GB2107322A

GB2107322A - Method of marking polymers by laser beam and polymers adapted therefor

Info

Publication number: GB2107322A
Application number: GB08128729A
Authority: GB
Inventors: Michael John Lock
Original assignee: Metal Box PLC
Current assignee: Crown Packaging UK Ltd
Priority date: 1981-09-23
Filing date: 1981-09-23
Publication date: 1983-04-27
Also published as: GB2107322B

Abstract

In a method of marking a polymer by irradiation with a laser beam, the polymer is modified before irradiation to increase its absorption of energy of the wavelength of the laser beam, for example by incorporating in the polymer an additive which has a strong absorption for energy of the wavelength of the laser beam, the additive suitably being a silicate such as calcium silicate.

Description

SPECIFICATION Methods of marking polymers by laser beam and polymers adapted therefor This invention relates to methods of marking polymers by irradiation with a laser beam and polymers adapted for such marking.

In the packaging and marketing of commodities such as foodstuffs, it is often desirable to mark the containers with information in an indelible manner, generally in code, such as the appropriate factory and batch codes. Laser coding systems are commercially available, using pulsed CO2 laser beams operating at a wavelength of 1 0.6y, which is in the infra-red range. Certain polymers, such as polyvinylchloride, can readily be marked by such a laser beam, but many of the polymers which are in general use for packaging foodstuffs, such as polypropylene, polyethylene, polystyrene and acrylonitrile-butadiene-styrene copolymers (ABS), do not yield a legible mark. A common characteristic of these latter polymers is olefinic type bonds in the polymer chain.Examination of the infra-red spectra of these polymers which do not yield legible marks shows very little absorption of energy at a wavelength of 1 0.6,u. In order for thermal damage to be caused to the polymer, so making a mark, the polymer must absorb energy at the wavelength of the incident radiation. Otherwise the beam energy will be transmitted and not used.

The present invention resides principally in a method of marking a polymer by irradiation with a laser beam, wherein the polymer is modified before irradiation to increase its absorption of energy of the wavelength of the laser beam.

Preferably the modification of the polymer is effected by incorporating in the polymer an additive which has a strong absorption for energy of the wavelength of the laser beam.

The invention is particularly applicable to the marking of polymers which have low absorption in the infra-red range of wavelengths, using a CO2 laser beam having a wavelength of 1 0.6y, in which case the additive is chosen to have a high absorption (e.g. over 50%) at the wavelength of 10.6y.

The additive may be chosen from a variety of materials having such a high absorption, but for polymers which are to be used as containers for foodstuffs it is essential that is should be nontoxic. It is also preferable that the additive should be capable of being blended into a master bath for incorporation into the polymer and that it should not impart a colour to the polymer on incorporation or on processing of the polymer prior to irradiation with the laser beam. With these requirements in mind, a silicate material has been found effective. The preferred additive is calcium silicate, which in the form of the mineral woolstonite has an absorption of 96% at a wavelength of 1 0.6y. The calcium silicate may be used in a synthetically precipitated form and may be incorporated in an amount of 1 to 5% by weight of the polymer.It has been found that with amounts over 5% the mark surprisingly becomes less visible. The preferred amount of calcium silicate is substantially 2% by weight. Other silicates which may be used include aluminium silicates, e.g. in the form of china clay.

It appears that the particles of the additive (e.g.

calcium silicate), which are distributed throughout the polymer, absorb energy from the incident laser beam and convert it into heat which produces a very high temperature in the polymer in their immediate locality, because the low thermal conductivity of the polymer means that there is no rapid route by which the energy can escape. The very high temperature may result in chemical bond cleavage of random weak links in the polymer chain, leading to associated formation of volatile low molecular weight fragments, or in other modes of local degradation of the polymer. Whatever the precise mechanism may be, the observed result in a visible marking on the surface of the polymer where the laser beam has been incident on it.

The invention also resides in a polymer adapted for marking by irradiation with a laser beam wherein the polymer is modified to increase its absorption of energy of the wavelength of the laser beam, preferably by incorporation in the polymer of an additive which has a strong absorption for energy of the wavelength of the laser beam.

In a specific example, calcium silicate in synthetically precipitated form was incorporated in a white pigmented polypropylene from a master batch so as to form 2% by weight of the polymer. Irradiation of the polymer by a pulsed beam from a commercially available CO2 laser, at a wavelength of 1 0.6u, produced a relief image on the polymer surface, silver grey in colour. The image stood about 40y above the original surface and the coloration was found to extend 40 to 5O below the original surface, resulting from the formation of a cellular structure which is consistent with the production of volatile fragments as referred to above.

Claims: (Filed on 2 July 1982) 1. A method of marking a polymer by irradiation with a laser beam, wherein the polymer is modified before irradiation to increase its absorption of energy of the wavelength of the laser beam.

2. A method according to claim 1 wherein the modification of the polymer is effected by incorporating in the polymer an additive which has a strong absorption for energy of the wavelength of the laser beam.

3. A method according to claim 2 using a polymer which has a low absorption in the infrared range of wavelengths and a laser beam which is produced by a CO2 laser and has a wavelength of 10.6u, wherein the additive has a high absorption at the wavelength of 1 0.6y.

