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Description  |
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The present invention is directed to flowable pressure compensating fitting
materials which are used in seating applications and other applications
wherein the human body is in close contact with the pressure compensating
fitting material. The compositions of the present invention have
particular application to medical seating uses, such as wheelchair
cushions and the like.
BACKGROUND
A number of viscous flowable pressure compensating fitting materials have
been developed in the prior art for a wide variety of utilities. Such
materials are generally stable so as to provide firm support and comfort
to the user while allowing the pressure compensating fitting material to
flow in response to continuous pressure in order to adapt to the
particular body part involved. Such compositions and pads based on such
compositions are described in U.S. Pat. Nos. 4,144,458; 4,229,546;
4,255,202; and 4,588,229, among others. The compositions described in
these patents are, generally speaking, comprised of a mixture of oil, wax,
and either glass microbeads or resin microbeads. The microbeads provide
the composition with some structure, but because of their relative light
weight, the microbeads reduce the density of the composition. Light weight
products are desirable and important, particularly in large cushions or
pads for wheelchairs wherein the weight of the cushion can be a major
factor. The mixture of oil and wax in the composition provides flowability
to the composition when subjected to pressure. The oil-wax mixture also
maintains a high viscosity in the overall composition, whereby the pad
will retain its shape in the absence of pressure.
The prior art products have worked quite well in most applications, but
when subjected to temperatures higher than room temperature, such as body
temperatures encountered in wheelchair cushion pads, wherein the
wheelchair is used as much as 18 or 20 hours per day, 365 days per year,
separation of the glass or resin microbeads from the oil-wax mixture
occurs to form a hard, non-flowable mass. Separation of the microbeads
from the oil-wax results in masses of microbeads which apparently "float"
on top of the oil-wax mixture, producing hard, non-flowable lumps which
cause discomfort to the user and potential skin breakdown(pressure sones).
The tendency of the microbeads to separate is also a problem in vertical
applications, such as wheelchair back cushions. Separation of these
components has also been observed in cushions which are left in closed
automobiles in the sunlight, wherein temperatures above 100.degree. F. are
encountered.
Sometimes the separation of the microbeads can be corrected by kneading the
pad to redisperse the microbeads in the oil-wax composition. However, when
the separation has progressed too far, hard lumps are formed which cannot
effectively be redispersed by kneading. Moreover, in many usages, the
fitting material is located in inaccessible places which makes it
difficult or impossible to knead the flowable pressure compensating
material back to its original dispersed form.
The prior art flowable compositions are also extremely temperature
sensitive. For example, a pad left in a cold car overnight will be
dangerously firm to sit on, as it can't conform to relieve pressure.
Conversely, a pad warmed to skin temperature (after 2 hours of sitting)
will develop a watery texture and no longer afford the same stability and
support as a pad at room temperature. Stability and support are prime
requirements in seating for disabled person, as they lose function when
sitting on unstable seating surfaces.
It is believed that many of the foregoing problems are related to the wax
component of the prior art flowable composition. The wax used in such
compositions is temperature sensitive so that it flows and tends to
separate from the microbeads at warmer temperatures. The temperature
sensitivity also results in a change in viscosity at low temperatures as
compared to relatively high temperatures.
THE PRESENT INVENTION
The present invention is based on the discovery that the wax can be removed
from the flowable pressure compensating fitting materials of the prior art
and a small amount of colloidal silica added to the composition to produce
a flowable fitting composition which is relatively insensitive to
temperature variation and which does not have the tendency to separate at
body temperature in the manner of prior art compositions.
The fitting material essentially consists of a compatible combination of
(1) a flowable containuous phase comprising oil as the principal
component, and (2) colloidal silica, such as Cab-O-Sil, uniformly
dispersed in the continuous phase. These components are homogeneously
mixed to provide an essentially homogeneous mixture which is flowable when
subjected to pressure, but which tends to retain its shape, at room
temperature, when pressure is removed.
