Complex Cure Kinetics of the Hydroxyl-Epoxide ...

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Extended isothermal runs were performed using a TAM Air isothermal microcalorimeter (TA instruments), as well as a. Q2000 Differential Scanning Calorimeter ...
Epon828

DEA

Diethanolamine (DEA) was used as a curative for a DGEBA epoxy (Epon 828, Momentive), mixed in a 100:12 (DGEBA:DEA) wt. ratio. Diethanolamine has three functional groups that are capable of reacting with the epoxide: a secondary-amine and two hydroxyls. The secondary-amine, or adduct forming reaction with epoxide is rapid, being catalyzed by the presence of the high concentration of hydroxyls [5], and is complete within 30 min under normal processing conditions (at 70°C). This results in an adduct that undergoes a further gelation-reaction to form the crosslinked network. The term gelation-reaction embodies a number of subreactions, each with their own more-or-less complicated kinetics. The gelation-reaction will be the focus of this work. The gelation-reaction is much slower than the initial adductforming reaction, and results in a glassy, crosslinked network after about 24 hrs (when cured at 70°C). The chemical details of this gelation reaction are less clear, involving the formation of a zwitterion transition state complex [1-3] as well as a complex mixture of hydrogen bonded complexes that can either accelerate or inhibit the reaction [4].

*Sandia National Laboratories, +New Mexico Tech

Reaction Mechanism The curing kinetics of DGEBA/DEA is more complicated than for most epoxy thermosets. There are at least three significant timetemperature regions involved each with their own characteristic reaction behavior. First is the adduct-forming reaction between the secondary-amine and epoxide groups (not shown). Second is the gelation-reaction at low temperature (below 70°C) between hydroxyl and epoxide groups through an (activated) anionic chain growth mechanism. Third is 0.02 the gelation-reaction at high 60 C temperature (above 70°C) between 70 C 90C 0.015 hydroxyl and epoxide groups 105 C through a direct (un-activated) 0.01 reaction. Observationally, the two types of gelation reactions are distinguished by the presence 0.005 or absence of a well defined 0 0 200 400 600 800 1000 maximum in the rate of reaction Time (min) (as measured by DSC).

Proposed Reactions For Gelation Reaction Rxn. 1 is the simple, un-activated reaction of epoxide with an alcohol. The reaction is irreversible since it involves the release of considerable energy in the opening of the epoxide ring. This reaction, along with the similar epoxide-epoxide reaction, is the dominant reaction at high temperature (greater than about 70°C) in (epoxy: alcohol: tertiary- Rxn. 1 O amine) mixtures such as X OH + R CH CH X O CH CH OH R in DGEBA/DEA X = R'' O CH CH n = 0,1,... [3, 5, 6, 7, 8]. R 2

2

2

n

Rxn. 2, the initiation reaction sequence, is the epoxide reaction with hydroxyl in the presence of tertiary amine at low temperature (less than about Rxn. 2 70°C). This three-molecule transition state complex can then open the epoxide ring. The resulting ammonium zwitterion has a negatively charged oxygen while the nitrogen retains a positive charge. R'' OH

O

R

CH CH2 +

R'3N

+

R'' OH

R

R'' OH R

O

O

CH2

CH CH2

R'3N

R'3N

+ R'' OH

CH R

OH

CH2

R'3N

CH2

R'' OH

+

CH

R'3N

+

Adduct-Forming Reaction The initial adduct-forming reaction (Rxn. 1) between the secondary amine group on the DEA and the DGEBA-epoxide group is rapid and scales in an Arrhenius manner with temperature. The reaction kinetics are relatively straightforward and have been fit to a Kamal equation by Adolf et al.[9] based on DSC data. The kinetic equation is well described by the Kamal functional form 𝑑𝛼 1 8 = 4 × 10 𝑒 𝑑𝑡 𝑠

