Fire retardant mechanism in intumescent ethylene vinyl aceta

Fire retardant mechanism in intumescent ethylene vinyl

acetate compositions

A.Riva a ,G.Camino a,b,*,L.Fomperie c ,P.Amigoue t c

a

Centro di Cultura per l’Ingegneria delle Materie Plastiche,V.T.Michel 5,15100Alessandria,Italy b

Politecnico di Torino,Sede di Alessandria,V.T.Michel 5,15100Alessandria,Italy

c NEXANS-NRC,170,Avenue Jean Jaure

`s,69353Lyon Cedex 7,France Received 11November 2002;accepted 6January 2003

Abstract

The thermal and combustion behaviour of an intumescent ?re retardant system based on Polyamide 6(PA6)and Ammonium

Polyphosphate (APP),used to improve ?ame retardant properties of poly(ethylene-co-vinyl acetate)(EVA),loaded with Mg(OH)2(MH)was examined.The study of the interactions between the additives introduced in EVA was focused in particular on the MH-APP interaction.The evolution of water from MH takes place at about 400 C,with a fair overlap with ammonia and water evo-lution from APP degradation.Ammonia evolution from APP is facilitated by the presence of MH,in their mixture heated alone or in the polymer matrix.UL94test shows that the interaction between MH and APP modi?es the combustion behaviour of the intumescent mixture.

#2003Elsevier Ltd.All rights reserved.

Keywords:EVA;Intumescence;Ammonium polyphosphate;Polyamide 6;Magnesium hydroxide;Flame retardancy

1.Introduction

Ethylene polymers and co-polymers are widely used in many ?elds,particularly in electrical engineering applications.Due to their chemical compositions,these polymers are easily ?ammable,and because of this,?ame retardancy of these materials is widely studied.The main approach used up to now to impart ?ame retardant properties to this class of polymeric materials has been the incorporation of additives,speci?cally of halogen compounds.The combustion products coming from these materials have a number of negative char-acteristics (corrosiveness,toxicity ...)that pushed the industry and the legislation to improve some new approaches to ?ame retardance [1–3].

One of these developing approaches is that of intumes-cence.The intumescence mechanism consists in creating on the polymer surface an expanded shield,able to reduce both the heat ?ux from the ?ame to the polymer matrix,responsible for the fuel production,and the transfer of fuel to the ?ame,limiting the spread of ?re [4,5].

Generally intumescent formulations consist of three ingredients:an acid source (phosphates,borates etc.),a carbonising compound (polyols,polyamides,poly-urethanes etc.),and a blowing agent (melamine and melamine compounds etc.).On heating,the acid source gives out a mineral acid,that takes part in the dehydration of the carbonising compound,that forms a cellular structure when the blowing agent decomposes [4–7].The association of PA6or other char forming polymers and APP as ?ame retardants for EVA and other thermoplastic polymers has already been reported [8–11].In this work we have studied the e?ect of com-bination of the intumescent system APP-PA6with MH which is a widely used ?re retardant in electrical cable sheeting materials.

2.Experimental 2.1.Materials

The following products were used:ethylene–vinyl ace-tate 24%copolymer (Elvax 265,DuPont,EVA),Poly-amide 6(UltramidB4BASF,PA6),magnesium hydroxide

0141-3910/03/$-see front matter #2003Elsevier Ltd.All rights reserved.

doi:10.1016/S0141-3910(03)00191-5

Polymer Degradation and Stability 82(2003)341–346

ad237aed0975f46527d3e1b2/locate/polydegstab

*Corresponding author.Fax:+39-0131-229-331.

E-mail address:giovanni.camino@proplast.it (G.Camino).

(Magni?n H10,Martinswerk,MH)and ammonium polyphosphate(Exolit AP422,Clariant,APP).

The compositions were prepared via a two step pro-cess to avoid APP degradation:in the?rst step EVA, PA6and MH were mixed at a temperature of235 C using a Brabender PLE Mixer,with roller blades,with a rotation speed of60rpm,for5min.In the second step, APP was added to the mixture and mixed at a tem-perature of180 C at60rpm for5min.

Samples for all the performed tests were prepared by pressing the material with an ATSFAAR hydraulic press at a pressure of200bar and a temperature of230 C.

2.2.TGA-FTIR analyses

The TGA-FTIR analyses were performed using a Perkin Elmer Pyris1TGA,coupled with a Perkin Elmer Spectrum GX Infrared Spectrometer equipped with an IR gas cell.The TGA and the FTIR spectrometer were coupled by a Perkin Elmer TG-IR Interface.

