melamine coatings

Eur. Polym. d. Vol. 19, pp. 11 18, 1983

0014-3057/S3'010011-08503.00/0 Copyright © 1983 Pergamon Press Ltd

Printed in Great Britain. All rights reserved

NITROXIDE KINETICS DURING PHOTODEGRADATION OF ACRYLIC/MELAMINE COATINGS J. L. GERLOCK,H. VAN OENE and D. R. BAUER Engineering and Research Staff Ford Motor Company, P.O. Box 2053, Dearborn. M1 48121, U.S.A. (Received 14 May 1982; in revisedJorm 5 July 1982) Abstract- Photooxidation of bis (2,2,6,6-tetramethylpiperidinyl-4)

sebacate to nitroxide has been followed by electron paramagnetic resonance in acrylic melamine coatings under a number of ultraviolet light exposure conditions. The strong relationships observed between nitroxide kinetic behaviour, coating composition and exposure condition suggest that nitroxide kinetics analysis could be useful in probing coating photodegradation. This approach, together with infrared spectroscopic measurements of chemical changes on degradation, has suggested a novel mechanism, viz. photo-assisted hydrolysis, to explain the observation that photodegradation in acrylic melamine coatings proceeds more rapidly under more humid conditions. Photo-assisted hydrolysis of melamine ether crosslinks yields formaldehyde which can promote free radical oxidation leading to a more rapid photodegradation of these coatings under humid conditions than under dry conditions.

INTRODUCTION Organic coatings used outdoors are subject to a number of environmental stresses (such as sunlight, heat and humidity) that can induce chemical and physical changes. The rates of these chemical processes and changes in physical properties under the action of environmental stresses is of considerable interest as they determine essentially the useful life or durability of the coating. The most trusted data on the weatherability of coatings is obtained by subjecting specimens to natural outdoor exposure [1]. This testing practice has several drawbacks. Years of natural exposure may be necessary to reveal weatherability differences between good and very good coatings. The choice of a test site and details of the testing procedure can significantly affect test results. For example, coatings perform differently at test sites in Florida (under humid conditions) than they do in Arizona (under dry conditions); further, the weather fluctuates at test locations from season to season and year to year [lb]. These difficulties have prompted many attempts to produce an artificial weather environment in the laboratory in which the deterioration of a coating is accelerated [2]. Commercial equipment is available. In all cases, the laboratory weathering environment is made more harsh than natural exposure, e.g. higher than ambient light intensity, temperature and humidity. The value of such accelerated tests has been debated for over 50 yr. Correlation between accelerated and natural exposure test results is found to be good for some coating systems but poor for others. The disparity in test behaviour may lie in the fact that good correlation requires at a minimum that reactions responsible for degradation during natural exposure retain their relative importance during accelerated exposure. Since the degradation reactions and their dependence on light intensity, temperature and humidity can be strong functions of the chemical structure and pigmentation of the coating, it is not

surprising that accelerated test results often do not parallel natural exposure results. Present work is aimed at developing techniques to assess the influence of exposure conditions on coating degradation chemistry in order to design improved accelerated tests. In this paper, the results of a series of survey experiments are described; they suggest that electron paramagnetic resonance (EPR) [3] measurements of nitroxyl radical kinetic behaviour can be used to monitor the influence of exposure conditions on coating degradation rates. We have determined qualitatively the effect of changes in humidity on the rate of photooxidation in our coatings. These results together with infrared spectroscopic measurements of chemical changes on degradation have suggested an unexpected degradation mechanism linking humidity and photooxidation. EXPERIMENTAL Two acrylic melamine coatings, designated A and B, were prepared. The coatings differ only in the comonomer composition of the acrylic copolymers. While both copolymers contained 30% by weight hydroxyethyl acrylate, the copolymer in coating A contained 25~; by weight styrene and the copolymer in coating B contained no styrene. The other comonomers were typical acrylate and methacrylate monomers including acrylic acid, butyl methacrylate and 2-ethylhexyl acrylate. The number-average molecular weight of both copolymers was around 2000. The crosslinker was a partially alkylated melamine (MeI-D of ref. [4]). The ratio of acrylic copolymer to crosslinker was 70:30. All coatings were cured at 130 for 20rain. The details of the analytical procedure developed to assay nitroxide concentration in intact, fully cured polymer films will be given [5] but a brief description is given here. Thin film samples, ~0.15 mm, were prepared on closely matched quartz sample plates measuring 37.0 _+ 0.05 mm x 10.5 +0.05mm x 1.0+0.02mm using a draw down casting procedure. Total film weight was typically 45 + 5 mg. A quartz sample holder was designed to position reproducibly sample plates in the resonance cavity of a Varian Associates E-3 spectrometer.

