Polyols as solvents - European Coatings › content › download › 60724 › 694532 › ... ·...
12
Quelle/Publication: Ausgabe/Issue: Seite/Page: European Coatings Journal 10/2006 49 Polyols as solvents A new route to NMP-free high-performing PUR-dispersions for various applications. Waterborne polyurethane dispersions (PUDs) combine excellent performance with low solvent content and can offer a wide range of properties. Until recently, almost all PUDs contained N-methyl pyrrolidone (NMP) which will in future be subject to labelling requirements. A new route to NMP-free PUDs uses polyols which themselves have solvent properties. These resins are shown to have excellent properties. Dirk Mestach. Waterborne polyurethanes are one of the first choices of binder for high quality and demanding applications. They can be formulated into environmentally acceptable coatings containing low amounts of volatile organic components (VOC). The nature of the polymeric backbone makes them both hard and flexible, and so ideal for use in scratch resistant, hard-wearing coatings. These properties are the result of inter- and intra-chain interactions between polyurethane chains, attributable to hydrogen bonding between the urethane and urea linkages in the polymer backbone. Waterborne polyurethanes can be used in both one- and two- component and radiation curing coatings and may be either thermoplastic or (self-) crosslinking. Production processes for polyurethane dispersions Regardless of the application, the general synthesis route for a polyurethane dispersion is that a polymeric diol, for example a polyester, polyether or polycarbonate diol, is reacted with a hydrophilising diol and a diisocyanate. Depending on the ratio between the diols and the diisocyanate, the polyurethane obtained can be either isocyanate (NCO) or hydroxy functional. Where the polyurethane is isocyanate functional, it is often referred to as a polyurethane prepolymer. The polyurethane is then dispersed into water either directly or by means of the phase-inversion emulsification process. If the polyurethane is NCO-functional it can be chain extended after dispersion using a diamine. Optionally, a chain stopper can be used to react with the NCO functionality. This chain stopper can be used to introduce functional groups, for example for crosslinking of the polyurethane after drying of the coating. Solvent choice for PUD manufacture is very limited In most waterborne polyurethanes used in the coatings industry, at least part of the hydrophilising diol is dimethylol propionic acid (DMPAc). This has a hindered carboxylic acid group which is less reactive than most acid groups, and it therefore reacts as a diol. The free acid group can be neutralised with a base to make the polyurethane water dispersible. Until recently, waterborne polyurethanes generally contained a certain amount of volatile organic solvents, which are used in the synthesis to reduce the (pre-)polymer viscosity and also to aid in dissolving DMPAc in the polyols prior to the addition of the diisocyanate. Not all solvents can be used for this as they have to be aprotic (non-reactive with isocyanates) and hydrolytically stable. This limits the choice to ketones, (cyclic) ethers and some amines and amides. The solubility of DMPAc in these solvents varies greatly, and until recently N-methyl-2-pyrrolidone (NMP) was the preferred solvent. The trouble with NMP A problem associated with NMP is the fact that it cannot be removed by distillation after dispersing the polyurethane in water. It therefore remains in the dispersion and functions as coalescing aid. The classification of NMP as "toxicologically questionable" is currently being discussed by the European Union. It is proposed that products containing more than 5% NMP will have to be labelled as being irritant and toxic (Xi: R36/37/38, T: R 61). In the United States, California's Proposition 65 also requires special labelling of products containing NMP while other states and countries may follow. There is therefore a worldwide need to replace or eliminate NMP from polyurethane dispersions. Alternative production routes - advantages and drawbacks Several approaches have been used to replace NMP in the manufacture of polyurethane dispersions. A straightforward replacement by N-ethyl-pyrrolidone (NEP) is suggested by some companies [1]. Even though it is claimed that NEP is a useful alternative and that its toxicological profile is favourable compared to that of NMP, this does not appear to be a sustainable option. Other manufacturers