Preferably, the composition also includes discrete, lightweight, sturdy,
hollow glass microbeads. As used herein, "glass" describes microbeads
including glassy, siliceous or ceramic microbeads.
The composition may include wax of the type used in the prior art
compositions, but generally it is essential to use an amount of wax
substantially lower than that used in conventional prior art compositions.
In the preferred composition, no wax is used.
THE COMPOSITION PROPORTIONS
The flowable, continuous phase oil, either with or without the wax, forms a
coherant, viscous, flowable material which preferably constitutes a major
portion, i.e., at least 50%, by weight of the composition.
The colloidal silica comprises a minor proportion of the overall
composition. In a composition made up only of oil and colloidal silica, up
to 20% of colloidal silica can be used, although the preferred level is
about 9% by weight. As glass microbeads are introduced into the
composition, the required level for the colloidal silica reduces. Also the
addition of wax to the continuous phase will reduce, somewhat, the
requirements for the colloidal silica.
The glass microbeads are discrete micro size particles, uniformly dispersed
or distributed throughout the continuous phase, and thus constitute a
discontinuous solid phase. Although up to 60% by weight of glass beads may
be used for some purposes, it is generally preferred to use about 30% by
weight of the glass microbeads in the composition. The glass microbeads
are not essential to the composition insofar as the stability and flow
characteristics are concerned, but the microbeads serve to reduce the
weight of the composition, and when used at the appropriate levels, the
glass microbeads do not interfere with the flow characteristics, the
rheology or stability of the composition.
THE OIL
The oil component of the fitting material may be a suitable natural,
synthetic, vegetable, or petroleum-based oil (neutral blending or bright
stock). In order to facilitate control of the flow characteristics of the
finished fitting material, it is important not to use unsaturated natural
or vegetable drying or semi-drying oils that are unsaturated in such a
manner or to such a level as to oxidize, thicken or harden significantly
(e.g., polymerize or cross-link) with time or conditions of storage or
use, which in many instances is reflected by the oil having an excessively
high or unsatisfactory iodine number.
The oil component, for example, may comprise petroleum-based oil, such as
exemplified by "Carnea" 21 oil, "Tufflo" 6204 oil, "Topco" Motor Oil,
S.A.E. 20 (supplied by the King Super Market, Boulder, Co.), R Industrial
#5-H oil [a neutral base oil supplied by American Oil Co. having a typical
viscosity of 300-320 sec. (S.U.V.) at 100.degree. F., typical flash point
of 450.degree. F., typical pour point (max.) of 0, and typical viscosity
index (min.) of 95], or suitable admixtures thereof. Moreover, if desired,
vegetable oil, such as coconut oil (e.g., SC 11518 coconut oil supplied by
Sargent-Welch Scientific Co., Skokie, Il., may be used, at least for some
purposes. "Tufflo" 6204 oil, supplied by Atlantic Richfield Company, is a
technical grade naphthenic-type white mineral oil having the following
typical properties:
______________________________________
Viscosity
100.degree. F., S.U.S.
1750
210.degree. F., S.U.S.
91
Gravity, A.P.I. 23.4
Specific Gravity 60/60
0.9135
Density, lbs./gal. 7.61
Color, Saybolt +20
Flash Point, .degree.F.
465
Pour Point, .degree.F.
+15
Aniline Point, .degree.F.
232
Refractive Index, 20.degree. C.
1.4945
Viscosity Gravity Constant
0.840
Molecular Weight 490
% Volatility, 22 hrs./225.degree. F.
0.3
U.V. Absorptivity, 260 mu
0.2
Acid No. 0.0
Aromatics, % 14
Saturates, % 86
Distillation, .degree.F.
I.B.P. 620
5% 745
50% 880
95% 1020
______________________________________
The pour point of the oil is a critical parameter, and the quantities of
the and microbeads must be adjusted to accommodate the oil pour point. In
the composition, the oil forms the continuous phase of the composition,
and to some extent, the flow characteristics of the composition depend
upon the oil and its pour point. Generally speaking, oils having a pour
point between zero and +15 degrees have been found suitable, although oils
having other pour points can be used if appropriate adjustments are made
to the composition.
