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New structure of diamine curing agent for epoxy resins with self-restoration ability: Synthesis and spectroscopy characterization
Accepted Manuscript New structure of diamine curing agent for epoxy resins with self-restoration ability: Synthesis and spectroscopy characterization Marialuigia Raimondo, Liberata Guadagno, Carlo Naddeo, Pasquale Longo, Annaluisa Mariconda, Anna Agovino PII:
S0022-2860(16)31116-4
DOI:
10.1016/j.molstruc.2016.10.060
Reference:
MOLSTR 23061
To appear in:
Journal of Molecular Structure
Received Date: 12 February 2016 Revised Date:
17 October 2016
Accepted Date: 19 October 2016
Please cite this article as: M. Raimondo, L. Guadagno, C. Naddeo, P. Longo, A. Mariconda, A. Agovino, New structure of diamine curing agent for epoxy resins with self-restoration ability: Synthesis and spectroscopy characterization, Journal of Molecular Structure (2016), doi: 10.1016/ j.molstruc.2016.10.060. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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New Structure of Diamine Curing Agent for Epoxy Resins with Self-Restoration Ability:
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Synthesis and Spectroscopy Characterization
Marialuigia Raimondoa*, Liberata Guadagnoa*, Carlo Naddeoa,
Department of Industrial Engineering - DIIn - University of Salerno
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a
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Pasquale Longob, Annaluisa Maricondab, Anna Agovinob
Via Giovanni Paolo II ,132 - 84084 Fisciano (SA), Italy (UE) b
Department of Chemistry and Biology - University of Salerno
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Via Giovanni Paolo II ,132 - 84084 Fisciano (SA), Italy (UE)
*Corresponding author
Marialuigia Raimondo and Liberata Guadagno
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Marialuigia Raimondo - e-mail: [email protected] phone +39 089964019
mobile +39 3293030935
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Liberata Guadagno - e-mail: [email protected] phone +39 089964114 mobile +39 3204213235
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Abstract The development of smart materials in aeronautical structures consisting of compounds based on epoxy resins having self-repair capability has been hampered by some criticalities. One of the main critical points is related to the impossibility to use primary amines (e.g.: 4,4′-diaminodiphenyl
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sulfone, DDS) as hardeners, because they can poison the catalyst responsible for the healing mechanisms. In this paper, the synthesis, characterization and some tests of applicability of a new hardener, the tetramethylated diaminodiphenyl sulfone (tm-DDS), are shown. The tm-DDS is able to rapidly react with epoxy resin, giving a composite material having some characteristics
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significantly better than composites hardened with different tertiary amines. The new hardener is able to increase the glass transition temperature (Tg) of about 90°C with respect to the more
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common hardener, ancamine K54, already used in self-healing epoxy formulations.
Keywords: Self-Healing Epoxy resins; Smart Materials; New Diamine Curing Agent Synthesis;
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Spectroscopy analysis; ROMP reaction
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1. Introduction Epoxy resins are fast gaining ground as preferred polymer to be used as carbon fiber-reinforced materials to manufacture large components able to sustain aerodynamic loads. The introduction of epoxy resins to the aeronautic industry was mainly driven by performance gains (better design flexibility, no corrosion, easy production process, etc.) and most of all by weight reduction. This
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last aspect is of relevance when considering issues related to the Energy Resources and Supply, costs in service and air pollution measures. This major advantage of the composites, the lightness, is also obtained saving and/or increasing the material strength in such a way as to make travelling on aeroplanes more comfortable for the crew and passengers in virtue of the possibility to slightly
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increase cabin pressures without to act on the structure weight. Unfortunately, composites possess also some properties which reduce their advantage with respect to traditional metal alloys. One
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inherent shortcoming is the poor impact damage resistance. Impact damage due to several factors such as unfavorable weather conditions, stones or rocks on the keel during landing, incidental contact during maintenance and bird strike may significantly affect the integrity of the composite structure of aircrafts in service [1].
Smart resins with self-healing capability offer many advantages with respect to traditional epoxy resins. As an instance, for aeronautical materials, inspection and maintenance are important aspects
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when considering the availability of aircraft for revenue flights. Modern airframe design is exploiting new exciting developments in materials and structures to construct ever more efficient air vehicle able to enable ‘smart’ maintenance including self-repair capabilities. Actually, also a very advanced design of an aircraft has to take required inspection intervals into account. An
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aircraft with inherent protective and smart abilities could help to significantly extend the inspection intervals, thereby increasing aircraft availability. Reduced aircraft ground time due to rapid repair
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following accidental damages are expected to strongly contribute: a) to save of maintenance costs; b) to extend the structural integrity and increase of the reliability structures. Recent studies have highlighted potential of self-healing polymer composites to be used as single structural materials and/or for sandwich structures [2-8]. Many self-healing resins, already proposed in literature, are based on microencapsulated systems [3-17]. In this system, the healing of microcracks is achieved by embedding a microencapsulated healing agent and a catalytic chemical trigger within the matrix. As soon as a crack breaks a microcapsule, the healing agent flows into the crack plane through capillary action. The healing monomer distributes in the crack plane and reacts with the embedded catalyst bonding the crack planes. Treatment of epoxy resins with curing agents or hardeners gives three-dimensional insoluble and infusible networks. Epoxy resins can be cured with a wide variety of curing agents which are essential components of an advanced composite system 3
ACCEPTED MANUSCRIPT providing a substantial contribution to the properties and the performance characteristics of the crosslinked finished products. Thus the choice of the curing agent is really crucial and needs to be considered very carefully. In fact, the final properties of the epoxy network are closely depending on the crosslinking agent, processing methods and curing conditions [18]. The impossibility to use as curing agent primary amines in combination with catalysts active in the Ring-Opening
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Metathesis Polymerization (ROMP) reaction constitutes a very strong impediment as regard the development of high performance self-repairing epoxy resins based on this design. In fact, primary aliphatic or aromatic amines used as hardeners for the cross-linking reactions of the epoxy matrix poison the catalysts used for the polymerization of the healing agent. Aliphatic amines, which
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rapidly react with epoxy resin, are representative room-temperature curing agents [19], moreover the resin crosslinked with aliphatic amines is characterized by low mechanical performance. Aromatic amines instead have been developed to achieve greater heat resistance and chemical
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resistance than those of aliphatic amines [20]. They require high temperature for the curing cycles because, having weaker basicity than aliphatic amine, slowly cure at room temperature due to the steric hindrance of aromatic rings. One of the most common is 4-4’-diaminodiphenylsulfone (DDS). In particular, the use of the aromatic amine DDS allows to obtain materials with excellent mechanical properties [21-24].
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This paper describes the synthesis and characterization of a new diamine curing agent (tetramethylated diaminodiphenylsulfone) for self-healing epoxy resins. The applicability conditions are also shown. The new hardener is able to enhance the Tg of the developed selfhealing material extending its applicability in many sectors of the structural materials (aeronautical,
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automotive, wind turbine industries etc..).
2. Experimental
2.1 Materials and methods The curing agents: 4,4-diaminodiphenyl sulfone (DDS) and ancamine K54 were purchased from Sigma-Aldrich and used as received. Tetrahydrofuran was thoroughly deoxygenated and dehydrated under nitrogen by refluxing over suitable drying agents. Deuterated solvent dimethyl sulfoxide (DMSO-d6) was purchased from Euriso-Top products. Proton Nuclear Magnetic Resonance (1H NMR) spectra were recorded on a Bruker AM250 and a Bruker AM300 operating at 250 and 300 MHz respectively. The 1H NMR chemical shifts are referenced to SiMe4 (δ = 0 ppm) using the residual proton impurities of the deuterated solvents as internal standards. Spectra are reported as follows: chemical shift (ppm), multiplicity and 4
ACCEPTED MANUSCRIPT integration. Multiplicities are abbreviated as follows: singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), and broad (br). Fourier Transform Infrared Spectroscopy (FTIR) spectra were performed at room temperature by using a Bruker Vertex 70 FTIR-spectrophotometer with a 2 cm-1 resolution (64 scans collected). Thermogravimetric analysis (TGA) was carried out in air and in an inert atmosphere of nitrogen
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using a Mettler TGA/SDTA 851 thermal analyzer. The temperature range was 30-1000°C at a heating rate of 10°C min-1.
