Small- to medium-sized companies often can not afford, and/or find it is too time consuming to have a safety, health, and environmental (SHE) expert employed to guide solvent selection. External consultants are an option, but many times they are too costly for smaller companies. Given this situation, many stay with their existing solvent formulations as long as they remain legal, even if they present possible safety and/or health risks of which they are unaware.
This article describes a new tool that can easily and more accurately evaluate the risk potential of solvents and aid in the selection of safer replacement solvents. The parameters describe a vapor hazard ratio (VHR) and a solvent hazard parameter (SHP), which are based on existing guidelines to classify solvents according to their hazard potential.
DiscussionNumerous lists have been established that serve as guides for the selection of safe solvents.4 These include VOCs; HAPs; Clean Air Act Amendments (CAAA) of 1990; the federal Resource Conservation and Recovery Act (RCRA); the federal Clean Water Act; and the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA). In addition to these lists, the Occupational Safety and Health Administration (OSHA) has set permissible exposure limits (PEL) on chemicals, and CGIH (Conference of Governmental Industrial Hygienists) has set chemical threshold limit values (TLV). It is easy to see how one can be overwhelmed when attempting to evaluate a solvent's hazard potential.
The TLV refers to airborne concentrations of substances and represents conditions under which it is believed that nearly all workers may be repeatedly exposed, day after day, without any adverse effects. Most TLVs are expressed as a time-weighted average (TWA) value, as this provides the most practical way of monitoring the airborne reagent for compliance with the limits. The method most commonly used in the United States to evaluate a solvent's health hazard potential is by comparison of the chemical threshold limit values (TLV). Other countries establish their own exposure limits for chemicals, which are used similarly to the TLVs in the United States.
Many times a comparison of TLV alone can mislead people on the safety aspects of using one solvent vs. another. For example, if Chemical A has a TLV of 100 ppm and Chemical B has a TLV of 10 ppm, a conclusion could be drawn that chemical A is safer. However, this may or may not be correct since the TLV describes only the exposure limit and not how fast that limit is achieved. The vapor pressures of the solvents also need to be considered in order to describe accurately which solvent is safer. To further complicate the issue, no TLV values exist for many of the commonly used solvents.
DuPont internally evaluates the safety of chemicals for workers and the environment. Through its Haskell Laboratory, the company has for the last 20 years been studying the acute, subchronic, reproductive, mutagenic and other toxicological effects of chemicals. From these studies, the acceptable exposure limits (AELs) are determined for chemicals. These are DuPont's internal exposure limits. AELs are guidelines based on informed judgment, and are not fine limits between safe and dangerous concentrations. They are not for use as relative toxicity indexes, limits for continuous uninterrupted exposure, or proof or disproof of health effects. TLVs for many chemicals have not been established, and for those chemicals AEL values are used to guide worker safety in DuPont, and will be used in this article.
The boiling point of a solvent is often overlooked by many people who do not recognize that a high exposure limit may be reached quite quickly at ambient temperature and pressure conditions. People feel much more comfortable working with a chemical that has an exposure limit of 100 ppm than one that has an exposure limit of 5 ppm. It is counter-intuitive to most people that the solvent with the 5 ppm exposure limit may be safer to work with because its limit is reached more slowly. The problem is that no easy method exists to help users evaluate solvents that is based on the exposure limit, and the rate at which the limit can be achieved at a given set of conditions. In Germany a parameter referred to as the vapor hazard ratio (VHR) is used, which is the quotient of the saturation concentration and the exposure limit,5 and addresses the previously described deficiency.
VHR = (saturation concentration, ppm) / (exposure limit, ppm)
The saturation concentration is the amount of chemical that is present at chosen conditions in a given volume of air, and is based on the vapor pressure of the solvent at the chosen conditions. It is easily calculated for any solvent using the following equation.
Saturation concentration, ppm = (vapor pressure, mmHg / 760)X106
The vapor hazard ratio reflects the risk potential of a solvent in terms of not only the exposure limit but also the rate at which that limit is achieved. Table 1 lists some of the more commonly used solvents and their associated VHR; solvents with lower VHR values have a lower potential health risk (are safer to use). For example, based solely on the exposure limit (TLV) one would consider acetone to have the lowest risk potential, but the VHR correctly shows that it has a much higher risk potential than a number of solvents. DuPont Dibasic Ester (DBE) solvent has a low exposure limit (as set by its AEL) but the VHR ratio correctly shows that it is one of the lowest potential health risk solvents.
