Ultraviolet Light Induced Reactions in Polymers

10 Ultraviolet Curing of Pigmented Coatings VINCENT D. McGINNISS

Downloaded by UNIV LAVAL on October 15, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch010

Glidden-Durkee, Div. of SCM Corp., Strongsville, Ohio 44136

It has now been well established that UV-curing of coatings can be achieved and there have been many references in the l i t e r a t u r e as to the development and descriptions of these coatings and the UV-curing art. (1,2) Most of these l i t e r a t u r e references describe commercial curing of clear f i n i s h e s as well as UVcurable inks. (3-6) This paper explores some of the parameters associated with non-ink low v i s c o s i t y pig mented UV-curable coatings. Pigmentation E f f e c t s on UV-Curable Coatings. Pig ments cannot be thought of as inert additives i n the UV-curing of opaque or colored coatings. The folowing are some of the considerations on the e f f e c t s of p i g ments during the curing process: (1) (2) (3) (4) (5) (6)

Light scattering (internal) or external r e f l e c t ance) and penetration of energy. (4) Refractive index and wavelength of l i g h t absorp tion of the pigment. Free r a d i c a l c a t a l y t i c a c t i v i t y and v i s c o s i t y effects. P a r t i c l e size and degree of dispersion. Amount of pigment and f i l m thickness in r e l a t i o n to hiding power and effects on cure speed. Photophysical properties of pigments. (7)

The r e f r a c t i v e index of a pigment at each wave length i s determined by i t s c r y s t a l structure. T i t a n ium dioxide pigments ( r u t i l e and anatase) d i f f e r from each other and from other white pigments or c r y s t a l l i n e substances, l i k e s i l i c o n dioxide, i n the proportion of radiant energy that i s transmitted, absorbed, or re135 In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


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f l e e t e d at each wavelength by a f i l m i n which i t i s contained. The maximum energy a b s o r p t i o n of Si02 i s about 150 to 200 nm and the maximum f o r anatase Ti02 i s i n the near UV r e g i o n and r u t i l e Ti02 i s nearer to the v i s u a l p o r t i o n of the spectrum ( F i g u r e s 1 and 2 ) . R e f r a c t i v e index i s h i g h e s t at a wavelength s l i g h t l y l o n g e r than the wavelength of maximum a b s o r p t i o n and d e c r e a s e s as the wavelength energy i n c r e a s e s ( F i g u r e 1). The o p t i c a l p r o p e r t i e s of r u t i l e and anatase Ti02 are d i f f e r e n t i n t h a t anatase i s more r e f l e c t i v e than r u t i l e at 380 nm and r u t i l e i s v e r y s t r o n g l y UV-absorbent at 380 nm ( F i g u r e 2).(JL) The o p t i c a l p r o p e r t i e s of the v a r i o u s pigments s t r o n g l y e f f e c t p h o t o s e n s i t i z e r a b s o r p t i o n and p h o t o c h e m i c a l a c t i v i t y . P h o t o s e n s i t i z e r s or P h o t o i n i t i a t o r s . The p r i m a r y component i n the U V - c u r a b l e c o a t i n g system i s the p h o t o s e n s i t i z e r or p h o t o i n i t i a t o r (a l i g h t s e n s i t i v e c a t a l y s t t h a t upon a b s o r p t i o n of energy r e s u l t s i n a f r e e r a d i c a l s p e c i e s t h a t can i n i t i a t e a c r y l i c monomer p o l y m e r i z a t i o n ) . (2) The f i r s t r u l e of p h o t o c h e m i s t r y i s t h a t i n order f o r a l i g h t s e n s i t i v e r e a c t i o n to take p l a c e , l i g h t energy, a t the a p p r o p r i a t e wavelength, must be a b s o r b ed by the r e a c t i n g m o l e c u l e ( p h o t o i n i t i a t o r ) . (i2.) In opaque or c o l o r e d c o a t i n g s t h e r e i s c o m p e t i t i o n between pigment and p h o t o c a t a l y s t i n t h a t the p h o t o i n i t i a t o r must (1)

