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J. Rad. Res. Appl. Sci., Vol.3 , No 2(B) , pp. 639- 654 (2010)

Melt Rheology, Thermal Stability and Flame Retardancy Properties of Electron Beam Irradiated Low Density Polyethylene Composites Magdy M. H. Senna Radiation Chemistry Department, National Center for Radiation Research and Technology, P.O.Box 29 Nasr City, Cairo, Egypt E-mail: magdysenna@hotmail.com Received: / /2010. Accepted: 16 /08 /2010.

ABSTRACT In this work, low density polyethylene (LDPE) composites based on flame retardant aluminum hydroxide (ATH) and maleic anhydride (MA) or poly (styrene-ethylene-butylene–styrene) triblock copolymer grafted with maleic anhydride (SEBS-g-MA) as compatiblizers were prepared by melt extrusion. The properties of unirradiated and electron beam irradiated LDPE composites were studied by rheology at molten state, thermal gravimetric analysis (TGA) and limited oxygen index (LOI). The melt rheological results indicated that the electron beam irradiated composites showed higher complex viscosity and shear shiner behavior than unirradiated samples. The LDPE/ATH /SEBS-g-MA composites have higher mechanical properties and thermal stability than pure LDPE or LDPE-MA composites. The LOI data revealed that the composite containing LDPE/ATH/SEBS-g-MA changed from combustible to self-extinguishing material. Key words: Electron beam irradiation; LDPE composites; Rheology; Flame

retardant

INTRODUCTION

Low density polyethylene (LDPE) is one of the most widely used polymers due to its balanced mechanical toughness, good chemical resistance, ease of processing and excellent electrical properties but the drawback of LDPE in some application is its somewhat low melting temperature (110-120 oC)(1). This disadvantage can be overcome if LDPE is crosslinked. The electron beam irradiation has been adapted as an efficient and pollution free way to improve the mechanical properties and thermal properties of irradiated polymers, and it has been carried out in wide range of applications for example in wire and cable

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 640

industry(2). This treatment usually induces three-dimensional network and also the irradiation influences greatly the basic nature of the polymer, such as molecular weight, crystalline degree, molecular structure and morphology(3). Safety requirements are currently become more and more drastic in terms of polymer reaction to fire and their flame resistance performance, while flame retardant additives such as halogenated additives are being phase out for their proven or suspected adverse effects on the environment. Many research were devoted to developing effective and environmentally friendly flame retardant systems for polymer materials(4,5). Flame retardant systems are intended to inhibit or to stop the polymer combustion process. In the function of their nature, the flame retardant system can either act physically (by cooling, formation of protective layer or fuel dilution) and/or chemically (reaction in condensed or gas phase).

The Flame retardancy of polymers can be improved by incorporating flame-retardant chemicals in two ways; first as additives, second as reactive. Additives are compounds, which are mechanically mixed with polymers (6). On the other hand, reactive are chemically bound as an integral part of polymer structures. There are six elements, which are particularly associated with flame retardancy of polymers, namely; boron, aluminum, phosphorous, antimony, chlorine and bromine. These elements retard flames in different ways (7). It is important to note that the addition of these elements may significantly influence mechanical, thermal and electrical properties. As a result, any investigation should include observing the influence of flame retardants on these properties. Aluminum hydroxide (ATH) is frequently used as fire retarding additive for polyolefin but unfortunately, it incompatible with LDPE and need a modification to improve the interfacial adhesion at the boundary surfaces between them(8). Fatty acids and their salts are frequently used to improve the dispersion of the flame retardant materials in polymers and consequently improve the mechanical properties(9).

In this work, the ATH is used as a flam retardant additive in absence and in presence of maleic anhydride (MA) or poly (styrene-ethylene-butylene–styrene) triblock copolymer grafted with maleic anhydride as compatiblizing agent to improve the dispersion of ATH in LDPE, also we can use the electron beam irradiation to retain the mechanical properties that deteriorate by ATH addition.

