Process for the production of aluminium hydroxide of improved thermal stability

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DESCRIPTION

Process for the production of aluminum hydroxide with improved thermal stability.

The invention relates to a process for the production of an aluminum hydroxide with improved thermal stability, which can be incorporated into plastic composite materials, the processing temperatures of which exceed 200 ° C.

Aluminum hydroxide (Al (OH) 3) is widely used as a flame retardant in polymer systems. Heating above a temperature of approx. 200 ° C, aluminum hydroxide begins to decompose into aluminum oxide and water. This reaction is endothermic, thus absorbing the heat from the polymer. The amount of water released corresponds to 34.6% by weight of the aluminum hydroxide. Both factors act to suppress a combustion process and reduce the formation of combustible gases by keeping the temperature falling.

To be useful as a flame retardant, aluminum hydroxide must remain stable during polymer processing. This makes it unsuitable for polymers that are processed above 200 ° C.

The prior art indicates that a progress towards this limitation is to preheat the aluminum hydroxide to remove some of the water that is released at elevated temperatures. However, there are inherent drawbacks to this method:

• The amount of water (steam) available for smoldering action is reduced.

• A portion of the (tri) aluminum hydroxide after heating above 200 ° C is converted to aluminum oxide hydroxide (AlOOH, “alumina monohydrate”) which contains only one third the amount of water and which is thermally stable up to approx. 520 ° C, that is, up to a temperature also above the useful range for effective smoldering of plastic materials.

• The partial removal of water as a means of stabilization against the subsequent rapid release of water vapor at elevated temperatures is accompanied by an increase in the specific surface area of ​​aluminum hydroxide. This increased surface is in the form of fine porosity that imparts hygroscopic properties to the surface of the thermally stabilized crystals. The increased specific surface area is further accompanied by higher oil absorption and higher viscosity in synthetic resins.

The problem to be solved by the present invention was to provide a simple and inexpensive process for the production of aluminum hydroxides of improved thermal stability, high water content and low specific surface area.

According to the invention, this problem must be solved by the process of claim 1.

It has been found that aluminum trihydroxide can be partially dehydrated at elevated temperatures without a concomitant increase in specific surface area and with little or no creation of aluminum oxide hydroxide, provided the residence time at the dehydration temperature is sufficient short. In accordance with the present invention, this is achieved by spray drying an aqueous suspension of ordinary aluminum hydroxide at a drying gas temperature (at the inlet) of 400 to 600 ° C.

Preferably, the temperature of the drying gas is 450 to 550 ° C.

The outlet gas temperature of the spray dryer is preferably 200 to 300 ° C.

The residence time of the aluminum hydroxide in the spray dryer is preferably 0.5 to 10 s, more preferably 1 to 5 s.

Preferably, the average particle size of the starting aluminum hydroxide is 2 µm or less. Advantageously, the coarser particles (eg> 10 µm) are removed earlier by the spray dryer. The suspension of the aluminum hydroxide used as the starting material can be produced by crystallization from the sodium aluminate liquor of the Bayer process, filtration and continuous washing without hot water, and re-suspension in a sufficient quantity of water to obtain a suitable suspension to feed the spray dryer. Typically the solids content of the suspension is 40 to 60% by weight.

The process of the present invention achieves the goal of improved thermal stability without significant increase in the specific surface area of ​​the aluminum hydroxide, compared to the starting material, and without a substantial decrease in loss over loss on calcination (LOI) values. . Furthermore, X-ray diffraction patterns reveal only the formation of trace amounts of aluminum oxide hydroxide (boehmite, AlOOH), thus indicating maximum retention of effective smoldering aluminum trihydroxide.

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The product of the present invention is of industrial importance especially in the area of ​​application of environmentally friendly printed circuit boards, where aluminum hydroxide can now be used to achieve UL-94 V0 fire classification, without the problems associated with the hygroscopic nature of prior art aluminum trihydroxide, no high viscosity levels and no reduction in smoldering effectiveness due to the presence of boehmite.

The following non-limiting examples illustrate the process of the present invention. The starting material in each example was aluminum hydroxide from the different batches of the Martinal type^® (manufacturer: Alusuisse Martinswerk GmbH, Bergheim, Germany).

