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Composites Basics: Materials
Introduction
Fiber Reinforced Polymer (FRP)
composites is defined as a
polymer
(plastic)
matrix, either
thermoset or
thermoplastic, that is reinforced (combined) with a
fiber or other
reinforcing material with a sufficient
aspect ratio
(length to
thickness) to provide a discernable reinforcing function in one or
more directions. FRP composites are different from traditional
construction materials such as steel or aluminum. FRP composites
are
anisotropic (properties only apparent in the direction of the
applied load) whereas steel or aluminum is
isotropic (uniform
properties in all directions, independent of applied load).
Therefore, FRP composite properties are directional, meaning that
the best mechanical properties are in the direction of the fiber
placement. Composites are similar to reinforced concrete where the
rebar is embedded in an
isotropic matrix called concrete.
Many terms have been used
to define FRP composites. Modifiers have been used to identify a
specific fiber such as Glass Fiber Reinforced Polymer (GFRP),
Carbon Fiber Reinforced Polymer (CFRP), and Aramid Fiber
Reinforced Polymer (AFRP). Another familiar term used is Fiber
Reinforced Plastics. In addition, other acronyms were developed
over the years and its use depended on geographical location or
market use. For example, Fiber Reinforced Composites (FRC), Glass
Reinforced Plastics (GRP), and Polymer Matrix Composites (PMC) can
be found in many references. Although different, each of
aforementioned terms mean the same thing; FRP composites.
Benefits
FRP composites have many benefits to their
selection and use. The selection of the materials depends on the
performance and intended use of the product. The composites
designer can tailor the performance of the end product with proper
selection of materials. It is important for the end-user to
understand the application environment, load performance and
durability requirements of the product and convey this information
to the composites industry professional. A summary of composite
material benefits include:
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Light
weight
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High
strength-to-weight ratio
-
Directional
strength
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Corrosion
resistance
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Weather
resistance
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Dimensional
stability
-
Radar
transparency
-
Non-magnetic
-
High
impact strength
-
High
dielectric strength (insulator)
-
Low
maintenance
-
Long
term durability
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Part
consolidation
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Small
to large part geometry possible
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Tailored
surface finish
Composition
Composites are composed of
resins,
reinforcements,
fillers, and
additives. Each of these
constituent materials or ingredients play an important role in the
processing and final performance of the end product. The resin or
polymer is the “glue” that holds the composite together and
influences the physical properties of the end product. The
reinforcement provides the mechanical strength. The fillers and
additives are used as process or performance aids to impart
special properties to the end product.
The mechanical properties and composition of FRP
composites can be tailored for their intended use. The type and
quantity of materials selected in addition to the manufacturing
process to fabricate the product, will affect the mechanical
properties and performance. Important
considerations for the design of composite products include:
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Type of fiber reinforcement
-
Percentage of fiber or fiber volume
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Orientation of fiber (0o, 90o,
+/- 45 oor a combination of these)
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Type of resin
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Cost of product
-
Volume of production (to help determine
the best manufacturing method)
-
Manufacturing process
-
Service conditions
Resins
The primary functions of
the resin are to transfer stress between the reinforcing fibers,
act as a glue to hold the fibers together, and protect the fibers
from mechanical and environmental damage. Resins are divided into
two major groups known as thermoset and thermoplastic.
Thermoplastic resins become soft when heated, and may be shaped or
molded while in a heated semi-fluid state and become rigid when
cooled. Thermoset resins, on the other hand, are usually liquids
or low melting point solids in their initial form. When used to
produce finished goods, these thermosetting resins are “cured”
by the use of a
catalyst, heat or a combination of the two. Once
cured, solid thermoset resins cannot be converted back to their
original liquid form. Unlike thermoplastic resins, cured
thermosets will not melt and flow but will soften when heated (and
lose hardness) and once formed they cannot be reshaped.
Heat
Distortion Temperature (HDT) and the
Glass Transition Temperature
(Tg) is used to measure the softening of a cured resin. Both test
methods (HDT and Tg) measure the approximate temperature where the
cured resin will soften significantly to yield (bend or sag) under
load.
The most common
thermosetting resins used in the composites industry are
unsaturated polyesters, epoxies, vinyl esters and phenolics. There
are differences between these groups that must be understood to
choose the proper material for a specific application.
Polyester
Unsaturated polyester resins
(UPR) are the workhorse of the composites industry and represent approximately 75% of the total resins used. To avoid any
confusion in terms, readers should be aware that there is a family
of thermoplastic polyesters that are best known for their use as
fibers for textiles and clothing. Thermoset polyesters are produced by the
condensation polymerization of dicarboxylic acids and difunctional
alcohols (glycols). In addition, unsaturated polyesters contain an
unsaturated material, such as maleic anhydride or fumaric acid, as
part of the dicarboxylic acid component. The finished polymer is
dissolved in a reactive monomer such as styrene to give a low
viscosity liquid. When this resin is cured, the monomer reacts
with the unsaturated sites on the polymer converting it to a solid
thermoset structure.
