NISTIR 6465

NEW APPROACHES TO THE DEVELOPMENT

OF FIRE-SAFE MATERIALS.

Alexander B. Morgan, Jeffrey W. Gilman, Marc Nyden, and Catheryn L. Jackson.

United States Department of Commerce

Technology Administration

National Institute of Standards and Technology
NISTIR 6465

NEW APPROACHES TO THE DEVELOPMENT

OF FIRE-SAFE MATERIALS.

Alexander B. Morgan, Jeffrey W. Gilman, Marc Nyden

Building and Fire Research Laboratory

and Catheryn L. Jackson

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD

February 2000

U.S. Department of Commerce

William M. Daley, Secretary

Technology Administration

Dr. Cheryl L. Shavers, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director
3
4

CONTENTS

ABSTRACT ..............................................................................................................................6

1.0 SYNTHESIS OF POLYETHERIMIDE-CLAY NANOCOMPOSITES. ........................7

1.1 Introduction........................................................................................................................7

1.2 Experimental Section. ........................................................................................................7

1.2.1 GENERAL PROCEDURES..................................................................................................7

1.2.2 SYNTHESIS OF POLYETHERIMIDE 1-CLAY NANOCOMPOSITES. .........................................8

1.3 Results.................................................................................................................................8

1.3.1 POLYETHERIMIDE SYNTHESIS. ........................................................................................8

1.3.2 THERMAL GRAVIMETRIC (TGA) DATA. .........................................................................10

1.3.3 WIDE ANGLE X-RAY SCATTERING (WAXS) RESULTS....................................................11

1.3.4 TRANSMISSION ELECTRON MICROSCOPY (TEM) RESULTS..............................................12

1.4 Conclusions On The Synthesis Of Polyetherimide-Clay Nanocomposites. ....................15

2.0 THERMALLY INDUCED CHANGES IN THE CONDENSED PHASE OF BURNING

POLYMERS ...........................................................................................................................16

2.1 Introduction......................................................................................................................16

2.2 Experimental. ...................................................................................................................16

2.3 Results...............................................................................................................................16

2.3.1 COMPUTATIONAL RESULTS...........................................................................................16

2.3.2 CONDENSED PHASE INFRARED ABSORBANCE RESULTS. ................................................17

2.4 Summary And Conclusions On Thermally Induced Changes In The Condensed Phase

Of Burning Polymers. .............................................................................................................20

3. Acknowledgments ..............................................................................................................21

4. Future Work.......................................................................................................................21

5. References...........................................................................................................................22
5

LIST OF FIGURES

Figure 1. Polyetherimide 1 .........................................................................................................9

Figure 2. Tethering of PEI 1 to montmorillonite surface via 12-amino-1-dodecanoic acid

ammonium salt................................................................................................................9

Figure 3. Synthesis of PEI 1-clay nanocomposites via in situ method.......................................10

Figure 4. TGA curves for PEI 1, PEI 1+10wt% MMT-C12, PEI 1+10wt% MMT-C12Acid.

Standard deviation for the wt% loss at 700 °C is: 0.25 for PEI 1, 0.49 for PEI 1+10wt%

MMT-C12, 0.62 for PEI 1+10wt% MMT-C12Acid. TGAs were run under N2 and at 10

°C/min...........................................................................................................................10

Figure 5. TGA curves (derivative) of PEI 1, PEI 1+10wt% MMT-C12, PEI 1+10wt% MMTC12Acid. Standard deviation for the peak temperature is: 0.73 for PEI 1, 0.3 for PEI

1+10wt% MMT-C12, 0.21 for PEI 1+10wt% MMT-C12Acid. TGAs were run under N2

and at 10 °C/min............................................................................................................11

Figure 6. TGA curves of MMT-C12, MMT-C12Acid. Standard deviation for the wt% lost at

700 °C were: 1.04 for MMT-C12, 0.22 for MMT-C12Acid. TGAs were run under N2 at

10 °C/min......................................................................................................................11

Figure 7. (A) WAXS plot of PEI 1 + 10wt% MMT-C12 and MMT-C12...................................12

Figure 8. (B) WAXS plot of PEI 1 + 10wt% MMT-C12Acid and MMT-C12Acid. ..................12

Figure 9. (A) Low magnification picture (14000×) of Polymer 1 + 10wt% MMT-C12. The

darker lines (a) are clay and the white areas (b) are holes in the TEM section................13

Figure 10. (B) Medium magnification picture (140000×) of Polymer 1 + 10wt% MMT-C12.

Note single layers present (a) as well as some multi-layer tactoids (b)...........................13

Figure 11. High magnification picture (600000×) of Polymer 1 + 10wt% MMT-C12 showing a

multi-layer tactoid. ........................................................................................................13

Figure 12. (A) Low magnification picture (14000×) of Polymer 1 + 10wt% MMT-C12Acid. The

small darker lines (a) are clay and the open white regions (b) are holes in the TEM

section...........................................................................................................................14

Figure 13. (B) Medium magnification picture (140000×) of Polymer 1 + 10wt% MMT-C12Acid.

This is a domain with many multi-layer intercalated tactoids. ........................................14

Figure 14. High magnification picture (300000×) of Polymer 1 + 10wt% MMT-C12Acid. Some

single layers (a) are present as well as multi-layer intercalated tactoids (b)....................14

Figure 15. Computational data for polymer decompostions. .....................................................17

Figure 16. Experimental set-up showing the fiber optic probe imbedded in the burning polymer.

