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Materials Research

Print version ISSN 1516-1439On-line version ISSN 1980-5373

Mat. Res. vol.22  supl.1 São Carlos  2019  Epub Sep 05, 2019

http://dx.doi.org/10.1590/1980-5373-mr-2018-0850 

Articles

Properties and Morphology of Polypropylene/Big Bags Compounds

Eduardo da Silva Barbosa Ferreiraa  * 
http://orcid.org/0000-0002-2670-2794

Caio Henrique do Ó Pereiraa 
http://orcid.org/0000-0003-3235-8501

Edcleide Maria Araújoa 
http://orcid.org/0000-0003-4906-864X

Elieber Barros Bezerraa 
http://orcid.org/0000-0003-1637-7761

Danilo Diniz Siqueiraa 
http://orcid.org/0000-0002-3533-513X

Renate Maria Ramos Wellenb 
http://orcid.org/0000-0002-3565-7366

aDepartamento de Engenharia de Materiais, Universidade Federal de Campina Grande, Campina Grande, PB, Brasil

bDepartamento de Engenharia de Materiais, Universidade Federal da Paraíba, João Pessoa, PB, Brasil


ABSTRACT

In this work, compounds based on two grades of polypropylene (PP), i.e., PP H503 or PP H103 with residues of Big bag fabrics (RBB) were processed in a corrotational twin screw extruder with RBB content ranging from 10% to 50% of the weight. Then, their thermal properties were investigated through Differential Scanning Calorimetry (DSC) and Thermogravimetry (TG); mechanical by tensile and impact; thermomechanic by heat deflection temperature (HDT); and their morphology by Scanning Electron Microscopy (SEM). With the aim to add maximum RBB content without considerable damage to the properties of neat PPs, it was observed that addition of 10%, 30% and 50% of RBB did not significantly interfere in the PP matrices, being the compound PP + 30% RBB which one presented more successful properties, leading to reused compounds with higher performance and lower costs at same time.

Keywords: Polypropylene; big bags; recycling

1. Introduction

Currently due to the quick increase in the use of polymeric products, the Statista in 2016 shows the worldwide polypropylene (PP) production was 73.8 million tons, producing a large amount of post-consumer waste, and according to Sustainable Businesses only a small proportion of this plastic waste is recycled or reused, while a large amount of it is disposed in landfills causing serious environmental damage1,2. This serious scenario has worried the society and specifically the scientific community who has researched solutions for reusing and recycling these materials once they result in great problems mainly when improperly discarded in the environment and consequently, become a harmful material to the ecological system3,4. Consequently, there is an urgent requirement to develop “new products” made from residues i.e., with the insertion of post-consumer resins, in order to minimize the use of the virgin polymer, reducing costs and the disposal of materials5-9.

Among post-consumer products, there are big bags made with PP, which are used to pack several products in various sectors of the production chain as bagging seeds for warehouses, packaging fertilizers, bagging inputs, among other applications. Its main feature is strength and durability compared to other packaging. Polypropylene was introduced in the market in 1954, nowadays it is one of the most important thermoplastic, representing one of the bestselling in the world10-12.

Even with its wide applications, traditional polymeric resins, such as PP, have negative characteristics, such as their slow decomposition rate after disposal; along with low density the high consumption and the daily disposal end up generating high volumes of waste that accumulate for long periods, reducing the useful life of landfills13. There is an estimative that 80% of the solid waste from sanitary landfills in Brazil consists of thermoplastics, of which PP represents 10%14,15.

The development of recycled plastic researches has allowed to discover quite surprising characteristics and innovations, associated to the advantages of material recovering, mostly processing polymer compounds. Researches carried out the production and application of polymer compounds based on PP and recycled materials13,16,17, presented good properties and the feasibility of PP recycling. However, although the subject is being explored, there is still a gap with works investigating uses of recycled PP from big bags in the development of new polymer compounds and products.

The aim of this work was to investigate compounds made with virgin PP upon addition of big bags (RBB) with contents ranging from 10% to 50% of the weight. These compounds were analyzed by thermal, mechanical, thermomechanical properties and the morphology, aiming to obtain the best content of RBB to be added in the developed compounds.

