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Polyamide-6/high-density polyethylene blend using recycled high-density polyethylene as compatibilizer: morphology, mechanical properties, and thermal stability.

INTRODUCTION

Compatibilization of immiscible binary blends during melting process has been extensively studied because it is an efficient way to produce polymeric materials with novel and targeted properties. Immiscible polymer blends present large particles of dispersed phase that are weakly adhered to the matrix and badly distributed within it. One strategy to reduce this problem is by promoting adequate interactions at the interface of the blend components.

The use of an appropriate compatibilizer promotes these interactions and consequently causes coalescence and a decrease in the interfacial tension between components, stabilizing the morphology with a fine dispersion of the minor phase and improves the phase adhesion (1). A strong specific interaction such as ion-ion, covalent, and hydrogen interaction is important to promote interfacial adhesion and to make the polymer blend more compatible (2).

Different compatibilization strategies have been used, one is the addition of block or graft copolymers containing reactive groups, which promote in situ compatibilization during the melt-blending process (1). Several articles about polyolefin-based blends are in the literature, using different compatibilizers, such as acrylic acid, phosphazene compounds (3), ionomers (4-6), and maleic anhydride (7-9). The morphological features of an incompatible blend, such as the size of the dispersed phase domains and the interfacial adhesion, also play an important role in its thermal and mechanical properties.

Polyolefins and polyamide-6 blends are extremely interesting, not only from a scientific point of view, but also for practical applications. High-density polyethylene (HDPE) is a commodity polymer which shows low water absorption and high impact strength. However, virgin HDPE do not have functional groups to interact with polyamide in blends. HDPE is one of the five most produced and consumed polymers in Brazil and in the world (10) and is mainly used in the packaging area; consequently, it is discarded in large amounts. On the other hand, polyamide-6 (PA6) is most frequently used in engineering applications and presents high water absorption, insufficient impact strength, good strength resistance compared with polyole-fins (11) and can react with many functional groups because it has amine end groups. Consequently, HDPE and PA6 present complimentary properties, but because of their differences in chemical nature and polarity, their blends are immiscible and exhibit a clear two-phase morphology, where the dispersed phase forms relatively large spherical droplets, and no adhesion between the phases exist (12).

An alternative to the use of conventional functionalized compatibilizers is the use of recycled polymers, which can have functional groups along the chain, generated during its aging process. These functional groups can promote favorable interactions with polyamide. The use of recycled polymers may reduce the cost of the blend, improve its mechanical properties, and reduce the waste of these materials (13). Literature reports the use of irradiated virgin HDPE, to promote functionalization, and to obtain similar compatibilization results (14).

In this work we present results of using recycled high-density polyethylene (rHDPE) in blends with virgin poly-amide-6 (PA6) or postindustry (piPA6). The blends were processed by extrusion-injection molding and characterized by morphological, mechanical, and thermal stability properties. Results indicate that rHDPE exerts a compatibilization effect in these blends.

EXPERIMENTAL

Materials

Polyamide-6 (PA6, Zytel[R] Du Pont, Sumare, Brasil, MFI = 24 g/10 min at 235[degrees]C, 0.325 kg), high-density polyethylene (HDPE, HB-0454[R] Braskem, Triunfo, Brasil, MFI = 0.3 g/10 min at 190[degrees]C, 2.16 kg), recycled high-density polyethylene (rHDPE, Proceplast, Sumare, Brazil, MFI = 6 g/10 min at 190[degrees]C, 2.16 kg, secondary recycling of containers produced by blow-extrusion), and postindustry polyamide-6 (piPA6, Raitek, Campinas, Brasil, primary recycling, industry scrap). Formic acid (85%, Merck) was used for Molau test.

FTIR of HDPE and rHDPE

HDPE and rHDPE were milled in a three-knife rotary mill (Rone, model NFA 1533) equipped with a 0.5-mm mesh sieve. The resulting powder was sieved to 150 mesh and maintained under vacuum by 12 h. The Fourier Transform Infrared, FTIR, measurements (Bomem, MB-100 Series) were made with KBr pellets in the 4000-450 [cm.sup.-1] region, with 4 c[m.sup.-1] resolution and 256 scans. The KBr was dried (100[degrees]C, 4 h) and the samples were prepared immediately before measurement.

Determination of the Drying Conditions of PA 6

The better drying conditions of PA6 were determined measuring the relative mass loss as a function of heating time, at 80, 90, 100, 110, and 120[degrees]C in a vacuum oven (Cole-Parmer model 5053-10, operating at 3.6 kPa).

