New aspects of the curing mechanism of anaerobic adhesives

New aspects of the curing mechanism of anaerobic adhesives St. Wellmann and H. Brockmann (University of Bielefeld, FRG) Received 10 June 1993; revised 2 November 1993

The curing mechanism of anaerobic adhesives catalysed by saccharin (1), N,Ndimethyl-p-toluidine (2) and cumene hydroperoxide (3) has been investigated in model reactions. As a result of these investigations, a new compound, formed by the interaction of the curing components saccharin (1) and N,N-dimethy/-p-to/uidine (2), w a s isolated and characterized. Further experiments which concentrated on relative gel times showed that this compound is the decisive catalyst for rapidly curing anaerobic adhesives.

Key words: anaerobic adhesives; curing mechanism; chemical interactions; model reactions; aminal; gel time

The expression 'anaerobic' for a type of singlecomponent acrylic adhesive is derived from biology and characterizes rapid curing in the absence of air and in the presence of metal ions. This process requires a low activation energy and therefore takes place at or below room temperature ~. Typical industrial uses of such adhesives are the locking of threaded assemblies, the sealing of threaded and flanged assemblies and the coaxial bonding of close-fitting metal parts. Modern applications are the assembly of firearms and fuel pumps, the assembly of electronic components for high-temperature exposure and bonding metal parts to glass in the automotive industry 2. Since the development of anaerobic adhesives in the 1960s, many anaerobic formulations have been patented, but there has hardly been any scientific literature about the curing mechanism of these adhesives. The majority of anaerobic formulations consist of bifunctional basic resins, i.e., triethylene glycol dimethacrylate, and curing systems based on tertiary amines, benzoic sulfimide (saccharin (1)) and cumene hydroperoxide (3) 3. The assumption of a radical polymerization mechanism is common to all publications on anaerobic adhesives, but the exact function of each of the curing components is not yet fully known 4. Thus, in the course of our investigations, we concentrated on the chemical interactions between cumene hydroperoxide (3), N,N-dimethyl-p-toluidine (2) and saccharin (1) as

components of the most common curing system with regard to possible reaction products. On this basis the studies focused not only on reactions which take place under curing conditions (anaerobic) but also on reactions which occur during storage of the adhesive (aerobic).

Experimental details Materials

N,N-Dimethyl-p-toluidine (2) was purified by distillation under reduced pressure; saccharin (1), 2,2,6,6-tetramethylpiperidine, cumene hydroperoxide (3) (80%) and triethylene glycol dimethacrylate were used as obtained from Janssen, Merck and Sichel, respectively. All solvents were purified by distillation. Dichloromethane was dried by heating for 4 h with phosphorous pentoxide and distillation in the presence of molecular sieve (3 A). Argon was dried by silica gel, phosphorous pentoxide and potassium hydroxide. A reaction of hydrogen peroxide with N,N-dimethyl-ptoluidine (2) in methanol yielded N,N-dimethyl-ptoluidine-tert-N-oxide (4), melting point (rap) = 35°C (Reference 5). Cumene alcohol (6) was produced by a Grignard reaction from acetophenone with methyl iodide, boiling point (bp) = 90°C, 18 torr (Reference 6). 2-Hydroxymethylsaccharin resulted from a reaction of formaldehyde (37% aqueous solution)

0143-7496/94/01/0047-09 © 1994 Butterworth-Heinemann Ltd INT.J.ADHESlON AND ADHESIVES VOL. 14 NO. 1 JANUARY 1994

