Structure/Property Analysis of Polyurethane Adhesives

Transcription

1 Structure/Property Analysis of Polyurethane Adhesives Charles Frazier Sustainable Biomaterials, Virginia Tech Wood-Based Composites Center Dakai Ren (Macromolecular Science & Engineering, Virginia Tech) Dow Chemical Company

2 Objectives Develop novel methods for structure/property analysis in moisture-cure polyurethane adhesives (PUR). Determine the relative suitability of analyzing: 1. Neat PUR films, or 2. Composite wood/pur specimens: a. excised from real bondlines, or b. failure surfaces from tested fracture specimens.

3 Materials Polymeric methylene(bisphenylisocyanate) (pmdi) NCO = 31%. Polypropylene glycol diols (PPG). 1. PPG2000: M n = 2000 g/mol, polydispersity = PPG400: M n = 400 g/mol, polydispersity = Curing catalyst 4,4 -(Oxydi-2,1-ethanediyl)bismorpholine JEFFCAT DMDEE. Southern yellow pine (Pinus spp.),l T R = mm; equilibrated RH= 65%, 25 C.

4 Synthetic Methods PUR prepolymers, mixture of PPG2000 and PPG400. NCO/OH = 5, 80 C (N 2, 3 h) until complete by FTIR (2270 cm -1 ). Liquid prepolymers stored N 2 / P 2 O 5 until cure; < 15 days. Catalyst DMDEE (0.5%) mechanically mixed with prepolymer under N 2 immediately prior to use in films or bondlines. PUR PPG2000/PPG400 Hard segment Theoretical Measured Prepolymer mass ratio content (%) NCO (%) NCO (%) PU / ± 0.04 PU / ± 0.02 PU / ± 0.09

5 Liquid PUR prepolymers. Analytical Methods 1. Size exclusion chromatography (SEC) in THF; universal calibration w/ polystyrene standards. 2. Differential scanning calorimetry (DSC); 20 mg equilibrated 90 C 5 min, heat to 20 C at 10 C/min. 3. Viscosity, 25 mm dia. parallel-plate geometry (gap=400 µm), steady-state flow, shear rate= s 1, under N 2.

6 Analytical Methods Cured neat films. PUR prepolymer cast onto dry polyethylene, 152 µm-gap film applicator; placed in RH = 65% chamber; RT cure 2 weeks. 1. Dynamic mechanical analysis (DMA), compressive-torsion, 8 mm dia. SS parallel-plates, N normal force, 3 C/min, 1 Hz, strain 0.1% within linear response, dry specimens or immersed in H 2 O. 2. Infrared spectroscopy (FTIR), attenuated total reflection of bulk film immediately after sectioning. 3. Water-vapor sorption; 10-mL vial W/ distilled H 2 O, rubber septum w/ copper wire passing through holding 10 mm 2 film; measure weight gain. 4. Atomic force microscopy (AFM), embedded in epoxy, cryo-ultramicrotome, phase images analyzed w/ image analysis software.

7 Analytical Methods Bonded specimens. Flat-sawn pine, 250 g/m 2 applied to one side, cold-press 0.69 Mpa (100 psi), 24 h, ripped into double cantilever beams. 1. Mode-I fracture similar to Gagliano & Frazier (Wood & Fiber, 2001, 33(3):377 85). 2. Fluorescence microscopy, 30 µm sections, stained 0.5% Safranin O, A Zeiss Axioskop microscope, mercury lamp, filter 360-nm excitation, 400-nm dichromatic mirror, 420-nm emission filter. 3. Infrared spectroscopy (FTIR), attenuated total reflection of failure surfaces immediately after fracture testing. 4. AFM as above.

8 Analytical Methods Bondlines excised from bonded fracture specimens; wood trimmed from bondline yielding 650 μm thick specimens. Adhesive Adhesive layer Penetration to Adhesive layer Penetration Total bondline thickness (µm) each side (µm) fraction (v %) fraction (v%) fraction (v%) PU PU PU DMA as described, dry specimens or immersed in H 2 O.

