Note: Descriptions are shown in the official language in which they were submitted.
CA 02358534 2001-06-20
WO 00/37242 PCT/US99R9990
STRUCTURAL FOAM COMPOSITE HAVING NANO-PARTICLE
REINFORCEMENT AND METHOD OF MAKING THE SAME
BACKGROUND OF THE INVENTION
Foamed plastics are plastics having reduced apparent densities due to the
presence of numerous cells disposed throughout the mass of the polymer. Rigid
foams usually produced at greater than about 320 kg/m3 density are known as
structural foams, and are well known in the art. Structural foams are commonly
used
in various aspects of manufacturing molded articles in which low density
polymer
lo materials are desirable. Cellular polymers and plastics are made by a
variety of
methods having the basic steps of cell initiation, cell growth and cell
stabilization.
Structural foams having an integral skin cellular core and a high strength to
weight
ratio are made by several processes, including injection molding and extrusion
molding, wherein a particular process is selected based upon product
requirements.
is . Injection molding of structural foams is usually conducted under either
low
pressure or high pressure conditions. For example, during the injection
molding
process, a chemical blowing agent is typically introduced to the polymer resin
melt in
the extrusion barrel of an injection molding machine. The temperature of the
extrusion barrel is increased under pressure, after which the pressure is
released,
20 injecting the polymer into a mold, permitting the chemical blowing agent to
generate
gas within the polymer. The expansion of the blowing agent pushes molten
polymer
material against the walls of the mold such that the material in contact with
the walls
has a higher density than the material toward the middle of the molded
article. This
establishes a density gradient wherein the outer surface areas of an injection
molded
25 article have a greater density than the core of the part due to more
foaming in the
CA 02358534 2001-06-20
WO 00/37242 PCT/US99/29990
2
center of the article. Thus, a gradient is established having smaller cells
present near
the mold surface with increasingly larger cells present toward the center of
the article.
The use of blowing agents permits short shooting during the molding process.
That is, because the blowing agent increases the volume of the expanding
polymer
composition, the mold is filled with less resin material than would be
required
without a blowing agent. Consequently, the density of the molded article may
be
reduced by about 10% to about 20% over articles molded without an incorporated
blowing agent. Use of less polymer resin has the advantage of decreasing the
weight
of the final molded product.
Initiation of cell formation and promotion of cells of a given size are
controlled by nucleation agents included in the polymer composition. The
nature of
cell-control agents added to the polymer compositions influence the mechanical
stability of the foamed structure by changing the physical properties of the
plastic
phase and by creating discontinuities in the plastic phase which allows the
blowing
agent to diffuse from the cells to the surrounding material. Typically, the
resulting
cells provide for a lightweight molded article, but do so at the expense of
impact
resistance. For example, nucleation agents often promote crystalline
structures within
the cooled polymer, which reduce impact resistance. Mineral fillers may be
added to
provide a large number of nucleation sites, but such fillers tend to serve as
stress
concentrators, promoting crack formation and decreasing the impact resistance
of
molded articles.
Poor impact resistance of structural foam articles may be improved by the
inclusion of glass fibers in the polymer melt during processing. However,
glass fibers
are generally too large to substantially reinforce the foam cells formed by
the bubble
structures. Glass fibers are often coated with sizing agents, which may induce
CA 02358534 2001-06-20
WO 00/37242 3 PCT/US99/29990
clumping and impair even dispersion of the fibers. In addition, the amount of
glass
fibers required to achieve reasonable impact resistance of structural foam
increases the
specific gravity of polymer used therein, thereby increasing the density of
the foamed
article. This defeats the purpose of using lightweight foamed articles in the
manufacture of, for example, automobiles, where lightweight components are
highly
desirable. Consequently, the levels of glass fibers in polymer compositions
for
foamed articles are kept relatively low, meaning impact resistance of the
molded
products is poor.
