Superinsulating Polyisocyanate Based Aerogels: A Targeted Search

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Superinsulating polyisocyanate based aerogels: a targeted search for the optimum solvent system Zhiyuan Zhu, Geert Snellings, Matthias M. Koebel, and Wim J. Malfait ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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ACS Applied Materials & Interfaces

Superinsulating polyisocyanate based aerogels: a targeted search for the optimum solvent system Zhiyuan Zhu 1, Geert Snellings 2, Matthias M. Koebel 1 and Wim J. Malfait 1,* 1

Laboratory for Building Energy Materials and Components, Swiss Federal Laboratories for

Materials Science and Technology, Empa, Überlandstrasse 129, 8600 Dübendorf, Switzerland. 2

Recticel N.V., Sustainable Innovation Department, Damstraat 2, Industriezone 7, 9230

Wetteren, Belgium.

KEYWORDS: polymer aerogel, solvent effects, Hansen solubility parameters, thermal conductivity, BET, density, solid-state NMR, FTIR

ABSTRACT: Polyisocyanate based aerogels combine ultra-low thermal conductivities with better mechanical properties than silica aerogel, but these properties critically depend on the nature of the gelation solvent, perhaps more so than on any other parameter. Here, we present a systematic study of the relationship between the polyurethane-polyisocyanurate (PUR-PIR) aerogel microstructure, surface area, thermal conductivity and density and the gelation solvent’s Hansen solubility parameters for an industrially relevant PUR-PIR rigid foam formulation. We

1 Environment ACS Paragon Plus


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first investigated aerogels prepared in acetone-dimethyl sulfoxide (DMSO) blends and observed a minimum in thermal conductivity (λ) and maximum in specific surface area for an acetone:DMSO ratio of 85:15 v/v. We then prepared PUR-PIR aerogels in 32 different solvent blends, divided into three series with δDispersion, δPolarity, and δH-bonding fixed at 15.94, 11.30 and 7.48 MPa1/2 respectively, corresponding to the optimum parameters for the acetone:DMSO series. The aerogel properties display distinct dependencies on the various solubility parameters: aerogels with low thermal conductivity can be synthesized in solvents with high δH-bonding parameters (above 7.2) and δDispersion around 16.3 MPa1/2. In contrast, the δPolarity parameter is of lesser importance. Our study highlights the importance of the gelation solvent, clarifies the influence of the different solvent properties and provides a methodology for a targeted search across the solvent chemical space based on the Hansen solubility parameters.

1. INTRODUCTION Aerogels are low-density, open-porous, predominantly mesoporous solids with a wide range of application in thermal insulation 1, 2, catalysis 3-5, oil-spill clean-up 6, CO2 capture 7-10, etc. Aerogels can be prepared from a wide variety of materials such as silica 11, 12, polyurethane 13-19, polyurea 20-22, polyimide 23, biopolymers 5, 24, 25, graphene 26, carbon nanotubes 27, various metals 4, 28

and hybrids thereof 29-33. Many aerogels are thermal superinsulators as their thermal

conductivity (λ) is lower than that of standing air (26 mW·m-1·K-1) because the pore sizes are small compared to the mean free path length of air (70 nm at ambient temperature), limiting heat conduction through the gas phase. In addition, the low solid fraction and the nanostructure and tortuosity of the skeleton limit the solid-state conduction. Silica aerogels are the best ambient


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pressure superinsulator, with the lowest thermal conductivity at ambient conditions known to man. On the order of 105 m3 of silica aerogel are produced annually by a range of producers in the form of powders, granulate and blankets, mostly for thermal insulation applications. Silica aerogels are also by far the most studied system academically, and the gelation, aging, hydrophobization and drying steps are well understood 34, 35. Polyisocyanate based aerogels are arguably the second most investigated system 13-22 and a recent target for commercialization (e.g. Slentite® by BASF and AirloyTM by Aerogel Technologies). Compared to silica, polyisocyanate based aerogels offer improved mechanical properties, and a subsequently lower dust release, but typically have a somewhat higher thermal conductivity. The properties of polyisocyanate based aerogels are determined by a variety of parameters such as the polyisocyanate and polyol blends or catalyst concentration 14, 16. The nature of the gelation solvent is also of particular importance 13, 22, but has not been reported on extensively in the literature. Here, we systematically determine the effect of the gelation solvent, and the Hansen solubility parameters 36 in particular, on the microstructure and properties of polyisocyanate based aerogels and present a new methodology for a targeted exploration of the solvent chemical space.

