Characterization of Highly Cross-linked Polymers - American

16 IR Spectroscopic Studies of Degradation in Cross-linked Networks Photoenhanced Hydrolysis of Acrylic-Melamine Coatings D. R. BAUER and L. M . BRIGGS

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: February 16, 1984 | doi: 10.1021/bk-1984-0243.ch016

Ford Motor Company, Dearborn, MI 48121

Infrared spectroscopy has been used to follow changes in the chemical crosslink structure of acrylic/melamine coatings during weathering as a function of ultraviolet light intensity, humidity, and coating composition. It has been found that crosslinks between the acrylic polymer and the melamine crosslinker hydrolyze rapidly during exposure. The hydrolysis rate increases both with increasing humidity and light intensity. The hydrolysis rate i s also a function of coating composition, being greatest for coatings composed of low molecular weight, low Τ acrylic polymers. The enhancement of hydrolysis by ultraviolet light has been attributed to the fact that melamine molecules have a weak absorbance in the near ultraviolet (300 nm). The excited state has been found to be more easily protonated than the ground state (by a factor of over 1000 for hexamethoxymethylmelamine). Since the first step in hydrolysis is protonation, hydrolysis should be more rapid in the excited state than in the ground state. g

Thermoset coatings c o n s i s t o f m a t e r i a l s which during cure form a c r o s s l i n k e d network that i s i n l a r g e part r e s p o n s i b l e f o r the p h y s i c a l p r o p e r t i e s o f the coating. As a coating weathers the c r o s s l i n k e d network may change. I n f r a r e d spectroscopy has been found t o be a valuable t o o l f o r monitoring c r o s s l i n k i n g i n acrylic copolymer melamine formaldehyde c r o s s l i n k e d coatings (1-9)· Melamine c r o s s l i n k e r s take part i n two main c r o s s l i n k i n g reactions (Table I) (5,10). Melamine alkoxy groups r e a c t with hydroxy groups on the polymer to form aerylic-melamine crosslinks. Melamine methyloi groups condense to form

0097-6156/84/0243-0271$06.00/0 © 1984 American Chemical Society Labana and Dickie; Characterization of Highly Cross-linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

HIGHLY CROSS-LINKED POLYMERS

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melamine-inelamine c r o s s l i n k s . The r e l a t i v e contributions of these r e a c t i o n s and t h e i r r a t e s depend p r i m a r i l y on the s t r u c t u r e of the melamine c r o s s l i n k e r . Coatings c o n t a i n i n g f u l l y a l k y l a t e d melamines require strong acids t o cure and under normal c o n d i t i o n s c r o s s l i n k s o l e l y by r e a c t i o n 1. Those containing partially a l k y l a t e d melamines undergo both r e a c t i o n s 1 and 2 and can be c a t a l y z e d by weak a c i d s . Extents o f r e a c t i o n i n these systems have been measured by f o l l o w i n g the disappearance o f a c r y l i c hydroxy, melamine methyloi, and melamine methoxy groups (5).


TABLE I .

CROSSLINKING AND HYDROLYSIS REACTIONS.

1.

N-CH -0-CH

2.

N-CH -0H +

3.

N-CH -0-R + H 0

4.

N-CH -0-CH

5.

N-CH 0H

2

3

!

2

3

+ H 0 2

-*

N-H

N-CH -0-R 2

—»