2. Methodology

2.1 Materials

Polypropylene H 103 with density of 0.905 g/cm3, MFR = 40 g/10 min (230 °C/2.16 kg) and polypropylene H 503 with density of 0.905 g/cm3, MFR = 3.5 g/10 min (230 °C/2.16 kg), both manufactured by Braskem. Big bag residue (RBB), supplied by a company located in Campina Grande - PB.

2.2 Preparation of RBB

To obtain a suitable granulometry to be fed into the extruder, a master made from big bag fabrics was produced in a MH-Equipment MH-50H high-speed homogenizer using 25g of material, which was homogenized for approximately 5 seconds (the camera temperature was checked being approximately 140 ºC), afterwards the output from the homogenizer was freely cooled to room temperature (~ 23 °C) and ground into a knife mill to obtain the granule shape.

2.3 Extrusion of Compounds

PP compounds were obtained using a modular interpenetrating corrotational twin screw extruder with L/D ratio of 40, model ZSK 18 mm, Werner-Pfleiderer of Coperion, temperature profile used ranged between 185 - 195 ° C, screw and feed rate were 250 rpm and 4 kg/h, respectively. After extrusion the output was granulated and dried in a vacuum oven at 80 °C for 24 hours. The drying stage was executed to make sure no moisture would be present in the pellets, once after extrusion, the material goes through a cooling bath with circulating water, before being pelleted. Table 1 shows extruded compounds in weight ratios (%).

Table 1. Compounds processed in this work. 

COMPOUNDS PP H103 (%) PP H503 (%) RBB (%)
PP H103 100 - -
PP H503 - 100 -
PP H103/RBB 90 - 10
PP H103/RBB 70 - 30
PP H103/RBB 50 - 50
PP H503/RBB - 90 10
PP H503/RBB - 70 30
PP H503/RBB - 50 50

Injected specimens for tensile, impact and HDT were produced according to ASTM D638, D256 (Type I) and D648, respectively, using an Arburg Injector, Model Allrounder 207C Golden Edition, operating at 180 °C in zone 1, 190 °C in zones 2 and 3, 210 °C in zone 4, the mold was kept at 20 °C.

2.4 Characterizations

To identify added fillings to RBB, it was calcinated in a muffle (model EDG3P-S) from EDG Equipment. Approximately 58g of material was heated from room temperature (23ºC) to 500ºC at a heating rate of 5ºC/min. The sample was kept at 500ºC for 120 minutes afterwards the muffle was turned off and allowed to cool down to room temperature. The residual powder was weighted after 24 hours and analyzed by X-ray Fluorescence. It was also submitted to chemical analysis applying the semi quantitative method, under nitrogen atmosphere. From residual powder, pressed samples were produced with 10 mm diameter.

Thermogravimetry (TG) analyzes were performed in a TGA 51H Shimadzu, using 5 ± 0.5 mg of sample. The heating rate was 10 °C/min, from room temperature (23ºC) to 500 °C, under nitrogen atmosphere with gas flow rate of 100 mL/min.

Differential scanning calorimetry (DSC) analyzes were performed in a Shimadzu DSC-50, samples were heated from room temperature (23ºC) to 300 ºC, and then cooled down to room temperature at a heating/cooling rate of 10 ºC/min, under nitrogen atmosphere with gas flow rate of 50 mL/min.

The tensile tests carried out according to ASTM D 638, Elastic Modulus, Tensile Strength and Elongation were measured. The tests were performed in a universal EMIC model DL10000, using a load cell of 200 kgf, with deformation rate of 50 mm/min, operating at room temperature. Presented results are an average of 10 tests.

The IZOD impact strength tests were performed on notched specimens, using a Resil 5.5 from Ceast and a pendulum of 2.75 J, according to ASTM D 256, at room temperature. Presented results are an average of 10 tests.

Heat deflection temperature (HDT) was obtained according to ASTM D 648, in a Ceast HDT 6 VICAT/N 6921.000, with a tension of 455 kPa, heating rate of 120 °C/h (method A). The temperature was determined after the sample having deflected 0.25 mm. Presented results are an average of 3 tests.

Scanning electron microscopy (SEM) images were captured from injected fractured surface using a Tescan Veja 3 at a voltage of 30 kV under high vacuum. Specimens were gold covered using a sputtering - Metallic Shimadzu - IC 50, using a current of 4 mA for 2 minutes.

