Method of making syntactic insulated containers

A method of insulating a substrate or a portion thereof by depositing a syntactic foam by a spraying, dipping or a variety of printing processes. The foam comprises void containing particles in expanded form, unexpanded form, or a mixture of these. The deposited foam is dried to remove solvents and cured to strengthen the binder which restrains the microspheres. Heating may also expand the unexpanded microspheres. A tie coat may be applied to promote adhesion between the substrate and the insulating syntactic foam. Multiple layers of the syntactic foam may be applied to increase the thickness of the resultant layer for providing a controlled amount of increased insulation.

BACKGROUND OF THE INVENTION 
CROSS REFERENCE TO RELATED APPLICATION 
This application is related to another U.S. patent application, having a 
filing date of Jun. 14, 1996, and Ser. No. 08/661,332 entitled "SYNTACTIC 
FOAM INSULATED CONTAINER", having common inventors and assignee and being 
incorporated herein by reference. 
FIELD OF THE INVENTION 
This invention relates to insulating a substrate with a syntactic foam, and 
more particularly to controlling the effectiveness of the insulation by 
controlling the composition and thickness of the foam in a deposition 
process. A typical application of this method is the making of insulated 
cups and containers. 
DESCRIPTION OF RELATED ART 
The production and use of disposable containers, such as cups, is well 
known with production measured in the billions of units annually. Because 
of this volume, the business is fiercely competitive and cost is measured 
in units of one thousand. A change of a few percent in cost can make a 
container unsaleable, particularly to large fast food chains. 
Paper and paperboard are widely used as materials in the production of 
disposable containers because they are inexpensive and amenable to very 
high volume production. They have limitations, however, particularly in 
containing hot liquids which are dispensed from coffee machines or 
supplied in fast food restaurants. The initial temperature of coffee 
poured into a disposable cup can approximate 200 degrees Fahrenheit, and 
the temperature achieved on the outside of the cup can make it painful to 
hold. A person's reaction to this pain can cause spillage, severe damage 
to the skin, and inevitable product liability litigation. 
Consequently there have been many attempts to provide improved insulation 
to paper containers. U.S. Pat. No. 5,363,982 to Sadlier shows a cup formed 
from one continuous sheet where the outer and inner shells are spaced 
apart by a corrugated layer which forms many air pockets between the 
shells to insulate the cup and provide greater strength. The blank from 
which the cup is formed is at least three times as long as that for a 
conventional cup, thereby adding to the cost. 
U.S. Pat. No. 5,226,585 to Varano discloses a double wall structure in 
which inwardly directed ribs from the outer surface maintain a gap between 
the walls to provide insulation. Again added material means added cost. 
U.S. Pat. No. 5,145,107 to Silver et al. teaches a double wall structure 
wherein the inner wall is connected to the outer one only at the lip and 
at the base of the cup. The walls have different tapers thereby defining a 
dead air space between them which provides insulation. 
Accordingly, there still exists a need for making an inexpensive material 
for a container which provides a controlled amount of heat transfer to 
warn the user that a hot liquid is contained therein, and which insulates 
sufficiently to protect against pain or burn. The method must also be 
compatible with high volume container making machinery. 
SUMMARY OF THE INVENTION 
The present invention relates to making a substrate with a controlled 
amount of insulation and incorporating that substrate into the manufacture 
of containers for hot or cold food products and liquids. The method needs 
to be inexpensive, so a layer of insulation is applied to one side of a 
substrate used to make a container, which is typically paper or paperboard 
to control cost, but the insulation could be applied to a single-walled 
plastic cup as well. The insulation is a syntactic foam, that is, a foam 
which incorporates insulating particles which are held in place by a 
binder. Greater insulation is obtained where the insulating particles are 
void containing particles which can be made from thermoplastic, thermoset, 
or inorganic materials which enclose an air space. The void containing 
particles may be of arbitrary shape and they may be applied to the 
substrate in expanded or unexpanded form. A subsequent heating operation 
may be used to expand previously unexpanded particles so that they contain 
voids. There are many different types of void containing particles and 
they may be used alone or in combination with each other to achieve a 
particular degree of insulation or other mechanical properties. These void 
containing particles are held in place by a binder, into which other 
ingredients may also be added to produce a color (pigment), to control 
viscosity (thickeners and solvents), and to control density (fillers and 
foaming agents). 
In one embodiment of the invention the foam is applied by depositing a 
first layer onto the substrate, drying it to remove any solvents which may 
be needed to control printing parameters, and curing a binder which keeps 
the void containing particles in place. This is often done by the 
application of heat which will also cause unexpanded void containing 
particles to expand. Some binders may be cured by radiation. A tie coat 
may be applied to the substrate to promote adhesion of the syntactic foam. 
