112

-2

Coatings Technology Handbook, Third Edition

112.3 Types of Coating

There are two main types of tablet coating done today: sugar coating and film coating; film coating is
the more popular. Coated tablets fall into three main subcategories depending on how the drug is released:
immediate release, enteric release, and sustained release:
Immediate-release coating systems, as the name implies, allow immediate release of the drug com-
pound to the body.
Enteric coatings are soluble only at a pH greater than 5 or 6. Thus, the drug is not released in the
stomach but in the small intestine. Enteric coatings are by far the most unreliable because of the
wide and unpredictable variance in gastric pH profiles. Gastric pH varies considerably based on
stomach content, age of the patient, and disease state.
Sustained-release coatings permit drug to dissolve slowly over a period of time. This helps to reduce
dosing intervals and improves therapeutic reliability.
Film coating can be carried out using either an organic solvent system, such as ethanol or methylene
chloride, or by using water as a solvent. The solvent film coating systems are fast disappearing because of
cost, environmental, and safety concerns. Most film coating carried out today is done with aqueous systems.

112.3.1 The Sugar-Coated Tablet

The sugar-coated tablet is the most elegant solid dosage form produced today. Its glossy appearance,
slippery feel, and sweet taste are unmatched by any other coated tablet. The sugar-coated tablet is also
the most difficult and time-consuming to produce. The tablet consists of a core upon which layer after
layer of coating material is slowly and carefully built up. In some cases this is done by hand and in other


112

-3

air throughput, are used almost exclusively. With greater air throughput, water evaporates more quickly,
thus speeding the process. Using automated techniques, tablets can be sugar coated in about 16 hours.

112.3.2 The Film-Coated Tablet

The film-coated tablet consists of a core around which a thin, colored polymer film is deposited. Thus,
a film-coated tablet gains about 3% of total tablet weight upon coating. The sugar-coated tablet undergoes
a 100% weight gain. Overall, film coating is a much faster procedure, and much less prone to error.
The basic film coating formula consists of a film former, a pigment dispersion, a plasticizer, and a
solvent. A variety of polymeric film formers can be used to coat tablets. By selecting the solubility
properties of the polymer, one can produce an immediate-release, an enteric-release, or a sustained-
release tablet.
The most popular immediate-release film formers are the water-soluble cellulose ether polymers. The
two most common are hydroxypropylcellulose (HPC) and hydroxypropylmethycellulose (HPMC). The
low viscosity grades of these polymers are employed in the coating formula to maximize polymer solids
concentration. Both these polymers are water soluble.
Water-insoluble film formers can also be used to prepare immediate-release coatings. These products
fall into two categories: cellulose ethers and acrylate derivatives. The most common cellulose ether is
ethylcellulose. This material is commercially available in two forms: as pure polymer and as an aqueous
dispersion. The pure polymer is generally dissolved in an organic solvent; the dispersion is delivered out
of an aqueous media. In both cases, a certain amount of water-soluble component (up to 50% of the
total polymer solids) is included in the coating formula, to provide immediate drug release.
The ethylcellulose and acrylate compounds are also used to formulate sustained-release products.
Again, a water-soluble component is included in the coating formula. However, the level is very low:
usually about 3% of total polymer solids. When the coated dosage form is exposed to water, the water-

,
Philadelphia: Lea & Febiger, 1st ed., 1970; 2nd ed., 1976; 3rd ed., 1986.
Osol, Arthur, Ed.,

Remington’s Pharmaceutical Sciences

. Easton, PA: Mack Publishing Company, 14th ed.,
1970; 15th ed., 1975; 16th ed., 1980; 17th ed., 1985.

DK4036_book.fm Page 3 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC

113

-1

113

Textiles for Coating

113.1 Yarns

113-

1
113.2 Fabrics

113-

2

and adhesion properties of backing materials made from these yarns. The amount of twist also determines
the mechanical properties, first of all the breaking force and extension of spun yarns.
Continuous filament yarns are made by extruding the fiber forming polymer (solution or molten
mass) through the holes in a spinneret. Filaments obtained by this way are long continuous fiber strands
of indefinite length. The number of filaments is determined by the number of holes in the spinneret.
Continuous filament yarns are characterized by a smooth, compact surface formed by parallel packing
of straight filaments with minimal air spaces between them. Yarn made from one continuous filament
is called monofilament yarn. Continuous filament yarns may be twisted or intermingled, to obtain
required degrees of compactness and structure.

