Blends of low-density polyethylene with nylon compatibilized with a sodium-neutralized carboxylate ionomer

European Polymer Journal 40 (2004) 2409–2420

EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj

Blends of low-density polyethylene with nylon compatibilized with a sodium-neutralized carboxylate ionomer Atchara Lahor a, Manit Nithitanakul b

a,*

, Brian P. Grady

b

a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, OK 73019, USA

Received 16 June 2004; accepted 1 July 2004 Available online 1 September 2004

Abstract An ethylene–methacrylic acid copolymer partially neutralized with sodium (Na-EMAA) was successfully used to compatibilize Nylon 6 (Ny6) and low-density polyethylene (LDPE) blends. The phase morphology and thermal behavior of these blends were investigated over a range of compositions using a variety of analytical techniques. The addition of small amounts (0.5 phr) of Na-EMAA improved the compatibility of Ny6/LDPE blends as evidenced by a significant reduction in dispersed phase sizes. TGA measurements demonstrated an improvement in thermal stability when NaEMAA was added to either LDPE or Ny6. DSC results of Ny6/Na-EMAA binary blends showed that with increasing Na-EMAA content, the crystallization temperature of Ny6 phase decreased indicating that Na-EMAA retarded crystallization of Ny6. TGA and DSC results indicate that chemical reactions might have taken place between Ny6 and NaEMAA, a hypothesis confirmed by the Molau test. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Sodium-neutralized carboxylate; Ionomers blend; Compatibilizer; Low-density polyethylene; Nylon 6

1. Introduction Blending of polymers is an excellent way for developing new materials with improved properties. Most polymers are incompatible and hence various morphologies can be realized via melt processing, for instance droplets or fibers in a matrix as well as stratified or co-continuous structures. The structures induced are usually thermodynamically unstable, and the mechanical properties of the blends are poor because of insufficient adhesion between phases. * Corresponding author. Tel.: +66 2 2184143; fax: +66 2 2154459. E-mail address: [email protected] (M. Nithitanakul).

Nylon (Ny) and polyethylene (PE) blends are thermodynamically immiscible and mechanically incompatible; however a compatible blend of the two components would possess interesting properties based on the complementary behavior of the individual components. A good example is food packaging, due to (1) the good oxygen barrier, very high strength, high heat resistance of the Ny and (2) the excellent moisture barrier, good flexibility, ease of processing of the PE [1]. The morphology and the properties of the immiscible blends can be enhanced by adding a third component, an interfacially active polymer, called a compatibilizer, which promotes physical and/or chemical interactions between the components. Compatibilizers for this system often contain carboxylic acid groups and are typically either a

0014-3057/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.07.004

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chemically modified ethylene homopolymer, i.e. via post-polymerization maleic anhydride grafting [2,3], or an ethylene copolymer, i.e. an ethylene–carboxylic acid copolymer [4–8]. Functionalization of polyethylene with a small amount of ionic groups is a particularly attractive way of compatibilizing nylon with polyethylene because the amide groups may interact with the ionic groups via hydrogen bonding, ion–dipole interactions or/and metal-ion coordination during melt blending [9,10]. The introduction of such specific interactions can improve compatibility and may promote miscibility of polyamide and polyethylene blend. Compatibility of Ny and PE blends has also been attributed to amidation reaction occurring between the terminal end groups of Ny and the carboxylic acid groups [11]. Copolymers of ethylene with monomers containing acid groups are important commercial products. These materials are sold commercially with either hydrogen or a metal cation as the neutralizing agent for the acid group. The latter are termed ionomers, and typically the amount of acid groups neutralized with a metal cation is less than stoichiometric, i.e. some of the acid groups are neutralized with a metal cation while others are neutralized by hydrogen. One common commercial type of ionomer is a copolymer of ethylene and methacrylic acid marketed by DuPont under the trademark SurlynTM. The two most common neutralizing cations are zinc (Zn2+) or sodium (Na+). The properties of zinc-neutralized and sodiumneutralized carboxylate ionomers are very different; for example sodium ionomers absorb significantly more water and tend to have higher fractional crystallinities than zinc ionomers. Zinc-neutralized ionomers have been studied as blend compatibilizers for the nylon–PE system extensively [12– 16]. The addition of compatibilizer has been shown repeatedly to increase compatibility between the two components, including improvements in mechanical properties [1], injection molding properties [15], barrier properties [16], as well as smaller dispersed domain sizes [1,12]. We are only aware of one study that investigated sodium-neutralized ethylene carboxylate ionomers as blend compatibilizers [17]. Although indications were that the sodium material could act as a blend compatibilizer, very little was done to quantify the interaction. In the present work, blends of nylon 6 (Ny6) and low-density polyethylene (LDPE) using sodium-neutralized copolymers of ethylene and methacrylic acid (NaEMAA) as a compatibilizer were produced. Binary blends of Ny6 and Na-EMAA, as well as Na-EMAA and LDPE, were also investigated. Attention was focused on the thermal behavior, crystalline structure, and phase morphology of these blends as a function of the compatibilizer content, and the effect of Na-EMAA with different types of neutralizing metal ions on Ny6/ LDPE blends were also investigated.

