Polymeric Foams and Foam Technology

vonDaniel Klempner, V Sendijarevic

Neuausgabe

Polymeric Foams and Foam Technology Klempner / Sendijarevic

schnell und portofrei erhltlich bei beck-shop.de DIE FACHBUCHHANDLUNG

Hanser Mnchen 2004

Verlag C.H. Beck im Internet:www.beck.de

ISBN 978 3 446 21831 4

Inhaltsverzeichnis: Polymeric Foams and Foam Technology Klempner / Sendijarevic

9.4 Processing Technology 323

Table 9.9 Property Comparison of Products Obtained by Free-Foaming vs. Celuka Foaming [4]

Properties Free-foam Celuka

Average density (g/cm3) 0.6 0.5

Surface hardness (Shore D) 42 69

COTE (in./in./C) 5.6 105 4.5 105

Impact resistance (kJ/m) 458 1148

Tensile strength (kPa) 10,045 21,726

Flexural strength (kPa) 20,305 25,097

COTE = Coefficient Of Thermal Expansion

Figure 9.12 Comparison of foam density distribution. (a) Free foam; (b) Celuka process

9.4.1.2 Effect of Process Parameters on FoamMorphology

Foam formation is the critical step in the process of foam extrusion. Most rigid PVCfoams are made with chemical blowing agents. Regardless whether the gases arechemical or physical blowing agents, the foaming mechanisms during extrusion foaminginvolve several stages, as was first proposed by Hansen [51] and extended by Reichert[52].

(a) (b)

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Upon merging with the polymer melt, the gases dissolve in the polymer melt under highpressure. When the melt exits the die, a sudden pressure drop causes a thermodynamicinstability of the gas/polymer solution. This leads to the formation of a separate gasphase. At this stage, a large number of bubbles are formed in the polymer matrix. Theseare the nuclei of the cells. These bubbles grow until the gas pressure inside the bubblesand the surface tension of the bubble walls are in equilibrium. The foam must harden andsolidify as soon as this equilibrium is reached, otherwise the cells will collapse. The foamstructure can be stabilized by rapid solidification of the melt during the cooling process.The cell nucleation, cell growth, and cell morphology are thus affected by:

The type and amount of gases available in the system, The temperature, pressure, viscosity, and elasticity of the melt before exiting the die, The geometry of the die, and The cooling rate outside of the die.

For a given formulation and equipment, these conditions boil down to control of melttemperature and shear effect control during extrusion. The following case willdemonstrate how these adjustable parameters influence the cell structure of rigid PVCfoams prepared with the free-foaming process.

9.4.1.2.1 Foam Density and Cell Size

The temperature of the polymer melt affects the foam density and cell size, thus affectingthe melt viscosity, the degree of fusion, and gas generation from the blowing agent. For asuccessful foaming process, the temperature of the melt must lie in an interval betweenvalues that are giving a sufficiently low viscosity of the melt to allow cell formation andexpansion and a sufficiently high viscosity of the melt to prevent cell collapsing [49].Figure 9.13 [51] indicates that the foam density gradually decreases with an increase ofthe temperature of the melt because it affects the decomposition of the CBA, until itreaches a minimum. A further increase in melt temperature, however, causes an increasein density because of too low a melt viscosity and too high a gas diffusivity. Otherresearchers working not only with PVC [14], but with other polymers [49], also observedthis U-shaped curve.

As shown in Fig. 9.14, the cell structure also changed significantly because of the variationof the melt temperature. In particular, when the temperature is low, the cells are either non-existent in some spots, or they are very small. As the temperature increases, the cells growand become more uniform because of more decomposition of CBA. At very high tempera-tures, however, large coalesced cells result in a non-uniform cellular morphology [53].

The other relevant variables in the extrusion-based foaming process are a variation of thescrew rotation speed (RPM) and stretching of the melt after exiting the die with respect tosemi-finished foams. Experimental results [53] revealed that as the screw RPM increased,the density decreased, reaching a minimum value before it increased again. This V-shapedrelationship between the density and the screw RPM was considered similar to the oneobtained for the density vs. the melt temperature because an increase in the screw RPMwould cause greater shear heating, resulting in a temperature increase. These temperaturechanges, rather than the screw speed alone, became a dominating factor governing thePVC foam density. However, when the melt temperature was held constant by adjusting the

9.4 Processing Technology 325

185 190 195 200 205

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Melt temperature, C

Density,g/cm

3

Figure 9.13 Effect of melt temperature on density of a rigid PVC foam [53]

Figure 9.14 Influence of melt temperature on the cell structure of a PVC foam [53]

(Courtesy of Society of Plastics Engineers)

barrel and die temperatures, the density increased with increase in the screw speed becauseof the lower residence time and the high back pressure, thereby preventing the blowingagent from realizing its full potential and thus generating a greater density.

The effect of stretching downstream of the extrusion line also possibly influences thedensity and the cell structure. The same study [53] indicated that under identicalprocessing conditions, the foam density was nearly independent of the downstreamstretching. This was attributed to the fast expansion and foaming taking place at the time

326 9 PVC Foams

when the melt exited the die. The cell shape, however, changed from fairly spherical tomore elongate, and the cell morphology became less uniform.

9.4.1.2.2 Cell Size Distribution (Formation of Large Voids)

The formation of large voids in the core of the profile cross section is one of the majorproblems in the extrusion of PVC foams. These voids are usually formed as the melt exitsthe die and they grow as the distance from the die increases.

Various studies [50, 53] identified a strong correlation between the tendency to formvoids and the temperature of the melt, die pressure, and cooling. It was found that as themelt temperature increased and the die pressure decreased, surprisingly, the tendency toform large voids decreased. In fact, for a given screw speed, there appeared to be acritical temperature below which large voids were formed [53].

Rabinovitch [53] attributed this to the fact that when PVC is premixed with additives itfuses at a higher temperature and its behavior then resembles that of thermoplastics. It isthus believed that the increased melt temperature improved the fusion, which, in turn,initially increased the melt strength. It was generally observed that fusion was improvedand thus cell opening was prevented. On the other hand, due to the further temperatureincrease, the melt strength decreased. If the pressure were higher, the pressure dropwould be higher as well. However, increased temperatures and reduced melt viscosityresulted in lower die pressure and less pressure drop at the die exit. This reduced the rateof gas expansion inside the cells, which are cooled and solidified, thus preventing cellrupture. This explanation does not simply imply, however, that a higher melt temperaturewould be beneficial for the foaming process. In fact, the thermal sensitivity of PVC isalways a limiting factor in the extrusion of PVC foams. Too high a melt temperature canalso cause collapse of over-grown cells, which results in large voids.

The screw speed also has a significant effect on the formation of large voids. It wasobserved [53] that as the screw speed increased, the amount of fusion increased becauseof the increased shear heating, but when the screw speed was too high, the melt was toosoft, and therefore, cells collapsed inside the extruded foam profile while the skin of theextruded PVC foam was solidified by cooling. Consequently, voids were easily formed,and the resulting foam density was higher.

