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Recent developments in radiation curing in the USA
0146-5724/90 $3.00 + 0.00 Pergamon Press plc
Radiat. Phys. Chem. Vol. 35, Nos 1-3, pp. 46-51, 1990 Int. J. Radiat. Appl. Instrum., Part C Printed in Great Britain
RECENT DEVELOPMENTS IN RADIATION CURING IN THE USA
Sam V. Nablo Energy Sciences Inc. 8 Gill Street Woburn, MA 01801 USA
ABSTRACT Radiation curing activities in the USA continue to focus strongly on the use of both UV and EB for the polymerization of silicone release coatings on paper and film. The high speeds of cure (200 m/minute) achievable at "room temperature" continue to make this the process of choice for the sillconlzatlon of both film and paper. Some of the process difficulties peculiar to this chemistry will be discussed. The success of the multi-color lithographic printing of web using single station electron curing has resulted in a major market for electron processors. The aesthetic and physical properties of the overprint varnishes used in this application are important and will be discussed. The relatlvely high costs of both the inks and varnishes will probably limit this application to folding carton use for foodstuffs and high quality products, with UV continuing to dominate in lower speed, less demanding applications. The application of electron initiated graft modlflcatlon of polymer materials, partlcularly for blologlcal/medlcal device appllcation, is~ showing a good rate of development as is the use of selective t r e a t m e n t of materials for functional modification of packaging films, particularly for gas permeability control. Some examples of these appllcatlons will be reviewed. Continued work with "deep" curing or vulcanization of composite structures, particularly in the elastomers fleld, will be discussed with a brief review of the continuing growth of electron processing in this industry, particularly for tires and roofing products.
KEYWORDS electron curing, electron processing, radiation crossllnklng, graft copolymerlzatlon, r.c. release products
RELEASE COATINGS There has been a great deal of interest in the use of radiation curable silicone coatings s i n c e t h e t e c h n o l o g y f o r b o t h UV and EB was f i r s t c o m m e r c i a l i z e d i n t h e m i d d l e o f t h e l a s t decade ( c a . 1975). These c o a t i n g s a r e t y p i c a l l y a p p l i e d to p a p e r f o r f i l m r e l e a s e s h e e t s , as i n t h e i r u s e f o r p r o t e c t i n g t h e p r e s s u r e s e n s i t i v e a d h e s i v e s on l a b e l s , t a p e s , e t c . The major requirements of such coatings are tha t they are we ll anchored to the re le a s e s he e t, t h a t t h e y p r o v i d e a s t a b l e r e l e a s e v a l u e t o t h e a d h e s i v e o v e r an a c c e p t a b l e r a n g e o f temperature and time, that their release values can be adjusted by minor formulation changes to satisfy the product release requirement, and that they demonstrate a modest dose to cure without undue or troublesome oxygen inhibition of the surface polymerization. The conventional heat cure process (Bickford and Jachmann, 1988) for these coatings utilizes condensation polymerization of an OS-difunctlonal silicone with a hydrogen functional silicone compound, applied out of solvents and catalyzed with some soluble tin derivative. Addition polymerization is also utilized with platinum group catalysis of the crosslinking of a vinylfunctional silicone again with a hydrogen functional silicone. I n h i b l t o r s m u s t be u s e d w h i c h r e l e a s e t h e c a t a l y z e r a t e l e v a t e d t e m p e r a t u r e s ; t h e p o i s o n i n g of t h e s e catalysts can cause serious problems in production. Since high temperatures are i n v o l v e d i n b o t h o f t h e s e p r o c e s s e s , e m b r l t t l e m e n t and r e m o i s t u r i z a t i o n of a p a p e r
46
7th International Meeting on Radiation Processing
47
substrate are usually involved, while their use on thermally sensitive film is impractical since dimensional stability of the release sheet is of great importance. No solventless thermal cure systems are currently available at cure temperatures below 200°F (Eckberg, 1988)--hence the high interest in radiation curables which offer the only proven route to ambient temperature cure. The radiation initiated processes (laser, UV, EB) involve the radical initiation of chain reactions of the acrylate groups which have been introduced into a linear polydimethylsiloxane. More recently (Eckberg, 1988), epoxy-functlonal polysiloxanes have been introduced by General Electric which are crosslinked by UV with the assistance of silicone miscible cationic photocatalysts. The major problems which have been faced in the commercialization of this technology lie in inerting and in the control of light volatile fractions produced during the curing process. For the most widely used Goldschmidt AG Tego ® Silicone acrylates (Weitemeyer et al, 1987), llne speeds of 30-40 m/min are realized with one 120 W/cm medium pressure mercury lamp with good inerting, and photolnitlator content in the 3-5% range. Higher initiator content can be used to overcome oxygen sensltivity but release properties suffer. For electron curing, current formulations from Goldschmidt AG require 2 megarads to cure at oxygen levels below a few hundred