Aging of low-density polyethylene in natural weather, underground soil aging and sea water: effect of a starch-based prodegradant additive.
Plastics have become an indispensable item in our daily life. Their specific properties such as light weight, easy processability, low cost, good resistance to chemicals and microorganisms, nonbiodegradable nature, and high mechanical properties have contributed to the increased use of plastics. The polyolefinic plastics are used widely in packaging and carry bags, which are disposed of mostly after one time usage. This causes accumulation of huge proportion of solid waste, which in turn posses a serious threat to our environment, adversely affecting the land vegetation, cattle life, marine life, and indirectly the human society. There are reports on the enhancement for the degradation of plastics especially polyethylene under various environmental conditions (1-7). It has been observed that the degradation of polyethylene can be accelerated by the presence of starch containing pro-oxidants (8-10). Starch is a natural polymer and is biodegradable in nature. During degradation starch is attacked by bacteria, thereby increasing the surface area of polyethylene available for degradation (11). Raquez et al. (12) studied the UV-degradation behavior of oxo-degradable LDPE/thermoplastic starch blend. The UV radiation resulted in the scission of bonds in polysaccharide units that resulted in the reduction of molecular weight of starch. Amin et al. (13) examined surface topography by scanning electron microscopy (SEM) of LDPE blended with starch-based additive before and after natural weather aging. They observed increase in proportion of leached out additives on the polyolefin surface with increase in aging time. Majid et al. (14) studied the effects of natural weathering of linear low-density polyethylene (LLDPE)/thermoplastic starch blends and observed leaching out of starch under natural weathering because of the effects of UV, sunlight, rain, and wind. The weathering of starch--LDPE blend under underground soil aging condition has been reported earlier by Danjaji et al. (15) and Ismail et al. (16). Torres et al. (17) studied the degradation of LDPE/banana starch blend under underground soil aging for 9 months and found that degradation of starch starts from the outer surface, thus gradually exposing the interior for further degradation. The weathering of starch--LDPE blend in the marine environment condition has been reported by Breslin et al. (18). They reported that the sea water temperature, biological activity, and low exposure to sunlight are responsible for the low rate of degradation. The effect of compatibilizers on the biodegradation and mechanical properties of starch/LDPE blends have been studied by Huang et al. (19). Literature survey shows that there are scanty reports on the aging behavior of LDPE under natural weathering, underground soil aging, and sea water environments.
This manuscript reports the results of studies on the effect of the starch based prodegradant additive, PSH, on the thermal, rheological, and mechanical properties of LDPE, under natural weathering, underground soil aging, and sea water aging. Morphology and surface topography of LDPE/PSH blends before and after aging have been investigated by SEM.
LDPE ([M.sub.w] = 92740; MF1 = 2 g/10 min), having a density of 0.92 g/[cm.sup.3], was obtained from Exxon Mobil Chemicals. PSH, the corn starch based pro-degradant additive, used for this study was obtained from Willow Ridge Plastics. PSH is reported to be a composite of linear low-density polyethylene (LLDPE).
LDPE was blended with PSH in a Brabender Plasticorder (Mixer 50E) at 140[degrees]C for 10 min. Mixing speed was set al 60 rpm. The extrudates was taken out from the mixer and molded to sheets of 2 mm thickness in a Carver press at 140[degrees]C and at pressure of 8 MT for 5 min. The experimental conditions used for the processing and molding of the samples were based on our preliminary study, which is not reported in this manuscript. The amount of PSH in its blend with LDPE was 25 wt%, as recommended by the supplier. Preliminary experiments on the effects of PSH loadings on the stress--strain properties of the aged samples also indicated that the degradation is higher for the blends having high amount (that is, 25 wt%) of Polystarch H (Fig. 1). It was also observed that high loading of starch in Polystarch H makes it difficult to process and mould the samples for mechanical and other tests. Accordingly, 25 wt% was chosen as the optimum loading for this study.
For the natural weather aging studies, LDPE and LDPE/PSH sheets were fixed on a steel rack at 450 with respect to the base of the rack fixed on the roof top of a seven storeyed building located in Dhahran, Saudi Arabia and facing to east to have the maximum exposure to sunlight. For the underground soil aging test, the samples were buried under soil in a vegetable garden at King Fahd University of Petroleum and Minerals campus, Dhahran, Saudi Arabia at a depth of 0.5 m. For the aging in sea water, the samples were immersed in the Arabian Gulf, Dhahran, Saudi Arabia (Latitude: 26[degrees] 17' 0" N, Longitude: 50[degrees] 12' 0" E), at a depth of 1 m from the surface, using a specially designed compartmentalized nylon net for holding samples under water with a long string attached to one end of the net for retrieving the samples after aging, and a heavy object attached to the other end of the net to keep the samples immersed under sea water. All the samples were exposed for 4 months from October 2011 onward and the temperature, humidity, and wind speed during these periods is given in Table 1. After 120 days of aging, the samples were retrieved, cleaned with distilled water, and dried in an air oven at a temperature of 30[degrees]C for 24 hr.
