Introduction To Six Modification Methods for Polyethylene

Polyethylene (PE) resin is a polymer formed by polymerization of ethylene monomers. The molecule of PE is a typical crystalline polymer with a long linear or branched chain structure. In the solid state, the crystalline and amorphous parts coexist. The crystallinity varies depending on the processing and processing conditions. Generally, the higher the density, the greater the crystallinity. The crystallinity of LDPE is usually between 55% and 65%, while that of HDPE is between 80% and 90%. PE has excellent mechanical processing performance, but its surface is inert and non-polar, resulting in poor printability, dyeing, hydrophilicity, adhesion, anti-static performance, and compatibility with other polar polymers and inorganic fillers. In addition, its wear resistance, chemical resistance, environmental stress cracking resistance, and heat resistance are poor, limiting its application range. Modification is needed to improve its performance and expand its application field.

 

01 Connection for technical modification

Grafting polymers almost do not change the PE skeleton structure, while also grafting polar monomers with various functions onto the PE main chain, which maintains the original characteristics of PE and adds new functions. It is a simple and effective method for PE polar functionalization.

The main methods for implementing grafting reactions include solution method, melting method, solid-phase method, and radiation grafting method.

(1) Solution method

Using toluene, xylene, chlorobenzene, etc. as reaction media in the liquid phase. PE, monomers, and initiators are all dissolved in the reaction medium, and the system is homogeneous. The polarity of the medium and the chain transfer constant of the monomers have a significant impact on the grafting reaction.

(2) Solid-phase method

React PE powder directly with monomers, initiators, interfacial active agents, etc. Compared with traditional methods, solid-phase method has advantages such as suitable reaction temperature, normal pressure, basic preservation of inherent polymer properties, no need for solvent recovery, simple post-treatment, high efficiency and energy conservation.

(3) Melting method

In the molten state, free radicals are generated through the thermal decomposition of the initiator, which in turn triggers the production of free radicals in the macromolecular chain. In the presence of grafting monomers, free radical copolymerization reactions occur, and then side chains are grafted onto the polymer macromolecular chain.

(4) Radiation grafting method

Radiation grafting surface modification includes γ Radiation β The principle of irradiation methods such as radiation and electron beam is to use polymers to generate free radicals after being irradiated, which then react with other monomers to form grafting polymerization reactions, thereby achieving the goal of surface modification. The main methods of radiation grafting modification include co irradiation method, pre irradiation method, and peroxide method.

02 Crosslinking modification

Crosslinking modification greatly improves the physical and mechanical properties of PE, and significantly improves its resistance to environmental stress cracking, corrosion, creep resistance, and weather resistance, thereby expanding its application range. The commercialized PEX (aluminum-plastic composite pipe) is a typical application of PE crosslinking. Crosslinking modification includes radiation crosslinking, chemical crosslinking, and silane crosslinking.

Radiation crosslinking: placing polyethylene in a radiation field, under high-energy radiation (mainly γ Under the action of radiation, X-rays, and electron beams, various active particles can be formed in solid polymers, triggering a series of chemical reactions, thereby forming a cross-linked three-dimensional network structure inside the polymer.

Chemical cross-linking: Free radicals generated by the decomposition of peroxides or azo compounds react with unsaturated points in PE molecules to form active centers. These active centers are connected by monomers to form chemical cross-linked PE.

Silane cross-linking: grafting silane containing unsaturated vinyl groups and easily hydrolyzable alkoxy functional groups onto the main chain of PE, and then hydrolyzing and shrinking under the action of water and silanol condensation catalyst to synthesize Si-O-Si cross-linking bonds, resulting in silane cross-linked PE.

03 Copolymerization (mixing) modification

(1) Copolymerization modification

The copolymerization modification of PE includes coordination copolymerization, such as ethylene propylene rubber (EPR), ethylene propylene diene monomer (EPDM), and copolymers of ethylene with 1-butene and 1-pentene; Free radical copolymerization of polyethylene, such as ethylene vinyl acetate copolymer (EVA); Ionic copolymerization, such as ethylene (methyl) acrylic acid copolymers, ethylene methacrylate glycerides (EGMA) copolymers, etc. Through copolymerization reactions, the flexibility of macromolecular chains can be altered or the original functional groups can be functionalized, acting as reactive compatibilizers.

(2) Blending modification

Blending modification is the blending of other resins, rubbers, or thermoplastic elastomers with PE to improve its toughness, impact resistance, printability, and oil barrier properties.

① High and low-density PE blending modification. Low density PE is softer and has lower strength; However, high-density PE has high strength and poor toughness. By blending the two, they can complement each other and produce PE materials with different hardness. The addition of LLDPE (Linear Low Density PE) or VLDPE (Extremely Low Density PE) to HDPE/LDPE blend system can improve its performance due to the co crystallization of LLDPE or VLDPE with HDPE and partial co crystallization with LDPE.

② Blending modification of PE and CPE (chlorinated polyethylene). After blending CPE with PE, introducing chlorine atoms into the blend can improve the flame retardancy of PE. Choosing appropriate compatibilizers can improve the compatibility between the two and avoid the potential degradation of product performance caused by other flame retardant methods. In addition, blending PE with CPE can also improve the printability and toughness of PE.

