Study On The Prepapration And Properties Of Degradable Plastics Derived From Glandless Cottonseed Protein

Environmentally friendly protein based bioplastics(PBBs)have attracted renewed attention over the last few decades across the world due to increased awareness of the environmental impact of petroleum based polymers and the rising costs of raw materials.Abundance of natural resources,low cost,easily modifiable properties and ease of manufacture make PBBs suitable alternatives to petroleum-based plastics,which are being considered in many environmentally sensitive industries,including agriculture(e.g.mulch films,greenhouse films,flower pots,planting pots,etc.),packaging(one-time or short-term use before disposal)and biomedical industries.Cottonseed protein(CP)extracted from cottonseed meal has abundant amino acid components and nutrition value,thus they are being used mainly as dairy cattle feed,and are not extensively used in non-food industries.To meet this need,glandless CP was utilized in this work,as a raw material,to prepare cottonseed protein derived bioplastics(CPBs).The results presented in this thesis could be valuable for further development of PBBs with demanding properties(high mechanical performance,desirable thermomechanical relaxations,resistance to wet envioronment,etc).A series of CPBs were prepared starting from glandless CP that was subjected to the processes of protein denaturation,plastilizing,cross linking and hot compression;they showed good mechanical strength and thermal stability,and a low degree of water absorption.Specifically,the optimum synthesis conditions of CPBs were firstly investigated,followed by the analysis of protein modification and cross linking mechanism,with a close view on changes of the microstructures or chemical structures.Then the thermal stability and thermal relaxation of thermal treated CPBs,and its interaction with molecule water were investigated,with emphasises on water transport kinetic behaviour and water states adopted within the CPBs network.In addition,we characterised detailed morporlogies,element composition,mechanical properties and biodegradabilities of various CPBs.The main results of these studies are as follows:The optimum CPBs synthesis conditions were obtained: initially,cottonseed protein was denatured using a urea solution,which was then adjusted to pH = 11 with NaOH solution,the aldehyde cross-linking agent added,and the mixture vacuum-dried for 10 h at 80 °C.Glycerol as plasticiser was then added homogeneously to the dried solid using both a high-speed mixer and a three roller mill.This mixture was conditioned in a desiccator at room temperature for 24 h and then hot pressed at 20 MPa,130 °C for 5 minutes.Urea denatures CP via both indirect and direct hydrogen bonding to water and to CP.The carbonyl groups in the cross-linking agents react with the amino groups in CP under alkaline and heated environment,resulting in the formation of an imine group,and thus forming a three dimensional cross-linked network.Specifically,methylene bridge formation is responsible for the formaldehyde cross-linked networks,whilst glutaraldehyde /glyoxal are able to react with protein molecules via imine covalent bond formation.The colour of the CPBs changed from yellow-brown before preparation,to golden-brown after hot compression molding.Interestingly,the different CPBs odor was not the same after hot press molding.Aldehyde cross linking treatment brings about a characteristic of ductile fracture whereas un-crosslinked network looks smooth and shows a characteristic of brittle fracture.Heterogeneity feature of CPBs is manifested by the following findings:(i)the presence of un-reacted,or partially reacted aldehydes with variable concentration in the CPBs networks;(ii)main elements--carbon,nitrogen,oxgen(atomic ratio= 4:1:1)are randomly distributed on the CPBs surface,together with a trace amount of other elements: magnesium,sulfur,phosphorus.Besides,the higher content of plasticiser(glycerol)in CPBs,the better the strain-to-break performance.Moreover,the cross-linked CPBs exhibited improved tensile strength,modulus,and micro hardness.CPBs biodegradability was verified both under natural storage and soil burial conditions.According to thermomechanical relaxation and thermal stability measurements,it was found that four different relaxation behaviours occurred in the cross-linked CPBs,namely protein denaturation(? relaxation),a weak ? transition associated with moisture absorption,and two glass transitions(? and ? transitions).In addition,increasing plasticiser content(glycerol)in the bioplastics can lead to a decrease in denaturation and ?-relaxation temperature of CP due to the reduced structural integrity and increased free volume.On the other hand,the use of different cross-linking agents has no obvious impacts on protein denaturation,but brings about a clear difference in storage modulus at a very low temperature,as well as an improved thermal stability.The results of CPBs water absorption test were analysed using theories of Fickian diffusivity,Liquid transport and Liquid permeability,to evaluate the liquid transit properties of the polymer network.The cross-linked bioplastics have a more compact,tightly bound structure,which helps retard the distance water can diffuse within,and hence,lowers total capacity of water that can be absorbed;so a lower amount of water absorption is obtained,compared with the non-linked sample.Values of the kinetic exponent n and characteristic constant of water absorption k vary from different bioplastics,with the n value increasing,whereas the k value decreases after addition of the cross-linking agent.Interestingly,a high diffusion coefficient D value for the cross-linked bioplastics indicates that water diffuses easier in the cross-linked networks according to the theory of Liquid transport.Furthermore,the findings based on the theory of Liquid permeability suggest that the presence of cross-linked structure promotes the physical transport process,including water diffusion and permeability,while that in the absence of cross linking,it is chemical interaction such as hydrogen bonding or van der Waals interactions that facilitate water transport within the protein.The results on the fusion of water,vaporisation and weight loss of all the water-absorbed bioplastics demonstrate that at least three different states of water are present within the sample while increasing water content.At lower concentrations,water is present in strongly-bond-to-polymer state where it is strongly connected to protein polymer chains,and thus cannot freeze.When the CPB system contains a content of water above a certain threshold,it appears as weakly-bond-to-polymer water,the intermediate state,suggesting a boundary where water molecules whose mobility is partially retarded could not sufficiently interact with the polymer chains to prevent the state of fusion,yet are able to crystallise.Lastly,excess water or bulk-like water,is free of interactions with the protein chains,and is a predominant fraction at higher water contents.In addition,the remaining un-reacted aldehyde in CPBs plays an important role in the formation of bound-to-polymer water,since they have a strong bonding strength with water as evidenced by infrared spectroscopy showing a reciprocal relationship.However,the employment of different types of aldehyde as cross-linking agent for CPBs synthesis has a marginal impact on the formation of different water states that different CPBs adopt.