Note: Descriptions are shown in the official language in which they were submitted.
12Z4~37~
STATIC REDUCTION IN MAGNETIC RECORDING CASSETTES
Field of the Invention
The present invention relates to a device and
method for the reduction of static charge buildup in
cassettes for housing magnetic recording tape. A non-
volatile, organic, conductive coating composition is
applied to the surfaces of components of the cassettes,
dramatically reducing static charge buildup within the
cassette.
Background of the Invention
The problems of triboelectric charging and other
forms of static charge buildup have long provided problems
in various commercial areas. A large number of various
methods have been developed over the years for dealing with
this problem. Amongst the various techniques used to
reduce electrostatic charge buildup are the application of
conductive coatings to surfaces, the inclusion of
conductive particulate matter or fibers within the composi-
tion, the matching of work functions of surfaces in movable
contact with each other, exterior grounding of surfaces
subject to charge, and even coating with compositions that
suppress spark discharge. Each of these procedures has its
own advantages and disadvantages, but are generally
regarded in the art as equivalent in their ability to
reduce static charge buildup.
Brief Description of the Invention
The present invention provides for an improved
method of reducing electrostatic charge buildup in
cassettes which house magnetic recording tape by the
application of non-volatile, electrically conductive,
organic coating compositions to surfaces of the cassettes
and the components of the cassette. The coating
composition is preferably applied to all surfaces which do
not directly contact the magnetic recording tape during
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storage or playback mode, although those surfaces may also
be coated. The coating composition i9 a non-integral,
preferably non-film-forming composition which is applied to
both interior and exterior surfaces and components in the
cassette.
Detailed Description of the Invention
Magnetic recording tapes are stored in various
types of cassettes for their use in various fields within
the industry. Magnetic tape containing cassettes are used
in computors, in audio recording, audio-video recording,
typewriting and other information storage and usage
mediums. Even within the various fields of use described
above, there are different formats in the industry.
Cassettes are thus variously available for both 8-track
(cartridges) and cassette audio recording (nominally 3.8 mm
tape), and both U-Matic (three-quarter inch nominal) and
VHS and BETA format (nominally one-half inch tape) audio-
video recording devices. Newer formats, such as the 8
millimeter audio-video format, are already being tested and
developed. These cassettes have traditionally been plagued
with a problem of electrostatic charging during both manu-
facture and use. During manufacture, the molding and
handling operations used to form the various parts of the
cassette have caused generation of triboelectric charges
within the cassette. When the cassette is then transported
to the operating station where the tape is inserted into
the cassette, significant problems are often encountered
because of the residual or developed charge on the
cassettes. Particulate matter, such as dust, is often
attracted into the cassette and is also deposited upon the
surface of the magnetic tape as it is inserted. The dust
causes both aesthetic and functional problems with the use
of the tape. During use of cassettes, static charge build-
up will again attract particulate matter into and onto the
cassette. The presence of dust on the tape itself can harm
both the tape and the magnetic heads which read the tape~
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The appearance of dust on the cassette and the tape pro-
vides an unattractive and worrisome appearance to the user.
As noted above, cassettes presently are made with
conductive materials included within the composition used
to form the various surfaces and components of the
cassettes. This moderately reduces the residual charge
which cassettes can maintain, but residual charging in
excess of 2000 volts is still quite common. The
application, according to the present invention, of a
non-volatile, conductive, organic coating composition to
the surfaces of and the components of a cassette which
houses magnetic recording tape has been found to be able to
consistently reduce the residual charge in cassettes to
less than 500 volts. Generally, electrostatic charging is
held to less than 200 or even less than 100 volts in such
cassettes. The residual charge is that charge retained on
the body after charging of the body and exposure to air at
20C and 30 percent relative humidity for 20 hours.
The conductive coating composition of the present
invention has been referred to as "non-volatile". In the
practice of the present invention this means that, after
evaporation of any solvents, less than 2 percent by weight
per week of the coating composition will evaporate off the
surface of an article at 25C and 40 percent relative
humidity. PreEerably less than 1 percent by weight per
week would evaporate and, more preferably, less than 0.1
percent by weight per week would evaporate at those
temperatures and conditions.