Algirdas Matukonis

Kaunas Technical University

DK4036_book.fm Page 1 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC
Woven Fabrics • Knitted Fabrics • Nonwovens

Textiles for Coating

113

-3

1 twill is arranged with the twill wale going in the reverse direction. Since the relative amount of interlacing
in the twill weave is less than in a plain weave, yarns can be packed closer, producing a thicker cloth. On
the other side, fewer interlacings diminish the interfiber friction, which contributes to a greater pliability,
softness, and wrinkle recovery of fabrics, but makes for lower strength. For backing manufacturing, a 2/
2 twill is widely used.
The satin weaves (Figure 113.3) have long yarn floats (over four yarns minimum) with a progression

the repeat of 5

×

5 yarns.

FIGURE 113.4

Four-layered fabric (derived from plain
weave).

FIGURE 113.5

Weft-pile fabric.

FIGURE 113.6

Warp-pile fabric (double-cloth method).

DK4036_book.fm Page 3 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC

113

-4

Coatings Technology Handbook, Third Edition

pile are interlaced around ground warp yarn, and in the warp-pile fabrics they are interlaced around
ground warp ends.

for warp and weft yarns; it expresses the relative tightness of the fabric concerned. The magnitude of

K

in fabrics intended for coating varies in the ranges of 50 to 140% and 40 to 130% for warp and wefts,
respectively. The mass per unit area (weight range) of fabrics depends on type and end use and varies
from 40 to 400 g/m

2

or more.
Among the wide range of mechanical characteristics, there are several determining the field of use of
coated woven fabrics. First, the fabric must have the required tensile strength and elongation. The tensile
strength of fabric as well as of yarns is expressed in terms of tenacity in specific units: centinewtons per
tex (cN/tex). Tenacity is calculated on the basis of the breaking force of a 50 mm wide strip and the
number of linear density of threads in the strained system.
For approximate calculations, it may be assumed that the breaking force of a loaded thread system is
expressed as the sum of the loaded yarn’s breaking force multiplied by a factor 0.8 to 1.2, depending on
the weave, thread count, type of fibers and yarns, finishing processes, and loading direction. In some
cases the conditional value of tenacity is evaluated on the basis of breaking force and the whole mass of
fabric strained (as in the case of nonwoven materials). The conditional values of breaking force and
breaking extension of woven fabrics of various types are represented in Table 113.1. The strength of high-
tech fabrics made from high tenacity fibers (polypropylene, polyethylene, aramid, and others) may be
much higher.
The tensile behavior under load of fabrics of different types — woven, knit, and nonwoven — is shown
There are also other characteristics of fabrics that determine the usefulness of these materials for
coating purposes. Important properties are tearing force, resistance to cyclic loading, bending stiffness,

TA BLE 113.1



-7

TA BLE 113.2

Comparison of Textile Properties

Properties

a

Fabric Type Thickness Porosity
Specific
Vo lume Roughness Tenacity
Braking
Extension
Tear
Force
Bending
Stiffness Compressibility
Elastic
Modulus

b

Drape
Coefficient
Shear
Strain
Woven L/M M L M H L/M H L/M L M L/M M

1
3
4
5
6
90°
90°
90°
+45 − 90°
90 − 45°
8

DK4036_book.fm Page 9 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC

113

-10

Coatings Technology Handbook, Third Edition

technology, and combined technology. All techniques of nonwoven manufacture are characterized by a
high operating speed and low fabric production costs in relation to conventional technologies.

113.2.3.1 Adhesive-Bonded Fabrics

Adhesive-bonded fabrics are made by the physical–chemical method, in which webs of fibers are strength-
ened by fiber-to-fiber adhesion. The web is prepared on special equipment based on carding or the
aerodynamic principle; such machinery is capable of producing webs with random or oriented fiber
distribution. The quality of the web determines to a large extent the quality of the nonwoven fabric.


