This invention relates to a composite foam structure having
a region of anisotropic strength and a region of isotropic strength, methods of
preparation of such composite foam structures and uses of such composite foam structures,
for instance in automobile energy absorbing applications, such as automobile bumpers.
Regulations in many countries around the world have been
promulgated which relate to energy absorbing articles or parts in automobiles. There
are regulations relating to the impact of the body to the interior of a vehicle.
In addition, some regulations relate to automobile bumpers which must protect the
automobile chassis from significant damage at low speed (that is 4-8 km/hr). Furthermore,
there are regulations that have been promulgated in Europe which relate to reducing
the number of injuries resulting from pedestrians being impacted by automobiles.
There are regulations that seek to limit or reduce the amount of emissions from
automobiles. One way to reduce the emissions is to lower the fuel consumption by
lowering the weight of vehicles, thus there is a move to replace heavier metal parts
with plastic and foam parts. As a result of these various regulations, foam materials
have found use in various energy absorbing structures or parts in automobiles, for
instance in headliners, pillars, doors and bumpers. U.S. Patent Publication US 2002/0121787
discloses a bumper system having a foam portion containing recesses extending through
a predetermined thickness of the foam. Disposed in the recesses are cylindrical
cell matrices which are configured to absorb energy from an impact on the bumper
from an external force. U.S. Patent Publication 2001/0035,658A1 (incorporated herein
by reference) shows a bumper fascia in which a portion of the fascia is filled with
an expanded bead foam. U.S. Patent No. 6,213,540 (incorporated herein by reference)
discloses the use of planks of anisotropic foam in front and rear-end systems. An
anisotropic foam exhibits exceptional energy management properties, such as a foam
made from a series of strands fused together with the strands oriented in the same
direction. When impacted, this foam absorbs energy by crushing and by using energy
to delaminate the foam strands from one another. The problem with the use of an
anisotropic foam is that once it has undergone a significant impact, the impact
causes the foam to deform and to lose its ability to absorb significant energy in
subsequent impacts. Furthermore, anisotropic foams made from oriented strands of
foam such as disclosed in U.S. Patent No. 6,213,540 could be fabricated cost effectively
into simple shapes only. In order to make complex shapes of such foams, a significant
amount of costly tooling and scrap are incurred. A significant problem with anisotropic
foams is that when subjected to low speed crashes, such as at 5 mph, the anisotropic
foam will absorb energy, but the foam part is not resilient, or recoverable, and
thus loses its ability to absorb similar levels of energy when the foam part is
subjected to a secondary impact. Thus, in many such situations it becomes necessary
to replace the foam structure after the initial impact. What is needed is a lightweight,
easily formed foam part which can absorb energy at slow speeds after multiple impacts.
Furthermore, what is needed is an energy absorbing system which will effectively
absorb energy at higher speeds, such as in pedestrian impacts.
The invention is a composite foam structure comprising
a first segment of an anisotropic thermoplastic foam and a second segment shaped
foam of a part comprising an isotropic foam wherein the anisotropic foam part and
isotropic foam part intersect along at least one common surface. Preferably, the
anisotropic foam comprises a plurality of extruded strands of foam which are fused
together wherein all of the strands of the foam are oriented in the same direction.
In another embodiment the invention is an automobile bumper
system comprising a bumper beam adapted to be attached to the chassis of a vehicle;
one or more composite structures according to the invention and a bumper fascia
which covers the bumper beam and the composite foam structures so that they are
not visible; wherein the isotropic foam part is disposed between the fascia and
the anisotropic foam portion of the composite structure. Preferably, the anisotropic
foam segment comprises a plurality of oriented strands of foam which are fused together,
which are attached to the bumper beam. More preferably the direction of oriented
strands of the anisotropic foam blocks is transverse to the longitudinal direction
of the bumper beam. The direction of orientation of strands of foam is preferably
perpendicular to the vertical surface of the bumper.
In another embodiment, the invention is a method of preparing
a composite structure according to the invention comprising placing a part of anisotropic
foam in a mold, placing thermoplastic beads impregnated with a blowing agent in
the remainder of the mold, injecting steam into the mold under conditions such that
the thermoplastic beads expand, melt at the surface, form a coalesced isotropic
foamed structure which bond to one or more surfaces of the anisotropic foam.
The composite foam structures of the invention illustrate
several advantages. First, the composite structures can be shaped in a cost-effective
manner without the need for costly and time-consuming machining processes. Furthermore,
the composite foam structures of the invention are resilient at low impacts, thus
allowing a part using structures of the invention to be subjected to a relatively
low impact without damaging the underlying structure. Further, such composite foam
structures exhibit the ability to absorb significant energy when subjected to high
speed pedestrian impacts.
- Figure 1 illustrates a bumper system utilizing the composite structures of the
- Figure 2 is a plot of recoverability as a function of anisotropic foam thickness.
- Figures 3 to 5 are representative plots of the affect of anisotropic foam thickness
on the quasi-static compressive stress versus strain response of composite foam
specimens which are thermally welded, adhesively bonded and taped together, respectively.
- Figures 6 to 8 are representative plots of the affect of anisotropic foam thickness
on the dynamic compressive stress versus strain response of composite foam specimens,
which are thermally welded, adhesively bonded or taped together, respectively.
As used herein an anisotropic foam means a foam which has
anisotropic strength properties. Anisotropic strength properties mean that the foam
structure has different impact strengths in different directions or along different
planes. As used herein, isotropic foam means foam that has substantially the same
strength properties in all directions. Substantially the same as used herein means
that the difference in strength properties in different directions is 10 percent
or less, preferably 5 percent or less, and most preferably 1 percent or less.
The anisotropic foam block can be prepared from any foam
which exhibits anisotropic strength properties. Further, it may be prepared by any
known process that provides an anisotropic foam or a foam having anisotropic properties.
Preferably, the foam is comprised of a thermoplastic polymer matrix.
