Radiation Curing of Polymeric Materials - American Chemical Society

Chapter 10

Effect of Monomer Structure on Concurrent Grafting During Radiation Curing 1

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Stephen J.Bett ,Paul A.Dworjanyn ,Barry A. Fields , John L. Garnett , Stan V.Jankiewicz ,and David F. Sangster 2

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Radiation Curing of Polymeric Materials Downloaded from pubs.acs.org by AUBURN UNIV on 04/17/17. For personal use only.

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Polycure Pty. Ltd., Brookvale, NSW 2100, Australia School of Chemistry, University of New South Wales, Kensington, NSW 2033, Australia Lucas Heights Research Laboratories, Division of Polymers and Chemicals, Commonwealth Scientific and Industrial Research Organisation, Menai, NSW 2234, Australia 2

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The role of multifunctional acrylates as additives in both grafting and curing reactions initiated by UV and ionising radiation is discussed. The model grafting system studied was styrene in methanol to the polyolefins. The role of multifunctional acrylates, particularly TMPTA and TPGDA, in accelerating these grafting processes is reported.Synergistic effects in grafting enhancement between these multifunctional acrylates and electrolytes is discussed The significance of monomer structure in these enhancement reactions is considered and a novel mechanism proposed for these processes. The relevance of this grafting work to the related fields of curing and cross-linking especially under electron beam conditions is discussed, particularly the role of multifunctional acrylates in the presence of commercial additives such as silanes. Using TMPTA and TPGDA as representative monomers, it is shown that concurrent grafting with curing can be important in UV and EB rapid polymersation work. Radiation curable formulations are essentially solvent f r e e . In order t o overcome problems associated with viscosity reduction of the formulation needed f o r a p p l i c a t i o n pruposes, monomers, especially the m u l t i f u n c t i o n a l a c r y l a t e s (MFAs) a r e u s e f u l . The presence o f such monomers a l s o influences t h e degree o f c r o s s linking and t h e speed o f cure i n the f i n i s h i n g process (1-4). One o f t h e major d i f f i c u l t i e s encountered i n these r a d i a t i o n r a p i d cure reactions i s to achieve good adhesion t o the substrate i n the v e r y s h o r t time a s s o c i a t e d w i t h t h e p o l y m e r i s a t i o n . The p o s s i b i l i t y t h a t c o n c u r r e n t g r a f t i n g c a n o c c u r w i t h c u r i n g e s p e c i a l l y on p l a s t i c s and cellulosic substrates assists the adhesion process. Typical of t h e MFAs used i n c u r i n g r e a c t i o n s i s t r i m e t h y l o l p r o p a n e t r i a c r y l a t e (TMPTA). When TMPTA i s u t i l i s e d as an a d d i t i v e i n analogous r a d i a t i o n grafting reactions, the g r a f t i n g y i e l d i s enhanced f o r a t y p i c a l system such as t h e c o p o l y m e r i s a t i o n o f s t y r e n e t o c e l l u l o s e and polyolefins initiated by s e n s i t i s e d UV and i o n i s i n g radiation