9 Viscosity (mpa s) Heat Flow (w/g) RI Intensity (mv) pmdi PPG2000 PPG400 A Results: Liquid Prepolymers PU8020 PU5050 PU Retention Volume (ml) While PU2080 had the lowest Mol. Wt. B PPG 400 PPG 2000 PU8020 PU5050 PU Temperature ( C ) higher hard segment content resulted in highest Tg, 10 6 PU8020 PU5050 PU and highest viscosity due to greater intermolecular interactions Shear Rate (1/s)

10 Relative Occurence (%) PUR8020: Neat Films & Bonded DCB Fracture Specimens PUR Film PUR/Wood Composite AFM phase images: A. PUR8020 bulk neat-film; B. PUR8020 within a wood-cell lumen, excised from bondline > 500 Soft Domain Size (nm 2 ) Error bars = ± 1 stan.dev.; n = 6, 2 images x 3 separate specimens. Wood alters size distribution of the dispersed soft phase. All three PUR s have a continuous hard phase.

11 Effective Penetration ( m) Bondline Thickness ( m) 160 Bonded DCB Fracture Specimens PU8020 PU5050 PU PU PU8020 PU5050 PU µm Clear differences in bondline thickness and adhesives penetration as related to viscosity.

12 Critical Fracture Energy (J/m 2 ) Mode-I Fracture Performance 400 Control Weathered Accelerated weathering: C vacuum water soak (10 mmhg, 1 hr); 2. Continue soaking (1 atm, 1 hr); 3. Dry (65 C, 22 h); 4. Steam (105 C, 1.2 bar, 2 hr) 5. repeat step PU8020 PU5050 PU2080 Error bars = ± 1 stan.dev.; n ~ 60 for each mean. Unweathered control exhibits little dependence on PUR composition. Surprising; perhaps indicative of continuous hard phase. Weather durability dependent upon PUR composition.

13 Normalized Absorbance Normalized Absorbance Infrared Spectroscopy of Carbonyl Region Neat films, interior PUR/wood failure surface PU2080 PU5050 PU Free Urethane H-bonded Urethane Free Urea Monodentate Urea PU8020_Film PU5050_Film PU2080_Film Bidentate Urea Free Urethane H-bonded Urethane Free Urea Monodentate H-bonded Urea Bidentate H-bonded Urea Wavenumber (cm -1 ) Wavenumber (cm -1 ) Average spectra; error bars = ±1 stan. dev. (n=6 films; n=18 composites); normalized by phenylene (1594 cm 1 not shown) with intensities of 1.0, 1.22, and 1.36 based on hard phase contents of PU8020, PU5050, and PU2080, respectively.

14 Absorbance Infrared Spectroscopy of Carbonyl Region PUR/wood failure surface PU2080 Control PU2080 Weathered PU5050 Control PU5050 Weathered PU8020 Control PU8020 Weathered Free Urethane H-bonded Urethane Free Urea Monodentate H-bonded Urea Bidentate H-bonded Urea Wavenumber (cm -1 ) Wavenumber (cm -1 ) Average spectra; error bars = ±1 stan. dev. (n=18); normalized by phenylene (1594 cm 1 not shown) with intensities of 1.0, 1.22, and 1.36 based on hard phase contents of PU8020, PU5050, and PU2080, respectively.

15 Infrared Spectroscopy of Carbonyl Region Infrared distinguishes neat films from bondline failure surfaces. Demonstrates wood/adhesive interaction. Impact of weathering detected on failure surfaces. No simple correlation of infrared to PUR weather durability. There is perhaps more useful information in this data but we did not analyze the subtleties.

