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OCR for page 62
62
_8 FEW YEARS AGO, who
would have dreamed that
Man aircraft could cir-
cumnavigate the earth without
landing or refueling? Yet in 1986
the novel aircraft Voyager did
just that (Figure 5.11. The secret
of Voyager's long flight lies in
advanced materials that did not
exist a few years ago. Much of
the airframe was constructed from
strong, lightweight polymer-fiber
composite sections assembled
with durable, high-strength ad-
hesives; the engine was lubri-
cated with a synthetic multicom-
ponent liquid designed to maintain
lubricity for a long time under
continuous operation. These
special materials typify the ad-
vances being made by scientists
and engineers to meet the de-
mands of modern society.
The future of industries such
as transportation, communications, electronics,
and energy conversion hinges on new and im-
proved materials and the processing technolo-
gies required to produce them. Recent years
have seen rapid advances in our understanding
of how to combine substances into materials
with special, high-performance properties and
how to best use these materials in sophisticated
designs.
Chemical engineers have long been involved
in materials science and engineering and will
become increasingly important in the future.
Their contributions will fall in two categories.
For commodity materials, which are nonpro-
prietary formulations with well-established
chemical compositions and property standards,
chemical engineers will help maintain U.S.'com-
petitiveness by creating and improving pro-
cesses to make these chemicals as pure as
possible and in high yields at the lowest possible
investment and operating costs. For advanced
materials, which are generally multicomponent,
often proprietary, compositions designed to have
very specific performance properties in specific
uses, the competitive edge will come from
chemical engineers who excel in controlling
FRONTIERS IN CHEUICAL ENGINEERING
FIGURE 5.1 The first airplane to circle the globe without refueling was
Voyager, which accomplished this feat in 1986. This novel aircraft was made
possible by high-performance lightweight materials and adhesives that were
used in its construction. Chemical engineering research is crucial to the design
of such new materials and their large-scale, efficient production. Copyright
1986 by Doug Shane Visions.
molecular conformation, microscopic and mac-
roscopic structure, and methods of combining
the components in a way that will maximize
product performance.
Chapter 4 discussed chemical engineering
challenges presented by materials and chemi-
cally processed devices for information storage
and handling. In this chapter, five additional
classes of materials are covered: polymers,
polymer composites, advanced ceramics, ce-
ramic composites, and composite liquids.
CHALLENGES TO CHEMICAL ENGINEERS
The revolution in materials science and en-
gineering presents both opportunities and chal-
lenges to chemical engineers. With their basic
background in chemistry, physics, and mathe-
matics and their understanding of transport
phenomena, thermodynamics, reaction engi-
neering, and process design, chemical engineers
can bring innovative solutions to the problems
of modern materials technologies. But it is
imperative that they depart from the traditional
"think big" philosophy of the profession; to
participate effectively in modern materials sci
OCR for page 63
POLYMERS, CERAMICS, AND COMPOSITES
ence and engineering they must learn to "think
small." The crucial phenomena in making mod-
ern advanced materials occur at the molecular
and microscale levels, and chemical engineers
must understand and learn to control such
phenomena if they are to engineer the new
products and processes for making them. This
crucial challenge is illustrated in the selected
materials areas described in the following sec-
tions.
Polymers
The modern era of polymer science belongs
to the chemical engineer. Over the years, poly-
mer chemists have invented a wealth of novel
macromolecules and polymers. Yet understand-
ing how these molecules can be synthesized and
processed to exhibit their maximum theoretical
properties is still a frontier for research. Only
recently has modern instrumentation been de-
veloped to help us understand the fundamental
interactions of macromolecules with them-
selves, with particulate solids, with organic and
inorganic fibers, and with other surfaces. Chem-
ical engineers are using these tools to probe the
microscale dynamics of macromolecules. Using
the insight gained from these techniques, they
are manipulating macromolecular interactions
both to develop improved processes and to
create new materials.
The power of chemical processing for con-
trolling materials structure on the microscale is
illustrated by the current generation of high-
strength polymer fibers, some of which have
strength-to-weight ratios an order of magnitude
greater than steel. The best known example of
these fibers, Kevlar~, is prepared by spinning
an aramid polymer from an anisotropic phase
(a liquid phase in which molecules are sponta-
neously oriented over microscopic dimensions).
This spontaneous orientation is the result of
both the processing conditions chosen and the
highly rigid linear molecular structure of the
aramid polymer. During spinning, the oriented
regions in the liquid phase align with the fiber
axis to give the resulting fiber high strength and
rigidity. The concept of spinning fibers from
anisotropic phases has been extended to both
solutions and melts of newer polymers, such as
63
polybenzothiazole, as well as traditional poly-
mers such as polyethylene. Ultrahigh-strength
fibers of polyethylene have been prepared by
gel spinning. The same concept, controlling the
molecular orientation of polymers to produce
high strength, is also being achieved through
other processes, such as fiber-stretching carried
out under precise conditions.
In addition to processes that result in mate-
rials with specific high-performance properties,
chemical engineers continue to design new pro-
cesses for the low-cost manufacture of poly-
mers. The UNIPOL process for the manufacture
of polyethylene is a good example of the con-
tributions of engineering research to polymer
processing. Polyethylene is probably the quin-
tessential commodity polymer. It has been man-
ufactured worldwide for decades, and current
U.S. production exceeds 15 billion pounds per
year. Considering the global capital investment
in existing plants for making polyethylene, it
could be argued that inventing a new process
for its manufacture is a waste of time and money.
Not so. Chemical engineers at Union Carbide
designed a proprietary catalyst that allowed
polyethylene to be made in a fluidized-bed, gas-
phase reactor operating at low temperature and
pressure (below 100°C and 21 Bar). The resulting
process produces a polymer with exceptional
uniformity and can precisely control the molec-
ular weight and density of the product. The
advantages of the process (including a low safety
hazard from the mild operating conditions and
minimal environmental impact since there are
no liquid effluents and unreacted gases are
recirculated) are such that, in 1986, UNIPOL
process licensees had a combined capacity suf-
ficient to supply 25 percent of the world's
demand for polyethylene. This is remarkable
market penetration for a new process technol-
ogy for a mature commodity, particularly in
light of the tremendous existing (and fully am-
ortized) worldwide capacity for polyethylene.
In 1985, Union Carbide and Shell Chemical
successfully extended the UNIPOL process to
the manufacture of polypropylene, another ma-
jor polymer commodity. Interestingly, the first
two licensees for the new polypropylene process
were a Japanese chemical company and a Ko-
rean petroleum company.