2. Experimental 2.1. Materials Ultramid B3 Nylon 6 (density 1.31 g/cm3) was supplied by BASF (Thailand) Co., Ltd. Low-density polyethylene, LD 1450J, was an injection molding grade polymer (density 0.914 g/cm3) graciously supplied by Thai Polyethylene Co., Ltd. Sodium-neutralized poly(ethylene-co-methacrylic acid) ionomer with 3.5 mol% methacrylic acid co-monomer was supplied by DuPont (USA). In this material, 54% of the acid groups were neutralized with sodium. 2.2. Blend preparation Pellets were mixed in a tumble mixer for 10 min followed by drying under vacuum at 60 °C for 24 h. The materials were blended in a Collin D-8017 T-20 twinscrew extruder using a screw speed of 40 rpm. The blends were extruded through a single strand die; the extrudates were cooled in a water bath, dried at ambient temperature and then pelletized. 2.3. Specimen preparation Test specimens were obtained using a Wabash compression press machine. Pellets were placed in a picture frame mold and preheated at 240 °C for 3 min between the plates without any applied pressure to allow for complete melting. After this period, a pressure of 10 tons was applied at the same temperature for 3 min. The sample was then slowly cooled to 40 °C under pressure. Test specimens were cut from the molded sheets using a pneumatic die cutter. 2.4. Scanning electron microscopy (SEM) Fracture micrographs were studied using a scanning electron microscope, JEOL (MP 152001), operated at 15–25 kV. The samples fractured under liquid nitrogen and were subjected to selective extraction of the LDPE and Na-EMAA ionomers phases by immersing in hot decalin to remove LDPE or in formic acid to remove Ny6. The specimens were then coated with gold to make samples electrically conductive. The number average diameter (dn) was calculated using Eq. (1), X X ðni d i Þ= ni ð1Þ dn ¼ where ni is the number of droplet and di is the diameter of the ith droplet. 2.5. X-ray diffraction (WAXS) Wide angle X-ray diffraction investigations of the neat Ny6, LDPE, and Na-EMAA ionomers as well as

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their blends were carried out at room temperature using a Rigaku Rint 2000 diffractometer equipped with a graphite monochromator and a Cu tube for generating CuKa radiation (k = 0.154 nm). Diffraction scans were collected using a scan speed of 5°/min between 2h values of 5° and 40° at tube settings of 40 kV and 30 mA.

to 250 °C and from this the melting temperature and fractional crystallinity were calculated. The crystallinity of the sample was determined from knowledge of the ratio of the melting enthalpy for 100% crystallinity of pure components. The absolute crystallinity of the blend was calculated using Eq. (2),

2.6. Thermogravimetric analysis (TGA)

vc ¼

A DuPont TGA 2950 thermogravimetric analyzer was used to collect thermogravimetric data. Samples (10 ± 0.5 mg) were heated to 30–600 °C at three different heating rates of 10, 20, 40 °C/min in a platinum pan under air.

DH
100% DH f
wt: fraction of component in the blend ð2Þ

where vc is the fractional crystallinity, DH is the melting enthalpy of the component present in the blends, DHf is the heat of fusion for 100% crystallinity of the pure component (190 J/g for Ny6 and 282 J/g for LDPE).

