Investigation of weight degradation
The biodegradability of tapes in the soil has been measured using the variability of the weight of the samples (Nejadian, 2018). Tables 1 and 2 show the results of variance analysis of weight changes and percentage of weight changes of normal and biodegradable tapes, six and eleven months after placement in soil and above soil, respectively. Based on Tables 1 and 2, the differences are significant in the amount of weight and percentage change of each treatment and their difference after six and eleven months.
Table 1
Analysis of variance results on the weight changes of samples after six and eleven months in soil
Time | Mean square | F | df | P-value |
Six months | 0.000 | 6.886 | 7 | 0.001 |
Eleven months | 0.000 | 12.122 | 7 | 0.000 |
Table 2
Analysis of variance results of the changes percentage in weight of samples after six and eleven months in soil
Time | Mean square | F | df | P-value |
Six months | 13.589 | 14.551 | 7 | 0.000 |
Eleven months | 77.394 | 34.926 | 7 | 0.000 |
Table 1
Table 2
Table 3 shows the results of comparing the mean weight changes of the samples and the percentage of weight change in each of the treatments six and eleven months after placement in the soil based on LSD test. As can be seen, weight change occurred over time in each of the samples. According to Table 3, this weight change was greater in the samples to which the oxo additive was added. Since Eq. 5 is given for biodegradability in soil, it should be expressed based on the percentage change in weight and therefore the highest percentage change in weight compared to the initial weight in the measurement after six months is related to BZ2 treatment with 4.74%. And after 11 months in BS2 treatment with 15.5% and the lowest amount of change after 6 months is related to NZ1 treatment with 0.31% and after 11 months is related to NZ2 treatment with 0.99%. Accordingly, it can be claimed that weight degradation has started in the treatments in which oxo additive has been used, and in the best case, after eleven months, it is about 15 times more than the normal tapes. It can also be said that the degradation in pure polyethylene treatments was close to zero. Antelava et al. (2020) compared the degradation of plastic bags made with oxo materials in simulated sunlight. They concluded that the weight of plastics made with oxo materials over time and their residence time decreases as the simulated temperature increases. They also stated that the more the samples are exposed to the air, the more brittle they become and the lower their polymer matrix, which is due to the presence of oxo materials. La Mantia et al. (2020) reported more and, of course, partial degradation of polyethylene containing oxo material than conventional polyethylene. Rodrigo (2013) also reported weight loss of propylene in the presence of oxo and stated that weight loss decreases over time, but a percentage of the polymer remains, which is the source of other additives that is present in the polymer structure. Therefore, this part of the research is consistent with the results of previous researches mentioned above.
Table 3
Mean comparison of weight changes in samples after six and eleven months in soil
Treatments | Mean of weight | | Percentage of changes |
Sixth months | Eleven months | | Sixth months | Eleven months |
NS1 | 0.3852 b | 0.3831 bd | | 0.83 b | 1.06 c |
NZ1 | 0.3882 bd | 0.3814 bd | | 0.31 b | 1.49 c |
BS1 | 0.4037 ae | 0.3905 ad | | 4.73 a | 7.83 b |
BZ1 | 0.4092 a | 0.3986 a | | 3.43 a | 5.92 b |
NS2 | 0.3858 bc | 0.3829 bd | | 0.35 b | 1.11 c |
NZ2 | 0.3931 bde | 0.3823 bd | | 0.32 b | 0.99 c |
BS2 | 0.4046 a | 0.3580 c | | 4.51 a | 15.50 a |
BZ2 | 0.4036 ae | 0.3961 a | | 4.74 a | 6.51 b |
Means that have one letter in common do not differ significantly at the 5% probability level of the LSD test. |
Table 3
Turbidity
Table 4 shows the results of comparing the mean before and after use and placement in the soil based on LSD test. The turbidity index is obtained according to the Eq. 6. These values are the amount of light absorbed by the samples at a given wavelength. In any case, no matter how much the value tends to zero, it indicates that the samples have deteriorated in terms of thickness, turbidity, and compactness of their constituents. In general, according to Table 4, the results of analysis of variance of treatments in each of the sampling times show a significant difference with each other. Therefore, it can be said that the structure of the tapes was affected.
