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Depth profiling of UV cured coatings containing photostabilizers by confocal Raman microscopy
Progress in Organic Coatings 35 (1999) 197±204
Depth pro®ling of UV cured coatings containing photostabilizers by confocal Raman microscopy W. Schrof*, E. Beck, R. KoÈniger, W. Reich, R. Schwalm BASF AG, Polymer Research Laboratory, D-67056 Ludwigshafen, Germany Received 15 July 1998; accepted 15 October 1998
Abstract The chemical imaging possibility of confocal Raman microscopy was used to characterize UV cured coatings layers. Depth pro®les of acrylate curing conversion were recorded in order to elucidate the interaction of photoinitator, photostabilizer, and irradiation source. Formulations containing acyl phosphine oxides resulted in a better through cure. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Ultraviolet curing; Confocal Raman spectroscopy; Chemical imaging; Depth pro®les; Curing; Ultraviolet stabilizers
1. Introduction Increasing amounts of radiation curable coating raw materials are used for the surface re®nement of furniture, wooden ¯oor coverings, paper etc. A low viscous formulation of reactive monomers and oligomers, photoinitiators, light stabilizers, pigments etc. without any solvents is cured after a simple deposition process by UV light or electron beam to become a protective coating [1±4]. The drying of the coating is completed within fractions of a second, due to the high reactivity of the compounds, which results in a three-dimensionally crosslinked polymer molecule with nearly no extractables (see Fig. 1). This technological advantage, however, simultaneously bears the disadvantage, that the reacted polymer as a crosslinked structure withstands every classical polymer characterization and classi®cation. As a consequence there is an urgent need for new and easily applicable measurement techniques to characterize these solid three-dimensionally crosslinked ®lms. In this context the method of real-time infrared spectroscopy (RTIR) has to be mentioned which was mainly developed by Prof. Decker, Mulhouse [4±6]. Thus the temporal reaction yield could be followed in the millisecond range by the decreasing -absorption of the acrylic ester double bonds while simultaneously curing with UV light. Important information on the polymerization rate, content of unreacted double bonds, inhibition by oxygen, polymerization quantum yield, etc. will be gained by evaluating the reaction * Corresponding author.
yield curves. The experimental re®nement of RTIR, rapid scanning FTIR, used at BASF AG allows the fast recording of a series of spectra which gives insight into the reaction kinetics of different reacting compounds. The method of RTIR faces dif®culties in the investigation of the depth pro®le of curing due to the integral transmission measurement. Extensive measurements at different sample thicknesses have to be performed to estimate the depth pro®le of curing in a relatively unprecise way. The present paper we will demonstrate how this problem can be solved by confocal Raman microscopy [7±11]. Depth pro®les of curing will be gained with excellent depth resolution without any special sample preparation. In the same way lateral pro®les of conversion can be measured with an even better spatial resolution of less than 1 mm. After a microtome cut preparation, this can be used to measure depth pro®les of samples optically not accessible due to pigments or other opaque ®llers. An inherent problem of UV curing technology is the interaction of photoinitiation and light stabilization. While UV light is needed to initiate the curing process, it is known to be a source of the unwanted long-term degradation of the crosslinked coating. Therefore light stabilizers have to be added to the coating formulation to ensure long-term functionality. Mostly UV absorbers (UVA) and sterically hindered amines are used for this purpose [1]. The in¯uence of such additives on the curing process will be investigated by confocal Raman microscopy. The perspective of confocal Raman microscopy to analyze a three-dimensional coating ®lm by lateral mappings or depth pro®les will be shown. Typical problems
0300-9440/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0300-944 0(99)00026-0
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W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
Fig. 3. Depth pro®le of curing of polyetheracrylate coating, the reactive acrylate groups are marked by arrows. Fig. 1. UV curing of acrylic coatings formulations.
in radiation curing can be addressed e.g. the inhibition of a radical polymerization reaction by oxygen, the limited polymerization reaction in deep layers due to the restricted penetration of UV light (Lambert±Beer), the reaction in pigmented coatings, the interaction of UV curing with UV additives, the distribution of UV stabilizers or pigments in the coating layer.
Fig. 2. Schematic layout of confocal Raman spectrometer.
