Power of success · isocyanate) and blocking agent can be easily calculated. Enthalpy values viable...
12
Quelle/Publication: Ausgabe/Issue: Seite/Page: European Coatings Journal 03/2004 22 Power of success Lower curing temperature of 1K-PUR-Systems with new blocking agents. Hermann Bach, Christoph Gürtler, Stephan Nowak. With the use of High Throughput techniques it has already investigated catalysts for 1K-PUR systems [1,2,5]. This methodology has now been extended onto the development of new blocking agents for isocyanate (1K-PUR) systems. The investigation shows that N-tert.-butyl-N-benzylamine cured at approximately 20°C lower than 3,5-dimethylpyrazole which represents the state of the art in high quality low curing 1K-PUR systems. It is believed that a high quality of a 1K-PUR system for curing temperatures of 120°C is now possible and that this new blocking agent can be used to coat thermally sensitive substrates. The goal of this research was to screen for and develop blocked polyisocyanates that can cure at 120°C (see Figure 1). 1K-PUR coatings are important to the coatings industry, specifically in the field of OEM- and coil coating [3]. Due to economic and application reasons lower curing temperatures are desirable. Presently, minimum curing temperatures of 140°C for a duration of 30 minutes are the lower limit with standard blocking agents. With blocking agents with a curing temperature of 120°C and little thermal yellowing additional applications could be covered and thermally sensitive materials as plastics could be coated. Research with highly automated workflow While searching for suitable blocking agents it is reasonable to investigate a huge number of blocking agents systematically using one method to ensure their comparability and to develop a structure-reactivity relationship. For the selection of candidates elements of the "rational design" of targets were employed and were coupled with a fully integrated, multiparallel, and partially automated workflow. The screening workflow incorporates a newly developed screening test for the determination of the relative degree of crosslinking of polymer films which is based on the elution of a dye from an organic coating film (see Figure 2). Eluted dye a measuring unit for crosslinking degree At first, a fluorescence dye is brought into solution in a coating formulation, afterwards a lacquer is generated and cured. Secondly, the polymer film is covered with an organic solvent. The solvent swells the polymer film and elutes the dye. The dye concentration in the organic solvent is proportional to the degree of swelling of the polymer film and hence proportional to the degree of crosslinking of the polymer film. The dye concentration is determined with a fluorescence reader. A low fluorescence signal indicates a high degree of crosslinking, a high signal reveals a low degree of crosslinking. Using this method different lacquers can be compared in regard to their crosslinking behaviour (relative test). Crosslinking temperature depends on experimental conditions For lacquers hardened at temperatures below the crosslinking temperature a relatively intense swelling of the lacquer is expected and hence a high dye concentration in the solvent. Lacquers hardened at a temperature above the crosslinking temperature the swelling of the polyurethane film and thus the concentration of the dye are lower (see Figure 3). A suitable blocking agent should shift the crosslinking temperature towards lower temperatures without affecting the distinctive step in the diagram. Absolute values for crosslinking temperatures are always a function of the recording method, since they depend on experimental conditions such as curing time and the substrate to be coated. Similar to other measuring methods we defined as the "crosslinking temperature" that characteristic point in the curves, where the decay of the fluorescence signal initially occurs. The crosslinking temperatures determined by this method correlate well with the results obtained from traditional techniques such as the dynamic mechanical analysis DMA [4], however, not in their absolute values but relatively referring to a reference value (e.g. for a specific blocking agent). Experimental and computation hand to hand Based on the data obtained in screening it was tried to establish a model for the structure reactivity relationship for the deblocking of different blocking agents. This could simplify the prediction of crosslinking temperatures. To adress such a problem, it is important to find a descriptor which is experimentally accessible on the one hand and easy to calculate on the other hand. Using semi-empirical methods (AM-1), the enthalpy of the reaction of the formation of the isocyanate (model substance butyl isocyanate) and blocking agent can be easily calculated. Enthalpy values viable descriptor These were correlated with the experimentally obtained crosslinking temperatures (see Figure 4). Using these calculation results the selection of additional candidates could be addressed. This was an iterative process, Figure 4 shows the correlation after the completion of the entire screening. As can be seen, the enthalpy of the deblocking reaction is a good descriptor for the crosslinking temperature. The correlation found is appropriate to make predictions for the curing temperature of blocking agents which have not yet been investigated. Curing temperatures can be made comprehensible The crosslinking temperatures for each blocking agent was recorded at least twice using the described method (typical error of +/-5°C). If not otherwise stated, the average values were given in the following schemes. The standard blocking agents were studied first (see Table 1). A comparison of the crosslinking temperatures for different blocking agents should be made only based on data that have been gathered using the described method. The experience shows, that the crosslinking temperatures in real applications are generally higher than those obtained with our rapid screening technique. A selection of the tested alcohols will be subsequently discussed. In the schemes it has been tried to indicate an increase of reactivity of the blocked isocyanate (which can be seen through a lower crosslinking temperature) and the decrease of the nucleophilicity or sterical demand by using coloured wedges. Above selected blocking agents with their Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Transcript of Power of success · isocyanate) and blocking agent can be easily calculated. Enthalpy values viable...
