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Flame Retardant Epoxy Resins containing Aromatic Poly(phosphonamides) K.L. Gordon, C.M. Thompson and R.E. Lyon High Performance Polymers 2010 22: 945 originally published online 13 May 2010 DOI: 10.1177/0954008310363164 The online version of this article can be found at: http://hip.sagepub.com/content/22/8/945

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Flame Retardant Epoxy Resins containing Aromatic Poly(phosphonamides) K. L. GORDON1 Advanced Materials and Processing Branch, NASA-Langley Research Center Hampton, VA 23681-2199, USA

C. M. THOMPSON National Institute of Aerospace, 6-A W. Taylor Street, MS 226, Hampton, VA 23666-1399, USA

R. E. LYON Federal Aviation Administration, William J. Hughes Technical Center, Atlantic City International Airport, NJ 08405, USA (Received 21 January 20101 accepted 21 January 2010)

Abstract: As part of a program to develop more survivable aircraft, flame-retardant epoxy resins were investigated for their potential as fire-resistant exterior composite structures for future subsonic commercial and general aviation aircraft. Four different poly(phosphonamide)s were prepared by low temperature NMR condensation and characterized by Fourier transform infrared spectroscopy, 1 H-NMR spectroscopy, 31 P-NMR, differential scanning calorimetry, viscometry and gel-permeation chromatography. The poly(phosphonamides) were used as toughening agents (with 4,41 -diaminodiphenyl sulfone) to partially cure a commercially available unmodified liquid epoxy resin. The resulting cured epoxy formulations were evaluated for water uptake, flame retardance and fracture toughness. The formulations show good flame retardation with phosphorus content as low as 1.6% by weight, but exhibited high moisture uptake compared to the baseline epoxy. The fracture toughness of the cured formulations showed no detrimental effect due to phosphorus content (2 1.5% P). The heat of combustion of the pyrolysis gases, h c = 23.5 3 1.3 kJ g41 for the poly(phosphonamide) formulations were essentially the same as the N,N,N 1 ,N 1 -tetraglycidylether of -4,41 -methylene dianiline/4,41 -diaminodiphenyl sulfone epoxy formulation, h c = 24 kJ g41 .

Key Words: Flame retardant, polyphosphonamides, epoxy resins

1. INTRODUCTION Epoxy resins are defined as a polymer/macromolecular resin containing more than one epoxide molecule group capable of forming tight cross-linked polymer structures [1].

High Performance Polymers, 22: 945–958, 2010 1 The Author(s), 2010. Reprints and permissions: 5 http://www.sagepub.co.uk/journalsPermissions.nav

DOI:10.1177/0954008310363164

946 K. L. GORDON ET AL.

They offer great versatility, low shrinkage, good chemical resistance, and outstanding adhesion and have applications in the fields of electronics (printed wiring boards and semiconductor encapsulation) and transportation (automotive, high speed trains, military and commercial aircraft) as composite structural and furnishing elements [2]. However, one of the major drawbacks of these materials is their flammability. Imparting flame retardancy to epoxy resins has been the focus of many research efforts in recent years [3–10]. Epoxy resins can be made fire retardant either by incorporating fire-retardant additives or by copolymerization with reactive fire retardants. Halogenated compounds, in conjunction with various inorganic materials, are used for additive-type flame retardancy for epoxies and are considered the state of the art. However, the generation of toxic and corrosive fumes during combustion, the reduction of thermal stability of polymers due to halogens, and environmental pressure to eliminate halogenated flame retardants, have caused the focus of industrial and academic research to shift to phosphorus-based fire retardants. Phosphorus compounds are known to impart flame retardancy via condensed phase and gas phase mechanisms [11]. In the condensed phase, phosphorus catalyzes char formation which protects the underlying material from heat and acts as a barrier to the release of fuel gases from the surface. When acting in the condensed phase as a char catalyst, phosphorus retards the spread of fire with minimal release of toxic gases [12]. In the gas phase, phosphorus acts as flame poison with phosphorous oxide radicals (PO species) participating in a kinetic mechanism analogous to that of halogens in flames [13, 14]. Gas phase activity is indicated by low heats of flaming combustion, the production of visible smoke and mineral acids (halogens), and high yields of carbon monoxide as a consequence of incomplete combustion of the fuel gases in the flame. Polymers are commonly made flame resistant when phosphorus is incorporated as an additive by simple physical blending or as a reactive co-monomer by introducing phosphorus-containing molecules into the polymer backbone by chemical means. Incorporation of phosphorus molecules into polymer chains ensures high homogeneity, in contrast to physically blended systems where additives tend to leach out and have a negative impact on mechanical properties and/or processability. Phosphorus can be incorporated either in the epoxy resin part or into the curing agents for two component thermosetting epoxy polymers [15]. The incorporation of phosphorus into the curing agent is the more efficient and economic of the two methods. Over the years, many studies have been reported on the preparation of phosphoruscontaining polymers. The preparation of poly(phosphonamide)s by direct condensation of phenyl phosphonic dichloride with diamines constitutes a major area of focus in previous studies. In 1968, Nielson and co-workers reported the preparation of low molecular weight poly(phosphonamide)s by the interfacial condensations of p-phenyl phosphonic dichloride with diamines [16, 17]. Similarly, Gutmann and coworkers also reported low molecular weight polymers via condensation reactions of the dichloride with diamines in solution [18–20]. To our knowledge, no poly(phosphonamide) high polymer has been reported to date. In this study, the feasibility of incorporating phosphorus into 177 6 C (350 6 F) cured epoxy formulations to provide fire-resistant polymer structural composites with little or no compromise in processing, handling, physical, and mechanical properties is examined.

