photocrosslinking and related properties of intumescent flame-retardant lldpe/eva/ifr blends

Research article

858

Received: 24 June 2009, Revised: 28 December 2009, Accepted: 26 February 2011, Published online in Wiley Online Library: 2 May 2011

(wileyonlinelibrary.com) DOI: 10.1002/pat.1980

Photocrosslinking and related properties ofintumescent flame-retardant LLDPE/EVA/IFRblends

Lei Yea, Qianghua Wua and Baojun Qua*

The photoinitiated crosslinking of halogen-free fl

Polym. Adv

ame retarded linear low density polyethylene/poly(ethylene-co-vinyl acetate) blends (LLDPE/EVA) with the intumescent flame retardant (IFR) of phosphorous-nitrogen compound(NP) in the presence of photoinitiator and crosslinker and their characterization of related properties have beeninvestigated by gel determination, heat extension test, cone calorimeter test (CCT), thermogravimetric analysis (TGA),Fourier transfer infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), mechanical properties measure-ments, limiting oxygen index (LOI), UL-94, and water resistance test. The data from the gel content and heat extensionrate (HER) show that the LLDPE/EVA/IFR blends filled with NP are readily crosslinked to a gel content of above 75% andthe HER values reach about 50% by UV-irradiation of 5 sec under suitable amount of photoinitiator and crosslinker.The data obtained from the CCT and LOI indicate that photocrosslinking can considerably decrease the heat releaserates (HRR) by 10–15%, prolongate the combustion time, and increase two LOI values for the LLDPE/EVA/NP blends UVirradiated for 5 sec. The results from TGA and the dynamic FTIR spectra give the evidence that the photocrosslinkedLLDPE/EVA/NP samples show slower thermal degradation rate and higher thermo-oxidative degradation temperaturethan the uncrosslinked LLDPE/EVA/NP samples. The morphological structures of charred residues observed by SEMgive the positive evidence that the compact charred layers formed from the photocrosslinked LLDPE/EVA/NP samplesplay an important role in the enhancement of flame retardant and thermal properties. The data from the mechanicaltests and water-resistant measurements show that photocrosslinking can considerably improve the mechanical andwater-resistant properties of LLDPE/EVA/NP samples. Copyright � 2011 John Wiley & Sons, Ltd.

Keywords: linear low density polyethylene/poly(ethylene-co-vinyl acetate) blend; intumescent flame retardant; photo-crosslinking

* Correspondence to: B. Qu, CAS Key Laboratory of Soft Matter Chemistry,Department of Polymer Science and Engineering, University of Science andTechnology of China, Hefei, Anhui 230026, China.E-mail: [email protected]

a L. Ye, Q. Wu, B. Qu

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science

and Engineering, University of Science and Technology of China, Hefei, Anhui

230026, China

INTRODUCTION

Polyethylene/poly(ethylene-co-vinyl acetate) blends are widelyused in wire and cable industry as insulating materials due totheir good mechanical and physical properties.[1,2] However, theflammability of these materials restricts their applications. Thisproblem can be solved by using flame retardant, such ashalogenated compounds with synergistic combination ofantimony trioxide. Unfortunately, there are serious disadvantagesabout the evolution of toxic gases and corrosive chemical fumethat choke people exposed to the toxicity and damage costlyequipment. So developing halogen-free flame retardant (HFFR)polymeric materials has become a potential trend.[3,4] And it hasbeen known for years that intumescent flame retardants (IFRs)are efficient in polyolefin (PO) and widely used as environmen-tally friendly halogen-free additives. Generally, intumescentformulations contain three active ingredients: an acid sourceas a catalyst, a carbonization compound, and a blowing agent.[5,6]

The proposed mechanism of intumescent retardant materials isthe formed cellular charred layers formed on heating acting as aphysical barrier on the surface, which slows down heat and masstransfer between the gas and condensed phase and protects theunderlying material from flux or flame. Melamine polyphosphate(MPP) and ammonium polyphosphate (APP) are commerciallyavailable and inexpensive NP compounds which commonly

