The present invention relates to beads comprising a polymer of a vinylaromatic compound, an expanding agent and at least 0.2 % by weight based on the weight of the uncoated bead, of a triazine derivative. In particular the present invention relates to beads comprising a polymer of a vinylaromatic compound, an expanding agent and a coating containing a triazine derivative, to expanded articles prepared therefrom and to a process for the preparation of said beads. More in particular the present invention relates to such beads wherein the polymer of a vinylaromatic compound is polystyrene. Conventional self extinguishing polystyrene foams comprise halogen containing compounds that act as fire retardant compound.

Hexabromocyclododecan (HBCD) as disclosed in US patent No. 4,520,136 is a representative example of a halogen containing fire retarding compound. The halogen containing fire retarding compound is located throughout the matrix of the polystyrene in the expandable polystyrene beads, since in the process for preparing the beads the polymerisation of styrene monomer is carried out in the presence of the halogen containing fire retarding compound. Halogen containing self extinguishing polystyrene foams are regarded with suspicion since they give rise to corrosive and toxic fumes under conditions of fire and recycling. It would thus be desirable to be able to provide self extinguishing polystyrene foams that contain fire retarding compounds that do not have the above mentioned disadvantages associated with halogen containing fire retarding compounds, whilst the processability of the expandable polystyrene beads and the mechanical properties of the foamed articles prepared therefrom are not affected to a significant extent. It is known in the art that triazine and triazine derivatives have fire retarding properties. They have been used for that purpose

in thermoplastic compositions. Such compositions are obtained by melt blending the triazine or triazine derivative with the thermoplastic polymer and optionally with other additives. For example, US patent No. 3,660,344 discloses self extinguishing polyamide moulding compositions comprising a polyamide and from 0.5 to 25% by weight, based on the weight of the moulding compositions, of melamine (2,4,6-triamino-l,3,5-triazine) , a melamine derivative or a melamine condensate. It is known in the art that, as compared to halogen containing fire retarding compounds, relatively large quantities of melamine are required for obtaining the desired level of fire retardancy and such high levels often negatively affect the mechanical properties of the objects obtained.

In order to solve this problem European patent application publication number 311,909 discloses thermoplastic compositions comprising an impact resistant polymer of a vinylaromatic compound (e.g. impact resistant polystyrene), a polyphenylene ether and a fire retarding composition containing a phosphorus containing organic compound and a triazine derivative.

European patent No. 3,799 (EP 3799) and European patent No. 69364 (EP 69,364) both describe beads comprising a styrene homopolymer, an expanding agent, a halogen containing fire retarding compound and of from 0.0001 to 0.1% by weight, based on the weight of the polymer, of a triazine derivative. The triazine derivative is dispersed throughout the matrix of the polymer of the bead, as is the halogen containing fire retarding compound. Such small amounts of triazine derivative are stated to have a positive effect on the processability of the beads and ultimately on the mechanical properties of the foamed article prepared therefrom. When the triazine derivative is used in such small amounts as mentioned above the fire retardancy of the foamed article is fully due to the presence therein of the halogen containing fire retarding compound.

Clearly the self extinguishing polystyrene foams of EP 3799 and 69,364 still suffer from the above mentioned disadvantages associated with the use of halogen containing fire retarding compounds. Up to date the use of triazine derivatives as fire

retarding compounds in expandable beads of polyvinylaromatic compounds such as polystyrene, has not been reported. The reason for that is the fact that, as stated above, it has not been possible to prepare beads comprising polystyrene, an expanding agent and at least a substantial amount of triazine or a derivative thereof (here the terms substantial amount are used to define such an amount that allows the measurement of a fire retarding effect according to ASTM- D 2862-87, that actually originates from the triazine derivative) finds its origin in that during the process for the preparation of the expandable beads (usually by means of a free radical initiated aqueous suspension polymerisation reaction of the styrene monomer) the triazine derivative does hardly dissolve in the vinylaromatic monomer phase, i.e. it will be present in the water phase instead. As a result thereof only a very small amount, at most about 0.1% by weight based on the weight of the uncoated bead, of the triazine derivative will end up in the polystyrene matrix of the beads.

