Taken from a Composites Fabricators Association document

Emissions during composites manufacturing

The monomers used in composites manufacturing serve an essential purpose: in the mould, they chemically react to "cross-link" the polymer molecules in the resin, transforming the liquid or paste-like mix of raw materials into a fully-cured, solid, strong, and durable product. Unfortunately, any time that uncured resin is exposed to the air, some monomer may evaporate. Once the curing stage is complete, essentially all of the monomers have reacted, and there will be no more evaporation.

Monomers used and emitted

Monomers are a class of chemicals that can react with each other to form long chains, or that can react with certain polymers to cross-link them together. It is this cross-linking action of monomers that make them essential in composites manufacturing.

While there are many different chemicals that are used as monomers, only a few are used in composites, and only one is used extensively: styrene. Styrene is a clear liquid with a distinctive odour. Styrene is used so extensively because it can be mixed with a wide variety of polymers to form workable resins, because it reacts readily and predictably to cross-link the polymers to form solid products, and because it imparts important chemical and physical properties to the products. Styrene is also a major industrial chemical, and so is lower in price than other monomers. The low cost of styrene helps make composite products affordable.

Another monomer that is sometimes used in composites is methyl methacrylate. It is used for products needing better resistance to sunlight, and for applications that require low smoke generation if exposed to fire. However, methyl methacrylate has a higher evaporation rate than styrene, and it is much more expensive than styrene.

A final group of monomers are those that have lower evaporation rates than styrene, for example para-methyl styrene and vinyl toluene. These monomers are used to replace some of the styrene in some resin formulae, and therefore reduce emissions. However, these monomers can change the physical and chemical properties of the resin, and in many cases, they also change the properties of the product. These monomers are also much more expensive than styrene.

Monomers are not solvents. The type of monomer and its concentration in the resin will impact the properties of the resin and of the final product. Changes in monomer type or concentration in a resin will change the physical and chemical properties of the products made with it.

Since styrene is more widely used and emitted by composites manufacturing operations at a much higher rate than the other monomers, the remainder of this report will address only styrene emissions. For those operations using and emitting significant amounts of other monomers, resin suppliers can provide the information needed to estimate emissions and assess any possible health impacts from exposure to these materials. In general, strategies employed to reduce styrene emissions will also reduce emissions of other monomers.

Levels of styrene emitted

The amount of styrene emitted by a composites manufacturing facility will be determined by many factors:

• What manufacturing process and resin application technique is used?
• How much resin is processed?
• What is the styrene content of the resin?
• What pollution prevention or capture-and-control technologies are used?

No two operations are likely to have the same emissions.

The most accurate way to determine how much styrene is emitted by a facility is to measure it. There are various procedures for measuring emissions, however, direct emission measurements (called "stack tests") are expensive, difficult to reproduce, and are seldom used for composites manufacturing operations. Instead of measuring emissions, most composites manufacturers and regulators use emission factors to estimate emissions. In general, emission factors are formulas that produce an emission estimate based on information about a manufacturing operation, like styrene content in the resin, use of pollution prevention techniques or capture-and-control, amount of resin processed, etc. Different emission factors are available for several of the different composites manufacturing processes. For example:

250,000 kg resin used x 35% styrene content in resin = 87,500 kg available styrene

87,500 kg available styrene X 5.5% (mid-point emission factor for pultrusion) = 4,813 kg styrene emissions

How accurate are emission factors?

The Unified Emission Factors (UEF) for open moulding are based on extensive research, and have proven to be usefully accurate in estimating emissions from actual open moulding operations. Given the cost and difficulty of performing stack tests on open moulding operations, the UEF represent the most useful and practical method of estimating emissions from the majority of these operations.

Emission factors for the other processes, however, may not be as accurate. In general, these other factors are reasonable assumptions based on limited data. They are widely used in the US because they are often the only feasible means for facilities to estimate emissions. However, unlike the UEF, they typically do not account for the use of various pollution prevention measures.

For some composites manufacturing processes, there are no widely accepted emission factors. This includes operations that employ pollution prevention technologies not accounted for in the EPA emission factors. Facilities using these processes can estimate emissions by using engineering calculations (emission estimates based on the physical properties of resin and styrene, surface areas of uncured resin, air flows, etc.), laboratory-scale measurements, or stack tests. Some facilities that employ processes for which there are emission factors may still choose to rely on these alternative methods, if they believe that these methods provide a more accurate estimate of emissions. Even the EPA's official emission factors should be used only when a facility lacks better information about emissions from its operations.

