CREEP AND IMPACT PROPERTIES OF PA6 WITH MONTMORILLONITE AND HALLOYISITE NANOPARTICLES. Robert VÁLEK a, Jaroslav HELL a

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1 CREEP AND IMPACT PRPERTIES F PA6 WITH MNTMRILLNITE AND HALLYISITE NANPARTICLES Robert VÁLEK a, Jaroslav HELL a a SVÚM, a. s., Podnikatelská 565, Praha 9 - Běchovice, Czech Republic, valek@svum.cz Abstract Results of a microstructure assessment, creep and impact testing of polyamide 6 nanocomposites are presented in the paper. Polyamide 6 matrix was reinforced by particles on base of a layered montmorillonite (MMT) and halloysite nanotubes (HNT). Experiments were focused on influence of different structure parameters of fillers on stiffness and toughness of thermoplastics composites. Mechanical properties of nanocomposites were compared with neat PA6 matrix in all cases. Microstructural investigations were performed using transmission electron microscopy (TEM) and wide angle x-ray scattering (WAXS). Experiments indicate relatively good dispersion of nanoplatelets of MMT in polyamide matrix. Results of experiments prove increasing of stiffness and strength of PA6 matrix reinforced by all types of applied fillers. Uniaxial tension creep tests under constant load at different stress levels were conducted. Results of creep tests clearly demonstrate, that a small amount of MMT and HNT prevents from growing creep deformation. Rigid nanoplatelets create barrier for conformation changes and motion of polymer chains. Determination of puncture impact behaviour was conducted at instrumented falling-weight machine. Results show a decrease of toughness of PA6/MMT and PA6/HNT nanocomposites in comparison with a neat PA6. Immobilisation of polymer chains by nanoparticles in those types of nanocomposites forbids growth of plastic deformation, which absorbs fracture energy. Toughness of the both materials strongly depends on moisture content. Influence of reinforcing particles and matrix-nanofiller adhesion to an embrittlement of nanocomposite will be discussed. Keywords: nanocomposite, microstructure, creep, impact properties 1. INTRDUCTIN Nanocomposite properties are notably influenced by size and shape of particles, dispergation quality of nanoparticles in polymer matrix and effectiveness of bonding between nanoparticles and matrix. Large contact surface between nanoparticle and polymer matrix causes effective immobilisation of polymer chains. Consequence is increase of modulus, stiffness, yielding point, hardness, toughness, mould shrinkage etc in comparison with neat polymer [1]. In addition physical properties such as thermal and electric conductivity, barrier properties and surface quality are better. Those positive changes are often linked with a decrease of toughness. Immobilisation of polymer chains by nanoplatelets forbids growth of plastic deformation, which absorbs fracture energy [2]. Moreover, in case of polyamide reinforced by nanoclay gets to transformation failure mechanism from shear yielding at neat PA6 to crazing [3]. 2. MATERIALS Experiments were realised on commercially available nanocomposite SCANCMP PA6 B140 N6, its thermoplastics matrix is PA6, which corresponds with commercial type SCANCMP PA6 B140. Both materials were produced by Polykemi AB. Filler is created by montmorillonite intercalated by quarter ammonium salt. Nanocomposite contains additives for better intercalation and exfoliation of clay and also small amount of wax for easier processing too. The producer quotes contents of nanofiler in range 3 5

2 wt%, this data correspond to contents determined by burning (3.6 wt%). Next studied type of nanofiller was halloysite, which occurs in the form of hollow tubules (halloysite nano tubes HNT) with diameter nm and length 0.5 to 10 microns. This mineral is main part of clay soil [4]. Halloysit was acquired in the form masterbatch with trade name Pleximer N TM, produced by NaturalNano. Pleximer N is consisted of 30 wt% HNT in polyamide Ultramid B3K (BASF). This blend can be mixed with additional matrix directly in an injection moulding machine. Masterbach was mixed with matrix polymer SCANAMID PA6 B140 and composite with 5 wt% HNT was prepared. Testing samples were injection moulded using a Battenfeld 750 CD injection moulding machine. Basic mechanical properties are presented in table below. Material σ y [MPa] E t [MPa] PA PA6/MMT PA6/HNT RESULTS AND DISCUSIN 3.1 Microstructure Microstructure of tested materials was studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). TEM samples were prepared by ultramicrotome from injection moulded testing samples. Plane of cross-section was perpendicular to melt flow direction during injection moulding. Majority of nanotubes is oriented parallel to direction of melt flow, for that reason HNT has elliptic shape on crosssection, Fig 1. TEM analysis proves very good dispergation and exfoliation of montmorillonite nanoplatelets in polyamide matrix, Fig 2. Length of particle was measured with aid of digital image analysis software ACC 5.0. Distribution of length particles corresponds to log-normal distribution, the modus is 32 nm, Fig ,4 nm relative frequency length of particles [nm] Fig 1. PA6/HNT Fig 2. PA6/MMT Fig 3. Distribution of particle length TEM provides informations about microstructure only from limited area, in the order of units of µm 2. Scanning electron microscopy was used for microstructure study in larger volume of materials. Testing samples with cross-section 4 x 10 mm were broken in liquid nitrogen and fracture surface was observed and documented

