Service Life Prediction of Organic Coatings - American Chemical Society

Chapter 16

Prediction of Coating Lifetime Based on FTIR Microspectrophotometric Analysis of Chemical Evolutions Jacques Lemaire and Narcisse Siampiringue

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: April 15, 1999 | doi: 10.1021/bk-1999-0722.ch016

Centre National d'Evaluation de Photoprotection, Ensemble Scientifique des Cézeaux, 63177 Aubière Cedex, France

Most of the world activity on research, development and control of polymer durability is still based on empirical techniques developed in the early ages of polymer uses. Those techniques should be critically analysed considering the advances of the fundamental understanding of these complex phenomena. In thefieldof coating ageing, empirism is prevalent in the mode of application of environmental stresses in laboratory conditions and in the degradation criteria used. A more rational approach is described which is based on the recognition of the chemical evolution mechanisms. Applications of the physico-chemical stresses and definition of the degradation criteria should be consistent with the identified mechanisms. Aliphatic and cycloaliphatic hydroxylated polyesters crosslinked with substituted melamins or condensed isocyanates are presented as examples of the "mechanistic approach" ; the identification of the two chemical routes which account for the mechanical detriment is based on FTIR and micro-FTIR spectrophotometric analysis, the later technique being used to observe specifically the chemical evolution of the elementary layers of the clear-coat and base-coat.

In thefieldof polymeric materials ageing, coatings appear as very special systems for many reasons, (i) The large values of the ratio surface to volume of these systems favour the detrimental effect of the main environmental stress i.e. the UV light, (ii) The geometrical characteristics of coatings have important consequences on the efficiency of photostabilisers. (iii) Coatings are often exposed in the form of multilayers systems, each elementary thin layer presenting a different photochemical evolution, (iv) As organic layers located on inorganic or metallic substrates, these systems are not easily analysed in situ, especially when the chemical composition varies heterogeneously along the light penetration, (v) Since coatings should protect the substrates, their own durability is therefore an essential property.

246

© 1999 A m e r i c a n C h e m i c a l Society

Bauer and Martin; Service Life Prediction of Organic Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

247 In the past 40 years, those special features have promoted a large use of empirical techniques at two levels. 1. the application of the physico-chemical stresses on the coatings, in laboratory conditions, has been essentially carried out as an exact simulation of environmental stresses to insure the relevancy of the artificial ageing ;


2. the degradation criteria have been based on the variations of macroscopic physical properties (gloss loss, microfissurations, aspect change, colorimetric variations, chalking...). Although the empiricism is present at a large extent in the field of polymer ageing, the part of empiricism has been so important in the prediction of coating lifetimes that the state of the art in this particular field has not progressed like for other polymeric systems. A critical analysis of the ageing units based on stresses simulation In the early 1950s, when failures of polymeric materials in use were observed, particularly in outdoor conditions, laboratory testing, that could reproduce the phenomena causing the degradation of the polymer, was urgently developed. In those years, the relevancy of the laboratory experiment was based on an exact qualitative and quantitative simulation of the physico-chemical stresses which exist in environmental conditions, applied during a time scale which was only somewhat shorter than the real lifetimes of the materials in use conditions. Indeed more or less exact simulation of the stresses implies that any acceleration of the degradation was unacceptable since it was anticipated that an accelerated degradation would be irrelevant. In the ageing units of the 50s, daylight was simulated using a carbon source, then a Xenon source emitting a continuous spectrum, rain was simulated by periodic water aspersion, day and night by dark and light periods ... Those principles are still respected in more than 90% ageing units functioning in 1997 (i.e. WeatherO-Meter and Xenotest), they however prompted the following critical comments : i) A high pressure Xenon arc emits a continuum from 240 nm to the IR, that continuum is normally filtered to avoid any radiation from 240 to 300 nm. Using Bisphenol A Polycarbonate as a solid actinometer sensitive to short wavelengths (300-330 nm) and to long wavelengths (X > 300 nm) with two different chemical consequences, it is very easy to demonstrate that the filtered Xenon source contains an excess of short wavelengths relatively to daylight [1]. A medium pressure Mercury arcfilteredby a borosilicate envelop and emitting discrete lines appears to be more relevant (this experimental result is not unexpected when vibrational relaxation in the condensed phase is considered). Xenon sources present a second inconvenience due to their short lifetimes (... and cost). ii) In the fifties, it was not understood that the complex chemical evolution of polymeric systems exposed to UV, heat, O2 (and H2O) involved some nonphotochemical processes and presented fairly large apparent activation energy. The control of the temperature of the exposed surface is therefore a strict prerequisite of any laboratory experiment. In simulators, an external "black body" more or less insulated indicates only a temperature which is not related to the actual temperature of the exposed surfaces of the samples, especially of clear samples.