parts per million in the cure zone. Since SiO 2 is readily formed in this region due to free Si and SiO in the treatment zone, the oxygen levels must be minimized. One of the major problems in successful commercialization of the EB process has been the control of SiO 2 formation so that its condensation on the window surface does not lead to loll overheating, stress and failure. These early difficulties are now sufficiently well understood to provide good system performance and have highlighted one of the major diagnostic problems of radiation curing of 02 sensitive coatings at elevated speeds; e.g. 200-400 m/mlnute. The treatment zone 02 analyzers are only providing an indication of the condition of the environment above the coating--not at its boundary layer interface where the important reaction kinetics are taking place. The result is that the 02 analyzer indication, at whatever its location, must be related to the quality of cure for a given web speed--inerting condition. As a result, the knowledge of the tolerable 02 level at a given speed (dose rate) is used to adjust the inerting conditions required for adequate cure. Weitemeyer and co-workers have discussed these and other important substrate-coating parameters.
WEB-OFFSET PRINTING The successes of electron and ultraviolet curing in web offset or lithographic printing constitute the major advance in "radiation" curing in the U.S. over the last two years. In the lithographic process, the image to be printed is formed on a metal plate and fixed to a plate cylinder. The image areas are at plate level and are ink receptive and water
Figure 1
Six color EB web offset line
48
SAM V. NABLO
repellant; the non-lmage areas are water receptive--when they are wetted with fountain solution they become ink repellent. The inked imaging areas on the plate are transferred to the rubber blanket cylinder and the image transferred to the web under pressure against the impression cylinder. The inks are highly viscous and hydrophobic, and typically are applled at thicknesses in the I-4 micron range, so are easily penetrated and cured by low energy electrons. For present folding carto$ applications, primarily for food packaging, an EB cured overprint varnish of a few gm/m ~ is usually used in the final print station, and it is typically applied with a flexographic type plate which protects the inked areas and leaves regions of unvarnished paperboard for later gluing of the container. One such 1.05 meter (42") system (UPC 19891 using a 6 color Hamilton press and Zerand diecutter is shown in Figure I, and provides an indication of the relatively small in-llne footprint of the curing station. These presses typically operate at speeds of 150 m/mlnute on a round the clock basis. There is no venting required with these 100% reactive inks and varnishes, and they are normally installed in air conditioned plants both for worker comfort as well as for paper/paperboard moisture content stabilization. For ~he system shown, inerting is accomplished with a nitrogen flow of some 2000 scfh (or 55 mJ/hr). The General Printing Ink Division of Sun Chemical Corporation has been the major supplier of these inks and varnishes in the US and their users report good satisfaction with color weight control at the printing stations and the excellent performance (abrasion/solvent resistance and gloss control) of the overprint varnish (GPI 19891. There are numerous techniques used for the "quality of cure" control of such EB/UV processed products. A simple methylethylketone rub test is the most commonly used, quite often in conjunction with a simple Crockmeter, an oscillatlng weighted head which presses a solvent loaded swab against the coated surface to be tested. More sophisticated sampllng tests will be developed and are being used where direct or indirect food contact is involved in the packaging appllcatlon. Although the majority of the 30 or so presses using electron curing are running roll:roll or roll:dle cut, the technology has been used for some time at Mitsumura Printing Co. Ltd. in Tokyo, Japan with a 1.0 meter x 0.65 m sheet feed offset Roland press. The configuration is shown in Figure 2. Sheet gripping using vacuum hold down is accomplished in the infeed A and outfeed C zones with sheet transfer to a stainless steel vacuum hold down conveyor in the central process zone B. Details of the inerting performance of this system which works well to sheet speeds of 300 fpm (90 m/mln), have been published (Nablo,
1988). ACCELERATOR
/SELFSHIELD
Figure 2
C
I
I
/METAL CONVEYOR CHAINS
/FABR IC CONVEYORBELTS
g
m
|NERTINGZONE
~
C'B
B I
I
8-A
A
Vacuum hold-down sheet stock Selfshield for electron cured lithographic printing
The dramatic increase in the use of ultraviolet for graphic arts in the United States is exceeding its growth in any other industry. A recent analysls by Omega Research Associates (19891 of these markets projects on annual rate of growth of 17.8% over the 1988-1993 period. Historically some 25% or 860 of the 3490 UV units installed by 1983 were in graphic arts which had risen to 30% or 2730 of the 9100 units by 1988. The referenced study projects that 35% or 6200 of the 17,700 units in the U.S. by 1993 will be in graphic arts. It seems likely that a comparable annual growth will take place in the much smaller segment of the graphic arts market occupied by electron beam curing.
