Flame retardant polymer nanocomposites

6 Pages·2007·0.963 MB·English

by  Morgan, Alexander B., Wilkie, Charles A

Flame Retardant Polypropylene Nanocomposites
QingliangHe,Tingting Yuan,and DaoweiDing
IntegratedCompositesLaboratory(ICL),DanF.SmithDepartmentofChemical
Engineering,LamarUniversity,Beaumont,Texas,U.S.A.
Suying Wei
DepartmentofChemistryandBiochemistry,LamarUniversity,Beaumont,Texas,
U.S.A.
ZhanhuGuo
IntegratedCompositesLaboratory(ICL),DanF.SmithDepartmentofChemical
Engineering,LamarUniversity,Beaumont,Texas,U.S.A.
Abstract
Inthisentry,theuniquefunctionofpolypropylene-graft-maleicanhydride(PP-g-MA)hasbeendemonstrated,
whichservesasasynergisttostimulatethecatalyticeffectofmagneticcobalt–cobaltoxidecore–shellnano-
particles (NPs) on reducing the flammability of polypropylene (PP). Specifically, through a one-pot wet
chemistry,PPpolymermatrixnanocomposites(PNCs)withinsitusynthesizedcobalt–cobaltoxidecore–shell
NPswerestabilizedby5.0wt%PP-g-MA,andtheheatcombustionparametersincludingheatreleaseratewas
reducedmorethan50%comparedwiththePPPNCswithonlytheseNPs.Eventhoughthesecobalt–cobalt
oxidecore–shellNPscanbesolelyusedasheatshieldingcomponentstodecreasethefirehazardsofPP,this
highlyeffectivesynergisticeffectofnonflame-retardantPP-g-MAonstimulatingthecatalyticflameretardant
PPwasrarelyreportedbefore.
INTRODUCTION main chains can effectively stabilize the magnetic NPs
through counteracting with the strong attraction forces
Transitional metals (oxides) have been extensively used among magnetic NPs.[15,16] Hence, this bottom-up
as synergist for flame retardant polymer nanocompo- method is able to compensate the weak filler–polymer
sites (PNCs);[1–3] however, the majority of the research interfacial adhesion for inert polymers like PP, which
focusedoncollaboratingwithtraditionalflameretardants favors the particle dispersion. More importantly, enhanc-
like intumescent systems[4–6] or clays[7] as major fillers ingthebondingatthepolymer–nanofillerinterfacefurther
forthepolymermatrix.Themechanismistopromotethe favors the improvements in thermal stability and flame
flame retardant efficiency of major flame retardant addi- retardancyofthetargetPNCs.[17,18]Nonetheless,howthe
tives by these transition metal (oxide) particles via cata- thermalstabilityandflameretardancywillbeimprovedis
lyticcarbonization.Forexample,cobalt,asawidelyused obscure. The catalytic effect of the transition metal or
catalyst, has been extensively studied in both academic metal oxide on the flame retardant treatment is widely
and industrial field because of its merits including a used during the 1990s;[3] however, the catalytic mecha-
wealth of structure-dependent catalytic properties.[8,9] nismisnotwellelucidated.ForPP-basedPNCswithtra-
Cobalt nanostructures were widely used as catalysts for ditionalflameretardantssuchasorganoclay,[17,19]carbon
reduction of nitrophenols[10] and synthesis of long-chain nanotubes (CNTs),[20–24] intumescent flame retardant/
hydrocarbonsandcleanfuels.[11]Amongthereportedver- CNT,[25,26] and layered double hydroxide PNCs,[27,28]
satile synthesis methods, in situ bottom-up approach by compatibilizer such as polypropylene-graft-maleic anhy-
thermal decomposing neutral organometallic precursors dride (PP-g-MA) is commonly used to improve the filler
such as dicobalt octacarbonyl in organic solvents is one dispersion.
