New polymer mortars containing polymeric wastes.Part 1. Microstructure and mechanical properties

M.C. Bignozzi, A. Saccani, F. Sandrolini*Dipartimento di Chimica Applicata e Scienza dei Materiali, Facolta` di Ingegneria, Universita di Bologna, Via Risorgimento 2, I-40136 Bologna, Italy

Received 8 March 1999; received in revised form 19 July 1999; accepted 6 August 1999

Abstract

Recycling industrial wastes as filler components and/or aggregates in polymer mortars makes these materials very interesting from anecological and safety point of view. In addition, the resulting materials have useful physical and mechanical properties. Four recycled fillers(powdered rubbers, tyre rubbers, micronized tyre fibers and milled electrical cable wastes) have been used to formulate new polymer mortars.The comparison of their mechanical properties and microstructures with those of a plain polymer mortar indicates that the presence ofrecycled waste affects the physicalmechanical behavior (compressive and flexural strengths, microstructure). The use of silane couplingagents has been also considered and its effect in leading to more compact materials is reported and discussed. q 2000 Elsevier Science Ltd.All rights reserved.

Keywords: Polymer mortars; A. Recycling; B. Mechanical properties

1. Introduction

Polymer mortars and concretes have obtained wideacceptance as materials for many applications thanks totheir versatility in formulation and processing combinedwith high-strength and rapid setting properties. Chemicaland corrosion resistance, ease of placement, long durability,low permeability, high damping and dynamic and thermalstability are some of the advantages that make these poly-mer composites suitable for precast components, bridgedeck overlays, artificial marbles, repair materials forconcrete structures and machine tool basements [111].Moreover, as great interest has been shown recently inconstruction materials with ecological and safety character-istics [1222], polymer mortars and polymer concrete arevery attractive because of their capability for using recycledwaste products. The importance of this new aspect of poly-mer composites has been developed primarily through twodifferent approaches. On the one hand, as the high cost ofthe resin is usually considered the main factor limiting thegrowth of polymer concrete and mortars, a lot of work[1518] has been done on the production of unsaturatedpolyester resins from recycled poly(ethyleneterephthalate)(PET). On the other hand, industrial wastes such as fly ash

[12,13], slag and wood chips [23] have been successfullyused for replacing or partially substituting the filler compo-nents in polymer mortars. However, only a few examples ofthe use of recycled organic fillers have been reported [1922,24].

The characteristics of particulate polymer compositesdepend upon different factors such as the type and theamount of the binder [25], the amount, size and type ofaggregates and fillers. Fine fillers such as calcium carbonateare largely used in polymer mortars and plastic materialsbecause of their low cost. The ability to fill the voidsbetween solid particles (thus replacing polymeric materials)and to be easily dispersed in the mixture makes fillers veryuseful in polymer mortar preparations. However, the higherstiffness and strengths obtained through filling are oftenassociated with drawbacks, such as reduced workabilityand processability.

The aim of the present work has been to prepare newpolymer mortars formulated with recycled plastic fillers toobtain materials with tailored mechanical and physical char-acteristics. For this purpose, some organic fillers, character-ized by different size and origin, have been used in themixture formulations. Powdered rubbers deriving fromacrylonitrilebutadiene rubber production, tyre rubber,micronized tyre fibers deriving from milled waste tyresand plastic granulates coming from milled waste electricalcables, were selected as recycled fillers for the work. To

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* Corresponding author. Tel.: 139-0520-93205; fax: 139-0520-93213.E-mail address: franco.sandrolini@mail.ing.unibo.it (F. Sandrolini).

further enhance the mechanical properties of the new poly-mer mortar composites, the use of coupling agents was alsoinvestigated in the presence of the organic fillers. It is wellknown that coupling agents usually act as modifiers of theinterfacial bond between the aggregate [26,27] and the poly-mer matrix, but their action as promoters of filler dispersionand modifiers of matrix morphology has to be considered[28,29]. In order to evaluate the effect of the filler nature andits size distribution in the composites and the use of acoupling agent, the mechanical properties and microstruc-tures of the new polymer mortars have been compared withthe behavior of a plain polymer mortar. Recycled polymericand rubber waste based materials can be also consideredvery attractive for their potential applications as noisebarriers and high skid resistance pavements, as recentlystudied [30,31]. Regarding this, the damping effect of theorganic filler additions in the new polymer mortar formula-tions is being studied and results will be published in afollowing paper.

