New high-throughput screening tool for the evaluation of pigmented UV-A curable coatings--a case study using low energy lamps.

Pigmented UV radiation-curable coatings have been a challenge to the coatings formulator for a long time. A critical match of pigment and photoinitiator package is crucial for good cure. Another issue facing the coatings chemist is the popularity of low energy lamps, which offer ease of handling, reduced safety concerns, and a low price tag. Obviously, the combination of curing pigmented coatings and using low energy lamps poses a special challenge.

To improve the pigmented coating development cycle, a rapid workflow was needed to screen formulations for cure under low energy UV-A radiation. For these types of applications, a high-throughput screening assay for discovering desirable photoinitiator packages was developed. Fluorescent dye-doped coatings cured under UV-A radiation were extracted with solvent. The relative degree-of-cure of the film was determined to be inversely related to a combination of the absorbance and fluorescence of the extractant. When this assay is used in conjunction with automated liquid handling and a spectrophotometer with an x-y scanning stage, the coatings formulator can analyze a large variable set in a short period of time. A primary screening of photoinitiators using this workflow suggests Lucirin[R] TPO-L, Darocur[R] 4265, and Irgacure[R] 2100 as optimum choices.

Keywords: Photoinitiators, fluorescence spectroscopy, solvent resistance, crosslinking, cure, UV, EB radiation cure, polyurethanes, isocyanate 

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UV radiation-curable coatings have been gaining popularity due to the potential of this technology to offer extremely low-VOC coating systems, high productivity, and energy savings. However, the curing of pigmented coatings by UV radiation has always been a challenge. (1-2) The secret of successful curing of a coating by UV radiation lies largely in both the initiation step, through which the free radicals are generated for the crosslinking reaction, and the radical propagation throughout curing. The efficiency of the initiation step in turn depends heavily on various factors, including the light source (wavelength, corresponding intensity, and dosage), the optical process (reflection, scattering, absorption, and transmission of the light), the quantum yield and optical absorbancy of the photoinitiator(s) (PI), and the pigment package. In a coating system where pigment- to-binder ratio (p/b) is high, achieving a good light absorbance efficiency and penetrating deep into the film is often difficult due to the interference from pigments/fillers. Recently, an even greater challenge presented itself with the introduction of low intensity UV-A light as the light source of choice for some applications. People are interested in low intensity UV-A light because of such potential benefits as ease of handling, reduced safety concerns, and lower equipment cost. (3) The lower energy lamp also allows for better penetration of the light into the film and promotes better through-curing. On the other hand, the longer curing time required could result in increased oxygen inhibition and thus, less efficient surface cure. Oxygen inhibition of the coating's surface is a concern in low energy light curing applications initiated with certain free radical photoinitiator systems, not including cationic initiators which do not suffer from oxygen inhibition. Efficient through-cure is difficult under high pigment-to-binder (p/b) ratios because the low energy radiation cannot easily penetrate through the coating. To develop a superior pigmented coating system for low intensity UV-A curing, it is very important for formulators to understand the effects of many variables on the curing process, including the secondary interactions, if any, among them. Facing the rapidly changing landscape of new equipment, new light sources, new photoinitiators, and/or pigments in today's market, it is a monumental task for a formulator to make the best choices of all these factors for an optimized coating system.

[FIGURE 1 OMITTED]

Fortunately, the relatively recent introduction of combinatorial and high-throughput screening methodology into material science research enables the rapid exploration of a large experimental space with multiple parameters. (4-6) This article describes such an effort in identifying the optimal photoinitiator package for a dual curing white urethane acrylate coating system using low intensity UV-A light.

EXPERIMENTAL

Preparation of Pigmented UV Formulation

A two-component dual cure formulation was prepared as follows. Polyol component consists of an unsaturated aromatic epoxy acrylate and an OH functional polyester. The polyisocyanate component contains an isocyanate bearing urethane acrylate. Each component (polyol and polyisocyanate) was prepared in a large batch and aliquots were taken to make the formulations with desired photoinitiator levels.

Polyol Blend                        Polyisocyanate Blend
Epoxy acrylate -- 413.55 g          Urethane acrylate -- 124 g
OH-functional polyester -- 206.7 g  Butyl acetate -- 76 g
Ti[O.sub.2] (rutile) -- 195 g       Butyl acetate -- 210 g

Photoinitiators

The photoinitiators were supplied as solids or liquids. All solid photoinitiators were dissolved in butyl acetate to yield a 10% solution by weight.

