无卤阻燃PP外文.doc

A review of flame retardant polypropylene fibres
Sheng Zhang,  and A. Richard Horrocks
Centre for Materials Research and Innovation (CMRI), Bolton Institute, Bolton BL3 5AB, UK
Received 12 March 2003; 
revised 26 August 2003; 
accepted 5 September 2003. ;
Available online 24 October 2003.

Abstract
Flame retardants for polypropylene (PP) and their potential suitability for use in fibre applications are reviewed. Five principal types of generic flame retardant systems for inclusion in polypropylene fibres have been identified as phosphorus-containing, halogen-containing, silicon-containing, metal hydrate and oxide and the more recently developed nanocomposite flame retardant formulations.
The most effective to date comprise halogen–antimony and phosphorus–bromine combinations, which while having limited performance also are falling environmental pressures. Alternatives are discussed as well as means of enhancing the effectiveness and hence usefulness of phosphorus–nitrogenformulations normally used at concentrations too high for fibre inclusion. Of special interest is the potential for inclusion of functionalised nanoclays and recent observations that certain hindered amine stabilisers are effective at concentrations of 1% or so.
Author Keywords: Polypropylene; Flame retardant; Fibres; Combustion; Phosphorus; Halogen; Silicon; Metal hydrate; Nanoclay; Nanocomposite
Article Outline
1. Introduction
1.1. The development of polypropylene fires
1.2. The properties of polypropylene
2. Thermal and combustion behaviour
3. Flame retardants for polypropylene and polypropylene fibres
3.1. Phosphorus-containing and intumescent flame retardants
3.1.1. Effect of heavy metal ions
3.1.2. Effect of silicon-containing species (see also Section 3.3)
3.2. Halogen-containing flame retardants
3.3. Silicon-containing flame retardants
3.4. Metal hydroxides and oxides (metal compounds)
3.5. Nanocomposites
3.6. Other methods
3.6.1. Grafting and coating
3.6.2. Hindered amine light stabilisers
4. Conclusions
References
1. Introduction
Polypropylene was the first synthetic stereo-regular polymer to achieve industrial importance [1] and it is presently the fastest growing fibre for technical end-uses where high tensile strength coupled with low-cost are essential features; it has shown consistent growth of about 5% per annum for the last 10 years [2]. In 1999, worldwide consumption of polyolefin fibres exceeded 5.5 million tonnes and they accounted for 18% of the world's synthetic fibre production [3]. Polypropylene fibres have been widely used in apparel, upholstery, floor coverings, hygiene medical, geotextiles, car industry, automotive textiles, various home textiles, wall-coverings and so on [4].
1.1. The development of polypropylene fires
The synthesis of highly crystalline isotactic polypropylene using stereospecific catalysts was patented in 1954 by Natta [5]. They used heterogeneous catalysts of the type discovered by Ziegler for the low-pressure polymerization of ethylene to yield linear high-density polyethylene. Commercial polypropylene production was initially undertaken by Montecatini and subsequently expanded by ICI Fibres who introduced their ‘Ulstron’ product in late 1950s [6]. However, because of patent restrictions associated with fibre production, fibrous polypropylene often appeared in the market in the form of tapes and filaments rather than fibres; it was not until the early 1960s that staple fibres started to be seen on the market [7]. In the early 1970s the emergence of extruded, orientated film technology led to an expansion of polypropylene end-uses, including tapes/slit-film and various fibrillated and fibrous products [1].
The monomer propylene is a hydrocarbon gas mainly produced from petroleum refining. The polypropylene chain comprises a monomer with an asymmetric carbon atom at the C2 position, –CH2CH(CH3)–, and hence the polymer may exist in three types (isotactic, syndiotactic and atactic) of molecular configurations depending upon the relative orientations of the methyl side groups [7]. Both isotactic and syndiotactic forms have methyl groups situated regularly with respect to adjacent groups along the molecular chain and have fibre-forming character due to their potential for creating order in the polymer structure. Currently, isotactic polypropylene is the main commercially available stereoisomer for use in oriented fibre films and tapes. A very recent EU patent, however, has described the properties of fibres when 0.5–50% by weight of syndiotactic polypropylene having a multi-modal molecular weight distribution is included with at least 50% by weight of an isotactic polypropylene [8].
