Oxygen indices

Many of the basic plastics require additives that will improve their resistance to supporting combustion. These improvements vary in degree, and the designer must be cautioned not to over specify the requirement for 

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Another property, used to compare the flammabiUty of textile fibers, is the limiting oxygen index (LOI). This measured quantity describes the minimum oxygen content (%) in nitrogen necessary to sustain candle-like burning. Values of LOI, considered a measure of the intrinsic flammabiUty of a fiber, are Hsted in Table 2 in order of decreasing flammabiUty. 

Table 2. Limiting Oxygen Index of Textile Fibers...

Antimony Oxide. The effect of antimony trioxide on the oxygen index of flexible poly(vinyl chloride) containing from 20 to 50 parts of plasticizer is shown in Figure 2. The flame resistance as measured by the oxygen index increases with the addition of antimony oxide until the oxygen index appears to reach a maximum at about 8 parts of Sb202. Further addition of antimony oxide does not have any increased beneficial effect. 

Fig. 2. The effect of antimony oxide on the oxygen index of poly(vinyl chloride) plasticized with dioctyl phthalate (DOP), (—...

Mixed Meta.1 Antimony Synergists. A similar increase in the oxygen index can be achieved using the mixed metal synergist Thermoguard CPA as illustrated in Table 8. 

Table 9. Oxygen Index of Flexible PVC Containing Antimony and Boron...

Fig. 3. A comparison of the effect of antimony oxide (-) and Firebrake ZB (---) on the oxygen index of PVC containing 50 phr DOP.

Molybdenum Oxide. Molybdenum compounds incorporated into flexible PVC not only increase flame resistance, but also decrease smoke evolution. In Table 10 the effect of molybdenum oxide on the oxygen index of a flexible PVC containing 50 parts of a plasticizer is compared with antimony oxide. Antimony oxide is the superior synergist for flame retardancy but has Httle or no effect on smoke evolution. However, combinations of molybdenum oxide and antimony oxide may be used to reduce the total inorganic flame-retardant additive package, and obtain improved flame resistance and reduced smoke. 

Fig. 4. Effect on limiting oxygen index (LOI) of (---) antimony trioxide (---) Flamtard S, and Flamtard H added to (a) the basic unfilled PVC...

Alumina Trihydrate. Alumina trihydrate is usually used as a secondary flame retardant in flexible PVC because of the high concentration needed to be effective. As a general rule the oxygen index of flexible poly(vinyl chloride) increases 1% for every 10% of alumina trihydrate added. The effect of alumina trihydrate on a flexible poly(vinyl chloride) formulation containing antimony oxide is shown in Figure 5. 

Fig. 5. Effect of alumina trihydrate on the oxygen index of flexible PVC (30) having 5 phr Tribase XL, 3 phr Sb202, 0.05 phr petroleum wax, and (—...

Table 11. Effect of Chlorine and Antimony Oxide on the Oxygen Index of Unsaturated Polyesters...

Table 12. Oxygen Index of Unsaturated Polyesters Containing Bromine ...

Sb202 Zinc borate Flame spread Oxygen index ... 

Table 14. Oxygen Index and Smoke Reduction of Unsaturated Polyester Containing Molybdenum Oxide and Antimony Oxide...

ASTM D2863-87 limiting oxygen index ease of extinction... 

Limiting Oxygen Index. The minimum concentration of oxygen in an O2/N2 mixture that supports combustion of a vertically mounted test specimen is called the limiting oxygen index (3,4). Test specimens are 0.65 x 0.3 cm x 12.5 cm. The principal advantage of this test is its reproducibiUty which makes it useful for quaUty control. The main disadvantage is that the results rarely correlate with the results of other fire tests. 

The limiting oxygen index of Tefzel as measured by the candle test (ASTM D2863) is 30%. Tefzel is rated 94 V-0 by Underwriters Laboratories, Inc., in their burning test classification for polymeric materials. As a fuel, it has a comparatively low rating. Its heat of combustion is 13.7 MJ/kg (32,500 kcal/kg) compared to 14.9 MJ /kg (35,000 kcal/kg) for poly(vinyHdene fluoride) and 46.5 MJ /kg (110,000 kcal/kg) for polyethylene. 

When exposed to fire. Teflon PEA contributes Httle in fuel value and is self-extinguishing when the flame is removed. The fuel value is approximately 5.4 MJ/kg (2324 Btu/lb). It passes the UL 83 vertical-flame test and is classified as 94VE-0 according to UL 94. The limiting oxygen index (LOI) by ASTM D2863 is above 95%. 

Elame-spread and smoke-density values, and the less often reported fuel-contributed semiquantitive results of the ASTM E84 test and the limited oxygen index (LOI) laboratory test, are more often used to compare fire performance of ceUular plastics. AH building codes requite that ceUular plastics be protected by inner or outer sheathings or be housed in systems aH with a specified minimum total fire resistance. Absolute incombustibHity cannot be attained in practice and often is not requited. The system approach to protecting the more combustible materials affords adequate safety in the buildings by aHowing the occupant sufficient time to evacuate before combustion of the protected ceUular plastic. 

DMPPO—polystyrene blends, because of the inherent flame resistance of the DMPPO component (oxygen index ca 29.5), can be made flame retardant without the use of halogenated additives that tend to lower impact strength and melt stabiUty in other polymers. Approximately one-half of total Noryl sales volume is in flame-retarded grades, ie, VO or VI in a 1.6-mm section (UL-94). 

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