Biomass-derived, functional step-growth polymers for coating
applications
Citation for published version (APA):
Noordover, B. A. J., Duchateau, R., Koning, C. E., & Benthem, van, R. A. T. M. (2011). Biomass-derived,
functional stepgrowth polymers for coating applications. In Proceedings of the 241st ACS National Meeting
-Biobased monomers, polymers and materials, Anaheim, USA (pp. 1-3).
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Published: 01/01/2011
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BIOMASS-DERIVED, FUNCTIONAL STEP-GROWTH POLYMERS FOR COATING APPLICATIONS.
Bart A.J. Noordover1, Robbert Duchateau1, Cor E. Koning1
and Rolf A.T.M. van Benthem2
1 Laboratory of Polymer Chemistry 2 Laboratory of Materials and Interface Chemistry
Department of Chemical Engineering and Chemistry Eindhoven University of Technology
P.O. Box 513, Helix, STO 1.40 5600MB, Eindhoven, The Netherlands
Introduction
Performance polymers derived from biomass represent a fascinating and increasingly important field of research, as such macromolecules offer differentiated material properties as compared to conventional polymers from fossil feedstock.1,2 The aim of our research is to understand the chemistry of
bio-based building blocks and the structure-property relations of the resulting novel polymers. For thermosetting polymer systems, some of the main challenges include enhancing the polymer functionality, mechanical performance and thermal stability. A range of fully aliphatic, 1,4:3,6-dianhydrohexitol (DAH) based polyesters and polycarbonates are presented, designed for thermosetting coating applications. In this paper, we focus on the reactivity of the DAH isomers under different reaction conditions as well as on methods to improve the polymer functionality.
Experimental
Materials. The 1,4:3,6-dianhydrohexitols (isosorbide, isoidide and
isomannide) were kindly donated by Roquette Frères and by Food and Biobased Research (Wageningen University & Research Centre). Succinic acid, 2,3-butanediol, trimethylolpropane, titanium(IV) n-butoxide were purchased from Acros Organics. Glycerol (99.5+ %), 1,3-propanediol (99.6+ %), ethyl chloroformate, phenyl chloroformate, zinc acetate, triglycidyl isocyanurate and dibutyltin dilaurate were bought from Aldrich. Triphosgene (bis(trichloromethyl) carbonate, 99+ %) was obtained from Fluka, 1,4-butanediol was puchased from Merck. An isophorone diisocyanate-based, ε-caprolactam blocked polyisocyanate (trade name: Vestagon B1530) was a gift from Degussa GmbH. A hexamethylene diisocyanate-based polyisocyanate was a gift from Bayer AG. All chemicals were used as received.
Instrumentation. Size exclusion chromatography (SEC) was used to
determine molecular weights and molecular weight distributions of polymer samples relative to polystyrene or PMMA standards. 1H NMR and 13C NMR
spectra were measured on a Varian Mercury Vx (400 MHz) spectrometer. The thermal stabilities of polymer samples were determined using a Perkin Elmer Pyris 6 TGA apparatus. DSC measurements were carried out with a DSC Q100 from TA Instruments. MALDI-ToF-MS measurements were performed on a Voyager DE-STR from Applied Biosystems. Calibrations were carried out with poly(ethylene oxide) standards for the lower mass range and polystyrene standards for the higher mass range. The mass accuracy was better than 0.2 Dalton and the mass resolution was approximately m/z 12,000. Spectra were recorded in reflector mode at positive polarity. Potentiometric titrations were carried out using a Metrohm Titrino 785 DMP automatic titration device fitted with an Ag titrode. Dynamic Mechanical Analysis (DMA) was carried out using a TA Instruments AR1000-N Rheolyst rheometer, having a parallel plate geometry. Cross-linking and coating performance at room temperature were evaluated using several characterization methods: acetone rub testing (if no damage is visible after more than 150 rubs, the coating has good acetone resistance), reverse impact testing at 100 kg.cm (ASTM D 2794) and pendulum damping test (ASTM D 4366, to determine König hardness).
