Note: Descriptions are shown in the official language in which they were submitted.
RADIATION-CURED MAGNE~IC
MEDIA AND PROCESS FO~ MAKING_SAME
DESCRIPTION
Technical Field
The invention relates to electron beam (~B)
cuxed magnetic recording media and to a process or
making such media.
Background Art
Magnetic media are composed of a nonmagnetic
substrate or support coated with a cured resin binder
containing dispersed finely divided magnetic partic-
lesO Normally the backing is made of plastic although
other materials such as paper, glass, or metal can be
used. Such magnetic media are ordinarily in the form
of a tape, belt, disc or the like. In this regard the
word "tape" is frequently used hereinafter to generic-
ally denote such media since tape is the most common
form v magnetic recording media. It will be under-
stood, however, that all forms of magnetic media are
included within the scope of this invention.
The binders used in tape are typically
curable high molecular weight thermoplastic polymers.
These binders are usually cured in the fluid state with
a chemical curing agent such as a diisocyanate. The
curing process causes crosslinking of the thermoplastic
polymer chains as well as other reactions involving the
diisocyanate.
674~3~
--2--
Chemical curing of tape binders has disadvan-
tages and drawbacks. The curing reaction is generally
-- unpredictable and i5 highly sensitive to temperature
variations, moisture, and stoichiometry. More impor
s tantly it generally provides a cured binder having a
lower than desired crosslink density~ It also results
in the curing agent being incorporated into the binder,
which agent does not directly contribute to the magnet-
ic or mechanical properties of the tape. Furthermore,
as compared to the EB curing process of the present in--
vention it requires more solvent and is more time con-
suming. Also, electron beam curing can bring about as~
ymmetric curin~ of the magnetic binder coating wherein
there is a crosslink density gradient across the coat-
lS ing thickness. Such curing cannot be achieved withchemical curing.
Radiation-induced curing of tape binders has
; also been suggested in the literature. US Pat No
3104983 teaches curing butadiene-acrylonitrile tape
binders with subatomic radiation. The present applic-
ants have found, however, that EB-cured hutadiene-a-
crylonitrile copolymer exhibits little crosslinking
relative to applicants' binder~ Japanese pa~ent pub-
lication no. 12423 (1972) describes an EB-cured
~5 magnetic tape in which the binder is composed of an
acrylate-methacrylate addition polymer that contains no
rective acrylate groups and an acrylate or dimethacryl-
ate monomer. The only component of this binder that is
~- suscep~ible to EB-curing is the monomerO It is
believed that the properties of such tapes will be
relatively inferior due, among other things~ to the
presence of methacrylate polymers which are known to
preferentially degrade when expo~ed to radiation.
--3--
. .
A prime object of the present invention is to
provide an EB-cured magnetic recording media having
improved magnetic and mechanical properties as compared
to the prior chemically cured or radiation-cured mag-
netic media. Another object is to provide a process
for making such media that involves no chemical curing
agents such as diisocyanates.
Disclosure of the Invention
; One aspect of the invention is a magnetic
medium comprising a nonmagnetic substrate coated with a
radiation-cured polymeric binder having magnetic par-
ticles dispersed therein characterized in that the
binder comprises a radiation-cured mixture of a high
molecular weight thermoplastic polymer and a radiation-
curable acrylate prepo~ymer.
A second aspect of the invention is a process
for making the above described magnetic medium compri-
~ing the steps of preparing a fluid mixture of a solu-
tion of a radiation-curable polymeric binder and mag
netic particles, coating a nonmagnetic substrate with
the fluid mixture, evaporating the solvent from the
coating to solidify the coating, calendering the dried
coated substrate, and exposing the dried coated
substrate to sufficient radiation to cure the coating
characterized in that the polymeric binder comprises a
mixt~re of a high molecular weight thermoplastic
polymer and a radiation-curable acrylate prepolymer.
:;
-~ Brief Description of_the Drawings
.
Figure l is a block diagram showing the manu-
facture of a magnetic tape according to the process of
.:
the present invention.
4~13
Figure 2 is a si~e view of a coating and
curing line wherein the curing is clone by means of an
electron beam.
