EoW July 2010

technical article

Improving themechanical propertiesof non-halogenated flame retardant compounds By Jeremy R Austin, Herbert S.-I Chao, Sartomer Company

Abstract Traditionally, plastic articles have been rendered flame retardant by the intro- duction of halogenated compounds, such as tetrabromobisphenol A, or TBBPA. Recently, a movement to non-halogenated flame-retardants has been the focus of academic and industrial research, but these safer alternative technologies have a deleterious impact on mechanical properties. Mineral fillers used as flame retardants require in excess of 60% by weight loading to fulfil flame requirements. In the current study, functionalised liquid polybutadienes (LPBD) are used to improve the elongation and tensile strength of aluminium trihy- drate (ATH) filled ethylene vinyl acetate (EVA) compounds. Pre-dispersion of the coupling agents onto ATH led to gains in elongation of greater than 200%. Low loadings of functionalities including maleic anhydride, epoxy and amine were proven to be most effective. Incorporation of a di-acrylic ionic monomer provided gains in tensile modulus unattainable by the LPBD materials. 1 Introduction Scientific studies have indicated that halogenated flame retardants (HFR) are widespread contaminants for the environment. Hazardous emissions from manufacturing, disposal or recycling of plastic articles containing HFRs pose such a serious threat that some HFRs have already been removed from electronics and household goods, and the European Union has ratified regulations governing the plastics industry to eliminate them. With similar legislature impending on all continents, several markets across the plastics industry are seeking alternative technologies.

compounds causes a chemical interaction between the carboxyl and hydroxyl groups in the polymer and filler respectively. Improving the interfacial adhesion was shown to improve both the thermal and mechanical properties. Similarly, Wang et al 6 introduced maleic anhydride grafted EPR into a PP-Mg(OH) 2 compound, and found that the EPR-g-MA resided exclusively at the interface. improved the dispersion of the filler, which was manifested as improved impact strength. Plentz et al 1 introduced an acrylic acid functional PP to their PP-ATH system, and demonstrated that improved interaction at the interface caused an increase in melt flow index and improved tensile and flex strength. In all three cases, the functionalised additives interacted with the filler to overcome the deleterious effects of high hydrated mineral filler loadings. materials have been investigated to overcome the deficiencies in flame retardant compounds containing ATH. The current evaluation examines the effect of low molecular weight functionalised liquid polybutadienes (LPBD) as interfacial modification agents in a 60% filled ethy- lene vinyl acetate (EVA) wire and cable (W&C) system. Feedback from industry has indicated that migrating to an ATH solution reduces the tensile strength, ductility and flow to such a degree that the material cannot function in W&C. Having a low molecular weight is thought to be advantageous to better seek and adhere to the filler surface, thereby enhancing the interfacial modification. The type and loading level of the functionality was varied to assess the appropriate chemistry to best improve EVA-ATH compounds. Encapsulating the Mg(OH) 2 Traditional functionalised

Several non-halogenated flame retardants (NHFR), such as ammonium phosphates, melamine compounds, nanoclays or hydrated minerals, have been identified. Aluminium trihydrate (ATH) is a recognised flame retardant filler for polymers, and is free from halogens. Typically, flame retardants act to delay ignition by depriving the fire of fuel, or suppressing the ignition temperature. ATH however, releases water vapour during decomposition, which is believed to withdraw heat from the substrate and dilute the fuel supply. Once charred, the residue of Al 2 O 3 inhibits migration of oxygen and volatile compounds released by the polymer that can further proliferate the exothermic reaction. In most applications, a simple replacement strategy can be employed, where one NHFR can replace an HFR. In some instances, such as the case of hydrated minerals such as aluminium trihydrate or magnesium hydroxide, the transition is more difficult. In order to achieve the required flame retardance high loadings of ATH are necessary, often in excess of 60% by weight. Once the volume fraction of inorganic filler exceeds 50% there is a marked deterioration of physical properties in the compound. Plentz et al 1 demonstrated that in PP compounds containing ATH there existed a relationship between the filler loading and aggregate size. This finding indicated that not only are physical properties compromised by the elevated filler loading, but that the ATH also would aggregate as the loading increased. Studies have shown that the addition of a functionalised polymer is an effective method to modify the inter- facial adhesion at the organic/inorganic boundary in polymer composites 2, 3,4 . Mai et al 5 demonstrated that incorporation of graft modified acrylic acid into PP-ATH

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EuroWire – July 2010