In Situ Cure of Cellulose Whiskers Reinforced Phenolic Resins

Transcription

1 In Situ Cure of Cellulose Whiskers Reinforced Phenolic Resins Hongzhi Liu - Washington State University & Marie-Pierre Laborie - University of Freiburg

2 utline Background bjective Materials & Methods Results Conclusions A representative TEM micrograph of negatively stained cellulose whiskers obtained by sulfuric acid hydrolysis of MCC.

3 Background Large majority of the work with cellulose nanowhiskers (CNWs) on thermoplastic or low modulus polymers Few studies with thermosetting resins Flexible, low Tg epoxy resins have been effectively reinforced by tunicate CNWs (Matos-Ruiz 2001) & MFC (Lu & Drzal 2008). A modest increase in tensile modulus (percolation network effect) is observed when reinforcing higher Tg epoxies (ca. 160 C ) with CNWs especially above the resin Tg (Tang & Weder 2010) CNWs affect the rheology and curing behavior of phenolformaldehyde resol resin (Hong et al., 2010)

4 Phenolic / Cellulose Fillers System Na CH 2 Na Strong interactions Resoles alter cellulose crystallinity (So et al. 1990) Molecular mechanics models (Pizzi. 1994) Cellulose catalyzes the cure of resoles (Chow 1969, Mizumachi et al ) CH 2 Na HCH 2 CH 2 Possible interactions Adsorption/ secondary interactions Covalent bonding (prolonged and high heating) H H H H H H H H H H H H H H n H H H H H H

5 Background Reinforcement effect of CNWs in polymer matrices is a function of: Percolation network formation, which requires H- bonding and connectivity between the CNWs CNWs dispersion in polymers & good stress transfer which requires some compatibility/ interactions between the polymer matrix and the CNWs Is there an optimum for reinforcement & balance between CNW/CNW and CNW/matrix interactions?

6 bjective Prepare well dispersed thermosetting nanocomposites by in situ polycondensation and cure of phenolic resoles with cellulose nanowhiskers Evaluate the influence of CNWs on resin properties Cure properties of resoles Thermal stability of cured composite Dynamic mechanical properties and relaxations (Tg )

7 Materials and Methods Microcrystalline cellulose CNW preparation from hydrolysis of MCC with sulfuric acid (Bondeson et al., 2006) Atomic force microscopy Transmission electron microscopy Resole / CNW mixture Preparation & morphology of bulk CNW/phenolic composites Atomic force microscopy Transmission electron microscopy Properties of phenolic composites CNW/ phenolic composites with a different cure history characterized by TGA, DMA, DSC Cured resole / CNW composite

8 Characterization of Cellulose Whiskers Atomic force microscopy a b -NanoScope IIIa, multimode SPM from Veeco Co. -Tapping mode -Etched silicon probes (k=46n/m, typical resonance frequency= 325kHz, radium of tips<10nm) -CNWs measured dimensions: L n /D n c d Scan size: 0.5µm Scan size: 1µm

9 Design of Formulations Liquid resole PF prepolymer: Durite SC-830A from Hexion Specialty Sample designation 25 C viscosity = 275cPs Solid content = 69 % ph = 8 Weight ratio of PF/CNXL a Formulation of CNXL-filled PF nanocomposites Volume fraction of CNXL b PF prepolymer (Durite SC-830A)(g) Solid content of PF prepolymer (wt%) 3.85 wt% CNXLs suspension (g) Solid content of 3.85 wt% CNXLs suspension (g) v-pf 100/ PF / PF / PF / a): The weight ratio was based on solid content between PF prepolymer and CNXLs suspension; b): During calculation, the densities of PF and CNXL were adopted to be 1.32g/cm 3 and 1.60g/cm 3, respectively.

10 Homogeneous Dispersion of CNWs in Liquid Resole Steps for homogeneous dispersion in liquid resole and in the cured resin: Concentration of aqueous CNW dispersion to ca. 3-5 wt% (TGA) Mixing of concentrated CNW dispersion with liquid resole Solvent exchange with DMF (long term stability at low temperatures) a) Flow birefringence of concentrated CNW suspensions (3.85wt%) observed between two crossed polarization filters, b) PF/CNW mixture in water, c) PF/CNW mixture in DMF.

11 Cured Bulk Composites Homogeneous & Transparent Phenolic /CNW Composites Staged cure 35mm 4mm 1mm bars a) pre-cure at 80 C for 38h; b) cure at 140 C under vacuum for 2h; c) post-cure at 185 C under vacuum for 1h.

