Enthalpy Relaxation in UV-Cured Epoxy Coatings - ACS Symposium

Chapter 21

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Enthalpy Relaxation in UV-Cured Epoxy Coatings Geoffrey A. Russell and W. Eugene Skiens Optical Data, Inc., 9400 Southwest Gemini Drive, Beaverton, OR 97005-7159

Development of structural ordering upon annealing i s a well-recognized phenomenon in glassy polymers. It has significant effects on modulus, impact resistance and other mechanical properties. Structural ordering can occur in thin films as well as in bulk materials. In this study it has been observed in UV-cured acrylated epoxy coatings. It has been characterized by quantitative differential thermal analysis (DSC) and by thermomechanical analysis (TMA). The extent of structural relaxation i s a function of cure dose, thermal history and type and level of comonomer. Because all of the formulations studied exhibited broad glass transitions beginning near ambient temperature, significant structural ordering occurred at room temperature. Relaxation rates decrease with increasing cure dose or upon addition of mobile multifunctional crosslinkers. Based upon these observations, it appears that structural ordering plays a significant role in the aging of UV-cured coatings.

It i s frequently observed that the properties of UV-cured films and c o a t i n g s change as a function of time a f t e r cure. This process can be a c c e l e r a t e d by a b r i e f thermal treatment. In the case of c a t i o n i c a l l y - c u r e d systems t h i s phenomenon i s g e n e r a l l y a t t r i b u t e d to the reaction k i n e t i c s of the system. The i n i t i a t i o n s t e p i s p h o t o c h e m i c a l l y - a c t i v a t e d and i s very r a p i d . The p r o p a g a t i o n s t e p i s s l o w e r , and r e q u i r e s time and/or thermal a c t i v a t i o n t o proceed to completion. S i n c e t h e r e are few c h a i n t e r m i n a t i o n r e a c t i o n s which affect c a t i o n i c systems, cure can proceed for extended periods after UV e x p o s u r e , as long as there are s t i l l r e a c t i v e species present.1'2 In the case of free r a d i c a l - i n i t i a t e d systems, the basis for t h i s p o s t - c u r e change i n p r o p e r t i e s i s l e s s c l e a r . Both the i n i t i a t i o n and propagation steps proceed r a p i d l y , even at ambient temperature. Furthermore, f r e e r a d i c a l s g e n e r a l l y are s h o r t - l i v e d s p e c i e s , and are s u b j e c t t o a number of r e a c t i o n s w i t h a t m o s p h e r i c s p e c i e s w h i c h consume them. Therefore, the changes i n p h y s i c a l p r o p e r t i e s observed i n a c r y l a t e d epoxies and other f r e e r a d i c a l - i n i t i a t e d c o a t i n g systems must b e a c c o u n t e d f o r by some o t h e r m e c h a n i s m . Since the changes i n p r o p e r t i e s occur at ambient temperature, i t i s unlikely that a chemical mechanism w i l l account for them. Enthalpy relaxation, a

0097-6156/90/0417-0284$06.00/0 ©1990 American Chemical Society

Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

21. RUSSELL &SKIENS

Enthalpy Relaxation in UV-Cured Epoxy Coatings

common phenomenon i n b u l k a m o r p h o u s g l a s s e s , a p p e a r s t o b e t h e p r i n c i p a l mechanism of p o s t - c u r e aging i n free r a d i c a l - i n i t i a t e d coatings. Enthalpy

