WO2000050506A1

WO2000050506A1 - Stable brominated polystyrene

Info

Publication number: WO2000050506A1
Application number: PCT/US2000/003473
Authority: WO
Inventors: James C. Gill; James L. Dever
Original assignee: Albemarle Corporation
Priority date: 1999-02-24
Filing date: 2000-02-10
Publication date: 2000-08-31
Also published as: AU3360300A

Abstract

A stable brominated polystyrene comprising a stabilizer in at least an effective amount to suppress the release of essentially all the thermally unstable halogen contained in the unstable brominated polystyrene, as generated at 300°C in 15 minutes. A process for the production of a stable brominated polystyrene comprising providing a source of brominated polystyrene having thermally unstable backbone halogen; and adding a stabilizer for the brominated polystyrene in at least an amount effective to suppress the release of essentially all the thermally unstable halogen contained in the brominated polystyrene, as generated at 300°C in 15 minutes.

Description

STABLE BROMINATED POLYSTYRENE

TECHNICAL FIELD

Brominated polystyrene is used as an additive to thermoplastics to impart lame retardant properties. The evolution of engineering thermoplastics has resulted in materials having 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 stability than that required in the past. Accordingly, this invention generally relates to a brominated polystyrene having improved stability. The invention also relates to a process for the stabilization of brominated polystyrene which overcomes the limitations of current technology by addition of a stabilizer to the brominated product.

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. The preferred process for making brominated polystyrenes has involved dissolving polystyrene in an appropriate solvent and reacting the polystyrene with a brominating agent using a catalyst which facilitates the introduction of bromine onto the aromatic rings. This basic process has many advantages over producing brominated polystyrenes by polymerization of brominated monomers. 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 commercially available brominating agents, bromine chloride or bromine. 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 monomers, it does not have the limitations of the other process. It is possible to achieve tribromination and approach bromine contents of 70%. 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 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 of this process, 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.

There are numerous references which describe a number of process variations for brominating polystyrene. All of these produce a brominated polystyrene product containing varying amounts of backbone halogen. Several examples of recent references are summarized below. 1. Ferro Corporation, the Assignee of record herein, has been engaged in the commercial production and sale of a brominated polystyrene, PyroChek® 68 PB, as a flame retardant additive for many years. The process for producing this product is described in U.S. 4,352,909. PyroChek 68PB typically contains 3500-6000 ppm of backbone halogen reported as HBr. This has an adverse effect on the thermal stability of this product.

2. In 1990, Dow Chemical was granted a patent (U.S. 4,975,496) on a new, improved process for brominating polystyrene. The product of this process contains at least 1500 ppm of backbone halogen as measured by a titration procedure.

3. In 1997, Albemarle was granted a patent (U.S. 5,677,390) on a process for brominating polystyrene which was claimed to represent an improvement over the Dow process. The brominated polystyrene from this process had a backbone halogen content of 2800-3600 ppm measured as HBr via a titration procedure. 4. Recently, Ferro Corporation obtained a patent, U.S.

5,637,650, on a process for brominating polystyrene which employs an additive which suppresses backbone halogenation. While this process can produce a brominated polystyrene with a backbone halogen content of less than 750 ppm measured as HBr, the product still does contain some backbone halogen.

From this summary it may be seen that the literature processes for brominating polystyrene produce a product with varying amounts of backbone halogen. It is further clear that this backbone halogen represents a site for thermal degradation which limits the utilization of brominated polystyrene as a flame retardant additive. The present invention has great value because it provides technology for production of a stable brominated polystyrene regardless of which process is employed to brominate the polystyrene. The invention solves the problem of thermal instability resulting from the backbone halogen content of brominated polystyrene. This, in turn, allows the continued use of brominated polystyrene as the flame retardant additive of choice for engineering thermoplastics even as the processing temperatures of these materials continue to increase.

SUMMARY OF INVENTION It is therefore an objective of the present invention to provide highly brominated polystyrenes which are stable.

It is yet another objective of the present invention to provide additives which, when added to brominated polystyrenes, greatly enhance the stability of the resultant brominated polystyrenes. It is yet another objective of the present invention to provide additives which when added to the brominated polystyrenes completely suppress the release of any backbone halogen content of the brominated polystyrene.

It is a further objective to provide additives which, when added to brominated polystyrenes, increase the temperature at which decomposition of these materials is initiated.

