Dynamic Mechanical Analyzer for Thermal ... - ACS Publications

4

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 19, 2016 | http://pubs.acs.org Publication Date: June 1, 1983 | doi: 10.1021/ba-1983-0203.ch004

Dynamic Mechanical Analyzer for Thermal Mechanical Property Characterization of Organic Coatings T H E O D O R E P R O V D E R , R I C H A R D M . H O L S W O R T H , and THOMAS H. G R E N T Z E R Glidden Coatings and Resins, Division of S C M Corporation, Strongsville, O H 44136

Dynamic mechanical analysis was used to study poly mer/end-use property relationships for chemical coat ings systems. The du Pont 981 Mechanical Analyzer (DMA) and a torsion pendulum gave comparable results for ex terior acrylic latex and styrene—acrylic—acrylonitrile terpolymer free films. DMA damping profiles of can coatings correlated with the coatings performance dur ing can manufacture. Gel coat DMA modulus and damp ing profiles correlated with the tendency toward en vironmental stress cracking. Combined DMA/DSC (dif ferential scanning calorimetry) techniques were used to analyze a high solids coating foam entrapment problem. Cure kinetics methodology using a single dynamic DMA temperature scan was developed to obtain kinetics pa rameters. Comparative reaction kinetics were obtained by DMA/DSC techniques for the curing of EPON 825 with bis(4-aminophenyl)methane.

JL

H E USEOF TORSION P E N D U L U M ANALYSIS (TPA)

for e v a l u a t i n g o r g a n i c

c o a t i n g s has b e e n w e l l d o c u m e n t e d (1-3). T h e f i r s t T P A i n u s e at t h i s laboratory was c a p a b l e of o b t a i n i n g m o d u l u s a n d loss profiles of t h i n (1—3 m i l ; 2 5 — 7 5 μπι) o r g a n i c c o a t i n g s as w e l l as t h i c k e r ( 1 0 — 2 0 m i l ; 2 5 0 - 5 0 0 ftm) g e l coat r e s i n s y s t e m s . T h i s i n s t r u m e n t is s h o w n i n F i g u r e s 1 a n d 2. T h e T P A c a n b e u s e d as a t o r s i o n b r a i d a n a l y z e r b y r e p l a c i n g the free f i l m w i t h a glass m a t or b r a i d to s u p p o r t coatings systems. T h i s use has b e e n d e m o n s t r a t e d for a t h e r m o s e t t i n g a c r y l i c p o w d e r c o a t i n g s s y s t e m as s h o w n i n F i g u r e 3 (4). T h e u s e o f t h e T P A was time c o n s u m i n g a n d r e q u i r e d constant operator attention. A n 0065-2393/83/0203-0077$06.00/0 © 1983 A m e r i c a n C h e m i c a l Society

In Polymer Characterization; Craver, Clara D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1983.


78

POLYMER CHARACTERIZATION

Figure 1. Torsion pendulum designed for free paint films. (Reproduced with permission from Réf. 1. Copyright 1966, Journal of Coatings Technology.) evaluation of the d u P o n t 981 d y n a m i c m e c h a n i c a l analyzer ( D M A ) s h o w e d t h a t t h e r e s u l t s w e r e c o m p a r a b l e to t h o s e o b t a i n e d o n t h e T P A for t h e same systems. T h e o p e r a t i o n o f the D M A d i d n o t r e q u i r e constant operator attention, thereby m a k i n g it an excellent i n s t r u m e n t for r o u t i n e t h e r m a l m e c h a n i c a l p r o p e r t y c h a r a c t e r i z a t i o n o f o r g a n i c coatings. Instrumentation

and

Methodology

Description of D M A Instrumentation. D y n a m i c mechanical analysis m e a s u r e s the d e f o r m a t i o n r e s p o n s e o f a s a m p l e w h i c h has b e e n s u b j e c t e d to o s c i l l a t o r y forces. T h e d u P o n t 981 D M A f o l l o w s t h e reso n a n t f r e q u e n c y (or r e l a t i v e m o d u l u s ) a n d e n e r g y d i s s i p a t i o n o f o r -



4.

