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Clean the oceans: decomposition of plastic through sunlight



One of the most significant inventions for modern society is plastic, i.e. synthetic polymers. Cheapness, ease of manufacture and variability of appearance and physical properties allowed plastic to spread around the world with incredible speed. There are many types of plastic, but all of them are present to one degree or another in our life every second, whether we want it or not: stationery, auto parts, household appliances, medical equipment, toys, packaging of grocery and household goods, etc. Naturally, such a wide distribution of this miracle material leads to the formation of a huge amount of plastic garbage, which is in no hurry to decompose, leading to an environmental disaster both on land and on water. In order to solve this problem,Scientists from Cornell University (USA) over the past 15 years have developed a new type of plastic that will meet all commercial requirements, but with prolonged exposure to ultraviolet light will decompose quickly enough. What does the new polymer consist of, how does UV radiation decompose it, and can this invention save the World Ocean from plastic suffocation? The answers to these questions await us in the report of scientists. Go.

Study basis


We have already discussed a study in which scientists calculated the amount of plastic debris in the oceans based on data on remote and uninhabited islands.

Nevertheless, it will not be superfluous to remind ourselves that in recent years the production of plastic has exceeded 350-400 million tons per year (according to various estimates). At the same time, a rather impressive part of plastic waste is in the waters of the oceans. Researchers remind us of a large Pacific garbage spot - a collection of garbage in the northern Pacific Ocean, whose area is about 1.6 million km 2. This garbage island has 79 thousand tons of plastic. Someone may suggest “catching” this island and sending garbage for recycling, but it is far from the only one. Specifically, this garbage spot occupies only a small fraction of about 4-12 million tons of garbage in the oceans and seas around the planet. While we will catch one garbage island, a new one, or even two, will have time to form. Not to mention the complexity of such a cleanup, due to weather conditions.

The principle of “clean where they do not litter” cannot yet be called a defining feature of our species. Therefore, scientists decided to create a material from which garbage will not last as long as from ordinary plastic (an ordinary plastic bag decomposes in the soil for about 100 years).

Researchers note that approximately 52% of the debris in the oceans is plastic fishing nets and other gear (data from 2015). These items not only contribute to the increase in the accumulation of garbage, but also lead to the death of marine life.

To convince the fishing industry to use something safer than plastic, the task is impossible. Plastic nets are cheap, it’s not a pity to lose them in the ocean (from an economic point of view), they can be easily and quickly produced, which means plastic will be used in the future.


Scheme number 1: the ideal way to decompose plastic.

If you can’t convince, then you can modify the existing material so that it is just as durable and practical, but it decomposes much faster (diagram above).

The leader among the polymers in the fishing industry is isotactic polypropylene (iPP from isotactic polypropylene ), high density polyethylene (HDPE from high-density polyethylene ), and polyamides.

Isotactic polypropylene is one of the types of polypropylene in molecular structure (the other two are syndiotactic and atactic). Isotactic has a high density (910 kg / m 3 ), a high melting point (up to 170 ° C) and high resistance to various chemicals. In other words, this type of plastic is very resistant, which is bad news for the environment.

High density polyethylene also has a high specific strength at a density of about 970 kg / m 3. This indicator is slightly different from ordinary polyethylene, but HDPE has more dense intermolecular bonds, which makes it stronger. The melting point of HDPE is 120 ° C.

Polyamide is a plastic with amide groups –CONH− in its main chain. This supplement makes it very durable, tough, viscous and resistant to external factors. The melting point and density depend on the particular type of polyamide, varying in the range from 178 ° C and 1 010 kg / m 3 to 220 ° C and 1 084 kg / m 3 .

One of the methods to accelerate the decomposition of plastics is the introduction of any dopants (additives) into their composition. However, the disadvantage of this technique is the high probability of initiation, i.e. chains of secondary undesirable chemical reactions.

Accelerating plastic decomposition by increasing its sensitivity to ultraviolet light is not a new idea. Similar developments have been going on since the 50s of the last century. The result was plastic (ethylene monoxide copolymer) used in the manufacture of beverage packaging. However, such material does not differ in the high strength required in fishing tackle. However, the very idea of ​​using UV has great potential.