2.7. Differential scanning calorimetry (DSC) 3. Results and discussion Thermal analysis was carried out in a Perkin-Elmer DSC 7 differential scanning calorimeter. The temperature was calibrated by measuring the melting temperature of indium. Samples (10 mg) were encapsulated in an aluminum pan, heated from 25 to 250 °C at a heating rate of 80 °C/min, held for 5 min at this temperature to remove thermal history, followed by cooling to 30 °C at 10 °C/min. The latter of these two steps represents the cooling part of the curve used to calculate crystallization temperatures. The sample was then heated at 10 °C/min

3.1. Scanning electron microscopy SEM micrographs of compression molded samples after cryogenic fracture of Ny6/LDPE binary blends, i.e. a blend prepared without any compatibilizer, are shown in Fig. 1. The presence of a dispersed phase, consisting of predominantly spherical droplets imbedded in a matrix, is clearly observed from the micrographs of the whole composition range. As expected, the adhesion

Fig. 1. Scanning electron micrographs of fractured surfaces of uncompatibilized Ny6/LDPE blends: (a) 20/80, (b) 40/60, (c) 50/50, (d) 60/40, (e) 80/20.

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additional carboxylic acids are available to interact with the nylon. Dispersed particles in the compatibilized blend ranged between 2 and 6 lm, i.e. five times smaller than in

between the Ny6 phase and the LDPE phase is poor, as confirmed by the surfaces of the remaining holes appearing to be very smooth [12]. The number average diameters ranged between 13 and 26 lm and are shown in Table 1. In order to determine the morphology better, micrographs are presented after the dispersed phase LDPE was extracted from the Ny6 matrix, i.e. 80/20 Ny6/ LDPE blend in Fig. 2. The addition of small quantities of Na-EMAA to Ny6/LDPE blends resulted in remarkable changes in the morphology. The particles of the dispersed phase become much smaller and less polydisperse. In fact, Fig. 3 seems to show a coating on the surface of the dispersed particle. Macknight and Lenz [11] as well as others have suggested two possible specific interactions between nylon and the ionomer: hydrogen bonding between the amide nitrogen on the nylon and the carboxylic acid on the ionomer, and a covalent amide bond between the primary amine at the end of the nylon chain and the carboxylic acid of the ionomer. This material, as in all commercial ethylene-carboxylate ionomers, was only partly neutralized with sodium;

Fig. 3. Scanning electron micrographs of fractured surfaces of 20/80 Ny6/LDPE blend compatibilized with Na-EMAA 0.5 phr.

Table 1 Number average diameter (lm) of dispersed phase size of blends Blend ratio (Ny6/LDPE)

Ny6/LDPE Ny6/LDPE with 0.5 phr Na-EMAA Ny6/LDPE with 1.5 phr Na-EMAA Ny6/LDPE with 5 phr Na-EMAA Ny6/Na-EMAA

20/80

40/60

50/50

60/40

80/20

15.8 2.6 1.7 2.6 1.6

26.1 3.9 2.5 2.0 2.2

24.8 6.0 2.7 2.8 1.9

13.0 2.1 2.3 1.4 1.9

17.3 2.5 1.7 1.7 0.8

Fig. 2. Scanning electron micrographs of fractured and etched surfaces of 80/20 Ny6/LDPE blends after immersion in hot decalin: (a) without Na-EMAA, (b) with 0.5 phr Na-EMAA, (c) with 1.5 phr Na-EMAA, (d) with 5 phr Na-EMAA.

the uncompatibilized blend. As shown in Fig. 2(b)–(d), only 0.5 phr of Na-EMAA is sufficient to produce the maximum reduction in dispersed phase sizes. No further decrease in phase size is achieved by adding more NaEMAA as shown in Fig. 4, consistent with the results obtained previously for zinc-EMAA ionomers [12,13]. The plateau presumably is due to saturation at the interface coupled with the processing technique used. In other words, the twin-screw extruder is able to reduce the minor phase to 2–6 lm and only 0.5 phr is necessary to stabilize 2–6 lm particles. The reduction in average droplet size is due to a suppression of coalescence; compatibilizer has little effect on an extruderÕs ability to breakup molten polymers into smaller domain sizes. SEM micrographs of the fractured and etched surfaces of Ny6/Na-EMAA blends are shown in Fig. 5. The dn of the dispersed phase particles in these blends was approximately 1–2 lm, smaller than in the Ny6/ LDPE/Na-EMAA blends. The smaller domain sizes vs. the ternary blend are presumably due to differences in viscoelastic properties of the two-component vs. the three-component blends. The size distributions were also narrower in the two-component blend vs. the threecomponent blend. As mentioned previously, a chemical reaction can occur between the primary amine groups and the car-

Avg. dispersed diameter (µm)

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20 15 10 5 0 0

1

2

3

4

5

Amount of Na-EMAA (phr)

Fig. 4. The dependence of the number average diameters measured as a function of the SurlynÒ content of 80/20 Ny6/ LDPE blends.

boxylic acid. The existence of this reaction was confirmed by the Molau test, which was performed by adding 85% formic acid to the blends [18–20]. The uncompatibilized blend gave, after stirring with formic acid and several hours of settling, a clean separation of a transparent solution of Ny6 from the upper layer of LDPE particles. Addition of Na-EMAA yields a third phase, which appears as a white colloidal suspension in the liquid phase.

