UNDERSTANDING WHY ADHESION IN EXTRUSION COATING DECREASES WITH DIMINISHING COATING THICKNESS, PART III: ANALYSIS OF PEEL TEST

Barry A. Morris DuPont Packaging and Industrial Polymers Wilmington, DE

ABSTRACT It is well known that in the extrusion coating process, peel strength to aluminum foil and other nonporous substrates decreases with decreasing coating thickness. The peel strength is found to be more sensitive to changes in thickness as the adhesion between the coating and substrate improves. An analysis of the peel test shows that changes in the critical dimension of the deformation region at the peel front may be responsible. INTRODUCTION

Extrusion coating involves extruding a molten polymer through a flat die onto a fast moving substrate and quenching with a cold roll. The performance of the resulting structure depends on a number of processing and polymer related properties. Of particular importance is the adhesion between the coating and substrate. As the coating thickness is reduced, the adhesive strength generally decreases. In part I of this study [1], we looked at adhesion to porous substrates and found adhesion is related to the amount of polymer that penetrates into the pores of the substrate. The processing parameters that most influence penetration were shown to be coating temperature and cooling in the nip. In part II [2], we examined adhesion to nonporous substrates such as aluminum foil. Cooling in the nip and stresses imposed during drawing were found to contribute to the reduction in peel strength with coating thickness.

Our analysis failed to discern the mechanism behind an observation from an earlier study [3] which showed that the sensitivity of an adhesive’s peel strength performance to changes in coating thickness increases as the bond strength of that adhesive increases. The earlier work involved blending an adhesion-enhancing (AE) additive into LDPE. As illustrated in Figure 1, the results of a statistically designed experiment (DOE) show that the modified LDPE has a steeper slope than LDPE alone when the peel strength to foil is plotted vs coating thickness. We will return to this DOE experiment in the present work. For simplicity, we will refer to it as the AE DOE.

In the present study, we will first show that these results are general; the peel strength of an acid copolymer, which has strong bonds to foil, is even more sensitive to changes in thickness than LDPE or the modified LDPE. We will turn to an analysis of the peel strength measurement for an explanation.

EXPERIMENT

We coated three resins of increasing acid functionality onto a foil laminate to confirm the earlier work showing that the peel strength to foil of better performing adhesives is more sensitive to changes in coating thickness. The three resins were LDPE, a blend of 20% AE modifier and 80% LDPE, and an ethylene acrylic acid (EAA) copolymer (9% AA, 10 MI). The AE modifier and EAA copolymer were supplied by DuPont. The coating conditions were:

• Die: Cloeren edge bead reduction die • Substrate: 13-µm OPET/19-µm tie/9-µm Al • Coating thickness: 20-50 µm • Coating temperature: 330°C for LDPE and AE-LDPE blend; 288°C for EAA • Die gap: 0.51 mm • Line speed: 122 m/min • Air gap: 127 mm • Chill Roll temperature: 10°C • Nip pressure: 0.4 MPa

The peel strength between the coating and foil was measured in the machine direction. The results are plotted vs coating thickness in Figure 2. As the strength of the adhesion increases from LDPE to AE+LDPE to EAA, the slope of the curve increases: the stronger the bond of the coating polymer to the foil, the greater is the sensitivity of the peel strength to changes in thickness.

ANALYSIS OF PEEL STRENGTH MEASUREMENT The peel test involves separating the layers at the desired interface, placing the “arms” into a tensile tester, and measuring the force required to pull the specimen apart. Peel strength is reported as the force divided by the width of the sample. Peel strength is known to be influenced by a number of factors, including pull speed, pull angle, temperature, thickness of the adhesive, thickness of the substrates, and the tensile and viscoelastic properties of the adhesive and substrates[4]. We hypothesize that the mechanics of the peel strength test is a critical factor for lower peel strength at thinner coating thickness.

We will follow the convention of Kinloch, Lau, and Williams [5] for the development of concepts key to this discussion.

Energy Analysis

An energy balance shows that the peel strength can be decomposed into contributions from the energy to create new surfaces (the fracture energy, Ga), the energy dissipated during tensile deformation of the peel arm (Ge), and the energy dissipated during bending of the peel arm near the peel front (Gdb). Mathematically, this can be written as:

θε cos1 −+++=

a

dbea GGG

b

P Eq 1

where P = peel force b = width of peel strip θ = peel angle εa = tensile strain in the peel arm Ga = fracture energy

Ge = energy dissipated during elongation of the peel arm Gdb = energy dissipated during bending of the peel arm.