TABLE 1. Variation in the climatic condition during this study. October November December January High temperature ([degrees]C) 42 35 28 23 Low temperature ([degrees]C) 24 15 10 11 Humidity (%) 50 54 45 70 Wind speed (km/h) 18.9 20.1 16.2 14.8
The tensile properties of the dumbbell-shaped samples were measured at 25[degrees]C as per ASTM D638 using Instron UTM (Model 5560) at a crosshead speed of 50 mm/min. Five samples were tested in each experiment and the average value has been reported.
The melting and crystallization behavior of LDPE and blends with PSH were determined by using DSC-Q1000, Universal V4.2E TA Instruments. Heating and cooling for both first and second cycles were done in nitrogen atmosphere at the rate of 10[degrees]C/min from 20[degrees]C to 180[degrees]C. The crystallinity of LDPE and the blends was calculated using the expression
% of crystallinity = ([DELTA][H.sub.fus]/[DELTA][h.sub.fus.sup.0]) x 100 (1)
where [DELTA][H.sub.fus] is the enthalpy of fusion of the LDPE--PSH blends and [DELTA][h.sub.fus.sup.0] is the enthalpy of fusion of the 100% crystalline LDPE. [DELTA][h.sub.fus.sup.0] of LDPE was taken as 287.6 J/g (20). The reported results are within the range of -[+ or -]5%.
Rheological measurements were performed using an advanced rheometrics expansion system (ARES). The measurements were carried out using cone and plate geometry (25 mm diameter and 0.1 rad cone angle) at 150[degrees]C in nitrogen atmosphere. Frequency sweeps with an angular velocity of 0.1-100 rad/s were performed in the linear viscoelastic regime at a strain of 10%. The samples were left to equilibrate for 5 min before each measurement.
Scanning Electron Microscopy (SEM)
The surfaces of the LDPE and its blend with Poly-starch H, both before aging and retrieved after aging, were examined under scanning electron microscope JEOL (Model JSM 5800LV) at a magnification of x500 and x1000.
To study the morphology of neat Polystarch N and LDPE/PSH blends before and after aging, the samples were subjected to cryogenic fracture followed by treatment in hot water at 80[degrees]C for 6 hr to etch out the starch phase in PSH. Samples were coated with a thin layer of gold in order to avoid sample charging during imaging and then examined under SEM. The photographs were taken at a magnification of x1000.
RESULTS AND DISCUSSIONS
Figure 1 shows that the maximum degradation on aging was observed in the case of 25 wt% PSH. Accordingly, this concentration was chosen in the subsequent studies and the following results deal with compositions containing 25 wt% of PSH. Figure 2A--E shows the SEM photomicrographs of the cryofractured hot water etched neat PSH and LDPE/PSH blend, before and after natural weather, underground soil aging, and sea water aging. It is evident that hot water extraction causes removal of the starch phase from the PSH showing a large number of voids on the surface (Fig. 2A). In the case of unaged blend with LDPE containing 25% of polystarch H (Fig. 2B) the number of voids decreases due to lesser proportion of starch in the composition. Natural weather aging (Fig. 2C) of the blend of polystarch H and LDPE shows erosion of the samples and removal of samples from the surface. Underground soil aging (Fig. 2D) shows plastic deformation of the matrix. In the case of sea water aging (Fig. 2E), however, the extent of erosion of the samples, as observed in the case of underground soil aging has increased causing detachment of larger segments.
The SEM images of the surface of LDPE and LDPE/PSH blend before and after aging in lower and higher magnifications are shown in Figs. 3A--D and 4A--D, respectively. Before aging, the LDPE surface was smooth and free of any kind of defects (Fig. 3A).
