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aesthetic closures, special-function closures, stoppers, and

overcaps.

Aesthetic Closure. The aesthetic closure is an important

sales-promotion aspect of the package. It is designed to

communicate clearly and powerfully by imagery. Original

private-mold glass stoppers used in fragrance bottles,

some with lavish sculptural representations, are an ex-

ample of this type. These are frequently the most expen-

sive forms of closure.

Special-Function Closures. These closures serve a spe-

cialized function in the marketplace. There are, for ex-

ample, closures that vent containers which sustain a

pressure buildup. Such pressures can cause the container

to rupture, or violently expel the product when the cap is

removed. Since venting closures can leak when the con-

tainer is not in an upright position, they must be used in

controlled circumstances. Many manufacturers require a

‘‘hold-harmless’’ agreement as a condition of sale for such

closures. Another special-function closure is the twist-off

closure for injectables, a two-piece aluminum cap used on

parenteral vials.

Stoppers. The wine and champagne industry is the

largest user of stoppers. Cork stoppers are standardized

by size and grades, the latter according to the degrees of

product vintage (28). Stoppers of natural rubber, synthetic

silicone rubbers, and thermoplastic materials provide

closure in some chemical and biological applications.

Rubber plug closures are crimped onto ampules with

metal bands and allow for the insertion of a hypodermic

needle in medical uses.

Overcaps. The overcap is a secondary cap designed to

protect the primary closure, dispenser, or ﬁtment of a

container. Metal or plastic overcap designs attach to the

container by friction-ﬁt or thread engagement, and they

are used to protect aerosol and dispensing ﬁtments. Over-

caps frequently double as measuring caps for mouthwash,

liquid detergents, and fabric softeners.

SEALING SYSTEMS

Though often the smallest aspect of a package, the seal is

responsible for keeping the entire concept intact. If the

seal is not maintained by the closure, liner, and container

working together, the success of the product is at stake.

Liners

Today’s lining material is either a single substance

(usually paperboard or thermoplastic) or a composite

material. Synthetic thermoplastic liners include foamed

and solid plastics of varying densities. A composite lining

material consists of a backing and a facing. The backing,

usually made of cellulose or thermoplastic, is designed to

provide the proper compressibility to affect the seal and

proper resiliency for resealing. Facing materials, repre-

senting the side of a composite liner that comes into direct

contact with the product, are numerous, as are the vari-

ables of product chemistry with which they must contend.

Generally, facing materials are thermoplastic-resin-coated

papers, laminated papers of foil or ﬁlm, or multilayer

types devised for special applications (see Closure liners).

Innerseals

The innerseal affords TE protection by sealing the mouth

of the container. Three common types are inserted by the

closure manufacturer into the cap (29). A waxed-pulp

backing and glassine innerseal is common within the

food industry. After the ﬁlling operation the container

runs under a roller system which applies an adhesive to

the lip of the container, and then the cap is applied. Upon

removal, the glassine adheres to the container while the

pulp backing remains in the closure. Pressure-sensitive

innerseals, generally a foamed polystyrene, adhere to the

lip upon application and require several hours to set.

Heat-induction innerseals are plastic-coated aluminum

foils, often adhered to a waxed pulp base liner. After the

cap is applied, the container passes under an electromag-

netic ﬁeld which causes the aluminum to generate heat.

The plastic facing on the aluminum subsequently melts

and adheres to the container.

Linerless Closures

Plastic linerless closures provide a positive seal in certain

circumstances, foregoing the need for intermediary mate-

rials and secondary liner-insertion operations. To many

Figure 14. Child-resistant ‘‘snap-off closure’’. (Courtesy of S . Kiefer

and B. Zemlo, Phoenix Closures, Inc.)

278 CLOSURES, BOTTLE AND JAR

packagers, the cost savings provided by the linerless

closure can be considerable. The seal of a linerless closure

is achieved by molded embossments forming diaphragms,

plugs, beads, valve seats, deﬂecting seal membranes, or

rings which press upon, grasp, or buttress the sealing

surfaces of the container. Over a dozen types of linerless

closures are in common use, each designed to provide a

seal at one or more critical sealing surfaces of the con-

tainer, which include the land surface, the inside edge of

the land surface, or the outside edge of the land surface.

Some form of land seal in conjunction with a valve or

ﬂange represents one type of effective linerless closure

design. The land is typically the most consistent sealing

surface. A land-seal ring can bite into plastic container

ﬁnishes or deﬂect on glass ﬁnishes. An inner buttress can

correct ovality problems in plastic containers by forcing

such off-round ﬁnishes back into proper shape.

CLOSURE MATERIALS

Closures are made of plastics, metal, or glass.

Plastic Closures

Molded plastic closures are divided into two groups:

thermoplastics (e.g., polyethylene, and polypropylene,

and polystyrene) and thermosets (e.g., phenolic resins

and urea components) (see Polymer properties). Thermo-

plastic materials can be softened or recycled by heat;

thermoset materials cannot be recycled once they are

molded.

Thermoplastics. In general, thermoplastic closures offer

the packager light weight, versatility of design, good

chemical resistance to a wide range of products, and

economical resins and manufacturing processes. Their

relative ﬂexibility is essential to contemporary closure

design with its emphasis on convenience and control

devices. Thermoplastics provide good application and

removal torque. They maintain a good seal and tend to

resist back-off. Unlike thermosets, thermoplastics can be

pigmented in the full-color spectrum in strong, faderesis-

tant intensities.

