SUMMARY
The end-of-life management of post-consumer plastic materials plays an important role in the development of sustainable polymer products. Over the last few years the growing effort to find environmentally more friendly solutions led to the support of biodegradable materials as an alternative to poly(hydrocarbon) (PE, PP, PS). In parallel PE, PP, PS full-carbon backbone thermoplastic polymers have been re-engineered by addition of pro-oxidants able to promote the carbon backbone oxidation eventually followed by backbone breakdown.
The major strategies in order to overcome the intrinsic recalcitrance of polyolefins to biological attack have been focused on the introduction of functional groups and chemical components (pro-oxidants) able to promote the formation of free radicals at carbon backbone susceptible to uptake oxygen with the formation of hydroperoxides. These last give raise to a free radical chain reaction leading to an abiotic, thermally and/or photophysical assisted breakdown of the polymer backbone with formation of oxidized groups. The oxidized fragments are vulnerable to microorganisms leading to a biotic phase with digestion of the chain fragments to CO2, H2O and cell biomass.
The production and consumption of plastics, in the last decade has recorded a remarkable increase in the scientific and industrial interest in environmentally degradable polymers and relevant plastic items (EDPs). Since the ultimate fate of EDPs has to be their conversion by microorganisms into metabolites such as CO2, H2O and new cell biomass (i.e. mineralization). The requirement of two steps, abiotic and biotic, in the degradation mechanism of oxo-biodegradable plastic items has recently inspired the definition and approval by the American Society for Testing and Materials (ASTM) of a Standard Guide ASTM D6954-04 “Standard guide for exposing and testing plastics that degrade in the
environment by a combination of oxidation and biodegradation”. Analogous initiative was
undertaken soon after the approval in 2002 of the EN 13432 norm on “Requirements for
packaging recoverable through composting and biodegradation – Test scheme and evaluation criteria for the final acceptance of packaging” by British Standard Institute (BSI)
as it was considered too discriminatory toward large consume plastic commodities. The norm BSI 8472 is in progress and when approved should give the input for an extend approval to EC- Countries.
known biostable full carbon backbone polymers convertible to eco-compatible plastic items imply the following steps:
Abiotic treatment meant to promote and assist the oxidative degradation under different
environmental conditions.
Biotic digestion of the oxidized polymer fragments.
Assessment of ultimate environmental fate of the analyzed samples and their impact on
toxicity.
Abiotic degradation studies, carried out under different test conditions, were performed in order to establish the role of pro-oxidant additives in enhancing the rate and extent of oxidation and evaluation of full-carbon backbone chain scissions as a prerequisite to promote the attack by microorganisms and finally to end up with biodegradation.
The propensity to oxidation in terms of rate and extent was found to be dependant upon the following abiotic parameters:
1) Type and amount of pro-oxidant.
2) Temperature at which the samples are exposed. 3) Outdoor exposure, time, temperature and light dose.
4) Exposure under static or dynamic conditions in oven in air atmosphere at controlled temperatures.
5) Exposure in air environment at controlled humidity level.
Combined effects, were also found to be dependent upon the cross-action of abiotic parameters and structural characteristics of the analyzed samples. Poly(ethylene) (LDPE, LLDPE, HDPE), Poly(propylene) (PP, BOPP), Poly(styrene) (HIPS, CPS).