Calorimetric analysis was performed with a Mettler DSC 822 differential scanning calorimeter in a flowing nitrogen atmosphere. The samples were analyzed in the temperature range of 0-300°C with
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a scan rate of 10°C min-1.
2.2 Synthesis of tetramethylated diaminodiphenyl sulfone tm-DDS
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In a three-necked flask equipped with magnetic stirrer, reflux condenser and dropping funnel, 250 mL of tetrahydrofuran (THF) and 1.5 g of DDS (6*10-3 mol, 248.30 g/mol) were introduced. Complete dissolution of the DDS was obtained after stirring for about 30 min. Subsequently, in the reaction flask, t-BuONa (6.77 g, 0.06 mol, 112 g/mol) was added. The reaction flask was placed in a water/ice bath (5°C) and drops of CH3I (3.73 mL, 0.058 mol, d=2.28 g/mol, 141.94 g/mol) were
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slowly added. The mixture was stirred for 24 h and quenched by the addition of saturated solution of ammonium chloride (NH4Cl, about 100 ml). The organic phase was then extracted with ethyl acetate (about 60 mL x 3), the combined extracts dried over MgSO4, filtered and concentrated in g, 60%). 1
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vacuo. The product was washed with ethyl ether to afford as a yellow powder (3.61·10-3 mol, 1.10
H NMR (300 MHz, DMSO-d6): δ = 7.58 (d, 4H), 6.72 (d, 4H), 2.95 (s, 6H).
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The chemicals NaH and CH3I, NaH and tBuONa were purchased from Sigma-Aldrich.
2.3 Epoxy Matrix Manufacture EB2 matrix was obtained by mixing the epoxy precursor (E) (trade name EPON 828) with an epoxy reactive diluent 1,4-butanediol diglycidyl ether (B) at a concentration of 80%:20% (by wt) epoxide to flexibilizer. The compounds E and B, both containing epoxy-moieties, were obtained by SigmaAldrich. After, the synthesized hardener tetramethylated DDS (product of the reaction of the fourth optimized procedure of synthesis), named “tm-DDS (4th synthesis)” was added to the EB2 epoxy mixture at a percentage of 18% of the total moles of salified epoxy groups, giving the mixture EB2+tm-DDS(4th synthesis 18%). The tm-DDS (4th synthesis) was completely solubilized by mechanical agitation into the EB2 formulation at the temperature of 120°C after 1 hour using an oil 5
ACCEPTED MANUSCRIPT bath. Samples based on the same epoxy matrix, containing different curing agents (tertiary amine “ancamine K54” and DDS), were also prepared to compare catalyst reactivity and glass transition temperature between the epoxy formulations solidified with different hardeners. The catalyst used in the epoxy matrix was Hoveyda-Grubbs’ 1st generation (HG1). The healing agent used in this work was a cyclic olefin 5-ethylidene-2-norbornene (ENB). HG1 catalyst and
2.4 HG1 catalyst based epoxy mixtures
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ENB were purchased from Sigma-Aldrich.
The sample EB2+tm-DDS(4th synthesis 18%)+HG1(7%) was obtained by solubilizing at molecular
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level by mechanical stirring in the mixture EB2+tm-DDS(4th synthesis 18%) at room temperature, the HG1 catalyst at a concentration of 7% (by wt) with respect to the epoxy mixture EB2. In the following, the sample EB2+tm-DDS(4th synthesis 18%)+HG1(7%) uncured refers to the initial
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liquid epoxy mixture before undergoing the curing process in the oven. Then, the sample EB2+tmDDS(4th synthesis 18%)+HG1(7%) was cured in a two-stage process. The first stage was carried out at a temperature of 80°C for 3 hours, while the second stage was carried out at the two temperatures of 150 and 170°C for 1 hour respectively, giving the sample EB2+tm-DDS(4th synthesis 18%)+HG1(7%) up to 150°C and up to 170°C respectively. Two different samples A and
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B were also prepared for comparison.
A) a sample based on the same matrix (EB2) was hardened with DDS which was added at stoichiometric concentration with respect to all the epoxy rings (E and B compounds). This sample was cured using the same curing cycle (up to 150°C) mentioned before. It is hereafter named with
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the acronym EB2+DDS+HG1(7%) up to 150°C.
B) a sample based on the same matrix (EB2) was hardened with ancamine and was cured in a two-
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stage process as described for the sample EB2+tm-DDS(4th synthesis 18%)+HG1(7%).
2.5 Evaluation of the catalytic activity in the tetramethylated DDS based epoxy mixture
To verify the catalytic activity of the HG1 catalyst in the tetramethylated DDS based epoxy mixture EB2+tm-DDS(4th synthesis 18%)+HG1(7%) (uncured and cured up to 150°C and up to 170°C) , spectroscopic investigation by FTIR was carried out. For this purpose, in the case of uncured sample, the obtained liquid mixture was stratified on a slide for light microscopy. Subsequently, two drops of ENB were added to the stratified mixture. In the case of cured sample up to 150°C and 170°C, the mixture was reduced in powder with a serrated blade and collected in a mortar. Two 6
ACCEPTED MANUSCRIPT drops of healing agent ENB (2 x 50 µL) were then added and dispersed into the powder sample and quickly analyzed on a KBr disk for FTIR investigation. The same procedure used to check the catalytic activity in the cured sample based on tm-DDS(4th synthesis) was carried out also for the following formulation EB2+DDS+HG1(7%) up to 150°C.
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3. Results and discussion
3.1 Synthesis of tetramethylated diaminodiphenyl sulfone. 1H NMR analysis
In this paper, the choice of methyl groups for the alkylation of DDS is due to the small steric
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hindrance of these substituents, as compared to other alkyl groups. This makes the free electron pair on the nitrogen certainly more available for crosslinking reactions. Tetramethylated
diaminodiphenyl
sulfone
(tm-DDS)
obtained
by
methylation
of
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diaminodiphenyl sulfone (DDS) (see Fig.1).
was
Four synthesis procedures have progressively been optimized in order to obtain the fully methylated DDS. Starting from a base ratio 1:5:5 of DDS:CH3I:NaH respectively and then 1:10:5, uncompletely methylated DDS was obtained. The same results were obtained using a ratio 1:6:6 of DDS:CH3I :t-BuONa whereas tetramethylated diaminodiphenyl sulfone (tm-DDS) was obtained by
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1:10:10 DDS:CH3I:t-BuONa ratio (see experimental part). Fig. 1 shows the scheme of the synthesis
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of tetramethylated diaminodiphenyl sulfone (tm-DDS).
Fig. 1. Scheme of the synthesis of tetramethylated diaminodiphenyl sulfone (tm-DDS).
The 1H NMR spectrum (DMSO-d6) of DDS, reported in Fig. 2, shows the following signals: 7
ACCEPTED MANUSCRIPT δ ppm 5.97 – Ha , δ ppm 6.57 – Hb, δ ppm 7.44 –Hc. In Fig. 2, also DDS formula for the signal
Hb
Ha
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Hc
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attributions is shown.
Fig. 2. 1H NMR spectrum (DMSO-d6) of diaminodiphenyl sulfone (DDS).
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The signals in the spectrum (see Fig. 3) indicate the obtainment of the desired product arising from the fourth synthesis procedure of tm-DDS, named “tm-DDS (4th synthesis)”.
tm-DDS (4th synthesis)
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-CH3
Hc
Hb
Fig. 3. 1H NMR spectrum of the new synthesized hardener obtained from the 4th synthesis procedure, named “tm-DDS (4th synthesis)”.
