AU2004201954A1

AU2004201954A1 - A fire retardant for thermoplastic and method of manufacture thereof

Info

Publication number: AU2004201954A1
Application number: AU2004201954A
Authority: AU
Inventors: Daidai Wu; Ying Ye; Weirui Zhang
Original assignee: Zhang Weirui Dr
Current assignee: Zhang Weirui Dr
Priority date: 2004-05-10
Filing date: 2004-05-10
Publication date: 2005-11-24

Description

TITLE OF THE INVENTION A fire retardant for thermoplastic and method of manufacture thereof FIELD OF THE INVENTION The present invention relates to a fire retardant compound and a method of producing thereof.

More specifically, the invention is directed to a fire retardant compound having a typical hydrotalcite structure with antinomy incorporated in it and a method of producing such a compound. This compound has endothermic properties to absorb considerable amounts of heat in association with the initial stages of a fire and consequently to release large amounts of water vapour and carbon dioxide during the higher temperatures.

BACKGROUND OF THE INVENTION It is known to this art that the addition of hydrotalcite into the thermoplastic resins imparts a good degree of fire resistance. For example, inventions with Patents Numbers US4085088 and US4729854 describe certain fire-retarding thermoplastic resins containing a magnesium-aluminum type of hydrotalcite. Inventions with Patents Numbers US5115010 and EP0694574 describe some fire-retarding polyamide and plastisol containing hydrotalcite.

Because of the lack of information, one would assume that only the basic magnesium-aluminum type of hydrotalcite was used.

Although the hydrotalcite has been mentioned in those patents, none of them has described the method of manufacturing it. A US invention (Patent No US5250279) describes a method of synthesizing hydrotalcite under hydrothermal conditions. Since the hydrothermal process involves elevated temperatures and pressures, thus equipments required for the process could be capital intensive.

In a Japanese invention (Patent No JP60231416), a method of precipitation was described.

The filtration stage of this process is difficult and usually time consuming.

DETAILED DESCRIPTION OF THIS INVENTION This invention provides a fire retardant composition that has a hydrotalcite structure and contains antimony, such a composition is able to absorb considerable amounts of heat energy and to release some fire-extinguishing gases during the heating. Once it has been incorporated in thermoplastic and thermosetting resins, those gases will intumesce the plastisol and form a fire-resistant surface-layer. This fire retardant combines the fire-retarding properties of both hydrotalcite and antimony oxide.

This invention is also to provide a method for the production of such a fire retardant composition.

Hydrotalcites are also named layered double hydroxides (hereafter called LDH) which have a layered structure. The basic layers are composed of octahedrons of divalent and trivalent metal ions hydroxides (ie M 2 OH and M 34 OH) which form positively charged layers.

Anions and water molecules fill the spaces between layers. Thus, they are also called anion clays. This layer-type structure can enhance the mechanical properties of plastics once it has been incorporated. The most common type of hydrotalcite is magnesium-aluminum type of carbonate hydroxide hydrate with a typical chemical formula of [Mg 3 Al(OH)s] 2

CO

3 nH 2

O.

Once the hydrotalcite is heated to 190 250 0 C, the first stage of thermal decomposition occurs. Water molecules from the structure will be released with large amount of heat energy. The peak of this reaction is at temperatures approximately 210°C with a weight loss of 12 15% and the heat absorption capacity of 410J/g. The second stage of thermal decomposition occurs at 380 450 0 C. At this stage, the hydroxy and carbonate radicals are released as water and carbon dioxide, the weight loss is 29 30% and the heat absorption capacity is 550J/g. Therefore, hydrotalcites release a large amount of carbon dioxide and steam which have good intumescent and fire-retarding properties. The disintegrated hydrotalcites are also called layered double oxides which still retain the layered structures and are good smoke reducers.

This invention describes a synthesized hydrotalcite with antimony incorporated in its structure. In this structure, Sb 3 are situated in the octohedrons, ie octohedrons of Sb 3

OH,

Mg 2 OH and A3+- OH form the basic layers ofhydrotalcite. The inter-layered anions can be carbonate (CO 3 2 or bicarbonate (HCO3-) or chloride Hydrotalcites containing the first two anions are suitable for halogen-containing plastics, whereas the third one is suitable for plastics not containing halogen.

