Effect of processing parameters on the cellular morphology and mechanical properties of...

Effect of Processing Parameterson the Cellular Morphology andMechanical Properties ofThermoplastic Polyolefin (TPO)Microcellular Foams

STEVEN WONG, HANI E. NAGUIB, CHUL B. PARKDepartment of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road,Toronto, Ontario, M5S 3G8, Canada

Received: April 25, 2008

ABSTRACT: In this study, the effects of processing parameters on the cellularmorphologies and mechanical properties of thermoplastic polyolefin (TPO)microcellular foams are investigated. Microcellular closed cell TPO foams wereprepared using a two-stage batch process method. The microstructure of thesefoamed samples was controlled by carefully altering the processing parameterssuch as saturation pressure, foaming temperature, and foaming time. Foammorphologies were characterized in terms of the cell density, foam density, andaverage cell size. Elastic modulus, tensile strength, and elongation at break of thefoamed TPO samples were measured for different cell morphologies. The findingsshow that the mechanical properties are significantly affected by the foamingparameters that varied with the cell morphologies. The experimental results canbe used to predict the microstructure and mechanical properties of microcellularpolymeric TPO foams prepared with different processing parameters. C© 2008Wiley Periodicals, Inc. Adv Polym Techn 26: 232–246, 2007; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20104

Correspondence to: Hani E. Naguib; e-mail: [email protected] grant sponsor: AUTO21 Network of Centres of

Excellence.Contract grant sponsor: Government of Ontario.

Advances in Polymer Technology, Vol. 26, No. 4, 232–246 (2007)C© 2008 Wiley Periodicals, Inc.

EFFECT OF PROCESSING PARAMETERS ON DIFFERENT PROPERTIES OF TPO FOAMS

KEY WORDS: Batch-foaming process, Mechanical properties, Microcellularfoam, Tensile testing, Thermoplastic polyolefin (TPO)

Introduction

M icrocellular foams are defined as foamshaving average cell sizes in the range 1–

10 µm and cell densities from 109 cells/cm3 to1015 cells/cm3. 1−3 Cells with such structures werefirst proposed by Suh et al. in the early 1980s.4

These microcellular foams were created to reducematerial usage and lower costs in certain polymerproducts in response to industrial needs.3 Typically,microcellular foams are known to exhibit high im-pact strength,2,3,5 high toughness,6 high stiffness-to-weight ratio,2,3 high fatigue life,7 high thermalstability,8 low thermal conductivity,3 and reducedmaterial weight and cost. Therefore, microcellularfoams have great potential for applications in suchindustries as automotive, aircraft, structural compo-nents, and packaging.

Microcellular foams have a large number of cells(bubbles) that have a size smaller than the criticalflaws that preexist in the polymer. According to theGriffith crack hypothesis,9 the stress concentrationaround the bubbles decreases as the cell size de-creases. Thus, there will not be a notable decrease inmaterial strength as the cell size in the foamed mate-rial becomes very small. Therefore, use of a microcel-lular foaming procedure should reduce the materialweight and costs without major compromise in me-chanical properties.

The microcellular foaming process used in thisstudy was performed as a two-stage batch process.In the first stage, polymer samples are saturated in ahigh-pressure chamber at room temperature with anonreactive gas such as nitrogen or carbon dioxide.In the second stage, the pressure is released and thesamples are quickly heated to a temperature highenough to soften the polymer. The thermodynamicinstability in the gas-saturated polymer causes cellnucleation and growth and results in foams with cellsizes in the order of micrometers.2−4,6,10,11

Polypropylene (PP) is widely used in many engi-neering applications. It has excellent physical andmechanical properties, as well as good recyclingability and is inexpensive. However, PP is brittle andhas poor mechanical properties on impact and at low

temperatures.12 Therefore, polymer alloys have beenused to enhance these mechanical properties. For in-stance, the addition of polyethylene terephthalate(PET) to PP could enhance the stiffness at highertemperatures13 and the addition of rubber such asethylene–propylene rubber (EPR) or Engage™ couldimprovement impact properties and scratch resis-tance of TPO materials. This rubber–toughened PPis called thermoplastic polyolefin (TPO).14,15