FLAME RETARDANT EPOXY RESINS 947

Scheme 1. Preparation of poly(phosphonamides).

A series of poly(phosphonamides) have been synthesized (scheme 1) as partial curing agents for epoxy resins and incorporate phosphorus into the cured polymer network.

2. EXPERIMENTAL 2.1. Materials

N,N,N 1 ,N 1 -Tetraglycidylether of -4,41 -methylene dianiline (TGMDA), lithium bromide, N 1 ,N 1 -dimethylacetamide (DMAC), and 4,41 -diaminodiphenyl sulfone (DDS) were pur-


chased from commercial sources and used as received. Phenyl phosphonic dichloride (PPDC) and pyridine were purchased from commercial sources and dried and distilled prior to use. 1-methyl-2-pyrrolidinone (NMP) Chromasolv Plus, HPLC grade solvent and a Fluka Polystyrene (GPC) Standard ReadyCal Set M(p) 1,500–6,500,000 were purchased from Sigma-Aldrich and used as received. All diamines were purchased from commercial sources and were recrystallized prior to use. Diamines were completely dissolved into hot ethanol and activated carbon was added. The solution was filtered by vacuum filtration. The filtrate was then poured into a second flask and placed into an ice bath to cool. Small amounts of water were then added to induce recrystallization. The crystals were filtered off and dried in a vacuum oven at 60 6 C for 12 h. 2.2. Characterization 1

H- and 31 P-NMR spectra were obtained on a Bruker 300 NMR spectrometer. The samples were dissolved in the appropriate deuterated (DMSO) solvent. Differential scanning calorimetry (DSC) was conducted on a Shimadzu DSC-50 thermal analyzer. Thermal scans were conducted at a rate of 20 6 C min41 . Infrared Spectra were obtained on an IR 300 Thermoelectron infrared spectrometer. Inherent viscosities of polymers were measured in a Cannon-Ubbelohde viscometer at 25 6 C in N 1 ,N 1 -dimethylacetamide 0.5 g dL41 concentration. The molecular weights were determined by gel permeation chromatography (GPC) using a Viscotek Model 203 Triple Detection Analysis (TDA) gel permeation chromatograph. The solvent used for chromatography was filtered (NMP) treated with 0.02 mol L41 LiBr. Solutions injected had concentrations of 5.0 mg mL41 . The dilute solutions were prepared within 24 h of injection into the GPC. The samples were filtered through a teflon 0.2 1m filter prior to each run. Sample runs were conducted at a flow rate of 1.0 mL min41 at a temperature of 60 6 C. GPC was performed on a three column bank consisting of a Viscotek Viscogel I series mixed bed high molecular weight column with an exclusion limit 2 10 M (106 ), in series with a Viscotek Viscogel I series mixed bed medium molecular weight column with an exclusion limit 2 200 K and a Viscotek Viscogel I series mixed bed low molecular weight column with an exclusion limit 2 20 K. The Viscotek Model 203 Triple Detector Analysis gel permeation chromatograph was equipped with a differential refractometer and a four capillary bridge viscometer detector. A universal calibration curve was generated using narrow molecular weight distribution polystyrene standards having molecular weights 1,500 to 5,330,000 g mol41 . 2.3. Polymer synthesis

Four different poly(phosphonamides) were prepared with a theoretical number-average molecular weight (Mn ) of 10 000 g mol41 by offsetting the stoichiometry of the aromatic diamine and PPDC using Carother’s equation. Benzoyl chloride was added at the end of each reaction as a monofunctional endcapper.

