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PROPELLANTS, AEROSOL

MONTFORT A. JOHNSEN

Montfort A. Johnsen and

Associates, Ltd., Danville,

Illinois

Propellants are an integral part of every aerosol product.

They may vary from about 0.5% to 100% of the total

composition, and even with speciﬁc products there may

be rather wide variations in propellant type and percen-

tage. The most basic role of the propellant is to provide

1038 PROPELLANTS, AEROSOL

pressure inside the dispenser, so that when the valve

actuator is pressed, the product emerges. But the propel-

lant ﬁlls a number of other rolls as well.

Propellents can be divided into classes according to

their generic makeup, pressures, and other attributes.

The hydrocarbon class includes propane, n-butane, iso-

butane, and perhaps n-pentane and isopentane. When

pure, these gas liquids are almost odorless, very low in

density, and quite inert chemically except for their ex-

treme ﬂammability. They are the workhorses of the aero-

sol industry since they are used in 80% of all U.S. aerosols.

Isobutane is the most popular due to it adequate but

intermediate pressure of 31 psig at 701F (21.4 bars at

21.11C). The next group has one member: dimethyl ether

(DME). DME has a pressure of 62.3 psi-g at 701F (4.30

bars at 21.11C), a low density, and exceptional solvency.

The third groups consists of the hydroﬂuorocarbons: HFC-

152a, HFC-134a, and HFC-227ea in order of popularity.

HFC-152a (1,1-diﬂuorethane ) is popular because it is not

a VOC (volatile organic chemical). HFC-134a (1,1,1,2-

tetraﬂuoroethnae) has a global warming potential about

11 times higher than HFC-152a and is highly restricted in

its availability. HFC-227ea (1,1,1,2,3,3,3,-heptaﬂuoropro-

pane) is almost unknown in the general aerosol industry.

The propellants mentioned above are sometimes called

liquid propellants or gas–liquids. The can be shipped by

tank car or tank trucks and can be stored in bulk tanks

having a capacity of 122,000 L (31,500 gal).

Both carbon dioxide and nitrous oxide are liquid pro-

pellants, since their critical points are slightly above room

temperature. The solubility of these gases varies greatly

with their formulation. The ﬁnal group of aerosiol propel-

lants is the permanent or noncondensable gas, typiﬁed by

nitrogen and puriﬁed compressed air (CAIR). Since nitro-

gen is virtually insoluble in water and only very slightly

available im organic solvents, it is often considered to act

as a gaseous piston, placing aersol concentrates under

pressure so they will be emitted when the valve is is

operated. Compressed air, which contains 77.52 vol%

nitrogen, has very similar properties.

For products in which the propellant is soluble (usually

anhydrous types), the propellant can be used to control

particle size. Increasing the percentage of propellant,

or the use of a higher-pressure propellant, will serve

to decrease the mean aerodynamic particle size of the

spray. The importance can be illustrated in the case of

insecticides. Sprays designed to kill ﬂying insects must

produce a preponderance of particles capable of ﬂoating

about in the air for signiﬁcant time periods. In the case of

particles larger than B50 mm, they would fall onto tables

and ﬂoors so rapidly that the insecticide would lose

potency—known as ‘‘knockdown’’ and cidality properties.

On the other hand, particles that are too small would tend

to ‘‘airﬂow’’ over or under the ﬂying insect. Some insect

sprays, more properly identiﬁed as bug killers, contain

only about 3–20% of propellant in the formula (depending

on propellant choice), and these produce coarse sprays

designed to coat baseboard areas where many crawling

insects travel. The formulas usually contain relatively

harsh toxicants that should not be inhaled to any signiﬁ-

cant degree.

The aerosol valve generally works hand-in-hand with

the propellant to produce the desired product presenta-

tion. Valves are important in controlling delivery rate.

They also have terminal oriﬁces that can determine the

width of the spray cone. Some of these oriﬁces come with

what is known as a mechanical breakup (MBU) insert,

which can take what might be a solid stream of product

and cause it to be emitted as a coarse spray. The function-

ality is rather similar to that of a garden-hose nozzle.