have modified the well-known acetone process [2]. An important problem associated with the acetone process is that the solubility of DMPAc in acetone is virtually zero. Neutralising the carboxylic acid group of DMPAc with triethylamine, however, raises its solubility considerably [3]. A drawback of this process is that large amounts of acetone have to be stripped from the polyurethane after it has been dispersed. In order to obtain an economically feasible process, the acetone has to be recycled and used again. As acetone has a very low flashpoint, some producers prefer to work with methyl ethyl ketone (MEK). Another way to increase the solubility of DMPAc in the reaction mixture is to cap the hydroxyl groups with ε -caprolactone. This modification converts the crystalline material into a soft waxy material with a low melting point and an enhanced solubility [4]. A drawback of this route is that the hard diisocyanate-dimethylol propionic acid-diisocyanate segment now becomes a soft segment. Therefore the polyurethane has to be completely redesigned in order to obtain the desired coating properties. A further way to produce solvent-free polyurethane dispersions is to use an ethylenically unsaturated monomer as a temporary solvent in the polyurethane synthesis. Esters of methacrylic acid are most suitable for this. These "solvents" are emulsion polymerised after the polyurethane is dispersed into water by adding a suitable initiator such as a persulphate or a hydroperoxide in combination with a reducing agent. Optionally, additional monomers can be added at this stage. This synthesis route leads to hybrid urethane-acrylic dispersions. This approach, however, does not solve the DMPAc solubility problem completely, because methacrylic esters are relatively poor solvents for DMPAc. The new aproach: Polyols assist in dissolving DMPAc A novel production route to NMP-free polyurethane dispersions has now been developed, which uses only small quantities of auxiliary process solvents. These solvents can be almost completely removed after production of the dispersion. The key to this process is the in-house development of polyol building-blocks that assist in Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Transcript of Polyols as solvents - European Coatings › content › download › 60724 › 694532 › ... ·...
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
Polyols as solvents
A new route to NMP-free high-performingPUR-dispersions for various applications.Waterborne polyurethane dispersions (PUDs) combineexcellent performance with low solvent content and can offera wide range of properties. Until recently, almost all PUDscontained N-methyl pyrrolidone (NMP) which will in future besubject to labelling requirements. A new route to NMP-freePUDs uses polyols which themselves have solventproperties. These resins are shown to have excellentproperties.Dirk Mestach.Waterborne polyurethanes are one of the first choices ofbinder for high quality and demanding applications. Theycan be formulated into environmentally acceptable coatingscontaining low amounts of volatile organic components(VOC). The nature of the polymeric backbone makes themboth hard and flexible, and so ideal for use in scratchresistant, hard-wearing coatings.These properties are the result of inter- and intra-chaininteractions between polyurethane chains, attributable tohydrogen bonding between the urethane and urea linkagesin the polymer backbone. Waterborne polyurethanes can beused in both one- and two- component and radiation curingcoatings and may be either thermoplastic or (self-)crosslinking.
Production processes for polyurethane dispersionsRegardless of the application, the general synthesis routefor a polyurethane dispersion is that a polymeric diol, forexample a polyester, polyether or polycarbonate diol, isreacted with a hydrophilising diol and a diisocyanate.Depending on the ratio between the diols and thediisocyanate, the polyurethane obtained can be eitherisocyanate (NCO) or hydroxy functional. Where thepolyurethane is isocyanate functional, it is often referred toas a polyurethane prepolymer.The polyurethane is then dispersed into water either directlyor by means of the phase-inversion emulsification process.If the polyurethane is NCO-functional it can be chainextended after dispersion using a diamine. Optionally, achain stopper can be used to react with the NCOfunctionality. This chain stopper can be used to introducefunctional groups, for example for crosslinking of thepolyurethane after drying of the coating.