Formulation Preparation

The base formulation was prepared by mixing 2.3 mL of the polyol blend with 1.25 mL of the polyisocyanate blend and the appropriate amount of photoinitiator. An aliquot (400 [micro]L of 1 x [10.sup.-3] M) of perylene dye in butyl acetate was also added to the formulation, and the formulations were mixed for 15 min prior to use.

Liquid handling technology (7) was used to transfer 80 [micro]L of the formulations prepared as described above into 7 mL shell vials. The liquid handler was equipped with the disposable pipetting tip option, and the tips were automatically changed to prevent cross contamination of the formulations. Three replicas of each sample were prepared. After copying, the films were flashed for 15 min at room temperature and for 15 min at 50[degrees]C to remove solvent.

The samples were then cured using a 415W H & S Autoshot lamp. (8) The samples were exposed to the lamp for 10 min at a height of 10 in. After the samples were cured, they were aged at room temperature for four hours to ensure sample relaxation and the completion of the majority of the isocyanate/polyol reactions. Residual isocyanate/hydroxyl content was not measured, but since this reaction is constant in every formulation it has no bearing on the variation in response measured. A film of approximately 0.03 g/[cm.sup.2] was formed upon curing.

[FIGURE 2 OMITTED]

Analysis

Liquid handling technology was used to pipette 900 [micro]L of toluene onto each film. After 10 min, the liquid handler transferred 100 [micro]L of this toluene to a black quartz glass micro titer plate cuvette having a transparent quartz glass bottom plate. (9) The fluorescence intensity and absorbance in each well was determined by employing a fluorescence/absorbance reader. (10) The amount of dye extracted, which is proportional to fluorescence intensity, is related to the degree of cure of the coating. Only a minimal amount of dye will be extracted from a well-cured film, which is reflected in a low fluorescence count. However, anomalously low fluorescence readings are observed when the supernatant contains a high concentration of pigment, a situation that can occur when extracting solvent from a poorly cured system. Low fluorescence is observed because light scattering, caused by a large amount of pigment particles, prevents accurate fluorescence reading by the detector. When an absorbance reading is also obtained from the supernatant of every sample, the false readings can be identified. A truly well-performing system will yield both low fluorescence and low absorbance values. This is described in Figure 1.

The parameters used in the sample analysis are shown in Table 1.

RESULTS AND DISCUSSION

Initial Screening

A series of initial screening experiments was undertaken to efficiently search the parameter space for an optimum photoinitiator package. The first screening experiment included 24 different photoinitiator combinations, which are shown in Table 2. Further variation was induced by including several concentrations for each photoinitiator combination. The primary purpose of this initial screening was to refine the experimental technique and note some general trends in photoinitiator performance. The levels chosen for a particular combination were taken from the manufacturers' guide formulations.

Table 3 gives a description of each of the photoinitiators that were included in the study. It is generally assumed that [alpha]-hydroxyketone ([alpha]-HK) type PIs promote UV cure in unpigmented systems, or surface cure only in pigmented systems. In contrast, acyl phosphine oxide (APO) type PIs typically promote through-cure in pigmented systems since they are designed to be excited by longer wavelength radiation. (11) Lower energy radiation can penetrate deeper into the coating, hence acyl phosphine oxide PIs near the substrate can be activated.

A design of experiments was chosen with four factors: photoinitiator 1 with eight levels (PI-1 = Irgacure 1850, Irgacure 1870, Darocur 4265, Irgacure 819, Irgacure 907, Irgacure 2022, Irgacure 2100, Lucirin TPO-L), photoinitiator 2 with seven levels (PI-2 = Irgacure 184, Irgacure 500, Irgacure 754, Irgacure 1850, Irgacure 1870, Irgacure 819, Irgacure 907), amount of photoinitiator 1 with five levels (%PI-1), and amount of photoinitiator 2 with four levels (%PI-2). Of the four factors in the experiment, PI-1 and PI-2 introduced the most response variation. The effect of PI concentration was of lesser importance when compared to photoinitiator identity even though the data showed, as expected, that the fluorescence intensity was inversely proportional to the concentration level of the photoinitiator(s) in general, indicating increased cure with the increased PI concentration.

