Melt Processing Of CO₂-based Plastics For Engineering Application

CO2-based plastic(PPC),an alternating copolymer of CO2 and propylene oxide,represents the lowest cost in all synthetic biodegradable plastics,because it contains more than 40 wt%of CO2 in mainchain.Such low cost characteristics allow-solving the low price performance issues that long-term exists in the biodegradable plastics industry.However,the industry application of PPC was far from satisfactory,since the material system based on PPC was still in early stage of development,let alone the relationship between the structure and property of PPC,which severely limits the development of PPC materials.The research in this thesis was based on Basic Science Center project from National Natural Science Foundation,which aims at developing tens of thousand applications of PPC products.For this purpose,it was necessary to conduct studies on the thermal stability of melt processing,melt blending behavior and melt processing engineering of products.Therefore,in this thesis,following researches were carried out,including the thermal stability of PPC in the melting process,the compatibility of PPC with biodegradable plastics like poly(butylene adipate-co-terephthalate)(PBAT),the water vapor and oxygen barrier performance,and engineering applications of PPC-based materials,etc.The main results are summarized as follows.1.The influence of PPC molecular chain structure on melt processing was studied.It was found that the content of ether segment in PPC chain,the catalyst residual and the shear strength were the key factors affecting the thermal stability of PPC.The PPC thermal degradation mechanism study showed that thermal oxygen degradation was resulted from the random chain scission of PPC.Through the addition of suitable heat stabilizer,the initial thermal decomposition temperature can be raised by 39℃,Introducing lubricant into PPC can also enhance the thermal stability of PPC during melt processing,due to less heat formation via reducing friction among PPC molecular chain.2.The compatibility of melt blending between PPC and PBAT was studied.It was found that the addition of compatibilizer with multi-epoxy group can enhance their compatibility.However,owing to the crosslinking reaction of the added compatibilizer,it is difficule to be well-mixed into the PPC/PBAT blends at low loading,giving batch instability issues.Therefore,PPC/PBAT block copolymers with different molecular weights were prepared as compatibilizers.Rrsults showed that the mixture of PPC/PBAT block copolymer with the lower molecular weight(Mn:4～6 kg/mol)and higher molecular weight(Mn:20～60 kg/mol)was more effective in improving the compatibility.For a typical polyblend with PPC:PBAT:PPC/PBAT block copolymer of 90:10:5(weight ratio),it showed tensile strength of 38.8MPa with elongation at break of 160%,indicating that the PPC/PBAT block copolymer was quite an effective compatibilizer for PPC/PBAT polyblend,especially using PPC as the dominant component.3.The terpolymerization of CO2,propylene oxide and allyl glycidyl ether was carried out to prepare reactive compatibilizer(C-PPC)with double bond on the side chain.The addition of C-PPC into PPC/PBAT polyblend can significantly improve the compatibility between PBAT and PPC.The water vapor and oxygen barrier properties of PBAT/PPC composite films increased by 51.9%and 33.5%,respectively,accompanied by an obviously improved mechanical properties.For example,addition of 1.0 wt%C-PPC into the PBAT/PPC(75:25)blends,the resulting mixture displayed an increased tensile strength(from 24.7 to 30.3 MPa)and elongation at break(from 858%to 1042%),indicating that reactive compatibilizer is an effective way for improving the compatibility of PPC/PBAT polyblend.4.The melt processing and engineering application of PPC/PBAT polyblend was studied for the preparation of biodegradable mulch.The resulting mulch showed excellent tensile strength(>20 MPa),elongation at break(>400%)and water vapor transmission(<400g/m2d),facilating research on the application of mulching film engineering for three consecutive years.In 2022,the demonstration application of 3000 mu PPC/PBAT biodegradable mulch film was carried out in Wuxiang,Shanxi province,developing overwintering millet biodegradable mulch.Meanwhile the effective coverage of cotton with mulching film in Xinjiang reached record high weathering period of 70 days,which is expected to be widely promoted across the country.Moreover,on the basis the merits of high gas barrier,strong heat seal strength and carbon dioxide affinity of PPC,this thesis reported PPC-based biodegradable postal express bag,bubble bag and foams,achieving PPC-based biodegradable products with important engineering application value.