Journal of Reinforced Plastics and Composites

Journal of Reinforced Plastics and Composites http://jrp.sagepub.com/

Comparative study on flame, thermal, and mechanical properties of HDPE/clay nanocomposites with MPP or APP Konrad Szustakiewicz, Barbara Cichy, Malgorzata Gazinska and Jacek Piglowski Journal of Reinforced Plastics and Composites published online 25 March 2013 DOI: 10.1177/0731684413481508 The online version of this article can be found at: http://jrp.sagepub.com/content/early/2013/03/25/0731684413481508

Published by: http://www.sagepublications.com

Additional services and information for Journal of Reinforced Plastics and Composites can be found at: Email Alerts: http://jrp.sagepub.com/cgi/alerts Subscriptions: http://jrp.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav

>> OnlineFirst Version of Record - Mar 25, 2013 What is This?

Downloaded from jrp.sagepub.com at Politechnika Wroclawska on April 2, 2013

Original Article

Comparative study on flame, thermal, and mechanical properties of HDPE/clay nanocomposites with MPP or APP

Journal of Reinforced Plastics and Composites 0(00) 1–13 ! The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684413481508 jrp.sagepub.com

Konrad Szustakiewicz1, Barbara Cichy2, Małgorzata Gazin´ska1 and Jacek Pigłowski1

Abstract The article reports on flammability, thermal, and mechanical properties of bimodal high-density polyethylene/clay nanocomposites modified with ammonium polyphosphate or melamine polyphosphate (MPP) flame retardants. Two types of clays were used as fillers for the composites—the first one was standard montmorillonite modified with quaternary ammonium salt (ZR2), and the second one was montmorillonite modified with aluminium hydrogen sulfate (ZG1). As a compatibilizer, maleic grafted polyethylene (Plb) was used. X-ray diffraction and transmission electron microscopy were used to characterize the layer structure of clays in the composites. The limiting oxygen index tests showed synergistic effect between both clays and MPP. Flammability was also examined using cone calorimetry technique. The influence of all the fillers on thermal stability (thermogravimetric analysis), crystallinity (differential scanning calorimetry and wide angle X-ray scattering techniques), and mechanical properties was also studied.

Keywords Polyethylene, clay, nanocomposites, flammability, WAXS, ammonium polyphosphate, melamine polyphosphate

Introduction Polyethylenes are widely used in many fields, i.e., in household, wires and cables, and automotive industry. It is generally known that polyethylene is highly combustible. For above mentioned application, flame retardation is needed. For conventional polymers with unimodal molecular weight distribution exists contradiction between mechanical properties and processability. It is known that polymer with better mechanical and physical properties can be obtained by increasing the average molecular weight, which also leads to poor processability due to high viscosity.1 Therefore, in recent years, a bimodal molecular weight distribution high-density polyethylene (HDPE) was developed. The polymer is composed of higher molecular weight and lower molecular weight fractions and exhibits good mechanical and physical properties as well as excellent processability.2 Many papers report on significant improvement of physical properties of polymer–clay nanocomposites over their neat polymer.3–7 There are many articles in

which authors emphasis the reduction of flammability of nanocomposites with layered silicates comparing to neat polymers. More than 250 articles about flame retardancy of polymer–clay nanocomposites were lately summarized by Kiliaris and Papaspyrides.8 Flame retardancy seems to be one of the most important advantage of the nanocomposites. It is known that all extraordinary properties of the nanocomposites are related to good dispersion of the clay in polymer matrix—in so called exfoliated hybrids.9 In such systems, clay platelets are separated and uniformly

1 Polymer Engineering and Technology Division, Wrocław University of Technology, Wrocław, Poland 2 Inorganic Chemistry Division, Fertilizers Research Institute, Gliwice, Poland

Corresponding author: Konrad Szustakiewicz, Polymer Engineering and Technology Division, Wrocław University of Technology, Wybrzez_e Wyspian´skiego 27, 50-370 Wrocław, Poland. Email: [email protected]


2

Journal of Reinforced Plastics and Composites 0(00)

dispersed, maximizing the surface interaction between polymer and clay. Nanocomposites obtained from polyolefins usually are modified with a polar compatibilizer to improve the clay exfoliation degree. Polyethylene nanocomposites are compatibilized with maleic anhydride grafted polyethylene10 or copolymers, i.e., ethylene vinyl acetate.11,12 In addition, the clay surface is typically modified with surfactant, to increase the affinity between the hydrophilic clay and hydrophobic polymer.18–20 Usually, to achieve high flame retardancy and nondripping composites, beside clay, additionally standard flame retardants (FRs) are also added to polymers. The most common FRs for polyolefins are metallic hydroxides such as aluminium trihydroxide,13 magnesium hydroxide14 and polyphosphates such as ammonium polyphosphate (APP) or melamine polyphosphate (MPP).15–17 In some cases, synergistic effect of flame retardancy between layered silicates applied along with conventional FRs is observed in thermoplastic materials.18 In this article, we present results of investigation on HDPE/maleic anhydride grafted polyethylene/clay nanocomposites using two types of clays—hydrophilic clay modified with flame retardant (aluminium hydrogen sulfate) and typical hydrophobic organoclay. As a reference sample the blend of HDPE with maleic anhydride grafted polyethylene was used, thus the influence of all fillers on the properties of the composites is evaluated, and the compatibilizer role is skipped. All the nanocomposites were also filled with 20 wt.% of polyphosphate FRs—APP as well as 20 wt.% of MPP and all the properties of all systems were compared.

