phosphorylated epoxy resin: effect of phosphorus content on the properties of laminates

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2003 21: 5Journal of Fire SciencesPreeti Jain, Veena Choudhary and I. K. Varma

Properties of LaminatesPhosphorylated Epoxy Resin: Effect of Phosphorus Content on the

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Phosphorylated Epoxy Resin:Effect of Phosphorus Contenton the Properties of LaminatesPREETI JAIN, VEENA CHOUDHARY* AND I. K. VARMA

Centre for Polymer Science and EngineeringIndian Institute of TechnologyDelhi, Hauz KhasNew Delhi – 110 016, India

(Received October 15, 2002)

ABSTRACT: The paper describes the properties of glass-fabric reinforcedcomposites based on novel phosphorylated epoxy matrix resins.Limiting oxygenindex as high as 80 were attained using such novel matrix resins. These resinswere prepared by curing diglycidyl ether of bisphenol-A (DGEBA) withphosphorylated amines such as bis(3-aminophenyl) methyl phosphine oxide(B), tris(3-aminophenyl)phosphine oxide (T), bis[3(30-aminobenzamidophenyl)]-methylphosphine oxide (MB), bis[3(40-aminobenzamido phenyl)]methylpho-sphine oxide (PB), tris[3(30-aminobenzamidophenyl)]phosphine oxide (MT) andtris[3(40-aminobenzamido phenyl)] phosphine oxide (PT). Phosphorus- contain-ing amide-acid amines were also used for curing and were prepared by reacting Twith 1,2,4,5-benzenetetracarboxylic acid anhydride(P); 3,30,4,40-benzophenone-tetracarboxylic acid dianhydride (Z) and 4,40-(hexafluoroisopropylidene)diphtha-lic acid anhydride (F). Interlaminar shear strength (ILSS), flexural strength andmodulus of composites (32� 3% resin content) based on B, T, MB, PB, MT, PTand state-of-the-art curing agent 4,40-diaminodiphenyl sulfone (D) were higherthan the composites prepared using amide-acid amines. Increase in ILSS wasobserved on heating at 180�C for 200 h. On hygrothermal ageing of the samplesfor 7 days, the ILSS values decreased but those cured with amide-acid aminesshowed the least change. A significant improvement in limiting oxygen index(LOI) was observed by increasing the phosphorus content of the composites.

*Author to whom correspondence should be addressed.

JOURNAL OF FIRE SCIENCES, VOL. 21 – JANUARY 2003 5

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KEY WORDS: phosphorylated epoxies, thermosetting resins, flame resistant,curing agents, DGEBA, amide-amines, imide-amines.

INTRODUCTION

THE USE OF fibre-reinforced composites in structural engineeringapplications has increased in the last two decades. Epoxy resins are themost important class of thermoset matrix resins because of the ease ofprocessing and excellent room temperature properties. Laminates basedon these resins have been widely used both in electronics and aerospaceindustries. Currently more than 90% of the thermosets used for printedwiring laminates are epoxy-based. However, flammability of these resinsis a major limitation in the areas requiring high flame resistance.Phosphorus-containing hardeners have shown in the past to increasethe flame resistance of DGEBA [1–10]. Synergistic combination ofphosphorus and nitrogen in the hardeners leads to significant improve-ment in flame resistance of cured DGEBA [11].

In our continued effort for improving the flame resistance of epoxyresins, we have reported synthesis and characterisation of novelphosphorus-containing amines as hardeners for epoxy resins, especiallyDGEBA [12–17]. The main objective of the present studies was toinvestigate the effect of such hardeners on the properties of glass fabricreinforced laminates. The amines used for this purpose are shown inScheme 1.

Mixed amines based on phosphorus containing amines B/T/MT and 4,40-diamino diphenyl sulfone (50 : 50 molar ratio) and TP, TZ, TF with Twere also used for curing of DGEBA. Such studies can yield usefulcorrelation between phosphorus content of resin and flame resistance.

The amide-acid linkage in TP, TZ and TF during curing of epoxychanges to imide linkage according to Scheme 2.

EXPERIMENTAL

Materials

Epoxy-compatible E-glass fabric RP8 (density 280 g/m2) of PilkingtonFibre glass Co. was used as reinforcement. Diglycidyl ether of bisphenol-A(DGEBA, Grade LY 556; epoxy equivalent 177, Hindustan Ciba GeigyLtd.) was used as matrix resin. T, B, MB, PB, MT, PT, TP, TZ and TFwere synthesized in the laboratory using the procedure describedelsewhere [8,14,17,18]. 4, 40-diaminodiphenyl sulfone (D) was procuredfrom Fluka.

