Yam, Kit L. (ed.). The Wiley encyclopedia of packaging technology

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as a result of the U.S. Clean Air Act of 1993, are being used

in place of CFC- and solvent-borne release agents.

FUTURE TRENDS

The ability of rotational molding to economically make

large, complex, hollow, and seamless products is becoming

more interesting to the industrial packaging industry.

Other attractive advantages of this process are its ability

to economically make low-volume production runs, the

wide range of resins that are available, and its multilayer

capabilities.

The rotational molding industry is growing rapidly. At

present, well over 200 rotational molding plants operate in

the United States.

Rotational molding technology is constantly improving.

Many new developments in rotational molding materials

are announced each year.

A great deal of technical information is readily avail-

able about rotational molding from the Association of

Rotational Molders. The industry organization offers

many technical brochures and copies of recent technical

papers.

1088 ROTATIONAL MOLDING

SEALING, HEAT

BARRY A. MORRIS

DuPont Packaging and

Industrial Polymers,

Wilmington, Delaware

DUNCAN DARBY

Packaging Science, Clemson

University, Clemson,

South Carolina

Most ﬂexible and semirigid packaging structures use

thermoplastic resins in their construction. When energy

is applied, the thermoplastic resins melt and fuse together

to form a seal. A heat seal can be deﬁned as a method of

closing a package using a thermoplastic material whose

temperature is elevated by some addition of energy that

results in heat. It is commonly accepted that, to achieve a

seal, heat (energy), time and pressure are required. En-

ergy is required to cause softening of the resin, pressure is

required to bring the surfaces into intimate contact, and

time is required to allow the heat transfer and molecular

motion. How the energy and pressure are supplied to the

seal area is what distinguishes the different methods of

sealing. This article describes how the energy and pres-

sure are supplied in ﬂexible and semirigid packaging in

order to achieve acceptable seals.

Flexible packaging materials can be classiﬁed into two

categories: multilayer packaging materials (see Laminat-

ing; Multilayer ﬂexible packaging) and monolayer ﬁlms

(see Films, plastic). Multilayer materials include lamina-

tions and coextruded ﬁlms. Monolayer ﬁlms consist of just

one thermoplastic material. Coextrusions consist of multi-

ple thermoplastic layers. Laminations are multilayer con-

structions of thermoplastics and/or nonthermoplastics,

including thermoplastic layers for sealing purposes.

When laminated ﬂexible materials are to be sealed, the

outer face of the material is generally nonthermoplastic

(such as paper or paperboard) or a thermoplastic with a

melting point sufﬁciently higher than that of the sealant.

Likewise, the outer face of a coextruded ﬁlm used for

sealing may be a thermoplastic with a melting point

higher than that of the sealant. In both cases, this makes

it possible to apply a heated bar (or other heated tooling)

directly to the outer face in order to get the energy to the

sealing interfaces and join the two package members.

Heated tooling cannot usually be applied directly to

monolayer unsupported ﬁlms because they melt and stick

to the surface of the sealing bar. The seal area is destroyed

in the process. For this reason, other methods of sealing

(impulse sealing, hot knife sealing, etc.) are used to seal

such materials.

If the package members are too thick and too insulat-

ing, other means must be used to get energy to the sealing

interfaces. Since heated tooling (commonly known as hot

bar) sealing uses the least expensive equipment, it is more

widely used for sealing than any other method. The other

methods described below are used in applications where

bar sealing is not suitable. When sealing thin materials

together, it may be sufﬁcient to introduce heat from one

side of the construction. When using thicker materials, or

if higher speeds are required with thinner materials, heat

may be introduced from both sides. The fundamental

principles in heat sealing are to provide heat at the

interfaces, provide pressure to bring them intimately in

contact, and complete a weld, all within an acceptable

time period. An exception to this is one of the forms of

radiant sealing, which relies on ﬁlm orientation and sur-

face tension as a substitute for applied pressure. Most of

the important sealing methods are listed in Table 1.

From a polymer science perspective, the heat seal

process consists of several steps. When the materials are

ﬁrst brought together, the surfaces are rough on a mole-

cular scale and no adhesion occurs. As the heat reaches

the interface, the surface begins to melt. The melting

helps smooth the surface and, along with the application

of pressure, creates more intimate contact for wetting and

bonding. With sufﬁcient time and temperature, molecular

segments may diffuse across the interface. The nature of

the attractive forces between molecular segments dictates

the amount of diffusion that is needed to build strength at

the interface. For nonpolar sealants such as polyethylene,

it is believed that enough diffusion must take place for the

segments to entangle. When polar forces are present, as in

EVAs, acid copolymers, and ionomers, less diffusion is

needed because hydrogen bonding and ionic forces help

build strength. After the heat is removed and the interface

cools down, co-crystallization may also help enhance the

seal strength.

During heat sealing, the weight of the product being

packaged or other forces (such as the natural spring-back

force of a gusseted area) may act to open the seal before it

has had a chance to solidify. The resistance to opening

forces while the seal is still hot is known as hot tack. Hot

tack strength is a function of (a) how much molecular

diffusion and entanglement has occurred in the seal area

and (b) the melt strength of the polymer. Sealants with

outstanding hot tack strength such as ionomers (see

Ionomers) may also have high hot tack strength over a

broad sealing temperature range. This can be important

for a variety of sealing operations with variable line

speeds and poor control of seal bar temperatures.

