fire retardance in polyamide-6. the effects of red phosphorus and radiation-induced cross-links

FIRE AND MATERIALS

Fire Mater. 25, 179–184 (2001)

DOI: 10.1002/fam.768

Fire Retardance in Polyamide-6. The Effects of RedPhosphorus and Radiation-induced Cross-links

A.I. Balabanovich1, G.F. Levchik1, S.V. Levchik1 and W. Schnabel2 *

1 Institute of Physical Chemical Problems, Belarussian State University, Leningradskaya 14, 220080 Minsk, Belarus2Hahn-Meitner Institut Berlin GmbH, Glienicker Str.100, D-14109 Berlin, Germany

Red phosphorus contained in non-irradiated polyamide-6 at concentrations up to 12.5 does not significantly improve

the fire resistance of the polymer. It weakens the thermal stability of the polymer as reflected by a decrease in the

onset temperature for mass loss from 4078C to 3638C, but causes char formation as indicated by mres ¼ 13wt%, the

non-volatile residue at 6008C. Irradiation of polyamide-6, loaded with red phosphorus, with 60Co-g-rays generatesintermolecular cross-links resulting in an improved fire resistance. However, the absorbed dose necessary to achieve

improvement is too high (>1MGy) from a practical point of view. This inconvenience is overcome by applying

triallyl cyanurate (TAC) as a cross-linking promoter. Typically, polyamide-6 containing 5wt% TAC and 12.5 wt%

red phosphorus exposed to a g-ray dose of 22 kGy yields a V-0 rating (in the mode A UL 94 test) concurrently with a

small increase in the onset temperature for mass loss and a drastic increase in the residue non-volatile at

6008C. Copyright # 2002 John Wiley & Sons, Ltd.

INTRODUCTION

In the course of our studies aimed at making polyamide-6 fire retardant, we have tested the effect of redphosphorus as a flame retardant additive in specimensthat were intermolecularly cross-linked with 60Co-g-rays. Reportedly, red phosphorus is an effective flameretardant for oxygen-containing polymers such aspolyesters, polyurethanes and polyamides. By contrast,it is only marginally active in the case of oxygen-freepolymers such as polyethylene and polystyrene.1�4

According to Taylor and Guest4 the relative quantitiesof red phosphorus to meet the requirements correspond-ing to V-0 rating in the UL 94 test (see below) are lessthan those of conventional flame retardants. A redphosphorus load level of 7% was noted to fulfil V-0requirements in the case of a (non specified) polyamide.4

But it seems that the flame retardance was attained onlyafter heating the red phosphorus-loaded polyamide at arelatively high temperature in the presence of oxygen (at1008C for 20 days).1 Regarding the decreased flamm-ability of oxygen-containing polymers such aspoly(methyl methacrylate) indications were found forthe reaction of red phosphorus with the carbonyl groupsas an important mode of action. The formation of cyclicanhydride and carboxylic acid groups is thought to beresponsible for the observed stabilization of the polymerbackbone toward thermal cleavage.3

Apart from a decrease in the flammability a fireretardant polymer should not release flaming drips uponburning. This requirement can be fulfilled frequently byintroducing intermolecular cross-links into the polymer.An elegant method of generating a three-dimensionalnetwork even in finally processed specimens consists of

