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E L S E V I E R PII : SO143--7496(96)00042-5

Int. J. Adhesion and Adhesives 17 (1997) 121-126~!;) 1997 E lsevier Science LimitedPrinted in Great Britain. All r ights reserved

0143-7496/97/$17.00

S p e c t r o s c o p i c a n d m e c h a n i c a l e v i d e n c e o f t h ei n fl u e n c e o f t h e s u b s t r a te o n a n a n a e r o b i ca d h e s i v e c u r eB . G e o r g e a *, F . T o u y e r a s a, Y . G r o h e n s b a n d J . V e b r e l aalut Chimie, 30, Aven ue d e I 'Obs ervatoire B P1559 25009 Besancon, Franceblnst i tut de C himie des Surfaces e t Interfaces-CNRS, 15, Rue Jean S tarcky BP247868057, Mulhouse, France(Accepted 5 Nove mb er 1996)

The ca ta ly t ic e ffec t of d i f fe rent substra tes-stee l, a luminium and copper-on the cur ing reac t ion o f astan dard anae robic adhesive is investigated by infrared micros copy in reflection mod e. Thus , kinetics ofthe cure reac t ion , s tudied from the decrease of the carbon-carbon double bonds absorpt ion peak, a requite different depending on the substra te under investigation. Am ong the studied m etals, co ppe r hasbeen foun d as the most reac t ive surface . T he assumed ro le of sacchar ine conta ined in the anaerobicadhesive w hich is able to oxidise metals into ions will be highlighted. Finally, an atte mp t will be made tocorre la te the k ine t ic results wi th the m echanica l performances of the adhesive jo in t measured by a s inglelap shearing test. Thu s, fast curing reaction o f the anae robic adhesive on a surface such as copp er leadsto p oor maximal s t ra in whi le lower surface react iv i ty may provide be t te r mechanica l performances of theadhesive joints. 1997 Elsevier Science Ltd( K e y w o r d s : a d h e s iv e ; a n a e r o b i c; m e t a l l i c c a t a l y s i s ; s p e c t r o s c o p y ; s h e a r i n g t e s t )

I N T R O D U C T I O NF o r t h e s e t w o l a s t d e c a d e s , t h e u s e o f s tr u c t u r a la d h e s i v e s i n i n d u s t r y d e v e l o p e d i n c r e a s i n g l y b e c a u s e o ft h e i r a d v a n ta g e s : l o w e r i n g o f p r o d u c t i o n c o s t s ,l i g h t n es s o f s tr u c t u r e s , e a s e o f a p p l i c a t i o n a n do c c u r r e n c e o f c o r r o s i o n l i m i t e d c o m p a r a t i v e l y t o t h em o r e u s u a l w a y s o f j o i n i n g p i e c es ( w e l d i n g , b o l t i n g ,c l i n c h i n g ) . T h e a n a e r o b i c a d h e s i v e s m e r e l y u s e d i na u t o m o t i v e i n d u s t r y 1 h a v e s o m e o t h e r i n t er e s t in gp r o p e r t i e s : t h e y c a n c u r e v i a a r e d o x r a d i c a lm e c h a n i s m a t r o o m t e m p e r a t u r e a s s o o n a s o x y g e n i sr e m o v e d . S o t h e s e o n e - p a r t s y s t e m s b a s e d o n a c r y l i cm o n o m e r s c u r e v e r y q u i c k l y w h e n c o n f i n e d b e t w e e nt h e m a t e r ia l s t o b e b o n d e d ( c o m m o n l y m e t a l s) 2 ' 3.