2. EXPERIMENTAL METHODS 2.1 Sample synthesis All polyisocyanate based aerogels were synthesized from the same polyurethane-modified polyisocyanurate (PUR-PIR) rigid foam formulation. This formulation was selected because its



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industrial relevance for the production of rigid foam insulation ensures that the raw materials are available from a range of manufacturers at a competitive cost. In addition, the presence of polyisocyanurate in addition to polyurethane (see below) leads to increased mechanical and thermal stability of the rigid foams and some of these benefits may transfer to the aerogels. The rigid foam formulation is prepared by reacting an excess of polyisocyanate, compared to a stoichiometric mixture, with polyfunctional isocyanate-reactive compounds in the presence of catalyst, blowing agent and additives. The exact nature of the PUR-PIR formulation is proprietary information, but it is prepared from a methylene diphenyl diisocyanate (MDI) based polyisocyanate that is reacted with a polyol blend in the presence of additives and a catalyst package. For the aerogel synthesis, the same rigid foam formulation is used, but without the blowing agent. Polymeric MDI from Huntsman, (Suprasec 5005) was used as polyisocyanate and the reaction is carried out at an isocyanate index of 300%, i.e. with a molar ratio of NCO-groups over reactive hydrogen atoms of three. The excess isocyanate results in the formation of isocyanurate in addition to urethane linkages (Fig. 1a). The PUR-PIR gels were prepared by a one-step sol-gel synthesis process. Two solutions were prepared separately at ambient temperature under stirring with a magnetic bar: the polyol blend (1.63 g) was dissolved in 25 mL solvent and Suprasec 5005 (3.37 g) was dissolved in 25 mL solvent, respectively, with negligible weighing errors. The above two solutions were then mixed together at ambient temperature under stirring for about 1 min, and poured into mold. The mold was sealed and kept at ambient temperature for 20 h. After gelation and aging, the wet gel was removed from the molds, washed with acetone 3 times over 2 days and then dried by supercritical CO2 extraction to avoid excessive shrinkage due to the strong capillary forces that occur during evaporative drying. Apart from the gelation solvent, all processing parameters were kept constant. PUR-PIR aerogels for a


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wide range of solvent blends of common aprotic solvents were synthesized and characterized (Table 1). The FTIR and NMR spectra are consistent with the PUR-PIR formulation that was used (Fig. 1).

Table 1. Hansen solubility parameters of common aprotic industrial solvents, common polyols, polyurethane 37, 38, water and CO2 37, 3837-3837, 3837, 3837, 3837, 3837, 38.

δDispersion

δPolarity

δH-bonding

Solvent

[MPa1/2]

[MPa1/2]

[MPa1/2]

Acetone

15.5

10.4

7.0

Propylene carbonate (PC)

20

18

4.1

Butanone

16

9

5.1

Acetonitrile

15.3

18

6.1

1-Methyl-2-Pyrrolidinone

18

12.3

7.2

Tetrahydrofuran (THF)

16.8

5.7

8

Dimethyl sulfoxide (DMSO)

18.4

16.4

10.2

Acetylacetone

16.1

11.2

6.2

Methyl ethyl ketone (MEK)

16

9

5.1

Butyrolactone

19

16.6

7.4

Poly(ethylene)oxide (PEO)

17.3

3.0

9.4

Poly(propylene)oxide (PPO)

16.3

4.7

7.4

Polyurethane (PEO+HDI)

17.6

3.5

9.0

Polyurethane (PPO+HDI)

16.8

4.4

6.8

Water

15.6

16.0

42.3

CO2

15.6

5.2

5.8


a PIR

1

0.5

0 1000

1500

2000

2500

3000

3500

4000

-1

Intensity [a.u.]

Wavenumber [cm ]

1

b

4 2+3

0.5

RCOOR'

6

RCH2OR' 1

5

* *

*

0 200

150

100

**

*

50

0

13

C Chemical shift [ppm]

Intensity [a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity [a.u.]


1

c

aromatic

RCH2OR' RCH 3

0.5

0 15

10

5

0

-5

1

H Chemical shift [ppm]

Figure 1. Spectral data of a PUR-PIR aerogel (sample #12 from Table 3). a) FTIR spectrum, note the isocyanurate peak near 1410 cm-1. b) 1H-13C CP MAS NMR spectrum: peaks 1-6 originate from the MDI part of the polymer (1: methylene, 2-5: aromatic carbons, 6: carbonyl in urethane linkages); the RCOOR’ bands are from the carboxyl groups from the polyester polyols; the RCH2OR’ bands from both the polyether and polyester polyols; the stars denote spinning sidebands. c) 1H MAS NMR spectrum: the RCH2OR’ band originates from the polyester and polyether polyols, the RCH3 band from the polyether polyol.


