Polyethylene teraphthalate (PET hereinafter) is a packaging material that can be readily blow-moulded into various thin-walled packaging containers. Such containers are used extensively throughout the food packaging industry. There is an ever- increasing requirement for the marking or coding of food packaging in particular and PET containers therefore. PET containers are readily marked by carbon dioxide (C02) lasers in the 8 to 12 micron wavelength region, but with varying mark qualities at specific wavelengths within that wavelength band.

For the normal gas isotopes of the carbon dioxide molecule (carbon-12 and oxygen-16), four major laser transition bands exist. These four bands are peaked at 9.27,9.55,10.25 and 10.59 microns. However, it is known (see IEEE Journal of Quantum Electronics, vol. 16 no. 11, November 1980, pp 1195-1198) that by modifying the lasing gas isotopes of C02 lasers (such as by using carbon-13 or carbon-14 with either oxygen-16 or oxygen-18) additional laser bands can be accessed. By varying the mirror reflectivities of a carbon dioxide laser resonator, laser operation in each of these four normal isotope wavelength bands (using carbon-12 and oxygen-16) can be obtained at the exclusion of the three others. Thus a carbon dioxide laser frequency can be chosen which has the best laser marking qualities for marking a given material, such as PET for example.

Figure 1 illustrates the transmittance of light through a thin PET substrate in the 8.0 to 12.0 micron range. Transmittance minima (or absorption peaks) can be seen at 8.90,9.30 and 10.35 microns. Lesser absorption peaks are seen at 8.35 and 9.62 microns. A very strong absorption peak lies just below 8.0 microns, but this it below the lower end of the normal 8.2 to 12 micron C02 laser band. In the 8.2 to 12.0 micron range accessible by carbon dioxide lasers, therefore, it would appear that the best marking should occur at 8.9 and 10.35 microns (the wavelengths having the best optical absorption peaks). The normal carbon dioxide laser output wavelength of 10.59 microns would seem to have the least optical absorption of the four normal isotope bands.

According to the present invention a method of marking PET material that has previously been mechanically stressed, the method comprising illuminating the PET

material with C02 laser light in the range of 9.2 to 9.4 micron wavelength. Preferably, the laser light has a wavelength of 9.3 microns, but may comprise a number of wavelengths within the range.

It has been found that marking PET material which retains a residual mechanical stress, by such a method, provides unexpectedly improved marks which provide enhanced reflection surfaces and thus'brighter'marks when illuminated with visible light than marks formed by marking at other wavelengths. Although marks formed with C02 laser light at 10.35 microns create deep marks, the enhanced reflectivity provides marks of greater practical use when the marks are coded dots or other indicia used for data recognition purposes.

Particularly, the invention may be used to mark PET material which has already been stretched, for example by blow-moulding. The method effects the production of what looks like a"crystalline-like"surface on PET bottles with surface-stretched material, such as that found on parts of the PET bottles after they are blow-moulded.

The pre-formed and/or non-blow-moulded parts of PET bottles still mark well at 9.3 microns but without the crystalline-like quality.

The unusual surface structure produced by the 9.3 micron laser beam on stretched or blow-moulded PET material appears to occur due to the thermal activation of certain physically-stretched hydro-carbon bonds in the PET matrix. Specifically, the molecular dynamics involved in the IR absorption at 9.3 microns for PET is most likely due to the stretching of the aromatic ring (Benzene)-(double bonded CO)-O-R (alkene/alkane group)] which is thermally-activated with absorption of the 9.3 micron radiation. The thermal activation of this particular bond while other bonds in the matrix remain intact causes the stretched material to relax asymmetrically into a complex web- like structure.

One example of a method and apparatus according to the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates the transmittance of light through a thin PET substrate in the 8.0 to 12.0 micron range; Figure 2 illustrates a C02 laser marking device as used to produce three rows of marks as shown in Figures 3 & 4, with a portion of the PET substrate magnified to show the rows of pits produced; and Figures 3 & 4 are photo-micrographs illustrating C02 laser marks produced at different speeds and wavelengths.

