Physical and structural characterization of blends made with polyamide 6 and gamma-irradiated polyethylenes

Radiat. Phys. Chem. Vol. 48, No. 2, pp. 207-216, 1996 Copyright© 1996ElsevierScienceLtd 0969-806X(95)00422-X Printed in Great Britain.All rights reserved 0969-806X/96 $15.00+ 0.00

Pergamon

PHYSICAL AND STRUCTURAL CHARACTERIZATION OF BLENDS MADE WITH POLYAMIDE 6 AND GAMMA-IRRADIATED POLYETHYLENES G. SPADARO, j D. ACIERNO, 2 C. DISPENZA, l E. C A L D E R A R O 3 and A. VALENZA 1 ~Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universit~i di Palermo, viale delle Scienze, 90128 Palermo, Italy, :Dipartimento di Ingegneria Chimica e Alimentare, Universith di Salerno, via Ponte Don Melillo, 84084 Fisciano (Sa), Italy and 3Dipartimento di Ingegneria Nucleare, Universit~i di Palermo, viale delle Scienze, 90128 Palermo, Italy Abstract--Morphological, calorimetric and rheological results of blends made with polyamide 6 and polyethylene gamma irradiated in air are presented. The polar oxidized groups grafted in the poliolefin chains through gamma-radiation induce "compatibilization" effects in the blends with a more uniform and finer distribution of the polyethylene "phase" in the polyamide matrix, with respect to blends made with the unirradiated polymer. This effect, observed with polyethylenes of different molecular structure, i.e. low density, linear low density and high density, was attributed to the presence of interactions among the functional oxidized groups of the polyethylene chains and the polyamide. Tests done at different mixing times indicate that short times are enough to allow these interaction to occur. Copyright © 1996 Elsevier Science Ltd.

INTRODUCTION

Polyethylene/polyamide blends are incompatible with poor properties and limited applications. There have been many efforts to modify the interfacial interactions and to induce "compatibilizing" effects; in particular one of the most used ways is to insert functionalized groups in the polyethylene chains which are able to react with the aminic and carboxylic groups of the polyamide (Liang et al., 1983; Chuang and Han, 1985; Utracki et al., 1986; Chen et al., 1988; Xanthos, 1988; Gaylord, 1989; La Mantia and Valenza, 1989; Serpe et al., 1990; Raval et al., 1991). An interesting way to graft functional groups in the polyethylene chains is the use of gamma-radiation processing in presence of air. The effects of this treatment on the polyethylenes has been extensively studied in a wide range of experimental conditions and many papers discuss the modifications induced in polyethylenes of different molecular structure in various experimental conditions, i.e. varying the integrated dose, the dose rate and the environment (Dole, 1972; Clegg and Collyer, 1991). In particular, papers of some of the authors concern the modifications induced in the molecular structure and in the thermal, mechanical and rheological behaviour (Spadaro et al., 1989; Spadaro et al., 1992a; Spadaro et al., 1993). In previous papers (Spadaro et al., 1992b; Valenza et al., 1993a, b) we have discussed blends made with a polyamide 6 and gamma-irradiated in air low density and linear low density polyethylenes. These materials show marked modifications in the morphology with respect to the corresponding blends made with the unirradiated polyolefin, i.e. a more

uniform distribution and a finer dispersion of the polyethylene particles in the polyamide matrix with an improvement in the mechanical behaviour for some irradiation conditions. These effects were generally attributed to some interactions among the oxidized functional groups formed in the polyethylene chains and the functional groups of the polyamide. Following these results, the study of the gammaradiation induced compatibilization effects in polyethylene/polyamide systems was completed using different polyethylenes (linear low density, low density and high density) at various irradiation and processing conditions. EXPERIMENTAL

Materials

Commercial polymeric samples have been employed, whose main molecular characteristics together with the code are reported in Table 1. Irradiation procedure and polyethylenes characterization

Before the irradiation, 1 mm thick polyethylene samples were prepared from pellets by compression moulding in a laboratory press. The polymer was kept at 180°C and 15MPa for 5min and rapidly cooled down to room temperature by cold water running through the press plates. For IR analysis samples 5 mm thick were prepared with the same procedure. The irradiation was performed in air at room temperature (about 25°C) by the IGS-3 (Calderaro

207

208

G. Spadaro et al. Table I. Characteristics of materials used Sample code LLDPE LDPE HDPE PA6

Mw • 103 128 108 130 62

Mw/M .

