Anal. Chem. 1998, 70, 3762-3765
Depth Distribution of Light Stabilizers in Coatings Analyzed by Supercritical Fluid Extraction-Gas Chromatography and Time-of-Flight Secondary Ion Mass Spectrometry F. Andrawes,* T. Valcarcel, G. Haacke, and J. Brinen
Cytec Industries Inc., 1937 West Main Street, Stamford, Connecticut 06904
The depth distribution of light stabilizers in coatings has been investigated by analyzing the stabilizer content of microtomed slices cut parallel to the coating surface. The analytical technique consists of extracting the unbound light stabilizers from each microtomed slice and determining the stabilizer concentration of the extract. Extraction was carried out using supercritical fluid, which yielded efficiencies as high as 96% with relative standard deviation of 4%. Samples in the 1-3 mg weight range were extracted. The extracts were analyzed using gas chromatography/nitrogen thermionic detection and timeof-flight secondary ion mass spectroscopy (ToF SIMS). In addition to analyzing the extract, ToF SIMS is also capable of analyzing the microtomed slices directly. Good agreement between results obtained with the chromatographic and ToF SIMS techniques was observed.
Previous investigations of the distribution of light stabilizers in coatings2,3 using the microtoming and extraction approach employed acetonitrile as extractant. The extractions were carried out in an ultrasonic bath and required 12 h. In the work described here, carbon dioxide supercritical fluid extraction (SFE) was utilized, which is much faster than organic solvent techniques. The extract was analyzed by using either gas chromatography (GC) or time-of-flight secondary ion mass spectrometry (ToF SIMS). ToF SIMS is fast, and, in addition to analysis of the extract, it can be used for direct analysis of microtomed slices. Quantitative direct analysis requires that one of the slices to be analyzed by another technique to provide a known additive concentration (e.g., SFE/GC). The samples investigated in this work included single-layer and two-layer automotive clearcoats on metal and thermoplastic olefin (TPO) substrates.
Light stabilizers are widely used as additives to reduce the photodegradation of plastics and coatings. In automotive coatings, light stabilizers are essential to obtain acceptable coating life. The stabilizer package in automotive coatings commonly consists of ultraviolet absorbers (UVA) and hindered amine light stabilizers (HALS). The UVAs convert UV energy into harmless heat energy, and HALSs inhibit polymer degradation via several mechanisms, including free radical termination and peroxide formation. In many cases, the addition of both UVA and HALS provides a synergistic effect. The open-pore structure of polymeric coatings facilitates migration of light stabilizers.1 The migration depends on additive characteristics such as polarity, size, and shape of the molecule. The nature of the polymer is equally important. Cross-link density, free volume, size, and shape of the pores are among the influential parameters. In multilayer coatings, e.g., automotive coatings, stabilizer migration occurs not only within individual layers but also between different layers, such as clearcoats and basecoats. When plastic substrates are used, stabilizer migration from the coatings into substrates also has been observed.1 By measuring the depth distribution of stabilizers in multilayer coatings systems, information concerning migration can be generated.
EXPERIMENTAL SECTION The experimental conditions and the chemicals employed in this work for SFE and GC are listed in Table 1. ToF SIMS spectra were obtained on a Charles Evans & Associates TRIFT spectrometer using bunched 15-keV 69Ga ions in microprobe mode. The Ga ion pulse width is approximately 1 ns, and the repetition rate of the ion source is 10 kHz. Ions are detected using a microchannel plate, followed by a single stop time-to-digital (TDC) converter. The spectrometer is capable of providing a mass resolution of 9000 at m/z 100. Charge compensation was effected with an electron flood gun, which was pulsed out of phase with the ion gun. All spectra were obtained under static SIMS conditions. Small slices of the microtomed sections, approximately 1 cm × 1 cm, were placed in the sample holders for examination. The ion gun was rastered over a 120-µm × 120-µm area, and spectra were obtained over a mass range from 1 to 800 mass units. Typically, at least three 5-min spectra were obtained for each sample slice. The films, generally 5-10 µm thick, were not flat (they were “chattered” as a result of the microtoming), and this presented a problem with charge compensation. The problem was minimized by carefully checking the peak widths of the peaks used for calibration (generally CH3, C2H3, C3H5, and C4H7). In
(1) Haacke, G.; Andrawes, F. F.; Campbell, B. H. J. Coatings Technol. 1996, 68 (855), 57-62.