Table 4
Analysis of variance results on sample turbidity after six and eleven months in soil
Time | Standard deviation | F | df | P-value |
Six months | 0.46 | 134.454 | 8 | 0.000 |
Eleven months | 0.714 | 269.582 | 8 | 0.000 |
Table 4
The results of comparing the average turbidity of the films are given in Table 5. The amount of light absorption, which is directly related to turbidity in all treatments has decreased compared to its initial value. As can be seen, this decrease was greater in the treatments in which oxo additive was used. According to Table 5, the highest decrease in light absorption after six months is related to BZ1 treatment and after eleven months is related to BS2. Also, the percentage of this decrease in treatments related to normal tapes after six months was much lower than other treatments, so that in NS1 and NS2 treatments, this value was almost zero between the sixth and eleventh months. Antelava et al. (2020) in their study concluded that over time in oxo plastics, light transmission increases and the polymer absorbs less light. This absorption of light is directly related to the polymer bonds in its matrix (Al-Salem et al., 2019a; Al-Salem et al., 2019b). In separate reports, they stated that light transmission in polyethylene samples containing oxo materials increased by about 63% since 12 months, and the opposite parameter, turbidity, has been decreasing. They also stated that light transmission in treatments whose polymer structure was damaged did not increase compared to each other. The results of this part of the research are consistent with the results of the above studies.
Table 5
Mean comparison results of samples turbidity after six and eleven months in the soil
Treatments | Mean of turbidity |
Sixth month | Eleventh month |
NS1 | 2.23 b | 2.06 b |
NZ1 | 2.27 b | 1.73 b |
BS1 | 1.53 ce | 1.1 ce |
BZ1 | 1.3 c | 0.9 de |
NS2 | 2.03 b | 2.03 b |
NZ2 | 1.9 be | 1.73 b |
BS2 | 1.43 ce | 0.87 de |
BZ2 | 1.4 c | 1.3 c |
N00 | 2.7 a | 2.7 a |
B00 | 2.7 a | 2.7 a |
Means that have one letter in common do not differ significantly at the 5% probability level of the LSD test. |
Table 5
Investigation of mechanical properties
Elongation at the point of rupture
Table 6 shows the results of the elongation analysis of variance at the point of rupture for two normal and biodegradable tapes after six and eleven months with the one-sample t-test. The two stages of sampling are not significantly different in terms of elongation index. However, according to Fig. 1, tapes made with oxo additive and ordinary tapes, whether in the subsoil or on the soil surface and under direct light or plant shade, in terms of elongation amounts increase over time. It has been found that this difference is more pronounced in tapes made with oxo additive. This could be due to a change in the structure of the polymer. Antelava et al. (2020) report that the percentage of elongation to rupture point in plastics containing oxo material decreases from 875–525% in 5 days and then it remains constant and no longer decreases. The results of La Mantia et al. 0(2020) showed that the elongation parameter at the rupture point for polyethylene film and polyethylene with oxo additive (after exposure to laboratory conditions and long-term UV radiation) 8 hours and at a temperature of 70 degrees) is not much different. The result of this section is consistent with the report of La Mantia et al. (2020).
Table 6
Analysis of variance results on elongation values at rupture point for both conventional and biodegradable tapes
Time | Mean square | F | df | P-value |
Six months | 6726.9 | 0.792 | 9 | 0.627 |
Eleven months | 12043.5 | 1.88 | 9 | 0.098 |
Table 6
Figure 2
Tensile strength at the point of rupture
Table 7 shows the results of analysis of variance for tensile strength values at the rupture point by one sample t-test. As can be seen, this parameter is not significantly different from the average unused tape in the treatments for normal tape in the sixth and eleventh months. On the other hand, in tapes made with oxo additive, although there is no significant difference in values after six months, but this difference is significant after eleven months. According to Fig. 3, the highest decrease in stress strength at the point of rupture in the treatment of biodegradable tapes is related to BS1 and BS2, respectively. According to Fig. 3, these values have not changed significantly in the normal tape treatment; therefore, it can be concluded that the structure of the degradable oxidizing polymer has changed and its resistance to stress is decreasing. La Mantia et al. (2020) and Vogt and Arne Kleppe (2009) as in the present study, showed a decrease in tensile strength in polyethylene films containing oxo materials compared to normal films after being placed in laboratory conditions.