Most thermoplastic closures are produced by injection

molding (see Injection molding), although some are made

by thermoforming (see Thermoforming). Polypropylene

and polyethylene account for about 90% of all thermo-

plastic closures.

Polypropylene. Polypropylene (see Polypropylene) has

unusual resistance to stress-cracking, an essential char-

acteristic of hinged closures. In thin hinged sections, it has

the quite remarkable property of strengthening with use.

The homopolymer has limited impact resistance, but it

can be modiﬁed for better performance. It has excellent

resistance to acids, alkalies, oils and greases, and most

solvents at normal temperatures. It has the best heat

resistance of the polyoleﬁns, with a high melting point

suitable for sterilized products, but it becomes embrittled

at low temperatures. Polypropylene has better printability

than polyethylene, but both are inferior to polystyrene or

thermosetting plastics in that respect. As a relatively rigid

molded material, it has outstanding emboss and deboss

potential for closure communications.

Low-Density Polyethylene. LDPE (see Polyethylene, low

density) is resilient and ﬂexible. It is relatively tasteless

and odorless, although some organoleptic problems are

more prevalent with LDPE than with polypropylene. It

provides outstanding moisture protection, but it is not a

good gas barrier. LDPE’s economy as a closure material is

provided by low-cost resins and relatively short injection-

molding cycle times. Though it is considered to have good

resistance to stress cracking, problems may occur in the

presence of certain chemicals such as detergents. Com-

munications embossments or debossments are good but

limited by the softness of the material.

High-Density Polyethylene. Compared to LDPE, HDPE

is stiffer, harder, and more impermeable (see Polyethy-

lene, high-density). It is tasteless, odorless, and impact-

resistant, but it will stress-crack in the presence of some

products such as detergents unless it is specially formu-

lated. Its heat resistance and barrier properties are super-

ior to LDPE. HDPE resin is more expensive than LDPE,

but it is still considered a relatively low-cost material. A

particular drawback to HDPE closures is a potential for

warpage and loss of torque.

Polystyrene. Polystyrene (see Polystyrene) is used for

about 10% of the closures produced today. Polystyrene

homopolymer is attacked by many chemicals, is very

brittle, has relatively low heat resistance, and does not

provide a good barrier against moisture or gases. Many of

the disadvantages of polystyrene are overcome by rubber

modiﬁcation and/or copolymerization.

Thermosets. Phenolic and urea compounds have a wide

range of chemical compatibility and temperature toler-

ances. Some thermosets can sustain subzero temperature

without embrittlement and can survive at temperatures

higher than 3001F (1491C) (30). The density and rigidity of

thermosetting plastics give the material its heavy weight

and guard against slippage over threads, a problem with

softer thermoplastics such as LDPE. Thermosets cannot

provide the color range or intensity of thermoplastics, but

they accept vacuum metalizing decoration in silver and

gold

with superior adhesion

qualities. During the mold-

ing process, thermosets undergo a permanent chemical

change and cannot be reprocessed as thermoplastics can.

Thermoset closures are manufactured by compression

molding (see Compression molding). Cycle time for ther-

mosets is generally longer than thermoplastics, (30–120 s),

depending upon thickness of the product and additives.

Phenolics. Phenol–formaldehyde closures are hard and

dense. They are the stiffest of all plastics, but are rela-

tively brittle and low in impact strength. The properties of

phenolics depend to a large extent upon the ﬁller material

used. Wood ﬂour improves impact resistance and reduces

shrinkage. Cotton and rag ﬁber additives increase the

CLOSURES, BOTTLE AND JAR 279

impact strength; asbestos and clay additives improve

chemical resistance. Phenolics are resistant to some dilute

acids and alkalies and attacked by others, especially

oxidizing acids. Strong alkalies will decompose phenolics,

but they have excellent solvent resistance. Their heat

resistance is outstanding. Phenolics cost less than ureas

and are easier to fabricate, but they are limited in color to

black and brown unless decorated.

Urea. Urea–formaldehyde is one of the oldest plastic

packaging materials, ﬁrst used in the early 1900s. The

resin produces extremely hard, rigid closures with excel-

lent dimensional stability. It has the highest mar resis-

tance of plastics discussed, but is the most brittle. Urea

compounds are odorless and tasteless, with good chemical

resistance. They are not affected by organic solvents but

are affected by alkalies and strong acids. They show good

resistance to all types of oils and greases. They will

withstand high temperatures without softening. Urea

compounds are available in white and a wide range of

colors, but with muted intensities compared to thermo-

plastics. Urea compounds, like phenolic resins, do not

build up static electricity, which leaves them free of

dust. They are the most expensive of the plastic closure

materials.

Metal

Metal caps, the strongest of closures, are used today for

general, vacuum, and pressurized applications. Tin-plate

and tin-free steel (see Tin mill products) are used in

the production of continuous thread, and vacuum press-

on closures, lugs, overcaps, and crown caps. The largest

market for steel closures is vacuum packaging. Aluminum

closures are primarily continuous thread caps and roll-on

designs.