(2) Bohnke, H.; Hess, E.; Avar, L. XIXth FATIPEC-Congress, Aachen, Germany, 1988; Congress Book Vol. 1, pp 335-347. (3) Bohnke, H.; Avar, L.; Hess, E. J. Coating Technol. 1991, 63 (799), 64-60.
3762 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
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Table 1. Experimental Conditions and Chemicals Employed in This Work instrument fluid density temperature flow rate equilibration time extraction time nozzle temp trap temp trap packing rinsing solvent thimble volume sample weight chromatograph column flow column temp injector temp detector
Supercritical Fluid Extraction Hewlett-Packard 7680T SFE unit CO2 (Air Products, analytical grade) 0.70 g/mL 120 °C 1 mL/min 2 min 25 min 45 °C 40 °C stainless steel toluene, 1 mL, at 40 °C 10 mL, packed with 6-mm glass beads 1-3 mg placed on the top of the glass beads Gas Chromatography Hewlett-Packard 5890 15 m × 0.53 mm i.d. DB1, 1.5 µm (J&W Scientific No. 125-1012) 15 mL/min of helium 100 °C for 1 min, heat to 300 °C at 8 °C/min 325 °C nitrogen thermionic detector at 325 °C
another approach, the SFE extract was deposited reproducibly onto a flat, inert surface and allowed to evaporate to dryness to provide high mass resolution spectra. Examples of this type of analysis will be shown. Details will be published elsewhere. Chemicals. Styrene/acrylic backbone (Joncryl 500 acrylic oligomer, S.C. Johnson Corp.), methylated-isobutylated melamine cross-linker (Cymel 1168 cross-linker, Cytec Industries Inc.) 2,4bis(2,4-dimethylphenyl)-6-(2-hydroxy-4-iso-octyloxphenyl)1,3,5-triazine (CYAGARD UV 1164, Cytec Industries Inc.), N-acetyl(2,2,6,6-tetramethylpiperidin-4-yl)dodecylsuccinimide (Sanduvor 3058), bis-(1,2,2,6,6-pentamethyl-4-pipiridyl) sebacate (Tinuvin 292, Ciba Geigy), and 2-(3,5-bis(1-methyl-1-phenyl)-2-(hydroxyphenyl)benztriazole (Tinuvin 900, Ciba Geigy) were used. Sample Preparation. The clearcoats investigated consisted of the styrene/acrylic backbone and the methylate diisobutylated melamine cross-linker. The backbone-to-cross-linker ratio was 65: 35 by weight. The catalyst was p-toluenesulfonic acid (CYCAT 4040, Cytec Industries Inc.) The coatings were drawn onto standard aluminum test panels, equilibrated in air, and cured in a forced-air convection oven. The cure conditions were 30 min at 120 °C. To study the diffusion of light stabilizers in the clearcoat matrix during curing, a two-layer system was designed.4 Clearcoat 1 was drawn first onto the substrate and cured for 10 min at 120 °C. Thereafter, clearcoat 2 was drawn on top of coat 1, and both layers were heated for 30 min at 120 °C. The two coats were identical in composition and thickness. Stabilizers were added to either layer 1 or 2. After curing, 5-cm × 5-cm squares were cut from the test panel, and the coatings were lifted off the substrate using a sharp razor blade. The samples were mounted on the support block of a Reichert-Jung Polycut E microtome (Reichert-Jung). Samples were sliced into 5-10 µm-thick sections parallel to the coating surface, but sections as thin as 1 µm are within the specifications of the instrument. (4) Haacke, G.; Brinen, J. S. Proc. 22nd FATIPEC Congr., Budapest 1994, 3, 182-191.
Figure 1. Gas chromatogram of 8 ppm TINUVIN 900, 11 ppm SANDUVOR 3058, and 8 ppm CYAGARD UV 1164 (two isomers). GC conditions are as described in the text.