Preferred foamable thermoplastic compositions which may
be used to prepare aniostropic foams include polyesters, polyamides, polyvinylchloride,
polyvinylidene chloride, polycarbonates, polystyrene resins, polyethylene, including
low density polyethylene, linear low density polyethylene, and high density polyethylene
(HDPE), polypropylene, and co-polymers of ethylene or propylene and a monoethylenically
unsaturated monomer copolymerizable therewith. Examples include copolymers of ethylene
and acrylic acid or methylacrylic acid and C1-4 alkyl esters or ionomeric
derivatives thereof; ethylene vinyl-acetate copolymers; ethylene/carbon monoxide
copolymers; anhydride containing olefin copolymers of a diene; copolymers of ethylene
and an alphaolefin having ultra low molecular weight (that is, densities less than
0.92 g/cc); blends of all of the above resins; blends thereof with polyethylene
(high, intermediate or low density); etc. Preferred polyolefins include polypropylene
homopolymers, impact grade polypropylene and copolymers of polypropylene that are
comprised of at least 50 percent propylene by weight. Other preferable polyolefins
include branched polypropylene homopolymer and branched copolymers of polypropylene.
The foamable thermoplastic polymer material or blend is
melt processed in a conventional manner by feeding, melting, metering it into a
conventional melt processing apparatus such as an extruder. A volatile blowing agent
and an optional cross linking agent are mixed with the thermoplastic polymer or
blend under a pressure suitable to form a flowable gel or admixture. A cross linking
agent may be added in an amount which is sufficient to initiate cross linking and
raise the pressure of the gel or admixture to less than that pressure which causes
melt fracture of the polymer to occur. The term "melt fracture" is used in the art
to describe a melt flow instability of a polymer as it is extruded through a die,
which flow instability causes voids and/or other irregularities in the final product.
It is also possible to add various additives such as inorganic fillers, pigments,
anti-oxidants, acid scavengers, ultraviolet absorbers, flame retardants, surfactants,
processing aids, extrusion aids, nucleating agents and blowing agents, and the like
as disclosed in U.S. Patent No. 6,213,540.
The amount of blowing agent mixed in the polymer is primarily
dependent upon the foam density that is desired. Densities of a foam according to
this invention are preferably in the range of 270 kg/m3 to 20 kg/m3,
more preferably 200 kg/m3 to 30 kg/m3, and most preferably
32 kg/m3 to 80 kg/m3. The foaming agent to achieve the foam
densities in the broadest range is in the range of 0.011 to 0.23 gram mole by weight
per 100 grams by weight of the thermoplastic resin or blend. In general, incorporation
of a greater amount of foaming agent results in a higher expansion ratio (the term
"expansion ratio" herein referred to means the ratio of the density of the resin
or blend to the density of the expanded product) and thus a lower foam density.
However, care must be taken not to incorporate an amount of foaming agent that causes
a separation between resin and foaming agent in the extruder and die. When this
happens, "foaming in the die" occurs, and the surface of the expanded product becomes
rough, generally producing an unsatisfactory product.
Other suitable methods of preparing and extruding foamable
thermoplastic blends to produce extruded profiles which may be useful in preparing
energy absorbing articles in accordance with the principles of this invention are
disclosed in U.S. Patent Nos. 5,348,795; 5,527,573 and 5,567,742, all of which are
hereby incorporated herein by reference.
In accordance with a particular aspect of this invention,
energy absorbing articles exhibiting anisotropic strength properties are prepared
by extruding a foamable thermoplastic gel through a die including a plurality of
orifices arranged such that contact between adjacent streams of molten extrudate
causes the surfaces thereof to adhere to form a unitary coalesced foam article,
wherein the polymeric component of the foamable gel comprises a polymer blend including
a major amount (greater than 50 percent by weight) of a homopolymer or copolymer
in which the majority of monomeric units are propylene monomeric units and a minor
amount (less than 50 percent by weight) of a polyethylene type resin modifier in
which the majority of monomeric units are ethylene monomeric units. It has been
discovered that the resulting coalesced strand foams produced from such polymeric
blends unexpectedly exhibit increased compressive strength, especially in directions
normal to the longitudinal direction of the coalesced strands (that is., the extrusion
direction), relative to coalesced strand foams made from polypropylene homopolymer
or from a blend of polypropylene homopolymer and an ethylene-propylene copolymer
resin modifier. The increased compressive strength, especially in directions normal
to the longitudinal direction of the coalesced strands, is attributable to improved
strand-to-strand adhesion caused by the addition of a minor amount of a polyethylene
type resin modifier which is a homopolymer or copolymer comprised entirely or mostly
of ethylene monomeric units. The polyethylene type resin modifiers will typically
have a melting point lower than that of the polypropylene homopolymer or copolymer,
and preferably have a melting point below 125°C. The polyethylene type resin
modifiers are comprised primarily of ethylene monomeric units, and more preferably
are comprised of at least 80 percent ethylene monomeric units by weight.
Examples of preferred polyethylene type resin modifiers
include low density polyethylene homopolymers and substantially linear ethylenic
polymers prepared using a metallocene catalyst as disclosed in U.S. 6,213,540 B1,
U.S. Patent Nos. 5,340,840, 5,272,236, 5,677,383 and 4,076,698.
The coalesced strand foam articles useful in this invention
may include missing strands or designed voids, that is, a profile or cross section
transverse to the extrusion direction which is discontinuous. Coalesced strand foam
articles having missing strands or designed voids can be prepared by extruding a
thermoplastic foamable gel through a die having a multiplicity of orifices arranged
in an array defining voids such that the extruded strands are joined at their extremities
to form a network defining voids. A die having a multiplicity of orifices which
is designed to produce coalesced strand foams which do not have missing strands
or designed voids can be modified to produce coalesced strand foams having missing
strands or designed voids by blocking off some of the orifices. Energy absorbing
articles having designed voids may be advantageously employed in certain applications
to allow manipulation of bulk density and softness (modulus control), and increase
air flow. Also, the designed voids may be used for creating raceways for wires,
optical fibers, and the like. Methods of forming stranded foam articles having designed
voids are described in U.S. Patent No. 4,801,484 which is incorporated by reference
Alternatively, the coalesced strand foam articles used
in this invention can be prepared with hollow foamed strands as described in U.S.