Modes o Car~ in Out the Invention
, Y g _ _ _
The principal polymeric component of the mag-
netic binder composition in terms of quanitity is a
high molecular weight thermopla~tic polymer. This
component is essential to obtaining a magnetic medium
having appropriate mechanical properties. These poly-
mers are typically linear ully polymerized homopoly-
mers or copolymers havi~g a weight average molecular
weight of at least about 50,000, usually in the range
100lO00 to 800,000 and more usually 100,000 to
300,000. Examples of such polymers are styrene-
butadiene copolymers, acrylonitrile-butadiene-
copolymers r vinylacetate-vinylchloride copolymers,
polyesters, polyamides, polycarbonates, polysulfones,
polyacrylates, polyacrylic acid, polyvinylacetal,
polyvinylbutyral, polyurethanes, and epoxy and phenoxy
resinsO Polyurethanes, both polyesterurethanes and
polyetherurethanes, are a preferred class of high
molecular weight thermoplastic polymers.
The other essential polymeric component of
the magnetic binder is a radiation-curable acrylic
prepolymer. As used herein the term "prepolymer" de-
notes low molecular weight partially polymerized mole-
cules, including molecules commonly called oligomers.
These prepolymeræ are preferably polyfunctional, that
is, they contain more than one reactive acrylate
group. Difunctional and triunctional acrylate pre
polymers are particularly pre~erred. Their weight
average molecular weight will usually be less than
about lO,000, more usually less than 5,000. They
~ 7'~8
: -5~
are susceptible to rapid radiation-induced crosslinking
using either nonparticulate ~ultraviolet, X-ray, or
~ gamma) radiation or particulate ( a -particles,
: electrons, ~-particles, protons) radiation~ Electron
beam radiation is preferred because i~s generation,
focussing, and shielding ar~ simple relative to other
forms of radiation. Examples of EB-curable acrylate
prepolymers that may be used in the mixture are
acrylated epoxy resins, acrylated urethanes, acrylated
10 alkyd urethanes, acrylated polycaprolactams, acrylated
polyethers, acrylated unsaturated acid modified drying
oils, and acrylated polyesters. Specific examples of
such prepolymers are
:
: 15 l. Epoxy/Acrylate
CH3
CH2-cH-cl-[o~cH2~m]n~~cH2-clH-cH2-o-~-c-~-Olp-cH2-fH-cH2
!~ ~ : HO CH 3 OH
. . .
20 _[o-~CH2)m]nO-ll-cH=cH2
~,, . O
. 2. Pol~ester/Ur hane/Acrylate
CH2=cH-lc~-[o-(cH2)m]no-cl-NH~ 1H-C~
~; 25 O CH3 O
: .
-[DO-AD]p-OO-c-NH-l-NH_lCl_[o-(c~2)m]n-o-s-cH=cH2
3. Polyether Acrylate
CH2=CH-C-0 ~CH2-CH-OI pfi-CH=CH2
. O CH3 O
/
,~ .
~6--
4~ Polyester/Acrylate
CH2=CH-C O(cH2)6[o~-(cH2)~ o_(cH2)6]poc C 2
O O O O
wherein
m = 1, 2, 3
n = 0, 1 (preferably 0)
p = 1, 2, 3 (preferably 1)
DO = 1,6-Hexanediol
AD = adipic acid
~= Phenyl or substituted phenyl
The ratio of the high molecular weiqht
thermoplastic polymer to the acrylate prepolymer in the
mixture can vary from 50:50 to 90:10 and is preferably
in the range of 60:4Q to 8Q:20 on a resin solids basis
by weight. In other words the acrylate prepolymer can
be from as little as 10~ to as much as 50~ of the total
polymer in the binder.
Minor amounts of other conventional additives
may be included in the magnetic binder composition if
desired. Examples of such a~ditives are: dispersants
such as lecithin, organic esters of phosphoric acid,
quaternary ammonium compounds, and other suractants to
aid in the deagglomeration and dispersal of the magnet-
ic particles; conductive pigments, such as conductive
carbon black, to reduce the electrical resistivity of
the tape; and lubricants to minimize head-tape
friction. The inclusion of materials, such as ~
methacrylate polymers, that are preferentially degraded
by radiation should be avoided. As indicated above,
the binder cQntains no chemical curing a~ent. L
~9L6~7~
-7-
The third essential ingredient in the magnet-
ic binder is finely divided magnetic particle~ Examp-
les of magnetic particles that are commonly used are
~( ferric oxides, doped iron oxides, chromium dioxide,
and elemental iron, cobalt and/or nickeL. Acicular
~ ferric oxide is most commonly usedl Particle size
should be such as to obtain a good dispersion of the
magnetic component in the mixture. The particle length
of the ~ ferric oxide will ~sually be in the range of
`10 0.2 to 1 ~ m and it will usually have an aspect ratio
of 5:1 to 10:1. It will normally constitute about 60%
to about 90% by weight of the ~.agnetic binder
composition after drying.