Figure 3 shows a schematic of the peel test geometry. Each of the energy contributions to the peel strength is a function of thickness of the peel arm, g.

Elongation of the Peel Arm

Ge is related to the thickness of the peel arm times the area under the stress strain curve:

∫=a

dgGe

ε

εσ0

Eq 2

Here σ is the stress. If εa is constant, Ge is directly proportional to the thickness. If εa is not constant (i.e., a function of thickness), the effect of thickness becomes nonlinear.

Bending of the Peel Arm

Gdb arises from bending of the peel arm and can be significant if plastic yielding occurs. Gent and Hamed [6] studied the plastic yielding of OPET film during bending. They note that peel strength often has been observed to go through a maximum at intermediate values of thickness. Gent concluded that at low thickness, plastic yielding occurs, but the total energy that is dissipated is small. As thickness is increased, more energy is dissipated: peel strength increases with Gdb. Eventually, the peel arm gets too stiff and less yielding occurs, which leads to a reduction in Gdb and peel strength. At high thickness, no yielding occurs and Gdb is small.

Gent also proposed that the adhesion strength at the interface can affect Gdb. Low adhesion is less likely to lead to yielding of the substrate. High adhesion can lead to large amounts of energy dissipation within the substrate in the highly deformed region near the detachment:

“Plastic deformation of the stripping member can thus lead to an increase in peel strength in two ways: i) directly, by an additional force representing the work required to propagate the bend in an elastic-plastic strip as peeling

proceeds, and ii) indirectly, by causing a larger deformation in the elastomeric substrate under the higher peel forces and bringing about greater energy losses in this layer as a result.”[6]

As we shall see, higher adhesion can also lead to a greater initial angle of peel, θo, which can lead to a greater Gdb. Recently, Pascal [7] discussed the effect of thickness on bending angle and peel strength in extrusion coating.

Fracture Energy

The fracture energy, Ga, can be decomposed into the product of two factors:

Ga = Wb(1 + Φ(R, T)) Eq 3

where Wb = work of adhesion or bonding and Φ(R, T) = local energy dissipation at the peel front.

Here Wb is the bond strength. It encompasses the thermodynamic and chemical adhesion that occurs at the interface. This term is typically quite small compared to the value of Ga but can be important because of its multiplying effect. In the AE – LDPE blend system, the addition of AE adds chemical functionality to the LDPE, which we postulate increases Wb.

The second term in equation 3, Φ, is the local energy that is dissipated as the detachment front, generally referred to as a crack, moves through. A zone of deformation is created in front of the crack, as illustrated in Figure 4. Because this deformation is related to the viscoelastic nature of the polymer, Φ is a function of rate (R) and temperature (T). Φ is also a function of thickness, up to a point. As illustrated in Figure 4, when the thickness of the adhesive is small, the local zone of deformation extends across the entire thickness of the adhesive. Indeed, the size of the zone is constrained by the thickness. Small increases in thickness result in larger zones of deformation. Hence, at low thickness, Ga increases with thickness. Above a critical value of thickness, gc, the size of the zone of deformation is small compared to the thickness (right side of Figure 4); further increases in thickness have no effect on Ga. This is illustrated in Figure 5.

The critical value of thickness, gc, is related to the tensile properties of the peel arm as well as the fracture energy:*

22

1

y

ac

EGg

σπ= Eq 4

where E = Young’s modulus σy = yield stress. The dependence of gc on Ga makes sense in lieu of Gent’s comments above. Higher values of Ga (through, for example, an increase in Wb) imply that the zone of deformation is larger, which in turn should increase the value of gc.

APPLICATION OF PEEL STRENGTH MODEL TO EXPERIMENTAL DATA

Our analysis of the mechanics of the peel test shows that thickness has a significant effect on the measured peel force. Moreover, both Gdb and Ga are a function of the adhesive strength, which may lead to an explanation of the behavior in Figures 1 and 2. To apply these concepts to the foil coating structures, we will utilize a model developed by Kinloch, Lau, and Williams [5]. They developed analytical expressions for Ge, Gdb, and Ga that we solved iteratively using a computer for conditions that simulate the AE DOE.

Inputs for the model include the imposed peel angle and the tensile properties of the peel arm (Young’s modulus, plastic yield strain, and work hardening parameter). Since Kinloch, et al., studied LDPE-foil laminates, we used their tensile data in our analysis. We assumed the addition of AE to LDPE does not significantly change its tensile properties. Our measurements were in a “T-peel” configuration, a geometry that is not explicitly solved for by the model. We used 90° as the imposed peel angle (θ) as an approximation.

