Polyfunctional Acrylic Nn-isocyanate Hydroxyurethanes as

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Polyfunctional Acrylic Nn-isocyanate Hydroxyurethanes as Photocurable Thermosets for 3D Printing Vitalij Schimpf,†,‡ Anne Asmacher,† Andre Fuchs,‡,§ Bernd Bruchmann,‡ and Rolf Mülhaupt*,†,‡ †

Freiburg Materials Research Center (FMF) and Institute for Macromolecular Chemistry, University of Freiburg, Stefan-Meier-Strasse 31, 79104 Freiburg, Germany ‡ JONAS - Joint Research on Advanced Materials and Systems, Advanced Materials & Systems Research, BASF SE, Carl-Bosch-Strasse 38, 67056 Ludwigshafen, Germany § BASF Schweiz AG, Mattenstrasse 22, 4057 Basel, Switzerland Macromolecules Downloaded from pubs.acs.org by UNIV OF OTAGO on 04/22/19. For personal use only.

S Supporting Information *

ABSTRACT: Liquid acrylic oligourethanes are components of photocurable thermoset resins for applications ranging from coatings to 3D printing technologies like stereolithography. Traditionally they are derived from isocyanates which are highly moisture sensitive and do not tolerate hydroxy groups. Herein we report on a versatile non-isocyanate route toward tailoring hydroxyurethane methacrylates (HUMA) and their oligomers for photo cross-linking and 3D printing. The key intermediate is (2-oxo-1,3-dioxolan-4-yl)methyl methacrylate, also referred to as methacrylated glycerol carbonate, obtained by the chemical fixation of carbon dioxide with glycidyl methacrylate. Upon aminolysis with di- and polyfunctional aliphatic amines, the ring-opening reaction of the cyclic carbonate group yields HUMA. No handling of isocyanates is required. The HUMA molecular architectures govern photo cure as well as thermal and mechanical properties. An alternative strategy toward molecular design of polyfunctional acrylics exploits chemical modification of the pendant hydroxy groups, e.g., by esterification with methacrylic acid anhydride. The resulting higher acrylate functionality accounts for improving Young’s modulus from 3160 to 4200 MPa and increasing the glass transition temperature from 86 to 173 °C with respect to HUMA-based formulations.

■

INTRODUCTION Additive manufacturing, alias 3D printing, converts the virtual objects of computer design into three-dimensional objects with complex shapes by adding layer on layer without requiring molds. Among 3D printing techniques stereolithography (SLA), digital light processing (DLP), and continuous liquid interface production (CLIP) make use of UV-curable polymer resins to fabricate 3D objects with excellent surface quality.1 Typical formulations resins for photoinitiated polymerization comprise monomers together with reactive diluents and oligomers to balance low resin viscosity, which is imperative for 3D printing, with good mechanical properties together with low shrinkage and warpage of printed parts.2 (Meth)acrylate-functional oligomers can be categorized in three main groups, namely epoxy acrylates, polyester (meth)acrylates and acrylic oligourethanes. The latter, in particular, are well-known for their excellent mechanical properties as well as their chemical resistance and therefore are widely applied in UV-curable coatings.3 Traditionally, they are tailored by the reaction of a polyol like hydroxyl-terminated polyether, polyester or polycarbonate, with an excess of a diisocyanate followed by the capping of the resulting isocyanate end groups with 2hydroxyethyl (meth)acrylate to render oligourethanes photoreactive.4 This synthetic pathway offers attractive design possibilities with respect to tailoring flexible, rigid and © XXXX American Chemical Society

polyfunctional (meth)acrylate building blocks. For instance, the incorporation of polyester polyols has improved the tensile strength of the cured polyesterurethanes owing to the strong intermolecular interactions via hydrogen bridges between the ester and the urethane groups in the polymer backbone.5 However, frequently covalent cross-link formation by photo cure accounts for massive shrinkage accompanied by warpage with increasing cross-link densities. Naturally, this highly undesirable effect becomes more pronounced with increasing content of reactive groups which is affected by both the molar mass and the functionality of photocurable acrylic oligourethanes.6 Moreover, the acrylate polymerization rate can be significantly increased in the presence of strong hydrogen bonding,7 which also accounts for higher resin viscosity and improved mechanical strength of the photocured thermosets.8 Nowadays, this isocyanate-based synthetic route toward (meth)acrylate-terminated oligourethanes is well established in industrial scale. Typically, commercially available acrylic oligourethanes exhibit molar masses ranging from 600 to 6000 g mol−1 and degrees of (meth)acrylate functionality varying from 2 to 6.9 Nevertheless this process requires the use of isocyanates Received: February 18, 2019 Revised: March 29, 2019

