POLYMERIC FOAMS SERIES
`
`' I LYIVIERIE
`
`E Science and Technology
`
`Shau-Tarng Lee, Chul B. Park.
`
`and N.S. Ramesh
`
`PAGE 1 OF 51
`
`BOREALIS EXHIBIT 1020
`
`

`
`Published in 2007 by
`CRC Press
`
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`
`9 2007 by Taylor tit Francis Group. LLC
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`
`Library of Congress Cataloging-in-Publication Data
`
`
`Lee. S.-T. (Shau-Tarng). i956-
`Polymcric foam 2 technology and science of polymeric foams I Shau-‘Iitmg Lee, Chul B. Park.
`N.S. Ramcsh.
`p. cm. -- (Polymeric foams)
`Includes bibliographical references and index.
`ISBN 0-8493-30'l5~0 (alk. paper)
`I. Plastic foams. l. Park. Chul B. ll. Ramesh. N.S. (Natantjan S.) III. Title. IV. Polymeric foams
`series.
`
`TPI I83.F6lA4 2006
`
`668.4'93~-dc22 2006043863
`
`informa
`‘lit lor & Francis Group
`is the Ace
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`
`Visit the ‘Baylor St Francis Web site at
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`PAGE 2 OF 51
`
`

`
`1
`
`‘i'jjj—
`
`Introduction to Polymeric Foams
`
`
`
`1.1 Basic Considerations on Foams, Foaming, and Foamed
`Polymers
`
`Matter generally assumes one of three forms (or phases): gas, liquid, or solid.
`Cases are essentially shapeless and formless, and naturally or artificially
`exist or co-exist with the other two phases, such as in sponge, cork, aerogel,
`cake, for example. In fact, gas molecules are capable of penetrating into a
`liquid or solid phase to create mixtures. Table 1.1 presents a summary of
`typical gas/ liquid and gas/solid mixtures.
`The word "foam” derives from the medieval German word vein: for "froth”
`[1]. "Foam" refers to spherical gaseous voids dispersed in a dense contin-
`uum. There are a number of common natural and artificial foamed products,
`ranging from foamed pumice to seat cushions [2,3] (see Table 1.2).
`
`TABLE 1.1
`
`Typical Two-Phase Systems
`
`Phenomena
`
`Terminology
`
`Gas bubbles on top of liquid
`C-as bubbles dispersed in liquid
`Liquid bubbles in liquid
`Liquid bubbles in solid
`Gas Bubbles in solid
`
`Froih
`Emulsion Bubble
`Emulsion Liquid
`lelation
`Foam
`
`TABLE 1.2
`
`Common Foamed Products
`
`Natural:
`Synthetic:
`Food:
`Plastic:
`Automotive:
`Sports:
`Medical:
`
`Pumice, Tree Trunk, Wood, Cork, Marine Organisms
`
`Steamed Rice, Flour Dough, Popped Cereal..
`Seat Cushion, Life Jacket, insulation Boar-d..
`Arm—rest. Liner, Bumper. .,
`Helmet Pad, Knee Protection, Surfing Board
`Tape, Gasket Seal...
`
` —
`
`PAGE 3 OF 51
`
`

`
`2
`
`Polymeric l-‘aams: 'Ii-chrzology and Science
`
`Foaming occurs when free gas molecules are converted into spherical
`bubbles, and typically takes place when the surrounding conditions change
`too abruptly to allow a smooth response from the system. Although, at
`modest changes, molecular diffusion may be adequate to restore the state of
`equilibrium, drastic changes usually preclude reaching the equilibrium
`through conventional transport mechanisms, such as diffusion and vapor-
`ization. Hence, foaming can be understood as a way of dissipating the
`disturbances in a given environment. However, foaming can also be regarded
`as a transition from a stable (homogeneous) state to a meta-stable or unstable
`(heterogeneous) state [4]. In the case of boiling, for example, under heating
`the stable liquid becomes the unstable bubbling which will stop when the
`liquid is exhausted to form a homogenized vapor state.