......................................................................................................................................18

Figure 17. The spectra obtained from nylon-6 just before ignition (dashed line) and in the

vicinity of the peak HRR (dotted line) are compared to a room temperature spectrum. ..19

Figure 18. The spectrum from nylon-6 (dashed line), which was measured just after ignition, is

compared to the spectrum of the nylon-6/clay nanocomposite (solid line) measured about

the same time into the burn............................................................................................20

Figure 19. 18-Crown-6 and 18-Crown-6 sodium cation (Na+) complex. ...................................21
6

New Approaches to the Development of Fire-Safe Aircraft Materials

Alexander B. Morgan, Jeffrey W. Gilman, Marc R. Nyden

Building and Fire Research Laboratory

and Catheryn L. Jackson

Materials Science and Engineering Laboratory

National Institute of Standards and Technology,

†

 Gaithersburg, MD 20899

ABSTRACT

Thermoplastic polyetherimide-clay nanocomposites were synthesized from 1,3-

phenylenediamine and bisphenol A dianhydride using an in situ approach. Two types of

organically treated clays were utilized to synthesize these nanocomposites. The two organically

treated clays were montmorillonite clays treated with the ammonium salts of n-dodecylamine or

12-amino-1-dodecanoic acid. The dispersion of the clay in the polyetherimide was analyzed by

wide-angle X-ray scattering and transmission electron microscopy. The results showed that the

clay treated with the ammonium salt of 12-amino-1-dodecanoic acid gave a well-dispersed

intercalated nanocomposite while the clay treated with the ammonium salt of n-dodecylamine

gave a well-dispersed immiscible blend. These nanocomposites were then analyzed by thermal

gravimetric analysis for their thermal stability.

Computational and experimental approaches were developed to explore the nature and

consequences of thermally induced changes that occur in the condensed phase of burning

polymers. A computational strategy for evaluating molecular weight distributions (which should

enable us to calculate the melt viscosity during burning) from molecular dynamics simulations

was developed based on a simple differential equation for the time dependence of the number

average degree of polymerization, x, derived by Boyd. The experimental effort, which we hope

to be able to use in validating the predictions of this model, was successful in obtaining real-time

measurements of the mid-infrared spectra of burning polymers. The condensed phase spectra of

nylon 6 and a nylon 6/clay nanocomposite were measured using a fiber optic reflectance probe

while they were burning on the cone calorimeter.

 

† This work was carried out by the National Institute of Standards and Technology (NIST), an agency of the U. S.

government, and by statute is not subject to copyright in the United States.
7

1.0 SYNTHESIS OF POLYETHERIMIDE-CLAY NANOCOMPOSITES.

1.1 Introduction

There is a desire to obtain zero heat-release rate (HRR) materials for aircraft interiors

because a major area of concern in the airline industry is making aircraft safer and more crashsurvivable for passengers. A prime cause of death and injury during aircraft crashes is leakage

of fuel into the fuselage, after which the fuel ignites, killing the passengers by flame or by

asphyxiation from noxious fumes released. About 20% of all deaths on US airlines between

1981 and 1990 occurred in crash-survivable accidents where fire was created upon impact.1 It is

further approximated that 40% of these deaths were caused by noxious fumes and toxic

combustion products from the burning of various plastic or polymeric material present on the

aircraft.1 The goal of our task within the Federal Aviation Administration (FAA) Interagency

agreement program is to develop zero HRR materials. The candidate polymers that could reach

this goal include polycarbonate and polyetherimides. Polymer clay nanocomposites have

attracted a great deal of interest due to their improved mechanical, thermal, and flammability

properties.2 The improved flammability properties have led our laboratory to undertake a large

research project in this area.3 Recently, reports on polyetherimide-clay nanocomposites4 led us

to pursue the synthesis of a thermoplastic polyetherimide (PEI). However, these PEI systems are

considered to be intractable, in that they are poorly soluble and their melting temperatures are so

high, thus prohibitive to facile engineering or aircraft interior components. Therefore, a lowermelting PEI would be of great interest.

There are several ways to make polymer clay nanocomposites. Solution mixing, melt

blending, and in situ polymerization can all yield polymer clay nanocomposites.5 Obviously,

solution mixing or melt blending are the easier routes towards making polymer clay

nanocomposites. However, these routes can be limited by the solubility of the polymer and the

dispersability of the clay. Using an organically treated clay will allow the clay to be more

miscible with a polymer matrix or in a solvent. The main problem with these organic treatments

is that their thermal stability limits their use in certain high melting polymers, such as aromatic

PEIs. Therefore melt blending can only occur if the organic treatment on the clay can survive

high temperature conditions. Currently there are no organic treatments which can survive typical

processing temperatures for thermoplastic aromatic PEIs (+350 °C). This leaves solution

blending and in situ routes as the ones most likely to succeed in producing a PEI-clay

nanocomposite. Our goal was to prepare a PEI treated clay (>50wt% clay) that was stable above

350 °C and could be melt-blended with more PEI to prepare a 10wt% clay PEI-clay

nanocomposite. We have recently synthesized thermoplastic polyetherimide clay

nanocomposites using an in situ polymerization, forming a polyamic acid-clay nanocomposite,

which was then imidized to give the polyetherimide clay nanocomposite.