(2)

absorb l i g h t energy i n the same r e g i o n of the ab s o r p t i o n spectrum as the pigment. In t h i s case, the molar e x t i n c t i o n c o e f f i c i e n t ( ε ) of the photoi n i t i a t o r must be l a r g e or a l a r g e c o n c e n t r a t i o n of p h o t o s e n s i t i z e r i s n e c e s s a r y to e f f e c t photo polymerization . not absorb l i g h t energy, or have a d i f f e r e n t ab s o r p t i o n spectrum, i n the same r e g i o n as the p i g ment. In t h i s case, the molar e x t i n c t i o n c o e f f i c i e n t of the p h o t o i n i t i a t o r may be s m a l l and s m a l l e r c o n c e n t r a t i o n s of p h o t o i n i t i a t o r s can be used.

In each of these c a s e s , the t h i c k n e s s of the c o a t i n g f i l m i s v e r y important ( i n k s 2 to 10 microns v e r s u s a f l u i d c o a t i n g of 0.5 to 2 m i l s ) on the amount of l i g h t energy absorbed by the p h o t o i n i t i a t o r . The amount of pigment, d i s p e r s i o n , and r e f l e c t a n c e of the p a i n t f i l m are a l s o important f o r l i g h t a b s o r p t i o n by the p h o t o c a t a l y s t .

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

UV Curing of Pigmented Coatings


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WAVELENGTH, MILUMICRONS Figure 2.


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In F i g u r e 3 are the p l o t s of a b s o r p t i o n v e r s u s wavelength of f o u r p h o t o i n i t i a t o r s used i n the UVc u r i n g of pigmented c o a t i n g s : M i c h l e r ' s Ketone d e r i v a t i v e s 4 , 4 - b i s ( d i e t h y l a m i n o ) benzophenone (DEABP), λ max 3 5 2 nm, ε = 4 0 , 7 0 0 ; Thioxanthone (TX), λ max 3 6 6 and 3 7 8 nm, 1


ε - 5 , 2 0 0 and 6,100 2-Chlorothioxanthone ε = 4 , 0 4 0 and 4,650

( 2 C T X ) , λ max

370

and

385

nm,

Benzanthrone (BZA), λ max 3 8 0 nm, ε - 1 0 , 9 0 0 In F i g u r e 4 one can see a comparison of photo s e n s i t i z e r a b s o r p t i o n λ max superimposed onto the UVv i s i b l e a b s o r p t i o n - r e f l e c t a n c e s p e c t r a of r u l t i l e t i tanium d i o x i d e . M i c h l e r ' s Ketone d e r i v a t i v e s (DEABP) absorb energy i n the same r e g i o n as the v e r y s t r o n g U V - a b s o r p t i o n of r u t i l e T 1 O 2 but these compounds have l a r g e e x t i n c t i o n c o e f f i c i e n t s i n t h i s r e g i o n and are a c c e p t a b l e p h o t o c a t a l y s t s f o r c u r i n g of a pigmented coating. Thioxanthone d e r i v a t i v e s (TX and 2 C T X ) have λ max v a l u e s near the a b s o r p t i o n edge of r u t i l e T 1 O 2 w h i l e benzanthrone (BZA) i s o u t s i d e of t h i s a b s o r p t i o n edge and absorb energy i n the v i s i b l e r e g i o n of the spectrum. A l l of these p h o t o a c t i v e compounds can be used to cure opaque c o a t i n g s systems. Some o t h e r f a c t o r s to c o n s i d e r i n the p h o t o c u r i n g of c o a t i n g s are s e l f quenching of p h o t o i n i t i a t o r s at h i g h c o n c e n t r a t i o n s and the v a r i o u s e f f e c t s of l i g h t i n t e n s i t y as w e l l as s p e c t r a l output of the l i g h t source. P i g m e n t a t i o n may a l s o quench p h o t o s e n s i t i v e i n i t i a t i o n r e a c t i o n s through energy t r a n s f e r of photop h y s i c a l d e a c t i v a t i o n pathways as w e l l as r a d i c a l t e r mination r e a c t i o n s . A f t e r l i g h t a b s o r p t i o n by the p h o t o c a t a l y s t * the next step i s p h o t o p r o d u c t i o n of f r e e r a d i c a l s t h a t i n i t i a t e p o l y m e r i z a t i o n of the a c r y l a t e u n s a t u r a t e d monomers, o l i g o m e r s , and polymers c o n t a i n e d i n the U V - c u r a b l e c o a t i n g system. Mechanisms of P h o t o i n i t i a t i o n 0

0

0

0



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,200 ,100

'4,040 4,650

1

340

I

1

I

I

360

380

WAVELENGTH

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I

1

400

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Figure 3.


