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The application of x-ray powder diffraction for the analysis of synthetic organic pigments. Part 1: dry pigments.

Abstract This paper presents x-ray powder diffraction data for over 200 synthetic organic pigments. These pigments, most manufactured in the last 130 years, are frequently found in modern works of art. Their identification is of interest in the field of art conservation for the purposes of dating works of art as well as making conservators and curators aware of issues with lightfastness and solubility. Most classes of these pigments, including [beta]-naphthol, Naphthol AS, mono- and di-arylide yellows, quinacridones, copper phthalocyanines, benzimidazolones, and perylenes give good diffraction data. Some pigments, including certain triarylcarbonium and some other metal containing pigments, especially aluminum containing pigments, were found not to diffract. X-ray powder diffraction is of great use in distinguishing polymorphs of pigments such as quinacridones and copper phthalocyanines.

Keywords Synthetic organic pigment, X-ray powder diffraction, Polymorphism, Copper phthalocyanines, Quinacridones

Introduction

The term "synthetic organic pigment" refers to manufactured colorants containing carbocyclic ring systems, many of which are aromatic. Some synthetic organic pigments contain a metal ion, but because of the carbon containing ring system, they are considered organic pigments. These pigments must also be differentiated from natural organic pigments that come from plant or animal sources, some of which are now man-made. Synthetic organic pigments refer to those pigments that have no counterparts in nature, but are manufactured.

Synthetic organic pigments span the entire color range from violet to blue, green, yellow, orange, and red. Most of the pigments, however, are yellow, orange, or red. Synthetic organic pigments dominate the colorant market, and are used in a variety of applications, including printing and pen inks, paints (including architectural, industrial, automotive, and artists'), and for coloration of textiles, rubber, and plastics.

These pigments exhibit a wide range of physical and chemical properties, including lightfastness, heat stability, solubility, and stability to recrystallization. The properties of a given class are usually fairly consistent, although this is not always the case. Different grades of an individual pigment might be manufactured for a particular property such as improved lightfastness or opacity.

The identification of synthetic organic pigments is of great interest in the field of art conservation. Many of the early synthetic organic pigments were azo pigments (containing the -N=N- linkage). Lightfastness was often problematic with these early pigments. From the point of view of art conservation, their identification in works of art may provide information about the original appearance. In addition, lightfastness information can help to make decisions about display. Since these synthetic organic pigments are used in materials such as textiles, inks, and plastics, there is concern about their lightfastness properties given that many modern works of art are multimedia. As these pigments have specific dales of patent, the earliest date of manufacture of a work of art is easily known. This information is of interest to conservators and curators for attribution and authenticity information as well as in helping to determine original materials from previous conservation treatments. Finally, as conservation treatments usually involve varnish removal or cleaning with solvents, knowledge of the solubility or sensitivity of a pigment to a certain solvent would be critical in planning a conservation strategy.

It is often very difficult to identify synthetic organic pigments. They have a much higher tinting strength than inorganic or mineral pigments, and so are frequently present in relatively small quantities. Large amounts of fillers and extenders, often barium sulfate or calcium carbonate, complicate analysis by spectroscopic techniques. Additionally, identification by traditional techniques used for mineral pigments, including polarized light microscopy is often impossible due to the small particle size. The structural similarity of many of the pigments can also make identification difficult.

A number of different analytical methodologies have been used for the identification of synthetic organic pigments, and include microscopic, spectroscopic, and chromatographic techniques. TEM has been applied to the analysis of arylide yellows. (1) Microchemical tests have also been used for various synthetic organic pigments, including exposure to strong acids or bases. (2), (3) A number of researchers have used spectroscopic techniques including infrared (4), (5) and Raman spectroscopy (6), (7) as well as solution spectrophotometry. (8-10) These analyses have been on both dry pigment samples and also from works of art. (11-17) Chromatographic techniques such as thin layer chromatography (TLC), (18-20) high performance liquid chromatography, (21) and pyrolysis gas chromatography/mass spectrometry (14), (22-24) have also been used, as well as mass spectral techniques. (25-27)

Problems exist with all of these methods of analysis. In general, chromatographic techniques can be problematic as some classes of synthetic organic pigments have limited solubility in solvents used for liquid chromatography, or are of insufficient volatility for gas chromatography or even pyrolysis gas chromatography. Fillers and extenders frequently complicate analysis by x-ray powder diffraction, infrared, or Raman spectroscopies. The most successful strategies for analysis have often employed a combination of techniques. (28)

The author has been exploring infrared spectroscopy and mass spectral techniques in the analysis of synthetic organic pigments to develop strategies for their identification in artists' materials. (27) A group of over 200 dry synthetic organic pigments was acquired from pigment manufacturers. * Most of the pigments were examined by Fourier-transform infrared spectroscopy and direct temperature-resolved mass spectrometry (DTMS), techniques that proved complementary in attempts to identify the pigment. Infrared spectroscopy provided information about the pigment class, and could, in some cases, be used to identify the individual dry pigment. Mass spectral analysis was very successful for most of the pigments, including pigments that, because of inherent stability due to conjugation, do not easily pyrolyze, such as quinacridones and copper phthalocyanines. DTMS was also useful for neutral organics and some organometallic pigments, but could not be universally applied to analyze all pigments.

Monochromatic x-rays interact with a crystalline material following a relationship referred to as Bragg's law

n[lambda] = 2d sin [theta]

where [lambda] is the wavelength of the x-ray, d is the interplanar distance, n is the order, and [theta] is the glancing angle of incidence of the x-ray beam. This diffraction takes place at angles and intensities that are characteristic of each crystalline material, and therefore, the diffraction pattern should serve to identify a compound. Of particular benefit to the field of art conservation are the facts that only very small samples are necessary for the analysis, and that the sample is not destroyed during the analysis and can be used for other analytical techniques. In addition, it is also useful in that there is essentially no sample preparation for the analysis.

Therefore, these analyses were expanded to examine the pigments by x-ray powder diffraction. While some of the pigments have been characterized by x-ray powder diffraction, many of them have not. * (29), (30) X-ray powder diffraction has been applied to the analysis of synthetic organic pigments, most notably arylide yellows. (31-35) Copper phthalocyanines have also been studied by x-ray diffraction (36) as well as the [alpha], [beta], and [gamma] forms of linear trans-quinacridone. (37), (38) This first paper addresses the diffraction patterns of the dry pigment samples, while the second paper addresses the analysis of over 50 commercial artists' paints.

Experimental

X-ray diffraction patterns were obtained using a Bruker D8 DISCOVER with GADDS microdiffractometer equipped with a Hi-Star area detector. Samples were not ground before analysis, but used as supplied by the pigment manufacturer. ** The sample was placed onto the surface of a zero background plate that was subsequently centered on the stage. Correct placement of the sample in XYZ space was achieved using a video microscope with laser-assisted focus. Beam conditions included a Cu anode at 45 kV and 35 mA to produce Cu K[alpha] radiation ([lambda] = 1.542 [Angstrom]) through a 500 [micro]m collimator in air. Reflections were collected using a two-dimensional general area diffraction detection system (GADDS) set up for a single run, five frames, coupled (step) mode, with XY oscillation (0.1 mm amplitude). Theta 1 and 2 starting angles were 8 and 8 with a frame width of 6. Runtime for each frame was 45 s. Debye ring data were integrated over chi. Integrated frame data were combined and background corrected using EVA (a Bruker proprietary diffraction pattern evaluation program). Phase identification was accomplished using EVA through a link to the International Center for Diffraction Data (ICDD) database on CD (Powder Diffraction File Release 2000) and the GADDS workstation hard drive. Corundum was used to calibrate the instrument. Relative intensities were calculated from peak heights.

