EP0934356A1

EP0934356A1 - Flame resistant polyolefin compositions

Info

Publication number: EP0934356A1
Application number: EP97911834A
Authority: EP
Inventors: Elisabeth S. Papazoglou; Frederick R. Scholer
Original assignee: FMC Corp
Current assignee: FMC Corp
Priority date: 1996-10-22
Filing date: 1997-10-21
Publication date: 1999-08-11
Also published as: CN1234047A; WO1998017718A1; JP2001502741A; AU4911997A

Abstract

The flame retardancy imparted to polyolefin resins by tris(trihaloneopentyl) phosphate is increased by the incorporation of a co-additive comprising a halogen containing compound which is stable at the processing temperature of the polymer. The preferred co-additives are other knowm flame retardants for polyolefins. The use of an appropriate combination of phosphate, flame retardant co-additive, and preferably on antimony trioxide or zinc borate synergist, provide polyolefin compositions which are non-blooming and achieve a V-O rating in the UL94 test.

Description

FLAME RESISTANT POLYOLEFIN COMPOSITIONS

TECHNICAL FIELD

This invention relates to flame resistant polyolefin compositions, to methods for improving the flame resistance of polyolefins, and to flame retardant additive compositions suitable for use in the aforesaid methods.

BACKGROUND

Polyoleflns such as polypropylene and copolymers of polypropylene find use in a variety of applications. In many of these it is preferred or mandatory to incorporate an additive into the polyolefin which improves its flame resistance (retardance) - sometimes abbreviated herein as "FR". A number of flame retardant additives are known and used but all suffer from the disadvantage that their incorporation detracts from the product's physical properties, may cause problems in processing the polymer and may shorten the useful life of the product. The use of flame retardant additives in polymers is usually a compromise between the desire for a particular degree of flame retardancy and the need to detract as little as possible from the desirable properties of the polymer.

One test which is commonly used to measure the flame retardancy of a polymer is the UL (Under writers Laboratory) 94 test. In the Underwriters Laboratory (UL) - Subject 94 vertical burn test a specimen (5.0 x ¹/₈ or V₁₆ inches) is exposed vertically to a Bbunsen flame for 10 seconds. The specimen is ignited at the bottom and burns up. If the specimen self-extinguishes within 30 seconds another 10 second application is made. Flaming droplets are allowed to fall on dry absorbent surgical cotton located 12 inches below the sample. If the average burning time is less than 5 seconds and the drips do not ignite the cotton the material is classified as 94V-0. If the time is less than 25 seconds and the drips do not ignite the cotton thisere material is classified 94V-1. If the sample is extinguishing but the cotton is ignited the material is classified as 94 V-2.

The production of a polyolefin and particularly a polypropylene composition which can achieve a V-0 rating in the UL94 test without detracting from the physical properties of the polymer has been an objective of the industry for many years. Particularly for moulded products used in the electrical industry such a composition would be very desirable. For many electrical application polymers which rate only V-2 in the UL94 test are currently employed.

A V-O rated polypropylene can be produced by incorporating known flame retardant additives. However, at least in the quantities needed to provide a V-O rated compositions these additives detract from the physical properties of the polymer. When used in these quantities these additives bloom from the polymer. Blooming or plating out is the separation of the additive from the polymer matrix which shows up as a surface film on the moulded specimen. It can occur during cooling of the article in the mold, or can be induced via heat ageing at elevated temperatures and extended times. The blooming issue is more than a simple appearance problem. The flame retardant additive diffusing out of the polymer creates problems as to the extent of the composition maintaining its FR performance over time. Many electrical applications of the flame retardant polymer required RTI (Relative Thermal Index) ratings, which involve 18 month heat ageing of the samples while maintaining physical properties and FR performance. If the additive blooms out of the polymer, this RTI rating cannot be achieved.