7th International Meeting on Radiation Processing
49
Its extension beyond web or sheet offset to the much larger markets of flexographlc or gravure printing does not seem llkely without major improvements in the economics of the process for these printing processes for which interstatlon curing is currently required. A major area of growth for electron curing lles in security (intaglio) printing (0'Brlen, 1987) where ultraviolet is inadequate. The intagllo process is slmilar to gravure except that the inks are wiped off the non-lmaglng areas of the plate with paper or with wiping cyllnders.
GRAFT MODIFICATION OF POLYMERS - F i b e r s ,
F i l m and Tow
T h e r e h a s b e e n a g r a d u a l l y i n c r e a s i n g l e v e l of a c t i v i t y i n t h e r a d i a t i o n g r a f t m o d i f i c a t i o n of p o l y m e r s o v e r t h e p a s t few y e a r s due t o an improved a w a r e n e s s of t h e p r o c e s s o r m a c h i n e r y now a v a i l a b l e f o r t h e t r e a t m e n t of t h e s u r f a c e s of f i l a m e n t a r y p r o d u c t s o r f i l m at high dose rates. A v e r y b r o a d b a s e o f r a d i a t i o n m o d i f i c a t i o n of p o l y m e r s was e s t a b l i s h e d o v e r t h e p a s t two d e c a d e s , l a r g e l y w i t h low d o s e r a t e ( I Gray p e r s e c o n d ) gamma-ray s o u r c e s . Only a few r a d i a t i o n g r a f t e d p r o d u c t s h a v e b e e n i n t r o d u c e d i n t o t h e m a r k e t p l a c e ( f i l t e r membranes ( T a n n y , 1984), b a t t e r y s e p a r a t o r s , e t c . ) l a r g e l y due t o t h e c o s t and c o m p l e x i t y o f t h e b u l k p r o c e s s i n g o f m a t t e r a t t h e d o s e s r e q u i r e d f o r g r a f t c o p o l y m e r i z a t i o n , u s i n g gamma-ray s o u r c e s . The d e v e l o p m e n t of e l e c t r o n beam e q u i p m e n t s u i t a b l e f o r t h e h i g h s p e e d t r e a t m e n t of f i l a m e n t a r y and f i l m p r o d u c t s ( a t I000 kGy p e r s e c o n d ) h a s i m p r o v e d t h i s s i t u a t i o n o v e r t h e p a s t d e c a d e so t h a t t h e i n d u s t r i a l u s e r ( i . e . c h e m i c a l e n g i n e e r ) i s a g a i n e v a l u a t i n g these sources for commercial applications. Among t h e m a j o r a r e a s o f a p p l l c a t l o n u n d e r s t u d y a r e f i b e r o r tow m o d i f i c a t i o n f o r i m p r o v e d s u r f a c e p r o p e r t i e s ( w e a r , s o l l r e l e a s e , hydrophillclty), m o d i f i c a t i o n o f non-woven f a b r i c s f o r i m p r o v e d a b s o r p t i v i t y (Vrelich, 1 9 7 9 ) , p a c k a g i n g f i l m m o d i f i c a t i o n f o r r e d u c t i o n o f g a s p e r m e a b l l l t y and h a z e , end improvement in f r a g r a n c e / f l a v o r r e t e n t i o n and s u r f a c e w e t t a b i l l t y (Wyman e t a l , 1989), and s t a t i c dissipative f i l m s f o r vacuum f o r m a b l e o r s h r i n k wrap a p p l l c a t l o n s (Keough, 1988). A v i e w o f a s m a l l (5 k i l o w a t t ) p i l o t d e s i g n e d f o r f i b e r / f i l a m e n t work i s shown i n F i g u r e 3. I t w i l l p r o v i d e 12,600 Mrad fpm (3800 Mrad mpm) i n t h e 52 p a s s c o n f i g u r a t i o n shown f o r t h i s 30 cm p i l o t ( N a b l o , 1988).