of the most commonly used approaches for synthesizing Previously, we have reported the use of PP-g-MA as
cobaltnanoparticles(NPs).[12,13]Besidestheadvantageof surfactantandstabilizerforsynthesizingmagneticcobaltand
size and shape control from the bottom-up approach, its Fe O NPswithtunablesize,morphology,crystallinestruc-
2 3
solution-based condition also facilitates using inert poly- tures, assembly patterns, and magnetic property.[13,29,30] In
mer such as polypropylene (PP) to serve as host to these this entry, we demonstrate a unique magnetic PNC, i.e., in
cobalt NPs for preparing PNCs.[13,14] The reason is situ formed cobalt–cobalt oxide core–shell NPs reinforced
because the steric repulsion from the PP hydrocarbon PP nanocomposites stabilized by PP-g-MA (two molecular
DekkerEncyclopediaofNanoscienceandNanotechnology,ThirdEditionDOI:10.1081/E-ENN3-120053555
Copyright©2015byTaylor&Francis.Allrightsreserved. 1
2 FlameRetardantPolypropyleneNanocomposites
weightsareselected,M =800andM =2500g/mole).The Table 1 TGA characteristics of the samples measured in
n n
detailed synthesis procedure is published by He et al.[14,31] nitrogen.
WehereemphasizeontheillustrationofhowthePP-g-MA
T (°C) Residueat
stimulates the catalytic effect of cobalt–cobalt oxide core– Composition inNi T (°C) 700°C(%)
2 max
shellNPsonreducingtheflammabilityofPP.
PurePP 421.0 475.7 0.0
PP/PP-g-MA(M =800) 391.0 481.4 0.0
n
DISCUSSION
PP/PP-g-MA(M =2,500) 415.3 477.3 0.1
n
PP/20.0%NPs 409.0 481.4 20.2
Fig. 1a and 1b shows the thermogravimetric analysis
PP/20.0%NPs/PP-g-MA 383.4 484.6 22.1
(TGA) curves of PP and the PNCs conducted under both
(M =800)
nitrogenandair,respectively.Thedetaileddataarelistedin n
PP/20.0%NPs/PP-g-MA 404.6 482.2 20.9
Tables 1 and 2. Here, the initial degradation temperature
(M =2,500)
(T ) is defined as a 5% weight loss of the tested sample, n
ini
whiletheT (obtainedfromthedifferentialthermogravi- Source:FromHe,Yuan,etal.[31]©2009TheRoyalSocietyofChemistry
max
(RSC).
metric analysis, DTG) is defined as the temperature when
the tested sample experiences maximum weight loss rate.
ThethermaldegradationofpurePPwasmainlyinitiatedby
with hydrogen transfer at the scission sites under
thethermalscissionsofaliphaticC–Cbackboneassociated nitrogen.[32]Specifically,purePPhasanone-stagethermal
degradationwithT of421.0°CandT of475.7°Cand
ini max
no char residue left at 700°C.[33] The addition of 5.0 wt%
PP-g-MAinthePPmatrix,decreasedT ,butincreasedthe
ini
T . When forming 20.0 wt% cobalt–cobalt oxide core–
max
shell NPs in the PP matrix (Table 1), T of the PNCs
ini
decreased by 12°C (from 421.0°C to 409.0°C) and T
max
increasedby5.7°C(from475.7°Cto481.4°C).When20.0wt%
cobalt–cobalt oxide core–shell NPs in situ are synthesized
in PP matrix in the presence 5.0% PP-g-MA, the PNCs
werefoundtohavesignificantlydecreasedT andslightly
ini
increased T . For the PP PNCs with 5.0% PP-g-MA
max
(M = 800) and 20.0 wt% particles, T decreased to
n ini
383.4°C and T increased to 484.6°C. Meanwhile, T
max ini
decreasedto404.6°CandT increasedto482.2°Cforthe
max
PP–5.0%PP-g-MA(M =2500)–20.0%particlessystem.
n
Thefinalresiduesat700°Care20.2,22.1,and20.9%inthe
PP PNCs without PP-g-MA, with PP-g-MA (M = 800),
n
and with PP-g-MA (M = 2500), respectively. The
n
decreased T indicated that earlier weight loss took place
ini
whenPP-g-MAwasintroducedinthePP/cobaltsystemand
the increased T suggested a better thermal stability at
max
Table2 TGAcharacteristicsofthesamplesmeasuredinair.