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 641

EXPERIMENTAL

Materials

Low density polyethylene (LDPE) in the form of pellets was received from Exxon Mobil Chemical, USA with grade No. LD 166BA. The apparent density and melt flow index were 0.923 g/cm3 and 13 g/10 min, respectively. Poly(Styrene-block-ethylene-butylene-block-Styrene) SEBS maleic anhydride graft copolymer (SEBS-g-MA) in pellets form was received from Asahi Kazei Chemicals, Japan. Tufted M-series, with grade No. M1943. The SEBS-g-MA has apparent density of 0.90 g/cm3, and melt flow index 8 g/10min. Aluminum hydroxide, ATH was of commercial grade (Apyral 40CD) with particle size 0.6~3.2 microns from Nabaltec Gmbh, Germany.

Preparation of LDPE composites

The composites were prepared by melt blending in Barbender plasticorder PL2100 Mixer with volume capacity of 50 cm3 at processing temperature of 185 oC and 60 rpm for 15 min. The films were obtained by compression molding under hot press (Saspol made by Costuruzioni Meccaniche, Italy) at temperature of 185 oC for 5 min. The pressure of hot press is kept constant at a 3000 kg/cm2 for all samples.

Electron beam irradiation

Irradiation was carried out using electron beam accelerator (1.5 MeV, 30 mA and 37.5 kW) facility installed at the National Center for Radiation Research and Technology, Cairo, Egypt. The current and conveyer speed were adjusted to give a total dose of 25 kGy. The higher irradiation doses were obtained by multiple passes. In the treatment with electron beam irradiation, the penetration of accelerated electrons is very important parameter, which is directly proportional to the irradiation energy of electrons. The penetration of electrons can be calculated in the case of irradiation from one side according the general known equation: Z = 0.33 (E - 0.2)

Where Z is the thickness in cm and E is the electron energy in MeV. According to the parameters of the used electron beam accelerator, the maximum penetration depth is calculated to be 4 mm. Thus in the case of electron beam irradiation of LDPE blend sheets of thickness 0.5 mm , the treatment is uniform across the thickness. Also, the opening of the electron beam window is 90 cm, which insures the uniform exposure of LDPE blend sheets of 10 cm in width.

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 642

Thermogravimetric Analysis (TGA)

The TGA studies were carried out using PerkinElmer (Pyris 6) at a heating rate of 10 0C/min under nitrogen atmosphere at temperature from 30 to 600 oC. The TGA thermograms were used to determine the different kinetic parameters.

Tensile mechanical properties

Mechanical tests including tensile strength and elongation at break points were performed at room temperature using Minimate Materials tester from Rheometric scientific, England, employing a crosshead speed of 20 mm/min. The mechanical properties of the samples were tested in the form of strips of 5 cm in length and 0.4 cm in width. The recorded value for each mechanical parameter is the average of three measurements according to ASTM D-638 standards.

Rheological measurements

Rheological measurements were performed using GEMINI-2 rheometer (Malvern, Bohlin Instruments Ltd, Worcestershire, UK) with a parallel plates geometry (plate diameter 25 mm, gap 1.1mm). Dynamic frequency sweep tests were executed in the frequency range of 0.01–100rad/s. The temperature for testing was 200 oC. The strain sweeps were first performed before dynamic testing to ensure that the frequency sweeps were within the linear viscoelastic and stable region.

Limiting oxygen index (LOI)

LOI tests were performed using an apparatus from Rheometric Scientific, UK and the recorded value for each sample is the average of three measurements in accordance with ASTM D 2863.

RESULTS AND DISCUSSIONS

Thermal gravimetric analysis (TGA)

One of the important factors affecting of the polymer combustion is the volatile fraction of the polymer fragment results from polymer decomposition, which when they mixed with oxygen in air to form flammable gaseous mixture. The life spam of combustion cycle depends on the quantity of liberated gaseous and liberated heat during combustion process. TGA is one of the important systems used to determine the thermal stability of polymeric materials over a

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 643

wide range of temperatures.

