Polyvinyl alcohol (PVA) is a water-soluble polymer with a large number of side hydroxyl groups on its molecular chain, making it also considered a large molecular polyol .
PVA is made by the polymerization and alcoholysis of vinyl acetate (VAc) monomer. According to the different raw materials, the preparation methods of PVA can be divided into two types: one is to prepare vinyl acetate from ethylene as the raw material; Another method is to prepare vinyl acetate from acetylene (carbide acetylene or natural gas acetylene) as raw material, and the final prepared vinyl acetate is then obtained through alcoholysis. Its performance is between plastic and rubber, with many excellent characteristics such as flexibility, oil resistance, chemical solvent resistance, gas barrier, and water resistance after special treatment, making it widely used.
China is the world's largest producer of PVA, currently accounting for approximately 50% of the world's production capacity. In 2021, the global production of PVA was approximately 1.25 million tons, of which a large portion was used in the textile fabric industry, especially in the nuclear industry, commonly used in the manufacturing of disposable rags and protective clothing.
Research on the biodegradation of PVA
At present, it is mainly concentrated in two directions. Firstly, from the perspective of materials science, PVA should be modified to make it more biodegradable; Another approach is to screen and domesticate PVA degrading strains or microbial communities from an environmental perspective. These two directions solve problems from different perspectives, and their ultimate goal is the same, which is to make PVA more biodegradable and have better environmental compatibility.
Modification of PVA for degradation purposes
At present, the PVA/starch blend system is being studied more extensively, but the physical properties of the blend film are largely related to the starch content. Taking 98% PVA and corn starch blend as an example, when the ratio of the two is 85:15, the tensile strength decreases by 32% after 18 months of soil burial; When the ratio of the two is 95:5, its tensile strength only decreases by 5%.
The relevant patent mentions the modification of PVA to have a dodecyl end group. The modified PVA is blended with modified starch (or starch), and its film undergoes considerable degradation in soil burial tests. For example, a film is formed by mixing ethylene vinyl alcohol copolymer terminated with dodecyl with oxidized starch. The ratio of the two can reach 25:75, and it can be buried in soil at 35 ℃ for 6 months. It is reported that it can be completely decomposed, but there is no clear evidence to indicate whether the presence of dodecyl end groups is conducive to the biodegradation of PVA. There are also reports of PVA modified by crosslinking with maleic ACID, and PVA films modified with Urea plasticizers have strong water resistance and good mechanical properties.
According to literature reports, PVA modified films mainly include: using banana starch (ST) and polyethylene glycol as raw materials, preparing biodegradable plastic films with good water resistance under the action of formaldehyde, gelatin, and borax crosslinking agents; By selecting and preparing polymers and plasticizers with complementary structures to PVA, controlling the aggregated structure of PVA to limit its crystallization, reduce its melting point, increase its thermal stability, and achieve the melting processing of PVA; Mix three drying components (starch, polyvinyl alcohol, talc powder) in a certain proportion, and add a certain amount of water while stirring. When the mixture turns into a moist but freely flowing powder, it is added to a Brabender42-mm twin-screw extruder for film blowing; Polyvinyl alcohol/silicon dioxide (PVA/Siq) blend homogenous membranes with different silicon dioxide content were prepared by sol-gel method using polyvinyl alcohol and tetraethyl orthosilicate (TEOS) as raw materials.
The influence of molecular weight and alcoholysis degree of PVA on its biodegradability
The concentration changes of PVA with an alcoholysis degree of 99% and polymerization degrees of 1300, 1500, and 1700 respectively in a biodegradable environment within 20 days. Under the same alcoholysis degree, the lower the molecular weight, the better the biodegradability. When the alcoholysis degree of PVA is within the range of 88%~99%, the effect of alcoholysis degree on the biodegradability of PVA is not significant under the same molecular weight. The influence of molecular weight on the biodegradability of PVA is greater than that of alcoholysis degree, and the molecular weight of PVA is the main factor determining its degradation rate.
The influence of crystallinity of PVA on its biodegradability
The degradation results of PVA membranes prepared by temperature control method in the biological environment are listed in Table 1. According to Table 1, the lower the crystallinity of PVA membrane, the higher its biodegradation rate. The reason is that the higher the crystallinity of PVA, the greater the intermolecular forces between the polymers, making it difficult for PVA molecular chains to form conformations that are conducive to enzyme action, and therefore less prone to degradation. The determination of the molecular weight of the remaining PVA membrane after 14 days of degradation showed that the lower the crystallinity of the membrane, the higher the molecular weight of the degraded residual PVA. This is because PVA, like other polymer materials, has a polydispersity of molecular weight. During the degradation process, small and medium molecular weight PVA is easy to detach from the PVA membrane and enter the solution, which is absorbed by bacteria for in vivo assimilation, resulting in an increase in the molecular weight of the remaining PVA membrane . This further indicates that molecular weight is the key factor determining PVA degradation .
Summary and Outlook: Reviewing the development history and current situation of PVA water-soluble biodegradable materials, we can see their important value and broad prospects in the field of environmental protection. Looking ahead to the future, we firmly believe that with the continuous progress of technology and the improvement of people's environmental awareness, PVA materials will be applied and promoted in more fields, making important contributions to the protection and sustainable development of the Earth's ecological environment. Let's work together to promote the research and application of PVA water-soluble biodegradable materials to new heights!
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