Alterations of molecular characteristics of polyethylene under the

Alterations of molecular characteristics of polyethylene under the influence of heat treatment Vigen G. Barkhudaryan

International Journal of Plastics Technology ISSN 0972-656X Volume 20 Number 2 Int J Plast Technol (2016) 20:231-240 DOI 10.1007/s12588-016-9151-6

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Author's personal copy Int J Plast Technol (December 2016) 20(2):231–240 DOI 10.1007/s12588-016-9151-6 RESEARCH ARTICLE

Alterations of molecular characteristics of polyethylene under the influence of heat treatment Vigen G. Barkhudaryan1

Received: 14 March 2014 / Accepted: 21 July 2016 / Published online: 30 July 2016 Ó Central Institute of Plastics Engineering & Technology 2016

Abstract The influence of exposure to heat treatment in the presence of air oxygen on structural transformations of low-density polyethylene have been investigated by methods of IR spectroscopy, viscometry, light scattering, turbidimetry and gelation measuring. It was established that the destruction and crosslinking processes of macromolecules proceed simultaneously with exposure to the heat treatment. In a range of exposures prior to gelation increase of mass average molecular mass and degree of branching of macromolecules, accompany the elevated temperatures. The results obtained are compared with the results of previously conducted similar studies of influence of UV and c-irradiation on low density polyethylene where the same experimental approach was applied. It has been concluded that the mechanisms of influence of c-, UV radiation and heat treatment on polyethylene are identical. Keywords Polyethylene Heat treatment Destruction Crosslinking

Introduction In the previous works the results of investigations of the influence of c- and UVirradiation on the molecular characteristics of low-density polyethylene (LDPE) were given depending on the samples’ thickness, intensity and dose [1–3]. A remarkable growth of average molecular mass of LDPE was established. The growth was linear for the mass average molecular mass and at the initial doses was mainly the result of the increase of macromolecular branching. In addition, it was shown, that the destruction and crosslinking processes of macromolecules proceed & Vigen G. Barkhudaryan [email protected] 1

Department of Molecular Physics, Yerevan State University, 1 Alex Manoogian St., 0025 Yerevan, Armenia

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Int J Plast Technol (December 2016) 20(2):231–240

simultaneously with the irradiation. A significant dependence of specified changes on irradiation intensity and irradiated samples’ thickness has been established [1–3]. The present work will describe the results of investigations of influence of moderate high temperature on the LDPE in dependence of heat treatment exposure where the same experimental approach was applied. As it is known elevated temperatures are one of the most frequent physical factors of influence on polymers. Prolonged or even short term exposure to elevated temperatures brings onset on further modification of the polymer, resulting in irreversible changes to the chemical and physical properties of the polymer [4]. The thermal destruction is defined by strength of chemical bonds in macromolecules and is supported by other actions on polymers such as radiation, light, oxygen, moisture etc. The break of chemical bonds occurs at certain amount and strength of thermal energy and in conditions promoting free radical processes. The literature review has shown that generally thermal ageing could lead to chain scission within the polymer and the formation of chain radicals. However, thermal destruction in its pure form is rarely realized, as on destructive processes are rendered strong influence even by traces of oxygen [5, 6]. Oxidation is the primary degradation process at elevated temperatures. The rate of degradation increases with the amount of oxygen presence. Taken from [4] following below Fig. 1 schematically illustrates the probable reactions after thermal processing of polyethylene: The polyethylene, particularly linear, is one of the most heat-resistant polymers. The reduction of its molecular weight comes above 290 °C [7]. However, the reactions of thermal destruction of polyethylene begin at considerably lower temperatures (even at room temperature in small quantity). At thermal oxidation of polyethylene, crosslinking of splinters of chains (i.e. formation of cross bonds such as simple ethers) occurs alongside with destruction. The increase of temperature naturally increases depth destruction of polyethylene [8]. Thermal destruction of