Styrene Toxicity

The information in this section is taken from work carried out by the Styrene Information and Research Centre (SIRC). SIRC is a non-profit organisation established in 1987 by companies involved in the manufacture or use of styrene. SIRC's mission is to evaluate existing data on potential health effects of styrene, and develop additional data where it is needed. SIRC has gained world-wide recognition as a source for information on styrene, thus helping to ensure that employee and public health is fully protected, and that regulatory legislation is based on sound science.

What is styrene?

Styrene is a clear, colourless liquid that is a component of materials used to make thousands of everyday products for home, school, work, and play. Styrene is used in everything from food containers and packaging materials to cars, boats, medical equipment, computers, and video games. Derived from petroleum and natural gas by-products, styrene helps create thousands of remarkably strong, flexible, and lightweight products, representing a vital part of our economy and quality of life.

The styrene used in these products is manufactured synthetically in petrochemical plants. However, styrene also occurs naturally in the environment and is present in many common foods, such as coffee, strawberries, and cinnamon.

Do I come in contact with styrene?

Most people are exposed to styrene every day in minuscule amounts that may be present in the air, or are found naturally in food. These generally are trace amounts, which were difficult to detect until recent technological advances. We also may recognise styrene by its distinctive odour when using certain products such as latexes, paints, and polyester resin solutions.

Is styrene harmful to health?

Styrene is not harmful in the very small amounts we sometimes may encounter in air or food. Someone working in an enclosed area with resin solutions containing styrene (patching the surface of a fibreglass boat, for example) may find the odour of styrene causes slight nausea. This goes away with exposure to fresh air, and there is no lasting effect.
In an important decision made in 1994 after an extensive assessment of its possible health and environmental effects, the Canadian government agencies Health Canada and Environment Canada concluded that styrene is "non-toxic." After a thorough review of health effects data and evaluation of potential human and environmental exposures, they found styrene "does not constitute a danger to human life and health" and "does not constitute a danger to the environment on which human life depends."

What about the scent of styrene around manufacturing plants?
Styrene's distinctive odour can be detected even when styrene is present at extremely low levels. People living near facilities that make or use styrene sometimes may notice a slight scent of it in the air. Neighbours can contact a plant's manager if they have concerns about odours.

What happens to styrene if it is released into the environment?
Extensive research shows that styrene exists only briefly in the environment; it is rapidly destroyed in the air and disappears quickly from soils and surface waters. Studies also have shown that styrene is not likely to occur in drinking water.

What about the health of workers exposed to styrene?
The health of workers in plants making or using styrene has been monitored for many years. Studies looking for long-term health effects related to styrene exposure have examined health records of over 50,000 workers exposed to styrene, going back nearly 50 years. These studies have not shown any statistically significant increases in long-term health problems of any kind attributable to styrene in these workers.

In most industrialised countries, there are strict regulations protecting worker health. In 1989, a safe exposure standard for styrene of 50 parts per million (ppm) over an eight-hour day was established. In years past, before effective monitoring systems were available, worker exposure to styrene (as well as other materials) often was greater than current exposure levels.

Is there a concern about a risk of cancer?
The styrene industry has invested many years of effort and funding to develop the most thorough and accurate information on possible cancer effects resulting from styrene exposure. The results of extensive health studies of workers in styrene-related industries collectively show that exposure to styrene does not increase the risk of developing cancer. Results of a two-year styrene inhalation study in rats, completed in 1996, also showed no increased incidence of cancer. The results of a mouse bioassay are currently being reviewed.

From a regulatory viewpoint, in 1989, the US National Institute for Occupational Safety and Health (NIOSH) reviewed the health data on styrene and concluded that styrene does not pose any cancer risk. An international panel of experts from the European Community reached the same conclusion in 1988. Canada decided in 1994 that styrene posed no carcinogenic risk in Canada. A draft 1996 risk assessment of styrene by the Health & Safety Executive of the United Kingdom also concluded that styrene does not pose a carcinogenic threat.

In 1987, the International Agency for Research on Cancer (IARC), in Lyon, France upgraded styrene's classification to a "possible" human carcinogen. This action has been disputed by many scientists because it was not based on new cancer data, but resulted from changes in the criteria for IARC classifications. However, IARC's charter stresses that their classifications are for hazard identification only - not to determine the risk of a given substance - and should not be used for regulatory purposes. In spite of substantial new human and animal data since 1987, showing no link between styrene exposure and cancer, IARC has maintained its classification. The styrene industry believes that the significant amount of available scientific data indicates this classification is not warranted, and continues to address IARC's decision.

