Automotive Clearcoats

Automotive Clearcoats

George Wypych Che111Tec Laboratories, Inc., Toronto, Canada

Fred Lee Atlas Electric Devices Co 11 Ipany, Chicago, USA

INTRODUCTION Preceding chapter indicated the need for specific information required to design experiment of material weathering. The aim of this paper is twofold: • generate and systematize information on degradation behavior of automotive coatmgs • provide an example of data selection in preparation for weathering studies The first reason is driven by the fact that such review of technology was not presented so far in spite of the fact that clear coats are of interest of many groups in industry, testing, and university research, including: automobile, motorcycle, bicycle, manufacturers; manufacturers of coatings for repairs; manufacturers of exterior metal parts; manufacturers of exterior plastic parts; manufacturers of polymer blends for automotive applications; compounders of plastics; niche markets for clear coats (office furniture, shelving, lighting fixtures, tool boxes, doors); raw material suppliers for coating manufacturers (polymers, curatives, stabilizers, catalysts, initiators, rheological additives, pigments); research institutes (development ofnew products, methods of testing, raw materials used for coatings); national testing institutes; standardization organizations; commercial testing laboratories; university research (development of new products, methods of testing, raw materials for coatings); environmental institutes (studies on environmental impact of degradation products); corrosion protection (research, manufacturers of protective chemicals); consultants in the area of weathering and ISO 14,000; military (research, engineering, quality control); aerospace (all aspects of exterior applications of coatings and plastics); others working in the similar fields. This long list shows that the number ofpeople and institutions involved is very large thus a comprehensive review of information that is currently scattered is required. As a long list of

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references shows, the currently available information is available in many sources - some of which are difficult to obtain. The information provided in this chapter should be updated in the next two years concerning that a very broad research on powder coatings is under way which will affect provided here list of materials used and the list of potential mechanisms of degradation. For the second purpose ofthis chapter, it is important to mention that the choice of automotive industry is adequate because it size warrants a large number of quality research and thus data. This allows to review all aspects of data required prior to weathering testing. It is also important to note that automotive coatings were recently developed from prone to failure technology to robust process which yields durable products. This successful conversion occurred in spite ofthe fact that the process was complicated by additional needs to eliminate or limit use of solvents which imposed many restrictions on the development process. It s also important to note that there are still large gaps in understanding which this contribution tries to point out to generate required research.

APPLICABLE STANDARDS EXPOSURE IN LABORATORY DEVICES

Table 1. Automotive exterior coatings • applicable standards for the laboratory testing Standard

Equipment

Irradiance, W1m2

Temperature °C

RH,%

SAE J1647

HID chamber

80

38-47

50

SAE J1960

Xenon-arc (water)

0.55 @340

38 and 70

95 and 50

SAE J2019 SAE J2020

Xenon-arc (air)

80 @300-400

38 and 47

95 and 50

Fluorescent UV

0.43 ~310

VDA 621-4 (German)

Xenon-arc

70 UV/SO dark 63 UV/I0 dark

LP-463PB-16-0 1 (Chrysler) LP-463PB-9-0 1 (Chrysler) BO 101-1 (Ford) GM9125P (GM)

Carbon-arc Humidity chamber Carbon-arc Carbon-arc Fluorescent UV

none

63-71 37.2-38.4 60-65 (BP)

98-100

60±2 70 UV/SO condo

Xenon-arc MO 135 (Nissan) BS AU 148 (British) JIS D0205 (Japanese)

Carbon-arc Xenon-arc Mercury lamp Carbon-arc Xenon-arc

63±3 or 83±3

50 and 90

89±3 and 38±3 63±3 or 83±3 63±3 or 83±3

50±5 50±S

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OUTDOOR EXPOSURE SAE J 1976 applies to outdoor exposures of automotive coatings and other exterior materials. Coating systems are exposed in panel racks (unbacked exposure) and black boxes.

SOLAR FRESNEL REFLECTOR APPARATUS SAE J 1961 applies to the use of concentrated radiation for exposure of automotive samples including coatings. Apparatus should be operated in dry, sunny climates receiving 3000-4000 h of sunshine. In addition to exposure during the day, specimens are sprayed in the night for 3 min in each 15 min. Two types of exposure are used: non-insulated and insulated (backed with plywood). In insulated exposure, the insulation is only used between November 1 and March 31.