1.2. The properties of polypropylene
The reason for the rapid expansion in production capacity for polypropylene is its advantage over polyethylene in cost and properties. An economic edge in raw material cost and the high efficiency catalysts have made polypropylene a very low-cost fibre-forming plastic material. A number of properties are responsible for the widespread usage of polypropylene. The general properties of isotactic polypropylene are shown in Table 1 [9].
Table 1. Properties of isotactic polypropylene

Fibre-forming atactic polypropylene is partially crystalline, i.e. it possesses a two-phase system comprising crystalline and non-crystalline regions. The molecular chains of crystalline isotactic polypropylene exist in helical coils having three monomer units per repeating helix with a length of 0.65 nm for each repeat unit. The methyl groups are arranged systematically around the helix forming three lateral rows about 120° apart and thus close packing is possible.
2. Thermal and combustion behaviour
The crystalline melting point of isotactic polypropylene with a crystallinity of around 45% and containing 90–95% isotactic material is quoted as 165 °C [10]. The Tg value of isotactic polypropylene ranges from ?6?130 to 25 °C depending on method of measurement and heat-annealing treatments [10]. Atactic polypropylene has a glass transition temperature (Tg) of ?6?112 to ?6?115 °C and no defined melting point. Table 2 shows the thermodynamical properties of polypropylene.
Table 2. The thermodynamical properties of polypropylene at 230 °C

H, enthalpy; S, entropy; P=pressure.
Because of its wholly aliphatic hydrocarbon structure, polypropylene by itself burns very rapidly with a relatively smoke-free flame and without leaving a char residue. It has a high self-ignition temperature (570 °C) and a rapid decomposition rate compared with wood and other cellulosic materials and hence has a high flammability. The heat of combustion for polypropylene was reported by Einsele et al. [11] to be 40 kJ/g and this is higher than many other fibre-forming polymers. Gurniak and Kohlhaas [12] investigated the combustibility tests carried out on four different backing fabrics: spunbonded polypropylene, woven polypropylene tape with nylon/polypropylene bonded staple fibre fabrics, spunbonded Bikofilament (polyester core, nylon sheath), and Freudenberg's Lutradur T5012 spunbonded polyester. They found that the lowest flammability was achieved by the spunbonded polyester product. This is a significant observation in that polypropylene competes with polyester in terms of tensile properties and price, but it does have inferior fire performance.
Polypropylene pyrolysis is dominated by initial chain scissions; consequently considerable research has been undertaken in the conversion of waste polypropylene into clean hydrocarbon fuels [13 and 14] or other valuable products such as lubricants [15 and 16]. The thermal degradation of both isotactic PP and atactic PP has been investigated under non-isothermal conditions. The maximum volatile product evolution temperature was 420 °C for atactic PP and 425 °C for the isotactic PP. The recovery of carbon as organic volatile products comprised dienes, alkanes, and alkenes. Major compounds are for instance C9 compounds, like 2-methyl-4-octene, 2-methyl-2-octene, 2,6-dimethyl 2,4-heptadiene, 2,4-dimethyl-1-heptene, 2-methyl-1-octene. The hydrogen content of pyrolysis products obtained by flash pyrolysis at 520 °C, indicates the magnitude of the flammability problem in term of its fuel-forming potential [17]. An abundance of unsaturated volatile fuel fragments renders the flame retardation problem even more severe as the longer, less-volatile molecules behave as secondary fuel sources, which decompose further [18 and 19].
Cool flame combustion of polypropylene at 350 °C leads to the formation of toxic compounds which can cause death in mice, probably because of incomplete combustion and CO formation [20]. While the fire hazard caused by textiles in general has been reviewed by Horrocks [21] and Christian [22], the particular hazard of polypropylene was noted in the Manchester Woolworth Fire of 1979 where polypropylene upholstery covers over polyurethane foam filling in a stacked furniture pile were identified as the first material ignited and were responsible for the rapid growth of that fire. The 12 deaths associated with this fire gave rise to the need to use flame retardant textiles in UK domestic furnishings for the first time in 1980 [23].
