Brominated polystyrene having improved thermal stability and color and process for the preparation thereof

Abstract

In a process for the suppression of backbone halogenation during the bromination of polystyrenes comprising the step of pretreating a solution of a polystyrene reactant comprising from about five to about 20 percent by weight of a polystyrene reactant, in a halogenated hydrocarbon solvent with at least about 0.1 percent by weight of an additive to suppress halogenation of the polystyrene backbone prior to brominating the polystyrene in the presence of a catalyst; adding a metal halide bromination catalyst to the solution, capable of effecting bromination of the ring without causing crosslinking of the polystyrene; adding to the solution from about 1 to about 3.4 moles of a brominating agent, per mole of polystyrene repeating units; reacting the polystyrene reactant with the brominating agent at a reaction temperature of from about -20°C C. to about 50°C C., and isolating the brominated polystyrene. A brominated polystyrene is also provided having a backbone halogen content of less than about 750 ppm and, a ΔE color as low as about 5.

Description

This application is a division of application Ser. No. 08/661,350, filed Jun. 14, 1996 now U.S. Pat. No. 5,637,650.

TECHNICAL FIELD

Brominated polystyrene is used as an additive to thermoplastics to impart flame retardant properties. In addition to thermal stability, it is necessary and desirable for these additives to impart essentially no color to the thermoplastic. The evolution of engineering thermoplastics has resulted in specialty polymers with much higher heat resistance and, as a result, a need to process these new materials at ever increasing temperatures. Because of higher and higher processing temperatures, the flame retardant additives used in these engineering thermoplastics must have a higher order of thermal stability and better color than that required in the past. Accordingly, this invention generally relates to a brominated polystyrene having improved color and thermal stability. More particularly, the invention relates to a process for the bromination of polystyrene which overcomes the limitations of current technology by use of an additive to suppress backbone halogenation.

BACKGROUND OF THE INVENTION

Reports of the use of brominated polystyrene as a flame retardant additive in thermoplastics extend back more than twenty-five years. In 1980, Ferro Corporation, the Assignee of record herein, introduced brominated polystyrene as a commercial flame retardant additive under the trade name PyroChek® 68PB. The process for producing PyroChek® 68PB is described in U.S. Pat. No. 4,352,909. This product has become a leading flame retardant additive for use in reinforced engineering thermoplastics. More recently, Great Lakes Chemical has introduced a second brominated polystyrene product, PDBS-80, to the marketplace. This product also finds its primary application in engineering thermoplastics.

Thus, there are currently two different synthetic routes available for the commercial production of brominated polystyrenes. Each process has certain advantages and disadvantages which should be noted in order to fully understand the significance of the present invention.

The process used to produce PDBS-80, the commercial product offered by Great Lakes Chemical, is described in U.S. Pat. No. 5,369,202. It involves four chemical steps starting from styrene monomer. The first step involves the addition of HBr across the double bond of the styrene in order to protect it. In the second step, this intermediate is brominated on the ring using conventional technology. Usually an average of two bromines are introduced. The second intermediate is then reacted with strong inorganic base. This eliminates hydrogen bromine from the bromoethyl group of the second intermediate, reforming the double bond to produce brominated styrene monomer. After purification, this monomer is polymerized to form the brominated polystyrene product. The entire process may be represented as follows:

This process has one significant advantage. It produces a brominated polystyrene which is essentially free of backbone halogen. This results in a product with very good thermal stability, good color, and good color stability. However, the process has two serious limitations which are major disadvantages when compared to the alternate process.

a. The process involves four distinct chemical reactions as well as several other unit operations. It is a complex process requiring a complex manufacturing facility with a high capital cost and the multiplicity of steps results in a long process. This process is inherently expensive.

b. Brominated styrene monomers are very reactive and difficult to handle. Ideally, a brominated aromatic flame retardant additive should have a high bromine content in order to have maximum efficiency and minimum cost. Thus, it would be preferable to produce and polymerize tribromostyrene monomer. However, this monomer is a highly reactive solid with low volatility. It is difficult to handle and polymerize and any residual monomer in the polymer would be difficult to remove. Consequently, this process tends to be limited to dibromostyrene as the maximum degree of bromination practical by this process. This limits the bromine content of the commercial brominated polystyrene (PDBS-80) to about 60%. Consequently, when used as a flame retardant additive, a relatively high use level is required to achieve flame retardance. This makes the product expensive to use. But of even greater concern to the user is the fact that high use levels cause deterioration of the important physical properties of the host resin. This result is frequently unacceptable to the user.