The thermal stability of any polymeric material is largely determined by the strength of the covalent bonds between the atoms forming the polymer molecules. The dissociation energy for the different covalent bonds C-H, C-C, C-O, O-H and C=C was reported to be 414, 347, 351, 464 and 611 kJ/mol, respectively. On the basis of these values, the average bond dissociation energy of LDPE and SEBS-g-MA is calculated to be 400, and 431 kJ/mol (10). Thus, it may be conclude that SEBS possesses higher thermal stability than LDPE and the composites of LDPE with any ratio of SEBS will eventually results in blends with higher thermal stability than pure LDPE.

The TGA thermograms and rate of thermal decomposition reaction for unirradiated and 100 kGy irradiated LDPE , LDPE/ATH, LDPE/ATH/MA and LDPE/ATH/ SEBS-g-MA composites are shown in Figs. 1and 2. Also, the different TGA kinetic parameters taken from these thermogrames are shown in Tables 1 and 2.

Based on the TGA study, few points can be summarized. L D P E / A T H / S B S - M A ( 7 0 / 1 5 / 1 5 % )

Fig. 1: TGA thermograms and rate of thermal decomposition

reaction for unirradiated LDPE composites

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 644

LDPE composites (Irradiated)

0 100 200 300 400 500 600

Wei

ght r

emai

ning

(%)

0

20

40

60

80

100

120

temp-LDPE (100%)LDPE/ATH (85/15%)LDPE/ATH/MA (84/15/1%)LDPE/ATH/SBS-MA (70/30/30%)

Temperature (oC)

300 350 400 450 500 550 600

Rat

e of

reac

tion

(mg/

min

)

-3

-2

-1

0

Fig. 2: TGA thermograms and rate of thermal decomposition

reaction for EB irradiated LDPE composites to a dose of 100 kGy

As shown in Figs. 1 and 2, the thermal decomposition of pure LDPE and LDPE composites goes through four stages, before or after electron beam irradiation. In the first stage from room temperature up to ~300 0C no weight loss is detected. Within the temperature range from 300 0C to ~ 400 0C, pure LDPE showed higher thermal stability than LDPE composites with less weight loss. The major weight loss occurs within the temperature range from 400 to 500 0C, in which the LDPE composites with MA and SEBES-g-MA showed higher thermal stability than pure LDPE. After irradiation, LDPE composites displayed higher thermal stability over all the studied range of temperatures as shown in Tables 1and 2 and TGA thermograms.

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 645

Table (1): Decomposition temperatures at different weight loss (%) for LDPE composites

LDPE composites Irradiation

dose ( kGy )

Decomposition Temperature (oC)

10% 20% 40% 60% 80% 90%

LDPE (100%) 0 414 422 437 452 460 513

100 424 433 444 462 475 540

LDPE/ATH (85/15%) 0 531 415 437 458 506 580

100 366 429 438 457 494 >550

LDPE/ATH/MA (84/15/1%)

0 384 424 447 465 512 580 100 361 412 437 458 507 600

LDPE/ATH/SEBS-MA (70/15/15%)

0 401 444 461 470 512 >560 100 390 445 462 472 511 >600

Table (2): Kinetic parameters of the thermal decomposition reaction of LDPE composites before and after EB-irradiation to a dose of 100 kGy

T1/2 Tpeak (oC)

Tendset (oC)

Tonset (oC)

Irradiation dose (kGy) LDPE composites

444 445 494 400 Unirradiated LDPE (100%) 455 469 502 419 100

447 322 446 347 495

289 403

Unirradiated LDPE/ATH (85/15%)

442 324 442 343 463

268 417

100

455 324 461 346 511

296 405

Unirradiated LDPE/ATH/MA (84/15/1%)