Solvent choice for PUD manufacture is very limitedIn most waterborne polyurethanes used in the coatingsindustry, at least part of the hydrophilising diol is dimethylolpropionic acid (DMPAc). This has a hindered carboxylic acidgroup which is less reactive than most acid groups, and ittherefore reacts as a diol. The free acid group can beneutralised with a base to make the polyurethane waterdispersible.Until recently, waterborne polyurethanes generallycontained a certain amount of volatile organic solvents,which are used in the synthesis to reduce the (pre-)polymerviscosity and also to aid in dissolving DMPAc in the polyolsprior to the addition of the diisocyanate.Not all solvents can be used for this as they have to beaprotic (non-reactive with isocyanates) and hydrolyticallystable. This limits the choice to ketones, (cyclic) ethers andsome amines and amides. The solubility of DMPAc in thesesolvents varies greatly, and until recentlyN-methyl-2-pyrrolidone (NMP) was the preferred solvent.
The trouble with NMPA problem associated with NMP is the fact that it cannot beremoved by distillation after dispersing the polyurethane inwater. It therefore remains in the dispersion and functions ascoalescing aid. The classification of NMP as "toxicologicallyquestionable" is currently being discussed by the EuropeanUnion. It is proposed that products containing more than 5%NMP will have to be labelled as being irritant and toxic (Xi:R36/37/38, T: R 61).In the United States, California's Proposition 65 alsorequires special labelling of products containing NMP whileother states and countries may follow. There is therefore aworldwide need to replace or eliminate NMP frompolyurethane dispersions.
Alternative production routes - advantages anddrawbacksSeveral approaches have been used to replace NMP in themanufacture of polyurethane dispersions. A straightforwardreplacement by N-ethyl-pyrrolidone (NEP) is suggested bysome companies [1]. Even though it is claimed that NEP is auseful alternative and that its toxicological profile isfavourable compared to that of NMP, this does not appear tobe a sustainable option.Other manufacturers have modified the well-known acetoneprocess [2]. An important problem associated with theacetone process is that the solubility of DMPAc in acetone isvirtually zero. Neutralising the carboxylic acid group ofDMPAc with triethylamine, however, raises its solubilityconsiderably [3].A drawback of this process is that large amounts of acetonehave to be stripped from the polyurethane after it has beendispersed. In order to obtain an economically feasibleprocess, the acetone has to be recycled and used again. Asacetone has a very low flashpoint, some producers prefer towork with methyl ethyl ketone (MEK).Another way to increase the solubility of DMPAc in thereaction mixture is to cap the hydroxyl groups with ε-caprolactone. This modification converts the crystallinematerial into a soft waxy material with a low melting pointand an enhanced solubility [4]. A drawback of this route isthat the hard diisocyanate-dimethylol propionicacid-diisocyanate segment now becomes a soft segment.Therefore the polyurethane has to be completely redesignedin order to obtain the desired coating properties.A further way to produce solvent-free polyurethanedispersions is to use an ethylenically unsaturated monomeras a temporary solvent in the polyurethane synthesis. Estersof methacrylic acid are most suitable for this. These"solvents" are emulsion polymerised after the polyurethaneis dispersed into water by adding a suitable initiator such asa persulphate or a hydroperoxide in combination with areducing agent.Optionally, additional monomers can be added at this stage.This synthesis route leads to hybrid urethane-acrylicdispersions. This approach, however, does not solve theDMPAc solubility problem completely, because methacrylicesters are relatively poor solvents for DMPAc.