47

with saccharin (1) and a subsequent recrystallization from ethanol, mp = 127'C (Reference 7). A reaction of p-toluidine with trimethyl orthoformiate resulted in Nmethyl-p-methyl-anilide and, by a subsequen! hydrolysis with hydrochloric acid, N-methyl-p-toluidine was obtained, bp - 9 7 C , 18 torr (Reference 8). A reaction of 2,2,6,6-tetramethylpiperidine with m-chloroperbenzoic acid yielded 2,2,6,6-tetramethylpiperidineN-oxyl free radical (10), mp 38°C (Reference 9). ~H nuclear magnetic resonance (NMR) spectra were recorded on a WP 80 F T - N M R spectrometer (Bruker), 13C NMR spectra by an AM 300 instrument (Bruker). Mass spectra were measured by a CH5 mass spectrometer (Varian Mat). General procedure for model reactions All model reactions were carried out in dichloromethane at room temperature and in equimolar concentrations of educts with reaction times of 12 h. Anaerobic reactions were carried out by degassing and flushing with argon, aerobic reactions by bubbling air through the solution for 1 h and leaving it in contact with air. In order to investigate model reactions the solvent was removed under reduced pressure and the residue was characterized by NMR spectroscopy and mass spectrometry (MS). Reaction of cumene hydroperoxide (31 with N,Ndimethyl-p-toluidine (2) in absence of air yielding N,Ndimethyl-p-toluidine-tert-N-oxide (4) A solution of l l mmol N,N-dimethyl-p-toluidine (2) and 11 mmol cumene hydroperoxide (3) was treated as described in the general procedure for anaerobic reactions. IH NMR (CDCI3, 80 MHz): 6 (ppm) - 7.84-7.12 (m, 9H, Ph); 3.46 (s, 6H, O N+-(CH3)2); 2.33 (s, 3H, Ph CH3): 1.57 (s, 6H, C(CH3)2). Reaction of N,N-dimethyl-p-toluidine-tert-N-oxide (4) with saccharin (1) catalysed by copper(I) chloride yielding 2,3-dihydro-2-(N-methyI-N-p-tolyl-)aminomethyl-3-one-benzisothiazole-l,l-dioxide (aminal, 5) A solution of l 1 mmol N,N-dimethyl-p-toluidine-tert-Noxide (4), 11 mmol saccharin (1) and a catalytic amount (0.1 mg) of copper(I) chloride was treated as described in the general procedure for anaerobic reactions. The numbering system for spectral assignment of 2,3-

(C~.~., N CH2 N): 38.6 {('p,, .... N CH~): 2{}.3 (('p, ..... Ph-CH3). MS (70 eV), m/e (relative intensity '!i,): 316 (3{}} [M' ]; 251 (5); 196 (10): 183 (1 I): 134 (90): 12(1 (54): 104 (31); 91 (26); 77 (21). Synthesis of 2,3-dihydro-2-(N-methyI-N-ptolyl-)aminomethyl-3-one-benzisothiazole-1,1 -dioxide (aminal, 5) 20 mmol 2-hydroxymethylsaccharin was suspended with 3.3 mmol phosphorous pentoxide in 10 ml of absolute dichloromethane. Then 20 mmol N-methyl-ptoluidine in 10 ml of absolute dichloromethane was added dropwise in the absence of air. After stirring for 30 rain the solution was filtered and the solvent was removed under reduced pressure. The pink residue was recrystallized from chloroform/petrol ether to yield 68% of a white powder, mp = 105cC. ~H NMR (CDCI3, 80 MHz): ~3(ppm) = 8.08 7.74 (m, 4H, Ph); AA'BB' (6AA, 7.1, 6UU, 7.0, k/ = 8.6 Hz, 4H, Ph); 5.48 (s, 2H, N C H 2 N): 3.13 (s, 3H, N C H)); 2.25 (s, 3H, Ph CH~). 13C NMR (CDCI31 75.5 MHz): 5 (ppm) = 159.2 (Cqu,,-~, C ~ O): 144.4 (Cqu,,-t, C-9); 137.7 (Cqu,rt, C-4); 134.9 (Ctm, C-5), 134.2 (Ct~t, C-8); 129.7 (Cto~t, C-2', C-if); 128.4 (Cquart, C-I'); 127.0 (Cqu~m, C-4'); 125.1 (Ct~t, C-7); 120.9 (Ctm, C-6); 114.3 (Ctm, C-3', C-5'): 59.5 (C .... N CH2 N); 38.6 (Cp~im, N CH3); 20.3 (Cp,.im. Ph CH3). MS (70 eV), m/e (relative intensity %): 316 (35) [ M " ] ; 251 (5): 196 (13); 183 (27); 134 (74): 120 (100); 104 (26): 91 (31): 77 (32). Model anaerobic adhesive The determination of relative gel times in an anaerobic adhesive system was investigated by curing model anaerobic systems in a self-constructed apparatus based on DIN 16945 (see Fig. 1). The sample is placed in a glass flask, which is flooded with argon. A glass stirrer magnetically coupled with a motor is moved up and