RESULTS AND DISCUSSION Supercritical fluid extraction (SFE) has been increasingly used for the analysis of chemical additives in polymers. It is rapidly replacing solvent and Soxhlet extraction.5-8 SFE is much faster and provides a clean and concentrated extract that is readily available for subsequent chromatographic or spectroscopic analysis. SFE lends itself well to the analysis of light stabilizers in coatings, as has been previously reported.9-11 When it is applied to the microtomed sections of a clearcoat, valuable information pertaining to additive distribution may be obtained. Sections between 5 and 10 µm thickness, weighing 1-3 mg, can be extracted and analyzed. For most additive extractions from polymer, high temperature and additional solvent might be required to fully extract the additives. In the case of thin sections, the additives are readily extractable in 10-15 min. Extraction time, density, and temperature of SFE were investigated for sections containing 1.9% CYAGARD UV 1164 and 1.2% SANDUVOR 3058. The samples examined were 10 µm-thick sections. Figure 1 shows a typical gas chromatogram for the compounds of interest. The results for different extraction periods are listed in Table 2. The effect of SFE extraction temperature was also investigated. For 25-min extraction time and 0.7 g/mL density, recovery was 100% for both components between 100 and 120 °C. For SFE density, at 120 °C and 25-min extraction time, the recovery was about 100% for both components between 0.5 and 0.7 g/mL. A comparison between solvent extraction using toluene at 80 °C for 4 h and SFE was also done. Five different sections from one topcoat in a one-layer system were analyzed by both techniques. The results are listed in Table 3. (5) Cotton, N. J.; Battle K. D.; Clifford, A. A.; Dowle, C. J. J. Appl. Polym. Sci. 1993, 48, 1607-1619. (6) Oudsema, J., W.; Pool, C., F. J. High Resolut. Chromatogr. 1993, 16, 198202. (7) Baner, L.; Buecherl, T.; Ewender, J.; Franz, R. J. Supercrit. Fluids 1992, 5, 213-219. (8) Terence, P.; Dowle, C. J.; Greenway, G. Analyst (London) 1991, 116, 1299. (9) Ryan, T. W.; Yocklovich, S. G.; Watkins, J. C.; Levy, E. J. J. Chromatogr. 1990, 505, 273-282. (10) Raynor, M.; W.; Bartle, K. D. J. Supercrit. Fluids 1993, 6, 39-49. (11) Simonsick, W. J., Jr.; Litty, L. L. ACS Symp. Ser. (Supercrit Fluid Technol.) 1992, 488, 288-303.
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Table 2. Recoveries of Two Samples at Different Extraction Times recoverya (%)
a
extraction time (min)
SANDUVOR 3058
CYAGARD UV1164
5 10 25
92 99 100
90 99 100
At 0.7 g/mL density and 120 °C.
Table 3. CYAGARD UV 1164 Concentration (% w/w) as Analyzed by Toluene Extraction and SFE toluene extraction
SFE
1.62 1.94 2.01 2.22 2.40
1.68 1.88 2.35 2.21 2.47
Figure 3. Depth distribution profile of SANDUVOR 3058 and CYAGARD UV 1164 in a two-layer system using SFE/GC. The additive was added only to the top layer from 0 to 100 µm. No additve was added between 100 and 160 µm.
Figure 4. Effect of adding a barrier to minimize migration of additives. Figure 2. Depth distribution profile of SANDUVOR 3058 and CYAGARD UV 1164 in a one-layer system using SFE/GC.
The precision of the SFE analysis was about 4% relative standard deviation for seven replicates for a sample containing 2.22% CYAGARD UV 1164. Depth profiles of light stabilizers in a one-layer system show a small amount of loss from the surface, as shown in Figure 2 for SANDUVOR 3058 and CYAGARD UV 1164. The small loss at the surface is probably due to vaporization and diffusion to the surface. The loss during curing was also detected by placing a steel strip coated with polymer containing additive inside the GC injection port at the polymer curing temperature. The front end of the GC column was kept in dry ice. After the curing period was completed, the metal strip was removed, the injection port temperature was raised to the operating temperature, and then the analysis began. The additives were detected. The estimated loss from the surface during curing is between 1% and 3%. In a two-layer system, the depth profile is significantly different from that in the one-layer system. The bottom layer (from 100 to 160 µm) was partially cured without additives. After the partial curing, a top layer (from 0 to 100 µm) containing 3% CYAGARD UV 1164 and 1.5% SANDUVOR 3058 was added and then cured. The first 30 µm shows some loss from the surface (Figure 3). 3764 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
This loss is similar to the loss from the surface in the one-layer system mentioned in Figure 2. Between 50 and 160 µm, there is a continuous decrease in the additive concentration, approaching linearity in part of the plot. While in the surface area of the coat the loss mechanism is surface vaporization and diffusion, the migration pattern between 50 and 160 µm is rather complex. It is facilitated by swelling.1 This occurs when the second layer (with additives) is applied. The solvent from this layer penetrates into the partially cured first layer, causing it to swell. This solventinduced swelling and subsequent escape from the bottom layer to the top layer are the key factors influencing the stabilizer migration. If the additive is added to either the top or the bottom layer, the additive will migrate to the layer that originally was free of additive. Additive migration is also observed when coatings are applied to plastic substrates. In such applications, most of the additive will migrate into the interior of the plastic base, and the stabilizers will lose their effectiveness. A barrier polymer layer can be used to prevent additive migration. Figure 4, shows the effect of utilizing such a barrier layer. While the migration of CYAGARD UV 1164 from the clearcoat into the TPO substrate is clearly indicated where no barrier layer is present, the barrier layer successfully stopped the additive migration.
