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Non-Electrical Measurement

Techniques for Assessing the

State of Coating Systems

Deterioration

Mary E. McKnight

Tinh Nguyen

Jonathan W. Martin

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

December 1985

Issued February 1 986

Prepared for; -QC al Civil Engineering Laboratory

2QQ Huemene, Ca 93043

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85-3293

1986
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N23 research INFCrwMATION CENTER

NBSIR 85-3293

NON ELECTRICAL MEASUREMENT

TECHNIQUES FOR ASSESSING THE

STATE OF COATING SYSTEMS

DETERIORATION

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Mary E. McKnight

Tinh Nguyen

Jonathan W. Martin

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

December 1985

Issued February 1986

Prepared for: Naval Civil Engineering Laboratory

Port Huemene, Ca 93043

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

ABSTRACT

ACKNOWLEDGEMENTS iv

LIST OF FIGURES vi

1. INTRODUCTION 1

2. CHARACTERIZATION PROCEDURES 1

2.1 DEGRADATION 1

2.1.1 Infrared Spectroscopy 1 2. 1.1.1 Ref lection/ Absorption Spectroscopy 2 2. 1.1. 2 Attenuated Total Reflection Spectroscopy 2

2. 1.1. 3 Diffuse Reflectance, Photoacoustic, and Emission

Spectroscopy 4

2.1.2 X-ray Photoelectron Spectroscopy 5 2.1.3 Electron Spin Resonance and Other Techniques 5 2.1.4 Measurements of Crosslinking . 6

2. 1.4.1 Swelling 6

2. 1.4. 2 Elastic Modulus 7 2. 1.4. 3 Chemical Analysis and Other Methods 7 2.1.5 Microscopy 7

2.2 COATING/ SUBSTRATE INTERFACE 7

2.2.1 Infrared Thermography 8

2.2.2 Thermal Wave Microscopy 8

2.2.3 Acoustic Emission 10

2.2.4 X-Ray Photoelectron Spectroscopy 10

2.2.5 Infrared Spectroscopy 11

2.2.6 Auger Electron Spectroscopy 11

2.2.7 Ellipsometry 11

2.3 ADHESION 12

2.3.1 Techniques for Measuring Bond Strength 12

2. 3. 1.1 Destructive Techniques 12

2. 3. 1.2 Nondestructive Techniques 13

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TABLE OF CONTENTS (Continued)

2.4 MECHANICAL PROPERTIES 13

2.4.1 Internal Stress 14

1 ,k .1 Residual Strain 15

2.4.3 Dynamic Tests 13

3.3. SUMMARY AND RECOMMENDATIONS 15

4.4. REFERENCES 17

iil
ABSTRACT

Nonelectrical methods used to characterize early degradation in coating systems were critically reviewed with respect to their ability to provide predictive

in-service performance data. The methods reviewed were classified into those

that measure chemical changes, coating/substrate interfacial changes, and

adhesion and mechanical properties. Although many methods are used to characterize

coating systems, only limited research has been done to relate early property

changes to in-service performance. Because of the complexity of the degradation

of coating systems, it was concluded that a combination of methods will be

needed to characterize early degradation in order that service-life prediction

of coating systems can be based on these measurements.

Keywords: coatings; protective; steel; characterization; test; service-life

prediction; review

IV
ACKNOWLEDGEMENT

This report was sponsored by the Naval Civil Engineering Laboratory, Port

Huenerae, CA. Technical coordination with the Naval Civil Engineering Laboratory

was provided by Dr. R.W. Drisko whose support the authors gratefully acknowledge.

V
LIST OF FIGURES

Figure 1. Schematic representation of infrared spectroscopy techniques , . 3

Figure 2. Schematic representation of thermal wave microscopy 9

Figure 3. Summary of characterization procedures with respect

to early detection of degradation 16

vi
1. INTRODUCTION

All organic coatings subjected to service environments degrade over time;

however, some coatings provide better protection and remain aesthetically

pleasing for a longer period of time than others. To select and specify these

coating systems, performance data are necessary. In the past, selection and

specification have often been based on in-service performance data, since these

data were available and were considered more reliable than that obtained

from short-term tests. As a result of environmental and health regulations and

improving coating technology, new coating systems have been introduced for

which in-service performance data are limited. Thus, there is an urgent need

for short-term testing procedures that will aid in the selection, specification,

and quality assurance of these coating systems.

To meet this need, the Naval Civil Engineering Laboratory (NCEL) [1] has

proposed three tasks that involve the development of improved 1) experimental

procedures for aging coating systems, 2) improved characterization of deterioration

of coating systems, and 3) improved statistically-based methods for experimental

design and analysis of data. As part of the first phase of this work, non- electrical methods of detecting and characterizing early deterioration of

coating systems were reviewed and are discussed in this report.

2. CHARACTERIZATION PROCEDURES

The deterioration of a coating system can be characterized at several levels

including bulk and surface degradation, integrity at the coating/substrate

interface, and adhesion and mechanical properties. Procedures for characterizing

coating systems at these different levels are discussed below.

