Note: Descriptions are shown in the official language in which they were submitted.
1~80S~9
RESILIENT PAD FOR KEYBOARDS
The present invention relates to keyboards and, in
particular, to an improved resilient pad for use in keyboards.
Keyboards are well-known structures. They are typically
used in computers, calculators, electronic typewriters,
and the like. As is well-known, the resilient pad is
used between the keys and the microswitches or other activating
devices. When a particular key is depressed, the function
of the pad is to give the user good "feel" while, at the
same time, having the resilience to return the key to
its original position. The pad serves the further function
of keeping dust, water, and other contaminants from the
operative part of the keyboard.
The state-of-the-art keyboard pads are made from
-silicone rubber. Silicone rubber has the desirable qualities
as hereinbefore set forth. However, it has been found
that, especially for some heavy-duty applications, silicone
rubber has the disadvantage of not having a long enough
flex life. Flex life is the number of cycles that the
rubber pad can withstand and still properly perform its
function.
Various attempts have been made to find a replacement
for the silicone rubber keyboard pad, especially where
a high flex life is required. One of the materials that
has been tried is EPDM. While EPDM has an excellent flex
life, it is generally not considered optimum as a keyboard
pad because of low resilience.
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The applicant has now discovered that an excellent
keyboard pad having all of the desirable properties of
a keyboard pad, including excellent resilience and excellent
flex life, can be obtained with a blend of EPDM and
5 a copolymer of propylene oxide and allyl glycidyl ether
such as that sold under the trademark Parel by Hercules.
The addition of Parel copolymer to EPDM substantially
improves the resilience of the pad without adversely affecting
the flex life of the pad. The ratio of EPDM to Parel
copolymer is suitably 10:90 to 90:10. It is preferred
that the materials be present in the ratio of 40:60 to
60:40 and best results have been obtained with a substantially
50:50 ratio of the two materials;
The term EPDM as used herein is a common industry
term which refers to a terpolymer of an ethylene propylene
diene monomer. In the terpolymer, the diene is present
in a comparatively small amount. Various dienes are in
common commercial use including, for example, 1,4 hexadiene;
ethylidene norbornene; and dicyclopentadiene. Of these,
we prefer to use the 1,4 hexadiene such as is present
A in DuPont's EPDM polymer sold under the name Nordel 1470.
As in normal rubber processing, the formulation includes
accelerators, typically sulfur compounds. Suitable accelerators
include zinc dibutyldithiocarbamate, tetramethylthiuram
disulfide, 2-mercaptobenzothiazole, and similar, known
accelerators. The formulation can further include antioxidants,
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e.g. nickel dibutyldithiocarbamate or polymerized 2,2,4-
trimethyl-1,2-dihydroquinoline; activators, e.g. a metal
oxide such as zinc oxide or magnesium oxide; curing agents,
e.g. a higher fatty acid such as stearic acid; sulfur;
and carbon black. Listed below are preferred and typical
levels for the various ingredients. The total amount
of rubber, i.e. the combination of EPDM and Parel copolymer,
is 100 parts. The other ingredients are listed in parts
by weight per 100 parts of the rubber component.
Typical Preferred
Inqredient Ranqe Range
EPDM 10-90 40-60
Parel copolymer ld-90 40-60
Antioxidant 1-3
Curing agent 0.75-1.5
Activator 3-25 10-20
Accelerators 2-5 2-3
Sulfur 0-3 1-2
Carbon black 0-20 5-15
In addition to the above-listed ingredients, the formulation
can include other ingredients, for example, an additional
rubber material such as polyisoprene or polybutadiene
may be employed. If an additional rubber material is
employed, it is preferred that it not be present in an
amount greater than 15 parts by weight of the EPDM-Parel
copolymer content. Other ingredients commonly used in
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rubber formulations can also be employed such as cure
retarders, processing aids, and the like.