Figure 3
52 pass festooner on 30 cm pilot
50
SAM V. NABLO
VULCANIZATION AND STABILIZATION OF COMPOSITES One of the major uses of medium electron curing systems (0.5-i MeV) is in the precure or stabilization of components (such as inner liners, body plies and chafer strips) (Bohm and Tveekrem, 1983). These stabilized components are cured by conventional thermal vulcanization in the "built" tire, providing improvements in quality and performance with less material. A typical ply construction is shown in Figure 4 in which the beam must penetrate 400-500 gm/m of SBR to reach and stabilize the location of the reinforcing cord or fabric. Single ply EPDM roofing material is produced commercially using electron vulcanization to replace heat and represents a major growth area for these processors.
~
~
\ \~
Reinforcin~ Cord SB Rubber
Figure 4
~
~
~
Bleed Cord
Green Tack Preserved
Electron Stabilization of SBR Ply Structure
REFERENCES
(1)
Bickford, R.H. and I. Jachmann (1988). Innovative and Unique Applications of Radiation Cured Silicone Release Coatings, Proc. RadTech '88, April 24-8, New Orleans, LA, 569-575; RadTech Internatlonal, 60 Revere Drive, Suite 500, Northbrook, IL 60062, USA.
(2)
Bly, J.H. (1983). Radiation Curing of Elastomers, Journal Ind. Irr. Tech. ~, i, 51.
(3)
Bohm, G.G.A. and J.O. Tveekrem (1982). The Radiation Chemistry of Elastomers and its Industrial Applications, Rubber Chemistry and Technology 55, 576.
(4)
Drelich, A.M. (1979). Method for Producing Bonded Nonwoven Fabrics Using Ionizing Radiation, U.S. patent 4,146,417, March 27.
(5)
Eckberg, R.P., Radiation Curable Epoxysilicone Coatings, ibid, 576-586.
(6)
General Printing Ink Division, Sun Chemical Corporation, Carlstadt, NJ 07072, USA, private communication.
(7)
Keough, A.H. (1988). Static Dissipative Rigid Plastic Sheets from EB Curing, Proc. RadTech '88, April 24-28, New Orleans, LA, 324; RadTech International, 60 Revere Drive, Suite 500, Northbrook, IL 60062 USA.
(8)
Nablo, S.V., M. Fishel and B.S. Quintal (1988). Developments in Product Handling for Selfshielded Electron Processing", RadTech '88, New Orleans, LA, April 24-28 (ESI TR-157).
631
Central
Avenue,
7th International Meeting on Radiation Processing
(9)
New Developments in Radiation Processing (1989). 5907 Penn Avenue, Suite 215, Pittsburgh, PA 15206.
51
Omega Research Associates Inc.,
(10) O'Brien, T.F. (1984).
Electron Beam Cylinder Wlpe Intaglio Inks, Proc. Radcure Europe, Munich, FRG, May 4-7; Soc. Mfg. Engineers Assoc. Pin. Proc. of SME, One SME Drive, P.O. Box 930, Dearborn, MI 48121.
(II) Tanny G.B. (1984). Method for Manufacturing Microporous Membrane, 4,466,931, August 21.
U.S.
Patent
(12) Universal Packaging Corporation, P.O. Box 918, Concord, NH 03301, USA; Hamilton Tool Company, Walnut at Ninth Street, Hamilton, OH 45011, USA, private communications.
(13) Weitemeyer, C., J. Jackmann, D. Allstadt, and H. Bras (1987).
Tego ® Silicone Acrylates RC for Release Coatings, 1987 Polymers, Laminations and Coatings Conference, San Francisco, CA, September 9-11, 343-353; TAPPI Press, Atlanta Technology Park, P.O. Box 105113, Atlanta, GA 30348, USA.
(14) Wyman, J.E.,
l.J. Rangwalla, and S.V. Nablo (1989). Electron Initiated Graft Polymerization on Polyolefin Film, Proc. ACS Symposium on Radiation Curing of Polymeric Systems, Dallas, Texas, April 10-12.