A range of raw materials
and processing techniques are available to achieve the desired
properties in the formulated or processed polyester resin.
Polyesters are versatile because of their capacity to be modified
or tailored during the building of the polymer chains. They have
been found to have almost unlimited usefulness in all segments of
the composites industry. The principal advantage of these resins
is a balance of properties (including mechanical, chemical,
electrical)
dimensional
stability, cost and ease of handling or
processing.
Unsaturated
polyesters are divided into classes depending upon the structures
of their basic building blocks. Some common examples would be
orthophthalic (“ortho”),
isophthalic (“iso”),
dicyclopentadiene (“DCPD”) and bisphenol A fumarate resins. In
addition, polyester resins are classified according to end use
application as either general purpose (GP) or specialty
polyesters.
Polyester producers have
proved willing and capable of supplying resins with the necessary
properties to meet the requirements of specific end use
applications. These resins can be formulated and chemically
tailored to provide properties and process compatibility.
Epoxy
Epoxy
resins have a well-established record in a wide range of composite
parts, structures and concrete repair. The structure of the resin
can be engineered to yield a number of different products with
varying levels of performance. A major benefit of epoxy resins
over unsaturated polyester resins is their lower
shrinkage. Epoxy
resins can also be formulated with different materials or blended
with other epoxy resins to achieve specific performance features.
Cure rates can be controlled to match process requirements through
the proper selection of hardeners and/or catalyst systems.
Generally, epoxies are cured by addition of an anhydride or an
amine hardener as a 2-part system. Different
hardeners, as well as
quantity of a hardener produce a different cure profile and give
different properties to the finished composite.
Epoxies are used primarily
for fabricating high performance composites with superior
mechanical properties, resistance to corrosive liquids and
environments, superior electrical properties, good performance at
elevated temperatures, good adhesion to a substrate, or a
combination of these benefits. Epoxy resins do not however, have
particularly good UV resistance. Since the viscosity of epoxy is
much higher than most polyester resin, requires a post-cure
(elevated heat) to obtain ultimate mechanical properties making
epoxies more difficult to use. However, epoxies emit little odor
as compared to polyesters.
Epoxy resins are used with
a number of fibrous reinforcing materials, including glass, carbon
and aramid. This latter group is of small in volume, comparatively
high cost and is usually used to meet high strength and/or high
stiffness requirements. Epoxies are compatible with most composite
manufacturing processes, particularly vacuum-bag molding,
autoclave molding, pressure-bag molding, compression molding,
filament winding and hand lay-up.
Vinyl Ester
Vinyl esters were developed to combine the
advantages of epoxy resins with the better handling/faster cure,
which are typical for unsaturated polyester resins. These resins
are produced by reacting epoxy resin with acrylic or methacrylic
acid. This provides an unsaturated site, much like that produced
in polyester resins when maleic anhydride is used. The resulting
material is dissolved in styrene to yield a liquid that is similar
to polyester resin. Vinyl esters are also cured with the
conventional organic peroxides used with polyester resins. Vinyl
esters offer mechanical toughness and excellent corrosion
resistance. These enhanced properties are obtained without complex
processing, handling or special shop fabricating practices that
are typical with epoxy resins.
Phenolic
Phenolics are a class of
resins commonly based on phenol (carbolic acid) and formaldehyde.
Phenolics are a thermosetting resin that cure through a
condensation reaction producing water that should be removed
during processing. Pigmented applications are limited to red,
brown or black. Phenolic composites have many desirable
performance qualities including high temperature resistance,
creep
resistance, excellent thermal insulation and sound damping
properties, corrosion resistance and excellent fire/smoke/smoke
toxicity properties. Phenolics are applied as adhesives or matrix
binders in engineered woods (plywood), brake linings, clutch
plates, circuit boards, to name a few.
Polyurethane
Polyurethane is a family of polymers with widely
ranging properties and uses, all based on the
exothermic reaction
of an organic polyisocyanates with a polyols (an alcohol
containing more than one hydroxl group). A few basic constituents
of different molecular weights and functionalities are used to
produce the whole spectrum of polyurethane materials. The
versatility of polyurethane chemistry enables the polyurethane
chemist to engineer polyurethane resin to achieve the desired
properties.
