Yong-Tao Yao，Guo-Jun Yan

(Science and Technology on Advanced Composites in Special Environments Laboratory，Harbin Institute of Technology，Harbin 15001，China)

1 Introduction

Pearlescent pigments are a classical type of luster pigments used for their ability to produce unusual optical effects.In the past few decades，these pigments have become popular in the creation of luster effects in decorative purposes(i.e.coatings，automotive paints，printed products or cosmetics)［1－3］. Currently，pearlescent pigments are also widely employed in security printing，optical filters or pharmaceutical field due to its unique color effects［4－7］.

The visual effects of pearlescent pigments are due to light interference，and pearlescent pigments usually contain thin platelets of semi-transparent materials which have high refractive index compared to the binder in which they are dispersed［8］. The pigments are lamella in form and become aligned to the coated surface in the binder film.The platelets consist of layers of inorganic materials;the incident light is reflected from the surfaces of platelets and phase boundaries in the multilayer.The light reflected directly from these platelets contributes the luster effects and causes the lightness，whilst multiple reflections give the depth of the color;besides，the interference colors occur when film thickness is in the range of the wavelength of visible light(shown in Fig.1)［9］.

Interference phenomena can be described that the light waves interact with each other destructively and constructively to cancelout or intensify certain wavelengths in the visible spectrum and lead to a range of colors being observed;it is responsible for iridescent colors and color change with viewing angle［10］.

In addition，absorption plays a part in luster pigments in case of the presence of the transparent inorganic pigments layers，e.g.Fe2O3on the platelets，which acts as the selectively light absorbing materials［11］.Color is observed when the incident light is absorbed or transmitted by the platelets［9］.Furthermore，in order to achieve maximize reflected light，the platelets must be extremely smooth.Non-fine particles or pigments with rough surface(or edges)can also diminish the affect of lustrous appearance［12］.

Fig.1 Optical effects in platelets layered pigments

Over the past three decades，the more researches had been concentrated on the study for natural fish silver，basic lead carbonate，bismuth oxychloride and titanium dioxide-mica etc.There is a lack of reporting the copper complex pearlescent pigment.In the present work，the copper complex pearlescent pigment is synthesized based on coupling diazotized anthranilic acid with naphthol AS-PH and cupric acetate.The characterization of final products is carried out by FTIR，DSC and SEM analysis.It has the advantages of the low cost，high luster，good hiding power，light stable and thermal stable，etc.

2 Experimental

2.1 Materials

Coupling components Naphthol AS-PH(99%)，sodium nitrite(pellets)，anthranilic acid(97%)，acetic acid(99%)，sulphonated caster oil(99%)and cupric acetate(98%)were all supplied by Ciba Specialty Chemicals.Hydrochloricacid(46% －48%)，sodium hydroxide pellets(98%)and DMF were supplied by Sigma-Aldrich Company Ltd.

2.2 Pigment Synthesis

1)(Direct)diazotization.

Anthranilic acid(6.85 g of 97%)was added to 15.2 g of HCl of 36%;5 mL water was added while stirring by using magnetic stirrer to give a thick paste which was stirred for 40 min and then was iced to 0－5℃.NaNO2(3.5 g in 15 mL water)was added dropwise over 10 min and allowed to work to an end point over 60 min.Then the solution was stored in fridge.

2)Coupling component.

NaOH(4.45 g of 47%)，sulphonated caster oil(1.38 g of 99%)and 150 mL water was heated to 75℃，and then the coupling component was added in portions until all solid had dissolved，whilst，NaOH was added dropwise to the mixture in order to complete dissolution.Then the solution was diluted，doubled in volume with water，and stirred while pH value was adjusted to range from 11－11.5.

3)Coupling method.

At room temperature，the diazo was added to the dissolved coupling component dropwise during 60 min giving a suspension which was adjusted for pH 10.It was stirred for 20 min until the pH value was controlled to range of 6.4－6.5 by addition of acetic acid of 20%，and then kept stirring for another 90 min.

