A New Silicone Flame-Retardant System for Thermoplastics

20 A New Silicone Flame-Retardant System for Thermoplastics R. BRUCE F R Y E Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: December 9, 1985 | doi: 10.1021/ba-1986-0211.ch020

Silicone Products Division, General Electric Company, Waterford, N Y 12188

The unique polymer blend of a silicone fluid, a silicone resin, and a metal soap effectively provides a flame-retardant system for polypropylene (PP). Optimum flame resistance depends on a good dispersion of the silicones and is optimized with poly(trimethylsilyloxy) silicate and a magnesium soap, magnesium stearate. This system also displays a synergism with decabromodiphenyl oxide but surprisingly is impeded by antimony oxide or high-filler loadings. When well compounded in PP, the silicone system offers the added benefits of impact resistance, processability, mold release, and gloss. In short, the silicone-PP blend has flame retardance and the impact resistance and processability usually associated with PP copolymers.

POLYOLEFINS POSSESS AN ATTRACTV IE BALANCE

of chemical, electrical, thermal, and mechanical properties; however, a major limitation is their flammability (I). Polypropylene (PP), in particular, requires large quantities of conventional flame retardants (antimony oxide and halogenated compounds) to achieve the U L 9 4 V - l and V-0 standards. These high additive levels lower the mechanical and electrical properties of the polymer (2). In addition, the high levels of halogen can generate corrosive gases, during processing and molding, and corrosive smoke, during burning. Thus, a flame-retardant system is needed that is less corrosive, operates at lower levels (maintaining mechanical properties), is nontoxic, and generates less smoke. Silicone fluids are widely used in the plastics industry as internal lubricants (processing aids), to reduce wear, and for mold release. Their nonvolatile counterpart, silicone rubber, provides elements of fire retardance for wire and cable insulation. Other attractive properties of silicone rubber are its dielectric strength, thermal stability, and low and noncorrosive smoke evolution (3). C o u l d a flame retardant for polyolefins be developed from the relatively inert silicone polymer? T h e goal of this work was to apply these advantages of silicone to the problem of providing flame-retardancy to PP first, and eventually to other thermoplastics. 0065-2393/86/0211/0337$06.00/0 © 1986 American Chemical Society

In Multicomponent Polymer Materials; Paul, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1985.

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MULTC IOMPONENT POLYMER MATERA ILS

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: December 9, 1985 | doi: 10.1021/ba-1986-0211.ch020

Experimental

Initial formula optimization was conducted by compounding on a heated two-roll mill at approximately 200 ° G , granulating, and compression molding at 195 ° C for 10 min. Optimized formulations were compounded on a co-rotating twin-screw extruder (Werner Pfleiderer) at 200 ° C , pelletized, dried 3 h at 100 ° C when aluminum trihydrate was present, and injection molded. The vertical burn tests were run on Ve-in. samples according to the UL94 bulletin except that the samples were not preconditioned at 50 % relative humidity. The oxygen index (OI) and mechanical properties were measured following the appropriate A S T M 1 test methods. The base resin for all experiments was a lightly stabilized isotactic PP homopolymer, Hercules Pro-Fax 6523. Magnesium stéarate was obtained from Synpro, decabromodiphenyl oxide (DBDPO) came from Great Lakes Chemicals (83R), and aluminum trihydrate was Solem's SB-632 grade.

Discussion Formula Optimization. Although silicones have been used in the past as flame-retardant agents (4-13), none of these formulations is effective enough to achieve a U L 9 4 V - l or V-0 rating for PP. According to M a c L a u r y and Holub (7), a silicone in combination with a metal soap can provide a degree of flame retardance to some thermoplastics including polyethylene (PE) and PP. Continuing this work, we confirmed that practically any high-viscosity silicone polymer in combination with a metal s t é a r a t e would give the same flame-retardant effect as the gum. Although lower viscosity silicones also show flame retardance in combination with a metal s t é a r a t e , they permit flaming drips. A rapid screening of various components indicated that the inclusion of a silicone resin seemed to improve flame retardance. A silicone resin is a polymer prepared from multifunctional silanes that provide branched structures. T h e importance of the resin was verified by running a rotating simplex optimization (14-17) and including the resin as one of the seven variables. Further optimization work led to the development of particularly effective silicone flame retardants. Table I lists four optimized formulations for PP. 1

American Society for Testing and Materials.

