S Tardio, M-L Abel, R H Carr, J F Watts Interaction of MDI with... · Adsorption Isotherms 14...
Transcript of S Tardio, M-L Abel, R H Carr, J F Watts Interaction of MDI with... · Adsorption Isotherms 14...
S Tardio, M-L Abel, R H Carr, J F Watts
Department of Mechanical Engineering Sciences
The Surface Analysis Laboratory
24 February 2015
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Polyurethanes
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Automotive
Furniture
Insulation
Coatings
Spray PU foams
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Adhesion of PU on metals
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Polyurethanes are widely used as adhesives in industry as they show excellent adhesion to metal oxide surfaces
Often the adhesion is due to intermolecular forces between
adherent and substrate: bonds
The understanding of adhesion phenomena is nowadays investigated by the characterization of the interfacial
interaction that occur when the adherent is applied to the substrate
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MDI*: a Polyurethane Monomer
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PU polymer chain is formed by urethane linkage. This bond is obtained from the reaction between an hydroxyl group and an
isocyanate
The great majority of di-isocyanate used for the polyaddition reaction is made by methylene diphenyl di-isocyanate*(MDI)
Methylene Diphenyl di-Isocyanate (MDI) Polyol
Polyurethane
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+
Project Aim
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Steel has been etched free of the oxide, exposed to air/water and subsequently exposed to solutions of MDI (at different concentration)
in acetone
MDI
Air + Clean Steel
H2O + Clean Steel
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Adsorption Isotherms
Stainless Steel Survey Spectra
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Air exposed Carbon contamination thickness: 0.3 nm
C % O % Fe % Cr %
16.5 47.5 32.2 3.8
Water exposed Carbon contamination thickness: 0.6 nm
C % O % Fe % Cr %
42.9 39.6 14.6 2.9
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Atomic %
Atomic %
Steel O1s High Resolution Spectra
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1) Hydroxide and oxide
layer on the surface
2) Hydroxide and water
layers thicker on the water
exposed steel
Air
250
Air
740
Water
250
Water
740
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Steel Summary
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Air Exposed Water Exposed
Fe enriched Fe enriched oxide
Cr enriched oxide
• Air exposed steel Fe enriched oxide. • Water exposed steel dissolution of Fe Cr enriched
oxide.
• Water exposed [OH] = 15.2% > air exposed [OH] = 11.2%
• Water exposed [OH] = 7.1% > air exposed [OH] = 3.3
Isocyanate Reactivity
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Stainless steel surface is metal oxides, metal hydroxides and adsorbed water
𝐑 − 𝐍𝐂𝐎 + 𝐇𝟐𝐎 → 𝐑 − 𝐍𝐇 − 𝐂𝐎𝐎𝐇 → 𝐑 − 𝐍𝐇𝟐 + 𝐂𝐎𝟐
𝐑 − 𝐍𝐂𝐎 + 𝐑 − 𝐍𝐇𝟐 → 𝐑 − 𝐍𝐇 − 𝐂𝐎 − 𝐍𝐇 − 𝐑
𝐑 − 𝐍𝐂𝐎 + 𝐑𝐈 − 𝐎𝐇 → 𝐑 − 𝐍𝐇𝐂𝐎𝐎𝐑𝐈
MDI
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Geometry Optimization: DFT*
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Gaussian 09 (Gaussian, Inc.)
has been used to calculate the most stable geometry for the MDI molecule: geometry in which the molecule energy is
minimum
Density Functional Theory* (DFT) Less demanding than Hartree-Fock methods
Ground state energy of the molecule
Energy is a functional of the electronic density Functional is not known: approximation
B3LYP functional employed, set of basis 6-31G
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Minimization Algorithm
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Starting geometry given is one optimized by classical- mechanical optimization method on GaussView (Gaussian, Inc.)