Minimisation of emissions

Two options can be considered when attempting to reduce the emissions from any manufacturing process:

• engineer the process so that it inherently emits less
• capture and control (destroy) the emissions that do occur.

Engineering the manufacturing process so that it emits less is called "pollution prevention." As the name implies, with pollution prevention the manufacturing process is modified so that it generates fewer emissions. Pollution prevention technologies can have a number of benefits: reducing emissions at the process is often less expensive than capturing and controlling them later; pollution prevention can reduce occupational exposures in addition to reducing environmental releases; and pollution prevention can avoid many of the "secondary" environmental impacts associated with some capture and control technologies.

However, pollution prevention technologies can have one major limitation. Since pollution prevention requires changing the manufacturing process, the feasibility of pollution prevention will depend on whether the modified process still makes good products. Many pollution prevention technologies are widely used throughout the composites industry, but all have their limitations. They all change the properties of the products being made. Whether these changes in properties can be tolerated will depend on the nature of the products and their ultimate use.

"Capture-and-control" technologies, on the other hand, do not interfere with the manufacturing process. In theory, capture-and-control means that you simply capture whatever happens to be emitted from a manufacturing operation, and then you direct these emissions to a device that converts them to some innocuous, non-polluting substance. However, these technologies can be very expensive to install and operate, they can consume large amounts of natural gas (a non-renewable natural resource), and they can have significant secondary environmental impacts, such as the generation of new pollutants.

Changes in manufacturing processes
There are basically two pollution prevention strategies that can be applied to composites moulding processes: use less monomer, or prevent exposure of the monomer to air. Emission reduction technologies included in the use-less-monomer group are low-monomer resins and filled resin systems. The second, prevent-exposure-to-air group includes low-emitting resin application technologies, suppressants, controlled spray, airflow management, and closed moulding processes.

Low-styrene resins
One way for some moulders to minimise the loss of styrene from their process is to use resins formulated with less styrene. Resin suppliers offer a variety of these resins.

Some of the low-styrene resins are made with special shorter-chain polymer molecules that permit lower monomer content while still maintaining workable and useful properties. Other low-styrene resins employ vinyl toluene, p-methyl styrene, or other non-styrene monomers; these other monomers have less evaporation when exposed to air. Finally, some low-styrene resins are made with combinations of short-chain polyesters and low-evaporation monomers.

While the use of low-styrene resin sounds as if it should be relatively simple, this technology is not for everyone. First, for many moulding processes, such as pultrusion and compression moulding, reducing the monomer content of a resin may not result in significant reductions in emissions. Further, reducing the styrene content of a resin means that it is a different resin. Using low-styrene resin may require the use of different formulas and application technologies. Products moulded using low-styrene resins will have different chemical and physical properties. Styrene, after all, is primarily a reactive monomer, not a solvent. Changing the amount of styrene in a resin, or changing the type of monomer used, will change the properties of the product, sometimes unacceptably.

Fillers
A second way to reduce styrene emissions is to use less resin. This might mean engineering a product so that it uses only as much resin as needed to provide the required properties, but more often this approach means adding an inert filler to the formula. Fillers reduce the amount of resin needed (thereby reducing monomer usage and consequently emissions), while possibly increasing product stiffness or fire resistance (depending on the type of filler used).

Typically, processing and product performance requirements will determine what type and how much filler can be used. Some suppressants, and some of the new non-atomised application technologies, may not be compatible with filled systems.

Low-emitting resin application technologies
While the emission reduction technologies discussed above work by decreasing the amount of styrene used to make a product, low emitting resin technology works by reducing the contact of resin with air.

As an analogy, to reduce the amount of water evaporating from a puddle, you could obviously mop up some of the water, so there would be less to evaporate. This would be analogous to the use-less-styrene technologies described above. If the water is needed, (like the monomer is needed to provide cross-links) you could reduce the surface area of the water by covering the puddle with a plastic sheet or by scooping the water into a bucket. This is analogous to the minimise-exposure-to-air technologies. For example, the various non-atomised application technologies like pressure-fed rollers and flow-applicators (also called flow coaters) work to reduce emissions by minimising the contact of resin to air. In traditional spray application of resin, very small "atomised" droplets of resin travel from the spray gun to the mould - or to the floor, walls, or filters of the spray booth - creating very high surface areas of wet resin, and consequently high emissions. By contrast, flow-applicators deliver streams of resin to the mould, rather than atomised droplets. The applied resin has a much lower surface area and therefore lower emissions. And with pressure-fed rollers, there is not even a stream of resin.

