Currently, the world is promoting plans to reduce the use of plastic bags. In August, China’s National Development and Reform Commission,
Ministry of Ecology and Environment and other nine departments jointly issued the Notice on Solidly Promoting Plastic Pollution Control (hereinafter referred to as “Notice”),
proposing a nationwide ban on the production, sale and use of disposable non-degradable plastic products in “two key areas”, and by the end of 2022, the By the end of 2022,
the annual use of disposable non-degradable mulch will strive to achieve zero growth.
According to China’s national policy requirements and the “Opinions on Further Strengthening Plastic Pollution Control” (Guo Fa [2020] No. 28) deployment,
the nationwide ban on the production, sale and use of plastic shopping bags less than 0.025 mm thick, disposable plastic tableware and express special plastic bags.
So, how degradable plastic bags are decomposed out of it?
In fact, many people do not know the decomposition of biodegradable plastic bags and where to go after decomposition?
First, from raw materials to finished products
As biodegradable plastic raw materials for petroleum-based, commonly used raw materials for plastic bags are LDPE, PP, PET, etc..
The principle is to add degradation additives to ordinary plastics. The main role in the production of degradation additives is stabilizer.
So generally need to add stabilizers to improve the solubility of raw materials, common stabilizers are carboxylated polypropylene (referred to as CPP),
acrylic acid / ester copolymer (referred to as ABS) and maleic anhydride graft acrylic polymer.
The amount of stabilizer added is generally 4% to 6%, and the amount added varies with different raw materials.
Stabilizers are mainly divided into two categories, one is the structure improvement type, mainly refers to the chemical composition of the material has an improvement effect,
and the other is the solvent-based structure improvement type. Structural improvement type is mainly through changing the surface properties of plastic particles to improve its mechanical properties,
so that it is easy to break, dissolve or produce bubbles during use, thereby reducing the mechanical strength and dimensional stability.
Stabilizers usually account for between 20% and 30% of the cost of CPP synthetic materials and are divided into two main categories: ① organic solvent stabilizers (IPW for short)
and ② structural modifier stabilizers (APF for short).
Different types of stabilizers can be selected according to different production processes, formulations, costs and other conditions.
In the production process, they need to be added to the raw materials to make them have a certain degree of solubility to lower the melting point and thus reduce the use temperature and other technical conditions to ensure that the biodegradable plastic products in the final packaging process will not be dissolved and affect their stability and appearance and other properties.
CPP materials are generally divided into two types: one is non-natural modified CPP and the other is degradable CPP.
Degradable plastics can be divided into three major categories: the first major category is traditional non-degradable plastics, such as polyethylene (PE), polypropylene (PP), etc.;
the second major category is degradable plastics, such as polylactic acid (PLA), polycaprolactone (PCL), etc.; the third major category is non-degradable plastics,
mainly refers to polylactic acid, dioctyl adipate malonate (PBAT/PGA) and so on these traditional non-degradable plastics.
The third category is non-degradable plastics, mainly refers to traditional non-degradable plastics such as PLA, PBAT/PGA and new degradable polymers such as PLA, PEF and PHA.
Second, the principle of decomposition
The main components of plastic bags are polyethylene (PE) and polypropylene (PP), under normal circumstances,
polyethylene molecules are difficult to oxidize with the oxygen in the air.
In fact, the principle of decomposition of additive-based degradation bags just the opposite of the process of making degradation bags.
The way it is under sunlight exposure or in a suitable temperature environment, through the stabilizer in the additive reverse decomposition,
making the degradation of the bag’s large molecules break into small molecules. Constantly broken, from plastic film split into plastic blocks,
plastic blocks split into plastic shreds, then into small particles, and finally split into small molecules.
Another easy way to get plastic bags to break down is to burn them. But when the plastic bags are put on the fire,
these burning plastic particles will slowly decompose into gas, while releasing oxygen.
These gases are mainly carbon dioxide, water and heat.
Carbon dioxide is the main component of the plastic bag generated when burning.
If the gas in the plastic bag is carbon dioxide, it will react violently with oxygen, so that the plastic bag produces a large amount of heat and hydrogen gas.
Plastic bags generate a lot of harmful gases and heat after combustion; when these harmful gases reach a certain amount will also poison people in nearby rooms (carbon monoxide,
carbon dioxide, sulfur dioxide, etc.); when the plastic bag is ignited will also cause fire accidents (including electrical equipment fires, forest fires, etc.).
This is the combustion is the reason why the method is not currently popular.
Starch degradation bag decomposition is much simpler, because the composition of this material is mainly starch and PBAT, PBAT through modification into esters,
just like the soap in our life, it is very easy to decompose and be gnawed by microorganisms, and will eventually decompose into water and carbon dioxide, without any pollution.
Third. Decomposition process
Additives + PE / PP such degradable bags, after sun exposure, its chemical molecules will gradually start a chemical reaction,
the process of large molecules break into small molecules.
Usually, this additive degradable plastic bags can not be directly incinerated, because most of its materials are still plastic,
incineration produces harmful substances.
This degradable material also can not be composted, because they are only large molecules into small molecules,
decomposition into small particles is still in the form of plastic molecules.
Only starch-based degradable bags can be treated by incineration, or by composting.
1、Incineration
(1) can be treated by incineration, the principle is: after incineration into inorganic material, harmless.
(2) The waste after incineration can also be reused because it can be incinerated at high temperature and composted at high temperature.
(3) Direct disposal of waste, i.e., direct landfill.
(4) As organic fertilizer applied to farmland, woodland or pasture; it can be composted and then used to grow crops, or composted and used as industrial raw materials.
(5) Directly landfilled as domestic waste.
(6) Put the starch-based degradation bag into the incinerator for incineration treatment.
2、Composting
The principle of composting is to benefit the microbial bacteria in the environment, the degradation bag will be decomposed and chewed away,
and eventually turned into carbon dioxide and water. Composting is only for starch-based degradation bags.
In addition, some of the faster degradation of degradation bags, after a period of natural degradation will also be disposed of by incineration.
The inorganic material produced by the incineration process can also be used as composting material.
In the United States, the treatment of degradation bag waste has two methods: one is the natural accumulation of degradation bag waste in the stage of biological treatment;
the other is the use of incineration method for biological composting.
These two methods are basically the same in principle, both through microorganisms to treat waste, but the specific treatment and principles are different.
The composting products produced by the two different methods are also different, but they can be used as fertilizer.
Fourth, where does it go after decomposition?
1. additive-based degradation bags.
This kind of degradation bag, after degradation note into small particles exist in nature. But they are still small plastic particles, just maybe we can not see it.
So it is recommended that the treatment of such degradation bags are still like ordinary plastic bags, recycling, and then processing benefits into ordinary plastic bags,
which also reduces the use of plastic, indirectly protect the environment.
2. Starch-based degradation bags.
This degradation bag is decomposed into water and carbon dioxide, if the compost or the right conditions in nature, they generally only exist in nature for about 6 months,
a long time then have about 1 year, it will be decomposed into carbon dioxide and water, or directly eaten by microbes in the environment.
It does not exist in nature for a long time causing environmental pollution.
Starch-based biodegradable bags, the true name is compostable bags, is not harmful,because it can totally decompost into Water and carbon dioxide.
The additive biodegradable bags, which also call oxo-biodegradable bags, finally became small pieces of particles. but it is still kind of plastic,
it is not as harmful as plastic, and some of them also eated by Microorganisms, Maybe it is not digested. Let us define it still has some harm,
but this harm is much less visible to the naked eye compared to plastic bags, what we do not see is still unknown.
