**FIBER REINFORCED POLYMER COMPOSITES APPLICATIONS IN USA**

**DOT - FEDERAL HIGHWAY ADMINISTRATION**

**Benjamin Tang, PE**

**Structural Engineer**

**HNG-32, Bridge Specialist Group**

**400 Seventh Street, S.W. Rm. 3203**

**Washington, D.C. 20590**

**January 6, 1997**

Published in the First Korea/U.S.A. Road Workshop Proceedings, January
28-29, 1997

**[History on Fiber Reinforced Polymer Composites]{.underline}**

AAlthough the concept of fiber reinforced materials can be traced back
to the use of straw as reinforcement in bricks manufactured by the
Isrealites in 800 B.C., and in more recent times to the use of short
glass fiber reinforcement in cement in the United States in the early
1930\'s, fiber reinforced resin matrix materials (or fiber reinforced
composites as we know them today) were not developed until the early
1940\'s@ \[1\]. In this paper the term composites is used in the context
of advanced fiber reinforced polymers (FRP=s).

After World War II, US manufacturers began producing fiberglass and
polyester resin composite boat hulls and radomes (radar cover). The
automotive industry first introduced composites into vehicle bodies in
the early 1950s. Because of the highly desirable light weight, corrosion
resistance, and high strength characteristics in composites; research
emphasis went into improving the material science and manufacturing
process. That effort led to the development of two new manufacturing
techniques known as filament winding and pultrusion, which helped
advance the composite technology into new markets. There was a great
demand by the recreation industry for composite fishing rods, tennis
rackets, ski equipment and golf clubs. The aerospace industry began to
use composites in pressure vessels, containers, and non-structural
aircraft components. The US Navy applied composites in mine sweeping
vessels, crew boats and submarine parts. The domestic consumers began
installing composite bath tubs, covers, railings, ladders and electrical
equipment. The first civil application in composites was a dome
structure built in Benghazi in 1968, and other structures followed
slowly.

The Composites Institute, that tracks the domestic economy, reported the
total consumption figures for 1994 and 1995 at more than three billion
pounds each year. Of the nine industries reported, the transportation
industry claimed 31% of the total market, thus, being the biggest
consumer of composites in both years. The preliminary figures for 1994
was 945.6 million pounds followed by 1995 with 984 million pounds. It is
predicted that orders for composites in the transportation sector will
exceed one billion pounds for 1996.

With the increasing demand for composites, new and improved
manufacturing processes such as pultrusion, resin transfer molding, and
filament winding were developed and implemented in the early 1990s. With
these enhancements in place, the current focus is to rebuild the US
transportation infrastructure using FRP composites for maintenance and
rehabilitation of existing bridges as well as new construction.

**[Civil Structural Applications Using Composites]{.underline}**

Many pedestrian bridge projects have been constructed throughout the
United States using pultruded composite structural shapes. In states
vulnerable to high seismic activity, concrete bridge columns are being
retrofitted with a composite filament winding to increase ductility.
Plate bonding using thin composite laminates to strengthen concrete and
steel bridge members has been demonstrated. Composite prestressed piles
are being applied to civil and marine structures in some of the coastal
states.

The first US advanced composite vehicular bridge superstructure was
dedicated into service on December 4, 1996 in Russell, Kansas. The deck
panels were shop-fabricated with composite honeycomb cells which are
sandwiched between two face sheets. These panels were then joined
together with epoxy in the field. Demonstration bridge projects are
being developed in other states such as Delaware, West Virginia and
California. Continued research projects using composite reinforcing bars
in concrete slabs are being studied in New Hampshire, Washington, D.C.
and Michigan. Composite prestressing tendons and stay cables are being
developed in Pennsylvania, Michigan, South Dakota and California.

**[Manufacturing Processes]{.underline}**

There are three basic manufacturing techniques in producing composite
structural products, with many variations and patented processes: 1) The
pultrusion process involves a continuous pulling of the fiber rovings
and mats through a resin bath and then into a heated die. The elevated
temperature inside the die cures the composite matrix into a constant
cross-section structural shape. 2) The filament winding process can be
automated to wrap resin-wetted fibers around a mandrel to produce
circular or polygonal shapes. 3) The layup process engages a hand or
machine buildup of mats of fibers that are held together permanently by
a resin system. This method enables numerous layers of different fiber
orientations to be built up to a desired sheet thickness and product
shape.