2. Experimental 2.1. Confocal Raman microscopy Confocal Raman microscopy combines the chemical information from vibrational spectroscopy with the spatial resolution of confocal microscopy [6±11] (see Fig. 2). For example the vibrations of acrylate groups from a con®ned measuring volume of about 1 mm 3 can be detected in backscattering geometry. A number of steps of technical improvement in modern Raman spectrometers resulted in short measuring times even with small confocal volumes: notch ®lters to selectively suppress laser Rayleigh scattering while transmitting Raman photons, high throughput single grating spectrographs and sensitive low-noise cooled CCD arrays for parallel detection of all Raman bands. Depth pro®les and lateral maps of characteristic Raman bands
Fig. 4. Depth pro®les of conversion of three polyetheracrylate formulations.
W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
199
Fig. 5. Conversion of polyetheracrylate PO as a function of depth (photo-initiator Irgacure 500, amine synergist, 10 m/min, UV power 120 W/cm). Intensities of reactive bands increase with depth.
can be recorded by moving the sample relatively to the focus of the microscope objective. This chemical imaging feature of confocal Raman microscopy can help to explore curing conversion or concentration gradients in coating layers originating from the formulation recipe or the curing process. The Raman intensities linearly scale with the concentrations. The Raman cross section, however, differs for every vibrating group. Therefore a calibration is needed to evaluate absolute concentrations from Raman spectra. Relative concentrations can be obtained from comparison of different spectra. The used Raman microscope LABRAM of the Dilor company is equipped with a He±Ne laser (632.8 nm, 10
mW laser power at the sample surface). A confocal volume down to 1 mm 3 can be selected by a microscope objective with a magni®cation 100 £ , numerical aperture (NA) of 0.8, and a working distance of nearly 4 mm (Olympus 100 ulwd) using a confocal aperture of 75 mm. A spectral resolution of 7 cm 21 is obtained using a grating with 600 grooves/mm or of 2 cm 21 with 1800 grooves/mm. Typical recording times are around 1 min for a single point measurement. Computer controlled movements of a x±y translational stage and a Piezo driven microscope objective can results in a lateral mapping or depth pro®ling of the crosslinking reaction, respectively. The coating formulations are doctor bladed onto glass substrates and UV cured. Then the ®lms can be investigated in the Raman microscope without further sample preparations. In case of limited optical penetration as with pigmented coatings thin sections can be prepared by microtomy. Thus depth pro®ling can be transformed to lateral mapping.
3. Measurements and discussion 3.1. Depth pro®ling to investigate oxygen inhibition
Fig. 6. Conversion of polyetheracrylate PO as a function of depth (logarithmic scale) for two photoinitiator systems, UV lamp running at 120 W/ cm.
One possible problem of UV curing of acrylic formulations is that the surface of the coating layer stays tacky while the bulk is fully hardened. Such an inhibition of the radical curing reaction is caused by the reaction of photoinitiator radicals and polymer radicals with dissolved and penetrating oxygen.
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Fig. 7. Depth pro®le of polyetheracrylate pigmented with 10% TiO2 (rutile). Intensities of reactive bands increase near the surface due to oxygen inhibition. Intense TiO2 bands are found below 700 cm 21.
R z 1O2 ! R 2 O 2 Oz where R z is the photoinitiator or polymer radical. The hydroperoxide radicals formed by this reaction are relatively stable and no longer active for polymerization. Thus the polymerization will be inhibited or slowed down
until no progress of the reaction will be found. Depth resolved Raman spectra for a UV cured polyetheracrylate coating are given in Fig. 3. In this case of a simple polyetheracrylate formulation unreacted acrylate bands can clearly be identi®ed in layers of about 6 mm from the surface (see arrows in Fig. 3), which are vanishing in deeper layers.
Fig. 8. Raman spectra of polyesteracrylate pigmented with 10% TiO2 (rutile and anatase modi®cations) each recorded at the coating surface and in a depth of 15 mm. Rutile and anatase can be distinguished easily by Raman spectroscopy. In contrast to Fig. 7, a concentration gradient is found for the TiO2 pigments with lower concentrations at the coating surface.
W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
201
Fig. 9. Raman spectra of cured polyurethane acrylates UA with and stabilizers (UVA, HALS), photoinitiator Irgacure 184 (4%) without light
UV curing is inhibited near the surface. Measurements performed in parallel with ®lms cured under inert atmosphere show a homogeneous curing through all depth layers. The lack of curing at the surface can be prevented by modi®cation with amines or by a paraf®n barrier layer (see Fig. 4). To compensate for optical penetration effects and for changes of the confocal volume at the surface the band intensities of the acrylate vibrations are normalized to
CH-vibrations not affected by the curing reaction. The effect of amine modi®cation and additives is clearly seen. Coating layers at the surface show the same curing conversion as bulk layers. This proves that confocal Raman microscopy is well suited to investigate oxygen inhibition near the surface of coatings. Other methods to suppress oxygen inhibition are: increased dosages of UV irradiation, curing under inert atmosphere or under protective foils, higher concentrations
Fig. 10. Depth pro®les of curing conversion of polyurethane acrylates UA with and without light stabilizers (UVA, HALS), two photoinitiator systems, and two irradiation dosages
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W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
Fig. 11. Lateral mapping of UVA concentration (arbitrary units) in an area of 30 mm, depth 10 mm. UVA content is 1.5%.
