Chapter 1 Radiation curing.pdf

7/25/2019 Chapter 1 Radiation curing.pdf

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MNL45 EB/Apr. 2002

Radiation uring

n t r o d u c t i o n

EFORETHE EARLY1 97 0S , t h e r e w a s l it tl e a p p a r e n t c o n c e r n o r e v e n a w a r e n e s s

o f t h e a m o u n t o f s o l v e n t th a t e n t e r e d t h e a t m o s p h e r e f r o m c o a t i n g s a n d a l-

l i e d p r o d u c t s . A s a r e s u l t , i n t h e e a r l y 1 9 7 0 s o v e r 9 0 % o f a p p l i e d i n d u s t r i a l

c o a t i n g s w e r e l o w s o l id s i n n a t u r e . T h a t i s, t h e y c o n t a i n e d f r o m 1 0% t o 20 %

b y w e i g h t s o li d s, e x c l u d i n g p i g m e n t s o r o t h e r s u s p e n d e d m a t e r ia l s, a n d t h e

r e m a i n d e r w a s s o l v e n t. I n d u s t ri a l c o a t i n g s a t t h a t t i m e w e r e t y p i f i e d a s b e -

i n g " l o w s o l id s a n d s o l v e n t b o r n e . " A r c h i t e c tu r a l c o a t i ng s , c o m m o n l y

k n o w n a s h o u s e p a i n t s a n d v a r n i s h , w e r e a ls o s o l v e n t b o r n e , a n d l a te x e s

w e r e r e a l ly j u s t c o m i n g i n t o c o m m e r c i a l r ea li ty . In t h o s e d a y s , m a n y p e o p l e

s a i d th a t l a te x e s d i d n o t h a v e t h e q u a l i t y o f o i l - b a s e d p a i n t s - - m a n y l at ex

p a i n t s d i d n o t . T o d a y , la te x h o u s e p a i n t s a r e p r e v a l e n t , b u t b e c a u s e o f t h ei r

e x c e l l e n t a p p l i c a t i o n c h a r a c t e r i s ti c s a n d p r o p e r t i e s , o i l - b a s e d p a i n t s s t il l h a v e

a s o l id p l a c e i n t h e m a r k e t . H o w e v e r , t h e ir s l o w d r y i n g n a t u r e a n d o d o r re -

m a i n a s d e fi c ie n c i e s. E v e n l a t e x p a i n t s a r e r e l a t i v e l y s l o w d r y i n g .

O n e w o u l d l ik e to t hi n k i n d u s t r y c o u l d r e a d il y s e e t h e m a n y a d v a n -

t a g e s o f c h a n g i n g f r o m a s in g le , h i g h - p o l l u ti n g t e c h n o l o g y t o a m u l t i te c h -

n o l o g y a r r a n g e m e n t t h a t w o u l d p r o v i d e f o r w a r d t h in k in g . T h i nk i n g t h at

w o u l d l e a d t o c o s t s a v i n g s a s w e l l a s r e s o u r c e c o n s e r v a t i o n , w h i l e b e i n g

e c o l o g i c al l y f r i e n d ly a n d s i m u l t a n e o u s l y p r o v i d i n g a h e a l t h ie r w o r k a n d

c o m m u n i t y e n v i r o n m e n t . A c t u a ll y , a t t h a t ti m e t h e s l o w m o v i n g c o a t i n g in -

d u s t r y c o u l d n o t o r w o u l d n o t s e e th e f u t u r e - - s a l e s a n d u s e o f s o l v en t

d o m i n a t e d t h e p i c tu r e .

H o w m u c h s o l v e n t w a s u s e d i n a c o a t i n g f o r m u l a t i o n i n t h e e a r l y 19 70 s?

W h e n t h e to t al s o li d s c o n t e n t w a s 1 0% , n i n e p o u n d s o f s o l v e n t w e r e u s e d

w i t h o n e p o u n d o f f in a l c o a t i n g m a t e r ia l . W h e n t h e t o ta l so l id s c o n t e n t w a s

2 0% , fo u r p o u n d s o f s o l v e n t w e r e u s e d w i t h o n e p o u n d o f f in a l c o a ti n g m a -

t e ri a l. T h e s e l a r g e q u a n t i t i e s o f r e l a t i v e l y i n e x p e n s i v e s o l v e n t r e s u l t e d i n ex -

c e l l e n t a p p l i c a t i o n c h a r a c te r i st i c s, f l o w , a n d l e v e l i n g o f th e l i q u i d c o a t i n g ,

a n d i n h i g h g l o s s, th i n , c o n t i n u o u s f i l m s o f t h e r e l a t i v e l y e x p e n s i v e , f in a l

s o l i d p o l y m e r i c c o a t i n g . P l e a s i n g d e c o r a t i v e a n d f u n c t i o n a l c o a t i n g s

r e

1

Copyright9 002 by AST M International ww w.astm.org


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RADI ATI ON CURING OF COATINGS

s u l te d . B u t h u g e q u a n t i t i e s o f s o l v e n t w e r e s p e w i n g i n t o th e a t m o s p h e r e , a n d

b o t h i n d u s t r y a n d t h e p u b l i c s e e m e d o b l i v i o u s to th i s fa c t.

A t t h i s t i m e t h e r e w e r e s e v e r a l r e a s o n s f o r t h e s t a t e o f a f f a ir s in t h e c o a t -

i n g i n d u s t r y b e i n g o n e o f s o l v e n t - b o r n e , l o w s o li d s. P e t r o l e u m - d e r i v e d s o l-

v e n t s a n d t h e e n e r g y , w h i c h i s a l s o f o s si l- f u e l b a s e d , n e e d e d t o e v a p o r a t e

t h e m w e r e i n e x p e n s i v e . T h i s is e a s y t o re a l i z e i f o n e r e c a ll s th a t , p r i o r t o t h e

e a r l y 1 9 70 s, o il a n d p e t r o l e u m p r o d u c t s h a d n o t y e t s e e n t h e e f f e c t o f c a r te l

p r ic in g . A t th e t im e , i f s o m e o n e h a d p r o p o s e d a s c e n a r io i n w h i c h t h e p ri c e

o f o il w o u l d i n c re a s e b y a f a c to r o f 1 0 t o 2 5 t im e s , t h e y w o u l d h a v e b e e n t o l d

t o q u i t w a s t i n g t i m e a n d p o s e s o m e t h i n g r ea li st ic . N o o n e w a n t e d t o im a g i n e

t h e e f fe c t s u c h p r i ci n g w o u l d h a v e o n t h e c o a t i n g i n d u s t r y o r o n i n d i v i d u a l s.

U n l e s s o n e l i v e d t h r o u g h i t, i t i s d i f f i c u l t t o v i s u a l i z e t h e p r e s s u r e s t h a t w e r e

b r o u g h t t o b e a r o n th e g e n e ra l p u b l i c a n d i n d u s t r y t h r o u g h d i c t a te d

p e t r o l e u m s h o r t a g e s t h a t e n d e d u p a s l o n g l i n e s a t g a s o l i n e s t a t i o n s a n d

m a r k e d l y i n c r e a se d p r i c es . N o t o n l y d i d c o n s u m e r s f e e l t h e s t re s s o f w a i t i n g

i n g a s o l in e l in e s, th e y f e lt t h e e v e n s h a r p e r s t re s s o f w o n d e r i n g if t h e y w o u l d

b e a b l e t o p u r c h a s e f o ss il f u e l - b a s e d p r o d u c t s a t a n y p r i c e to h e a t t h e ir h o m e s

a n d d r i v e t o t h e ir p l a c e o f e m p l o y m e n t o r o n v a c a t io n . I n th e c o a t i n g i n d u s -

t ry , c o m p a n i e s t h a t h a d l ar g e d r y i n g o v e n s b e c a m e c o n c e r n e d w i t h t h e p r o b -

l e m o f w h a t t o d o i f t h e y c o u l d n ' t o p e r a t e t he i r o v e n s - - t h e y k n e w h e a t in g o f

h o m e s w o u l d c o m e f irs t. T h e re w a s e n o u g h o il a n d n a t u r a l g a s , b u t i t w a s

o n l y a v a i l a b l e a t a h ig h , c a r t e l - se t p r ic e . A n d t h e p r i c e o f o i l w e n t u p a n d u p

i n w h a t s e e m e d a n e v e r - e n d i n g s pi ra l.

