Journal of Food Engineering 105 (2011) 530–536

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Journal of Food Engineering

journal homepage: www.elsevier .com/locate / j foodeng

Development of a novel method to measure the film thickness of cured can coatings

Janine Wagner a,⇑, Thomas Moschakis b, Phillip V. Nelson a, Bronislaw L. Wedzicha a

a School of Food Science and Nutrition, University of Leeds, Leeds LS2 9JT, UKb Department of Food Science and Technology, School of Agriculture, Aristotle University, GR-541 24 Thessaloniki, Greece

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 December 2010Received in revised form 11 February 2011Accepted 14 March 2011Available online 21 March 2011

Keywords:Film thickness measurementCan coatingConfocal laser scanning microscopy

0260-8774/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2011.03.017

⇑ Corresponding author. Tel.: +30 2310 383536; faxE-mail address: jean.i@gmx.de (J. Wagner).

A new technique was developed for measuring the thickness of can coatings applied to a metal substrate.A metal disc with a known radius creates a small indent in the coating, stopping exactly when the coatingthickness is cut through. The indent length is employed to calculate the coating thickness. Accurate mea-surement of thin films is achieved as the indent is much larger than the respective film thickness. Thethickness of epoxy phenolic coatings on tinplate were measured and the results were validated by micro-graphic cross section analysis and by confocal laser scanning microscopy (CLSM). The coating thicknessesmeasured using the indentation method coincide with the film thicknesses measured by CLSM, whereasdiscrepancies were observed using the micrographic cross section analysis. The new technique, presentedin this paper, uses simple equipment, is inexpensive, fast and easy to use, and is suitable for film thick-ness measurements in laboratories and production plants.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Coatings for food and beverage cans are thin solid polymericfilms with thicknesses of 5–10 lm (Oldring and Nehring, 2007).Film thickness is an important parameter which determines, to-gether with other factors, the quality of metal protection (Turner,1998a). As part of our on-going studies to understand and modelthe migration process from can coatings and their interactionswith foods, it was critical to have an accurate and convenientway to measure the thickness of the many coatings under study.

Various methods have been employed to measure the filmthickness of thin films on metal substrates, and they are classifiedmainly as destructive and non-destructive methods. Destructivemethods can be divided into destructive-gravimetric and otherdestructive methods. Turner (1998b) reports that destructive-gravimetric methods are likely to give the most accurate results.This is because can coatings are usually very thin films, whichmakes it difficult to measure their film thickness by non-destruc-tive methods. However, the destructive-gravimetric method,which is basically scraping the coating of the substrate, only deter-mines the average film weight rather than the film thickness at aparticular point, which may vary significantly. Other minimaldestructive methods, such as the Gardner needle thickness gage,have been developed (Sward, 1972), but are only limited applicableto measure accurately very thin films, such as can coatings.

Non-destructive methods are manifold and based on differentprinciples. Most of the techniques need calibration with samples

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of known thicknesses, such as magnetic and electromagnetic thick-ness gauges, which are described in detail by Wenzler (1995). Forthe measurement of film thicknesses with ultrasound the densityof the coating, as well as the material properties of the substratehave to be known (Dixon et al., 2006).

Using a microscope to measure the film thickness is not some-thing new; i.e. it has been used for many years. The ‘classical meth-od’ of determining film thickness employs wax or a resin, in whichthe sample is embedded or mounted (Wenzler, 1995). A cross-sec-tion of the sample is prepared by grinding and polishing the sur-face. At the end of that procedure, the film thickness can bemeasured using a conventional stereoscope with a calibratedgraticule.

A more advanced and very powerful microscopy technique isconfocal laser scanning microscopy (CLSM), which is mostly oper-ated in three modes; transmission, reflectance, and fluorescence.Fluorescence is the predominant mode for the microscopy of foodmaterials and biological applications (Blonk and Vanaalst, 1993;Ferrando and Spiess, 2000). It is clearly a versatile method as onecan examine structures (Chen et al., 2004, 2006; Hepler and Gun-ning, 1998; Macierzanka et al., 2006; Nayebzadeh et al., 2006),but also dynamic processes (Moschakis et al., 2005, 2006). Themeasurement of film thickness has been carried out on variousmaterials, such as the cuticle of apples (Veraverbeke et al., 2001)and liquid thin polymer films (Yeong et al., 2006).