THE COLLOIDAL SILICA
The colloidal silica which forms a component of the composition should have
a particle size between about 5 millimicrons through 200 millimicrons. The
preferred range of particle sizes is from about 7 to 50 millimicrons.
Suitable colloidal silicas are described in the patent literature. Other
colloidal material, such as alumina, could also be used.
The preferred colloidal silica is sold under the trademark Cab-O-Sil.
Cab-O-Sil is silicon dioxide produced by the hydrolysis of silicon
tetrachloride vapor in a flame of hydrogen and oxygen. The process
produces spheres having extremely small particle size with an enormous
surface area. The diameter of the spheres are varied through process
parameters, but have averages in the 7 to 30 millimicron range. This
produces grades with final surface areas ranging from 400 square meters
per gram to 100 square meters per gram, respectively. The molten spheres,
which are formed during the hydrolysis process, termed primary particles,
collide and fuse with one another to form branched, three dimensional,
chain-like aggregates. As the aggregates cool below the fusion temperature
of (approximately 1710.degree. C.) further collisions result in some
reversable mechanical entanglement or agglomeration. Further agglomeration
also takes place in the collection process. Residual hydrogen chloride is
reduced to less than 200 ppm by calcination. The bulk density of the
product is normally 2 pounds per cubic foot, although, depending upon the
handling, may be as much as 4 pounds per cubic foot.
Commercially available grade of colloidal silica, sold under the trademark
"Cab-O-Sil PTG", has been used successfully in the practice of the present
invention and is used in the Examples which follow.
Although several Cab-O-Sil grades can be used, Cab-O-Sil PTG is the
preferred grade, as is Cab-O-Sil M5. The non-rheological grades of
Cab-O-Sil, such as MS7, MS75, and MS55, are not preferred.
Cab-O-Sil PTG has a surface area of approximately 200 meters per gram, a
primary particle diameter of about 14 millimicrons and a standard bulk
density of 2 pounds per cubic foot.
Applicant does not wish to be bound by the theory by which the composition
of the present invention works, but it is postulated that as used in the
composition of the present invention, the colloidal silica does not create
any chemical bonding with any of the other components. The hydroxyl groups
on the surface of the colloidal silica appear to set up a physical
interlocking, possibly through a form of hydrogen bonding, to maintain the
composition in its physical location unless it is subject to some external
pressures. Advantageously, the composition which includes the colloidal
silica, such as Cab-O-Sil PTG has a high viscosity while at rest, but when
subject to high shear, has a lower viscosity. This enables the composition
of the present invention to be handled and loaded into envelopes by
subjecting the composition to mixing, i.e., shearing forces, without the
need to heat the composition.
THE GLASS MICROBEADS
The microbeads used in the fitting composition are preferably discrete,
hollow, microbeads. It is preferred to have microbeads which are of low
density in order to reduce the weight of the overall composition.
Accordingly, the hollow, spherical glass microbeads are preferred. If
desired, other suitable micro-shapes or micro-forms may be used, such as
solid glass or cellular forms of glass, but the advantage of low weight is
sacrificed to some extent by using such forms. The micro-size of the
hollow glass microbeads may be within the size range, for example, of
about 10 to 300 microns.
The density of the glass microbeads should be, for example, about 0.05 to
0.70 gm./cc., or, more particularly, about 0.25 to 0.45 gm./cc. It is
essential, however, that the density of the glass microbeads be lower than
the combined density of the other components, namely, the wax-oil phase,
and therefore the glass microbeads are considered to be a density-reducing
component of the fitting material.
Glass microbeads sold by 3M under the trade designation B-37 have been
successfully used in the compositions of the present invention. The B-37
microbeads have nominal density of 0.37 gm./cc., a range of density
between 0.34 and 0.40 gm./cc., with a calculated bulk density of 0.19-0.27
gm./cc. These beads have approximate isostatic compressive strength of
2,000 psi. In addition to the B-37 microbeads, B-38 and B-25 microbeads
may also be used. The B-37 microbeads are the preferred microbeads. Other
sources of microbeads may be used, including eccospheres, hollow,
unicellular microbeads sold by Emerson & Cuming, Inc. of Canton, Mass.
under the designation 1G-25 glass microbeads.