The automotive industry has been increasinglyusing TPO alloys in exterior and interior applica-tions, such as bumper fascias, claddings, and wireinsulation, because of its excellent weatherability,low density, and inexpensive.12,14,16 However, TPOfoams have not been utilized because the technolo-gies for foaming TPO materials using batch- orinjection-foaming processes have not yet been devel-oped. There is nonetheless a significant amount of in-terest in developing foamed TPO products since re-placing solid TPO with a foamed counterpart couldsignificantly reduce material costs and fuel con-sumption.

The mechanical properties of microcellularfoams processed with the two-stage batchmethod are rarely reported. Tensile behav-iors of microcellular foamed polymer suchas polycarbonate,17 polyvinyl chloride (PVC),3,18

polymethyl-methacrylate (PMMA),2 polysulfone,polyethersulfone, and polyphenylsulfone19 were in-vestigated in earlier work. Recently, the tensile be-havior of various unfoamed TPO composites werealso investigated by Lee et al.,20 Mehta et al.14 Mishraet al.,12 and Liu and Kontopouloua.21 However, therelationship of cell morphologies and tensile me-chanical properties of microcellular TPO foams withspecific compositions of PP and Engage have notbeing studied before.

The purpose of this study was to investigatethe relationships of the processing, structure, andproperties for microcellular TPO foams. Microcel-lular closed-cell TPO foams were processed usingthe aforementioned two-stage batch approach. Ni-trogen was used as the physical blowing agentto process the microcellular TPO foams. By care-fully choosing the processing parameters, differentcell morphologies of microcellular foams could be

Advances in Polymer Technology DOI 10.1002/adv 233


successfully controlled. Tensile tests were conductedto investigate the relationships between the foam-ing parameters, foam morphologies, and the me-chanical properties. The microcellular TPO foamswere prepared with different processing parame-ters (including saturation pressure, foaming temper-ature, and foaming time) to examine their effectson the mechanical properties of the foam structure.Elastic modulus, tensile strength, and elongation atbreak of the foamed TPO samples were character-ized as a function of foaming parameters. The re-sults of this study should allow designers to pre-dict the microstructure and mechanical propertiesof microcellular TPO foams prepared from differ-ent sets of processing parameters for their specificapplications.

Experimental

MATERIALS

The TPO materials were composed of different ra-tios of PP and Engage 8130. PP7805 (80 dg/min MFI)and Engage 8130 (13 dg/min MFI) were supplied byExxon (Calgary, Canada) and DuPont (Mississauga,

Heated press

Temperature controller

Foamed sample

2) Foaming stage

N2 gas

Syringe pump

Pressure chamber

1) Saturation stage

Sample

) )

FIGURE 1. The schematics of batch-foaming process.

Canada) Companies, respectively. The PP and En-gage were blended by a twin-screw extruder to thedesired combinations. The number suffix of the TPOindicates the percentage of PP in the resultant TPOcomposite. For instance, the TPO70 examined in thisstudy consisted of 70 wt% of PP. Three different TPOcomposites (TPO90, TPO70, and TPO50) were testedin this study. The TPO materials were hot compres-sion molded into 3-mm thick panels by using a hy-draulic heated press (Carver Inc., Wabash, IN) for5 min under 5 tons of pressure at 185◦C. The fi-nal TPO specimens were obtained by punching anASTM (D638-03, type V) die on the TPO panel. Thephysical blowing agent used was nitrogen suppliedby BOC Inc. (Mississauga, Canada).