In those cases where sufﬁcient dissolved propellant is

available to create a spray, as the liqueﬁed gas bursts into

the gaseous state, the use of an MBU sprayhead can serve

to make the spray particles somewhat ﬁner and the spray

pattern more uniform and elegant. MBU sprays are gen-

erally quieter than other types.

Most propellants are relatively insoluble in water, and

thus in water-based products, but can be emulsiﬁed into

them if some surfactant is added. This is the basis of

aerosol foams: shaving creams, mousses, foam charcoal

starters, and so on. Typically, 4–8% of hydrocarbon pro-

pellant is emulsiﬁed in the soap solution by gentle shaking

before use. If one uses a spray-type actuator instead of a

foam spout, foaming sprays can be generated. Hard-sur-

face cleaners, textile spot removers, oven cleaners, and

similar products are formulated this way.

About 45–50% of all aerosol valves have vapor-tap

oriﬁces:

tiny holes pinned

or drilled through the body, so

that both liquid and gas enter the body chamber when the

valve is actuated. The result is a spray with much lower

delivery rate, a ﬁner particle size, and a somewhat louder

sound. Since much of the emerging propellant is already

in the gas phase, the spray is warmer than that of regular

sprays. This can be important in the case of underarm

deodorants, antiperspirants, body sprays, and medicated

foot sprays. The use of vapor-tap valves also allows liquid

oriﬁces to be larger. Vapor-taps are always seen on anti-

perspirants, partly to keep the ﬁnely powdered aluminum

chlorhydrate astringent from clogging valve oriﬁces. The

vapor-tap feature permits the use of large, clog-resistant

oriﬁces, but without the stigma of excessive delivery rates.

One important consideration with vapor-tap valves is

that sufﬁcient propellant must be used to keep it from

running low, due to the continual short-circuiting of the

gas phase. For example, depending on the relative size of

the liquid-tap and the vapor-tap oriﬁces, a dispenser that

starts with 45% propellant could end up with 35% pro-

pellant. This might mean a discernably coarser spray and

lower delivery rate. A ﬁnal interesting feature of vapor tap

valves is that, if the liquid and vapor oriﬁces are the same

diameter, the aerosol will spray just the same way, if it is

inverted. In such cases the regular aerosol types would

only emit propellant gas.

HISTORY

The earliest aerosol propellants were carbon dioxide,

followed by vinyl chloride, isobutane, dimethyl ether,

and a few other gases. In 1938, when the chloroﬂuorocar-

bons (CFCs) became commercially available, these very

low-odor, nonﬂammable, and relatively inexpensive gases

PROPELLANTS, AEROSOL 1039

virtually displaced all the others. Most early aerosols used

about 85–90% CFCs—from the heavy ‘‘bug bombs’’ of

World War II to the consumer aerosols that ﬁrst appeared

about 1948 in the United States. After 1954 very low-cost,

low-odor hydrocarbons became available, and propane, n-

butane, and isobutane began to be used to propel water-

based products, such as window cleaners.

The very light-density hydrocarbon liqueﬁed gases

simply sat on top of the product as a discrete liquid layer,

conferring a rather constant pressure. Under such condi-

tions, the product itself ﬂowed up the dip tube when the

dispenser was actuated, turning into the desired coarse

spray as it passed through the MBU valve oriﬁce (see

Figure 1). This type of operation had been impossible with

the CFCs, since they are about 1.4 times as dense as water

and would have formed a bottom layer of almost pure

propellant. Actuating the valve would have swept the

propellant up the diptube—and even if the diptube had

been shortened, to extend only to the bottom of the

product layer, tilting the dispenser during use would still

have caused propellant discharge.

The window cleaner, oven cleaner, basin, tub, and tile

cleaner, and similar products typically ran about 4–6%

isobutane propellant (7.3–10.9 vol%). The top layer would

diminish in size when the headspace was enlarged during

sprayout. But a certain amount of propellant was also

added as a contingency against the very real probability of

occasional consumer misuse, such as trying to spray the

can when it was severely tilted or even upside down.

