EpoxyDinorbornene Spiro Orthocarbonate System

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Epoxy-Dinorbornene Spiro

Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: May 5, 1990 | doi: 10.1021/ba-1990-0227.ch015

Orthocarbonate System Fourier Transform Infrared Spectroscopy and Dynamic Mechanical Testing Hatsuo Ishida and John Nigro Department of Macromolecular Science, Case Western Reserve University, Cleveland, O H 44106-1712

Polymerization of an epoxy resin in the presence of dinorbornene spiro orthocarbonate was studied with both Fourier transform infrared spectroscopy and dynamic mechanical spectrometry. The kinetic study indicates that the epoxy and the spiro orthocarbonate compound polymerize at similar rates; however, most of the spiro orthocarbonate compound reacts before the gel point of the epoxy resin, and thereby negates the volume expansion effect of the spiro orthocarbonate compound. The activation energy of the reaction of the spiro orthocarbonate compound was found to be 18.1 kcal/mol based on first-order kinetics.

D

INORBORNENE

SPIRO O R T H O C A R B O N A T E * (see Scheme I) counteracts the

detrimental stress formation associated with cure-induced shrinkage of epoxy resins. During polymerization, the double-ring diether breaks open (1-5). This ring scission provided by the cross-linking reaction results in the breakage of two covalent bonds for every linkage (intermoleeular bond) formed. Bailey and co-workers (1, 2, 4, 5) contend that this molecular " u n packing" manifests itself in an expansion that counteracts cure shrinkage. *The current Chemical Abstracts index name of this compound is orthocarbonic acid, cyclic bis(5-norbornen-2-ylidenedimethylene) ester.

0065-2393/90/0227-0259$06.00/0 © 1990 American Chemical Society

Craver and Provder; Polymer Characterization Advances in Chemistry; American Chemical Society: Washington, DC, 1990.

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\ Scheme I. Molecular structure of dinorbornene spiro orthocarbonate before and after polymerization.

Thus, the spiro orthocarbonate acts primarily as a shrinkage-inhibiting agent. Reduced shrinkage leads to a reduction in internal resin stress. Although resin shrinkage occurs in all phases of cure, only that occurring while the resin is relatively solid is responsible for stress formation. For Baileys theory to accurately account for the observed stress reduction, a major portion of the spiro orthocarbonate must polymerize after the resin system has gelled. In an alternative mechanism postulated by Shimbo et al. (6), the orthocarbonate's expansion properties have little to do with its effectiveness. Shimbo determined that most stress is formed during the cooling stage from the glass transition temperature (T ) to the ambient temperature (T) and that the amount of residual stress in the cured resin is directly proportional to this difference (ds /d[T - T] is a constant; s is resin residual stress). Consequently, a reduced difference between T and T should result in a lower-stressed resin. Subsequently they demonstrated that this material (the orthocarbonate) consistently reduces both cured resin T and the residual internal stress accordingly. g

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g

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In an attempt to expose the true mechanism of this phenomenon, we used Fourier transform infrared (FTIR) spectroscopy coupled with dynamic mechanical testing to investigate the spiro orthocarbonate-epoxy resin system. Basic IR transmission was used to follow the orthocarbonate's ringopening reaction with time. The resulting reaction kinetics were compared with the rheological data obtained via dynamic mechanical spectrometry (DMS). The coordinated F T I R - D M S approach is useful to elucidate the


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Epoxy-Dinorbornene Spiro Orthocarbonate System

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true polymerization behavior of this system and to demonstrate the complimentary nature of the two techniques.