Steel Closures. Steel closures are of two materials: tin-

plate and tin-free steel. Tin-plate closures are plated steel

with a thin coating of tin on both sides that helps protect

the base steel from rust and corrosion. There are limita-

tions to tin’s protective abilities, however, because tin

plate is susceptible to rust when exposed to high humidity.

Additional coating operations offer increased protection.

Tin plate is graded according to temper. Temper T-1 is soft,

and T-6 is quite hard. Closures can be fabricated in any

temper, but are predominantly T-2 to T-5. The more

common base weights used include single-reduced 80-

and 90-lb (36.3- and 40.8-kg) with a cost-reducing trend

toward double-reduced 55- and 65-lb (25- and 29.5-kg)

plate (15).

Tin-free steel shows promise of becoming the dominant

steel closure material. Crown closures are now made

primarily of tin-free steel produced as single-reduced

stock in a 90-lb (40.8-kg) plate weight for conventional

crowns and a lighter, 80-lb (36.3-kg) plate for twist-off

crowns.

Aluminum Closures. Light weight, malleability, and

resistance to atmospheric corrosion characterize alumi-

num closures. Some products more corrosive to aluminum

than tin plate require special coatings for optimum pro-

tection (31). The composition of aluminum alloys varies

according to intended use with up to 5% magnesium and

lesser elements such as manganese, iron, silicon, zinc,

chromium, copper, and titanium (32).

Metal-Overshell Closures. Steel, aluminum, copper, or

brass shells slip over plastic closures to form composite

‘‘overshell’’ caps. Freed from ﬁnish contours, the smooth,

often-tall sidewalls of polished and burnished metals

provide aesthetically pleasing characteristics much in

demand by the cosmetic and fragrance industries. There

is a greater willingness to pay a premium for appearance

in these industries because the closure assumes a greater

role in sales promotion.

Glass

Glass stoppers are used in commercial glassware and in

cosmetic and fragrance packaging. Frequently, a polyethy-

lene base cap assists in friction-ﬁtting the stopper into the

bottle. Stoppers for the premium fragrance industries

represent superlative designs in molded glass.

CLOSURE SELECTION

Selection

Who selects and speciﬁes the closure depends upon the

size, nature, and organization of a company. It may be a

president or general manager, a brand manager, package

engineer, purchasing agent, or a packaging committee.

General guidelines for closure selection are provided

below by ‘‘The 5 Cs of Closure’’ (33).

Containment

The essential requirements of containment are product

compatibility and the ability to provide functional protec-

tion. This objective is reached by evaluated choices in

closure method, type, material, and sealing system. De-

termining the sealing system, for example, may involve

decisions as to whether lining materials or a linerless

closure will resist permeation of the product to standards.

Other important variables arise in the interaction

of closure and container and how they affect the efﬁcacy

of engagement and seal. Torque considerations include

seal pressure (the amount of pressure exerted on sealing

surfaces) and strip torque (the torque at which a closure

slips over the container threads). As torque is affected by

different coefﬁcients of friction between the liner surface

and the container, as well as by materials used in closure

manufacture, each closure system should be individually

evaluated to ensure it meets applicable performance

criteria.

Convenience

Opportunities for convenient dispensing may begin with a

containment closure that provides a reduced number

of turns, or broader ‘‘knurls’’ on the wall of the closure

skirt to provide surer opening and closing. Convenience

280 CLOSURES, BOTTLE AND JAR

closures provide many options including simple spouts,

plug-oriﬁce snap caps, and, at the mechanically complex

end of the continuum, variable-dispersion sprayers and

pumps. The method of closure engagement, the require-

ments of containment, the type of sealing system required,

and the premium placed upon convenience will determine

options in dispensing closures.

Control

A variety of substances are mandated by law to be pack-

aged as tamper-evident or child-resistant. Cost and seal-

ing needs will determine options in control closures, as

well as whether secondary sealing systems are required.

More consumer complaints result from inadequate open-

ing and closing of product containers than any other

package function. Careful review and testing of control-

closure and lining system can prevent potential access

problems with elderly or handicapped consumers.

Communications

The shelf appearance of a closure is perceived as a

reﬂection of product quality. The closure communicates

this by style and brand signature. In addition it often gives

Table 1. Common Finishes and Descriptions

Finish Designation Description

120 Two-lug Amerseal quarter-turn ﬁnish

140 Four-lug Amerseal quarter-turn ﬁnish

160 Six-lug Amerseal quarter-turn ﬁnish

326 Pour-out snap cap CT combination

327 Snap cap CT combination

400 Shallow CT ﬁnish

401 Wide sealing surface on 400 ﬁnish

405 Depressed threads of 400 ﬁnish at

mold seam

410 Medium CT concealed-bead ﬁnish

415 Tall CT concealed-bead ﬁnish

425 8- to 15-mm shallow CT

430 Pour-out CT

445 Deep S CT ﬁnish

450 Deep CT Mason ﬁnish

460 Home-canning jar ﬁnishes

600 Beverage crown ﬁnish

870 Vacuum side seal pry-off

1240 Vacuum lug-style ﬁnish

1337 Roll-on pilferproof ﬁnish

1600 Roll-on ﬁnish

1620 Roll-on pilferproof ﬁnish

1751 Twist-off vacuum seal

Table 2. CT Closure Finishes and Pitch

Finish Description Sizes (mm) Threads per inch (Pitch per centimeter)

