Plastics   Mechanical Properties
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Plastics Mechanical Properties

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Plastics   Mechanical Properties Plastics Mechanical Properties Presentation Transcript

  • Mechanical Properties
    • Mechanical Properties of Viscoelastic Materials
    • Stress / Strain Behavior
    • Creep
    • Toughness
    • Reinforcement, Fillers, Modifiers
  • Properties and product performance
    • Materials
    • Properties
    • Availability
    • Cost
    The successful design of a product depends on the synergy of the design , manufacturing and materials
  • Why look at polymer chemistry, structures and properties ?
    • What we really want to learn is ….
      • How to make a plastic part that delivers the required life cycle performance at the best cost and meets regulations
    GOAL = Product Performance
  • Plastics – from chemistry to performance Polymer Chemistry Material microstructure Material properties Product Performance
  • Intermolecular Attraction Forces
    • The performance of a plastic part depends on the attraction forces between polymeric chains
    • These forces increase as chain length increases
    • These forces are stronger as the chain to chain distance decreases
      • Force  1/ d & Force  n
  • ‹ #› Bonding Energy & Distance Bond energy Bond length
  • Intermolecular Attraction Forces
      • Force  1/ d
    Intermolecular forces Intermolecular distance, d Intermolecular distance, d d
  • Intermolecular Attraction Forces
      • Force  n
    Intermolecular forces Degree of polymerization, n
  • Mechanical Properties & M W
    • Higher MW means stronger intramolecular interactions which means better mechanical properties
    Intermolecular forces Molecule length  to molecular weight
  • Differences in Properties
    • Leathery
    • More soluble
    • Transparent
    • Low shrinkage
    • Tough
    Amorphous
    • Melt
    • Less soluble
    • Opaque
    • High shrinkage
    • Rigid
    Crystalline
  • Mechanical Properties - stiffness Amorphous stiffness temperature T g T g – glass transition temperature
  • Mechanical Properties - stiffness Temperature stiffness Semi crystalline T g T m – melt temperature T m
  • Mechanical Properties - stiffness T m T m T g stiffness Temperature Amorphous plastic Semi crystalline plastic
  • Physical Properties 15%GF Polyester, PBT 1400 nylon 6/6 13%GF Density 0.0509   lb/in ³ 0.0444   lb/in ³ Water Absorption 0.1   % 1.1 % Linear Mold Shrinkage 0.005   in/in 0.006 in/in
  • Plastic Mechanical Properties
  • Secondary Bonds
    • These bonds are physical in nature, there are no chemical changes happening and they are weaker than chemical bonds
    • Hydrogen Bonds
    • Entanglement
    • Van der Waals
    Increasing strength
  • Intermolecular Attraction Forces
    • The performance of a plastic part depends on the attraction forces between polymeric chains
    • These forces increase as chain length increases
    • These forces are stronger as the chain to chain distance decreases
      • Force  d & Force  n
  • Elastic Behavior of Solids
    • Stress : Applied force per unit area
    • Strain : Displacement of sample
    • Stress/Strain = E (Young’s Modulus)
    • Large modulus E : Stiff materials
    • Constant modulus- -linear S/S curve: Hookean material (like most metals or ceramics)
  • Stress/Strain Curve Linear Elastic Material
  • Types of Forces
    • Pulling on the end: Tensile
    F F
  • Types of Forces
    • Rotational: Torsion
    T T
  • Types of Forces
    • Pushing and sliding: Shear
  • Stress Testing
    • Tensile Test
    A F L Stress,  = F / A Strain,  =  L / L
  • Tensile Testing Results Stress vs Strain For plastics the rate of stress applied affects the material’s response
  • Elastic Behavior of Solids
    • Stress: Applied force per unit area
    • Strain: Displacement of sample
    • Stress/Strain = E (Young’s Modulus)
    • Large modulus E : Stiff materials
    • Constant modulus--linear S/S curve: Hookean material (like most metals or ceramics)
  • Solid Materials
    • A Solid can be defined as a state of the material where the deformation of the part is a function of the load applied to it
    •  = f (force)
    • Elastic behavior - Small deformations then return to original shape
    • Virtually all applied energy retained and used to rebound
    • Forces typically normalized for sample area
  • Mechanical Response as a function of Time ‹ #› F F F time input  time displacement
  • Elastic Solid Model ‹ #› F F   k Spring constant or stiffness k
