Polyethylene Foam

Chapter 1: Properties of Polyethylene Foams

Polyethylene foam is a valuable closed-cell thermoplastic foam material. A closed-cell foam consists of tiny dense cells enclosed by its walls. The cells sit close to each other, but they are not interconnected. Hence, closed-cell foams like polyethylene foams are ideal for blocking the flow of liquids and gases. They are resistant to air, moisture, and chemical penetration. Their structure makes them stronger, denser, and more rigid than open-cell foams. Polyethylene foam remains a lightweight and flexible material despite its closed-cell structure.

Polyethylene foams are impact-resistant and highly resilient, critical for cushioning applications. Their low thermal conductivity makes them suitable insulating materials. They are soft, non-abrasive, and resistant to dusting, tearing, and degradation. They have high dimensional stability allowing them to withstand several fabrication processes. They also have antimicrobial properties that inhibit the growth of bacteria, molds, and other microorganisms. These properties make polyethylene foams fit for many personal and industrial uses. Special properties are imparted to polyethylene foams by blending chemical agents or additives followed by a series of treatments.

Chapter 2: Types of Polyethylene Foams

The types of polyethylene foams are the following:

Cross-Linked Polyethylene (XLPE) Foams

In XLPE foams, the constituent molecules connect by molecular bonds, forming chained connections from straight carbon chains. The cross-linking of the molecules creates a strong and rigid structure. XLPE foams are more durable and resistant to chemical, gas, and moisture permeation. They have greater thermally and dimensionally stability, insulating properties, and shock and vibration resistance. However, the production of cross-linked structures removes the recyclability of polyethylene foam as a thermoplastic.

XLPE foams can be formulated to be fire-retardant, static-dissipative, or conductive.

Cross-linking may be carried out by either of the following methods:

• Chemical cross-linking involves adding cross-linking agents and inflating agents to the polyethylene resins during molding or extrusion. The molded or extruded foam is subjected to further heat treatment to induce cross-linking and expansion. Chemical cross-linking produces fine and dense cells, making the foam tougher and more durable. However, the surface of chemically cross-linked polyethylene foams is rough and may not be aesthetically pleasing.
• Physical cross-linking involves an electron beam process followed by an oven treatment to induce cross-linking and expansion. It also produces a fine cellular structure, but the cells are larger than those produced by chemical cross-linking. The foams produced by this method are more consistent. The cross-links are weaker, making them more resilient. Physically cross-linked foams have smoother surfaces and are more aesthetically pleasing than chemically cross-linked foams.

XLPE foams are used when thicker sections of foam material are needed. Their applications include expansion joints, gaskets, padding material, insulating materials, orthopedic bracing, and protective packaging for sensitive medical devices. Due to their low water permeability and high buoyancy, they are suitable as a material for floating devices.

Expanded Polyethylene (EPE) Foams

EPE foams are made by applying heat and pressure to the polyethylene resin, blowing agents, and additives inside an autoclave in order to form tiny beads. These beads are cooled and later used as feed material for injection molding. They are melted and poured into the mold cavities. EPE foams are characterized by their high strength-to-weight ratio and high thermal resistance. Unlike XLPE, EPE foams are recyclable.

Extruded Polyethylene Foams

Extruded polyethylene foams are foams having constant cross-sections that are produced by an extrusion process. They are manufactured by forcing the molten polyethylene resin and additives over the hole of the extruder die, making a long and continuous polyethylene foam. The continuous foam is then cooled and rolled into coils or cut into specific thicknesses or lengths.

Low-Density and High-Density Polyethylene Foams

Polyethylene foams can be manufactured based on the density requirements of a specific application. The varied foam densities of commercial polyethylene foams are brought by the foaming method utilized. For instance, LDPE plastics can be manufactured into low or high-density polyethylene foams.

• Low-density polyethylene foams have more voids in their cellular structure; hence, their density is relatively lower. These foams are softer and have better thermal insulating materials than high-density foams. They are also valued for their buoyancy and water resistance. However, their thickness decreases due to prolonged exposure to compressive stress. Low-density polyethylene foams are commonly used as a packaging material.