The transfer line was heated to220 C,while the IR gas cell was heated to230 C to avoid condensation of degradation products inside the gas cell.The samples size was between25and30mg.The samples were heated from50to600 C with an heating rate of10 C/ min under a nitrogen?ow of30ml/min.The nitrogen ?ow was switched on10min before the beginning of the analysis,keeping the furnace closed,to get a stable IR background.1IR spectrum/ C was collected to draw a good evolution pro?le.

2.3.UL-94tests

The?ammability behaviour of the intumescent mix-tures was investigated by the UL-94test on3mm thick specimens ignited from bottom in the vertical con?gur-ation.The best ranking is V-0when burning time is short and there is no dripping of?aming particles, whereas the worst corresponds to‘‘not classi?ed’’when the sample burns for more than30seconds or up to the holding clamp at125mm from the ignition point(see ASTM D3801/00for detailed procedure).The measurement of the burning rate of the materials was based on the time of combustion of the bottom8cm of the specimen,and was measured to integrate evaluation of those specimens that did burn up to the holding clamp and could not be classi?ed.The higher is t8,the lower the burning rate and hence the?re retardant behaviour is better for samples showing higher t8values.

2.4.FTIR analyses

The FTIR analyses were performed with a Perkin Elmer Spectrum GX FTIR spectrometer,equipped with a Multiscope FTIR microscope and a Micro-ATR germanium crystal.3.Results and discussion

3.1.TGA-FTIR studies

TGA-FTIR analyses were carried out on all the mix-ture components to identify the IR signals that could be used to monitor the degradation of the di?erent com-pounds on heating the samples under nitrogen.We found that for all the mixture components except that for MH a typical IR signal was present.

For EVA,the acetic acid evolution at about350 C could be monitored,by the C?O stretching vibration of the carboxylic group(1797cmà1),while for PA6, caprolactam evolution at about470 C was monitored, by the C?O stretching vibration of the amidic group (1710cmà1).Finally for APP NH3evolution at about 380 C is detected by the absorbance at966cmà1,where the typical ammonia gas phase doublet is found.

The analysis of all these evolving products in the dif-ferent mixtures allowed us to investigate the mutual in?uences between the various components.

As MH degradation takes place with evolution of water alone,it was not possible to?nd an IR absor-bance useful for monitoring its degradation.The TGA curve of MH heated to600 C showed two main weight loss steps with two weight loss rate maxima at about 400and510 C respectively.The?rst degradation step is well known to be due to water release,that is respon-sible for the FR mechanism of inorganic hydroxides [12].The second might be due to the thermal degrada-tion of an impurity,as analyses carried out on analytical grade MH did not show this degradation step.

Fig.1reports the results of the TG-FTIR carried out on pure APP at10 C/min under nitrogen?ow.In the upper part of the?gure the weight loss curve(TG)and its derivative curve(DTG)are reported(solid and dashed line respectively),while the ammonia evolution pro?le (dotted line,Absorbance),that represents the intensity of the IR absorbance peak found at966cmà1is reported in the bottom part of Fig.1.Ammonia evolution as a func-tion of temperature,in terms of IR absorption at966 cmà1,is shown by the dotted curve in Fig.1,upper part. It can be seen that a fair overlap of the DTG and NH3evolution curves exists,meaning that ammonia represents a considerable part of the evolved gases from APP in the temperature range300–450 C.In the lower part of the?g.is reported the IR spectrum of the gases evolving at385 C,where the maximum ammonia evo-lution is taking place.It can be noticed that after the ammonia evolution peak the DTG curve doesn’t decrease to zero,meaning that other products are still evolving,while the NH3signal is disappearing.This fact can be related to evolution of water,which is another degradation product of APP in this temperature range, as reported in the literature[13,14],but could not be monitored by means of FTIR spectroscopy as H2O

342 A.Riva et al./Polymer Degradation and Stability82(2003)341–346

doesn’t give a sharp IR signal.After water and ammo-nia release the degradation of the polyphosphate net-work takes place,beginning at about 550 C [14],as shown by the DTG increase.The IR spectra of the products evolved during this phase were not collected to avoid damaging the IR gas cell.