**[Characteristics of Composites]{.underline}**

The mechanical properties of composites depend on many variables such as
fiber types, orientations, and architecture. The fiber architecture
refers to the preformed textile configurations by braiding, knitting, or
weaving. Composites are anisotropic materials with their strength being
different in any direction. Their stress-strain curves are linearly
elastic to the point of failure by rupture. The polymeric resin in a
composite material, which consists of viscous fluid and elastic solids,
responds viscoelastically to applied loads. Although the viscoelastic
material will creep and relax under a sustained load, it can be designed
to perform satisfactorily. Composites have many excellent structural
qualities and some examples are high strength, material toughness,
fatigue endurance, and light weight. Other highly desirable qualities
are high resistance to elevated temperature, abrasion, corrosion, and
chemical attack.

Some of the advantages in the use of composite structural members
include the ease of manufacturing, fabrication, handling, and erection.
Project delivery time can be short. It took the Russell county engineer
one day to install the deck panels in the first vehicular composite
bridge. Composites can be formulated and designed for high performance,
durability and extended service life. They have excellent
strength-to-weight ratios. If durability can be proven to last 75 years,
composites can be economically justified using the life-cycle cost
method.

Some of the disadvantages in the use of composites in bridges are high
first cost, creep, and shrinkage. The design and construction require
highly trained specialists from many engineering and material science
disciplines. The composites have a potential for environmental
degradation, for examples, alkalis= attack and ultraviolet radiation
exposure. There are very little or nonexistant design guidance and/or
standards. There is a lack of joining and/or fastening technology.
Because of the use of thin sections, there are concerns in global and
local buckling. Although the light weight feature may be an advantage in
the response to earthquake loading, it could render the structure
areodynamically unstable. In manufacturing with the hand layup process,
there is a concern about the consistency of the material properties.

**[What Is Really in Composites?]{.underline}**

As the name implies, advance fiber reinforced polymer composites is made
of fiber reinforcements, resin, fillers, and additives. The fibers
provide increased stiffness and tensile capacity. The resin offers high
compressive strength and binds the fibers into a firm matrix. The
fillers serve to reduce cost and shrinkage. The additives help to
improve not only the mechanical and physical properties of the
composites but also workability. The discussions that follow immediately
will explain the basic functions and behaviors of the constituents.

**[Fiber Reinforcements]{.underline}**

The fiber is an important constituent in composites. A great deal of
research and development has been done with the fibers on the effects in
the types, volume fraction, architecture, and orientations. The fiber
generally occupies 30% - 70% of the matrix volume in the composites. The
fibers can be chopped, woven, stitched, and/or braided. They are usually
treated with sizings such as starch, gelatin, oil or wax to improve the
bond as well as binders to improve the handling. The most common types
of fibers used in advanced composites for structural applications are
the fiberglass, aramid, and carbon. The fiberglass is the least
expensive and carbon being the most expensive. The cost of aramid fibers
is about the same as the lower grades of the carbon fiber. AOther
high-strength high-modulus fibers such as boron are at the present time
considered to be economically prohibitive@ \[14\].

-   ***Glass Fibers***

> The glass fibers are divided into three classes \-- E-glass, S-glass
> and C-glass. The E-glass is designated for electrical use and the
> S-glass for high strength. The C-glass is for high corrosion
> resistance, and it is uncommon for civil engineering application. Of
> the three fibers, the E-glass is the most common reinforcement
> material used in civil structures. It is produced from
> lime-alumina-borosilicate which can be easily obtained from abundance
> of raw materials like sand. The fibers are drawn into very fine
> filaments with diameters ranging from 2 to 13 X 10^-6^ m. The glass
> fiber strength and modulus can degrade with increasing temperature.
> Although the glass material creeps under a sustained load, it can be
> designed to perform satisfactorily. The fiber itself is regarded as an
> isotropic material and has a lower thermal expansion coefficient than
> that of steel.