248 iii) Experimental results have often shown that the water aspersion in most standardised cycles simulates very heavy and frequent rains. In moderate weathering conditions, the oxidised layers are built up from March to September. Hydrolysis or mechanical abrasion of these layers occurs ratherfromSeptember to March when the oxidation progressed only very slowly. Moreover water aspersion simulates mostly the mechanical role of rain and far less the physical and chemical influences of water (hydrolysis of some oxidation photoproducts, extraction of hydrosoluble stabilisers, matrices swelling etc ). iv) In most cycles, dark period has been introduced to favour the migration to the surface of stabilisers whose depopulation in the superficial layers could be too fast during UV exposure. Dark periods have beenfittedwith additives which were generally migrating fast. Presently, the most migrating additives are not migrating that fast (compare for example monomelic HALS and BHT). During the fairly short dark periods, the depopulation at the surface could not be any more compensated. It should be emphasised too that photo-oxidation during the light period and migration during the dark period are two dynamic processes which could not be generally accelerated within the same factor. v) Since every environmental stress is applied in the same experiment, analysis of the degradation origin remains difficult. Only correlations (which means relationships no one could account for) can be observed between weathering data and laboratory data. From those comments, it could be concluded that any experimental procedure based on simulate units could be only pragmatically accepted, considering the large number of the corresponding units in function. An objective analysis of that testing of organic coatings is made even more difficult by the empiricism prevalent too in the choice of degradation criteria. To evaluate the degradation of coating, the following criteria are extensively used: - loss of gloss related to the extent of superficial microfissuration ; - aspect changes evaluated through visual inspection, colorimetry or spectrocolorimetry. Aspect changes result indeed from superficial microfissurations, formation and bleaching of yellowing photoproducts, formation and photo-oxidation of fluorescent photoproducts and bleaching of dyes and pigments. Any aspect change could be a very complex information since each phenomenon should obey a different kinetic low . - fissuration of the coatingfilm(mosaic formation); - chalking. As illustrated in the next section, a better understanding of the complex ageing phenomena could be gained considering the chemical evolution of polymeric systems through ageing. The macroscopic physical changes listed above could be indeed accounted for by the different chemical evaluations identified. Up to the recent years, however, it appears that the analytical difficulties which were inherent to coatings, have inhibited such an effort. The physical changes have not been often related to the chemical changes.


249


Towards a rational approach based on the analysis of chemical evolutions When a polymeric material is submitted to the environmental stresses, which should be ranked as rather moderate, the degradation of most physical properties is due to chemical ageing. Therefore, through weathering, the polymer should be considered as a "photochemical reactor" in the presence of light and as a "thermal reactor" in its absence [2-5]. The exact nature of the chemical events responsible for the physical detriment should be understood (in the sense which will be explained later on). The analysis and the follow-up of the chemical evolutions, occurring as well in artificial conditions as in natural weathering or in real use conditions, have two advantages : i) it affords a very strict relevancy control based on the comparison of the on-going chemistry; ii) since the chemistry extent could be related to the exposure duration either in average conditions (weathering) or in well-defined conditions (artificial ageing), the lifetime in laboratory conditions could be converted into lifetime in use conditions. Analysis of the chemical evolution of a polymeric material submitted to light, heat, O2, H2O, etc. ..., appears to be complex for the following reasons : - only evolutions in the solid state are relevant and analysis should be carried out in that solid state (specially to examine the stability of intermediate products); - chemical evolution at very small extent should only be considered. When the extent of the chemistry exceeds 0.5 to 1%, the loss of physical properties is complete and it becomes useless to study the further reactions occurring in the fragments (except when the "ultimate" fate of polymeric materials is examined for the safe of environmental protection); - chemical evolution includes indeed many mechanisms of various importance and it is required to identify the transformations which account for the physical detriment. Usually, the most important route involves a photo-oxidation or a thermo-oxidation mechanism whose products are formed in concentrations high enough to be observed in vibrational spectrophotometries, even in local zones (FTIR, micro-FTIR, FT-Raman, micro-FT Raman, FTIR with photoacoustic detector, ATR, ATR-H). The extent of the chemical evolution is determined from the accumulation in the matrix of a "critical product" which : - should measure the main detrimental route of the matrix and therefore, should be formed from a chain scission process ; - should be chemically and photochemically inert in the matrix ; - should not diffuse out; - should accumulate linearly with the exposure time until the complete loss of physical properties insuring the function ; since the complete physical detriment is observed at low extent, the system is "at initial time". From the correlation which appears in accelerated conditions between the variations of physical properties and the variations of the critical product concentration, the lifetime of the polymeric materials is determined in artificial conditions and


