Protective coatings for a magnetic tape sensor

Several materials have been proposed as protective coatings for magnetic sensing heads in magnetic tape readers. Some of these, known for hardness, are not very wear resistant. Others that seem to wear well tend to spall. The invention is a polymer coating that wears exceptionally well, can be formed conveniently to encapsulate the magnetic sensing elements, can be photocured and therefore photopatterned, and is inexpensive. The polymer is composed of a mixture of urethane-modified acrylates and methacrylates.

FIELD OF THE INVENTION 
This invention relates to a code conversion apparatus of the kind used for 
reading magnetic tape codes. More specifically the invention is directed 
to long wearing magnetic sensing heads for credit card readers. 
BACKGROUND OF THE INVENTION 
Credit card readers have been devised which rely on magnetic codes stored 
on the card in the form of binary data. A simple and reliable code scheme 
is a bar code known as the "Aiken code." In this scheme binary data is 
stored on the magnetic tape in the form of magnetic domain transistors 
spaced along the tape. The presence or absence of a transistor (actually 
an interface between oppositely poled magnetic domains) indicates a binary 
one or zero. Since this data is represented specifically along the tape 
axis the reading speed must be carefully controlled at a uniform rate to 
coincide with a clocking arrangement incorporated into the reader. This 
requires typically a motorized arrangement to feed the card passed the 
sensing head of the reader. 
Systems are available in which timing data is incorporated into the 
magnetic tape so the card reader can sense the instantaneous rate of tape 
travel and adjust accordingly the clocking circuit in the reader. By 
comparison with the system described here, such systems are unnecessarily 
complex and have difficulty in responding if the card reading rate is 
interrupted or grossly varied. 
A simple credit card reader that overcomes the foregoing, at least in part, 
is described and claimed in application Ser. No. 553,528 filed Feb. 27, 
1975 in the name of G. E. Moore, Jr. In this device two sensing elements 
are spaced one-half cell length apart to provide in response to the code 
information independent outputs which when applied to simple logic 
circuitry provide separate representations of the data and the timing 
information. The logic circuitry includes decisional circuitry responsive 
to outputs from the two elements for generating an output stream 
containing timing information. The timing information in conjunction with 
the outputs of both of the sensor elements is used to generate a separate 
output stream containing data. 
In a preferred embodiment, a pair of magnetoresistive sensor elements are 
spaced one-half cell length apart in the path of the code. Each element 
responds independently to the regular transitions at the beginning and at 
the end of a cell and also to the irregular transitions (data information) 
stored in the center of the cell. Due to the spacing between elements, an 
output is received from the two elements simultaneously only when a data 
bit (a binary one) is stored. Only one element is activated when no data 
bit (a binary zero) is stored. The electronic output is independent of the 
speed at which the code passes the sensor. 
A simple electronic circuit processes the outputs of the two elements 
typically for transmission to a remote computer. The outputs are applied 
to an OR circuit, the output of which is employed both to enable the data 
from the elements to be applied (via an AND gate) to a shift register and 
to provide a clock pulse for the shift register. The operation provides 
enabling pulses with widths reflective of the movement of the credit card 
and the effect is that the output from at least one of the elements is 
stored in the shift register whenever the corresponding bits of the code 
arrive at the sensor. 
The operation depends on the close proximity of the elements to one 
another. In order to achieve the desired proximity, magnetoresistive 
sensor elements are formed by photolithographic techniques and are 
connected electrically in parallel to provide independent indications of 
the code as required. The constraint on the magnetoresistive elements 
impose design criteria realized in a novel approach herein. Each element, 
for example, includes a plurality of subelements having a prescribed 
growth or shape anisotropy. The subelements are connected electrically in 
series and respond to the presence of a transition of the code by the 
rotation of the magnetization therein. 