T h e c a t a ly t i c d e c o m p o s i t i o n o f in i t i a t o r m o l e c u le s b ym e t a l l i c c a t i o n s , a r i s i n g f r o m s u b s t r a t e s , p r o v i d e s f r e er a d i c a l s a b l e t o i n d u c e t h e r e a c t i o n . T h i s c a t a l y s e dr e a c t i o n r e q u i r e s a b o u t f o u r t i m e s l e s s e n e r g y ( 1 0 -1 5 k c a l / m o l ) t h a n a t h e r m a l l y i n d u c e d r e a c t io n . A si n i ti a t io n g e n e r a l ly p r o c e e d s t h r o u g h t h ed e c o m p o s i t i o n o f a n h y d r o p e r o x i d e , s u c h a s c u m e n eh y d r o p e r o x i d e 4 , t h e f o l l o w i n g r e l a t i o n s c a n b e w r i t t e n :

RO OH + M e "+ ~ RO + M e ("+1~+ + O H (a )

* C o r r e s p o n d e n c e t o : I u t C h i m i e , 3 0 , A v e n u e d e l ' O b s e r v a t o i r eB P 1 5 5 9 2 5 0 0 9 B e s a n c o n ( F r a n c e )

RO OH + Me ('+11+ --+ RO 0 + Me "+ + H + (b )T h e r e a c t i o n ( b ) p r o d u c e s r a d i c a l s w h i c h a r e l e s se f f i c ie n t o n t h e i n i t i a t i o n s t a g e t h a n t h e R O r a d i c a l s 5.N e v e r t h e l e s s , t h i s r e a c t i o n i s c r u c i a l t o r e g e n e r a t e t h ec a t i o n s a t t h e i r l o w e r o x i d a t i o n s t a t e . I n d e e d , t h i sr e g e n e r a t i o n i s e s s e n t i a l t o m a i n t a i n t h e i n i t i a t i o n r a t ea t a h i g h l e v e l i n f o r m u l a t i o n s w h e r e a n i n h i b i t i n gs y s t e m i s p r e s e n t . C o n s e q u e n t l y , i n o r d e r t o e n s u r e t h ec o n t i n u o u s r e d u c t i o n o f c a t io n s t o t h e i r l o w e ro x i d a t i o n s t a t e , a s y n e r g i s t i c a c c e l e r a t i n g s y s t e m h a s t ob e e m p l o y e d 6 . T h u s , a c h a r g e - t r a n s f e r c o m p l e xb e t w e e n c o n s t i t u e n t s o f t h e a c c e l e r a t in g s y s t e m , i n t h ep r e s e n c e o f m e t a l l i c i o n s , h a s b e e n p r o p o s e d b yO k a m o t o 7, B e a u n e z et al. 4 a n d D o u r n e l 8 . B e s i d e s,W e l l m a n n et al. 9 h a v e a c h i e v e d t h e i s o l a t i o n o f a n e wc o m p o u n d , a m i n a l , w h i c h r e s u l t s f r o m t h e r e a c t i o n o fs a c c h a ri n a n d N , N - d i m e t h y l - p - t o l u i d i n e . W h a t e v e r t h ea c c o u n t e d a s s u m p t i o n , t h e s y n e r g i s t i c e f f e c t o f t h ec a t a l y t i c s y s t e m m a i n l y a f f e c t s t h e r e l a t i o n ( 2 ) b yp r o v i d i n g s p e c i e s w h i c h a r e s t r o n g r e d u c t a n t s t o w a r d st h e h y d r o p e r o x i d e . F u r t h e r m o r e , Y a n g et al. 10. 11 haves h o w n , u s i n g F T - I R , t h a t t h e n a t u r e o f th e s u b s tr a t e,a s w e l l a s t h e s u r f a c e t r e a t m e n t , g r e a t l y i n f l u e n c e t h ec u r e r a t e b u t a l s o t h e f i n a l c u r e d e g r e e o f a n a n a e r o b i ca d h e s i v e . T h u s c o p p e r h a s b e e n f o u n d t o b e t h e m o s ta c t i v e s u r f a c e f o r i n i t i a t i n g t h e p o l y m e r i s a t i o n .I n th i s w o r k , F T - I R m i c r o s c o p y w i ll be s h o w n a s ap o w e r f u l t o o l f o r i n v e s t i g a t i n g t h e a d h e s i v e / s u b s t r a t e

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I n f luence o f subs t ra te on an aerob i c adhes i ve : B . George et al .

interface. Moreover, absorption spectroscopy will beuseful for probing the availability of the surfacemetallic ions and the mechanical performances will beinvestigated through a single lap shearing test. Thiswork will not focus on the chemistry of the anaerobicadhesive cure which is well documented in theliterature cited above. Therefore, the originality of ourwork will be of the search for relations between thereactivity of the adhesive formulation and theavailability of the surface cations and also between thisreactivity and the mechanical performances of the joints.