The great advantage of the non-destructive methods is that theydo not damage the film, which can then be used for further inves-tigation. It is also important for industry to have a method for qual-ity assurance that can be build into the production line withoutdestroying the product. However, it is also clear that those tech-

J. Wagner et al. / Journal of Food Engineering 105 (2011) 530–536 531

niques are rather expensive and, thus, not always practical for re-search laboratories.

This paper describes a fast, accurate, cost-effective and minimaldestructive technique for measuring the film thickness of a cancoating on a conductive substrate (tinplate). Validation of the re-sults, obtained by this new method, was carried out by comparisonwith microscopic methods (CLSM and resin-embedment method).

2. Experimental

2.1. Materials

The coating used for this study was an epoxy phenolic coating.The raw materials for the preparation of the coating were suppliedby Valspar (Grüningen, Switzerland). The generic epoxy phenoliclacquer was composed of solid epoxy resin (�27 wt.%), phenolicresins (�24 wt.%) and solvents (�49 wt.%). The solid epoxy resinwas first dissolved in the aromatic solvent, obtaining a liquid epoxyphenolic prepolymer. The epoxy phenolic prepolymer was checkedfor absence of any solid epoxy particles before proceeding with thelacquer preparation. The final generic epoxy phenolic lacquer wasobtained by mixing the liquid epoxy phenolic prepolymer with thephenolic resins, solvents, as well as a wetting agent, a wax additiveand the catalyst at room temperature. In contrast to the formula-tions used in industry, the lacquer used for this study was of basiccomposition, and some additives, mainly waxes, were left out.

Laser beam

Objective

CLSM

Laser beam

Objective

CLSM

2.2. Sample preparation

To prepare the cured, solid coating, the epoxy phenolic lacquerwas applied to an industrial tinplate with the dimension of170 mm � 80 mm (Impress, La Flèche, France) by means of a barcoater and subsequently cured (polymerized) in a standard oven(LTE Scientific, Oldham) at 200 �C for 10 min. Different coatingthicknesses (�8–25 lm) were obtained by using different bar coat-ers. To avoid heat loss when opened, the standard oven was insu-lated with glass wool (covered with aluminium foil), leaving anarrow slot (5 cm � 30 cm) for inserting the metal panel.

The term sample, used throughout this paper, refers to the fullycured epoxy phenolic coating on the tin plate. A guillotine wasused to cut the sample into a smaller size. When cutting the sam-ple, the distortion of the metal could not be avoided. In order forthe metal not to cover the coating at the cut edge and thus, possi-bly disturbing the thickness measurements of the coating, the cut-ting process was conducted in a way that the blade of the guillotinefirst cuts the coating and then the metal. Approximately 10 smallcoated metal flags (15 mm � 60 mm) were obtained from the ori-ginal coated metal panel (170 mm � 80 mm).

Cutting edge

Depth (µm)

Focal plane used for measuring the film thickness

Metal

Coating

Cutting edge

Depth (µm)

Focal plane used for measuring the film thickness

Metal

Coating

Fig. 1. Confocal laser scanning microscopy: Experimental setup. CLSM was oper-ated in fluorescence and reflection mode with an Argon laser (excitationwavelength = 488 nm). The reflection mode visualised the metal, whereas thefluorescence mode visualised the coating. The focal depth used for carrying our themeasurements in x–y plane was ca. 50 lm. The focal depth was chosen far awayfrom the cut edge of the coated panel, where the coating had fractured unevenly,due to the cutting process.

2.3. Determination of the film weight of the coating

To determine the film weight, a coated metal panel with aknown area was weighed, followed by the softening of the coatingin an appropriate solvent (acetone) in order to facilitate its removalfrom the substrate (metal). Removal of the coating was realised byabrasion using a metal sponge. After removal of the coating, the pa-nel was re-weighed and the film weight was calculated using thefollowing equation:

Fw ¼ ðW1 �W2Þ � A�1

where, Fw is the film weight of the coating in g m�2, W1 is theweight of the panel and the coating in g, W2 is the weight of the pa-nel without the coating in g, A is the area of the panel in m2.