Polyurethanes appear in an amazing variety of
forms. These materials are all around us, playing important roles
in more facets of our daily life than perhaps any other single
polymer. They are used as a coating, elastomer, foam, or
adhesive. When used as a coating in exterior or interior finishes,
polyurethane’s are tough, flexible, chemical resistant, and fast
curing. Polyurethanes as an elastomer have superior toughness
and
abrasion is such applications as solid tires, wheels, bumper
components or insulation. There are many formulations of
polyurethane foam to optimize the density for insulation,
structural sandwich panels, and architectural components.
Polyurethanes are often used to bond composite structures
together. Benefits of polyurethane adhesive bonds are that they
have good impact resistance, the resin cures rapidly and the resin
bonds well to a variety of different surfaces such as concrete.
Summary of Resins
The
resins in thermoset composites are an important source of
properties and process characteristics. One of the great design
strengths of composites is the multiple choice of resins. In order
to make effective use of these choices, designers and product
specifiers should be familiar with the properties, advantages and
limitations of each of the common composite resins. It is common
to use the resources of the resin manufacturers laboratories to
determine the best resin or an application.
Reinforcements
The primary function of
fibers or reinforcements is to carry load along the length of the
fiber to provide strength and stiffness in one direction.
Reinforcements can be oriented to provide tailored properties in
the direction of the loads imparted on the end product.
Reinforcements can be both natural and man-made. Many materials
are capable of reinforcing polymers. Some materials, such as the
cellulose in wood, are naturally occurring products. Most
commercial reinforcements, however, are man-made. Of these, by far
the largest volume reinforcement measured either in quantity
consumed or in product sales, is
glass
fiber. Other composite
reinforcing materials include
carbon,
aramid, UHMW (ultra high
molecular weight) polyethylene, polypropylene,
polyester and
nylon. Carbon fiber is sometimes referred to as
graphite
fiber.
The distinction is not important in an introductory text, but the
difference has to do with the raw material and temperature at
which the fiber is formed. More specialized reinforcements for
high strength and high temperature use include metals and metal
oxides such as those used in aircraft or aerospace applications.
Development of
Reinforcements- Fibers
Early
in the development of composites, the only reinforcements
available were derived from traditional
textiles and
fabrics.
Particularly in the case of glass fibers, experience showed that
the chemical surface treatments or “sizings” required to
process these materials into fabrics and other sheet goods were
detrimental to the
adhesion of composite polymers to the fiber
surface. Techniques to remove these materials were developed,
primarily by continuous or batch heat cleaning. It was then
necessary to apply new “coupling agents” (also known as
finishes or surface treatments), an important ingredient in sizing
systems, to facilitate adhesion of polymers to fibers,
particularly under wet conditions and fiber processing.
Most reinforcements for
either
thermosetting or
thermoplastic resins receive some form of
surface treatments, either during fiber manufacture or as a
subsequent treatment. Other materials applied to fibers as they
are produced include resinous binders to hold fibers together in
bundles and lubricants to protect fibers from degradation caused
by process
abrasion.
Glass
Fibers
Based on an alumina-lime-borosilicate
composition,
“E” glass produced fibers are considered the
predominant reinforcement for polymer
matrix composites due to
their high electrical insulating properties, low susceptibility to
moisture and high mechanical properties. Other commercial
compositions include
“S” glass, with higher strength, heat
resistance and modulus, as well as some specialized glass
reinforcements with improved chemical resistance, such as AR glass
(alkali resistant).
Glass fibers used for
reinforcing composites generally range in diameter from 0.00035”
to 0.00090” (9 to 23 microns). Fibers are drawn at high speeds,
approaching 200 miles per hour, through small holes in
electrically heated bushings. These bushings form the individual
filaments. The filaments are gathered into groups or bundles
called “strands.” The
filaments are attenuated from the bushing, water and air cooled,
and then coated with a proprietary chemical binder or
sizing to
protect the filaments and enhance the composite laminate
properties. The sizing also determines the processing
characteristics of the glass fiber and the conditions at the
fiber-matrix interface in the composite.
Glass is generally a good
impact resistant fiber but weighs more than carbon or aramid.
Glass fibers have excellent characteristics, equal to or better
than steel in certain forms. The lower
modulus requires special
design treatment where stiffness is critical. Composites made from
this material exhibit very good electrical and thermal insulation
properties. Glass fibers are also transparent to radio frequency
radiation and are used in radar antenna applications.