Experimental Details Epoxy resin (diglycidyl ether of bisphenol A (DGEBA), Shell Epon 828) was heated to 100 °C; 10 phr (parts per hundred parts of resin) based on the weight of epoxy resin was used. Finely ground crystalline dinorbornene spiro orthocarbonate was added with vigorous magnetic stirring. The system was stirred under these conditions for 1-2 h. The resulting dispersion was then allowed to cool to room temperature. After reaching ambient temperature, 10 phr of B F • M E A catalyst was mixed in (MEA, monoethylamine, Anchor 1115, is complexed with the highly reactive B F to impart stability). The system was degassed under reduced pressure before any curing studies were initiated. Infrared samples were prepared by placing the resin described in the IR transmission cell. At least 12 spectra were obtained at discrete intervals during three isothermal cures at temperatures of 80, 90, and 100 °C. All spectra are presented in absorbance mode and were taken with 200 scans. Sample and reference spectra were obtained at a resolution of 4 c m on a double-beam spectrophotometer (Digilab FTS-20) equipped with a mercury-cadmium telluride (MCT) detector cooled with liquid nitrogen. Resin prepared in an identical manner was subjected to rheological testing in a dynamic mechanical spectrometer (Rheometrics RMS-800). The parallel-plate configuration with a plate diameter of 25 cm and a gap of 2 mm was used in the dynamic cure mode to provide resin viscosity as a function of time during a 100 °C cure. Other relevant settings include a strain of 0.5%, strain rate of 10 per minute, and a sampling rate of one measurement per minute. Because of memory limitations, three sequential curves (obtained sequentially on the same sample) were pieced together to cover the entire cure range. 3


3

1

Results and Discussion The IR cell, which accommodates low-viscosity liquids, was used to maintain a constant thickness of easily flowing material in the sampling beam. The resulting chronological spectra were analyzed, and a band growth at 1750 c m " was assigned to the C = 0 stretching mode of the carbonate formed during the double-ring-opening polymerization reaction (Figure 1). 1

A n absorption at 1891 c m " , also seen in Figure 1, was assigned as a summation band because of the aromatic nature of the bisphenol A component of the epoxy resin and was used as an internal reference to correct for variations in sample thickness. Spectral manipulation software was employed to evaluate peak areas, and the ratio A /A was taken, where A is the absorbance at the wavelength indicated by the subscript. The plateau value toward the end of the reaction was assumed to be 100% conversion of the double-ring spiro orthocarbonate, and a chemical conversion profile was calculated and is shown in Figure 2. The conversion-vs.-time data were tested for first-order kinetics and found to fit the model well through 95% of the reaction data (Figure 3). 1

1 7 5 0

1 8 9 1



Reaction rate constants (k) were calculated as the slope from the firstorder plots and were used to construct an Arrhenius plot. The reaction rate constants are 1.11 X IO" m i n at 100 °C (R = 0.99), 5.42 X 10~ m i n at 90 °C (R = 0.98), and 2.82 X I O " m i n " at 80 °C (R = 0.99), where R represents the correlation coefficient. From this result, an activation energy was calculated for the spiro orthocarbonate's ring-opening polymerization reaction. The resulting value of 18.1 kcal/mol compares quite favorably with those values generally accepted for most epoxy polymerizations (10-25 kcal/ mol) (7) and indicates a cure compatibility of the two compounds (dinorbornene spiro orthocarbonate and epoxy resin). 2

1

3

3

1

1

A comparison with the epoxide cure data obtained at 100 °C shows that spiro orthocarbonate and epoxy have much the same reaction kinetics (Figure 4). The orthocarbonate's activation energy is in the appropriate range, a result indicating that the monomer polymerizes and expands as the epoxy resin polymerizes and shrinks. Postcuring the slurry at 100 °C for 1200 min indicated that the expansion is approximately 90-92% completed before the plateau region is reached at 360 min. It seems reasonable to ask how much of this (expansion) takes place after the epoxy had reached its gel point and could effectively combat residual stress formation.



