Thermomechanical and some mechanical properties of irradiated films prepared from polymer blends

Polymer Degradation and Stability 23 (1989) 217-226

Thermomechanical and Some Mechanical Properties of Irradiated Films Prepared from Polymer Blends

L. Minkova* & M. Nikolova~ Scientific and Industrial Enterprise of Electron Processing of Materials, 34, 30-20 Str., 1324 Sofia, Bulgaria (Received 26 April 1988; accepted 9 May 1988)

ABSTRACT Non-oriented films prepared from EVA-iPP and LDPE-iPP blends, irradiated with .fast electrons, have been examined by thermomechanical analysis ( T M A ) and some mechanical properties (tensile strength and elongation at break) have been determined. The irradiation was carried out at low doses (5-12 Mrad) which brings about the formation of 50~50% gel content in the material The thermomechanical curves of the irradiated films exhibit a plateau of a high elastic material after the segregation melting of the components, whose elastic modulus E depends on the composition and the irradiation dose. The density of cross-linking, evaluated qualitatively from E, increases with increase of D and the increase of the EVA and LDPE contents in the blends. The tensile strength does not depend on the blend composition and the dose, while the elongation at break decreases with increase of D. The peculiarities in the dependence ~f the thermomechanical and mechanical properties on the dose and the composition have been explained by the formation of a statistical network with a broad distribution of the lengths between cross-links. The properties of the samples depend on the density of cross-linking, which is determined in turn by the balance between crosslinking and scission in the separate components. * Present address: Central Laboratory for Polymers, Bulgarian Academy for Sciences, Sofia 1040, Bulgaria. To whom all correspondence should be addressed. 217

Polymer Degradation and Stability 0141-3910/89/$03-50 ~3 1989 Elsevier Science Publishers Lid, England. Printed in Great Britain

218

L. Minkova, M. Nikolova

INTRODUCTION Polymer blends have recently become an important new area of commercial application and scientific research. It has been demonstrated in a large number of patents that the properties of films prepared from various blends of polyethylene, ethylene-vinyl acetate copolymers (EVA), polypropylene (PP), etc., are often more valuable than those of the parent homopolymers. Almost no data can be found in the literature on the effect of irradiation on the phase composition and structure of films prepared from polymer blends. However, there have been a number of investigations on the radiation effect in films prepared from homopolymers. For example, in some of our previous studies on low density polyethylene (LDPE) films irradiated with fast electrons up to doses of 40 Mrad we have established that crosslinking takes place predominantly in the amorphous regions and at the crystallite surfaces. / The melting temperature and the crystallinity of the films do not change with irradiation dose D. After the melting of the films a highly elastic material is formed whose shrinking modulus exhibits a linear increase with increase of D. 2 Compared with LDPE films, those prepared from isotactic PP (iPP) are more liable to chain-scission and their mechanical properties deteriorate. 3'4 We have shown that in thin iPP films, Buplen type, the degradation predominates above 3 Mrad. 5 It has also been shown that EVA-copolymers are liable to cross-linking under irradiation. 6 The mechanical properties of y-irradiated bulk samples of iPP-EVA blends have been investigated v and also those of bulk samples of LDPE-iPP blends. 8'9 The changes in the tensile strength and the elongation at break with the irradiation dose and blend composition have been followed, as well as the respective stress-strain curves. The aim of the present paper is to describe the thermomechanical and some of the mechanical properties of tubular films, extruded from LDPE-iPP and EVA-iPP polymer blends, irradiated with fast electrons up to doses of 5-12 Mrad. In order to study the effect of irradiation on the temperature of melting and the Young's modulus at elevated temperatures we have used the method of thermomechanical analysis. We have also followed the change in the tensile strength and the elongation at break with increase of irradiation dose and the changes in the film composition.

EXPERIMENTAL Polymer films, 100pm thick, have been investigated which were prepared from polymer blends with the compositions shown in Table 1. The EVA copolymer is Lupolen V5510 SX (BASF) with a melt index of

Thermomechanical and mechanical properties of irradiated films

219

TABLE 1 Compositions of Polymer Blends

No.

