Pigmented Coatings Cured with Visible Light - ACS Symposium

Chapter 16

Pigmented Coatings Cured with Visible Light 1,3

2,4

Barbara F. Howell , Anita de Raaff , and Thomas Marino

2

1

Carderock Division, Naval Surface Warfare Center, Annapolis Detachment, Annapolis, MD 21402-5067 Spectra Group, Ltd., 1722 Indian Wood Circle, Maumee, OH 43537

Downloaded by CORNELL UNIV on July 21, 2016 | http://pubs.acs.org Publication Date: November 1, 1997 | doi: 10.1021/bk-1997-0673.ch016

2

In order to develop an environmentally friendly, visible light curable, pigmented coating which can be applied for touch up purposes to minimally prepared steel surfaces, use of photobleaching photoinitiators has been investigated. Additional requirements are that the coating does not yellow during aging and that it can be applied to a cold surface. Photoinitiators containing BAPO [bis (2,6-dimethoxybenzoyl) -2,4,4trimethylpentyl-phosphine oxide] and DIBF (5,7diodo-3-butoxy-6-fluorone) were tested. DIBF can initiate curing by free radical reactions as well as by cationic means. It is used with an amine and the cationic photoinitiator OPPI (4octyloxyphenylphenyl-iodonium hexafluoroantimonate) for radical initiation, but for cationic initiation only OPPI and DIBF are needed. A cycloaliphatic epoxy has been rapidly cured cationically with this pair and only a small amount (0.1%) of DIBF photoinitiator is required. Performance test results are excellent indicating that through cure has been achieved. The purpose of work d e s c r i b e d here i s t o develop a c o a t i n g which can be used on naval v e s s e l s f o r touch up purposes while a ship i s underway. R a d i a t i o n curable coatings are of s p e c i a l i n t e r e s t because of the extremely low l e v e l of v o l a t i l e organic compounds emitted during c u r i n g . To be u s e f u l the c o a t i n g should be Navy gray and s u i t a b l e f o r a p p l i c a t i o n t o c o l d s u r f a c e s . When used i n confined spaces a f u r t h e r l i m i t a t i o n i s that the a p p l i c a t i o n should not produce ozone and f o r some Naval a p p l i c a t i o n s mercury i s not 3

Current address: 6694 Flamingo Road, Melbourne Village, FL 32904

4

Current address: General Electric Corporation, Antwerpen, Belgium

© 1997 American Chemical Society

Scranton et al.; Photopolymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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allowed. Photobleaching p h o t o i n i t i a t o r s (2), which are a c t i v a t e d by v i s i b l e l i g h t , have r e c e n t l y become commercially a v a i l a b l e . These substances photobleach d u r i n g the c u r i n g process which e l i m i n a t e s the y e l l o w i n g of a pigmented c o a t i n g that would otherwise occur with a p h o t o i n i t i a t o r absorbing l i g h t at long u l t r a v i o l e t o r short v i s i b l e wavelengths. Photobleaching a l s o allows l i g h t t o penetrate more deeply i n t o the c o a t i n g . Photobleaching photoinitiators investigated were BAPO [bis(2,6dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, mixed 1/4 with 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP)], as w e l l as DIBF (5,7-diiodo-3-butoxy-6-fluorone).