It is another objective of the present invention to provide additives which, when added to brominated polystyrenes, significantly reduce the amount of discoloration experienced when brominated polystyrenes are processed at temperatures of 300°C or higher. Finally, it is an objective of the present invention to provide a process for stabilizing highly brominated polystyrenes.

In general, a process for the production of a stable brominated polystyrene comprises providing a source of brominated polystyrene having thermally unstable backbone halogen; and adding a stabilizer for the brominated polystyrene in at least an amount effective to suppress the release of essentially all the thermally unstable halogen contained in the brominated polystyrene, as generated at 300 °C in 15 minutes.

The present invention also provides a stable brominated polystyrene, comprising a stabilizer in at least an effective amount to suppress the release of essentially all the thermally unstable halogen contained in the brominated polystyrene, as generated at 300°C in 15 minutes.

The availability of highly brominated polystyrenes with enhanced stability will allow the continued use of these materials as excellent flame retardant additives in engineering thermoplastics.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing figure is a graph depicting the relationship between backbone halogen and the amount of an additive employed to stabilize brominated polystyrene required to completely suppress backbone halogen.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION Engineering thermoplastics (ETP's) have enjoyed tremendous growth during the past twenty-five years. One reason is that this class of materials, particularly the reinforced grades, have excellent heat resistance which makes them particularly useful for continuous use at elevated temperatures. In recent years, the development of new engineering thermoplastics has focused on materials with ever increasing heat resistance. This has been accomplished by creating new polymers with higher glass transition temperatures (Tg) or higher melting points (Tm). However, the development of these new materials has not been without complications. As the Tg or Tm increased substantially, the temperatures required to process these materials also underwent substantial increases. Fifteen years ago, ETP's were rarely processed at melt temperatures approaching 300°C. Today, that processing temperature is quite common and new materials may now go as high as 350°C when being processed. In order to be useful, all the additives used to formulate the newer ETP's must have sufficient thermal stability to survive the higher processing temperatures. This is especially true of the halogen containing additives used to impart flame retardance to ETP's. If these additives have inadequate thermal stability, they will degrade when processed at high temperatures and liberate hydrogen chloride and/or hydrogen bromide which are very corrosive. If these materials are released during processing they may degrade the ETP or, at the very least, cause corrosion of the equipment used to process the thermoplastic. If this latter process occurred, it could cause serious damage to the equipment. This, in turn, would result in major expenses to repair the equipment and a loss of production time on the equipment. Obviously, a user of halogenated flame retardant additives would be greatly concerned about the thermal stability of the additives that they use.

The backbone halogen content of brominated polystyrene, reported as parts per million of HBr equivalent, can vary from less than 250 ppm as reported in U.S. 5,637,650 to greater than 7500 ppm, as is sometimes encountered in the commercial product, PyroChek® 68PB. The exact amount which might be encountered varies as a function of the bromination process employed, the specific reaction conditions, the nature of the brominating agent, and the quality of the polystyrene being brominated.

While the present invention will produce a stable brominated polystyrene regardless of the process used to brominate the polystyrene, it should be obvious that the amount of additive required for full stabilization will vary as a function of the amount of backbone halogen content in the specific brominated polystyrene. However, what is not obvious is the fact that the relationship between the amount of backbone halogen in the brominated polystyrene and the amount of additive required for complete stabilization is not linear.

This is illustrated in the following table for a natural hydrotalcite marketed as DHT-4A, an additive available from Mitsui & Co., USA Inc. TABLE I

ADDITIVE AMOUNT FOR INCREASING LEVELS OF BACKBONE HALOGEN

BACKBONE HALOGEN CONTENT OF ADDITIVE REQUIRED FOR COMPLETE BROMINATED PS SUPPRESSION

1060 ppm HBr Equivalent 0.6%

1900 ppm HBr Equivalent 0.8%

4000 ppm HBr Equivalent 1.0%

As can be seen, it required 0.6% of the additive DHT-4A to fully suppress the backbone halogen content of a brominated polystyrene with 1060 ppm HBr equivalent backbone halogen content. However, only 0.2% more additive was required to suppress a second 900 ppm HBr and, in the third brominated polystyrene, only an additional 0.2% additive was required to suppress the additional 2100 ppm HBr contained in the third material. It is not clear why the relationship between backbone halogen content and the amount of additive required for suppression appears to be non-linear. It is possible that the explanation might be as simple as the difficulty in uniformly dispersing a very small amount of a powdered additive in a very large amount of brominated polystyrene. It may be due to the fact that the additive/brominated polystyrene interaction is a solid-solid interaction. The exact reason is unimportant. The key consideration is that the relationship is not linear.

