Flame-Retardant Formulations for HIPS and Polyolefins Using Chlorinated Paraffins Abstract Chlorinated paraffins are one the lowest cost flame-retardants available. Solid chlorinated paraffins have been used to flame retard many polymers, but some applications were limited due to the thermal stability of the chlorinated paraffin. A solid chlorinated paraffin with improve thermal stability is now available and can be used as a flame retardant for HIPS and polyolefins. Formulations, flame retardant results and physical properties are discussed. Introduction Chlorinated paraffins are a low cost flame retardant with only the hydrated metal oxides being less expensive. They are manufactured by reaction of a paraffin wax with elemental chlorine either neat, in a solution, or in an emulsion. Product and property variations are induced by changes in raw materials, duration and conditions of chlorination, work-up and stabilization. Based on bond dissociation energies, chlorinated paraffins theoretically should have stability up to 300 C. However, due to the presence of some imperfections in the product, the maximum recommended processing temperatures for chlorinated paraffins is 230 C. Selection of the most linear paraffin, the optimum carbon chain length and the correct chlorine content yields chlorinated paraffins with optimum properties for a polymer. The optimum carbon chain length is C 22 -C 26 and the optimum chlorine content is 72%. Chlorinated paraffins have a synergistic effect with Group V metal oxides, especially antimony trioxide. In addition, since they lack chromophores, they have better UV stability than do brominated aromatic fire retardants. Finally, chlorinated paraffins are additive flameretardants with partial solubility in thermal plastics. Therefore, they do not bloom and have an influence on physical properties such as heat deflection temperature and percent elongation. Synthesis In general, a wax is treated with molecular chlorine to yield chlorinated paraffin Don Stevenson, Vic Lee, Daryl Stein, and Tarang Shah, Dover Chemical, 3676 Davis Road, Dover, OH 44622
and HCl as shown in equation 1. The addition of one chlorine atom per carbon results in a product with a 73% chlorine content. In practice the chlorine content for a solid product can C 20 H 42 + 20 Cl 2 C 20 H 22 Cl 20 + 20 HCl (1) (73% Chlorine) range from 70-74% with 72% being a typical value. The physical properties of the product depend on the chain length of the wax, the chlorine content, and the manufacturing process. The effect of chain length and chlorine content on physical properties of a chlorinated paraffin can be seen Table 1. Here the chlorinated paraffin with the longer chain length has a higher softening point compared with one with the same chlorine content but shorter chain length. In addition, as the chlorine content increased, the softening point for both the short and Softening Point C Table 1. The Effect of Chlorine Content on Softening Point Wt% Chlorine Softening Point C C 18-22 C 22-26 70 90 105 71 95 110 72 100 120 73 115 135 74 130 160 long chain paraffins also increased. The manufacturing process can play an important part in determining physical properties and stability of a chlorinated paraffin. As can be seen the Table 2, a change from the Table 2. The Effect of the Manufacturing Process on Physical Properties Standard Process New Process Chlorez 700 SSNP Chlorine Content, wt% 71-72 72-73 Softening Point, C 105 130 Thermal Stability HCl after 4h at 175 C Color Stability 15 minutes at 200 C 0.1 0.01 black yellow standard manufacturing process to a new proprietary one, leads to improvement in physical properties, especially improved thermal stability. The improved thermal stability permits use of chlorinated paraffins in polymers such as polyethylene, polypropylene, and HIPS. In addition, it offers benefits such as
No polyhalogenated biphenyls or dioxins. Low cost. Improved melt flow. Better U-V stability than aromatic brominated flame-retardants. Mechanism of Flame Retardance Many have shown that chlorinated paraffins act as a flame retardant by releasing HCl, which poisons the flame. This vapor gas inhibition is greatly enhanced when chlorinated paraffins are used with Group V metal oxides, especially antimony trioxide. Although antimony trioxide (ATO) alone does not work as a flame retardant, it has been proposed that antimony oxychloride is formed by the reaction of antimony trioxide with hydrochloric acid and with further reactions antimony trichloride is formed, which can act both as a radical scavenger to disrupt oxidation and also as a vapor barrier above the condensation phase to smother the flame (eqs. 2-5). Sb 2 O 3 + 2HCl 2SbOCl + H 2 O ca. 250 C. (2) 5 SbOCl Sb 4 O 5 Cl 2 + SbCl 3 245-280 C. (3) 4Sb 4 