AEROSOLS

by John J. Moran, edited by Bob Bath

Raabe Corp., Menomonee Falls, W/s

Aerosols are, by far. the most convenient package that we use on an everyday basis; just touch a button and product is dispensed until we decide to stop dispensing it. Aerosols are clean, portable, require no setup, and save time and money. The metal-finishing industry uses aerosols to clean. dcgrease, dust off. prime, paint, undercoat, lubricate, etc. The package has only been on our store shelves for SO year5 but has become a mainstay in our live\. The following passage will describe some technical aspects of aerosol packages. Among the subject matter is the effect of aerosols on the ozone. how propellants make an aerosol work, VOC issues. proper storage of aerosols. and safety.

PROPELLANTS

Propellants are any group of compounds that can exert preysure when held inside a scaled container at room temperature. There are two general classifications: liquefied gas propellants and compressed gas propellants. Industry defines liquefied gases as propellants that can be compressed into a liquid atate with less than 125psig pressure while at room temperature. These propellants are typically handled in their liquid Ftate, but the propellants gasify as boon as they are released from their pressurized container. The details of this gasification process are given later in this section. Table I lists many of the common liquefied propellant5 available to the aerosol industry. along with some of their properties.

Compressed gases are defined as compounds that cannot be compressed into a liquid state at room temperature without applying more than 125 psig pressure. and which usually are handled in their gaseous state. Compressed gases used within the aerosol industry are inert nonUammable gases and include carbon dioxide (CO?). nitro&en (N2) nitrous oxide (N,O), and air.

The mechanism that controls compressed gas propellant5 is rather simple. An aerosol is charged with a specified amount of pressurized gas that gives the aerosol its initial pressure. The gas pushes on the contents of the aerosol and forces the product out of the valve when it is opened. The force of the spray and the degree of atomization are dictated by the initial pressure and by the sprayhead’s mechanical breakup. Industrial products that may use gaseou\ propellants include carburetor and choke cleaners, brake cleaners, metal degreasers. elcctron- its cleaners, and lubricant\.

Gaseous propellants have two major drawbacks: (I) their gaseous nature prebents them from achieving a substantial degree of atomization, and (2) pressure continuously drops throughout the life of the can a\ the propellant is exhausted. Liquid propellants are somewhat immune to these problems because the mechanism that controls the pressure of a liquid IS much different than that of a compressed gas. So, why are liquefied propellants different’?

Liquefied Gas Propellants A liquid propellant. such as A-70 (&butane/propane), starts boiling at -46°F. When

A-70 propellant is in an open container at room temperature, it boils vigorously hut generates no pressure because the vapor is free to escape into the environment. When the liquid is trapped inside a closed container, such as an aerosol can, the vapor has no place to go. This i\ how pressure buildup begins.

The propellant ih still mostly liquid because the closed container only allows a small amount of A-70 to turn into vapor before the system equilibrates. The gaseous molecules have

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CFC- I2 (dichlorodlfluorornethane)

CFC- I I (trichlorotluoroethnne)

HCFC-22 (chlorodltluoroethane)

HFC- 114~3 (tetmfluoroerhanc)

HFC- l52a (difluoroethane)

Hydrocarbon blend A-? I (IOO’Z Isohutane)

Hydrocarbon blend A-70 (SO% propane/SO% luohutane)

Hydrocarbon blend A-85 (70% propane/30% I\ohutane)

Hydrocarbon blend A-108 (100% propane)

DME (dimethyl ether)

Yel; NO

Yes No

Ye NO

NO Yt%

Banned for general use smce 197X. Good solvency, nnnilammahle. moderate prcsurz

Bannad Car general use s,ncc 1978. Good solvency, ntmflammahle. very low pre~ure

WIII he phased out by 2001. Very high pre\\ure. nonflamn~ahlc. poor solvency. very linntcd usgc

Next genemtion of propellant\. E\scntially nontlammahle. poor solvency, expcnrive