T (°C) Residueat
ini
Composition air T (°C) 550°C(%)
max
PurePP 266.5 328.0 0.0
PP/PP-g-MA(M =800) 280.4 359.9 0.0
n
PP/PP-g-MA(M =2,500) 266.5 375.0 0.0
n
PP/20.0%NPs 330.6 432.0 25.0
PP/20.0%NPs/PP-g-MA 345.0 443.3 24.5
(M =800)
n
PP/20.0%NPs/PP-g-MA 330.4 435.2 26.4
Fig.1 TGAcurvesofpurePPandthePNCsunder(a)nitrogen
(M =2,500)
and(b)airatmosphere. n
Source: From He, Yuan, et al.[31] ©2009 The Royal Society of Source:FromHe,Yuan,etal.[31]©2009TheRoyalSocietyofChemistry
Chemistry(RSC). (RSC).
FlameRetardantPolypropyleneNanocomposites 3
highertemperatures.ThedecreasedT indicatesadifferent
ini
degradation pathway, in which more gas volatiles were
generated from initial thermal degradation. The probable
reason is because the chain scissions took place at low
temperature inthePP PNCs inthepresenceof PP-g-MA.
Comparedwiththermaldegradationundernitrogen,the
thermal oxidative stability of the polymer material was
prominentlyreducedbyoxidativedehydrogenationaccom-
paniedbyhydrogenabstraction.[34]TheT ofpurePPwas
ini
drasticallyreducedto266.5°Cinaircomparedwiththatof
421.0°C in nitrogen, and T also reduced to 328.0°C
max
(Table 2). T and T increased to 330.6°C and 432.0°C
ini max
when 20.0% cobalt–cobalt oxide core–shell NPs were
formedinPPmatrix,whichwere64.1and104.0°Chigher
than those of pure PP. The oxidative residue of the PP/
cobalt PNCs at 550°C was 25.0%, indicating that the fur-
theroxidationofthesecobalt–cobaltoxidecore–shellNPs Fig. 2 Heat release rate curves of pure PP, PP/PP-g-MA, PP/
20.0% cobalt PNCs, and PP/20.0% cobalt PNCs stabilized with
takes place under high temperature in air. It is probably
because that the cobalt–cobalt oxide core–shell NPs act twodifferentmolecularweightPP-g-MAs.Insetisenlargedinitial
low temperature decomposition between 100°C and 400°C
aseffectiveheatbarrierandthussignificantlyenhancedthe
(S:M =800,L:M =2500).
thermaloxidative stability ofthePPmatrix throughdefer- Sourcne: From He, Ynuan, et al.[31] ©2009 The Royal Society of
ringoxidativedegradation.Moreimportantly,T andT
ini max Chemistry(RSC).
werefurtherincreasedto345.0°Cand443.3°C(14.4°Cand
11.3°ChigherthanPP/cobaltor78.5°Cand115.3°Chigher
thanpurePP)when5.0%PP-g-MA(M =800)wasadded indicated a flame retardancy effect of these in situ
n
in the PP-20.0 wt% cobalt–cobalt oxide core–shell NPs obtained cobalt–cobalt oxide core–shell NPs. A barrier
system. However, the PP-g-MA (M = 2500) was found effect from these cobalt–cobalt oxide core–shell NPs is
n
tohavelimitedeffectinfurtherincreasingtheT andT believed to be responsible for this flame retardancy sim-
ini max
ofthePPmatrixwhenaddedintoPPPNCswith20.0wt% ilar to the enhanced thermal oxidative stability. When
cobalt–cobaltoxidecore–shellNPs.Theenhancedthermal the PNCs were exposed under high temperature, heat
oxidative stability by PP-g-MA (M = 800) is obviously and mass transfers between gas and condense phases
n
attributed to the enhanced interfacial adhesion effect were slowed down by an insulating layer formed from
betweenPPandcobalt–cobaltoxidecore–shellNPs,which these NPs, which suppressed the fast decomposition of
requires more energy to decompose the PP/PP-g-MA/ the PP matrix.[35] Therefore, the lower flammability in
cobalt complex in air; meanwhile, this effect is limited by terms of HRR reduction suggests a slower speed of
thefewerbondingthroughPP-g-MA(M =2500)onthese combustible volatiles generated from the random chain
n
cobalt–cobalt oxide core–shellNPs.[14] scission of PP backbones in the presence of cobalt–
Microscalecombustioncalorimetry(MCC)wasutilized cobalt oxide core–shell NPs.