Carbon
Fibers
Carbon
fiber is created using polyacrylonitrile (PAN),
pitch or rayon
fiber precursors. PAN based fibers offer good strength and modulus
values up to 85-90 Msi. They also offer excellent compression
strength for structural applications up to 1000 ksi. Pitch fibers
are made from petroleum or coal tar pitch. Pitch fibers extremely
high modulus values (up to 140 Msi) and favorable
coefficient of
thermal expansion make them the material used in space/satellite
applications. Carbon fibers are more expensive than glass fibers,
however carbon fibers offer an excellent combination of strength,
low weight and high modulus. The
tensile strength of carbon fiber
is equal to glass while its modulus is about three to four times
higher than glass.
Carbon fibers are supplied
in a number of different forms, from continuous filament
tows to
chopped fibers and mats. The highest strength and
modulus are
obtained by using unidirectional continuous reinforcement.
Twist-free tows of continuous filament carbon contain 1,000 to
75,000 individual filaments, which can be woven or knitted into
woven roving and hybrid fabrics with glass fibers and aramid
fibers.
Carbon fiber composites are
more brittle (less strain at break) than glass or aramid. Carbon
fibers can cause galvanic corrosion when used next to metals. A
barrier material such as glass and resin is used to prevent this
occurrence.
Aramid Fibers (Polyaramids)
Aramid fiber is an aromatic
polyimid that is a man-made organic fiber for composite
reinforcement. Aramid fibers offer good mechanical properties at a
low density with the added advantage of toughness or damage/impact
resistance. They are characterized as having reasonably high
tensile strength, a medium modulus, and a very low density as
compared to glass and carbon. The tensile strength of aramid
fibers are higher than glass fibers and the modulus is about fifty
percent higher than glass. These fibers increase the impact
resistance of composites and provide products with higher tensile
strengths. Aramid fibers are insulators of both electricity and
heat. They are resistant to organic solvents, fuels and
lubricants. Aramid composites are not as good in
compressive
strength as glass or carbon composites. Dry aramid fibers are
tough and have been used as cables or ropes, and frequently used
in ballistic applications.
Reinforcement
Forms
Regardless of the material, reinforcements
are available in forms to serve a wide range of processes and
end-product requirements. Materials supplied as reinforcement
include
roving, milled fiber, chopped strands, continuous, chopped
or thermoformable mat. Reinforcement materials can be designed
with unique fiber architectures and be preformed (shaped)
depending on the product requirements and manufacturing process.
Multi-End and Single-End Rovings
Rovings
are utilized primarily in
thermoset compounds, but can be utilized
in thermoplastics. Multi-end rovings consist of many individual
strands or bundles of
filaments, which are then chopped and
randomly deposited into the resin
matrix. Processes such as
sheet
molding compound (SMC), preform and spray-up use the multi-end
roving. Multi-end rovings can also be used in some
filament
winding and
pultrusion applications. The single-end roving
consists of many individual filaments wound into a single strand.
The product is generally used in processes that utilize a
unidirectional reinforcement such as filament winding or
pultrusion.
Mats
Reinforcing
mats are usually described by weight-per-unit-of-area. For
instance, a 2 ounce
chopped strand mat will weigh 2 ounces per
square yard. The type and amount of
binder that is used to hold
the mat together dictate differences between mat products. In some
processes such as
hand
lay-up, it is necessary for the binder to
dissolve. In other processes, particularly in
compression
molding,
the binder must withstand the hydraulic forces and the dissolving
action of the matrix resin during molding. Therefore, two general
categories of mats are produced and are known as soluble and
insoluble.
Woven, Stitched, Braided
& 3-D Fabrics
There
are many types of fabrics that can be used to reinforce resins in
a composite. Multidirectional reinforcements are produced by
weaving, knitting, stitched or braiding continuous fibers into a
fabric from
twisted and plied
yarn. Fabrics refer to all
flat-sheet, roll goods, whether or not they are strictly fabrics.
Fabrics can be manufactured utilizing almost any reinforcing
fiber. The most common fabrics are constructed with fiberglass,
carbon or aramid. Fabrics are available in several weave
constructions and thickness (from 0.0010 to 0.40 inches). Fabrics
offer oriented strengths and high reinforcement loadings often
found in high performance applications.
Fabrics
are typically supplied on rolls of 25 to 300 yards in length and 1
to 120 inches in width. The fabric must be inherently stable
enough to be handled, cut and transported to the
mold, but pliable
enough to conform to the mold shape and contours. Properly
designed, the fabric will allow for quick wet out and wet through
of the resin and will stay in place once the resin is applied.
Fabrics, like rovings and chopped strands, come with specific
sizings or binder systems that promote
adhesion to the resin
system.
Fabrics
allow for the precise placement of the reinforcement. This cannot
be done with milled fibers or chopped strands and is only possible
with continuous strands using relatively expensive fiber placement
equipment. Due to the continuous nature of the fibers in most
fabrics, the strength to weight ratio is much higher than that for
the cut or chopped fiber versions. Stitched fabrics allow for
customized fiber orientations within the fabric structure. This
can be of great advantage when designing for shear or torsional
stability.