Next grnemtion of propellants. tlammablc, low aolvcncy. expmhive

Low prersure (31 psi at 70°F). moderate soluhility, extremely tlammahle

Moderate pres\u~-e (70 psi at 70°F). modemtc aoluhllity. extremely flammahlc

Moderate presure (85 pai at 70°F). moderate soluhllity. extremely flammable

Hqh pre\\orc , I08 p\, at 70°F). moderate soluhdity, extremely flammable

Excellent \oluhility. u\ed extensively in water-haxd S~rmula~, extremely flammable

lots of kinetic energy because they are held in a confined space at a temperature that is 116°F higher than their natural boiling point, and they want to get out. Kinetic energy translates into linear motion and a molecule makes a straight path outward. Of course. it hits a well and i!, bounced into another wall where it bounces off again until a set rate of collisions per second is established.

Weight and accelerated speed combine to creutc an outward force each time a molecule hits a wall. This creates pressure, which is measured in units of pounds per square inch. In this case it measures the pounds of force that each square inch of the can wall is subjected to by the relentless battering of high-energy propane and isobutanc molecules trying to get out.

It may not seem that so much pressure can be created by tiny molecules repetitively bouncing off the inside walls of a can. After all, propane molecules arc very light and don’t have a lot of force behind them when they strike an object; however, consider what is happening in the vapor phase of an average I h-fl.oz. can at room temperature:

l 7.100,000,000,000.000,000.000 (7. I sextillion) molecules are present. That’s more than the number of people there would be on one trillion Earths.

. Each molecule i$ moving at 850 mph and collides with a wall 9,500 times every second.

l 67.300.000,000,000,000,000,000,000 (67.3 septillion) collisions occur every second.

Taking these factors into consideration. it now should seem more likely that propane and isobutane molecules bumping into the can’s walls can indeed be responsible for generating all that pressure. But don’t forget, despite the extraordinarily large numbers. the net result is that the moving molecules create only 70 lb of pressure at room temperature.

To review brietly, molecules turn into vapor and build energy because they are being held inside a space that is too confining. They increase in speed and collide with the walls of the can. pushing outward. This outward push is called pressure and is measured in pounds per square inch (psi).

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Better Atomization and Constant Pressure Throughout the Can’s Life The pressure buildup process that differentiate\ liquid propcllants from gnseou~

propellants help5 tn provide superior atomiration and i\ also responsible for providing constant pressure throughout the life of the can. Again, only a small portion (about 0.6 g) of propellant is in its gaseous form when an ae~-nwl ib fil-st sp~myed. The remainder is in B liquid state and i\ intimately blended with the product that is being dispensed.

When the can is sprayed. product and liquid propellant are pushed through the lalve due to the can’s pressure. Similar to u gaseous propellant. pressure and mechanical breakup in the

sprayhead cause the product to atomize: however, additional atomization I\ provided when liquid propellant (mixed with product) hit\ the atmosphere and immcdlately turns into a vapor. The intermixed propellant crupth insidc each droplet and vaporizes it during a wry rapid. highly expansionary evaporation process. This step i\ missing ft-om gaseous propellant\ because they are already gawou\ and have no expansionary evaporative stage. Thi\ added hick is what allo~cs high-viwosity item\ to be atomized, providing the user with a w~ooth. even application. Some example\ of products that benefit firom liquid propellant\ include paints, ;Lutomntive undercoating. furniture polish. air freshener, ail- dusters. and portable tire inflators.

Constant pressul-c 1s maincaincd hecuu\e liquid pl-npcllunt!, habe the ahilit) to replenish the vapor phase of an aerosol until the can ih nearly empty. As the volume of the product decreases. the empty apace (vapor phase) increases. Physical laws of sases dictate that Ichen all else IS held constant, an increase in volume mu\t he accompanied hy eithcl- a drop in pressure or a proportional incl-case in the numhcr of molrculc~ that inhabit the \pacc. Gaseou\ propellants have a predetermined amount of gaseous moleculrs and mwt drcreaw in prcswl-c ;I\ the can is used. Liquefied propellants are able to replenish the vapor phase \\ith additional molecules from the liquid phase. keeping the pressure inside the can cnn\tant unlil the can i\ very nearly empty. Only then doe\ the demand for additional vaporous moleculc~ cuceed the liquid’s ability to pro\~dc them. and the pre\wl-e finally begins tn dccrcaw.

