Polyethylene oxygen index

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. 

We have recently evaluated the chlorendic imide/hindered phenol for its effect on the oxygen index of polyethylene, and we found only a miniscule increase, not considred statistically significant, in comparison to the same loading of chlorine as chlorendic anhydride. We believe that if the antioxidant approach to flame retardancy is to be successful, special high temperature antioxidant structures must be designed for this purpose. 

Figure 15.16. Limiting oxygen index of polyethylene vs. concentration of filler. [Adapted, by permission, from Yeh J T, Yang H M, Huang S S, Polym. Degradat. Stabil., 50, No.2, 1995, 229-34.]...

Stackman [29] carried out a study to find systems suitable for reducing the flammability of polyethylene terephthalate (PET) and poly-1,4-butylene terephthalate (PBT) while retaining the chemical and physical properties of the original polymers. The additives used were phosphine oxides, phosphonates and phosphates and their activity was assessed by means of an oxygen index test. Most of the phosphorus esters were found to be volatile under the blending conditions and both the halogenated phosphorus esters and halogenated derivatives of phosphorus oxide proved to be ineffective as flame retardants. 

Intumescence of polyethylene (PE) and PP has been reviewed using as intumescent agents zeolites [36], melamine phosphate and pentaerythritol [37]. Ammonium polyphosphate-pentaerythritol [38], zinc borate and ammonium polyphosphate [39], and APP [40], limiting oxygen index (LOI) [36, 37], cone calorimetry [36] and the UL 94 test [25, 36, 37] have all been used in these studies. 

Figure 6.1 A comparison of the effect of calcium carbonate and aluminium hydroxide fillers on the oxygen index of a filled polyethylene...

Very importantly, there is a significant correlation between the char residue (at 850°C) and the oxygen index of the polymers, as illustrated in Figure 14.32 (117). Polymers such as polyethylene and polyisoprene leave little or no char residue. Note that carbon itself leaves a 100% char residue, important for the carbon nanotube discussion below. 

Figure 14.32 The flame resistance of polymeric materials, indicated by the oxygen index. 1, polyformaldehyde 2, polyethylene, polypropylene 3, polystyrene, poiyisoptene 4, polyamide 5, cellulose 6, poly(vinyl alcohol) 7, poly(ethylene terephthalate) 8, polyactylonitrile 9, poly(phenylene oxide) 10, polycarbonate 11, aromatic nylon 12, polysulfone 13, Kynol 14, polylmide 15, carbon. Polymers producing large values of char residue are more fire resistant.

It has been found that magnesium hydroxide appears to have a special synergy with polyethylene vinyl acetate [119], resulting in high levels of flame retardancy as measured by the limiting oxygen index tests (LOI) [120]. The effect of the magnesium hydroxide on the LOI of a polyethylene vinyl acetate (18% of vinyl acetate content) is presented in Table 3.3.1. 

We have found an extreme combustion behavior for a system polypropylene-polypropylene-co-polyethylene [238], It can be explained in terms of oxidative degradation of polymer materials. We studied the features of autooxidation and combustibility of blends of isotactic polypropylene (PP) and ethylene-propylene copolymers (PP-co-PE) as thin films (50-80 ). ASTM D2863 (ISO 4589 Part 2) was used to determine the oxygen index (LOI) for polymer films 140 by 52 mm. 

Polymers which can be largely degraded at relatively low temperature (eg. less than 400 ) to give monomer, dimer or other low molecular weight combustible volatiles gave low oxygen index values. Thus many of the important commodity polymers such as polyethylene, polypropylene, polyformaldehyde, polystyrene and polymethyl methacrylate will have oxygen indices of less than 21. 

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