In the Laboratory

1526 Journal of Chemical Education • Vol. 76 No. 11 November 1999 • JChemEd.chem.wisc.edu

Effect of Plasticizers on the Properties of Polystyrene Films W

Jeffrey Csernica* and Alisha BrownDepartment of Chemical Engineering, Bucknell University, Lewisburg, PA 17837; *csernica@bucknell.edu

Background

Even though polymeric materials inherently exhibit awide range of attractive properties, most commercial plasticsand rubbers are in fact mixtures of polymers with one or morechemical additives, which modify and improve final charac-teristics. Important classes of additives include UV-stabilizers,flame retardants, antistatic agents, and plasticizers (1).

Plasticizers add flexibility to rigid polymers, renderingthem less hard and more resistant to impact (i.e., less brittle).By becoming dissolved and intimately mixed with the long-chain molecules of the polymer, plasticizers disrupt the sec-ondary bonds that hold the polymer chains to one anotherand create more room for chain motion. Successful plasticizersare typically organic compounds that have a lower molecularweight than the host polymer to aid dissolution, yet exhibitlow enough volatility to prevent rapid evaporation and lossof the desired effects.

Polystyrene (Fig. 1) is a hard, rigid, clear plastic thatenjoys a wide variety of applications in items such as toys, cups,office supplies, packaging materials, and surface coatings. In

this experiment, the effectsof different amounts andtypes of plasticizers on thephysical properties of poly-styrene films (hardness, im-pact resistance, and glasstransition temperature) areinvestigated.

A drawback to conducting mechanical property tests ofplastics in classroom laboratories is that many require fabri-cation of samples with specialized geometries (e.g., tensile testspecimens). Here, by focusing on the surface-coating appli-cation, rapid techniques for preparing films on substrates andsimple hands-on test methods that are intuitively satisfyingto the students can be employed.

This laboratory (or a part of it) is appropriate in anyclass that contains an introductory treatment of polymers.Note that the experiment can be supplemented with relatedprojects, such as the synthesis and molecular weight deter-mination of polystyrene (2, 3) or the synthesis of a compoundthat can be used as a plasticizer (4).

Experimental Procedures

We used polystyrene pellets with an average molecularweight of 190,000 g/mol (Scientific Polymer Products, Inc.,Ontario, NY). Alternative sources might include samplessynthesized by students in an earlier experiment or commercialproducts such as clear cups or expanded polystyrene foam cups(composition can be verified by checking for the PS recyclingsymbol). We obtained three plasticizers, also from ScientificPolymer Products: diethyl phthalate (DEP); diethylene glycoldibenzoate (DEGD); and tri-n-butyl citrate (TNBC). Thesecompounds are liquids at ambient conditions.

Sample solutions are prepared in xylene. These includea plain polystyrene solution and mixtures containing variouslevels of plasticizers, up to 1:1 by weight (polymer to plasticizer).For attainment of relatively constant film thickness betweensamples, a consistent concentration of polystyrene in thesesolutions should be maintained. For the 190,000 g/mol pellets,25 wt % is recommended. Ten milliliters of each solution issufficient to make several replicate samples.

Films are prepared on 7.5 × 15-cm steel test panels (TheQ-Panel Co., Cleveland, OH). Panels with a “ground surface”provide good adhesion and consistent test results. With thepanel on a flat level surface such as a glass plate, ca. 3 mL ofsolution is deposited at one end and spread over the panelsurface using a “drawdown bar”. This steel bar provides aspecific clearance, which leaves behind a film of uniformthickness when drawn along the panel. We used a bar with aclearance height of 0.75 mm. Drawdown bars are availablecommercially (Paul N. Gardner Co. Inc., Pompano Beach,FL). A glass rod with several layers of tape wrapped aroundeach end makes an inexpensive and satisfactory alternativefor the purposes of this experiment.

After solvent evaporation, clear solid coatings will remainon the panels. Our samples were left in the hood overnightand then heated at 50 °C for 24 h. Forming clear plastic filmsby this method can itself be an enlightening experience forsome, especially if the starting polystyrene appeared opaque,as is the case for powder or expanded foam.

Films are tested for hardness using the pencil hardnesstechnique, which is described in detail in an ASTM standardtest protocol (5). We maintain the following range of standard2-mm drawing leads and lead holders for this experiment(listed in order of increasing hardness):

4B, 3B, 2B, B, HB, F, H, 2H, 3H

The lead is prepared by light grinding, normal to a pieceof 400 grit abrasive paper, so that a flat surface is obtained.To conduct the test the lead holder is gripped tightly and thetip is placed on the polymer film surface, pointing it towardthe tester at an angle of approximately 45°. With steadypressure, the tester then pushes down and pulls the leadtoward himself or herself in an attempt to cut the film.Enough downward force must be exerted so that either thefilm is cut down to the steel panel surface, or the leadcrumbles. The hardness of the film is reported as that of thehardest lead which cannot cut or gouge the film.

The impact resistance of the films is tested by means of adrop weight tester. In our apparatus, a coated panel, polymerfilm side up, is placed on a steel support ring with an innerdiameter of 1.6 cm. Guided by a vertical tube, a cylindricalprojectile weight (either 0.9 or 1.8 kg) with a 1.25-cm diam-eter hemispherical tip is allowed to drop onto the panel fromvarious heights. The greatest drop height we can attain is 115cm. Commercial devices of this type corresponding to anASTM test protocol (6 ) are available (Paul N. Gardner Co. Inc.,

n

Figure 1. Polystyrenerepeat unit.

