Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-02-18
2004-03-16
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, C523S216000, C106S436000, C106S466000, C106S479000, C106S480000, C106S482000
Reexamination Certificate
active
06706785
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the field of laser sensitive pigments for laser marking of plastics. Laser marking techniques may be used to replace conventional ink-printing techniques in marking plastic objects for identification or safety purposes, including markings such as date or batch codes, bar codes, and serial numbers. Laser marking is a clean and rapid process which produces permanent, rub-fast, scratch proof, and solvent resistant markings. The laser marking process does not generate waste or involve the use and disposal of hazardous solvents. Laser sensitive pigment additives having broad application with a variety of laser types and resin types would facilitate the laser marking process.
SUMMARY OF THE INVENTION
In accordance with the current invention, it has been discovered that laser sensitive pigments comprising particle substrates coated with metal oxides have wide ranging utility in the laser marking of plastics. The laser sensitive pigments may be used in conjunction with a wide variety of plastic compositions and in conjunction with different laser types.
As used herein, laser sensitive pigments means pigments that are capable of absorbing sufficient laser energy to generate a mark on a plastic substrate containing the pigment.
The metal oxide coating comprises at least one marking component which is CuO, MoO
3
, WO
3
, V
2
O
5
, Ag
2
O, or PbO
2
. The metal oxide coating may also comprise a host substrate component of either TiO
2
or SnO
2
or a mixture of TiO
2
and SnO
2
.
Laser marks on plastic are typically the result of carbonization of irradiated polymer material. In most instances, satisfactory laser marking quality cannot be achieved with the polymer material alone because carbonization does not occur effectively or selectively. The polymer material either does not absorb enough laser energy for carbonization to take place, or the polymer material becomes excessively burned at the surface when laser energy is applied. Laser sensitive additives can enhance the quality of laser marking. However, because the mechanisms behind laser marking are different for different laser types, additives specific to the laser type have typically been employed to obtain optimal laser marking results. CO
2
lasers have a laser frequency in the mid-infrared (IR) region (&lgr;=10600 nm), and preferred additives for CO
2
lasers have strong vibrational energy absorption in the mid-IR region and are highly heat resistant. The additives act as secondary heaters that absorb laser energy and raise the temperature of the surrounding polymer to very high levels, resulting in carbonization of the polymer. The laser markings are primarily the result of thermal processes. With ultra-violet (UV) excimer lasers (&lgr;<400), laser marks are mainly realized via photochemical processes. Nd:YAG lasers have wavelength frequencies in between the two extremes for CO
2
lasers and UV lasers, with frequencies at 1064 nm (near IR) and at 532 nm (visible) for an optically doubled frequency. For Nd:YAG lasers, both vibrational energy absorption and photochemical processes are necessary to achieve high quality marks.
As discovered herein, certain particle substrates coated with certain metal oxides may be employed as laser sensitive pigments for use in laser marking of plastics using different types of lasers. Sensitivity of the pigments to a particular laser type is primarily attributable to individual components of the coated substrates, although the other components may enhance laser sensitivity and marking contrast. For example, sensitivity to a CO
2
laser is primarily attributable to the particle substrate. Sensitivity to near IR and visible lasers is primarily attributable to the marking component(s) of the metal oxide layer, namely CuO, MoO
3
, WO
3
, V
2
O
5
, Ag
2
O, or PbO
2
and sensitivity to UV lasers is primarily attributable to the host component(s) of the metal oxide layer, namely SnO
2
or TiO
2
. Moreover, TiO
2
or SnO
2
, or a combination thereof, in the coating layer enhances the contrast of the CO
2
laser marks compared with uncoated mica. This result may be due to the fact that the coating oxide layer increases the selectivity of the laser induced carbonization and, as a result, its yield. The oxide layer may also become partially reduced upon laser irradiation, forming dark-colored sub-oxides which increase the marking contrast.
The laser sensitive pigments of the invention are relatively thermally stable and exhibit good dispersability and minimal color effects. During calcination, the metal oxide coating layer crystallizes into nanometer sized crystals after calcination. Due to their small particle size, they show relatively high transparency and are therefore compatible with other colorants used in the final applications. Additionally, because the nanometer-sized particles are imbedded onto micrometer-sized particles, they can be processed more easily as micrometer-sized particles.
A preferred substrate particle is a platelet-shaped substrate. Although particle substrates with other geometries may be employed in accordance with the invention, platelet-shaped substrates are preferred because they tend to orient themselves when dispersed in a plastic matrix so that their larger surface faces are parallel to the object surfaces. This orientation maximizes the efficiency of the pigment particles to couple laser energy. The use of particle substrates is preferred over substrate free powders. A thin layer coating of metal oxides on transparent substrates reduces the necessary loading levels in comparison to substrate free powders, and thereby reduces the color effect of the oxides. A reduced color effect from the oxides makes the laser sensitive pigment additive more compatible with other colorants. Coated substrates are also more easily dispersed within the resin composition than substrate free powders.
Platelet-shaped substrates include, but are not limited to, the following materials: natural or synthetic mica such as muscovite, phlogopite, and biotite; other sheet silicates, such as talc, kaolin or sericite; glass platelets, silica flakes and alumina flakes. Mica particles are preferred because of its relatively high absorption of laser energy, and therefore a higher sensitivity when used in conjunction with CO
2
lasers than other platelet-shaped substrates. Wet-ground muscovite is a preferred mica substrate. Mica particle sizes, as measured by light scattering methods, are preferably in the range of about 1 to about 150 &mgr;m, more preferably in the range of about 5 to about 100 &mgr;m, and most preferably in the range of about 10 to about 50 &mgr;m. The particle size range may affect marking quality in that large particles tend to provide a higher contrast in the mark, but also decrease the definition and smoothness of the mark, particularly for fine marks.
The components of the coating layer are chosen to optimize laser sensitivity. Combinations of CuO with MoO
3
or WO
3
showed the best laser sensitivity marking contrast for the Nd:YAG laser, although any single component of these oxides also yielded fairly good marks. V
2
O
5
may also be used in lieu of MoO
3
or WO
3
, although the use of V
2
O
5
may add a much stronger color to the material, which may be incompatible for use with light colored systems. Ag
2
O and PbO
2
may be used as marking components in the metal oxide layer. However, their use may be constrained because of their higher toxicity. These marking oxides may be used alone as the coating layer on mica flakes. Preferably, however, the marking oxides are further mixed with TiO
2
or SnO
2
or a combination thereof, wherein the TiO
2
, SnO
2
, or the TiO
2
/SnO
2
combination forms a host structure for the mixed oxide. The use of a host structure is advantageous in that the coating layer is more strongly bound to the substrate because TiO
2
and SnO
2
exhibit greater adherence to substrates than other oxides. The use of a host structure is also advantageous in that it increases the thermal stability of the mi
Cain Edward J.
Millen White Zelano & Branigan P.C.
Rona/EMI Industries, Inc.
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