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//-->.pos {position:absolute; z-index: 0; left: 0px; top: 0px;}Development of recycled polymer compositesfor structural applicationsA.-M. Hugo1, L. Scelsi*1, A. Hodzic1, F. R. Jones2and R. Dwyer-Joyce1This paper is concerned with the formulation of composite materials for structural or semistructuralapplications using thermoplastic polymer waste. The mechanical and thermal properties of aproprietary blend of recycled polymers with a range of different fillers were investigated. Theeffect varied with the aspect ratio of the filler and the mode of loading. Spherical calciumcarbonate gave a marginal improvement in modulus. Plate-like mica produced a significantincrease in modulus without reduction in strength. Glass fibres caused a significant increase inmodulus and strength while decreasing the linear coefficient of thermal expansion. Hybridsystems containing glass fibre and a lower aspect ratio filler were also investigated to obtain amaterial system which combines high properties and reasonably low cost. It was found thataddition of small quantities of mica to glass fibre reinforced blends exhibited a significant synergyin tensile strength and modulus.Keywords:Recycled, Thermoplastic, Mica, Glass fibre, Polymer blend, CompositesThis paper is part of a special issue on Latest developments in research on composite materialsIntroductionIn 2005, the annual consumption of plastic materials wasnearly 44 million tonnes for Western Europe alone, and ithas been rapidly growing.1Recycling of plastics hastherefore become a worldwide environmental priority.The main barriers to plastics recycling are the high cost ofthe recycled products relative to their performance andthe difficulty to identify satisfactory markets for thesematerials.2In the UK, recycling targets for plastics of26% for 2008 have been set by legislation.3Any post-useplastic packaging contributes to this target, which has ledto a focus on the most cost effective and easilyrecoverable streams, such as industrial and commercial,rather than domestic, packaging.Traditionally, recovered plastics are separated intopolymer types and used to produce ‘second grade’pellets. However, the range of applications for thesepellets is limited due to their reduced properties and topossible contamination, which prevents their use in foodcontact applications. An alternative approach, exploredin this study, consists of upgrading plastic recyclates bythe addition of rigid fillers to improve their structuralproperties and to make them suitable for long term(semi)structural applications.Polymers have many advantages compared to con-ventional materials for structural applications, especiallyif their stiffness and strength can be improved. Usingrecycled materials reduces cost significantly; however,1Department of Mechanical Engineering, The University of Sheffield,Sheffield S1 3JD, UK2Department of Materials Science and Engineering, The University ofSheffield, Sheffield S1 3JD, UK*Correspondingauthor, email scelsi.lino@gmail.compolymer formulation and production can be challengingto overcome the natural variability in feedstock.2Semicrystalline polyolefins have many desirable proper-ties for structural applications: good toughness, highfatigue resistance, chemical resistance, non-toxicity inthe environment, high electrical resistivity, low waterabsorption, good corrosion resistance, UV stability,lifetime of up to 50 years and recyclability. Conven-tional building products have higher stiffness, bettercreep resistance and lower coefficient of thermal ex-pansion (CTE). This study investigates the use of fillersto improve the properties of a proprietary blend ofplastics.The polymer blend examined in this work consistsmainly of post-industrial plastic waste. Post-industrialwaste is the scrap from industrial processes, e.g. yarnbobbins, jerry cans, end of runs, misprinted pots, etc. Itis usually clean and segregated by type, which presentsconsiderably less batch to batch property fluctuationscompared to domestic polymer waste. The blend is aproprietary polymer formulation containing both semi-crystalline and amorphous thermoplastics in order toachieve a good balance between toughness and stiffness.Although details on blend composition cannot bedisclosed, all the components were commodity polymerstypically very abundant in landfills, and the resultspresented in this study are generally relevant for therecycling of commingled polymer