Fig. 5. Scanning electron micrographs of fractured surfaces of Ny6/Na-EMAA blends: (a) 20/80, (b) 40/60, (c) 50/50, (d) 60/40, (e) 80/ 20.

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Fig. 6. Scanning electron micrographs of fractured surfaces of LDPE/Na-EMAA blends: (a) 20/80, (b) 40/60, (c) 50/50, (d) 60/40, (e) 80/20.

a b c

Intensity

SEMs of the fractured surfaces of the Na-EMAA/ LDPE binary blends at various blend ratios are shown in Fig. 6. One phase materials are observed for the 20/ 80, 40/60, and 80/20 blend ratios (Fig. 6(a), (b), (e)), whereas two phases, though highly adherent, are clearly visible for the other two ratios (Fig. 6(c), (d)). The compatibility of these blends is attributed to the ethylene segments of Na-EMAA.

d e f

3.2. X-ray diffraction WAXS patterns of melt-crystallized samples for Ny6/ Na-EMAA are shown in Fig. 7, for LDPE/Na-EMAA blends are shown in Fig. 8, and for 50/50 Ny6/LDPE blends with or without Na-EMAA are shown in Fig. 9. Pure Ny6 gave pronounced 2h peaks at 20.2° and 23.5° associated with a1-form (0 0 2) and a2-form (2 0 0) crystal structure, respectively. A characteristic peak at 21.4° specific to the c form was not found for the pure nylon, unlike in DSC experiments where the characteristic c endotherm was present. Presumably this difference is due to the difference in thermal history between the WAXS samples and DSC samples. Whether the peak at 21.4° is present for the blend material is impossible to determine because of overlapping polyethylene reflections. For pure LDPE, the scan showed two crystalline peaks at 21.4° and 23.7° associated with the (1 1 0) and (2 0 0) reflections of the orthorhombic unit cell of poly-

g

10

20

30

40

2 theta (deg.)

Fig. 7. WAXS patterns of Ny6/Na-EMAA blends: (a) Ny6, (b) 80/20, (c) 60/40, (d) 50/50, (e) 40/60, (f) 20/80, (g) NaEMAA.

ethylene. Pure Na-EMAA specimen showed the same peaks at slightly different angles. For blends, peak positions at similar angles were observed, indicating that the crystalline unit cells were not affected by blending. 3.3. Thermogravimetric analysis The Arrhenius equation can be used to fit the kinetic behavior of degradation (Tables 2 and 3):

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Table 2 Kinetic parameters of pure components and binary blends

Intensity

a b c

f g

20

2 theta (deg.)

Ea (kJ/mol)

ln A (ln min1)

380.2 311.8 354.6

175.7 122.0 102.5

34.3 27.1 21.7

Ny6/Na-EMAA blend ratio 20/80 389.5 40/60 386.9 50/50 391.5 60/40 388.6 80/20 388.5

170.2 139.8 155.0 125.9 139.4

32.8 27.4 30.2 24.9 27.4

LDPE/Na-EMAA blend ratio 20/80 369.0 40/60 360.6 50/50 352.5 60/40 345.7 80/20 335.7

154.4 255.0 243.5 226.8 189.8

31.0 50.4 48.8 46.2 39.4

Pure component Ny6 LDPE Na-EMAA

d e

10

Degradation temperature at 0.1 conversion (°C)

30

40

Fig. 8. WAXS patterns of LDPE/Na-EMAA blends: (a) LDPE, (b) 20/80, (c) 40/60, (d) 50/50, (e) 60/40, (f) 80/20, (g) Na-EMAA.

a Intensity

b c

Table 3 Kinetic parameters of Ny6/LDPE blends

d e f

10

20

30

40

2 theta (deg.)