Another factor affecting void formation is the cooling after the melt exits the die [50].Due to the low thermal conductivity of PVC, a broad temperature profile exists in theextrudate. Insufficient cooling will leave the center of the extrudate hot, which results inweak cell walls prone to collapse under the pressure of hot gases inside the cells. This iswhy the voids are more visible for foams made by the free-foaming process than forthose made by the Celuka process.

9.4.1.2.3 Surface Smoothness

The surface appearance of PVC foams is greatly influenced by the extrusion parameters,especially when using the free-foaming process. Extrudates of rigid PVC foams are usuallyrough at low temperatures, become smooth at intermediate temperatures, and rough again athigh temperatures. The roughness appearing at low temperatures is mainly associated withinsufficient fusion and melt fracture, whereas the roughness appearing at high temperatures

9.4 Processing Technology 327

is due to sticking and cell rupture on the surface caused by shear at the die wall [53]. Withthe exception of fusion at the limit of an extruder heating system, a higher screw speedresults in increased shear rates, which usually causes an increased roughness.

9.4.1.3 Influence of Extrusion Equipment

Although single- and twin-screw extruders intended for processing solid PVC can beused for processing of foamed PVC products, specially designed extruders for PVCfoams are also in use. The screw configuration of the extruder should allow plasticizationof the compound to occur as rapidly as possible and at a relatively low melt temperatureso that the melt prevents the escape of already evolved gas through the throat of thehopper [1]. As the PVC melt containing gases are conveyed under high pressure throughthe screw channels, the decompression zones must be avoided to prevent bubbleformation [2], or the decomposition temperature of the CBA must be higher than the melttemperature in the decomposition zones.

The design of the dies deserves special attention. The cross-sections of the die shoulddecrease uniformly down to the die outlet. This helps to keep the melt pressure above thegas pressure or vapor pressure of the blowing agent almost to the end of the die. The dielength should be kept as short as possible to ensure that the melt pressure in the die ismaintained above the gas pressure as long as possible. Otherwise, premature foaming ofthe melt in the die will occur. The premature foaming is undesirable because it willdestroy the cells at the extrudate surface.

Typically, the calibration unit and cooling bath must be longer for foamed products thanfor solid ones because of the lower heat conductivity of cellular products versus solidproducts. Many new devices promoting the cooling processes have recently becomeavailable on the market, in response to demands for high output foam extrusion.

9.4.1.4 Recently Developed Extrusion Equipment for Rigid PVC Foams

Significant developments have been made in the area of extrusion equipment for rigidPVC foams during the last decade. The general trends are aimed towards increasing theflexibility and efficiency of the process. To meet the increasing demands on theequipment, multiple improvements along the extrusion line both upstream anddownstream have been introduced.

A trend in the core-foamed pipe industry is to use generic formulations for foamed andnon-foamed products, thanks to the improvement of upstream feeding systems. The newsystem can precisely add chemical blowing agents on-line [54], thereby avoiding theneed for making a compound exclusively for each foamed product. Usually, thisapproach results in a slightly heavier pipe because the PVC formulation is not optimizedfor greater density reduction.

At the heart of an extrusion line is an extruder(s). Currently, most of the vinyl foam sheetand pipe products use twin-screw extruders, whereas in the production of rigid foamprofiles, the single-screw extruder is most frequently found [4]. Although single-screwextruders are still much more inexpensive and simpler, one of the recent trends is to extendthe use of counter-rotating twin-screw extruders because of the following benefits [55]:

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Higher and steadier feeding rates of solids Twin-shaft drive to power more sequenced unit operations Better dispersive and distributive mixing of nucleating agents and other additives Fast and even incorporation of blowing agents Better capacity of cooling and heating control

The output of an extrusion line can be as high as 2000 kg/h for the free-foaming processwith a large extruder [56]. However, the increased output rates can lead to higher torques,which can produce greater wear on screws and gearboxes. As a result, several extrusionmachinery manufacturers have redesigned screws to handle this higher torque ordesigned gears to distribute the torque more efficiently [57]. Most recently, a new in-lineprocess capable of extruding sheets and profiles using direct-gas foaming with twin-screw extruders is under development [58].

The cooling of the foaming PVC melt is another major limiting issue in increasing theoutput rates of rigid PVC foams. Therefore, more technological changes are occurring tothe downstream equipment than to the primary extrusion machinery. It has been foundthat parts can cool much faster with water spray than conventional water baths [4],because the water mist removes heat through evaporation cooling and avoids becomingan insulator as in a water bath.

The dry calibration system [59] is another important development to the downstreamequipment for complex profiles. The newer cryogenic or nitrogen cooling equipmentreplaces the conventional chillers, water baths, and so forth. The major disadvantage ofthe cryogenic equipment is its cost. One possibility is to combine water cooling withnitrogen cooling. Water-cooling depends on the temperature difference between thecooling water and the product, and the volume of water in contact with the part. Withnitrogen cooling, the temperature difference is large and the circulation velocity of thegas over the part is high. A hybrid system could provide a more cost-efficient alternativebecause water can cool products efficiently at product temperatures over 120 C, andnitrogen cooling is effective below 120 C.

An inexpensive development to a downstream system includes extending the coolinglines significantly to produce higher production rates. Calibrators have gone from 24 in.to 48 in. long and flood tanks from 20 to 30 ft plus; in some cases, an additional 10 ft ofspray tank is added. This can, however, result in as much as a 50% increase in the outputrates [60].

It is worthwhile to mention that the extrusion dies play a critical role in efficientlyproducing high-quality foams, and this is an active research area in the field of foamextrusion. Because of the empirical methods used and the proprietary nature of designdetails, unfortunately, no reports are available in the open literature on recentdevelopments in this area. If interested, readers may refer to the latest version of bookson die design [61] and extrusion [62] for a general understanding of this field.

9.4.2 Injection Molding of PVC Foams

Over the years [1], rigid PVC foams have been commonly injection-molded into partssuch as residential ceilings, wall stock, and camping and lawn furniture. However, some

9.4 Processing Technology 329

injection molded PVC foams have been replaced by other materials. For example,molded plasticized PVC foams with a density of 600900 kg/m

3were used principally in

shoe soles. Nowadays, these applications of injection-molded PVC foams have beenlargely replaced by other polymers, mainly due to the longer cycle time needed for themanufacture of PVC foams and partially because of the issues related to plasticizers. As aresult, rigid PVC foams have been replaced with polypropylene in numerous applicationswhile EVA and other soft polymers have replaced flexible PVC foams.

