Polyolefin Foam Extrusion Overview

Most commercial foam products can be classified as high-density foams or low-density foams. High-density foams have densities of 75% to 90% of the un-foamed polymer and are used in more permanent applications in furniture, cabinetry, material handling, industrial, construction, consumer, automotive, marine, and consumer markets. Low-density foams have densities of 10% to 20% of the un-foamed polymer. Low-density foams may be flexible or rigid, depending on the flexibility or rigidity of the base polymer. Flexible low-density foams are used for sound and heat barriers, for shock absorption, and for floatation applications. Rigid low-density foams are used for sanitary piping and conduit, pipe, and millwork. Thermoformed rigid high-density foams are used for food trays, containers and other packaging.

There are a few different technologies available for producing polyolefin foams by extrusion processing, but chemical foaming and physical foaming are the main foaming technologies. Chemical foaming involves the use of a chemical that reacts to produce gas through decomposition. Chemical foaming agents may be endothermic or exothermic depending on their composition. Azodicarbonamide (AZO) is the most widely used exothermic chemical foaming agent. It decomposes around 200°C to yield 220 cm³/g of blowing gas. About 65% of the initial dosage of AZO remains as a residue consisting of cyanuric acid, urazole, and biuret.

4,4’-oxybis (benzene sulfonylhydrizde) (OBSH) is another widely used exothermic chemical foaming agent. Its decomposition temperature is 155°C and its blowing gas yield is 125 cm³/g. Because of its lower decomposition temperature, it is mainly used in lower melting polymers like EVA, LDPE and flexible PVC.

5-phenyl tetrazole (5-PT) is used in higher melting point engineering and high-performance polymers such as PBT, PET, PSU and PES because its decomposition temperature is between 250°C and 300°C.

Endothermic chemical foaming agents are generally alkali carbonate mixtures that decompose over a broader temperature range than exothermic foaming agents. Some endothermic foaming agents are used with moisture sensitive polymers like PET because they do not give off moisture. Since many of the decomposition products of endothermic foaming agents are chemically benign, they are used for food contact or medical applications.

Physical foaming agents can be either permanent or atmospheric gases or volatile liquids. Nitrogen and carbon dioxide are two common gases used to foam thermoplastics. These foaming gases must first be dissolved into the polymer to produce fine cells. Foam density depends on the amount of gas that is dissolved into the polymer, so its solubility in the polymer is especially important. Because nitrogen and carbon dioxide have limited solubility in most thermoplastics, they are not able to produce very low-density foam.

Volatile liquids like butane and pentane, evaporate at the appropriate conditions to produce the foaming gas. These gases are relatively inexpensive and are easily transported and pumped into the extrusion equipment. They are, however, flammable, and so care needs to be taken not only in their handling, but in storing and handling the finished foam products. Chlorofluorocarbons which were commonly used in the past are no longer used because they are known to cause depletion of the earth’s ozone layer.

This document will focus on the physical foaming of polyolefins used to produce extruded foams with densities below 200 kg/m³.

FOAM INGREDIENTS

The main ingredients in extruded polyolefin foam are the polymer itself and the foaming agent. Common polyolefins include LDPE, HDPE, EVA, polyethylene copolymers, and polypropylene, and the physical foaming agent is generally n-butane, iso-butane, n- pentane, iso-pentane, cyclopentane or a combination of these. The amount of a physical foaming agent needed can be up to about 15%. The other ingredients include nucleating agents and cell stabilizers. Nucleating agents are fine particle size (1 to 20 microns) talc, calcium carbonate or other inorganic particulate and are generally added at 1% or less. The nucleating agent provides irregular surfaces, cracks, and crevices to which the dissolved gas molecules migrate and begin bubble growth.

Cell stabilizers are required to stabilize the foam and to balance the outward migration of the foaming gas and the inward migration of air. The most common cell stabilizer is glycerol monostearate (GMS), but titanates have also been used. The typical loading level for cell stabilizers is 1% or less.

THE FOAMING PROCESS

In order to produce high quality foam, the foaming gas must be thoroughly dissolved and uniformly dispersed in the polymer melt. Bubbles start to form on the nucleating agent. However, if the melt pressure exceeds the pressure necessary to keep the gas in solution, bubbles cannot form. Typically, the melt pressure in the extruder is high until just before the melt exits the die. As soon as the polymer melt exits the die, the pressure drops below the solubility pressure and microbubbles start to form on the nucleating agents. Once these microbubbles form, they grow rapidly, typically in less than 1 second, to a size of about 100 microns. Processing of the polymer with low melt temperature can help to stabilize the bubble growth. As the bubble grows, the blowing gas is depleted in the melt immediately surrounding the growing bubble. Blowing gas in the melt then diffuses toward the bubble site, and bubble growth is controlled by the rate at which the gas can reach the bubble. The expansion rate of the foam starts to decrease because of decreasing gas concentration. For low density foam, the sheet or profile continues to expand freely after exiting the die. Free expansion of the foam yields very uniform, symmetrical cells with cell walls of uniform thickness.

Newly produced foam may not be very dimensionally stable because as the foam cools, the internal cell gas pressure drops below atmospheric pressure. If the foam cannot support the pressure difference between atmospheric pressure and cell gas pressure, it will collapse or shrink. If air diffuses into the foam faster than the foaming gas can diffuse out, the foam may expand or grow. The foam dimensions will eventually stabilize once the atmospheric pressure and cell gas pressure are equal. To help control and balance this outward and inward diffusion of foaming gas and air, cell stabilizers are incorporated into the foam formulation.

PROCESSING CONSIDERATIONS FOR PHYSICAL FOAMING

Extrusion foaming of polyolefin materials can be conducted using a tandem extruder or a single screw extruder. In tandem extrusion the first (primary) extruder is usually a standard single screw extruder where the polymer and other additives are melted and mixed and then the blowing gas is introduced under pressure via a hole (gas port) in the barrel wall. The polymer melt with the blowing gas is then transferred to the secondary extruder where the melt temperature is reduced (to allow for stable foaming) while keeping the melt under pressure to prevent premature foaming prior to coming out of the die.

In single screw extrusion, a long, two-stage screw is used. The length to diameter ratio (L/D) is often in the range of 40 to 60. The first part of the screw is used for melting and mixing the polymer and other additives. The foaming gas is injected in a port between the first and second stage of the screw where there is a short mixing section followed by cooling of the melt.

Extruder barrel temperatures for physical foaming of ethylene-based polyolefins are set to achieve a melt temperature of around 120°C. Too high of a melt temperature (greater than 145°C) often results in poor foaming and foam instability. Basically, high melt temperatures would cause the foaming gas to quickly diffuse out resulting in high foam expansion followed by shrinkage.

Extruded polyolefin foam is often produced in the shape of a hollow tube, followed by slitting to produce a flat sheet. This sheet must then be cooled before rolling or cutting and stacking.

When physically foaming polyolefins using butane or pentane gas, care must be taken when storing the finish foam products to avoid the potential of a fire. Finished foam products can give off butane or pentane gas over time until they are completely off-gassed. Because of this, the foam should be stored in a well-ventilated area or fire-proof room away from any sources of flame or sparks which could ignite the butane or pentane gas and cause a fire.