[FIGURE 3 OMITTED]

Of the four photoinitiators that were used as blend partners, Lucirin TPO-L blends showed the most favorable responses. Figure 2 shows that the average responses of formulations with Lucirin TPO-L were low (i.e., favorable), although they may not be statistically different from the Darocur 4265 blends, which also performed well. Blends containing Lucirin TPO-L or Darocur 4265 typically performed better than formulations with only one photoinitiator, pointing to a synergistic effect with these photoinitiators. Blend combinations with Irgacure 2100 appeared to perform particularly poorly when compared to the performance of Irgacure 2100 alone. It is important to point out that the differences in average responses of many of these photoinitiator packages may not be statistically significant, so Figure 2 should only be used to draw some general conclusions. Additionally, absorbance readings for all formulations were not obtained in this primary screening.

Subsequent Screenings

Based on the results of the initial screening, subsequent photoinitiator screenings were carried out with additional PIs. The photoinitiator factors and levels used in several rounds of experiments are given in Table 4. Because photoinitiators evaluated in the primary screening may have been subsequently re-evaluated and were always included in the final data analysis, these initial factors are included again in Table 4.

Table 5 gives a description of each of the additional photoinitiators not previously mentioned in Table 3.

Two responses, fluorescence and absorbance, were obtained for all of the combinations tested after the primary screening. Ideally, formulation optimization should involve analyzing both responses simultaneously. One way of doing this is by choosing lead formulations or identifying trends from a scatter graph where the responses are plotted along the x and y axes, as shown in Figure 3. For clarity, only single photoinitiator formulations are shown on this plot. In this graph, the ideal formulations would be found in the lower left hand corner, where both responses are minimized. Within this quadrant, formulations can be selected to achieve the desired blend of properties. A visual scan of Figure 3 indicates that Lucirin TPO-L or Darocur 4265 may be the best choices for PI-1.

Another technique for optimizing on multiple responses involves creating a combined "cure response" value. This cure response (CR), shown in equation (1), is a sum of the standardized response values for a given formulation. CR is minimized in ideal systems, which have a low fluorescence response and a low absorbance response. The terms are standardized (12) so that they can be compared.

CR = [w.sub.f] ([yi, fluorescence - [bar.y] flourescence]/[sigma] fluorescence) + [w.sub.a] ([yi, absorbance - [bar.y] absorbance]/[sigma] absorbance) (1)

The addition of coefficients to the terms in the cure response equation allows for an unequal weighting of the importance of the responses. Weighting coefficients are probably warranted, since more emphasis should be placed on the fluorescence term. Figure 4 shows a plot of the weighted cure response as a function of photoinitiator blend where [w.sub.f] = 0.7 and [w.sub.a] = 0.3. In this plot, only those formulations with a CR < -0.3 are shown. Based on the data shown in Figure 4, Lucirin TPO-L appears to be the best PI for this system.

[FIGURE 4 OMITTED]

A complimentary visualization is given in Figure 5, where only those photoinitiator packages with two photoinitiators are shown. Blends with Lucirin TPO-L or Darocur 4265 with various secondary photoinitiators appear to be appropriate choices for cure, if the concentration of PI-1 is kept to [less than or equal to] 3% and the concentration of PI-2 is kept to [less than or equal to] 1. This graph also indicates that the improved blending partners to produce better cure are Irgacure 819, Irgacure 1850, Irgacure 184, and Irgacure 1870.

There are many factors to consider when trying to rationalize why certain photoinitiators performed better than others in this study. Photoinitiators absorb photons in a single event and generate radicals that initiate cure. The degree of cure of the coating is influenced by the energy absorption and depth dose profile of the photoinitiator. Ideally, the emission spectrum of the light source should overlap the absorbance spectrum of the photoinitiator. In this experiment, a filter was used with the UV-A lamp to prevent radiation greater than 420 nm from reaching the sample. This lamp produces minimal light intensity at wavelengths less than 300 nm.

Consideration of the pigment package is also important since the pigment can absorb, scatter, and reflect part of the incoming radiation. Ideally, a photoinitiator that absorbs where the pigment does not should be chosen. A photoinitiator that absorbs in the lower energy UV-A region can be a good choice, as the rutile Ti[O.sub.2] used in this experiment absorbs radiation below ~360 nm. (14) Light in the visible region is reflected and/or scattered by this pigment. The pigment particle's shape, size, and concentration determine the extent of the reflection/scattering. Rutile Ti[O.sub.2] is expected to reflect about 50% of the incoming radiation at around 400 nm. The graph in Figure 6 shows the effects of the filter and pigment on limiting the radiation that is available to excite the photoinitiator(s). This plot shows a window between 365-400 nm that is optimal in this system for photoex-citation.

