Understanding All Six Polyethylene Modification Methods

Understanding All Six Polyethylene Modification Methods

Polyethylene (PE) resin is a polymer formed by the polymerization of ethylene monomers. PE molecules exhibit a linear or branched chain structure and are typical crystalline polymers.

In the solid state, crystalline and amorphous regions coexist. The degree of crystallinity varies depending on processing conditions and pretreatment, with higher density generally corresponding to greater crystallinity.

LDPE typically has a crystallinity of 55–65%, while HDPE ranges from 80–90%. Although PE possesses excellent mechanical processing properties, its inert and non-polar surface results in poor printability, dyeability, hydrophilicity, adhesion, antistatic properties, and compatibility with polar polymers and inorganic fillers. Additionally, its wear resistance, chemical resistance, environmental stress crack resistance, and heat resistance are limited, restricting its application scope. Modification methods are employed to enhance its performance and expand its applications.

1. Graft Modification

Graft modification preserves the backbone structure of PE while introducing polar functional monomers, retaining PE’s inherent properties while adding new functionalities. It is a simple and effective method for polar functionalization.

Grafting methods include:
(1) Solution method: Uses solvents like toluene, xylene, or chlorobenzene to dissolve PE, monomers, and initiators, forming a homogeneous system. Solvent polarity and chain transfer constants significantly influence grafting.
(2) Solid-phase method: Directly reacts PE powder with monomers, initiators, and surfactants. Advantages include moderate reaction temperature, ambient pressure, retained polymer properties, no solvent recovery, and energy efficiency.
(3) Melt method: Generates radicals via thermal decomposition of initiators in the molten state, enabling radical copolymerization with grafting monomers to form side chains.
(4) Radiation grafting: Utilizes γ-rays, β-rays, or electron beams to generate free radicals on polymers, which then react with monomers for surface modification. Techniques include co-irradiation, pre-irradiation, and peroxide methods.

2. Crosslinking Modification

Crosslinking enhances PE’s mechanical strength, environmental stress crack resistance, corrosion resistance, creep resistance, and weatherability. Commercial examples include PEX (used in aluminum-plastic composite pipes). Methods include:
(1) Radiation crosslinking: Exposes PE to high-energy rays (γ-rays, X-rays, or electron beams) to form a 3D network structure.
(2) Chemical crosslinking: Uses peroxides or azo compounds to generate radicals that create crosslinks via unsaturated sites in PE.
(3) Silane crosslinking: Grafts silicon-containing vinyl and alkoxy groups onto PE, followed by hydrolysis and condensation to form Si–O–Si crosslinks.

3. Blend (Copolymer) Modification

(1) Copolymerization

Modifies PE through:
(a) Coordination copolymerization: E.g., ethylene-propylene rubber (EPR), ethylene-propylene-diene monomer (EPDM), or copolymers with 1-butene/1-pentene.
(b) Free radical copolymerization: E.g., ethylene-vinyl acetate (EVA).
(c) Ionic copolymerization: E.g., ethylene-(meth)acrylic acid copolymers or ethylene-glycidyl methacrylate (EGMA). These introduce reactive functional groups or improve chain flexibility.

(2) Blending

Blends PE with other resins, rubbers, or elastomers to enhance toughness, impact resistance, printability, or oil barrier properties. Examples:
(a) HDPE/LDPE blends: Combines HDPE’s strength with LDPE’s softness. Adding LLDPE/VLDPE improves performance via co-crystallization.
(b) PE/CPE blends: Chlorinated PE (CPE) improves flame retardancy and printability.
(c) PE/EVA blends: Enhances flexibility, transparency, and gas permeability but reduces mechanical strength.
(d) PE/rubber blends: E.g., with butyl/natural/SBR/EPDM rubber to boost impact resistance.
(and)PE/PA blends: Polyamide (PA) improves oxygen/hydrocarbon barrier properties. Polar group grafting (e.g., C=O, –COOH) enhances compatibility.

4. Filling Modification

Incorporates inorganic/organic fillers to reduce costs or alter properties. Divided into:

(1) General Filling

Inorganic fillers (e.g., CaCO₃, talc, kaolin) improve rigidity, heat resistance, and dimensional stability but may reduce mechanical properties. Surface treatment (e.g., coupling agents or maleic anhydride-grafted PE) enhances adhesion. Organic fillers include straw/wood fibers.

(2) Functional Filling

Targets non-mechanical properties:
(a) Biodegradable PE: Starch additives enable microbial degradation.
(b) Conductive PE: Blends with carbon black/metal powders for antistatic/EMI shielding applications.
(c) Flame retardant PE: Achieved via halogen/Sb₂O₃, organic phosphates, or inorganic fillers (e.g., Al(OH)₃/Mg(OH)₂).

5. Reinforcement Modification

Uses reinforcing materials (e.g., glass/synthetic fibers, whiskers) or self-reinforcement (molecular alignment via processing).
(1) Glass fiber reinforcement: Improves strength/heat resistance; interfacial modifiers enhance bonding.
(2) Synthetic fibers: Lighter/higher-strength alternatives (e.g., PAN, PA, PVA, aramid fibers).
(3) Whiskers: High-strength materials (e.g., CaCO₃ or potassium titanate whiskers).

6. Nanoparticle Modification

Nanomaterials (particle size

Nano-montmorillonite, ZnO, Al₂O₃, or clay-modified PE.