FLAME RETARDANT EPOXY RESINS 949 2.3.1. Direct polycondensation of PPDC and 3,31 -diaminobenzophenone (PPA-1)

In a three-necked, round-bottom flask equipped with a nitrogen inlet, addition funnel, drying tube, condenser, and mechanical stirrer, 3,31 -diaminobenzophenone 20.19 g (0.095 moles) and 150 mL of pyridine, were added and allowed to stir under nitrogen gas at room temperature. The diamine solution was cooled to a temperature of 0–5 6 C, and 17.72 g (0.09091 moles) of PPDC in 50 mL of pyridine were added dropwise to the diamine solution over a period of an hour. After the completion of the addition, the temperature of the reaction was taken to 120 6 C and stirred overnight. The endcapper, benzoyl chloride, was added in excess and allowed to stir for an additional 2 h. The polymer solution was precipitated into deionized water and pulverized in a Waring Blendor. The polymer was filtered and dried in a vacuum oven at 120 6 C for 16–24 h. FT-IR: 1289 cm41 (P=O), 881 cm41 (P– N), and 3393 cm41 (–NH–). 1 H-NMR (d6 –DMSO): 3 = 6.8–8.0 ppm (m, 4H, phenyl), (m, 5H, phenyl), 3 = 10.5 ppm (s, 5H, acyl). 31 P-NMR (d6 –DMSO): 3 = 8.56 ppm. 4inh = 0.12 measured in DMAC at 25 6 C. 2.3.2. Direct polycondensation of PPDC and 4,41 –thiodianiline (PPA-2)

In a three-necked, round bottom flask equipped with a nitrogen inlet, addition funnel, drying tube, condenser, and mechanical stirrer, 4,41 –thiodianiline 18.9911 g (0.0878 moles) and 150 mL of pyridine were added and allowed to stir under nitrogen gas at room temperature. The diamine solution was cooled to a temperature of 0–5 6 C and 16.5547 g (0.0849 moles) PPDC in 50 mL of pyridine were added dropwise to the diamine solution over a period of an hour. After completion of the addition, the temperature of the reaction was taken to 120 6 C and stirred overnight. The endcapper benzoyl chloride was added in excess and allowed to stir for an additional 2 h. The polymer solution was precipitated into deionized water and pulverized in a Waring Blendor. The polymer was filtered and dried in a vacuum oven at 120 6 C for 12–16 h. FTIR: 1265 cm41 (P=O), 918 cm41 (P–N), and 3166 cm41 (–NH–). 1 H=NMR (d6 –DMSO): 3 = 6.5–8.0 ppm (m, 4H, phenyl), (m, 5H, phenyl), 3 = 10.34 ppm (s, 5H, acyl). 31 P-NMR (d6 –DMSO): 3 = 8.57 ppm. 4inh = 0.17 measured in DMAC at 25 6 C. Mn = 4160 g mol41 and Mw = 8146 g mol41 (as determined by GPC.) 2.3.3. Direct polycondensation of PPDC and 4,41 -diaminodiphenyl sulfone (PPA-3)

In a three necked round bottom flask equipped with a nitrogen inlet, addition funnel, drying tube, condenser, and mechanical stirrer, 20.0 g (0.09431 moles) 4, 41 4diamino diphenyl sulfone and 150 mL of pyridine were added and allowed to stir under a strong flow of nitrogen gas at room temperature. The diamine solution was cooled to a temperature of 0–5 6 C. 17.72 g (0.09091 moles) of PPDC in 50 mL of pyridine were added dropwise to the diamine solution over a period of an hour. After the completion of the addition, the temperature of the reaction was taken to 120 6 C and stirred overnight. The endcapper benzoyl chloride was added in excess and allowed to stir for an additional 2 h.