of photoinitiators or synergists reacting with the formed hydroperoxiradicals. 3.2. Depth pro®ling to investigate imperfect reaction conversion in deep layers Due to Lambert±Beer's law of absorption the penetration of UV light to deeper layers is limited. As a consequence the crosslinking conversion in deeper layers is reduced resulting in weaker ®lm cohesion. This problem can occur not only in thick ®lms but also with reactive formulations being soaked into the pores of the substrate. Confocal Raman spectroscopy reveals this effect in transparent coatings without further sample preparation down to depth layers of 0.5 mm. In the case of opaque ®lms microtome cuts are needed. Afterwards the depth pro®le can be measured by lateral mapping of the conversion. In Fig. 5, Raman spectra of polyetheracrylate PO recorded in different depth layers are given. The Raman intensities of reactive groups normalized
Fig. 12. Lateral mapping of double bond density concentration (arbitrary units) in an area of 30 £ 30 mm, depth 10 mm
Fig. 13. Depth pro®les of curing conversion of polyesteracrylate containing increasing concentrations of light stabilizers (UVA, ®xed concentration of photoinitiator.
to bands not affected by the crosslinking reaction (1450 cm 21) vary with depth. Fig. 6 shows the conversion of the crosslinking reaction as a function of depth. The logarithmic scale allows a demonstration of oxygen inhibition directly at the coating surface and a reduced conversion in deeper layers some hundred mm below surface. 3.3. Depth pro®ling to investigate reaction conversion in pigmented layers Pigmented formulations are a challenge not only for optical measurement techniques but also for UV curing because of limited penetration of light. The formulation of Fig. 7 contains 10% TiO2 pigments. In this case Raman microscopy can explore the curing conversion in a layer of about 30 mm near to the surface. The spectra show an inhibition reaction at the surface while the deeper layers exhibit
Fig. 14. Depth pro®les of curing conversion of polyesteracrylate formulations containing increasing concentrations of photoinitiator, ®xed concentrations of light stabilizers.
W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
Fig. 15. Depth pro®les of curing conversion of polyesteracrylate formulations containing varying concentrations of UVA and HALS to separate the in¯uence of UVA and HALS, ®xed photoinitiator concentration 4%.
a better curing. A reduction of curing conversion due to restricted UV penetration can not be found in the investigated depth layer of 30 mm. Additional measurements with microtomed coatings show a reduced curing conversion in deeper layers which is more pronounced at higher pigment loadings. 3.4. Depth gradients of pigments in cured coatings Confocal Raman microscopy can not only be used to elucidate the depth dependence of curing, but can also monitor gradients of additives. In the case of TiO2 loaded coatings, Raman spectroscopy can distinguish rutile and anatase modi®cations (Fig. 8) by the shift of the anatase bands to lower frequencies. The anatase loaded coating exhibits a slightly higher conversion than the rutile loaded one. The example of Fig. 8 shows a depth dependence of the TiO2 concentration. In this polyesteracrylate formulation, which was not optimized, a sedimentation of the pigment seems to occur which results in a relative lower pigment concentration at the coating surface. A similar behavior is not found in the example of Fig. 7.