Experimental

1

Materials Oligomers and Resins. Oligomers and r e s i n s used were: E b e c r y l 3700 epoxy a c r y l a t e , Radcure S p e c i a l t i e s , Inc., Smyrna, GA 30080; Unicarb, which i s 73% UVR 6128 (a c y c l o a l i p h a t i c epoxy), 26.5% p o l y o l and 0.5% S i l w e t L-7602 (See under miscellaneous compounds), Union Carbide Chemicals and P l a s t i c s Co., Inc., Danbury, CT 06817-0001; and Heloxy 505 M o d i f i e r , S h e l l Chemical Co., Houston, TX 77210. Monomers. Monomers used were: IBOA (isobornyl a c r y l a t e ) , TMPTA (trimethylol-propane t r i a c r y l a t e ) , and TRPGDA ( t r i p r o p y l e n e g l y c o l d i a c r y l a t e ) a l l three from Radcure S p e c i a l t i e s , Inc., Smyrna, GA 30080; TMPTMA. ( t r i p r o p y l e n e g l y c o l t r i m e t h a c r y l a t e , Sartomer Co. West Chester, PA 19283; Photomer 4017, 1,6-hexanediol d i a c r y l a t e , Henkel Corp., Ambler, PA 19002-3498; and Epon 828, the d i g l y c i d y l ether of bisphenol A, S h e l l Chemical Co., Houston, TX 77210 v i a a d i s t r i b u t o r . P h o t o i n i t i a t o r s . P h o t o i n i t i a t o r s used i n c l u d e : Darocur 1173 and Darocur 4265 which are 2-hydroxy-2-methyl-1-phenyl1-propanone; Irgacure 907, which i s 2-methyl-l[4(methylthio)phenyl]-2-(4-morpholinyl)-l-propanone(MMMP) ; and CGI 1700 which contains BAPO [bis (2, 6-dimethoxybenzoyl) 2,4,4-trimethylpentylphosphine oxide] mixed 1/4 with HMPP (2hydroxy-2-methyl-l-phenyl-l-propanone) a l l from Ciba Geigy, Hawthorne, NY 10532; FX-512, a sulfonium p h o t o i n i t i a t o r , 3-M Industrial Chemical Products Div. , St. Paul MN 55144-1000; DIBF, 5,7-diiodo-3-butoxy-6-fluorone (H-Nu 470), Spectra Group, L t d . , Maumee, OH 43537; OPPI, 4-octyloxyphenylphenyliodonium hexafluoroantimonate, G. E. S i l i c o n e s , Waterford, NY 12188; and DIDMA, N,N-dimethyl-2,6*To describe procedures adequately, it is occasionally helpful to identify commercial products and equipment. In no case does such identification imply Naval Surface Warfare Center recommendation or endorsement, nor does it imply that the item is necessarily the best available for the purpose.


16.

HOWELL ET AL.

221

Pigmented Coatings Cured with Visible Light

diisopropylaniline, 39568-1427.

First

Chemical

Corp.,

Pascagoula,

MS

Photosensitizer. The photosensitizer ITX, isopropylthioxanthone, was obtained from Aceto Corp., Lake Success, NY 11042-1215. Pigments. Pigments employed i n c l u d e TiONA T i 0 , SCM Chemicals, l o t 25-JKSJ, Baltimore, MD 21202; lampblack, l o t 97 P f i z e r M i n e r a l s and Pigments Div., New York, NY 10017; manganese f e r r i t e and copper chromite, Ferro Colors, Ferro Corp., P i t t s b u r g h , PA 15204.


2

Miscellaneous compounds. Other m a t e r i a l s used i n c l u d e : FC-171, f l u o r o c a r b o n s u r f a c t a n t , 3M I n d u s t r i a l Chemical Products D i v i s i o n , S t . Paul MN 55144-1000; Byk 306, Bykchemie USA, W a l l i n g f o r d , CT 06492; Polyol (polyc a p r o l a c t o n e t r i o l a p o l y e s t e r p o l y o l ) , and Silwet L-7602 (polyalkylene oxide m o d i f i e d p o l y d i m e t h y l s i l o x a n e ) , both from Union Carbide Chemicals and P l a s t i c s Co., Inc., Danbury, CT 06817-0001. Equipment. Lamps used i n c l u d e an RC-500 xenon lamp obtained from Xenon Corp., Woburn MA. 01801; a Portacure 1000F/1500F mercury lamp, American U l t r a v i o l e t , Murray H i l l , N.J. 07974; and a Fusion Systems F300 u l t r a v i o l e t lamp system operated with a V-bulb. Test Procedures. Coatings were t e s t e d by the f o l l o w i n g procedures. For Crosshatch adhesion, ASTM-D3359, a s p e c i a l s c r i b e was employed which had 11 sharp p o i n t s spaced approximately 1 mm apart. The f i l m was scored twice with t h i s t o o l so that scratches were p e r p e n d i c u l a r . S p e c i a l tape was then p l a c e d f i r m l y over the scored area and p u l l e d o f f . The number of squares of f i l m c o a t i n g remaining were counted, and t h i s provided a numerical b a s i s f o r measuring surface adhesion. No squares are removed f o r a s a t i s f a c t o r y coating. For impact t e s t s , ASTM-02 794, a b a l l of known mass was dropped a f i x e d height through a tube, impacting the coated metal panel. I n d i r e c t impact r e s i s t a n c e i s reported as the f r a c t i o n of p a i n t removable ( p i c k o f f ) from the impact s i t e when the panel i s struck on the uncoated s i d e . Direct impact r e s i s t a n c e i s r e p o r t e d as the p i c k o f f at the impact s i t e when the panel i s struck on the coated s i d e . Optimally, no c o a t i n g i s loosened by t h i s t e s t . For the balanced beam-scrape adhesion t e s t , ASTM-2197, weights were added t o a commercial balanced beam t e s t e r . The kind of s c r a t c h produced by the s t y l u s with the d i f f e r e n t loadings was recorded and served as a measure of f i l m hardness. Coatings which are not scratched to bare metal by 500 g are d e s i r a b l e . For the s a l t fog t e s t , ASTM-B117, panels were p l a c e d on a rack i n a s a l t fog spray f o r 500 or 1000 hours at a