irradiating the polymer with high energy radiation suchas electron beam radiation or 60Co-g-rays. Principally,this method can be applied to all polymers thatpredominantly undergo cross-linking upon irradiation.Although polyamide-6 belongs to this group of poly-mers,5,6 radiation-induced cross-linking is not veryfavourable because of the rather low 100 eV yield forcross-linking GðXÞ ¼ 0:67 and the fact that simulta-neously main-chain cleavage occurs with a 100 eV yieldGðSÞ ¼ 0:67. The latter process counteracts networkformation. For these reasons the absorbed dosenecessary to prevent the release of flaming drips frompolyamide-6 specimens is too high to be regarded aspotential practical application. This problem can beovercome, however, to a large extent by loading thepolyamide with a cross-linking promoter such as triallylcyanurate, TAC. According to Stenglin7 TAC enhancesradiation-induced cross-linking of polyamides signifi-cantly and absorbed doses much lower than for the neatpolymer have to be applied to generate an insoluble,three-dimensional network in TAC-filled polyamide-6.The experiments carried out in this work demonstratethat TAC containing polyamide-6 specimens loadedwith red phosphorus and having been subjected toirradiation with 60Co-g-rays perform satisfactorily instandard fire retardance tests. Details are reportedbelow. With regard to high energy radiation-inducedcross-linking of polymers loaded with red phosphorustwo papers by Chinese researchers are notable.8,9 Thepapers report that polyethylene and copolymers onolefin basis that contain both Al(OH)3 and redphosphorus become flame resistant upon g-irradiation.This was inferred from a high oxygen index and the factthat there was no release of flaming drips from theirradiated filled polymer.

Received September 2000Copyright # 2002 John Wiley & Sons, Ltd. Accepted 30 November 2001

*Correspondence to: Dr W. Schnabel, Hahn-Meitner Institut Berlin GmbH, Glienicker Str.100, D-14109 Berlin, Germany.Contract/grant sponsor: German Academic Exchange Service

EXPERIMENTAL

Materials

Polyamide-6 was a commercial product of Khimvolo-kno, Belarus (Mn¼ 3:5� 104). Stabilized red phos-phorus (Safest) was obtained from Italmatch (Italy)and used as received. Triallyl cyanurate (TAC) waspurchased from Aldrich and used as received.

O

OO CH2 2 CH CH2

CH2 CH CH2

CHCHCH

C

N

N N

C C

TAC

2

Sample preparation

Polyamide/additive samples were prepared under nitro-gen in a closed mixer operating at 60 rpm at 2408C. Testrods (10.0� 0.6� 0.3 cm) for LOI determination and(10.0� 1.2� 0.2 cm) for fire retardancy tests were cutfrom slabs formed from the melt.

Irradiation of samples

The specimens were irradiated under argon or air with60Co-g-rays at a dose rate Drabs of 2 kGy/h using aradiation source from the Hahn-Meitner Institute.

Characterization of irradiated samples

Fire retardancy tests. Combustion tests were performedaccording to two modes, A and B. Mode A correspondsessentially to the vertical UL 94 protocol10 (Under-writers’ Laboratory Bulletin 94 vertical burning test).According to this protocol the sample rod was placed ina holder in a vertical position and the lower end of therod was contacted by a methane flame for 10 s thusinitiating burning. A second ignition was made afterself-extinguishing of the flame at the sample. Theburning process was characterized by times t1 and t2pertaining to the two ignitions. Times t1 and t2 denotethe time between ignition and self-extinguishing of theflame. Moreover, it was noted whether flaming dripswere released from the sample during times t1 and t2.The observations made during the tests serve to classifythe samples into three groups as shown in Table 1.

Mode B corresponds to the horizontal UL 94protocol10 (Underwriters’ Laboratory Bulletin 94 hor-izontal burning test). It was applied if samples failed topass the mode A test. According to the protocol of mode

B horizontally positioned test specimens were ignited bya methane or natural gas burner for 30 s. HB (horizontalburning) classification was achieved if the burning ratebetween reference marks did not exceed 76mm/min andif the flame extinguished before the 76mm mark wasreached.

Determination of the limiting oxygen index. The oxygenindex, OI, was determined using a Stanton-Redcroftapparatus following the standard ASTM procedure(ASTM 2863-77). The limiting oxygen index, LOI,denotes the minimum concentration of oxygen in anoxygen/nitrogen atmosphere necessary to sustain aflame at a top-ignited vertical test specimen.11

Thermogravimetric analysis. Thermogravimetric ana-lyses (TGA) were carried out on the samples using aMettler 3000 Thermoanalyzer at a heating rate of 108Cper min.

Determination of gel content and swelling ratio. The gelcontent of the irradiated samples was determined byextraction with formic acid for 10–12 h. It denotes theratio of the mass of the sample after extraction to themass before extraction. The swelling ratio was deter-mined from the ratio of the mass of the swollen sampleobtained immediately after extraction to that of thethoroughly dried sample.