12 Cured Bulk Composites Scanning Electron Microscopy Pure PF Increasing CNW content Composite Magnification 5,000 Magnification, 50,000

13 Properties of Cured CNW/Resoles CURE PRPERTIES THERMAL STABILITY FLEXURAL PRPERTIES

14 Cure Properties Modulated DSC2620 from TA Instruments High-pressure crucibles Heating to 300 C at 2.0 C/min, 5.0 C/min, 10 C/min & 20.0 C/min, No obvious influence of CNWs on the main exotherm and peak temperature of CNWs

15 Influence of CNWs on the Cure Properties of Phenolics CNWs remarkably increased the heat of reaction and the resin degree of conversion Tendency for decrease in Ea with addition of CNW? -ln(φ/t p 2 ) 15.5 a ln R= ( φ T ) = E RT ln( RA/ E) p a p 1/T p Sample T p1 ( C) a T p ( C) T p2 ( C) a H cure (J/g) b Ea (KJ/mol) PF 148.3± ± ± PF2.5CN 147.0± ± ± PF5.0CN 146.9± ± ± PF7.5CN 146.1± ± ±

16 Influence of CNWs on the Cure Properties of Phenolics 1. Catalytic effect of CNWs 2. Higher degree of conversion in presence of CNWs

17 FT-IR spectroscopy No evidence of interactions between CNWs and phenolic resin FT-IR spectra (a) PF; (b)pf with 5 % CNW; (c) the difference spectra (b) and (a); (d) control CNW

18 Thermogravimetric Analysis of PF/CNW Composites Below 230 C, no pronounced difference in thermal stability of PF resin after addition of CNW Above 250 C, slightly inferior thermal stability of PF upon addition of CNW TGA thermogram obtained under heating at 10 o C/min

19 Thermal Stability of Composites Comparison of the degradation temperature at 5%, 10% weight loss and the char yield of v-pf and its nanocomposites with varying loading levels of CNXL Systems Temperature of Char yield Temperature of 5% weight loss,t d5 ( o 10% weight loss, at 400 o C C) T d10 ( o C) (wt%) v-pf PF2.5CN PF5.0CN PF7.5CN CNXL powder Note: heated from 50 o C to 400 o C at a heating rate of 10 o C/min under nitrogen atomosphere. T d5 is not significantly affected by CNW addition T d10 & char yield are slightly decreased in presence of CNWs up to 5% CNW loading, thermal stability of composite is very close to that of neat resin

20 Dynamic Mechanical Analysis Single-cantilever beam E' (MPa) 35mm 4mm 1mm bars Heating scan from 35 o C to o C at 2 o C/min & 1Hz 0 Q800 from TA Instruments v-pf1 PF2.5CN3 PF5.0CN3 PF7.5CN2 Specimens post-cured at 140 C Residual cure Two main relaxations: β relaxation at ca 150 C Tanδ Temperature ( o C) v-pf1 PF2.5CN3 PF5.0CN3 PF7.5CN2 Tg at ca 250 C Temperature ( o C)

21 Dynamic Mechanical Analysis Specimens postcured at 140 C E' (MPa) v-pf PF2.5CN PF5.0CN PF7.5CN Moderate reinforcement & proportional to CNW content up to 5 wt% Temperature ( o C) Slight increase in relaxation temperatures up to 5 wt% CNW loading Termperature ( C) T ( C) Tg( C) wt% CNW

22 Dynamic Mechanical Analysis v-pf3 PF2.5CN2 PF5.0CN2 PF7.5CN2 Single-cantilever beam E' (MPa) Specimens post-cured at 185 C Moderate reinforcement (ca. 80%) & proportional to CNW content No more residual cure Tanδ v-pf3 PF2.5CN2 Temperature ( o C) PF5.0CN2 PF7.5CN Temperature ( o C)

23 Dynamic Mechanical Analysis Specimens post-cured at 185 C Reinforcement is best described by the Halpin- Kados Model No clear trend or change in Tg with CNW addition

24 Summary -Well dispersed phenolic resole/ CNW composites using DMF - Possible catalytic effect of CNWs on resin cure - Higher degree of conversion reached in presence of CNWs - Additional bond formation? - Phenolic / cellulose covalent bonds? - Moderate reinforcement observed up to 5 wt % CNW loading - Reinforcement is best described by Halpin-Kados model - strong filler / matrix interactions compared to filler/filler interactions

25 Acknowledgements Dr. Rajan Srinivasacharya in Hexion Specialty Chemicals Co. for kindly providing us liquid resole PF prepolymer Stephanie Pitts, undergraduate student at Washington State University Boeing Inc. for funding