One class of flame retardant additives which has been proposed for use in polyolefin compositions is the tris (trihaloneopentyl) phosphates especially tris (tribromo neopentyl) phosphate. These additives are melt blendable into polypropylene and thereby attractive in so far as they have minimal impact upon the properties and the processing of the polymer. They can be very easily extruded with polypropylene in preblends or through separate feeders. Being free flowing powders they readily mix with polypropylene and at processing temperatures melt and mix with polypropylene resin giving a uniform product. Their exceptional heat stability allows processing stability and storage stability as well as performance permanence in the molded specimens. However, even when used in combination with antimony trioxide a well known and conventionally employed synergist for halogenated flame retardants these additives can only achieve a V-2 rating in the UL 94 test, at acceptable addition levels. A V-O can be reached by increasing the amount of flame retardant and antimony trioxide to very high levels but such compositions are not commercially useful.

Japanese Patent Application Hei-8-32797 describes flame retardant polymer compositions comprising tris (tribromoneopentyl) phosphate, a second halogen containing flame retardant and an anti-oxidant. The anti-oxidant is said to be an essential ingredient of these compositions in order to impart good color protection to the product. The organic halogen compound is said to be present to impart flame retardancy. Although a wide variety of polymers are stated to be useful in the compositions the examples of the patent application utilise ABS and HIPS polymers. These compositions achieve a V-O rating in the UL94 test but these are polymers which can attain a V-O rating using any halogenated flame retardant providing sufficient flame retardant is incorporated. The present invention is directed to flame retardant polyolefin compositions. DISCLOSURE OF THE INVENTION

We have now discovered that the use of tris (trihaloneopentyl) phosphates such as tris (tribromo neopentyl) phosphate in combination with a relatively small quantity of certain other halogenated co-additives results in a synergistic interaction between the phosphate and the halogenated co-additive. Moreover, when the combinations are used in the presence of an acceptable quantity of a synergist such as antimony trioxide, a polyolefin composition which can achieve V-0 rating in the UL 94 test is produced.

The compositions of this invention do not require the presence of an anti-oxidant to provide an acceptable color in the finished product. Moreover they may be non-blooming, i.e. the flame retardants/additives do not exude from the polymer during processing to give an unacceptable sticky product which is a common problem in other known flame retardant polyolefin compositions.

The halogenated co-additive which is used in combination with the tris (trihaloneopentyl) phosphate is one having at least one halogen atom bound to an aliphatic carbon atom as part of its molecular structure. Preferably the co-additive is one which has at least one hydrogen atom bound to a carbon atom, which is in the beta position in relation to at least one halogen atom The additive will be selected so as to be compatible with the polyolefin, sufficiently non-volatile as to be non blooming, and not to detract from the other useful properties of the polymer in the quantities in which it is used. Although a large number of compounds might be useful, preferred compounds are those already known to be useful as additives in polyolefin compositions. The co-additive is preferably a flame retardant other than a tris (trihaloneopentyl) phosphate. Preferably the co-additive is a brominated flame retardant. A large number of halogenated compounds are known to be useful as flame retardants in polymers. Those which contain only halogen atoms bound to atoms (usually carbon atoms) which form part of an aromatic ring system are not useful in the compositions of this invention Those which comprise halogen atoms bound to an atom which does not form part of an aromatic ring are potentially useful

A large number of flame retardant additives for polyolefms have been disclosed One useful review is included in "Thermoplastic Polymer Additives - Theory and Practice" edited by John T Lutz Jr published by Marcel Dekker Inc, 270 Madison Avenue, New York (1989) the relevant disclosures of which are hereby incorporated by reference