Patent No. 4,755,408 and U.S. Patent No. 4,952,450.
By forcing the extrudable mixture through the extrusion
die the invention, one obtains (a) a plurality of separately extruded and thereafter
coalesced hollow foam strands or (b) a combination of a plurality of separately
extruded and thereafter coalesced solid foam strands and a plurality of separately
extruded and thereafter coalesced hollow foam strands. In a preferred embodiment,
a grouping of solid, coalesced foam strands forms one part of a composite cellular
foamed structure and a grouping of hollow, coalesced foam strands forms a second
part of a composite cellular foamed structure. The blowing agent composition at
least partially determines the temperatures for each of the process steps. Preparation
of such a composite foamed structure preferably employs a die that has a plurality
of first orifices or orifice sets that yield hollow foam strands and a plurality
of second or single orifices that yield solid foam strands.
Where the extrusion die provides for both solid and hollow
foam strands, die apertures or orifices for both the solid and hollow foam strands
may take on nearly any geometric shape so long as the shapes yield, as appropriate,
the respective solid and hollow foam strands. Suitable geometric shapes include
round, square, polygonal, x-shapes, cross-shapes and star-shapes. Selection of a
particular shape or combination of shapes allows production of a cellular foamed
structure with a specific profile or shape. The shape is preferably round or circular,
especially for the hollow foam strands. Where the cellular foam structures of the
invention comprise both hollow foam strands and solid foam strands, the geometric
shapes, while preferably the same, may differ without departing from the scope and
spirit of the invention.
The extruded thermoplastic foams exhibiting anisotropic
strength property may also be prepared as extruded planks. Extruded thermoplastic
foam planks may be pulled through a slit die at generally any rate of speed commonly
used, and can be stretched or pulled from the die by any means known in the art,
such as by pulling with opposing belts, nip rollers and like take-up means.
Although a particularly preferred method of preparing the
energy absorbing articles exhibiting anisotropic strength properties involves extruding
a foamable thermoplastic gel through a die including a multiplicity of orifices
arranged such that contact between adjacent streams of molten extrudate causes the
surfaces thereof to adhere and form a unitary coalesced foam article, other methods
known to those skilled in the art may be employed to form thermoplastic foam articles
exhibiting similar or equivalent anisotropic strength properties. Alternative methods
of forming foam articles exhibiting anisotropic strength properties are disclosed
in U.S. Patent No. 6,213,540B1, see column 7, line 64 to column 9, line 58, incorporated
herein by reference.
Although preparation of the extruded thermoplastic foams
useful in the invention preferably employs a heated extruder, skilled artisans can
readily substitute other apparatus that accomplish the same purpose. U.S. Patent
No. 5,817,705 and U.S. Patent 4,323,528, the teachings of which are incorporated
herein by reference, discloses one such apparatus. This apparatus, commonly known
as an "extruder-accumulator system" allows one to operate a process on an intermittent,
rather than a continuous, basis. The apparatus includes a holding zone or accumulator
where the foamable composition remains under conditions that preclude foaming. The
holding zone is equipped with an outlet die that opens into a zone of lower pressure,
such as the atmosphere. The die has an orifice that may be open or closed, preferably
by way of a gate that is external to the holding zone. Operation of the gate does
not affect the foamable composition other than to allow it to flow through the die.
Foam expansion following extrusion of a foamable composition
from the extrusion die suitably takes place under normal atmospheric conditions.
The anisotropic strength properties of any of the previously
described embodiments of the invention may be further enhanced or augmented by incorporation
of continuous and/or discontinuous reinforcing fibers which may be oriented length
wise in a direction in which high impact resistance is desired. Suitable reinforcing
fibers include synthetic fibers, such as aramid, polyester, and polyolefin fibers;
natural fibers, such as sisal; ceramic fibers; glass fibers; metallic fibers; and
the like, as disclosed in U.S. Patent No. 6,213,540B1 at column 10, lines 1 to 15,
incorporated herein by reference.
The anisotropic foam can be fabricated into any desired
shape. In a preferred embodiment the anisotropic foam is a plank, sheet or block
of foam as most processes for preparing anisotropic foam form the foam in a plank,
sheet or block. Such shapes generally have a square or rectangular shape and have
multiple flat surfaces or faces.
The isotropic foam can be any foam which has isotropic
properties prepared by any process which prepares a foam having isotropic properties.
Preferably, the foam is prepared from a thermoplastic or thermoset matrix. Preferably,
the foam is thermoplastic polymer based foam. Generally, any of the thermoplastic
material that can be used to prepare anisotropic foam may be used to make isotropic
foam. The selection of the process used to make a foam determines whether an isotropic
or anisotropic foam is prepared.
In the embodiment wherein the foam used is made from a
thermoset polymer, the thermoset polymer may be a polyurethane, polyurea, polyisocyanurate
or polymerized epoxy resin matrix foam. Such foams can be prepared by well-known
means in the art. Preferably, the thermoset foams are prepared from reactive systems
wherein such reactive systems are poured or injected into a mold and allowed to
cure to form the desired shape. A polyurethane foam preferably comprises the reaction
product of an organic isocyanate; a compound containing isocyanate reactive active
hydrogen groups; a catalyst; a blowing agent, for instance water; and optionally,
one or more additives such as surfactants, crosslinkers, chain extenders, pigments,
stabilizers, fungistats, bacteriostats, fillers and flame retarding agents.