447 316 457 346 496

285 402

100

467 322 477 457 510

282 440

Unirradiated LDPE/ATH/SBS-MA (70/15/15%)

467 336 476 358 511

314 445

100

The rate of thermal decomposition reaction for unirradiated or irradiated LDPE goes through one stage, while the rate of thermal decomposition of LDPE composite goes through multiple steps with the different temperatures of the rate of decomposition (Tpeak, T onset and T endset) as shown in Table 2. It can be seen that the addition of ATH decreases the thermal stability of LDPE at the temperature range of (300-400 0C) and this may be attributed to the decomposition of Al(OH)3 to H2O and Al2O3 and these fragments are important in flame retarding properties (11 ).

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 646

It is clear that the electron beam irradiation improves the thermal stability of LDPE and LDPE composite due to the crosslinking process that induced in LDPE irradiation (12). Also, the crosslinking is expected to improve the fire retardancy properties of LDPE.

Van Krevelen developed a correlation for the temperature of half-decomposition (Td1/2), which is defined as the temperature at which the loss of weight during pyrolysis reaches 50 % of its final value (at a constant rate of temperature rise(13). In this correlation, Td1/2 is approximated in terms of the ratio of the molar thermal decomposition, function (Yd1/2) divided by molecular weight (M) per repeat units:

Td1/2 = Yd1/2 / M.

It should be noted that Yd1/2 is estimated by group contribution and is expressed in Kelvin Kg/mol. Based on this correlation, the calculated Td1/2 value for LDPE was found to be 414 oC. As shown in Table 2, the experimental results of Td1/2 for the different blends are deviated with the theoretical calculations. This deviation was differing from sample to another and this deviation was arising from, the phase separation, irregularities in the matrix and the crosslinking in LDPE.

The results for SEBS are in accordance with the theoretical values. Meanwhile, the limited macromolecular chain mobility as a result of the formation of intermolecular hydrogen bonds, as well as the association between the macromolecules of the blend components is also expected to increase the thermal stability.

Mechanical properties

Most plastic materials are used because they have desirable mechanical properties at an economic cost. For this reason, the mechanical properties may be considered the most important physical properties of polymers for most applications. There are many factors, which affect the mechanical behavior of such materials. The chemical composition, molecular weight, crosslinking and branching, crystallinity and crystal morphology, copolymerization, plasticization, molecular orientation and fillers are factors affecting the mechanical properties..

As shown in Figs. 3 and 4, the compounding of LDPE with ATH or ATH and MA results in lower the mechanical properties (tensile strength and elongation percentage at break) of LDPE.

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Irradiation Dose

unirradiated 50 kGy 100 kGy

Tens

ile stren

gth (M

Pa)

0

5

10

15

20

25

30

LDPE (100%)LDPE/ATH (85/15%)LDPE/ATH/MA (84/15/1%)LDPE/ATH/SEBS-g-MA (70/15/15%)

Fig. 3: Effect of electron beam irradiation on the tensile strength

properties of LDPE composites

Irradiation Dose

unirradiated 50 kGy 100 kGy

Elon

gatio

n at bre

ak (%

)

0

200

400

600

800

LDPE (100%)LDPE/ATH (85/15%) LDPE/ATH/MA (84/15/1%)LDPE/ATH/SEBS-g-MA (70/15/15%)

Fig. 4: Effect of electron beam irradiation on the elongation

at break properties of LDPE composites

This decrease would result from the uncompatability of ATH and MA in the LDPE matrix. Also, on the other hand, the LDPE composites containing SEBS-g-MA as compatibilizer, the mechanical properties were improved with the respect to composites containing ATH and ATH/MA. The presence of

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 648

SEBS-g-MA improves the compatibility between LDPE and ATH as shown in the following scheme:

Also, the EB irradiation improves the mechanical properties of LDPE and LDPE/SEBS-g-MA/ATH whereas, the LDPE/ATH and LDPE/ATH/MA the properties were decreased by irradiation.