0097-6156/90/0417-0128$06.00/0 o 1990 American Chemical Society

10. BETT ET Al*

Concurrent Grafting During Radiation Curing

129


(5). Under these c o n d i t i o n s t h e r e appears t o be a r e l a t i o n s h i p between r a d i a t i o n grafting and c u r i n g , common m e c h a n i s t i c pathways o c c u r r i n g i n both systems ( 4 ) . It i s t h e purpose o f t h i s paper t o examine t h e r o l e o f f u n c t ionality o f MFA and MFMA i n t h e enhancement p r o c e s s i n r e a c t i o n s using t h e c o p o l y m e r i s a t i o n o f s t y r e n e i n methanol t o t h e p o l y o l e f i n s under the i n f l u e n c e o f UV as model system. The r e s u l t s w i l l be extrapolated to i o n i s i n g r a d i a t i o n work. I n a d d i t i o n , s y n e r g i s t i c effects involving these MFAs and o t h e r a d d i t i v e s i n both UV and radiation grafting processes w i l l be r e p o r t e d . The s i g n i f i c a n c e of t h e data i n UV and e l e c t r o n beam c u r i n g r e a c t i o n s w i l l be discussed. Experimental The grafting p r o c e d u r e s have p r e v i o u s l y been o u t l i n e d ( 6 ) . Monomers used were t r i m e t h y l o l p r o p a n e t r i m e t h a c r y l a t e (TMPTMA), p r o p y l eneglycol t r i a c r y l a t e (PGTA), 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 (TPGDA), allyl m e t h a c r y l a t e (ΑΜΑ), t r i e t h y l e n e g l y c o l d i m e t h a c r y l a t e (TEGDMA), tetrahydrofurfuryl methacrylate (TFMA) and TMPTA. Benzoin ethyl ether (BEE) was used as s e n s i t i s e r . P o l y e t h l e n e was low d e n s i t y f i l m (0.12mm) w h i l e p o l y p r o p y l e n e was i s o t a c t i c f i l m (0.10mm). The model monomer g r a f t i n g system used was t h e c o p o l y m e r i s a t i o n of styrene i n methanol t o t h e p o l y o l e f i n s as r e p r e s e n t a t i v e t r u n k polymers. In a t y p i c a l experiment isotactic polypropylene film ( 5 x 3 cm, t h i c k n e s s 0.10mm) which had p r e v i o u s l y been S o x h e l t extracted i n benzene f o r 72h was immersed i n s o l u t i o n (25mL) o f monomer i n s o l v e n t c o n t a i n i n g t h e n e c e s s a r y additives including photosensitizer (UV o n l y ) . F o r t h e UV i r r a d i a t i o n s l i g h t l y s t o p p e r e d pyrex tubes were u t i l i z e d . These were p o s i t i o n e d i na rotating rack s u r r o u n d i n g a 90W medium p r e s s u r e mercury vapor lamp. A t t h e completion of the i r r a d i a t i o n , the polypropylene strips were quickly removed from t h e tubes t o minimize p o s t - i r r a d i a t i o n e f f e c t s , washed i n cold methanol, e x t r a c t e d i n benzene i n a Soxhlet f o r 72h t o remove homopolymer, then dried a t 40*C f o r s e v e r a l hours to constant weight, t h e percentage g r a f t b e i n g c a l c u l a t e d as t h e percentage i n c r e a s e i n weight of the grafted strip. A similar procedure was used f o r the i o n i s i n g radiation grafting work, experiments being performed i n t h e spent f u e l element and c o b a l t 60 facilities of the A u s t r a l i a n Atomic Energy Commission, dose r a t e s b e i n g determined by f e r r o u s s u l f a t e d o s i m e t r y (G(Fe)=15.6). For the s w e l l i n g studies of styrene i n polyethylene, f i l m samples o f polyethylene of uniform size and weight were s w o l l e n in m e t h a n o l i c s o l u t i o n s o f t r i t i a t e d s t y r e n e which had been p r e p a r e d by a unique tritium catalytic exchange method ( G a r n e t t , J . L . and Long, M.A., U n i v e r e s i t y o f NSW, Sydney - u n p u b l i s h e d d a t a ) . The s w e l l i n g r e a c t i o n was performed i n a water b a t h a t 25*C. A t c o m p l e t i o η o f the s w e l l i n g , s u b s t r a t e s were r a p i d l y b l o t t e d d r y and immed iately immersed i n an e x t r a c t i o n solution o f 1,4-dioxane. A f t e r one week t h e dioxane solutions were sampled and s t y r e n e c o n t e n t determined by l i q u i d scintillation c o u n t i n g . Repeated e x t r a c t i o n s did not g i v e further yield o f monomer. T h i s method was found t o be convenient and extremely sensitive. R e s u l t s a r e e x p r e s s e d as mg s t y r e n e absorbed p e r gram o f f i l m w i t h a d e t e c t i o n l i m i t o f


130

RADIATION CURING OF POLYMERIC MATERIALS

one mg s t y r e n e p e r gram o f f i l m and a r e p r o d u c i b i l i t y , based on t e n samples p e r data p o i n t , o f 4= 1%. Counting o f t r i t i a t e d s t y r e n e solutions was a c h i e v e d using a low background, double headed c o i n cident liquid scintillation counter. Bray's s o l u t i o n was used as liquid scintillator. For the r a d i a t i o n rapid cure experiments, appropriate resin mixtures containing oligomers, monomers, f l o w a d d i t i v e s and s e n s i t isers (UV) were a p p l i e d t o the s u b s t r a t e as a t h i n c o a t i n g , the material p l a c e d on a conveyor b e l t and then exposed t o the UV and EB s o u r c e s . The time taken t o observe c u r e f o r each o f the samples was then measured on a r e l a t i v e basis. The UV system used was a Primarc Minicure u n i t w i t h lamps o f 200W p e r i n c h . Two EB f a c i l i t i e s were u t i l i s e d namely a 500KeV N i s s i n machine and a 175KeV ESI u n i t . Results MFA A d d i t i v e E f f e c t s

i n G r a f t i n g I n i t i a t e d by UV.

Inclusion of t r i f u n c t i o n a l acrylate monomers i n g r a f t i n g s o l u t i o n s in additive amounts ( 1 % v/v) g e n e r a l l y r e s u l t e d i n a very large increase i n percent graft, consistent with previous preliminary work w i t h TMPTA ( 5 , 7 ) . TMPTA was m a r g i n a l l y s u p e r i o r t o TMPTMA with PGTA l e s s e f f e c t i v e ( T a b l e I ) . The magnitude o f t h e enhancement was p a r t i c u l a r l y significant a t the s t y r e n e c o n c e n t r a t i o n c o r r e s p onding t o the Trommsdorff peak ( 8 ) . When l i t h i u m n i t r a t e i s added to the monomer s o l u t i o n , a s y n e r g i s t i c e f f e c t i n g r a f t i n g enhancement with the a c r y l a t e monomers i s observed a t c e r t a i n s t y r e n e concen t r a t i o n s ( T a b l e I I ) c o n s i s t e n t a g a i n w i t h e a r l i e r TMPTA data ( 5 ) . Table I .

