Polymer Science and Engineering The Shifting Research Frontiers (1994) / Chapter Skim
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3. Manufacturing: Materials and Processing
3. Manufacturing: Materials and Processing
Pages 65-115
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From page 65... ...
Polymers constitute a high-value-added part of the petroleum customer base and have led to increasing international competition in the manufacture of commodity materials as well as engineering thermoplastics and specialty polymers. Polymers are now produced in great quantity and variety.
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From page 66... ...
As shown in Table 3.1, man-made plastics, fibers, and rubber accounted for U.S. production of about 71 billion pounds in 1992 (Chemical & Engineering News, 1993)
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From page 67... ...
Plastics The word "plastic" is frequently used loosely as a synonym for "polymer," but the meaning of "polymer" is based on molecular size while "plastic" is defined in terms of deformability. Plastics are polymeric materials that are formed into a variety of three-dimensional shapes, often by molding or melt They retain their shape when the deforming forces are removed, unlike some other polymers such as the elastomers, which return to their original shape.
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From page 68... ...
These materials generally have greater heat resistance and better mechanical properties than the less expensive commodity thermoplastics and, therefore, are used in more demanding applications, such as aircraft, automobiles, and appliances. A major area of development is
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From page 69... ...
Development of new molecular structures has dominated this sector. Polymer chains with quite rigid backbones have liquid crystalline order, which offers unique structural properties as described below.
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From page 70... ...
SOURCE: Reprinted with permission from Chemical & Engineering News (1991)
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From page 71... ...
MANUFACTURING: MATERIALS AND PROCESSING Linear )
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From page 72... ...
One source estimates that the number of "grades" of existing polymers tripled during the 1980s (Chemical & Engineering News,
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From page 73... ...
An important consideration with respect to rigid thermosetting networks is the extensively studied interrelationship between reactivity, "elation, and vitrification. As the reaction proceeds, the glass transition temperature rises to meet the reaction temperature, and the system vitrifies; that is, the motion of the main chain stops.
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From page 74... ...
. SOURCE: Reprinted with permission from Chemical & Engineering News (1991)
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From page 75... ...
processing of high-performance polymeric matrix resins, particularly for structural adhesives and composite structures. An approach for "toughening" that has been investigated over the last 10 years involves the incorporation of either rubbers or reactive engineering thermoplastics into networks, such as epoxies, to develop a complex morphology.
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From page 76... ...
Elastomers are typically amorphous, network polymers with lower cross-link density than thermoses plastics. Most thermosets can be made to function as elastomers above their glass transition temperatures.
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From page 77... ...
These materials combine some of the most intriguing properties of liquid crystalline molecules of low molecular weight with the elastomeric properties of polymeric networks. Materials with this unique behavior should be exploited.
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From page 78... ...
The fluoroelastomers are another class that is under intense development. Polyphosphazenes have rather low glass transition temperatures in spite of the fact that the skeletal bonds of the chains are thought to have some double-bond character.
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From page 79... ...
Acetate Acrylic Nylon Olefin Polyester 0.50 0.44 2.ss 1.99 3.s8 SOURCE: Data from Chemical & Engineering News (1993)
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From page 80... ...
These parameters control the extent of molecular chain orientation (time to orient/time to relax) and the degree of crystallinity achieved during spinning.
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From page 81... ...
or the use of supercritical carbon dioxide as a polymerization and/or spinning solvent will become more commercially attractive. The reduction of offspecification production will become more important as the cost of waste disposal increases and as easy-to-reclaim fibers grow in importance (e.g., biodegradable cellulosics produced without organic solvents, or polytethylene terephthalate)
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From page 82... ...
They are mentioned in Egyptian hieroglyphics, in the Bible, and in the writings of the early natural philosophers. The physical strength of an assembly made by the use of adhesives, known as an adhesive joint, is due partly to the forces of adhesion, but primarily to the cohesive strength of the polymeric materials used to formulate the adhesive.
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From page 83... ...