PHOTOPOLYMERIZATION

222

temperature of 49 °C. Amount and k i n d of b l i s t e r i n g was assessed by the method of ASTM D-714. The best coatings do not b l i s t e r o r r u s t d u r i n g t h i s exposure. B l i s t e r i n g , ASTM-D-714, was assessed according t o the number and s i z e of b l i s t e r s as i l l u s t r a t e d i n t h i s ASTM procedure. Number 6 b l i s t e r s are approximately 1 t o 2 mm i n diameter, and f o r dense b l i s t e r i n g 50% o r more of the panel surface i s covered by b l i s t e r s . T e s t P a n e l P r e p a r a t i o n . G r i t - b l a s t e d , s t e e l Q-panels, 3 χ 5 i n were s t o r e d i n p r o t e c t i v e paper i n s i d e a d e s i c c a t o r . Just p r i o r to use they were r i n s e d with a 1/1 (by volume) mixture of mineral s p i r i t s and methyl e t h y l ketone, allowed to dry, then r i n s e d with methyl e t h y l ketone. Coatings were a p p l i e d with a brush and thinned t o uniform thickness with a number 24 h e l i c a l l y wound (Meyer) rod.


2

R e s u l t s and D i s c u s s i o n . Early Investigations. Work has been conducted a t the Carderock D i v i s i o n to develop a s u i t a b l e c o a t i n g f o r Naval a p p l i c a t i o n s such as touching up a r u s t y area i n s i d e a tank. For the c o a t i n g t o cure w e l l i t i s necessary f o r some component of the mixture t o absorb r a d i a t i o n at a wavelength longer than 380 nm below which t i t a n i u m d i o x i d e absorbs s t r o n g l y , Figure 1. The f i r s t combination t r i e d was ITX (isopropyl-thioxanthone) and MMMP (2-methyl-l[4(methylthio)-phenyl]-2-(4-morpholinyl)-1-propanone). ITX has a maximum absorbance at 386 nm and i s known to t r a n s f e r energy e f f i c i e n t l y t o MMMP, a p h o t o i n i t i a t o r , which can t r i g g e r the f r e e r a d i c a l r e a c t i o n of the a c r y l a t e containing r e s i n components. T h i s combination produced c u r i n g i n the presence of pigment, but the coated panel became s l i g h t l y yellowed a f t e r 500 h i n the s a l t f o g . Therefore use of other p h o t o i n i t i a t o r s was i n v e s t i g a t e d . BAPO. The p h o t o i n i t i a t o r , BAPO, absorbs throughout the UV but a l s o i n the long UV and short v i s i b l e l i g h t r e g i o n s . It has the f o l l o w i n g s t r u c t u r e ,

and i t s long UV-short V i s absorbance spectrum i n shown i n Figure 2. When exposed t o l i g h t , i t d i s s o c i a t e s a t the carbonyl groups forming two f r e e r a d i c a l s and one b i r a d i c a l . A f t e r d i s s o c i a t i o n , i t l o s e s i t s yellow c o l o r and i n i t i a t e s a chain r e a c t i o n with the a c r y l a t e group which l i n k s molecules together producing the cured c o a t i n g . In a d d i t i o n t o proper reagent s e l e c t i o n , i n order t o


16. HOWELL ETAL.


100


5

s

π

1

1

1

r

80 60 h

§ 4 0 h w

ω

T i 0 Pigment Absorption Spectrum 2

20

320

F i g u r e 1.

