Background of the Invention A rubber modified styrenic resin is excellent in mold processability and mechanical properties, therefore, the resin has been widely applied to electric or electronic goods and office supplies. However, the disadvantage could be observed when the rubber modified styrenic resin is employed to heat-emitting products, because the styrenic resin is extremely easily flammable. Therefore, the methods for improving the flame-retardant property of the rubber-modified styrenic resin have been developed.

A widely known method for flame retardancy is that a halogen-containing compound is added to a rubber modified styrene-containing resin to give a good flame-retardant property. The examples of the halogen-containing compounds used in the method above are, for example, polybromodiphenyl ether, tetrabromobisphenol-A, epoxy compounds substituted by bromine. In addition, an antimony-containing compound is commonly used along with the halogen-containing compound in order to increase flame retardancy.

However, the disadvantages could be observed that the halogen-containing compound results in the corrosion of the mold itself by the

hydrogen halide gases released during the molding process and is fatally harmful due to the toxic gases liberated in case of fire. Especially, since a polybromodiphenyl ether, mainly used for a halogen-containing flame retardant, can produce toxic gases such as dioxin or difuran during combustion. So, a major concern in this field is to develop a flame retardant which is prepared without a halogen-containing compound.

U. S. patent No. 3,639, 506 discloses resin composition using mono aromatic phosphoric acid ester such as triphenylphosphate to a blend of styrenic resin and polyphenylene ether resin. However, deterioration of heat resistance and juicing crack phenomenon are observed during molding process due to triphenyl phosphate since the triphenyl phosphate is highly volatile.

In order to solve the above volatility problem, Japanese Patent Laid-open No.

7-043769 discloses that the use of phosphoric acid ester compound having a substituent containing 12 to 25 carbon atoms derived from TPP to a rubber-reinforced styrenic resin (HIPS), which may obtain anti-dripping flame-retardancy. Further, Japanese Patent Laid-open No. 5-1079 discloses an aromatic diphosphate which contains phenyl linkage in its structure. However, in these cases, a large amount of flame retardant is needed to obtain anti-dripping flame-retardancy, which leads to deterioration of heat resistance.

Accordingly, the present inventors have developed a flame retardant thermoplastic resin composition which has anti-dripping flame retardancy by employing oxaphosphorane compound to rubber modified styrenic resin. The thermoplastic resin composition of the present invention does not show deterioration of heat resistance or volatility problem.

Objects of the Invention An object of the present invention is to provide a flame retardant thermoplastic resin composition with good flame retardancy by using oxaphosphorane compound as a flame retardant to a rubber modified styrenic

resin.

Another object of the present invention is to provide a flame retardant thermoplastic resin composition with good properties, such as impact strength and heat resistance.

A further object of the present invention is to provide an environmentally friendly and non-toxic flame retardant thermoplastic resin composition which does not contain a halogen-containing compound.

Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.

Summary of the Invention The flameproof thermoplastic resin composition according to the present invention comprises (A) 100 parts by weight of a rubber modified styrenic resin containing (al) 20 to 100 % by weight of graft copolymer prepared by graft-polymerizing 5 to 65 parts by weight of a rubber polymer, 35 to 95 parts by weight of an aromatic vinyl monomer, 1 to 20 parts by weight of a monomer copolymerizable with said aromatic vinyl monomer and 0 to 15 parts by weight of a monomer for providing processability and heat resistance; and (a2) 0 to 80 % by weight of copolymer prepared by polymerizing 60 to 90 parts by weight of an aromatic vinyl monomer, 10 to 40 parts by weight of a monomer copolymerizable with said aromatic vinyl monomer and 0 to 30 parts by weight of a monomer for providing processability and heat resistance; (B) 0.1 to 15 parts by weight of an oxaphosphorane compound; and (C) 0 to 20 parts by weight of an aromatic phosphoric acid ester compound.

Detailed Description of the Invention (A) Rubber-modified styrenic resin

The rubber modified styrenic resin according to the present invention is a polymer wherein rubber phase polymers are dispersed in the form of particles in a matrix obtained by polymerizing an aromatic vinyl monomer and a vinyl group-containing monomer, which can be polymerized therewith, in the presence of a rubber phase polymer. Such rubber-modified styrenic resin is prepared by a known method such as emulsion polymerization, suspension polymerization or bulk polymerization, and is conventionally produced by an extrusion with a styrene-containing graft copolymer resin and a styrene-containing copolymer resin.

In a bulk polymerization, both a styrene-containing graft copolymer resin and a styrene-containing copolymer resin are prepared together in one process. In other polymerizations, a styrene-containing graft copolymer resin and a styrene-containing copolymer resin may be prepared separately. In either case, the contents of rubber in a final rubber-modified styrenic resin to the total weight of the base resin are preferably in 5 to 30 % by weight.