SUMMARY It is interesting to note that the national standards are not playing an essential role in testing of automotive coatings. Only Britain and Japan have national standards. The British standard is old (1969) and probably not frequently used. The laboratory testing is mostly based on SAE standards which allow for the use of all three weathering devices (carbon-arc, xenon-arc, and fluorescent UV). It is important to note that only Xenon-arc device offers full control of all weathering parameters (irradiance, temperature, and humidity) which are specified in the SAE standards.

GENERAL DISCUSSION OF TRENDS Quality of automotive finishes, legal requirements, and environmental concerns were the driving factors for changes in automotive coatings. 1 During 1950-1970, oven-dried alkyd-melamine, monocoat, straight-shade, coatings were in the common use. In the period of 1970-1990, the evolution of paint technology was gradually leading toward a more complex system of automotive finishes which eventually included low-solids, solvent-borne basecoats and alkyd-melamine clear coats, later replaced by high-solids basecoats and acrylic-melamine clear coat with UV/HALS stabilizers. These systems included metallic basecoat. During the most recent times, several new solutions were introduced, including water-borne basecoats with urethane clear coats. Even more recently, water-borne basecoats were combined with powder clear coats. The above short introduction indicates three major trends:

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Period

Action

Drivers

1950-1970 development of new technology of coatings

quality

1970-1990 development of clear coat technology

quality, appearance, durability

1990-

development of water-borne and powder coatings

environment, legislation

The period of 1970-1990 was especially instructive in stressing importance of testing with a special emphasis on weathering testing. During this time, many failures occurred, indicating that both long-term performance predictions and quality control must include weathering testing, considering that failure is very expensive.

PERFORMANCE CONDITIONS Automotive coatings meet variable environmental conditions due to the widespread use of cars in different climatic conditions. Table 2 gives a list of essential parameters.

Table 2. Typical parameters of performance of automotive coatings. Parameter UV radiation

Average value wavelength: 295-380 nm irradiance: 0.35 W/m 2 @340 nm

Major influences photochemical conversions photooxidation degradation of metallic effect

Temperature as a function of air -60 -:- 100°C (up to 115°C) temperature, IR, and color

combined degradation activity increased rate of reactions caused by other parameters

Humidity

stress due to thermal movement hydrolysis

10 -:- 100%

non-oxidative photodegradation mar (acid etch) stress due to change of volume Wetness

1-40% total time

extraction hydrolysis permeation to interface

Pollutants and fog

pH of fog as low as 2

surface erosion mar (acid etch) hydrolysis crack initiation

Acid rain (dew)

pH as low as 1 pH of dew as low as 2

surface erosion mar (acid etch) hydrolysis crack initiation deposition of salts into clearcoat

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Parameter

Average value

Major influences

Dust particles

widely variable

absorb moisture and acids embedment into clearcoat

Salt (deicing, coastal)

surface etching

delamination corrosion shrinkage

Evaporation of volatile components

surface roughening crack initiation

Pancreatine (bird droppings)

surface etching

MODES OF FAILURE Table 3 gives a list and analysis of modes of failure. Table 3. Modes of failure of automotive coatings in relationship to causes and essential parameters of weathering involved in the failure. Mode of failure Causes Parameters Gloss loss photoxidative processes caused by combination of parameters; correlation UV wavelength (18 months in Florida)38 strongly depends on the control and simulation of conditions of degradation;2o,21 irradiance level initial loss is due evaporation of volatiles'" (1700 h Xenon arc )38 temperature humidity shrinkage Yellowing (2500 h Xenon arc i" Adhesion loss (2 years in Florida)21

Cracking

chemical conversion of certain chemical groups in some resins; some hardeners UV radiation increase probability;" more visible with lighter (white basecoat) colors temperature partially attributable to photochemical processes but becomes visible due to UV radiation stress causing delamination (sources of stress - variable temperature and temperature moisture intake) moisture pH see adhesion, water spots, and surface erosion

see adhesion loss

(2 years in Floridai l

Mar (a few months )26

formation of fine scratches due to the environmental effects (associated defects: UV radiation deformation and spotting); car washing, in-plant polishing and exposure are precipitation (pH) main causes; typical reasons are photochemical damage, droplet's swelling, and abrasion solid particle deposits 19,26 , H 20 concentration