4)Laking and solvent treatment.

The suspension was filtered and the press-cake was transferred to a conical flask.150 mL DMF was added and the suspension was heated about 70℃.44 gram of 98% of cupric acetate was added and the reaction was mass heated to reflux and held for 90 min.The suspension was then allowed to cool to room temperature and left standing for 2 d.The dried solid was filtered off and washed with ethanol until the washings were colorless.The final products(washed pigments)were air dried.

3 Results＆Discussion

3.1 Mechanism ofPigmentSynthesisand Morphology of the Final Product

Step 1－3 indicates the reaction scheme of diazotization and thecoupling mechanism，where Naphthol AS-PH and the potential coupling components are shown in Fig.2.The components varied with both nature and positions of R groups give the effects of final products on both physical and chemical properties.For example，in case of Naphthol AS-PH as a coupling component，in presence of substitute group‘－ OEt’，give a greenish color in shade;Naphthol AS-BS with‘－ NO2’group shows a bright red shade pigment;Naphthol AS-D with the methyl group gives a brownish color in shade and Naphthol AS-ITR with‘MeO-and Cl-groups’presents the brownish color，etc.

Step 1 Scheme of diazotization，as following:

Fig.2 Coupling components

In this project，Naphthol AS-PH as a coupling component，the final products give a greenish color in shade(shown in Fig.3).The copper coupling process was carried out in presence of DMF，and this provides ease of coupling reaction because of the absence of dipole forces from impurity(e.g. water). The presence of copper gives dramatic effect on the pigment performance，especially in the solvent fastness due to the increase of molecule weight［13］.For example，the final product can be washed with ethanol.

Fig.3 Macroscopic image of copper complex pigment，prepared as described in the experimental section

3.2 Purity Test by DSC

The purity and thermophysical properties of the copper complex pigments and coupling product were carried out by Different Scanning Calorimetry(DSC DuPont 2000 thermal analyzer)analysis.The 10 mg of the samples were pressed and fitted into aluminum pans with lids.The atmosphere was dry by nitrogen gas，and the heating rate was 10 ℃ ·min－1and the temperature range was between 50－400℃throughout the investigation.

DSC curves for coupling product(Naphthol ASPH)and the copper complex pigments are shown in Fig.4，respectively.Fig.4 indicates a negative double peak(with peaks at 65.36℃and 84.39℃)and a positive peak(at 302.79℃)are obtained in Naphthol AS-PH DSC curve.For the double peak of absorption heat in the Naphthol AS-PH，it may be caused by the loss of moisture，and the positive peak corresponding to its heat decomposition.DSC curve of the copper complex pigment remains nearly constant until 310℃and only one positive peak appear at 332.76℃.It reveals the purity of copper complex products is very high and also appears the excellent heat stability compared with Naphthol AS-PH due to the adding of copper atom.

3.3 Confirmation of the Metal Complex by FT-IR

A Perkin Elmer 1725X Fourier Transform Infra-Red Spectrometer(FT-IR)，supplied by Perkin Elmer Ltd.UK，was used to analyze the starting materials(NaphtholAS-PH)and the resulting pigment products，in order to confirm the chemical structure of the copper complex pigment obtained.FT-IR spectra of the samples are shown in Fig.5.

Naphthol AS-PH was used as the background for the FT-IR measurements.It is evident from Fig.5 that-OH group and Ph-N=N-Ph group are present in Naphthol AS-PH and copper complex pigment samples as indicated by the peaks located at 2800－3000 cm－1and 1442 cm－1，respectively.It matches the structure characterize of copper complex pigment(shown in Fig.3).

Fig.4 DSC of Naphthol AS-PH and copper complex pigment

Evidently，the amount of covalence bond created between the-OH group，Ph-N=N-Ph group and cupric ion after copper coupling reaction，the corresponding peaks of final product are reasonably dispread or decreased compared with Naphthol AS-PH as indicated in Fig.5.Based on the FT-IR and DSC analysis，it was thought that the metal complex has been achieved.

