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Jul 22, 2009 - Dynamic mechanical analysis was used to study polymer/end-use property relationships for chemical coatings systems. The du Pont 981 ...
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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.

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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 -

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

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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 -

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

80

POLYMER CHARACTERIZATION

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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 -

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

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

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

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1979,

oo

-s

ci

M H > r

Ο < υ

82

POLYMER CHARACTERIZATION

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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

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

4.

PROVDER ET AL.

CO ZD -J D Û

Ο

Dynamic

Mechanical

83

Analyzer

G(t,T)

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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

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

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92

POLYMER CHARACTERIZATION

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

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

4.

PROVDER ET AL.

Dynamic

Mechanical

93

Analyzer

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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.

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

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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

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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.

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