Influence of Physical Aging on the Time-Dependent Properties of

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Downloaded by EAST CAROLINA UNIV on January 3, 2018 | http://pubs.acs.org Publication Date: June 8, 1983 | doi: 10.1021/bk-1983-0221.ch009

Influence of Physical Aging on the TimeDependent Properties of Network Epoxies and Epoxy-Matrix Composites ERIC S. W. KONG Stanford University/NASA-Ames Joint Institute for Surface and Microstructure Research, Stanford University, Department of Materials Science and Engineering, Stanford, CA 94305, and NASA Ames Research Center, Materials Science and Applications Office, Moffett Field, CA 94035 Volume r e l a x a t i o n and enthalpy r e l a x a t i o n processes have been known to occur i n amorphous, glassy materials f o r many years (1). This r e l a x a t i o n phenomenon involves a continuation of vitrification a f t e r the material is quenched into the glassy s t a t e . Struik, who coined the term "Physical Aging" ( 2 ) , demonstrated that the chain mobility of macromolecules is not quite zero even though the polymer is stored in the glassy state (3). S p e c i f i c a l l y , when stored at temperatures between the glass t r a n s i t i o n temperature, Tg, and the highest secondary t r a n s i t i o n temperature below Tg, Τβ , the material approaches i n an asymptotic fashion from a quenched, non-equilibrium state to an aged, equilibrium glassy s t a t e . Thus, physical aging i s viewed as a recovery process that a r i s e s from a thermodynamic drive to a t t a i n equilibrium (see Figure 1 ) . Physical aging has been known to a f f e c t the properties o f thermoplastics (4,5). In f a c t , a majority o f e f f o r t s so f a r have been focused on the study of this recovery phenomenon in l i n e a r macromolecular systems (1-5). The e f f e c t s o f physical aging on the time-dependent properties o f crosslinked polymer systems or thermosets have been by and large ignored u n t i l recently (6). This paper represents a summary o f findings thus f a r on how p h y s i c a l aging influences the d u r a b i l i t y of crosslinked epoxies and the carbon-fiber-reinforced epoxy composites (7,8). Experimental The epoxy used i n t h i s study was F i b e r i t e 934 r e s i n supplied by P i b e r i t e Corporation, Winona, Minnesota. The main constituents of t h i s r e s i n are 63.2% by weight o f t e t r a g l y c i d y l 4,4'-diaminodiphenyl 0097-615 6/ 8 3/0221-0171 $06.25/0 © 1983 American Chemical Society

Bauer; Epoxy Resin Chemistry II ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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Figure 1. O r i g i n of physical aging as explained from a thermodynamic point of view. Τ i s the glasst r a n s i t i o n temperature; Τ Λ i s the highest secondaryt r a n s i t i o n temperature below T^; and T i s the sub-T^ annealing temperature. a


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f

methane (TGDDM), 2 5 . 3 $ of the c r o s s l i n k i n g agent 4 , 4 diaminodiphenyl sulfone (DDS), 1 1 . 2 $ of d i g l y c i d y l orthophthâlate, and about 0.4$ of boron t r i f l u o r i d e / ethylimine catalyst ( 9 ) . The neat r e s i n was prepared by casting. The epoxy material was f i r s t subjected to d e g a s i f i c a t i o n at 85 C inside a vacuum oven. The softened l i q u i d was then poured into a preheated mold. The curing schedule was 121°C f o r 2 h and 177°C f o r 2 . 5 h, followed by a slow cooling at ca. 0.5 C per m|n. to room temperature. Symmetrically reinforced (-45 ) Q Thornel 300/Fiberite' 934 laminates were fabricated from prepreg tapes obtained from P i b e r i t e Corporation. The d e t a i l s on specimen f a b r i c a t i o n are described elsewhere ( 8 ) . With tïïe exception of f i v e specimens (which were