AEROSOLS AND THE ENVIRONMENT

A cnmm~n misconception ih that aerosols damage the ozone laqer and that they htill contain chloroiluorocarhons (CFCs). the subamces that have been found to destroy stratospheric ozone. The \ ilst majwity of aerosol products doe\ not harm the wane Ia)el- and habe not contained CFCs \incc 197X.

Widespread we of CFCI prompted scientists to study the long-term effects of thi\ new family of molecules. In 197-l Rowland and Molina’ published llmr findings regardinc CFCs and damage to the wane layer. By thi\ time CFCs had ewpandcd into air condizoning. refrigerant\. safety solvents, acrowl5. and sty!-ofoam manufacture. Rowland and hlolinu found that this family of m~leculcs i\ very stable. doesn’t react easily with other n~olecules, and Inhts in the amwphrre for decades. They also found that wane will caux them to react in the upper strutwphrre.

Brielly. the behavior of the molecule ih a\ follows. The chlwine atom\ m the CFC molecule are held very tightly by carbon-chlor-ine hondh. making it difficult for the Imolecule to react with other molecules. The CFC molecule hurvivw inlact until it reache\ the upper stratosphere where a chlorine atom i\ removed from the molcculr due to ahwrptlon ol ultraviolet radiation. The freed chlorine atom makes conIact with an wane molecule (0%) breaking ith ring struclure and de\troyinp the wont molecule. The partial mechanism for the reaction of Freon-l 2 with o/one is given a\ follows:

CCI,F, + UV radiation --f CCIF, 1 Cl

Cl + 0, -Cl0 + 0,

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Scientists still don’t know how efficient the reaction is, but Rowland and Molina’s findings were sufficient to brincg the matter before the United Nations Environmental Pro&ram. As a result. the U.S. EPA banned the use of CFCs in all consumer aerosol products in 1978. Thi\ was followed in 1987 by the Montreal Protocol, which called for a slow phase-out of CFCs in other segments of industry, including non-CFC ozone depletors such as I, 1, I -trichloroethane and HCFC-22. Thi\ phase-out ih nearly complete, and very few aerosol

products contain ozone-depleting compounds. The ruling allowed CFC usage to con~inur only in hpecializrd products where no known

substitute wah available. but even these exceptions are being quickly phased out. Exemptions were granted to bronchial inhalers because of the extensive toxicological reviews needed before a new non-CFC formulation will he able to bc approved. Exemptions wcrc also given to certain products manufactured for the U.S. government. the military. and products that arc used in the aerospace industry. Aerospace continues to llse CFCs because substitute product% may damage delicate electrical equipment that is crucial to a manned space flight’s survival: however. the aerosols that we use evrl-y day do not contain CFCI and do not conrribute to the destruction of the ozone. All consumer products (hairspray, deodorant, whipping cream. lubricants. cleaners. polishes. spray paint. air fresheners, IO name a few) have been without CFCa for year\. If you want to be certain that your product doe\ not contain CFCs. check the back of the aerosol can’s label. The can will clearly indicate that it does not contam CFCs and does no harm to the ozone.

The few exceptions to the rule will eventually succumb due to the emergence of new high-tech <ubstitutes. The new generation of substitutes uses hydrofluorocarbons (HFCs) when an inert. low-flammability propellant is necessary. Hydrofluorocarbons do not contain chlorine atoms (note the absence of the prefix chloro-) and do not react with stratospheric ozone. providing an ozone-friendly alternative to CFCs. HFCs include HFC-I 52a (ditluoro- ethane) and HFC-I 34a (tetrutluoroethane). Many of the other non-CFC substitutes (propane and isobutane blends, dimethyl ether. and the compre\aed gases) have been used in consume, products for years and have proven to he good genera-purpose substitutes for CFC technology.

