Network controllers - European Coatings · Network controllers Using crosslinkers to tailor...
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Quelle/Publication: Ausgabe/Issue: Seite/Page: European Coatings Journal 12/2005 26 Network controllers Using crosslinkers to tailor properties of UV powder coatings. Mircea Manea, Keith Ogemark, Lennart S. Svensson. An alternative chemistry for UV curable powder coatings is described, in which unsaturated polyesters having maleic/fumaric reactive double bonds are combined with crystalline crosslinkers bearing unsaturated reactive groups. Varying the level of crosslinker in the formulation provides control over coating properties. Powder coating technology offers some very important application benefits such as: - No intermediate sanding steps; - Material reclamation providing 95% powder usage; - A VOC free process; - No waste treatment. Combining UV curing with powder coating introduces a number of additional benefits: - Separation between melting and curing processes gives undisturbed levelling; - Application to a complete board in one step in a vertical line; - No cooling of the coated object required; - Good adhesion on different substrates; - Design with good Out door durability (ODD). Nevertheless, UV curing imposes some limitations in terms of geometry (it is difficult to avoid shadow areas on complex shapes) and also in terms of its chemistry. An alternative approach to UV powder formulation which offers greater control over the final properties of the coating may be of considerable value. Several UV powder chemistries are in use Although there are a few limitations (such as a very narrow range in the mixing ratio between components), several different types of UV curing powder coatings are now commercially available: - Maleic/fumaric polyester combined with vinyl ester; - Acrylate or methacrylate functional polyesters; - Unsaturated polyester combined with acrylate functional polyurethane; - Acrylate functional polyacrylic combined with acrylate functional polyurethane; - Maleic/fumaric polyester combined with allyl functional polyester. Crosslinkers can provide closer control of properties The best approach in formulating a coating is using a main binder which determines its primary properties such as discolouration, outdoor stability and mechanical properties and a crosslinker to control the network density of the coating. In thermoset powder coatings, crosslinkers such as TGIC and β-hydroxyl amides are used in this way. However, this is not the case with the UV curing chemistries referred to above. A crosslinker should offer properties specific to an application: low molecular weight, high functionality and physical properties compatible with the application. Crosslinkers for UV powder coatings should ideally match the properties of those used for thermoset powder coatings. The crosslinking performance of three acrylic and one allyl derivative which are suitable for use in UV curing powder coatings has therefore been studied. The chemicals chosen were as follows; their structures and other information are shown in Table 1: - Pentaspiroglycol diacrylate (PSGDA); - Hexahydro-1,3,5-tris(1-oxo-2-propenyl)-1,3,5-triazine (1,3,5-triacryloyl-hexahydro-triazine); - Zinc acrylate; - Pentaspiroglycol diallyl ether (PSGDE). Three crosslinkers are synthesised in the laboratory The triazine product above is commercially available as "M 1530" from Daito Chemix. The other three crosslinkers were synthesised according to the procedures described below. Synthesis of pentaspiroglycol diacrylate (PSGDA) Crude pentaspiroglycol was produced by reaction of pentaerythritol with hydroxy-pivalic aldehyde. 98.5% pure pentaspiroglycol was then obtained by recrystallisation. 0.60 mols pentaspiroglycol were reacted with 9 mols ethyl acrylate in the presence of 265 ml n-heptane, 500 ppm phenothiazine, 1000 ppm nitrobenzene and 1% tert-butyl titanate ("Tyzor TBT") in a four-neck round bottom flask. The reaction was performed under reflux, removing a part of the distillate at a ratio of 5:1 until the vapour temperature rose to 85°C. The blend was cooled to 5°C and the pentaspiroglycol diacrylate isolated by filtration. The raw product was recrystallised from solution in ethanol, washed with n-heptane and dried overnight at 60°C. The purity of the prepared product was 98% at 77% yield. Pentaspiroglycol diallyl ether (PSGDE) - a novel compound Pentaspiroglycol diallyl ether (which has not yet been quoted in literature) was produced from 98.5% pure pentaspiroglycol obtained as above. 