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3.2 FTIR analysis Fourier Transform Infrared Spectroscopy (FTIR) is a sensitive technique particularly for identifying the organic molecular structures of new reaction products. The products obtained from the different synthesis procedures were analyzed by FTIR investigations (Fig. 4). The substitution of the Ha hydrogen (see Fig. 2) with methyl groups can be
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followed in different spectral ranges. In particular, primary amines display two weak absorption bands (N-H stretching vibrations): one at 3471 cm-1 and the other near 3373 cm-1. These bands represent, respectively, the “free” asymmetrical and symmetrical N-H stretching modes. The band at 3240 cm-1 is the Fermi resonance band with overtone of the band at 1617 cm-1 related to the N-H
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bend (scissoring). Secondary amines show a single weak band in the 3350-3310 cm-1 region (see pink spectrum). The absence of these signals and the appearance of the following signals: a) the
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sharp band near 2800 cm-1 characteristic of sp3 C-H stretches of the N-methyl groups; b) the two bands occurring at 2962 cm-1 and 2872 cm-1 (the first of these results from the asymmetrical stretching mode in which two C-H bonds of the methyl group are extending while the third one is contracting -νasCH3-, the second arises from symmetrical stretching -νsCH3- in which all three of the C-H bonds extend and contract in phase) confirm the obtaining of the desired products. Other evidences of the obtaining of tetramethylated diaminodiphenyl sulfone, in the products of the
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synthesis, arise from the appearance of the C-N stretch of tertiary aromatic amines at ~ 1360-1310 cm-1 , from the absence of the bands a 1617 cm-1 already commented before, from the appearance of a) the out-of-plane C-H bending vibration in the range between 900 and 675 cm-1, and b) the
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symmetrical bending vibration (δsCH3) at 1375 cm-1.
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3471
1
DDS tm-DDS(3rd synthesis) tm-DDS (4th synthesis after elimination of impurities)
2872
0.6
2800
0.4
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Absorbance
0.8
2962
0.2
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0 -0.2
3200 2700 2200 Wavenumber (cm-1)
1700
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3700
Fig. 4. FTIR spectra of DDS, and products obtained from the different synthesis procedures
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(spectral range 1700 cm-1 - 4000 cm-1).
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DDS tm-DDS(3rd synthesis) tm-DDS (4th synthesis after elimination of impurities)
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1360-1310
2.2
900-675
1617
1.4
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1373
0.6
-0.2 1400 900 Wavenumber (cm-1)
400
Fig. 5. FTIR spectra of DDS, and products obtained from the different synthesis procedures (spectral range 400 cm-1 - 1800 cm-1). 10
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3.3
Calorimetric
investigation:
Thermogravimetric
analysis
(TGA)
and
Differential Scanning Calorimetry (DSC) 3.3.1 Thermogravimetric analysis (TGA)
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TGA analysis was carried out in order to evaluate the thermal stability of the products obtained from the different synthesis procedures. Fig. 6 shows the thermogravimetric curves of these products. The product obtained from the 4th synthesis, after elimination of impurities (see brown
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curves), shows a beginning of degradation in air at ~ 300°C, a value very similar to the initial DDS.
Fig. 6. TGA curves of the products obtained from different synthesis procedures.
3.3.2 Differential Scanning Calorimetry (DSC) The Differential Scanning Calorimetry (DSC) is a fundamental tool in thermal analysis because it monitors heat effects associated with phase transitions and chemical reactions as a function of temperature and is a very informative method in physical characterization of a compound. DSC is 11
ACCEPTED MANUSCRIPT also useful to determine the melting point. DSC curves of the same products are shown in Figs 7 and 8. The substitution of hydrogen atoms with methyl groups causes a strong increase in the melting peak of the new hardener agent, in particular the product obtained from the 4th synthesis, after elimination of impurities, shows the melting peak between 260-275°C (see Fig. 8). This high
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melting temperature did not constitute a problem for its solubility in the epoxy mixture.
Fig. 7. DSC curves of the products obtained from different synthesis procedures in the range 0°C300°C.
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150°C-300°C.
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Fig. 8. DSC curves of the products obtained from different synthesis procedures in the range
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3.3.3. Differential Scanning Calorimetry (DSC) - Epoxy formulations hardened with tm-DDS(4th synthesis) and ancamine.
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The technique of differential scanning calorimetry was also used to obtain information on thermal transitions such as the glass transition temperature of the resins hardened with tm-DDS. The result was compared with the resin hardened with ancamine which is the most common tertiary amine used as hardener in self-healing epoxy systems because it does not deactive the Grubbs' catalyst [48,16]. The polymerization mechanism of the new hardener agent (tm-DDS) is supposed to be that typical of tertiary amines; it induces the direct linkage of epoxy groups to one other. The reaction mechanism is supposed to be as follows (see scheme of Fig. 9):
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Fig. 9. Scheme of the reaction mechanisms of tm-DDS with the epoxy precursor (EPON 828).
The reaction can occur at both ends of the diglycidyl ether molecule, hence a crosslinked structure will be built up. The reaction mechanisms are similar to the reactions of R2N- groups of ancamine
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with the initial epoxy precursors. In this last case, the epoxy group also reacts with hydroxyl groups
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according to the scheme of Fig. 10.
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Fig. 10. Scheme of the reaction of -OH group of the ancamine with the epoxy precursor (EPON
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828).
Despite the similarity in the main reaction mechanisms of tm-DDS and ancamine, the new
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sinthesized hardener “tm-DDS(4th synthesis)” is able to give composites characterized by higher glass transition temperature (Tg). Fig. 11 shows the differential scanning calorimetric (DSC) curves in dynamic regime of the sample solidified with the new synthesized hardener “EB2 tm-DDS(4th synthesis) HG1” and the sample hardened with ancamine “EB2 Ancamine HG1”.
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Fig. 11. DSC curves of the epoxy samples solidified with two different hardeners tm-DDS(4th synthesis) and ancamine.
Considering the DSC curves, a Tg of ~ 75°C is observed for the sample EB2 Ancamine HG1
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hardened with ancamine, whereas a Tg of ~ 165°C is observed for the sample EB2 tm-DDS(4th synthesis) HG1 hardened with tetramethylated DDS. The new hardener is able to increase the glass transition temperature of about 90°C.
It is worth noting that for the two different samples the amount of hardener agent was calculated
curing cycle.
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considering the same number of unshared electron pair on the nitrogen atoms and using the same
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This different behaviour is most likely due to the greater rigidity of the new synthetized hardener with respect to the ancamine.
This is a very interesting result from an industrial point of view because high glass transition temperatures are frequently requested to fullfill some of the main industrial requirements of structural materials in several fields (aeronautics, marine industry etc.. ).
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3.3.4 Evaluation of the catalytic activity inside the epoxy mixture containing tetramethylated DDS To verify the catalytic activity of the HG1 catalyst in the tetramethylated DDS based uncured and cured (up to 150°C and up to 170°C) epoxy mixture EB2+tm-DDS(4th synthesis 18%)+HG1(7%), spectroscopic investigation by FTIR was carried out. The control of the catalytic activity was
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performed by evaluating the formation of the metathesis product. For the sample EB2+tm-DDS(4th synthesis 18%)+HG1(7%) uncured, after the addition of two drops of ENB to the stratified mixture, a thin solid film of metathesis product was immediately obtained. Therefore, spectroscopic investigations highlighted that the presence of tm-DDS hardener did not alter the catalytic activity
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of the HG1. The spectrum of the obtained film PMET EB2+tm-DDS(4th synthesis 18%)+HG1(7%) uncured (see Fig. 12) shows a peak at 966 cm-1 attributable to ring-opened poly(ENB) [25], which
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is an indication of the formation of the metathesis product. This peak is assigned to the trans substituted alkenes, characteristic of the ring opened cross-linked product [poly(ENB)] providing evidence that the catalyst embedded in the resin is active in the ROMP reaction and hence of the fact that the activity of the catalyst was not compromised by the chemical nature of the oligomers, and the treatments of mechanical mixing.
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PMET EB2+tm-DDS(4th synthesis 18%)+HG1(7%) uncured
0.425
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0.475
966 cm-1
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Absorbance
0.525
0.375 0.325 0.275
1275
1175 1075 975 Wavenumber (cm-1)
875
Fig. 12. FTIR spectrum of the solid film (metathesis product) obtained by polymerization of ENB inside EB2+tm-DDS(4th synthesis 18%)+HG1(7%) uncured mixture.