HEATED TOOLING SEALING (COMMONLY CALLED HOT

BAR SEALING)

Heated tooling sealing is the most widely used method for

sealing. It is commonly called hot bar sealing or bar

sealing, but there are many different geometries beyond

the bar that use heated metal to affect the seal. Seal

tooling geometries include ﬂat bars, serrated bars, rings,

rectangular shapes, ﬂat or shaped platens, heated rollers,

1089

and others. This type of sealing is used to make and seal

pouches, as well as for sealing lidding to blisters, cups, and

trays in form ﬁll and seal equipment.

When very long seals are to be made, it is essential that

the seal bars be designed to avoid any deﬂection in order

to ensure uniform pressure throughout the length of the

seal. Since it is important to avoid wrinkles in seals,

means are frequently provided to stretch out the seal

area of the packaging materials in order to ensure that

they are ﬂat when sealed. Another approach is to have

mating serrations incorporated into the seal bar faces,

which will stretch the packaging materials, and, hopefully,

remove any wrinkles. Care must be taken to ensure that

the serrations do not puncture the ﬁlms during sealing.

Serrated bars are used where good mechanical strength is

required and the risk of some tiny leaks in the seal can be

tolerated.

When hermetic and/or liquid-resistant seals are de-

sired, they can be made by a ﬂat-faced heater bar, opposed

by a bar that has a resilient surface. Silicone rubber is

generally used for this purpose. It is best if the resilient

surface is curved when viewed from the end of the seal bar.

When the bars come together, they ﬁrst create a line of

pressure throughout the length of the seal bar. As the bars

Table 1. Sealing Methods

Bar Concept: Jaw sealer with one or two heated opposed bars.

Applications: Manufacture and closing of laminated pouches, cup lidding, form/ﬁll/seal packaging (e.g., potato-chip

bags).

Band Concept: Two moving bands backed by heated and cooled metal jaws.

Applications: Sealing ﬁlled pouches, including those made from unsupported materials.

Impulse Concept: Jaw sealer backed with resilient silicone rubber. Electric current ﬂows through Nichrome ribbon stretched over

one or both surfaces and covered with high-temperature release ﬁlm or fabric.

Applications: Sealing tacky materials, unsupported thermoplastic ﬁlms (eg, frozen vegetable bags).

Wire or knife Concept: Hot wire or knife seals and cuts ﬁlm.

Applications: Bagmaking and bagclosing; overwrap for toys, records.

Ultrasonic Concept: Tooling hammers or rubs materials together at high frequency, generating heat for sealing.

Applications: Sealing biaxially oriented ﬁlms, thick webs, aluminum foil, rigid container components.

Friction Concept: Frictional heat generated by rubbing components together generates heat for sealing.

Applications: Assembly of round containers, sealing ends of strapping.

Gas Concept: Hot air or gas ﬂame applied to both surfaces and removed; molten surfaces are then pressed together.

Applications: Manufacture and closing of polyethylene-coated paperboard milk containers.

Contact Concept: Plate is placed between surfaces to be sealed and is then withdrawn, and the molten surfaces are pressed

together.

Applications: Sealing ends of strapping, tubing.

Hot-melt Concept: Continuous strip or dots of hot, molten thermoplastic sealant are applied between two surfaces just before

pressing them together. The hot melt contains sufﬁcient heat to effect seal, and surfaces absorb heat to cool melt

rapidly.

Applications: Paperboard containers, peelable seals, case packers.

Pneumatic Concept: Heated ﬁlm is sealed to another surface by application of air pressure.

Applications: Skin packaging.

Dielectric Concept: High-frequency electric ﬁeld melts materials held under pressure.

Applications: Unsupported PVC, PVC-coated paper (not for polyoleﬁns).

Magnetic Concept: Gasket with iron-containing compound pressed between surfaces, assembly placed in magnetic ﬁeld.

Applications: Heavy-gauge polyoleﬁns (e.g., cap liners or lids).

Induction Concept: Alternating current is induced in metallic foil, usually aluminum, which heats and melts surfaces pressed

against the foil.

Applications: Tamper-evident closure liners; lids.

Radiant Concept: Infrared radiation sealing without pressure.

Applications: Sealing uncoated highly oriented ﬁlms and nonwovens, including polyester, nylon, and polyoleﬁns.

Solvent Concept: Solvent liqueﬁes surfaces that are then pressed together.

Applications: Sealing conﬁgurations where heat may degrade the thermoplastic or is not practical to apply.

1090 SEALING, HEAT

close further, this line broadens into a band. The pressure

along the initial line is at a maximum and at a minimum

along the edges of this band. This ensures that the

optimum sealing pressures are present somewhere in

between. In the event that drops of liquid are in the seal

area, this pressure proﬁle expanding from a line will tend

to push these droplets out of the seal area. If the droplets

contain water and they are not pushed out of the seal area,

they become steam and rupture the seal. As an alternative

to curving the section of the resilient bar, the heated metal

bar can be curved. This is generally not done since it is

more difﬁcult to maintain a curved section than a ﬂat

section on a metal seal bar. It is also important that the

edges of metal seal bars be gently rounded to avoid

puncturing the packaging materials. Figures 1 and 2

show the best constructions used for straight seal bars.