Woven
fabrics are fabricated on looms in a variety of weights, weaves,
and widths. In a plain weave, each fill yarn or roving is
alternately crosses over and under each warp fiber allowing the
fabric to be more drapeable and conform to curved surfaces. Woven
fabrics are manufactured where half of the strands of fiber are
laid at right angles to the other half (0o to 90o).
Woven fabrics are commonly used in boat manufacturing.
Stitched
fabrics, also known as
non-woven, non-crimped, stitched, or
knitted fabrics have optimized strength properties because of the
fiber architecture. Woven fabric is where two sets of interlaced
continuous fibers are oriented in a 0o and 90o
pattern where the fibers are
crimped and not straight. Stitched
fabrics are produced by assembling successive layers of aligned
fibers. Typically, the available fiber orientations include the 0o
direction (warp), 90o direction (weft or fill), and +45o
direction (bias). The assembly of each layer is then sewn
together. This type of construction allows for load sharing
between fibers so that a higher modulus, both
tensile and
flexural, is typically observed. The fiber architecture
construction allows for optimum resin flow when composites are
manufactured. These fabrics have been traditionally used in boat
hulls for 50 years. Other applications include light poles, wind
turbine blades, trucks, busses and underground tanks. These
fabrics are currently used in bridge decks and column repair
systems. Multiple orientations provide a quasi-isotropic
reinforcement.
Diagram
of Stitched Triaxial and Quadraxial Fabrics
Braided
fabrics are engineered with a system of two or more yarns
intertwined in such a way that all of the yarns are interlocked
for optimum load distribution. Biaxial braids provide
reinforcement in the
bias direction only with fiber angles ranging
from ±
15o to ±
95o. Triaxial braids provide reinforcement in the bias
direction with fiber angles ranging from ±
10o to ±
80o and axial (0o) direction.
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Biaxial Braided
Fabric |
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Triaxial Braided
Fabric |
Unidirectional
Unidirectional
reinforcements include tapes,
tows, unidirectional tow sheets and
rovings (which are collections of fibers or strands). Fibers in
this form are all aligned parallel in one direction and uncrimped
providing the highest mechanical properties. Composites using
unidirectional tapes or sheets have high strength in the direction
of the fiber. Unidirectional sheets are thin and multiple layers
are required for most structural applications.
Strength
Relation to Fiber Orientation [Schwarz (1992B)]
In
some composite designs, it may be necessary to provide a corrosion
or weather barrier to the surface of a product. A surface veil is
a fabric made from nylon or polyester that acts as a very thin
sponge that can absorb resin to 90% of its volume. This helps to
provide an extra layer of protective resin on the surface of the
product.
Surface veils are used to improve the surface appearance
and insure the presence of a corrosion resistance barrier for
typical composites products such as pipes, tanks and other
chemical process equipment. Other benefits include increased
resistance to abrasion, UV and other weathering forces. Veils may
be used in conjunction with
gel coats to provide reinforcement to
the resin.
Prepreg
Prepregs are a ready-made material made of
a reinforcement form and polymer matrix. Passing reinforcing
fibers or forms such as fabrics through a resin bath is used to
make a prepreg. The resin is saturated (impregnated) into the
fiber and then heated to advance the curing reaction to different
curing stages. Thermoset or thermoplastic prepregs are available
and can be either stored in a refrigerator or at room temperature
depending on the constituent materials. Prepregs can be manually
or mechanically applied at various directions based on the design
requirements.
Summary of Reinforcements
The
mechanical properties of FRP composites are dependent on the type,
amount, and orientation of fiber that is selected for a particular
service. There are many commercially available reinforcement forms
to meet the design requirements of the user. The ability to tailor
the fiber architecture allows for optmized
performance of a product that translates to weight and cost
savings.
Other Matrix Constituents
Fillers
Use of inorganic fillers in
composites is increasing. Fillers not only reduce the cost of
composites, but also frequently impart performance improvements
that might not otherwise be achieved by the reinforcement and
resin ingredients alone. Fillers can improve mechanical properties
including fire and smoke performance by reducing organic content
in composite laminates. Also, filled resins shrink less than
unfilled resins, thereby improving the dimensional control of
molded parts. Important properties, including water resistance,
weathering, surface smoothness, stiffness, dimensional stability
and temperature resistance, can all be improved through the proper
use of fillers.
The thermosetting resin
segment of the composite industry has taken advantage of the
properties of fillers for many years. More recently, the
thermoplastic industry has begun to make widespread use of
inorganic fillers. Breakthroughs in chemical treatment of fillers
that can provide higher filler loadings and improved laminate
performance are accelerating this trend.