Rheological Properties

As for most polyolefin, it has proven that the three dimensional network structure will form when the polymer are irradiated. The structure of crosslinked network was determined with the rheological properties measurements where the viscosity of molten polymers are affected by their molecular weight and their molecular distribution and it will has both frequency and temperature dependency . Figs. 5-8 show the storage modulus (G') (energy stored and can be recovered), loss modulus (G") and the viscosity (η) for unirradiated and irradiated LDPE composite measured at 200 0C. From the figure it can be seen the plateau region of G”_ (ώ) indicated that a three- dimensional cross-linking network structure formed in the composites after irradiation. From Figs. 5-8, it could be seen that the frequency dependence of G” showed a similar trend. As in high (ώ) regime,G” of the unirradiated composite was much higher than those of the irradiated ones. The complex viscosity (η*) of the unirradiated composite and the irradiated ones vs. frequency(ώ) was shown in Figs. 5-8.

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 649

Frequancy (Hz)

0.01 0.1 1 10 100

Mod

ulus

(Pa)

1e+2

1e+3

1e+4

1e+5

1e+6

Com

plex

vis

cosi

ty

1e+1

1e+2

1e+3

1e+4

1e+5

(G")- unirradiated (G')- unirradiated (G")- 100 kGy (G')- 100 kGy (h*')- unirradiated (h*)- 100 kGy

Three dimensional crosslinking network

Fig. 5: Melt rheological properties of unirradiated and EB-irradiated

LDPE (100%) measured at 200 0C. Storage modulus (G’), Loss modulus (G”) and Complex viscosity (η*)

Frequancy (Hz)

0.01 0.1 1 10 100

Mod

ulus

(Pa)

1e+3

1e+4

1e+5

Com

plex

vis

cosi

ty

1e+2

1e+3

1e+4

1e+5

(G") - 100 kGy (G') - 100 KGy (G") - Unirrad. (G')- Unirrad(*) -100 kGy(*) -unirrad

Fig. 6: Melt rheological properties of unirradiated and EB-irradiated

LDPE/ATH composites (85/15%) measured at 2000C. Storage modulus (G’), Loss modulus (G”) and Complex viscosity (η*)

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 650

It could be seen that the irradiated composites displayed a much higher complex viscosity and a pronounced shear thinning behavior. The complex viscosity increased with increasing irradiation dose, inferring that the gel content increased with increasing irradiation dose. But the unirradiated composite showed more pseudo-Newtonian behavior in low (ώ) regime.

Frequancy (Hz)

0.01 0.1 1 10 100

Mod

ulus

(Pa)

1e+2

1e+3

1e+4

1e+5

Com

plex

vsc

osity

1e+2

1e+3

1e+4

1e+5

1e+6

(G')- unirradiated (G")- unirradiated (G")- 100 kGy(G')- 100 kGy(h*)- unirradiated (h*)- 100 kGy

Fig.7: Melt rheological properties of unirradiated and EB-irradiated LDPE/ATH/MA (84/15/1%) composites measured at 200oC. Storage

modulus (G’), Loss modulus (G”) and Complex viscosity (η*)

Flame retardant properties

LOI is one of the most important screening and quality control method used in plastic industry. The value of the LOI is defined as the minimal oxygen concentration [O2] in the oxygen/nitrogen mixture [O2/N2] that either maintains flame combustion of the material for 3 min or consumes a length of 5 cm of the sample, with the sample placed in a vertical position (the top of the test sample is inflamed with a burner).

