Abstract:
A novel flame-retardant material is prepared by copolymerizing pentabromobenzyl-acrylate with a metal salt of a compound of Formula I: ##STR1## wherein R 1  is H or lower alkyl, and R 2  is selected from H, lower alkyl and ##STR2## The novel flame retardant has improved thermal stability and is useful in a variety of uses, e.g. as a flame retardant in engineering plastics and styrenic.

Description:
FIELD OF THE INVENTION 
     The present invention relates to novel flame-retardant compositions having improved thermal stability, to plastic materials containing it, and to a method for improving the thermal stability of synthetic resins. More particularly, the invention refers to flame-retardant compositions containing copolymers of poly-pentabromobenzyl acrylate (PBB-PA). 
     BACKGROUND OF THE INVENTION 
     PBB-PA has been known for some time, and is a valuable flame-retardant material, useful in a number of synthetic resins. PBB-PA is prepared by the polymerization of pentabromobenzylester acrylate (PBB-MA), e.g., by the process described in DE 25 27 802. The PBB-PA polymeric flame-retardant material is incorporated into the synthetic resin to which it is desired to impart flame-retardant properties, by techniques known in the art. 
     The use of this valuable flame-retardant material, however, has been somewhat limited so far because of its limited thermal stability. Thermal stability, as will be known to the skilled person, is an important property both in the lifetime of the synthetic resin which embodies the flame-retardant material, as well as during its preparation. The use of many flame-retardant materials is limited, because the compounding temperatures required in order to incorporate it into a given resin are too high, and the flame-retardant material is not thermally stable enough to permit its compounding at such temperatures. Consequently, flame-retardant materials which would be efficient in certain resins are not used, because it is impossible to effect the compounding operation, without adversely affecting the flame-retardant material. 
     SUMMARY OF THE INVENTION 
     It has now been found, and this is an object of the present invention, that it is possible to provide copolymers based on PBB-MA, which possess improved thermal stability, and which thus can be employed with a number of synthetic resins requiring high compounding temperatures. It has further been surprisingly found that it is possible, by relatively slight modifications, to obtain the advantages provided by the improved characteristics of the flame-retardant material, without causing the final compound which incorporates it substantially to depart from the behavior of a final compound embodying substantially pure PBB-PA. This, as will be apparent to the skilled person, is very important, because it opens the door for a wide use of the product obtained according to this invention. 
     Thus, the invention is primarily directed to a novel flame-retardant material comprising a copolymer resulting from the copolymerization of pentabromobenzyl acrylate with a metal salt of a compound of Formula I: ##STR3## and wherein R 1  is H or lower alkyl, and R 2  is selected from H, lower alkyl and ##STR4## According to a preferred embodiment of the invention, the metal, generally indicated by M in this specification, is selected from Mg, Zn, Na and K. Still more preferably, the metal salt is selected from the group consisting essentially of sodium acrylate, magnesium acrylate, zinc acrylate, potassium acrylate, bis-sodium-maleate, and their mixtures. 
     The amount of metal salt contained should normally not exceed 50% by mole of the total content of the flame-retardant material. 
     Thus, the copolymer of the invention will be constituted of the repetitive units shown in formula II ##STR5## wherein R 1 , R 2  and M have the meanings defined above, and the ratio x/n is equal or less than 1. 
     It should be noted that the improvement in thermal stability properties has a maximum, and that the addition of too much salt may impair the properties of the resulting product. Thus, for instance, when sodium acrylate is employed, the amount of sodium acrylate employed should preferably not exceed 7%, and should more preferably be comprised between 1.5 and 9 wt. %, still more preferably, about 3 wt. %. 
     The invention, as will be understood by the skilled chemist, also embraces a method for imparting thermal stability to a PBB-PA-based polymer, which method comprises copolymerizing PBB-MA with a compound of Formula I, as hereinbefore defined. Thus, any of the known polymerization processes which polymerize PBB-MA to obtain PBB-PA can be easily modified, according to the invention, by adding a compound of Formula I to the polymerization mixture, to obtain a copolymer as herein described. 
     Flame-retarded synthetic articles which contain a flame-retardant material according to the invention, of course also form part of this invention. 
     The process and advantages of the invention and of the resulting novel flame-retardant composition will now be further illustrated through the following examples, which are provided for the purpose of illustration and are not intended to constitute a limitation of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Example 1 
     Preparation in solution 
     To a 50 ml, three-necked flask containing 0.72 g (0.01 mole) of acrylic acid there are added 0.95 g of NaHCO 3  which were previously slurried in about 1 ml of water. This addition results in the evolution of CO 2  gas, and when evolution is completed methylethyl ketone, 25 ml, is added, together with 22.4 g PBB-MA (0.04 mole). Nitrogen is bubbled through the solution for a few minutes. The flask, which is equipped with a stirrer and a condenser is put into a thermostatic bath preheated to 100°-105° C. During the first minutes, the mixture is very viscous and is difficultly stirred, but the materials then dissolve almost completely and stirring becomes easier. At this stage, the initiator, benzoyl peroxide (0.04 g in crystalline form), which has been previously dissolved in 1 ml methylethyl ketone is added. The mixture obtained is heated for about one half hour, during which time a heavy precipitate is formed. When this period has lapsed, the mixture is cooled to room temperature, diluted with methylene chloride and vacuum-filtered and washed again with methylene chloride. 22.5 g of a dry solid are obtained, with a yield in excess of 95%. The above process was carried out with 8 different PBB-MA/sodium acrylate ratios, and the resulting product was tested for thermal stability. Thermal stability was tested as follows: heating with a heating rate of 50° C. per minute in a nitrogen atmosphere up to 290° C. and then keeping the sample at 290° C. for one half hour. Weight losses resulting from this procedure were recorded. The material tested in the thermal stability test was previously dried to constant weight. 
     The result for 10 experiments, together with the exact compositions, are reported in Table I below. The results clearly indicate a substantial improvement of the thermal stability of the product, throughout the entire range of sodium acrylate concentrations. Optimal results were obtained with about 2 to about 8 wt. % sodium acrylate in PBB-MA. It should be noted that the weight loss for pure PBB-PA under the test conditions was between 40-75%, while optimum conditions brought this weight loss to values as low as 3%. 
     