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ACCEPTED MANUSCRIPT Fig. 13 shows FTIR spectra of the cured sample up to 150°C and 170°C EB2+tm-DDS(4th synthesis 18%)+HG1(7%) powder treated with ENB. FTIR analysis of the cured sample highlights a signal at 966 cm-1 characteristic of ring-opened poly(ENB) in the spectrum of the sample cured up
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to 150°C whereas no signal of metathesis product is observed for the sample cured up to 170°C.
Fig. 13. FTIR spectrum of the sample obtained by polymerization of ENB inside EB2+tm-DDS(4th
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synthesis 18%)+HG1(7%) mixture after curing process.
3.3.4.1 Evaluation of the catalytic activity inside the epoxy mixture containing DDS
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In this section, in order to demonstrate the effectiveness of the catalytic activity of the new synthesized catalyst tm-DDS(4th synthesis) with respect the DDS, the FTIR spectrum of the cured sample EB2+DDS+HG1(7%) after the treatment with ENB is shown in Fig. 14. No signal of metathesis product was found for this sample cured up to 150°C, clearly proving that the primary aromatic diamine DDS poison the catalyst responsible for the healing mechanisms. No signal of metathesis product was also observed for the same uncured sample, in opposite to the result shown in Fig. 12 where an intense peak of the metathesis product is observed for uncured mixture corresponding to the sample hardened with tm-DDS (EB2+tm-DDS(4th synthesis 18%)+HG1(7%)). These results highlight that the ROMP catalyst is completely deactivated (also in the fluid epoxy mixture) in the epoxy precursors containing DDS. This is an expected result, in fact, 18
ACCEPTED MANUSCRIPT in spite of the fact that the implementation of ruthenium catalyzed olefin metathesis has benefitted from a broad functional group tolerance, nitrogen bases as the DDS hardener have remained challenging often requiring catalyst protection to achieve good yields with reasonable catalyst loadings. In the case of the hardener tm-DDS, no nitrogen bases can poison the catalyst because the polymerization mechanisms, as illustrated in the scheme of reaction of Fig. 9, consume the
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unshared electron pair of nitrogen atoms already in the first stage of polymerization at low
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temperature.
Fig. 14. FTIR spectrum of the sample EB2+DDS+HG1(7%) cured up to 150 °C after the treatment
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with ENB.
Another relevant information that can be inferred from the performed experimental tests is related to the intensity of the signal at 966 cm-1 in the spectrum of the sample cured with tm-DDS(4th
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synthesis) up to 150°C (see Fig. 13). It is weaker than the intensity of the peak in the same uncured sample (see Fig. 12); this is a clear evidence that part of the catalyst is deactivated during the curing at high temperature. In fact, while no unshared electron pair of nitrogen atoms can poison the catalyst, the oxirane rings of the epoxy precursors can survive up to high temperature (150-180 °C). The deactivation of the catalyst for high curing cycle is due to its dispersion method (it was dispersed at molecular level). These results are in accord with other results [17] dealing with the catalysts dispersed at molecular level in the epoxy mixture. Experiments performed in these last years to better understand the reason of the deactivation of ROMP catalysts have highlighted that when catalyst complex (Hoveyda-Grubbs’1st generation catalyst) is solubilized in the form of molecular complex in the resin, an equimolecular reaction between the epoxide ring and the 19
ACCEPTED MANUSCRIPT alkylidene of the ruthenium complex occurs determining its deactivation [16]. Experimental analysis have highlighted that solid catalyst particles in the epoxy mixtures at high temperature retain an intact heart of catalyst particles which are not deactivated in contact with the epoxy rings of the matrix. [16]. In this regard, work is in progress to apply the same strategy used in literature [26] to open epoxy rings at low temperature in such a way to avoid the equimolecular reaction
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between catalyst and epoxy rings which deactivates the catalyst at high temperature (equal or higher than 120°C). On the other hand, a considerable part of the future work will also focus on new
Conclusions
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catalyst structures that are electronically or sterically predisposed to greater amine tolerance.
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The synthesis, characterization and some tests of applicability of a new hardener, the tetramethylated diaminodiphenyl sulfone (tm-DDS), are shown. The tm-DDS is able to rapidly react with epoxy resin, giving a composite material having some characteristics significantly better than materials treated with different tertiary amines such as ancamine. Despite the similarity in the main reaction mechanisms of tm-DDS and ancamine, the new sinthesized hardener “tm-DDS(4th
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synthesis)” is able to give composites characterized by higher glass transition temperature (Tg). In fact, the new hardener is able to increase the glass transition temperature of about 90°C with respect to that obtained using the ancamine as curing agent preserving, at the same time, as it occurs with ancamine, the catalytic activity of HG1 catalyst solubilized at molecular level. Work is ongoing to
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study in detail the influence of the curing cycle on epoxy mixtures containing ROMP catalyst in
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combination with this new hardener.
Acknowledgments
The research leading to these results has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under Grant Agreement N° 313978.
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References [1] Decadal Survey of Civil Aeronautics: Foundation for the Future. Steering Committee for the Decadal Survey of Civil Aeronautics, Aeronautics and Space Engineering Board, Division on Engineering and Physical Sciences, National Research Council National Academies Press, (2006) -
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[5] L. Guadagno, M. Raimondo, C. Naddeo, P. Longo, A. Mariconda. Self-healing materials for structural applications, Polymers Engineering & Science, 54 (4) (2014) 777-784. DOI:10.1002/pen.23621
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efficiency and dynamic mechanical properties of self-healing epoxy systems, Smart Materials and. Structures, 23 (4) (2014) 045001 (11pp).
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[7] L. Guadagno, P. Longo, M. Raimondo, C. Naddeo, A. Mariconda, A. Sorrentino, V. Vittoria, G. Iannuzzo, S. Russo. Cure behavior and mechanical properties of structural self-healing epoxy resins, Journal of Polymer Science, Part B: Polymer Physics 48 (23) (2010) 2413-2423. DOI: 10.1002/polb.22139 [8] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N. Brown, S. Viswanathan. Autonomic healing of polymer composites, Nature 409 (2001) 794-797. DOI:10.1038/35057232 [9] E. N. Brown, M. R. Kessler, N. R. Sottos, S. R. White. In situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene, Journal of Microencapsulation 20 (6) (2003) 719-730. 21
ACCEPTED MANUSCRIPT DOI: 10.1080/0265204031000154160 [10] B. Aïssa, D. Therriault, E. Haddad, W. Jamroz. Self-Healing Materials Systems: Overview of Major Approaches and Recent Developed Technologies, Advances in Materials Science and Engineering 2012 (2012) Article ID 854203, 17 pages. DOI:10.1155/2012/854203
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[11] H. Li, R. Wang, H. Hu, W. Liu. Surface modification of self-healing poly(urea-formaldehyde) microcapsules using silane-coupling agent, Applied Surface Science 255 (5) (2008) 1894-1900. DOI: 10.1016/j.apsusc.2008.06.170
[12] R. Wang, H. Li, H. Hu, X. He, W. Liu. Preparation and characterization of self-healing
Applied Polymer Science 113(3) (2009) 1501-1506. DOI: 10.1002/app.30001
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microcapsules with poly(urea-formaldehyde) grafted epoxy functional group shell, Journal of
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[13] L. Yuan, G-Z. Liang, J-Q. Xie, S-B. He. Synthesis and characterization of microencapsulated dicyclopentadiene with melamine–formaldehyde resins, Colloid and Polymer Science 285 (7) (2007) 781-791. DOI: 10.1007/s00396-006-1621-5
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formaldehyde microcapsules containing ENB-based self-healing agents, Macromolecular Materials and Engineering 294(6-7) (2009) 389-395. DOI: 10.1002/mame.200900015
[15] X. Liu, J. K. Lee, M. R. Kessler. Microencapsulation of Self-healing Agents with Melamine-
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Urea-Formaldehyde by the Shirasu Porous Glass (SPG) Emulsification Technique, Macromolecular Research 19 (10) (2011) 1056-1061. DOI: 10.1007/s13233-011-1009-3
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[16] M. Raimondo, P. Longo, A. Mariconda, L. Guadagno. Healing agent for the activation of selfhealing function at low temperature, Advanced Composites Materials 24 (6) (2015) 519-529. DOI: 10.1080/09243046.2014.937135 [17] A. Mariconda, P. Longo, A. Agovino, L. Guadagno, A. Sorrentino, M. Raimondo. Synthesis of ruthenium catalysts functionalized graphene oxide for self-healing applications, Polymer 69 (2015) 330-342. DOI:10.1016/j.polymer.2015.04.048 [18] R. Jain, P. Kukreja, A. K. Narula, V. K. Chaudhary. Studies of the curing kinetics and thermal stability of epoxy resins using a mixture of amines and anhydrides, Journal of Applied Polymer Science 100 (5) (2006) 3919-3925. 22
ACCEPTED MANUSCRIPT DOI: 10.1002/app.22769 [19] S.G. Prolongo, Gilberto del Rosario, A. Urena. Comparative study on the adhesive properties of different epoxy resins, International Journal of Adhesion and Adhesives 26 (3) (2006) 125-132. DOI:10.1016/j.ijadhadh.2005.02.004 [20] N. A. Razack, L. A. Varghese. The Effect of Various Hardeners on the Mechanical and
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Thermal Properties of Epoxy Resin, International Journal of Engineering Research & Technology 3 (1) (2014) 2662-2665.