360 400 W A V E L E N G T H (nm)

440

Titanium d i o x i d e a b s o r p t i o n spectrum.

320

Figure 2.

360 400 W A V E L E N G T H (nm)

440

BAPO absorbance spectrum.


223


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PHOTOPOLYMERIZATION

produce a pigmented c o a t i n g which can be r e a d i l y cured with u l t r a v i o l e t or v i s i b l e l i g h t i t i s necessary to choose a lamp which emits s t r o n g l y at a wavelength that i s not absorbed by the t i t a n i u m d i o x i d e pigment, that i s at wavelengths longer than approximately 380 nm. A xenon lamp has a strong output i n the u l t r a v i o l e t above t h i s wavelength and emission remains strong throughout the v i s i b l e r e g i o n . However, commercially a v a i l a b l e xenon lamps are more expensive than other kinds of lamps, and a reasonably p r i c e d u n i t cures a patch 2 χ 3 i n which makes i t j m p r a c t i c a l f o r cure of a much l a r g e r area such as 2 χ 2 f t . A mercury lamç i s commercially a v a i l a b l e which can cure an area 4 χ 10 i n , and i t has a lamp u n i t that i s l i g h t weight enough to be hand h e l d so that i t can be moved over a s u r f a c e to cure a c o a t i n g . Moreover, ozone f r e e lamps are a v a i l a b l e f o r t h i s u n i t which e l i m i n a t e s problems with ozone generated when work i s done i n a c o n f i n e d space. However, mercury lamps have a weak output i n the long UV-short V i s s p e c t r a l region, Figure 3, and f o r some Naval purposes, mercury i s not allowed. Another p o s s i b l e lamp a l t e r n a t i v e i s a Fusion Systems lamp f o r which a v a r i e t y of bulbs are a v a i l a b l e with s p e c t r a l outputs i n d i f f e r e n t s p e c t r a l regions. Spectra of the bulbs a v a i l a b l e f o r Fusion Systems lamps are shown i n Figure 3. By comparing Figures 1, 2, & 3, i t can be seen that the V-bulb emits i n a s p e c t r a l r e g i o n i n which t i t a n i u m d i o x i d e does not absorb s t r o n g l y and that the BAPO absorbs r a d i a t i o n at these wavelengths so that l i g h t can penetrate into the c o a t i n g . A gray-pigmented epoxy acrylate formulation has been s u c c e s s f u l l y cured with use of BAPO/HMPP and the V-bulb. I n i t i a l l y , carbon b l a c k was added to the UV-curing formulation to produce a gray c o l o r , but i t was found that t h i s substance i s a f r e e r a d i c a l absorber which i n t e r f e r e s with the UV-curing process. Following a suggestion made by Lucie ( M i c r o l i t e Corp., personal communication, 1990.) manganese f e r r i t e b l a c k s p i n e l was used i n s t e a d of carbon black to produce the gray c o l o r when mixed with t i t a n i u m d i o x i d e . I t was found that when s p i n e l comprised 10% of the pigment, the d e s i r e d gray c o l o r was produced, and c u r i n g occurred much more r e a d i l y than with the carbon b l a c k . D i s p e r s i o n of the pigment was done by b a l l m i l l i n g i n t o a mixture of the epoxy a c r y l a t e (Ebecryl 3700) thinned 2/1 by weight with r e a c t i v e d i l u e n t . Since the commercially a v a i l a b l e epoxy a c r y l a t e is extremely v i s c o u s , i t i s necessary to t h i n the f o r m u l a t i o n with a r e a c t i v e d i l u e n t . A mixture with s a t i s f a c t o r y v i s c o s i t y f o r brush-on a p p l i c a t i o n contains about 45% d i l u e n t . Of the d i l u e n t s tested, TMPTA (trimethylolpropane t r i a c r y l a t e ) proved most s u c c e s s f u l . The f o r m u l a t i o n which performed best contained 1% BAPO/HMPP p h o t o i n i t i a t o r , 50% a c r y l a t e e s t e r of bisphenol A epoxy (Ebecryl 3700), 45% r e a c t i v e d i l u e n t , 5% pigment, and a few drops of the f l u o r o c a r b o n s u r f a c t a n t FC-171.