The greater the percentage of surfaces of
components on and within the cassette that are coated with
the organic, conductive coating compositions of the present
invention, the greater the general reduction in electro-
static charging. This does not necessarily mean that
greater amounts of the coating composition better reduce
the electrostatic charging, but rather that it i9 important
to insure the coating of as much surface area as possible
in and on the cassette. rrhus both interior and exterior
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surfaces of the cassette, reels, spools, structural
supports, and other parts of the cassette should be coated.
As the coating composition is likely to come in contact
with the magnetic tape, which is not necessarily preferred,
the coating composition should be selected so as to not be
detrimental to the performance and structural intearity of
the magnetic recording tape (e.g., it should not be a
strong solvent for the structural component of the tape).
The coating composition should generally be applied so as
to provide a dry (without solvent) coating weight of
between 0.5 (0.08 micrograms per square centimeter) and 250
micrograms per square inch (40 micrograms per square
centimeter) as an average coating weight. Because the
coating composition will not necessarily spread uniformly
over the coated surface, or because of surface structure
variations (such as an embossed non-glare surface), it is
the average coating weight that must be considered in this
range. If less than this amount of material is applied,
insufficient antistatic protection will be provided. If
greater than this amount is applied, no further antistatic
protection is obtained, and an undesirable, highly greasy
appearance will be provided to the coated article.
Preferably a coating weight of 1 - 100 micrograms per
square inch (0.16 to 16 micrograms per square centimeter)
is used or more preferably 1 - 50 micrograms per square
inch (0.16 to 8 micrograms per square centimeter). In the
most preferred practice of the present invention, 5 - 30
micrograms per square inch of the coating composition is
used.
The organic coating composition useful in the
practice of the present invention may be any non-volatile,
non-integral, organic, electrically conductive composition.
It is highly preferred that the coating is non-integral as
this provides the greatest static reduction. sy
"non-integral" it is meant that the composition does not
form a self-supporting solid film when air dried, as it
would be when present on the surface of the cassette. The
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term non-integral therefore excludes the provision of a
solid, thermoplastic or non-liquid conductive coating
composition to the surface of the cassette. Useful
conductive coating compositions include the many antistatic
agents and systems already known in the art including long
chain alkyl quaternary amines, long chain alkyl quaternary
phosphines, fluorinated antistatic materials, polymers
having pendant antistatic, ionic groups such as quaternary
amine groups, and other ionic, organic materials known for
antistatic purposes in the art. The preferred material
according to the practice of the present invention for use
as an antistatic coating composition is the composition of
U.S. Patent No. 4,313,978. This particular composition
provides extremely long lasting and high efficiency
antistatic protection for cassettes. That antistatic
composition comprises a fluorinated anionic surfactant
which is an amine salt of an acid containing a fluorinated
organic radical (as defined herein) and an antistatic agent
which is an ionic salt of an amine. It has been found that
the combination of the fluorinated surfactant and anti-
static agent can provide a reduction in static charging at
very low application amounts.
The ~urfactants used in that preferred composi-
tion of the present invention are anionic fluorocarbon
surfactants which are the amine salts of acids containing a
fluorinated organic radical. The preferred surfactants may
generally be depicted by the formula:
Rf-A-X Z+
in which
Rf represent a fluorinated organic radical (as
defined herein),
A represents a bond or a divalent linking group,
X~ represents an acid anion, and
Z~ represents a quaternary ammonium cation.
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The fluorinated organic radical Rf is defined as
a radical which is a saturated, aliphatic radical having
from 2 to 20 and preferably at least 3 carbon atoms, the
skeletal chain of which may be straight, branched or, if
sufficiently large, cycloaliphatic. The skeletal chain may
be interrupted by divalent oxygen or trivalent nitrogen
atoms bonded only to carbon atoms provided the radical does
not contain more than one heteroatom, i.e., nitrogen or
oxygen, for every two carbon atoms in the skeletal chain
the radical being fully fluorinated with the exception that
it may contain hydrogen or chlorine atoms as substituents
provided that not more than one atom of either is present
in the radical for each carbon atom. Preferably, the
fluoroaliphatic radical is a perfluoroalkyl radical having
a skeletal chain that is straight or branched.