Compounds for injection-molded PVC foams contain a resin with the same range of Kvalue as their non-foamed counterparts. The blowing agents are exothermic, and include4,4'-oxybis(benzenesulfonylhydrazide) (OBSH) alone or mixed with ADC, or ADCmixed with zinc oxide (as a catalyst). The compound is plasticated in a screw injectionunit and injected directly into the closed mold cavity. To prevent premature gas evolutionand consequent loss of gas, the temperature in the screw conveyor zone must be lowerthan the decomposition temperature of the CBA. The CBA decomposes duringplastication of PVC, releasing gas that dissolves in the pressurized melt. The gas mustremain dissolved in the melt until the melt is injected into the mold, so the plasticatingbarrel, nozzle, and sprue are under a back pressure greater than the pressure of the CBAgas. A shutoff valve at the nozzle, which can be a sliding shutoff or an externallycontrolled gate or globe valve, prevents the emergence of the pressurized mixture of meltand CBA through the sprue. The melt containing the CBA under pressure is injected asrapidly as possible into the mold cavity, thereby producing foams with a uniform andfine-celled structure. The melt coming into contact with the cool mold walls hardensimmediately into a dense skin, while an inner foamed core forms. The obtained structureis thus of integral-skin type with a dense outer layer and a foamed core [1].

No holding pressure control is needed, since this would increase the density. Theinjection mold is usually cooled to about 20 C. Higher temperatures lead to smoothersurfaces, a reduction in the thickness of the outer skin, and thereby a lower averageoverall density, but also requires longer cooling times [1].

9.4.3 Foaming of PVC at Atmospheric Pressure

The use of various atmospheric-pressure foaming processes is different from that of thehigh-pressure process, because the process of foam formation depends on threeparameters [1]:

The rate of gelation and extensibility of the melt The rate of gas formation during the gelling process The rate of heat transfer during the foaming of the plastisol

In a typical foaming process at the atmospheric pressure [59], a plastisol containingCBAs is coated on a substrate. The substrate is then transported through a conveyor ovenheated to a high temperature (typically 200 C). Once in the oven, foam expansion occurswhen the plastisol reaches the decomposition temperature of the blowing agent while theplastisol undergoes gelation. The foam formation depends on the three rates listed above.In practice, the foaming process must be synchronized with the gelation process tocontrol the cell morphology.

330 9 PVC Foams

Figure 9.15 Effect of temperature on the gelation process

During the gelation process, the rheological properties of the plastisol change drastically.Figure 9.15 shows a typical curve of viscosity versus temperature together with thestructural changes of the plastisol. At ambient temperature, the plastisol represents adispersion of PVC particles in a plasticiser. The viscosity decreases initially between 25and 50 C. Above 50 C, the viscosity increases rapidly, particularly between 70 and90 C. At 50 C, the particles begin to be solvated by the plasticiser. The migration of theplasticiser into the resin particles leads to their swelling until all the plasticiser isabsorbed by the particles. At this point the plastisol is converted into a dry mixture withno cohesive strength. The viscosity reaches a maximum between 90 and 140 C while thegel state is developed. Above 140 C, the particles begin to fuse into a homogeneousmelt. At 150180 C, the mass is converted into a thermoplastic melt. At thesetemperatures, the PVC takes the form of a high-viscosity melt. If the blowing agent is aCBA, it begins to decompose at about 170185 C in the presence of catalysts. Theelasticity of the melt at the time of CBA decomposition determines the proportion ofopen and closed cells in the foam.

The key strategy for gaining a desired cell density and cell structure is to control thefollowing factors for matching the gelation and foaming process [59, 60]:

Rate of heat transfer Temperature of oven Dwell time Gas velocity over the surface of the plastisol to be foamed Nature of the participating PVC resins and additives

9.4 Processing Technology 331

Many analytical methods (turbidity, electron microscopy, light scattering, etc.) have beenused to monitor these processes [60, 63], but it is still a challenge to obtain precise dataon each stage of the gelation process: solvating, swelling, dissolution, melting ofcrystallites, etc. In a recent study [59] the processing parameters still had to be linkedwith visual quality control in order to estimate the significance of different factors. Also,a number of practical works with respect to making different plastisol products fordifferent considerations has been demonstrated in [1].

9.4.4 Microcellular PVC Foams

9.4.4.1 Foamed PVC and Blowing Agents

The current technology for extrusion processing of high-density rigid PVC foamsinvolves the use of CBAs. However, besides their relatively high cost, the use of CBAs isassociated with some other limitations of technical nature, such as:

It is difficult to achieve on-line adjustment of foam densities without changing theconcentration of the CBA.

CBA residuals in the extrudate can hinder the process of recycling scrap. The migration of a CBA within the dry-blended mix during transport may cause

density variations in the final product.

The decomposition temperatures of some CBAs may make them not applicable tosome processes.

Conventional PBAs such as halocarbons (CFCs and HCFCs) and hydrocarbons are sub-ject to strict environmental regulations today. An alternative group of inert-gas blowingagents such as carbon dioxide (CO2), nitrogen (N2), and argon (Ar) are growing in use,since they are more environmentally friendly. Dey et al. [34] developed a novel approachto the extrusion of high-density, rigid PVC foams using commercial compounds withinert-gas PBAs (CO2 and Ar). The process was developed on a segmented single-screwextruder and provided an alternative to conventional processing methods using CBAs.

Microcellular foaming technology provides a potential solution to the high cost and thereduced temperature performance by significantly reducing the cell size and increasingthe cell density. PVC products can be successfully foamed while maintaining superiorphysical properties [64] with the presence of acrylic modifiers [65] that are commonlyadded to PVC foams to improve their toughness. It is worth noting that the toughness ofconventionally foamed rigid PVC is dramatically lowered with the foamed structurealthough the cost and weight of rigid PVC foams are advantageous over solid products.However, with a decrease in cell size, the weakened properties of PVC foams can becompensated for significantly [64].

9.4.4.1.1 Processing of Microcellular Foams

The concept of improving the performance (i.e., the properties) by creating a microcellularstructure in a polymer was put forth by Suh [66] at the Massachusetts Institute ofTechnology in the early 1980s. Microcelluar foams, characterized by their small cell size

332 9 PVC Foams

(less than 10 m) and a large cell-population density (higher than 109 cells/cm3) havedemonstrated the benefits of reduced material usage and lowered weight, while enhancingthe impact strength, toughness, fatigue life, and thermal stability [7, 64, 6771]. All of theseadvantages offer the possibility of replacing solid polymers in applications where the fullstrength of the solid polymer is not needed. This is especially true for many PVC productsused in the building and construction industries, where load bearing is not the primaryfunction of products such as window frames, decking, door trims, and so forth.

Over the last decade substantial research and development have been conducted tounderstand the processing conditions that lead to the production of microcellular plastics[5, 7, 24, 33, 65, 66, 7280]. Microcellular foams were initially produced in a batch processand later in continuous extrusion and injection molding systems. The fundamental mecha-nism for producing a very fine cell structure comprises dissolving gas into the polymermatrix and then inducing thermodynamic instability to nucleate a large number of bubbles.This instability can be created by rapidly dropping the pressure [20, 33, 73]. The growth ofthe nucleated bubbles is controlled and the bubbles are ultimately stabilized [72, 79].

9.4.4.1.2 Batch Microcellular Processing

Microcellular polymers were first produced in a batch process [73]. A schematic of thebatch processing principle is shown in Fig. 9.16.

