How To Produce Filter Foam?

1. Filter foam refers to polyurethane foam that has undergone membrane removal, leaving only the skeletal network structure. It is widely used in air conditioning and automotive filtration materials, catalyst carriers, waste oil recovery, and diving water treatment.

2. The production of filter foam is based on the previously discussed flexible polyurethane foam, which is further processed. Currently, there are four main processing methods: alkaline treatment, hot air method, steam hydrolysis, and explosion method.

The alkaline method has the advantages of a simple process, easy-to-obtain raw materials, and quick setup. However, it produces significant waste and results in major loss of foam properties, making it difficult to produce thick filter foam.

The explosion method preserves foam characteristics well and generates no waste, but it is difficult and dangerous to operate.

Steam hydrolysis and hot air methods also cause significant loss of foam properties, though they are safer and produce no waste.

The following is a detailed explanation of two methods:

(1) Alkaline Treatment Process:

First, polyurethane foam with high open-cell content is cut into specified shapes and thicknesses as required by the user. Then, it is immersed in 8% to 25% aqueous sodium hydroxide or potassium hydroxide solution. Under suitable temperature and after a certain hydrolysis time, the foam membranes rupture. Afterward, the foam is squeezed to remove alkali, neutralized with a weak acid, washed with clean water, squeezed, and dried to obtain the final filter foam product.

Higher alkaline concentration and temperature result in shorter reticulation time but greater physical damage to the foam. Lower concentration and temperature require longer time but cause less physical loss. For thicker foams, slightly longer treatment allows the alkali to penetrate deeply, improving uniformity.

Example: For polyester-based polyurethane foam, use a 10% sodium hydroxide solution at 50°C, with an immersion time of about 10 minutes. Exact conditions should be determined experimentally.

To enhance efficiency and reduce physical loss, alcohol-based solubilizers and surfactants may be added. A typical formulation might include:

Isopropanol 15 parts, Ethylene glycol 20 parts, Sodium hydroxide 15–20 parts, Water 60 parts.

With this solution, treatment can be conducted at only 30°C for 30–60 seconds, producing foam with minimal physical damage, close to that achieved by the explosion method.

(2) Explosion Method Process:

The explosion method uses combustible gases such as acetylene, alkanes (liquefied gas), or hydrogen mixed with air or oxygen to form a combustible mixture within the explosive limits. This mixture is ignited remotely via electric spark, generating high temperatures and shock waves that melt the foam membranes, creating a reticulated structure.

Example of Hydrogen/Oxygen Explosion Process:

A pressure-resistant steel explosion chamber (up to 2 MPa) is constructed, equipped with upper and lower inlets/outlets, a safety valve, a vacuum pump valve, and a spark igniter.

The foam to be reticulated is placed in the chamber, which is sealed and evacuated. Hydrogen and oxygen are introduced through the inlets and mixed uniformly via a static mixer. After closing the valves, the gases are left to diffuse evenly into the foam for about 2 hours. Then, remote ignition triggers the explosion. The resulting foam is removed and placed in a well-ventilated area to allow residual gases and byproducts to dissipate naturally before proceeding to further processing.

Precautions in Hydrogen/Oxygen Explosion Process:

①The hydrogen-oxygen ratio varies depending on the pore size and wall thickness of the foam.

②Excess hydrogen results in incomplete membrane removal; excess oxygen may char the foam.

③Uniform gas mixing and diffusion are critical to ensure consistent product quality.

④A safety valve must be installed to prevent accidents.