Results

X-ray powder diffraction patterns were collected for over 200 dry pigment samples. These include duplicate samples from both the same and different pigment manufacturers. Therefore, when the same pigment from the same supplier is listed more than once in the tables, the samples were obtained at different times. Wherever possible, the pigments were independently characterized by FTIR and DTMS techniques to verify structures. A CD with graphical data for the pigments is available on request.

Table 1 lists the five most intense lines of the arylide yellow pigments. Formerly known as Hansa Yellows, this is a class of monoazo pigments that have acetacetarylamide coupling components. These pigments are widely used in paints, colored pencils, crayons, and drawing inks and are characterized by good (not exceptional) lightfastness and high solubility in most organic solvents. The two most commonly used pigments historically in this class for artists' materials have been PY1 and PY3. *
Table 1: Diffraction data for arylide yellow pigments

CI pigment name  Supplier          d spacings (intensity)
(number)

PY1 (11680)      Heubach   10.11 (35), 5.68 (26), 5.19 (25), 3.77
                           (24), 3.28 (100)

PY1 (11680)      Sun       10.2 (85), 7.25 (23), 5.69 (27), 3.78
                           (24), 3.28 (100)

PY1:1            Avecia    10.3 (89), 7.31 (21), 5.75 (23), 3.78
                           (23), 3.29 (100)

PY2 (11730)      Sansui    9.44 (10), 7.70 (60), 6.55 (33), 3.57
                           (12), 3.38 (100)

PY3 (11710)      Heubach   6.88 (12), 4.61 (14), 3.61 (14), 3.31
                           (100), 3.04 (12)

PY3 (11710)      Sun       6.88 (26), 4.60 (18), 3.31 (100), 3.25
                           (50), 3.03 (18)

PY4 (11665)      EC        10.4 (22), 6.36 (49), 6.23 (82), 3.51
                           (27), 3.20 (100)

PY6 (11670)      Kremer    10.3 (56), 7.28 (39), 5.72 (33), 4.60
                           (23), 3.30 (100) (a)

PY65 (11740)     Sun       8.48 (100), 7.46 (35), 6.87 (11), 4.82
                           (13), 3.36 (84)

PY65 (11740)     Sun       8.47 (75), 7.48 (27), 4.48 (18), 3.46
                           (30), 3.37 (100)

PY73 (11738)     Clariant  8.06 (36), 4.17 (40), 3.82 (27), 3.52
                           (80), 3.25 (100)

PY73 (11738)     Sun       8.19 (38), 4.18 (38), 3.83 (25), 3.52
                           (78), 3.25 (100)

PY74 (11741)     Ciba      11.7 (17), 7.43 (100), 4.97 (14), 3.49
                           (28), 3.33 (82)

PY74 (11741)     Kremer    11.9 (13), 7.54 (100), 5.04 (15), 3.52
                           (38), 3.35 (88)

PY74 (11741)     Sun       7.48 (72), 4.99 (14), 3.56 (17), 3.49
                           (32), 3.33 (100)

PY75 (11770)     Lansco    14.6 (13), 7.33 (33), 4.30 (39), 3.56
                           (12), 3.36 (100)

PY75 (11770)     Sun       14.8 (22), 7.38 (55), 7.06 (31), 4.31
                           (39), 3.36 (100)

PY97 (11767)     Clariant  9.89 (100), 8.21 (50), 3.84 (28), 3.48
                           (37), 3.25 (33)

P01 (11725)      Hoechst   11.2 (41), 7.82 (95), 6.98 (60), 3.40
                           (71), 3.35 (100)

(a) Sample does not match ICDD, appears to be PY1


Table 2 presents the five most intense lines for the diarylide yellow and orange pigments. These pigments are disazo products of 3,3'-dichlorobenzidine with two equivalents of acetoacetarylide and are widely used in the printing ink industry. Diarylide yellows are not as lightfast as the monoarylide yellows, but have better heat and solvent fastness.
Table 2: Diffraction data for diarylide yellow and orange pigments

CI pigment name  Supplier            d spacings (intensity)
(number)

PY12 (21090)     Sun       8.37 (100), 4.60 (19), 3.50 (89), 3.35
                           (28), 3.11 (27)

PY13 (21100)     Ciba      11.7 (21), 8.06 (88), 5.21 (22), 3.99 (24),
                           3.32 (100)

PY13 (21100)     Sun       11.6 (23), 7.96 (95), 5.20 (23), 3.97 (26),
                           3.30 (100)

PY14 (21095)     Sun       10.5 (26), 7.68 (79), 5.35 (26), 3.57 (29),
                           3.32 (100)

PY14 (21095)     Sun       10.5 (29), 7.74 (86), 5.37 (27), 3.58 (32),
                           3.33 (100)

PY14 (21095)     Magruder  10.5 (34), 7.71 (81), 5.25 (27), 3.56 (30),
                           3.33 (100)

PY16 (20040)     Clariant  33 (20), 9.94 (27), 6.93 (71), 5.30 (24),
                           3.44 (100)

PY17 (21105)     Sun       7.94 (100), 4.41 (20), 3.69 (33), 3.39
                           (50), 3.12 (14)

PY17 (21105)     Sun       7.99 (100), 4.43 (22), 3.72 (38), 3.41
                           (54), 3.14 (17)

PY55 (21096)     Albion    9.27 (58), 6.40 (29), 4.42 (26), 4.26 (28),
                           3.31 (100)

PY81 (21127)     Clariant  7.25 (58), 5.62 (15), 5.04 (21), 3.99 (10),
                           3.47 (100)

PY83 (21108)     Sun       13.0 (28), 9.02 (36), 8.53 (49), 6.67 (24),
                           3.50 (100)

PY83 (21108)     Sun       13.0 (39), 8.55 (37), 6.63 (22), 3.51
                           (100), 3.24 (33)

PY83 (21108)     Magruder  13.0 (20), 8.43 (30), 3.49 (100), 3.23
                           (31), 3.11 (25)

PY126 (21101)    Clariant  8.48 (100), 7.61 (35), 6.09 (25), 3.51
                           (87), 3.36 (31)

PY127 (21102)    Lansco    8.40 (95), 5.70 (20), 4.60 (24), 3.50
                           (100), 3.34 (33)

PY155 (200310)   Clariant  8.75 (68), 7.91 (21), 5.29 (19), 4.59 (23),
                           3.31 (100)

PO16 (21160)     Ciba      7.50 (29), 7.17 (43), 6.30 (42), 3.59 (20),
                           3.19 (100)

PO34 (21115)     Sun       8.84 (97), 6.01 (14), 5.41 (14), 4.76 (13),
                           3.40 (100)