The process used by Ferro Corporation to produce its brominated polystyrene flame retardant additive. PyroChek® 68PB is described in the aforementioned U.S. Pat. No. 4,352,909. This process has many advantages over the process which involves the production and polymerization of brominated styrene monomer. Some of these include:

a. The process involves only a single chemical reaction, the bromination of commercially available polystyrene dissolved in a commercially available solvent using a commercially available brominating agent, bromine chloride. The process can be carried out in a simpler plant with a much lower capital cost. This process is inherently less expensive than the production of brominated polystyrene by the preparation and polymerization of brominated styrene monomer.

b. Because the process never involves the formation and handling of brominated styrene monomer, it does not have the limitations of the other process. It is possible to achieve tribromination and approach bromine contents of 70%. Since the brominating agent is less expensive than the polystyrene raw material, this actually reduces the cost of the product. Further, higher bromine contents result in lower use levels to achieve flame retardance. This reduces costs. But of even greater importance, reduced use levels result in better retention of physical properties of the host resin.

c. The process allows the use of a wide variety of polystyrenes and this, in turn, allows for the production of a variety of brominated polystyrenes. Further, general purpose, crystal polystyrene is produced in very large volumes in every part of the developed and developing world. This makes it readily available and inexpensive.

Notwithstanding the many advantages this process has over the process for making brominated polystyrene from monomer, a disadvantage exists which is beginning to limit the value and versatility of this product. In particular, while the process puts most of the bromine on the aromatic ring of the polystyrene, it also puts a small but significant amount of bromine and chlorine on the backbone. Typically, the amount of halogen, reported as HBr, on the backbone is 5000-6000 ppm, as measured by a test procedure described in detail hereinbelow. This backbone halogen is the direct cause of the limited thermal stability of brominated polystyrenes produced in this manner and is the direct cause of both its problems regarding initial color and color stability during thermal processing. Under the conditions of thermal processing, the backbone halogen of the current brominated polystyrenes produced in this manner may be released causing corrosion of processing equipment and degradation of the host resin. The formation of unsaturation in the backbone of the brominated polystyrene also leads to a loss of good color during processing. Since the technology trend in engineering thermoplastics is to higher and higher processing temperatures, the current brominated polystyrenes produced in this manner are becoming less acceptable in newer applications.

When brominated polystyrene is employed as a flame retardant additive in thermoplastics, its color is a property of primary importance to the manufacturer of the thermoplastic materials. The thermoplastic manufacturer desires to produce the thermoplastic articles in a wide range of colors. The more highly colored an additive, the more difficult it becomes to match (produce) a broad range of colors. The more lightly colored the additive, the easier it becomes to produce a wide range of colors. Therefore, in view of the needs of the manufacturer of thermoplastic parts, and in view of the inadequacy of prior art processes to produce a highly brominated polystyrene having the desired light color characteristics, a need exists for a highly brominated polystyrene with an improved light appearance as manufactured so that the end user can formulate a wide range of colors and thereby better meet the needs and demands of the marketplace.

SUMMARY OF INVENTION

It is therefore, an object of the present invention to provide a process for highly brominating polystyrenes which allows the direct bromination of polystyrene to produce a product with excellent thermal stability, excellent color, good color stability, and a minimum of backbone halogen.

It is another object of the present invention to provide a process which can be carried out in the existing facilities for the bromination of polystyrene without modification, without any additional capital investment, and with an absolute minimum of increase in raw material cost.

It is yet another object of the present invention to provide a process which utilizes an additive for the suppression of backbone halogenation, thereby allowing the operator to obtain highly brominated polystyrenes having improved thermal stability and color.

It is another object of the present invention to provide a highly brominated polystyrene having improved color and thermal stability and with less backbone halogen.

At least one or more of the foregoing objectives, together with the advantages thereof over existing prior art forms, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.

In general, a process for the suppression of backbone halogenation during the bromination of polystyrenes comprises pretreating a solution of a polystyrene reactant comprising from about five to about 20 percent by weight of a polystyrene reactant, in a halogenated hydrocarbon solvent with at least about 0.1 percent by weight of an additive to suppress halogenation of the polystyrene backbone, prior to brominating the polystyrene in the presence of a catalyst; gradually adding a metal halide bromination catalyst to the solution, capable of effecting bromination of the ring without causing crosslinking of the polystyrene; adding to the solution from about 1 to 3.4 moles of a brominating agent, per mole of polystyrene reactant repeating units; reacting the polystyrene reactant with the brominating agent at a reaction temperature of from about -20°C C. to about 50°C C., and isolating the brominated polystyrene.