`The dense phase surrounding the gas bubbles can be further strengthened
`(e.g., by cooling) to retain the useful effects of the dynamically intensive
`foaming process and thereby deliver stable foamed products. The hardening
`process must proceed faster than the condensation of the gas phase othewvise
`bubbles may shrink back to liquid state. In certain cases, the surrounding
`may be inadequate to hold the shrinkage-induced vacuum. it tends to col-
`lapse back to the unfoamed state. In other words, timing is very important.
`Foaming is thus a very unique technology, turning unstable foaming into a
`stable and useful product.
`The foaming methodology usually consists of introducing a gaseous phase
`into a melt, then foaming the gas, and subsequently solidifying the melt
`before gaseous bubbles condense or collapse back to a liquid state. Gas
`bubbles are generated in a spherical shape by virtue of either entrainment
`or nucleation. Since the spherical form has the lowest surface energy for a
`given volume, it is the ideal shape for the weak (gaseous) phase to sustain
`within a dense (liquid or solid) phase. As illustrated in Figure 1.1, the gas
`is considered to be the weak phase, and therefore it has to counterbalance
`the summation of both the surrounding pressure and structural force, F, in
`order to survive in the form of a bubble. Thus:
`
`Pu: l’+t-‘/A
`
`(1.1)
`
`
`
`I’. and A, denote the bubble pressure, surrounding pressure, and
`where, l’,.,
`bubble surface area, respectively. When the surrounding phase is in a liquid
`state, Equation (1.1) becomes:
`
`P” '3 P
`
`where, o and R" represent the surface tension and bubble radius, respectively.
`As Table 1.3 makes evident, there are various pos.'~:ible ways of introducing
`external gas molecules into a given volume occupied by liquids or solids.
`When gas exists in a dense matrix,
`the gas molecule is small enough
`to establish itself in structural holes of the surrounding liquid or solid
`
`PAGE 4 OF 51
`
`

`
`Introduction to Polymeric Foams
`
`3
`
`\ ‘L \
`
`\ \ \ \
`
`v I
`
`I
`
`/\'
`
`._,\\V‘ ‘,l\l \ X l\\I.,K]<)\,\.--__--_..------\J
`
`FIGURE 1.1
`Gaseous bubbles in a dense matrix.
`
`TABLE 1.3
`
`
`Methods of Implementing Gas into Liquid and Solid
`
`Dissolve gas by pressure and/or mixing into solid and liquid
`Blow gas into melt. such as: gas-assisted injection molding
`Chemical reaction to evolve gas in liquid or melt
`Partial decomposition of molten polymer
`Blend encapsulated gas phase in polymer into solid or liquid for further expansion
`in processing
`Reaction with liquid to form bubble, such as: sodium bicarbonate in water
`
`Entrain air into liquid for high-temperature or low-pressure (vacuum) processing
`
`However, for a bubble to be produced, a sufficient number of gas molecules
`must gather in order to overcome the resistance of the surrounding matrix.
`In this context, an important parameter known as the critical bubble radius,
`can be determined according to the following equation:
`
`K, = 20/(PB - P)
`
`(1.3)
`
`Hence, if the bubble radius is smaller than the critical radius, we can infer
`that the surface tension force is high enough to cause the gas clusters to
`collapse. in a state of ultimate equilibrium, I’, should be the same as P, and
`therefore, Rt, becomes an infinity. This indicates that a spherical shape would
`not be achieved because any bubble size is smaller than the infinite critical
`radius. But when the equilibrium is destroyed, the system will go through
`a series of non-equilibrium states to reach another equilibrium, and in this
`pnocess, bubbling may occur. For instance, when a lower pressure is sud-
`denly applied (i.e., P < P.,) to an equilibrium condition, bubbling becomes a
`way to re-establish another equilibrium. Bubbling is an aggressive way of
`generating a significant amount of interfacial area for diffusion, which in
`
`3
`
`PAGE 5 OF 51
`
`

`
`4
`
`Polymeric Foams: Technology and Science
`
`turn dissipates any energetic inequalities. This dynamic phenomenon in an
`unstable state is very natural and continual bubbling (e.g., boiling), and
`bubble growth (e.g., polymeric foaming) will occur until the system becomes
`stabilized. At any event, this dynamic state can be timely harnessed for useful
`applications.