Volatile Organic Compounds The aerosol industry, like everybody else. is faced with the dilemma of complying with

volatile organic compound (VOC) requirements and staying below the maximum levels permitted. VOCs are becoming a big issue across the United States because they contribute to the formation of ground-level o.zone. which is a key ingredient in the creation of smog. Ground-level ozone. unlike stratospheric ozone. is not beneficial to our overall health and safety and actually contributes to the pollution problem that wx are all trying to combat. The earth’s oLone situation creates quite a paradox: we are fighting to preserve our stratospheric Ozone levels while we are trying to eradicate grounrf-level ozone.

The Clean Air Act stipulates maximum allowable amour~~s of gl-ound-lcuel ozone. which leads to the declaration ofnonattainment zones in areas whcrc the ozone levels are higher than their standards. Thehe area\ must make efforts to reduce VOC emissions from consumer products, industrial u\e, and automobile emissions. As a result. aerosols and other products must lower the percentages of VOC’s. Many aerosols state the VOC percentage either on the label or on the bottom of the can. Aerosol VOC calculations are very simple to perform: the weight of the VOCs is divided by the overall weight of the product, and the resulting fraction is converted into a percentage. as follows:

Weight Q VOC = (weight of VOC/overall weight of product) X 100%

Table II shows VOC limits of aerosol paint categories, as stipulated by the California Air Resources Board (CARB). At the present time, CARB is the primary body regulating VOCs in aerosol coatings. This situation may change in the future. The U.S. EPA is currently compiling data that will allow them to establish standards for VOC limit\ nationally. Of

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Tahle II. California Air Resource Board Maximum Percentage Volatile Organic Compounds hy Weight

80 YS x0

x0 xx xx

Xl 41 60 65 45 40

1s 7s ho

50 50 7(1

course, states will still have control over theil- policy, hut thi\ action will probably estahli\h

greater parity between the various states’ VOC limits.

Aerosol formulators are meeting the new VOC limits by using VOC-exempt solvent!, and

by increasing solids. Like everybody else trying to rel‘ormulate to a lower VOC level, there arc some seriuu pitfall%. A\ discussed earlier, an increase in viscosity lead\ to pow

atomization, which aerowl paint needs to aid in good application. Atomization is I-educed &hen increased holids or poor wlvency of VOC-exempt wlvcnts causes the viscosity 01‘ the

product to increase.

The exemption of acetone as a VOC has allowed aerowl formulators to mot-e easily meet

VOC standards in many of the categories in Table II. Acetone i\ being u\ed in a majority of

aerosol paint products and may reach levels as high as 25 IO 30% in the final product.

Recycling Many aerosol organizations are undertaking projects to help educate conaurnerh and

communities about the recyclability of acrosoI\. Communities are slowly beginning to accept empty steel aerosol cans as recyclable materials. a large step in improving our en~ironmrnt

and reducing the amount of materials that make their way into landfills. Check with your local

collection subice or recycling center to SW il they will accept yaw empty xrov~l~ ;I\;

rrcyclnhle mctul.

SAFETY AND STORAGE

NFPA 308 Code The National Fire Protection Associution (NFPA) has eablished Code NFPA 3OB’ to

“provide minimum requirements for the prcvcntmn and control of fires and explosions in

facilities that manufacture. store. or display aerosol products.”

An end uwr need only hc concerned about guidclinea for \toragc. as it is not very likely that end users are also manut’actmws of aerosol products. Storage guidclinc\ hcgin by

classifying wrowls into one of three cateporics (Lcvcls I, II. and III) according to the

product’s chemical heat of combustion. Level I aerosols have the Ieat chemical cornhustion

potential and Level III has the greatest potential. The categories are del‘inrd a\ follows:

. Level I aerosol\: From 0-X,hOO BTU/lb (O--20 kJ/p)

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l Level II aerosols: From 8.600-I 3.000 BTU/lb (20-30 kJ/g) l Level III aerools: Above 13,000 BTU/lb (>30 kJ/p)

The chemical heat of combustion is the summation of the weighted heats of combustion for the individual component% The following is taken from Appendix A of reference 2.