In the case of LDPE, natural weather aging (Fig. 3B) causes high extent of crazing and erosion. Furthermore, the detached samples are visible on the surface. Roy et al. (6) reported that the degradation of LDPE by UV created cracks and grooves on the polymer surface and the effect was more pronounced for the samples having prodegradant. In the case of underground soil aging (Fig. 3C) the extent of erosion is much less and the detached samples are not visible on the surface. Similar observation was made by Kiatkamjornwong et al. (7) while studying the morphology of LDPE after underground soil aging for 2 months. In the case of sea water aging (Fig. 3D), there is no ploughing and crazing instead there is erosion on the surface along the weakly developed crazing path along with plastic deformation. Sudhakar et al. (3) reported an increase in the roughness of LDPE after sea water aging for 6 months. In the case of LDPE/PSH blend, natural weather aged (Fig. 4B) shows extensive erosion of the sample along the crazing path along with eroded particles seen on the surface. Similar trend was observed by Majid et al. (14) during the natural weathering of linear low-density polyethylene (LLDPE)/thermoplastic starch blends due to the leaching out of starch under natural weathering due to the effects of UV, sunlight, rain, and wind. Underground soil aged samples (Fig. 4C) shows a mix up of crazing scratches and voids on the surface. Danjaji et al. (15) found perforations on the surface of LPDE/starch blend after 3 months of underground soil aging, and attributed to the same leaching out of starch leading to surface agglomerates and cracks. Similarly, Ismail et al. (16) in his studies observed the formation of holes because of leaching out of starch and fungal growth in the case of LDPE/thermoplastic starch blends under underground soil aging. Sea water aged sample (Fig. 4D), however, shows extensive crazing with lesser extent of eroded debris on the surface. Danjaji et al. (15) reported that natural weathering causes the maximum degradation when compared with enzymatic hydrolysis and underground soil aging in the case of starch filled linear low-density polyethylene.
The DSC heating curves for LDPE and the blend before and after aging of 120 days under various environmental conditions are given in Fig. 5. Polystarch H shows a major peak at 120[degrees]C with a hump due to the presence of LLDPE component. The main peak and the shoulder are also visible in the blend but the peak temperature and the hump temperature shift to lower temperature and come closer to the peak of LDPE, presumably due to partial compatibility between LDPE and LLDPE. The [T.sub.m] values remained unaffected after aging under different environments. However, aging caused significant changes in [DELTA][H.sub.fus] and percentage of crystallinity of the LDPE and the blend (Table 2). Incorporation of the prodegradant additive caused a fall in the [DELTA][H.sub.fus] and crystallinity of LDPE. It is interesting to note that there is an increment in the percentage crystallinity of LDPE when aged under underground soil aging and sea water, but remained unaffected in natural weather aging. However, in the case of the blend, the [DELTA][H.sub.fus] and percentage crystallinity increased on natural weather aging, but remaining almost unaffected in the case of underground soil aging and sea water aging. Because PSH consists of photo-degradant additives, its blend with LDPE undergoes chain scission, which occurs mostly in the amorphous region of the polyolefins (21), increasing thereby the percentage of crystallinity. Kyrikou et al. (22) also reported chain scission and crosslinking in the polymer matrix as a result of photochemical degradation of linear low-density polyethylene film with Ciba Envirocare AG100 as pro-oxidant. This is in conformity with the effects of aging on stress--strain properties and rheological behavior, as discussed in the following section.
TABLE 2. Heat of fusion and percentage crystallinity from DSC studies on LDPE and LDPE/PSH blends containing 25 wt% of PSH before and after natural weather, underground soil, and sea water aging for 120 days. [DELTA][H.sub.fus] (J/g) Crystallinity (%) Materials Unaged Natural Underground Sea Unaged Natural weather soil water weather LDPE 110 108 123 123 38 38 Blend 96 120 88 103 33 42 Materials Underground Sea soil water LDPE 43 43 Blend 31 35
The stress--strain plots for the LDPE and its blend with PSH before and after aging of 120 days under various environmental conditions are given in Fig. 6. Incorporation of the prodegradant additive caused a drop in mechanical properties, but the fall in properties became phenomenal after aging. Results of stress--strain measurements are summarized in Table 3. It is evident that aging of LDPE caused marginal drop in tensile strength, but the elongation at break marginally increases. However, it is noteworthy that aging resulted in remarkable changes in the stress--strain behavior of the blend of LDPE with PSH. The percentage elongation at break for the blend decreased sharply on aging and this effect is most pronounced in the case of natural weather aging and the samples behaved more like a brittle material. This indicates that the effects of the prodegradant present in the PSH on the degradation of LDPE and the surface erosion of starch are pronounced in the case of natural weather aging, when compared with underground soil aging and sea water aging. The prodegradants function by producing free radicals thereby increasing the chain scission in LDPE (23). Furthermore, the presence of sand wind and high temperature can cause erosion of the polymer and the additive present on the polymer surface (11), (24) which, in turn, increases the surface area of LDPE available for further degradation to occur. However, after underground soil aging and sea water aging, the decrease in the percentage of elongation for the blend was less pronounced, as compared with the natural weather aging. During underground soil aging, the pro-degradant present in PSH cannot function in the absence of sunlight. But the starch present in PSH leaches out from the surface of the blend, which helps to increase the surface area of the LDPE available for further degradation (25), apart from its role in microbial activity. In sea water aging, the nature and extent of degradation are known to depend on the temperature, salinity, pH, microbial population, and the dissolved amount of oxygen in sea water (26), (27). In the case of sea water aging, the availability of oxygen and sunlight that is essential for the initiation of photo-degradation is less when compared with natural weather aging (28). Furthermore, absorption of water by starch in the PSH is likely to cause leaching out of starch from the bulk material (14). Besides oxygen concentration, the ambient temperature is also low in the case of underground soil aging and sea water aging as compared with natural weather aging. Gijsman and Sampers (29) reported that both oxygen concentration and temperature influence the degradation of polyethylene. It is worthwhile to note that underground soil aging and sea water aging caused a decrease in both tensile strength and elongation at break of the blend (Fig. 6b). This is believed to be due to chain scission. In the case of natural weather aging, the elongation at break of the blend decreased, but the tensile strength registered an increase, presumably due to partial crosslinking of the broken down chains (30).
