Research On Rotational Foaming Process Of Polyethylene And Controllable Mechanism Of Cell Structure

The development of high-value products of synthetic resin has become an important part of China’s strategic emerging industries,one of the top ten areas of"Made in China 2025"and one of the six major areas of the Science and Technology Innovation Board,and it is also an important direction for China’s industrial upgrading and future industries.Rotational foaming materials have promoted the upgrading of polyolefin industry,reducing carbon emissions,and realizing diversified applications,as well to meet the high-performance needs of material functionalization in the fields of large-scale weapon packaging,modern intelligent logistics,advanced marine equipment,and 5G new energy.In order to solve the pain points of the industry such as single material of large special-shaped complex structural products and difficult green recycling,the products can achieve lightweight emission reduction and low carbon insulation functions.The high-value polyethylene materials for rotational foaming process will strongly support the key needs of major equipment and high-end manufacturing,as well as meet the development of national strategic emerging industries.In the area of rotational foaming technology on polyethylene,how to achieve efficient preparation of pressure-free rotational foaming process by accurately controlling the diffusion rate of gas in the polyethylene melt,it is necessary to explore the influence of environmental factors such as temperature and pressure on gas diffusion,so as to seek the best operating conditions.Meanwhile,in order to control the cell structure of polyethylene to realize the new application of high-rate micro-pore size foaming materials,it is necessary to study the effects of different resin matrices,and new foaming agent systems on the cell structure,as well as the kinetic mechanism in the foaming process,so as to provide theoretical support for the preparation of high-performance polyethylene rotational foaming materials.This paper aims to address the technical bottlenecks in the polyethylene rotational foaming process and achieve the high-quality foamed products.Based on the characteristics of the rotational molding process and the principle of chemical atmospheric pressure foaming,the research on the mechanism of cell structure regulation in high-performance polyethylene rotational foaming process and products was conducted.A new method for controlling polyethylene foaming by adjusting melt viscoelasticity was proposed,which could effectively control the foaming behavior by precisely adjusting the viscoelasticity of the melt.In addition,a new idea of using multi-component polyethylene blend design to regulate the morphology of bubble cells was proposed,and to accurately regulate the morphology of bubble cells by designing polyethylene composition of different components.Moreover,a new strategy for controlling the decomposition behavior of blowing agents by specific component combination was discovered,which could better control the decomposition process of blowing agents by selecting appropriate blowing agent components for combination.By using freeze-drying technology,a new technology for preparing supported blowing agent systems was created,which achieved smaller and more uniform pore sizes in polyethylene foamed products,and the prepared multi-size pores are expected to expand the application range of rotational foamed products.The research results of this thesis were concluded as following:（1）A new method for preparing high expansion ratio and high degree of cross-linked foamed products using rotational foaming process controlled by melt viscoelasticity was proposed.The effects of multiple factors on the cross-linking kinetics and melt viscoelasticity of polyethylene,as well as the influence of cross-linking degree on the structure and properties of cross-linked polyethylene,were studied.The effects of molecular weight and density of polyethylene on the cross-linking behavior of polyethylene were studied by rheological and empirical methods.The results showed that high cross-linked expanded polyethylene products can be prepared by controlling the high tanδ value of the polymer melt（≥5）in the early stage of blowing agent decomposition,and the low tanδ value（≤0.6）in the later stage of foaming agent decomposition.It provided reliable scientific and technological support for the realization of high degree of cross-linked low-density polyethylene foam products.The effects of chemical blowing agents,foaming agents and cross-linking agents on the density and cross-linking degree of foamed products in polyethylene compositions with high melt flow index were studied by the experimental method of central composite design.The simple and practical mathematical empirical equations were obtained to reliably describe the relationship between the properties of foaming materials and the formula variables reliably.This empirical model can also use to predict the optimal formulation to obtain the minimum foam density and the maximum degree of cross-linking.（2）A novel tailored polyethylene blend with a unique chain structure for rotational foaming was creatively proposed and demonstrated,and the effects of crystallization temperature and viscosity–temperature sensitivity on foaming were also investigated.The polyethylene blends with few chain branches in the low-molecular-weight part and many chain branches in the high-molecular-weight part effectively improved the crystallization temperature and the viscosity-temperature sensitivity for better prevention of coalesce and collapse during the foam-shaping stage.Higher crystallization temperature and higher viscosity-temperature sensitivity effectively increased the melt strength around bubble cells during the foam-shaping stage to better prevent coalescence and collapse.The experimental polyethylene blends with a higher melt index(MI_2.16≥8 g/10 min)first produced a larger expansion ratio,around 8,with smaller cell sizes（<600μm）and a narrow cell size distribution.A higher melt index is very important for rotational foaming when fabricating complex-structure products.（3）A novel blowing agent for polyethylene resin was designed and synthesized by one-pot method.The novel NH₄H₂PO₄/AZ composite blowing agent system（PAZ）can achieve a lower decomposition temperature and a milder decomposition rate than azodicarbonamide（AZ）due to thermal coupling effect and phosphoric acid catalysis.PAZ blowing agent generated more small-sized cell nucleation during the thermal decomposition.And the fewer thermal decomposition byproducts improved the pretty surface of end-used polyethylene foam products during the rotational foaming process.Compared with pure AZ,PAZ was an excellent blowing agent for the preparation of polyethylene foam product.The cell diameter of polyethylene foam product can be reduced significantly using PAZ blowing agent.Therefore,the flexural modulus of polyethylene foam product produced with PAZ is 50%higher than that produced with pure AZ.Additionally,PAZ as blowing agent of polyethylene resin can provide a better demolding performance and much more perfect surface than AZ.This work provided an important approach for tuning foaming behavior of chemical blowing agent during industrial applications.（4）To address the functional demands for multi-sized cells in rotational foamed products,research on the preparation and foaming mechanisms of supported foaming agent systems was carried out.Although the surface of the physically adsorbed supported foaming agent system contains nano-aerogel particles,it does not alter the decomposition temperature of the foaming agent.The use of chemically in-situ supported foaming agents can enhance the rigidity of the final foam used,with an increase of 30%in bending modulus;however,it is still challenging to produce multi-level cell structures.In order to meet the functional requirements of multi-size bubble cells in rotational foamed products,a new method for the preparation of chemical in-situ supported foaming agent system for gel network freeze-drying was proposed.Silica-based gel chemically in-situ supported foaming agents can decompose controllably at lower temperatures and release foaming gases,which is beneficial for improving the melt strength of the polyethylene in rotational foaming.This achieves effective regulation of the multi-sized cell structure in polyethylene rotational foamed products,providing significant scientific and technological support for the development of functionalized polyethylene rotational foamed products.