Photodegradation of Automotive Paints - ACS Publications

27 Photodegradation of Automotive Paints

Downloaded by MONASH UNIV on December 15, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch027

P. C. KILLGOAR, JR., and H. VAN OENE Research Staff, Ford Motor Co., Dearborn, Mich. 48121

Retention of gloss, color stability and resistance to embrittlement under outdoor conditions are some of the important requirements of automotive paints. Changes in gloss can be related to photochemical degradation and subsequent removal of low molecular weight fragments of the polymer binder used in the paint. New paints are usually tested for their chalk resistance and gloss retention properties by outdoor exposure in Florida or other locations for a period of two years. Therefore, there is a two years waiting period between the completion of the paint development work and the actual use of that paint. In order to speed up paint development, accelerated testing methods have been devised. One such test method is the 500-2000 hour exposure test in the weather-o-meter using high intensity light. Accelerated weathering techniques do not always correlate well with actual outdoor exposure data, presumably the spectral distribution of the light being different from sunlight. Moreover, the actual weathering process depends on the details of the environment such as temperature, humidity and pollution levels (1), the influence of which cannot be duplicated in an accelerated test. The process of weathering is not understood in great detail because of the complexity of paint formulations in that polymer composition, crosslinking agent, catalysts, pigments and additives all influence the photochemical processes. Loss of gloss during outdoor exposure can be due to inadequate pigment dispersion (2). Some of the surface treatments of pigments used to obtain good pigment dispersion may also be detrimental in outdoor exposure (3). Since chalking is essentially a surface phenomena, any segregation of components of the paint during evaporation or the extensive use of surface active additives which can cause the composition of the surface region to be significantly different from that of the bulk will also influence weathering properties. To predict the gloss retention behavior of any paint, one must know the photochemical processes taking place and the 407 In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


408

UV L I G H T

INDUCED

REACTIONS

IN

POLYMERS

morphology of the paint film. Morphology of the paint film and the state of pigment dispersion can be determined by electron microscopy (2). The study of photochemical changes i s usually more d i f f i c u l t . Infrared spectroscopy successfully used for the study of photo-oxidation of polyethylene (4) i s not sufficiently sensitive to study acrylic type polymers used i n automotive paints. The chemical compositions of the paint film are too complicated to allow studies of their photochemistry by most of the available techniques. In this work we have concentrated on the development of a method which can be used to study the chemical aspects of photo degradation, specifically photoinitiâtion under conditions of illumination which do not d i f f e r greatly from those experienced during outdoor exposure. This method can be adapted to study photo-oxidation also. The photoinitiated degradation of a typical melamine crosslinked acrylic enamel studied by using this technique w i l l be discussed. Chemistry of Photodegradation of Acrylic Polymers The majority of the work on the fundamental processes of photodegradation of acrylic polymers has been carried out by Grassie and co-workers (5-7). For radiation with wavelength λ > 300.0 nm photodegradation i s initiated by absorption of a photon by the carbonyl group of the ester side chains. Subse quent fragmentation depends on local stereochemistry and tempera ture. The principal modes of fragmentation are described by the Norrish I and Norrish II processes (8). The Norrish I process can be represented by: ^CH

hv

C(x)w

-

9

'v/v/CR 2

C(x)^

+ -C0 R

Ka)

^CH,

0(χ)^

+

Kb)

2

o

-OR

COfollowed by •C0 R

C0 t

0

o

v^CH - C ( x ) ^ 2

+ R-

2(a)

·* HC0 R (formate ester) 9

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

2(c)

27.

Photodegradation of Automotive Paints

KiLLGOAR AND VAN OENE

409

In the primary process C 0 , CO or formate esters are formed plus a polymeric macroradical and ester side chain fragments R- and 2

R0-.

The Norrish II process involves the participation of 3 hydrogen of the alcohol segment or a γ hydrogen on the polymer backbone i n the formation of a six member ring through hydrogen bonding with the carbonyl oxygen. This can be represented as: Η ^CH

2

0

C = Ο Downloaded by MONASH UNIV on December 15, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch027

Ç-CH

CH - C ( x ) ^

V

CRo

N

2

Y

Ç ( x ) ^ 3(a) C00H

+ H C = CHRi 2

H

CH - C ( x ) ^

C = CH +

2

0

I

2

3(b)

Y C ( x ) ^ ^ HÇÎx)^ C HO

N

O^OR

0R

Hence the Norrish II process leads to formation of an alkene [3(a)] or chain scission [3(b)]. The fate of the polymeric macroradical depends on the sequence distribution of the copolymer. Consider the macroradical: v^CH,


4(a)

410

U V L I G H T INDUCED REACTIONS

IN

POLYMERS

followed by depropagation ÇH

ÇH

3

ÇH

3

ÇH

3

3

- CH - C. + wC. + CH = Ç + 2 I I I C0 R C0 R C0 R C0 R

ΐ

0

4(b)

0

2

2

2

2

2

For X = Y = H (an acrylic sequence) one obtains: 1)

Chain scission: Η


I

-* ^ C H

2

Η

Η

- ÇH - CH - C = CH + . Ç - CH 2

2

2

2

C0 R

C0 R

2

C™>

5(a)

C0 R

2

2

followed by intermolecular transfer H

H - CH - Ç.