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
Power of success
Lower curing temperature of 1K-PUR-Systems with newblocking agents.Hermann Bach, Christoph Gürtler, Stephan Nowak.With the use of High Throughput techniques it has alreadyinvestigated catalysts for 1K-PUR systems [1,2,5]. Thismethodology has now been extended onto the developmentof new blocking agents for isocyanate (1K-PUR) systems.The investigation shows that N-tert.-butyl-N-benzylaminecured at approximately 20°C lower than3,5-dimethylpyrazole which represents the state of the art inhigh quality low curing 1K-PUR systems. It is believed that ahigh quality of a 1K-PUR system for curing temperatures of120°C is now possible and that this new blocking agent canbe used to coat thermally sensitive substrates.The goal of this research was to screen for and developblocked polyisocyanates that can cure at 120°C (see Figure1).1K-PUR coatings are important to the coatings industry,specifically in the field of OEM- and coil coating [3]. Due toeconomic and application reasons lower curingtemperatures are desirable. Presently, minimum curingtemperatures of 140°C for a duration of 30 minutes are thelower limit with standard blocking agents. With blockingagents with a curing temperature of 120°C and little thermalyellowing additional applications could be covered andthermally sensitive materials as plastics could be coated.
Research with highly automated workflowWhile searching for suitable blocking agents it is reasonableto investigate a huge number of blocking agentssystematically using one method to ensure theircomparability and to develop a structure-reactivityrelationship.For the selection of candidates elements of the "rationaldesign" of targets were employed and were coupled with afully integrated, multiparallel, and partially automatedworkflow. The screening workflow incorporates a newlydeveloped screening test for the determination of therelative degree of crosslinking of polymer films which isbased on the elution of a dye from an organic coating film(see Figure 2).
Eluted dye a measuring unit for crosslinking degreeAt first, a fluorescence dye is brought into solution in acoating formulation, afterwards a lacquer is generated andcured. Secondly, the polymer film is covered with an organicsolvent. The solvent swells the polymer film and elutes thedye. The dye concentration in the organic solvent isproportional to the degree of swelling of the polymer film andhence proportional to the degree of crosslinking of thepolymer film. The dye concentration is determined with afluorescence reader. A low fluorescence signal indicates ahigh degree of crosslinking, a high signal reveals a lowdegree of crosslinking. Using this method different lacquerscan be compared in regard to their crosslinking behaviour(relative test).