(ZR2) (CEC 85 meq/100 g, d001 ¼ 2.1 nm), modified with N,N-didecyl-N,N-dimethylammonium chloride, with organic content of 20 wt.% was supplied by ZGM Zebiec, Poland; it is known that best improve of exfoliation can be obtained by using ‘‘two tails’’ on ammonium ion modified organoclay,20 (2) NanoBentÕ ZG1 (ZG1) (CEC 85 meq/100 g, d001 ¼ 1.6 nm), modified with aluminium hydrogen sulfate was also supplied by ZGM Zebiec. APP intumescent flame retardant having P2O5 50 wt.% and N2 of 21 wt.% under trade name Budit 3167 obtained from Budeheim, Spain was used as a commercial flame retardant. MPP was obtained and characterized in our laboratory according to technology described in the patent.21

Experimental Materials

In the second step, the melamine orthophosphate was heated in muffle furnace (1 h, 330 C), with addition of urea, and MPP was obtained (equation (2)):

Bimodal HDPE Hostalen GC 7260 having melt flow rate of 8 g/10 min (190 C/2.16 kg) and density d ¼ 0.963 g/cm3 was supplied by Basell Orlen Polyolefins (Plock, Poland). Maleic anhydride grafted polyethylene (Plb) Polybond 3009 having melt flow rate of 3–6 g/10 min (190 C/2.16 kg), density of d ¼ 0.950 g/cm3, and grafting level of maleic anhydride (MAH) ¼ 1 wt.% obtained from Chemtura was used as a compatibilizer. As a reference sample, HDPE with 20 wt.% of Plb was used (HDPE/Plb). As it was showed,19 the exfoliation of the clay depends on anhydride grafted polyethylene content in the composite, thus the level of the Plb content in this article was set at constant level of 20% in all composites. In this approach, polyethylene content in the composite rises as fillers content decrease. Two type of clays were used in the study: (1) organobentonite NanoBentÕ ZR2

Preparation of melamine polyphosphate MPP was obtained in two step reaction according to equations (1) and (2). In the first step, melamine (2,4,6triamine-1,35-triazine) was treated with phosphoric acid (melamine: orthophosphoric acid molar ratio was 1.1:1 (equation (1)), and melamine orthophosphate was obtained.

The P2O5 content in obtained MPP equals 31 wt.% and nitrogen of 42%. The MPP was grinded and separated using 100AFG fluidized bed opposite jet


Szustakiewicz et al.

3

mill (Hosokawa Alpine Germany) to obtain particle size up to 20 mm and unimodal particle size distribution.

Processing All the composites were extruded using Brabender DSE 20/40 (D ¼ 20 mm, L/D ¼ 40) corotating twin-screw extruder with screws rotational speed of 250 per min and a temperature profile of 190–190–190–180 C (from hopper to die). The screws profile was presented in our previous work.22 The composites of HDPE/Plb/FR were extruded in one step extrusion. The nanocomposites containing clay and FRs were compounded according to the procedure consisting of two steps: (1) in the first step, masterbatch of Plb with 30 wt.% of ZR2 (or ZG1) was extruded, (2) in the second step, the masterbatch was diluted with HDPE/Plb or HDPE/ Plb/FR. The compatibilizer content in the nanocomposites was set at 20 wt.% and the clay content at 2 wt.%. All obtained composites are summarized in Table 1. After extrusion process, the extrudates were cooled down in water to 30 5 C and pelletized. Samples for tensile tests and limiting oxygen index (LOI) tests were prepared by Arburg 221 M injection molding machine using a barrel temperature of 170–180–200–215–220 C (from hopper to die) and an injection pressure of 1600 bar. Samples for cone calorimeter test of dimension 4 mm 100 mm 100 mm were compressed molded (190 C, 3 min).

Measurements The tensile properties of all composites were determined according to PN-EN ISO 527-1. Tests including tensile modulus, tensile strength, stress, and strain at break were conducted using universal strength machine

Table 1. Composition of samples and their codes. Sample code

HDPE Plb ZR2 ZG1 MPP APP (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%)

HDPE/Plb

80

20

–

–

–

–

HDPE/Plb/MPP

60

20

–

–

20

–

HDPE/Plb/APP HDPE/Plb/ZR2

60 78

20 20

– 2

– –

– –

20 –

HDPE/Plb/ZG1

78

20

–

2

–

–

HDPE/PlbZR2/MPP

58

20

2

–

20

–

HDPE/Plb/ZG1/MPP 58

20

–

2

20

–

HDPE/Plb/ZR2/APP

58

20

2

–

–

20

HDPE/Plb/ZG1/APP

58

20

–

2

–

20

HDPE: high-density polyethylene; MPP: melamine polyphosphate; APP: ammonium polyphosphate.

Tira Test 2705. All the tensile properties were averaged from at least 20 specimens. Wide angle X-ray scattering (WAXS) experiments were done at room temperature on Rigaku Ultima IV diffractometer (Bragg–Brentano geometry) with Ni filtered CuKa ( ¼ 1.54178 A˚) radiation generated by sealed X-ray tube. The radiation source was powered by a generator operated at 40 kV and 30 mA. Data were collected in a continuous-scan mode (scanning speed of 1 /min) within the range of 2y from 1.1 to 60 . Organobentonites interlayer distance was calculated according to Braggs’s law. Samples for WAXS measurements were prepared by compression molding (190 C, 3 min). Differential scanning calorimetry (DSC) measurements were carried out on Mettler Toledo DSC 821 apparatus. All the HDPE samples were heated to the melt for ensuring uniform conditions at the beginning (40 to 180 C, heating rate 10 C/min), held at 180 C for 5 min to destroy all crystallites, then cooled to 40 C with cooling rate of 10 C/min. The temperature program was repeated, and the second scan was recorded. The DSC method was used to determine the influence of all the filler on melting temperature (Tm), degree of crystallinity (Xc), and heat of fusion (Hm). The percent of crystallinity was calculated based upon a perfect crystal heat of fusion of 290 J/g.23 Flammability was characterized using LOI and the cone calorimetry method according to ISO standard 5660 with a horizontal specimen position using an FTT cone calorimeter. Heat flux was set at 50 kW/m2, which is equivalent to a trash-can-fire heat flux.24 The standard uncertainty of the cone calorimetry was up to 10%. Exhaust flow was set at 24 l per s, and the spark was continuous until the sample ignited. The specimens for LOI tests were injection moulded and for the cone calorimeter tests (4 mm 100 mm 100 mm and mass of 34–38 g) were prepared by compression moulding (190 C, 3 min). Melt mass flow rate (MFR) measurements were conducted at 190 C/2.16 kg using Zwick (Germany) plastometer. Transmission electron microscopy (TEM) pictures were taken to evaluate structure of silicate in the composites. All composites were cut at room temperature into slices of maximum thickness of 0.5 mm using Leica ultramicrotome. Pictures were taken using EM900 microscope (Zeiss, Germany) at magnification of 15,000. Thermogravimetric analysis (TGA) was carried out to examine thermal stability of the composites. The experiments were conducted using Mettler–Toledo TGA/SDTA 851 under air atmosphere at gas flow rate of 30 ml/min. A heating process was performed from 20 C to 900 C at the constant rate of 10 K/min.