The aerosol industry ﬁrst learned about the ‘‘ozone/

CFC’’ issue around 1974, at a time when various CFC

propellants were used in about 53% of all aerosols. There

was disbelief, since the evidence was nonempirical and

speculative at ﬁrst. But in 1978 the EPA and FDA acted to

ban the CFCs from all but medical aerosols—while not

doing anything about their continuing use in refrigera-

tion, air conditioning, and all other applications. (The

United States was also the only country in the world to

take such an action.)

Without the ability to continue using nonﬂammable

CFCs for hair sprays and virtually all other aerosols,

marketers turned to about the only viable alternatives:

the liqueﬁed petroleum gases (LPGs) (e.g., propane,

n-butane, and isobutane). Elaborate reformulation

programs were needed to evolve products of acceptably

low ﬂammability while still maintaining compatibility

with active ingredients. One problem involved net weight

per can. In the case of antiperspirants, which had used

90% CFC in the formula, the reformulated products now

used B68–85% of very low-density hydrocarbons—gener-

ally isobutane. Since the density of the new antiperspir-

ants was about 0.68 g/mL (at room temperature),

compared to about 1.38 g/mL for the CFC predecessors,

marketers could ﬁt only about half the previous product

weight into their cans. Consumers thought the cans were

‘‘only half full,’’ and this sparked a degree of discontent-

ment that acted to plummet the sales of this particular

product to about 25–30% of what they had been, prior to

1978.

In 1989 the U.S. Congress passed thr Clean Air Amend-

ments Act (CAAA), which among many things directed the

U.S. EPA to conﬁrm the probable ozone depletion status

of hydoﬂuorocarbons propellants, the Halons, and a num-

ber of polychlorinated solvents. By 1993, all of these

propellants and solvents were banned. In the years

that followed, the aerosol industry had to cope with

two more propellant sanctions. The ﬁrst came in a ﬁnding

that many VOCs, when released into the air, could directly

increase the level of tropospheric ozone. This problem

was the most intense in southern California. The Califor-

nia Air Resources Board (CARB) wanted reformulation of

virtually every aerosol to reduce the VOC content. CARB

plans to cascade the VOC contents of aerosols until

at least 2023. The ﬁnal, and still developing, threat relates

to the global warming issues. The HFCs have been at the

center of this issue. HFC-134a is primarily used in me-

tered dose inhalants and life safety aerosols—for example,

car tire sealants and inﬂators, air conditioner reﬁll

units, pepper sprays, boat horns, electronic de-dusters,

and electric circuit board freezant testers. It has a rather

high GWP, having a potency 1300 times that of

carbon dioxide. HFC-152a is more important for aerosol

purposes. It is used in underarm products and hairsprays.

It is a mild global warming agent with a potency of 120

times that of carbon dioxide. HFC-227ea has had

very little impact on the aerosols industry. See Table 1

for the globabl warming potentials of the various

propellents.

By 2005, regulatory committees accelerated their pace

of activities. European aerosol associations banned pro-

duction of HFC-152a aerosols, although not much was

used at the time. In 2010 or 2011, The European Economic

Vapor

tap

Valve stem

Housing pedestal

Actuator

Terminal orifice

Stem gasket

Crimp

Cup

Body

Spring

Tailpiece and

liquid orifice

Barb

Propellant

vapor

Can

(stylized)

Propellant layer

(hydrocarbon)

Concentrate

(water-based)

Dip tube

Figure 1. Water-based aerosol spray system.

1040 PROPELLANTS, AEROSOL

Community will vote whether to ban most HFC propel-

lants Denmark and Austria have already voted. In 2007,

the Province of Manitoba, Canada, enacted a ban on all

aerosls containing HFC-134a.

California’s new Public Law AB-32 requires the 25% of

reduction of global warming gases by the year 2020

(translates to 70 106 metric tons). The Climate Action

team has been formed to implement this goal. They will

target any gas mentioned in the Kyoto Protocol, such as

HFCs, nitrous oxide, and carbon dioxide in the case of

aerosols. They may allow the use of HFC-134a, but will

ban the introduction of any new products. They may also

investigate the possibilty of replacing HFC-134a with

HFC 152a, or a future propellant, HFC-16 (ﬂuoroethane),

which has an extremely low potency of just 12 times that

of carbon dioxide and a half-life of 110 days. A number of

clarifying amendments to the Act are scheduled in late

2008.