2.1 DEGRADATION

Exposure of coating systems to degradation factors, such as UV radiation,

elevated temperature, temperature changes, moisture, and salts can change both

the exposed surface and the bulk properties of a coating. Such property

changes often stem from chemical changes in the polymeric components of a coating. Thus, characterizing degradation of the polymeric portion of the

coating should provide useful data for predicting coating performance. In this

section, methods which are capable of characterizing early changes in the

bulk and surface properties of a coating are discussed.

2.1.1 Infrared Spectroscopy

The powerful combination of specific compound identification and functional

group characterization has made infrared spectroscopy (IRS) the dominant tool

for studying chemical changes in coatings and related materials [2]. The

recent improvements in dispersive infrared spectrometers and the development of

Fourier transform infrared spectroscopy have increased the capability of IRS to

study deterioration of coating materials [3,4]. For polymers in solution and

transparent free films, transmission IRS is the method of choice. However, for

other samples, IRS combined with reflection/absorption (RA) , total attenuated

reflection (ATR), diffuse reflection (DR), photoacoustic (PA), or emission (EM)

1
spectroscopy ere more suitable methods. All of these techniques are nondestructive

and require little material. The principle of each is shown schematically in figure

1 and discussed below.

2. 1.1.1 Reflection/Absorption Spectroscopy

Infrared reflection/absorption (IR-RA), also referred to as external reflection,

is a nondestructive technique for studying chemical species on reflective

surfaces and the chemical constituents at the interface between a thin film and

a reflective metal surface. IR-RA is not suitable for studies of thick opaque

coatings or of transparent coatings on non-reflective substrates. In IR-RA

spectroscopy, the spectrum of a surface film is determined by comparing the

reflectivity of uncoated and coated specimens (figure 1-a and 1-b) [5] . The

maximum interaction between the sample and the incident radiation occurs when

the radiation is parallel polarized and strikes the surface at near glancing

angles (80-89 degrees). Under optimum conditions, an IR-RA spectrum of a thin

film (< 50 nm) is 20-50 times stronger than that obtained by the conventional

IR transmission technique. For thicker films (> 50 nm) , a single reflection

and an angle of incidence of 45 degrees are sufficient to obtain good spectra.

RA spectra are complex functions of numerous factors [6] , such as the refractive

indices of the films and metals, the absorption constant and the thickness of

the film, and the angle of incidence. Thus, the shape, position and intensity

of strongly absorbed bands obtained by IR-RA are quite different from those

obtained by IR transmission.

IR-RA has been used to study the structure and orientation of monolayers

adsorbed on the surface of polished metals [7] and for studying chemical

changes in films resulting from degradation. For example, it was used 1) to

aid in identifying the role of the metal substrate on the mechanisms of

photolytic, thermal, and environmental degradation of a polycarbonate coating

[8]; 2) to study thermal, oxidative, and photo-degradation of various epoxy

resins [9,10]; 3) to study the degradation of epoxy, polybutadiene, and polyurethane

coatings on steel exposed to a corrosive environment [11,12]; and 4) to

identify chemical reactions occurring during the zinc-phosphate coating of iron

[13]. It has also been used extensively for studying the oxidation of polyethylene,

polybutadiene, and poly ( 1-butene ) [14,15,16,17].

2. 1.1. 2 Attenuated Total Reflection Spectroscopy

A widely used technique for studying the chemical changes on the surface of

coatings is infrared attenuated total reflection spectroscopy (ATR). ATR is

also known as internal reflection spectroscopy, multiple internal reflection

spectroscopy, or sometimes as frustrated multiple internal reflection spectroscopy.

ATR is a versatile technique that can be used to study the chemistry of thin

and thick films on a wide range of substrate materials. Harrick [18] developed

the technique, and discussed its theory and application.

In ATR spectroscopy, radiation is introduced into a prism (internal reflection

element) that is in intimate contact with the sample (figure 1-c). The

radiation is introduced into the prism at an angle above the critical angle

to achieve total internal reflection. As the incident wave is absorbed by the

2
i

a

Cl i

w i

I

e.

Figure 1. Schematic representation of infrared spectroscopy techniques: a. and

b, reflectance/absorption, c. attenuated total reflectance, d. diffuse

reflectance, e. emission, f. photoacoustic, where I is the incident beam, R is the reflected beam, ©is the angle of incidence, ni is the refractive index

of the coating, E is the emission, and KBr is the material of the transparent

window.

3
sample, the reflected radiation loses energy, giving rise to an ATR spectrum.

The depth to which the radiation penetrates into the sample is usually a few

micrometers but depends on the refractive indices of the prism and sample, the

angle of incidence, and the wavelength of the incoming radiation. For ATR to

be successful, good surface contact between the sample and the internal

reflection element is essential. Soft, flexible coatings, which make continuous

contact with the prism, produce excellent spectra, but hard or rough coatings

give poor and nonreproducible spectra since achieving intimate contact is difficult.

IR-ATR has been widely used for various analytical problems in coatings and

related materials [19,20]. Reports of its use as a sensitive surface analytical

method for investigating the degradation of coatings are abundant. For

example, it has been used for studying the oxidation, decomposition, and

plasticizer leaching of a number of thermoplastic coatings, and for studying

the rupture of crosslinks in thermosets exposed to accelerated and outdoor

environments [21]. ATR has also been used to study accelerated and natural

weathering degradation of poly (vinyl chloride), polyurethane, epoxy, and alkyd

coatings [22,23,24], and the UV degradation of ethyl cellulose [25]. Other

applications of ATR include determining the relationships between changes in

spectra and service life of coatings [26] , changes in the spectra and the

adhesion properties of coatings as a function of exposure [27,28], and detecting

the formation of corrosion products derived from chelating agents at the

metal/coating interface [29].