2. Experimental procedure

2.1. Materials

Commercially available orthophthalate type unsaturatedpolyester resin (FS0956/AMV/I SIRESTER) was used asbinder with a 50% dimethylphthalate solution of 2-butanoneperoxide as catalyst. The resin has a styrene content of31 wt%. Normal silica sand (UNI EN 196-1), with a sizedistribution 0.0752.0 mm, was used as the base inorganicaggregate to reproduce the standard preparations of cementmortars. Commercially available calcium carbonate (Cari,Bologna, Italy) with an average size of 2.5 mm was used as afine filler. Tyre rubber (TR) (Remida, Modena, Italy),micronized tyre fibers (MTF) (Remida, Modena, Italy),waste electrical cable (EC) (Metalferro, Teramo, Italy)and powdered rubber (PR) (Reevs, Lodi, Italy), a byproductof rubber production, were used as recycled organic fillers.Their properties are given in Table 1. Micronized tyre fibersare mainly composed of polyester fibers ( < 50%), rayon( < 20%) and nylon ( < 30%), powdered rubbers arebased on acrylonitrilebutadiene rubber while waste elec-trical cables are chiefly formed from polyvinylchloride,polyethylene and some thermosets.

Titanium(IV)(triethanolaminato)isopropoxide (Aldrich),

3-aminopropyltriethoxysilane (Aldrich), 3-glycidoxypro-pyltrimethoxysilane (Aldrich) and Silanogran V (Kettlitz)were used as coupling agents.

2.2. Specimen preparation

In a previous experimental study [24] the maximumamount of polymeric waste replacing an equal volume ofinorganic aggregate in mortar formulations, still keeping aworkable mixture and without a marked reduction inmechanical properties, was found to be 3 vol%. Thisamount of sand has been replaced by a different organicfiller in all mixtures. Polymer mortars, with recycled plasticfillers, were prepared according to the following procedure.Sand, organic fillers and calcium carbonate were firstlymixed in a Hobart mixer (UNI EN 196-1) and afterwardsthe unsaturated polyester resin premixed with the catalyst,was added. The binder formulation and the mix proportionsof the mortars studied are reported in Table 2: 3 vol% ofsand was replaced by an equivalent volume of recycled fillerand the samples were designated, respectively, EC3, PR3,TR3 and MTF3. Polymer mortar without any organic fillerwas also prepared as a control sample and designated by S.

The influence of coupling agents on the mechanical prop-erties was investigated using treated aggregates. Couplingagents titanium(IV)(triethanolaminato)isopropoxide, 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxy-silane and Silanogran V were added directly to the sandand were dispersed by intimate contact of the sand grainswhen mixed manually. 1% (based on the resin content) ofthe coupling agent was used. Preliminary experimentalstudies were carried out to determine which couplingagent had the best performance. It was observed that themost effective from a mechanical and microstructuralpoint of view was 3-glycidoxypropyltrimethoxysilane.Accordingly, five samples designated Sp, EC3p, PR3p,TR3p and MTF3p were prepared using 3-glycidoxypropyl-trimethoxysilane according to the procedure describedabove. Their composition was equivalent to that of speci-mens S, EC3, PR3, TR3 and MTF3, apart from the presenceof the silane.

Prismatic mechanical test specimens 40 mm 40 mm 160 mm of all the mixtures were molded and cured for14 days at room temperature and ,40% RH, and storedunder these conditions until tested.