Melt polycondensation of DAHs. The monomers were weighed into a
250 mL round bottom glass flange reactor. The reactor was fitted with a vigreux column and a Dean-Stark type condenser to collect the condensation product. During the first part of the synthesis, the setup was continuously flushed with inert gas to limit oxidation and facilitate the removal of the low MW condensation product. While stirring, the reaction temperature was increased gradually to its maximum value (depending on the type of reaction and monomers). After several hours of processing at atmospheric pressure, vacuum processing was started at Tmax with typical pressures ranging from 1 –
5 mbar. After vacuum processing for several hours, the polymer was discharged from the reactor and left to cool and solidify.
Solution phosgenation of DAHs. Copolycarbonates were synthesized
by phosgenation of DAH and comonomers using triphosgene. The reactants were weighed into a 250 mL round-bottom flask and subsequently dissolved in a mixture of 1,4-dioxane and dichloromethane. Pyridine diluted with dichloromethane was added dropwise at 0 ºC while stirring. Subsequently, after 20 hours of stirring at room temperature, the reaction mixture was concentrated to half its original volume by applying vacuum and poured into cold methanol. The precipitated polycarbonate crude was isolated by filtration and purified by dissolution/precipitation from dichloromethane/methanol. Upon isolation, the polycarbonate was dried overnight at 40 °C in vacuo.
Curing of biomass-based polymer resins. Polyesters and polycarbonates
were cured using conventional epoxy or polyisocyanate curing agents. A solution of 0.3 – 0.5 g of polymer and 1.05 molar equivalent of the cross-linker (calculated from the COOH- or OH-values, determined by titration) in 1 mL of N-methyl-2-pyrrolidone was prepared (when using Vestagon B1530, 0.5 wt% dibutyltin dilaurate was added). A wet film of approximately 250 µm thickness was applied onto an aluminum panel followed by curing at 180-200 ºC during 10-30 minutes (depending on the curing agent) under nitrogen, resulting in films having thicknesses between 30 and 100 µm.
Results and Discussion
DAH-based copolyesters. Depending on the orientation (endo or exo,
Figure 1) of the OH-groups of the DAH isomers isosorbide (IS), isoidide (II) and isomannide (IM), the reactivity of the monomers are known to vary depending on the reaction conditions.3-7
Figure 1. The dianhydrohexitols, showing intramolecular H-bonding for
endo-oriented OH-groups.
For melt polycondensations, reactivity differences are demonstrated by the reactions of succinic acid (SA) with IS, II and IM respectively (Figure 2, Table 1). Clearly, polymerization of II yields higher molecular weights than IS and IM when using the same reaction conditions, indicating that the exo-oriented OH-group is more reactive under these conditions. Apart from the reactivity differences, we also found that IM is less thermally stable than the other two DAHs, leading to partially cross-linked and, hence, insoluble products. IM is therefore regarded to be unsuitable for melt polycondensation reactions performed at high temperatures.
Figure 2. Synthesis poly(isohexide succinate)s. Table 1. Properties of poly(isohexide succinate)s.
entry composition (NMR) Tg [1] [ºC] Tm [2] [ºC] Mn[3] [g/mol] PDI [4] 1 SA/IS [1:1.11] 56.5 - 3000 2.0 2 SA/II [1:1.08] 73.4 175.4 3900 2.2 3 SA/IM [1:1.0] 46.0 147.5 2400 [4] 2.2
[1] DSC, heating/cooling rate: 10 ºC/min, second heating curve [2]
second heating curve at a heating rate of 10 ºC/min, after cooling at 2 ºC/min.
[3] SEC in HFIP, using PMMA standards [4] only partially soluble in HFIP
Whereas IS yields fully amorphous polyesters, semi-crystallinity was observed for the II- and IM-based counterparts. The Tg, Tm and degree of
crystallinity can be controlled by introducing biomass-derived comonomers
IS II IM
such as 1,3-propanediol, 2,3-butanediol or glycerol. In SA-based polyesters, the DAH-content must be above 60 mole% (of the total diol amount) to ensure sufficiently high Tg-values (i.e. larger than 40 ºC). As mentioned, the
functionality (usually, hydroxyl or carboxylic acid end-groups) of resins used in thermosetting coating systems is crucial. While OH-functional polymers could easily be prepared by adding an excess of diol compounds, synthesizing COOH-functionalized polyesters was rather troublesome. Due to the relatively low reactivity of DAH-moieties present at the chain end, polymers prepared using an excess of SA were found to contain too many hydroxyl end-groups (according to titration data), hampering effective network-formation during curing reactions.
