Reactive Adhesives - American Chemical Society

9 Reactive Adhesives JOSEPH W. PRANE

Downloaded by RUTGERS UNIV on May 30, 2018 | https://pubs.acs.org Publication Date: November 27, 1979 | doi: 10.1021/bk-1979-0113.ch009

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A variety of two component adhesives and sealants are currently available for applications requiring the ultimate i n bonding and sealing efficiency and performance. Included among these conventional systems are: epoxies, urethanes, polyesters, silicones, polysulfides and some modified phenolics and acrylics. Although their performance can be and usually i s quite satisfactory, most are limited by pot life and fixturing requirements - and the need for thorough substrate treatment and cleanliness. An unusual type of two component system has recently been introduced i n the adhesive industry (1). These are called reactive adhesives, second generation acrylics, toughened acrylics, modified acrylics (2) or "honeymoon adhesives" (in Europe). Basically, the system i s comprised of two liquid components - an oligomeric liquid blend (usually acrylic), and a dilute solution of an often proprietary free radical initiator or activator. Application methods are similar to those used for contact adhesives; the oligomeric portion i s applied by brush, roller or spray to one surface, and the initiator solution i s applied i n a thin (quasi-monomolecular) film to the other surface. The two films are non-air inhibited and may be left open, unbonded for many hours with no adverse effect on subsequent bonding operations. After mating, the surfaces may be positioned for up to two minutes before the reaction starts and the bond forms. Mild contact pressure i s maintained for 5-10 minutes; 75% of the ultimate bond strength i s reached within one hour. Lap shear strengths of 3500 psi and peel strengths of 20 pli have been achieved i n model systems. 0-8412-0523-X/79/47-113-107$05.00/0 © 1979 American Chemical Society

Seymour; Plastic Mortars, Sealants, and Caulking Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

108

P L A S T I C

M O R T A R S ,

S E A L A N T S ,

A N D

C A U L K I N G

C O M P O U N D S

Compositions o f these adhesives are suggested i n a number o f r e c e n t p a t e n t s (3-12)· A l l o f t h e s e r e a c t i v e a d h e s i v e p a t e n t s i n d i c a t e e s s e n t i a l l y t h e same c o n c e p t : an e l a s t o m e r i s c o l l o i d a l l y d i s p e r s e d i n a monomer, o r a monomer/oligomer/polymer s o l u t i o n . The system i s t h e n p o l y m e r i z e d u s i n g a f r e e r a d i c a l mechanism. What o c c u r s i s a r a p i d , " i n s i t u p o l y m e r i z a t i o n of a ( t y p i c a l l y ) m e t h y l m e t h a c r y l a t e system, toughened b y e l a s t o m e r i c domains w h i c h have "beer, i n c o r p o r a t e d i n t o t h e s t r u c t u r e by g r a f t i n g .


n

Many v a r i a t i o n s a r e p o s s i b l e i n t h e n a t u r e o f t h e e l a s t o m e r , p o l y m e r , monomer ( o r monomers), s t a b i l i z e r s , a c t i v a t o r s and i n i t i a t o r s ( r e d o x , S c h i f f ' s b a s e , e t c . ) - and i n t h e d i s t r i b u t i o n o f t h e components between t h e a d h e s i v e b a s e and t h e a c c e l e r a t o r p o r t i o n . Monomers are f a i r l y standard - m o s t l y methyl methacrylate (and m e t h a c r y l i c a c i d ) , w i t h s o f t e r monomers s u c h as b u t y l a c r y l a t e sometimes u s e d f o r Τ g c o n t r o l . Some systems may u s e s m a l l amounts o f p o l y f u n c t i o n a l a c r y l a t e s t o reduce v a p o r p r e s s u r e ; however, t h i s c a n l e a d t o b r i t t l e g l u e l i n e s . F o u r d i f f e r e n t e l a s t o m e r domain m a t e r i a l s have b e e n r e p o r t e d : p o l y c h l o r o p r e n e , chlorοsulfonated p o l y e t h y l e n e , n i t r i l e r u b b e r and u r e t h a n e ( 1 3 ) » One c o m p o s i t i o n f o r a r e a c t i v e a d h e s i v e i s shown i n r e f . ( 9 ) . T h i s p a t e n t d e s c r i b e s b o t h a c o n v e n t i o n a l twop a r t , p r e - c a t a l y z e d a d h e s i v e system - and an a d h e s i v e / a c t i v a t e d p r i m e r system. I n t h e l a t t e r system, a p r i m e r (containing the polymerization catalyst) i s f i r s t applied t o one o r b o t h o f t h e s u r f a c e s t o be j o i n e d . F o l l o w i n g t h i s , a s o l u t i o n o f o l i g o m e r / p o l y m e r i n a v i n y l i c mono mer i s a p p l i e d t o a t l e a s t one o f t h e s u r f a c e s . The two compositions f o l l o w : US P a t e n t 5,890^07

