IMPACT PROPERTIES OF POLYMERIC NANOCOMPOSITES WITH DIFFERENT SHAPE OF NANOPARTICLES. Robert VALEK a, Jaroslav HELL a

Transcription

1 IMPACT PROPERTIES OF POLYMERIC NANOCOMPOSITES WITH DIFFERENT SHAPE OF NANOPARTICLES Robert VALEK a, Jaroslav HELL a a SVUM, a. s., Podnikatelska 565, Prague, Czech Republic, valek@svum.cz Abstract Results of a microstructure assessment and impact testing of thermoplastic nanocomposites are presented in the paper. Experiments were focused on influence of different structure parameters of fillers on toughness of thermoplastics composites. Mechanical properties of nanocomposites were compared with neat polymeric matrix in all cases. Montmorillonite, halloysite, carbon nanotubes and precipitated calcium carbonate were used to reinforce polymeric matrix. The above-mentioned four reinforcing materials are possible to sort to three groups in accordance with shape: nanoplatelets (montmorillonite MMT), nanofibres (halloysite nanotubes HNT, carbon nanotubes CNT) and isometric nanoparticles (calcium carbonate). Determination of impact behaviour was conducted at Charpy impact tester and instrumented falling-weight machine. Results show a decrease of toughness of PA6/MMT, PA6/HNT and PA6/CNT nanocomposites in comparison with a neat PA6. Toughness of these materials strongly depends on moisture content. Nanofillers doesn t influence toughness of polypropylene copolymer in negative way. Influence of a reinforcing particles and adhesion between matrix-nanofiller to embrittlement of nanocomposite will be discussed. Keywords: nanocomposite, microstructure, impact properties 1. INTRODUCTION Material and structural parameters of nanofiller such as shape, size, aspect ratio, concentration etc play important role for properties of nanocomposites. Quality of interface between particles and matrix are no less important. Presence of rigid filler usually deteriorates strength and toughness of polymeric materials [1]. Quality dispersion of nanoparticles in matrix plays key role for an improvement of impact properties of nanocomposites. Dispersed systems are generally in thermodynamically non-equilibrium state. Surface free energy reduction is driving force for termination of dispersion state and aggregations growth (coagulation) [2]. Suitable treatment of particle surface can prevent coagulation of particles during compounding of the polymer composites [3]. Nanocomposites are possible to sort by geometric shape of reinforcement to: - Nanonoparticle-reinforced composite (CaCO 3, TiO 2, SiO 2 ) - Nanofibre-reinforced composite (carbon nanotubes - CNT, halloysite - HNT) - Nanoplateled-reinforced composite (organically modified clay, expanded graphite), Fig 1. Fig. 1 Geometric shape of reinforcement

2 2. RESULTS AND DISCUSSION 2.1 Materials Polypropylene homopolymer (PP-h), polypropylene copolymer (PP-c) and polyamide 6 were used as the matrices polymers. Precipitated calcium carbonate was used like representative of equiaxial nanofiller. Surface of CaCO 3 was organically modified. Details about treatment of particle surface are guarded in this time. Halloysite nanotubes (HNT) and carbon nanotubes (CNT) represent particles with fibrous shape. And organically modified clay montmorillonite (MMT) is typical representative of the layered filler. Summary of all materials used for matrices and nanofillers are presented in table below. Table 1 Matrix Tradename Producer PP-c PPC MFI 3 Polykemi AB PP-h Moplen HP 4 R LyondellBasell PA6 SCANAMID PA6 B14 Polykemi AB PA6 Ultramid B3K BASF Filler CaCO 3 Socal 31 Solvay Carbon nanotubes CNT Plasticyl PP 21 Nanocyl Halloysite - HNT Pleximer NaturalNano Montmorillonite - MMT Polykemi AB Testing specimens for all experiments were prepared by injection moulding technique on Battenfeld 75 CD machine. 2.2 Microstructure Microstructure of nanocomposite materials was investigated by transmission and scanning electron microscopy. Fig. 2 MMT nanoplatelets in PP-c matrix. Fig. 3 MMT nanoplatelets in PA6 matrix.

3 notched impact strength [kj/m 2 ] notched impact strength [kj/m 2 ] , Brno, Czech Republic, EU Fig. 4 PP-c + 7wt% CaCO 3 Fig. 5 PP-h + 1wt% CNT 2.3 Impact properties Charpy notched impact strength and puncture impact behaviour were measured on nanocomposites and neat matrices. Rectangular bars of 8 x 1 x 4 mm 3 were sawn from injection-molded sheets. The testing samples were taken parallel to melt flow direction. Notches were prepared by a Notchvis device of Ceast. Testing discs with diameter 8 mm and thickness 3 or 4 mm were cut off from injection-molded sheets for puncture impact tests. 2.4 Notched impact strength Generally, polypropylene homopolymer exhibits considerably brittle material (a cn = 1,9 kj/m 2 ) in comparison with polypropylene copolymer (a cn = 64 kj/m 2 ), Fig 6 and Fig 7. The results of the notched Charpy impact strength versus filler concentration of PP-h composites with different types of nanofiller are shown in Fig Content of HNT or CaCO 3 [wt%] Polypropylene - homopolymer CaCO 3 - treated HNT CNT 8 Polypropylene - copolymer Content of CNT [wt%] Fig. 6 Impact strength versus filler concentration. neat 4%MMT 5%HNT 7%CaCO 3 Fig. 7 Impact strength of PP-c and its nanocomposites.

