Mechanical propertiesof freestanding PET-film

Axel van ZweedenInternship MT05.39

Ir. M.J. van den Bosch

July, 2005

Contents

1 Introduction 2

2 Influence of HCl on PET 32.1 Separating HCl from steel . . . . . . . . . . . . . . . . . . . . . . . 32.2 Testing the specimens . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Preparation of specimens 63.1 Punching specimen . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2 Spraying specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3 Advantages and disadvantages . . . . . . . . . . . . . . . . . . . . . 7

4 Strain-rate dependency 84.1 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.2 Numerical models . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.2.1 Geometries . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.2.2 Thickness and place of load . . . . . . . . . . . . . . . . . . 10

4.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.3.1 Numerical models . . . . . . . . . . . . . . . . . . . . . . . . 114.3.2 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.3.3 Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5 Obtaining strain-fields 145.1 Strain-field of the bulk material . . . . . . . . . . . . . . . . . . . . 145.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.3 Obtaining strain-field with light microscope and Aramis . . . . . . 165.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6 Conclusion and recommendations 18

1

Chapter 1

Introduction

The production process of a beverage can includes several forming and decoratingsteps, like deep drawing, wall ironing, applying protective films and prints. Theuse of polymer coated steel, Protact, is a way to reduce production costs. Thismaterial consists of a steel sheet that is covered with a polymer (PET) film onboth sides. In order to succesfully integrate polymer coated steel into the produc-tion process, it is necessary to know and understand properties and behaviour ofthe materials involved. To verify whether the currently used material model andmaterial parameters for the polymer are correct, tensile experiments and strain-field measurements are carried out and compared to simulation results.

For such research the polymer film has to be separated from the steel. The ques-tion rises whether the polymer is influenced by this proces in a negative way. Thisis described in chapter 2. After this test, PET taken from Protact can be tested;the preparation of specimens is explained in chapter 3. It is known that manypolymers show strain-rate dependent behaviour. In chapter 4 it is explained howit is tested experimentally. After this a numerical simulation of this test is carriedout also, for comparison. See section 4.2. Finally it is tested whether it is pos-sible to obtain a strain-field in bulk material and locally where it stretches in anon-homogenous way and thus necks. This is described in chapter 5. After eachgroup of experiments, results are given. In the end a final conclusion is drawn andrecommendations are made.

2

Chapter 2

Influence of HCl on PET

2.1 Separating HCl from steel

The PET is extruded onto the steel under certain process conditions, thereforethe mechanical properties are not necessarily identical to PET films created in adifferent process. So, the only way to test the properties of the PET film, is byseparating it from the steel. Mechanical separation without damaging the PET

Figure 2.1: Cross section of Protact

film is impossible. Therefore a chemical processis used to dissolve the steel and thechromium layers with hydrochloric acid (HCl) as a reagent. The steel dissolves asdescribed by the following chemical reaction (for convenience non-ferro componentsof the steel are omitted):

Fe(s) + 2 H+(aq) −→ Fe2+

(aq) + H2(g) (2.1)

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The chromium layer dissolves by a comparable chemical reaction.

2.2 Testing the specimens

Corus claims that PET is not affected by this HCl treatment. Corus’ claim isinvestigated by comparing specimens that are treated in two different ways: onegroup of specimens is subjected to hydrochloric acid for two weeks, the other groupdoes not get the acid treatment. The PET that is used in this experiment is madeby melting PET granulate and pressing it into thin films. Finally, all specimensare subjected to a uniaxial tensile test. For this purpose a Kammrath & Weisstensile stage with a 20 N loadcell is used. Of all the specimens the thickness ismeasured at five places. Finally the recorded data is processed with Matlab.

2.3 Results

Comparing the results of normal and HCl-specimens shows that their propertiesdo not differ. See figure 2.2. The graphs of the HCl-specimens are indicated witharrows. Two HCl-specimens fail early compared to the rest; this seems to be aninfluence of HCl, but it is not. This is due to the fact that HCl-specimens arehandled and thus damaged more, leading to early failure; moreover other PET-specimens, taken from Protact, that experienced an acid treatment do not showthis failure.

