Material Properties Tensile and impact behavior of ...

1 Material PropertiesTensile and impact behavior of polypropylene /low densitypolyethylene blendsR. Strapasson, Amico*, Pereira, SydenstrickerMechanical Engineering Department, Federal University of Parana (UFPR), Box , Curitiba-PR, BrazilReceived 18 November 2004; accepted 7 January 2005 AbstractBlends of polypropylene (PP) and low- density polyethylene (LDPE) may contribute to make recycling more economicallyattractive. The aim of this work was to make PP/LDPE blends (0/100, 25/75, 50/50, 75/25 and 100/0 w/w) via injection moldingcarried out under various injection temperatures and to evaluate their Tensile and impact Properties . The blends yielded tensilestress strain curves very dependent on their composition, especially regarding elongation at break and the presence of irregular behavior for the 50/50 w/w blend is reported.

2 Nevertheless, a linear variation of the yield strength and elasticmodulus with the blend composition was observed. The behavior of the blend was also very dependent on processingtemperature. Addition of 25% of LDPE to the PP may result in similar degradation of its mechanical Properties to that caused bya108C processing temperature increase. Statistical analyses proved valuable when reporting results concerning Elsevier Ltd. All rights :Low- density polyethylene ; polypropylene ; Blends; Tensile and impact properties1. IntroductionAs the economy achieves global status, many factorsregarding the competitiveness of a nation come underinvestigation. More recently, together with important areassuch as technology advancement and technology transfer,issues related to sustainable development and environmentpreservation are receiving increasing attention from theworld of the mechanical recycling of polymersinclude reduction of oil and energy consumption comparedwith the synthesis of virgin polymers, reduced disposal ofplastic waste in municipal garbage and generation ofemployment and income.

3 The recycling of industrial scrapis an ongoing successful practice due to the low level ofcontamination. However, recycling of municipal plasticwaste is often an arduous task due to the fact that thismaterial is usually a mixture of several polymers, whichmakes processing more difficult and also limits the numberof potential applications[1].The most abundant plastics in Brazilian municipal wasteare polypropylene (PP), poly(ethylene-terephthalate) andpolyethylene (PE), the latter being available in differentgrades such as low density polyethylene (LDPE), linear lowdensity polyethylene (LLDPE) and high density polyethy-lene (HDPE).A few methods have been used to classify plastics frommunicipal post-consumer waste.

4 With the flotation method,two fractions are obtained: a lighter fraction, floating onwater, and a heavier fraction. The former is essentiallyconstituted of LDPE, HDPE and PP[2], polyolefins thatexhibit of similar density . It is uneconomic to separate themusing alcohol solutions in a subsequent stage. Thus, theusual practice in small recycling units in Brazil is toindiscriminately mix different amounts of PE and PP duringrecycling, leading to incompatible blends of varying andpoor at good performance materials, it is important toconsider processing conditions, blend composition and itsPolymer Testing 24 (2005) 468 $ - see front matterq2005 Elsevier Ltd. All rights *Corresponding author.

5 Tel.:C55 41 361 3430; fax:C55 41 ( Amico). behavior in the solid state from the melt. While thecrystallization of a homopolymer is controlled by nuclea-tion, spherulite growth, rate of cooling and degree of supercooling, crystallization behavior of polymer blends is morecomplex due to the existence of a second component,usually resulting in an incompatible mixture. Blends of PPand LLDPE (20/80 w/w), for instance, are partially miscibleand its crystallization is controlled by nucleation anddiffusion[3].The incompatibility between LDPE and PP has alreadybeen reported by various authors[4,5], followingmicroscopy and calorimetric studies. In LDPE rich blends,a heterogeneous PP dispersion in the LDPE matrix producestwo phases in the melt.

6 The low interfacial adhesionbetween the phases is responsible for a decrease inmechanical Properties especially related to its morphology,including impact strength, strain at break and ductile tobrittle transition. According to Shanks[6], the immiscibilitybetween the phases makes the rule of mixtures ineffective inpredicting some Properties of overcome this difficulty, the use of various couplingagents have been reported. Amongst others[7 9], Yang[10]showed that the addition of a commercial ethylene/propylene block copolymer improved the ductility ofLDPE/PP blends, particularly for PP rich blends. Bertin[5]studied and characterized virgin and recycled LDPE/PPblends and the use of compatilizing agents, such asethylene-propylene-diene monomer copolymer (EPDM) orPE-g (2-methyl-1,3-butadiene)graft copolymer, to enhancetheir impact strength and elongation at break.

7 Although thismay solve the compatibility problem, the use of compati-bilizers adds cost to the recycled product, usually resultingin loss of interest from the recycling this work, the evaluation of Tensile and impactproperties of PP/LDPE blends was carried out to investigatethe composition range for better mechanical performanceand also to define the impact of PE addition on PP forcomposition adjustment of blends used in a commercialrecycling unit in Almirante Tamandare /PR, Materials and methodsPolypropylene (H301-Braskem) and low density poly-ethylene (BC 818-Braskem) were used. The specific gravityof the PP is and that of the LDPE is g/cm3, withmelt flow index of and g/10 min, PP, pure LDPE and their blends were processed inan injection-molding machine with various PP/LDPE weight contents, namely 100/0, 75/25, 50/50, 25/75 and0/100.

8 These blends are called PP100, PP75, PP50, PP25and PP0, respectively, throughout the text, and each of themwas processed at several injection temperatures (170, 180,190 and 2008C).For the evaluation of the blend mechanical Properties , Tensile tests were performed on an EMIC universal testingmachine (Model DL10000), in general accordance withASTM D638. Data for yield strength, elastic modulus andelongation at break were obtained in tests carried out at acrosshead speed of 5 mm/min. For low elongations, anEMIC extensometer having a gage length of 25 mm wasused. impact tests were performed on a PANTEC equipment(model PW-4), in general accordance with ASTM 10 and 20 measurements were taken for eachexperimental condition, and the reported results include themean values and their standard statistical analysis of variance of Tensile and impactresults has been carried out using commercial software.

9 Aone-way ANOVA and a series of Tukey HSD post hoc wereused to check for statistical difference among groups (forp! ).3. Results and Tensile testsThe stress strain curves for the various blends injected at170, 180, 190 and 2008C are shown inFigs. 1 4,respectively. The 170 and 2008C curves for the PP0 (pureLDPE) were not included since there were flow difficultiesrelating to the high viscosity of the melt for the former andsevere degradation for the latter, both resulting in non-homogeneous test of these figures shows the importance ofcontrolling the injection temperature, for example,elongation at break for pure polypropylene (PP100)decreases from 650% to less than 10%, when thetemperature reaches 1908C, indicating severe fact,Figs.

10 3 and 4demonstrate PP degradation withsevere changes on curve profiles and on yielding. Therefore,the retrieved data for the 190 and 2008C injectiontemperature should not be directly compared to dataobtained for the other injection of these figures also shows for the PP50 avery distinct behavior . Yielding is not seen for any injectiontemperature, and very low yield strength and elongation atbreak are found, in comparison to the other 1. Stress strain curves for the different blends injected Strapasson et al. / Polymer Testing 24 (2005) 468 473469 Surprisingly, further increase in the LDPE content causes arecovery of elongation at break yielding for the lowertemperatures.