Examples of known commercially available flame retardants which may be useful in the compositions of this invention include tetrabromo bis phenol-A-bis (2,3 dibromopropyl ether) [which is available from the Great Lakes Chemical Corporation as PE-68, tetrabromo bis phenol-S bis (2,3 dibromopropyl ether (which is available as Non-Nen-52 from Manac Inc of Japan), adducts of hexachloropentadiene and cyclo octadiene (one of which is available as Dechlorane Plus from Oxychem Inc, ethylene bis (bis bromonorborane) dicarboximide ( which is available from Albermarle Corporation as BN451 ), dibromoethyldibromocylclohexane which is available from the Ethyl Corporation as BCL 462, pentabromo chlorocyclohexane which is available from the Ethyl Corporation as FR651 P, FM 836--a halogenated phosphate ester which is sold by the Great Lakes Corporation, and halogenated paraffins especially chloroparaff s and bromoparaffms such as those available from the Ferro Corporation as Bromchlor

At least some of the flame retardant co-additives can be effective alone as flame retardant additives for polyolefms When used in large quantities, e g more than 10% by weight of the composition, these co-additives in combination with antimony trioxide (sometimes abbreviated herein as "autimony") at a 2 1 or 3 1 weight ratio of additive to antimony, can achieve a V-O rating in the UL 94 test When used in these quantities these co-additives bloom from the polymer

We have discovered that when these co-additives are used in relatively small quantities, e g from 0 5 to 5 0% by weight of the compositions, in combination with a tris (trihaloneopentyl) phosphate and preferably a conventional flame retardant synergist such as antimony trioxide, the flame retardance of the composition is improved to a degree which is greater than would be produced by the presence of the same quantity of the co-additive alone, i e the tris (tribromoneopentyl) phosphate and the co-additive exhibit a synergistic interaction This means that by combining small amounts of the co-additives with tris (tribromoneopentyl) phosphate and antimony trioxide, non blooming UL-94 V-0 rated polyolefin compositions can be achieved Such compositions are believed to be novel and constitute one aspect of this invention

The co-additives which are useful in the compositions of this invention are preferably those which are stable to processing at the temperature used to process the compositions Thus certain co-additives such as hexabromocyclo-dodecane (HBCD), although they fulfill the criteria of having at least one hydrogen bound to a carbon atom which is in the Beta position relative to at least one hydrogen atom, are less preferred for use in polypropylene compositions of this invention because they degrade at the temperatures at which polypropylene is processed Such degradation is obvious in terms of volatile emission during processing, and also discoloration of the plastic part This problem is especially acute in the production of complex parts involving high residence times in the mold, or where high pressures are involved

The tris (trihaloneopentyl) phosphate ester flame retardant useful in the compositions of this invention preferably comprises a compound selected from tris (tπchloroneopentyl) phosphate, tris (chlorodibromo) neopentyl phosphate, tris (dichlorobromo) neopentyl phosphate, tris- (tribromoneopentyl) phosphate and any mixtures of two or more thereof. The most preferred flame retardant is tris (tribromoneopentyl) phosphate. This compound is sold under the trademark and designation "Reoflam PB 370" by the FMC Corporation.

Accordingly, in a first aspect this invention is a polyolefin resin composition having improved flame retardant properties based on the presence in the resin of at least one tris (trihaloneopentyl) phosphate and at least one halogen containing co-additive having at least one halogen atom bound to an aliphatic carbon atom as part of its molecular structure, which co-additive is stable at the temperature at which the polyolefin is processed. Preferably these co-additives have at least one hydrogen bound to a carbon which is adjacent to that carrying a halogen substituent.

The polyolefins useful in this invention (sometimes also referred to as

"polyolefin resins") may be derived from a variety of monomers especially from propylene, ethylene, butene, isobutylene, pentene, hexene, heptene, octene, 2-methyl propene, 2-methyl butene, 4-methylpentene, 4-methyl hexene, 5-methyl hexene, bicyclo (2,2,1 )-2-heptene, butadiene, pentadiene, hexadiene, isoprene, 2,3 dimethyl butadiene, 3,1 methyl pentadiene 1 ,3,4 vinyl cyclo hexene, vinyl cyclohexene, cyclopentadiene, styrene and methyl styrene. The polyolefins include copolymers produced from any of the foregoing monomers and the like, and further include homopolymer blends, copolymer blends, and homopolymer-copolymer blends.