Copyright © 2011 Joh. Technol. 201 , 2 –32 8 85 58 6

combined with a carbonization agents, such as pentaerythritol(PER), as IFRs for POs. According to the studied in ourlaboratory,[7–9] the phosphorus-nitrogen compound NP is anexcellent IFR additive that performs both as a carbonizingcompound and a blowing agent in PO.However, major disadvantages of HFFR materials are to

deteriorate mechanical properties and can not resist hightemperature, which are hard to meet the requirements of somespecial applications, especially for the thermal stability at hightemperature. Various crosslinking methods, such as high-energyirradiation (60Co g-rays and electron beam), organosilane,peroxide chemical crosslinking, and photocrosslinking havebeen widely used to improve the thermal and mechanicalproperties of polymeric materials.[10–17] Compared with other

n Wiley & Sons, Ltd.

Figure 1. UV emission spectrum of ‘‘D type’’ lamp.

INTUMESCENT FLAME-RETARDANT LLDPE/EVA/IFR BLENDS

methods, photocrosslinking has some advantages, such as highlyefficient, easily to handle, and low investment cost, etc.Photocrosslinking of PO has been industrially applied formanufacturing the PE-insulated crosslinked wire and cable formany years.[14] However, till date, few studies have been reportedon the photocrosslinked IFR PE/EVA/IFR blends.The present work is devoted to study the photocrosslinking

and related properties of LLDPE/EVA/IFR blends by geldetermination, heat extension test, cone calorimeter test(CCT), thermogravimetric analysis (TGA), Fourier transfer infrared(FTIR) spectroscopy, scanning electron microscopy (SEM),mechanical measurements, limiting oxygen index (LOI), UL-94,and water resistance test. The main interesting of the presentwork is to develop a new photocrosslinked PO/HFFRmaterial thatcan be used in the manufacture of HFFR wires and cables.

85

EXPERIMENTAL

Materials

LLDPE with amelting flow index of 2.0 g per 10min and a numberaverage molecular weight of 32 000 gmol�1 was supplied byZhongyuan Petrochemical Company, China. EVA copolymercontaining 28wt% vinyl acetate was purchased from SumitomoChemical Co. Japan. PER and photoinitiator benzophenone (BP)were obtained from Shanghai Reagent Plant No.1. Crosslinkertrially isocyanurate (TAIC) was obtained from Anhui Institute ofChemical Engineering. The phosphorus-nitrogen compound NPcontaining 15.6wt% P, 27.5wt% N with a particle size of400meshes from Zhuhai Deco Chemical Reagent Corporationwas used as IFR additives. APP and MPP were supplied byZhengjiang Xingxing Chemical Industry Co., Ltd.All the above chemicals are commercial products and used as

received.

Sample preparation

The polymer matrix used was a blend of LLDPE and EVA in a massratio of 7:3 commonly employed in the wire and cable industry.And APP or MPP is combined with PER as IFR, and their weightratio is 3:1 according to the literatures.[18,19] Then the filledcomposites with different amount of NP, APP, MPP, PER, BP, andTAIC were processed using a two-roll mill at 1408C for 10min. Andthen the mixtures were hot pressed into sheets with 1 and 3mmthick sheets at 1508C using a hot press.The LLDPE/EVA/NP samples with a given amount of LLDPE,

EVA, and NP are marked as LLDPE/EVA/NPn where n indicates theweight of NP in the LLDPE/EVA/NP blends. For example, theLLDPE/EVA/NP35 sample means 70 phr LLDPE, 30 phr EVA, and35 phr NP.

Sample Irradiation

The samples were pre-heated and then UV-irradiated in LC6BBenchtop Conveyer equipped with F300SQ lamp system (‘‘D’’bulb, the light intensity 1800mwcm�2, its emission spectrumshown in Fig. 1, which was supplied by the manufacturer)constructed by Fusion UV system, USA. The distance betweenF300SQ lamp and the surface of sample was 5.3 cm. Theirradiation time was controlled by the speed of conveyer belt. Thepre-heated temperature was usually 1208C unless statedotherwise.