PROPELLANT CHEMISTRY

Propellants must meet numerous requirements to be ideal

for aerosols. Many of these are as follows:

1. Reasonable boiling point and vapor pressure

2. Reasonable cost

3. Acceptable toxicology proﬁle

a. Acute

b. Subacute

c. Chronic

4. Acceptable low odor

5. Acceptable environmental aspects

a. Minimum effect on stratospheric ozone

b. Minimum effect on tropospheric ozone effects

6. Flammability consideration

7. Stability consideration

a. Hydrolysis

b. Alcoholysis

c. Polymerization

8. Solvent properties

a. Useful in some formulas

b. Sometimes detrimental to elastomers

9. Purity

The CFCs came close to being ideal, except for their great

chemical stability, which brought about the free-radical

depletion of stratospheric ozone. Current propellants are

all considerably less than ideal. The hydrocarbon gases

(LPGs) indirectly act to produce tropospheric ozone, or

smog, but their major drawback is their extreme ﬂamm-

ability. Dimethyl ether is rather uniquely water-soluble,

but it is also ﬂammable and has such a strong solvent

action that gaskets may be attacked and corrosion possi-

bilities are enhanced. Carbon dioxide is nonﬂammable,

but acidic if added to water. In any event, no more than

about B3–6% can generally be added before aerosol

pressures reach the regulatory limits imposed on aerosols

by the U.S. Department of Transportation. These and

other general properties of the available propellants are

given in Table 2.

A selection of the physical properties of aerosol propel-

lants is presented in Table 3 (hydrocarbons), Table 4

(DME and HFC-152a), and Table 5 (compressed gases).

The propellants may be mixed with other propellants, or

with various aerosol concentrates, in order to achieve any

reasonable product pressure. Entrapped air, present in all

aerosol dispensers to varying degrees, adds about 6–21 psi

(0.41–1.45 bar) to the propellant pressure.

The usual aerosol pressures vary within B30–70 psig

at 701F (2.06–4.83 bar at 211C). In the special cases of CO

and N

O propellants, pressures are typically 85–95 psig at

701F (5.86–6.55 bar at 211C), and where nitrogen or

compressed air (CAIR) are used, pressures may be as

high as 141 psig at 701F (9.72 bar at 211C). All these

ranges are designed to keep the aerosol pressure at or

below 180 psig at 1301F (12.41 bar at 55.41C), which is the

Table 1. Global Warming Potentials (GWP) of Aerosol

Propellants

Propellant Lifetime (yr) GWP (100 yr ITH)

Carbon dioxide B65 1.000

Propane 0.03 nil

Butanes 0.01 nil

Dimethyl ether (DME) 0.02 nil

HFC-152a 1.5 140

HFC-134a 14 1300

HFC-227ea 41 3300

Nitrous oxide 120 320

Table 2. General Propellant Comparisons

Hydrocarbons DME HFC Compressed Gases

Flammability Flammable Flammable Flammable and nonﬂammable Nonﬂammable

Toxicity Low Low Low Low

Solvency Poor Good Poor Poor

Density Low Low Intermediate —

Solubility in water Low High Low Low

Environmental concerns VOC VOC GWP None

Cost Low Low High Low

Abbreviations: DME, dimethyl ether; HFC, hydroﬂuorocarbon; VOC, volatile organic compound (as deﬁned by regulators); GWP, global warming potential.

PROPELLANTS, AEROSOL 1041

limit set by the U.S. Department of Transportation (DOT)

for the strongest of the three standard aerosol cans.

Pressures increase very rapidly with increasing tempera-

ture for those formulas that contain large amounts of

lower pressure propellants, such as isobutane. On the

other hand, nitrogen, which often has a negligible solubi-

lity in the concentrate, increases in pressure more or less

in harmony with Charles’ law.

