This invention relates to laser marking polymeric surfaces and especially to the use of lasers having a wavelength in the infrared region for example about 5 to 15 micrometres. Of particular interest is the marking of surfaces by a carbon dioxide gas laser, a carbon monoxide laser or a semiconductor laser, and especially a carbon dioxide laser. Such lasers normally have a wavelength of 10.6 micrometres, and include the transversely excited atmospheric pressure (TEA) CO ₂ laser and the scanning type (continuously oscillating or pulse oscillating) CO ₂ laser. Such lasers will normally operate for marking purposes at fluences in the range of from 4 to 60 Jem- ² and especially from 8 to 30 Jem- ² *

.Although such lasers have been employed for marking a number of different surfaces, it has been proved very difficult to form legible markings on surfaces of certain fluoropolymers by this means. It is believed that this is due at least in part to the fact that such polymers have a small but significant absorption at the wavelength of interest (10.6 micrometres) with the result that the polymer pyrolyses under the action of the laser leaving no residual char to provide a contrast for the mark.

It has been proposed to employ non-black inorganic lead compounds together with a borate, silicate or phospate to improve the

contrast CO ₂ laser marked materials in EP-A-345,032. However the use of such materials suffers from the fact that very high levels of the inorganic compounds must be employed, normally from 20 to 50% of the total composition in order to provide a high level of contrast. This has the disadvantage not only that the physical properties of the composition suffer but the electrical properties can also suffer due to the high salt loading.

We have found that certain organic lead compounds, in particular lead carboxylates, may be employed to enhance the contrast of a laser mark on certain fluoropolymer surfaces. Thus, according to one aspect the invention provides a method of marking an article which comprises exposing it to a laser having a wavelength in the infrared region, characterised in that the article has a surface formed from a composition comprising a fluoropolymer that contains hydrogen atoms and containing a lead carboxylate.

Preferably the lead carboxylate is unsaturated and more preferably it is formed from a carboxylic acid having two or more carboxylic acid groups, especially one having two carboxylic acid groups. The salt may be derived from an aromatic or aliphatic carboxylic acid. The oxidation state of the lead is preferably +2. .Although, it is possible to employ a soluble lead carboxylate, it is preferred for it to be insoluble in order to prevent deterioration of the surface when exposed to water or water vapour. The preferred lead carboxylates include tetrabasic lead fumarate and dibasic lead phthalate.

The lead carboxylates may be, and preferably are, employed in relatively low concentrations. For example the composition preferably contains up to 10%, and especially up to 7% by weight of the lead carboxylate. At levels substantially above 10% of the salt deterioration of the physical properties of the polymer composition may occur. The composition preferably contains at least 2%, and especially at least 4% of the lead carboxylate in order to produce a mark with sufficient contrast.

The lead carboxylate salts do not in general absorb significantly at the far infrared wavelengths employed by the laser. The composition absorbs the laser radiation by virtue of the small, but significant, absorption by the polymer. .Although it is possible to include an energy absorbing additive, for example a borate or phosphate, the inclusion of such a material normally makes no difference to the marking characteristics of the surface and will also normally detrimentally affect the physical properties of the polymeric composition. Preferably, therefore, the total content of marking additives in the composition (eg. lead carboxylate, borate etc) is not more than 15%, more preferably not more than 12%, most preferably not more than 10% and especially not more than 7% by weight, based on the total weight of the composition.

Fluoropolymers that may be employed in the present invention include thermoplastic and elastomeric fluoropolymers, for example tetrafluoroethylene copolymers such as ethylene-tetrafluoroethylene copolymer (ETFE) and tetrafluoroethylene-propylene copolymer; vin lidine fluoride homo- and copolymers such as polyvinylidine fluoride and vinylidine fluoride hexafluoropropylene copolymer and the like. Preferred polymers are ethylene-tetrafluoroethylene copolymers. The fluoropolymer may, if desired, contain other halogens, eg. ethylene- chlorotrifluoroethylene copolymer. However, polymers containing no hydrogens such as polytetrafluoroethylene and fluorinated ethylene propylene copolymer are not included.

For some applications ^" the polymer composition may be crosslinked. They may be crosslinked by any convenient method, for example by irradiation or, by chemical crosslinking using, for example, a peroxide. The peroxide may be absorbed on an inert carrier such as a calcium carbonate, carbon black, or kiesleguhr; however, the weight of the carrier is not included in the above range.

Preferably, however, the polymer compositions of the present invention are crosslinked using high energy radiation. Radiation dose levels to achieve crosslinking according to the present invention may range from about 20 to 800 kGy or more, but a dose of about 50 to 500 kGy

is preferred. For most purposes a dose of about 80 to 200 kGy will be effective.

