Recycling of degraded polyethylene: Blends with nylon 6

Recycling of degraded polyethylene: Blends with nylon 6

Polymer Degradation and Stability 36 (1992) 131-135 Recycling of degraded polyethylene: Blends with nylon 6 F. P. La Mantia & D . Curto Dipartimento ...

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Polymer Degradation and Stability 36 (1992) 131-135

Recycling of degraded polyethylene: Blends with nylon 6 F. P. La Mantia & D . Curto Dipartimento di lngegneria Chimica dei Processi e dei Materiali, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy (Received 25 February 1991; accepted 11 March 1991)

Recycled polyethylene can be used for blending with nylon to obtain blends with mechanical properties better than those of blends with virgin polyethylene. Moreover, the mechanical properties of these blends are very close to and, in some cases, better than those of the pure nylon and those of blends made with functionalized polyolefines. The carbonyl groups of the photo-oxidized polyethylene react with amino end-groups of nylon 6, giving rise to graft copolymers which act as interfacial agents between the two incompatible phases. Large degrees of photo-oxidation of the polyethylene and intense mixing improve the properties of the blends.

INTRODUCTION

the ~ O groups of the photo-oxidized polyethylene react with the amino end-groups of the polyamidic phase giving rise to graft copolymers which act as interfacial agents. These copolymers improve the compatibility between the two phases and thus the characteristics of the blends. This goal is usually achieved by using expensive functionalized polyethylenes obtained by reaction with acrylic acid or maleic anhydride. 9-11 All these results suggest the possibility of using recycled polyethylene from greenhouse films as a natural funtionalized polymer to prepare PE/Ny blends with good mechanical properties. The aim of this paper is to present the mechanical properties of PE/Ny blends made from various recycled samples of polyethylene and using various processing conditions.

Plastic films from greenhouses, mostly made from low-density polyethylene (PE), are recycled by means of well-assessed processes x,2 to obtain polymeric materials usually used for manufacturing films and moulded objects with poor mechanical properties. Of course, a material of low value is obtained by these operations. The major problem which arises in using these degraded films results from the photo-oxidation undergone by the material during its life. Photo-oxidation processes change both the structure and morphology of the polyethylene, resulting in a deterioration of mechanical properties) -7 The major changes in the structure are due to the formation of polar groups (carbonyl, carboxyl, etc.) (as a result of the combined action of ultraviolet (UV) radiation and oxygen. In previous work, 8 we demonstrated that photo-oxidized polyethylene can be blended with nylon 6 (Ny) to obtain materials with mechanical properties better than those of blends prepared from nylon 6 and virgin polyethylene. Indeed,

EXPERIMENTAL The materials used in this work were samples of virgin polyethylene (V) and nylon 6 (Ny), and two samples of recycled polyethylene (R) obtained from highly photo-oxidized greenhouse films. The main physico-chemical properties of the raw materials are presented in Table 1. The blends, with Ny content 80% w/w, were

Polymer Degradation and Stability 0141-3910/92/$05.00 © 1992 Elsevier Science Publishers Ltd. 131

132

F. P. La Mantia, D. Curto

Table 1. Physico-chemicai properties of raw materials

Sample

Supplier

V

Montedipe

250

7.2

R1

--

--

--

--

-37 000

-2-1

R2 Ny

Snia

Mwx 10-3

Mw/Mn -Gel % 0 40 56 --

prepared by melt extrusion in a Brabender laboratory single-screw extruder (D = 1 9 m m , L/D=25) at 1 0 0 r e v m i n -~ and at a die temperature of 260°C. The R2/Ny blend was also prepared by melt mixing the h o m o p o l y m e r s in the same Brabender Plasticorder equipped with a mixer head (model W 50 EH) at 260°C and 100 rev min -1 for 15 min. Structural determinations

The gel contents of the two samples of recycled polyethylene were determined by means of a Soxhlet extractor. Approximately 0 . 3 g of the photo-oxidized polyethylene sample was exposed to refluxing p-xylene for 48 h. Samples of all the blends, fractured under liquid nitrogen, were observed with a Philips Model 505 scanning electron microscope. The surfaces of the specimens were coated with gold. Molau tests 12 were carried out by dissolving 200 mg of sample in 10 ml of 80% formic acid. Mechanical