O 5 Cl 2 5Sb 3 O 4 Cl + SbCl 3 410-475 C. (4) 3Sb 3 O 4 Cl 4Sb 2 O 3 + SbCl 3 475-565 C. (5) Even though chlorine has a lower flame retardant efficiency than bromine, chlorinated paraffins can be tailored with flame retardant synergists and brominated aromatics to give optimum flame retardant performance. CHLOREZ 700-SSNP is a solid chlorinated paraffin, especially developed and optimized for use in HIPS and polyolefins. Sample Preparation All formulations were compounded in a lab size 1600 cc Banbury type internal mixer. The body temperature was set at 200 C, and the rotor temperature was set at 180 C. All ingredients were added to the Banbury at low speed (77 RPM) except for Chlorez 700 SSNP. The materials were fluxed and allowed to mix for two minutes. The Chlorez was then added in three approximately equal portions. The formulation was then mixed for an additional five minutes at low speed before it was removed, sheeted on a 2-roll mill, and granulated when cooled. The granulated material was injection molded on a Battenfeld injection molder (Model 30 plus) into test specimens (5 x 0.5 x 0.125 and 5 x 0.5 x 0.0625 test bars). The HDPE and the PP samples were injection molded at 180 C with a mold temperature of 16 C. The HIPS samples were molded at 210 C with a mold temperature of 28 C. All test specimens were conditioned at 70 F and 50% RH for at least 40 h before testing.
Chlorinated Paraffins in Polyethylene The effect of improved stability for Chlorez 700 SSNP can be seen in Figure 1. In this example, two grades of chlorinated paraffin were compounded into polyethylene along with antimony trioxide (ATO). The samples were then injection molded into test specimens. Before molding, however, the samples were allowed to stand in the barrel of the injection molder at 200 C for periods of five and ten minutes before. The color (YI) of the molded specimens was taken as a measure of the stability of the chlorinated paraffin. The specimens with Chlorez 700 SSNP developed significantly less color than did the ones with standard chlorinated paraffin. With an improved grade of chlorinated paraffin in hand, an experimental program was undertaken to develop flame retardant formulations for injection moldable HDPE, which is a difficult material to flame retard, and still maintain acceptable physical properties. In this program, a total of 54 formulations were evaluated, which used two grades of HDPE, eight impact modifiers, and ten other additives (flame retardant synergists or polymer stabilizers). The notched Izod impact test was used as a screening tool, since the impact strength is a measure of the usability of the material. If no significant improvement in impact resistance was observed, then no further work on a formulation was carried out. In order to obtain good impact properties it was necessary to use an impact modifier, and it quickly became clear that only one of the eight impact modifiers gave significant impact improvement at the loadings necessary to achieve the desired flame resistance. In addition, only one of the two grades of HDPE had improved impact properties at the required loadings. Thus in order to develop a flame retarded HDPE with acceptable physical properties, great care has to be exercised not only in the type of impact modifier used, but also in the grade of HDPE used. Figure 2 lists three formulations from this work that achieved a V-0 rating for 1/8 and 1/16 test specimens as compared with three that did not meet these requirements. The major difference between them is the presence of a small amount of PTFE as a drip suppressant. The three formulations with PTFE compare the recommendations for an all Chlorez formulation, a 1:1 blend of Chlorez and decabromodiphenyl ether (deca) formulation, and an all deca based formulation. Since the loading levels are similar, the all Chlorez formulation is significantly less expensive than the other two formulations. Figure 3 shows some of the physical properties for the three flame retarded formulations compared with those of the base resin. Although all three formulations adequately maintain physical properties, the ones with Chlorez show some plasticization effects with lower heat deflection temperatures compared with the deca only formulation. However, the all Chlorez formulation has better tensile and flexural properties than does the all deca formulation. In addition, the all Chlorez formulation has a lower density. Chlorinated Paraffins in Polypropylene In this work a series of twelve formulations were tested which used Chlorez, ATO, ZnS, and an impact modifier to obtain a product with good physical properties and flame