0.05 mol pentaspiroglycol were added to 152g freshly distilled dimethyl formamide in a 3-neck round bottom flask and heated to 40°C. Then 0.05 mol NaH was added to the solution and hydrogen gas started to evolve. After 5 minutes' reaction time 0.1 mol allyl chloride was added. The temperature was raised to 60°C to complete the reaction. The blend was cooled to 40°C and a further 0.08 mol NaH was added in small portions to control the exotherm. After 5 minutes 0.07 mols allyl chloride were further added at 53°C and the temperature was raised to 60°C. After 30 minutes at 60°C an additional 0.16 mol allyl chloride was added. After 6.5 hours the reaction was complete, with precipitation of NaCl. 100 g water was added and the pH was adjusted to 6.5 with 0.3g 10% HCl solution. The organic phase was further extracted twice with xylene, washed with water and dried at 50°C under vacuum. The crystals obtained were re-crystallised twice from warm methanol. Pentaspiroglycol diallyl ether of 97% purity was obtained at 72% yield. Synthesis of zinc acrylate Zinc acrylate was prepared by the reaction between acrylic acid (3 mols) in 216 g water and zinc oxide (0.5 mols) in the presence of 200 mg 4-methoxy phenol and strong air bubbling. The reaction blend was vacuum distilled (10mm Hg) at 40°C. Small amounts of toluene were added and removed by vacuum in order to completely eliminate all water. 114.5 g product containing approximately 31% Zn was obtained. Crosslinking properties are evaluated A polyester binder resin was produced based on the raw materials shown in Table 2. Crosslinking performance was evaluated on blends at different ratios (by mol) of double bond C=C in the polyester against the acrylic unsaturation in Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
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Network controllers
Using crosslinkers to tailor properties of UV powdercoatings.Mircea Manea, Keith Ogemark, Lennart S. Svensson.An alternative chemistry for UV curable powder coatings isdescribed, in which unsaturated polyesters havingmaleic/fumaric reactive double bonds are combined withcrystalline crosslinkers bearing unsaturated reactive groups.Varying the level of crosslinker in the formulation providescontrol over coating properties.Powder coating technology offers some very importantapplication benefits such as:- No intermediate sanding steps;- Material reclamation providing 95% powder usage;- A VOC free process;- No waste treatment.Combining UV curing with powder coating introduces anumber of additional benefits:- Separation between melting and curing processes givesundisturbed levelling;- Application to a complete board in one step in a verticalline;- No cooling of the coated object required;- Good adhesion on different substrates;- Design with good Out door durability (ODD).Nevertheless, UV curing imposes some limitations in termsof geometry (it is difficult to avoid shadow areas on complexshapes) and also in terms of its chemistry. An alternativeapproach to UV powder formulation which offers greatercontrol over the final properties of the coating may be ofconsiderable value.
Several UV powder chemistries are in useAlthough there are a few limitations (such as a very narrowrange in the mixing ratio between components), severaldifferent types of UV curing powder coatings are nowcommercially available:- Maleic/fumaric polyester combined with vinyl ester;- Acrylate or methacrylate functional polyesters;- Unsaturated polyester combined with acrylate functionalpolyurethane;- Acrylate functional polyacrylic combined with acrylatefunctional polyurethane;- Maleic/fumaric polyester combined with allyl functionalpolyester.
Crosslinkers can provide closer control of propertiesThe best approach in formulating a coating is using a mainbinder which determines its primary properties such asdiscolouration, outdoor stability and mechanical propertiesand a crosslinker to control the network density of thecoating. In thermoset powder coatings, crosslinkers such asTGIC and β-hydroxyl amides are used in this way. However,this is not the case with the UV curing chemistries referredto above.A crosslinker should offer properties specific to anapplication: low molecular weight, high functionality andphysical properties compatible with the application.Crosslinkers for UV powder coatings should ideally matchthe properties of those used for thermoset powder coatings.The crosslinking performance of three acrylic and one allylderivative which are suitable for use in UV curing powdercoatings has therefore been studied. The chemicals chosenwere as follows; their structures and other information areshown in Table 1:- Pentaspiroglycol diacrylate (PSGDA);
- Hexahydro-1,3,5-tris(1-oxo-2-propenyl)-1,3,5-triazine(1,3,5-triacryloyl-hexahydro-triazine);- Zinc acrylate;- Pentaspiroglycol diallyl ether (PSGDE).
