Phosphorus-Containing Polyurethanes with Inherent Flame-Retardant Properties

Halogenated additive materials are currently predominant for increasing flammability levels. PentaBDE and OctaBDE, two types of polybrominated diphenyl ether (PBDE) halogenated flame retardants, were banned in 2004. DecaBDE, the third compound, was banned on 1 April 2008 following a decision by the European Court of Justice. Despite their use in North America through 2010, the use of these compounds was prohibited in the bedding and electronics markets. Today, inorganic-based additive materials are also used as additives in polymers to impart flame-retardant properties. For example, zinc borate is used as an additive in polymers and paints. Inorganic additives have certain disadvantages due to aggregate formation within the polymer matrix, poor dispersion resulting from phase separation, and wear effects on extruder machinery. Additionally, surface leaching over time affects the environment as waste. This environmental problem also applies to low-molecular-weight organic-based additives. In addition to inorganic additives, due to the effects of halogen-based additives on the environment and human health, there is a need for new flame-retardant materials. Countries have developed new standards for flammability levels, taking special care for environmental conditions and human health, and issuing new regulations within this scope, resulting in increased research efforts on flame retardants. Currently, research efforts are focused particularly on polymers having inherent flame-retardant properties without the need for flame-retardant additives.

Flammability and Its Importance

Fire occurs when a combustible material receives heat from any source, reaches its ignition temperature, undergoes a chain and exothermic reaction with oxygen, and burns in an uncontrolled manner. Following combustion, varying amounts of heat, light, smoke, and fire gases are produced depending on the nature of the combustible material. If the fire cannot be controlled or extinguished, these released elements burn everything in the area and adjacent spaces through conduction, convection, and radiation [1]. Combustion is a rapid and powerful exothermic reaction. Viewing this reaction broadly, combustion is a system containing fuel and oxidation reactants. For combustion to occur, these two reactants must be together at the molecular level. Due to this molecular interaction in combustion, combustion systems exhibit different behaviors depending on the mixing amount and manner of the reactants [2]. Fire is an important problem we may encounter at any time and anywhere, and without necessary precautions, it can cause loss of property and life. As is known, polymer-based products, electrical and electronic materials are an important part of our lives, and these materials are highly flammable in a way that can cause fires, loss of life and property.

An electrical device that has shorted, a cigarette, a cable, or a spark from any source can turn our lives upside down in an instant. For example, if a fire starting from a chair spreads to other items in the room, it can spread rapidly.

As the number of burning items and materials increases, the room temperature will gradually rise. As temperature continues to rise, the heated combustible gases can cause the fire to spread throughout the entire room or house. And for this reason, it becomes impossible to escape the room and save your life. However, if the chair that we said started the fire had been produced with flame-retardant materials, it could have prevented or slowed the spread of fire to the room. This could provide enough time for the person in the room at that moment to escape. To prevent or minimize this, flammability, that is, flame-retardant materials are needed [3]. Fires can occur anywhere in the world, leading to human deaths, material losses, and the loss of invaluable art and historical artifacts. According to experts, the use of flame retardants largely prevents fire spread. According to 2009 data from the NFPA (National Fire Protection Association), there were 1,348,500 fires in the United States, 3,010 citizens died, 17,050 citizens were injured, and USD 12.5 million in damage occurred [4]. Considering statistics, fire incidents in Turkey show an increase of 15% to 20% annually [1].

In textiles, transportation and domestic technical textiles, in the production of upholstery fabrics, preference for fibers that do not emit toxic gas when burned and have high ignition temperatures, use of materials resistant to high temperatures and difficult to ignite in furniture, hot gas filtration, ropes, cables and fabrics, and many other uses of flame-retardant materials have prevented numerous losses of property and life due to fire.