Trademark

Manufacturer

4.4 9.8 10.5 2.2

BF2211 BB2700 ZB5015 ADS40

ENICHEM ENICHEM EN1CHEM SNIA

et al., 1980), a panoramic 3000 Ci 6°Co irradiator. The irradiation conditions were 10, 25 and 50 kGy at 0.1 kGy/h and 50 kGy at 0.04 kGy/h. The dose rates were measured by the Fricke dosimeter; a variance of 5% in the radiation absorption was accepted. IR analysis was made by a Perkin Elmer 1420 infrared spectrophotometer on 5 0 # m microtomed samples. All the polyethylenes were subjected to extraction in a soxhlet extractor in order to determine the presence of insoluble fractions. The samples were exposed to xylene close to its boiling point for 72 h. Thermal analysis was carried out by a Perkin Elmer DSC-7. The samples were heated up to 240°C and then cooled down to -10°C. Both the heating and cooling ramps were performed at 10°C/min. The crystallization temperature, T¢, and the crystallization enthalpy, AH¢, have been taken as the peak temperature and the area under the peak respectively. Rheological measurements were made with a Rheometrics Dynamic Analyser (RDA2) used in the dynamic mode with the plate and plate geometry (R 12.5 mm) at 240°C. -=

]o 5

~0 • [] • •

L ~A~,A L., ~A

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Unirradiated Irr. 10 kGy-0.1 kGy/h Irr. 25 kGy-0.1 kGy/h Irr. 5 0 k G y - 0 . 1 kGy/h I rr. 50 kGy-0.04 kGy/h

- -'"..

m\ m,,,, 10 2

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II

tilid

10 0

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10 2

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co (rad/s) Fig. 1. Flow curves for the unirradiated and irradiated LLDPE samples.

Blending procedure and blends characterization

Blends were prepared by a Brabender Plasticorder mod. PLE 330 at 240°C. The polyamide was carefully dried before mixing. The composition was 25/75 wt/wt (polyethylene/polyamide 6) in all cases and the mixing time, /mix, was usually 15 rain. Some blends were also prepared at trnix= 5 and 30 min. Pure materials were subjected to the same treatment. Thermal analysis and rheological tests were performed as for pure polyethylenes. SEM micrographs were obtained using a Philips Scanning Electron Microscope. The samples, fractured under liquid nitrogen, were also subjected to selective extraction of the polyethylene component by boiling xylene for 2 days, then were coated with gold under vacuum to make them electrically conductive.

RESULTS AND D I S C U S S I O N

Blends with L L D P E irradiated under different conditions

Before discussing of the blends, first we look at the effect of gamma-radiation on the pure LLDPE. As reported in Dole (1972) and Clegg and Collyer (1991), the irradiation of the polyethylene causes chain branching, crosslinking and oxidative degradation with formation of polar oxidized groups grafted to the polymer chains, whose extent depends

on the irradiation conditions. The oxidation can be controlled by the diffusion of the atmospheric oxygen inside the bulk of the material, depending on the irradiation dose rate and the thickness of the sample. On the basis of the quoted observations (Spadaro et al., 1993), we have considered irradiation conditions in which LLDPE essentially undergoes oxidative degradation. Furthermore IR analysis of microtomed samples (Spadaro et al., 1993) shows that the concentration of oxidized groups is almost uniform along the whole thickness of the irradiated polymer. Gel extraction confirms the absence of significant amount of crosslinking. In Fig. l LLDPE flow curves are shown for all the irradiation conditions. The main effects are the following: the increase of the slope of the curves for the irradiated polymers and the decrease of the melt viscosity increasing the dose and decreasing the dose rate. The enhancement of the non-Newtonian behaviour is related to the chain branching phenomenon (Acierno et al., 1985), whereas the molecular weight decrease causes the lowering of the melt viscosity. In particular the results indicate that the chain branching is the main effect up to the irradiation dose of 25 kGy, whereas the oxidative degradation prevails at 50 kGy and 0.04 kGy/h. At 50 kGy and 0.1 kGy/h the two phenomena are mostly balanced.