203
polymer. From Fig. 10 one can learn that light stabilzers reduce reaction conversion, especially at low UV irradiation doses and in deeper layers. Replacing a small amount of photoinitiator Irgacure 184 by the long wavelength absorbing TPO (0.5% out of 4%) helps to reduce insuf®cient curing in deeper layers. The distribution of the UV absorber molecules can also be investigated by Raman mapping. Fig. 11 demonstrates for an area of 30 £ 30 mm a nearly uniform UVA concentration in a depth layer of 10 mm below the surface. Additional depth pro®les don't show any gradients either. Lateral maps of conversion show a similar homogeneity (Fig. 12). These lateral investigations of curing conversion may reveal inhomogeneous irradiation conditions which can be caused by varying distances to the UV lamps or by shadowing effects. A more detailed investigation has been carried out with a polyesteracrylate formulation with varied concentrations of photoinitiators (Darocure 1173 and TPO), UVA (Tinuvin 400) and sterically hindered amines (Tinuvin 292). The ®lms of a thickness of 50 mm have been cured by a mercury bulb (120 W/cm, 10 m/min). In Fig. 13 the photoinitiator (D1173) concentration is kept constant at 4% while the amount of light stabilizers (UVA: HALS 2:1) is increased from 0.2% UVA to 4% UVA. As a result, the curing conversion decreases. UVA concentrations of 2% and more yield drastic reductions of curing conversion. The depth pro®le of conversion proves that higher UVA concentrations are accompanied by oxygen inhibition at the coating surface and less bulk conversion which is caused by lower UV curing intensities in deeper layers due to Lambert±Beer's law of absorption. In Fig. 14 a similar result is observed by variation of the photoinitiator concentration while keeping the amount of light stabilizers constant. Confocal Raman microscopy manifests a massive reduction of curing in deeper layers if
3.5. Interaction of UV curing with UV light stabilizers Many coatings formulations contain UV additives to stabilize coatings against long-term degradation by UV light. These additives can in¯uence the curing conversion of radiation curing, because they can absorb UV light, scavenge or inactivate radicals needed for polymerisation. Confocal Raman microscopy can monitor the distribution of these additives and their effect on curing. Fig. 9 plots the Raman spectra of two polyurethane acrylates UA, with and without light stabilizers (UVA, HALS). Whereas the aromatic UV absorber can be detected by its characteristic bands (Fig. 9) even at low concentrations around 1%, the aliphatic HALS compound (concentration 1%) is hidden by the intense Raman bands of the crosslinked
Fig. 16. Depth pro®les of curing conversion of polyesteracrylate formulations containing varying relative concentrations of the two photoinitiators D1173 and TPO (together 4%), the concentrations of UVA and HALS are kept ®xed.
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conversion in deep layers or at the coating surface will occur. The non-destructive optical determination of concentration gradients in coatings on the original substrate opens fundamental new insights into daily problems in radiation curing. Craters or inhomogenities of leveling can be analyzed directly on the spot for their chemical or physical origins.
the concentration of photo-initiator becomes comparable or lower than the UVA concentration. In order to separate the in¯uence of UVA and HALS stabilizers on curing, depth pro®les of curing conversion are taken from coatings containing only UVA and HALS, respectively. Fig. 15 clearly shows that reduced curing is mainly caused by UVA absorption reducing the UV illumination, especially in deeper layers, wheras HALS only produces a minor effect. Small amounts of the long wavelength absorbing photoinitiator TPO in combination with D1173 can improve the curing conversion in deeper layers, even in presence of light stabilizers (see Fig. 16). Higher concentrations of TPO, on the other hand, can lead to increased oxygen inhibition at the surface.
S. Fischer, K. Menzel, F. VoÈllinger, R. Horn, M. MuÈller, and K.-H. Joost are thanked for sample preparation and UV curing, M. MuÈller for measurements and data evaluation.
4. Conclusion and outlook
References
The method of confocal Raman microscopy allows a simple and fast characterization of UV cured coatings by depth pro®ling and lateral mapping of functional groups with a spatial resolution up to 1 mm 3. Typical problems in radiation curable coatings were successfully investigated:
[1] J.P. Fouassier, Photoinitiation, Photopolymerization and Photocuring, Hanser, MuÈnchen, 1995. [2] L. HaÈubling, W. Reich, E. Keil, Kunststoffe 86 (1996) 354. [3] S.P. Pappas, Radiation Curing: Science and Technology, Plenum Press, New York, 1992. [4] C. Decker, K. Moussa, J. Coat. Technol. 62 (1990) 55. [5] C. Decker, J. Polym. Sci. A: Polym. Chem. 30 (1992) 913. [6] C. Decker, in: V.V. Krongauz, A.D. Trifunal (Eds.), Processes in A Photoreactive Polymers, Chapman and Hall, New York, 1995, pp. 34. [7] T. Wilson, Confocal Microscopy, Academic Press, London, 1990. [8] B. Schrader, Infrared and Raman Spectroscopy, VCH, Weinheim, 1995. [9] L. Markwort, B. Kip, E. Da Silva, B. Roussel, Appl. Spectrosc. 49 (1995) 1411. [10] K.P.J. Williams, G.D. Pitt, D.N. Batchelder, B.J. Kip, Appl. Spectrosc. 48 (1995) 232. [11] M. Claybourn, A. Luget, K.P.J. Williams, in: M.W. Urban, T. Provder (Eds.), Multidimensional Spectroscopy of Polymers, 598, ACS Symp. Ser, 1995, pp. 41.
