Flame-retardant polypropylene compositions

Economical manufacturing methods of mass production and improvements to the properties of finished products have in many applications greatly helped to replace traditional materials, such as metals or wood, with plastics and rubbers. In particular, polypropylene (PP) is the fastest growing commodity plastic world-wide. It has found its place in many sectors such as building, transportation (automotive, railways, etc.), electrical engineering (electrical/household appliances, housings, etc.) or paper industry. 

The use of organic pol)nner systems, such as PP, which are flammable, leads to greater fire risks and thus to the growing importance of flame retardancy. Recent events have shown the need to improve fire resistance. One can notice as an example, the PP fire at the BASF plant (March 1995 in Teeside, UK) which has been described as one of the largest fires ever seen in the UK during peacetime (more than 10 000 tonnes of PP were consumed) [1]. This clearly indicates the need for a comprehensive appraisal of all aspects of the combustion of polymers and the ways to prevent it. Reductions in the propensity of organic materials to ignite or emit dense and/or toxic fumes are equally important. 

Ignition of poI)aners occurs either spontaneously or from an external source if the concentration of volatile combustible products evolved by pyrolysis or thermo-oxidative degradation of the polymer is within the flammability limits. A self-susfeiined combustion cycle is then triggered. 

This is driven by the heat of the flame promoting the pyrolysis of the polymer. The process will continue as long as the heat transmitted to the polymer is sufficient to keep its rate of thermal degradation above the level required to feed the flame, otherwise it will extinguish. The self-sustained combustion cycle occurs in both the condensed and gas phase. This means that in order to extinguish the flame by depressing the rate of chemical and/or physical processes taking place in one or both phases, polymers have to contain a variety of additives that may act as fire retardants or they have to be modified (chemical or physical modifications) to resist fire [2]. 

There are therefore three main techniques to create flame retarded polymers  

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Dvir, H., Gottlieb, M., Daren, S., and Tartakovsky, S. Optimization of a flame-retarded polypropylene composite. Composites Science and Technology, 63, 1865-1875 (2003). 

In some cases, flame-retarded polypropylene should not contain any halogen, as prescribed in France for switch-boxes that have to conform to the rating V-0. A composition of Montefluos under the trade name of MF 80 as flame-retardant is used. ... 

The materials were compounded by twin-screw extruder (6 =45, L/D=30, Ikegai). Flame-retardant polypropylene (PP) composition contains 3wt% of TBS-DBP as flame-retardant and lwt% antimony oxide as synergist plus some fillers. Filler content was varied in a range of 0 - 20 wt%. The details of composition are listed in Table 2. As the reference materials, talc filled neat PP-based composites were also prepared as listed in Table 3. All the test pieces used for flammability test were molded by injection molding. 

Marosi, G., Anna, P., Balogh, I., Bertalan, G., Tohl, A., and Maatoug, M. A. 1997. Thermoanalytical study of nucleating effects in polypropylene composites. 3. Intumescent flame retardant containing polypropylene. Journal of Thermal Analysis 48 717-26. 

Marosfoi, B., Szabo, A., Kiss, K., and Marosi, G. 2009. Use of organosilicone composites as flame retardant additives and coating for polypropylene. In Fire Retardancy of Polymers, eds. Kandola, K. and Hull, R. Cambridge, U.K. Royal Society of Chemistry, pp. 49-58. 

Because reactive types of flame retardants are polymer-specific, their application is limited. There are several reactive flame retardants, specifically produced and all different in composition. For example, there is a 25% pelletized concentrate of antimony pentoxide, bromine and polypropylene resin of various melt flow indices, which is geared to PP fibers for textiles and carpets,... 

Natural fiber-reinforced polyolefins are commonly apphed to automotive and constmction applications. The most abundantly used additive is fire retardant. Flammability is an important factor that often limits the application of composites to a specified field. Magnesium hydroxide is the most common flame retardant material used in the constmction industry. This filler responds well to surface modifiers and decomposes by an endofliermic reaction that releases water at temperatures close to the polymer degradation temperature as show in Eq. 6.1. Rothon et al. [78] studied the effects of magnesium hydroxide on polypropylene as a flame retarder of 60 % by weight. The smdy found less heat emission at 100 kWm after 6 min of fire exposure compared to filled PP without Mg(OH)2 at 500 kWm. ... 

Figure 12.10 shows the release of volatiles during thermal degradation of polypropylene. Mica reinforces the polypropylene stability, while halogenated flame retardant reduces it. Both composite polymers are compared with the pure polymer. The thermal stability of low, medium and high-density polyethylenes has its linkage to the number of branches in the main polymer chain. It is of interest that the flammabihty expressed in the amount of oxygen consumed follows the similar tendency (Fig. 12.11). 

Work based in Hungary and France has been examining the claims for a siloxane compoimd to act as a synergist for the protection of polyolefins. Organoboroxo-siloxane (OBSi) is added with APP and pentaerythritol to polypropylene. The material appears to increase the viscosity of the polymer composition and provide some plasticity to the resultant intumescent char. The plasticity allows for better prevention of char cracking and so provides improved flame retardancy. The increased melt viscosity is created by the product of BSi-pentaerythritol, formed during the compound preparation, and the improved char plasticity is the result of products formed at high temperatures from BSi and APP. 

Expandable graphite has potential as the flame retardant in polypropylene-flax composites under consideration for vehicle trim, along with other natural fibres, and also in building applications. 

Realinho, V., Haurie, L., Antunes, M., Velasco, J.I. Thermal stability and fire behaviour of flame retardant high density rigid foams based on hydromagnesite-filled polypropylene composites. Compos. B Eng. 58, 553-558 (2014)... 

Jeencham R, Suppakarn N, Jarukumjom K. Effect of flame retardants on flame retardant, mechanical, and thermal properties of sisal fiber/polypropylene composites. Compos Part B 2014 56 249-53. 

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