A

DOI: 10.1021/acs.macromol.9b00330 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Synthetic pathways toward urethane methacrylate (UMA) and hydroxyurethane methacrylates (HUMA) via the conventional isocyanatebased route (top) and via the non-isocyanate route (bottom). methacrylate (GMA, 97%, 100 ppm MEHQ), 2,6-di-tert-butyl-cresol (BHT, 99%), tetrabutylammonium bromide (TBAB, 99%), 4,7,10trioxa-1,13-tridecanediamine (TODA, 98%), 4,9-dioxa-1,12-dodecanediamine (DODA12, 99%), 1,8-diamino-3,6-dioxaoctane (DODA8, 98%), 1,3-cyclohexandiyldimethanamin (CDMA, 98%), m-xylylenediamine (XDA, 99%), isophorone diamine (99%, mixture of cis and trans), and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO, 97%) were obtained from Sigma-Aldrich. Magnesium oxide (99%, nanoparticles, 20 nm) was purchased from ABCR. Samples of Jeffamine THF-170 (PTHFA, 1700 g mol−1), Jeffamime T3000 (JT3000, 3000 g mol−1), and Jefffamine T403 (JT403, 400 g mol−1) were kindly provided by Huntsman. Carbon dioxide (N45) was obtained from Air Liquide. Priamine 1075 (206 [mg KOH]/g) was kindly supplied by Croda and used without purification. Methods. NMR spectra were recorded in deuterated chloroform on an ARX 300 spectrometer from Bruker at room temperature. The chemical shifts were referenced to the solvent signals. DSC measurements were performed using a Perkin-Elmer Pyris 1 with a heating and cooling rate of 20 K min−1 in the temperature range between 0 and 200 °C. Tensile testing was performed on a Zwick Z005 (Ulm, Germany, ISO 527−1/2) with a drawing speed of 5 mm min−1. The mechanical properties such as elastic modulus, tensile strength and breaking elongation were extracted from measurements at 21 °C by taking the statistical average of three to six test specimens (5A), which were conditioned before testing (24 h, 21 °C, const. humidity). The viscosities were measured on a MARS from Thermo Scientific using a plate−plate setup at various shear rates from 0.1 to 100 s−1 (100 steps, logarithmic, 5 s per step, 3 s integration time, 25 °C) and the final viscosity received as an average over all 100 values. Significant shearthinning was usually not observed, as indicated by the standard deviations. 3D Printing was performed on a Totem3D from 3DLABS with a layer thickness of 100 μm (405 nm UV-LED, 1.5 s exposition time per layer, 2 s exposition time per layer for the first four layers), followed by a postcure in a BB Cure Oven from MeccatroniCore (405 nm UV-LED, program: little, 10 min, 40 °C). Preparation of Glycerol Carbonate Methacrylate. Glycidyl methacrylate (GMA, 1045.0 g, 7.3514 mol), 2,6-di-tert-butylcresol (BHT, 5.2 g, 0.024 mol), and tetrabutylammonium bromide (TBAB, 10.5 g, 0.0326 mol) were placed in a stainless steel reactor and put under 30 bar of carbon dioxide. The mixture was heated to 100 °C and stirred at 500 rpm for 24 h. The reaction product was used without further purification. η(GCMA) = 57.4 ± 1.4 mPa·s. Preparation of DAP_MMA. 1,5-Diaminopentane (DAP, 4.205 g, 41.15 mmol) and methyl methacrylate (MMA, 4.204, 41.99 mmol) were heated under air atmosphere for 5 h at 60 °C. Synthesis of Hydroxyurethane Methacrylates. The di- or triamine and the respective amount of GCMA were placed in a threenecked flask, and a mechanical stirrer was attached. In case of short

which demand special safety precautions due to their high toxicity and moisture sensitivity.10−13 To comply with green chemistry standards and to design reactive oligomers which are not available via these isocyanate-based routes, the development of isocyanate-free synthetic pathways are highly desirable. A green alternative to acrylic oligourethanes was introduced by Han et al., who reacted biobased diamines with ethylene carbonate to yield bis(hydroxyethyl)carbamates which in turn where polymerized with itaconic acid through polycondensation to produce UV-curable oligoesterurethanes.14 In particular, the high reactivity of isocyanates is also limiting the possibilities to introduce functional groups like hydroxyl groups into the polyurethane backbones. In contrast to the common polyurethane synthesis, non-isocyanate polyurethanes (NIPU) exploit the ring-opening reaction of multifunctional cyclic carbonates with polyamines to yield linear, branched, and cross-linked nonisocyanate polyhydroxyurethanes (PHU).15−18 For instance, Endo et al. applied 2-isocyanatoethyl methacrylate to functionalize the hydroxyl groups in the backbone of various PHUs, thus producing a linear polyurethane chain with pendent methacrylate groups.19 Mathias et al. as well as Wang et al. prepared NIPU methacrylates by reacting amines with ethylene carbonate and subsequently introducing the methacrylate group through esterification of the resulting alcohol groups using methacrylic acid anhydride.20 Herein we present a versatile non-isocyanate route toward liquid hydroxyurethane methacrylates (HUMA) prepared in a solvent-free single step process from di- or polyamines and glycerol carbonate methacrylate (GCMA). The latter is obtained either by carbonation of glycidyl methacrylate with carbon dioxide or by esterification of glycerol carbonate with methacrylic acid anhydride. As seen in Figure 1, upon aminolysis of GCMA the primary aliphatic amines are readily converted into UV-curable acrylic hydroxyurethanes. Opposite to the common urethane acrylate syntheses, this pathway does not require toxic and moisture-sensitive isocyanates. Moreover, the resulting hydroxyurethane allow for further functionalization of the oligourethane backbone.