Figure 9.16 Schematic illustration of batch microcellular processing technology

In this process, a polymer sample is placed in a high-pressure chamber connected to a gasreservoir. The gas can be either nitrogen (N2) or carbon dioxide (CO2). The polymersample absorbs the gas and, after a sufficient time, reaches a saturation state. When thesample is fully saturated with the gas, the pressure is rapidly decreased to cause a suddendrop in the solubility of the gas in the polymer. This initiates a thermodynamic instability,which drives nucleation of billions of microcells. After the pressure is released, thepolymer specimen is transferred into a heated fluid bath to promote foaming.

In the batch process, cell nucleation is governed mainly by the saturation pressure (orpressure drop) and cell growth is governed by the heating temperature and time. Hence,the number of nucleated cells and the foam density can be controlled independently. It

9.4 Processing Technology 333

must be noted that in a batch process, the foaming temperature is generally chosen to bethe lowest to make the cell growth step easily controllable [20, 33]. When the nuclei aregenerated, their growth is retarded due to the high stiffness of the polymer matrix at lowtemperatures. However, this prevents the deterioration of the cellular morphology of thefoamed product through coalescence of the cells. By modulating the temperature and thetime of exposure to heat, the growth of the cells can be controlled.

Because of the low rate of gas diffusion into the polymer at room temperature, a verylong time is required for the saturation of the polymer with gas, which is the majordisadvantage of the batch process. For instance, the diffusivity (D) of CO2 in anythermoplastic at room temperature is typically 5 108 cm2/s [67]. The diffusion timecan be estimated using Eq. 9.2 [74]:

D

htD

4

2

(9.2)

where, h is the thickness of the sample. Thus, for a sample with a thickness of 1 mm, thediffusion time will be about 12 hours. Clearly, the total processing time is quite long, andthus the batch process would not be cost-effective.

9.4.4.1.3 Continuous Microcellular Processing

In order to overcome the shortcomings of the batch process, a cost-effective, continuousmicrocellular extrusion process was developed based on the same concept of thermo-dynamic instability as in the batch process [75, 76, 79]. A schematic of microcellularextrusion processing is shown in Fig. 9.17. In this process, a much shorter time is neededfor dissolving the gas in the polymer. After the polymer is plasticated and is in a moltenstate in the extrusion barrel, a metered amount of gas is delivered to the polymer meltwhere it is made to dissolve completely.

Figure 9.17 Schematic of continuous microcellular processing

9.4.4.2 Processing Microcellular PVC Foams

Microcellular foamed PVC represents a particularly good alternative to achieve a givendegree of strength and to lower the thermal conductivity with less material (cost saving)because of the unique properties of microcellular foams. Therefore, the production of

334 9 PVC Foams

foamed PVC would lead to the creation of a new class of materials with reduced cost,density, and enhanced mechanical properties.

Matuana et al. [64] investigated the structure and mechanical properties of microcellularfoamed PVC. Microcellular foamed structures were produced from PVC by usingsupercritical CO2 in the batch process under high pressure followed by rapidly decreasingthe solubility of CO2 in the samples as described above [64]. The void fraction of themicrocellular foamed PVC was controlled by tailoring the foaming temperature andfoaming time. Tensile and impact tests were performed on the foamed PVC to investigatethe dependence of these properties on the void fraction of foamed specimens. Thenotched Izod impact strength of microcellular foamed PVC increased as the void fractionincreased. When the void fraction was 80%, the notched Izod impact strength of foamedPVC was four times as high as that of non-foamed PVC. However, the tensile strengthand modulus decreased as the void fraction increased. On the other hand, Juntunen et al.[5] presented somewhat different results. They observed that the impact strength of themicrocellular PVC foam decreases linearly with relative density. Further research needsto be conducted to comprehensively derive the structure-property relationships formicrocellular foamed PVC.

Holl et al. [24, 81] investigated the effect of the presence of commonly used additives onthe processing and structure of microcellular PVC foams. It was found that the presenceof additives leads to a polydispersed cell structure with large variations in cell sizes. Thesolubility of CO2 in the additives and in the lubricants was found to be lower than in thePVC matrix. The blowing agent, CO2, also acted as a plasticizer due to its small size andminimal interaction with the polymer. The presence of additives resulted in no adverseeffect on the overall foam growth dynamics.

The continuous production of microcellular foamed PVC for commercial applications hasalso been successful. Vanvuchelen et al. [65] and Blizard et al. [80] developed thin-walled microcellular PVC foam products using the MuCell microcellular foam process[82] with an existing commercial-scale rigid PVC extrusion line. Supercritical fluids ofCO2 or N2 were introduced into the extruder as blowing agents as shown in Fig. 9.17. Thescrew and die were modified to meet the microcellular foaming requirements. Thefoamed thin-wall PVC profiles with wall thickness of about 0.45 mm had a specificgravity ranging from 0.81.2.

9.4.5 PVC/Wood Flour Composite Foams

Another emerging technology is PVC/wood-flour composite foams and microcellularPVC/wood-flour composite foams. Plastic/wood (or wood-flour) composites represent anemerging class of materials that combine the favorable performance and cost attributes ofboth wood and thermoplastics. During the past two decades, these materials havereceived substantial attention in scientific research (see Chapter 14 for details). Mostwood-plastic composites can be fastened, sanded, stained, and machined in the same wayas natural wood without the need to invest in new equipment. Although polyethylene-based wood-flour composites are more popular than PVC-based composites because ofthe higher heat resistance [6] and lower resin costs, there are several advantages of PVC-based wood-flour composites. Firstly, the PVC-based wood-flour composites have out-standing paintability whereas polyethylene-based or polypropylene-based wood-fiber

9.5 Mechanical Property Analyses and Test Standards 335

composites have very poor paintability. Little, if any, treatment is needed to make thepaint stick to the PVC-based composites. Secondly, being a polar polymer, PVC canbond fairly well to the wood-flour even without introducing a coupling agent, althoughuse of silane as a coupling agent improves their bonding significantly [8387]. Incontrast, polyolefins (i.e., polyethylene and polypropylene) adhere less well to wood-flour so that modification or compatibilization would be absolutely required. However, inorder to improve the mechanical properties of PVC/wood-flour composites and thefoamability, surface treatment of wood flours is strongly recommended [7, 20, 33].Matuana et al. [83] studied the effectiveness of surface treatment on the wood-flours ofPVC/wood-flour composites by investigating the adhesion between PVC and laminatedwood veneers. Their results indicated that the adhesion between PVC and wood veneerswas significantly improved when wood veneers were treated with amino-silane, and thatmatching the surface tension is not sufficient to ensure good adhesion between PVC andwood veneers. The role of surface acid-base properties of plasticized PVC and cellulosefibers on the mechanical properties of the composites was also examined [8486]. Acid-based pair interactions have been found to be a valuable parameter in the design ofsurface modification strategies intended to optimize the tensile strength of thePVC/wood-flour composites.

The combination of the PVC/wood-flour composite technology and the microcellularfoaming technology was also studied. In order to compensate for the lowered impactstrength and ductility of wood flour composites due to the incorporation of wood flour inthe PVC matrix, Matuana et al. successfully introduced a microcellular-foamed structureusing a batch process [20, 33]. The property investigation of these materials indicatedthat the microcellular structures improved the impact strength of PVC/wood-flourcomposites dramatically while lowering the density of the artificial wood to the desiredrange of 0.6 to 0.8 g/cm

3[7].