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64
Polymer Composites
Polymer composites consist of high-strength
or high-modulus fibers embedded in and bonded
to a continuous polymer matrix (Figure 5.21.
These fibers may be short, long, or continuous.
They may be randomly oriented so that they
impart greater strength or stiffness in all direc-
tions to the composite (isotropic composites),
or they may be oriented in a specific direction
so that the high-performance characteristics of
the composite are exhibited preferentially along
one axis of the material (anisotropic compos-
ites). These latter fiber composites are based
on the principle of one-dimensional microstruc-
tural reinforcement by disconnected, tension-
bearing"cables" or"rods."
To achieve a material with improved prop-
erties (e.g., strength, stiffness, or toughness) in
more than one dimension, composite laminates
can be formed by bonding individual sheets of
anisotropic composite in alternating orienta-
tions. Alternatively, two-dimensional reinforce-
ment can be achieved in a single sheet by using
fabrics of high-performance fibers that have
been woven with enough bonding in the cross-
overs that the reinforcing structure acts as a
connected net or trusswork. One can imagine
that an interdisciplinary collaboration between
WHISKERS
OR SHORT
FIBERS
FABRIC
,,, /
v
LONG
FIBERS
~ ,' ,' ,' ,''~
O () () Oo/
LAMINATE
FIGURE 5.2 Fibers that are either very strong or very stiff
can be used to reinforce polymers and ceramics. The
resulting materials, known as composites, usually have one
of the structures depicted in this figure. Clockwise from the
upper left, reinforcement may be accomplished by embed-
ding randomly oriented fibers, by orienting fibers along a
particular axis, by assembling reinforced layers into lami-
nates, or by embedding fabrics of reinforcing fibers in the
material.
FRONTIERS IN CHEMICAL ENGlNEERl\G
chemical engineers and textile engineers might
lead to ways of selecting the warp, woof, and
weave in fabrics of high-strength fibers to end
up with trussworks for composites with highly
tailored dimensional distributions of properties.
First-generation polymer composites (e.g.,
fiberglass) used thermosetting epoxy polymers
reinforced with randomly oriented short glass
fibers. The filled epoxy resin could be cured
into a permanent shape in a mold to give
lightweight, moderately strong shapes.
The current generation of composites is being
made by hand laying woven glass fabric onto a
mold or preform, impregnating it with resin,
and curing to shape. Use of these composites
was pioneered for certain types of military
aircraft because the lighter airframes provided
greater cruising range. Today, major compo-
nents for aircraft and spacecraft are manufac-
tured in this manner, as are an increasing
number of automobile components. The current
generation of composites are being used in
automotive and truck parts such as body panels,
hoods, trunk lids, ducts, drive shafts, and fuel
tanks. In such applications, they exhibit a better
strength-to-weight ratio than metals, as well as
improved corrosion resistance. For example, a
polymer composite automobile hood is slightly
lighter than one of aluminum and more than
twice as light as one of steel. The level of energy
required to manufacture this hood is slightly
lower than that required for steel and about 20
percent of that for aluminum; molding and
tooling costs are lower and permit more rapid
model changeover to accommodate new de-
signs. Polymer composite hoods and trunk lids
are commercial on the 1987 models of one major
U.S. automobile line, and the early problems
of higher manufacturing cost and of achieving
adequate production have been largely over
come.
The mechanical strength exhibited by these
composites is essentially that of the reinforcing
glass fibers, although this is often compromised
by structural defects. Engineering studies are
yielding important information about how the
properties of these structures are influenced by
the nature of the glass-resin interface and by
structural voids and similar defects and how
microdefects can propagate into structural fail
OCR for page 65
POLYMERS, CERAMICS, AND COMPOSITES
ure. These composites and the information gained
from studying them have set the stage for the
next generation of polymer composites, based
on high-strength fibers such as the aramids.
Advanced Ceramics
For most people, the word "ceramics" con-
jures up the notion of things like china, pottery,
tiles, and bricks. Advanced ceramics differ from
these conventional ceramics by their composi-
tion, processing, and microstructure. For ex-
ample:
· Conventional ceramics are made from nat-
ural raw materials such as clay or silica; ad-
vanced ceramics require extremely pure man-
made starting materials such as silicon carbide,
silicon nitride, zirconium oxide, or aluminum
oxide and may also incorporate sophisticated
additives to produce specific microstructures.
· Conventional ceramics initially take shape
on a potter's wheel or by slip casting and are
fired (sintered) in kilns; advanced ceramics are
formed and sintered in more complex processes
such as hot isostatic pressing.
· The microstructure of conventional ce-
ramics contains flaws readily visible under op-
tical microscopes; the microstructure of ad-
vanced ceramics is far more uniform and typically
is examined for defects under electron micro-
scopes capable of magnifications of 50,000 times
or more.
Advanced ceramics have a wide range of
application (Figure 5.3~. In many cases, they
do not constitute a final product in themselves,
but are assembled into components critical to
the successful performance of some other com-
plex system. Commercial applications of ad-
vanced ceramics can be seen in cutting tools,
engine nozzles, components of turbines and
turbochargers, tiles for space vehicles, cylinders
to store atomic and chemical waste, gas and oil
drilling valves, motor plates and shields, and
electrodes for corrosive liquids.
Because advanced ceramics provide key com-
ponents to other technologies for major im-
provements in performance, their impact on the
U.S. economy is much greater than is indicated
by their sales figures. Ceramic components used
65
in turbines permit the construction of engines
that operate at much higher temperatures than
metallic engines, thus greatly increasing their
thermodynamic efficiency and compactness.
Ceramic liners and other ceramic components
in diesel engines provide added benefits, such
as the elimination of the need for water cooling
and the prompter ignition of the fuel. An in-
vestment in wear-resistant ceramic cutting tools
can be more than repaid by the decrease in
downtime for sharpening or replacing a dulled
or worn metallic tool.
Given these advantages, it is not surprising
that market forecasts for advanced ceramics
(including ceramic composites) are optimistic;
in fact, sales in the year 2000 are predicted to
be $20 billion. The market for advanced ce-
ramics in heat engines is slated to grow by 40
percent per year to a total of $1 billion in 2000.
The use of advanced ceramics is predicted to
grow 16 percent per year over the next 5 years,
and sales for automotive applications are fore-
cast to increase from $53 million per year in
1986 to $6 billion per year by the end of the
century.