Na-EMAA content (phr)

Ny6/ LDPE blend ratio

Degradation temperature at 0.1 conversion (°C)

Ea (kJ/mol)

ln A (ln min1)

0

20/80 40/60 50/50 60/40 80/20

271.5 292.7 301.9 318.9 359.7

48.1 71.9 83.0 70.1 117.1

12.6 17.2 19.3 16.2 24.2

0.5

20/80 40/60 50/50 60/40 80/20

342.2 355.9 359.9 358.1 395.7

186.8 285.4 370.3 232.0 190.6

38.6 56.7 72.3 46.2 36.3

1.5

20/80 40/60 50/50 60/40 80/20

336.9 385.0 378.7 365.3 393.5

136.9 476.0 451.1 444.4 168.1

29.0 89.0 85.2 85.9 32.4

5.0

20/80 40/60 50/50 60/40 80/20

355.6 349.2 360.0 354.1 395.5

420.0 322.5 241.0 252.2 148.1

82.4 64.3 47.7 50.3 28.8

Fig. 9. WAXS patterns of 50/50 Ny6/LDPE blends: (a) LDPE, (b) 5 phr, (c) 1.5 phr, (d) 0.5 phr, (e) 0 phr of Na-EMAA, (f) Ny6.

da=dt ¼ f ðaÞA expðEa =RT Þ

ð3Þ

where T is the temperature at a specific weight loss, a is the reacted fraction at time t, f(a) is a function of a depending on the reaction mechanism. The reacted fraction a is normally defined as: a¼

ðW i  W Þ ðW i  W f Þ

ð4Þ

where Wi is the initial weight of sample, Wf is the final weight of sample and W is the weight at a particular time. The Flynn and Wall method is one of the simplest methods to estimate the values of the kinetic parameters. This approach chooses an arbitrary extent of reaction, in this case a = 0.1 and determines the temperature at which this extent of reaction is obtained for different heating rates. Fig. 10 shows the TGA thermograms of pure Ny6, LDPE, Na-EMAA, and the 50/50 Ny6/LDPE with

and without 5 phr of Na-EMAA. Pure LDPE showed an initial degradation temperature at around 280 °C.

A. Lahor et al. / European Polymer Journal 40 (2004) 2409–2420

90 80 % Weight Loss

70 60 50 40 Pure Nylon6 Pure LDPE Pure Na-EMAA 50/50/5 Ny6/LDPE/Na-EMAA 50/50/0 Ny6/LDPE/Na-EMAA

30 20 10 0

100

200

300

400

500

600

Fig. 10. TGA curves at 10 °C/min of neat polymers and 50/50 Ny6/LDPE and Ny6/LDPE with 5 phr Na-EMAA.

Degradation temperature at 0.1 fractional conversion (˚C)

Above this temperature, free radicals are generated leading to sequential degradation and breakdown of the main chain due to the thermal decomposition of the covalent C–C bond at a higher temperature [21]. In the case of pure Ny6, a minor weight loss, associated with loss of bound water was initially observed, followed by major weight loss above 360 °C, attributed to the breakdown of the main chain, evolving water, NH3, CO2, hydrocarbon fragments, and CO [22]. The thermal stability, as measured by the temperature at which a = 0.1, was much lower than that predicted by the rule of mixing in all composition ranges for uncompatibilized Ny6/LDPE blends as shown in Fig. 11. Thermal stability is very different when NaEMAA was added into Ny6/LDPE blends. A positive deviation from simple additive mixing rule occurs, and a strong increase in activation energy is present indicating that the addition of Na-EMAA led to a higher ther-

420 400 380 360 340 320 300

Mixing rule Na-EMAA 0.5 phr Na-EMAA 1.5 phr

280

Na-EMAA5.0 phr Na-EMAA 0 phr

260 0

10

20

30

40

50

60

70

80

90

100

mal stability of these blends. An increase in thermal stability has been found previously in similar systems, specifically blends of Nylon 6 with ethylene–acrylic acid copolymers [5]. Only 0.5 phr of Na-EMAA was enough to improve the thermal stability of the 80/20 Ny6/LDPE blends, as the degradation temperature at 395.7 °C was slightly higher than for pure Ny6 (380 °C). The thermal stability of compatibilized blends is slightly dependent on Na-EMAA concentration. Fig. 12 shows that blends of Na-EMAA with either Ny6 or LDPE increased the thermal stability vs. that predicted via a simple mixing rule indicating the improvement that the ionomer has on the individual blend components. The degradation temperature of LDPE/Na-EMAA blends increased with increasing amounts of Na-EMAA, but the level of Na-EMAA did not affect the degradation temperature of the Ny6/ Na-EMAA blends although the degradation temperature was significantly higher in the blends than in the pure Ny6 material.