`It should be noted here that when the bubble is very tiny in size, the bubble
`pressure is relatively high in order to sustain the surface tension force.
`Although the buoyancy force is low because of the small bubble size, the
`high internal bubble pressure renders the foaming process highly unstable.
`These unstable bubbles expand in size, which in turn dissipates the pressure
`gradient across the interface. if the surrounding resistance is low, buoyancy
`becomes manifest, like when the bubbles in soda or in soapy water are
`collected on top of the liquid. The rupture of the bubble walls would be
`inevitable as the bubbles meet and the thinning of the bubble walls progres-
`sively occurs because of the material drainage in the cell walls.
`In a dense
`syrup. bubble movement is stifled by the surrounding viscous phase. As a
`consequence, bubbles appear to have a much longer lifespan, creating a
`greater opportunity for the viscous phase to solidify into a ”permanent"
`gas/solid structure.
`Polymers represent a very special group of materials which are quite
`different from, for example, closely packed materials, such as glass, metal,
`rock, or wood. "Poly" in Greek means "multitude/’ while the suffix "mer”
`signifies unit (i.e., monomer). in fact, a polymer consists of many long-chain
`polymers entangled and/or bonded by functional groups and van der Waals
`forces. Conversely, polymers can be made by prompting reactions between
`functional groups or by free particle addition to form a long-chain polymer.
`'l'he former type of polymer normally possesses a network structure, which
`foams after a heat-ind uced reaction is complete. As a result, its fluidity would
`no longer be thermally responsive, especially once a 3-D structure is formed.
`By contrast, the latter kind of polymer changes from a solid to a molten state,
`and vice versa, in response to temperature changes. Thus, polymers (or
`plastics) are categorized as thermoplastics or thermosets according to the
`way they respond to thermal variations.
`if thermosets are somehow exposed to elevated temperatures, their poly-
`mer chains become susceptible to forming a network structure; they are then
`no longer thermo-reversible. However, when polymers possess various chain
`lengths held together by van der Vaal forces and chain entanglements, they
`demonstrate a unique visco—elastic behavior, as illustrated in Figure 1.2.
`Unlike crystals, which are t-Iiaracterizt-d by a sharp change of state occur-
`ring at a given inciting point, thermoplastics convert from a solid to a liquid
`state within a specific temperature window. Below the glass—transition tem-
`perature, T,._, polymer chains are basically frozen. As the temperature exceed-‘~
`Tu, chain movement begins to increase. if a polymer possesses crystallir\il)’-
`it tends to melt at the melting point to form a polymeric melt. liecausr Of
`their thermo—reversible nature, thermoplastics are the preferred rinses Of
`materials for processing purposes. in the molten state, they can blend dlld
`
`PAGE 6 OF 51
`
`

`
`I
`
`,
`,mrIim1t0 Polyrncric Foams
`H I‘! I
`
`5
`
`Modulus
`
`'
`|
`'
`
`Tum: 84 l('n‘Iperstnn'
`
`HGURT 1.2
`
`Viscovlastic nature of Polymer: variation of elastic in time and temperature domain.
`
`co-exist with other polymers or gases. Moreover, the visco-elastic nature of
`thcrmuplastics endows them with the appropriate strength (or resistance) to
`control the unstable foaming phenomena and thereby facilitate a plausible
`path to reach a stable foamed plastic product [5].
`it is important to note that thermoplastic and thermoset foams are manu-
`factured according to very different processes. The successful development
`of thermoplastic foams is strongly dependent on the kind of equipment that
`determines the capacity to process the thermoplastic having unique ther-
`mally reversible characteristics with a gas that is introduced. Essentially,
`foaming is a pressure- and temperature-controlled phenomenon. On the
`Other hand, the production of thermoset foams seems to hinge more on the
`materials "formula" and their reaction kinetics. Table 1.4 compares thermo-
`sets and thermoplastics, and their respective foaming mechanisms [6].