The preferred polyolefins are polypropylene and polyethylene, including atactic, syndiotactic and isotactic polypropylene and polyethylene, low density polyethylene, high density polyethylene, linear low density polyethylene, block copolymers of ethylene and propylene, and random copolymers of ethylene and propylene. These polyolefins may be produced using a variety of catalytic processes. The polyolefins useful in this invention may be produced by any of these processes including metallocene catalysed processes. The polymers may have a range of melt indexes (Ml) but will typically have Ml values in the range 4 to 30. The invention finds particular applications in polymers which are fabricated into finished articles by molding processes.

The compositions of this invention will normally comprise from 1 % to 20 % by weight of tris (trihaloneopentyl) phosphate, preferably from 3% to 10% by weight of the phosphate.

The ratio of the weight of tris (trihaloneopentyl) phosphate to the weight of co-additives may vary through a wide range, e.g., say from 10:1 to 1 :10 and preferably from 4:1 to 1 :4. In the preferred embodiments this ratio will be in the range 4 :1 to 1 :2 and most preferably 4:1 to 1 :1.

The compositions of this invention preferably comprise at least one conventional synergist such as antimony trioxide, sodium antimonate, antimony pentoxide, zinc stannate, hydroxystannate and zinc borate, or any mixtures of two or more thereof. The preferred conventional synergists are antimony trioxide and zinc borate. The zinc borate synergist should have a suitably high degradation temperature for use in the compositions of this invention. An example of a suitable commercially available zinc borate is the product sold as Firebrake 415.

In a preferred embodiment the ratio of the weight of the conventional synergist (antimony trioxide) to the weight of co-additive in the composition is in the range 1 :5 to 1 :1 and more preferably in the range 1 :3 to 1 :1. The synergistic interaction between the phosphate flame retardant and the co-additive is increased if the amount of synergist is within these preferred ranges. Larger amounts of the conventional synergist may be employed but in general this is less preferred. The quantity of tπs-(trιhaloneopentyl) phosphate, co-additive flame retardant and synergist employed may be optimised using routine experimentation to achieve particular goals in a particular polymer The nature of the polymer and the degree of flame retardancy desired exert a significant effect The cost of polymer and its intended use also exert an influence on the amounts of tπs-(tπhaloneopentyl) phosphate, co-additive flame retardant and synergist, which are employed

The compositions of this invention may be compounded using techniques well known in the art It is important to achieve uniformity of the formulation if the optimum flame retardant performance is to be obtained The use of a twin screw extruder is preferred to the use of a single screw extruder It is important to keep the extrusion temperature above the melting points of the polyolefin and additives The extrusion temperature should not be so high as to accentuate the difference between the viscosities of the polyolefin and the additives Extrusion temperatures below 230°C are generally preferable

The use of concentrated batches (also known as "masterbatches") of polyolefin and additive or additives and subsequent letting down (dilution) of this concentrate with polyolefin is another route to achieving a well mixed product with uniform dispersion A suitable masterbatch composition will comprise the polyolefin, from 5 to 20% (preferably from 10 to 20%) by weight of the tris (trihaloneopentyl) phosphate, and from 10 to 20% by weight of the co-additive Such masterbatch compositions are believed to be novel and constitute another aspect of this invention

The invention is illustrated by the following examples Example A

The formulations having the compositions shown in Table 1 were extruded using a Haake twin screw extruder with simultaneous addition of all components The polypropylene used had a Melt Index of 12 The temperatures employed are 200°C in a flat temperature profile Injection molding of the UL and Oxygen Index specimens was carried out in a Battenfeld injection molding machine at a 190°C-200 °C -200 °C -200 °C -210 °C profile All formulations exhibited stable processing with no surging or torque variations during extrusion No volatiles were emitted during processing at the extrusion or the injection mold No discoloration or other signs of thermal degradation occurred during processing

