The first step in recycling expensive metal filters and parts is to remove the polymer or major contaminant. In determining the appropriate depolymerization method, consideration must be given to the physical and chemical physiognomies. Each plays a role when deciding which polymer removal method is best for a metal part or filter.
Polymer chain structures can affect solubility.
Generally, the molecular weight and degree of crosslinking will decrease solubility, whereas, chain branching can increase the solubility. Some cross-linked polymers will swell as solvents fill the voids among the many chain linkings within the polymer. Depending on the construction of the filter, the swelling could damage the media. For polymers that have crystalline and amorphous states, the amorphous form may be more soluble than the crystalline forms in a particular solvent.
Generally, as the polymer’s molecular mass increases, so does its melting point. However, chain structure greatly affects melting.
The proximity of polar groups may set up hydrogen bonding among points on the chain.
Attractive forces between points on the chain increases melting point. For example, increasing the number of –CH2- groups between the polar amide linkages on a polyamide chain decreases the melting point of higher polyamides.
Some polymers can be melted at temperatures that are not detrimental to the material of construction of the filter, whereas, others may degrade or decompose without melting. It is important to realize that some polymers, such as thermosets, will not melt.
Flow characteristics of the melt:
If melting is an option, then knowing the flow characteristics of the polymer will determine whether melting will be helpful in removing the polymer from the filter.
If burnout procedures are necessary, allowing the bulk of the polymer to flow/melt out of the filter will reduce the risk of damage to the filter at oxidation temperatures.
If the contaminating material is the result of a suspension of polymeric particles, as opposed to, a coating of polymer from a hot melt process, ultrasonics could be useful in removing the particles.
For this method of polymer removal, it will be necessary to know the optimum frequency, time, temperature, bath medium, and orientation of the part for the ultrasonic process.
When polymers react with a particular solvent, water, or aqueous solution, the chain breaks down into smaller units. The byproducts of the reaction and how they are handled must be considered.
For example, using glycols to breakdown polyester will produce low molecular weight compounds, which are flammable. These “low boilers” must be collected and contained according to EPA guidelines.
Parameters of the reaction: If the polymer reacts with a chemical, it will be necessary to know the optimum concentrations, soak times, temperatures, etc.
Response of Heat:
If there are no solutions or solvents that can be used for depolymerization, then the alternative would be some type of heat source.
Hot air ovens with blanketing controls: Care must be taken when using hot air ovens for depolymerization. Polymers will burn, char, decompose, etc when exposed to high temperatures. What happens will depend on the characteristics of that polymer. In any case, since polymers are organic, there is an exothermic reaction involved once the polymer is exposed to temperatures in excess of 750°F. The heat produced can drive the part temperature to 800°F and greater. At these high temperatures, problems, as described in an earlier section, can occur with the particular alloy being used.
Vacuum pyrolysis ovens: The benefit of these ovens is that they operate at low oxygen potential which reduces the possibility of burning when the temperatures get into the oxidation range for carbon. As a result, charring may occur or possibly sublimation depending on the polymer characteristics. In any case, if the temperature exceeds 800°F for an extended period of time, sensitization and other problems with the metal substrate can occur.
For those polymers that sublime, the sublimation temperatures can be adjusted based on temperature and pressure.
Steam processes: If the polymer hydrolyzes, then steam processes with attention to temperature, pressure, and pressure changes can facilitate polymer removal through hydrolysis, as well as, melting and/or mechanical means. Depending on the steam process used, the type or amount of post-depolymerization cleaning will vary. An important benefit of steam cleaning is that the operating temperatures do not necessarily reach temperature ranges that could result in sensitization of 300-Series stainless steel media.
Molten Salt Bath processes: Depending on the type of polymer and off-gases produced during depolymerization, molten salt bath depolymerization may be a possibility. However, since most polymers are a viable source of burning, the proper afterburners and containment equipment are necessary in order to operate safely and meet environmental regulations. Using molten salt baths for depolymerization will require that the filters or parts go from a hot ambient air environment directly into a bath that could be anywhere from 400⁰F-900⁰F. Considerations must be given to the metallurgy and robustness of the part to withstand going through a 300°F-800°F temperature differential upon immersion. Any organic material on or embedded in the media may possibly ignite while being immersed. Once immersed in the salt bath, the salt will act as a heat sink in the event of exothermic.
There are a variety of ways to remove polymer safely and effectively from filters and parts. It is important to understand the construction and restraints of the filter media or parts, along with the chemistry and characteristics of the polymer being removed. To read more about this subject, download Susie Reynolds’ whitepaper, “Methods of Polymer Removal.” If your facility needs professional help removing polymer from its equipment, parts or filters, contact Carolina Filters today to speak with a professional.