Table 4. Comparison of Concentration Results for CYAGARD UV 1164 Obtained by ToF SIMS and SFE/GC for Individual Microtomed Sections section no.
concentration (%) SFE/GC
ToF SIMS
1 2 3 4 5 6 7 8 9
2.72 2.88 2.90 2.98 3.10 3.03 2.83 2.63 2.72
2.86 2.88a 2.91 3.14 2.92 2.81 2.71 2.58 2.54
average
2.89
2.81
a For this qualitative comparison, ToF SIMS was calibrated using SFE extraction of section 2 analyzed by GC.
Figure 5. ToF SIMS spectra of microtomed film containing Sanduvor 3058 and Cyagard UV 1164.
While the SFE/GC analysis of microtomed sections is very useful, it requires extraction followed by subsequent analysis. In contrast, ToF SIMS is a one-step technique. This surface analytical technique can be used to provide fast qualitative information on the microtomed film directly. In addition to positive identification of the additives present, ToF SIMS also can be a semiquantitative and quantitative technique. SIMS in general is not easy to quantify because of matrix effects. However, semiquantitative measurements may be made by comparison of ratios of peak areas. Thus, the ratio of the area under the peak of the additive divided by the area from a peak attributed to the polymer or the total ion counts may be used to follow the additive distribution. Since mass spectral information covering a wide mass range is always obtained, the presence of unexpected components is generally detected and can provide the analyst with additional information. Figure 5 shows the ToF SIMS spectrum from a microtomed section of a clearcoat film containing both CYAGARD UV 1164 and hindered amine light stabilizers. The top portion of the spectrum shows the range from m/z 0 to 600. The low-mass region (m/z 0-40) was blanked for most of the acquisition to minimize detector saturation effects. Most of the peaks observed up to m/z 250 are due to the polymer matrix. The lower spectrum, from m/z 340 to 600, shows peaks attributable to the additives. The peaks from the molecular ion of CYAGARD UV 1164 (M ) C33H39N3O2) are centered at m/z 510. The cluster of masses seen around m/z 509 is the result of M, M+, M-, and, to a lesser extent 13C isotope. The binning presentation for broad mass range distorts the relative peak intensity. Mass resolution greater than or equal to 6000 is obtainable and is obtained in this work. The other peaks observed in this region are contributions from HALS. To monitor the distribution of the CYAGARD UV 1164, spectra of the individual microtomed sections are obtained. A minimum of three spectra were obtained for each section. The data were analyzed by measuring the integrated intensity under the envelope centered at m/z 509, normalized by the peak area from a peak
Figure 6. Correlation between SFE/GC and ToF SIMS of Tinuvin 292 in a single-layer clearcoat.
from the polymer at m/z 163 (C6H7N6) obtained in the same spectrum. Alternatively, the total ion intensity from m/z 60 to 800 could be used. The depth profile produced from these data is qualitatively similar to that obtained by SFE/GC. The ToF SIMS and GC results are compared in Table 4. The supercritical fluid extract can also be analyzed directly by ToF SIMS. Figure 6 shows the results obtained by SFE/GC and SFE/SIMS for the analysis of Tinuvin 292. A good agreement between the two techniques is evident. For this qualitative comparison, ToF/SIMS was calibrated using SFE extraction of section 2 analyzed by GC. CONCLUSIONS The combination of microtoming and SFE techniques is a viable analytical tool for studying the behavior of light stabilizers. These techniques can be useful in investigating many other analytical problems. For example, depth profiles of residual monomer or residual volatiles can be investigated. ToF SIMS is a fast qualitative technique for the identification of the additives present. The ToF SIMS experiment takes less than 5 min. It may also be used as a quantitative technique if the additive in the film is calibrated using other techniques, such as SFE/GC. Received for review February 13, 1998. Accepted June 26, 1998. AC980171A Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
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