Filler Types
There are a number of
inorganic filler materials that can be used with composites
including:
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Calcium
carbonate
is the most widely used inorganic filler. It is available at low
cost in a variety of particle sizes and treatments from
well-established regional suppliers, especially for composite
applications. Most common grades of calcium carbonate filler are
derived from limestone or marble and very common in automobile
parts.
-
Kaolin
(hydrous
aluminum silicate) is the second most commonly used filler. It is
known throughout the industry by its more common material name,
clay. Mined clays are processed either by air flotation or by
water washing to remove impurities and to classify the product for
use in composites. A wide range of particle sizes is available.
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Alumina
trihydrate
is frequently used when improved fire/smoke performance is
required. When exposed to high temperature, this filler gives off
water (hydration), thereby reducing the flame spread and
development of smoke. Composite plumbing fixture applications such
as bathtubs, shower stalls and related building products often
contain alumina trihydrate for this purpose.
-
Calcium
sulfate
is a major flame/smoke retarding filler used by the tub/shower
industry. It has fewer waters of hydration, and water is released
at a lower temperature. This mineral filler offers a low cost
flame/smoke retarding filler.
Other commonly used fillers include:
Using Fillers in Composites
When used in composite
laminates, inorganic fillers can account for 40 to 65% by weight.
They perform a function similar to silica fume in concrete. In
comparison to resins and reinforcements, fillers are the least
expensive of the major ingredients. These materials are nevertheless
very important in establishing the performance of the composite
laminate for the following reasons:
-
Fillers reduce the
shrinkage of the composites part.
-
Fillers influences the fire resistance of
laminates.
-
Fillers lower compound cost by diluting more
expensive resin and may reduce the amount of reinforcement required.
-
Fillers can influence the mechanical strengths of
composites.
-
Fillers serve to transfer stresses between the
primary structural components of the laminate (i.e., resin and
reinforcement), thereby improving mechanical and physical
performance.
-
Uniformity of the laminate can be enhanced by the
effective use of fillers. Fillers help maintain fiber-loading
uniformity by carrying reinforcing fibers along with the flow as
resin is moved on the mold during compression molding.
-
Crack resistance and crack prevention properties are
improved with filled resin systems. This is particularly true in
sharp corners and resin-rich areas where smaller particles in the
filler help to reinforce the resin in these regions.
-
The combination of small and medium filler particles
helps control compound
rheology at elevated temperatures and
pressures, thereby helping to ensure that compression molded parts
are uniform.
-
Low-density fillers are used extensively in marine
putty and the transportation industry. They offer the lowest cost of
filled systems, without the increases of weight that affect the
performance of the final product.
Surface Treatments Improve Some Fillers
Some fillers are chemically
modified by treating the surface area of the particles with a
coupling agent. These
coupling agents help to improve the chemical
bond between the resin and filler and can reduce resin demand.
Summary of Fillers
Effective use of fillers in
composites can improve performance and reduce cost. In today’s
market, many of the filler systems being sold are providing several
different properties for the composite in one filler system.
Flame/smoke, shrink control, weight management and physical
properties are often modified by using a designed filler package
that has a blend of specialty and commodity fillers. Product and
technical information exists and is readily available to those
interested in this very important composite ingredient that affects
material, process and design and cost.
Additives and Modifiers
A
wide variety of additives are used in composites to modify materials
properties and tailor the laminate’s performance. Although these
materials are generally used in relatively low quantity by weight
compared to resins, reinforcements and fillers, they perform
critical functions.
Additive Functions
Additive used in thermoset
and thermoplastic composites include the following:
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Low
shrink/low profile:
when parts with smooth surfaces are required, a special
thermoplastic resin, which moderates resin shrinkage, can be added
to thermoset resins.
-
Fire
resistance:
Combustion resistance is improved by proper choice of resin, use of
fillers or flame retardant additives. Included in this category are
materials containing antimony trioxide, bromine, chlorine, borate
and phosphorus.
-
Air
release:
most laminating resins,
gel coats
and other
polyester resins might entrap air during processing and application.
This can cause
air voids and improper fiber wet-out. Air release
additives are used to reduce such air entrapment and to enhance
fiber
wet-out.
-
Emission
control:
in open mold applications, styrene emission suppressants are used to
lower emissions for air quality compliance.
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Viscosity
control:
in many composite types, it is critical to have a low, workable
viscosity during production. Lower viscosity in such filled systems
is usually achieved by use of wetting and dispersing additives.
