Published August 13, 2020
Before its hazards were known, N-Methylpyrrolidone (NMP) was used as an industry standard solvent for various applications. The toxicity and increasing environmental and safety concerns regarding NMP have led to its removal from many products, and it is currently being phased out and banned in many countries’ manufacturing processes. Some solutions attempt to address worker exposure rather than the solvent itself, so it is important to understand the chemical components, uses, dangers, and alternatives.
What is NMP?
N-Methylpyrrolidone or N-Methyl-2-Pyrrolidone is a colorless, high boiling, high flash point, and low vapor pressure chemical solvent that is completely miscible with water at all temperatures. These attributes are highly desired in many manufacturing processes. It is commonly used in the petrochemical and plastics industry as a solvent, and in the manufacturing.
As a water-miscible organic solvent, NMP has high solvency, resulting in its popularity as a surface treatment of textiles, resins, and metal-coated plastics, and as a paint stripper. It is a versatile cleaning agent that is used in the semiconductor manufacturing process and in the electrode coating process in the fabrication of lithium ion batteries.
After NMP is used, it must be removed, recovered, and recycled to prevent its release into the air as a pollutant. Due to its high boiling point, NMP is recovered through a separate cooling and evaporation process.
NMP in Everyday Products
NMP is also used in the manufacturing of consumer products, such as pigments, cosmetics, insecticides, herbicides, and fungicides. NMP can be found in many households, as it is used in some nail polish removers, hair dyes, mascaras, inkjet printer inks dyes, and arts and crafts products. It is found in many paints, floor cleaners, adhesives, herbicides, and pesticides. Surprisingly, NMP is also present in certain brands of children’s clothing, footwear, bibs, bedding, and toys.
From consumer products, NMP is most commonly absorbed through the skin. Many of these products have various percentages of NMP present, which influences the potential irritability of the product. Items with low percentages of NMP are less likely to have a harmful effect on consumers; however, it is still recommended to avoid products with NMP.
Commercially, one large application of NMP is in the coating of electrodes to manufacture lithium ion batteries. A typical cathode manufacturing line uses the polymer polyvinylidene fluoride (PVDF) dissolved in NMP mixed with carbon and active material to make a slurry that is applied to the current collector with slot die coater.
NMP is also used as a solvent in semiconductor manufacturing. It is used for photoresist stripping, developing, and metal lift-off. NMP effectively removes the photoresist residue without causing surface abrasions. While NMP has advantageous commercial uses, its toxicity is still concerning to many companies and consumers.
Exposure to NMP can have several adverse health effects. The EPA describes NMP as a developmental toxicant, which is a toxin that interferes with growth, development, and behavior as a result of environmental insult. NMP can irritate the skin and eyes and can make it difficult to breathe by irritating the lungs and respiratory system. Extended exposure to NMP can also cause kidney and liver damage, neurotoxicity, and reproductive toxicity. For pregnant women or women who may become pregnant, any exposure to NMP can result in fetal development problems, including decreased fetal weight and even death.
Although some risks can be mediated by the use of chemical resistant gloves, the EPA finds that the use of these gloves and respirators is not sufficient in reducing the health risks for people who work with NMP for more than four hours per day.
OSHA, the US Occupational Safety and Health Administration for the US government limits worker exposure to NMP exposure to 10 ppm (parts per million) in an eight-hour day. Likewise, the state of California sets the Permissible Exposure Limit (PEL) at 1 ppm or 4 mg/m3. Since it is readily absorbed through the skin, workers exposed to NMP must have a level of NMP below 100 mg/L in their urine tested immediately after their shift. All of this information can be found on the chemical’s SDS (Safety Data Sheet), which is generally accessible on the internet, but must be supplied by the manufacturer to any customer. The chemical industry designates the CAS number 872-50-4. It is much easier to search for safety data using the CAS number.
The GHS (Globally Harmonized System) of designating chemical safety for NMP has the following statements:
- Flammable liquids (Category 4) H227 Combustible liquid.
- Skin irritation (Category 2) H315 Causes skin irritation
- Eye irritation (Category 2A), H319 Causes serious eye irritation.
- Reproductive toxicity (Category 1B), H360 May damage fertility or the unborn child
- Specific target organ toxicity – single exposure (Category 3),
- H335 May cause respiratory irritation
In the category system, for this comparative safety scale, category 4 would be mild and category 1 would be extremely dangerous.
Alternatives to NMP
Due to environmental and health safety concerns, several alternatives to NMP are currently being explored. To mimic all the attributes of NMP, some manufacturers are experimenting with combining different solutions. Instead of using harmful chemicals in the manufacturing of lithium ion batteries, Miltec UV developed a binder system that contains no NMP in its coating process. Miltec UV’s process is faster, cheaper, uses less space and less energy, and requires no solvent recovery to produce electrodes.
Cost, recovery, disposal, and environmental concerns are all minimized with the elimination of NMP’s volatile compounds. Miltec’s alternative process eliminates the need for the costly NMP solvent recovery process and reduces the environmental footprint. A UV manufacturing system costs 1/10th or less than that of a thermal drying system that uses NMP (with PVDF). While it is not a solvent free process, the solvents used do not require the expense of capture and recovery, and they evaporate much faster than NMP. Since there is no NMP present in Miltec UV binders, employees involved in the production process work in a much safer environment.
Written by Miltec UV Interns Kaylee Towey and Kelsie Oshinsky with input from Dr. John Arnold Miltec UV Director of R&D Battery Division.