The major limitations to the broader use of adhesives in industry are the extreme sensitivity of adhesive bonding to surface conditions and the lack of a nondestructive quality control method. Adhesive technology can be solidly advanced by the synthesis of new monomers and polymers that extend the range of applicability of adhesive bonding.
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From page 84... ...
There are several driving forces for blending two or more existing polymers. Quite often, the goal is to achieve a material having a combination of the properties unique to each of the components, such as chemical resistance and toughness.
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From page 85... ...
In this blend, the polycarbonate brings toughness, which is augmented at low temperatures by the impact modifier, while the poly~butylene terephthalate) brings the needed chemical resistance to survive contact with gasoline, oils, and greases.
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From page 86... ...
Elastomers with low glass transition temperatures are needed to impart toughness at low use temperatures, while thermal and oxidative
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From page 87... ...
A wide range of future needs encompasses synthesis, characterization, processing, testing, and modeling of important polymer matrix composite systems. In general, the future of polymer matrix composites is bright.
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From page 88... ...
Advanced polymer matrix composites have been used for more than 20 years, for example, on the B-1 bomber and for many top-of-the-line Navy and Air Force jet fighters. For military purposes, the high performance and stealthiness of composites have often outweighed issues of durability and even safety.
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From page 89... ...
Liquid Crystalline Polymers The liquid crystalline nature of stiff polymer molecules in solution was predicted by Onsager in 1947, further refined by Flory in 1956, and experimentally verified through aramid investigations at the Du Pont Company in the 1960s. Flory suggested that as the molecular chain becomes more rodlike, a critical aspect ratio is reached, above which the molecules necessarily line up to pack efficiently in three dimensions.
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From page 90... ...
More versatile than the lyotropic polymers, these nematic copolyesters (Figure 3.7) are amenable to uniaxial processing, such as fiber formation, and three-dimensional processing, such as injection molding, utilizing essentially conventional thermoplastic processing techniques.
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From page 91... ...
An extensive literature exists, and interesting concepts such as self-reinforcing composites and molecular composites have been developed to describe immiscible and miscible liquid crystal polymer-containing blends. Major problems encountered in this technology include: · Inherent immiscibility of mesogenic and conventional polymers, leading to large-scale phase separation; · Strong dependence of blend morphology (properties)
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From page 92... ...
These diverse functions can be achieved partly because the permeability to small molecules via a solution-diffusion mechanism can be varied over enormous ranges by manipulation of the molecular and physical structure of the polymer. The polymer that has the lowest known permeability to gases is bonedry polyvinyl alcohol)
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From page 93... ...
There are certain polymer molecular structures that provide good barrier properties; however, these structural features seem invariably to lead to other problems. For example, the polar structures of polyvinyl alcohol)
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From page 94... ...
, or PET, involved innovative developments in processing for increasing molecular weight (solid-state reaction) and for fabrication (injection-blow molding)
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From page 95... ...
The skin and the substructure may be integral, made of the same material. The method to fabricate such asymmetric membranes was discovered in the 1960s and was first applied to make reverse osmosis membranes and later to make gas separation membranes.
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From page 96... ...
Currently most of the efforts are devoted to developing reverse osmosis membranes and processes for removal of organic pollutants, rather than salt, from water. Gas separation is clearly one of the most active and promising areas of membrane technology for polymer science and engineering (Figure 3.81.
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From page 97... ...
Also shown (bottom) is a cut-off section of a bundle of thousands of tiny hollow fibers made of polysulfone embedded in an epoxy tube sheet that fits into each tubular module shown.
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From page 98... ...
Until the early 1970s most coatings contained only 15 to 30 percent paint solids, the remaining 70 to 85 percent being organic solvents, which were released as air pollutants when the films dried. Since then, reduction of solvent emissions has been the most important single driving force for technology change.
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From page 99... ...
For example, a high-Tg (glass transition temperature) core and a low-Tg skin in the latex particles provide relatively hard coatings (related to high Tg)
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From page 100... ...