3. Results and Discussion

3.1 Calcination and X-Ray Fluorescence - FRX

From X-Ray Fluorescence and chemical analysis, the present oxides in PP compounds were identified as shown in Table 2. Calcium oxide (CaO) was identified in the highest content, approximately 88%, followed by titanium dioxide (TiO2) with 4.1% and magnesium oxide (MgO), around 3.7%.

Table 2. Chemical analysis performed by X-ray fluorescence of RBB

OXIDES CONTENT (%)
Cao 87.81
TiO2 4.13
MgO 3.69
SiO2 1.15
ZnO 1.06
SO3 0.93
Al2O3 0.83
K2O 0.28
SrO 0.06
CuO 0.04
ZrO2 0.02

These three oxides represent more than 95% of the mineral fillers added to virgin PP during big bags processing to obtain the fabrics of big bags. These additives may act as pigments providing white color (CaO), ultraviolet resistance (anti-uv) (TiO2), nucleation, among other properties, probably they behave as external agents during recycling of RBB implying in changes in virgin PP18-19.

3.2 Thermogravimetry (TG)

The thermal stability of PP H103, PP H503, RBB, and its compounds was investigated by TG, Figures 1 and 2 display TG plots.

Figure 1 TG curves of PP H103, PP H503 and RBB

Figure 2 TG curves of PP H103 (a) and PP H503 (b) compounds with 10, 30 and 50% of RBB

From TG plots in Figure 1 is verified that PP H103, PP H503 and RBB have distinct characteristics. Decomposition temperatures are possibly associated to the difference in the viscosity between PPs, where PP H503 being more viscous has a higher molecular weight and consequently greater macromolecular entanglements, leading to a higher energy consumption to break the chains, therefore to higher degradation/decomposition temperatures as already observed by Rjeb et al. (2005)20. The plot for RBB displays weight loss in higher temperature than those observed for PP H103 and PP H503 what can be attributed to the mineral fillers/additives as evidenced by FRX analysis18-19.

Figure 2 presents TG plots of PP compounds with 10, 30 and 50% of RBB. It can be verified that upon addition of 30% RBB the thermal stability of both PPs significantly increased, it was even higher than those observed for compounds with 50% RBB, probably resulted due to an overhead effect of the added mineral fillers18-19.

3.3 Differential Scanning Calorimetry (DSC)

Figure 3 presents DSC scans of PP H103, PP H503 and RBB, from these scans, melting and crystallization temperatures (Tm and Tc), melting and crystallization enthalpies (ΔHm and ΔHc), and degree of crystallinity (Xc) were evaluated and are presented in Table 3.

Figure 3 DSC curves of PP H103, PP H503 and RBB acquired during heating (a) and cooling (b) at 10ºC/min. 

Table 3. Melting and crystallinity parameters of PP H103, PP H503 and RBB

COMPOUNDS MELTING MELT CRYSTALLIZATION
Tm (°C) Xc (%) ΔHm (J/g) Tc (°C) ΔHc (J/g)
PP H103 167 57 78.58 120 101.07
PP H503 166 63 87.42 117 100.6
RBB 173 35 45.24 120 44.99
PP H103+ 10%RBB 166 60 82.64 120 102.4
PP H103 + 30%RBB 167 57 79.09 118 97.9
PP H103 + 50%RBB 165 54 74.65 117 94.21
PP H503+ 10%RBB 166 68 94.39 117 107.7
PP H503 + 30%RBB 165 61 84.28 118 101.5
PP H503 + 50%RBB 166 56 77.49 117 96.51

Tm = melting peak temperature; ΔHm = melting enthalpy; Xc = degree of crystallinity, measured as Xc = ΔHm/Δmº; Tc = Crystallization peak temperature; ΔHc = crystallization enthalpy; ΔHmº = equilibrium melting enthalpy assuming PP crystal 100% crystalline, 138 J/g. Libano et al. (2011)23.

Melting endotherms related to fusion of crystals are observed during heating, in Figure 3a. The melting peak of RBB is displayed at relatively higher temperatures, possibly due to the formation of larger and/or more stable crystals due to the stretching during processing leading to a greater orientation or even due to the presence of mineral fillers as evidenced in Table 2 12,18,19.