These additives facilitate the wet-out and dispersion of fillers
resulting in lower viscosity (and/or higher filler loading).
-
Electrical
conductivity:
most composites do not conduct electricity. It is possible to obtain
a degree of electrical conductivity by the addition of metal, carbon
particles or conductive fibers. Electromagnetic interference
shielding can be achieved by incorporating conductive materials.
-
Toughness:
can be enhanced by the addition of reinforcements. It can also be
improved by special additives such as certain rubber or other
elastomeric materials.
-
Antioxidants:
plastics are sometimes modified with antioxidants, which retard or
inhibit polymer
oxidation and the resulting degradation of the
polymer.
-
Antistatic
agents:
are added to polymers to reduce their tendency to attract electrical
charge. Control of static electricity is essential in certain
plastics processing and handling operations, as well as in finished
products. Static charges on plastics can produce shocks, present
fire hazard and attract dust. The effect of static charge in
computer/data processing applications, for example, is particularly
detrimental.
-
Foaming
agents:
are chemicals that are added to polymers during processing to form
minute cells throughout the resin. Foamed plastics exhibit lower
density, decrease material costs, improve electrical and thermal
insulation, increase strength-to-weight ratio and reduce shrinkage
and part warping.
-
Plasticizers:
are added to compounds to improve processing characteristics and
offer a wider range of physical and mechanical properties.
-
Slip
and blocking agents
provide surface lubrication. This results in reduced coefficient of
friction on part surfaces and enhances release of parts from the
mold.
-
Heat
stabilizers:
are used in thermoplastic systems to inhibit polymer degradation
that results from exposure to heat.
-
Ultraviolet
stabilizers:
both thermoset and thermoplastic composites may used special
materials which are added to prevent loss of gloss,
crazing,
chalking, discoloration, changes in electrical characteristics,
embrittlement and disintegration due to ultraviolet (UV) radiation.
Additives, which protect composites by absorbing the UV, are called
ultraviolet absorbers. Materials, which protect the polymer in some
other manner, are known as ultraviolet stabilizers.
Catalysts, Promoters, Inhibitors
In
polyesters, the most
important additive is
catalyst or
initiator. Typically, organic
peroxide such as methylethylketone peroxide (MEKP) is used for room
temperature cured processes, or benzoyl peroxide is added to the
resin for heat-cured molding. When triggered by heat, or used in
conjunction with a promoter (such as cobalt napthenate), peroxides
convert to a reactive state (exhibiting free radicals), causing the
unsaturated resin to react (cross-link) and become solid. Some
additives such as TBC (tertiary butyl catechol) are used to slow the
rate of reaction and are called inhibitors. Accelerators such as DMA
(dimethyl aniline) speed curing.
Colorants
Colorants are often used in
composites to provide color throughout the part. Additives can be
mixed in as part of the resin or applied as part of the molding
process (as a gel coat). Also, a wide range of coatings can be
applied after molding.
Release Agents
Release agents facilitate
removal of parts from molds. These products can be added to the
resin, applied to molds, or both. Zinc stearate is a popular mold
release agent that is mixed into resin for compression molding.
Waxes, silicones and other release agents may be applied directly to
the surface of molds.
Thixotropic agents
In
some processes such as hand lay-up or spray-up, thixotropic agents
may be used. When “at rest”, resins containing thixotropic
agents remain at elevated viscosities. This reduces the tendency of
the liquid resin to flow or drain from vertical surfaces. When the
resin is subjected to shear, the viscosity is reduced and the resin
can be easily sprayed or brushed on the mold. Fumed silica and
certain clays are common thixotropic agents.
Summary
Additives and modifier
ingredients expand the usefulness of polymers, enhance their
processability or extend product durability. While additives and
modifiers often increase the cost of the basic material system,
these materials always improve cost/performance.
Core
Materials for Sandwich Structures
Bonded
sandwich structures
have been a basic component of the composites industry for over 45
years. The concept of using relatively thin, strong face sheets
bonded to thicker, lightweight core materials has allowed the
industry to build strong, stiff, light and highly durable structures
that otherwise would not be practical. This technology has been
demonstrated in boats, trucks, and building panels. A 3% weight
increase can increase the flexural strength and stiffness by a
magnitude of 3.5 times and 7 times respectively if cores and skins
are properly chosen. The structure then acts more or less
monolithically.
The
most common comparison made is that of a composite sandwich to an
I-beam. The panel skins, like the flanges of the I-beam, carry the
stresses imposed by use. The stresses are transferred between the
top and bottom skins through shear stresses that run through the
core or web of the I-beam. The purpose of an I-beam is to lessen the
weight required to support a given load in bending. Since the
highest stresses are carried at the extremities, both the top and
bottom of the I-beam, the center section can be much narrower in
width in relation to the flanges. In a sandwich structure, the core
will generally have the same width and length dimensions as the
skins, but can be much weaker than the skins since it primarily
experiences shear stresses. Care must be taken in design to ensure
that the shear carrying capability of the expected loads does not
exceed both the core and the adhesive.