Trough DFT methods the energy of the molecule at this geometry is calculated
Berny algorithm applied: First derivatives calculation Hessian matrix approximation: curvature of the surface information Geometry closer to a minimum of energy Iteration
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4,4MDI Optimized Geometry
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x axis rotation
y axis rotation
4,4MDI Optimized Geometry
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x-z plane
y-z plane x-y plane
Adsorption Isotherms
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Adsorption at solid-liquid interface
Langmuir equation (in simplest case of monolayer chemisorption)
𝒄
𝒙=
𝟏
𝒃Γ𝒎+
𝒄
Γ𝒎
Where: 𝒄 is the solution concentration
𝒙 is the adsorbate concentration 𝒃 is the ratio of the rate constants for adsorption and desorption
Γ𝒎 is the monolayer coverage
𝒃 = 𝒃𝟎𝒆𝒙𝒑(𝑸
𝑹𝑻)
𝒃𝟎 is a frequency factor 𝑸 is the adsorption interaction energy
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Adsorption Isotherms
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3.2
3.4
3.6
3.8
4
0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%
[N]
ato
mic
%
MDI solution concentration vol%
Nitrogen vs MDI concentration
Water
Air
42
43
44
45
46
47
48
49
50
0% 2% 4% 6% 8% 10%
[C]
ato
mic
%
MDI solution concentration vol%
Carbon vs MDI concentration
Water
Air
Water exposed stainless steel isotherm appears to develop over the air exposed one
Greater capacity for MDI for the former substrate
C/N > C/N in pure MDI
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Adsorption Isotherms
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3
3.5
4
4.5
0.000% 0.001% 0.010% 0.100% 1.000% 10.000% 100.000%
[N]
ato
mic
%
MDI solution concentration vol%
Nitrogen vs MDI concentration in log scale
Water
Air
Oscillating behavior at lower solution concentration already observed in the past*
This may be a result of the formation of MDI dimers, or of the
particular surface employed
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*P. Dastoor et al. Surf. Interface Anal. 1997, 25, 931–936 *R.Grilli et al. Int. J. Adhes. Adhes. 2011, 31, 687–694
*K. Shimizu et al. RSC Adv., 2013, 3, 10754–10763
Langmuir Plots
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y = 0.2498x + 2E-05
R² = 1
0
0.005
0.01
0.015
0.02
0.025
0.03
0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%
c/[N
]
c
Langmuir Plot (water exposed sample)
y = 0.2496x + 5E-05 R² = 0.9999
0
0.005
0.01
0.015
0.02
0.025
0.03
0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%
c/[N
]
c
Langmuir Plot (air exposed sample)
𝒄
𝒙=
𝟏
𝒃Γ𝒎+
𝟏
Γ𝒎𝒄
𝒃 = 𝒃𝟎𝒆𝒙𝒑(𝑸
𝑹𝑻)
Slope of the straight line same for both: Same number of adsorption sites
Intercept with the ordinate bigger for air exposed sample:
Bigger heat of adsorption for the water exposed sample
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Monolayer
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Overlayer thickness: Data from plateau point of isotherms
Beer-Lambert equation 𝑰𝒄 =𝑰𝒄
∞ 𝟏 − 𝒆 −𝒅/𝝀𝒄𝒄𝒐𝒔𝜽
𝒅 = thickness of the MDI film
𝑰𝒄 = intensity carbon signal in the sample
𝑰𝒄∞= intensity carbon signal of an infinite layer
𝝀𝒄=attenuation length 𝜽= angle of acquisition
Air exposed sample thickness: 1.1 nm Water exposed sample thickness: 1.2 nm
In the range of the molecule dimension (0.4-1.4 nm)
Monolayer of MDI on both isotherms
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Potential Orientation
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The thickness of the adsorbed layer compared to the molecule dimension seems to suggest for both samples a geometry of this
kind
The molecule seems to interact with the substrate with one end
Potential Bond Type
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𝐑 − 𝐍𝐂𝐎 + 𝐌 − 𝐎𝐇 → 𝐑 − 𝐍𝐇𝐂𝐎𝐎𝐌
Can this kind of reaction occur?
With the formation of this kind of compound?
Some answers will be given in the next talk!
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Potential Bond Type Geometry
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≈ 0.4 <1.1 nm
≈1.1 nm
≈50-600
Bridge orientation not possible!
One end bonding consistent with the geometry found!
Conclusions
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• Polyurethanes are widely used as adhesives in industry, their adhesion to metal oxides is really important .
• PU is formed by polyaddition reaction of MDI with polyols. Some adhesive formulations are based on PU monomers.
• Adhesion studies of MDI to steel exposed to air and exposed to water has been carried out.
• Adsorption Isotherms on the two types of sample show that MDI has greater capacity for steel which has been previously exposed to water and contains more hydroxyl groups.
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Conclusions
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• The adsorption appears to be of the Langmuir type.
• The thickness of the adsorbed layer and the dimension of the molecule obtained by geometry optimization matches confirming the presence of a monolayer.
• The comparison between the dimension of the molecule and the thickness of the adsorbed layer suggest the link with one end of the MDI molecule.
• Hypothesised covalent bond between isocyanate and metal hydroxide.
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