PROVDER ET A L .

Dynamic

Mechanical

Analyzer

79

Figure 2. Lower sample holder and phonograph cartridge torque sensing unit. (Reproduced with permission from Réf. 1. Copyright 1966, Journal of Coatings Technology.) g a n i c c o a t i n g s y s t e m s as a d y n a m i c f u n c t i o n o f t i m e o r t e m p e r a t u r e . F r o m these measurements the m o d u l u s a n d tan δ can be calculated i n t h e t e m p e r a t u r e r a n g e f r o m —150 t o 5 0 0 ° C . F i g u r e 4 shows a scheme of the d u P o n t 981 D M A . T h e s a m p l e a r m s are f i x e d to the r i g i d b l o c k v i a l o w - f r i c t i o n f l e x u r e p i v o t s . A c o m p o u n d resonance system is f o r m e d b y c l a m p i n g the sample b e t w e e n the arms. A n e l e c t r o m e c h a n i c a l t r a n s d u c e r is u s e d to d r i v e the active a r m w h i l e the c o u n t e r w e i g h t e d passive a r m is u s e d for p h y s i c a l s u p -


80



0.2

0.004-fc

1 25

1 50

L_Z 75

1 100

TEMPERATURE

| 125

(°C)

1 150

L_J 175

Figure 3. TBA relative rigidity as a function of temperature for a thermosetting acrylic powder coating. Key: A, powder fusion; B, T of uncuredfilm; C, end of melting; D, onset of gel; E, cure effected; and F, T of cured film. (Reproduced with permission from Ref. 4. Copyright 1974, Journal of Coatings Technology.) g

g

port. H o r i z o n t a l angular d i s p l a c e m e n t of the active a r m about the p i v o t f l e x u r e p r o d u c e s a f e w t e n t h s o f a m i l l i m e t e r d e f l e c t i o n at t h e s a m p l e . T h u s , t h e s a m p l e i s p l a c e d i n flexural s t r e s s . A f t e r r e m o v i n g t h e d i s p l a c i n g force, the s a m p l e goes into resonant o s c i l l a t i o n . T h e o s c i l l a t i o n a m p l i t u d e a n d f r e q u e n c y are s e n s e d b y a l i n e a r v a r i a b l e d i f f e r ential transformer ( L V D T ) . T h e sample's natural resonant frequency o f o s c i l l a t i o n is d i g i t a l l y d i s p l a y e d o n t h e f r o n t p a n e l o f t h e 9 8 1 D M A a n d i s a l s o p l o t t e d as a d y n a m i c f u n c t i o n o f t i m e o r t e m p e r a t u r e . N o r m a l l y , w h e n a s a m p l e is d e f o r m e d a n d r e l e a s e d , t h e s a m p l e w i l l o s c i l l a t e at its r e s o n a n t f r e q u e n c y w i t h a d e c r e a s i n g a m p l i t u d e o f o s c i l l a t i o n . T h e D M A sends t h e s i g n a l f r o m t h e L V D T to t h e D M A d r i v e r c i r c u i t r y , w h i c h f e e d s t h e s i g n a l b a c k to t h e e l e c t r o m e c h a n i c a l t r a n s d u c e r to m a i n t a i n a f i x e d a m p l i t u d e o f o s c i l l a t i o n . T h e p o w e r r e q u i r e d to m a i n t a i n t h i s f i x e d a m p l i t u d e o f o s c i l l a t i o n i s r e l a t e d to t h e d a m p i n g c a p a c i t y o f the m a t e r i a l , a n d is p l o t t e d a l o n g w i t h the r e s o n a n t f r e q u e n c y as a d y n a m i c f u n c t i o n o f t i m e a n d t e m p e r a t u r e (5, 6 ) . E x p e r i m e n t a l C o n d i t i o n s for D y n a m i c M e c h a n i c a l A n a l y s i s . The d u P o n t 981 D M A was u s e d i n conjunction w i t h the 990 t h e r m a l anal y z e r p r o g r a m m e r / r e c o r d e r to o b t a i n t h e e x p e r i m e n t a l r e s u l t s . T h e m e c h a n i c a l r e s p o n s e o f a l l s a m p l e s w a s r e c o r d e d as a d y n a m i c f u n c -



