EVALATING THE MECHANICAL PROPERTIES OF COMMINGLED WASTE PLASTICS: A MIXTRE STDY MICHAEL D. MONFORE AND ROBERT L. MORGAN Rbert Mrgan & Cmpany Battle Creek Michigan PAL V. ENGELMANN Western Michigan niversity Kalamaz Michigan ABSTRACT The study presented demnstrates the mdeling f a cmmingled waste plastics blend. The apprach is based n statistically designed mixture experiments and regressin analysis. Thrugh the evaluatin f a specific array f test pints mathematical mdels and respnse surface plts were develped which describe the degree f influence and relatinship f the frmulatin variables. INTRODCTION A majr challenge t the recycling f plastics waste is the utilizatin f mixed plymers. Physical and ecnmic limitatins ften make separatin f these materials impractical. nder suitable prcessing cnditins cmpsite materials f desirable engineering prperties may be created frm mixed and/r cntaminated plymers. Hwever little published wrk has mdeled the effects f frmulatin and prcessing variables n the perfrmance f varius recycled blends. This has limited pprtunities t ptimize and predict end prduct perfrmance. This has in turn restricted the ability f engineers t design and prduce high value prducts. T investigate the practicability f mdeling the mechanical prperties f waste stream plastics experiments were develped. These studies evaluated majr cmpnents f the plastics waste stream including recycled plyethylene (PE) plyprpylene (PP) and plystyrene (PS). MIXTRE DESIGNS Statistically designed experiments encmpass a wide range f strategies and techniques. Mixture experiments which invlve blends f materials are smewhat unique in their design and analysis. In a mixture the cmpnent's levels always sum t. This impses a special set f cnstraints n the design since the cntrlled variables are n lnger cmpletely independent. Thse familiar with regressin analysis will recgnize this vilates the imprtant assumptin f independence amng the "independent" variables. Frtunately this des nt present a serius barrier t analysis and several ptins exist fr analysis f the data [1-3]. A classical designed experiment invlving factrial arrangements f three variables (i.e. A Band C) may be gemetrically presented as a cube with each variable represented by a different dimensin (Fig. 1). An experiment t mdel these variables wuld explre and map this three-dimensinal space. In a mixture design the special cnstraint A + B + C = reduces the design by ne dimensin (i.e. frm a three-dimensinal cube t a tw-dimensinal plane). The nly regin satis- 317
FACTOR A 0:: f- <t: LL O%C '- 0% B F ACTO R B - L ' FACT ORC 0% A FIG.3 3 COMPONENT MIXTRE FACTOR A FIG. 1 FIG.2 THREE FACTOR DESIGN SPACE ------ -------------------- - ". : c A +:\3 + )- - -- - --- - -------_1- FACTOR A MIXTRE CONSTRAINT PLANE - - fying the cnstraint is a triangular plane within the riginal cube space (Fig. 2). Three cmpnent mixture designs are therefre graphically represented by a triangular plane r surface where each apex is a different cmpnent at its level (Fig. 3). The experiments discussed in this paper used a three-cmpnent mixture design as their basic unit. EXPERIMENTAL PROCEDRE The verall investigatin was cmpsed f tw majr phases. Phase 1 included evaluatin f bth frmula- 0:: f- <t: LL tin and prcessing variables n a Killin 31.75-mm single screw extruder. The frmulatin wrk was replicated acrss bth virgin and recycled resins. Phase 2 expanded the frmulatin wrk n a 50-mm Davis Standard single screw extruder. All studies evaluated blends f PE PP and PS. This paper cncentrates n the frmulatin wrk f Phase 2. The Phase 1 experiment included replicatin f the basic mixture design acrss three extruder screw speed treatments (30 70 and 110 rpm). Results f this wrk were discussed by Engelmann et al. [4]. Recycled resin material fr Phase 2 was btained frm a variety f pst-cnsumer surces. Within each resin type several surces were utilized s that each resin represented a general mix f available grades. N cleaning r drying f the materials was perfrmed. All materials were grund and cmbined int the 10 unique blends required by the design. N special additives r cmpatibilizers were used in the experiment. In this experiment the samples were first extruded. The 50-mm Davis-Standard extruder was equipped with an lefin mixing screw. The samples were then cmpressin mlded t achieve the desired final frm. A 45-metric tn Dake cmpressin mlder and a cast Ampc brnze ally mld were used t mld the extrudate int test plaques. The mld was a simple flash-type and utilized water cling. The mld prduced a prduct with a thickness f 3.175 mm the standard fr the subsequent ASTM tests. Samples were cut frm the cmpressin mlded extrudate and tested fr tensile strength (ASTM 0638) izd impact strength (ASTM 0256) and flexural mdulus (ASTM 0790). The tensile strength test draws r elngates the sample t its "break" pint n a stress/strain curve. Tensile strength is the lad at break divided by the riginal crsssectinal area. Izd impact strength measures the 318
