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ENVIRONMENTAL HEALTH & SAFETY

 

Frequently Asked Questions About Aerogel Insulation:

  1. What is aerogel?
  2. What is Aspen Aerogels’ nanotechnology?
  3. Will handling Aspen Aerogels’ products expose me to harmful amounts of nanoparticles?
  4. What health studies have been done on synthetic amorphous silica?
  5. What are the health effects of aerogel dust?
  6. What are the exposure limits for aerogel dust?
  7. How do you dispose of waste aerogel blanket?
  8. What is the combustibility of aerogel dust?
  9. What are the handling guidelines?
1. What is aerogel?

Aerogel is a synthetically produced amorphous silica gel (distinctly different from crystalline silica) impregnated into a non-woven flexible fabric substrate, offering the twin benefits of extreme thermal performance and a flexible blanket form. To meet customer demands, Aspen Aerogels may use a variety of aerogel additives and fabric substrate materials. Aspen Aerogels’ products utilize nanoporous structures that minimize the three mechanisms of thermal transport, resulting in thermal conductivity values in the 12 - 20 mW/m-K range at 40°C.

The primary difference between crystalline silica and the amorphous silica involves the lack of long range atomic order in amorphous silica that is seen in crystalline silica (see pictures below). The high degree of repeating atomic order in crystalline silica gives rise to its physical properties and the distinctive faceted morphology of its crystals.

Amorphous Silica

Crystalline Silica


Scanning electron micrograph of
silica aerogel (amorphous silica)

 


Scanning electron micrograph of
quartz (crystalline silica)



Lack of long range atomic order in
amorphous silica particles




Long range atomic order
of quartz crystals



2. What is Aspen Aerogels’ nanotechnology?

First, “nanotechnology” is not a process or product form and should not be confused with the term nanoparticles.” The ASTM subcommittee on Nanotechnology has issued detailed definitions for each of these terms in the ASTM standard E 2456-06 (available for free at http://www.astm.org/Standards/E2456.htm):

  • Nanotechnology, n A term referring to a wide range of technologies that measure, manipulate, or incorporate materials and/or features with at least one dimension between approximately 1 and 100 nanometers (nm). Such applications exploit the properties, distinct from bulk/macroscopic systems, of nanoscale components.
  • Nanoparticle, n In nanotechnology, a subclassification of ultrafine particle with lengths in two or three dimensions greater than 0.001 micrometer (1 nanometer) and smaller than about 0.1 micrometer (100 nanometers) and which may or may not exhibit a size-related intensive property.

Second, the “nanotechnology” aspect of Aspen’s blanket products relates to the nanometer scale voids incorporated into the amorphous silica gel matrix. Thus, Aspen’s materials are considered nanotechnology from a void space, rather than a discrete, nanoparticle basis. The void space is what gives rise to the exceptional insulation performance of our materials. The structure of the aerogel matrix is continuous and homogeneous throughout the volume of our products formed by a highly aggregated amorphous silica network. Materials that are shed as dust from our products typically have dimensions averaging around a tenth of a millimeter. Such particles are much larger than a nanometer (10-9 meter) or the accepted definition of nanoparticles (see for example the ASTM standard E 2456-06 discussed above). The pores (or air space) of the aerogel structure typically range between 2 and 50 nanometers. However, it takes tremendous amounts of energy to separate the aerogel particles from their highly aggregated native state, and once separated they tend to rapidly reaggregate into larger particles.

Finally, independent laboratory analyses of the dust shed from Aspen Aerogels’ products, including studies using laser diffraction, dynamic light scattering, and scanning electron microscopy, confirm that Aspen Aerogels products do not contain detectable quantities of dispersible nanoparticles (see definition above). For example, the particle size distribution graph below (Figure 1) was measured for material released from Cryogel via tearing, ripping, cutting, flexing, abrading, and shaking stresses. These stresses are approximately representative of “rough” handling that these products might receive on a job site during transportation, fabrication or installation. The lowest detectable particle size in this case was 8 microns (the detection size limit was 20 nm for the equipment and method utilized).


Figure 1. Particle size distribution for dust removed from Cryogel 5201 by tearing, ripping, cutting, flexing,
abrading, and shaking.