Scientists came to the conclusion that isotactic poly (propylene oxide) or iPPO can ideally fit the role of the foundation of a new plastic, since it is structurally very similar to regular iPP. A higher level of tact (identical repeating stereochemical chains) in the iPP increases its crystallinity, which leads to an increase in mechanical strength.

Consequently, high tactfulness can also be achieved in PPO by increasing strength. Moreover, the presence of ether bonds will provide sensitivity to photochemical degradation due to UV radiation.

Commercial production aPPO uses dual metal cyanide catalysts in combination with alcohol chain transfer agents (CTAs) to produce low molecular weight polymers with low dispersion (Đ). However, maintaining such control over the structure and at the same time obtaining high tact is problematic for the synthesis of iPPO.

Therefore, scientists decided to use enantioselective catalysts that selectively polymerize one enantiomer while simultaneously dissolving a less reactive epoxide. As a result, several high levels of stereoselective Co and Cr catalysts were obtained.stereoselectivity * (up to> 99%).
Stereoselectivity * - when during the course of a chemical reaction the formation of one stereoisomer prevails over another.
Highly isotactic poly (propylene oxide) can be synthesized from enantiomeric * propylene oxide by chain transfer polymerization ( A in Scheme 2).


Scheme 2: synthesis of iPPO.
Enantiomers * - a pair of stereoisomers, which are mirror images of each other, not compatible in space.

Stereoisomers are chemical compounds that have the same structure, but differ in the spatial arrangement of atoms.
For this type of synthesis, a catalyst is required that provides mutual enrichment, as well as an enantiomonomer, which makes it practically impossible for production on a large scale.

Enantioselective chain transfer polymerization can also result in a highly isotactic polymer, where tact is determined solely by catalyst selectivity ( B in Scheme 2).

Isoselective polymerization of the chain provides a unique polymer architecture in which both monomers ( C in Scheme 2) are included in the stereo block * configuration (SB, i.e., stereoregular block polymer).
Stereoregular polymers * - polymers in the macromolecules of which an asymmetric carbon atom is present.
Despite the variety of iPPO synthesis options, little is known about the mechanical properties of this material. Therefore, scientists decided to synthesize different variants of highly isotactic iPPOs and study its mechanical strength and photodegradation.

Research results


In its enantiomeric and racemic form, complex 1 was used to polymerize the enantiomeric and racemic PO to create each stereoregular iPPO configuration (Table 1).


Table No. 1: synthesis of various stereoregular configurations of highly isotactic iPPO using 1 and 1,6-hexanediol.

Enantio-enriched (S) -iPPO was obtained from (S) -1 and (rac) -PO in the presence of 1,6-hexanediol (1,6-HD) as CTA. Its enantiomeric form was synthesized from (S) -PO using (rac) -1 under similar reaction conditions. Enantiomeric ®-iPPO was obtained in the same way using ®-PO. Finally, mixing equal amounts of enantiomeric (S) - and ®-iPPO in solution gave a highly isotactic (rac) -iPPO. As expected, all synthesized forms of iPPO have similar values ​​of Mn (molecular weight) and Đ (dispersion).


Image No. 1

Each stereoregular form of iPPO was subjected to uniaxial elongation, followed by the removal of non-reactive PO and dimethoxyethane (DME).

Each material exhibited an elastic reaction to a sharp peak in yield, followed by a flat region, and then an increase in stress to failure.

As expected, enantiomeric (S) - and ®-iPPOs have the same tensile profile with an elastic modulus of ~ 290 MPa, a yield strength of ~ 12 MPa, and a tensile strength (UTS) of 75 MPa ( 1A ).

Although an enantiomerically enriched (with more than one enantiomer) (S) -iPPO has minor errors due to the insignificant inclusion of ®-PO, its elastic response is comparable to enantiomerically pure (only one enantiomer) (S) -iPPO at large deformation ( 1B) It has a slightly reduced modulus of elasticity and yield strength of 248 and 11 MPa, respectively. Variants of (rac) -iPPO and (SB) -iPPO showed the lowest rates of decrease in strength (4 and 10 MPa, respectively) under tension ( 1C ).

Lower strength (SB) -iPPO may be due to its low isotacticity. However, the structure of its isotactic triads (mm) is similar to that of enantiomerically enriched (S) -iPPO, which exhibits the same tensile strength as enantiomerically pure iPPO.


Isotactic triad (mm) of a polypropylene molecule. The mass fraction of isotactic (mm) triads is a general quantitative measure of tact assessment.

