Aromatic polyester, represented by PET, has found wide application in production of chemical fibers, packaging and other industries due to its excellent chemical stability, good mechanical and hygienic properties, and transparency. At present, production and sales of polyester still maintain a high growth rate, especially in field of carbonated beverage packaging: with breakthrough in research on barrier properties of polyester, application of polyester in fields of beer, food and cosmetics will make its market further expanded. However, PET polyester waste is difficult to decompose naturally. Polyester bottles exist in an environment with a humidity of 45% -10% and a temperature of 20 ° C for 30-40 years, and lose their properties only by 50%, under same conditions, polyester films can exist for 90-10 years. . For this reason, a large amount of polyester waste will put a lot of pressure on environment.
Recycling polyester waste can not only solve problem of environmental pollution, but also ensure full use of resources. This is a method pioneered by countries around world. Various PET-based polyester processing technologies have been developed. A simple recovery could be to clean polyester waste and remelt it to produce relatively low-grade products such as toys and detergents; Relatively high-grade polyester recovery is to use polyester materials with large polycondensation sizes Molecules can be decomposed, repolymerized, or used as chemical raw materials, etc., in addition, there are oil technologies, fuel recovery, and incineration together with other waste to generate energy.
However, recycling and reusing polyester waste may not be final solution to problem of environmental pollution. Firstly, there are certain restrictions on polyester waste that can be recycled: they contain a large amount of additives or contain other impurities that are difficult to remove, they have been recycled many times, they are very difficult to recycle and reuse. ; secondly, a large number of polyester products that are inconvenient to collect, such as agricultural film, garbage bags, etc., are not suitable for recycling; Finally, items that are too expensive to recycle or have no recyclable value do not deserve to be recycled. At present, it is necessary to change ability of polyester used in production of such products to degrade in environment so that waste can naturally degrade into low molecular weight products over a period of time in nature and finally return to material. cycle of nature.
Effectively controlling lifespan of PET polyester waste in nature and preventing it from polluting environment is very beneficial to improve environmental performance of PET polyester materials and thus contribute to their long-term development.
1. Factors affecting degradability of aromatic polyester materials
Environmental degradability refers to property that macromolecular chains of polymeric materials can gradually break down over a period of time under natural conditions without human intervention, and finally break down into small molecules that can be absorbed by nature. Cellulose, polylactic acid and aliphatic polyester are environmentally degradable materials. Light, heat, water and microorganisms in nature are all factors that cause degradation of polymeric materials. According to different types of polymers and specific natural environmental conditions, main causes of degradation are very different, but basically they are a complex and mutuallyIt is synergistic effect of various causes. Environmental degradation is a complex process with many aspects.
Under natural conditions, due to lack of powerful sources of light and heat, decomposition of polyester materials occurs mainly due to hydrolysis and microbial decomposition. In other words, degradability of polyester materials in environment is related to hydrophilicity of materials, that is, ability to undergo hydrolysis and susceptibility to damage by microbial enzymes. PET polyester hydrolysis is breaking of macromolecular ester bonds under action of water, biodegradation is also breaking of macromolecular chains caused by destruction of ester bonds by microbial enzymes (such as bacteria, algae, etc.). Hydrolysis and microbial degradation of polyester PET are often associated with each other and mutually stimulate each other. The presence of biological enzymes can significantly accelerate course of hydrolysis, and hydrophilicity of materials is a necessary condition for microbial enzymes to decompose polymeric materials. Therefore, modified PET that promotes hydrolysis is equally effective for biodegradation.
Chemical factors that affect degradation characteristics of materials include hydrophilicity, morphological structure, relative molecular weight, and polymer composition. The polymers are highly hydrophilic, easily hydrolyzed, and also prone to biodegradation by microorganisms. Hydrolytic enzymes have a strong effect on ester, amide, and carbamate bonds. The area is more susceptible to damage by water and microorganisms. ; molecular chain is soft and glass transition temperature is low, which promotes degradation; degradability also increases with decreasing relative molecular weight; composition of polymers, such as blending and copolymerization, can also affect its degradability.
PET polyester contains ester bonds that are easily damaged by microbial enzymes and water molecules. In molten state, presence of traces of moisture can cause rapid cracking of polyester bonds. For this reason, moisture content of chips must be strictly controlled in production and processing of polyester. However, under normal conditions, PET polyester has good chemical stability and is difficult to decompose under natural conditions. This is due to regular structure of main chain of PET macromolecules and aromatic rings included in main chain. The existence of an aromatic ring increases polarity of regular molecular chain, reduces its flexibility and improves crystallization characteristics. A higher degree of crystallinity inhibits hydrolysis because water is less likely to enter crystalline phase. PET is a semi-crystalline polymer, initial stage of its decomposition occurs in a relatively loose amorphous region and at edge of crystallineth region, after hydrolysis of binding molecular chains between crystalline particles and cracking of chains, amorphous region will be Further crystallization significantly increases degree of crystallinity, thereby preventing further hydrolysis, on other hand, rigidity of molecular chain increases and mobility of macromolecules inevitably decreases, which is reflected in a higher glass transition temperature, which also makes polymer more resistant to hydrolysis, reduces sensitivity. Therefore, in contrast to molten state, degradation in solid state becomes a complex process dependent on activity and penetration of molecular chains.