Regarding the exotherms of melt crystallization, Figure 3b PPs and RBB presented single peaks with bell shape translating as crystallization without discontinuities. PP H503 presented Tc in 117 °C slightly lower than PP H103 (120 ºC) and RBB (120 ºC), most likely due to the higher viscosity leading to an entanglement state lowering the macromolecular packaging, as was also verified during melting.

PP H103 and RBB have higher Tc, which may be associated to smaller polymer chains. PP H103 has higher MFR (40 g/10 min), this fact may improve crystallization (nucleation and growth), as already reported by Oliveira et al. (2012)21. In addition, the presence of impurities can contribute to a higher crystallization rates, they may act as activation centers for new crystals22.

Concerning the degree of crystallinity as shown in Table 3 samples presented close values, being PP H503 subtly less crystalline, probably due to higher molecular weight, making difficult the macromolecular packaging and thus the crystallization.

Figures 4 and 5 present DSC scans for PP compounds with 10%, 30% and 50% of RBB. Both crystallization and melting are displayed as bell shape, taking place as processes without discontinuities. Interesting characteristic is verified where even upon addition of 50% of RBB the thermal properties are preserved. In Table 3, thermal parameters are presented, evidencing similar data between neat and compounded PP.

Figure 4 DSC curves of PP H103 compounds acquired during heating (a) and cooling (b) at 10ºC/min. 

Figure 5 DSC curves of PP H503 compounds acquired during heating (a) and cooling (b) at 10ºC/min. 

3.4 Mechanical Properties

Mechanical properties of neat PP H103 and PP H503 and their compounds with RBB in contents of 10%, 30% and 50% are presented in Table 4. Related to Elastic Modulus data observed for PP H103 it is higher than for P H503 providing higher stiffness and lower flexibility24,25. Upon addition of RBB, compounds presented Elastic Modulus between 10% and 20% higher than neat PPs, presence of mineral fillers as the main contribution for this. Regarding Tensile Strength, investigated compounds display similar values, while PP H503 presented higher Impact Strength, on average data were 30% higher than compounds with PP H103. In general, addition of RBB leaded to an improvement of impact, possibly mineral fillers provide better energy absorption18,19,11. The results obtained with tensile and impact experiments give evidence that addition of RBB to neat PP provide compounds with higher performance at lower cost.

Table 4. Mechanical properties of PP H103, PP H503 and PP/RBB systems. 

COMPOUNDS ELASTIC MODULUS (MPa) TENSILE STRENGTH (MPa) IMPACT STRENGTH (J/m)
PP H103 602.6 ± 17.8 33.2 ± 0.6 21.64 ± 1.3
PP H503 578.4 ± 10.7 34.5 ± 0.2 31.29 ± 2.5
PP H103 + 10% RBB 610.3 ± 3.5 33.2 ± 0.4 21.97 ± 1.3
PP H103 + 30% RBB 681.0 ± 6.1 33.2 ± 0.3 24.21 ± 2.8
PP H103 + 50% RBB 659.8 ± 7.6 33.1 ± 0.2 23.08 ± 3.3
PP H503 + 10% RBB 691.7 ± 11.4 33.3 ± 0.4 32.24 ± 1.4
PP H503 + 30% RBB 648.9 ± 31.0 32.9 ± 0.5 31.69 ± 2.9
PP H503 + 50% RBB 677.5 ± 18.4 33.0 ± 0.5 29.98 ± 2.3

3.5 Heat Deflection Temperature (HDT)

In order to evaluate the thermomechanical behavior of PP H103 and PP H503 their compounds with 10%, 30% and 50% of RBB, were submitted to HDT test and data are shown in Table 5. For PP H103, it can be observed, despite addition of RBB, HDT data kept invariable, at approximately 93 °C.

Table 5. HDT of PP H103, PP H503 and PP/RBB compounds. 