waste streams.Previous studies from Rutgers University4have shownthat it is possible to achieve polymer blend morphologieswith high mechanical properties without the addition ofcompatibilisers through correct blend formulation andprocessing.The effect of filler on the mechanical properties willdepend upon its chemical composition, particle shapeßInstitute of Materials, Minerals and Mining 2011Published by Maney on behalf of the InstituteReceived 16 September 2010; accepted 19 September 2010DOI 10.1179/1743289810Y.0000000008Plastics, Rubber and Composites2011VOL40NO6/7317Hugo et al.Recycled polymer composites for structural applicationsand size, size distribution, specific surface area, sur-face chemistry, interparticle spacing and extent ofagglomeration.5Higher aspect ratio fillers give greaterreinforcement and produce higher stiffness, heat distor-tion temperature and creep resistance.The typical spherical fillers are calcium carbonate,clay, glass beads, carbon black and alumina trihydrate.Among these, calcium carbonate is the most widely usedfiller as it is readily available and of low cost.5It reduceswarpage, increases modulus and, in virgin materials,reduces the cost of the material. In such applications,strength is normally reduced slightly. Impact toughnessis also reduced, with the exception of very fine additivegrades, which can act as impact modifiers.5Stearatecoatings are often used to improve surface bonding anddispersion. The type of polymer is also important wherefiller/matrix interfaces are considered. For example, thecoated filler increased the impact toughness in poly-propylene (PP) homopolymer; however, it decreased thetoughness in high density polyethylene and PP.6Plate-like fillers are better reinforcements than sphe-rical fillers. Examples are talc, mica and kaolin.7Modulus, shrinkage, warpage and heat distortiontemperature have been improved by the addition of allthese fillers to polymers. However, tensile strength,impact strength and elongation at break tend todecrease.8Mica has an aspect ratio only rivalled by fibrousmaterials. For good bonding to non-polar plastics, itneeds to be silane treated or mixed with maleicanhydride modified polymers. Most commercial appli-cations do not justify the addition of expensive silanetreatment.8Mica has low CTE and good weatheringperformance.7A synergy was found by some authors9,10when adding low quantities of mica to glass fibrereinforced polyolefins to increase modulus, improvedimensional stability and reduce cost. The increase inproperties was attributed to a positive effect of mica onthe fibre–matrix adhesion.Fibre fillers have the highest aspect ratio and givesignificant reinforcement. Examples are glass, carbon,straw, flax, hemp and kenaf. The degree of reinforce-ment is significantly affected by fibre modulus, aspectratio, length and orientation in the product. Glass fibreis the most common reinforcement for polymers. Asreported by several industrial and academic studies, itcan be used to upgrade recycled thermoplastics into longlife products.11It improves strength, stiffness, fracturetoughness and heat resistance.5,9An increase in the heatdeformation temperature from 60 to 150uC for a 40wt-% loaded PP has been reported.5Titanate or silanecoatings and maleic anhydride or acrylic acid couplingagents are required for optimum fibre–matrix bonding.Fibre lengths.0?5mm are required for optimumstrengthening, and the properties are dramaticallyimproved above 1 mm.A study on 30 wt-% long glass fibre PP showed thatthe addition of 20 wt-%CaCO3to the PP matrix gave anincrease of 10% in tensile modulus. Such an increaseexceeded the modulus enhancements predicted by therule of mixtures and was therefore attributed tosynergistic interactions between the glass fibres andCaCO3. However, tensile strength and fracture tough-ness decreased.9Short glass fibre and mica have beenshown to increase stiffness and reduce warpage.10In astudy of mica filled PP based glass mat thermoplastic,the addition of up to 15 wt-% mica enhanced the fibre–matrix adhesion while improving the tensile, flexural andimpact properties.10Fillers naturally have a lower CTE than polymers.The CTE of filled compounds can depend upon particlesize, distribution and specific surface area.12Increasingthe interfacial area increases the constriction of thematrix and decreases the CTE. However, poor adhesionbetween filler and matrix can lead to an increase in thethermal expansion coefficient.13For some