Quelle/Publication: Ausgabe/Issue: Seite/Page: European Coatings Journal 05/2003 14 Crosslinking mechanisms Binder systems for parquet flooring applications. André S. Harmsen, Peter L. Jansse, Miranda Vermeer, Edward v/d Hoogen, Nicole v/d Werf-Willems Mechanical properties and chemical resistances of water-based coating systems for parquet flooring are different with and without the use of an external crosslinking system. Fatty acid modified urethanes are known for their already excellent level of performance such as high gloss, hardness, abrasion and chemical resistance, warm wood wetting effect and moderate yellowing characteristics when used as one component coating. Still, additional crosslinking can give some increase in properties, in particular in black heel mark resistance. However, it my be questionable wether these improvements outweigh the toxicologically unfavourable profile associated with these types of crosslinking systems. Aqueous urethane and urethane acrylate dispersions are well known binder systems for a broad range of coating applications. The chemical structure of a urethane, resulting in a high level of hydrogen bonding, is responsible for hardness, abrasion and chemical resistance, already at molecular weights as low as 50,000g/mole. Black heel mark and scratch resistance can generally be improved when the network is covalently crosslinked in addition to the polymer chain interaction through hydrogen bonding. Therefore, the addition of a crosslinker could further enhance the properties of linear systems. To show this, several urethane and urethane acrylate systems were tested as sole binders in a parquet coating formulation and also as formulations to which external crosslinkers were added. Hybrid systems achieve desired properties The process of preparing waterdispersed urethanes is extensively described in the literature. The urethane prepolymer mixing process (Figure 1) is the most widely applied route towards waterborne urethanes and subsequently, urethane acrylate hybrid systems, allowing the use of a broad variety of building blocks to achieve the desired properties. The category of autoxidatively curable polyurethanes (Figure 2), in which polyunsaturated fatty acids or oils are incorporated, is specifically described to show that a high level of coating performance can already be achieved with these systems, without the use of additional crosslinking. Three different crosslinkers are possible The most commonly used crosslinkers for water-based systems are polyaziridines, polycarbodiimides and polyisocyanates, which have to be mixed in as a second component resulting in a formulation with limited potlife. The crosslinking reaction of the former two takes place upon drying of the coating, when pH drops as a result of evaporation of the neutralising agent. Carbodiimide and aziridine functionality will then react with the carboxylic acid groups resulting in a crosslinked network. In the case of polyisocyanates, the isocyanate group will react with pendant hydroxyl (and less likely carboxyl) functionality or water generating an amine, which will react further with isocyanate to form a partially interpenetrating network. All three crosslinkers still contain traces of starting materials, which can represent a health concern during application. Crosslinking mechanisms Polyaziridine (PA) often causes allergic reactions This still widely used crosslinking principle, introduced in the late 1970's, is based on the use of a multifunctional aziridine compound synthesized from either trimethylolpropane triacrylate or pentaerithrytol tetraacrylate and ethyleneimine or propyleneimine. It provides plasticisation of the binder in combination with a wide cure profile. Potlife typically ranges from 2 to 24 hours depending on the nature of incorporation of the carboxylic acid groups present and the pH of the system. During drying, the crosslinking starts because of evaporation of water and neutralising agent, resulting in a drop in pH. The aziridine nitrogen atom will be protonated facilitating the hydroxylgroup to ring open the aziridine three-membered ring, resulting in a β-aminoester bond (Figure 3). The extend of crosslinking depends on the resulting mobility of the film when T g rises as a result of the crosslinking and the level of premature hydrolysis of aziridine groups making them unavailable for further reaction. Polyaziridine crosslinkers show a positive Ames test whereas a certain percentage of general users develop allergic reactions when applying this crosslinking system. Despite this unfavourable toxicity