Styrene (% v / v ) 20 30 40 50 60 70 a

P h o t o g r a f t i n g o f Styrene i n Methanol t£ i n presence o f T r i f u n c t i o n a l Monomers

Β 5 24 45 31 22 15

B+TM 95 282 513 285 180

-

G r a f t (%) B+T 100 297 496 283 281

-

Polypropylene

B+P 50 113 289 131 98

-

l r r a d i a t e d 8 h a t 24cm from 90W lamp a t 20°C; B= BEE ( U w/v); TMlTMA( 17„ v / v ; Τ = TMPTA ( 1 % v / v ) ; Ρ = PGTA ( 1 % v / v ) .

TM=

With the lower functionality MFAs and MFMAs, namely TPDGA, TEGDMA, ΑΜΑ and TFMA, t h e g r a f t i n g enhancement i s decreased when compared w i t h the t r i f u n c t i o n a l series i n Tables I and I I , the difunctional a c r y l a t e s b e i n g more e f f e c t i v e than t h e i r m o n o f u n c t i o n a l analogues ( T a b l e I I I ) . When l i t h i u m n i t r a t e i s used as a c o - a d d i t i v e with the d i f u n c t i o n a l acrylates, grafting i s enhanced f u r t h e r a t certain styrene c o n c e n t r a t i o n s . In t h i s group of difunctional acrylates, the TPGDA r e s u l t i s particularly important s i n c e t h i s is one o f the most w i d e l y used monomers i n r a d i a t i o n r a p i d c u r i n g work.

10. BETTETAL. T a b l e II»

131


S y n e r g i s t i c E f f e c t o f T r i f u n c t i o n a l Monomers w i t h L i t h i u m N i t r a t e i n Photografting Styrene to Polypropylene Graft

Styrene (% v / v ) 20 30 40 50 60

L 1.9 1.4 1.7 2.0 4.5

B+L 10 38 20 14 14

B+L+TM 111 376 274 144 93

(%) B+L+T 126 529 337 221 81

B+L+P 55 147 146 73 46


a

l r r a d i a t i o n c o n d i t i o n s and symbols as i n T a b l e I ; r e l e v a n t data a l s o i n T a b l e I . ; L = l i t h i u m n i t r a t e (0.25M)

T a b l e III»

Styrene (% v/v)

Synergistic Effect of Monomers w i t h Lithium to Polypropylene

N.A.

A

5 24 45 31 22

36 136 331 98

59 305 88 40

-

-

20 30 40 50 60

A+L

TE 25 61 217 132 104

control

Difunctional and M o n o f u n c t i o n a l Nitrate i n P h o t o g r a f t i n g Styrene

G r a f t (%) TE+L TP 105 296 141 101 64

κ N.A.

17 99 339 167 118

κ

κ TF+L

TF

10 36 116 53 40

80 280 118 65 27

11 38 181 112 76

I r r a d i a t i o n c o n d i t i o n s and symbols as i n T a b l e I ; N.A/= no a d d i t i v e ; A = ΑΜΑ ; TE = TEGTMA ; TP = TPGDA Irradiated

16h; TF = TFMA.

Other co-additives i n addition t o l i t h i u m n i t r a t e have a l s o been discovered (9) t o a c c e l e r a t e these grafting processes. Typical of these i s m i n e r a l a c i d and r e p r e s e n t a t i v e s y n e r g i s t i c UV g r a f t i n g data f o r TMPTA and s u l f u r i c a c i d i s shown i n T a b l e IV · S u l f u r i c acid i s a p o l a r s u b s t r a t e and behaves i n a manner s i m i l a r t o l i t h i u m n i t r a t e i n these g r a f t i n g r e a c t i o n s . T a b l e IV.

S y n e r g i s t i c E f f e c t o f TMPTA and A c i d as A d d i t i v e s i n UV G r a f t i n g S t y r e n e t o P o l y e t h l e n e a

Styrene α v/v) 20 30 40 50 60

Graft TMPTA 28 52 321 412 133

N.A. 28 101 189 124 37

(%) +

H 14 126 193 107 31

I r r a d i a t e d 24 h a t 24cm from 90W lamp a t 20°C BEE ( 1 % w/v) s e n s i t i s e r ; TMPTA ( 1 % w/v) ; methanol N.A. = no a d d i t i v e ; H = H S0,(0.2M). +

9

TMPTA+H 41 78 266 525 188

solvent;

+

132



Additives are also useful in radiation r a p i d cure work. Thus i n formulations used i n UV and e l e c t r o n beam (EB) c u r i n g , additives are utilised to control slip, gloss, f l o w e t c . T y p i c a l of these are the fluorinated e s t e r and the s i l a n e used i n the p r e s e n t study (Table V). When these commercial additives are i n c l u d e d i n the monomer g r a f t i n g s o l u t i o n , the r e s u l t s show t h a t the s u r f a c e a c t i v e f l u o r i n a t e d compound enhances g r a f t whereas the s i l a n e i s a r e t a r d e r , presumable due to the r e p u l s i o n e f f e c t of the s i l i c o n atom. I n c l u s i o n of TMPTA i n the grafting solution containing a l l of the above commercial additives leads to large enhancement in graft, the effect of the TMPTA overcoming the retarding p r o p e r t i e s of the silane. Table

V.