113.2.3.2 Spunbonded Fabrics

The manufacture of spunbonded nonwoven fabrics consists of combining the preparation of webs with
the production of man-made fibers. The whole sequence of operations, such as melt extruding and
drawing of continuous filaments, arranging them on a moving collecting surface, forming a web, and
bonding together by means of adhesive, thermobonding, or needlepunching, may be done in one process.
Var ious man-made fibers may be used for the production of spunbonded nonwovens: viscose, polyester,
nylon, polypropylene, polyethylene, and polyurethane fibers. The main spunbonded nonwoven properties
depend on filament properties (linear density, tenacity, elongation, crimp, micromorphology), filament
arrangement, and bonding parameters. These types of nonwoven fabrics are produced in weight range
of 15 to 125 g/m

2

. The use of randomly arranged continuous filaments contributes to a higher tear and
tensile strength (5 to 8 cN/tex) in all directions, and also to good handle (see Table 113.2). By use of
modified spunbonding techniques, it is possible to obtain fabrics with special properties, including those
required for coating products (greater elasticity, elongation, and air permeability).
Tr ade names of spunbonded fabrics include Cerex (nylon 6 6), Reemay (polyester), Typar (polypro-
pylene), and Tyvek (polyethylene).
tenacity
breaking force
mass per unit length
=

DK4036_book.fm Page 10 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC
Ta ble 113.2) depend highly on binder content, fiber type, and fiber orientation in the web. It is possible


weight range of fabric is 200 to 1500 g/m

2

. The mechanical properties depend mainly on fiber charac-
teristics, interfiber friction, web weight, and fabric finish treatment. Fibers of all types, and their blends,
may be used for production of needlepunched fabrics. The strength of needlepunched fabrics varies in
the range of 2 to 5 cN/tex. To increase fabric strength, additional backing in the form of a woven, knitted
fabric or a film may be used. It is also possible to produce patterned colored fabrics by means of colored
layers and by needling fibers from the top layer through the surface layer, making loops on the face of
the fabric. Needlepunched fabrics with such special properties as flame retardancy, conductivity, and
elasticity may be produced by using corresponding components.

113.2.3.4 Spunlaced Fabrics

Spunlaced fabrics are made by entanglement of the fibers in the web by means of streams of high pressure
water jets. The web obtains the required bonding, which influences the strength, handle, drape, and air
permeability of fabrics, the fluid fiber entangled fabrics may be made in the weight range of 20 to 70 g/
m

2

and with a tenacity of 1.5 to 2.5 cN/tex. Polyester, polyamide, and other fibers may be used.

113.2.3.5 Stitchbonded Fabrics

There are several techniques for producing stitchbonded fabrics. The stitchbonding of fibrous web carried
out by Arachne (Czechoslovakia), Maliwatt (East Germany), and VP (USSR) is widely known. The web
of natural or man-made fibers prepared by the carding process, with oriented or random arrangement
of fibers, is stitched with yarns by means of warp knitting technology units. This technology is therefore

113

-13

McConnell, R. L., M. F. Meyer, F. D. Petke, W. A. Haile, “Polyester adhesives in nonwovens and other
textile applications,”

J. Coated Fabrics, 16

(1), 199–208 (1987).
“Vliesstoffe auf der Techtextil ’86,”

Chemiefasern/Textilindustrile,
36

, 88, 581–587 (1986).

DK4036_book.fm Page 13 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC

114

-1

114

Nonwovens as

and knit fabrics made out of spun yarns, mainly of cellulosic base. Properties of such fabrics were sharply
defined and limited. They could be varied somewhat by changing the weave pattern, the yarn size, and
the weight of the product. As a whole they were thick, had poor tear strength when coated, tended to
lint, were uneven, had comparatively rough surfaces, and had holes or voids where the yarns intersected
— poor properties when very thin and even coatings were needed.
Initial nonwovens of the carded and random air-lay type composed of synthetic fibers were an improve-
ment in some respects but not all. Carded unidirectional webs were of good quality even at medium to
low weights, but they were stiff and had too high an elongation for some end uses, as well as poor cross-
directional strength and poor tear strength. Random air-lay fabrics had good isotropic strength and fair
tear strength at low binder levels, but their quality was too poor for use as a coating substrate at anything
lower than a weight of 85 g/m

2

.
The introduction of finer deniers and continuous filament yarns in woven and knit fabrics used in
coating substrates overcame some of the deficiencies of the spun yarn woven and knit fabrics, such as
evenness of cover, roughness of surface, and minimum thickness. They also were an improvement over
the initial nonwovens used, especially in regard to strength, strength/weight ratios, drape, and conform-
ability for molding. However, these materials are much more expensive than either nonwovens or spun
yarn wovens and knits.