Typically, the reduced strength of structura] foams may be at least partially
io offset by increasing the wall thickness of molded articles. Increasing wall
thickness
requires more raw materials per unit molded, thereby increasing the cost of
production.
U.S. Patent number 5,753,717 to Sanyasi discloses a method of producing
foamed plastics with enhanced physical strength. The structural foams of
Sanyasi
utilize CO2 in combination with an adjustment in the extrusion temperature of
molten
polystyrene resins to improve foam strength. This process, however, does not
improve the foam strength of other types of resins, and is not suitable for
enhancing
the strength of articles for use in, for example, automotive trim.
Structural foam automotive parts historically have inconsistent surface
2o appearances due to variations in the density of the polymer near the skin
or surface of
these molded articles. The imperfections in the surfaces of molded structural
foam
articles usually limits the usage of these foam products to non-appearance
(e.g.,
hidden or non-visible) parts or parts in which the surface has been textured.
Examples
of these structural automotive interior trim products include interior door
pane]
structural members, instrument panel retainers, interior seat backs covered
with fabric,
CA 02358534 2001-06-20
WO 00/37242 4 PCT/US99/29990
load floors in the storage compartments of vehicles, side wall trim and the
like. Some
pickup truck beds can be made from structural foam. All of these products
require
reduced density and good impact resistance.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome the problems delineated
hereinabove. In accordance with this object, the present invention provides a
structural foam article suitable for use as automobile trim. The article (and
hence the
composition forming the article) comprises at least one thermoplastic; about
2% to
io about 15% by volume reinforcing particles having one or more layers of
0.7nm-1.2
nm thick platelets, wherein more than about 50% of the reinforcing particles
are less
than about 20 layers thick, and wherein more than about 99% of the reinforcing
particles are less than about 301ayers thick; and there is at least one
blowing agent
present in a range from about 0.5% to about 10% by weight. The automotive trim
component is constructed and arranged to be both lightweight and strong,
exhibiting
good impact resistance.
It is a further object of the present invention to provide a method which
overcomes the problems delineated above. Accordingly, there is provided a
method
of producing structural foam articles which comprises preparing a melt of at
least one
thermoplastic having about 2% to about 15% by volume reinforcing particles.
The
reinforcing particles have one or more layers of 0.7nm-1.2nm thick platelets,
wherein
more than about 50% of the reinforcing particles are less than about 20 layers
thick.
More than about 99% of the reinforcing particles are less than about 30 layers
thick.
The melt comprises at least one blowing agent present in a range from about
0.5% to
about 10% by weight. The polymer melt is subjected to a molding process,
wherein
CA 02358534 2001-06-20
WO 00/37242 5 PCT/US99/29990
the molding process is a process selected from the group consisting of
injection
molding and extrusion molding.
This and other objects of the invention can be more fully appreciated from the
following detailed description of the preferred embodiments.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, reinforcing nanoparticle fillers are
added in levels of only a few percent by volume to polymer compositions prior
to
molding into the article. As a result, the impact resistance of molded
articles made of,
for example, polyolefins, is improved. For example, automobile splash guards
and
fender liners may utilize greater amounts of recycled polypropylene when
combined
with reinforcing nanoparticles to create strong molded parts, thereby
requiring less
higher cost virgin polymers and using as much as 30% less material overall due
to
improved strength. Use of lower cost, reinforced materials for the interior
trim of an
automobile is an effective way to provide impact resistant components without
negatively affecting the production cost per automobile.
The automotive parts manufactured in accordance with the present invention
comprise a composite material of a polymer having dispersed therein
reinforcement
fillers in the form of very small mineral reinforcement particles. The
reinforcement
filler particles, also referred to as "nanoparticles" due to the magnitude of
their
dimensions, each comprise one or more essentially flat platelets. Generally,
each
platelet has a thickness of between about 0.7-1.2 nanometers. The average
platelet
thickness is approximately 1 nanometer.