Degradation temperature at 0.1 fractional conversion (˚C)

100

Degradation temperature at 0.1 fractional conversion (˚C)

2416

400

390

380

370

360

350

Mixing rule Ny6/Na-EMAA blends

0

10

20

30

40

50

60

70

80

90

100

% Ny6

380 370 360 350 340 330 320 Mixing rule LDPE/Na-EMAA blends

310 300

0

10

20

30

40

50

60

70

80

90

100

% Na-EMAA

% Ny6

Fig. 11. Degradation temperature at 0.1 fractional conversion of Ny6/LDPE blends.

Fig. 12. Degradation temperature at 0.1 fractional conversion of Ny6/Na-EMAA blends (top) and LDPE/Na-EMAA blends (bottom).

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vc (%)

f 50

100

150

200

250

Temperature (oC)

Fig. 13. DSC thermograms of Ny6/Na-EMAA blends: (a) Ny6, (b) 80/20, (c) 60/40, (d) 50/50, (e) 40/60, (f) 20/80, (g) NaEMAA.

– 218.9 222.4 218.7 216.0 219.0 222.0

a c

– – 213.7 209.8 208.4 210.2 215.7 – 207.6 199.0 203.3 200.6 203.3 207.3 14.6 12.9 13.3 13.3 13.8 14.3 – 41.1 29.1 22.5 18.7 15.5 8.1 – 95.2 95.9 96.5 96.0 96.0 95.9 – 84.0 85.3 85.1 85.1 87.9 89.2 – – 5.7 9.6 28.0 39.3 49.2 67.0 – 167.0 184.5 180.8 179.3 181.1 187.1 – 176.2 188.0 185.5 183.9 185.5 191.4 58.7 38.4 30.4 32.8 22.8 8.9 –

Onset (°C) DHc (J/g) Tc (°C) DHc (J/g) Tc (°C)

62.3 63.0 63.5 67.6 68.8 72.5 – 68.0 71.6 72.6 75.4 76.2 83.0 –

Tm (°C)

DHm (J/g) g

Onset (°C)

e f

Onset (°C)

b c d

Na-EMAA

a

Heating 250

Ny6

200

Na-EMAA

150 Temperature ( oC)

Normalized heat flow endo up (W/g)

100

Table 4 Thermal properties of Ny6/Na-EMAA blends as a function of blend composition

c

Cooling

b

Ny6/Na-EMAA (wt.%)

Normalized heat flow endo up (W/g)

a

0/100 20/80 40/60 50/50 60/40 80/20 100/0

Tm (°C) Onset (°C) vc (%)

Ny6

Tc of Ny/Sur blends

50

– 11.6 24.7 27.2 33.9 43.9 56.6

DHm (J/g)

Pure Ny6 gave a crystallization temperature, Tc, of 187 °C and two distinct melting temperatures (Tm) at 215 and 222 °C. Normally the melting endotherm of pure Ny6 appears as a main peak at 221 °C and a shoulder at 213 °C, corresponding to a- and c-form crystals, respectively [7]. Pure LDPE had a crystallization temperature (Tc) of 88.8 °C and a Tm of 105.2 °C while the pure Na-EMAA ionomer had a Tc of 62.3 °C and a Tm of 94.7 °C. When Ny6/Na-EMAA blend samples were cooled from the melt, the crystallization exotherm of Ny6 shifted to lower temperature with the addition of NaEMAA as shown in Fig. 13 and Table 4. This result indicates that Na-EMAA inhibits Ny6 crystallization most

d e f g

– 30.5 32.5 28.6 29.7 28.9 29.8

3.4. Differential scanning calorimetry

Normalized heat flow endo up (W/g)