RESULTS AND DISCUSSION

Radiation-induced cross-linking

Cross-linking of neat PA-6. Upon irradiation under anargon or air atmosphere the unloaded PA-6 specimensbecame partially insoluble indicating the formation of athree-dimensional network. The absorbed dose forincipient gelation exceeded 0.57MGy. Notably, oxygenhad only a marginal effect, if any, on the formation ofintermolecular cross-links. The data are presented inTable 2.

Cross-linking of PA-6 loaded with TAC. Cross-linkingwas promoted by loading PA-6 with 5wt% triallyl

Table 1. Classification of samples tested according to the UL 94

protocol

Rating t1 (s) t2 (s) Sðt1 þ t2Þa (s) Dripping

V0 510 510 550 NoV1 530 530 5250 NoV2 530 530 5250 Yes

aSum of t1 and t2 values recorded in 10 tests.

Table 2.60Co-g-ray-induced cross-linking of neat polyamide-6

Dabsa

(MGy)Atmosphere at

irradiationGel content (%) Swelling

ratio (%)

0.285 Ar 00.390 Ar 00.572 Ar 00.723 Ar 55 36300.912 Ar 50 1670

0.146 Air 00.571 Air 00.955 Air 44 36501.854 Air 78 2860

aAbs. dose rate: Drabs ¼ 2 kGy/h.

A.I. BALABANOVICH ET AL.180

Copyright # 2002 John Wiley & Sons, Ltd. Fire Mater. 25, 179–184 (2001)

cyanurate. As can be seen from Table 3 a gel contentvalue close to the saturation level was attained at therather low absorbed dose of about 0.005MGy, i.e. thegel dose, which was not determined exactly, wassomewhat lower than 0.005MGy. The fact that Dgel isabout two orders of magnitude lower than in the case ofneat PA-6 is in accordance with the finding reported byStenglin7 that TAC acts very efficiently as a cross-linking promoter. It can also be seen from Table 3 thatthe swelling ratio decreased with increasing absorbeddose. Since the swelling ratio is inversely proportional tothe cross-link density, this result indicates that under theinfluence of high energy radiation an intermolecularnetwork was formed in polyamide 6 and that the cross-link density increased with increasing absorbed dose. Asin the case of neat PA-6, oxygen did not noticeablyaffect radiation-induced cross-linking.

Combustion tests

Polyamide-6 loaded with red phosphorus. Upon loadingPA-6 with red phosphorus to up to 10wt% the oxygenindex increased somewhat, but the specimens still

released flaming drips during burning and the UL 94 testyielded V-2 rating as can be seen from Table 4. Thisrating is at variance with the V-0 rating reported byTaylor and Guest for an unspecified polyamide loadedwith only 7wt% red phosphorus.4

Irradiation with 60Co-g-rays improved to some extentthe fire resistance of the polyamide-6 under investigationas is shown in Table 5. The limiting oxygen indexincreased to values of 27.0 and 28.8 at a red phosphorusloading of 10.0 and 12.5wt%, respectively, when theabsorbed dose was increased up to the rather high valueof about 3MGy. Since the irradiated samples failed topass the vertical UL 94 test the fire retardancy waschecked with the aid of the mode B test. It can be seenfrom Table 5 that, as a rule, the burning time tended todecrease with increasing absorbed dose. Notably, thespecimens became non-dripping.

In conclusion, loading with red phosphorus did notlead to a significant improvement of the fire resistanceof polyamide-6 if applied at levels which did notdeteriorate the physical properties of the polymer.Radiation-induced intermolecular cross-links affectedthe fire retardancy. However, the improvement wasrather meagre and the absorbed dosage necessary toachieve improvement was so high that the physicalproperties of the polyamide were worsened. Therefore,red phosphorus loading and irradiation with 60Co-g-rays alone were not sufficient to make polyamide-6 fire-resistant. For this reason, the usage of a cross-linkingpromoter permitting the generation of an intermolecularnetwork in a large yield at a low absorbed dosageappeared to be indicated. The relevant experiments aredescribed below.