Oxygen Index (Ol) is defined as the minimum concentration of oxygen, expressed as volume percent, in a mixture of oxygen and nitrogen that will just support flaming combustion of a material initially at room temperature under specified conditions Although oxygen index cannot always be successfully correlated with large scale testing, it is one of the most useful and widely used small scale tests for screening formulations The Oxygen Index was measured according to the methods described in ASTM D-2863 The equipment used for measuring oxygen index consists of a heat resistant glass tube with a brass base A specimen holder supports the specimen and holds it vertically in the center of the column A tube with a small orifice having an open gas flame suitable for inserting into the open end of the column is used as an ignition source The dimensions of the specimens are 5" long, 1/4" wide and 1/8" thick The specimen is clamped vertically and its top is ignited with an ignition flame The flow valves are calibrated to introduce the desired oxygen concentration into the column The specimen is allowed to burn for a specified time or length of sample, whichever occurs first. For solid plastic samples the criteria are 3 minutes burn time or 2" of sample, whichever occurs first The test is repeated until a critical oxygen concentration is reached (This is the lowest oxygen concentration that meets the above criteria).

Ol (percent) = (100 x O₂)/(O₂ + N₂)

where O₂ = volumetric oxygen flow rate at the determined concentration

and N₂ = volumetric flow of nitrogen

The Ol increases with the thickness of the specimen. In these examples the Oxygen Index values were determined using 1/8" thick specimens unless otherwise stated.

Table 1

Example 1 Example 2 Example 3 Example 4 Example 5

Tris

(tribromoneopentyl)

Phosphate 10 10

Non-Nen-52

RESULTS

UL-94 @ 1/8" V-0 V-0 V-2 V-2 V-2

UL-94 1/16" V-0 V-0 V-2 V-2 V-2

Oxygen Index 31.2 29.2 27.1 26.3 26.9

Examples 1 and 2 are compositions of this invention. Example 3, 4 and 5 are comparative examples in which no co-additive is used. In example 3 a typical level of phosphate of 5% along with 2.5% antimony (a ratio of 2:1 flame retardant to antimony, very common in brominated FRs) yields a V-2 rating in both 1/8" and 1/16" samples. Increasing the level of phosphate to 10% (Example 5) does not change the rating. The Oxygen Index also remains unchanged. Increasing phosphate to 10% and using a very high antimony content (1:1 ratio) (Example 4) is not helping the UL rating and reduces the oxygen index. Therefore from examples 3,4 and 5 it is clear that acceptable usage levels of phosphate cannot produce V-0 rating in polypropylene at either 1/8" or 1/6" thickness samples.

Example 1 a blend of 3% Non-Nen 52 with 5% phosphate and 5% antimony (Example 1) gives V-O at both 1/16" and 1/8" thicknesses. The increased Oxygen Index of the formulation in Example 1 , (01=31.2) also shows the improvement in flame retardant activity. In example 2 a further increase in Non-Nen 52 and antimony at the level of 6% antimony and 4% Non-Nen 52, along with 5% phosphate does not improve the Ol further, but maintains the UL V-O rating at both 1/8" and 1/16".

Samples from Examples 1 , 2 and 3 were then heat aged for 72 hours at 60 °C in an oven. None of the samples showed evidence of blooming. A comparative sample containing 8% Non-Nen 52 with 4% antimony (a V-O formulation) and no phosphate showed significant signs of blooming (white surface film or "chalking" ) after 24 hours at 60 °C .

Example B

A series of compositions were made up and tested in the same manner as is described in Example A

Table 2

L.λαι 1 ifji 6 Example 7 Example 8

Tris(tribromoneopentyl) phosphate 5 — 5

Sb₂O₃ 5 9 —

Non-Nen 52 3 4 2.5

RESULTS

UL-94 @1/8" V-0 V-0 V-2

UL-94 @ 1/16" V-0 V-0 V-2

Blooming after 48 hours at 70 °C:

NO YES NO

These results show that although the use of high levels of antimony oxide and Non-Nen-52 (Example 7) can produce a polymer having a V-0 rating that polymer does bloom upon aging. Use of tris (tribromoneopentyl) phosphate with Non-Nen-52 in the absence of antimony (Example 8) results in a non-blooming composition which achieves a V-2 rating. Use of tris (tribromoneopentyl) phosphate with antimony oxide and Non-Nen-52 (Example 6) again results in a composition which is non-blooming and achieves a V-O rating.