Research efforts have also been made to implement the continuous microcellular plasticstechnology with PVC/wood-flour composites. Although fine-celled PVC/wood-flourcomposite foams have been successfully obtained in a continuous extrusion process [17,88], limited success has been reported to date because of the unavoidable volatileemissions from wood flour at elevated temperature in an extruder [8991]. Furtherresearch is required to produce uniformly distributed microcellular structures inPVC/wood-flour composites. Interested readers can refer to Chapter 14 for the details.

9.5 Mechanical Property Analyses and Test Standards

The determination of mechanical properties of PVC foams is necessary for both academicstudy and industrial practice. The mechanical properties of rigid PVC foams arecontrolled by complex formulations and processing conditions. In industrial practice, theproperties of foams have to be quantitatively tested based on recognized standards forquality control and commercial purposes. It is therefore helpful to have information onthese standards at hand. Since the results obtained from these analyses are very beneficialfor the design and/or selection of the most suitable foams for the desired applications,improving the accuracy of these analyses is still an active challenge for researchers andengineers in this field.

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9.5.1 Mechanical Properties

Both application and market data indicate that it is more difficult to understand themechanical properties of rigid PVC foams than those of flexible foams. The mechanicalproperties of rigid PVC foams are the responses of the foamed materials to mechanicalforces. Given the viscoelastic nature of polymers, these properties are influenced by theway forces are applied as well as the inherent properties of the materials. The mechanicalproperties that will be discussed in this section are for rigid PVC foams.

Often encapsulated by a solid PVC layer (skin), rigid PVC foams can be regarded as acombination of solid walls and foamed cores. The mechanical properties of solid PVCproducts are predictable based on their particular formulations and the three-dimensionalnetwork of PVC in the products [12]. The perfection of the network can be measured bysimple but reliable methods such as the acetone test in which a poorly fused PVC will showsigns of surface deterioration [53]. The effects of the foamed structure on mechanicalproperties were investigated using both experimental and theoretical approaches [7, 63,9296]. These studies revealed that the key factors influencing the mechanical properties ofthe foamed structure include foam density, cell geometry, and cell imperfections.

9.5.1.1 Effect of Foam Density

Figure 9.18 [92] illustrates typical compressive stress-strain curves of PVC commercial foamsat various foam densities. All the stress-strain curves show a linear-elastic regime at low stressvalues followed by a long plateau of roughly constant stress, truncated by a regime of steeplyrising stress. The yield point becomes less obvious as the foam density decreases.

(1)

(2)

(3)

0.1

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

PVC foam

(1) pf=0.08 g/cm3

(2) pf=0.06 g/cm3

(3) pf=0.03 g/cm3

Strain

Stress(M

Pa)

Figure 9.18 Effects of foam density on compressive stress-strain response of PVC foams [92]

9.5 Mechanical Property Analyses and Test Standards 337

PVC foam

(1) pf=0.08 g/cm3

(2) pf=0.06 g/cm3

(3) pf=0.03 g/cm3 (1)

(2)

(3)

0.050

0.5

1.0

1.5

2.0

2.5

3.0

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

Strain

Stress(M

Pa)

Figure 9.19 Effects of foam density on tensile stress-strain response of PVC foams [92]

Typical effects of the foam density on the tensile stress-strain behavior of commercialPVC foams are shown in Fig. 9.19 [92].

Clearly, tensile deformation is different from compression. These foams are plastic incompression but brittle in tension. The brittleness in tension is likely caused by the stress-concentrating effect of cracks nucleated at weak cell walls or pre-existing flaws andpropagating catastrophically, resulting in cell wall collapse and fast fracture. There is lessdamage in compression, because the stress tends to close the cracks rather than openthem. The relative thickness of the cell walls increases with rising foam density. Thismakes it more difficult for the cell walls to align with the tensile axis (or the compressiveaxis), since the resistance to cell wall bending increases. Therefore, the elastic modulusincreases in both cases. The brittle stress increases in tension. The compressive yield andplateau stresses increase as the foam density increases. Also, the compressive strain atwhich densification starts decreases with higher foam densities [92].

The effects of foam density on its mechanical properties are best expressed in terms ofthe relative density. The relative density (f/s) is the most important structuralcharacteristic of a foamed plastic, where the density of the material from which the foamis made is denoted by , while f and s are suffixes indicating foamed or solid,respectively. The relationship between the relative yield strength (f /s) and the relativedensity can be expressed as [93]:

))(1()(3.02/3

s

f

s

f

s

f

+= (9.3)

338 9 PVC Foams

where is the friction of the material in cell edges and can be determined by scanningelectronic microscopy, and (1- ) is the friction of wall surface of a cell [92].

9.5.1.2 Effect of Cell Geometry

The cell geometry plays a significant role with regards to the mechanical properties ofPVC foams, especially to those having a relative density lower than 0.1. The dominantuse of these low-density foams is in sandwich constructions where the PVC foams areused as core materials. However, PVC structural foams possess some unique featuresarising from their mechanical properties, to which classic continuity mechanics for solidisotropic materials does not apply. An example is that negative Poissons ratios could bederived by classic mechanics [94]. To establish a relation between Young's modulus,shear modulus, and Poisson's ratio, a simplified closed-cell model was developed [95],based on the elasticity theory and consideration of a closed cell of cubic foam. The newrelation includes the effect of volume fraction of cell wall to total volume of the foamwhile Young's modulus and shear modulus are derived in terms of the volume fractionand Poisson's ratio. This relationship can be expressed as:

( )

+++=

2

222

1

)(1)1(

12

w

ww

w

w

f

f

G

E(9.4)

where E, G, and represent the Youngs modulus, shear modulus and Poissons ratio ofthe foam, respectively; fw is the volume fraction of the cell wall to the cell; and w isPoissons ratio of the cell walls.

Equation 9.4 indicates that the mechanical behavior of a foamed material is greatlyassociated with its cell geometry and the material properties of the cell wall. Thisequation is useful for the selection of foams intended for used in a sandwich structure,where foams will more likely be subjected to tensile stresses than shear stresses. The ratio(G/E) can also be used to control the final volume of the polymeric structural foamsduring the manufacture of sandwich constructions, as the structural foam, which containsa high modulus ratio, has an advantage over the others [96]. The above equation predictsthat for low-density foams, the mechanics of the foam are mainly determined by theshape of the cell rather than by its material, as shown in Fig. 9.20. In this figure, theG/E values are almost constant at fw = 0.1, regardless of the wall property (Poissonsratio).

This theory is supported by the literature data [95]. These data indicate that the Poisson'sratio of the foam radically varies from its initial value of about 0.3 down to a value of 0.1while the foam is deformed from zero strain to its elastic limit. Beyond the elastic limit,the material yields and exhibits collapse of the cells, and the Poisson's ratio of the foamin this region becomes approx. 0.45. These facts were checked against the correspondingstress-strain curve for the foam with fixed/free boundary conditions at a low stresslevel. The results revealed that the modulus of PVC structural foams strongly depends onthe initial cell geometry while the Poisson's ratio varies as a result of the deformationhistory.

