The Use of Solvents in the Plastics Industry and Safer Alternatives
Hao Pan, UMass Lowell
The global plastics demand is expected to reach 334.83 million tons by 2020, representing a market of USD 654.38 billion according to a new study by Grand View Research. Inc [1]. As of 2013, the global plastics demand was 233.75 million, thus the compound annual growth rate (CAGR) will be 5.3% from 2014 to 2020. If you’ve been checking out this site, you know that plastics materials have many advantages compared to metal and other materials such as light weight, ease of processing, more cost efficiency and more. There is no doubt that the plastics market will keep growing in the future. But there are potential sustainability concerns behind this promising market. One major worry is recycling; people have realized that the plastics can persist in the environment for hundreds of years and will cause serious environmental problems without proper disposal. Besides that, the use of a very large amount of solvents in plastics industries is another issue which is lesser known to the public.
The term “solvent” is applied to a vast number of different substances. They are defined as “substances that dissolve other substances to form a solution.” Solvents have found widespread use in plastics industries from the synthesis of raw materials to the end-use of products. For example, solution polymerization is a widely used technique to produce raw resins, which uses a solvent as the chemical reaction medium. A significant source of toxicity in polymer products occurs when solvents are used either as diluents or to enable of the application of the polymer (e.g., as a paint or adhesive). However, these applications make the use of toxic solvents an inescapable concern when we use plastics products. According to Zion Market Research [2], the global solvents market was valued at USD 26.80 billion in 2016 and is expected to reach USD 33.0 billion in 2022, growing at a CAGR of 4.0% between 2017 and 2022.
Figure 1. solvent demand by product, 2015
As shown in Figure 1 [3], In the US, alcohols such as ethanol, n-butanol, isopropanol and methanol are major solvents in use, followed by hydrocarbons, glycols, ketones and others. Among all end-uses, paints and coatings accounts for about 46% of the total use (see Figure 2 [4]). Most paints and coatings are polymer emulsions, which require the use of organic solvent. Growth in the construction and automotive industries also leads to increasing demand for paints and coatings. Other uses for solvents include pharmaceuticals, adhesives, inks, house care, chemical processing, metal and agriculture (some samples are given in Figure 3).
Figure 2: Solvent end use
Figure 3: Solvent Use Application: Paint (5)
Figure 3: Solvent Use Application: Adhesives (6)
Figure 3: Solvent Use Application: Pharmaceutical (7)
During the production and usage, there are potential dangerous including fire, explosion, erosion and most importantly, the damage to human bodies. Millions of people are exposed to solvents and health hazards associated with solvent exposure include toxicity to the nervous system, reproductive damage, liver and kidney damage, respiratory impairment, cancers [8]. Solvents as well as other chemicals additives may enter human body by breathing, skin or eye contact and swallowing. The effects of solvents can be acute and chronic. Due to the difficulty to recognize so many kinds of solvents, symbols have been systemically developed to show the hazards (see Figure 4 [9]). For example, ethanol, one of the most common used solvents, are labeled as 3 in red, 2 in blue and 0 in yellow, indicating it’s flash point (the temperature leading to vapor to ignite) is below 100F, moderate damage to heath and it is stable. Many websites have database for looking up the hazards of solvents and the requirements when working with them. For example, from higher hazard substances [10], chemicals that pose the most serious threats to health and the environment are listed, also, toxic substances control act [11] and integrated risk information system [12] can be used to look up those high hazard solvent and additives.
Figure 4: Hazordous Materials Warning Symbols (9)
There are several strategies to overcome the drawbacks of toxic solvent use. One of them is to develop water-based emulsion systems. Paints are composed of mainly three materials: resins, which form a coating with pigments that provide color and luster as the components in the coating, and solvent auxiliary agents, which, as volatile components that do not form part of the coating, are included to thin the materials and help them cover more smoothly. Using a water-based system can be an effective way to reduce the use of volatile organic solvent. The main difficulty of applying a water-based system is that it requires removal of water, requiring heating up to 100oC or a long dry time. this can be conquered by tuning volatility to make water-based paints and coatings dried at lower temperature. Our research group works on making semi-conductive polymer aqueous colloidal systems which are safer and more environmentally friendly compared with conventional toxic solvent-based systems. For more information, you can read our paper, "Effect of surfactant conjugation on structure and properties of poly (3-hexylthiophene) colloids and field effect transistors."
Another strategy is to develop bio-based solvent. Most bio-based solvents are made from sugar, corn, or beets. “Bio-based” doesn’t mean they are non-toxic at all, they can be still toxic during use. They are chosen primarily because they don’t release toxic byproducts and volatile organic compounds (VOC) during manufacturing. And they are safer for use owing to their high boiling point and low miscibility. Many green solvents have become popular in the recent years. These solvents are mainly used for coatings, paints, printing inks, personal care products, cosmetics, sealants, and pharmaceuticals, and therefore, sustainable solvents suppliers see the demand for bio-based solvents growing fast. Hopefully, a combination of government regulations and increased consumer awareness will ensure that safer and more sustainable solvents will replace hazardous materials used in coatings today.
References:
1. https://www.prnewswire.com/
3. https://www.inkworldmagazine.com
4. Pereira, Carla SM, Viviana MTM Silva, and Alirio E. Rodrigues. "Ethyl lactate as a solvent: properties, applications and production processes–a review." Green Chemistry 13.10 (2011): 2658-2671.
5. https://www.eastbayexpress.com