2.3. Characterization

Measurements of the mechanical strengths (bothcompressive, s c, and flexural, s f), were carried out on allsamples using an Amsler Wolpert testing machine(200 kN): at least five measurements for each compositionwere made. Dynamic elasticity modulus was determined byultrasonic measurements using a Controls Ultrasonic TesterE46. Pore size distribution measurements were carried outwith a mercury porosimeter, Carlo Erba 2000, equippedwith a macropore unit. Microstructural observations were

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Table 1Properties of recycled organic fillers

Filler Size (mm) Densitya at 238C (g/cm3)

TR 0.0751.0 1.31MTF 1.38EC 310 1.33PR 0.1250.5 1.20

a Calculated by volumetric titration [32].

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Table 2Mix proportions of polyester mortars

Sample Binder FS0956 (wt%)a Sand (wt%)a CaCO3 (wt%)a Coupling agent (wt%)b Recycled filler type Recycled filler (wt%)a Replaced sand (vol%)c

S 12.0 76.00 12.0 EC3 12.0 74.80 12.0 EC 1.2 3PR3 12.0 74.90 12.0 PR 1.1 3TR3 12.0 74.80 12.0 TR 1.2 3MTF3 12.0 74.70 12.0 MTF 1.3 3Sp 12.0 75.88 12.0 1 EC3p 12.0 74.68 12.0 1 EC 1.2 3PR3p 12.0 74.78 12.0 1 PR 1.1 3TR3p 12.0 74.68 12.0 1 TR 1.2 3MTF3p 12.0 74.58 12.0 1 MTF 1.3 3

a Based on the whole composition.b Based on the resin content.c Vol% of replaced sand by recycled filler (sand density has been assumed equal to 2.5 g/cm3).

performed on the undisturbed fracture surface resultingfrom flexural tests with a Philips 501 B scanning electronmicroscope (SEM): phases recognition was aided by the useof EDAX 9800 micro-analysis. Disks (46 mm thick), forelectrical characterization, were obtained from flexural testfragments by sawing. A suitable three-terminal configura-tion was deposited on the disks by gold evaporation invacuo. Electrical volume conductivity, at 258C and 50 ^10% RH, was measured according to ASTM D257 underan electrical field of 1 kV/cm, applied by a Keithley 237power supply. Currents flowing through the samples wererecorded as a function of time up to 3600 s with a Keithley617 electrometer. A detailed description of cells, instrumen-tation and measuring procedures is reported elsewhere [3335].

3. Results and discussion

Mechanical characteristics of samples EC3, TR3, PR3and MTF3, containing 3 vol% of different plastic fillers

are reported in Table 3 and compared to the mechanicalproperties of the control polymer mortar, S. The compres-sive (s c) and flexural (s f) strengths of the mixtures inves-tigated are 87108 MPa and 2531 MPa, respectively, andthe dynamic elastic modulus 23.828.2 GPa. All the speci-mens prepared with the recycled fillers show mechanicalcharacteristics lower than the control sample S, butcompressive strength values are higher than the compres-sive strength of an ordinary Portland cement (OPC) mortar(about 50 MPa after 28 days curing, with a water/cementratio of about 0.5 [36,37]). Concerning the flexural strength,the effect of the filler does not depend on its nature, althoughfor sample EC3 the decrease in compressive strength andelastic modulus is less evident than those observed for theother organic filler based composites.

The mechanical properties of samples EC3p, TR3p, PR3pMTF3p and Sp, prepared using the coupling agent 3-glyci-doxypropyltrimethoxysilane, are also reported in Table 3and compared to the similar polymer mortars prepared with-out using the organo-silane compound. As can be observed,there is an overall increase in both the above flexural andcompressive strengths due to the silane treatment of thesand, even if it is more remarkable for the polymer mortarscontaining organic based fillers than for the referencesample. Polymer mortars containing 3 vol% of electricalcables based fillers (EC3p) show the highest above flexuralstrength increase (,32%), whereas samples containing3 vol% of tyre rubbers based fillers (TR3p) show the highestabove compressive strength increase (,12%). In addition,the mechanical properties of samples prepared with thecoupling agent are in some cases almost the same as thoseof the untreated plain polymer mortar S that was used as thereference sample.