4


endo

134°C 132°C

127°C

HDPE

Q/m[W/g]

HDPE/Plb

100

Plb

110

120

130

140

150

160

T [°C]

Figure 1. DSC patterns of HDPE, Plb and HDPE/Plb blend. DSC: differential scanning calorimetry; HDPE: high-density polyethylene.

Results and discussion DSC analysis of HDPE/Plb Before discussing the nanocomposites, the properties of neat HDPE, Plb, and HDPE/Plb blend were examined. The melting temperatures of all three systems determined by DSC are shown in Figure 1. Plb has lower melting temperature (127 C) than HDPE (134 C) due to maleic anhydride groups that disrupt the ordered crystalline structure of HDPE.20 The blend of HDPE with 20 wt.% Plb has lower temperature than neat HDPE (132 C) and that means that both polymers are miscible. In further studies, HDPE/Plb blend was chosen as the reference sample.

Structure of the nanocomposites characterized by XRD and TEM All layered fillers and nanocomposites containing modified montmorillonites were investigated using X-ray diffraction (XRD) technique. The aim of this study was the analysis of layered structure of both clays before and after compounding with HDPE/Plb (or HDPE/Plb/FR). The XRD patterns for the composites filled with ZR2 clay are shown in Figure 2. There are no differences between d001 of ZR2 filler and the Plb/ZR2 masterbatch. In both cases, maxima are located at 2y ¼ 4.85 that corresponds to d001 ¼ 1.8 nm. Similar pattern was recorded for HDPE/Plb/ZR2 system.

The main differences between this curve and the one recorded for Plb/ZR2 system are related to clay content: Plb/ZR2 contains 30 wt.% and for HDPE/Plb/ ZR2 the content of the clay is 15-fold lower (2 wt.%). In addition, HDPE/Plb/ZR2 maximum diffraction peak is located at 2y ¼ 5.00 and that means that the d-spacing distance of ZR2 clay in HDPE/Plb/ZR2 system is slightly lower than in the case of neat ZR2 and Plb/ZR2 system. In this case, d001 ¼ 1.77 nm. This shift to higher diffraction angle (and lower d001) could indicate degradation of the surfactant, causing the clay galleries to collapse.25 The scattered intensities originating from layered structure of the clay for both composites containing 2 wt.% of ZR2 and 20 wt.% of APP or MPP (HDPE/Plb/ZR2/MPP and HDPE/Plb/ZR2/ APP) are much weaker than in the corresponding composite with 2 wt.% of ZR2 but without FRs (HDPE/ Plb/ZR2). This indicates that extrusion process of HDPE/Plb/ZR2 composites in the presence of MPP or APP FRs leads to exfoliation of the clay in above mentioned composites. Similar effect was observed for PP/Plb/clay/APP system in our previous work.26 In the case of composites containing ZG1 clay, the situation is similar (Figure 3). The d-spacing distance for neat ZG1 d001 ¼ 1.54 nm (2y ¼ 5.75 ). For the masterbatch Plb/ ZG1 the XRD maximum peak is observed at 2y ¼ 5.93 , so the d-spacing distance of the clay in the Plb/ZG1 masterbatch is slightly lower than for neat ZG1 d001 ¼ 1.54 nm. Further decrease of d001 is observed for HDPE/Plb/ZG1 composite



5

ZR2

4000

d001=1,80 nm

Plb/ZR2 (masterbatch) HDPE/Plb/ZR2 HDPE/Plb/ZR2/APP HDPE/Plb/ZR2/MPP

3000

d001=1,77 nm 2000

I[a.u]

1000

0 2

3

4

5

6

7

8

2θ [°]

Figure 2. XRD patterns of composites filled with ZR2 clay. XRD: X-ray diffraction.

ZG1b

4000

Plb/ZG1 (masterbatch) HDPE/Plb/ZG1 HDPE/Plb/ZG1/APP

3000

d001= 1,50 nm

HDPE/Plb/ZG1/MPP

d001= 1,54 nm 2000 d001= 1,45 nm

I[a.u]

1000

0 1

2

3

4

5

6

7

8

2θ [°]

Figure 3. XRD patterns of composites filled with ZG1. XRD: X-ray diffraction.

(d001 ¼ 1.45 nm) and another difference comparing to Plb/ZG1 system is 15-fold lower clay content in HDPE/Plb/ZG1. In the case of composites filled with ZG1 clay also addition of MPP or APP (HDPE/Plb/ ZG1/MPP and HDPE/Plb/ZG1/APP composites) to extrusion process facilitates the exfoliation of the clay. This effect is particularly interesting because no

amphiphilic compound for the surface modification of the clay was used. In addition, TEM pictures were made to assess the degree of exfoliation of ZG1 and ZR2 clays in polymer nanocomposites (Figure 4(a) and (b)). It is known that addition of HDPE-g-MAH to the polymer matrix improves the exfoliation of the clay. The degree of


6


Figure 4. TEM pictures of (a) HDPE/Plb/ZG1 and (b) HDPE/Plb/ZR2 composites. TEM: transmission electron microscopy; HDPE: high-density polyethylene.

25

25,8

24,8 23,0

25,2

23,5

19,8

19,8

HDPE/Plb/ZG1

20,8

HDPE/Plb

0

HDPE/Plb/ZR2/MPP

HDPE/Plb/ZR2/APP

HDPE/Plb/MPP

HDPE/Plb/APP

5

HDPE/Plb/ZR2

10

HDPE/Plb/ZG1/MPP

15

HDPE/Plb/ZG1/APP

LOI, %

20

24,0

Figure 5. Limiting oxygen index (LOI) of HDPE nanocomposites. HDPE: high-density polyethylene.

exfoliation increase as more HDPE-g-MAH is added to HDPE, however, above 20–25 wt.% little change in the extent of exfoliation is observed.27 In the presented pictures, some agglomerates of few clay platelets can be observed. This indicates that in both cases (composites with 2% of ZR2 and with 2% of ZG1 clay), the composites are not fully exfoliated.