Suitable polyisocyanates may be used in the preparation
of polyurethane based thermoset foams are described in U.S. Patent No. 6,028,122
at column 3, lines 31 to 56, relevant portions incorporated herein by reference,
see also U.S. Patent No. 6,127,443 at column 6, lines 33 to 51, relevant portions
incorporated herein by reference. Suitable polyols useful in the preparation of
polyurethane foams are well known to those skilled in the art and include those
disclosed in U.S. Patent No. 6,028,122 at column 3, lines 58 to column line 5, relevant
portions incorporated herein by reference. Useful catalysts in preparing polyurethane
foams are disclosed in U.S. Patent No. 6,028,122 at column 7, line 6 to line 17;
U.S. Patent No. 6,127,443 at column 6, line 1 to 16; and in U.S. Patent No. 6,316,514
at column 8, lines 1 to 38, relevant portions incorporated herein by reference.
Useful blowing agents for the preparation of polyurethane foams are disclosed in
U.S. Patent No. 6,316,514 at column 7, lines 37 to 56, relevant portions incorporated
herein by reference. Other suitable additives in polyurethane foam preparation formulations
are disclosed in U.S. Patent No. 6,028,122 at column 7, lines 17 to 50, U.S. Patent
No. 6,127,443 at column 5 line 35 to Column 6, line 32; and in U.S. Patent No. 6,316,514
at column 8, line 39 to column 9, line 53.
To produce foams useful in the present invention, the polyisocyanates
(a), the polyols, (b) and, if desired, further compounds, (c) bearing hydrogen atoms
which are reactive toward isocyanates are reacted in such amounts that the equivalence
ratio of NCO groups of the polyisocyanates (a) to the sum of the reactive hydrogen
atoms of the components (b) and, if used, (c) is 0.7-1.25:1, preferably 0.90-1.15:1.
Polyurethane foams are advantageously produced by the one-shot method, for example,
by means of the highpressure or low-pressure technique, in open or closed molds,
for example, metallic molds. The continuous application of the reaction mixture
onto suitable conveyor belts for producing slabstock foam is also customary. The
polyols, chain extenders, additives, isocyanates, etc. are mixed together and introduced
generally into an open or closed mold. The mixing may take place entirely in the
mixhead, that is, using a multi-stream, high pressure mixhead, or the isocyanate-reactive
components and isocyanate components may be separately blended into respective A
(isocyanate) and B (resin active hydrogen component) sides, and mixed in the mixhead.
Water is generally included as a reactive blowing agent, for example, in amounts
of from 0.1 weight percent or higher relative to the total weight of the ingredients,
to 2 weight percent or lower. Preferably, water is employed in amounts of 0.1 weight
percent to 2 weight percent, more preferably 0.2 weight percent to 1.5 weight percent.
Foam densities are preferably in the range of 270 kg/m3 to 30 kg/m3,
more 200 kg/m3 to 50 kg/m3, and most preferably 65 kg/m3.
In a preferred embodiment the isotropic foams are prepared
such that the foams are closed-cell foams so as to prevent water absorption by the
foams. Skilled artisans are familiar with techniques for preparing closed cell foams.
In another embodiment the isotropic foam is thermoplastic
foam that may be made by conventional extrusion processes such as those disclosed
in U.S. Patent No. 3,960,792; U.S. Patent No. 4,636,527 and U.S. Patent No. 5,106,882,
incorporated herein by reference. In another embodiment the isotropic foam is bead
foam. Bead foam comprises expanded fused bead pellets that have a cellular microstructure.
In general, the beads used to make bead foams are prepared by contacting blowing
agent and molten thermoplastic polymer and forming pellets or beads upon cooling
of the mixture. The beads comprise a polymer with foaming agent or blowing agent
entrained therein. Processes for preparation of such beads are disclosed in U.S.
Patent No. 5,026,736, at column 1, line 19 to column 4, line 16, incorporated herein
by reference. In preferred embodiments the beads used to make the bead foam are
pre-expanded which means that they are exposed to an operation that partially expands
the bead. Processes for expansion of such beads are well known in the art. Processes
for pre-expanding the beads are disclosed in U.S. Patent No. 5,454,703 at column
1, lines 30 to 55, incorporated herein by reference. Bead foams are prepared at
contacting the thermoplastic beads containing blowing agent in a mold of the desired
shape exposing them to heat, such as by passing steam through the bed of beads to
cause release of the blowing agent and foaming and expansion of the beads. Usually
in this process the outer surfaces of the beads become molten and the adjacent beads
then fuse together as the beads expand. Specific processes for preparing such bead
foams can be found in U.S. Patent No. 5,454,703 at column 1, lines 58 to column
2, line 24, incorporated herein by reference.
The composite structures of the invention are prepared
by contacting a segment, such as a block, plank or shaped part, of an anisotropic
foam with a shaped part of isotropic foam along one common face, surface or the
like, and thereafter securing such common face, surface or the like together. The
anisotropic foam segment can be contacted on more than one side with isotropic foam.
The number of sides or faces of anisotropic foam segment in contact with isotropic
foam segment is primarily dictated by the application for which the composite structure
is used. In one embodiment the anisotropic foam, block or plank is contacted on
all but one surface or face with anisotropic foam. In another embodiment the isotropic
foam completely encapsulates the anisotropic foam.
The anisotropic foam block can be foamed or formed into
the desired shape in which it is used, or the anisotropic foam block may be cut
and shaped into the desired shape of the energy absorbing article. The composite
structure may be prepared by any process that results in the preparation of the
desired composite of the desired shape. The anisotropic block and isotropic part
may be joined by hot air welding, steam welding, radio frequency welding, adhesives,
mechanical fasteners, friction fit, or the like, to form a composite structure.
In this manner, composite structures having unusual shapes, or containing portions
of various densities may be prepared. In a more preferred embodiment the anisotropic
foam is placed in a mold and the material used to make the isotropic foam is placed
around it and thereafter processed to form isotropic foam around the anisotropic
foam. In the embodiment wherein the anisotropic foam is a thermoset foam, the reactive
components which are used to prepare the thermoset foam are flowed in around the
anisotropic foam and allowed to react to form an anistropic foam adjacent to the
desired surfaces of the anisotropic foam. Such a foam can be formed on from one
to all sides, faces or surfaces of the anisotropic foam. Alternatively, the anisotropic
foam can be partially encapsulated with the isotropic foam. In another embodiment
the isotropic foam can be molded separately from the anisotropic foam wherein the
isotropic foam is formed with a cavity into which the anisotropic foam can be inserted.