                       TABLE I______________________________________Thermal Stability of Product Polymerized in SolutionExperiment   gr Na Acrylate/                 Mole Na Acrylate/                               WeightNo.     100 gr PBB-MA Mole PBB-MA   loss (%)______________________________________1       0             0             542       0             0             40-543       0             0             754       0.5           0.03          185       1.0           0.06          106       2.1           0.12           37       4.2           0.24           38       8.4           0.50           39       12.6          0.76           610      16.8          1.00           7______________________________________ 
    
     It should be noted that the results for pure PBB-PA are erratic, giving weight losses ranging from 40% to 75% when all conditions are kept substantially constant. This does not happen with the compositions of the invention, in which only insignificant fluctuations of this value are noted. 
     The compositions according to the invention have been fully characterized as being of structure II, both by elemental and spectroscopic analyses. By way of illustration, the results for the material obtained according to Experiment No. 10, (16.8% Na-Acrylate) are reported below. 
     % Na:theoretical: 4.11, found: 3.94. % Br:theoretical: 59.75, found: 59.33. 
     FT-IR spectra of the product were prepared using KBr as a standard. One such spectrum is shown in FIG. 1, in which the characteristic peaks of the salt are seen at 1318 cm -1  and at 1564 cm -1 . The spectrum of pure PBB-PA (Experiment No. 2) is also shown in FIG. 2 for the purpose of comparison. 
     Example 2 
     Thermal Polymerization in an Extruder 
     A mixture of PBB-MA and sodium acrylate in the desired proportions, with and without a radical initiator, were polymerized in an extruder. The polymerization temperature was between 140°-240° C. The residence time in the extruder was 1-10 minutes. The polymer obtained in this process was ground to a powder, before thermal stability was evaluated. Thermal stability was evaluated as in the previous experiment, and the results are shown in Table II below. It can be seen from the results of this table that no substantial difference was seen between the product polymerized in solution, and that heat-polymerized in an extruder. 
     