[FIGURE 5 OMITTED]

Thus, one would expect that photoinitiators that absorb strongly in the UV-A region between 365-400 nm would be the best performers. It has also been suggested that the [[lambda].sub.max] for PI absorption should be somewhat shifted from the [[lambda].sub.max] for lamp emission so that all of the incident radiation is not absorbed at the surface of the coating. This would be most important when curing thick films with a large extinction coefficient. The degree of cure will vary at different depths of the film for most systems. The radiation dose versus depth profile is inversely related to the photon penetration profile. To a first approximation, photon absorption follows the Lambert-Beer law, where the number of photons present at depth l is related to the optical absorbancy. Other factors that influence the rate of cure are the yield of radicals per absorbed photon, and the number of photons absorbed per unit time. These two factors control the quantum yield of a photoinitiator, which is the amount of monomer molecules consumed per photon absorbed. Quantum yield was not determined in this project.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Absorbance spectra were obtained for 11 of the photoinitiators used in this study. (15) One way to visualize the overlap of photoinitiator absorbance and lamp emission is shown in Figure 7. A variable that is created from the overlap term of PI absorbance and light emission was plotted versus the wavelength for six selected photoinitiators. Photoinitiators that maintained a high overlap term from 360-400 nm are Irgacure 819, Lucirin TPO-L, Darocur 4265, and Irgacure 2100. This correlated with the results from high throughput screening experiments (Figures 2 and 3). Due to yellowing issues, Irgacure 819 was one of four photoinitiators that was not utilized at the highest (5%) concentration.

However, this overlap term alone falls short of explaining the synergistic effect of the blend. Nervertheless, this study confirms that although there is still a potential need for the development of new and improved photoinitiators for pigmented UV-A curable paints, there are options already available for curing white pigmented systems under these conditions. The use of parallel screening tools facilitates and speeds up the development of the right initiator package. Specifically, a comprehensive picture of average behavior and discoloration can be analyzed to find the optimum for a given system. Using the right mixture of photoinitiators, the challenge of using low energy UV-A lamps in white paints can be solved.

CONCLUSIONS

A high-throughput screening study was used to select photoinitiators for a dual curing urethane acrylate white coating system suitable for low energy UV-A curing. Based on several levels of experiments, suggested photoinitiators for this particular system were Lucirin TPO-L, Darocur 4265, and Irgacure 2100. These photoinitiators absorbed radiation in the spectral region that coincided not only with the lamp's output, but also with the window where Ti[O.sub.2] did not absorb, scatter, or reflect light significantly. There were some degrees of synergistic effects when these PIs were blended with selected partners (Irgacure 819, 1850, 1870, and 184). This allowed for the use of lower total photoinitiator concentrations.

Although these photoinitiator choices can be rationalized, they are not necessarily the choices the coatings' formulator would have made a priori. Thus, the results from the high-throughput screening provide the UV coatings chemist with a large amount of relevant information that enables further optimization of the formulation.

ACKNOWLEDGMENT 

The authors thank Ciba, BASF, and Lamberti for providing samples of photoinitiators used in this study. Our gratitude also goes out to Mr. Phil Lunney for his support on statistical experimental design, Mr. Larry Galeza for formulation support, Mrs. Peggy Burke for analytical support, and Mr. Mike Dvorchak for helpful discussions.

References

(1) Lowe, C., "UV & EB Pigmented Coatings," in Chemistry and Technology of UV & EB Formulation for Coatings, Inks & Paints, Wiley & Sons, London, Vol. IV, pp. 24-43 and the references therein, 1997.

(2) Dvorchak, M.J., "UV Curing of Pigmented High-Build Wood Coatings Based on Non-Air-Inhibited Unsaturated Polyesters," J. COAT. TECHNOL., 67, No. 842, 49 (1995).

(3) Dvorchak, M.J., Strazisar, S., and Bach, H., "Automotive UV-A Curable Refinish Primers and Clear Coats-Fact or Fiction?," Radtech Report (2004).

