GB1575542A - Antistatic laminates - Google Patents

Description

(54) ANTISTATIC LAMINATES (71) We, THE BRITISH PETROLEUM COMPANY LIMITED, of Britannic House Moor Lane, London, EC2Y 9BU, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to antistatic laminates suitable for use as a protective barrier against liquids and vapours of gases which are susceptible to ignition on exposure to electrostatic discharge either during storage or transportation.

Prior art polymer sheets and films have had desirable mechanical and barrier properties but these also have a very high surface and high volume resistivity. This leads to the generation and accumulation of electrostatic charges which render such films and sheets unsuitable for use in some environments and is critical if the film or sheet is to be used in environments where explosive gaseous mixtures may occur. One of the techniques suggested to offset this disadvantage is to coat the base film with a conductive material but this is unsatisfactory because the coating tends to lift from the base film thus reducing the effectiveness of the system. Another suggested method is to incorporate conductive additives into the film during manufacture but may result in a reduction in mechanical properties and particularly in its ability to be joined by heat sealing.

It has now been found that some important electrical properties of such films and sheets may be improved to safe levels without substantially impairing their mechanical and physical properties and in many cases the mechanical properties will be improved.

Accordingly, the present invention is a laminate comprising at least one layer of a polymer sheet or film and at least one layer of a scrim of electrically conductive epitropia fibres, said scrim having a mechanical size below 10 cm2.

The laminates of the present invention may be prepared from sheets or films of any of the well-known polymers depending upon the desired mechanical and physical properties.

Examples of polymers that may be used include polyesters, polyvinyl fluoride, polyvinyl chloride, polyethylene, fluorinated polyethlene and polypropylene.

The scrim of electrically conductive material is of a mesh not greater than 10 cm2, suitably between 3 mm2 and 10 cm2, preferably between 6 mm2 and 5 cam2 Such mesh sizes approximately represent a rate of electrostatic discharge of about 70 nC for a 10 cm2 mesh down to below 10 nC for a 1 cm3 mesh.

By using a scrim having a mesh size lying within a specific range it can be ensured that only small areas, between the strands of the scrim, are available to generate charge which may then be dissipated rapidly by connecting the conductive scrim to a suitable earthing point.

The scrim may be made up of any electri- cally conductive epitropic fibre which contains very fine particles of carbon embedded in the surface of the fibre. This fibre containing the fine carbon particles is preferably of the same polymer as that used for the sheet or film with which it is to be laminated. One such example is the bonding of an epitropic polyester fibre with a polyester film. The sheet or film and scrim which are bonded together to form a laminate may however be made from dissimilar polymers depending upon the end use of the laminate.

The bonding of a layer of the polymer sheet or film with the scrim of electrically conductive material may be carried out in any conventional manner. It is preferable to laminate the layers by heat since the scrim material melts and flows under the sealing conditions. This produces a fairly flexible, impermeable laminate. The laminating may be carried out either by impulse

Material Surface Charge Comments Resistivity Transfer ohmlsquare . nC Control 1 Melinex *X301 1.25 x 1013 &commat; 100 V 52-390 Hazardous in gasolinetair heterofilm (polyester) environment. Control 2 Melinex *X301 1 x 1014 &commat; 100 V 390 Hazardous in gasolinelair laminated. to environment. Lamination condi Melinex X301 tions have not improved values. Laminate of Example 1 Melinex *X301 X301 side 1.0 x 1013 < lOnC Charge transfer values suggest laminated to a scrim &commat; 1 OOH hazard is considerably reduced of polyester fibres Scrim side resistance < 1 OnC on both sides. containing 3% epi- 6.15 x 10 to 5.3 x 108 tropic fibres ohm Laminate' of Example 2 Melinex*X301/ 1 x 101 3 &commat; 100 V < lOnC As above. epitropic scrim/ Melinex X301

* Registered Trade Mark heat sealing or by ultrasonic sealing in order to produce a strong bond. Alternatively, the scrim layer may be bonded to the polymer sheet or film by using an adhesive compound.

When using an adhesive for joining two lami nates in order to fabricate structures it must be ensured that the electrical continuity is main tained across the weld. This may be achieved by using a carbon filled adhesive compound.

For most applications a laminate comprising one layer of the polymer sheet or film and one layer of the scrim of electrically conductive material is adequate. However, if added strength and permeability characteristics are requred, a laminate may be formed wherein the scrim of electrically conductive material is sand wiched between two layers of the polymer sheet or film. The laminate may also be pro duced from one layer of polymer sheet or film sandwiched between two layers of the electri cally conductive scrim.

Laminates of the present invention can be used not only in producing safe gasoline vapour membranes for use as seals or storage tanks as claimed and described in our British Patent Specification Serial No. 1543579 but may also find wider applications such as in gasoline transfer hoses, plastic floating decks on gaso line or benzene storage tanks, safety work wear and carpets.

The invention is further illustrated with reference to the following Examples.