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2.2 Characterization The microstructure was investigated by scanning electron microscopy (SEM) with a FEI Nova NanoSEM 230 at an acceleration voltage of 10 kV. The envelope density was determined by the mass and volume calculated from the sample dimensions and its uncertainty is estimated to be on the order of 5% relative. The nitrogen sorption isotherms (77 K) of selected samples were determined with a Triflex instrument (Micromeritics) after degassing at 120°C for 17 hours at 0.02 mbar. The specific surface area was derived from the sorption isotherms by the BrunauerEmmet-Teller (BET) method 39 with a reproducibility within 10 m2/g, although the accuracy is lower (on the order of 20 m2/g) as the derivation of the BET surface area from the nitrogen sorption data depends on multiple assumptions. The average pore diameter was approximated by 4·VPore/SBET, where VPore is the pore volume and equal to 1/ρ – 1/ ρSkeleton, rather Barret-JoynerHalenda (BJH) analysis because of the well-known limitations of the latter for aerogel materials 40

. The skeleton density is approximated by 1.2 g/cm3, the expected density for our PUR-PIR

formulation. The thermal conductivity was determined using a miniature guarded hot plate device designed for samples of low conductivity materials 41, 42 with a conservative estimate of the uncertainty of 1 mW·m-1·K-1. Fourier transform infrared FTIR spectra were collected with a Bruker Tensor 27 spectrometer in attenuated total reflection (ATR) mode using a diamond crystal. 1H and 1H-13C cross polarization (CP) NMR spectra were collected with a Bruker AV III HD spectrometer equipped with a 7.9 Tesla magnet. The samples were spun in 2.5 and 4 mm zirconia rotors at magic angle spinning rates of 24 and 10 kHz for the 1H and 1H-13C CP spectra, respectively. For the 1H spectrum, 16 scans were acquired with a pulse delay of 7.5 s (T1=1.3 s). For the 1H-13C cross polarization spectrum, 40000 scans were acquired with a delay of 1.5 s.



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3. RESULTS AND DISCUSSION

3.1 DMSO-acetone blends We initially investigated the effect of solvent composition on aerogel properties for different DMSO-acetone mixtures where XDMSO is the volume fraction of DMSO in the solvent mixture, both common solvents for isocyanate based aerogel synthesis 17. The samples prepared in solvents with XDMSO 7.2 MPa1/2. The density strongly increases as δDispersion increases: all aerogels prepared in solvent blends with δDispersion16.5 MPa1/2 result in densities above 0.250 g/cm3 (Fig. 6b, Fig. 7b). The strong correlation between density and δDispersion (Fig. 6d) is most likely responsible for the minimum in λ at intermediate values for δDispersion (Fig. 6c), as it is well known that aerogels with an intermediate density typically have the lowest λ 21, 43 (also see Section 3.3). The density dependence on the hydrogen bonding parameter is weaker compared to the dispersion parameter, but aerogels with δH-bonding> δPolarity, commensurate with the expected interaction strength between the solvent and PUR-PIR molecules and gel surfaces. Note that the use of the one-dimensional Hildebrand solubility parameter, which is related to the 2 2 2 Hansen solubility parameters through the equation δ Hildebrand , = δ Dispersion + δ H2 − bonding + δ Polarity

does not provide the necessary level of detail to explore the solvent space for these PUR-PIR aerogel applications (Fig. 6g,h). In contrast, the exploration of the three-dimensional Hansen solvent parameter space resulted in the discovery of solvent systems leading to PUR-PIR aerogels with lower thermal conductivities compared to the initial DMSO-acetone system, most notably DMSO-acetonitrile (#12, λ=18.6 mW·m-1·K-1) and THF-acetone (#14, λ=19.5 mW·m1

·K-1) (Table 3) and these system present further targets for optimization. Particularly the THF-

acetone system is of interest as one of the goals of the study was to find alternatives for DMSO.