Blends Preparation

Raw materials (PA6, HDPE, and rHDPE) and blends of PA6/HDPE and PA6/rHDPE with 25, 50, and 75 wt% of PA6 were prepared using either a single-screw (Wortex, Campinas, Brazil, L/D = 30, D = 32 mm, temperature profile of 230-255[degrees]C, from feed to dye) with a Maddock mixing screw or a co-rotating twin-screw extruder (APV model MPC/V30, L/D = 13, D = 29 mm, temperature profile of 230-255[degrees]C, from feed to dye). The screw rotation was fixed at 100 rpm in both extruders. The piPA6 and its blends using similar compositions and extrusion conditions were processed in the twin-screw extruder. The blends are refereed in the text as nPA6/mHDPE, where n corresponds to the weight percent of PA6 or piPA6 and m to the weight percent of HDPE or rHDPE.

The pellets obtained were injection molded (Arburg model All Rounder M-250, Lassburg, Germany) using a temperature profile from 230 to 255[degrees]C, mold temperature of 20[degrees]C, 110 GPa injection pressure and 20 s of mold cooling time. Mold cavities provided Type I test specimens for tensile properties analysis, according to ASTM D 638-02, test specimens for Izod impact strength (ASTM D 256-02) and test specimens for flexural properties analysis (ASTM D 790-02). PA6 and piPA6 were vacuum dried (Cole Parmer, model 5053-10, 3.6 kPa of pressure) at 120[degrees]C by 6 h before process and injection.

Blend Characterization

Injection molded test specimens of raw materials and respective blends were cryofractured and surface morphology was analyzed by scanning electron microscopy, (SEM; Jeol model JSM6360LV, 25 kV) after gold/palladium (8:2) coating using a Bal-Tec, Mult Coating System MED020. Typically, over 300 particles and several regions of the fractured surface were analyzed. Size domain distributions and number average diameters ([D.sub.n]) were analyzed using the Image-Pro Plus[R] 3.0 software and Microcal Origin[R].

Molau tests were performed by stirring ca. 100 mg of milled samples (average size of ~1 mm) in 25 mL of 85% formic acid, storing the test tubes for 24 h, and measuring the turbidity of the solutions with Alem Mar (Sao Paulo, Brazil) turbidimeter model TAM 300 A at ~30[degrees]C (15), (16).

The mechanical properties of the materials were analyzed by measuring the tensile and flexural strength in EMIC (Sao Jose dos Pinhais, Brazil) model DL 2000 test equipment, according to ASTM D 638 and D 790, respectively. Izod pendulum impact strength was measured in a Tinius Olsen model 92T equipment (Horsham), using notched test specimens, according to ASTM D 256, method A. The impact strength measurements were made at room temperature using a pendulum with impact energy of 5.54 J. All mechanical measurements and the standard deviation calculations were made using 10 test specimens.

A TA Instruments model 2050 analyzer was used to measure the thermogravimetric curves. Samples (10 [+ or -] 0.5 mg) were heated from 30 to 800[degrees]C at heating rate of 10[degrees]C/min under argon atmosphere at a flow rate of 50 mL/min.

RESULTS AND DISCUSSION

Drying Conditions of PA6

PA6 drying conditions (time and temperature) were studied before processing (see Fig. 1), because literature shows values of heating time from 4 to 24 h and temperatures from 80 to 120[degrees]C (17-22). In this work, PA6 drying conditions were chosen based on the necessary time for stabilization of mass loss as a function of temperature.

[FIGURE 1 OMITTED]

Figure 1 show that, the weight loss at 110 and 120[degrees]C, related to the water content in the PA6, occurs in a shorter time. Therefore the conditions 120[degrees]C for 6 h, were chosen to dry PA6 before all extrusion and injection processes.

Morphological Properties

The surface morphology of the cryogenically fractured injection-molded samples of the PA6/HDPE blends, in transversal orientation in relation to the molten flow, was studied by SEM. Figure 2 shows SEM photomicrographs of blends of PA6 with HDPE or rHDPE processed in the single-screw extruder. The morphology of all blends containing 25 and 75 wt% of PA6 exhibited spherical particles dispersed in the matrix. A cocontinuous morphology was observed for blends containing 50 wt% of PA6.

