microcellular pp vs. microcellular pp/mmt nanocomposites: a comparative study of their mechanical...

7/31/2019 Microcellular PP vs. Microcellular PP/MMT Nanocomposites: A Comparative Study of their Mechanical Behavior

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2/8IPP_ipp-2375 27.5.11/stm media kthen

S. J. A. Rizvi, N. Bhatnagar: Microcellular PP vs. Microcellular PP/MMT Nanocomposites

In this paper, the effect of microcellular foaming on me-chanical properties of neat polypropylene and PP/MMT (2 %by wt.) nanocomposites was studied.

2 Experimental

2.1 Materials

Polypropylene (REPOL H110MA, Reliance Industries Ltd.,[6]) having a MFI of 11g/10 min, as per ASTM D-1238 stan-dard is used as matrix material. Maleic anhydride grafted poly-propylene (MA-g-PP) is used as the compatibilizer. The MFIofMA-g-PP is 52.5 g/10 min. Montmorillonite nanoclay (Clo-site 15A) was obtained from Southern Clay Products Inc.,USA. This was used as the nano-filler for PP-nanocompositepreparation. As informed by the manufacturing company, Clo-site15A was organically modified with dimethyl dyhyroge-nated to allow quaternary ammonium ion, and the tallow com-position is about 65 % C18, 30 % C16 and 5% C14. Thecation exchange capacity (CEC) of Closite 15A is 125 mEq/100 g; the corresponding organic modifier content is about

40 wt.%.

2.2 Preparation of Test Specimens

For comparative study ofthe effect of nanoclay and microcel-lular foaming on the mechanical properties of polypropylene(PP), the following sets ofspecimens were prepared;1. Neat solid polypropylene.2. Microcellular foamed polypropylene.3. Solid Polypropylene/MMT nanocomposite (2 % by wt.).4. Microcellular foamed polypropylene/ MMT nanocompos-

ite.

2.2.1 Compounding of PP/MMT Nanocomposites

The melt intercalation offers a simple way of preparing nano-composites. However, care has to be taken to fine tune thelayered silicates surface chemistry in order to increase the sili-

cate compatibility with the polymer matrix. Many studies haveshown that the polar interactions of polymer and clay surfaceplay a critical role in achieving particle dispersion. For non-po-lar polymers, e.g., PP, a polar compatibilizer such as maleicanhydride modified PP (PP-g-MA) is commonly added to im-prove the compatibility ofPP and clay and thus the clay nano-particle dispersion. All reported studies on PP nanocompositefoams were synthesized in this manner. Processing conditions

such as shear rate and mixing have profound effects on thestructure evolution of polymer nanocomposites by melt inter-calation and these effects are still not well understood.

In present study PP/nanocomposites were prepared via meltblending route in a twin-screw extruder from Thermo Fischer.A co-rotating twin-screw extruder with the characteristics;screw diameter 16 mm, barrel length 1.08 m, L/D 40, numberof heating cylinders 6 and maximum screw speed 1 000 min 1.

Pellets ofPP and MA-g-PP were tumble mixed with the nano-clay (Closites15A) and simultaneously introduced into thehopper. Screw speed of300 min 1 was kept constant for man-ufacture o f nanocomposite material. The concentration ofMMT nanoclay in nanocomposite was maintained at approxi-mately 2.0 wt.% level. Ratio between the nanoclay and MA-

g-PP was 1 : 1.The extrudate was cut into pellets and then ovendried before being injection molding.

2.2.2 Injection Molding of Test Specimens

Injection molding oftest specimens were carried out on micro-cellular injection molding machine manufactured by Batten-feld Austria (model HM 40/ 210), see Fig. 1. Temperature pro-file in the barrel was maintained at 40, 165, 190, 220, and2408C from hopper to the nozzle end. The actual melt tempera-ture was 2158C. A cycle time of60 s including cooling time of45 s was common for all molding operation.

This machine is capable of producing parts with microcellu-lar foamed structure as well as solid parts. A family mold wasused for preparation of tensile, flexural and impact test speci-men as per ASTM D638-03 (Type-I), ASTM D790 and D256standards. Physical blowing agents N2 gas (above its criticalpoint) was used in microcellular injection molding process.Super critical N2 is termed as super critical fluid (SCF), which

2 Intern. Polymer Processing XXVI (2011) 4

Fig. 1. Microcellular injection molding ma-chine (A), shut-off nozzle (B) and ASTM testspecimen mold (C)A)

B)

C)


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The X-ray diffraction (XRD) was carried for the solid andfoamed specimen of PP/MMT nanocomposite. The diffracto-grams (I vs. 2h) for solid and foamed specimen of PP/MMTnanocomposite are shown in Fig. 5. The very small peak at2.73nm in solid PP/MMT nanocomposite followed by peakat 1.34 nm justifies the good dispersion of nanoclay in compa-tibilized polypropylene matrix. In case of foamed PP/MMTnanocomposite, absence of peak around 2.7 nm indicates bet-ter dispersion on nanoclay. Further the shift of peak to left(at 1.35 nm) in foamed PP/MMT nanocomposite, confirms

the increase in d-spacing. During the melt processing (micro-cellular injection molding), the reduced viscosity of PP ma-trix, caused by plasticization effect of super critical nitrogen,helps the dispersion of nanoclay and leads to exfoliated nano-structure.

