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Materials: extruding thermoplastic foams with a non-CFC blowing agent.

The chlorofluorocarbons CFC-11, CFC-12, and CFC-114 have been used as blowing agents for extruded polystyrene and polyethylene foams for many years. Their unique combination of properties- non-flammability and low-order toxicity coupled with their polymer-adaptive properties of appropriate solubility and volatility-is conducive to the safe and economical production of foams. However, theoretical and practical scientific assessments suggest that CFCs deplete the ozone layer through reaction with stratospheric ozone. Consequently, restrictions have been imposed on CFC production and use.

The Montreal Protocol, an international agreement for regulation of CFCs, was signed in September 1987. Its first scheduled control period-calling for reduction in production of CFCs -11,-12, -113,-114, and -115 to 1986 levels-began in July 1989. Subsequent reductions in CFC production to 80% and then 50% of 1986 levels are scheduled for July 1993 and July 1998.

in August 1988 the U.S. Environmental Protection Agency (EPA) issued its Final Rule on Protection of Stratospheric Ozone, which generally follows the regulatory theme of the Montreal Protocol. Based on new scientific evidence of reduced ozone levels over Antarctica and the findings of the Ozone Trends Panel, released in March 1988, the EPA also issued an Advance Notice of Proposed Rulemaking. Its purpose was twofold: to consider eliminating possible windfall profits for CFC producers by supplementing the allocated production quota system with a regulatory fee; and to speed up the phaseout of CFCs by encouraging development of CFC alternates.

In late 1989 the U.S. Congress passed the Omnibus Budget Reconciliation Act of 1989, which imposed an excise tax on certain ozone-depleting chemicals, that is, CFCs and halons, and on imports of products made with or containing such chemicals. CFCs used as foam blowing agents are taxed at $1.37/lb this year, which will increase to $2.65/lb in 1993 and 1994. CFCs used in rigid insulation foams are exempt this year and have a reduced schedule of taxation from 1991 to 1993.

A United Nations Environmental Programme meeting, scheduled for next month in London, will review and revise as necessary the provisions of the Montreal Protocol. A total CFC phaseout by the year 2000 is very likely.

As a result of all this activity, producers of extruded foam have been prompted to seek environmentally safe and economically acceptable alternative blowing agents. One such alternative, chlorodifluoromethane (HCFC-22), a commercially available product, is being used alone and in blends in the extrusion of polystyrene and polyethylene foams. This article discusses HCFC-22 as an environmentally safe blowing agent; it also considers properties of alternates, their application and toxicity requirements, lab and plant trial evaluations, and conversion to alternates.

Direct injection Process.

In this process, a physical blowing agent, for example, dichlorodifluoromethane (CFC-12) or a hydrocarbon such as pentane, is metered continuously under high pressure into the heated melt section of an extruder. The blowing agent dissolves in the polymer and gasifies only when the mixture is expelled into the atmosphere at the exit die. Useful blowing agents must have adequate, but limited, solubility in the polymer melt and good change-of-state properties to create a foamed structure. Other requirements include U.S. Food and Drug Administration (FDA) approval where food packaging is involved.

CFC Alternates

Some of the approaches proposed for significantly reducing CFC emissions in extruded polystyrene and polyethylene foam production include using:
* carbon dioxide (C[O.sub.2]) as a second blowing
* CFC/hydrocarbon (HC) blends;
 HCFCS; and
* hybrid blowing agents, such as HCFC/
 CFC blends or HCFC/HC/C[O.sub.2] blends.

HCFCs have at least one carbon-hydrogen bond in their molecular structure, which makes them less chemically stable than CFCs. As a result, HCFCs have significantly lower atmospheric lifetimes than CFCs and, hence, significantly lower stratospheric ozone depletion potentials. In which the properties of relevant CFCs and candidate HCFCs aTe listed, the molecular structures of HCFCs also result in their having much lower "greenhouse warming potentials."

Three of the alternates are nonflammable and two-HCFC-142b and HFC-152a-are less flammable than hydrocarbons. (The flammability properties of CFC, HCFC, or HFC blowing agents are not intended to reflect the fire hazards presented by any resultant cellular or foamed plastic products.) The alternates also have a low order of toxicity. The toxicities of HCFC-and 124 HFC-134a are currently under study.