Hc( product) = I:[]% X Hc(I)l

where: Hc = chemical heat of combustion, kJ/g; I% = weight fraction of component “I” in product; and He(I) = chemical heat of combustion of component “I” kJ/g.

The manufacturer has the responsibility to calculate the flammability level and mark the outer cartons accordingly. Cartons that xc not marked are considered to hc Level 111 aerosols until they can be properly classified.

Cla\Gfication levels place limits on the number of aerosol\ that can he stored in general-purpose (nonsprinklered) warehouses. Nonsprinklered warehouses may house up to

1,000 lb net weight of Level III aerosols or up to 2,500 Ih of Level II aerosols, with the combined weight of Level II and III aerosols not exceedin, _._ ~7 3 500 lb. Amounts above these limits must be stored in special sprinklered warehouxx that comply with NFPA standards. as outlined in the NFPA 30B Code book. Many users do not even come close to approaching these numbers, which means that most aerosol users will he able to store aerosol cans safely without making special provisions. Remember to check your local fire codes and your insurance company‘s minimum standards to see if they (uprrscde this code, as the NFPA 30B code is merely a minimum standard that i?l often not as stringent as local codes. Warehousers need not worry about Level I aerosol storage, as thcrc is no maximum for these aerosols, as defined in the NFPA 3OB Code hook.

Miscellaneous Storage Tips Aerosols are pressurized containers that are often extremely flammable. Improperly

storing aerosol cans can lead to an unsafe situation that could result in fire. injury, or death. First, be certain IO store can\ in a dry environment. Damp or wet areas will cause the cans to corrode, leading to a weakenins of the cans’ sidewalls that may even completely permeate the

133

metal. The cans sidewalls and seams are guaranteed to withstand the cans’ pressure under normal storage conditions but they may fall It they are weakened.

Also. it is important to avoid temperature extremes. Aerosols 9lould not he stored in areas where the temperature is above 120°F because it create\ an unsafe situation due to an increase in preswre. which ma) cause the aerosol’~ dome or bottom chime to expand (wc Fig. I). In extreme cases the aerowl can uxplodc. Domes and bottom chimes iw engineered to

expand under extreme prcsure to probide more headspaw inside the can in the hopr of deterring a potentially hazardous situation. Expansion should he considered a warning \ign that the can may have dangerously high presure. These cons should only he handled wth extreme caution and should be disposed of immediately.

Cold-temperature storage can freeze wme \~;lter-containinE aerosols but will not permanently affect moht wlvcn-based products. A showterm effect of cold-weather wrage i\ that your aerosol will have poor atomizntion and mny not spray at all. Cold cans \hould be \harmcd before \prayin g hy placing the cans in a temperate room until they are at room temperature. This will ensure that the product will paform as it wa\ designed. Do not uw heaters or dryer\ to speed the process. as this only creates a potentially hazardous situation.

Some Useful Tips for Safely Using Aerosols Avoid temperature extremes: aerosols operate optimally when hetwcn 60 and XS’F.

Cover nearby item to protect them from over\pray. Clear the valve by inverting the can and spra) ins until clear mist come\ nut. This will keep the val\e clean and prevent clogging. UK

adequate ventilation. a\ aerosols are highly atomized and evaporate very quickly. Rememhc~ to u\e the proper safety equipment that is recommended by your shop. keep spark XXII-CC~ and open llamc away from the arca when spraying. and follo\v all instructions on the can’\ label.

REFERENCES

I. Rowland. F.S. and M.J. Molinu, “Stratospheric Sink for Chlorolluoromethanes: Chlorine Aton-Catalyaed Destruction of Orone.” ,Var~rw. 1249(5360): 8 I O-8 12; I973

2. NFPA 30B Code for the Manufacture and Storage of Aerosol Product\. 1993 edition. Chapter I Section 2

3. Johnson, M.A.. “The Aerorol Handbook.” 2nd edition. Wayne Dorlantl Co.. Mendhull. N.J.: I982

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