■

EXPERIMENTAL SECTION

Materials. 4-Methacryloylmorpholine (ACMO, 98%) was purchased from TCI. 1,5-Diaminopentane (DAP) and the urethane acrylate Laromer UA 9089 were kindly provided by BASF SE. Glycidyl B


Article

Macromolecules Table 1. Weight Portions and Reaction Conditions of Hydroxyurethane Methacrylate (HUMA) Syntheses amine

GCMA

HUMA

m [g]

n [mmol]

m [g]

n [mmol]

T [°C]

t [h]

mf(CO2)a [wt %]

PTHFA-G JT3000-G JT403-G TODA-G DODA12-G DODA8-G DAP-G DAP-G-60 PA-G XDA-G CDMA-G IPDA-G

251.61 258.62 53.09 64.91 60.67 46.64 18.94 2.130 31.80 14.85 16.51 35.38

153.5 94.83 115.0 294.6 297.0 314.7 157.6 20.83 116.77 109.0 116.1 207.8

48.55 41.45 54.36 93.23 93.56 99.61 49.65 7.889 18.40 34.51 34.63 65.45

255.8 218.8 284.6 491.1 494.9 524.5 262.7 41.65 97.40 182.21 193.4 346.3

100 100 100 100 100 100 100 60 100 100 100 100

9.5 37 24 1.5 1.5 1.2 1.3 26.5 3.5 1.5 1.5 11

3.8 3.2 11.7 13.7 14.1 15.8 16.9 18.3 8.5 16.2 16.5 15.1

a

Mass fraction of chemically bound carbon dioxide.

Table 2. Acrylic Resins and the Respective Weight Portions of Their Acrylate Components resina

oligomer

m(oligomer) [g]

m(ACMO+TPO)b [g]

ACMO_Laromer ACMO_PTHFA-G ACMO_JT3000-G ACMO_JT403-G ACMO_TODA-G ACMO_DODA12-G ACMO_DODA8-G ACMO_DAP-G ACMO_PA-G ACMO_XDA-G ACMO_CDMA-G ACMO_IPDA-G ACMO_DODA12-G5 ACMO_DODA12-G5/GDMA

Laromer UA 9089 PTHFA-G JT3000-G JT403-G TODA-G DODA12-G DODA8-G DAP-G PA-G XDA-G CDMA-G IPDA-G DODA12-G5 DODA12-G5/GDMA

42.89 4.682 5.248 4.236 4.983 4.807 5.185 4.565 5.368 5.957 5.008 4.962 3.774 4.223

65.98 7.207 8.060 6.510 7.664 7.332 7.976 7.023 8.253 9.153 7.713 7.634 5.811 6.516

a

Oligomer/ACMO/TPO mixture in a 39:59:1 ratio. bACMO/TPO mixture in a 59:1 ratio. Preparation of DODA12-G5/GDMA. DODA12-G (NuE = 3.85 mmol g−1, 9.8356 g, 37.867 mmol nucleophiles), methacrylic anhydride (6.4270 g, 41.690 mmol, 1.1 equiv) and magnesium oxide (0.2124 g, 1.3 wt %) were vigorously stirred for 3 h at 100 °C under air atmosphere using a mechanical stirrer to yield DODA12-G5/MA. The turnover of methacrylic anhydride was monitored with 1H NMR spectroscopy. After full conversion, the amount of methacrylic acid was determined with a 1H NMR spectrum using 1,4-dimethoxybenzene as internal standard (methacrylic acid equivalent MAE = 2.72 mmol g−1). DODA12-G5/MA (12.604 g, 34.282 mmol methacrylic acid), and glycidyl methacrylate (5.360 g, 37.71 mmol, 1.1 equiv) were vigorously stirred for 5 h at 100 °C under air atmosphere using a mechanical stirrer to yield DODA12-G5/GDMA. Full turnover of the epoxy groups was confirmed with 1H NMR spectroscopy (Supporting Information, Figure S22). Preparation and Curing of Acrylate Resins. The resin formulations consist of 4-methacryl-oylmorpholine (ACMO) and a respective oligomer (Laromer UA 9089 or a HUMA from Table 1) in a 59:39-ratio. Additionally, 1 wt % diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) was applied as photoinitiator. For the preparation of the acrylic resins, TPO was first dissolved in the respective amount of ACMO, and subsequently, the oligomer homogenized with this solution according to the predefined ratio. In case of JT403-G, DAP-G, XDA-G, CDMA-G, and IPDA-G, the homogenization was performed through mechanical stirring at 70 °C. The other samples were homogenized at room temperature using a SpeedMixer DAC 150.1 FV from Hausschild (2 min, 2500 rpm). The curing of the casted samples was performed under a mercury-vapor lamp (400 W, 280−700 nm, 10 cm distance, 2 × 10 min from both

diamines like TODA, DODA12, DODA8, DAP, XDA, and CDMA, an ice bath was used during the initial stirring for 5−10 min, since the exothermic heat of the reaction would otherwise cause acrylate polymerization to occur. Afterward, the flask was put into a preheated oil bath and the reaction performed under vigorous stirring in order to mix air bubbles into the reaction mixture and to increase surface contact with the air atmosphere as a mean to inhibit acrylate polymerization. The weight portions and reaction conditions are summarized in Table 1. All products were used without further purification. Amine equivalents (AE) of JEFFAMINES were determined via 1H NMR using naphthalene as internal standard. AE (PTHFA) = 1.22 mmol g−1, AE (JT3000) = 1.1 mmol g−1, and AE (JT403) = 6.5 mmol g−1. The calculation of the nucleophile equivalent NuE (number of nucleophiles per gram) of DODA12-G was calculated according to eq 1. In this equation the number of nucleophiles describes the sum of all hydroxyl groups and secondary amines in the HUMA. NuE =

f × n(amine) − n(GCMA) m(amine) + m(GCMA

(1)

f: functionality of the amine (number of amine groups per molecule) Preparation of DODA12-G5. DODA12-G (NuE = 3.85 mmol g−1, 17.145 g, 66.009 mmol nucleophiles), methacrylic anhydride (11.192 g, 72.601 mmol, 1.1 equiv), and magnesium oxide (0.3684 g, 1.3 wt %) were vigorously stirred for 4 h at 100 °C under air atmosphere using a mechanical stirrer. The reaction product was then dissolved in 50 mL of dichloromethane and washed four times with 1 M K2CO3 solution and once with distilled water. The organic phase was then dried over MgSO4 and the solvent evaporated under reduced pressure. C