Aerosols will buckle (permanently deform) and ﬁnally

burst if overheated. The label almost invariably warns

against heating higher than about 1201F. Some labels list

1301F. Bursting aerosol cans pose a severe hazard and may

be life-threatening. In some cases the contents may also

ignite, causing an additional hazard. A limited number of

aerosol cans have ﬁtments designed to allow the contents to

be released in a controlled fashion if they are overheated.

For three-piece ETP cans, about 10 small indentations may

be notched into the top of the top double seam. The can is

designed so that the dome sections will evert (or ‘‘jump up,’’

somewhat) well below the minimum bursting pressure.

When this occurs, the work-hardened indentations will re-

form into little apertures, releasing the overheated product.

This is known as the pressure-relief mechanism (PRM).

Aluminum cans and others without a top double seam

may be relieved by means of various ﬁxtures in the base.

Marketers have a concern that gas will be inadver-

tently dispensed. As a rule, a negligible amount of pro-

pellant is lost by leakage, through the valve or past the

junction of valve cup and can bead. The consumer may

hold the can in such a position that the dip tube end

protrudes into the headspace, particularly near the end of

the can, when little liquid is left and for a product such as

surface sprays when the dispenser is tilted strongly down-

ward to spray (see Figure 2). These episodes will act to

partly depressurize the dispenser quickly changing the

properties of the product. In severe cases, all products may

not be delivered at the time the internal pressure sinks to

zero (see Figure 2). There are two ways to solve this

problem. The older way is to ﬁll the can to 50 vol% rather

than the usual 75–90 vol%. With a relatively large, well-

pressurized headspace to start with, the drawdown pres-

sure will be reduced as the dispenser is used up. The

second is to use an initial higher pressure, which is not

always possible because of the U.S. DOT ceiling of 180 psig

at 1301F (12.41 bars at 54.41C). A DOT exempted can

would be required.

SAFETY

The aerosol industry had to learn, over the years, how

to safely handle ﬂammable propellants and gas them

into dispensers. They must be manipulated in closed

systems. In larger ﬁlling plants, ﬂammable propellants

are stored in 1 to B10 bulk tanks, ranging up to 30,000 gal

(113,500 L) in nominal capacity. Two-inch (51 mm) double-

strength steel pipes lead from the bottom of each tank past

valves to a low-pressure pump. From there the line may

lead to a blending station, followed by a manifolding unit,

to direct the liqueﬁed gas (or mixture) to the appropriate

gas house. In this enclosure, which is often outside the

main establishment because of the potential hazards, the

gas supply goes to piston-operated gassers. Two types are

used. The earliest, now called a ‘‘through-the-valve’’ (T-t-

V) gasser, charges the propellant backward through the

valve and into the sealed can. The liquistatic pressure

ranges from about 600 to 950 psig (41–66 bars), and the

temperature is often controlled to about 1041F (401C) for a

number of technical reasons. Many valves are designed to

backﬂow propellant at very high rates, so that a typical

18-head rotary T-t-V gasser may attain speeds of up to

B350–400 cans per minute.

The other gassing machine performs three consecutive

functions: (i) After lifting the lossely inserted valve

slightly, it draws a partial vacuum on the can; (ii) then it

adds the liquid propellant under high pressure, around

Table 3. Properties of the Hydrocarbon Propellants

Propane Isobutane n-Butane

Formula C

i-C

n-C

Molecular weight 44.1 58.1 58.1

Boiling point, 1F 43.7 10.9 31.1

Vapor pressure, psig

701F 109 31 17

1301F 257 97 67

Density (g/mL), 701F 0.50 0.56 0.58

Solubility in water (wt%), 701F 0.01 0.01 0.01

Kauri–butanol value 15 17 20

Flammability limits in air (vol%) 2.2–9.5 1.8–8.4 1.8–8.5

Flash point, 1F 156 117 101

Table 4. Properties of DME and HFC-152a

DME HFC-152a

Formula CH

OCH

CHF

Molecular weight 46 66

Boiling point, 1F 12.7 12.5

Vapor pressure, psig

701F6362

1301F 174 176

Density (g/mL), 701F 0.66 0.91

Solubility in water (wt%), autogenous

pressure

34 1.7

Kauri–butanol value 60 11

Flammability limits in air (vol%) 3.4–18 3.9–16.9

Flash point, 1F 42 o58

1042 PROPELLANTS, AEROSOL

the periphery of the lifted valve cup; and (iii) ﬁnally, it

presses the cup ﬁrmly down on the can bead and performs

a crimping (swaging) operation to hermetically seal the

can. A minor disadvantage of this machine is that above

3.0 mL of liquid propellant is ﬂashed off when the ﬁlling

head lifts off the sealed can. This must be exhausted

through a ﬂoor-sweeping ventilation system.