A portion of the same sample of dust described above (see Figure 1) was subjected to severe compressive and shear stresses in a Spex “Shatterbox” planetary grinding mill. The particle size distribution shifts to lower average size, but still does not generate detectable quantities of dispersible nanoparticles as seen in Figure 2 below. In this case, the lowest detectable particle size was 0.6 microns (or 600 nm). This aggressive milling process, used industrially to make the finest powders from very hard materials, applies forces to materials far beyond any experienced in typical application handling. As can be seen in Figure 2 below, the particle size distribution maxima shifted to a smaller particle size as expected, but no nanoparticles were detected as a result of the milling. This behavior is representative of the other products tested using this protocol. Similar results are seen with both sampling as a dry aerosol sampling or as particulate dispersions in solvent aided by ultrasonic agitation.


Figure 2. Particle size distribution for dust obtained from Cryogel (see Figure 1 above) and subsequently
treated by aggressive particle milling stresses.

 

   


3. Will handling Aspen Aerogels’ products expose me to harmful amounts of nanoparticles?

As explained above, we have not detected any nanoparticles shed from our products, even after subjecting the product aerogel to immense crushing and shear forces. While no product can be accurately claimed to contain zero nanoparticles, our technology is based on a nanoporous, finely divided silica gel structure that is NOT nanoparticulate in terms of dust shed from the material. We manufacture and sell products that can be handled safely using conventional methods and personal protective equipment that allow users to comply with current or proposed regulations for material exposure from around the world. These exposure limits refer to those governing use of amorphous silica, titanium dioxide or any other added component of our products.



4. What health studies have been done on synthetic amorphous silica?

The United Nations Organization for Economic Co-operation and Development (OECD) has been characterizing the hazardous properties of High Production Volume (HPV) chemicals. The “Screening Information Data Set” (SIDS) for synthetic amorphous silica was released in 2004. The SIDS report concluded that synthetic amorphous silica (SAS) is a low priority for further study. Excerpts from the OECD SIDS Human Health conclusions are included below(1):

Absorption, disposition, elimination: SAS forms [CAS No 7631-86-9] are rapidly eliminated from the lung tissue during and after prolonged inhalation exposure of experimental animals with no disproportionate disposition occurring in the mediastinal lymph nodes, whereas crystalline forms exhibit a marked tendency to accumulate and persist in the lung and lymph nodes. Intestinal absorption of SAS appears to be insignificant in animals and humans. There is evidence of ready renal elimination of bioavailable fractions.

Acute toxicity: Following inhalation exposure of rats to the highest technically feasible concentrations of 140 to ~2000 mg/m3 SAS, no lethal effects were observed. Oral and dermal administration of SAS and amorphous silicates failed to cause mortality at the highest doses tested: LD0 values ranged from 3300 to 20000 mg/kg in rats.

Irritation and sensitization: Synthetic amorphous silica and silicates are not irritating to skin and eyes under experimental conditions, but may produce dryness following prolonged and repeated exposure.

No sensitization experimental data are available on the synthetic amorphous silicas and silicates. However, there is a long work history with these materials at industrial scales. Data collected from industrial hygiene surveillance over the last 50 years do not indicate any potential for skin sensitization. Given the inherent physico-chemical properties and ubiquitous nature of this class of compounds, there is no structural alert to indicate a sensitizing potential.

The US EPA reviewed several toxicity studies for synthetic amorphous silica including four acute toxicity studies (acute oral LD50 in the rat, acute inhalation LC50 in the rat, primary eye irritation in the rabbit, and primary dermal irritation in the rabbit); four mutagenicity studies, and an oral toxicity study. The US EPA summary of these study results was as follows(2):

1. Acute toxicity studies. No mortalities were observed for the oral and inhalation studies. For the primary eye irritation study, there was no corneal opacity or iridial irritation in any of the eyes. For the dermal study, there was no dermal irritation at 72 hours. For the acute toxicity study, the oral LD50 is >5,000 milligrams/kilograms (mg/kg). For the acute inhalation study, the LC50 is >2.08 mg/L. All studies are toxicity category IV.

2. Mutagenic studies. In all four studies there was no indication of any mutagenic activity associated with exposure to silica, amorphous, fumed (crystalline free).

3. Oral toxicity of fumed silica. There were no mortalities or clinical signs. There was no significant difference between the test group and the control group with respect to silica concentration in the carcass.

Based on their analysis of the reviewed studies, the US EPA concluded the following(3):

“Silica, amorphous, fumed (crystalline free) has a demonstrated lack of toxicity. The acute toxicity studies are toxicity category IV. The mutagenicity studies are negative. Silica, amorphous, fumed (crystalline free) is not classifiable, as to its carcinogenicity however, given its amorphous nature, it is not expected to pose a carcinogenic risk. Silicas are considered to be inert when ingested, and due to the high molecular weight it is unlikely to be absorbed through the skin. There should be no concerns for human health, whether the exposure is acute, subchronic, or chronic by any route.”