Although the VHR correctly describes the risk potential based on exposure limits, it fails to describe other hazards that should be considered to ensure the safest workplace possible. A common concern when using solvents is the risk of fire. Every year numerous injuries occur as the result of a fire involving a solvent. We feel this is an additional parameter that an employer should consider to more accurately describe the risk potential of a solvent. The National Fire Protection Association (NFPA) has established a set of criteria for evaluating the fire hazards of solvents and other materials. It is based on the susceptibility of a material to burning. The following rating is based on the NFPA criteria, and ratings for any solvent/chemical can be found on the MSDS for that chemical.
0 - Materials that will not burn.
1 - Materials that must be preheated before ignition (flash pt. above 100F).
2 - Materials that must be moderately heated or exposed to relatively high ambient temperatures before ignition (flash point between 73-100F).
3 - Materials that can be ignited under almost all ambient temperature conditions (flash point below 73F (22.8C) and boiling point above 100F).
4 - Materials that rapidly or completely vaporize at ambient conditions, and burn readily (flash point below 73F (22.8C) and boiling point below 100F).
The solvent hazard parameter (SHP) is the VHR multiplied by the fire hazard rating, and is another method for assessing the risk potential of a solvent since it combines the health exposure and fire hazards. A listing of the SHP values for a number of the more common solvents is shown in Table 1, with the lowest SHP values corresponding to the lowest risk potential for a chemical (safest to use). The table is organized such that the lowest SHP solvent is at the top, and the highest SHP solvent is at the bottom.
It is important to state that the SHP values and VHR values do not completely describe all the hazards or parameters that an employer should consider. Other possible hazards/factors include chemical incompatibility, reaction with the process or other solvents, and the biodegradability of the solvent.
One must also consider that many chemicals have ceiling limits of exposure. The exposure limit also does not describe the health hazard effects to an individual if the exposure limit is exceeded. For example, one chemical's TLV could be set to protect workers from skin irritation while another could be set due to toxicity. Employers/workers, in addition to looking at the VHR and SHP (which are based on exposure limits) should consult the MSDS to determine the human health effects associated with exceeding the exposure limit. The Superfund Amendments and Reauthorization Act (SARA) requires that some chemicals be disclosed as toxic under SARA 313. The chemicals that require disclosure under SARA 313 are indicated as such in Table 1. Solvents with a low VHR and SHP (safer solvents) are not listed on SARA 313 (with a few exceptions) consistent with the indication of a solvent's safety given by these metrics.
The following examples show how the VHR and SHP can be easily and proactively used to develop safer solvent formulations. Hansen solubility parameters are used to classify solvents in terms of their nonpolar, polar and hydrogen bonding characteristics.6 They provide a systematic method that can be used to search for substitute solvents, or determine the solubility of a resin in a solvent or solvent blend. Figure 1 shows the location of a number of the commonly used solvents in terms of their Hansen polar and hydrogen bonding characteristics.
Replacement of a Solvent Blend in a Paint Stripping ApplicationA typical paint stripping blend (labeled current blend in Figure 2) is shown in Table 2.
An exposure limit (TLV or AEL) for a liquid mixture can be calculated on a time-weighted average exposure basis assuming the atmospheric concentration is similar to that of the original mixture (all the liquid mixture eventually evaporates). When the percent composition (by weight) is known the exposure limit of the mixture is determined by the following equation.
(where fn is the weight fraction of component n and TLVn is the exposure limit of component n.)
However using a TLV exposure limit for a liquid mixture presents the same limitations when trying to evaluate the potential health risk of the mixture as in the case of a pure chemical. Following is an alternative method using the VHR in place of the TLV to more accurately reflect the health risk a solvent mixture poses.
Using a solvent formulation program (DuPont as well as many other companies offer such services to customers), the Hansen parameters for the above hypothetical solvent blend is found to be nonpolar 8.6, polar 2.3 and hydrogen bonding 3.2. Figure 2 shows the location of the blend on a solubility map. The Hansen parameters describe the solvency of the solvent blend and it is desired that a replacement solvent/blend have similar Hansen parameters to perform adequately. Table 1 reveals that although methylene chloride and toluene have TLV values that are mid-range compared to the other solvents, their VHR and SHP reveal these solvents to have a high potential health risk. Generally, the replacement solvent blend must have similar solvency (Hansen parameters) to perform adequately. As is almost always the case there is no single correct answer for a replacement solvent in a blend. A few possible answers are shown in Table 3.
Comparison of the VHR and SHP values shows the proposed formulations to be solvent blends of lower hazard potential than the current blend. Calculation of the exposure limits for the solvent blends confirms the conclusion shown in Table 4.
The Hansen parameters for the proposed solvent blends 1-3 are shown in Figure 2.
A solvent blend SHP can be calculated by either measuring or calculating the flash point for the solvent blend and using the ratings previously described in this article to assign a fire flammability rating. Then multiply the flammability rating by the VHR for the blend.