profile, the use of aziridine crosslinkers still survives due to the versatility of the chemistry involved. Polycarbodiimides (PC) react rapidly at RT In order to circumvent the healthrisk associated with polyaziridine compounds, the development of new crosslinking techniques started in early 1980. In 1983, Union Carbide Corporation introduced the polycarbodiimide chemistry for the coatings industry. As with polyaziridine compounds, polycarbodiimides react rapidly at ambient conditions with bindersystems containing carboxylic acid functionality (Figure 3). Being derived from diisocyanate chemistry, polycarbodiimide technology is expensive and traces of monomeric diisocyanates can be present, which represent a healthrisk, though be it from a different nature then associated with polyaziridines. Polyisocyanates (PI) have to be waterdispersable A third route to achieving crosslinking via a two component system, is the use of waterdispersable polyisocyanates in coatings systems of which the binder carries hydroxy or amino functionality. A large number of these crosslinkers have been introduced in the last 15 years, of which the chemistry originates from polyisocyanates applied in solvent based systems e.g automotive applications. The polyisocyanates, which very often are allophanates or triisocyanurates derived from hexamethylenediisocyanate, are made waterdispersable by introducing a polyethyleneoxyde chain of defined length into the molecule. In order not to introduce too much watersensitivity into the coating, the optimum level of waterdispersable groups in the crosslinker has been established. Bubbles of CO 2 have to be removed The crosslinking mechanisme is based on the reaction of an isocyanate group with a hydroxyl, carboxyl or amino group attached to the polymer backbone (Figure 3). Due to restricted potlife the use of aminogroups is not preferred. The major drawback from the use of polyisocyanates is the competing reaction with water, which is present in abundance. Although the reaction of the isocyanate group with water generates an aminogroup, which will react rapidly with another isocyanate group, crosslinking efficiency will decrease because the stoechiometry has changed. Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
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Crosslinking mechanismsBinder systems for parquet flooring applications.André S. Harmsen, Peter L. Jansse, Miranda Vermeer,Edward v/d Hoogen, Nicole v/d Werf-WillemsMechanical properties and chemical resistances ofwater-based coating systems for parquet flooring aredifferent with and without the use of an external crosslinkingsystem. Fatty acid modified urethanes are known for theiralready excellent level of performance such as high gloss,hardness, abrasion and chemical resistance, warm woodwetting effect and moderate yellowing characteristics whenused as one component coating. Still, additional crosslinkingcan give some increase in properties, in particular in blackheel mark resistance. However, it my be questionablewether these improvements outweigh the toxicologicallyunfavourable profile associated with these types ofcrosslinking systems.Aqueous urethane and urethane acrylate dispersions arewell known binder systems for a broad range of coatingapplications. The chemical structure of a urethane, resultingin a high level of hydrogen bonding, is responsible forhardness, abrasion and chemical resistance, already atmolecular weights as low as 50,000g/mole.Black heel mark and scratch resistance can generally beimproved when the network is covalently crosslinked inaddition to the polymer chain interaction through hydrogenbonding. Therefore, the addition of a crosslinker couldfurther enhance the properties of linear systems. To showthis, several urethane and urethane acrylate systems weretested as sole binders in a parquet coating formulation andalso as formulations to which external crosslinkers wereadded.
Hybrid systems achieve desired propertiesThe process of preparing waterdispersed urethanes isextensively described in the literature. The urethaneprepolymer mixing process (Figure 1) is the most widelyapplied route towards waterborne urethanes andsubsequently, urethane acrylate hybrid systems, allowingthe use of a broad variety of building blocks to achieve thedesired properties. The category of autoxidatively curablepolyurethanes (Figure 2), in which polyunsaturated fattyacids or oils are incorporated, is specifically described toshow that a high level of coating performance can alreadybe achieved with these systems, without the use ofadditional crosslinking.
