Fundamental Research on Material Structure-Property Relationship in HDPE Blow Molding Process

Blow molding is one of the most widely used processes in the production of rigid packaging for food, household industrial chemicals, personal care products, agricultural chemicals, and pharmaceuticals.

Modern blow molding technology originated from glass blowing. Today, there are many types of thermoplastic resins used for blow-molded containers, including polyolefin materials such as PE, PP, PVC, PC, and PET. Among them, PE has become the preferred material for blow-molded container production due to its excellent rheological properties, superior mechanical strength, and chemical resistance during melt processing.

The most important characteristic of PE is its relatively low melting point, while maintaining ideal solid-state properties at room temperature. Due to its excellent thermal stability, PE can be repeatedly processed, allowing for reprocessing or recycling with minimal changes to its physical properties.

PE also possesses excellent flexibility, durability, and chemical inertness, making it an ideal container material for holding highly corrosive chemicals. As a semi-crystalline material, the size of its crystalline and amorphous regions significantly affects the physical properties of blow-molded products, such as stiffness, gas barrier properties, and hardness. By controlling variations in structural parameters and solid-state morphology, a wide variety of PE products can be manufactured.

Catalysts, monomers, and modifiers, as well as polymerization reactors and reaction conditions, all affect the molecular structure, molecular weight, and composition of PE. Ethylene, the main monomer of PE, is primarily produced from fossil fuels such as crude oil and natural gas, but can also be obtained from renewable bio-based raw materials such as sugarcane, agricultural residues, and waste-derived oils (such as waste cooking oil).

Ethylene monomers polymerize in a reactor to produce PE resin. High-pressure reactor processes primarily produce low-density polyethylene (LDPE) resin through free radical polymerization under high temperature and pressure conditions. High-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) can be produced using solution polymerization, slurry polymerization, and gas-phase polymerization processes.

Most blow-molding grade HDPE resins are typically produced using slurry or gas-phase processes. In classic slurry reactors, polymerization takes place in a liquid medium (diluent). Gas-phase processes (such as Dow's UNIPOL™ PE process, Innovene, Spherilene, etc.) polymerize under solvent-free conditions, resulting in excellent product consistency and odorless/tasteless properties, making them suitable for direct-contact food packaging applications.

Catalyst molecules (small metals or non-metals) help lower the activation energy of any chemical reaction and have always been at the heart of innovation in polymerization technology. In the commercial production of PE resin, catalysts play a crucial role: they react with ethylene to form intermediates, and then ethylene molecules are added sequentially, gradually "growing" into longer PE chains.

When only ethylene participates in polymerization, the final product is a homopolymer. Industrial production of PE often uses other α-olefin monomers, such as 1-butene, 1-hexene, and 1-octene. These comonomers can insert into the growing PE chain to form a short-chain branched structure. The degree of short-chain branching is a key factor determining the physical properties of PE resin, including density, stiffness, resistance to environmental stress cracking, impact strength, and hardness.

PE resins with a broad molecular weight distribution can be used to improve melt processing properties in blow molding applications. Chromium-based catalysts are most commonly used for these broad molecular weight distribution products.

The PE industry also uses other types of catalysts, including Ziegler-Natta catalysts and single-active-site catalysts. These catalysts are often used to produce PE resins with a narrower molecular weight distribution to achieve high compositional uniformity and superior physical properties, but are less commonly used in the production of PE resins for blow molding.

PE resins can be divided into three main categories: HDPE, LLDPE, and LDPE. The applications of these resins vary globally, but generally, HDPE and LLDPE are consumed far more than LDPE. Approximately 12% of global PE resin production is used in blow molding.

In the production of HDPE resin, α-olefin comonomers are typically added in small quantities or not at all. This results in highly linear PE chains with very few or no side branches. Upon cooling from the amorphous molten state, the highly fluid linear PE chains can recombine into ordered and denser regions, known as crystals or crystalline regions.

LLDPE resin has a high degree of short-chain branching, which disrupts the regularity of the chains and interferes with the crystallization process. The resulting solid structure has relatively low crystallinity, leading to lower melting point, density, and stiffness, but higher resistance to environmental stress cracking and impact strength.

LDPE resin is a type of PE with a highly random chain structure, typically characterized by highly long-chain branching or "multi-branched" structures. LDPE resin has low rigidity and poor gas barrier properties, but it is an ideal choice for blow-molded extrusion bottles that require softer or more flexible designs.

HDPE, LLDPE, and LDPE resins are suitable for various blow molding bottle applications. Figure 1 illustrates the core properties of these PE resins and their typical end-use blow molding bottle types.