• High-density polyethylene foams have smaller and thicker cellular walls. They are characterized by their high compressive and tensile strength, fatigue resistance, and low thermal shrinkage. They are more preferred than low-density foams for heavy-duty applications. They are used as sport underlays and cushions for shoes and couches.

Chapter 3: Production of Polyethylene Foams

Below are the stages involved in a closed cell foaming process when observed at the molecular level:

• Gas Dissolution. Gas is dispersed in the molten polymer until saturated, forming a single-phase polymer-gas solution. It is critical that the gas has sufficient solubility in the molten polymer matrix.
• Cell Nucleation. The gas solubility limit in the polymer matrix is decreased by abruptly increasing the temperature or decreasing the pressure. The thermodynamic instability of the mixture during this process leads to the phase separation of the dissolved gas and the molten polymer, forming the cell nuclei in the matrix. The cell nuclei continue to multiply.
• Cell Growth.Cell growth is performed in a controlled condition. The cells increase in size due to the gas's combined heat and mass transfer. The gas expands inside the cells continuously, thus increasing the volume of the matrix and producing the foam. The cells are allowed to expand until the desired volume is reached.
• Cell Stability.At its cell nucleation and growth stage, a foam is thermodynamically unstable. The formation of cells increases the polymer-gas solution's free energy, which is unstable with the environment. There are several methods to stabilize foams. The addition of surfactants lowers the free energy of the foam. It increases the viscosity at its surface, thereby stabilizing the foam. Cooling also favors the stabilization of the foam. After this stage, the foam now has a strong and durable hollow cellular structure.

The primary raw materials in the production of polyethylene foams are as follows:

Polyethylene Resins

It is the main component of polyethylene foams. Low-density and high-density polyethylene (LDPE and HDPE) resins are the most common types of polyethylene manufactured into foams. LDPE foams are lightweight, elastic, water-resistant, and low cost. On the other hand, HDPE foams are stronger and tougher than LDPE foams. In all methods of polyethylene foam production, the resins are melted and combined with agents and additives, which modify the original properties of polyethylene.

Blowing Agents (or Foaming Agents)

A blowing agent is primarily responsible for supplying the gas needed to grow fine cellular structures in the polyethylene matrix. Blowing agents are categorized into chemical and physical blowing agents:

• Chemical blowing agents liberate the gas needed for the foaming process through thermal decomposition or a chemical reaction of a reactive blowing agent. The gas is unreactive with the polyethylene matrix. Reactive blowing agents may exhibit an endothermic or exothermic reaction and usually produce nitrogen or carbon dioxide gas.

Organic blowing agents are known to enhance the foaming process because they constantly produce dispersible gas. They also make uniformly-sized bubbles.

Some examples of chemical blowing agents are azodicarbonamide (exothermic), sodium bicarbonate (endothermic), zinc carbonate (endothermic), and acylhydrazide (thermal decomposition).

• Physical blowing agents are in a gaseous form. They are introduced directly into the polyethylene melt. Examples of physical blowing agents are carbon dioxide, nitrogen, argon, water, air, and chlorofluorocarbons.
• Expandable beads are composed of a thermoplastic acrylic resin polymer with hollow spherical microparticles at their core. As the beads are heated and depressurized, they expand with the polyethylene resin, achieving an excellent foaming effect.

The common thermoplastic foaming techniques utilized in the production of polyethylene foams are the following:

Batch Foaming

Batch foaming is suitable for relatively low production orders. It is used for testing novel foam formulations which is challenging to support in continuous foaming machines. Batch foaming is normally carried out in an autoclave and series of thermal baths. Thus, they are less expensive and take less time to set up.