  ---------------------------------- ------------------ ------------------
  Typical Properties                 E-Glass            S-Glass

  Density (g/cm^3^)                  2.60               2.50

  Young=s Modulus (GPa)              72                 87

  Tensile Strength (GPa)             1.72               2.53

  Tensile Elongation (%)             2.4                2.9
  ---------------------------------- ------------------ ------------------

[Table 1 \[Ref. 5\]]{.underline}

-   ***Aramid Fibers***

> These are synthetic organic fibers consisting of aromatic polyamides.
> The aramid fibers have excellent fatigue and creep resistance.
> Although there are several commercial grades of aramid fibers
> available, the two most common ones used in structural applications
> are Kevlar7[^1] 29 and Kevlar7 49. The Young=s Modulus curve for
> Kevlar7 29 is linear to a value of 83 GPa but then becomes slightly
> concave upward to a value of 100 GPa at rupture; whereas, for Kevlar7
> 49 the curve is linear to a value of 124 GPa at rupture (see Table 2).
> As an anisotropic material, it=s transverse and shear modulus are an
> order of magnitude less than those in the longitudinal direction. The
> fibers can have difficulty achieving a chemical or mechanical bond
> with the resin.

  ---------------------------------- ------------------ ------------------
  Typical Properties                 Kevlar 29          Kevlar 49

  Density (g/cm^3^)                  1.44               1.44

  Young=s Modulus (GPa)              83/100             124

  Tensile Strength (GPa)             2.27               2.27

  Tensile Elongation (%)             2.8                1.8
  ---------------------------------- ------------------ ------------------

[Table 2 \[Ref. 5\]]{.underline}

-   ***Carbon Fibers***

> The graphite or carbon fiber is made from three types of polymer
> precursors \-- polyacrylonitrile (PAN) fiber, rayon fiber, and pitch.
> The tensile stress-strain curve is linear to the point of rupture.
> Although there are many carbon fibers available on the open market,
> they can be arbitrarily divided into three grades as shown in Table 3.
> They have lower thermal expansion coefficients than both the glass and
> aramid fibers. The carbon fiber is an anisotropic material, and its
> transverse modulus are an order of magnitude less than its
> longitudinal modulus. The material has a very high fatigue and creep
> resistance.

  ---------------------- -------------- -------------- -------------------
  Typical Properties     High Strength  High Modulus   Ultra-High Modulus

  Density (g/cm^3^)      1.8            1.9            2.0 - 2.1

  Young=s Modulus (GPa)  230            370            520 - 620

  Tensile Strength (GPa) 2.48           1.79           1.03 - 1.31

  Tensile Elongation (%) 1.1            0.5            0.2
  ---------------------- -------------- -------------- -------------------

[Table 3 \[Ref. 5\]]{.underline}:

> Since its tensile strength decreases with increasing modulus, its
> strain at rupture will also be much lower. Because of the material
> brittleness at higher modulus, it becomes critical in joint and
> connection details, which can have high stress concentrations. As a
> result of this phenomenon, carbon composite laminates are more
> effective with adhesive bonding that eliminates mechanical fasteners.

**[Resin Systems]{.underline}**

The resin is another important constituents in composites. The two
classes of resins are the thermoplastics and thermosets. A thermoplastic
resin remains a solid at room temperature. It melts when heated and
solidifies when cooled. The long-chain polymers do not chemically cross
link. Because they do not cure permanently, they are undesirable for
structural application. Conversely, a thermosetting resin will cure
permanently by irreversible cross linking at elevated temperatures. This
characteristic makes the thermoset resin composites very desirable for
structural applications. The most common resins used in composites are
the unsaturated polyesters, epoxies, and vinyl esters; the least common
ones are the polyurethanes and phenolics.

-   ***Unsaturated Polyesters***

> The unsaturated polyester amounts to about 75% of all polyester resins
> used in USA. It is produced by the condensation polymerization of
> dicarboxylic acids and dihydric alcohols. The formulation contains an
> unsaturated material such as maleic anhydride or fumaric acid which is
> a part of the dicarboxylic acid component. The formulation affects the
> viscosity, reactivity, resiliency and heat deflection temperature
> (HDT). The viscosity controls the speed and degree of wet-out
> (saturation) of the fibers. The reactivity affects cure time and peak
> exotherm (heat generation) temperatures. High exotherm is needed for a
> thin section curing at room temperature and low exotherm for a thick
> section. Resiliency or flexible grade composites have a higher
> elongation, lower modulus, and HDT. The HDT is a short term thermal
> property which measures the thermal sensitivity and stability of the
> resins.
>
> The advantages cited in the unsaturated polyester are its dimensional
> stability and affordable cost. Other advantages include ease in
> handling, processing, and fabricating. Some of the special
> formulations are high corrosion resistant and fire retardants. This
> resin is probably the best value for a balance between performance and
> structural capabilities.