1.2 Experimental Section.‡

1.2.1 General Procedures.

All organic treated clays (sodium montmorillonite: Na-MMT, montmorillonite treated

with n-dodecylamine ammonium salt: MMT-C12, montmorillonite treated with 12-

 

‡ Certain commercial equipment, instruments, materials or companies are identified in this paper

in order to adequately specify the experimental procedure. This in no way implies endorsement

or recommendation by NIST.
8

aminododecanoic acid ammonium salt: MMT-C12Acid) were provided by Southern Clay

Products. These clays were dried for 2 hours at 110 °C in a forced air oven prior to use. N,N’-

Dimethylacetamide (DMAc) was purchased from Sigma-Aldrich Inc., distilled over CaH2, and

stored over 3Å molecular sieves under N2 prior to use. Bisphenol A dianhydride (BPADA) and

polyetherimide 1 were obtained from General Electric Corporation and contained 5wt% phthalic

anhydride to control molecular weight of polymers using BPADA as a monomer. 1,3-

Phenylenediamine was purchased from Aldrich, vacuum distilled and stored under N2 prior to

use. All other solvents were purchased from Acros Organics and were used as received.

Thermal gravimetric analysis (TGA) data were collected on a SDT-2960 from TA Instruments.

Wide angle X-ray scattering (WAXS) data was collected on a Phillips diffractometer using Cu

Kα radiation, (λ=0.1505945 nm). The polyetherimide samples were ground to a particle size of

less than 40 μm. Bright field transmission electron microscopy (TEM) images of polyetherimide

1/layered silicate (clay) nanocomposites were obtained at 120 kV, at low dose conditions, with a

Phillips 400T electron microscope. The samples were ultramicrotomed with a diamond knife on

a Leica Ultracut UCT microtome at room temperature to give 70 nm thick sections. The sections

were transferred from water to carbon-coated (type B) Cu grids of 200 mesh. The contrast

between the layered silicates and the polymer phase was sufficient for imaging, so no heavy

metal staining of sections prior to imaging was required.

1.2.2 Synthesis of Polyetherimide 1-Clay Nanocomposites.

To a 250 mL round bottom flask equipped with a magnetic stirbar was added 1,3-

phenylenediamine (0.77 g, 7.15 mmol). The flask was then sealed under a N2 atmosphere with a

rubber septum. DMAc (50 mL) was then added and the solution was allowed to stir at R.T. for 1

hour. The septum was then removed, BPADA (3.72 g, 7.15 mmol) was added quickly, and the

septum was replaced. This reaction mixture was allowed to stir at R.T. for 1 hour. An organic

treated clay (MMT-C12 or MMT-C12Acid, 0.5 g) and 50 mL of DMAc were added to a separate

100 mL round bottom flask equipped with a magnetic stirbar. This mixture was heated to 90 °C

for 5 hours to give a tan-colored translucent suspension. The septum on the flask containing the

polyamic acid of polymer 1 was removed, and the MMT/DMAc suspension was added. The

resulting polyamic acid/clay mixture was allowed to stir at R.T. overnight (approximately 12

hours). Upon completion of the reaction, the DMAc was removed via rotary evaporation and the

resulting brown product was dried in vacuo. The material was then heated to 250 °C for 2 hours

in a vacuum oven to give polyetherimide 1-clay nanocomposites.

1.3 Results

1.3.1 Polyetherimide Synthesis.

Several attempts were made to synthesize a polyetherimide-clay nanocomposite. Initially

we tried using PEI 1 (Figure 1) and sodium montmorillonite (Na-MMT) via melt blending. We

chose Na-MMT because of its high thermal stability, which would allow it to survive

polyetherimide 1’s typical processing conditions of 350 °C. However, since Na-MMT has no

organic treatment, it is highly immiscible in PEI 1, and no intercalation or exfoliation of the clay

occurred using melt blending. WAXS was used to determine this result. Specifically, WAXS

showed no change in the basal d-spacing for Na-MMT, indicating that no polymer had

intercalated between the layers of the clay. We also tried solution blending of 1 and Na-MMT

using methylene chloride, 1,3-dioxolane and N,N-dimethylacetamide (DMAc). WAXS showed

that solvent intercalated, but no intercalation or exfoliation of 1 occurred. We then tried the in
9

situ approach of making polymer clay nanocomposites, using two organically treated clays.

Since DMAc is the solvent of choice for the polymerization of 1, we chose two organically

treated clays which were shown by light scattering to disperse in DMAc.6 The two clays were

montmorillonite treated with the ammonium salt of n-dodecylamine (MMT-C12) or the

ammonium salt of 12-amino-1-dodecanoic acid (MMT-C12Acid). The MMT-C12Acid has the

potential for reacting with the monomers during polymerization, creating a potential “tethered”

polymer clay nanocomposite (Figure 2).

O O

N N

O

O O

O

n

1

Figure 1. Polyetherimide 1

N

H

O

H3N N

O

O

O

O

N

polymer

O

O

Figure 2. Tethering of PEI 1 to montmorillonite surface via 12-amino-1-dodecanoic acid

ammonium salt.