Article

Macromolecules

Figure 2. Reaction between GCMA and 1 equiv of a primary amine at 60 °C affords 90% of a hydroxyurethane group via ring-opening aminolysis of the cyclic carbonate and 10% of a Michael addition product, which accounts for the presence of residual cyclic carbonate remaining in the reaction product.

Figure 3. 1H NMR spectra of 1,5-diaminopentane (DAP), glycerol carbonate methacrylate (GCMA), and their reaction products after 5 min and 23 h (DAP_MMA) at 60 °C. sides) with a subsequent thermal postcure (30 min, 150 °C) in an oven. The respective weight portions are shown in Table 2. 3D Printing of ACMO_DODA12-G. A mixture of 4-methacryloylmorpholine (ACMO, 63.43 g, 59 wt %), DODA12-G (41.93 g, 39 wt %), and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO, 2.154 g, 2.0 wt %) was homogenized at room temperature using a mechanical stirrer. The resin was poured into the vat of the Totem3D printer from 3D LABORATORIES and the printing performed according to the parameters described in the methods sections above.

(TBAB, 1 wt %) as a catalyst at 100 °C. In order to prevent acrylate polymerization at elevated temperatures and reduced oxygen concentrations, additional stabilizer BHT (0.5 wt %) was added. The product was received as a colorless liquid (57.4 ± 1.4 mPa·s) and used without purification. Assuming full conversion and no side reactions, as verified by 1H NMR spectroscopy (Supporting Information, Figure S1), the cyclic carbonate equivalent (CCE, amount of cyclic carbonate groups per gram) was calculated to be 5.29 mmol g−1, taking into account the respective weight portions, as TBAB and BHT account for the only impurities. When reacting GCMA with 1 equiv of primary amines in the temperature range varied between 60 and 100 °C, two major reaction products were observed, as illustrated in Figure 2. Cyclic carbonate aminolysis is the favored reaction path yielding the hydroxyurethane methacrylate but only accounts for about 90% of the amine consumption. The remaining 10% of amine groups react via Michael addition to

■

RESULTS AND DISCUSSION Synthesis of Non-isocyanate Hydroxyurethane Methacrylates. The key intermediate (2-oxo-1,3-dioxolan-4-yl)methyl methacrylate, also referred to as methacrylated glycerol carbonate or glycerol carbonate methacrylate (GCMA), was prepared similar to the procedure reported in the literature for other cyclic carbonates.21 Preferably, glycidyl methacrylate was reacted with carbon dioxide in a stainless steel reactor using 30 bar carbon dioxide pressure and tetrabutylammonium bromide D


Article

Macromolecules

Figure 4. Reaction between GCMA and 0.6 equiv of a 1,5-diaminopentane to yield a hydroxyurethane methacrylate oligomer DAP-G (mtheo = 0.5). The ring-opening may produce a secondary (shown) or primary hydroxyl group (not shown).

Figure 5. Simplified structures of hydroxyurethane acrylate (HUMA) oligomers prepared from GCMA and the respective di- or triamine with 20% excess of amine groups. The respective HUMA can be categorized into large/flexible (left), short/flexible (middle), and short/stiff (right) building blocks.

S16 (Supporting Information) on the right allows for a definite identification of the Aza-Michael addition product signals, which can also be seen in the 1H/13C-HSQC-NMR spectrum of DAP-G-60 (Figure S16, left). The proton signals 6−11 in Figure 3 were assigned on the basis of this data. The remaining carbonate signal 2 in Figure 3 accounts for about 10%. Since all primary amines are already consumed (Supporting Information, Figure S16), the reaction comes to a halt at this point. These cyclic carbonate end groups are the reason for the reduced acrylate functionality (amount of acrylate groups per molecule). It was found that by performing this reaction with an amine excess of 20% cyclic carbonate turnover increases markedly while the excess amine groups are consumed by the (slower) Aza-Michael reaction. The latter affords oligomers as illustrated in Figure 4. The parameter m describes the degree of oligomerization and can be theoretically calculated (Supporting Information, p S27) to m = 0.5 for a reaction with 0.6 equiv of any given diamine. While this oligomerization also occurs with stoichiometric amount of the amine, the excess amine naturally intensifies this effect and affords products with higher molar mass and therefore also higher viscosities. In order to counter the latter and ensure an adequate stirring of the mixture, the reaction temperature was set to 100 °C. In order to inhibit the acrylate polymerization, the