Fig.5 FT-IR spectrum of starting material(Naphthol ASPH)and the copper complex pigment

3.4 SEM Analysis of the Particle Surface

Fig.6 presents the SEM image of copper complex pigments particles， the uniformity and order arrangement of primary particles is observed.The primary particles join together face to face，which gives high density of the particles.This structure can give pigment excellentfastnessproperty.Itowesthis structure to the strong intra-molecular force arising from the three dimensional co-ordinal bonding due to the presence of copper in the pigment molecules.In addition，the image shows the uniform particle size distribution and the average particle size is around 15 μm.Larger particles are better reflectors leading to higher brilliance and brightness［14］. The copper complex structure can be formed successfully.

Fig.6 SEM image of the copper complex pigment

In this research，the criticalfactors are temperature and pH in synthesis the copper complex pearlescent pigment with desired crystal.For example，in the first step of diazotization，it is the exothermic nature ofthe reaction，combined with the heat sensitivity of most diazonium salts;thus，the diazotization reaction has to be carried out under the cooling condition in order to avoiding diazonium salts by thermal decomposition.However，the control of the degree of pH is another essential factor in achieving the smooth reaction and reducing the by-product during the azo coupling process.

4 Conclusions

In this study，the copper complex pearlescent pigment with greenish color was successfully synthesized by the two steps diazotization and azo coupling.The crystal phases and the microstructure of the copper complex pearlescent pigment powders were characterized through a combination of a FT-IR and SEM analysis after purification.The uniform particle size distribution was obtained by SEM analysis and the average particle size is around 15 μm.The physic and the optical property of pigments have been dramatically changed by the introduction of the copper to the pigmentmolecule.The nature and position of substitute group ataromatic ring has also been demonstrated as the important factor on the copper complex pigment performance since the increase of the molecular weight［13］.

［1］Greenstein L M.Pigment Handbook.New York:John Wiley＆ Sons Press，1998.

［2］Gunter B.Industrial Inorganic Pigments，5.3 Luster Pigments.New York:Wiley-VCH Press，1998.213 －221.

［3］Pfaff G，Reynders P.Angle-dependent optical effects deriving from submicron structures of films and pigments.Chemical Reviews，1999，99(7):1963－1981.

［4］Macleod H A.Thin-film Optical Filters.Bristol:Adam Hilger Ltd.Press，1986.

［5］Knoth M，Miehe G，Fuess H.Characterization of pearl luster pigments.Advanced Engineering Materials，2005，7(10):928－931.

［6］Stengl V，Subrt J，Bakardjieva S.The preparation and characteristics of pigments based on mica coated with metal oxides.Dyes and Pigments，2003，58(3):239 －244.

［7］Song G B，Liang J K，Liu F S.Preparation and phase transformation of anatase-rntile crystals in metal doped TiO2/muscovite nanocomposites.Thin Solid Films，2005，491(1/2):110－116.

［8］Pfaff G，Gabel P，Kieser M，et al.Special Effect Pigments.Hannover:Vincentz Press，2008.

［9］Bamfield P.Chromic Phenomena Technological Applications of Colour Chemistry.Cambridge:RSC Press，2001.324 －325.

［10］Nadal M E，Early E A.Color measurements for pearlescent coatings.Color Research ＆ Application，2004，29(1):8－42.

［11］Thimsen E，Formal F L，Grǎtzel M，et al.Influence of plasmonic Au nanoparticles on the photoactivity of Fe2O3electrodes for water splitting.Nano Letter，2011，11(1):35－43.

［12］Liang X J，Xu H Q，Chen J，et al.Research of mica/Fe3O4pearlescentpigmentby co-precipitation.Glass Physics and Chemistry，2011，37(3):330 －342.

［13］Christie R M.Colour Chemistry.Cambridge:RSC Press，2001.160.

［14］Maile F J，Pfaff G，Reynders P.Effect pigments-past，present and future.Progress in Organic Coatings，2005，54(3):150－163.