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0 0.5 1 1.5 2 2.5−10

0

10

20

30

40

50

60

Displacement [mm]

Nor

mal

str

ess

[MP

a]

Figure 2.2: Stress-displacement graph of HCl-experiments

5

Chapter 3

Preparation of specimens

3.1 Punching specimen

Specimens are created out of the PET film by means of a punching process. Theirgeometry is shown in figure 3.1. To protect the PET from damaging it is putbetween some sheets of paper: one on top and two on the bottom. Then specimensare punched out of the film.

Figure 3.1: Dimensions of a (sprayed) specimen

3.2 Spraying specimens

The specimens that are used to obtain a strain-field have to be sprayed with paintor graphite so they show a fine pattern of spots. Two factors are important forspecimen preparation: position of the specimen and distance between nozzle ofthe sprayer and specimen. Drops of different sizes leave the nozzle when spraying,but only small drops give the desired fine pattern. The best result is obtainedwhen the specimen is placed vertically; therefore is sticked to a piece of paper withtape. This indicated with an arrow in figure 3.2. Moreover the distance betweenspecimen and nozzle must be large enough (0.6-1.0 m). Then only blobs that have

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almost the desired size, circa 50–100 µm, reach the specimen.

Figure 3.2: Specimen and sprayer orientation and spraying distance

3.3 Advantages and disadvantages

Another important factor for specimen preparation is the used colourant. Threedifferent colourant are used: graphite and two kinds of paint. The used graphiteis Kontakt Chemie Graphit 33, which gives darkgrey blobs on the specimen. It isquite simple to get a fine pattern with graphite. The spots are small, as desired.A disadvantage is little adhesion of graphite: accidently touching the specimendestroys the pattern.

Two kinds of paint were used: black paint, Mipa acryl Autospray en white paint,Helling Eindringprufverfahren Standard Chek Entwickler medium nr. 3. It isharder to get a good pattern with paint, but the adhesion is better. Enlargingthe distance can have a favourable influence on the spraying result, if it is not toolarge. In the last case drops need too much time to reach the specimen, so thesolvent evaporates; then adhesion is worse. If the drops are not properly attachedto the specimen, they might not follow the deformation exactly. This will lead toincorrect strain field measurements.

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Chapter 4

Strain-rate dependency

4.1 Experiments

It is investigated whether the PET experiences strain-rate dependent behaviourat speeds achieved during tensile tests in the micro-tensile stage. The used micro-tensile stage stage cannot provide a constant strain-rate. So tests are peformedat constant velocity. The tests are carried out in the Kammrath & Weiss tensilestage with a 20 N loadcell. The following speeds are used: 1, 5, 10 en 20 µm/s.Because of the small displacement, stretch ratio λ of the specimen is circa 1; thismeans that the equation of linear strain (4.1) is applicable to estimate strain forthe specimen as a whole:

εl = λ− 1 (4.1)

Half of the wide part of a specimen is clamped, so the initial length is 27 mm Thestrain of the specimen for an displacement of 1.5 mm is 0.056. The strain-rate isestimated by dividing the speed by the initial length. This gives a strain-rate of3.7·10−5 s−1, 1.9·10−4 s−1, 3.7·10−4 s−1 and 7.4·10−4 s−1 respectively. Of coursethis is not a good estimation of the strain-rate in the transition zone between bulkmaterial and necked material. Strain and thus strain-rate are much higher there.

Specimen preparation is described in chapter 3. For a good comparison, thick-nesses of the specimens are measured at several places. With this information theinitial cross-section is known, to convert forces into stresses.

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4.2 Numerical models

4.2.1 Geometries

As mentioned before, numerical models of the strain-rate dependency tests arerun, which are verified by the experiments. This is done to check the materialmodel and the used parameters. Hereto a 3D model of the PET film is created inMSC.Marc. The Leonov material model is used to describe the PET.