  • Elastic Solid – Microstructural Behavior ‹ #› The applied force straightens polymer chain segments F F The polymer chain segments return back to a more disorder and stable configuration when force is removed
  • Viscous Behavior
    • A fluid can be defined as a state of the material where the RATE of DEFORMATION of is a function of the load applied
    • d  dt  = f (force)
  • Viscous Behavior
    • Typically applied to liquids; arises from entanglement
    • Flow Resistance = Viscosity
    • Stress causes velocity gradient with time and distance: Shear Rate
    • Stress = Viscosity * Shear rate, for a Newtonian liquid
  • Viscous Behavior
    • Typically applied to liquids; arises from entanglement
    • Flow Resistance = Viscosity
    • Stress causes velocity gradient with time and distance: Shear Rate
    • Stress = Viscosity * Shear rate, for a Newtonian liquid
  • Cone and Plate cone polymer
  • Stress and Shear Rate Polymer melt stationary plate moving plate force Shear stress
  • Newtonian Fluid
    • Linear shear-rate with stress:
      Slope =  viscosity Example – water  =  shear stress  =  shear rate
  • Newtonian / NonNewtonian
    • Non-Newtonian (non-linear) types
      • Pseudoplastic : Shear-thinning , most plastics
      • Dilatant : Shear-thickening
      pseudoplastic dilatant
  • Rheometry Experiments Experiment H Re-Grind PC Melt Rheology at 550 o F
  • Effects of Time and Temperature
    • Compared to other materials, the properties of plastic are more sensitive to the time (how long) at which they are observed and measured
    • The properties are also sensitive to the temperature they are being observed and measured
  • Elastic Solid – Microstructural Behavior
    • One of the most microstructural features of polymers or plastics is that they try to keep the level of disorder or entropy as high as possible
    • The preference for high entropy is the driving force for the polymer chains to spring back once the force is removed
  • Mechanical Response as a function of Time
    • Fluid Like Behavior
    F time  time input response No recovery
  • Viscous Fluid Model F F  d  dt C Viscous Damping Constant
  • force force
    • The polymer chain rub against the nearby chains.
    • This frictional is proportional to the rate of deformation
    Heat is generated
  • The plastic part is subjected to a tensile force The plastic part is increases its length L, when the force is removed it will not spring back – this a permanent deformation The plastic part is increases its length L F F L o , original length F F L new = L o +  L L new = L o +  L permanent
  • Mechanical Response as a function of Time
    • Viscoelastic Like Behavior
    F time  time input response recovery
  • Viscoelastic Solid Model F   (t) time
  • Viscoelastic Solid, Microstructural Behavior Polymer chain segments are stretched by the force, this is the elastic element of the model As the Polymer chain segments are stretched there is friction between these chain segments – this is the viscous damping element When the force is removed, the chains return to the original state – during this motion, there is also friction Heat is generated
  • General Viscoelastic Model F C E K E C P K E – elastic stretching of chain segments C E – friction between chain segments (very small) C P – friction between complete polymer chains
  • Maxwell Viscoelastic Model F K E C P K E – elastic stretching of chain segments C P – friction between complete polymer chains
  • Viscoelastic Behavior
    • Continuum of liquids and solids is continuum of viscous to elastic behavior
    • Disentanglement is time dependent
      • Elastic and Viscoelastic materials tend to be stiffer at high shear rates (short time)
    • Viscous properties: energy dissipation in the mass--long range, long time
    • Elastic properties: Molecular stretching, bending--short range, short time
  • Mechanical Response & Intermolecular Forces
    • The same plastic can have the mechanical response of
      • An Elastic Solid
      • A Viscoelastic Solid
      • A Viscoelastic Fluid
      • A Viscous Fluid
    • The particular mechanical response depends on the intermolecular forces
  • Deborah’s Number
    • The Deborah number is a dimensionless number , used in rheology to characterize how "fluid" a material is.
    • It is defined as the ratio of a relaxation time , characterizing the intrinsic fluidity of a material, and the characteristic time scale of an experiment
    • The smaller the Deborah number, the more fluid the material appears.