The present invention now provides beads comprising a polymer of a vinylaromatic compound, an expanding agent and at least 0.2% by weight of a triazine derivative. In particular the present invention relates to beads comprising a polymer of a vinylaromatic compound, an expanding agent and a coating containing a triazine derivative. The beads according to the present invention preferably comprise an amount of the triazine derivative in the range of from 0.2 to 10% by weight, preferably in the range of from 0.5 to 2% by weight, based on the weight of the uncoated beads. When the beads comprise less 0.2% by weight of triazine derivative it is difficult to measure a significant fire retarding effect originating from the triazine derivative using the ASTM-D 2862-87 method, whereas when the beads comprise more than 10% by weight of the triazine derivative the breaking strength of the foam prepared from such beads may be adversely affected. The foams prepared from the beads according to the present invention show a significant fire resistance expressed as Limiting Oxygen Index (LOI), measured according to ASTM-D 2862-87.

Thus the beads according to the present invention may contain a lesser amount of halogen containing fire retarding compound for obtaining the same level of fire resistance of the foam prepared therefrom as foams prepared from beads that do not comprise a coating comprising a triazine derivative and that are otherwise the same. Beads according to the present invention that do not contain halogen containing fire retarding compounds are particularly preferred.

The manufacture of foams from beads according to the present invention can be done according to well known methods in the art that involve essentially two steps (a) pre-expansion of the beads and (b) heating the pre-expanded beads in a mould to form a cellular structure therein. For ease of handling and in order to obtain satisfactory mouldings it is desirable that the beads are free flowing and that their tendency to agglomerate before during and after pre-expansion is avoided as far as possible. Therefore the beads according to the present invention preferably comprise a coating that overcomes such problems.

A coating composition suitable for that purpose is disclosed in Great Britain patent specification No. 1,408,267 and comprises (a) a glyceryl C16-C20 carboxylate and (b) a metal stearate. The glycerylcarboxylate may be derived from any monocarboxylic acid containing 16-20 carbon atoms. For economic reasons it is preferred to use a stearate as the glycerylcarboxylate because stearic acid is a readily available naturally occurring material.

The carboxylate may be a mono,- di-, or tri-ester or a mixture of these esters. Particularly preferred are the glyceryl monoesters, for example glyceryl monostearate. The carboxylate may be present in the coating in an amount in the range of from 0.01 to 1.0% by weight, preferably of from 0.005 to 0.1% by weight, based on the weight of the uncoated bead. The metal stearate may be water soluble or water insoluble. Though the water soluble stearates such as sodium or potassium stearate improve the flow characteristics of the pre-expanded particles, the improvement is more significant when

water insoluble stearates are used such as calcium or zinc stearates, the latter being particularly preferred.

The metal stearate may be present in the coating in an amount in the range of from 0.005 to 0.1% by weight based on the weight of the uncoated bead. Apart from the class of common coating compositions as disclosed in GE 1,408,267 use may be made of other common coating compositions comprising one or more of the following compounds: sucrose esters, wax, starch, calcium acetate, polyethers, ammonium perfluoroalkylphosphate, ammonium perfluoroalkylamide phosphate, metal salt of glycerophosphoric acid, fluorinated vinylmonomer, polysiloxane paraffin oil sorbitane, monoparmitate, silicone oil, styrene monomer modified silicone oil, hydrogenated castor oil, dipentaerithritride, sorbitan fatty acid ester, hydrogenated rice bran oil, sucrose ester, propyleneglycol fatty acid ester, cooking oil, fatty acid bisamide, thiodifatty acid esters, hydrogenated bean oil, glycerin, magnesium oxide, di-2- ethylhexyl phthalate, dibutylsebacate, polyoxyethylene-lauryl ether, polyoxyethylene alkyl (phenol) ether, polyoxyethylene alkyl ester, liquid paraffin, diphenyl siloxane, cationic surfactants, non-ionic surfactants, pentaerithritride, ethylene-bis-stearic amide, hydroxy stearyl triglyceride and silicone oil.

The beads according to the present invention comprising an expanding agent and a coating containing a triazine derivative thus preferably comprise a coating that, in addition, also contains a common coating composition as exemplified above. Thus the preferred beads according to the present invention either comprise a one layer coating containing the triazine derivate and the common coating composition, or alternatively a coating of two or more layers, whereof at least one layer comprises the common coating composition and at least one layer comprises the triazine derivative.