Ammonia evolution from APP is related to acidic site formation involved in the intumescence phenomena,as already reported in the literature [13].As MH is a base,we were interested in analysing the interaction between these two components,to investigate whether it could lead to suppression of the FR behaviour of the intumes-cent mixture or not.Our ?rst approach was to carry out TG-FTIR on mixtures of APP and MH,characterised by di?erent APP/MH ratios,comparing experimental curves with those expected from the behaviour of APP and MH heated separately.For example,if no interaction was present between the two compounds,only one weight loss step should take place,as shown by the dotted line in Fig.2for a 50–50%mixture.Only one DTG peak should be found at a temperature of about 385 C,as both the water evolution from MH and the ammonia and water evolution from APP take place in the temperature range 280–460 C.The second DTG peak expected at 496 C is related to the degradation of MH,while the DTG curve increase above 500 C is related to the phosphate net-work degradation.

When we ran the TG experiment on the mixtures of MH and APP we found some considerable di?erences with the calculated curves.For example,for the 50–50%mixture the expected single peak was substituted by three peaks in the range 250–500 C,as shown in Fig.2by the solid line.A further di?erence is represented by the disappearance of the peak at 496 C,related to MH,and of the polyphosphate network degradation (>500 C),related to APP.These di?erences show that an interaction between MH and APP occurs on heating.Only ammonia is shown by FTIR to be evolved on heating the mixture with an absorption pro?le,as a function of temperature,revealing that only the ?rst weight loss step is related to NH 3release,as shown in Fig.3by the dotted line (IR absorbance at 966cm à1)compared to the solid line that represents the DTG curve of the mixture.The peak of ammonia evolution in the mixture (330 C)appears to be earlier if compared to that of pure APP in Fig.1(385 C).The two follow-ing DTG peaks can be attributed to water evolution due to the interaction between MH and the hydroxy groups left after ammonia release from APP (Scheme 1a and b ),as no gas phase IR signal was associated with these degradation steps,showing the evolution of other pro-ducts.The reactions suggested in scheme 1are related to magnesium phosphate formation during the thermal degradation of the MH/APP mixture,which is in agreement with the thermal stability of the residue at 600 C.

A number of TGA-FTIR experiments was carried out on mixtures containing APP,MH and the

EVA-PA6

Fig.1.TGA-FTIR of pure APP under nitrogen,heating rate 10 C/min.Upper part:solid line:TG curve;dotted line:NH 3evolution (A.U.);dashed line:DTG curve.Lower part:FTIR spectrum of gases evolving at 385

C.

polymer matrix.The results are reported in Fig.4,where a comparison between di?erent mixtures char-acterized by di?erent APP/MH ratios and di?erent overall ?ller concentrations is reported.The compar-ison is focused on the ammonia evolution pro?le,recorded as absorbance at 966cm à1.All the ammonia detected on heating derived from APP,as NH 3evol-ving from PA6was found not to be detectable in these conditions,PA6being about 7%of the polymer matrix.The absorbance curves here reported in Fig.4were obtained from samples containing MH/APP ratios 4,1,0.25and 0,respectively dash–dot,dashed,dotted and solid line.The overall ?ller content for these samples is 50,60,50and 60%respectively.Looking at the ?gure it can be noticed that the pre-sence of MH causes an earlier of the ammonia evolu-tion that is not related to the MH percentage into the composition,as demonstrated by the peaks of the B,C and D curves,all found at the same temperature,about 30 C lower than that of curve A.The tem-perature of the maximum ammonia evolution found for the mixtures containing both APP and MH dis-persed in the polymer matrix is the same found for the mixtures of the additives alone.Whereas in the absence of MH APP evolves ammonia at a tempera-ture 20 C lower in the polymer matrix (comparison between Figs.1and 4,with APP ammonia evolution peaks at 380and 360 C).

3.2.FTIR analyses

The formation of magnesium phosphate is in agree-ment with comparison of the IR spectra of MH,APP and the residue of their mixture after heating at 600 C under nitrogen ?ow,reported in Fig.5.The solid line represents the pure APP FITR–ATR spectrum,the dashed line is related to pure MH spectrum and the dotted line is obtained from the residue.It can be noticed that the –OH stretching vibration of MH at 3688cm à1disappears,as does the broad band due to

NH 4+stretching vibration from 2600to 3300cm

à1

and the NH 4+bending vibration at 1415cm

à1

because of evolution of water and ammonia on heating.The shift of P ?O,P–O and P–O–P phosphate vibrations at 1246,1062and 1012cm à1should be due to the formation of magnesium phosphate.3.3.Flammability behaviour