Figure 4. Scheme

DISPLAY

COUNTERWEIGHTS |

from

990 TA

Ref. 5. Copyright

PROGRAMMER

SAMPLE THERMOCOUPLE

DRIVEN ARM

PASSIVE A R M

DAMPING DISPLAY

HEATER

of du Pont 981 DMA. (Reproduced with permission Ε. I. du Pont de Nemours i? Co., Inc.)

FREQUENCY

CONTROL PANEL & ELECTRONICS HOUSING

ELECTROMECHANICAL TRANSDUCER

981 DMA


1979,

oo

-s

ci

M H > r

Ο < υ

82



t i o n o f t e m p e r a t u r e at a h e a t i n g rate o f 5 ° C / m i n . T h e s a m p l e s w e r e c o o l e d to s u b a m b i e n t t e m p e r a t u r e s b y p u r g i n g the s a m p l e c h a m b e r w i t h l i q u i d nitrogen. After the sample temperature reached approxi m a t e l y - 1 0 0 ° C , t h e l i q u i d f l o w rate w a s r e d u c e d a n d a d r y n i t r o g e n p u r g e o f 5 L / m i n w a s d e l i v e r e d to t h e s a m p l e c o m p a r t m e n t e n v i r o n ment simultaneously. T h e sample preparation procedure and clamp i n g arrangement were d e p e n d e n t on the type of coating investigated, as w e l l as t h e i n f o r m a t i o n d e s i r e d f r o m t h e a n a l y s i s . T h e r e f o r e , t h i s i n f o r m a t i o n w i l l b e d i s c u s s e d later for the specific c o a t i n g s y s t e m investigated. E x p e r i m e n t a l C o n d i t i o n s for Differential Scanning Calorimetry (DSC). T h e d u P o n t 990 t h e r m a l analyzer programmer/recorder a n d t h e 9 1 0 D S C c e l l m o d u l e w e r e u s e d to o b t a i n t h e e x p e r i m e n t a l r e sults. T h e s a m p l e e n v i r o n m e n t was e i t h e r n i t r o g e n or c o m p r e s s e d air at a f l o w rate o f 5 0 m L / m i n . T h e s a m p l e w e i g h t w a s k e p t i n t h e 0 . 5 - m g range. H e r m e t i c a l l y s e a l e d s a m p l e p a n s w e r e u s e d for reactions ex h i b i t i n g a w e i g h t loss. Reaction

Kinetics

I n p r e v i o u s studies ( 7 - 9 ) , a m e t h o d was d e v e l o p e d for d e t e r m i n i n g quantitative reaction kinetics b y D S C w i t h the use of a single d y n a m i c t e m p e r a t u r e scan (one t h e r m o g r a m ) . I n this c h a p t e r the m a t h e m a t i c a l a p p r o a c h w a s e x t e n d e d to o b t a i n q u a n t i t a t i v e r e a c t i o n kinetics of cure b y d y n a m i c m e c h a n i c a l analysis b y the use of a single d y n a m i c temperature scan. T h e d u P o n t D M A c a n b e u s e d to f o l l o w the m e c h a n i c a l r e s p o n s e o f a m a t e r i a l b e f o r e , d u r i n g , a n d after c u r e . H o w e v e r , i n m o s t c a s e s , t h e t h e r m o s e t t i n g s y s t e m m u s t b e s u p p o r t e d b y a n i n e r t m a t e r i a l (i.e., a w o v e n fiberglass braid) to f o l l o w the b u i l d u p of m e c h a n i c a l p r o p erties d u r i n g cure. D u r i n g a d y n a m i c temperature scan, the D M A frequency profile (relative modulus) w i l l increase i n the c u r i n g t e m p e r a t u r e r e g i o n for a t h e r m o s e t s y s t e m . T h i s i n c r e a s e i n r e l a t i v e m o d u l u s , w h i c h c a n b e r e l a t e d to t h e f r a c t i o n a l extent or d e g r e e o f c u r e , F(t,T), i s d e f i n e d b y t h e f o l l o w i n g e q u a t i o n a n d s c h e m a t i c a l l y i l l u s t r a t e d i n F i g u r e 5: F(,,T)