The role of the coupling agent is known to be not only tocreate physicalchemical bridges between the polyestermatrix and sand by forming interpenetrating networks

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Table 3Mechanical properties and standard deviations of modified and unmodifiedcomposite polyester resin mortars

Sample s f (MPa) s c (Mpa) E (GPa)

S 31.1 ^ 1.7 108.8 ^ 2.6 28.1 ^ 0.7EC3 23.7 ^ 1.9 98.4 ^ 2.9 26.5 ^ 0.1PR3 25.6 ^ 1.7 86.9 ^ 1.9 24.4 ^ 0.2TR3 25.1 ^ 2.1 89.4 ^ 1.2 24.7 ^ 0.1MTF3 25.3 ^ 1.9 87.8 ^ 2.1 23.8 ^ 0.7Sp 33.9 ^ 2.2 117.1 ^ 2.6 27.1 ^ 0.7EC3p 31.3 ^ 2.0 107.4 ^ 2.2 27.2 ^ 0.1PR3p 32.5 ^ 1.3 90.6 ^ 1.8 25.0 ^ 0.4TR3p 30.6 ^ 1.4 100.4 ^ 1.8 25.6 ^ 0.3MTF3p 27.7 ^ 2.6 93.4 ^ 3.0 23.8 ^ 0.5

Fig. 1. FT-IR spectra of the coupling agent 3-glycidoxypropyltrimethoxysilane at different times.

promoting adhesion, but also to increase the filler dispersionby wetting the particulate surfaces to uniformly spread theresin during the mixing [2629]. In order to investigatefurther which was the prevalent effect here, FT-IR measure-ments on 3-glycidoxypropyltrimethoxysilane were made,after contact for different times with the sand at room con-ditions: the hydrolysis of the methoxy groups linked to theSi atom should have occurred and the SiOH band shouldhave appeared under these conditions, which reproduced theexperimental procedure used in polymer mortar preparation.After 1 h, the presence of the peak characteristic of theSiOH bond (35003600 cm21) was not detected (Fig. 1).Similar FT-IR spectra (not reported), without the evidenceof the SiOH peak, were obtained for the coupling agentafter its contact with the recycled organic fillers. This resultappears to indicate that, in the present study, the prevalenteffect of the coupling agent is the promotion of filler disper-

sion. Figs. 2 and 3 show the results of mercury intrusionmeasurements for all the samples. All the materials havelower porosity compared to the conventional cementitiousbased composite that usually ranges between 50 and70 mm3 of Hg intruded volume per gram [36]. The additionof organic fillers increases the overall porosity accordinglyto a more difficult procedure in sample preparation due to aless workable mixture: however, the use of the couplingagent always leads to more compact materials. In order toevaluate the effect of microstructural modifications in therange of smaller porosity, Figs. 4 and 5 report mercuryintrusion in pores of size smaller than 0.1 mm. In thisporosity range the coupling agent has almost no effect onthe plain polymer mortars (which are however the leastporous of all) and on the samples containing tyre rubber(TR3p) and powdered rubber (PR3p) based fillers. The silanetreatment leads to lower porosity for polymer mortars

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Fig. 2. Total intruded volume for samples S, Sp, EC3, EC3p, MTF3 and MTF3p.

Fig. 3. Total intruded volume for samples PR, PR3p, TR3 and TR3p.

formulated with micronized tyre fibers (MTF3p) comparedto the relevant untreated sample (MTF3), whereas theresults are reversed for electrical cable (EC3p) based fillersbelow a pore size of 0.01 mm.