The lower mm triad content observed in (SB) -iPPO is characteristic of its block microstructure. But for enantiomerically enriched (S) -iPPO, this is due to the random inclusion of the less preferred enantiomer.

(SB) -iPPO maintains the approximate yield strength of all other forms of iPPO, however, its tensile behavior diverges during strain hardening. With continuous uniaxial elongation, semi-crystalline polymer chains often rebuild after irreversible deformation, which leads to crystallization, otherwise called mechanical hardening. It was found that it is this process that leads to a decrease in tensile strength of (SB) -iPPO.

Scientists note that in many polymers, fluctuations in stress (strain) are observed, which is usually inherent in metals. Previously, it was rather difficult to establish the cause of such vibrations, however, there is a theory that this is due to the formation of micro-cracks and cavities in the polymer structure.


Image No. 2

Earlier studies of stress fluctuations showed their dependence on temperature, strain rate, and the nature of the material. However, due to the relatively low melting point of iPPO (68 ° C), the temperature dependence of the vibrations in this work was not studied. Instead, the dependence of the strain rate of vibrations was studied in the strain range from 0.10% to 1000% (graph above).

An increase in the strain rate above 100% led to the attenuation and disappearance of the dentate response (indicated by a dotted line), but also to a decrease in the tensile strength.

To determine the upper value of the iPPO tensile strength, various variants of enantiomerically enriched (S) -iPPO were synthesized. The polymers were synthesized with molecular weights ranging from 49 to 206 kDa. Each sample showed the same approximate tact and yield strength (about 12 MPa).


Image No. 3

An increase in molecular weight above 100 kDa led to a slight improvement in tension. The occurrence of a loss of tensile strength was also detected when the molecular weight decreased below 104 kDa.

Having determined the limiting value of ultimate strength for iPPO, scientists conducted a comparative analysis of this polymer with commercially available plastics (iPP, HDPE and nylon-6.6).


Image No. 4

iPPO showed a relatively low yield strength, but the tensile strength was higher than that of commercial polymers (graph above).

The iPPO under study is strong enough, which is extremely important for the fishing industry. It remains to check how susceptible it is to sunlight. Naturally, the light intensity varies depending on the season, weather conditions and time of day, but for experimental purposes, it was decided to use constant radiation, but less intense than it would be in natural conditions.

The sample was exposed to UV radiation (365 nm) for 30 days at an intensity of 250 μW / cm 2 . Decomposition was evaluated by gel permeation chromatography (GPC) to quantify the change in polymer molecular weight.


Image No. 5 The

sample began to decompose after 3 days, as evidenced by a drop in molecular weight of 9 kDa (graph above, blue markers). After 15 days, the molecular weight has already lost 29 kDa. The final result of the experiment (after 30 days) showed that the molecular weight of the polymer decreased from 93 kDa to 21 kDa.

In parallel, the decomposition of the same polymer was evaluated, but without UV radiation (graph above, red markers). As expected, after a full 30 days, the molecular weight has not changed, which indicates the absence of degradation of the material.

In this case, the main mechanism of decomposition can be considered autooxidation. Low molecular weight antioxidants are often used as radical stabilizers in commercial polyesters and polyurethanes. Manipulating these additives will allow you to control the life of the polymer.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists .

Epilogue


In this work, scientists were able to develop a new polymer that exhibits characteristics comparable to commercial polymers, but can decompose under the influence of sunlight. Of course, in the future it is planned to conduct several more studies in order to gain full control over the decomposition process, because no one needs fishing nets that decompose from the first day of use.

The amount of plastic used can be reduced, but this can lead to other problems, both in the economy and in the environment. Scientists believe that the problem must be considered from a different angle - if you can’t refuse plastic, then you need to change it so that the use of plastic does not harm the environment.

However, it is worth noting that the presence of this kind of research does not mean that you can be careless in the matter of plastic waste, its sorting, etc. It is impossible to rely on scientists, thinking that they will all “resolve”. Indeed, the creation of a new super-plastic is still a long way off, and garbage islands in the oceans are still floating. Therefore, each person should, as far as possible, contribute to the common cause of preserving the environment. If we are not able to correct the environmental damage caused by previous generations, then at least we should not aggravate it.

Thank you for your attention, remain curious and have a good working week, guys. :)

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