400 Shallow continuous thread 18, 20, 22, 24 8 (3.2)

28, 30, 33, 35, 38, 40 6 (2.4)

43, 45, 48, 51, 53, 58 6

60, 63, 66, 70, 75, 77 6

83, 89, 100, 110, 120 5 (2.0)

410 Medium continuous thread 18, 20, 22, 24 8

28 6

415 Tall continuous thread 13, 15 12 (4.7)

18, 20, 22, 24 8

28 6

425 Shallow continuous thread 8, 10 14 (5.5)

13, 15 12

430 Pour-out continuous thread 18, 20, 22, 24 8

28, 30, 33, 38 6

445 Deep ‘‘S’’ continuous thread 45, 56, 58, 63, 73, 75 6

77, 83 5

450 Deep CT Mason ﬁnish 70, 86, 96, 132 4 (1.6)

455 CT for 455 glass ﬁnish 28, 33, 38 8

460 Home canning jars 70, 86 4

470 CT for GPI 470 glass ﬁnish 70, 86 4

480 CT for GPI 480 glass ﬁnish 24, 28, 33, 38 6

485 Deep ‘‘S’’ ﬁtment cap 28, 33, 35, 38, 40 6

43, 48, 53, 63

490 Deep ‘‘S’’ larger ‘‘H’’ ﬁtment cap 18, 20, 22, 24, 28, 30 8

33, 35, 38, 43, 48, 63

495 CT for GPI 495 glass ﬁnish 28, 33, 38 8

SP 100 CT for plastic SP-100 ﬁnish 22, 24, 26, 28, 38 8

SP 103 CT for plastic SP-103 ﬁnish 26 8

SP 200 CT for plastic SP 200 ﬁnish 24, 28 6

SP 444 CT for plastic SP 444 ﬁnish 24, 28, 33, 38, 43, 45, 48 6

53, 56, 58, 63, 73, 75 6

70 4

83 5

CLOSURES, BOTTLE AND JAR 281

a detailed list of ingredients, and sometimes instructs on

disengagement. The larger the closure the more it may

augment the label in communicating ingredients or nutri-

tional information. Today’s closure not only communicates

visually, but audibly as well. Steel vacuum button closures

‘‘pop’’ to conﬁrm the freshness of a product, and polypro-

pylene plug-oriﬁce types ‘‘snap’’ when the seal is engaged.

Determining the kinds of communications required, and

selecting the graphic options which maximize readibility

and impact, specify the ﬁnal appearance, or point-of-

purchase impact, of the closure.

Cost

Some cost considerations depend on production require-

ments. Thermoplastic-mold costs, for example, are gener-

ally more expensive than those for thermosets, but faster

cycles and resin economy may prove more economical in

larger volumes. A cost savings can be realized through the

use of linerless closures if they can maintain seal integrity.

Lightweighting the closure by selection of an appropriate

material has helped to reduce transportation costs for

packagers. Another cost consideration is whether a ‘‘stock’’

or a ‘‘privately tooled’’ closure is required. A privately tooled

closure is far more expensive to produce, but, again, the

packaging concept or production volumes may ‘‘economize’’

it. There is a trend toward specifying stock closures with

market-tested designs. Many manufacturers and distribu-

tors offer extensive stock-closure lines to the packer.

CLOSURE SPECIFICATION

A closure is designated by a series of numbers and/or

letters. An example is the designation 48–400. The ﬁrst

number refers to the inside diameter of the closure as

measured in millimeters. Common closure diameters

range from 22 to 120 mm.The second set of numbers, the

400, is the ﬁnish designation.

The ‘‘ﬁnish’’ of a closure is its thread design, and the

size, pitch, proﬁle, length, and thickness of the engage-

ment threads on plastic and metal closures and contain-

ers. Today there are over 100 glass-ﬁnish designations for

a great variety of glass containers (see Table 1). Series

designations for the most popular CT closures are 400 and

425 for shallow continuous thread designs, 410 for med-

ium CTs, and 415 for tall CTs. For all glass ﬁnishes,

tolerances have been established by the Glass Packaging

Institute, whose closure committee became the Closure

Manufacturers Association (CMA) in 1980. Voluntary

standards for closures have been issues by CMA which

include closures for both glass and plastic container

ﬁnishes (see Table 2) (34).

Closures developed for glass containers were used for

plastic containers when the latter were introduced, but it

was soon realized that the contour of a glass bottle thread is

not an optimum proﬁle for the plastic bottle. It does not

provide accurate closure centering on the ﬁnish, nor does it

permit higher capping torques required to provide a positive

seal on plastic containers (35). The Dimensional Subcom-

mittee of the Society of the Plastics Industry developed

speciﬁc ﬁnish dimensions, tolerance, and thred contours for

blown plastic bottles. The two basic contours are the M-style

and the L-style. Where a typical glass thread is rounded in

contour, the M-style thread engaging surfaces are angled at

101 and the L-style is angled at 301. Both contours increase

sealing abilities for closures on plastic bottles.

Four critical closure dimensions are represented by

the four letters T, E, H, and S (see Figure 15). T is the

dimension of the root of the thread inside the closure. E is

the inside dimension of the thread in the closure. H is the

measurement from the inside top of the closure to the

bottom of the closure skirt. S is the vertical dimension

from the inside top of the closure to the starting point of

the thread. These critical closure dimensions and toler-

ances for metal and plastic closures designed for glass and

plastic containers are represented in the voluntary stan-

dards for closures as issued by CMA.