form secondary amines, which exhibit much lower reactivity toward cyclic carbonates as opposed to primary amines. As determined from the respective 1H NMR spectra, the cyclic carbonate consumption in the bulk reaction of 1,5diaminopentane (DAP) and GCMA at 60 °C after 5 min and 3, 6, and 23 h accounted for 78%, 86%, 87%, and 90%, respectively. The 1H NMR spectra of DAP and GCMA as well as their reaction product DAP-G-60 after 5 min and 23 h at 60 °C are shown in Figure 3. While the reaction is very fast in the beginning, the reaction rate slows down drastically after the first 5 min. This can be explained with the Aza-Michael addition turning primary amines into unreactive secondary amines and consequently prevents full cyclic carbonate conversion. Unfortunately, the amine consumption cannot be properly monitored due to new emerging signals in the range between 2.3 and 2.9 ppm, which are attributed to the Aza-Michael addition product. In order to back this hypothesis, we mixed DAP with 1 equiv of methyl methacrylate (MMA) and heated the mixture under stirring for 5 h at 60 °C. The resulting product DAP_MMA showed almost no methacrylate groups anymore (Supporting Information, Figure S17) due to the formation of the Michael addition product which showed the corresponding signals in the 1H NMR spectrum in accordance with literature.22 The 1H/13C-HSQC-NMR spectrum of DAP_MMA in Figure E


Article

Macromolecules

increase the density of hydroxyl groups. As a consequence, DAPG, IPDA-G, XDA-G, CDMA-G and trifunctional JT403-G exhibit much higher viscosity values which could not be measured with the used setup. IPDA-G even turned out to be a brittle solid. The etheramine-based HUMAs in Table 3 showed viscosity values between 7.3 and 156 Pa·s. Fatty acid based dimer Priamine 1075 (PA) yielded the HUMA PA-G with 208 Pa·s and was the only compound free from ether groups with a reasonable viscosity at room temperature due to long, flexible methylene sequences. Nevertheless, it is obvious that ether groups have a more significant influence with respect to lowering the viscosity. Hydroxyurethane Methacrylates as Components of UV-Curable Acrylic Resins. To evaluate the influence of these HUMAs on the viscosity of a basic resin formulation and the mechanical properties of the cured samples, various mixtures of 4-acryloylmorpholine (ACMO) and the respective HUMA were prepared according to a 59:39-ratio, together with 1 wt % of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) as a photoinitiator. The resin viscosities are summarized in Table 4 and all lie well below 1300 mPa·s which as a rule of thumb represents the upper limit for most commercial SLA- and DLPbased 3d printers. The highest values between 630 and 880 mPa· s are represented by the mixtures containing HUMAs like DAPG, XDA-G, CDMA-G, and IPDA, which lack flexible ether groups. Etheramine-based HUMAs JT3000-G, TODA-G, DODA12-G and DODA8-G afford values in the range from 190 to 310 mPa·s, which are close to the viscosity of the commercial reference of 160 Pa·s and are well-suited for application in 3D printing. Polyetheramine-based PTHFA-G and JT3000-G act as flexibilizers and reduce stiffness when compared to Laromer UA 9089 as a reference. While both flexible HUMAs have similar effect on the mechanical properties, JT3000-G clearly fosters lower resin viscosity when compared to PTHFA-G and therefore would be the favored component for 3D printing. The lower viscosity of JT3000-G is a consequence of the branched structure and higher frequency of flexible ether groups in the molecule structure, while the trifunctional nature affords higher cross-linking densities compared to PTHFA-G, thus compensating for the flexibilizing effect and achieving similar stiffness at lower resin viscosity. Short and rigid building blocks like the cycloaliphatic IPDA-G and CDMA-G or the aromatic

reaction was performed under air and under vigorous mechanical stirring to increase the mixture’s exposure to the air atmosphere. On the basis of these reaction conditions (100 °C, 20% excess of amine groups), a number of hydroxyurethane acrylate (HUMA) oligomers were prepared. Figure 5 gives an overview of the respective (simplified) structures and their sample codes. The respective 1H NMR spectra are shown in the Supporting Information (pp S2−S9) and showed cyclic carbonate turnover of typically greater than 98% after 1.2 to 37 h as a function of the used amine. Reaction time (shown in Table 1) typically increased with higher molar mass of the amine which reduced the overall density of functional groups and with steric hindrance as present for example in Jeffamines with methyl substituent in α position to the amine. This pathway facilitates the isocyanate-free preparation of urethane-based acrylate building blocks containing 3.2−16.9 wt % (see Table 1) of chemically bound carbon dioxide, thereby offering green alternatives to conventional urethane acrylates. Table 3 gives a summary of the measured viscosity values at 25 °C in comparison to a commercial urethane acrylate benchmark Table 3. Viscosity Values of Various HUMAs and Laromer UA9089 oligomer

η(25 °C)a [Pa·s]

Laromer UA 9089 JT3000-G PTHFA-G TODA-G DODA12-G DODA8-G PA-G

20.9 ± 0.7 7.3 ± 0.1 43 ± 4 73 ± 3 88 ± 2 156 ± 5 208 ± 5

Determined with a plate−plate rheometer (from 0.1 to 100 s−1, 100 steps, logarithmic, 5 s per step, 3 s integration time). a

(Laromer UA 9089 by BASF). With the only exception of JT3000-G, which contains long, flexible poly(propylene oxide) segments and exhibits a viscosity of 7.3 Pa·s, all hydroxyurethane methacrylates (HUMAs) showed higher viscosity values as compared to this benchmark. This is attributed to the presence of HUMA’s hydroxyl groups which cause higher viscosity due to hydrogen bonding. The values tend to increase with lower amount of flexible groups and with smaller molecule size, which