                       TABLE II______________________________________Thermal Stability of Product Polymerized by ExtrusionExperiment           gr Na Acrylate/                              WeightNo.      Catalyst    100 g PBB-MA  loss (%)______________________________________1        none        3             62        Luperox-D-1 3             4    0.1%3        Luazo AD    3             4    0.05%______________________________________ 
    
     The product obtained in Experiment No. 1 was analyzed to give: % Na:Calcd: 0.73, found: 0.50. % Br:Calcd: 69.66, found: 69.31. 
     Example 3 
     Copolymerization with Different Salts 
     Example 1 was repeated, using a number of different salts, to show the effect of different compounds of Formula I on the thermal stability of the resulting product. The results are shown in Table III below, from which it can be appreciated that all salts tested give an appreciable improvement, as compared with the 40-75% weight loss of the product which does not contain any copolymerized salt. Small fluctuations in the thermal stability between different salts are not such as to substantially change this conclusion. 
     
                       TABLE III______________________________________Thermal Stability of Different Copolymers         gr Co-Monomer/                      Weight lossCo-Monomer    100 gr PBB-MA                      (%)______________________________________Mg-Acrylate   2            8Zn-Acrylate   2            9Bis-Na-Maleate         3            13K-Acrylate    2            5______________________________________ 
    
     It should be noted that, for the case of sodium acrylate which is believed to be the most convenient of the comonomers employed, when using up to 20 wt. % of sodium acrylate, a polymer is obtained which is a white flowable and homogeneous powder. Amounts higher than 20 wt. % result in two different products, a white powder and an amorphous product which has not been identified, but which is believed to be a polyacrylic acid or a salt thereof. These limiting amounts, therefore, are only related to the nature of the product obtained, and may vary when different salts or compounds of Formula I are employed. 
     Example 4 
     Two different compositions based on high-impact polystyrene (HIPS) were prepared, to evaluate the effect of the thermally stabilized PBB-PA on the properties of the final product, as compared with conventional PBB-PA. 
     The contents of the two compositions tested, containing also conventional additives, as well as the results of the tests, are shown in Table IV below. It can be seen that the Na-acrylate-containing copolymer, while remaining V-O rated, according to UL-94, also improves the results for the Notched Izod Impact and the HDT. 
     
                       TABLE IV______________________________________Comparison of FR and Mechanical Properties withCompositions of the Art in HIPS              Comp. 1  Comp. 2Component/Parameter              %        %______________________________________HIPS Vestyron 638  82.7     82.7PBB-PA             12.9     --PBB-PA containing  --       12.93% NaAcrylate(Exp. 2, Example 2)Antimony Trioxide  3.3      3.3Tinuvin P          0.2      0.2Irganox PS 800     --       0.2Irganox 245        0.2      0.2AC 400 A (Allied)  0.5      0.5UL 94 (3.2 mm) Rate              V-O      V-ONotched Izod Imact (J/m)              57       69HDT (°C.)   58       62Br %               9.3      9.0______________________________________ 
    
     Example 5 
     Two mixtures of pure PBB-PA, one with 5 wt % Na Acetate, and the other with 5 wt % Na Polyacrylate were prepared. The thermal stability of these mixtures was measured as described in Example 1, to show that non-copolymerized mixtures do not improve the thermal stability. The results are shown in Table V below: 
     
                       TABLE V______________________________________Thermal Stability of Mixtures                           Wt ratio                                  WeightExperiment                      Comp. 1/                                  lossNo.     Component 1              Component 2  Comp. 2                                  (%)______________________________________1       PBB-PA     Na Acetate   95%    512       PBB-PA     Na Polyacrylate                           95%    33______________________________________ 
    
     As is seen from Table V, the weight losses of these mixtures are of the same order of magnitude as for pure PBB-PA. 
     Example 6 
     In order to further illustrate the effect of the metal salt on the thermal stability of PBB-PA, a product obtained according to the procedure described in Example 2, Experiment No. 1, was treated to remove the metal ion, and the treated and untreated materials were compared for thermal stability. The treatment was effected by stirring 32.5 gr of the copolymer in 150 ml of aqueous HCl solution 1:5, for two days. After filtration the cake was washed with distilled water until neutral, and was then washed with methanol. After drying, 28.9 gr of the copolymer were recovered. 
     The product was tested for weight loss by TGA, as described hereinbefore. TGA was run for 30 minutes, and the results are shown in Table VI below. 
     
