water especially during polymer treating is eliminated. Further, said reaction uses excess boric acid with respect to stoichiometric ratio (molar ratio). Molar ratio of boric acid: zinc oxide is from 4: 1 to10: 1.

In a preferred embodiment of the invention, the process comprises the step of coating crystalline zinc borate at a fourth temperature value (preferably 80-90°C) by means of a chemical component (e.g. fatty acids, metal salts of fatty acids, coupling agents of silane/titanium etc.) with a ratio of 0.5-5% of the amount of crystalline zinc borate. Thanks to that, crystalline zinc borate which is the final product is modified so that its compliance with polymer systems (e.g. PVC, PE, PP) is improved.

Thanks to the process provided by the present invention, a crystalline zinc borate having a particle size of 4 pm or less is obtained and the degree of flame formation/expansion retardant and smoke suppression effect thereof is increased. Production costs are decreased since the reaction water rich in boric acid and the washing water which are stored at the stock tanks are utilised repeatedly during the process. In addition, coating the crystalline zinc borate preferably with chemical components increases compatibility with polymer systems. This, in turn, minimises the decrease in mechanical properties of the polymer systems.