Solid solutions and precipitation in flame retardant polystyrene-tetrabromoxylene systems

Solid solutions and precipitation in flame retardant polystyrene-tetrabromoxylene systems A. Siegmann, M. Narkis* and A. Dagan Plastics Institute, Center for Industrial Research (CIR) Ltd, POB 311, Haifa, Israel (Received 7 December 1973) Flammability and certain mechanical properties of high impact and general purpose polystyrene-tetrabromoxylene (TBX) systems were studied. TBX was found to be an efficient fire retardant and did not affect significantly basic mechanical properties of the polymer in proportions up to 10phr. It has been shown that polystyrene-TBX mixtures can form solid solutions. On annealing above the To, supersaturated solid solutions precipitate crystalline TBX from the amorphous matrix, affecting the optical properties and softening temperatures of the system. INTRODUCTION Tetrabromoxylene (TBX), a new product developed by Bromine Compounds Ltd in Israel, has been evaluated as a fire retardant for polystyrene (PS). TBX contains a high percentage (76~) of bromine, melts at 250°C and crystallizes in the form of needles. It is soluble only in a small selective group of solvents and can be compounded homogeneously with molten thermoplastic materials. The high thermal stability of TBX permits easy processing with thermoplastics. Halogenated compounds are well known as fire retardants 1 in plastic materials; however, their addition very often results in a decrease in mechanical properties~. The mode of dispersion, interaction, compatibility, etc. play an important role in the mechanical behaviour. The common practice is to choose a system where a balance between mechanical properties and flammability is achieved. This work concentrates on the system PS-TBX, which has been found intriguing from aspects other than mere fire retardancy. For example, transparent compatible amorphous PS-TBX systems can be obtained from which TBX can be precipitated. EXPERIMENTAL Polystyrene pellets (high impact, Lustrex HT88, Monsanto or general purpose Hostyrene A1301, Hoechst) were mixed with various amounts of TBX in a Sigma blade kneader (Werner and Pfliederer, Germany) at 220°C. TBX was added into the molten polymer and mixed for 10min. After cooling to room temperature the solid mixture was granulated (Dreher granulator, Germany). ASTM specimens for testing of mechanical and physical properties and flammability were injection moulded with an Esgo laboratory scale injection moulding machine at 220°C (mould at room temperature). A * Also with the Department of ChemicalEngineering, Technion-Israel Institute of Technology,Haifa, Israel.

few specimens were injection moulded at 270°C, i.e. above the melting temperature of TBX (250°C). In addition sheets were compression moulded at 220°C. Samples were annealed, below and slightly above the Tg of the polymer in an air oven. The following properties were measured: tensile and flexural (Instron machine), Izod impact (Zwick pendulum Impact Tester 5102), Vicat softening point (Zwick Model 4204), burning (burning test--ASTM D 635-68 and oxygen index test--ASTM E 4-64) and flow (melt flow indexer, Tinius Olsen Thermosyne).

RESULTS AND DISCUSSION The flammability of high impact polystyrene (HIPS) containing various concentrations of TBX was studied. As shown in Table 1, TBX changes significantly the flammability of HIPS. At a loading of 3 phr, the polymer is rated as a burning material, at 6phr it is a selfextinguishing material, and a non-burning material is obtained at 10phr TBX. As a bromine carrier compound TBX is similar to other bromine compounds as far as the effect of the bromine content on the flammability of the polymer is concerned 1, a. Antimony oxide, a well known synergistic additive to halogenated compounds, does not improve the burning resistance of HIPS containing 6phr TBX, in agreement with other literature data 4. Polymers characterized by oxygen indices of 21 and below and 27 and above are considered 5 as burning and non-burning polymers respectively, in agreement with the results shown in Table 1. The effect of TBX on some mechanical properties of HIPS can be summarized as follows: both the modulus and ultimate tensile strength are practically unchanged and average values found are 3.5xl051bf/in 2 and 35001bf/in z (llbf/inZ=6874.76N/m 2) respectively for the concentration range of 0 to 10phr TBX. Figure 1 shows that the ultimate tensile elongation and flexural

POLYMER, 1974, Vol 15, August

Flame retardant polystyrene-TBX systems: A. Siegmann et aL Table I Flammability evaluation of polystyrene containing TBX and antimony oxide Burning characteristics ASTM D635-68 classification

Sample

Extent of burning (cm)

HIPS Burning HIPS +3phr TBX Burning HIPS +6phr TBX Self-extinguishing HIPS +10phr TBX Non-burning HIPS Self-extinguishing +2phr Sb408+6 phr TBX HIPS Self-extinguishing +4phr Sb406+6phr TBX HIPS Self-extinguishing +6phr SbaOe+6phr TBX

Oxygen Index, n Burning rate (ASTM E4-64) (cm/min)

