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European Coatings Journal
05/2007
156
Blends and hybrids
New waterborne acrylic and/or polyurethane dispersionbinders for VOC compliant parquet coatings.A look at the cross-linking chemistry behind the waterbornebinders developed for use in VOC-complaintenvironmentally friendly parquet-coatings. These bindershave an acrylic or a polyurethane backbone or can behybrids of both. High performance parquet coating systems,both for professional or do-it-yourself application, can beformulated using these binders.Dirk Mestach.** Corresponding Author. Contact: Dr. Dirk Mestach, NuplexResins b.v., Synthesebaan 1 / P.O. Box 79, 4600 ABBergen op Zoom, Netherlands, Tel. + 31 164 276 582, Fax:+ 31 164 276 502, [email protected] floors are currently in fashion. In addition to factoryfinished parquet, many such floors are coated afterinstallation. The parquet lacquers used for this must meet anumber of criteria: they must be non-yellowing and have ahigh chemical resistance. Furthermore they must havereasonable abrasion resistance. As far as the application isconcerned, they must be fast drying and have a low odour.Changes in the regulations concerning the emission oforganic solvents in the European Community have led to agrowing interest in the use of polymer dispersions inhigh-performance, waterborne coating systems.Such coating systems have been in use since themid-1980s. Currently they are exhibiting their highest rate ofgrowth and within a short time will become the leadingproducts for on-site, parquet-coating systems. The mainreason for their success is that they have well-balancedperformance, combined with a low environmental impact.Polymers used as binders for waterborne parquet lacquerscan be based on either acrylic or polyurethane backbones ormay be hybrid polymers consisting of acrylic andpolyurethane chains grafted onto each other. They can bephysically drying, but in order to have a high performancelevel, a cross-linking mechanism is usually required.
New waterborne acrylics: low MFFT yet excellenthardnessWaterborne, one-component acrylic, self-crosslinkingpolymer dispersions are made by means of emulsionpolymerisation. In this process, acrylic monomers arepolymerised in an aqueous medium containing surfactantmicelles. In advanced products, these surfactants havebeen replaced by polymeric stabilisers. One-pack systemsuse a combination of carbonyl-hydrazide cross-linking withmorphology control of the particles [1], [2]. A newself-crosslinking acrylic dispersion (XL) has been developedthat combines a low minimal film-formation temperature (15°C) with excellent hardness development. This binder canbe used as such for the formulation of high performanceparquet lacquers. In order further to improve properties suchas hardness or abrasion resistance, it can be combined withfunctional organic or inorganic nano-particles. Two typeswere worked on: a hydrophobically modified colloidal silicaand a high-Tg, non-film forming, acrylic nano-particle (MFFT> 70 °C).Three semi-gloss parquet lacquers with a VOC of 122 g/lwere formulated (see Table 1) The coatings were appliedonto glass (dry film thickness 40 microns). The increase inhardness was determined as a function of the drying time (atambient temperature). Modification with hard nano-particleshad a pronounced effect on the hardness build-up eventhough the VOC of all lacquers was identical. Abrasion
resistance of the lacquers was tested using a Taber Abrader(CS-17 test wheel, 1000 cycles, 100 g load) after sevendays of drying at ambient temperature. As can be seen fromtable Table 2, the modification of XL with nano-particles hasa positive effect.
NMP-free waterborne PUDPolyurethane dispersions offer a good balance betweenchemical resistance and mechanical properties. This is aresult of their chemical structure, consisting of both hard andsoft building blocks and the occurrence of hydrogen bondingbetween the urethane and urea groups in the polymerbackbone [1]. Conventional polyurethane dispersionscontain N-methyl pyrolidone (NMP) that is used as aprocess solvent to facilitate the incorporation of dimethylolpropionic acid, an anionically stabilizing diol, into thepolymer backbone. Because of its high boiling point, NMPcan not be removed from the dispersion. This material isunder severe regulatory pressure; therefore a novelsynthesis process was developed to produce NMP-freepolyurethane dispersions. Using this new process, apolyester-based chain extended high molecular weightpolyurethane dispersion was developed (PU). Suchmaterials are most frequently used in combination withacrylic dispersions. The main purpose is to improve themechanical properties of the acrylics and also to make moreeconomical formulations.
PUD/acrylic blends: close to pure PUR performanceIn the following one component formulations, we thereforeevaluated blends of PU with XL and also with athermoplastic acrylic dispersion (TP). Both acrylics have aMFFT of about 15 °C.Blends were made with 30 % (w/w) of XL or TP and a blendcontaining up to 70 % XL. The blend of PU with 70 % TPwas not compatible, as it gave hazy films. The actualformulations used are given in Table 3.It has to be noted that the levels of co-solvent used in theformulations are not constant. The amount was adjusted toensure proper film formation with the binder combinationused.The varnishes were applied onto glass (dry film thicknessca. 30 microns). The films were allowed to dry at ambienttemperature (23 °C) and the hardness was measured afterone and seven days. The results are shown in Figure 1.The coatings were also applied onto oak veneer (150microns wet film thickness) and the dust-dry and tack-freetimes were recorded. The results are shown in Figure 2.It is surprising to notice that the addition of 30 % TP-1 doesnot influence the drying times, even though the level ofco-solvent in the polyurethane-acrylic blend is higher.Blends with the self-crosslinking acrylic have longer dryingtimes although still sufficiently short for this application.The chemical resistance of the dried varnishes wasdetermined as two layers on oak veneer, which were testedafter drying for seven days at ambient temperature.Blending of the thermoplastic acrylic with PU did not showmuch improvement in the chemical properties. Indeed, someresistance properties even deteriorated. Blends with theself-crosslinking acrylic on the other hand showed significantimprovements in performance. Especially the blendcomposed of 30 % PU and 70 % XL performed well. Finally,the abrasion resistance was determined (Taber Abrader,test-wheel CS-17, load 1000 grams). The pure polyurethaneformulation obviously had the best abrasion resistance, butadding an acrylic dispersion only had a minimal negative
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effect. The difference between the abrasion results of theblended binders was negligible.