Crosslinking temperature depends on experimentalconditionsFor lacquers hardened at temperatures below thecrosslinking temperature a relatively intense swelling of thelacquer is expected and hence a high dye concentration in
the solvent. Lacquers hardened at a temperature above thecrosslinking temperature the swelling of the polyurethanefilm and thus the concentration of the dye are lower (seeFigure 3). A suitable blocking agent should shift thecrosslinking temperature towards lower temperatureswithout affecting the distinctive step in the diagram. Absolutevalues for crosslinking temperatures are always a function ofthe recording method, since they depend on experimentalconditions such as curing time and the substrate to becoated.Similar to other measuring methods we defined as the"crosslinking temperature" that characteristic point in thecurves, where the decay of the fluorescence signal initiallyoccurs.The crosslinking temperatures determined by this methodcorrelate well with the results obtained from traditionaltechniques such as the dynamic mechanical analysis DMA[4], however, not in their absolute values but relativelyreferring to a reference value (e.g. for a specific blockingagent).
Experimental and computation hand to handBased on the data obtained in screening it was tried toestablish a model for the structure reactivity relationship forthe deblocking of different blocking agents. This couldsimplify the prediction of crosslinking temperatures. Toadress such a problem, it is important to find a descriptorwhich is experimentally accessible on the one hand andeasy to calculate on the other hand. Using semi-empiricalmethods (AM-1), the enthalpy of the reaction of theformation of the isocyanate (model substance butylisocyanate) and blocking agent can be easily calculated.
Enthalpy values viable descriptorThese were correlated with the experimentally obtainedcrosslinking temperatures (see Figure 4). Using thesecalculation results the selection of additional candidatescould be addressed. This was an iterative process, Figure 4shows the correlation after the completion of the entirescreening. As can be seen, the enthalpy of the deblockingreaction is a good descriptor for the crosslinkingtemperature. The correlation found is appropriate to makepredictions for the curing temperature of blocking agentswhich have not yet been investigated.
Curing temperatures can be made comprehensibleThe crosslinking temperatures for each blocking agent wasrecorded at least twice using the described method (typicalerror of +/-5°C). If not otherwise stated, the average valueswere given in the following schemes.The standard blocking agents were studied first (see Table1). A comparison of the crosslinking temperatures fordifferent blocking agents should be made only based ondata that have been gathered using the described method.The experience shows, that the crosslinking temperatures inreal applications are generally higher than those obtainedwith our rapid screening technique.A selection of the tested alcohols will be subsequentlydiscussed. In the schemes it has been tried to indicate anincrease of reactivity of the blocked isocyanate (which canbe seen through a lower crosslinking temperature) and thedecrease of the nucleophilicity or sterical demand by usingcoloured wedges. Above selected blocking agents with their
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
crosslinking temperatures are listed. The first temperaturevalue indicates the crosslinking temperature with 1% DBTLas a catalyst, the second the value without catalyst.
Volatility influences crosslinking temperatureThe investigation shows - what is well known - that thevolatility of alcohols has a big influence on the crosslinkingtemperature. Alcohols having an electron-poor hydroxylgroup (e.g. benzylic alcohol) have a higher crosslinkingtemperature. In most cases the volatility decreases. Theaddition of a co-ordination site for the catalyst in thealcoholic blocking agent as seen in glycol or aminoethanol(e.g. fufuryl alcohol, Figure 5) leads to a lowering of thecrosslinking temperature when a bismuth carboxylate isused as a catalyst. The improved co-ordination witha-hydroxy carboxylicacid esters with this catalyst seems toovercompensate the electron withdrawing effect of the estersubstituent. Blocking agents with an acetal structure show arelatively low crosslinking temperature, however, the acetaldecomposes during the blocking process and releasesformaldehyde in small amounts. They are, therefore,non-desirable as blocking agents. With salicylic acid esters itis difficult to obtain complete blocking of the isocyanate.
Attractive or non-attractive blocking agentsThe heteroaromatic blocking agents, based on orsynthesized from pyrazoles exhibit suitable crosslinkingtemperatures which cannot be lowered with additionalsubstituents. The 5-tert.-butyl-1,3,4-oxadiazol-2(3H)-one hasa crosslinking temperature which is approximately 30°Chigher than pyrazole types and is, therefore, not attractiveas a blocking agent. Imidazoles tend to yellow during thebaking process as does the 2,5-dimethylpyrazolinone.Electron rich structures and in most cases nucleophilicaromatic cores lead to lower crosslinking temperatures.Normally, in cases of an electron poor substitution pattern(see saccharin, Figure 6) higher crosslinking temperaturesresult.