`Foaming is a unique phenomenon that can be effectively used in a number
`of applications. For example, the tremendously increased surface area in
`foams makes them suitable for effective mass transfer in separation opera-
`tions such as foam—enhanced devolatilization [7].
`in another example, a
`Separation tower can literally be reduced to a unit operation in a plasticator
`Where a foamed structure can be induced and destroyed to achieve the
`removal of the undesired volatile contents from the polymeric melt. Another
`(“-Xample of a natural foam at work is the human lung,
`in which cellular
`tissues exist to enhance gas exchange. The cell wall strength substantially
`
`TABLE 1.4
`
`Fomning Mechanism Comparison between Thermoplastic and Thermoset Foam
`
`
`
`Gas within System
`
`Foaming
`
`Stabilization
`
`Thermoplaslics
`
`Dissolution or
`Decomposition
`l’hc-rmoscl
`(‘hemical Reaction
`(ins Evolution
`|’olymerizntion, X-linking and
`
`Cooling
`
`Supt-rsaturation
`
`Cooling
`
`gj
`
`PAGE 7 OF 51
`
`

`
`Polymeric Foams: Technology and Science
`
`rnonary tissue. By extension,
`ffects the efficiency of gas exchange in the pol
`»ngth and thus enhance the
`exercising our muscles can improve tissue strc
`healthy transfer of gas [8].
`However, the presence of gaseous voids in the product will reduce the
`high insulation properties of the foamed struc-
`heat transfer rate due to the
`ture. This can cause serious concerns about the safety on account of reaction
`heat accumulation [9]. The presence of voids can also be a clisadvantage from
`an aesthetics and /or mechanical strength point of view.
`
`6 a
`
` {?'?:
`
`1.2 History of Polymeric Foams
`Over the years, the foam processing of plastics has become the subject of
`much discussion, attracting extensive and intensive research and develop-
`ment efforts within the disciplines of chemistry, material and engineering
`[10-13]. A plastic itself possesses a unique property/weight ratio, and its
`thermal dependency makes it quite different from other materials. it is no
`wonder that plastics constitute their own distinct material industry. Foamed
`plastics indeed represent an important extension of the polymer property
`spectrum and have an enhanced property/weight ratio, therefore offering
`some unique advantages [14].
`The production of both thermoplastic and thermosetting materials has
`contributed appreciably to the development of the polymeric foam industry,
`which itself dates back to the first half of the 20th century. it used to be the
`case that some polymers would be virtually tested with a gas in laboratory
`environments, and then implemented in pllot- and commercial-scale foam-
`ing processes. However, the unique properties of forums have opened up the
`plastics industry to a wide variety of possible applications; this has been the
`driving force behind the accelerated development of foaming technologies
`during the second half of the 20th century.
`The development of various technologies for polymer synthesis and more
`recently, of newly designed polymer processing equipment, was the key
`factor that propelled the development of polymeric foams between the 19505
`and the 19705. Based on this infrastructure, dedicated efforts from scientists
`and engineers around the world resulted in an increased understanding of
`foaming mechanisms and in enhanced techniques for efficient loam produc-
`tion. After the 19805, increasing insight
`in environmental
`issues of both
`polymeric materials and blowing agent contributed further to the reinforce
`ment of the foam industry. Although it still appears diversified, significant
`efforts have been made in the industry towards bridging the gap between
`a purely scientific and a purely practical approach |l4,l5|. Table l.5 presCfll5
`a number of common methods for making polymeric foams I15-’l 7], whereas
`Table 1.6 highlights the crucial milestones in the development of polymvric
`foams [18--31].
`
`
`
`PAGE 8 OF 51
`
`

`
`lulroduclian to Polymeric Foams
`
`7
`
`TABLE 1-5
`pnlymeric Foam Methodology
`.'.|“.,.muplastic:
`Extrusion Foaming. Injection Molding Foaming, Bead Foaming,
`Rotational Molding Foaming, Compression Molding Foaming, Oven
`Heat Foaming, Coex Foaming
`Reactive Foaming, Reactive Mold Foaming, Reactive (or Reaction)
`Injection Molding (RIM) Foaming
`
`l‘hcrmosel‘.