I

dihydro-2-( N-methyI-N-p-tolyl-)aminomethyl-3-onebenzisothiazole-1,l-dioxide (aminal, 5) is shown below.

O

,

I

2-4

7~ " - ~ . / g ~ s

6

?

x

..//"~r~ u ~

=

6'

J

s'

5

IH NMR (CDCI3, 80 MHz): 5 (ppm) = 8.08-7.78 (m, 4H, Ph); AA'BB' (0AA, = 7.10, 5BB, = 6.94, 3j = 8.8 Hz, 4H, Ph); 5.48 (s, 2H, N - C H 2 N); 3.13 (s, 3H, N-CH3); 2.25 (s, 3H, Ph-CH3). 13C NMR (CDCI3, 75.5 MHz, J-modulated): 6 (ppm) = 159.0 (Cquart, C==-O); 144.4 (Cquart, C-9); 137.0 (Cqu~.-t, C-4); 135.0 (Cte~t, C-5); 134.3 (Ct:rt, C-8); 129.7 (Cten, C-2', C-6'); 128.4 (Cq,an, C-I', C-4'); 124.9 (Ctert , C-7); 120.7 (Ct~rt, C-6); 114.3 (Ct~rt, C-3', C-5'); 59.5

48

INT.J.ADHESlON AND ADHESIVES JANUARY 1994

Fig. 1 Apparatus for determination of relative gel times based on DIN 16945

down inside the sample. At the moment of gelation the glass stirrer stops moving and thus interrupts the magnetic coupling as well as an electrical circuit connected to a stop watch. Gel time determinations were carried out with model anaerobic adhesives consisting of 1.35 mmol saccharin (1) and 1.35 mmol N,N-dimethyl-p-toluidine (2) dissolved in 20.2 ml of triethylene glycol dimethacrylate. These components were dissolved in triethylene glycol dimethacrylate by vigorously stirring the mixture for 2 h in the presence of air. To this solution (basic solution), which was stored at 7”C, different amounts of aminal (5) in the range 0.02 to 0.5 mmol were added and the resulting solutions stored at 7°C. Shortly before gel time determination, 0.1 ml cumene hydroperoxide (3) and 0.05 ml of 5 x lop4 M ethanolic copper acetate solution were added to 4.5 ml of these solutions. Subsequently their gel times were compared with those of the basic solution. For reactivity investigations, solutions of 1.35 mmol saccharin (I), 1.35 mmol N,N-dimethyl-p-toluidine (2) or 1.35 mmol aminal (5), each dissolved in 20.2 ml of triethylene glycol dimethacrylate, were prepared. Gel time determinations were carried out as described previously. Results

and discussion

The chemical interactions between the components cumene hydroperoxide (3), N,N-dimethyl-p-toluidine (2) and saccharin (1) were examined in separate reactions. N,NDimethyl-ptoluidine (3)