EVA (weight %)

LDPE (weight %)

iPP (weight %)

1

89-85

--

l0

2

69.85

--

30

3

49.85

--

50

4

39.85

--

60

5

84-85

15

6

--

79.85

20

7

--

69.85

30

54-85

45

8

18.4g/10min and density of 954kg/m 3 at 23°C. The LDPE is Ropoten FV 03223 type, Bulgarian production, with a melt index of 0"907 g/10 rain (190 °, 5 kg) and a density of 920 kg/m 3 at 23c~C. The iPP is Buplen type 7623, Bulgarian production, with a melt index of 4 g/10 min (230 °, 2.16 kg) and a density of 903 kg/m 3. All the blends contain 0.15 weight % of stabilizer. Melt blending of the granules was carried out on a Brabender plasticorder at 180-225°C. The tubular films were extruded at 145-190°C and cooled slowly to room temperature. The irradiation was carried out on a linear electron accelerator E-250 in air to doses of 5-12 Mrad. The gel content of the samples was determined by extraction of the soluble component with xylene and weighing the residue. The samples were placed in platinum mesh baskets and immersed in a Soxlet extractor with boiling xylene. After the extraction was complete the samples were placed in an oven at about 70-90°C and dried for 6 h to constant weight of the residue. The thermomechanical investigations were carried out on a UIP70 apparatus made in the USSR. The thermomechanical curves were obtained by the penetration method with a constant load of 0"3 MPa and a heating rate of 2"5°C/min. The high elasticity Young's modulus, E, was determined from the thermomechanical curves using the formula: 1°

E-- F/A L?° where Fis the load, A is the loaded area and Lo and L are the initial and final lengths of the sample. For all samples, L o was 4000 pm. L was determined from the height of the high elastic plateau on the thermomechanical curves

220

L. Minkova, M. Nikolova

after the melting of the separate components. E was calculated at the temperature of the high elastic plateau according to the time-temperature superposition principle. 10,~x Mechanical testing of the films was carried out at 25°C according to the Bulgarian standard 10086-82, on an Instron Universal Testing Machine. The test samples were standard dumb-bell shaped pieces. The tensile strength and the elongation at break were determined in two directions-along the axis of extrusion and perpendicular to it. The values of these properties in the two directions were almost the same so we have used the average. This is probably due to the lack of any orientation of the films, established by the WAXS method.

RESULTS A N D DISCUSSION The gel content does not change much within the dose range 5-12 Mrad for the various compositions of the two types of blends--LDPE-iPP and EVA-iPP. At 5 Mrad it is about 50% and reaches 60% at 11.32 Mrad. At that dose the iPP content in both types of blends brings about a reduction of the gel content from 63% to about 54%. That is, the gel content in the irradiated films, which is a measure of the relationship between the number of crosslinks and chain-scissions in the material, 12 hardly depends on the composition of the blends and the dose D within the interval mentioned. The simultaneous processes of cross-linking and chain-scission result in the formation of a material with a final degree of cross-linking of 50-60%. The thermomechanical curves of the non-irradiated and irradiated samples prepared from blends Nos 1-4 (Table 1) are given in Fig. 1. In Fig. 2 the respective curves for blends Nos 5-8 (Table 1) are shown. For the nonirradiated films prepared from blends in which the iPP concentration is below 50% there is an increase in the steepness of the thermomechanical curves from which the temperature of melting of EVA (Fig. la,b) and LDPE (Fig. 2a,b,c) can be determined (those are the components with the lower melting temperature in the respective blends). A second increase in the steepness of the curves of the non-irradiated films appears only for those blends in which the concentration of iPP is above 50%, then its melting temperature can be determined (Fig. lc,d; Fig. 2d). This is due to the specific character of the thermomechanical method, according to which the temperature of melting is determined by the hardness of the structures capable of withstanding the external load. 13 The thermomechanical curves of all non-irradiated films exhibit a deformation which rapidly increases after the melting of the components of the blends, i.e. a viscous liquid melt is formed.

Thermomechanical and mechanical properties of irradiated films

221

d

15001 c

13801

1

540

12601

3 ~20

11/.,01 10201

3 4

60 i

a

ia

1

2

$4o I 4201

1}

._g

300

6O

_-...----zz=.~ j,

a

2

b

1

~5-

540

420

3001

300

180

180

3

60

60

-z 30

90

150

30

90

150 T~eC

Fig. 1. Thermomechanical curves: blend No. 1: curve 1, 0 M r a d ; curve 5'33Mrad; curve 3, 10Mrad; curve l l.32Mrad, b, blend No. 2: curve

30

a,

2, 4, 1, 0 M r a d ; curve 2, 5.33Mrad; curve 3, 10 Mrad; curve 4, 11"32 Mrad. c, blend No. 3: curve 1, 0 M r a d ; curve 2, 5.33 Mrad; curve 3, 10Mrad; curve 4, l l ' 3 2 M r a d , d, blend No. 4: curve 1, 0 M r a d ; curve 2, 5"33 Mrad; curve 3, 10 and 11.32 Mrad.

gO

150

30

gO

150 TfC

Fig. 2. Thermomechanical curves, a, blend No. 5" curve l, 0 M r a d ; curve 2, 5'33Mrad; curve 3, 10Mrad; curve 4, l l . 3 2 M r a d , b, blend No. 6: curve l, 0 M r a d ; curve 2, 5 . 3 3 M r a d ; curve 3, l 0 Mrad; curve 4, 11"32 Mrad. c, blend No. 7: curve l, 0 M r a d ; curve 2, 5.33Mrad; curve 3, 10Mrad; curve 4, l l ' 3 2 M r a d , d, blend No. 8: curve l, 0 M r a d ; curve 2, 5'33 Mrad; curve 3, l 0 and 11-32 Mrad.

