Studies on the Interfacial Structure and Properties of Starch/Polycaprolactone Composites
Supervisor:wu qiang xian
The continuous development of plastic products in recent years has led to the replacement of traditional materials such as metals by polymeric materials.This is because polymers materials are resistant to corrosion,light in weight,cheap,easy to mold and many other advantages.However,the wide use of polymer materials is becoming a nuisance due to the the formation of "white pollution"which has a great impact on the environment because these polymer materials in the natural environment are difficult to be degraded.In addition,oil which is the main source of polymer materials is a nonrenewable resource,and the depletion of the oil resources has a great impact on the development of plastic industry.Therefore,in order to solve these increasingly serious environmental pollution problems,as well as getting rid of the dependence on oil resources,the use of natural biodegradable polymer and synthetic degradable polymer materials is a hot spot in the near future.Starch is a kind of natural renewable resource,which is obtained by the solar energy,use of water and carbon dioxide.Starch has been attracted the attention of many researchers because of its wide source,rich in content,low price and it could be degraded completely in natural environment.As a kind of natural polymer,starch can be used as a raw material to reduce the dependence on petroleum resources in the plastic industry.At the same time,the use of starch as raw materials for the preparation of plastic products can fully be degraded in a variety of environments and will not cause environment pollution.After heat processing of starch to a certain shape,the thermoplastic starch(TPS)formed can have the mechanical properties of manufactured plastic material.However,when starch is used as a continuous phase in materials,the performance of the final product is often not conducive,because the higher starch content leads to poor mechanical properties,poor stability and high water sensitivity of the final product.Like many polymer materials,the polycaprolactone(PCL)which is prepared from plant resources,is completely biodegradable.Thus,PCL can be an effective substitute for petroleum based polymers.Therefore,TPS is usually blended with PCL,with the hope of preparing of starch/PCL composite with good properties.However,the hydrophilic starch and the hydrophobic PCL are thermodynamically incompatible.Sometimes,the product obtained by blending starch and PCL have good mechanical properties,however due to the instability of the hydrogen bonds formed between starch and PCL the resultant mechanical properties of the composites decrease significantly with increasing starch content.So the use of a compatibilizer,and chemical modification of starch or PCL are used to improve the compatibility between starch and PCL.Moreover,chemical modification of starch or PCL will inevitably use tetrahydrofuran(THF),dimethyl sulfoxide(DMSO)as organic solvent,and may cause environmental pollution.Also,the addition of compatible agent such as maleic anhydride(MAH)which only reacts with a group of starch dispersed phase or PCL continuous phase has low reaction efficiency.Therefore,it is necessary to develop a simple,but efficient and environmentally friendly method to improve the compatibility and interaction between starch and PCL,in order to prepare high performance of starch-PCL composite materials.The main innovative achievements of this work include the following:(1)the first time to form a new type of coating layer which is not only compatible with the starch granules but also compatible with the PCL polymer in the starch-PCL composite,which is beneficial to the improvement of the properties of the composite material.(2)By changing the molecular weight and the structure of the hard segment and the soft segment of polyurethane in the composite,different structure of polyurethane prepolymer was synthesized.The influence of the structure of the polyurethane on the interface structure and properties of the starch-PCL composite was studied.(3)Also by changing the molecular weight of starch in composite materials preparation of maltodextrin-PCL composite,the effect of starch molecular weight on the interface structure and properties of the composite was studied.(4)Utilized the structure of coating layer set up to improve the compatibility between hydrophilic wood powder and hydrophobic PVC and also prepared wood-PVC wood plastic composites with excellent performance.(5)The melting method adopted for the coating layer without using organic solvent was green,simple,effective and environment friendly,and a suitable requirement for industrialization.The main contents and conclusions of this thesis are briefly described in the following aspects:(1)Generally hydrophilic natural polymer and hydrophobic polymer are thermodynamically incompatible and instability of their blends leads to poor mechanical properties.In this work,we found that PCLPU which contains large number of isocyanate groups(NCO)could form a coating layer on the surface of starch and react with the hydroxyl group of starch to form urethane bond,which could improve the compatibility between starch and coating layer.On the other hand,the PCL component in the coating layer and PCL matrix had good compatibility due to physical crosslinking.Therefore,the addition of PCLPU could obviously improve the interaction and compatibility between starch and PCL,and the thermal stability and mechanical properties of the composites were obviously improved.In addition,such composite materials can be simply and quickly processed in the fast growing industries through melting to form a compatible structure with improved mechanical properties and maintenance of the thermal characteristic of the plastic.(2)By changing the molecular weight of the soft segment of the PCLPU,we found that the size of the molecular weight of PUP soft segment had a significant effect on the structure and properties of the starch-PCL composite.With decreasing molecular weight of the PCLPU soft segment,the amount of NCO increased,the number of urethane bonds between PCLPU and starch increased,and the compatibility between starch and PCLPU was better.On the other hand,the physical crosslinking between PCL and PCLPU was stronger,and the compatibility between PCLPU and PCL was better with decreasing molecular weight of PCLPU soft segment.Therefore,the use of low molecular weight PCL diol as PCLPU soft segment could form a stronger coating layer between starch and PCL in preparing the composite for better performance.