During the heating process, water molecules in this antimony-containing hydrotalcite will be released at approximately 200°C, which will carry away a large amount of heat and intumese the plastics and thus reduce the heat conducting ability of the plastics. Once the temperature has reached to 350C, this antimony-containing hydrotalcite starts the second stage of disintegration. Again, it will absorb a large amount of heat and release a large amount of fire-retarding gases. At the same time, the antimony will combine with the halogen atoms in the plastics or react with its inter-structure halogen atoms and become the antimony halogenides which further enhances the fire-retarding abilities. Therefore, this antimony-containing hydrotalcite has fire-retarding properties from both hydrotalcite and antimony oxide (Sb20 3 This antimony-containing hydrotalcite is deduced from a layered double oxide (hereafter called LDO). The principle of using magnesium hydroxide (Mg(OH) 2 or magnesium oxide (MgO) with aluminum hydroxide (AI(OH) 3 to produce LDO and hydrotalcite is that Mg(OH) 2 and MgO are slightly soluble in water. Therefore, during the wet grinding process, they can be thoroughly mixed with AI(OH) 3 at the molecular scale. This mixture will form a 2

I

solid solution (ie LDO) after calcination at high temperatures. After the hydrolysis in a solution of inorganic salt, this LDO will turn into a hydrotalcite.

Properties of the final product are affected by the ratio of Mg(OH) 2 and AI(OH) 3 in the initial raw materials. In general, the product has a more stabile crystal structure if the Mg(OH) 2 contents are higher. However, the final product has a better absorption capability if the

AI(OH)

3 contents are higher. The preferred molar ratios for Mg(OH) 2 to AI(OH) 3 are between 2:1 to 5:1.

The Mg(OH) 2 in the raw materials can be either a chemical product or from the natural mineral of brucite, since the latter has the chemical composition of Mg(OH) 2 It can be replaced by MgO in the synthesizing process, as MgO can be hydrolyzed into Mg(OH)2 during the wet grinding. The MgO can be either a chemical product or from the natural mineral of magnesite after it has been calcined at 400 600 0 C. The other raw material is

AI(OH)

3 which can be either a chemical product or from the natural mineral of gibbsite, since the later has the chemical composition of AI(OH) 3 According to the physical properties of raw materials used, a different amount of water is added into the mixture. The mixture should be a dense slurry. If not enough water is added into the mixture, then the mixture will become lumpy and thus poor grinding will result.

However, if too much water is added, then it will require more energy in the next stage of the process and also higher wearing of the equipments. The preferred amount of water added into the mixture is 3 5 times of the solid mass.

Because Mg(OH) 2 and MgO are slightly soluble in water, so they can be thoroughly mixed with AI(OH) 3 at the molecular scale through the wet grinding process. The best grinding time will depend on the physical properties of the raw materials used. When using the natural minerals as the raw materials, the minimum grinding time should be 12 hours or more; whereas when chemical products are in use, in order to conserve energy, the grinding time can be reduced to 10 hours.

The ground mixture should be de-watered through sedimentation- filtration or centrifugation.

It will then be dried preferably at room temperature, otherwise dried at temperatures less than The mixture is aged during the drying process. The aged mixture will be calcined at temperatures less than 750 0 C for 2 5 hours, preferred temperatures are between 550 to 650 0 C. The purpose of calcination is to convert the mixture into a solid solution. The calcined product is a LDO.

Naturally occurring stibnite (Sb 2

S

3 can be the source of antimony. Using stibnite can reduce the cost of raw materials and simplifies the process. The stibnite ore or concentrate should be treated in a cylindrical slurry tank with a depth of 1 to 2 times of its diameter. A mechanical stirrer is located in the middle of the tank to ensure the homogeneous mixing of ore or concentrate with the sulphide solution. Sulphides can be either one of the following: e.g. lithium sulphide (Li 2 sodium sulphide (Na 2 potassium sulphide (K 2 S) and

I

ammonium sulphide ((NH 4 2 or the mixture of several of them. For the cost concerns, Na 2 S is recommended. The stibnite will react with the S2- ions in the solution and become soluble thioantimonite: Sb2S, +3S 2 2SbS 3 Theoretically, at the completion of the dissolution process, the stibnite used and thioantimonite in the solution should have the same molar weights. However, from the results of laboratory tests, only 60% of the theoretical value has been achieved. Therefore, excess sulphides should be added to ensure that stibnite be totally dissolved. If the sulphide used is Na 2 S, the amount of sulphide required is approximately the same as that weight of antimony in ore or concentrate. If not enough sulphide is added, then the dissolution of stibnite will be incomplete. When too much of excess amount of sulphide is added, it will increase the production cost.