Flammability The LOI has been determined to find volume content of oxygen in oxygen–nitrogen mixture at which the material still burns. It is clear that the greater the LOI value,

the lower is flammability of the material. It is known that HDPE-g-MAH/‘‘two tailed’’ organoclay nanocomposites have higher LOI than neat HDPE-gMAH.28 As seen in this study (Figure 5), addition of 2% of ‘‘two tailed’’ organoclay causes increase of LOI from 19.8 for neat HDPE/Plb to 20.8 (HDPE/Plb/ ZR2). That means, the presence of the ZR2 clay reduces the flammability of the polymer. This effect was described by Zanetti and Costa29 and related to accumulation of the silicate on the surface of the burning polyethylene. In this way, a protective barrier to heat and mass transfer is created. It is noteworthy that the flammability reduction effect (LOI) is not



7

Figure 6. Samples of (a) HDPE/Plb, (b) HDPE/Plb/ZG1, and (c) HDPE/Plb/ZR2 after 1 min from ignition in air atmosphere. HDPE: high-density polyethylene.

observed for HDPE/Plb/ZG1 system. In this case, the LOI value is the same as for HDPE/Plb system (19.8). To explain this effect, the combustion process was carried out also in air atmosphere. Two photographs have been made after 1 and 2 min from the ignition. The results of the experiments are shown in Figures 6 and 7. As can be seen, molten polyethylene runs down (drips) in the case of HDPE/Plb system (Figures 6(a) and 7(a)). For the HDPE/Plb/ZR2 composite, char on the surface of the specimen is observed. The char protects the sample against dripping (Figures 6(c) and 7(c)). The burning process of the HDPE/Plb/ZG1 composite is comparable to HDPE/Plb system (Figures 6(b) and 7(b)). It seems that molten HDPE/Plb/ZG1 composite drips down even faster than molten HDPE/Plb. The dripping material heats up the specimen which collapses after 2 min from the ignition (Figure 7(b)) and no char formation is observed. The explanation of this process is related to the modifiers used for modification of both clays. In the case of ZR2 clay, ‘‘two tailed’’ organic modifier was used for the surface modification, so the surface of the ZR2 clay is hydrophobic. In the case of ZG1 clay, the aluminium hydrogen sulfate was used. The aluminium hydrogen sulfate does not change the surface of the clay from hydrophilic to hydrophobic. The hydrophilic surface of the clay has much lower affinity to the surface of hydrophobic polyethylene and during the combustion process no char formation is observed. Flame retardancy of nanocomposites is connected to structure of the clay. The HDPE/Plb/ZR2 composites have higher d001 value, which indicates better exfoliation of the clay in HDPE/Plb/ZR2

Figure 7. Samples of (a) HDPE/Plb, (b) HDPE/Plb/ZG1, and (c) HDPE/Plb/ZR2 after 2 min from ignition in air atmosphere. HDPE: high-density polyethylene.

system comparing to HDPE/Plb/ZG1 nanocomposite, where flame retardancy was not achieved (in LOI tests). Interestingly, the specimen of HDPE/Plb/ZR2 which burns slower and without dripping, burns with brighter flame comparing to HDPE/Plb and HDPE/Plb/ ZG1. It is known that modification of polyethylene with MPP or APP leads to reduction of flammability of polyethylene.30 The results describe in this work confirm that. The HDPE/Plb modified with 20% of phosphorus containing additives, MPP and APP, leads to increase of the LOI values. The LOI value for the HDPE/Plb/APP is 24.8 and for the HDPE/Plb/MPP is 23.0 (Figure 5). The combination of APP with ZR2 leads to further increase of LOI value. It seems that the typical additive effect is observed in the case of HDPE/Plb/ZR2/APP composite. The LOI value in this case is of 25.8 (1 unit of LOI increasement in HDPE/Plb/ZR2 composite and 5 units of increasement of LOI in HDPE/Plb/APP system comparing to HDPE/Plb). In this case, the LOI increase is caused by two factors: ZR2 clay reinforced structure that hinders the transfer of heat and APP intumescent char formation. The combination of these two effects makes the material burn slowly. The APP/ZG1 combination in HDPE/Plb does not lead to increasement of LOI. In this case, the LOI value (24) is even smaller than in the case of HDPE/Plb/APP (24.8). It is worth mentioning that the LOI value is not always proportional to the amount of used additives.31 In this case, no protection structure of ZG1 clay was obtained. When the ZG1/MPP (or ZR2/MPP) combination of fillers is added to the HDPE/Plb system, the synergistic


8


effect is observed. In the case of ZG1/MPP additives, the LOI rises to 23.5 (0.5 of benefit unit), but in the case of ZR2/MPP fillers, 1.2 of LOI beneficial units is observed (the LOI for HDPE/Plb/ZR2/MPP is 25.2). Cone calorimeter method was used to measure the flammability of the HDPE-based composites under firelike condition. For the polymeric fire retarded materials, the best systems should have lower peak heat release rate (PHRR), longer time to PHRR, lower values of a HRR and mass loss rate (MLR), and longer time to ignition (TTI). All the measured parameters are listed in Table 2. By adding 2% of ZR2 organoclay to HDPE/Plb system, the PHRR of the nanocomposite was significantly reduced by 51%. The effect was reported by Yang and coworkers.32 In cited article, the PHRR of the 2% filled nanocomposite was reduced by 54%. The main difference between the two studies was heat flux, 35 kW/m2 versus 50 kW/m2. The PHRR of HDPE/Plb/ ZG1 composite is reduced by 64% comparing to HDPE/ Plb system. Usually, the PHRR is decreased as more clay is added to polyethylene, however, in experiments presented in the article, much more reduction of the PHRR was obtained by adding to polyethylene 2% of clay modified with aluminium hydrogen sulfate, than in the same system filled with 2% of organoclay. There is no agreement in TTI changes in the PE/clay composites comparing to neat PE. In some articles, TTI of the composites was longer,33,34 but in some cases, the composites ignite faster.34 In presented results, TTI of the composite filled with organophilic clay rises, but in the system with hydrophilic clay, the TTI decrease comparing with HDPE/Plb. The main difference between HDPE/Plb/ZR2 and HDPE/Plb/ZG1 is observed in average HRR (aHRR) and average MLR (aMLR). In the case of HDPE/Plb/ZR2, slight reduce of aMLR

