Water absorption and mechanical properties of starch foam trays ...

www.ital.sp.gov.br/bjWater absorption and mechanical properties of starch foam trays impregnated with starch acetateSCHIMIDT, V. C. R. and LAURINDO, J. B.2.2 Scanning electron microscopy (SEM)Micrographs of the foam tray samples were obtainedusing a model XL-30 (Philips, Holland) scanning electronmicroscope. Before obtaining the micrographs, the foamtray samples were coated with a thin gold layer using amodel SCD 005 (BAL-TEC, Switzerland). All samples wereexamined using an accelerating voltage of 20 kV.2.3 Tray water absorptionA comparative water absorption study wasperformed using a modified procedure based on the ABNTNBR NM ISO 535 Standard (1999), which was derivedfrom the Cobb method. It consisted of a gravimetricalanalysis in which a sample (with a known dimension of25 × 50 mm) was weighed before and after immersion indistilled water for 1 min. The water adhered to the samplesurface was removed with absorbent paper.2.4 Measurement of the mechanical propertiesThe mechanical properties of the foam trays weredetermined using the tensile strength test in accordancewith the ASTM D828-97 standard test method. Before themechanical tests, the foam tray samples were conditionedfor 5 days at ambient temperature (about 25 °C) and a relativehumidity of 75% (obtained using a saturated solutionof sodium chloride). The conditioning is important becausethe water absorbed by the materials acts as a plasticizingagent, modifying the material properties (SHOGEN et al.,1998; SOYKEABKAEW et al., 2004). The model TA.XT2iTable 1. The conditions used for sample impregnation.SamplecodesStarchacetate/chloroform(g.mL –1 )Impregnationtime(min)*PressureconditionA = non -- - -impregnated sampleAP, 1:3,05 min 1:3 5 APAP, 1:3,10 min 1:3 10 APAP, 1:3,30 min 1:3 30 APVP, 1:3,10 min 1:3 10 VPAP, 1:5,05 min 1:5 5 APAP, 1:5,10 min 1:5 10 APAP, 1:5,30 min 1:5 30 APVP, 1:5,10 min 1:5 10 VPAP, 1:10,05 min 1:10 5 APAP, 1:10,10 min 1:10 10 APAP, 1:10,30 min 1:10 30 APVP, 1:10,10 min 1:10 10 VP* Pressure condition: AP = atmospheric pressure; and VP = vacuumpulses, followed by a period at atmospheric pressure.texture analyser (SMS, Surrey, UK) with the 25 N loadcell was used to determine the mechanical properties ofthe foam tray samples. The tensile strength and elongationat break were determined from the tension tests. Inthe tension tests, foam tray samples with dimensions of25 × 100 mm were fixed to the machine, and the testsperformed at 2 mm/s.The force at break and relative deformation weredetermined from the puncture tests. The puncturetests were performed using a spherical probe (21 mmdiameter), previously fixed in an annular space with adiameter of 80 mm, as sketched in Figure 1. The sphericalsteel probe was built specially for these mechanical tests,to avoid sharp edges that could influence the materialbreak. During the tests, the probe was moved throughthe sample, from top to bottom, with a speed of 1 mm/s,until 30 mm below the sample level. The load applied was0.5 N and the tests were performed with 10 repetions foreach sample. For this test, the relative deformations atbreak were represented by Equation 1.∆d= LDwhere ∆L = the probe vertical shift, from the moment ittouched the sample to the break point; D = diameter ofsample under test and d = relative deformation of thesamples at break.2.5 Water sorption isothermsThe foam tray samples were previously dried andconditioned in recipients with different relative humidities(RH = 7 to 90%), obtained using saturated salt solutionsat constant temperature (25 °C). The samples wereSphericalprobeFigure 1. Spherical probe and base used to fix the sampleduring the puncture tests.(1)Samplesecuring screwBraz. J. Food Technol., v. 12, n. 1, p. 34-42, jan./mar. 2009 36