Fig. 1 Reactions after thermal processing of polyethylene

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polyethylene at high temperatures is well-investigated [3–27]. However, there are not enough works in the literature devoted to the influence of moderate temperatures on polythene. Competing reactions of chain scission and crosslinking are expected to occur in polyethylene samples, where at high temperatures, chain scission is dominant. There are many papers concerning the thermal degradation of polyolefin materials [4–6, 8–17]. It is well documented that the degradation of polymers begins during the initial heat processing stage during product manufacture [4]. Other reactions that polymers can undergo include crosslinking and the formation of double bonds. Polyethylene undergoes chain scission, branching and cross linking, which occur as competitive reactions [17–27]. The comparison of influence of c-, UV-irradiations and temperature on polyethylene shows that no principal difference is present between chemical changes of macromolecules under the above-stated influences. Moreover, in [19] the c-, photo and thermally initiated oxidation of polyethylene is investigated by IRspectroscopy method and is shown that the products formed at different kinds of oxidation practically do not differ from each other and represent hydroperoxydes, spirits, ketons, carbon acids and complex ethers. We investigated the influence of moderate high temperature (100 °C) on LDPE. In our opinion, the temperature that is insufficient for thermal destruction of LDPE can intensify oxidation processes and thus stimulate molecular transformations in polymers.

Experimental section Details of the sample preparation are presented in [1]. LDPE 200 lm thick films have been studied. Thermostating was carried out in thermostat, in the presence of air, at 100 °C. After a certain time of thermostating the samples were removed, investigated at room temperature and then thermostating was proceeded. The IR-spectroscopy, viscometry, light scattering and turbidimetry investigations of all the samples were conducted in the range of exposures in which they were practically completely soluble. Conclusion about full solubility was based on the absence of insoluble portion after filtration of solutions through glass filter No. 3. Intrinsic viscosity measurements by Ostwald’s capillary viscometer (the capillary’s diameter 0.64 mm) were performed in cis decalin at 70 ? 0.01 °C. The efflux time of solvent was 100.2 s. The solvent and solutions were filtered through a No. 3 glass filter (at 70 °C) before the measurements. The following formula was used for the relation of molecular mass and intrinsic viscosity [2] ½g ¼ 6:8 104 M0:675 Details of all experiments are presented in [1–3].

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Results and discussion The spectrophotometry method fixes small growth of crystallinity, determined on doublet 720–731 cm-1, and caused by recrystallization and before recrystallization processes. Then, with increase of exposure of heat treatment, a very slow recession of the degree of samples crystallinity is observed. In spectra of absorption of samples the occurrence and synchronous growth of bands 1633 and 1720 cm-1 in an interval of heat treatment 400–1000 h responsible for formation C=O groups owing to oxidation, and also carbonyl groups of ceton type, ethers and complex ethers is observed simultaneously. It specifies that in an initial stage the oxidation occur at the non-saturated sections. C–O oscillations of ether and complex ether groups become at 1175 cm-1 as a wide band of absorption, which occurs after *1000 h of heat treatment, and probably further increase of absorption in the band of 1700–1800 cm-1 occurs at the expense of accumulation of ethers and complex ethers group. If assumed that the *1000 h heat treatments start the crosslinking process, a conclusion arises that at thermal ageing crosslinking is mainly carried out by means of oxygen bridge bond. An interesting fact is that in the absorption area of C=O groups (1700–1800 cm-1) the band of 1733 cm-1 is absent, concerning to carbonyl groups of aldehyde type. It testifies that LDPE heat ageing at 100 °C the destruction of macromolecules is insignificant. In Fig. 2 turbidimetry curves of initial and heat aged samples of various durations are given (Samples thickness 200 lm). As it is shown, bimodality of turbidimetry curves is observed for the heat-aged samples. It shows two ageing concomitant processes—destruction and crosslinking. Chain scission during degradation results in the continual breaking of polymer chains and shortening of molecules length. The

Fig. 2 The turbidimetryc curves of LDPE for the exposures (hour): circle-0, closed circle-100, triangle500