(by 7%) and quite significant of aHRR (by 24%) comparing to HDPE/Plb is observed. The reduction of both parameters is much more significant for HDPE/Plb/ ZG1 system. The aHRR is reduced by 53%, and aMLR is reduced by 38% comparing to unfilled blend. It seems that HDPE/Plb/ZR2 system burns more rapidly—time of combustion lower around 100 s than for the HDPE/Plb/ZG1. In addition, around 130 kW/m2 higher value of aHRR for HDPE/Plb/ ZR2 system is observed. This causes higher value of HDPE/Plb/ZR2 aMLR in comparison with HDPE/ Plb/ZG1. The effect is caused by differences in structural stability of both systems. HDPE/Plb/ZR2 system seems to be more stable during combustion comparing to HDPE/Plb/ZG1. The effect of stabilization of polyolefins with organically modified clay during combustion process was described previously.35 The PHRR and aMLR values of composites HDPE/ Plb/APP and HDPE/Plb/MPP are reduced remarkably comparing to neat HDPE/Plb and both nanocomposites HDPE/Plb/ZR2 and HDPE/Plb/ZG1. Unfortunately, APP and MPP filled systems have shorter TTI comparing to HDPE/Plb. It seems that better combustion parameters—longer combustion time, lower aHRR and aMLR were obtained for HDPE/Plb/APP than for HDPE/Plb/MPP. The better combustion parameters of HDPE/Plb/APP were achieved by the addition of APP flame retardant and formation of intumescent structure during combustion process.36 Combination of APP with ZG1 clay leads to further reduction of flame parameters comparing to HDPE/ Plb/APP. In fact, the composite has the lowest flame parameters from all discussed in the article. The further flame reduction effect is not observed in the case of system filled with APP and ZR2.

Table 2. Data recorded in cone calorimeter experiments of HDPE/Plb/clay nanocomposites modified with different fire retardants (50 kW/m2). Pro´bka

TTI (s)

TT (s)

PHRR (kW/m2) (, %)

HDPE/Plb HDPE/Plb/ZR2 HDPE/Plb/ZG1 HDPE/Plb/MPP HDPE/Plb/APP HDPE/Plb/ZR2/APP HDPE/Plb/ZG1/APP HDPE/Plb/ZR2/MPP HDPE/Plb/ZG1/MPP

51 75 43 34 44 36 32 34 33

129 385 478 659 838 684 910 665 541

2476 1213 902 446 346 349 262 404 428

(100) (49) (36) (18) (14) (14) (11) (16) (17)

tPHRR (s)

aHRR (kW/m2) (, %)

aMLR (g/(s m2)) (, %)

150 205 195 240 275 240 165 180 280

444 338 208 206 143 163 111 195 221

18.9 17.5 11.8 9.2 5.5 4.6 3.7 8.5 9.9

(100) (76) (47) (46) (32) (37) (25) (44) (50)

(100) (93) (62) (49) (29) (24) (20) (45) (52)

HDPE: high-density polyethylene; MPP: melamine polyphosphate; APP: ammonium polyphosphate; TTI: time to ignition; TT: total time of combustion; aHRR: average heat release rate; PHRR: peak HRR; tPHRR: time to PHRR; aMLR: average mass loss rate.



9

Combination of MPP with ZG1or ZR2 in HDPE/ Plb does not lead to further significant reduction of flame parameters. In fact, HDPE/Plb/ZG1/MPP system has slightly higher values of aMLR and aHRR than corresponding composite without the clay (Table 2). In addition, for HDPE/Plb, HDPE/Plb/ZR2, and HDPE/Plb/ZG1 sharp HRR peaks are observed, while in all systems with MPP and APP the HRR curves became more flat, and have no sharp peaks (Figures 8 and 9). This means that HDPE/Plb and nanocomposites without FRs burn more rapidly.

HRR [kW/m2]

2500

Flux=50 kW/m2

HDPE/Plb HDPE/Plb/ZG1 HDPE/Plb/ZR2

2000 1500 1000 500 0

0 50

100 150 200 250 300 350 400 450 500 Time [s]

Morphology The melting temperature (peak values), heat of fusion, and percent crystallinity for all examined composites and neat HDPE, Plb, and HDPE/Plb blend are shown in Table 3. In addition, percent crystallinity was calculated from WAXS curves of the composites. An example of deconvoluted WAXS curve is shown in Figure 10. In general, there were no significant changes observed in melting point of all samples. The variation of melting point was only 1–3 C (HDPE/Plb reference sample) suggesting that the addition of clays and FRs does not significantly influence the melting point behavior of the composites. Only slight increase of degree of crystallinity is observed for the HDPE/Plb/clay systems in comparison with neat HDPE/Plb blend. The results are in good agreement with those described in the literature.37 In this article, two effects can be observed: slight increase (up to 4%) of crystallinity in composites with only one kind of filler (ZR2, ZG1 clays or MPP, APP FRs) comparing to HDPE/Plb, and slight decrease of crystallinity of composites with two kind of fillers (MPP/ZG1, MPP/ZR2, APP/ZG1, and APP/ ZR2). Combination of clay with flame retardant leads to better exfoliation in HDPE/Plb blend. In these cases, many single platelets of the clay can be dispersed in the entire volume of the blend. Single platelets are hindrances for mobility of polymer chains in the composites and that leads to hinder the growth of nuclei. The results of crystallinity obtained from DSC are confirmed by the WAXS calculations.

Figure 8. HRR of HDPE/Plb/clay nanocomposites. HRR: heat release rate; HDPE: high-density polyethylene. Table 3. Degree of crystallinity of the composites obtained from WAXS and DSC. 600

Flux=50 kW/m2

HDPE/Plb/MPP HDPE/Plb/ZG1/MPP HDPE/Plb/ZR2/MPP HDPE/Plb/APP HDPE/Plb/ZG1/APP HDPE/Plb/ZR2/APP

HRR [kW/m2]

500 400 300 200 100 0 0

200

400

600

800

1000

Time [s]

Figure 9. HRR of HDPE/Plb/clay nanocomposites modified with MPP or APP. HRR: heat release rate; HDPE: high-density polyethylene; MPP: melamine polyphosphate; APP: ammonium polyphosphate.