Examples ( w e i g h t s i n grams) 1

Methyl methacrylate 85 Methacrylic acid, g l a c i a l 15 Ethylene g l y c o l dimethacrylate 2 C h l o r o s u l f o n a t e d p o l y e t h y l e n e *_ 100 (202)

Cumene h y d r o p e r o x i d e Ν, N-dime t h y l a n i l i n e A c c e l e r a t o r 808**

6 2 210

2 85 15 2 100 (202)

0Λ

202.4 ***


9.

109

Reactive Adhesives

P R A N E

Notes:

* C o n t a i n s 4-3% CI and 54- m i l l i m o l e s o f s u l f o n y l c h l o r i d e p e r 100 g. p o l y m e r ** A m i x t u r e o f b u t y r a l d e h y d e and condensation products

aniline


*** Example 2 i s a r e a c t i v e a d h e s i v e system. The a c c e l e r a t o r i s a p p l i e d as a p r i m e r w i t h a c o t t o n swab t o t h e s t e e l t e s t s u b s t r a t e , and t h e n wiped t o a t h i n f i l m w i t h a p i e c e o f cloth. A n o t h e r r e f e r e n c e (11) d e s c r i b e s a c r y l i c / u r e t h a n e systems - w i t h o p t i o n a l epoxy c o n t e n t ( f o r improved heat r e s i s t a n c e ) . T y p i c a l compositions from the patent follow: US P a t e n t 5*994,764-

Examples ( p a r t s b y w e i g h t ) IV

I

Component A Polyester-based polyurethane* Acrylic acid Methyl methacrylate D i g l y c i d y l e t h e r o f B i s p h e n o l A** Benzoyl peroxide Hydro quinone

33 33

-5

33 10

46 6

0.1 105.1

5 0.1 100.1

10 90 100

10 90 100

Component Β Dimethyl Methanol Notes:

aniline

* e.g. E s t a n e 5712 ** e.g. Epon 828

(B.E.

Goodrich)

( S h e l l C h e m i c a l Co.)

Component Β i s s p r a y e d as a v e r y t h i n f i l m on c l e a n ( o r o i l y ) s t e e l and a l l o w e d t o e v a p o r a t e f o r 2 m i n u t e s . Component A i s a p p l i e d t o t h e o t h e r s t e e l s u b s t r a t e i n a f i l m t h i c k n e s s o f 3-10 m i l s . The two s u r f a c e s a r e mated u n d e r c o n t a c t p r e s s u r e . S h e a r s t r e n g t h s o f 3200 p s i were a c h i e v e d , w i t h p e e l s t r e n g t h s on t h e o r d e r o f 25-30 p l i .

Most c u r r e n t a p p l i c a t i o n s f o r r e a c t i v e a d h e s i v e s a r e as s t r u c t u r a l a d h e s i v e s ( 1 6 ) . They compete w i t h h i g h p e r f o r m a n c e e p o x i e s and u r e t h a n e s and, i n c e r t a i n c a s e s , w i t h a n a e r o b i c s and c y a n o a c r y l a t e s - i n s t r i n g e n t a e r o space, a i r c r a f t , e l e c t r i c a l , e l e c t r o n i c , marine, i n s t r u ment and o t h e r u s e s (1,2,14). The r e a c t i v e a d h e s i v e s do n o t r e q u i r e t h e s o p h i s t i c a t e d f i x t u r i n g u s u a l l y needed