Polyamide-6 containing 5wt% TAC loaded with redphosphorus. Polyamide-6 specimens containing 5wt%TAC that were loaded with red phosphorus at contentsup to 12.5wt% performed unsatisfactorily in the

Table 3. 60Co-g-ray-induced cross-linking of polyamide-6

loaded with 5wt% TAC

Dabsa

(MGy)Atmosphere atirradiation

Gel content (%) Swellingratio (%)

0.0056 Ar 78 11800.0152 Ar 83 9000.0292 Ar 90 7500.046 Ar 89 6900.099 Ar 82 5800.141 Ar 90 4700.239 Ar 89 6102.052 Ar 79 570

0.0052 Air 83 10900.0154 Air 88 8600.0294 Air 93 5900.046 Air 88 5800.146 Air 93 4100.333 Air 86 470

aAbs. dose rate: Drabs ¼ 2 kGy/h.

Table 4. Combustion tests of polyamide-6 loaded with red

phosphorus

P (red)loading (wt%)

LOIa ModeA testb Rating

Dripping t1c (s) t2

c (s)

0 22.9 Yes 13 9 V-25 22.9 Yes 23 16 V-27.5 23.4 Yes 23 6 V-2

10 24.0 Yes 20 4 V-2

aLimiting oxygen index.bUnderwriters’ Laboratories Schedule 94, Standards for Testsfor Flammability of Plastic Materials for Parts in Devices andAppliances.cAverage burning time.

Table 5. Combustion tests on cross-linked polyamide-6 loaded

with red phosphorus, effects of 60Co-g-irradiation

P (red)loading (wt%)

Dabsa (MGy) LOIb Mode B testc

Dripping td (s) Ratinge

10 0 24.2 Yes 20 HB10 0.57 24.9 No 2 HB10 1.060 25.8 No 10 HB10 2.249 25.3 No 12 HB10 3.201 27.0 No 4 HB

12.5 0 26.4 No 7 HB12.5 0.99 26.1 No 2.5 HB12.5 1.82 27.2 No 2.5 HB12.5 2.42 28.7 No 4.5 HB12.5 2.91 28.8 No 1 HB

aAbs. dose rate: Drabs ¼ 2 kGy/h.bLimiting oxygen index.cUnderwriters’ Laboratories Schedule 94, Standards for Testsfor Flammability of Plastic Materials for Parts in Devices andAppliances.dAverage burning time.eHB, horizontal burning classification.

FIRE RETARDANCE IN POLYAMIDE-6 181


combustion tests. The UL 94 rating corresponded to V-2. Flaming drips were released during combustion, andthe limiting oxygen index was not affected by thephosphorus load within the error limits. This can be seenfrom Table 6, where the results obtained with specimensloaded with 10 and 12.5wt% red phosphorus arepresented.

Notably, radiation-induced cross-links affect thecombustion performance as can be seen from Table 7,where the results obtained at a red phosphorus contentof 10wt% are presented. There was no release offlaming drips from the irradiated specimens, even at thequite low absorbed dose of 22 kGy. However, there wasno significant change in the limiting oxygen index.Interestingly, irradiation improved the flammability ofthe specimens unless the phosphorus content exceeded10wt%. In those cases the flame did not self-extinguishin an acceptable time interval. However, as can be seenfrom Table 8, irradiated specimens loaded with12.5wt% red phosphorus were self-extinguishing, andthe UL 94 rating corresponded to V-0 in the dose rangebetween 0.022 and 0.344MGy. In conclusion, redphosphorus at a load of 12.5wt% was an effective fireretardant for polyamide-6 containing 5wt% TAC,provided the polymer was intermolecularly cross-linkedby exposure to high energy radiation.