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Table 3. Influence of Hansen solubility parameters on PUR-PIR aerogel properties. δ Solvent 1

Solvent 2

δ

Polarity 1/2

δ

H-bonding 1/2

λ

Density [g/cm3]

[mW/(m.K)]

0.414

34.3

9

DMSO

10

DMSO

4.65

ethyl acetate

45.35

16.04

6.33

7.48

0.164

cracked

11

DMSO

1.45

butyrolactone

48.55

18.98

16.59

7.48

no gelation

/

12

DMSO

16.75

acetonitrile

33.25

16.34

17.46

7.47

0.226

18.6

13

DMSO

27.70

PC

22.30

19.11

17.11

7.48

0.528

cracked

14

THF

24.00

acetone

26.00

16.12

8.14

7.48

0.205

19.5

15

THF

41.00

MEK

9.00

16.66

6.29

7.48

0.293

21.3

16

THF

17.50

ethyl acetate

32.50

16.15

5.44

7.48

0.158

23.8

17

THF

6.50

butyrolactone

43.50

18.71

15.18

7.48

no gelation

/

18

THF

36.25

acetonitrile

13.75

16.39

9.08

7.48

0.241

21.3

19

THF

43.25

PC

6.75

17.23

7.36

7.47

0.486

30.9

20

DMSO

26.25

THF

23.75

17.64

11.32

9.16

cracked

/

21

DMSO

27.00

ethyl acetate

23.00

17.20

11.29

8.82

cracked

/

22

DMSO

15.50

MEK

34.50

16.74

11.29

6.69

0.268

21.9

23

acetonitrile

5.90

acetone

44.10

15.48

11.30

6.90

0.213

cracked

24

acetonitrile

22.75

THF

27.25

16.12

11.30

7.14

0.278

20.6

25

acetonitrile

23.60

ethyl acetate

26.40

15.56

11.29

6.69

0.166

35.5

26

acetonitrile

12.75

MEK

37.25

15.82

11.30

5.36

foam

36.8

27

acetone

45.17

PC

4.83

15.94

11.135

6.72

0.161

35.1

28

acetone

6.50

MEK

43.50

15.94

9.185

5.35

0.179

29.7

29

acetone

33.25

THF

16.75

15.94

8.83

7.34

0.197

21.4

30

acetone

43.79

butyrolactone

6.21

15.93

11.17

7.05

0.187

23.8

31

acetonitrile

43.25

PC

6.75

15.93

18.00

5.83

0.156

34.5

32

acetonitrile

4.63

MEK

45.37

15.94

9.83

5.19

0.174

32.1

33

acetonitrile

28.85

THF

21.15

15.93

12.80

6.91

0.179

24.3

34

acetonitrile

41.40

butyrolactone

8.60

15.94

17.73

6.33

0.149

33.3

35

acetonitrile

39.75

DMSO

10.25

15.94

17.67

6.95

no gelation

/

36

ethyl acetate

48.40

PC

1.60

15.93

5.71

7.11

no gelation

/

37

ethyl acetate

16.25

MEK

33.75

15.94

7.80

5.79

foam

/

38

ethyl acetate

43.25

THF

6.75

15.94

5.35

7.31

no gelation

/

39

ethyl acetate

47.89

butyrolactone

2.11

15.94

5.78

7.21

no gelation

/

40

ethyl acetate

47.40

DMSO

2.60

15.94

5.88

7.36

foam

/

MEK

Volume [mL] 26.25

Dispersion 1/2

Volume [mL] 23.75

[MPa ] 17.14

[MPa ] 12.52

[MPa ] 7.52

The uncertainties on density and λ are estimated at 5% relative and 1 mW·m-1·K-1, respectively.



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Density [cm3/g]

λ [mW.m-1.K-1] 0.5

a

40

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b

0.4

30

0.3 0.2

20 15.5

16

16.5

0.1 15.5

17

16

1/2

δ Dispersion [MPa

16.5

17 1/2

]

δ Dispersion [MPa

]

0.5

c

40

d

0.4

30

0.3 0.2

20 5

6

7

8

0.1

5

6

1/2

δ H-bonding [MPa

]

δ H-bonding [MPa

]

0.5

f

0.4

30

0.3 0.2

20 5

10

0.1

15

5

10

1/2

δ Polarity [MPa

40

8 1/2

e

40

7

15 1/2

]

δ Polarity [MPa

]

0.5

g

0.4

30

h

0.3 0.2

20 18

20

22

24

26

1/2

δ Hildebrand [MPa

]

0.1

18

20

22

24

26

1/2

δ Hildebrand [MPa

]

Figure 6. Aerogel thermal conductivity (left) and density (right) as a function of the Hansen solubility parameters. Data are plotted for four series of solvent blends, with δDispersion=15.94 (red triangles and red plane), δPolarity=11.30 (black diamonds and grey plane) and δH-bonding =7.48 MPa1/2 (blue squares and blue plane); the cyan circles correspond to the samples for the DMSOacetone mixtures. The bottom graphs plot the aerogel properties as a function of δHildebrand.