A laser marking apparatus, used to illustrate the invention, comprises three CO2 lasers10,11,12 arranged to operate in a three bands of wavelengths (a) between 9.2

and 9.4 microns, (b) at 10.3 microns, and (c) at 10.6 microns respectively as shown in Figure 2. CO2 laser apparatus of this general type is well known and will not be described in detail. However, the laser beams 10,11,12 are, as shown, focussed onto a moving PET substrate 20 (which, for example, may be a PET bottle or the like and the required markings are made by relative movement of the focus of the laser beam over the surface of the substrate 20 and by repeated firing of the laser to produce a three series or rows of closely spaced pits 30,31,32.

Figure 3 is a photo-micrograph of marks (pits) 30,31,32 formed on a PET container using the three separate lasers of substantially equal laser output power, with all three lasers producing approximately 25 watts of peak optical power, at 9.3 (actually a range between 9.2 and 9.4), 10.3 and 10.6 microns respectively at a speed of 2.5 ms- 1, with the lasers fired for 125 ps of'on'time followed immediately by 2751ls of'off'time.

The on/off times were then repeated continuously. As can clearly be seen, the 10.3 micron wavelength laser created the deepest pits 31 compared to the other two wavelengths. At a speed of 5 mus-' (see Figure 4) the 10.6 micron laser does not create a mark at all, while both the 9.3 and 10.3 micron lasers produced good marks 30,31.

However, of special interest in Figures 3 and 4 is the unusual dot structure of the 9.3 micron laser-produced pits compared to the pits made with 10.3 and 10.6 micron lasers. At 2.5 ms', both the 10.3 and 10.6 micron lasers produce smooth pits 31,32 with the 10.3 micron pit being deeper (as expected from the optical absorption data in Figure 1). On the other hand at both 2.5 ms'and 5 ms', the 9.3 micron laser produces speckled or striated pits 30. The surface pattern was not related to optical mode structure, but has been found to be characteristic of the 9.3 micron laser interacting at high intensity with the PET material.

This unexpected physical effect of the 9.3 micron laser interacting with the PET material produces a highly enhanced mark (pit) 30 with a nearly crystalline quality. Thus it appears that the complex optical absorption spectrum of PET in the 8.0 to 9.7 micron range can be utilized for enhancing the laser marking of PET containers by utilizing lasers operating in the wavelength range of 9.2 to 9.4 micron.

The use of lasers to mark on PET containers in the 8.0 and 9.7 micron wavelength band allows for two major technical advancements. Firstly, the highly "crystalline-like"structure of the individual laser dots creates a highly enhanced reflection surface thus causing a brightening of the mark. This"crystalline-like"mark surface structure is apparently caused by the inhomogeneous absorption of the laser radiation in the aforesaid wavelength band creating a type of striated melting of the PET structure. This observation is supported by the complex structure of the PET

absorption spectrum of Figure 1 wherein the complexity of the spectrum between 8.0 and 9.7 microns indicates inhomogeneous absorption due to a range of different optically absorbing PET sub-components. This is especially noticeable for marks made at high speeds (greater than 2.5 mi').

Secondly the local scattering effect of the of the 9.3 micron laser pits appears to reduce the ability of the laser to puncture holes through a given thickness of container. (Note that hole puncturing of PET containers by laser marking is a very undesirable condition.) Experiments indicate that this tolerance to hole puncturing is improved from between 25% to 50% compared to the standard 10.6 micron C02 laser as measured by the number of laser pulses needed to drill a hole through the wall of the PET container at the two wavelengths. A series of focused 150 microsecond laser pulses were fired using approximately 25 watts of laser power in both lasers. The PET sample was 0.30 mm thick. A ZnSe focusing lens of 7.0 cm was used to focus the beams on the PET target. Approximately 14 to 16 pulses of laser energy were required to puncture the PET at 10.6 microns whereas approximately 20 to 22 pulses of laser energy were required to puncture the PET at 9.3 microns. These are clearly major technical advances with significant market advantages.