    • De = relaxation time / observation time
  • Effects of Time and Temperature – Silly Putty
    • When heated, it becomes more like a viscous fluid
    • The longer a load is applied, the more it will act like a viscous fluid
    • When the temperature is lowered, it becomes more like a solid
    • The shorted the load is applied, the more it will act as a solid
  • time Tension in a part Relaxation curves Increasing temps Initial Tension t o Length after time t o 1 2 3 3 2 1
  • l Stress Relaxation
  • Mechanical Response and Intermolecular Forces
    • The same plastic can have the mechanical response of
      • An Elastic Solid
      • A Viscoelastic Solid
      • A Viscoelastic Fluid
      • A Viscous Fluid
    • The particular mechanical response depends on the intermolecular forces
  • Mechanical Response and Intermolecular Forces
    • When the intermolecular forces are very high, the chains are held together tightly
    • The only possible motion is the stretching and spring back of short chain segments
    • Therefore the plastic acts as an Elastic Solid
  • Mechanical Response and Intermolecular Forces
    • When the intermolecular forces are not so strong, the chains are held together less tightly and they are more separated
    • When force is applied, longer segments can stretch and there is friction between these chains – Viscoelastic Solid
  • Mechanical Response
    • The mechanical response a plastic part depends on intermolecular forces
      • % of crystallinity
      • Temperature
      • Hydrogen Bonds
      • Molecular Weight
      • Chain to chain distance
      • Entanglements
  • Temperature and Mechanical Response
    • Mechanical Response & T g
  • Load Rate & Mechanical Response
    • Stress / Strain Curve
     Increase of Strain rate 
  • Dynamic Mechanical Analysis From TA Instruments
  • Dynamic Mechanical Analysis
  • Dynamic Mechanical Analysis From TA Instruments
  • Dynamic Mechanical Analysis From TA Instruments
  • Dynamic Mechanical Analysis Viscous elastic response From TA Instruments
  • Viscous elastic response
  • Viscous elastic response
  • Dynamic Mechanical Analysis
  • Creep
    • Small, constant load, long time
    • Results from stretching and uncoiling /disentanglement
    • Opposed by strong intermolecular forces and crosslinking
    • It’s a function Temperature, time, load
    force force Heat Chains flow by each other
  • From TA Instruments
  • From TA Instruments
  • Creep Viscoelastic Model  (t) time Critical time For this load, there has not been enough time to start the viscous motion F C E K E Permanent Deformation – creep For this load, the viscous motion has started
  • Creep – Temperature, Time and Load
    • The critical time is f(temperature, load)
    • The higher the Temp, the lower t critical – this is because the higher temp makes the material more viscous like
    • The higher the Load, the lower t critical – This is because higher loads can start whole displacement in shorter periods of time
  • Creep – time, temp loads
  • Impact Strength and Toughness
    • Toughness : Absorb energy without breaking
    • Related to area under stress/strain curve
    • Toughness experiments mostly short time, e.g., impact strength
  • Plastic Toughness
    • The amorphous region can deform more and absorb more energy
    crystalline amorphous
  • Depends on material ability to absorb energy Stress/strain curve Area underneath Stress/strain curve is the measure of impact  strain  stress
  • Toughness is not Strength
    • Tough: High elongation, low modulus.
      • High M W
      • Low Intermolecular Strength
      • Rubber, slight cross linking
    • Brittle: Low elongation, high modulus
      • Crystalline
      • High degree of Xlinked rigid
      • High Intermolecular Strength
  • Degree of Crosslinking & Toughness Tough Strong
  • Small Additives Get between Polymer chains This increases d and can make the degrade properties Example – excess of colorant can weaken a plastic part
  • Reinforcements
    • Different polymer chains are attracted to the reinforcement
    • Works as a bridge to attract polymer chains that normally would not interact
    • This makes the properties better
  • Melt Flow Rate (ASTM D1238) Given a resin's MFR,will the part fill properly?