Another low-emitting resin application technology is resin impregnation, where glass mats are mechanically fed through a resin bath, and then applied to the mould. Again, there is no spray (or stream) of resin through the air, and more of the resin is delivered to the mould.

These technologies have their limitations. None can practically be used for gel coat, since this material needs to be applied in a very uniform layer that can be accomplished only with atomised spray. Pressure-fed rollers may not transfer resin to the mould fast enough to keep up with production in many shops, and, like resin impregnation, are not useful with chopped reinforcement. Flow-applicators may have problems with filled resin systems, although suppliers are working to overcome this limitation.

Suppressants

A second way of reducing contact of resin with air is through the use of suppressants. These materials, which are typically waxes, form a layer on the surface of the resin while it is curing. This layer reduces the evaporation of monomer. Suppressants work only when the resin is left undisturbed, so that the layer of wax can be formed on the surface. During resin application and roll-out, fresh resin and monomer is continuously exposed to the air, and suppressants are not effective in reducing emissions. In a typical spray-up/roll-out/curing process, however, about one-quarter of the emissions comes from the curing stage, and suppressants can be very effective in reducing these emissions.

Suppressants have to be matched to the resin system employed, and they may not work as effectively with filled resin systems. Resin and suppressant suppliers can help select the most effective combinations of materials. Moulders have often found that they need to sand or grind the surface of parts if additional laminate layers are to be applied after curing. This problem with "secondary bonding" is because the waxy suppressant layer on the surface of the part, which reduced monomer evaporation during curing, can later act like a release agent and prevent adequate bonding between the laminate and the subsequently applied resin.

Controlled spray

For the atomised spray application of resin or gel coat, the use of a controlled spray program is one of the most effective ways of reducing the surface area of the wet resin, thereby reducing contact of resin with air, and consequently reducing emissions. A controlled spray program consists of three components:

• proper calibration of spray guns, so that the lowest tip pressure that still delivers an adequate fan pattern is used (higher pressures result in much more "misting" of resin and evaporation of monomer)
• training operators to handle spray guns and apply resin using techniques designed to minimise overspray
• use of containment flanges at mould perimeters, which prevent the deposition of high surface area resin layers on the floor and walls.
  

Actually, controlled spray can fit into both of the general emission reduction technologies: a controlled spray process uses less resin and monomer - because more of it is delivered to the mould - and it reduces the exposure of resin to air.

Airflow management

Air has a limited ability to hold monomer vapour, and once this limit is reached, no more monomer will evaporate, unless the monomer-laden air is replaced with fresh air. So, in theory, allowing a monomer-laden layer of air to remain at the resin surface can reduce monomer evaporation. In practice, airflow must be reduced to very low levels to achieve any real impact on monomer evaporation. Tests have shown that reducing airflow across wet open moulds does not significantly reduce emissions in open moulding, probably because airflow can not be practically reduced enough over an open mould.

In other processes, however, emission reductions of up to 60% can be achieved. Testing has shown significant emission reductions from the use of covers or enclosures over wet process areas in pultrusion, continuous lamination, and SMC compounding. And tests with pultrusion lines have shown emission reductions just by reducing general ventilation flows.

Of course, users of this technology must still comply with occupational exposure limits. The enclosures mentioned above typically allow access to the process only through hand-sized doors ("hand-ways" rather than "man-ways"). And general ventilation can be reduced only if occupational exposure levels are still below the required levels.

Finally, airflow management - through the use of lids or covers - is also very effective in reducing emissions from resin mixing, storage, and transfer vessels.

Closed moulding

Closed moulding is a family of composites moulding processes where liquid resin is not exposed to the air. This group includes resin transfer moulding, resin infusion moulding, compression moulding, and injection moulding. These technologies can have very low emissions. However, process economies often severely limit the application of these technologies. Compression moulding is feasible only when large production runs are needed (say, 50,000 parts per year), and where the capital is available to pay for the large presses and complex moulds.

Resin infusion and resin transfer moulding, however, are sometimes useful and economically feasible for products made with open moulding. These processes find use when customers demand - and are willing to pay for - higher quality products, and where high volume production is not needed.

Capture-and-control technologies

There are two parts to capture-and-control:

• " capturing the emissions and transporting them to some central point
• " controlling them by converting them to some non-polluting substance.
  