Preferably, the anisotropic foam and isotropic foam can
be contacted and adhered to one another by heat welding, an adhesive, adhesive tape
or friction fit. In the embodiment where heat welding is used, the surface of the
anisotropic foam and the isotropic foam to be contacted are heated until the polymer
at the surfaces is in the molten state, the two parts are contacted and then as
the polymer surface cools the parts are then welded together. For instance, in the
embodiment where the parts are made from polypropylene, the polypropylene surface
can be melted at 450°F, (232°C), for 30-60 seconds and thereafter contacted.
In another embodiment the anisotropic and isotropic foam
parts can be contacted along the desired surfaces and adhered together using an
adhesive composition. Adhesives known in the art may be employed to adhere isotropic
and anisotropic foam parts together. Useful adhesives include thermoset adhesives
such as polyurethane resins and epoxies and thermoplastic adhesives such as polyethylenes,
polypropylenes, ethylene copolymers; propylene copolymers; and the like. Among useful
adhesives are those taught in U.S. Patent Nos. 5,460,870 and 5,670,211. The adhesive
may be applied by any means known in the art such as by spraying, coating, or in
film form. Preferred adhesives are thermoplastic because of their lower cost and
potential recyclability. Preferably, the adhesive used is a low energy surface plastic
adhesive. By low energy surface plastic is meant materials that have a surface energy
of less than 45 mJ/m2, suitably less than 40 mJ/m2 and desirably
less than 35 mJ/m2 including, by way of example polypropylene and polyamide.
If desired, the surface of the foam segments may be treated or primed to improve
adhesion prior to application of the adhesive. Preferably the foam segments are
not subjected to treatment or priming and the adhesive is applied directly to the
surface of the foam segments.
In a preferred embodiment, the adhesive is a low energy
surface plastic adhesive which comprises a polymerizable composition comprising
an organoborane/amine complex and one or more of monomers, oligomers or polymers
having olefinic unsaturation which is capable of polymerization by free radical
polymerization. Optionally, the adhesive may additionally comprise a compound which
causes the complex to disassociate so as to release the borane to initiate polymerization
of one or more of monomers, oligomers or polymers having olefinic unsaturation.
Where a compound which causes the complex to disassociate is employed, it is kept
separate from the complex until initiation of polymerization is desired. The polymerizable
composition which contains the disassociating agent may be cured at any desired
temperature, such as at, or near, ambient temperature and below ambient temperature.
Such low energy surface adhesives comprise preferably acrylate
based adhesive compositions wherein polymerization of the acrylate adhesive material
is initiated using an alkylborane amine complex. Such alkylborane amine complex
further enhances the adhesion of the alkyacrylate to low energy surface adhesive
materials such as thermoplastic foam materials. The adhesive is preferably derived
from a polymerizable composition comprising
- i) an organoborane/amine complex;
- ii) one or more of monomers, oligomers or polymers having olefinic unsaturation
which is capable of polymerization by free radical polymerization; and, optionally
- iii) a compound which causes the said complex to disassociate so as to release
the borane to initiate polymerization of one or more of monomers, oligomers or polymers
having olefinic unsaturation.
Adhesives and polymerizable compositions disclosed in Application
No. PCT/US00/33806 and WO 2003/038006 and U.S. Patent Publication 2002-0033227;U.S.
Patent Publication 2002-0058764 and 2002-028894; U.S. Patent No. 5,106,928; U.S.
Patent No. 5,143,884; U.S. Patent No. 5,286,821; U.S. Patent No. 5,310,835; U.S.
Patent No. 5,376,746; U.S. Patent No. 5,539,070; U.S. Patent No. 5,616,796; U.S.
Patent No. 5,621,143; U.S. Patent No. 5,681,910; U.S. Patent No. 5,686,544; U.S.
Patent No. 5,718,977; U.S. Patent No. 5,795,657 and U.S. Patent No. 5,883,208, all
incorporated herein by reference are especially preferred for use in the present
invention to bond the structural member and reinforcing member together.
The organoborane used in the complex is a trialkyl borane
or an alkyl cycloalkyl borane. Preferably such borane corresponds to Formula 1:
)3 Formula 1
wherein B represents Boron; and R2 is separately in each occurrence a
C1-10 alkyl, C3-10 cycloalkyl, or two or more of R2
may combine to form a cycloaliphatic ring. Preferably R2 is C1-4
alkyl, even more preferably C2-4 alkyl, and most preferably C3-4
alkyl. Among preferred organoboranes are tri-ethyl borane, tri-isopropyl borane
The amines used to complex the organoborane compound can
be any amine or mixture of amines which complex the organoborane and which can be
decomplexed when exposed to a decomplexing agent. Preferred amines include the primary
or secondary amines or polyamines containing primary or secondary amine groups,
or ammonia as disclosed in Zharov U.S. Patent No. 5,539,070 at column 5, lines 41
to 53, incorporated herein by reference, Skoultchi U.S. Patent No. 5,106,928 at
column 2, line 29 to 58, incorporated herein by reference, and Pocius U.S. Patent
No. 5,686,544 at column 7, line 29 to Column 10 line 36, incorporated herein by
reference; ethanolamine, secondary dialkyl diamines or polyoxyalkylenepolyamines;
and amine terminated reaction products of diamines and compounds having two or more
groups reactive with amines as disclosed in Deviny U.S. Patent No. 5,883,208 at
column 7, line 30 to column 8 line 56, incorporated herein by reference. With respect
to the reaction products described in Deviny the preferred diprimary amines include
alkyl diprimary amines, aryl diprimary amines, alkyaryl diprimary amines and polyoxyalkylene
diamines; and compounds reactive with amines include compounds which contain two
or more moieties of carboxylic acids, carboxylic acid esters, carboxylic acid halides,
aldehydes, epoxides, alcohols and acrylate groups.