Water spots

occurs due to deposition of inorganic salts into the surface of clear coats (initial acid rain (pH) UV exposure under dry and cool conditions limits the process );25 formation of hydrolysis microscopic blisters and clear coat cracking forms the so-called defect of UV radiation "unremovable water spots,,17

temperature

296 Mode of failure Surface erosion

Oil staining Substrate corrosion

Weathering of Plastics

Causes Parameters acid rain in combination with dust collection (dust absorbs pollutants) and UV radiation photooxidation; pancreatine related surface damage mostly occurs with freshly oxygen produced cars 1 dissolved acids pancreatine polluted motor oil containing carbonaceous products of degradation" used oil loss of barrier properties, transport of ions to interface with metal deicing salt saltwater particles

The above list of modes of failures indicates that failure is generally caused by a combination of factors which sets the important criteria of testing: • parameters of exposure must precisely imitate conditions of performance • reproduction of conditions depends on the precise control of several parameters (not just UV radiation) • method of exposed specimen testing determines result. The length of time to encounter failure is given as a general information to illustrate premature failures of selected formulations.

CHEMICAL COMPOSITION Automotive coatings are applied for two groups of substrates: metal and plastic. The following diagrams best explain component elements of the coating systems:

Clearcoat Basecoat Primer Electrocoat Phosphate METAL Phosphate Electrocoat

Clearcoat Basecoat Primer PLASTIC

It is easy to predict that the clearcoat must be designed to withstand environmental impact (effect of parameters of performance). For this reason, the emphasis is given to the clearcoat in this report. The general literature' lists currently used clearcoat systems, which include: one-component acrylic-melamine, one-component polyurethane, two-component polyurethane, one-component waterborne, one-component powder. Powder coatings are still on the stage of development and extensive testing thus some data should be updated in future.

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The weathering performance (durability) depends on the chemical composition which must include all components of the mixture since each component, even used in very small quantity, may contribute to photochemical degradation. In order to describe composition, recent patents'"!" obtained by the major manufacturers of these materials were analyzed to construct a list of individual components given below. Components of automotive clearcoats: Polymeric materials: Powder coafings: 4 7,48,55,58

• copolymer of methacrylate, methyl & butyl methacrylate, and styrene with epoxy functionality cured with diacid or uretdione (HDI, IPDI) • polyacrylate polyol (MMA, esters of acrylic & methacrylic acid, styrene) OH group functionality polyester polyol (dialcohol + diacid) cured with aliphatic or cycloaliphatic ketone (ketoxime) polyisocyanate or isocyanurate • polyester (hydroxymethacrylate, n-butylacrylate, MMA, neopentyl glycol, and dicarboxylic acid) with OH functionality, polyacrylate containing hydroxyl group cured with HMDI blocked with 1,2,4-triazole • acrylic copolymer (styrene, methacrylic acid, butyl & methyl methacrylates cured with crosslinker of carboxylic groups (epoxides or oxazolines) Solve 11 t-con fa i11 i11g: 47-54,56,57,59,60

• acrylic resin OH terminated alkoxysilyl group-containing copolymer (urethane or siloxane bonding) cured by reaction of hydroxyl group from acrylic resin with alkoxysilyl • acrylic resin with OH functional groups cured with aminoplast (condensate of formaldehyde and urea, thiourea or melamine) Resimene 755 from Monsanto or Cyme I 1130 (methylate melamine-formaldehyde cond.) • acrylic polymer with OH groups microgel based on acrylic cured with aminoplast or polyisocyanate (2-colnponent system) • organosilane polymer (styrene, methacryloxy propyltrimethoxy silane, and trimethylcyclohexyl methacrylate) acrylic polyol (styrene, alkyl methacrylate, hydroxy alkyl acrylate) - macrogel urea by reaction of Resimene 755 from Monsanto or Cymel 1130 (methylate melamine-formaldehyde cond.) • polyol (caprolactone copolymerized with 1,4-cyclohexanedimethanol) star polymer (ehyleneglycol dimethacrylate, methyle, benzyl and 2-hydroxyethyl methacrylates) cured with isocyanurate or aminoplast (Cymel 1133) • acrylic polymer (styrene, alkyl methacrylate, hydroxy alkyl methacrylate) with OH functionality polyol (caprolactone copolymerized with 1,4-cyclohexanedimethanol) cured with isocyanate (triphenyhnethane triisocyanate or trimer of hexamethylene diisocyanate

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Weathering of Plastics

• acrylic polymer (styrene, MMA, alkyl methacrylate, alkyl acrylate) crosslinking acrylic (the same but containing glycidyl) • acrylic resin aminoplast (Cymel 1130) • acrylic polymer (hydroxypropyl acrylate, styrene, butyl acrylate, butyl methacrylate, acrylic acid) cured with aminoplast (CymeI 1130) In summary, the following polymeric materials will be analyzed in the section discussing chemical mechanisms of degradation: • acrylic polymers and copolymers • polyurethanes • aminoplasts The importance of this analysis is to include typical chemical groups in order to predict potential products of degradation.

