The Role Of Static Biaxial Stretching Machine In The Application And Development Of UHMWPE Film

  UHMWPE (ultra-high molecular weight polyethylene) is a thermoplastic engineering plastic with high impact resistance, wear resistance, self-lubrication, anti-aging, etc., and has excellent comprehensive performance. UHMWPE diaphragm is a diaphragm material with excellent performance. It can be applied to wastewater treatment, lithium battery diaphragm, aerospace, national defense and military industry, municipal construction, petrochemical industry, and even new energy materials, medical and health care and other industries.

  The molecular structure of UHMWPE is exactly the same as that of ordinary PE, but the molecular weight of the two is different. The molecular weight of ordinary PE is in the range of (5~30)x104, while the molecular weight of UHMWPE is as high as 1 million. The slender molecular chains are arranged in the same direction and entangled with each other. Due to the mutual entanglement between the macromolecular chains, the macromolecular chains can effectively transfer the load to the main chain I, so that UHMWPE has many excellent comprehensive properties that ordinary PE does not have: excellent wear resistance, self-lubrication, impact resistance and other mechanical properties. UHMWPE has low production cost and is an engineering material with ideal cost performance. Its application field is constantly expanding and its output is constantly increasing. The comparison of the performance of UHMWPE and ordinary PE is shown as follows.

  At present, due to the difficulty in processing UHMWPE, it is impossible to use conventional processes such as blow molding to prepare it, and the dry process cannot effectively control the parameters such as the porosity of the diaphragm. Therefore, the thermally induced phase separation method (i.e., wet method) is used to prepare UHMWPE lithium battery diaphragms with excellent performance. UHMWPE generally adopts a biaxial stretching process, including: batching, extrusion, casting biaxial hot stretching, extraction, drying, heat setting and winding. Biaxial stretching includes biaxial synchronous stretching and biaxial asynchronous stretching. It is a process of coordinated control of thermal effect and mechanical force. Its influencing factors include stretching temperature, stretching rate and stretching multiple. By stretching the film or cast sheet longitudinally and transversely, the structure and size of the film are changed, the mechanical properties of the film are improved, and its microstructure and physical properties are optimized.

  The static biaxial stretching machine provides accurate and precise feedback for experimental data through scientific process design and continuous improvement of production methods.

  The static biaxial stretching machine applies precisely controlled longitudinal and transverse stretching forces to make the film uniformly expand in two dimensions, thereby achieving fine control of its structure and performance. Just as the team of Academician Qu Jinping and Professor Zhang Guizhen from the National Engineering Research Center for New Polymer Forming Equipment of South China University of Technology recently strategically introduced spherical inorganic particles (SiO2 microspheres) into the "polymer (ultra-high molecular weight polyethylene, UHMWPE)-porogen (liquid paraffin, LP)" system, they developed a simple and continuous "phase separation-biaxial stretching" technology, and innovatively realized a micro-sandwich porous structure (MAMS) on the polymer film.

Technical principle:

  By strategically introducing spherical inorganic particles (SiO2 microspheres) into the "polymer (ultra-high molecular weight polyethylene, UHMWPE)-porogen (liquid paraffin, LP)" system, a simple and continuous " "Phase Separation-Biaxial Stretching" technology innovatively realizes a micro-sandwich porous structure (MAMS) on the polymer membrane. Utilizing the highly entangled characteristics of UHMWPE molecular chains, the tight polymer skeleton formed can effectively wrap SiO2 microspheres and LP microdroplets. Among them, SiO2 microspheres play a key supporting role in the biaxial stretching process, causing the LP droplets to form an ellipsoid shape when squeezed under the UHMWPE skeleton.

  Under the action of biaxial stretching, the lamellar crystals of UHMWPE slip significantly and are gradually replaced by oriented fiber crystals, forming a staggered network structure, and the crystallinity of UHMWPE is also improved. Due to the supporting role of SiO2 microspheres, this process is also accompanied by the formation of an ellipsoid-like micro-sandwich structure. The highly oriented fiber crystals and high crystallinity significantly improve the mechanical strength of the polymer porous membrane. Even at 60wt% of inorganic particles and 68% porosity, the porous membrane exhibits excellent mechanical properties (tensile strength 25 MPa and tear strength 88.9 N/mm).

  Shorten development cost and time: The static biaxial stretching machine can quickly produce UHMWPE film samples with different properties for R&D personnel to conduct performance testing and verification, shorten the cycle from design to R&D, and can more accurately predict and regulate the performance of the film changes, reducing the cost and time of trial and error caused by improper process parameters.

Optimized film structure: Through static biaxial stretching, the microporous structure inside the UHMWPE film is further optimized, the micropore distribution is more uniform, and the pore size is controllable, thereby improving the air permeability and permeability of the film. This is particularly critical for lithium battery separators that require good breathability and help improve the electrochemical performance of the battery.

  Improved mechanical properties: The molecular chains of UHMWPE films are oriented and aligned under the action of two-way forces, forming a closer crystal structure. This structural change significantly improves the tensile strength, puncture strength and tear resistance of the film, making the UHMWPE film more stable when withstanding external impact or pressure.

Improved optical properties: The surface flatness and optical transparency of biaxially stretched UHMWPE film are improved. This change not only improves the aesthetics of the film, but also broadens its application in optical films, transparent packaging and other fields.   More than 60% of spherical inorganic particles are filled during stretching, showing a stronger light scattering efficiency, and reaching a peak value when the aspect ratio is ≈1.9, providing sufficient interface for effective sunlight reflection, taking into account high performance and weather resistance. A large number of pore structures and inorganic particles are added, and it still has excellent mechanical strength under the support of high porosity and high inorganic particle content.

  In the future, with the continuous advancement of materials science, mechanical manufacturing and automation control technology, the role of static biaxial stretching machine in the application and development of UHMWPE film will be more prominent. The application of new technologies for ultra-high molecular weight polyethylene UHMWPE film promotes the development of static biaxial stretching machine in a more precise direction, and the new process also makes higher requirements for traditional static biaxial stretching machine.