www.ital.sp.gov.br/bjWater absorption and mechanical properties of starch foam trays impregnated with starch acetateSCHIMIDT, V. C. R. and LAURINDO, J. B.40%, compared to the non-impregnated samples. Thiscan be explained by the filling up of the porous spacesin the material by starch acetate solution and by thecoating on the material surface. Figure 8 shows the morehomogeneous starch acetate film formed at the materialsurface for VP, 1:3,10 min. Moreover, since starch acetateis hydrophobic (LAROTONDA et al., 2005), the impregnatedmaterial became less hygroscopic, contributing toa reduction in water uptake. Similar results were reportedby Larotonda et al. (2003) and Matsui et al. (2004).Matsui et al. (2004) reported data obtained withcassava bagasse-Kraft paper composites impregnatedwith starch acetate/chloroform solutions (concentrationsImpregnation conditionVP, 1:3,10 minVP, 1:5,10 minVP, 1:10,10 minAP, 1:3,30 minAP, 1:5,30 minAP, 1:10,30 minAP, 1:3,10 minAP, 1:5,10 minAP, 1:10,10 minAP, 1:3,05 minAP, 1:5,05 minAP, 1:10,05 minnon impregnated34%40%64%63%66%67%69%72%70%72%72%73%60 80 100 120 140 160 180 200 220 240Water absorption (g.cm 2 )100%Figure 4. Influence of impregnation on tray water absorption.of 1:5 and 1:10 (g.mL –1 ), for 5 and 10 min). The impregnationdecreased water absorption by the material byabout 75%. The water uptake by commercial Kraft paper(75 g.m –2 ) impregnated with starch acetate/chloroformsolution, decreased by 56.4% for the samples impregnatedusing vacuum pulses and 11.4% for the samplesimpregnated under atmospheric pressure (LAROTONDAet al., 2003).Shogren et al. (2002) studied the preparationof starch foam trays with the addition of hydrophobiccompounds to improve the water absorption resistance.Trays produced with the addition of paraffin wax,vegetable oil, silicone oil, resin, aryl acid ester, citric acid,butane-tetra-carboxylic acid, succinic acid and ethyleneglycol resin failed to reduce the water absorption. Onlymonoester citrate presented a significant decrease inwater absorption.The values obtained for tensile strength and elongationat break of the impregnated and original foamtrays are shown in Figures 5a and b. The results obtainedwith the impregnated foam trays were compared withnon-impregnated samples and with commercial EPS(expanded polystyrene) trays. Impregnation had a smallinfluence on the mechanical properties mentioned above.Similar results were reported by Matsui et al. (2004) forcomposites formed from cassava bagasse and Kraftpaper, impregnated with starch acetate/chloroform solutions(concentrations of 1:5 and 1:10 (g.mL –1 ) for 5 and10 min). These authors reported that impregnation didnot influence the tensile strength and elongation at breakof the samples. Larotonda et al. (2003) reported thatimpregnation of Kraft paper with starch acetate/chloroformsolutions (concentrations of 1:5 g.mL –1 ) increasedthe material tensile strength 1.5 times.Tensile strength (MPa)654321aElongation at break (%)2.52.01.51.00.5b0EPSnon impregnatedAP, 1:3,05 minAP, 1:3,10 minAP, 1:3,30 minAP, 1:5,05 minAP, 1:5,10 minAP, 1:5,30 minAP, 1:10,05 minAP, 1:10,10 minAP,1:10,30minVP, 1:3,10 minVP, 1:5,10 minVP, 1:10,10 min0.0EPSnon impregnatedAP, 1:3,05 minAP, 1:3,10 minAP, 1:3,30 minAP, 1:5,05 minAP, 1:5,10 minAP, 1:5,30 minAP, 1:10,05 minAP, 1:10,10 minAP,1:10,30minVP, 1:3,10 minVP, 1:5,10 minVP, 1:10,10 minFigure 5. Foam trays impregnated with a starch acetate/chloroform solution: a) Tensile strength; and b) Elongation at break.Braz. J. Food Technol., v. 12, n. 1, p. 34-42, jan./mar. 2009 38

www.ital.sp.gov.br/bjWater absorption and mechanical properties of starch foam trays impregnated with starch acetateSCHIMIDT, V. C. R. and LAURINDO, J. B.Force at break (N)1009080706050403020100EPSnon impregnatedVP, 1:3,10 minVP, 1:5,10 minVP, 1:10,10 minAP, 1:3,05 minAP, 1:5,05 minAP, 1:10,05 minAP, 1:3,10 minAP, 1:5,10 minAP, 1:10,10 minAP, 1:3,30 minAP, 1:5,30 minaAP, 1:10,30 minRelative deformation0.1350.1200.1050.0900.0750.0600.0450.030EPSnon impregnatedVP, 1:3,10 minVP, 1:5,10 minVP, 1:10,10 minAP, 1:3,05 minAP, 1:5,05 minAP, 1:10,05 minAP, 1:3,10 minAP, 1:5,10 minAP, 1:10,10 minAP, 1:3,30 minAP, 1:5,30 minbAP, 1:10,30 minFigure 6. Foam trays impregnated with a starch acetate/chloroform solution: a) Force at break, N; and b) Relative deformation atbreak, δ.The environmental aspects of using chloroform asthe solvent must be carefully evaluated. For industrial use,it is important to evaluate the residual concentration of thissolvent in the foam trays and also find ways to recoverthe chloroform evaporated during the vacuum pulses(impregnation) and foam tray drying procedures.The puncture tests results for the impregnated andnon-impregnated foam trays are presented in Figures 6aand b. In general, the samples impregnated with the moreconcentrated solutions (1:3 and 1:5 g.mL –1 ) presentedrelatively less deformation than samples impregnated withthe lower concentration (1:10 g.mL –1 ). This behaviour canbe explained by the greater starch acetate impregnationobtained with the more concentrated solutions.Moisture sorption isotherms of the impregnatedand non-impregnated foam trays are shown in Figure 7.The GAB model fitted the experimental sorption data forthe impregnated and non-impregnated samples well. Thevalues for the GAB constants are shown in Table 2. Themoisture content of the monomolecular layer was higherfor non-impregnated samples. A plausible explanation forthis result is the coating of both the sample surface andpart of its porous space by starch acetate, which is hydrophobic(LAROTONDA et al., 2005). Figure 8 shows theSEM micrographs of the surface of an impregnated foamtray, and some aspects of the thin acetate film that coatthe sample surface can be observed. The film presenteda dense structure, interspersed with cracks, due to thebrittle characteristic of starch acetate (LAROTONDA et al.,2004, 2005). Similar film coatings were reported by Matsuiet al. (2004) and Larotonda et al. (2005).Figure 9 shows micrographs of cross-sections ofthe impregnated foam trays. It is impossible to identify theEquilibrium moisture (gw/gss)0.400.300.200.100.000.00 0.20 0.40 0.60 0.80 1.00GAB Impregnated sampleImpregnated sampleWater activityTable 2. Values for the GAB constants.Constants Non-impregnatedsampleGAB Non-impregnated sampleNon-impregnated sampleFigure 7. Sorption isotherms for the impregnated and nonimpregnatedfoam trays.Impregnatedsample(1:10 g.mL –1 )K 0.9613 0.9989Xo 0.0269 0.0168C 17.981 11.574R 2 (Adjusted) GAB 0.9941 0.9645starch acetate film inside the structure, but details of thefibres and calcium carbonate distribution can be seen,as well as the voids formed by the arrangement of thematerials formed during the heating process.Braz. J. Food Technol., v. 12, n. 1, p. 34-42, jan./mar. 2009 39