From above analysis of factors controlling degradability of polyester PET, it can be seen that in order to improve degradability of polyester PET, it is necessary to reduce crystallization properties and glass transition temperature of polymer. Lowering glass transition temperature of polyester can improve mobility of molecular chains and reduce energy required to change state, thereby increasing susceptibility of polyester to hydrolysis. The reduction in crystallinity can allow water molecules or microorganisms to effectively enter interior of material and attack its weak ether bonds.
2. Ways to Improve Degradability of PET-Based Polyester
The way to reduce crystallinity of polyester can start not only by controlling post-processing technology of polymer materials, but also to a certain extent change initial rigidity and regularity of PET macromolecules through concept of molecular design. . The crystallization properties of PET-based polyesters can be fundamentally changed by introduction of flexible third monomeric structural units containing special functional groups. Introduction methods mainly include addition of modified third monomers for copolymerization and reaction blending with aliphatic polyesters.
When PET undergoes esterification and polycondensation, a small amount of third monomer is co-condensed with other feedstocks. Since amount of modified third monomer added is small, typically only 2-5%, and most of them are dihydric alcohols, cocondensation reaction conditions do not change much. Several representative third monomers suitable for copolymerization to improve degradability of PET, such as: polyethylene glycol, polytetramethylene glycol, 2,3-dihydrocarbyl-1,4-butanediol, 2,2-dihydrocarbyl-1,3-propanediol.
Polyethylene glycol and polytetramethylene glycol are linear oligomers of diols. After copolymerization with PET, introduction of flexible chains not only appropriately reduces initial rigidity and regularity of PET macromolecular chains, but also increases hydrophilic ether bonds, which are easily damaged by water and microorganisms; 2,3-dihydrocarbyl-1,4-butanediol and 2,2-dihydrocarbyl-1,3-propanediolThey are diols containing side hydrocarbon groups, which can effectively increase distance between molecules and reduce mutual attraction between molecules, thereby reducing crystallization properties of polymers.
The introduction of a small amount of a third monomer into polyester backbone can have a large impact on sensitivity of polyester to hydrolysis. The PET-polybutylene glycol copolymer begins to hydrolyze in a 10% dilute NaOH solution at 70°C within 4 hours, while PET containing a small amount of polyethylene glycol monomer can completely dissolve in 4 hours under same conditions. The difference in its hydrolysis performance is due to fact that polyethylene glycol contains more ester bonds with strong hydrophilic ability and is easily damaged by water molecules. Studies of degradability of these two types of copolyesters show that presence of enzymes and an increase in content of third monomer can greatly accelerate decomposition reaction, and rate of decomposition reaction is related to hydrophilicity and regularity of polymer.
2) Reactive mixing
Reactive blending is a polymer modification technique that has gained a lot of attention in research and manufacturing circles in recent years. For PET polyester modification, reactive blending is blending of aromatic PET polyester and easily hydrolysable aliphatic polyester to carry out transesterification reaction. The chemical reactions involved include molecular alcoholysis or acidolysis between molecules, transesterification reactions between molecules or intramolecules, etc., can be expressed as :
Third monomeric polymers suitable for modifying PET polyesters by this method include: polycaprolactam (PCL), polylactic acid (PLLA), polyglycocolic acid (PCA) and polycaprolactam, ethylene glycol diate (PEA), etc.
The mixing reaction can be carried out in a molten state or in a heated solid state. Mixing in molten state contributes to thorough mixing of components and flow of frontal exchange reaction. However, when distribution point of a component is above decomposition temperature of other component, it can only be carried out in solid state below decomposition temperature of component. Factors affecting frontal exchange reaction during blending include affinity between components and blending conditions such as blending temperature, time, component viscosity, and catalyst. In early stage of mixing, mixture is first mixed, then a microscopically tolerant structure is formed and crystallization disappears. Sufficient interesterification will help mixture system in metastable state gradually reach degree of miscibility. Only by selecting appropriate blending components and processing conditions to complete transesterification reaction can modified PET retain its excellent physical and mechanical properties and make it degradable in environment.
Reaction mixing of PET and PEA is carried out at 265°C and reacted under vacuum for 3 hours after two polymers have reached molten state. The intrinsic viscosity of blended product is 0.68 dl/g and melting point is 230°C, which is same temperature as unmodified PET, in addition, this blended product does not react at room temperature, but is exposed to water with a pH value of 7 at 80 °C Thereafter, there may be significant weight loss.
Improving environmental degradation of PET aromatic polyesters by reactive blending appears to be more effective than copolymerization. However, choice of suitable third monomers is very limited, since melting point of PET is relatively high, and a favorable temperature for mixing reaction must be carried out in molten state of components. In addition, whether it is copolymerization or reactive blending, how to make modified product environmentally degradable while maintaining or even exceeding original excellent performance by selecting a suitable third monomer, adjusting addition amount and injection method, etc. is under control, which will become a hot research area. in future.
In addition, by adjusting polymer molding process and controlling crystallinity of polymer, thereby reducing degree of compaction of its structure, it is also possible to improve degradability of PET polyesters in environment.