Particularly preferred are beads that have a coating of two layers, since in general they provide better foam properties. Such beads come in two configurations, one of which having an inner layer comprising the triazine derivative and an outer layer comprising the common coating composition, the other having an inner layer

comprising the common coating composition and an outer layer comprising the triazine derivative.

Beads of the former configuration are particularly preferred, since in general they will provide better processability. The triazine derivative may be any one known in the art, in particular any one of the triazine derivatives mentioned in the above referred to EP 3799 and EP 69364, such as 2,4-diamino-l,3,5-triazines substituted by 6-methyl, 6-ethyl, 6-propyl, 6-isopropyl, 6-nonyl, 6-heptadecyl, 6-cyclopentyl, 6-cyclohexyl, 6-cyclohexylmethyl, 6-phenyl, 6-(3-pyridyl-) , 6-benzyl; 2, ,6-triamino-l,3,5-triazines substituted by N-phenyl, N-dodecyl, N-octadecyl, N,N'-diphenyl, N,N*,N"-triphenyl or N-cyclohexyl; and melamine condensation products. Preferably, the triazine derivative is melamine since it provides an excellent performance at a reasonable price. The polymer of a vinylaromatic compound is preferably i) a homopolymer of styrene, alpha methyl styrene, vinyltoluene or of an other substituted styrene monomer or ii) a copolymer of any one of the aforementioned monomers.

Expanding agents present in the bead according to the invention cause the bead to expand or foam when it is heated. The expanding agent is a gaseous or liquid organic compound that does not dissolve the polymer of the vinylaromatic compound at the levels employed and that has a boiling point or boiling range below the softening point of the polymer. Suitable expanding agents are for example the aliphatic hydrocarbons either alone or in admixture containing from 2 to 6 carbon atoms such as propane, butane, n-pentane, isopentane, cyclopentane, hexane, cyclohexane; halogenated derivatives of aliphatic hydrocarbons containing from 1 to 3 carbon atoms such as the various chloro- and fluoro derivatives of methane ethane and of ethylene such as dichlorordifluoromethane and 1,2,2-trifluro-l,1,2- trichloroethane. A blend of n-pentane and isopentane is particularly preferred. The expanding agent is used in an amount ranging from 3 to 15% by weight, preferably between 4 and 8% by weight, based on the weight of the uncoated bead. The uncoated beads of vinylaromatic

monomer are obtainable by methods known in the art, preferably by free radical initiated suspension polymerization of the vinyl¬ aromatic monomer in the presence of an expanding agent and optionally other components that one wishes to include in the matrix of the resulting polymer, by means of a free radical initiator preferably an organic peroxide such as t-butylperbenzoate, t-butyl- peroctoate, dibenzoylperoxide, t-butylperacetate.

Azo compounds, such as azobisisobutyronitril, may also be used as free radical initiators. Examples of the above-mentioned other components that may be present in the reaction mixture are: plasticizers, UV or heat stabilising agents, pigments and other fire retardant compounds.

The diameter of the beads thus prepared usually ranges of from 0.6 to 2.3 mm. The coating subsequently applied to the beads may consist of one or more layers, each layer can applied to the bead by any conventional method, for example by dry coating (tumbling in a ribbon blender), coating from a slurry or coating from a solution in a readily vaporising non-aqueous liquid, provided the liquid does not have any solvent action on either the bare, non expanded, bead or any coating layer that has already been applied to said bare bead.

Examples of suitable liquids are lower alcohols, such as methanol, ethanol, isopropylpropanol or an aliphatic hydrocarbon such as hexane, heptane or petroleum ether. The dry coating technique being particularly preferred since it gives the best results and since it is the easiest technique. Of course it will be understood that at the end of the coating process the solids of the coating composition may not be distributed over the beads in an entirely even coating and that some beads may contain slightly more than others.

The beads according to the invention were found to undergo the pre-expansion and moulding processes without clogging or other interference with the equipment that transfers the pre-expanded beads to the mould.

The expanded articles (foams) obtainable from the beads according to the invention were found to have satisfactory mechanical properties, indicating that the triazine derivative does not have a significant negative effect on the processability of the bead nor on the mechanical properties of the foam obtainable therefrom.