Ideally the gassing facility should be a strongly con-

structed room located at least 2 m (6.5 ft) from the main

establishment. Since hydrocarbon gases will be released

in this room, it is vital to install an adequate ventilation

system. The best approach is to add ‘‘make-up’ air through

ﬂoor level registers along one wall of the the gashouse to

remove it through registers a the bottom of the opposite

wall. The ventilation should ﬂow in a laminar fashion

across the entire ﬂoor. The heavy hydrocarbon vapors will

be captured and drawn up into the exhaust system at a

point 3 m (10 ft) above the rooﬂine. A gas sensor is used to

provide assurance that excessive hydrocarbon vapors are

not being produced. Frictonal sparks or static disharges

can be detected by a double electric eye installation, which

can turn on a water deluge system. In warm climates, the

Table 5. Physical Properties of Compressed Gases

Carbon Dioxide Nitrous Oxide Nitrogen CAIR

Formula CO

Molecular weight 44.0 44.0 28.0 29.1

Boiling point, 1F 108.4 126.4 320.8 320

Boiling point, 1C 78 88 196 196

Critical temperature, 1F 87.8 98.6 233 231

Critical temperature, 1C 31.0 37.0 147 146

Solubility in water

0.82 0.60 0.016 0.017

Flammability None None None None

Liquid density (g/mL at 701F, or 21.11C) 0.81 0.83 N/A N/A

Volume of gas per volume of liquid water, at 70

F, or 21.1

C and 1 atmosphere of absolute pressure.

Concentrate

Propellant gas

Propellant

liquid layer

Diptube is about

180° from dir. dot

Diptube is about

90° from dir. dot

(bends forward o

backward)

Diptube curvature conforms to

the dirrectional dot

Influence of diptube position

Normal

spray

Propellant

liquid to gas

Propellant

gas

The propellant layer has a volume of about

7% that of the concentrate layer.

Figure 2. Inﬂuence of diptube posi-

tion.

PROPELLANTS, AEROSOL 1043

gashouse may be replaced by open air gassing arrange-

ments. Unfortunately, there are many aerosols lines

where the gassing operation is done inside the main

establishment. This is common in Japan. Other than

excellent maintenance and normal ventilation, there is

no provision for possible leakage of liquid hydrocarbons

from pipes, hoses, valves or other equipment. Firms in the

United States that cannot construct an outside gashouse,

opt to enclose the gassing operation in very strong steel

cubicles of minimum size equipped with excellent ventila-

tion, gas sensors, and other engineering safety features.

The U.S. EPA is interested in plant emissions of volatile

organic compounds (VOCs), especially in the case of facil-

ities located in VOC nonattainment areas, such as larger

cities. If one considers the loss of (say) isobutane from an

intermediate to large aerosol plant, running a total

of 1000 cans per minute; at 3.0-mL loss per can, the total

loss will be B47.5 gal (180 L) per hour, or 380 gal per 8-h

shift. The EPA has required some plants to dramatically

reduce their emission rates, either by changing gassers,

adding conservation accessories to their U-t-C gassers, or

collecting the bulk of the released gas and compressing it

to liquid in a ‘‘gasser waste’’ storage tank. In some cases

the ‘‘gasser waste’’ may be used as heating fuel for the

plant boiler.

The extreme ﬂammability of hydrocarbon gases has

necessitated equipping gas houses with sophisticated sen-

sing devices, halon-powered extinguishing equipment, de-

luge systems , regular and emergency ventilation attributes ,

and a complex electroprotective system. Employees must be

carefully trained in ﬁre safety. There is a minor but

signiﬁcant trend to depopulate gas houses, so that, if a

catastrophe occurs, one or more lives will not be jeopardized.