PREPARATION OF MICROCELLULARFOAM

Microcellular foaming experiments were per-formed in a two-stage batch process. A schematicof the batch-foaming process is shown in Fig. 1. Inthe first stage, the TPO samples were saturated inthe pressure chamber with N2 gas under room tem-perature. A syringe pump (ISCO model 206D) wasused to increase the gas cylinder pressure to the de-sired saturation pressures of 13.8 MPa (2000 psi) and

234 Advances in Polymer Technology DOI 10.1002/adv


27.6 MPa (4000 psi). The saturation times for TPOmaterials were estimated by N2 gas uptake data un-der both saturation pressures. The gas uptake datawere obtained by periodically measuring the weightof the samples during the saturation process. Thesegas uptake data were mainly used for the purposeof estimating the time required to saturate the TPOsamples. By observing the gas uptake amount, theweight of the TPO samples became constant after60 h of saturation process. As a result, we concludedthat the time require to reach full saturation is ap-proximately 3 days. After that, the foaming step wascarried out by removing the gas-saturated samplesfrom the pressure chamber and putting it in a heatedhydraulic press in which the platens are heated to thedesired foaming temperatures for specified foamingtimes without using a mold. Previous study had alsoused the heated hydraulic press to foam microcellu-lar polymeric materials.22 To allow free expansionin the heated press, the top surface of the sampleremained free of contact from heated platen. As aresult, the combination of pressure and temperaturechanges resulted in the rapid solubility drop in thesamples, inducing cell nucleation and cell growth.All specimens were foamed less than 5 min after be-ing taken out of the pressure chamber. Afterward,the samples were quenched in cold water to fix themicrostructure of the resultant foams.

SAMPLE CHARACTERIZATION

The TPO foam morphology was characterized byutilizing a scanning electron microscope (SEM, JOELJSM-6060). The samples were cooled in liquid nitro-gen and fractured to produce a clean and intact sur-face with minimum plastic deformation. They werethen gold coated by using a sputter coater for en-hanced conductivity. The average cell size and celldensity were analyzed by utilizing the ImageJ soft-ware from National Institutes of Health, Bethesda,MD. The cell density was calculated as the number ofcells per unit volume with respect to the unfoamedpolymer. The cell density was calculated from Im-ageJ software by using the following equation:

N =( n

A

)3/2× ρP

ρ f(1)

where n is the number of cells in the defined areaA, and ρp and ρ f are the density of the unfoamedand foamed polymer, respectively. The foam densitywas measured by the buoyancy method. The relativefoam density is defined as the ratio of the foamed

density to the unfoamed polymer density. The vol-ume expansion ratio is defined as the ratio of theunfoamed polymer density to the foamed density.

TESTS OF MECHANICAL PROPERTIES

The tensile mechanical properties of microcellularTPO foams were tested, following the ASTM D638procedure on an Instron 4465 mechanical testing ma-chine at room temperature. TPO foam samples usedfor tensile testing had a thickness ranging from 3mm to 4 mm, depending on the expansion ratio ofthe sample under different foaming conditions. Thedisplacement rate of the crosshead was 10 mm permin.23 Tensile strength was calculated as the loadforce divided by the initial cross-sectional area of thespecimen. Strain was calculated as the elongationdivided by the initial length of a specimen. Threetensile properties were determined from the resul-tant stress–strain curves for each foaming condition,namely modulus of elasticity, stress at break, andstrain at break. The modulus of elasticity was ob-tained by calculating the slope of the stress–straincurves in the elastic region. The elongation at breakof a sample was calculated in terms of percent elon-gation. A minimum of five specimens was tested foreach set of foaming conditions. The average valuefor each foaming condition is reported in this paper.

Results and Discussion

Closed cell TPO foams with average cell sizesin the microcellular level were obtained. The ef-fect of processing conditions on the foam morpholo-gies was investigated. The foam morphologies werecharacterized in terms of their cell density, foam den-sity, and average cell size. A detailed discussion isprovided on the effect of each processing condition:saturation pressure, foaming time, and foaming tem-perature, as well as the effect of TPO composition.The elastic modulus, tensile strength, and elongationat break are presented for different cell morphologiesof the microcellular TPO foams.