The brominated polystyrene is stabilized according to the present invention by the addition of an effective amount of a stabilizer to suppress the release of essentially all the thermally unstable halogen contained in the brominated polystyrene. As used herein, this specification and the claims below, the recitation of "an effective amount of stabilizer to suppress all the thermally unstable halogen contained in the brominated polystyrene" means the addition or use of a sufficient level of stabilizer in the brominated polystyrene to prevent release of essentially all of the thermally unstable halogen confirmed to be in the brominated polystyrene, as defined in the thermal stability test procedure set forth hereinbelow.

By "essentially all of the thermally unstable halogen" is meant that none has been generated in the brominated polystyrene, while heating for 15 minutes at 300°C. i.e., 0 ppm. Where the standard of performance can tolerate a product with some thermally unstable halogen present, one to several hundred ppm of thermally unstable halogen can remain and an effective amount of stabilizer would be that amount capable of reducing the thermally unstable halogen to such levels. It is also within the present invention to reduce thermally unstable halogen by only several hundred ppm from the amount initially present if the desire of the end user is to have such a brominated polystyrene. In addition, suppression of "all" backbone halogen is a function of measuring the amount released while heating the product at 300°C for 15 minutes.

More specifically, in defining the essential property of the additives covered by this invention, this can best be accomplished by defining additive level requirements at various backbone halogen contents. The additives of this invention will completely suppress the backbone halogen content of brominated polystyrenes containing 1000 ppm backbone equivalent HBr or less at use levels of less than 0.7%. The additives of this invention will completely suppress the backbone halogen content of brominated polystyrenes containing 2000 ppm backbone equivalent HBr or less at use levels of less than 0.9% of additive. For the stabilization of brominated polystyrenes with backbone halogen contents greater than 2000 ppm, the amounts of the additives of this invention required for full stabilization can be easily determined experimentally. The relationship between additive amount and backbone halogen is depicted in the drawing figure which clearly reveals that the relationship is non-linear.

While the foregoing amounts are the least or minimum amounts effective to stabilize the release of thermally unstable halogen, it is to be understood that greater amounts are not precluded. Thus, the practice of the invention may involve the use of considerably higher amounts, for example, 2 to 6 percent by weight. Preferably then, unless or until cost becomes a factor, at least an effective amount is employed and greater amounts are optional. Of course, as the level of additive is increased, it is with the understanding that either the thermally unstable halogen is totally suppressed or, greater levels of the thermally unstable halogen can be suppressed. Before proceeding further, it is important to note that stabilization is not achieved simply by employing an acid acceptor to scavenge HX liberated when the brominated polystyrene (BrPS) decomposes. A key factor in this invention is the fact that many materials which are very basic and have a high capacity to absorb acid are not very good as BrPS stabilizers. For example, one gram of calcium oxide is capable of neutralizing 35.6 millimoles of acid while one gram of hydrotalcite is capable of reacting with only 3.3 millimoles of acid. Yet, as shown by the data in Table II, hydrotalcite is much more effective than calcium oxide in stabilizing BrPS. Using a brominated polystyrene having a backbone halogen of 1900 ppm as a control (Ex. No. 1), calcium oxide was added in an amount of 1 percent by weight (Ex. No. 2), and hydrotalcite was added in an amount of 0.8 percent by weight (Ex. No. 3).

TABLE II

BROMINATED POLYSTYRENE BACKBONE HALOGENATION Exp. Additive HBr Color (after 15

Equivalent min at 300°Q (ppm)

1 - 1900 66

2 CaO (1%) 900 47

3 Hydrotalcite (0.8%) 0 39

It is significant to note that calcium oxide only suppressed about one-half of backbone halogen while hydrotalcite completely suppressed it. When the BrPS was heated to 300°C as the control, color, measured as ΔE, described hereinbelow, was very high at 69, making the product unacceptable in such applications. When calcium oxide was added to this brominated polystyrene, the color was 47. Yet when hydrotalcite was added, the color did not increase as it had for the unstabilized brominated. Instead, the color was stablilized.