Radiat. Phys. Chem. Vol. 35, Nos 1-3, pp. 46-51, 1990 Int. J. Radiat. Appl. Instrum., Part C Printed in Great Britain
RECENT DEVELOPMENTS IN RADIATION CURING IN THE USA
Sam V. Nablo Energy Sciences Inc. 8 Gill Street Woburn, MA 01801 USA
ABSTRACT Radiation curing activities in the USA continue to focus strongly on the use of both UV and EB for the polymerization of silicone release coatings on paper and film. The high speeds of cure (200 m/minute) achievable at "room temperature" continue to make this the process of choice for the sillconlzatlon of both film and paper. Some of the process difficulties peculiar to this chemistry will be discussed. The success of the multi-color lithographic printing of web using single station electron curing has resulted in a major market for electron processors. The aesthetic and physical properties of the overprint varnishes used in this application are important and will be discussed. The relatlvely high costs of both the inks and varnishes will probably limit this application to folding carton use for foodstuffs and high quality products, with UV continuing to dominate in lower speed, less demanding applications. The application of electron initiated graft modlflcatlon of polymer materials, partlcularly for blologlcal/medlcal device appllcation, is~ showing a good rate of development as is the use of selective t r e a t m e n t of materials for functional modification of packaging films, particularly for gas permeability control. Some examples of these appllcatlons will be reviewed. Continued work with "deep" curing or vulcanization of composite structures, particularly in the elastomers fleld, will be discussed with a brief review of the continuing growth of electron processing in this industry, particularly for tires and roofing products.
KEYWORDS electron curing, electron processing, radiation crossllnklng, graft copolymerlzatlon, r.c. release products
RELEASE COATINGS There has been a great deal of interest in the use of radiation curable silicone coatings s i n c e t h e t e c h n o l o g y f o r b o t h UV and EB was f i r s t c o m m e r c i a l i z e d i n t h e m i d d l e o f t h e l a s t decade ( c a . 1975). These c o a t i n g s a r e t y p i c a l l y a p p l i e d to p a p e r f o r f i l m r e l e a s e s h e e t s , as i n t h e i r u s e f o r p r o t e c t i n g t h e p r e s s u r e s e n s i t i v e a d h e s i v e s on l a b e l s , t a p e s , e t c . The major requirements of such coatings are tha t they are we ll anchored to the re le a s e s he e t, t h a t t h e y p r o v i d e a s t a b l e r e l e a s e v a l u e t o t h e a d h e s i v e o v e r an a c c e p t a b l e r a n g e o f temperature and time, that their release values can be adjusted by minor formulation changes to satisfy the product release requirement, and that they demonstrate a modest dose to cure without undue or troublesome oxygen inhibition of the surface polymerization. The conventional heat cure process (Bickford and Jachmann, 1988) for these coatings utilizes condensation polymerization of an OS-difunctlonal silicone with a hydrogen functional silicone compound, applied out of solvents and catalyzed with some soluble tin derivative. Addition polymerization is also utilized with platinum group catalysis of the crosslinking of a vinylfunctional silicone again with a hydrogen functional silicone. I n h i b l t o r s m u s t be u s e d w h i c h r e l e a s e t h e c a t a l y z e r a t e l e v a t e d t e m p e r a t u r e s ; t h e p o i s o n i n g of t h e s e catalysts can cause serious problems in production. Since high temperatures are i n v o l v e d i n b o t h o f t h e s e p r o c e s s e s , e m b r l t t l e m e n t and r e m o i s t u r i z a t i o n of a p a p e r
46
7th International Meeting on Radiation Processing
47
substrate are usually involved, while their use on thermally sensitive film is impractical since dimensional stability of the release sheet is of great importance. No solventless thermal cure systems are currently available at cure temperatures below 200°F (Eckberg, 1988)--hence the high interest in radiation curables which offer the only proven route to ambient temperature cure. The radiation initiated processes (laser, UV, EB) involve the radical initiation of chain reactions of the acrylate groups which have been introduced into a linear polydimethylsiloxane. More recently (Eckberg, 1988), epoxy-functlonal polysiloxanes have been introduced by General Electric which are crosslinked by UV with the assistance of silicone miscible cationic photocatalysts. The major problems which have been faced in the commercialization of this technology lie in inerting and in the control of light volatile fractions produced during the curing process. For the most widely used Goldschmidt AG Tego ® Silicone acrylates (Weitemeyer et al, 1987), llne speeds of 30-40 m/min are realized with one 120 W/cm medium pressure mercury lamp with good inerting, and photolnitlator content in the 3-5% range. Higher initiator content can be used to overcome oxygen sensltivity but release properties suffer. For electron curing, current formulations from Goldschmidt AG require 2 megarads to cure at oxygen levels below a few hundred parts per million in the cure zone. Since SiO 2 is readily formed in