2

2

2

+

N-CH -0H

2

2

+ ROH

-

2

+CH =0 + H 0 2

2

N-CH -0H • ROH 2

* +

N-CH -N

+ CHjOH

N-CH -0H + CH OH 2

3

CH =0 2

Since the r e a c t i o n s are r e v e r s i b l e and i n v o l v e hydroxy groups, c r o s s l i n k i n g i n melamine c o n t a i n i n g coatings can be s e n s i t i v e to h y d r o l y s i s . H y d r o l y s i s o f melamine c r o s s l i n k e r s has been s t u d i e d i n s o l u t i o n ( 1 1 ) and i n cured acrylic/melamine coatings subjected t o condensing and non-condensing humidity (12). I t was found ( 1 2 ) that both aerylic-melamine bonds and unreacted melamine methoxy groups can hydrolyze t o y i e l d hydroxy groups and melamine methyloi groups ( r e a c t i o n s 3 and 4 ) . The rate of hydrolysis depends on the following variables: temperature (the a c t i v a t i o n energy f o r hydrolysis i s 22 kcal/mole), the l e v e l o f a c i d i n the c o a t i n g ( h y d r o l y s i s i s a c i d catalyzed), the concentration o f water i n the c o a t i n g (which i n t u r n i s a f u n c t i o n o f the degree o f c r o s s l i n k i n g o f the coating, the molecular weight o f the polymer and the g l a s s t r a n s i t i o n temperature o f the polymer), and the s t r u c t u r e o f the melamine crosslinker (under weak a c i d c o n d i t i o n s , partially alkylated melamines hydrolyze some 30 times f a s t e r than f u l l y alkylated melamines). The melamine methylol group produced on h y d r o l y s i s can e i t h e r s e l f condense to form a melamine-melamine c r o s s l i n k ( r e a c t i o n 2) o r deformylate t o y i e l d an amine ( r e a c t i o n 5 ) . For

Labana and Dickie; Characterization of Highly Cross-linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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coatings c r o s s l i n k e d with p a r t i a l l y a l k y l a t e d melamines, both r e a c t i o n s 2 and 5 were observed. Since r e a c t i o n 2 forms crosslinks, i t can compensate f o r a c r y l i c melamine bond hydrolysis. Even though network chemistry may be d r a s t i c a l l y a l t e r e d by h y d r o l y s i s , the c r o s s l i n k d e n s i t y and o v e r a l l p h y s i c a l p r o p e r t i e s may not be g r e a t l y a f f e c t e d (12). In p a r t i c u l a r , w e l l cured coatings do not g e n e r a l l y lose g l o s s or show other signs o f weathering when they are subjected only to condensing humidity. For coatings c r o s s l i n k e d with f u l l y a l k y l a t e d melamines, no significant melamine-melamine bond formation was observed on h y d r o l y s i s though f o r most c o n d i t i o n s studied the amount of h y d r o l y s i s was s m a l l . These experiments were performed i n the absence o f l i g h t . It i s generally believed that photooxidation i s the primary source o f weathering i n these coatings (13), though i t has been noted that coatings l o s e p h y s i c a l p r o p e r t i e s more r a p i d l y when weathered i n humid c o n d i t i o n s than dry c o n d i t i o n s (14). Recent ESR studies (15) have demonstrated that photooxidation r a t e s i n acrylic/melamine coatings increase with i n c r e a s i n g humidity. It was a l s o reported (15) that the r a t e o f disappearance of methoxy groups ( r e a c t i o n 2) a t constant humidity increased with increasing ultraviolet l i g h t i n t e n s i t y . I t was speculated that the r a t e o f h y d r o l y s i s was enhanced by u l t r a v i o l e t l i g h t and that the o x i d a t i o n o f formaldehyde r e l e a s e d as a by-product o f the h y d r o l y s i s contributed t o the increased r a t e of photooxidation observed under humid c o n d i t i o n s (15). I t has a l s o been reported that h y d r o l y s i s o f acrylic/melamine coatings occurs during n a t u r a l exposure (16). I t i s the purpose o f t h i s paper to f u r t h e r c h a r a c t e r i z e the changes i n c r o s s l i n k s t r u c t u r e that occur on weathering. In p a r t i c u l a r , a d e t a i l e d study o f the dependence o f the r a t e o f methoxy disappearance on humidity and l i g h t i n t e n s i t y i s presented which v e r i f i e s the photoenhancement o f h y d r o l y s i s . The e f f e c t o f changes i n the composition o f the acrylic/melamine c o a t i n g and the e f f e c t o f common p h o t o s t a b i l i z e r s on the r a t e o f photohydrolysis has been determined. Finally, possible mechanisms f o r photoenhanced h y d r o l y s i s are discussed. Experimental

Coating Formulation, The a c r y l i c copolymers used i n t h i s study were prepared by conventional free r a d i c a l polymerization. The monomer compositions, molecular weights and g l a s s transition temperatures are given i n Table I I . Coatings A-G were comprised o f a c r y l i c polymers A-G and a p a r t i a l l y a l k y l a t e d melamine (Mel-D of Ref. 7 ) . Coating G' was comprised o f a c r y l i c polymer G and hexamethoxymethylmelamine. The r a t i o o f polymer to c r o s s l i n k e r



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was 70:30 i n a l l cases. Coatings were cured by baking a t 130 C f o r 20 minutes (0.1$ p-toluene s u l f o n i c a c i d was used to c a t a l y z e the cure o f c o a t i n g G ) . 1

TABLE I I .