EFFECT OF SATURATION PRESSURE

To study the effect of saturation pressure on thecell morphologies, the other two processing param-eters were fixed. The foaming temperature was se-lected to be 150◦C. Figure 2 shows that the cell den-sity increased as the saturation pressure increased



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FIGURE 2. Effect of saturation pressure and foamingtime on the cell density of microcellular foamed: (a)TPO90, (b) TPO70, and (c) TPO50. Saturationconditions: 2000 and 4000 psi. Foaming temperature:150◦C.

from 2000 psi to 4000 psi. As the saturation pressureincreased, the solubility of N2 in the TPO matrix alsoincreased under room temperature. This resulted inan overall increase of N2 gas content in the saturatedgas–polymer mixture. The increased amount of gasin turn increased the number of nucleation sites andtherefore the cell density was increased as well.2,19

Thus, the increased cell density proved that the solu-bility of the gas under various pressures was a major

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FIGURE 3. Effect of saturation pressure and foamingtime on the average cells size of microcellular foamed: (a)TPO90, (b) TPO70, and (c) TPO50. Saturationconditions: 2000 and 4000 psi. Foaming temperature:150◦C.

factor in controlling the foam nucleation. The high-est saturation pressure (4000 psi) was selected forthe other experiments since the cell density was thehighest. The effect of saturation pressure on the av-erage cell size is shown in Fig. 3. It can be observedthat the average cell size decreased as the saturation



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FIGURE 4. Effect of saturation pressure and foamingtime on the relative density of microcellular foamed: (a)TPO90, (b) TPO70, and (c) TPO50. Saturationconditions: 2000 and 4000 psi. Foaming temperature:150◦C.

pressure increased. Since the number of nucleationsites increased as the pressure increase, the increasedcell density resulted in the decrease of average cellsize. Previous studies reported that the averagecell size decreased with increasing saturation pres-sure for microcellular PMMA,24 polystyrene,25 andPMMA–polystyrene composite26 foams. Figure 4

shows that the relative foam density decreased as thesaturation pressure increased. It was found that asthe saturation pressure increases, the amount of N2content in the polymer matrix also increases, whichleads to a higher volume expansion, causing the den-sity of the TPO foams to decrease.

EFFECT OF FOAMING TIME

The effects of foaming time on the cell morpholo-gies of microcellular TPO foams are presented inFigs. 5–7. To study the effect of foaming time, thefoaming temperature and saturation pressure werefixed at 150◦C and 4000 psi, respectively.

As shown in Fig. 5, the average cell size increasedas the foaming time increased. This phenomenon il-lustrated that the cell nucleation stage was dominantduring the early foaming stage. According to thedynamics of foam growth,10 the rate of cell growthdecreases as the foaming time increases because theconcentration of N2 in the gas–polymer matrix de-creases. For TPO70 and TPO50, much of the cellgrowth takes place within the first 60 s of foam-ing. Beyond this time, the foam structure does notappear to change significantly. As a result, the foam-ing time is a major factor in controlling cell growth.To produce high cell density foam with a fine aver-age cell size, the foaming time should be carefullycontrolled.2 The foaming time of 30 s was selected forthe following discussion since the largest cell den-sity was obtained with this foaming time under vari-ous foaming temperatures for TPO70. Analyzing thegraphs in Fig. 7, it can be seen that the foam densityof TPO70 foams decreased as the foaming time in-creased, since the volume expansion ratio increasedas the cell size increased. Similar results were ob-served for TPO90 and TPO50.

The foaming time and its effect on the engineer-ing stress–strain curve of TPO70 can be observedfrom Fig. 10. More additional graphs illustratedin Figs. 12–14 show the effect on elastic modulus,tensile strength, and the elongation at break forTPO90, TPO70, and TPO50, respectively. The me-chanical properties such as elastic modulus and ten-sile strength decreased with foam density. These re-sults were expected, since a decrease in relative foamdensity means that the actual material per unit vol-ume decreases and the material reduce its weight.However, the elongation at break of the TPO foamsincreased as the foam density decreases at first. Oneexplanation for this phenomenon is that it is a resultof the small bubbles blunting the crack tips that in-creased the energy needed to propagate the crack.2



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FIGURE 5. Effect of foaming temperature and foamingtime on the average cell size of microcellular foamed: (a)TPO90, (b) TPO70, and (c) TPO50. Saturationconditions: 4000 psi under room temperature.