Heated tooling sealing is also used for applying lidding

to cups and trays. The upper heated tool is shaped to

match the shape of the rim of the container being covered.

The bottom tool is shaped to ﬁt under the rim and support

the container. In order to assure good seals, it is essential

that uniform seal pressure be obtained around the entire

rim of the container. Factors contributing to nonuniform

pressure are variations in the thickness of the rim of the

container and warping of the opposed sealing tools. Using

a resilient sealing surface under the rim of the container

can correct these deﬁciencies. Another approach is to

design the rim of the container to be curved upward in

section. When the seal is made, this curved rim deﬂects

much like a resilient backup member.

For some applications, continuous seals can be made by

passing materials between heated rollers. Unfortunately,

the dwell time for actual sealing under pressure between

rotating rollers is extremely short. Rotary sealing works

well for low-speed machines and for highly conductive

materials, as well as in applications where preheating can

be utilized.

Modern heated tooling systems use thermostatic con-

trols, maintaining a constant temperature in the tooling.

Rheostatically controlled systems on older lines still exist,

but for packages with thermoplastic outer layers, thermo-

static control offers better performance.

Heated tooling sealing is by far the most common form

of sealing. This is due to (a) the low costs of resistance

heaters and steel and (b) the versatility with respect to

seal shape. The most important disadvantage of heated

tooling sealing is the fact that the seal leaves the tooling

with residual heat. If any pressure is exerted on the seal

after it is made, the still-molten polymer in the seal area

can creep, even to the point of opening. In this type of

situation, the hot tack of the polymer becomes important.

HOT-WIRE AND HOT-KNIFE SEALING

Hot-wire and knife sealing are similar to heated tooling

sealing in that the seal system is heated by resistance-type

heaters. The wire (or knife) is mounted in or on the surfaces

of a heated bar and is heated by conductive contact with the

bar. The relatively small surface of contact causes all of the

force of sealing to be transmitted through a small area,

thus pressure in the seal area becomes very high. High

temperatures, along with these high pressures, allow this

type of sealing system to be an effective cutting method, as

well as a sealing system. Hot wire and hot knife sealing are

adaptable to very high-speed operations and are used for

both sealing and cutting apart polyethylene bags on bag-

making equipment (see Bags, plastic). Unsupported ﬁlms,

when trim sealed by this method, tend to form a strong

bead in their seal areas due to surface tension and orienta-

tion. This method is also used to a limited degree with

laminated constructions. Hot-wire cutoff is also used in

ﬁlm-dispensing equipment and with L bar sealers used on

shrink-wrapping machines.

BAND SEALING

In band sealing, a ﬁlled pouch is introduced between two

moving bands, which are pressed together by heated bars

(see Figure 3). The heat passes through the bands and into

the pouch material, softening it for sealing. As the pouch

continues along between the bands, the bands are next

pressed together by chilled bars that withdraw heat from

the pouch seal through the bands. The bands then progress

to release the pouch. Band sealing is fast and widely used

Figure 1. Bar sealer.

Figure 2. Rounded resilient bar.

SEALING, HEAT 1091

for closing pouches ﬁlled with product. It is important that

the pouch mouth be ﬂattened before entering the bands if

wrinkles in the seals are to be avoided. Band sealing

provides a continuous method for sealing, avoiding the

problems found in sealing between rotating heated tooling

sealers. It also provides cooling for the seals, overcoming the

main disadvantage of heated tooling sealing.

IMPULSE SEALING

Impulse sealers have the same general conﬁguration and

mechanical construction used for bar sealers. The differ-

ence between the two systems lies in the sealing jaws (see

Figure 4). Each of the opposed jaws in an impulse sealer is

generally covered with a resilient surface, such as silicone

rubber. A taut Nichrome ribbon is then laid over the

resilient jaw and covered with an electrically insulating

layer of thin heat-resistant material, such as silicone-

rubber-coated ﬁberglass, Teﬂon

ﬂuoropolymer-coated

ﬁberglass, or Teﬂon

-coated Kapton

polyimide ﬁlm

(DuPont). A pouch mouth is placed between the jaws,

and the jaws are closed. An electric current passes

through the Nichrome ribbon for a brief period of time

and is then turned off. While the seal jaws remain closed,

heat is withdrawn from the pouch mouth and the ribbon

through the resilient jaw surfaces, as well as through the

jaws. Next, water recirculating through a tube behind

the impulse ribbon cools the plastic to set the heat seal.

The jaws are then opened and the sealed pouch is removed.

This permits the sealing of constructions that have low hot

tack. It also permits the sealing of unsupported thermo-

plastic materials, which would stick to heated bars and pull

the seal open when heated bars are opened.

The advantage of impulse sealing over heated tooling

sealing is that the seal is cooled to achieve adequate

strength before the jaws are opened. The material that

covers the Nichrome ribbon has release properties, so that

it does not stick to unsupported thermoplastics or to

many of the coatings that are used on the outside of

packaging laminations. Such coatings include PVDC, ink,

and varnish. Although impulse sealing does not have the

versatility of heated tooling, some vendors offer shaped

impulse-sealing ribbons for securing lids to cups and trays.