-   ***Epoxies***

> The epoxies used in composites are mainly the glycidyl ethers and
> amines. The material properties and cure rates can be formulated to
> meet the required performance. Epoxies are generally found in marine,
> automotive, electrical and appliance applications. The high viscosity
> in epoxy resins limits it use to certain processes such as molding,
> filament winding, and hand lay-up. The right curing agent should be
> carefully selected because it will affect the type of chemical
> reaction, pot life and final material properties. Although epoxies can
> be expensive, it may be worth the cost when high performance is
> required.

-   ***Vinyl Esters***

> The vinyl ester resins were developed to take advantage of both the
> workability of the epoxy resins and the fast curing of the polyesters.
> The vinyl ester has higher physical properties than polyesters but
> costs less than epoxies. The acrylic esters are dissolved in a styrene
> monomer to produce vinyl ester resins which are cured with organic
> peroxides. A composite product containing a vinyl ester resin can
> withstand high toughness demand and offer excellent corrosion
> resistance.

-   ***Polyurethanes***

> Polyurethanes are produced by combining polyisocyanate and polyol in a
> reaction injection molding process or in a reinforced reaction
> injection molding process. They are cured into very tough and high
> corrosion resistance materials which are found in many high
> performance paint coatings.

-   ***Phenolics***

> The phenolic resins are made from phenols and formaldehyde, and they
> are divided into resole and novolac resins. The resoles are prepared
> under alkaline conditions with formaldehyde/phenol (F/P) ratios
> greater than one. On the contrary, novolacs are prepared under acidic
> conditions with F/P ratios less than one. Resoles are cured by
> applying heat and/or by adding acids. Novolacs are cured when reacting
> chemically with methylene groups in the hardener. The phenolics are
> rated for good resistance to high temperature, good thermal stability,
> and low smoke generation.

**[Fillers]{.underline}**

Since resins are very expensive, it will not be cost effective to fill
up the voids in a composite matrix purely with resins. Fillers are added
to the resin matrix for controlling material cost and improving its
mechanical and chemical properties. Some composites that are rich in
resins can be subject to high shrinkage and creep and low tensile
strength. Although these properties may be undesirable for structural
applications, there may be a place for their use.

The three major types of fillers used in the composite industry are the
calcium carbonate, kaolin, and alumina trihydrate. Other common fillers
include mica, feldspar, wollastonite, silica, talc, and glasses. When
one or more fillers are added to a properly formulated composite system,
the improved performance includes fire and chemical resistance, high
mechanical strength, and low shrinkage. Other improvements include
toughness as well as high fatigue and creep resistance. Some fillers
cause composites to have lower thermal expansion and exotherm
coefficients. Wollastonite filler improves the composites= toughness for
resistance to impact loading. Aluminum trihydrate improves on the fire
resistance or flammability ratings. Some high strength formulations may
not contain any filler because it increases the viscosity of the resin
paste. High viscosity resins may have a problem wetting out completely
for composite with heavy fiber reinforcement. A filler should not be
used with fiber volume greater than 50% for the sheet molding composite
production method.

**[Additives]{.underline}**

A variety of additives are used in the composites to improve the
material properties, aesthetics, manufacturing process, and performance.
The additives can be divided into three groups \-- catalysts, promoters,
and inhibitors; coloring dyes; and, releasing agents. Their roles are as
simple as their names imply, and they need no further discussion here.

**[Design Considerations]{.underline}**

Professor Steenkamer and his coauthors at the University of Delaware
stated it well when they wrote: AThe development of a composite is a
complex process that requires the simultaneous consideration of various
parameters such as component geometry, production volume, reinforcement
and matrix types and relative volumes, tooling requirements, process and
market economics, etc. Every decision made during the product
development process is intricately related to a set of three interacting
decision=s areas (i.e., materials, processing, and configuration)@
\[8\].

