Use of Submicron Inorganic Flame Retardants in Polymeric Systems-antimony trioxide

Flame retardants composed of solid inorganic particles generally have an adverse effect on the physical and aesthetic properties of polymers and synthetic fibers. Colloidal sized particles, however, maintain aesthetic and physical qualities and provide flame retardant properties. This paper discusses the advantages of antimony oxide flame retardant particles having a size less than 0.1 micron. Particular attention will be given to polyolefin systems, the importance of choosing a halogen, and the effects on antimony oxides on a polymer processing system.

Advances in fine denier polypropylene fiber processing have opened the carpet face and wall covering markets to this versatile polymer. Aesthetically pleasing polypropylene structural products, such as waste baskets and other containers, have also found a niche in the marketplace.

At the same time, today’s consumer is demanding flame retardant goods. Manufacturers need a product that flame retards the polymer and also maintains its physical and aesthetic properties.

There currently are two systems that effectively flame retard polymers: halogenated systems and non-halogenated systems. Many manufacturers would prefer a non-halogenated system, such as magnesium hydroxide, aluminum trihydrate, ammonium phosphate, etc. principally because halogenated systems have received negative publicity. Non-halogenated systems, however, require loadings of up to 60% of flame retardant, and the physical and aesthetic properties of the base polymers are negatively affected as a result. In the case of fine-denier fibers, a usable fiber could not be produced with these high flame retardant loadings.

Halogenated systems offer the advantage of lower loadings to achieve the desired levels of flame retardancy. In fact, several fiber manufacturers have dictated that no more than 8% active FR ingredients can be used in a finished fine denier fiber. Most halogenated FR compounds and all non-halogenated systems, however, cannot meet this criteria.

Antimony oxide and halogenated organic compounds combine to produce a synergistic action that flame retards plastics at desirable loading levels. Many combinations of antimony oxide and halogenated additive systems are available.

An ideal flame retardant system would also be easily processed and would safeguard the physical and aesthetic properties of a polymer. It would incorporate a melt-blendable halogenated additive with a submicron antimony oxide particle. This combination should yield an acceptably flame-retarded product with good tensile strength, impact resistance and elongation. The finished product should also be untinted/translucent.

An antimony pentoxide powder that disperses to colloidal size (.03 micron) particles is the only FR additive that meets all these criteria. A detailed comparison of colloidal-sized antimony pentoxide versus antimony trioxide (the smallest particle size commercially available) is given in Table 1.


Table 1 – Physical Properties of Antimony Pentoxide vs. Trioxide


Figure 1 shows the visual impact of a submicron antimony pentoxide particle on a 1.5 denier polypropylene fiber vs. antimony trioxide. The antimony pentoxide particle occupies only 0.2% of the cross-sectional area of the fiber vs. 7% for antimony trioxide.

RESULTS AND DISCUSSION

We tested the desirability of several potentially acceptable antimony oxide/halogenated additive systems for polypropylene fiber and translucent products. The antimony oxides used in this screening study were:

1. Nyacol® ADP480 – a powder which disperses to colloidal size (.03 micron) particles in nonpolar hydrocarbons; and
2. Antimony trioxide powder.

Both antimony oxides were individually compounded into a flame retardant concentrate with each of five halogenated additives:

1. Brominated aromatic ester (63% Br)
2. Brominated polystyrene (60% Br)
3. Brominated polystyrene (66% Br)
4. Brominated aromatic compound (66% Br)
5. Chlorinated paraffin (74% Cl)



We produced concentrates of each combination to test the complete dispersion of the ingredients when the product was let-down into the polymer. This is critical in fine-denier fiber applications.

The concentrates contained 50% active FR ingredients in a carrier of polypropylene. This carrier was chosen to maintain the physical properties for fine denier fiber applications.

Table 2 summarizes the flame tests results of this initial screening of FR compounds. Only one halogen/antimony oxide compound exhibited flame retardancy according to the UL-94 vertical flame test. That one system is antimony pentoxide or trioxide with a brominated aromatic compound.

The antimony pentoxide compounds were all rated V-2 according to UL-94 with afterflame times ranging from 0 to 3.8 seconds, depending on FR concentration. The trioxide compounds were rated V-0 through FAIL depending on FR concentration. The V-0 ratings achieved by trioxide at high loading levels (8 and 12%) were probably due to the rheology of the polymer being changed as a result of trioxides larger particle size, which reduced the quality of drips as well as reducing their flaming characteristics.