In table 1the UL-94test results pide the samples into two groups,one reaching the V-0rating (short burning time,no dripping),the other being ‘‘not classi-?ed’’(n.c.).Looking at table 1it can be noticed that the best ?re retardant behaviour (V-0rating)is obtained with the mixtures containing at least 30%of APP (see No.2,4)or 60%of MH (see No.3)used alone.Addi-tion of MH to APP reduces its ?re retardant activity

as

Fig.3.TGA-FTIR results for experiments carried out on 50%w/w APP/MH physical mixture under nitrogen,heating rate 10 C/min.Upper part:solid line:TG curve;dotted line:NH 3evolution (A.U.);dashed line:DTG curve.Lower part:FTIR spectrum of gases evolving at 330

C.

Table 1

Intumescent ad237aed0975f46527d3e1b2position and UL-94performances.Polymer matrix:EVA (93%)+PA6(7%)Sample Filler t 1a sec t 2b sec t 1+2c sec t 8d sec Flaming dripping UL94ranking Total ?ller Content (%)APP/MH ratio

130Y n.c.f 0260%APP 000n.m.e NV-060360%MH 0 2.2 2.2n.m.e NV-060430%APP 0 2.6 2.6n.m.e NV-030530%MH 36Y n.c.f 30630%APP 3

83

86

n.m.e Nn.c.f 60130%MH 720%APP 66Y n.c.f 40120%MH 810%APP 46

Y

n.c.f

20110%MH 940%APP 0 2.3 2.3n.m.e NV-050410%MH 10

10%APP 0

60

60

n.m.e

Y

n.c.f

50

1/4

40%MH

a t 1=burning time after ?rst ignition.

b t 2=burning time after second ignition.

c t 1+2=overall combustion time.

d t 8=tim

e for 80mm combustion i

f the specimen burns until the clamp.

e n.m.(not measured)=t 8was not measured i

f the sample did self extinguish.f

n.c.=not classi?ed.

A.Riva et al./Polymer Degradation and Stability 82(2003)341–346345

shown by increase of combustion times in sample 6as compared to sample 4.An excess of APP is required if used with MH as compared to samples in which is used alone (see Nos.4,6,9)

By considering also the t 8values (related to the com-bustion rate),it is possible to distinguish between samples not classi?ed because they burned for more than 30s but able to give self-extinguishing of the ?ame (only t 1and t 2recorded),and samples not classi?ed because they burned to the clamp (only t 8recorded).The ?re behaviour of the mixtures appears to be better when the APP/MH ratio is di?erent from 1,and gets better with increasing the ratio.The comparison between samples Nos.6,7,8,(APP/MH=1)shows that the ?ame retardant behaviour gets better as the total ?ller amount rises (No.6self extin-guished,N 8burned to the clamp faster than No.7).Nos.9and 10show that with the same overall amount of APP+MH (50%)when APP/MH is higher than 1(No.9)the sample is V-0classi?ed,while if APP/MH is lower than 1(No.10)we have a n.c.sample,though with better behaviour than Nos.6,7,8(shorter self extinguishing time).A reason for this behaviour could be found in the sali?cation e?ect caused by MH on the acidic sites of APP,able to inhibit its ?ame retardant e?ect,and on APP being the most e?cient component as shown by comparison of mixtures 2–3and 4–5.

4.Conclusions

This study has shown that APP and MH interact on heating when introduced into an intumescent mixture

based upon EVA 24%VA,PA6and the e?ects on ?re retardant behaviour and thermal degradation of the polymer matrix of their interaction.

APP and MH react on heating forming salts at the acidic sites left on APP after NH 3release,and this redu-ces the ?ame retardant e?ectiveness of the additives.The TGA-FTIR analyses carried out on the intumes-cent mixtures showed that a facilitation of the forma-tion of the acidic sites on APP can be caused even by a low MH quantity.This sort of catalytic e?ect might be used to impart ?ame retardancy to materials with decomposition temperatures lower than that of EVA,by shifting the action of the acid source to match the charring and blowing e?ects of the intumescent additive to the decomposition of the polymer matrix.

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Fig.5.FTIR spectra of pure APP (solid line),pure MH (dashed line)and the residue left after heating their 50%w/w physical mixture at 600 C under nitrogen ?ux.

346 A.Riva et al./Polymer Degradation and Stability 82(2003)341–346

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