The properties of some of the auxiliary blowing agents that may be used in hybrid-type formulas to improve foam processing and/or economics. Laboratory and plant trials have shown the utility of C[O.sub.2] in extruded foam production.

CFC-12 to HCFC-22 Conversion for PS Foam

Although HCFC-22 has a lower boiling point than CFC-12 (-40.8 [degrees] C vs. -29.8 [degrees] C), it is a nonflammable liquified gas, has low-order toxicity, and is available from several producers. Also, based on molecular-weight considerations, less HCFC-is 22 required than CFC-12 to achieve comparable extruded foam densities.

Evaluations of HCFC-22 vs. CFC-12 in PS foam sheet extrusion on a lab-scale 1.25/1.5-in tandem extruder produced the following results.
* No significant changes in extruder processing
 conditions were required to
 produce visually acceptable foam when
 using HCFC-22.
* Less HCFC-22 than CFC-12 was required
 to produce foams equivalent in
* HCFC-22-based foam exhibited
 slightly coarser cell structure than
 CFC-12-based foam at the same nucleating
 agent (talc) level.
* HCFC-22 foams exhibited less post-expansion
 growth than CFC-12 foams.

Post-expansion growth was determined by simply measuring the changes in dimensions of a small sample of foam after exposure to 120 [degrees] C for 2 min. The latter result suggests that less "puff-up" may be obtained during thermoforming HCFC-based 22-PS foam sheet, and increased thicknesses of the initial foam sheets may be required to achieve thermoforming properties comparable with those of CFC-based 12-foams. The lower expansion gain with HCFC-22 results from its rapid diffusion out of PS foam.

Plant Trials

Several successful plant trials were conducted using HCFC-22 alone and in blends with HCs and C[O.sub.2] for the commercial production of extruded PS foam sheet goods. This sheet was used as the base stock for thermoforming egg cartons, meat trays, and other food service and packaging products. Adaptation of production parameters was guided by the results of the lab trials, that is, lower HCFC-22 requirement, slightly higher nucleator content for fine cell structure, and increased sheet thickness for thermoforming.

The processing conditions of the PS foam sheet produced on a commercial 4.5/6-in tandem extrusion line employing a 7-in-diameter die and 86-in-circumference mandrel. The results of this trial confirmed that lower concentrations of HCFC-22 and greater sheet thicknesses were required for thermoforming.

The results of additional production trials conducted on the same equipment using a resin feed of 875 lbs/min (2/3 virgin, 1/3 regrind). These and other production trials indicated the utility, latitude, and economics of using HCFC-22 as a substitute for CFC-12.

A Successful Conversion.

The food service and packaging industry has already phased out the use of CFC-12 in extruded PS foam and has converted to HCFC-22. The smooth changeover was due to excellent cooperation among government, business, and environmental groups.

Members of this industry produce PS foam plates, institutional trays, fast-food containers, egg cartons, meat trays, and certain types of cups, so one major hurdle was obtaining the approval of the FDA for use of HCFC-22 in these food-contact products. Diligent action by foam manufacturers, HCFC-22 producers, and the Foodservice and Packaging Institute in supplying information to the FDA and analyzing foam products for residual HCFC-22 content contributed to the rapid acceptance of "food-grade " HCFC-22. A 1987 ruling by the FDA recognized the use of HCFC-22 for food-service items as acceptable.

In March 1988 the FDA accepted a petition proposing that food-grade HCFC-22 be affirmed as "generally regarded as safe" for all PS food packaging. This allowed foam producers to use this blowing agent for egg carton and meat tray stock as long as the residual HCFC-content 22 was less than 5 parts per million at the time of food contact. The length of time for PS foam to reach 5 ppm of residual HCFC-22 depends upon thickness and storage conditions.
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Title Annotation:Engineering resins: innovation, development, realization
Author:Dwyer, F.J.; Zwolinski, L.M.; Thrun, K.M.
Publication:Plastics Engineering
Date:May 1, 1990
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