Composites manufacturers must consider both the capture and the control parts when deciding if capture-and-control is workable for their operations. For many composites manufacturers, simply capturing the emissions is not feasible. Composites moulding operations are often large and widely distributed throughout a building, making the collection and transportation of the emissions prohibitively expensive. In addition, the need keep styrene levels below 50ppm to protect workers often complicates the task of collecting emissions. However, for those composites manufacturing operations where capturing emissions may be feasible, three basic types of control technology can be considered: absorption, oxidation, and biological treatment.

Adsorption

Adsorption uses a substance such as charcoal, which has a surface that allows monomer vapour present in an emission stream to adsorb (or adhere) to the material, removing the monomer from the air. Heat is later applied to the material, removing the adsorbed monomer.

While suppliers of adsorption technologies claim that they can work for controlling composites manufacturing emissions, some in the industry have not had good success with this technology. Some of the adsorption units tested caught fire, and others stopped functioning because the adsorbing media became plugged with monomer. Additionally, the adsorbing media must be regularly treated to remove the adsorbed material, possibly creating emissions or solid waste.

Oxidation

Oxidation is the most common capture-and-control technology used by composites manufacturing facilities. While there are different types of oxidisers available (basically: thermal oxidisers, catalytic oxidisers, and oxidisers paired with pre-concentrators), they all use heat to convert monomer vapour to carbon dioxide (CO2) and water. While CO2 is not considered toxic, it does increase the retention of heat by the atmosphere and contributes to global warming. In addition, most of the oxidisers consume large amounts of natural gas. Burning natural gas creates nitrogen oxides, one of the primary constituents of smog. Catalytic oxidisers use special materials ("catalysts") to reduce the temperatures needed to convert the monomers to CO2 and water, and they therefore use less natural gas. Catalytic oxidisers tested in composites manufacturing plants have failed, however, because substances present in the exhaust stream poisoned the catalysts.

Oxidisers paired with pre-concentrators may also use less natural gas, because a more-concentrated stream of monomer is sent to the oxidiser, and less fuel is needed to achieve the high temperatures. Two composites manufacturing operations recently started using oxidisers paired with pre-concentrators to control emissions.

Whatever the type of oxidiser considered, the major limitation will be cost. Purchasing and installing even the smallest of the available units will likely cost at least £800,000 and another £800,000 will be required every year to buy fuel (natural gas) and operate and maintain the unit. These costs exceed the total capital and operating costs of many composites manufacturers.

Biological treatment

Another control option available to composites manufacturers uses micro-organisms to "digest" monomers, converting the monomer present in the captured emission stream into CO2 and water. In the presently installed units, the emission stream is passed over a moist bed of peat or similar material, and monomer in the air is absorbed into the water where it is consumed as food by the micro-organisms. According to the suppliers of this technology, the peat (or other media) can be disposed of every few years as non-hazardous waste.

Although the operating expenses for biological treatment should be less than oxidation, this technology will likely be too expensive for many composites manufacturing operations. Additionally, it has not been in use long enough to fully evaluate its long term effectiveness or reliability. Still, compared to oxidation, biological treatment offers the promise of lower costs and little or no secondary environmental impact.

Measures to be taken by the composites industry

Promotion of good environmental practices by the composites manufacturing industry through its educational programs, by working in partnership with regulators is essential.

Education of the composites manufacturing industry about its health and environmental impacts, and about opportunities and technologies for reducing emissions has to be a priority.

In the US, when Congress amended the Clean Air Act in 1990, Maximum Achievable Control Technology (MACT) standards for the composites industry were established. These MACT standards are to require most composites manufacturing facilities to achieve levels of control equivalent to the best controls currently in use in the industry. Many composites manufacturing facilities have already adopted the kinds of pollution prevention technologies that are likely to be required under MACT.

Best practice

Best practices come in two basic forms.

Best technology/work practices are specific technologies or work practices that can be used to achieve environmental or occupational safety objectives. For example, controlled spray may be one of the best work practices a company employs to reduce emissions, while flow coaters might be a best technology practice used by another company.

Best management practices are management tools or systems. For example, as a best management practice, a company might base a supervisor's annual bonus on whether their shift has met its target for reducing waste. In addition to incentive programs, best management practices might also include training, employee participation in goal setting and problem solving, and bench marking.

It must be noted that best technology/work practices are always defined in relation to a specific company. For example, what might be the best way to minimise waste at one company may be very different from the best methods of achieving the same objective at another company. In each case, what is "best practice" will depend on the availability of resources, what materials are processed, how they are processed, what products are made, local community or regulatory requirements, and other factors.

Aesthetics

Emissions

Environmental Profile

Life Cycle Assessment

Recycling

Resource Efficiency

Whole Life Costing

Copyright & Disclaimer