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140

Some b a s i c (1) (2)

mechanisms of p h o t o i n i t i a t i o n a r e :

D i r e c t photofragmentation into free r a d i c a l s . E l e c t r o n t r a n s f e r f o l l o w e d by p r o t o n t r a n s f e r and f o r m a t i o n of f r e e r a d i c a l s .

In d i r e c t p h o t o f r a g m e n t a t i o n , the t h i o x a n t h o n e d e r i v a t i v e absorbs l i g h t at about 380 nm which r e s u l t s i n h o m o l y t i c c l e a v a g e of the methylene-halogen or s u l f o n y l - h a l o g e n bonds of the parent m o l e c u l e to produce



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free r a d i c a l species, ( i i ) In e l e c t r o n t r a n s f e r r e a c t i o n s , the t h i o x a n t h o n e d e r i v a t i v e s absorb l i g h t at 380 nm and then forms an e x i p l e x i n which an e l e c t r o n i s t r a n s f e r r e d from the n i t r o g e n atom to the e x c i t e d c a r b o n y l of the t h i o x a n thone. T h i s e x i p l e x then undergoes p r o t o n t r a n s f e r f o l l o w e d by p r o d u c t i o n of i n i t i a t i n g f r e e r a d i c a l s . ( 1 2 ) M i c h l e r ' s ketone d e r i v a t i v e s absorb UV-energy a t 360 nm and forms an e x i p l e x , through e l e c t r o n t r a n s f e r of the n i t r o g e n atom, w i t h a ground s t a t e benzophenone molecule. T h i s e x i p l e x then decomposes i n t o f r e e r a d i c a l s p e c i e s v i a p r o t o n t r a n s f e r and r e d u c t i o n of the benzophenone i n t o a b e n z p i n a c o l r a d i c a l , (QSC)x 360 T

nm F i l t e r

or pigment

(TX)i

65+

E

(kcal/mole) 60+

free radical formation

(QSC)

(TX) Thioxanthone (TX) (TX)Q

(TX) * 3

hv

donor

+

œ

Quinoline S u l fonyl Chloride (QSC)

(TX)3* e x c i t e d

(ground

state

state

donor

acceptor)

>

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Sooci "excited acceptor Transfer 2

Energy

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142

POLYMERS

In t h i s type of energy t r a n s f e r r e a c t i o n , t h i o xanthone (TX) i s the o n l y a c t i v e l i g h t a b s o r b i n g s p e c i e s (cut o f f f i l t e r s or t i t a n i u m pigments s t r o n g l y absorb r a d i a t i o n below 360 nm) c o n t a i n e d i n the r e a c t i v e p a i n t system. Thioxanthone alone does not p h o t o i n i t i a t e p o l y m e r i z a t i o n of v i n y l u n s a t u r a t e d monomers. Q u i n o l i n e s u l f o n y l c h l o r i d e (QSC) absorbs l i g h t at a p p r o x i m a t e l y 310-330 nm which r e s u l t s i n h o m o l y t i c cleavage of the s u l f o n y 1 - c h l o r i d e bond to produce i n i t i a t i n g f r e e r a d i c a l s p e c i e s ( c l e a r r e a c t i v e systems). In f i l t e r e d or pigmented c o a t i n g systems a l l l i g h t t r a n s m i t t e d at 310-330 nm i s absorbed by the f i l t e r s or pigment so d i r e c t a b s o r p t i o n or e x c i t a t i o n by QSC i s not a l l o w e d . The o n l y way a p h o t o c h e m i c a l i n i t i a t i o n can take p l a c e i s through the l i g h t a b s o r p t i o n of energy (370-380 nm) by TX which can then t r a n s f e r i t s absorbed energy to a ground s t a t e QSC m o l e c u l e and r e s u l t i n f r e e r a d i c a l f o r m a t i o n (see diagram ) . < i l ) The t r i p l e t energy f o r TX i s 65 k c a l / m o l e and the t r i p l e t energy f o r QSC i s 60 k c a l / m o l e (ΔΕ = 5 k c a l / mole e x c e s s ) which meets one of the b a s i c r e q u i r e m e n t s f o r e f f i c i e n t energy t r a n s f e r i . e . Et donor (TX) > E a c c e p t o r (QSC). (±1> t

