Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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100
Optical
Fiber
Coatings
1
.O
INTRODUCTION AND BACKGROUND
The concept
of
high speed data transmission on
a
beam
of
light was demonstrated by
Alexander Graham Bell
in
1880.
His “photophone” used mirrors and sunlight to transmit
low quality voice messages. Today’s technology has developed to a point of passing
extremely high data rates
(>2
Gbit/s) unrepeated through
a
high purity quartz fiber for
distances exceeding
100
km.
Optical fibers offer many advantages over copper conductors and are gaining accep-
tance for the following reasons:
1.
They carry more signals for greater distances without repeaters. A fiber with
a
1
GHz bandwidth can transmit several thousand simultaneous telephone calls
through use
of
state-of-the-art multiplexing techniques. Undersea cables have
repeaters spaced as widely as
30-55
km, whereas copper cables require signal
regeneration ever
2-5
km.
2.
Electromagnetic interference does not occur.
3.
Cross-talk is eliminated because the optical signal is maintained
in
fiber and
4.
Weight and volume
of
fiber cables are greatly reduced (for the same signal
“short-circuits” do not occur.
carrying capacity).
These advantages have led to the development of numerous optical fiber telecorn-
munication networks, with an increasing awareness that the initial growth of this technol-
ogy for long-distance trunk lines will continue
to
accelerate
as
fiber replaces copper wire
in the subscriber loop (residential and business installations).
1
.l
Optical Waveguide Principles
In
any transpaent material, light travels more slowly than in a vacuum. The ratio of these
speeds is the index
of
refraction
(H).
When light, traveling in a material
of
one refractive
843
844
LAWSON
index, strikes a material having a lower refractive index, the light is bent back toward the
higher refractive index material. This phenomenon can be used
to
guide a light down a
high purity glass rod,
if
the rod is clad with a material having a lower refractive index.
Current telecommunication fibers employ a doped (germanium, phosphorus, etc.) silica
core in combination with a lower refractive index
(n
=
1.46)
silica cladding. The core
will generally vary in thickness from
8
to
100
pm, whereas the outside diameter of the
cladding is usually 125 km. When made from pure silica, the resultant fibers have excellent
strength (to
14,000
N/mm’)’, but they rapidly degrade as a result
of
the development
of
microscopic scratches, which are subject
to
growth, resulting in breakage when stressed
(stress corrosion).
2.0
COATING
The application of
a
protective coating is required
to
preserve the optical fiber’s strength
and to protect it from lateral deformation, which can result in a reduced light signal
(attenuation).
Attenuation is one of the major concerns of a fiber user. This is a measure of the
light lost in passing through the fiber and is described in electronic terms as noise (i.e.,
decibels per kilometer: dB/km. This
loss
results from light scattering due
to
fluctuations
of glass density, absorption due to impurities, and radiant signal
loss
due
to
microbending,
the result of unequal forces or distortions of the glass fiber core.
Long-term durability of the fiber and its ability
to
transmit a signal of high fidelity
is of paramount importance. Lifetimes
of
25-40 years are generally expected, even though
the anticipated environments vary from tropical climates to arctic winters to undersea
water pressures. In addition, the fiber may be exposed to groundwater (having a pH of
2- 12), solvents, steam, stress, and rapidly fluctuating temperatures. Under conditions
of
high stress and humidity, silica glass is known
to
undergo rapid stress corrosion through
a
process of hydrolysis, resulting in premature breaks of the communication link. To avoid
this difficulty, manufacturers minimize flaws and conditions during fiber manufacture. To
protect the glass, a protective buffer coating is applied within one or two seconds
of
drawing, or as soon as the glass has cooled to below 100°C. The buffer coating is designed
to protect the glass from being scratched. However, it is mandatory that the coating be
free of any particulate matter, which may cause microscratches on the glass surface,
forming a site for crack propagation.
2.1
Handleability
To keep up with increasing demand, the coating or coating system must lend itself to high
production speeds. During the commercial development period
of
the late 1970% typical
production speeds varied from
0.5
to about
1
m/sec. At these slow speeds several coatings
were found to be acceptable. Two-component polysiloxanes were found to offer the best
overall properties, namely good strength, and minimal temperature dependence (their flexi-
bility and modulus remain largely unchanged over a temperature range of
-
55
to
+
85°C).