Relaxation

i n Bulk

Polymeric

Glasses

S t r u c t u r a l r e l a x a t i o n has been observed i n a wide v a r i e t y of o r g a n i c , inorganic and m e t a l l i c glasses.3"13 It gives rise t o systematic changes i n such mechanical properties as modulus, impact strength and heat capacity as a function of the thermal history of t h e 4 5 1 3 m a t e r i a l . ' ' " ' The b a s i s f o r s t r u c t u r a l r e l a x a t i o n i s t h e nonequilibrium nature of t h e glassy state due t o supercooling e f f e c t s observed at any f i n i t e cooling rate. S t r u c t u r a l r e l a x a t i o n has been shown t o b e a n o n l i n e a r , n o n e x p o n e n t i a l p r o c e s s 1 0 due t o t h e f a c t that the enthalpy of a glass i s a function of the entire thermal history of the glass rather than a state variable which i s independent o f t h e path by which t h e g l a s s was formed. Differential s c a n n i n g c a l o r i m e t r y (DSC) i s a w i d e l y - u s e d method f o r s t u d y i n g 3 5 9 structural relaxation i n polymeric glasses. ' " It i s convenient i n t h a t p r o p e r c a l i b r a t i o n o f t h e DSC i n s t r u m e n t a l l o w s d i r e c t determination of t h e heat c a p a c i t y of t h e sample as a f u n c t i o n of temperature. A l s o , t h e sample c a n be annealed f o r v a r y i n g p e r i o d s i n t h e DSC p a n a n d r e - a n a l y z e d c o n v e n i e n t l y . The extent of s t r u c t u r a l relaxation i s determined by the area of the "excess enthalpy" peak which appears as t h e annealed sample i s f i r s t heated from below Tg through the glass transition region. T h i s i s shown s c h e m a t i c a l l y i n Figure 1. The e x c e s s e n t h a l p y peak may appear b e l o w , i n the middle or a t t h e end o f t h e g l a s s t r a n s i t i o n r e g i o n , depending upon t h e s p e c i f i c material system. I t s area i s a function of the entire thermal h i s t o r y of t h e g l a s s a f t e r i t i s quenched from t h e rubbery state. As s t r u c t u r a l r e l a x a t i o n occurs, impact strength decreases1 and modulus i n c r e a s e s . 3 While t h e work reported t o date has dealt w i t h s t r u c t u r a l o r d e r i n g i n b u l k p o l y m e r i c g l a s s e s , t h e same s t r u c t u r a l r e l a x a t i o n p r o c e s s e s ought t o o c c u r i n amorphous glassy films as well, and should have a s i g n i f i c a n t e f f e c t on t h e i r properties. Experimental A l l t h e formulations used i n our experiments u t i l i z e d a d i a c r y l a t e d B i s p h e n o l A epoxy, Novacure 3700 ( I n t e r e z , I n c . ) , as t h e base resin. Four m u l t i f u n c t i o n a l a c r y l a t e s were used t o modify c u r e behavior and physical properties. They were: trimethylolpropane t r i a c r y l a t e (TMPTA), p o l y b u t a n e d i o l d i a c r y l a t e (PBDDA), pentaerythritol t r i a c r y l a t e (PETA) a n d d i p e n t a e r y t h r i t o l t r i a c r y l a t e (DPEPA). The b a s e r e s i n , TMPTA a n d PETA w e r e s u p p l i e d b y I n t e r e z , I n c . PBDDA w a s obtained from A l c o l a c Chemical. DPEPA was s u p p l i e d b y S a r t o m e r . A l l were commercial grades, and were used without f u r t h e r p u r i f i c a t i o n . The p h o t o i n i t i a t o r u s e d f o r a l l e x p e r i m e n t s w a s D a r o c u r 1 6 6 4 (EM Industries), a m i x t u r e o f a r o m a t i c ketones whose a b s o r p t i o n spectrum i s shown i n F i g u r e 2 . I t was chosen because i t i s well-matched t o 1 4 the emission spectrum o f t h e F u s i o n "V" lamp w h i c h was used a s t h e source of i r r a d i a t i o n . The formulations used a r e summarized i n Table I. F o r m u l a t i o n s were p r e p a r e d b y d i s s o l v i n g t h e base r e s i n i n 100 p h r o f 2-butanone, t h e n a d d i n g t h e p h o t o i n i t i a t o r a n d a n y monomers t o t h e resin solution. F i l m s w e r e c a s t f r o m t h e r e s i n s o l u t i o n o n t o 7 5 urn polyethylene terephthalate (PET) s h e e t s , d r i e d i n a c o n v e c t i o n oven a t 60°C f o r 15 m i n u t e s t o remove r e s i d u a l s o l v e n t , and cured. A LESCO C612 conveyor system w i t h F u s i o n Systems F450-10 i r r a d i a t o r was used f o r curing. A 300 W/inch F u s i o n "V" lamp was used i n a l l experiments. Cure dose was v a r i e d by changing b e l t speed a t constant irradiator power.