Albert G. Hoyle

Hoyle Associates

DK4036_book.fm Page 1 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC
Spunbonded Webs • Carded Unidirectional Webs • Carded,
Cross-Lapped, Needlepunched Webs • Air-Lay

115-

3
115.4 Permanent Inks

115-

4
115.5 Dyes Used in Permanent Ink Systems

115-

4
115.6 Current and Future Aspects

115-

4
The types of inks manufactured and their applications are so varied. The following is a general classifi-
cation of inks and the colorants used in them. Generally, the main two colorant classifications are dyes
and pigments — the main difference being that dyes are soluble while pigments are not. Some of the
ink areas a dye supplier can focus on are ink-jet inks, marker inks for children including highlighter and
disappearing inks, writing inks, stamp pad inks, ballpoint pen inks, ribbon inks, permanent inks, and
artists’ inks. Appropriate dyes must be specifically qualified and developed for each type of ink. The dyes
listed below must be tested in the ink system to conform to low insolubility levels, purity, viscosity, surface
tension, strength, shade, and solubility.

115.1 Ink-Jet Inks

Ink-jet inks can be water or solvent based. Many dyes used in other areas mentioned in this article are

•Food
•Cleaners
•Cosmetics
•Drugs
•Wax
•Textiles
•Detergents
•Coatings
•Leak detection
• Plastics
•Paper
•Leather
•Candles
• Shoe polish

DK4036_book.fm Page 5 Monday, April 25, 2005 12:18 PM
© 2006 by Taylor & Francis Group, LLC

116

-1

116

Gravure Inks

116.1 Introduction

116-


gravure markets — publication, packaging, and product (or specialty).
Publication gravure is an exceptionally high speed, four-color process printing method, the primary
function of which is the reproduction of text and pictures. The substrate printed is a very thin, generally
low-basis weight paper. The primary end products include catalogues, magazines and newspaper inserts.
Packaging gravure is a somewhat slower variation of the process using the same mechanics but not based
solely on four-color work. The substrate range is also much wider — including film and foil as well as
paperboard and paper label. Spot colors and coatings are often included. In packaging, the ultimate
printed product is a package, in which the printing not only decorates the product but may also serve a
functional purpose, such as a barrier. Product printing, like packaging, is relatively low speed. Substrates
range from plastics to metals to paper. The end products include floor coverings, swimming pool liners,
postage stamps, and wood grain materials for furniture or wall covering.
The process is based on printing from a recessed image that is engraved or etched into a metal cylinder.
The cylinder is placed into a pan containing the ink. Excess ink is removed by use of a metal or plastic
blade, and the ink left in the cells is then transferred to the substrate.
In recent years, there have been many pressures for changing the process. Some of these changes are
being government regulation driven and some are cost driven. Print quality in gravure is quite high, and
the challenge has been to respond to the need for change without loss of quality. Another area of challenge
is the recent upsurge in Flexo print volume. As Flexo print quality has improved, and improved markedly,
many jobs previously printed gravure have moved to Flexo. This is primarily a cost function when quality
is perceived as equal or mutually satisfactory.
As changes in gravure have occurred, the inks have had to evolve as well. We are, therefore, seeing
many changes in the solvents, resins, and additives used for gravure.
In gravure, whether publication or packaging, the amount of ink transferred to the substrate depends
on the cell volumes and configurations, the substrate used, and the ink formulation. The actual print
strength obtained depends on the colorant, the ratio of colorant to vehicle, and the viscosity of the applied
material. The gravure system of using an engraved cylinder and wiping off excess ink gives very high
print quality and positive control over the process. The process lends itself to long runs, but cylinder
costs can be high.

Sam Gilbert