J.L. GERLOCK et al.

12

Experiments with primary standard dinitroxide VI doped films revealed a linear relationship between ESR signal peak height and nitroxyl function concentration over the concentration range 10 -7 10 -5 mol/g. Nitroxide assay error taking into account contributions from sample positioning error, spectrometer tuning error and error due to volatilization of dinitroxide VI during cure is estimated to be less than + 10%. Infrared spectra (i.r.) were recorded with a Perkin-Elmer 265 spectrophotometer on coating samples cast on polished stainless steel plates using a specular reflection accessory. Coating thickness was ~ 5 microns for the i.r. experiments. Dew points were measured with an ORTEC dew point apparatus. Bis(2,2,6,6-tetramethylpiperidinyl-4) sebacate, I, other hindered amine light stabilizer additives and a benzotriazole ultraviolet absorber were generously supplied by Ciba Geigy. These materials were recrystallized prior to use. Synthesis of the 4-amino-2,2,6,6-tetramethyl piperidine-l-oxy free radical urea of 4,4' diphenyl methane diisocyanate, VI, will be described I-5"i. A variety of ultraviolet (u.v.) exposure conditions have been used. These are designated as natural, u.v.-a, u.v.-b and u.v.-c. Natural exposure consisted of weather existent in Southern Michigan during the time period April-June, 1981, U.V.-a exposure is that conventionally used in QUV weatherometers (ASTM-G-53-770, namely 8 hr u.v. light at 60 ° followed by 4 hr condensing humidity at 50°. U.V.-b exposure consists of the conventional u.v.-a exposure in the absence of a condensing humidity cycle. Humidity is higher here (dew point = 27 °) than ambient because water at ~50 ° is normally stored in the bottom of the chamber of the weatherometer for subsequent use in the condensing humidity cycle. U.V.-c exposure resembles u.v.-b exposure except that all water was drained from the weatherometer. Humidity during the u.v.-c exposure was two to three times lower (dew point 13°C + 5) than that during u.v.-b exposure. Light intensity during u.v.-c exposure was twice that of u.v.-a and u.v.-b exposures.

I O0 8o '7"-" • 60 0

,z,

%

20

Survey experiments HALS additives are rapidly oxidized to nitroxides in coatings exposed to u.v. For example, when coating A doped with l~o by weight HALS additive, bis (2,2,6,6-tetramethylpiperidinyl-4) sebacate, I, is

II

II

I

1

I

1

20

30

40

50

Fig. L Formation of nitroxide II from amine ! in coating A containing 1% by weight amine I during outdoor exposure.

?'---I

50G

=

!

I

Fig. 2. EPR spectrum of nitroxide II in coating A.

280 r

240

Hindered amine light stabilizer (HALS) additives are excellent photostabilizers [6] particularly when used in c o m b i n a t i o n with a u.v. light absorbing additive such as a benzotriazole [7]. N u m e r o u s solution [8] a n d polymer [9] experiments suggest that HALS based nitroxyl radicals are key intermediates in the chemistry of photostabilization. To determine whether or not nitroxyl radical formation and decay kinetics reflect the rate of p h o t o o x i d a t i o n of coatings, survey experiments were performed varying coating composition, exposure conditions a n d H A L S structure. The data from these experiments were interpreted using a simple kinetic model relating nitroxide behaviour and photooxidation.