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of CNF-based resins, Nanotechnology 24 (30) (2013) 305704 (10pp). DOI:10.1088/0957-4484/24/30/305704
[22] L. Guadagno, M. Raimondo, V. Vittoria, L. Vertuccio, C. Naddeo, S. Russo, B. De Vivo, P.
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Lamberti, G. Spinelli, V. Tucci. Development of epoxy mixtures for application in aeronautics and aerospace, RCS Advances 4 (30) (2014) 15474-15488. DOI: 10.1039/C3RA48031C
[23] L. Guadagno, M. Raimondo, L. Vertuccio, M. Mauro, G. Guerra, K. Lafdi, B. De Vivo, P. Lamberti, G. Spinelli, V. Tucci. Optimization of graphene-based materials outperforming host
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matrices, RSC Advances 5 (46) (2015) 36969-36978.
[24] M. Raimondo, S. Russo, L. Guadagno, P. Longo, S. Chirico, A. Mariconda, L. Bonnaud, O. Murariu, Ph. Dubois. Effect of Incorporation of POSS compounds and phosphorous hardeners on
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thermal and fire resistance of nanofilled aeronautic resins, RSC Advances 5 (15) (2015) 1097410986.
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[25] H. Balcar, Z. Bastl. Ring Opening Metathesis Polymerization of Norbornene by WCl6: Formation of WCl5 and WCl4 and Their Catalytic Activity, Collect. Czech. Chem. Commun. 61 (9) (1996) 1353-1359.
doi.org/10.1135/cccc19961353 [26] L. Guadagno, P. Longo, M. Raimondo, C. Naddeo, A. Mariconda, V. Vittoria, G. Iannuzzo, S. Russo. Use of Hoveyda–Grubbs’ second generation catalyst in self-healing epoxy mixtures, Composites Part B: Engineering 42 (2) (2011) 296-301. doi:10.1016/j.compositesb.2010.10.011
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Highlights
• A new diamine curing agent (tm-DDS) for self-repairing resins has been synthesized.
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• The synthesized tm-DDS allows to obtain self-healing resins with increased Tg.
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• The new hardener fulfills the industrial requirements of structural materials.
S0022-2860(16)31116-4
DOI:
10.1016/j.molstruc.2016.10.060
Reference:
MOLSTR 23061
To appear in:
Journal of Molecular Structure
Received Date: 12 February 2016 Revised Date:
17 October 2016
Accepted Date: 19 October 2016
Please cite this article as: M. Raimondo, L. Guadagno, C. Naddeo, P. Longo, A. Mariconda, A. Agovino, New structure of diamine curing agent for epoxy resins with self-restoration ability: Synthesis and spectroscopy characterization, Journal of Molecular Structure (2016), doi: 10.1016/ j.molstruc.2016.10.060. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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New Structure of Diamine Curing Agent for Epoxy Resins with Self-Restoration Ability:
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Synthesis and Spectroscopy Characterization
Marialuigia Raimondoa*, Liberata Guadagnoa*, Carlo Naddeoa,
Department of Industrial Engineering - DIIn - University of Salerno
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a
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Pasquale Longob, Annaluisa Maricondab, Anna Agovinob
Via Giovanni Paolo II ,132 - 84084 Fisciano (SA), Italy (UE) b
Department of Chemistry and Biology - University of Salerno
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Via Giovanni Paolo II ,132 - 84084 Fisciano (SA), Italy (UE)
*Corresponding author
Marialuigia Raimondo and Liberata Guadagno
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Marialuigia Raimondo - e-mail: [email protected] phone +39 089964019
mobile +39 3293030935
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Liberata Guadagno - e-mail: [email protected] phone +39 089964114 mobile +39 3204213235
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Abstract The development of smart materials in aeronautical structures consisting of compounds based on epoxy resins having self-repair capability has been hampered by some criticalities. One of the main critical points is related to the impossibility to use primary amines (e.g.: 4,4′-diaminodiphenyl
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sulfone, DDS) as hardeners, because they can poison the catalyst responsible for the healing mechanisms. In this paper, the synthesis, characterization and some tests of applicability of a new hardener, the tetramethylated diaminodiphenyl sulfone (tm-DDS), are shown. The tm-DDS is able to rapidly react with epoxy resin, giving a composite material having some characteristics
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significantly better than composites hardened with different tertiary amines. The new hardener is able to increase the glass transition temperature (Tg) of about 90°C with respect to the more
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common hardener, ancamine K54, already used in self-healing epoxy formulations.
Keywords: Self-Healing Epoxy resins; Smart Materials; New Diamine Curing Agent Synthesis;
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Spectroscopy analysis; ROMP reaction
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1. Introduction Epoxy resins are fast gaining ground as preferred polymer to be used as carbon fiber-reinforced materials to manufacture large components able to sustain aerodynamic loads. The introduction of epoxy resins to the aeronautic industry was mainly driven by performance gains (better design flexibility, no corrosion, easy production process, etc.) and most of all by weight reduction. This
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last aspect is of relevance when considering issues related to the Energy Resources and Supply, costs in service and air pollution measures. This major advantage of the composites, the lightness, is also obtained saving and/or increasing the material strength in such a way as to make travelling on aeroplanes more comfortable for the crew and passengers in virtue of the possibility to slightly
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increase cabin pressures without to act on the structure weight. Unfortunately, composites possess also some properties which reduce their advantage with respect to traditional metal alloys. One
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inherent shortcoming is the poor impact damage resistance. Impact damage due to several factors such as unfavorable weather conditions, stones or rocks on the keel during landing, incidental contact during maintenance and bird strike may significantly affect the integrity of the composite structure of aircrafts in service [1].