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Figure 7. Thermal conductivity (a) and density (b) of PUR-PIR aerogels as a function of the δHbonding

and δDispersion parameters. Each marker is labeled with the corresponding thermal

conductivity (a) and density (b). In addition, the marker color correlates with the aerogel properties: a) white and black correspond to 18 and 43 mW·m-1·K-1, respectively; b) white and black correspond to 0.140 and 0.540 g/cm3, respectively.

3.3 Density dependent thermal conductivity The thermal conductivity of the PUR-PIR aerogels displays a minimum for densities around 0.220 g/cm3 (Fig. 8). The higher λ for lower density samples is related to the larger pores (Fig. 3) resulting in a higher gas phase conductivity; the higher λ at higher densities is related to the higher heat conduction through the solid skeleton. This type of behavior is typical for many classes of aerogel 21, 45. A minimum in λ for similar densities around 0.2 g/cm3 was previously



observed also for polyurea aerogels 21, but the minimum occurs at lower densities for silica aerogels (around 0.120 g/cm3) 42, 46, presumably because silica is distributed more finely in space compared to PUR-PIR systems, resulting in a higher surface area (~800 m2/g for silica aerogel compared to 7.4 MPa1/2 and intermediate δDispersion parameter (between 15.8 and 16.7 MPa1/2). Most industrial aprotic solvents have lower δH-bonding parameters (Table 1) and the requirement for a high δH-bonding parameter imposes the use of DMSO or THF (Table 1), or an alternative solvent with high δH-bonding, as part of the solvent blend. The observation that other studies on polyurethane aerogels also use solvent blends containing either DMSO or THF 17 may indicate that our optimum solvent systems may also be useful for related PUR or PUR-PIR formulations. However, a Hansen solubility parameter study for polyurea aerogels identified solvent systems with δH-bonding between 3 and 5 MPa1/2 as optimal 22, in stark contrast to our data on the PUR-PIR system. Thus, a solvent optimization for each specific formulation is highly advisable and our study provides the methodology for such a search. Note that resorcinol-formaldehyde gels and other important organic gel systems have chemistries that are even further removed from our formulation than the polyurea system, and our results may not be transferable to these systems.



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4. CONCLUSIONS We systematically investigated PUR-PIR aerogel properties as a function of the Hansen solubility parameters for a rigid-foam-based PUR-PIR formulation. The nature of the solvent was found to have a dramatic effect on the sol-gel process, e.g. gelation versus precipitation, and aerogel properties, with variations in density and thermal conductivity by a factor of three and two respectively. Although each polymer formulation will have its own ideal solvent 13, 22, the observed importance of the δDispersion and δH-bonding parameters compared to the δPolarity parameter is likely to be transferable to other polyisocyanate based formulations. Our search for alternative solvent systems, based on the Hansen solubility parameters, enabled us to identify solvent systems that not only avoid the use of industrially less favorable solvents, but also lead to superior material properties. The Hansen solubility parameter based search method detailed in this study provides a useful approach for a targeted exploration of the solvent chemical space coupled to a specific polyisocyanate gel system.

ASSOCIATED CONTENT The supporting information contains TGA data for sample sample #6 (prepared in DMSO:acetone 15:85).

AUTHOR INFORMATION Corresponding Author * Wim J. Malfait, [email protected], +41 58 765 4983


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Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here.

Author Contributions Z.Z., M.M.K, G.S. and W.J.M designed the study. Z.Z. performed all synthesis and characterization. G.S. selected the PUR-PIR formulation and provided the industry perspective. W.J.M. wrote the first draft of the manuscript which was modified through the comments of the other authors. All authors have approved the final version of the manuscript.

Funding Sources The authors are grateful to the Flemish agency ‘Agentschap Innoveren en Ondernemen’ for providing financial support of this research.

ACKNOWLEDGMENT We thank Dr. Shanyu Zhao for the BET measurements and his assistance with the supercritical drying.

ABBREVIATIONS PUR-PIR, polyurethane-polyisocyanurate; DMSO, dimethyl sulfoxide; THF tetrahydrofuran; PC, propylene carbonate; MEK, methyl ethyl ketone; SEM, scanning electron microscopy; BET,



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Brunauer-Emmet-Teller; ATR, attenuated total reflection; CP, cross polarization; VOCs, Volatile Organic Compounds; MDI, methylene diphenyl diisocyanate; SCFD, supercritical fluid drying.

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