[FIGURE 2 OMITTED]

As expected, blends containing HDPE show large particles of several micormeter, dispersed phase with large variation of domain size, particles detached from the matrix and absence of surface adhesion of the disperse phase in the matrix, indicating poor interfacial adhesion (Fig. 2; 75PA6/25HDPE, 50PA6/50HDPE, and 25PA6/75HDPE). On the other hand, when HDPE was replaced by rHDPE in the same proportions, a decrease in the size domain of the disperse phase was observed, (Fig. 2; 75PA6/25rHDPE, 50PA6/50rHDPE, and 25PA6/75rHDPE). Similar results were observed for blends prepared using a twin-screw extruder or using piPA6.

Morphology development in immiscible blends is dependent on the processing conditions and, sometimes, is unstable under system perturbations, like another melt processing. Consequently, this perturbation can promote the coalescence of the disperse phase. The morphological instability of the polymer blend can influence its practical use. It is important to note that, in this work, the polymer blends were processed by two melt processing (extrusion and injection molding), with previous drying and the micrographs were taken after the second processing. Thus, it is possible to state that the blends morphology is stable and adequate for practical use.

Morphological characteristics of immiscible blends, like size domains and surface adhesion, strongly affect their mechanical properties (23). The optimal mechanical properties often rely on an average disperse-phase diameter at the submicrometric level (24), (25). So the distribution of domain size of the disperse phase was analyzed to observe the effect of rHDPE.

The size distribution of the disperse phase for blends with 25 and 75 wt% of PA6 processed in the single or twin-screw extruders is shown in Fig. 3. The number-average disperse phase diameter ([D.sub.n]) for PA6/HDPE blends processed in the single and twin-screw extruder are shown in Table 1. These values were calculated to support the results shown in the Fig. 3. Blends processed by single-screw extruder (using PA6 as matrix), twin-screw extruder (using PA6 as matrix), and twin-screw extruder (using piPA6 as matrix) show [D.sub.n] of HDPE domains of 4.6, 6.6, and 15.3 [micro]m, respectively. When HDPE was replaced by rHDPE, the [D.sub.n] was reduced to 1.1, 4.1, and 1.46 [micro]m, respectively. On the other hand, for blends composed by HDPE as continuous phase, the [D.sub.n] of PA6 domains were 6.0, 6.6, and 15.1 [micro]m. When HDPE matrix was replaced by rHDPE these values were reduced to 3.0, 0.77, and 1.1 [micro]m, respectively (see Fig. 3 and Table 1). The reduction of disperse phase size does not depend on the matrix, in the case of HDPE or PA6. The higher relative reduction occurred for blend 25piPA6/75rHDPE (~92%). However, the smaller value of [D.sub.n] occurred for the blend 25PA6/ 75rHDPE processed in the twin-screw extruder (0.77 [micro]m).

[FIGURE 3 OMITTED]
TABLE 1. Number-average dispersed phase diameter ([D.sub.n]) for blends
using PA6 with 25 and 75 wt% of HDPE or rHDPE.

                            [D.sub.n] PA6         [D.sub.n] HDPE
                          domains/[micro]m       domains/[micro]m
                           (25PA6/75HDPE)         (75PA6/25HDPE)

HDPE, single-screw          6 ([+ or -] 2)      4.6 ([+ or -] 0.4)
extruder

rHDPE, single-screw       3.0 ([+ or -] 0.1)   1.10 ([+ or -] 0.03)
extruder

HDPE, twin-screw          6.6 ([+ or -] 0.1)    6.6 ([+ or -] 0.1)
extruder

rHDPE, twin-screw        0.77 ([+ or -] 0.02)   4.1 ([+ or -] 0.1)
extruder

HDPE/piPA6, twin-screw   15.1 ([+ or -] 0.5)   15.3 ([+ or -] 0.7)
extruder

rHDPE/piPA6, twin-screw  1.10 ([+ or -] 0.03)  1.46 ([+ or -] 0.06)
extruder


The reduction of PA6 disperse phase size domain in the rHDPE matrix observed in this work is similar to works using copolymers as coupling agents, such as acrylic acid (3), (9), (21) glycidyl methacrylate (9), (26), (27), maleic anhydride (9), (21), (27), (28), ionomers (21), and blends compatibilized by ultrasonic extrusion (29). Cha-treenowat et al. (5) observed that the use of 9 wt% of maleic anhydride grafted HDPE in the PA6/HDPE blend promotes a reduction of disperse phase diameter to 2 [micro]m. The best result reported in the literature is for blends with 20 wt% of LDPE functionalized with 6 wt% of acrylic acid dispersed in PA6 matrix, which present disperse phase diameter from 0.1 to 0.2 [micro]m (24).