THE WAX
The optional wax component, for example, may be a suitable natural,
mineral, petroleum-based synthetic, vegetable, or animal wax (includes
insect wax), such as beeswax [e.g., SC 10979 beeswax (yellow), supplied by
Sargent-Welch Scientific Co., Skokie, Ill.], or microcrystalline wax.
The wax component, as such, although deformable under pressure, is
substantially incompressible, as such, and is an essentially non-flowable
solid at temperatures below about 110.degree. F., but softens and flows at
higher temperatures.
Particularly good results are obtained with microcrysalline wax.
Microcrystalline wax is a mixture of solid hydrocarbons derived from
petroleum. More particularly, it is a mixture of alkylated naphthenes
(saturated cycloparaffins) and isoparaffins (branched chains) with varying
amounts of normal paraffins (straight chains). Microcrystalline waxes
generally have an average molecular weight of about 580 to 700, although
in some instances the molecular weight of paraffin wax generally is much
lower, usually ranging from about 260 to 420 and having individual
molecules varying from about 20 to 30 carbon atoms. As the same implies,
microcrystalline wax refers to the crystalline size when in the solid
state, and has crystals that are much smaller than those of paraffin wax
(sometimes referred to as macrocrystalline wax). Microcrystalline wax
generally has good low temperature flexibility, adhesion and excellent
resistance to moisture penetration under conditions of stress and strain,
and, therefore, generally is preferred over paraffin wax, particularly
when the fitting material is used in boots for use at low temperatures
(e.g., ski boots).
In the fitting material, the preferred wax is HM 1319 wax, a
microcrystalline wax-based, hot melt adhesive product formulated and
supplied by HB Fuller Company, St. Paul, Min. That wax product is light
yellow, has a typical "application" temperature of 180.degree. F. to
200.degree. F., and has a typical ring and ball softening point of
174.degree. F. to 176.degree. F. It essentially consists of 85% by weight
of microcrystalline wax, and 15% by weight of "Tufflo" 6204 oil.
THE ENVELOPE
The envelope in which the flowable, pressure compensating fitting material
is confined may be fabricated from any flexible material which is inert
with respect to the flowable, pressure compensating material and the
components thereof.
Generally, the envelope may be formed of a variety of flexible and pliable
materials that provide a protective barrier for the flowable, pressure
compensating fitting material. Preferably the material used for the
envelope is heat sealable to provide a substantially impervious barrier to
the flow or seepage therethrough of the components of the confined
flowable, pressure compensating material. If the envelope is formed of a
synthetic resinous film, the film should be flexible or elastomeric, both
at ambient room temperatures and the temperatures at which the product is
used, which can be in the vicinity of 100.degree. F. It is important that
the envelope material be durable and retain its flexible, plaible
properties over extended period of use. The various envelope materials
described in the prior art could be used as the envelope for the pressure
compensating materials of the present invention.
THE MIXING PROCESS
The flowable compositions of the present invention may be produced using a
variety of mixing processes. For example, the microbeads may be first
placed in a mixing bowl. The silica is then added on top of the microbeads
without mixing. Finally, the oil is added and all three components are
mixed together in one step to form a homogeneous composition. This process
is quicker and causes less "fly-away" of the microbeads, as compared to
other mixing processes.
THE EXAMPLES
The following Examples will serve to illustrate the preparation of several
flowable compositions within the scope of the present invention. Selected
prior art compositions are described for comparative purposes. It is
understood that these Examples are set forth merely for illustrative
purposes and many other compositions and methods of producing flowable
compositions are within the scope of the present invention. Those skilled
in the art will recognize that compositions containing other levels of
colloidal silica may be similarly prepared.