^

-> ^

2

C0 R

C0 R

2

- CH - CH 2

C0 R

2

5(b)

2

C0 R

2

2

The radical remaining can undergo termination by H abstrac tion or recombination leading to further crosslinking. For X = CH and Y = H (an alternating sequence of acrylic and methac rylate monomers). 3

1)

Chain scission: ÇH

H

3

ÇH

ÇH

3

-

Δ EXPERIMENTAL POINT

(0

Ο CALCULATED POINT • EXPERIMENTAL AND


CALCULATED POINT EQUAL

0

J_ 38

Η

_J_

76

J_ 114 152 190 228 266 304 342 380 TIME (MINS.)

Figure 5. Comparison of experimental data for C0 with a nonlinear least squares fit of this data to Equation 14 2


417


418

UV LIGHT

INDUCED

REACTIONS

IN

POLYMERS

due to a different kinetic process being responsible for the generation of the molecules producing these fragments. This of course i s not unreasonable because the type of molecules which can generate these fragments are alcohols and esters (e.g. formates) which are probably formed in a two step process. The computer f i t of the CO2 data makes one confident that with the proper kinetic assumptions an analytical expression can be derived to f i t the data, and therefore kinetic parameters arrived at. One complication exists in this process, the M/E = 31 peak, for example, may have contributions from more than one molecule (e.g. CH3OH and CH4CO2). That being the case the curve would be a sum of two or more kinetic processes. This can be overcome by the use of high resolution mass spectrometry. With high resolution capability one can find unique masses which are attributable to each molecule and from that intensity and a library of mass spectral data (9) one can "deconvolute" the mass spectrum into i t s individual components. We currently have the a b i l i t y to acquire such data and are developing the software needed to handle i t . In low resolution finding unique masses for each fragment i s considerably more d i f f i c u l t i f not impossible. However, there i s another approach which may be taken. If one examines the reaction scheme, a prediction of which molecules w i l l be generated can be made. Then u t i l i z i n g a library of mass spectral data a series of simultaneous equations in contribution to intensity of given mass spectral peaks can be written (12). The only assumption necessary for this i s we know a l l the molecules contributing to each peak of interest. Such a procedure was followed with our data. Based on the prepolymer composition i t was decided that Butanol, Butane, Butene, methanol and methyl formate would "be the most l i k e l y degradation products in addition to the CO2 and CO. U t i l i z i n g the M/E = 56, 43, 41 and 31 peaks, a set of simultaneous equations were solved for the composition of the degradation products ob served at the end of the experiment. Table I l i s t s this composition. TABLE I Calculated composition of degradation products from acrylic paint at 25°C found at end of experiment Component Butanol Butane Butene Methanol Methyl formate

Calculated Intensity 3.4 4.4 2.8 2.0 1.3

χ χ χ χ χ

10 10 10 10 10

5 5 5 5 5


27.

KiLLGOAR A N D V A N OENE


419

Using this composition the intensity of several other peaks not used i n the solution of the simultaneous equations and assumed to originate from these degradation products only were calculated. The results are tabulated i n Table II. TABLE II Calculated vs. observed intensity for mass spectral peaks assumed to originate from the degradation products of Table I.


M/E 27 29 39

Calculated Intensity 4.3 χ 10 4.9 χ ΙΟ 2.2 χ 10

5 5

5

Observed

Intensity

4.4 χ 10 β.Ο χ 10 2.5 χ 10

5 5

5

Good agreement with the M/E = 27 and 39 peaks i s obtained, but there i s a f a i r l y large discrepancy with the 29 peak. This indicates that there i s another degradation product we have not considered contributing to the 29 peak. This may be a melamine fragment or some other molecule. In any event i t i s clear the low resolution data can be used to gain insight into the role played by the chemistry of the film in the degradation as well as giving kinetic information. Examination of our data for peaks due to monomer which might be formed from unzipping reactions showed no detectable amount being produced. This i s not unexpected because the prepolymer composition i s approximately 50% aerylate type monomers and, therefore, degradation by unzipping i s unlikely. Conclusions It has been demonstrated that the sensitivity of the mass spectrometer makes i t ideally suited for application to the study of polymer photodegradation. The basic understanding of the system makes i t possible to extract useful kinetic and chemical information about the degradation reactions. Work on the roles of temperature, chemical composition and network structure i s in progress. Acknowled gments The assistance, discussions and suggestions rendered by D. Schuetzel i n the development of this experimental technique, by R. Ullman for his advice i n the theoretical development and M. E. Heyde for her assistance i n a l l phases of this work i s gratefully acknowledged.