As mentioned above, one of the measures that can be taken to minimize loss of property and life due to fire is the use of non-flammable or fire-delaying materials in industry and daily life. Many countries have developed various directives to establish an acceptable minimum level of fire protection. In particular, one of the basic requirements of building materials is to have flame-retardant properties that will assist safety measures in case of fire. This matter is considered highly important by the European Union, and various regulations (such as the Building Materials Regulation 89/106/EEC) have been developed. According to these regulations: a) The load-bearing capacity of the constructed building shall not decrease during a fire for a certain period. b) In the event of fire in the building, the spread of fire and smoke shall be limited. c) The spread of fire to adjacent buildings shall be limited. d) Building occupants shall be able to leave the building or be rescued by other means. e) The safety of rescue teams shall be taken into account. In Turkey, the "Regulation on Fire Protection of Buildings" was published in the Official Gazette dated 19 December 2007. The purpose of this regulation is to define the task descriptions of public institutions and organizations and to establish the procedures and principles of measures and action plans to be taken regarding fire (before, during, and after).

Flame-Retardant Materials

Currently, the use of flame retardants is increasing significantly, particularly because plastics occupy an important place in our daily lives. These materials are generally composed of bromine, chlorine, metal hydroxides, nitrogen-based, phosphorus-containing, or mixed structures. Annual consumption of flame retardants exceeds 1.5 million tons in the global market. The cost is approximately USD 2.4 billion. The flame-retardant market is growing at an annual rate of 5%. The effectiveness of flame retardants also depends on the quantities of chemicals used. The flame retardant used can be inorganic and/or organic in structure. These chemicals can sometimes be used alone and sometimes as dual formulation additives. An important problem that emerges here is that these flame retardants must be well distributed within the polymer matrix. While halogen-based flame retardants mix well with the polymer matrix, problems such as aggregate formation in the matrix and matrix surface exudation (leaching over time) are encountered, particularly with inorganic-based flame retardants. The main problems encountered in this context include loss of flame-retardant properties in the composite material, reduction in mechanical strength values, and threats to the environment and/or human health through leaching over time. While bromine-based flame retardants are primarily used today, their use is restricted due to observed side effects. PCB (polychlorinated biphenyls), one of the first flame retardants used, was banned in 1977 due to its toxic effects.

As of 2008, the European Union banned the use of many PBDE (polybrominated diphenyl ethers).

Worldwide, problems related to flame retardants, their environmental impacts, accumulation of inorganic substances in organisms, and the potential for this accumulation to poison humans and the environment over time, as well as unwanted gases produced during combustion, are issues of concern. The use of materials containing chlorine and bromine-based flame retardants has been periodically restricted or banned since 2000 because it causes the formation of gases harmful to the environment and humans during combustion. This has driven the chemical industry to search for next-generation flame-retardant materials with good distribution in polymers and low likelihood of creating toxic waste upon combustion. Generally, reactive-type flame retardants consist of halogens and phosphorus. Examples of phosphorus compounds used include red phosphorus, phosphine, phosphine oxide, phosphite, phosphonate, and phosphates as flame retardants. Examples of halogenated types include chlorinated and brominated flame retardants. Despite their effectiveness and widespread use, halogenated flame retardants (bromide and chloride) have been restricted by the European Commission (EC) because they form highly toxic, carcinogenic furans and dioxins during fire. Polybrominated biphenyl (PBBs), a halogenated flame retardant, has been banned in Europe, Japan, and the United States. Interest in halogen-free flame retardants has been increasing in recent years. Phosphorus-containing flame retardants are gaining importance as replacements for halogenated flame retardants because they produce less toxic gas during combustion.

Flame-Retardant Properties of Phosphorus-Based Compounds

Phosphorus-containing compounds have long been used as flame retardants in polymeric materials [5]. Examples of phosphorus compounds used include red phosphorus, phosphine, phosphine oxide, phosphite, phosphonate, and phosphates as flame retardants [6]. Phosphorus-containing flame retardants can be added to the polymer matrix as an inorganic additive or can be covalently bonded to the polymer main chain to impart flame-retardant properties to the material. During heating or combustion, phosphorus-containing flame retardants in solid phase react with oxygen present in the polymer chain or matrix. As a result of the polymer's interaction with the flame, phosphorus compounds form phosphoric acid, and following dimerization of these molecules, water molecules are released and pyrophosphate structures are formed (Figure 1) [7].

The released water molecule dilutes the gas phase, reducing the concentration of oxidizers that cause combustion. Phosphoric acid and pyrophosphonic acid also catalyze the formation of carbon-carbon double bonds, promoting the formation of aromatic structures.