110

P L A S T I C

M O R T A R S ,

S E A L A N T S ,

A N D

C A U L K I N G

C O M P O U N D S


by t h e e p o x i e s and u r e t h a n e s , n o r t h e need f o r h e a t c u r e . R e a c t i v e a d h e s i v e s have t h e f o l l o w i n g c o m b i n a t i o n o f advantageous p r o p e r t i e s : - S i m p l e no-mix a p p l i c a t i o n - B r o a d a p p l i c a b i l i t y t o a wide v a r i e t y o f s u b s t r a t e s metal, p l a s t i c ) w i t h minimal surface p r e p a r a t i o n - Tolerance f o r oil-contaminated surfaces - R a p i d b o n d i n g a t room t e m p e r a t u r e - E x c e l l e n t bond f l e x i b i l i t y - H i g h p e e l s t r e n g t h , l a p s h e a r s t r e n g t h and i m p a c t resistance - Long open t i m e - Wide l a t i t u d e i n c u r e r a t e - A b i l i t y t o bond h e a t - s e n s i t i v e s u b s t r a t e s - Low s h r i n k a g e d u r i n g p o l y m e r i z a t i o n - E x c e l l e n t r e p r o d u c i b i l i t y and r e l i a b i l i t y - Good h e a t r e s i s t a n c e - Excellent solvent resistance - Good low- and h i g h - t e m p e r a t u r e p r o p e r t i e s - F o r m u l a t i n g f l e x i b i l i t y f o r s p e c i f i c end use requirements To d a t e , o t h e r t h a n h i g h u n i t c o s t , t h e l i m i t a t i o n s o f r e a c t i v e a d h e s i v e s a r e few; however, t h e s e may be c r i t i c a l for certain applications: - S t o r a g e and s h i p p i n g s t a b i l i t y i s p o o r o v e r 20°C. Many o f t h e c u r r e n t p r o d u c t s on t h e market a r e h a n d l e d i n r e f r i g e r a t e d c o n t a i n e r s , u n d e r 4°C. - Bond l i n e t h i c k n e s s i s l i m i t e d , e.g. 3-10 m i l s i s common, w i t h p o o r c u r e o v e r 50-60 m i l s - because o f r e d u c e d a c t i v a t o r d i f f u s i o n . However, a t h i g h t h i c k n e s s e s , t h e c o s t of t h e a d h e s i v e i s a l r e a d y excessive. - S u r f a c e s must be f l a t ; t h e low v i s c o s i t y a d h e s i v e s y s t e m has p o o r g a p - f i l l i n g p r o p e r t i e s - P o t e n t i a l (and a c t u a l ) t o x i c i t y p r o b l e m s , due t o monomers and a c t i v a t o r s . T o x i c i t y . One o f t h e p r i n c i p a l d e t e r r e n t s t o t h e g r o w t h o f r e a c t i v e a d h e s i v e s has been t h e t o x i c i t y (and p o t e n t i a l c a r c i n o g e n i c nature) of the i n g r e d i e n t s : 1. Monomers. M e t h y l m e t h a c r y l a t e and m e t h a c r y l i c a c i d have w e l l r e c o g n i z e d t o x i c i t y c h a r a c t e r i s t i c s as r e g a r d s i n h a l a t i o n , i n g e s t i o n and s k i n c o n t a c t . Compos i t i o n s c o n t a i n i n g l a r g e q u a n t i t i e s of these m a t e r i a l s have b e e n r e j e c t e d f o r use on c e r t a i n p r o d u c t i o n l i n e s , e.g. a u t o m o t i v e a s s e m b l y . ( I n t e r e s t i n g l y , t h e use o f d i m e t h a c r y l a t e s , as i n r e f . ( f t ) , w h i c h have l o w e r v a p o r p r e s s u r e , may ease t h i s s i t u a t i o n , a t t h e expense o f


9.

PRANE

Reactive Adhesives

w e t t i n g and

111

o i l penetration.)

2. I n i t i a t o r s , A c t i v a t o r s , A c c e l e r a t o r s . D i m e t h y l a n i l i n e , t h e p r e f e r r e d a c t i v a t o r i n most p a t e n t s , i s s t r o n g l y s u s p e c t e d t o "be a c a r c i n o g e n . A s s u c h , i t w i l l be u n d e r s c r u t i n y b y TSCA - T o x i c S u b s t a n c e s C o n t r o l A c t . G e n e r i c a l l y s i m i l a r m a t e r i a l s , e.g. dimethyl-pt o l u i d i n e , d i i s o p r o p a n o l - p - t o l u i d i n e , a l s o mentioned i n p a t e n t s , may a l s o s u b j e c t t o s i m i l a r r e s t r i c t i o n s . S c h i f f s b a s e s ( s u c h as t h e A c c e l e r a t o r 808 p r e v i o u s l y described) are a p p a r e n t l y l e s s t o x i c .