The present invention also provides a brominated polystyrene having a backbone halogen content of less than about 750 ppm and, a ΔE color as low as about 5.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

Brominated polystyrene, marketed primarily as PyroChek® 68PB by Ferro Corporation, the Assignee of record herein, has been a leading flame retardant additive for engineering thermoplastics for many years. As currently produced, it can contain anywhere between 3000 to 6000 ppm and typically contains 5000 to 6000 ppm of backbone halogen, measured as HBr. This latter factor is the primary source of the poor thermal stability of the additive which, in turn, is a problem at higher processing temperatures. Furthermore, it has a Ser. No. 08/663,657everyN_2g) N_2(g) (flow rate=0.5 SCFH) for five minutes, then placed in the salt bath deep enough to surround the entire sample for 15 minutes. The sample is withdrawn from the bath and purged for another five minutes. The test tube containing the pyrolysed sample is removed and replaced with a clean empty test tube. This test tube with the N_2g N_2(g) purge is submerged in the salt bath for five minutes to flush out any residual HX.

After the test tube is rinsed, the gas dispersion tubes are carefully removed and rinsed with deionized (di) H₂O, keeping N_2(g) flow through the test tube during the rinse. Begin with the last collection flask and work back to the first. After all dispersion tubes are out, the empty test tube is removed. The Viton® tubing connecting each of the flasks is also rinsed with di H₂O. The contents of the flasks are combined and quantitatively transferred to bottles, rinsing with di H₂O, until the operator is ready to do titrations (described below). The solutions can be stored in these bottles with caps if the solution is kept alkaline. Two or three lest tubes containing no sample are run as blanks before the first sample each day of testing in order to verify that there is no residual HX in the system.

Once the samples have been pyrolysed and the HX gases collected, the bottled solutions are titrated in the analytical lab using a Metrohm 670 titroprocessor with an Ag combination electrode. Each sample solution is acidified with a 1:2 solution of HNO₃; Dl di H₂O, to a pH <7, and then filtrated titrated with standardized AgNO₃ to a potentiometric equivalence point. The parameters for the filtration titration are those which are recommended in the manual for the titroprocessor. Variations of those parameters are left to the discretion of the operator. The results are reported in duplicate as ppm HBr HCl, and ppm HBr Equivalents.

Calculations

ppm HBr=(Ep 1 mL * N_titrant * molecular wt, HBr * 1,000,000)/(wt. of Sample * 1000)11

ppm HCl=[(Ep2 mL-Ep1 mL) * N_titrant * molecular wt. HCl * 1,000,000]/ (wt. Sample * 1000)

ppm HBr Eq={[Ep2 mL-Ep1 mL) * N_titrant * molecular wt. of HBr * 1,000,000]/ (wt. Sample * 1000)}+ppm HBr where Ep=end point volume in mL and N_titrant=Normality of AgNO₃

Experimental Procedure

PyroChek® PB68 was produced in the laboratory by dissolving general purpose polystyrene in ethylene dichloride (EDC). Antimony trichloride (5% by weight based on polystyrene charged) was added as a catalyst. Then the brominating agent, bromine chloride, containing 10% EDC, was added gradually while maintaining the reaction temperature at 20°C C. The total reaction time was five hours to produce the product described hereinabove.

In the present invention, the process is identical to the general process above, with one important exception and that is, prior to the addition of the catalyst and the initiation of bromination, the backbone halogenation suppression additive was added to the solution of EDC and polystyrene and the solution was agitated for 30 minutes. Thereafter, the general process was followed.

In the first series of experiments the level of BHSA employed was varied to determine the effect on product quality. All experiments were carried out at 20°C C. with a 5 hour reaction time. The solvent was ethylene dichloride. The bromination catalyst was SbCl₃ used at a level of 0.023 moles per mole of polystyrene. The polystyrene solution containing the BHSA was agitated for 30 minutes before adding the bromination catalyst and initiating BrCl addition. Example No. 1 was a Control, Made according to U.S. Pat. No. 4,352,909, without any BHSA. Color was determined as Total Color Difference (ΔE), using the Hunter L, a, b scales, for product solution in chlorobenzene, 10 percent by weight concentration versus chlorobenzene, according to the formula: $\mathrm{\&Delta;}\&it;\&it;E=\sqrt{{\left(\mathrm{\&Delta;}\&it;\&it;L\right)}^2+{\left(\mathrm{\&Delta;}\&it;\&it;a_L\right)}^2+{\left(\mathrm{\&Delta;}\&it;\&it;b_L\right)}^2}$

Results are reported in Table I.