16. HOWELL ET AL.


Hg BULB


π D BULB

VBUL3

Q BULB

200 Figure 3.

250

300 350 NANOMETERS

400

450

500

Emission s p e c t r a f o r v a r i o u s lamps


225


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PHOTOPOLYMERIZATION

Several r e a c t i v e d i l u e n t s were t e s t e d which include trimethylolpropane t r i a c r y l a t e (TMPTA), t r i p r o p y l e n e g l y c o l d i a c r y l a t e (TRPGDA), i s o b o r n y l a c r y l a t e (IBOA) , and 1,6h e x a n e d i o l d i a c r y l a t e (HDODA) . Since TMPTA i s a t r i a c r y l a t e , i t provides more c r o s s l i n k i n g than the other d i l u e n t s and produces a very hard g l o s s y c o a t i n g . However with TMPTA, i t was d i f f i c u l t to get good through cure, e s p e c i a l l y i f the c o a t i n g thickness exceeded 60 micrometers. Through cure was measured with the cross hatch adhesion t e s t . I f the c o a t i n g has not been cured next to the metal surface i t w i l l be removed by the tape i s a p p l i e d f o r t h i s t e s t . Control of c o a t i n g thickness, as i s done with the Meyer rod, i s essential. The Meyer rod has a s o l i d metal core which i s h e l i c a l l y wrapped with wire of known diameter. A number 24 rod gives a cured c o a t i n g about 50 micrometers t h i c k . Through cured TMPTA c o n t a i n i n g formulations performed w e l l i n impact, balanced beam, Crosshatch adhesion, and s a l t fog t e s t s as shown i n Table I. I t a l s o s u c c e s s f u l l y withstood s i x months exposure to s a l t water or to UV l i g h t without l o s s of c o a t i n g i n t e g r i t y , although c h a l k i n g was observed. These r e s u l t s are s a t i s f a c t o r y f o r the intended a p p l i c a t i o n . It was found, when an attempt was made to conduct ship t r i a l s that the p a i n t a p p l i e r would be r e q u i r e d to wear long sleeves, gloves, s a f e t y g l a s s e s , and a p r o t e c t i v e face shield. I t i s a l s o a requirement that an eyewash fountain be nearby. These requirements were imposed because a c r y l a t e s are s k i n , eye and r e s p i r a t o r y t r a c t i r r i t a n t s , but the problem l i e s mostly with the a c r y l a t e d r e a c t i v e d i l u e n t r a t h e r than with the a c r y l a t e d r e s i n (2,3). To avoid these problems, tests have been performed with use of trimethylolpropane t r i m e t h a c r y l a t e (TMPTMA) as a replacement f o r TMPTA. TMPTMA has been shown to be much l e s s of an i r r i t a n t than TMPTA, and i s used i n dental r e s t o r a t i o n s . When used as p a r t of the r a d i a t i o n c u r i n g coating mixture, TMPTMA was found to cure more slowly than TMPTA and i t i s more d i f f i c u l t to get through cure. After testing many p h o t o i n i t i a t o r combinations, a s u c c e s s f u l formulation was produced which contained ITX, BAPO/HMPP, and N,Ndimethyl p - t o l u i d i n e combined one part to 1.7 parts hexanediol d i a c r y l a t e by weight. T h i s combination made up 6.5% of the t o t a l UV c u r i n g mixture which a l s o contained 40 percent TMPTMA, 48.5% epoxy a c r y l a t e and 5% pigment by weight. Although the V-bulb works w e l l with BAPO/HMPP, i t i s a metal h a l i d e bulb and contains mercury. In a d d i t i o n i t i s microwave powered, which provides quick s t a r t up and high l i g h t i n t e n s i t y , but a high speed fan i s r e q u i r e d f o r adequate c o o l i n g . T h i s makes the lamp assembly cumbersome and heavy ( i t weighs s i x t e e n pounds) . In a d d i t i o n s h i e l d i n g from microwave r a d i a t i o n i s provided by covering the end of the lamp assembly u n i t with a very f i n e tungsten wire mesh. The mesh could be e a s i l y broken i f the lamp u n i t were to be moved about to cure a v e r t i c a l surface so that the p r o t e c t i o n from microwaves would be l o s t .



e

f

0

g

400g

200g,

c

500g

Balanced Beam (Mass to scratch to bare metal)

Overall wrinkling Moderate rusting Dense #6 blisters Bleaching

Few to medium no. 2 blisters A little rust around the edges Bleaching

No. 2 medium dense Least amount of rust

d

Salt F o g 500 h, 49 ° C

Cured with a V - b u l b . C u r e d with a tungsten halogen bulb.