Solvents • xylene • Solvesso 100 • methanol • butanol, iso-butanol • mineral spirits • heptane • butyl acetate • ethyl acetate • methyl ethyl ketone • acetone • dipropyleneglycol monomethylether • methyl amyl ketone • hexyl acetate Initiators various initiators used in polymerization of acrylic resins UV stabilizers • HALS (Tinuvin 144,292) • UV absorbers (Tinuvin 400, 900, 1130) Catalysts • tin (most frequently DBTL) • amIne

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Automotive Clearcoats

Flow/rheology modifiers • Perenol F30 and F45 - polyacrylates • Modaflow PIlI - polybutyl acrylate • polydimethyl siloxane oil • Byk 361, 323 & 325 - polyacrylates • BYK 306 - polyether modified dimethyl polysiloxane Other components • fume silica • phosphites

EFFECTS OF PROCESSING Processing effects are given in Table 4.

Table 4. Process parameters, their potential effects, and induced modes of failure. Process parameter Altered composition Production in spring and summer

Potential effect Induced mode of failure durability, quality of finish all modes of failure possible increased acid etch 25 which can be compensated by cracking exposure to UV under dry, cool conditions delamination mar

Reduced rotation speed of spraying random orientation of metal flakes, orange pee128,36 bells Residual moisture in the basecoat Dust in plant3o ,43

•popping, fuzziness, wrinkling, poor gloss28

lower durability cracking cracking

craters; cars need to be repainted with different paint potential corrosion (more initiator) faster degradation

Higher temperature of bakinz'"

degradation products

Lower film thickness f

in solvent-base paints shorter life, in powder paints corrosion uneven finish (particle size too close to film cracking thickness)

Particle size"

surface defects

cracking

Humiditv" Spray gun orientation"

gloss (lower durability)

delamination

thickness uniformity

cracking

Paint volume output vs. line speed':'

thickness

corrosion

chromophores

mar'

cracking

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Weathering of Plastics

Numerous effects can be induced by the method of processing and precision of equipment operation. At the same time, it should be considered that probabilities of these inconsistencies in production are very low because automotive companies have invested in very sophisticated equipment which prevents such artifacts. It is very essential to note that many of these failures are related to film uniformity and that film uniformity can completely change coating performance in relationship to its durability." These effects are discussed further in the next section. Similarly, errors in composition may seem very remote since paint manufacturers are very experienced. At the same time, present coatings (clear coat/base coat) are very risky in automotive applications because of their weathering properties. Previously used coatings deteriorated in a gradual process initiated by a loss of gloss. It was therefore possible to obtain early warning that particular paint (batch) does not work. In the case of modern paints, this warning does not exist, only catastrophic failure (cracking, peeling) suddenly occurs without much detectable difference in performance prior to the failure. Under these circumstances, precise control of coatings prior to their application makes good business practice, considering that in-field failure is very expensive.

MECHANISMS OF FAILURE Many aspects of degradation must be analyzed to reach expected understanding which allows one to pinpoint chemical changes contributing to the modes of failure included in the Table 3 and to find candidate methods which can predict failure. Some of these data can be found in the existing literature l ,17-26,46,6 1-76 and some mechanisms are still not fully understood. First, we need to analyze the mechanisms of degradation of individual polymers which are used for the production of clear coats as listed above. These polymers include: acrylic polymers and copolymers, polyurethanes, and aminoplasts. The analysis is performed to select the most important reactions which determine durability of automotive coatings. Figure 1 shows typical reactions of acrylic resins. These reactions are itnportant for all three types of resins used in automotive clear coats because they all contain acrylic backbone but differ in the method of chain extension (cure). Acrylic resins are UV stable. They are only degraded because of presence of photoinitiators from impurities. The initial step of photochemical degradation consists of macroradical formation. This first step opens numerous possibilities such as chain scission, crosslinking, formation of hydroperoxides, and formation of carbonyl groups. It is important to mention that there is a general agreement that these changes take place but the kinetics of these changes varies. For example, one research group presents data indicating decrease in carbonyl group formation." In other paper," there is an experimental evidence of a reverse trend. This information is very essential to follow degradation because

