Chemical Engineering Department Polymer Technology (64572) Eng. Shadi Sawalha
Transcript
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Chemical Engineering Department Polymer Technology (64572) Eng.
Shadi Sawalha
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To acquire fundamental chemical and physical information on the
synthesis, production and characterization of polymer materials To
appreciate the breadth of polymer properties and applications, and
to learn in depth about polymers in a particular application area
Course Objectives
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Course contents Introduction to Polymers and Plastics
Introduction, Polymer structure and synthesis, Solid properties of
polymers, Mechanical properties, Rheological properties, Processing
of thermoplastics Thermoplastics Introduction, Polymer categories,
Additives, Polymer blends. Thermosets Materials and applications,
Processes Elastomers Introduction, Differences and similarities
between Plastics and elastomers, Types of elastomers, Properties of
elastomers, valcunizable elastomers, thermoplastic elastomers.
Plastics Additives Stabilizers, Fillers and reinforcements,
Coupling agents, Plasticizers, Lubricants and processing aids,
Foaming agents, Flame retardants, Colorants, Antistats, Organic
peroxides, Polymer blends, Miscellaneous additives Plastics
Recycling Introduction, Recycling collection, Recycling processing,
Design issues, Legislation, Biodegradable plastics
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Textbook Charles A. Harper, Handbook of Plastics Technologies,
McGraw-Hill, 2006 Reference Crawford R. J., Plastics Engineering,
Butterworth Heinemann, 3 rd edition, 2002
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Learning Outcomes and Competences At the end of this course
students should be able to; Have a knowledge about polymer
definition, structure, properties and types Understand
polymerization reactions Identify polymer types ( Thermoplastic,
thermoset, elastomers) and their applications Be familiar with
polymer processes and utilize the suitable process for needed final
product Select appropriate plastic additives according to required
functions in the final product Carry out the principles of
recycling to protect environment and save money.
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CHAPTER 1 INTRODUCTION TO POLYMERS AND PLASTICS Plastics are an
important part of everyday life; products made from plastics range
from sophisticated products, such as prosthetic hip and knee
joints, to disposable food utensils. The reasons for the great
popularity of plastics are the wide range of properties and ease of
proceesing which may due to: o Varying the atomic composition of
the repeat structure o Varying molecular weight and molecular
weight distribution o The presence of side chain branching, via the
lengths and polarities of the side chains o The degree of
crystallinity can be controlled through the amount of orientation
imparted to the plastic during processing, through
copolymerization
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CHAPTER 1 Polymers differ from the other materials in a variety
of ways but generally exhibit lower densities, thermal
conductivities, and modulii
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CHAPTER 1 Polymeric materials are used in a vast array of
products: o In the automotive area, they are used for interior
parts and in under-the-hood applications
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CHAPTER 1 o Packaging applications are a large area for
thermoplastics, from carbonated beverage bottles to plastic wrap
Application requirements vary widely but, luckily, plastic
materials can be synthesized to meet these varied service
conditions. It remains the job of the part designer to select from
the array of thermoplastic materials available to meet the required
demands
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CHAPTER 1 POLYMER STRUCTURE AND SYNTHESIS A polymer is prepared
by stringing together a low molecular weight species (monomer;
e.g., ethylene) into an extremely long chain (polymer; in the case
of ethylene, the polymer is polyethylene) much as one would string
together a series of bead to make a necklace
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CHAPTER 1 The chemical characteristics of the starting low
molecular weight species will determine the properties of the nal
polymer When two low different molecular weight species are
polymerized, the resulting polymer is termed a copolymerfor
example, ethylene vinylacetate
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random alternating block graft
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CHAPTER 1 Plastics can also be classied as either
thermoplastics or thermosets Thermoplastic a high molecular weight
polymer that is not crosslinked. It can exist in either a linear or
branched structure (has Vander Wals bonds between chains) Upon
heating, thermoplastics soften and melt, allowing them to be shaped
using plastics processing equipment Have ceiling temperature Scrap
can be recovered and recycled ( can be reprocessed) Thermoset has
all of the chains tied together with covalent bonds in a three
dimensional network (crosslinked) will not ow once crosslinked (
formed by chemical reactions) Can withstand high temperatures Cant
be recycled easily ( not by heating
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Chapter 1 Polymerization Reactions There are two primary
polymerization approaches: step-reaction polymerization and
chain-reaction polymerization Polymers synthesized by step reaction
typically have atoms other than carbon in the backbone. Examples
include polyesters and polyamides Chain-reaction polymers typically
contain only carbon in their backbone and include such polymers as
polystyrene and polyvinyl chloride.