2. 1.1.

3

Diffuse Reflectance, Photoacoustic, and Emission Spectroscopy

Diffuse reflectance (DR), emission (EM), and photoacoustic (PA) Fourier

transform infrared (FTIR) spectroscopies have also been used for detecting

chemical changes on coating surfaces [30,31]. The use of these techniques in

the mid-IR (2.5 to 50pm) is now possible because of the improved sensitivity

of FTIR spectrometers. In DR spectroscopy, light incident upon a solid or powdered surface is diffusely scattered and the scattered light is collected and

directed to the detector (figure 1-d) [32]. The DR technique requires little

sample preparation and has good sensitivity. In addition, DR spectra can be

obtained from very small amounts of material. However the depth of penetration

is wavelength dependent and the particle size of the powder must be uniform for

quantitative studies. Diffuse spectroscopy has been used to study organosilane

coupling agents on glass fibers [33], curing of epoxy resins [34], and coatings

on paper substrates [35].

Emission spectroscopy (EM) is based on the principle that the radiation emitted

is equal to that absorbed (figure 1-e) [36]. EM is particularly useful for

obtaining spectra of remote samples and samples at high temperatures. Emission

spectra can be obtained at room temperature, but the quality of spectra is

better for samples having temperatures in the range of 50-150“C. The interference

of background radiation is an inherent disadvantage of EM. In coatings related

applications, EM has been used to determine the oxide formed during high

temperature oxidation of metals [37].

Photoacoustic infrared spectroscopy (PA) is based on the generation of acoutsic waves when a sample is exposed to modulated radiation (figure 1-f) [38]. In

dispersive infrared instruments, choppers are used to modulate the incident

4
infrared radiation. But in Fourier transform infrared spectrometers, no special choppers are needed since the interferometer acts as a modulator. In this

technique, the sample is placed in a closed cell containing a sensitive

microphone which is contructed with a window that is transparent to infrared

radiation. When radiation is absorbed by the sample, the temperature of the

sample surface increases, thereby heating the gas causing an increases in gas

pressure. Thus, modulated radiation absorbed by the sample produces a pressure

modulation in the gas and a sound is generated at the frequency of the radiation

modulation and is detected with the microphone. By scanning over the wavelength

range of interest, a PA spectrum is produced. The advantages of PA are that little

sample preparation is required, it is relative insensitive to sample surface

morphology, and it is suitable for studying surface phenomena. Disadvantages

of PA include a low signal-to-noise ratio making long scan times necessary and

difficulty in obtaining quantitative data. Information obtained from PA

spectra has been compared with that obtained from diffuse reflectance or attenuated

total reflectance [39]. In addition to obtaining spectra of powdered inorganic

and organic material, foamed polymers and polymer films [40], PAS has been used

for depth profiling within a film and for obtaining spectra of materials

beneath thin films [41].

2.1.2 X-ray Photoelectron Spectroscopy

In X-ray photoelectron spectroscopy (XPS), also commonly known as electron

spectroscopy for chemical analysis (ESCA) , binding energies of electrons

ejected from molecules close to the surface as a result of X-ray bombardment

are measured [42]. This method provides information on the structure, chemical

bonding, and elemental composition of the surface of a sample. The typical

sampling depth of XPS is 2 nm. Thus, the technique complements other traditional

techniques, e.g., infrared spectroscopy (penetration depth up to 5pm), microscopy,

and surface free energy determinations, for surface characterization of organic

solids. The disadvantages of XPS include the need for an ultra-high vacuum,

sensitivity to surface contamination, and inability to provide the detailed chemical

information that can be obtained with IR spectroscopy.

Applications of XPS for investigating chemical changes on polymer surfaces due

to surface modifications or weathering are extensive [43,44]. It has also been

used for studying the degradation of coatings [45] , corrosion of base metals

[46], and the deterioration of metals protected by coatings [24,47,48].

2.1.3 Electron Spin Resonance and Other Techniques

Electron spin resonance spectroscopy (ESR), also known as electron paramagnetic

resonance spectroscopy (EPR), provides information on the structure and

concentration of free radicals formed in polymers as a result of degradation

[49]. It is based on the splitting of otherwise degenerate electron energy

states by an external magnetic field. Although ESR is capable of detecting

small concentrations of radicals, ESR measurements often must be made at low

temperatures and the spectra are usually difficult to interpret. ESR has been

used to follow the radical processes in the UV radiation of polymers [50,51]

and the photodegradation of coatings [24,25].