E f f e c t of TMPTA i n Presence of Commercial O r g a n i c Photografting Styrene to Polypropylene' Additives on

Graft

Styrene

a 20 30 40 50 60

3

v/v)

B+U 5 30 46 19 13

(%)

B+Si

B+FE

5 18 31 13 9

5 23 53 16 19

B+Additives+T 260 588 711 368 283

I r r a d i a t i o n c o n d i t i o n s and symbols as i n T a b l e I; U = u r e a ( 1 % w/v) ; S i = s i l a n e ( 1 % v / v ) ; Z-6020 ex-Dow; FE = f l u o r i n a t e d a l k y l e s t e r ( 1 % v / v ) , FC-430 ex-3M; Τ = TMPTA ( 1 % v/v)

A d d i t i v e E f f e c t s w i t h MFAs i n G r a f t i n g Radiation.

I n i t i a t e d by

Ionising

When i o n i s i n g r a d i a t i o n i s used as s o u r c e of i n i t i a t i o n f o r g r a f t i n g i n s t e a d of UV (10-14), analogous a d d i t i v e e f f e c t s to those p r e v i o u s l y discussed, have been found. Thus inclusion of sulfuric acid in methanolic solutions of styrene leads to an enhancement in copolymerisation to a polyolefin, such as p o l y e t h y l e n e , when irradiated by c o b a l t - 60 gamma r a y s ( T a b l e V I ) . Lithium s a l t s when used as additives a l s o i n c r e a s e the g r a f t i n g yields for the same r e a c t i o n , lithium perchlorate being more efficient that lithium nitrate (Table VII ). A comparison of the effectiveness of acid versus l i t h i u m p e r c h l o r a t e i n the same grafting reaction is shown in Table V I I I , the s a l t b e i n g more effective than a c i d only at the lower monomer concentrations i.e. 25%. When MFAs such as TMPTA are included in the monomer s o l u t i o n enhancement in grafting to polyethylene i s observed at c e r t a i n styrene concentrations (Table IX). In the p r e s e n c e of both a c i d and TMPTA as additives, a s y n e r g i s t i c e f f e c t i n the same g r a f t i n g reaction i s observed (Table IX), consistent with the UV d a t a , thus these additive effects appear to be a g e n e r a l phenomenon in UV and r a d i a t i o n g r a f t i n g p r o c e s s e s .

10. BETT ET AL. Table VI.

A c i d Enhancement i n R a d i a t i o n G r a f t i n g to P o l y e t h y l e n e i n Methanol

Styrene (7o v/v) 10 20 30

40 50 60 80



133

Styrene

Graft (7o) H+ 40 118 267 156 120 98 73

N.A. 30 84 244 150 114 92 71

N.A. = no a d d i t i v e ; H+ = H SO (0.1M); 4 5 Dose r a t e = 3.30 χ 10 rad/h; Dose = 2.0 χ 10 r a d ; Temperature 29 C.

Table V I I .

E f f e c t of L i t h i u m S a l t s on R a d i a t i o n G r a f t i n g o f S t y r e n e i n Methanol t o P o l y e t h y l e n e

Styrene (7o v/v)

N.A.

15 20 25 30 35 40

34 60 78 142 139 117

3

Graft ( 7 o ) C10. 4

Li

N0

3

28 68 122 102 91 80

42 77 140 160 116 92

N.A. = no a d d i t i v e ; [.ithium s a l t s (0.2M) ; 5 Dose r a t e = 3.30 χ 10 r a d / h r ; Dose = 1.8 χ 10 r a d ; Temperature 20 C

Table

VIII.

Comparison o f A c i d w i t h L i t h i u m S a l t on R a d i a t i o n G r a f t i n g o f Styrene i n Methanol t o P o l y e t h y l e n e

Styrene (% v/v)

N.A.

15 20 25 30 35 40

31 64 103 187 193 150

a

Li

Graft ( 7 o ) H S0 2

Li

4

C10. 4 44 81 192 196 140 114

32 70 148 240 212 157

H S 0 (0.1M) ; L i C 1 0 (0.2M); N.A. = no a d d i t i v e ; R a d i a t i o n c o n d i t i o n s as i n T a b l e V I I 2

4

4

134



T a b l e IX.