The development of the advanced composite technology is an engineer=s
dream for innovative design and application. The characteristics of a
composite can be tailored and designed to meet any desired
specifications. Most of the information and design data available on
composites are in the aerospace applications, but they are protected
under the guise of proprietary systems and/or military classified
documents. Unlike conventional isotropic materials of steel and
concrete, there are no readily available design charts and guidelines to
help the structural engineer. When it comes to working with composites
as opposed to conventional materials, as the author has discovered, the
difference can be as dramatic as night and day.

The challenge in applying composites is for one to understand the
behavior of not only the constituents in the composites but also the
completed end product in the way they respond to an applied load. Since
a separate design specification for composites bridges is not yet
available, existing bridge design guidelines may have to be used with
some caution. Under the current American Association of State Highway
and Transportation Officials (AASHTO) LRFD bridge design specifications,
the philosophy is one of a probability-based limit state approach. The
four basic limit states that is applicable in bridge design with
advanced composite materials are the service, fatigue and fracture,
strength, and extreme event.

The service limit states dictate the level of deformation and crack
width under normal service conditions for a bridge to perform
satisfactorily during its service life. The fatigue and fracture limit
states restrict the stress range under normal service conditions within
an expected number of load cycles. They are to limit crack growth under
repetitive loading and to prevent fracture during the design life of a
bridge. The strength limit states are to ensure that both the global and
local strength and stability are provided to resist the statistically
significant load combinations as experienced by a bridge during its
design life. Some overstress and structural damage may be inevitable,
but the overall integrity of the structure will not be compromised. The
extreme event limit states ensure the structural survival of a bridge
during a major earthquake or unusual collision force.

Based on some of the design data obtained from completed composite
bridge structures to date, the deflection and/or local buckling govern
composite design. With the inherent low section modulus of a composite
structural member and critical high stress demand in structural
applications, a designer should consider the following features
carefully in his design:

-   Avoid abrupt thickness change in components

-   Take advantage of geometrical shapes

-   Take advantage of hybrid systems

-   Use bonded assemblies and joints

-   Provide good details for connected joints

-   ***Avoid abrupt thickness change in components***

> In steel or concrete design, an increase in the plate or flange
> thickness will usually keep the stresses under control. Although this
> concept also works for composites, it is inefficient for a composite
> member to follow suit by increasing its overall part thickness.
> Because composites are viscoelastic materials, it is undesirable to
> create high stress risers. An understanding in the stress flow of a
> structural member will help a designer tailor the parts= thicknesses
> locally and avoid abrupt changes in its geometry.

-   ***Take advantage of geometrical shapes***

> In most design using composites, the stress level is very low. An
> optimal design in composites balances the stress, deflection, and
> stability with the use of flanges, ribs, stiffeners, honeycomb or
> box-cells, or tubes to maximize the stiffness of the section. By
> placing flanges farther apart at the top and bottom of a hollow core,
> the section modulus can be designed to span longer structures. By
> proportioning and orienting the cells adequately, local buckling can
> be eliminated and material stiffness can be increased.

-   ***Take advantage of hybrid systems***

> By taking advantage of the high stiffness in concrete and the high
> strength in composites, concrete filled carbon composite tubes for
> piles and main superstructure members in bridges are found to be very
> cost effective. Structural timber beams reinforced with composites in
> strategic locations have demonstrated an increase in the beam
> capacity. Prestressing tendons in concrete beams and decks are being
> studied in South Dakota and Michigan. Pultruded carbon FRP composite
> laminates bonded to steel beams and concrete slabs are being
> considered for strengthening of bridges. Composite fiberglass rods
> replacing reinforcing bars in concrete bridge decks are being studied
> in West Virginia. With any of these hybrid systems, the designer
> should account for the difference in the strains of each material
> affecting the compatibility of the total unit.

-   ***Use bonded assemblies and joints***

> Much work needs to be done in developing good joints to assemble the
> composite members. The successful use of the epoxy adhesive technology
> from the aerospace industry has been transferred to many recent civil
> structural applications. The concept of using epoxied shear transfer
> toggle strips has been demonstrated in two composite bridges in the
> United Kingdom. Plate bonding using epoxy adhesive on thin laminates
> to strengthen civil structures is seen as a promising application. The
> column wrapped with carbon tows (sheets) will be as strong as the
> epoxy bonded overlapping splice. The ability to advance the composite
> technology in civil structures will depend on the integrity and
> durability of these joints.

