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Plastics and Properties Important in

ExtrusionChapter 4

Professor Joe Greene CSU, CHICO

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

Polymer Chains

- Average Molecular Weight
- Polymers are made up of many molecular weights or

a distribution of chain lengths. - The polymer is comprised of a bag of worms of the

same repeating unit, ethylene (C2H4) with

different lengths some longer than others. - Example,
- Polyethylene -(C2H4)-1000 has some chains (worms)

with 1001 repeating ethylene units, some with

1010 ethylene units, some with 999 repeating

units, and some with 990 repeating units. - The average number of repeating units or chain

length is 1000 repeating ethylene units for a

molecular weight of 281000 or 28,000 g/mole .

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

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

States of Thermoplastic Polymers

- Amorphous- Molecular structure is incapable of

forming regular order (crystallizing) with

molecules or portions of molecules regularly

stacked in crystal-like fashion. - A - morphous (with-out shape)
- Molecular arrangement is randomly twisted,

kinked, and coiled

States of Thermoplastic Polymers

- 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

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

Types of Polymers

- Amorphous and Semi-Crystalline Materials

- PVC Amorphous
- PS Amorphous
- Acrylics Amorphous
- ABS Amorphous
- Polycarbonate Amorphous
- Phenoxy Amorphous
- PPO Amorphous
- SAN Amorphous
- Polyacrylates Amorphous

- LDPE Crystalline
- HDPE Crystalline
- PP Crystalline
- PET Crystalline
- PBT Crystalline
- Polyamides Crystalline
- PMO Crystalline
- PEEK Crystalline
- PPS Crystalline
- PTFE Crystalline
- LCP (Kevlar) Crystalline

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.

Elastomers

- Elastomers are materials capable of large elastic

deformations with elastic elongation gt 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

Flow Behavior of Plastic Melts

- Viscosity
- Defined as the materials resistance to flow
- Most important property of plastics for

processing - Low viscosity materials flow easily e.g. water,

syrup, olive oil - High viscosity materials flow very slowly when

heated most plastics, e.g., LDPE, HDPE, PP, PS,

PU, Nylon, PET, PBT, etc. - Units are Pascal-seconds (Metric N/m2-sec),

Poise (Englishlb/ft2-sec) - Viscosity can be reduce by
- flowing faster (increasing shear rate)
- increasing temperature

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

Plastic

Plastic Resin

Melt Index and Viscosity

- Melt index for common materials
- Material Temp Mass
- Polyethylene 190C 10 kg
- Nylon 235C 1 kg
- Polystyrene 200C 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

Melt Index and Molecular Weight

- Melt Index is indication of length of polymer

chains - Molecular Weight is a measurement of the length

of polymer chains - Melt index MW properties
- High melt index high flow short chains
- Low melt index low flow long chains
- Table 3.1 Melt Index and Molecular Weight of PS
- Mn Melt Index (g/10min)
- 100,000 10.00
- 150,000 0.30
- 250,000 0.05
- T200C with mass 5 kg

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

F

Cross Sectional Area A

A

F

A

F

?

? µd? /dt

Deformed Shape

F

Some Greek Letters

- Nu ?
- xi ?
- omicron ?
- pi ?
- rho ?
- sigma ?
- tau ?
- upsilon ?
- phi?
- chi ?
- psi ?
- omega?

- Alpha ?
- beta ?
- gamma ?
- delta ?
- epsilon ?
- zeta ?
- eta ?
- theta ?
- iota ?
- kappa ?
- lamda ?
- mu ?

Effect of Shearing

- Shear flows are present in plastic processing
- In shear flow (tangential flow), layers of the

plastic move at different velocities. - Rate of shearing is called the shear rate
- shear rate velocity/thickness
- Thin gaps high shear rates
- High flow rates high shear rates

Viscosity

- Viscosity is defined as a fluids resistance to

flow under an applied shear stress, Fig 2.2 - 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

P

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.