③ Blending modification of PE and EVA. The blend of PE and EVA (ethylene vinyl acetate) has been widely recognized for its excellent flexibility, transparency, good breathability, and printability. But at the same time, the mechanical strength of the product has decreased.

④ Blending modification of PE and rubber. Blending HDPE with rubber materials such as butyl rubber, natural rubber, styrene butadiene rubber, ethylene propylene rubber, etc. can significantly improve its impact performance.

⑤ PE and PA (polyamide) blend modification. Adding PA to PE can improve its barrier properties against oxygen and hydrocarbon solvents. However, due to the differences in molecular structures, the compatibility between PA and PE is poor, and it is necessary to improve their compatibility by introducing polar groups or adding compatibilizers to PE.

04 Filling modification

Filling modification is the addition of inorganic particles to a thermoplastic resin matrix, which reduces the raw material cost of plastic products or significantly changes the performance of plastic products. Sacrificing certain performance while significantly improving others. For the convenience of discussion, filling modification is divided into general filling and functional filling.

(1) General filling modification

General filling is limited to changes in the mechanical properties of PE. The inorganic fillers used to fill PE include calcium carbonate, talc powder, kaolin, barium sulfate, calcium silicate, and silica. Calcium carbonate filled PE composite materials can reduce product costs, improve rigidity, heat resistance, and dimensional stability. However, the poor interfacial adhesion between inorganic filler calcium carbonate and non-polar polymer PE leads to a decrease in the mechanical and flow properties of the material. The interface adhesion can be improved by adding coupling agents or coating calcium carbonate with MPEW (maleic anhydride grafted polyethylene oligomer). Common organic fillers for filling PE include straw fiber, wood powder fiber, etc.

(2) Functional filling modification

Filling modification mainly aims to improve the effects of plastics in terms of light, electricity, magnetism, combustion, etc., rather than just changes in mechanical properties. This type of filling modification is called functional filling. Functional filled polyethylene includes biodegradable polyethylene, conductive polyethylene, and flame retardant polyethylene.

Biodegradable polyethylene: Starch can be modified and added to PE to produce starch plastic. After being buried in soil, it has microbial degradability due to the presence of starch. Research has shown that PE/starch degradable plastics can not only be directly utilized by microorganisms as a carbon source, but can also be corroded by microbial secondary metabolites.

Conductive polyethylene: A new type of conductive functional material can be obtained by combining insulating polyethylene resin with conductive fillers (such as carbon black and metal powder). This type of material has important theoretical research value and has extremely broad application prospects in many fields such as anti-static, conductive, freely controlled surface heating agents, electromagnetic shielding, etc.

Flame retardant polyethylene: ① Add halogen flame retardant and use it in combination with antimony trioxide; ② Add organic acids, ammonium phosphate, tribromobenzene, etc. ③ Add inorganic fillers with flame retardant properties, such as Al (OH) 3, Mg (OH) 2, etc.

05 Enhanced modification

The filling modification with enhanced effect is called reinforcement modification, and the selected reinforcement materials include glass fiber, synthetic fiber, whiskers, etc. For the convenience of discussion, self strengthening modification is also included in this category.

Self strengthening modification does not add any filling materials, but through special molding methods and special design of mold flow channels, the flow velocity gradient of PE melt is increased, resulting in parallel orientation of molecular chains, which helps to generate elongated chain crystals, fully tapping the inherent potential of the material, and developing polyethylene products with mechanical properties comparable to engineering plastics. Due to the absence of any filling material, there is no need to consider the compatibility issue between polyethylene and the filling material.

Utilizing low-cost and easily available high-strength glass fiber reinforced PE to improve its mechanical strength and heat resistance, making it an engineering plastic. The research results indicate that the addition of interface reaction reagents and their grafted products with PE during the composite process can chemically interact or crosslink with the surface of glass fibers and their silane, significantly improving the interfacial bonding and mechanical properties of composite materials.

Synthetic fibers can also be used as reinforced fillers, as they have lower density and higher strength than glass fibers. Synthetic fibers that can be used for PE modification include polyacrylonitrile fibers, polyamide fibers, polyvinyl alcohol fibers, aromatic polyamide fibers, etc.

Whiskers, as a new type of material, have advantages such as high strength, high modulus, good thermal insulation performance, and good compatibility with the matrix. Therefore, they can also be used as reinforcing agents. Commonly used include calcium carbonate whiskers, potassium titanate whiskers, etc.

06 Nanoparticle modification

Nanomaterials refer to materials with an average particle size below 100nm and particle size within the interface area between atomic clusters and macroscopic objects. Due to surface effects, volume effects, etc., nanoparticles have many novel physical and chemical properties. Polymer/inorganic nanoparticle composites based on polymers have good mechanical, optical, electrical, magnetic and other properties, which can form important multifunctional new materials. Nanotechnology for polymer modification has become a frontier in materials science research. Among the nano modified PE materials, there are: nano montmorillonite modified PE, nano zinc oxide modified PE, nano alumina modified PE, and nano clay modified PE.

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