A detailed work [97] on multi-axial characterization and modeling of a rigid PVC foamindirectly observed the importance of cell geometry for mechanical properties by

9.5 Mechanical Property Analyses and Test Standards 339

confirming the orthotropic nature in stiffness and strength of the foam. This work alsofound that the Tsai-Wu failure criterion [98] is in agreement with the experimental resultsfor predicting the biaxial stress.

-0.5 -0.25 0 0.25 0.5

0

0.25

0.5

0.75

0

w

G/E

=0

fw=0.1

Figure 9.20 Modulus ratio of non-deformed foam with 10% of the volume occupied by the solid

vs. Poissons ratio of the cell wall w(Legend: isotropic solid, = 0.1, = 0.3, = 0.5)

9.5.1.3 Effect of Cell Imperfections

In many cases, the cell structure of PVC foams is not perfect. The most seriousimperfections are large, uneven voids in rigid foams. The voids cause fatal failure offoams exposed to stresses. It is such an obvious problem that practical users do not acceptfoams with visible uneven voids, however, researchers have shown little interest instudying the effects on PVC foams in detail [99].

The mechanical properties of PVC foams without large uneven voids may be affected byvarious types of cell structure imperfections, such as wavy distortion of cell walls,variations in cell wall thickness, non-uniform cell shapes, and so forth. Based on Kelvinsmodel [100], a model was developed in a recent study for predicting the effects of threekinds of PVC foam imperfections. It indicated with supportive evidence that:

Stiffness is the most sensitive to wall waviness: a moderate waviness of cell walls canreduce the foam stiffness by 50%.

Stiffness is insensitive to thickness variations of the cell wall. Shape variations of the cells have little influence on the stiffness of PVC foams.

It should be noted that all discussions here dealt with foams as such, but not with skin-containing foams. Because of the much higher strength of the skin compared to that ofthe foamed core, and because of the great complexity of structure-property relationshipsof PVC foams with skins, the product strength used to be calculated based on the skinlayer only. However, this trend is changing recently, as more and more researchers havestudied foam failure in sandwich structures [92, 95, 100104].

340 9 PVC Foams

9.5.2 ASTM and ISO Standards

In order to test and evaluate the properties of PVC foams, it is more important to ensure thatthe tests and judgments are based on widely accepted standards. Experts in this field aroundthe world have made a lot of efforts to develop these standards. Below is a comprehensivelist of standards from both ASTM [105] and ISO Institute [106] for PVC foams. Thesestandards are well accepted globally by both industry and academic institutes.

9.5.2.1 ASTM Standards

D3576-98 Standard Test Method for Cell Size of Rigid Cellular Plastics D2856-94 (1998) Standard Test Method for Open-Cell Content of Rigid Cellular

Plastics by the Air Pycnometer

D2126-99 Standard Test Method for Response of Rigid Cellular Plastics to Thermaland Humid Aging

D1623-78 (1995) Standard Test Method for Tensile and Tensile Adhesion Propertiesof Rigid Cellular Plastics

D1622-98 Standard Test Method for Apparent Density of Rigid Cellular Plastics D3204-93 (1998) Standard Specification for Preformed Cellular Plastic Joint Fillers

for Relieving Pressure

D6226-98e1 Standard Test Method for Open Cell Content of Rigid Cellular Plastics D2842-01 Standard Test Method for Water Absorption of Rigid Cellular Plastics D1621-00 Standard Test Method for Compressive Properties of Rigid Cellular Plastics D3748-98 Standard Practice for Evaluating High-Density Rigid Cellular Thermo-

plastics

9.5.2.2 ISO Standards

ISO 844:2001 Rigid cellular plastics Determination of compression properties ISO 845:1988 Cellular plastics and rubbers Determination of apparent (bulk)

density

ISO 1183:1987 Plastics Methods for determining the density and relative density ofnon-cellular plastics

ISO 1183-3:1999 Plastics Methods for determining the density of non-cellularplastics Part 3: Gas pycnometer method

ISO 1209-1:1990 Cellular plastics, rigid Flexural tests Part 1: Bending test ISO 1209-2:1990 Cellular plastics, rigid Flexural tests Part 2: Determination of

flexural properties

ISO 1663:1999 Rigid cellular plastics Determination of water vapor transmissionproperties

ISO 1922:2001 Rigid cellular plastics Determination of shear strength (available inEnglish only)

ISO 1923:1981 Cellular plastics and rubbers Determination of linear dimensions

9.6 References 341

ISO 1926:1979 Cellular plastics Determination of tensile properties of rigidmaterials

ISO 2796:1986 Cellular plastics, rigid Test for dimensional stability ISO 2896:2001 Rigid cellular plastics Determination of water absorption (available

in English only)

ISO 4590:2002 Rigid cellular plastics Determination of the volume percentage ofopen cells and of closed cells (available in English only)

ISO 4651:1988 Cellular rubbers and plastics Determination of dynamic cushioningperformance

ISO 4897:1985 Cellular plastics Determination of the coefficient of linear thermalexpansion of rigid materials at sub-ambient temperatures

ISO 4898:1984 Cellular plastics Specification for rigid cellular materials used in thethermal insulation of buildings

ISO 4898:1984/Add 1:1988 Phenol-formaldehyde cellular plastics (RC/PF) ISO 6187:2001 Rigid cellular plastics Determination of friability (available in

English only)

ISO 7616:1986 Cellular plastics, rigid Determination of compressive creep underspecified load and temperature conditions

ISO 7850:1986 Cellular plastics, rigid Determination of compressive creep ISO 8873:1987 Cellular plastics, rigid Spray-applied polyurethane foam for thermal

insulation of buildings Specification

ISO 9054:1990 Cellular plastics, rigid Test methods for self-skinned, high-densitymaterials

ISO 9772:2001 Cellular plastics Determination of horizontal burning characteristicsof small specimens subjected to a small flame

ISO 6453:1985 Polymeric materials, cellular flexible Polyvinylchloride foamsheeting Specification

ISO 7203-1:1995 Fire extinguishing media Foam concentrates Part 1:Specification for low expansion foam concentrates for top application to water-immiscible liquids

ISO 7203-2:1995 Fire extinguishing media Foam concentrates Part 2:Specification for medium and high expansion foam concentrates for top application towater-immiscible liquids

ISO 7203-3:1999 Fire extinguishing media Foam concentrates Part 3: Specificationfor low expansion foam concentrates for top application to water-miscible liquids.