Their durability is good, and their low modulus properties provide a soft coating, which
protects the glass core from physical distortions and microbending. Major deficiencies of
the silicones include relatively limited application and cure rates, high hydrogen genera-
tion, limited pot life (when catalyzed), and a high coefficient of surface friction, which
requires overcoating. Other early coating systems included cellulose-based lacquers, Tef-
lon extrusions, and polybutadiene rubbers.
OPTICAL FIBER COATINGS
045
2.2
Coating Requirements
In
1977 the demand for increased volume and productivity resulted
in
the development
of ultraviolet light curable acrylate coatings, which met the needs for strength, longevity,
and performance while also providing production speeds of
5
&sec and faster.’
“I
By using
combinations of acrylate-functional oligomers and monomers, it is possible
to
formulate
coatings having a wide range of useful properties, including
Modulus,
1
-
1000
MPa
(
125-
150,000 psi)
Elongation, 10-250%
Hardness,
35
Shore
A
to 70 Shore
D
Tensile strength, 0.5-40 MPa (70-6000 psi)
Low water absorption
(<3%)
It has long been theorized’ that an optimal fiber coating system should consist of a
very soft, low modulus, particulate-free coating against the glass to provide for surface
protection and microbending resistance. Microbending is the attenuation
of
the light signal
caused by small bends of the core, resulting from nonuniform stresses on the fiber. The
primary coating is followed by a harder, tough, secondary buffer to provide mechanical
protection and water resistance, while offering a hard, slick surface that is compatible
with subsequent cabling operations.
It is desirable that these coatings be applicable in-line, at maximum possible draw
rates, and at several different outside diameters. Coating thicknesses may be from
3
p
to nearly 400 km.
The properties usually desired and generally provided by multifunctional acrylate
coatings include the following.”
1.
Primary buffer coatings
Good adhesion to the glass. Buffer removal or stripping can be accomplished
with the use of solvents such as methylene chloride, or short lengths of coating
can be removed mechanically. Another construction might have minimal bond-
ing to the glass to facilitate rapid, safe mechanical stripping of the buffer during
installation.
Low tensile modulus over a range of operating temperatures
(
-
55°C to
+
85°C).
Long-stability, with excellent resistance to oxidation, hydrolysis, saponification,
and ultraviolet light.
A
satisfactorily high index
of
refraction,
so
that the coating can be applied using
laser forward-scattering for concentricity control, and an index of refraction
sufficiently higher than the cladding that light launched into the glass cladding
will be stripped from the fiber’ (mode stripping)
(W
>
1.48).
Low glass transition temperature (T,) to allow the primary coating
to
function
without going through a glass transition phase that will cause differential expan-
sion and contraction, resulting
in
microbending.
Low generation of hydrogen during the fiber’s installed lifetime.
Low surface free energy (i.e., low coefficient of friction and tack) as to allow
for easy handling in the cable plant.
Low surface tack, to prevent binding or coupling of the coated fiber to the walls
of a “loose tube” cable.
Good intercoat adhesion characteristics when overcoated with other components
2.
Secondary buffer coatings
846
LAWSON
of a cable system (i.e., tape or extrusion coatings such as nylon, Hytrel Tefzel,
etc.).
Excellent resistance
to
environmental factors such as water, acids, bases,
sol-
vents, and fungus growth.
High modulus
to
resist external lateral forces that will deform the light path.
High
T,
to
avoid differential expansion and contraction during thermal cycling.
Low volatility
to
resist outgassing during hot extrusion applications to tightly
buffered cable processes and long-term changes
in
physical properties.
The ability to be colored or to accept coloring, which permits individual fiber
identification.
Resistance to the chemical gels used to fill “loose tube” cables.
2.3
Composition
of
Fiber Coatings
More than
75%
of the world’s quartz optical fiber is coated with acrylate-based materials
and cured with
UV
light. The classical reaction chemistry uses photoinitiators to absorb
photons generated by medium pressure mercury, xenon, or doped mercury lamps. lkm’
of a wavelength of
300-390
n
is generally sufficient for
full
cure in less that
1
second).