www.ital.sp.gov.br/bjWater absorption and mechanical properties of starch foam trays impregnated with starch acetateSCHIMIDT, V. C. R. and LAURINDO, J. B.abAcc.V Spot15.0 kV 5.0Magn Det WD250x SE 10.9100 mAcc.V Spot15.0 kV 5.0Magn Det WD250x SE 10.9100 mcdAcc.V Spot15.0 kV 5.0Magn Det WD250x SE 11.0100 mAcc.V Spot15.0 kV 5.0Magn Det WD250x SE 9.815 F100 mFigure 8. SEM micrographs of foam tray surfaces: a) VP, 1:3,10 min; b) AP, 1:3,10 min; c) AP, 1:5,10 min; and d) non-impregnatedsample.abAcc.V Spot15.0 kV 5.0Magn Det WD100x SE 10.9200 mAcc.V Spot15.0 kV 5.0Magn Det WD60x SE 9.8200 mcdAcc.V Spot15.0 kV 5.0Magn Det WD60x SE 9.2200 mAcc.V Spot15.0 kV 5.0Magn Det WD100x SE 10.0200 mFigure 9. SEM micrographs of foam tray cross-sections: a) VP, 1:3,10 min; b) AP, 1:3,10 min; c) AP, 1:5,10 min; and d) nonimpregnatedsample.Braz. J. Food Technol., v. 12, n. 1, p. 34-42, jan./mar. 2009 40

www.ital.sp.gov.br/bjWater absorption and mechanical properties of starch foam trays impregnated with starch acetateSCHIMIDT, V. C. R. and LAURINDO, J. B.process parameter, structure and properties. Industrial Cropsand Products, Amsterdam, v. 16, n. 1, p. 69-79, 2002.SOYKEABKAEW, N.; SUPAPHOL, P.; RUJIRAVANIT, R.Preparation and characterization of jute and flax reinforcedstarch-based composite foams. Carbohydrate Polymers,Kidlington, Oxford, v. 58, n. 1, p. 53-63, 2004.THIEBAUD, S.; ABURTO, J.; ALRIC, I.; BORREDON, E.; BIKIARIS,D.; PRINOS, J.; PANAYIOTOU, C. Properties of fatty-acid ester ofstarch and their blends with LDPE. Journal of Applied PolymerScience, Hoboken, v. 65, n. 4, p. 705-721, 1997.TIEFENBACHER, K. F. Starch-based foam materials: use anddegradation properties. Journal of Macromolecular Science:Pure Applied Chemistry, Lowell, v. 30, n. 9-10, p. 727-731,1993.WHISTLER, R. L.; HILBERT, G. E. Mechanical properties ofcorn-starch of film from amylose, amylopectin, and whole starchtriacetates. Industrial & Engineering Chemistry, Washington,v. 36, n. 9, p. 796-798, 1944.WOLFF, I. A.; OLDS, D. W.; HILBERT, G. E. Triesters of cornstarch, amylose, and amylopectin. Industrial & EngineeringChemistry, Washington, v. 43, n. 4, p. 911-914, 1951.XU, Y.; HANNA, M. A. Preparation and properties of biodegradablefoams from starch acetate and poly (tetramethylene adipate-coterephthalate).Carbohydrate Polymers, Kidlington, Oxford,v. 59, n. 4, p. 521-529, 2005.Braz. J. Food Technol., v. 12, n. 1, p. 34-42, jan./mar. 2009 42