SEM was used to observe the interfacial adhesion; the fracture of the disperse phase, when HDPE is used, is interparticle and the borders are detached, Fig. 2. In contrast, when rHDPE is used the fracture is transparticle, which is; the particles and the matrix appear fractured along the same plane, indicating an efficient adhesion between the polymer matrix (rHDPE) and the disperse phase (PA6), Fig. 4.

[FIGURE 4 OMITTED]

Generally, the reduction of disperse phase dimensions to submicrometric and a good adhesion between the phases results in improved mechanical properties (25). The results reported in this work indicate that rHDPE shows a relevant compatibilization effect. This effect occurs because rHDPE present polar groups, as evidenced by the absorption at 1712 [cm.sup.-1] in the FTIR spectrum, Fig. 5, assigned to carbonyl groups formed upon thermo oxidation of the polymer backbone. These polar groups interact with the carboxyl or amine end groups of PA6.

[FIGURE 5 OMITTED]

Similar results were observed by Desidera and Felisberti (30) using recycled PA66 dispersed in virgin and recycled LDPE. A narrow distribution of disperse phase size was also observed when rHDPE was used in the blends (Fig. 3, Table 1), also typical of compatible immiscible blends (31), (32). When rHDPE is the matrix, the reduction of the disperse phase domain size is dramatic (Table 1). On the other hand, when PA6 is the matrix, a greater variation of disperse phase size distribution was observed. This result can be attributed to the lower viscosity of PA6 compared with HDPE (6), (33). The viscosity ratio is an important factor for the reduction of disperse phase size; the maximum reduction occurs when the matrix and disperse phase viscosities are similar (34-36).

Molau Test

This test is related to fractionated dissolution and is usually applied as a qualitative test to indicate the formation of graft or block copolymers between polyolefins and polyamides, which act as surfactants and stabilize the colloidal suspension of polyolefin particles into the formic acid solution (9), (13), (16). In most cases, the polymer solubility can change as a function of its molecular weight. Therefore, this method can be used for this blend, because no significant change in molecular weight occurs (3).

Figure 6 show the photography of mixtures of 75PA6/25HDPE blends in formic acid. The mixtures containing HDPE showed a clear separation between the solid polyolefin onto the liquid surface and the almost clean polyamide solution in formic acid. The low turbidity observed in these solutions can be attributed to the formation of a small amount of a compatibilizing copolymer, formed by reaction of the polyamide and the polyolefin chains due to the shear tensions during melt processing (15). On the other hand, the mixtures containing blends with rHDPE showed an increased turbidity. Similar to the results obtained for blends compatibilized with functionalized copolymers (3), (9), (17) and ionomers (2), For all blends, a positive deviation could be observed from the additive rule. Moreover, the replacement of HDPE by rHDPE promoted an increase of mixture turbidity, from 3.5- to 5-fold, except for blends containing piPA6. This increase of mixture turbidity is similar to the results reported by other authors using copolymeric compatibilizers (13), (20). This behavior reinforces the hypothesis of the compatibilizing effect of rHDPE.

[FIGURE 6 OMITTED]

Mechanical Properties

Tensile Strength. The average values and deviation of tensile strength are shown in Table 2. It could be verified that, in general, a higher PA6 amount causes a higher tensile strength for both PA6/HDPE and PA6/rHDPE blends. In addition, all samples exhibited negative deviation from the additive rule, which is a typical behavior for immiscible polymer blends (37), (38). Other authors reported a positive deviation from the additive rule for PA6,6/HDPE, when powdered HDPE was superficially irradiated with a low energy ion beam under [O.sub.2] atmosphere (14), generating polar groups on the polyolefin surface. This introduction of polar groups onto the polyolefin surface should be similar to the process that occurs during the aging and the reprocessing of rHDPE.
TABLE 2. Tensile strength (TS) values for homopolymers and blends.