Sample

Enthalpy of fusion WAXS DSC Xc (WAXS) Xc (DSC) Tm ( C) H (J/g)

HDPE Plb HDPE/Plb HDPE/Plb/ZG1 HDPE/Plb/ZR2 HDPE/Plb/APP HDPE/Plb/MPP HDPE/Plb/ZG1/APP HDPE/Plb/ZG1/MPP HDPE/Plb/ZR2/APP HDPE/Plb/ZR2/MPP

82.6 76.0 80.3 81.8 80.7 80.5 81.0 74.5 74.0 74.3 73.5

76.1 71.2 73.0 75.7 74.5 76.4 77.0 71.4 72.9 74.7 71.0

134.0 127.0 132.0 133.4 133.5 132.2 134.8 132.4 135.3 133.1 133.6

220.65 206.54 211.69 215.02 211.86 177.31 178.68 161.54 164.91 168.86 160.58

HDPE: high-density polyethylene; MPP: melamine polyphosphate; APP: ammonium polyphosphate; WAXS: wide angle X-ray scattering; DSC: differential scanning calorimetry.


10


25 000 HDPE/Plb R2=0,987 20 000

15 000

10 000

I[a.u]

5 000

0 10

20

30

40

50

60

2θ [°]

Figure 10. Representative deconvoluted WAXS curve of HDPE/Plb. HDPE: high-density polyethylene; WAXS: wide angle X-ray scattering.

Thermal stability

Table 4. TGA data for flame retardants and HDPE composites.

TGA is one of the most popular technique for evaluation the thermal stability of different materials. To understand the thermal degradation of all the composites and the amount of residual char, we compared thermal degradation of all composites and FRs. TGA data of all the materials are listed in Table 4. Neat HDPE/Plb system begins to decompose at 362 C (5 wt.% mass loss is marked as beginning of the process T5) in air. Thermal stability of the HDPE/Plb/ ZR2 nanocomposite is slightly higher in whole investigated range than HDPE/Plb, while HDPE/Plb/ZG1 system starts to decompose in lower temperature than HDPE/Plb blend. It is known that clay layers have good barrier properties, which can improve the thermal stability of polymer–clay nanocomposites. In some cases, quaternary ammonium salts in the organically modified montmorillonites can favor decomposition according to Hofmann elimination reaction38 and products can also catalyze the degradation of the polymer. In discussed cases, the composite with hydrophobic clay modified with quaternary ammonium salt has higher degradation temperature. In the case of the composite with hydrophilic clay modified with aluminium hydrogen sulfate, slight reduction of thermal stability comparing to neat HDPE/Plb blend is observed. This effect is related to lower thermal stability of the clay.

Sample

T5 ( C) T10 ( C) T30 ( C) T50 ( C)

APP MPP HDPE/Plb HDPE/Plb/ZG1 HDPE/Plb/ZR2 HDPE/Plb/MPP HDPE/Plb/APP HDPE/Plb/ZG1/APP HDPE/Plb/ZR2/APP HDPE/Plb/ZG1/MPP HDPE/Plb/ZR2/MPP

324 396 362 348 365 380 360 357 365 378 387

341 408 376 359 379 394 374 366 394 385 400

419 505 401 381 406 433 425 444 442 413 437

540 585 423 411 426 464 459 454 462 449 473

TGA: thermogravimetric analysis; HDPE: high-density polyethylene; MPP: melamine polyphosphate; APP: ammonium polyphosphate.

The APP and MPP FRs influence the thermal stability of composites much more than clays. One of the reason is higher content of the FRs in composites. In general, two behaviors are observed. First, composites with MPP have higher thermal stability in whole temperature range in comparison with HDPE/Plb. Second, addition of APP results in reduction of T10 (10 wt.% of mass loss) parameter of HDPE/Plb/APP comparing



11

Table 5. Tensile properties of HDPE/Plb/clay composites with APP and MPP. Sample

Tensile modulus (MPa)/%

Tensile strength (MPa)/%

Elongation at break (%)

MFR (190 C/216 kg)

HDPE/Plb HDPE/Plb/ZG1 HDPE/Plb/ZR2 HDPE/Plb/APP HDPE/Plb/MPP HDPE/Plb/ZG1/APP HDPE/Plb/ZG1/MPP HDPE/Plb/ZR2/APP HDPE/Plb/ZR2/MPP

826 33/100 843 25/102 871 32/105 1089 21/132 1167 20/141 1152 21/139 1096 16/133 1095 16/133 1238 20/150

24.0 0.2/100 24.4 0.2/101 24.6 0.3/103 24.8 0.2/103 27.0 0.2/113 25.3 0.1/105 25.9 0.2/108 23.3 0.2/97 23,6 0.2/98

24 2 30 4 30 4 20 3 14 2 17 2 13 1 15 2 91

5.0 5.2 5.4 3.7 3.9 4.0 4.3 3.6 3.3

HDPE: high-density polyethylene; MPP: melamine polyphosphate; APP: ammonium polyphosphate; MFR: mass flow rate.

to HDPE/Plb blend. This effect is a result of lower thermal stability of APP in comparison with MPP. The MPP flame retardant thermal stability T5 is of 72 C and T10 is of 67 C higher than corresponding values of APP. The combination of flame retardant (MPP or APP) with clay (ZR2 or ZG1) leads to further changes in thermal stability. As it was expected, the highest thermal stability in whole temperature range was obtained for HDPE/Plb/ZR2/MPP composite. In this case, the composite contains combination of fillers with highest thermal stability, for example, T5 of the system is of 387 C which is 25 C higher than neat blend HDPE/Plb and 7 C higher than HDPE/Plb/MPP composite. The addition of ZR2/APP also leads to increase thermal stability in comparison with HDPE/Plb and with HDPE/Plb/APP. Addition of 2% of ZG1 to HDPE/ Plb/APP leads to slight reduction of thermal stability (up to few degree). The same effect was observed in the combination of ZG1 with MPP. In this case, all thermal parameters are slightly lower than for HDPE/Plb/ MPP. It is due to lower thermal stability of modifier of ZG1 clay.