The invention is further described by means of the following examples which by no means are intended to restrict the scope of the present invention. Examples 1 to 5 and comparative example A

1 kg batches of uncoated expandable polystyrene beads (having a diameter in the range of 0.7 to 1.0 mm) and comprising about 6% by weight based on the weight of the uncoated bead, of a mixture of n- pentane and iso-pentane as the expanding agent, were tumbled in a ribbon blender (a 3.6 litre Herman Linden machine for 1 kg bathes) for 10 minutes with a common coating composition comprising glycerol monostearate, glycerol tristearate and zinc stearate in a weight ratio of 5:5:2, until the uptake of the composition by the beads was 0.4% by weight based on the weight of the uncoated beads (calculated on the basis of the quantity of coating powder in the blender before and after the coating exercise) .

Subsequently the coated beads were coated with a separate layer of melamine in a similar fashion up to levels indicated in the accompanying table. The level of melamine on the bead was determined according to atomic absorption spectroscopy of the foam prepared therefrom.

After the coating procedure was completed the beads were immediately removed from the blender and pre-expanded in a continuous pre-expander using a steam/air mixture to a target bulk density of about 20 g/1 using an arbitrary steaming time of

18 seconds. The actual densities achieved were all in the range of 18.7 to 19.2 g/1 which is indicative of the fact that the triazine derivative does not affect the pre-expansion process.

The preexpanded beads were allowed to mature overnight in an air permeable silo and were machine moulded into tiles of 300 mm by

300 mm by 50 mm in a steam heated enclosed mould operating at 0.5 to 1.2 bar gauge. The tiles were dried for two days at 70 ^β C and then conditioned and tested for cross-breaking load measurement. The maximum cross-breaking strength (at break) was determined for each tile using a standard three point cross breaking test whereof the results are included in the accompanying table.

From this table it follows that the triazine derivative does not have a significant effect on the maximum breaking load of the foam. The fire resistance of the foams was measured in accordance with ASTM D-2862-87 on band saw fragments of the tiles referred to above. The resulting Limiting Oxygen Index (LOI) values are present in the accompanying table, from which it can be concluded that the foams prepared from beads according to the invention comprising a coating of two layers, the outer layer containing melamine, show improved fire resistance as compared to foams prepared from beads that only differ from said ones according to the present invention in that they do not comprise said outer layer. Example 6 and 7 and comparative example B In a process similar to the one described above coated beads containing 5 %wt of the same blowing agent and having a density of about 25 g/1 were produced. The beads of comparative example B were coated with one layer comprising the common coating composition referred to above (amount of common coating composition on the bead: 0.4% by weight based on the weight of the uncoated bead) . The bead of example 6 was coated with one layer comprising the above common coating composition and melamine in a 50:50 weight ratio (amount of melamine on the bead: 0.2% by weight based on the weight of the uncoated bead) . The bead of example 7 was coated with two layers; an inner layer of a common coating composition (amount of common coating composition on the bead: 0.4% by weight based on the weight of the uncoated bead) and the outer layer of melamine (amount of melamine on the bead: 0.2% by weight based on the weight of the uncoated bead) .

The maximum cross breaking strength and the fire resistance of each of the foams was determined in the same manner as described above and the data are included in the accompanying table. From the LOI data presented in said table it follows that foams prepared from beads according to the invention comprising a coating of one layer comprising an equal amount of common coating composition and melamine have improved fire resistance as compared to foams prepared from beads that only differ from the said ones according to the invention in that their one layer coating solely comprises common coating composition.

It must further be noted that in the ASTM 2862-87 method an absolute difference in LOI of less than 0.1% is not considered significant.

TABLE

Example melamine %wt max. breaking LOI on beads load in N# in vol%

6 %wt blowing agent, density 18.7-19.2 g/1

A 0.0 1086 (0.9) 19.3

1 0.5 1051 (0.8) 19.8

2 1.0 1059 (0.9) 20.0

3 2.0 1063 (0.9) 20.2

4 5.0 1068 (0.6) 20.4

5 10.0 1051 (0.8) 20.8

5 %wt blowing agent, density 24.2-24.8 g/1

B 0.0 1335 (1.0) 17.8

6 0.2 1444 (1.0) 18.1

7 0.2 1317 (1.0) 18.0

# the values in brackets indicate the steam pressure in bar gauge.