EXPERIMENTALM at er i a l sA standard anaerobic adhesive, used in the automotiveindustry, is chosen from a large range of commerciallyavailable products. Thus, Loctite 648, purchased fromLoctite Corporation, is mainly composed by adimethacrylic ester, an initiator, accelerators andstabilizers. This formulat ion is more specifically madeof a urethane modified dimethacrylate prepolymerwhich formula is:

O2C = C(CH3) - CO0 - (CO2)n - N H - C O-R - CO - NH- (CH2)n - COO - (CH3)C = CH2

Where R is the polyol structure resulting from thereaction with an isocyan atoalkyl methacrylate~2. Sizeexclusion chromatography, performed on thisanaerobic adhesive, has provided the ester methacrylatemolecular weight that are: Mn=4 74, Mw = 1022g/moland polydispersity=2,14. These results show that themonomer cited above is prepolymerised which leads tothis polydispersity. The initiator commonly used iscumene hydroperoxide. The accelerating system ismade of a tertiary aryl alkylamine, such as N,Ndimethyl-p-toluidine13, coupled with saccharin, alsonamed orthobenzosulfimid acid. Finally, the freeradical stabilizer is generally quinone, and moreprecisely 1,4 benzoquinone 14 which acts, in thepresence of oxygen, as radical scavenger 15.

The main physicochemical characteristics are asuperficial energy (7) of 37,16mJ/mm 2 as determinedwith a LAUDA liquid tensiometer, a viscosity of

500mPa.s. at 25C and a density of 1.14 as providedby the supplier.Three metallic active substrates, steel, aluminium andcopper, have been supplied from Goodfellow and theircompositions are given in Table 1. These substrates arepolished, in order to have a roughness of the sameorder. This roughness checked by profilometry hasprovided values ranging from 4 to 5,5/~m. The surfacesare then degreased by dipping the lamellas in atrichloroethane ultrasonic bath. After 10 minutes, theyare rinsed with distilled water and acetone, then driedat room temperature and stored in a dessicator undervacuum.TechniquesThe apparatus used was a Fourier Transform InfraredSpectrometer connected to an optical microscope(MAGNA IR spectrometer 550, NICOLET, fitted withsoftware OMNIC). 200 scans were collected in order toimprove the signal to noise ratio. Fo r the studies of thecure of the bulk adhesive, a 10#m thick adhesive filmwas held between two lamellas of KBr. The spectrawere recorded in transmission mode at regular intervalsof time. The studies of the cure of the adhesive at asurface were performed by applying a thin layer (30-50/tin) on the different metallic substrates and leftstanding at room temperature. The infrared spectrawere recorded in reflection microscopy and thespectrum of a metallic sample, without adhesive, wasused as a reference.

The polymerization reaction was studied byfollowing the variations of intensity of the absorptionpeaks corresponding to the stretching of double bondsC = 0 and C = C. For our purpose, the degree o f curewas calculated at different intervals of time using thefollowing equation:

c~ = [(A 0- A, ) / Ao] x 100 (1)where A0 is the ratio of the area of the peak around1640 cm -l (C = C stretching band) by the area of thepeak around 1720 cm-1 (C = 0 stretching band) at timet=0 (time required for the recording of the firstspectrum, chosen as reference time) and At the ratio ofthe areas of the same two peaks at time t.In order to study the influence of saccharin on the

