An investigation on rheological and impact behaviour of high density and ultra high molecular weight polyethylene mixtures

Pergamon PII:

SOO14-3057(96)00115-2

Eur. P&n. J. Vol. 33, No. 1, pp. 97-105, 1997 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0014-3057/97 $17.00 + 0.00

AN INVESTIGATION ON RHEOLOGICAL AND IMPACT BEHAVIOUR OF HIGH DENSITY AND ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE MIXTURES A. BOSCOLO

BOSCOLETTO,

R. FRANCO,*

M. SCAPIN

and M. TAVAN

Centro Ricerche EniChem, Pot-to Marghera, Venezia, Italy (Received 24 July 1995; accepted in final form 8 December

1995)

Abstract-Blends of high density polyethylene (HDPE) and ultra high molecular weight polyethylene (UHMWPE), with a content of UHMWPE up to 20% wt, were prepared using two different types of processing apparatus, a single screw extruder and an internal mixer, as a function of mixing time. A detailed morphological investigation revealed that UHMWPE is only partially dissolved, even under the most intensive and extended mixing conditions. Evaluation of melt viscosity of the blends showed a double effect of UHMWPE due to its slight dissolution and to its presence as a filler. A further analysis of the experimental data, confirmed that UHMWPE, with respect to its initial content, dissolved up to about 3% only. The dissolution effect is also validated by both impact strength behaviour, increasing with the amount of dissolved UHMWPE, and a crystallization kinetics study by differential scanning calorimetry (DSC). Copyright 0 1996 Elsevier Science Ltd

having the following characteristics: d = 0.94 g/cm), MVI < 0.01 cm’/(lO min) and [q] = 12 dL/g according to ASTM 4020. Samples named HI + H9 were prepared using a Haake Rheocord 90 plastograph, equipped with two high shear rotors, in the following conditions: initial temperature of chamber T, = 19O”C, mixing time up to 30min, rotor speeds of SO, 100 and 200rpm. Composition, MVI, viscosity and operative conditions are summarized in Table 1. The polymers for the blends were introduced in one step, as referred by Kyu and co-workers [IO, 111. The sample series named BAI + BA6 and BBl + BB6, as reported in Table 2, were prepared using a Brabender PL2000 PlastiCorder. For the first series (BA) the apparatus was equipped with single screw extruder (Mod. g-320) with a 19/25 D barrel and a 30/2 L/D round capillary die. Working parameters were chosen as follows: barrel temperature Ts = 190°C screw speed u = 100 rpm. The second series (BB) were obtained by reprocessing the first one using the apparatus in the mixing head configuration, equipped with a couple of high shear W50 mixer roller type rotors. In this case, working temperature was 19OC, rotor speed 100 rpm and mixing time 30 min.

INTRODUCIION

Interest towards UHMWPE has grown in recent years as a result of the useful set of properties (such as high toughness, abrasion resistance and autolubricant behaviour) that this polymer can offer. Several studies have been performed on characterization [l-6] and technological application [7, 81 of the material, which, however, cannot be easily processed by means of conventional techniques, like injection moulding and extrusion, because of its extremely high viscosity. In the present work, UHMWPE was added to HDPE in the range t&20% wt. Evaluation of this blend, in comparison with previous investigations [9] points out the double role of UHMWPE, both as dispersed and as dissolved phase in the HDPE matrix. The HDPEjUHMWPE mixtures were examined as a function of both mixing time and UHMWPE content. Dissolution degree was analysed with respect to rheological and impact behaviour. Investigations of morphology and crystallinity of the two phases were also performed.

EXPERIMENTAL

Rheological and mechanical measurements MVI measurements were performed by using a Gottfert MPS-2 apparatus, according to ASTM D1238, at 190°C and 2.16 kg; from these values, apparent viscosities were calculated. Specimens for mechanical characterization were obtained from 3.2 mm thick compression moulded sheets. A tensile test was performed, according to ASTM D1708, by means of an Instron 4505 dynamometer, working with a cross head speed of lOmm/min. Sample toughness was measured at 23°C according to ASTM D256 (notched Izod impact test), using a Ceast digital pendulum.

PROCEDURE

Materials and blending procedure HDPEjUHMWPE blends were prepared with two commercial polymers: HDPE pellets (EniChem EracleneTM MM95) with density d = 0.95 g/cm3, melt volume index (MVI) = 4.8 cm3/( 10 min), [nl = 1.5dL/g according to ASTM 4601 and UHMWPE powder

*To whom all correspondence

should

be addressed. 97

98

A. Boscolo

Boscoletto

et al.