5
other less common methods used for studying the degradation of coatings include

luminescence, nuclear magnetic resonance (NMR) and UV-visible spectroscopies,

gas chromatography (GC), GC-FTIR, mass spectroscopy (MS), thermal analysis, and

surface free energy determinations,

2.1.4 Measurements of Crosslinking

Chemically reactive species, such as free radicals, are capable of reacting

with one another or^ with themselves to form a crosslinked (3-dimensional)

polymer. Although crosslinking is a useful reaction for increasing the

thermal and chemical stabilities of thermoplastics, it may be undesirable when

the reaction occurs in-service. Examples of undesirable property changes

caused by crosslinking reactions include increased modulus, increased glass

transition temperature [52,53] (making the material more brittle), decreased

compatibility of polymers and plasticizers (causing plasticizers to exude),

film shrinkage, cracking, and film delamination. For thermoset coatings, where

the degree of crosslinking is already high, further crosslinking will result in

undesirable property changes similar to those in thermoplastics. Insufficient

crosslinking also may result in poor coating performance [54]. Thus, information

on the structure of crosslinked polymers, such as crosslink density or the

molecular weight of chains between crosslinks, is important for predicting the

performance of a coating.

Characterization of cross-linked polymers is more difficult than that of linear

polymers. Typically, only lightly-crosslinked polymer structures above their

glass transition temperature have been characterized. Research to characterize

highly-crosslinked polymers or lightly-crosslinked polymers at temperatures below

their glass transition temperature has been limited [55]. Techniques for

characterizing crosslink density include solvent-induced swelling, elastic

modulus and chemical analysis. These techniques provide a measure of the

average crosslink density and are briefly described below.

2. 1.4.1 Swelling

If an uncrosslinked polymer is soluble in a liquid, then the same polymer when

crosslinked will swell in this liquid. The theory of swelling behavior of

lightly-crosslinked polymers in solvents is fairly complete [56]. The swelling

ratio is generally expressed as

Swelling ratio = q = volume of swollen gel . volume of unswollen gel (2)

The number average molecular weight between crosslinks, M , is directly related

to the swelling ratio. For M >> 5000, the relationship is quite good. But

for any polymer-solvent system, the generalization that as M decreases the

swelling decreases appears to hold. Hence, swelling measurements may be useful

in characterizing the network structure of highly-crosslinked polymers such as epoxy or polyurethane coatings.

6
2. 1.4. 2 Elastic Modulus

Based on kinetic theory, the equilibrium shear modulus of elasticity, G, at

temperatures well above Tg is related to the crosslink density by [56]

G = dRT/M <1-2M /M >, (3)

where d is the density, T is the absolute temperature, R is the gas constant,

and M is the molecular weight of the uncrosslinked polymer. Although this

equation is valid only for lightly-crossed linked polymers, the elastic moduli

of highly-crosslinked polymers appear to be nearly independent of the chemical

structure of the network and to depend primarily upon the tightness of the

newwork structure [56]. Thus, the elastic moduli of highly-crosslinked

polymers may provide a good empirical assessment of the degree of crosslinking.

2. 1.4. 3 Chemical Analysis and Other Methods

Chemical methods used to characterize crosslink density are based on the

principle that if one knows the concentration of a crosslinking agent, and if

it reacts completely according to one's hypotheses of the chemical reaction,

then it is possible to estimate the average M . Among chemical methods that

have been employed with limited success are U^, IR, X-ray, Raman and NMR

spectroscopies [57].

Other methods of determining degree of crosslinking involve measurements of

glass transition temperature, creep [58], and mechanical damping [59]. These

techniques are largely empirical. In spite of the lack of good theoretical

justification, these methods are useful because of their sensitivity and ease of

measurement, especially if they are calibrated by other techniques such as swelling or shear modulus.

2.1.5 Microscopy

In addition to overall surface deterioration of coatings, localized surface

defects such as craters, pinholes, and vortex cells mar the appearance of

coatings and may become sites for coating failures. Microscopy, both light and

scanning electron microscopy (SEM) , has been used extensively to follow

changes in surface appearance and to detect and analyze surface defects [60].

For example, light microscopy has been used to monitor changes in time of the

surface topology of epoxy coatings exposed in laboratory salt spray and

sulfuric acid immersion tests [51]. Other studies have reported on the effects

of chalking on the microscopic surface appearance of coatings [61,62,63], the

detection of craters in coating films [64], and the spread of corrosion beneath

an acrylic film [65].

2.2 COATING/ SUBSTRATE INTERFACE

Deterioration of many coating systems begins at the interface between the

coating and the substrate. Thus, methods that can be used to detect changes at

the interface may provide information on early deterioration of a coating

system. Many techniques have been used to study the metal/ coat ing interface

7
[66,67], In addition, research is being done to expand the use of established

surface analytical techniques, such as secondary ion mass spectroscopy (SIMS)

and variable angle ultrasoft X-ray fluorescence spectrometry, to inter facial

studies [68,69], Nondestructive and destructive techniques that have been used

to assess changes at the coating/substrate interface are discussed below,

2.2.1 Infrared Thermography

Corrosion products on steel substrates and air- or water-filled blisters

beneath intact coatings can be detected nondestructively using infrared

thermography [70], In this procedure, coated specimens are heated to a temperature 20-50° C above ambient temperatures and evaluated using a thermographic

camera. Due to differences in emissivity and thermal conductivity, corrosion spots

and blisters can be distinguished from nondeteriorated areas of the coating

system.