In some cases it may be desirable to add to the crosslinkable polymer composition a co-agent to assist in the crosslinking reaction. Such co-agents usually contain multiple unsaturated groups such as alkyl or acrylic esters. While their mode of action is not known with certainty, it is believed that they react with the initial radical formed on the polymer backbone to form a more stable radical, which undergoes a coupling reactions to form crosslinks more readily than chain scission reactions.

The co-agent can be for example, N,N'-m(phenylene)- dimaleimide, trimethylolpropane trimethylacrylate , tetraallyloxyethane, triallyl ' cyanurate, triallyl isocyanurate, tetramethylene acrylate, or polyethylene oxide glycol dimethacrylate. The amount of the co-agent is preferably up to about 5 parts by weight per 100 parts of the polymer composition and preferably from 1 to 3 parts by weight per 100 parts of the polymer composition.

Crosslinked polymer compositions according to the present invention may be used in a wide range of applications, and the preferred compositions find particular application as electrical insulation or jacketting in electrical wire or cable or in electrical harnesses. For example they may be used for primary insulation and/or primary jackets in electrical wires as described in GB Patent No. 1,592,979 or they may be used as extruded cable jackets.

Compositions according to the invention, especially crosslinked ones, may be used for the production of heat-recoverable articles for a wide variety of purposes. A heat-recoverable article is one which is in a dimensionally unstable condition and is capable of altering its physical form upon the application of heat alone to assume a dimensionally heat stable condition. Preferably the article is elastomeric in both its heat unstable and heat stable forms. Heat-recoverable articles may be produced for example deforming an article under heat and pressure

from an original dimensionally heat stable form to a dimensionally heat unstable form from which it is capable of recovery towards it original form upon the application of heat alone. Heat-recoverable articles and methods for their production are described, for example in US Patents Nos. 2,027,962, 3,086,242 and 3,957,372. Heat-recoverable articles that are employed specifically for marking purposes are described in US patent Nos. 3,994,731 and 4,349,404.

Alternatively the compositions may be employed in non- recoverable applications, such as for labels and other markers that are intended to be affixed to a surface either before or after marking. The compositions may be provided in the form of a liquid coating composition, eg. as an aqueous dispersion, that is intended to be applied to an existing surface in order to enable it to be laser marked.

Other polymers may be blended with the composition according to the invention. In the broadest aspect of the invention any polymer may be blended with the composition provided that it does not remove the ability to be laser marked according to the invention. Preferred polymers for blending with the compositions include ethylene homo- and copolymers eg. low, medium or high density polyethylene and ethylene vinyl acetate copolymers (or so-called vinyl acetate ethylene copolymers which have a higher vinyl acetate content), ethylene acrylic copolymers, polyamides, and polyesters for example polybutylene terephthalate.

The composition may contain one or more additional materials, for example fillers, flame retardants, anti-oxidants, ultraviolet stabilizers, processing aids„ pigments, fungicides or the like. Preferably, however, the total content of non-polymeric material is not more than 20% by weight and especially not more than 12% by weight, based on the total weight of the composition.

The following Examples illustrate the invention:

Examnle 1

Samples of ethylene-tetrafluoroethylene containing various lead compounds were prepared by blending the components on a laboratory rabender mixer at 290°C and pressing into lmm thick plaques. The plaques were then marked using a CO ₂ laser at fluences 6 between 8 and 30 Jem ^-2 . The results are shown in Table 1 from which it can be seen that tetrabasic lead fumarate gave the mark with the greatest contrast. It appeared that some degradation of the dibasic lead phosphite occured at the processing temperature.

IA£L£ 1

Dibasic lead phthalate 10 10

Tetrabasic lead fumarate 10

Dibasic lead phosphite 10

Colour of test sample light white cream dark grey grey

Mark formed none faint dark faint

Example 2

Plaques of various polymers were prepared containing 5% -dibasic lead phthalate and were subjected to a CO ₂ laser marker as described in Example 1. The results are shown in Table 2 from which it can be seen

that marks could be formed only with ethylene-tetrafluoroethylene and ethylene chlorotrifluoroethylene.

TABLE 2

. a- plfi fl (Comparative)

Example 1 was repeated using a range of organic salts of metals other than lead, and ethylene-tetrafluoroethylene. The results are shown in Table 3.

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TABLE 3

Calcium salt of phthalic acid 10

Aluminium salt of 2,6 NDA 10

Zinc salt of 2,6 NDA 10

Calcium salt of 2,6 NDA 10

Aluminium salt of terephthalic acid 10

Zinc salt of terephthalic acid 10

Calcium salt of terephthalic acid 10

Zinc salt of pyromeliitic acid 10

Aluminium salt of ISO-terephthalic acid 10

Zinc salt of ISO- terephthalic acid

Colour of test sample white Light white brown grey grey grey white white light white white grey grey

Mark formed none none none none none none none none none none none none