(2) and cumene hydroperoxide

First, the interactions between N,N-dimethyl-ptoluidine (2) and cumene hydroperoxide (3) were investigated. In the absence of air, cumene hydroperoxide (3) oxidizes N,N-dimethyl-p-toluidine (2) to the corresponding N-oxide (4) and cumene alcohol (6) is formed. However, this reaction is

quenched in the presence of air, indicating its radical nature. With the he\! of previous investigations by Horner and Anders , who studied the accelerated decomposition of peroxides by tertiary amines, a mechanism for this formation of N-oxide (4) can be proposed (see Fig. 2). The formation of cumene alcohol (6) in this process is confirmed by the studies of Humphreys’ I. However, owing to his reaction conditions (lOO’C, 48 h), Humphreys isolated a series of further reaction products which are not formed under usual conditions (room temperature,
(2)

The interactions between saccharin (1) and N,Ndimethyl-p-toluidine (2) are equivalent to those of an acid and a base, and the resulting salt (7) is in equilibrium with its free components in solution (see Fig. 3). On one hand, this salt formation enhances the concentration of dissolved saccharin (l), which as a free acid is hardly soluble in organic solvents; on the other hand, it diminishes the amount of N,N-dimethylp-toluidine (2) available for cumene hydroperoxide (3) decomposition and thus restrains its reactivity. Furthermore, on account of its acidic character, saccharin (1) is able to dissolve metals by forming metal salts. Saccharin (11 and Noxide

(4)

The question of possible reactions of N-oxide (4) led to an investigation of the effect of saccharin (1) on Noxide (4) in the presence of metal ions. In the course of

3

Fig. 2

Reactionof cumene

hydroperoxide

(3) with fV,N-dimethyl-ptoluidine

(2) in the absence of eir

INT.J.ADHESION

AND ADHESIVES JANUARY

1994

49

oH3

//--N

HN

+

2

H3C

L

I~ m H

N\

CH3

O/./ \~)

I

1

Fig. 3 Interactions between N,N-dimethyl-p-toluidine (2) and saccharin (1) O

Investigations on colouration

In the presence of air N,N-dimethyl-p-toluidine (2) is oxidized to the radical cation (9) (see Fig. 6) of the Wurster-type salt. The existence of this radical can be proved by trapping it with 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO, 10) (see Fig. 6). The formation of this radical cation is accompanied by an intense colouration of its solutions, as already described in the literature for radical cations of Wurster-type salts ~3. Consequently, the intense purple colour of N,N-dimethyl-p-toluidine (2)/saccharin (1) solutions is due to the formation of the N,N-dimethylp-toluidine radical cation (9) because the colour disappears immediately after addition of the TEMPO radical (10). This result is contrary to a report in which the formation of a charge transfer complex between saccharin 1(41)and N,N-dimethyl-p-toluidine (2) is suggested Thus, using knowledge about the formation of this radical cation (9) and with the help of investigations about the autoxidation mechanism of tertiary aromatic amines 15, a mechanism for the aerobic formation of aminal (5) and N-oxide (4) can be formulated (see Fig. 7). The radical cation ~9) is stable in acid solution in very low concentrations L and gives rise to an intense colouration of such solutions. In this case the stability is ensured by saccharin (1) because of its acidic character. If a certain concentration of radical cations (9) is exceeded, surplus radicals will be quenched by oxygen to give ~-aminomethylene hydroperoxide (11). This intermediate reacts rapidly with N,N-dimethyl-ptoluidine (2), yielding c~-aminomethylene alcohol (12) and N-oxide (4). In a subsequent reaction step ~-