Table I. Formulations Using the Silicone Flame Retardant Compound

Polypropylene (Pro-Fax 6523) Silicone fluid and resin (SFR100, GE) Magnesium stéarate Decabromodiphenyl oxide Talc Aluminum trihydrate Oxygen index a

Low-Halogen V-l*

Nonhalogen V-l a

74.3 9.5 4.4 6.9 5.0

78.5 10.7 3.6

27

23

—

_

7.2 —

V-0

5V

54.3 9.2 4.3 11.8

47.7 10.5 4.7 13.5

20.4 27

23.3 27

a

—

Values in percent.


a

—

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339


Properties. T h e final properties of the optimized formulations were quite dependent on the degree of compounding. Although most of the opti mization experiments were conducted on a heated two-roll mill, an intermeshing co-rotating twin-screw extruder gave the most consistent results. In addition, injection-molded parts were superior to compression-molded ones for mechanical properties, appearance, and flame retardance. The formulas listed in Table I achieved the primary objective of V - l V-0 flame retardance with a silicone system. A n important side benefit was impact resistance (see Table II). The PP homopolymer (Pro-Fax 6523) tested in our laboratory had a Gardner impact resistance (1-in. ring, 5/s-in. dart, 2 V 2 i n . x 2 V 2 i n . χ Ve i n . injection-molded plate) of approximately 18 i n . lbs, and the failure mode was cracking on the unimpacted side. T h e same homopolymer containing any of the silicone flame-retardant formu lations listed in Table I had a Gardner impact resistance of greater than 160 in. lbs (some plates achieved 250 in. lbs), and the mode of failure was a ductile puncture. T h e improved impact resistance was also present at low temperature, for example, at - 40 ° C the V - 0 plates withstood 25 i n . lbs. The notched Izod impact resistance also increased with the siliconeadditive package, from 0.5 for the homopolymer to 1.6 for the silicone sys tem. In addition to flame retardance and impact resistance, the silicone system also provided improved processability, improved mold fill, and gloss.

Table II. Properties of Formulations Properties

PP

Low Halogen

Nonhalogen

HALa/ ATHhc

HAL0/ ATHbc

UL94 Gardner impact (RT) (in. lbs) Gardner impact (-40 °C) (in. lbs) Density Notched izod Tensile at yield (psi) Tensile at break (psi) Percent elongation at break HDT d (264 psi) (°F) Flexural modulus (psi) Dielectric constant (1 kHz) Dielectric strength (V/mil) Volume resistivity ( χ 1015) Limiting oxygen index

burn

V-l

V-l

V-0

5V

160-200

160-200

160-200

160-200

18

—

—

—

1.01 1.6 3700

3700

360 140 2.2 χ 105

290 130 2.2 χ 105

250 140 2.1 χ 105

2.5 χ 105

_

1.61

_

2.50

650

—

17

—

811 22.9 27

0.97

25 1.148 1.14 3010 2220

0.904 0.5 4800 2900

—

—

— —

23

290

—

775

—

1.197 2730 2200 195 130

—

2.42 790

2.85 27

1.6 27

a Halogen. 6 Aluminum trihydrate. cSee Table I for composition. dHeat distortion temperature.


340

MULTC IOMPONENT POLYMER MATERA ILS The improved processability is evident in the increase in melt index

( A S T M D-1238) from 4 to 10 when the silicone and other components were added (low-halogen V - l composition). High-temperature gel-phase chromatography showed that the increased melt index was not due to PP degradation; the relative molecular weight did not change. In trichlorobenzene with polystyrene standards, the relative PP weight-average molecular


weight was 205,000 before compounding and 209,000 after compounding. The improved mold fill was demonstrated by using a channel-flow mold. Including the silicone package increased the flow from 8.50 in. to 10.75 i n . Synergism. T h e synergism between silicone-magnesium s t é a r a t e and D B D P O was documented by using oxygen index (OI) values. T h e graph in Figure 1 plots O I versus the flame-retardant additive concentration in Pro-Fax 6523 PP. A l l of the formulations were compounded on a two-roll mill at < 400 ° F and compression molded. Curves A , B, and F confirm the M a c L a u r y - H o l u b (7) original findings and extend them to the silicone-resin system. That is, neither the silicone nor the magnesium s t é a r a t e alone in PP confers any flame retardance. The combination inhibits PP burning. Curves F , E , and H demonstrate three important points. First, the combination of the silicone and resin with magnesium s t é a r a t e in PP (i.e., a nonhalogen system) causes a significant rise in O I with concentration; at 25%, this combination attains an O I of 23. Second, D B D P O alone in PP only achieves an O I of 20 at the same 25 % concentration. A n d third, a 1:1 mixture of silicone-magnesium s t é a r a t e and D B D P O produces a dramatically higher O I (OI equals 31) than either material alone at the same concentration. This effect is synergism. In addition, curve D shows that barium s t é a r a t e is much less effective than magnesium s t é a r a t e in this system in PP. Flame retardance in the U L 9 4 test follows approximately the same pattern. Curves H and G show that D B D P O is more effective in the siliconemagnesium s t é a r a t e system than the chlorinated cycloaliphatic compound known as Dechlorane Plus (Occidental Chemical Company). Mechanism. Polyolefins in general burn without char formation. The silicone flame retardant induces a slightly intumescent char in PP, and this solid-phase reaction appears to be a major mode of flame retardance. The brittle char presumably inhibits heat conduction to the plastic and fuel flow to the flame. A stearate-type moiety seems necessary to make the magnesium compatible with the silicone, and this metal soap probably serves as a siloxane rearrangement catalyst at high temperatures promoting silicate char. T h e M Q resin is a rather unique silicone-soluble form of silicate. M Q resin is the common name for poly(trimethylsilyloxy) silicate. M stands for