From the curves o f the irradiated films, however, two melting temperatures can be obtained (Figs 1 and 2). The melting temperatures determined from all the curves are summarised in Table 2. For blend Nos 1-4 Tml denotes the melting temperature of EVA and for Nos 5-8, that of LDPE. Tm z is the melting temperature of iPP. It can be seen from the values in Table 2 that the temperatures of melting o f EVA and LDPE do not depend on the composition and the dose. This confirms the suggestion that mixing and irradiation do not influence the perfection of the crystallites in LDPE and EVA. 2'v-9 The melting temperature ofiPP does not depend on the composition of the blends. There is a slight decrease, however, for the irradiated films compared with the nonirradiated films. This is an indication that the perfection of the crystallites is influenced by irradiation more obviously in iPP than in LDPE and EVA. 9 On the thermomechanical curves o f all the irradiated films, a well defined high elastic plateau is noticeable above the melting temperature of iPP; that is, the system has passed into a high elastic state. The hardness of the

222

L. Minkova, M. Nikolova

TABLE

2

Melting Temperatures from Thermomechanical Curves No.

0 Mrad

5"33 M r a d

10 M r a d

11"32 M r a d

Tm 1

Tm 2

Tm 1

Tm 2

Tm 1

Tm 2

Tm x

Tm 2

1

116

--

2 3 4 5 6 7 8

119 118 119 106 109 110 114

-152 150 ---150

114 118 118 118 110 110 112 112

140 144 144 143 140 140 144 144

118 119 118 117 109 109 111 112

140 140 143 144 142 142 144 143

118 119 118 117 109 109 111 112

140 140 143 t44 142 142 143 143

irradiated films is due to network formation as a result of irradiation.13 The height of the plateau depends on the composition of the blends and the dose D. We have tried to evaluate qualitatively the density of cross-linking as a result of irradiation by determining the modulus E at the temperature of the high elastic plateau. According to the kinetic theory of high elasticity14 E is proportional to the cross-link density vE. With increase of vE, i.e. with reduction of the average length of molecular segments between two points of cross-linking ~tc, the high elastic modulus E increases in value.l°'15 The relationships between the high elastic modulus and the composition of the EVA-iPP and LDPE-iPP blends are shown in Figs 3 and 4 respectively. In both types of blends E; that is, vE, increases with increase of D. This is greatest in films with the lowest content of iPP. An increase in the amount of iPP brings about a large decrease of E at 10-11 Mrad, while at E,MI~I E,MPc

. . . . . .

108 2

5-

3

4-

2

5

3-

4

2-

2 I

1

10

20

30

40

50

60

i" PP~weig hi o/o

Fig. 3. Relationship between the high elastic modulus E and film composition for E V A - i P P blends: curve 1, 5'33 Mrad; curve 2, 10 and 11'32 Mrad.

~

10

.

20

30

40

50

i PP, weigh~ */o

Fig. 4. Relationship between the high elastic modulus and film composition for L D P E - i P P blends: curve 1, 5"33Mrad; curve 2, 10 Mrad; curve 3, 11'32 Mrad.

Thermomechanical and mechanical properties o f irradiated films

223

5 M r a d this decrease is negligible. A possible explanation of this could be the degradation of iPP at low doses, while EVA and L D P E are predominantly cross-linked. 7'9 The modulus E o f the E V A - i P P blends is almost twice that for the L D P E - i P P films at the same irradiation dose (11-32 Mrad) and at the same iPP content (10o15%). It could therefore be stated that EVA is more liable to cross-linking than L D P E ~6 and the density of cross-linking in it is higher than that in L D P E for the same dose. The gel content of all the samples is a b o u t 50060%, that is, the degree o f cross-linking within the interval 5-12 Mrad is a b o u t 50-60%, but the crosslinking density depends on the composition of the blends and the irradiation dose D. The tensile strength values O'max (MPa) for all the films are summarised in Table 3. It can be seen from Table 3 that composition and irradiation dose hardly influence the tensile strength, o-. It is not to be expected that within this low dose range, the tensile strength would undergo any large changes with change in the blend composition, as shown by other authors. 9 On the other hand, the tensile strength should increase with increase in cross-link density. ~° According to the relationships between the modulus E, the composition and the dose D (Figs 3 and 4), O'max should increase with D for a single composition, and it should decrease with increase of the concentration of iPP in films irradiated to the same dose. A probable explanation for the behaviour of trmax could be the following: the increase in the density of crosslinking in the films leads to an increase in the distribution of the lengths of the segments between two points of cross-linking A~tc. This determines the non-uniform distribution of the tension along the chains. The shorter segments, which are more strained, break down earlier and the tension is distributed between the other chains, gradually leading to their breaking as well. As a result the values of O'max decrease, x° The creation of a chemical TABLE 3

Tensile Strength of Blends No.