COMPOUNDS HDT (ºC)
PP H103 93.37 ± 1.1
PP H503 89.87 ± 0.5
PP H103 + 10% RBB 93.37 ± 0.7
PP H103 + 30% RBB 93.80 ± 1.7
PP H103 + 50% RBB 93.63 ± 1.3
PP H503 + 10% RBB 95.70 ± 1.1
PP H503 + 30% RBB 98.63 ± 1.7
PP H503 + 50% RBB 97.23 ± 0.1

For PP H503, compounds with RBB presented higher HDT, being the highest PP H503 + 30% RBB, this increase may be associated to an increase in the Elastic Modulus as above observed leading to higher energy requirement to change shape, what can be translated in materials with longer life cycles and better performance.

According to McCaffrey et al. (2018)26, HDT is the temperature in which a pronounced decrease in mechanical properties is verified, i.e., higher HDT greater thermal resistance. Therefore, as above presented RBB leaded to better thermal resistance.

3.6 Scanning Electron Microscopy (SEM)

Figures 6 and 7 present SEM images of PP H103 and PP H503, and compounds with 10%, 30% and 50% of RBB. Disperse RBB aggregates are observed in PP matrices, however segregation is absent, what could conduct to lower mechanical performance. According to these images is verified both neat PPs and compounds presented ductile fracture with rough fracture surfaces and energy absorption mechanisms working properly agreeing with Impact Strength analyses behavior.

Figure 6 SEM images of (a) PP H103, (b) PP H103 + 10% RBB, (c) PP H103 + 30% RBB and (d) PP H103 + 50% RBB

Figure 7 SEM images of (a) PP H503, (b) PP H503 + 10% RBB, (c) PP H503 + 30% RBB and (d) PP H503 + 50% RBB

4. Conclusions

Polypropylene compounds with big bag residue (RBB) were melt extruded, afterwards their thermal, mechanical; thermomechanical and morphological properties were investigated. From the results, it was verified RBB recycling is economically viable, saving raw material, i.e., neat PP and energy. PPs morphological character was kept after adding RBB in contents up to 50% of the weight and higher thermal stability was observed in the compounds. There was also an increase in the Elastic Modulus with no significant change in tensile and impact strengths. Summing up, compounds with 30% of RBB present satisfactory properties and are economically viable, and can be used for sealing caps; toys; food packaging for food and cosmetics, products for general use, electronic devices.

5. Acknowledgement

The authors thank Labmat (Laboratory of Materials Engineering/CCT/UFCG) for tensile, impact strength, thermogravimetry and differential scanning calorimetry experiments, CNPq, MCTIC/CNPq and CAPES/PNPD for the financial support.

6. References

1 Leblanc R. An Overview of Polypropylene Recycling. New York: Sustainable Business; 2018. Available from: <https://www.thebalancesmb.com/an-overview-of-polypropylene-recycling-2877863>. Access in: 26/04/2019. [ Links ]

2 Statista. Production of polypropylene worldwide in 2016 by region (in million tons). New York: Statista: The Statistics Portal; 2018. Available from: <https://www.statista.com/statistics/732167/distribution-of-polypropylene-consumption-worldwide-by-region/>. Access in: 26/04/2019. [ Links ]

3 de Faria PC, Wisbeck E, Dias LP. Biodegradação de polipropileno reciclado (ppr) e de poli (tereftalado de etileno) reciclado (petr) por Pleurotus ostreatus. Matéria (Rio de Janeiro). 2015;20(2):452-459. [ Links ]

4 França DC, Morais DD, Bezerra EB, Araújo EM, Wellen RMR. Photodegradation Mechanisms on Poly (ε-caprolactone)(PCL). Materials Research. 2018;21(5):e20170837. [ Links ]

5 Nunes SG, da Silva LV, Amico SC, Viana JD, Amado FDR. Study of Composites Produced with Recovered Polypropylene and Piassava Fiber. Materials Research. 2017;20(1):144-150. [ Links ]

6 Forlin FJ, Faria JAF. Considerações Sobre a Reciclagem de Embalagens Plásticas. Polímeros. 2002;12(1):1-10. [ Links ]

7 Battistelle R, Viola NM, Bezerra BS, Vilarelli ID. Caracterização física e mecânica de um compósito de polipropileno reciclado e farinha de madeira sem aditivos. Matéria (Rio de Janeiro). 2014;19(1):7-15. [ Links ]