Three different crosslinkers are possibleThe most commonly used crosslinkers for water-basedsystems are polyaziridines, polycarbodiimides andpolyisocyanates, which have to be mixed in as a secondcomponent resulting in a formulation with limited potlife. Thecrosslinking reaction of the former two takes place upondrying of the coating, when pH drops as a result ofevaporation of the neutralising agent. Carbodiimide andaziridine functionality will then react with the carboxylic acidgroups resulting in a crosslinked network. In the case ofpolyisocyanates, the isocyanate group will react withpendant hydroxyl (and less likely carboxyl) functionality orwater generating an amine, which will react further withisocyanate to form a partially interpenetrating network.All three crosslinkers still contain traces of starting materials,which can represent a health concern during application.
Crosslinking mechanisms
Polyaziridine (PA) often causes allergic reactions
This still widely used crosslinking principle, introduced in thelate 1970's, is based on the use of a multifunctional aziridinecompound synthesized from either trimethylolpropanetriacrylate or pentaerithrytol tetraacrylate and ethyleneimineor propyleneimine. It provides plasticisation of the binder incombination with a wide cure profile. Potlife typically rangesfrom 2 to 24 hours depending on the nature of incorporationof the carboxylic acid groups present and the pH of thesystem. During drying, the crosslinking starts because ofevaporation of water and neutralising agent, resulting in adrop in pH. The aziridine nitrogen atom will be protonatedfacilitating the hydroxylgroup to ring open the aziridinethree-membered ring, resulting in a β-aminoester bond(Figure 3). The extend of crosslinking depends on theresulting mobility of the film when Tg rises as a result of thecrosslinking and the level of premature hydrolysis ofaziridine groups making them unavailable for furtherreaction.Polyaziridine crosslinkers show a positive Ames testwhereas a certain percentage of general users developallergic reactions when applying this crosslinking system.Despite this unfavourable toxicity profile, the use of aziridinecrosslinkers still survives due to the versatility of thechemistry involved.
Polycarbodiimides (PC) react rapidly at RTIn order to circumvent the healthrisk associated withpolyaziridine compounds, the development of newcrosslinking techniques started in early 1980. In 1983, UnionCarbide Corporation introduced the polycarbodiimidechemistry for the coatings industry. As with polyaziridinecompounds, polycarbodiimides react rapidly at ambientconditions with bindersystems containing carboxylic acidfunctionality (Figure 3). Being derived from diisocyanatechemistry, polycarbodiimide technology is expensive andtraces of monomeric diisocyanates can be present, whichrepresent a healthrisk, though be it from a different naturethen associated with polyaziridines.
Polyisocyanates (PI) have to be waterdispersableA third route to achieving crosslinking via a two componentsystem, is the use of waterdispersable polyisocyanates incoatings systems of which the binder carries hydroxy oramino functionality. A large number of these crosslinkershave been introduced in the last 15 years, of which thechemistry originates from polyisocyanates applied in solventbased systems e.g automotive applications. Thepolyisocyanates, which very often are allophanates ortriisocyanurates derived from hexamethylenediisocyanate,are made waterdispersable by introducing apolyethyleneoxyde chain of defined length into the molecule.In order not to introduce too much watersensitivity into thecoating, the optimum level of waterdispersable groups in thecrosslinker has been established.
Bubbles of CO2 have to be removedThe crosslinking mechanisme is based on the reaction of anisocyanate group with a hydroxyl, carboxyl or amino groupattached to the polymer backbone (Figure 3). Due torestricted potlife the use of aminogroups is not preferred.The major drawback from the use of polyisocyanates is thecompeting reaction with water, which is present inabundance. Although the reaction of the isocyanate groupwith water generates an aminogroup, which will react rapidlywith another isocyanate group, crosslinking efficiency willdecrease because the stoechiometry has changed.
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Moreover this hydrolysis reaction also results in theformation of carbondioxyde, which could lead to bubblesand subsequent surface defects of the coating. As withpolycarbodiimides, polyisocyanates may still contain someresidual diisocyanates, which have been dimerised orreacted with other molecules to obtain the desiredproperties.