CLOSURE TRENDS

Closure concepts seem to have changed more in the last

40 years than in the last 4000 years. Yet changes in state-

of-the-art concepts, materials, and manufacturing processes

do not represent the real driving forces behind today’s

closure developments. The industry is consumer-driven.

More and more , a premium is readily paid for convenience.

The industry has also been awakened, sometimes with great

shock and alarm, to the powers of human foible, which

demands a redeﬁnition of access control.

Functional Trends

Today’s consumer has been characterized as oriented to-

ward health, diet, appearance, longevity, and convenience

Figure 15. T, E, H, and S dimensions.

282 CLOSURES, BOTTLE AND JAR

(36). Households with two working spouses, increased

single households, and retiree households all command a

market for convenience packaging (37). The dispensing

closure is no longer a functional appendage, but is seen

as an integral part of the total package (38). Today’s

convenience closure is time- and labor-saving. It prevents

spills, leaks, and drips. It provides measured-dose dispen-

sing and can visually signal tampering (39). As plastic

containers continue to penetrate the food market, the

closure will broaden squeeze-dispensing. Other functional

trends include the expansion of TE food packaging, larger

closure sizes, increased use of stock caps to avoid private-

mold costs, and new concepts in linerless closure design.

Since innovative packaging can increase market share,

special emphasis is being placed on improved tamper-

evidence, child-resistance, and convenience designs. These

functions will no doubt become more and more integrated

into one cap. Closures are now being marketed which

provide for both TE and CR.

Conclusion

The ductility of plastics accounts for the fast progress of

plastic closures, which will undoubtedly take a still-larger

share of the market in years ahead (40,41). Polypropylene

represents the largest volume and highest growth plastic

material with 200 million (10

) lb (90,700 tons) used in the

production of plastic closures in 1984, an increase of 33%

within 4 years (42). Polystyrene has shown slight growth

in recent years, with 72 million lb (32,700 tons) used in

closure production in 1984. Closures accounted for 66

million lb (29,900 tons) of HDPE consumption, an amount

that has remained relatively stable over recent years.

LDPE (37 10

lb or 16,800 tons) and PVC (35 10

lb or

15,900 tons) have both remained stable in recent years. As

for thermosets, 15 10

lb (6800 tons) of phenolic resins

and 11 10

lb (5000 tons) of urea compounds were used

in closure production in 1984 reﬂecting little growth in

recent years.

Metal thread-engagement closures continue to assume

a position in the food and pharmaceutical industires due

to new fabricating, plating, and light-weighting tech-

nologies which will keep steel and aluminum closures

competitive.

BIBLIOGRAPHY

J. F. Nairn and T. M. Norpell, ‘‘Closures, Bottle and Jar’’ in

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Cited Publications

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2. Ibid., pp. 210–212.

3. H. Higdon, ed., The Phoenix Flame (a house magazine of the

Phoenix Cap Company, Chicago, now Phoenix Closures,

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4. A. Lief, A Close-Up of Closures, Glass Container Manufac-

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(85)-1, U.S. Department of Commerce, Washington, D.C.,

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158–160 (1984).

9. J. F. Hanlon, Handbook of Package Engineering, McGraw-

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10. ‘‘Tamper-Evident Packaging Requirements,’’ Fed. Reg.

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11. J. B. Carroll, Memorandum from the Closure Manufacturers

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12. B. Knill, Food Drug Packag. 48, 5 (1984).

13. ‘‘Quantity and Value of Metal and Plastic Closures,’’ Current

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14. Ref. 5, p. 152.

15. Steel Cans, Report by the Committee of Tin Mill Products

Producers, American Iron and Steel Institute, Washington,

D.C., 1984, p. 8.

16. Ref. 9, Section 6, p. 20.

17. Ref. 4, p. 29.

18. P. Zwirn, ‘‘The Crown is Still King,’’ Can. Packag. 37(6), 24

(1984).

19. ‘‘The Best Ideas in Packaging,’’ Food Drug Packag. 39, (1984).

20. C. G. Davis, Packag. Technol. 27, 27 (1982).

21. R. Heuer, Packaging, 29, 34 (April, 1984).

22. R. L. Harris, ‘‘Closures, Dispensing Systems,’’ Packag. Encycl.

29,

157 (1984).

23. T

. T. Williams, A History of Technology, Clarendon Press,

Oxford, 1978, p. 1411.

24. ‘‘Tamper-Resistant Packaging Requirements,’’ Fed. Reg.

47(215), 50444 (1982).

25. H. Forcinio, Food Drug Packag. 29, 35 (1984).

26. Child Resistant Packaging, American Society for Testing and

Materials, Philadelphia, pp. 20–21.

27. ‘‘CPSC Replies to Queries on CR Rules Compliance,’’ Food

Drug Packag. 40, 20 (1985).

28. Ref. 9, Section 9, p. 21.

29. Options for Successful Medical/Pharmaceutical Packaging,

Bulletin PB-484, Phoenix Closures, Inc., Naperville, IL., April

1984, p. 2.

30. L. Roth, An Introduction to the Art of Packaging, Prentice-

Hall, Englewood Cliffs, NJ, 1981, p. 153.