Table I. Comoosition. treatment and rheoloeical characteristics of H samoles HDPE [wt%]

Samples Hl HZ H3 H4 H5 H6 Hi7 H8 H9

100 100 100 100 90 90 90 90 90

UHMWPE [wt%] 0 0 0 0 IO IO IO 10 IO

Mixing rate [RPM1 50 50 100 200 50 50 SO 100 200

Mixing time [min]

MVI [cm’/10 min]

v* [Pa sl

5 30 30 30 3 15 30 30 30

4.9 5.1 5.2 5.3 2.9 2.3 1.9 1.4 I.1

2160 2080 2040 2000 3650 4600 5570 7650 9620

*Viscosity value obtained from measure of MVI. at constant shear stress of 19.39 kPa.

Optical and electron microscopes The homogeneity level of blends and UHMWPE phase distribution were examined by means of Zeiss FOMI III microscope, working with cross-polarized light on 1 pm thick microtomed sections, directly extracted from blends. The diameter distribution of UHMWPE powder was obtained by a Kontron Vidas image analyser interfaced to a microscope via CCD camera and operating in a semi-automatic manner. Due to experimental resolution and sample features, an inferior limit of 20 pm for the dimensional analysis was fixed. Micro-morphological analysis of fracture surfaces, from notched Izod, was performed by using a digital scanning electron microscope Zeiss DSM 950. Differential scanning calorimerr~ Differential scanning calorimetry (DSC) measurements were performed by using a Perkin-Elmer DSC 7 instrument. Samples of about 15 mg were analysed at heating and cooling rates of S”C/min under a nitrogen flow of 35 mL/min in the temperature range between 50 and 150°C. In order to analyse the crystallization kinetics of the blend, the samples were first heated up to 150°C (first scan), held at this temperature for 3 min, cooled down to 50°C and then re-heated (second scan).

RESULTS Rheological

and impact behaviour

and HDPEjUHMWPE mixtures processed by the Haake plastograph (H series), gave a first insight into the rheological properties of blends. All the rheograms showed an initial torque peak, due to the material feeding, followed by a rapid decrease within the first 1.5 min and then by a second lower decrease down to achieve a plateau value. The peak can be explained Pure

HDPE

considering the transition from the solid to the liquid state, through the following different steps: fluid with solid suspended polymer phase, fractured materials or semi-fluid, and pasty-like material [12]. The building up of both torque momentum and temperature profile with increasing rotor speed was measured. Constant values were achieved after 8 min but with higher absolute values of torque and temperature, in the case of HDPEjUHMWPE blends, owing to the presence of the UHMWPE phase. In order to collect information about blend modifications induced by processing, samples were prepared at the same rotor speed (50 rpm) and different mixing time. The MVI data, reported in Table 1, are substantially constant in the case of pure HDPE (Hl + H2) and decrease for HDPE/ UHMWPE system as a function of mixing time (H5 + H7). This could indicate on one hand a substantial stability of the HDPE matrix and, on the other, an increase of compatibility between the two phases with the occurrence of interdiffusion phenomena. This fact can also be confirmed by observing the behaviour of MVI at constant mixing time and increasing rotor speed (H2 + H4 and H7 + H9). Optical micrographs of samples HS and H7, represented in Figs l(a) and (b), respectively, show the presence of a UHMWPE dispersed phase and, in particular, the morphological differences in the particle surfaces due to different mixing treatment. While the first picture exhibits a rough particle border, similar to that observed by SEM in the original UHMWPE powder, the second one indicates a smooth ellipsoidal profile. In Fig. l(a) there is also evidence of a good surface wettability of particles, which are completely

Table 2. Composition and rheological characteristics of B samples Extruder UHMWPE iwt%l 0

2.5 5 IO 15 20

Brabender Plasti-Corder PL2Otl configurations Mixer head

Sample

MVI Icn?/lO min]

BAl BA2 BA3 BA4 BA5 BA6

4.7 4.2 3.8 2.9 2.2 1.4

2250 2520 2780 3650 4810 7560

Sample

MVI [cm’/10 min]

BBI BB2 BB3 BB4 BBS BB6

4.9 3.8 2.8 1.1 0.4 0.1

*Viscosity value obtained from measure of MVI, at constant shear stress of 19.39 kPa.