250


afterwards, converted into the lifetime in weathering conditions using a predetermined acceleration factor of the corresponding chemistry. Acceleration of ageing cannot be fundamentally justified by practical reasons although users of polymeric materials are prompted to demand and accept quick testing. Indeed acceleration should be recommended for a very different reason associated with the impossibility to extrapolate the data collected in non-accelerated conditions. The usual techniques of homogeneous kinetics cannot be applied to handle the chemical transformation of a polymer matrix through ageing. It is indeed very difficult to obtain a reliable expression of the evolution rate of a solid polymer because of the complexities of the occuring reactions (short chain reactions, solid state processes, conjugation between chain reactions and step reactions, copropagaters, etc ...) and more important because of heterogeneity. Kinetic treatments were more used to rationalise the data collected in laboratory conditions than to predict the lifetime in use conditions [6-7]. As consequence of the limitation of any kinetic treatment, it is strongly recommended to observe the chemical and the physical evolution of the polymer until its life end. This life end can only be reached after a reasonable period of time in artificial accelerated conditions. After many years of experiments on polymer photo-ageing in accelerated and in environmental conditions and examinations of the corresponding mechanisms, the following principles could be proposed : a) it is possible to provoke accelerated chemical evolutions in solid polymers which obey the same mechanisms as the non-accelerated evolutions. b) acceleration should be limited to the level where chemical distortions are observed (like enhancement of cross-linking due to biradical recombination). c) photo-ageing acceleration should be due only to high light intensity (and not to shorter wavelengths) and to high temperature ; the temperature increase should however be limited by the fact that photothermal transformation should exceed largely any pure thermal conversion. d) acceleration is only allowed when one unique dynamic process controls the ageing in natural and in artificial conditions. In many outdoor uses of polymeric systems, photo-oxidation is the main detrimental mechanism. It is therefore required to accelerate photo-oxidation without observing, in artificial conditions, some irrelevant control due to an oxygen starvation effect or to a stabiliser migration. The example of cross-linked saturated polyesters Presently, aliphatic or cycloaliphatic saturated polyesters with lateral or terminal hydroxyl groups are often used as clear-coats or base-coats in the automotive industry. They are processed at high temperature through cross-linking with melamine derivatives or with condensed isocyanates. The replacement of the aromatic units by saturated units in the polyester chain favours a photostability increase, either in the form of transparent matrix (clear-coat), or in the form of pigmented matrix (base-coat). This photostability can indeed be improved through


251


the optimisation of the macromolecular structure or through additivation. However, durability problems have appeared in the automotive industry and this failure has prompted some fundamental research on the detriment mechanisms [8-11] and some developments on photostabilisation [12-14]. Non-hydroxylated saturated aliphatic polyesters were observed to have a very long lifetime, exposures during several thousand hours in the severe conditions of a SEPAP 12.24 unit have not induced any significant oxidation. Industrial hydroxylated saturated polyesters, used as coatings, contain indeed either some aromatic units as contaminants, or some aliphatic double bonds added on purpose. These reactive sites are able to induce some fast oxidation of the polyester chain. Cross-linked polyesters are even more reactive since the new bonds formed in the cross-linking reactions are highly photo-oxidable, as shown in the results presented in the next sections. The more usual cross-linkers are substituted melamines, like methoxy-methylated melamines:

CH -N> 2

H

CH OCH 2

3

or condensed isocyanates with low volatility like trimers (isocyanurates) or oligomers. The isocyanate groups could be blocked and the blocking agents BH are eliminated at high temperature during the cross-linking process :

O +BH

R is aliphatic or cycloaliphat


252 Based on micro-FTIR spectrophotometry, micro-Raman spectrophotometry, or FTIR spectrophotometry with photoacoustic detection (FTIR-PAS), the control of the condensation reaction of the hydroxyl groups either through trans-etherification* (melamines) or through reactions with the isocyanate groups, could be carried out in the different elementary layers of the clear-coat and the base-coat. Using FTIR-PAS technique, a non -destructive analysis can be used on the solid substrate (polymeric or metallic).


Photo-oxidation mechanism of melamine-cross-linked polyesters. The photo-oxidation mechanism of aliphatic and cycloaliphatic polyesters cross-linked with methoxy-methylated melamine has been recently examined in Laboratoire de Photochimie** [15]. A simplified structure of the network is presented on scheme 1, the maxima of the main IR absorption bands are indicated on : H

V

1 7 3 5 c m

.

H

^N^^

1

N

\_M

|| ^1550 cm *»

CH

2

NJ P—C—0-CH -CH —O—CH 2

2

2

Jl JJ jjj

CH -Q—CH 3

H

T »j

~ „ , P= Polyester chain

815cm -> H

2

913 cm *»

Scheme 1 In the experimental conditions of the artificial accelerated photo-ageing of the SEPAP 12.24 unit, the polymeric network is involved in a photo-induced oxidation, the chemical chromophoric defects absorbing the UV light. The most reactive site, in the presence of peroxy radicals formed photochemically, was observed to be the ethylene group -CH2- located in the a-position of the oxygen and the nitrogen atoms: ?

y~

V-N-CH — 0 2

+ r0

2

•

N

n

1 ^ — N — C H - 0 — + r0 H 2

The radical-initiated oxidation mechanism is presented on scheme 2, the various intermediate photoproducts being determined by FTIR spectrophotometry (with or

* Meanwhile the trans-etherification proceeds in the cross-linking operation, self condensation of melamine occurs, the solid network is indeed very complex like an interpenetrated network (IPN) ** P. Delorme, PhD Doctorate, University Blaise Pascal (Clermont-Ferrand, France), Dec. 1995


253 without derivatization reactions). This mechanism implies the oxidation scission of the cross-links -NH-CH2-O- and a radicalar attack on the polyesters chains with the formation of acidic and anhydride groups.

yv 1

\ — N

h815 \ cm-

2

792 1

ih

cm"

1

V

I H A l l (])—N-C—O—R

0

@—N—CH—O—R


O-R

V = N - C H —
i£-

NH^ R

II


I

o

h

(polyester)

h

I

I

°s A

I

y

^ K

I

V

T

I

v

(polyester)

°

f

Scheme 3 where R' is an aliphatic or cycloaliphatic unit presenting a CH2 or CH group in the a position of the urethane groups. The mechanism of photo-oxidation again in the experimental conditions of the SEPAP 12.24 unit is presented on scheme 4 and involves both the oxidation of the urethane groups and the oxidation of the polyester chains. As indicated in this scheme 4, acylurethanes are formed in the oxidation of urethane groups. These groups are easily hydrolysed, at room temperature, water has therefore a complementary detrimental effect since it destructs a cross-link bond [16]. It should be however emphasized that the primary degradation occurs as oxidation of the urethane groups, water is just revealing a potential detriment. In a testing protocol, when degradation is evaluated from the consumption of urethane groups, experimentation in "dry" conditions is relevant. However, if degradation is evaluated from the variation of any physical property (mechanical, or aspect ...), application of water as a chemical stress is a pre-requisite for relevancy. It should be added, that the influence of water is not conjugated with the influence of UV, heat or O2, since water is hydrolysing the pre-formed oxidation products. A simple immersion in water following a "dry" photo-ageing test should reveal the detrimental effect of water. It is therefore shown that the understanding of the ongoing chemical phenomena makes easier any testing in laboratory conditions. The photo-oxidation mechanism observed in accelerated artificial ageing has been observed too throughout weathering in climatic station, or in normal use conditions.