The compositions used in the Examples were prepared as follows:
The required amount of oil was placed in a mixing bowl. Microbeads, in the
appropriate quantity, were added thereto, care being taken to avoid
releasing the microbeads in the air. After the oil and microbeads had been
completely mixed, the Cab-O-Sil was added with mixing required to get the
desired viscosity. These compositions were placed, with no heating, in
polyethylene bags and tested as is set forth in the Examples.
The oil used is a technical grade of naphthenic-type white mineral oil sold
under the trademark "Tufflo" 6204.
The colloidal silica used in all of the Examples was Cab-O-Sil PTG grade,
described above.
For each of the following Examples, B37 microbeads obtained from Minnesota
Mining and Manufacturing were used.
The wax used was a microcrystalline wax comprising a mixture of solid
hydrocarbons derived from petroleum. This wax is available under the
indentification HM1319 from the HB Fuller Company of St. Paul, Minn.
EXAMPLE 1
The composition of Example 1 was as follows:
______________________________________
COMPONENT AMOUNT (% BY WEIGHT)
______________________________________
Oil 61.5
Colloidal Silica
1.0
(Cab-O-Sil PTG)
Microbeads 30.0
(B-37 from 3M)
Wax 7.5
______________________________________
This composition was slightly stiffer than the prior art composition
illustrated by Example 7.
EXAMPLE 2
The composition of Example 2 was as follows:
______________________________________
COMPONENT AMOUNT (% BY WEIGHT)
______________________________________
Oil 63.5
Colloidal Silica
2.0
(Cab-O-Sil PTG)
Microbeads 30.0
(B-37 from 3M)
Wax 4.5
______________________________________
This composition was slightly stiffer than the prior art composition
illustrated by Example 7.
EXAMPLE 3
The composition of Example 3 was as follows:
______________________________________
Component AMOUNT (% BY WEIGHT)
______________________________________
Oil 65.0
Colloidal Silica
2.0
(Cab-o-Sil PTG)
Microbeads 30.0
(B-37 from 3M)
Wax 3.0
______________________________________
This composition was slightly stiffer than the prior art composition
illustrated by Example 7.
EXAMPLE 4
The composition of Example 4 was as follows:
______________________________________
COMPONENT AMOUNT (% BY WEIGHT)
______________________________________
Oil 67.0
Colloidal Silica
3.0
(Cab-O-Sil PTG)
Microbeads 30.0
(B-37 from 3M)
Wax 0
______________________________________
This composition feels identical to the prior art composition illustrated
by Example 7.
EXAMPLE 5
The composition of Example 5 was as follows:
______________________________________
COMPONENT AMOUNT (% BY WEIGHT)
______________________________________
Oil 66.0
Colloidal Silica
4.0
(Cab-O-Sil PTG)
Microbeads 30.0
(B-37 from 3M)
Wax 0
______________________________________
This composition was slightly stiffer than the prior art composition
illustrated by Example 7.
EXAMPLE 6
The composition of Example 6 was as follows:
______________________________________
COMPONENT AMOUNT (% BY WEIGHT)
______________________________________
Oil 91.0
Colloidal Silica
9.0
(Cab-O-Sil PTG)
______________________________________
EXAMPLE 7
The composition of Example 7 was as follows:
______________________________________
COMPONENT AMOUNT (% BY WEIGHT)
______________________________________
Oil 58.0
Microbeads 30.0
(B-37 from 3M)
Wax 12.0
______________________________________
TEMPERATURE SENSITIVITY TEST
The composition of each of Example 1 through 7 were subjected to testing
for temperature sensitivity. Several portions fo the composition from each
Example (50 grams of the composition) were placed in polyethylene bags and
sealed. One bag from each Example was placed in a refrigerator held at
about 45.degree. F. to observe the effects of cold or low temperatures.
Another bag from each Example was placed in an oven at 105.degree. F. to
determine the effect of elevated temperatures (above room temperatures).
It is believed that the 105.degree. F. temperature exceeds slightly the
temperature generated in a wheelchair cushion (i.e., 95.degree. F.) which
is used for more than 2 or 3 hours.