0 Mrad

5"33 Mrad

10 Mrad

11"32 Mrad

1 2 3 4 5 6 7 8

29 -29.3 34.5 22'2 24.7 25"9 27-5

27"5 21"5 23"1 24"7 20.5 21.4 25.5 23"5

21"7 26"1 21"5 23.5 22"1 21.8 22"2 25"3

24 22'8 22"8 23"8 20.4 20-9 24 24"5

L. Minkova, M. Nikolova

224

700

5001 500

2

5O0 400

2

400

300

4 10

20

30

40

50

3OO

60

i PP, weight %

10

4 20

30

40

50

i PPpweight %

Fig. 5. Dependence of elongation at break on film composition for EVA-iPP blends: curve 1,0 Mrad; curve 2, 5-33 Mrad; curve 3, 10Mrad: curve 4+ 11'32 Mrad.

Fig. 6. Dependence of elongation at break on film composition for L D P E iPP blends: curve 1,0 Mrad, curve 2, 5.33 Mrad; curve 3+ 10Mrad; curve 4, l l'32Mrad.

network with a broad distribution of the segment length between points of cross-linking is quite probable in films extruded from polymer blends, some components of which are liable to cross-linking, while others are liable to chain-scission under irradiation. The way in which the elongation at break, e, depends on the composition of blends is shown in Figs 5 and 6 for EVA-iPP and LDPE-iPP, respectively. While O'max (the tensile strength) is not a sensitive parameter, e is quite sensitive to the film structure. 9"t v In both types of sample s exhibits the wellknown 7 - 9.12 rapid decrease with increase of dose D. In the L D P E iPP films (Fig. 6) the decrease of E with the increase of D is more obvious in the blends with higher iPP content (30~,0%), obviously due to chain-scission predominating in iPP within that dose interval. In blends with higher LDPE content the cross-linking nature of the latter is the decisive factor for the behaviour of ~ with increase of D. For the EVA-iPP films (Fig. 5) the decrease ofe with increase of D is more clearly expressed in blends where the content ofiPP is low (10-30%). It is obvious that in samples containing more than 50% EVA, the behaviour of e is determined by the liability to crosslinking of the latter, while for samples containing about 50-60% iPP, its degradation under irradiation is decisive for the behaviour ofe with increase of D.

CONCLUSIONS The irradiation with low doses (5-12 Mrad) of fast electrons of non-oriented films of L D P E - i P P and EVA iPP blends leads to the formation of a 50-60% gel fraction in the material. The non-irradiated and irradiated films are two-phase--they exhibit two melting temperatures. As a result of the

Thermomechanical and mechanical properties o1+irradiated.films

225

simultaneous processes of cross-linking (predominating in L D P E and EVA) and chain-scission (predominating in iPP), a non-uniform statistical network is formed with a broad distribution of lengths between cross-links. The density of cross-linking, qualitatively evaluated from the values of the high elastic modulus determined above the melting temperatures, depends on the blend composition and irradiation dose. Increase in EVA and L D P E concentrations in blends irradiated to the same dose and/or increase of D for a given composition leads to an increase in the density of cross-linking. It is higher for films containing about 10% EVA than for films containing the same quantity of LDPE, irradiated to the same dose. This also brings about a greater decrease of elongation at break with increase of D for films containing 10% EVA than for films containing 10% LDPE. The influence of blend composition and D on the tensile strength is insignificant. As a result of the investigations carried out, the following conclusions may be drawn: the thermomechanical properties of non-oriented EVA-iPP and L D P E - i P P films with various compositions, irradiated to 5-12 Mrad, are determined by the balance between cross-linking and chain-scission in the separate components, which behave in different ways when subjected to irradiation.

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13. Tejtelbaum, B., Thermomechanical Analysis of Polymers, Nauka, Moscow, 1979, p. 162. 14. Flory, P. J., Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953; Treloar, L. R. G., The Physics of Rubber Elasticity (2nd edn), Oxford University Press (Clarendon), London and New York, 1958. 15. Minkova, L., Stamenova, R., Tsvetanov, Ch. & Nedkov, E., J. Polym. Sci.-Phys., (in press). 16. Wilski, H., Rad. Phys. Chem., 30 (1987) 1. 17. Robertson, R. E. & Paul, D. R., J. Appl. Polym. Sci., 17 (1973) 2579.