4 notched impact strength a cn [kj/m 2 ] Load [N] , Brno, Czech Republic, EU The impact strength of composites on the base of PP-h appears as function of filler concentration. Welldispersed organically treated nanoparticles of CaCO 3 considerably increase impact strength of PP-h matrix. PP-h with 15wt% CaCO 3 shows about 58% higher impact strength in comparison with neat matrix. The increase of toughness may be due to microplastic deformation creating around the CaCO 3 particles. The filling of PP-h matrix by halloysite nanotubes has got positive effect to toughness too. The increase of the toughness isn t so considerable like in case of CaCO 3 nanoparticles. Composite on the base PP-h filled by carbon nanotubes showed the decrease of notched impact strength in comparison with neat matrix. Reason of unfavourable impact properties is worse dispergation of CNZ in PP-h. Microscopic investigation detected absolutely unsuitable microstructure; nanocomposite has got heterogenic structure from macroscopic point of view. Large pieces of unprocessed masterbatch were observed in PP-h matrix by optical microscopy, Fig 5. Neat PP-c matrix represents materials with high Charpy impact strength. Notch impact strength of neat matrix and nanocomposites is compared on Fig 7., content of nanoparticles was in all three composites similar about 5 wt%. Results clearly show, nanoparticles with high aspect ratio have got unfavourlable influence on notch impact strength of composites on the base of the PP-c. It is commonly known, that very fine spherulitic structure has positive effect to toughness of polymers. Despite strong nucleating effect of nanoparticles, PP-c/MMT and PP-c/HNT have got lower toughness than neat PP-c with coarse spherulites. This finding indicates that mechanism of embrittlement of PP-c/MMT and PP-c/HNT may be caused by a concentration of stress in matrix close to nanoparticle with high aspect ratio, Fig 2. Moreover immobilisation of polymer chains by nanoplatelets in this type of nanocomposite forbids growth of plastic deformation, which absorbs fracture energy. These high aspect ratio high modulus nanoparticles don t cause by mechanism overbridge of crack or this mechanism is marginal during crack growth. Rigid nanoparticles CaCO 3 with aspect ratio close to 1 increase notch impact strength of PP-c matrix. Important condition for the increasing toughness is qualitative distribution of CaCO 3 in PP-c matrix, without creation of coarse agglomerates of nanoparticles, Fig PA6 PA6/MMT PA6/5%HNT PA6/1%CNT 6 4 PP-c PP-c + 7wt% CaCO 3 (treated) PP-c + MMT (3 mm) PP-c + 5wt% HNT (3 mm) 2 2 m = 2 kg v = 4.4 m/s moisture content [%] Deflection [m] Fig. 8 Charpy notched impact strength of PA6 and its nanocomposites vs. moisture content. Fig. 9 Puncture impact behavior of PP-c and its nanocomposites.

5 Charpy notched impact experiment was conducted on neat PA6 and PA6 composites filled by MMT (microstructure on Fig 3.), HNT and CNT. Testing samples were conditioned to a few levels of moisture content before testing. Dependence of Charpy notched impact strength on moisture content is shown in Fig 8. Dry PA6 and dry PA6 with MMT, HNT and CNT are brittle and their values of notch toughness are almost similar. Values of notch impact strength a cn are rising with the increasing of humidity, but for PA6 is enhancement much higher then for nanocomposites. Nanoparticles are decreasing rate of moisture absorption as well as decreasing their impact toughness. Results indicate, that nanoplatelets and nanofibres don t cause to overbridge of crack, but due to their aspect ratio cause like stress concentrator. Moreover MMT nanoplatelets in water condition are swelling; therefore enlarge their size. These size rising induce strain in matrix. Effect of nanoparticles is become negative. Therefore notch impact strength of PA6 nanocomposite after wetting is not increasing so rapidly then for neat PA6. Results of instrumented puncture impact testing of neat PP-c matrix and composites with treated calcium carbonate, MMT and HNT clearly show, that presence of a nanofiller with different shape, aspect ratio and size haven t noticeable influence to change of toughness, Fig. 9. Interaction between inorganic nanoparticles and polypropylene copolymer matrix does not cause embrittlement of composites. Toughness of these composites is completely determined by properties of neat matrix. Interpretation of this phenomenon can be non-polarity of PP-c matrix, thus poor interaction on interface matrix-nanoparticle. Weak interaction matrixfiller create better condition for development of plastic deformation during impact action. CONCLUSIONS A few types of nanocomposites with different types of matrices and nanofillers were fabricated and characterised by electron and light microscopy. Polypropylene copolymer and polypropylene homopolymer were chosen like typical representative of non-polar matrices. Polyamide 6 represented matrix with polar character of macromolecular chains. Well-dispersed nanoparticles of CaCO 3 and HNT up to the content about 15wt% increase Charpy impact strength of PP-h matrix. Only inorganic particle with aspect ratio 1 improve Charpy impact strength of PP-c. Under multi-axial impact loading, nanoparticles with different shape do not affect the toughness of polypropylene copolymer negatively. Preparation of nanocomposites with CNT is problematic even at very low degree of filling. Impact properties nanocomposites with CNT ar negatively influenced by inapt processing. Significant drawback of PA6 nanocomposites is their unsuitable impact properties. An embrittlement of PA6 nanocomposite filled by particles with high aspect ratio represents a limiting factor for wider application of this perspective material. ACKNOWLEDGEMENTS This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic, project no. OC936. LITERATURE [1.] ROTHON R. N.: Particulate-filled polymer composites, Rapra Technology Ltd., UK, 23 [2.] SHCHUKIN E. D. et al: Colloid and Surface Chemistry, Elsevier, Netherlands, 21 [3.] WANG Y. et al: Interfacial interactions in calcium carbonate polypropylene composites. 2: Effect of compounding on the dispersion and the impact properties of surface-modified composites, Polymer Composite, Vol. 25 (24), p. 451