All samples which contained wax (Examples 1, 2, 3 and 7) showed
significantly greater sensitivity to temperature at 45.degree. F., i.e.,
they were hard and unflexible and had very low flowability as compared to
those without wax (Examples 4, 5 and 6). Those samples which had no wax
showed no appreciable viscosity difference at 45.degree. F. as compared to
room temperature, whereas the prior art composition, Example 7, was
notably stiffer at 45.degree. F.
At higher temperatures the samples with wax (Examples 1, 2, 3 and 7) were
considerably less viscous and more watery than the samples which contained
no wax. The lack of temperature sensitivity in the compositions containing
no wax is self-evident.
SEPARATION TESTING
To measure the tendency of the compositions to separate, several
polyethylene "Zip-Lock" pouches approximately 2".times.5" were filled with
45 grams of the composition of Examples 1 through 7 and hung vertically at
room temperature, in an oven at 111.degree. F. and in an oven at
158.degree. F.
ROOM TEMPERATURE TESTING
None of the samples showed separation despite being hung at room
temperature for 5 months.
TESTING AT 111.degree. FOR 4 MONTHS
The tests run at 111.degree. F. for 4 months was run because it accelerates
the effect of sitting on a cushion and warming it to 95.degree. F. It is
estimated that this test is equivalent to sitting on the cushion 12 hours
per day for about 1.5 years.
The prior art sample (Example 7) showed significant separation. The top of
the pouch became too stiff and unflowable with a 1 inch pool of oil/wax
mixture at the bottom of the pouch. This result was judged less desirable
for use in cushions for long term seating application.
The composition of Example 1 showed some separation, wherein the oil/wax
mixture migrated to the bottom of the pouch leaving a stiff agglomeration
of microbeads at the top.
The products of Examples 2 and 3 (2% Cab-O-Sil) showed some separation, but
less than the formula of Example 1.
Examples 3, 4 and 5 (containing 3% and 4% Cab-O-Sil, but no wax) showed no
evidence of separation.
The use of Cab-O-Sil seems to prevent separation almost entirely when used
at 3% and 4% concentration with no wax. Based on the results of the tests,
the formula of Example 4 is preferred, because the viscosity at room
temperature is closer to the prior art product, which is the desirable
viscosity.
The compositions of the present invention, when confined in envelopes, may
be used in a variety of applications, including wheelchair seats and other
cushions, patient supporting system designed for beds or in orthopedic
applications, pressure relief pads juxtaposed against body casts, supports
and cushions, chair backs, ski boots, grips and other uses wherein the
pressure compensating pad is used in close proximity to portions of the
human body.
TESTING AT 158 .degree.
A test was developed to exaggerate the affects of leaving a flowable pad in
a car closed up in the noon-day sun, where the temperatures can elevate to
in excess of 120.degree. F.
Samples were made up in the manner described above and placed in the oven
at 158.degree. F. (70.degree. C.) for 7 days. The formulas with 3% and 4%
Cab-O-Sil, and no wax (Examples 4, 5 and 6), showed no visible separation
and did not evidence any significant change in viscosity. However, when
the pouches were opened, there was some separation observed in that the
fluid at the bottom of the pouch was more oily than the fluid at the top.
However, the fluid at the top was still fluid enough to be acceptable.
The prior art composition showed extreme separation which was visible
without opening the pouch.
The compositions containing Cab-O-Sil (Examples 1 through 3) and some wax
showed some separation, but significantly less than the prior art (Example
7). The compositions containing 1% or 2% Cab-O-Sil with some wax (Examples
1 through 3) were sufficiently stable that the flowable composition could
be readily reconstituted by kneading the composition within the pouch. In
other words, the separation was not as severe as the prior art composition
(Example 7).
The forms of invention herein shown and described are to be considered only
as illustrative. It will be apparent to those skilled in the art that
numerous modifications may be made therein without departure from the
spirit of the invention or the scope of the appended claims.
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