A c o m p l i c a ti n g e f f ec t s u p e r i m p o s e d o n t h e p r i c e / a v a i l a b i l it y p i c t u re

w a s t h e f a c t t h a t s o l v e n t s w e r e a n e x c e l le n t m e d i u m f o r r e d u c i n g t h e v is c o s -

i ty o f h i g h m o l e c u l a r w e i g h t p o l y m e r s t o a n a p p l i c a t i o n v i s co s i ty . A t t h a t

t im e , m o s t p o l y m e r s u s e d f o r c o a t in g s h a d h i g h m o l e c u l a r w e i g h t . T h is w a s

a n e c e s s a r y r e q u i r e m e n t t o a c h i e v e t h e h ig h - q u a l i t y p e r f o r m a n c e c h a r a c te r -

i st ic s e x p e c t e d f r o m t h e l i q u id c o a t in g s t h a t w e r e m o s t o f t e n u s e d a s la c q u e r s.

T h i s fa c t o r w a s t h e m a j o r r e a s o n l a rg e q u a n t i t i e s o f s o l v e n t w e r e u s e d i n fo r -

m u l a t i o n s . T h e n , a l m o s t o v e r n i g h t , o i l c o s t a n d a v a i l a b i l it y b e c a m e a s e r i o u s

p r o b l e m . T o s a t is f y e c o l o g ic a l c o n c e r n s o f c e rt a in g r o u p s , t h e m a t t e r w a s e x -

a c e r b a t e d w h e n t h e g o v e r n m e n t s e t r e q u i r e m e n t s o n t h e c o a ti n g i n d u s tr y .

T h e i n d u s t r y w a s t o q u i c k l y f o r m u l a t e 8 0% s o li d s co a t in g s . T h a t i s, t h e y w e r e

t o c h a n g e f r o m u s in g a b o u t f o u r o r m o r e p o u n d s o f so l v e n t f or e a c h p o u n d

o f f in a l c o a t i n g to o n e - f o u r t h o f a p o u n d o f s o l v e n t p e r e a c h p o u n d o f f in a l

c o a t i n g - - a f a c to r o f 1 6 o r m o r e d e c r e a s e

I t w a s i n th i s c r u c i b l e o f m a r k e t p r e s s u r e s , t h e C l e a n A i r A c t o f 19 70 1

a n d e x i st in g o l d k n o w - h o w t h a t th e n e w t e c h n o l o g i e s o f w a t e r - b o r n e , h i g h -

s o li d s, p o w d e r , a n d r a d i a t i o n - c u r a b l e c o a t i n g s , a l o n g w i t h t h e v a r i o u s s u b -

1The USA Environmental Protection Ag ency's Clean Air A ct of 1970 was am ended in 1977

and 1990,bu t at the time unde r discussion, it was the original act that was the driving force.


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Radiation Curing

c a t e g o r i es s u c h a s t w o - p a c k a g e h i g h s o l id s , w a t e r d i l u t a b l e c o a t i n g s , h i g h e r -

s o l i d s c o a t i n g s , a n d t h e l ik e , w e r e b o r n .

A s m i g h t b e e x p e c t ed , th e r e w e r e m a r k e d l y d i f fe r e n t o p i n i o n s r e g a r d i n g

e a c h o f t h e s e n e w t e c h n o lo g i e s . M a n y t h o u g h t s o l v e n t - b o r n e s y s t e m s w o u l d

n e v e r b e re p l a c e d , a n d i n t h e l o n g r u n , t h e y w e r e l o se rs . O t h e r s t h o u g h t w a -

t e r -b o r n e c o a t in g s w e r e t h e a n s w e r . T h e s e w e r e u s u a l l y p e o p l e w h o w e r e i n

t h e a r c h i t e c t u r a l la t e x - c o a t i n g b u s i n e s s , a n d t h e y f e lt i t w o u l d b e e a s y t o d e -

v e l o p i n d u s t r i a l l a t ex c o a t in g s . H i g h - s o l i d s c o a t in g s s e e m e d a n e a s y w a y t o

m e e t t h e d e m a n d s o f t h e C l e a n A i r A c t . A l l o n e h a d t o d o w a s d e c r e a s e

m o l e c u l a r w e i g h t a n d h i g h s o l i d s c o u l d b e a c h ie v e d . T h e y t e m p o r a r i l y f o r-

g o t o r j u s t d i d n o t k n o w m o l e c u l a r w e i g h t - p r o p e r t y r e la t io n s h i p s , a n d t h e y

f o u n d t h a t s y s t e m s t o b e s a t i sf a c t o r i l y c r o s s - l in k e d n e e d e d f u n c t i o n a l i t y s u it -

a b l y lo c a t ed o n e a c h m o l e c u l e t o e n s u r e t h e y w o u l d b e i n c o r p o r a t e d i n to t h e

f in a l c o a t in g f i l m n e t w o r k . O t h e r s t h o u g h t p o w d e r a n d r a d i a t i o n w o u l d b e

i m p o r t a n t , b u t t h e s e w e r e m o r e d i f fi c u l t t o a c h i e v e a n d o f t e n d i d n o t r e c e iv e

a s m u c h a t t e n t io n f r o m t h e i n d u s t r y a t t h a t t i m e .

I n t h e e n d a ll t h e n e w t e c h n o l o g i e s f o u n d a p l a c e in i n d u s t r y . T h e o n l y

p a r t s o f i n d u s t r y t h a t d i d n o t g a i n f r o m t h e c h a n g e s b r o u g h t a b o u t b y t h e

n e w t e c h n o l o g ie s w e r e t h o s e w h o r e f u s e d to b e f o r w a r d - l o o k i n g a n d c l u n g

t o t h e o l d w a y s . N e w c o m p a n i e s c a m e i n to b e i n g , n a m e s c h a n g e d , a n d c o n -

s o l i d a t io n s t o o k p l a c e a s th e n e w t e c h n o l o g i e s f o u n d t h e i r n i c h e i n th e m a r -

k e t p la c e . T o d a y , r a d i a t i o n c u r i n g i s w e l l e s ta b l i s h e d a n d a p p e a r s t o b e p o i s e d

t o m a k e g r e a t s t r i d e s i n t o t h i s m a r k e t [1].

W h a t is R a d i a t io n C u r i n g a n d W h y i s it U s e d

R a d i a t i o n c u r i n g o f c o a t i n g s , i n k s , a d h e s i v e s , a n d s e a l a n t s 2 i s t h e e f f e c t i v e l y

i n s t a n t a n e o u s c o n v e r s i o n o f a u s u a l l y l i q u i d , l o w v i s c o s i t y f o r m u l a t i o n o f i n -

g r e d i e n t s t h a t h a s b e e n a p p l i e d t o a s u b s t r a t e a n d e x p o s e d t o a r a d i a t i o n

s o u r c e in t o a p o l y m e r i z e d a n d c r os s - l in k e d m a s s w i t h d e c o r a t iv e a n d / o r

f u n c t i o n a l p r o p e r t i e s . T h i s p r o c e s s u s u a l l y r e s u l t s i n a n e f f e c t iv e l y c o m p l e t e

c o n v e r s i o n o f t h e l i q u i d f o r m u l a t i o n i n t o a s o l i d , u s e fu l p r o d u c t , i .e ., r a d i a -

t i o n - c u r e d c o a t i n g s a r e e s s e n ti a l ly 1 0 0 % s o li d s i n n a t u re . R a d i a t i o n a s u s e d

i n t h e r a d i a t i o n c u r i n g i n d u s t r y i s u s u a l l y u l t r a v i o l e t r a d i a t i o n ( p h o t o n s o r

p h o t o c u r i n g ) o r e l e c t ro n b e a m e n e r g y ( e l ec t ro n s o r E - b e a m c u r in g ) , h o w -

e v e r , c o h e r e n t o r l a s er r a d i a t i o n a n d v i s ib l e r a d i a t i o n 3 o r l i g h t a r e a l s o u s e d .