In the Laboratory

JChemEd.chem.wisc.edu • Vol. 76 No. 11 November 1999 • Journal of Chemical Education 1527

Pompano Beach, FL), although homemade and alternatedesigns would be suitable for this experiment. The energy ofthe impact is taken to be the product of the projectile weightand the drop height. Films are examined after each drop tocheck for failure, indicated by visible cracking (a magnifyingglass can aid crack detection). The “impact energy” of thesample is reported as the highest energy at which the sampledoes not fail. Two sample plates usually provide area forenough drops to determine the impact energy within a heightrange of 5 cm.

To further explain the phenomenon of plasticization usinga standard analytical tool, the glass transition temperature (Tg)of these films can be studied by differential scanning calo-rimetry (DSC) (7 ). Tg is the temperature at which amorphouspolymers are considered to go from being rigid to being flexibleor rubbery. The increases in polymer chain mobility and freevolume caused by incorporation of a plasticizer typicallyresult in a Tg decrease, and plasticizer effectiveness is oftencharacterized by this measurement. For DSC study, filmsprepared on glass plates can be readily removed with a razorblade after solvent evaporation.

ResultsAll three plasticizers cause measurable reductions in film

hardness over the concentration range studied, indicatingsuccessful disruption of secondary bonding forces betweenthe polymer molecules. Test results (Fig. 2) show the effects ofdifferent plasticizer types and amounts. Note that the lowestreported value on the plot of “<4B” indicates that even oursoftest pencil cut the film. The TNBC was the most effective atdecreasing film hardness, producing a substantial reductioneven at 9 wt %, the lowest content investigated. Samples con-taining DEGD exhibited a more gradual hardness decrease withconcentration, with a drastic drop at the 50 wt % level, whileDEP showed a limited effect, even at high concentration.

Results of the impact energy tests are contained in Figure3. Note that the points listed as “20 J” did not fail at ourupper limit of measurement. Polystyrene films containing noplasticizer were quite brittle, registering an impact energyof less than 0.5 J. All three plasticizers caused measurableincreases in impact energy, indicative of increased polymerchain mobility. Literature comparison (8) can be made to

other types of impact testingapplied to plasticized polysty-rene, which demonstratesimilar reductions in brittle-ness. As in the hardness mea-surement, effects of bothplasticizer type and contentare apparent. TNBC andDEGD were able to causevery substantial changes inimpact energy, TNBC exhib-iting the most rapid increasewith concentration. In con-trast, the effect of DEP wasmore gradual and less exten-sive at the higher levels.

Figures 2 and 3 reveal a wide range of attainable mechani-cal properties and possible combinations of hardness andimpact energies. This demonstrates the importance of carefulselection and control of chemical additives in order to tailorpolymer properties to meet specific applications.

Table 1 contains measured depressions in polystyrenefilm Tg from an unplasticized value of 89 °C, obtained usingDSC at a heating rate of 15 °C/min. As expected, substantialdecreases were observed with incorporation of plasticizer, andthe extents of Tg depression with plasticizer content are con-sistent with ranges reported in the literature (9). Over thecomposition range investigated, the changes in Tg caused byDEP incorporation were more gradual than those seen forTNBC and DEGD. This behavior partially corroborates theearlier observation that DEP had the most limited effect onhardness and impact energy. However, a common relationshipfor all plasticizers between mechanical characteristics andTg is not apparent. For example, DEP does cause large Tgdepressions at high levels, with relatively small associatedproperty changes. This highlights the danger of using Tg asthe sole predictor of all effects of such additives in polymers.

NoteWSupplementary materials for this article are available on JCE

Online at http://jchemed.chem.wisc.edu/Journal/issues/1999/Nov/abs1526.html.

Figure 2. Plasticizer effect on pencil hardness of polystyrene films.

1

2

3

4

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DEGD

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Wt % plasticizer in film

Figure 3. Plasticizer effect on impact energy of polystyrene films.

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1528 Journal of Chemical Education • Vol. 76 No. 11 November 1999 • JChemEd.chem.wisc.edu

Literature Cited

1. Alper, J.; Nelson, G. L. Polymeric Materials; American ChemicalSociety: Washington, DC, 1985; pp 39–40.

2. Quaal, K. S.; Wu, C.-N. In National Educator’s Workshop: Up-date 93; NASA Conference Publication 3259; National Aero-nautics and Space Administration: Hampton, VA, 1993; pp219–237.

3. Slough, G.A. J. Chem. Educ. 1995, 72, 1031.4. Caspar, A.; Gillois, J.; Guillerm, G.; Savignac, M.; Vo-Quang, L.

J. Chem. Educ. 1986, 63, 811.

5. ASTM D-3363; American Society for Testing and Materials:Philadelphia, 1990.

6. ASTM D-2794; American Society for Testing and Materials:Philadelphia, 1990.

7. Rosen, S. L. Fundamental Principles of Polymeric Materials, 2nded.; Wiley-Interscience: New York, 1993; pp 104–106.

8. Campbell, W. ANTEC Conference Proceedings, Vol. 3; Societyof Plastics Engineers: Brookfield, CT, 1998; pp 3455–3460.

9. Immergut, E. H.; Mark, H. F. In Plasticization and Plasticizer Pro-cesses; Gould, R. F., Ed.; Advances in Chemistry 48; AmericanChemical Society: Washington, DC, 1965; pp 1–26.