6 P. JAIN ET AL.




PH N NH

O

CH

P

OH N NH

NH

Bis(3-aminophenyl)methylphosphine oxide (B) [246]

Tris(3-aminophenyl)phosphine oxide (T) [323]

P

OArCOHN NHCOAr

CHNH

CHNH

NH

NH

NH

NH

X

X = Ar = Bis[3(3'-aminobenzamidophenyl)]methylphosphine oxide (MB) [484]

X = Ar = Bis[3(4'-aminobenzamidophenyl)]methylphosphine oxide (PB) [484]

X = Ar = Tris[3(3'-aminobenzamidophenyl)]phosphine oxide (MT) [680]

X =Ar = Tris[3(4'-aminobenzamidophenyl)]phosphine oxide (PT)[680]

Where

,

,

,

,

O

C C

CF

CF

P

OH N

NH

C

C

O

C

O

NH

OHHO

HNP

O

NH

C

C

O

C

O

NH

OHHOP

ONH

NH

HN

Where

Scheme 1. Representative structures of phosphorus-containing amine hardeners:The letter designations of these amines are shown in parenthesis. Formulated molarmasses of amines are given in square brackets.

C

OO

C

CA r

O

C

O

N H

O HH O

H NRH N R N H C

OO

C

CA r

O

C

O

NRH N R N HN

Scheme 2. Reaction sequence showing the imidisation of amide-acid into imide.

Effect of Phosphorus Content 7




Fabrication of Glass Fabric Reinforced Composites

Eight plies of glass cloth were coated uniformly with a DMF solutionof DGEBA and stoichiometric amounts of curing agent. The prepregsthus obtained were left at ambient conditions overnight and then driedat 100�C in an air oven for 1 h. The prepregs were then stacked togetherbetween two teflon sheets and placed in a compression-mouldingmachine (Carver Laboratory Press). The cure cycle involved heatingat 100�C (contact pressure, 1 h), 125�C (138 MPa, 1 h), 150�C (345 MPa,1 h), 175�C (483 MPa, 1 h), 200�C (690 MPa, 2 h) and then post-curingwas done at 200�C for 2 h in an air oven.

The composites thus prepared have been designated by adding a prefixC to the letter designation of the amine. For example the glass fabricreinforced DGEBA prepared by curing DGEBA with amine T, MT or TPhave been designated as CT, CMT or CTP respectively. Laminates basedon mixed amines such as B or T with D have been designated as CBD orCTD respectively while those based on TP or TF with T have beendesignated as CTPT or CTFT respectively.

Testing of Laminates

Resin content of the laminates was determined by the concentratednitric acid method. The flexural properties and interlaminar shearstrength (ILSS) were measured according to ASTM D-790 and ASTM D2344-76 respectively using a Zwick tensile testing machine, model Z010.An average of 5 specimens is reported. Flame resistance characteristicsi.e. limiting oxygen index (LOI) and smoke density of composites weredetermined according to ASTM D-2863-77 and ASTM D-2843 respec-tively.

ILSS of the samples was also determined after

(i) Isothermal aging at 180�C for 200 and 500 h in an air oven.(ii) Hygrothermal aging for 7 days. For this purpose the laminates were

placed on wire gauze kept above water in a desiccator for a week. Anaverage of five specimens was taken.

RESULTS AND DISCUSSION

In the present studies the molar masses of amines were significantlydifferent. Therefore the quantity of stoichiometric amount of aminesneeded for curing DGEBA ranged from 33 phr (D, B or T) to 65 phr (MB,

8 P. JAIN ET AL.




PB, MT or PT), to 91–132 phr (TP, TZ, TF). This fact has to be takeninto consideration while discussing the properties of laminates.

The prepregs based on DGEBA and amine B, D, T, MB, MT, PBand PT were tacky while those containing amide-acid amines (TP, TFand TZ) were non-tacky and more rigid. This may be due to higher phr(91–132) of amines used for curing DGEBA.