‘1’ [Pa sl 2160 2780 3780 9620 26400 106000

An investigation

I Fig.

1. Micrographs

of HDPE-UHMWPE

99

I of (aI) H5 and (B) H7 samples.

embedded; in addition to this feature, Fig. l(b) makes clear the effect of an extended action of shear stress on the system. Further effects of shear stress action are detectable from the variation of dimensional distribution of particles with a decrease of both maximum diameter value, from

Fig. 2. Micrographs

mixtures

300 to 200 pm, and the corresponding minim1 Irn value, as a function of mixing. This diame ter reduction could be attributed to the formation of collapsed particles with a lower surface al+ea and containing a small amount of HDPE diffus ;ed in the early stage of blending. In fact, Ithe

of native

UHMWPE

powder.

A. Boscolo Boscoletto et al.

100 ALI”-

o . 50 RPM A . 100 RPM 0 n 200 RPM

g : S m 2 0 8 z

4

A1 /j/l 100 90-

0

10

20

30

Mixing time (min) Fig. 3. Notched Izod of H series vs mixing time at different rotor speeds. Open symbols refer to pure HDPE, while full symbols refer to HDPEjUHMWPE mixture. Error bars are two standard deviations long.

micrographs of Fig. 2 show, at two different magnifications, the morphology of the particle surface before blending; UHMWPE particles appear to be composed of a large number of smaller units with the presence of micro-voids among them. This permits the good wettability previously observed and the HDPE incorporation. Therefore, the UHMWPE in the blend plays an important role both as high molecular weight chain tails dissolved in the HDPE continuous phase [9] and as particles suspended phase. In Fig. 3 the Izod impact data of H series samples are reported as a function of mixing time and different rpms. In the case of pure HDPE (open symbols), a near-constant Izod value is measured. On the contrary, the HDPEjUHMWPE specimens (full symbols) exhibit an increase of considering longer mixing time, Izod value, achieving up to about 200 J/m for samples H7 + H9.

20(

. 50 RPM . 100 RPM . 200 RPM 3 2 s c? m g lO( 0 5 9( z 8(

:,

,

,

2

3

-‘)f$,;,,,

7( 1

4

5

6

7

MVI (cc/l0 min) Fig. 4. Notched Izod of H series vs MVI at different ro~ur speeds. Open symbols refer to pure HDPE, while full symbols refer to HDPEjUHMWPE mixture. Error bars are two standard deviations long.

Fig.

5. Micrographs

of H5 sample, cations.

at different

magnifi-

An investigation of HDPE-UHMWPE This value is about twice that obtained from pure HDPE. Figure 4 shows that these Izod values are related with MVI reported in TabIe 1 through a log-log linear dependence. Morphological investigations were performed on the fracture surface of specimens; in Figs S(at(c) the main aspects of the H5 specimen, relative to the early stage of blending, are displayed. It is evident on one hand that there is a good interfacial affinity of the UHMWPE particle, perfectly embedded in the HDPE matrix phase, as previously mentioned, and, on the other hand, a high plastic deformation in the central zone due to impact [Fig. 5(a),(b)]. Moreover, in the image at higher magnification [Fig. 5(c)], the formation of circular concentric striations, which are thicker and thicker moving from the external zone to the central one, and giving rise to the central prominence, clearly appears. On the basis of observations, the mechanism of these thefracture can be briefly described in the following picture.

This fracture behaviour can be explained in terms mechanism, in which the of a “crack-bridging” dispersed particles are plastically deformed and then torn [13]. The micrograph of Fig. 6, relative to the H7 specimen show, in agreement with previous observations and the described fracture mechanism, the ellipsoidal shape of the particle, due to the prolonged action of shear stress. On the basis of these observations the toughness improvement for H7 samples might have been

600 -

500 -

400

_$ 2 p

300

101

mixtures o BA series l BB series

;;;;

I 0

I 5

I 10

I 15

I 20

Weight % UHMWPE Fig. 7. Notched Izod of BA and BB series vs UHMWPE % wt. Error bars are two standard deviations long.