Comparative Example 1

Martinal aluminum hydroxide samples were heated^® OL-104 (Alusuisse Martinswerk, GmbH, Bergheim, Germany) with a specific surface area of ​​3.75 m² / g in an oven at various temperatures for different periods of time. The sample size was 100 g in each case. After heat treatment, the loss on ignition (LOI) and the specific surface area of ​​each sample were measured and the X-ray diffraction patterns were recorded. The results obtained are given in Table 1.

TABLE 1

<td>Temperature [° C]</td><td>Time [h]</td><td>LOI [wt%]</td><td>Surface Area [m²/ g]</td>

<td> 180</td><td> 1</td><td> 34,4</td><td> 3,85</td>

<td> 180</td><td> 2</td><td> 34,4</td><td> 3,92</td>

<td> 180</td><td> 4</td><td> 33,6</td><td> 4,10</td>

<td> 200</td><td> 1</td><td> 33,5</td><td> 4,47</td>

<td> 200</td><td> 2</td><td> 33,1</td><td> 5,30</td>

<td> 200</td><td> 1</td><td> 32,7</td><td> 6,76</td>

<td> 200</td><td> 2</td><td> 30,7</td><td> 34,2</td>

The data indicate that the low temperature-extended time combination leads to increased specific surface area as loss on ignition decreases.

In all cases, aluminum oxide hydroxide was formed as evident from X-ray diffraction.

Comparative Example 2

Martinal scattered^® untreated OL-104 and the aluminum hydroxide obtained in Example 1 by treatment at 200 ° C / 1 hour, in Synolite® 0020-N-2 unsaturated polyester resin (DSM BASF Structural Resins), using a fill level 150 phr (parts percent resin). Comparative viscosity measurements were made at 23 ° C using an HBT Brookfield Viscometer with Spindle 2 at 10 rpm. The results are given in Table 2.

TABLE 2

<td>Aluminum hydroxide</td><td>Viscosity | Pa-s |</td>

<td>Martinal® OL-104 (untreated)</td><td> 150</td>

<td>Martinal® OL-104 (200 ° C / 1h)</td><td> 176</td>

The results show that the heat treatment increased the viscosity of the resin / aluminum hydroxide mixture by almost 20%.

ES 2 201 030 T3

Example 1

About 5 kg of an aqueous suspension of Martinal were spray dried^® OL-104 (53% by weight solids) in a Niro type spray dryer (Niro A / S, Soeberg, Denmark) ("Production Minor" type) at a slurry feed rate of approx. 5 kg / h and an air flow of 300 kg / h. The inlet gas temperature was 480 ° C and the outlet temperature was approx. 200 ° C with a solid retention time of approx. 3 seconds. Approximately 2.5 kg of product was recovered from the dryer. The product had a specific surface area of ​​4.3 µm²/ g and a loss on calcination of 33.2% by weight (starting material: 4.5 m²/ g and 34.6% by weight, respectively), that is, the specific surface area was within the experimental error of the invariant method.

The rheological properties of the product were determined as described in Comparative Example 2. The results are given in Table 3.

TABLE 3

<td>Aluminum hydroxide</td><td>Viscosity | Pa-s |</td>

<td>Martinal® OL-104 (untreated)</td><td> 165</td>

<td>Martinal® OL-104 (treated)</td><td> 160</td>

The results show that spray drying had no significant effect on the rheological properties of the product.

Example 2

Essentially the same procedure as in Example 1 was adopted, except that production was gradually raised to the industrial level. This time, a Niro dryer (type SD-0080-CN / CR) was used with a production of 150 to 450 kg per hour with an air flow of approx. 1000 kg / h. The starting material Martinal^® OL-104 (specific surface area 4.6 m²/ g) was suspended in water to give approx. 50% solids by weight. The air inlet temperature to the dryer could be varied between 400 and 580 ° C. The outlet temperature varied between 200 and 300 ° C. The total retention time in the reactor was 2-10 s. The dried product was packed in plastic lined paper bags.

The characteristics of the product were as follows:

Loss on ignition (% by weight) 32.5

Specific surface area (m²/ g) 4.8

Oil absorption (ml / 100 g) 28

Viscosity in synthetic resin (Pa ^ s) 160