systems (e.g.silica filled epoxy composites), decreasing the fillercrystallinity decreases the CTE.12The majority of plastics are inherently flammable.Reduced flammability is desirable for many buildingapplications and often mandatory for products designedfor indoor use. Conventional halogenated systems arebanned in many applications due to smoke toxicity andenvironmental concerns. Zero halogen systems areavailable, such as alumina trihydrate and magnesiumhydroxide. Loadings of 60 wt-% are required to producethe same level of flame retardancy to the detriment ofstrength and impact resistance. Intumescent phosphor-ous based systems require,30wt-% loading. At thislevel, the mechanical properties are still affected.5ExperimentalMaterialsThe polymer used was a proprietary blend of commonamorphous and semicrystalline recycled polymers. Theplastics were shredded and granulated to 10 mm sizedflakes and then tumble mixed. The blend had a melt flowindex of 11?1 g/10 min at 230uC. Four different addi-tives were compounded with the blend.(i) Omyalene 102M calcium carbonate from OmyaUK: an 86 wt-% stearic acid coated chalkwhiting in a polyolefin carrier. The particleshave an aspect ratio of 1 and an average particlediameter of 2mm.The specific surface area is2?5 m2g21according to BET ISO 4652(ii) Micro Mica W160 from Norwegian Talc ASand distributed by Omya: a muscovite withaspect ratio 20 : 1 and a median particle size of13?5mm(wet analysis Malvern Mastersizer X)or 4?2mm(X-ray analysis Sedigraph 5001). Thespecific surface area is 6?8 m2g21according toBET ISO 4652(iii) 3299 EC13 chopped strand glass fibre fromPPG Industries: a silane treated fibre of 14mmdiameter and 4?5 mm length. For additionalcoupling, 2% Bondyram 1001 maleic anhydridemodified homo-polypropylene from Polyramwas added(iv) 58578-M1-300 Superex POV0-HF flame retar-dant masterbatch from Americhem, a proprie-tary halogen free intumescent flame retardant inlow density polyethylene carrier.Table 1 shows the additive combinations used with therecycled blend in this study.The flame retardant was added at a suitable level togive UL 94 V0 rating.14Sample preparationThe materials were compounded using a Berstorff ZE25co-rotating twin screw extruder with a temperature318Plastics, Rubber and Composites2011VOL40NO6/7Hugo et al.Recycled polymer composites for structural applicationsprofile of 180–210uC and a speed of 430 rev min21. Thecompounded material was pelletised into 5 mm longpellets. Standard test specimens were injection mouldedusing a NegriBossi V55-200 with a 62 ton maximumclamp force. The temperature profile was 200–230uC.The mould was not cooled.Mechanical testingSpecimens were tested using a Hounsfield HK100-S.Type 2 ISO 1367 dog bone specimens were tensile testedat a speed of 5 mm min21. ASTM D 970 flexuralspecimens (127612?763?2 mm) were tested to flexuralthree-point bend test with a span of 51?2 mm at a speedof 2 mm min21. The support noses had a diameter of5?95 mm. The loading nose had a diameter of 6?3 mm.Linear CTETwo different methods were used to measure the linearCTE. A Perkin Elmer Diamond thermomechanicalanalyser was used over the range of220–60uCand ata ramp rate of 2uC min21. Measurement of the changein length of the flexural test bars after conditioning at218and 55uC was carried out using a standardlaboratory oven and freezer. Vernier callipers were usedto measure the change in dimensions.Scanning electron microscopyFlexural test bars were dipped in liquid nitrogen,clamped in a vice and fractured by a hammer blow.The samples were carbon coated during the samplepreparation procedure. The fracture surfaces wereexamined using a Philips XL 40 in secondary electronand backscattered electron modes.1 Tensile and flexural modulus of systems developed inthis study:atensile modulus andbflexural modulusof particle reinforced recycled polymersResultsCaCO3and mica were added to the recycled polymerblend to evaluate the potential property improvementsthat can be achieved using low cost fillers. The proper-ties resulting from the addition of a fire retardant arealso reported for comparison. Subsequently, the effect ofglass fibre reinforcement was evaluated to assess whetherthe higher enhancement in properties justifies theiradditional cost and processing complexity. A furtherstep was the investigation of hybrid systems containingglass fibre and a lower aspect ratio filler in order toobtain a wider range of property enhancement andfurther improvement of