2.4. Confocal laser scanning microscopy (CLSM)

The sample preparation consisted of preparing the coated paneland cutting a sample of a smaller size (2 � 2 cm), which the instru-ment could accommodate. The experiments were carried out usingan upright Leica TCS SP2 confocal laser scanning microscope(CLSM), mounted on a Leica Model DM RXE microscope base, oper-ated in fluorescence and reflection mode with an Argon laser (exci-tation wavelength = 488 nm). Two different detection channelswere used to collect the signal from the sample at the same timein reflection as well as in fluorescence mode. The two separateimages from the two detection channels were superimposed tocreate the combined image. The direction of the laser beam wasapplied to the cutting edge of the coated metal panel, so that theresulting image showed coating and metal side by side (see Fig. 1).

The substrate (metal) was easily distinguished from the coating,as the reflection mode only visualised the metal, whereas the fluo-rescence mode visualised the epoxy phenolic coating due to thefact that the coating is auto-fluorescent. The focal depth used forcarrying out the measurements in x–y plane was adjusted toapproximately 50 lm. When cutting the samples into size thecoating on the cut edge fractured. Thus, the focal depth was chosento be as far away as technical possible (�50 lm) from the cut edgeof the coated panel, to reduce the influence of the fractured area onthe measurement of the film thickness. The size of the images wasadjusted to 1024 � 1024 pixels. A 20x dry-objective of numericalaperture 0.7 was used to visualise the coating and the metal. Tocreate an image eight scans were averaged and the images wereprocessed with the Leica ‘QWin’ software. The image data wereconverted from pixels to micrometers. The maximum resolutionin confocal microscopy in x–y direction is approximately 0.2 lm.The film thickness of the sample was obtained by averaging fourmeasurements across the sample. Two individual samples(15 mm � 60 mm) obtained from one panel (170 mm � 80 mm)were analysed and the results were averaged.

2.5. Resin-embedment method

In addition, the resin-embedment method was used to measurefilm thickness. The coated panel was embedded into a thermosetresin. The specimen was cut and the cut edge was polished care-

532 J. Wagner et al. / Journal of Food Engineering 105 (2011) 530–536

fully to obtain a cross section of the sample. The film thickness wasmeasured by means of a conventional stereoscope. Two individualsamples (15 mm � 60 mm) obtained from one panel (170 mm �80 mm) were analysed. This procedure was repeated for four dif-ferent film thicknesses. The measurements were carried out at Im-press (La Flèche, France) and the results shown were kindlyprovided by Impress.

2.6. Stylus indentometer

The stylus-indentation method used for measuring the filmthickness was developed and designed in-house. A metal stylusof �5 mm diameter with a cone angle of 120� was used to createan indent in the coating by application of a force. A stable appara-tus was needed to perform the indentation, which was achieved byusing steel blocks, as illustrated schematically in Fig. 2A. The styluswas attached to a flexible metal strip and moved up and down bymeans of a screw, which was operated by finger action. By movingthe stylus down, towards the sample, an indent was formed whenthe stylus penetrated the coating. It is important to note that thestylus did not rotate with the screw, but remained immobile. Thediameter of the indent was then used for calculating the coating’sthickness.

The further the stylus moved into the coating, the larger becamethe indent that was formed in the coating. Thus, it was importantfor the accuracy of the measurement that the penetration of thecoating by the stylus was immediately stopped when reachingthe substrate. To achieve this, advantage was taken of the fact thatboth, the stylus and the substrate were conductive, and an electri-cal circuit was applied between stylus and substrate. This electriccircuit was closed when the stylus had cut through the coatingand touched the substrate, which was immediately indicated bythe attached ammeter (see Fig. 2B). At this point, the screw wasno longer turned to avoid the stylus penetrating the sample fur-ther, and subsequently, the stylus was moved upwards to releasethe sample. During the indentation procedure any sliding of thesample was avoided by placing it on a non-slippery surface(Parafilm�).