The dominant drawback of impulse sealing is high

maintenance cost. The impulse ribbons slowly deteriorate

and the release coverings over the ribbons degrade, re-

quiring frequent replacement. On a production basis,

impulse sealing should be used only where heated tooling

sealing does not do the job properly. Additionally, impulse

sealing is less versatile than heated tooling sealing. Since

impulse sealing applies current across both ends of a wire

or ribbon, this system does not lend itself well to round or

rectangular sealing systems, and it is not really suitable

for platen sealing.

ULTRASONIC SEALING

In ultrasonic sealing, the sealing energy is transmitted to

the seal interface by mechanically hammering or rubbing

the packaging materials together at high (ultrasonic)

frequencies. Frequencies typically range between 20 and

40 kHz. The mechanical energy transforms to heat at the

interface through friction, viscoeleastic response, or some

combination of the two. This type of sealing is useful to

seal materials that are too thick to permit conductive heat

transfer through them for sealing. Ultrasonic sealing has

been proven to be a good system for sealing through

contamination. Because the seal tooling is not heated,

the system also offers advantages over heated tooling

sealing with respect to cleaning. If a product gets on an

ultrasonic sealing horn, it does not oxidize. Ultrasonic

welding is the only method used for welding aluminum foil

in its production.

The energy delivery in ultrasonic systems begins with a

converter, which uses piezoelectronics to convert electrical

energy into mechanical energy. The vibrations are then

transmitted through a stack and into the horn. The horn is

analogous to the sealing tool in other forms of sealing in

Figure 4. Impulse sealer.

Figure 3. Band sealer.

1092 SEALING, HEAT

that it is the surface that contacts the sealable packaging.

Like heated tooling sealing, ultrasonic sealing is versatile

with respect to shape. It can be used for pouches, cartons,

cup/lidding, tray/lidding, and blister/lidding systems. Some

tooling limitations exist for very long seals, although some

ultrasonic sealer manufacturers can compensate with mul-

tiple converter/stack systems driving a single horn.

The biggest disadvantage to ultrasonic sealing is its

cost. The seal tooling (horn) must be carefully designed to

properly transmit the mechanical energy to the sealing

interface. These tools are typically designed using ﬁnite

element analysis. When a tool is worn or damaged,

restoring its performance requires precise milling. Addi-

tionally, the tooling systems must, of course, be resilient

with respect to high-speed vibration. This cost is justiﬁ-

able in some applications and not in others.

FRICTION SEALING

Friction sealing is the name of a family of sealing systems

that utilize mechanical energy resulting in friction as the

energy source. In spin welding, the top and bottom halves

of cylindrical containers may be easily joined. The bottom

member is held against rotation by a friction brake; the

top member is pressed against the bottom member and

rotated. The heat generated by friction at the interface

between the two halves melts their surfaces, and the

viscous interface causes the bottom half to rotate in unison

with the upper half. This produces the seal. Thermoplas-

tics with surfaces that become somewhat slippery when

heated are best suited to friction welding. Thermoplastics

that become dough-like present problems. A thermoplastic

with a very low coefﬁcient of friction may not generate

enough friction to be a viable candidate for spin welding.

Spin welding is generally the most economical way to

make spherical shapes for packaging. It is also used to

manufacture two-piece cups.

There are other forms of friction welding used in

packaging. Seals are also made between the ends of

package strapping, which are rubbed together at high

speed until the interface melts, causing them to weld.

Friction welding is also used to manufacture plastic pallets.

HOT AIR/HOT GAS/FLAME SEALING

Hot air and hot gas sealing can be used in applications

where paperboard and thicker thermoplastic materials

offer too much resistance to the conduction of sealing

heat. In this case, heat is applied directly to the surfaces

to be sealed by a gas. The gas may be hot air, a more inert

heated gas (nitrogen or CO

), or a gas ﬂame. An important

application of this method is the manufacture of paper

containers for dairy products. Closing sealable tubes after

ﬁlling is another application where hot air sealing is

commonly used. Hot air sealing can also be used to seal

bags of landscaping goods, due to the thickness of

the material and the risk of damage to the seal jaws.

The primary advantage of these sealing techniques is the

ability to heat the sealable surface without dependence on

the conduction of heat. The main disadvantage of this type

of sealing is the risk of overheating and oxidizing the

sealing surfaces.

In ﬂame sealing, a gas ﬂame is played on the surfaces

about to be sealed, after which they are clamped together

between chilled bars. Hot air and hot gas sealing utilize

the same technique, but use heated gas rather than

ﬂame. Sufﬁcient sealant must be used on the surfaces in

order to ﬁll all the ﬁssures formed at the joints in the

paperboard.

CONTACT SEALING

Another technique to heat the sealing surface rather than

depend on conduction is contact sealing. In this case, one

or both of the sealing interfaces are contacted by a heated

surface before being pressed together. This technique

replaces the forced convection heat transfer of hot air or

hot gas sealing with conductive heat transfer. The heated

surfaces include plates, ﬂat bars, tapered bars, and even

shaped tooling. This has been used for strapping as an

alternative to friction sealing. Some modern ﬁtment seal-

ing machines use this technique for preheating the sur-

face of pouch ﬁtments before they are sealed to the pouch

material. The main disadvantage of this sealing process is

tacking of the sealant materials to the heated surface.

PTFE coatings may be employed to prevent this.