Tab l e 1 C h a r a c t e r i s t i c s o f m e t a l l i c s u b s t r a t e s ( c o p p e r , s te e l a n d a l u m i n i u m ) u s e d i n t h a t s t u d yC o p p e r o f h i g h l ev e l o f p u r i t y ( 9 9 , 9 9 % )A 1 P S i C r C o P b S n N i Z n F e

< 0,0001 < 0,0001 < 0,0001 < 0,0001 < 0,0001 < 0,0001 < 0,0001 < 0,0001 < 0,0001 0,001S t e e l X C 3 8C M n C u T i A 1 S i S P0 ,34 0 ,5 < 0 ,4 < 0 ,01 0 ,015 0 ,1 0 ,020 < 0 ,030 ,4 2 0 ,8 0 ,0 0 4 0 ,4 0 ,0 4 0

A l u m i n iu m A - U 4 GF e S i C u Z n M g M n T i< 0 , 5 0 ,3 3 ,5 < 0 ,1 0 ,5 0 ,3 < 0 ,20 ,8 4 ,5 1 0 ,8

1 22 I NT . J . A D H E S I O N A N D A D H E S I V E S V o l u m e 1 7 N u m b e r 2 19 97

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Influence o f substrate on anaerobic adhesive: B. George e t a l .

m e t a l l i c s u r f a c e s , s a m p l e s o f t h e t h r e e m e t a l s w e r ed e g r e a s e d , a s e x p l a i n e d a b o v e , a n d t h e n i m m e r s e d i ne t h a n o l i c s o l u t i o n s o f s a c c h a r i n e a t d i f fe r e n tc o n c e n t r a t i o n s . S a m p l e s i m m e r s e d i n e t h a n o l w i t h o u ts a c c h a r i n e w e r e u s e d a s r e f e r e n c e s . T h e c o n c e n t r a t i o no f m e t a ll i c io n s i n s o lu t i o n w a s d e t e r m i n e d b y m e a n so f a t o m i c a b s o r p t i o n s p e c t r o m e t e r P e r k i n E l m e r 3 1 0 0o r i o ni c c h r o m a t o g r a p h W a t e r s a s a f u n c ti o n o f ti m e .

M e c h a n i c a l p e r f o r m a n c e s o f t h e a d h e s i v e w e r e t e s te do n t h e t h r e e m e t a l li c s u b s tr a t e s . T h e j o i n t w a s m a d e o ft w o l a m e l l a s , w h i c h d i m e n s i o n s a r e 5 0 x l 0 x l 0 m m .T h e s a m p l e s w e r e m a i n t a i n e d u n d e r p r e ss u r e , 1 0 k g /c m 2 a p p l i e d f o r 3 h o u r s , a n d s t o r e d f o r 2 4 h o u r s w h i c hw a s t i m e r e q u i r e d t o r e a c h t h e f i n a l r e s i s t a n c e . T h es i n g le l a p s h e a r i n g t e s t i s c a r r i e d o u t b y c o m p r e s s i o n a ta co n s ta n t s pe e d o f 2 m m / m i n o n a n I N S T R O N 1121c o m p r e s s i o n d e v i c e f i t t e d w i t h a m e a s u r i n g c e l l . T h em a x i m a l r e s i s t a n c e f o r t h i s t y p e o f s o l l ic i t a t i o n ~ b mo y sd e f i n e d i n e q u a t i o n 2 a s

qSmoy = F,~,x /A (2 )w h e r e F m , x i s t h e c h a r g e a p p l i e d e x p r e s s e d i n N a n d At h e s u r f a c e o f t h e b o n d .

T h e s a m p l e s w e r e o b s e r v e d a f t e r r u p t u r e w i t h ac l a s s i c a l o p t i c a l m i c r o s c o p e i n r e f l e c t i o n ( L E I C Am e t a l l u x 3 ) c o u p l e d w i t h a c o l o u r v i d e o p r i n t e r i no r d e r t o d e f i n e t h e f a c e s o f th e r u p t u r e .