In the rubber modified styrenic resin, a graft copolymer resin can be used alone or in combination with a copolymer resin in consideration of compatibility thereof.

(a,) Graft copolymer resin The graft copolymer of the present invention is prepared by graft - polymerizing rubber polymer, aromatic vinyl monomer, copolymerizable monomer with said aromatic vinyl monomer and monomer which provides processability and heat resistance; Examples of the rubber polymer are diene rubbers such as polybutadiene, poly (styrene-butadiene), poly (acrylonitrile-butadiene), etc; saturated rubbers in which hydrogen is added to said diene-containing rubber; isoprene rubbers; acryl rubbers such as a polybutyl acrylic acid; and a terpolymer of ethylene-propylene-diene (EPDM). It is preferable to use a diene-containing rubber, more preferably a butadiene-containing rubber. The content of rubber polymer in the

graft copolymer resin is preferably in the range of 5 to 65 parts by weight based on the total weight of a graft copolymer resin.

Examples of aromatic vinyl monomer are styrene, a-methyl styrene, p-methyl styrene, etc. In the above examples, styrene is the most preferable. The content of aromatic vinyl monomer in the graft copolymer resin is preferably in the range of 35 to 95 parts by weight based on the total weight of a graft copolymer resin.

At least one copolymerizable monomer may be introduced and applied to the aromatic vinyl monomers. It is preferred that the copolymerizable monomer is a cyanide vinyl-containing compound such as acrylonitrile or an unsaturated nitrile-containing compound such as methacrylonitrile. The copolymerizable monomer is used in an amount of 1 to 20 parts by weight.

In addition, in order to give good characteristics of processability and heat resistance, other monomers such as acrylic acid, methacryl acid, maleic anhydride and N-substituted maleimide can be added in the graft polymerization. The amounts of the monomers are in the range of 0 to 15 parts by weight based on the graft copolymer resin.

To acquire good impact strength and surface appearance when said graft copolymer is prepared, the average size of rubber particles is preferably in the range of from 0. l to 4 am (a2) Copolymer resin The copolymer resin of the present invention is prepared copolymerizing aromatic vinyl monomer, copolymerizable monomer with the aromatic vinyl monomer, and monomer which provides processability and heat resistance depending on the ratio and compatibility between monomers except rubber in the graft copolymer.

The examples of the aromatic vinyl monomer are styrene,

- methylstyrene, p-methylstyrene, etc. Styrene is the most preferable. The aromatic vinyl monomer in the total copolymer resin is contained in the amount of 60 to 90 parts by weight.

At least one copolymerizable monomer may be introduced and applied to the aromatic vinyl monomers. The examples of the copolymerizable monomer are cyanide vinyl group-containing compounds such as acrylonitrile and unsaturated nitrile-containing compounds such as methacrylonitrile. It is preferable that 10 to 40 parts by weight of the copolymerizable monomer to the total copolymer is employed.

In addition, 0 to 30 parts by weight of other monomers such as acrylic acid, methacrylic acid, maleic anhydride and N-substituted maleimide may be added and copolymerized thereto.

The examples of the rubber-modified styrenic resin (A) used in the present invention are acrylonitrile-butadiene-styrene (ABS) copolymer resin, acrylonitrile-ethylenepropylene rubber-styrene (AES) copolymer resin, acrylonitrile-acryl rubber-styrene (AAS) copolymer resin, and so on.

The rubber modified styrenic resin (A) of the present invention is prepared by mixing 20-100 % by weight of the graft copolymer resin (al) with 0-80 % by weight of the copolymer resin (a2).

(B) Oxaphosphorane compound The oxaphosphorane compound of the present invention is represented by the following chemical Formula (I):

wherein Rl is hydrogen, C 4 alkyl or C6-10 aryl; R2 and R3 are independently of each other hydrogen or Cl4 alkyl ; and n is 1-3.

Examples of the oxaphosphorane compound having the structural formula (I) include 2-methyl-2, 5-dioxo-1-oxa-2-phosphorane and 2-phenyl-2, 5-dioxo-l-oxa-2- phosphorane. The oxaphosphorane compound (B) of present invention may be used alone or in combination as a mixture. And the oxaphosphorane compound (B) is used in the amount of from 0.1 to 15 parts by weight, preferably 1 to 10 parts by weight per 100 parts by weight of base resin.

(C) Aromatic phosphoric acid ester compound The aromatic phosphoric acid ester compound used in the present invention is a compound having the following structural formular (II) :

wherein R3, R4 and Rs independently of one another are hydrogen or Cl 4 alkyl ; X is a C6 20 aryl group or alkyl-substituted C6 20 aryl group that are derivatives from a dialcohol derivative such as resorcinol, hydroquinol and bisphenol-A; and n is 0-4.