                         Single-screw (PA6)

PA6/HDPE       TS (MPa) HDPE          TS (MPa) rHDPE

100/0      65.0 ([+ or -] 0.2)    65.0 ([+ or -] 0.2)
 75/25       36 ([+ or -] 10)       48 ([+ or -] 1)
 50/50       22 ([+ or -] 0.2)      37 ([+ or -] 1)
 25/75       17 ([+ or -] 1)      29.5 ([+ or -] 0.3)
  0/100      24 ([+ or -] 2)      20.6 ([+ or -] 0.2)

                        Twin-screw (PA6)

PA6/HDPE    TS (MPa) HDPE          TS (MPa) rHDPE

100/0      73 ([+ or -] 3)        73 ([+ or -] 3)
 75/25     43 ([+ or -] 1)        51 ([+ or -] 1)
 50/50     26 ([+ or -] 1)        41 ([+ or -] 2)
 25/75     18 ([+ or -] 1)        27 ([+ or -] 1)
  0/100    23 ([+ or -] 0.1)    22.0 ([+ or -] 0.3)

                        Twin-screw (piPA6)

PA6/HDPE      TS (MPa) HDPE          TS (MPa) rHDPE

100/0        72 ([+ or -] 5)        72 ([+ or -] 5)
 75/25       51 ([+ or -] 3)        55 ([+ or -] 4)
 50/50       32 ([+ or -] 1)        43 ([+ or -] 4)
 25/75       26 ([+ or -] 1)      33.0 ([+ or -] 0.4)
  0/100    23.0 ([+ or -] 0.1)    22.0 ([+ or -] 0)


The use of rHDPE in the blends increased the tensile strength for all compositions in comparison with HDPE. This result is similar to that obtained for blends compatibilized by copolymeric agents (37). Scaffaro et al. (3) prepared PA6/HDPE blends, compatibilized with ethylene copolymers functionalized with acrylic acid and epoxy or oxazoline compounds, and reported an increase of tensile strength from 19 to 25 MPa. Lopez-Quintana et al. (34) prepared blends of PA6 and metalocenic polyethylene; compatibilized with metalocenic polyethylene co-polypropylene rubber functionalized with maleic anhydride, and reported an increase of tensile strength from 8 to 10 MPa. Comparing the above mentioned results with those listed in Table 2 it could be verified that the replacement of HDPE matrix by rHDPE matrix, for blends processed in the single-screw extruder, promoted an increase of tensile strength from 17 to 30 MPa.

This result represents approximately a twofold increase in comparison with the initial value, which is a noticeable effect. The highest tensile strength values were observed for piPA6/rHDPE blends; on the other hand PA6/HDPE blends showed the lowest values and, in most cases, the addition of the PA6 exhibited an antagonistic effect, reducing the tensile strength values to lower than those for HDPE. Intermediate tensile strength values, for PA6/rHDPE blends in comparison with the homopolymers, were similar to those observed for blends compatibilized with functionalized copolymers (8).

Tensile strength is a limiting property and is therefore proportional to the adhesion level at the polymeric interfaces, so the obtained results indicate that rHDPE and piPA6 increase the interfacial adhesion in comparison with HDPE and PA6.

Flexural Modulus. Mechanical properties of blends are related to morphology and intermolecular interactions of the components (34). The average flexural modulus (E) values for homopolymers and their respective blends are show in Table 3. The PA6/rHDPE and PA6/HDPE blends show intermediate values of flexural modulus compared with the homopolymers. The replacement of HDPE by rHDPE does not change the blend's rigidity. The blends processed in the twin-screw extruder showed higher modulus values than the blends processed in the single-screw extruder. This can be produced by the higher shear stress of the twin-screw in comparison with the single-screw extruder, causing HDPE degradation by cross-linking.
TABLE 3. Flexural modulus (E) for homopolymers and blends.

                       Single-screw (PA6)

PA6/HDPE       E (MPa) HDPE           E (MPa) rHDPE

100/0      1830 ([+ or -] 60)     1830 ([+ or -] 60)
 75/25     1390 ([+ or -] 50)     1368 ([+ or -] 57)
 50/50     1098 ([+ or -] 53)     1100 ([+ or -] 47)
 25/75      780 ([+ or -] 120)     903 ([+ or -] 89)
  0/100     578 ([+ or -] 73)      621 ([+ or -] 103)

                          Twin-screw (PA6)

PA6/HDPE        E (MPa) HDPE          E (MPa) rHDPE

100/0      2378 ([+ or -] 156)    2378 ([+ or -] 156)
 75/25     1723 ([+ or -] 45)     1598 ([+ or -] 50)
 50/50     1361 ([+ or -] 72)     1274 ([+ or -] 27)
 25/75      869 ([+ or -] 80)      910 ([+ or -] 44)
  0/100     630 ([+ or -] 87)      776 ([+ or -] 29)

