BLOWING AGENTS AND INSULATION FUNDAMENTALS
As Figure 8.22 shows, the manufacturer and consumer have a significant stake in the choice of blowing agent that is made. The manufacturer must meet regulatory, cost, and quality requirements in production. At the same time, the economics of ownership are affected greatly by insulation efficiency [42-44]. In addition to these prosaic considerations, there is the recognized influence blowing agents can have on the global environment. There is particularly recognition that certain blowing agents that were previously ubiquitous are capable of depleting ozone present in the upper atmosphere [45^17]. While ozone in the lower atmosphere is regarded as a pollutant, in the upper atmosphere (stratosphere) ozone serves as a necessary filter on high energy UV-B radiation. In 1973, Chemists F. Rowland and M. Molina showed that the high vapor pressure and stability of chlorotluorocarbons, commonly used blowing agents and propellants, could be dissociated by UV radiation, releasing a chlorine atom, that proceeded to breakdown ozone . The removal of ozone from the upper atmosphere, responsible for filtering of UV radiation would result in a greater flux of UV radiation at the Earth's surface increasing the potential for skin cancer and cell damage to living things.
TABLE 8.9 Properties of rigid polyurethane foam blowing agents, several of which are banned or scheduled for phase out
GWP, relative global warming potential; ODP, relative ozone depletion potential.
In 1987, an international treaty was signed  to protect the ozone layer by phasing out production and use of chemicals capable of depleting stratospheric ozone. Since then, it has undergone numerous revisions and updates and has been ratified and upheld by almost all nations. Among the revisions, the protocols have recognized that in addition to their potential for ozone depletion (ODP), chemicals useful for blowing agents can be scrutinized for their contributions to global warming potential (GWP) with several blowing agents showing orders of magnitude more GWP than C02. Recent attention has also focused on the atmospheric lifetime of blowing agents. Since the effect of this treaty and its revisions has a major effect on the industrial practice of making polyurethane foams for insulation, it is of major interest to practitioners of polyurethane science.
Table 8.9 is a partial compendium of blowing agents that have been used for preparation of rigid polyurethane insulation foams. Other blowing agents have been explored and have been mixed in attempts to obtain synergistic effects or dilute the different negative issues one agent has without harming the positive attributes that may come in the pairing.