The growing global importance of chemical safety is changing the manufacturing and utilization priorities of all chemical products including coatings. Life cycle concerns in regards to environmental effects of commercial chemical products are also gaining attention from governmental agencies, non-governmental groups and the public. This is leading to the use of more intrinsically safe and bio-based solvents in coatings.
One of the important distinctions when discussing this topic is the difference between bio-safe (non-toxic) and bio-based (based on renewable raw materials) although a particular solvent could meet both definitions. The best approach is development and commercialization of solvents that are safe in use and minimize environmental impact through the sustainable practices during manufacturing, application, recycling and disposal.
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Methods which minimize the environmental impact of chemical manufacturing and use are outlined in the 12 Design Principles of Green Chemistry, as defined by the American Chemical Society.1 These principles can be grouped into “Reducing Risk” and “Minimizing the Environmental Footprint”. Two of the principles apply particularly well to bio-safe and bio-based solvents.
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Safer Solvents and Auxiliaries
Safer solvents reduce toxic materials, improve process safety and minimize generation of hazardous waste. Solvents used in industrial chemical manufacturing are often carried over into the formulated products delivered to the end user. Reducing or eliminating hazardous materials makes the production process, as well as the final products, safer.
Use of Renewable Feedstocks
Utilizing renewable raw materials is the first step in reducing the environmental impact of industrial chemical production. Renewable materials use plant-based feedstocks which might be purpose grown, surplus material from food production or by-products from some other manufacturing process such as paper production.
Bio-safe and Bio-Based Solvents in the Coatings Industry
Many bio-safe and bio-based solvents are familiar to the coatings formulator because they have been in common use for decades. Other solvents are new to the coatings formulary. Many of these novel solvents are able to be economically utilized in coatings because of new lower cost manufacturing processes.
Cyrene™
Cyrene™ is both low toxicity and uses plant-based raw materials derived from cellulose wastes from paper manufacturing. This material, (-)-dihydrolevoglucosenone
((1S,5R)-6,8-Dioxabicyclo[3.2.1]octan-4-one) CAS Reg. No. 53716-82-8 is a high boiling (BP 203°C) solvent that can be used as an alternative to toxic dipolar aprotic solvents. Two solvents that are especially easily replaced with Cyrene™ are N-Methylpyrrolidinone (NMP) and Dimethylformamide (DMF).
Recent advances have made the economics of the synthesis of Cyrene™ from cellulose cost-effective for industrial products. In coatings, one of the most common uses of NMP is as a solvent during manufacturing of waterborne polyurethane dispersions also known as W/B PUDs. This class of polymers is used to formulate high performance coatings for various markets.
Since PUDs are waterborne systems, they can be formulated into low volatile organic compound (VOC) containing coatings. However, recently the residual NMP from synthesis of the polymer backbone is problematic due to its toxic properties. A non-toxic, bio-based solvent like Cyrene™ overcomes this hurdle and allows the use of PUD technology with fewer restrictions.
Soybean Solvents
The uses of soybean derivatives keep expanding beyond tofu and alkyds. Soy products are found everywhere from meatless burger patties that appear to “bleed” to solvents for architectural and industrial coatings.
Methyl soyate is a mixture of fatty acid methyl esters derived from soybean oil which can be used as a substitute for many chlorinated and hydrocarbon solvents. Methyl soyate is not classified as a hazardous substance under RCRA or as a hazardous air pollutant (HAP). The high boiling point means that Methyl soyate is considered a zero or very low VOC component for coatings formulation in many jurisdictions.
Other Coatings Solvents
Several other solvents in common use for coatings manufacturing can be made from bio-based raw materials. These include n-butanol, acetone and propylene glycol. These solvents are not necessarily non-toxic or bio-safe, but are generally considered to be of low to medium toxicity and are not considered hazardous air pollutants. As demand for products based on renewable materials increases, these bio-based versions of common solvents become more cost competitive.
Bio-safe and bio-based solvents are increasingly in demand for both industrial and consumer products. New processes and economies of scale are making them more cost effective and widespread in use.
References:
American Chemical Society: 12 Design Principles of Green Chemistry
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