Uniform microstructure is crucial to the su-
perior performance of advanced ceramics. In a
ceramic material, atoms are held in place by
strong chemical bonds that are impervious to
attack by corrosive materials or heat. At the
same time, these bonds are not capable of much
"give." When a ceramic material is subjected
to mechanical stresses, these stresses concen-
trate at minute imperfections in the microstruc-
ture, initiating a crack. The stresses at the top
of the crack exceed the threshold for breaking
the adjacent atomic bonds, and the crack prop-
agates throughout the material causing a cata-
strophic brittle failure of the ceramic body. The
reliability of a ceramic component is directly
related to the number and type of imperfections
in its microstructure.
As the requirements for greater homogeneity
in ceramics become more stringent, and the
scale at which imperfections occur becomes
smaller, the need for chemical processing of
ceramics becomes more compelling. Traditional
approaches to controlling ceramic microstruc-
ture, such as the grinding of powders, are
reaching the limits of their utility for microstruc
OCR for page 66
66
wee,
resistance
Low thermal
cxDension
Ret rac~ori ness
I nsula~ion
..:r ~
f ~ Thermal
is' ' \conductivitV
H igh \
strength A
/''.. ."..''~..'.'.~..',,',2.
#. , . ~ ~ . ~ .... ~ Hi i. ~
Advanced
ceramics
Biological
compatibility
tural control. Chemical engineers have an un-
paralleled opportunity to contribute their ex-
pertise in reaction engineering to problems that
are in need of new analytical, synthetic, and
processing tools. These include sol-gel process-
ing and the use of chemical additives in ceramic
processing.
FRONTIERS IV Cl~iE.~L 3ElYGIlYEERING
strength /
Optical
condensing
F I uorescence
Transl ucence
., ~ .,.,./
,' Optical
, ~ cow conductivity
.,.,.,.~.,..,y. ~
~ Electrical \
Electrical conductivity
Semiconductivity
Piezoelectric
Dielectric
Magnetic am
FIGURE 5.3 The myriad functions, properties, and applications of advanced
ceramics. Reprinted from High-Technology Ceramics in Japan, National
Materials Advisory Board, National Research Council, 1984.
Sol-Gel Processing
The use of sol-gel techniques to prepare
ceramic powders has recently attracted much
interest in academia and industry. Sol-gel tech-
niques involve dissolving a ceramic precursor
(e.g., tetramethyl orthosilicate) in a solvent and
OCR for page 67
POLYMERS, CERAMICS' AND COMPOSITES
67
FIGURE 5.4 Stages in sol-gel processing are captured by a new electron microscopy
technique. (1) Spherical particles tens of nanometers across can be seen in a
colloidal silica sol. (2) Addition of a concentrated salt solution initiates "elation.
(3) The gelled sample, after drying under the electron beam of the microscope'
shows a highly porous structure. Courtesy, J. R. Bellare, J. K. Bailey, and M. L.
Mecartney, University of Minnesota.
subjecting it to a carefully controlled chemical
reaction, hydrolysis (Figure 5.41. When the
hydrolysis products first appear as a separate
phase, they are fine colloids consisting of small
particles, some with radii as small as a few
nanometers. This colloidal suspension (the sol)
further reacts and polymerizes to form a porous
high-molecular-weight solid (the gel) that con-
tains the solvent as a highly dispersed fluid
component in its internal network structure.
Removal of the solvent leaves behind solids
with a wide variety of macrostructures depend-
ing on the solvent and the way in which it was
removed. These macrostructures can be sin-
tered to convert them to dense ceramics.
Sol-gel techniques are of interest because
they can be used to prepare powders with a
narrow distribution of particle size. These small
particles undergo sintering to high density at
temperatures lower by several hundred degrees
centigrade than those used in conventional ce-
ramic processing. Sol-gel processes may also
be used to prepare novel glasses and ceramics
such as
· ceramics with novel microstructures and
distributions of phases,
~ amorphous powders and dried gels that can
be processed without crystallization to fully
dense amorphous materials whose synthesis
might not otherwise be possible,
· materials with controlled degrees of poros-
ity and possibly tailored surfaces within pores,
and
· ceramics with surfaces modified to alter
their response to mechanical forces or to pro-
mote their adhesion to other materials.
Sol-gel processes also allow the manufacture of
preforms that, upon sintering, collapse to a final
product with the proper shape.
There are many unresolved problems in sol-
gel processing, many of which revolve around
the poorly characterized chemistry of the pro-
cess. Understanding and controlling the poly-
merization reactions that produce the gel are
key challenges, as are characterizing and opti-
mizing both the removal of fluid from the gel
and the subsequent sintering of the porous solid
to a fully dense ceramic body. Solving these
problems will make sol-gel processing the pro-
cess of choice for the synthesis of a wide variety
of ceramics, glasses, and coatings.
Chemical Additives in Ceramic Processing
Another area to which chemical engineers
can contribute is the use of chemical additives
OCR for page 68
68
to improve the properties of ce-
ramic materials. For example,
zirconium oxide can form a meta-
stable state in ceramic bodies
that is denser than its normal
state. The incorporation of suit-
able chemical additives stabilizes
the metastable state sufficiently
to allow the fabrication of parts
containing it. When a crack forms
in such a ceramic part, the zir-
conium oxide region at the crack
tip changes to the less dense
form. The resulting expansion
blunts the crack tip and stops its
propagation (Figure 5.51. This
strategy for using a chemical ad-
ditive to improve ceramic resis-
tance to cracking is called trans-
formation toughening.
Ceramic Composites
Like polymer composites, ce-
ramic composites consist of high-
strength or high-modulus fibers
embedded in a continuous ma-
trix. Fibers may be in the form
of "whiskers" of substances such
as silicon carbide or aluminum
oxide that are grown as single crystals and that
therefore have fewer defects than the same
substances in a bulk ceramic (Figure 5.6~. Fibers
in a ceramic composite serve to block crack
propagation; a growing crack may be deflected
to a fiber or might pull the fiber from the matrix.