In the simulations only 18

of an actual specimen is modelled, because of x- and y-and z-symmetry. The necking process is provoked by introducing a notch. Thenit will be known where necking will initiate. The mesh is finer near the neckingarea than elsewhere, to describe the stress-field more accurate. See figure 4.1Two different geometries are used in this research: one is rectangular, the other ismore like an actual specimen. See figure 4.1 and 4.2.

Figure 4.1: Geometry 1: rectangular mesh

Figure 4.2: Geometry 2: adapted mesh

The part of an actual specimen that is modelled is indicated in figure 4.3. Thearea surrounded by a solid line is modelled in geometry 1. The part indicatedwith a dashed line is included in geometry 2. Notice that just the area of aspecimen’s clamp part that was freestanding in the experiments is modelled ingeometry 2. The strain-rates in geometry 1 are not the same as in the experimentsdue to a smaller initial length (20 mm): for 1, 5, 10 and 20 µm/s they were5.0·10−5 s−1, 2.5·10−4 s−1, 5.0·10−4 s−1 and 1.0·10−3 s−1 respectively. The strain-rates for geometry 2 were the same as for the experiments. The strain in geometry1 and 2 was 0.075 and 0.056 respectively.

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Figure 4.3: Modelled part of an actual specimen

4.2.2 Thickness and place of load

From former tests of this model, it is known that the number of elements in z-direction (thickness) does not make much difference for the results, because planestress conditions are present. If the evolution of the thickness reduction is to becaptured accurately, more elements over the thickness are required. Because onlythe strain-fields are compared with experiments, one element is sufficient here.The place where the load is applied does matter, because it directly affects thecomputed strain. This effect is made clear in figure 4.4. The right graph shows

0 0.5 1 1.50

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Displacement [mm]

Ten

sile

forc

e [N

]

Figure 4.4: Effect of the location of the applied load

what happens, if the load is applied on the left side of the fine mesh. The middlegraph is the result of putting the load on the left of the less fine mesh; the leftgraph appears if the load is put on the far left of the mesh. This makes clear thatYoung’s modulus, maximum force and slope of the graph after necking differ fordifferent locations of the load. This is due to the amount of elastic energy that is

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stored in the specimen during the load process. If a smaller part of the specimenis deformed, less elastic energy is stored and so less elastic energy is suddenlyreleased when necking initiates.

4.3 Results

4.3.1 Numerical models

The results of simulations show a clear result: the maximum force and force levelafter necking increase. See figure 4.5. The adapted geometry does not makemuch difference for the results: the graph is almost identical. The general trendis the same: maximum force and force level increase too. Due to problems thesimulation did not reach an elongation of 1.5 mm. A lack of convergence causedthese problems. See figure 4.6.

0 0.5 1 1.50

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Displacement [mm]

Ten

sile

forc

e [N

]

Figure 4.5: Force-displacement graph of rectangular model

4.3.2 Experiments

As expected, PET shows speed dependent behaviour in the experiments. Seefigure 4.7. The maximum force and force level after necking increase for higherspeed. The graph of 20 µm/s differs from this trend. Probably because of thesmall number of tests.

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0 0.5 1 1.50

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Displacement [mm]

Ten

sile

forc

e [N

]

Figure 4.6: Force-displacement graph of adapted model

0 200 400 600 800 1000 12000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Displacement [µm/s]

For

ce [N

]

20 µm/s

10 µm/s

5 µm/s

1 µm/s

Figure 4.7: Force-displacement graph of strain-rate experiments

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4.3.3 Comparison

The maximum force in the 20 µm/s is the highest in the simulations. This isdifferent in the experiments. Probably this difference is due to the small amountof tested specimens. The simulations do not show a stable force level after neckingas in the experimental results. Because the simulations stopped too early, such astable force level could not be reached.

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Chapter 5

Obtaining strain-fields

5.1 Strain-field of the bulk material

Deformation of a specimen during a uniaxial tensile test consists of two phases:the first phase lasts until necking starts: the strain is more or less homogenous.The second phase starts when necking initiates: the strain-field becomes non-homogenous. First the bulk material is studied, which is the part of a specimenthat does not neck. It is tried to obtain an evolving strain-field by performing auniaxial tensile test and recording the deformation of the sprayed specimens. Awhite piece of paper is put underneath the specimen for a good black and whitecontrast. The strain-fields are determined by using Aramis, which is a DigitalImage Correlation (DIC) package. In the experiments pictures of the uniaxialtensile test are taken with a constant interval.