this region due to free Si and SiO in the treatment zone, the oxygen levels must be minimized. One of the major problems in successful commercialization of the EB process has been the control of SiO 2 formation so that its condensation on the window surface does not lead to loll overheating, stress and failure. These early difficulties are now sufficiently well understood to provide good system performance and have highlighted one of the major diagnostic problems of radiation curing of 02 sensitive coatings at elevated speeds; e.g. 200-400 m/mlnute. The treatment zone 02 analyzers are only providing an indication of the condition of the environment above the coating--not at its boundary layer interface where the important reaction kinetics are taking place. The result is that the 02 analyzer indication, at whatever its location, must be related to the quality of cure for a given web speed--inerting condition. As a result, the knowledge of the tolerable 02 level at a given speed (dose rate) is used to adjust the inerting conditions required for adequate cure. Weitemeyer and co-workers have discussed these and other important substrate-coating parameters.
WEB-OFFSET PRINTING The successes of electron and ultraviolet curing in web offset or lithographic printing constitute the major advance in "radiation" curing in the U.S. over the last two years. In the lithographic process, the image to be printed is formed on a metal plate and fixed to a plate cylinder. The image areas are at plate level and are ink receptive and water
Figure 1
Six color EB web offset line
48
SAM V. NABLO
repellant; the non-lmage areas are water receptive--when they are wetted with fountain solution they become ink repellent. The inked imaging areas on the plate are transferred to the rubber blanket cylinder and the image transferred to the web under pressure against the impression cylinder. The inks are highly viscous and hydrophobic, and typically are applled at thicknesses in the I-4 micron range, so are easily penetrated and cured by low energy electrons. For present folding carto$ applications, primarily for food packaging, an EB cured overprint varnish of a few gm/m ~ is usually used in the final print station, and it is typically applied with a flexographic type plate which protects the inked areas and leaves regions of unvarnished paperboard for later gluing of the container. One such 1.05 meter (42") system (UPC 19891 using a 6 color Hamilton press and Zerand diecutter is shown in Figure I, and provides an indication of the relatively small in-llne footprint of the curing station. These presses typically operate at speeds of 150 m/mlnute on a round the clock basis. There is no venting required with these 100% reactive inks and varnishes, and they are normally installed in air conditioned plants both for worker comfort as well as for paper/paperboard moisture content stabilization. For ~he system shown, inerting is accomplished with a nitrogen flow of some 2000 scfh (or 55 mJ/hr). The General Printing Ink Division of Sun Chemical Corporation has been the major supplier of these inks and varnishes in the US and their users report good satisfaction with color weight control at the printing stations and the excellent performance (abrasion/solvent resistance and gloss control) of the overprint varnish (GPI 19891. There are numerous techniques used for the "quality of cure" control of such EB/UV processed products. A simple methylethylketone rub test is the most commonly used, quite often in conjunction with a simple Crockmeter, an oscillatlng weighted head which presses a solvent loaded swab against the coated surface to be tested. More sophisticated sampllng tests will be developed and are being used where direct or indirect food contact is involved in the packaging appllcatlon. Although the majority of the 30 or so presses using electron curing are running roll:roll or roll:dle cut, the technology has been used for some time at Mitsumura Printing Co. Ltd. in Tokyo, Japan with a 1.0 meter x 0.65 m sheet feed offset Roland press. The configuration is shown in Figure 2. Sheet gripping using vacuum hold down is accomplished in the infeed A and outfeed C zones with sheet transfer to a stainless steel vacuum hold down conveyor in the central process zone B. Details of the inerting performance of this system which works well to sheet speeds of 300 fpm (90 m/mln), have been published (Nablo,
1988). ACCELERATOR
/SELFSHIELD
Figure 2
C
I
I
/METAL CONVEYOR CHAINS
/FABR IC CONVEYORBELTS
g
m
|NERTINGZONE
~
C'B
B I
I
8-A
A
Vacuum hold-down sheet stock Selfshield for electron cured lithographic printing
The dramatic increase in the use of ultraviolet for graphic arts in the United States is exceeding its growth in any other industry. A recent analysls by Omega Research Associates (19891 of these markets projects on annual rate of growth of 17.8% over the 1988-1993 period. Historically some 25% or 860 of the 3490 UV units installed by 1983 were in graphic arts which had risen to 30% or 2730 of the 9100 units by 1988. The referenced study projects that 35% or 6200 of the 17,700 units in the U.S. by 1993 will be in graphic arts. It seems likely that a comparable annual growth will take place in the much smaller segment of the graphic arts market occupied by electron beam curing.
