June 1994 Additives for Polymers

Phenol Co, Tokyo, for the production of Irganox and Irgafos stabilizers. Ciba controls 58.5% of the new company, Shanghai Ciba Gao-Qiao Chemical Company Limited, Pudong, Shanghai. The plant will start up in 1997. Contact: Ciba Additives Division, Seven Saline Drive, Hawthorne, NY 10.532, USA. Tel: i-1-914-347-4700.

Bayer merges inorganics

Bayer’s UK business groups covering pigments and ceramics and inorganic chemicals have been merged to form the Inorganic Products Business Group, selling silicones, speciality chemicals, fluorides, titanium dioxide, speciality pigments, glass fibres, iron oxide pigments and enamels. Contact: Buyer plc, Bayer House, Strawberry Hill, redbud, Berkshire, RG13 IJA, UK. Tel: + 44-635-563389. Fax: + 44-635 563513.

The flame retardant and smoke suppression mechanisms for alumina trihydrate and magnesium hydroxide

Alumina trihydrate (ATH) is used as a flame retardant in many applications including thermosets, elastomers and thermoplastics. Magnesium hydroxide Mg(OH),, is used as a flame retardant in thermoplastic and wire and cable applications. Both ATH and magnesium hydroxide are fine white powders that are available in a variety of particle sizes and particle size distributions. ATH particle sizes range from less than one micron to 80-100 microns median particle diameter: magnesium hydroxide is available from one micron to 4.5 microns. They can be surface modified with a variety of coupling agents and dispersion aids commonly used in the plastics processing industries.

Both are commonly used at high weight percent loadings. Loading levels of 100 or more parts by weight of flame retardant to 100 parts by weight of resin are not uncommon. The high loading levels are necessary to achieve a variety of flame retardance and smoke suppression properties.

However, one must formulate properly to achieve good processability and end use physical properties of the polymer. The proper selection of flame retardant particle size, dispersion techniques, surface modification, resin selection and moulding conditions wilt help to meet end use production requirements.

Alumina trihydrate Alumina trihydrate, also called hydrated alumina, is chemically aluminium hydroxide, Al(OH),. The thermogravimetric curve for ATH shows that it decomposes at 203°C (4OO’F) to yield alumina and water. The chemical reaction is: 2 Al(OH), -----> Al,O, + 3H,O The decomposition reaction is endothermic, heat absorbing, with an enthalpy of decomposition of -280 callgm. The water formed due to the ATH decomposition reaction is 34.6% by weight of the ATH. The endothermic decomposition is the flame retardant mechanism for ATH when it is incorporated into polymers. Combustion is affected in various ways due to the decomposition of ATH: I) the endo~ermic decomposition absorbs heat energy, thus making ignition and combustion more difficult; 2) the water released due to decomposition dilutes the combustible gases and acts as a vapour barrier to prevent oxygen from reaching the flame, and 3) it has been theorized that along with the charring products formed during combustion, the aluminium oxide that is produced forms an insulating barrier protecting the polymer from fire.

Magnesium hydroxide The flame retardant mechanism for magnesium hydroxide, Mg(OH),, is similar to

O1994 Elsevier Science Ltd 9

Additives for Polymers June 1994

that of ATH. However, magnesium hydroxide decomposes at 332°C (630”F), to yield magnesium oxide and water. The chemical reaction is: Mg(OH)* -----> MgO + HZ0 The decomposition reaction is endothermic with an enthalpy of decomposition of -328 cal/gm. The water formed due to the Mg(OH)* decomposition reaction is 3 1 .O% by weight of the Mg(OH),. The endo~ermic decomposition reaction is the flame retardant mechanism for magnesium hydroxide. The water that is formed acts to dilute the combustible gases and acts as a vapour barrier to prevent oxygen from reaching the flame.

The smoke suppression property of both alumina trihydrate and magnesium hydroxide is not clearly understood. It has been theorized that the smoke suppression property is due to the dilution effect of the water vapour on the combustible gases or due to a char formation effect on the polymer and the prevention of large soot particle formation. {Source: Solem Division. J.M. Huber Cop, 4940 Peachtree I~~triai ~o~ievard, Suite 340, Norcross, GA 30071, USA. Tel: +I- 404-441-l 301. Fax: + l-404-368-9908).

Thermal stabilization of high- temperature engineering thermoplastics

The nature of the kinetics of high-tempera~re oxidation of polyalkaneimide and polysulphone with its dependence on the O2 pressure, with low values of the effective activation energy of 0, abso~tion and volatile production liberation (up to 100 kj/mal), and with similarity of structure formation to homolytic acylation, indicates a radical chain mechanism of the process. Phosphorus-containing compounds are universal inhibitors of the thermal oxidation of high-temperature polymers. Aromatic phosphates or phosphites, in particular, are characterized by the optimum stabilizing effect in the processing of polyalkaneimide

and polysulphone. The addition of these compounds lowers the 0, absorption rate by a factor of 1.5-3 and improves the thermal stability of the polymer melts at 300-350°C.

Besides the function of inhibition, phosphorus-containing compounds, as complexing agents, carry out partial thermal stabilization, combining transition metal (manly iron) impurities. The effectiveness of phosphorus-containing compounds is always relatively higher in specimens with an increased content of impurities, but even stabilization does not bring the properties of these specimens to the level of the indices of polymers with a high degree of purification and blocking of labile end groups.

The introduction of phosphorus-containing compounds not only increases the stability of polysulphone and pofyalkaneimide during processing, but also slows down their ageing characteristics. As with a high-temperature process in a melt, solid-phase oxidation, determining the kinetics of ageing at a temperature of up to 200°C is inhibited by additions of phosphorus-containing compounds. (4 refs). Int. Polymer Science & Technology, 1993, 20, (l2), T/45-4.5. ~ra~~ted jkom: Plasticheskie Massy, 1993, (21, 21).

ENVIRONMENTAL ISSUES

European Directive on additives in plastics

Doubts have been expressed at the proposed short-cuts in preparing a draft EU Directive listing ‘permitted additives for plastics’ intended to come into contact with foodstuffs. The British Plastics Federation has written to the UK Minist~ of Agriculture, Fisheries and Food expressing concern.

For an additive to be included in the EU list, a toxological dossier has to be submitted to

10 *I994 Elsevier Science Ltd