To be suitable for practice of the present invention a stabilizer will possess the following properties: a. It should be water insoluble or non-extractable from the host resin when used in an flame retardant (FR) system. It should not be moisture sensitive, should not hydrolyze, etc. This is critical to avoid negative impact on electrical properties when BrPS is used as an FR. b. It should have excellent thermal stability as it may be processed at temperatures of up to about 340°C when incorporated into various

ETP's. The higher temperatures can be a factor in increasing throughput rates and accordingly, productivity. c. It should not react with host resins when present in BrPS as an FR. Also, there should be no accompanying degradation of physical properties brought about by the presence of the BrPS. d. It should stabilize the thermal color of BrPS. e. It should have good efficiency as a stabilizer, that is, work at low levels to insure against deleteriously affecting physical properties of the host ETP. It is critical that a stabilizer for BrPS be capable of effective performance at very low use levels. When the BrPS is used as a flame retardant, it is crucial that additives which have the potential for reacting with the host resin be eliminated or incorporated at such low levels that they have little or no effect on resin properties. As noted hereinabove, many materials that are basic are capable of neutralizing acid. However, they frequently show low efficiency in stabilizing BrPS or are water soluble or moisture sensitive or have poor thermal stability. Accordingly, these materials cannot be viewed as practical stabilizers for BrPS.

There are many materials that will be suitable as stabilizers and, as an example, suitable additives according to the present invention include natural hydrotalcite (DHT 4A), available from Mitsui & Co, USA Inc.; synthetic hydrotalcite (Hysafe 510 and 539) available from JM Huber Corp. and (L55 Rll) available from Reheis Ireland; zeolites (Valfor 100, a type A zeolite), available from PQ Corporation; zinc complexes including zinc oxide pentaerythritol (ZnPE 8136) available from Ferro Corp. and prepared according to U.S. Pat. No.5,576,452, the subject matter of which is incorporated herein by reference, and zinc amino acid complexes (Zn amino acid 8102) available from Ferro Corp. and of the type described in U.S. Pat. No. 4,425,280, the subject matter of which is incorporated herein by reference; di-ammonium phosphate; melamine and, compounds similar to the foregoing. It is to be noted however, that di-ammonium phosphate is a water sensitive compound and that melamine may react with some ETP's and thus, these would not be suitable for all uses.

The effect of these additives on BrPS having a backbone halogen content of 1900 ppm, and on a BrPS having a backbone halogen content of 1060 ppm, is shown in the following table. The amount required for total backbone halogen suppression is expressed as Amount Required, weight percent.

TABLE III

BROMINATED POLYSTYRENE

BACKBONE HALOGEN CONTENT - 1900 PPM HBR EQUIVALENT

ADDITIVE PERFORMANCE

Ex. No. ADDITIVE AMOUNT REQUIRED (WT%)

4 Zinc/Amino Acid Complex 0.8

5 Hydrotalcite DHT-4A 0.8

6 Hydrotalcite Hysafe 510 0.6

7 Hydrotalcite Hysafe 539 0.6

8 Hydrotalcite L55 Rll 0.8

9 Melamine 0.6

10 Zinc Oxide/Pentaerythritol 0.6 Complex

BACKBONE HALOGEN CONTENT - 1060 PPM HBR EQUIVALENT

Ex. No. ADDITIVE AMOUNT REQUIRED (WT%)

11 Hydrotalcite DHT-4A 0.6

12 Hydrotalcite Hysafe 510 0.4 13 Zeolite Valfor 100 0.6

14 Melamine 0.4

As is known, thermal stability of brominated polystyrene is superior for polystyrene products that are essentially ring halogenated versus those containing backbone halogen in addition to ring halogen. Accordingly, when brominating polystyrene, the ideal result is to place 100% of the halogen on the aromatic ring of the polystyrene and to have no halogen on the backbone of the polymer. By way of explanation, bonds between benzyl ic (backbone) carbon atoms and halogen atoms are less thermodynamically stable than bonds between aromatic ring carbons and halogen atoms. For example, the bond dissociation energy of a C(benzylic) Br bond is 51 kcal/mole while that of a C(aromatic) Br bond is 71 kcal/mole. This means that a C(benzylic) Br bond will break down at a lower temperature than the C(aromatic) Br bond. When this occurs, the very corrosive hydrogen bromide is released and a double bond is formed. As the number of double bonds in the backbone increases, the color quality of the brominated polystyrene will diminish. Hence backbone halogenation is to be avoided.