S y n e r g i s t i c E f f e c t o f A c i d and TMPTA as A d d i t i v e s i n G r a f t i n g S t y r e n e t o P o l y e t h y l e n e F i l m I n i t i a t e d by Ionising Radiation

Styrene (% v/v)

N.A.

H+

30 40 50 60 70

37 76 109 89 68

51 81 134 73 62

a

G r a f t (%) TMPTA 39 73 137 105 59

+

H +TMPTA 54 106 181 101 95

+

N.A. = no a d d i t i v e ; H = H S0, (0.2M); TMPTA ( 1 % v / v ) ; D o s e r a t e =4.1 χ 10 rad/hr; Dose = 2.4 χ 10 ras 2

5

Discussion Mechanism o f MFA

Additive Effect

i n UV

and R a d i a t i o n G r a f t i n g

The present results suggest t h a t the enhancement e f f e c t due t o MFAs i n both UV and i o n i s i n g r a d i a t i o n work i s a g e n e r a l phenomenon. Previously the enhancement had o n l y been observed w i t h TMPTA and divinylbenzene (5) i n very p r e l i m i n a r y s t u d i e s w i t h both UV and ionising r a d i a t i o n i n i t i a t i n g systems. C o n s i s t e n t w i t h the mechanism previously proposed for this enhancement e f f e c t ( 5 ) , the p r e s e n t results may be attributed to branching and c r o s s - l i n k i n g of the polystyrene c h a i n s which result when compounds w i t h more than one polymerisable group are present. According to t h i s theory, branching and cross-linking occur when one polymerisable group of the polyfunctional monomer has been bonded t o a growing p o l y styrene c h a i n . T h i s growing c h a i n has then a c q u i r e d u n s a t u r a t e d functionality and reactivity because one or more polymerisable groups a r e still present on the polyfunctional monomer. These unsaturated groups may then participate in extra polymerisation or scavenging reactions with a d j a c e n t growing p o l y s t y r e n e c h a i n s , l e a d i n g t o a h i g h l y c r o s s - l i n k e d network w i t h many of the p o l y s t y r e n e chains having numerous b r a n c h i n g p o i n t s . Such a model can e x p l a i n why the graft copolymer y i e l d i n c r e a s e s so d r a m a t i c a l l y i n the presence of p o l y f u n c t i o n a l monomers. Mechanism o f A c i d and

Salt Effect

i n UV

and R a d i a t i o n G r a f t i n g .

An important observation recently made c o n c e r n i n g a c i d and s a l t effects i n UV and radiation grafting to p o l y e t h y l e n e i s t h a t i n the s w e l l i n g o f p o l y e t h y l e n e i n the presence o f m e t h a n o l i c s o l u t i o n s of styrene, p a r t i t i o n i n g of styrene into polyethylene i s s i g n i f i c antly improved by the inclusion o f m i n e r a l a c i d or lithium salt in the grafting solution. Styrene l a b e l l e d w i t h t r i t i u m was used f o r these s o p h i s t i c a t e d experiments which i n d i c a t e t h a t most s w e l l i n g occurs w i t h i n the f i r s t few minutes o f exposure o f backbone polymer

10. BETTETAK

135


to solution (Table X ) . At 25° C, a t l e a s t 80% o f the s w e l l i n g is achieved during the f i r s t hour, t h e system a s y m p t o t i c a l l y approaching equilibrium after 10 hours ( T a b l e X I ) . I n each s e r i e s of data, the l i t h i u m salt i s more e f f i c i e n t than the s u l f u r i c acid i n enhancing the p a r t i t i o n i n g o f monomer i n t o the p o l y e t h y l e n e . As r e p r e s e n t a t i v e T a b l e X.

V a r i a t i o n i n Styrene A b s o r p t i o n by P o l y e t h y l e n e w i t h time: I n i t i a l S w e l l i n g Behaviour

Time o f S w e l l i n g

Styrene


(mg (Minutes)

_ H S0

N.A. 0 0.08 0.5 2.0 4.0

. ο equilibrium

Absorption

styrene / g polyethylene) 2

0 5.1 11.9 24.9 38.0 45.4

0 5.6 12.9 31.3 43.1 54.4

0 5.3 12.4 27.2 41.2 54.0

S t y r e n e i n methanol ( 3 0 % v / v ) was used as model monomer s o l u t i o n ; N.A. = no a d d i t i v e ; H S 0 ( 0 . 1 M ) ; L i C10 (0.2M) V a l u e determined a f t e r 13 hours s w e l l i n g ; Temperature 25 C. 2

b

LiCIO, 4

4

4

4

monomer s o l u t i o n s f o r these s w e l l i n g experiments, 30% s t y r e n e i n methanol was chosen s i n c e f o r m&st r a d i a t i o n c o n d i t i o n s , peak g r a f t i n g and enhancement o c c u r s around t h i s r e g i o n . T a b l e XI*

V a r i a t i o n i n Styrene A b s o r p t i o n by P o l y e t h y l e n e w i t h Time: Asymptotic Approach t o E q u i l i b r i u m

