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This invention relates to a marking method for a polyolefin
resin.
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Molded articles made of a polyolefin resin, such as personal
articles, domestic appliances, interior and exterior parts and
engine parts of automobiles, are often marked with letters, patterns,
symbols, etc.
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Such marks can be put on the molded articles by applying
thermosetting or UV-curing ink, but ink marking methods are of low
productivity, take time for ink application and baking and involve
many steps. Besides, ink marks lack durability, and tend to fall
off due to insufficient ink adhesion and insufficient resistance
to solvents or chemicals. Productivity could be improved by
sticking an adhesive label having marks onto a resin molded article,
but the durability of the adhesive label is similarly insufficient.
In addition, the inks or adhesives must be removed from the molded
articles to be recycled, which has reduced the applicability for
recycling.
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On the other hand, marking by laser beam processing,i.e.,
laser marking can be carried out easily and rapidly to achieve
markedly improved productivity. In laser marking, a laser beam is
applied to a resin molded article having incorporated therein a black
pigment which synchronizes with the wavelength of the laser beam to
cause the black pigment to burn and evaporate rapidly. As a result,
only the black pigment of the laser beam-irradiated area is released
to present a contrast between the irradiated area (marked area) and
non-irradiated area (background). According to this technique,
since the marking is to release only the pigment, the marks are very
excellent in solvent resistance, chemical resistance and durability,
and the thus marked articles are highly practical for recycling.
For these reasons, various laser marking techniques have been
studied recently.
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For example, JP-A-1-254743 (the term "JP-A" as used herein
means an "unexamined published Japanese patent application")
teaches that incorporation of titanium dioxide and carbon black into
a resin makes laser marking possible. However, when this method
is applied to a polyolefin resin composition, the laser marks formed
have a brown or light brown color. The background being black, the
marks lack a sufficient contrast to the background and have
insufficient visibility.
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Various proposals have been made in order to make laser
marks white. For example, JP-A-4-246456 discloses a technique in
which carbon black and/or graphite having high thermal conductivity
is incorporated into a polyester resin so as to provide white laser
marks. JP-A-7-238210 teaches that an epoxy resin composition
containing carbon black, an antioxidant, and a blue colorant in
specific ratios provides white laser marks. However, application
of these techniques to a polyolefin resin composition fails to
achieve sufficient whitening of laser marks.
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A polyolefin resin composition for laser marking which
comprises a polyolefin resin and a black pigment mainly comprising
a metal oxide, which composition is capable of forming
a white mark with improved visibility is also known (see JP-A-10-273537).
However, since a black pigment comprising a metal
oxide, which is more expensive than carbon black, has less
coloring power than carbon black, it should be added in a
larger amount, which leads to an increase of costs.
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An object of the present invention is to provide a laser
marking method for a molded article of a black-colored polyolefin
resin composition by which a white mark having improved visibility
in clear contrast against the black background can be formed.
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This object has been achieved by the surprising finding
that incorporation of carbon black having specific properties
into a polyolefin resin makes the polyolefin resin capable of forming
a white mark with improved visibility on its surface upon being
irradiated with a laser beam.
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The present invention provides a method for marking a
polyolefin resin comprising irradiating a polyolefin resin
composition containing 0.1 to 1.0 part by weight of carbon black
having an average secondary particle size of not smaller than 150 nm
per 100 parts by weight of the polyolefin resin composition with
a YAG laser.
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In the following, preferred embodiments the invention shall be
illustrated.
[I] Polyolefin Resin Composition
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The polyolefin resin composition which can be used in the
present invention comprises a polyolefin resin and carbon black.
(1) Polyolefin Resin
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The polyolefin resin used in the present invention is not
particularly limited, and those generally used in polyolefin molded
products can be used. Suitable polyolefin resins include ethylene
resins, such as ethylene homopolymers and ethylene copolymers, e.g.,
ethylene-α-olefin (e.g., propylene) copolymers; propylene resins,
such as propylene homopolymers and propylene-α-olefin random or
block copolymers; and other α-olefin resins, such as polybutene-1,
poly-4-methylbutene-1, poly-3-methylbutene-1, and poly-4-methylpentene-1.
In addition, olefin copolymers comprising
ethylene or propylene and copolymerizable monomers, such as
unsaturated carboxylic acids or derivatives thereof (e.g., acrylic
acid, methyl methacrylate, ethyl acrylate, and maleic anhydride),
aromatic unsaturated monomers (e.g., styrene and α-methylstyrene),
vinyl esters (e.g., vinyl acetate and vinyl butyrate), vinylsilanes,
etc.; and saponification products or metal-ionized products of
these copolymers are also useful. These polyolefin resins can be
used either individually or as a mixture thereof.
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Preferred of them are ethylene resins, such as ethylene
homopolymers and ethylene-propylene copolymers; and propylene
resins, such as propylene homopolymers, propylene-ethylene random
copolymers, propylene-ethylene block copolymers, and propylene-ethylene-butene
copolymers. The ethylene content of the
ethylene-propylene copolymer mainly comprising ethylene is about
60 to 95% by weight, and that of the propylene-ethylene random or
block copolymer mainly comprising propylene is about 0.5 to 20% by
weight.