In addition to linear polyester chains, cyclic structures are also formed when polymerizing the DAHs with SA (Figure 3), limiting the average functionality. To increase the number of end-groups, branching agents such as glycerol or trimethylolpropane were added to the reaction mixture.
Figure 3. MALDI-ToF-MS spectrum of poly(isoidide succinate), showing
linear OH-functional (A) as well as cyclic (C) species.
To prepare carboxylic acid-functional polyesters with average functionalities larger than 2, linear OH-functional polymers were end-capped with citric acid (Figure 4). In this way, a single OH-functionality can be converted to a double COOH-functionality. The reaction proceeds via an intermediate citric acid anhydride moiety, facilitating reaction at moderate temperatures (approx. 150 ºC). It was found that 80–85 % of the CA residues present at the polymer chain end had only reacted with one of its COOH groups, while some chain extension and branching was also observed. The doubling in functionality was also reflected in the titration data (i.e. the resulting acid value was almost twice the original hydroxyl value).
Figure 4. Reaction of DAH end-group with citric acid to yield
COOH-functional polyester.
DAH-based copolycarbonates. The three DAH isomers were used to
synthesize copolycarbonates. Initially, the polymers were obtained through phosgenation of the diol / polyol monomers using triphosgene in solution (Figure 5). In contrast to previously discussed results from melt polycondensations, the endo-oriented OH-groups present in IS and IM showed higher conversion than their exo-oriented counterparts in pyridine-catalyzed reactions of DAHs with phosgene. This is caused by the intramolecular H-bonding, enhancing the nucleophilic character of the endo-hydroxyls. The molecular weights of the prepared polycarbonates were controlled by adapting the stoichiometry of the reaction. The Tg of the polymers depends on the ratio
between the rigid DAHs and the more flexible comonomers such as 1,3-propanediol or glycerol.
Figure 5. Synthesis of DAH-based copolycarbonates using triphosgene.
This synthetic procedure was prone to side-reactions, such as reaction with water. Also, it was found that several different types of end-groups were formed depending on reaction stoichiometry, conditions and work-up procedure. This is illustrated by Figure 6, which shows partial MALDI-ToF-MS spectra of two different batches of poly(isosorbide carbonate) prepared at slightly different IS:triphosgene ratios. Spectrum I shows masses corresponding to linear chains with two OH-functionalities and some cyclic structures. When increasing the amount of triphosgene with approx. 10 % (spectrum II), no OH-functional species or cyclics are found. Instead, several different groups were formed, including methyl / ethyl carbonate end-groups resulting from reactions of chloroformate end-end-groups with the solvent during reaction or the work-up. Also, some decarboxylation was observed, yielding ether linkages. The balance between molecular weight and functionality proved to be rather difficult to tune and differs depending on the types of comonomers used. Although polycarbonates with satisfactory MWs and end-group structures can in principle be prepared, the phosgenation route is not preferred.
Figure 6. Sections of the MALDI-ToF-MS spectra of poly(isosorbide
carbonate)s prepared at IS:triphosgene ratios of: I) 1:0.33; II) 1: 0.38.
Observed species: (A) linear chains, two hydroxyl end-groups; (B) linear chains, one methyl carbonate and one methyl ether end-group; (C) cyclic chains; (D) linear chains, two methyl carbonate groups; (E/F) linear chains, one or two ethyl carbonate end-groups instead of methyl carbonate end-end-groups, as a result of using dichloromethane stabilized with ethanol.
Figure 7. 1H NMR spectrum of isoidide bis(ethyl carbonate), recorded in
chloroform-d.
To achieve better control over the polymer composition, molecular weight and end-group structure, additional series of experiments were
performed to prepare copolycarbonates from dialkyl and diaryl carbonate derivatives of the DAH isomers. The three different DAH bis(phenyl carbonate)s were prepared according to a previously described procedure.8,9 In
addition, IS and II bis(ethyl carbonate)s were prepared (Figure 7). These DAH derivatives were subsequently reacted with primary diols / polyols to form OH-terminated copolymers. This procedure did indeed give improved control over especially the polycarbonate end-groups. Although formation of cyclic polymer chains could not be avoided, we were able to prepare otherwise fully OH-functional materials. To counter the loss of functionality, small amounts of polyols such as glycerol were incorporated.