PO34 (21115)     Sun       8.83 (94), 5.97 (11), 5.48 (12), 4.73 (12),
                           3.41 (100)


Diffraction data for [beta]-naphthol pigments are presented in Table 3. These are a group of red pigments based on the coupling of a substituted aniline with [beta]-naphthol. Historically, they were among the first pigments produced, and are relatively inexpensive. In general, these pigments have poor solvent fastness and often poor lightfastness. Important members of this class include dinitroaniline orange (PO5), Toluidine red (PR3), chlorinated para red (PR4), parachlor red (PR6), and the BON pigments (PR48, PR52, PR57, and PR63).
Table 3: Diffraction data for [beta] naphthol pigments

CI pigment name    Supplier             d spacings (intensity)
(number)

PR1 (12070)      Haagen       6.23 (100), 5.40 (54), 3.44 (55), 3.33
                              (55), 3.24 (51)

PR3 (12120)      Clariant     7.98 (89), 6.77 (25), 6.47 (45), 3.39
                              (54), 3.30 (100)

PR3 (12120)      Kremer       10.2 (29), 8.08 (97), 6.51 (45), 3.42
                              (64), 3.32 (100)

PR3 (12120)      Sun          7.97 (45), 6.46 (27), 5.80 (14), 3.40
                              (55), 3.29 (100)

PR4 (12085)      Clariant     6.69 (23), 6.33 (15), 4.95 (25), 3.38
                              (100), 3.26 (17)

PR6 (12090)      Clariant     7.90 (33), 6.96 (23), 6.46 (24), 3.37
                              (59), 3.30 (100)

PR48:1           Sun          18.4 (100), 9.25 (16), 6.25 (13), 4.58
(15865:1)                     (15), 3.57 (38)

PR48:2           Avecia       17.7 (100), 5.80 (17), 4.81 (23), 4.09
(15865:2)                     (19), 3.41 (61)

PR48:2           Magruder     17.4 (24), 5.62 (27), 4.79 (52), 4.08
(15865:2)                     (58), 3.39 (100)

PR48:2           Sun          18.1 (70), 5.64 (29), 4.79 (52), 4.08
(15865:2)                     (53), 3.40 (100)

PR48:3           Lansco       18.4 (100), 7.83 (26), 5.74 (19), 4.91
(15865:3)                     (30), 3.44 (48)

PR48:3           Sun          18.7 (100), 7.84 (14), 5.82 (16), 4.88
(15865:3)                     (27), 3.44 (61)

PR48:4           Ciba         19.0 (13), 8.08 (85), 5.86 (71), 4.21
(15865:4)                     (74), 3.44 (100)

PR49 (15630)     Cyanamid     16.3 (80), 8.21 (100), 7.33 (35), 3.49
                              (68), 3.33 (45)

PR49:1           Sun          17.3 (100), 8.38 (94), 7.70 (52), 4.12
(15630:1)                     (39), 3.50 (54)

PR49:1           Sun          19.5 (74), 17.5 (93), 8.39 (100), 4.12
(15630:1)                     (43), 3.50 (85)

PR49:1           Magruder     17.6 (100), 8.29 (84), 7.57 (56), 4.87
(15630:1)                     (46), 4.09 (55)

PR49:2           Sun          16.5 (67), 8.80 (100), 4.26 (44), 3.57
(15630:2)                     (42), 3.44 (45)

PR49:2           Sun          16.7 (48), 8.71 (100), 4.36 (59), 3.57
(15630:2)                     (60), 3.43 (59)

PR49:2           Magruder     16.6 (46), 8.80 (100), 4.26 (65), 3.58
(15630:2)                     (61), 3.42 (63)

PR52:1           No supplier  18.0 (41), 4.78 (39), 4.06 (34), 3.41
(15860:1)                     (100), 3.22 (33)

PR52:1           Magruder     18.8 (100), 6.20 (9), 4.78 (14), 4.08
(15860:1)                     (14), 3.39 (28)

PR52:1           Sun          18.3 (34), 4.80 (44), 4.69 (30), 4.07
(15860:1)                     (50), 3.40 (100)

PR53:1           Sun          16.9 (69), 15.7 (100), 5.44 (58), 4.31
(15585:1)                     (39), 3.44 (38)

PR53:1           Magruder     15.6 (100), 6.24 (48), 5.45 (62), 4.35
(15585:1)                     (57), 3.43 (66)

PR57 (15850)     Ciba         17.8 (100), 5.84 (17), 4.77 (28), 3.42
                              (34), 3.25 (23)

PR57:1           Sun          17.9 (100), 5.89 (23), 4.78 (37), 3.43
(15850:1)                     (45), 3.26 (29)

PR57:1           Sun          17.6 (90), 4.75 (66), 4.16 (58), 3.42
(15850:1)                     (100), 3.26 (65)

PR57:1           Magruder     17.7 (86), 5.88 (46), 4.76 (77), 3.43
(15850:1)                     (100), 3.26 (70)

PR58:4           Sansui       11.3 (84), 5.67 (48), 5.09 (100), 3.29
(15825:4)                     (78), 2.542 (54)

PR60:1           Sun          13.7 (100), 7.03 (28), 4.64 (29), 3.62
(16105:1)                     (25), 3.48 (28)

PR60:1           Sun          13.9 (100), 6.99 (30), 4.70 (33), 3.65
(16105:1)                     (32), 3.52 (33)

PR63:1           Sun          18.7 (100), 14.7 (29), 8.20 (35), 4.14
(15880:1)                     (45), 3.46 (42)

PR63:2           Sansui       18.2 (100), 8.23 (39), 4.43 (24), 4.23
(15880:2)                     (54), 3.47 (60)

PO5 (12075)      Ciba         7.50 (29), 7.17 (43), 6.30 (42), 3.59
                              (20), 3.19 (100)

PO46 (15602)     Sun          18.2 (86), 15.4 (100), 3.87 (16), 3.55
                              (17), 3.37 (20)

PO46 (15602)     Sun          17.9 (78), 15.0 (100), 3.83 (57), 3.52
                              (59), 3.31 (67)

PO46 (15602)     Magruder     18.2 (41), 15.2 (84), 3.88 (100), 3.57
                              (93), 3.37 (100)


Table 4 lists the diffraction data for the Naphthol Reds. Also sometimes called the Naphthol AS pigments, this is a large class of azo red pigments. They are azo derivatives of 2-hydroxy-3-naphthoic acid. These pigments are of moderately good lightfastness and high tinting strength, and they are extensively used in automotive, architectural, and artists' paints.
Table 4: Diffraction data for Naphthol AS pigments

CI pigment name     Supplier             d spacings (intensity)
(number)

PR2 (12310)      Heubach          10.7 (100), 4.93 (33), 4.43 (24),
                                  3.36 (62), 3.25 (82)

PR5 (12490)      Avecia           7.40 (25), 6.13 (41), 5.50 (31),
                                  3.57 (32), 3.36 (100)

PR7 (12420)      ICI              11.8 (34), 7.19 (42), 6.78 (34),
                                  4.74 (24), 3.37 (100)

PR8 (12335)      HY               11.5 (100), 7.60 (59), 4.65 (74),
                                  4.17 (77), 3.25 (94)