Polymer Mn Τ $§TY $BMA $BA $MMA $EHA

ACRYLIC COPOLYMER COMPOSITION A 1700 •27 0 68 0 0 0

Β 6400 9 25 43 0 0 0

C

D

3900 -26 0 68 0 0 0

3600 18 25 0 0 43 0

Ε 4500 -9 15 0 53 0 0

F 2500 -11 25 43 0 0 0

G 2700 -13 25 23 0 0 20

STY s Styrène BMA = Butylmethacrylate BA = B u t y l a c r y l a t e MMA = Methylmethacrylate EHA s E t h y l h e x y l a c r y l a t e A l l copolymers contain 30$ by weight hydroxyethylacrylate and by weight a c r y l i c a c i d .

2$

Weathering. Samples were exposed to constant u l t r a v i o l e t (UV) light i n a modified weatherometer which allowed the independent c o n t r o l o f a i r temperature and humidity. A i r temperature was maintained a t 60 ± 1 C. The humidity was c o n t r o l l e d by c o n t r o l l i n g the temperature o f the water i n the bottom of the weatherometer and r a p i d l y c i r c u l a t i n g a i r i n the weatherometer to e s t a b l i s h e q u i l i b r i u m . Weathering was s t u d i e d a t h u m i d i t i e s with the f o l l o w i n g dew p o i n t s : 50 ± 1 C (UV:50), 25 C ± 1 C (UV:25), and -40 ± 5 C (UV:-40). The UV:-40 exposure c o n d i t i o n was achieved by removing the water from the bottom of the weatherometer and rapidly circulating dry air in the weatherometer. Standard FS-20 sunlamps were used. The peak wavelength was 300nm. L i g h t i n t e n s i t y was v a r i e d with n e u t r a l density filters (intensity = 1 denotes use of no f i l t e r , 0.5 denotes use o f a 50$ t r a n s m i s s i o n f i l t e r , 0.1 denotes use of a 10$ t r a n s m i s s i o n f i l t e r ) . The UV l i g h t i n t e n s i t y without f i l t e r s was around 1 raw/cm . I n f r a r e d Measurements. Thin (10 micron) coatings were cast on KRS-5 p l a t e s and cured. I n f r a r e d s p e c t r a were obtained i n transmission u s i n g a N i c o l e t F o u r i e r transform IR.


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Results

aM


275

Piscu§sjpn

The changes i n network structure that occur on hydrolysis i n the dark were clearly elucidated i n the infrared (12). The reappearance of acrylic hydroxy functionality was used to measure the hydrolysis rate of acrylic-melamine bonds. For coatings containing partially alkylated melamines, prolonged hydrolysis resulted i n the rupture of virtually a l l of the original acrylic melamine bonds. The disappearance of methoxy was used to follow the hydrolysis of unreacted methoxy groups. For f u l l y alkylated melamines, the rate of hydrolysis of unreacted methoxy groups was identical to that of acrylic melamine bonds. For partially alkylated melamines, the rate of hydrolysis of unreacted methoxy groups was slower than that for acrylic-melamine bonds and the rate of hydrolysis decreased with increasing hydrolysis. This can be explained by the fact that different methoxy groups on partially alkylated melamines have different reactivities. More reactive methoxy groups w i l l both crosslink and hydrolyze more rapidly leading to the observation that acrylic melamine bonds hydrolyze more rapidly than unreacted methoxy groups. The most reactive groups hydrolyze f i r s t and as hydrolysis proceeds, the rate of hydrolysis slows as less reactive groups begin to hydrolyze. The increase of a relatively weak band at 1350 em was used to monitor semi-quantitatively the formation of melamine- melamine crosslinks. This band was only observed i n coatings crosslinked with partially alkylated melamines. The spectral changes that occur on photodegradation are more complex than those for hydrolysis i n the dark (Figure 1). In addition to hydrolysis-like changes (appearance of hydroxy groups, disappearance of methoxy groups and appearance of melamine-melamine crosslinks) there are changes i n other parts of the spectrum. In particular, there are significant changes i n the carbonyl part of the spectrum with the appearance of two new bands at 1770 cm and 1710 cm . Similar changes have been observed i n the photooxidation of polybutylacrylate (17). The band at 1710 cm i s most likely due to car boxy l i e acid formation; the band at 1770 cm was ascribed to lactone formation (17) but may also be i n part due to peracid or perester formation. The increase i n intensity of these bands can be used as a qualitative measure of the rate of photooxidation. Although the changes i n the IR spectrum due to hydrolysis are readily apparent i n the spectrum of coatings degraded under UV:50 conditions, other photodegradation processes can make quantitative measurements of hydrolysis d i f f i c u l t . For example, various photooxidation reactions lead to the formation of hydroxy groups. These hydroxy groups make i t impossible to quantify the amount of acrylic hydroxy groups which are generated on