Three crosslinkers are synthesised in the laboratoryThe triazine product above is commercially available as "M1530" from Daito Chemix. The other three crosslinkers weresynthesised according to the procedures described below.
Synthesis of pentaspiroglycol diacrylate (PSGDA)Crude pentaspiroglycol was produced by reaction ofpentaerythritol with hydroxy-pivalic aldehyde. 98.5% purepentaspiroglycol was then obtained by recrystallisation.0.60 mols pentaspiroglycol were reacted with 9 mols ethylacrylate in the presence of 265 ml n-heptane, 500 ppmphenothiazine, 1000 ppm nitrobenzene and 1% tert-butyltitanate ("Tyzor TBT") in a four-neck round bottom flask.The reaction was performed under reflux, removing a part ofthe distillate at a ratio of 5:1 until the vapour temperaturerose to 85°C. The blend was cooled to 5°C and thepentaspiroglycol diacrylate isolated by filtration. The rawproduct was recrystallised from solution in ethanol, washedwith n-heptane and dried overnight at 60°C. The purity of theprepared product was 98% at 77% yield.
Pentaspiroglycol diallyl ether (PSGDE) - a novelcompoundPentaspiroglycol diallyl ether (which has not yet beenquoted in literature) was produced from 98.5% purepentaspiroglycol obtained as above.0.05 mol pentaspiroglycol were added to 152g freshlydistilled dimethyl formamide in a 3-neck round bottom flaskand heated to 40°C. Then 0.05 mol NaH was added to thesolution and hydrogen gas started to evolve. After 5 minutes'reaction time 0.1 mol allyl chloride was added. Thetemperature was raised to 60°C to complete the reaction.The blend was cooled to 40°C and a further 0.08 mol NaHwas added in small portions to control the exotherm. After 5minutes 0.07 mols allyl chloride were further added at 53°Cand the temperature was raised to 60°C.After 30 minutes at 60°C an additional 0.16 mol allyl chloridewas added. After 6.5 hours the reaction was complete, withprecipitation of NaCl. 100 g water was added and the pHwas adjusted to 6.5 with 0.3g 10% HCl solution.The organic phase was further extracted twice with xylene,washed with water and dried at 50°C under vacuum. Thecrystals obtained were re-crystallised twice from warmmethanol. Pentaspiroglycol diallyl ether of 97% purity wasobtained at 72% yield.
Synthesis of zinc acrylateZinc acrylate was prepared by the reaction between acrylicacid (3 mols) in 216 g water and zinc oxide (0.5 mols) in thepresence of 200 mg 4-methoxy phenol and strong airbubbling. The reaction blend was vacuum distilled (10mmHg) at 40°C. Small amounts of toluene were added andremoved by vacuum in order to completely eliminate allwater. 114.5 g product containing approximately 31% Znwas obtained.
Crosslinking properties are evaluatedA polyester binder resin was produced based on the rawmaterials shown in Table 2. Crosslinking performance wasevaluated on blends at different ratios (by mol) of doublebond C=C in the polyester against the acrylic unsaturation in
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the crosslinker. Performance was monitored by UV-FTIR(Fourier transform infra-red spectroscopy), UV rheology andUV-DSC (differential scanning calorimetry).