The fluorinated organic radical is linked to the
acid anion either by a direct bond or through the divalent
linkage A. Preferably the chain of the linking group A is
composed of carbon atoms although heteroatoms, e.g.,
nitrogen or oxygen, may be present provided they do not
interfere with the ionic nature of the surfactant.
Preferred linking groups are alkylene groups.
Particularly suitable acid anions X~ include
carboxylic acid and sulfonic acid groups.
Examples of suitable fluorinated organic radical
containing anions Rf-A-X- include 5 to 18 carbon atom
perfluorinated alkyls such as perfluoropentyl sulfonate and
perfluoro-octylcarboxylate.
The quaternary ammonium groups Z+ are derived
from the corresponding amine. Suitable amines include
aliphatic amines and aliphatic cyclic amines, which may
optionally be substituted with substituents which will not
affect the ionic nature of the surfactant. Preferably the
amines contain 2 to 12 carbon atoms. Suitable amines
include piperidine, dimethylaminoethanol, morpholine,
triethanolamine and triethylamine.
--7--
Other suitable surfactants include those
containing 2 or more acid anions and quaternary ammonium
cations in which the acid anions are bonded directly to the
fluorinated organic radical or via one or more linking
groups. Preferably the fluorinated organic radical is
pendant although it may be present within the molecule as
in the case of the amine salts of (C2F4COOH)2 and
(C2F4SO3H)2. Anionic surfactants containing two or more
fluorinated organic radicals may also be used.
The antistatic agent used in the preferred compo-
sitions is an ionic salt of an amine. The compoundi have
the property that they induce conductivity to the surface
upon which they are applied. Suitable amines include those
from which the quaternary ammonium groups Z+ are derived.
The anionic portion of the antistatic agent may be chosen
from a wide variety of anions including halide, sulfate,
aryl sulfonate, aliphatic sulfonate, aryl carboxylate and
aliphatic carboxylate. The anions may contain further
substituents providing they do not affect the antistatic
properties of the compound, for example, the presence of
nitrogen atoms and highly fluorinated radicals is
undesirable in the anion.
Specific examples of anions include:
C7H7SO3-
so4-
Cl-
CH3(CH2)8C02
C6H5C2
CH3C02-
The fluorinated surfactant and antistatic agent
may be derived form the same or different amines.
The compo~itions are preferably applied from a
single solution. Suitable solvents include lower alcohols,
e.g., ethanol and isopropanol, which may be diluted with a
low boiling fluorocarbon. Preferably the solvent is chosen
such that the fluorinated surfactant and antistatic agent
have substantially the same solubility so that the dried
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coating contains the same ratio of constituents as in the
applied solution. If there is a substantial difference in
the solubilities non-uniform coatings may result.
The ratio of fluorinated surfactant to antistatic
agent in a composition depends upon the intended use.
The concentration of the solutions vary according
to their intended use. Preferably the anion molar percent
of the fluorinated organic radical is between 1 and 50~,
preferably 1.8 and 47.9%, and most preferably between 15
and 40 percent of the mixture of the two ingredients. The
present invention has been found to be independent of the
specific antistatic composition used, although some, of
course, perform better than others because of their
physical properties.
The composition of the cassette is immaterial to
the practice of the present invention, since triboelectric
charging can occur on substantially all solid surfaces.
Generally, however, only synthetic resin compositions are
used for the major structural components of cassettes.
These may be either thermoplastic or thermoset resins.
Amongst the various resins normally used in the production
of cassettes acrylonitrile-butadiene-styrene plastic,
styrene-acrylonitrile resin, polystyrene, polycarbonates,
high impact polystyrene and medium impact polystyrene,
polyvinyl acetal and polyesters are generally used. Of
course, dyes, Eillers, molding aids and the like may be
included within these compositions. Surprisingly, the
inclusion of equal or larger quantities of antistatic
materials into the composition used for structural portions
of the cassette do not reduce static charging nearly as
well as the applied coatings of the present invention, even
where the same materials are used.