-   ***Provide good details for connected joints***

> Discontinuities within a structural system can be a designer=s
> nightmare. Special attention must be given to the local stress flow,
> overall load path, and joint lines that create weak links or porosity
> introduced during the manufacturing process. Other irregularities
> introduced during the cutting/drilling and fit up process must be
> evaluated. It is important to select proper fasteners. Certain
> composites with high flexural modulus are very brittle and have a
> tendency to granulate; they would not be suitable with screws. The
> ability to connect the components into a structural system will enable
> composites to go far in civil applications. It needs a technological
> breakthrough from the current thinking of using nuts and bolts to
> connect its members.

**[Conclusion]{.underline}**

There has been a great deal of development in the composite technology.
It is an exciting time for civil engineers to be involved with
composites. Both the US Government and private institutions are funding
many demonstration bridge projects to show that advanced composite
materials can be applied to rebuilding our highway infrastructure.

There is much to be learned about composites. Manufacturers, bridge
owners, government officials, academia, researchers and contractors need
to work together. Trade secrets should be honored and respected to the
extent that information is provided to bridge owners to understand and
evaluate the behavior of their structures. The bridge owner has been and
will continue to be held responsible for the safety of the traveling
public during the service life of a structure. Current laws require
bridge owners to inventory and rate their bridges. The owners need to
know the pertinent information and data that are used in bridge design,
manufacturing, and fabricating. The AASHTO, ASTM, and American Concrete
Institute have established numerous technical committees to develop
design specifications, guidelines, standards, testing methods and
methodologies. The Composites Institute which represents the composite
industry has been coordinating the industry=s effort to develop product
design manuals, improve manufacturing processes, and collect test data.
Numerous universities are offering research and design courses in
composites. There are ample opportunities for civil engineers to
participate and contribute to this growing technology.

**[References]{.underline}**

\[1\] SIEBLE, F. & KARBHARI, V; A paper titled [Advanced Composites for
Civil Engineering Applications in the United States]{.underline},
University of California, San Diego, CA.

\[2\] BALLINGER, Craig; \"[Composites poised to make inroads as Highway
Structural Materials]{.underline},\" The Journal: Roads and Bridges;
April 1994 Issue, pp. 40-44.

\[3\] SMITH, W; [Resin Systems]{.underline}; Delaware Composites Design
Encyclopedia (DCDE), Volume 3; 1990.

\[4\] LINDSAY, Karen; [What is a Composite?]{.underline}; The Journal on
Composites Design and Application, Fall 1995 Issue; pp 29-30.

\[5\] ZWEBEN, C.; [Introduction to Mechanical Behavior and Properties of
Composites Materials]{.underline}; DCDE, Volume 1; 1989.

\[6\] ABEREGG, Dale; [SMC Design Guidelines for Structural
Applications]{.underline}, The Journal: CDA Summer 1996 Issue; pp 22.

\[7\] \"[Plastics and Composites in Construction]{.underline},\" The ENR
Journal, November 7, 1994.

\[8\] STEENKAMER, D.A.; WILKINS, D.J.; KARBHARI, V.M.; [Resin Transfer
Molding: Part 1 - Materials and Preforming]{.underline}; Nov. 1992; CCM,
UDEL, Newark, DE.

\[9\] ABEREGG, Dale, [SMC Design Guidelines for Structural
Applications]{.underline}, The Journal: CDA Summer 1996 Issue, pp. 22.

\[10\] WOLFE, Joanne, [Tips for Using Fasteners]{.underline}, The
Journal: CDA Summer 1996 Issue, pp. 34.

\[11\] LINDSAY, Karen; [State-of-the-Industry: 1995-1996]{.underline};
The Journal: CDA February 1996 Issue, pp. 12-16.

\[12\] FHWA Training Manual in LRFD Design of Highway Bridges, Modjeski
and Masters, Inc. October 1994.

\[13\] REISCH, Marc S.; [Advanced Polymer Composite Makers Look Toward
Infrastructure Market]{.underline}; March 25, 1996, C & EN.

\[14\] PODOLNY, Walter, Jr.; [Winds of Change and Paradigms of
Obsolescence]{.underline}; Proc., The National Steel Bridge Symposium,
October 15-17, 1996, Chicago, Illinois; The National Steel Bridge
Alliance, Chicago, Illinois.

[^1]: Kevlar7 is a registered trade mark of E.I. du Pont de Nemours &
    Co.