WO2000037242A1

WO2000037242A1 - Structural foam composite having nano-particle reinforcement and method of making the same

Info

Publication number: WO2000037242A1
Application number: PCT/US1999/029990
Authority: WO
Inventors: Phillip S. Wilson
Original assignee: Magna International Of America, Inc.
Priority date: 1998-12-21
Filing date: 1999-12-17
Publication date: 2000-06-29
Also published as: CA2358534A1; CA2358534C; AU2365900A

Abstract

A structural foam article suitable for molding into automobile trim, the article comprising at least one thermoplastic; about 2 % to 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; at least one blowing agent present in a range from about 0.5 % to about 10 % by weight. A method of producing structural foam articles comprising this structural foam is also disclosed.

Description

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

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.

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,

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

article have a greater density than the core of the part due to more foaming in the 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 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 structural foams may be at least partially

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

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 panel

structural members, instrument panel retainers, interior seat backs covered with fabric, 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

about 15% by volume reinforcing particles having one or more layers of 0.7nm-l .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 30 layers 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 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 should be within this range. If too many 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 particle

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

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. 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 93/1 1 190, each of which is hereby incorporated by reference. For additional

background, the following are also incorporated by reference: 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. 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 (30 layers or platelets) in thickness, and that more than about 50% of the

particles should be less than about 20 nanometers (20 layers 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

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 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 optimal. Inclusion 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 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 CO₂, nitrogen, helium, argon and air. Organic agents include volatile organics and halogenated hydrocarbons, such as chlorofluorocarbons, and

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 CO and O₂ in the presence of heat. Preferably, at least

one blowing agent is present in the polymer composition (and hence the molded

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

compositions to promoting bubble formation during processing of polypropylenes. 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

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.

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 least about 10¹⁴ per 1% loading of reinforcement

particles. Where the majority (>50%) particles are one platelet thick and have an approximate total particle size of about 1.2nm x 200nm x 300nm or less, the potential nucleation sites is about 2 x 10'² per 1 % loading of reinforcement particles. In the

broad aspect of the invention, it is contemplated that there exists at least 10" particles

for each 1 % 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, 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, 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.

Claims

What is claimed is:

1. A structural foam article comprising:

(a) at least one thermoplastic;

(b) about 2% to about 15% by volume reinforcing particles, each of said

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 30 layers thick; and

(c) at least one blowing agent present in a range from about 0.5% to about

10% by weight.

2. A method of producing structural foam articles comprising:

(a) preparing a melt of at least one thermoplastic having about 2% to 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, wherein more than about 99% of the reinforcing particles are less than

about 30 layers thick, and said melt comprising at least one blowing agent present in a

range from about 0.5% to about 10% by weight; and

(b) subjecting the polymer melt to a molding process, wherein the molding

process is a process selected from the group consisting of injection molding and

extrusion molding.