`._.7———
`
`TABLE 1.6
`
`Highlights of Polymeric Foam Developments
`Authors or
`
`Time
`
`W31
`
`I937
`W41
`194-1
`N45
`W52
`I954
`
`1959
`1962
`19fi2
`W67
`
`Contents
`
`Companies
`
`References
`
`Foamcd Polystyrene
`
`Munters and
`Tandlac-rg
`Dr. Otto Bayer
`Foamed Polyurethane (PU)
`Johnson, F. I...
`Foamed Polyethylene
`Extruded Polystyrene Foam Dow Chemical
`Rigid PU Foam
`Germany
`Flexible PU Foam
`Germany
`Expandable Bead
`Stastncy and
`Goeth
`
`U.S. Pat. 2,023,204 [18]
`
`l<.C. Frisch [19]
`US. Pat. 2,256,483 I20]
`[21]
`PU at Farben, Report 1122 [22]
`K, C. Frisch [19]
`U.S. Pat. 2,681,321 [23]
`
`Rigid PU Foam Produce
`PS Foam Injection Molding
`Extruded Ethylene Foam
`'l\~ln Screw for Foam
`llrt. Fat. 1,152,306
`
`lCl
`Beyer et al.
`Rubens et al.
`Spa, L. M. P.
`
`G. Woods I16]
`US. Pat. 3,058,161 [24]
`U.S. pat. 3,067,147 [25]
`It. Pat. 795,393 [26]
`
`‘
`,
`
`1967
`1968
`1972
`
`ABS foam; Injection Mold
`Rigid lsocyanurate Foam
`Extruded Propylene Foam
`
`SP1 12th Ann. Cont. |27l
`G. Woods [16]
`U.S. Pat. 3,637,458 [28]
`
`1982
`
`Accumulator Extnlsion
`
`U.S. Pat. 4,323,529 [29]
`
`Woollard, 1').
`lCl
`Parrish, R. G.
`(DuPont)
`Collins, F.
`(Valcour)
`Japan Styrene
`Paper
`1990 Xanthos, D. 2000 I31] PET Foam Extrusion Shell/l’etlit'e"‘
`
`
`1984
`
`PI’ Molded Foam Article
`
`lap. Pat. 59-3731 [30]
`
`
`
`
`
`The steady growth of polymeric foam consumption in the last several
`decades is solid evidence of the importance of foam to our society. Foam
`consumption surpassed metal usage in the mid 1980s, and similarly, plastics
`have replaced numerous traditionally wooden products. Figure 1.3 shows
`the property ranges for different materials used in the products [32,33].
`
`L
`
`PAGE 9 OF 51
`
`

`
`Polymeric Foams: Technology and Science
`
`-3 ~——+?o———o
`0
`0.5
`l
`l.5
`3.
`._ E E
`‘as
`‘DD{lam
`
`2
`
`25
`
`A:
`a
`
`3
`
`35
`
`--
`--
`gg.:.§<o -a
`5 °‘.‘—&:,
`5* s
`
`4
`
`45
`
`-%
`.9
`
`Log materials and their density (kg/m7)
`
`FIGURE 1 .3
`
`Variation of thermal conductivity and Young's modulus for different materials in log-log scale
`with density as the independent ordinate (data collected lrom [32] and [33]).
`
` ?
`
`1.3 Foam Structure and Properties
`
`A polymeric foam possesses unique physical, mechanical, and thermal prop-
`erties, which are governed by the polymer matrix, the cellular structure and
`the gas composition. When a gaseous phase is being dispersed in a spherical
`form within a polymeric matrix, a composite structure is naturally formed,
`and the properties of this composite are determined by its constituents and
`their distributions. Since the weight of gas is negligible, the properties of
`gas/polymer composite often volumetrically depend on the participating
`components. The density is a typical example of this, especially in cases
`where the bubble phase dominates. However, the thermodynamic properties
`such as the specific heat, the equilibrium constant, and the heat conductivity,
`would still remain gravimetrically dependent on the individual elements,
`i.e., by weight of each element.