PR9 (12460)      Rowney           14.4 (20), 11.2 (17), 7.11 (17),
                                  4.68 (24), 3.33 (100)

PR12 (12385)     Avecia           11.4 (49), 6.73 (31), 5.68 (17),
                                  3.75 (18), 3.33 (100)

PR14 (12380)     Clariant         12.3 (27), 8.64 (40), 6.82 (30),
                                  6.09 (16), 3.34 (100)

PR17 (12390)     Sun              12.2 (100), 5.98 (11), 5.34 (13),
                                  4.14 (11), 3.33 (60)

PR17(12390)      Sun              12.2 (88), 6.01 (14), 5.37 (25),
                                  4.14 (25), 3.33 (100)

PR18 (12350)     ICI              11.8 (41), 8.28 (17), 6.55 (51),
                                  3.78 (18), 3.41 (100)

PR21 (12330)     HY               6.35 (40), 4.50 (35), 4.13 (36),
                                  3.27 (100), 3.02 (37)

PR22 (12315)     Sun              11.4 (100), 7.19 (21), 4.57 (32),
                                  4.26 (22), 3.32 (86)

PR22 (12315)     Magruder         11.5 (100), 8.98 (14), 4.57 (25),
                                  4.31 (18), 3.34 (64)

PR23 (12355)     Lansco           12.6 (99), 9.31 (16), 6.23 (40),
                                  5.29 (19), 3.25 (100)

PR23 (12355)     Sun              12.6 (94), 9.39 (18), 6.28 (36),
                                  5.31 (19), 3.24 (100)

PR112 (12370)    Clariant         7.30 (17), 5.88 (39), 4.41 (19),
                                  3.64 (20), 3.35 (100)

PR146 (12485)    Lansco           16.1 (25), 8.01 (14), 4.93 (32),
                                  4.41 (37), 3.31 (100)

PR146 (12485)    Magruder         7.87 (21), 6.76 (16), 4.88 (37),
                                  4.38 (47), 3.29 (100)

PR147 (12433)    Clariant         15.0 (100), 6.73 (85), 4.98 (27),
                                  4.33 (37), 3.32 (78)

PR170 (12475)    Albion           12.2 (57), 7.76 (27), 5.75 (19),
                                  4.86 (29), 3.47 (100) (a)

PR170 (12475)    Kremer           12.3 (37), 10.7 (24), 7.82 (21),
                                  4.96 (13), 3.50 (100)a

PR170 (12475)    Sun              12.2 (26), 7.71 (14), 5.81 (9), 4.86
                                  (9), 3.46 (100) (a)

PR170 (12475)    Sun              12.1 (34), 7.76 (26), 5.76 (20),
                                  4.87 (27), 3.47 (100) (a)

PR170 (12475)    Winsor & Newton  12.2 (29), 7.70 (23), 5.80 (11),
                                  4.89 (11), 3.48 (100) (a)

PR187 (12486)    Clariant         13.8 (76), 7.64 (50), 5.10 (100),
                                  3.81 (57), 3.43 (85)

PR188 (12567)    Clariant         12.8 (100), 8.38 (78), 5.28 (51),
                                  3.54 (55), 3.26 (99)

PR253 (12375)    Clariant         11.7 (88), 6.72 (69), 5.19 (100),
                                  3.54 (74), 3.43 (74)

PR256 (124635)   Clariant         8.63 (46), 6.31 (35), 5.69 (54),
                                  4.80 (40), 3.33 (100)

(a) PR170 samples are all [gamma]-phase except for Kremer sample which
is [beta]-phase


The five most intense lines for benzimidazolone pigments are listed in Table 5. These are a class of yellow, orange, and red pigments that were first produced in the 1960s. The pigments contain the 5-aminocarbonyl benzimidazolone functionality coupled to either an arylide-type structure or a Naphthol AS-type structure. These pigments have excellent lightfastness properties and exceptional resistance to solvents. They are used in both automotive and artists' paints.
Table 5: Diffraction data for benzimidazolone pigments

CI pigment name  Supplier            d spacings (intensity)
(number)

PY120 (11783)    Clariant  8.62 (31), 6.41 (10), 5.40 (13), 4.51 (13),
                           3.36 (100)

PY151 (13980)    Clariant  8.56 (16), 5.10 (29), 4.57 (20), 4.22 (21),
                           3.24 (100)

PY151 (13980)    Kremer    8.67 (21), 5.11 (32), 4.59 (24), 4.24 (24),
                           3.25 (100)

PY154 (11781)    Clariant  7.21 (31), 6.61 (41), 4.85 (33), 4.47 (26),
                           3.40 (100)

PY175 (11784)    Clariant  9.02 (100), 6.85 (26), 4.34 (38), 3.40
                           (46), 3.29 (74)

PY180 (21290)    Clariant  13.5 (83), 9.33 (48), 6.63 (81), 4.56 (48),
                           3.50 (100)

PY181 (11777)    Clariant  4.85 (65), 4.59 (70), 4.08 (88), 3.53 (87),
                           3.17 (100)

PY194 (11785)    Clariant  13.4 (45), 6.70 (78), 5.06 (42), 3.82 (61),
                           3.40 (100)

PO36 (11780)     Sun       7.28 (22), 6.13 (36), 4.92 (17), 3.93 (21),
                           3.25 (100)

PO36 (11780)     Sun       7.27 (20), 6.09 (32), 4.93 (18), 3.93 (21),
                           3.24 (100)

PO60 (11782)     Kremer    6.72 (58), 4.66 (68), 4.17 (61), 3.46
                           (100), 3.28 (66)

PO62 (11775)     Kremer    8.13 (47), 5.82 (39), 5.49 (19), 4.77 (28),
                           3.24 (100)

PR175 (12513)    Clariant  11.4 (36), 9.89 (100), 7.23 (33), 4.88
                           (33), 3.27 (74)

PR175 (12513)    Kremer    20.2 (33), 11.4 (51), 9.87 (100), 4.90
                           (25), 3.28 (72)

PR176 (12515)    Talens    11.7 (100), 6.81 (46), 4.95 (56), 4.77
                           (34), 3.57 (43)

PR185 (12516)    Clariant  10.4 (100), 8.35 (18), 5.08 (17), 3.98
                           (11), 3.32 (90)

PR208 (12514)    Clariant  10.9 (66), 8.82 (20), 6.33 (76), 5.71 (16),
                           3.38 (100)


Diffraction data for disazo condensation pigments are shown in Table 6. This pigment class, first synthesized in the 1950s, is specifically designed to be of high molecular weight which serves to reduce their solubility. As with the benzimidazolones, there are two main classes of these pigments, an arylide yellow type and a Naphthol AS type. These pigments have excellent solvent fastness and lightfastness. Due to their expense of manufacture, they are used for high-end automotive, architectural, and artists' applications.
Table 6: Diffraction data for disazo condensation pigments

CI pigment name  Supplier            d spacings (intensity)
(number)

PY128 (20037)    Ciba      13.3 (42), 4.25 (61), 3.86 (59), 3.44 (82),
                           3.31 (100)

PR144 (20735)    Ciba      10.3 (53), 4.93 (28), 4.27 (16), 3.77 (13),
                           3.26 (100)