285

RADIATION

286

CURING

OF POLYMERIC

MATERIALS

TEMPERATURE (°C) FIGURE 1: Schematic and annealed samples

r e p r e s e n t a t i o n o f DSC c u r v e s f o r q u e n c h e d showing s t r u c t u r a l r e l a x a t i o n e f f e c t s .

Absorption curve

Wave length

-- 10 mg/100 ml £ 0 . 0 1 % = 50 mg/100 m! £ 0.05% - 100 mg/100 ml £ 0 . 1 % measured in ethanol; cell thickness 1 cm FIGURE

2:

Absorption

spectrum

of

Darocure

1664

photoinitiator.


21.

RUSSELL &SKIENS


A f t e r i r r a d i a t i o n , s a m p l e s w e r e p e e l e d f r o m t h e PET substrates and a n a l y z e d b y q u a n t i t a t i v e d i f f e r e n t i a l t h e r m a l a n a l y s i s (DSC) and t h e r m o m e c h a n i c a l a n a l y s i s (TMA) u s i n g a M e t t l e r TA3000 t h e r m a l a n a l y s i s s y s t e m w i t h DSC30 a n d TMA40 m o d u l e s . TABLE

Is

Formulation

Comonomer

Epoxy

A c r y l a t e ( l ) Type(2) Level Novacure 3700 lOOphr

(1) (2)

(3)

None None TMPTA TMPTA PBDDA PBDDA PETA PETA DPEPA DPEPA

15phr 30phr 15phr 30phr 15phr 30phr 15phr 30phr

Parameters

Photoinitiator(3) Sample Level 2phr 5phr 2phr 2phr 2phr 2phr 2phr 2phr 2phr 2phr

Designation N37D2 N37D5 N37T15D2 N37T30D2 N37PB15D2 N37PB30D2 N37PE15D2 N37PE30D2 N37DP15D2 N37DP30D2

Bisphenol A epoxy d i a c r y l a t e (Interez, Inc.) TMPTA » T r i m e t h y l o l p r o p a n e triacrylate; PBDDA * P o l y b u t a n e d i o l diacrylate; PETA - p p e n t a e r y t h r i t o l triacrylate; DPEPA = d i p e n t a e r y t h r i t o l pentaacrylate P h o t o i n i t i a t o r = D a r o c u r e 1664 (EM Industries)

Samples f o r FTIR a n a l y s i s were c a s t from d i l u t e 2-butanone s o l u t i o n o n t o K B r p l a t e s , t h e n d r i e d f o r 15 m i n u t e s a t 6 0 ° C i n a c o n v e c t i o n oven. The a b s o r p t i o n s p e c t r a of t h e unexposed f i l m s were recorded f r o m 4 0 0 0 - 6 0 0 c m - 1 u s i n g a N i c o l e t 5DXB F T I R s p e c t r o m e t e r . Spectra w e r e o b t a i n e d a t 2 cm-1 r e s o l u t i o n u s i n g 128 c o - a d d e d s c a n s p e r sample. E a c h s a m p l e w a s t h e n i r r a d i a t e d a t 18 m / m i n a n d t h e n r e analyzed. T h i s p r o c e s s was r e p e a t e d u n t i l a t o t a l of six i r r a d i a t i o n s was a c c u m u l a t e d . Percent e x t r a c t i b l e s were determined by p e e l i n g cured f i l m s from the PET s u b s t r a t e , h e a t i n g e a c h s a m p l e i n a c o n v e c t i o n o v e n f o r 30 minutes at 60°C t o remove v o l a t i l e s , and w e i g h i n g t h e d r i e d f i l m . E a c h f i l m was t h e n p l a c e d i n a s c i n t i l l a t i o n v i a l and e x t r a c t e d with 2 - b u t a n o n e f o r 30 m i n u t e s on a w r i s t - a c t i o n s h a k e r . Films were removed i n t a c t from the s o l v e n t and d r i e d t o c o n s t a n t weight at 60°C in vacuo, then re-weighed to determine weight loss. Results