From the foregoing it is evident that in this equipment the magnetic 
sensing head is an important element and it is vital that the integrity of 
the magnetic sensing elements and the space between them be precisely 
preserved. This requires a protective layer into which the sensing 
elements are embedded. The layer should be of a material that is 
nonmagnetic (so as not to interfere with the magnetic data being sensed) 
durable and in which the sensing elements can be rigidly suspended by a 
simple casting or potting process. 
Various kinds of protective coatings for magnetic sensing heads have been 
proposed in the art. Well recognized are the requirements for hardness and 
durability, for the sensor should withstand exposure to a tape which 
contains hard particles of magnetic oxide that can be severely abrasive 
over a period of continued use. 
U.S. Pat. No. 3,249,700 issued May 3, 1966 to S. Duinker et al proposes a 
glass protective layer. Glasses, aside from their tendency to crack and 
devitrify, do not possess outstanding wear resistance. 
U.S. Pat. No. 3,417,386 issued Dec. 17, 1968 to R. A. Schneider, suggests a 
metal or metal alloy coating or a carbide coating. However, metal and 
alloy coatings, some of which are known for hardness, are not very wear 
resistant. Carbides tend to spall and gall. When the latter occurs the 
magnetic tape of the credit card processed through the machine is often 
damaged. Materials allegedly superior to these are described in U.S. Pat. 
No. 3,665,436 issued May 23, 1972 to J. J. Murray, et al. They purpose 
ceramic layers such as chromium oxide. Such materials, typically applied 
by plasma plating, have been used in the industry with some success. 
However, we have now discovered a protective coating for use with credit 
card readers, and advantageously with the reader described herein, that 
exhibits improved durability, can be applied with a solvent free coating 
process (in the spirit of OSHA standards), can be patterned by 
photodeposition or screen printing prior to curing to final hardness, and 
is inexpensive. The coating is a polymer of one or more of the following 
monomers: 
(a) 1-butane carbamic acid 2-methacryloyloxyethyl ester, 
(b) 1-butane carbamic acid 2-acryloyloxyethyl ester, 
(c) a mixture of isomers 2,2,4 - and 2,4,4,- trimethyl-1, 6-hexane 
dicarbamic acid di(2-methacryloyloxyethyl)ester, 
(d) a mixture of isomer as above in (c) in which the esters are both 
2-acryloyloxyethyl, 
(e) di(4-cyclohexylcarbamic acid 2methacryloyloxyethyl ester)methane, 
(f) the same as in (e) in which the esters are 2-acryloyloxyethyl. 
The monomers may be prepared by first reacting a hydroxy substituted ester 
of acrylic or methacrylic acid with a mono- or difunctional isocyanate. 
The reaction of the hydroxyl group with the isocyanate yields a urethane. 
The reaction product can be polymerized readily by heat, light or electron 
beams. Photo-initiated polymerization implies that the coatings can be 
patterned by standard lithography. The polymerization advantageously 
involves a high degree of cross-linking of the monomers until the system 
becomes essentially saturated (above 70%). This class of materials and 
techniques for their preparation are described in U.S. Pat. No. 3,479,328 
patented Nov. 18, 1969. 
In a preferred form of the invention the coating comprises a polymerized 
mixture of mono- and difunctional monomers advantageously in a weight 
ratio of mono- to difunctional monomer of 9:1 to 4:6. Thus with the 
monomers a-f, a or b could be mixed with any of c through f. Normally the 
acrylates would be mixed and the methacrylates mixed so that the most 
typical examples would be: a with c or e and b with d or f. Ternary 
mixtures are also useful. 
One of the properties of interest is the toughness of the polymer. This 
characteristic is a measure of the amount of energy the material can 
absorb without losing structural integrity. The latter limitation is 
referred to in terms of the yield point and the break point. The preferred 
range of weight ratios of the mono- and difunctional monomers have to do 
with this characteristic of the resulting polymer. If the weight ratio 
exceeds 9:1 the yield point of the polymer is lower than desired although, 
with certain formulations and for some applications, mixtures that exceed 
that ratio may still be found useful. As the ratio falls below 4:6 the 
resulting polymer becomes hard and brittle, and breaks before reaching a 
yield point. However again, special cases may allow a departure from this 
preferred limit.

DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a top view of a conventional credit card of a kind for reading 
with the apparatus of this invention; 
FIGS. 2 and 3 are magnified top views of portions of the magnetic tape of 
the credit card of FIG. 1 showing the arrangement of the magnetic states 
in the tape; 
FIG. 4 is a fragmentary projection view of a sensor arrangement for reading 
the credit card of FIG. 1; 
FIG. 5 is a sectional view of the magnetic sensing head assembly and a 
suitably coded credit card; and 
FIG. 6 is a prospective view of the magnetic sensing head assembly. 
DETAILED DESCRIPTION 
FIG. 1 shows a typical plastic credit card 10 which bears an elongated 
magnetic tape 11. The tape includes a series of bars each forming a 
separate cell. In the absence of recorded information, the successive 
cells are magnetized in alternating directions along the axis of the tape 
as indicated by the oppositely directed arrows 12 in the figure. Each 
interface between a pair of oppositely magnetized cells is indicated by a 
broken line 13 normal to the axis of the tape. Each broken line 
corresponds to a change (or transition) in the magnetization direction 
within the tape. 
FIG. 2 shows a section of tape 11 in an enlarged view. The arrows 12 
indicate three separate cells defining two adjacent transistions 13. These 
transitions occur at regular intervals along the tape and define a 
constant cell length C therebetween. Data information is stored in a cell 
by including in the center of the cell a reversal in the direction of 
magnetization. FIG. 3 shows such an additional transition in the center of 
a representative cell defined by a pair of adjacent broken lines 13. This 
transition divides a cell into two regions and represents a data bit (viz: 
a binary one). The irregular transition is represented in the figure by a 
dotted vertical line as viewed. Note that the magnetization in regions 15 
and 16 of FIG. 3 are reversed from the directions shown for these regions 
in FIG. 2 in order to store the data bit and to still maintain the 
end-of-cell transition. Apparatus to code credit cards of this type in 
this manner is in common use and is operative normally to so maintain the 
cell length constant. 
FIG. 4 shows code conversion apparatus (a credit card reader) in accordance 
with one embodiment of this invention for reading a credit card bearing a 
magnetic tape coded as described in FIGS. 1, 2, and 3. The card is placed, 
tape down, against a surface 20 and abutting a guide 21. Typically the 
card is inserted in a slot defined by surface 20 and a plate 22 secured to 
guide 21 to press the card against surface 20. FIG. 4 also shows 
magnetoresistive sensor apparatus 25 inserted into the surface 20 and 
level therewith. Apparatus 25 is responsive to the transitions of tape 11 
as the card is moved to the left as indicated by arrow 26 in the figure. 
Plate 22, in pressing card 10 against surface 20, ensures close proximity 
between the tape and the sensor apparatus to provide suitable output 
signals from that apparatus. 
The sensor apparatus includes first and second magnetoresistive elements 30 
and 31 astride the path along which tape 11 moves. 
The sensor apparatus is shown in greater detail in FIGS. 5 and 6. In FIG. 5 
the magnetic detectors 50 and 51 are shown arranged on a rigid substrate 
52 which may be glass, ceramic or other suitable non-magnetic material. 
The magnetic elements 50 and 51 are of an appropriate magnetic material 
such as permalloy. Overlying the sensing elements and the support 
substrate is the protective coating 53 in accordance with the invention. 
The credit card 54 carrying a coated magnetic tape 55 is shown positioned 
so as to communicate magnetically with the magnetic sensing elements. The 
magnetic sensing head assembly is shown in prospective in FIG. 6. Numerals 
designating similar elements are repeated in FIG. 6. FIG. 6 shows 
additionally gold leads 60 and 61 interconnecting the magnetic elements 50 
and 51 with a bonding area 62 near the edge of the assembly. This is 
simply an area free of polymer coating giving access to the external 
decoating circuitry. As indicated before the protective coating 53 is 
formed of one of the urethane modified acrylic polymers mentioned 
previously. 