`The characteristics of polymeric foams are determined by the following
`structural parameters: cell density, expansion ratio, cell size distribution.
`open-cell content, and cell integrity. These cellular structural parameters are
`governed by the foaming technology used in processing, and the foaming
`technology often heavily depends on the type of polymer to be foamed. 1"
`other words, different polymers display diiferent properties, and W115
`distinct processing systems are required to accommodate these discrepancies
`[34,35]. This is why various foaming technologies, such as bold‘
`
`y
`
`‘
`
`PAGE 10 OF 51
`
`8
`
`3 ~
`
`2
`
`'
`
`0
`
`iv‘:
`:
`E
`2:
`S
`§
`E
`5
`E -I
`09
`-5 -2
`an
`o.1
`
`0 Thermal conductivity
`
`o Young's modulus
`"“"‘
`
`.
`9'
`I
`
`’
`
`
`
`‘3
`
`7
`
`5
`
`«E
`E
`2
`‘J
`.2
`» i
`E
`~21:
`E
`-l
`>‘
`-3 8’
`-J
`
`-5
`
`

`
`Introduction to Poll/men'c Foams
`
`9
`
`
`
`FIGURE 1.4
`
`Heal and mass transfer across the polymeric foam at different cell size distributions.
`
`(single—stage, multi-stage), semi-continuous, and continuous processes, have
`been gradually developed over the years for each specific polymer foam.
`Different polymer foams exhibit different properties depending on the char-
`acteristics of foams which can range from soft to stiff modulus, resilient to
`tough behavior, low to high hysterisis, and mono to multi—model cell distri-
`bution. in this context, it would be desirable to discuss the fundamental
`relationships between the structure and the properties before discussing the
`foaming methodology and application diversification.
`When the bubble phase is limited, the polymer mainly dictates the prop-
`erties of the foam. As illustrated in Figure 1.4, the bubble’s existence detours
`the direction of the heat transfer because the convection heat transfer in the
`gas phase of the bubble is negligible. However, when the bubble phase
`dominates (as in the case of a highly expanded foam), the polymer’s ability
`to hold the gas within the integral cell
`is higher, and yet,
`the polymer
`contributes less to the foam’s final properties. Therefore, in many instances,
`the contributions of the gas phase could be dominant in determining the
`properties of the foams. For example, the energy absorption ability of low-
`density polyethylene foams is governed by the gaseous phase. Namely, the
`dispersion of many grown cells results in a superior absorption capability
`when compared with the same amount of gas encapsulated in a polymeric‘.
`film, such as in a sealed bag or pillow.
`Not only are the amount and distribution of cells critical parameters in
`establishing the final property configuration of a given polymeric foam, but
`the nature of the cells (open cell versus closed cell) similarly plays an impor-
`tant role in determining the properties, which, in turn, dictate the foam's
`
`PAGE 11 OF 51
`
`

`
`ll)
`
`Polymeric Foams: Technology and Science
`
`possible applications. When the integrity of the cell wall is poor and does
`not hold the gaseous phase together,
`it instead prompts interconnections
`with the neighboring cellls). Then.-fore. under deformation, the gas compres-
`sion, which is a key component dictating the mechanical response of the cell,
`may no longer be effective. Despite an eventual quick recovery, after the
`deformation has been released, both the compressive strength and the energy
`absorption ability would be weaktmed. Other mechanical properties would
`also be adversely affected. However, open cells have capillaries that allow
`for unique fluid absorption features; this is a desirable feature, for instance,
`in the wet meat industry. Open cell foams also perform well in sound-
`deadening applications, where sound waves attenuate after numerous
`bounces.
`On the other hand, the cell size exerts a great impact in disturbance dis-
`tribution. According to the studies of the microcellular foams with a cell size
`in the order of 10 microns, small cells provide an improved energy absorption
`capability, which will be further described in a later section. it is well known
`that the insulation ability of polyurethane and polystyrene foams strongly
`depends on the cell size. The smaller the size, the higher the insulation, partly
`because of the reduced radiation effects in the cell during heat transfer [36].