9.6 References

1 Brathun, R., Zingsheim, P., in Handbook of Polymeric Foams and Foam Techno-logy,Klempner, D., and Frisch, K., (Eds.) (1991), Hanser, Munich, Chap. 10, p. 245

2 Schipper, P., Black, J., Dymeck, T., J. Vinyl Additive Tech., (1996), 2(4), p. 304

342 9 PVC Foams

3 Thomas, N.L., Prog. Rubber Plast. Technol., (1998), 14(3), p. 129

4 Patterson, J., SPE ANTEC Techn. Papers, (2002), 48, Paper # 792

5 Juntunen, R.P., Kumar, V., Weller, J.E., Bezubic, W.R., J. Vinyl Additive Tech.,(2000), 6(2), p. 93

6 Patterson, J., J. Vinyl Additive Tech., (2001), 7(3), p. 138

7 Matuana, L.M., Park, C.B., Balatinecz, J.J., Polym. Eng. Sci., (1998), 38(11), p.1862

8 Thomas, N.L., Quirk, J.P., Cell. Polym., (1997), 16(5), p. 364

9 Cibitt, J., UK Patent 2288143, (1995)

10 Haruna, S., JP Patent 2002012691, (2002)

11 Huang, S., CN Patent 1307075, (2001)

12 Titow, W.V., PVC Plastics, (1990), Elsevier Applied Science, London

13 Summers, J., J. Vinyl Additive Tech., (1997), 3(2), p. 130

14 Blundell, D.J., Polymer, (1979) 20, p. 934

15 Katchy, E.M., J. Appl. Polym. Sci., (1983), 28, p. 1847

16 Rabinovitch, E.B., Summers, J.W., J. Vinyl Technol., (1980), 2, p. 165

17 Summers, J. W., J. Vinyl Technol., (1981) 3, p.107

18 Summers, J. W., Rabinovitch, E. B., J. Vinyl Technol., (1991) 13, p. 54

19 Matuana, L.M., Mengeloglu, F., J. Vinyl Additive Tech., (2001), 7(2), p. 67

20 Matuana, L.M., Park, C.B., Balatinecz, J.J., Polym. Eng. Sci., (1997), 37, p. 1137

21 Kim, K.U., Kim, B.C., Hong, S.M., Park, S.K., Int. Polym. Proc., (1989), 4, p. 225

22 Thomas, N.L., Eatsup, R.P., Roberts, T., Plastics, Rubber and Composite.Processing and Applications, (1994), 22, p. 115

23 Patterson, J., J. Vinyl Additive Tech., (1998), 4(1), p. 26

24 Holl, M.R., Ma, M., Kumar, V., Cellular Polym., (1998), 17(4), p. 271

25 Thomas, N.L., Harvey, R., J. Vinyl Additive Tech., (1999), 5(2), p. 63

26 Patterson, J., Szamborski, G., J. Vinyl Additive Tech., (1995), 1(2), p. 148

27 Heck III, R., J. Vinyl Additive Tech., (1998), 4(2), p. 113

28 Zhang, H., M.A.Sc. Thesis, University of Toronto, (1999)

29 Hurnik, H., in Plastics Additives Handbook, Gaechter, R. and Mueller, H., (Eds.)(1985), Hanser, Munich, p. 619

30 Thomas, N.L., Eatsup, R.P., Quirk, J.P., Plastics, Rubber and Composite.Processing and Applications, (1997), 26, p. 47

31 Kim, K.U., Park, T.S., Kim, B.C., J. Poly. Eng., (1986), 7, p. 1

32 Luebke, G., Rapra Conference Proceedings, Blowing Agent Systems: Formulationsand Processing, (1998), Paper #11/7, Rapra Technology Ltd, Shawbury,Shrewsbury, UK

33 Matuana, L.M., Park, C.B., Balatinecz, J.J., J. Cellular Plast., (1996), 32(5), p. 449

34 Day, S.K., Jacob, C., Xanthos, M., J. Vinyl Additive Tech., (1996), 2(1), p. 48

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35 Shutov, F.A., in Handbook of Polymeric Foams and Foam Technology Klempner,D., Frisch, K. (Eds.), (1991), Hanser, Munich, Chap. 17

36 Decker, R.W., J. Vinyl Additive Tech., (1996), 2, p. 121

37 Szamborski, G., Pfenning, J.L., J. Vinyl Technol., (1992), 14(1), p. 105

38 Ide, F., Okano, K., Pure and Appl. Chem.,(1981), 53, p. 489

39 Pfenning, J.L., Ross, M., PVC 90, (April, 1990), Paper #10, Brighton, UK

40 Kitai, K., Holsopple, P., Okano, K., J. Vinyl Technol., (1992), 14 (1), p. 211

41 Haworth, B., Chua, L., Thomas, N.L., Plastics, Rubber and Composite- Processingand Applications, (1994), 22, p. 159

42 Seers, J. K. Darby, J. R., The Technology of Plasticizers, (1982), John Wiley andSons, New York. p. 152

43 Velickovic, S.J., Stojkov, D., Popovic, I.G., Brankov, K., Cvorkov, L., J. VinylAdditive Tech., (2002), 8(2), p. 159

44 Thomas, N.L., Quirk, J.P., Plastics, Rubber and Composite Processing andApplications, (1995) 24 (2), p. 89

45 Dunkelberger, D.L., Herman III, H.R., J. Vinyl Additive Tech., (1996), 2(1), p. 44

46 Neuman, R., Experimental Strategies for Polymer Scientists and Plastics Engineers,(1997), Hanser, Munich

47 MODDE 6.0, (2002), Umetrics Inc., USA

48 JMP 6.0, (2002), SAS Institute Inc., USA

49 Burt, J.G., J. Cellular Plast.,(1978), 14, p. 341

50 Ishii, T., Kuratani, Y., Shimomura, K., Horioka, M., Yoshiki, S., JP Patent55039360, (1980)

51 Hansen, R.M., Martin, W.M., J. Polym. Sci., (1965), 38, p. 325

52 Reichert, U., Kunststoffe-Rundschau, (1977), 10, p. 443

53 Rabinovitch, E.B., J. Vinyl Additive Tech., (1996), 2(1), p. 48

54 ONeill, M., Modern Plastics International, (1997), online

55 Thiele, W., Foams 2000, (2000), p. 35, Parsippany, NJ, USA

56 Kager, M., Vinyltec 2000, CD ROM, Philadephia, USA, (2000)

57 Naitove, M., Plastics Technology, online, November 2001

58 Davis-Standard Corp., Plastics Technology, online, June 2002

59 Bergounhon, P., Plastics, Rubber and Composites, (1999), 28(7), p. 317

60 Vrentas, J.S., Vrentas, C.M., J. Appl. Polym. Sci., (1998), 67, p. 2093

61 Michaeli, W., Extrusion Process for Plastics and Rubber, Hanser, (1992)

62 Lee, S.T., Principles of Plastic Extrusion, Technomic, (2000)

63 Kwak, S.Y., J. Appl. Polym. Sci., (1995), 55, p. 1683

64 Matuana, L.M., Park, C.B., Balatinecz, J.J., Cellular Polymers, (1998), 17(1), p. 1

65 Vanvuchelen, J., Perugin, C., Deweerdt, M., Chen, L., Burnham, T., J. CellularPlastics, (2000), 36, p. 148

344 9 PVC Foams

66 Suh, N.P., Personal Communications, MIT-Industry Polymer Processing Program,(1980)

67 Doroudiani, S., Park, C.B., Kortschot, M.T., Polym Eng Sci, (1996), 36, p. 2645

68 Collias, D.I., Baird, D.G., Polym Eng Sci, (1995), 35(14), p.1167 and 1178

69 Seeler, K.A., Kumar, V., J Reinforced Plast Comp, (1993), 12, p. 359

70 Shimbo, M., Baldwin, D.F., Suh, N.P., Polym Eng Sci, (1995), 35, p. 1387

71 Glicksman, L., Notes from MIT Summer Session Program 410S, Cambridge, MA(1992)

72 Park, C.B., Behravesh, A.H., Venter, R.D., in: Polymeric Foams: Science andTechnology, K. Khemani (Ed.), (1996), p. 115, ACS, Washington