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Specific examples of suitable polyolefin resins are
high-density polyethylene, medium-density polyethylene, low-density
polyethylene, linear low-density polyethylene, branched
low-density polyethylene, ethylene-propylene copolymers,
propylene homopolymers, propylene-ethylene random copolymers,
propylene-ethylene block copolymers, propylene-ethylene-butene
copolymers, polybutene-1, poly-4-methylbutene-1, poly-3-methylbutene-1,
and poly-4-methylpentene-1.
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Particularly suitable polyolefin resins are propylene
resins, such as propylene homopolymers and propylene-ethylene block
or random copolymers. Propylene-ethylene block copolymers are
especially preferred.
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The polyolefin resin usually used in the present invention
should preferably have a melt flow rate (MFR) of about 0.1 to
300 g/10min, more preferably about 1 to 150 g/10 min.
(2) Carbon Black
(a) Types
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Carbon black is classified according to the process of its production into
furnace black, channel black, thermal black, etc. and by raw material
into acetylene black, ketjen black, oil black, gas black, etc. Any
of these carbon black species can be used in the present invention.
Acetylene black and ketjen black that have high electrical
conductivity are particularly preferred.
(b) Particle Size (average primary particle size)
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It is usually preferred for the carbon black used in the
present invention to have an average primary particle size of not
smaller than 30 nm, preferably from 40 to 150 nm, still preferably
from 60 to 120 nm.
(c) Aggregate (average secondary particle size)
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It is important for the fine carbon black particles having
the above particle size to agglomerate to form secondary particles
(aggregate) having a diameter of not smaller than 150 nm, preferably
from 150 to 1,000 nm, still preferably from 200 to 500 nm. If carbon
black having an average secondary particle size (average aggregate
size) of smaller than 150 nm is used, the mark assumes a pale brown
color having reduced visibility.
Measurement Method:
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The primary and secondary particle sizes of carbon black
can be measured as follows.
Measurement of Average Primary Particle Size:
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The maximum diameters of selected particles are measured
under an electron microscope to obtain a number average.
Measurement of Average Secondary Particle Size:
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A frequency distribution curve of Stoke's equivalent
diameter of secondary particles is prepared according to a
centrifugal sedimentation method by means of a disc centrifuge
manufactured by Joyes Loebl Co., G.B. and the 50% diameter of the
curve is read.
(3) Compounding Amount
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The carbon black is used in an amount of 0.1 to 1.0 part
by weight, preferably 0.1 to 0.7 part by weight, particularly
preferably 0.2 to 0.5 part by weight, per 100 parts by weight of
the polyolefin resin composition. If the amount of the carbon black
is less than 0.1 part, absorption of laser energy is insufficient
for making marks. On the other hand, more than 1 part of carbon
black absorbs excessive laser energy to generate excessive heat
while being released, which will cause the resin to change its color.
It follows that the marked area turns light brown due to scorching
and is poorly visible.
(4) Additional ingredients
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If desired, the polyolefin resin composition used in the
present invention can contain inorganic fillers, such as talc and
glass fiber.
(a) Inorganic Filler
Talc
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Talc which can be used in the present invention
preferably has an average particle size of not greater than 5
µm, more preferably from 0.5 to 3 µm, and a specific surface
area of preferably not less than 3.5 m2/g, more
preferably from 3.5 to 6 m2/g. The average particle size is obtained
as a 50% diameter of a cumulative distribution curve determined
according to a liquid phase sedimentation photo-extinction method
by use of, e.g., Model CP manufactured by Shimadzu Corp. The
specific surface area is measured by an air permeation method by
use of, e.g., Model SS-100 (constant pressure type) manufactured
by Shimadzu Corp. Talc to be used is prepared by, for example, dry
grinding followed by dry classification.
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For the purpose of improving dispersibility in the
polyolefin resin, talc can be treated with various surface treating
agents, such as organic titanate coupling agents, organic silane
coupling agents, fatty acids, fatty acid metal salts, and fatty acid
esters.
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Talc is preferably added in an amount of 1 to 60 % by weight
based on the polyolefin resin composition.
Glass Fiber
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Glass fiber which can be used in the present invention
includes glass fiber having been treated with silane coupling agents,
such as aminosilanes (e.g., γ-aminopropyltriethoxysilane),
epoxysilanes (e.g., γ-glycidoxypropyltrimethoxysilane) and
vinylsilanes (e.g., vinyltrichlorosilane).
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Glass fiber having an average fiber diameter of 5 to 25 µm,
particularly 8 to 15 µm, is preferred. If the fiber diameter is less
than 5 µm, the productivity of strands as well as glass fiber-reinforced
resin is considerably reduced and the production costs
are increased. Glass fiber thicker than 25 µm tends to have a too
broad distribution of residual fiber length, which deteriorates
the appearance of the molded article, and the aspect ratio of
glass fibers is diminished so that the degree of improvement
in mechanical properties such as flexural modulus is reduced.