5 Fig. 4 Aminal (5)

this reaction a new compound, which has not yet been described in the literature, was formed. Its structure was determined by NMR, MS and by independent chemical synthesis as aminal (5) (see Fig. 4). Based on published results for reactions of tertiary amine-N-oxides with iron salts ~z, a mechanism for the formation of aminal (5) is suggested (see Fig. 5). This figure shows the copper-catalysed decomposition of Noxide (4) to the reactive intermediate (8), the imminium cation, which reacts with saccharin (1) to yield aminal (5). Similar to the formation of N-oxide (4), aminal (5) is also obtained by the reaction of saccharin (1) and N,N-dimethyl-p-toluidine (2) in the presence of air. Thus aminal (5) represents a further component of the curing system which has to be taken into account. According to these investigations, both the anaerobic as well as the aerobic formation of N-oxide (4) and aminal (5) are possible. However, due to the presence of monomer as a potential reaction partner for the anaerobically formed radicals, the formation of aminal (5) and N-oxide (4) under these conditions is of minor importance. Thus, in real adhesive systems, aerobic formation is the main source for N-oxide (4) as well as for aminal (5).

CH 3 H 3 C ~ ~ + ~ O

-

+

Cu(I)

+

2 H*

CH3

+ Cu(II)

]. H"

+ H20

~CH3

I cu(,)

N--CH2~ N

CH3

~

o -

H +

5 Fig. 5 Mechanism of the copper-catalysed reaction of N-oxide (4) with saccharin (1)

50

INT.J.ADHESION AND ADHESIVES JANUARY 1994

=CH I .

o

8

H3C

"

H3C

CH3

merely partly polymerized were not recorded by this method. The results of these investigations are given in Fig. 8 and Table 1. Fig. 8 shows that the addition of one part of aminal (5) to three parts of saccharin (1) and three parts of N,N-dimethyl-p-toluidine (2) shortens the gel time to one-third that of the original system. However, account must be taken of the fact that, since aminal (5) may be formed aerobically (as described in Fig. 7), a certain amount of aminal (5) is formed during preparation of the basic solution for gel time determinations (in which saccharin (1) and N,N-dimethyl-p-toluidine (2) are dissolved in the presence of air). This means that even those samples indicated as having 'zero aminal (5) concentration' in Fig. 8 contain at least some aminal (5). This assumption was confirmed by further gel time determinations, in which the contact of N,N-dimethylp-toluidine (2) and saccharin (1) in the presence of air was avoided until shortly before making the measurement, so that no aminal (5) could be formed before curing. The resulting gel times were greater than 2 h. This fact was the first indication of the importance

CH3 H3C OI CH3

9 10 Fig. 6 Radical cation (9) of N,N-dimethyl-p-toluidine (2) and TEMPO (10) as trapping reagent

aminomethylene alcohol (12) reacts with saccharin (1), yielding aminal (5). Effect of aminal (5)

The effect of the new compound on the curing of anaerobic adhesives was elucidated by determining relative gel times of model anaerobic adhesives in a self-constructed apparatus, based on DIN 16945. It was the intention of these gel time determinations to measure the time necessary for complete curing of anaerobic model systems, because only this is relevant for anaerobic adhesives. Therefore, systems which were

,c.3 H3C

÷

*'--H " N CH3

2

H3C

O 1

"+

o;

CH3

9

c~ 11 IIsC

N~CHs 2

CH3

CH3

4

12

0

x0

1

0

Fig. 7 Mechanism of aerobic formation of aminal (5) and N-oxide (4)

INT.J.ADHESlON A N D ADHESIVES J A N U A R Y 1994

51

25

2O

0

0

0,1

0,2 concentration

0,3

0,4

0,5

0,6

of am±nal [mmol]

Fig. 8 Dependence of the gel times of model anaerobic adhesives on concentration of aminal (5)

Table 1. Dependence of gel times of model anaerobic adhesives on aminal (5) concentration Aminal (5) concentration (mmol)

Gel time (min)

