Failure Analysis: Large Format Bags

Toward a material neutral specification for

heavy duty shipping sacks

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Objectives

• Multi-wall paper bags Specification and Test Methods

• Other Specifications and Test Methods– Heavy duty plastic bags– Other bags

• Suggest a Sustainable Formula for Success– Engineering properties– Performance-based Specifications

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Context• Multi-wall paper bags– Mature Industry– Widespread applications– Renewable/degradable resource

• Heavy duty plastic bags– Polymer improvements frequent – Cost driven down-gauging

• Other bags• Textile bags-new materials• Premium performance

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Marketplace• Commodities– Little product differentiation– Much packaging conformity

• FMCGs– Market differentiation– Package formats support segments and trends

• Sustainability Trend– Renewable– Compostable– Source reduced

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Marketplace

Commodity

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Segmented Market

Woven

Paper Plastic

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Segmented Marketplace: Pet Food

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$0.69/lb

$0.64/lb

$1.37/lb

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Specification: Multi-wall paper bags

• “Kraft Paper”

– Min No. Walls– Min (∑ basis weights)– Min avg (of each wall) dry

tensile strength

– Min avg (of each wall) dry tearing strength

– Min avg outerwall (wet strength Kraft) wet tensile strength

• “Extensible heavy duty Kraft Paper”– Min No. Walls– Min (∑ basis weights)– Min avg (of each wall) dry

tensile energy absorption strength (CD)

– Min avg (of each wall) dry tearing strength (MD+CD)

– Min avg outerwall (wet strength Kraft) wet tensile strength (CD)

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UU-S-48 Version F: 1973

Strength

Tear

WetStrength

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End Drop Test-CD stress

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Force = mass acceleration

Energy = Force x

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Flat Drop Test-MD stress

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Force = mass acceleration

Energy = Force x

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Force: kx

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Incr

easi

ng F

orce

(S

tres

s)

X(strain)

Break

Strength

Elongation

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Energy: Forcex

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Incr

easi

ng F

orce

(S

tres

s)

X(strain)

BreakSteeper: more energy faster

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Multiwall bags: springs in series

1 1 1Keq K1 K2

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• #4x “Heavy duty shipping sack extensible kraft paper”– = 3 Walls– ∑ basis weights 180– Avg dry tensile energy

absorption* ( 19 CD; 57 MD+TD)

– Min avg dry tearing strength– Min avg outerwall wet tensile

strength (CD)* Foot pounds

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Strength so far• Bag’s product accelerates until it stops.• That force elongates bag material until

“limit” is exceeded.Greater mass: more forceHigher drop: more force

• If limit is tensile strength, then bag fails!• If not, product remains contained

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Tear•Measures the force –perpendicular to the plane of the paper–required to tear through a specified distance

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Notched Tear v. Other•Notched tear–Force applied to thickness of the paper (z)

• Initiating tear–At edge (all 3 dimensions)–Puncture• 2 stages

1. Force to bend/break paper’s x-y plane2. Force to break thickness

• Rate /surface area dependent

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Tear so far• More complex than strength• How tear initiated is key• Modeling requires understanding use

context and review of risks.

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Wet Strength• Strength of paper does vary with moisture

content.• Suggests other environmental factors may

influence strength of other materials.–Temperature–Chemicals

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Stress/Strain of Plastic Films

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Cross Direction

Machine Direction

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Tensile Energy Absorption of OPP

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Cross Direction177%

Machine Direction 100%

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Textiles• Variability in all dimensions• Design for worst case results in general over

specification

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Textile Tear Testing• Variability in all dimensions–Breaking strands–Breaking bonds between strands–Re-orienting strands

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5 mil hexene lldpe film v. minimum

MD 1% Secant (lb)Tensile @ Yield MD (lb)

Tensile @ Yield TD (lb)

Ultimate Tensile MD (lb)

Puncture Break Energy (in-lb)

Ultimate Tensile TD (lb)Break Elongation MD (%)

Break Elongation TD (%)

Elmendorf Tear MD (g)

Elmendorf Tear TD (g)

Dart Drop Method A (g)

Peak Puncture Force (lb)

-400%

100%

600%

5 mil film

Normalized Minumum C6 LLDPE

Abuse Resistance

StrengthStiffness

After Fiscus, 2005

5 mil hexene lldpe film v. minimum

• Commodity resin optimum– Stiffness: marginally adequate– Strength: slightly over specified– Tear/abuse: mostly over specified

• Converter option for trade off– Blend in another resin

• greater stiffness• Maintain other properties • Down gauge to reduce cost

– Coextrusion for structural effect (“I-beam”)

What Next?

• Strength:– Empirical Values-We (paper/plastic/textile) know

what works – Theoretical Values-Finite element analysis

• Tear & Environment:– What we want to protect against

How?

• Industry committee leadership• Compliance labs-compile empirical data base• MSU: theoretical analysis of strength

dynamics.

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Thank you!

Questions or Comments?

How can I help?

tdunn@flexpacknology.com404-376-4866

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