Inorganic Polymers The materials described above are made up primarily of polymer molecules based on covalently bonded chains in which carbon is the principal element. Polyethylene, polystyrene, poly~methyl methacrylate)
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From page 102... ...
Glass transition temperatures as low as -100°C have been achieved, and elastomeric performance over a wide temperature range is characteristic of this family. Fluoroalkoxy substituents yield hydrocarbon-resistant materials that could be useful as fuel lines, o-rings, and gaskets in demanding environments.
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From page 103... ...
The use of these inorganic polymers as ceramic precursors is important because the precursor can be spun into a fiber that yields fibrous ceramics following processing. In fact, polysilanes are the basis of a process commercialized by Nippon Carbon Company to produce continuous NICALON~ silicon carbide fibers for use in such applications as fiber-reinforced ceramic composites.
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From page 104... ...
New synthetic routes such as "molecular building block" approaches to multicomponent ceramics will be explored to prepare superconductor, ferroelectric, nonlinear optical, and ionic conducting phases, primarily in thin film form. The use of sol-gel processing to prepare "tailored" porous materials for applications in sensors, membranes, catalysts, absorbents, and chromatography is an especially attractive area of research and development.
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From page 105... ...
Some examples of new materials are special blends of existing polymers, polymer composites with fiber reinforcement, and liquid crystalline polymers. Some of these new materials are expensive and may be difficult to form into desired shapes; however, they are of value to the defense and aerospace industries in applications in which weight and performance are more important than cost and processibility.
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From page 106... ...
Among the high-performance plastics that have been introduced to meet the demands of the high-precision market are the thermotropic liquid crystalline polymers and low-viscosity versions of high-temperature materials such as polyetherimides and polyaryl sulfones. Advances in processing are occurring at a rapid rate as on-line sensing, computing, and process feedback allow control and optimization of the molding process that were undreamed of only a few years ago.
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From page 107... ...
Stretch blow molding processes allow control of the development of chain orientation and crystalline structure for materials such as polyethylene terephthalate) to gain better barrier properties.
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From page 108... ...
has been widely studied, in order to understand the morphological transformations and because it can now be drawn into one of the highest specific tensile moduli and strengths in both fiber and film form. Solution Processing Not all polymers can be fabricated by the convenient and economical melt processing techniques; nor is this desirable in some instances.
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From page 109... ...
Nevertheless, such fabrication methods are the only option available for certain materials of critical importance. Interest in polymers with very high heat resistance and other special attributes is likely to increase the need for solution processing technology, because by the nature of their properties such materials cannot be melt fabricated.
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From page 110... ...
Powder coatings are applied to various substrates by a number of techniques, including fluidized beds and flame heating. Powdered liquid crystalline polymers can be "extruded" below their melting points in much the same way as metal powders
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From page 111... ...
while minimizing solvent emissions. These are sophisticated materials in which fine polymer particles are formed by dispersion polymerization in a nonaqueous environment in which dispersant polymer chains prevent coalescence of the particles through steric stabilization.
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From page 112... ...
Future work in this area is expected to be extensive and more limited by the nature of the physical models available as the trend moves from simple process questions that can be answered by simple phenomenological models to complex product questions that require more detailed molecular models. The most common driving force for developing process models is to aid in design.
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From page 113... ...
. A great deal of the current research and development on polymeric materials and associated processes is being driven by environmental considerations.
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From page 114... ...
Ease of fabrication into dimensionally precise parts with high-quality surface finish is one driving force. · For polymer-based materials being used in highly critical structural applications, there is need for a better understanding of the mechanical, chemical, and environmental factors that affect their useful lifetimes and for methodologies to predict lifetimes in complex situations.
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From page 115... ...
The systems approach, in comparison with the traditional "compartmentalized" approach, facilitates rapid identification of the critical material and processing parameters and aids manufacturing procedures. · The relationship between processing and properties, particularly for complex polymer systems such as blends, composites, and liquid crystalline polymers, is not fully developed.
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