R E S U L T S A N D D I S C U S S I O N

T h e i n f r a r e d s p e c t r a o b t a i n e d f r o m t h e t r a n s m i s s i o nm e a s u r e m e n t s a r e s h o w n i n Figure 1. T h e d e c r e a s e o f1.4

1.2

0. 8

0,1~ 0.6 s t e e l X C 3 8 - _ a l u m i n iu m A U 4 G . T h iss e n s i t i v i t y t o t h e o x i d a t i o n , w h i c h i n c r e a s e s t h ea v a i l a b i l i t y o f i o n s a t t h e s u r f a c e , g r e a t l y a f f e c t s t h ec u r e s p e e d o f a n a e r o b i c f o r m u l a t i o n s . T h i s m a y b e o n eo f t h e m a i n r e a s o n s w h y t h e c u r in g r a t e i s m u c hg r e a t e r o n c o p p e r t h a n o n s t e e l a n d a l u m i n i u m .

D o e s a n y c o r r e l a t i o n e x i s t b e t w e e n k i n e t i c s o fp o l y m e r i z a t i o n a n d t h e m e c h a n i c a l p e r f o r m a n c e s ? B yo b s e r v i n g t h e r e s u l t s f r o m Table 3, w e n o t e t h a t t h em a x i m a l s t r a i n ~ r e c o r d e d i n t h e c a s e o f c o p p e r i s v e r yl o w ( 3 ,9 2 M P a o n l y ) a n d t h e r u p t u r e i s 1 0 0 % c o h e s i v et y p e i n t h e a d h e s i v e . V e r y f a s t c u r i n g k i n e t i c s r e s u l t i nt he f o r m a t i o n o f a p o l y m e r n e t w o r k n o t h o m o g e n e o u sa l l a l o n g t h e b o n d l i n e : a t t h e i n t e r f a c e a d h e s i v e / m e t a l ,t h e r e a c t io n i s f a v o u r e d a n d t h e p o l y m e r is f o r m e d o fv e r y s h o r t m a c r o m o l e c u l a r c h a i n s t i g h t l y c r o s s l i n k e d .I n t h e j o i n t i t s e lf , t h e d e n s i t y o f n e t w o r k d e c r e a s e ss l ig h t l y s in c e m o v e m e n t s o f c h a i n s i s p r o g r e s s i v e l yh i n d e r e d . T h e s t r a i n s r r e c o r d e d f o r t h e s a m p l e s w i t hs te e l a n d a l u m i n i u m a r e n e a r ly o f t h e s a m e o r d e r( 1 1, 5 4 a n d 9 , 2 3 M P a , r e s p ec t iv e l y ) a n d a b o u t t w i c eg r e a t e r t h a n t h e v a l u e c o n c e r n i n g c o p p e r s a m p l e s .S i n c e t h e c u r i n g r e a c t i o n p r o c e e d s s l o w e r o n t h e s e t w os u b s t r a te s , w e c a n a s s u m e t h a t a m o r e h o m o g e n e o u sp o l y m e r n e t w o r k w i t h a l o w e r c r o s s l i n k i n g d e n s i t y i sa c h i e v e d . O u r a s s u m p t i o n c a n b e r e l a t e d t o t h eF i g u r e 6 . T h e p h o t o g r a p h s ( G - 7 0 t im e s ) w h i chr e p r e s e n t t h e s u r f a c e o f t h e a d h e s i v e a f t e r t h e t e s t w a sp e r f o r m e d h i g h li g h t d i f fe r e n t t y p e s o f r u p t u r e . O nc o p p e r , t h e r u p t u r e a p p e a r s t o b e b r i t t l e w h e r e a s o na l u m i n i u m o r s te e l, t h e r u p t u r e i s m o r e d u c t i le , i n d e e dt h e d e n s i t y o f r ip p l e s i n t h e t e s t i n g d i r e c t i o n i s m u c hm o r e g r e a t e r f o r t h e f o r m e r c a s e c o m p a r e d t o t h el a tt e r. P o l y m e r c u r e d o n a l u m i n i u m a n d s t e el s a m p l e sc a n s t o r e m o r e e n e r g y b e f o r e r u p t u r e b e c a u s e i t sn e t w o r k i s f l ex i b le . C o n s e q u e n t l y , t h e r u p t u r e o c c u r s i nt h e z o n e o f e la s ti c d e f o r m a t i o n f o r c o p p e r s a m p l e s a n di n p l a s t i c z o n e f o r a l u m i n i u m o r s t e e l s a m p l e sr e s p e c t i v e l y a s s h o w n o n Figure 7.