DIELECTRIC SEALING (ALSO KNOWN AS RF SEALING)

In dielectric sealing, used with polar materials such as

PVC and APET, a high-frequency electric ﬁeld generates

heat for sealing. The members are pressed together be-

tween a cold bar, generally made of brass, and a cold ﬂat

metal surface. An alternating electric ﬁeld is generated

between the bar and the lower surface. The ﬁeld alter-

nates at several megahertz, hence the ‘‘radio frequency’’

name. The dipoles in the polymers vibrate in response to

the alternating electric ﬁeld. Molecular resistance to the

vibration, which is pronounced in large molecules such as

polymers, causes the energy to dissipate into heat and

ultimately melt the polymer. The main bodies of the

members are kept cold to withdraw any residual heat

from the materials that have been sealed. This technique

is commonly used for sealing of theft-prevention clam-

shells found in the electronics departments of retail stores.

The primary disadvantage of dielectric sealing is the

requirement for a polar molecule. Nonpolar materials

such as polyethylene and polypropylene do not respond

adequately to an alternating electrical ﬁeld.

INDUCTION SEALING (ALSO KNOWN AS RF SEALING)

Induction sealing utilizes the property of some metals to

heat inductively, using an alternating magnetic ﬁeld. The

magnetic ﬁeld is alternated in polarity at radio frequen-

cies. The magnetic ﬁeld induces electric currents in the

metal. The metal resists the ﬂow sufﬁciently to heat

resistively, eventually building enough temperature to

SEALING, HEAT 1093

conduct out of the metal and into nearby thermoplastic

sealing materials.

A common use of induction sealing is to produce a

tamper-evident seal over the top of a bottle. The alumi-

num foil-based cap seal is generally incorporated under

the closure liner (see Closure liners). The assembly is

placed in the alternating ﬁeld, and a current is induced in

the foil. This current heats the foil, and energy is con-

ducted into the sealant, sealing it to the bottle.

In another variant of this type of sealing, a gasket,

shaped to ﬁt between the sealing faces of the two package

members, is made from the same thermoplastic as the

package, but has milled into it an iron-containing com-

pound that has a high hysteresis loss. The package por-

tions are pressed together with the gasket between their

sealing surfaces and placed in a magnetic ﬁeld oscillating

at a frequency high enough to melt the gasket. This, in

turn, melts the sealing surfaces and causes them to weld

together. As an alternative to a gasket, an iron-containing

material can be pre-applied to one or both of the sealing

surfaces, producing the same result.

RADIANT SEALING

There are two completely different techniques that can be

classiﬁed as radiant sealing. The ﬁrst is similar to hot air/

hot gas sealing in that the packaging material is heated

and then subsequently subjected to pressure. This type of

radiant sealing is typically used to close pouches on high-

speed horizontal form ﬁll and seal lines such as those used

to package sugar. The top seal travels between two radiant

heat bars to soften the polymer. At the end of the radiant

heating zone, two crimp wheels close the package. The

radiant heat bars are set much higher in temperature

than that needed for conductive heating due to the lower

effectiveness of radiant heating. These high temperatures

and the fact that the material is not supported in the

radiant heat zone means that some sort of thermally

stable material needs to be part of the construction.

Typically on this type of machine, paper is contained in

the structure.

In the other form of radiant sealing, the two surfaces

are held in contact by two cooled jaws with ends of the

material extending beyond the jaws. These exposed ends

are subjected to radiant heat to the point of fusing the

package closed. This form of radiant sealing makes it

possible to seal many materials used in packaging that

might be considered either difﬁcult or impossible to seal

without coatings (see Figure 5). Polyester ﬁlm (see Film,

oriented polyester), long used in packaging constructions

and many nonwoven (see Nonwovens) materials, can be

sealed by this method. These include: Tyvek

(DuPont), a

nonwoven polyoleﬁn used in packaging and for protective

garments; Reemay

, a nonwoven polyester, used in ﬁlters;

Cerex

, a nonwoven nylon, used as a carpet backing and

in ﬁlters; and nonwovens made from polypropylene and

other base materials. This method can also used for

sealing other oriented ﬁlms, such as polypropylene and

polyethylene. OPET pouches are commercially made for

document preservation. OPET is approved by the Library

of Congress for use in contact with valuable documents. In

addition to making pouches, this sealing method can also

be used to produce continuous seals to edge-join rolls of

materials at high speed.

HOT-MELT SEALING

Hot-melt sealing is performed by depositing either a con-

tinuous strip, or dots, of a thermoplastic material on a

packaging member, just before the next member is pressed

against it. It is used as an alternative to glue, since the

packaging members quickly remove heat from the melt,

making it tacky enough to hold the members together

quickly to prevent the springback forces from opening

them up (in sealing the covers of a shipping container).

With glue, the members must be held together until enough

of the glue’s solvent is absorbed by the packaging material

to make it tacky. Hot melts are also used where applied

heat might damage a packaging member or where peel-

ability is desired (see Adhesives; Waxes).

PNEUMATIC SEALING

Pneumatic sealing is used for skin packaging (see Carded

packaging), where pressure from the atmosphere pushes a

hot, tacky ﬁlm into close conformity with an object and

seals the ﬁlm to a substrate on which the object is placed.

It is also used in hermetic skin packaging for fresh red

meat. The energy source for this type of sealing comes

from the polymer being sealed, which has been preheated.

SOLVENT WELDING

Solvent welding is used to join together package conﬁg-

urations that may not lend themselves to heat sealing.