Autoxidation with catalysts is suitableOne way to achieve one component crosslinking is theincorporation of unsaturation in the polymer backbone bymaking use of unsaturated oils or fatty acids. Thistechnology can be successfully applied in waterbornepolyurethane synthesis when the fatty acid groups areintroduced via functionalized diols. The crosslinking densitycan be varied by changing the amount of functional groups.The relatively slow cure takes place when the coatingsystem reacts with atmospheric oxygen upon drying afterthe water has evaporated. The use of suitable catalysts likemanganese, cobalt or zirconium salts will speed up thecrosslinking reaction (Figure 4). Much is known from thetraditional alkyd technology of the types of oils, which givethe best results in terms of cure speed, wood coloration andyellowing tendency. If the right oil type is chosen incombination with aliphatic diisocyanates, polyurethanes forthe wood flooring market with excellent appearance can bedeveloped.
Azomethine chemistry to achieve self-crosslinkingAnother way of achieving self-crosslinking is the applicationof azomethine chemistry. This can be accomplished byintroducing keto- or aldehyde functionality together withamino or hydrazide groups in one system. The crosslinkingreaction starts when upon drying the pH drops and protonsbecome available that will help intermediates to shift theequilibrium towards azomethine formation (Figure 3). Asfinal crosslinking mechanism, the use of epoxysilanes isdescribed. This principle is based on initially reacting theepoxygroup of the crosslinker with the carboxylic acid groupin the polymer backbone. In this way pendant silanefunctionality is created, which can act as a reactive siteunder changing pH conditions (Figure 3).Upon drying of the polymer film, the pH drops triggering thehydrolysis of the alkoxysilane into a hydroxysilane, whichthen condenses to a siloxane bond after reaction with asecond hydroxysilane available on the polymer backbone.The evaporation of the water from the film helps theequilibrium to shift towards the formation of the irreversiblesiloxane bond.
Mechanical and chemical properties are investigatedAn important way of looking at binder systems for theparquet flooring market including refinish, is to distinguishbetween mechanical properties like black heel markresistance (BHMR, NeoResins method), abrasion resistance(F510 - 78 ASTM and SIS 923509), hardness (DIN 53157/NEN 5319) on one hand and chemical properties like stainresistance against a number of household chemicals andproducts (DIN 68861 (IB)) on the other. The development ofa number of these properties were followed in time up to 28days after application; also a number of properties weremeasured, after a defined drying period at ambienttemperature (RT). Gloss measurements were performedaccording to DIN 67350.
Polyaziridine improves mechanical properties ofurethane acrylate systemsBHMR and scratchresistance are measured in time on"Opacity testcharts 2C", after applying a 120mm wet film by
means of a wire rod and allowing the film to dry at RT.Abrasion resistance is measured via de Taber falling sandmethod after applying 3 layers of 150mm wet on "LenetaP121-10N" testchart and 4 weeks drying at RT. Pendulumhardness is measured in time on glass after applying a80mm wet film, which is allowed to dry at RT.The results show that BHMR development of fatty acidmodified polyurethanes is accelerated by an externalpolyaziridine crosslinker, when the starting BHMR value ofthe uncrosslinked system is relatively low. If however theinitial BHMR value is already at a high level, the addition ofan external crosslinker will give no further improvement. Thesame effect can be seen for the urethane acrylate hybridsystems; for the urethane acrylate blend systems however,a dramatic improvement in BHMR can be achieved upon theaddition of a polyaziridine crosslinker (Figure 5a). Thethermoplastic nature of the acrylate part dominates whenmeasuring the various mechanical properties. In this caseeither the acrylate particles are crosslinked reducing thethermoplasticity or the polyaziridine reacts with carboxylicacid functionality from both polymer systems, building acomplete polymer network.It is shown that excellent BHMR values can already beobtained with fatty acid modified polyurethanes with anelastic polymer backbone, which has the lowest level of hardsegments (PU1) even without the use of dryer salt. It isinteresting to note that addition of the polyaziridinecrosslinker initially can give a plasticizing effect in particularin PU2 and U/A1. Pendulum hardness has reducedsignificantly after 28 days versus the reference, whencrosslinking has reached its maximum (Figure 5b).