The preferred aspect ratio, which is the largest dimension divided by the
thickness of each particle, is about 50 to about 300. At least 80% of the
particles
CA 02358534 2001-06-20
WO 00/37242 6 PCT/US99129990
should be within this range. If too maliy particles have an aspect ratio above
300, the
material becomes too viscous for forming parts in an effective and efficient
manner.
If too many particles have an aspect ratio of smaller than 50, the partic]e
reinforcements will not provide the desired reinforcement characteristics.
More
preferably, the aspect ratio for each particle is between 100-200. Most
preferably at
least 90% of the particles have an aspect ratio within the 100-200 range.
The platelet particles or nanoparticles are derivable from larger layered
mineral particles. Any layered mineral capable of being intercalated may be
employed
in the present invention. Layered silicate minerals are preferred. The layered
silicate
lo minerals that may be employed include natural and artificial minerals. Non-
limiting
examples of more preferred minerals include montmorillonite, vermiculite,
hectorite,
saponite, hydrotalcites, kanemite, sodium octosilicate, magadite, and
kenyaite. Mixed
Mg and Al hydroxides may also be used. Various other clays can be used, such
as
claytone H.Y. Among the most preferred minerals is montmorillonite.
To exfoliate the larger mineral particles into their constituent layers,
different
methods may be employed. For example, swellable layered minerals, such as
montmorillonite and saponite are known to intercalate water to expand the
inter layer
distance of the layered mineral, thereby facilitating exfoliation and
dispersion of the
layers uniformly in water. Dispersion of layers in water is aided by mixing
with high
shear. The mineral particles may also be exfoliated by a shearing process in
which the
mineral particles are impregnated with water, then frozen, and then dried. The
freeze
dried particles are then mixed into molten polymeric material and subjected to
a high
sheer mixing operation so as to peel individual platelets from multi-platelet
particles
and thereby reduce the particle sizes to the desired range.
CA 02358534 2006-08-02
7
The polymer composites of the present invention are prepared by combining the
platelet mineral with the desired polymer in the desired ratios. The
components can be
blended by general techniques known to those skilled in the art. For example,
the
components can be blended and then melted in mixers or extruders.
Additional specific preferred methods, for the purposes of the present
invention,
for forming a polymer composite having dispersed therein exfoliated layered
particles are
disclosed in U.S. Patent Nos. 5,717,000, 5,747,560, 5,698,624, and WO
93/11190.
Additional background information is disclosed in the following U.S. Patent
Nos.
4,739,007 and 5,652,284.
Generally, expandable plastic formulations include polystyrenes, poly(vinyl
chlorides), polyethylene, polyurethanes, polyphenols and polyisocyanates. A
preferred
thermoplastic is used, and based on the selection of thermoplastic determines
the
temperature at which foaming commences, the type of blowing agent used and the
cooling conditions required for dimensional stabilization of the foam.
Preferably, the
thermoplastic used in the present invention is a polyolefin or a homogenous or
copolymer
blend of polyolefins. The preferred polyolefin is at least one member selected
from the
group consisting of polypropylene, ethylene-propylene copolymers,
thermoplastic
olefins (TPOs), and thermoplastic polyolefin elastomers (TPEs). For high
performance
applications, engineering thermoplastics are most preferred type of
thermoplastic. Such
engineering thermoplastic resins may include polycarbonate (PC), acrylonitrile
butadiene
styrene (ABS), a PC/ABS blend, polyethylene terephthalates (PET), polybutylene
terephthalates (PBT), polyphenylene oxide (PPO), or the like.