                       Twin-screw (piPA6)

PA6/HDPE       E (MPa) HDPE          E (MPa) rHDPE

100/0      2426 ([+ or -] 80)    2426 ([+ or -] 80)
 75/25     1910 ([+ or -] 73)    2055 ([+ or -] 267)
 50/50     1234 ([+ or -] 44)    1275 ([+ or -] 29)
 25/75      918 ([+ or -] 65)     945 ([+ or -] 36)
  0/100     630 ([+ or -] 87)     776 ([+ or -] 29)


The replacement of PA6 by piPA6 does not change the flexural modulus of the blends with rHDPE. On the other hand, for blends with HDPE random changes of this property were observed.

Albano et al. (37) reported an inverse behavior of the Young's modulus for these blend components. They attributed the lower PA6 Young's modulus to higher humidity absorption and lower degree of crystallinity PA6 in comparison with HDPE.

Impact Strength. Figure 7 shows curves of the notched Izod impact strength (IS) for homopolymers and respective blends. Average values obtained of IS for HDPE (processed in the single or twin-screw extruder) is 250 J/m, for rHDPE processed in the single-screw extruder is 38 J/m, for rHDPE processed in the twin-screw extruder is 70 J/m, for PA6 (processed in a single or twin extruder) is 54 J/m and for piPA6 is 36 J/m.

[FIGURE 7 OMITTED]

All PA6/HDPE blends processed in the single or twin-screw extruder showed poor impact performance compared to the impact strength of HDPE (250 J/m), Fig. 6. This is due to the immiscibility of the system. These blends show impact strength similar to the values of the PA6 phase and present a negative deviation from the additive rule. rHDPE presents lower value of IS (38 or 70 J/m depending of the process machine) compared with HDPE. In the case of the blends with rHDPE, the values of IS are close to the additive rule. Only for 25PA6/ 75rHDPE and 75PA6/25rHDPE blends, processed in the single-screw extruder, the IS values exceeded the additive rule and a synergy was observed. It would be expected that an increase of the interfacial tension would increase the blend toughness, but the IS value of rHDPE is low and similar to PA6, so the rHDPE is not acting as a toughening agent.

In general, a considerable variation of IS values was observed for some compositions. This behavior could be attributed to differences in the macromolecular relaxation process for the injection molded material along the flow direction. The "skin-core" structure may cause significant changes in the mechanical properties along the flow direction. The higher melt viscosity, the lower mold temperature, and shorter packing time, restrict the macromolecular relaxation and enhance the differences in morphologies and properties at the near and far parts of a mold (8). Thus, it is quite reasonable to consider that the significant dispersion of the impact strength values can be due to the changes on "skin-core" structure along the flow direction.

Thermogravimetry. The thermogravimetric results show that PA6 initially exhibits a low weight loss associated to bound water, followed by major weight loss above 360[degrees]C due to the breakdown of the PA6 main chain, with evolution of water, [NH.sub.3], [CO.sub.2], hydrocarbon fragments, and CO (2); HDPE also exhibit a single major weight loss with onset at 350[degrees]C.

Analysis of the TGA curves and their first derivatives indicate the onset temperature of weight loss ([T.sub.5%]) (which determines the upper temperature limit at which polymers can be processed or manufactured), the temperature of maximum weight loss rate ([T.sub.max]) (which is a measure of the long-term stability of the polymer) (37), and the residue percentage after decomposition. These data are listed in Table 4 for homopolymers and blends.
TABLE 4. Values of onset temperature of mass loss ([T.sub.5%]),
temperature of maximum rate of mass loss ([T.sub.max]), and residue
percentage (Res), obtained from the TGA curves analysis.