Resin content of the laminates was found to be in the range of32� 3%. The laminates based on amines D, B, T and amide-amines MB,MT, PB, PT had flexural strength in the range 212–334 MPa, flexuralmodulus in the range 17–24 GPa and ILSS in the range 17–25 MPa.Composites based on triamine T (CT) had �17% lower flexural strengthand modulus as compared to composite based on diamines B (CB) or D(CD) which had almost comparable flexural strength and modulus. ILSSof the composites CD, CB and CT was comparable. Similar behaviourwas observed in laminates CMT, CPT or CMB, CPB. Composites basedon DGEBA cured with amide-amines containing m-linkages (i.e. CMTand CMB) exhibited inferior flexural strength and flexural modulus ascompared to their para analogues i.e. CPB/ CPT. This can be explainedon the basis of rigidity imparted by p-linkages.

The mechanical properties of the laminates based on epoxy matrixcured with amide-acid amines were inferior than those based on aminesD, T, B and amide-amines MB, PB, MT and PT. For example, flexuralstrength was in the range of 178–270 MPa, flexural modulus in therange of 15–22 GPa and ILSS in the range of 14–21 MPa. This may bedue to the fact that higher phr of the amide-acid amines (91–132 phr)compared to amines D, B or T (�33 phr) or amide-amines (� 65 phr) wasused for curing of DGEBA, which may be responsible for a decrease inthe epoxy content of the cured resin.

It would be worthwhile to compare the mechanical properties ofthese laminates based on DGEBA cured with phosphorus containingamines with the state-of-the-art resin systems obtained by using 4,40-diaminodiphenyl sulfone (D). Bar charts showing flexural strength,flexural modulus, inter laminar shear strength of various laminatesbased on P-containing amines relative to amine D are shown inFigures 1–3.

It is obvious that flexural strength of all the laminates was inferiorwith respect to the conventionally used amine except in CB wheremarginal improvement was observed. Partial replacement of amine T byD increased flexural strength (CT vs. CTD).

Higher values of flexural modulus were obtained in CB, CMB, CPB,CPT and CTPT compared to CD laminate (Figure 2). ILSS was higher inCMB, CPB and CPT laminates. Thus it is possible to select appropriate





P-containing amine or mixed amines to fabricate laminates with bettermechanical properties than the state-of-the-art materials.

The mechanical properties of laminates based on DGEBA cured withamide-acid amine TF or mixed amine TF and T were similar. Addition ofamine T to amide-acid amine TP (50 : 50 molar ratio, sample CTPT) didnot affect the flexural properties of the laminates, but 26% increase inILSS was observed. Composite having mixed amine TZ and T (CTZT)had �22% higher flexural strength and modulus and �27% higher ILSScompared to composite CTZ.

Figure 2. Relative flexural modulus of laminates fabricated from DGEBA cured withP-containing amines.

Figure 1. Relative flexural strength of laminates fabricated from DGEBA cured withP-containing amines.

10 P. JAIN ET AL.




Smoke Density and LOI of the Laminates

Limiting Oxygen Index of the laminates based on commercial amine Dwas found to be lowest (30%). Composites based on phosphorylatedamines showed better flame resistance than conventional amine D. Barcharts showing the relative change in smoke density and LOI (relative toamine D) are shown in Figures 4 and 5 respectively.

Figure 4. Relative smoke density of laminates fabricated from DGEBA cured withP-containing amines.

Figure 3. Relative ILSS of laminates fabricated from DGEBA cured with P-containingamines.





Presence of the phenyl group in amine T compared to thermally labilemethyl group in amines B increases the aromaticity of amine T. Higheraromatic content increases the char residue, which in turn increasesthe flame resistance. Increasing char formation limits the productionof combustible gases and decreases the exothermicity of the pyrolysisreaction and thermal conductivity of the burning materials andconsequently limits the flammability of the materials. Thus higherLOI value was obtained in composite CT compared to CB.

Composite CMB and CPB contain thermally labile methyl group andexhibited �35% higher smoke density than corresponding compositesCMT and CPT based on amines MT and PT. Laminates CMB and CMTwhich differ from composites CPB and CPT respectively only in the typeof linkage present in the amine showed similar LOI and smoke densityvalues. Thus presence of the m- or p-linkage in the amine hardener doesnot affect the flame resistance of cured epoxies.

Composites CTD, CBD and CMTD showed LOI values in between theLOI values of composites obtained by using pure amines as curingagents.

CTP composite prepared by using amide-acid amine showed highersmoke density and LOI value than CTZ and CTF. Addition of amine Tto these amines did not affect the smoke density in composite CTPT,however an increase in smoke density was observed in compositesCTZT and CTFT. A reduction in LOI values was observed in compositesCTPT and CTFT while no change was observed in CTZT by addition ofamine T to these amines.