=

ψ

±

^

(1,

w h e r e G G , a n d G are t h e r e l a t i v e m o d u l u s r e a d i n g s at t h e o n s e t o f c u r e , at a g i v e n t i m e a n d t e m p e r a t u r e d u r i n g t h e c u r i n g p r o c e s s , a n d after t h e c u r i n g p r o c e s s has c e a s e d , r e s p e c t i v e l y . A s s u m i n g that t h e f u n c t i o n a l f o r m o f the c u r e c u r v e f o l l o w s the g e n e r a l n t h o r d e r rate expression: 1 ?

2


4.

PROVDER ET AL.

CO ZD -J D Û

Ο

Dynamic

Mechanical

83

Analyzer

G(t,T)


LU > THERMOSET

THERMO LU

PLASTIC

STATE

STATE

TEMPERATURE Figure 5. DMA relative

LU Lt

THERMOPLASTIC STATE "

CURE REGION •

-4-

-100

0 100 TEMPERATURE (°C)

200

Figure 14. DMA relative modulus curves as a function of for high solids coatings during the curing process. Key: foam entrapment; and , good film.

temperature , with



92


TEMPERATURE

(°C)

Figure 15. DMA relative modulus curves as a function of temperature for high solids coatings after cure. Key is the same as in Figure 14. w h i c h does not e x h i b i t defects d u e to f o a m e n t r a p m e n t , is m o r e flexible w i t h a higher molecular weight between cross-links. T h i s w o u l d b e e x p e c t e d to o c c u r u p o n d e c r e a s i n g t h e c a t a l y s t l e v e l . T h e d e c r e a s e i n catalyst l e v e l m a y a c c o u n t for the e l i m i n a t i o n of foam e n t r a p m e n t b y r e d u c i n g the l e v e l of volatile e v o l u t i o n o c c u r r i n g d u r i n g the melamine cure. D M A / D S C C u r e K i n e t i c s . T h e D S C m o n i t o r s t h e h e a t flow i n t o a n d out of the sample d u r i n g the c u r i n g process. O n the other h a n d , the D M A monitors rigidity b u i l d u p d u r i n g cure. T h e D M A kinetics r e s u l t s a r e s e n s i t i v e to v o l a t i l i z a t i o n , m o l e c u l a r w e i g h t b e t w e e n cross-links, backbone rigidity, viscosity, and functional group reactivity. Therefore, b y u s i n g kinetics results from different techniques it s h o u l d b e p o s s i b l e to g a i n i n s i g h t i n t o t h e c h e m i c a l a n d p h y s i c a l factors a f f e c t i n g t h e c u r i n g p r o c e s s . T h e D M A experiment r e q u i r e d the use of a fiberglass b r a i d supp o r t for t h e e p o x y c u r e r e a c t i o n . R e a c t i o n k i n e t i c p a r a m e t e r s f o r t h e curing of a narrow molecular weight distribution epoxy, E P O N 825, w i t h bis(4-aminophenyl)methane by the single d y n a m i c scan D S C a n d D M A m e t h o d s are r e p o r t e d i n T a b l e I. T h e r e a c t i o n k i n e t i c s r e s u l t s d o n o t a g r e e , as o n e m i g h t e x p e c t , b e c a u s e e a c h m e t h o d i s s e n s i t i v e to d i f f e r e n t p h y s i c a l p h e n o m e n a . T h e d i f f e r e n c e s b e t w e e n the fractional c o n v e r s i o n (by D S C analysis) a n d the d e g r e e of cure (by D M A a n a l y s i s ) are s h o w n i n F i g u r e 16. T h e t w o c u r v e s i n F i g u r e 16


4.

PROVDER ET AL.