                                        Single-screw (PA6)

               [T.sub.5%] [degrees]C  [T.sub.max]/[degrees]C  Res/%

PA6                     363                     449           1.45
piPA6                   --                      --            --
75PA6/25HDPE            369                     447           1.51
50PA6/50HDPE            377                     447           1.44
25PA6/75HDPE            362                     469           0.81
HDPE                    347                     457           0.97
75PA6/25rHDPE           372                     448           1.66
50PA6/50rHDPE           372                     446           1.67
25PA6/75rHDPE           373                     466           1.54
rHDPE                   380                     463           1.63

                                         Twin-screw (PA6)

               [T.sub.5%]/[degrees]C  [T.sub.max]/[degrees]C  Res/%

PA6                     366                     446           1.18
piPA6                    --                       --          --
75PA6/25HDPE            368                     447           1.67
50PA6/50HDPE            366                     449           1.19
25PA6/75HDPE            362                     463           1.41
HDPE                    351                     442           1.02
75PA6/25rHDPE           370                     454           1.10
50PA6/50rHDPE           377                     445           1.67
25PA6/75rHDPE           365                     460           1.52
rHDPE                   373                     456           1.5

                                        Twin-screw (piPA6)

               [T.sub.5%]/[degrees]C  [T.sub.max]/[degrees]C  Res/%

PA6                      --                       --          --
piPA6                   368                     443           4.42
75PA6/25HDPE            362                     445           3.50
50PA6/50HDPE            375                   443/461         3.02
25PA6/75HDPE            367                     464           2.17
HDPE                    351                     442           1.02
75PA6/25rHDPE           367                     444           3.39
50PA6/50rHDPE           382                   444/473         3.30
25PA6/75rHDPE           365                     462           2.55
rHDPE                   373                     455           1.75


In terms of onset weight loss ([T.sub.5%]), blends with cocontinuous morphology (50PA6/50HDPE) present higher thermal stability in comparison with blends with globular morphology (25 PA6/75HDPE and 75PA6/25HDPE). Replacement of the HDPE by rHDPE promoted a slight increase in the thermal stability of the blends, except for blends containing 50 wt% of each component prepared the in single-screw extruder and for blends containing piPA6 as dispersed phase. A similar increase in thermal stability was reported for blends compatibilized with functionalized copolymers (34) and ionomers (2). On the other hand, Albano et al. (37) verified a decrease of the thermal stability in this blend when HDPE was replaced by rHDPE.

In terms of the thermal decomposition process it could be verified, by means of the first derivative curves and [T.sub.max] data, that the blends presented two major decomposition processes, which are related to the decomposition processes of each component; PA6 at lower temperature and HDPE at higher temperature. For blends containing 75 and 50 wt% of PA6, the peak related to HDPE appeared as a shoulder. For blends containing 25 wt% of PA, the opposite was observed. This result indicates that the thermal decomposition process of each component of the blend occurs independently of each other. On the contrary to that observed for thermal stability, replacement of HDPE by rHDPE did not affect the maximum temperature of decomposition.

In terms of the percentage residues, a tendency could be verified for a positive deviation from the additive rule and a higher residue percentage for rHDPE and piPA6 in comparison to the pristine components was observed. This was expected because they contain additives. The percentage residue ranged from 0.8 wt% (75PA6/25HDPE) to 3.5 wt% (75piPA6/25rHDPE).

CONCLUSIONS

In this work we studied the properties of blends of high-density polyethylene (HDPE) and polyamide-6 (PA6) and the effect of its replacement by recycled materials, namely, recycled high-density polyethylene (rHDPE) and postindustry polyamide-6 (piPA6), respectively. It was verified that the replacement of the virgin by the recycled polymers improved the mechanical properties and slightly increased the thermal stability of the blend. These results are attributed to the compatibilizing effect of rHDPE, which is a consequence of the presence of polar groups on the polyolefin backbone generated through thermo oxidative aging. Other evidences of the compatibilizing effect of rHDPE were the size domain reduction in the blends, verified by SEM, and the higher turbidity into the formic acid mixtures. These results indicate an interesting perspective for future application of recycled polymers.

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Marcio R. Vallim, Joyce R. Araujo, Marcia A. Silva Spinace, Marco-A. De Paoli

Laboratorio de Polimeros Condutores e Reciclagem, Instituto de Quimica, Universidade Estadual de Campinas, C. P. 6154, 13084-971, Campinas; SP, Brazil

Correspondence to: Marco-A. De Paoli; e-mail: mdepaoli@iqm.unicamp.br Contract grant sponsor: FAPESP; contract grant number: 04/15084-6; contract grant sponsor: CNPq (to M.A.P.).

DOI 10.1002/pen.21439
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Author:Vallim, Marcio R.; Araujo, Joyce R.; Spinace, Marcia A. Silva; Paoli, Marco-A. De
Publication:Polymer Engineering and Science
Article Type:Technical report
Date:Oct 1, 2009
Words:5436
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