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overall result is that there are more short chain molecules present and the average molecular weight is reduced whilst the distribution of weights broadens. Therefore, the distribution curve is seen to shift to the left with a lower and broader peak. Indeed, which the increase of heat exposure time the precipitate thresholds shift to the left with testifies of increase of high molecular fraction in a sample. Crosslinking due to degradation leads to chain branching. Here the free radical polymer chains terminate by bonding to other free radical chains, resulting in larger branched molecules. Consequently the average molecular weight increases, giving shift to the right for the curve. Viscometric and light scattering investigations were done and LDPE molecular characteristics were calculated exactly by methods and ways detailed in [1–3]. The w , ðR2 Þ1=2 and calculated [g]l, g, mw for the values of experimental [g]g, M investigated exposures are given in the Table 1. v) The dependence of the ratio of intrinsic viscosity’s (consequently and M thermally aged ([g]g) and initial ([g]g initial) samples of 200 lm thick samples from exposure of heat treatment are shown in Fig. 3. As it can be seen from the given diagram, with increase of exposure a small growth of this ratio is observed. At the same time, achievement of gel point after certain exposure time (*1000 h) testifies that an increase of molecular mass is taking place, owing to formation of intermolecular cross bonds. In fact further investigations of aged samples by the method of light scattering have fixed a considerable growth of mass average w ) during irradiation. The dependencies of the ratio of mass molecular mass (M w ) and initial (M w initial ) 200 lm thick samples average molecular mass of aged (M on exposure time are shown in Fig. 3. ðR2 Þ1=2 also has a similar behavior. The Table 1 Intrinsic viscosity is experimental ([g]g) and calculated ([g]l) values, viscosity-average v ), mass average molecular mass (M w ), mean square radii of gyration (ðR2 Þ1=2 ), molecular mass (M branching index (g) and mass-average number of branching per molecule (mw ) of aged LDPE 200 lm thick samples for various exposure time Exposure (h)

[g]g, (dl g-1)

v 105 M

w 105 M

0

1.33

0.75

2.84

338

3.26

0.407

10.3

2

1.33

0.75

–

–

3.26

–

–

4

1.34

0.76

–

–

3.26

–

–

10

1.34

0.76

–

–

3.26

–

–

20

1.30

0.72

–

–

3.18

–

–

40

1.33

0.75

–

–

3.20

–

–

60

1.30

0.72

–

–

3.20

–

–

100

1.36

0.77

3.08

348

3.38

0.390

11.1

200

1.40

0.80

3.32

356

3.53

0388

11.6

300

1.46

0.84

3.50

375

3.71

0386

11.7

500

1.54

0.88

3.80

399

3.86

0.387

11.5

700

1.62

0.93

4.20

430

–

0.377

12.1

900

1.72

0.98

4.68

463

4.49

0.371

12.5

˚) ðR2 Þ1=2 (A

[g]l, (dl g-1)

g

mw

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Fig. 3 The dependence of the ratio [g]g/[g]g function of exposure

initial,

w =M w initial and ðR2 Þ1=2 =ðR2 Þ1=2 LDPE as a M initial

w as function of M w Fig. 4 LDPE samples ðR2 Þ1=2 =M

dependencies of the ratio of mean-square radii of gyration aged (ðR2 Þ1=2 ) and initial 1=2

(ðR2 Þinitial ) samples plotted against exposure time are shown in Fig. 3. As it can be w =M w initial seen from Fig. 3, during the heat treatment the rate of change of M noticeably exceeds the rate of change [g]g/[g]g initial. These phenomena can be connected with the change of form of macromolecules w fðM w Þ, given in Fig. 4, during the heat treatment. The dependence ðR2 Þ1=2 =M