These results show that the toughness of the resul-tant TPO foams was improved by introducing micro-cellular structures with high cell density in the ma-terial. The elongation at break decreased at longerfoaming times since the N2 gas molecules woulddiffuse out of the polymer matrix under extendedfoaming time.

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FIGURE 6. Effect of foaming temperature and foamingtime on the cell density of microcellular foamed: (a)TPO90, (b) TPO70, and (c) TPO50. Saturationconditions: 4000 psi under room temperature.

EFFECT OF FOAMING TEMPERATURE

The effects of foaming temperature on the cellmorphologies of microcellular TPO70 foams are pre-sented in Figs. 5–9. To study the effects of foamingtemperature, the foaming time and saturation pres-sure were fixed at 30 s and 4000 psi, respectively.

Figures 5–7 show the effect of foaming tempera-ture on the cell density and average cell size of themicrocellular TPO foams. For TPO70, the cell density



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FIGURE 7. Effect of foaming temperature and foamingtime on the relative density of microcellular foamed: (a)TPO90, (b) TPO70, and (c) TPO50. Saturationconditions: 4000 psi under room temperature.

did not change notably when the foaming time wasfixed at 30 s. The cell density mostly remains in theorder of 1010 for both TPO90 and TPO 70, and re-mains in the order of 109 for TPO 50 over the first60 s of foaming time. The average cell size increasedas the foaming temperature increased from 130◦C to170◦C. This phenomenon arose because the mobil-ity of the polymer chains increased and the polymerviscosity decreased19; furthermore, the movement of

N2 gas was more active at high temperatures, result-ing in the promotion of cell growth.2 Similar resultscan be observed in TPO90 and TPO50 from Fig. 5 toFig. 7.

Figure 7 shows the effects of foaming temperatureand foaming time on the relative foam density of themicrocellular TPO foams. The relative foam densitydecreases as the foaming temperature is increased,reaching a minimum of about 0.75 at approximately170◦C for TPO70, a relative foam density of 0.9 at180◦C for TPO90 and a relative foam density of 0.8at 150◦C for TPO50. These results were consistentwith the increase in both the average cell size andcell density as mentioned before.

On the other hand, Fig. 11 shows the effect offoaming temperature on the engineering stress–strain curve of microcellular TPO foams. The elasticmodulus and the tensile strength decreased as thefoaming temperature increased. Similar results areobserved for TPO90 and TPO50. In general, the elon-gation at break increased as the foaming temperatureincreases. As a result, the property of elasticity wasimproved by introducing a microcellular structurein the material.

As the foaming temperature further increased,cell density, average cell size decreased. The elasticmodulus, tensile strength, and relative foam densityincreased quickly. For TPO70, the relative foam den-sity increased from the minimum of 0.75 at 170◦Cto 0.85 at 180◦C. The reason for this phenomenonis that, as the foaming temperature approaches themelting point of the polymer, the mobility of thepolymer chains increases whereas the chain stiff-ness and viscosity decrease. This means that N2 gasmolecules diffused out of the polymer matrix in-stead of supporting the desired cell nucleation andgrowth.19 As a result, the cells in the foam structurecollapse as the foaming temperature approaches themelting point of the polymer material. Similar re-sults are observed for TPO90 and TPO50 in Figs. 5–7,but this phenomenon occurred at different foamingtemperature that was depended on the melting pointof various TPO composites.

EFFECT OF TPO COMPOSITION

The effects of TPO composition on the cell mor-phologies of microcellular TPO foams are presentedin Figs. 5–7. The average cell size increased as thePP content in the TPO composite decreases. Fromthe results of the N2 uptake curves at room tempera-ture, the solubility of N2 in the TPO matrix increasedas the PP content in the TPO decreases. This resulted



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FIGURE 8. Effect of foaming temperature on the average cell size and cell density of microcellular foamed TPO70.Saturation conditions: 4000 psi. Foaming time: 30 s.

in an overall increase of N2 gas content in the satu-rated gas–polymer mixture as the PP content in theTPO composite decreases. In addition, the increasedamount of rubber phase in the TPO matrix also re-duced the stiffness of the material, which in turnincreases the average cell size in the microstructure.However, the cell density decreased as the PP con-tent in the TPO composite decreases.