The diameter of the indent (d) was measured using a conven-tional stereoscope with a calibrated graticule. The diameter ofthe indent was used for calculating the coating’s thickness as

d= diameter of the indent (measured with a stereoscope)

120° 60°

90°

30°a

b= d/2

c

αadjacent

oppo

site

d= diameter of the indent (measured with a stereoscope)

120° 60°

90°

30°a

b= d/2

c

αadjacent

oppo

site

Stylus

coating

metal

Force

No current flows

Indentation

Current flows

coating

metal

d

coating

Indent (top-view)

Stylus

coating

metal

Force

No current flows

Indentation

Current flows

coating

metal

d

coating

Indent (top-view)

Coating

(a) Stylus (120°)

Substrate (stainless steel)

(d) Screw

Non-slippery surface (Parafilm®)

(c)

(c)

(c)(b) Flexible metal strip

Coating

(a) Stylus (120°)

Substrate (stainless steel)

(d) Screw

Non-slippery surface (Parafilm®)

(c)

(c)

(c)(b) Flexible metal strip

(A) (B)

(C)

d= diameter of the indent (measured with a stereoscope)

120° 60°

90°

30°a

b= d/2

c

αadjacent

oppo

site

d= diameter of the indent (measured with a stereoscope)

120° 60°

90°

30°a

b= d/2

c

αadjacent

oppo

site

Stylus

coating

metal

Force

No current flows

Indentation

Current flows

coating

metal

d

coating

Indent (top-view)

Stylus

coating

metal

Force

No current flows

Indentation

Current flows

coating

metal

d

coating

Indent (top-view)

Coating

(a) Stylus (120°)

Substrate (stainless steel)

(d) Screw

Non-slippery surface (Parafilm®)

(c)

(c)

(c)(b) Flexible metal strip

Coating

(a) Stylus (120°)

Substrate (stainless steel)

(d) Screw

Non-slippery surface (Parafilm®)

(c)

(c)

(c)(b) Flexible metal strip

(A) (B)

(C)

Fig. 2. (A) Illustration of the indentometer. Stylus with a cone angle of 120� (a),flexible metal stripe (b), steel blocks (c), screw (d) operated by finger action. (B)Stylus indentometer geometry and function. The diameter of the indent (d) wasmeasured using a stereoscope. (C) Film thickness calculations using the stylusindentometer. The diameter of the indent (d) was used for calculating the coating’sthickness (a) by using the equation tan a = a/b.

shown in Fig. 2C. The indent left by the stylus was assumed to bea triangle with an angle c = 120�. The triangle was split into tworight triangles with angle a = 30�. The adjacent is known andequals the radius of the indent. The opposite is the film thicknessof the coating and can be calculated by the following equation:

a ¼ tan a� b

where, a is the film thickness, a is the angle (30�), b is the radius ofthe indent.

2.7. Disc indentometer

The disc-indentation method used for measuring the film thick-ness is an improved version of the stylus-indentation method,using a metal disc of 10 mm diameter instead of a stylus to createan indent into the coating and was also developed in-house. Thedisc was carefully pushed into the coating in the same manner asdescribed for the stylus-indentation method, stopping exactlywhen the coating thickness was cut through and the disc madeelectrical contact with the metal substrate. Finally, the disc wasmoved upwards to release the sample. During the indentation pro-cedure any sliding of the sample was avoided by placing it on anon-slippery surface (Parafilm�).

The length of the indent was measured using a conventionalstereoscope with a calibrated graticule. Subsequently, the lengthof the indent was used to calculate the film thickness of the coating(see Fig. 3). To obtain the film thickness of the sample, three mea-surements were taken at different points across the sample andaveraged. Eleven individual samples (15 mm � 60 mm) obtainedfrom one panel (170 mm � 80 mm) were analysed and averaged.

3. Results and discussion

3.1. Stylus indentometer

In this method for measuring the film thickness, a steel styluswith a cone angle of 120� was employed to form an indent in thematerial (coating) by application of force on the indenter. Subse-quently, the diameter of the indent was measured and convertedinto film thickness of the coating as described in detail in the Sec-tion 2. However, this method has a serious drawback as can beseen in Fig. 4C. Material builds up around the edges of the indent

r

b

a

r- Radius (5 mm)b- ½ indent

Calculation:a2= r2- b2

r- a= x x= (Film thickness in mm)

Shape of the indent

MetalCoating

Metal wheel

MetalCoating

ra

bx

r

b

a

r- Radius (5 mm)b- ½ indent

Calculation:a2= r2- b2

r- a= x x= (Film thickness in mm)

Shape of the indent

MetalCoating

Shape of the indent

MetalCoating

Metal wheel

MetalCoating

ra

bx

Fig. 3. Explanatory drawing of the film thickness measurement using the discindentometer. The film thickness is calculated from the length of the indent createdby the disc. The know variables are (r), which is the radius of the disc and (b), whichis 1/2 indent length. The variable (a) is calculated by using the Pythagoreanequation a2 + b2 = r2. The film thickness (x) is calculated by subtracting (a) from (r).

