Moisture-Temperature Effects on the Dynamic Mechanical Properties

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Downloaded by CALIFORNIA INST OF TECHNOLOGY on October 31, 2017 | http://pubs.acs.org Publication Date: August 29, 1982 | doi: 10.1021/bk-1982-0227.ch006

Moisture-Temperature Effects on the Dynamic Mechanical Properties of Epoxy Polymers WAYNE J. MIKOLS1 and JAMES C. SEFERIS

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Polymeric Composites Laboratory, Department of Chemical Engineering, University of Washington, Seattle, WA 98195

Dynamic mechanical properties are used in this work to develop an understanding of how epoxy based networks respond to different hygrothermal environments. A fundamental understanding of why property changes occur as a result of hygrothermal exposure is provided. Numerous efforts have focused upon the nature of moisture transport of epoxy systems. Previous-sorption desorption work demonstrated that equilibrium moisture levels in an epoxy system can be related to thermodynamic states (1,2,3). Transient and equilibrium dynamic mechanical experiments are performed in this work with two epoxy systems TGEBA-TETA and N-5208. These experiments provide insight into the nature and extent that network changes have on the dynamic mechanical properties as a result of hygrothermal cycling. Experimental Epoxy film samples were prepared with the DGEBA-TETA and N-5208 resins. Detailed information on the DGEBA-TETA and N-5208 resin cure and sample preparation has been previously provided (2,4). Films of the N-5208 resin were made with a curing agent concentration of 25 PHR-DDS (4); 14 PHR ΤΕΤΑ was used for curing the DGEBA epoxy (2). Dynamic mechanical properties for films were measured on strip samples cut to nominal 0.02 cm x 0.3 cm x 6.0 cm dimensions. Two different types of dynamic mechanical experiments were performed. First, the temperature dependence of "equilibrium" dynamic mechanical properties for all epoxy samples were obtained

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Currently with Shell Development Co. in Houston, T X Author to whom correspondence should be addressed

0097-6156/83/0227-0095$06.00/0 © 1983 American Chemical Society

May; Chemorheology of Thermosetting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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CHEMORHEOLOGY OF THERMOSETTING P O L Y M E R S

w i t h the Rheovibron DDVII as p r e v i o u s l y d e s c r i b e d ( 5 ) . These experiments were performed a t a frequency of 11 Hz over the temperature range beginning near -150°C t o j u s t above the s p e c i f i c sample Τ · A h e a t i n g r a t e of l°C/min. was employed f o r a l l experiments. The samples f o r " e q u i l i b r i u m " -Rheovibron experiments were f i r s t allowed to e q u i l i b r a t e i n a s p e c i f i e d hygrothermal environment. They then were loaded i n t o the Rheovibron environmental chamber and maintained i n an atmosphere of dry n i t r o g e n d u r i n g the h e a t i n g process imposed by the experiment. The data obtained were recorded i n the form of the t r a d i t i o n a l t a n δ moduli ( E and E") and dynamic mechanical compliances (S* and S") ( 5 ) . The second type of dynamic mechanical experiment i n v o l v e d c o l l e c t i o n of " t r a n s i e n t " -Rheovibron data a t a f i x frequency as a f u n c t i o n of time a f t e r a change i n e i t h e r the moisture o r thermal environment. The experimental apparatus designed f o r t h i s purpose i s d e p i c t e d i n F i g u r e 1.


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Discussion E q u i l i b r i u m Dynamic Mechanical Data. Dynamic mechanical p r o p e r t i e s of both the DGEBA-TETA and the N-5208 epoxy systems e x h i b i t c h a r a c t e r i s t i c t r a n s i t i o n s observed i n many polymeric m a t e r i a l s . F i g u r e s 2a and 2b i l l u s t r a t e " e q u i l i b r i u m " dynamic mechanical t a n 6 as a f u n c t i o n of temperature f o r samples saturated a t d i f f e r e n t moisture l e v e l s . At h i g h temperatures, the e x h i b i t e d α t r a n s i t i o n i s a s s o c i a t e d w i t h the m a t e r i a l ' s g l a s s t r a n s i t i o n . Near -50°C, both of these epoxies e x h i b i t a f a m i l i a r low temperature $ t r a n s i t i o n . The low temperature $ t r a n s i t i o n i n g l a s s y polymers i s o f t e n a r e s u l t of segmental c h a i n m o b i l i t y t r i g g e r e d by the system s thermal c h a r a c t e r i s t i c s . T h i s low temperature peak i s q u i t e broad i n d i c a t i n g that a wide spectrum of motion types and/or a c t i v a t i o n energies are c o n t r i b u t i n g t o the t r a n s i t i o n (7). The i n c r e a s e i n magnitude of the low temperature t a n 6 a s s o c i a t e d w i t h the 3 - t r a n s i t i o n peak can be r a t i o n a l i z e d due t o a p l a s t i c i z a t i o n o f the epoxy network. However, the s i g n i f i c a n t d i f f e r e n c e s i n the t a n