likely due to the reduced mobility of Ny6 because of interactions with the ionomer. This effect has been seen previously with Ny6 after mixing with the unionized form of ethylene–acrylic acid (EAA) copolymers [6,7]. Tc of Na-EMAA increased with increasing Ny6 content, suggesting that Ny6 was nucleating ethylene crystallization and enhancing the ethylene crystallization rate. Without nucleation, ethylene crystallization would probably have slowed due to the reduced mobility caused by the interaction of acid segments with the nylon. In subsequent heating curves, the addition of Na-EMAA dropped the melting temperature of the Ny6 with the exception of the sample with 40% Ny6; this sample seems to be an outlier. A reduction in intensity of the shoulder at 213 °C of Ny6 relative to the main peak at 220 °C is observed indicating a reduction in the amount of c form. This effect is exactly opposite to that found for blends of Ny6 with ethylene–acrylic acid; EAA was found to promote the c form of Ny6 [7]. As shown in Table 5 and Fig. 14, vc of the LDPE phase drops dramatically with an increase in Na-

a b c d e f g

50

100

150 Temperature ( oC)

200

Normalized heat flow endo up (W/g)

41.1 14.5 11.7 5.5 4.5 0.8 – 6.0 9.1 9.9 10.7 12.1 17.3 – 3.4 10.3 13.9 18.2 27.2 48.9 58.7 22.9 29.9 21.1 16.0 6.5 – 68.0 69.8 70.5 69.6 69.7 96.4 – – 77.5 93.0 92.3 92.2 92.1 92.3

– 74.5 89.6 88.8 89.0 89.1 88.8

– 0.5 21.0 29.2 35.7 47.4 58.8

62.3 65.5 65.8 65.5 65.6 65.0 –

DHm (J/g) DHc (J/g) Onset (°C)

250

a b c d e f g

50

100

150

200

250

Temperature ( oC)

0/100 20/80 40/60 50/50 60/40 80/20 100/0

Onset (°C)

Tc (°C)

DHc (J/g)

Tc (°C)

84.0 81.0 84.0 84.8 88.9 86.5 – – 106.0 106.4 105.5 104.4 104.4 105.2 – 101.0 101.1 101.0 97.4 99.7 100.2

95.2 95.2 95.5 95.0 92.2 93.7 –

DHm (J/g) Tm (°C) Onset (°C) Tm (°C) Onset (°C)

vc (%)

Na-EMAA LDPE

Heating

Na-EMAA LDPE

Cooling LDPE/Na-EMAA (% wt)

Table 5 Thermal properties of LDPE/Na-EMAA blends as a function of blend composition

14.6 6.4 6.9 3.9 4.0 1.3 –

A. Lahor et al. / European Polymer Journal 40 (2004) 2409–2420

vc (%)

2418

Fig. 14. DSC thermograms of LDPE/Na-EMAA blends: (a) LDPE, (b) 80/20, (c) 60/40, (d) 50/50, (e) 40/60, (f) 20/80, (g) Na-EMAA.

Table 6 Thermal properties of Ny6/LDPE blends as a function of blend composition Ny6/LDPE (wt.%)

Amount of Na-EMAA (phr)

Cooling

Heating

Ny6 Onset (°C)

LDPE Tc (°C)

DHc (J/g)

Onset (°C)

Ny6 Tc (°C)

DHc (J/g)

Onset (°C)

LDPE Tm (°C) a

vc (%)

Onset (°C)

Tm (°C)

DHm (J/g)

vc (%)