We synthesized the polyetherimides from 1,3-phenylenediamine and bisphenol A

dianhydride (BPADA) using DMAc as the solvent (Figure 3). The 1,3-phenylenediamine and

BPADA was allowed to stir for 1 hour at room temperature to make a polyamic acid. The

organically treated clay was then added to give a polyamic acid-clay nanocomposite. The

solvent was then removed and the polyamic acid was imidized to the polyetherimide in a vacuum

oven at 250 °C for 2 hours. The resulting material was then analyzed by thermal gravimetric

analysis (TGA) for its thermal stability.

O O

O O

O

O O

O

NH2

NH2

O O

1. DMAc, 30 min.

2. BPADA (1.0 eq.), 1 h

3. MMT-C12 or

MMT-C12 Acid

(10wt%), 5 h

4. Heat, 250

0C, 2 h

BPADA

N N

O

O O

O

n

Polyetherimide 1-clay nanocomposite
10

Figure 3. Synthesis of PEI 1-clay nanocomposites via in situ method.

1.3.2 Thermal Gravimetric (TGA) data.

Based on the above TGA data shown in Figures 4 and 5, it appears that the organic

treatment on MMT-C12 and MMT-C12Acid begins to decompose at lower temperatures,

resulting in a lower char yield for these nanocomposites. This is also reflected in the TGA data

for MMT-C12 and MMT-C12Acid with no PEI present (Figure 6). There are some other

potential reasons for these lower char yields. 1,3-Phenylenediamine, one of the necessary

monomers for the synthesis, is very susceptible to oxidation. This can result in lower molecular

weight polyetherimide and incomplete imidization, thus creating defects in the polymer chain.

The current method7 used to synthesize polymer 1 is to use 1,3-phenylene bis(3-nitromaleimide)

and the sodium salt of bisphenol A. These monomers are not susceptible to oxidation. This

polymerization utilizes a nucleophilic addition reaction to facilitate polymerization which leads

directly to polymer 1 rather than a polyamic acid. This route has an advantage over the polyamic

acid route in that it does not have to undergo an imidization after initial polymerization, thus

keeping the number of potential defects in the polymer backbone low.

43.26% wt% lost@700 C,PEI1

(3.657mg)

485.46°C

50.17% wt%lost@700C,PEI1+MMTC12Acid

(2.079mg)

240.47°C

46.40% wt% lost@700 C-PEI1+MMTC12

(3.919mg)

45

55

65

75

85

95

105

Weight (%)

30 80 130 180 230 280 330 380 430 480 530 580 630 680

Temperature (°C)

––––––– PEI 1

– – – – PEI 1 + 10wt% MMT-C12

––––– · PEI 1 + 10wt% MMT-C12Acid

Universal V2.6D TA Instruments

Figure 4. TGA curves for PEI 1, PEI 1+10wt% MMT-C12, PEI 1+10wt% MMT-C12Acid.

Standard deviation for the wt% loss at 700 °C is: 0.25 for PEI 1, 0.49 for PEI 1+10wt% MMTC12, 0.62 for PEI 1+10wt% MMT-C12Acid. TGAs were run under N2 and at 10 °C/min.
11

537.40°C

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Deriv. Weight (%/°C)

30 80 130 180 230 280 330 380 430 480 530 580 630 680

Temperature (°C)

––––––– PEI 1

– – – – PEI 1+10wt%MMT-C12

––––– · PEI 1+10wt%MMT-C12Acid

Universal V2.6D TA Instruments

Figure 5. TGA curves (derivative) of PEI 1, PEI 1+10wt% MMT-C12, PEI 1+10wt% MMTC12Acid. Standard deviation for the peak temperature is: 0.73 for PEI 1, 0.3 for PEI 1+10wt%

MMT-C12, 0.21 for PEI 1+10wt% MMT-C12Acid. TGAs were run under N2 and at 10 °C/min.

16.72% wt%lost@700 C-MMT-C12

(0.4779mg)

19.63% wt%lost@700 C-MMT-C12Acid

(0.9811mg)

75.0

77.5

80.0

82.5

85.0

87.5

90.0

92.5

95.0

97.5

100.0

Weight (%)

30 80 130 180 230 280 330 380 430 480 530 580 630 680

Temperature (°C)

––––––– MMT-C12

– – – – MMT-C12Acid

Figure 6. TGA curves of MMT-C12, MMT-C12Acid. Standard deviation for the wt% lost at

700 °C were: 1.04 for MMT-C12, 0.22 for MMT-C12Acid. TGAs were run under N2 at 10

°C/min.

1.3.3 Wide Angle X-Ray Scattering (WAXS) Results.

Using WAXS we investigated how well the clay was dispersed in PEI 1. WAXS plots

are shown below (Figures 7-8). WAXS indicated that the d-spacing (basal distance between

montmorillonite layers) in MMT-C12Acid increased from 1.36 nm to 1.92 nm, following the in

situ polymerization. This indicated that an intercalated PEI nanocomposite had been
12

synthesized. However, no d-spacing change occurred when MMT-C12 was used during the

synthesis of the polyetherimide. This would indicate that an immiscible blend had been

synthesized. However, when these materials were examined by TEM we observed significant

dispersion and exfoliation of some of the layered-silicate.