60 60 % pickoff 5 50

0%

40

40 30 % pickoff 0 50 30 % pickoff 10 10

f

b

Impact test Front Back (in-lbs)

5%

D I B F is 5,7-diiodo-3-butoxy-6-fluorone.

g

Cycloaliphatic epoxy III DIBF

e

Epoxy acrylate, DIBF , TMPTA

Epoxy acrylate BAPO/HMPP TMPTA

3

Crosshatch Adhesion (Percent removed)

Table I. Comparison of Resins and Photoinitiators


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DIBF. Spectra Group, working i n c o n j u n c t i o n with CDNSWC, has conducted many t e s t s with the p h o t o i n i t i a t o r DIBF o r i g i n a t e d at the Bowling Green State U n i v e r s i t y by Neckers and coworkers (4). DIBF (5,7-diiodo-3-butoxy-6-fluorone, H-Nu 470) i s of s p e c i a l i n t e r e s t because the very l a r g e molar a b s o r p t i o n c o e f f i c i e n t at 470 nm 30,200 allows low concentrations to i n i t i a t e photopolymerization (4) . I t has the f o l l o w i n g chemical s t r u c t u r e .

I

DIBF has the absorbance spectrum shown i n Figure 4. Coating mixtures c o n t a i n i n g t h i s p h o t o i n i t i a t o r can be cured with an intense tungsten halogen lamp, such as a p r o j e c t o r bulb. When DIBF i s i r r a d i a t e d i t forms an e x c i t e d s t a t e which has a very low absorbance, i . e . , i t photobleaches (1) . T h i s allows l i g h t to penetrate deeply i n t o the c o a t i n g . Because v i s i b l e l i g h t i s not e n e r g e t i c enough to break chemical bonds, d i r e c t p r o d u c t i o n of f r e e r a d i c a l s by the p h o t o i n i t i a t o r does not occur. Instead when c a t i o n i c i n i t i a t i o n i s needed, as f o r r e a c t i o n with epoxies, DIBF i s used i n c o n j u n c t i o n with an iodonium compound such as 4octyloxyphenyl-phenyliodonium hexaf luoroantimonate (OPPI) . It has been proposed that when i r r a d i a t e d , DIBF and OPPI i n t e r a c t to form a c a t i o n i c s p e c i e s . The DIBF OPPI combination has been shown to e f f i c i e n t l y cure a wide v a r i e t y of epoxies including cycloaliphatics. With t h i s p h o t o i n i t i a t o r i t i s p o s s i b l e to cure bisphenol A epoxies such as Epon 828 q u i c k l y without the need f o r a c r y l a t i o n of the epoxy. Cycloa l i p h a t i c epoxies were of s p e c i a l i n t e r e s t because they were expected to r e a c t much f a s t e r than bisphenol A type epoxies. Those t e s t e d i n c l u d e 3,4-epoxycyclohexylmethyl3',4 -epoxycyclohexyl-carboxylate (UVR 6110), bis(3,4 epoxy-cyclohexylmethyl) adipate (UVR 6128), and 1,2-epoxy4-vinylcyclohexane ( v i n y l cyclohexene o x i d e ) . I t was found that the v i n y l cyclohexene oxide reacted r a p i d l y , but work with i t was d i s c o n t i n u e d because i t has a f a i r l y high vapor pressure (2 t o r r at 20 °C) , an intense odor, and the p h o t o i n i t i a t o r does not d i s s o l v e i n t h i s r e s i n . A promising f o r m u l a t i o n was produced which contains the c y c l o a l i p h a t i c epoxy UVR 6110. The formulation contains 73% of the epoxy, 26.5% P o l y o l , 5% Silwet L7602 0.1% DIBF, 2.5% OPPI and 5-10 drops FC-171 i n 100 g of 1