2 Coatings, inks, adhesives, and sealants can be an d are cured w ith radiation. In this book,

the word coatings will be taken to be generic to all of these products, unless on e is specifically

dealing with one o f the others. Currently, radiation curing of sealants has limited u se. This is pri-

marily d ue to the n eed for curing th rou gh a con siderable thickness of sealant material. Readers

interested in radiation curing of sealants might consider the information in the topic Thick Sec-

tion Curing fou nd in Chapter 11.

3 Webster's New Collegiate Dictionary defines light as that p art of the electromagn etic spec-

trum of radiation th at is visible to the hu m an eye.


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RAD IATI ON CURING OF COATINGS

"Radcure" is an adjective derived to describe anything that has to do with ra-

diation-curing technology--applications, equipment, materials, and so on.

When a material is exposed to radiation, it is "irradiated. " Although radia-

tion and its natu re will be discussed in a following section, a few words about

it are appropriate at this time.

The electromagnetic spectrum is made up of electromagnetic waves of

radiation that are identified by fluctuations of electric and magnetic fields. It

comprises the total range of electromagnetic wavelengths, k, and extends

from essentially zero to infinity. If numbers are to be placed on these wave-

lengths, it could be said that the y range from about 10 -13 to 103 meters. Cos-

mic rays, gamma rays, and hard X-rays are on the low end of the range, and

microwave, radio, radar, television, and electric waves are on the high end of

the range [2,3]. Contained within the electromagnetic spectrum is the visible

portion of the spectrum, wavelengths f rom 400 to 760 nanometer s or from vi-

olet to red, that is known as "light," and the invisible portion of the spectrum

just beyon d the violet end of the visible region that is known as "ultraviolet"

and often referred to as "ultraviolet light." The ultraviolet region of wave-

lengths extends from about i to 400 nanometers, and it contains a region from

about 15 to 280 nanometers known as "Ultraviolet C," another from 280 to

320 nm known as "Ultraviolet B," and another region from 320 to 400 nm

known as "Ultraviolet A"[4]. Ultraviolet B is radiation that causes the phe-

nomenon known as "sunburn" and prolonged exposure to it over many

years can cause skin cancer. Ultraviolet C radiation is very powerful and is

used to sterilize surfaces because it has the ability to kill bacteria and viruses.

The range f rom 800 to about 30 000 nanometers is known as the in frared re-

gion of the spectra. Above this begins the microwave region and so on.

RadTech International has som ewhat different, but similar, designations of

the ultraviolet radiation ranges. These can be fo und in the RadTech Glossary

of Terms at the end of this volume.

Why does industry use the radiation curing process? Many of the at-

tributes of this process can be summarized as:

9 Low capital inves tment

9 Low energy consumption and cost

9 High solids and elimination of solvents

9 Plastics and other heat sensitive substrates can be coated and cured

9 Rapid cure and high productivi ty

9 Multiple operations are possible

9 Space requirements are low

9 Use of existing equipment whe n retrofitting radiation equipmen t

9 High degrees of cross-linking and, therefore, potent ial for imp roved abra-

sion, chemical, and stain resistance.

Capital investmen t is low for ultraviolet curing or photocuring equip-

ment. The equ ipment usua lly can be designed to meet specific needs, and no


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Radiation Curing

d e a d o r u n u s e d c u r i n g a re a s n e e d b e in v o l v e d . I f e l e c t r o n b e a m t e c h n o l o g y

is i n v o l v e d , c a p it a l i n v e s t m e n t c a n b e h i g h , b u t i t is u s e d w h e n h i g h , l a rg e -

v o l u m e p r o d u c t i o n c o u p l e d w i t h v e r y h ig h l in e s p e e d s i s i n v o l v ed . N e w

s m a l l- sc a le e l e c tr o n b e a m s y s t e m s h a v e e x p a n d e d t h e e le c t ro n b e a m t ec h -

n o l o g y i n t o t h e r e a l m o f r e la t iv e l y m o d e s t c os t.

R a d i a t i o n - c u r in g s y s t e m s h a v e l o w p o w e r r e q u i r e m e n t s , a n d l it tl e h e a t -

i n g o f t h e s u b s t r a t e t a k e s p la c e . N o l a r g e o v e n s a r e r e q u i r e d f o r e v a p o r a t i o n

o f s o l v e n t o r w a t e r . T h e s e f a c t o rs r e s u l t i n l o w o p e r a t i n g c o s ts , a fa c t o r t h a t

t a k e s o n e v e r i n c re a s i n g i m p o r t a n c e a s t h e p r i c e o f f u e l u n d e r g o e s m a r k e d i n-

c r e a se s , a s i t d i d i n 2 00 0. A n a d d i t i o n a l b e n e f i t is t h a t t h e s y s t e m s r u n c o o l

a n d th is i m p r o v e s t h e w o r k i n g e n v i r o n m e n t f o r e m p l o y e e s b y m a k i n g a m b i -

e n t t e m p e r a t u r e s q u i t e r e a s o n a b l e a n d e a s i er t o co n t r o l . It is al so e a s i e r a n d

le ss t i m e - c o n s u m i n g t o c h a n g e o v e r a f i n is h i n g l in e f r o m o n e f o r m u l a t i o n o r

d e s i g n p a t t e r n t o a n o t h e r , w h i c h r e s u l t s in in c r e a s e d c o s t sa v i n g s a n d p r o -

d u c t i v i t y .

A s m e n t i o n e d e a r li e r , r a d i a t i o n - c u r i n g s y s t e m s a r e e i th e r 1 00 % s o l id s o r

v e r y h i g h s o l id s i n n a t u r e . V O C 4 is n e a r z e r o , s in c e a ll o f t h e f o r m u l a t i o n r e -

a c ts o r r e m a i n s e f f e c t iv e l y w i t h t h e f i n a l fi lm . F o r p a r t i c u l a r e n d u s e s , t h e r e

a r e s p e c i a l i z e d f o r m u l a t i o n s t o w h i c h s o l v e n t is a d d e d f o r v i s c o s i ty r e d u c -

t io n , b u t i t is u s u a l l y e a s y t o s t a y w i t h i n r e g u l a t o r y l im i t s o r t o f it t h e e q u i p -

m e n t w i t h a s o lv e n t r e c o v e r y s y s te m . W a t e r - b o r n e r a d i a t i o n c u r a b le s y s t e m s

a r e a l s o u s e d .

H e a t - s e n s i t i v e s u b s t r a t e s s u c h a s p l as t ic s , p r i n t e d c i rc u i t b o a r d s a n d a s-

s e m b l ie s , p a p e r , w o o d , a n d t h e l ik e c a n b e c o a t e d a n d c u r e d w i t h r a d i a t i o n

t e c h n o l o g y . T h e r e a s o n f o r th i s is t h a t t h e r e is li tt le b u i l d u p o f h e a t i n t h e s u b -

s t ra t e w h e n t h i n c o a t i n g f i l m s a r e i n v o l v e d . I f t h i ck f i lm s a r e p r e p a r e d , j u d i -

c i o u s s e l e c t i o n o f f o r m u l a t i o n i n g r e d i e n t s - - p a r t i c u l a r l y p h o t o i n i t i a t o r

l e v e l - - a n d l in e s p e e d c a n b e u s e d a s c o n t ro l s to r e m o v e c o n c e r n s a b o u t h e a t

b u i l d u p . T h i s f a c to r c a n b e o f p a r t i c u l a r u t i l i ty i n t h e e l e c tr o n i c s i n d u s t r y .