The capability of this technique to distinguish corroded from non-corroded

areas increases as the temperature of the substrate increases , but decreases as the coating thickness increases [71]. For example, corrosion spots on steel are easily detected under a 0.15 mm (0.006 in.) thick layer of pigmented epoxy

coating, when the steel panel is heated to 60° C, but barely detectable when the

panel temperature is 40°C. Similar results were obtained for pigmented

alkyd coatings on steel substrates. Corrosion spots could not be detected

under zinc-rich inorganic coatings that were on the order of 10 micrometers

thick.

Air-filled blisters, that may or may not be detected visually, beneath organic

coatings on wood and metals are easily detected thermographically . Water-filled

blisters under coatings on metals can be detected but are difficult to distinguish

in thermographic images from corrosion spots.

The spatial resolution of the method depends on the area of the field of view

of the camera. Up to a point, the smaller the imaging area, the smaller the

defect that can be detected. For example, when the imaging area is about 20 mm

by 20 mm, rust spots or blisters having a 0.5 mm diameter can be detected.

Advantages to the use of infrared thermography include its ease of operation,

interpretation, and analysis of images [72]. The major disadvantage of

infrared thermography is that a coating must be transparent to radiation in the

wavelength range of the thermographic detector in order for corrosion products

to be detected beneath the coating; blisters are detected under opaque coatings.

2.2.2 Thermal Wave Microscopy

Thermal wave microscopy (TWM) can be used to detect minor variations in thermal

properties of materials. The basic processes involved in a TWM experiment are schematically illustrated in figure 2 [73]. The sample is exposed to a focused, modulated beam such as a laser or an electron beam such as one in a scanning electron microscope. As the surface is scanned by the beam, energy is

absorbed by the sample causing local periodic heating and thermal expansion of

the surface. The periodic heating results in the formation of diffuse highly-damped

thermal waves, whose wavelength is a function of the thermal conductivity,

8
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Figure 2. Schematic representation of thermal wave microscopy

9
density, and specific heat of the material and the modulation frequency of the

beam. Thermal waves behave similarly to other propagating waves in that

features of materials different from the bulk will reflect and scatter them.

Images of these features are obtained by detecting the thermoacoustic waves produced in the sample by the thermal waves.

The resolution of thermal wave microscopy is controlled by the spot size of the

beam and the thermal wavelength. The resolution obtained for thermal conductors

ranges from 0.2 mm for a modulation frequency of 100 HZ to 200 nm for a modulation frequency of 100 MHz. For poor thermal conductors, e.g., organic

coatings, the resolution ranges from 0.02 mm at 100 HZ to 20 nm at 100 MHz. The

depth of penetration of thermal waves is controlled by the signal-to-noise

ratio and the thermal wavelength. The depth of penetration is typically about

4 times the spatial resolution.

In early investigative work [74], small corrosion spots (5pm diameter) under

thin (2.5pm) pigmented alkyd coatings and thicker (10pm) clear coatings were observed. This method provides good resolution of defects, but may have

limitations imposed by scanning electron microscopy, that is, sample size, high

vacuum, and heating effects. The use of a laser beam, instead of an electron beam,

may solve these problems.

2.2.3 Acoustic Emission

Acoustic emission (AE) is a technique that has been used for detecting crack

formation and propagation in materials. It has been used extensively for

evaluating crack growth in metals. Recently, the technique has also been

applied to characterizing coating deterioration [75]. The principle of the

technique is that any sudden movement within a material results in the release

of acoustic energy which may be detected by a piezoelectric device.

The signals produced are complex, although in principle they can be characterized

by their shape, frequency, amplitude, and the decay rate. Frequency and

amplitude distributions were measured in the reported work on coatings. In

these experiments, acoustic signals generated by pulling paint/substrate

samples in tension were detected. Differences in acoustic emissions of an alkyd coating on aluminum foil were detected between unexposed samples and

samples exposed for 300 hours and 600 hours in a xenon-arc weathering machine

[75]. Differences in acoustic emissions were also observed between dry

coatings and wet coatings (coatings tested immediately upon removal from a 100%

relative humidity environment) and between epoxy coatings applied to phosphated

and nonphosphated steel after weathering [75]. Research to detect acoustic

emissions associated with the formation of interfacial corrosion and the subsequent

microcracking of the paint above the corrosion products have not, as yet, been

successful [76,77].

2.2.4 X-Ray Photoelectron Spectroscopy

X-Ray photoelectron spectroscopy (XPS), a sensitive surface analytical technique

discussed in section 2.1.2, has also been used for interfacial studies [47].

In most of these studies, the coating was removed from the substrate and the

interfacial areas of both the substrate and coating film were examined.

10
Although a destructive method, it has provided useful information in the

analysis of corrosion- induced adhesion loss of coatings and adhesives. For

example, Dickie and coworkers [16] used XPS to show that, for some coatings,

corrosion- induced delamination occurs within the coating close to the interface.

They also found that the rate of delamination could be decreased by increasing

the polymer crosslink density or by phosphating the steel surface. Other

applications of XPS include studying 1) the interaction between iron oxide

surfaces and polybutadiene coatings [78], 2) the degradation of coatings on galvanized steel [47], 3) the adhesion mechanism of chromated and nonchromated

tin protected by organic coatings [79], and 4) adhesion improvement and

corrosion reduction of coated metals associated with pretreatments [80].