² the inhibition of the polymerization reaction by oxygen; ² the limited polymerization reaction in deep layers due to the restricted penetration of UV light (Lambert±Beer); ² the reaction in pigmented coatings; ² the effect of UVA and HALS light stabilizers on the curing conversion, spatial distribution of UV absorbing additives. In formulations with light stabilizers a sensitive balance between photoinitiators, UV absorbers and illumination conditions has to be adjusted. Otherwise a reduced curing
Acknowledgements
Depth pro®ling of UV cured coatings containing photostabilizers by confocal Raman microscopy W. Schrof*, E. Beck, R. KoÈniger, W. Reich, R. Schwalm BASF AG, Polymer Research Laboratory, D-67056 Ludwigshafen, Germany Received 15 July 1998; accepted 15 October 1998
Abstract The chemical imaging possibility of confocal Raman microscopy was used to characterize UV cured coatings layers. Depth pro®les of acrylate curing conversion were recorded in order to elucidate the interaction of photoinitator, photostabilizer, and irradiation source. Formulations containing acyl phosphine oxides resulted in a better through cure. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Ultraviolet curing; Confocal Raman spectroscopy; Chemical imaging; Depth pro®les; Curing; Ultraviolet stabilizers
1. Introduction Increasing amounts of radiation curable coating raw materials are used for the surface re®nement of furniture, wooden ¯oor coverings, paper etc. A low viscous formulation of reactive monomers and oligomers, photoinitiators, light stabilizers, pigments etc. without any solvents is cured after a simple deposition process by UV light or electron beam to become a protective coating [1±4]. The drying of the coating is completed within fractions of a second, due to the high reactivity of the compounds, which results in a three-dimensionally crosslinked polymer molecule with nearly no extractables (see Fig. 1). This technological advantage, however, simultaneously bears the disadvantage, that the reacted polymer as a crosslinked structure withstands every classical polymer characterization and classi®cation. As a consequence there is an urgent need for new and easily applicable measurement techniques to characterize these solid three-dimensionally crosslinked ®lms. In this context the method of real-time infrared spectroscopy (RTIR) has to be mentioned which was mainly developed by Prof. Decker, Mulhouse [4±6]. Thus the temporal reaction yield could be followed in the millisecond range by the decreasing -absorption of the acrylic ester double bonds while simultaneously curing with UV light. Important information on the polymerization rate, content of unreacted double bonds, inhibition by oxygen, polymerization quantum yield, etc. will be gained by evaluating the reaction * Corresponding author.
yield curves. The experimental re®nement of RTIR, rapid scanning FTIR, used at BASF AG allows the fast recording of a series of spectra which gives insight into the reaction kinetics of different reacting compounds. The method of RTIR faces dif®culties in the investigation of the depth pro®le of curing due to the integral transmission measurement. Extensive measurements at different sample thicknesses have to be performed to estimate the depth pro®le of curing in a relatively unprecise way. The present paper we will demonstrate how this problem can be solved by confocal Raman microscopy [7±11]. Depth pro®les of curing will be gained with excellent depth resolution without any special sample preparation. In the same way lateral pro®les of conversion can be measured with an even better spatial resolution of less than 1 mm. After a microtome cut preparation, this can be used to measure depth pro®les of samples optically not accessible due to pigments or other opaque ®llers. An inherent problem of UV curing technology is the interaction of photoinitiation and light stabilization. While UV light is needed to initiate the curing process, it is known to be a source of the unwanted long-term degradation of the crosslinked coating. Therefore light stabilizers have to be added to the coating formulation to ensure long-term functionality. Mostly UV absorbers (UVA) and sterically hindered amines are used for this purpose [1]. The in¯uence of such additives on the curing process will be investigated by confocal Raman microscopy. The perspective of confocal Raman microscopy to analyze a three-dimensional coating ®lm by lateral mappings or depth pro®les will be shown. Typical problems
0300-9440/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0300-944 0(99)00026-0
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W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
Fig. 3. Depth pro®le of curing of polyetheracrylate coating, the reactive acrylate groups are marked by arrows. Fig. 1. UV curing of acrylic coatings formulations.
in radiation curing can be addressed e.g. the inhibition of a radical polymerization reaction by oxygen, the limited polymerization reaction in deep layers due to the restricted penetration of UV light (Lambert±Beer), the reaction in pigmented coatings, the interaction of UV curing with UV additives, the distribution of UV stabilizers or pigments in the coating layer.