301

Automotive Clearcoats

Formation of radicals CH3

CH3

I

I

This reaction may lead to formation of hydroperoxides which may

-CH,-C-CH,-C-

I

-

COOH

I

COOH

either decompose producing carbonyl groups or become precursors of further chain scission. The examples below show two reactions CH3

CH3

-CH2-~-CHz-fCOOH

+

CH3

that affect molecular weight:

CH3

-CH2-~-CH2-{-

+ t:OOH

COOH

COOH

chain scission (molecular weight reduction): -CH.,-~H-CH.,-

tO~H

----+

-C H2-

CH=C H2 +

-r.

H

COOH

crosslinking (molecular weight increase)

Formation ofmacro radicals and subsequent reactions affecting molecular weight occur also (in similar sequence of reactions) in esters: -CH.,-CH-

I

COOR

-eo

coo.

I

-CH2-CH-

or

-CH2-CH-

+

I

or

-C H2-CH-

+

tOOH

The above reactions give examples offormation ofmacro radicals which only occur due to abstraction ofa side group with a help ofphoto initiators rather than by a direct action ofUV itself(bonds involved are UV stable to sunlight radiation). These reactions also show that carbonyl groups are lost in the process of photolytic degradation, although they can be also formed from decomposition of hydroperoxide as represented by the following reaction:

-CH.,-CH-

I

OOH

Figure 1. Typical reactions of acrylic resins.

-CH2-~-

o

+ ~H

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Weathering of Plastics

this is one of the basic measurements. Since there is an agreement among most research groups that correlation (between, for example, natural and laboratory exposures) requires verification ofmechanism, it is difficult to reconcile this statement with the fact that such different estimations of fundamental product of degradation exist. More comments on this subject are included below. Hydroperoxide concentration depends on two competing reactions: oxidation of macroradicals and decomposition of hydro peroxide by heat or UV. Here is one important parameter of weathering - temperature - which plays an essential role in the studies of these materials. Depending on temperature, reaction may take different course. Figure 2 shows another potential anomaly in the course of degradation which occurs when wrong wavelength of light is used in the studies. During such reaction main chain scission occurs which never happens in the outdoor environment. CH3 I

CH3 I

+

-CH2-C-CH2-CI

0= COCH3

I

COOCH3

Figure 2. Reaction not typical of outdoor exposures of acrylic resins.

In polyurethane clear coats, urethane linkages are formed. Figure 3 shows two potential reactions which may take place at urethane linkage. Both reactions have low probability which is most likely the most important reason for which urethane coatings are used more frequently than aminoplasts. Especially, in regard to acid etching, polyurethanes are superior to other coatings which have either ether or ester Iinkagcs" Cleavage of C-N bond: -O-C-NHII

o

--+

.

o-cII a

Hydrolysis:

Figure 3. Typical reactions of urethane bonding."

The mechanism of acid etching of melamine cured systems is given in Figure 4. Presence of water and acid causes hydrolysis of ether linkage which changes molecular weight and thus physico-mechanical properties of coating."

Automotive Clearcoats

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OH

OH

OH

+

HO OH

Figure 4. The mechanism of acid etching."

These changes prompted some research groups20,23,25 to conduct extended studies especially in connection with field observations that cars produced during fall or winter have more durable paint than those produced in spring and summer. Figure 5 explains perceived mechanism. If car is painted in winter, the coating cures at dry, cool conditions which ultimately leads to the last compound to the left in the 2 nd row. These changes do not cause a change in molecular weight ofpolymer forming coat. If the hydroperoxide (compound at the right ofthe 2 nd row) is decomposed by UV or heat then changes eventually lead to hydrolysis which weakens coating (last formula at the left of the 3rd row). Similar coating protection can be achieved by controlled exposure of coating to UV. The proposed mechanism helps to understand some problems with melamine coatings. In addition, it indicates importance of other parameters of weathering such as temperature, humidity, and acid rain. In summary, one may observe that some progress was made in qualitative understanding ofchemistry ofautomotive degradation. At the same time, there is still deficiency in quantitative data - necessary to control mechanisms during an experiment (outdoor, laboratory, or correlation of both).

INTERRELATIONS BETWEEN THE PERFORMANCE CO-NDITIONS, THE MODES OF FAILURE, AND THE CHEMICAL MECHANISMS OF DEGRADATION Table 5 lists these interrelationships for the modes offailure from Table 3, typical parameters of performance from Table 2, and information included in the literature on the mechanisms of chemical degradation in relationship to failure modes.