One of the latest technological breakthroughs in the PE industry is polymodal PE, whose molecular structure design gives the material flexible plasticity and a better balance of performance. Multi-reactor technologies (such as Dow's UNIPOL™ II process and Spherilene C) can produce PE resins with a bimodal molecular weight distribution: the low molecular weight component is designed to maximize crystallinity or rigidity, while the high molecular weight component is designed to maximize comonomer content or improve toughness, resistance to environmental stress cracking, and post-molding melt processing properties (i.e., preform orifice expansion and melt strength).

Multimodal PE resins (such as Dow's CONTINUUM™ bimodal HDPE products) help drive sustainability goals in the blow molding industry. Bimodal resins can be designed to have higher density while maintaining excellent resistance to environmental stress cracking and drop impact. Containers made with bimodal PE resins can be lightweight while maintaining physical properties, allowing for the incorporation of more PCR HDPE resin in blow-molded containers, and enabling them to withstand high environmental stress cracking applications.

Many physical properties of PE resins are crucial for blow-molded containers. Most of these properties can be found in the material data sheets provided by the supplier. Table 1 lists the physical properties of common PE resins, along with an explanation of their correlation with container performance and their importance to the application.

In blow molding, most material properties are interrelated. Density and melt flow index are key indicators for predicting other physical properties. For example, using HDPE resin can improve container rigidity, but its resistance to environmental stress cracking and impact strength may decrease. PE resins with high melt flow index have better flowability in the molten state and allow for greater extrusion, while resins with lower melt flow index exhibit the best solid-state properties, including resistance to environmental stress cracking, impact strength, and melt strength. Figure 2 illustrates the interaction between these physical properties and density and melt flow index.

Blow-molded containers can achieve multi-layered structures through co-extrusion processes, integrating different polymer layers with specific barrier properties, mechanical properties, or appearance characteristics. For example, barrier plastics such as ethylene-vinyl alcohol copolymer (EVOH) or polyamide (PA) can be combined with PE resin to form multi-layered structures, ultimately enabling containers to be used in applications requiring good chemical or gas barrier properties, such as food, pharmaceutical, agrochemical packaging, and gasoline containers.

However, due to differences in polarity and chemical properties, most unmodified PE resins and barrier plastics are incompatible, resulting in insufficient adhesion between layers in multilayer structures. The layers easily separate, affecting the structural integrity of the container. To prevent delamination, a third material with both non-polar and polar properties can be added to multilayer containers to promote good adhesion between PE and the barrier layers. In the PE industry, this type of "surfactant" resin is called an adhesive or bonding resin.

Depending on the type of plastic required for assembly in multilayer containers, various compatibility chemistry approaches are available. The polar groups of these molecules can interact with the functional layers via ionic, covalent, or even hydrogen bonds (Figure 3). The most commonly used bonding resin is PE modified with polar functional groups (e.g., acid anhydrides) (e.g., Dow BYNEL™ bonding resin). Functional groups such as maleic anhydride can be grafted onto PE resin. These ester/anhydride groups can be adsorbed onto polar polymers such as EVOH and PA, forming strong covalent or hydrogen bonds. The backbone of the connecting molecules remains PE, allowing for strong interactions with other PE layers.

Ionic polymers, as commonly used functional polymers, can form very strong electrostatic interactions with reactive groups. Dow's SURLYN™ ionic polymer is a typical example, prepared by neutralizing PE acid copolymers with metal salts. Typical applications of this type of resin can endow materials with very strong physical properties, such as excellent abrasion resistance and toughness.

SURLYN™ ionomer possesses unique optical properties, making it suitable as a surface material for blow-molded containers, thereby enhancing their gloss and scratch resistance. These properties are particularly advantageous for visually appealing personal care and cosmetic packaging. This ionomer achieves a unique balance between optical transparency and mechanical durability—an advantage unmatched by conventional PEs—while maintaining excellent processability in blow molding applications.

PE blow molding is a fundamental process in the packaging industry, and it continues to evolve with ongoing innovation in resin design and processing technologies. Core properties such as density and melt flow index remain key indicators for predicting material performance. HDPE, with its linear structure and crystalline characteristics, can be used to create containers that combine strength, lightweight, and excellent processability. Multimodal PE resins combine low-molecular-weight components (for increased stiffness) and high-molecular-weight components (for increased toughness and corrosion resistance), enhancing the flexibility of container design. These resins, such as Dow Chemical's CONTINUUM™ bimodal HDOE resin, further enhance design flexibility by integrating rigidity, toughness, and processability. These bimodal resins also support lightweight design and increase the use of recycled materials, aligning with the industry's sustainability goals.

Furthermore, the introduction of multilayer co-extrusion technology, materials engineering, and specialty polymers (such as Dow's BYNEL™ adhesive resin and SURLYN™ ionomer) continues to expand the functionality and aesthetics of blow-molded containers. As sustainability and performance requirements evolve, a deep understanding of the behavior and processing principles of PE resins remains crucial for innovation in blow molding technology.