2.2.5 Infrared Spectroscopy

Both reflection/absorption (RA) and attenuated total reflection (ATR) have been

shown to be sensitive characterization techniques for interfacial studies. The

literature on the applications of infrared spectroscopy to the study of

polymer/metal interfaces has been reviewed recently [4,81]. In addition to the

studies of polymer degradation discussed in section 2. 1.1.1, RA spectroscopy

has been used in studies of the orientation of polymer molecules on metal

surfaces [82,83] and the interactions between polymer molecules and metal

surfaces [84 ,85 ]

.

2.2.6 Auger Electron Spectroscopy

Auger electron spectroscopy (AES) measures the Auger electrons emitted from a surface, induced by electron bombardment of the surface [86]. It is another

surface analytical technique that has often been used as a destructive method

for studying the coating/metal interfacial region. Changes in the type and

concentration of elements present on the surfaces examined have been determined.

The sampling depth of AES is about 2 nm and the spatial resolution is on the

order of 100 nm. AES is sensitive to elements with atomic number greater than

2. A disadvantage of AES is that it may cause decomposition of metal oxides

and organic materials due to the high energy of the electron beam. Nevertheless,

van Ooij [47] has used this technique, in combination with x-ray diffraction and

XPS, to study the mechanisms of failure of zinc-rich primers on steel and the

adhesion of paint to galvanized steel.

2.2.7 Ellipsometry

Ellipsometry is an optical technique that can be used to measure the thickness

of a metal-oxide film beneath a coating by determining the change in the state

of polarization that light undergoes when it is reflected by the metallic

surface [87]. Although chemical information is not obtained using ellipsometry,

Ritter [88] has shown that ellipsometry combined with electrochemical measurements

can be used to follow changes in the metal-oxide film thickness and detect the

development of remote cathodic areas under a transparent film, thereby aiding

in the understanding of the mechanisms of delamination of coatings on steel.

11
2.3 ADHESION

For a coating to perforin any of its functions, it is necessary that the coating

remain bonded to the substrate. To characterize changes in bonding strength,

it is helpful to understand the mechanisms by which debonding occurs. Several

mechanisms have been proposed to explain the reduction or loss of bonding of

coatings to substrates. For coatings on metals, these mechanisms are related

to displacement of the coating by water at the interface [89], commonly

referred to as adhesive failure, and to debonding associated with the high

concentration of alkali at the interface due to corrosion reactions, commonly

referred to as cathodic delamination. The reported mechanisms of cathodic

delamination include displacement of the coating by aqueous solutions [90,91],

degradation of the coating resin adjacent to the interface [92,93], and

degradation of the protective metal-oxide film [94,95]. The mechanism has also

been reported to depend on the resin type [96,97], the corrosion environment

[98,99] and the pretreatment [100]. Although factors controlling cathodic

delamination have not been completely determined [101,102], it appears that the

chemical properties of the resin and the corrosion environment are the primary

factors controlling the mechanisms of cathodic delamination.

Much less work has been done in studying the mechanism of adhesion loss of

coatings on non-metallic substrates. Water displacement, degradation of the

resin by alkali, and deterioration of the substrate surface, such as UV

oxidation of wood, are the primary proposed mechanisms of debonding of coatings

on wood and concrete [103,104,105].

2.3.1 Techniques for Measuring Bond Strength

In this section, methods to measure a so-called "practical" adhesion of coating

systems are discussed [106]. Practical adhesion combines factors associated

with adhesive and cohesive failure and other properties of the coating system.

These methods have been reviewed recently by Bullet and Prosser [107]. Often

these methods do not provide results that can be used to distinguish between

adhesive or cohesive failure of the coating system but they do provide a technique for assessing change in practical bonding strength of a coating

system as it ages and may be useful in the selection and specification of

coatings

.

2. 3. 1.1 Destructive Techniques

Several standard testing procedures are available for destructive adhesion

testing. These include the "x" and cross-cut tape tests, the knife stripping

tests, and direct pull-off tests, such as ASTM D3359, ASTM D2197 and ISO 4632,

respectively. Results of tape testa tend to be highly subjective. However,

they are widely used in the paint industry due to their simplicity and their ease of sample preparation. Pull-off tests, in which either laboratory universal

testing machines or portable devices are used, are commonly used methods to

measure tensile adhesion [108,109]. Although these techniques yield quantitative

results, the results are seldom reproducible [110]. Several factors influence

the results including substrate stiffness, loading fixture design, rate at

which the assembly is pulled apart, secondary adhesive, and assembly alignment

[111,112]. Because a loading fixture is cemented to the coating surface with

12
an adhesive, this method is suitable to in situ measurements in which the

adhesive will cure.

Another destructive adhesion test is the "blister test" [113]. In this test,

gas pressure is applied to the underside of a coating through a small hole in

the substrate. The pressure required to lift a coating from the substrate is

measured and the interfacial energy is calculated. Based on a limited amount

of research, the reproducibility of this method appears to be good [114]. However,

rather elaborate sample preparation is required.

2. 3. 1.2 Nondestructive Techniques

One nondestructive technique used to follow the deterioration of a coating/substrate

bond is the ultrasonic-impedance technique [115]. In this test, a shear wave

pulse of 5 to 30 MHz is transmitted obliquely to the coated surface of a substrate with nearly homogeneous reflectance properties. It is reflected off

the surface opposite the prism surface from which it was transmitted and

returned to the piezo-electric crystal, via the surface of oblique incidence.