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r e s i n with 5% pigment. Coating l a y e r s 100 micrometers t h i c k c o u l d be cured i n open a i r with the V-bulb and 4 passes under the lamp when the b e l t speed was 6 f t / m i n . The UVR 6110 f o r m u l a t i o n was a p p l i e d to g r i t b l a s t e d s t e e l Q panels. A f t e r cure i t was found that with the Crosshatch adhesion t e s t there was no p u l l o f f . In the impact t e s t (D2794) , with 6.8 j of impact, 95% of the c o a t i n g remained on the f r o n t impact and 50% on the back impact s u r f a c e . With the balanced beam (ASTM D2197), 400 g were r e q u i r e d to s c r a t c h to bare metal. These r e s u l t s are considered s a t i s f a c t o r y . However, i n s a l t fog (49 °C, 1000 h) the c o a t i n g performed l e s s w e l l . At the end of 1000 h, the panels showed o v e r a l l w r i n k l i n g , moderate r u s t i n g , dense #6 b l i s t e r s and b l e a c h i n g . By way of comparison, a standard two p a r t epoxy c o a t i n g shows no w r i n k l i n g , r u s t i n g or b l i s t e r i n g under these c o n d i t i o n s . Another f o r m u l a t i o n t e s t e d contained 30% of the UVR 6110 f o r m u l a t i o n (above) and 70% of a mixture c o n s i s t i n g of 73% UVR 6128, 26.5% P o l y o l and 0.5% S i l w e t L7602 with the same percentages of the FC-171 and pigment as the f i r s t formulation. There was no removal of c o a t i n g i n cross adhesion, no loosened c o a t i n g a f t e r 6.8j impact, and 1000g were r e q u i r e d on the balanced beam to s c r a t c h to the metal surface, s a t i s f a c t o r y r e s u l t s . However, a f t e r 500 h i n the s a l t fog there were many r u s t streaks and #6 dense or medium dense b l i s t e r s ; u n s a t i s f a c t o r y performance. In a d d i t i o n there was b l e a c h i n g of the pigment. Bisphenol A type epoxies such as Epon 828, were a l s o cured with the DIBF OPPI combination. When a m o d i f i e r , Heloxy 505 (a low v i s c o s i t y polyepoxide m o d i f i e r ) was added, v i s c o s i t y was reduced and adhesion to the metal surface and impact r e s i s t a n c e were improved as compared with the bisphenol A only. S u r p r i s i n g l y cure was f a s t e r than with the Union Carbide c y c l o a l i p h a t i c r e s i n s , but c h a r r i n g of the r e s i n d u r i n g cure was a problem. When used with N , N - d i m e t h y l - 2 , 6 - d i i s o p r o p y l a n i l i n e (DIDMA), the DIBF-OPPI p h o t o i n i t i a t o r can be used to cure systems which react by a f r e e r a d i c a l mechanism. I t has been suggested that the amine donates an e l e c t r o n to the DIBF c o n v e r t i n g i t to the r a d i c a l anion while forming an amine r a d i c a l c a t i o n which then l o s e s H to become an amine free r a d i c a l . Although the OPPI i s known to enhance r a d i c a l curing, i t s r o l e has not been c l e a r l y d e f i n e d . The amine r a d i c a l species formed upon i r r a d i a t i o n r e a c t s with a c r y l a t e double bonds forming a c r y l a t e r a d i c a l s which cause cross l i n k i n g (curing) (4) . Epoxy a c r y l a t e (Ebecryl 3700, 50%) combined with 25% TMPTA, 25% IBOA and with DIBF-OPPI-DIDMA pigment and FC-171 as above was a l s o used to coat panels. These panels showed no removal of c o a t i n g i n the cross hatch adhesion t e s t . With 6.8j there was no loosened c o a t i n g on the f r o n t a l impact surface, and only 30% was loosened on the back surface. Five hundred grams were r e q u i r e d to s c r a t c h to bare metal with the balanced beam. Performance was +