Hirschler [24] studied the fire hazard and toxic potency of the smoke from burning polypropylene in 1987. Grand [25] investigated the effect of experimental conditions on the evolution of combustion products of polypropylene by using a modified toxicity test apparatus. In 2000, Shemwell and Levendis [26] studied the particulate (soot) emissions from burning polypropylene and four other plastics. Results showed that both the yields and the size distributions of the emitted soot were remarkably different for the five plastics burned. Soot yields increased with an increase of the nominal bulk (global) equivalence ratio (φ). Combustion of polystyrene yielded the highest amounts of soot (and most highly agglomerated), several times more than the rest of the polymers. Emissions from PE and PP were remarkably similar to each other, and each produced very low emissions at φ≤0.5 where excess oxygen prevails, but emissions increased drastically with φ, and most of the soot was very fine (70–97% of the mass was 2 μm or smaller, depending on φ).
Subsequently, the pre-combustive oxidative behaviour of commercial PP has been evaluated by simultaneous thermogravimetric analysis (TGA), differential thermal analysis (DTA) and pressure differential scanning calorimetry (PDSC) by Riga et al. [27]. They found that thermal oxidation was a consequence of rapid carbonyl formation initiated at the labile tertiary hydrogen atoms in the chain.
Polypropylene oxidizes more readily than polyethylene because of the lability of its tertiary hydrogen atom. It was reported that the uncatalysed, uninhibited oxidation kinetics and the overall energy of activation of both polyolefins were similar and that the oxidation rate of polypropylene was about 30 times faster than that of polyethylene. Several mechanisms were postulated to account for some of these volatile products [10 and 28]. The following volatile products have been identified during the oxidation of polypropylene: water, formaldehyde, acetaldehyde, acetone, methanol, hydrogen, hydrogen peroxide, carbonmonoxide and carbon dioxide.
Decomposition of the polypropylene chain is also oxidatively sensitized and occurs by breaking the weaker bonds at the polymer surface in the presence of chemisorbed oxygen. The peroxy radicals are formed by the propagating oxidative reaction at the reaction zone. In the inner oxygen-free zone, C–C bond scission begins resulting in formation of shorter radicals and biradicals. At high temperature (>350 °C) the dehydrogenation of the polymer radicals occurs yielding alkene species as discussed above.
3. Flame retardants for polypropylene and polypropylene fibres
While polypropylene fibres may be treated with flame retardant finishes and back-coatings in textile form with varying and limited success [21], the ideal flame retardant solution for achieving fibres with good overall performance demands that the property is inherent within the fibre. This review will concentrate only on systems that can be incorporated within polypropylene polymers, fibres and filaments.
Flame retardant formulations for polypropylene have been established for nearly 40 years and the first patent was probably that published on September 15th, 1964 by Blatz [29], in which polypropylenes were rendered flame retardant by the incorporation of a halogen compound (1,2-dibromo-4-(α,β-di-bromoethylcyclohexane) and metal oxide (Sb4O6) a free radical initiator and a dispersant into the thermoplastic melt.
The flame retardancy of polypropylene may be achieved in one of the following ways [30 and 17]:
(1)by changing the pyrolysis reactions to form a carbonaceous char, which will block heat transformation and hence reduce the volatile formation and protect the polymer from further oxidation. This method poses real challenges for polypropylene because the polymer has no reactive side chains thereby preventing char formation following elimination of these groups; (2)by inhibiting the initiating radicals in the pre-flame and flame zones; typically, antimony–halogen based additives are effective here; (3)by adding some hydrated inorganic additives which decompose endothermically and release water, and hence withdraw heat from the substrate and dilute the combustible volatiles; (4)by modifying the chemical structure of the polymer to change its decomposition procedure and/or to improve the compatibility with other flame retardants; (5)by addition of char-forming additives preferably having an intumescent property.
An acceptable flame retardant for polypropylene and especially fibre-forming grades, should have the following features:
?6?1 It should be thermally stable up to the normal polypropylene processing temperature (<260 °C). ?6?1 It should be compatible with polypropylene and have no leaching and migratory properties. ?6?1 The additive should retain its flame retardant properties when present in the fibre. ?6?1 It should also reduce the toxicity of gas and smoke during burning to an acceptable level. ?6?1 It should be present at a relatively low level (typically less than 10% w/w) to minimize its effect on fibre/textile properties as well as cost.
Most flame retardants act either in the vapour phase or the condensed phase through a chemical and/or physical mechanism to interfere with the combustion process during heating, pyrolysis, ignition or flame spread stages. Halogen-containing species, often in combination with some phosphorusand antimony-containing flame retardants, usually act in the vapour phase by capturing the radicals to interrupt the exothermic oxidative flame chemical processes and thus suppress combustion. However, phosphorus-containing species typically a