In one preferred embodiment, the amine comprises a compound
having a primary amine and one or more hydrogen bond accepting groups, wherein there
are at least two carbon atoms, preferably at least three, between the primary amine
and hydrogen bond accepting groups. Preferably, an alkylene moiety is located between
the primary amine and the hydrogen bond accepting group. Hydrogen bond accepting
group means herein a functional group that through either inter- or intramolecular
interaction with a hydrogen of the borane-complexing amine increases the electron
density of the nitrogen of the amine group complexing with the borane. Preferred
hydrogen bond accepting groups include primary amines, secondary amines, tertiary
amines, ethers, halogen, polyethers, thioethers and polyamines. Among preferred
amines of this class are dimethylaminopropyl amine, methoxypropyl amine, dimethylaminoethylamine,
dimethylaminobutylamine, methoxybutyl amine, methoxyethyl amine, ethoxypropylamine,
propoxypropylamine, amine terminated polyalkylene ethers (such as trimethylolpropane
tris(poly(propyleneglycol), amine terminated)ether), and aminopropylpropanediamine.
In another embodiment the amine is an aliphatic heterocycle
having at least one nitrogen in the heterocycle. The heterocyclic compound may also
contain one or more of nitrogen, oxygen, sulfur or double bonds. In addition, the
heterocycle may comprise multiple rings wherein at least one of the rings has nitrogen
in the ring.
In yet another embodiment, the amine which is complexed
with the organoborane is an amidine. Any compound with amidine structure which decomplexes
from the alkylborane when exposed to decomplexing agent or heat may be used. Among
preferred amidines are 1,8-diazabicyclo[5,4]undec-7-ene; tetrahydropyrimidine; 2-methyl-2-imidazoline;
and 1,1,3,3-tetramethylguanidine, and the like.
In yet another embodiment, the amine that is complexed
with the organoborane is a conjugated imine. Any compound with a conjugated imine
structure, which decomplexes from the alkylborane when exposed to decomplexing agent
or heat may be used. The conjugated imine can be a straight or branched chain imine
or a cyclic imine.
In another embodiment the amine can be an alicyclic compound
having bound to the alicyclic ring a substituent containing an amine moiety. The
amine containing alicyclic compound may have a second substituent that contains
one or more nitrogen, oxygen, sulfur atoms or a double bond. The alicyclic ring
can contain one or two double bonds. The alicyclic compound may be a single or multiple
ring structure. Preferably the amine on the first substituent is primary or secondary.
Preferably the alicyclic ring is a 5 or 6 membered ring. Preferably functional groups
on the second substituent are amines, ethers, thioethers or halogens. Included in
amine substituted alicyclic compounds is isophorone diamine and isomers of bis(aminoethyl)
Compounds capable of free radical polymerization which
may be used in the polymerizable compositions include any monomers, oligomers, polymers
or mixtures thereof which contain olefinic unsaturation which can polymerize by
free radical polymerization. Such compounds are well known to those skilled in the
art. Mottus, U.S. Patent No. 3,275,611, provides a description of such compounds
at column 2, line 46 to column 4, line 16. Examples of preferable acrylates and
methacrylates are disclosed in Skoultchi, U.S. Patent No. 5,286,821 at column 3,
lines 50 to column 6, line 12 and Pocius, U.S. Patent No. 5,681,910 at column 9,
line 28 to column 12, line 25.
The organoborane amine complexes useful for polymerization
of the compounds having moieties capable of free radical polymerization require
the application of a decomplexation agent or heat to displace the amine from the
borane and initiate free radical polymerization. The displacement of the amine from
the alkylborane can occur with any chemical for which the exchange energy is favorable,
such as mineral acids, organic acids, Lewis acids, isocyanates, acid chlorides,
sulphonyl chlorides, aldehydes, and the like. Preferred decomplexation agents are
acids and isocyanates. Polymerization may also be initiated thermally. The temperature
at which the composition is heated to initiate polymerization is dictated by the
binding energy of the complex. Generally the temperature used to initiate the polymerization
by decomplexing the complex is 30°C or greater and preferably 50°C or
greater. Preferably the temperature at which thermally initiated polymerization
is initiated is 120°C or less and more preferably 100°C or less. Any heat
source that heats the composition to the desired temperature can be used, provided
the heat source does not negatively impact the components of the composition or
In the embodiment wherein the anisotropic foam and the
isotropic foam are held together by friction fit, the anisotropic foam is partially
encapsulated by the isotropic foam such that the fit is tight and friction prevents
easy removal of the anisotropic foam form contact with the isotropic foam.
The relative amount of anisotropic and isotropic foam used
depends upon the application and the requirements of the application. The requirements
of energy absorbing efficiency and recoverability are balanced. If higher energy
absorbing efficiency is required more anisotropic foam is desired. On the other
hand if more recoverability is desired more isotropic foam is desired. To get a
good balance of the properties the relative thickness of each of the two foams is
preferably from 40 percent or greater of the total thickness each and more preferably
45 percent or greater. To get a good balance of the properties the relative thickness
of each of the two foams is preferably 60 percent or less of the total thickness
and more preferably 55 percent or less of the total thickness.
The composite structures of this invention have broad applicability
in transportation, shipping containers, building and construction, as well as automotive
applications. Automotive applications include energy absorbing structures for front
and rear end systems, doors, pillars, headliners and instrument panel components.
The composite structure of the invention demonstrates good resilience upon being
subjected to low speed impact wherein there is minimal visible damage to the anisotropic
foam part of the structure. Furthermore, at higher speed impacts, the anisotropic
foam structure provides excellent energy absorption characteristics thereby protecting
the remainder of the vehicle and the occupants of the vehicle from the impact. In
a more preferred embodiment the absorbing structures of the invention are utilized
in automobile bumpers.
