Barbara CICHY ? Fertilisers Research Institute, Inorganic Chemistry Division ?IChN? in Gliwice, Poland

Abstract:
The article reviews the most significant halogen free flame retardants applied in polymers. The production of ortophosphate, pyrophosphate and melamine polyphosphate in accordance with the technique eveloped at IChN Gliwice has been discussed. The effect of melamine polyphosphates as flame retardants in selected polymers (PP, PE, PA6) has been specified.

Please cite as: CHEMIK 2013, 67, 3, 214-219

Introduction

Organic synthetic polymers are the core of cutting-edge materials that supersede traditional ones. Their significance is on the rise as they are light-weight, readily processed and cost-efficient. Ever more appealing materials are obtained by virtue of an ever broader spectrum of marketed polymers and auxiliary materials. In 2011 the production of synthetic plastics worldwide has risen by 10 mln t (increase of 3.7%) and reached a level of approximately 280 mln t. 22% of global plastic production occurs in Europe. The demand of European manufacturers for plastic has risen to 47 mln t, constituting a 1.1% rise as opposed to the previous year. The production of plastic in Poland satisfies market demands only partially. Poland imports a considerable net amount of synthetic plastic. The statistical data show that the surplus of import over export amounts to nearly 1.4 mln t, bringing about a negative balance of trade in Poland estimated at over 2.2 bln (2,200,000,000) EUR. With such a result, Poland holds the sixth position in Europe. Plastic consumption per capita in our country amounts to approx. 66 kg, much below the consumption levels in more developed European countries (over 100 kg). The largest amounts are used by the packaging industry (31.5%) as well as construction industry (29%) [1].

The broad use of synthetic polymers has altered our lives, daily commodities such as furniture, household appliances, electronic devices, and construction materials have become lighter, cheaper, more accessible and subject to a more dynamic development. However, synthetic polymers are particularly flammable and the common application of polymer materials instead of traditional materials has reduced fire safety. The experts assess that in the 70?s of the 20th century evacuation time from a house amounted to 17 minutes as against 3 minutes currently [2]. Organic material combustion, the structure of which is based on carbon chains, cannot be excluded; it may only be limited to the acceptable level. Wherever necessary for the safety of humans, polymer materials modified by means of flame retardants are used, if fire occurs.

Flame retardants

Flame retardants (FR) that reduce the rate of combustion are constituted by chemical compounds or the mixtures thereof that deter the time to ignition and decrease the rate of pyrolysis or the oxidation of polymers on contact with a flame. The onset of thermal degradation of carbon polymers requires the supply of a relatively minor amount of energy that breaks the covalent bond C-C. The level of energy required for most polymers ranges between 200-400 kJ/mol. It is a prerequisite for the combustion process to be sustained that the amount of heat transferred by the flame to the sample ensures at least a constant stream of volatile products of pyrolysis that penetrate the flame. In the event of fire, the flames consume an expanding area, the intensity of the heat stream is on the rise and the entire process is an autocatalytic reaction. The mechanism involves volatile flammable combustion products which are essential here. The flame retardant agent systems may either rely on physical (cooling, forming of a protective layer or fuel dilution) or chemical (condensed phase reaction or gas) phenomena, depending on their properties. They may impact various stages of polymer combustion (heating, pyrolysis, ignition, thermal degradation propagation [3].

The global market demand for flame retardants applied for polymers in 2011 reached the level of 2,2 mln t/year. The largest market of flame retardant agents features construction materials and products, electric devices, transportation means as well as furniture and interior fittings sectors. The increasing stringency of safety and flammability regulations as well as the soaring figures of synthetic polymer products are conducive to an ever broader application of materials with a reduced flammability that may be obtained by the addition of flame retardants to polymers. The global demand for fire retardant additives is expected to rise by 6.1% annually until 2014 [2, 4].

Halogen free flame retardants

Flame retardants may be divided in terms of either being additive, namely not showing reaction with base polymers, or reactive that are usually combined with the polymer in the course of synthesis (monomers or polymer precursors) or at a post-reaction stage. Flame retardant additives are incorporated into the polymer chain. The implementation of the REACH regulation has given rise to systematic study of the impact of chemical substances on the life standard and health of humans as well as on the environment. Among the substances classified as hazardous and recalled from the market are some agents heretofore applied as fire retardants. Brominated bisphenols have been identified as PBT agents (persistent, bioaccumulative, toxic). Under the UNEP Stockholm Convention of 2009 it has been prohibited to apply some halogen free retardants and this restraint was enforced in Poland as well as in Europe in 2011. There have been safety-related objections as regards the application of some volatile organic phosphates but they have not been recalled from the market so far [4÷6]. Organic halogen free FR prove superior not only in terms of high performance but also with respect to the suitability for combining with polymer matrix ? as a result of their chemical affinity. The area of the interface between the dispersed phase (filler) and the type of interaction between the continuous and the dispersed phase are two important factors defining the properties of polymers. Higher shape factor and reduced transverse dimension of the filler particles yield a larger effective area of the filler particle as well as an increase of total of interactions between the polymer matrix and the filler particles. As a result, the composite usually demonstrates improved mechanical properties but at the expense of the deterioration of the rheological characteristics of the polymer. The morphological structure of a FR additive, namely the dimensions and geometry of the particles thereof, affects the mechanical properties of the polymer as well as its ?fitness for processing?. The requirements of small size, low shape factor as well as a minimum effective area may be assumed for polypropylene and polyamide fillers [7]. The objective is to identify flame retardants and the systems thereof that remain efficient at low content in polymer.