and

Discussion

A s i n g l e p a s s t h r o u g h t h e UV c u r e s y s t e m c o n v e r t e d a l l t h e f i l m s t o a tack-free state. DSC t h e r m o g r a m s w e r e o b t a i n e d o n s a m p l e s c u t from e a c h f i l m t o d e t e r m i n e t h e e f f e c t of c u r e dose and c o m p o s i t i o n on Tg and other p r o p e r t i e s . The DSC t h e r m o g r a m f o r S a m p l e N 3 7 D 2 , t h e base r e s i n w i t h o u t added comonomer, i s shown i n F i g u r e 3 ( a ) . The sample e x h i b i t s a broad g l a s s t r a n s i t i o n w i t h an onset at 30°C. At 41°C an a d d i t i o n a l endotherm i s observed, superposed on the g l a s s t r a n s i t i o n . The endotherm i s complete a t 63°C. The g l a s s t r a n s i t i o n i s c o m p l e t e at 90°C. When t h e s a m p l e i s q u e n c h e d a n d i m m e d i a t e l y r e h e a t e d , a broad g l a s s t r a n s i t i o n i s observed, extending from 28°C t o 92°C, but no endotherm i s o b s e r v e d , as seen i n F i g u r e 3 ( b ) . If the sample i s a l l o w e d t o a n n e a l a t 20°C f o r 25 h o u r s , t h e e n d o t h e r m i s a g a i n o b s e r v e d i n t h e DSC t h e r m o g r a m , but i t s area i s s i g n i f i c a n t l y


RADIATION CURING OF POLYMERIC MATERIALS


21. RUSSELL &SKIENS


reduced, as seen i n Figure 3 ( c ) . Similar behavior i s ex a l l the compositions tested. DSC d a t a f o r Tg o n s e t , m i d f i n a l temperature a r e summarized i n Table I I , along with the endotherm observed during the f i r s t heating of each samples were a n n e a l e d a t 20°C f o r a p p r o x i m a t e l y one week f i r s t DSC a n a l y s i s . TABLE I I :