In one embodiment the invention is an automobile bumper
system comprising a bumper beam adapted to be attached to the chassis of a vehicle;
one or more composite structures of the invention and a bumper fascia which covers
the bumper beam and the composite foam structures so that they are not visible;
wherein the isotropic foam part is disposed between the fascia and the anisotropic
foam portion of the composite structure, (preferably the anisotropic foam comprises
a plurality of oriented strands of foam which are fused together which are attached
to the bumper beam wherein the direction of oriented strands of the anisotropic
foam blocks is transverse to the longitudinal direction of the bumper beam). Preferably,
more than one composite structure is attached to the bumper beam. In this embodiment
it is preferable that the composite structure be oriented such that the isotropic
foam is on the outside of the structure and the anisotropic foam is closest to the
bumper beam. Preferably, the highest strength direction of the anisotropic foam
is perpendicular to the bumper beam and is oriented in a direction protruding from
the bumper beam. The composite foam structures in preferred embodiments can be located
in front of parts of the vehicle which need particular protection, for instance
in front of the chassis rail so as to protect the chassis rail of the vehicle. The
composite structure of the invention may be attached to the bumper beam by any known
means of attachment including by using mechanical fixturing or adhesives, such as
low energy surface adhesives. Preferred adhesives for bonding composite structures
of the invention to the bumper beam include the low energy surface adhesives described
hereinbefore. Due to packaging restrictions, the composite foam structures of the
invention when used in bumpers are preferably 75 mm or less and preferably between
50 mm and 75 mm. In one embodiment the thickness of the ansiotropic foam is 25 mm
or greater, and more preferably 30 mm or greater. Preferably the thickness of the
anisotropic foam is 50 mm or less and more preferably 40 mm or less. In one embodiment
the thickness of the isotropic foam is 25 mm or greater, and more preferably 30
mm or greater. Preferably the thickness of the isotropic foam is 50 mm or less and
more preferably 40 mm or less. Such thicknesses refer to the relative thicknesses
in the direction of the anticipated impact. In one embodiment the foam in the bumper
can be completely anisotropic.
Figure 1 illustrates a bumper system utilizing the composite
structures of the invention. In particular Figure 1 illustrates a bumper system
(11). The bumper system has three foam blocks (12) which are fastened to a bumper
beam (13) which is affixed to two chassis rails (14). Each of the foam blocks has
an anisotropic (15) and an isotropic (16) portion with the strongest being the direction
of highest strength protruding from the bumper beam in the direction of away from
the bumper beam. Covering the entire structure is a bumper fascia (17).
The foams preferably have compressive strengths in the
range useful for energy absorbing applications, that is, from 50 kPa 2000 kPa, preferably
50 kPa to 1500 kPa, and more preferably 70 kPa to 1000 kPa. The efficiency and recovery
of the foam are key factors in foam performance. Factors such as compressive strength,
energy management, compression, etc., are measured by or calculated from measurements
of foam properties according to ASTM D-1621 "Standard Test Method For Compressive
Properties of Rigid Cellular Plastics." The test methodology employs an Instron
Model 55R 112S with a 100 kN load cell. Crosshead speed is 2.54 mm/min per 25.4
mm of sample thickness. Samples are nominal 101.6 mm x 101.6 mm x 25.4 mm. All sample
length and width dimensions are measured to three significant digits using Mitutoyo
Digimatic™ calipers, and thickness measurements made using a Mitutoyo Digimatic™
indicator type IDF-150E. All samples are conditioned for at least 40 hours at 23°C
and 50 percent relative humidity prior to testing under standard laboratory conditions.
The Instron tester collects force or load data continuously during deflection such
that force (N) is collected versus deflection (mm).
Compressive stress (kPa) is calculated from the force divided
by the sample cross-sectional area. Compression (percent) is calculated from the
deflection divided by the original sample thickness multiplied by 100. Sample compression
continues to 80 percent or until the load limit of the instrument is reached Compressive
strength is calculated in accordance with section 9.3.3 of ASTM D1621-00. Recovery,
R, (percent) is calculated from the sample thickness measured 5 min after compression
to 80 percent divided by the original sample thickness multiplied by 100.
The following examples are provided to illustrate the invention,
but are not intended to limit the scope thereof. All parts and percentages are by
weight unless otherwise indicated.
A series of quasi-static and dynamic compression tests
were performed on composite foam blocks which consisted of varying levels of 46.6
kg/m3 isotropic expanded polypropylene (EPP) bead foam and 37.3 kg/m3
anisotropic polypropylene (PP) foam. The density for each material was reported
as the mean of five samples that were measured and weighed to compute the specimen
density. In addition, composite blocks were joined together using the following
methods to simulate potential manufacturing methods: no adhesive interface, Low
Energy Surface Adhesive (LESA) interface, and thermally bonded interface. For those
samples that were not adhered, the perimeter of the blocks were simply joined with
masking tape to simulate the anisotropic foam block being press fit into an isotropic
foam cavity. Thermally bonded samples were prepared by placing the interfacial surface
of each block on the surface of a 232°C polytetrafluoroethylene (PTFE) coated
plate. The samples were held against the surface of the plate for approximately
thirty seconds after which they were then manually pressed together to allow the
molten interface to cool.
Samples as described above were tested according to GM216M
- Recoverable Energy Absorbing Foam Specification (General Motors Corporation specification).
This specification defined the minimum physical requirements for a cellular plastic
product known as recoverable energy absorbing polyurethane foam. This product's
main application was as an energy absorbing material, used in conjunction with other
materials and structures to meet the requirements of United States Federal Motor
Vehicle Safety Standard (FMVSS) 201U and FMVSS 214, or for energy absorbing bumpers
per FMVSS 581. This material may be used for energy absorbing applications where
recoverable foam is desired, such as knee bolsters, instrument panel topper pads,
airbag doors, side impact door bolsters, and head impact countermeasures.
Per Table 2 (Specific Physical Requirements) of GM216M,
recoverability was to be measured using a 50-millimeter (mm) foam cube. Prior to
testing, the initial thickness (ti) of the foam specimen is to be measured
to the nearest 0.03 mm. On a non-perforated surface, the specimen was then to be
compressed to 75 percent of its original thickness at a rate of 101.6 mm/min. The
final thickness (tf) was to be measured to the nearest 0.03 mm approximately
five minutes after removal of the load. Recoverability, in percentage, was then
computed from the following equation:
The minimum level of recoverability accepted per the GM216M
specification was 80 percent.