Free radicals, which are produced by this reaction, cause rapid addition polymerization
of the unsaturated acrylate groups. The proprietary chemistry is
in
the design and choice
of
the acrylate oligomers and reactive monomers. The oligomers are generally based
on
a
range of polyesters, polyethers, and epoxy polymers, which have been esterified with
acrylic acid (coreacted with a diisocyanate and a hydroxyfunctional acrylate). The reactive
diluents are both mono- and multifunctional. Solvents are used only rarely for viscosity
control. Stabilizers, inhibitors, flow aids, and a wide assortment of additives are used to
gain required properties. Secondary buffers may include pigments or soluble dyes for
color identification.
Two-component polymethylsiloxane coatings are used in decreasing quantities for
quartz telecommunication fiber.
A
phenyl variation provides the high refractive index
required for a primary buffer, but at a significant cost disadvantage. The usual low refrac-
tive index makes silicones the coatings of choice for most all-plastic fiber and high
loss,
“plastic clad silica”
(PCS)
fiber. Physical properties of the silicones are excellent, but
line speeds are generally limited
to
speeds well below
2
ndsec. Silicones are also difficult
to mechanically remove from glass fiber for splicing, because they leave a residue.
Fluorine- and silicone-modified acrylates are used for specialty applications.
as
are
polyimides for high temperature resistance.
A
new class
of
inorganic, hermetic coatings
is being commercialized to provide stronger. more durable fiber. These include various
vapor-deposited coatings such as reactive silica and titanium nitrides and carbides, as well
as molten aluminum. Gold and indium have also been used.
REFERENCES
1.
C.
I.
Schlef, ct
al.
J.
Rtrtlitrt.
Curing.
p.
1
I,
April 1982.
2.
U.
C.
Paek,
and
C.
M.
Schroeder,
Appl,
Opt.
V
120,
(23/1) pp.
4028-4034,
December 1981.
3.
K.
R.
Lawson,
and
0.
R.
Cutler,
J.
Raditrt.
Curing.
pp.
4-10,
April 1982.
4.
G.
A.
Prrr;v,
“Pcrformance of acrylate-coated single-modc fibers,” Proceedings of Intcrwire
‘87,
Atlarlttr,
Oct.
27,
1987.
OPTICAL FIBER COATINGS
a47
5.
J. A. Jefferies. and
R.
J.
Klniber,
Wrsforrl
Electric
Olg.,
pp.
13-23.
October
1980.
6.
K.
R.
Lawson, “Advances in UV cured coatings for optical fibers,”
in
Proccw/irlSqs
of’Rrcc1.
Crdrc.
Ec4ropr
‘X?.
FC83-264.
Society of Manufacturing En&’ wuxrs.
7.
K.
R.
Lawson,
R.
E.
Ansel,
md
J.
J.
Stanton, “UV cured coatings for optical fibers,“ in
Proc~rrlir~gs
of
F!fth
Rtrrlirrtior~
Curing
Cor!frrerrc?.
Association
of Finishing Processes-Soci-
ety of Manufacturing Engineers.
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101
Exterior
Wood
Finishes
William
C.
Feist
Consultnnt.
Mitldleror~.
Wiscormirz
1
.O
INTRODUCTION
Wood exposed outdoors undergoes a number of physical and chemical phenomena mostly
caused by moisture influences. sunlight, and temperature. The degradation of wood by
any biological or physical agent modifies some of the organic components of wood. These
components are primarily polysaccharides (cellulose and hemicelluloses) and polypheno-
lics (lignin). Extractives are also present
in
relatively small quantities. and their concentra-
tion determines color, odor, and other nonmechanical properties of a wood species.
A
change in the organic components may be caused by an enzyme, a chemical, or electromag-
netic radiation, but invariably the net result is a change in molecular structure through
some chemical reaction.
The relative effects
of
various energy forms on wood indoors and outdoors are
compared in Table
1.
The most serious threat to wood indoors comes from thermal energy,
and outdoors, from weathering-the combination of chemical, mechanical, and light ener-
gies. The weathering
of
wood is not
to
be confused with wood decay (rot), which results
from organisms (fungi) acting in the presence of excess moisture and air for an extended
period. Under conditions suitable for decay, wood can deteriorate rapidly, and the result
is far different from that observed for natural outdoor weathering.
Weathering can be detrimental to the surfaces and appearance of wood. Thus weath-
ering must be taken into account when considering the preservation and protection of
outdoor wood. Being a product
of
nature, wood is also subject
to
biological attack by
fungi and insects. Most of these stressing factors, influencing factors, and weathering
effects interact and influence
a
finished wood surface (Fig.
1).