PR166 (20730)    Ciba      9.17 (100), 4.59 (44), 4.48 (59), 3.31
                           (61), 3.22 (49)

PR214 (200660)   Clariant  10.3 (55), 9.13 (12), 4.95 (17), 3.74 (12),
                           3.26 (100)

PR220 (20055)    Ciba      12.2 (100), 8.59 (23), 6.61 (14), 4.61
                           (28), 3.35 (79)

PR221 (20065)    Ciba      23 (40), 12.1 (100), 5.18 (35), 3.69 (15),
                           3.30 (88)

PR242 (20067)    Clariant  10.3 (37), 6.86 (21), 4.77 (27), 3.80 (12),
                           3.35 (100)


The five most intense lines for pyrazolone pigments are listed in Table 7. These orange, yellow, and red pigments have few members of commercial importance. They have good lightfastness and adequate solvent fastness, except for PY100 (also known as tartrazine yellow), which has poor solvent and lightfastness.
Table 7:  Diffraction data for pyrazolone pigments

CI pigment name  Supplier            d spacings (intensity)
(number)

PO13 (21110)     Ciba      7.21 (100), 4.93 (51), 4.65 (25), 3.38
                           (68), 3.29 (57)

PO13 (21110)     Kremer    7.21 (100), 4.94 (58), 4.68 (30), 3.39
                           (88), 3.30 (71)

PO34 (21115)     Sun       8.84 (97), 6.01 (14), 5.41 (14), 4.76 (13),
                           3.40 (100)

PO34 (21115)     Sun       8.83 (94), 5.97 (11), 5.48 (12), 4.73 (12),
                           3.41 (100)

PR38 (21120)     Sun       10.2 (70), 5.00 (16), 3.81 (22), 3.39
                           (100)

PR41 (21200)     Sansui    8.76 (74), 6.33 (13), 5.88 (36), 5.17 (8),
                           3.41 (100)


Table 8 lists the diffraction data for the blue and green copper phthalocyanine pigments. These pigments are the predominant blue and green organic pigments in use today and they have excellent fastness properties. Copper phthalocyanine blue exists in five crystalline polymorphs, two of which are used in paints. The green copper phthalocyanines are made by halogenation of the parent blue pigment.
Table 8: Diffraction data for phthalocyanine pigments

CI pigment name  Supplier            d spacings (intensity)
(number)

PB15:0 (74160)   Lansco     12.8 (100), 12.2 (89), 5.60 (34), 3.34
                            (44), 3.22 (41)

PB15:1 (74160)   Avecia     12.8 (100), 12.1 (87), 5.68 (31), 3.55
                            (34), 3.30 (64)

PB15:1 (74160)   Kremer     13.2 (100), 12.3 (80), 5.80 (13), 3.58
                            (12), 3.56 (20)

PB15:2 (74160)   Avecia     13.1 (100), 12.2 (83), 3.57 (32), 3.32
                            (58), 3.24 (40)

PB15:3 (74160)   Avecia     12.6 (100), 9.67 (75), 3.74 (33), 3.40
                            (20), 2.936 (13)

PB15:3 (74160)   Magruder   12.8 (85), 9.72 (100), 3.78 (54), 3.42
                            (33), 2.943 (26)

PB15:3 (74160)   Sun        12.6 (100), 9.64 (92), 3.74 (55), 3.40
                            (34), 2.939 (30)

PB15:4 (74160)   Avecia     12.6 (83), 9.57 (100), 3.73 (59), 3.39
                            (26), 2.931 (22)

PB15:6 (74160)   Kremer     11.5 (68), 9.49 (100), 5.02 (15), 3.74
                            (15), 3.13 (17)

PB16 (74100)     BASF       13.1 (100), 11.9 (83), 5.91 (70), 3.59
                            (57), 3.40 (78)

PB76             Dainippon  14.1 (44), 3.62 (13), 3.43 (31), 3.30
                            (100)

PG7 (74260)      Avecia     14.3 (32), 3.33 (100), 2.883 (29), 2.830
                            (27), 2.605 (29)

PG7 (74260)      Sun        3.32 (100), 2.894 (25), 2.658 (29),
                            2.607 (36), 2.480 (28)

PG36 (74265)     Avecia     5.27 (48), 3.51 (85), 2.816 (49), 2.741
                            (50), 2.650 (100)


The diffraction data for quinacridone pigments are listed in Table 9. These pigments span the color range of gold, orange, maroon, scarlet, red, magenta, and violet and were first manufactured commercially in the 1950s. Quinacridones exist in a number of crystalline modifications and also as solid solutions. These pigments have excellent lightfastness properties and are used in many applications including automotive and artists' paints, printing inks, and for the coloration of plastics.
Table 9: Diffraction data for quinacridone pigments

CI pigment name    Supplier            d spacings (intensity)
(number)

P048             Ciba          10.3 (100), 6.28 (65), 3.64 (93), 3.49
                               (36), 3.30 (74)

P049 (564800)    Kremer        10.2 (100), 6.25 (51), 3.65 (74), 3.47
                               (42), 3.29 (74)

P049 (564800)    Kremer        10.2 (100), 6.32 (71), 3.67 (93), 3.50
                               (59), 3.30 (94)

PR122 (73915)    Sun           15.8 (100), 7.90 (19), 6.29 (50), 3.48
                               (21), 3.25 (46)

PR122 (73915)    Sun           15.6 (46), 7.92 (40), 6.32 (90), 3.50
                               (48), 3.26 (100)

PR122 (73915)    Kremer        16.0 (100), 7.96 (13), 6.34 (27), 3.51
                               (14), 3.29 (32)

PR122 (73915)    Kremer        13.7 (86), 6.47 (82), 3.78 (18), 3.39
                               (100), 3.12 (14) (a)

PR202 (73907)    Ciba          16.3 (44), 5.88 (33), 5.42 (12), 3.79
                               (32), 3.20 (100) (b)

PR202 (73907)    Sun           16.6 (19), 5.82 (25), 5.41 (15), 3.82
                               (37), 3.20 (100) (b)

PR206 (73900     Ciba          13.6 (27), 10.6 (87), 6.36 (97), 3.66
and 73920)                     (100), 3.33 (58)

PR207 (73900     Talens        14.0 (45), 6.75 (85), 3.68 (25), 3.47
and 73906)                     (34), 3.35 (100)

PR209 (73905)    Clariant      14.8 (23), 7.42 (19), 6.84 (52), 6.41
                               (12), 3.38 (100)

PV19 (73900)     Avecia        15.1 (58), 7.47 (20), 5.53 (35), 4.03
                               (20), 3.28 (100) (c)

PV19 (73900)     Sun           13.6 (100), 6.66 (34), 6.44 (93), 3.73
                               (19), 3.37 (98) (c)

PV19 (73900)     Sun (violet)  15.2 (38), 7.50 (19), 5.48 (32), 4.04
                               (20), 3.28 (100) (c)

(a) Kremer sample, pink shade, matches with [gamma]-trans-quinacridone
(b) Both samples of PR202 are [gamma]-phase
(c) Avecia and Sun violet shade [beta]-phase, Sun is [gamma]-phase