Smart resins with self-healing capability offer many advantages with respect to traditional epoxy resins. As an instance, for aeronautical materials, inspection and maintenance are important aspects
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when considering the availability of aircraft for revenue flights. Modern airframe design is exploiting new exciting developments in materials and structures to construct ever more efficient air vehicle able to enable ‘smart’ maintenance including self-repair capabilities. Actually, also a very advanced design of an aircraft has to take required inspection intervals into account. An
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aircraft with inherent protective and smart abilities could help to significantly extend the inspection intervals, thereby increasing aircraft availability. Reduced aircraft ground time due to rapid repair
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following accidental damages are expected to strongly contribute: a) to save of maintenance costs; b) to extend the structural integrity and increase of the reliability structures. Recent studies have highlighted potential of self-healing polymer composites to be used as single structural materials and/or for sandwich structures [2-8]. Many self-healing resins, already proposed in literature, are based on microencapsulated systems [3-17]. In this system, the healing of microcracks is achieved by embedding a microencapsulated healing agent and a catalytic chemical trigger within the matrix. As soon as a crack breaks a microcapsule, the healing agent flows into the crack plane through capillary action. The healing monomer distributes in the crack plane and reacts with the embedded catalyst bonding the crack planes. Treatment of epoxy resins with curing agents or hardeners gives three-dimensional insoluble and infusible networks. Epoxy resins can be cured with a wide variety of curing agents which are essential components of an advanced composite system 3
ACCEPTED MANUSCRIPT providing a substantial contribution to the properties and the performance characteristics of the crosslinked finished products. Thus the choice of the curing agent is really crucial and needs to be considered very carefully. In fact, the final properties of the epoxy network are closely depending on the crosslinking agent, processing methods and curing conditions [18]. The impossibility to use as curing agent primary amines in combination with catalysts active in the Ring-Opening
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Metathesis Polymerization (ROMP) reaction constitutes a very strong impediment as regard the development of high performance self-repairing epoxy resins based on this design. In fact, primary aliphatic or aromatic amines used as hardeners for the cross-linking reactions of the epoxy matrix poison the catalysts used for the polymerization of the healing agent. Aliphatic amines, which
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rapidly react with epoxy resin, are representative room-temperature curing agents [19], moreover the resin crosslinked with aliphatic amines is characterized by low mechanical performance. Aromatic amines instead have been developed to achieve greater heat resistance and chemical
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resistance than those of aliphatic amines [20]. They require high temperature for the curing cycles because, having weaker basicity than aliphatic amine, slowly cure at room temperature due to the steric hindrance of aromatic rings. One of the most common is 4-4’-diaminodiphenylsulfone (DDS). In particular, the use of the aromatic amine DDS allows to obtain materials with excellent mechanical properties [21-24].
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This paper describes the synthesis and characterization of a new diamine curing agent (tetramethylated diaminodiphenylsulfone) for self-healing epoxy resins. The applicability conditions are also shown. The new hardener is able to enhance the Tg of the developed selfhealing material extending its applicability in many sectors of the structural materials (aeronautical,
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automotive, wind turbine industries etc..).
2. Experimental
2.1 Materials and methods The curing agents: 4,4-diaminodiphenyl sulfone (DDS) and ancamine K54 were purchased from Sigma-Aldrich and used as received. Tetrahydrofuran was thoroughly deoxygenated and dehydrated under nitrogen by refluxing over suitable drying agents. Deuterated solvent dimethyl sulfoxide (DMSO-d6) was purchased from Euriso-Top products. Proton Nuclear Magnetic Resonance (1H NMR) spectra were recorded on a Bruker AM250 and a Bruker AM300 operating at 250 and 300 MHz respectively. The 1H NMR chemical shifts are referenced to SiMe4 (δ = 0 ppm) using the residual proton impurities of the deuterated solvents as internal standards. Spectra are reported as follows: chemical shift (ppm), multiplicity and 4
ACCEPTED MANUSCRIPT integration. Multiplicities are abbreviated as follows: singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), and broad (br). Fourier Transform Infrared Spectroscopy (FTIR) spectra were performed at room temperature by using a Bruker Vertex 70 FTIR-spectrophotometer with a 2 cm-1 resolution (64 scans collected). Thermogravimetric analysis (TGA) was carried out in air and in an inert atmosphere of nitrogen
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using a Mettler TGA/SDTA 851 thermal analyzer. The temperature range was 30-1000°C at a heating rate of 10°C min-1.
Calorimetric analysis was performed with a Mettler DSC 822 differential scanning calorimeter in a flowing nitrogen atmosphere. The samples were analyzed in the temperature range of 0-300°C with
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a scan rate of 10°C min-1.
2.2 Synthesis of tetramethylated diaminodiphenyl sulfone tm-DDS
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In a three-necked flask equipped with magnetic stirrer, reflux condenser and dropping funnel, 250 mL of tetrahydrofuran (THF) and 1.5 g of DDS (6*10-3 mol, 248.30 g/mol) were introduced. Complete dissolution of the DDS was obtained after stirring for about 30 min. Subsequently, in the reaction flask, t-BuONa (6.77 g, 0.06 mol, 112 g/mol) was added. The reaction flask was placed in a water/ice bath (5°C) and drops of CH3I (3.73 mL, 0.058 mol, d=2.28 g/mol, 141.94 g/mol) were
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slowly added. The mixture was stirred for 24 h and quenched by the addition of saturated solution of ammonium chloride (NH4Cl, about 100 ml). The organic phase was then extracted with ethyl acetate (about 60 mL x 3), the combined extracts dried over MgSO4, filtered and concentrated in g, 60%). 1
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vacuo. The product was washed with ethyl ether to afford as a yellow powder (3.61·10-3 mol, 1.10
H NMR (300 MHz, DMSO-d6): δ = 7.58 (d, 4H), 6.72 (d, 4H), 2.95 (s, 6H).
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The chemicals NaH and CH3I, NaH and tBuONa were purchased from Sigma-Aldrich.
2.3 Epoxy Matrix Manufacture EB2 matrix was obtained by mixing the epoxy precursor (E) (trade name EPON 828) with an epoxy reactive diluent 1,4-butanediol diglycidyl ether (B) at a concentration of 80%:20% (by wt) epoxide to flexibilizer. The compounds E and B, both containing epoxy-moieties, were obtained by SigmaAldrich. After, the synthesized hardener tetramethylated DDS (product of the reaction of the fourth optimized procedure of synthesis), named “tm-DDS (4th synthesis)” was added to the EB2 epoxy mixture at a percentage of 18% of the total moles of salified epoxy groups, giving the mixture EB2+tm-DDS(4th synthesis 18%). The tm-DDS (4th synthesis) was completely solubilized by mechanical agitation into the EB2 formulation at the temperature of 120°C after 1 hour using an oil 5
ACCEPTED MANUSCRIPT bath. Samples based on the same epoxy matrix, containing different curing agents (tertiary amine “ancamine K54” and DDS), were also prepared to compare catalyst reactivity and glass transition temperature between the epoxy formulations solidified with different hardeners. The catalyst used in the epoxy matrix was Hoveyda-Grubbs’ 1st generation (HG1). The healing agent used in this work was a cyclic olefin 5-ethylidene-2-norbornene (ENB). HG1 catalyst and
2.4 HG1 catalyst based epoxy mixtures
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ENB were purchased from Sigma-Aldrich.
The sample EB2+tm-DDS(4th synthesis 18%)+HG1(7%) was obtained by solubilizing at molecular
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level by mechanical stirring in the mixture EB2+tm-DDS(4th synthesis 18%) at room temperature, the HG1 catalyst at a concentration of 7% (by wt) with respect to the epoxy mixture EB2. In the following, the sample EB2+tm-DDS(4th synthesis 18%)+HG1(7%) uncured refers to the initial
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liquid epoxy mixture before undergoing the curing process in the oven. Then, the sample EB2+tmDDS(4th synthesis 18%)+HG1(7%) was cured in a two-stage process. The first stage was carried out at a temperature of 80°C for 3 hours, while the second stage was carried out at the two temperatures of 150 and 170°C for 1 hour respectively, giving the sample EB2+tm-DDS(4th synthesis 18%)+HG1(7%) up to 150°C and up to 170°C respectively. Two different samples A and
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B were also prepared for comparison.
A) a sample based on the same matrix (EB2) was hardened with DDS which was added at stoichiometric concentration with respect to all the epoxy rings (E and B compounds). This sample was cured using the same curing cycle (up to 150°C) mentioned before. It is hereafter named with
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the acronym EB2+DDS+HG1(7%) up to 150°C.
B) a sample based on the same matrix (EB2) was hardened with ancamine and was cured in a two-
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stage process as described for the sample EB2+tm-DDS(4th synthesis 18%)+HG1(7%).