0.00 0.02 0.05 0.11 0.21 0.32 0.50

19.56 15.79 13.61 11.30 9.42 6.88 6.46

of aminal (5) formation with regard to complete curing of the adhesive. This importance has been confirmed by comparing the reactivity of aminal (fi) with that of saccharin (1) and N,N-dimethyl-p-toluidine (2) in gel time determinations. These studies showed that curing does not take place in the absence of aminal (fi) (see Fig. 9). On the other hand, the combination of all components aminal (fi), saccharin (1), N,N-dimethyl-p-toluidine (2) and cumene hydroperoxide (3)~--results in the shortest gel times (see Fig. 8). In the course of these reactivity investigations it was discovered that the N-oxide (4) has no accelerating effect on curing. Furthermore, Fig. 9(a) shows that the concentration of free radicals produced by the amine (2) induced decomposition of cumene hydroperoxide (3) (see Fig. 2) is not sufficient for curing. The extremely long polymerization time of the saccharin (1) amine (2) system in Fig. 9(c) shows that curing requires the presence of aminal (fi), which is obviously formed in this system due to residual oxygen dissolved in the reaction solution. This assumption is confirmed by the fact that aminal (5) alone is capable of effecting curing of the monomer four times faster (Fig. 9(d)) than the system described in Fig. 9(c). Additional proof for the importance of aminal (5) is given by a special patented technique of preparing an anaerobic adhesive 16. In a first step the monomer is saturated with oxygen in the presence of saccharin (1) and N,N-dimethyl-p-toluidine (2). The following addition of cumene hydroperoxide (3) results in a highly stable as well as extremely reactive system. The

52

INT.J.ADHESION AND ADHESIVES JANUARY 1994

scientific explanation l'or this procedure i,~ gix.cn m Fig. 7. Obviously the additional stability is achieved b 3 the formation of N-oxide (4), which prevents the carl~ decomposition of cumene hydroperoxide (3) by N,Ndimethyl-p-toluidine (2). Tile reason l\)r tile relnarkablc reactivity is the influence of aminal (5), which is I'ormed in higher concentrations during this oxygenating technique. Investigations by Okamoto ~4 confirm our assumption. He showed that the polymerization rate of the anaerobic system was significantly decreased when replacing N,N-dimethyl-p-toluidine (2) by secondary and modified tertiary amines or when replacing saccharin (1) by N-substituted saccharins and open chain analogues of saccharin (I). Aminal (5), which is obviously of highest importance for a rapid curing, can of course not be formed in these systems.

Function of aminal (5)

On the one hand, it is possible to trace back the function of aminal (5) to its ability to form metal complexes and thus to provide the metal ions required in the course of curing. The complex formation constant of aminal metal complexes is larger than that of the corresponding saccharin metal complexes, as proved by the precipitation of saccharin (1) from a saccharin metal salt solution after the addition of aminal (5). On the other hand, it can be shown that aminal (5) is a rather strong reducing agent. Reducing agents are necessary l\~r the system, because active radicals are produced only by reduced metal ions from cumene hydroperoxide (3) 4 (see Fig. 10(a)). Metal ions in higher oxidation states only produce inactive radicals that are not able to initiate polymerization (see Fig. 10(b)). Some authors 4- 17 suggest the amine (2) to be the reducing agent, but our experiments have shown that only aminal (5) is able to reduce metal ions in this system. Aminal (5) reduced Cu(ll) to Cu(I) in the Fehling reaction, decolourized a KMnO4 solution and produced metallic silver from an Ag(I) solution. All these tests were negative with N,N-dimethyl-p-toluidine (2) and saccharin (!). Fig. 11 shows a proposed mechanism for the reduction of Cu(ll) by aminal (5). Following this mechanism, the metal reduction by aminal (5) generates three reactive species, the imminium cation (8), the saccharin radical (15) and the copper(I) ion, which all lead to further reactions.