−𝐸𝐴 𝑅𝑇

0.1 + 𝛼 1.5 1 − 𝛼

1.5

Gelation Reaction The slower gelation reaction is clearly not simple (as shown in the previous figure for temperatures between 65°C and 95°C). The extent of reaction at a high temperature cannot be simply superimposed on a lower temperature by shifting the time. Similar behavior can be seen in the IR measurements in the following figure. Here, the initial reaction rates seem plausible enough with the higher temperature (80°C) reaction consuming epoxide more quickly than in the lower temperature case (70°C). 1.0 As the consumption of epoxide 0.8 continues over time, a 70°C crossover of the 80⁰C and 70⁰C 0.6 80°C rate curves occurs. A plausible 0.4 explanation of this behavior is 0.2 in the decreased stability of the 0.0 zwitterion initiation sites at high 0 2 4 6 8 10 Time [hours] temperature (Rxn. 2-4).

R'' O

CH

R

Rxn. 3, indicates the propagation steps. The hydroxyl concentration remains constant throughout propagation, and the zwitterion/ alkoxide complex is seemingly stable as high as 70°C for DGEBA/DEA.

Rate of the gelation reaction at a cure temperature of 70°C. The lines are DSC results scaled to IR 0.004 units. Two of these DSC curves are mDSC and one is conventional 0.003 DSC. The points are found from taking numerical derivatives of IR 0.002 data tracking the epoxide peak. The scaling factors differ slightly 0.001 from DSC to DSC curve. The un-scaled peak heights were 0.0152±0.0005 W/g. 0 0 5 10 15 time (hrs) The location of the peak is 3.98±0.01 hrs. The total heat of reaction is 240±5 W/g.

CH CH2

R'3N

O

Consistency of Reaction Rate for DSC And IR

O

Chemical Kinetics

Epoxy 4529cm-1 [Conversion]

Background

Windy Ancipink†, John D. McCoy†, Jamie M. Kropka*, Mathias C. Celina

dH/dt (IR units/hr)

When diethanolamine (DEA) is used as a curative for a diglycidyl ether of bisphenol-A (DGEBA) epoxy, a rapid “adduct-forming” reaction of epoxide with the secondary amine of DEA is followed by a slow “gelation” reaction, of epoxide with hydroxyl and other epoxide. At low temperature, the gelation reaction is activated by the tertiary amine in the adduct, while at high temperature, the reaction becomes non-activated. In the current study, the kinetics of the gelation reaction was investigated with Differential Scanning Calorimetry, with Infrared Spectroscopy, and with Microcalorimetry. It is shown that the kinetic characteristics of the DGEBA/DEA system are similar to other tertiary-amine activated epoxy reactions and that the chemistry of DGEBA/DEA is consistent with the anionic polymerization model previously proposed for this class of materials. The principle result obtained is the upper stability temperature of the gelation reaction.

Complex Cure Kinetics of the Hydroxyl-Epoxide Reaction in DGEBA Epoxy Hardened with Diethanolamine

Heat flow (W/g)

Abstract

SAND2016-2069C

R

Epoxy band consumption behavior at 4529 cm-1 (near-IR data) showing the non- superimposability of the gelation reaction of 828 with DEA.