Imidazoles tend to yellowAminic blocking agents can be broadly varied in theirelectronic and sterical parameters. An electron rich amineseems to be as suitable as one with a high stericalhindrance. Consequently, low crosslinking temperatures areobtained with diisopropylamine (DIPA) andtert.-butylmethylamine (Figure 6). However, both tend toyellow during overbake. Another interesting blocking agentwould be the 4,4-dimethyloxazolidine. This compoundcontains an amino acetal structure which reduces thenucleophilicity of the nitrogen and thus increases thenecessary crosslinking temperatures compared to e.g.DIPA. If the sterical demand and the nucleophilicity of thenitrogens is too large (example tetramethylpiperidine), thenthe crosslinking reaction takes place at rather lowtemperatures. Furthermore, in the presence of a catalyst thecrosslinking occurs even at room temperature. Thesesystems are not storage stable after formulation.
Favourable properties with N-tert. butyl N-benzylamineDuring the investigations a new compound has beenidentified that combines a favourable curing temperaturewith very good coating properties:N-tert.-butyl-N-benzylamine. Its curing temperature isapproximately 20°C lower compared to 3,5-dimethylpyrazolewith an equally low thermal yellowing [6]. The curingbehaviour of N-tert.-butyl-N-benzylamine compared to DMPis demonstrated in Figure 8.Using this new blocking agent in combination withHDI-trimer "Desmodur N3300" and IPDI-trimer "Desmodur
Z4470" two new blocked polyisocyanates were synthesized.These products have a somewhat lower viscosity comparedto DMP-blocked products and show very low colouration(below 20 hazen). Regarding the coating properties, such aschemical resistance, pendulum hardness and especiallythermal yellowing, N-tert.-butyl-N-benzylamine (BEBA)blocked systems were comparable to DMP blockedisocyanates. Furthermore, the amount of catalyst required isonly 0.2% weight of solids (as compared to 1% for DMPblocked systems) and the curing temperature for practicalapplication can be up to 20°C lower. Gloss and reflowproperties as well as the results from weathering(Jacksonville) are comparable to DMP blocked systems.
AcknowledgementsThe authors acknowledge the important contributions of Dr.C. Niederalt to the calculations of the reaction enthalpies.They also thank Dr. J. Rawlins for his very useful commentson the manuscript.
Reference[1] R. F. Service, Science, 1997, 277, 474. B. Jandeleit, D.J. Schaefer, T. S. Powers, H.W. Turner and W. H Weinberg,Angew. Chem. Int. Ed., 1999, 38, 2494. R. Dagani,Chemical & Engineering News, 2000, 78 (20), 66[2] D. A. Wicks, H. Bach, Proceedings of the Water-Borne &Higher-Solids and Powder Coatings Symposium, February6-8, 2002, New Orleans, LA, USA[3] D. A. Wicks, Z. Wicks Jr., Progress in Organic Coatings1999, 36, 148-172[4] T. Engbert, E. König, E. Jürgens, FARBE&LACK07/1996, Curt R. Vincentz Verlag, Hannover[5] H. Bach, C. Gürtler, S. Nowak, FARBE&LACK 2002,108(4), 30-41, Curt R. Vincentz Verlag, Hannover[6] (Db value for overbaking (30 min) on a solvent bornebasecoat at an overbake temperature of 120°C+20°C: 0.2;at 140°C+20°C: 0.7; compare DMP at 140°C+20°C: 0.6; onwaterborne basecoat thermal yellowing with both blockingagents is about 0.1)
Result at a glanceUsing a High Throughput Screening method based on theelution of a dye, the curing temperatures of lacquers withmore than 60 blocked isocyanates based on "Desmodur N3300" in the presence of two catalysts (DBTL and Bismuth2-ethylhexanoate) were determined. For comparison fourstandard blocking agents (3,5-dimethylpyrazole,diisopropylamine, butanone oxime, and e-caprolactam) weretested. Thus, a broad survey of different blocking agentswas possible, supporting the direct comparison of theproperties of blocking agents and providing insight into theirstructure-reactivity relationship.