276

hydrolysis. The rate of disappearance of methoxy functionality can be measured relatively easily. There are two possible mechanisms which lead to a decrease i n the methoxy band during weathering: hydrolysis and abstraction of methoxy hydrogens by free radicals produced photochemically. By measuring the rate of disappearance of methoxy as a function of light intensity and humidity, i t i s possible to separate these two mechanisms. The disappearance of methoxy for Coating G i s shown as a function of humidity in Figure 2 and as a function of UV light intensity i n Figure 3· In a l l cases except dark hydrolysis (discussed above) the disappearance of methoxy functionality obeys simple f i r s t order kinetics. The rate constants for Coating G and Coating G' are given i n Table III as a function of humidity and light intensity.

TABLE III.

RATES OF METHOXY LOSS (x10

3

1

~ ). f

COATING G

COATING G INTENSITY 1.0 0.5 0.1 0.0

UV:-40 1.2 0.8 0.35

1.9 1.1 0.55 0.15

UVtSQ.

UV:5Q 3.8 2.3 1.15 0.6

3.3 1.7 0.4 0.03

Hydrolysis should not occur during UV:-40 exposure and the rate of disappearance of methoxy during this exposure should give a good measure of hydrogen abstraction rates. The methoxy group was found to disappear under UV-DRY conditions at a rate proportional roughly to the square root of the intensity. No formation of melamine-melamine bonds was observed during UV:-40 exposure indicating that the disappearance of methoxy did not lead to the formation of methyloi groups which could subsequently crosslink. Both of these findings are consistent with the loss of methoxy being due to photochemically produced radicals abstracting a hydrogen from the methoxy group (hydrogens alpha to ethers are easily abstractable). The rate of disappearance of methoxy groups during humid exposure was much greater than that for the dry exposure. It was greater than the sum of the UV-DRY rate and the dark hydrolysis rate indicating a photoenhancement of the hydrolysis. To a good approximation the rate constant for the disappearance of methoxy can be given by the following expression: K

K

meth = hyd

x

HUM χ (I + D) + K

a b s

χ I

1

(1)


BAUER AND BRIGGS

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. Λ - s llrVA^ Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: February 16, 1984 | doi: 10.1021/bk-1984-0243.ch016

_J

I 3000

ι

I 2000 cm"

ι

I 1000

I

1

Figure 1· I n f r a r e d s p e c t r a o f cured and degraded (230 hours, UV:50) samples o f Coating G. Peaks o f i n t e r e s t i n c l u d e : A, A c r y l i c hydroxy; B, Melamine methyloi; C, 1710 cm* ; Q Melamine-melamine c r o s s l i n k ; E,Melamine methoxy; Ç Melamine t r i a z i n e r i n g ; G, Styrene.

0

100 200 300 400 500 600 700 HOURS EXPOSURE

Figure 2. F r a c t i o n o f methoxy remaining versus hours exposure f o r c o a t i n g G under UV:50 ( Ο ) » UV:25 ( Ο ), and UV:-40 ( Δ ) exposure c o n d i t i o n s . L i g h t i n t e n s i t y = 1.