Changes in the intensity of the reflected pulse appear to correlate with

changes in coating properties including adhesion. This technique has been used

to follow the cure, setting [116], and adhesion failure [117] of various types

of coatings in controlled environments on a quartz substrate. Although the

results of early work were encouraging, its use appears to be limited, since the

technique requires that the substrate have nearly uniform reflectance properties.

In another nondestructive method, reflected acoustic pulse, delaminations were observed in an epoxy/ steel coating system [118]. By scanning the transducer

over the specimen, coating delaminations were detected and the thickness of the

coating (0.2 mm to 0.6 mm) and the depth of each delamination were determined.

Funke [119] proposed another nondestructive adhesion test, the cross-over time

test. In this test, water up-take of fixed and free films placed in a high

humidity environment is measured as a function of time. The time at which the

up-take of water by the fixed film equals that of the free film is called the

cross-over time. Loss of adhesion is assumed to be associated with the greater

up-take of water of the fixed film. Thus, the cross-over time may indicate

when the coated metal begins to lose adhesion if the primary mechanism of

adhesion failure is water displacement of the coating.

2.4 MECHANICAL PROPERTIES

Mechanical properties of coatings are known to be related to the chemical

composition of a coating [120]. Thus, changes in mechanical properties that

accompany aging may be useful indicators of early coating deterioration and

predictors of service life. Researchers have measured changes in mechanical

properties, such as internal stress, internal strain, tensile stress-strain,

and dynamic moduli, in efforts to relate these changes to durability. Results

of their works are discussed below.

13
2.4.1 Internal Stress

As a coating cures, via solvent-loss, crosslinking, or oxidation, stresses are developed in the coating system. These stresses have been measured to range

from 3.0 to 14.0 MPa (500 to 2000 psi) [121,122,123]. As the coating system

ages, the strength of the film usually decreases and, in some cases, the

stresses in the film increase. Failure of the film occurs when the internal stress

exceeds the adhesive or cohesive strength of the film. Typical failure modes

include cracking, peeling, and blistering.

Internal stress can be measured in several ways. The most common techniques

use a freely supported cantilever beam [124,125,126], a clamped cantilever beam

[122,124,127,128], or photoelastic material in which either the substrate

[129,130] or the polymer [131,132] is the photoelastic material. Internal

stress equations for the freely supported and clamped cantilever beams have been

derived by Corcoran [124] using plate theory. For a freely supported or clamped cantilever beam, the coating is applied to one side of a metallic gauge

stock and allowed to cure. As the coating cures, stresses develop in the film

causing the metallic substrate to deflect. Deflection is usually measured with

a microscope.

Knowing the moduli of elasticity, Poisson's ratios, and the thicknesses of the

metal and coating, and the deflection of the gauge stock, one can compute

internal stress. Results of internal stress measurements from clamped and

freely supported cantilever beam experiments compare well [127,128].

Photoelastic methods have been used mainly by Russian researchers. In these

methods, a glass or transparent amorphous polymer is used as a photoelastic

substrate for the coating. As the coating cures, stresses develop in the

photoelastic material changing its birefringent pattern. The principal

stresses in the photoelastic material are computed from this birefringent

pattern [ 133 ] .

Many material variables affect internal stress including the resin type,

molecular weight, modulus of elasticity, Poisson's ratio, filler concentration,

plasticizer concentration, and coating thickness. Of these variables, the

effects of plasticizers, including the solvent, fillers, and coating thickness

on internal stress have been studied most. In general, as plasticizers

are lost, the magnitude of the internal stress in the film increases [121,134].

In contrast, up to a critical concentration level, as the concentration of

filler increases, the magnitude of the stress in the coating tends to increase

[127,128,135]. At concentrations above this critical level, internal stress

decreases. Coating thickness also influences internal stress. As the thickness

of the coating increases, the magnitude of the stress at the interface between

the substrate and the coating increases [130, 136]. Environmental variables,

such as temperature, humidity, and ultraviolet radiation, also affect the

internal stress in the coating. In general, the higher the temperature or relative humidity, the lower the stress in the film [122]. Ultraviolet

radiation tends to cause scissions on the surface of the polymer which effectively

reduce the coating thickness. The longer the exposure to ultraviolet radiation,

therefore, the greater the decrease in the internal

stress in the coating [122].

14
In related procedures, strength and elongations are typically measured using

free films, although coatings are often exposed on tin substrates. In this

case, free films are obtained by amalgamating the tin prior to testing. Both

strength and elongation typically decrease with weathering, with a greater

decrease in elongation than strength. Thus, elongation is often the preferred

indicator of deterioration [137,138].

2.4.2 Residual Strain

Residual strain is relatively easier to measure than internal stress and is

used as an alternate method to determine internal stress in coatings

[135,139,140,141,142]. The rationale behind measuring residual strain is that

the coating will not fail as long as the residual strain remains below or does

not exceed the interfacial work of adhesion [142].

Residual strain has been determined in two ways: 1) theoretically using plate

theory [143], and 2) experimentally by applying a coating to a tin substrate,

amalgamating the tin, and then observing dimensional changes in the film

[139]. Limited available data shows that residual strain increases as solvent

is lost and decreases as filler concentration increases [135,139].