Fig. 2. Schematic layout of confocal Raman spectrometer.
2. Experimental 2.1. Confocal Raman microscopy Confocal Raman microscopy combines the chemical information from vibrational spectroscopy with the spatial resolution of confocal microscopy [6±11] (see Fig. 2). For example the vibrations of acrylate groups from a con®ned measuring volume of about 1 mm 3 can be detected in backscattering geometry. A number of steps of technical improvement in modern Raman spectrometers resulted in short measuring times even with small confocal volumes: notch ®lters to selectively suppress laser Rayleigh scattering while transmitting Raman photons, high throughput single grating spectrographs and sensitive low-noise cooled CCD arrays for parallel detection of all Raman bands. Depth pro®les and lateral maps of characteristic Raman bands
Fig. 4. Depth pro®les of conversion of three polyetheracrylate formulations.
W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
199
Fig. 5. Conversion of polyetheracrylate PO as a function of depth (photo-initiator Irgacure 500, amine synergist, 10 m/min, UV power 120 W/cm). Intensities of reactive bands increase with depth.
can be recorded by moving the sample relatively to the focus of the microscope objective. This chemical imaging feature of confocal Raman microscopy can help to explore curing conversion or concentration gradients in coating layers originating from the formulation recipe or the curing process. The Raman intensities linearly scale with the concentrations. The Raman cross section, however, differs for every vibrating group. Therefore a calibration is needed to evaluate absolute concentrations from Raman spectra. Relative concentrations can be obtained from comparison of different spectra. The used Raman microscope LABRAM of the Dilor company is equipped with a He±Ne laser (632.8 nm, 10
mW laser power at the sample surface). A confocal volume down to 1 mm 3 can be selected by a microscope objective with a magni®cation 100 £ , numerical aperture (NA) of 0.8, and a working distance of nearly 4 mm (Olympus 100 ulwd) using a confocal aperture of 75 mm. A spectral resolution of 7 cm 21 is obtained using a grating with 600 grooves/mm or of 2 cm 21 with 1800 grooves/mm. Typical recording times are around 1 min for a single point measurement. Computer controlled movements of a x±y translational stage and a Piezo driven microscope objective can results in a lateral mapping or depth pro®ling of the crosslinking reaction, respectively. The coating formulations are doctor bladed onto glass substrates and UV cured. Then the ®lms can be investigated in the Raman microscope without further sample preparations. In case of limited optical penetration as with pigmented coatings thin sections can be prepared by microtomy. Thus depth pro®ling can be transformed to lateral mapping.
3. Measurements and discussion 3.1. Depth pro®ling to investigate oxygen inhibition
Fig. 6. Conversion of polyetheracrylate PO as a function of depth (logarithmic scale) for two photoinitiator systems, UV lamp running at 120 W/ cm.
One possible problem of UV curing of acrylic formulations is that the surface of the coating layer stays tacky while the bulk is fully hardened. Such an inhibition of the radical curing reaction is caused by the reaction of photoinitiator radicals and polymer radicals with dissolved and penetrating oxygen.
200
W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
Fig. 7. Depth pro®le of polyetheracrylate pigmented with 10% TiO2 (rutile). Intensities of reactive bands increase near the surface due to oxygen inhibition. Intense TiO2 bands are found below 700 cm 21.
R z 1O2 ! R 2 O 2 Oz where R z is the photoinitiator or polymer radical. The hydroperoxide radicals formed by this reaction are relatively stable and no longer active for polymerization. Thus the polymerization will be inhibited or slowed down
until no progress of the reaction will be found. Depth resolved Raman spectra for a UV cured polyetheracrylate coating are given in Fig. 3. In this case of a simple polyetheracrylate formulation unreacted acrylate bands can clearly be identi®ed in layers of about 6 mm from the surface (see arrows in Fig. 3), which are vanishing in deeper layers.
Fig. 8. Raman spectra of polyesteracrylate pigmented with 10% TiO2 (rutile and anatase modi®cations) each recorded at the coating surface and in a depth of 15 mm. Rutile and anatase can be distinguished easily by Raman spectroscopy. In contrast to Fig. 7, a concentration gradient is found for the TiO2 pigments with lower concentrations at the coating surface.