8 Ignatyev IA, Thielemans W, Beke B. Recycling of polymers: A review. ChemSusChem. 2014;7(6):1579-1593. [ Links ]

9 Araújo LMG, Morales AR. Compatibilization of recycled polypropylene and recycled poly (ethylene terephthalate) blends with SEBS-g-MA. Polímeros. 2018;28(1):84-91. [ Links ]

10 Martins AF, Suarez JCM, Mano EB. Produtos poliolefínicos reciclados com desempenho superior aos materiais virgens correspondentes. Polímeros. 1999;9(4):27-32. [ Links ]

11 Machado JCV. Reologia e Escoamento de Fluidos: Ênfase na Indústria de Petróleo. Rio de Janeiro: Editora Interciencia; 2002. [ Links ]

12 Holzschuh GG. Controle de Qualidade na Indústria de Ráfia Padronização e Otimização dos Processos. [Dissertation]. Santa Cruz do Sul: University of Santa Cruz do Sul; 2009. [ Links ]

13 Fernandes BL, Domingues AJ. Caracterização mecânica de polipropileno reciclado para a indústria automotiva. Polímeros. 2007;17(2):85-87. [ Links ]

14 Rodrigues A, Carvalho BM, Pinheiro LA, Bretas RES, Canevarolo SV, Marini J. Effect of compatibilization and reprocessing on the isothermal crystallization kinetics of polypropylene/wood flour composites. Polímeros. 2013;23(3):312-319. [ Links ]

15 Silva EA, Moita Neto JM. Possibilidades de melhorias ambientais no processo de reciclagem do polietileno. Polímeros. 2016;26(n.esp):49-54. [ Links ]

16 Santos LS, Silva AHMFT, Pacheco EBAV, Silva ALN. Estudo do efeito da adição de PP reciclado nas propriedades mecânicas e de escoamento de misturas de PP/EPDM. Polímeros. 2013;23(3):389-394. [ Links ]

17 Matei E, Râpă M, Andras ÁA, Predescu AM, Pantilimon C, Pica A, et al. Recycled Polypropylene Improved with Thermoplastic Elastomers. International Journal of Polymer Science. 2017;2017(1):7525923. [ Links ]

18 Wypych G. Handbook of Fillers. Toronto: ChemTec Publishing; 2016. [ Links ]

19 Yu M, Huang R, He C, Wu Q, Zhao X. Hybrid Composites from Wheat Straw, Inorganic Filler, and Recycled Polypropylene: Morphology and Mechanical and Thermal Expansion Performance. International Journal of Polymer Science. 2016;2016(1):2520670. [ Links ]

20 Rjeb M, Labzour A, Rjeb A, Sayouri S, Claire Y, Périchaud A. TG and DSC studies of natural and artificial aging of polypropylene. Physica A: Statistical Mechanics and its Applications. 2005;358(1):212-217. [ Links ]

21 Oliveira RVB, Ferreira CI, Peixoto LJF, Bianchi O, Silva PA, Demori R, et al. Mistura polipropileno/poliestireno: um exemplo da relação processamento-estrutura-propriedade no ensino de polímeros. Polímeros. 2012;23(1):91-96. [ Links ]

22 Canevarolo Jr SV, coord. Técnicas de Caracterização de Polímeros. São Paulo: Artliber; 2003. [ Links ]

23 Líbano EVDG, Visconte LLY, Pacheco EBAV. Propriedades Térmicas de Compósitos de Polipropileno e Bentonita Organofílica. Polímeros. 2012;22(5):430-435. [ Links ]

24 Agrawal P, Oliveira SI, Araújo EM, Melo TJA. Effect of different polypropylenes and compatibilizers on the rheological, mechanical and morphological properties of nylon 6/PP blends. Journal of Materials Science. 2007;42(13):5007-5012. [ Links ]

25 La Mantia FP. Mechanical properties of recycled polymers. Macromolecular Symposia. 1999;147(1):167-172. [ Links ]

26 McCaffrey Z, Torres L, Flynn S, Cao T, Chiou BS, Klamczynski A, et al. Recycled polypropylene-polyethylene torrefied almond shell biocomposites. Industrial Crops and Products. 2018;125:425-432. [ Links ]

Received: December 08, 2018; Revised: April 30, 2019; Accepted: June 23, 2019

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