Both processes absorb energy, slowing the
propagation of the crack. The strength, stiffness,
and toughness of a ceramic composite is prin-
cipally a function of the reinforcing fibers, but
the matrix makes its own contribution to these
properties. The ability of the composite material
to conduct heat and current is strongly influ-
enced by the conductivity of the matrix. The
interaction between the fiber and the matrix is
also important to the mechanical properties of
the composite material and is mediated by the
chemical compatibility between fiber and matrix
at the fiber surface. A prerequisite for adhesion
between these two materials is that the matrix,
FRONTIERS IN CHEMICAL ENGINEERING
FIGURE 5.5 Zirconia ceramics can be made stronger and less brittle by using
chemical additives to stabilize a more compact tetragonal structure that does
not naturally occur at room temerature. When such a phase is subjected to
stress, it can change phases, expanding to the monoclinic structure. This
expansion fills any stress-initiated crack and prevents it from moving. This
micrograph shows a zirconia ceramic composed of lozenge-shaped grains.
pore with grain boundaries radiating from its top and base dominates the
picture. Courtesy, National Physical Laboratory (United Kingdom).
in its fluid form, be capable of wetting the fibers.
Chemical bonding between the two components
can then take place.
Ceramic matrix composites are produced by
one of several methods. Short fibers and whis-
kers can be mixed with a ceramic powder before
the body is sintered. Long fibers and yarns can
be impregnated with a slurry of ceramic particles
and, after drying, be sintered. Metals (e.g.,
aluminum, magnesium, and titanium) are fre-
quently used as matrixes for ceramic composites
as well. Ceramic metal-matrix composites are
fabricated by infiltrating arrays of fibers with
molten metal so that a chemical reaction be-
tween the fiber and the metal can take place in
a thin layer surrounding the fiber.
As with advanced ceramics, chemical reac-
tions play a crucial role in the fabrication of
ceramic composites. Both defect-free ceramic
fibers and optimal chemical bonds between fiber
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POLYMERS, CERAMICS, ANID COMPOSITES
FIGURE 5.6 This is a fractured sample of a ceramic
composite (alumina with 30 volume-percent silicon carbide
whiskers). The lighter regions of circular or cylindrical
shape are randomly oriented whiskers protruding from the
fractured surface. The rod-like depressions in the surface
mark places where whiskers nearly parallel with the fracture
were pulled out. Courtesy, Roy W. Rice, W. R. Grace and
Company.
and matrix are required for these composites to
exhibit the desired mechanical properties in
use. Engineering these chemical reactions in
reliable manufacturing processes requires the
expertise of chemical engineers.
Composite Liquids
A final important class of composite materials
is the composite liquids. Composite liquids are
highly structured fluids based either on particles
or droplets in suspension, surfactants, liquid
crystalline phases, or other macromolecules. A
number of composite liquids are essential to the
needs of modern industry and society because
they exhibit properties important to special end
uses. Examples include lubricants, hydraulic
traction fluids, cutting fluids, and oil-drilling
muds. Paints, coatings, and adhesives may also
be composite liquids. Indeed, composite liquids
are valuable in any case where a well-designed
liquid state is absolutely essential for proper
delivery and action.
All composite liquids are produced by the
69
chemical processing industries, and chemical
engineers face continuing challenges in tailoring
their end-use properties. Some of these chal-
lenges are illustrated in the following examples:
· Motor lubricants are complex liquid com-
posites in which components provide different
performance characteristics. The basic com-
ponent is a hydrocarbon oil with a fixed boiling
range. It must have sufficient viscosity at engine
operating temperatures to prevent the friction
and wear of moving surfaces, but must be fluid
enough below freezing temperatures for winter
start-up. Viscosity modifiers are high-molecu-
lar-weight polymers that reduce the temperature
coefficient of viscosity (viscosity index). Sus-
pended colloidal particles of calcium or mag-
nesium carbonate are added to neutralize engine
acids and are stabilized by adsorbed polymers
and surfactants to prevent coalescence. Solids
dispersants are low-molecular-weight polymers
with functional groups that pick up carbon
particles generated in combustion and maintain
them in suspension. At low temperatures, the
waxes (straight-chain paraffin hydrocarbons) in
the lubricant form long crystals to set up a solid
gel. To prevent this, low-molecular-weight poly-
mers, called pour point depressants, are added
to co-crystallize with the wax; the resulting
smaller crystals do not gel. Finally, there are
antiwear additives and antioxidants to reduce
engine wear and deposits. Lubricants with out-
standing viscosity indexes enable an engine to
start when the lubricant temperature is as low
as -40°C and yet operate well when the lubri-
cant temperature is as high as 200°C. Other
additives allow broadening the temperature range
further by providing increased thermal and ox-
idative stability. The use of synthetic base oils
allows still broader ranges of operating temper-
atures, up to 500°C.
~ Advanced adhesives are composite liquids
that can be used, for example, to join aircraft
parts, thus avoiding the use of some 30,000
rivets that are heavy, are labor-intensive to
install, and pose quality-control problems. Ad-
hesives research has not involved many chem-
ical engineers, but the generic problems include
surface science, polymer rheology and ther-
modynamics, and molecular modeling of ma
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70
serials near or at interfaces. The scientific and
engineering skills needed are very similar to
those needed for polymer composites and mul-
ticomponent polymer blends. The time-tested
mechanical methods developed for joining met-
als are not satisfactory for composite and other
advanced materials, and chemical engineers
skilled in interracial science are well qualified
to contribute to this area.
· Another class of liquid composites is that
of coating compositions used to deposit thin
films on a substrate or other films; these com-
posites have evolved from typical paints and
varnishes into multilayer films in which each
layer contributes specific properties to the en-
semble. Such films may be paints used for
sealing and decorative purposes, films used for
printing or packaging purposes, or multilayer
products used in recording tapes and photo-
graphic products. All are based on generic
scientific principles that include many common
elements from thermodynamics, polymer sci-
ence, rheology, and fluid mechanics.
Liquid composites seldom behave as New-
tonian fluids. These complex mixtures usually
contain macromolecules, suspended particles,
and surfactants. They are frequently multi-
phase, and changes in phase composition or
formation of new liquid phases may occur over
the range of operating conditions. Phase com-
position may be shifted by chemical reaction,
by shear forces, or simply by changes in tem-
perature or pressure. Liquid-liquid and liquid-
solid equilibria are crucial. Detailed molecular
understanding of the interactions among such
components as surfactants, polymers, and par-
ticles is essential for the rational design of liquid
composites. Much of this design is now accom-
plished by informed empiricism, which is useful
for the incremental improvement of current
products but inadequate for major changes and
innovation.
INTERNATIONAL COMPETITION
The potential markets for the advanced ma-
terials discussed in this chapter are lucrative,
and most nations that possess the technological
infrastructure needed to invent, develop, and
FRONTIERS l^\ C~EMICAL EAGINEERING
understand these materials are mounting major
efforts to exploit these developing markets.