TABLE 3. Tensile properties of LDPE and LDPE/PSH blend containing 25 wt% of PSH before and after natural. underground soil and sea water ageing for 120 days. Tensile strength (MPa) Percentage elongation (%) Materials Uruged Natural Underground Sea Unaged Natural weather soil water weather LDPE 15.3 [+ 12.5 [+ 14.1 [+ or 14.4 338 [+ 347 [+ or -] or -] -] 0.4 [+ or or or -] 0.09 1.0 -] 0.7 -]2.12 10.3 Blend 11.4 [+ 12.2 [+ 10.2 [+ or 9.3 [+ 118 [+ 24 [+ or or -] or -] -] 0.6 or -] or -] -] 3.9 0.10 0.4 0.2 6.8 Materials Underground Sea soil water LDPE 379 [+ or -] 404 [+ 13.4 or -] 9.8 Blend 82 [+ or -] 79 [+ 6.3 or -] 7.2
The variation in the storage and loss modulus of the LDPE/PSH blends at different frequencies is given in Fig. 7. As frequency increases the storage and loss modulus of the blend increases. However, the aging caused a decrease in these properties at higher frequency and the effect is prominent in the case of natural weather aging. The dependence of dynamic viscosity of LDPE and its blend with PSH before and after aging of 120 days at 10 Hz is given in Fig. 8. The dynamic viscosity of LDPE decreased after natural weather aging, whereas there is no variation in viscosity in the case of underground soil aging and sea water aging. This is ascribed to the chain scission in LDPE during natural weather aging. However, in the case of the blend, the dynamic viscosity decreased under all aging conditions and the effect is also most pronounced in the case of natural weather aging. As discussed earlier, the simultaneous action of prodegradant in the presence of sunlight and the erosion of starch from the blend surface accelerate the chain scission (31). The severity in changes in properties under different aging conditions follow the order, natural weather > underground soil aging > sea water.
Incorporation of the pro-degradant additive (PSH) to LDPE enhanced the degradation of LDPE, which resulted in the reduction of percentage of elongation and dynamic viscosity for the samples especially under natural weather aging conditions. The degrading effect is less pronounced in the case of underground soil aging and sea water aging conditions. The decrease in mechanical properties is attributed primarily to the chain scission of the polyolefin and leaching of starch from the prodegradant additive in the blend. Photo-oxidative degradation in LDPE is believed to be occurring in the amorphous region of the polyolefin resulting in an increase in the crystallinity. Aging also resulted in the increase in the microcracks and crazes in LDPE, whereas scratches, microcrazing, and erosion became prominent in its blend with PSH under soil and sea water aging because of the leaching out of starch in the PSH component from the blend.
Thanks are due to Deanship of Scientific Research and Chemical Engineering Department of King Fahd University of Petroleum and Minerals for providing the necessary facility in this study.
Correspondence to: P.A. Sreekumar; e-mail: email@example.com P.A. Sreekumar is currently at Department of Chemical and Process Engineering Technology, Jubail Industrial College, Al-Jubail, 31961, Kingdom of Saudi Arabia.
Published online in Wiley Online Library (wileyonlinelibrary.com). 0 2013 Society of Plastics Engineers
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M. Elanmugilan, (1) P.A. Sreekumar, (2) N.K. Singha, (1) Mamdouh A. AI-Harthi, (2), (3) S.K. De (2)
(1) Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, West Bengal, India
(2) Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, P.O. Box 5050, Dhahran 31261, Kingdom of Saudi Arabia
(3) Center for Research Excellence in Petroleum Refining and Petrochemicals, King Fahd University of Petroleum and Minerals, PO. Box 5040, Dhahran 31261, Kingdom of Saudi Arabia
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|Author:||Elanmugilan, M.; Sreekumar, P.A.; Singha, N.K.; Al-Harthi, Mamdouh A.; De, S.K.|
|Publication:||Polymer Engineering and Science|
|Date:||Nov 1, 2013|
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