*The critical thickness of the peel arm is that thickness which equals the radius of the plastic zone at the crack tip, ry. From Kinloch and Young

[8], a stress analysis for plane stress gives ry = (KI/σy)2/2π, where KI is the stress intensity factor. KI is related to the fracture energy by Ga = KI

2/E for plane stress. Therefore, ry = GaE/(2πσy

2).

Figure 6 shows the results of the model for LDPE. The peel strength values (converted to the units of energy per area) as a function of thickness are taken from the AE DOE results (bottom curve in Figure 1). The results show Ge is small, reflecting our experimental observation that the peel arms did not elongate beyond the yield point. Ga is large but not very dependent on thickness. Gdb has the greatest dependence on thickness.

Figure 7 shows the results for the 20% AE, 80% LDPE blend (top curve from Figure 1). Again Ge is small and will be ignored for the rest of this discussion. Here both Ga and Gdb increase substantially with thickness. This may account for the steeper peel strength curve in Figure 1.

Contribution of Fracture Energy to Peel Strength Reduction

Comparing Figures 6 and 7, we see that Ga for the blend is a strong function of thickness, whereas Ga for LDPE is not. Using equation 4, we estimate gc to be around 10-20 µm for LDPE and 50-80 µm for the AE blend. Since our coating thickness was in the range of 20 to 50 µm, this suggests that the LDPE coatings are near the plateau region of Figure 5, whereas the AE-blend coatings are still on the slope. This is illustrated in Figure 8. Increasing the bonding of the polymer to the substrate by adding chemical functionality increases gc. Over the thickness range of interest, this change in critical thickness helps explain the sensitivity of the peel strength to thickness. An estimate of gc for the EAA coating in Figure 2 of around 80-120 µm further validates this.

Contribution of Bending Energy to Peel Strength Reduction

As noted earlier, Gdb increases with increasing thickness for both LDPE and the AE-LDPE blend (see Figures 6 and 7). The slope of the AE-blend curve, however, is considerably higher. Following the argument of Gent, it appears the addition of AE to LDPE increases the energy dissipated during bending of the peel arms. This is achieved through a greater anchoring effect. A comparison of the initial peel angles (θo – see Figure 3) supports this. As shown in Figure 9, model calculations indicate that the peel angle remains constant for the AE blend coating as the thickness is increased. For the LDPE, coating the initial peel angle decreases as thickness is increased. For a given thickness, the peel arm of the AE-blend coating is pulled back at a higher angle than for LDPE, expending more energy during the peel strength measurement.

CONCLUSIONS

The mechanics of the peel strength test help explain the low peel strength performance of thin coatings. The energy dissipated during bending of the peel arm decreases with decreasing thickness. Moreover, both the fracture energy and bending dissipation are strongly influenced by adhesion. Greater adhesion increases the initial peel angle, resulting in greater bending energy, and the size of the deformation zone, leading to a steeper rise in fracture energy with increasing thickness. This may account for the divergence in slopes in our experiments; the greater bond strength provided by the chemical functionality in the AE blend and acid copolymer gives rise to a steeper climb in peel strength with increasing thickness than for LDPE.

PRACTICAL IMPLICATIONS

This work shows that increasing bond strength between the polymer coating and substrate does not always result in higher peel strength. The peel strength depends on many factors beyond the bond strength, including the local energy dissipation (Φ) at the peel front, the local peel angle, and whether the polymer yields or elongates. These factors depend on the stiffness, thickness, viscoelastic response, and yield stress of the polymer. The physical properties, in turn, do not affect each component of the peel strength in the same way. For example, reducing stiffness may reduce the energy for bending the peel arm, but it may increase the fracture energy. The situation becomes even more complicated for extrusion laminations where the properties of the second substrate may come into play.

Despite these complexities, our findings suggest some general strategies for increasing the peel strength at low coating thickness. For LDPE, increasing the bond strength by increasing oxidation (increasing temperature or using ozone treatment) or adding modifiers may help. But as we saw with the AE modifier work, this may be most effective for thick coatings. Without changing the physical properties, the improved bond strength may not be enough to significantly improve the peel strength of thin coatings. Acid copolymers have even stronger bond strength to aluminum foil than modified polyethylene. They are less crystalline than LDPE and, hence, have lower modulus and yield stress. Thus, their peel strength may be higher than LDPE at a given thickness (see Figure 2), even though they may be more sensitive to changes in thickness. Further changes in physical properties can be obtained by incorporating soft comonomers such as acrylates or acetates into the ethylene backbone. EMA, EVA,

or acid terpolymers (ethylene-acid-acrylate) are examples. Primers may be needed for some of these polymers to achieve good bond strength to the substrate.