(4) Wicks, D. and Bach, H., "High Throughput Screening for Formulations," Coatings World, 10, 38-47 (2002).

(5) Wicks, D. and Bach, H., "High Throughput Screening for Formulations-Part II," Coatings World, 11, 28-33 (2002).

(6) Webster, D., "High Throughput Workflow for the Development of Coatings," JCT COATINGSTECH, 1, No. 6, 34 (2004).

(7) Utilized Genesis RSP liquid handling robot, from Tecan U.S., Inc., www.tecan.com.

(8) UV-A400 model from H & S Autoshot Manufacturing Co. Ltd., 49 Mountainview Road North, Georgetown, Ontario L7G 4J7, Canada.

(9) Bach, H. and Guertler, C., PCT Int. Appl. WO 2002042745, 2002.

(10) Utilized SpectraFluor reader, from Tecan U.S., Inc., www.tecan.com.

(11) Key Product Selection Guide, Ciba, 2003.

(12) Standardized values measure how many standard units away from the mean the [y.sub.i] response is. These values are "unitless."

(13) At the p/b levels used in this experiment, Irgacure[R] 819 can be used at low concentrations without significant yellowing.

(14) Mehnert, R., Pincus, A., Janorsky, I., Stowe, R., and Berejka, A., UV & EB Curing Technology and Equipment, Vol. 1, 49, 1998.

(15) As measured in the Bayer MaterialScience labs.

Stephanie Strazisar, Margaret Kendi, Thomas Facke, Leone Hermans-Blackburn, and Xudong Sharon Feng -- Bayer MaterialScience*

Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27-29, 2004, in Chicago, IL.

* 100 Bayer Rd., Pittsburgh, PA 15205.

Table 1 -- Fluorescence and Absorbance Parameters Used on Sample
Analysis (a)

                        Fluorescence  Absorbance

Excitation wavelength   430 nm         --
Emission wavelength     535 nm         --
Gain                     70            --
Flashes                  20            20
Measurement wavelength   --           620 nm

(a) Utilized SpectraFluor reader, from Tecan U.S., Inc., www.tecan.com.

Table 2 -- Photoinitiators and Photoinitiator Loading Levels for
Primary Screening

Photoinitiator Package (a)   Photoinitiator Level (b)

Irgacure[R] 1850             1-4%
Irgacure 1870                1-4%
Darocur[R] 4265              1-5%
Irgacure 819                 1-4%
Irgacure 907                 1-4%
Irgacure 2022                2-5%
Irgacure 2100                2-5%
Lucirin[R] TPO-L             2-5%
Irgacure 2100/Irgacure 184   (2-3%)/(1-2%)
Irgacure 2100/Irgacure 500   (2-3%)/(1-2%)
Irgacure 2100/Irgacure 754   (2-3%)/(1-2%)
Irgacure 4265/Irgacure 184   (2.5-5%)/(0.25-0.5%)
Irgacure 4265/Irgacure 1850  (2.5-5%)/(0.25-0.5%)
Irgacure 4265/Irgacure 1870  (2.5-5%)/(0.25-0.5%)
Irgacure 4265/Irgacure 819   (2.5-5%)/(0.25-0.5%)
Irgacure 4265/Irgacure 907   (2.5-5%)/(0.25-0.5%)
Irgacure 819/Irgacure 184    (1%)/(1-2%)
Irgacure 819/Irgacure 500    (1%)/(1-2%)
Irgacure 819/Irgacure 754    (1%)/(1-2%)
Lucirin TPO-L/Irgacure 184   (2.5-5%)/(0.25-0.5%)
Lucirin TPO-L/Irgacure 1850  (2.5-5%)/(0.25-0.5%)
Lucirin TPO-L/Irgacure 1870  (2.5-5%)/(0.25-0.5%)
Lucirin TPO-L/Irgacure 819   (2.5-5%)/(0.25-0.5%)
Lucirin TPO-L/Irgacure 907   (2.5-5%)/(0.25-0.5%)

(a) All photoinitiators from Ciba Specialty Chemicals, www.cibasc.com,
except Lucirin TPO-L from BASF Corporation, www.basf.de.
(b) On total solids.