Figs. 6 and 7 show SEM microstructures of S and Spsamples. Both samples show a similar compact matrixwith a few voids in the surface near to the aggregatesthus indicating that the coupling agent appears not toinfluence the microstructure in this case. Figs. 8 and 9refer to polymer mortars EC3 and EC3p and, Figs. 10and 11 to samples MTF3 and MTF3p. Organic particleswere recognized by their morphology and with the aid oflocal EDAX analysis: again the microstructure of themodified samples containing the coupling agent are simi-lar to the relevant unmodified polymer mortars. However,a slightly more compact microstructure can be observedfor EC3p in Fig. 9 and a better adhesion of the filler phaseis shown by the decrease of the voids between matrix andorganic particles. A similar behavior can be observed in

Fig. 11 for MTF3p sample, where the coupling agentseems to improve the adhesion between all the differentphases present in the composite material. Figs. 12 and 13refer to samples PR3 and PR3p containing powderedrubber: again, though the filler appears to be wellembedded in the matrix, the coupling agent leads to amore compact microstructure.

In several polymer mortar applications (conduits, outdoorinsulators, etc.) electrical properties are extremely impor-tant: therefore the volume electrical conductivity of the newformulations have been determined at room temperature.Electrical conductivity of unmodified and modified samplesis reported in Figs. 14 and 15 as a function of time aftervoltage application. All samples, regardless of their compo-sition, behave as insulating materials showing large char-ging transients as a function of time. Electrical conductivityvaries from 1:7 10214 to 3:1 10216 S=cm for polymermortars untreated with the coupling agent, whereas conduc-tivity values of the samples containing the silane compound

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Fig. 4. Total intruded volume for porosity whose dimension is lower than 0.1 mm for samples S, Sp, EC3, EC3p, MTF3 and MTF3p.

Fig. 5. Total intruded volume for porosity whose dimension is lower than 0.1 mm for samples PR3, PR3p, TR3 and TR3p.

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Fig. 6. SEM observations of sample S (160 , marker 10 mm).

Fig. 7. SEM observations of sample Sp (160 ).

Fig. 8. SEM observations of sample EC3 (40 , marker 10 mm).

Fig. 9. SEM observations of sample EC3p (40 ).

Fig. 10. SEM observations of sample MTF3 (320 , marker 10 mm).

Fig. 11. SEM observations of sample MTF3p (320 , marker 10 mm).

are 1:7 10213 to 3:3 10215 S=cm: The organic filleraddition results in a slight volume electrical conductiv-ity increase for samples TR3 and EC3 that can beconsidered negligible, whereas for MTF3 and PR3 theincrease is more evident and it is presumably owed toimpurities contained in the relevant organic fillers. Thecoupling agent reduces the porosity of the samples, butincreases the electrical conductivity of all samples: thelatter effect may be ascribed to the introduction of ioniccharge carriers induced by the long-term hydrolysis of3-glycidoxypropyltrimethoxysilane.

4. Conclusions

The influence of organic recycled fillers in new polymer

mortar formulations has been investigated from amechanical and microstructural point of view. The follow-ing conclusions may be drawn:

Compressive and flexural strengths of the resin compo-sites prepared using recycled organic fillers decrease withreference to a plain polymer mortar, but they are stillhigher than the mechanical properties shown by an ordin-ary Portland cement (OPC) mortar. However, the addi-tion of recycled organic fillers increases sample porosity,particularly at the interphases between the resin/extendermatrix and the filler grains.

The use of 3-glycidoxypropyltrimethoxysilane treatedaggregates improves the mechanical behavior of thenew polymer composites and decreases the porosity,thus leading to more compact materials. As the observedporosity reduction is higher in the macroporosity range,the coupling agent appears to mainly act as a wettingagent.

The presence of recycled organic fillers and couplingagent leads to higher electrical conductivity values incomparison with those of a plain polymer mortar: theorganic filler introduction has a minor influence on theelectrical conductivity than the addition of the couplingagent in the composite formulations. However, all thecomposite materials obtained can be still classified asinsulators.

Acknowledgements

Financial support of Consorzio R&S (Carpi, Modena,Italy) and MURST 60% are gratefully acknowledged.

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Fig. 12. SEM observations of sample PR3 (1250 ).

Fig. 13. SEM observations of sample PR3p (1250 ).

Fig. 14. Volume conductivity of samples S, EC3, TR3, PR3, MTF3.

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