I m p r o v e d e c o n o m i c s a n d e ff ic i en c y r e s u l t f r o m t h e i n s t a n t a n e o u s c u r-

i n g c h a ra c te r is t ic s o f r a d i a t i o n - c u r e d s y s t em s . D e p e n d i n g o n t h e n a t u r e o f

t h e f o r m u l a t i o n b e i n g c u r e d , l i ne s p e e d s o f h u n d r e d s o f f e e t p e r r a i n c a n b e

e a s i l y a c h i e v e d [7]. T h i s r e s u lt s i n i n c r e a s e d p r o d u c t i o n . W h e n t h is is c o u -

p l e d w i t h t h e fa c t t h a t m u l t i p l e o p e r a t i o n s c a n b e a c c o m p l i s h e d i n a s i n g le

p a s s t h r o u g h a f i n i sh i n g l in e , p l a n t o u t p u t i s o f t e n d o u b l e d , t r ip l e d , o r m o r e

o n a l i n e t h a t is n o l o n g e r a n d o f t e n s h o r t e r b y 1 00 t o 20 0 % , o r m o r e t h a n a

c o n v e n t i o n a l f in i s h i n g l in e . T h a t i s, a s u b s t r a t e c a n b e p r i n t e d i n o n e o r m o r e

c o l o rs , t h e i n k c u r e d , o v e r c o a t e d w i t h a c l e a r f in i sh , c u r e d , t u r n e d o v e r in a

4 VOC is art abbreviation for Volatile Organic Com pounds that is used by the USA Envi-

ronmen tal Protection Ag ency (EPA). VOC is defined as Any organic compou nd that partici-

pates in atmosph eric photochem ical reactions; that is, any organic compoun d other than those

the Adm inistrator designates as having negligible photochem ical reactivity. Details about VO C

and ho w it is measured can be fou nd in the literature [5, 6].


6/13

RADI ATION CURING OF COATINGS

F e r ri s w h e e l a r r a n g e m e n t , a n d t h e n c o a t e d a n d c u r e d o n t h e r e v e r s e s id e .

O f c o u r s e , e l i m i n a t i o n o f s o l v e n t a l s o r e p r e s e n t s a c a l c u l a b l e c o s t s a v i n g s .

S a v i n g s r e s u l t b e c a u s e t h e r e i s n o n e e d f o r so l v e n t , h a n d l i n g s o l v en t , e v a p o -

r a t i n g s o l v e n t , r e c o v e r i n g s o l v e n t , a n d f o r s a f e l y a n d e c o l o g i c a l l y s o u n d d i s -

p o s i n g o f r e c o v e r e d s o l v e n t i n s o m e m a n n e r .

I n g e n e r a l , s p a c e r e q u i r e m e n t s a r e m u c h l e s s , o f t e n 2 5 - 5 0 % l e s s , f o r a r a -

d i a t i o n - c u r i n g l i n e t h a n f o r a c o n v e n t i o n a l c o a t i n g l i n e . T h e m a i n r e a s o n f o r

t h i s is t h a t n o la r g e o v e n s a r e re q u i r e d f o r e v a p o r a t i o n o f s o l v e n t a n d / o r w a -

t er . I n a d d i t io n , t h e r e i s n o n e e d f o r a s o l v e n t r e c o v e r y s y s t e m . A n a d d i t i o n a l

s p a c e - s a v i n g f e a t u r e o f r a d i a t i o n - c u r a b l e c o a t i n g s i s t h e i r 1 0 0 % s o l i d s n a t u r e .

L e s s d r u m s o f f o r m u l a t e d c o a t i n g a r e r e q u i r e d w h e n s o l v e n t is e l i m i n a t ed ,

w h i c h a l so r e s u lt s i n d e c r e a s e d h a n d l i n g .

A t t i m e s , e x i s t i n g c o a t i n g l i n e s c a n b e r e t r o f i t t e d w i t h r a d i a t i o n - c u r i n g

e q u i p m e n t . T h i s ca n b e a c c o m p l i s h e d w i t h m i n o r a d a p t a t i o n s a n d c o s t s, p a r -

t ic u l a rl y w i t h p h o t o c u r i n g e q u i p m e n t , w h i c h c a n b e v e r y v e r s a ti le i n n a t u r e .

I t c a n b e r e a d i ly u n d e r s t o o d t h a t ra d i a t i o n - c u r in g l in e s c a n r u n m u c h

f a s te r th a n c o n v e n t i o n a l c o a t i n g l in e s. W h e n t h is i s c o u p l e d w i t h t h e m u l t i -

p l e o p e r a t i o n s t h a t c a n b e a c c o m p l i s h e d o n a s i n g l e p a s s t h r o u g h t h e l i n e ,

m a r k e d i n c r e a s e s in p r o d u c t i v i t y c a n b e a t ta i n e d . F o r th e s e a n d t h e o t h e r re a -

s o n d i s c u s s e d , r a d i a t i o n c u r i n g i s a te c h n o l o g y t h a t is c u r r e n t l y i n v o g u e a n d

t h a t w i ll b e e x p e r i e n c i n g g r o w t h o f a b o u t 1 0 % c o m p o u n d e d i n t h e f u t u r e [ 8].

a d i a t i o n

I n t h e a b o v e , t h e r e w a s a b r ie f i n t r o d u c t i o n t o r a d i a t i o n a n d i ts r e l a t i o n s h i p

t o t h e e l e c t r o m a g n e t i c s p e c t ru m . R a d i a t i o n is t h e p r o p a g a t i o n a n d / o r e m i s -

s i o n o f r a y s t h r o u g h s p a c e , w h i c h is e f f e c t iv e l y a v o i d , o r t h r o u g h a m e d i u m ,

s u c h a s a ir , i n t h e f o r m o f e i th e r w a v e s o r p a r t ic u l a t e e m i s s i o n . J u st w h a t d o e s

t h is m e a n ?

M o s t p e o p l e t h i n k o f a c t iv a t i n g m o l e c u l e s b y u s i n g t h e r m a l e n e r g y a s

t h e n o r m a l w a y f o r c h e m i c a l p h e n o m e n a t o o c c u r. T h a t i s, t e m p e r a t u r e e le -

v a t i o n i s u s e d t o p u s h m o l e c u l e s u p a h i ll o r o v e r a b a r r i e r a n d a l lo w t h e m t o

a t t a in a d i f f e re n t , h i g h e r e n e r g y s t a te . F o r e x a m p l e , t h e c o n v e r s i o n o f s o m e -

t h i n g f r o m t h e l i q u id s t at e t o t h e g a s e o u s s t a te i s u s u a l l y a c c o m p l i s h e d w i t h

t h e u s e o f t h e r m a l e n e r g y . I n t h e c o a t in g i n d u s t r y , m a n y t h e r m a l p r o c e s s e s

o r t h e r m a l l y a c t i v a t e d re a c t i o n s a re u s e d . S u c h p h e n o m e n a u s u a l l y u s e t h er -

m a l e n e r g y , w h o s e s o u r c e is a n i n c r e a s e o f t e m p e r a t u r e , t o in c r e a se o n e o r

m o r e o f t h e r o t a ti o n a l, t r an s l at io n a l , a n d v i b r a t io n a l e n e r g y s t a te s f r o m a

g r o u n d s t a te t o th e u p p e r r a n g e o f t h e ir e n e r g y d i s t r ib u t i o n [9]. O n e o f t h e

s im p l i st ic p r o c e s s e s u s e d i n th e c o a t i n g i n d u s t r y is e v a p o r a t i v e d r y i n g o f a

l a c q u e r t h a t u s u a l l y i s a c c o m p l i s h e d w i t h s u p p l i e d h e a t o r t h e r m a l e n e r g y .

T h e l i q u e f ac t i o n a n d f l o w o f p o w d e r c o a t i n g s i s a l so a c c o m p l i s h e d w i t h t h e r -

m a l e n e r g y . A n o t h e r s p e c if ic e x a m p l e i s t h e c r o s sl in k i n g r e a c t io n o f h e x a m -


7/13

Radiation Curing 7

e t h o x y m e l a m i n e w i t h h y d r o x y l f u n c t i o n a l i t y t h a t i s a c c o m p l i s h e d b y s u p -

p l y i n g t h e r m a l e n e r g y t o t h e s y s te m . V a r i o u s o t h e r t h e r m a l p r o c e s s e s u s e d

i n t h e c o a ti n g i n d u s t r y s h o u l d b e r e a d i l y a p p a r e n t t o r e a d e r s.