`The other vital consideration is the open-cell content: it seems to decrease
`as the cell size decreases. Nonetheless. when large foam expansion occurs
`(e.g., over 10 times), the cell morphology greatly contributes to the overall
`thermal and mechanical performance of the foams.
`The amount of residual blowing agent in the cells affects the insulating
`property of the polyurethane foam and the extruded or head polystyrene
`insulation foams. Because of the low diffusivity of llct-‘Cs and l-ll-‘Cs in the
`polymeric matrix, a blowing agent with a high insulating property does not
`diffuse out quickly and any remaining blowing agent in the cells and cell
`structures will determine the overall insulating property of the foam. Since
`the blowing agent eventually is replaced by air (as described in the next
`section], the insulating property of the polymeric foam decreases with time.
`
`
`
`1.4 Blowing Agents
`Any gas can serve as a blowing agent, but not every gas can be easily
`implemented in the foaming process. In fact, some gases are friendlier than
`others, in terms of solubility, volatility, and diffusivity.
`it appears that the
`blowing agent's quality, quantity, and nature are key factors in the produc-
`tion of a foamed structure with certain desired properties. The blowing ngfnl
`governs the selection of the foaming methodology, which more often than
`not becomes a limiting factor in industrial practice. It should he noted here
`that most blowing agents are easier to introduce into a polymer in M1)‘
`foaming equipment than air although the foam will consist of polymer and
`
`1
`
`.‘
`1
`
`l
`'
`
`PAGE 12 OF 51
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`

`
`Introduction to Polymeric Foams
`
`11
`
`—
`
`C )- ~<--:3~ oogsoo
`
`CIC
`
`0 U
`
`Solid
`
`Gas
`
`Foam (solid 1- gas)
`
`Foam (solid 9 air)
`
`FIGURE 1.5
`Ccnefill thermoplastic foaming path; gas from without to within the polymer. eventually re-
`placed by air.
`
`air ultimately. Once the foam is formed and exposed to air, most blowing
`agents are replaced by air as time passes. in most polymeric foams, the role
`of the blowing agent is like that of a "catalyst”—it participates in the process
`(see Figure 1.5). From the perspective of the blowing agent, the foaming
`process can be seen as a succession of three steps: implementation, liberation,
`and evacuation. in simple terms, (i) implementation refers to the process
`used for introducing predetermined quantities of an external gas into the
`polymer matrix to form a polymer/ gas solution; (ii) liberation suggests the
`conversion of the polymer/ gas solution, which is characterized by non-
`differentiated (invisible) structural elements, into a fully-diflerentiated (vis-
`ible) cellular structure; and (iii) evacuation signals the transformation of the
`polymer foam from a blowing—agent-filled to an air—filled state. The respec-
`tive key mechanisms are illustrated in Figure 1.6.
`
`Volume
`
`Stage:
`Function:
`
`l implementation
`l Gas into poi mar
`Y
`
`Key
`mechanism:
`
`I Mixing
`|
`
`: Liberation
`Saturation
`Gas bubblesin
`Pol merl as
`Y
`8
`I homogenization I polymer
`l Dissolution
`l Foaming
`I
`N
`
`I Evacuation
`Gas bubbles in
`I polymer
`I Permeation
`
`l
`
`FIGURE 1.6
`Polymer volume variation as foaming progresses; function and key mechanism attached.
`
`5
`
`PAGE 13 OF 51
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`
`12
`
`Polymeric Foams: Technology and Science
`
`
`
`Temperature
`
`FIGURE 1.7
`P-T-V change of foam extrusion from A to D; A: low V, low I’,
`C: high V, low I’, high T, D: high V, low P low T.
`
`low T., B: low V, high P, high T,
`
`foaming can be characterized by state change
`(see Figure 1.7). Once the raw plastic material (State A) is heated and pres-
`e loam structure is developed
`surimd, a blowing agent is added (State 8). Th
`by lowering the pressure (State C), and finally, a foam product is yielded by
`cooling the polymer matrix. It is the blowing agent that plays a major role
`in changing these states.