In general, the polar ethylene copolymers, such as acid copolymers and EMA, have the right combination of stiffness, yield stress, and chemical functionality to improve peel strength over LDPE at low coating weights. One word of caution is that increasing the peel strength does not necessarily imply improved performance in a specific application.

KEY WORDS

Adhesion, peel strength, extrusion coating, foil, coating weight, fracture mechanics

REFERENCES

1. B. A. Morris, “Understanding Why Adhesion In Extrusion Coating Decreases With Diminishing Coating Thickness, Part I: Penetration of Porous Substrates,” 2005 TAPPI PLACE Conference, Las Vegas, NV..

2. B. A. Morris, “Understanding Why Adhesion In Extrusion Coating Decreases With Diminishing Coating Thickness, Part II: Nonporous Substrates,” 2006 TAPPI PLACE Conference, Cincinnati, OH.

3. B. A. Morris and H. T. Thai, “Improving the Adhesion of LDPE to Aluminum Foil Through Blending,” Annual Technical Conference – Society of Plastics Engineers, 62, 1101-1105 (2004).

4. S. Wu, Polymer Interface and Adhesion, Marcel Dekker, Inc., New York, 1982.

5. A. J. Kinloch, C. C. Lau, and J. G. Williams, “The Peeling of Flexible Laminates,” International Journal of Fracture, 66, 45-70 (1994).

6. A. N. Gent and G. R. Hamed, “Peel Mechanics for An Elastic-Plastic Adherend,” J. Applied Polymer Sci, 21, 2817-2831 (1977).

7. J. Pascal, “Ultraversatile Adhesives to Broaden the Possibilities of Extrusion Lamination,” 2006 TAPPI PLACE Conference, Cincinnati, OH.

8. A. J. Kinloch and R. J. Young, Fracture Behavior of Polymers, Elsevier Applied Science Publishers, London, 1985.

The advice contained herein is based upon tests and information believed to be reliable, but users should not rely upon it absolutely for specific applications since performance properties will vary with processing

conditions. It is given and accepted at user’s risk, and confirmation of its validity and suitability in particular cases should be obtained independently. The DuPont Company makes no guarantees of results and

assumes no obligation or liability in connection with its advice. This publication is not to be taken as a license to operate under, or recommendation to infringe, any patents

Figure 1: Results of DOE varying coating thickness, coating temperature, time in the air gap (TIAG) and % AE modifier in blend with LDPE. Here temperature and TIAG are kept constant and peel strength as a function of thickness and % AE are plotted based on a statistical model of the experimental results. From Morris and Thai [3].

Figure 2: Results of Coating Trial

Peel Strength to Aluminum FoilOPET/tie/9-um Al/Coating

122 m/min, 127 mm air gap, 75 ms TIAG, 330 C Melt Temp (288 C for EAA)

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50 60

Coating Thickness, microns

Pee

l Str

engt

h, g

/25m

m

LDPE

20% AE, 80% LDPE

EAA

Figure 3: Peel Strength Test Geometry

Peel TestP

θ0

θg

Figure 4: Illustration of the Relative Size of the Deformation Zone

Viscoelastic Deformation Zone in Front of Crack Relative to Thickness of Peel Arm

Figure 5: Theoretical Effect of Thickness on Ga

Effect of Thickness on Ga

Adhesive Thickness

Ga

gc

Figure 6: Results of energy analysis for LDPE coatings. Peel strength and thickness data taken from AE DOE model (Figure 1) with 80 ms TIAG and 310°C coating temperature. Peel Energy model parameters: E = 150 MPa, alpha = 0.087, ey = 6.2%, θ = 90°.