T h e r m a l p r o c e s s e s s u c h a s t h e s e a r e i n e ff ic i e nt in t e r m s o f e n e r g y u s a g e

a n d o f t h e a m o u n t o f f in a l c o a ti n g a p p l i e d t o a s u b s t r a t e f o r a g i v e n u n i t o f

e n e r g y . A m o r e e f f ic i e n t w a y t o s u p p l y a n d u t il iz e t h e e n e r g y n e c e s s a r y t o ef -

f e c t a c h a n g e t o a n a c t i v a t e d o r r e a c t i v e s t a t e is t o b r i n g t h e m o l e c u l e s d i r e c t l y

i n to c o n t a c t w i t h p a c k e t s o f e n e r g y s u p p l i e d b y s o u r c e s s u c h a s n e u t r o n s ,

B - r a y s o r h i g h v e l o c i t y e l e c t r o n s , o ~ -p artic le s, o r t h e q u a n t a o f e l e c t r o m a g n e t i c

e n e r g y t e r m e d p h o t o n s a n d s u p p l i e d b y t h at p o r t i o n o f t h e e l ec t ro m a g -

n e ti c s p e c t r u m k n o w n a s u l tr a v i o l e t ra d i a t i o n o r b y l ig h t. A d i s t in c t a d v a n -

t a g e o f p h o t o c h e m i c a l l y i n d u c e d r e a c t i o n s o v e r t h e r m a l l y i n d u c e d r e a c t io n s

i s s e l e c ti v i ty . D i f f e r e n t a n d p a r t i c u l a r r e a c t i o n s o f t h e s a m e s u b s t a n c e c a n b e

p r o m o t e d b y i r r a d i a ti n g i t w i t h d i f f e r e n t f re q u e n c i e s o f u lt r a v i o le t o r v i si b le

r a d ia t io n . I n v e s t i g a t io n o f t h e r e a c ti o n s c a u s e d b y s u c h e n e r g y - s u p p l y i n g

p r o c e s s e s is t e r m e d r a d i a t i o n c h e m i s t ry . T h e p r o c e s s u s e d to e f fe c t c u r e o f

c o a t in g s , i nk s , a d h e s i v e s , a n d s e a l a n t s w i t h r a d i a t i o n c h e m i s t r y is k n o w n a s

r a d i a t i o n c u r i n g . I f o n l y u l t r a v i o l e t o r v i s i b le r a d i a t i o n i s u s e d , t h e p a r ti c -

u l a r p r o c e s s is k n o w n a s p h o t o c h e m i s tr y . I n i n d u s t ry , th e t w o m a i n m e t h -

o d s o f ef fe c t in g r a d i a t i o n c u r i n g a r e e l e c t r o n b e a m s a n d u l t ra v i o l e t r a d ia t io n .

E q u i p m e n t f o r c u r in g w i t h t h e se t w o m e t h o d s w i ll b e d i s c u s s e d in C h a p t e r

2.

A n e l e c tr o n b e a m i s a s t r e a m o f e l e c t ro n s t h a t a r e e m i t t e d b y a s o u rc e .

T h e e l e c tr o n s a ll m o v e a t th e s a m e s p e e d a n d i n t h e s a m e d i r ec t io n . A n e l ec -

t r o n b e a m , w h i c h i s s i m i l a r t o a c a t h o d e - r a y t u b e , p r o d u c e s t h e s t r e a m o f

e l e ct ro n s . T h e c a t h o d e i s h e a t e d t o g e n e r a t e e l e ct r o ns , a h i g h - v o l t a g e s y s t e m

t h e n d i r e c t s t h e e l e c tr o n s tr e a m , a n d e l e c t r o m a g n e t s g u i d e t h e s t r e a m t o t h e

s u b st ra t e . T h is t e c h n iq u e w a s m a i n l y u s e d f o r h i g h v o l u m e p r o d u c t i o n b e -

c a u s e o f e q u i p m e n t c o s t a n d s iz e. I n a g e n e r a l s e n s e , se l e c ti o n b e t w e e n u l tr a -

v i o le t a n d e l e c tr o n b e a m c u r i n g i s d e p e n d e n t o n f a c t o rs s u c h a s f il m th i ck -

n e s s, p i g m e n t a t i o n o r o p a c it y , p r o d u c t i o n s ca le o r v o l u m e , a n d s im i l a r it em s .

P h o t o c h e m i s t r y i s a s u b d i v i s i o n o f r a d i a t i o n c h e m i s t r y , a n d i t i s t h e s ci -

e n c e o f t h e c h e m i c a l c h a n g e s t h a t a r e i n i t ia t e d b y u l t r a v i o l e t a n d v i s i b l e r a -

d i a t i o n o n m o l e c u le s . T h e p a r t ic u l a r w a v e l e n g t h r e g i o n o f u l t ra v i o l e t t h a t is

o f i n t e r e s t i n th e c o a t i n g a r e a i s f r o m a b o u t 1 0 0 t o 40 0 n m , b u t m o r e p r a c t i-

c a ll y f r o m a b o u t 2 00 to 4 00 n m . I t s h o u l d b e u n d e r s t o o d t h a t i n c i d e n t u l tr a -

v i o l e t r a d i a t io n c a n b e t r a n s m i t te d , a b s o r b e d , o r r e f r a c te d , but i t i s on ly ab

sorbed radiat ion th at can prod uce a chem ical change

i n m o l e c u l e s . T h i s i s t h e f i r s t

l a w o f p h o t o c h e m i s tr y , w h i c h i s k n o w n a s t h e G r o t th u s s - D r a p e r L a w . C e r -

t ai n c o m p o u n d s c a n a b s o r b p a r ti c u l a r u l t ra v i o l e t w a v e l e n g t h s o r e n e r g y

p a c k e t s a n d a r e c o m p l e t e l y t r a n s p a r e n t t o o t h e r u l t r a v i o l e t r a ys . A n o t h e r

w a y o f s t a t in g t h i s i s t h a t i f a m o l e c u l e d o e s n o t a b s o r b r a d i a t i o n o f a p a rt i c -

u l a r f re q u e n c y , it w i l l n o t u n d e r g o a p h o t o c h e m i c a l c h a n g e w h e n c o n t a c te d

w i t h r a d i a t i o n o f t h a t p a r ti c u l a r f r e q u e n c y . T h e a m o u n t o f r a d i a ti o n a b -


8/13

RAD IAT ION CURING OF COATINGS

s o r b e d b y a c o m p o u n d is p r o p o r t i o n a l t o t h e th i c k ne s s o f t h e c o m p o u n d ' s

s u r r o u n d i n g s . T h e s e c o n d l a w o f p h o t o c h e m i s t r y , o r S ta r k- E in s te in L a w ,

t el ls u s t h a t o n l y o n e m o l e c u l e o f a r a d i a t io n a b s o r b i n g s y s t e m w i l l b e a c ti -

v a t e d f o r e a c h p h o t o n o f r a d i a t io n a b s o r b e d b y a c h e m i c a l s y s te m . T h e q u a n -

t i ty o f p h o t o c h e m i c a l r e a c t io n t h a t t a k e s p l a c e i s d e t e r m i n e d b y t h e p r o d u c t

o f t i m e o f e x p o s u r e a n d t h e r a d i a t i o n i n te n s it y .

T h e f a c t t h a t r a d i a t i o n i s e n e r g y c a n b e s i m p l y a n d r e a d i ly e x e m p l i f ie d .