In order to disperse the magnetic particles
and apply the magnetic binder composition as a thin
coating to the nonmagnetic film substrate the polymeric
components are dissolved in a common solvent such as
tetrahydrofuran, cylcohexanoner methyl ethyl ketone,
toluene, and methyl isobutyl ketone that will evaporate
rapidly. The polymer concentration in the solution
will typically be in the range o 0.05 to 0.20 mg/ml~
This solution, containing the homogeneously dispersed
magnetic particles, is applied to the magnetic
substrate using conventional coating machinery at a
thickness in the range of about 2.5 to 15 ~m. After
the coating is applied, the coated substrate is dried
to evaporate off the solvent leaving a solid coating
that is dry to the touch.
After the solvent is evaporated from the
'''! 30 coating the coated substrate is calendered and then
exposed to radiation of sufficient energy and dose to
cure the magnetic binder composition. The strength of
the radiation will depend upon a number of factors such
as the percentage of the acrylate prepolymer in
the zoating, the activity or crosslinkability of the
acrylate prepolymer, the thickness of the coating and
the duration of exposure. As indicated above, electron
beam radiation is preferred. UV radiation is the least
desirable since its use will normally require inclusion
of photoinitiators in the binder an~ it is highly
absorbed by additives such as pigments. Preferably, an
electron beam energy of no more than 300 KeV is employ-
ed since higher energies do not result in a better cure
of the binder and may cause damage to many magnetic
tape substrate materials. The dose can vary from 1 to
15 Mrad.
The chemical reactions that occur during the
curing are primarily radiation-induced free radical
reaction~, the most important of which are the direct
crosslinking of high molecular weight thermoplastic
polymer chains via hydrogen abstraction from the chains
and crosslinking of those chains via polyfunctional
prepolymer links. Other competing reactions are addi-
tion polymerization of the prepolymer molecules and
grating of the prepolymer molecules onto the thermo-
plastic polymer chains. These reactions result in an
EB-cured tape that has improved mechanical properties
as compared to prior tapes.
Referring now to the drawings, Figure 1 shows
the general plan for manufacturing a magnetic tape u-
tilizing the present invention. Although this partic-
ular figure shows the manufacture of a tape, it is ob-
vious that the same technique could be used to manufac-
ture other magnetic media by making suitable modiica-
tions as are well-known to those skilled in the art.
At 3, a coating mixture is prepared as is later de-
scribed in the examples. This mixture is then coated
at 5 on a tape utilizing well-known tape coating tech~
nlques. Before the tape has dried, it is ordinarily
oriented as at 7 by passing it through a strong
magnetic ield. At 9 the tape is passed through a
conventional drying oven which may be followed by
S burnishing or similar operations. The tape is then
calendered at 11 and at this point the tape is dry,
i.e. the binder is in a solid, thermoplastic state.
The tape is now passed through an electron beam curing
apparatus at 13 wherein the crosslinking reaction(s)
take place. The tape may then be slit at 15, burnished
at 17 and then spooled at 13. All of these operations
are conventional in the tape makin~ field and are well-
known to those skilled in the art except step 13 which
consists of passing a tape through a device wherein it
is exposed to an electron beam.
Figure 2 shows a typical election beam curing
process wherein an electron beam generator 21 is provi-
ded with suitable shielding 23 and ~5. The tape is
passed under the generator 21 and between the shields
23 and ~5 so that the electron beam 29 impinges on the
tape.
As is mentioned above, applicants' EB-cured
binder exhibits much more crosslinking than EB-cured
prior art binders. In order to demonstra~e ~his a
number of films of diferent polymers were prepared and
the elastic modulus was tested before and after being
subjected to an electron beam treatment. The elastic
modulus of the free film is used here as a measure to
reflect the crosslinkability or extent of crosslinking
- 30 of a polymer when subjected to an electron beam. Ex-
ample 1 shows the results which were obtained.
:; .
7~
--10--
Example l--Elastic Modulus of the Free Film
EB Elastic
Sample Dosage Modulus
- No. Film_Composition _ (Mrad) (X104 kPa)
lA Phenoxy resin 0 212
lB " 5 lgO
2A Butadiene-Acrylonitrile 0 0.17
copolymer
2B " ~ 5 0.21
3A Polyurethane (Estane 5701~Fl)l 0 3.08
3B " 5 3.12
4A EB curable acrylate prepolymer 0 0c45
(Celred 36001~ 50~ and poly-
urethane (Estane~5701-Fl) 50%
4B " 5 59.1
1~ A linear polyesterurethane which is sold
by B.F. Goodrich. Its properties are:
Typical ASTM
i 20 Value No.