Polym. Adv. Technol. 2012, 23 858–865 Copyright © 2011 John

Measurements

Gel content was measured by extracting the UV irradiatedsamples for 48 hr with boiling xylene. The solvent was renewedafter the first 24 hr. Three samples were analyzed to determinethe average gel content for a given set of irradiation conditions.The gel content can be calculated as the flowing formula: gel%¼ (weight of the samples after extraction/weight of thesamples before extraction)� 100.The heat extension measurement was done to measure the

extension of samples using a static loading at a fixedtemperature. Measurements were carried out in an oven at2008C with normal air conditions. The sample loading was20N cm�2. The extension between two marks was determinedafter 15min heating of sample in the oven, the heat extensionrate (HER) (l) is measured by the equation of l¼ L/L0� 1, whereL0 is the original length between two marks on the sample beforeheat extension, and L is the actual length between two marksafter heat extension.The CCTs were carried out using a Redcroft Cone Calorimeter

following the procedure defined in ISO 5660 on 4mm thick100� 100mm2 plaques. Each specimen was wrapped inaluminum and exposed horizontally to an external heat flux of35 kWm�2. The experimental error of data from the conecalorimeter was about 5%.The SEM of the char layers was analyzed by a JEOL JSM-6700F

Field emission SEM. The specimens were previously coated with aconductive layer of gold.The dynamic FTIR spectra were recorded with a MAGNA-IR 750

spectrometer (Nicolet Instrument Co., USA). The film sampleswere placed in a ventilated oven kept at 4008C with temperaturefluctuation of�18C for dynamically measuring the FTIR spectra insitu during thermo-oxidative degradation.The LOI value was measured using an HC-2 type instrument

(Nanjing Analytical Instrument Factory, China) on the sheets of120� 6.5� 3mm3 according to ASTM D2863-77. To estimate theerror and to check the repeatability in the LOI test, we measuredthe LOI value five times for each sample. The values werereproducible to within �0.5%.The UL-94 vertical test was carried out using a CFZ-1 type

instrument (Nanjing Analytical Instrument Factory, China) on thesheets of 127� 12.7� 3mm3 according to ASTM D635-77.The TGA was performed on a Shimadzu TGA-50H thermo

analyzer at a scan rate of 108Cmin�1 under the air flow rate of2� 10�5m3min�1.The tensile strength (TS) and the elongation at break (EB) were

measured with a Universal Testing Machine DSC-5000 (Shimadzu,

wileyonlinelibrary.com/journal/patWiley & Sons, Ltd.

9

L. YE, Q. WU AND B. QU

860

Japan) at the crosshead speed of 25mmmin�1 according to theChinese Standard GB 2951.6-82.In order to determination of water resistance of LLDPE/EVA/

IFRs blends, the specimens used for flammability test were put indistilled water at 708C and were kept at this temperature for168 hr. The treated specimen was subsequently taken out, driedin vacuum, and evaluated by burning test (LOI and UL-94). Thewater resistance of compositions was valued by the decreasingdegree of LOI value after treated with water.

RESULTS AND DISCUSSION

Flame retardant efficiency of different IFRs in the LLDPE/EVAblends

LOI and UL-94 tests are widely used to determine theflammability of FR materials. The LOI values of LLDPE/EVAblended with APPþ PER, MPPþ PER, and NP were shown inTable 1. It can be seen that the LOI values of LLDPE/EVA/NP blendsare 1–2 higher than those of LLDPE/EVA/APPþ PER or LLDPE/EVA/MPPþ PER samples at the same IFR contents. And the LOIvalue of LLDPE/EVA/NP sample is 30% and passes the V-0 ratingwhen the NP loading reaches 35 phr.Water resistance test of flame retardant polymeric materials

with phosphorus-containing additive is of practical importancebecause the formation of phosphoric acid species with water ormoisture of atmosphere can cause the decomposition of the FRpolymeric materials and deteriorate their physical properties,especially the flame retardant properties. Table 1 also lists the LOIvalues and UL-94 rating of different amount of IFRs samplestreated with water at 708C for 168 hr. After treated with water, theLOI values of all the samples decreased and the UL-94 ratingsshows the other samples can not pass the UL-94 rating except forthe LLDPE/EVA/NP sample with above 35 phr NP. These factsshow that a certain amount of flame retardants are leached outfrom the samples. But their reducing degrees are different, asshown in Table 1. The decrease in degree of LOI values of LLDPE/