Fig. 1. Molau tests. From left to right: V/Ny, R1/Ny and R2/Ny prepared by melt extrusion.

determinations

Tensile property measurements were carried out by means of an Instron machine (Model 1122) at room temperature. A crosshead speed of 5 cm min -1 and a gauge length of 3 cm were used in all measurements. The specimens used for tensile tests were cut from a sheet obtained by compression moulding at 240°C (180°C for the pure polyethylene). All the results reported are averages of at least 10 measurements. Impact strength was determined on notched samples using a Fractoscope (CEAST) in the Izod mode. Before testing, the specimens were equilibrated under ambient conditions ( T = 2 0 ° C and 60% R.H.) for at least 3 days.

RESULTS

AND

DISCUSSION

The possibility of obtaining polymeric materials with good mechanical properties from incom-

Fig. 2. Molau tests. From left to right: R2/Ny prepared by melt extrusion; R2/Ny (Ny 20%) and R2/Ny prepared by melt mixing.

Recycling of degraded polyethylene

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patible blends depends on the presence of suitable compatibilizing agents. In general, two different kinds of compatibilization have been exploited, i.e. the addition of copolymers that are partly miscible in both phases and the addition of a functionalized polymer that is miscible in one phase and that can react with the other phase. The copolymers formed during blending between one phase and the functionalized copolymer act as interfacial agents between the two polymers. This latter type of compatibilization can be achieved directly by using a functionalized polymer (for example, nylon and maleic grafted polypropylene). As shown previously,8 the possibility of obtaining compatibilized blends of polyethylene and nylon using a photo-oxidized PE sample has been demonstrated. The Molau test 12 can be used to verify the presence of PE/Ny copolymer.

Figure 1 shows the results of Molau tests on the following blends: V/Ny, R1/Ny and R2/Ny prepared by extrusion. The solution is clear for the V/Ny blend consisting of a nylon 6 solution in formic acid. The upper part is a suspension of polyethylene particles. The tests on the blends with recycled (photo-oxidized) polyethylene show, in contrast, a turbidity which represents a suspension of colloidal particles. This colloidal suspension must be due to the existence of polyethylene/nylon graft copolymers formed during melt extrusion by chemical reactions between the carbonyl groups of the photooxidized polyethylene and amino groups of nylon 6. The turbidity of the suspension increases as the degradation of the polyethylene is increased. The R2 sample contains larger amounts of C=O groups. Figure 2 shows Molau tests for the same blend, R2/Ny, prepared both by extrusion and

(a)

(b)

(c) (d) Fig. 3. Scanningelectronmicrographs: (a) V/Ny; (b) R1/Ny; (c) R2/Ny prepared by melt extrusion; (d) R2/Ny prepared by melt mixing,

134

F. P. La Mantia, D. Curto

by melt mixing. The blend prepared by melt mixing shows more intense turbidity. For comparison, a Molau test on an R2/Ny blend (20% Ny) is also shown. At this low content of nylon no turbidity is observed. Scanning electron micrographs of the same samples are illustrated in Fig. 3(a)-(d). In the blend with virgin PE (Fig. 3(a)), the polyethylene particles have average dimensions ranging from 5 to 30#m. Furthermore, very limited adhesion between the two phases is observed. The micrographs of the samples with recycled PE show that the dimensions of the discrete phase decrease with increasing degradation of the polyolefine (Fig. 3(b) and (c)) and the severity of the mixing process (Fig. 3(c) and (d)). Moreover, the adhesion improves with these same parameters. In particular, the blend with R2 prepared by melt mixing (Fig. 3(d)) shows an almost homogeneous phase. The results indicate that the formation of graft copolymers increases with: (i) the degree of photo-oxidation of the polyethylene; (ii) the content of nylon; (iii) the intensity of the mixing. The presence of graft copolymers, acting as 'bridges' between the two phases, can improve the mechanical properties of the compatibilized blends with respect to those of the noncompatibilized blends. Modulus, tensile strength, elongation at break and impact strength of all the blends are reported in Figs 4-6. It is interesting to note that modulus (Fig. 4) and impact strength (Fig. 6) increase when recycled degraded polyethylene is used and, for blends with the same components, when the 140 120 100

~, 80 E E 60 LU

[~ITS

120 I00

'E3 E

~2

5O ~0

V/Ny

I

~0

R1/N~'(extr)