2.5 Evaluation of the catalytic activity in the tetramethylated DDS based epoxy mixture
To verify the catalytic activity of the HG1 catalyst in the tetramethylated DDS based epoxy mixture EB2+tm-DDS(4th synthesis 18%)+HG1(7%) (uncured and cured up to 150°C and up to 170°C) , spectroscopic investigation by FTIR was carried out. For this purpose, in the case of uncured sample, the obtained liquid mixture was stratified on a slide for light microscopy. Subsequently, two drops of ENB were added to the stratified mixture. In the case of cured sample up to 150°C and 170°C, the mixture was reduced in powder with a serrated blade and collected in a mortar. Two 6
ACCEPTED MANUSCRIPT drops of healing agent ENB (2 x 50 µL) were then added and dispersed into the powder sample and quickly analyzed on a KBr disk for FTIR investigation. The same procedure used to check the catalytic activity in the cured sample based on tm-DDS(4th synthesis) was carried out also for the following formulation EB2+DDS+HG1(7%) up to 150°C.
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3. Results and discussion
3.1 Synthesis of tetramethylated diaminodiphenyl sulfone. 1H NMR analysis
In this paper, the choice of methyl groups for the alkylation of DDS is due to the small steric
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hindrance of these substituents, as compared to other alkyl groups. This makes the free electron pair on the nitrogen certainly more available for crosslinking reactions. Tetramethylated
diaminodiphenyl
sulfone
(tm-DDS)
obtained
by
methylation
of
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diaminodiphenyl sulfone (DDS) (see Fig.1).
was
Four synthesis procedures have progressively been optimized in order to obtain the fully methylated DDS. Starting from a base ratio 1:5:5 of DDS:CH3I:NaH respectively and then 1:10:5, uncompletely methylated DDS was obtained. The same results were obtained using a ratio 1:6:6 of DDS:CH3I :t-BuONa whereas tetramethylated diaminodiphenyl sulfone (tm-DDS) was obtained by
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1:10:10 DDS:CH3I:t-BuONa ratio (see experimental part). Fig. 1 shows the scheme of the synthesis
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of tetramethylated diaminodiphenyl sulfone (tm-DDS).
Fig. 1. Scheme of the synthesis of tetramethylated diaminodiphenyl sulfone (tm-DDS).
The 1H NMR spectrum (DMSO-d6) of DDS, reported in Fig. 2, shows the following signals: 7
ACCEPTED MANUSCRIPT δ ppm 5.97 – Ha , δ ppm 6.57 – Hb, δ ppm 7.44 –Hc. In Fig. 2, also DDS formula for the signal
Hb
Ha
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Hc
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attributions is shown.
Fig. 2. 1H NMR spectrum (DMSO-d6) of diaminodiphenyl sulfone (DDS).
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The signals in the spectrum (see Fig. 3) indicate the obtainment of the desired product arising from the fourth synthesis procedure of tm-DDS, named “tm-DDS (4th synthesis)”.
tm-DDS (4th synthesis)
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-CH3
Hc
Hb
Fig. 3. 1H NMR spectrum of the new synthesized hardener obtained from the 4th synthesis procedure, named “tm-DDS (4th synthesis)”.
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3.2 FTIR analysis Fourier Transform Infrared Spectroscopy (FTIR) is a sensitive technique particularly for identifying the organic molecular structures of new reaction products. The products obtained from the different synthesis procedures were analyzed by FTIR investigations (Fig. 4). The substitution of the Ha hydrogen (see Fig. 2) with methyl groups can be
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followed in different spectral ranges. In particular, primary amines display two weak absorption bands (N-H stretching vibrations): one at 3471 cm-1 and the other near 3373 cm-1. These bands represent, respectively, the “free” asymmetrical and symmetrical N-H stretching modes. The band at 3240 cm-1 is the Fermi resonance band with overtone of the band at 1617 cm-1 related to the N-H
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bend (scissoring). Secondary amines show a single weak band in the 3350-3310 cm-1 region (see pink spectrum). The absence of these signals and the appearance of the following signals: a) the
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sharp band near 2800 cm-1 characteristic of sp3 C-H stretches of the N-methyl groups; b) the two bands occurring at 2962 cm-1 and 2872 cm-1 (the first of these results from the asymmetrical stretching mode in which two C-H bonds of the methyl group are extending while the third one is contracting -νasCH3-, the second arises from symmetrical stretching -νsCH3- in which all three of the C-H bonds extend and contract in phase) confirm the obtaining of the desired products. Other evidences of the obtaining of tetramethylated diaminodiphenyl sulfone, in the products of the
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synthesis, arise from the appearance of the C-N stretch of tertiary aromatic amines at ~ 1360-1310 cm-1 , from the absence of the bands a 1617 cm-1 already commented before, from the appearance of a) the out-of-plane C-H bending vibration in the range between 900 and 675 cm-1, and b) the
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symmetrical bending vibration (δsCH3) at 1375 cm-1.
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3471
1
DDS tm-DDS(3rd synthesis) tm-DDS (4th synthesis after elimination of impurities)
2872
0.6
2800
0.4
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Absorbance
0.8
2962
0.2
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3200 2700 2200 Wavenumber (cm-1)
1700
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Fig. 4. FTIR spectra of DDS, and products obtained from the different synthesis procedures
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(spectral range 1700 cm-1 - 4000 cm-1).
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DDS tm-DDS(3rd synthesis) tm-DDS (4th synthesis after elimination of impurities)
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2.2
900-675
1617
1.4
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Absorbance
1373
0.6
-0.2 1400 900 Wavenumber (cm-1)
400
Fig. 5. FTIR spectra of DDS, and products obtained from the different synthesis procedures (spectral range 400 cm-1 - 1800 cm-1). 10
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3.3
Calorimetric
investigation:
Thermogravimetric
analysis
(TGA)
and
Differential Scanning Calorimetry (DSC) 3.3.1 Thermogravimetric analysis (TGA)
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TGA analysis was carried out in order to evaluate the thermal stability of the products obtained from the different synthesis procedures. Fig. 6 shows the thermogravimetric curves of these products. The product obtained from the 4th synthesis, after elimination of impurities (see brown
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curves), shows a beginning of degradation in air at ~ 300°C, a value very similar to the initial DDS.
Fig. 6. TGA curves of the products obtained from different synthesis procedures.
3.3.2 Differential Scanning Calorimetry (DSC) The Differential Scanning Calorimetry (DSC) is a fundamental tool in thermal analysis because it monitors heat effects associated with phase transitions and chemical reactions as a function of temperature and is a very informative method in physical characterization of a compound. DSC is 11
ACCEPTED MANUSCRIPT also useful to determine the melting point. DSC curves of the same products are shown in Figs 7 and 8. The substitution of hydrogen atoms with methyl groups causes a strong increase in the melting peak of the new hardener agent, in particular the product obtained from the 4th synthesis, after elimination of impurities, shows the melting peak between 260-275°C (see Fig. 8). This high
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melting temperature did not constitute a problem for its solubility in the epoxy mixture.
Fig. 7. DSC curves of the products obtained from different synthesis procedures in the range 0°C300°C.
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150°C-300°C.
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Fig. 8. DSC curves of the products obtained from different synthesis procedures in the range
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3.3.3. Differential Scanning Calorimetry (DSC) - Epoxy formulations hardened with tm-DDS(4th synthesis) and ancamine.
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The technique of differential scanning calorimetry was also used to obtain information on thermal transitions such as the glass transition temperature of the resins hardened with tm-DDS. The result was compared with the resin hardened with ancamine which is the most common tertiary amine used as hardener in self-healing epoxy systems because it does not deactive the Grubbs' catalyst [48,16]. The polymerization mechanism of the new hardener agent (tm-DDS) is supposed to be that typical of tertiary amines; it induces the direct linkage of epoxy groups to one other. The reaction mechanism is supposed to be as follows (see scheme of Fig. 9):
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Fig. 9. Scheme of the reaction mechanisms of tm-DDS with the epoxy precursor (EPON 828).
The reaction can occur at both ends of the diglycidyl ether molecule, hence a crosslinked structure will be built up. The reaction mechanisms are similar to the reactions of R2N- groups of ancamine
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with the initial epoxy precursors. In this last case, the epoxy group also reacts with hydroxyl groups
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according to the scheme of Fig. 10.