Table 1. The LOI values and UL-94 results of LLDPE/EVA/IFR sampletest

SamplesIFRs content

(phr) LOI (%)

Pure LLDPE/EVA — 18LLDPE/EVA/(APPþ PER) 20 23

30 2735 2940 30

LLDPE/EVA/(MPPþ PER) 20 2230 2635 2840 30

LLDPE/EVA/NP 20 2430 2835 3040 31

wileyonlinelibrary.com/journal/pat Copyright © 2011 John

EVA/APPþ PER or LLDPE/EVA/MPPþ PER samples is about 2–4,which shows poor water resistance of these samples. This isbecause PER is a polyol, which is easily dissolved in water.However, the LLDPE/EVA/NP samples show better waterresistance, the decreasing degree of LOI values is only 1, andthere is no influence on the UL-94 rating after treated with water.Above results indicated that the NP not only has higher flameretardant efficiency on LLDPE/EVA blends but also has betterwater resistance. So we select NP as IFR for LLDPE/EVA blends infollowing sections and investigate the photocrosslinking charac-terization of LLDPE/EVA/NP blends.

Characteristics of photocrosslinking of LLDPE/EVA/NPblends

Figure 2 shows the changes of gel content with the irradiationtime for pure LLDPE/EVA, LLDPE/EVA/NP20, and LLDPE/EVA/NP35samples with and without TAIC in the presence of BP. It can beseen that the gel contents of samples rapidly increase withincrease in irradiation time. The gel contents can reach 60, 58, and56% for the pure LLDPE/EVA, LLDPE/EVA/NP20, and LLDPE/EVA/NP35 samples with only BP after UV-irradiated for 5 sec,respectively. However, the gel contents quickly increase to about82, 79, and 77% for the above samples with both 1 phr BP and1 phr TAIC, respectively. So it can be found that addition of amultifunctional crosslinker (TAIC) greatly accelerates the cross-linking process. It has been reported that TAIC can form highyields of radicals during UV-irradiation and improve thecrosslinking density of POs.[14] And it also can be seen fromFig. 2 that the gel contents decrease with increasing the NPcontent at the same irradiation time, which is probably due to thereflection effect of NP particles in the LLDPE/EVA/NP sampledecreases the UV energy absorption of photoinitiator and theamount of radicals initiated crosslinking.Figures 3 and 4 show the dependence of gel content on the

concentration of BP and TAIC for LLDPE/EVA/NP35 sample UVirradiated for 5 sec, respectively. As shown in Fig. 3, the optimum

with different amount of IFRs before and after water resistance

UL-94

(After treated with hot water)

LOI (%) UL-94

Decreasingdegree ofLOI values

fail — — —fail 20 fail 3fail 23 fail 4V-0 26 fail 3V-0 27 fail 3fail 20 fail 2fail 23 fail 3V-0 25 fail 3V-0 27 fail 3fail 23 fail 1fail 27 fail 1V-0 29 V-0 1V-0 30 V-0 1

Wiley & Sons, Ltd. Polym. Adv. Technol. 2012, 23 –858 865

Figure 2. Gel contents of LLDPE/EVA/NP blends with different

UV-irradiation time.Figure 4. Effect of TAIC concentrations on the gel content of LLDPE/EVA/

NP35 blends: BP 1 phr; irradiation time 5 sec.


concentration of BP is about 1 phr for LLDPE/EVA/NP35 sample. Afurther increasing the amount of photoinitiator does not increasethe gel content. The sample with 2 phr BP shows lower gelcontent. This can be interpreted as being due to two effects.[14]

The BP in the ground state and triplet state absorbs UV in theregion of n–p transition that will screen the UV light and act as an‘‘autoretardant.’’ In addition, some ketyl radicals formed byphotoinitiators will combine with polymer radicals and preventthe photoinitiated crosslinking of POs. Both effects are expectedto increase with the higher BP concentration. Moreover, theselection of a suitable amount of crosslinker is also important, asshown in Fig. 4. An optimal amount of TAIC for gel formation is1 phr. The further addition of TAIC has only a minor effect. The gelcontent of LLDPE/EVA/NP35 sample even decreased at 2 phr TAIC.This situation suggests that excessive TAIC can form a low-molecular-weight TAIC homopolymer, which is easily extractedby xylene. Therefore, the TAIC homopolymer reduced thecrosslinking between TAIC and POs and thus caused a reductionof gel content, and the similar results have also observed inprevious works.[14,20