EXAMPLES Example 1 A polyethylene terephthalate fflm Melinex X301 (Registered Trade Mark) was coated on one side with a low molecular weight polyester adhesive. A scrim of polyester fibre interwoven with epitropic fibres and having a 1 cm2 mesh was heat sealed by impulsive sealing to the film 1800C for 3 minutes, then cooled under pressure to produce the laminate.

Example 2 The process of Example 1 was repeated except that the scrim of polyester fibre interwoven with epitropic fibres was sandwiched between two layers of the same polyethylene terephthalate film to produce the laminate.

These laminates according to Examples 1 and 2 were compared with a simple untreated Melinex X301 (Registered Trade Mark) film (Control 1) and a laminate of two layers of the same film without the scrim (Control 2). These were tested for their electrical characteristics and the results are tabulated in the table shown above: Temperature of measurement 22"C Relative Humidity 43 per cent WHAT WE CLAIM IS: 1. A laminate comprising at least one layer of a polymer sheet or film and at least one layer of a scrim of electrically conductive epitropicfibres, said scrim having a mesh size below 10 cm2.

2. A laminate according to Claim 1 wherein a layer of the scrim is sandwiched between two layers of a polymer sheet or film.

3. A laminate according to Claim 1 or 2

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. Material Surface Charge Comments Resistivity Transfer ohmlsquare . nC Control 1 Melinex *X301 1.25 x 1013 &commat; 100 V 52-390 Hazardous in gasolinetair heterofilm (polyester) environment. Control 2 Melinex *X301 1 x 1014 &commat; 100 V 390 Hazardous in gasolinelair laminated. to environment. Lamination condi Melinex X301 tions have not improved values. Laminate of Example 1 Melinex *X301 X301 side 1.0 x 1013 < lOnC Charge transfer values suggest laminated to a scrim &commat; 1 OOH hazard is considerably reduced of polyester fibres Scrim side resistance < 1 OnC on both sides. containing 3% epi- 6.15 x 10 to 5.3 x 108 tropic fibres ohm Laminate' of Example 2 Melinex*X301/ 1 x 101 3 &commat; 100 V < lOnC As above. epitropic scrim/ Melinex X301 * Registered Trade Mark heat sealing or by ultrasonic sealing in order to produce a strong bond. Alternatively, the scrim layer may be bonded to the polymer sheet or film by using an adhesive compound. When using an adhesive for joining two lami nates in order to fabricate structures it must be ensured that the electrical continuity is main tained across the weld. This may be achieved by using a carbon filled adhesive compound. For most applications a laminate comprising one layer of the polymer sheet or film and one layer of the scrim of electrically conductive material is adequate. However, if added strength and permeability characteristics are requred, a laminate may be formed wherein the scrim of electrically conductive material is sand wiched between two layers of the polymer sheet or film. The laminate may also be pro duced from one layer of polymer sheet or film sandwiched between two layers of the electri cally conductive scrim. Laminates of the present invention can be used not only in producing safe gasoline vapour membranes for use as seals or storage tanks as claimed and described in our British Patent Specification Serial No. 1543579 but may also find wider applications such as in gasoline transfer hoses, plastic floating decks on gaso line or benzene storage tanks, safety work wear and carpets. The invention is further illustrated with reference to the following Examples. EXAMPLES Example 1 A polyethylene terephthalate fflm Melinex X301 (Registered Trade Mark) was coated on one side with a low molecular weight polyester adhesive. A scrim of polyester fibre interwoven with epitropic fibres and having a 1 cm2 mesh was heat sealed by impulsive sealing to the film 1800C for 3 minutes, then cooled under pressure to produce the laminate. Example 2 The process of Example 1 was repeated except that the scrim of polyester fibre interwoven with epitropic fibres was sandwiched between two layers of the same polyethylene terephthalate film to produce the laminate. These laminates according to Examples 1 and 2 were compared with a simple untreated Melinex X301 (Registered Trade Mark) film (Control 1) and a laminate of two layers of the same film without the scrim (Control 2). These were tested for their electrical characteristics and the results are tabulated in the table shown above: Temperature of measurement 22"C Relative Humidity 43 per cent WHAT WE CLAIM IS:

1. A laminate comprising at least one layer of a polymer sheet or film and at least one layer of a scrim of electrically conductive epitropicfibres, said scrim having a mesh size below 10 cm2.

2. A laminate according to Claim 1 wherein a layer of the scrim is sandwiched between two layers of a polymer sheet or film.

3. A laminate according to Claim 1 or 2

wherein the polymer sheet is a polyester sheet.

4. A laminate according to any one of the preceding claims wherein the scrim has a mesh size of between 3 mm2 and 10cm2.

5. A laminate according to Claim 4 wherein the scrim has a mesh size ofbetween 6 mm2 and 5 cm2.

6. A laminate according to any of the preceding claims wherein the scrim is bonded to the layer of a polymer sheet by heat sealing.

7. A laminate according to Claim 1 as hereinbefore described with reference to the Examples.

8. Laminates as claimed in any of the preceding claims whenever used as gasoline vapour membranes.