The primary functions of any wood finish (e.g., paint, varnish, wax, stain, oil) are
to protect the wood surface, help maintain appearance, and provide cleanability. Unfinished
wood can be
used
outdoors without protection. However, wood surfaces exposed
to
the
weather without any finish are roughened by photodegradation and surface checking,
change color, and slowly erode.
849
850
Table
1
Relative Effects of
Various
Energy Forms on Wood
Indoors Outdoors
Form of Degree Degree
energy Result of effect Result of effect
Thermal
intense Fire Severe Fire Severe
slight Darkening of color Slight Darkening of color Slight
Light" Color change Slight Extensive color changes Severe
Chemical degradation (especially lignin) Severe
Mechanical Wear and tear Slight Wear and tear Slight
Wind erosion Slight
Surface roughemng Severe
Defiberization Severe
Chemical Stsuning Slight Surface roughening Severe
Discoloration Slight Defiberization Severe
Color change Slight Selective leaching Severe
Color change Severe
Strength
loss
Severe
'
Both
vlsible and ultraviolet
light.
Stressing elements
Influencing factors
Conditions
of
weather exposure
Quality
of
design
Properties
and application
of finish
Maintenance
Weathering effects
U
Malm
nlemen¶s
and
eflecls
Figure
1
Stresslng factors, influencing factors, and weathering effects that contribute to wood-
finish performance.
EXTERIOR
WOOD
FINISHES
851
Wood and wood-based products in a variety of species, grain patterns, textures, and
colors can be effectively finished by many different methods. Selection of the finish will
depend on the appearance and degree of protection desired and on the substrates used.
Also, different finishes give varying degrees of protection,
so
the type, the quality, the
quantity, and the application method
of
the finish must be considered when selecting and
planning the finishing or refinishing of wood and wood-based products.
Satisfactory performance of wood finishes is achieved when thorough consideration
is given
to
the many factors that affect these finishes. These factors include the properties
of the wood substrate, characteristics of the finishing material, details of application, and
severity of exposure. Some of these important considerations are reviewed in this chapter.
Additional sources of detailed information are listed in the references at the end of this
chapter.
2.0
EXTERIOR SUBSTRATES
2.1
Wood Properties and Weathering
Wood is a natural biological material and as such its properties vary
not
only from one
species
to
another but within the same species. Some differences can even be expected
in boards cut from the same tree. The natural and manufacturing characteristics of wood
are important influences on finishing characteristics and durability.
The properties of wood that vary greatly from species to species are density, grain
characteristics (presence
of
earlywood and latewood), texture (hardwood or softwood),
presence and amount of heartwood or sapwood, and the presence
of
extractives, resins,
and oils. The density
of
wood
is
one
of
the most important factors that affect finishing
characteristics. Excessive dimensional change in wood constantly stresses
a
film-forming
finish such as paint and may result
in
early failure of the finish. Density varies greatly
from species
to
species and is important because high-density woods shrink and swell
more than do low-density woods. The paintability of various softwoods and hardwoods
is related to natural wood characteristics of density, presence
of
latewood, and texture,
and to manufacturing characteristics such as ring orientation.
Wood surfaces that shrink and swell the least are best for painting. For this reason,
vertical- or edge-grained surfaces (Fig.
2)
are far better than flat-grained surfaces of any
species, especially for exterior use, where wide ranges of relative humidity and periodic
wetting can produce equally wide ranges of swelling and shrinking. Table
2
lists the
painting and weathering characteristics of softwoods and hardwoods.
Because the swelling of wood is directly proportional to density, low-density species
are preferred over high-density species for painting. However, even high-swelling and
dense wood surfaces with
a
flat grain can be stabilized with a resin-treated paper overlay
(overlaid exterior plywood and lumber) to provide excellent surfaces for painting. Medium-
density, stabilized fiberboard products with a uniform, low-density surface or paper overlay
are also good substrates for exterior use. However, edge-grained heartwood
of
western
redcedar and redwood are the woods most widely used for exterior siding and trim when
painting is desired. These species are classified
in
group
I,
those woods easiest to keep
painted (Table
2).
Edge-grained surfaces of all species are considered excellent for paint-
ing, but most species are available only as flat-grained lumber.
Species classified
in
groups
I1
through
V
(Table
2)
are normally cut
as
flat-grained
lumber, are high in density and swelling, or have defects such
as
knots or pitch. The