2

to be tested a l l specimens

i n the as-cast or as-fabricated condition), were postcured for 16 h at 250 °C,

followed by a slow cooling to room temperature at a rate of ca. 0.5 0 per min. Testing was performed on the f i v e as-postcured specimens. The other postcured specimens were heated to 260 C f o r 20 min. and then immediately air-quenched to room temperature inside a desiccator. Five of these as-quenched specimens were tested. The others were sub-Tg annealed i n nitrogen atmosphere at 140 C f o r time increments of 1 0 , 10 , 1 0 ^ , 1 0 ' , and 1(r min. Time-zero f o r the sub-Tg annealing experiment was taken as the time when a mercury thermometer placed adjacent to the specimens reached the temperature of 140 C. At each decade of aging time, f i v e specimens were removed from the environmental chamber and stored inside a desiccator at room temperature p r i o r to t e s t i n g . To demonstrate the "Thermoreversibility" of physical aging (2), the following requenching procedure was carried out."" Some of the specimens aged f o r 10 min. were heated to 260 C, which i s above the epoxy Tg, f o r 20 min. and then air-quenched to room temperature inside a desiccator. Pive of these requenched specimens were tested immediately, while the rest were subjected to "re-aging" at 1402c in.nitrogen f o r the time increments of 1 0 , 10 , 10 , 10 , and up to lO^min. At l e a s t f i v e specimens were tested f o r each decade of re-aging time. A Perkin-Elmer DSC-2 d i f f e r e n t i a l scanning calorimeter was used to measure the heat capacity of the neat epoxies. The DSC analysis was performed i n nitrogen at a heating rate of 10 C per min. The DSC was coupled to a "scanning-auto-zero" u n i t f o r baseline optimization. Each specimen was measured from 50 C to 280 C. The specimens were discs of 5 mm i n diameter



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cut from 0.8 mm thick resin-sheet cast using the method described above. The enthalpy relaxation measurements were made by superimposing the f i r s t and second scans f o r each specimen using a data analysis suggested by Wyzgoski (10). An InsFFron model 1122 was used f o r the stress relaxation experiments. Dog-bone-shaped specimens were prepared i n accordance to ASTM D1708-66. The specimens v/ere 22.25 mm long ( l i n e a r section of the dog-bone-shaped specimen), 4.75 mm wide, and c a . 1.5 mm t h i c k . The r e s i n components were prepared as described above and poured into a preheated Dow Corning s i l i c o n rubber RTV 3110 mold. In the stress relaxation experiment, the specimens v/ere stretched within a 2-sec i n t e r v a l to an elongation of c a . 1$. The stress l e v e l was then monitored as a function of time under constant elongation. The r e s u l t s are given as the percent of stress relaxation i n the f i r s t 10 min. of the experiment. At least f i v e specimens were tested for each decade of sub-Tg annealing time. Dynamic mechanical analysis was c a r r i e d out using a low-frequency, f o r c e d - o s c i l l a t i o n DMA 981 unit ( Ε . I. du Pont de Nemours and Company) interfaced with a KINC 11 computer ( D i g i t a l Corporation, Marlboro, Massachusetts). Measurements were made i n nitrogen atmosphere from -100 C to 300 C at a heating rate of 5 C per min. The specimens fr the dynamic mechanical experiment were rectangular (-45 ) « laminates with a length of 23.8 mm, width 12.7 mm and t y p i c a l t h i c k ness of 1.0 mm. Thermal mechanical analysis was performed using a Perkin-Elmer TMS-2 u n i t . Thermal expansion behavior was monitored i n helium atmosphere from 50 C to 260 C at a heating rate of 5 C per min. The specimens were discs of 6 mm i n diameter cut from a 2.5 mm thick resin-plate.