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N-5208

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6. MiKOLS A N D SEFERis

Moisture-Temperature Effects

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comparison i s made between dynamic mechanical l o s s compliance data and epoxy moisture content i n F i g u r e 4. The a b s c i s s a f o r t h i s graph corresponds t o the apparent e q u i l i b r i u m molar content of water i n the epoxy per mole of n i t r o g e n i n the makeup batch of uncured epoxy r e s i n . The o r d i n a t e i s a measure of the d i f f e r e n t i a l area under the ω peak i n the L o g ^ ( S " ) versus temperature curve from F i g u r e s 3a or 3b. For each epoxy system, the d i f f e r e n t i a l area i s d e f i n e d as the d i f f e r e n c e between the Log^CS") versus temperature curves a t a given moisture content and a t zero moisture content. Numerical i n t e g r a t i o n i s done between these two curves under the ω t r a n s i t i o n . I t i s of i n t e r e s t t o note t h a t f o r both epoxies the d i f f e r e n t i a l area appears to i n c r e a s e s t e a d i l y w i t h the molar r a t i o of moisture to n i t r o g e n content. Since each of the epoxies c o n t a i n v a r y i n g amounts of primary, secondary, and t e r t i a r y amines, the f a c t that both epoxy systems do not f a l l on the same l i n e i s expected. Sulfone groups i n the N-5208 system f u r t h e r complicate a n a l y s i s of the hypothesized r e l a t i o n s h i p . U s e f u l i n s i g h t can be gained by examining q u a l i t a t i v e l y the r e l a t i o n s h i p s between v a r i o u s s e t s of data of F i g u r e 4. For the d i f f e r e n t i a l area under the curve f o r the DGEBA-TETA epoxy sample, i t i s observed that as the moisture content per mole of n i t r o g e n i s i n c r e a s e d from z e r o , the d i f f e r e n t i a l area under the l o s s compliance versus temperature curve i n c r e a s e s . Near an a b s c i s s a value of 0.6, the mechanical property data appears t o p l a t e a u . A d d i t i o n a l i n c r e a s e s i n epoxy moisture content p l a y no f u r t h e r r o l e i n ω t r a n s i t i o n l o s s compliance property changes. T h i s should indeed occur i f a l l a v a i l a b l e amine hydrogen bonding s i t e s i n the m a t r i x become s a t u r a t e d . A d d i t i o n a l moisture enters the network without i n t r o d u c i n g a d d i t i o n a l network hydrogen bonding or mechanical property changes. The 25 PHR-DDS N-5208 epoxy sample covers a broader range of mole-moisture per mole-nitrogen v a l u e s i n F i g u r e 4. However, at an a b s c i s s a value of 0.8, the d i f f e r e n t i a l l o s s compliance area does not appear s a t u r a t e d . The higher f u n c t i o n a l i t y of the N-5208 components r e s u l t i n a cured network w i t h fewer t e r t i a r y amine l i n k s ( 6 ) . T h i s , i n c o n j u n c t i o n w i t h the presence of the DDS s u l f o n e groups, can be used to r a t i o n a l i z e the l a c k of a p l a t e a u i n the d i f f e r e n t i a l area curves of the 25 PHR-DDS epoxy data. Transient Dynamic Mechanical Data. Data from F i g u r e s 2a and 2b f o r both the DGEBA-TETA and the N-5208 epoxy systems suggest that moisture serves t o i n c r e a s e the r e l a t i v e molecular m o b i l i t y of c h a i n segments w i t h i n the network s t r u c t u r e . T h i s i s r e a d i l y apparent from the i n c r e a s e s i n the t a n δ v a l u e s . However, these f i g u r e s provide no i n f o r m a t i o n regarding the manner i n which these changes occur. I n s i g h t i n t o the network changes which



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CHEMORHEOLOGY OF THERMOSETTING

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Moisture-Temperature Effects on the Dynamic Mechanical Properties

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