The polymer solution was precipitated into deionized water and pulverized in a Waring Blendor. The polymer was filtered and dried in a vacuum oven at 120 6 C for 16–24 h. FTIR: 1289 cm41 (P=O), 915 cm41 (P–N), and 3258 cm41 (–NH–). 1 H-NMR (d6 –DMSO): 3 = 6.5–8.0 ppm (m, 4H, phenyl), (m, 5H, phenyl), 3 = 10.6 ppm (s, 5H, acyl). 31 P-NMR (d6 –DMSO): 3 = 8.57 ppm. 4inh = 0.12 measured in DMAC at 25 6 C. Mn = 3097 g mol41 and Mw = 5381 g mol41 (as determined by GPC.) 2.3.4. Direct polycondensation of PPDC and 4,41 -oxydianiline (PPA-4)

In a three-necked, round bottom flask equipped with a nitrogen inlet, addition funnel, drying tube, condenser, and mechanical stirrer, 20.24 g (0.1 moles). 4,41 -oxydianiline and 150 mL of pyridine were added and allowed to stir under a strong flow of nitrogen gas at room temperature. The diamine solution was cooled to a temperature of 0–5 6 C. 18.91 g (0.097 moles) of PPDC in 50 mL of pyridine were added dropwise to the diamine solution over a period of an hour. After the completion of the addition, the temperature of the reaction was taken to 120 6 C and stirred overnight. The endcapper benzoyl chloride was added in excess and the reaction mixture was allowed to stir for an additional 2 h. The polymer solution was precipitated into deionized water and pulverized in a Waring Blendor The polymer was filtered and dried in a vacuum oven at 120 6 C for 16–24 h. FT-IR: 1206 cm41 (–P=O), 922 cm41 (–P–N–), and 3064 cm41 (–NH–). 1 H-NMR (d6 –DMSO): 3 = 6.7–8.0 ppm (m, 4H, phenyl), (m, 5H, phenyl), 3 = 10.28 ppm (s, 5H, acyl). 31 P-NMR (d6 –DMSO): 3 = 8.9 ppm. 4inh = 0.28 measured in DMAC at 25 6 C. Mn = 2074 g mol41 and Mw = 6817 g mol41 in (as determined by gel-permeation chromatography.). 2.4. Neat epoxy resin plaque preparation

Circular resin plaques were prepared by blending TGMDA with 80% stoichiometry of the curing agent at room temperature. The formulations were heated to and maintained at 2 90 6 C with periodic stirring for 1–4 h. The formulations were then degassed for 1–2 h in a vacuum oven at 90 6 C and later cured for 4 h at 100 6 C followed by a 2 h post-cure at 177 6 C. 2.5. Preparation of cured epoxy/poly(phosphonamide) resin formulations

Circular resin plaques were prepared by blending TGMDA with 47 phr of the curing agent with 20 phr loading for the poly(phosphonamides) at room temperature. The poly (phosphonamides) were dissolved in a small amount of DMAC prior to mixing with the liquid epoxy resin. The formulations were heated to and maintained at temperatures of 90, 100, and 110 6 C with periodic stirring for 1–4 h. The formulations were then degassed for 3–4 h in a vacuum oven at 90–100 6 C and later cured for 4 h at 100 6 C followed by a 2 h post-cure at 177 6 C.


2.6. Flammability testing 2.6.1. Flame resistance

A flame resistance test was conducted by placing a cured epoxy specimen approximately 1.5 cm 7 1.5 7 0.4 cm in a propane torch flame for 10 s and noting the time required for the sample to self-extinguish upon removal from the flame. 2.6.2. Microscale combustibility

Pyrolysis combustion flow calorimetry (microscale combustion calorimetry) was conducted on 2–5 mg samples according to a standard method at a heating rate of 1 6 C s41 to 800 6 C with nitrogen flowing at 80 cm3 min41 through the pyrolyzer [21]. The pyrolysis gases and nitrogen exiting the pyrolyzer are mixed with 20 cm3 min41 of oxygen prior to entering the combustor for 10 s at 900 6 C. Water from the combustion process is scrubbed from the gas stream and the oxygen depletion and flow rate are measured to calculate the specific heat release rate of the sample. Four thermal combustion properties are measured during the 15 minute test: the heat release capacity HRC (J (g-K)41 ), which is the maximum heat release rate (W g41 ) divided by the heating rate (K s41 ), the total heat released by combustion of the fuel gases per unit initial mass of sample HR (J g41 ), the temperature at peak heat release rate Tp ( 6 C) and the anaerobic pyrolysis residue (char yield), which is the mass fraction of sample remaining after heating to 800 6 C in nitrogen during the test. Reported thermal combustion properties are the average of three measurements on each composition. 2.7. Mechanical properties 2.7.1. Plane strain fracture toughness

