Hyperbranched urethane-acrylates Hyperbranched urethane-acrylates based on alkoxylated hydroxy acrylates combine high molecular weight, excellent reactivity and good coating properties, such as hardness, flexibility and chemical resistance. These compounds are good candidates for a range of UV-curing applications. Branko Dunjic, Srba Tasic, Branislav Bozic. Hyperbranched polymers are highly branched macromolecules with a large number of end groups. Their tree-like structure has been shown to result in some unique properties that are very different from conventional linear or slightly branched polymers [1]. Hyperbranched polymers have high solubility and low melt and solution viscosity compared to linear polymers of similar molecular mass. These properties make them attractive in many application fields, especially for coatings, where low viscosity in combination with high functionality and high molecular weight can give enhanced coating properties [2]. The use of hyperbranched polymers in UV-curable applications has been described in several papers [3, 4]. Acrylated hyperbranched polyesters based on bis-methylol propionic acid have lower viscosity compared to linear UV-curable resins of similar molecular weight and cure very rapidly (even without the photoinitiator). They have good chemical resistance, scratch resistance and low shrinkage upon curing. Due to their high functionality, the films obtained are very hard but brittle, so they have poor flexibility. To the authors' knowledge, no papers have been published about urethane-acrylates based on hyperbranched polymers. In previous work the preparation of urethane-acrylates based on hyperbranched aliphatic polyesters partially modified with short chain saturated fatty acids was presented [5]. In the present work the synthesis of new hyperbranched urethane-acrylates (HBUA) based on alkoxylated hydroxy functional (meth)acrylate monomers is investigated. Introduction of a flexible alkoxylated spacer between a compact hyperbranched core and crosslinkable groups reduces steric hindrance by moving the unsaturated groups away from hyperbranched core and increase its reactivity. At the same time, higher molecular weight oligomers give good performance of cured coatings. Materials used The purchased materials used in HBUA synthesis were: isophorone diisocyanate (IPDI, Hüls); 2-hydroxyethyl acrylate (2-HEA), polyethyleneglycol(6) monoacrylate (PEA6), polypropyleneglycol(6) monoacrylate (PPA6) and polypropyleneglycol(5) monomethacrylate (PPM5S, Laporte Performance Chemicals); hexanediol diacrylate (HDDA, BASF); 2,2-bis(methylol) propionic acid (Bis-MPA) and ditrimethylolpropane (DITMP, Perstorp AB); dibutyltin dilaurate (DBTDL, Merck), and 1-hydroxy-cyclohexyl-phenyl-ketone ("Irgacure 184", Ciba). All chemicals were used as received. Synthesis of urethane-acrylate oligomers An adduct of IPDI and hydroxyalkyl (meth)acrylate was prepared by dropping an equimolar amount of hydroxyalkyl (meth)acrylate into a 250ml 3-necked round-bottom flask containing 0,1mol of IPDI and catalytic amount of DBTDL. The flask was equipped with a mechanical stirrer, dropping funnel, water condenser and a thermometer. The reaction mixture was stirred at below 40 C over 2h. Then, an appropriate amount of hyperbranched polyester (HBP,G2) dissolved in THF (20wt.%) was added to the flask. The reaction mixture was stirred at 70 C until the complete reaction of the NCO groups, which was confirmed by FTIR analysis - the disappearance of the peak at 2267cm -1. After evaporation of the solvent (THF) a clear viscous liquid was obtained. Measuring the cured oligomers The complex dynamic viscosity (h*) of oligomers diluted with 20wt.% HDDA were measured with a Rheometrics mechanical spectrometer "RMS-605" operating in rate sweep mode, using a cone and plate geometry at 30 C. Dynamic mechanical properties of cured oligomers were analysed using Rheometrics "RMS-605" in the temperature sweep mode (at frequency of 1Hz). Differential scanning calorimetry (DSC) was performed using a Perkin Elmer "Pyris 6 DSC" analyser at a heating rate of 10 C/min under nitrogen. HBUA oligomers were mixed with HDDA (20wt.%) and "Irgacure 184" (4wt.%) and drawn on metal plates at a film thickness of 40 ± 5µm. The films were cured using 2" UVPS metal halide lamp (80W/cm). Hardness of coatings was determined by Persoz pendulum. The flexibility of the coatings was determined by measuring the Erichsen indentation. Synthesis of hyperbranched urethane-acrylates Most urethane-acrylate oligomers are synthesised by reaction of the linear polyols (polyester or polyether type) with diisocyanate and hydroxyalkyl acrylate (usually 2-hydroxyethyl acrylate) and give flexible films upon curing. New urethane-acrylate oligomers based on hyperbranched polyesters and alkoxylated hydroxy functional (meth)acrylate monomers have recently been demonstrated. A hyperbranched polyester of the second generation (HBP,G2) with ditrimethylolpropane as a core and dimethylolpropionic acid as a branching unit was prepared by a procedure described in reference [6]. The hydroxy-functional hyperbranched polyester obtained was used as a polyol core for synthesis of hyperbranched urethane-acrylates. The synthesis of urethane-acrylates