attributed to either a more efficient impact energy absorption from UHMWPE particles or modified and enhanced mechanical properties of the HDPE matrix as a consequence of the interdiffusion phenomena [ 141. In order to better investigate the effect of dispersed UHMWPE particles and their dissolution in the system, the B-blend series were prepared by Brabender apparatus as a function of LJHMWPE content. Figure 7 shows a roughly linear behaviour of Izod values vs UHMWPE percent weight fraction, but with a higher slope in the case of BB series with respect to BA. From these data it seems reasonable that the increase in toughness is due principally to the fraction of UHMWPE dissolved into the HDPE matrix. In analogy with the previous observations, Izod values were related to MVI and the result is represented in Fig. 8. The obtained results, in the H and B series, also considering different relative Izod values due to the processing apparatus, suggest a detailed evaluation of the amount of UHMWPE really dissolved, as reported in Ref. [9].

o BA series l BB series

MVI (ccl10 min)

Fig. 6. Micrograph of H7 sample

Fig. 8. Notched lzod of BA and BB series vs MVI at different rotor speeds. Error bars are two standard deviations long.

102

A. Boscolo Boscoletto ef al. (a)

(b)

50

A

h, = l/[ 1

20 -

- (w/A)]

A = 0.519, sA = 0.003

0 Single screw extr. l

.

Plastograph

40 -

blend series BA series BB series

0 Dry l

A

30 -

15 -

2

6 20 -

A 10 A * T 0.05 w

0

Fig. 9. (a) Relative

Interpretation

‘r 0.10

Y 0.15

9 0.20

0

I 5

I 10

UHMWPE

I 15

c (g/dl)

of dry blend, BA and BB series vs UHMWPE content; (b) relative viscosity increment vs UHMWPE content purely due to its dissolution.

viscosity

of rheological

data

In order to evaluate dissolution of the UHMWPE phase, melt viscosity of the blends could be described by the following equation ? = r?ef(cp)g(c)

(1)

where ‘lo represents the Newtonian viscosity of pure HDPE matrix and f and g are related to the presence of UHMWPE as dispersed phase and as dissolved ultra-high molecular weight tail in the HDPE matrix, respectively. In Fig. 9(a) the relative viscosity values Q are reported as a function of UHMWPE volume fractions cp for the three following systems: dry physical blend of powders, BA and BB series. Assuming that during the MVI measurement of the dry physical blend of powders no dissolution of UHMWPE occurred, the qr values of the dry physical mixtures were fitted by means of the Pierce equation [ 151

UHMWPE content higher than 10% wt, with respect to physical dry blend values. This fact could indicate that also in poor mixing conditions a migration of HDPE chains into UHMWPE phase and/or an initiating dissolutive process is present at a higher UHMWPE content. This effect may also be observed, with more emphasis, in the BB series samples; in this case, higher values of q1 were measured at higher residence time and more critical mixing conditions than those obtained for the BA series. On the basis of this evidence, and assuming that no further change in dimensional distribution of UHMWPE particles occurs after the first stage of mixing, an estimation of the amount of dissolved UHMWPE was attempted. At first, qCexperimental values of BA and BB series were divided by the corresponding dry blend ones [Fig. 9(a)] to calculate the g(c) factors, represented in Fig. 9(b). Then, the following equation g(c) = 1 +

c[rll~exp(l.l9c[ql0),

(2) where cp is the volume fraction of suspended phase and A represents the maximum packing volume fraction. The non-linear interpolation of experimental data gives an A value of 0.519 with a standard deviation of 0.003; the result is qualitatively in agreement with theoretical values** of maximum packing values [16], also considering the fact that A depends on dimensional distribution of the particles and on their shape [ 171. Experimental data in Fig. 9(a), relative to the BA series, show a positive deviation of viscosity for **For example, in the case of homogeneous spheres in a cubic lattice,

drn assumes

the value of 0.524.

(3)

derived for concentrated polymeric solutions at the &Flory condition [9], was utilized to estimate, point by point, the UHMWPE dissolute percentage content. In our case, [U]S= 3.39 [dL/g] and c [g/dL] represent, respectively, 0 intrinsic viscosity of

Table 3. Amounts c (total) [wt%] 0 2.5 5 10 15 20

of UHMWPE effectively dissolved mixtures

c (total)

c (BA dissolved)

in BA and BB

C (BE dissolved)