Single-edged notched bend (SENB) specimens were tested following ASTM D 5045-99 [22] Cured epoxy formulations having dimensions of approximately 6.35 cm 7 1.30 cm 7 1.02 cm were cut from an epoxy resin plaque. A crack was initiated with a razor blade and specimens were tested at a crosshead speed of 0.51 mm/min on a Korros Data test stand equipped with a 0.5 kN (45.5 kg) load cell [23]. Three to five specimens of each resin formulation were tested at room temperature. 2.8. Moisture uptake

Cured epoxy resin/poly(phosphonamide) formulation plaques were broken into fragments. Fragments of each of the cured resin plaques were dried in a vacuum oven at 110 6 C for 24 h to remove any absorbed moisture and then placed in a closed chamber saturated with water vapor (water boil) for 72 h. The weight of each of the fragments were measured prior to and after the water boil.

952 K. L. GORDON ET AL. Table 1. Polyphosphonamides physical properties, 10 000 g mol41 .

4inh (dL g41 ) a

Tg (6 C) b

1

0.12

149

2

0.17

157

3

0.12

158

4

0.28

155

Polyphosphonamide PPA

a b

Diamine

Inherent viscosity measured in DMAC at 25 6 C. Thermal analysis was conducted on a Shimadzu DSC-50 thermal analyzer, third thermal scan.

Table 2. Molecular weights of diamine based poly(phosphonamides).

Polymers PPA-2 PPA-3 PPA-4

Theoretical: Mn 7 103 10 10 10

Mn by GPC 7 103 4.2 3.1 2.1

Mw by GPC 7 103 8.1 5.4 6.8

Mw /Mn 2.0 1.7 3.3

3. RESULTS AND DISCUSSION 3.1. Synthesis and characterization of polyphosphonamides

Aromatic diamine based poly(phosphonamide) PPA-1 through PPA-4 thermoplastics were synthesized by reacting phenyl phosphonic dichloride with the respective diamine in pyridine solvent. Pyridine also serves as the acid acceptor in the polymerization. Four different poly(phosphonamide)s were prepared. The polymers were characterized by 1 H-, 31 PNMR, DSC, FT-IR spectroscopy, viscometry, and GPC. The results obtained are listed in tables 1 and 2. The IR absorption peaks of the P–N bond appeared at 922 cm41 for PPA-4, at 918 cm41 for PPA-2, at 881 cm41 for PPA-1, and at 915 cm41 for PPA-3. Other specific absorption

FLAME RETARDANT EPOXY RESINS 953 peaks appeared at 1200 cm41 (–P=O– bond), and 3250 cm41 (–NH–). One major peak was found in 31 P-NMR spectra for the polymers, PPA-1, -2, -3, and -4 (3 2 8.5 ppm), whereas a single peak for phenyl phosphonic dichloride displays a single peak at 36.5 ppm. All of the polymers had similar 1 H-NMR spectra (phenyl peaks : 3 2 7.0–8.0 ppm). Also, an amine peak was observed at 3 2 4.5–6.0 ppm when the polymers were amino terminated and/or when there was small amounts of unreacted diamine present in polymer. The four poly(phosphonamide)s were prepared with a target number-average molecular weight (Mn ) of 10 000 g mol41 by offsetting the stoichiometry of the aromatic diamine and phenyl phosphonic dichloride using Carother’s equation. In all cases, the inherent viscosities were lower than expected indicating low molecular weight. In table 2, the molecular weights determined by quantitative GPC are compared with the calculated or target molecular weight for the polymers. The results indicate that the obtained molecular weights are not close to target molecular weight. This may be due to poor solubility of the products in the solvent pyridine. Although, the various diamines readily dissolved upon addition into pyridine, as the propagation of the polymer chain proceeded upon addition of PPDC, the solubility appeared to decrease, and the product precipitated out of solution thus prematurely terminating and impeding growth of the polymer chain. N 1 ,N 1 dimethylacetamide, 1-methyl-2-pyrrolidinone, and dimethylformamide could not be used as solvents, because phenyl phosphonic dichloride reacts with the solvents to form a complex [17]. Another cause may be due to moisture and impurities in the starting reagents. Ultimately, precise control of molecular weight could not be achieved in this system. Additionally, it is possible that some macrocycles formed as indicated by small, broad peaks on the GPC chromatograms at long retention times. 3.2. Cured epoxy resin plaques