At high temperatures, ortho- and pyrophosphoric acid convert to metaphosphoric acid [(O)P(O)(OH)] and their oligomers [(PO3H)n]. Phosphoric acid reacts with the carbon compound present in the medium, forming a protective layer on the polymer surface. Additionally, specific phosphorus-based flame retardants such as metal phosphinates can create phosphorus and phosphate radicals in the gas phase of combustion, negatively affecting combustion. Surface migration of low-molecular-weight flame retardants and their effects on the environment and human health also apply to phosphorus compounds. For example, it has been observed that the hydrolytic stability and plasticizing effect of red phosphorus decreases over time following its addition to the polymer matrix. In addition to phosphorus-based additives, phosphorus structures covalently bonded to the polymer chain are becoming a focal point of both academic and industrial interest. The covalent incorporation of phosphorus into the polymer chain will lead to the development of polymers with inherent flame-retardant properties without the need for another additive (non-leaching, high vapor pressure). For example, arylphosphonate-based polymers have been shown to have higher thermal stability than aliphatic phosphorus-containing polymers, and also higher hydrolytic stability than polyphosphates [8, 9]. For this reason, the synthesis of arylpolyphosphonate-based polymers has been emerging recently.

In another study, arylphosphonate structures were obtained by subjecting hydroxybenzoin and phenylphosphonic dichloride to a condensation reaction, and a significant increase in the thermal stability of the resulting polyester was observed [9]. (Figure 2)

Polymers with oligomeric structures containing phosphonate esters can also be used as additives. For example, the addition of 5% Poly(sulfonyldiphenylene phenylphosphonate) to the PET formulation resulted in an increase in the LOI value of PET from 21 to 30, and success in the UL-94 V-0 test [8]. In our project work, diols and acid-functional reactive phosphorus groups with different phosphorus content are synthesized and applied in polyurethane and polyester applications to develop polymeric and/or composite materials that provide inherent flame-retardant properties without the use of flame-retardant additives. (Figure 3)

Conclusion

The behavior of materials with and without flame retardants during fire and the performance of flame retardants when used in combination have been described. However, due to surface migration and loss of properties of flame-retardant products made with additive materials over time, research is being directed toward polymeric structures with inherent flame-retardant properties. Within this scope, the most important solution in studies conducted is aimed at polymers demonstrating inherent flame-retardant properties without the addition of additives. The covalent bonding of additives to the polymer matrix or the incorporation of flame-retardant structures into monomers and their use in polymerization are among the leading new strategies for "non-flammable polymers." In our study, polymers containing phosphorus groups were synthesized and their flame-retardant properties were examined. Not only in insulation materials but also in various plastic derivatives as flame-retardant improving additives, phosphorus-containing polymeric flame-retardant materials are among the important studies being conducted today. Flame-retardant-enhancing and phosphorus-containing reactive new additives are being developed for use in plastic derivatives in diverse fields such as textile materials (bedding, clothing, curtains), car seats and panels, lamp accessories, and electrical cables. These additives can be covalently bonded to the polymer matrix, or they can appear as oligomeric/polymeric phosphorus-containing materials.

Acknowledgements

We thank Flokser A.Ş. for polyurethane foam formulation and LOI, UL94 studies. We thank TÜBİTAK (Project No: 114Z666) and EU COST Action CM1302 Committee for supporting our project work. Prof. Dr. Tarık Eren / Department of Chemistry - Yıldız Technical University Melek Dede / Graduate Student - Department of Chemistry - Yıldız Technical University

References

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[8] Chen, L. and Wang, Y-Z., (2010). "Aryl Polyphosphonates: UsefulHalogen-Free Flame Retardants for Polymers", Materials, 3,4746-4760.

[9] Emrick, T.; Ranganathan, T.; Coughlin, E.B.; Farris, R.J.; Zilberman,J., (2011). "Deoxybenzoin based antiflammable polyphosphonates and poly(arylate-phophonate) copolymer compounds, compositions and related methods of use", U.S. Patent 7,863,400 January 4.