278


ο 11 0

1

1

100

200

ι

1 300

HOURS

400

ι 500

ι 600

I 700

EXPOSURE

Figure 3· F r a c t i o n o f methoxy remaining versus hours exposure f o r c o a t i n g G under UV:50 exposure c o n d i t i o n s . L i g h t i n t e n s i t y = 1.0 ( O h 0.5 ( Ο ), 0.1 ( ù ) , and 0.0

( C )·


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279

where K. . and Κ . are r a t e constants, HUM i s the humidity, and I i s ïne l i g h t i n t e n s i t y . The product JL . χ HUM χ D g i v e s a measure o f the dark h y d r o l y s i s r a t e while t h e p r o d u c t K . χ HUM χ I gives a measure o f the photoenhanced h y d r o l y s i s r a t e . For Coating G, comparison o f the r a t e s o f methoxy l o s s under UV:-40 and UV:50 exposures and the observation s i g n i f i c a n t formation o f melamine-melamine bonds during UV:50 exposure suggests that most of the l o s s o f methoxy group i s due to h y d r o l y s i s and that acrylic-melamine bond h y d r o l y s i s i s a l s o photoenhanced. The relative increase i n h y d r o l y s i s with l i g h t i n t e n s i t y was much greater f o r the f u l l y a l k y l a t e d melamine than f o r the p a r t i a l l y a l k y l a t e d melamine p r i m a r i l y due to the very slow r a t e o f dark h y d r o l y s i s f o r the f u l l y a l k y l a t e d melamine. In f a c t , values o f hvd Coating G and Coating G implying t h a t for photoenhanced h y d r o l y s i s , a l l methoxy groups (and by inference a l l acrylic-melamine bonds) are equally r e a c t i v e . This i s c o n s i s t e n t with the observation o f simple f i r s t order k i n e t i c s for photoenhanced h y d r o l y s i s o f Coating G. I t should be noted that the band a s s o c i a t e d with melamine-melamine c r o s s l i n k s was not observed f o r Coating G' exposed t o UV:50 c o n d i t i o n s . Either the methylol groups do not r e a c t as e f f i c i e n t l y f o r the f u l l y a l k y l a t e d melamine o r the band a s s o c i a t e d with melamine-melamine bond formation i s s p e c i f i c t o p a r t i a l l y a l k y l a t e d melamines. y


fa

K

a

r

e

v

e

r

y

s

i

m

i

l

i

a

r

i

n

1

The r a t e constants f o r the disappearance o f methoxy groups for Coatings A-G during exposure to UV:50 and f u l l l i g h t are summarized i n Table IV. The r a t e constants increase with decreasing g l a s s t r a n s i t i o n temperature and molecular weight o f the a c r y l i c copolymer. Since the concentration o f water i n the coating should increase with decreasing glass t r a n s i t i o n temperature and e f f e c t i v e c r o s s l i n k density, the behavior o f these r a t e constants i s c o n s i s t e n t with h y d r o l y s i s being the primary mechanism f o r methoxy disappearance (note: i t has been previously shown i n Ref. 7 that lowering the a c r y l i c copolymer molecular weight r e s u l t s i n a decrease i n e f f e c t i v e c r o s s l i n k density). In a l l cases, a s u b s t a n t i a l increase i n the band a s s o c i a t e d with melamine-melamine s e l f condensation was observed a l s o implying that i n these coatings under t h i s exposure the primary mechanism f o r l o s s o f methoxy i s h y d r o l y s i s . In Ref. 15 it was suggested that h y d r o l y s i s i n f l u e n c e s the r a t e o f photooxidation through the oxidation of formaldehyde, a h y d r o l y s i s by-product. To t e s t t h i s hypothesis, photooxidation r a t e s (as monitored by the increase i n absorbanee a t 1710 cm" ) were measured under standard QUV exposure c o n d i t i o n s f o r two samples o f c o a t i n g G one o f which had been hydrolyzed e x t e n s i v e l y in the dark t o reduce the number o f hydrolyzable groups. I t was found that the r a t e of photooxidation was slower f o r the prehydrolyzed sample c o n s i s t e n t with the above hypothesis. It i s a l s o i n t e r e s t i n g t o note that q u a l i t a t i v e l y , the r a t e s o f



280

photooxidation o f Coatings A-G roughly c o r r e l a t e w i t h the r a t e s of methoxy l o s s . Further work t o determine the exact relationship between hydrolysis and photooxidation is in progress.