Summary o f DSC R e s u l t s on Cured F i l m s CURED 1 X t 18m/min

FORMULATION Tg

Δ

Tg

Tg

A

42 48 46 49 47 48 41 43 42

First

58 69 74 74 60 59 66 54 72

3.4 3.0 2.7 2.2 1.9 2.3 2.2 2.7 2.4

Heating

50 43 40 47 43 44 39 44 35

Cured \2 Χ β 12 m/min sTg

Tg

C O C O (J/9) C O N37D2 N37T15D2 N37T30D2 N37PB15D2 N37PB30D2 N37PE15D2 N37PE30D2 N37DP15D2 N37DP30D2

hibited by point and the area of sample. A l l prior to the

CO CO 48 53 58 58 50 57 66 74 56

Second Htg

60 59 65 53 50 61 68 54 47

Pirst

60 67 82 61 71 72 79 59 37

Δ

H

tt

(J/g) 1.7 1.3 1.7 1.2 1.6 1.4 1.9 1.3 1.0

Beating

Tg

ATg

CO CO 60 59 64 62 49 62 68 54 57

64 57 78 90 79 72 91 80 34

Second Htg

The b r e a d t h o f t h e g l a s s t r a n s i t i o n c a n be e x p l a i n e d by t h e i m m i s c i b i l i t y of t h e a c r y l i c and epoxy components i n t h e f i l m . Since the two components would phase separate i f they were not c o v a l e n t l y linked, their glass transition reflects the relaxation spectra of the a c r y l i c component and t h e epoxy component. The endotherm observed i n annealed samples r e f l e c t s the s t r u c t u r a l rearrangement which i s occurring i n the glassy state. This type of endothermic t r a n s i t i o n i s f r e q u e n t l y observed i n aged o r annealed samples of amorphous 3 5 6 polymeric glasses. ' ' I t has been termed an "excess enthalpy" peak, and i s caused by s t r u c t u r a l o r d e r i n g and d e n s i f i c a t i o n w i t h i n t h e amorphous g l a s s . The presence i n t h e a c r y l a t e d epoxy f i l m s o f t h e a c r y l i c component, with a relaxation onset at or below ambient temperature, provides a mechanism f o r s t r u c t u r a l r e l a x a t i o n a t ambient conditions. In order to determine the rate of structural relaxation i n the cured f i l m s , Sample N37D2 was a n n e a l e d f o r v a r y i n g p e r i o d s a t 2 0 ° C a n d t h e n r e - a n a l y z e d by DSC. The a r e a of t h e e x c e s s e n t h a l p y peak was determined by a simple geometric i n t e g r a t i o n technique. A plot of the excess enthalpy, Hex, versus the r e c i p r o c a l of the annealing t i m e w a s c o n s t r u c t e d , a s s h o w n i n F i g u r e 4. Extrapolation to i n f i n i t e t i m e ( t - 1 = 0) p r e d i c t s t h a t S a m p l e N37D2 w o u l d h a v e a Hex of 1.8-1.9 J/g at equilibrium, with a t l / 2 for structural relaxation o f 25 hours a t 20°C. E x a m i n a t i o n o f t h e d a t a i n T a b l e I I shows that Hex d e c r e a s e s w i t h i n c r e a s i n g c u r e d o s e . A t l o w UV d o s e , the a d d i t i o n o f comonomer c a u s e s a d e c r e a s e i n t h e e x c e s s e n t h a l p y peak. A t h i g h e r UV d o s e , a d d i t i o n o f comonomer h a s l i t t l e o r n o e f f e c t o n Hex. T h e r m o m e c h a n i c a l a n a l y s i s (TMA) i s a v e r y u s e f u l t e c h n i q u e for determination of the properties of coatings and t h i n f i l m s . I n many cases i t c a n be used t o determine t h e Tg of a f i l m which i s t o o t h i n o r t o o a d h é r a n t t o t h e s u b s t a t e t o permit a n a l y s i s by DSC. However, i n t e r p r e t a t i o n o f TMA r e s u l t s c a n b e c o m p l i c a t e d b y t h e e f f e c t s of sample c u r l d u r i n g c u r i n g coupled w i t h volume r e l a x a t i o n effects. T h i s i s i l l u s t r a t e d b y t h e TMA t h e r m o g r a m o f S a m p l e N 3 7 T 1 5 D 2 , cured 2 X a t 12 m / m i n , w h i c h i s s h o w n i n F i g u r e 5 . Instead of a monotonie i n c r e a s e i n t h e r m a l e x p a n s i o n upon h e a t i n g , t h e a s - c u r e d sample shown i n Figure 5(a) e x h i b i t s a net decrease i n thickness beginning at 27°C and c o n t i n u i n g u n t i l 90°C. Above 90°C t h e r a t e o f t h e r m a l expansion


289

290



21.

RUSSELL &SKIENS

FIGURE

5:

TMA


thermograms

of

N37T15D2

cured

2

Χ

β

12m/min


291


292

increases and a net expansion occurs. The sample annealed 1500 hours at 20°C e x h i b i t s s i m i l a r behavior, a s shown i n F i g u r e 5 ( b ) . However, when t h e sample shown i n F i g u r e 5 ( b ) i s quenched from 130°C a n d i m m e d i a t e l y r e h e a t e d , no c o n t r a c t i o n i s o b s e r v e d , a s shown i n F i g u r e 5(c). The Tg determined by t h e i n t e r s e c t i o n of t h e g l a s s y state and rubbery s t a t e p o r t i o n s o f t h e thermogram i s 67°C f o r t h e quenched sample. T h i s i s s l i g h t l y h i g h e r than t h e v a l u e o f 59°C determined b y DSC. However, t h e range o f temperature over which t h e c o n t r a c t i o n occurs i n t h e annealed samples agrees w e l l with t h e range over which enthalpy r e l a x a t i o n i s observed by DSC. While quantitative e s t i m a t i o n o f e n t h a l p y r e l a x a t i o n i s n o t p o s s i b l e f r o m t h e TMA thermogram, Tg c a n be determined from t h e thermogram o f annealed thin f i l m specimens. Care must b e e x e r c i s e d i n i n t e r p r e t a t i o n o f t h e thermograms o f annealed samples, however, as structural relaxation w i l l be superposed on thermal expansion once t h e sample i s heated above t h e onset o f t h e g l a s s t r a n s i t i o n . S i n c e b o t h UV d o s e a n d t h e c o m p o s i t i o n o f t h e f i l m h a d a n e f f e c t o n the degree of s t r u c t u r a l r e l a x a t i o n observed, i t was important t o determine the effect of these variables on t h e extent of cure as well. The degree o f cure was determined by two methods: FTIR analysis t o determine t h e loss of v i n y l unsaturation as a function of c o m p o s i t i o n a n d UV d o s e , a n d s o l v e n t e x t r a c t i o n t o d e t e r m i n e t h e extent of crosslinking i n the cured film. The v i n y l u n s a t u r a t i o n peak a t 810 c m ' 1 was u s e d a s a measure o f t h e extent of reaction.15"18 V i n y l group disappearance was a l s o c o r r e l a t e d w i t h changes i n t h e i n t e n s i t y o f t h e a c r y l a t e H2C=CHs t r e t c h i n g i n t h e 1620-1640 cm"1r e g i o n . (17) FTIR s p e c t r a were o b t a i n e d a s a f u n c t i o n o f UV d o s e f o r s a m p l e s N37D2, N37D5 a n d N37T30D2. The spectrum o f each unexposed sample was s t o r e d and used for a l l subsequent analyses. Each sample was then exposed by p a s s i n g i t t h r o u g h t h e UV s o u r c e a t 18 m/min, a n d a new F T I R s p e c t r u m w a s obtained. Spectra were obtained a f t e r 1, 2 , 4 and 6 passes through the source. The spectrum of each exposed sample was then subtracted from t h e spectrum o f t h e unexposed sample. A t y p i c a l difference s p e c t r u m i s shown i n F i g u r e 6 . The peak a t 810 cm"1 i n t h e difference spectrum was then integrated using t h e software supplied with the spectrometer. Percent conversion was then c a l c u l a t e d a s :