It is also used for joining materials that are either

not susceptible to heat sealing, or may be damaged

by the application of heat (e.g., shrink sleeves). In this

type of sealing, the solvent dissolves the polymer,

which increases the mobility of the chains. In this case,

Figure 5. Radiant sealer.

1094 SEALING, HEAT

chains can diffuse across the interface not because of

heat, but because the polymer chains are more mobile

in solution. When the solvents dry, the chains are left

entangled. Solvent welding is commonly used for shrink

sleeves. In the case of shrink sleeves, solvent is applied to

one side of the material, which is folded over to create the

weld. Its primary disadvantage is that the materials to be

used must be reasonably soluble in commonly used

solvents.

SELECTING A SEALING METHOD

For laminations and coextrusions with high-temperature

outer layers, heated tooling sealing is the least expensive

of all sealing methods. It should be considered ﬁrst before

other methods. If sticking to or contamination of the seal

bar is a problem, the following should be tried out before

abandoning heated tooling sealing:

1. Coating or impregnating the seal bar with Teﬂon

ﬂuoropolymer (DuPont).

2. Periodically wiping the seal bar with a silicone

grease.

3. Mounting a release material, such as Kapton

polyimide ﬁlm (DuPont), between the seal bar and

the packaging material.

If all of these fail, alternate sealing methods should be

considered next. This article is not suited to be a complete

reference for a seal method selection, but important items

to be considered and possible sealing methods include:

Does the seal need to be cooled before the pressure is

released? (Band/impulse sealing)

Does the seal need to separate the packages also? (Hot

knife & hot wire sealing)

Is the package material very thick? (Ultrasonic, hot air/

gas, contact)

Is the package material thick and aluminum foil-

based? (Induction)

Does the sealer need to cut through signiﬁcant con-

tamination? (Ultrasonic)

Is this a cap seal application with aluminum? (Induc-

tion sealing)

Is the package material polar (dielectric)?

Is the package material an oriented ﬁlm? (Second

radiant sealing method)

When packaging materials are sealed, the package and its

function dictate the properties to be achieved. Mechanical

strength is important, since the seals should not fail under

normal handling. Hermetic integrity is important in food,

pharmaceutical, medical, and chemical packaging.

Although small wrinkles in a seal area may have slight

effect on its mechanical strength, they are completely

unacceptable if a hermetic-type package is required. The

best approach to avoid seal wrinkles is to hold the seal

region of the construction under tension during sealing. If

this is impractical, increasing the thickness of the sealant

portion of a lamination may ensure good seals, even

though small wrinkles may be present.

Incorporating EVA in polyethylene, or substituting

ionomer (see Ionomers) as a sealant, results in improved

ﬂow properties and better tolerance of small wrinkles. The

most reliable seals for hermetic packaging consist of true

fusion welds between the opposed sealant members of a

lamination. A true weld can be identiﬁed by pulling apart

the lamination and noting where seal failure occurs. If it

occurs along the interface between the materials, a true

weld was not attained. If failure migrates from the inter-

face, through the sealant, and to the outer portion of the

lamination, a homogeneous seal has been achieved (see

Multilayer ﬂexible packaging).

If a peelable seal is desired, one should understand the

risks of relying on control of temperature and/or pressure

to achieve peelability. Seals made in this way may be

peelable, but they can come apart by themselves with

handling. Another way to achieve dependable peelability

is to have an interface formulated to be peelable. There are

three general strategies for achieving peelable seals. One

is to ‘‘contaminate’’ the sealant so that it does not bond as

strongly to itself during the seal process. This is known as

an interfacial seal since it fails along the sealant-to-

sealant interface. A second strategy is to control the

adhesion between the sealant layer and the layer adjacent

to it in a multilayer structure. As the seal is pulled apart,

the opening forces burst through the sealant layer and the

seal fails at the sealant-adjacent layer interface. This is

known as a delamination or burst peel peelable seal. The

initial seal strength is controlled by the thickness and tear

strength of the sealant and the steady-state seal strength

by the adhesion between layers. Delamination-type peel

seals typically are less sensitive to seal bar temperature as

a result. The third strategy is to design a sealant that has

poor cohesive strength; failure occurs within the sealant

layer as the seal is opened. This is known as a cohesive-

type mechanism.

SEAL TESTING

The American Society of Testing and Materials (ASTM)

has set up standards for the testing of the seal strength of

ﬂexible barrier materials. Their designation for the test-

ing procedure is ASTM F88-06 and F2029-00. Hot tack

testing is covered by ASTM F1921-98. There are also

European standards such as those from the Deutsches

Institute for Normung (DIN): DIN 55-529, 55-543, and 55-

445. These German standards will likely be incorporated

into European Union ISO standards at some time in the

future.

These standards describe how the specimens are to be

prepared, along with the equipment and methods to be

employed for testing. Tests in accordance with these

standards are extremely useful in comparing the seal

strength achieved with various materials under consid-

eration for a particular application. Seal strength and hot

tack, however, are not the only criteria to be evaluated for

many applications. Although some seals may test well in

the laboratory, they may fail in service. Reasons for this

SEALING, HEAT 1095

failure include absence of a true fusion weld of the

interfaces of the materials being sealed, among other

reasons. Occasionally, unwelded seals perform well under

mechanical testing, but fail in the actual applications.