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OXYGEN INDEX

10

« 5

1

0

» 10

1 15

1 20

1 25

WEIGHT % ADDITIVE IN POLYPROPYLENE Figure 1. Oxygen index data demonstrate synergistic flame retardance in PP for silicone and magnesium stéarate (F), and for silicone, magnesium stéarate, and DBDPO. Additives: A, silicone; B, magnesium stéarate; C, alicyclic chloride; D, 3:1 silicone-barium stéarate; Ε, DBDPO; F, 3:1 silicone-magnesium stéarate; G, 3:1:4 silicone-magnesium stearatealicyclic chloride; and H, 3:1:4 silicone-magnesium stearate-DBDPO.

a monofunctional silane, and Q represents a tetrafunctional silane (quaternary). Antimony oxide surprisingly decreases the flame retardance of this system. Mineral fillers also prolong the extinguishing time, perhaps by forming discontinuities in the char.

Conclusions W e have found that the combination of a linear polydimethylsiloxane and a branched silicone resin together with magnesium s t é a r a t e and other optional components can improve flame retardance in PP and, perhaps, other thermoplastics. Experimental formulations have been developed that can


342

MULTC IOMPONENT POLYMER MATERA ILS

meet the Underwriters Laboratory U L 9 4 Bulletin criteria. In addition to flame retardance, the silicone package dramatically improves the impact resistance of P P and adds the conventional silicone properties of improved processability, improved moldability, gloss, and good electrical (insulat ing) properties. T h e best and most consistent properties require thorough compounding, preferably by using a twin-screw extruder.

Acknowledgments Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: December 9, 1985 | doi: 10.1021/ba-1986-0211.ch020

I wish to thank E . Lovgren, R. Reed, and R. Ronda for compounding, molding, and testing assistance. I am also indebted to A . Torkelson, J. Golba, and M . M a c L a u r y for helpful discussions.

Literature Cited 1. Schwarz, R. J. In "Flame Retardance of Polymeric Materials"; Kuryla, W . C.; Papa, A . J., Eds.; Dekker: New York, 1973; Vols. 1-2. 2. Hayes, W . K.; Lesniewski, J. M . Plast. Eng. 1981, 37, 25-31. 3. Lipowitz, J. "Flammability, Smoke, Toxicity, and Corrosive Gases of Electric Cable Materials," U.S. Dept. of Commerce, NTIS AD/A-065047, 1978, p. 99. 4. Luce, J. B.; Vaughn, H . A . U.S. Patent 4 235 978, 1980. 5. Betts, J. E . ; Holub, F. F. U.S. Patent 4 123 586, 1978. 6. Betts, J. W . ; Holub, F. F. U.S. Patent 4 247 446, 1981. 7. MacLaury, M . R.; Holub, F. F. U.S. Patent 4 273 691, 1981. 8. Bialous, C . Α.; Luce, J. B.; Mark, V . U.S. Patent 3 971 756, 1976. 9. Japanese Kokai Tokkyo Koho 81 93,207 of Showa Electric Wire and Gable Co., Ltd., 1981. 10. Japanese Kokai Tokkyo Koho 81 20,041 of Showa Electric Wire and Cable Co., Ltd., 1981. 11. Japanese Kokai Tokkyo Koho JP 81 166,246 of Furukawa Electric Co., Ltd., 1981. 12. Moody, A . G . ; Pennick, R. J. U.S. Patent 4 265 801, 1981. 13. Weise, C.; Wolfer, D . ; Patzke, J. U.S. Patent 4 390 656, 1983. 14. Hendrix, C . Chemtech 1980, 10, 488. 15. Deming, S. N . ; Morgan, S. L . Anal. Chem. 1973, 45, 178A. 16. Long, D . E . Anal. Chim. Acta 1969, 46, 193. 17. Leggett, D . J. J. Chem. Educ. 1983, 60, 707. RECEIVED for review November 15, 1984. ACCEPTED February 19, 1985.


A New Silicone Flame-Retardant System for Thermoplastics

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