73 Martini, J., Waldman, F.A., Suh, N.P., SPE ANTEC Technical Papers, (1982), 28,p. 674

74 Behravesh, A.H., Ph.D. Thesis, University of Toronto, (1998)

75 Park, C.B., Baldwin, D.F., Suh, N.P., Polym Eng Sci, (1995), 35(5), p. 432

76 Park, C.B., Suh, N.P., Polym Eng Sci, (1996), 36(1), p. 34

77 Behravesh, A.H., Park, C.B. Venter, R.D., Cellular Polym., (1998), 17(4), p. 309

78 Matuana, L.M., Ph.D. Thesis, University of Toronto, (1997)

79 Park, C.B., Behravesh, A.H., Venter, Polym Eng Sci, (1995), 38(11), p.1812

80 Blizard, K., Chen, L., Straff, R., Deweerdt, M., Mullie, D., Technical Paper, TrexelInc. (HTTP://WWW.TREXEL.COM/TECHPP/PVCFM300.HTML)

81 Holl, M.R., Kumar, V. Ma, M., SPE, ANTEC 96, (1996), p. 1908

82 WWW.TREXEL.COM

83 Matuana, L.M., Balatinecz, J.J., Park, C.B., Polym. Eng. Sci., (1998), 38(5), p. 765

84 Matuana, L.M., Woodhams, R.T., Balatinecz, J.J., Park, C.B., Polym. Comp.,(1998), 19(4), p. 446

85 Matuana, L.M., Balatinecz, J.J., Park, C.B., Woodhams, R.T., Wood Fiber Sci.,(1999), 31(2), p. 116

86 Matuana, L.M., Balatinecz, J.J., Park, C.B., Sodhi, R.N.S., Wood Sci. Tech., (1999),33 (4), p.259

87 Riedl, B., Matuana, L.M., in Encyclopedia of Surface and Colloid Science, A.Hubbard (Ed.), Marcel Dekker, N.Y, (2002), p. 2842

88 Mengeloglu, F., Matuana, L.M., J. Vinyl Additive Tech., (2001), 7(3), p. 142

89 Rizvi, G.M., Pop-Iliev, R., Park, C.B., J. Cellular Plast., (2002), 38(5), p. 367

90 Rizvi, G.M., Park, C.B., Lin, W.S., Guo, G., Pop-Iliev, R., Polym. Eng. Sci.,accepted, July 2002

91 Guo, G., Rizvi, G.M., Park, C.B., Lin, W.S., Foams 2002, (2002), p. 153

92 Lin, H.R., Polym. Testing, (1997), 16, p. 429

93 Gibson, L.J., Ashby, M.F., Cellular Solids: Structure and Properties, (1988)Pergamon Press, New York

94 Nordstand, T., Carlsson, L., Allen, H.G., Composite Structure, (1994), 27, 317

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95 Li, G.Q., 28th Int. SAMPE Tech. Conf., (1996), p.281, Seattle, WA, USA

96 Burman, M., Zenkert, D., Int. J. Fatigue (1997), 19, p. 551

97 Gdoutos, E.E., Daniel, I.M., Wang, K-A, 15th Am. Soc. Comp. Tech. Conf., (2000),CD ROM, College Station, TX, USA

98 Tsai, S. W., Wu, E. M., J. Comp. Mat., (1971), 5, p. 58

99 Bledzki, A.K., Gassan, J., Kurek, K., J. Cellular Plast., (1996), 32(3), p. 224

100 Thomason, W. (Lord Kelvin), Phil. Mag., (1887), 24(151), p. 503

101 Grenestedt, J., Mat. Res. Soc. Symp. Proc., (1998), 521, p. 3

102 Abot, J. L., Daniel, I. M., The American Society for Composites, 16th Tech. Conf.,(2001), p. 66

103 Li, X.M., Carlsson, L. A., J. Sandwich Structures and Materials (1999), 1, p. 60

104 Shih, W. K., Jang, B. Z., J. Reinforced Plast. Comp., (1989), 8, p. 270

105 American Standards of Tests and Materials, WWW.ASTM.ORG, (2002)

106 International Standard Organization Institute, WWW.ISO.ORG, (2002)

346 9 PVC Foams

10 Epoxy Foams

DR. LINDA A. DOMEIER

10.1 Introduction

Epoxy foams, while known for decades, remain specialty products within the thermosetfoam family and are used in much lower volumes than polyurethane foams and otherlower cost alternatives. Inert or reactive blowing agents must be added to the epoxy resinto provide gas for the blowing process, although extractable void formers and mechanicalfrothing can also be used. Epoxy foams are generally rigid and are often used whengreater heat resistance, solvent resistance, adhesion or a more closely controlled foamingaction are required than is available with polyurethane foams. Epoxy foams also avoidthe potential health issues associated with isocyanate sensitization in the processing ofurethanes, a factor cited in many cases as a reason for the use of epoxies. Anadvantageous feature of epoxy foams is the wide variety of epoxy resins and curingagents which are available and which can be used to tailor the final product performance.

It is important to distinguish between syntactic epoxy foams and cellular epoxy foams.Syntactic epoxies are the most common type of epoxy foam and utilize hollowmicrospheres of glass, carbon, thermoplastic or other materials to reduce the resindensity. Any matrix resin can be used as the binder in syntactic systems, but epoxies are acommon choice. These widely used materials are discussed in Chapter 17 on syntacticfoams. As is noted below, some epoxy foams have combined the features of bothsyntactic and cellular foams in the same material and certain syntactic fillers can even beused as blowing agents. The foams discussed in this chapter, however, are primarily theblown or cellular epoxies.

Commercial applications of epoxy foams have included the reinforcement of automotivepanels, electronics encapsulation, retrofitting damaged buildings, fiber reinforcedcomposite structures and others discussed below.

10.2 Epoxy Chemistry and Formulations

A variety of epoxy resins and curing agents are available to the epoxy formulator andpermit a range of heat resistance, toughness and other properties in the cured material.The chemistry of this extremely versatile class of materials is thoroughly discussed inmany references including recent [1] and more classic [2] books as well as chapters inmany polymer texts.

Uncured epoxy formulations typically include one or more epoxy resins, one or morecuring agents and modifiers such as fillers and cure accelerators. In the case of epoxyfoams, the formulations would also generally include a blowing agent and surfactants tocontrol the cell morphology. During cure, either at room or elevated temperature, a ring