TABLE IV.

RATES OF METHOXY LOSS ( x 1 0


COATING A Β C D Ε F G Exposure c o n d i t i o n s :

J

1

hr" )

Κ metn 3.9 1.5 3.2 1.2 2.5 3.0 3.8 UV:50, l i g h t i n t e n s i t y = 1

There a r e a t l e a s t two p o s s i b l e explanations f o r the increase i n the apparent r a t e o f h y d r o l y s i s w i t h l i g h t i n t e n s i t y . One p o s s i b i l i t y i s that photooxidation produces c a r b o x y l i e acids which c a t a l y z e the h y d r o l y s i s . Another i s that melamines e x c i t e d by UV l i g h t hydrolyze more r a p i d l y than ground s t a t e melamines. To t e s t these p o s s i b i l i t i e s , the r a t e o f disappearance o f methoxy was f o l l o w e d f o r s o l u t i o n s o f hexamethoxymethylmelamine i n water buffered t o pH 7 and i n methanol f o r both l i g h t and dark exposure. The r e s u l t s are shown i n Figure 4. The observed h y d r o l y s i s behavior i n the dark i s s i m i l i a r t o t h a t observed by Berge e t a l (11), and from comparisons o f the r a t e s i n the two experiments an a c t i v a t i o n energy o f 21±2 kcal/mole can be determined i n e x c e l l e n t agreement w i t h the a c t i v a t i o n energy p r e v i o u s l y determined f o r h y d r o l y s i s i n coatings (12). The r a t e of methoxy l o s s i n the presence o f l i g h t was some 6 times f a s t e r than the dark h y d r o l y s i s r a t e . Since the pH was constant, t h i s can not be a t t r i b u t e d t o c a r b o x y l i c a c i d formation. Since the r a t e was zero i n the methanol s o l u t i o n s , i t can not be a t t r i b u t e d to hydrogen a b s t r a c t i o n o r some other photochemical process. The degree o f enhancement by UV l i g h t was s m a l l e r i n the s o l u t i o n experiments than i n the coatings i n part due t o the f a c t t h a t the l i g h t i n t e n s i t y was lower i n the s o l u t i o n experiments. For h y d r o l y s i s t o be enhanced by the absorption o f UV l i g h t , the e x c i t e d s t a t e must be s u f f i c i e n t l y long l i v e d f o r h y d r o l y s i s to occur and the r a t e o f h y d r o l y s i s i n the e x c i t e d s t a t e must be f a s t e r than that i n the ground s t a t e . A weak a b s o r p t i o n i n the



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r e g i o n ~300nm has been p r e v i o u s l y reported f o r melamine r e s i n s and assigned to a s i n g l e t to t r i p l e t t r a n s i t i o n (18). The absorption s p e c t r a of both the protonated and the non-protonated forms of hexamethoxymethylmelamine are shown i n Figure 5. S i m i l i a r s p e c t r a of the p a r t i a l l y a l k y l a t e d melamine are shown i n Figure 6. I t i s c l e a r that the absorption peak ~300nm s h i f t s to higher wavelength on p r o t o n a t i o n . One p o s s i b l e e x p l a n a t i o n for t h i s s h i f t i s that the e x c i t e d s t a t e of the melamine i s more e a s i l y protonated than the ground s t a t e . If this effect i s responsible f o r a l l of the s h i f t , the s h i f t s i n pKa of the e x c i t e d s t a t e r e l a t i v e to the ground s t a t e are c a l c u l a t e d to be +3*2 and +1.6 (±0.5) f o r hexamethoxymethylmelamine and the p a r t i a l l y a l k y l a t e d melamine r e s p e c t i v e l y . In both cases the e x c i t e d s t a t e i s much more e a s i l y protonated. Since the f i r s t step i n h y d r o l y s i s i n v o l v e s p r o t o n a t i o n , i t can be i n f e r r e d that h y d r o l y s i s may indeed be f a s t e r i n the e x c i t e d s t a t e . The l a r g e r enhancement observed i n Coating G r e l a t i v e to Coating G i s c o n s i s t e n t w i t h the l a r g e r s h i f t i n pKa observed f o r the f u l l y a l k y l a t e d melamine. Adding the pKa s h i f t to measured v a l u e s of the pKa of ground s t a t e melamines (19) i n d i c a t e s that the pKa i n the e x c i t e d s t a t e i s v i r t u a l l y i d e n t i c a l f o r both melamines (4.5-5.0). This r e s u l t i s c o n s i s t e n t w i t h the f a c t that s i m i l i a r v a l u e s of were observed f o r Coatings G and G . 1