Thermal gravimetric analysis

PA-6 loaded with red phosphorus . Figure 1 shows TGAcurves recorded with unloaded PA-6 and with PA-6loaded with red phosphorus. Evidently, the phosphorusload reduced the thermal stability as indicated by adecrease in the onset temperature for degradation. Thetemperature for 10% mass loss, T10%, was taken as ameasure for the onset of degradation and typical valuesare listed in Table 8. T10% decreased from 4078C for neatPA-6 to 3638C for PA-6 loaded with 5wt% redphosphorus. Interestingly, a further increase in thephosphorus load to 7.5 and 10wt% did not affect theonset temperature further. It can be also seen fromFig. 1 and Table 9 that red phosphorus had a strongeffect on the char forming tendency. Whereas neat PA-6volatilized completely upon heating to 6008C, there wasa non-volatile residue at 6008C of mres ¼ 13wt% at aphosphorus load of 5wt%. This mres value increased to20 and 21wt% at phosphorus loads of 7.5 and10.0wt%, respectively.

PA-6 loaded with 5wt% TAC and red phosphorus. WhenTGA curves for PA-6 containing 5wt% TAC wererecorded it turned out that TAC did not affect thethermal degradation of PA-6. Additional loading with


with red phosphorus (10wt%) and TAC (5wt%),

effects of60Co-g-irradiation

Dabsa (MGy) LOIb Mode B testc

Dripping td (s) Ratinge

0 24.2 Yes 8 HB0.022 23.2 No >30 NC0.051 23.8 No >30 NC0.134 25.1 No >30 NC0.344 25.4 No >30 NC0.435 25.4 No 10 HB

aAbs. dose rate: Drabs ¼ 2 kGy/h.bLimiting oxygen index.cUnderwriters’ Laboratories Schedule 94, Standards for Testsfor Flammability of Plastic Materials for Parts in Devices andAppliances.dAverage burning time.eHB: horizontal burning classification, NC: not classified.

Table 6. Combustion tests on polyamide-6 containing 5wt%

TAC

P(red)loading (wt%)

LOIa ModeA testb Rating

Dripping t1c (s) t2

c (s)

0 Yes10 24.2 Yes 8 4 V-212.5 24.8 Yes 19 2 V-2

aLimiting oxygen index.bUnderwriters’ Laboratories Schedule 94, Standards for Testsfor Flammability of Plastic Materials for Parts in Devices andAppliances.cAverage burning time.

Figure 1. TGA curves for neat PA-6 (curve 1) and PA-6 containing 5and10wt% red phosphorus (curves 2 and 3, respectively).


with red phosphorus (12.5 wt%) and TAC (5wt%).

Effects of60Co-g-irradiation

Dabsa (MGy) LOIb ModeA testc Rating

Dripping t1d (s) t2

d (s)

0 24.8 Yes 19 2 V-20.022 24.2 No 1 8 V-00.051 No 1 4 V-00.134 No 2 5 V-00.344 25.0 No 2 3 V-00.435 25.0 No 1 8 V-1

aAbs. dose rate: Drabs ¼ 2 kGy/h.bLimiting oxygen index.cUnderwriters’ Laboratories Schedule 94, Standards for Testsfor Flammability of Plastic Materials for Parts in Devices andAppliances.dAverage burning time.



red phosphorus resulted in a decrease in the onsettemperature and in the formation of a non-volatileresidue. As can be seen from Table 10 the values forT10% and mres are quite similar to those found in theabsence of TAC.

TGA curves for irradiated PA-6 specimens. Irradiationwith g-rays of PA-6 specimens loaded only with redphosphorus turned out to affect the TGA curves onlymarginally. The onset temperature decreased somewhatand char formation was increased to a small extent only,even at a very high absorbed dose (c. 3MGy) (Table 10).

However, irradiation with g-rays affected significantlythe thermal degradation of polyamide-6 containing thecross-linking promoter TAC thus indicating the strongeffect of radiation-induced cross-links. Typical TGAcurves, recorded with PA-6 loaded with 5wt% TAC and12.5wt% red phosphorus, are displayed in Fig. 2. It isseen that irradiation caused a small increase in the onsettemperature and a drastic increase in the non-volatileresidue at 6008C.