Table 10 lists the diffraction data for the primarily red perylene and perinone pigments. Perylenes are diimides of perylene tetracarboxylic acid, while perinones are made from naphthalene-1, 4, 5, 8-tetracarboxylic acid. They are characterized by excellent fastness properties and are used in industrial, automotive and artists' paints, printing inks, and in the coloration of plastics.
Table 10: Diffraction data for perylene and perinone pigments

CI pigment name  Supplier            d spacings (intensity)
(number)

PR149 (71137)    Clariant  17.5 (45), 6.34 (100), 4.64 (33), 3.65
                           (41), 3.45 (46)

PR178 (71155)    BASF      13.8 (20), 6.55 (29), 4.57 (100), 3.69
                           (30), 3.07 (36)

PR179 (71130)    Sun       10.7 (52), 7.74 (41), 7.28 (100), 6.59
                           (52), 3.24 (99)

PR190 (71140)    Bayer     14.9 (52), 9.25 (38), 6.53 (100), 3.87
                           (24), 3.57 (15)

PR224 (71127)    Bayer     10.0 (29), 7.05 (74), 3.55 (30), 3.24
                           (100), 3.10 (10)

PV29 (71129)     Bayer     8.82 (14), 7.43 (100), 3.57 (43), 3.30
                           (75), 2.953 (17)

PV29(71129)      Sun       8.63 (7), 7.35 (59), 3.56 (52), 3.28 (100),
                           2.939 (24)

P043 (71105)     Clariant  11.4 (46), 8.03 (53), 6.97 (100), 3.53
                           (86), 3.25 (41)

PR194 (71100)    Kremer    10.1 (53), 9.11 (61), 6.59 (100), 3.65
                           (42), 3.28 (94)


Diffraction data for the rest of the pigments examined is found in Table 11. This includes pigments such as triarylcarbonium, isoindolinone and isoindoline, diketopyrrolo pyrrole, thioindigoids, and various other pigments. The class of pigment is listed in the table. (39), (40)
Table 11: Diffraction data for miscellaneous pigments

CI pigment name       Class           Supplier   d spacings (intensity)
(number)

PB60 (69800)     Indanthrone          Avecia     7.74 (100), 7.07 (38),
                                                 3.61 (19), 3.44 (53),
                                                 3.26 (57)

PB60 (69800)     Indanthrone          Kremer     7.77 (100), 7.07 (50),
                                                 3.61 (24), 3.45 (77),
                                                 3.27 (84)

PG1 (42040:1)    Triarylcarbonium     Haagen     6.35 (46), 5.33 (56),
                                                 5.14 (56), 4.78 (93),
                                                 3.69 (100)

PG4 (42000:2)    Triarylcarbonium     ICN        13.9 (68), 6.98 (49),
                                                 5.30 (46), 4.89 (71),
                                                 4.64 (100)

PG8 (10006)      Metal complex        Albion     13.7 (100), 7.92 (55),
                                                 6.69 (43), 5.14 (26)
                                                 4.58 (66)

PO51 (597030)    Pyranthrone          Kremer     11.4 (39), 5.67 (47),
                                                 3.71 (32), 3.51 (100),
                                                 3.24 (37)

PO61 (11265)     Isoindolinone        Ciba       6.91 (56), 3.70 (61),
                                                 3.50 (46), 3.36 (100),
                                                 2.746 (49)

PO61 (11265)     Isoindolinone        Kremer     6.89 (41), 3.70 (62),
                                                 3.50 (46), 3.37 (100),
                                                 2.753 (44)

PO64 (12760)     Azoheterocycle       Ciba       7.50 (29), 7.17 (43),
                                                 6.30 (42), 3.59 (20),
                                                 3.19 (100)

PO67 (12915)     Pyrazoloquinazolone  BASF       10.3 (30), 6.49 (29),
                                                 5.42 (16), 5.17 (19),
                                                 3.31 (100)

PO68 (486150)    Metal complex        Clariant   13.2 (91), 9.94 (62),
                                                 7.17 (56), 3.48 (70),
                                                 3.25 (100)

PO69 (56292)     Isoindoline          Kremer     7.28 (20), 6.12 (39),
                                                 4.89 (24), 4.21 (23),
                                                 3.25 (100)

PO69 (56292)     Isoindoline          Kremer     16.1 (56), 4.20 (54),
                                                 3.75 (39), 3.65 (35),
                                                 3.21 (100)

PO71 (561200)    DPP                  Ciba       14.6 (74), 6.78 (30),
                                                 6.23 (30), 6.09 (30),
                                                 3.28 (100)

PO73 (561170)    DPP                  Kremer     15.3 (90), 5.49 (48),
                                                 5.18 (42), 3.81 (100),
                                                 3.26 (69)

PO74             Azo                  Clariant   5.09 (38), 4.57 (66),
                                                 3.84 (39), 3.34 (100),
                                                 3.18 (74)

PO79             Azo metal salt       Engelhard  14.7 (89), 4.97 (67),
                                                 4.74 (64), 3.54 (78),
                                                 3.45 (100)

PR42 (21210)     Disazo               Ciba       15.0 (49), 6.39 (54),
                                                 5.93 (25), 3.58 (32),
                                                 3.22 (100)

PR81:1           Triarylcarbonium     EC         14.6 (100), 13.8 (98),
(45160:1)                                        3.23 (14)

PR81:2           Triarylcarbonium     Sun        14.8 (100), 13.6
(45160:2)                                        (100), 9.68 (44)

PR83:1           Anthraquinone        EC         11.6 (100), 9.83 (44),
(58000:1)                                        8.43 (31), 5.72 (33),
                                                 3.76 (50)

PR88 (73312)     Thioindigo           Kremer     10.0 (28), 6.61 (37),
                                                 3.57 (16), 3.32 (100),
                                                 3.24 (66)

PR168 (59300)    Anthanthrone         Clariant   9.73 (25), 4.86 (35),
                                                 3.64 (20), 3.52 (32),
                                                 3.35 (100)

PR168 (59300)    Anthanthrone         Kremer     9.70 (21), 6.99 (15),
                                                 4.83 (31), 3.48 (39),
                                                 3.34 (100)

PR173            Rhodamine, Al salt   Talens     10.6 (100), 7.22 (72),
(45170:3)                                        3.61 (42), 2.826 (36),
                                                 2.355 (33)

PR 177 (65300)   Anthraquinone        Ciba       8.81 (63), 7.31 (96),
                                                 6.54 (54), 3.84 (49),
                                                 3.35 (100)

PR181 (73360)    Thioindigo           Anstead    9.33 (100), 4.56 (42),
                                                 3.31 (40), 3.12 (64),
                                                 2.480 (52)

PR254 (56110)    DPP                  Ciba       3.60 (23), 3.44 (39),
                                                 3.14 (100), 2.864
                                                 (46), 2.776 (34)

PR255 (561050)   DPP                  Ciba       13.7 (100), 6.02 (73),
                                                 3.73 (33), 3.38 (75),
                                                 3.14 (48)

PR257 (562700)   Metal complex        Clariant   7.76 (64), 3.46 (100),
                                                 3.38 (88), 2.704 (24),
                                                 2.512 (22)