Polymers and Polymer Composites
The Panel on Advanced Materials of the NSF
Japanese Technology Evaluation Program
(JTECH) issued a report in 1986 assessing the
status and direction of Japanese research and
development efforts in several high-technology
polymer areas.' The panel noted that the major
Japanese chemical companies already manufac-
ture most of the commercially available poly-
mers and that "since 1970, there has been an
increasing flow of upgraded technology from
Japan to the United States." For engineering
plastics and resins, the panel judged the United
States to be ahead in basic research (although
the lead is diminishing), on a par with Japan in
advanced development, and behind Japan in
product implementation.
The JTECH panel also compared the U.S./
Japanese position in high-strength/high-modulus
polymer research and development. Its conclu-
sions substantially agree with the following
statements, drawn from a recent report of the
NAS/NAE/IOM Committee on Science, Engi-
neering, and Public Policy.2 The United States
has a strong position in the development of
high-strength polymers, but comparable activity
in this area exists in Japan. The Japanese
Ministry of International Trade and Industry
(MITI) has designated the development of a
"third-generation" fiber as a government-sub-
sidized project, beginning in 1983 and targeted
for practical application by 1988. Because of
the overwhelming importance of the load-bear-
ing fibers in composites, the field is sensitive to
breakthroughs in stronger fibers. Such a break-
through could come from Japan; for example,
a Japanese group was first to patent a process
for making high-strength fibers from polytethylene
terephthalate) .
Much of the technology used for manufac-
turing carbon fibers in the United States is
licensed from Japanese companies. The high
level of Japanese carbon-fiber technology sug-
gests that Japanese companies may produce
many of the expected future advances in these
materials.
OCR for page 71
POLYMERS, CERAMICS, A\D COMPOSITES
The United States currently has a strong
position in composite manufacturing and pro-
cessing technology and leads the world in de-
veloping major applications in aircraft, sporting
goods, and automotive components. There is
growing overseas activity in composites tech-
nology, however, particularly among European
aircraft companies.
Ceramics and Ceramic Composites
Japan is the United States' chief competitor
in ceramics. There is a widespread but false
-perception that Japan leads the United States
in ceramics in general. Nevertheless, it is clear
that the Japanese effort in ceramics is compre-
hensive and long term and that several Japanese
companies lead their U.S. industrial counter-
parts in specific technologies. There is tremen-
dous enthusiasm in Japan for the potential of
ceramics, and a recent report of a U.S. visiting
team to Japan3 reached the following conclu-
sions:
· The Japanese are committed to vigorously
developing and dominating the field of advanced
ceramics. They have put in place a well-inte-
grated national effort primarily based on gov-
ernment-industry interactions.
· Industrial management commitment to long-
term research and development appears to be
more solid in Japan than in the United States.
· Japanese research is focusing increasingly
on fundamental research issues. Government-
inspired basic research programs are being put
in place, and Japan's position as a net consumer
of basic ceramics research may change in the
coming years.
Given the potential future importance of ce-
ramics in areas as diverse as electronics (see
Chapter 4), machine tools, heat engines, and
superconductors (see Chapter 4), the United
States can ill afford to surrender technical lead-
ership to its competitors. The dominant trend
in the field is toward materials with finer micro-
structures, fewer defects, and better interac-
tions at interfaces (particularly in composites).
Chemical processes provide important tools to
capture the promise of ceramics for the benefit
71
of our society and to maintain our international
competitive position in technology.
Composite Liquids
The field of composite liquids has not received
much attention outside the industries associated
with specific liquid products (e.g., the petroleum
industry). In areas such as lubrication, the
United States has clear technological leader-
ship. The situation is less clear for liquid crystals
and adhesives, where there is greater compe-
tition from Europe and Japan.
INTELLECTUAL FRONTIERS
A wide variety of chemical engineering re-
search frontiers involve advanced materials and
belong in the mainstream of academic chemical
engineering departments. The following list of
frontiers ranges from the molecular level to the
systems level.
Microscale Structures and Processes
The study of materials has traditionally cen-
tered on the influence of molecular composition
and microstructure on mechanical, electrical,
optical, and chemical properties. At the molec-
ular level are a variety of research frontiers that
can profitably draw chemical engineers into
close collaboration with physical and theoretical
chemists. They include the following research
areas.
New Concepts in Molecular Design of
Composite Materials
The toughest challenge and the greatest op-
portunity in chemical engineering for high-per-
formance materials lie in the development of
wholly new designs for composite solids. Such
materials are typified by composites reinforced
by three-dimensional networks and truss-
works microstructures that are multiply con-
nected and that interpenetrate the multiply con-
nected matrix in which they are embedded. In
such materials, both reinforcement and matrix
are continuous in three dimensions; the com-
posite is bicontinuous. Geometric prototypes of
OCR for page 72
72
such structures are found in certain liquid crys-
tals and colloidal gels and in bones and shells.
The challenge is to break away from today's
technology, to go beyond today's research in
two-dimensional fabric-like reinforcement, and
to determine how to create truly three-dimen-
sional microstructures and design, construct,
and process them so as to control the properties
of the composite.
A potentially promising area is "molecular
composites," in which the fiber and its sur-
rounding matrix have the same composition and
differ only in molecular structure or morphol-
ogy. This might involve forming the composite
from very stiff, linear polymer molecules, some
of which are aligned during the forming step as
reinforcing crystallites in the amorphous re-
gions the matrix. An analogous ceramic com-
posite may be envisioned. There are difficult
engineering problems to be solved in learning
how to control the orientation of the crystalline
regions and the ratio between crystalline and
amorphous regions in the material.
The Role of Interfaces in Materials
Chemistry
Two general problems relate to the role of
interfaces in advanced materials. The first is
simply that we do not have the theory or the
computational or experimental ability to under-
stand the interatomic and microscopic interac-
tions at the interfaces between components of
an advanced material, on which its properties
are critically dependent. There is a general need
for research on processes at interfaces and on
the structure-property-performance relation-
ships of interfaces.
The second problem relates to the role that
interfaces play in mediating chemical reactions
in the synthesis of composite materials. This
problem has three parts, which are illustrated
here for polymeric composites.
· First, in composites with high fiber con-
centrations, there is little matrix in the system
that is not near a fiber surface. Inasmuch as
polymerization processes are influenced by the
diffusion of free radicals from initiators and
from reactive sites, and because free radicals
FRONTIERS iN CHEMICAL ENGINEERING
can be deactivated when they are intercepted
at solid boundaries, the high interracial area of
a prepolymerized composite represents a radi-
cally different environment from a conventional
bulk polymerization reactor, where solid bound-
aries are few and very distant from the regions
in which most of the polymerization takes place.