31. Ref. 9, Section 7, p. 16.

32. Ref. 9, Section 7, p. 31.

33. ‘‘The 5 C’s of Closure,’’ Marketing Communications Bulletin,

Phoenix Closures, Inc., Naperville, IL., December 1, 1984.

34. CMA Voluntary Standards, Closure Manufacturers Assoc.,

McLean, VA, 1984.

35. J. Szajna, Food Drug Packag. 29, 12 (1984).

36. A. J. F. O’Reilly, Food Drug Packag. 29, 72 (1984).

37. H. K. Foster, Food Drug Packag. 29, 46 (1984).

38. Ref. 21, p. 36.

CLOSURES, BOTTLE AND JAR 283

39. B. Miyares, Food Drug Packag. 29, 72 (1984).

40. R. Graham, Food Drug Packag. 29, 42 (1984).

41. H. Peter Aleff, ‘‘Comparison of Thermoplastic with Thermoset

Closures,’’ Packag. Technol. 12(2), 18 (1982).

42. ‘‘Materials 1985,’’ Mod. Plast., 63(1), 69 (1985).

General References

H. L. Allison, ‘‘High Barrier Packaging: What are the Options?,’’

Packag. 30, 25 (1985).

J. Agranoff, ed., Modern Plastics Encyclopedia, Vol. 60, McGraw-

Hill, New York, 1984. Note: The Engineering Data Bank

section contains extensive data on properties of plastics, along

with design data, chemicals and additives information, and

data on molding machinery.

L. Barail, Packaging Engineering, Reinhold Publishing Corpora-

tion, New York, 1954.

The Closure Industry Report, Closure Manufacturers Association,

McLean, VA, fall 1983 and summer 1984.

E. F. Dorsch, ‘‘Closures and Systems,’’ Packag. Encycl. 30, 126–

128 (1985).

R. C. Grifﬁn, Jr., and S. Sacharow, Principles of Packaging

Development, The Avi Publishing Co., Westport, CT., 1972.

K. Hannigan, ‘‘Baby Food Closure Grows Up,’’ Chilton’s Food

Engineering, Chilton Company, Radnor, PA, 1984.

G. Jones, Packaging: A Guide to Information Sources, Gale

Research Company, Detroit, MI, 1967.

Kline’s Guide To The Packaging Industry, Charles H. Kline & Co.,

Inc., Fairﬁeld, NJ, 1979.

E. A. Leonard, Packaging Economics, Books for Industry, New

York, 1980.

‘‘Mack-Wayne Introduces New One-Piece Dispensing Closure and

New Breakaway Closure,’’ Packag. Technol. 14(1), 34 (1984).

‘‘Packaging: Big Changes Ahead,’’ Prepared Foods 153, 60–70

(1984).

‘‘Plastics Hit the Market with Tamper Evident Closures,’’ Chil-

ton’s Food Eng. 56, 60 (1984).

N. C. Robson, ‘‘The State of the Anti-Tampering Art,’’ Packag.

Technol. 13, 26 (1983).

S. Sacharow, A Guide to Packaging Machinery, Books for Industry,

New York, 1980.

S. Sacharow, A Packaging Primer, Books for Industry, New York,

1978.

B. Simms, ed., ‘‘Closures, Dispensers and Applicators,’’ Packag.

Encycl. 29 (1984).

CLOSURES, BREAD BAG

G. E. GOOD

Kwik Lok Corporation

INTRODUCTION

Packaging bread in polyethylene bags is an almost uni-

versal method of bread packaging. Customer preference

for easy opening and reclosing the package inﬂuenced the

development of bag closures at the very start of bread

bagging. Closing by tape, which had been used in connec-

tion with early bagging methods, was quickly discarded

when the bulk of the bakery production lines changed to

the new polyethylene bagging and closing equipment.

Tape was neither convenient for the consumer nor fast

and dependable for the bakers. Two types of closures and

automatic equipment systems have emerged as the stan-

dards of the baking industry. Developed simultaneously,

these are the wire tie and the plastic-clip closure.

WIRE TIES

The predominant automatic wire-tie equipment now being

used by the United States baking industry is from the

Burford Corporation, Maysville, Oklahoma. Different

models are required to attach to the different types of

installed bread and bun baggers. This type of automatic

bag-closing equipment is limited to about 60 packages per

minute. The wire ties used in automatic application are

4 in. (10.2 cm) long. The wire comes on reels of 6000 ft

(1829 m), 18,000 closures per reel, and is cut to length as it

is applied to the package. Colors are available for color-

coding purposes (see Figure 1).

The wire tie has a left- or right-hand twist, depending

on the production equipment. Wire ties are available with

all-plastic, laminated plastic/paper, along with all-paper

covers over the wire core. The quality of the wire tie must

be carefully maintained; otherwise, separations of the

laminations occur, leaving the bare wire exposed. Source

of the wire ties is Bedford Industries, Worthington, Minn.

PLASTIC-CLIP CLOSURE

Plastic-clip closures are produced from medium-impact

polystyrene (see Polystyrene). The material has a resi-

liency that allows it to return to its original shape after

many reuses by the consumer (see Figure 2). They are

Figure 1. A wire tie for a bread bag.