255

1 T

H X>300nm >

•

I

—CH-N-C—O—CH 2

ro;

II

O

—CH —N-C—0-CH 2

2

o

O URETHANE CROSS-LINK 1525 cm-l H

H PH

I

—CH-N—C—O—CH

I

II

—CH-N—C—O—CH -

OCT

2

I

2

II

2

o


OOH O HYDROPEROXIDE

H

H

I

I

OH*

—C-N-C—O—CH 2

o

o cage reaction H

OH

—C—N—C—O—CH II

O

+ H 0-

2

-CH —O—C—NH

2

II

2

O ACID 3260 cm-l 1704 cm-l 1670 cm-l

O

ACETYLURETHANE 1850-1750 cm-l

ACIDS (-COOH) 3260 cm- ; 1704 cm" 1

POLYESTER C H A I N

o

2

2

O URETHANE 1620 cm-l

1

A L C O H O L S (-OH) 3480 cm-i

/ ° A N H Y D R I D E S (^^ji 1850-1750 cm-i \

° \ ) /

Scheme 4 Conclusions Photoageing experiments carried out in laboratory conditions on coating would afford relevant data and fairly correct lifetime prediction (within the errors range of environmental lifetimes), when 3 conditions are obeyed : - the chemical evolution mechanisms, and especially the chemical route, which is controlling the detriment of the functional physical property, should be identified ; - the physico-chemical stresses which provoke the identified chemical evolution should be applied at levels which are not modifying the observed mechanism; - the detriment should be evaluated from the extent of the controlling reactions


256 through the accumulation of a selected critical photoproduct. In most coatings exposed in outdoor conditions, photo-oxidation is the most important degradation process. Therefore, the extent of photo-oxidation should be evaluated using vibrational spectrophotometries (IR or Raman) and measuring the formation rate of the critical products specifically in the clear-coat or in the base-coat.


It is fairly important not to consider the variations of the aspect of the samples exposed in artificial conditions more or less accelerated. As detailed in the previous sections, aspect variations result from different types of photo-reactions which are not accelerated within the same factor. Variations of aspect are extremely important in actual use conditions, variations of aspect could be very misleading in artificial testing.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

RIVATON A., SALLET D. and LEMAIRE J., Polym. Degrad. Stab. 1986, 14, 1 LEMAIRE J., Pure and Appl. Chem. 1982, 15, 1432 LEMAIRE J., ARNAUD R., GARDETTE J.L., LACOSTE J. and SEINERA H., Kunststoffe 1986, 76, 149 LEMAIRE J., ARNAUD R. and LACOSTE J., Acta Polym. 1988, 39, 27 LEMAIRE J., Chemtech 1996, 10, 42 SOMERSALL A.C. and GUILLET J.E., Polym. Stab. and Degrad., ACS Symposium Series, P. Klemchuk Ed. 1985, 211 GEUSKENS G., DEBIE F., KABANKA M.S. and NEDELKOS G., Polym. Photochem. 1984, 5, 313 MIELEWSKI D.F., BAUER D.R. and GERLOCK J.L., Polym. Degrad. Stab. 1991, 33, 93 GERLOCK J.L. and BAUERD.R.,J. Polym. Sci. Polym. Phys. 1984, 22, 447 GERLOCK J.L., MIELEWSKI D.F. and BAUER D.R., Polym. Degrad. Stab. 1988, 20, 123 BAUER D.R. and MIELEWSKI D.F., Polym. Degrad. Stab. 1993, 40, 349 BAUER D.R., GERLOCK J.L., MIELEWSKI D.F., PAPUTA PECK M.C. and CARTER III R.O., Polym. Degrad. Stab. 1990, 27, 271 BAUER D.R., J. Coat. Technol. 1994, 66(57), 835 MIELEWSKI D.F., BAUER D.R. and GERLOCK J.L., Polym. Degrad. Stab. 1993, 41, 323 DELORME P., PhD Doctorate, Université Blaise Pascal, Clermont-Ferrand (France), December 12. 1995 WILHELM C., PhD Doctorate, Université Blaise Pascal, Clermont-Ferrand (France), January 7. 1994


Service Life Prediction of Organic Coatings - American Chemical Society

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