In forming the pad of the present invention, the
two rubber materials, i.e. the EPDM and the Parel copolymer,
are first mechanically mixed to homogenize them. It has
been found to be important to mix for a sufficient period
of time at an elevated temperature to obtain substantially
complete homogenization of the two materials. It has
been found that this can be accomplished by mixing the
k 0 two materials in a Banbury mixer at 150-200F for two
minutes. After the two polymers have been blended together,
the other ingredients except for the accelerators and
sulfur are added to the Banbury'mixer. The other ingredients
are preferably added se~uentially in order to insure good
mixing in accordance with normal rubber processing procedures.
The addition of the further materials suitably takes place
at a temperature of up to about 300F and is suitably
over a period of four minutes.
After the completion of the first stage, i.e. the
mixing of all of the ingredients except for the accelerators
and the sulfur (if present), the material is cooled.
It is then mixed again, suitably in a Banbury~mixer, for
2-3 minutes at a temperature of up to about 200F with
the addition of the accelerators and sulfur. The mixed
material is then made into a preform, suitably by extrusion.
The extruded preform is then made into the appropriate
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shape for the particular pad application, suitably by
transfer, injection, or compression molding. The part
is preferably post cured after molding, suitably at about
225F for about two hours.
These and other aspects of the present invention
are illustrated in the examples which follow:
EXAMPLE 1
In this example the formulation consisted of the
following ingredients with each of the ingredients beinq
listed as parts by weight based on the total rubber content:
Inqredient PPHR
EPDM 50
Parel copolymer 50
. Nickel dibutyldithio-
carbamate 0.5
Polymerized 2,2,4-
trimethyl-1,2-
dihydroquinoline 0.5
Stearic acid
Zinc oxide 15
Zinc dibutyldithio-
carbamate
Tetramethylthiuram
disulfide 0.25
2-mercaptobenzothiazole
Spider sulfur 1.5
Carbon black 5
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A The EPDM and Parel copolymer were mixed in a Banbury
mixer for two minutes and the temperature rose to about
200F during the mixing. The antioxidant, stearic acid,
zinc oxide, and carbon black were added seguentially and
mixing was continued for another four minutes and the
temperature rose to about 300F. The material was subsequently
reintroduced to the Banbury mixer and was mixed for two
minutes at a temperature below about 200F. During this
mixing the accelerators ~zinc dibutyldithiocarbamate,
tetramethylthiuram disulfide, and 2-mercaptobenzothiazole)
and the sulfur (spider sulfur) were added. After removal
from the Banbury mixer, the mixture was extruded into
a suitable preform and the preform was then formed into
a keyboard pad in a transfer molding operation. The keyboard
pad was cured for two minutes at 350-360F. The pad was
post cured at 225F for two hours.
The finished pad had the following physical properties:
Property Value
Specific gravity 1.07
Hardness, Shore A 50
Tensile strength above 400 p.s.i.
Elongation above 350%
Elastic modulus
at 100% above 150
The pad was measured for change in peak tactile between
compression and relaxation to determine the hysteresis
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of the rubber. The value is expressed as ~ grams and
indicates the resilience of the material. A material
with complete resilience has a value of ~ O grams. The
pad of this example was found to have a value of ~8 grams.
The pad was also found to have excellent flex life.
EXAMPLE 2
For comparison purposes, a keyboard pad was made
in the same way as Example 1 except that, in this case,
the rubber component was only EPDM and no Parel copolymer
was present. While the flex life of this product was
also excellent, it was found that its resilience was
~19 grams, i.e. much worse than the resilience of the
pad of Example 1.
EXAMPLE 3
Example 1 is repeated using 10 parts EPDM and 90
parts Parel copolymer. Acceptable results are obtained.
EXAMPLE 4
Example 1 is repeated using 90 parts EPDM and 10
parts Parel copolymer. Acceptable results are obtained.
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EXAMPLE 5
Example 1 is repeated using 40 parts EPDM and 60
parts Parel copolymer. Results comparable to those of
Example 1 are obtained.
EXAMPEE 6
Example 1 is repeated using 60 parts EPDM and 40
parts Parel copolymer. Results comparable to those of
Example 1 are obtained.
It will be understood that the claims are intended
to cover all changes and modifications of the preferred
embodiment of the invention herein chosen for the purpose
of illustration which do not constitute departure from
the spirit and scope of the invention.
