This invention relates to polyester compositions, and particularly to those based on thermoplastic polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate .

Thermoplastic polyesters are commonly used in moulding compositions, and when flame retardant grades are required they are typically flame retarded with additives containing halogens. The level of flame retardancy obtained from such materials can be very good and VO ratings in the Underwriters (UL) Flammability test are readily achieved. A problem with such compositions, however, is that the resistance to electrical tracking of thermoplastic polyesters containing these additives is very low. (Typical values of Comparative Tracking Index - CTI - for a UL VO material would be around 200 volts.)

It has also been proposed to use red phosphorus as a flame retardant in PET and PBT compositions, whereby some improvement in electrical tracking resistance may be achieved.

US Patent No 4,314,927 proposes the use of red phosphorus and melamine cyanurate in moulding compositions of Nylon 66 to improve the heat stability and lower the level of phosphorus required to achieve VO ratings. It gives no guidance as to the electrical tracking resistance of the compounds disclosed.

We have now found that red phosphorus and melamine cyanurate may be used together in glass reinforced thermoplastic polyester compositions to enhance both flame retardancy and electrical tracking resistance.

Thus according to the present invention a polyester moulding composition comprises a thermoplastic polyester, reinforcing fibres and as fire retardant red phosphorus with melamine cyanurate.

The thermoplastic polyester may be, for example, polyethylene terephthalate (PET) or polybutylene terephthalate (PBT) or a polyester containing at least 80 per cent by weight of repeating units derived from ethylene terephthalate or butylene terephthalate.

The amount of thermoplastic polyester in the composition is preferably in the ranee 35 to 75 per cent by weight of the total composition.

As flame retardant red phosphorus is an essential ingredient of the polyester moulding composition but the red phosphorus is preferably treated in order to reduce its handling difficulties, since red phosphorus alone can be ver dangerous in handling. Thus the red phosphorus ma ^¬ be carried on a polymeric carrier, and/or may be encapsulated ith a suitable substance such as a polymer or resin.

Preferably the amount of red phosphorus in the composition is 1 to 15 per cent by weight of the total composition.

The composition contains reinforcing fibres, for example short lengths of inorganic reinforcing fibre such as glass fibre, although other types of fibre which will stand up to the processing conditions to which the composition is subjected may be used. The amount of reinforcing fibre in the composition is preferably in the range 15 per cent to 45 per cent by weight of the composition.

Fillers may also be used in the composition, for example kaolinitic clays, talc, glass spheres, wollastonite , calcium carbonate, and when used in conjunction with the

reinforcing fibres have the advantage of giving a composition with less warping and less anisotropic properties.

The composition may also contain a nucleant and the nucleant may be either a metal salt of an organic acid, for example a metal stearate, acetate or benzoate, or a finely divided material which does not melt at or below the melting point of polyethylene terephthalate, for example microtalc. The preferred nucleant is a group 1 metal stearate, or microtalc with a particle size of less than 20 microns.

Preferably the group 1 metal stearate is sodium stearate, which is a known nucleant for polyethylene terephthalate and gives easier nucleation of the polyester. The nucleant is preferably used in an amount no more than 1% -by weight of the composition, particularly in the range 0.1 to 0.7 per cent by weight. However, if a finely divided solid nucleant is used it may be used in greater proportion, eg up to six parts by weight per 100 parts by weight total polymer, and may be used, if desired, in addition to a nucleant of the other type.

In the compositions of this invention the melamine cyanurate functions to enhance the flame retardancy of the system with an important improvement in the electrical

tracking resistance of the moulded composition. The amount of melamine cyanurate used may be in the range from 1 per cent to 20 per cent, preferably 4 per cent to 15 per cent, by weight of the composition.

Other additives such as mould release agents, stabilisers etc may be used in the composition as desired.

The invention will now be described in more detail, by way of example only, by means of the following Examples.

