Aluminum and aluminum alloy are more and more used to replace steel and many
non-metallic materials. Aluminum is a metallic element which is easy to react
with oxygen, so aluminum products can be formed a natural oxidation film whose
thickness is about 0.25x10-2 μm but this film has many weaknesses,
for example, its corrosion resistance and rubbing resistance is very poor (Du
et al., 2011) and products are single silvery white. The corrosion
resistance of aluminum and aluminum alloys can be greatly increased by forming
a thick oxide layer through the anodizing process in which the aluminum sample
is anodically polarized in an appropriate electrolyte to form a thicker oxide
layer than that of the naturally formed film (Cramer and
Covino, 2003), then we can use different dyes to colored it.
At present, the aluminums color quality control mainly applies visual
assessment, therefore the quality of the products fluctuates greatly. On the
other hand, in other fields such as frabic dyeing, technologies using computers
to measure color are ripe (Moiz et al., 2010),
through the man-machine dialogue we can carry out color measurement. These technologies
have many advantages: high speed, high accuracy, not being controlled by exterior
light and there is no visual difference between people. If they can be introduced
into the field of coloring metals, we can make colour management and quality
detection modernization, decrease cost shorten the cycle of production and effectively
improve the production efficiency.
THE METHOD OF THE COMPUTERIZED COLOR MEASURING INSTRUMENT
The structure: As Fig. 1 shows, it is the structure of
a computerized color measuring system, the light produced by the light source
catches the sample via an integral sphere and reflected light from the sample
is divided by the monochrometer which spreads the light into a spectrum, then
enters into a slit as monochromatic light in order of decreasing wavelength,
at last, it can be caught by a photoelectric detector, then be converted to
an electrical singal which a chart or computer records.
||The structure of the color measuring system. 1: Sample, 2:
Monochrometer, 3: Light source, 4: Integral sphere, 5: Photoelectric, 6:
A/D converter and recorder and 7: Slit
Transforming the electrical signals into digital codes that the computer can
process by a A/D converter and combining with the spectral energy distribution
of the light source, we can work out the spectral reflectance ρ of each
Calculative about color depth -K/S: The Kubelka-Munk equation provides
for calculation of a ratio, K/S, from measured reflectance values as is shown
in Eq. 1.
where, ρ∞ is the minimum fractional reflectance at a
specific wavelength and the bigger the K/S value, the deeper the color.
Calculative about color difference: According to the spectral reflectance
ρ of each measured wavelength, combined with Eq. 2-4,
the tristimulus values X, Y and Z of the sample can be worked out.
From the X, Y, Z color tristimulus values, lightness index L*, redness-greenness
index a* and blueness-yellowness index b* can be calculated following the equations
where, X0, Y0 and Z0 refer to the tristimulus
values of the illuminant which is used in the instrument. Total color difference,
Δ ECIE, is the actual distance in CIELAB color space between
samples and its calculation can be listed as following:
|| The relationship between ΔECIE and fading
|| The relationship between SSR and staining color fastness
Tests for color fastness: Color fastness is an important indicator to
measure the color quality of colored products and the changes of non-ferrous
metals in the application are closely involved with rubbing condition. Fastness
to crocking can be divided into two catalogs: fading color fastness and staining
color fastness. We can calculate the ΔECIE value between standard
examples and test samples, then according to Table 1 fading
color fastness is obtained, as for staining color fastness we should use the
ΔECIE value and Eq. 9 to work out SSR, then
according to Table 2 staining color fastness is obtained.
MATERIALS AND METHODS
The experiments were carried out using a type of aluminum named 6063 (3x10
cm, thinkness 1 mm). Concentrated sulfuric acid, concentrated nitric acid, phosphoric
acid, sodium hydroxide, sodium carbonate, sodium silicate sodium phosphate,
sodium dodecylsulphonate, ammonium dihydrogen phosphate, acetic acid, aluminium
oxide, the above chemical agents were purchased from J and K Chemical Co (China).