The polymer molecular weight distribution and
cross-link density produced under such diffu-
sion--controlled conditions will differ apprecia-
bly from those in bulk polymerizations.
· Second, the molecular orientation of the
fiber and the prepolymer matrix is important.
The rate of crystal nucleation at the fiber-matrix
interface depends on the orientation of matrix
molecules just prior to their change of phase
from liquid to solid. Thus, surface-nucleated
morphologies are likely to dominate the matrix
structure.
~ Third, the ultimate mechanical properties
of a composite will be strongly influenced by
the degree to which the matrix wets the fiber
surface and by the degree of adhesion between
the two after curing. Both phenomena depend
on intimate details of the surface science of the
two phases, about which little is known.
Molecular modeling techniques, augmented
by careful measurements of the structure of the
interracial regions, hold promise for elucidating
details of these three aspects of interracial
control of matrix polymerization.
Understanding the Molecular Behavior of
Complex Liquids
Basic understanding of the liquid state of
matter still lags behind that of the solid and
gaseous states. Our knowledge of interactions
in multicomponent liquids containing macro-
molecules and suspended solids is extremely
limited. Thus, the study of complex liquids,
including polymer solutions, sots, gels, and
composite liquids, is a significant challenge for
chemical engineers.
The ability to predict liquid-liquid and liquid-
solid equilibria in complex systems is still rather
undeveloped, in part because of the lack of
systematic and molecularly interpreted experi-
mental information. Considerable research has
OCR for page 73
POLYMERS, CERAMICS, AND COMPOSITES
been conducted on the behavior of liquids near
their critical points, on lower critical solution
phenomena, on spinodal decomposition, and on
related dynamics such as the growth and mor-
phology of new phases, but generalized corre-
lations and connections of theory to practice
are few.
Molecularly motivated empiricisms, such as
the solubility parameter concept, have been
valuable in dealing with mixtures of weakly
interacting small molecules where surface forces
are small. However, they are completely inad-
equate for mixtures that involve macromole-
cules, associating entities like surfactants, and
rod-like or plate-like species that can form
ordered phases. New theories and models are
needed to describe and understand these sys-
tems. This is an active research area where
advances could lead to better understanding of
the dynamics of polymers and colloids in so-
lution, the theological and mechanical proper-
ties of these solutions, and, more generally, the
fluid mechanics of non-Newtonian liquids.
Chemical Dynamics and Modeling of
Molecular Processes
Chemical dynamics and modeling were iden-
tified as important research frontiers in Chapter
4. They are critically important to the materials
discussed in this chapter as well. At the molec-
ular scale, important areas of investigation in-
clude studies of statistical mechanics, molecular
and particle dynamics, dependence of molecular
motion on intermolecular and interracial forces,
and kinetics of chemical processes and phase
changes.
Mechanistic studies are particularly needed
for the hydrolysis and polymerization reactions
that occur in sol-gel processing. Currently, little
is known about these reactions, even in simple
systems. A short list of needs includes such
rudimentary data as the kinetics of hydrolysis
and polymerization of single alkoxide sol-gel
systems and identification of the species present
at various stages of gel polymerization. A study
of the kinetics of hydrolysis and polymerization
of double alkoxide sol-gel systems might lead
to the production of more homogeneous ce-
ramics by sol-gel routes. Another major area
73
for exploration is the chemistry of sol-gel sys-
tems that might lead to nonoxide ceramics.
The Intimate Connection Between Materials
Synthesis and Processing
Materials synthesis and materials processing
have classically been thought of as separate
activities, and in the days of simple, homoge-
neous materials, they were. But today's com-
plex materials are bringing these two areas
closer together in research and in practice. Four
outstanding intellectual challenges demonstrat-
ing this connection are described in this section.
Processing of Complex Liquids
Complex liquids are ubiquitous in materials
manufacture. In some cases, they are formed
and must be handled at intermediate steps in
the manufacture of materials (e.g., sots and gels
in the making of ceramics, mixtures of monomer
and polymer in reactive processing of poly-
mers). In other cases (e.g., composite liquids),
they are the actual products. Understanding the
properties of complex fluids and the implications
of fluid properties for the design of materials
processes or end uses presents a formidable
intellectual challenge.
Complex liquids seldom behave as classical
Newtonian fluids; thus, analysis of their behav-
ior requires a thorough understanding of non-
Newtonian rheology. The importance of this
knowledge is illustrated by the following two
examples:
· The problem of processing complex liquids
while they are undergoing rapid polymerization
is an important challenge in reactive polymer
processing (e.g., reactive injection molding and
reactive extrusion). In these processes, the
viscosity of a reaction mixture, as it proceeds
from a feed of monomers to a polymer melt
product, may change by 7 decades or more in
magnitude. Fluid mixtures flowing into a mold
of complicated geometry may exhibit large tem-
perature gradients from the highly exothermic
chemical reactions taking place and significant
spatial variations in viscosity and molecular
weight distribution.
OCR for page 74
· Rheology is especially important to the
understanding of composite liquids in their many
applications as products (e.g., lubricants, sur-
face coating agents, and additives for enhanced
oil recovery and drag reduction). Such liquids
usually contain polymers, and their behavior is
frequently viscoelastic under use conditions.
While data on linear response and relatively
mild shear flows are available for nonassociating
polymer solutions in the relevant ranges of
molecular size and concentration, far fewer data
are available on liquid systems that contain
particles or micelles, particularly those in which
there are strong interparticle interactions.
Knowledge of the fluid mechanics of ordered
liquids is similarly sparse. Information on the
response to rapid shear flows and extensional
flows, even in simple polymer solutions, is very
limited. Thus, we are far from having depend-
able equations from which models of such fluids
could be developed and farther still from a
generalized molecular understanding of the
structure-property relations of these fluids and
from extrapolations of the flow patterns and
stress distributions in such fluids in geometries
close to those in which they are used. For
example, there is significant divergence between
theoretical prediction and empirical observation
of the flow of lubricants in journal bearings.
Even if satisfactory equations of state and
constitutive equations can be developed for
complex fluids, large-scale computation will still
be required to predict flow fields and stress
distributions in complex fluids in vessels with
complicated geometries. A major obstacle is
that even simple equations of state that have
been proposed for fluids do not always converge
to a solution. It is not known whether this
difficulty stems from the oversimplified nature
of the equations, from problems with numerical
mathematics, or from the absence of a laminar
steady-state solution to the equations.