Physical and structural characterization of blends

Irradiationconditions Dose

(kGy) 0 l0

25 50 50

209

Table 2. Thermal analysisresultsfor the pure materials LLDPE HDPE LDPE

PA6

D o s e rate

Tc

AH¢

Tc

AH~

Tc

AHc

Tc

AHc

(kGy/h) 0 0.1 0.1 0.1 0.04

(°C) 98 98 97 97 96

(J/g) 115 105 120 120 130

(°C) 117

(J/g) 222

(°C) 98

(J/g) 121

(°C) 189

(J/g) 63

114

203

98

121

Calorimetric data, reported in Table 2, reflect the molecular modifications discussed above. In particular, in comparison with the unirradiated polymer, the crystallization temperature, To, is almost unaffected by the irradiation conditions. On the contrary some modifications are present in the crystallization enthalpy, AHc. It is lower for the irradiation condition of 10 kGy and 0. l kGy/h, whereas it increases up to values comparable to the unirradiated one for the irradiation conditions of 25 and 50kGy and 0.1 kGy/h; finally it is higher for 50kGy and 0.04 kGy/h. These results reflect the previously discussed molecular modifications induced in the LLDPE by gamma-radiation. Let us discuss now blends made with PA6 and irradiated LLDPE. Figure 2(a--e) shows SEM micrographs for all the blends prepared with t ~ = 15 rain. Starting from the micrograph of the blend made with LLDPE irradiated at 25 kGy we observe an almost uniform morphology where the dispersed phase is not detectable. However looking at the etched surfaces [Fig. 3(a--e)] we see that the main effect is a lowering of the particles dimension of the dispersed LLDPE phase in the PA6 matrix, together with a more uniform distribution. The increase of the oxidation and the decrease of the melt viscosity of the irradiated LLDPE enhance this effect. Flow curves for LLDPE/PA6 blends are shown in Fig. 4. The complex viscosity for blends made with irradiated polyethylene is always higher than the viscosity of the blend made with the unirradiated polymer. This rheological behaviour can be interpreted considering two different parameters. The increase of the LLDPE viscosity, related to molecular modifications, at low doses agrees with the higher values of the melt viscosity for the corresponding blends. The marked increase for the blend made with LLDPE irradiated at 50 kGy and 0.04 kGy/h can be attributed to the presence of some interactions among

the oxidized groups of the LLDPE and the polyamide. The calorimetric results of the blends are reported in Table 3. Thermograms always show two crystallization peaks clearly related to the crystallization of the polyamide and the polyethylene phases respectively, thus indicating the absence of cocrystallization phenomena, in good agreement with the "two phase" morphology of these blends. For all blends the crystallization temperature and enthalpy of the polyamide component do not show significant modifications with respect the pure polymer (see Table 2). Some effects can be observed for the LLDPE component. In particular the crystallization temperature is always higher than the corresponding value of the pure polymer, possibly because of the nucleating effect of the solid polyamide. Furthermore Tc decreases increasing the concentration of the oxidized groups, thus indicating that their interactions with the functional groups of the polyamide hinder the crystallization process.

Blends with different polyethylenes and PA6 We discuss now the influence of the molecular structure of the polyethylene considering blends made with PA6 and LLDPE, LDPE and HDPE irradiated at 50 kGy and 0.1 kGy/h. We have chosen this irradiation condition as the best compromise between chain branching and oxidative degradation for LLDPE (see Fig. 1). The effect of irradiation on flow curves of both pure HDPE and LDPE is shown in Fig. 5(a, b). The decrease of the melt viscosity indicates a prevailing effect of the oxidation, which is more marked for high density polyethylene. A more careful investigation of the gamma-radiation induced molecular modifications of the three different polyethylenes was studied elsewhere (Spadaro et al., 1993, 1994). In particular in Fig. 6 we show the carbonyl index, measured in arbitrary units, as a

Table 3. Thermal analysisresultsfor LLDPE/PA6blends Irradiationconditions LLDPE/Pa6 Dose (kGy)

Dose rate (kGy/h)

0 10

0 0.1

25 50 50

0.1 0.1 0.04

Tc (PA6) (°C) 189 190 190 189 190

AHc (PA6) (J/g) 62 63 64 66 66

Tc (LL) (°C) 108 107 108 105 105

AHc (LL) (J/g) 110

115 120 120 125

210

G. Spadaro et al.

Fig. 2. SEM micrographs for the LLDPE/PA6 blends: (a) unirradiated; (b) 10 kGy; 0.1 kGy/h; (c) 25 kGy, 0.1 kGy/h; (d) 50 kGy, 0.1 kGy/h, (e) 50 kGy, 0.04 kGy/h.