T h e f o l lo w i n g e x a m p l e s q u i c k ly a n d v i v i d ly d e m o n s t r a t e t h a t u l tr a v io l e t a n d

v i s ib l e r a d i a t i o n a r e e n e r g y . It i s w e l l k n o w n t h a t s u n l i g h t c o n t a i n s w a v e -

l e n g t h s f r o m b o t h t h e u l t r a v i o l e t a n d t h e v i s i b l e s p e c t r a . A l m o s t e v e r y o n e

h a s t a k e n a m a g n i f y i n g l e n s a n d f o c u s e d th e s u n ' s r a y s o n t o p a p e r o r a s i m -

i la r m a t e r i a l . W h e n t h i s is d o n e , i t i s o b s e r v e d t h a t t h e f o c u s e d b e a m o f

b r o a d - s p e c t r u m r a d i a t i o n q u i c k l y ra i se s t h e t e m p e r a t u r e o f t h e p a p e r t o t h e

k i n d l i n g p o i n t, a n d s m o k e b e g i n s t o ri se . A n o t h e r e x a m p l e i s t h e f a ct t h a t

w h e n t h e u lt ra v i o le t r a y s i n su n l i g h t b o m b a r d o u r s k in , t h e y c a u s e r e d d e n -

i n g o r t a n n i n g a s w e l l a s o t h e r l e s s a p p a r e n t , l o n g - t e r m c h a n g e s . T h e f a c t t h a t

u l t r a v i o l e t r a y s a r e n o t a b s o r b e d o r r e f l e c t e d b y t h e s a m e m e d i a a s o t h e r

f o r m s o f e n e r g y , s u c h a s v i s ib l e o r li g h t e n e r g y , is a p p a r e n t f r o m t h e s u n -

b u r n o n e c a n o b t a i n o n a c l o u d y d a y . T h e u l t r a v io l e t r a y s a r e t r a n s m i t t e d

t h r o u g h t h e c l o u d m o i s t u r e , w h e r e a s m u c h o f t h e o t h e r s u n ' s r a y s a r e a b -

s o r b e d o r re f l e c te d b y t h e c l o u d , a n d a d a r k e n i n g o r d i m m i n g e f fe c t a n d a

c o o l i n g e f f e c t a r e o b s e r v e d .

O t h e r c o m m o n l y k n o w n , b u t p e r h a p s n o t r ea l iz e d a s s u ch , p h o t o c h e m -

ic a l r e a c t io n s i n c l u d e p h o t o g r a p h y w h e r e i n l i g h t c o n v e r t s s il v e r b r o m i d e t o

m e t a l li c s il v er , v i s i o n i n w h i c h a p r o t e i n is i s o m e r i z e d b y r a d i a t i o n i n th e

r e ti n a o f t h e e y e , a n d p h o t o s y n t h e s i s w h e r e i n p h o t o n s f r o m s u n l i g h t a r e a b -

s o r b e d b y c h l o r o p h y l l a n d m a k e e n e r g y a v a i l a b le f o r t h e fo r m a t i o n o f v a r i-

o u s c h e m i c a l s s u c h a s c a r b o h y d r a t e s , v a n i l l in s , p h e n o l i c s , a n d s o o n . T h u s , it

i s r e a d i l y a p p a r e n t t h a t u l t r a v i o l e t a n d v i s i b le r a d i a t i o n , a s w e l l a s o t h e r p o r -

t io n s o f t h e e l e c tr o m a g n e t i c s p e c t r u m , a r e e n e r g y . H o w e v e r , a t th i s p o i n t t h e

m a g n i t u d e o f t h e e n e r g y , it s n a t u r e , a n d i ts m o d e o f t r a n s m i s s i o n a r e n o t a p -

p a r e n t .

Sim pl i f i e d C la s s ic to M o de r n L ig ht R a dia t io n Ene r g y Th e o r y

I n t h e S e v e n t e e n t h C e n t u r y , t h e r e w e r e t w o c o n c e p t s u s e d t o d e s c r i b e t h e

n a t u r e o f l ig h t a n d u l t ra v i o l e t r a d ia t io n . I n o n e o f t h e s e c o n c e p t s , N e w t o n

p o s e d a c o r p u s c u l a r o r p a r ti c le t h e o r y f o r su c h r a d i a ti o n . T h i s t h e o r y w a s

g e n e r a l l y a c c e p t e d u n t i l a l m o s t th e e n d o f t h e E i g h t e e n t h C e n t u r y . A t t h is

t im e , H u y g e n s p o s e d t h e se c o n d c o n c e p t , a n d i t c o n s i d e r e d s u c h r a d ia t io n

t o b e w a v e - l i k e i n n a t u r e . S t u d i e s c o n c e r n e d w i t h t h e d i ff r a c t io n , in t e r fe r -

e n c e , r e fl ec t io n , a n d r e f ra c t io n p h e n o m e n a t h a t o c c u r w i t h l i g ht t o o k p l a c e


9/13

Radiat ion Cu ring 9

i n t h e e a r l y N i n e t e e n t h C e n t u r y , a n d t h e re s u l t s o f t h e se s t u d i e s c o u l d b e

b e s t e x p l a i n e d b y i n v o k i n g t h e H u y g e n s w a v e t h e o r y . F u r t h e r c r e d e n c e

w a s g i v e n t o t h e w a v e - l i k e c h a r a c te r t h e o r y w h e n i t w a s f o u n d t h a t , i n a

v a c u u m , l ig h t t ra v e l s a t t h e s a m e v e l o c i t y as t h a t o f o t h e r e l e c t r o m a g n e t i c

r a d i a t i o n , n a m e l y 3 101~ c m 9 - 1 .

I n t h e 1 86 0s , M a x w e l l p r e s e n t e d h i s e l e c t r o m a g n e t i c t h e o r y , in w h i c h r a -

d i a t i o n e n e r g y i s d e s c r i b e d a s a c o m b i n a t i o n o f o s c i l l a t i n g e l ec t ri c a l a n d m a g -

n e t ic f i e ld s p o s i t i o n e d a t r i g h t a n g l e s o r n o r m a l t o e a c h o t h e r. T h e n t h e e n -

e r g y p r o p a g a t e s t h r o u g h o t h e r w i s e e m p t y s p a c e o r a m e d i u m w i t h t h e

v e l o c i ty o f l ig h t in t h e d i r e c ti o n o f p r o p a g a t i o n . M a x w e l l ' s t h e o r y g a v e f u r -

t h e r a c c e p t a n c e to t h e w a v e t h e o r y o f H u y g e n s . H o w e v e r , e v e n t h o u g h s u c h

p h e n o m e n a c o u l d b e e x p l a in e d b y o n l y i n v o k i n g t h e w a v e t h e o ry , th e p h o -

t o e l e c t r i c e f f e c t 5 a n d o t h e r l i g h t c h a r a c t e r is t ic s c o u l d n o t b e e x p l a i n e d w i t h

o n l y th e w a v e t h e o r y . T o u n d e r s t a n d t he s e p h e n o m e n a , i t w a s n e c e s s a ry to

b r i n g i n to p l a y t h e c o r p u s c u l a r o r p a c k e t t h e o ry .

S t u d i e s b y P l a n c k [10] a t t h e b e g i n n i n g o f t h e T w e n t i e t h C e n t u r y i n d i-

c a t e d t h a t a b o d y r a d i a t i n g l i g h t w i t h a f r e q u e n c y v w o u l d r a d i a t e e n e r g y

o n l y i n w h o l e n u m b e r m u l t i p l e s o f h v , w h e r e h , P l a n c k ' s c o n s t a n t = 6 .6 24

1 0 27 e r g " S, i s t h e p r o p o r t i o n a l i t y b e t w e e n e n e r g y , E , a n d f r e q u e n c y w i t h :

E = h u = h c / A

w h e r e c i s th e v e l o c i t y 6 o f l ig h t i n a v a c u u m a n d k is th e w a v e l e n g t h . P l a n c k ' s

h y p o t h e s i s a l so s t a t e d t h a t a b o d y c o u l d o n l y a b s o r b e n e r g y i n e v e n n u m b e r

m u l t i p l e s o f h r . A f e w y e a r s l a t e r E i n s t e i n [12] d e v e l o p e d h i s r e l a ti v i ty e x -

p r e s s i o n ,

E = m c 2

w h e r e i n m i s m a s s a n d c i s a s p r e v i o u s l y d e f i n e d . H i s e f fo r t s e x t e n d e d

P l a n c k ' s t h e o r y , a n d t h e y i n d i c a te d t h a t e l e c t r o m a g n e t i c r a d i a t i o n is q u a n -

t i z e d a n d m a d e u p o f d i sc r e te q u a n t a o r p h o t o n s e a c h o f w h i c h , a s a u n i t o f

e n e r g y , h a s a v a l u e h r . T h u s , a p h o t o n is a q u a n t u m o f e l e c t ro m a g n e t i c e n -

e r g y o f a s in g l e w a v e l e n g t h , i .e ., o f a s in g l e m o d e o f d i r e c t io n a n d p o l a r i z a -

5 W hen a p hoton impinges onto a surface and is absorbed, an electron m ay be emitted from

the surface. This emission of an electron is the means b y which p ho ton s can be detected a nd is

kn ow n as the photoelectric effect.