Weathering of Plastics

304

,

,

/ N H

N.J-. N

\ I,.. II N~N~N

A.

)

/

hv

-.

\ N

0

~

t

~

/

\

N .... H

N

II

N~N

)

/

o

o

\

\

R

/

N

R

N

r-; 0

~

~ ·,H N

\A)l

/N

N

N

0

,,0·

A

0) \

R

! this branch applies to winter production

!

this branch applies to summer production Figure 5. Photooxidation mechanisms of melarnine."

hv or heat

0

~

Automotive Clearcoats

305

Table 5. Mode of failure versus parameters involved and chemistry of changes. Mode of failure

Parameters involved

Gloss

UV radiation

Chemical changes involved loss of amide and urethane (PU), loss of ether and triazine not well resolved

(melaminej." carbonyl decrease," carbonyl increase and crosslink scission correlates with hydroperoxide concentration," irradiance level

increased irradiance does not always accelerates degradation

temperature

increase in carbonyl & decrease in triazine on temp. increase by lO°C23

humidity

melamine photooxidation rate increase with humidity increasing"

sulphuric acid

loss of amide (PU), loss triazine (melaminer"

shrinkage

loss of volatiles':'

Yellowing

UV radiation

no specific data; affected by weathering equipment (QUV different than W_O_M)74

temperature

no specific data

Adhesion loss

UV radiation

oxidation of lower layer (basecoat),"

tem perature

no specific data

moisture pH

accelerated by combined action ofUV and pH (decreasingj"

Cracking

UV radiation

no specific data generally related to photooxidation but no data and correlation with gloss

decrease':' temperature

Mar

Water spots

Surface erosion

no specific data

moisture

no specific data

pH

see UV radiation

UV radiation

affects crosslinking loss (no specific data)

precipitation (pH)

accumulation of dust particles helps to retain moisture and acid"

abrasion

car washing resistance correlates with Taber tesr" and stress-strain19

H 20 absorbed

concentration of water in film depends on hydrophobicity of film"

temperature

increases water permeability; coating may have higher temp. than T

acid rain (pH)

pH affects surface etching rate,17,26 several acids in composition17

particle embedding

no specific data

moisture

no specific data

UV radiation

effect confirmed, 17 no specific data

UV radiation

effect confirmed, 17 no specific data

oxygen

no specific data

dissolved acids

increases with pH decreasing.lv"

pancreatine

new paint mostly affected,' no specific data

Q

Oil staining

used oil

staining due to carbonaceous materials," no specific data

Substrate corrosion

deicing salt

most severe cases are due to the loss of environmental protection due to the damage of coating: mechanical or photochemical"

saltwater particles

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Weathering of Plastics

There are numerous publications available which deal with the subject (36 publications references during 2 two years) and extensive information available on qualitative reasons for automotive coating degradation. The quantitative data are still very scarce. From the above list, it is easy to note that only a few chemical changes can be selected as a base for quantitative measurement of the degradation rate (based on existing data). Gloss change is the most investigated mode offailure and perhaps there is a possibility to select methods of chemical analysis which may correlate with gloss. At the same time, experts 1S,24 in the field clearly indicate that gloss loss is not the major problem of clear coat/base coat systems. Moreover, it is indicatcd" that good gloss retention cannot preclude catastrophic failure of coating which occurs by peeling and cracking. Frequently, these last two failures are described as "unpredictable". This seems to signalize the nature of the problem of the lack of correlation which is discussed in more detail below. Similar systems are used for coating plastic parts of an automobile. They also include clear coat/base coat system. Several current publications deal with this subject. 42,65,67,68 Two directions are taken into consideration: development of directly paintable and adherable polyolefin compounds and preparation of TPQ for painting. If the first direction prevails in future (more novel solution) then weathering aspects will be described by similar relationships. If the second method prevails then preparation method of a surface must be included. These methods include: chemical oxidation, corona discharge, flame treatment, plasma treatment, UV/benzophenone surface degradation, and adhesion promoters. Except for the last method, all methods used affect photodegradation since all these methods induce potential defects which may initiate further degradation which must be accounted for. Finally, the above discussion included only chemistry of degradation. At the same time, it is well known that the structure offilm (unevenness, defects, orange peel, problems offlow, problems with sintering ofpowder coatings) has essential bearing on its degradation. There is no data to report on this matter and thus there is a clear need for extensive research in this area, considering that initial defects in the film surface can alone ruin chances to obtain correlation in experimental work.