PUD/acrylic hybrids: avoiding compatibility problemsUnfortunately it was demonstrated above that blends do notalways offer the best solution, as the polyurethanedispersion and the acrylic polymer are not alwayscompatible. Instead of blending the polyurethane dispersionwith an acrylic dispersion, it is possible however to makehybrid polymers where the urethane and the acrylic arecontained in the same particle. These hybrids can be madeby swelling a polyurethane with acrylic monomers and then,using the emulsion polymerization process, produce theacrylic polymer inside the polyurethane particle. In thehybrid particles, the urethane and acrylic polymer areintimately mixed. Using monomers that have a functionalgroup that is reactive towards groups present on theurethane, covalent grafting of the acrylic onto the urethaneis possible. Also functional monomers for crosslinking afterfilm formation, such as carbonyl-functional monomers, canbe incorporated into the hybrid polymer resulting inself-crosslinking, urethane-acrylic dispersions.The hybrids were, in general, synthesized by swelling PUwith a mixture of acrylic monomers (methyl methacrylate,butyl acrylate), using diacetone acrylamide (DAAA) ascarbonyl-functional monomer and glycidyl methacrylate asgrafting monomer, which reacts with acid groups in thepolyurethane.In Figure 3, an atomic force microscopy micrograph of a filmcast from a self-crosslinking urethane-acrylic dispersion isshown. It is obvious that there is only one type of particlepresent. This is evidence that both the acrylic and thepolyurethane are contained in the same particle.Dynamical mechanical thermal analysis (DMTA) was usedto study the mechanical properties after crosslinking. Eventhough only one type of particle can be seen using AFM, theDMTA plot shows different transitions, suggesting acore-shell-like morphology.A number of parameters were varied in the design of theurethane-acrylic hybrids to optimize the performance inparquet lacquers:- The urethane-acrylic ratio and influence of Tg of the acrylicpolymer- The concentration of carbonyl functional monomer in theacrylic polymer- The effect of concentration grafting monomer in the acrylicpolymerUrethane-acrylic hybrids were made with a varying ratiobetween the urethane and the acrylic polymer. Properties ofthe experimental products are given in Table 4.These hybrid dispersions were formulated in a simplelacquer using 3 % butyl glycol and 1 %tributoxy-ethylphosphate to ensure proper film formation andapplied onto glass (dry layer thickness 40 microns) anddried for seven days at ambient temperature. In Figure 4 thePersoz hardness is plotted as a function of the Tg of theacrylic polymer and as a function of the urethane-acrylicratio (at a constant Tg). Varying both parameters at thesame time gave the opportunity to optimize the hybridcomposition.The concentration of carbonyl groups in the acrylic polymercan also influence hardness build-up as can be seen inFigure 5. It has to be stressed that the self-crosslinkingmechanism needs about one week of drying at ambienttemperature to reach full conversion. Final hardness istherefore always higher then when measured initially.
Grafting increases the crosslink densityFinally, the effect of grafting was investigated by varying the
concentration of glycidyl methacrylate in the acrylic polymer.Both the initial and the final hardness were improved whenthe degree of grafting was increased. When thepolyurethane becomes covalently grafted to the acrylic, itwill also become part of the cross-linked network; thereforethe sol-fraction in a cured film is expected to decrease. Oneway of determining this is testing the acetone resistance of adried film. The results given in Figure 6 indicated that thegrafting effectively increases the cross-link density.
References:[1] D. Mestach, Polym. Paint Col. J Vol 190 No 4431: 28,2000[2] D. Mestach, F. Loos, Proc. XXIV FATIPEC Congress,Vol. B: 91, 1998[3] D. Dieterich, Prog. Org. Coatings, 9, 281-340 (1981)
Results at a glance- The demand for waterborne coatings for parquet flooring isgrowing due to EU legislation restrictions.- Waterborne polymer dispersions based on either acrylic orpolyurethane backbones have been found to be the answer.- A new self-crosslinking acrylic dispersion has beendeveloped that combines a low minimal film-formationtemperature with excellent hardness development.- Polyurethane dispersions offer a good balance betweenchemical resistance and mechanical properties, but areunder severe regulatory pressure. Blends with acrylicdispersions are needed.- As an alternative, acrylic-polyurethane hybrids offer a goodsolution, which avoids compatibility problems sometimesencountered in physical blends.
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 EuropeanCoatingsConference "Parquet Coatings IV" Berlin, 9/10 November2006
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Figure 1: Hardness development for polyurethane and polyurethane-acrylic blendparquet varnishes.
Figure 2:Dust-dry and tack-free times of polyurethane and polyurethane-acrylic blendparquet varnishes.
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Figure 3: Atomic force microscopy on a film cast from a self-crosslinkingurethane-acrylic dispersion (left: topographic, right: tapping mode).
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Figure 4: Effect of (a) the Tg of the acrylic part and......
.... (b) the urethane-acrylic ratio in the hybrid dispersion on the Persoz hardness.
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Figure 5: Effect of diacetone acrylamide in acrylic polymer on the Persoz hardnessdevelopment.
Figure 6: Effect of the percentage of grafting monomer in the hybrid on acetoneresistance.
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