W e s a w t h a t v e r y fa s t k in e t ic s o f p o l y m e r i z a t i o n d on o t i n d u c e v e r y h ig h m e c h a n i c a l p e r f o r m a n c e s b u t t h ec o n t r a r y i s n o t n e c e s s a r i ly t r u e . A s a m a t t e r o f f a c t , t h eh i g h e r p e r f o r m a n c e s a r e r e c o r d e d f o r s t e e l f o r w h i c ht h e r e a c t i o n o f p o l y m e r i z a t i o n p r o c e e d s n e v e r t h e l es s

Table 3 Mecha nical performances of the adh esive no metallicsubstrates.F= strength applied, A = adhesion area, r = max imal strainFme an A(cm2) r(Mpa) Type of(daN) rupture (%cohesive)

Copper 113.5i4.0 2.88 3.94 100(in adhesive)Steel XC38 202.0-1-5.4 1.75 11.54 70Aluminium AU4G 219.0~26.2 2.37 9.23 60

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In f l u e n c e o f s u b s t r a t e o n a n a e r o b i c a d h e s i v e : B . G e o r g e et a l .adhesive copper

adhesive alumi ni um A U4G

2 5 0

20 0

Z.~ 150

100i1

50

00.1 0.2 0.3De f orma t ion ( ra m)

t0.4

F i g u r e 7 Strains recorded as a function of the relativedeform atio n.at copper, aluminium AU-4G, steel XC38.

adhesive steel XC38

a d h e s i v e s . F r o m t h e r e s u l t s p r o v i d e d b y t h i st e c h n i q u e a n d a s h e a r i n g s i n g l e - l a p t e s t , t h e r o l e o fm e t a l l i c c a t a l y s i s o n c u r i n g k i n e t i c s o f a s a m ea n a e r o b i c a d h e s i v e a p p l i e d o n c o p p e r , s t e e l a n da l u m i n i u m h a s b e e n h i g h l i g h t e d . O u r r e s u l t s s e e m t os h o w t h a t a t i g h t r e l a t i o n s h i p e x i s t s b e t w e e nc o h e s i v e p r o p e r t i e s a n d k i n e ti c s o f p o l y m e r i z a t i o nw h i c h is d e p e n d e n t u p o n i n t e r a c t i o n s b e t w e e nf o r m u l a t i o n a n d s u b s t r a t e s . A ( t o o ) f a s t r e a c t i o nr e s u l t s i n s a m p l e s w i t h p o o r m e c h a n i c a l p r o p e r t i e s( c a s e o f c o p p e r ) .

F i g u r e 6 Profiles of rupture for the adhesive cured on the threesubstrates.

f a s t e r t h a n o n a l u m i n i u m . B e s id e s , t h e t y p e o f r u p t u r e( % o f c o h e s i o n ) s h o w s t h a t a d h e r e n c e o n s t e e l i s h i g h e rt h a n o n a l u m i n i u m . I t i s p o s s i b l e t h a t t h e s u p e r f i c i a lc h a r a c t e r i s t i c s o f s u b s t r a t e s a s s u r f a c e e n e r g y f o re x a m p l e h a v e t o b e t a k e n i n t o a c c o u n t i n o r d e r t oe xp la in the s e d i f fe re nc e s .C O N C L U S I O NT h e i n f r a r e d m i c r o s c o p y a p p e a r e d t o b e a v a l u a b l et e c h n i q u e t o a n a l y s e t h e p o l y m e r i z a t i o n o f

R E F E R E N C E S

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