3.2 Tensile Properties

The tensile tests were carried out at room temperature. Instron5582 testing machine equipped with a 100 kN load cell wasused. Tensile testing was conducted according to ASTMD638-03 (Type-I) with a crosshead speed of 5 mm/min for all

the samples. Five samples were tested for each type ofsample.Foamed samples of polypropylene and PP/nanoclay werestored for at least 10 to 15 days before testing so that dissolvednitrogen gas should escape from polymer matrix. This step isimportant because tensile testing with freshly molded micro-cellular specimen may lead to error in computation of % elon-gation at breakand toughness because of internal plasticizationeffect of physical blowing agent N2. This step is also valid fortesting of flexural and impact properties. Fig. 6 shows the ten-sile plot for PP and PP/nanoclay in foamed and solid condi-tions.

3.2.1 Tensile Modulus

Polymer nanocomposite foams exhibit substantially improved

propertiesc

ompa

red to their nea

t polymer foa

mc

ounterpa

rts.The tensile modulus ofPP/clay nanocomposite foams has been

measured and compared to neat PP foam. As shown in Fig. 8,

the nanocomposite foams exhibit a much higher modulus as com-

pared to neat PP foam and moderately higher than solid neat PP.

A high tensile modulus means that the material is rigid morestress is required to produce a given amount ofstrain. In poly-mers, the tensile modulus and compressive modulus can be ina close or wide range. This variation may be 50 % or more, de-pending on resin type, reinforcing agents, and processing meth-ods. The tensile and compressive moduli are often very closefor metals. Fig. 7 shows the slope of load extension plots forPP and PP/nanoclay in foamed and solid conditions. Highestmodulus is shown by PP/nanoclay in solid condition. Foamed

PP shows the lowest modulus.It can be seen from Fig. 8 that foaming has adverse effect on

tensile modulus. Addition of nano particles result into enhance-ment in tensile modulus however foamed PP/ nanoclay sam-ples show higher modulus than neat polypropylene by 5 %.


Fig. 4. TEMofPP/MMTnanocomposite

Fig. 5. XRD ofsolidand foamedPP/MMTnanocomposite

Fig. 6. Extension vs. load plotforPP andPP/nanoclay in foamedandsolidconditions

Fig. 7. Effectof foaming on tensile modulus


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3.2.2 Toughness

Toughness can be defined as ability of material to absorb me-chanical (or kinetic) energy up to failure. Toughness can befound by taking the area under the stress-strain curve. Fig. 9shows the stress-strain curve for PP and PP/nanoclay underfoamed and solid conditions.

Toughness was calculated and values are plotted in Fig. 10.Highest toughness is found in neat polypropylene. It can beconcluded from the Fig. 10 that microcellular foaming leadsto flexibility and hence reduction in toughness. Further it canbe noted that nano particles enhances the modulus of matrixand reduces the maximum strain as shown in Fig. 15, thereforethe area under the stress-strain curve reduces leading to re-

duced toughness value for both solid and foamed PP/MMTnanocomposite in comparison to their neat PP counterparts re-spectively.

3.2.3 Energy to Yield Point and Break Point

Energy required for deformation of tensile specimen within

elastic limit and at the rupture point is called Energy to YieldPoint and Energy to Break Point. Fig. 11 and 12 showsthese values for PP and PP/nanoclay in foamed and solid con-ditions. Energy at yield point and break point decreases withfoaming. Since foaming increases the flexibility therefore en-ergy requirement to cause rupture is low. Similar effect can beobserved for PP/nanoclay nanocomposite. But very low reduc-tion in yield energy value for foamed and solid PP/nanoclaycomposite material is observed.

The foamed samples of neat PP and PP/MMT nanocompos-ite show contradictory observation in Fig. 11 and 12. The en-ergy requirement for the deformation upto yield point, forfoamed PP/MMT is much higher (0.539 J) than that of foamedneat PP (0.371 J). But when these samples are deformed untilbreak point, the neat PP foam dissipates more energy (9.566 J)as compared to PP/MMT nanocomposite (7.97 J). But whenthis contradictory information is analyzed in conjunction withstress vs. strain curve (Fig. 9) and toughness data (Fig. 10), itcan be explained that the foamed PP/MMT nanocomposite of-fers a stiff resistance to deformation (until yield point) due toenhancement in modulus by the incorporation of nanoclay intoPP matrix. Beyond the yield point, the plastic deformation setsin and more ductile porous matrix of neat PP foam exhibits lar-ger elongation (Fig. 15) hence higher toughness (Fig. 10) thanfoamed PP/MMT nanocomposite. Addition of nanoclay in PP

IPP_ipp-2375 27.5.11/stm media kthen


Intern. Polymer Processing XXVI (2011) 4 5

Fig. 8. Tensile modulus values forPP andPP/nanoclay in foamedandsolidconditions

Fig. 9. Effectof foaming on toughness

Fig. 10. Toughness values forPP andPP/nanoclay in foamedandsol-idconditions

Fig. 11. Energy at yield point

Fig. 12. Energy atbreak point




matrix improves the tensile modulus that reflects in term of en-hanced ultimate tensile strength (Fig. 13) and strength at breakpoint (Fig. 14) for PP/MMT nanocomposites under solid andfoamed conditions. On the other hand the ductility of matrix issacrificed for improvement in stiffness due to the addition ofnanoclay in PP matrix. Since toughness is total energy requiredfor the rupture that can be quantitatively estimated as area un-der the stress/strain curve, a ductile matrix will exhibit highertoughness than a stiff matrix with lesser elongation at break

point.