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w can be considered as the criteria of this change. The evident decrease of ðR2 Þ1=2 =M indicates the increase of macromolecules branching during the heat treatment. However, it is considered more reliable to estimate the branching by the relation of w and viscosity and molecular mass, because of the difference in averaging of M ðR2 Þ1=2 measured by the light scattering method. Thus, in Fig. 5 the relation [g]l/ w ) and [g]g/[g]g initial * f (M w ) for 200 lm thick samples are [g]l initial * f (M w ) is shown. As it can be seen from the corresponding curves, [g]g/[g]g initial * f (M visibly deviating from the [g]l/[g]l initial * f (Mw ). This fact indicates the increase of branching during heat treatment. It is evident, that the increase of molecular mass of samples occurs mainly due to increase of branching. The increase trend of branching during heat treatment is evident from Table 1 too. The results of gel content definition as a function of exposure time for the 200 lm thick films of LDPE are given in Fig. 6. Competing reactions of chain scission and crosslinking are expected to occur in polyethylene samples, where at high temperatures, chain scission is dominant. Heat ageing brings about the onset of further modification of the polymer, resulting in irreversible changes to the chemical and physical properties of the polymer. Predominantly polyethylene samples experience crosslinking, leading to increases in molecular weight. Competing reactions of chain scission and crosslinking occur in polyethylene samples, this is temperature dependent on high temperatures chain scission is dominant owing to faster movement of the polymer chains [4–7]. The data trend in Fig. 6 specifies that the relative magnitudes of chain destruction and chain crosslinking processes when both occur simultaneously, determine the extent of crosslinking relative to destruction during the initial exposure. However, as the trend indicates, the destruction-to-crosslinking speeds ratio rises up to a constant value. It might be caused by a highly crosslinked rigid structure inhibiting

Fig. 5 LDPE [g]l/[g]l

initial

and [g]g/[g]g

initial

w as function of M

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Fig. 6 LDPE gel fraction percentage as function of exposures (hour)

further crosslinking reaction. After *1000 h of heat treatment gel fraction becomes apparent, obviously owing to the formation of intermolecular oxygen bonds between oxidized groups of macromolecules which correlates well with the results of IR-spectroscopy researches described above.

Conclusion The IR-spectrometry, viscometry, light scattering, turbidimetry and gelation measuring investigations of thermal aged LDPE in the presence of air oxygen testify that during heat treatment in molecular ensemble of a polymeric material there are essential irreversible changes. The chain destruction and crosslinking processes take place simultaneously. As a result, the correlations of molecular fractions of LDPE are changed. At the initial exposure of heat treatment, the crosslinking process is prevailing. However, in further stages of exposure the ratio of destruction-to-crosslinking frequencies is increasing to constant values allegedly due to formation of highly cross-linked rigid structures inhibiting further crosslinking reaction. The gelation measuring of thermal aged LDPE shows the generation of gel fraction It was obtained, that by the heat treatment growth the speed of mass-average molecular mass is higher than that intrinsic viscosity values. The phenomena are explained by the growth of degrees of macromolecules chain branching. The qualitative comparison of the results of the influence of c-, UV-irradiation and high temperature on LDPE indicates that both destruction and crosslinking processes occur simultaneously in all forms of influence. The influence is limited only by diffusion of oxygen during the c-irradiation and heat treatment, when it is limited both by diffusion of oxygen and penetration of radiation during the UV-

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irradiation. Consequently, the molecular transmissions are deeper and evident in case of c-irradiation, as soon as the polymer is transformed in the whole volume, whereas the molecular transmissions in case of UV-irradiation are proceeding in surface layer area. However, as all three discussed factors are extremely sensitive to oxygen content in the system, it is possible to assume that the main effects are displayed in superficial layers of the material. Regardless of degradation mechanism of the polymer, these irreversible changes affect the overall properties of the polymer including melting flow, molecular weight, viscosity and mechanical strength. Reduction in molecular weight (chain scission) leads to shorter polymer chains and decreases all mechanical properties. Increase in molecular weight (crosslinking) yields stiffer materials; a slight reduction in molecular weight (chain scission) leads to shorter polymer chains, increase in molecular weight (crosslinking) yields stiffer materials. All this testifies that the changes of molecular ensembles in all discussed cases occur in accordance with certain trends that are in line with the general patterns of ageing processes.

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