Retardant additives prevail among marketable halogen free flame retardants. The following may be specified, in the order of significance:
? non-organic hydroxides, e.g. of aluminum or magnesium
? non-organic and organic phosphate compounds, including phosphates, phosphites, red phosphorus
? nitrogen compounds, including melamine and its salts
? retardants and intumescent retardant systems, characterised by various chemical structure, predominantly with a polyphosphate content
? mineral and synthetic silicon compounds, including modified aluminum-silicon nanofillers [3].

Melamine phosphates as FR additives

Phosphorus and nitrogen compounds, including polyphosphates, are of significance on the market of halogen free flame retardants. The most popular and best described among them is ammonium polyphosphate. Melamine salts have gained less popularity but show a growing importance. Melamine (2,4,6-triamine-1,3,5-triasine) is crystalline solid with melting point at 345oC, and nitrogen content as high as 67%. It sublimates already at 350oC because of high heat adsorption rates. At a slightly higher temperature, it undergoes degradation, releasing ammonium and forming temperature resistant condensates. Both melamine and its salts such as cyanurate, oxalate, phthalate, borate as well as the most popular phosphates are applied as flame retardants. Of interest are the following: orthophosphate, diphosphate (or pyrophosphate) and melamine polyphosphate [1, 8]. Melamine polyphosphate has a lower phosphate content than the popular ammonium polyphosphate (approx. 30% as opposed to approx. 60% in ammonium polyphosphate) but contains significantly more nitrogen. The nitrogen bound within a stable melamine ring is released from melamine phosphates and transfers to gaseous phase at temperatures higher than 300oC, at the expense of ambient heat absorption.

The synthesis of melamine phosphate [9], pyrophosphate and polyphosphate involves the following reactions [1, 8], specified here in a simplified form:

Subsequent products of the condensation of phosphates differ mainly in terms of thermal resistance. MPP is designed mainly for polymers processed at higher temperatures, e.g., polyamide. The synthesis of melamine polyphosphate following the IChN [10] methodology has been demonstrated in Figure 1 which shows a model of a technical process developed by means of a derivatograph manufactured by Metler-Toledo. Initially, a sample (MP) was heated at the rate of 1.3oC/minute, with the material subjected to the temperature of 300oC for 75 minutes. Figure 1 indicates the respective differential effects of mass loss (as established by means of differential thermal gravimetry ? DTG) accompanying the transformation process: MP into MDP and, subsequently, into MPP. Melamine polyphosphate, derived in an analogous way and pulverised to the particle size range of 5-10 ?m, was deployed for the purpose of the preparation of multiple compositions of various polymers (PEHD, PA6, PP, EVA) that served to obtain granulates and samples for flammability tests. Thus, the robustness of MPP as a flame retardant was tested and appropriate mechanical properties of the products and rheological compositions were provided. A selection of the results of the flammability tests have been provided in Table 1 as well as in Figure 2 [11, 12].

The properties of polymer materials, including flame and fire resistance as well as the their behaviour in case of fire, are determined by the parameters of all the components of the polymer product: polymer, fillers, flame retardants and other additives [3, 13]. The efficacy of flame retardants may be estimated by various standards, for instance, basing on the heat release rate (HRR) as well as the amount and type of toxic products of polymer pyrolysis and combustion. The heat release rate of polymer materials directly correlates with mass loss rate during heating and is a function of the amount of heat energy yielded by the flame and transferred to the non-burning surface of the material. Apart from an increased flame resistance of the materials, the following are required: reduced smoke emission and the elimination of toxic combustion products from the process [1, 3, 13]. Laboratory tests of the flammability of polymer materials differ to a considerable extent. The attempts of establishing conformity criteria between respective types of tests for either flame retardant polymer materials considered globally or for particular polymer group have failed. The most significant tests in practical terms are: the oxygen index LOI established in accordance with ISO 4589 as well as the counterparts thereof: ASTM D 2863 and PN-EN ISO 4589-2:2006, UL-94 test as well as cone calorimeter. LOI method proves the most universal as it may be applied for a spectrum of flammable and non-flammable materials. The LOI oxygen index is defined as the lowest oxygen content (expressed as volumetric %) that is sufficient for sustaining the combustion of a polymer with a candle-like flame with the initial temperature equal to room temperature [13]. Hence, materials with LOI levels exceeding standard oxygen content in air should be self-extinguishing. The UL 94 method has been set by the American Underwriters Laboratories as a standard for tests of flammability and fire safety of polymers used in equipment and devices. The UL 94 HB (horizontal flame propagation) test investigates the course of the combustion of a horizontally oriented element from a polymer, while the more sophisticated UL 94 V (vertical flame propagation) test ? the course of the combustion of a vertically oriented element. The Polish statutory counterpart is the standard PN-EN ISO 9773:2003. The Polish standard evaluates the glow duration and route; moreover, it requires that there are no fragments detached from any burning or glowing sample and falling onto cotton pieces underneath the samples (for grades V0 and V1). This is the most straightforward test for the assessment of industrial materials that is of little use for the purpose of determination of material characteristics or the mechanism of combustion.