Time o f S w e l l i n g

Styrene (mg

Absorption

styrene / g polyethylene)

(Hours)

0 0.5 1 2 12 20

N.A

H S0

0 38.8 40.5 42.5 45.6 45.4

0 42.8 44.3 45.8 53.3 54.0

2

4

LiCIO, 4 0 43.8 47.0 48.0 54.8 54.4

C o n d i t i o n s as i n T a b l e X. The s w e l l i n g behaviour e x h i b i t e d i n Tables Χ, XI i n d i c a t e t h a t a major p o r t i o n o f t h e e q u i l i b r i u m styrene absorption occurs i n the f i r s t f i v e minutes o f exposure. I t i s t h i s e a r l y , r a p i d a b s o r p t i o n phase t h a t i s r e p r e s e n t a t i v e o f the d i f f u s i o n s i t u a t i o n o p e r a t i v e during the similtaneous g r a f t i n g process. U s i n g t h i s time scale of five minutes, the v a r i a t i o n i n s t y r e n e a b s o r p t i o n f o r monomer


136

c o n c e n t r a t i o n s i n t h e r e g i o n o f the Trommsdorff peak i n the p r e s e n c e of t h e two e l e c t r o l y t e s has been examined and the r e s u l t s shown in Table X I I . The data indicate that at styrene concentrations o f 30% v/v, H S 0 2

Table XII.

4

E f f e c t o f E l e c t r o l y t e s on S t y r e n e A b s o r p t i o n i n t o P o l y e t h y l e n e from S o l u t i o n s near the Trommsdorff Region


Styrene cone. (% v / v )

15 20 25 30 35

C o n d i t i o n s as i n T a b l e X.

Styrene Absorption (mg s t y r e n e / g p o l y e t h y l e n e ) N.A.

H S0

20.0 27.4 33.3 37.9 45.6

20.8 28.6 35.0 42.4 48.0

2

4

LiClO. 4 22.1 30.3 36.8 44.2 52.6

Exposure time 5 m i n u t e s .

increases styrene absorption by 12% over a d d i t i v e f r e e s o l u t i o n s Under the same c o n d i t i o n s , 0.2M L i C l O ^ i n c r e a s e s s t y r e n e a b s o r p t i o n by 17%. C l e a r l y these results support the h y p o t h e s i s t h a t the presence of e l e c t r o l y t e s s i g n i f i c a n t l y a f f e c t s the c o n c e n t r a t i o n of monomer w i t h i n t h e g r a f t r e g i o n . Based on the above results and more e x t e n s i v e e v i d e n c e r e c e n t l y reported (15 - 17) a new model has been proposed t o e x p l a i n t h e effect of e l e c t r o l y t e s i n enhancing radiation grafting. I n any grafting system a t any one time, t h e r e i s an e q u i l i b r i u m c o n c e n t r a t ion o f monomer absorbed w i t h i n the g r a f t i n g r e g i o n o f the backbone polymer. T h i s g r a f t i n g r e g i o n may be c o n t i n u a l l y changing as g r a f t i n g p r o c e e d s . Thus i n grafting styrene to polyethylene during the i n i t i a l p a r t o f the r e a c t i o n , t h e g r a f t i n g r e g i o n w i l l be e s s e n t i a l l y olefinic i n nature, however, as r e a c t i o n p r o c e e d s , the g r a f t i n g region w i l l become more s t y r e n a t e d . The degree t o which monomer will be absorbed by t h i s grafting r e g i o n w i l l t h e r e f o r e depend on the c h e m i c a l structure o f the r e g i o n a t the s p e c i f i c time of grafting. The s w e l l i n g data i n Tables X - XII i n d i c a t e that increased partitioning of monomer o c c u r s i n t h e g r a f t r e g i o n when a p p r o p r i a t e e l e c t r o l y t e s a r e d i s s o l v e d i n the b u l k g r a f t i n g s o l u t i o n . Thus h i g h e r concentrations o f monomer a r e a v a i l a b l e f o r g r a f t i n g at a particular backbone polymer site i n t h e presence o f t h e s e additives. The e x t e n t o f t h i s improved monomer p a r t i t i o n i n g depends on the p o l a r i t i e s o f monomer, s u b s t r a t e and s o l v e n t and a l s o on the concentration of acid. I t i s thus the e f f e c t o f these i o n i c species on p a r t i t i o n i n g which i s e s s e n t i a l l y r e s p o n s i b l e f o r the observed increase i n radiation grafting yields i n the p r e s e n c e of such a d d i t i v e s . In the s p e c i f i c i n s t a n c e s where i o n i s i n g r a d i a t i o n is the i n i t i a t i n g source, radiolytically produced free radicals are formed and, i n the p r e s e n c e of e l e c t r o l y t e s such as a c i d p a r t icularly, could lead t o enhancement i n the g r a f t i n g p r o c e s s e s ,

10. BETTETAL.


137

however such p r o c e s s e s do not appear t o be the predominant pathway for increasing grafting yields i . e . the p a r t i t i o n i n g phenomenon would still be expected t o p r e v a i l under i o n i s i n g r a d i a t i o n conditions.