4. A method according to claim 3 wherein the additive is a silicate.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Methods of marking polymers by laser beam and polymers adapted therefor This invention relates to methods of marking polymers by irradiation with a laser beam and polymers adapted for such marking. In the packaging and marketing of commodities such as foodstuffs, it is often desirable to mark the containers with information in an indelible manner, generally in code, such as the appropriate factory and batch codes. Laser coding systems are commercially available, using pulsed CO2 laser beams operating at a wavelength of 1 0.6y, which is in the infra-red range. Certain polymers, such as polyvinylchloride, can readily be marked by such a laser beam, but many of the polymers which are in general use for packaging foodstuffs, such as polypropylene, polyethylene, polystyrene and acrylonitrile-butadiene-styrene copolymers (ABS), do not yield a legible mark. A common characteristic of these latter polymers is olefinic type bonds in the polymer chain.Examination of the infra-red spectra of these polymers which do not yield legible marks shows very little absorption of energy at a wavelength of 1 0.6,u. In order for thermal damage to be caused to the polymer, so making a mark, the polymer must absorb energy at the wavelength of the incident radiation. Otherwise the beam energy will be transmitted and not used. The present invention resides principally in a method of marking a polymer by irradiation with a laser beam, wherein the polymer is modified before irradiation to increase its absorption of energy of the wavelength of the laser beam. Preferably the modification of the polymer is effected by incorporating in the polymer an additive which has a strong absorption for energy of the wavelength of the laser beam. The invention is particularly applicable to the marking of polymers which have low absorption in the infra-red range of wavelengths, using a CO2 laser beam having a wavelength of 1 0.6y, in which case the additive is chosen to have a high absorption (e.g. over 50%) at the wavelength of 10.6y. The additive may be chosen from a variety of materials having such a high absorption, but for polymers which are to be used as containers for foodstuffs it is essential that is should be nontoxic. It is also preferable that the additive should be capable of being blended into a master bath for incorporation into the polymer and that it should not impart a colour to the polymer on incorporation or on processing of the polymer prior to irradiation with the laser beam. With these requirements in mind, a silicate material has been found effective. The preferred additive is calcium silicate, which in the form of the mineral woolstonite has an absorption of 96% at a wavelength of 1 0.6y. The calcium silicate may be used in a synthetically precipitated form and may be incorporated in an amount of 1 to 5% by weight of the polymer.It has been found that with amounts over 5% the mark surprisingly becomes less visible. The preferred amount of calcium silicate is substantially 2% by weight. Other silicates which may be used include aluminium silicates, e.g. in the form of china clay. It appears that the particles of the additive (e.g. calcium silicate), which are distributed throughout the polymer, absorb energy from the incident laser beam and convert it into heat which produces a very high temperature in the polymer in their immediate locality, because the low thermal conductivity of the polymer means that there is no rapid route by which the energy can escape. The very high temperature may result in chemical bond cleavage of random weak links in the polymer chain, leading to associated formation of volatile low molecular weight fragments, or in other modes of local degradation of the polymer. Whatever the precise mechanism may be, the observed result in a visible marking on the surface of the polymer where the laser beam has been incident on it. The invention also resides in a polymer adapted for marking by irradiation with a laser beam wherein the polymer is modified to increase its absorption of energy of the wavelength of the laser beam, preferably by incorporation in the polymer of an additive which has a strong absorption for energy of the wavelength of the laser beam. In a specific example, calcium silicate in synthetically precipitated form was incorporated in a white pigmented polypropylene from a master batch so as to form 2% by weight of the polymer. Irradiation of the polymer by a pulsed beam from a commercially available CO2 laser, at a wavelength of 1 0.6u, produced a relief image on the polymer surface, silver grey in colour. The image stood about 40y above the original surface and the coloration was found to extend 40 to 5O below the original surface, resulting from the formation of a cellular structure which is consistent with the production of volatile fragments as referred to above. Claims: (Filed on 2 July 1982)

1. A method of marking a polymer by irradiation with a laser beam, wherein the polymer is modified before irradiation to increase its absorption of energy of the wavelength of the laser beam.

4. A method according to claim 3 wherein the additive is a silicate.

5. A method according to claim 4 wherein the additive is calcium silicate.

6. A method according to claim 5 wherein the calcium silicate is incorporated in an amount of 1% to 5% by weight of the polymer.

7. A method according to claim 6 wherein the calcium silicate is incorporated in an amount of substantially 2% by weight of the polymer.

8. A method according to any one of claims 2 to 7 wherein the additive is incorporated in the polymer from a master batch.

9. A method according to any one of claims 3 to 8, wherein the polymer is a polypropylene, a polyethylene, a polystyrene or an acrylonitrilebutadiene-styrene copolymer.

10. A polymer adapted for marking by irradiation with a laser beam, wherein the polymer is modified to increase its absorption of energy of the wavelength of the laser beam.

11. A polymer according to claim 10 wherein the modification comprises the incorporation in the polymer of an additive which has a strong absorption for energy of the wavelength of the laser beam.

1 2. A polymer which normally has a low absorption in the infra-red range of wavelengths, adapted for marking by irradiation with a CO2 laser beam of 1 0.6y wavelength by incorporation of an additive having a high absorption at the wavelength of 1 0.6zz.

13. A polymer according to claim 12 wherein the additive is a silicate.

1 4.-A polymer according to claim 13 wherein the additive is calcium silicate.

1 5. A polymer according to claim 14 wherein the calcium silicate is present in an amount of 1% to 5% by weight of the polymer.

1 6. A polymer according to claim 1 5 incorporating substantially 2% by weight of the calcium silicate.

17. A polymer according to any one of claims 12 to 16, consisting principally of polypropylene, polyethylene, polystyrene or an acrylonitrilebutadiene-styrene copolymer.