Biodegradable shoe covers are gradually replacing traditional plastic ones, driven by both environmental policies and market demand. Biodegradable materials, represented by polylactic acid (PLA), are substantially different from traditional materials such as polyethylene (PE) and polypropylene (PP) in their manufacturing processes. These differences cover everything from raw material processing to molding to reprocessing.
I. Raw Material Processing: From Chemical Synthesis to biologicalextraction
Traditional Materials (PE/PP):
Polyethylene and polypropylene are petroleum-based thermoplastics. For example, polyethylene requires ethylene polymerization to regulate molecular weight distribution by controlling catalyst types and reaction conditions,such as temperature and pressure. Raw material must be dried strictly (moisture content ≤0.01%) to avoid bubble defects caused by moisture evaporation during processing. Polypropylene production requires propylene polymerization at high temperatures (240-275°C), and the addition of fiberglass or mineral fillers can improve its mechanical properties.
Biodegradable Material: lactic acid PLA was extracted from maize, cassava and other PLA renewable resources by microbial fermentation and synthesized in two steps:
Lactic acid oligomerization: Lactic acid is dehydrated and condensed at 150 ° C to180°C to form oligomers.
Prolactin open-loop polymerization: In the presence of an initiator, the oligomer are broken down into prolactin, which is then purified by recrystallization and ring-opening polymerization to form high molecular weight PLA.
This process requires strict control lactide purity ≥ 99.5%, otherwise the polymerization reaction will cease or product molecular weight will be insufficient. In addition, PLA feedstock must avoid prolonged exposure to high temperatures to prevent hydrolysis and degradation.
ii. Molding Process: from Single Melting to Multi-Technology Integration
Traditional materials (PE/PP): PE/PP shoe covers are mainly produced by blow molding or hot pressing:
Blow Molding: Molten PE/PP is extruded into a tubular preform, then sealed in a mold and inflated and solidified after blowing. This process is suitable for hollowed-out shoe sheaths, but wall thickness uniformity (deviation ≤5%) must be controlled to avoid local defects.
Thermal molding: Polyethylene/ polypropylene sheets is heated to a softening point and then bonded to the mold by vacuum adsorption or mechanical pressure. The process is efficient (single cycle ≤ 10 seconds), but the die is expensive and suitable for simple geometry.
Biodegradable materials: Plastic shoe covers need to strike a balance between material properties and environmental requirements. Common processes include:
Melted Spinning Composite Molding: PLA fiber is first prepared from molten spun and then compounded with fabric substrate to form the main body of the shoe cover. The process involves the addition of nanoparticles (e.g., silica) or the regulation of spinning speed (≥3000m/min) to resolve the contradiction between the high viscosity and thermal sensitivity of PLA melt and fiber performance.
3D printing: PLA filaments are deposited layer by layer using fused deposition modeling (FDM) technology. The process allows complex structures such as non-slip textures to be created at once, but requires control of printing temperature (190-220) and interlayer adhesion strength to avoid layering and cracking.
Two, two steps injection molding stretch method: first the main body of the shoe sleeve injection molding, then thermal stretching, to form anti-slip texture. The process requires the addition of plasticizers (such as polyethylene glycol) to PLA to reduce brittleness, while controlling the stretch ratio (≤3:1) to prevent breakage.
III. Post-treatment: from simple cutting to functional handling
Traditional Materials (PE/PP):
Post-treatment of PE/PP shoe covers consists mainly of cutting and sealing:
Ultrasonic welding: High frequency vibration melts the edge of the shoe sleeve to achieve seamless sealing and prevent liquid infiltration.
Electrostatic dust removal: high voltage electric highvoltage electric field dust removal shoe removal shoe sleeve surface dust, in line with the requirements of clean room.
Printed logos: Brand information is added using heat transfer or screen printing techniques, but the content of volatile organic compounds in the ink must be controlled.
Biodegradable materials:
PLA shoe cover reprocessing needs to balance functionality and environmental protection:
Nanocoating treatment: Spray titanium dioxide nanoparticles on the surface of the shoe cover for self-cleaning and antibacterial effects. This process requires control of coating thickness (≤50nm) to avoid affecting degradation rate.
Laser engraving of anti-skid textures: Laser etching of micron textures such as rhombus or corrugated on PLA surfaces increases friction coefficient (≥0.6). The process is chemical-free, but laser power (≤50W) needs to be optimized to prevent material from carbonizing.
Accelerated bio-enzymatic degradation treatment: In industrial composting environments, Pre-treatment with ultraviolet light or cellulase can shorten the degradation cycle of PLA shoe covers (from 6 to 3 months).
IV. INTRODUCTION INTRODUCTION Deeper process differences Traditional PE/PP shoe sleeve manufacturing processes focus on mass production and cost control, with efficiency achieved through standardized processes (e.g. blow molding cycles ≤15 seconds). The design of PLA shoe covers requires a balance between performance optimization and environmental constraints:
Material property adaptation: The brittleness and thermal sensitivity PLA require precise control of process parameters (such as temperature and pressure) to avoid processing defects;
Functional integration requirements: additional functions such as anti-skid and anti-bacterial need to be implemented through composite processes, increasing technical complexity;
Degradation cycle management: Post-treatment must ensure that the shoe covers maintain performance during their useful life and degrade rapidly after disposal.
Conclusion: Process innovation drives a sustainable future. Traditional PE/PP shoe covers differ from PLA shoe covers in production process, essentially reflecting the difference in technology between petroleum and bio substrates. With the development of the "dual carbon" goal, the process optimization of biodegradable materials such as melt spinning efficiency and 3D printing precision will become the focus of industry attention. In the future, through a combination of material modification (e.g., PLA/PBAT blending) and process innovation (such as microfoaming molding), biodegradable shoe covers are expected to achieve comprehensive breakthroughs in performance, cost, environmental protection, and provide China solutions for global plastic pollution control.