Combined E f f e c t o f A c i d s and S a l t s w i t h MFAs. The results show t h a t the pathways wereby the e l e c t r o l y t e s ( a c i d s and s a l t s ) and MFAs enhance UV and r a d i a t i o n g r a f t i n g a r e d i f f e r e n t , thus the two c l a s s e s o f a d d i t i v e s can a c t i n c o n c e r t t o g i v e a p p r e c i a b l y i n c r e a s e d c o p o l y m e r s a t i o n y i e l d s . T h i s o b s e r v a t i o n i s important in a p r e p a r a t i v e c o n t e x t s i n c e , i n the presence o f these a d d i t i v e s , lower total radiation doses are required to achieve a p a r t i c u l a r percentage g r a f t , thus r a d i a t i o n d e g r a d a t i o n e f f e c t s on the backbone polymers a r e m i n i m i s e d d u r i n g g r a f t i n g . In a d d i t i o n , the enhancement due t o these s y s e r g i s t i c e f f e c t s reaches a maximum a t the Trommsdorff peak which i s the r e g i o n where the l e n g t h o f the g r a f t e d c h a i n s is also a maximum. Thus, these a d d i t i v e s can i n f l u e n c e the n a t u r e and s t r u c t u r e o f the copolymer former and may l e a d t o the s y n t h e s i s o f new p o l y m e r i c m a t e r i a l s . With respect to absolute grafting yields, TMPTA and TMPTMA demonstrate the h i g h e s t enhancement i n UV g r a f t i n g o f a l l the MFAs and MFMAs s t u d i e d . A l t h o u g h t h e i r g r a f t i n g p r o f i l e s a r e almost coincident and both monomers a r e p r e s e n t a t the same c o n c e n t r a t i o n , TMPTMA has a h i g h e r m o l e c u l a r weight, thus i t i s more e f f e c t i v e on an e q u i v a l e n t s b a s i s than TMPTA i n i n c r e a s i n g g r a f t i n g y i e l d . This difference may reflect t h e h i g h e r hydrocarbon f u n c t i o n a l i t y of t h e TMPTMA, l e a d i n g t o h i g h e r c o m p a t i b i l i t y w i t h the o l e f i n i c polypropylene, resulting i n i n c r e a s e d s w e l l i n g o f t h i s backbone polymer, higher absorption o f monomer and enhanced grafting reactivity. S i g n i f i c a n c e o f MFA G r a f t i n g Work i n C u r i n g A p p l i c a t i o n s R a d i a t i o n r a p i d c u r e (RRC) p r o c e s s i n v o l v e c u r i n g oligomer/monomer mixtures i n a f r a c t i o n of a second e i t h e r under ( i ) a h i g h e l e c t r o n field from a low energy EB machine o r ( i i ) s e n s i t i s e d UV from high pressure sources o f up t o 400 w a t t s / i n c h i n t e n s i t y . I n these RRC systems the a b i l i t y t o a c h i e v e c o n c u r r e n t g r a f t i n g w i t h cure is important i n order t o a c h i e v e good a d h e s i o n t o many s u b s t r a t e s e.g. cellulosics and p l a s t i c s such as the p o l y o l e f i n s . I n the present grafting s t u d i e s monomers such as TMPTA, TMPTMA and TPGDA have been d e l i b e r a t e l y used s i n c e these m a t e r i a l s a r e common components i n RRC f o r m u l a t i o n s . Such monomers e x h i b i t a d u a l f u n c t i o n in c u r i n g p r o c e s s e s , namely f a s t r a t e s o f p o l y m e r i s a t i o n and c r o s s linking. TMPTA and TPGDA a r e used predomonantly f o r f a s t cure whereas TMPTMA i s slower t o cure but i s u t i l i s e d as a h i g h l y e f f i c i e n t c r o s s - l i n k i n g agent e s p e c i a l l y w i t h EB. The present data show t h a t TMPTA i s a v a l u a b l e a d d i t i v e f o r acceleratinggrafting reactions involving monomers such as s t y r e n e to the p o l y o l e f i n s with both UV and i o n i s i n g r a d i a t i o n . I n c l u s i o n o f monomers l i k e TMPTA i n RRC f o r m u l a t i o n s o f f e r s the p o t e n t i a l of enhancing concurrent grafting with c u r e . T h i s concept i s particularly r e l e v a n t t o those RRC f o r m u l a t i o n s where a d d i t i v e s