resistance. As can be seen in Figure 4, a minimum of 25% Chlorez is necessary to attain a V- 0 rating for both 1/8 and 1/16 test specimens. As was the case with HDPE, the use of an impact modifier was necessary to obtain the desired physical and flame retardant properties. Figure 5 shows the notched Izod impact results for formulations with 25% Chlorez. As the figure indicates, a minimum of 10% impact modifier is needed to maintain good impact resistance. However, once the amount of impact modifier goes beyond 15%, the impact resistance actually decreases as the sample becomes soft and rubbery. In fact, the sample does not break, but actually flexes away from the impact hammer. The impact modifier also cause changes in percent elongation (Figure 6) and tensile strength (Figure 7). Once the level of 15% is reached the percent elongation sharply increases while the tensile strength decreases. On the other hand, the impact modifier has only a small effect on melt flow (Figure 8). What is really noteworthy is how the impact modifier affects the burn properties (Figure 9). With a Chlorez concentration of 25%, the optimum impact modifier concentration was at about 10% for good burn results. At lower or higher impact modifier concentrations, reduced flame resistance was observed. This effect is not clearly understood, but it appears that there is a concentration range in which the impact modifier actually seems to hold the sample together during burning so the tendency to drip is reduced. Unfortunately, at higher concentrations afterglow becomes a problem. At still higher concentrations the sample becomes rubbery, and flame time and the afterglow increase further. Thus it appears that there is not only a minimum amount of fire retardant required but also an optimum amount of impact modifier required as well. Chlorinated Paraffins in HIPS In order to develop flame resistant formulations for HIPS a designed experiment with 25 formulations was carried out in which Chlorez, FR 1808, ATO, and ZnS were used as the ingredients in the flame retardant package. FR 1808 (octabromotrimethylphenyl indane from Ameribrom) was used for its low blooming properties. In this design there were five variables and two constraints: Variable A: FR 1808 between 3.4% and 15.4% Variable B: Chlorez 700 SSNP between 3.4% and 15.4% Variable C: Antimony trioxide between 0.8% and 7.7% Variable D: ZnS between 0.8% and 7.7% Variable E: HIPS between 69.2% and 82.9% Constraint 1: 13.7% < (A + B) < 20.5% Constraint 2: 3.4% < (C + D)< 10.2 The test was whether or not a V-0 rating could be obtained for a 1/16 test specimen. The results were that 10 formulations gave a V-0 rating and 10 formulations burned. The remaining five formulations gave either a V-1 or a V-2 rating. An examination of the results indicates that if A+B > 18.5 and C > 2.5 then a V-0 rating will be obtained. Variable D did
not show any strong effect for flame resistance, although other work shows that it does suppress afterglow. The utility of this rule can be seen in Figure 10 where the results for five formulations are given. As can be seen in the figure, formulations 1 and 2 are nearly identical except that the amounts of Chlorez and FR 1808 are reversed. In both formulations A + B > 18.5 and C > 2.5, therefore both gave a V-0 rating. On the other hand, formulations 3 and 4 burned, because C < 2.5. Interestingly, in formulations 3 and 4, variable D was at the maximum allowed by the design yet it had little influence on flame resistance. Finally, formulation 5 where A + B < 18.5 and C > 2.5 also burned. The physical properties were also measured for HIPS and are given in Figure 11. All the formulations had adequate physical properties. However, one physical property that was not in the design was UV light stability. In Figure 12, there is a comparison of color stability for Chlorez 700 SSNP vs deca. In this work, the samples were exposed to intense fluorescent light (a UV source) and the changes in color were measured. As can be seen in the figure, the sample with Chlorez clearly had better light stability than did the one with deca. Conclusions With the proper selection of raw materials and process conditions, an improved grade of chlorinated paraffin can be produced. With improved thermal stability, it was possible to use this material in HDPE, PP, and HIPS. With careful attention to the selection of resin grades and impact modifiers as well as the amounts of fire retardant additives, it was possible to develop formulations that gave a V-0 rating for 1/16 test specimen and still maintain acceptable physical properties. The advantages of chlorinated paraffins as a flame retardant are that it is low cost, does not bloom, and has good UV stability. Acknowledgements The authors would like to acknowledge the generous help and assistance of Joanne Reneker, Ed Wykoff, and Sridhar Siddhamalli in the preparation of this paper.