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s a t i s f a c t o r y f o r these three t e s t s as was found f o r the c y c l o a l i p h a t i c epoxies. However, a f t e r 1000 h i n the s a l t fog, there was extensive r u s t b l e e d and dense #6 b l i s t e r i n g as w e l l as b l e a c h i n g of the pigment, u n s a t i s f a c t o r y performance. A problem with t h i s p h o t o i n i t i a t o r i s that c u r i n g i s very slow with commercially a v a i l a b l e lamps such as a mercury lamp o r a V-bulb, bulbs which do not emit s t r o n g l y at the 470 nm absorbance maximum of DIBF. A Q bulb works best, but i s not optimal. At present there i s no commercially a v a i l a b l e , intense visible lamp f o r c u r i n g areas as l a r g e as 4 χ 6 i n q u i c k l y . Therefore, work i s being undertaken t o develop a tungsten halogen lamp which w i l l cure a l a r g e r area. A s u l f u r lamp i s being considered as an a l t e r n a t i v e as w e l l . The c u r i n g lamp should be p o r t a b l e and should not c o n t a i n mercury. T h i s type of lamp w i l l not pose the eye hazard a s s o c i a t e d with a UV e m i t t i n g bulb.


2

Borate p h o t o i n i t i a t o r s . Workers a t Spectra Group, L t d . (5) have r e c e n t l y ' i n v e s t i g a t e d use of borate compounds (tetramethylammonium t r i p h e n y l b u t y l borate (1) , and b u t y r y l c h o l i n e t r i p h e n y l b u t y l b o r a t e (5) i n c o n j u n c t i o n with DIBF, OPPI and DIDMA.

borate (1)

borate (5)

These borate compounds a c t as e l e c t r o n donors and i n t e r a c t with DIBF generating a b u t y l r a d i c a l which can i n i t i a t e a c r y l a t e p o l y m e r i z a t i o n . With a urethane acrylate, e i g h t y seven percent acrylate double bond conversion has been found t o occur with 5 sec exposure t o a 75 W tungsten bulb when weight percents of the three p h o t o i n i t i a t o r components are 0.05% DIBF, 0.43% OPPI and 0.5% borate 5. Comparable r e s u l t s were obtained with epoxy acrylates. Borate 1 i s l e s s s o l u b l e i n an a c r y l a t e f o r m u l a t i o n than i s borate 5 which may account f o r the g r e a t e r e f f e c t i v e n e s s found f o r borate 5 i n i n c r e a s i n g curing rate. Although no c o a t i n g performance t e s t s have been performed f o l l o w i n g borate a s s i s t e d c u r i n g , the performance i s expected t o be comparable t o that of other acrylate coatings.


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Conclusions. Photobleaching v i s i b l e l i g h t p h o t o i n i t i a t o r s have been t e s t e d f o r cure of gray pigmented coatings and found t o produce coatings which meet performance requirements. The best r e s u l t s so f a r have been obtained by use of BAPO/HMPP i n an epoxy a c r y l a t e . Although work with DIBF has not been optimized, i t i s c l e a r that i t i s f e a s i b l e t o cure r a d i a t i o n curable coatings with v i s i b l e l i g h t such as that emitted by a tungsten halogen lamp. Use of v i s i b l e l i g h t i s d e s i r a b l e i f a p o r t a b l e l i g h t c u r i n g source i s t o be used because of the reduced eye hazard and the f a c t that these lamps do not c o n t a i n mercury. However, a t the present time there i s no commercially a v a i l a b l e p o r t a b l e l i g h t source meeting the performance requirements. Since a c r y l a t e s are s k i n , eye and r e s p i r a t o r y system i r r i t a n t s , i t would be d e s i r a b l e to f i n d a system which d i d not c o n t a i n these groups. Methacrylates are being t e s t e d as p o s s i b l e a l t e r n a t i v e s , but cure i s much slower than with acrylates. Acknowledgement. Funding support f o r t h i s work has been r e c e i v e d from O f f i c e of Naval Research

the

Literature Cited. 1. Pappas, S. P., In Radiation Curing Science and Technology; Pappas, S. P . , E d . , Plenum Press: New York, NY, 1992, p 9. 2. American Industrial Hygiene Assoc. J. 1981, 42, pp853 f f . 3. Andrews, L . S.; Clary, J. J. J. Toxicology & Env. Health 1986, 4. Marino, T. L.; Martin, D . ; Neckers, D. C. Adhesives Age, 1995, pp22 ff. 5. Marino, T. L.; De Raaff, A. M.; and Neckers, D. C. RadTech'96 North America UV/EB Conference Proceedings, A p r i l 28 - May 2, 1996, pp 7-16.


Pigmented Coatings Cured with Visible Light - ACS Symposium

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