5.2 Results

As described in chapter 3, two different colourants were used for spraying thespecimens. Black paint gave little success despite a quite fine pattern of spotson the specimen; Aramis could not compute a strain-field in the bulk materialfor the whole tensile test, because Aramis could not correlate successive picturesrepeatedly. The use of graphite was successful. Aramis succeeded in computing astrain-field before and after necking. Images showed a maximum strain of almost3 % before necking which dropped to less than 1 % after necking. See figure 5.1.Figure 5.2 shows clearly that problems occur for the necked part of the specimen.In the neck there is no fine pattern, so Aramis cannot correlate it.

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0 0.5 1 1.5 2 2.5 3 3.5−0.5

0

0.5

1

1.5

2

2.5

3

Displacement [mm]

Str

ain

[%]

Figure 5.1: Strain as a function of displacement for the point indicated on theright

Figure 5.2: The fine pattern disappears in the neck

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5.3 Obtaining strain-field with light microscope

and Aramis

To determine the non-homogenous strain in the neck, an uniaxial tensile test isperformed under a light microscope. Hereto specimens are sprayed very fine, toobtain the desired resolution of the strain-field. Images are taken by a cameramounted on top of the light microscope, which are processed by Aramis after-wards. Not the whole specimen can be visualized under the light microscope dueto the magnification. So before starting an experiment, it is tried first to get anindication where necking will start. Then the microscopes camera is focused onthis area.

The specimen gets out of focus after starting the tensile test due to stretchingof the specimen. This can be corrected by stretching the specimen with the tensilestage until circa 10 % of maximum force, after which the camera can be focused.Then the tensile stage is moved to its original starting position, after which the ac-tual tensile test can be performed. The tensile tests are performed with a Debenmicro-tensile stage, with a velocity of 1 mm/min. The white paint is used forspraying the specimen and for contrast a black paper is put underneath. Thelight microscope uses a 5 x magnification and polarized light, which prevents lightscattering of the transition between bulk material and neck. Then two hundredpictures are taken of the specimen that is subjected to an uniaxial tensile test, witha time interval of one second. Afterwards all pictures are processed with Aramis,which uses the 2D mode, to obtain the strain-field of necking PET.

5.4 Results

The method used for obtaining a strain-field in the bulk material was not succesfulfor determination of a strain-field in necking PET. During necking the strainsincrease enormously, making the fine pattern disappear. See figure 5.2. ThereforeAramis could not correlate spots that entered the neck. So a different approachhad to be used. As described before, a tensile test with a light microscope was thealternative. Instead of graphite, black and white paint were used. The combinationof the latter colourant with a black paper underneath gave the best pictures.They were much clearer due to the magnification of the light microscope, whichis promising. See figure 5.3. Nevertheless processing the pictures in Aramis gavelittle success. Now it is tried to let Matlab compute a strain-field of necking PET.

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Figure 5.3: Almost invisible transition in white-sprayed specimen

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Chapter 6

Conclusion and recommendations

The HCl-experiments showed that hydrochloric acid does not affect the mechani-cal properties of PET. Therefore the rest of the experiments could be carried outwithout a negative effect of acid.

The strain-rate experiments showed an increasing force, if the speed was increased.In the experiments only the graph for 20 µm/s differed. Probably more experi-ments will give the right graph. The simulations can be improved, if the time stepis decreased, but this will require more CPU-time. Also two different loadcases inone model can be applied: until necking just the fine mesh is loaded; this preventsa storage of elastic energy in the rest of the model, which causes problems in thesimulation; then after necking the whole model can be subjected to the load. Athird way of improvement is using a finer mesh. More CPU-time is required thenas well.

The strain-field determination was succesful for the bulk material. Although thepictures taken by the light microscope’s camera were promising. Probably by usingMatlab, a strain-field for necking PET can be obtained too.

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