TABLE I
EFFECT OF BHSA LEVEL ON PRODUCT QUALITY
		BHSA Level(1)		HBr Equivalent
	Ex. No.	(Moles/Mole PS)	Color (ΔE)(2)	(ppm)(3)
	1	0	14.5	6000
	2	0.0055	9.6	912
	3	0.027	6.8	531
	4	0.055	7.3	673
	5	0.082	8.5	568
	(1)BHSA is TiCl4 in all experiments.
	(2)Color was measured as a 10% solution in chlorobenzene. The ΔE was determined by comparison with the color of pure chlorobenzene.
	(3)Amount released in 15 minutes at 300°C C.

It will be noted that backbone halogenation was decreased as the amount of BHSA was increased and, that the content of backbone halogen was decreased by an order of magnitude over the Control. Color was also improved over the Control by the use of a BHSA.

In the next series of experiments, the level of BHSA employed was constant and the effect of agitation time was varied to determine the effect on product quality. Example No. 1 was the Control from Table I, without any BHSA. All experiments were carried out at 20°C C. with a 5 hour reaction time. The bromination catalyst was SbCl₃ used at a level of 0.023 moles/mole of polystyrene. The solvent was ethylene dichloride.

TABLE II
EFFECT OF AGITATION TIME
	BHSA
	Level(1)	Agitation		HBr
	(Moles/	Time(2)	Color(3)	Equivalent(4)
Ex. No.	Mole PS)	(Minutes)	(ΔE)	(ppm)
1	None	0	14.5	6000
6	0.082	0	15.2	1002
7	0.082	30	8.5	658	568
(1)BHSA is TiCl4 in all experiments.
(2)Time between addition of BHSA and addition of bromination catalyst and initialation of BrCl addition.
(3)Color was measured as a 10% solution in chlorobenzene. The ΔE was determined by comparison with pure chlorobenzene.
(4)Amount released in 15 minutes at 300°C C.

The use of a BHSA without prior agitation greatly reduced the level of backbone halogenation. However, the best results were obtained where the solution of polystyrene and BHSA was stirred together for at least 30 minutes.

In the next series of experiments, the brominating agent bromine was employed. Example No. 8 was another Control from Table I, without any BHSA. All experiments were carried out at 20°C C. with a 5 hour reaction time. The bromination catalyst was SbCl₃ used at a level of 0.023 moles/mole of polystyrene. In the experiment with the BHSA, the solution of polystyrene and BHSA was agitated for 30 minutes before initiation of bromination. The solvent was ethylene dichloride.

TABLE III
WITH BROMINE AS THE BROMINATING AGENT
		BHSA Level		HBr Equivalent
	Ex. No.	(Moles/Mole PS)1	Color (ΔE)2	(ppm)3
	8	0	21.2	5939
	9	0.082	10.5	750
	1BHSA is TiCl4
	2Color was measured as a 10% solution in chlorobenzene. The ΔE was determined by comparison with pure chlorobenzene.
	3Amount released in 15 minutes at 300°C C.

As evident from the foregoing results, the use of a BHSA is effective when bromine is used as the brominating agent.

In the next series of experiments, the effect of different solvents was considered, using the same amount of BHSA. Example No. 1 was the Control from Table I, without any BHSA. All experiments were carried out at 20°C C. with a 5 hour reaction time. The bromination catalyst was SbCl₃ used at a level of 0.023 moles/mole of polystyrene. The polystyrene solution containing the BHSA was agitated for thirty minutes before adding the bromination catalyst and initiating BrCl addition.

TABLE IV
EFFECT OF SOLVENT
				HBr
	BHSA Level1			Equivalent
Ex. No.	(Moles/Mole PS)	Solvent	Color (ΔE)2	(ppm)3
10	0.082	ClCH2CH2Cl	8.6	602
11	0.082	ClCH2CH2Cl	8.5	568
12	0.082	CH2Cl2	11.7	624
13	0.082	CH2Cl2	11.5	658
1	None	ClCH2CH2Cl	14.5	6000
1BHSA is TiCl4 in all experiments.
2Color was measured as a 10% solution in chlorobenzenz. The ΔE was determined by comparison with the color of pure chlorobenzene.
3Amount released in 15 minutes at 300°C C.

In the next series of experiments, two other materials were employed as the BHSA and compared against titanium tetrachloride and the Control, Example No. 1 from Table I, without any BHSA. All experiments were carried out at 20°C C. with a 5 hour reaction time. The bromination catalyst in each experiment was SbCl₃ at a use level of 0.023 moles per mole of polystyrene. The level of BHSA was 0.082 moles per mole of polystyrene. The solvent was ethylene dichloride. The polystyrene solution containing the BHSA was stirred for 0.5 hours before adding the bromination catalyst and initiating the addition of BrCl.