The foam provides a controlled degree of insulation by adjusting its 
thickness, particle type, and composition, so that a user has enough 
sensation to know that a hot or cold liquid is inside the container, yet 
not enough sensation to cause discomfort. These containers may be used for 
all types of foods and liquids, such as soups, frozen foods that are 
reheated, ice cream, and so on. 
In another embodiment of the invention, a layer of syntactic foam is 
deposited on a first surface of a substrate, the foam is dried, cured, and 
another layer is deposited over the syntactic foam for the functional 
purpose of: impeding moisture, resisting abrasion, improving appearance, 
promoting adhesion, or adding thermal insulation. A tie coat may be 
applied to the first surface to promote adhesion. A functional material 
may be applied to the second surface of the substrate to provide a 
moisture, vapor, or gas barrier. 
In yet another embodiment of the invention, making a container comprises 
coating a first side of a substrate with a layer of a thermoplastic, 
printing a first layer of a syntactic foam upon a second side of the 
substrate, drying the syntactic foam to remove any solvents present, 
heating it to cure a binder which restrains the void containing particles 
contained therein, die cutting the substrate supporting the thermoplastic 
layer and the syntactic foam in the form of a blank, forming the blank to 
make a preform of a container, and heating the preform to seal the 
thermoplastic layer to selected areas of the preform, thereby creating the 
container. Not all containers are heat sealed. Some are locked together or 
they may be glued together like a clamshell. The adhesives may range from 
heat seals, hot melts, pressure sensitive, and cold set types which are 
well known in the art of making containers. 
The insulating layer may be applied in various configurations. The foam may 
be applied in a continuous layer on the substrate or container, or the 
syntactic foam may nearly cover the blank from which the cup is made 
except for edge areas which are designated for sealing the sidewall to 
itself and the bottom of the cup and the formation of the rolled rim at 
the top of the cup. The syntactic foam may be applied as a pattern of 
matrix elements, which may be dots, lines, quadrangles, arcs, letters, 
symbols, or any other fanciful configuration. The interrupted pattern 
saves material, yet still keeps fingers away from the sidewall of a cup 
because the air spaces between the pattern elements are limited so that 
fingers can not descend between the elements to touch the sidewall. 
A tie coat may be interspersed between the insulating pattern elements and 
the substrate to promote adhesion. It may be continuous over most of the 
blank and pattern elements are printed thereon or it may also be 
patterned. 
A second layer may be applied over the syntactic foam for a functional 
purpose of: impeding moisture penetration, resisting abrasion, enhancing 
appearance, promoting adhesion, or adding insulation. 
Multilayer applications are contemplated for all the embodiments, and a 
combination foam employing void containing particles together with a 
foaming agent may produce a spongy coating which also contains void 
containing particles. 
The void containing particles may be in expanded form or unexpanded form, 
or in a combination of both forms as applied. For unexpanded microspheres 
a heating step expands them.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The use of disposable containers reaches billions of units per year in the 
home and in the fast food industry. These containers are most often in the 
form of drinking cups which are made from paper, paperboard, or plastic, 
and to be competitive in the marketplace they must be very inexpensive. 
Paper is an inexpensive material, and it is also well adapted to high 
volume production. 
However, paper or paperboard is not a sufficient thermal insulator when 
used for the sidewalls of a container which stores hot liquids. For 
example, coffee can be made and dispensed at temperatures near 200 degrees 
Fahrenheit. A common experience is lifting a paper container from a coffee 
dispensing machine and burning the fingers on the sidewall of the 
container. 
A variety of configurations have been proposed to improve the insulation 
properties of the sidewall of paper containers by using double walled 
sidewalls which trap air and reduce heat transfer. These approaches can 
add a factor of three times the material cost or complicate the rolling 
and sealing operation. 
The present invention uses a single layer wall with an added layer of 
insulation on the external side of the sidewall. The layer may be printed 
on the outside wall in a single coat, or in multiple coats, in a 
continuous pattern or in a matrix pattern of lines, dots, or any other 
fanciful pattern. High volume printing, in moderately thick layers as used 
in the present invention, is well known in the graphic arts industry. 
U.S. Pat. No. 4,902,722 given to Melber describes the use of syntactic foam 
graphic arts print media. Syntactic foams have cells which are preformed 
by way of incorporating small hollow particles into them, rather than by 
making the foam by the expansion of a blowing agent. The particles are 
called microspheres or microballons by various inventors and they have 
particle sizes ranging from approximately 10 to 100 microns in the graphic 
arts industry, and even larger when they are used in the molded parts 
industry. 
U.S. Pat. No. 5,120,769 given to Dyksterhouse, et al. teaches the 
preparation of syntactic foams and suitable powder compositions. This is 
an excellent tutorial describing the geometry of microspheres, the 
surrounding polymer binder, and the use of solvents, surfactants, 
thickeners, and fillers. The materials disclosed in this patent are 
incorporated herein by reference. 
