Green Solvents: An Ideal Solution?

Oct 21, 2015

Driven by regulation and concerns about environmental preservation and resource depletion, the solvent industry is facing a general push towards green innovation, but what does it mean to be “green”?

Solvents are integral components in a vast number of processes and products, and are used in a wide array of industrial applications including: agrochemicals, printing inks, paints and coatings, pharmaceuticals, and household and industrial cleaners.  This $30 billion industry is entering a new era of regulatory and end-user scrutiny and, as a result, solvent selection is becoming increasingly complex.  Solvent consumers may no longer select a solvent solely based on its physical capability to perform the required function.  Environmental and worker safety implications play an increasingly instrumental role in solvent selection, and consumers are starting to turn towards greener replacements to conventional solvents.  Selection of the right solvent has always had the power to increase competitiveness, but environmental benefits and worker/user safety will be the key to product differentiation and better margins as the industry enters into this era of improved environmental awareness.  Maintaining process compatibility and solvent functionality will continue to be the top priority when selecting a green solvent replacement, but financial performance and environmental performance need not be mutually exclusive.

The use of volatile conventional solvents and the associated emissions of ozone precursors and hazardous air pollutants have prompted numerous regulatory initiatives over the past 20 years.  Certain volatile solvents also contribute to indirect greenhouse gas emissions.  In fact, solvent use has been implicated as one of the largest contributors to the chemical industry’s total environmental footprint.  Moreover, handling of toxic or flammable solvents may require costly personal protective equipment and other expensive safety measures.  In order to protect workers and users, worker exposure limits and other restrictions for solvent use are becoming increasingly stringent.  Governmental regulations have been the leading impetus driving the industrial shift towards “greener” solvent use, though sustainable business initiatives and increased consumer awareness have also been influential drivers in their own right. 

Green solvents currently represent around 10 percent of the total solvent market.  Green solvent consumption growth through 2014 was assisted by the higher prices of conventional solvents that began escalating in 2004 as a result of record high feedstock pricing.  Despite current low oil and gas prices, Nexant forecasts green solvents growth to markedly outpace the conventional solvents market in its special report: “Green Solvents: An Ideal Solution?”.

What Makes a Solvent “Green”?

The term “green” is often used to express the goal of minimizing the environmental impact resulting from the use of a particular substance.  However, the definition of “green” is rather fluid, and the term is used loosely and extensively in marketing materials.

There are many criteria that form the basis upon which a solvent may be labelled “green”, including, but not limited to: a renewable feedstock, status as non-VOC (or functionally non-VOC), a high flash point, non-toxic, biodegradable, and recyclable.

Given that a solvent may possess many or only one of these attributes, how can the industry measure the greenness of a solvent in order that it can be compared with another?  To answer this question, Nexant developed a method to quantitatively assess the relative “greenness” of solvents; the Nexant Solvent Sustainability Index, or NSSI.

Nexant Solvent Sustainability Index Methodology

The NSSI evaluates solvents on eight parameters in order to determine its relative greenness: feedstock, toxicity, biodegradability, VOC status, odor, fire hazard, vapor pressure, and reactivity.  A solvent with perfect greenness in all categories will receive an NSSI of 100 percent, e.g. water.  Less green alternatives are rated on a sliding scale down to zero. 


Popular “Green” Solvents

The figure below provides a sample of the 62 solvent assessments that Nexant completed in its recent special report: “Green Solvents: An Ideal Solution?”.

Ethanol is the oldest and most successful biorenewable chemical product in commercial production. Aside from its more pervasive end-uses in fuels and beverages, ethanol is an important solvent in detergents, cosmetics, lotions, soaps, shampoos, and other consumer products.  Solvent applications for ethanol account for a very small percentage of total demand.  The majority of future demand growth will likely continue to come from fuel applications, while demand for solvent applications will be more stable.  Petrochemical ethanol has an NSSI rating of 44 percent; bioethanol is rated at 54 percent.

Acetone is a conventionally sourced petrochemical product with broad solvent demand in paints, coatings, adhesives, inks, manufacturing, pharmaceutical, and food applications.  Bio-acetone is commercially available at significant scale at a very small premium and is naturally present in many fruits and vegetables.  The acetone market is dominated by solvent, methyl methacrylate (MMA) and BPA demand, and is centered in China, the US, and Western Europe.  Acetone’s NSSI is 55 percent, or 65 percent for bio-acetone.

Glycerol can be used as a solvent in a variety of applications, including anti-freeze, botanical extracts, and in the food industry.  Glycerol has the potential to be a suitable replacement for a number of solvent applications.  Though its use as a solvent has been limited in the past, demand has increased recently.  Importantly, glycerol is produced as a byproduct to biodiesel, and abundant biorenewable supply is available at a relatively low cost.  Nexant finds that glycerol stands up quite well as a green solvent, with petro-derived glycerol scoring an NSSI of 71 percent and biorenewable glycerol an NSSI of 81 percent.

Ethyl Acetate is regarded as one of the least toxic of industrial organic solvents.  As a solvent, ethyl acetate finds use in a wide range of applications, including printing inks, varnishes, and car care chemicals and in the production of enamels, plastics, and rubber.  Ethyl acetate has seen above average growth in the solvents sector, as it has substituted for hazardous air pollutants (HAPs) such as MEK and toluene, particularly in the developed world.  However, the outlook is for lower growth, as a result of the ongoing reductions in volatile organics that has led manufacturers to formulate away from solvent based coatings. Ethyl Acetate received an NSSI rating of 49 percent, with its biorenewable counterpart 10 points higher.

d-Limonene has been widely touted as a green solvent as it is a major component of the oil extracted from citrus rinds.  The past decade has seen the use of d-limonene expand enormously, and a great deal of d‑limonene goes into making paint solids.  By far the largest growth sector has been the use of d-limonene in cleaning products for both industrial and household applications.  However, supply is unstable and entirely dependent upon citrus fruit production, and is thus susceptible to supply chain challenges.  Though d‑limonene is advertised widely as being a green solvent, there is evidence to suggest that it is not actually as green as perceived, having quite notable aquatic toxicity ratings, and a high tendency to form ground-level ozone.  d-Limonene has an NSSI of 58 percent.

Implementing a Green Solvent Replacement

In choosing an alternative solvent or technology, the end-user is faced with a number of considerations, such as one-off conversion costs, changes in operating costs, regulations associated with the use of the substitute, worker safety, effectiveness, and process compatibility.  In short, solvent replacement is complicated, and might take several years.  Therefore, when considering making a solvent containing process or product more sustainable, Nexant recommends that consumers follow a step-wise approach as shown in the figure below: 


The combined effects of regulation and producer/consumer benefits garnered from environmental innovations in the solvent industry make the broad-spectrum switch to green solvents inevitable. Profitable growth will come from companies that align their sustainable solvent offerings with industry needs, key regulatory drivers and consumer preferences.


Dr. Iwamoto and Ms. Boisvert are analysts in Nexant’s London and New York offices who co-authored Nexant’s Special Report “Green Solvents: An Ideal Solution?”.