0

500

1000

1500

2000

2500

3000

3500

4000

0 5 10 15 20 25 30

Polymer 1 + 10wt% MMT-C12

MMT-C12

Counts

Two Theta

1.36 nm

1.36 nm

A. 

0

500

1000

1500

2000

2500

3000

3500

4000

0 5 10 15 20 25 30

Polymer 1 + 10wt% MMT-C12Acid

MMT-C12Acid

Counts

Two Theta

1.36 nm

1.92 nm

B.

Figure 7. (A) WAXS plot of PEI 1 + 10wt% MMT-C12 and MMT-C12.

Figure 8. (B) WAXS plot of PEI 1 + 10wt% MMT-C12Acid and MMT-C12Acid.

1.3.4 Transmission Electron Microscopy (TEM) Results.

TEM analysis tends to support these findings, but shows that the clay is well dispersed in

both systems, including the immiscible MMT-C12/1 system. Some holes (white areas) are

present in the microtomed thin films (Figure 9, 12); these are caused by knife damage during

cutting and are not inherent in the nanocomposite material. In Figures 9-14, the darker lines are

the clay layers and the polymer matrix is gray. TEM for MMT-C12/1 (Figure 9) shows that

where multi-layer tactoids exist, the clay layers are tightly packed together, but there are no

large, undispersed clay tactoids. A fair number of single layers were also observed (Figure 10).

This indicates that the MMT-C12 was well dispersed and exfoliated in the DMAc before

polymerization, but no polymer chains intercalated between the layers of this material during

polymerization. Individual exfoliated layers in the solvent were encased in polymer, while

multi-layer tactoids were similarly encased, leaving their original d-spacing intact. Thus, the

polymerization occurred outside of the clay layers, giving the mixtures seen in figures 9-11.

TEM of the MMT-C12Acid/1 system indicate that the clay was well dispersed, and some

polymer intercalated in between the clay layers (Figures 12, 13). Not as many single layers were

seen in this system, but some did exist. The MMT-C12Acid/1 system is a mixed dispersion

system, having some intercalated domains, but also some single layers, making it neither fully

intercalated nor exfoliated. MMT-C12/1 also fits into this mixed dispersion category, as it has

single layers indicating exfoliation, but multi-layer tactoids indicating immiscibility.
13

A. B.

Figure 9. (A) Low magnification picture (14000×) of Polymer 1 + 10wt% MMT-C12. The

darker lines (a) are clay and the white areas (b) are holes in the TEM section.

Figure 10. (B) Medium magnification picture (140000×) of Polymer 1 + 10wt% MMT-C12.

Note single layers present (a) as well as some multi-layer tactoids (b).

Figure 11. High magnification picture (600000×) of Polymer 1 + 10wt% MMT-C12 showing a

multi-layer tactoid.
14

A. B.

Figure 12. (A) Low magnification picture (14000×) of Polymer 1 + 10wt% MMT-C12Acid. The

small darker lines (a) are clay and the open white regions (b) are holes in the TEM section.

Figure 13. (B) Medium magnification picture (140000×) of Polymer 1 + 10wt% MMT-C12Acid.

This is a domain with many multi-layer intercalated tactoids.

Figure 14. High magnification picture (300000×) of Polymer 1 + 10wt% MMT-C12Acid. Some

single layers (a) are present as well as multi-layer intercalated tactoids (b).
15

1.4 Conclusions on the Synthesis of Polyetherimide-Clay Nanocomposites.

We were able to successfully synthesize a PEI-nanocomposite using MMT-C12Acid via

an in situ approach. However, this nanocomposite did not have the improved thermal stability

we had hoped to achieve via this route. To evaluate the flammability properties of this

nanocomposite using the cone calorimeter we will need to prepare larger quantities of this

material. The synthetic route used to make this nanocomposite has its drawbacks. The first

drawback is due to the low decomposition temperature of the organic treatment on the clay.

Secondly, the oxidative instability of the diamine may have produced a polymer which had

several defects in the polymer backbone. These two drawbacks most likely led to the lower than

expected char yields and lower thermal stability relative to pure PEI 1. Further, the thermally

unstable organic treatment on the clay may also have contributed to the low char yield. We are

currently investigating new high temperature organic treatments for clays, which may allow the

more commercially friendly approach of thermoplastic blending to be used in the preparation of

a polyetherimide nanocomposite. We will also look at other polyetherimides other than polymer

1 which may lend themselves to easier synthetic routes.
16

2.0 THERMALLY INDUCED CHANGES IN THE CONDENSED PHASE OF BURNING

POLYMERS

2.1 Introduction

On the basis of evidence obtained in previous FAA sponsored investigations conducted in

this laboratory, it now appears that the transport of volatile thermal degradation products through

the condensed phase (as distinguished from the chemistry involved in producing these

compounds) is the critical factor that determines the flammability of polymer/clay

nanocomposites and other materials under consideration for future use in aircraft cabins. Thus,

our focus this year was on using both computational and experimental tools to explore the

changes that occur in the condensed phase when polymers are burned.

2.2 Experimental.

The fiber optic set-up used in the experiments consisted of a sapphire probe (300 μm in

diameter and 10 cm in length) which was mounted on a steel rod and connected at both ends to

zirconium fluoride (ZrF) cables. These cables were connected to focusing optics, consisting of a

pair of off-axis parabolic mirrors, that launch an evanescent wave at the source and capture it at

the detector. A reflectance spectrum results from the attenuation of the evanescent wave due to

absorption of the polymer (and its degradation products) in the immediate vicinity of the probe.