Experimental The same e x p e r i m e n t a l c o n d i t i o n s were used as p r e v i o u s l y d e s c r i b e d i n r e f e r e n c e 13 f o r the photo p o l y m e r i z a t i o n of m e t h y l m e t h a c r y l a t e (MMA) with 4,4 b i s ( d i e t h y l a m i n o ) benzophenone (DEABP) l x l 0 ~ M i n com b i n a t i o n w i t h 3X10~2M benzophenone. The d i l a t o m e t r i c i r r a d i a t i o n equipment i s shown i n F i g u r e 5. This apparatus a l l o w s the f o l l o w i n g of r a t e s of p h o t o p o l y m e r i z a t i o n (Rp) of MMA i n the presence of v a r i o u s con c e n t r a t i o n s of pigments and a l s o a l l o w s measurement of changes i n l i g h t i n t e n s i t y ( I ) as % t r a n s m i t t a n c e ( Τ ) . The pigment used i n these experiments was u n t r e a t e d c h l o r i d e process r u t i l e titanium dioxide (Glidden P i g ments D i v i s i o n , B a l t i m o r e , Md.). T h i s was o b t a i n e d as a water s l u r r y (pH 1-3) and was screened b e f o r e u s i n g . U l t r a v i o l e t s p e c t r a of v a r i o u s p h o t o s e n s i t i z e r s ( i n THF) were taken w i t h a P e r k i n Elmer 350 spectrophoto meter . f

2

0

Results

and

Discussion

One of the u n u s u a l f e a t u r e s of the U V - c u r i n g of pigmented c o a t i n g s i s t h a t the apparent r a t e of p o l y m e r i z a t i o n (Rp) appears to be somewhat f a s t e r than the



10.

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Curing of Pigmented Coatings