function of the distance from the external surface for polyethylene samples irradiated at 50 and 0.1 kGy/h. We observe a marked difference among the gammaoxidized HDPE and the other polymers. For HDPE the oxidation essentially occurs in the external layers up to a depth of about 250 #m, whereas crosslinking phenomena occur in the bulk. This is in agreement with solubility tests which indicated the formation of about 10% of gel. For LDPE and LLDPE, on the contrary, an almost constant concentration of oxidized groups up to a depth of about 500 p m was seen

and they were completely soluble. This different effect was attributed to the higher crystallinity degree for HDPE which causes a diffusion controlled kinetics of the oxidative reactions with a consequent less homogeneous distribution of the oxidized groups in the bulk of the polymer. The calorimetric results are reported in Table 2. We can say that for LDPE the graft of the irregular oxidized groups in the polymer chains does not significantly modify the crystallization ability of the branched chains. On the contrary, for HDPE, the

Physical and structural characterization of blends

211

Fig. 3. SEM micrographs for the LLDPE/PA6 etched blends: (a) unirradiated; (b) I0 kGy; 0.1 kGy/b; (c) 25 kGy, 0.1 kGy/h; (d) 50 kGy, 0.1 kGy/h, (e) 50 kGy, 0.04 kGy/h. oxidized groups grafted to the linear chains decrease T¢ and the presence of gel lowers the crystallization degree. SEM micrographs for the blends made with all the irradiated polyethylenes are shown in Fig. 7. The irradiation causes qualitatively similar modifications in the morphology, i.e. a lowering of the polyethylene particles dimension and a more uniform distribution in the polyamide matrix. A more careful looking evidences some differences of the morphology for HDPE/PA6 blend: in particular

we observe a less homogeneous dispersion of the polyethylene "phase" in the polyamide. This effect can be related to the before discussed distribution of the oxidized groups in the irradiated HDPE samples and to the presence of crosslinking. Flow curves, reported in Fig. 8(a) and (b), agree with the presence of interactions between the irradiated polyethylenes and the polyamide, which cause a melt viscosity increase despite the corresponding low values of the pure irradiated polymers.

212

G. Spadaro et al. 105

Blends prepared at different mixing time O LLDPE unirradiated • L L D P E irr. 10 k G y - 0 . 1 k G y / h

1:2 LLDPEirr. 25 kGy-0.1 kGy/h • LLDPEirr. 50 kGy-0.1 kGy/h • LLDPEirr. 50 kGy-0.04 kGy/h 10 4

10 3

10 2

i l lliHtl

10 -1

,.,.,.I

,

10 0

,...,.I

101

I i lllllll

10 2

10 3

tO(rad/s) Fig. 4. Flow curves for LLDPE/PA6 blends.

In Tables 4 and 5 the calorimetric data of these blends are reported. The most evident result is a decrease of the AH~ for the blend containing irradiated LDPE. This appears as an indication of a strong interaction in the solid state.

The change of the mixing time allows to obtain additional informations about the interactions observed in the system. In Fig. 9 SEM micrographs of the blends made with PA6 and both unirradiated and irradiated LLDPE at three different mixing times are shown. The mixing time does not affect the morphology of the blends. The samples made with the unirradiated polymer always show the typical morphology of incompatible blends. An improvement of the two component distribution was observed in the blends made with the irradiated polyethylene, whatever the mixing time. The mixing time affects the rbeological behaviour as shown in Fig. 10. To better discuss these results we report in Fig. l I ~/* as a function of the mixing time at to = 0.1 rad/s. For blends made with the unirradiated polymer we observe a slight increase of the viscosity with t ~ and we relate this effect to thermo-mechanical oxidation. For blends made with irradiated LLDPE a sudden increase at t~ax= 5min is obtained, whereas for the other mixing times this effect is less evident. This means that in blends made with irradiated polyethylene short mixing times are enough to allow the interactions between the functional groups of the two polymer components to occur, while the increase of tmix causes a mechanical destruction of the interactions, maintaining the dimensions of the particles of the dispersed phase.

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10 |

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10 3

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tO (rad/s) Fig. 5. (a) Flow curves for the unirradiated and irradiated HDPE samples. 0a) Flow curves for the unirradiated and irradiated LDPE samples.

i

I

103 0

200

400

600

800

Depth (gm) Fig. 6. Carbonyl index vs the depth for the all the polyethylenes irradiated at 50 kGy, 0.1 kGy/h.