Disk shaped samples (approximately 7.5 cm in diameter) of the polymers were placed on top of

the probe which fit snugly into a groove that had been previously milled to facilitate a good

contact between the polymer melt and the optical fiber. Although the sapphire probe can

withstand temperatures up to 2000 °C, the ZrF cables are sensitive to heat and had to be

protected by water-cooled jackets constructed by winding plastic tubing around a copper pipe.

As an added protection, these jackets were also wrapped in aluminum foil.

2.3 Results

2.3.1 Computational Results

A strategy for evaluating molecular weight distributions (which should enable us to

calculate the melt viscosity during burning) from molecular dynamics (MD) simulations was

developed based on a simple differential equation for the time dependence of the number average

degree of polymerization, x, derived by Boyd. The relevant equation is

2

( R)x

I

k

i

k

dt

dx

= − +

where ki and kI are the rate constants for random scission and intermolecular hydrogen transfer

and R is the steady-state concentration of free radicals. On the basis of previous results8, which

are summarized in Figure 15, the Ahrrenius constants for ki in polypropylene are Ea = 323 ± 25

kJ/mol and A = 1.5 x 1015 s

-1. These values are in line with experimental values for bond

scission reactions in small gas phase molecules and are consistent with TGA measurements of

the global rate of mass loss during the thermal degradation of polypropylene. We are now in the

process of performing additional simulations to obtain values for kI.
17

1/T (K)-1

0.00032 0.00034 0.00036 0.00038 0.00040 0.00042 0.00044 0.00046

ln(k(T))

-11

-10

-9

-8

-7

-6

-5

Molecular dynamics data

Regression Line

slope = -38871 K

intercept = 7.308

 Random Scission Reactions

1/T (K)-1

4.8e-4 5.0e-4 5.2e-4 5.4e-4 5.6e-4 5.8e-4 6.0e-4 6.2e-4 6.4e-4 6.6e-4 6.8e-4

ln(k(T))

-5

-4

-3

-2

Molecular dynamics data

Regression Line

slope = -9263 K

intercept = 1.720

 Propagation Reactions

1/T (K)-1

4.8e-4 5.0e-4 5.2e-4 5.4e-4 5.6e-4 5.8e-4 6.0e-4 6.2e-4 6.4e-4 6.6e-4 6.8e-4

ln(k(T))

-6

-5

-4

-3

-2

-1

Molecular dynamics data

Regression Line

slope = -19563 K

intercept = 8.053

Termination Reactions

)

237528 5.3 10 exp( ( )

( )

1 13

RT

x

dt

dm t

m t

= − −

Experimental Values from Bockhorn et. al.,

Journ Anal. and Appl. Pyrolysis 46, (1998), 1-13.

Ea = 220000 J/mol

A = 1.9 x 1013 s

-1

Figure 15. Computational data for polymer decompostions.

2.3.2 Condensed Phase Infrared Absorbance Results.

Condensed phase absorbance spectra of nylon-6 and a nanocomposite of nylon-6 and

montmorillonite clay were acquired using a Midac Illuminator spectrometer‡ during flammability

measurements on the Cone Calorimeter. The nylon-6 and nylon-6/clay nanocomposite (mass

fraction clay, 5 %) were obtained from UBE industries and used as received. The samples were

exposed to a radiant heat flux of 35 kW/m2 and ignited when there was sufficient volatilization to

sustain gas phase combustion. Spectra were measured continuously until the flames selfextinguished, which was about 15 minutes (from the time the samples were exposed to the

radiant heat) for the pure polymer and almost twice as long for the nanocomposite. A photograph

showing the probe assembly and the burning polymer during an experiment is presented in figure

16.

The spectral window of the measurements was limited to the region extending from about

2250 cm-1 to 4000 cm-1 due to the transmission properties of the fiber optics. This frequency

range encompasses absorption due to C-H, N-H, and O-H stretching modes, which are active in
18

the amines, amides and carboxylic acids produced in the thermal decomposition of nylon

polymers9. Each spectrum was signal-averaged by co-adding 256 scans over the specified

frequency range. This corresponds to a time resolution of about 30 s per spectrum at a frequency

resolution of 4 cm-1.

Figure 16. Experimental set-up showing the fiber optic probe imbedded in the burning polymer.

The spectra obtained early in the Cone Calorimeter experiment (i.e., before ignition) and

in the vicinity of the peak HRR are compared to a spectrum of nylon-6 measured using a

diamond reflectance probe at room temperature in figure 17. These spectra are similar, but not

identical, to spectra obtained from solid residues collected at various stages in the thermal

decomposition of nylon-6 carried out on a thermogravimetric analyser.10 The spectrum measured

before ignition closely resembles the reference spectrum of nylon-6 except for an apparent

broadening of the peak centered at 3295 cm-1, which is probably an effect of the elevated

temperature. The reference spectrum was measured at about 25 °C, whereas the temperature of

the melt at this point in the Cone Calorimeter experiment was probably in excess of 300 °C. The

unresolved absorbance in the region between 3050 cm-1 and 3200 cm-1, which is evident in both

the reference spectrum and in the spectrum prior to ignition, is probably due to the monomer (εcaprolactam) and its cyclic oligomers which have peaks centered at about 3055 cm-1 and 3185

cm

-1. There is also an overtone of the amide II band in the spectrum of nylon-6 at about 3085

cm

-1 which is sensitive to the conformation of the polymer chains.11
19

3357

2921

2856

3295

0.0

 0.1

 0.2

 0.3

 3400 3300 3200 3100 3000 2900 2800

ν(cm-1)

A

3190

3085

Figure 17. The spectra obtained from nylon-6 just before ignition (dashed line) and in the

vicinity of the peak HRR (dotted line) are compared to a room temperature spectrum.