DAH-based poly(ester urethane) and poly(carbonate urethane) coatings. In the previous sections of this paper, we have described ways to
prepare OH- and COOH-functionalized polyesters and polycarbonates based on monomers from renewable resources. Although the DAH isomers pose some challenges in terms of reactivity (depending on the reaction conditions), suitable procedures were developed to yield the desired polymeric structures. A wide range of polymers was thus prepared, having molecular weights typically ranging from 2000-6000 g/mol and Tg-values between 40 and 85 ºC.
In addition, we were able to control the type and amount of end-groups, making these polymers suitable candidates for application in thermosetting coating systems.
The COOH-functional polyesters prepared through citric acid modification were cured with several types of curing agents including triglycidyl isocyanurate, mixtures of diglycidyl terephthalate and triglycidyl trimellitate and a tetrafunctional compound containing activated OH-groups. Apart from the carboxylic acid/epoxy curing reaction, a slightly different curing mechanism was also anticipated. It is likely that part of the citric acid end-groups, having two carboxylic acid moieties available, will form five-membered anhydrides during curing. Residual OH-groups will react with these anhydrides, forming an ester bond and one remaining carboxylic acid, which can then react with the epoxy curing agent (Figure 8). Such acid anhydride/epoxy curing systems are known to afford higher reaction rates, conversions and crosslink densities and are less sensitive to variations in the stoichiometry of the curing formulations.10 In these citric acid modified
systems, rapid curing was observed, resulting in chemically and mechanically stable coatings. The cross-linking reactions could be effectively performed at temperatures as low as 150 ºC while achieving good flow and film formation.
Figure 8. Citric acid anhydride / epoxy curing mechanism.
OH-functional polyester and polycarbonate resins were cured with polyisocyanate curing agents (Figure 9), resulting in cross-linked poly(ester urethane) and poly(carbonate urethane) networks. Table 2 summarizes the performance of the different types of coatings in terms of solvent and impact resistance. In general, the obtained coatings are glossy, transparent films with a smooth surface, indicating sufficient flow of the polymer resins. Polymer networks formed using curing agent A show better chemical and mechanical stability than those cured with B. This is thought to be caused by the reduced combined reactivity of the IPDI moieties with the secondary, bulky DAH end-groups. In addition, dynamic mechanical analysis performed during the curing reaction demonstrated that a higher plateau modulus was reached when curing II-based polyesters (90 MPa) as compared to their IS-based equivalents (55 MPa), indicating an enhanced cross-link density. These results suggest that the
exo-oriented OH-groups of II are more accessible for reaction with IPDI.
Figure 9. Polyisocyanate curing agents based on HMDI having free
NCO-groups (A) or based on IPDI having ε-caprolactam blocked NCO-NCO-groups (B).
Table 2. Performance of coatings based on DAH polyesters and
polycarbonates.
polymer topology curing agent solvent resistance impact res. [100 kg.cm] polyesters linear A + + B +/- - branched A + + B + +/- polycarbonates linear / branched A + + B + -
+ = passes test, +/- = minor damage, - = severe damage
Conclusions
Polyesters and polycarbonates based on three 1,4:3,6-dianhydrohexitols, isomers in combination with diol and/or polyols, were prepared through different polycondensation methods. Depending on the reactivity of the DAH
endo- and exo-oriented DAH OH-groups, the synthetic procedures were
optimized to achieve polymer functionalities, molecular weights and thermal properties suitable for application in thermosetting coating systems. The polymers were subjected to detailed molecular characterization, affording information concerning reactivity differences between the different DAH isomers, chain topology and end-group structures. Curing of the polymer resins using various curing agents leads to glossy, transparent, hard coatings having excellent chemical and mechanical stability.
Acknowledgements. The authors would like to thank the Dutch Polymer
Institute (DPI) for supporting this research. Also, we thank Food & Biobased Research (Wageningen University and Research Centre) and Roquette Frères for generously providing us with 1,4:3,6-dianhydrohexitols and for fruitful discussions.
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