The heat extension can provide a quick determination for thecrosslinking density of samples. The lower values of HER mean

Figure 3. Effect of BP concentrations on the gel content of LLDPE/EVA/

NP35 blends: irradiation time 5 sec.


86

the higher crosslinking degree of the samples.[14,20] The heatextension results from the LLDPE/EVA/NP20 and LLDPE/EVA/NP35samples are listed in Table 2. It is found that the unirradiatedsamples failed the test immediately. The LLDPE/EVA/NP20 andLLDPE/EVA/NP35 samples UV irradiated for 1 sec still failed the testbut maintained for several minutes. With increase in theirradiation time to 3 sec the samples passed the test and theHER values are 100 and 105% for LLDPE/EVA/NP20 and LLDPE/EVA/NP35 samples, respectively. With further increasing theUV-irradiation time to 5 sec, the HER values rapidly decreased to40 and 45% for LLDPE/EVA/NP20 and LLDPE/EVA/NP35 samples,respectively. Above results shown that LLDPE/EVA/NP blend canbe photocrosslinked easily and the crosslinking degree is satisfiedwith the need for wire and cable.

Effect of photocrosslinking on the flame retardantproperties

Table 3 listed the LOI and UL-94 results of LLDPE/EVA/NP samplesirradiated by 5 sec before and after treated with water at 708C for168 hr. It can be seen from Table 3 that the photocrosslinkedLLDPE/EVA/NP samples increase two LOI values when compari-son with those of the corresponding unirradiated samples (referTable 2). But photocrosslinking dose not effect on the UL-94rating. However, the LOI values of all samples decrease 0.5 aftertreated with water, which is much lower than those of theunirradiated samples (refer Table 1). These results indicate thatwater resistance property of photocrosslinked samples is muchbetter than that of uncrosslinked sample, which can be attributedto decreasing the migration of the FR additive to the surfacerestricted by the crosslinking network structures.The CCT based on the oxygen consumption principle is a

powerful tool for measuring the fire-retardancy of polymers inthe combustion process to evaluate their dynamic flameproperties. Although the CCT is a small-scale test, the obtainedresults have been found to correlate well with those obtainedfrom a large-scale fire test and can be used to predict thecombustion behavior of materials in real fire.[21] For instance,the heat release rate (HRR) is a very important parameter, andcan be used to express the intensity of fire.[22] A high flameretardant system normally shows a low HRR value.


1

Table 2. Hot extension test results of photocrosslinked LLDPE/EVA/NP blends at 2008C under 20N cm�2 load for 15min

Irradiation times (sec) 0 1 2 3 4 5

LLDPE/EVA/NP20 Failed immediately Failed in 1min 190% 100% 70% 40%LLDPE/EVA/NP35 Failed immediately Failed in 2min 200% 105% 75% 45%

Figure 5. Dynamic curves of HRRs versus burning time for LLDPE/EVA/NP35 blend with different irradiation time.


862

Figure 5 presents the dynamic curves of HRR versus time forLLDPE/EVA/NP35 samples UV irradiated for 0, 3, and 5 sec,respectively. It can be found from Fig. 5 that all the samples showtwo main HRR peaks at about 20 and 500 sec. Apparently the twopeak intensities of photocrosslinked samples are lower than thatof the sample without UV-irradiation, especially for the secondHRR peak at 500 sec. The second peak intensities of thephotocrosslinked LLDPE/EVA/NP35 samples decrease quicklyfrom 282.5 kWm�2 for the unirradiated sample to 257.2 and239.5 kWm�2 for the samples UV-irradiated for 3 and 5 sec,respectively. At the same time, the appearance time of thesecond HRR peak has been prolonged to 527 and 561 sec ofthe samples UV-irradiated for 3 and 5 sec from 497 sec of theunirradiated sample, respectively. Moreover, it can be seen fromFig. 5 that the burning time of the photocrosslinked LLDPE/EVA/NP35 samples has prolonged about 50 sec compared with theunirradiated sample.So it can be concluded that the flame retardant property of