1

C o n s i d e r a b l e a c t i v i t y i s underway t o d e v e l o p nont o x i c a c t i v a t o r and a c c e l e r a t o r systems f o r t h e s e room temperature c u r i n g r e a c t i v e adhesives. I t i s o f i n t e r e s t t o n o t e t h a t p r o g r e s s i n a n a l o g o u s systems has b e e n made i n d e n t a l r e s e a r c h - i n work on a c r y l i c a d h e s i v e s f o r r e p a i r and f o r a c r y l i c d e n t u r e s . I n one o f s e v e r a l p a p e r s (15), w o r k e r s d e s c r i b e c o m p o s i t i o n s o f complex, e x o t i c a c r y l i c monomers c u r e d w i t h s t a n d a r d and modif i e d S c h i f f s b a s e s and w i t h complex a r o m a t i c amines. Two o f t h e l a t t e r a r e : N,N-dimethyl-sym-m-xylidene and N , N - d i m e t h y l - p - t - b u t y l a n i l i n e . T o x i c i t y o f t h e s e materi a l s i s b e l i e v e d t o be s i g n i f i c a n t l y l o w e r t h a n t h a t o f dimethylaniline. 1

B r o a d a c c e p t a n c e o f r e a c t i v e a d h e s i v e s w i l l be c o n t i n g e n t upon t h e development o f systems w i t h a c c e p t a b l e t o x i c i t y l i m i t s . U n t i l t h e n , a p p l i c a t i o n s w i l l be l i m i t e d to those i n s e l e c t e d , h i g h performance adhesive m a r k e t s . However, t h e i r t e c h n o l o g y has i m p o r t a n t i m p l i c a t i o n s f o r adhesives o f the f u t u r e . LITERATURE CITED: 1. Adhesives Age, 1976, 19, (9), 21-24; "SecondGeneration A c r y l i c Adhesives" 2. H a r r i e s , R.W., Adhesives Age, 1978, 21, (2), 19-22; "New Developments in Low-Energy Reactive Adhesives" 3. Bader, E.; U . S . Patent 3,333,025; July 25, 1967, to Deutsche Gold-und-Silber Scheideanstalt 4. Toback, A.S.; U.S. Patent 3,616,040; Oct. 26, 1971, to L o c t i t e Corp. 5. Owston, W . J . ; U . S . Patent 3,725,504; Apr. 3, 1973, to Lord Corp. 6. Owston, W . J . ; U . S . Patent 3,832,274; Aug. 27, 1974, to Lord Corp. 7. Owston, W . J . ; U . S . Patent 3,838,093; Sept. 24, 1974, to Lord Corp.



112

PLASTIC MORTARS, SEALANTS, AND CAULKING COMPOUNDS

8· Owston, W . J . , Howard, D . D . ; U . S . Patent 3,873,640; Mar. 25, 1975, to Lord Corp. 9. Briggs, Jr., P.C., M u s c h i a t t i , L.C.; U.S. Patent 3,890,407; June 17, 1975, to DuPont 10. Owston, W . J . ; U . S . Patent 3,970,709; July 20, 1976, to Lord Corp. 11. Wolinski, L.E.; U . S . Patent 3,994,764; Nov. 30, 1976, to Pratt & Lambert, Inc. 12. S k o u l t c h i , M.M.; U . S . Patent 4,052,244; Oct. 4, 1977, to National Starch and Chemical Corp. 13. Prane, J.W.; Polymer News, 1977, 3, (6), 308-310; "About System C l a s s i f i c a t i o n s and Certain TwoComponent Systems" 14. Wilkinson, T.L.; Adhesives Age, 1978, 21, (6), 2023; "Acrylic Adhesives: New Way to Bond Aluminum Auto Parts" 1 5 · Bowen, R.L.; Argentar, H.; Journal of Dental Research, 1971, 50, (4),923-928; "Amine Accelerators f o r Methacrylate Resin Systems" 16. Zalucha, D . ; Adhesives Age, 1979, 22, (2), 21-23; "New A c r y l i c s S t r u c t u r a l l y Bond Unprepared Metals" RECEIVED June 4, 1979.


Reactive Adhesives - American Chemical Society

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