(A)

(B)

coating

metal

Stylus

A

B

Material pushedout by stylus

(A)

(B)

coating

metal

Stylus

A

B

Material pushedout by stylus

(C)

Fig. 4. Schematic representation of the crater-formation observed with the stylusindentometer. Picture (A) represents the ideal situation, showing the ‘real’ diameterwith no crater formation taking place. Picture (B) represents the real situation,where the diameter is falsified by the material build up around the indent.Picture (C) shows the actual image (top view) of a crater formed by indentationwith the stylus into a generic epoxy anhydride coating.

Fig. 5. Top view of the indent formed by using the disc indentometer in a genericepoxy anhydride coating. The length of the indent is representative of the coatingsfilm thickness. The measurement is not disturbed by the material build up at thesides.

Table 1Length of the indent in mm and the respective film thickness in lm, determined withthe disc indentometer, obtained from coatings prepared using different bar coaters.The value of standard deviation was calculated from 33 measurements across onepanel with the dimensions 16.5 � 6 cm.

Bar coater Length of indent in mm Film thickness in lm

3 0.57 ± 0.03 8.2 ± 0.24 0.72 ± 0.03 12.9 ± 0.96 0.90 ± 0.03 20.3 ± 0.68 1.00 ± 0.03 24.8 ± 1.3

J. Wagner et al. / Journal of Food Engineering 105 (2011) 530–536 533

and forms a crater-like structure as a result of the indentation pro-cess. Thus, it is very difficult to determine the diameter of the in-dent and subsequently the film thickness with high accuracy.

Fig. 4A illustrates an ideal case with no material build uparound the stylus, where the diameter of the indent can be usedto calculate the film thickness of the coating. Fig. 4B illustratesschematically the actual case, where coating material is pushedout by the stylus and the measured diameter of the indent (B) ismuch larger compared to the ‘real’ diameter (A). An improvementof the method was necessary to give satisfactory results.

3.2. Disc indentometer

In order to make the film thickness measurements more accu-rate the stylus-indentation method was improved and a metal discwas used instead of the conical stylus. The advantage of this formof indentation is clearly evident from a typical picture of the indentas shown in Fig. 5. The build up of material is at the side of the cut.Since the length of the indent is measured, the material build updoes not disturb or falsify the measurement. The strength of thismethod lies in the fact that the length of the measured indent isapproximately 40–70 times larger than the respective film thick-ness (Table 1), which results in a high accuracy.

The error associated with measuring the length of the cut is re-lated to the shallowest depth at each end of the cut that is discern-ible by eye under the stereoscope. The error attached to measuringthe length of the indent using the stereoscope was approximately0.03 mm at each side of the cut. As the film thickness is calculatedfrom the cut length, this systematic error has to be accounted for.Thus, the error of film thickness measurement, arising from thesystematic error of the cut length measurement is approximately0.04 lm (the resolution of a conventional microscope is 0.2 lm).This systematic error is independent of the film thickness, and is

influenced by the illumination of the sample, the scale of the eye-piece, as well as the accuracy of the observer. It is clear, however,that with decreasing thickness of the film, the influence of this sys-tematic error increases. Additionally, increasing the diameter ofthe disc could result in an even smaller error of the film thicknessmeasurement. A source for a random error could be the sliding ofthe coated panel during the indent formation. Sliding was pre-vented by placing the sample on a non-slippery surface (Parafilm�)during the indentation procedure. Another source of potential er-ror is any non-uniformity of the surface of the metal (tinplate) sub-strate. If this is not completely flat (smooth) then the disc couldstrike a high-spot of the metal surface, complete the electrical cir-cuit early, and so underestimate the film thickness.