Face sheets can be of
almost any material. In the composites industry, the most common
face sheets are glass and carbon. The common core materials are
foam, syntactic foam,
honeycomb, and balsa wood.
Some core materials can be shaped, such as a waffle pattern
or corrugation to achieve the desired mechanical properties.
Honeycomb Sandwich Construction
A cost-effective and
superior sandwich construction uses end-grain balsa wood. This
material has exceptional bond, high impact and fatigue resistance
with excellent strength/stiffness and lightweight properties. Balsa
wood is “mother nature’s” honeycomb material. Balsa has a
high-aspect ratio and directionally aligned cells such that the
grain is oriented in the direction of the maximum stress. Balsa has
a proven track record in products such as pleasure boat hulls,
military aircraft, vehicles, and corrosion-resistant industrial
tanks.
Laminated Sandwich Construction with Balsa Wood
Adhesives
Adhesives are used to
attach composites to themselves as well as to other surfaces.
Adhesive bonding is the method of choice for bonding
thermoset
composites and is sometimes used for thermoplastic composites. There
are several considerations involved in applying adhesives
effectively. The joint or
interface connection must be engineered to
select the proper adhesive and application method to ensure bond
strength. Careful surface preparation and cure are critical to bond
performance.
Adhesives should be used in
a joint design where the maximum load is transferred into the
component using the loading characteristics of the adhesive and the
composite material. The most common adhesives are acrylics, epoxies,
and urethanes. A high-strength bond with high-temperature resistance
would indicate the use of an epoxy, whereas a moderate temperature
resistance with good strength and rapid cure might use an acrylic.
For applications where toughness is needed, urethane might be
selected.
Gel
Coats
Gel
coats are considered resins but have a very special purpose. A gel
coat is a specially formulated polyester resin incorporating
thixotropic agents to increase the gel coat’s viscosity and
non-sag properties, fillers for flow properties, pigments to give
the desired color, and additives for specific application
properties, such as gel time and cure. Gel coats are primarily used
for contact molding (hand or spray lay-up). The gel coat, usually
pigmented, provides a molded-in finished surface that is weather and
wear resistant. The gel coat helps in hiding the glass reinforcement
pattern that may show through from the inherent resin shrinkage
around the glass fibers. Considerations used for the proper
selection of a gel coat are compatibility of the underlying FRP
materials to ensure good adhesion of the gel coat, as well as the
operating environment.
The
most common current usage of gel coats in “in-mold
applications.” That is, the gel coat is sprayed into the mold and
the laminate is applied behind it. Adhesion of the laminating resin
to the gel coat is a critical issue. Thickness of the gel coat can
vary depending on the intended performance of the composite product.
Gel coats are typically applied by spray application to
approximately 16-20 mils wet film thickness. While gel coats do not
add any structural strength to the FRP part, gel coats should be
resilient. Gel coats should be able to bend without cracking. They
should be resistant to thermal cracking (cracking that may occur
with dramatic changes in temperature). The primary measurements of
resilience are flexural modulus and elongation. Gel coats should be
UV stable and pigmented sufficiently to provide good opacity.
Gel
coats are used to improve weathering, filter out ultraviolet
radiation, add flame retardancy, provide a thermal barrier, improve
chemical resistance, improve abrasion resistance, and provide a
moisture barrier. Gel coats are used to improve the product
appearance such as the surface of a boat hull or golf cart. A unique
benefit of gel coats is that they are supplied in many colors by the
incorporation of pigments per the specification of the engineer.
References
Hollaway,
Leonard (Editor), 1994, Handbook of Polymer Composites for
Engineers, Woodhead Publishing, Cambridge, England.
Kaw, Autar K., 1997, Mechanics of Composites
Materials, CRC Press, New York, NY.
Miller, Tara, 1998, Introduction to Composites, 4th
Edition, Composites Institute, Society of the Plastics Industry,
New York, NY.
Murphy,
John, 1998, Reinforced Plastics Handbook, Elsevier Science,
Oxford, England.
Richardson,
Terry, 1987, Composites: A Design Guide, Industrial Press,
New York, NY.
Rosato,
Dominick V., 1997, Designing with Reinforced Plastics, Hanser/Gardner,
Cincinnati, Ohio.
Schwarz,
M.M., 1992, Composite Materials Handbook, McGraw Hill, Inc.,
New York.
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