| Processing methods and conditions have significant influence on the final structure and properties of polymer films. The films produced by the stretching processing methods, such as biaxially oriented polypropylene (BOPP) and biaxially oriented polyethylene terephthalate (BOPET) films, own excellent mechanical and optical properties. Also, the production efficiency of stretching processing is very high. As a consequence, the stretching processing method has evolved into the main process of producing polymer films with high performance. Nevertheless, there still exist great challenges in the film stretching industry:the technology of biaxial stretching polyethylene(BOPE) has not been developed in the world so far, despite that PE occupies the largest proportion among all the materials of producing polymer films; how to improve the production efficiency and rate of fine variety of functional film, such as lithium-ion battery separator, is another challenge in the film stretching industry. Worse still, studies of the film stretching process are still lacking and the physics behind the process are poorly understood. On one hand, film stretching is a multi-step process, which contains multidimensional molecular parameters, processing conditions as well as aggregated structure parameters. Sovling the technical problems in the film stretching process is to search the optimal solution in the multidimensional parameter space, which is a great challenge. On the other hand, studies about the process of biaxial stretching of polymer films need high requirement for the research instrument. Therefore, researchers managed to avoid such research work due to the limitation of research contions.This thesis focuses on the physical problems behind the film stretching process. The objective of this thesis is to build the experiment roof for in situ study of the processing physics of polymer film stretching, design and construct a series of online research equipments simulating the producing process of film stretching in industry. On the basis of the in situ research platform, formation and evolution of the microstructure during film stretching process can be tracked, revealing the processing principle of film stretching. On the other hand, the platform can be used to provide guidance to solve the engineering problems encountered by film companies producing packaging films, li-ion battery separator and optical films, etc., to develop new formula as well as optimize the process parameters. The main results and conclusions of the thesis are as follows:(1) The experiment line of three layer coextrusion casting was designed and constructed, which can be used to prepare monolayer of threelayer precursor films or casting sheets of stretched films. The extruder unit of the line contains two sets of single screw extruders, automatical charging system, filter, melt pump, distributor and die, etc. With respect to the casting unit, the rotation of casting rolls or heat setting rolls are driven by servo motor with high accuracy, ensuring the precise control of the rotation speed of rolls. Also, the rolls are decorated with spiral flow channel, along which, conduction oil can circulate, making the temperature of roll surface homogeneous. Programmable logic controller is employed to control the casting unit, therefore, the operating process is very simple. The line has been successfully developed and the usability of the line has been verified. On the basis of the extrusion casting line, the precursor film to make li-ion battery separator has been prepared, which was stretched to obtain microporous membranes with homogeneous distribution of micropore. Meanwhile, the line possesses the tensile function, which can be used to simulate the continuous cold and hot stretch process producing li-ion battery separator by "dry uniaxial tensile craft" or the machine direction orientation (MDO) process of biaxial stretching.(2) A novel constrained uniaxial tensile apparatus was designed and constructed, which can be combined with synchrotron radiation ultrafast X-ray scattering technique. Acting as the basic stretching unit, constrained uniaxial tensile process can be used to simulate almost all the film stretching process, such as biaxial sequential stretch and longitudinal multi-stage stretching along MD. Stretch is carried out through two motor driven clamps in the MD and scissor-like clamps in the transverse direction (TD) keeping the sample width constant. All parameters related to film stretching manufacturing, such as temperature, draw ratio and stretching speed can be set independently, making the apparatus an effective instrument to explore the relationship between processing parameters and structure. When combined with synchrotron radiation ultrafast X-ray scattering, structural evolution of polymer films (crystallinity, degree of orientation and long period of lamellar stacks) during stretching can be obtained. Combining the mechanical information (stress and strain) during film stretching process recorded by a tension sensor, the relationship between processing and structure can be established.