CA 02358534 2001-06-20
WO 00/37242 8 PCT/US99/29990
The exfoliation of layered mineral particles into constituent layers need not
be
complete in order to achieve the objects of the present invention. The present
invention contemplates that at least 99% of the particles should be less than
about 30
nanometers (301ayers or platelets) in thickness, and that more than about 50%
of the
particles should be less than about 20 nanometers (201ayers or platelets) in
the
thickness direction. Preferably, at least 90 % of the particles should have a
thickness
of less than 5 layers. Also, it is preferable for at least 70% of the
particles should have
a thickness of less than 5 nanometers. It is most preferable to have as many
particles
as possible to be as small as possible, ideally including only a single
platelet. Particles
io having more than 30 layers behave as stress concentrators and should be
avoided, to
the extent possible.
Generally, in accordance with the present invention, each of the automotive
parts that can be manufactured in accordance with the principles of the
present
invention should contain nanoparticle reinforcement in amounts less than 15%
by
volume of the total volume of the part. The balance of the part is to comprise
an
appropriate thermoplastic material, a blowing agent and optionally, suitable
additives.
If greater than 15% by volume of reinforcement filler is used, the viscosity
of the
composition becomes too high and thus difficult to mold. Preferably, the
amount of
reinforcing nanoparticles is greater than 2% by volume (as lower amounts would
not
achieve the desired increase in strength) and less than 15%. More preferably,
the
nanoparticles comprise less than 13% and greater than 3% of the total volume
of the
part for extrusion molding.
Preferably, relatively rigid injection molded trim parts comprise
reinforcement
particles of the type described herein at about 2-10% of the total volume of
the part,
with the balance comprising the thermoplastic substrate. It is even more
preferable for
CA 02358534 2001-06-20
WO 00/37242 9 PCTIUS99/29990
these interior panels to have reinforcement particles of the type contemplated
herein
comprising about 3%-8% of the total volume of the part. For some applications,
inclusion of about 3%-5% reinforcing nanoparticles is optima]. l;nclusion of
more
than 10% nanoparticles tends to increase the viscosity of the composition to
point
which impairs injection molding.
Blowing agents incorporated into the compositions according to the invention
govern the amount of gas generated during polymer processing and molding, and
thus
control the density of the final product. The type of agent used determines
the rate of
gas production, the pressure developed during gas expansion, and the relative
amount
jo of gas lost from the system to the amount of gas retained within the cells.
Blowing
agents may be either physical or chemical agents; chemical agents are
preferred.
Chemical agents may be organic or inorganic compounds. Commonly used inorganic
blowing agents include C02, nitrogen, helium, argon and air. Organic agents
include
volatile organics and halogenated hydrocarbons, such as chlorofluorocarbons,
and
]5 hydrochlorofluorocarbons, although their use is diminishing due to
environmental
concerns. Volatile organic compounds include aliphatic hydrocarbons, such as
propane, n-butane, neopentane, hexane, and the like. Preferred blowing agents
are azo
compounds which produce CO2 and 02 in the presence of heat. Preferably, at
least
one blowing agent is present in the polymer composition (and hence the molded
20 article) in a range from about 0.5 % to about 10%, more preferably about
0.5% to
about 4 % by weight. Combinations of more than one blowing agent may be used.
Additives or cell control agents heavily influence the nucleation of foam
cells
by altering surface tension of the polymer system or by serving as nucleation
sites
from which cells can grow. Nucleation agents are often added to polymer
25 compositions to promoting bubble formation during processing of
polypropylenes.
CA 02358534 2001-06-20
WO 00/37242 PCT/US99/29990
Nucleation agents can be selected to develop cells of a particular pore size.
Suitable
nucleating agents include metal aromatic carboxylates, sorbitol derivatives,
inorganic
compounds and organic phosphates. Examples are aluminum hydroxyl di-p-t-butyl
benzoate, dibenzylidene sorbitol, magnesium silicate (talc), sodium 2,2'-
methylene
5 bis (4,6-di-t-butylpheyl) phosphate and zinc oxide. Inorganic nucleation
agents are
often chemically modified to improve dispersion throughout the polymer
composition.