Dynamic

Mechanical

93

Analyzer


T a b l e I. Reaction Kinetics for the C u r i n g of E p o n 825 w i t h Bis(4-aminophenyl)methane

Method

Activation Energy (kjimol)

Reaction Order

Arrhenius

DSC DMA

58.6 109.7

1.05 1.10

Frequency Factor (s' ) 1

8.9 χ 1 0 7.8 x 1 0

4

1 2

were obtained b y using E q u a t i o n 6 along w i t h the D S C a n d D M A k i n e t i c parameters i n T a b l e I. T h e c a l c u l a t e d degree of cure profile b y D M A a n a l y s i s i n d i c a t e s a f a s t e r r e a c t i o n t a k i n g p l a c e at 1 5 0 °C t h a n i s indicated by D S C analysis. T h e discrepancy b e t w e e n the D S C a n d D M A kinetic results m a y b e d u e to t h e v i t r i f i c a t i o n o f t h e s a m p l e b e f o r e c o m p l e t e c h e m i c a l r e a c t i o n has b e e n a c h i e v e d . A l s o , i t is p o s s i b l e t h a t t h i s d i f f e r e n c e is d u e to t h e D M A ' s s e n s i t i v i t y to t h e r i g i d i t y o f t h e a m i n e m o l e c u l e bis(4-aminophenyl)methane. Further work i n determining isothermal v i t r i f i c a t i o n p o i n t s is p l a n n e d . Conclusions D y n a m i c m e c h a n i c a l a n a l y s i s is a v a l u a b l e t e c h n i q u e for s o l v i n g p r o d u c t i o n p r o b l e m s a n d a i d i n g coatings failure analysis. T h i s tech-

H

0

1 30

1 60

1 90

1 120

1 150

1

Li.

TIME (min) Figure 16. Comparison of DMA degree of cure curve and DSC frac tional conversion curve for an epoxy—amine reaction at an isothermal temperature of 150 °C.


94

POLYMER

CHARACTERIZATION

n i q u e also is u s e f u l for e s t a b l i s h i n g r e l a t i o n s h i p s b e t w e e n e n d - u s e performance properties

and fundamental

thermal—mechanical

pa

rameters. T h e use of d y n a m i c mechanical analysis cure kinetics studies c a n provide

i n s i g h t i n t o t h e c h e m i c a l a n d p h y s i c a l factors affecting t h e

c u r i n g process. T h e k i n e t i c parameters c a n b e u s e d to s i m u l a t e o v e n bake conditions. T h i s w i l l a l l o w coatings chemists to better


a n d formulate coatings to a c h i e v e d e s i r e d performance

design

properties.

Literature Cited 1. Pierce, P. E.; Holsworth, R. M. J. Paint Technol. 1966, 38 (496), 263. 2. Pierce, P. E.; Holsworth, R. M. J. Paint Technol. 1966, 38 (501), 585. 3. Bender, H.S.J.Paint Technol.1969,41 (535), 445. 4. Zicherman, J. B.; Holsworth, R. M. J. Paint Technol. 1974, 46 (591), 55. 5. Ε. I. du Pont de Nemours & Co., Inc., Scientific and Process Instruments Division, "Instruction Manual—981 Dynamic Mechanical Analyzer"; Wilmington, D E , 1979. 6. Murayama, T. In "Dynamic Mechanical Analysis of Polymeric Material"; Elsevier: New York, 1978; pp. 51, 52. 7. Grentzer, T. H.; Holsworth, R. M.; Provder, T.; Kline, S. "Proceedings of the Tenth North American Thermal Analysis Society Conference," Oct. 26-29, 1980, Boston, MA; p. 269. 8. Grentzer, T. H.; Holsworth, R. M.; Provder, T. Org. Coat. Plast. Chem. 1981, 44, 673. 9. Kah, A. F.; Koehler, M. E . ; Grentzer, T. H.; Niemann, T. F.; Provder, T. Org. Coat. Plast. Chem. 1981, 45, 480. RECEIVED for review October 14, 1981. ACCEPTED March 1, 1982.


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