OC(CH2)CO

I I0

EXPOSURE, DAYS

RESULTS AND DISCUSSION

(I)

40

2OO

160

), %

c

120

8O

200

600

I000

I

I

1400

1800

EXPOSURE, HOURS

Fig. 3. Formation of nitroxide II in coatings doped with 1% by weight amine I; (a) Coating A during u.v.-b exposure, (b) Coating B during u.v.-b exposure, and (c) coating A with 1% by weight benzotriazole u.v. absorber during u.v.-b exposure.

h u =- p _ coating "

II (CHz)(:'O II Tr

N--O"

Nitroxide kinetics exposed to sunlight a strong EPR signal from nitroxide 11 (9 h, i) can be detected within a day. The signal is stable in the dark at room temperature for months. With continued exposure the intensity of the signal from nitroxide I! builds to a steady level, maintained for the duration of the experiment (2 months) (Fig. 1). The EPR spectrum of II (Fig. 2) is that of an immobilized nitroxide [10]. The mobile spectrum of II can be obtained by swelling the coating with either ethyl acetate or methylene chloride. No dinitroxide (? = NO') was observed. Similiar results have been reported for polypropylene films doped with 4-hydroxy-2,2,6,6-tetramethylpiperidine and exposed to n.y.

13 240

200 b

160 ' •7 " "

a

o z

120

_o 8o 4O

Ill]. The dependence of nitroxide kinetic behaviour on coating composition and on the presence of u.v. absorbing additives is illustrated in Fig. 3. Curves a and h depict the formation and decay of II in coatings A and B respectively under identical u.v. exposure conditions (u.v.-at. One per cent of I was used in both coatings. In both cases, II builds rapidly to a maximum during u.v. exposure, and then decays. The concentration of 1I is higher in coating A than in coating B at all times. This indicates that the amount of nitroxide detected is sensitive to relatively small differences in the coating composition. The highest level of nitroxide detected corresponds to 8% conversion of HALS amine to nitroxide. Curve c (Fig. 3) results when 1% by weight of a benzotriazole u.v. absorber is added to coating A along with 1% of I. The u.v. absorber reduces the amount of II detected, and greatly extends the duration of maximum concentration. This behaviour mimics that observed during natural exposure (Fig. 1) and is consistent with the fact that u.v. absorbers reduce the rate of photoinitiation of radicals in coatings [12]. Curves a and b (Fig. 4) show the effect on nitroxide concentration of exposure conditions (u.v.-b vs u.v.-c).

\ (2)

\CH

~' " °

z " - v - ~ n 2 '"

8

/ N

.N

.._.

P

CH20H + PCHzOH

Y /\

H20~

CH20H

/

I

N

÷ CH30H

I

\c.oH

+ H20 H2C=O

/

/

N

N

N'-(

12 14 16

N

/ ~CHE-O_CH3

(3)

I0

The fact that less II is produced in the relatively dry u.v.-c exposure, despite the fact that the light intensity was higher in that exposure, suggests an unexpected link between the free radical photooxidation chemistry of these coatings and their reactions with water. The reactions of acrylic melamine coatings with water have recently been examined by Bauer [13]. It was shown that ether crosslinks hydrolyze to yield parent copolymer hydroxyl groups and melamine methylol groups, reaction 2. Excess melamine methoxy groups react similarly, reaction 2. The melamine methylol groups may then either condense, reaction 3, or eliminate formaldehyde, reaction 4. In the present work, i.r. spectra of coating A have been recorded at various exposure times for u.v.-a and u.v.-c exposure conditions. Many spectral changes were observed. In particular, the rate of disappearance of the melamine methoxy group (reaction 2) was monitored by comparing absorbances at 915 cm 1 (methoxy band) and

N

(4)

6

Fig. 4. Formation of nitroxide I1 in coatings doped with 1% by weight amine I: (at Coating A during u.v.-c exposure, and (b) Coating A during u.v.-b exposure.

N.

N -...y/N .. / N

4

EXPOSURE, HOURS

\

/

/ _T(-';T N,,~N

2

~,,'N-',)~ ~H + H2C=O N~,~N

J.L. GERLOCKet al.