The LOI is expressed as:

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 651

Frequancy (Hz)

0.01 0.1 1 10 100

Mod

ulus

(Pa)

1e+3

1e+4

1e+5

1e+6

Com

plex

vis

cosi

ty

1e+1

1e+2

1e+3

1e+4

1e+5

1e+6(G')- unirradiated (G")- unirradiated(G")- 100 kGy(G')- 100 kGY(h*)- unirradiated(h*)- 100 kGy

Fig. 8: Melt rheological properties of unirradiated and EB-irradiated LDPE/ATH/SEBS-g-MA (70/15/15%) composites measured at 2000C. Storage modulus (G’), Loss modulus (G”) and Complex viscosity (η*)

The Flame response of unirradiated LDPE composite in comparison with 100 kGy irradiated samples was evaluated by limited oxygen index (LOI) test. It can be seen from Table 3, that the LOI values were increased with addition of inorganic filler to LDPE, but this increase was higher in cease of adding ATH or ATH with compatibilizing agent.

Table (3): LOI for unirradiated and electron beam irradiated LDPE composites

LDPE composites Irradiation dose

( kGy ) LOI

LDPE (100%) 0 17.0

100 17.5

LDPE/ATH (85/15%)

0 18.7

100 17.8

LDPE/ATH/MA (84/15/1%) 0 18,5

100 18.9 LDPE/ATH/SEBS-MA

(70/15/15%) 0 18.8

100 21.9

Magdy M. H. Senna. /J. Rad. Res. Appl. Sci., Vol. 3 , No. 2(B) (2010) 652

Also, irradiation of these composites improves the flame retardancy properties than irradiated LDPE alone. The LOI value of LDPE/SEBS-g-MA/ATH- 100 kGy was more than 21 which indicates the flame retardance improvement of this composite with low level of ATH and irradiation. Materials with an LOI below 21 are classified as ‘combustible’’ whereas those with an LOI above 21 are classified as ‘‘self-extinguishing’’, because their combustion cannot be sustained at ambient temperature without an external energy contribution and the higher LOI the better the flame retardant property.

CONCLUSION

The melt rheological results indicated that the irradiated samples showed higher complex viscosity than unirradiated samples and have shear shiner behavior. The mechanical properties and thermal stability of LDPE composites were higher in case of (SEBS-g-MA) than other composites. The LOI reveals that the composite containing LDPE/ATH/SEBS-g-MA is changed from combustible material to self-extinguishing, because their combustion cannot be sustained at ambient temperature without an external energy contribution.

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4. Shui-Y. L. and Hamerton I., (2002) Recent developments in the chemistry of halogen-free flame retardant polymers, Prog. Polym. Sci. 27:1661.

5. Nshuti C. M., Hossenlopp J. M. and Wilkie C. A., (2009) Comparative study on the flammability of polyethylene modified with commercial fire retardants and a zinc aluminum oleate layered double hydroxide, Polym. Degrad. and Stab., 94:782.

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magnesium hydroxide, Polym. Degrad. Stab.; 54:383.

8. Hippi U., Mattila J., Korhonen M. and Seppälä J.,(2003), Compatibilization of polyethylene/aluminum hydroxide (PE/ATH) and polyethylene/magnesium hydroxide (PE/MH) composites with functionalized polyethylenes, Polymer, 44:1193.

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11. Liu H., Fang Z. , Peng M. Shen L. and Wang Y. (2009) ; The effects of irradiation cross-linking on the thermal degradation and flame-retardant properties of the HDPE/EVA/magnesium hydroxide composites; Radiat. Phys. chem.78:922.

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13. Van Krevelen, D. W. (1990), Properties of Polymers, 3rd ed.; Elsevier: Amsterdam,; Chapter 21.

14.

اإلشعاعیةاإلشعاعیةبحوث بحوث مجلة المجلة ال والعلوم التطبیقیةوالعلوم التطبیقیة

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والثبات الحراري ومقاومة الحریق لمتراكبات البولى اثیلین الخواص الريولوجية منخفض الكثافة و المشعع بالمعجالت األلكترونیة

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قسم الكیمیاء األشعاعیة –المركز القومى لبحوث وتكنولوجیا األشعاع

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