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Fig. 10. Scheme of the reaction of -OH group of the ancamine with the epoxy precursor (EPON
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828).
Despite the similarity in the main reaction mechanisms of tm-DDS and ancamine, the new
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sinthesized hardener “tm-DDS(4th synthesis)” is able to give composites characterized by higher glass transition temperature (Tg). Fig. 11 shows the differential scanning calorimetric (DSC) curves in dynamic regime of the sample solidified with the new synthesized hardener “EB2 tm-DDS(4th synthesis) HG1” and the sample hardened with ancamine “EB2 Ancamine HG1”.
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Fig. 11. DSC curves of the epoxy samples solidified with two different hardeners tm-DDS(4th synthesis) and ancamine.
Considering the DSC curves, a Tg of ~ 75°C is observed for the sample EB2 Ancamine HG1
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hardened with ancamine, whereas a Tg of ~ 165°C is observed for the sample EB2 tm-DDS(4th synthesis) HG1 hardened with tetramethylated DDS. The new hardener is able to increase the glass transition temperature of about 90°C.
It is worth noting that for the two different samples the amount of hardener agent was calculated
curing cycle.
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considering the same number of unshared electron pair on the nitrogen atoms and using the same
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This different behaviour is most likely due to the greater rigidity of the new synthetized hardener with respect to the ancamine.
This is a very interesting result from an industrial point of view because high glass transition temperatures are frequently requested to fullfill some of the main industrial requirements of structural materials in several fields (aeronautics, marine industry etc.. ).
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3.3.4 Evaluation of the catalytic activity inside the epoxy mixture containing tetramethylated DDS To verify the catalytic activity of the HG1 catalyst in the tetramethylated DDS based uncured and cured (up to 150°C and up to 170°C) epoxy mixture EB2+tm-DDS(4th synthesis 18%)+HG1(7%), spectroscopic investigation by FTIR was carried out. The control of the catalytic activity was
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performed by evaluating the formation of the metathesis product. For the sample EB2+tm-DDS(4th synthesis 18%)+HG1(7%) uncured, after the addition of two drops of ENB to the stratified mixture, a thin solid film of metathesis product was immediately obtained. Therefore, spectroscopic investigations highlighted that the presence of tm-DDS hardener did not alter the catalytic activity
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of the HG1. The spectrum of the obtained film PMET EB2+tm-DDS(4th synthesis 18%)+HG1(7%) uncured (see Fig. 12) shows a peak at 966 cm-1 attributable to ring-opened poly(ENB) [25], which
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is an indication of the formation of the metathesis product. This peak is assigned to the trans substituted alkenes, characteristic of the ring opened cross-linked product [poly(ENB)] providing evidence that the catalyst embedded in the resin is active in the ROMP reaction and hence of the fact that the activity of the catalyst was not compromised by the chemical nature of the oligomers, and the treatments of mechanical mixing.
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PMET EB2+tm-DDS(4th synthesis 18%)+HG1(7%) uncured
0.425
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0.475
966 cm-1
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Absorbance
0.525
0.375 0.325 0.275
1275
1175 1075 975 Wavenumber (cm-1)
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Fig. 12. FTIR spectrum of the solid film (metathesis product) obtained by polymerization of ENB inside EB2+tm-DDS(4th synthesis 18%)+HG1(7%) uncured mixture.
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to 150°C whereas no signal of metathesis product is observed for the sample cured up to 170°C.
Fig. 13. FTIR spectrum of the sample obtained by polymerization of ENB inside EB2+tm-DDS(4th
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synthesis 18%)+HG1(7%) mixture after curing process.
3.3.4.1 Evaluation of the catalytic activity inside the epoxy mixture containing DDS
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In this section, in order to demonstrate the effectiveness of the catalytic activity of the new synthesized catalyst tm-DDS(4th synthesis) with respect the DDS, the FTIR spectrum of the cured sample EB2+DDS+HG1(7%) after the treatment with ENB is shown in Fig. 14. No signal of metathesis product was found for this sample cured up to 150°C, clearly proving that the primary aromatic diamine DDS poison the catalyst responsible for the healing mechanisms. No signal of metathesis product was also observed for the same uncured sample, in opposite to the result shown in Fig. 12 where an intense peak of the metathesis product is observed for uncured mixture corresponding to the sample hardened with tm-DDS (EB2+tm-DDS(4th synthesis 18%)+HG1(7%)). These results highlight that the ROMP catalyst is completely deactivated (also in the fluid epoxy mixture) in the epoxy precursors containing DDS. This is an expected result, in fact, 18
ACCEPTED MANUSCRIPT in spite of the fact that the implementation of ruthenium catalyzed olefin metathesis has benefitted from a broad functional group tolerance, nitrogen bases as the DDS hardener have remained challenging often requiring catalyst protection to achieve good yields with reasonable catalyst loadings. In the case of the hardener tm-DDS, no nitrogen bases can poison the catalyst because the polymerization mechanisms, as illustrated in the scheme of reaction of Fig. 9, consume the
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unshared electron pair of nitrogen atoms already in the first stage of polymerization at low
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temperature.
Fig. 14. FTIR spectrum of the sample EB2+DDS+HG1(7%) cured up to 150 °C after the treatment
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with ENB.
Another relevant information that can be inferred from the performed experimental tests is related to the intensity of the signal at 966 cm-1 in the spectrum of the sample cured with tm-DDS(4th
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synthesis) up to 150°C (see Fig. 13). It is weaker than the intensity of the peak in the same uncured sample (see Fig. 12); this is a clear evidence that part of the catalyst is deactivated during the curing at high temperature. In fact, while no unshared electron pair of nitrogen atoms can poison the catalyst, the oxirane rings of the epoxy precursors can survive up to high temperature (150-180 °C). The deactivation of the catalyst for high curing cycle is due to its dispersion method (it was dispersed at molecular level). These results are in accord with other results [17] dealing with the catalysts dispersed at molecular level in the epoxy mixture. Experiments performed in these last years to better understand the reason of the deactivation of ROMP catalysts have highlighted that when catalyst complex (Hoveyda-Grubbs’1st generation catalyst) is solubilized in the form of molecular complex in the resin, an equimolecular reaction between the epoxide ring and the 19
ACCEPTED MANUSCRIPT alkylidene of the ruthenium complex occurs determining its deactivation [16]. Experimental analysis have highlighted that solid catalyst particles in the epoxy mixtures at high temperature retain an intact heart of catalyst particles which are not deactivated in contact with the epoxy rings of the matrix. [16]. In this regard, work is in progress to apply the same strategy used in literature [26] to open epoxy rings at low temperature in such a way to avoid the equimolecular reaction
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between catalyst and epoxy rings which deactivates the catalyst at high temperature (equal or higher than 120°C). On the other hand, a considerable part of the future work will also focus on new
Conclusions
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catalyst structures that are electronically or sterically predisposed to greater amine tolerance.
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The synthesis, characterization and some tests of applicability of a new hardener, the tetramethylated diaminodiphenyl sulfone (tm-DDS), are shown. The tm-DDS is able to rapidly react with epoxy resin, giving a composite material having some characteristics significantly better than materials treated with different tertiary amines such as ancamine. Despite the similarity in the main reaction mechanisms of tm-DDS and ancamine, the new sinthesized hardener “tm-DDS(4th
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synthesis)” is able to give composites characterized by higher glass transition temperature (Tg). In fact, the new hardener is able to increase the glass transition temperature of about 90°C with respect to that obtained using the ancamine as curing agent preserving, at the same time, as it occurs with ancamine, the catalytic activity of HG1 catalyst solubilized at molecular level. Work is ongoing to
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study in detail the influence of the curing cycle on epoxy mixtures containing ROMP catalyst in
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combination with this new hardener.
Acknowledgments
The research leading to these results has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under Grant Agreement N° 313978.
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Highlights
• A new diamine curing agent (tm-DDS) for self-repairing resins has been synthesized.
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• The synthesized tm-DDS allows to obtain self-healing resins with increased Tg.
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• The new hardener fulfills the industrial requirements of structural materials.