Viscosity

- For non-Newtonian fluids (plastics), Shear stress

is proportional to velocity gradient and the

viscosity function. - 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. Measured in - Cone-and-plate viscometer
- Capillary viscometer
- Brookfield viscometer

Viscosity

- Kinematic viscosity,? , is the ratio of viscosity

and density - Viscosities of many liquids vary exponentially

with temperature and are independent of pressure - where, T is absolute T, a and b
- units are in centipoise, cP

Viscosity Models

- Models are needed to predict the viscosity over a

range of shear rates. - Power Law Models (Moldflow First order)
- Moldflow second order model
- Moldflow matrix data
- Ellis model

Viscosity Models

- Models are needed to predict the viscosity over a

range of shear rates. - Power Law Models (Moldflow First order)
- where m and n are constants.
- If m ? , and n 1, for a Newtonian

fluid, - you get the Newtonian viscosity, ?.
- For polymer melts n is between 0 and 1 and is the

slope of the viscosity shear rate curve. - To find constants, take logarithms of both sides,

and find slope and intercept of line

Shear Thinning or Pseudoplastic Behavior

- Viscosity changes when the shear rate changes
- Higher shear rates lower viscosity
- Results in shear thinning behavior
- Behavior results from polymers made up of long

entangles chains. The degree of entanglement

determines the viscosity - High shear rates reduce the number of

entanglements and reduce the viscosity. - Power Law fluid viscosity is a straight line in

log-log scale. - Consistency index viscosity at shear rate 1.0
- Power law index, n slope of log viscosity and

log shear rate - Newtonian fluid (water) has constant viscosity
- Consistency index 1
- Power law index, n 0

Effect of Temperature on Viscosity

- When temperature increases viscosity reduces
- Temperature varies from one plastic to another
- Amorphous plastics melt easier with temperature.
- Temperature coefficient ranges from 5 to 20,
- Viscosity changes 5 to 20 for each degree C

change in Temp - Barrel changes in Temperature has larger effects
- Semicrystalline plastics melts slower due to

molecular structure - Temperature coefficient ranges from 2 to 3

Viscosity

Temperature

Viscous Heat Generation

- When a plastic is sheared, heat is generated.
- Amount of viscous heat generation is determined

by product of viscosity and shear rate squared. - Higher the viscosity higher viscous heat

generation - Higher the shear rate higher viscous heat

generation - Shear rate is a stronger source of heat

generation - Care should be taken for most plastics not to

heat the barrel too hot due to viscous heat

generation

Thermal Properties

- Important is determining how a plastic behaves in

an extruder. Allows for - selection of appropriate machine selection
- setting correct process conditions
- analysis of process problems
- Important thermal properties
- thermal conductivity
- specific heat
- thermal stability and induction time
- Density
- Melting point and glass transition

Thermal Conductivity

- Most important thermal property
- Ability of material to conduct heat
- Plastics have low thermal conductivity

insulators - Thermal conductivity determines how fast a

plastic can be processed. - Non-uniform plastic temperatures are likely to

occur. - Long times are needed to equalize temperatures
- Channel is 20 mm in diameter, it may take 5 to 10

minutes for temperatures to equalize - Typical residence is 30 seconds.
- Results in high temperature melt stream persists

all through the die and causes non-uniform flow

at the die exit and a local thick spot in

extruded product.

Specific Heat and Enthalpy

- Specific Heat
- The amount of heat necessary to increase the

temperature of a material by one degree. - Most cases, the specific heat of semi-crystalline

plastics are higher than amorphous plastics. - The amount of heat necessary to raise the

temperature of a material from a base temperature

to a higher temperature is determined by the

enthalpy differences between two temperatures. - If you know the starting temperature (room T) and

the ending temperature (die exit) then we can

determine the energy required to heat plastic

material. - Enthalpy to heat of PVC from Room T to 175C is

150 kW.hr/kg or for 100 kg/hr (220lbs/hr) the

minimum power is 5 kW (6.7 HP) - LDPE is much higher enthalpy than PVC, or it

takes more energy to heat up and cool down than

PVC

Specific Heat and Enthalpy

- Specific Heat
- The amount of heat necessary to increase the

temperature of a material by one degree. - Most cases, the specific heat of semi-crystalline

plastics are higher than amorphous plastics. - If an amount of heat is added ?Q, to bring about

an increase in temperature, ?T. - Determines the amount of heat required to melt a

material and thus the amount that has to be

removed during injection molding. - The specific heat capacity is the heat capacity

per unit mass of material. - Measured under constant pressure, Cp, or constant

volume, Cv. - Cp is more common due to high pressures under Cv

Specific Heat and Enthalpy

- Specific Heat Capacity
- Heat capacity per unit mass of material
- Cp is more common than Cv due to excessive

pressures for Cv - Specific Heat of plastics is higher than that of

metals - Table

Thermal Stability and Induction Time

- Plastics degrade in plastic processing.
- Variables are
- temperature
- length of time plastic is exposed to heat