PR264 (561300)   DPP                  Ciba       21 (100), 4.95 (43),
                                                 4.78 (35), 3.36 (29),
                                                 3.20 (15)

PR276            Azo metal salt       Engelhard  14.7 (68), 12.4 (100),
                                                 11.7 (86), 4.09 (79),
                                                 4.03 (74)

PR277            Azo metal salt       Engelhard  14.9 (67), 12.9 (100),
                                                 11.9 (54), 4.11 (51),
                                                 3.61 (58)

PR279            Thiazine indigo      Clariant   6.15 (46), 3,98 (16),
                                                 3.68 (45), 3.22 (100),
                                                 3.07 (29)

PR280            Monoazo              Engelhard  10.1 (100), 8.98 (78),
                                                 5.81 (51), 3.92 (59),
                                                 3.36 (51)

PV23 (51319)     Dioxazine            Avecia     15.2 (100), 8.61 (63),
                                                 3.79 (20), 3.45 (66),
                                                 3.11 (15) (a)

PV23 (51319)     Dioxazine            Sun        15.1 (72), 8.62 (77),
                                                 3.80 (29), 3.45 (100),
                                                 3.10 (25) (a)

PV37 (51345)     Dioxazine            Ciba       13.5 (100), 8.50 (71),
                                                 5.04 (43), 3.79 (28),
                                                 3.55 (35)

PV51             Monoazo              Engelhard  14.9 (100), 13.4 (69),
                                                 4.25 (80), 3.33 (31),
                                                 2.810 (36)

PV52             Monoazo              Engelhard  13.1 (100), 8.96 (49),
                                                 7.21 (42), 4.30 (71),
                                                 3.44 (53)

PV53             Monoazo              Engelhard  14.3 (64), 9.88 (99),
                                                 8.91 (100), 5.74 (52),
                                                 3.89 (55)

PY24 (70600)     Flavanthrone         ICC        8.10 (100), 7.35 (38),
                                                 3.52 (35), 3.40 (57),
                                                 3.30 (91)

PY61 (13880)     Monoazo salt         Albion     8.02 (100), 5.10 (29),
                                                 4.24 (27), 3.50 (70),
                                                 3.17 (28)

PY62 (13940)     Monoazo salt         Lansco     9.95 (26), 8.10 (100),
                                                 4.06 (19), 3.45 (48),
                                                 3.37 (31)

PY62 (13940)     Monoazo salt         Sun        9.90 (26), 8.13 (100),
                                                 3.46 (82), 3.38 (36),
                                                 3.18 (25)

PY62:1           Monoazo salt         Heubach    9.94 (28), 8.12 (100),
                                                 3.46 (100), 3.37 (43),
                                                 3.17 (31)

PY109 (56284)    Isoindolinone        Ciba       9.46 (27), 7.50 (33),
                                                 3.41 (100), 3.20 (27),
                                                 2.845 (51)

PY110 (56280)    Isoindolinone        Ciba       7.08 (48), 4.12 (24),
                                                 3.54 (100), 3.25 (24),
                                                 2.801 (22)

PY129 (48042)    Metal complex        Ciba       12.5 (75), 9.72 (100),
                                                 8.81 (79), 4.99 (18),
                                                 3.30 (31)

PY138 (56300)    Quinophthalone       BASF       7.21 (47), 6.86 (66),
                                                 3.46 (100), 3.43 (99),
                                                 3.20 (57)

PY139 (56298)    Isoindoline          Clariant   9.70 (44), 7.14 (22),
                                                 5.41 (30), 3.37 (100),
                                                 3.22 (46)

PY150 (12764)    Metal complex        Golden     9.83 (100), 4.82 (60),
                                                 5.37 (65), 3.74 (48),
                                                 3.25 (36)

PY153 (48545)    Metal complex        W and N    10.9 (36), 9.94 (45),
                                                 7.13 (100), 5.46 (33),
                                                 3.68 (40)

PY168 (13960)    Azo salt             Ciba       8.14 (91), 5.39 (25),
                                                 4.09 (20), 3.39 (100),
                                                 3.18 (28)

PY169 (13955)    Azo salt             Toyo       8.02 (100), 5.00 (31),
                                                 4.02 (31), 3.53 (68),
                                                 3.18 (17)

PY173 (561600)   Isoindolinone        Clariant   11.5 (56), 6.29 (75),
                                                 5.02 (63), 3.67 (78),
                                                 3.23 (100)

PY183 (18792)    Isoindoline          Albion     4.87 (71), 4.42 (56),
                                                 4.26 (35), 3.46 (100),
                                                 3.04 (40)

PY205            Azo metal salt       Engelhard  6.39 (68), 5.40 (62),
                                                 3.79 (64), 3.72 (93),
                                                 3.60 (100)

PY206            Azo metal salt       Engelhard  16.5 (55), 7.15 (30),
                                                 5.92 (36), 5.65 (36),
                                                 3.46 (100)

PY209            Azo metal salt       Engelhard  14.0 (100), 11.0 (85),
                                                 6.07 (41), 3.66 (34),
                                                 3.42 (89)

PY209:1          Azo metal salt       Engelhard  13.4 (69), 11.3 (56),
                                                 10.7 (100), 3.33 (77),
                                                 3.30 (73)

PY210            Disazo               Engelhard  10.4 (55), 9.05 (80),
                                                 5.04 (31), 4.38 (100),
                                                 3.40 (7)

PY212            Azo metal salt       Engelhard  8.97 (35), 6.13 (100),
                                                 4.20 (50), 3.59 (49),
                                                 2.994 (45)

PY213 (11775)    Monoazo              Clariant   9.72 (66), 6.92 (14),
                                                 5.92 (13), 4.55 (23),
                                                 3.37 (100)

PY214            Disazo               Clariant   8.44 (68), 6.54 (60),
                                                 4.84 (20), 4.10 (25),
                                                 3.38 (100)

(a) PV23 samples are [beta]-phase


There were a number of pigments that were found not to diffract. These include: PB1 (triarylcarbonium), PB52 (anthraquinone), PB63 (indigo, sulfonic acid), PR90:1 (phthalein), PR169 (triarylcarbonium), PR172 (tetraiodofluoresceine), PR174 (phthalein), PV1 (triarylcarbonium), PV3 (triarylcarbonium), PV3:1 (triarylcarbonium), PV5 (triarylcarbonium), PV27 (triarylcarbonium), PY100 (monoazopyrazolone), and PY104 (naphthalene sulfonic acid, Al salt). Some other pigment samples were found to be contaminated with barite (barium sulfate), making analysis by x-ray powder diffraction impossible.

Discussion

The technique of x-ray diffraction, at first glance, looks promising for the analysis of dry synthetic organic pigments. Distinctive diffraction patterns were obtained for most of the pigments examined. While diffraction data for some of the pigments were found in the ICDD database, many were not, and they should serve as references for future analysis. In general, good agreement was found with existing data in the ICDD database. On occasion, there were examples where intensities were off a bit compared to the data in ICDD, so that the most intense line found was not that of the ICDD standards. Those pigments include PO13, PO16, PO43, PR5, PR58:4, PR144, PR149, and PY128. Common sources of differences in intensities between ICDD reference patterns and matches with samples are due to a variety of reasons. These may include absorption, wavelength differences, and differences in preferred orientation.