420

UV

L I G H T INDUCED REACTIONS I N P O L Y M E R S

Literature Cited 1. Hoffman, E. and Saracz, A., J. Oil Col. Chem. Assoc., 55, 1079 (1972). 2. Colling, J . H., Cracker, W. E . , Smith, M. C., and Dunderdale, J., J. Oil Col. Chem. Assoc., 54, 1057 (1971). 3. Perera, D. Υ., and Heertjes, P. M., J . Oil Col. Chem. Assoc., 54, 774 (1971). 4. Tamblyn, J . W., Newland, G. C., and Watson, M. T., Plastics Technol., 4, 427 (1958). 5. Grassie, N. and Farrish, E . , Europ. Poly. J., 3, 627 (1967). 6. Grassie N., Torrence, B. J . D., and Colford, J . B., J . Poly. Sci. Al, 7, 1425 (1969). 7. Grassie, N. and Jenkins, R. H., Europ. Poly. J., 9, 697 (1973). 8. Calvert, J . G. and Pitts, J . Ν., "Photochemistry", John Wiley and Sons, Inc., New York (1966). 9. ASTM Committee E-14 on Mass Spectrometry, "Index of Mass Spectral Data", ASTM, Philadelphia, Pa. (1969). 10. Calvert, J . G and Pitts, J . Ν., "Photochemistry", John Wiley and Sons, Inc., New York, p. 434 (1966). 11. Dye, J . L. and Nicely, V. Α., J . Chem. Ed. 48, 443 (1971). 12. Willard, H. H., Merritt, L. L., J r . , and Dean, J . Α., "Instrumental Methods of Analysis", D. Van Nostrand Co., Inc., Princeton, N. J., p. 449 (1965). 13. Bateman, Η., "Tables of Integral Transforms Vol. I.", McGraw-Hill Book Co., Inc., New York, pp. 245-6 (1954).


;

APPENDIX I SOLUTION OF DIFFERENTIAL EQUATION DESCRIBING TIME DEPENDENCE OF PRODUCT DETECTION I t i s assumed that the f i l m absorbs l i g h t by Beer's law I(x)

= I exp(-ax)

(1)

where the v a r i a b l e s are defined i n the t e x t of t h i s paper. I t i s a l s o assumed that the degradation products a r e formed by a pseudozero th order r e a c t i o n : 3c(x,t) 3t

=

ΦΙ

(2)

The products formed a r e removed from the f i l m by d i f f u s i o n . The r a t e of d i f f u s i o n from the f i l m i s given by

(3)


27.


KiLLGOAR A N D V A N OENE

421

The product distribution within the film i s given by the diffusion equation

91

ο

d X


Solving equation 4 for c(x,t) and then solving equation 3 yields the rate of product into the chamber. The boundary conditions imposed to solve these equations are c(x,o) = 0 c(~,t) = 0 c(o,t) = 0

(5)

The Laplace transform i s used to solve equation 4 where: pt

c(x,P) =/°°c(x,t) e " 'ο hence / J

o

Γ Jo

. dt

-pt 9c (x, at

e

2

9 c(x,t) 3x^ e

' /

(6)

dt

A e

-

a x e

-

p t

P t

+

dt

(7)

0

Solving

y

r

°°-pt 9c(x,t) dt ce~

Γ /

D

pt

= 0

e

9x

-ptl

,

/"

Since for t = 0

l!|ix^. - P t

0

[

d t

=

*

D

z

Γ

-pt ,. _

c(x,o) = 0

C

.-Pt

=

„

z

3x /

-

0

Λ 9x

z

and

IΙ

A

-αχ Ae

-pt ,^ 1 « -αχ e dt = — Ae

'ο

Ρ

Therefore the transformed d i f f e r e n t i a l equation becomes: A X

~ 1 A ~ - pc = - — Ae 9x^ ρ The complementary solution to equation 8 i s D

7ΓΤ

/ON (8)

1 2 C

=

AJ°)

X

,„ 2 v

+ B e - (

D

X

)


(9)

422

UV L I G H T INDUCED REACTIONS I N

00

since as χ ->

c ->· o, A

1

POLYMERS

must be zero.

The particular solution to equation 8 i s c = Fe"

a x

(10)

substituting into 8 one obtains 1 F = ρ

A ρ -Da

z


and the complete solution i s

Ix F

c-Be"( )

+

A

, , e" p(p -Da ) n

2

a x

(11) '

z

K ± ±

eliminating one of the constants with the boundary condition c(o,t) = 0 we obtain: 1 -pr

-

A

-αχ

Ρ(Ρ-Μ )

C

2

6

A

-V/D

" P(P -D«*)

/

,

C

(

1

2

Λ0 Λ )

Using the tables of transforms (13) we rewrite equation 12 i n terms of χ and t:

_1 A Da t Γ ax [ . - « . ( i ^ 2 Da " 2

2

e

+

2

«

)

*

.

-

«

^

^

.

.


«

î

)

]

Photodegradation of Automotive Paints - ACS Publications

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