Tensile properties Usually polymer composites containing large amount of fillers (in our case FRs) differ in tensile properties comparing to neat polymer. Flame retardancy of polymers should not be at the expense of mechanical parameters. The results of the tests are summarized in Table 5. Tensile properties for the HDPE/Plb/clay systems differ slightly comparing to reference sample HDPE/Plb. In fact, the only tensile parameter that changes in both HDPE/Plb/clay systems comparing to HDPE/Plb, is strain at break. In the composites the value of strain at break is 25% higher than for HDPE/Plb. It is generally known that organically

modified montmorillonite (OMMT) has rather slight influence on tensile properties of polyethylene at low filler content,39 however, the properties depend strongly on the clays and the compatibilizer content.40 It is also known that the greatest reinforcement of tensile modulus of polyethylene/clay nanocomposites is achieved using so called ‘‘two-tailed organoclay’’ (in our case, the ZR2 clay modified with N,N-didecyl-N,N-dimethylammonium chloride belongs to two-tailed organoclay group).41 This explains higher values of tensile modulus for HDPE/Plb/ZR2 system comparing to HDPE/Plb/ZG1 composites. The modulus of APP or MPP filled systems with 20 wt.% filling level is higher 32% and 41%, respectively, comparing to HDPE/Plb system. For the HDPE/Plb/MPP also tensile strength is 10% higher than for corresponding composite filled with APP. The influence of all fillers on tensile properties is more complicated in the case of HDPE/Plb/clay/FR systems. Generally, in this article, the tensile modulus rises in all composites with FR and clay. This is typical phenomenon observed in binary composite systems polymer-filler, where the filler is fully wetted by the polymer. In addition, higher exfoliation level of the clay in HDPE/Plb/clay/FR influence higher modulus of the systems and lower elongation at break comparing to both HDPE/Plb/clay and both HDPE/Plb/FR systems.

Conclusions Nanocomposites obtained from blend of bimodal HDPE with maleic anhydride grafted polyethylene and two kinds of clays (hydrophobic clay modified with quaternary ammonium salt or hydrophilic clay modified with aluminium hydrogen sulfate) were melt extruded with APP or MPP FRs. Flame tests revealed that clay modifier (organic or inorganic) is the most important factor affecting LOI, MLR, and HRR.


12


The hydrophobic two-tailed organoclay acts as an antidripping agent in polyethylene, while addition of hydrophilic clay improves dripping of polyethylene during combustion. In the case of HDPE/Plb/hydrophilic clay system much lower values of MLR and HRR were obtained comparing to HDPE/HDPE-gMAH/hydrophobic clay composite. Based on the results presented in this article it is clear, that usage of complementary two flammability test methods (LOI and cone calorimeter) can lead to proper conclusion about flammability of the material. In addition, synergistic effect of flame retardancy between both used layered silicates and MPP was observed. WAXS showed that extrusion of polyethylene-clay composites in the presence of flame retardant (APP or MPP) leads to improvement of exfoliation of the clay. Tensile tests revealed that addition of 2% of clay have slight effect on tensile properties, while addition of 20% of flame retardant (MPP or APP) increases mainly tensile modulus. Thermal stability of flame retarded HDPE/HDPE-g-MAH composites depends mainly on the thermal stability of used flame retardant. In the case of HDPE/HDPE-g-MAH/MPP, thermal stability is much higher in the whole measured temperature range comparing to HDPE/HDPE-g-MAH/APP as well as HDPE/HDPE-g-MAH blend, because of higher thermal stability of MPP. As it was expected, composites filled with flame retardant have higher viscosity (lower MFR) in comparison with HDPE/Plb and both nanocomposites, however, decrease of MFR for composites containing 20–22% of fillers is not drastic. Funding This work was financially supported by the Ministry of Science and Higher Education of Poland under grant number NN209 186538.

References 1. Vega JF, Munoz-Escalona A, Santamaria A, et al. Comparison of the rheological properties of metallocenecatalyzed and conventional high-density polyethylenes. Macromolecules 1996; 29: 960–965. 2. Shen HW, Luan T, Xie BH, et al. Rheological behaviors and molecular weight distribution characteristics of bimodal high-density polyethylene. J Appl Polym Sci 2011; 121: 1543–1549. 3. Kawasumi M, Hasegawa N, Kato M, et al. Preparation and mechanical properties of polypropylene-clay hybrids. Macromolecules 1997; 30: 6333–6338. 4. Hasegawa N, Okamoto H, Kato M, et al. Preparation and mechanical properties of polypropylene-clay hybrids based on modified polypropylene and organophilic clay. J Appl Polym Sci 2000; 78: 1918–1922. 5. Giannelis EP. Polymer-layered silicate nanocomposites: synthesis, properties and applications. Appl Organomet Chem 1998; 12: 675–680.