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Glass fiber strands usually consist of 100 to 5,000
filaments, preferably 300 to 3,000 filaments, still preferably 500
to 2,000 filaments. As to the glass composition, alkali-free glass,
such as E glass, is preferred. The glass fiber length is usually
2 to 20 mm, preferably 3 to 10 mm, still preferably 4 to 9 mm,
particularly preferably 5 to 8 mm. Glass fiber is preferably used
in an amount of 1 to 70% by weight, particularly 10 to 40% by weight,
based on the polyolefin resin composition.
(b) Other additional ingredients
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If desired, the polyolefin resin composition can further
contain other additives as far as the effects of the present
invention are not impaired. Useful additives include phenol type,
sulfur type or phosphorus type antioxidants; benzophenone type or
benzotriazole type weathering agents; nucleating agents, such as
organoaluminum compounds, ultraviolet absorbers, organophosphorus
compounds, and sorbitol compounds; and dispersants.
[II] Marking
(1) YAG laser
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In the present invention it is essential to use a YAG laser for
laser marking. A YAG laser is a solid state laser using an yttrium-aluminum-garnet
(Y3Al5O12) generally doped with about 1% Nd3+ and has
near infrared output at a wavelength of 1.06 µm It is capable of
pulse oscillation on excitement with light of a xenon flash lamp
and continuous oscillation on excitement with continuous light from
a tungsten iodine lamp, a krypton arc lamp, etc.
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If a carbon dioxide gas laser is used in place of a YAG laser,
the surface of the molded article is only etched with little release
of the irradiated pigment so that the resulting mark lacks clear
contrast against the background and is not clearly visible.
(2) Irradiation
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Laser marking on a molded article with a YAG laser can be
carried out by imagewise scanning the molded article with a laser
beam or irradiating the molded article with a laser beam through
a mask. The output of the laser may be continuous or pulsating
(normal or Q switch pulses).
[III] Use
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The mark thus formed on the polyolefin resin, being white
in color, is distinctly visible in clear contrast against the
background. Therefore, the present invention is suitably applied
to marking on various polyolefin molded articles, such as personal
articles, domestic appliances, interior and exterior parts and
engine parts of automobiles, with letters, patterns, and symbols.
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The present invention will now be illustrated in greater
detail with reference to Examples.
EXAMPLES 1 TO 4 AND COMPARATIVE EXAMPLES 1 TO 6
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A molded article of the polyolefin resin composition shown
in Table 1 below was marked with a YAG laser or a carbon dioxide
gas laser under the following conditions, and the contrast between
the mark and the background and the visibility of the mark were
evaluated in accordance with the following methods. The results
obtained are shown in Table 1.
YAG Laser Marking Conditions:
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- Apparatus:
- Laser Marker Engine SL475H, manufactured by
NEC Corporation
- Wavelength:
- 1.06 µm (Nd:YAG laser)
- Frequency:
- 10 Hz
- Output:
- 6 W
- Aperture:
- 2.0 mm
- Scanning speed:
- 700 mm/sec
Carbon Dioxide Gas Laser Marking Conditions:
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- Apparatus:
- Xymark™, manufactured by Lumonix·Pacific Co.
- Wavelength:
- 10.6 µm
- Output:
- 100 W
- Scanning speed:
- 20 m/min (330 mm/sec)
Method of Evaluation of Laser Marking Properties:
1) Contrast
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The lightness of the mark and the background was measured
with MMP-300A manufactured by Nihon Denshoku Kogyo K.K. to obtain
a lightness difference (ΔL) . The greater the difference, the
clearer the contrast.
2) Visibility
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The visibility of the mark was organoleptically evaluated
with the naked eye and rated A (good), B (poor) or C (very poor).
| | Example | Comparative Example |
| | 1 | 2 | 3 | 4 | 1 | 2 | 3 | 4 | 5 | 6 |
| Composition (part by wt.) |
| Polypropylene resin | 100 | 100 | 60 | 80 | 100 | 100 | 60 | 80 | 100 | 100 |
| Carbon black A | 0.3 |
| Carbon black B | | 0.3 | 0.3 | 0.3 | | | | | 1.2 | 0.3 |
| Carbon black C | | | | | 0.3 |
| Carbon black D | | | | | | 0.3 | 0.3 | 0.3 |
| Talc | | | 40 | | | | 40 |
| Glass fiber | | | | 20 | | | | 20 |
| Irradiation Laser | YAG | YAG | YAG | YAG | YAG | YAG | YAG | YAG | YAG | CO2 |
| Results of Evaluation |
| L value on background | 14 | 16 | 24 | 17 | 13 | 11 | 19 | 13 | 8 | 16 |
| L value on mark | 50 | 56 | 48 | 52 | 43 | 38 | 32 | 37 | 30 | 20 |
| Contrast (ΔL) | 36 | 40 | 24 | 35 | 30 | 27 | 13 | 24 | 22 | 4 |
| Visibility | A | A | A | A | B | B | C | B | B | C |
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The marking method of the present invention provides a
polyolefin resin molded article having highly visible marks such
as letters, patterns, and signals in clear contrast against the
background.