Trition x-100 non-ion dispersion agent, OP-10 emulsifying agent, dispersing
agent NNO were provided from Yancheng institute of technology. We also prepared
a ultra-fine azo-dye acid red G by high-energy planetary ball mill (The average
diameter of dyes was 135 nm and D90 was 206 nm).
||The structure of anodic oxidization system. E: DC electrical,
A: Source, V: Voltage meter, K: Switch, R: Electric resistance, 1: Cathode,
2: Anode and 3: Oxidation tank
|| The interface of the color measuring software
The technology route of aluminum oxidation and coloring: Degreasing→Alkaline
washing→Chemical polishing→Anodic oxidization→Coloring→Water
The experiment device of anodic oxidization is shown in Fig.
K/S value test: Using Coloreye7000A computer colour measuring system
(Gretag Macbeth, U.S.A) to test K/S value, The software interface is shown as
Color fastness test: Using Y(B)571-II Preset Rubbing Color Fastness
Tester (Wengzhou Da Rong Textile Standard Instrument Factory, China) to test
RESULTS AND DISCUSSION
Influence factors including dyeing temperature (T), the thickness of the oxidized-layer
(δ), pH value and dyeing time (t), which affected the qualities of aluminum
coloring were studied, respectively, their results are shown in Table
|| The effects of temperature on dyeing performance
|pH = 5, t = 30 min, δ = 16±0.5 μm
|| The effects of film thickness on dyeing performance
|pH = 5, t = 30 min, T = 60°C
|| The effects of dyeing time on dyeing performance
|pH = 5, T = 60°C, δ = 16±0.5 μm
|| The effects of pH value on dyeing performance
|T = 60°C, t = 30 min, δ = 16±0.5 μm
Table 1 indicates that at first K/S and color fastness increase
with the increasing heat treatment temperature, dyestuff particles easily spread
into the oxidation film and the dyeing rate is accelerated, K/S and color fastness
is improved but when Temperatures are above 70°C, they show a downward trend,
this phenomenon is casued by the speeding up rate of hydration reaction on the
oxidation film (Ye et al., 2009) and dyeing performances
can be suppressed, even be halted, because the oxidation film is in a half-closed
state. According to the data listed in the Table 3, the best
dyeing temperature will be 60°C.
The thickness of oxidation film can be adjusted by changing anode voltages,
in a certain range, improving voltage can accelerate the growing rate and the
thickness of the oxidation film but it is necessary to realize that too high
voltages result to the breakdown of the film. Table 4 indicates
that at first K/S and color fastness increase with the increasing film thickness,
thick oxide membrane can dye deeper color than thin oxide membrane because the
relationship is positive correlation between the thickness of oxidation film
and the ability of the dyes absorbance (Guo, 2004)
and thin oxide membrane is easy-to-wear but when the thickness reaches a certain
degree, the color depth will tend to be flat and the color fastness will decrease
due to two factors: firstly, the aggregation of the dye on the surface of the
film, which will lead to floating color (Walker et al.,
2001), secondly too high thickness results that some parts of the film is
easy to drop from the matrix (Zhou, 2011).
Table 5 indicate that K/S and color fastness increase with
the increasing dyeing time, general speaking, the longer the dyeing time, the
deeper the product color, furthermore too shorter dyeing time will cause bigger
color difference but too long dyeing time is also not suitable because the product
color wont be deeper by the
way extending the time after 30 min, at this time the adsorption capacity of
oxidation film is close to saturation.
The adsorption ability of oxidation film raised with the dropping of pH value.
That is because high concentration of H+ can provide larger surface
which has a lot of positive charge, absorbing anion dyes at acid conditions
but too low pH value will lead to lower the color fastness and during the sealing
process the color will be easy to partly lost. Furthermore, too low pH value
will dissolve oxidation film and form precipitation or partly make dyestuffs
lose their efficiency (Jung et al., 2009). According
to the data in the Table 6, the appropriate pH is in the range
The optimum products were obtained in solution with pH of 5, dyed at 60°C
for 30 min and the film thickness was controlled within 16±0.5 μm.
The technology parameters must be tightly controlled during the aluminum coloring
process and computer color measuring system should be utilized to monitor and
adjust the processing technology, thus the best quality products can be obtained.
Only using modern maintenance and scientific management, we can improve the
product quality, increase productivity and compress costs.
This project was supported by Yancheng zhongzhan Co.LTD and Yancheng Institute