The new aproach: Polyols assist in dissolving DMPAcA novel production route to NMP-free polyurethanedispersions has now been developed, which uses only smallquantities of auxiliary process solvents. These solvents canbe almost completely removed after production of thedispersion. The key to this process is the in-housedevelopment of polyol building-blocks that assist in
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
dissolving the DMPAc.Using this new process, several types of polyurethanedispersions were developed:- Chain extended high molecular weight polyurethanedispersions ("Setaqua PU-1")- Low molecular weight fatty acid modified polyurethanedispersions ("Setaqua PU-2");- Self-crosslinking urethane-acrylic dispersions ("SetaquaUA").High molecular weight polyurethane dispersions are mostfrequently used in combination with acrylic dispersions,where their main purpose is to improve the mechanicalproperties of the acrylics. Common applications can befound in industrial wood coatings. The chain extendeddispersions, however, can also be used as the main bindersin both one- and two-component coatings. Applicationsinclude parquet, furniture and plastic coatings. Cross-linkerssuch as polyaziridines or water-dispersible polyisocyanatescan be used to enhance their performance.Fatty acid modified polyurethane dispersions are a veryversatile class of binders that find application in bothdecorative and industrial coatings. Because of theirrelatively low molecular weight, they offer excellent flow andlevelling. In wood primers or sealers, they allow goodwetting and penetration of the wood. When used fordecorative applications, such as in do-it-yourself trim paints,they offer good wet-edge and open time.Because of the fatty acid modification, the hardness andchemical resistance properties build up after drying of thecoating. Most of these dispersions still contain free hydroxylgroups which can be used for reaction with, for example,water-dispersible polyisocyanates. Once again this booststhe performance of the coating.Self-crosslinking urethane-acrylic dispersions offersynergistic properties compared to simple physical blends ofpolyurethane and acrylic dispersions. Their applications areagain numerous, ranging from parquet finishes to decorativeenamels. Some application examples will now be given forthese novel binders.
Blends with acrylics produce good parquet finishesParquet floors are currently in fashion. In addition to factoryfinished systems, many parquet floors are coated afterinstallation. The lacquers used must meet a number ofcriteria: they must be non-yellowing and have a highchemical resistance, as well as fair abrasion resistance.From the application point of view, they must be fast dryingand have a low odour.These lacquers can be based solely on a polyurethane, butmost commonly this is combined with an acrylic dispersion.Not only does this result in the improvement of a number ofcoating properties, but it also brings economic benefits.In the trial one-component formulations, blends of the newchain extended PUD were produced with a self-crosslinking,surfactant-free acrylic ("Setaqua XL") and a thermoplasticacrylic ("Setaqua TP") dispersion. Both acrylic dispersionshave a minimum film formation temperature of about 15°C.Test lacquers were based either on pure polyurethane, ablend with 30 % (w/w) of either of the acrylics and a blendcontaining 70 % of the self-crosslinking acrylic. (Thecorresponding blend with 70 % thermoplastic acrylic washazy and not compatible). The formulations used are givenin Table 1.It should be noted that the levels of cosolvent used in theformulations was not constant, but was based on theminimum film formation temperature (MFFT) of the bindercombination. Films of the varnishes were applied onto glass(dry film thickness ca. 30 µm). The films were allowed to dryat ambient temperature (23°C) and the hardness was
measured after one and seven days. These results areshown in Figure 1.The coatings were also applied onto oak veneer (150 µmwet layer thickness) and the dust-dry and tack-free timeswere recorded. The results are shown in Figure 2.It is quite surprising to see that the addition of 30%thermoplastic acrylic does not affect the drying times, eventhough the level of cosolvent in the blend is higher. Blendswith the self-crosslinking acrylic have longer drying times,although still short enough for this application.The chemical resistance properties of the dried varnisheswere determined (on oak veneer, two coats after drying forseven days at ambient temperature). These results areshown in Table 2.The blend with the thermoplastic acrylic does not offer muchadvantage with respect to chemical properties, and someresistance properties even deteriorate. Blends with theself-crosslinking acrylic, on the other hand, offer interestingimprovements in properties. The blend containing the higherlevel of acrylic performs particularly well.