FIGURE 9. SEM micrographs for various foaming temperatures of microcellular foamed TPO70. Saturation conditions:4000 psi. Foaming time: 30 s.

Analyzing the graphs in Fig. 7., the relative foamdensity of TPO foams decreased from the minimum0.9 to 0.75 as the PP content in the TPO compos-ite decreases from 90% to 70%, respectively. How-ever, the relative foam density of TPO50 increasedto the minimum of 0.8. This is due to the cell den-sity of TPO50 significantly reduced to the minimumof 109 cells/cm3, which affects the overall volume



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FIGURE 10. Effect of foaming time on the engineeringstress–strain curves of microcellular foamed: (a) TPO90,(b) TPO70, and (c) TPO50. Saturation conditions: 4000psi. Foaming Temperature: 150◦C.

expansion of the microcellular foamed TPO50. Over-all, the cell morphologies of microcellular TPOfoams vary with the TPO composition under thesame processing parameters. However, the effects ofprocessing parameters on the cell morphologies ofthe different TPO composites share a similar trend.

The effects of TPO composition on the elasticmodulus, tensile strength, and the elongation at

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FIGURE 11. Effect of foaming temperature on theengineering stress–strain curves of microcellular foamed:(a) TPO90, (b) TPO70, and (c) TPO50. SaturationConditions: 4000 psi. Foaming time: 30 s.

break of TPO foams can be observed from Figs. 12–14. The mechanical properties such as elastic modu-lus and tensile strength decreased, as the PP contentin the TPO composite deceases. By comparing theunfoamed TPO composites, the elastic modulus de-creased from 400 MPa (TPO90) to approximately 80MPa (TPO50). On the other hand, the tensile strengthdecreases from 20 MPa (TPO90) to approximately



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(c)

FIGURE 12. Effect of foaming temperature and foamingtime on the tensile modulus of microcellular foamed: (a)TPO90, (b) TPO70, and (c) TPO50. Saturationconditions: 4000 psi under room temperature.

6 MPa (TPO50). These results were expected sincethe PP acts as a plastic segment, which contributesmost of the tensile mechanical strength of the TPOcomposites.

From Fig. 14, the elongation at break, however,increased as the PP content in the TPO compositedecreases. These results shown that the elastomericsegment contributes the elastomeric properties. Theresults have shown that the TPO composite couldimprove the mechanical properties such as impactproperties and scratch resistance.

0

5

10

15

20

25

0 10 20 30 40 50 60 70Foaming time (s)

Ten

sile

str

engt

h (M

Pa

)


(a)

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70Foaming time (s)

Ten

sile

str

engt

h (M

Pa

)


(b)

0

2

4

6

8

10

0 10 20 30 40 50 60 70Foaming time (s)

Ten

sile

str

engt

h (M

Pa)


(c)

FIGURE 13. Effect of foaming temperature and foamingtime on the tensile strength of microcellular foamed: (a)TPO90, (b) TPO70, and (c) TPO50. Saturationconditions: 4000 psi under room temperature.

EFFECT OF RELATIVE FOAM DENSITY

Microcellular foam material was used to mini-mize material consumption for various engineeringapplications. The relative foam density of the micro-cellular foam describes the amount of material thathas been consumed compared to the unfoamed ma-terial. Thus, a desired relative foam density of themicrocellular foamed TPO can be obtained from thefoaming process by altering specific parameters. As



0

10

20

30

40

50

0 10 20 30 40 50 60 70Foaming time (s)

% E

long

atio

n


(a)

0

10

20

30

40

50

0 10 20 30 40 50 60 70Foaming time (s)

% E

long

atio

n


(b)

0

20

40

60

80

100

0 10 20 30 40 50 60 70

Foaming time (s)