There are some obvious differences between the spectrum obtained in the vicinity of the

peak in the HRR and the spectrum measured before ignition. In particular, there is a shift in the

center of the N-H stretching vibration from about 3300 cm-1 to 3357 cm-1. This is consistent with

spectral features that have been attributed to conformational changes resulting in the disruption

of hydrogen bonds.12 In this case, the hydrogen bonds involve the carbonyl oxygen and amide

hydrogen on adjacent anti-parallel chains.13 There is also a shoulder extending from about 3400

cm

-1 to 3450 cm-1 that may be due to the formation of carboxylic end-groups resulting from the

hydrolysis of peptide bonds in the interior of the polymer chains.

The spectra obtained from nylon-6 and the nylon-6/clay nanocomposite, which were both

measured about 4 minutes into the burn (after ignition but before the peak HRR), are compared

in figure 18. The spectrum from the nanocomposite, although noisier, presumably because the

melt did not adhere as well to the sapphire probe, exhibits features similar to those observed in

the spectrum of the pure polymer. This observation is consistent with previous results from

computer simulations14 and electron microscopy15 which indicate that the same products are

produced during the thermal degradation of both materials, but, in the case of the nanocomposite,

they become trapped in a char-like residue. This hypothesis will be examined further in future

experiments, where both the condensed and gas phase spectra corresponding to nylon-6 and the

nylon-6/clay nanocomposite will be monitored at critical points along their HRR curves.
20

Figure 9. The spectrum from nylon-6 (dashed line), which was measured just after ignition, is compared to the

2922

2854

 0.0

 0.1

 0.2

 0.3

 3400 3300 3200 3100 3000 2900 2800

 3330

A

ν(cm

-1)

Figure 18. The spectrum from nylon-6 (dashed line), which was measured just after ignition, is

compared to the spectrum of the nylon-6/clay nanocomposite (solid line) measured about the

same time into the burn.

2.4 Summary and Conclusions on Thermally Induced Changes in the Condensed Phase of

Burning Polymers.

Our computer program for simulating the thermal degradation of polymers was validated

by comparing predicted values for the Ahrrenius constants associated with the thermal

degradation of polypropylene to experimental values obtained from thermal gravimetric analysis.

This approach to the prediction of the global rate of mass-loss from thermally degrading

polymers was formally extended to provide a description of thermally induced changes in the

molecular weight distribution of the polymer melt. This will facilitate a better understanding of

the transport phenomena that are critical in determining the flammability of aircraft materials.

Changes in the condensed phase mid-infrared (MIR) spectra of nylon-6 and a nylon6/clay nanocomposite were monitored in real-time during flammability measurements conducted

on the Cone Calorimeter. The spectra obtained during the burning process were of sufficiently

high quality to show the progression of the material in contact with the probe from molten

polymer to thermal degradation products. The spectral features are consistent with the increasing

formation of ε-caprolactam as a result of depolymerization reactions. On the basis of these

results, we have concluded that the use of MIR fiber optics in conjunction with a fourier

transform infrared (FTIR) spectrometer to monitor changes in the condensed phase chemistry of

burning polymers is both feasible and useful. In the future, we hope to use this tool to in

conjunction with the molecular modeling effort to identify major degradation products of
21

burning polymers and thereby to elucidate the chemistry responsible for the heat release at

critical points along the corresponding HRR curves.

3. Acknowledgments

The authors wish to thank the Federal Aviation Administration under interagency

agreement No. DTFA03-99-X-9009. We also wish to thank General Electric Corporation for the

gift of bisphenol A dianhydride and polymer 1 (UltemTM). We also wish to thank Southern Clay

Products for the donation of montmorillonite clays; Na-MMT, MMT-C12, and MMT-C12Acid.

Finally we wish to thank Dr. James Cline of NIST for use of XRD facilities.

4. Future Work

While we were able to synthesize a polyetherimide nanocomposite, the resulting thermal

properties were not improved. We need to scale up the preparation of PEI 1-nanocomposite to

evaluate its flammability in the Cone Calorimeter. However, the preparation of more material is

limited by the synthetic routes available and by the organic treatment on the clay. We believe

that the weakest part of trying to make a polyetherimide nanocomposite is the organic treatment

on the clay. Most ammonium organic treatments are rarely stable beyond 250 °C. This limits

their use to polymers which can be processed below 250 °C. Therefore making nanocomposites

of thermoplastic engineering plastics such as Ultem (1) or polycarbonate is very difficult to

accomplish. We had hoped that the solvent blending approach would work, but due to the

limited solubility of Ultem this approach failed.