7th International Meeting on Radiation Processing
49
Its extension beyond web or sheet offset to the much larger markets of flexographlc or gravure printing does not seem llkely without major improvements in the economics of the process for these printing processes for which interstatlon curing is currently required. A major area of growth for electron curing lles in security (intaglio) printing (0'Brlen, 1987) where ultraviolet is inadequate. The intagllo process is slmilar to gravure except that the inks are wiped off the non-lmaglng areas of the plate with paper or with wiping cyllnders.
GRAFT MODIFICATION OF POLYMERS - F i b e r s ,
F i l m and Tow
T h e r e h a s b e e n a g r a d u a l l y i n c r e a s i n g l e v e l of a c t i v i t y i n t h e r a d i a t i o n g r a f t m o d i f i c a t i o n of p o l y m e r s o v e r t h e p a s t few y e a r s due t o an improved a w a r e n e s s of t h e p r o c e s s o r m a c h i n e r y now a v a i l a b l e f o r t h e t r e a t m e n t of t h e s u r f a c e s of f i l a m e n t a r y p r o d u c t s o r f i l m at high dose rates. A v e r y b r o a d b a s e o f r a d i a t i o n m o d i f i c a t i o n of p o l y m e r s was e s t a b l i s h e d o v e r t h e p a s t two d e c a d e s , l a r g e l y w i t h low d o s e r a t e ( I Gray p e r s e c o n d ) gamma-ray s o u r c e s . Only a few r a d i a t i o n g r a f t e d p r o d u c t s h a v e b e e n i n t r o d u c e d i n t o t h e m a r k e t p l a c e ( f i l t e r membranes ( T a n n y , 1984), b a t t e r y s e p a r a t o r s , e t c . ) l a r g e l y due t o t h e c o s t and c o m p l e x i t y o f t h e b u l k p r o c e s s i n g o f m a t t e r a t t h e d o s e s r e q u i r e d f o r g r a f t c o p o l y m e r i z a t i o n , u s i n g gamma-ray s o u r c e s . The d e v e l o p m e n t of e l e c t r o n beam e q u i p m e n t s u i t a b l e f o r t h e h i g h s p e e d t r e a t m e n t of f i l a m e n t a r y and f i l m p r o d u c t s ( a t I000 kGy p e r s e c o n d ) h a s i m p r o v e d t h i s s i t u a t i o n o v e r t h e p a s t d e c a d e so t h a t t h e i n d u s t r i a l u s e r ( i . e . c h e m i c a l e n g i n e e r ) i s a g a i n e v a l u a t i n g these sources for commercial applications. Among t h e m a j o r a r e a s o f a p p l l c a t l o n u n d e r s t u d y a r e f i b e r o r tow m o d i f i c a t i o n f o r i m p r o v e d s u r f a c e p r o p e r t i e s ( w e a r , s o l l r e l e a s e , hydrophillclty), m o d i f i c a t i o n o f non-woven f a b r i c s f o r i m p r o v e d a b s o r p t i v i t y (Vrelich, 1 9 7 9 ) , p a c k a g i n g f i l m m o d i f i c a t i o n f o r r e d u c t i o n o f g a s p e r m e a b l l l t y and h a z e , end improvement in f r a g r a n c e / f l a v o r r e t e n t i o n and s u r f a c e w e t t a b i l l t y (Wyman e t a l , 1989), and s t a t i c dissipative f i l m s f o r vacuum f o r m a b l e o r s h r i n k wrap a p p l l c a t l o n s (Keough, 1988). A v i e w o f a s m a l l (5 k i l o w a t t ) p i l o t d e s i g n e d f o r f i b e r / f i l a m e n t work i s shown i n F i g u r e 3. I t w i l l p r o v i d e 12,600 Mrad fpm (3800 Mrad mpm) i n t h e 52 p a s s c o n f i g u r a t i o n shown f o r t h i s 30 cm p i l o t ( N a b l o , 1988).