14

810cm -1 (triazine ring band) [-14]. Hydrolysis rates in the absence of u.v. have been determined as a function of temperature and humidity [13]. As shown in Fig. 5, the observed rate of disappearance of the methoxy group was 7 times faster under u.v.-a exposure conditions than the rate estimated [13] for u.v.-a temperature and humidity conditions. Apparently, u.v. accelerates the rate of disappearance of the melamine methoxy group. The rate of disappearance of the methoxy group during u.v. exposure is sensitive to the humidity level, being slower in the drier u.v.-c exposure than in the u.v.-a exposure. This suggests that methoxy group loss during u.v. exposure is a hydrolytic process which is somehow enhanced by u.v. light• The mechanism of this apparent photoenhanced hydrolysis is currently under investigation. The addition of 1% by weight amine I to coating A reduces the rate of photoenhanced hydrolysis but does not change the overall chemistry. A possible link between coating hydrolysis and free radical oxidation chemistry involving hydrolysis products will be presented later. To examine the influence of HALS additive structure on nitroxide kinetics, experiments were conducted on a variety of N-alkylated analogues of I. It was found that N-alkylated HALS additives yield lower concentrations of nitroxide than amine I in all cases. Ancillary solution experiments revealed that these amines can undergo a ring opening to yield nitroxides other than II. For example, 1,2,2,6,6-pentamethylpiperidine, III, a model N-alkylated HALS additive, reacts with p-nitroperbenzoic acid in methylene chloride to yield a methyl substituted nitroxide tentatively assigned to IV [15] as well as nitroxide V [16], reaction 5. The relative amounts of nitroxides IV and V have not been determined. Since the chemistry of

Fortunately sufficient model system solution work has been reported to provide excellent chemical guidelines• A simple reaction scheme, accommodating much of the observed nitroxide kinetic behaviour, is given below.

Initiation: 0

0

P~--OR

KI p~. +'OR

(6)

Propagation: Peracyl radical formation: 0

0 • + 02

(7)

K~,PCO0.

Nitroxyl formation: O

0

K~o , > N O ' + PCOH

P~O0" + >NH

(8a)

Polymer attack: 0

rl

K,o , Polymer oxidation products

P C O 0 ' + PH

(8b)

Nitroxyl decay." ~NO" + Y'

(9)

K,0,NOY

O

H

(10)

>NO" + P(~OO' --, ?

CH 3 CH 3.

I

CH 3

0

(51

CH3~ "IT[

-t-

CH3

CH2CI2

IV

V

A N = 15.6 G

A N = 15.6 G

A H CH3= I 1.7 G

these additives was more complicated than that of I, the use of these compounds in our kinetic studies was abandoned.

Nitroxide recycling: ~ N O Y - Z ~ >NO" + Y"

Nitroxide kinetics The results of these survey experiments appear to confirm a direct relation between nitroxide kinetics and coating degradation chemistry. Translation of nitroxide kinetic data into coating degradation rate data is necessarily empirical at this stage. That is, little direct information about reaction intermediates can be obtained in fully crosslinked coating systems.

(11)

O

L(

> N O Y + PCOO'----, >NO" + ?

(12)

Ester homolysis, reaction 6, is written in accord with the high ester content (6 × 10-3 tool/g) of these coatings and the well known photochemistry of esters [-17]. Formation of low molecular weight alcohols corresponding to RO' + PH ~ ROH + P' and form-

Nitroxide kinetics I,O

ation of CO by

15 1,..,

0.9 0.8'

O

0.7

R C ' - , R' + CO

(.9 0 6

has been confirmed by mass spectral analysis of gases released from acrylic melamine coatings during u.v. photolysis E18]. Formation of peracyl radicals, reaction 7_, by reaction of acyl radicals with oxygen is expected in an oxygen saturated system (1 x 10 -3 mol/g) [19] reacting at low temperature (less than 60°). The use of peracyl radical as a key intermediate provides a ready link between hydrolysis, reactions 2-4, and photolysis chemistry since hydrolysis yields formaldehyde, a likely peracyl radical precursor reactions 13-14, H2C~----O + R - - , HC---O + RH

Z

z

0.5 0.4

>X O

~_ 03 ILl

z O

~O 0.2 i,

(13)

O II

HC~---O + 0 2 --, H ~ O O '

0.1 0

(14)

~N" + ROO' ---*NOOR ) N O O R ~ ) N O " + RO'

I 400

I 600

I 800

I I000

1200

EXPOSURE TIME (hours)