Conclusions

Rxn. 3

O

Our initial study of the curing of 828/DEA shows that this nontraditional epoxy can be understood as resulting from an anionic addition polymerization with the typical initiation, propagation, and termination steps. Both the tertiary-amine and the hydroxyl groups are essential for forming the zwitterion from an epoxide group, and this initiates the polymerization. Since the system has a high concentration of hydroxyls, the proton transfer to the Rxn. 4 is the termination reaction. A reaction of an alkoxide with a solvent is common, forming an alkoxide that can then attack (and Rxn. 4 bond to) another epoxide group. This then forms a crosslink positively charged zwitterion OH OH CH CH between DGEBA entities. The attacked epoxide group probably tertiary-amine to form an ether + X O + R' N R' N CH X O CH will then transfer its charge to another alkoxide and the process bond and free the (neutral) amine. R R will continue. The ratio of chain transfer to homo-polymerization X = R'' O CH CH is difficult to assess and depends on temperature and degree of n = 0,1,... R cure, with chain transfer becoming dominate as the Results of Complex Reactions concentration of epoxide decreases while that of hydroxyl stays As a consequence of these complex kinetics, weak ternary amines constant. such as triethylamine and the DGEBA-DEA adduct, show epoxideAcknowledgments & References hydroxyl-amine reaction rates which are non-superimposable 1) Fernandez-Francos X. European Polymer Journal 2014;55:35-47. with temperature and conversion 2) Rozenberg BA. Advances in Polymer Science 1986;75:113-165. state, as is particularly evident at 3) Laird RM and Parker RE. Journal of the Chemical Society B-Physical Organic elevated temperature (above 1969(9):1062-&. 4) St. John NA and George GA. Progress in Polymer Science 1994;19(5):755-795. 70°C for the case of DGEBA/ 5) Shechter L, Wynstra J, and Kurkjy RP. Industrial and Engineering Chemistry DEA). This is in contrast with the 1956;48(1):94-97. relatively predictable kinetic 6) Shechter L and Wynstra J. Industrial and Engineering Chemistry 1956;48(1):8693. behavior observed for the 7) Doszlop S, Vargha V, and Horkay F. Periodica Polytechnica-Chemical Engineering amine-epoxide reaction. 1978;22(3):253-275. R'' O

R

+

CH CH2

R'' O CH CH2 O R

R'' O CH CH2 O R

R'' O CH CH2 O CH CH2 O

+

R

O

R

R''

R

CH CH2

O CH CH2

O

R''

O CH CH2

n

R

R

+

+

R'' OH

R'' O

2

OH

n

2

3

20

3

2

320 280

TAM Air 80C DSC 80C

240 200

Hint J/g

Extended isothermal runs were performed using a TAM Air isothermal microcalorimeter (TA instruments), as well as a Q2000 Differential Scanning Calorimeter (TA Instruments). Both studies were performed under isothermal conditions on 828/DEA mixtures. The mixtures were “cleared” by hand, mixing for 10 min at 70C to permit the initial adduct forming reaction to near completion before preparing the samples. The TAM Air experiments were performed over a range of temperatures between 30°C and 90°C, and the DSC experiments were performed over a temperature range of 50°C and 110°C. DSC experiments were performed in modulated mode (mDSC) with a period of 1 min. and an amplitude of 0.1°C. Infrared Spectroscopic studies were performed using a GladiATR with a Bruker Equinox 55 spectrometer for isothermal cures. The intensity of the 914 cm-1 epoxide band was tracked [3]. The results were fit to a smooth curve and then differentiated to compare to the microcalorimeter and the DSC results.

The use of microcalorimetry allowed for the use of large samples which enhanced the signal at long times permitting the reaction to be tracked at low temperatures for weeks to months. In the present study, the peak heights and peak locations are in good agreement with DSC and IR results; however, presumably as a consequence of the more complete capture of heat by the TAM Air, total heat of reaction was roughly 20% higher than for the DSC. TAM Air 60C DSC 60C

160 120 80 40

0

Top: The total heat of reaction in TAM Air vs. DSC at 60°C and 80°C. Bottom: The DSC plots for the top graph were given the same multiplier of 1.18 for 60°C and 80°C and all plots were divided through by the total heat of reaction of 320 J/g. This is done assuming the TAM Air is capturing the total heat, and full reaction is occurring at 80°C, while the reaction at 60°C is stopped by vitrification before the full reaction can occur.

1

TAM Air 80C DSC 80C

0.8

Extent of Reaction

Experimental Methods

Inconsistency With Microcalorimitry Results

n

Top: these Sandia-legacy results are shown for the adduct-forming reaction. Bottom: The extent of reaction for the gelation reaction at a high temperature cannot be simply superimposed on a lower temperature by shifting the time

TAM Air 60C DSC 60C

0.6

0.4

0.2

0 10

100

1000 Time (min)

4

10

10

5

8) Bowen DO and Whiteside RC. Advances in Chemistry Series 1970;92:48-59 9) Sandia Website: http://www.sandia.gov/polymer-properties/828_DEA.html Figures of molecular structures from Wikipedia and www.polysciences.com Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.