Crosslinking is not in most time effectiveThe use of polycarbodiimide shows different results; theplasticizing effect after addition of this crosslinker is notobserved and it is shown too that BHMR is not improved forthe urethane acrylic blend systems U&A1 and 2 (Figure 6aand b). In these systems it is questionable wether the morehydrophobic carbodiimide crosslinking is capable of buildinga bridge between the two polymer systems, which arepresent as separate acrylic and urethane particles.Crosslinking by means of polyisocyanate and epoxysilanegives a reduction in BHMR for most polymer systems,except for the elastic polyurethane system PU1, indicatingthat crosslinking is only partially effective. If the aim isBHMR improvement, these two types of crosslinkers shouldnot be the first choice (Figure 7).It is demonstrated (Figure 8), that taber abrasion values,when measured after 4 weeks drying at room temperature,hardly show an improvement when the polymer systems arecrosslinked, except for the hydroxy functional acrylic AOH,when it is combined with polyisocyanate or polyaziridine.Due to its complete thermoplastic nature, this acrylate hasno resistance against abrasion when applied as singlebinder system. The fatty acid modified urethanes alreadyhave very acceptable taber abrasion values which do notreally require additional crosslinking.
BHMR and scratch resistance indicate thermoplasticbehaviourThe study shows that only the use of polyaziridinecrosslinking has a noticeable effect on scratch resistance ifthe value for the uncrosslinked system is relatively low(Figure 9). The scratch resistance measured after 4 dayscure at RT of the fatty acid modified urethane PU2 andurethane acrylate blends U&A1 and 2 can be improvedremarkably. In the first case, crosslinking is not complete yetas shown by the hardness development (Figure 5b). Anexternal crosslinker will assist in quickly building up of a
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polymer network, which gives a step change in resistanceagainst scratch after 4 days cure at RT. The urethaneacrylate blends will show separate acrylate particles in aurethane continous phase. This is the reason why thesedried films show to much thermoplastic behaviour when theyare not crosslinked for which both BHMR and scratchresistance are good indicators.
Crosslinking with polyaziridine enhances chemicalresistanceTo determine the effect of crosslinking of the various bindersystems on the resistance against alcohol, spottests with 48% ethanol were performed on coated mahogany panels (3layers of 80mm wet) after 2 weeks drying at roomtemperature (Figure 10). The level at which the coating wasaffected was judged after recovery period of 16 hours. Theresults clearly show that polyaziridine crosslinking is themost effective technique, bringing binder systems with lessresistance against ethanol, to their maximum achievablevalue. Even though polycarbodiimide crosslinking is alsobased on the reaction with the carboxylic acid groups,ethanol resistance is hardly improved (PU1 and AOH).Polyurethane system PU1, which has the lowest level offatty acid functionality, can be greatly improved in ethanolresistance when crosslinked with polyisocyanate.Not surprisingly, the hydroxy functional acrylic AOH whencrosslinked with polyisocyanate (like polyaziridine), shows astep change in ethanol resistance.
Results at a glanceWhen applying any external crosslinker to autoxidativelydrying urethanes, the mechanical properties and chemicalresistances will only be marginally improved. The highcrosslink density, which develops after a minimum of 4 days,gives the film sufficient resistance against wear andchemical attack. It is remarkable that the fatty acid modifiedurethane PU1 with the lowest level of fatty acid functionalityand hard segments has the fastest BHMR development,even without dryer salt. Already after one day drying at roomtemperature, the maximum value is almost achieved.Pendulum hardness, in this case at a medium level, hasshown not to be a measure for BHMR. It appears thatsufficient toughness combined with flexibility cancompensate a lower level of autoxidation, in cases whereBHMR is required in the first days after application.The use of polyaziridine crosslinking does have a noticeableeffect on scratch resistance, if the value of the uncrosslinkedsystem is relatively low. This is the case for the fatty acidmodified urethane PU2 and urethane-acrylate blend U&A1and 2.This study has shown, that parquet coatings with the rightmechanical properties and chemical resistances can beachieved without the need for environmentally unfriendlycrosslinking systems.