SPECIMEN TESTING Some existing standards define testing method which should be used for the evaluation ofexposed specimens. These methods include: • description of changes to appcarancet'v':' • comparison with original • testing according to material specification" • color change 3,4,13 • glossll,13

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• physical properties 11 • mechanical properties] ],]3 The above methods are important for the final product evaluation but do not have any predictive value which can be used in the design of weathering experiment which may help to establish correlation. There are several analytic methods which are used to follow a degradation rate: • FTIR to determine carbonyl, melamine crosslink density, and amide II in PU 77,78 • photoinitiation rate (PIR) based on ESR measurement" • hydroperoxide tiration80 • surface composition by XPS 39 • orange peel by image analysis:" All the above methods are suitable and they can eventually contribute to the selection of laboratory weathering conditions. At the same time, the methods have some important deficiencies. ESR measurement is time-consuming and expensive. FTIR and XPS methods are affected by the surface contamination of a specimen which is especially important in outdoor exposures. Carbonyl determination does not allow to distinguish between carbonyl loss, due to degradation of carboxyl and ester groups, and carbonyl gain due to hydroperoxide decomposition. Hydroperoxide titration gives reliable data but there are always two competing processes during degradation: hydroperoxide formation and hydroperoxide decomposition. It is therefore difficult to determine extent of photochemical reaction. From the above, a clear need for a further search of chemical analytic method is needed. In addition to the study of selective chemical change there is a need to assess distribution of changes. The so-called "catastrophic (without warning) failure" clearly indicates that a part of a mechanism of cracking must be related to the changes in crystalline structure which makes material increasingly non-uniform to cause sudden (unexpected) crack. Several other opportunities still exist for monitoring the degradation. One is described in the separate chapter of this book. Clearcoats retain their properties due to a high addition of UV stabilizers. Therefore the method of monitoring the concentration of active stabilizers is another useful approach. Recent developments in image analysis allow for simultaneous monitoring of gloss, color change, and surface changes such as formation of haze, microcracking, delamination, etc. This methods tested for sealants applications'" have proven to be very efficient in durability assessment. If there is a choice between direct determination of defects and indirect (such as chemical analysis) the direct method should always be selected since it provides information on changes directly responsible for perceived failure. The chemical analysis is still very useful because it allows to confirm mechanisms ofchange - useful in remediation of the problem.

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Weathering of Plastics

EXPECTED LIFETIME For the lack of correlation with studies conducted in laboratory the only requirement used for OEM coatings is that of 5 years Florida exposure without failure. All other methods are still auxiliary techniques used more to accumulate data and experience than as a screening procedure. There is a clear need to develop an expected lifetime in Xenon-arc Weather-a-Meter and EMMAQUA, even ifbased on energy assumption as a starting point. If such standard is not clearly stated (and results not compared with) false expectations regarding laboratory exposures will always exist. Bauer" recently suggested a new approach to the prediction of durability of a painted car and in view of these considerations such standard is essential.

NATURAL EXPOSURE The precise guidelines can be found in SAE standard." Coating systems are exposed in the panel exposure racks and black boxes. Alternative method of outdoor exposure includes the use of solar Fresnel reflector apparatus." Environmental data include: total solar radiation, total UV, optionally selected wavelength radiation, and time of wetness. In Fresnel reflector exposures, it is necessary to determine radiant exposure, elapsed exposure time, black panel temperature, and spray cycle.

LABORATORY EXPOSURE The summary of standardized laboratory methods of exposure is given in Table 1. It can be additionally mentioned that there is an interest in extending laboratory methods to include the effect of acid rain on weathered coatings. Interesting modification of SAE J 1960 is reported." Panels were removed for 1 h three times a week and sprayed with solution (pH=3.2) of mixture of sulphuric, nitric, and hydrochloric acids in proportions 1:0.3:0.17.

CORRELATIONS The situation is well characterized by two statements included in Bauer's paper: 18 "Given the complex photodegradation chemistries that occur in these coating systems, a lack of correlation between outdoor exposures and conventional accelerated tests, which employ harsh exposure conditions, is not surprising."