Table 1 shows the dimension of the length of the indent as wellas the respective, calculated film thickness. The total error of themeasurement includes the systematic and random errors of themeasurement itself as well as the error of applying the film evenly.In other words, the total error given in Table 1 is a sum of differenterrors from different sources. Since the systematic error is verysmall (0.04 lm) the predominant error originates from the randomerrors (applying the film, non-uniformity of the metal substrate,sliding of the coated panel).

3.3. Comparison of the disc-indentation method with other methods

3.3.1. Resin-embedment methodThe results of the film thickness measurements using the resin-

embedment method are displayed and compared to the results ofthe disc-indentation method (Fig. 6). It can be seen that there isa noticeable difference between the results with the disc-indenta-tion method giving higher results than the resin-embedmentmethod. The difference between the film thickness measurementswith the two methods seems to increase with increasing filmthickness. It is not clear why the measurements deviate in this

3 4 6 80

5

10

15

20

25

Film

thi

ckne

ss (

µm)

Bar coater (no.)

Fig. 6. Comparison of the film thickness (lm) obtained by the resin-embedmentmethod (filled) and the disc-indentation method (white) as a function of bar coaterused. The error bars are the standard deviation.

534 J. Wagner et al. / Journal of Food Engineering 105 (2011) 530–536

way and why the deviation seems to increase with the film thick-ness measured. The resin-embedment method was developed sothat fracturing of the coating is minimised (Wenzler, 1995). How-ever, it is possible that the coating fractured while the sample wascut by the guillotine. In order to obtain accurate film thickness val-ues, the polishing process has to remove the fractured material;otherwise a lower value for the film thickness might be observed.As already mentioned, the accuracy of the resin-embedment meth-od with respect to measuring the film thickness is very muchdependent on the polishing process as well as on the resolutionof the microscope used, which, for a conventional microscope ismaximum 0.2 lm. The systematic error of the film thickness mea-surement using the disc indentometer was 0.04 lm for our system.This shows that measuring the film thickness with the disc inden-tometer should give more accurate results compared to the resin-embedment method.

3.3.2. Confocal laser scanning microscopy (CLSM)An advantage of the epoxy phenolic coating is that it is auto-

fluorescent. This made it possible for the CLSM to distinguish be-

Fig. 7. CLSM micrograph of a cross section of a metal panel (red colour) coated with afractured coating visible, expressed by yellow colour (combined image), where metalobserved. (For interpretation of the references in colour in this figure legend, the reader

tween the metal (substrate) and the coating, where the metal isvisualised by the red colour and the coating by the green colour.A particular advantage of the CLSM is the ability to observe onlya ‘slice’ of the sample’, without the destruction of the sample. How-ever, it soon became clear that it was important at which focalplane the ‘slice’ was examined. Fig. 7 demonstrates the importanceof the location of the focal plane during the film thickness mea-surements. Fig. 7A shows the focal plane at 10 lm depth, whereit can be seen that the coating (green colour) is not uniform, butseems fractured. In the combined image it can be seen that thecoating is covering the metal (red colour), which is expressed bythe yellow colour. The fractured coating is the result of the cuttingprocess during sample preparation, which unfortunately could notbe avoided. By lowering the depth of the focal plane to 50 lm, theproblem was overcome (Fig. 7B). It can be seen that the coating atthis depth is no longer fractured and its thickness can be measuredwith confidence. Hence, it was not necessary to go through theenormous effort of polishing to gain a neat cross section of coatingand metal.

The very accurate and advanced method of CLSM was used todetermine the film thickness of the epoxy phenolic coating. Fig. 8shows micrographs of four epoxy phenolic coatings of differentfilm thickness, ranging from �8 lm to �24 lm. It can be seen thatespecially for the very thin film (�8 lm) the film is not very uni-form, which seems to be mainly a result of the non-uniformity ofthe metal surface. In the case of can coatings this non-uniformitymight even result in a failure of the coating, e.g. spoilage of thefood stuff.