`1'he fundamental features of a blowing agent are its capability to migrate
`throughout the polymer, out of the polymer, and away from the polymer.
`in other words, the thermodynamic, kinetic, and transport properties of the
`gas are of vital importance at different stages of the process. Therefore, a
`balanced view should be used in analyzing and selecting a proper blowing
`agent in order to produce a desired foam structure out of a polymer. In
`general, chemical reactions and physical mixing are the main methods of
`introducing a blowing agent into the polymeric melt. The former implies
`crenting a reaction-induced gas in the solution, whereas the latter assumes
`gas dissolution into the polymeric melt. Table 1.7 presents the key properties
`that should be considered when selecting a blowing agent.
`
`
`
`1
`
`PAGE 14 OF 51
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`

`
`(ntroduction to Polymeric Foams
`
`13
`
`TABLE 1.7
`
`Blowing Agent Property Considerations
`J
`Aspect
`Thermodynamic properties
`
`Properties
`Volatility, Solubility, Molecular Weight, liquilibrium
`Constant, Surface Tension, Boiling Point, Specific Volume
`at 511’
`
`Transport Properties
`Processing and Environment
`
`Application:
`
`Diffusivity, Permeability
`Piasticization, Flammability, Stability, Tbxieity, Ozone,
`Warming
`Odor, FDA, Conductivity
`
`
`
`1.5 Environmental Issues and Technical Challenges
`
`In 1927, Midgley succeeded in synthesizing halogenated carbon compounds,
`or what we now commonly refer to as chlorofluorocarbons (CFCs), to replace
`actionl ammonia as refrigerants in Europe. Since then the non-toxic, non-
`flammable, and stable natures of CFCs have attracted wide and deep interest
`from various industries [37-39]. In 1988, the global annual production of
`CFCs exceeded ten billion pounds; a very substantial portion of this amount
`was used for foam production. However, the stable CFCs tend to release
`chlorine under ultraviolet (UV) exposure in the high atmosphere, which in
`turn retards the ozone formation reaction. As a result of the environmental
`threat posed by CFCS, the Montreal protocol was established in 1987 in an
`effort to officially enforce the emission control and usage of CPCs. In addi-
`tion, heavy taxes were proposed and put into effect to be associated with
`CFC purchase to reduce consumption. In the early 1990s, l-lydrochlorofluo-
`rocarbons (HCFCs) were first introduced as a replacement for CFCs. How-
`ever, according to scientific findings, HCFCs also released chlorine, which
`continued to contribute to ozone deterioration. At the same time, the global
`warming phenomenon was receiving increased attention. Also known as
`"the green-house effect," global warming is the consequence of slowing
`down the dissipation of heat from the earth's surface. Earth scientists and
`meteorologists predict that the earth's surface temperature will continue to
`increase over the next centuries. In consequence, new or existing blowing
`agents must conform to environmentally sound standards, and it was finally
`decided to include I-ICFCs on the phase-out list.
`The inclusion of a large molecular weight physical blowing agent with a
`low diffusivity is necessary in the production of low density (high expansion)
`polymeric foams. This is especially true in the case of thermoplastic foams.
`Since the previously used CFC-based and I-{CFC-based blowing agents are
`environmentally hazardous, new blowing agents have been searched. How-
`ever, the development of a reliable and safe alternative blowing agent should
`
`g
`
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`14
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`Polymeric Foams: Technology and Science
`
`satisfy both environmental concerns as well as lS$lll'S of product perfor
`mance. Although the polyurethane foaming process creates .1 sufficient
`amount of blowing gases, most of these gases possess at higher thermal
`conductivity than CFCs, and provide either a low-energy efficiency or a
`bulkier unit. Therefore, the present search for new physical blowing agents
`with certain desired properties (see the list of properties in Table 1.7) con-
`tinues. Currently, it appears that hyrdrofluorocarbon (HFC), hydrocarbon
`(HC), and their blends are the main candidates for the new blowing agent.
`HCs are readily available from petroleum cracking and are very compatible
`with most potynu-rs, however they