#

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Results and Discussion In an e a r l i e r report (8), the e f f e c t of sub-Tg annealing on the mechanical properties of carbonf i b e r - r e i n f o r c e d epoxy laminates was discussed. In short, i t was found that the ultimate-tensi.le-strength, strain-to-break, and s t a t i c toughness of (-45 )/ symmetrically reinforced TGDDM-DDS epoxy composites decreased with incresing sub-Tg annealing time at 140 C. In the present study, the k i n e t i c s of aging have been followed at 140 C i n nitrogen atmosphere using a variety of techniques. It i s i n t e r e s t i n g to note that no weight loss was observed i n these materials during the sub-Tg annealing experiments. In this r e p o r t , S


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r e s u l t s from four d i f f e r e n t techniques w i l l be discussed, namely, d i f f e r e n t i a l scanning calorimetry, stress r e l a x a t i o n , dynamic mechanical a n a l y s i s , and thermal mechanical a n a l y s i s . D i f f e r e n t i a l Scanning Calorimetry. The thermal analysis technique of d i f f e r e n t i a l scanning calorimetry has been amply demonstrated i n the past to be a u s e f u l method to follow the k i n e t i c s of enthalpy r e l a x a t i o n i n polymers (4,6,1C),11 ). Figure 2 shows the DSC scans of the fulTy-crossTinked epoxy specimens which were quenched from above Tg and then subjected to aging at 140 C. The f u l l l i n e i s the f i r s t scan, and the dotted l i n e represents the second scan taken r i g h t a f t e r rapid cooling from the i n i t i a l one. The following observations v/ere made: 1. The enthalpy r e l a x a t i o n peak appears near the onset of the t r a n s i t i o n from the g l a s s y state to the rubbery s t a t e . This peak appears only a f t e r 10 min. of aging at 140 C'. 2. During sub-Tg annealing, the r e l a x a t i o n peak has both s h i f t e d to a higher temperature and grown i n magnitude (see Figure 2). 3. This recovery phenomenon i s thermoreversible. Upon re-aging the material which was cooled from above Tg, the r e l a x a t i o n peak w i l l reappear and grow with time (see Figure 3). As mentioned e a r l i e r , i t i s possible to measure the r e l a x a t i o n enthalpy using procedures suggested i n the l i t e r a t u r e (£,JK)). Figure 4 shows the enthalpy l o s s during the p h y s i c a l aging versus logarithmic sub-Tg annealing time at 140 C. I t i s apparent that there i s a l i n e a r r e l a t i o n s h i p between t h i s enthalpy r e l a x a t i o n process and the logarithmic aging time. In an e a r l i e r report (8), a l i n e a r r e l a t i o n s h i p was also observed between The drop i n ultimate t e n s i l e behavior and the logarithmic aging time i n a s i m i l a r TGrDDK-DDS epoxy system. Stress Relaxation Experiments. Figure 5 shows the stress r e l a x a t i o n curves of the f u l l y - c r o s s l i n k e d epoxies with 10, 10 , 10^, 10 , and 10 min. of sub-Tg annealing at 140 C. As thermal aging progresses, the i n i t i a l s t r e s s l e v e l increases and the r e l a x a t i o n rate decreases. The higher i n i t i a l stress i s caused p r i m a r i l y by the higher modulus (as also confirmed by dynamic mechanical analysis)but i s also p a r t i a l l y due to l e s s r e l a x a t i o n during the s t r e t c h i n g period. The r e l a x a t i o n rate i s measured as a percent of stress r e l a x a t i o n during the f i r s t 10 min. of the experiment. p