certain properties throughsynergistic effects.Mechanical propertiesThe effect of each filler is dependent on the method ofloading. The particulate fillers increase tensile andflexural moduli significantly (Fig. 1). The higherenhancement of tensile modulus for mica filled systemsis consistent with the literature. Mica has a far higheraspect ratio than calcium carbonate, which increases thecontact area between the mica and the matrix and leadsto a more significant effect on properties. The increasedsurface area enables improved stress transfer to thefiller.12In addition, mica has a higher tensile modulus(over 100 GPa) compared to CaCO3(35 GPa).Tensile strength decreases, and flexural strengthincreases slightly (Fig. 2). The reduction of tensilestrength for the CaCO3filled systems indicated poorinterfacial adhesion for this system. The stearic acidcoating on CaCO3generally improves dispersion, buthas no or limited coupling effect.5For the mica filledsystem, the tensile strength was practically unchanged,as mica possesses better reinforcing ability than calciumcarbonate and typically does not depress strengthconsiderably.5In flexure, the stress is maximum at thesurfaces. The force is compressive on the loaded surfacewith an equal and opposite tensile stress on the oppositesurface. The increase in flexural strength for bothparticulate filled systems was in turn attributed to thecompressive component of the mechanical response. Thecompressive strength of filled systems tends to increaseeven for uncoupled systems. This is consistent withTable 1 Additive combinations compounded with proprietary recycled plastic blend used in this study (GF: glass fibre,M: mica, FR: flame retardant)Amount of additive in compound/wt-%20%CaCO3202015401520%MicaFlameretardant15%GF15%GFz5%C55153053015%GFz5%M30%GF30%GFz5%MAdditiveCalcium carbonateMicaGlass fibreFlame retardant masterbatchPlastics, Rubber and Composites2011VOL40NO6/7319Hugo et al.Recycled polymer composites for structural applications2 Tensile and flexural strength of systems developed inthis study:atensile strength;bflexural strength ofparticle reinforced recycled polymers3 Tensile and flexural modulus of fibre reinforced materi-als developed in this study:atensile modulus;bflex-ural modulus of recycled systemsprevious studies on filled thermoplastics, which reportedthe compressive strength to be directly proportional toYoung’s modulus.13The addition of an intumescent flame retardantincreases tensile modulus, but is very detrimental tostrength and causes an unexpected decrease in flexuralmodulus. The proprietary masterbatch (40 wt-%) wasrequired to give the required improvement on flamm-ability properties. This high level would be expected tohave a significant effect on mechanical properties.Intumescent flame retardant systems are not reportedto have a reinforcing effect, plus their hydrophilic naturecreates a poor interfacial bond with hydrophobicpolymers. Studies have reported an increase in modulusand heat deflection temperature, but a decrease inimpact strength and other mechanicals.5Couplingagents have been studied, showing improvements inmechanical properties without a detrimental effect onflammability.15,16Elongation in tension was reduced for all fillers, inparticular for glass fibres. However, elongation at breakfor all systems was above 3?5%. Such values ofelongation at break are sufficient to guarantee asatisfactory performance in semistructural applications,which are generally designed for stiffness with highsafety factors.17Glass fibre significantly increased the strength andmodulus of the recycled polymer blend (Figs. 3 and 4).Owing to the cost and processing limitations for therecycled composite, the maximum amount of glass fibreincorporated in the product was 30 wt-%. A secondaryfiller was added to the glass fibre reinforced systems tofurther enhance the structural properties withoutincreasing the cost. Calcium carbonate had a similareffect in the glass filled blend as with the pure polymerblend. In both cases, the addition of calcium carbonatecaused a slight increase in the mechanical properties,except for the tensile strength.The addition of small proportions of mica to the glassfibre reinforced blend resulted in an increase, rather thana decrease (observed for CaCO3), in tensile strength.Since 20 wt-% of mica alone did not alter the tensile4 Tensile and flexural strength of fibre reinforced sys-tems developed in this study:atensile strengths;bflexural