LLDPE/EVA/NP35 samples can be greatly improved by photo-

Table 3. Flammability of photocrosslinked LLDPE/EVA/NP blends

Samples

NP contentUV-irradiated for

5 sec

(phr) LOI (%) UL-94

LLDPE/EVA/NP20 20 26 failLLDPE/EVA/NP30 30 30 failLLDPE/EVA/NP35 35 32 V-0LLDPE/EVA/NP40 40 33 V-0


crosslinking. This is because the crosslinked materials easily formconjugated aromatic rings charred structures during combustion,which result in making combustion become difficult.[23]

Thermal stability of photocrosslinked LLDPE/EVA/NPsamples

Figure 6 shows the TGA and DTG curves of LLDPE/EVA/NP35sample with different irradiation time. It can be seen from Fig. 6that the thermal degradation of all three LLDPE/EVA/NP35samples occurs in three steps. The first degradation step atthe 200–2608C temperatures ranges can be assigned to thepyrolysis of phosphorus-nitrogen compound NP. The seconddegradation step at the 300–4008C temperatures rangescorresponds to the loss of acetic acid from EVA. And the thirddegradation step at the 400–5508C temperatures ranges is due tothe degradation of polyethylene chains and the volatilization ofthe residual polymer. Apparently the photocrosslinked LLDPE/EVA/NP35 samples show slower degradation rate and have higherthermal degradation temperatures than the unirradiated sample,as shown in the DTG curves of Fig. 6 B. The pyrolysis temperatureof main resins from the samples UV irradiated for 3 and 5 sec areabout 6–138C higher than that of the unirradiated sample. Forinstance, the main chain pyrolysis temperatures are 434 and4418C for the sample irradiated for 3 and 5 sec, which are 6 and138C higher than that of the unirradiated samples, respectively.These results suggest that the thermal stability of LLDPE/EVA/NPblends can be enhanced considerably by photocrosslinking.Figure 7 shows the changes of dynamic FTIR spectra with

different thermo-oxidative degradation time of LLDPE/EVA/NP35sample before and after photocrosslinking. The detailed assign-ments of FTIR spectra in Fig. 7 are presented in Table 4 togetherwith corresponding references. By comparison with Fig. 7 (a) and(b), it can be seen that the dynamic FTIR spectra almost the samefor the LLDPE/EVA/NP35 samples before and after photocrosslink-ing. The absorption peaks at 2920, 2850 cm�1 and 1461,1375 cm�1 have been assigned to the asymmetric or symmetricvibration and deformation vibration of aliphatic group CH2 or

before and after water resistance test

Flame retardant properties

After treated with hot water

LOI (%) UL-94 Decreasing degree of LOI values

25.5 fail 0.529.5 fail 0.531.5 V-0 0.532.5 V-0 0.5


Figure 6. TGA (A) and DTG (B) curves of LLDPE/EVA/NP35 sample with

different irradiation time.


CH3, respectively, while the absorption peaks at 1740, 1240, and1020 cm�1 are assigned to the C––O and C–O deformationvibration in an ether. The above peak intensities decreaserapidly with increasing pyrolysis time. The various peaks at800–1400 cm�1 observed from Fig. 7 (a) and (b) can beenassigned to the various phosphorus oxides and phosphate–carbon complexes, such as P–O vibration in P–O–P groups,stretching mode of P–O–C, and symmetric vibration of PO3

groups, as reported in our previous work.[7] However, it can befound that the disappearance time of the aliphatic groups inFig. 7 (a) and (b) is quite different. The peaks at 2920 and2850 cm�1 almost disappeared after about 70 sec for theunirradiated LLDPE/EVA/NP35 sample in Fig. 7 (a). While the

Figure 7. Dynamic FTIR spectra with different thermo-oxidative time at 4008Cfor 5 sec.


disappearance of corresponding peaks need about 90 sec forthe irradiated LLDPE/EVA/NP35 sample in Fig. 7 (b), which givethe positive evidence that photocrosslinking can enhance thethermal stability of LLDPE/EVA/NP sample.