The typical chemical reaction for the LDO for absorbing thioantimonite ions in this invention can be expressed as: 3Mg Al 2 09 35H 2 0 2SbS3- 2[Mg 3 Al(OH) 8 (SbS 3 4HzO The amount of thioantimonite absorbed by the LDO is restricted by the above chemical reaction. Considering the interferences from S 2 OH- and CO 2 left in the solution, excessive amount of LDO should be added. The actual amount of LDO required should be about 10 times of the antimony in the solution (in weight). After the absorption process, a small amount of SbS 3 3 and S 2 ions will still be left in the solution. Therefore, the solution should be re-cycled and pumped back to the slurry tank.

As a result of the absorption of thioantimonite ions, the colour of LDO becomes bluish grey.

After drying, it will be calcined in an oven at a temperature between 500 to 750C. The oven should have sealed walls and a long flue to facilitate the recovery of volatiles. Typical reaction during the roasting is: [Mg 3

A(OH)

8 3 (SbS)4H 2 0 602 3Mg 3 AlSbo..330 3SO, T +16H 2 0 T The thioantimonite radicals absorbed by the LDO are disintegrated and turned into antimony oxide which is merged into the molecular structure of LDO. The SO 3 in the flue gas can be recovered for the production of sulphuric acid as indicated in the following equation: SO, H 2 0 H 2

SO

4 The calcined product is still LDO. In order to increase the antimony concentration in the LDO, the above mentioned absorption and calcination processes can be repeated several times.

The antimony-containing LDO can be converted into the antimony-containing hydrotalcite after hydrolysis in carbonate or chloride solution. Typical reactions in the solution are: Mg 6 (Al, Sb) 2 09 (9 n)H 2 0 CO 2 [Mg 3 (Al, Sb)(OH), ]2 CO, nH 2 0 2(OH) Mg, (Al, Sb) 2 09 (9 n)H 2 0 2CI [Mg 3 (Al, Sb)(OH) 8 ]2 C 2 nH 2 O 2(OH) In those reactions, ratios of LDO to carbonate and chloride radicals are 1:1 and 1:2 respectively. In order to ensure the completion of reaction, moles of carbonates should be 1.2 1.5 times of LDO, whereas chlorides should be 2.4 3 times of LDO. Different types of salts have little effect on the final product from the hydrolysis process. In considering the production costs, sodium carbonate (Na 2

CO

3 or sodium bicarbonate (NaHCO 3 is the preferred choice as the carbonate salt and sodium chloride (NaCI) is the preferred choice as the chloride salt. The hydrolysed product is de-watered and then washed at least twice to eliminate all the excessive salts. It will then be dried at room temperature, or dried at temperatures less than 90 0 C. The final product is an antimony-containing hydrotalcite.

The carbonate-containing hydrotalcite is suitable for halogen-containing plastics or resins, whereas the chloride-containing hydrotalcite is suitable for plastics or resins that do not contain halogen atoms.

EXAMPLES

Examples of the process of this invention in the laboratory are described as follows: EXAMPLE 1: Preparation of LDO using Brucite and Gibbsite 1. Placed 174g of brucite (Mg(OH) 2 and 78 g of gibbsite (AI(OH) 3 into the ball mill with 750g of water and ground for 18 hours.

2. Filtered the mixture and dried it at 3. Placed the dried mixture into a muffle furnace and calcined at 600 0 C for 3 hours.

4. Cooled the calcined product to room temperature and ground to <200 mesh. The final product is a magnesium-aluminum type of LDO.

EXAMPLE 2: Preparation of LDO using Magnesite and Gibbsite 1. Calcined 168g ofmagnesite (MgCO 3 at 400 0 C for 3 hours.

2. Mixed 78g of gibbsite with the calcined magnesite and placed the mixture into the ball mill with 500 g of water and ground for 14 hours.

3. Filtered the mixture and dried it at 80 0

C.

4. Placed the dried mixture into a muffle furnace and calcined at 600 0 C for 3 hours.

Cooled the calcined product to room temperature and ground to <200 mesh. The final product is LDO. The final product is a magnesium-aluminum type of LDO.

EXAMPLE 3: Preparation of LDO using Magnesite and Aluminum Hydroxide 1. Calcined 168g of magnesite (MgCO 3 at 400°C for 3 hours 2. Mixed 78g of aluminum hydroxide (AI(OH) 3 with the above calcined magnesite and placed the mixture into the ball mill with 500 g of water and ground for 14 hours.