When there is not a true fusion weld, portions of the

packaged product may ﬁnd their way out of the package

through the seal. For seals that must be reliable, particu-

larly where sterility must be ensured in a package

(see Healthcare packaging), sections of the seals should

be examined under the microscope. Unless examination

shows that the interface between the sealant portions of

the laminations has been completely obliterated, the seals

should be suspect; with a true fusion weld, there is no

visible interface remaining.

The exception to this is peelable seals. The only way to

ensure that these are acceptable for the long term is to

subject them to rough-handling testing for extended

times, exceeding the environmental extremes likely to be

encountered. If heat processing is involved, good sense

dictates that the tests be carried out for at least twice the

period of time that the product is heat processed. Tests

should also be conducted to simulate environmental pres-

sure ﬂuctuations at something in excess of the maximum

temperature expected to be used in processing. For the

packaging of chemicals, or other corrosive products, test-

ing should be carried out with ﬁlled packages, subjected to

rough handling, elevated temperatures, and any other

severe conditions that might be experienced.

When dealing with ﬂexible material constructions,

even small wrinkles in the seal area produce a statistically

signiﬁcant percentage of leakers. Aside from visual in-

spection, no satisfactory automatic system has yet been

devised to isolate packages with small seal wrinkles,

although a great deal of time and money has been spent

in this area. When developing a packaging system where

seal integrity is paramount, everything should be done in

the design of the system and its equipment to eliminate

chances for seal wrinkles. The best way is to keep the

packaging materials under tension in two directions while

they are being sealed. This can do more to achieve success

in eliminating seal wrinkles than any other single effort

that might be made. Frequently, design of the packaging

can be improved for the sole purpose of reducing the

possibility of wrinkles in the seals.

Packaging testing can be also be conducted to evaluate

the seal integrity of packages. Typically, this involves

vacuum testing or pressurization testing. These tests

can be conducted to the point of failure or set to test for

pressure decay within the package. Vacuum tests can be

conducted dry or under water. Pressure tests can be

conducted by compression or inﬂation. In inﬂation pres-

sure tests, the packages can be restrained or unrestrained.

Additional options include dye leak tests and testing for

leaking of gas out of or into the package.

Attempts have also been made to adopt online tests

for seal integrity. These include optical comparators, de-

ﬂection tests, infrared tests to evaluate the heat

proﬁle, and ultrasonic imaging techniques. All of these

techniques have advantages and disadvantages. None are

considered to give complete assurance of well-sealed

packages.

BIBLIOGRAPHY

W. E. Young, ‘‘Sealing, Heat’’ in The Wiley Encyclopedia of

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

New York, 1986, pp. 574–578.

R. D. Farkas, Heat Sealing, Reinhold, New York, 1964.

D. Grewell, A. Benetar, and J. Park, eds., Plastics and Composites

Welding Handbook, Hanser Publishers, Munich, Germany,

2003.

W. Nietzert, Welding and Heat-Sealing of Thermoplastic Films.

Zechner and Huthig Verlag, Speyer am Rhein, Germany, 1965.

W. Whiteside, PKGSC 401 Packaging Machinery Class Notes,

Clemson University, Clemson, SC, 2007.

D. Darby, Sealing Technology for Packaging Processes Seminar

Notes, Clemson University, Clemson, SC, 2007.

D. Considine, Ultrasonic Packaging Sealing-Technology and Ap-

plications Review, Herrmann Ultrasonics, Bartlett, IL, March

22, 2007.

General References

W. E. Young, ‘‘Sealing’’ in W. C. Simms, ed., Packaging Encyclo-

pedia, Cahners Publishing, Boston, 1984.

K. R. Osborn and W. Jenkins, Plastic Films: Technology and

Packaging Applications, CRC Press, Boca raton, FL, 1992.

SHEET, PETG

JOHN WININGER

Eastman Chemical Co.

Kingsport, Tennessee

PETG copolyester is a clear amorphous polymer with a

glass-transition temperature (T

)ofapproximately811C

(1781F) as determined by differential scanning calorimetry

(DSC). It is manufactured by an addition of a second glycol

to polyethylene terephthalate (PET) to eliminate crystal-

lization; therefore, all PETG-ﬁnished products are amor-

phous. PETG has a number-average molecular weight (M

)

of approximately 26,000. As with all thermoplastic polye-

sters, PETG is subject to hydrolysis (during processing) in

the melt state. Although some reduction in molecular

weight is a normal consequence of melt extrusion, insufﬁ-

cient drying of PETG can result in an excessive inherent (or

intrinsic) viscosity (IV) breakdown that is reﬂected in lower

physical properties—particularly impact strength.

A harmful degree of hydrolytic degradation can be

prevented if PETG is dried in a dehumidifying dryer at

651C (1491F) for 4 h to reduce the moisture level to less

than 0.08% before processing.

The molecular weight of either PETG pellets or ex-

truded sheet can be correlated to IV, which is normally

determined with a capillary viscometer; however, because

this procedure is complex and the required solvents are

hazardous, a simpler melt-ﬂow-rate test is preferable. The

resulting ﬂow-rate number can later be converted (via the

calibration curves) to the more standard polyester IV units.

1096 SHEET, PETG

Thermal and electrical properties of PETG copolyester

are shown in Tables 1 and 2.