There is a graphic demonstration of the stability of ring bromine vs backbone bromine. It is possible, on a laboratory scale, to produce polyftri- bromostyrene) from tribromostyrene monomer. Brominated polystyrene made in this way contains no backbone halogen. It contains 70.3% bromine, all of which is on the ring. On the other hand, brominated polystyrene made commercially via the process of U.S. 4,352,909 contains backbone halogen. A thermal stability test procedure, detailed hereinbelow, involves heating the brominated polystyrene for 15 minutes at 300°C and measuring the total amount of hydrogen halide liberated during the test period. When this is done on the commercial brominated polystyrene, 3000-6000 ppm of HBr equivalent are liberated. When the same test is carried out on poly(tribromostyrene) made from monomer no HBr was detected. This shows that aromatic bromines are 100% stable at 300°C while backbone halogen is sufficiently unstable to be eliminated well below 300°C.

As a supplier of halogenated flame retardant additives, Ferro Corporation, the Assignee of record, believed that it was very important to develop a simple screening test which would allow for the evaluation of the relative thermal stability of various halogenated flame retardants. Conceptually, the procedure is very simple. A carefully measured sample (2.00 + 0.01g) of additive is exposed to a temperature of 300 °C for fifteen minutes. All of the acidic gases (HCI, HBr) generated during the period are collected in a standard aqueous solution of NaOH. This solution is acidified to a pH < 7 and then is titrated with standardized silver nitrate using a potentiometric titrimeter. This measures the parts per million of HCI and HBr that were released during the heating period. In the interest of simplicity, the ppm of HCI are converted into ppm HBr, this is added to the amount of HBr already measured, and the resultant number is reported as ppm of HBr equivalent. The larger the amount of HBr equivalent reported, the less thermally stable is the given additive. An additive releasing 0 ppm of HBr equivalent would have the best thermal stability. A detailed description of the test procedure follows.

THERMAL STABILITY TEST PROCEDURE The apparatus was assembled in a fume hood. A 2.00 + 0.01g sample was weighed in a 20 x 150mm tared test tube. Three 250mL sidearm filter flasks were filled with 150-170mL of 0.1 N NaOH (enough to completely cover the frit) containing phenolphthalein (2% w/v solution in 3A EtOH), and were connected with Viton^® tubing. This allowed the acidic gases generated by a sample in the test tube to be passed through the aqueous NaOH, thus trapping the HBr and/or HCI (HX). The test tube containing the sample was fitted with a number 2 neoprene stopper with a 1/16" inlet and a 7mm outlet for Teflon^® tubing. The sample was purged with N2₍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 was withdrawn from the bath and purged for another five minutes. The test tube containing the pyrolysed sample was removed and replaced with a clean empty test tube. This test tube with the N2(g) purge was submerged in the salt bath for five minutes to flush out any residual HX.

After the test tube was rinsed, the gas dispersion tubes were carefully removed and rinsed with deionized water, keeping N2(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 were out, the empty test tube was removed. The Viton^® tubing connecting each of the flasks was also rinsed with deionized water. The contents of the flasks were combined and quantitatively transferred to bottles, rinsing with deionized water, until the operator was ready to conduct titrations (described below). The solutions can be stored in these bottles with caps if the solution is kept alkaline. Two or three test tubes containing no sample were run as blanks before the first sample each day of testing in order to verify that there was no residual HX in the system. Once the samples had been pyrolysed and the HX gases collected, the bottled solutions were titrated in the analytical lab using a Metrohm 670 titroprocessor with an Ag combination electrode. Each sample solution was acidified with a 1:2 solution of HNO3; deionized water, to a pH <7, and then titrated with standardized AgNθ3 to a potentiometric equivalence point. The parameters for the titration are those which are recommended in the manual for the titroprocessor. Variations of those parameters were left to the discretion of the operator. The results are reported as ppm HBr HCI, and ppm HBr Equivalents. Calculations:

ppm HBr = (Ep1 mL * Ntftrant * molecular wt. HBr * 1,000,000) / (wt. of Sample * 1000)11 ppm HCI = [(Ep2 mL - Ep1 mL) * Ntftrant * molecular wt. HCI * 1,000,000] / (wt. Sample * 1000) ppm HBr Eq = {[Ep2 mL - Ep1 mL) * N_tjtrant * molecular wt. of HBr * 1,000,000] / (wt. Sample * 1000)} + ppm HBr where Ep = end point volume in mL and Nfjfran = Normality of AgNθ3

In addition to characterizing the thermal stability as a function of backbone halogen content as just described, it is possible and useful to characterize the thermal stability of brominated polystyrenes as a function of the temperature at which thermal decomposition starts to occur. The higher this temperature, the more stable the brominated polystyrene. This temperature may be determined through the use of differential scanning calorimetry (DSQ.