W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
201
Fig. 9. Raman spectra of cured polyurethane acrylates UA with and stabilizers (UVA, HALS), photoinitiator Irgacure 184 (4%) without light
UV curing is inhibited near the surface. Measurements performed in parallel with ®lms cured under inert atmosphere show a homogeneous curing through all depth layers. The lack of curing at the surface can be prevented by modi®cation with amines or by a paraf®n barrier layer (see Fig. 4). To compensate for optical penetration effects and for changes of the confocal volume at the surface the band intensities of the acrylate vibrations are normalized to
CH-vibrations not affected by the curing reaction. The effect of amine modi®cation and additives is clearly seen. Coating layers at the surface show the same curing conversion as bulk layers. This proves that confocal Raman microscopy is well suited to investigate oxygen inhibition near the surface of coatings. Other methods to suppress oxygen inhibition are: increased dosages of UV irradiation, curing under inert atmosphere or under protective foils, higher concentrations
Fig. 10. Depth pro®les of curing conversion of polyurethane acrylates UA with and without light stabilizers (UVA, HALS), two photoinitiator systems, and two irradiation dosages
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W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
Fig. 11. Lateral mapping of UVA concentration (arbitrary units) in an area of 30 mm, depth 10 mm. UVA content is 1.5%.
of photoinitiators or synergists reacting with the formed hydroperoxiradicals. 3.2. Depth pro®ling to investigate imperfect reaction conversion in deep layers Due to Lambert±Beer's law of absorption the penetration of UV light to deeper layers is limited. As a consequence the crosslinking conversion in deeper layers is reduced resulting in weaker ®lm cohesion. This problem can occur not only in thick ®lms but also with reactive formulations being soaked into the pores of the substrate. Confocal Raman spectroscopy reveals this effect in transparent coatings without further sample preparation down to depth layers of 0.5 mm. In the case of opaque ®lms microtome cuts are needed. Afterwards the depth pro®le can be measured by lateral mapping of the conversion. In Fig. 5, Raman spectra of polyetheracrylate PO recorded in different depth layers are given. The Raman intensities of reactive groups normalized
Fig. 12. Lateral mapping of double bond density concentration (arbitrary units) in an area of 30 £ 30 mm, depth 10 mm
Fig. 13. Depth pro®les of curing conversion of polyesteracrylate containing increasing concentrations of light stabilizers (UVA, ®xed concentration of photoinitiator.
to bands not affected by the crosslinking reaction (1450 cm 21) vary with depth. Fig. 6 shows the conversion of the crosslinking reaction as a function of depth. The logarithmic scale allows a demonstration of oxygen inhibition directly at the coating surface and a reduced conversion in deeper layers some hundred mm below surface. 3.3. Depth pro®ling to investigate reaction conversion in pigmented layers Pigmented formulations are a challenge not only for optical measurement techniques but also for UV curing because of limited penetration of light. The formulation of Fig. 7 contains 10% TiO2 pigments. In this case Raman microscopy can explore the curing conversion in a layer of about 30 mm near to the surface. The spectra show an inhibition reaction at the surface while the deeper layers exhibit
Fig. 14. Depth pro®les of curing conversion of polyesteracrylate formulations containing increasing concentrations of photoinitiator, ®xed concentrations of light stabilizers.
W. Schrof et al. / Progress in Organic Coatings 35 (1999) 197±204
Fig. 15. Depth pro®les of curing conversion of polyesteracrylate formulations containing varying concentrations of UVA and HALS to separate the in¯uence of UVA and HALS, ®xed photoinitiator concentration 4%.
a better curing. A reduction of curing conversion due to restricted UV penetration can not be found in the investigated depth layer of 30 mm. Additional measurements with microtomed coatings show a reduced curing conversion in deeper layers which is more pronounced at higher pigment loadings. 3.4. Depth gradients of pigments in cured coatings Confocal Raman microscopy can not only be used to elucidate the depth dependence of curing, but can also monitor gradients of additives. In the case of TiO2 loaded coatings, Raman spectroscopy can distinguish rutile and anatase modi®cations (Fig. 8) by the shift of the anatase bands to lower frequencies. The anatase loaded coating exhibits a slightly higher conversion than the rutile loaded one. The example of Fig. 8 shows a depth dependence of the TiO2 concentration. In this polyesteracrylate formulation, which was not optimized, a sedimentation of the pigment seems to occur which results in a relative lower pigment concentration at the coating surface. A similar behavior is not found in the example of Fig. 7.