[g/dLl

k/dLl

[g/dLl

0 2.38 4.76 9.52 14.27 19.01

0% 0.00 0.02 0.03 0.06

0.04 0.09 0.23 0.34 0.56

An investigation of HDPE-UHMWPE Table 4. DSC analvsis

132.0 138.6 132.4 133.0

162 173 190 189

*Crystallinities

were calculated

55 59 65 64

112.7 117.3 117.2 119.2

Interpretation

Second scan

184 130 196 197

130.6 129.0 134.6 134.5

using AHr = 293J/g of pure HDPE crystal,

UHMWPE in HDPE solvent and its concentration. The concentration values so obtained are reported in Table 3; it may be observed that only a low percentage, up to 3% of the UHMWPE originally introduced in the blend, is effectively dissolved. of DSC curves

In order to analyse the crystallization kinetics and obtain information on the role of processing, DSC experiments were performed, whose results are reported in Table 4. Pure HDPE exhibits the maximum endotherm peak at 132.O”Cand 130.6”C for first and second scans, respectively, while UHMWPE gives 138.6”C and 129.O”C. This behaviour of UHMWPE could be explained on the basis of a structure modification of nascent polymer from an initial extended-chain configuration to a folded-chain one, with a relative reduction of crystallinity [I]. Maximum exotherm peaks at 112.7”C and 117.3”C were found for crystallization of HDPE and UHMWPE. BA6 and BB6 samples gave single melting endotherm peaks at about 133°C and 135°C for

103

samoles

Crystallization

First scan

HDPE UHMWPE BA6 BB6

of indicated

mixtures

194 125 206 209 reported

66 43 70 71

in Ref. 1211.

first and second scans, respectively. Also, the normalized areas under the endotherm curve and full width half maxima (FWHM) for each respective scan had approximately the same values for the two samples, and a sightly higher enthalpy value at the second scan indicates a higher crystallinity. The fact that BA6 and BB6 samples gave single melting endotherm peaks at the same temperature could be explained by considering either that HDPE and UHMWPE components present second scan endothermic peaks at similar temperature or that a partial co-crystallization could have occurred, as observed by Ueda et al. [18] for UHMWPE/LDPE or by Kyu and Vadhar [lo] for samples prepared by means of bi-steps method. The crystalhnity values AX relative to the first and second scans, reported in Table 4, summarize these results. Figure 10(a) shows the DSC crystallization curves for pure HDPE in comparison with UHMWPE. UHMWPE exhibits an exothermic peak at higher temperature than HDPE, but with lower crystallinity, as indicated by AH values in

(4

; -2.0-

----

HDPE UHMWPE

-2.q

80

,

,

100

120

(b)

3

2

-15-

;;

g -2.0-

-

BA6

----

BB6

-25 -

Temperature

(“C)

Fig. 10. Comparison between DSC crystallization curves: (a) UHMWPE and HDPE; (b) BA6 and BB6 blends. EPJ 33,1-E

A. Boscolo Boscoletto

104

Table 4. The initial part of the curve, starting at 126”C, could be related to the crystallization of ordered structures not completely destroyed by previous fusion, while the following sharp change of slope at 122°C emphasizes a change in crystallization kinetics. Figure 10(b) displays the DSC crystallization curves for HDPEjUHMWPE BA6 and BB6 blends. In the case of BA6 the exotherm peak exhibits a convoluted peak profile due to mutual effects of HDPE and UHMWPE: the maximum temperature value at 1 17.2cC corresponds to that of pure UHMWPE, but a shift of HDPE polymer contribution towards higher crystallization temperature is seen from the shoulder centred at 115°C. The other important feature of this ycurve is given by a second prominent shoulder centred at 122°C that can be related to the more perfect crystal population of UHMWPE observed, but with higher intensity due to HDPE crystallizing over. The appearance of this dual polymer contribution at the exothermic peak could indicate bothpoor mixing and separate crystal formation in agreement with the findings of Vadhar and Kyu

1111. On the contrary, the exotherm peak of the BB6 sample appears as a single peak shifted to higher temperature with a maximum at 119.2”C, having an onset crystallization temperature at 123”C, as for the BA6 sample. This behaviour points out that massive HDPE crystallization occurs at a higher rate than the pure UHMWPE as result of a nucleating action with shifting of HDPE crystallization temperatures. In fact, it is also possible to note that the crystallization end profile of the BB6 exotherm peak is superimposed on that of UHMWPE to indicate the severe influence of UHMWPE on blend crystallization. Moreover, the normalized areas under the exotherm and endotherm curves for both BA6 and BB6 samples are larger than those calculated from additivity rule to indicate a positive UHMWPE effect for crystallinity development in the blend. From these data, nucleating action by UHMWPE and co-crystallinity between the two polymers can be inferred. Two different types of UHMWPE nucleating centres are present: one is given by crystallizing chains at the particle surface that give rise, at a lower mixing time, to a major contribution to the HDPE crystallization fastening, the other is given by the dissolved molecules, resulting in a proportional relationship to the blending time and diffusion process.