Processing of Powders
One route to better ceramic powders, sol-gel
processing, has already been described in this
chapter. There are, however, many other pos-
sible routes to improved ceramic powders. These
FRONTIERS IN CHE10~AL ENGINEERi\G
routes include refinements of older processes,
such as precipitation and thermal decomposi-
tion, as well as newer processes, such as plasma
processing and chemical vapor deposition. The
nucleation and particle-growth processes in such
systems need to be described quantitatively to
enable better process development and scale-
up. Chemical engineering frontiers include the
development of new chemical processes for
producing ceramic raw materials, such as sub-
micron, spherical, uniform powders, and high-
strength fibers and whiskers.
Chemical engineers could also work to devise
processes to improve the flow characteristics
of powders after they are formed. Such research
would help control agglomeration of particles
in subsequent processing steps as well as facil-
itate the production of compacted ceramic pre-
forms. For example, gas-solid chemical reac-
tions might be used to tailor the chemical
composition of powders. As another example,
better methods of compounding powders with
binders might be achieved by processes that
mix powders with suitable binders in a liquid
and then spray dry the resulting suspension.
Powder processing is also one element in the
engineering of grain boundaries in large, com-
plex parts. Such engineering would allow sin-
tering ceramics to full density without degrading
oxidation resistance and long-term strength.
Processing of Polymers
Other important research challenges confront
chemical engineers in the area of polymer pro-
cessing. One concerns the interactions of poly-
mers with their environment. For example,
contacting a glassy polymer with a solvent or
swelling agent may lead to unusual diffusion
characteristics in the polymer, stress formation,
crazing, or cracking. Such phenomena are poorly
understood because glassy polymers may ex-
hibit complex viscoelastic behavior in the pres-
ence of a liquid or during their second-order
(glass) transitions. The study of diffusion in
glassy polymers is a virgin research area for
chemical engineers. A better understanding of
polymer-solvent interactions could have impor-
tent payoffs in the development of positive
resists for microcircuit manufacture (see Chap
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POLYMERS, CERAMICS, AND COMPOSITES
ter 4), because the dissolution characteristics
of polymeric resists are crucial to their appli-
cation and removal during microlithography.
The focus of research on engineering ther-
moplastics with enhanced mechanical, thermal,
electrical, and chemical properties has shifted
away from synthesizing novel polymers toward
combining existing polymers. Multicomponent
polymer blends pose interesting and challenging
new problems for chemical engineers. Many
multicomponent polymeric melts are homoge-
neous at processing temperatures but separate
during cooling. Judicious choice of stress levels
during cooling and of the cooling rate can effect
changes in the structure and morphology of the
end product and hence in its properties.
The fluid prepolymer in which the load-bear-
ing fibers of a polymer composite are placed
undergoes further polymerization and cross-
linking during the thermal curing of the com-
posite. The chemical reactions that occur during
curing are exothermic and are difficult to con-
trol. Some regions in the composite material
react adiabatically while others lose heat by
conduction to their surroundings. The resulting
point-to-point variations in polymer matrix mo-
lecular weight and cross-link density result in
changes in the composite's properties and qual-
ity. We need to better understand and control
these variations in well-characterized processes
and to deduce how to change the geometry of
the finished object or the distribution of fibers
within it to compensate for the variations in
polymer structure that might inherently arise
. .
during processing.
Process Design and Control
Because processing conditions and history
have such an important influence on the con-
formation and properties of materials, there is
a need to develop models and systems for the
measurement and control of materials manufac-
turing processes so that processes can be better
designed, more precisely controlled, and auto-
mated. Opportunities for chemical engineers in
process design and control, including advanced
mathematical modeling of polymer processing,
are explored in depth in Chapter 8.
~5
It is particularly important to study process
phenomena under dynamic (rather than static)
conditions. Most current analytical techniques
are designed to determine the initial and final
states of a material or process. Instruments
must be designed for the analysis of materials
processing in real time, so that the crucial
chemical reactions in materials synthesis and
processing can be monitored as they occur.
Recent advances in nuclear magnetic resonance
and laser probes indicate valuable lines of de-
velopment for new techniques and comparable
instrumentation for the study of interfaces,
complex liquids, microstructures, and hierar-
chical assemblies of materials. Instrumentation
needs for the study of microstructured materials
are discussed in Chapter 9.
Fabrication and Repair of Materials Systems
Advanced materials systems based on poly-
mers, ceramics, and composites are constructed
by assembling components to create structures
whose properties and performance are deter-
mined by the form, orientation, and complexity
of the composite structure. The properties of
these assemblages are determined not by the
sum of weighted averages of the components
but rather by synergistic effects in intercon-
nected phases. For this reason, the study of
fabrication of hierarchical assemblages of ma-
terials, as well as the study of mechanisms for
repairing defects in assembled structures, must
be supported by fundamental research.
Designing Systems from the
Molecules on Up
Successful systems design and fabrication
depend on understanding the connections be-
tween microscale phenomena and macroscale
behavior of materials. For example, with suf-
ficient insight into intermolecular interactions,
appropriate models, and the computational power
of supercomputers, it may be possible to predict
changes in macromolecular configurations when
loads are imposed on polymers or changes in
the properties of a material as a result of
OCR for page 76
76
branching or cross-linking the material's ma-
cromolecular structure.
A related problem in composites is the need
to design optimal fiber orientations for a com-
posite part given the set of stress vectors and
levels to which the part will be subjected. These
design considerations would be useful in de-
signing airframe components such as parts for
the tail, wing, or fuselage. A similar problem is
assessment of the performance penalties that
might result from imperfections in manufacture.
Solutions to these problems lie in the realm
of computer-aided design and manufacturing
(CAD/CAM). This area of technology is being
developed rapidly by mechanical engineers, but
the problems encountered include many that
are logical extensions of polymer process en-
gineering. Interdisciplinary collaborations be-
tween mechanical and chemical engineers should
be fostered for problems where chemical ex-
pertise would be valuable. Just as chemical
engineers of a previous era contributed exten-
sively to the knowledge of heat and mass trans-
fer by collaborating with mechanical engineers,
so are they now well positioned to contribute
to composites CAD/CAM and to the education
of students who may one day use and oversee
these processes in industry.