Morphologic structures of charred layers fromphotocrosslinked LLDPE/EVA/NP sample

It is important to study the relationship between the structure ofchar residues and the flame retardant and thermal properties ofpolymeric materials. In burning, an instantaneous char formed onthe surface of IFR-POs can act as a thermal insulation, whichfacilitates the extinction of the flame. Such a char prevents thecombustible gas from feeding the flame and also separatesoxygen from the burning material. Figure 8 presents the SEMimages of the intumescent charred layers from the LLDPE/EVA/NP35 samples before and after photocrosslinking. The charredlayers of the LLDPE/EVA/NP35 sample before photocrosslinkingare constituted of many closed cells, but these cells are cracked,as shown in Fig. 8 (a). Therefore, the heat and flammable volatilescould penetrate the char layer into the flame zone duringburning. However, the charred layers of the LLDPE/EVA/NP35sample after photocrosslinking (Fig. 8b) shows more compactwithout visible cracks, which can effectively limit the heat andmass transfer between polymer and flame. The above resultsshow that photocrosslinking is in favor of enhancing the flameretardant and thermal properties of samples.

Mechanical properties of photocrosslinked LLDPE/EVA/NPsample

Figure 9 shows the changes of mechanical properties withdifferent UV irradiation time of LLDPE/EVA/NP35 sample. It can beseen that the TS values rapidly increase to 14.8MPa of the sampleUV irradiated for 5 sec from 10.5MPa of the unirradiated sample.This can be attributed to the increase of crosslinks density in thecrosslinked sample. Therefore, the TS of the samples can beimproved by photocrosslinking. In contrast, the correspondingvalues of EB greatly decrease from the original 1010% to 780%after 5 sec UV-irradiation. These results can be attributed to theformation of the crosslinking network structures which make themacromolecules more rigid, as reported in our previousliterature.[20]

: (a) LLDPE/EVA/NPwithout UV irradiation, (b) LLDPE/EVA/NP UV irradiated


863

Figure 8. SEM images of the char samples obtained from the LLDPE/EVA/NP sample before (a) and after photocrosslinking (b).

Figure 9. Changes of the TS and EB of LLDPE/EVA/NP35 blends withdifferent UV irradiation time.

Table 4. Assignments of dynamic FTIR absorption peaks in Fig. 7

Wavenumber (cm�1) Assignment References

2920 yas of C–H in CH2 or CH3[24,25]

2850 ysym of C–H in CH2 or CH3[24,25]

1700–1800 C––O stretching vibration [26,27]

1020 C––O vibration in a ether [26,27]

1461 g-CH2[25]

1375 g-CH3[25]

1350–1360 Stretching mode in P––O[25]

1150–1300 Stretching mode of P–O–C [25,27]

1088 ysym of P––O in P–O–P [25,26]

1018 ysym of PO2 and PO3 in phosophate–carbon complexes [25]

880 yas of P––O in P–O–P [25,26]


864

CONCLUSIONS

The LLDPE/EVA blends filled with suitable amount of IFR NP canbe easily photocrosslinked in the presence of suitable amount ofinitiator and crosslinker to obtain the satisfied crosslinkingdegree for application the flame retardant cable. The results fromgel content and heat extension show that about 75% gel content


and 40% HER can be obtained by 5 sec irradiation from theLLDPE/EVA/NP sample. The data from the flame retardantproperties by comparison of before and after the water resistancetests shows that photocrosslinking can improve the LOI andwater resistance of LLDPE/EVA/NP sample. The data obtainedfrom the CCT data indicate that photocrosslinking considerablydecreases the HRRs of the LLDPE/EVA/NP sample. The TGA anddynamic FTIR spectra give the evidence that the photocros-slinked LLDPE/EVA/NP samples show slower thermal degradationrate and higher thermal degradation temperature than theuncrosslinked LLDPE/EVA/NP sample. The structures of charredresidues observed by SEM give the positive evidence that thecompact structure of charred layers formed from the photo-crosslinked LLDPE/EVA/NP sample effectively enhance the flameretardant and thermal stability properties. The mechanical testsshow that the mechanical properties of LLDPE/EVA/NP blends aremarkedly improved by photocrosslinking.