ExperimentalCoating formulations consisting of a polyacrylate polyol andthe blocked polyisocyanate were produced with an NCO/OHratio of 1:1 and with a catalyst quantity of 1 weight% solidsemploying DBTL (dibutyltin dilaurate), bismuth2-ethylhexanoate and no catalyst. As the isocyanatecomponent a blocked hexamethylendiisocyanate trimer,"Desmodur N3300"* was used with 50% solids. Thestandard polyol component "Desmophen A 870"*, apolyacrylate polyol, was used. Dosage was donevolumetrically. The systematic investigation of blockingagents comprised more than 60 blocked isocyanatessynthesised. The standard blocking agents3,5-dimethylpyrazole (DMP), diisopropylamine, and ε-caprolactame were used for comparison with thesynthesized products. In addition to the crosslinking
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
temperature, thermal yellowing was also analyzed asanother important property. The experimental procedure isdescribed in [5]. The lacquers were tempered for 30 mineach at constant temperatures between 80°C and 190°C (in10°C gradation). After cooling and standing over night theelution test was performed. At the end of the residence timethe organic solvent was stirred, pipetted and read out in afluorescence reader. Dosage and pipetting were performedusing a pipetting station. The lacquers were produced inglass vials as substrates which were arranged matrices of24 in racks of teh dimensions of the MTP (micro titer plates).The fluorescence analysis was performed in transmissionmode in a black glass MTP cuvette with transparent bottom. * Trademark: Bayer MaterialScience
The authors:> Dr. Hermann Bach studied chemistry in Marburg andEdinburgh and achieved his doctoral degree 1998 with Prof.Urs P. Wild (ETH Zürich, physical chemistry). He joinedBayer in 1999 (Central Research Department) and workedfor the research department of Bayer Corp. in Pittsburgh,USA. He is now engaged in the New Technologiesdepartment.> Dr. Christoph Gürtler studied chemistry in Bonn and Berlin(TU) and achieved his doctal degree 1996. After a postdocat the MIT he joined Bayer (Central Research Department)and is now working for the research department of BayerMaterialScience, Business Unit Coatings, Adhesives andSealants. He is focused on the development of newisocyanates, 1K-PUR systems and catalysts for solvent andwater borne coating systems.> Dr. Stephan Nowak achieved his doctoral degree 1998 inatomic physics, optics and surface physics at the Universityof Constance. After a two year post doctoral stay at theInstitut d'Optique Théorique et Appliquée in Orsay nearParis/France, Nowak joined Bayer in 2000. He developedand operated the high-throughput testing lab for coatings.Since 2003 he is responsible for the physical testingfacilities for coatings for the region EMEA ofBayerMaterialScience/business development.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
Figure 1: Chemistry of a 1K-PUR coating system.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
Figure 2: Dye elution test for the determination of the relative degree of crosslinking ofpolymer films.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
Figure 3: Measuring principle and specification of the crosslinking temperature fordifferent blocking agents. (a.u. = arbitrary units) .
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
Figure 4: Crosslinking temperature versus calculated reaction enthalpy of thedeblocking reaction for the non catalysed case. The correlation coefficient is R >0.9.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
Figure 5: Crosslinking temperatures of alcoholic blocking agents. Crosslinkingtemperatures are listed with bismuth catalysts/ with DBTL/without or with
DBTL/without catalyst, starting from lower crosslinking temperatures (left) to highertemperatures (right) .
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
Figure 6: Curing temperatures of heterocyclic blocking agents.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
Figure 7: Curing temperature of aminic blocking agents.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
Figure 8: Crosslinking temperature curves for N-tert.-butyl-N-benzylamine and3,5-dimethylpyrazole in comparison (a.u. = arbitrary units).
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
03/2004
22
.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