CONCLUSIONS

The effect of red phosphorus and radiation-inducedintermolecular cross-links on the fire resistance ofpolyamide-6 was studied. The red phosphorus contentwas kept relatively low (up to 12.5wt%) to prevent thedeterioration of the physical properties of the polymer.It turned out that red phosphorus contained innon-irradiated polyamide-6 did not lead to a significantimprovement of the fire resistance of the polymer.However, radiation-induced intermolecular cross-linksimprove the fire resistance. But in the absence of across-linking promoter, the absorbed dosage necessaryto achieve improvement was so high that the physicalproperties of the polyamide worsened because ofsevere chemical modifications differing from cross-linking. Therefore, red phosphorus loading and irradia-tion with 60Co-g-rays alone were not sufficient tomake polyamide-6 fire-resistant. A different situationwas met, however, if a cross-linking promoter wasapplied. Actually in the presence of a cross-linkingpromoter (5wt% triallyl cyanurate) intermolecularcross-links were quite effectively introduced into thepolymer upon irradiation with g-rays, i.e. the geldose was about two orders of magnitude lower than inthe absence of TAC. Therefore, it was possibleto improve the fire resistance of polyamide-6 containingTAC by irradiation with g-rays. Optimum improvementwas obtained at a red phosphorus load of 12.5wt%at an absorbed dose of 22 kGy. In this case testspecimens were found to be self-extinguishingand the UL 94 rating corresponded to V-0. Moreover,thermal gravimetric analysis showed that irradiationcaused, in this case, a small increase in the onsettemperature and a drastic increase in the residuenon-volatile at 6008C.

Acknowledgement

The financial support of German Academic Exchange Service(DAAD) for this work is gratefully acknowledged.

REFERENCES

1. Peters EN. Flame Retardancy of Polymeric Materials, KurylaWC, Papa AJ (eds.) Marcel Dekker: New York, 1979; 113.

2. Granzow A. Acc. Chem. Res. 1978; 11: 177.3. Brown CE, Wilkie CA, Smukalla J, Cody Jr RB, Kissinger JA.

J. Polym. Sci. Polym. Chem. Ed. 1986; 24: 1297.

4. Taylor DB, Guest R. Use of Red Phosphorus as a FlameRetardant. Fire Retardant Engineering Polymers and Alloys:San Antonio, Texas, 1989; 133.

5. Zimmermann J. In The Radiation Chemistry of Macromole-cules, Dole M (ed). Academic Press: New York, 1973; p. 121.

Table 9. Thermal gravimetric analysis of PA-6, the effect of red

phosphorus loading

P (red) loading (wt%) T10%b (8C) mres

a (wt%)

0 407 05.0 363 137.5 363 2010.0 363 21

aNon-volatile residue at 6008C.bTemperature for 10wt% mass loss.

Table 10. Thermal gravimetric analysis of PA-6 containing

5wt% TAC, the effect of red phosphorus loading

P (red) loading (wt%) T10%b (8C) mres

a (wt%)

0 407 07.5 359 1610.0 359 2112.50 359 21

aNon-volatile residue at 6008C.bTemperature for 10wt% mass loss.

Figure 2. TGA curves for PA-6 loaded with 5wt% TAC and 12.5wt%red phosphorus recorded before and after irradiation to various ab-sorbed doses, as indicated at the curves.

FIRE RETARDANCE IN POLYAMIDE-6 183


6. Lyons BJ, Glover LC Jr. Radiat. Phys. Chem. 1990;35: 139.

7. Stenglin U. Plastverarbeiter 1991; 42: 56.8. Wang JH, Liu WY, Chen J. Nucl. Techniques (China) 1995;

18: 444.

9. Ma YZ, Hou HJ. Nucl. Techniques (China) 1997; 20:408.

10. Troitzsch J. International Plastics Flammability Handbook,2nd edn. Hanser: M .uunchen, 1990.

11. Fenimore CP, Martin FJ. Mod. Plastics 1966; 44: 141.



fire retardance in polyamide-6. the effects of red phosphorus and radiation-induced cross-links

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