EC

HIP

100

200

300

400

500

600

700

20 25 30 35 40 45

Thickness, µm

Peel Strength to Al Foil

Low %AE = 0Middle %AE = 10High %AE = 20

Temperature, C = 310TIAG, msec = 80

Pee

l Str

engt

h, g

/25m

m

EC

HIP

100

200

300

400

500

600

700

20 25 30 35 40 45

Thickness, µm

Peel Strength to Al Foil

Low %AE = 0Middle %AE = 10High %AE = 20

Temperature, C = 310TIAG, msec = 80

Pee

l Str

engt

h, g

/25m

m

EC

HIP

100

200

300

400

500

600

700

20 25 30 35 40 45

Thickness, µm

Peel Strength to Al Foil

Low %AE = 0Middle %AE = 10High %AE = 20

Temperature, C = 310TIAG, msec = 80

EC

HIP

100

200

300

400

500

600

700

20 25 30 35 40 45

Thickness, µm

Peel Strength to Al Foil

Low %AE = 0Middle %AE = 10High %AE = 20

Temperature, C = 310TIAG, msec = 80

Pee

l Str

engt

h, g

/25m

m

Peel Energy for LDPE

0

50

100

150

200

250

300

350

0 10 20 30 40 50

Thickness, microns

En

erg

y p

er A

rea,

J/m

2

Ga

Ge

Gdb

Gtot

Figure 7: Results of energy analysis for 20% AE, 80% LDPE coatings. Peel strength and thickness data taken from AE DOE model (Figure 1) with 80 ms TIAG and 310 °C coating temperature. Peel Energy model parameters: E = 150 MPa, alpha = 0.087, ey = 6.2%, θ = 90°.

Peel Energy for (20% AE+ 80% LDPE) Blend

0

50

100

150

200

250

300

350

0 10 20 30 40 50

Thickness, microns

En

erg

y p

er A

rea,

J/m

2

Ge

Gdb

Ga

Gtot

Figure 8: Scenario where Ga for LDPE and AE-LDPE blends diverge because of differing values of gc.

Effect of Thickness on Ga

0 20 40 60 80

Adhesive Thickness, um

Ga

AE + LDPE Blend

LDPE

Figure 9: Comparison of initial peel angle (θο) for LDPE and AE-LDPE blend. Peel strength and thickness data taken from AE DOE model (Figure 1) with 80 ms TIAG and 310°C coating temperature. Peel Energy model parameters: E = 150 MPa, alpha = 0.087, ey = 6.2%, θ = 90°.

θθθθo vs. Thickness

0

5

10

15

20

25

30

35

0 10 20 30 40 50

Thickness, microns

Init

ial P

eel A

ng

le,

deg

rees

LDPE

80% LDPE, 20% AE

2007 PLACE Conference September 16-20

St Louis, MO

Understanding Why Adhesion in Extrusion Coating Decreases with

Diminishing Coating ThicknessPart III: Analysis of Peel Test

Presented by:Barry A. MorrisSr. Technology AssociateDuPont Packaging and Industrial Polymers

ECH

IP

100

200

300

400

500

600

700

20 25 30 35 40 45

Thickness, um

Peel Strength, g/25mm

Low %AE = 0.0Middle %AE = 10.0High %AE = 20.0

Temperature, C = 310.0TIAG, msec = 80.0

Motivation for WorkMorris and Thai, TAPPI PLACE 2004

Rapid Increase with Thickness

Slow Increase with Thickness

Part I: Porous Substrates

• Well known that adhesion decreases with coating thickness to paper

• Examined 3 mechanisms– Time in air gap– Cooling in air gap– Cooling in nip

• Conclusions– Adhesion is related to penetration– Penetration is influenced by polymer rheology,

coating temperature and cooling in the nip

Part II: Non-Porous Substrates

• Relooked at adhesion to foil• Examined 4 mechanisms

– Time in the air gap– Cooling in the air gap– Cooling in the nip– Stress from drawing

• Last two have impact on adhesion but do not explain the different slopes

Outline• Introduction• Experimental Results• Analysis of Peel Test

– Fracture mechanics– Modeling of results– Insight

• Conclusions• Practical Implications

Extrusion Coating/Lamination

Nip Roll Chill Roll

Die

Line Speed

Air gap

Substrate

Not drawn to scale

Experiment• Purpose: Coat 3 resins of increasing

acid functionality onto foil to validate previous findings

• Resins:– LDPE– Blend of 20% AE modifier, 80% LDPE– EAA (9%AA, 10 MI)

Process Conditions and Set-up• Die: Cloeren edge bead

reduction die• Substrate: 13-μm

OPET/19-μm tie/9-μm Al• Coating thickness: 20-

50 μm• Coating temperature:

330 °C for LDPE and AE-LDPE blend; 288 °C for EAA

• Die gap: 0.51 mm • Line speed: 122 m/min• Air gap: 127 mm• Time in the air gap: 75

ms• Chill Roll temperature:

10 °C• Nip pressure: 0.4 MPa

Peel Strength to Aluminum FoilOPET/tie/9-μm Al/Coating

122 m/min, 127 mm air gap, 75 ms TIAG, 330 ºC Melt Temp (288 ºC for EAA)

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50 60

Coating Thickness, microns

Peel

Str

engt

h, g

/25m

m

LDPE

20% AE, 80% LDPE

EAA

Analysis of Peel Strength Test

Peel TestP

θ0θ

g

Peel Strength• Measure of force to pull the specimen apart• Contributions from

– Energy to bend the peel arm, Gdb

– Energy to elongate the peel arm, Ge

– Energy to create new surfaces, Ga

θε cos1 −+++

=a

dbea GGGbP

Elongation of the Peel Arm

∫=a

dgGe

ε

εσ0

Elongation of Peel Arm

Strain

Stre

ss

Ge = thickness x area under curve

ε a

Bending of Peel Arm• Bending energy goes through a maximum with

increasing thickness.• Increased bonding at interface increases bending

energy.• Angles can be affected as well.

Yielding, but low thickness

No yielding at high thickness

Bending Energy

Thickness

Gdb

Fracture EnergyGa is the energy required to create new surfaces.

Ga = Wb(1 + Φ(R, T))

Wb is affected by changes in chemical functionality.

Φ is a function of the viscoelastic properties of the polymer

Effect of Thickness on GaViscoelastic Deformation Zone in Front of Crack and

Thickness of Peel Arm

Effect of Thickness on Ga

Adhesive Thickness

Ga

gc

221

y

ac

EGg

σπ=

Fracture Mechanics

• Thickness affects each component of the Peel Strength

• Stronger bond strength results in greater deformation and peel energy

Analysis of Peel Strength

*Kinloch, Williams, and Lau, Int J Fracture, 66, 45-70 (1994)

Using Model Developed by Kinloch,et al.*

LDPE

0

50

100

150

200

250

300

350

10 20 30 40 50

Thickness, microns

Ener

gy p

er A

rea,

J/m

2

Gtot

Ga

Gdb

Ge

Analysis of Peel StrengthUsing Model Developed by Kinloch, et al.

(20% AE+ 80% LDPE) Blend

0

50

100

150

200

250

300

350

10 20 30 40 50

Thickness, microns

Ener

gy p

er A

rea,

J/m

2

Gtot

Ga

Gdb

Ge

Analysis of Peel Strength

LDPE

0

50

100

150

200

250

300

350

10 20 30 40 50

Thickness, microns

Ener

gy p

er A

rea,

J/m

2

Gtot

Ga

Gdb

Ge

(20% AE+ 80% LDPE) Blend

0

50

100

150

200

250

300

350

10 20 30 40 50

Thickness, microns

Ener

gy p

er A

rea,

J/m

2

Gtot

Ga

Gdb

Ge

Bending Energy Comparison

Gdb vs. Thickness

0

20

40

60

80

100

120

140

10 20 30 40 50

Thickness, microns

Gdb

, J/m

2

LDPE

80% LDPE, 20% AE

Initial Peel Angleθo vs. Thickness

0

5

10

15

20

25

30

35

10 20 30 40 50

Thickness, microns

Initi

al P

eel A

ngle

, deg

rees

LDPE

80% LDPE, 20% AE

Fracture Energy ComparisonGa vs. Thickness

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50

Thickness, microns

Ga,

J/m

2

LDPE

20% AE, 80% LDPE

Scenario of Why Ga has Different Slopes

Effect of Thickness on Ga

0 20 40 60 80Adhesive Thickness, μm

Ga

gc for AE + LDPE Blend is 50-80 μm

gc for LDPE is 10-20 μm

Conclusions• New experimental results validate earlier results

– Slope ↑ with chemical functionality

• Analysis of peel strength explains slope– Each component of PS is affected by thickness

– Greater bond strength increases the sensitivity to thickness by

• Increasing the bending energy and initial peel angle

• Increasing the natural thickness of the zone of deformation at the peel front

Practical Implications• PS is a complex function of bond strength and

physical properties– Physical properties interact differently with the

various contributions to PS– Example: Reducing stiffness may decrease Gdb but

increase Ga

• Tactics– Acid copolymers - enhanced chemical functionality

(bond strength) and reduced crystallinity (stiffness) over LDPE

– Softer polymers such as EVA’s, EMA’s and acid terpolymers may be good choice for some applications.

Thank YouPRESENTED BY

Barry A. MorrisSr. Technology AssociateDuPont Packaging and Industrial Polymers

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