Figure 3
52 pass festooner on 30 cm pilot
50
SAM V. NABLO
VULCANIZATION AND STABILIZATION OF COMPOSITES One of the major uses of medium electron curing systems (0.5-i MeV) is in the precure or stabilization of components (such as inner liners, body plies and chafer strips) (Bohm and Tveekrem, 1983). These stabilized components are cured by conventional thermal vulcanization in the "built" tire, providing improvements in quality and performance with less material. A typical ply construction is shown in Figure 4 in which the beam must penetrate 400-500 gm/m of SBR to reach and stabilize the location of the reinforcing cord or fabric. Single ply EPDM roofing material is produced commercially using electron vulcanization to replace heat and represents a major growth area for these processors.
~
~
\ \~
Reinforcin~ Cord SB Rubber
Figure 4
~
~
~
Bleed Cord
Green Tack Preserved
Electron Stabilization of SBR Ply Structure
REFERENCES
(1)
Bickford, R.H. and I. Jachmann (1988). Innovative and Unique Applications of Radiation Cured Silicone Release Coatings, Proc. RadTech '88, April 24-8, New Orleans, LA, 569-575; RadTech Internatlonal, 60 Revere Drive, Suite 500, Northbrook, IL 60062, USA.
(2)
Bly, J.H. (1983). Radiation Curing of Elastomers, Journal Ind. Irr. Tech. ~, i, 51.
(3)
Bohm, G.G.A. and J.O. Tveekrem (1982). The Radiation Chemistry of Elastomers and its Industrial Applications, Rubber Chemistry and Technology 55, 576.
(4)
Drelich, A.M. (1979). Method for Producing Bonded Nonwoven Fabrics Using Ionizing Radiation, U.S. patent 4,146,417, March 27.
(5)
Eckberg, R.P., Radiation Curable Epoxysilicone Coatings, ibid, 576-586.
(6)
General Printing Ink Division, Sun Chemical Corporation, Carlstadt, NJ 07072, USA, private communication.
(7)
Keough, A.H. (1988). Static Dissipative Rigid Plastic Sheets from EB Curing, Proc. RadTech '88, April 24-28, New Orleans, LA, 324; RadTech International, 60 Revere Drive, Suite 500, Northbrook, IL 60062 USA.
(8)
Nablo, S.V., M. Fishel and B.S. Quintal (1988). Developments in Product Handling for Selfshielded Electron Processing", RadTech '88, New Orleans, LA, April 24-28 (ESI TR-157).
631
Central
Avenue,
7th International Meeting on Radiation Processing
(9)
New Developments in Radiation Processing (1989). 5907 Penn Avenue, Suite 215, Pittsburgh, PA 15206.
51
Omega Research Associates Inc.,
(10) O'Brien, T.F. (1984).
Electron Beam Cylinder Wlpe Intaglio Inks, Proc. Radcure Europe, Munich, FRG, May 4-7; Soc. Mfg. Engineers Assoc. Pin. Proc. of SME, One SME Drive, P.O. Box 930, Dearborn, MI 48121.
(II) Tanny G.B. (1984). Method for Manufacturing Microporous Membrane, 4,466,931, August 21.
U.S.
Patent
(12) Universal Packaging Corporation, P.O. Box 918, Concord, NH 03301, USA; Hamilton Tool Company, Walnut at Ninth Street, Hamilton, OH 45011, USA, private communications.
(13) Weitemeyer, C., J. Jackmann, D. Allstadt, and H. Bras (1987).
Tego ® Silicone Acrylates RC for Release Coatings, 1987 Polymers, Laminations and Coatings Conference, San Francisco, CA, September 9-11, 343-353; TAPPI Press, Atlanta Technology Park, P.O. Box 105113, Atlanta, GA 30348, USA.
(14) Wyman, J.E.,
l.J. Rangwalla, and S.V. Nablo (1989). Electron Initiated Graft Polymerization on Polyolefin Film, Proc. ACS Symposium on Radiation Curing of Polymeric Systems, Dallas, Texas, April 10-12.