Table 3

Example 9 Example 10 Tris (tribromoneopentyl) phosphate 5

Sb₂O₃ 3

BT-93 5

RESULTS

UL-94 @1/8" V-2 V-2 UL-94 @ 1/16" V-2 V-2

The results show that a co-additive (BT-93), which does not have halogen atoms attached to an aliphatic carbon atom as part of its molecular structure is not effective in improving the flame retardancy imparted by the tris (tribromoneopentyl) phosphate alone. BT-93 is an abbreviation for ethylene bis(tetrabromophthalimide) sold by Albermarle Corporation as Saytex BT-93.

Example C

A polypropylene of Ml=12 is used as the base resin, namely Profax 6323 from Montell. Table 4

Tris(tribromoneo-p 3% 3.5% 3% 3% 3% entyl) phosphate

Non Nen 52 4% 4% — 2%

PE-68 — 4.5% 4.5% 2%

Sb₂O₃. 1.75% 1.875% 3.75% 1.875% 1.75%

Zinc Borate 1.75% 1.875% 1.875% 1.75%

Oxygen Index 31.2 31.2 31.5 31.2 31.8

UL testing

1/8" Rating V-0 V-0 V-0 V-0 V-0

ABT (1/8")^* 0 0 0 0 0

1/16" Rating V-2 V-2 V-0 V-2 V-0

ABT (1/16")^* 0 0 0 0 0

^* ABT = Average Burning Time of the 5 samples tested during the UL - 94 test. A time of zero seconds indicates that the samples extinguish immediately. The above Table demonstrates the sensitivity of the formulation to the specific load levels of FR components and synergists.

Example D

This example illustrates the use of Zinc Borate (ZnB) to substitute 40-60% of the antimony synergist.

The benefit of substituting Zinc Borate for antimony is the lighter final weight of a plastic part due to the specific gravity difference between Zinc Borate and antimony.

Table 5

Composition 1 2 3 4 5 6 7 8 9

Tris 4 4 4 4 3 3 5 3 3 (tribromoneopentyl) phosphate

PE - 68 (%) - — 2 2 5 5 4.5 2

Non-Nen 52(%) 4 4 4 2 2 — — — 2

Sb₂O₃ 3.5 2.5 1.75 2 2.5 3.75 1.75

ZnB 1.5 2.5 2 2 1.75 2 2.5 — 1.75

Oxygen Index 30.1 30.2 30 29.5 31.8 29.8 30 31.5 31.8

All the above formulations meet the V-0 requirement of the UL-94 test in both 1/8 and 1/16" thicknesses with zero average burning time.

Example E - Blooming Test

Molded plaques of the desired formulations (2"x2"x1/8") are oven aged at 100C (or any other test temperature) in a recirculating oven for a certain period of time (8 and 28 days).

Then the plaques may or may not have visible blooming on the surface. The plaques are placed into a 400 ml beaker containing approximately 50 ml of dichloromethane for 3 minutes under continuous stirring. Dichloromethane does not dissolve or chemically attack polypropylene. The short period of time is chosen so that only surface species are dissolved and none from the bulk of the polymer.

The blooming component is dissolved into the solvent and transferred to a pre-weighed cup. The solvent is allowed to evaporate and the remaining residue is weighed. This residue is assumed to be totally bloomed flame retardant material.

