Laser-markable plastics

Details Laser-markable plastics Laser-markable plastics

2000-12-18

2002-04-23

Cain, Edward J. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C523S210000, C524S495000, C524S496000

Reexamination Certificate

active

06376577

ABSTRACT:

The present invention relates to laser-markable plastics which comprise, as dopant, graphite particles having one or more coatings.
The identification marking of products is becoming increasingly important in almost every branch of industry. For example, it is frequently necessary to apply production dates, expiry dates, bar codes, company logos, serial numbers, etc. to plastics or plastic films. These markings are currently mostly executed using conventional techniques, such as printing, hot-stamping, other stamping methods or labelling. However, in particular for plastics, increasing importance is being attached to the contactless, very rapid and flexible method of marking with lasers. With this technique it is possible to apply graphic inscriptions, such as bar codes, at high speed, even to non-planar surfaces. Since the inscription is located within the plastics article itself, it is durably abrasion-resistant.
Many plastics, e.g. polyolefins and polystyrenes, have hitherto been difficult or impossible to mark with a laser. A CO
2
laser which emits infrared light in the region of 10.6 &mgr;m brings about only very weak, hardly legible marking on polyolefins or polystyrenes, even using very high power. In the case of polyurethane elastomers and polyether ester elastomers, no interaction occurs with Nd-YAG lasers, but engraving occurs using CO
2
lasers. It is not permissible for the plastic to reflect or transmit all of the laser light, since there is then no interaction. Nor must excessively strong absorption take place, however, since in this case the plastics evaporate and all that remains is an engraving. The absorption of the laser beams, and therefore the interaction with the material, depends on the chemical structure of the plastic and on the laser wavelength used. It is often necessary to add appropriate additives, such as absorbers, in order to render plastics laser-inscribable.
Besides CO
2
lasers, Nd:YAG lasers are increasingly being used for the identification marking of plastics by laser. The YAG lasers usually used give a pulsed energy beam with a characteristic wavelength of 1064 nm or 532 nm. The absorber material must have pronounced absorption in this specific NIR region in order to exhibit an adequate reaction during fast inscription procedures.
The use of graphite as dopant in laser-marking is known, for example from EP 0 053 256 or WO 94/12352. These markings are pale markings on a black plastics surface. A disadvantage of graphite, however, is that laser inscription produces merely a greyish-brown inscription on a black background, and the inscription then lacks adequate contrast.
It was an object of the present invention, therefore, to find laser-markable plastics which when exposed to laser light can give a white marking with high contrast. The filler should increase the absorption of the laser energy and/or cause the production of gas when impacted by a laser, thus improving the contrast by additionally forming a foam.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
Surprisingly, it has been found that the laser-markability of plastics, in particular the contrast of the marking, can be improved if use is made of coated graphite particles, such as graphite coated with CaCO
3
, CaSO
4
, Fe
3
O
4
or Turnbull's blue. In comparison with uncoated graphite, the markings achieved are significantly paler, and at the same time the energy densities are lower.
By adding the coated graphite at concentrations of from 0.01 to 4% by weight, based on the polymer system, preferably from 0.3 to 2.5% by weight, and in particular from 0.5 to 2% by weight, high contrast is achieved during laser-marking of thermoplastics.
The concentration of the dopant in the plastic is of course dependent on the polymer system used. The small proportion of coated graphite does not substantially alter the polymer system, and has no effect on its processability.
It is also possible for colorants to be added to the plastics, permitting any type of variation in colour and at the same time ensuring retention of the laser-marking. Suitable colorants are colour pigments, white pigments, black pigments, effect pigments and in particular coloured metal oxide pigments, and also organic pigments and dyes.
Uncoated graphite is available commercially, e.g. from Merck KGaA. The graphite here may be lamellar or of irregular shape.
Suitable graphite lamellae have an average particle size of from 0.1 to 200 &mgr;m, in particular from 0.5 to 20 &mgr;m, and irregularly shaped graphite particles have an average diameter of from 0.1 to 200 &mgr;m, in particular from 1 to 50 &mgr;m.
A coating is applied to the graphite particles in a manner known per se. For example, in the case of Turnbull's blue the coating can take place by treating an aqueous suspension of the graphite at an elevated temperature, preferably>50° C., and at a suitable pH, preferably<6, with an aqueous K
3
[Fe(CN)
6
] solution and with an aqueous iron(II) salt solution, with stirring. The pH is held constant by simultaneous titration with a base. The coated pigment is filtered off with suction, washed and dried. It is often advisable to add anionic and/or nonionic surfactants to improve properties.
The layer thickness is from 10 to 500 nm, preferably from 20 to 200 nm, in particular from 20 to 150 nm.
Graphite coated with CaCO
3
, CaSO
4
, Turnbull's blue or (Sn,Sb)O
2
is particularly suitable for laser-marking. Intermediate layers of SiO
2
, of SnO
2
and/or of TiO
2
may also be introduced if desired.
Particularly preferred dopants have the following compositions:
Graphite(substrate)+TiO
2
+CaCO
3
Graphite(substrate)+TiO
2
+CaSO
4
Graphite(substrate)+TiO
2
+Turnbull's blue
Graphite(substrate)+TiO
2
+SiO
2
+(Sn,Sb)O
2
Graphite(substrate)+SnO
2
+carbon black+TiO
2
Graphite(substrate)+Turnbull's blue.
Any of the known thermoplastics, e.g. those described in Ullmann, Vol. 15, p. 457 et seq., Verlag VCH, may be used for the laser-marking. Examples of suitable plastics are polyethylene, polypropylene polyamides, polyesters, polyester esters, polyether esters, polyphenylene ethers, polyacetal, polybutylene terephthalate, polymethyl methacrylate, polyvinyl acetal, polystyrene, acrylonitrile-butadiene-styrene (ABS), acrylonitrile-styrene-acrylate (ASA), polycarbonate, polyether sulfones, polyether ketones, and also mixtures and/or copolymers of these.
The incorporation of the coated graphite lamellae within the thermoplastic takes place by mixing the plastics pellets with the dopant, followed by shaping with exposure to heat. During incorporation of the dopant, the plastics pellets may, if desired, be treated with adhesion promoters, organic polymer-compatible solvents, stabilizers and/or surfactants resistant to the operating temperatures used. The doped plastics pellets are usually produced by placing the plastics pellets in a suitable mixer, wetting these with any additives, and then adding and incorporating the dopant. The plastics are generally pigmented by way of a colour concentrate (masterbatch) or compound. The resultant mixture may then be directly processed in an extruder or injection-moulding machine. The mouldings formed during the processing have a very homogeneous dopant distribution. Laser-marking with a suitable laser then takes place.
The invention also provides a process for producing the laser-markable plastics of the invention, characterized in that a thermoplastic is mixed with the dopant and then shaped with exposure to heat.
The method of inscription by the laser is such that the specimen is placed in the path of a pulsed laser beam, preferably an Nd:YAG laser. Inscription by a CO
2
laser, e.g. by using a mask technique, is also possible. The desired results can also be achieved by other conventional types of laser whose wavelength is within the region of high absorption of the pigment used. The mar

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