Batch foaming can be a pressure-induced or temperature-induced process:

Pressure-Induced Batch Foaming

In pressure-induced batch foaming, the molten polyethylene resin is saturated with the blowing agent in an autoclave operating at high pressure. Then, the gas-saturated polyethylene sample is depressurized to atmospheric pressure by opening the relief valve of the vessel. The sudden pressure drop induces cell nucleation and subsequent growth. The foam is then allowed to expand to a certain volume. Finally, the foam is allowed to cool by air or a solvent for cell stabilization.

Temperature-Induced Batch Foaming

In temperature-induced batch foaming, the gas dissolution stage is performed in a high-pressure autoclave (2-5 MPa) but at a lower temperature (10-250C). Then, the gas-saturated polyethylene sample is taken out of the autoclave chamber and immersed in hot oil, water, or glycerin bath (80-1500C) for a specific duration. Plunging the sample into a high-temperature fluid triggers cell nucleation and subsequent growth. The foam can then expand to a certain volume while in the hot fluid. Finally, the foam is quenched into cold water or a solvent bath for cell stabilization.

Foam Extrusion

Foam extrusion is a continuous foaming process that produces foams with a constant cross-section such as sheets, rods, and tubes. In a foam extruder, the polyethylene pellets and additives are fed from the hopper into the barrel consisting of several heating zones. The blowing agent is introduced into the polyethylene melt at the hopper or at a specific point in the barrel. The polyethylene resin is melted inside the barrel. The extruder screw exerts high pressure to push the melt through the barrel and over the hole of the extruder die. As the melt leaves the hole of the extruder die, it experiences a sudden pressure drop which induces cell nucleation and growth. The screw speed, discharge rate, and extruder zone temperatures are set at optimal values. After cell growth, the foam is cooled, stabilized, and advances to downstream processes (e.g., cutting).

Foam extrusion is suitable for high production orders.

Foam extrusion can either be a physical or a chemical foaming process depending on the type of blowing agent utilized:

• Physical extrusion foaming involves the injection and dissolution of a supercritical fluid, usually carbon dioxide or nitrogen, in the polyethylene melt at a certain point in the barrel. A supercritical fluid is a highly compressed fluid that has the properties of a liquid and a gas. This fluid reduces the melt viscosity. The nucleation of the melt is prevented due to the high pressure inside the barrel. Cell nucleation and growth happen after the foam leaves the extruder die.
• In chemical extrusion foaming, the blowing agent is introduced in the hopper together with the polyethylene pellets and additives. The blowing agent thermally decomposes inside the barrel due to the high temperature of the melt. The melt temperature must be high enough to decompose the blowing agent completely. Otherwise, the undecomposed blowing agent will form agglomerates which can clog the melt filter or cause voids in the foam structure. The incomplete decomposition can also lead to poor cell morphology and surface quality. The barrel pressure must also be sufficient to keep the liberated gases dissolved until it exits the extruder die.

Foam Injection Molding

Foam injection molding is a large-scale batch-mode foaming process. Like foam extruders, a foam injection molding machine consists of a barrel with zone heaters, a reciprocating screw, and feeding mechanisms for the resin and the blowing agent. The blowing agent is introduced into the polyethylene melt either at the hopper (chemical blowing agents) or at a specific barrel point (physical blowing agents). The melt is pushed from the barrel with sufficient injection pressure and speed until it reaches the nozzle. The melt experiences a sudden pressure drop as it leaves the nozzle and fills the mold cavities. Thus, cell nucleation and growth are initiated. The foam expands to the shape of the mold cavities and dwells for a set duration. The mold halves are then opened, and the foam is taken out from the mold cavities. Finally, the excess materials (e.g., flashes) around the foam product are trimmed. This cycle is repeated to accommodate the production orders.

Foam injection molding can produce foams with complex shapes accurately. However, the tooling and the energy requirements can be expensive.

Chapter 4: Polyethylene Foam Products

The following are some of the products made from or uses polyethylene foams:

Polyethylene Foam Insulation

Polyethylene foam insulation is an effective temperature barrier valued for its high flexibility, durability, and resistance to corrosion, weathering, moisture, and rupture. These foams are typically laminated with aluminum foil to increase their heat reflectivity and durability. They can also help in vibration and sound-proofing. Due to their flexibility and low density, polyethylene foams can be easily installed in roofs, ceilings, walls, floors, and pipes. Installation is even made easier for insulation foams backed with adhesives. Polyethylene foam insulation with fire retardant properties is also available commercially.