DSC measures cure speed and TgUV-DSC was performed in a "Mettler" DSC. A 15 mgsample, in an open 40µl DSC crucible, was placed in theDSC with a glass cover at 140°C isothermal. The oven waspurged with nitrogen during the UV operation. After the 5minutes required to reach thermal equilibrium a hand held"UVGL25" lamp was switched on for 5 minutes. (Conditions:52mm above the crucible, peak irradiation at 365nm, powerlevel 1.3 mW/cm2). After the isothermal UV curing wasperformed, the samples were submitted to a normal DSCtest from -10 to +150°C to determine the glass transitiontemperature (Tg) after curing. Figure 1 shows the Tg as afunction of the double bond molar ratio between binder andcrosslinker.
UV-FTIR confirms crosslinking reactionsThe samples were melted onto a KBr disk placed on a140°C heated plate, then covered by another KBr disk, sothat the sample formed a film between the two discs. Thesample was placed in an isothermal holder at 140°C andpositioned in the FTIR instrument using the same hand-heldlamp as above. FTIR spectra were recorded for up to 8seconds UV exposure.No significant changes could be detected in the UV-FTIRcurves from blend A (the reference) and B. Blend C showedpeaks at 809cm-1 and 1635cm-1 that decreased with UVirradiation.The 809cm-1 peak derives from the PSGD while the 1635cm-1 peak derives from the C=C bonds in the polyester.Blend D has peaks at 828cm-1 and 1646cm-1 that decreasewith UV irradiation. The 828cm-1 peak derives from the zincacrylate and the 1646cm-1 from the C=C bonds in thepolyester.
UV rheology follows viscosity changes during cureTo follow the rheology changes during curing, a new UVrheology method was developed. 0.2g samples were placedin a "Stresstech" rheometer with 22mm plate/platemeasuring system held at 140°C with a gap of 0.5mm. Theupper plate was made from a 22mm diameter glass disk0.1mm thick glued to a rod that fits onto the rheometer. UVfrom a "UVGL25" UV lamp hits the sample at 45° at 15cmdistance.The rheometer was set to oscillation mode with stress =500Pa. The delay time for each measurement was set to 0".Measurements were done at intervals of 3 seconds. Theresults are shown in Table 3.
Crosslinker improves standard polyester coatingTrials were then carried out with a commercial UV curingunsaturated polyester formulation. Results are shown inTable 4. It should be noted that "Pulverad 3010" used inthese experiments is a maleic unsaturated polyester fromInterchem Hellas which produces a network only whencombined with either a crosslinker or an allyl modifiedpolyester.The PSG diacrylate had a positive impact by improving theflow and crosslinking, as shown by the resistance to MEKsolvent.
Crosslinker controls speed and degree of final curePSGDA is rated as having the highest reactivity in powdercoatings formulations as measured by UV-DSC, followed byzinc acrylate and the triazine. The PSGDE gave nocrosslinking at all.
The high reactivity of PSGDA can also be seen in the UVrheology tests (Figure 2). The onset time is a little longerthan for the triazine, but the network is crosslinked at ahigher degree (shown by the lower phase angle and higherstorage modulus G').The zinc acrylate crosslinks more slowly than the triazineinitially, but shows both lower phase angle and higherstorage modulus G' at the end of the measurement.The changes detected by UV-FTIR during curing wererelatively small.Coatings using a crystalline acrylic ester may be easilyformulated using any unsaturated polyester of themaleic/fumaric type as a starting point The addition of asmall quantity of PSG diacrylate may also improveperformance in commercial systems of lower reactivity. Thevalue of this approach is confirmed by the crosslinking levelshown in terms of double MEK rubs and Tg of cured films inTable 4.