Specific Gravity 1.20 D12-27
Hardness, Durometer 87 D-676
Tensile Strength (kPa)52,000 D-412
.~ Modulus at 30~ Elongation (kPa) 10,300 D-412
~' ` 25 Elongation (%) 575
Graves Tear (g/cm) 62,000 D-624
. Low-Temperature Brittleness
Freeze Point (C) -62 D 746
. Ge~man Low-Temperature
.~ : 30 Freeze Point (C) -28 D-1053
~- Taber Abrasion (mg loss)
(CS17 Wheel, 1000 g weight,
5000 cycles)5 D1044-49T
Processing Stock Temperature (C) 171
., .
i7~
2. A fast curing diacrylate ester of a
bisphenol A type epoxy resin which is sold hy Celanese
Chemical Company. Its properties are:
.
Visco~ity @ 25QC (cps) 250,000
Density g/cc 1.18
% Free acrylic acid 5 maximum
Gardner color 0.1
Flash point (C) >90
` % Active 100
Hydroxyl value 200
It can be seen from the above that conven-
tional tape binders such as phenoxy resins, butadiene-
acry}onitrile copolymers and polyurethane resins under-
wen~ little change when being subjected to an electron
beam, while a composition made in accordance with the
present invention, as is shown in samples 4A and 4B
wherein 50% of an acrylate prepolymer was used in
combination with the high molecular weight resin,
:20 underwent a very dra~tic change in elastic modulus.
Example 2--Formulation of EB-cured Scm Video Tape
Into a jar mill containing just enough one cm steel
balls to be covered by the ingredient solution, was
. added the:following ingredients:
1515 gm of acicular ~ ferric oxide
7.8 gm of alumina powder
6201 gm of carbon black
~: 43~4 gm of lecithin
31.9 ym of melamine type resin
12.4 gm of butoxyethyl stearate
-12-
71.1 gm of Estane 5714-Fl pol~urethane3
290 gm of methyl ethyl ketone
290 gm of tetrahydrofuran
680 gm of cyclohexanone
3. This polyurethane is a member of a family
of polyurethane resins which are made by reacting p,p'-
diphenylmethane diisocyanate, adipic acid and
butanediol-1,4 in such proportions that all of the
0 isocyanate groups have reacted to give a substantially
unreactive polymer. It is sold by B.F. Goodrich and
has the following characteristics:
Specific Gravity................................... 1.21
Hardness (Durometer A)~ .88
Tensile Strength at 23~C (kPa)................ ~. 40,000
300% modulus at 23C (kPa)..................... O~ 8,500
Taber abrasion resistance (gram loss--CS17
wheel, 1000 gr/wheel 500 rev.)................. 0.0024
The above ingredients were milled for 48
hours, and after which was added a solu~ion containing
the following ingredients:
130 gm of Estane 5714-Fl
85.5 gm of EB-curable acrylate prepolymer (Celred
3701)4
220 gm of tetrahydrofuran
,,
:! 210 gm of cyclohexanone
410 gm of methyl ethyl ketone
-13-
4. A nonvolatile diacrylate ester of a
bisphenol A epoxy resin, which is sold by Celanese
Chemical Company. Its typical properties are listed
below:
Viscosity @ 25C (cps)850,000
Density, g/cc 1.2
Free Acrylic AcidLess than 1%
Hydroxyl Value 232
Color 5 maximum
Flash point ~C) 90
After ~he addition, the final mix was then
milled or an additional six hours, followed by separa-
tions, filtration, coating, drying, calendering ana
elec~ron beam curing at a dose of 10 Mrad.