As little as 0.58 ﬂ oz (9.6 g) of isobutane, poured into an

open-top 55-gal (200-L) steel drum and mixed with the air

in the drum, will produce about a 500-L ‘‘ﬁreball’’ if

ignited. As little as 2.0 g, poured into the drum but

unmixed, will produce ﬂames that ﬁll most of the drum

if ignited. The following data show that the hydrocarbons

cannot be detected by odor and are substantially heavier

than air:

1. Odorless

2. Colorless

3. Heavier than air

a. Two times

4. Narrow limits of ﬂammability

a. LEL 2%

b. HEL 10%

5. Low ignition temperature

a. 825–9001F

6. Expansion

a. Liquid to vapor = 1–270

This means that they have a tendency to collect in ﬂoor-

level areas. Mixtures of 1.86–2.04 vol% gas in air are

potentially ﬂammable. Fortunately for the industry, prac-

tically all hydrocarbon-type aerosols are used for only a

few seconds at a time. But for those paints, total-release

indoor insect foggers, and a few other products, where

large amounts are sprayed at a time, good ventilation or

dilution is quite important, and such admonitions will be

seen on the can labels.

CONCLUSIONS

As can be seen, the technology of aerosol propellants is far

from simplistic A fair number of personal care products

may have as many as four different propellants, all

contibuting to the desired properties and quality of the

ﬁnal formulation. Each propellant has its own proﬁle of

pressure, density, water solubilty, cost and other proper-

ties. They must be used to their greatest advantage by

formulators and packaging experts.

BIBLIOGRAPHY

General References

The Aerosol Guide, 8th edition, Consumer Speciality Products

Associations, Washington, D.C., 1995, 10th edition, CD to be

published in 2009.

Aerosol Propellents: Considerations for Effective Handling in the

Aerosol Plant and Laboratory, Aerosol Divison, Consumer

Speciality Products Association, Inc., Washington, D.C., 1999.

M. A, Johnson, ‘‘Carbon Dioxide—Our Oldest and Newest Pro-

pellant,’’ Spray Technol. Mktg. (September 2007).

PULP, MOLDED

EDWIN H. WALDMAN

Keyes Fibre Company

Updated by Staff

INTRODUCTION

Like the paper-making process itself, the origins of pulp

molding are pretty well obscured by antiquity. One as-

sumes that artisans in early Oriental, Egyptian, and

Greco-Roman civilizations who were involved with

screen-felting of vegetable ﬁbers into sheets of paper

rapidly discovered beneﬁts that derived from embossing

the screening material to obtain contours. It is believed

that ornate wall- and ceiling-molding artifacts of those

civilizations were fabricated in this manner. With the

rapid development of the paper industry around the

turn of the twentieth century, disposable molded pulp

products began to ﬁnd their way into the mainstream of

commercial life as protective vehicles for transport of

fragile foods such as pies and eggs. The economics of

manufacture of pie transfer plates and egg trays have

kept these unglamorous products in the marketplace.

Although certainly in the mature part of the viability

curve, they remain of considerable commercial interest.

1044 PULP, MOLDED

FABRICATION

The term ‘‘molded pulp’’ is used to describe three-dimen-

sional packaging and food-service articles that are manu-

factured by forming from an aqueous slurry of cellulosic

ﬁbers into discrete products on screened, formaminated

molds in a process analogous to continuous-sheet cylinder-

board papermaking. It is not unusual to ﬁnd the pulp-

molding process grouped with other converting processes

such as compression molding of ﬁber-reinforced resin

parts and pressboard converting because of the similari-

ties in ﬁnishing steps and common markets, but the

molded-pulp process and its products are fundamentally

unique. Two basic methods of fabrication are used: plain

molding and precision molding (see Figure 1).

Products of plain molding are as fundamental as the

triangular corner protector pads used in furniture and

appliance packing crates, and they are as complex as

Figure 1. The molded-ﬁber process.