Physicaland structuralcharacterizationof blends LLDPE/PA6

213 HDPE/PA6

LDPE/PA6

Fig. 7. SEM micrographs for LLDPE/PA6, LDPE/PA6 and HDPE/PA6 blends. (a) Unirradiated polyethylene,(b) 50kGy, 0.1 kGy/h. (a) 105

----~

HDPE

• Irr. 50 kGy-0.1 kGy/h 0 Unirradiated

-

10 4

~.. 103 102 10-1

J

i iJllliJ

i

i Jlllzll

100

i

i Liliitl

101

i

i llllJl]

102

103

o~(rad/s)

(b) 105 ----

LDPE

_~3.0~ _

0 Unirradiated

104 ~

~-- 103 102 10-1

I

I llllJll

,

100

,~JJ~,l

~

101

, ,,,,,,I

,

102

J ,,t,,,I

103

O~(rad/s) Fig. 8. (a) Flowcurvesfor HDPE/PA6blend. (b) Flowcurvesfor LDPE/PA6blend.

214

G. Spadaro et al.

Fig. 9. SEM micrographs for LLDPE/PA6 blends at different mixing times: (a) 5 ,rnin, (b) 15 min, fc) 30 min.

215

Physical and structural characterization of blends Table 4. Thermal analysis results for LDPE/PA6 blends Irradiation conditions

LDPE/PA6 Dose rate (kGy/h) 0 0.1

Dose (kGy) 0 50

Tc (PA6) (°C) 191 190

AHc (PA6) (J/g) 62 63

Tc (LD) (°C) 100 101

AHc (LD) (J/g) 120 90

Table 5. Thermal analysis results for HDPE/PA6 blends Irradiation conditions

HDPE/PA6 Dose rate (kGy/h) 0 0.1

Dose (kGy) 0 50

Tc (PA6) ('C) 191 191

AHc (PA6) (J/g) 61 65

Experimental results reported in this work confirm t h a t the m o r p h o l o g y o f polyamide/polyethylene blends can be significantly modified by grafting polar oxidized groups in polyethylene chains t h r o u g h g a m m a - i r r a d i a t i o n in air. Some differences are observed varying the structure of the polyethylene. W e see in particular a less h o m o g e n e o u s dispersion o f the polyethylene " p h a s e " in the matrix for H D P E / P A 6

In~m'm,,,m~. ~.A A

,.

•

Tmi x 30 m i n

:i ;m:',:mm::

~ _ ~ ,

\m \

O Tmix 30 min 102 10 -n

J

I []L]l[[ 100

1

0

] 5

I 10

I 15

] 20

[ 25

I 30

Tmix (min)

\m\ m

•

t~ t~ a_

Fig. 11. Complex viscosity at co = 0.I rad/s vs the mixing time for LLDPE/PA6 blends.

X~~O,~.

Tmix 5rain

0 Unirradiated LLDPE A Irradiated LLDPE

IE+03

m~,.

n

AHc (HD) (J/g) 130 135

1E+04

CONCLUDING REMARKS

10 4

T~ (HD) (°C) 116 126

J IIIIIIJ

I I llIttll

i01

102

[

L InnJlll 103

(rad/s) Fig. 10. Flow curves for polyethylene/polyamide blends at different mixing times; the open symbols refer to blends with unirradiated LLDPE and filled symbols to blends with irradiated LLDPE.

blends a n d we relate this b e h a v i o u r to the not uniform distribution of the functional groups. The modifications o f the m o r p h o l o g y due to the " c o m p a t i b i l i z a t i o n " p h e n o m e n a affect the rheological b e h a v i o u r o f the blends. T h e r m a l analysis always shows the presence o f two distinct crystalline phases without cocrystallization p h e n o m e n a with slight effects in the crystallization t e m p e r a t u r e a n d enthalpy for some experimental conditions. Finally tests performed at different mixing times indicate t h a t very short times are e n o u g h to allow the previously discussed interactions to occur.

Acknowledgements--This work has been partially financially supported by MURST and partially financially supported by C.N.R. Progetto Finalizzato Chimica Fine II. The authors wish to thank Dr L. Sarcinelli for carrying out the IR analysis reported in Fig. 6.

216

G. Spadaro et al. REFERENCES

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