Table 4. Material Characteristics of Photo-Cured Acrylic Resins Consisting of 59:39 Ratio of 4-Acryloylmorpholine (ACMO) and a Respective Oligomer with 1 wt % of Diphenyl(2,4,6-trimethylbenzoyl)phosphine Oxide (TPO) as a Photoinitiator acrylic resin

η(25 °C)a [mPa·s]

Young’s modulusb [MPa]

σmaxb [MPa]

εbreakb [%]

Tgc(DSC) [°C]

ACMO_Laromer ACMO_PTHFA-G ACMO_JT3000-G ACMO_JT403-G ACMO_TODA-G ACMO_DODA12-G ACMO_DODA8-G ACMO_PA-G ACMO_DAP-G ACMO_XDA-G ACMO_CDMA-G ACMO_IPDA-G

160 ± 10 750 ± 20 190 ± 30 565 ± 8 254 ± 18 310 ± 30 250 ± 20 267 ± 12 794 ± 14 630 ± 11 710 ± 40 880 ± 20

2660 ± 50 869 ± 17 1000 ± 100 3070 ± 40 2540 ± 180 3160 ± 40 3600 ± 500 2140 ± 40 3610 ± 70 5700 ± 600 4400 ± 300 3990 ± 60

71.9 ± 1.5 20.5 ± 1.7 21.9 ± 0.9 57 ± 5 67 ± 3 81 ± 7 85 ± 2 59.1 ± 1.0 61 ± 3 61 ± 16 71 ± 14 46 ± 7

6.1 ± 0.9 74 ± 11 90 ± 40 2.1 ± 0.3 12 ± 5 4.2 ± 1.4 3.9 ± 0.4 14 ± 6 1.8 ± 0.1 1.6 ± 0.5 2.0 ± 0.5 1.2 ± 0.2

97 n.v. n.v. 84 78 86 95 90 90 115 114 120

Determined with a plate−plate rheometer (from 0.1 to 100 s−1, 100 steps, logarithmic, 5 s per step, 3 s integration time). bTensile testing (ISO 527 1/2, 5 A, 5 mm min−1). cDSC (10 K min−1, second heating cycle); n.v. = not visible. a

F


Article

Macromolecules

Figure 6. Functionalization of DODA12-G with methacrylic acid anhydride. Subsequent addition of glycidyl methacrylate (GMA) consumes the methacrylic acid side product to afford the reactive diluent glycerol dimethacrylate (GDMA).

Figure 7. 1H NMR spectra (CDCl3) of DODA12-G (top) and DODA12-G5 (below).

transition temperature of up 120 °C. While the highest stiffness comes from the aromatic building block, it should be noted that

XDA-G afford high stiffness as represented by high Young’s moduli in the range from 3990 to 5700 MPa and high glass G


Article

Macromolecules

markedly lower with respect to that of the nonfunctionalized DODA12-G (88 Pa·s). The functionalization of the hydroxyl groups clearly accounts for lower intermolecular interactions and therefore lowers the viscosity. Added to the previously described photocurable resin formulation, these compounds showed a similar influence on the resin viscosity, as is apparent from Table 6. Furthermore the higher acrylate functionality of DODA12-G5 over DODA12-G significantly increased both Young’s modulus from 3160 to 4200 MPa and the glass transition temperature from 86 to 173 °C. Similar effects are encountered with DODA12-G5/GDMA as a consequence of higher network density. In the case of both DODA12-G5 and DODA12-G5/GDMA, the high acrylate density caused noticeable warpage of the testing specimen (Supporting Information, Figure S53) which was attributed to a reduced tensile strength. Such effects have to be counterbalanced by adding higher molar mass oligomers as warpage interferes with the printing process. Nevertheless, the results in Table 6 show to what extent HUMA may increase the potential range of material properties via functionalization. 3D Printing of HUMA. Using a commercial, DLP-based 3D printer, multiple structures were printed with the exemplarily selected HUMA-based acrylate resin ACMO_DODA12-G. Figure 8a illustrates the additive manufacturing process. The resulting printed objects are displayed in Figure 8b−d using a model of the Eiffel Tower, a double helix and a basket, respectively.

this resin mixture turned bright yellow after UV cure while the cycloaliphatic building blocks remained colorless (Supporting Information, Figure S52). Short and flexible etheramine-based HUMAs DODA12-G and DODA8-G offer a compromise between a low viscosity and mechanical performance. The formulation with DODA8-G showed a Young modulus of 3600 MPa and a tensile strength of 85 MPa, which are markedly increased as compared to the commercial reference. At the same time the viscosity of the resin increased only from 160 to 250 Pa· s. Functionalization of Hydroxyurethane Methacrylates by Esterification. Nucleophilic hydroxy groups and secondary amines are present in HUMA and can be functionalized for example by esterification with carboxylic anhydrides like methacrylic acid anhydride (MAA). On the basis of DODA12-G, this functionalization was performed at 100 °C within 3 h using magnesium oxide (1.3 wt %) as a catalyst, as illustrated in Figure 6. The number of nucleophiles, namely the hydroxyl groups and the secondary amines per gram of DODA12-G, was calculated from the respective weight portions of its synthesis. The consumption of the nucleophiles can be monitored via 1H NMR spectroscopy as illustrated in Figure 7. The signals 3, 4, and 8 correspond to protons near the hydroxyl groups and the signals 5, 6, and 7 belong to protons adjacent to the secondary amines (Supporting Information, Figures S7, S8, S19, and S20). The esterification of the hydroxyl groups goes along with a marked low field shift of the adjacent protons. As shown in Figure 6, this synthetic pathway affords methacrylic acid as a side product which would be undesirable in a 3D printing photocurable resin, as it is monofunctional, volatile, and toxic and has an unpleasant odor. It was therefore removed by liquid−liquid extraction to afford the polyfunctional DODA12-G5. In another attempt, the methacrylic acid was subsequently consumed by the reaction with glycidyl methacrylate (GMA) to afford the reactive diluent glycerol diamethacrylate (GDMA), as is illustrated in Figure 6. The resulting product DODA12-G5/GDMA contains 44 wt % of GDMA and has a viscosity of 8.1 Pa·s, as shown in Table 5.

