The Effect of Seal Width & Material Stiffness on Peel Strength (F88 Technique C) Dan Burgess 28 Oct, 2015 1 Agenda Effect of Seal Width Influence of Material Stiffness Conclusions
2 Background Info (Seal Width) Observations When seal width is challenged (reduced) for design verification testing it has been noted that in some cases lower seal strength values have resulted. When using ASTM technique C (180 degree w. backing plate) for heat seal peel testing, seal strength is reduced once the tail of the sample reaches a length such that the sample effectively goes into a 90 degree peel condition during the test. 3
Phases of Peel Test Initial Phase Change in bending film geometry + any plastic deformation of film Steady State (separation occurs) Loss of Bending Component (Force Reduced
due to change in Geometry) Force Displacement Elastic bending energy contributes during initial phase. Once the steady-state is reached, the dissipative mechanisms are in play (e.g. work of separation, any plastic energy dissipated during bending of the film, etc.) 4 Materials Tested
Poly-Tyvek Pouch Poly Web Thickness: 4 mil Low Density Polyethylene Youngs Modulus: 0.11 - 0.45 (109 N/m2) Foil Pouch Thickness: 6.2 mil Aluminum Youngs Modulus: 69 (109 N/m2) 5 Results (Varying Widths) Material:
TOP WEB: 4 mil LDPE BOTTOM WEB: UNCOATED TYVEK 1073B 6 Seal Strength Spec: 1.0 lb/in Side Seal (0.063 width) Repeatability Material: TOP WEB: 4 mil LDPE BOTTOM WEB: UNCOATED TYVEK 1073B 7
Seal Strength Spec: 1.0 lb/in Side Seal (0.032 width) Repeatability Material: TOP WEB: 4 mil LDPE BOTTOM WEB: UNCOATED TYVEK 1073B 8 Seal Strength Spec: 1.0 lb/in Results (Varying Widths) Foil Pouch
Material: 6.2 mil foil composite 9 Seal Strength Spec: 1.0 lb/in Conclusions Total energy (area under the curve) required to completely peel the sample is reduced Work = Force x Distance Smaller seal widths may not reach a steady state
The seal strength of low modulus (flexible) materials are less affected by reductions in seal width Poly-Tyvek seals maintain their observed strength until they are almost completely peeled because the geometry of the materials does not change until very late in the execution of the test The seal strength of high modulus (stiff) materials such as foil films are more affected by reductions in seal width as they are more likely to move to a 90 degree peel state 10 Background Info (Material Stiffness)
L Film A Back Plate fs Film B fn Very flexible material Stiff material
As material stiffness increases, the amount of force attributed to the normal force (fn) increases and shear force (f s) decreases, due to changes in the geometry associated with the peel mechanism 11 Actual Force Recorded for Foil Film sample Chart A Foil 90 vs 180 35 30
180 Peel 90 Peel Force (N) 25 20 15 10
5 0 1 11 21 31 41
51 Extension 12 61 71 81 91
Can we increase force w/o changing the seal? 13 Stiffened material Control 14 Higher force for
Stiffened Sample Change in L 15 Force plateau occurs L has reached steady state
16 Force drops L begins to increase 17 Shorter plateau 90 degree position
18 Force increases again as bend energy is added from bending the bottom web (Tyvek) 19 Conclusions (Seal Strength) Material stiffness can play a significant role in the output of peel test using technique C. It is possible to raise the observed seal strength of a sample by
increasing its stiffness this is a function of the positon the material is in! Peel strength recorded by the 180 degree peel test method measures the bond strength between the two substrates being separated from each other and the bend force required to achieve web separation! This is noted in ASTM F88 significance and use section 4.2 A portion of the force measured when testing materials may be a bending component and not seal strength alone. For a good example of this in the test method refer to unsupported vs. 180 methods FIG X1.1 of ASTM F88 20 So what does all this mean?
1. 2. 3. Never mix techniques (A,B,C) Know that increasing the stiffness of the material under test will affect the result if it is bent in to place for testing If there is a need to minimize the effect of stiffness Clamp the stiffer material in the lower jaw Use technique A or B 4. 5.
6. Be sure to record which material is clamped in each grip When using this method, understand how a reduction in seal width may affect your resultsa lower value may not mean the bond has changed! Consider the interaction between the test method and minimum seal width specification Example: minimum seal width may be based on the point at which the width of the seal at starts to negatively affect observed seal strength during peel testing. 21 Questions?
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