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"It is clear that the predicting free radical photooxidation requires measurement ofboth K (reaction constant) and hydroperoxide concentration." These two statements include several important messages: • harsh exposure conditions • complex photodegradation chemistry • measurement Further discussion concentrates on these subjects. It is absolutely certain that the industry needs to accelerate testing. Otherwise, product improvement will be slow. There are two options which can be exploited to achieve this goal: • increase values of quantities involved in photodegradation • find "magnifying glass" The first option was tried for many years. Various equipments and sets of parameters were tested and, since correlation is still not available, failed. Most researchers in the field of durability of materials agree today that acceleration of testing cannot be done by modifying test environment. Also, the reason is clear - complex photodegradation chemistry does not allow to predict what such changes in parameters will affect. It is thus clear that one has to simulate in laboratory conditions typical of natural environment. There is no particular barrier in equipment which would not allow to achieve consistent control of • radiation wavelength • radiation intensity • temperature (composite of air temperature, infrared, and specimen color) • humidity The above are the main parameters controlling photodegradation and they can be controlled with precision (see two other chapter in this book on application of different equipment to studies of automotive coatings). The current developments in weathering devices allow one to run any complex program, such as for example, close simulation of seasonal effects. There are two environmental parameters which are not currently simulated in weathering devices. These are stress and pollutants but at the same time there are many methods to include them using the existing equipment (one example was discussed in the previous section). The key to the further development is to find methods which allow to verify if the chosen program of weathering conditions allows to follow degradation in the outdoor environment. In order to achieve this, the future work should concentrate on the understanding of degradation mechanisms rather than looking for universal new machines for testing as discussed in one recent publication."

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In order to use the second method (nicknamed "magnifying glass") designed to shorten testing time (or time of decision point), two directions of studies are needed: • understanding a chemical mechanism of degradation • establishing a consistent indicator of degradation useful in measuring kinetics of degradation. There were made comments on this subject in the recent paperr" "There is little reason to suspect that comparable composition changes should have comparable physical repercussions in coatings from different chemical families." "Weathering tests based on chemical composition change rates provide no information about the physical repercussions of the chemical changes. Therefore, these tests can make no comment on the physical tolerance of clearcoats to the chemical composition changes they undergo, leading to possible erroneous conclusions regarding their durability." The above comments suggest that the fact of detecting a certain concentration of, for example, carbonyl groups does not mean that a coating, regardless of its formulation, is bound to fail. At the same time, it is possible to observe that the particular coating fails when it attains a particular composition of carbonyl groups, providing that the conditions of degradation (determining the mechanisms of photochemical changes) where the same. This sets the goals for experiment design which may offer correlation: • prior to the experiment the chemical mechanisms of degradation were sufficiently understood to select a measurable quantity which allows to check that mechanisms of degradation are the same in two correlated environments • physical parameters are chosen to have close proximity of exposure conditions • a measurable quantity allows to detect early changes which have been found to signalize particular failure. In automotive coatings, this stage was not reached yet in spite of extensive effort. One reason is that, in most studies, goals too difficult to achieve were set. In the most extensive studies, attempts were made to find universal method, whereas there are no universal changes for, say, polyurethanes and melamine cured acrylics. On a surface, they produce the same hydroperoxides, carbonyls but these concentrations "can make no comment on physical tolerance of clearcoars". There are many examples in automotive coatings which show that focusing on a particular problem helps to solve it. When difference between summer and winter products was observed, the problem was solved as described in Figure 5. When the hydrolysis of aminoplasts was discovered as described by Figure 4, polyurethane coatings gained markets. Many years ago, coatings were degrading because some undesirable solvents

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were used which where then eliminated. UV stabilizers were not giving protection and this problem was eliminated because new stabilizers were introduced to assure their lower volatility during coating baking. The first powder coating was developed long time ag0 55 and it did not perform because today's rheological additives and UV stabilizers were not available to support idea. Present powder coatings are close to the required performance. All these examples show that well focused effort can produce results. The second reason can be related to very rapid changes in automotive coatings which did not allow to stabilize situation. Before any required mechanisms were found, a new range of products was introduced and work had to be repeated. The third reason is related to the fact that too many exploratory research works were needed to scrutinize the test methods which can be useful. It seems that it is a matter of time when proper correlations will be developed. In order for this to happen, the approach must include fundamental analysis of the problem which can be narrowed down to • exposure should simulate conditions found in environment of material performance • verification of these conditions should be established by the use analytical factor which confirms that chemical changes are the same in compared studies • the modes of failures of interest should be related to the chemical changes which can be easily measured.

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