3 by SEM. Results showed on existence of coarse aluminosilicate clusters, often larger than 10 µm, which could act like stress concentrators. This fact was proved at both tested materials, Figs. 4 and 5. Quality dispersion and exfoliation of aluminosilicate in polymer matrix is still problematic process, even in commercial available nanocomposites. Fig 4. PP/MMT Fig 5. PP/HNT 3.2 Creep tests Constant loading creep tests in uniaxial tension configuration were performed on creep machines own construction. Sample elongation was observed like change of distance between clamping jaws with accuracy mm. Creep machines were placed in air conditioned room, where the temperature was hold on 23 ± 1 C. For creep testing was used adapted tension testing sample, type A, according to IS Testing samples were conditioned to equilibrium moisture content about 1.5 wt% before testing. Creep experiments were conducted at and 5 different levels of applied stresses. Applied stresses were in range from 8 to 20 MPa in the case of the neat PA6 and 12 to 26 MPa for PA6/MMT and 15 to 23 MPa for PA6/HNT nanocomposite. In graphs on Figs. 6, 7 and 8 creep curves are plotted like dependence of creep rate on creep strain. All experiments were interrupted in primary creep stage between 2500 to 3500 hours. Under those conditions is not possible to leads creep experiments to secondary creep stage. 1x10 3 1x10 3 creep rate ε. [%/h] PA 6 8 MPa 10 MPa 12 MPa 15 MPa 20 MPa creep rate ε. [%/h] PA6/MMT 12 MPa 15 MPa 20 MPa 23 MPa 26 MPa strain ε [%] Fig 6. Creep curves of neat PA strain ε [%] Fig 7. Creep curves of PA6/MMT

4 creep rate ε. [%/h] 1x10 3 PA6/HNT PA6/ 5% HNT 15 MPa 18 MPa 20 MPa 23 MPa creep rate at strain 2% [%/h] PA6 PA6/MMT / MMT PA6/HNT PA6/5%HNT strain ε [%] Fig 8. Creep curves of PA6/HNT 1x applied stress [MPa] Fig 9. Stress dependence creep rate at strain 2% Results of creep tests cleary show that small amount of montmorillonite nanoplatelets and halloysite nanotubes in PA6 matrix effectively prevents from growing creep deformation, Fig 7 and 8. Neat PA6 matrix creeps much easily, fig 6. Reason is giant surface between nanoparticles and polymer matrix. Rigid nanoplatelets create barrier for conformation changes and motion of polymer chains [5]. Moreover polar character of amide groups in PA6 matrix has good affinity to inorganic reinforcement owing to hydrogen bonds. Both composites have got similar contents of reinforcing phase and from chemical point of view are identical matters [4], so it is possible to suppose similar bonding between matrix and nanoparticles. Reinforcing phases only differ in shape and size of nanoparticles, fig 1 and 2. Graphical dependence of creep rate at strain 2% on applied stress is presented on Fig 9 for all tested materials. Graph (Fig 9) obviously demonstrates improving influence of organoclays to creep resistance, neat PA6 creeps more than two orders of magnitude faster than PA6/MMT and PA6/HNT nanocomposites under all loading conditions. Neat PA6 and PA6/MMT don't obey on by Norton's law, it means dependence of creep rate on applied stress hasn't linear course in logaritmic axes. There is small but quantifiable difference between creep properties of PA6/MMT and PA6/HNT. Nanocomposite PA6/HNT creeps slightly easily than PA6/MMT, fig 9. This small difference is possible to explain by different shape and size of reinforcing phases. Well dispersed and isolated montmorilonitte particles have got much larger interphase surface in comparison wit halloysite and so prevent to conformation changes of macromolecules much effectively during long time loading. 3.3 Impact properties Determination of puncture impact behaviour was conducted at own construction instrumented falling-weight impact machines, in conformity with standard ČSN EN IS Penetration of test specimen is realised by dropping of indentor with diameter 20 mm and weight 20 kg to testing sample. Testing specimens with dimension 60 x 60 x 3 mm were during test attached to support ring. Indentor fell down to testing specimen from 0.5 m by velocity 3.13 m/s. Dry PA6 as well moist PA6 had high toughness at multiaxial impact loading.