Thermal oxidation was particularly effective in the case of PE and PP samples, whereas only minor effects were ascertained in the case of PS. The rate and extent of oxidation of PE/PP samples were positively affected by both temperature and oxygen partial pressure, whereas a slight drop in rate and extent of oxidation was found to be associated to the humidity level in the case of PE, but not in the case of PP samples. On the other hand, sunlight outdoor exposure (3 months late spring/early summer) resulted less efficient in promoting oxidation of the analyzed LLDPE samples. This behavior can be attributed in a first instance to the ambient temperature monitored during the test that was in any case below 35°C. On the contrary, thermal degradation behavior of samples previously submitted to outdoor sunlight exposure appeared to be different from that exhibited by the pristine samples submitted only to thermal degradation tests. The absence of induction phases in the oxidation processes of
light exposed samples was evident during the tests. These observations may suggest that the initiation of the oxidation process, as promoted by light irradiation, positively affected the rate of oxidation once the samples were submitted to a thermal treatment. It can be therefore suggested that the combination of UV radiation and temperature was capable to promote oxidative degradation of the tested LLDPE samples containing pro-oxidant additives. In particular, the level of oxidation, is promoting the increase of the amount of the solvent extractable fraction as well as a significant decrease of the relevant molecular weight that was found to correlate with the extent of oxidation (carbonyl index), as determined by FT-IR spectroscopy. This holds true particularly in the case of PP and PE samples, thus providing evidence on the statistical random scission of the polymer chains, according to Norrish I and/or Norrish II, which was accompanied by the formation of substantial amounts of low molecular weight fractions extractable by different solvents. On the contrary, in the case of PS samples, the random chain scissions does not seems to occur in spite of the presence of tertiary carbon atoms in the main chain in 1-3 positions. Instead sub-terminal oxidation and relevant release of oxidized polyaromatic moieties might be the main degradation mechanism occurring for the outdoor exposed PS samples.
In addition, GPC determinations showed that the molecular weights of solvent extractable fractions from abiotically degraded PE and PP samples are fairly low (0.4-1.9kD) and compatible with their potential vulnerability by natural occurring microorganisms. The results obtained during thermal and photo degradation tests are therefore demonstrating that the polyolefin matrices can be effectively oxidized by using pro-oxidant additives based on transition metal organic salts, as well as that the rate and extent of the oxidation processes is depending upon the environmental conditions.
A study was undertaken on the oxo-biodegradable materials preparation of LDPE/alkali lignin (AL) blends in abscnce or presence of pro-oxidant additives. The obtained results, even within the limits of the number of the samples investigated, are therefore ultizing natural auto-oxidazable and biodegradable components. This may represent an useful intriguing implementation in affecting the propensity to oxo-degradation of the re-engineered polyolefins composites.
The biodegradation propensity of abiotically pre-aged (thermal and outdoor exposed) and pristine polyolefin samples have been ascertained in aqueous and soil burial conditions as aimed at establishing the mineralization rate and extent of several polymeric materials, as well as to ascertain the progress of polymer oxidation and degradation of full carbon
backbone polymers by natural occurring microorganisms. The microbial consumption of oxidized fractions present in abiotically degraded PE and PP films was confirmed by the decrease (30-35%) in the COi values of the films submitted to the biodegradation test with respect to the starting pre-treated samples. The microbial degradation and assimilation was particularly effective in the case of solvent extracted fractions from PE and PP degraded samples. Nevertheless the higher propensity to microbial assimilation of linear oxidized fractions coming from PE with respect to fairly high branched PP fractions was observed in accordance to the role of sterical effects of side chains in refraining the microbial attacks. During soil burial respirometric tests it was also ascertained the potential for the ultimate biodegradability of polyolefins (LLDPE, PP and PS) previously exposed to abiotic degradation tests (thermal and/or outdoor).Finally it has been interestingly found that single soil borne microbial species are capable to promote the oxidation of pro-oxidant loaded LLDPE samples once the process has been initiated by relatively mild degrading conditions to which the samples have been exposed, such as those related to a few months outdoor exposure. The information pertaining the level of thermal and photo-oxidation required to achieve an effective and sustained biodegradation of full-carbon backbone polymers is critical for the design of polyolefin-based products and predicting their environmental fate. The research activity undertaken during the present PhD thesis provides important information with respect to synergistic effects of microbial/enzymatic attack and physical-chemical parameters in promoting the degradation of partially oxidized full-carbon backbone polymers, thus allowing for a better design of oxo-biodegradable materials to be really and ultimately biodegraded under different natural environments.