■

CONCLUSION A variety of photocurable polyfunctional hydroxyurethane methacrylates (HUMA) are readily tailored by aminolysis of methacrylated glycerol carbonate (GCMA), produced by chemical fixation of the greenhouse gas carbon dioxide with glycidyl methacrylate. The Aza-Michael addition as a minor side reaction enables the oligomerization in the presence of a slight excess of amine groups via advancement reaction. Opposite to the conventional synthesis of urethane methacrylates, this facile solvent-free one-step process does not require any toxic and water-sensitive isocyanates. As a function of the amine type, between 3.2 and 16.9 wt % carbon dioxide are incorporated into HUMA. Furthermore, the absence of highly reactive isocyanates enables one to introduce hydroxy groups in the polyurethane backbone. They account for enhanced intermolecular interactions paralleled by increased resin viscosity and markedly improved mechanical properties. Moreover, by esterification of hydroxy groups with methacrylic acid anhydride the acrylate functionality of HUMA significantly increases. The residual methacrylic acid is quantitatively converted with glycidyl methacrylate into glycerol dimethacrylate which serves as reactive diluent and eliminates the need for tedious methacrylic acid recovery. The resulting higher acrylate functionality improves both Young’s modulus from 3160 to 4200 MPa and

Table 5. Viscosity Values As Determined from a Plate−Plate Rheometer oligomer

η(25 °C)b [Pa·s]

DODA12-G DODA12-G5 DODA12-G5/GDMAa

88 ± 2 42 ± 3 8.1 ± 0.3

a

Contains 44 wt % of glycerol dimethacrylate (GDMA). bDetermined with a plate−plate rheometer (from 0.1 to 100 s−1, 100 steps, logarithmic, 5 s per step, 3 s integration time).

In the absence of the reactive diluent the functionalized DODA12-G5 still exhibited a viscosity of 42 Pa·s which is

Table 6. Material Properties of Cured Acrylic Resins Consisting of 59:39 Ratio of 4-Acryloylmorpholine (ACMO) and a Respective Oligomer with 1 wt % of Diphenyl(2,4,6-trimethylbenzoyl)phosphine Oxide (TPO) as a Photoinitiator acrylate resin

η(25 °C)a [mPa·s]

Young’s modulusb[MPa]

σmaxb [MPa]

εbreakb [%]

Tgc(DSC) [°C]

ACMO_DODA12-G ACMO_DODA12-G5 ACMO_DODA12-G5/GDMA

310 ± 30 159 ± 5 84 ± 6

3160 ± 40 4200 ± 600 3900 ± 200

81 ± 7 68 ± 10 65 ± 9

4.2 ± 1.4 3.1 ± 0.9 2.2 ± 0.4

86 173 150

Determined with a plate−plate rheometer (from 0.1 to 100 s−1, 100 steps, logarithmic, 5 s per step, 3 s integration time). bTensile testing (ISO 527 1/2, 5 A, 5 mm min−1). cDSC (10 K min−1, second heating cycle). a

H


Article

Macromolecules

Figure 8. 3D printing of HUMA ACMO_DODA12-G. (a) Additive manufacturing process, (b) model of the Eiffel Tower (L × W × H = 42 mm × 42 mm × 90 mm), (c) double helix (D × H = 32 mm × 42 mm), and (d) basket (L × W × H = 43 mm × 32 mm × 33 mm).

■

the glass transition temperature from 86 to 173 °C with respect to HUMA-based resins. Hence, the molecular engineering of HUMA improves the performance of photocurable resins which hold great promise for 3D printing applications. Moreover, the use of biobased feed stocks combined with chemical carbon dioxide fixation contributes to better sustainability of resins tailored for 3D printing applications.

■

ABBREVIATIONS HUMA, hydroxyurethane methacrylate; UMA, urethane methacrylate; SLA, stereolithography; DLP, digital light processing; UV, ultraviolet; NIPU, non-isocyanate polyurethane; PHU, polyhydroxyurethane; NuE, nucleophile equivalent

■

ASSOCIATED CONTENT

* Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00330. 1 H and 13C NMR spectra, DSC, tensile testing, and rheological experiments (PDF)