Curing mechanism (see also Fig. 12)

Based on the assumption of a polymerization mechanism initiated by radicals, the results of our investigations suggest a synergistic interaction between the individual components of an anaerobic curing system, which ensures a rapid and complete curing. The decomposition of cumene hydroperoxide (3) into active radicals is catalysed by metal ions in low oxidation states. These are dissolved by aminal (5) and saccharin (1) while in a higher oxidation state and then reduced by aminal (5). Formation of aminal (5) takes place by reaction of N,N-dimethyl-p-toluidine (2) and saccharin (1) in the presence of oxygen. In this reaction N-oxide (4) is also produced, which gives rise to additional stability of the system. By reducing metal

H3C~N~1 I'3

monomer

no cudng

cu(,) cumene hydml:~mxide(3)

2 O

monomer

no curing

Cu(ll) cumene hydroperoxide (3)

O

O ~mer

NH + H3C

N\

curing after 2:15 h

cu(ll)

2'%

cumer~

hydm~mxide (3) amine (2) added shortly before gel time detem~ination

O monomer

/7-.-o.2-., '~---t Fig. 9

5

J! .J

O/~8k~ ",.~"

curing after 34 min

cu(ll) cumene hydm~m:~de (3)

Reactivity investigations based on gel time determinations

ions, aminal (5) decomposes into the imminium cation (8) and saccharin radical (15). The latter is able to react with monomer, while the imminium cation (8) reacts with saccharin (1) to yield new aminal (5). That is, the aminal is regenerated and is again available for new reduction cycles. N,N-Dimethyl-p-toluidine (2) is also able to initiate the decomposition of cumene hydroperoxide (3), in the course of which active radicals are produced. According to Fig. 5, the N-oxide (4) also decomposes into active radicals in the presence of metal ions. Thus, the aminal (5) produced in an aerobic reaction is the decisive catalyst for curing anaerobic adhesives, because only aminal (5) in the presence of saccharin (1), N,N-dimethyl-p-toluidine (2) and cumene hydroperoxide (3) generates a 'pool' of radicals necessary for a complete and rapid curing. The curing mechanism is summarized in Fig. 12.

a ~~H3OOH

Conclusions

The curing mechanism of anaerobic adhesives depends on radicals which are quenched in the presence of air and generated by metal ions in the absence of air. Rapid and complete curing is ensured by combining the radical initiator cumene hydroperoxide (3) with saccharin (1) and N,N-dimethyl-p-toluidine (2). In the presence of air saccharin (1) and N,N-dimethyl-ptoluidine (2) lead to the formation of a new compound, aminal (5), which forms metal chelates and is an excellent reducing agent for metal ions. Thus, saccharin (1) and aminal (5) provide soluble metal ions from the surface of the substrate, which are reduced by aminal (5) to low oxidation state. These ions then generate active radicals from cumene hydroperoxide (3). N,NDimethyl-p-toluidine (2) acts as a co-initiator apart

+ Cu(I)

+ Cu(ll) + O H -

CH3

CH3

3

13

. Cu(,

+ Cu(I) + H +

CH3 3

CH3 14 (inactive)

Fig. 10 Radical decomposition of cumene hydroperoxide (3)

INT.J.ADHESlON AND ADHESIVES JANUARY 1994

53

--

?H3

O \\

Cu(,)

A minaI-MetaI-Complex

o H3C~N~---CH2

+ cu(t)

8

o

o

15

Fig. 11 Reduction of Cu(ll) by aminal (5)

o

~ /

~H3

~

~ /CH3 NH + H3C--(X\ //~---N ~CH3

--OOH CH 3 3

1

02 HzO

O ~ A /--\ I ° /"~', ~ H3C--'k\ //'----~--Cm--N Jl .J CH~

~

/--\

CH3

i+

H3C~-~k\ //"~N~--CH2

Cu(It)

Cu(t)

--0" 13 CH3 Monomer

> Polymer

Fig. 12 Curing mechanism

from metal ions. Therefore, the essential step in the anaerobic adhesive curing mechanism is the formation of aminal (5), which is spontaneous in the presence of air and guarantees rapid and complete curing.

months of financial support. Furthermore, the authors thank Dipl. Chem. V. Selig and Dr B. Meyer-Roscher for the development of the apparatus used for the determination of relative gel times. This paper is dedicated to Professor Dr E.V. Dehmlow on the occasion of his 60th birthday.