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usage of such materials as s t r u c t u r a l components i n the aerospace and automotive industry (15), parameters such as physical aging time and temperature must be taken into consideration i n the p r e d i c t i o n of longterm d u r a b i l i t y and r e l i a b i l i t y of polymer-matrix composites. The r e s u l t s from t h i s i n v e s t i g a t i o n can be summarized as follows: Enthalpy relaxation has been observed i n TGDDM-DDS network epoxies. Thermoreversible nature of enthalpy relaxation has been demonstrated i n the network glasses of TG-DDM-DDS epoxy. The rate of stress relaxation i n neat TG-DDM-DDS thermoset r e s i n decreases with sub-Tg annealing time. The secondary mechanical damping of epoxy matrix i n Thornel 300/Fiberite 934 composite decreases with physical aging time. Thermoreversibility of physical aging has been demonstrated i n the time-dependent rate of stress relaxation as well as i n the damping behavior of the p - t r a n s i t i o n . Volume relaxation has been observed i n F i b e r i t e 934 epoxies by monitoring the thermal expansion behavior. Thermoreversible nature of volume r e l a x a t i o n has been observed i n TG-DDM-DDS epoxies. Synopsis Matrix-dominated properties of a network epoxy and i t s carbon-fiber-reinforced composite have been found to be affected by sub-Tg annealing i n nitrogen atmosphere. Postcured symmetrically reinforced specimens of Thornel 300 carbon-fiber/Fiberite 934 epoxy composite as well as neat F i b e r i t e 934 epoxy r e s i n v/ere quenched from above Tg and given a sub-Tg annealing at 140 C for times up to 2 months (10 m i n . ) . The damping behavior, stress-relaxation r a t e , and the enthalpic state of the matrix materia,! were found to decrease as functions of sub-Tg annealing time. Thermal expansion behavior of the r e s i n was also observed to be affected by aging. No weight loss v/as observed during sub-Tg annealing. The time-dependent change i n physical properties i s explained on the basis of free-volume collapse that i s related to the volume recovery process of the non-equilibrium glassy network of epoxy. Physical aging i n network epoxy has been demonstrated to be "thermoreversible". The aging k i n e t i c s v/as followed by d i f f e r e n t i a l P


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scanning calorimetry, s t r e s s - r e l a x a t i o n t e s t s , dynamic mechanical a n a l y s i s , and thsrmal mechanical analysis.


Acknowledgments The author would l i k e to thank h i s co-workers i n generating part of the data reported i n t h i s paper. Thanks are due to Susanna Lee, John L a i and Mark Rosenberg f o r t h e i r t e c h n i c a l support. The author also thank Michael Adamson, Linda"Clements, Theodore Sumsion and Howard Nelson f o r t h e i r valuable comments and constructive reviews. A grant (NCC 2 - 1 0 3 ) from NASA to Stanford U n i v e r s i t y i s g r a t e f u l l y acknowledged.

Literature Cited 1. 2. 3.

Kovacs, A.J. J . Polym. Sci., 1958, 3 0 , 131. Struik, L.C.E. "Physical Aging in Amorphous Polymers and Other Materials"; Elsevier Scientific Publishing Company: Amsterdam, 1978. Struik, L.C.E. Annals New York Acad. Sci., 1976, 279,

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5. 6. 7. 8. 9.

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Petrie, S.E.B. "Polymeric Materials: Relation ships between Structure and Mechanical Behavior"; Amer. Society for Metals: Metals Park, Ohio, 1975, p. 5 5 . Golden, J.H.; Hammant, B.L.; Hazell, E.A. J . Appl. Polym. Sci., 1967, 1 1 , 1571. Ophir, Z.H.; Emerson, J.Α.; Wilkes, G.L. J . Αppl. Phys., 1978, 4 9 , 5032. Kong, E.S.W.; Wilkes, G.L.; McGrath, J.E.; Banthia, A.K.; Mohajer, Y.; Tant, M.R. Polym. Eng. Sci., 1981, 2 1 , 9 4 3 . Kong, E.S.W. J . Appl. Phys., 1981, 5 2 , 5921. May, C.A.; Fritzen, J.S.; Whearty, D.K. "Exploratory Development of Chemical Quality Assurance and Composition of Epoxy Formulations"; Lockheed Missiles and Space Company: Sunnyvale, California, Air Force Technical Report: AFML-TR-76-112 ( 1 9 7 6 ) .

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Wyzgoski, M.G. J . Appl. Polym. Sci., 1980, 25,

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Meier, D.J., Ed. "Molecular Basis of Transitions and Relaxations" Gordon and Breach Science


KONG

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Publishers: London, Midland Macromolecular Monographs, V o l . 4 (1978). B a i l e y , R.T.; North, A . M . ; Pethrick, R.A. "Molecular Motion i n High Polymers" Oxford University Press: Oxford, 1981. Beardmore, P.; Harwood, J . J . ; Kinsman, K.R.; Robertson, R.E. Science, 1980, 208, 833. December 2, 1982


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Influence of Physical Aging on the Time-Dependent Properties of

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