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Chapter 1 Molecular Weights Unlike low molecular weight
species, polymeric materials do not possess one unique molecular
weight but rather a distribution of weights
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Chapter 1 Molecular Weights M w is larger than or equal to M
n
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Degree of Polymerization, n n = number of repeat units per
chain n i = 6 mol. wt of repeat unit iChain fraction
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Chapter 1 Viscosity Average One of the oldest methods of
measuring the average molecular weight of polymers is by solution
viscosity. The viscosity-average molecular weight, M v, lies
somewhere between the number average and the weight average Thus we
see that for a particular application only a certain molecular
weight range is practical for a given polymer. This range is a
compromise between optimum properties and ease of processing 1.
Most of the practically useful polymers have a DP between 200 to
2000, corresponding to a molecular weight range from 20,000 to
200,000.
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1.3 Solid Properties of Polymers Glass Transition Temperature
(T g )
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Crystallization and Melting Behavior (T m ) In its solid form,
a polymer can exhibit different morphologies, depending on the
structure of the polymer chain as well as the processing
conditions: o Amorphous: Random unordered structure(chains
entangled like spaghetti) ex. Polystyrene o Crystalline: ordered
regular structure. Ex: polypropylene and polyethylene which are
semicrystalline o Crystallininty can be controlled by different
synthetic methods ( Zeigler Natta catalyst) The amount of
crystallinity actually present in the polymer depends on a number
of factors, including the rate of cooling, crystallization
kinetics, and the crystallization temperature. Thus, the extent of
crystallization can vary greatly for a given polymer and can be
controlled through processing conditions
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1.4 Mechanical properties The mechanical behavior of polymers
is dependent on many factors, including polymer type, molecular
weight, and test procedure Polymeric material behavior may be
affected by other factors such as test temperature and rates. This
can be especially important to the designer when the product is
used or tested at temperatures near the glass transition
temperature, where dramatic changes in properties occur
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Viscoelasticity Polymer properties exhibit time-dependent
behavior, meaning that the measured properties are dependent on the
test conditions and polymer type
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Failure Behavior The design of plastic parts requires the
avoidance of failure without overdesign of the part, leading to
increased part weight The type of failure can depend on
temperatures, rates, and materials Materials that fail at rather
low elongations (1 percent strain or less) can be considered to
have undergone brittle failure Failure typically starts at a defect
where stresses are concentrated. Once a crack is formed, it will
grow as a result of stress concentrations at the crack tip. Many
amorphous polymers will also exhibit what are called crazes. Crazes
appear to look like cracks, but they are load bearing
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Ductile failure of polymers is exhibited by yielding of the
polymer or slip of the molecular chains past one another This is
most often indicated by a maximum in the tensile stress-strain test
or what is termed the yield point. Above this point, the material
may exhibit lateral contraction upon further extension, termed
necking Molecules in the necked region become oriented and result
in increased local stiffness Material in regions adjacent to the
neck are thus preferentially deformed, and the neck region
propagates. This process is known as cold-drawing (see Fig. 1.11).
Cold drawing results in elongations of several hundred
percent.
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Under repeated cyclic loading, a material may fail at stresses
well below the single cycle failure stress found in a typical
tensile test This process is called fatigue and is usually depicted
by plotting the maximum stress versus the number of cycles to
failure Fatigue tests can be performed under a variety of loading
conditions as specied by the service requirements Thermal effects
and the presence or absence of cracks are other variables to be
considered when the fatigue life of a material is to be
evaluated