The health effects of synthetic amorphous silica are significantly different from the health effects of crystalline silica. No evidence of silicosis has been found from epidemiological studies of workers with long-term exposure to intentionally manufactured synthetic amorphous silica(4). From a health standpoint, a significant different between crystalline and amorphous silica could be lung clearance. Studies of various animal species have shown that amorphous silica products can be completely cleared from the lungs(5).

The International Agency for Research on Cancer (IARC) considers synthetic amorphous silica to be not classifiable as to its carcinogenicity to humans (Group 3).

(1) United Nations Environmental Programme (UNEP), Organization for Economic Co-operation and Development (OECD) Screening Information Data Set (SIDS) Initial Assessment Report, Synthetic Amorphous Silica, July 23, 2004.

(2) Federal Register, Vol. 67, No. 94, May 15, 2002, 34616-34620.

(3) Federal Register, Vol. 67, No. 94, May 15, 2002, 34616-34620.

(4) Merget, R et al, “Health hazards due to the inhalation of amorphous silica”, Arch Toxicol (2002) 75: 625-634, November 29, 2001.

(5) Warheit, David, “Inhaled Amorphous Silica Particulates: What Do We Know About Their Toxicological Profiles?”, Journal of Environmental Toxicology and Oncology, 20(Suppl. 1) 133-141 (2001).



5. What are the health effects of aerogel dust?

Handling of aerogel blankets will produce dust. Aerogel dust exposure can produce the following effects:

— A sensation of dryness to skin
Irritation to eyes, skin, respiratory tract

These effects are not unique to aerogels, but are consistent with the handling of a host of dusty materials. When inhaled in sufficient amounts, any dust or particulate will cause respiratory effects. Excessive exposure to any type of dust can cause skin or mucous membrane irritation by chemical or mechanical action or rigorous skin cleaning.



6. What are the exposure limits for aerogel dust?

The answer depends on the country in which the materials are being used and the relevant regulations for handling materials that can generate respirable amorphous silica containing titanium dioxide dust. For example, because the percentage of crystalline silica in aerogel is 0%, OSHAs particulate limits of 15 mg/m3 (total dust) and 5 mg/m3 (respirable dust) apply to Aerogel exposure. The US National Institute for Occupational Safety & Health (NIOSH) standard for amorphous silica is 6 mg/m3. The German MAK for amorphous silica is 4 mg/m3 (inhalable fraction). Please consult your local governing regulatory information for guidance and/or recommendations for exposure limits.

It should be recognized that actual exposure levels to dust will depend on how the material is handled as well as where it is being handled (outdoors versus confined spaces). Availability of ventilation and other engineering controls will also impact actual exposures. Industrial hygiene studies conducted during intensive aerogel fabrication operations typically show total measured dust concentrations ranging from 0.2 to 5 mg/m3 and respirable dust concentrations ranging from <0.1 to 1.2 mg/m3.



7. How do you dispose of waste aerogel blanket?

Aspen Aerogels’ insulation blanket is composed primarily of synthetic amorphous silica impregnated onto a fabric material. The final product does not contain any liquid material. Scrap aerogel blanket can be disposed of in landfills that are approved to accept industrial waste. Scrap aerogel blankets will generate dust during landfill operations. Industrial waste landfills should be informed of the potential for dust generation during the waste approval process. Scrap materials from Aspen’s Manufacturing operations are disposed in a licensed, landfill.

To meet customer demands, Aspen Aerogels may use a variety of aerogel additives and fabric substrate materials. These additives may include materials such as carbon black, titanium dioxide and aluminum oxide. Aerogel blankets do not meet any of the characteristics of a US EPA hazardous waste [40 CFR Part 261, Subpart C].



8. What is the combustibility of aerogel dust?

An independent laboratory measured the Minimum Explosible Concentration (MEC) for aerogel dust to be 575 g/m3 per ASTM E1515-03, “Standard Test Method for Minimum Explosible Concentration of Combustible Dusts.” The laboratory also performed a Minimum Ignition Energy (MIE) test on the dust per ASTM E2019-03, “Standard Test Method for Minimum Ignition Energy of a Dust Cloud in Air.” No ignition was observed at 28.8 Joules Capacitor Stored Energy, the maximum capacitor stored energy delivered by Electrostatic Discharge Tester used for the test.



9. What are the handling guidelines?

Aerogel blankets should be handled as described on the product information page and on the Material Safety Data Sheets. Current MSDS are also available by contacting your Aspen Aerogels representative.

 

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