% E

long

atio

n


(c)

FIGURE 14. Effect of foaming temperature and foamingtime on the percent elongation of microcellular foamed:(a) TPO90, (b) TPO70, and (c) TPO50. Saturationconditions: 4000 psi under room temperature.

seen in Fig. 7, by drawing a horizontal line parallelto the x-axis at the desired relative foam density, thedesired relative foam density of microcellular TPOfoam can be produced by various combinations offoaming time and foaming temperature. Similarly,various relative foam densities of microcellular TPOfoams can be achieved by drawing a vertical lineparallel to the y-axis at the desired foaming timeon Figs. 5c, 6c, and 7c. In this manner, a set of con-stant relative foam densities of microcellular TPO

0

50

100

150

200

250

300

350

400

450

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Ten

sile

mod

ulus

(M

Pa)

(a)

0

50

100

150

200

250

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Ten

sile

mod

ulus

(M

Pa )

(b)

0

20

40

60

80

100

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Ten

sile

mod

ulus

(M

Pa)

(c)

FIGURE 15. Effect of relative density on the tensilemodulus of microcellular foamed: (a) TPO90, (b) TPO70,and (c) TPO50.

foams can be achieved by altering the processingparameters.

Figure 8 also shows that a desired average celldensity of the microcellular foamed TPO70 can beobtained by altering the foaming parameters sincethe cell densities were not notably affected by var-ious foaming temperatures when the foaming timewas fixed at 30 s. Figure 9 presented the SEM micro-graphs of the TPO70 foams in this experiment.

The graphs in Figs. 15–17 show the effect of rela-tive foam density of the microcellular TPO foams on



0

5

10

15

20

25

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Ten

sile

str

engt

h (M

Pa

)

(a)

0

2

4

6

8

10

12

14

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Ten

sile

str

engt

h (M

Pa

)

(b)

0

1

2

3

4

5

6

7

8

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Ten

sile

str

engt

h (M

Pa)

(c)

FIGURE 16. Effect of relative density on the tensilestrength of microcellular foamed: (a) TPO90, (b) TPO70,and (c) TPO50.

tensile modulus, tensile strength, and elongation atbreak. Figures 18 and 19 also show the effect on rel-ative tensile modulus and relative tensile strength,respectively. It was observed that the tensile modu-lus and tensile strength decreased nonlinearly as therelative foam density decreased. The tensile modu-lus and tensile strength values decreased approxi-mately by half for TPO90, TPO70, and TPO50, as therelative foam density decreased from 1 (unfoamed)to the minimum.

0

20

40

60

80

100

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

% S

trai

n

(a)

0

20

40

60

80

100

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

% S

trai

n

(b)

0

20

40

60

80

100

0.70 0.75 0.80 0.85 0.90 0.95 1.00 Relative density

% S

trai

n

(c)

FIGURE 17. Effect of relative density on the elongationat break of microcellular foamed: (a) TPO90, (b) TPO70,and (c) TPO50.

For TPO70, comparing the unfoamed and foamedmaterials, the tensile strength decreased approxi-mately 25% as the relative foam density decreased25% from 1.0 to 0.75 as shown in Fig. 16. Althoughthis observation shows a one-to-one reduction ratioin tensile strength and relative foam density, once thematerial is foamed, the tensile strength decreasedonly approximately 13% as the relative foam den-sity decreased 22% from 0.96 to 0.75. The rate ofdecrease in tensile strength reduced significantly af-ter the material was foamed. Similar results were



0.0

0.2

0.4

0.6

0.8

1.0

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Rel

ativ

e te

nsile

mod

ulus

(a)

0.0

0.2

0.4

0.6

0.8

1.0

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Rel

ativ

e te

nsile

mod

ulus

(b)

(c)

0.0

0.2

0.4

0.6

0.8

1.0

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Rel

ativ

e te

nsile

mod

ulus

FIGURE 18. Effect of relative density on relative tensilemodulus of microcellular foamed: (a) TPO90, (b) TPO70,and (c) TPO50.