203
polymer. From Fig. 10 one can learn that light stabilzers reduce reaction conversion, especially at low UV irradiation doses and in deeper layers. Replacing a small amount of photoinitiator Irgacure 184 by the long wavelength absorbing TPO (0.5% out of 4%) helps to reduce insuf®cient curing in deeper layers. The distribution of the UV absorber molecules can also be investigated by Raman mapping. Fig. 11 demonstrates for an area of 30 £ 30 mm a nearly uniform UVA concentration in a depth layer of 10 mm below the surface. Additional depth pro®les don't show any gradients either. Lateral maps of conversion show a similar homogeneity (Fig. 12). These lateral investigations of curing conversion may reveal inhomogeneous irradiation conditions which can be caused by varying distances to the UV lamps or by shadowing effects. A more detailed investigation has been carried out with a polyesteracrylate formulation with varied concentrations of photoinitiators (Darocure 1173 and TPO), UVA (Tinuvin 400) and sterically hindered amines (Tinuvin 292). The ®lms of a thickness of 50 mm have been cured by a mercury bulb (120 W/cm, 10 m/min). In Fig. 13 the photoinitiator (D1173) concentration is kept constant at 4% while the amount of light stabilizers (UVA: HALS 2:1) is increased from 0.2% UVA to 4% UVA. As a result, the curing conversion decreases. UVA concentrations of 2% and more yield drastic reductions of curing conversion. The depth pro®le of conversion proves that higher UVA concentrations are accompanied by oxygen inhibition at the coating surface and less bulk conversion which is caused by lower UV curing intensities in deeper layers due to Lambert±Beer's law of absorption. In Fig. 14 a similar result is observed by variation of the photoinitiator concentration while keeping the amount of light stabilizers constant. Confocal Raman microscopy manifests a massive reduction of curing in deeper layers if
3.5. Interaction of UV curing with UV light stabilizers Many coatings formulations contain UV additives to stabilize coatings against long-term degradation by UV light. These additives can in¯uence the curing conversion of radiation curing, because they can absorb UV light, scavenge or inactivate radicals needed for polymerisation. Confocal Raman microscopy can monitor the distribution of these additives and their effect on curing. Fig. 9 plots the Raman spectra of two polyurethane acrylates UA, with and without light stabilizers (UVA, HALS). Whereas the aromatic UV absorber can be detected by its characteristic bands (Fig. 9) even at low concentrations around 1%, the aliphatic HALS compound (concentration 1%) is hidden by the intense Raman bands of the crosslinked
Fig. 16. Depth pro®les of curing conversion of polyesteracrylate formulations containing varying relative concentrations of the two photoinitiators D1173 and TPO (together 4%), the concentrations of UVA and HALS are kept ®xed.
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conversion in deep layers or at the coating surface will occur. The non-destructive optical determination of concentration gradients in coatings on the original substrate opens fundamental new insights into daily problems in radiation curing. Craters or inhomogenities of leveling can be analyzed directly on the spot for their chemical or physical origins.
the concentration of photo-initiator becomes comparable or lower than the UVA concentration. In order to separate the in¯uence of UVA and HALS stabilizers on curing, depth pro®les of curing conversion are taken from coatings containing only UVA and HALS, respectively. Fig. 15 clearly shows that reduced curing is mainly caused by UVA absorption reducing the UV illumination, especially in deeper layers, wheras HALS only produces a minor effect. Small amounts of the long wavelength absorbing photoinitiator TPO in combination with D1173 can improve the curing conversion in deeper layers, even in presence of light stabilizers (see Fig. 16). Higher concentrations of TPO, on the other hand, can lead to increased oxygen inhibition at the surface.
S. Fischer, K. Menzel, F. VoÈllinger, R. Horn, M. MuÈller, and K.-H. Joost are thanked for sample preparation and UV curing, M. MuÈller for measurements and data evaluation.
4. Conclusion and outlook
References
The method of confocal Raman microscopy allows a simple and fast characterization of UV cured coatings by depth pro®ling and lateral mapping of functional groups with a spatial resolution up to 1 mm 3. Typical problems in radiation curable coatings were successfully investigated:
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² the inhibition of the polymerization reaction by oxygen; ² the limited polymerization reaction in deep layers due to the restricted penetration of UV light (Lambert±Beer); ² the reaction in pigmented coatings; ² the effect of UVA and HALS light stabilizers on the curing conversion, spatial distribution of UV absorbing additives. In formulations with light stabilizers a sensitive balance between photoinitiators, UV absorbers and illumination conditions has to be adjusted. Otherwise a reduced curing
Acknowledgements