Examples of preparation techniques of these materials are given the 
following: 
EXAMPLE 1 
210.5 grams of butyl isocyanate were added dropwise with stirring over a 3 
hour period to 294.3 grams of 2-hydroxyethyl methacrylate stabilized with 
1200 ppm of hydroquinone monomethyl ether. The temperature was raised to 
70.degree. C. Heating and stirring were continued for 2 hours after 
addition was completed. The reaction product was 1-butane carbamic acid 
2-methacryloyloxyethyl ester. 
EXAMPLE 2 
145.3 grams of butyl isocyanate and 180.7 grams of 2-hydroxyethyl acrylate, 
stabilized with 400 ppm of hydroquinone monomethyl ether, were reacted 
together for 6 hours at 75.degree. C. to give 1-butane carbamic acid 
2-acryloyloxyethyl ester. 
EXAMPLE 3 
98.5 grams of trimethylhexamethylene diisocyanate and 130 grams of 
2-hydroxyethyl methacrylate were reacted together with stirring for 6 
hours at 70.degree. C. to give a mixture of 2,4,4 - and 2,2,4 - 
trimethyl-1, 6-hexane dicarbamic acid di(2-methacryloyloxyethyl)ester. 
Other materials may be added to the four monomers indicated above to 
facilitate polymerization and/or to improve some property of the 
formulation or of the final polymer. These other materials may have as 
much as 25 percent of the total formulation. In this class of additives 
are: 
(a) thermal polymerization initiators, such as those described in Free 
Radicals by William A. Pryor, McGraw-Hill Book Company, N.Y., 1966 and 
references cited therein 
(b) photochemical polymerization initiators, such as those described in 
Light-Sensitive Systems by Jaromir Kosar, John Wiley and Sons, Inc., N.Y., 
1965 and references cited therein 
(c) viscosity builders and thixotropic agents, such as the polymers 
described and/or finely divided silica 
(d) wetting and antisettling agents, such as Nuosperse and Modaflow 
(e) pigments and dyes 
(f) stabilizers, such as hydroquinone t-butyl catechol, or hydroquinone 
monomethyl ether 
(g) fire retarding materials, such as antimony oxide or compounds of 
phosphorous, carbon and/or the halogens 
(h) plasticizers, such as diesters of phthalic acids and 
(i) deodorants, such as alcohols, to mask the odor or unreacted isocyanates 
(j) inert mineral fillers such as silica, alumina, and chalk. 
EXAMPLE 4 
A mixture of 30 part by weight of material prepared from Example 1 and 10 
part of material prepared from Example 2, with or without 5 parts of fumed 
silica, and 0.5 parts of benzoin methyl ether were coated, either spin or 
roller, onto the detector substrate. 
The coating is cured by a short exposure 30 sec to actinic radiation from a 
medium pressure mercury arc 40 m Wcm.sup.-.sup.2 at 365 nm in a nitrogen 
or air atmosphere. The coating may be further cured thermally, e.g. at 
150.degree. C. for half an hour. When a pattern is desired in the coating, 
a mask can be aligned over the coated sample and exposure made through the 
mask. The mask should be transparent in those areas in which the coating 
is desired and opaque in the areas which should be uncoated. The regions 
on the sensor head that correspond to the opaque areas of the mask can be 
cleaned of monomers after exposure by washing with an organic solvent such 
as benzene or toluene. Such a region appears in FIG. 6 over the bonding 
area at the edge of the structure. 
Various additional modifications and extensions of this invention will 
become apparent to those skilled in the art. All such variations and 
deviations which basically rely on the teachings through which this 
invention has advanced the art are properly considered to be within the 
spirit and scope of this invention.