Pure Ny6 Pure LDPE

0 0

191.4 –

187.1 –

67.0 –

– 92.3

– 88.8

– 58.8

208.2 –

215.7 –

222.0 –

56.6 –

29.8 –

– 100.2

– 105.2

– 48.9

– 17.3

20/80

0 0.5 1.5 5.0

187.8 189.7 188.7 187.8

183.0 185.1 182.6 181.8

11.3 7.3 5.4 6.1

95.5 94.9 95.3 93.8

91.6 91.3 91.5 90.3

44.1 44.5 45.5 39.4

208.9 208.5 207.4 206.4

211.7 211.5 210.2 207.0

220.0 220.0 219.5 219.2

10.1 8.2 8.7 8.9

26.6 21.7 23.2 24.6

102.3 101.5 101.7 100.2

108.0 106.7 107.4 106.0

37.8 35.8 34.6 32.3

16.8 15.8 15.3 14.2

40/60

0 0.5 1.5 5.0

188.9 189.6 188.8 188.4

185.1 185.0 183.6 183.8

23.2 24.8 16.1 15.1

97.2 94.4 94.2 93.1

93.0 90.3 90.5 89.6

32.7 41.2 33.8 27.4

206.7 209.1 207.1 206.7

213.9 212.2 210.5 210.2

219.9 220.5 220.0 219.2

20.0 21.8 19.5 17.6

26.3 28.8 26.0 24.3

102.9 101.9 101.7 99.9

106.7 107.4 107.0 105.0

25.6 27.2 25.6 23.5

15.1 16.0 15.0 13.5

50/50

0 0.5 1.5 5.0

189.4 189.7 191.2 190.7

184.5 185.5 186.5 185.8

32.1 28.8 46.5 25.6

96.9 94.3 97.7 94.2

92.5 90.1 92.0 90.8

22.2 32.8 11.0 26.0

206.3 208.8 206.9 209.3

216.4 211.5 213.9 211.7

221.4 219.9 221.0 220.2

28.3 25.0 39.1 24.3

29.8 26.4 25.9 26.0

100.9 101.1 100.0 101.1

107.5 106.2 105.9 105.9

20.0 24.3 11.6 22.3

14.2 17.1 20.1 15.6

60/40

0 0.5 1.5 5.0

189.9 189.7 189.2 190.3

185.8 185.8 184.5 186.6

36.6 34.9 33.7 27.6

99.8 94.2 93.8 93.3

94.0 90.3 90.1 90.3

15.8 22.7 20.8 16.2

206.6 209.8 210.7 206.4

214.9 212.9 213.4 212.4

220.5 220.9 221.0 220.5

29.1 31.1 28.8 28.8

25.5 27.4 25.6 26.5

96.8 101.3 101.3 99.1

105.9 106.5 106.2 103.7

15.9 20.1 18.0 15.4

14.1 17.6 15.6 12.7

80/20

0 0.5 1.5 5.0

191.8 191.2 187.8 191.2

188.3 186.5 184.3 187.6

48.0 46.5 45.9 46.0

98.6 97.7 96.9 96.0

93.6 92.0 91.6 91.3

6.9 11.0 9.4 7.1

210.1 206.9 213.6 210.0

– 213.9 216.4 215.7

220.0 221.0 222.9 222.9

34.4 39.1 36.8 37.9

22.6 25.9 24.6 26.2

100.4 100.0 100.6 100.3

106.9 105.9 106.7 106.2

8.3 11.6 10.8 9.4

14.6 20.1 18.1 14.0

A. Lahor et al. / European Polymer Journal 40 (2004) 2409–2420

c

DHm (J/g)

2419

2420

A. Lahor et al. / European Polymer Journal 40 (2004) 2409–2420

EMAA content for the binary blends, and the same is true for the ionomer with the incorporation of LDPE. This statement seems to be supported by WAXS results, although difficulties in baseline determination complicate the issue. The reason for this behavior is not known, although difficulties in baseline determination make the claim regarding a drop in crystallization tenuous for the ionomer particularly. Surprisingly, the positions of the peaks in cooling behavior are not affected by the blend composition except in the high content ionomer material. Other than a slight reduction in the c-phase melting temperature, no consistent observations could be made for the ternary blends as shown in Table 6, i.e. the effect on crystallization parameters of adding ionomer to a LDPE/Ny6 blend was small if not negligible.

4. Conclusion The ethylene–methacrylic acid copolymer partially neutralized with sodium (Na-EMAA) had been shown to be an effective compatibilizer for Ny6/LDPE blends by a morphological and thermal investigation. Addition of small amounts of Na-EMAA reduces the dispersed phase size by approximately a factor of 5, and as little as 0.5 phr of Na-EMAA was sufficient to produce a maximum reduction of dispersed phase size. The ionomer thermally stabilized binary blends with either Ny6 or LDPE as well as ternary blends with both materials; the 80/20 Ny6/LDPE compatibilized with 0.5 phr of Na-EMAA gave the highest decomposition temperature, ca. 16 °C higher than pure Ny6. The presence of Na-EMAA decreased the crystallization temperature and melting temperature of Ny6 in binary blends, indicating that Na-EMAA retards nylon crystallization. However, the effects of this retardation are not seen in the ternary blends, nor in the LDPE/NaEMAA blends. Although not from the same base resin nor the same neutralization level, the Na+ carboxylate ionomers were more effective compatibilizers than the Zn2+ carboxylate ionomers due to the lower dispersed phase size and higher thermal stability of the resulting blends.

Acknowledgment The authors are thankful to TPE Co., Ltd., BASF Co., Ltd., and DuPont (USA) for providing the materials for carrying out the present work.

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