to evaluate the fire hazards of the PP and its PNCs by SynergisticeffectinreducingHRRwasobservedwhen
investigating the heat combustion parameters including adding 5.0% PP-g-MA in the PP-20.0 wt% cobalt–cobalt
heatreleasecapacity(HRC),heatreleaserate(HRR),peak oxide core–shell NPs system (Fig. 2). HRC and PHRR
heat release rate (PHRR), temperature at PHRR (T ), wereobservedtodecreasesignificantly,i.e.,PHRRfurther
PHRR
andtotalheatrelease(THR).Fig.2depictstheHRRcurves decreasedto532.4W/ginthecaseofPP–20.0wt%cobalt–
asfunctionoftemperature.ThehigherPHRRonematerial cobalt oxide core–shell NPs PNCs with PP-g-MA (M =
n
behaves under a specific heat flux, the more dangerous it 2500)orto500.8W/ginthecaseofPP-g-MA(M =800).
n
willactunderafireaccident.PurePPisahighlyflammable Meanwhile, THR further slightly decreased from 27.7 to
material with a measured PHRR value of 1513.0 W/g. 25.6kJ/gwith5.0wt%PP-g-MA(M =2500)orto25.1kJ/g
n
When forming cobalt–cobalt oxide core–shell NPs in PP with 5.0 wt% PP-g-MA (M = 800) in PP-20.0% cobalt–
n
PNCs,PHRRdecreasedfrom1513.0to1024.0W/g(more cobaltoxidecore–shellNPsPNCs.Althoughconventional
than32%reduction),THRdecreasedfrom40.6to27.7W/g synergisticeffectincludingnitrogen–phosphorus,[33,36,37]
(more than 31.8% reduction) and the initial decomposi- phosphorus–silicon,[38,39] or nitrogen–phosphorus–
tion temperature was enhanced upon adding these cobalt– silicon[40]on flame retardant PNCs has been extensively
cobaltoxidecore–shellNPs(Fig.2).Obviously,excluding studied, the synergistic effect between PP-g-MA and
the dilution effect of 20.0 wt% noncombustible cobalt– cobalt–cobalt oxide core–shell NPs have rarely been
cobalt oxide core–shell NPs in PP matrix, an additional reported. In addition, when PP-g-MAwas added in the
12.3% decrease in HRR and 11.8% decrease in THR PP–20.0% cobalt–cobalt oxide core–shell NPs system, a
4 FlameRetardantPolypropyleneNanocomposites
broadHRRpeakwasobservedduringtheinitialdecompo- MCC clearly. Therefore, the degradation and real-time
sition stage (around 100–300°C) and T decreased to weightlossprocessescanbedemonstratedsimultaneously.
PHRR
471.0°C(shownintheinsetofFig.2). ItisnoticedfromFig.2thattheadditionof5.0%PP-g-MA
X-ray photoelectron spectroscopic (XPS) analysis was barely decreased the initial thermal degradation tempera-
further investigated to identify the atomic composition of tureofPP,while20.0%cobalt–cobaltoxidecore–shellNPs
solidcharresiduesofthesePPPNCsaftertheMCCtestand delayed the initial degradation of PP as evidenced by the
thus to determine flame retardancy mechanism. It was higherthermaldegradation temperature than that of pure
observedthatthecarbonspeciesmasspercentageincreased PP (no detectable HRR increase before 400°C, shown
sharply from 61.80% for the PP–20.0 wt% cobalt–cobalt in the inset of Fig. 2). However, the degradation of
oxide PNCs to 71.08% for PP PNCs in the presence of PP–5.0% PP-g-MA–20.0% cobalt–cobalt oxide core–
PP-g-MA (M = 2500) and to 80.92% in the presence of shell NPs PNCs was definitely altered by the evidence
n
PP-PP-g-MA (M = 800), corresponding to 83.16%, of broad HRR peak appeared in the range of 130–310°C
n
89.43%, and 90.89% carbon atomic percentage, respec- (inset of Fig. 2) and ~18.0% weight loss within the
tively. This proves that the combination of cobalt–cobalt thermal degradation temperature of 100–310°C (dashed
oxide core–shell NPs and PP-g-MA in the PP matrix can rectangle zone in Fig. 3).