Table 6

Heat Aging at 100C

Composition

Tris (tribromoneopentyl) — phosphate

PE-68

Nonene52

Antimony 1.5 4.5 4.5

UL-94 V-2 V-0 V-0 V-0 V-2 V-0

Bloom after 8 days (g) .0278 .032 .016 .018 .014 .021

Bloom after 28 days (g) .033 .055 .022 .025 .02 .025

Bloom with inhibitor .0075 .025 .012 .014 .010 .016 8 days (metallocene)

Bloom with inhibitor .009 .06 .017 .02 .01 .017 28days(metallocene)

Formulation 2 has visible bloom after 8 days and is not acceptable. Formulation 5 is considered non-blooming. Formulation 1 is marginal.

Claims

1. A composition comprising at least one polyolefin resin, at least one tris - (trihaloneopentyl) phosphate flame retardant and at least one co-additive which is a halogenated flame retardant having at least one halogen atom attached to an aliphatic carbon atom as part of its molecular structure.

2. The composition of claim 1 wherein the co-additive has at least one hydrogen atom bound to a carbon atom which is in the beta position in relation to at least one halogen atom.

3. The composition of claim 2 wherein the co-additive is tetrabromobisphenol-A-bis(2, 3, dibromopropyl) ether, tetrabromobisphenol-S-bis (2,3 dibromopropyl) ether, a Diels-Alder adduct of hexachloropentadiene and cyclooctadiene, ethylene bis (dibromonorborane) dicarboximide, dibromoethyldibromocyclohexane, or a halogenated paraffin.

4. The composition of claim 1 wherein the tris -(trihaloneopentyl) phosphate is tris-(tribromoneopentyl) phosphate, tris -(dibromochloro) neopentyl phosphate, tris - (bromodichloro) neopentyl phosphate, or tris - (trichloroneopentyl) phosphate.

5. The composition of claim 4 wherein the tris - (trihaloneopentyl) phosphate is tris - (tribromoneopentyl) phosphate.

6. The composition of claim 1 wherein the polyolefin resin is polypropylene, polyethylene, a block copolymer of ethylene and propylene, or a random copolymer of ethylene and propylene.

7. A composition of claim 6 wherein the polyolefin resin is polypropylene.

8. A composition of claim 7 wherein the polypropylene is a molding grade of polypropylene.

9. The composition of claim 1 which further comprises at least one flame retardant synergist selected from antimony trioxide, antimony pentoxide, zinc stannate, sodium antimonate, zinc hydroxystannate, and zinc borate.

10. A composition of claim 9 wherein the flame retardant synergist is antimony trioxide.

11. A composition of claim 9 wherein the flame retardant synergist is zinc borate.

12. The composition of claim 4 which comprises from 1 % to 20% by weight of the tris - (trihaloneopentyl) phosphate.

13. The composition of claim 12 which comprises from 3% to 10% by weight of tris - (trihaloneopentyl) phosphate.

14. A composition of claim 12 which comprises from 0.5% to 5.0% by weight of a co-additive halogenated flame retardant.

15. A composition of claim 14 wherein the ratio of the weight of tris (trihaloneopentyl) phosphate to the weight of the co-additive halogenated flame retardant is in the range 4:1 to 1 :2.

16. A composition according to claim 13 further comprising antimony trioxide.

17. A composition of claim 14 further comprising zinc borate.

18. A composition of claim 14 wherein the ratio of the weight of antimony trioxide to the total weight of the tris (trihaloneopentyl) phosphate and the co-additive halogenated flame retardant is in the range 1 :5 to 1:1.

19. A composition of claim 1 which is rated V-O in the UL94 flammability test.

20. A composition of claim 1 which is non-blooming.

21. A method for improving the flame retardancy of a polyolefin resin composition containing a tris (tri haloneopentyl) phosphate flame retardant, which comprises incorporating into the composition as a co-additive a halogenated flame retardant having at least one halogen atom attached to an aliphatic carbon atom as part of its molecular structure.

22. A masterbatch composition of claim 1 comprising 5 to 20% by weight of tris (trihaloneopentyl) phosphate and from 10 to 20% by weight of the co-additive.