U.S. Pat. No. 5,385,778 given to Deviney et al. shows the use of 
microspheres to improve the mechanical toughness of plastic molded parts. 
The microspheres interrupt the propagation of cracks in high performance 
plastic structures. In this case the particle size of the microspheres 
ranges from 0.1 to 50 microns. 
U.S. Pat. No. 5,244,613 given to Hurley et al. describes the production of 
reinforced moldings formed in a reaction injection molding operation. The 
addition of microspheres improves the mechanical properties by reducing 
density, lowering volume costs, improving impact resistance, and reducing 
shrinkage. The reaction injection molding process has become important in 
the production of external automotive body parts. 
The present invention preferably incorporates graphic arts printing 
processes and the use of void containing particles in syntactic foams to 
control thermal properties, rather than the mechanical properties 
described above. However, the foam could also be applied by spraying or 
dipping. 
Referring to FIG. 1A there is shown a one exemplary embodiment of a 
container 10 made in accordance with the present invention method. The 
container 10 is typically made from a blank 20, shown in FIG. 1B, which is 
cut from a substrate 22 which may be plastic but more typically is paper 
or paper board, and preferably solid bleached sulfate ranging in thickness 
from 10 mils to 26 mils. The substrate is coated one side with an inner 
coating, which is typically a thermoplastic, and preferably polyethylene 
whose thickness ranges from 0.5 mils to 2.0 mils. The coating of paper 
with polyethylene is well known and substrate stock of this type is 
available commercially from several sources. The most common purposes of 
the inner coating are to seal the paper or paperboard to make it 
impervious to liquids, vapors, aromas, and gases; and to seal the 
container at its seams and at the intersection of the sidewall with the 
base. The impervious inner layer ensures the freshness of the product 
contained and it also prevents the penetration of gases such as carbon 
dioxide for frozen food containers. This method of construction is given 
by way of example because the method of construction may vary with the 
type of container. Not all containers are heat sealed. Some are locked 
together or they may be glued together like a clamshell. The adhesives may 
range from heat seals, hot melts, pressure sensitive, and cold set types 
which are well known in the art of making containers. Machines which seal 
the base and sidewall are well known and are adapted to high volume 
production. The outer surface 24 of the sidewall supports an insulating 
syntactic foam coating 26, which does not cover the entirety of the blank 
22, but which leaves a frame uncoated so that the inner coating can seal 
the sidewall. 
In one embodiment of the invention, outlined in FIG. 2, a substrate 
material such as plastic, paper, or paperboard is prepared by applying a 
tie coat to it in a first step 30, depositing a syntactic foam over the 
tie coat in next step 32, heating the syntactic foam to remove any 
solvents which might be needed to adjust printing parameters in a further 
step 34, and curing the syntactic foam by heating it, or by applying 
radiation for a radiation cured system in another step 36. Depositing a 
second layer, or multiple layers, of syntactic foam over the first layer 
in a next step 38 provides a controlled amount of increased insulation. 
The syntactic foam may also contain a foaming agent, and the step of 
heating the syntactic foam 34 will increase its volume and lower its 
density providing for a softer feel and a changed thermal conductivity. 
These steps are well adapted to coating paper or paperboard meant for the 
making of containers. 
The syntactic foam coating may be deposited by any process such as spraying 
or withdrawing the substrate from a bath. The deposition may also be by 
any printing process such as offset, gravure, flexographic, rotary screen, 
wire rod, air knife, spray, and others. Preferably the printing is done 
through a rotary metal screen. Rotary screen printing is well adapted to 
high volume production, typical machines being furnished by Stork Brabant, 
Charlotte, N.C. A coating of 10 to 30 mils in thickness may be achieved in 
a single pass and thicker coatings are possible with multiple 
applications. The function of the syntactic foam coating is to provide a 
controlled amount of insulation. By controlling the composition and 
thickness of the coating sufficient thermal protection to the user is 
achieved while minimizing the material cost of the container. The function 
of the outer coating is to provide a controlled amount of insulation so 
that the user can hold the container comfortably, yet know that a hot or 
cold liquid is contained inside. The outer coating contains void 
containing particles which can be made from thermoplastic, thermoset, or 
inorganic materials. A void containing particle comprises an outer shell 
of arbitrary shape which surrounds any medium of lower thermal 
conductivity or lower density. The void containing particle may be in 
expanded or unexpanded form. An unexpanded void containing particle will 
expand with the application of heat which expands the encapsulated medium 
and also softens the surrounding outer shell until the pressure of the 
medium is balanced by restraining forces in the outer shell. The void 
containing particles may range from 0.1 microns to 200 microns in size in 
either form. Typical examples of the materials of the outer shell would be 
polyvinylidene copolymers or glass. The largest particle size in the 
application process being limited by the openings in the metal mesh of the 
printing machine. If unexpanded microspheres are applied to the blank, a 
subsequent heating process may be employed to expand them. This might be 
coupled with the sealing operation. Experiments have also shown that 
multiple applications of unexpanded microspheres, separated by an 
expansion step provide better insulation than a single application of the 
same thickness. The binder holding the microspheres may be any suitable 
synthetic or natural binder including aqueous based, solvent based, high 
solids, or 100% solids materials, such as radiation cured systems, which 
are mentioned in the references. Additional ingredients may be added to 
the formulation, such as: pigments or dyes for coloring, fillers/extenders 
of organic or inorganic materials, surfactants for dispersion or rheology, 
thickeners and solvents to control viscosity for optimized application, 
foaming agents to control the density of the coating, additives like waxes 
or slip aids, and plasticizers and other ingredients common to the 
formulation of coatings. In a preferred embodiment, the resin is acrylic, 
the void containing particles are synthetic microspheres whose particle 
size distribution ranges from 10 to 20 microns, and whose weight fraction 
in the wet mix ranges from 1 to 20 percent. The microspheres are available 
from Akzo Nobel, Duluth, Ga. Other additions may include: inorganic silica 
beads which are available as "Sil-cell" brand from Silbrico Corporation, 
Hodgkins, Ill., or coated microspheres which are available as "Dualite" 
brand from Pierce and Stevens, a division of Pratt and Lambert, Buffalo, 
N.Y. 