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conventional clear coatings. One example i n the l i t e r a t u r e i s the thermal and p h o t o c h e m i c a l p o l y m e r i z a t i o n s of MMA i n the p r e s e n c e of s i l i c a g e l pigments. These r e s u l t s show t h a t the a d d i t i o n of c o l l o i d a l s i l i c a to MMA markedly d e c r e a s e s the time r e q u i r e d to complete the p o l y m e r i z a t i o n . These e f f e c t s c o u l d be caused by the change i n v i s c o s i t y of the d i s p e r s i o n due to the s i l i c a because an i n c r e a s e i n v i s c o s i t y would be expected to reduce the r a t e c o n s t a n t ( k t ) f o r t e r m i n a t i o n and e f f e c t the o v e r a l l r a t e of p o l y m e r i z a t i o n (Rp k t - 1 / 2 ) . The a c c e l e r a t i o n of the r a t e of p o l y m e r i z a t i o n or c o n v e r s i o n was shown to occur at c o n c e n t r a t i o n s of s i l i c a where t h e r e was l i t t l e e f f e c t on v i s c o s i t y of the d i s p e r s i o n . Another mechanism i s the s t a b i l i z a t i o n of f r e e r a d i c a l s on the pigment s u r f a c e which reduces mutual t e r m i n a t i o n r e a c t i o n s of growing r a d i c a l c h a i n s but the Authors found no e v i dence of polymer g r a f t i n g onto the s i l i c a s u r f a c e . The s i l i c a pigment was thought to e x e r t some s o r t of c a t a l y t i c e f f e c t on the r a t e of p o l y m e r i z a t i o n . (jJi) In the p h o t o p o l y m e r i z a t i o n of MMA i n the presence* of r u t i l e T i 0 i t was n o t i c e d ( F i g u r e 6) t h a t as one i n c r e a s e s pigment c o n c e n t r a t i o n t h e r e i s a c o r r e s p o n d ing d e c r e a s e i n l i g h t t r a n s m i s s i o n ( I ) through the reaction c e l l . The R f o r the r e a c t i o n shows a s l i g h t i n c r e a s e up to a l i m i t i n g v a l u e of pigment c o n c e n t r a t i o n at which p o i n t the R d e c r e a s e s c o r r e s p o n d i n g to the d e c r e a s e i n I . T h i s r e l a t i v e i n c r e a s e of Rp at d e c r e a s i n g Io c o u l d be a measure of c a t a l y t i c a c t i v i t y or l i g h t r e f l e c t a n c e and s c a t t e r i n g of the pigment. C l e a r or pigmented l i q u i d p h o t o c u r a b l e coatings can be a p p l i e d to a f l e x i b l e s u b s t r a t e ( m e t a l , p o l y e t h y l e n e or f i l t e r paper) and the r e l a t i v e r a t e of cure or network f o r m a t i o n as a f u n c t i o n of changes i n dynamic m e c h a n i c a l p r o p e r t i e s v e r s u s exposure time can be determined. F i g u r e s 7 and 8 c o n t a i n a diagram and the energy c a l c u l a t i o n s f o r a simple f l a s h lamp c i r c u i t . This xenon f l a s h lamp ( L l ) was used to cure f i l m s on a t o r s i o n pendulum ( F i g u r e 9 ) . The theory and manufact ure of c o n v e n t i o n a l t o r s i o n a l pendulums have been des c r i b e d elsewhere but a simple diagram and b a s i c c a l c u l a t i o n s f o r the pendulum are shown i n F i g u r e s 9 and 1 0 . ? ' Both pigmented and c l e a r c o a t i n g s were cured by t h i s t e c h n i q u e e i t h e r w i t h f l a s h or steady s t a t e i l l u m i n a t i o n from a 450watt medium p r e s s u r e mer cury lamp ( L i ) . F i g u r e 11 shows a sample t r a c e of the t o r s i o n a l pendulum b e f o r e and a f t e r i r r a d i a t i o n . Be f o r e i r r a d i a t i o n the c o a t i n g i s a f l u i d v i s c o u s l i q u i d and e x h i b i t s a h i g h degree of damping (peak to peak 2

0

p

p

0

1


UV L I G H T INDUCED REACTIONS IN

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TO RECORDER

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CONSTANT TEMPERATURE BATH MAGNETIC STIRRER Figure 5.

I

3

5

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[PIGMENT] X

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I0 (g) 2

Figure 6.


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V —VOLTMETER S 3 - HIGH VOLTAGE SWITCHES S - SPARK GAP C — 20 KV 10 )JF CAPACITOR L - XENON FLASH LAMP XENON CORP h

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C - S T O R A G E CAPACITY IN FARADS V - V O L T A G E ON C IN VOLTS C= IO]JF V* 5 0 0 0 VOLTS Ε = 125 J

P - A V E R A G E POWER DELIVERED TO FLASH TUBE F - F L A S H E S PER SECOND FLASH DURATION TO 5 0 % PEAK s 2 0 4 0 JJ SEC. QUANTA FLASH ( 2 0 0 - 4 0 0 nm) S 10 QUANTA Figure 8.




LOWER SAMPLE HOLDER Figure 9.

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Δ-LOG DECREMENT Figure 10.


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Figure 12.


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UV L I G H T INDUCED REACTIONS