Acknowledgements

The authors thank Dr A. GroB for fruitful discussions stimulating these investigations and the Institut ffir Angewandte Materialforschung, Bremen, FRG, for five

54

INT.J.ADHESION A N D ADHESIVES J A N U A R Y 1994

References 1

Stemper, D.J. 'Curing characteristics of anaerobic sealants and adhesives' Brit Polym J 15 (March 1983) pp 34--39

2 3 4

5

6

7 8

9

10

11

Hauser, M. and Loft, T.J. 'Anaerobic and modified acrylic adhesives' Adhesives Age 23 (December 1980) pp 21-24 Penczek, P. 'Neue Entwicklungen bei Acrylatklebstoffen' Adhesion 32 No 4 (1988) pp 24-29 Boeder, C,W. 'Anaerobic and structural acrylic adhesives' in "Structural Adhesives" edited by S.R. Hartshorn (Plenum Press, New York, 1986) pp 217-233 analogous to Tietze, L.F. and Eicher, Th. 'Bildung und Umwandlung heterocyclischer Verbindungen' in "Reaktionen und Synthesen" (Georg Thieme-Verlag, Stuttgart, 1981) pp 294-295 analogous to Becker, H.G.O. et al. 'Reaktionen mit metallorganischen Verbindungen' in "Organikum, 15th edition" (VEB Deutscher Verlag der Wissenschaften, Berlin, 1981) p 623 Zinner, H., Zelck, U. and Rembarz, G. 'Mannich-Basen des Saccharins' J Prakt Chem 8 No 4 (1959) pp 150-155 analogous to Tietze, L.F. and Eicher, Th. 'Darstellung und Umwandlung funktioneller Gruppen' in "Reaktionen und Synthesen" up. cit. pp 76-77 analogous to Doser, K. 'NitroxyI-Radikale, Nitroxide' in "Methoden der organischen Chemie', (Houben-Weyl), 4th edition, volume E16a/1 (Georg Thieme-Verlag, Stuttgart, 1990) p 399 Homer, L. and Anders, B. 'Uber den polaren und radikalischen Reaktionsverlauf der Umsetzung von Diacetylperoxid mit (prim., sek., und tert.) Aminen, Phosphinen, Arsinen, Thio~ithern und •&,thern' Chem Ber95 (1962) pp 2470-2484 Humphreys, R.W.R. 'Chemistry of accelerators for curing anaerobic adhesives - - reaction of N-N-dimethylaniline derivates with cumene hydroperoxide' Polym Sci Techno129 (1984) pp 482-485

12

13

14 15

16 17

Ferris, J.P., Gerwe, R.D. and Gapski, G.R. 'Detoxication mechanism. III. The scope and mechanism of the iron-catalyzed dealkylation of tertiary amine oxides' J Org Chem 33 (1968) pp 3493-3498 Michaelis, L., Schubert, M. P. and Granick, S. 'The free radicals of the type of Wurster's salt' J Am Chem Soc 61 (1939) pp 1981-1992 Okamoto, Y, 'Anaerobic adhesive cure mechanism--I' J Adhesion 32 (1990) pp 227-235 Howard, J.A. and Yamada, T. 'Absolute rate constants for hydrocarbon autoxidation. 31. Autoxidation of cumene in the presence of tertiary amines' J Am Chem Soc 103 (1981) pp 71027106 Patent cited in Stamper, D.J. 'Curing characteristics of anaerobic sealants and adhesives' Brit Po/ym J 15 (1983) pp 34-39 Lees, W.A. 'The science of acrylic adhesives' Brit Po/ym J 11 (1979) pp 64-71

Authors The authors are with the University of Bielefeld, Germany. Correspondence should be addressed to St. Wellmann.

INT.J.ADHESlON AND ADHESIVES JANUARY

1994

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