% conversion

=

ΓASlOcm"1 ,η=0Ί - ΓASlOcm"1. n=l ,2.4.61 [A810cm"A, n=0]

R e s u l t s a r e summarized i n F i g u r e 7. I n a l l c a s e s , i n c r e a s e d UV exposure l e d t o increased conversion over t h e dose range studied. The r a t e o f c o n v e r s i o n a n d t h e maximum e x t e n t o f c o n v e r s i o n v a r i e d s i g n i f i c a n t l y among s a m p l e s . S a m p l e N37D2 h a d t h e l o w e s t c u r e rate and lowest extent o f conversion (40%). Increasing t h e photoinitiator l e v e l t o 5 p h r (Sample N37D5) i n c r e a s e d c u r e r a t e a n d i n c r e a s e d t h e e x t e n t o f c o n v e r s i o n t o 5 0 % . A d d i t i o n o f 3 0 p h r o f TMPTA t o a f o r m u l a t i o n w i t h 2 p h r o f p h o t o i n i t i a t o r was even more e f f e c t i v e i n increasing the cure rate and t h e f i n a l extent of conversion (57%). It i s i n t e r e s t i n g t o note, however, t h a t t h e maximum d e g r e e o f conversion observed f o r any sample was approximately 60%. There a r e two p o s s i b l e e x p l a n a t i o n s f o r t h i s : 1. 2.

t h e s a m p l e s c o n t a i n some s p e c i e s i n a d d i t i o n t o t h e v i n y l group which h a s an absorbance a t 810 c m " 1 ; o r a s c u r i n g p r o c e e d s a s i g n i f i c a n t number o f t h e unsaturated species i n t h e system a r e immobilized and a r e unable t o react further.

T h e f a c t t h a t t h e a b s o r b a n c e o f t h e a c r y l a t e g r o u p i n t h e C=C s t r e t c h i n g region p a r a l l e l s changes i n t h e v i n y l group absorbance


ν© N37D5 Sample spectrum, difference FTIR 6s FIGURE

ο Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


294

Δ • ο

N37T30D2 N37D5 N37D2

0 UV D o s e FIGURE

7:

(n p a s s e s

% Conversion

at

(by FTIR)

18

m/min)

versus

UV

dose


21.