ACKNOWLEDGEMENTSThe authors would like to thank Dr. S. Lundmark (Perstorp),Stavros Apostolatos (Interchem), Dr. Ronan Van Rjisbergen(Oxyplast), Dr. Ralph Nussbaum (IKEA), Carmen Fourar(Protech) for their help, and also the European Communityfor financial support of this project (Growth project LOTECGRD 1-2000-25541).
references[1] J. Ericsson, M. Nilsson, S. Lundmark, L. Svensson, S.Jönsson, K. Lindgren, Synthesis and photoinducedcopolymerization of novel maleimides, combined with vinyland/or styrene ethers, Radtech 2000, Technicalproceedings, pp 173-195, Baltimore, USA 2000[2] R. Subramanian, C. J. Sullivan, Unsaturated polyestersin low temperature cure powder coatings, Powder coatingsEurope 2002, Congress Papers, pp 231-239, Nürnberg[3] Research disclosure 463047, November 2002[4] A. Matsumoto, Polymerisation of multiallyl monomers,Prog Polym Sci. 26 (2001) 189-257[5] C. E. Schildknecht, Allyl compounds and their polymers,Wiley Interscience 1973[6] M. Johansson, A. Hult, Influence of allyl ethers in coatingresins, J. of Polymer Chemistry, vol 29, 1639-1644(1991)[7] H. U. Pohl, H.-J. Traenckner, H. Grünewald, AnalytischeUntersuchungen an photochemisch vernetzbarenallylethermodifizierten Fumarsäureestern, Die AngewandteMakromolekulare Chemie 98, 1981 1-34 (nr.1516)[8] N. Rehnberg, A. Harden, S. Lundmark, A. Manea, L.Svensson, Allyl ethers in the thiol-ene reaction, ACS,Chicago, Polymer preprints, 42(2) August 26-31, 2001,701-702[9] J. W. Knapczyk, Polyallyl glycidyl ether resins for veryfast curing high performance coatings, Dallas, October 6,1987, 65th Annual Meeting of the FSCT[10] T. Nishijima, S. Hata, Allyl ester oligomer and itsapplications, Showa Denko KK, November 2001[11] D. Burget, C. Mallen, J-P. Fouassier, M. Manea, S.Lundmark, Kinetics and photochemistry of visible lightinduced polymerization of allyl ether, Radtech 2003, Berlin,ECJ, 12/ 2003.
Results at a glance- UV curable powder coatings are normally crosslinked byreaction between two components whose ratio must beclosely controlled. This type of chemistry is less flexible thanthe binder/crosslinker systems used in thermoset powdersand many other coatings.- Four compounds were evaluated as crosslinkers for use inUV curable powder coatings based on unsaturated polyester
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of the maleic/fumaric type as the main binder.- Pentaspiroglycol diacryate (PSGDA) had the highestreactivity of these four compounds and produced the mosthighly crosslinked networks on curing.- The addition of a small quantity of PSGDA may alsoimprove performance in commercial UV powder systems oflower reactivity.
The authors:> Mircea Manea got his Polymer Chemistrys Diploma 1976in Bucharest. After several positions in the coatings industryhe joined Perstorp Specialty Chemicals where he has beenSenior R&D Associate since 1998. He was Associatedprofessor at the Institute of Chemical Engineering inBucharest (RO) between 1978 and 1987. He publishednumerous patents/publications and is a member of theSwedish Paint Federation.> Lennart Svensson took his degree as Chemical EngineerM.Sci in Lund 1970. He enrolled the Perstorp Company1975 as R & D Manager for the Swedstab daughtercompany and stayed in this position during the time thecompany was owned by Perstorp (1978-1982). In 1983 hestarted at the Analytical Chemistry laboratory as managerfor the Spectroscopy, Thermal analysis, Wet chemistry andProduction laboratories at Perstop Specialty Chemicals AB.Since 1994 he worked at the same laboratory as SeniorAnalytical Chemist.> Keith Ogemark graduated as M.Sci. in chemistry at theTechnical High School in Helsingborg (Sweden) in 1964. Heenrolled the Agricultural Analytical Institute (1965-1969) inKristianstad (Sweden). Since 1969 forward Keith hasworked in R&D for Perstorp AB, Perstorp CoatingsIntermediates AB and Perstorp Specialty Chemicals AB assynthesis chemist for fine monomer synthesis and coatingsbinders applications. Keith is author of several patents andpublications.
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Figure 1: UV-DSC results for different crosslinkers and addition rates.
Figure 2: UV rheology results for blend C (open markers) and blend G (filled markers).
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