6 Usually the velocity o f light is thou gh t of as a constant; how ever, in Janu ary 2001, it was

reported that physicists were able to stop l ight and then send i t on i ts wa y at 3 10~~cm 's 1 .

Atom s of gas were magnetically chilled to w ithin a few millionths of a degree fro m absolute

zero. Tw o laser bea m s tha t operated at different frequencies were directed into the g as, wherein

it im printed a pa ttern into the orientation of the spinning atom s of sodium and rubidium. As the

intensity decreased, the probe l ight dim m ed a nd then vanished, b ut s ti ll wa s imp rinted on the

metals wh ere it wa s frozen or stored. When the control be am w as restored, the light stored in the

spinning a tom s w as reconstituted an d continued on its w ay [11].


10/13

10 R A D I A T I O N C U R I N G O F C O A T I N G S

tion. 7 From these investigations, the modern concept of radiation emerged as

energy with a dual nature. Radiation moves t hrou gh space in wave packets

(corpuscles) that have a spect rum of energies (i.e., wavelengths) wi th wave-

like properties.

Earlier it was pointed out t hat an advan tage of curing with ultraviolet or

visible radiat ion is selectivity. For a change to take place, the product of the

frequency of the radiation that is absorbed by a molecule and Planck s con-

stant must exactly match the energy that separates the ground state of the

molecule , Eg, and the excited state, Ee.

E = h v = E~ - Eg)

If the frequency is changed, the magni tude of E changes; and if a state

Ee - E~ equal to the ne w magni tude of E exists for the molecule, the molecule

will be transformed to this new and different state. Thus, by varying the fre-

quency of radiation impinging on a molecule, it is possible to get photo-

chemical reactions that are completely different from each other, assuming a

number of excited states exist for the molecule. As was ment ion ed earlier, the

electromagnetic spectrum encompasses a very broad range of wavelengths

that are between about 10 -13 to

1 3

meters [18].

M a g n i t u d e o f U l t r a v io l e t a n d V i s i b l e R a d i a ti o n n e rg y

With this picture of radiation and how it functions, we can now turn to cal-

culation of the magn itud e of the energy associated with ultraviolet and visi-

ble radiation. As described above, Planck s constant is the propor tiona lity

constant between the energy of a photon and wavelength, and the energy at

7 T h i s h a s b e e n a s i m p l i s ti c d i s c u s s i o n o f t h e r e l a ti o n s h i p b e t w e e n r a d i a t i o n a n d e n e rg y . I t

i s b e y o n d t h e s c o p e o f t h is b o o k t o d e a l w i t h t h e t r a n s i t io n f r o m t h is in f o r m a t i o n i n t o d e t ai ls o f

t h e v a r i o u s t h e o r i e s i n v o l v e d . H o w e v e r , a s h o r t d i s c u s s io n m a y h e l p d i r e c t i n t e r e s t e d r e a d e r s in

u n d e r s t a n d i n g r a d i a t i o n a n d p r o v i d e l i t e r a tu r e r e f e re n c e s t h a t ar e p e r t i n e n t t o t h e t op ic .

R u t h e r f o r d [1 3] t h e o r i z e d t h a t th e a t o m w a s a d e n se , c h a r g e d n u c l e u s w i t h s u r r o u n d i n g o r -

b i t i n g e l ec t ro n s . T h i s p i c tu r e r e q u i r e d t h e o r b i t i n g e le c t r o n s t o l o se e n e r g y t h r o u g h r a d i a t i o n ,

a n d t h u s , s p i r a l i n t o t h e n u c l e u s . B o h r [ 14 ] a lt e r e d R u t h e r f o r d s p i c t u r e b y p l a c i n g t h e e l e ct r o n s

i n t o f ix e d o r b i ts t h a t h a d d i f f e re n t e n e r g y l ev e ls . H i s t h e o r y o f t h e h y d r o g e n a t o m h a d t h e s ol e

e l e c tr o n i n t h i s a t o m o c c u p y i n g p a r t i cu l a r , d i s c re t e e n e r g y l e v el s w i t h t h e e n e r g y s t at e d e te r -

m i n e d b y a p r i n c ip l e q u a n t u m n u m b e r , n , w i t h n h a v i n g a w h o l e i n t e g e r v a l u e. T h e t h eo r y

w o r k e d w e l l f o r s in g l e e le c t r o n m a t e r ia l s , b u t i t w a s n o t c a p a b l e o f d e a l i n g w i t h a t o m s t h a t c o n -

t a i n e d m o r e t h a n o n e e l e c tr o n . W h e n t h e e l e c tr o n m o v e d f r o m a h i g h e r e n e r g y l ev e l, E 2, t o a

l o w e r l e ve l, E l, a q u a n t u m o f r a d i a t i o n w a s e m i t te d . T h e f r e q u e n c y o f t h e e m i t t e d r a d i a t i o n w a s

g i v e n b y E = E 2 - E1 =

hr

A b o u t t w o d e c a d e s l a t er , i n 1 92 4, d e B r o g l ie [1 5] s u g g e s t e d t h a t t h e d u a l n a t u r e o f r a d i a t i o n

m i g h t a ls o b e t r u e f o r m a t te r . T h a t i s, a n y m o v i n g c o r p u s c l e, r a d i a t i o n o r m a t t e r , h a s a w a v e -

l e n g t h a s s o ci a te d w i t h i t a n d t h a t a s im i l a r r e l a ti o n s h i p e x i s t s b e t w e e n m o m e n t u m a n d t h e

w a v e l e n g t h o f b o d i e s o t h e r t h a n l i g h t q u a n t a w i t h ~ =

h/mv

T h i s w a s l a t e r c o n f i r m e d f o r e l ec -

t r o n s b y T h o m s o n a n d o t h e r s [ 1 6,1 7].


11/13

Radiation uring

11

a n y p a r t i c u l a r w a v e l e n g t h , E ( c a n b e c a l c u l a t e d f r o m

= I q [ 6 6 2 6 s e c ] [ luantum

T h i s e n e r g y i s t h e s p ec i fi c e n e r g y a s s o c i a t e d w i t h t h e w a v e p a c k e t t h a t

h a s a w a v e l e n g t h / t . Q u i t e o f te n , c h e m i s ts a r e i n te r e s t e d i n th e e n e r g y t h a t is

a s s o c ia t e d w i t h a m o l e o f m a t e r i a l o r w i t h t h e e n e r g y a s s o c i a te d w i t h a m o l e

o f q u a n t a . T h i s q u a n t i t y o f e n e r g y h a s b e e n t e r m e d a n " E i n s t e i n " a n d is e q u a l

t o A v o g a d r o ' s n u m b e r , 6 .0 23 10 23, t im e s t h e e n e r g y a s s o c i a t e d w i t h a p a r -

t ic u l a r w a v e l e n g t h . S o , i f w e m a k e t h i s c h a n g e a n d s u b s t i t u t e f o r th e v e l o c it y

o f li g ht , t h e e x p r e s s i o n b e c o m e s

[ 6 .6 2 5 6 1 0 -2 7 e rg 9 s ec ]

