聚合物涂层中烘干过程进行浓度分析检测方案(激光拉曼光谱)

Polymers 05 Concentration profile measurements in polymericcoatings during drying by means ofInverse-Micro-Raman-Spectroscopy (IMRS) W. Schabel*， P. Scharfer， 1. Ludwig， M. Muller， M. Kind， Institute of Thermal Process Engineering，University Karlsruhe (TH)， Germany There are many important industrialapplicationsfiforsolvent and waterborneascoatings suchhfoilss for LCD panels，varnishes， adhesives， paper coatings and evenbio-medical applications such as transdermalpatches. Coatings on these products anddevicesare often1formedfrom dissolvedcoating components that are applied to thesubstrate material and then subsequentlydried to create the desired end material. The drying process of the coating oftendetermines the performance and quality of theresulting product. In order to design andcontrol such coating and drying processes，online moisture andd solvent profilemeasurements during the coating formationbecome of significant interest. The University Karlsruhe (TH) has applied ameasuringj technique calleddIlInverse-Micro-Raman-Spectroscopy (IMRS) to a ConfocalRaman Spectrometer (LabRAM INV -HORIBAJobin Yvon) which was manufactured wherebythe standard upright LabRAM microscope wasreplaced by an Inverted Microscope samplingsystem (in this case an Olympus IX50). Figure 1 Inverse-Micro-Raman-Spectroscopy (IMRS)， a new technique tomeasure concentration profilessin polymercoatings and films， e.g. during a drying process. Withthis arrangement， a free space wascreated above the sample which allowed agreater flexibility in sampling orientation and inthe accommodation of environmental controlcells. In this case a special cell incorporatingan air flow channel was installed to enable thecharacteristics and kinetics of the drying of apolymer coating to be studied in-situ. The：preliminary work obtained intthiscollaboration showed many advantages in theuse of Raman spectroscopy for the study ofdrying characteristics in coating materials.Subsequently this has led to a much largerproject funded by the DFG (German ResearchFoundation) on "Thin-Film-Drying". The work has most recently revealed that thelMRS technique enables the measurement ofsolvent content within thin films online， in-situand with a high spatial resolution of up to 1 pm.Temporal and kinetic resolution of up to 0.5 sis also obtained through this method andquantitative measurements can be acquired bythe use of specific calibration procedures. Instrumentation The modified LabRAMINV used】 in thesestudies utilises the standard backscatteringRaman geometry whereby the laser beam istransferred through the optical microscope andirradiates the sample. The backscattered lightis then collected with the same objective andtransmitted into the integrated spectrometer.Since the inverted geometry is provided by theLabRAM INV， the sample is illuminated frombelow with the focused laser spot moved intothe sample volume by means of a piezo Z-axisnano-positioning system. Translation of thepiezo device enables data to be collected at adepth of up to 200 microns. Figure 2 The HORIBA Jobin Yvon LabRAM/Nv The design of the rest of the system is then thesame as that for the more conventional Ramanmicroscope with the scattered light directedthrough a holographic notch-filter to removethe Rayleigh scattered light and transmittingthe Raman signal and the spectrometer with aair cooled detector CCD. The confocal pinhole within the system isresponsible forthe control of thespatialfiltering and ensuring that only light from acontrolled and desired sampled volume bedetected. The use of such a precise spatialand depth control mechanism is importantwith such measurements. Without suchadevice spectral artefacts and components fromscattered light that is generated from above orbelow the desired position within the samplewould also be detected and degrade thequality of data. The difference in the refractive index (RI)caused by the air gap between sample andobjective is minimised by using an objectivewith a high NA and an immersion oil (placedbetween the glass cover slip and objective).The immersion oil has the same refractiveindex as the glass and a similar one to thepolymer and so minimises the differences inthe refractive indices for these media. Thecloser the refractive indices of immersion oil，glass and sample， the better is the spatialresolution one obtains inside the polymer film. For the sample preparation a liquid polymersolution is cast on a thermostated copper platewith a glass slide in the centre of the plate.Detailed information of this procedure can befound in reference [1，2]. Investigations with solvent borne coatings In figure 3 the Raman spectra in differentlayers of a solvent borne polymer coatingduring drying are shown. 