TABLE V
REPRESENTATIVE ADDITIVES PROVIDING SUPPRESSION OF
BACKBONE HALOGENATION
				HBr Equivalent
	Ex. No.	BHSA	Color (ΔE)1	(PPM)2
	1	None	14.5	6000
	14	TiCl4	8.5	568
	15	SnCl4	9.7	262
	16	BCl3	13.7	267
	1Color was measured as a 10% solution in chlorobenzene. The ΔE was determined by comparison with the color of pure chlorobenzene.
	2Amount released in 15 minutes at 300°C C.

It will be noted that backbone halogenation was decreased and color was also improved over the Control by the use of all three BHSA materials. Again, the content of backbone halogenation was decreased by an order of magnitude over the Control when a BHSA was employed.

In view of the foregoing results, the brominated polystyrene of the present invention provides a bromine content of at least about 66 percent by weight; backbone halogen, conventionally between 3000 to 6000 ppm, is reduced by as much as 95% and preferably 80 to 95% to less than about 750 ppm and preferably 250 ppm, and, a ΔE color conventionally between 13 and 16, is reduced by as much as 60% and preferably 40 to 60% to less than about 7 and as low as about 5. Thermal stability of the brominated polystyrene is assured and is increased because the backbone halogenation is so much lowered compared to conventional brominated polystyrenes.

Thus it should be evident that the process of the present invention is highly effective in preparing a brominated polystyrene having improved thermal stability and color.

Based upon the foregoing disclosure, it should now be apparent that the use of the process described herein will achieve the objectives set forth hereinabove. It is, therefore, to be understood that any variations evident fall within the scope of the claimed invention and thus, the selection of specific component elements can be determined without departing from the spirit of the invention herein disclosed and described. In particular, the brominating agent, catalysts and reaction temperatures and times and other reaction conditions according to the present invention are not necessarily limited to those discussed herein. Nor, is practice of the present invention necessarily limited to the use of titanium tetrachloride, tin tetrachloride or boron trichloride as the additive to suppress backbone halogenation during the bromination of polystyrenes. Thus, the scope of the invention shall include all modifications and variations that may fall within the scope of the attached claims.

Claims

1. A brominated polystyrene product made by the process comprising:

pretreating a solution of a polystyrene reactant comprising from about five to about 20 percent by weight of said polystyrene reactant, in a halogenated hydrocarbon solvent with at least about 0.1 percent by weight of an additive to suppress halogenation of the polystyrene backbone, prior to brominating the polystyrene in the presence of a catalyst;

subsequently adding a metal halide bromination catalyst to said solution, capable of effecting bromination of the ring without causing crosslinking of the polystyrene;

adding to said solution from about 1 to about 3.4 moles of a brominating agent, per mole of polystyrene repeating units;

reacting said polystyrene with said brominating agent at a temperature of from about -20°C C. to about 50°C C., and isolating the brominated polystyrene.

2. A brominated polystyrene having a backbone halogen content of less than about 750 ppm and, a ΔE color as low as about 5.

3. A brominated polystyrene, as in claim 2, having a bromine content of at least about 66 percent by weight.

4. A brominated polystyrene, as in claim 2, wherein said brominated polystyrene reactant is produced from a polystyrene having a weight average molecular weight of from about 500 to about 1,500,000.

5. A brominated polystyrene, as in claim 4, wherein said polystyrene reactant is selected from the group consisting of homopolystyrene, polystyrene oligomers, halogenated polystyrenes and alkylated polystyrene.

6. A brominated polystyrene process as in claim 2, wherein said a backbone halogen content is 580 ppm.

7. A brominated polystyrene process as in claim 2, wherein said ΔE color is 8.5.

8. A flame retardant additive comprising the brominated polystyrene, as in claim 2.

9. A brominated polystyrene flame retardant which is a derivative of polystyrene and which has a bromine content of at least about 66 wt % and a ΔE color of less than about 7.8 to as low as 5.

10. The brominated polystyrene flame retardant of claim 9 wherein the ΔE color is less than about 6.8 to as low as about 5.

11. The brominated polystyrene flame retardant of claim 9 wherein the ΔE color is less than about 7 to as low as about 5.

12. A brominated polystyrene flame retardant which is a derivative of polystyrene and which has a bromine content of at least about 66 wt %, a ΔE color of less than about 7.8 to as low as 5 and a backbone halogen content from about 250 to about 750 ppm.

13. The brominated polystyrene flame retardant of claim 12 wherein the ΔE color is less than about 6.8 to as low as about 5.

14. The brominated polystyrene flame retardant of claim 12 wherein the ΔE color is less than about 7 to as low as about 5.