EXAMPLES

Flammability Testing

The method used was the standard Underwriters Laboratories test method UL94. VO means that the specimens burnt for less than 10s after removal of flame, that the total burn time for ten applications of the flame on five specimens was less than 50 seconds. In addition no flaming drops are permitted. In VI classification no specimen burns for longer than 30 seconds after the application of the flame and the total burning time for 10 specimens is less than 250 seconds. No flaming drops are permitted. The same criteria apply in V2 classification except that some flaming drops are permitted.

In addition to these classifications, the average burning time for each application of flame was calculated and the occurrence of non-flaming drops was noted (10 specimens).

Electrical Tracking Resistance Testing

The test equipment used is that of the Comparative Tracking Index test of the European Standard IEC 112 (DIN 53480). An aqueous solution containing 0.1% NH Cl was dropped at 30 second intervals on to the surface of the mouldings between two electrodes carrying the applied voltage. Platinum electrodes were used, with trips at 0.5 amps and 2 sees.

Results are expressed in the tables below for each applied voltage as "failures", or "passes" at 50 drops total.

Examples 1 and 2

Two compositions of polyethylene terephthalate (PET) were prepared with and without melamine cyanurate for comparison.

The compositions were made by first blending the ingredients shown (as parts by weight) in Table I, and then fully compounded in a Baker Perkins twin-screw extruder.

TABLE I

Ingredient Example 1 Example

PET (D217)

Coated Red Phosphorus

Glass Fibre

Nucleant/Mould Release Agent

Melamine Cyanurate "C"

Stabiliser

In each example the red phosphorus used (MASTERFLAM P70S) was encapsulated in a polyamide carrier (70% by weight of red phosphorus). The glass fibres were grade 429YZ from Owens Corning Fibreglass, chop length 4.5mm. The nucleant/mould release agent was NAI/LOXIOL masterbatch and the stabiliser was IRGAF0S PEPQ. NAI is a nucleant/polymer (linear low density polythene) supplied by Goodyear. L0XI0L is pentaer thritol tetrastearate supplied by Henkel.

Sample mouldings were made by injection moulding from each composition on a standard BIPEL 60/26 machine using a barrel temperature of 270°C.

These were tested for flammability and for Electrical Tracking Resistance. The results of these tests are given below in Table II.

TABLE II Test Example 1 Example 2

Flammability - UL94 test a) At 1.5mm

Class VO VO

Average Burn Time (sees) 0.7 0

b) At 3mm

Class VO VO

Average Burn Times (sees) 0.1 0

Electrical Tracking Resistance

At 400V Fail Pass

(at 6 and (5 passes at

18 drops) >52 drops)

Examples 3 and 4

Two compositions of polybutylene terepthalate (PBT) were prepared with and without melamine cyanurate for comparison.

The compositions were made by first blending the ingredients shown (as parts by weight) in Table III, and then fully compounded in a Baker Perkins twin-screw extruder.

TABLE III

Ingredient Example 3 Example 4

PBT (TQ9)

Coated Red Phosphorus

Glass Fibre

Nucleant/Mould Release Agent

Melamine Cyanurate "C"

Stabiliser

In each example the red phosphorus used (MASTERFLAM P70S) was encapsulated in a polyamide carrier (70% by weight of red phosphorus). The glass fibres were grade 429YZ from Owens Corning Fibreglass, chop length 4.5mm. The nucleant/mould release agent was NAI/LOXIOL masterbatch and the stabiliser was IRGAF0S PEPQ. NAI is a nucleant/polyraer (linear low density polythene) supplied by Goodyear. L0XI0L is pentaerythritol tetrastearate supplied by Henkel. Sample mouldings were made by injection moulding from each composition on a standard BIPEL 60/26 machine using a barrel temperature of 270°C.

These were tested for flammability and for Electrical

Tracking Resistance exactly as for Examples 1 and 2. The test results were very similar; Example 3 failed and Example 2 passed.