S Tardio, M-L Abel, R H Carr, J F Watts Interaction of MDI with... · Adsorption Isotherms 14 Adsorption at solid-liquid interface Langmuir equation (in simplest case of monolayer

S Tardio, M-L Abel, R H Carr, J F Watts

Department of Mechanical Engineering Sciences

The Surface Analysis Laboratory

24 February 2015

1

Polyurethanes

2

Automotive

Furniture

Insulation

Coatings

Spray PU foams

Tuesday, 24 February 2015

Adhesion of PU on metals

3

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


MDI*: a Polyurethane Monomer

4

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


+

Project Aim

5

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


Adsorption Isotherms

Stainless Steel Survey Spectra

6

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


Atomic %

Atomic %

Steel O1s High Resolution Spectra

7

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


Steel Summary

8


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

9

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


Geometry Optimization: DFT*

10

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


Minimization Algorithm

11

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


4,4MDI Optimized Geometry

12


x axis rotation

y axis rotation

4,4MDI Optimized Geometry

13


x-z plane

y-z plane x-y plane


14

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



15

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

%


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



16

3

3.5

4

4.5

0.000% 0.001% 0.010% 0.100% 1.000% 10.000% 100.000%

[N]

ato

mic

%


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


*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

17

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


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


Potential Orientation

19 Tuesday, 24 February 2015

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!


Potential Bond Type Geometry

21 Tuesday, 24 February 2015

≈ 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.


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.


Tuesday, 24 February 2015 24

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S Tardio, M-L Abel, R H Carr, J F Watts Interaction of MDI with... · Adsorption Isotherms 14 Adsorption at solid-liquid interface Langmuir equation (in simplest case of monolayer