3.2.4 Ultimate Tensile Strength

The maximum stress a material can withstand when subjectedto tension. It is the maximum stress on the stress strain curve.Fig. 13 and 14 show the ultimate tensile strength (MPa) andstrength at break values for PP and PP/nanoclay in foamedand solid conditions. It has been observed that ultimate tensilestrength and strength at break decreases with foaming. Similarfindings were published by Hwang et al. (2009). The respectivevalues of ultimate tensile strength and strength at break in-

creases with incorporation of nanoclay for both foamed andsolid samples.

3.2.5 Maximum Percentage Strain

The ability of material to undergo elongation can be expressedby the term maximum % strain. Higher value of max. % strainreflects the ability of material to elongate more before finalrupture or break up. Fig. 15 shows the maximum % strain forPP and PP/nanoclay in foamed and solid conditions.

3.3 Flexural Properties

The flexural strength ofa material is defined as its ability to re-sist deformation under lateral loading. For materials that de-form significantly but do not break, the load at yield, typicallymeasured at 5% deformation/strain of the outer surface, is re-ported as the flexural strength or flexural yield strength. Thetest beam is under compressive stress at the concave surfaceand tensile stress at the convex surface. The value representsthe highest stress experienced within the material at its momentofrupture. In a bending test, the highest stress is reached on thesurface of the sample.

For the measurement of flexural properties ofPP and PP/na-noclay (foamed and solid samples) ASTM D790 standard wasfollowed. Flexural testing was carried out on a three point flex-ural testing attachment supplied by Zwicks universal testingmachine. All the tests were performed at room temperature(258C maintained by air conditioner). Cross head speed was50 mm/min and the support separation was 70 mm for all flex-ural tests. Fig. 16 and 17 shows the flexural modulus and


Fig. 13. Ultimate tensile strength (MPa) values of PP and PP/nano-clay underfoamedandsolidconditions

Fig. 14. Strength atbreak (MPa) values ofPP andPP/nanoclay underfoamedandsolidconditions

Fig. 15. Maximum % Strain for PP and PP/nanoclay in foamed andsolidconditions

Fig. 16. FlexuralModulus forPP andPP/nanoclay in foamedandsol-idconditions

Fig. 17. Flexural Strength forPP andPP/nanoclay in foamedandsol-idconditions


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Lee, L. J., et al., Polymer Nanocomposite Foams, Compos. Sci.Tech., 65, 2344 2363, (2005),DOI:10.1016/j.compscitech.2005.06.016

Leszczynska, A., et al., Polymer/Montmorillonite Nanocompositeswith Improved Thermal Properties Part I Factors Influencing Ther-mal Stability and Mechanisms ofThermal Stability Improvement,Thermochim Acta, 453, 75 96 (2007),DOI:10.1016/j.tca.2006.11.002

Okamoto, M.: Macromolecular Engineering in Precise Synthesis, Ma-terials Properties, Applications, Wiley-VCH, Weinheim (2007)

Pinnavaia, T. J., Beall, G. W., Polymerclay Nanocomposites, JohnWiley and Sons, New York (2000)

Rohlmann, C. O., et al., Comparative Analysis of NanocompositesBased on Polypropylene and Different Montmorillonites, Eur.Polym.J., 44, 2749 2760, (2008),DOI:10.1016/j.eurpolymj.2008.07.006

Shyh-shin Hwang et al., Effect of organoclay on the mechanical/ ther-mal properties of microcellular injection molded polystyrene-claynanocomposites, Int. Communications in Heat and Mass Transfer,36, 799 805 (2009),DOI:10.1016/j.icheatmasstransfer.2009.06.011

Utracki, L. A., Clay-containing Polymeric Nanocomposites, Shawburi:Rapra Technology, ?New Dehli? (2004)

Waldman, F.A., The Processing of Microcellular Foam, Phd Thesis,Department of Mechanical Engineering, Massachusetts Institute ofTechnology, Cambridge, MA, (1982)

Yuan, M. J., et al., Microcellular Nanocomposite Injection MoldingProcess, SPE ANTEC Tech. Papers, 691 695 (2003)

Date received: May 12, 2010

Date accepted: February 20, 2011

BibliographyDOI 10.3139/217.2375Intern. Polymer ProcessingXXVI (2011) 4; page 18 Carl Hanser Verlag GmbH & Co. KGISSN 0930-777X

You will find the article and additional material by enter-ing the document number IPP2375 on our website atwww.polymer-process.com


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