16 H 2 O Absorption (% mass increase) Water Vapor Absorption in Neat Films 15 PU8020 PU5050 PU Greater hard phase content results in lower water absorption. Correlates to greater weather durability in fracture testing Time (days)

17 Storage modulus (Pa) Storage modulus (Pa) tan 1st Heat DMA Neat Films Dry Films Films Saturated/Immersed in H 2 O PU8020 Film PU5050 Film PU2080 Film Temperature ( C) tan 10 7 PU8020 Film/water PU5050 Film/water PU2080 Film/water Temperature ( C) Average DMA 1st heat PUR films (3 C/min, 1 Hz); error bars = ±1 standard deviation (n=3).

18 Storage modulus (Pa) Storage modulus (Pa) 1st Heat DMA Neat Films Dry Films Films Saturated/Immersed in H 2 O PU8020 Film/water PU5050 Film/water PU2080 Film/water 10 6 PU8020 Film PU5050 Film PU2080 Film Temperature ( C) Temperature ( C) Average DMA 1st heat PUR films (3 C/min, 1 Hz); error bars = ±1 standard deviation (n=3).

19 tan 1st Heat DMA Neat Films Dry Films Films Saturated/Immersed in H 2 O PU8020 PU5050 PU PU8020 Film/water PU5050 Film/water PU2080 Film/water tan Temperature ( C) Temperature ( C) Average DMA 1st heat PUR films (3 C/min, 1 Hz); error bars = ±1 standard deviation (n=3).

20 1st Heat DMA Neat Films Water immersion DMA reveals reduced stiffness in all PUR s, consistent with water vapor absorption. Lowest hard-segment content (PU8020): Dramatically plasticized. Steaming temp. exceeds hard-phase softening temp. High hard-segment content (PU5050, PU2080): Remains stiff during steam treatment. PU5050, mechanical damping increased near steaming temp.

21 Storage Modulus (Pa) tan Storage Modulus (Pa) 1st Heat DMA of Weathered & Unweathered Specimens Neat Films Excised Bondlines 1E7 A) 8E6 6E6 4E6 PU8020 Film Control PU8020 Film Weathered A) PU8020 Control PU8020 Weathered tan 2E Temperature ( C) Temperature ( C) Average dry-dma 1st heating scans of control and weathered PUR films (left) & bondlines (right) (3 C/min, 1 Hz.); error bars = ±1 standard deviation (n=3). Weathering caused minor damping change in neat films. Weathering caused much greater damping change in bondline specimens.

22 Storage Modulus (Pa) tan Storage Modulus (Pa) 1st Heat DMA of Weathered & Unweathered Specimens Neat Films Excised Bondlines 1E7 B) 8E6 6E6 4E6 PU5050 Film Control PU5050 Film Weathered A) 10 7 PU5050 Control PU5050 Weathered tan 2E Temperature ( C) Temperature ( C) Average dry-dma 1st heating scans of control and weathered PUR films (left) & bondlines (right) (3 C/min, 1 Hz.); error bars = ±1 standard deviation (n=3). Weathering caused little/no damping change in neat films. Weathering caused minor damping change in bondline specimens.

23 1st Heat DMA of Weathered & Unweathered Specimens For lowest hard-segment content (PU8020): Films: weathering caused little change in mechanical damping. Bondlines: weathering caused large reduction in damping. For high hard segment content (PU5050, PU2080): Films: weathering caused little change in mechanical damping. Bondlines: weathering caused little change in damping.

24 Summary AFM & infrared spectroscopy demonstrate significant wood/pur interaction, suggesting superiority of composite specimens. Water vapor absorption of neat films correlated to fracture durability. Water-submersion DMA of neat films correlated to fracture durability. DMA of weathered vs unweathered specimens suggests that bondlines specimens may be superior to neat films. Conclude that both neat films and bondline specimens are valuable for PUR structure/property analysis. Do these correlations have real meaning? More analysis required to determine causality.

25 Acknowledgements The Wood-Based Composites Center, a National Science Foundation Industry/University Cooperative Research Center. The Virginia Tech Graduate School.