138


are needed to achieve specific properties such as gloss, s l i p and flow. T y p i c a l of such commercial a d d i t i v e s are the f l u o r i n a t e d alkyl ester and s i l a n e used i n the p r e s e n t study (Table V ) . When these commercial additives are included i n a monomer g r a f t i n g solution, the results show t h a t the surface active fluorinated compound enhances g r a f t whereas the s i l a n e i s a r e t a r d e r , presumably due to the repulsion e f f e c t o f the s i l i c o n atom. However the data show t h a t i f TMPTA i s i n c l u d e d i n such g r a f t i n g s o l u t i o n s , d r a m a t i c increases in graft are observed even i n the presence of the s i l a n e r e t a r d e r (Table V ) . The m e c h a n i s t i c r o l e of TMPTA i n RRC formulations is thus more c o m p l i c a t e d than h i t h e r t o c o n s i d e r e d . TMPTA not o n l y speeds up c u r e and c r o s s - l i n k i n g , i t can also markedly a f f e c t the occurrence of concurrent g r a f t i n g d u r i n g c u r e . Hence c a r e f u l choice o f MFA i n RRC mixtures could lead to g r a f t enhancement during cure w i t h improved p r o p e r t i e s i n the f i n i s h e d product. The difficulty w i t h RRC mixtures incorporating a range of such a d d i t i v e s i s that the final material i s a compromise i n properties o f the components i . e . each a d d i t i v e w i l l not n e c e s s a r i l y perform at maximum e f f i c i e n c y because of possible detrimental interactions with the o t h e r components i n the system. For example, TMPTA, a l t h o u g h one of the f a s t e s t c u r i n g MFAs, i s g e n e r a l l y not used commercially because i t i s too b r i t t l e i n p o l y m e r i s e d form for many f l e x i b l e applications. I t a l s o possesses a questionable Draize v a l u e which r e f l e c t s the degree to which TMPTA i n f l u e n c e s skin irritancy f a c t o r s . As a consequence, TPGDA i s frequently preferred to TMPTA i n RRC f o r m u l a t i o n s as a r e a c t i v e diluent»TPGDA p o s s e s s e s none of the above d i s a d v a n t a g e s o f TMPTA, however i n accelerating grafting reactions i t i s l e s s e f f i c i e n t than TMPTA. Even w i t h lower r e a c t i v i t y i n g r a f t i n g , the overall properties of TPGDA are a suitable compromise f o r RRC f o r m u l a t i o n s . TPGDA can thus be a v a l u a b l e a d d i t i v e i n RRC work l e a d i n g to enhancement in concurrent g r a f t i n g d u r i n g c u r e and y i e l d i n g improved p r o p e r t i e s i n the f i n a l p r o d u c t . Acknowledgments The authors thank the Australian I n s t i t u t e of Nuclear Science and Energy, the A u s t r a l i a n Research Grants Committee, the A u s t r a l i a n Nuclear S c i e n c e and Training Organisation and P o l y c u r e Pty L t d . for financial assistace. Literature 1. 2. 3. 4. 5. 6.

Cited Garnett, J.L. J. Oil.Col. Chem.Assoc. 1982, 65, 383 Pappas, S.P. UV Curing; Science and Technology; Technology Marketing Corpn. : Norwalk, Conn., 1978 Senich, G.A.; Florin, R.E. Rev. Macromol. Chem. Phys. 1984, C24(2), 234 Bett, S.J.; Garnett, J.L. Proc.Radcure Europe '87, Assoc. Finishing Processes, Munich, West Germany, 1987 Ang, C.H.; Garnett, J.L.; Levot, R.; Long, M.A. J.Polym.Sci. Polym. Lett.Ed. 1983, 21,257 Garnett, J.L. Radiat. Phys.Chem.1979,14,79

10. BETT ET AL. Concurrent Grafting During Radiation Curing 7. 8. 9. 10.


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15. 16. 17.

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Micko, M.M.; Paszner, J. J.Rad. Curing. 1974, 1(4), 2 Odian, G.; Sobel, M.; Rossi, Α.; Klein, R. J. Polym.Sci. 1961, 55,663 Dworjanyn, P.Α.; Garnett, J.L. J. Polym. Sci Polym. Lett.Ed. 1988,26,135 Geacintov, N.; Stannett, V.T. ; Abrahamson, E.W.; Hermans, J.J. J.Appl.Polym.Sci. 1960, 3, 54 Arthur, J.C. Jr. Polym. Preprints 1975,16,419 Kubota, H.; Murata, Y.; Ogiwara, Y. J.Polym.Sci. 1973,11,485 Tazuke, S.; Kimura, H; Polym.Lett. 1978,16, 497 Hebeish, Α.; Guthrie, H.T. The Chemistry and Technology of Cellulosic. Copolymers; Springer-Verlag: Berlin Heidelberg, 1981. Chappas, W.J.; Silverman, J. Radiat.Phys.Chem. 1979,14,847. Garnett, J.L.; Jankiewicz, S.V.; Long, M.Α.; Sangster, D.F. J.Polym.Sci. Polym. Lett.Ed.1985,23,563 Garnett, J.L.; Jankiewicz, S.V.; Lavot, R.; Sangster,D.F. Radiat.Phys.Chem. 1985,25,509

RECEIVED October 27,

1989

Radiation Curing of Polymeric Materials - American Chemical Society

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