Since the organic treatment is crucial to the successful preparation of a polymer-clay

nanocomposite, we are currently looking into different types of organic treatments with thermal

stabilities above 250 °C. One treatment currently under investigation is to use 18-crown-6 (2),

(Figure 19) which can complex with the sodium cations on sodium montmorillonite (Na-MMT).

18-Crown-6 is more thermally stable than ammonium cations, and further, it can be used on NaMMT without removing the sodium cations. Na-MMT is the most thermally stable version of

montmorillonite, so to try and leave it intact by complexing organophilic ligands to the sodium

cations may be an approach which would yield success.

2

O

O

O

O O

O

 

O

O

O

O O

O

Na

+

18-Crown-6 Sodium

Cation Complex

Figure 19. 18-Crown-6 and 18-Crown-6 sodium cation (Na+) complex.

We will also be collaborating with NASA-Langley on the preparation of polyetherimide

nanocomposites. Specifically, we will be using a NASA synthesized polyetherimide, LARC

TPI. NASA Langley facilities has prepared experimental resins which have high thermal

stability, but low melting (processing) temperatures. This will allow the synthesis of

nanocomposites which can be prepared by melt blending methods rather than the more difficult

in situ techniques.
22

We plan to continue our work on the development and validation of MD_REACT8

modeling software with a focus on further refining our capability to use this model to simulate

thermally induced changes in the condensed phase of burning polymers.

5. References

 

(1) Lyon, R. E. “Memorandum: FAA Advanced Fire-Safe Aircraft Materials Research

Program.” 1994, p. 1.

(2) (a) Gianellis, E. P. Advanced Materials 1996, 8, 29. (b) Kojima, Y.; Usuki, A.; Kawasumi,

M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J. Mater. Res. 1993, 8, 1185. (c)

Wang, Z.; Pinnavaia, T. J. Chem. Mater. 1998, 10, 1820. (d) Burnside, S. D.; Giannelis, E. P.

Chem. Mater. 1995, 7, 1597. (e) Lee, J.; Takekoshi, T.; Giannelis, E. P. Mat. Res. Soc. Symp.

Proc. 1997, 457, 513.

(3) (a) Gilman, J. W.; Kashiwagi, T.; Lichtenhan, J. D. SAMPE Journal 1997, 33, 40. (b)

Gilman, J. W.; Kashiwagi, T.; Brown, J. E. T.; Lomakin, S.; Giannelis, E. P.; Manias, E.

Proceedings of 43rd Inter. SAMPE Symp. And Exib. 1998, 1053. (c) Gilman, J. W.; Kashiwagi,

T.; Nyden, M.; Brown, J. E. T.; Jackson, C. L.; Lomakin, S.; Giannelis, E. P.; Manias, E.

Chemistry and Technology of Polymer Additives. The Royal Society of Chemistry, Cambridge,

1999, 249-265. (d) Gilman, J. W. App. Clay Sci. 1999, 15, 31.

(4) (a) Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 573. (b) Tyan, H.; Liu,

Y.; Wei, K. Chem. Mater. 1999, 11, 1942. (c) Tyan, H.; Liu, Y.; Wei, K. Polymer 1999, 40,

4877.

(5) (a) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694. (b) Vaia, R. A.;

Vasudevin, S.; Krawiec, W.; Scanlon, L. G.; Giannelis, E. P. Adv. Mater. 1995, 7, 154. (c)

Giannelis, E. P. Adv. Mater. 1996, 8, 29.

(6) (a) Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Poly. Sci, Part A 1993,

31, 2493. (b) Yang, Y.; Zhu, Z.; Yin, J.; Wang, X.; Qi, Z. Polymer 1999, 40, 4407.

(7) This industrial technique is used by General Electric Inc. to make Ultem (polymer 1). The

monomers for this synthetic route are not commercially available and the nuances of the

chemistry are best known to GE.

(8) Nyden, M. R., International Aircraft Fire and Cabin Safety Research Conference, November

16-20, Atlantic City, NJ (1998).

(9) Levchik, S. V.; Weil, E. D.; Lewin, M. Polym. Int. 1999, 48, 1.

(10) Levchik, S. V.; Costa, L.; Camino, G. Polym. Degrad. and Stab. 1992, 36, 229.

(11) Silverstein, R. M; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic

Compounds, (John Wiley and Sons, NY, 1963), Third Edition, p. 108.

(12) de Haseth, J. A.; Andrews, J. E.; McClusky, J. V.; Priester, R. D. Jr.; Harthcock, M. A.;

Davis, B. L. Appl. Spectrosc. 1993, 47, 173.

(13) Holmes, D. R.; Bunn, C. W.; Smith, D. J. J. Polym. Sci., 1955, 17, 159.

(14) Nyden, M. R.; Gilman, J.W. Computational and Theoretical Polymer Science 1997, 7, 191.

(15) Gilman, J. W.; Kashiwagi, T.; Nyden, M. R.; Brown, J. E. T.; Jackson, C. L.; Lomakin, S.;

Giannelis, E. P.; Manias, E. "Flammability Studies of Polymer Layered Silicate Nanocomposites:

Polyolefin, Epoxy, and Vinyl Ester Resins," in: Chemistry and Technology of Polymer Additives,

S. Al-Malaika, A. Golovoy, and C.A. Wilkie, Ed., (Blackwell Science Ltd., Oxford, 1999).