Table 3 -- Photoinitiator Descriptions

Photoinitiator  Type (a)

Irgacure 1850   BAPO & [alpha]-HK
Irgacure 1870   BAPO
Darocur 4265    [alpha]-HK & MAPO
Irgacure 819    BAPO
Irgacure 907    [alpha]-AK
Irgacure 2022   BAPO & [alpha]-HK
Irgacure 2100   PO
Lucirin TPO-L   BAPO
Irgacure 184    [alpha]-HK
Irgacure 500    [alpha]-HK & benzophenone
Irgacure 754    phenylglyoxylate

(a) BAPO = bis acyl phosphine oxide, MAPO = mono acyl phosphine oxide,
[alpha]-AK = [alpha]-aminoketone, [alpha]-HK = [alpha]-hydroxyketone,
PO = phosphine oxide.

Table 4 -- Photoinitiators and Photoinitiator Loading Levels for Further
Screenings

Photoinitiator Package (a)   Photoinitiator Level (b)

CAA 0869                     2-5%
Darocur 4265                 1-5%
Esacure ONE                  5%
Esacure 1001M                2-5%
Esacure EDB                  2-5%
Esacure ITX                  2-5%
Irgacure 184                 5%
Irgacure 1850                1-4%
Irgacure 1870                1-4%
Irgacure 2100                2-5%
Irgacure 819                 1-4%
Irgacure 907                 1-4%
Irgacure 2022                2-5%
LFC 1103                     2-5%
LFC 1689                     2-5%
LFC 1699                     2-5%
LFC 1849                     2-5%
LFC 1891                     5%
LFC 1900                     5%
LFC 2006                     5%
Lucirin TPO-L                2-5%
Darocur 4265/Esacure ONE     (2.5%)/(1-2%)
Esacure EDB/Esacure ITX      (2-5%)/(0.25-1.5%)
Darocur 4265/Irgacure 184    (2.5-5%)/(0.25-2%)
Irgacure 2100/Irgacure 184   (2-3%)/(1-2%)
Irgacure 819/Irgacure 184    (1%)/(1-2%)
Lucirin TPO-L/Irgacure 184   (2.5-5%)/(0.25-2%)
Darocur 4265/Irgacure 184    (2.5%)/(2%)
Darocur 4265/Irgacure 1850   (2.5-5%)/(0.25-2%)
Lucirin TPO-L/Irgacure 1850  (2.5-5%)/(0.25-2%)
Darocur 4265/Irgacure 1870   (2.5-5%)/(0.25-0.5%)
Lucirin TPO-L/Irgacure 1870  (2.5-5%)/(0.25-0.5%)
Irgacure 2100/Irgacure 500   (2-3%)/(1-2%)
Irgacure 819/Irgacure 500    (1%)/(1-2%)
Irgacure 2100/Irgacure 754   (2-3%)/(1-2%)
Irgacure 819/Irgacure 754    (1%)/(1-2%)
Darocur 4265/Irgacure 819    (2.5-5%)/(0.25-2%)
Lucirin TPO-L/Irgacure 819   (2.5-5%)/(0.25-2%)
Darocur 4265/Irgacure 907    (2.5-5%)/(0.25-0.5%)
Lucirin TPO-L/Irgacure 907   (2.5-5%)/(0.25-0.5%)
Darocur 4265/LFC 1891        (2.5%)/(1-2%)
Darocur 4265/LFC 1900        (2.5%)/(1-2%)
Darocur 4265/LFC 2006        (2.5%)/(1-2%)

(a) All Irgacure and Darocur photoinitiators were obtained from Ciba
Specialty Chemicals, www.cibasc.com, all Esacure photoinitiators and CAA
0869 were obtained from Lamberti Spa, www.esacure.com, and Lucirin TPO-L
from BASF Corporation, www.basf.de.
(b) On total solids.

Table 5 -- Photoinitiator Descriptions

Photoinitiator  Type

CAA 0869        phenylglyoxylate
Esacure ONE     [alpha]-HK (a)
Esacure 1001M   [beta]-ketosulfone derivative
Esacure EDB     amine coinitiator
Esacure ITX     thioxanthone
LFC 1103        [beta]-ketosulfone derivative
LFC 1689        [beta]-ketosulfone derivative
LFC 1699        [beta]-ketosulfone derivative
LFC 1849        phenylglyoxylate
LFC 1891        [alpha]-HK
LFC 1900        [alpha]-HK
LFC 2006        [alpha]-HK

(a) [alpha]-HK = [alpha]-hydroxyketone.