Table 2 – Flame Test Summary of Polypropylene





ADP480 is a colloidal-sized antimony pentoxide

A summary of the physical property test results of the UL-94 acceptable materials is given in Tables 3 and 4. The results with 1/8" thick test pieces show the antimony pentoxide and trioxide to be reasonably comparable from the perspectives of elongation and tensile strength. It would be expected that the larger trioxide particles would have a negative effect on these characteristics as the thickness of the test piece decreased.

The Izod impact data, however, show that the material with pentoxide has a significant advantage at all loading levels. In fact, the Izod data for polypropylene processed with antimony pentoxide-based flame retardants are comparable to the Izod result for virgin PP.


Table 3 – Physical Property Summary



ASTM states that tensile strength and elongation at break value for unreinforced polypropylene plastics generally are highly variable due to inconsistencies in necking of the center section of the test bar. Tensile strength and elongation at yield are more reproducible.

3


ASTM D638

Figure 2 shows the advantage of the smaller antimony pentoxide particles. The Izod data is 38 to 75% better for the Nyacol ADP480 compounds than for the trioxide-based compounds. As with elongation and tensile strength data, we strongly believe that as the test piece thickness decreases, the difference between pentoxide- and trioxide-based compounds will become even more exaggerated in favor of pentoxide-based compounds.

The antimony pentoxide- and trioxide-based flame retardant compounds processed equally well at all loading levels except at 12%, where the trioxide-based flame retardant compounds processed more easily. The larger trioxide particles may have absorbed the halogen material and thereby prevented puddling or slippage in the throat of the extruder. Ease of processing is probably a moot issue, however, since the industry standards for FR loading levels are expected to be no higher than 8%.

Figure 2 – ADP480 vs. Trioxide-based compounds



Table 5 shows the color effects of the flame retardant additive on the polymer. This data is reported as a L’a’b’ total color difference as measured on a Minolta CR-200 Chroma Meter with virgin polymer as the base standard. The color is reported in the standard CIE 1976 L’a’b’ notation. Figure 3 graphically shows a dramatic difference between Nyacol ADP480-based compounds and those processed with trioxide. ADP480 has less pigmenting or whitening effect than trioxide on the base polymer.


Table 5 – Color Effect of Additive on Polymer – Unpigmented
V Slightly Translucent

A comparison of antimony pentoxide versus trioxide reveals a color difference at 4% loading of 34.2 versus 58.7, respectively. At 4% loading the FR additive system utilizing antimony pentoxide shows that the test piece at 1/8 inch thickness begins to exhibit some translucency (L’a’b’ delta of 43.7). The level of translucency for the antimony pentoxide compounds increases as the loading level drops to 1% (L’a’b’ delta of 19.4). The antimony trioxide compounds are opaque at all loading levels except at 1% loading, where the test pieces exhibit only slight translucency (L’a’b’ delta of 35.7), but the material fails UL-94.

One of the curiosities of physics, which we will not attempt to explain, is that very small and very large particles have low hiding power, or opacity. There is, on the other hand, an optimum size for maximum opacity. For antimony trioxide, the 0.5 – 1 micron particles provide maximum opacity. Figure 4 dramatizes the translucence at 4% loadings of antimony pentoxide vs. antimony trioxide when compounded with virgin polypropylene. The affects of a 2.5% FR compound pentoxide vs. trioxide are also shown.







Differences in opacity levels in unpigmented environments help explain why less color concentrate is required to obtain a given color in an antimony pentoxide vs. trioxide flame retarded material. Generally a polymer system that uses antimony trioxide as a flame retardant will require an average of four times more pigment to achieve a particular shade than a polymer system that contains antimony pentoxide. Even higher proportions of pigment are needed to achieve dark red and blue tones when antimony trioxide is involved (see Figure 5). Dollar savings for pigments can be significant when antimony pentoxide is used for flame retardancy.





SUMMARY

Our goal in undertaking this study was to identify FR systems which could be used to flame retard fine denier PP fiber and/or "translucent" PP products. Our results indicate that an FR system based on colloidal sized antimony pentoxide best achieves this goal. This antimony pentoxide FR systems achieved good physical properties and translucency (aesthetics) as well as excellent flame retardancy. Our next step is to take this FR system and determine its effects in direct fiber production and other areas requiring translucency or reduced pigment loadings.




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