The cone calorimeter test ensures a more thorough analysis of fire resistance parameters of the material. Undisputedly, the most common parameter applied for the assessment of fire resistance of polymer materials is heat release rate (HRR). A calorimeter makes it possible to determine the heat release rate as a function of combustion time (HRR), mass loss rate relative to time (MLR), total heat release rate (THR), effective heat of combustion (HOC) as well as the measurements of the contrast attenuation coefficient resulting from the measured smoke density and the content of the released CO and CO2. Most flame retardants reduce heat release rate (HRR) as well as mass loss rate (MLR) of the sample but do not affect total heat release rate (THR) [3, 4, 7, 11, 13].

Melamine phosphates may serve as the base of intumescent systems acting as protective agents against fire by virtue of the formation of a temperature resistant layer that shows temperatureinduced expansion [3, 8, 11, 14]. MPP has proved especially efficient in polypropylene-based intumescent systems, as evidenced by heat release rate reduced by virtue of MPP-based intumescent system (Fig. 2). This system also ensures the flammability grade V-0 as determined by means of UL- 94 test.

Conclusions

In view of higher consumer and industry expectations of polymer materials, and also demands for a reduced material flammability as well as the pressure to use environmentally-friendly and humanfriendly flame retardants, melamine phosphates are an appealing option on the market. Their practical significance is on the rise. Most promising results in terms of the reduction of the flammability of polymer materials can be seen for polypropylene and polyamide. There has been a growing body of literature on the application of melamine phosphates in complex retardant systems, also for polymers other than polyamides and polyolephines.

Literature

1. www.plasticseurope.pl/, Accessed: January 2013
2. http://www.cefic-efra.com/, February 2013
3. Laoutid F., et al. Materials Science and Engineering: R: Reports, 2009, 63 (3,29), 100.
4. http://www.pinfa.org/, Accessed: July 2012
5. www.echa.europa.eu/, Accessed: January 2013
6. ECHA European Chemicals Agency, Justifcation for the draft recommendation of inclusion in annex XIV, Substance name: Hexabromocyclododecane (HBCDD) and all major diastereoisomers identified, 14.01.2009
7. Dufton P., Flame Retardants for Plastics Market Report, Rapra Technology Limites, 2003, ISBN: 1-85P57-385-1.
8. Cichy B., Łuczkowska D., Nowak M., Władyka-Przybylak M., Polyphosphate Flame Retardant with Increased Heat Resistance, Ind. Eng. Chem. Res., 2003, 42(13), 2897
9. Cichy B., Kużdżał E., Kinetic Model of Melamine Phosphate Precipitation, Ind. Eng. Chem. Res., 2012, 51, 16531.
10. Patent application ref. no. P.395656 (2011)
11. Cichy B., Kużdżał E., Rymarz G., Gajlewicz I., Uniepalniające działanie soli melaminy w kompozycjach kopolimerem etylenu i octanu winylu, Przem. Chem., 2012, 91/11, 2257.
12. Cichy B et al., Sprawozdanie z projektu badawczego NN209 186538, nr IChN 4480, unpublished
13. G. Jankowska, W. Przygocki, A. Włochowicz, Palność of polymer materials, WNT Warszawa 2007.
14. Chen Y., Wang Q., Reaction of melamine phosphate with pentaerythritol and its products for flame retardation of polypropylene, Polym. Adv. Technol. 2007; 18: 587.

Note: the study of flammability presented herein was performed at the Institute of Polymer and Colorant materials, Branch in Gliwice.
Barbara CICHY ? Ph.D. (Eng), works at the Fertilisers Research Institute, Inorganic Chemistry Division ?IChN? in Gliwice as an assistant professor. She is Head of Department of Inorganic Synthesis and Environmental Protection. Scientific interests: inorganic technology, especially phosphate- and polyphosphate-based methods as well as industrial waste and environmental protection.
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