3. Filtered the mixture and dried it at 4. Placed the dried mixture into a muffle furnace and calcined at 600 0 C for 3 hours.

Cooled the calcined product to room temperature and ground to <200 mesh. The final product is a magnesium-aluminum type of LDO.

EXAMPLE 4: Preparation of LDO using Magnesium oxide and Aluminum Hydroxide 1. Placed 84g of magnesium oxide (MgO) and 78g of aluminum hydroxide (AI(OH) 3 into the ball mill with 600 g of water and ground for 6 hours.

2. Filtered the mixture and dried it at 80 0

C.

3. Placed the dried mixture into a muffle furnace and calcined at 600 0 C for 3 hours.

EXAMPLE 5: Preparation of Antimony-containing LDO with Stibnite Concentrate 1. Placed 350g of pulverized stibnite (contains 20% of Sb) in a beaker together with 105 g of (NH 4 2 S, 20g of NaOH and 1000 mL of water. Continuously stirred for 10 minutes, so the stibnite was totally dissolved.

2. Filtered the solution and washed the solid phase twice with water. Retained the filtrate and mixed with the washed water.

3. Added 1900g of LDO from the above Examples into the solution and continuously stirred for 10 hours.

4. Filtered the mixture and dried the solid at temperatures less than 80 0

C.

Placed the dried mixture into a muffle furnace and calcined at 550 0 C for 4 hours. The final product is an antimony-containing magnesium-aluminum type of LDO.

EXAMPLE 6: Preparation of Antimony-containing LDO with pure Stibnite 1. Placed 100g of pulverized pure stibnite in a beaker together with 120 g of Na 2 S, 20g of NaOH and 1000 mL of water. Continuously stirred for 10 minutes, so the stibnite was totally dissolved.

2. Added 1900g of LDO from the above Examples into the solution and continuously stirred for 10 hours.

3. Filtered the mixture and dried the solid at temperatures less than 80 0

C.

4. Placed the dried mixture into a muffle furnace and calcined at 550 0 C for 4 hours. The final product is an antimony-containing magnesium-aluminum type of LDO.

EXAMPLE 7: Preparation of Antimony-containing LDH with Antimony-containing LDO 1. Dissolved 125g of sodium carbonate (Na 2

CO

3 into 1000mL of water.

2. Added 350g of antimony-containing LDO produced from Example 5 into the solution and continuously mixed for 6 hours.

3. Filtered the mixture.

4. Washed the solid with 1000ml of water, then filtered the slurry again. Repeated this washing process twice.

Dried the solid at temperatures less than 70 0 C. The final product is an antimony-containing magnesium-aluminum type of LDH.

EXAMPLE 8: Preparation of Antimony-containing LDH with Antimony-containing LDO 1. Dissolved 250g of sodium bicarbonate (NaHCO 3 into 1000mL of water.

2. Added 300g of antimony-containing LDO produced from Example 5 into the solution and continuously mixed for 8 hours.

3. Filtered the mixture.

Dried the solid at temperatures less than 80C. The final product is an antimony-containing magnesium-aluminum type of LDH.

EXAMPLE 9: Preparation of Antimony-containing LDH with Antimony-containing LDO 1. Dissolved 250g of sodium chloride (NaC1) into 1500mL of water.

2. Added 180g of antimony-containing LDO produced from Example 5 into the solution and continuously mixed for 5 hours.

3. Filtered the mixture.

4. Washed the solid with 2000ml of water, then filtered the slurry again. Repeated this washing process twice.

EXAMPLE 10: Preparation of Antimony-containing LDH with Antimony-containing LDO 6. Dissolved 200g of potassium chloride (KC1) into 1500mL of water.

7. Added 160g of antimony-containing LDO produced from Example 5 into the solution and continuously mixed for 5 hours.

8. Filtered the mixture.

9. Washed the solid with 2000ml of water, then filtered the slurry again. Repeated this washing process twice.

Dried the solid at temperatures less than 80°C. The final product is an antimony-containing magnesium-aluminum type of LDH.

EXAMPLE 11: Preparation of Antimony-containing LDH with Antimony-contained LDO 11. Dissolved 200g of ammonium chloride (NH 4 C1) into 2000mL of water.

12. Added 120g of antimony-containing LDO produced from Example 5 into the solution and continuously mixed for 6 hours.

13. Filtered the mixture.

14. Washed the solid with 1000ml of water, then filtered the slurry again. Repeated this washing process twice.