PULP, MOLDED 1045

hinge-lidded, self-locking, one-dozen egg cartons en-

hanced by hot afterpressing and graphic print applica-

tions. Between these extremes are products such as berry

punnets, peat pots, produce prepackaging trays, and egg

and apple-locator trays (see Figure 2). Such products have

commercial relevance because they are produced on

highly automated and productive machine modules (W20

tons (W18.1 metric tons) per day), are nestable for efﬁcient

transport to users, are low in density in comparison to

converted paperboard products (see Paperboard), and are

fabricated from some of the least-costly raw materials

available (recycled mechanical cellulosic ﬁbers, ground-

wood, sphagnum peat ﬁber, etc).

Precision-molded processing differs from its higher-

speed forerunner in drying methodology. Forming of the

product and the removal of mechanically bound water by

vacuum-assisted compaction is essentially the same; but

at that point, ﬁnal drying is accomplished by step- or

continuously applied heat between matching mold sur-

faces (see Figure 3). Precision-molded products include

disposable plates, dishes, bowls, and specialty products

such as loudspeaker membranes and cones (see Figure 4).

These products are typically denser, smoother, and more

exactly dimensioned and contoured than their free dried

counterparts. Virtually any plain-molded product can also

be made in this manner, but they would be more costly

because of the complexity of tooling and higher electrical

energy required.

Many internal treatments are given to the slurries to

render them more valuable to their users. By blending

ﬁbers that have received different process and reﬁning

treatments, ‘‘green’’ matrix strength can be enhanced;

drainage, internal bond strength, and shrinkage can be

altered; and biodegradability of products after or during

use can be controlled. Colloidal rosin or wax emulsions are

commonly added together with paper-making alum to

convert hydrophilic ﬁber into the water-repellent products

essential to food handling. Fluorocarbon chemicals in

concert with cationic retention aids can be added to impart

repellency to low-surface-tension oil or greasy liquids.

Fertilizers, dyestuffs, ﬂame retardants, and modiﬁed

starches or wet-strength resins are among other additives

that are also commonly added internally where speciﬁc

end-use effects are desired.

Secondary treatments are sometimes given to nondis-

posable molded-pulp products. Cafeteria serving trays

have been made by after-pressing laminates of molded-

ﬁber preforms loaded with thermosetting polymers. Lug-

gage shells, automobile trunk wells, glove compartments,

and door liners are made this way. The hot afterpressing

and decorative printing of egg cartons also represents

secondary treatment that enhances printability and

product automation. Colored molded packaging can be

Figure 2. Examples of plain-molded

products.

Figure 3. The Chinet precision-molding machine.

1046 PULP, MOLDED

produced using a dyed pulp. Decorative ﬁnishes can be

applied by spray gun. Text (such as brand and end-user’s

names) and symbols (such as the recyclable logo or trade-

marks) can be incorporated into the mold to produce an

embossed or debossed effect.

An example of secondary treatment is the lamination of

a thin thermoplastic ﬁlm to one surface of molded-pulp

trays by vacuum-thermoforming techniques. Products of

this type are being used for frozen dinners because of their

‘‘dual ovenability’’ (i.e., their suitability for use in micro-

wave and convection ovens).

The use of computer-aided design has facilitated mold

design and enabled more complicated designs to be repro-

duced than before (1).

APPLICATIONS

Molded-pulp packaging provides a cost-effective solution

for the packaging of a wide range of products, many of

which are fragile such as eggs, fruits, and vials. Clam-

shell containers are used for eggs and bottles. Molded pulp

packaging provides an important use of recovered paper

and paperboard. Molded pulp is based on a renewable

resource and provides the environmentlal beneﬁts of

recyclability and biodegradability.

BIBLIOGRAPHY

Edwin H. Waldman, ‘‘Pulp, Molded’’ in The Wiley Encyclopedia of

Packaging Technology, 1st edition, John Wiley & Sons, New

York, 1986, pp. 559–561; 2nd edition, 1997, pp. 791–794.

Cited Publication

1. C. Hogarth, ‘‘Moulded-Pulp Packaging’’ in M. J. Kirwan, ed.,

Paper and Paperboard Technology, Blackwell Publishng,

London, 2005, Chapter 14.

Figure 4. Examples of precision-molded machine products.

PULP, MOLDED 1047