Unstressed sheet extruded from PETG copolyester

exhibits good resistance to dilute aqueous solutions of

mineral acids, bases, salts, soaps, aliphatic hydrocarbons,

alcohols, and a variety of oils.

Halogenated hydrocarbons, low-molecular-weight ke-

tones, and aromatic hydrocarbons will swell or dissolve

the plastic.

A plastic sheet extruded from PETG copolyester has

good transparency, low haze, and high surface gloss. It has

high stiffness and good impact strength. The physical

properties for a 3-mm (0.12-in.) PETG sheet are shown

in Table 1.

PETG COPOLYESTER SHEET EXTRUSION

PETG copolyester can be extruded into a clear, tough,

amorphous sheet at melt temperatures that usually range

within 230–2501C (446–4821F). The sheet thickness is

limited only by the cooling capacity of the processing

rollstock and/or the capability of the downstream takeoff

equipment, but otherwise it might typically range from

1 mm (0.04 in.) to 13 mm (0.50 in.) or thicker.

The extrusion of PETG copolyester is straightforward,

and good results can be expected when suitable equipment

and proper procedures are used, which include pellet

drying, melt extrusion into sheet, and nip polishing on a

chrome-plated three-roll stack to ensure an excellent sur-

face ﬁnish and good transverse-direction gauge control.

Machine-direction gauge control is dependent on the

uniformity of extruder output. Excellent uniformity can

be obtained using a gear pump with PETG, but its use is

especially suggested when regrind is included in the

PETG feedstock.

At a given speed, a gear pump provides a constant

volumetric rate of copolymer melt to the sheet die. The

pressure between the extruder discharge and the entrance

to the gear pump is controlled by varying the extruder

screw rpm. The variation in extruder output caused by

changes in the percentage of regrind or particle shape and

size are isolated from the extrusion, die, and the effect on

the plastic sheet gauge is minimized. For example, a gear

pump set at a suction pressure of 6.9 MPa (1000 psi) would

typically control PETG sheet gauge to approximately 74%.

A barrier screw that is cored for water cooling in the

feed zone is typically used to extrude the PETG sheet.

Industry practice for all polyesters, including PETG copo-

lyester, usually requires an extruder screw with a mini-

mum length/diameter (L/D) ratio of 24:1. As the L/D ratio

increases (e.g., 30:1), output capacity (lb/h) and output

stability (/+ lb/h) will improve for PETG copolyester, if a

suitably designed extrusion screw is used.

Melt ﬁltration is normally used to trap contaminants

and/or other types of particulate matter such as ‘‘ﬁnes,’’

which can be associated with regrind use. A simple screen

pack assembly is often used with PETG copolyester, but

several different types of automated ﬁltration systems are

commercially available for polyesters.

SECONDARY FABRICATION OF PETG SHEET

The manufacture of plastic articles from PETG copolyester

sheet normally involves secondary operations such as saw-

ing, drilling, bending, decorating, and assembling. PETG

sheet can be worked with most tools that are suitable for

use with wood or metal; however, the tool speed normally

must be reduced to prevent friction melting.

PETG copolyester sheet thickness of r2.5 mm (0.10 in.)

can be cut effectively with either a power shear (straight

cut) or with a steel rule die (irregular cut). The blade-to-

bed clearance and alignment for the shear knife edge is an

important consideration when cutting PETG sheet. Addi-

tionally, a steel rule die press must have a very accurate

stroke to prevent damage to the cutting die. Saw cutting is

a preferred technique for sheet thicknesses of W2.5 mm

(0.10 in.).

FORMING

Hot Bending. PETG copolyester sheet can be bent

around a small radius by preheating an area on both sides

with an electric strip heater and then quickly bending the

sheet along the heated line. Sheet thicknesses of W3.2 mm

(0.13 in.) should be turned periodically during the heating

cycle.

Cold Bending. Unheated PETG sheet of r2.5 mm

(0.10 in.) thick can be successfully brake formed at bend-

ing

rates somewhat slower

than heated sheet.

Thermoforming. Several different thermoforming tech-

niques can be used to force a PETG sheet, once heated,

into the shape of a mold by mechanical force, vacuum

force, or by air pressure. Both male (plug) and female

(cavity) molds can be used. Low-cost plaster molds, cast

aluminum molds, and/or water-cooled steel molds are

commonly used. Typical forming processes include va-

cuum, drape, and matched mold. Thermoformed items

include light ﬁxtures, tote trays, housewares, toys, and a

variety of different transparent enclosures.

PETG Sheet Assembly. Several solvents, cements, and

adhesives can be used effectively with PETG copolyester

sheet products. Sheet edges, once aligned, are softened by

applying cement and clamping until dry.

A PETG copolyester sheet is often fabricated with

mechanical fasteners. This technique is preferred if fre-

quent disassembly of a product is required. Common

mechanical fasteners include screws, rivets, bolts, clips,

hinges, and dowels. This may be a concern, however, if

security is a consideration.

PETG Sheet Finishing. PETG copolyester sheet is best

sanded wet to avoid frictional heat buildup. An 80-grit

silicon carbide could be initially used, followed by progres-

sively ﬁner abrasives (e.g., 280, 400, and 600).

PETG Sheet Decoration. PETG copolyester sheet can be

hotdie-stamped through a ﬁlm carrier that contains either

SHEET, PETG 1097