■

REFERENCES

(1) Ligon, S. C.; Liska, R.; Stampfl, J.; Gurr, M.; Mülhaupt, R. Polymers for 3D Printing and Customized Additive Manufacturing. Chem. Rev. 2017, 117, 10212−10290. (2) Ligon-Auer, S. C.; Schwentenwein, M.; Gorsche, C.; Stampfl, J.; Liska, R. Toughening of photo-curable polymer networks: A review. Polym. Chem. 2016, 7, 257−286. (3) Wang, X.; Soucek, M. D. Investigation of non-isocyanate urethane dimethacrylate reactive diluents for UV-curable polyurethane coatings. Prog. Org. Coat. 2013, 76, 1057−1067. (4) Maurya, S. D.; Kurmvanshi, S. K.; Mohanty, S.; Nayak, S. K. A Review on Acrylate-Terminated Urethane Oligomers and Polymers: Synthesis and Applications. Polym.-Plast. Technol. Eng. 2018, 57, 625− 656. (5) Liao, F.; Zeng, X.-r.; Li, H.-q.; Lai, X.-j.; Zhao, F.-c. Synthesis and properties of UV curable polyurethane acrylates based on two different hydroxyethyl acrylates. J. Cent. South Univ. 2012, 19, 911−917. (6) Gao, L.; Zhou, L.; Fang, S.; Wu, C. New copoly(urethanemethacrylate)s obtained by adjusting the content of the poly(1,2propanediol ortho -phthalate): Transparent, thermal, and mechanical properties. J. Appl. Polym. Sci. 2013, 128, 3900−3905. (7) Lee, T. Y.; Roper, T. M.; Jönsson, E. S.; Guymon, C. A.; Hoyle, C. E. Influence of Hydrogen Bonding on Photopolymerization Rate of Hydroxyalkyl Acrylates. Macromolecules 2004, 37, 3659−3665. (8) Lemon, M. T.; Jones, M. S.; Stansbury, J. W. Hydrogen bonding interactions in methacrylate monomers and polymers. J. Biomed. Mater. Res., Part A 2007, 83A, 734−746. (9) Kunwong, D.; Sumanochitraporn, N.; Kaewpirom, S. Curing behavior of a UV-curable coating based on urethane acrylate oligomer: the influence of reactive monomers. J. Sci. Technol. 2011, 33, 201−207. (10) Niyogi, D.; Kumar, R.; Gandhi, K. S. Water blown free rise polyurethane foams. Polym. Eng. Sci. 1999, 39, 199−209.

S

AUTHOR INFORMATION

Corresponding Author

*(R.M.) E-mail: [email protected]. Telephone: +49 761 203 6273. ORCID

Vitalij Schimpf: 0000-0002-9232-0479 Rolf Mülhaupt: 0000-0003-2804-3486 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support of JONAS−Joint Research Network on Advanced Materials and Systems, BASF SE, Ludwigshafen, Germany. We are thankful to our co-workers Victor Hugo, Pacheco Torres, Kristin Lehmann, and Carolin Guth for their enthusiastic support. I


Article

Macromolecules (11) Guan, J.; Song, Y.; Lin, Y.; Yin, X.; Zuo, M.; Zhao, Y.; Tao, X.; Zheng, Q. Progress in Study of Non-Isocyanate Polyurethane. Ind. Eng. Chem. Res. 2011, 50, 6517−6527. (12) Peters, J. M. Studies of isocyanate toxicity. Proc. R. Soc. Med. 1970, 63, 372−375. (13) Allport, D. C.; Gilbert, D. S.; Outterside, S. M. MDI and TDI: Safety, Health and the Environment: A Source Book and Practica; John Wiley & Sons, 2003. (14) Han, L.; Dai, J.; Zhang, L.; Ma, S.; Deng, J.; Zhang, R.; Zhu, J. Diisocyanate free and melt polycondensation preparation of bio-based unsaturated poly(ester-urethane)s and their properties as UV curable coating materials. RSC Adv. 2014, 4, 49471−49477. (15) Maisonneuve, L.; Lamarzelle, O.; Rix, E.; Grau, E.; Cramail, H. Isocyanate-Free Routes to Polyurethanes and Poly(hydroxy Urethane)s. Chem. Rev. 2015, 115, 12407−12439. (16) Blattmann, H.; Fleischer, M.; Bähr, M.; Mülhaupt, R. Isocyanateand phosgene-free routes to polyfunctional cyclic carbonates and green polyurethanes by fixation of carbon dioxide. Macromol. Rapid Commun. 2014, 35, 1238−1254. (17) Rokicki, G.; Parzuchowski, P. G.; Mazurek, M. Non-isocyanate polyurethanes: Synthesis, properties, and applications. Polym. Adv. Technol. 2015, 26, 707−761. (18) Ochiai, B.; Inoue, S.; Endo, T. One-pot non-isocyanate synthesis of polyurethanes from bisepoxide, carbon dioxide, and diamine. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 6613−6618. (19) Ochiai, B.; Sato, S.-I.; Endo, T. Crosslinkable polyurethane bearing a methacrylate structure in the side chain. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 3400−3407. (20) Assumption, H. J.; Mathias, L. J. Photopolymerization of urethane dimethacrylates synthesized via a non-isocyanate route. Polymer 2003, 44, 5131−5136. (21) Fleischer, M.; Blattmann, H.; Mü lhaupt, R. Glycerol-, pentaerythritol- and trimethylolpropanebased polyurethanes and their cellulose carbonate composites prepared via the non-isocyanate route with catalytic carbon dioxide fixation. Green Chem. 2013, 15, 934. (22) Bosica, G.; Abdilla, R. Aza-Michael Mono-addition Using Acidic Alumina under Solventless Conditions. Molecules (Basel, Switzerland) 2016, 21.

J


Polyfunctional Acrylic Nn-isocyanate Hydroxyurethanes as

Recommend Documents