E(~) = q u a n t u m

r

X [ 6 : 0 2 3

1 0 2 3

q u a n t u m ] [ 2 . 9 9 7 9 x 1 0 1 ~

c m ] [ ]

m o l e ] [ ~ e c k

[ 1 1 9 6 3 e 8 c m ] [ l

E (x ) = m o l e

w i t h t h e w a v e l e n g t h e x p r e s s e d i n c en t im e t e r s . S i nc e t h e w a v e l e n g t h is u s u -

a l ly e x p r e s s e d i n n a n o m e t e r s , a n d s i n c e o n e j o u l e , J, i s e q u a l t o i 10 7 e r g s ,

t h e e x p r e s s i o n f o r e n e r g y c a n b e e x p r e s s e d a s

[ 1 .1 9 6 3 X 108 e r g 9 c m ] [ 10 7 n m ]

E(x) = L m o l e j l _ ~ j

I 1 j ~ ] I X - ~ J 1 "1 96 3 1 08 j ~

1 1 07 e r g s = X m o l e

i n t e r m s o f j o u l e s p e r m o l e a n d a s

[ 1 . 1 9 6 3 x 1 08 j o u l e s ] [ . 1 k c a l ] _ 2 . 8 59 2 1 0 4 k c a l

E (x ) = [ k m o l e J [ 4 1 8 4 j o u l e s J k m o l e

i n t e r m s o f k il o c a l o r ie s p e r m o l e . V a l u e s o f E (~ ) a t v a r i o u s w a v e l e n g t h s a r e

g i v e n in T a b l e 1.1 . T h e e n e r g y r e q u i r e d t o r u p t u r e a d o u b l e b o n d a n d f o r m a

T A B L E 1 .1 . M a g n i t u d e o f E n e r g y a t V a r i o u s W a v e l e n g t h s .

W A V E L E N G T H ,

t

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

7 0 0

E N E RG Y P E R M O L E

JOULES 10 - s

1 1 . 9 6

5 , 9 8

3 . 9 9

2 . 9 9

2 . 3 9

1 , 71

KILOCALORIES

2 8 6 . 0 0

1 4 3 . 0 0

9 5 . 3

7 1 . 5

5 7 . 2

4 0 . 9


12/13

2 R A D I A T I O N C U R I N G O F C O A T I N G S

free radical depends on the particular chemical molecule receiving the en-

ergy. It is usua lly on the order of 10-80 Kcal per mole (0.42-3.35 x 105

J/mole) ) [19], which m ay be compared with the values in Table 1.1.

P h o t o n P r o d u c t io n

A phot on is the q uan tum of electromagnetic energy of a single wavelength.

Photons are produc ed in the ultraviolet and visible radiation regions by

9 spontaneous or stimulated radia tion transitions between electronic energy

levels in atoms or molecules,

9 recombination of two radiative bodies as in electron-ion recombination,

and

9 electron retardation wherein radiation is emitted when an electron is

forced to deviate from its normal path as occurs when an electron passes

near a positive ion.

If it is possible for an atom to exist in two energy states, E~ and E2 with

E 2 ~ E l

there are three possible radia tion processes that can connect the two

energy levels. There can be a simple, uncomplicated absorption in which the

ato m at energy level E1 is placed in a radia tion field that contains wavelength

K, where K =

h c / E 2 - E l ) .

Here, it may absorb a pho ton a nd be excited to a

higher energy level E2 Induce d or stimulated emission occurs when an atom

that exists in energy state E2 is placed in a rad iation field that contains wave-

length K, where again ;~ is given b y h c / E 2 - El). A photo n may impinge on

the atom and cause it to emit a pho ton that is coherent with the impinging

photon. When this happens, the energy level drops to El. Spontaneous emis-

sion is a third method of photon generation. In this process, an atom in a

higher energy state E2 can spontaneousl y emit a photon and drop to energy

l e v e l E 1 These mechanisms lead to the photons or quanta that activate pho-

toinitiators that, as we shall see in ensuing chapters, cause polymerizat ion of

reactive species in the formation of radiation~cured coatings.

E l e c t r o n s

It is impor tant to point out that electrons are not included in the electromag-

netic spectrum. Electrons differ from photons in that they are particulate and

not wave-like in nature, and they have mass. Because of their mass, acceler-

ated electrons as fo rmed by an electron beam apparatus have the ability to

penetrate matter. These phenomena, as well as the depth of penetrat ion into

materials, are further discussed in Chapter 2. Photons, which are included in

the electromagnetic spectrum, can also penetrate matter. The depth of pene-

tration depends on their energy, wave, and particulate nature.


13/13

Radia t ion Cur ing 1 3

e f e r e n c e s

[1] Kolesk e, J . V. , Ra dT ech 2000,

Paint and C oating hldustry Ma gazine,

Vol. 16, No. 6 , June

2000, p. 34.

[2 ] Kins t l e , J . F ., Ra d ia t ion Po ly me r iza t io n ,

Paint and Varnish Production,

Vol. 63, No . 6 , June

1973, p. 17.

[3 ] L ake , R . T ., U l t rav io le t C ur in g o f Org an ic Co a t ings , Radiation Curing, Vol. 10, No. 2, 1983,

p. 18.

[4 ] Dav is , T . W . , Ul t rav io le t Ra d ia t ion , Microso ft| Encarta| On line Encyclopedia 2000.

[5] Glossary or Air Pollution Control of Industrial Coating Operations, E P A - 4 5 0 / 3 -8 3 - 0 1 3R , E n v i -

r o n m e n t a l P r o t e c t io n A g e n c y , W a s h i n g t o n , D C , D e c e m b e r 1 98 3.

[6] Br ezi nsk i, J. J., Ma nual on Determination of Volatile Organic Compou nds in Paints, Inks, and Re-

lated Co ating Produc ts,

M N I A , A S T M I n t e r n a t i o n a l , W e s t C o n s h o h o c k e n , P A , 1 99 3.

[7 ] Schaeffer , W . B ., UV Cu rab le M ate r ia l s Res pon se and i t s Re la t ion sh ip to Pow er Leve l an d

L a m p S p e c t ra ,

Proceedings, Volum e 1, RadT ech 90-North Am erica,

M arc h 25-29 , 1990, p . 29.

[8] L a w s o n , K . , S t a t u s o f U V / E B C u r i n g i n N o r t h A m e r i c a - 2 0 0 0 ,

RadTech 2000,

Bal t imore ,

MD , A pr i l 12 , 2000 .

[9] M oore , W. J ., Physical Chem istry, 2n d ed . , P ren t ice Ha l l , Inc . , NJ , 1955 .

[10] Planck, M., Anna len der Physik, Vo l. 1, 1900, p. 69; Annalen der Physik, VoL 4, 1901, p. 553.

[ 11 ] P h y s i c i s t s B r i n g L i g h t t o a S t o p T h e n S e n d I t o n I t s W a y , S c i- T e c h, www.foxnews.com, a n -

ua ry 19, 2001.

[12] E in s te in , A . ,

Annalen der Physik,

Vol. 17,1905 , p . 132.

[13] Rutherford, E. , Philosophical M aga zine, Vo l. 21, 1911, p. 669.

[14] Bohr , N. ,

Philosophical M aga zine,

Vo l. 26, 1913, p. 476.

[15] de Broglie, L., Philosophical Ma gazin e, Vol. 47, 1924, p. 446.

[16] T ho m ps on , G . P .,

Proceedings Ro yal So ciety,

A l1 7, 600, 1928.

[ 17 ] D a v i s s o n , C . a n d G e r m e r , L . H . ,

Physics Review,

Vo l. 31, 1927, p. 705.

[ 18 ] C o m m i s s i o n I n t e r n a t i o n a l e d e l 'E c l a i ra g e , I n t e r n a t i o n a l L i g h t i n g V o c a b u l a r y , C IE , P a r i s,

1970.

[1 9] R i d y a r d , A n d r e w , A c c u r a t e, H i g h P o w e r U V S p e ct ra l M e a s u r e m e n t , Proceedings o f

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Chapter 1 Radiation curing.pdf