Peracyl radical formation can also account for the rapid oxidation of HALS amine function to nitroxide (reaction 8a). The peracid oxidation of amine to nitroxide, reaction _9, is well documented [20]. The peracid oxidation of substituted hydroxyl amines ~NOY, to nitroxide, reaction 12 has been reported (8g). The exact radical species responsible for nitroxide formation is not known. Radicals other than peracyl could account for the nitroxide kinetics seen (for example reactions 15-17). > N H + R'--* ~ N ' + RH

I 200

Fig. 5. Fraction of melamine methoxy groups left in coating A vs hours of exposure for u.v.-a exposure (O) and u.v.-c exposure (D). - . . . . indicates the fraction methoxy left in the absence of u.v. light for the temperature and humidity conditions of the u.v.-a exposure [13].

If a steady state assumption is made for O

O

(15) P~"

(16)

and

PCOO'

using reactions _6, 7, _8 and 9, and ignoring reactions 10-12, an expression can be obtained for nitroxide behaviour during the early stages of u.v. exposure in terms of a balance between the rates of competing reactions, Eqn (19).

(17)

The formation of substituted hydroxyl amines, ) N O Y , by the nitroxyl radical scavenging, reaction 9, and the thermolytic dissociation of ~ N O Y compounds to nitroxide, reaction 11 is known in solution (9a). Radical scavenging by nitroxide in coatings is demonstrated by experiments wherein films doped with persistent dinitroxide, VI, were irradiated with

d[~NO'] d~

1+

KI K20 K3o[ ~NO" ] K1o[PH ] K2102]

U.V.

o "0--

NHCNH

CH 2

NHCNH

VI Upon irradiation, the EPR signal from VI decays rapidly (Fig. 6). The decay is near second-order in accord with a diffusion controlled reaction between VI and Y' [21]. The possibility of nitroxide excited state hydrogen atom abstraction, reaction 18, has not been ruled out as a second source of nitroxide loss [22]. >NO'+

PH--* > N O H + P"

(18)

× [

[>NH]

Klo[PH]

K2o 1 + K~o[pH ] [ ~ N H ]

K2o

K3° x [K2 O2~

[ )NO'] ] J

(19)

16

J.L. GERLOCK et al.

I . O . , xI . O ' ~

800

600

b I"o

,

Z~ tZ,

l. 0"

400

%

ab

2O0

O. I

SLOPE

I 2

4

6

I0

8

EXPOSURE, HOURS

I

K2oEV~OO'[]>NH]/K,o[P~OO'EPH] ] is the ratio of the rates at which P C O O O ' either oxidizes amine to nitroxide or attacks the coating; and O

O

II

II

] C[>NO']

(20)

where the rate factor A is just the rate of initiation times B. During the early stages of reaction when the concentration of amine changes little, Eqn (20) is of the form: d[>NO'] _ a - b[>NO'] dt

E>No'l, l

[ >NO'],.~,]

(21)

A[>NH] 1 + B[>NH]

-bt.

1

B

1

J

I

l

]

I

0.1

0.2

0.3

0.4

0.5

I

14

0.6

tool/q

Fig. 8. Determination of rate factors A and B.

such that a plot of 1/a vs 1 / [ > N H ] (initial) yields a curve with slope 1/A and intercept B/A (Fig. 8). The rate factor C is just b times A. Putting values for A, B and C (derived for u.v.-b exposure) back into the original equation yields: d[>NO'] - 9.87 x 10-3(hr -1) dt [>NH]

(22)

[>NH] x 105 Exposure mol/g u.v.-b

(24)

(25)

Table l. Summary of constants under different exposure conditions

(23)

1

i + 9.35 x 103 [ > N H ]

- 19.8 [ > N O ' I ]

the quantity 1/a is given by: a = X + A [>NH]

I 12

O.02

x

Values ofb can be obtained by conducting nitroxide formation experiments over a range of initial amine concefitrations, Fig. 7. Plotting the left hand side of Eqn (22) vs time yields lines with slope - b . Since d[>NO']/dt is zero at [ > N O ' ] = max, values of a may also be obtained from a = b [ > N O ' ] ( m a x ) at the various amine concentrations. Since the quantity a is given by: a -