Differential scanning calorimetry (DSQ is one of several routine thermal analytical techniques. In this method, a sample is heated at steadily increasing temperatures using fixed heating rates usually expressed in degrees per minute (e.g. 10°C per minute). Any thermal transitions, either endothermic or exothermic which are experienced by the sample show up as a deviation from the base line formed by plotting time against temperature. Since materials such as brominated polystyrene usually undergo exothermic decomposition, the use of DSC will de ine the temperature of on-set of decomposition as the point at which a significant exotherm is generated resulting in a significant deviation from the base line. Thermal analytical techniques became established, accepted analytical tools more than thirty years ago. Today, a number of companies manufacture and market instrumentation for carrying out thermal analysis. Some of these include Mettler, Perkin-Elmer, and TA Instruments, Inc.

In the data provided hereinbelow, color was determined as Total Color Difference (ΔE), using the Hunter L, a, b, scales, for product solutions in chlorobenzene, 10 percent by weight concentration versus chlorobenzene, according to the formula:

Finally, the amount of color generated when brominated polystyrene is heated at 300 °C for fifteen minutes provides a further indication of the thermal stability of brominated polystyrene. The more color development, the less stable is the brominated polystyrene.

EXPERIMENTAL The stabilizers of the present invention may be blended with the brominated polystyrenes by any of the methods commonly used for the intimate mixing of two powders, one of which is present in small quantities. On a commercial scale, efficient blending can be achieved using a variety of blenders. Some of those which may be employed include ribbon blenders, double cone blenders, V-type mixers, and horizontal cylinder mixers.

On a laboratory scale, intimate mixing could be achieved by use of a mortar and pestle or by using a small ribbon blender. All experimental results presented in this application were generated on a laboratory scale. A. Suppression of Backbone Halogen

These results were already reported in Table III and are repeated here for convenience and completeness. The amount required for total backbone halogen suppression is expressed as Amount Required, weight percent.

TABLE III

BROMINATED POLYSTYRENE

BACKBONE HALOGEN CONTENT - 1900 PPM HBR EQUIVALENT

ADDITIVE PERFORMANCE

EX. NO. ADDITIVE AMOUNT REQUIRED (wτ%)

4 Zinc/Amino Acid Complex 0.8

5 Hydrotalcite DHT-4A 0.8

6 Hydrotalcite Hysafe 510 0.6

7 Hydrotalcite Hysafe 539 0.6

8 Hydrotalcite L55 Rll 0.8

9 Melamine 0.6

10 Zinc Oxide/Pentaerythritol 0.6 Complex

BACKBONE HALOGEN CONTENT - 1060 PPM HBR EQUIVALENT

EX. NO. ADDITIVE AMOUNT REQUIRED (WT%)

11 Hydrotalcite DHT-4A 0.6

12 Hydrotalcite Hysafe 510 0.4

13 Zeolite Valfor 100 0.6

14 Melamine 0.4

B. Increased Thermal Stability as Measured bv DSC The procedure for these determinations has already been described elsewhere in this application. The effectiveness of the stabilizers on enhancement of thermal stability of brominated polystyrenes has been demonstrated two ways using DSC. The data in Table IV demonstrates that the thermal stability of a given brominated polystyrene increases as a function of the amount of stabilizer added. At a level (0.6%) where this particular BrPS has its backbone halogen fully suppressed, its thermal stability is comparable to that of pure poly (tribromostyrene) made from monomer.

TABLE IV THERMAL STABILITY BY DIFFERENTIAL SCANNING CALORIMETRY (DSQ (BROMINATED POLYSTYRENE - BACKBONE HALOGENATION CONTENT 1900 PPM HBR EQUIVALENT, ADDITIVE - HYDROTALCITE H YSAFE 510)

Ex. No. Additive Level Temperature of

Weight % Onset of Decomposition (°Q

15 0 348

16 0.2 360

17 0.4^(a) 373^(a)

18 0.6 372

19 Standard-Pure 373

Poly (tribromostyrene)

(from monomer)

(a) This data point might appear to be inconsistent however, the 0.4% additive level was sufficient to essentially suppress all backbone halogen. This resulted in the decomposition temperature indicated.