facilitate the char formation at high temperature com- When exposed to heat at elevated temperature from
bustion. Possible reasonisthat theanhydride groups from 80°C to 650°C, the inert thermal degradation of PP was
PP-g-MA promoted carbonization catalytically during the initiated mainly by chain scission and chain transfer, and
chain scission and chain transfer process, thus increasing then reductionsinmolecular weightwerefirstobservedat
the char yield. In order to further elucidate the synergism 227–247°C and gas volatiles became significant above
betweenPP-g-MAandcobalt–cobaltoxidecore–shellNPs 302°C. Finally, ignition of PP was observed at a surface
in reducing the flammability of PP, a fast thermal degra- temperature of 337°C,[41] which is consistent with the ini-
dation test was performed by TGA using the same condi- tial HRR jump at ~330°C observed from MCC. Mean-
tions as MCC measurements—a heating rate of 60°C/min while, the addition of 5.0% PP-g-MA has limited
(1°C/sec)undernitrogen(TGAandDTGcurvesshownin influence on initiating the degradation of the PP matrix.
Fig.3).MCCmeasurementsherewereperformedusingan With adding 20.0% cobalt–cobalt oxide core–shell NPs,
inertsamplethermaldegradationproceduretopyrolyzethe only heat barrier effect was found to reduce the HRR
sample into combustible gas volatiles followed by a non- throughslowingthereleaseofgasvolatiles.[35]Whenadd-
flamingoxidation ofthese volatiles. The fastthermal deg- ing PP-g-MAwith cobalt–cobalt oxide core–shell NPs in
radationbyTGAcanillustratethedynamicsampleweight the PP matrix, the catalytic effect was found to lower the
loss under temperature ramping at a constant high heating initial thermal degradation temperature of the resulted
rate(1°C/sec);meanwhile,theDTG(%/°C)fromtheinset PNCs (from 330°C for pure PP to ~100–130°C for the
of Fig. 3 can reproduce the thermal degradation stage of PNCs) and lead to a smaller HRR in the range of 100–
310°Cduetoasmallamountofgasvolatilesreleasedfrom
bulkmaterial.Probablemechanismincludesrandomchain
scission of C–C bond of PP backbone to generate hydro-
carbonradicalsduringinitialdecomposition,theformation
oflowerhydrocarbonssuchaspropylenefromfurtherdeg-
radation of these hydrocarbon radicals, the β-scission and
abstraction of H radicals from other hydrocarbons to pro-
duceanewhydrocarbonradicalsduringpropagationstage,
and finally the disproportionate or recombination of two
radicals as termination reaction.[42] Meanwhile, slightly
similar to a “smoldering,” a substantial fraction (~18.0%)
ofthetotalmassofPP/cobaltPNCsinthepresenceofPP-g-
MAwas consumed at 100–310°C. Therefore, only small
amount of heat release in a slow speed was generated,
effectivelydecreasing thetotalavailablegas fuels, which
wouldgeneratelargequantityofheatifplacedunderhigh
temperatures.Onecanalsoobservethatthepeakwidthof
thePP/PP-g-MA/cobaltPNCswasmuchwiderthanthose
of pure PP, PP/PP-g-MA, and PP/Co PNCs (Fig. 2), fur-
Fig.3 TGAandDTGcurvesofPP/20.0%cobaltPNCsandPP/
therindicatingalongercombustionperioduponintroduc-
20.0%cobaltPNCsstabilizedwithtwoPP-g-MAsunderaheating
rateof1°C/sec. ingthePP-g-MAandcobalt–cobaltoxidecore–shellNPs
Source: From He, Yuan, et al.[31] ©2009 The Royal Society of inthePPmatrix.Thisisanothersignoflowfirehazardfor
Chemistry(RSC). the PP/PP-g-MA/cobalt–cobalt oxide core–shell NPs
FlameRetardantPolypropyleneNanocomposites 5
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ThisprojectwassupportedbytheLamarUniversity.Partial
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financial support from National Science Foundation—
Sun,L.;Young,D.P.;Wei,S.;Guo,Z.Magneticallysoftand
Chemical and Biological Separations (CBET: 11-37441)
hardpolypropylene/cobaltnanocomposites:Roleofmaleic
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