RUSSELL &SKIENS

Enthalpy Relaxation in UV-Cured Epoxy Coating?

tends to discount the former explanation. Recent work on polymerization of methyl methacrylate to high degrees of conversion i n undiluted systems tends to support the l a t t e r e x p l a n a t i o n . 1 9 In the case of bulk polymerization of methyl methacrylate at 0°C, a r a p i d i n c r e a s e i n p o l y m e r i z a t i o n was o b s e r v e d as t h e v i s c o s i t y o f the system increased (the well-known Trommsdorff e f f e c t ) . However, as the reaction proceeded to higher degrees of conversion (approximately 70%), monomer m o b i l i t y i n t h e s y s t e m d e c r e a s e d , p o l y m e r i z a t i o n rate d e c r e a s e s , and c o m p l e t e c o n v e r s i o n was n e v e r o b t a i n e d . In UV-curing s y s t e m s we m i g h t e x p e c t t o s e e s i m i l a r b e h a v i o r , e s p e c i a l l y w h e n m u l t i f u n c t i o n a l monomers a r e u s e d . As soon as one of the unsaturated groups i s incorporated into a growing chain, the p r o b a b i l i t y that the other unsaturated groups i n the molecule w i l l encounter either another c h a i n r a d i c a l or an a c t i v e p h o t o i n i t i a t o r fragment are reduced. As cure proceeds, the f r a c t i o n of unsaturated s p e c i e s remaining which are capable of d i f f u s i o n over s i g n i f i c a n t distances becomes v a n i s h i n g l y s m a l l . Even i f they encounter an a c t i v e i n i t i a t o r fragment, the probability that they can participate i n a r e a c t i o n w i t h a n o t h e r monomer g r o u p i s t h u s d e c r e a s e d , a n d t h e r e a c t i o n stops w e l l short of complete conversion. T h i s may a c c o u n t for the free r a d i c a l s observed i n cured a c r y l a t e d epoxy f i l m i n a 1 5 recent study. Data f o r percent e x t r a c t i b l e s versus cure dose are summarized i n Table III f o r S a m p l e N37D2 a n d N37T30D2. The maximum w e i g h t loss o b s e r v e d was 2%, even a t low c u r e dose and low p h o t o i n i t i a t o r concentration. This i n d i c a t e s that the samples are highly c r o s s l i n k e d , even though t h e i r FTIR s p e c t r a i n d i c a t e that they contain significant residual unsaturation. Loss of m o b i l i t y of the reactive groups during cure e f f e c t i v e l y l i m i t s the degree of conversion, even though e s s e n t i a l l y a l l of the low molecular weight s p e c i e s have been i n c o r p o r a t e d i n t o the network. TABLE

I l l s

Butanone E x t r a c t i b l e s versus A c r y l a t e d Epoxy Films

UV

Formulation N37D2

N37T30D2

(1) (2)

Dose

(1)

Cure

Dose

for

% Extracted

1 1 2 2

X X X X

§ § § §

18m/min 12m/min 12m/min 12m/roin

0.8% 1.6% 0.8% 0.5%

1 1 2

X § X § X §

18m/min 12m/min 12m/min

2.0% 1.1% 2.0%

Films were cured u s i n g Fusion F450-10 lamp 30 m i n u t e e x t r a c t i o n w i t h 2-butanone

irradiator

(2)

with

"V"

Conclusions A c r y l a t e d epoxy systems undergo s i g n i f i c a n t changes i n thermomechanical p r o p e r t i e s on s t a n d i n g at room t e m p e r a t u r e . Other s t u d i e s 1 5 ' 2 0 have shown t h a t f r e e r a d i c a l s a r e v e r y l o n g - l i v e d i n UVcured systems kept under i n e r t atmospheres, and t h a t polymerization can c o n t i n u e f o r extended p e r i o d s due t o t h e low m o b i l i t y of the residual unsaturated species at higher conversions. In t h i s study, however, the f i l m s were exposed to a i r immediately a f t e r i r r a d i a t i o n , which should e f f e c t i v e l y quench the residue free radicals. Therefore, these changes i n p r o p e r t i e s cannot be f u l l y accounted for