Acknowledgements

This project has been supported by the support item of Ministryof Science and Technology of China (the item No.:2007BAE27B00), which is gratefully acknowledged. This work isalso supported by the National Natural Science Foundation of



China (No. 20704040) and National Key Technology R&D Programof China (NO. 2007BAE27B01).

REFERENCES

[1] M. Holmes, Plast. Addit. Compd. 2004, 6(3), 32–36.[2] J. A. Brydson, Plastic Materials. Butterworth, London, 1982.[3] S. Y. Lu, I. Hamerton, Prog. Polym. Sci. 2002, 27, 1661–1721.[4] A. K. Sen, B. Mukheriee, A. S. Bhattacharya, L. K. Sanghi, P. P. De, K.

Bhowmick, J. Appl. Polym. Sci. 1991, 43, 1673–1694.[5] H. L. Vandersall, J. Fire Flamm. 1971, 2, 97.[6] M. Le Bras, S. Bourbigot, Fire Retardancy of Polymers- The Use of

Intumescent (Eds. M. Le Bras, G. Camind, S. Bourbigot, R. Delobell,).The Royal Society of Chemistry, Cambridge, 1998, 64.

[7] Q. Wu, B. J. Qu, Polym. Degrad. Stab. 2001, 74, 255–261.[8] R. C. Xie, B. J. Qu, Polym. Degrad. Stab. 2001, 71(3), 375.[9] R. C. Xie, B. J. Qu, Polym. Degrad. Stab. 2001, 71(3), 395.[10] A. Gheysari, Behjat. Eur. Polym. J. 2001, 37, 2011.[11] P. Ghosh, D. Dev, A. Chakrabarti, Polymer 1998, 38, 6175.[12] H. G. Scott, British Patent 1 286 460, 1968.[13] B. J. Qu, W. F. Shi, B. Ranby, Polym. Eng. Sci. 1995, 35, 1005.


[14] B. J. Qu, B. Ranby, J. Appl. Polym. Sci. 1993, 48, 701–709.[15] B. A. Sultan, M. Palmlof, Rub. Comp. Appl. 1994, 21, 65.[16] E. J. Lawton, A. M. Bueche, J. S. Balwit, Nature 1953, 172, 76.[17] Y. T. Shich, T. H. Tsai, J. Appl. Polym. Sci. 1998, 69, 255.[18] S. Bourbigot, L. B. Michel, S. Duquesne, M. Rochery,Macromol. Mater.

Eng. 2004, 289, 499–511.[19] P. Lv, Z. Z. Wang, K. L. Hu, W. C. Fang, Polym. Degrad. Stab. 2005, 90,

523–534.[20] D. H. Yao, B. J. Qu, Q. H. Wu, Polym. Eng. Sci. 2007, 47, 1761–

1767.[21] M. M. Hirschler, Heat release from plastic materials in heat release in

fire. In Heat Release in Fire (Eds. V. Babtauskas, S. Grayson), Elsevier,London, 1992, 375.

[22] U. Wickstrom, U. Goransson, Full-scale/bench-scale correlations ofwall and ceiling linings. In Heat Release in Fires (Eds. V. Babtauskas, S.Grayson), Elsevier, London, 1992, 375.

[23] H. D. Lu, Y. Hu, J. F. Xiao, Q. H. Kong, Z. Y. Chen, W. C. Fan, Mater. Lett.2005, 59, 648–651.

[24] S. Bourbigot, B. M. Le, R. Delobel, Carbon 1995, 33(3), 283.[25] M. Bugajny, S. Bourbigot, Polym. Int. 1999, 48, 264–270.[26] The Adrich Library of Infrared Spectra Edition III Charles J.[27] R. Kwiatkowski, A. Wlochowicz, J. Mol. Struct. 2000, 516, 57–69.


865

photocrosslinking and related properties of intumescent flame-retardant lldpe/eva/ifr blends

Documents