284 CLOSURES, BREAD BAG

furnished with various aperture sizes to accommodate the

different polyethylene bag widths and thicknesses. The

proper selection of aperture size can result in an almost

air-tight package. Colors are available for color-coding

purposes. The plastic-clip closures are provided in reels

of 4000 or 5000 closures per reel.

The Striplok closures and closure-applying equipment

are manufactured by the Kwik Lok Corporation, Yakima,

Washington. The equipment is simple and dependable,

with production speeds up to 120 bakery bags per minute.

The Kwik Lok Corporation manufactures two basic ma-

chines that attach to any of the common bagging machines

from Formost Packaging Machines, Inc., Woodinville,

Washington; United Bakery Equipment, Compton, Cali-

fornia; and AMF, Inc., Union Machinery Division, Rich-

mond, Virginia. By mounting an imprinter on the closing

machine, imprinting can be done on the plastic clip. Many

bakeries are required to print on the package the price per

pound, the unit price of the package, and the ‘‘sell by’’ date

(see Code Marking and Imprinting). Printing on the

polyethylene bag lacks legibility and is often obscured by

the package graphics. Three sizes of Striplok closures are

available to facilitate compliance with regulatory coding

and dating requirements.

BIBLIOGRAPHY

G. E. Good, ‘‘Closures, Bread, Bag’’ in M. Bakker, ed., The Wiley

Encyclopedia of Packaging Technology, 1st edition, John Wiley

& Sons, New York, 1986, pp. 185–186; 2nd edition, A. J. Brody

and K. S. Marsh, eds., 1997, p. 220.

COATING EQUIPMENT

STEFANO FARRIS

Food Science and Microbiology,

University of Milan, Milan,

Italy

Coating is deﬁned as the process of applying at least one

layer of a ﬂuid or melt substance onto the surface of a

Figure 2. A plastic-clip closure for a bread bag.

COATING EQUIPMENT 285

material (paper, plastic, metal) in the form of ﬁlms, sheets,

or shaped structures (i.e., bottles and jars). The coating

process differs from laminating, which encompasses all

processes in which two or more plastic or nonplastic

materials (paper, aluminium foil, cellophane) are com-

bined together to form a composite multilayers structure.

Bonding between the different layers can be achieved by

means of several techniques, including thermal or chemi-

cal treatment, use of tie layers (adhesives), and curing

systems. As for the lamination, the main advantage aris-

ing from the coating process is obtaining composite struc-

tures having speciﬁc properties and characteristics not

available in any of the individual parts taken alone. Some

functional performances that can be improved, for in-

stance, include mechanical properties, barrier to gas and

water vapor, and acceptance of printing and labeling. In

addition, it is a common practice to use multilayered ﬁlms

for food packaging applications for an economical reason.

In fact, combining different ﬁlms often lead to a reduction

of the costs that should be incurred if monolayered

structures were used for the same purpose. This is be-

cause each different single thin layer can function in a

speciﬁc way (e.g., as a barrier against light or gases, as a

heat or cold sealant, as a protection against abrasion),

differently from the performances of a single layer.

Even if similar to other processes (e.g., lamination),

some prerequisite properties of materials are speciﬁc for

the coating method. These include: the capacity to be

applied from high-speed rolls without forming foam; the

ability to be spread uniformly and quickly over the wide

surfaces of fast-moving substrates; generating fairly

strong initial bonds to enable handling in the coating

machinery; and rapid conversion to the ﬁnal desired forms

as well as the retention of necessary bonding after drying

or curing. In addition, the coating process is useful and

suitable only when very thin layers have to be applied

(from about 0.5 to 15 mm). For thickness greater than

15 mm, solution coatings or dispersions are not often

applied, except by different subsequent steps. Finally,

coating when applied must be sufﬁciently ﬂuid to be

spread into an evenly thin layer across the web. For that

reason, the only polymers practical as coatings are those

that can be made directly by emulsion polymerization.

Therefore, coatings are applied as solutions in organic

solvents, as aqueous solutions or emulsions, as a hot melt

(solid molten or softened by heat), or as a reactive liquid

that solidiﬁes by a polymerization reaction induced either

thermally or by radiation. Extrusion coating, which is

similar to hot-melt coating, is discussed separately (see

Extrusion Coating).

Coating equipment and processes have been reviewed

in several books (1–7). As a result of new materials being

recently developed and the always increasing demand

for high-performance solutions, an interest in the world

of coatings has rapidly increased, and the opportunity

of using new deposition techniques has become broad

and various. Consequently, different equipments have

been developed, and summarizing them in few pages

will not be an easy task. Dedicated and updated textbooks

provide, in an exhaustive manner, detailed information

(8–10).

Coating is usually applied to the web material wound

in rolls. This process requires unwinding the web, apply-

ing coating, drying, and then rewinding the web again. For

this purpose, different coating and handling equipment

are required. Coating equipment is intended as the collec-

tion of machines and devices used to put into practice the

related process. It consists of four main components: (1) a

surface treatment device, (2) a coating head, (3) a dryer or

other coating solidiﬁcation station, and (4) web handling

equipment and accessories (drives, winders, edge guides,

coating thickness controls, etc.).