6. Lee H, Fasulo PD, Rodgers WR, et al. TPO based nanocomposites. Part 2. Thermal expansion behavior. Polymer 2006; 47: 3528–3539. 7. Takahashi S and Paul DR. Gas permeation in poly(ether imide) nanocomposite membranes based on surface-treated silica. Part 1: without chemical coupling to matrix. Polymer 2006; 47: 7519–7534. 8. Kiliaris P and Papaspyrides CD. Polymer/layered silicate (clay) nanocomposites: an overview of flame retardancy. Prog Polym Sci 2010; 35: 902–958. 9. Giannelis EP. Polymer layered silicate nanocomposites. Adv Mater 1996; 8: 29–35. 10. Rezanavaz R, Aghjeh MKR and Babaluo AA. Rheology, morphology, and thermal behavior of HDPE/Clay nanocomposites. Polym Compos 2010; 31: 1028–1036. 11. Liu H, Song PA, Fang ZP, et al. Thermal degradation and flammability properties of HDPE/EVA/C(60) nanocomposites. Thermochim Acta 2010; 506: 98–101. 12. Marini J, Branciforti MC, Alves RMV, et al. Effect of EVA as compatibilizer on the mechanical properties, permeability characteristics, lamellae orientation, and long period of blown films of HDPE/Clay nanocomposites. J Appl Polym Sci 2010; 118: 3340–3350. 13. Chen XL, Jiao CM and Zhang J. Thermal and combustion behavior of ethylene-vinyl acetate/aluminum trihydroxide/Fe-montmorillonite composites. Polym Eng Sci 2012; 52: 414–419. 14. Hippi U, Mattila J, Korhonen M, et al. Compatibilization of polyethylene/aluminum hydroxide (PE/ATH) and polyethylene/magnesium hydroxide (PE/ MH) composites with functionalized polyethylenes. Polymer 2003; 44: 1193–1201. 15. Gao HL, Hu S, Han HC, et al. Effect of different metallic hydroxides on flame-retardant properties of low density polyethylene/melamine polyphosphate/starch composites. J Appl Polym Sci 2011; 122: 3263–3269. 16. Nyambo C, Kandare E and Wilkie CA. Thermal stability and flammability characteristics of ethylene vinyl acetate (EVA) composites blended with a phenyl phosphonateintercalated layered double hydroxide (LDH), melamine polyphosphate and/or boric acid. Polym Degrad Stab 2009; 94: 513–520. 17. Jiao CM and Chen XL. Flammability and thermal degradation of intumescent flame-retardant polypropylene composites. Polym Eng Sci 2010; 50: 767–772. 18. Chen YJ, Fang ZP, Yang CZ, et al. Effect of clay dispersion on the synergism between clay and intumescent flame retardants in polystyrene. J Appl Polym Sci 2010; 115: 777–783. 19. Spencer MW, Cui LL, Yoo Y, et al. Morphology and properties of nanocomposites based on HDPE/HDPEg-MA blends. Polymer 2010; 51: 1056–1070. 20. Hotta S and Paul DR. Nanocomposites formed from linear low density polyethylene and organoclays. Polymer 2004; 45: 7639–7654. 21. Cichy B. Sposo´b wytwarzania bezhalogenowego opo´zńiacza palenia tworzyw sztucznych, zwlaszcza tworzyw poliolefinowych. Patent RP 395656, 2011.



13

22. Szustakiewicz K, Gazinska M, Kiersnowski A, et al. Polyamide 6/organomontmorillonite nanocomposites based on waste materials. Polimery 2011; 56: 397–400. 23. Flory PJ and Vrij A. Melting points of linear-chain homologs—normal paraffin hydrocarbons. J Am Chem Soc 1963; 85: 3548–3553. 24. Schartel B and Hull TR. Development of fire-retarded materials—interpretation of cone calorimeter data. Fire Mater 2007; 31: 327–354. 25. Shah RK, Hunter DL and Paul DR. Nanocomposites from poly(ethylene-co-methacrylic acid) ionomers: effect of surfactant structure on morphology and properties. Polymer 2005; 46: 2646–2662. 26. Szustakiewicz K, Kiersnowski A, Gazinska M, et al. Flammability, structure and mechanical properties of PP/OMMT nanocomposites. Polym Degrad Stab 2011; 96: 291–294. 27. Spencer MW, Cui LL, Yoo Y, et al. Morphology and properties of nanocomposites based on HDPE/HDPEg-MA blends. Polymer 2010; 51: 1056–1070. 28. Minkova L and Filippi S. Characterization of HDPEg-MA/clay nanocomposites prepared by different preparation procedures: effect of the filler dimension on crystallization, microhardness and flammability. Polym Test 2011; 30: 1–7. 29. Zanetti M and Costa L. Preparation and combustion behaviour of polymer/layered silicate nanocomposites based upon PE and EVA. Polymer 2004; 45: 4367–4373. 30. Manzi-Nshuti C, Hossenlopp JM and Wilkie CA. Comparative study on the flammability of polyethylene modified with commercial fire retardants and a zinc aluminum oleate layered double hydroxide. Polym Degrad Stab 2009; 94: 782–788. 31. Su SP, Jiang DD and Wilkie CA. Poly(methyl methacrylate), polypropylene and polyethylene nanocomposite formation by melt blending using novel polymericallymodified clays. Polym Degrad Stab 2004; 83: 321–331.

32. Zhao CG, Qin HL, Gong FL, et al. Mechanical, thermal and flammability properties of polyethylene/clay nanocomposites. Polym Degrad Stab 2005; 87: 183–189. 33. Huang GB, Zhu BC and Shi HB. Combination effect of organics-modified montmorillonite with intumescent flame retardants on thermal stability and fire behavior of polyethylene nanocomposites. J Appl Polym Sci 2011; 121: 1285–1291. 34. Zhang JG and Wilkie CA. Preparation and flammability properties of polyethylene-clay nanocomposites. Polym Degrad Stab 2003; 80: 163–169. 35. Chen Y, Guo Z and Fang Z. Relationship between the distribution of organo-montmorillonite and flammability of flame retardant polypropylene. Polym Eng Sci 2012; 52: 390–398. 36. Lu HD, Hu Y, Li M, et al. Effects of charring agents on the thermal and flammability properties of intumescent flame-retardant HDPE-based clay nanocomposites. Polym Plast Technol Eng 2008; 47: 152–156. 37. Zanetti M, Camino G, Thomann R, et al. Synthesis and thermal behaviour of layered silicate-EVA nanocomposites. Polymer 2001; 42: 4501–4507. 38. Coskunses FI and Yilmazer U. Preparation and characterization of low density polyethylene/ethylene methyl acrylate glycidyl methacrylate/organoclay nanocomposites. J Appl Polym Sci 2011; 120: 3087–3097. 39. Golebiewski J, Rozanski A, Dzwonkowski J, et al. Low density polyethylene-montmorillonite nanocomposites for film blowing. Eur Polym J 2008; 44: 270–286. 40. Morawiec J, Pawlak A, Slouf M, et al. Preparation and properties of compatibilized LDPE/organo-modified montmorillonite nanocomposites. Eur Polym J 2005; 41: 1115–1122. 41. Shah RK, Kim DH and Paul DR. Morphology and properties of nanocomposites formed from ethylene/methacrylic acid copolymers and organoclays. Polymer 2007; 48: 1047–1057.