In order to evaluate the effect of phosphorus content of laminateson LOI, DGEBA was cured with amines D, B, T; amines with amide

Figure 5. Relative limiting oxygen index of laminates fabricated from DGEBA curedwith P-containing amines.

12 P. JAIN ET AL.




linkages MB, PB, MT, PT; amines with amide-acid linkage TP, TZ, TFand mixed amines. A linear relationship was observed with diamines(Figure 6) and triamines (Figure 7) with following correlations.

LOI¼ 25 P (%)þ 29.2 laminates CD, CB, CBD, CMB and CPBLOI¼ 46 P (%)þ 23.3 laminates CT, CTD, CMT, CPT, CMTD,

CTP, CTZ, CTF, CTPT, CTZT and CTFT

Epoxy resin cured with MB, PB, MT and PT had lower flameresistance compared to B and T cured systems. This may be due to the

Figure 7. Effect of phosphorus content on LOI of laminates cured with triamines.

Figure 6. Effect of phosphorus content on LOI of laminates cured with diamines.





decrease in the phosphorus content from B and T to MB, PB and MT,PT respectively. Composites based on amide-acid amines TP or TBshowed highest LOI value of 78.6% and lowest smoke density value of30%. Amide-acid amines on curing give imide linkages, which isresponsible for enhancing the thermal stability of the resins and hencethe char residue [15,17]. Thus higher LOI and smoke density in suchsystems could be due to the presence of phosphorus as well as thermallystable imide linkages.

Effect of Isothermal Aging on ILSS

Effect of isothermal aging on ILSS was determined by heating thecomposites at 180�C for 200 h and 500 h. A bar chart showing the changein ILSS relative to unaged samples is shown in Figure 8. An increase inILSS was observed on isothermal aging of laminates. This may be due toresidual curing which occurred on heating. An increase of 41% after200 h and 45% after 500 h was observed in some of these laminates.Major increase in ILSS was observed in first 200 h, after that ILSSincreased only marginally.

Effect of Hygrothermal Aging on ILSS

The results of effect of hygrothermal aging on ILSS are summarizedin Table 1. A maximum decrease of 48% in ILSS was observed whencomposites were exposed to hygrothermal aging for 7 days. The decrease

Figure 8. Relative change in ILSS after isothermal aging.

14 P. JAIN ET AL.




in ILSS after hygrothermal ageing was least in case of composites CTPand CTZ, i.e. those cured with amide-acid amines. This reduction inmechanical properties on water absorption may be attributed to thedegradation of a fabric–matrix interfacial bond.

An increase in weight of the laminates indicative of moistureabsorption was observed and this leads to debonding thereby causinga decrease in ILSS. Composites CD, CB, CT, CBD and CTD showed amoisture absorption of 0.02–0.36%. Composites based on amide-aminesi.e. CMB, CPB, CMT, CPT and CMTD showed 0.32–0.62% moistureabsorption. Amide-acid amines are highly polar and their compositesshowed maximum moisture absorption of 1.0–2.5%.

CONCLUSIONS

These studies show that the flame resistance of epoxy resin can beenhanced significantly by using phosphorus-containing amines. Thepresence of thermally stable linkages in amines (e.g. imide moieties)further enhances the LOI. Deterioration in mechanical properties oflaminates was observed by using higher phr of amines. By properselection of hardeners an optimum resin system can be obtained withgood mechanical properties and excellent flame resistance.

Table 1. Effect of hygrothermal aging on ILSS of laminates.

SampleDesignation

ResinContent (%)

% MoistureAbsorptionAfter 7 days

Decrease in ILSSAfter Hygrothermal

Aging for 7 days (%)

CD 31.0 0.23 23CB 31.0 0.02 2CT 33.2 0.34 34CBD 30.2 0.03 7CTD 30.7 0.36 36CMB 29.0 0.32 10CMT 31.1 0.51 17CPT 29.6 0.45 48CMTD 30.3 0.62 40CTP 35.2 1.23 1CTZ 33.6 1.08 1CTF 34.4 2.37 15CTPT 34.9 1.24 31CTZT 33.8 1.70 22CTFT 34.2 2.39 43





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phosphorylated epoxy resin: effect of phosphorus content on the properties of laminates

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