Chemical Processing in the Fabrication of
Materials Systems
One might imagine that the fabrication of
materials systems involving polymers, ce-
ramics, and composites would be principally a
concern of mechanically oriented materials en-
gineers. This is not true. For example, the
mechanical attachment of composites to other
materials (e.,g., metal parts) by drilling holes in
the composite and attaching mechanical fas-
teners can alter and degrade the performance
of the composite. In a number of situations,
joining and fabrication processes involving
chemical reactions with the material will be
needed in systems fabrication.
Fundamental research to support materials
assembly and fabrication probably centers on
the science and technology of adhesion, al-
though research on mechanical assembly driven
by chemical action, such as the self-assembly
FRONTIERS IN CHE.~ICAL ENGINEERING
of large molecules or particles, also holds prom-
ise for solving some fabrication problems.
Detection and Repair of Flaws in
Materials Systems
A central problem in complex materials sys-
tems of any kind involves testing to detect
flaws, analysis to predict their effect on re-
maining service life of the system, and repair
strategies to overcome them. For the structural
materials discussed in this chapter, these prob-
lems are uncharted territory in need of explo-
ration by chemical engineers.
There is a general need for nondestructive
test methods capable of determining whether
the manufacturing process for a polymer, ce-
ramic, or composite has achieved the desired
microstructure. Chemical engineers can profit-
ably contribute to interdisciplinary efforts to
develop such test methods. For example, one
type of defect in a composite arises because the
placement of the fibers is different from what
was intended. This may reflect perturbations in
the filament-winding operations on a mandrel
or fiber movement during curing in response to
differential stresses. The processing expertise
of chemical engineers could be useful in devel-
oping instrumentation to detect such flaws dur-
ing the manufacturing process so that automatic
control and correction of the process can be
invoked to avoid or compensate for flaws.
There is also a need for methods to predict
the effects of flaws and the remaining service
life of a flawed or degraded part in use For
example, because of limited basic knowledge
about composites, structures based on them are
now overdesigned for considerably greater mar-
gins for error than those required for metal
structures, thereby losing some of the inherent
superiority of composites.
Finally, attempts to repair composite struc-
tures will become increasingly common in future
years as the use of composites spreads. At this
point, a fundamental repair science for com-
posites is completely lacking. Since such strat-
egies are likely to depend heavily on chemical
reactions to heal breaks and flaws, chemical
engineers should be at the forefront of this
emerging field.
OCR for page 77
POLYMERS, CERAMICS, AND COMPOSITES
IMPLICATIONS OF RESEARCH
FRONTIERS
Chemical engineers are already equipped to
pursue the frontiers outlined in this chapter.
The core undergraduate curriculum provides
both a science base and an engineering knowl
edge base for approaching problems in materials
phenomena and processing. At the same time,
undergraduate chemical engineering students
would benefit from a broader exposure to prob
lems in materials science and engineering. What
is needed is a better integration of such problems
into the curriculum at all levels. This is best
achieved by developing better instructional ma
terial and example problems for existing courses
in thermodynamics, transport, and reaction en
gineering. What is not needed is a proliferation
of general, encyclopedic materials courses for
undergraduates.
All the scientific and engineering disciplines
involved in materials research are in need of
better instrumentation and facilities. Suitable
equipment for chemical engineers interested in
materials questions might include the following:
solid-state NMR spectrometry;
spin-echo NMR spectrometry;
Raman spectroscopy;
secondary ion mass spectrometry;
X-ray photoelectron spectroscopy;
laser light scattering;
advanced dynamic rheometers;
computer-controlled, fully equipped poly- 2
mer~zation reactors;
· directional irradiation devices; and
· dynamic mechanical property measure
ment equipment.
Special efforts to help academic institutions
acquire these instruments are needed. Future
chemical engineers will be required to under
stand the design and operation of sophisticated
equipment in the analysis of materials proper
ties. An early exposure to these techniques is
highly desirable, and is probably indispensable
to quality research at the graduate level.
The Matenals Research Laboratones (MRLs),
sponsored by the NSF, have been one mecha
nism for providing instrumentation and facilities
support to small groups of principal investiga
77
tors with interests in materials. There is a
perception in the chemical engineering com-
munity that MRLs are more physics directed,
and probably not open to significant participa-
tion by chemical engineers. One way of ad-
dressing this problem would be for NSF to
target more funds to the Division of Materials
Research with an emphasis on interdisciplinary
and process studies. Another way might be to
develop mechanisms intermediate between MRLs
and ERCs that would promote engineering re-
search on materials.
A final goal for improving the chemical en-
gineenng contribution to materials research would
be to develop focused continuing ec~ucat~on
programs to help qualified chemical engineers
move aggressively into materials-related areas.
Such courses might take a number of forms.
The AIChE might take the lead in sponsoring
short courses within the context of its existing
continuing education program. Universities might
provide complementary, more intense exposure
to the problems and opportunities in materials
research by initiating special workshops, mas-
ters degree programs, or sabbaticals for indus-
trial researchers.
NOTES
JTECH Panel Report on Advanced Materials in
Japan. La Jolla, Calif.: Science Applications In-
ternational Corp., 1986.
National Academy of Sciences-National Academy
of Engineering-Institute of Medicine, Committee
on Science, Engineering, and Public Policy. "Re-
port of the Research Briefing Panel on High-
Performance Polymer Composites," in Research
Briefings 1984. Washington, D.C.: National Acad-
emy Press, 1984.
3. National Research Council, National Materials
Advisory Board. High-Technology Ceramics in
Japan (NMAB-418~. Washington, D.C.: National
Academy Press, 1984.
SUGGESTED READING
C. G. Gogos, Z. Tadmor, D. M. Kalyon, P. Hold,
and J. A. Biesenberger. "Polymer Processing: An
Overview." Chem. Eng. Prog., 83 (6), June 1987,
49.
National Research Council, Engineering Research
Board. "Materials Systems Research in the United
OCR for page 78
States," in Directions in Engineering Research.
Washington, D.C.: National Academy Press, 1987.
D. R. Uhlmann, B. J. J. Zelinski, and G. E. Wnek.
"The Ceramist as Chemist Opportunities for New
Materials." Mat. Res. Soc. Symp. Proc., 32, 1984,
59.
iFlR0NTIFIRS IN AL ERG
U.S. Congress, Office of Technology Assessment.
New Structural Materials Technologies: Oppor-
tunities for the Use of Advanced Ceramics and
Composites A Technical Memorandum (OTA-
TM-E-321. Washington, D.C.: U.S. Government
Printing Office, 1986.
Representative terms from entire chapter:
advanced ceramics