295

296


by changes i n u n s a t u r a t i o n or c h e m i c a l s t r u c t u r e , since they are thermally reversible. The most p r o b a b l e s o u r c e o f t h e changes observed i n t h i s study i s structural relaxation i n the cured f i l m s . The b r o a d g l a s s t r a n s i t i o n s e x h i b i t e d by a l l samples i n d i c a t e a degree of i n c o m p a t i b i l i t y between the a c r y l i c and epoxy components of the system studied. The b r o a d r e l a x a t i o n , w h i c h e x h i b i t s an o n s e t at or near ambient temperature, provides a mechanism by which annealing can occur at ambient temperature, even though the midpoint of the Tg i s 50-70°C. The maximum e x t e n t o f a c r y l i c g r o u p r e a c t i o n o b s e r v e d a f t e r c u r e was approximately 6 0 % , e v e n t h o u g h t h e f i l m s c o n t a i n e d l e s s t h a n 2% extractible material. This indicates that a s i g n i f i c a n t fraction of the a c r y l i c groups are s t e r i c a l l y hindered and unable t o be incorporated into a c t i v e network chains. Instead they act as additional chain ends, i n c r e a s i n g free volume w i t h i n the cured f i l m . This provides an a d d i t i o n a l d r i v i n g force for s t r u c t u r a l relaxation during annealing. The a b i l i t y of a c r y l a t e d epoxy f i l m s t o undergo s t r u c t u r a l relaxation at ambient c o n d i t i o n s has s i g n i f i c a n t i m p l i c a t i o n s on such f i l m p r o p e r t i e s as impact s t r e n g t h , adhesion, hardness and abrasion resistance. I n c o m p a r i n g p r o p e r t i e s among f o r m u l a t i o n s , i t i s important t h a t a l l the f i l m s or c o a t i n g s t e s t e d be subjected t o the same t h e r m a l h i s t o r y p r i o r t o a n a l y s i s . While quenched samples w i l l y i e l d t h e most c o n s i s t e n t r e s u l t s , t h e i r p r o p e r t i e s w i l l d i f f e r s i g n i f i c a n t l y from those of annealed or aged f i l m s . Acknowledgments The for and

a u t h o r s w i s h t o t h a n k M r . R o b e r t G. Marx and M s . t h e i r h e l p i n p r e p a r a t i o n of formulations and i n thermal analysis measurements.

L a u r a T. Onstott performing FTIR

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15. 16. 17. 18. 19. 20.

Gaube, H.G. RadCure '86: Conf. Proc., 15-27(1986) Lapin, S.C. RadTech '88: Conf. Proc., 395(1988) Tribone, J.J.; O'Reilly, J.M.; Greener, J. Macromolecules, 19, 1732(1986) Kovacs, A.J. Fortschr. Hochpolym.-Forsch. 3, 394(1963) Marshall, A.S.; Petrie, S.E.B. J.Appl.Phys. 46, 4223(1975) Berens, A.R.; Hodge, I.M. Macromolecules 15, 756(1982) Hodge, I.M.; Berens, A.R. Macromolecules 15, 762(1982) Hodge, I.M.; Huvard, G.S. Macromolecules 16, 371(1983) Hodge, I.M. Macromolecules 16, 898(1983) Hodge, I.M. Macromolecules 20, 2897(1987) Privalko, V.P.; Demchenko, S.S.; Lipatov, Y.S. Macromolecules 19, 901(1986) Krzewski, R.J.; Labovitz, M.; Sieglaff, C.L., pp.67-76 in "Structure and Properties of Amorphous Polymers", A.G. Walton (Ed.), Studies in Physical and Theoretical Chemistry, Volume 10, Elsevier Scientific Publishing Company, Amsterdam, 1980. Flick, J.R.; Petrie, S.E.B., pp.145-171, ibid. Levine, L.S.; Ury, M.B. RadCure '86: Conf. Proc., 1-1 (1986) Decker, C.; Moussa, Κ. Polymeric Material Science and Engineering: Proceedings, 55, 552 (1986) Kosnik, F.J.; Schweri, R.J. RadCure '86: Conf. Proc., 921(1986) Bellamy, L.J. "Alkenes", Chapter 3 in The Infrared Spectra of Complex Molecules, Volume One, Third Edition, Chapman and Hall, London, 1975. Anderson, D.G. RadTech '88: Conf. Proc., 513(1988) Sack, R.; Schulz, G.V.; Meyerhoff, G. Macromolecules 21, 3345(1988) Kloosterboor, J.G., "Advances in Polymer Science", Volume 84, "Polymers in Electronics, pp 46-59 (1988).

RECEIVED

September 13,

1989


Enthalpy Relaxation in UV-Cured Epoxy Coatings - ACS Symposium

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