(3) In order to simulate the biaxial stretching process, we designed and constructed a biaxial stretching machine, which contains three main subassemblies: the stretching frame, air heating circulation system and the control system. As for the stretching frame, two pairs of orthogonal guide rails move along MD and TD independently, driven by servo motors with high precision. Under the drive of rhombic driving mechanism, pneumatic clamps move along the rails, stretching the polymer films. Air heating circulation system is employed to heat the preheating oven as well as the stretching oven, which can not only achieve the rapid heating inside the cavity, but also ensure the homogeneous distribution of temperature inside the cavity. With regard to the control system, a LabVIEW program is compiled to stretch the samples, record the tension information in two directions during stretching, make the stretching frame move among different stations. The technical parameters of the machine can be comparable to the commercial apparatus. Compared with the commercial one, the outstanding feature of the machine is that the size in the vertical direction is small. As a result, the machine can be combined with the wide angle and small angle X-ray scattering with a vertical layout constructed by our group for in situ study. On the basis of the biaxial stretching apparatus, the processing principles of film stretching can be investigated. Also, the laboratory studies based on the apparatus can provide extremely useful insight into the manner that new formulations and film structures will behave on larger equipment, thus saving both time and effort on these larger scale experiments.(4) Based on the homemade constrained uniaxial tensile apparatus, the influence of draw ratio on the crystallization behavior of high density polyethylene (HDPE) film under constrained and free uniaxial stretchng modes were studied with in situ synchrotron radiation small and wide angle X-ray scattering (SR-SAXS, SR-WAXS). Under constrained mode, the sample width kept constant during stretching while the sample can shrink freely along TD during stretching under free mode. Under the constrained mode, the SAXS results show that slightly oriented lamellae, highly oriented lamellae and micro-fibrils formed in the initial crystallization stage along the draw ratio, which can be defined into three regions. Interestingly, the WAXS results show that the crystal morphology are completely twisted lamellae, partially twisted lamellae and flat one in the three regions, respectively. As for the free uniaxial streching, the initial crystal structures show similar features to those of constrained stretching, which can aslo be allocated to three regions along the draw ratio range. However, the draw ratio window of region â…¡ is very narrow, and the micro-fibrils feature of region â…¢ appears at a rather small draw ratio. The reason is that constraint in TD for constrained mode prevents molecular chains from orientating towards MD to some degree. As polymer film requires a balance of structure and properties in two dimensions, micro-fibrils with chains strongly oriented in one direction are not a desirable structure for film. Delaying the formation of micro-fibril under constrained mode can enlarge the processing window, which is more favorable for film processing.(5) Using the biaxial stretching machine mentioned above, effects of stretching modes on the structure of films were investigated. The SAXS results show that the lamellae in casting sheets have no obvious orientation while stongly oriented lamellae are formed after constrained and free uniaxial stretching. Also, the lateral size of lamellae under constrained mode is larger than that under free mode. As for simultaneous and sequential biaxial stretching, the lamellae have no preferential orientation towards certain direction. The DSC results show that the melting point of crystals in stretched films under all modes is higher than that in cast sheets.(6) Cooperating with the film company, we have developed a kind of BOPP film with high softness, which can be used as protection film of flexible printed circuit board (FPCB), increasing the additional value of BOPP film. Also, the method to characterise the softness of polymer films is established. In the process of laboratory research, two measures are taken to improve the softness of BOPP films. From the angle of formula, adding soft components such as low density polyethylene (LDPE), ethylene propylene diene monomer (EPDM) into PP matrix can take effect. As for the influence of processing conditions, decreasing the draw ratio in MD, reducing stretching speed, increasing draw temperature can reducing orientation level of crystals along MD, improving the film softness. According to the formula developed in the lab, we successfully developed BOPP film with great softness in the production line, which meets the requirements of the downstream customers.The main innovations involved in this thesis:(1) The proportion of each layer in casting sheets produced by the three-layer coextrusion casting line ranges continuously from 10%:80%:10% to 30%:40%:30%. Also, apart from the general screws for PP and HDPE extrusion, the extruders are equipped with dedicated screws for PET, LLDPE (linear low density polyethylene) and PVDF (polyvinylidene fluoride) extrusion. As a consequence, the line can be used to extrude almost all the polymer materials.(2) The constrained uniaxial stretching apparatus can simulate almost all film stretching process, acting as a basic unit. Combined with ultrafast synchrotron radiation X-ray scattering technique, the structural evolution and stress-strain information during stretching can be obtained, which provides an effective method to reveal the processing mechanism of film stretching as well as establishing the relationship between processing and structure.(3) The biaxial stretching machine can be combined with X-ray scattering with a vertical layout, which makes online study of the biaxial stretching process possible for the first time.(4) The influence of draw ratio and stretching modes on crystal morphology of HDPE films was investigated using in situ study method, which helps explore the processing window of MD for producing BOPE films.(5) The relationship between stretching modes and structure of polymer films was established, providing guidance to control the structure and property of films.(6) Under the guidance of laboratory evaluations, the BOPP films with high softness have been successfully developed on production line, achieving the perfect combination of the laboratory research and industrial production. |
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