strength modulus of recycled systems320Plastics, Rubber and Composites2011VOL40NO6/7Hugo et al.Recycled polymer composites for structural applications5 Linear CTE for systems developed in this studystrength of the material (Fig. 2), it was concluded that asynergistic interaction took place between mica andglass fibre reinforcement. Such a positive effect wasobserved both at 15 and 30 wt-% glass fibre loading andwas particularly marked during the tensile tests. Theaddition of 5 wt-% mica into 30 wt-% glass fibrereinforced blend resulted in an increase in tensilestrength of 20%. For the same material system, an evenmore remarkable synergy was observed in terms oftensile and flexural modulus, which increased by 35 and7% respectively. An explanation for this effect has beenproposed in the section on ‘Discussion’.Linear CTEThermomechanical analysis measurements producedcomplex results due to the number of transitions forthe separate polymers. Measurement of the test speci-mens produced reasonably consistent results (Fig. 5).The standard deviation was appreciable due to the smallchanges in size. Particulate fillers appeared to increasethe linear CTE, while it was significantly reduced in thepresence of fibre reinforcement. The increase in CTE forthe particulate systems can be attributed to the pooradhesion between these fillers and the polymer matrix.13The reduction in CTE for the glass fibre reinforcedsystems is consistent with a strong coupling betweenglass fibres and the polymer matrix, which was achievedby the addition of silane coating and maleic anhydridegrafted PP.6 Scanning electron microscopy images showing fracturesurface morphology of 30 wt-% glass fibre and 5 wt-%mica filled compound:asecondary electron mode andbbackscattered electron mode showing filler distributionScanning electron microscopyScanning electron microscopy showed a well dispersedblend of different polymers. The orientation of the fibresin the direction of process flow (perpendicular to thefracture surface) can be observed at the fractured surface(Fig. 6). The calcium carbonate addition showed gooddistribution with a little agglomeration (Fig. 7). Themaximum agglomerate size observed was below 10mm(Fig. 7b).DiscussionStearate coated calcium carbonate behaved as predictedfrom the earlier reported literature.5The modulus wasincreased by 24%, and the strength was decreasedslightly. In flexural mode, the modulus was increasedby 40%, and the strength was also increased slightly. Thehigher improvement in flexural properties was due to thecombination of tensile and compressive modes.The uncoated mica resulted in an increased reinforce-ment effect as expected with the increase in aspect ratio.The tensile modulus was increased by 86%, and thestrength decreased marginally. This improvement is inline with other studies that reported 50–100% higherproperties compared to talc or calcium carbonate, withlittle or no reduction in impact strength.5In flexuralmode, the modulus increased by 114% and strengthincreased slightly.The silane treated glass fibre with maleic anhydridepolypropylene compatibiliser significantly improved thestrength and modulus of the blend, as predicted. The15 wt-% glass fibre increased the tensile strength by 70%and the elastic modulus by 63%. The flexural strengthwas again increased by 60% and the flexural modulus by210%. The 20 wt-% mica increased the tensile modulusto the same degree as 15 wt-% glass fibre, howeverwithout the increase in strength. Mica could be used asan alternative to glass fibre for certain applications. The30 wt-% glass fibre increased the tensile strength by 85%and the modulus by 240%. The flexural strength wasincreased by 115% in this case, and the flexural modulusby 445%, as expected in a well oriented and consolidatedglass fibre composite.The addition of 5 wt-% calcium carbonate to 15 wt-%glass fibre in the recycled polymer blend increased thetensile and flexural moduli by further 20%, while thestrength was decreased in tension. The 15 wt-% glassfibre reinforced PP (GRPP) samples reinforced with5 wt-% mica presented similar property enhancements,with the difference that mica improved the tensilestrength. However, a much more pronounced synergisticeffect was observed for the addition of mica to 30 wt-%filled glass fibre blend. For this system, mica improvedthe tensile modulus by as much as 35% and flexuralPlastics, Rubber and Composites2011VOL40NO6/7321zanotowane.pl doc.pisz.pl pdf.pisz.pl hannaeva.xlx.pl