Diffraction was useful in determining which pigments were mislabeled. For example, a sample of PO5 from a vendor was actually found to be PO16, and the corresponding sample from the same vendor labeled PO16 was determined to be PO5. One of the samples of PR48:2 (yellow shade) was found not to match an expected pattern for this pigment. Other samples of PR48:2 did not exactly match the expected pattern, but were consistent one to the other. A sample of PR122 (pink shade) was found to actually be the linear trans-quinacridone, [gamma]-phase instead of the expected 2,9-dimethyl quinacridone. A sample of PY6, an arylide yellow, appears to match the diffraction pattern of PY1, a more commonly used arylide yellow. A sample labeled PG10 was found not to be this pigment, although its identity could not be determined. The sample of PR18, a Naphthol Red, did not match that of the ICDD, although its mass spectrum is consistent with the structure of PR18.

Many pigments that have very similar infrared spectra within the same class are easily differentiated by this technique. For example, the Naphthol AS pigments PR2 and PR21 are structurally quite similar (see Fig. 1), but have completely different XRD patterns, as shown in Table 4. These two pigments have virtually superimposable infrared spectra, although their mass spectra are distinctive due to the double chlorination on the donor aromatic ring of PR2.

[FIGURE 1 OMITTED]

The same holds true for other pigments within classes of synthetic organic pigments. For example, Fig. 2 shows overlays of diffraction data for three commonly used monoarylide yellow pigments, PY73, PY74 and PY75. These are pigments introduced more recently than the monoarylides PY1 and PY3 and they are used extensively in paints due to greater light and solvent fastness. It is clear that they are easily discernible by x-ray powder diffraction. As shown in Fig. 3, the structurally similar diarylide yellows can be distinguished as well. Members of this class are characterized by an intense reflection at 2[theta] of 27, but there are many other characteristic reflections for the individual members of this class.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Figure 4 shows an overlay of the powder diffraction patterns of the [beta]-naphthol pigments PR1, PR3, and PR4. These small molecular weight red pigments are very similar in chemical structure, but have distinctive powder diffraction patterns. Powder diffraction patterns of three representative Naphthol AS pigments are displayed in Fig. 5. These include the pigments PR9, PR112, and PR170. Figure 6 displays the powder diffraction patterns for the benzimidazolones PY151, PY154, and PO36, while the pyrazolones PO13 and PR41 are shown in Fig. 7.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

A number of synthetic organic pigments are available in more than one polymorphic form. Polymorphs are different crystalline arrangements of the same compound. In general, although polymorphs have identical chemical composition, they have different physical properties such as color, hardness, melting point, and density and usually one polymorphic form is more stable. Variations in synthesis conditions can produce different polymorphic forms. Many pigments exist in polymorphic forms, but the most common are the copper phthalocyanines and quinacridones.

Copper phthalocyanines are known to exist in at least five crystalline modifications. The two most important in terms of pigments are the [alpha]-form (a red shade blue) and the [beta]-form (a green shade blue). The [alpha]-form is less stable and can be stabilized by partial chlorination. (39), (40) In all of the crystalline modifications, the planar copper phthalocyanine molecules are slacked and the polymorphs differ in how the stacks are arranged relative to one another. Figure 8 shows an overlay of the diffraction patterns of two common copper phthalocyanine pigments, PB15:1 and PB15:4. PB15:1 is the phase-stabilized [alpha] copper phthalocyanine and PB15:4 is the phase-stabilized [beta]-form. These polymorphic pigments are clearly distinguishable by x-ray powder diffraction.

[FIGURE 8 OMITTED]

The two most common phases of the unsubstituted linear trans-quinacridone (PV19) used as pigments are the [beta]- and [gamma]-forms. The [beta] modification is a reddish violet shade, while the [gamma] form is a bluish red shade. Figure 9 shows an overlay of the diffraction patterns of two of the polymorphs of PV19. The sample labeled violet matches up with the diffraction pattern of the [gamma]-phase, while the other sample matches with the [beta]-phase.

[FIGURE 9 OMITTED]

X-ray powder diffraction can also be used to detect solid solutions. These are not physical mixtures, where the diffraction pattern would be additive; the pattern of a solid solution is different. It can, however, be similar to the pattern of one of the components. The most common examples in synthetic organic pigments are the quinacridones, in particular mixtures with quinacridone quinones, and unsubstituted linear trans-quinacridone with 2,9-dichloroquinacridone. Figure 10 shows an overlay of the diffraction patterns of PO48, PO49, and PR206. All three of these components are mixtures of the unsubstituted linear trans-quinacridone with the corresponding quinacridone quinonc (see Fig. 11), but in varying amounts. PR206 is mostly the unsubstituted linear trans-quinacridone. The orange pigment PO48 consists of almost equal amounts of the two species, while the pigment PO49 contains more of the quinacridone quinone.* The three pigments have similar, but not identical diffraction patterns, and they are distinctly different than that of the parent linear trans-quinacridone. Due to their greater similarity in amounts of components, the patterns of PO48 and PO49 are the most similar.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

Conclusions

Organic pigments are generally poorer diffractors of x-rays than mineral pigments. Therefore, not only do some of the pigments not diffract but certain dry pigments that contain fillers such as barium sulfate do not provide useful diffraction data for the synthetic organic pigment. This is also evident from the high tinting strength of these pigments, which mean that they are present in lower concentrations than their mineral or inorganic counterparts.

In addition, crystals of small particle size, such as many synthetic organic pigments, lead to patterns where the diffracted lines are broadened and the peak heights are diminished.

As it applies to dry pigments, x-ray diffraction is a useful technique. The technique can also be useful for identification of polymorphs.

Acknowledgments The author is grateful to Tom Learner, currently of the Getty Conservation Institute, and Jamie Martin, Orion Analytical, for providing pigment samples. In addition, Michael Palmer, National Gallery of Art, provided instruction and assistance with the x-ray diffractometer.

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* The samples were either provided by Tom Leaner, currently at the Getty Conservation Institute or James Martin of Orion Analytical.

* Powder diffraction file, International Centre for Diffraction Data (ICDD), 12 Campus Blvd., Newtown Square, PA 19073-3273.

** It has been reported that grinding can cause phase transitions or conversions to amorphous forms. See reference (38).

* The Colour Index system of nomenclature is used in this paper. Each pigment is given a Colour Index name (such as Pigment Yellow 3 or PY3) and a Colour Index number (such as 11710 for PY3).

* These results were determined by DTMS and are consistent with the color of the pigments. PR206, consisting of the mixture with the major amount being the quinacridone is a violet-maroon color. The more maroon pigment PO48 has more of the quinacridone component, while the more orange- brown pigment PO49 has more of the quinacridone quinone component.

S. Q. Lomax (*)

Scientific Research Department, National Gallery of Art, DCL-SR, 2000B South Club Drive, Landover, MD 20785, USA

e-mail: s-lomax@nga.gov

J. Coat. Technol. Res., 7 (3) 331-346, 2010

DOI 10.1007/s11998-009-9206-0
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