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Time (minutes) Figure 2. Conversion of the double-ring structure of spiro orthocarbonate in epoxy at 100 °C. T h e rheological data p r e s e n t e d i n F i g u r e 5 w e r e o b t a i n e d at 100 °C b y d y n a m i c m e c h a n i c a l spectroscopy. A s p l o t t e d , the data indicate that r e s i n viscosity b e c o m e s significant after 250 m i n of heat treatment a n d increases i n a regular m a n n e r u p to 700 m i n . D u r i n g this t i m e , the r e s i n is a h i g h l y viscous m a t e r i a l a n d is i m p e r v i o u s to r e s i d u a l stress formation because it can flow i n response to i m p o s e d forces. F r o m 700 m i n , the viscosity increased drastically. T h i s b e h a v i o r i n d i cates a transition from a viscous to a m o r e g e l l e d state. T h e viscosity a s y m p totically approaches an i n f i n i t e value that has b e e n extrapolated to occur at 8 5 0 - 9 0 0 m i n . A l t h o u g h gelation is generally associated w i t h i n f i n i t e viscosity, the r e s i n can n o longer be c o n s i d e r e d viscous after 750 m i n . I R c u r e data indicate that 9 0 - 9 2 % of the orthocarbonate's d o u b l e - r i n g - o p e n i n g reaction is c o m p l e t e b y 360 m i n , l e a v i n g o n l y 8 - 1 0 % for c u r e b e y o n d this t i m e . M o s t l i k e l y , a major p o r t i o n of the 8 - 1 0 % is e x p e n d e d i n the 3 6 0 - 7 5 0 - m i n range, l e a v i n g little o r n o n e for expansion w h e n the r e s i n is w e l l g e l l e d . A p r e d o m i n a n t a m o u n t of the spiro orthocarbonate c o m p o u n d s h o u l d react after 360 m i n a n d c o n t i n u e to o p e n (ring-opening expansion) w e l l past 750 m i n i f it is to r e d u c e shrinkage a n d to p r o v i d e r e s i d u a l stress r e d u c t i o n v i a the expansion m e c h a n i s m . M o s t of the orthocarbonate has b e e n e x p e n d e d w h i l e


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3.5 -

350

Time (minutes) Figure 3. First-order kinetic plot of the spiro orthocarbonate in epoxy at 100 ° C . the system is viscous and is thus ineffectual in combating residual stress formation. The 8-10% that reacts after 360 min probably cannot contribute considerably to the reduction of this stress.

Conclusions The data suggest that the orthoearbonate's expansion abilities do not act to reduce epoxy cure-induced stress formation as proposed by Bailey et al. (1, 2,4,5). Instead the experimental evidence reported here supports the theory postulated by Shimbo (6), in which this additivity effectively alters resin thermal properties such that less cure shrinkage is converted into residual stress. In this sense, then, the spiro orthocarbonate may not be unique; any substance that can reduce epoxy resins glass transition temperature without altering cured resin integrity should provide for similar resin performance. Along with helping to clarify the spiro orthocarbonate-epoxy resin interaction, this study presented a combined study of FTIR and D M S . It is hoped that the complementary nature of the rheological and IR properties and the utility of such an approach will prove useful in the future study of other systems.



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Time (minutes) Figure 5. Viscosity of the spiro orthocarbonate-epoxy system at 100 °C.


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Acknowledgments The authors gratefully acknowledge the financial support of the Center for Adhesives, Sealants, and Coatings (CASC), Case Western Reserve University, Cleveland, Ohio.


References 1. Bailey, W. J . ; Iwama, H . ; Tsushima, R. J. Polym. Sci. 1977, 56, 117. 2. Bailey, W. J.; Siago, K. Polym. Prepr. 1980, 21, 4. 3. Piggott, M . ; Lam, P. W.; Lim, J. T.; Woo, M . S. Comp. Sci. Technol. 1985, 23, 247. 4. Bailey, W. J.; Sun, R. L . ; Katsuki, H . ; Endo, T.; Iwama, H . ; Tsushima, R.; Saigo, K.; Bittrito, M . M. In Ring-Opening Polymerization; ACS Symposium Series No. 59; American Chemical Society: Washington, D C , 1977; p 38. 5. Bailey, W. J.; Endo, T. J. Polym. Sci. 1978, 60, 17. 6. Shimbo, M . ; Ochi, M.; Inamura, T.; Inoue, M. J. Mater. Sci. 1985, 20, 2965-2972. 7. Antoon, M . , Ph.D. Thesis, Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio; 1977. RECEIVED

26, 1989.

for review February 14, 1989.

ACCEPTED

revised manuscript September


EpoxyDinorbornene Spiro Orthocarbonate System

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