Aside from homes and buildings, polyethylene foam insulation is also found in pipes, refrigerators, and cold boxes.

Anti-Static Foams

Anti-static polyethylene foams prevent the build-up of static electricity on electronic devices and protect them from electrostatic discharge. They provide cushioning and minimize the effects of shock and vibration during transportation and handling. They also protect the device from moisture, heat, and accidental spills. Hence, they are ideal for packaging electronic and semiconductor devices. Anti-static polyethylene foams are typically pink in color and available in sheet form.

Foam Tapes

Sheet polyethylene foam can serve as a carrier for the adhesive film in tapes. Polyethylene foam tapes are available in one-sided or double-sided types. One-sided foam tapes have an adhesive film covering one side of the polyethylene sheet. They are used in gaskets, cushioning, packaging, and insulation applications. On the other hand, double-sided foam tapes have an adhesive film covering both sides of the polyethylene sheet. They are used in mounting and bonding applications. Polyethylene foam tapes vary in thickness and softness. They are more durable than tapes consisting of plastic film and paper carriers. They are widely used in HVAC equipment, homes, and buildings.

Fire Retardant Foam Sheets

Polyethylene foams are naturally combustible, like wood and paper. They are treated with synergistic antimony oxide-halogen or phosphorus-halogen systems to make them fire retardant. However, there are environmental and health concerns on the use of halogens in the anti-flaming methods of polyethylene foams. Halogens can generate large amounts of toxic fumes.

However, mold processing techniques and halogen-free additives are being developed to impart fire retardant properties to polyethylene foams. Magnesium hydroxide is being examined to see if it can be a viable substitute for halogen-containing fire retardant additives. Moreover, high-temperature melting, chemical cross-linking, and mold processing are optimized to develop a fire retardant, low-density polyethylene foam with ethylene-vinyl acetate.

Laminated Foams

Sheets of polyethylene foams and other materials can be laminated together to form thicker sections. Laminating can also help increase mechanical stability, cushioning, insulating properties, and durability of polyethylene foams.

Polyethylene Foams in Furniture

Polyethylene foams can provide comfort and enhance ergonomics. They can be used as padding and cushioning for chairs, couches, beds, car seats, arm and headrests, and other furniture. However, open-cell foams are softer and more comfortable than polyethylene foams.

Polyethylene Foams in Flotation Devices

Polyethylene foams have high buoyancy (the tendency of an object to float) and low water permeability. Hence, they are ideal in flotation devices such as swimming noodles, life jackets, buoyancy aids, and lifeguard floats. They may also be found beneath the straps of backpacks.

Conclusion

• Polyethylene foams are closed-cell thermoplastic foams. They are valued for their durability, high strength-to-weight ratio, high buoyancy, and resistance to chemicals, weathering, and rupture.
• The types of polyethylene foams are cross-linked polyethylene foams, expanded polyethylene foams, and extruded polyethylene foams. Polyethylene foams may be low-density or high-density foam.
• At the molecular level, the stages of the foaming process are gas dissolution, cell nucleation, cell growth, and cell stabilization.
• The primary raw materials for polyethylene foam production are polyethylene resins (LDPE or HDPE) and blowing agents. Additives are blended to impart special properties to the foam product.
• Blowing agents may be a chemical blowing agent (reactive or thermal decomposition agents), a physical blowing agent (gas or supercritical fluid), or expandable beads.
• The common foaming techniques used in producing polyethylene foams are batch foaming (pressure-induced and temperature-induced), foam extrusion, and foam injection molding.
• Examples of polyethylene foam products are insulation foams, anti-static foams, foam tapes, fire retardant foam sheets, laminated foams, and paddings for furniture and flotation devices.