Table 7
Analysis of variance results on stress strength at rupture point for both conventional and biodegradable tapes
Treatments | Time | Mean differences | T | df | P-value |
Conventional tape | Six months | -0.2667 | -0.142 | 11 | 0.890 |
Eleven months | 1.533 | 0.884 | 11 | 0.396 |
Biodegradable tape | Six months | -8.158 | -2.98 | 11 | 0.007 |
| Eleven months | -7.683 | -5.034 | 11 | 0.000 |
Table 7
Figure 3
Tensile strength at the yield point
Table 8 shows the results of analysis of variance for the tensile strength values at the yield point with one sample t-test. This parameter has no significant difference in the treatments related to normal and biodegradable tapes in the 6th and 11th months. Therefore, in terms of the changes of this parameter during the 11-month period, it is not possible to draw conclusions about destruction or non-destruction in all treatments.
Table 8
Analysis of variance results on stress strength at yield point for both conventional and biodegradable tapes
Treatments | Time | Mean differences | T | df | P-value |
Conventional tape | Six months | 0.235 | 0.685 | 11 | 0.508 |
Eleven months | 0.659 | 1.651 | 11 | 0.127 |
Biodegradable tape | Six months | -0.092 | -0.237 | 11 | 0.817 |
| Eleven months | 0.354 | 1.042 | 11 | 0.320 |
Table 8
Young module
Table 9 shows the results of analysis of variance for Young modulus values with one sample t-test. As can be seen, this parameter in the treatments for normal tape in the sixth and eleventh months is not significantly different from the average of not used normal tape. On the other hand, in tapes made with oxo additive, although no significant difference in values is observed after six months and in some treatments, the Young module has increased, but this difference is significant after eleven months. And the amount of Young module has decreased compared to unused biodegradable tape. According to Fig. 4, the highest decrease in Young module values in the treatment of biodegradable tapes in the sixth month is related to BS1 treatment and after eleven months is related to BZ2 treatment. Therefore, tapes made of degradable oxidizing polymer have changed after eleven months, and of course the amount of Young module in them is decreasing. Since the Young module shows the tolerable amount of polymer stress up to the yield point, it can be concluded that degradation is occurring in these treatments. Antelava et al. (2020) reported that the Young module of polyethylene films containing oxo materials gradually increased after 15 days and under UV radiation, and suddenly decreased between the 15th and 20th day. Rodrigues et al. (2013), in their research on the bags used in stores made of oxo materials and after 45 days in the incubator, stated the Young module of these bags which has increased from 33.341 MPa to 17.417 MPa and has decreased by about 50%. The results of Lamantia et al. (2020) showed that the Young module for polyethylene and polyethylene films with oxo additive after exposure to laboratory conditions and UV irradiation for 8 hours at the temperature of 70 ° C has dropped from 206 MPa to 183 MPa. In general, the results of the present study are the same as the results of other studies in this section.
Table 9
Yang modulus analysis results for both conventional and biodegradable tapes
Treatments | Time | Mean differences | t | df | P-value |
Conventional tape | Six months | 157.5 | 1.40 | 11 | 0.189 |
Eleven months | 156.8 | 1.42 | 11 | 0.184 |
Biodegradable tape | Six months | 158.3 | 1.37 | 11 | 0.196 |
| Eleven months | 104.3 | -4.99 | 11 | 0.000 |
Table 9
Figure 4
Toughness
Table 10 shows the results of analysis of variance of toughness values by one sample t-test. As can be seen, this parameter is not significantly different from the average of normal unused tape in the treatments related to normal tape in the sixth and eleventh months. Six months and after eleven months it has changed and is decreasing. According to Fig. 5, the highest decrease in toughness in the treatment of biodegradable tapes in both the sixth and eleventh months is related to the BS1 treatment. Toughness defines the amount of energy required for failure and rupture. Therefore, it can be concluded that by changing the structure of polymers in tapes containing oxo additives, these materials are degraded. The results of this section are consistent with the results of Rodrigues et al. (2013), where the amount of toughness of the bags made with oxo materials has decreased from 0.069 J to 0.013 J after 45 days in the incubator.
Table 10
Analysis of variance results on toughness for both conventional and biodegradable tapes
Treatments | Time | Mean differences | t | df | P-value |
Conventional tape | Six months | 84.03 | 0.739 | 11 | 0.476 |
Eleven months | 85.45 | 0.989 | 11 | 0.344 |
Biodegradable tape | Six months | 95.91 | -2.492 | 11 | 0.030 |
| Eleven months | 65.50 | -10.99 | 11 | 0.000 |
Table 10
Figure 5