Factors Affecting Crystallinity and Material Properties of Polymers, Slides of Software Engineering

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1

Plastics and Properties Important

in Extrusion

Chapter 4

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2

Chapter 4 Objectives

• Topics

• Main types of plastics
• Flow properties
• Thermal properties
• Help
  • Select appropriate machines for extrusion
  • Set proper processing conditions
  • Analyze extrusion probelms

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4

Main Type of Plastics

• Polymers are carbon-based materials made up of

very long molecules

• Polymers

• Thermoplastic: Melt and flow upon heating
  • Can be reheated and flow again
  • When cooled behaves as a solid
  • Very suitable for recycling
• Thermoset: React and cross-link (set-up) upon heating
  • Can be heated only once.
  • Material is not easily recycled

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5

Amorphous and Crystalline Plastics

• Thermoplastics are further classified based upon

molecular arrangement of polymer chains

• Amorphous: (without shape)
  • Polymer chains are random arrangement
• Crystalline
  • Polymer chains form regular pattern

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• Crystalline- Molecular structure forms regular order

(crystals) with molecules or portions of molecules regularly

stacked in crystal-like fashion.

• Very high crystallinity is rarely achieved in bulk polymers
• Most crystalline polymers are semi-crystalline because

regions are crystalline and regions are amorphous

• Molecular arrangement is arranged in a ordered state

States of Thermoplastic Polymers

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Factors Affecting Crystallinity

• Cooling Rate from mold temperatures

• Barrel temperatures

• Injection Pressures

• Drawing rate and fiber spinning:

Manufacturing of thermoplastic fibers

causes Crystallinity

• Application of tensile stress for

crystallization of rubber

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Liquid Crystalline Plastics (LCPs)

• The molecules of LCPs are rod-like structures

organized in large parallel domains, not only in the

solid state but also in the melt state.

Mechanical Properties PEEK LCP Polyester Nylon 6, Density, g/cc 1.30-1.32 1.35 - 1.40 1.13-1. Tensile Strength, psi

10,000 – 15,000 16,000 – 27,000 14, Tensile Modulus, psi

500K 1,400K - 2,800K 230K – 550K Tensile Elongation, %

30% - 150% 1.3%-4.5% 15%-80% Impact Strength ft-lb/in^ 0.6 – 2.2^ 2.4 - 10^ 0.55 – 1. Hardness R120 R124 R CLTE 10 -6^ mm/mm/C

40 - 47 25-30 80 HDT 264 psi 320 F^ 356F -671F^ 180F Docsity.com

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Elastomers

• Elastomers are materials capable of large elastic

deformations with elastic elongation > 200%

• Conventional: vulcanizable
  • polyisoprene, polybutadiene, polychloroprene, polyisobutylene
• Thermoset elastomers: cross-linking reaction
  • polyurethane, silicone
• Thermoplastic elastomers: physical linking
  • olefinic, TPO
  • urethane, TPU
  • etherester, TPE
  • copolyester, TPE
  • styrenic, TPR Docsity.com

13

Melt Index

• Melt index test

• Measures the flow of a material at a

temperature and under a load or weight.

• Procedure (ASTM D 1238)
  • Set the temperature per the material type.
  • Add plastic pellets to chamber. Pack with rod.
  • Place mass (5Kg) on top of rod.
  • Wait for the flow to stabilize and flow at constant rate.
  • Start stop watch
  • Measure the flow in a 10 minute interval
  • Repeat as necessary

Mass

Temp

Plastic

Plastic Resin

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Melt Index and Viscosity

• Melt index for common materials

Material Temp Mass

• Polyethylene 190°C 10 kg
• Nylon 235°C 1 kg
• Polystyrene 200°C 5 kg

• Melt Index is indication of Viscosity

• Viscosity is resistance to flow

• Melt index flow properties

• High melt index = high flow = low viscosity
• Low melt index = low flow = high viscosity

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16

Stresses, Pressure, Velocity, and Basic Laws

• Stresses: force per unit area
  • Normal Stress: Acts perpendicularly to the surface: F/A
    • Extension
    • Compression
  • Shear Stress,  : Acts tangentially to the surface: F/A
    • Very important when studying viscous fluids
    • For a given rate of deformation, measured by the time derivative d /dt of a small angle of deformation , the shear stress is directly proportional to the viscosity of the fluid

Cross SectionalArea A A F A F

F

Deformed Shape

F

 = μd /dt Docsity.com

17

Some Greek Letters

• Alpha: 
• beta: 
• gamma: 
• delta: 
• epsilon: 
• zeta: 
• eta: 
• theta: 
• iota: 
• kappa: 
• lamda: 
• mu: 
  • Nu: 
  • xi: 
  • omicron: 
  • pi: 
  • rho: 
  • sigma: 
  • tau: 
  • upsilon: 
  • phi:
  • chi: 
  • psi: 
  • omega: Docsity.com

19

Viscosity

• Viscosity is defined as a fluid’s resistance to flow under an

applied shear stress, Fig 2.

• The fluid is ideally confined in a small gap of thickness h

between one plate that is stationary and another that is

moving at a velocity, V

• Velocity is u = (y/h)V
• Shear stress is tangential Force per unit area,

 = F/A

Stationary, u=

Moving, u=V V

x

y Y= 0

Y= h P

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Viscosity

• For Newtonian fluids, Shear stress is proportional to velocity

gradient.

• The proportional constant, , is called viscosity of the fluid

and has dimensions

• Viscosity has units of Pa-s or poise (lbm/ft hr) or cP
• Viscosity of a fluid may be determined by observing the

pressure drop of a fluid when it flows at a known rate in a

tube.

     dy

du yx

LT

M

Ln shear rate, 

Ln 

0.01 0.1 1 10 100

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