5 Testing sheets were damaged by ductile fracture associated with high absorption impact energy during crack initiation as well as during crack propagation, Fig 10a. Considerable brittlement was observed at PA6/MMT and PA6/HNT composites under the same experiment condition, degree of brittlement depends on moisture contents. Dry composites PA6/MMT and PA6/HNT are completely brittle and failure to shatters under multiaxial impact loading, red curves on Fig 10b and 10c. Maximum force is seven times lower in comparison with neat PA6 matrix. Nanocomposite PA6/MMT with 1.7 wt% H 2 absorbed high quanta of energy in state of crack initiation, after reaching maximum force the brittle crack growths unstably, Fig 10b. Main reason of brittlement of PA6/ organoclay nanocomposite is immobilization of macromolecular chains by nanoparticles forbid growth of plastic deformation, which absorbs fracture energy [2] a PA6 b PA6/MMT c dry dry 2 % H % H 2 8 % H 2 PA6/HNT dry 3 % H 2 9 % H 2 Load [N] Deflection [m] Fig 10. Impact curve for neat PA6 a) Pa6/MMT nanocomposite b) and PA6/HNT nanocomposite c) Composite PA6/HNT with contents of absorbed water above 3 wt% shows completely ductile damage, Fig 10c. Results of impact tests indicate on existence of threshold value of moisture. Material demonstrates ductile damage above this threshold value, mechanism of moisture like inner lubricant prevails over mechanism of brittlement caused by nanoparticles. Next experiment will be realised to complete results. 4. CNCLUSIN Polyamide composites filled by nanoparticles on the base organically improved clays prove considerable improving of creep properties. Creep rate at selected strain of polyamide composites is about two to three order of magnitude lower in comparison wit neat PA6 matrix. This result was achieved at low degree of filling of polyamide matrix by clay nanoparticles. Nanoparticles hinder to conformation changes and motion of macromolecules during long time loading. Experiments prove, that size, shape and amount of nanoparticles and quality of interphase [6] affect the creep resistance. Impact properties of neat PA6 are independent on moisture contents in multiaxial impact test. Immobilisation of polymer chains by rigid particles prevents from developing of a plastic deformation, which absorb fracture energy. Dry nanocomposites are completely brittle, toughness increase with increasing moisture contents. Nanocomposites become weaker with increasing moisture contents, materials put up lower resistance, which is indicasted by decreasing of maximum force. From certain threshold moisture both tested nanocomposites exhibit ductile fracture.

6 Acknowledgement The authors would like to thank Ministry of education, youth and sport (project MSM and project CST MP 0701 C09036) for their financial support. LITERATURE [1] THSTENSN, E. T. et al: Nanocomposites in contex, Composite Science and Technology, 2005, 65, nr. 3-4, page 491 [2] CHAN, C. M., et al., Polypropylene/calcium carbonate nanocomposites, Polymer, 2002, vol. 43, page 2981 [3] Gruber, F., In proceeding: Chancen der Nano-Technologie in der Kunststofftechnik. Würzburg, 2002, page D/1 [4] HLAVÁČ, J, Fundamentals of the silicate technology, 2. ed., Prague, 1988, page 116 (in Czech) [5] Zhang, Z. et al: Creep resistant polymeric nanocomposites, Polymer, 2004, 45, nr. 10, page 553 [6] Hell, J., et al: Tensile creep and toughness of pp with aluminosilicates and CaC 3 nanoparticles, From Conference Proceedings Nanocon 09. Rožnov pod Radhoštěm: TANGER, 2009, page 132