] I0

0.01

the ratio of the rate at which PCO" either reacts with nitroxide or with oxygen. If we assume that the value of this latter ratio is small relative to unity, a good assumption since diffusion of oxygen is rapid, then Eqn (19) reduces to:

[

I 8

0.03

K30 [ > N O ' ] [PC'l/K2 [O2] [PC'],

In 1

I 6

0.07' O.06 05 I"6- 0.0.04

O

which integrates to

I 4

Fig. 7. Determination of b. Concentration of amine I is 0.5, 1.0 and 2,0% by weight for curves a, b and c respectively.

For example,

d[>NO'] [ [>NH] dt - A 1 + B[>NH]

] 2

EXPOSURE, HOURS

Fig. 6. Decay of dinitroxide V in coating A during, (a) natural exposure, and (b) u.v.-b exposure.

O

b

u.v.-c Natural

2,16 4,44 8.49 4.44 8.49 4.24 4.24

u.v. abs

1.0% 1.0%

A × 103 hr

B × 10 -3 g/mol

C x 10-1

9.87 9.87 9.87 1.83 3.0 6.16 0.42

9.35 9.35 9.35 9.35 9.35 9.35 9.35

2.09 1.97 1.88 1.87 2.09 1.96 1.98

Nitroxide kinetics 240

2o0 -

~ ,O'--

C

0~

b

/o" /

160

--

/ /

Q

,z, i20 ~

-o -

/

/

o/

I ?/ 80 -- / /

~5__o---

o

,,ce" 40 [/I

I 1 2

4

6

I

I

1

I

I

8

I0

12 14

16

17

tire monitor of the rate of photodegradation in these coatings. Using an empirical model of photooxidation to interpret nitroxide kinetics, a parameter has been derived which characterizes the severity of the weathering exposure. It has been found that humidity during u.v. exposure plays a key role in the rate of photodegradation of acrylic melamine coatings. Photoassisted hydrolysis of ether crosslinks has been proposed to explain this dependence. The direct relationship demonstrated between nitroxide chemistry and coating degradation chemistry, together with the sensitivity of electron paramagnetic resonance, may assist in a more rapid evaluation of coating performance under near ambient exposure conditions.

EXPOSURE , HOURS

Fig. 9. Agreement between experimental © and calculated curves . . . . . using fixed values for rate factors A. B and C, for nitroxide II formation during u.v.-b exposure of coating A.

These particular values of A, B and C fit the data from which they were derived (Fig. 9). According to Eqn 19, the rate factor A should be a measure of the severity of the exposure while the factors B and C being closely linked to coating composition should remain constant for a given coating. When nitroxide formation is followed in a single coating under differing exposure conditions, the early stages of formation can be fitted well with only small changes in the "constants" B and C, Table 1, even when benzotriazole u.v. absorber is added. In the absence of benzotriazole, the A factor does not change over the amine concentration studied. The increase in A factor with increasing amine concentration in the presence of benzotriazole is not presently understood. The ratio of A factors for u.v.-b exposure to that measured under outdoor exposure yields a value near 20. This value is similiar to the ratio of the rate of gloss loss during u.v.-a exposure to that during Florida exposure for acrylic melamine coatings both with and without HALS additives [23]. This suggests that it may be possible to develop relationships between coating degradation rates as reflected by nitroxyl kinetics and coating weatherability. The ratio of A factors for relatively wet and dry exposure conditions (u.v.-b/u.v.-c = 1.6) is comparable to the ratio of the rate of photo-enhanced hydrolysis as measured by i.r. under similiar exposure conditions (u.v.-a/ u.v.-c = 1.74). This suggests that photo-enhanced hydrolysis may play a key role in the photodegradation of acrylic melamine coatings. This finding may help to clarify the well known fact that coatings usually appear to be more durable when tested in Arizona (dryl than when tested in Florida (wet) even though the u.v. intensity is higher in Arizona [la].

CONCLUSION

Electron paramagnetic resonance measurements of nitroxyl radical formation and decay in acrylic melamine coatings doped with hindered amine light stabilizers have been shown to be a rapid and sensiLPJ. 19,1

i~

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