IN POLYMERS

d i s t a n c e s a r e l a r g e and t h e r e i s a l a r g e r a p i d d e c r e a s e i n a m p l i t u d e ) . A f t e r i r r a d i a t i o n the c o a t i n g under goes c u r i n g which i n f l u e n c e s the r e l a t i v e r i g i d i t y of the sample and the degree of damping becomes lower (peak t o peak d i s t a n c e s a r e s h o r t e r and t h e r e l a t i v e change i n amplitude of the waveform i s s m a l l e r or shows l e s s damping).(19) In F i g u r e 12 i s a t y p i c a l c u r i n g curve f o r a c o a t ing as a f u n c t i o n of r e l a t i v e r i g i d i t y (K V p ) , l o g decrement (£n Σ /Am+1) and f l a s h i r r a d i a t i o n . The c o a t i n g (O.lg sample), 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 (PETA) s e n s i t i z e d w i t h 2% η-butyl e t h e r of b e n z o i n (BEB) or 2% benzophenone (BP)/3% m e t h y l d i e t h a n o l a m i n e (MDEOA), was cured between t h i n p o l y e t h y l e n e s h e e t s and as t h e c u r i n g r e a c t i o n took p l a c e , r e l a t i v e r i g i d i t y stayed f a i r l y constant u n t i l a f i n a l breaking point was reached a f t e r 5 i r r a d i a t i o n f l a s h e s (5,000 v o l t s / flash). A damping i n c r e a s e o c c u r r e d a f t e r i n i t i a l r a d i a t i o n then d e c r e a s e d r a p i d l y at t h e onset of further gelation. In pigmented c o a t i n g s the r e l a t i v e i n c r e a s e of change i n r i g i d i t y or d e c r e a s e i n damping as a f u n c t i o n of exposure time f o r d i f f e r e n t pigments a t e q u a l PVC f o l l o w t h e o r d e r : S 1 O 2 > anatase Ti02 > r u t i l e T i 0 2 . The r e l a t i v e changes i n r i g i d i t y (K 1/p ) or l o g de crement a r e a measure of cure f o r t h e c o a t i n g . The d a t a a t t h i s time i s o n l y q u a l i t a t i v e but e f f o r t s a r e b e i n g made to develop t h e t o r s i o n a l pendulum-UV-curing t e c h n i q u e s and to q u a n t i f y r e l a t i o n s h i p s between r e l a t i v e r i g i d i t y , damping, g e l - p o i n t , v i t r i f i c a t i o n , and i r r a d i a t i o n i n t e n s i t y or exposure time f o r c l e a r as w e l l as pigmented p h o t o c u r a b l e c o a t ings . F u r t h e r work i s planned t o determine t h e exact parameters a s s o c i a t e d w i t h p h o t o c u r i n g of pigment c o a t i n g s and the development of more e f f i c i e n t p h o t o i n i t i ators. 2


An

2

References 1. 2. 3.

4.

K i n s t l e , J . , Paint Varnish Prod., 17, June (1973). Special Radiation Cure, Issue Paint Varnish Prod., August (1974). Bassemir, R. W. and Bean, A. J . , paper presented at 26th Annual Meeting of TAGA, St. Paul, Minn., May 13-15, 1974; to be published i n TAGA Pro ceedings. Wicks, Z. W., paper presented at 14th Annual Coatings Symposium, North Dakota State u n i v e r s i t y , Fargo, North Dakota, June 3-5, (1974).


10. MCGiNNiss

5. 6. 7. 8. 9. 10.


11. 12. 13. 14. 15. 16. 17. 18. 19.


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Vanderhoff, J . W., i b i d . Huemmer, T. F., Journal of Radiation Curing, 9, July (1974). A l l e n , Ν. S., et al, Polymer Letters, 12, 241 (1974). Patterson, D., "Pigments", E l s e v i e r Publishing Co., New York, (1967). McGinniss, V. D., and Dusek, D. Μ., J . Paint Tech., 46, 23 (1974). Turro, N. J . , "Molecular Photochemistry", W. A. Benjamin, Inc., New York, (1967). McGinniss, V. D., U. S. Patent 3,827,956; 3,827,957; 3,827,958; 3,827,959; 3,827,960 (1974). Sander, M. R., Osborn, C. L., and Trecker, D. J . , J. Polymer S c i . , 10, 3173 (1972). McGinniss, V. D., and Dusek, D. M., Am. Chem. Soc. Div. Polymer Chem. Preprints 15, 480 (1974). McGinniss, V. D., Paper presented at 14th Annual Coatings Symposium, North Dakota State University, Fargo, North Dakota, June 3-5, (1974). McGinniss, V. D., U. S. Patent 3,857,769 (1974). Manley, T. R., and Murray, B., European Polymer Journal, 8, 1145 (1972). Pierce, P. E., and Holsworth, R. Μ., J . Paint Tech., 38, 263 (1966). Nielsen, L. E., "Mechanical Properties of Poly mers", Reinhold Publishing Corp., New York, (1962). Gillham, J . Κ., J . Macromol. S c i . Phys., 89 (2), 209 (1974).


Ultraviolet Light Induced Reactions in Polymers

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