Quasi-Static Compression Test Equipment
All quasi-static compression tests conducted in accordance
with GM216M were performed on a low-rate servohydraulic test system available from
Materials Test System (MTS). The servohydraulic test system was equipped with a
0-22.2 kiloNewton (kN) load cell and a 0-125 mm linear variable differential transformer
(LVDT) to measure transient force and displacement respectively. All foam samples
were placed in a compression test fixture consisting of two flat aluminum plates
and four polished shafts guided on linear bearings. The aluminum plates were substantially
larger than the surface area of the foam specimen to ensure that the sample was
compressed uniformly throughout the test.
The sample thickness was measured before and five minutes
after removal of the load using digital calipers, available from Mitutoyo America
Corporation, with a full-scale range of 150 mm. Five replicate tests were performed
on each series of foam samples.
Three sets of samples were run. In one the isotropic foam
was adhesively bonded to the anisotropic foam and in a second the foams were thermally
bonded and in the third the foam parts were taped together. Several tests were performed
using a total mm thickness of the sample with from 0 to 50 mm of anisotropic foam
at 10 mm increments. The results of the testing were compiled in Table 1.
No Adhesive Interface
LESA Adhesive Interface
A plot of GM216M recoverability (percent) as a function
of anisotropic foam thickness was plotted in Figure 2 for all three attachment methods.
The data clearly demonstrated that 80 percent recoverability
may be achieved with approximately 20-30 millimeters of anisotropic foam thickness,
or 40-60 percent of the total composite thickness. Moreover, the data also demonstrated
that there is very little effect of the method of joining on recoverability.
Quasi-Static Compressive Response
In contrast to recoverability, one method to characterize
the compressive response of any foam was to compute its constitutive response in
the form of a stress versus strain curve. The transient stress, &sgr;, and strain,
&egr;, of any given material may be computed from the following equations
where F is the transient force as measured by the load cell, A was the surface area
of the foam specimen, &Dgr;t was the change in thickness of the foam sample, and
t0 was the original thickness of the foam sample prior to testing. Several
tests were performed using a total mm thickness of the sample of 50 mm with from
0 to 50 mm of anisotropic foam at 10 mm increments with the remainder being isotropic
foam. Representative plots of the effect of anisotropic foam thickness on the quasi-static
compressive stress versus strain response of composite foam specimens were illustrated
in Figures 3 to 5.
Figure 3 showed the plot for thermally welded parts. Figure
4 showed the plot for adhesively bonded parts and Figure 5 showed the plot for taped
The specimens tested had a size of 50 mm x 50 mm x 50 mm
and the actuator velocity was 101.6 mm/min.
The quasi-static compression stress levels at 70 percent
strain for each tested attachment system at the various anisotropic foam thicknesses
tested were illustrated in Table 2.
Quasi-Static Compression Stress Levels @ 70Percent Strain
No Adhesive Interface
For high-speed impacts, engineers typically desire materials
that were capable of stabilizing the loading, or stress, as intrusion, or strain,
increases. Thus, engineers often refer to this desired attribute as a square wave
response. The quasi-static compressive stress versus strain curves clearly demonstrated
that increasing the anisotropic foam content results in a decrease in the transient
stress at higher strain levels (that is >30 percent). Thus, this type of response
indicated an improvement in performance as composite thickness was held constant
or the ability to decrease composite thickness while achieving equivalent performance
to samples containing lower levels of anisotropic foam.
Dynamic Compression Test Equipment
In an effort to characterize the compressive response of
composite foam blocks subjected to dynamic test conditions, a series of compression
tests were performed with a 6.8 kilogram aluminum sled equipped with a 12.5 mm thick,
150 mm square faced plate.
The sled was equipped with a 0-500 g piezoresistive accelerometer
to record transient deceleration and a 0-22.2 kN piezoelectric load cell. The output
from the piezoresistive accelerometer was transmitted to a digital strain conditioner
whereas the output from the piezoelectric load cell was transmitted to a charge
amplifier. The output from the strain conditioner and charge amplifier were then
transmitted to a sixteen channel analog input card. The sled was guided along a
low friction track via roller bearings. The sled was propelled using a high-rate
servohydraulic test system available from MTS. The targeted impact velocity for
this series of experiments was 6.7 meters per second (m/s).
Dynamic Compressive Response
Composite foam blocks were subjected to a series of high-speed
compression tests with a targeted impact velocity of 6.7 m/s. Five replicate tests
were performed for each joining method using samples that were approximately 75
mm x 75 mm x 60 mm (thickness). This specific thickness was selected to reflect
the desire of European automobile manufacturers to meet the requirements of pending
pedestrian impact legislation without an increase in today's bumper energy absorbing
foam thickness of approximately 60 mm. Several tests were performed using a total
thickness of the sample of 60 mm with from 0 to 60 mm of anisotropic foam at 10
mm increments with the remainder being isotropic foam. The impact velocity was 15
mph. A representative plot of the effect of anisotropic foam thickness on the dynamic
compressive stress versus strain response of thermally bonded composite foam specimens
was illustrated in Figures 6 to 8. Figure 6 illustrated the data from a thermally
bonded interface, Figure 7 illustrated the data from an adhesively bonded interface
and Figure 8 illustrated the data from parts taped together.
The dynamic compressive stress versus strain curves clearly
demonstrated that increasing the anisotropic foam content results in a decrease
in the transient stress at higher strain levels (that is >50 percent). Thus,
this type of response indicated an improvement in performance as composite thickness
is held constant or the ability to decrease composite thickness while achieving
equivalent performance to samples containing lower levels of anisotropic foam.
The compression stress levels at 70 percent strain for
each tested attachment system at various anisotropic foam thicknesses tested were
illustrated in Table 3.
Dynamic Compression Stress Levels @ 70 Percent Strain
No Adhesive Interface