SURFACE TREATMENT DEVICES

Some webs (e.g., polyoleﬁn ﬁlms) show low surface en-

ergies such that the application of a coating is very

difﬁcult to achieve. For that reason, these substrates

must be treated before the coating process, in order to

modify their surface energy level and hence increase the

probability of a satisfactory bond between them and the

coated substances. The same treatments are needed for

other processes such as printing or laminating. The most

important concept is that after the treatment, the surface

energy of the polymer (i.e., its surface tension) must be

higher than that of the coated substance to accomplish the

right bond between components.

Different methods can be employed for surface treat-

ment: corona discharge, gas plasma, etching with chemi-

cals of continually moving webs, and gas ﬂame treatment

of molded objects for example. The most widely used is

corona discharge, which consists of impinging a high

energy electrical discharge on a surface, leading to a

surface more suitable to adhere to other substances.

Plastic ﬁlm is unrolled between both a metal roll and a

high voltage source, producing a voltaic arc (also called

corona) which generates oxidation and other effects on the

surface. In most cases, webs are treated in line (Figure 1),

just before they undergo the speciﬁc coating process. This

is because of the disappearing nature of the treatment,

which effect gradually dissipates over time. Ozone is a

byproduct of the corona-discharge method, and provision

must be made for its removal.

Different methods can be used in practice for evaluat-

ing the goodness of the treatment, such as ink retention,

contact angle, and water spreading. However, the most

widely

used is that

referring to the ASTM standard

method (11). Further methods have been successfully

set, as the simple and rapid one to be used in production

proposed by Sprecher (12).

COATING HEADS

The coating head accomplishes two functions: applying

the coating to the substrate and distributing a metered

amount uniformly over the surface. Metering may be

combined with the coating application, or it may be

carried out separately immediately following the coating

deposition. There are many designs of coating heads, but

they fall into three major categories: (1) roll coaters; (2)

286 COATING EQUIPMENT

knife, blade, and bar coaters; and (3) hot melt coating.

General characteristics of most important coating heads

are shown in Table 1.

Roll Coaters

Roll coaters are most widely used and can be subdivided

into several major types: direct, reverse, gravure and

calender coaters.

Direct Roll Coaters. In this case, substance is deposited

by contact between the moving web and the rotating

applicator roll. The roll picks up coating from a speciﬁc

source, either a different roll (transfer roll) or a bath, and

then transfers it onto the web surface. The web carrying

roll and the coating applicator roll rotate in the same

direction. The thickness of the coating can be modiﬁed by

the speed of both the application roll and the web, and also

by considering the physical properties of the coating and

those of the web. In fact, the more viscous the coatings and

the lower the speeds for their application, the thicker will

be the ﬁnal dry coatings. Conversely, the thinnest will be

obtained with low-viscosity coatings applied at high

speeds. The coating is split evenly between the roll and

the substrate as shown in Figure 2. Coating splitting

produces an uneven surface, which sometimes can be a

problem. For that reason, a steel backing roll covered

by rubber represents a useful tool for accommodating

some irregularities in thickness of the substrate. This is

particularly evident when working with ﬁbrous webs (e.g.,

paper). They offer a sponge-like surface that quickly

absorbs water from wet coating. On one hand, this speeds

the drying of coating; on the other hand, it may interfere

with the achievement of smooth and not porous coatings.

As a result, several primers of polymers can be used in

order to avoid the coating penetrate into the substrate as

paper. On the contrary, plastic ﬁlms do not absorb water or

solvent-dissolved coatings. However, also for this kind of

substrate, primers are often used to improve the adhesion

between the substrate itself and the coating—that is, to

bond the coating to the ﬁlm. Many different coating heads

utilize the direct roll-coating principle. A squeeze-type roll

coater is shown in Figure 3. Direct roll coaters can be used

to coat sheeted material as shown in Figure 4. The coating

is metered in a gap between accurately machined meter-

ing and applicator rolls. Sheets are fed through a nip

formed by applicator and carrier rolls. Direct roll coaters

are used widely for packaging materials: paper sizing,

paper color coating, overcoating of blister packaging

board, and heat seal coating.

Kiss roll coaters apply the coating to the web from a pan

(Figure 5). The amount of coating deposited is not con-

trolled by the coater; therefore, kiss roll coaters usually

employ a metering device, such as wirewound rod, to

remove the excess coating. Such a rod is installed imme-

diately after the coater and is a part of the coating head.

Kiss roll coaters can be also run in the direction opposite to

the web travel. In such cases the web wipes the roll clean.

Transfer roll coaters of modern design are used for

deposition of very lightweight coatings accurately (Figure

6). One of the important uses is the application of 100%

Figure 1. Corona treating equipment mounted on a coating line.

Note the voltaic arc (the small clearest zone in the center).

(Courtesy of Metalvuoto Spa, Roncello, Italy.)

Table 1. Characteristics of Various Coaters

Coating System Deposit Weight Range (g m

2

) Coating Speed Range (m min

1

) Viscosity Range (cP)

Knife-over-roll 10–100 150 1000–30,000

Floating knife 2–30 150 500–15,000

Air knife 3–30 500 100–1000

Blade 5–20 100–1200 2000–10,000

Wirewound rod 5–20 150 100–2500

Reverse roll 25–300 150 300–20,000

Kiss/squeeze 10–30 300 50–2000

Transfer roll 0.3–50 300 300–150,000

Gravure 2–25 10–400 100–10,000

Slot oriﬁce 20–600 300 400–200,000

COATING EQUIPMENT 287