In Table V, the data demonstrates that thermal stability is a function of backbone halogen content and that when each stabilizer is used at an effective level, the thermal stability of the brominated polystyrene is virtually identical to that of poly(tribromostyrene) made from monomer on a laboratory scale. This latter material is free of backbone halogen. TABLE V THERMAL STABILITY BY DSC

Ex. No. Brominated Polystyrene Additive Weight % Temperature of Onset of Backbone Halogen Decomposition (°Q (ppm HBr Equivalent)

20 1000 None 0 358

21 1900 None 0 348

22 4000 None 0 325

23 1900 Hydrotalcite HySafe 539 0.6 381

24 1900 Hydrotalcite - DHT4A 0.8 373

25 1900 Melamine 0.4 368

19 Standard-Pure None 0 373

PolyfTribromostyrene)

(from monomer)

„„-„_^ 20 PCT US00/03473 00/50506 "^tw

C. Stabilization of Thermal Color

The stabilizers of the present invention provide stability to the thermal color of brominated polystyrenes. The procedure for establishing thermal color has already been described.

Stabilization of thermal color has been demonstrated two ways. In Table VI, the effect on thermal color of a range of stabilizers in comparison with the thermal color of the same brominated polystyrene which contains no stabilizer. It is evident that all stabilizers employed greatly reduce the amount of color development observed.

TABLE VI

STABILIZATION OF THERMAL COLOR (ΔE₃₀₀) (BROMINATED POLYSTYRENE - BACKBONE HALOGENATION CONTENT 1900 PPM HBR EQUIVALENT)

EXP. ADDITIVE AMOUNT THERMAL COLOR (ΔE₃₀₀) WEIGHT

%

15 None 0 66

26 Hydrotalcite-Hysafe 1.0 41.3 510

27 Hydrotalcite - Hysafe 1.0 39.6 539

28 Zeolite-Valfor 100 1.0 47.5

29 Zinc/Amino Acid 1.0 45.9 Complex

30 ZnO/PE Complex 1.0 45.8

31 Melamine 1.0 47.6

32 Di-ammonium 1.0 46.1 phosphate

In Table VII the effect of various levels of a single stabilizer on a single brominated polystyrene is presented. As can be seen, the amount of color MISSING AT THE TIME OF PUBLICATION

O 00/50506 22

Based upon the foregoing disclosure, it should now be apparent that the use of the stabilizers 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 zeolites or synthetic and natural hydrotalcites as the additives to provide stability to brominated polystyrenes. Thus, the scope of the invention shall include all modifications and variations that may fall within the scope of the attached claims.

Claims

CLAIMS What is claimed is: 1. A stable brominated polystyrene comprising a stabilizer in at least an effective amount to suppress the release of essentially all the thermally unstable backbone halogen contained in said unstable brominated polystyrene, as generated at 300°C in 15 minutes.

2. A stabilized brominated polystyrene as in claim 2, wherein said brominated polystyrene is manufactured from polystyrenes selected from the group consisting of homopolystyrene, polystyrene oligomers, halogenated polystyrenes and alkylated polystyrenes.

3. A stabilized brominated polystyrene, as in claim 2, wherein said stabilizer selected from the group consisting of synthetic and natural hydrotalcites, zeolites, organozinc complexes, di-ammonium phosphate, melamine and mixtures thereof.

4. A process for the production of a stable brominated polystyrene wherein said brominated polystyrene is produced comprising: providing a source of brominated polystyrene having thermally unstable backbone halogen; and adding a stabilizer for said brominated polystyrene in at least an amount effective to suppress the release of essentially all thermally unstable halogen contained in said brominated polystyrene, as generated at 300°C in 15 minutes.

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

6. A process as in claim 4, wherein said stabilizer is selected from the group consisting of synthetic and natural hydrotalcites, zeolites, organozinc complexes, di-ammonium phosphate, melamine and mixtures thereof.

7. A process as in claim 4, wherein said unstable brominated polystyrene includes a backbone halogen content of from about 250 to about 7500 ppm HBr equivalent.

8. A brominated polystyrene product made according to the process of claim 4.