R2/I~ ,(extr)

R2/Nv (mix)

140 120

1

100

) eo ~ 6o 4o 2o 0

Vl Ny

R1/Ny (extr)

R2/Nv (extr)

R2/Ny (mix)

Fig. 6. Impact strength, IS, for all the blends investigated.

intensity of mixing is increased (mixer vs extruder). Tensile strength (Fig. 5) is significantly improved only when the blending is performed in the mixer. The elongation at break (Fig. 5) is greatly reduced when recycled PE is used. In this case, however, it must be kept in mind that elongation at break dramatically decreases with photo-oxidation. In Table 2 the mechanical properties of all the polyethylene samples are reported. Whereas modulus, tensile and impact strength are only slightly influenced by photooxidation, elongation at break decreases with increasing degradation. Therefore, the elongation at break of the blend with recycled

40

properties samples

of

polyethylene

Sample

E (MPa)

ob (MPa)

eb (%)

V R1 R2

160 170 210

12 10 7

490 180 110

I v/Ny

R1 / Ny (extr)

R21Ny (extr)

R2/Ny (mix)

Fig. 4. Modulus, E, for all the blends investigated.

0

Fig. 5. Tensile strength, TS, and elongation at break, EB, for all the blends investigated.

Table 2. Mechanical

2C

~EB

Recycling of degraded polyethylene Table 3. Mechanical properties of blends

Sample

E(MPa) Ob(MPa) eb(%) IS(Jm 1)

Ny Ny/FPE" Ny/R2 (mixer)

75 79 120

65 52 46

180 40 25

95 70 135

FPE, functionalizedpolyethylene; dry blends (taken from Ref. 13). polyethylene decreases drastically because the elongation at break of the polyethylene is greatly decreased. Nevertheless, the blend with R2 obtained by melt mixing shows a value higher than that of the same blend obtained by extrusion, because of the larger amount of compatibilizer formed during processing. A final comment can be made on the mechanical properties of the blend containing recycled PE compared with those of pure nylon and of a blend containing a functionalized polyethylene (FPE) as in Table 3.13 The mechanical properties of the blends with the more degraded polyethylene are very similar to and, in some cases, better than those of the pure nylon, except for the elongation at break. Moreover, the mechanical properties of this blend are very similar to and, in some cases, better than those of the blend with FPE, although the results for this latter material are for a dry sample.

CONCLUSIONS It has been demonstrated that the use of degraded polyethylene in blends with nylon can

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give rise to polymeric materials with mechanical properties which are very similar to (and, in some cases, better than) those of pure nylon or of blends with functionalized polyolefines. This result suggests the possibility of using recycled PE to lower the cost of polyamides or, in some cases, to replace expensive functionalized polyolefine used in blends with nylon.

ACKNOWLEDGEMENT

This work has been supported financially by CNR Grant 90.04208.MZ78.

REFERENCES

1. Leidner, J. C., Polymer Waste. Dekker, New York, 1981. 2. La Mantia, F. P., Macplas International, May (1990) 53. 3. Heacock, J. F., Mallory, F. B. & Gay, F. B., J. Polym. Sci., 6 (1968) 2921. 4. Adams, J. M., J. Polym. Sci., 8 (1970) 1279. 5. Amin, M. U., Scott, G. & Tillekeratne, L. M. K., Europ. Polym. J., 11 (1975) 85. 6. La Mantia, F. P., Radiat. Phys. Chem., 23 (1984) 699. 7. La Mantia, F. P., Eur. Polym. J., 20 (1984) 10. 8. Curto, D., Valenza, A. & La Mantia, F. P., J. Appl. Polym. Sci., 39 (1990) 865. 9. Ide, F. & Hasegawa, A., J. Appl. Polym. Sci., 18 (1974) 963. 10. IUing, G., In Polymer Blends: Processing Morphology and Properties, ed. E. Martuscelli, R. Palumbo & M. Kryszewski, Plenum, New York, 1980, p. 167. 11. Chuang, U. K. & Han, C. D., J. Appl. Polym. Sci., 30 (1985) 2457. 12. Molau, G. E., J. Polym. Sci., A3 (1965) 1267; Kolloid Z. Z. Polym. 238 (1970) 493. 13. Chuang, U. K. & Han, C. D., J. Appl. Polym. Sci., 30 (1985) 165.