The chosen nucleation agent will influence the mechanical properties of the
polymer
composition, and should be selected accordingly. For example, some fillers
induce
crystallization of polymers, which impairs impact resistance of molded
articles.
10 The nanoparticles of the invention also advantageously behave as nucleating
agents in polymer compositions. The extremely small size of these reinforcing
particles permits them to be evenly dispersed throughout the polymer
composition.
Accordingly, the extremely small size and even distribution of the
nanoparticles
provides for between about 20 to about 100 times more potential nucleation
sites
within the polymer composition than can be achieved in an equivalent volume
using
larger, standard nucleation agents.
Specifically, for each 1% loading of nanoparticles by volume, there exists a
minimum of at least about 10" particles, and hence potential nucleation sights
(one for
each particle), per cubic centimeter of structural foam, where more than 50%
of the
reinforcement particles are less than about 20 platelets thick, and wherein
the majority
of reinforcement particles have a total particle size of less than about 20nm
x 200nm x
300 nm. Where the majority (>50%) of particles are one platelet thick and have
an
approximate total particle size of about 1.2nm x 50nm x 75nm or less, the
potential
nucleation sites increases to at lcast about 1014 per 1% loading of
reinforcement
particles. Where the majority (>50%) particles are one platelet thick and have
an
CA 02358534 2001-06-20
WO 00/37242 PCT/US99/29990
11
approximate total particle size of about 1.2nm x 200nm x 300nm or less, the
potential
nucleation sites is about 2 x 10" per l% loading of reinforcement particles.
In the
broad aspect of the invention, it is contemplated that there exists at least
10" particles
for each l% loading of nanoparticles per cubic centimeter of structural foam,
with the
balance of the cubic centimeter being formed from the other constituent
components
of structural foam, such as thermoplastic material, blowing agent, and
optionally, at
least one additive.
When about 90% of the nanoparticles in the composition are less than 5 nm in
thickness, a more preferred uniform distribution of the particles occurs in
the resin,
jo which translates into evenly distributed gas bubble formation during blow
molding. A
reduction to near elimination of clusters of nucleation agent can be achieved,
accordingly. The advantage to nanoparticle nucleation is the near elimination
of
nucleation stress concentrators in concert with substantial reinforcement of
foam cells,
which is not possible with existing nucleation agents.
In addition to nucleating agents, other additives may optionally be included
in
the polymer composition to improve processability. For example, aging
modifiers,
such as glycerol monostearate, are useful additives in polymer compositions
for
molding. Aging modifiers are typically present in an amount from about 0.5% to
about 5% polyolefin resin. Lubricants may also be present to enhance extrusion
of the
polymer composition during molding. Other additives include pigments, heat
stabilizers, antioxidants, flame retardants, ultraviolet absorbing agents and
the like.
Reinforced articles of the invention exhibit improved properties over non-
reinforced articles. For example, polyethylene articles having 5%
nanoparticles by
volume, wherein 90% of the particles have 5 or fewer layers, increased
flexural
modulus by 2.5 to about 3 times over compositions lacking reinforcing
nanoparticles,
CA 02358534 2001-06-20
WO 00/37242 PCT/U599/29990
12
as measured under ASTM D790 test conditions. This 5% nanoparticle polyethylene
composition exhibited > 200% elongation to rupture. By contrast, about 25%
glass
fiber reinforcement is required in such articles to achieve an equivalent
modulus.
Polypropylene articles according to the invention showed about a 60%
improvement
in flexural modulus over articles lacking reinforcement nanoparticles. Thus,
the use
of reinforcing nanoparticles according to the invention provides articles
having good
flexural stiffness.
The specific gravity of structural foams having reinforcing nanoparticles is
typically 22.5% lower than in materials lacking a blowing agent, which is 50%
less
dense than the blow molded foams known in the art.
It should be appreciated that the foregoing description is illustrative in
nature
and that the present invention includes modifications, changes, and
equivalents
thereof, without departure from the scope of the invention.