Commercial aerospace structural epoxy formulations contain TGMDA as the base epoxy and DDS as the curing agent and are cured at 177 6 C. These formulations also generally contain additional components such as a thermoplastic toughening agent, diepoxy, and in some cases, a catalyst. The following formulations are modifications of the commercial aerospace epoxy in which phosphorus is introduced using polymers PPA 1-4 as partial curing agents with the amine curing agent, DDS, and the epoxy resin, TGMDA. Formulation 1, F1, in table 3 is the control formulation in which TGMDA is cured with an 80% stochiometric amount of DDS. Formulation 1 is used as the baseline to compare properties with the phosphorus-containing epoxy formulations. All formulations were cured for 4 h at 100 6 C and post-cured for 2 h at 177 6 C. 3.2.1. Formulations with diamine-based phosphorus-containing polymers, F2-F5

Formulations 2-5, F2-F5, include the different poly(phosphonamides) PPA 1-4 with DDS in their respective quantities. Each of the cured formulations had a calculated phosphorus


Table 3. Moisture uptake and flame test data of poly(phosphonamides)/TGMDA formulations.

Formulation

Diamine

DDS (phr)

PPA (phr)

P (%)

DDS

47

0

0

Water uptakeb (%w/w) 2.6

F2

47

20

1.1

4.3

3.0 s burn

F3

47

20

1.1

4.5

1.0 s burn

F4

47

20

1.0

9.1

3.0 s burn

F5

47

20

1.2

8.5

2.0 s burn

F1

a b

Flame testa Sustained burn

Sample placed in propane torch flame for 10 s and removed. After 72 h in water at 100 6 C.

content in between the range of 1.0–1.2 wt.%. All of the cured formulations were reddish brown in color and transparent. 3.3. Moisture uptake

Water uptake studies were conducted to determine the amount of water uptake of the cured epoxy/poly(phosphonamide) formulations. Water uptake is known to have an adverse effect on the overall glass transition temperature and mechanical properties of these materials. Additionally, water uptake increase has been observed in epoxy resins containing phosphorus [24]. The pristine formulation, F1, had an average water uptake of 2.6%, which is comparable with that of commercial systems. Formulation, F2, containing 20 phr of the 3,31 -diamino benzophenone based poly(phosphonamide) had a higher water uptake of 4.3% in comparison to F1, but the lowest water uptake of all of the poly(phosphonamides) screened. Formulation F3 was next with a water uptake of 4.5% whereas the formulations of F4 and F5 both had water uptakes greater than 8.0%. The varying water uptake of the formulations appear to be a function of the pendant polar functional group associated with each of the respective diamines of the perspective


Table 4. Plain-strain fracture toughness data.

Formulations F1 F2 F3 F4 F5

K 1c , (MPa m152 3 1 SD) 0.62 3 0.04 0.93 3 0.10 0.71 3 0.08 0.62 3 0.05 0.70 3 0.09

poly(phosphonamide)s in addition to the varying phosphorus content in each of the formulations. Polar groups in hydrophilic polymers have an affinity for water pickup. The calculated phosphorus content for the formulations ranged in between 1.0–1.2 wt.%. Due to the high water uptakes of formulations F4 and F5, the formulations can not be considered for use in structural epoxy formulation. 3.4. Epoxy resin/poly(phosphonamide) formulation, F2–F5, mechanical properties

Mechanical properties for the different epoxy resin/poly(phosphonamide) formulations, F2–F5, were measured to evaluate the effect of phosphorus content on K 1c . The K 1c values allocated for the formulations F2, F3, F4, and F5 were higher than the fracture toughness of the baseline formulation (shown in table 4). 3.5. Flammability 3.5.1. Flame resistance