1

d

F i n a l l y , the e f f e c t on the r a t e of l o s s of methoxy f u n c t i o n a l i t y under UV:50 exposure of the a d d i t i o n of t y p i c a l p h o t o s t a b i l i z e r s i s shown i n Figure 7· The benzotriazole (CGL-900 from Ciba-Giegy) i s a UV absorber and reduces the r a t e of photohydrolysis by reducing the light intensity. Surprisingly, it was found that when b i s ( 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n y l - 4 ) sebacate (a hindered amine l i g h t s t a b i l i z e r - CGL-770, Ciba-Giegy) was added to c o a t i n g G a t a l e v e l of 2%, the r a t e of photoenhanced h y d r o l y s i s was reduced by a f a c t o r of 2. The mechanism f o r t h i s e f f e c t i s not known, though at l e a s t two explanations are p o s s i b l e . One i s t h a t the amine n e u t r a l i z e s the a c i d that c a t a l y z e s the h y d r o l y s i s . Another i s r e l a t e d to the f a c t t h a t n i t r o x i d e s , produced on o x i d a t i o n of the amine, are known t r i p l e t quenchers (20). I f the n i t r o x i d e quenches the e x c i t e d s t a t e of the melamine, the r a t e of photoenhanced h y d r o l y s i s would be reduced. I t i s i n t e r e s t i n g to note that the r a t e of methoxy l o s s i s decreased more by u s i n g a combination of UV absorber and hindered amine than by using e i t h e r s t a b i l i z e r alone. Conclusion

I n f r a r e d spectroscopy can be used to monitor changes i n the chemical structure of acrylic/melamine coatings during photodegradation. The p r i n c i p a l changes i n the crosslink



282


Figure 4. F r a c t i o n o f methoxy remaining versus exposure f o r hexamethoxymethylmelamine i n water a t pH UV l i g h t exposure ( Ο ) and dark exposure ( • ) a t Also shown i n hexamethoxymethylmelamine i n methanol s i m i l i a r exposures ( & )·

320 300 WAVELENGTH (ran)

hours 7for 55 C. under

280

Figure 5· Absorbance o f hexamethoxymethylmelamine versus wavelength f o r protonated and nonprotonated melamine.



ο ιI

0

I

I

I

I

1

1

1

100 200 300 400 500 600 700 HOURS EXPOSURE

Figure 7· F r a c t i o n o f methoxy remaining versus hours exposure f o r Coating G under UV:50 exposure c o n d i t i o n s (light intensity = 1). No stabilizer (O), 2% Benzotriazole UV absorber ( Ο ), 2% Hindered Amine L i g h t S t a b i l i z e r ( ^ ), 1% B e n z o t r a i z o l e and λ% Hindered Amine

( 7 )· Labana and Dickie; Characterization of Highly Cross-linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

284

HIGHLY CROSS-LINKED

POLYMERS

s t r u c t u r e are the h y d r o l y s i s o f acrylic-melamine c r o s s l i n k s and the subsequent formation o f melamine-melamine c r o s s l i n k s . The r a t e o f h y d r o l y s i s d u r i n g exposure depends on both the humidity and the u l t r a v i o l e t l i g h t i n t e n s i t y as w e l l as the c o a t i n g composition. The enhancement by u l t r a v i o l e t l i g h t has been a t t r i b u t e d t o a s i n g l e t - t r i p l e t t r a n s i t i o n by the melamine molecule.

Literature Cited


1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

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RECEIVED

August 29, 1983


Characterization of Highly Cross-linked Polymers - American

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