Download - Heavy Duty Shipping Sacks
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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
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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”)
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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
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How?
• Industry committee leadership• Compliance labs-compile empirical data base• MSU: theoretical analysis of strength
dynamics.