The results of the disc-indentation method and the CLSM resultsare compared in Fig. 9. A linear relationship between the results ofthe two methods was observed with a slope of 1.02 ± 0.03, indicat-ing an agreement between the two methods. To further investigatethe relationship, an analysis of variance (ANOVA) was carried outto compare the two methods. No statistical difference betweenthe film thickness results obtained by the CLSM and the disc inden-tometer was found (p = 0.157). Thus, the measurements carried outby the CLSM support the validity of the disc-indentation method asa tool of measuring the film thickness.

The error in film thickness seems to increase with increasingfilm thickness (Fig. 9). As the systematic error (the error of measur-ing the indent) should decrease with increasing film thickness, it is

generic epoxy phenolic coating (green colour). (A): Focal plane (depth) = 10 lm –is covered by the coating. (B): Focal plane (depth) = 50 lm – no fractured coating

is referred to the web version of this article.)

Fig. 8. Film thickness of a generic epoxy phenolic coating measured with CLSM in fluorescence mode at a focal plane (depth) of 50 lm. The metal, which is below the coating,was not visualised. Film thickness: (A) �8 lm, (B) �13 lm, (C) �20 lm and (D) �24 lm.

J. Wagner et al. / Journal of Food Engineering 105 (2011) 530–536 535

implied that the random error (e.g. applying the film) increaseswith increasing film thickness. This hypothesis, however, needsfurther experimentation before a conclusion can be drawn.

3.3.3. Film weightFig. 10 compares all obtained film thicknesses measured in this

investigation to the respective film weights of the samples. The lin-

8 12 16 20 24 28

8

12

16

20

24

28

Dis

c-in

dent

atio

n m

etho

dfi

lm t

hick

ness

(µm

)

Confocal method- film thickness (µm)

Fig. 9. Comparison of the film thickness in lm obtained by the CLSM and the disc-indentation method. No statistical difference was found for the two differentmethods.

ear relationship confirms the correlation between the film weightand the film thickness measured by the different techniques.

From this linear relationship it was assumed that the density ofthe dry coating is not affected by its film thickness; i.e. it seemsthat the structure/composition of the coating does not significantlychange with the film thickness. Thus, it was possible to extract thedensity of the epoxy phenolic can coating formulated for this studyby using the following equation:

4 8 12 16 20 24 28

4

8

12

16

20

24

28

Film

wei

ght

(g m

-2)

Film thickness (µm)

Fig. 10. Relationship between film weight and film thickness measured by differentmethods: disc-indentation method (h, broken line), CLSM (s, full line), resin-embedment method (4, dotted line).

536 J. Wagner et al. / Journal of Food Engineering 105 (2011) 530–536

y ¼ m x

where, y is the film weight of the coating in g m�2, m is the densityof the coating in g cm�3, x is the film thickness in micrometres.

A density of 1.03 g cm�3 was calculated for the epoxy phenoliccan coating from the values obtained by the disc indentometer, adensity of 1.05 g cm�3 by using the CLSM and a value of1.23 g cm�3 by using the resin-embedment method. Unfortunately,density values of a dry epoxy phenolic coating could not be foundin the literature.

4. Conclusions

In this study, a new method of measuring coating thickness on aconductive substrate was developed. The method is based onindenting or cutting into the coating by means of a small metaldisc, and named disc-indentation method. The length of the cutwas then used to calculate the respective film thickness of thecoating. The results obtained by the disc-indentation method werecompared to results obtained by micrographic cross section analy-sis. The results of the disc-indentation method are in agreementwith those from the micrographic cross section analysis by CLSM,but not with those obtained by the resin-embedment method.The reasons for the discrepancy are still unclear.

The advantages of this new method are manifold, such as highaccuracy at submicron level due to the unique measuring tech-nique. The technique can be applied easy and fast without theuse of solvents. Moreover, there is no need for knowing the coatingproperties in detail and also no need for calibration (except the ste-reoscope used). The method is applicable to pigmented and non-pigmented coatings on conductive substrates.

Acknowledgements

The authors would like to thank Philippe Tourelle for the filmthickness measurements by resin-embedment method. This workwas carried out as a part of a Defra LINK project FQS45 New Tech-nologies and Chemistries for Food Can Coatings. Funding by Defraand matching funds in kind from Valspar Corporation, ImpressGroup and H.J. Heinz are gratefully acknowledged. The contentsof this paper are the responsibility of the authors alone and shouldnot be taken to represent the views of the supporting organisations.

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