The incorporation of phosphorus at the 1% level by weight in the form of poly(phosphonamides) produce self-extinguishing compositions with TGMDA epoxy as shown in table 3. The poly(phosphonamides) were comparable in fire retardant efficiency to phosphonates such as poly(m-phenylenemethylphosphonate) (PMP) and 9,10-dihydro-9-oxa10-phosphaphenanthrene-10-oxide (DOPO), which produced self extinguishing UL 94 V-0 ratings [25] in TGMDA/DDS epoxies at 1–1.5% phosphorus by weight [26, Levchik, S.V., private communication]. In multifunctional (Novolac) epoxies cured with dicyandiamide, the PMP and DOPO phosphonates produced self-extinguishing UL 94 V-0 ratings in glass fabric laminates at 2 wt.% phosphorus [27]. 3.5.2. Combustibility

The thermal combustion properties for the poly(phosphonamide) epoxy formulations are shown in table 5. The heat release capacity (HRC), which is the primary indicator of flammability, decreased by about 20% compared to F1 for the poly(phosphonamide) formulations F2–F5 even though HR did not change significantly. The lower HRC is a con-


Table 5. Thermal combustion properties of poly(phosphonamide) formulations.

Formulation

F1 F2 F3 F4 F5

HRC (J (g-K)41 )

HR (kJ g41 )

Tp (6 C)

238 3 4 199 3 22 196 3 9 193 3 10 182 3 10

17.5 3 0.3 17.0 3 0.6 17.3 3 0.3 16.3 3 0.1 17.0 3 0.4

404 3 2 372 3 4 385 3 2 383 3 4 384 3 5

Char yield (% w/w) 27 3 1 32 3 2 27 3 1 30 3 1 27 3 2

P in compound (% w/w)8 0 1.5 1.5 1.4 1.6

8

Calculation based omatomic composition of thermoplastic repeat unit incorporated at 20 phr (16.77% w/w) in TGMDA-DDS epoxy resin.

sequence of the heat of combustion of the fuel gases being released over a broader temperature range in the F2–F5 formulations because the thermal decomposition temperature Tp was lower. The heat of combustion of the pyrolysis gases was h c = HR/(1 – char fraction) = 24 kJ g41 for the TGMDA/DDS epoxy formulation F1, which is essentially the same as that of the poly(phosphonamide) formulations F2–F5, h c = 23.5 3 1.3 kJ g41 . These data suggest that the chemical composition of the fuel gases and char were not changed significantly by the poly(phosphonamides). All of these changes in thermal combustion properties are consistent with a mechanism for self-extinguishing behavior in flame tests due to char swelling (intumescence) [28]. It is known that phosphorus catalyzes dehydration of cellulosic and epoxy polymers via cleavage of hydroxyl groups and that this dehydration occurs at temperatures below the thermal decomposition temperature of the pure polymer but above the boiling temperature of the evolved water [28]. The elimination of water vapor during thermal decomposition has a blowing effect that results in char swelling/intumescence which thermally insulates the underlying polymer and thus prevents burning which yields self-extinguishing epoxy formulations [24].

4. CONCLUSIONS Four different poly(phosphonamide)s were successfully synthesized and used as partial curing agents for a TGMDA/DDS aerospace epoxy. Characterization using FT-IR, 1 HNMR, and 31 P-NMR confirmed the chemical structures of the phosphorus-containing polymers. The poly(phosphonamides) were successfully used as toughening agents (with DDS) to partially cure a commercially available unmodified liquid epoxy resin. The fracture toughness of the cured formulations showed no detrimental effect due to phosphorus content (2 1.5% P). The formulations showed good flame retardation with phosphorus content as low as 1.6% by weight, but exhibited high moisture uptake relative to the baseline epoxy. The heat of combustion of the pyrolysis gases, h c = 23.5 3 1.3 kJ g41 for the poly(phosphonamide) formulations were essentially the same as the baseline TGMDA/DDS epoxy formulation F1, h c = HR/(1 – char fraction) = 24 kJ g41 , suggesting


that the chemical composition of the fuel gases and char were not changed significantly by the poly(phosphonamides). Acknowledgements. The authors acknowledge and appreciate Mrs Patricia Davis of NASA LaRC for collection of GPC data and Mr Paul Hergenrother, Dr John Connell, and Dr Emilie Siochi of NASA LaRC for their valuable discussion. The use of trade names of manufacturers does not constitute an official endorsement of such products or manufacturers, either expressed or implied, by the National Aeronautics and Space Administration or the Federal Aviation Administration.

NOTE 1. Author to whom correspondence should be addressed: [email protected]

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High Performance Polymers

High Performance Polymers

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