3.1 2.14

m -2 0 1.8

17'5 25 26 29 95

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1.6

6300

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Figure I Flexural strength (O) and ultimate tensile elongation ( 0 ) of high Impact polystyrene containing various amounts of TBX

strength decrease monotonically by increasing the TBX content in the studied range of up to 10phr TBX. A different behaviour is shown in Figure 2 where the Izod impact resistance initially increases up to about 6phr TBX and then decreases. Figure 2 also shows that the addition of antimony oxide to HIPS containing 6phr TBX reduces the impact resistance in the same manner as would further addition of TBX (beyond 6 phr) change the impact resistance. The effect of TBX on Vicat softening point (VSP) of general purpose polystyrene (GPPS) is shown in Figure 3. The softening temperature decreases with TBX content, attains a minimum at about 20phr and then increases gradually. The addition of antimony oxide in the concentration range studied does not affect the VSP or Tg (the latter was measured with a DuPont 990 Thermal Analyzer). All the samples prepared for the VSP test by injection moulding were visually transparent up to a content of 17.5 phr TBX, while samples containing 20 phr and above were opaque (HIPS could not be used

500

P O L Y M E R , 1974, Vol 15, A u g u s t

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Total additive concentration(phr) Figure 2 Izod impact resistance of high impact polystyrene

containing TBX (O) and 6phr TBX and various amounts of antimony oxide ( x )

for transparency studies owing to its opacity). Upon annealing of GPPS-TBX samples below their Tq, no changes in VSP and impact resistance have been found. However, upon annealing at 100°C (i.e. above Ta) the samples containing more than 10phr TBX became opaque and their softening temperature increased as shown in Figure 3. The observed mechanical behaviour of HIPS containing TBX indicates that TBX can be considered in these systems as a high melting solid plasticizer. High melting solid plastieizers such as tri(p-t-butylphenyl)phosphate (m.p. 99"5°C), dieyclohexylphthalate (m.p. 65°C) and triphenyl phosphate (m.p. 49°C) have been reported previously6. For example, tri(p-t-butylphenyl)phosphate gives compositions with ethylcellulose that are tougher and harder than corresponding ones made with conventional liquid plastieizers. The systems studied in this work behave similarly to the systems described above as far as toughness (impact resistance),

Flame retardant polystyrene-TBX systems: A. Siegmann et al.

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Figure 3 Vicat softening point of general purpose polystyrene containing various amounts of TBX before ( 0 ) and after (©) annealing

modulus and yield strength, are concerned. TBX, being a solid plasticizer reduces the elongation to break (Figure 1) of ductile HIPS whereas liquid compatible plasticizers usually result in greater elongations. Experimental data on the effect of solid plasticizers on the mechanical behaviour of brittle polymers, such as GPPS, are not available. The Vicat softening temperature behaviour shown in Figure 3 suggests that up to about 20phr TBX, the system, as prepared, is a solid solution whereas at higher TBX loadings it forms a second discrete solid phase. The solute decreases the VSP while the excess, forming the second dispersed rigid phase, increases the VSP. The 20phr minimum in VSP corresponds approximately to the transition concentration from visually transparent to opaque systems. The behaviour of the PS-TBX system is similar to the well known nitrocellulose-camphor (m.p. 176°C) system which is characterized by a discontinuity in the elongation-camphor content curve 7 at 35 ~o camphor. It has been suggested that a nitrocellulose-w.amphor complex is formed up to a saturation concentration of 35 Y/oocamphor and all excess camphor is unbound. The discontinuity at 35 results from the transition between two different dispersion modes which affect the mechanical response of the system. The 20phr minimum shown in Figure 3 and the transition in the optical properties were initially surprising, since the room temperature saturation solubility of TBX in styrene monomer is only about 6~o. It is clearly difficult to assume a higher solubility in the polymer than in the monomer. It is suggested that under the sample preparation conditions, injection moulding of a hot molten mixture into a cold mould (actually a quenching process), a solid solution or a supersaturated solid solution is formed. At the processing temperature (220°C), which is well below the melting temperature of TBX (250°C), TBX (in the concentration range studied) is completely dissolved and upon quench-

ing a glassy solid solution is obtained. The solid solution is considered here as supersaturated for TBX contents higher than about 5 phr (the exact saturation solubility is not known). The opaque samples containing more than 20 phr TBX were further studied under an optical microscope (magnification of 240x) and dispersed needle shaped crystallites were observed, as shown in Figure 4. Pure TBX crystallizes also in the form of needles about 100 times larger. Samples containing more than 20 phr TBX and injection moulded at 270°C (20°C above the m.p. of TBX) also exhibit needle shaped crystals. Rheological measurements at a constant shear stress, z = 7 × 104dyne/cm ~, at 220°C have shown that the addition of 25phr TBX reduces the melt viscosity of the polymer by a factor of 3.5. In another experiment an opaque sample containing 20 phr TBX turned transparent upon heating to 150°C, showing that the needles are redissolved, and became opaque upon cooling to room temperature, showing reprecipitation. These observations support the view that TBX is dissolved in the polymer at processing conditions used in this work. Thus the needle shaped crystallites found in the polymer were crystallized during the quenching process and are not residuals of the original TBX crystals. Joseph et aL s studied the in situ crystallization of acetanilide (m.p. 113°C), or anthracene (m.p. 216°C) from amorphous styrene-acrylonitrile copolymer. They have reported the effect of the dissolved low-molecular weight materials on the polymer glass transition, using this effect to determine phase diagrams and crystallization kinetics. Supersaturated solutions are thermodynamically unstable and tend to approach their equilibrium compositions 0. This can be accomplished by thermal treatment which is a time dependent process. In our studies samples were annealed at 100°C, i.e. above Tg, for 1 h.