PE 10000 25 -I rn ::J (j) SO 500 s 0 50 343333 @ 176617 @ 171766 0100 0 C9-----e---- 00100 PP 0 SO 50 PS NOTE: ALL VALES ARE IN PERCENT AND INDICATE THE PE:PP:PS RA TIO FIG. 4 PHASE 2 DESIGN FIG. 5 -. TENSILE STRENGTH PLOT tughness f a sample r its ability t withstand a sharp blw such as that frm a hammer. Flexural mdulus is a measure f the flexural strength r stiffness f a sample. Ten unique blends and tw replicates were prcessed during the experiment. The test pints may be seen in Fig. 4. Blend rder was randmized and PE purges were used between blends. A marker dye was used t identify transitins between runs. All prcessing cnditins were held cnstant thrughut the experiment. Apprximately seven samples were cllected and mlded within each blend. Fur t five samples per blend were tested fr each ASTM prcedure. Replicates were made f all dimensinal measurements t cntrl measurement errr. Study data were subsequently analyzed by leastsquares regressin analysis. The sftware packages ECHIP and SAS were used fr the analysis duties. Frmulatin mdels and graphs were develped frm the regressin equatins. RESLTS AND DISCSSION Quadratic r secnd rder plynmial mdels were fund t adequately explain variatin in all mechanical prperties evaluated. The generic mdel suggested by Cx [2] was: where: R = respnse r dependent variable. aj = parameters f the mdel. XI = PE X2 = PP X3 = PS An alternate mdel suggested by Scheffe [3] was als evaluated. This mdel was f the frm: Bth mdel types yielded similar results. All further discussin specifically relates t the frmer mdel. R square values were 0.87 0.84 and 0.92 fr tensile impact and flexural mdels respectively. Frtunately sample variatin thrughut the experimental regin was relatively hmgeneus. In general results cmpared favrably t Phase 1 wrk by Engelmann et al. [4]. Since results are frequently difficult t interpret in the mdel frm respnse surface plts were develped t better understand the relatinships. Three dimensinal plts fr tensile strength izd impact and flexural mdulus are presented in Figs. 5-7 respectively. It was evident that specific cmbinatins f the three resins significantly influenced mechanical perfrmance f the prduct. The mst ntable feature is the nnlinearity present in the blending relatinships f the three recycled plastics. Cmbining the resins des nt result in simple linear changes in mechanical prperties. In additin prminent interactins between resins were bserved. 319
FIG. 6 IZOD IMPACT STRENGTH PLOT. n i5-0 -"" 1500 1000 -r"] ru x c -; OJ I :3 0 0- C C l :3 LJ OJ will nt fall belw 40 Jim as the PE : PP rati is varied. Alternately at higher levels f PS dramatic changes in impact strength will result frm variatin in the PE : PP rati. Anther area f interest n the impact respnse surface is a "flat spt" in the plane centered arund the 70% PE : 10% PP : 20% PS blend. In this regin the blend may vary in cmpsitin by as much as 8% withut appreciable variatin in impact perfrmance. Analysis f the flexural mdulus data indicated a linear PE effect quadratic PE and PS effects and a prminent interactin between PE and PS. Additin f PS t PE initially lwers the flexural mdulus until the blend is abut 30% PS at which pint the flexural mdulus begins t rise rapidly. The level f PP had little r n effect n flexural mdulus. The respnse surface graphs als shw that blends f these three plastics can have perfrmance similar t r even superir t that f individual "pure" resins. Blends may als be cnstructed which brrw characteristics frm each f the individual resins. Hwever the respnse is ften nnlinear and slight changes in blend cmpsitin can result in large perfrmance changes. The mechanisms behind blend perfrmance were nt evaluated in this study. Wrk by Appelbaum et al. [5] Nsker et al. [6] and thers has suggested the presence f beneficial hetergeneus phase mrphlgy in sme blends. Favrable perfrmance f sme cmmingled blends may we mre t mechanical bnding than chemical bnding. CONCLSIONS FIG. 7 FLEX RAL MOD LS PLOT Analysis f the tensile strength data suggested all three resins had significant individual effects n blend perfrmance. Linear effects were bserved fr PE and PS. Quadratic effects were bserved fr PP and PS. An interactin between PP and PS was als apparent. Blends cmpsed primarily f intermediate levels f PP and PS demnstrated lwer tensile perfrmance as cmpared t ther blends. Results fr izd impact strength included significant PE and PS linear effects PP and PS quadratic effects and a pp PS interactin. It is interesting t nte the varius frmula management scenaris which can be evaluated frm these data. Fr example if PS is held t levels f abut 30% r less izd impact strength Results f this study suggest designed experiments and statistical mdeling have ready applicatin fr studying the perfrmance f cmmingled resin blends. The resultant mdels and graphs can be used t develp a detailed understanding f bth frmulatin and prcessing effects. This infrmatin may then be used t ptimize and predict the mechanical prperties f end prducts. The availability f perfrmance and reliability data will prvide the basis t develp higher value prducts frm mixed waste plastics. ACKNOWLEDGMENTS We extend ur sincere thanks t Eric Dawkins Jeff McInerney and Marc Vanderki fr their wrk in building the tling prducing the samples and perfrming mechanical tests. We wuld als like t thank Perfrmance Allys fr prviding the Ampc brnze allys t prduce the cmpressin mld. 320
REFERENCES [I] Crnell J. A. Experiments With Mixtures 2nd editin New Yrk New Yrk: Jhn Wiley & Sns 1990. [2] Cx D. R. "A Nte n Plynmial Respnse Functins fr Mixtures" Bimetrika 58 1 1971 155-159. [3] ScheWe Henry. "Experiments with Mixtures" Jur. Ry. Statist. Sc. (B) 20 34+-360. [4] Engelmann P. V. Dawkins E. W. and McInerney J. A. "Extrusin Cmpressin f Cmmingled Resin Blends: A Mixture Viability Experiment" SPE RETEC 1991. [5] Applebaum M. D. Nsker T. J. Renfree R. W. Mrrw D. R. and Van Ness K. E. "Prperties f Refined Reinfrced Cmpunded Pst-Cnsumer Plastics" SPE ANTEC 1991. [6] Nsker T. J. Renfree R. W. Mrrw D. R. Van Ness K. E. and Dnaghy J. J. "Prperties and Mrphlgies f Recycled Plystyrene/Curbside Tailings Materials" SPE ANTEC 1991. 321