                       TABLE VI______________________________________Thermal Stability of Washed and Unwashed ProductsCopolymer   Na Content  Br (%)  Weight Loss (TGA), %______________________________________Unwashed   0.496%      69.31   6.3Washed  8.5 ppm     70.83   34.5______________________________________ 
    
     Example 7 
     The same PBB-PA/Na Acrylate copolymer of Example 4 was used to prepare flame-retarded glass fiber reinforced polyethylene terephthalate (GFR PET ex Allied Corp.). The properties of the resulting compositions are reported in Table VII. 
     
                       TABLE VII______________________________________Properties of 30% GFR PETComponent/Parameter %______________________________________FR                   9.8PET (PETRA 130)     85.3ANTIMONY TRIOXIDE    4.9(ex CAMPINE)UL 94 (0.8 mm) Rate VOUL 94 (0.8 mm) Rate 19.0Max Flame (sec.)UL 94 (0.8 mm) Rate 29.0Total Flame (sec.)UL 94 (0.8 mm) Rate 0Number Ignit. CottonIzod Notched Impact (J/m)               49.0HDT (°C.)    162Br %                 7.0______________________________________ 
    
     Example 8 
     Example 7 was repeated, but using 30% GFR polybutylene terephthalate (PBT - ex G.E.P.). The results are shown in Table VIII. 
     
                       TABLE VIII______________________________________Properties of 30% GFR PBTComponent/Parameter %______________________________________FR                  11.4ANTIMONY TRIOXIDE    5.7PBT (Valox 420)     82.8UL 94 (0.8 mm) Rate VOUL 94 (0.8 mm) Rate  2.0Max Flame (sec.)UL 94 (0.8 mm) Rate 10.0Total Flame (sec.)UL 94 (0.8 mm) Rate 0No. Ignit. CottonIzod Notched Impact (J/m)               92.0HDT (°C.)    202.4Br %                 8.0______________________________________ 
    
     Example 9 
     Example 7 was repeated, but using Nylon 6 (CAPRON 8200 HS, ex Allied Corp.). The results are shown in Table IX. 
     
                       TABLE IX______________________________________Properties of Nylon 6Component/Parameter %______________________________________FR                  15.7ANTIMONY TRIOXIDE    5.2CAPRON 8200 HS      77.5Mg. Stearate         0.5Hostaflon TF 3202    1.0UL 94 (1.6 mm) Rate VOUL 94 (1.6 mm) Rate  8.0Max Flame (sec.)UL 94 (1.6 mm) Rate 19.0Total Flame (sec.)UL 94 (1.6 mm) Rate 0No. Ignit. CottonBr %                11.0______________________________________ 
    
     Example 10 
     Example 7 was repeated, but using Acrylonitrile-butadiene-styrene terpolymer (ABS) ex Borg Warner, U.S.A. The results are reported in Table X below. 
     
                       TABLE X______________________________________Properties of ABSComponent/Parameter %______________________________________FR                  20.0ANTIMONY TRIOXIDE    4.2ABS                 75.8UL 94 (1.6 mm) Rate VOUL 94 (1.6 mm) Rate  1.0Max Flame (sec.)UL 94 (1.6 mm) Rate  2.0Total Flame (sec.)UL 94 (1.6 mm) Rate 0No. Ignit. CottonIzod Notched Impact (J/m)               98.0HDT (°C.)    90.0Br %                14.0______________________________________ 
    
     The above description and examples have been given for the purpose of illustration, and are not intended to be limitative. Many modifications can be carried out in the invention, without exceeding its scope.