8=40°C Figure 3 Raman spectra atdifferentdepthsoffapolymericsolventitcoating(polyvinylacetate-toluene))during9the dryingprocess， measured with the IMRS-technique(left)： at the beginning of a drying process (t= Os， initial film thickness： 150 um) (right)： after t = 60s of drying. The solvent(=1010 cm-1) evaporates and the film shrinksto 50 pm. In order to obtain quantitative solvent contentdata， calibration measurements are needed. Acalibrationncan beeeasily/ performedacquiring Raman spectra ofcalibration set of samples prepared withdifferent but known concentrations in sealedoptical cells. The calculated solventconcentration profiles from measured data (likethe ones displayed in figure 3) are shown infigure 4. The symbols indicate the locallydetermined solvent content (gram solvent pergram polymer) in the film during the dryingprocess. The lines are simulation results fromdrying model calculation. Simulation resultscan be fitted to the measured values in order topredict thee Cdryingbehaviour of technicalprocesses. Ini addition， the shrinkage of the coatingsurface is also plotted. Towards the end ofthe drying process， steeper concentrationprofiles could be measured as a dry skin at thesurface is formed. The diffusion of the solventthrough this skin controls the entire dryingprocess and even after 3 days of drying asignificant solvent content underneath the filmsurface couldstillbbe measured.Thisphenomenon， in the coating industry， known as"skinning"， could be shown here for the firsttime experimentally in thin film coatings.Further investigations avewith the measurementtechnique and more detailed information aboutthe results can be found in reference [3]. Figure 4 Measured concentrationprofiles in a polymer solution (polyvinylacetate-toluene) during the drying process (symbols).The quantitative solvent contents (gsolvent/gpolymer) in the film are determined from theraw data (e.g. figure 3) by using a measuredcalibration factor [1]. Investigations with Water based Coatings A major issue for the coating industry is todevelopnew waterborne latex systems toreplacethemore commonsolvent basedpolymeric coatings. Large scale applicationssuch as paints， adhesives and other appliedcoatings would benefit from the use of systemswith a lower environmental impact， but whichmatch the properties and drying behaviour(shown in figure 4) of solvent-based polymericfilms. Waterborne decorative ttopcoatsgenerally stillshow inferiro rapplicationproperties compared to solvent-based paintsand the quality of the final polymer film is oftencharacterised by a poorer finish， or micro-cracking， and poorer resistance to dissolutionby water. A variety of physicochemical parameters suchas a polymer’s glass transition temperature(Tg)， the composition of the latex dispersionand a polymer’s physical characteristics arelikely to influence the application and final filmproperties in such a coating. To date， thespecificc effectsof theseparameters inwaterborne systems is largely unknown. In thisprogramme of research some of the very firstexperimental data on factors influencing theapplication properties and final film qualityhave been obtained using the IMRS technique.Asi an example， figure 5a， 5b show re-dispersion experiments of dry latex films withdifferent glass transition temperature Tg. Thesoft latex (fig. 5a) has formed a smooth andnon-porous film structure， which makes waterintrusion impossible， whereas for the harderlatex (fig. 5b) film formation is incomplete andwater can diffuse into the film and evenaccumulates at the film-substrate interface. Figure 5 Redispersion experiments of a drypolymer film (latex dispersion)with waterMeasurements performed with IMRS. a) from "soft" latex (low Tg) b) from "hard"latex (high Tg) In summary， the coupling of the power ofconfocal Raman microscopy to the invertedsampling geometry has enabled detailedinvestigations to be made of solvent and waterbased coating systems， providing importantinformation on the processes and chemistrythat occurs at the coating interface and within. ( References： ) ( 1. Schabel V W.， Ludwig gI.， K ind M.： Measurements of Concentration Profiles i n Polymeric Solvent Coatings by Means of an Inverse-Confocal-Micro-Raman-Spectrometer- Initial results， Drying Technology， V ol. 2 2， N os.1&2， 285-294，2004 ) ( 2. SchabelW.：Inverse MikroRaman Spektroskopie-eine neue Messmethode zurUntersuchung lokaler Stofftransportvorgangein dunnen Filmen， Folien und Membranen，Chemie Ingenieur Technik. 2005 ) ( 3. Schabel W .， S charfer P .， Muller M.， Ludwig1. ， Kind M.； Messung und S imulation vonKonzentrationsprofilen b e i de r Trocknungbinarer Polymerlosungen； Chemie Ingenieur T echnik， v ol 75， Heft 9 ； p . 1336-1344， 2003 ) ( Corresponding author： E-Mail： ) Schabel@tvt.uka.de ORIBAExplore the future ORIBAExplore the future In summary, the coupling of the power of confocal Raman microscopy to the inverted sampling geometry has enabled detailed investigations to be made of solvent and water based coating systems, providing important information on the processes and chemistry that occurs at the coating interface and within.