Study On The Structure And Properties Of PET Foam And Its Sandwich

Cellular materials are commonly used in a variety of applications,including sound insulation,thermal insulation,packaging,filtration,cushioning,sports,and bioengineering,as well as core materials in sandwich structure.These materials offer numerous advantages,such as low density,good barrier properties,impact resistance,and strong mechanical properties.Polyethylene terephthalate(PET)foam has several advantages,including an environmentally friendly manufacturing process,easy recycling,a high heat deflection temperature,relatively good mechanical properties,and low raw material costs or recycled PET materials can be used.PET foam is gradually replacing thermoplastic rigid foam,such as polyurethane(PU)and polyvinyl chloride(PVC),in wind turbine blades,construction,and automotive applications.PET foam possesses a distinctive honeycomb-like internal structure that directly impacts its performance.The relationship between the cell structure of PET foam and its compression,tensile,and shear properties has not been thoroughly studied.Therefore,it is important to investigate this relationship to better understand the foam’s performance.PET foam is often used as a core layer in sandwich structures and there is a need for further clarification of a simple,efficient and optimised design methodology for PET foam sandwich structures used as flat panels in non-primary load bearing applications.PET foam sandwich structures are typically laminated with FRP or metal sheets,and the lamination process is intermittent and inefficient.To meet the rapidly growing market demand for PET foam sandwich panels,a more efficient continuous preparation method is required.This paper clarifies the reasons for the formation of the internal elongated honeycomb structure of PET foam by analysing the equipment and process of PET foam.Establishment of a continuous manufacturing process for PET foam/glass-reinforced polypropylene sandwich structures and manufacture of PET foam/glass-reinforced polypropylene(PP)sandwich panels.To systematically characterise the internal structure and compression,bending and shear properties of PET foams,to explain the mechanism between structure and performance,to consider the effects of density and structural anisotropy on performance,to establish a model of the relationship between structure and performance of PET foams,and to provide a reference for optimising the performance of PET foams.To establish a structural optimisation design method for PET foam sandwich flat plate structures under bending loads in non-critical load bearing areas,and to provide design references for industrial applications of PET foam sandwich structures.The main points of each part of the study are as follows:(1)Internal tensile structure of PET foam and its sandwich structure continuous manufacturing process.Processing technology affects the internal structure of the foam,the structure directly affects the performance of the foam,PET foam in the extrusion by the tensile action,the formation of extrusion along the direction of the tensile state of the structure of bubble holes,resulting in the PET foam extrusion direction of the mechanical properties of the other direction of 2-4 times.A continuous and efficient process for the production of PET foam sandwich structures has been established using a continuous Teflon press and process to replace the existing interstitial production process for PET foam sandwich structures.The use of multi-layer hot melt adhesive film to solve the problem of low peel strength(2.93N-mm/mm)between PET foam polar materials and glass-fibre reinforced polypropylene non-polar materials,improved PET foam/glass-fibre reinforced polypropylene sandwich structure peel strength of 38.8N-mm/mm,reaching the level of traditional PET foam/glass-fibre reinforced polypropylene sandwich structure.(2)Characterisation of the internal structure and compression,tensile and shear properties of PET foams and study of the mechanism of structure on performance.A systematic study of the structure and properties of the foams was carried out,and the results showed that the overall structure of the vesicles is characterised by strong randomness,although there are also certain regularities.The length to diameter ratio and the size of the vesicles become smaller with increasing density,and the sphericity and wall thickness become larger with increasing density.The pattern of variation of vesicle structure with density in the extrusion direction is more significant,while the pattern of variation of vesicle structure with density in the two non-extrusion directions is not significant.The compression,tensile and shear properties of the foam increase with increasing density,the mechanical properties in the extrusion direction are significantly higher than those in the other two directions,and the properties in the two non-extrusion directions are basically close to each other.PET foam is analysed as a transversely isotropic material,as its structure and properties are similar in both non-extruded directions.If the deformation mechanism of the foam under compressive,tensile or shear loading is dominated by tensile deformation,the value of n is close to 1 when analysed using Ashby’s law of proportions,and close to 2 when the deformation mechanism is dominated by bending.(3)Modelling the structure-property relationship of PET foams taking into account density and cell structure anisotropy.The tetrahedral and tensile tetrahedral microstructure theories were used to analyse the’structure-property’ relationship of PET foams,and the results showed that due to the high randomness of the internal structure of the foams,the microstructure model ’structure-property’ constitutive equations over-simplify the structure of the vesicles,leading to a large error in predicting the properties of PET foams,with most of the relative errors higher than 30%.Considering the strong randomness and some regularity of the internal structure of PET foam,and with reference to the basic conclusions of microstructure theory,we constructed the single-direction(without considering structural anisotropy)and structural anisotropy ’structure-sex’ model of PET foam from the image-only perspective,and formed the ’structure-sex’ model of PET foam,which predicted values with high accuracy compared with experimental values.The relative error between most of the model predictions and experimental values is less than 5%,and the accuracy of the model predictions is high.(4)Research into the optimum design method for the core structure of PET foam sandwich panels used in non-critical flexural load-bearing areas.The experimental results show that the sandwich structure should focus on the yield constraint of the face layer,the shear constraint of the core layer and the stiffness constraint from the design perspective.Based on the basic theory of sandwich structure,the optimisation objective of the sandwich structure is the best level of lightweighting,and the optimisation parameters are the thickness of the face layer and the thickness of the core layer.The core structure optimisation problem consists of constraints,design inputs,optimisation objectives and optimisation parameters,and is solved by the Lagrange multiplier method to obtain five key design points: The optimal design point of the face yielding constraints,the optimal design point of the stiffness constraints,the intersection of the face yielding constraints with the stiffness constraints,the intersection of the core shear constraints with the face yielding constraints and the intersection of the core shear constraints with the stiffness constraints,and to form the core structure Optimal design method.The optimisation method was validated by experiments and the error was less than11% and the design method was highly reliable.