    • Test conditions are not real world processing conditions
    • Different weights and temperatures used for different resins
      • - Comparison of different resins is not 1 to 1
      • - Only relative comparisons are possible
    • Single-Point Data vs. Rheological Curves
    Len Czuba August 2006
  • Melt Flow Rate Test Apparatus: Resin
  • Melt Flow Index # grams of flow per 10 minutes Weighted Plunger Barrel Molten Pellets Extrudate Orifice Heater Band Dynisco LMI 4000 Len Czuba August 2006
  • Tensile Strength (ASTM D638)
    • Cross-head speed not standardized
    • Specimen thickness can be anything up to 0.55"
    • Specimen gating not standardized
    • How did the specimen fail:
      • - Ductile ?
      • - Brittle ?
    Len Czuba August 2006
  • Tensile Strength (ASTM D638)
    • Ultimate tensile strength
    • Tensile modulus
    • Tensile elongation
  • Tensile Strength Test Apparatus:
  • Impact Resistance ASTM D256 Is this a relevant impact test for your device?
    • Five Different Methods
      • - Izod (Methods A, C, and D)
      • - Charpy (Method B)
      • - Unnotched (Method E)
    • Cannot correlate results from different methods
    • Specimen toughness highly dependent on notch size
    • Specimen preparation not standardized
  • Impact Resistance Test Apparatus: Len Czuba August 2006
  • Impact Resistance Test Specimen: Len Czuba August 2006
  • Summary
    • Elastic, Viscoelastic, Viscous
    • Stretch/bend vs entanglement
    • Tensile, Compressive, Flexural, Torsional, Shear
    • Stress/strain performance
    • Strength/toughness
    • Effect of modification on properties
  • Summary
    • Plastic materials behave as elastic solids, viscous fluid or a combination of both
    • The mechanical behavior of plastics depends on factors such as:
      • Intermolecular forces
      • Temperature
      • Time load is applied
  • Summary
    • There are many important mechanical properties that must be considered for processing and use, such as
      • Tensile strength
      • Impact Resistance
      • Creep
      • Use Temperature
      • Processing Temperature
  •  
  • Design Example
    • Reduce Costs of the system without reducing quality or compromising safety
    • This part works mostly in bending
     = Mc / I old
  • Load and Material Interaction
    • Normally, we want the material property to be higher than the value actually applied to the material – example yield stress
      • material yield stress > applied stress
      • Safety Factor = material yield / load
    • Caution – loads and property values are probabilistic not deterministic
  • Load and Material Interaction Failure probability
  • Load ave = 6000, std dev = 1000 PET ave = 12000, std dev = 1000 Example - Material Properties & Loads
  • Load ave = 6000, std dev = 2000 PET ave = 12000, std dev = 1000 Solution 1 - use the same part for another application
  • Load ave = 6000, std dev =1000 PET ave = 12000, std dev 1500 Solution 2 - get cheaper materials
  • Load ave = 6000, std dev = 1000 Regrind PET ave = 9500, std dev = 1200 Solution 3 - get cheaper materials, use regrind
  • 25% regrind 75% virgin Solution 4 - get cheaper materials, use regrind + virgin
  • Solution 4 - get cheaper materials, use regrind + virgin
  • Solution 4 - get cheaper materials, use regrind + virgin
  • Solution 5 - Redesign Part
    • The stress depends on the MC / I
      • where C - is the distance from the neutral axis & I is the moment of inertia of area
    • We can redesign the part to reduce the C / I ratio so that even with the if M is the same, we reduce the stress
  • Solution 5 - Redesign Part Existing cross section New cross section
  •  
  •  = Mc / I new
  • Possible Solutions
    • Add a great % of virgin material, or use 100 % virgin
      • Good Quality Control
      • Increases Solid Waste Problem
      • Increases Production Costs, not added value
      • No new jobs are created by importing resin
  • Possible Solutions
    • Increase the use of regrind and redesign the part
      • Reduces Solid Waste Volume
      • Reduces Costs
      • Can generate more jobs in PR
      • Reduces production time (material sources are closer by )
  • Design Summary
    • Most recycled materials will have lower properties than virgin materials
    • Virgin material can be combined with recycled material to improve properties
    • Increasing the use of regrind reduces costs
  • Design Summary
    • The best way to take advantage of the low cost of recycled and compensate for the lower performance is by redesign of the part