These and other aspects of the present invention
will be shown in the following non-limiting examples.
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Example 1
The following formulation was prepared in our
laboratory for spray application to plastic components:
AdditivesParts by Weight
Piperidinium toluene sulfonate 0.057
Piperidinium perfluoro-octyl sulfonate 0.023
Polyethylene glycol 200 0.020
Volatile Vehicle
Freon TF~ 75.0
Ethyl alcohol 25.0
Properties: Clear Solution of 0.1% by weight non-volatile.
The solution was applied from a Binks Model 69 air spray
gun to ABS plastic components of the VHS format. The elec-
trical potential due to the electrostatic surface charge of
each component was measured before and after spray treat-
ment by means of a Monroe Model 175 electrostatic
voltmeter. Results were as follows:
Before Treatment - 2000 to 18,000 volts.
After Treatment - 200 volts maximum.
The treated parts were subjected to the following condi-
tions:
1) Rubbed briskly with Nylon carpet.
2) Sl:ored in 5% relative humidity cabinet for
three days.
Results: Charge level for both conditions
remained below 200 volts.
The laboratory experiments were expanded to plant studies
wherein technical feasibility of spray application to total
assemblies was demonstrated. Evaluation of the treated
assembled cassettes (VHS) confirmed the laboratory findings
that electrostatic charges were reduced to an acceptably
low level (less than 300 volts).
Competitive analyses were run on commercially
available cassette products and the findings published;
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namely, that the subject invention treatment reduced
cassette body charges below 100 volts whereas all commer-
cially available cassettes were orders of magnitude higher
(at 2000 - 11,000 volts).
Substantially identical results were obtained
using isopropyl alcohol in place of the ethyl alcohol as
solvent.
Example 2
The following tests were performed on six
different commercially available VHS format cassettes. The
cassettes were first electrostatically charged to about
13,000 volts. Electrometer probes were placed adjacent to
each of the cassettes (within one-quarter inch) to measure
static electricity bleed-off at 76F and 42% relative
humidity. The charge was measured at various time
intervals (at least initially, 1, 2, 3, 4, 5 and 20 hours)
and recorded. It was determined by analysis that cassettes
C, D, E and F contained antistatic ingredients in the
plastic composition of the cassette. The lowest level of
static charge for any of the cassettes, even after 20 hours
of air discharge, was 1550 volts.
~e~
The cassette B of Example 2 was used in the test
procedure of Example 2 after being coated with the
following antistatic coating compounds according to the
present invention:
a) cationic, long-chain alkyl (greater than
C-12) fatty acid condensate (SandotexOA)
b) quaternary amine sulfonic acid derivative
(Avitex~E)
c) vinyl polymer with quaternary amine pendant
groups (DOW~ ECR 34)
d) dimethyl allyl ammonium chloride
(Calgon~ E1515), and
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e) piperidinium toluene sulfonate (1 part by
weight) and piperidinium perfluorooctyl
sulfonate (4 parts) as a 0.1~ by weight
solution in isopropanol.
Each of the antistatic compositions was found to
reduce the residual static charge on the VHS cassettes to
less than 500 volts in less than 20 hours. The last
composition (e) displayed a residual voltage of less than
100 volts.
Examples 4-7
The effect of antistatic materials in the molding
composition was compared to the same materials on the
molded surface.
An acrylonitrile-butadiene-styrene molding resin
at a melt temperature of 470F was injected at 950 p.s.i.
into a cassette mold at 150F. In the first run, no
antistatic agents were included in the molding resin. The
three following runs respectively contained 0.01% by
weight, 0.10% by weight and 1.0% by weight of the
antistatic system of Example 1. The residual change was
measured in the same manner as Example 2 and are compared
in the Table below with the coated cassette of Example
3(e).
Included
Antistat;cCoated
Example (% by weight) Antistatic Charge
4 o 0 >2000
0.01 0 >1700
6 0.1 0 >1500
7 1.0 0 >1800
3(e) 0 <0.01% <200
The superiority of coated antistatic agent to
include antistatic agent is apparent.