(3)We also found that the change of the type of PCLPU hard segment could significantly change the mechanical properties and thermal stability of the composite.The different kinds of isocyanates used in preparing the PCLPU affected the degree of phase separation between PCLPU soft and hard segment,and the level of phase separation between PCLPU soft and hard segment directly affected the physical crosslinking between PCLPU and PCL.When the PCLPU with hard segment from the aromatic isocyanates was compared with PCLPU with hard segment from the aliphatic,the degree of phase separation in the PCLPU with hard from the aromatic isocyanate was low due to the stronger interaction between their soft and hard segments.As a result,the effect ofphysical crosslinking in the composites containing PCLPU with hard segment from aromatic isocyanate was stronger than the composites containing PCLPU with hard segment from the aliphatic isocyanate.However,because the NCO content in the different kinds isocyanate structure in PCLPU is not very different,the interaction between the different PCLPU and starch was also not very different.Thus,PCLPU of different hard segment structure was the main factor that influenced the structure and properties of composite materials.Therefore,we found that the hard segment structure of the aromatic PCLPU could better improve the compatibility between starch and PCL,so that the composite exhibited better thermal stability and mechanical properties.(4)Beside the effect of PCLPU on the structure and properties of the composite materials,the content of starch in the composites has a very important effect on the structure and properties of the composite materials.Therefore,we used aromatic isocyanate and PCL diol to synthesize PCL based polyurethane prepolymer.Subsequently,different proportions of PCL and starch were reactively blended to prepare starch-PCLPU-PCL composite.We found that with increasing starch content in the composite,the hydrogen bond interaction between starch and PCL was enhanced,and the compatibility between starch and PCL in the composite became better.With higher component of the starch content,the crystallinity of starch-PCLPU-PCL composite decreased and the melting point increased.Through our analysis of the mechanical properties,we found that by increasing the quantity of PCL in the composite we could obtain a composite material with high elongation at break.However,the fracture strength of the composite increased exponentially when the starch:PCL:PCLPU mass ratio was 7:2:1,while the elongation at break was quite appreciable.At the same time,higher starch content also reduces the cost of the composite materials to some extent.So,it could thus be concluded that 7:2:1 is a good ratio of starch:PCL:PCLPU in the preparation of starch-PCLPU-PCL composite,in order to produce a comprehensive composite material with better performance.(5)Based on our theory,starch is not the only polymer that can be used as a dispersed phase in preparing such composite materials.Any other natural polymer rich in hydroxyl groups can be blended with PCLPU and PCL to prepare composite materials with better performance.Therefore,we used maltodextrin a hydrolyzed product of natural starch as the dispersed phase,and blended the maltodextrin with PCL and PCLPU to prepare maltodextrin-PCL composite materials.It was observed that the interaction between maltodextrin and PCL was successfully improved by the PCLPU.The thermal stability and the mechanical properties of the maltodextrin-PCL composites were significantly improved compared with the composites without the addition of PCLPU.However,because maltodextrin has poorer thermal stability and mechanical properties than starch,the thermal stability and mechanical properties of the maltodextrin-PCL composites were poorer than the starch-PCL composites.This showed that the properties of a composite are also influenced by the natural polymer as a dispersed phase in the composite.It was also proved that our starch-PCLPU-PCL composite system is not only suitable for starch,also suitable for other kinds of natural polymer materials,such as maltodextrin,cellulose,chitin,sodium alginate,etc.The composite material can have different properties based on the characteristics of the natural polymer used as the dispersed phase,which indicated that the molecular weight of starch has a significant effect on the properties of the composite materials.(6)In order to further prove that our composite system can significantly improve the compatibility between the hydrophilic natural polymer and a hydrophobic polymer,we used poly butylene adipate polyurethane prepolymer(PBAPU)as a compatibilizer to prepare wood flour-PVC composite.On one hand,the interaction between PBAPU and wood flour could be increased by the reaction between NCO and hydroxyl groups,and on the other hand,the interaction between PBAPU and PVC has good compatibility because of the plasticizing effect of the PBAPU.Therefore,the addition of PBAPU could obviously improve the interaction and compatibility between wood flour and PVC.Compared with the wood flour-PVC composites without PBAPU,the mechanical properties of the composites were significantly improved after the addition of PBAPU.With increasing content of wood flour,the mechanical properties and hydrophobicity of the materials decreased and the thermal stability was improved.Therefore,PBAPU as a new type of compatible agent can effectively improve the interaction and compatibility between wood flour and PVC.In this paper,a simple and efficient method was used to prepare the starch-PCLPU-PCL composite with the polyurethane(PUP)as the compatibilizer.It is believed that the structure of the coating layer had an important impact to solving the problem of incompatibility between the hydrophilic natural polymer and the hydrophobic polymer.On this basis,we prepared wood-PVC composite material,which could effectively improve the compatibility between hydrophilic wood flour and hydrophobic PVC,and also improve the performance of the wood-PVC composite.At the same time,the method of melting used in forming the coating structure without using any organic solvent in the process of preparing the composite material was green and environment friendly.In addition,the preparation process for the composite material was simple and has a short reaction time which could be applied in industries for large-scale production without the discharge of waste materials.This work did not only provide a theoretical basis and guidance for solving the problem of incompatibility between hydrophilic natural polymer and hydrophobic polymer in composite materials,but also provide a theoretical basis for the preparation of new types of composite materials with high performance.