2.4.3 Dynamic Tests

Dynamic mechanical tests, such as torsional braid analysis (TBA) and those in

which a sample is flexed or stressed at a constant rate are being used to study

the curing processes of coatings and follow changes in mechanical properties

during aging [137,144]. In torsional braid analysis (TBA) tests, torsional

oscillations are induced in the specimen and the natural frequency and decay of

these oscillations are measured. From these measurements, storage and loss

modulus properties are calculated. TBA measurements have been used for

determining the time for thermo-oxidative embrittlement of poly (phenylene

oxide) [144]. Other dynamic flexing tests, such as cyclic bending [136], have

been used with limited success in predicting cracking in exterior exposures on wood

.

3. SUMMARY AND RECOMMENDATIONS

Techniques for detecting early changes in chemical properties of coating

systems are summarized in figure 3. Of these techniques, electron spin

resonance (ESR) and fluorescence appear to be most sensitive. The information

obtained from these techniques, however, is often complex, hard to interpret,

and qualitative. X-ray photoelectron spectroscopy (XPS) provides early

detection of surface deterioration, but sample preparation and measurement

technique are difficult. Infrared spectroscopy provides more detailed chemical

information than ESR, fluorescence, or XPS, but it is less sensitive to

chemical changes. Very little chemical information can be deduced from SEM

,

surface free energy, and gravimetric techniques.

Adhesion and mechanical properties are known to be related to service life.

However, present methods to measure practical adhesion are subjective and the

observations are often hard to reproduce. Analytical techniques that can

15
TESTING TIME

(hours)

A

Visible changes

(gloss, cracks, color)

Artificial weathering combined

with physical methods

Outdoor weathering io^-

Artificial weathering io^- 8EM, TEM

UV-VIS

iq2-

IR (FTIR, ATR, RA, PA?)

Surface tension, gravimetric

10- XPS

1- On-line MS

Fluorescence lifetime

ESR

0.1-

Figure 3. Summary of characterization procedures with respect to

their ability to detect early degradation of coatings.

16
detect changes in interfacial bonds may be more suitable for assessing changes

in adhesion than practical adhesion measurements. In situ mechanical property

measurements are possible and limited research indicates that changes in some mechanical properties may be useful in predicting the service life of coating

systems

.

Among the techniques used to assess deterioration of the interface, infrared

thermography and thermal wave imaging combined with image analysis appear to be

the most promising. These techniques complement each other in that infrared

thermography can assess corrosion and blistering under thick coatings, as well

as visible engineering properties, such as corrosion, blistering and cracking,

while thermal wave imaging can detect micrometer sized defects under thin

coatings. Reflectance infrared spectroscopy can be used to obtain detailed chemical

information of thin coating/ref lective metal substrate systems while destructive

techniqes such as X-ray photoelectron and Auger spectroscopies provide complementary

chemical information.

Many of the analytical techniques described in this report have been used

successfully to characterize the degradation and predict the durability of

relatively pure poljnneric materials [145]. Unfortunately, applications of

these techniques to the study of coating systems is limited. Since coating

systems are usually more complex than polymers, it is difficult to assess the

relative merits of each of the techniques for characterizing coating system

degradation.

In order to develop charaterization procedures that can be used to improve

selection, specification, and quality assurance of coating systems, it is

necessary to develop 1) procedures that can be used to quantitatively evaluate

the engineering properties of a coating system, 2) procedures that are capable

of quantitatively detecting early changes in a coating system which can be

related to engineering properties, and 3) mathematical relationships between

the changes in the chemical properties and the engineering properties. It is

unlikely that a single characterization technique will satisfy all three

requirements; instead the success of this endeavor will be greatly increased if

a systematic investigation of degradation of coatings is used in which the

factors causing degradation are unraveled one at a time.

4.

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23
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24
NBS»n4A tREV. 2-«C) U.S. OEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

PUBLICATION OR

REPORT NO.

NBSIR 85-3293

2. Performing Organ. Report No. 3. Publication Date

FEBRUARY 1986

4. TITLE AND SUBTITLE

Non-Electrical Measurement Techniques for Assessing the State of Coatings Systems

Deteriorati on

5. AUTHOR(S)

M.E. McKnight, T. Nguyen, J.W. Martin

S. PERFORMING ORGANIZATION (If joint or other than N6S. see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. N0002585WR06000

8. Type of Report & Period Covered

NBSIR FY85

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

Naval Civil Engineering Laboratory, Port Hueneme, CA 93043

ATTN: Code LOS

10.

SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Nonelectrical methods used to characterize early degradation in coating systems were

reviewed and critiqued with respect to their abilityto provide predictive in-service

performance data. The methods -reviewed were classified into those that measure

chemical changes, coating/substfate interfacial changes, and adhesion and mechanical

properties. Although many methods are used to characterize coating system degradation,

very limited research has been done to relate early property changes to in-service

performance. It was concluded that because of the complexity of the degradation of

coating systems, a combination of methods will be needed to characterize early

degradation to the extent that service-life prediction of coating systems can be

based on these measurements.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key

deterioration; evaluation; nieasurement; paints, protective coatings

words by semicolon s.

13. AVAILABILITY 14. NO. OF

V PRINTED pages

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Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $9.95

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