(residence time) - Plastics degrade when exposed to high

temperatures - high temperature more degradation
- degradation results in loss of mechanical and

optical properties - oxygen presence can cause further degradation
- Induction time is a measure of thermal stability.
- Time at elevated temperature that a plastic can

survive without measurable degradation. - Longer induction time better thermal stability
- Measured with TGA (thermogravimetric analyzer),

TMA

Thermal Conductivity

Q

T?T

T

- Most important thermal property
- Ability of material to conduct heat
- Plastics have low thermal conductivity

insulators - Thermal conductivity determines how fast a

plastic can be processed. - Non-uniform plastic temperatures are likely to

occur. - Where, k is the thermal conductivity of a

material at temperature T. - K is a function of temperature, degree of

crystallinity, and level of orientation - Amorphous materials have k values from 0.13 to

0.26 J/(msK) - Semi-crystalline can have higher values

Thermal Stability and Induction Time

- Plastics degrade in plastic processing.
- Variables are
- temperature
- length of time plastic is exposed to heat

(residence time) - Plastics degrade when exposed to high

temperatures - high temperature more degradation
- degradation results in loss of mechanical and

optical properties - oxygen presence can cause further degradation
- Induction time is a measure of thermal stability.
- Time at elevated temperature that a plastic can

survive without measurable degradation. - Longer induction time better thermal stabilty
- Measured with TGA (thermogravimetric analyzer),

TMA

Thermal Stability and Induction Time

- Plastics degrade in plastic processing.
- Induction time measured at several temperatures,

it can be plotted against temperature. Fig 4.13 - The induction time decreases exponentially with

temperature - The induction time for HDPE is much longer than

EAA - Thermal stability can be improved by adding

stabilizers - All plastics, especially PVC which could be

otherwise made.

Density

- Density is mass divided by the volume (g/cc or

lb/ft3) - Density of most plastics are from 0.9 g/cc to 1.4

g/cc_ - Table 4.2
- Specific volume is volume per unit mass or

(density)-1 - Density or specific volume is affected by

temperature and pressure. - The mobility of the plastic molecules increases

with higher temperatures (Fig 4.14) for HDPE. PVT

diagram very important!! - Specific volume increases with increasing

temperature - Specific volume decrease with increasing

pressure. - Specific volume increases rapidly as plastic

approaches the melt T. - At melting point the slope changes abruptly and

the volume increases more slowly.

Melting Point

- Melting point is the temperature at which the

crystallites melt. - Amorphous plastics do not have crystallites and

thus do not have a melting point. - Semi-crystalline plastics have a melting point

and are processed 50 C above their melting

points. Table 4.3 - Glass Transition Point
- Point between the glassy state (hard) of plastics

and the rubbery state (soft and ductile). - When the Tg is above room temperature the plastic

is hard and brittle at room temperature, e.g., PS - When the Tg is below room temperature, the

plastic is soft and flexible at room temperature,

e.g., HDPE

Thermodynamic Relationships

- Expansivity and Compressibility
- Equation of state relates the three important

process variables, PVT - Pressure, Temperature, and Specific Volume.
- A Change in one variable affects the other two
- Given any two variables, the third can be

determined - where g is some function determined

experimentally. - Reference MFGT242 Polymer Flow Analysis Book

Thermodynamic Relationships

- Coefficient of volume expansion of material, ?,

is defined as - where the partial differential expression is the

instantaneous change in volume with a change in

Temperature at constant pressure - Expansivity of the material with units K-1
- Isothermal Compressibility, ?, is defined as
- where the partial differential expression is the

instantaneous change in volume with a change in

pressure at constant temperature - negative sign indicated that the volume decreases

with increasing pressure - isothermal compressibility has units m2/N

PVT Data for Flow Analysis

- PVT data is essential for
- packing phase and the filling phase.
- Warpage and shrinkage calculations
- Data is obtained experimentally and curve fit to

get regression parameters - For semi-crystalline materials the data falls

into three area - Low temperature
- Transition
- High temperature

Temperature, C

PVT Data for Flow Analysis

- Data is obtained experimentally and curve fit to

get regression parameters - For amorphous there is not a sudden transition

region from melt to solid. There are three

general regions - Low temperature
- Transition
- High temperature

Temperature, C

PVT Data for Flow Analysis

- The equations fitted to experimental data in

previous PVT Figures 2.11 and 2.12 are - Note All coefficients are found with regression

analysis - Low Temperature region
- High Temperature Region
- Transition Region