The printed syntactic foam may be in the form of a pattern comprising a 
matrix of elements such as dots, lines, quadrangles, arcs, alphabetical 
letters symbols, or the like. The common properties among them being that 
(1) the interrupted pattern saves material, and (2) the pattern elements 
are sufficiently close together to prevent fingers from descending between 
the pattern elements to touch the sidewall. The insulation properties of 
the pattern are therefore controlled by the thickness and composition of 
the material comprising the pattern, together with the ratio of the area 
of the elements of the pattern to the area of the substrate. Area ratios 
may range from 10 to 100 per cent. The lower end of the coverage ratio 
being controlled by the maximum distance between pattern elements which 
will support a finger away from the substrate. 
In another embodiment of the invention, outlined in FIG. 3, a tie coat may 
be interspersed between the substrate and the syntactic foam (40). A 
syntactic foam insulating coating is applied to a first surface of a 
substrate (42), dried to remove any solvents which remain (44), and cured 
to strengthen the foam (46). At least one additional layer may be applied 
over the syntactic foam for a functional purpose (48) which may be to: 
impede moisture penetration, provide added thermal insulation, resist 
abrasion, improve appearance, or promote adhesion. A second surface may be 
coated with a functional material (50). 
In yet another embodiment of the invention, outlined in FIG. 4, a syntactic 
foam insulated container is made by applying a tie coat to a first side of 
a substrate to promote adhesion between the substrate and a subsequent 
layer of syntactic foam (60), depositing a first layer of a syntactic foam 
upon a first side of the substrate to create an insulating layer (62), 
drying the syntactic foam to remove any solvents (64), curing the 
syntactic foam by heating it or by irradiating it to keep the void 
containing particles fixed (66), depositing at least one layer of 
syntactic foam over the first layer to provide a controlled degree of 
insulation (68), coating a second side of the substrate with a layer of a 
functional material to create a sealing layer (70), cutting the substrate 
supporting the functional material layer and the syntactic foam to prepare 
a blank (72) which is adapted to make the container, forming the blank for 
obtaining a preform of a container (74), and sealing the functional 
material to selected areas of the preform (76). The syntactic foam may 
contain void containing particles ranging from 0.1 microns to 200 microns 
in particle size, a binder, a pigment, or a foaming agent, used singly or 
in combination. Typical temperatures for expanding the unexpanded void 
containing particles range from 150 to 400 degrees Fahrenheit. The binder 
can cure under similar conditions depending on its composition. Ideally, 
both are accomplished in one step (66). 
The printing may provide a first layer of syntactic foam which is 
continuous over a major part of the substrate, leaving enough of a border 
for sealing purposes, or the first layer of syntactic foam may be a 
pattern comprising pattern elements. 
The syntactic foam may have void containing particles in expanded or 
unexpanded form, used singly or in combination. 
Printing a second layer of unexpanded microspheres with a rotary screen 
printer over the first layer will increase the insulation. Multiple layers 
could achieve 120 mils or more in thickness. Each layer may be expanded in 
an intervening step, or a few layers may be expanded in one step. 
Changes and modifications in the specifically described embodiments can be 
carried out without departing from the scope of the invention. In 
particular, in any of the embodiments above, multilayers comprised of 
previously expanded void containing particles are also within the scope of 
the invention. A coating containing microspheres plus a foaming agent 
could provide a spongy coating. Various combinations of underlayers and 
patterns are also within the scope of the invention.