can be divided into two steps: synthesis of an adduct from IPDI and hydroxyalkyl (meth)acrylate monomer as a first step, and in second step modification of polyol core (HBP,G2) with IPDI based adduct obtained in the first step. In this study a series of urethane-acrylates were synthesised with different hydroxy-functional (meth)acrylate monomers. The chemical structures of the hydroxy-functional (meth)acrylate monomers used are given in Table 1. The reaction diagram of the synthesis of hyperbranched urethane-acrylate based on PEA6 is presented in Figure 1. All other polymers were obtained in the same way (by the procedure already described), with a different hydroxyalkyl (meth)acrylate. The composition of synthesised urethane-acrylates is given in Table 2. Complex dynamic viscosities of hyperbranched urethane-acrylates The complex dynamic viscosities (h*) of urethane-acrylate oligomers diluted with 20wt.% of HDDA determined at 30 C are given in Figure 2. It can be seen that the type of hydroxyalkyl (meth)acrylate monomer used for end-capping of HBP,G2 have a considerable effect on the viscosity of urethane-acrylate oligomers. The hyperbranched urethane-acrylate based on 2-HEA (H2(HEA) 8 ) have the highest viscosity (285Pas at 1Hz), much higher than conventional urethane-acrylates. All the other oligomers
have much lower viscosities and exhibit Newtonian behaviour, i.e. no shear thinning. HBUA based on alkoxylated (meth)acrylate monomers have lower concentration of polar urethane groups, which reduce the density of hydrogen bonding as well as the final viscosity of the oligomers. The values of h* at 30 C and 1 Hz are given in Table 2. Dynamic mechanical analysis of cured oligomers Viscoelastic properties of UV-cured oligomers, such as T g and shear modulus were determined by dynamic mechanical analysis in a temperature-scanning mode. Glass transition temperature, T g, was determined as the temperature of the maximum on tan δ peak. Figure 3 shows the tan δ curves of UV-cured hyperbranched urethane acrylate oligomers mixed with 20 wt.% HDDA. The type of hydroxyalkyl (meth)acrylate used as well as the type of unsaturated end groups (acrylate or methacrylate) has a considerable effect on the T g and the shape of tan δ curve. Hyperbranched urethane acrylate based on 2-HEA (H2(HEA) 8) has the highest T g (95 C). The broad tan δ curve also indicates that the obtained network is very inhomogeneous, i.e. there is a broad distribution of chain lengths between crosslinks. Introduction of a flexible spacer (polyethylene oxide or polypropylene oxide) between a compact hyperbranched core and crosslinkable groups increases the mobility in the cured samples and decreases the T g. Hyperbranched urethane-acrylates based on PEA6 and PPA6 have lower T g (55 and 31 C respectively) as compared to urethane-acrylates based on 2-HEA. This is a consequence of not only increasing flexibility of the oligomers but also of the decrease in both: concentration of crosslinkable acrylate groups and concentration of polar urethane groups compared to H2(HEA) 8 oligomer (Table 2). The type of functional groups (acrylate or methacrylate) will also determine the properties of cured oligomers. It is known that methacrylate functional oligomers generally exhibit a higher T g compared to acrylate functional oligomers due to a stiffer structure of the methacrylate group. By comparing the T g of the cured oligomers based on PEA6 and PPM5S (which have similar concentration of unsaturated groups) it can be seen that PPM5S (methacrylate end groups) based oligomer has higher T g (73 C). The values of T g for the cured oligomers determined as (tan δ) max or (G") max are given in Table 3. Properties of UV-cured coatings The mechanical properties of UV-cured oligomers were determined by measuring the coating properties such as pendulum hardness (Persoz hardness) and flexibility (Erichsen cupping). The solvent resistance of cured coatings was determined by MEK rub testing. All urethane-acrylate oligimers were diluted with 20wt.% of HDDA and contained 4wt.% of photoinitiator "Irgacure 184". The coat films, applied on metal plates at typical thickness of about 40µm, were cured by 2, 5, 10 and 15 successive passes under the UV-lamp at belt speed of 10m/min. The obtained films were kept for 1 hour at room temperature before testing. The mechanical properties of the cured coatings are summarized in Table 4. One of the phenomena that is a limiting factor for obtaining good UV-curable coating properties is oxygen inhibition [7, 8]. Oxygen reacts with free radicals (obtained after photoinitiator decomposition) giving peroxy-radicals which are insufficiently reactive to continue the polymerization. As a result, tacky surface of the film is obtained. This problem can be overcome in different ways: curing under inert atmosphere (N 2 or CO 2 ), using amine synergists or UV-lamps of high intensity. The film characteristics (surface tackiness) after 2 passes under a UV-lamp in an air atmosphere were evaluated in order to determine the reactivity of synthesised urethane-acrylate oligomers. It can be seen that oxygen inhibition (surface tackiness) influences the properties of coatings based on propoxylated hyperbranched urethane-acrylates (H2(PPA) 8 and H2(PPM) 8 ). The coating based on H2(HEA) 8 did not show oxygen inhibition effect. This is probably due to several overlapping effects such as high viscosity which prevent oxygen diffusion in deeper surface layers and high acrylate concentration (see Table 1). Ethoxylated hyperbranched urethane-acrylate (H2(PEA) 8 ) is more reactive due to the presence of abstractable hydrogens in an a-position to the ether links. The coating based on H2(PEA) 8 did not show an oxygen inhibition effect even after only one pass under UV-lamp. This is probably due to their high reactivity. However, the overall effect of reactivity and the appropriate coating viscosity results in good surface curing. In case the coatings were affected by oxygen inhibition, the tacky surface layer (about 1-5µm) was removed by wiping with ethanol. In order to keep constant curing conditions, the films that did not show oxygen inhibition were also wiped with ethanol prior to further testing. After UV-exposure of HBUA, highly crosslinked coatings were formed. As for the T g, also the mechanical properties of the cured coatings change with the structure and the chemical composition of used urethane-acrylates. The influence of the number of passes under the UV-lamp (i.e. dose) on the film hardness is illustrated in Figure 4. The hardness of the coatings and the T g are known to be related. They both depend on the crosslink density of the cured film, but also on the structure of UV-curable resin. By plotting the hardness values as a function of the T g obtained by DSC an almost linear relationship was found (Figure 5). In the same Figure, the flexibility data were plotted against T g temperature. It can be seen that flexibility of the coatings having T g above room temperature is significantly reduced. Only the H2(PEA) 8 based coating showed a good compromise between hardness and flexibility. This can be attributed to the flexible structure of polyethyleneglycol chains between crosslinks. Hyperbranched urethane-acrylates - some further possibilities The hyperbranched urethane-acrylates presented here can be further tailored to a match specific end user's needs. The remaining OH groups could be used for further modifications. In order to decrease the amount of extractables present in cured coatings (photoinitiator fragments or unreacted photoinitiator) some remaining OH groups were modified with photosensitive compounds such as xanthates [9], thus combining the concept of controlled radical polymerization and dendritic polymers. The photoinitiator moieties attached to the hyperbranched core give reactive oligomers and low extractable matter in UV-cured coatings. This is one of the topics of current work which will be published soon. References [1] Y. H. Kim, J. Polym. Sci., Part A: Polym. Chem., 36, (1998), 1685 [2] B. Pettersson, Pigment Resin Technology, 25, No.4, (1996), 4 [3] M. Johansson, A. Hult, J.Coat. Tech., 67, No. 849, (1995), 35 [4] M. Johansson, T. Glauser, G. Rospo, A. Hult, J. Appl. Polym. Sci. 75, (2000), 612
[5] E. Dzunuzovic, S. Tasic, B. Bozic, D. Babic, B. Dunjic, J. Serb. Chem. Soc., accepted for publication. [6] E. Malmström, M. Johansson, A. Hult, Macromolecules, 28, (1995), 1698 [7] C. Decker, A. Jenkinks, Macromolecules, 18, (1985), 1241 [8] C. Decker, Polym. Int., 45, (1998), 133. [9] International Patent Pending, PCT/GB2003/003239 Result at a glance - The synthesis and UV-curing of multifunctional urethane-acrylates based on hyperbranched polyesters have been investigated. - Hyperbranched urethane-acrylates based on flexible alkoxylated hydroxy functional (meth)acrylate monomers were found to be good candidates for UV-curing applications. - The UV-cured films of these oligomers combine a high crosslinking density with flexible segments between crosslinks, which results in a good compromise between hardness and flexibility. - The problems associated with UV-curing under air, such as- tacky surface due to oxygen inhibition and related poor mechanical properties, might be overcome by using these high molecular weight hyperbranched urethane-acrylates. The authors: > Dr Branko Dunjic is Chief Scientific Officer of Duganova Ltd, part of the biggest south-east European paint manufacturer, Duga Paints, Belgrade, Serbia, which is in charge of development and application of new materials and technologies, specialized for radiation-cured coatings. > Srba Tasic is researcher in Laboratory for Synthesis in Duganova, responsible for development of hyperbranched oligomers for radiation-cured coatings. > Branislav Bozic is researcher in Laboratory for Application in Duganova, responsible for application of new concepts of polymer synthesis, such as controlled radical polymerization, CRP, in coatings industry.
Figure 1: Synthesis of hyperbranched urethane-acrylate, H2(PEA)8. Figure 2: Complex dynamic viscosities ofhyperbranched urethane-acrylates.
Figure 3: tan delta curves of UV-cured hyperbranched urethane-acrylates. Figure 4: Hardness of coatings as a function of number of passes under the UV-lamp.
Figure 5: Hardness and flexibility as a function of Tg.
Table 1: Chemical structure of hydroxy-functional (meth)acrylate monomers.
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