NMP-free metal primersAir-drying, fatty acid modified polyurethane dispersions arevery suitable as industrially applied metal coatings. So far,however, these types of coatings have often containedNMP. A low VOC NMP-free metal primer formulation wasdeveloped and its main properties were studied. Goodadhesion is crucial for a primer, and the tests showed thatadhesion was excellent on virtually all metal substrates.Primers based on this resin, designated "Setaqua PU-2" caneasily be overcoated with both one- and two- componentwater-borne and solvent-borne topcoats. Even the primeralone shows very good salt spray resistance (Table 3).
Urethane-acrylic hybridsUrethane-acrylic dispersions offer advantages over simpleblends of a polyurethane dispersion and an acrylicdispersion. Because the acrylic part is polymerised in thepresence of the dispersed polyurethane, grafting reactionsoccur, resulting in the formation of true hybrid particleswhere the polyurethane and acrylic polymer chains arepresent in one particle. This is clearly shown in Figure 3,where atomic force microscopy picture of a film cast fromsuch a hybrid is shown.The acrylic-urethane hybrid polymer is modified withcarbonyl groups in order to cross-link via the reaction with apolyhydrazide component. As described in numerousprevious papers, it takes about one week at ambienttemperature for such a system to reach full conversion.Dynamical Mechanical Thermal Analysis (DMTA) was usedto study the mechanical properties after crosslinking (seeFigure 4).Even though only one type of particle can be seen usingAFM, the DMTA plot shows different transitions, suggestinga core-shell like morphology.A clear furniture lacquer can be formulated using theformulation shown in Table 4. The coating, applied at a wetlayer thickness of 150 µm, showed a Persoz hardness of150 s after only one day of drying at ambient temperature.After one week, hardness had increased to 200 s, indicatingthat full crosslinking had taken place. Chemical resistanceproperties were tested after 2 and 7 days of drying. Theresults are given in Table 5. As the table shows, resistanceproperties are quite satisfactory after short drying times andexcellent after full cure has been obtained.
REFERENCES[1] K. Ott et al, Patent Application WO2005/090447 A2 toBASF AG, 2005.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
[2] The Bayer Scientific Magazine, issue 17, p 92, 2005(Internet:http://www.research.bayer.com/medien/pages/4003/wood_coating.pdf)[3] H. Schurmann, J. Bung, H. VanAlsten, US Patent4,096,127 to Akzona Inc., 1978.[4] R. L. Scriven, W. Chang, US Patents 4,066,591 and4,147,679 to PPG Industries Inc. 1979.
Results at a glance- Waterborne polyurethanes (PUDs) combine excellentperformance with low solvent content and can be producedwith a wide range of properties.- Until recently, almost all PUDs contained N-methylpyrrolidone (NMP) which will in future be subject to labellingrequirements.- A new route to the production of NMP-free PUDs has beendeveloped, by using novel polyols which themselves act assolvents.- Test results on these resins and their blends with acrylicdispersions show that a good range of properties can beobtained in wood finishes and metal primers.- Hybrid systems, in which acrylates are polymerised in thepresence of the PUD, can provide even better performancethan blends of PUD and acrylic resins.
The author:-> Dr. Dirk Mestach obtained his doctorate in polymerchemistry at the University of Gent (Belgium). In 1989 hejoined Akzo Nobel, first in Belgium in the coatings divisionand later on with Akzo Nobel Resins, today Nuplex Resins,in the Netherlands. He has been active in the developmentof waterborne binders for the coatings and printing inksindustry. At present he is R&D manager at Nuplex Resins.This paper was presented at the European CoatingsConference "Polyurethanes for High Performance CoatingsIV", Berlin, 23/24 March 2006
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
Figure 2: Dust-dry and tack-free times as well as co-solvent amounts of parquetvarnishes.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
Figure 3: Atomic force microscopy views of a waterborne urethane acrylic hybrid resin("Setaqua UA"); left: topographic, right: tapping mode.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
Figure 4: Dynamic mechanical thermal analysis on the waterborne urethane acrylichybrid resin "Setaqua UA" (frequency 11 Hz) after drying at ambient temperature for 7
days.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
10/2006
49
.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