Characteristics of the binders used in the studyIn the investigation, 3 different fatty acid modifiedpolyurethanes are used next to 2 urethane acrylate hybridpolymers, a hydroxyfunctional acrylate and 2 urethaneacrylate blends (Table 1). Each binder system is tested assole binder and in combination with 4 external crosslinkersfor a number of coatings properties according to DINstandard. Except for the use of the epoxysilane crosslinkerwhereby the quantity is based on the acid number of thebinder, the additions per crosslinker, based on practical usein the industry, are done on a fixed weight basis on polymersolids: 6% of a 2/1 dilution in water of the polyaziridine, 10%of a 1/1 dilution in water of the polycarbodiimide and 10% ofthe polyisocyanate as 65% solution in N-methylpyrrolidone;
per mg KOH/gram binder solids, 0.04% epoxyfunctionalsilane was added.The three fatty acid modified polyurethanes contain anincreasing level of unsaturation and hard segments goingfrom PU1 to PU3. The type of alkydfunctional diol is alsodifferent in each system. The urethane acrylate hybridsystems U/A1 and 2 as well as the urethane acrylate blendsU&A1 and 2 all have a urethane to acrylate ratio of 1:1. Allbinder systems were diluted to 32 % solids before testingthem as such or in combination with the differentcrosslinkers.
LIFELINES-> André Harmsen, Neoresins b.v. Waalwijk/Netherlands,studied Organic Chemistry and Chemical Engineering. Heworks as an Industry Manager at NeoResins.-> Peter Jansse, Avecia bv, NeoResins/ Netherlands,studied Organic Chemistry and Chemical Engineering. Heworks as R&T Manager at NeoResins.-> Miranda Vermeer, Avecia bv, NeoResins/ Netherlands,studied MBO Chemistry. She works as Application Chemist.-> Edward v/d Hoogen, Avecia bv, NeoResins/ Netherlands,studied MBO Chemistry. He works as a Lab TechnicianApplications Flooring/ Construction at NeoResins.-> Nicole v/d Werf-Willems, Avecia bv, NeoResins/Netherlands, studies MSc Polymer Technology. She worksas a Lab Technician R&T at NeoResins.This paper was presented at the European CoatingsConference "Parquet Coatings II", 14 and 15 November2002, Berlin/Germany.
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Figure 1: The urethane prepolymer mixing process.
Figure 2: Autoxidation in polyurethanes by introducing unsaturation.
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Figure 3: Various crosslinking mechanisms.
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Figure 4: Mechanism of autoxidation.
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Figure 5: Black Heel Mark Resistance (a) and Pedulum Hardness (b) development intime with and without polyaziridine crosslinking.
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Figure 5: Black Heel Mark Resistance (a) and Pedulum Hardness (b) development intime with and without polyaziridine crosslinking.
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Figure 6: Black heel mark resistance (a) and pendulum hardness (b) development intime without and with polycarbodiimide.
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Figure 6: Black heel mark resistance (a) and pendulum hardness (b) development intime without and with polycarbodiimide.
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Figure 7: Black heel mark resistance after 4 days cure at RT without and withcrosslinking.
Figure 8: Taber abrasion values (falling sand, 1000 rev's) after 4 days cure at RTwithout and with crosslinking.
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Figure 9: Scratch resistance after 4 days cure at RT without and with crosslinking.
Figure 10: Ethanol (48%) resistance on mahogany after 2 weeks cure at RT without andwith crosslinking.
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