CONCLUSIONS

presence of UHMWPE particles as a filler, revealed a partial dissolution of the higher molecular weight phase. Analysis of rheological data showed, for the samples obtained by the most intensive shearing treatment, a dissolution of UHMWPE up to 2.6% wt of the original amount. DSC analysis confirmed this dissolution, by indicating the occurrence of co-crystallization between the two PE phases and a faster rate of HDPE crystallization induced by the presence of UHMWPE. Moreover, the data indicate that the occurrence of separate crystallization or co-crystallization is strictly dependent on the experimental conditions of mixing for the UHMWPE/HDPE blends. Morphological examinations of fractured surfaces revealed very good adhesion between UHMWPE particles and the HDPE matrix, and a large deformation of the dispersed phase induced fracture propagation. Nevertheless, the by UHMWPE chains dissolved in the HDPE matrix seem to play the main role in toughening, since a rather good correlation between the amount of the dissolved UHMWPE and impact strength was found. Comparing Izod vs MVI for the H, BA and BB series with literature data for commercial grades of pure HDPE (EracleneTM), the general trend of increase of toughness with viscosity is preserved [19, 201. So, the effect of UHMWPE dissolution on toughness seems to be overwhelming with respect to its presence as dispersed phase.

REFERENCES

1. Wang, X. and Salovey, R., J. Appl. Pol.ym. Sci., 1987, 34, 593. 2. Zacharariades, A. E., Polym. Eng. Sci.. 1985, 25, 147. 3. Zacharariades, A. E., Polym. Eng. Sci., 1986. 26, 658. 4. Zacharariades, A. E., J. Polvm. Sri. Phvs. Edn, 1983, 21, 821. 5. Wang, X., Li, S. and Salovey, R., J. Appl. Polym. Sci., 1988, 35, 2165. 6. Lupton, J. M. and Regester, J. W., J. Appl. Polym. Sri., 1974, 18, 2407. 7. Gauvin, R., Nguyen, Q. X. and Chalifoux, J. P., Polym. Eng. Sri., 1987, 27, 524. 8. Birnkraut, W. H., Braun, G. and Falbe, J., J. Appl. Polym. Sci.: Polym. Sci. Symp., 1981, 36, 79. 9. Dumoulin, M. M., Utracki, L. A. and Lara, J., Polym. Eng. Sci., 1984, 24, 117. 10. Kyu, T. and Vadhar, P., J. Appl. Polym. Sci., 1986,32,
HDPEjUHMWPE blends were analysed, paying particular attention to the effects of UHMWPE content and mixing procedure on rheological and impact properties. The morphological analysis showed the presence of UHMWPE particles, whose dimensional distribution was comparable with that of native powder. An increase of melt viscosity with respect to pure HDPE matrix, which cannot be justified by the

ef al.

12. 13. 14. 15.

Vadhar, P. and Kyu, T., Polym. Eng. Sci., 1987, 27, 202. Shih, C. K., Tynan, D. G. and Denelsbeck, D. A., Polym. Eng. Sci., 1991, 31, 1670. Pearson, R. A. and Yee, A. F., Polymer, 1993, 34, 3658. Wu, S., Polymer Inlerface and Adhesion. M. Dekker, New York, 1982. Dealy, J. M. and Wissbrun, K. F., Melf Reology and ifs Role in Plastic Processing. Van Nostrand Reinhold, New York, 1990.

An investigation of HDPE-UHMWPE 16. Utracki, L. A., Polymer Alloys and Blends. Hanser, Munich, 1989. 17. Malkin, A. Y., Adu. Polym. Sci., 1990, 96, 82. 18. Ueda, H., Karasz, F. E. and Farris, R. J., Polym. Eng. SC;., 1986, 26, 1483.

mixtures

105

19. EniChem EracleneTM (HDPE), Technical Bulletin, 1991. 20. Margolies, A. F., SPE Journal, 1971, 27, 44. 21. Mandelkern, L. and Fatou, J. C., J. Polym. Sci., 1965, B3, 803.