observed for TPO90 and TPO50. These results showthat the weight reduction of TPO materials can beachieved without sacrificing much on the mechan-ical properties by introducing microcellular struc-ture in TPO materials. Furthermore, the elongationat break increased linearly as the relative foam den-sity decreased. The elongation at break of TPO70increased from 27% to 45% as the relative foam den-sity decreased from 1 to 0.75. Similar results wereobserved in TPO90 and TPO50 in Fig. 17. These re-

0.0

0.2

0.4

0.6

0.8

1.0

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Rel

ativ

e te

nsile

str

engt

h

(a)

0.0

0.2

0.4

0.6

0.8

1.0

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Rel

ativ

e te

nsile

str

engt

h

(b)

0.0

0.2

0.4

0.6

0.8

1.0

0.70 0.75 0.80 0.85 0.90 0.95 1.00

Relative density

Rel

ativ

e te

nsile

str

engt

h

(c)

FIGURE 19. Effect of relative density on relative tensilestrength of microcellular foamed: (a) TPO90, (b) TPO70,and (c) TPO50.

sults show that the elasticity was improved by in-troducing a microcellular foam structure in the TPOmaterials.

Overall, the results of this study show that a de-sired relative foam density and cell density withassociated mechanical properties for three differentTPO composites can be obtained by carefully alter-ing the processing parameters such as saturationpressure, foaming temperature, and foaming timeaccording to the structure–property relationships.



Conclusion

In this study, a two-stage batch process was usedto prepare closed cell microcellular TPO foams.The effects of processing parameters on the cellu-lar morphologies of microcellular TPO foams wereinvestigated. The experimental results show thatthe average cell size increased with foaming timeand foaming temperature; however, it decreased asthe saturation pressure increased. The cell densityincreased with increasing saturation pressure andfoaming temperature, but decreased with increas-ing foaming time. The relative foam density of themicrocellular TPO70 foams was inversely propor-tional to saturation pressure, foaming temperature,and foaming time. A minimum relative foam den-sity of 0.75 was obtained under a saturation pressureof 4000 psi for TPO70. Higher saturation pressuresare necessary to further decrease the relative foamdensity of the microcellular TPO foams.

Mechanical properties of the microcellular TPOfoams are directly related to the processing param-eters. The results show that the tensile strength ofTPO70 decreased only approximately 13% as the rel-ative foam density decreased 22%. Similar resultswere observed in other two TPO composites. There-fore, the weight reduction of TPO material can beachieved without sacrificing much of the mechan-ical properties by introducing microcellular struc-ture in TPO materials. Under the same processingparameters, the cell morphologies and mechanicalproperties of TPO foams vary with the TPO compo-sition. However, the results show that the effects ofprocessing parameters on the cell morphologies andmechanical properties of the different TPO compos-ites share a similar trend.

In summary, the microstructure of TPO foamscan be controlled effectively by carefully altering thefoaming parameters. Lightweight TPO material for

various applications can be achieved with variousfoam densities and cell densities, producing diversemechanical properties of microcellular TPO foams.

References

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92, 928.15. Lu, J.; Wei, G.; Sue, H. J Appl Polym Sci 2000, 76, 311.16. Shah, S.; Kakarala, N. ANTEC 2000, 1095.17. Kumar, V.; Weller, J. E. ANTEC 1991, 1401.18. Kumar, V.; Weller, J. E.; Montecillo, R. J Appl Polym Sci 1992,

86, 1692.19. Lee, H.; Fasulo, P. D.; Rodgers, W. R. Polymer 2005, 46, 11673.20. Liu, Y.; Kontopouloua, M. ANTEC 2006, 248.21. Kumar, V.; Nadella, K.; Li, W. ANTEC 2003, 1722.22. ASTM International D638-03 2005.23. Goel, S. K.; Beckman, E. Cell Polym 1993, 12, 251.24. Wang, J.; Cheng, X.; Zheng, X. J Polym Sci 2003, 41, 368.25. Han, X.; Shen, J.; Wingert, M. ANTEC 2006, 2695.


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