Where n is 0, the compound represented in the structural formular (II) is triphenyl phosphate, tri (2,6-dimethyl) phosphate, and the like, and where n is 1, the compounds include resorcinolbis (diphenyl) phosphate, resorcinolbis (2,6-dimethyl phenyl) phosphate, resorcinolbis (2,4-ditertiary butyl phenyl) phosphate, hydroquinolbis (2,6-dimethyl phenyl) phosphate, hydroquinolbis (2, 4-ditertiary butyl phenyl) phosphate, and the like. The compounds can be used alone or in combination therewith.

In the present invention, the aromatic phosphoric acid ester can be used in the amount of 0 to 20 parts by weight, preferably 0.1 to 15 parts by weight, more preferably 0.1 to 6 parts by weight per 100 parts by weight of base resin.

Other additives may be contained in the resin composition of the present invention. The additives include heat stabilizers, anti-oxidants, light stabilizers, inorganic or organic pigments or dyes and/or inorganic filler. The additives are employed in an amount of 0 to 30 parts by weight as per 100 parts by weight of base resin (A).

The present invention may be better understood by reference to the following examples that are intended for the purpose of illustration and are not to be construed as in any way limiting the scope of the present invention, which is defined in the claims appended hereto.

Examples

The components to prepare flameproof thermoplastic resin compositions in Examples 1-3 and Comparative Examples 1-2 are as follows: (A) Rubber modified styrenic resin (a,) Graft-copolymer resin 50 parts of butadiene rubber latex, 36 parts of styrene, 14 parts of acrylonitrile, and 150 parts of deionized water were mixed. To the mixture, 1.0 part of potassium oleate, 0.4 parts of cumenhydroperoxide, 0.2 parts of mercaptan-containing chain transfer agent, 0.4 parts of glucose, 0.01 parts of ferrous sulfate hydrate, and 0.3 parts of sodium pyrophosphate were added. The blend was kept at 75 °C for 5 hours to obtain g-ABS latex. To the g-ABS latex, 0.4 parts of sulfuric acid was added, coagulated and dried to obtain rubber modified polystyrene resin (g-ABS) in a powder form.

(a2) Copolymer resin 75 parts of styrene, 25 parts of acrylonitrile, and 120 parts of deionized water were mixed. To the mixture, 0.2 parts of azobisisobutylonitrile (AIBN) 0.4 parts of tricalciumphosphate and 0.2 parts of mercaptan-containing chain transfer agent were added. The resultant solution was heated to 80C for 90 minutes and kept for 180 minutes. The resultant was washed, dehydrated and dried.

Styrene-acrylonitrile copolymer (SAN) was obtained.

(B) Oxaphosphorane compound 2-methyl-2, 5-dioxo-1-oxa-2-phosphorane with a melting point of 102-104 °C was used.

(C) Aromatic phosphoric acid ester compound

Resorcinol bis (2,6-dimethylphenyl) phosphate by Daihachi Chemical of Japan (product name: PX200) was used.

Examples 1-3 The components as shown in Table 1 were mixed and the mixture was extruded at 180-250 °C with a conventional twin screw extruder in pellets. The resin pellets were dried at 80 °C for 3 hours, and molded into test specimens using a 6 oz injection molding machine at 180-280 °C and barrel temperature of 40-80 °C.

The flame retardancy of the test specimens was measured in accordance with UL94VB with a thickness of 1/8"and 1/12"respectively. The impact strength was measured according to Izod impact strength ASTM D-256 A (1/8"notch). The heat resistance was measured according to ASTM D-1525 under 5 kg.

Comparative Examples 1-2 Comparative Examples 1 was conducted in the same manner as in Example 1 except that the oxaphosphorane compound was not used. Comparative Examples 2 was conducted in the same manner as in Example 1 except that the aromatic phosphoric acid ester compound was used as a flame retardant instead of the oxaphosphorane compound. The test results are presented in Table 1.

Table 1 Comparative Examples Examples 2 3 4 1 2 (a,) 2 3 32 25 32 (A) Rubber modified styrenic resin (a2) 68 68 68 75 68 68 (B) Oxaphosphorane compound 1 3 2 2-- (C) Aromatic phosphoric acid ester compound--2--6 UL 94 flame retardancy (1/12") V-2 V-2 V-2 V-2 Fail V-2 UL 94 flame retardancy (1/8") V-2 V-2 V-2 V-2 Fail V-2 Izod impact strength (kgf-cm/cm) 25 23 21 18 34 20 Vicat Softening Temperature (VST, C) 94 92 92 93 94 88 As shown above, the resin compositions employing a oxaphosphorane compound as a flame retardant show good flame retardancy without deterioration of impact strength and heat resistance compared to comparative examples 1-2.

The present invention can be easily carried out by an ordinary skilled person in the art. Many modifications and changes may be deemed to be with the scope of the present invention as defined in the following claims.