Utilizing the same general procedures as
outlined in Fxample 2 and the standar~ procedure of
sandmilling, additional magnetic media were made and
tested as follows:
Example 3--Hlgher Output of EB-cured
High Energy Instrumentation Tape
Binder Compasition
G162-71 polyure~hane
stan 5701 Fl)/halogenated polymer
(Ratio: 75/25)
G162-84A polyur ~ hane/EB-curable acrylate prepolymer
(Estan ~ 5701-Fl/Celred 3600)
(Ratio: 60/4~)
G162-84B polyur~hane/EB-curable acrylate prepolymer
(Estane 5701-Fl/(Chempol acrylate
prepolymer)5
(~atio: 60/40)
-14-
5. A solvent-free epoxyacrylate re~in which
contains active acrylic unsaturation in the polymer
molecule. It i5 sold by Freeman Company and has the
- following properties:
Polymer solids, % by weight..... O................... ~. loa
Reactive monomer, % by weight........... ........... none
UV Photoinitiator, % by weight.. ~....... .~..................... none
Acid number.... ~.O................................ 3-10
` Color.......... O.............. ...... 1-4
Viscocity as supplied, Centipoise....... 4000 6000 at 60C
10 ' Density g/cc...O...... 1400-1800 at 70C
1.17-1.20
Output6 dB ~ indicated
CuringFrequency (MHz)
Binder Method` 0.2 1.0 1.5 2.0
- ___ _
G162~71 chemical +1.4 +1.3 ~1.6 +1.9
G162-84A EB curing ~2.0 ~3.0 ~3.5 ~4.9
G162-84B EB curing +1.8 ~2.7 +3.4 +4.0
6. Output was mea~ured by Ampex ER-2000 at
the indicated frequencies. A higher number indicates
~- higher output~ and better tape. The reerence tape was
-~ ~emorex 716 tape~
,
Example 4--Higher Output o~_EB-Cured_Flop~ Disk
- Bin~er _omposition _ _
~ G162-41 polyure~hane
- ~Estan 701
` :
G162-82C polyurethane/EB-curable acrylate prepolymer
(Estane~5701-Fl/Celred 3600)
(Ratio: 55/45)
-15-
A
output7 ~)
Binder Curing Method 00-2F 34-2F _00-lF 34-lF
G162-41 chemical 96 94 96 92
G162-82C EB-cured llO 107 115 108
7. The output was measured by 3-Phenix
Certifier. The higher the percentage, the better the
tape. 100~ was the reference percentage.
Example 5--Better Performance_of _B-cuxed_
_. Vi~
~inder Composition
G162 47 polyurethane
(Estane~S714-Fl)/phenoxy resin (PKHH~3
(Ratio: 67/33)
G162-85C polyurethane
(Estane~ 714-Fl)/EB-curable acrylate
prepolymer (Celred 3701)
~Ratio: 70/30)
8. A phenoxy resin made from bis-phenol-A
and epichlorohydrin, sold by Union Carbide Chemical
Company under the trade name of Bakelite phenoxy resin
-~ PKH~, and having the followin~ properties:
.
j, :
'`'., '
-16-
Specîfic Gravity............................ O.... 1.18
Viscosity of 40~ solids in MEK, BrookEield
RVF, 20 rp~ NoO 5 spindle,.. ~..~........ ~........... 5,500 to
7,700 cps.
Reduced Viscosity
(0~2 g/100 ml dimethylformamide).......... ~Ø4 to 0.6
~ltimate Tensile Strenyth...................... G2,000 to
65,000 kPa
Ultimate Tensile Elongation.................. 50% to 100~
Softening Temperature.............................. 100C
Permeability (25 micron free film at 25C)
Water Vapor (24 hrs/645cm2)......s............ 138 g/mm
Oxygen (24 hrs/645 cm2)................... ...... 200 to
310 cc/mm
Carbon Dioxide (24 hrs/645 cm2)........... ...... 590 to
1180 cc/mm
Bulking Value............................... ... 1.18 g/cc
Perform nce
Chroma Video Binder
SNR10 SNRll Dura- Act-
Bind- Curing (Ref. (Ref. bilityl2 ivity
er Method Gloss 50.0dB) 47.BdB) Detector
G162
47 chemical 74 -1.0 0 2l~40/ 4.0 6.0
10/4
G162-
85C EB-cured 93 +2.0 +1,0 2'30"/ 1.6 3.0
`- 15~/4
9. Gloss reading was used to indicate the
smoothness of the tape surface, the higher the reading,
the smoother the surface.
~17-
10. Chroma SNR (signal to noise ratio) was
measured by using spectra analyzer; -1.0 means ldB
worse than the referen~e, +200 means 2dB better ~han
the reference.
11, Video SNR was m~asured by Rhode & Schwaz
meter. O means reference/ +1~0 means 1.0 dB better
than reference.
12. Binder durability was measured by Ampex
VR-2000
1st Number represents the time length of
measurement, e.g~ 2'; measured for 2 minutes
2'30" measured for 2-1/2 minutes
2nd Number represents percentage shed on the head
3rd Number represents percentage shed on the drum
4th Number represents general rating 1-10, lower
the number, the betterO
. 13. Activity Detector was measured by home-
made instrument using electrical reading to test the
physical flaw of the tape. Using a scale oE 0 to 10,
lower the number, the better.
. ~ .