Figure 4 Transmission optical micrograph of polystyrene film containing 25phr TBX

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501

Flame retardant polystyrene-TBX systems: A. Siegmann et aL All the transparent samples containing 10 to 20phr TBX became opaque and their VSP shifted to higher values as shown in Figure 3. The shifted values reflect the amount of TBX which is rejected from the solution and forms a second crystalline phase upon annealing. This process has a multiple effect on the VSP, namely stiffening by reducing the plasticizer concentration and simultaneously adding rigid particles into the system. A different behaviour is observed for samples containing up to 10phr TBX since their transparency and VSP were not affected by annealing at the same conditions (see Figure 3). The shape of the VSP curve for annealed samples is as qualitatively expected based on the observed phenomenon of precipitation. It should be noticed that the minimum in this curve occurs at 10phr TBX which is somewhat above saturation. This result stems from the different precipitation conditions in these supersaturated solutions compared to simple supersaturated liquid solutions. In the former, approach to equilibrium composition is much slower, thus the composition at the minimum is controlled by the specific precipitation conditions, and is expected to vary with annealing temperature and period. In addition, by cooling from 100°C the amorphous sample will rapidly pass through Tg which is accompanied by a dramatic viscosity increase; thus the approach to room temperature equilibrium solubility is hindered. It is important to emphasize that an enhanced precipitation will occur from crystalline polymers owing to rejection of solute from the lattice. One can conclude that annealing at 100°C for 1 h of GPPS containing more than 10phr TBX results in the formation of a two phase system with about 10phr TBX in solid solution with all excess TBX as a discrete crystalline phase. The solubility of TBX in the softening temperature range (70-90°C, see Figure 3) is higher than at room temperature. Hence, the 10 phr minimum in the annealing curve of Figure 3 does not necessarily coincide with the minimum (or maximum) in a room temperature property such as impact resistance, elastic modulus etc. As shown previously in Figure 2, the impact resistance reaches a maximum at the lower concentration of 6 phr TBX which is a decrease as expected. It is important to point out that one would expect a better solubility of TBX in GPPS than in HIPS. The impact data are for HIPS while the VSP data are for GPPS--a fact which also supports the results. An interesting result relevant to our system is mentioned by Doolittle6. It is concerned with triphenyl

502 POLYMER, 1974, Vol 15, August

phosphate, a crystalline solid plasticizer (m.p. 50°C) which has flame retarding properties. This material tends to crystallize when used in high proportions enough to exude from films of cellulose ester plastics. In summary, it has been shown that polystyrene-TBX mixtures can form solid solutions, a physical system which is known in metal alloys9. Such solid solutions are not limited to the pair polystyrene-TBX8 and may be expected to occur also in other polymeric systems where chemical affinity between the components exists. Precipitation of the non-polymeric solid component will occur in systems of limited solubility under proper conditions. By mixing polystyrene and TBX at elevated temperatures and quenching to room temperature a supersaturated solid solution can be formed. At a temperature below the solvus curve the solid solution is no longer structurally stable and tends to reject solute molecules from itself. It is assumed that during annealing, clusters which had been previously formed grow to form stable nuclei, followed by growth into precipitate particles from the addition of solute molecules from the matrix. The practical consequences of these phenomena are not yet exploited in polymers and call for further studies.

ACKNOWLEDGEMENTS The authors extend their thanks to Drs I. Saxs and E. Wurman of Bromine Compounds Ltd for supplying the materials and for permission to publish this work, and to Professor A. T. DiBenedetto for helpful discussions during his stay at the Center for Industrial Research.

REFERENCES 1 Lyons, J. W. 'The Chemistry and Uses of Fire Retardants', Wiley, New York, 1970 2 Nametz, R. C. Ind. Eng. Chem. 1967, 59, 99 3 Narkis, M., Grill, M. and Leeser, G. J. Appl. Polym. Sci. 1969, 13, 535 4 Cooper, A. U S Pat. 3 039 991 5 Fenimare, C. P. and Martin, F. J. Mod. Plast. 1966, 43 (11), 141 6 Doolittle, A. K. 'The Technology of Solvents and Plasticizers', Wiley, N e w York, 1954 7 McNaily, J. G. and Sheppard, S. E. :. Phys. Chem. 1931, 35, 2498 8 Joseph, J. R., Kardos, J. L. and Nielsen, L. E. J. AppL Polym. Sei. 1968, 12, 1151 9 'Precipitationfrom Solid Solution', A S M , Cleveland, 1959