Magnetic recording media

Magnetic recording media comprise a non-magnetic base and one or more magnetic layers consisting of an organic binder matrix containing finely divided magnetic material, the said binder matrix being obtained by electron beam curing of a mixture of from 60 to 100% by weight of a polyurethane acrylate polymer possessing polymerizable acrylate double bonds and from 0 to 40% by weight of an acrylate monomer and/or an acrylate prepolymer and/or an N-vinyl monomer.

The present invention relates to magnetic recording media which comprise a 
non-magnetic base and one or more magnetic layers consisting of an organic 
binder matrix containing finely divided magnetic material, the said binder 
matrix being obtained by electron beam curing of a mixture of from 60 to 
100% by weight of a polyurethane acrylate polymer possessing polymerizable 
acrylate double bonds and from 0 to 40% by weight of an acrylate monomer 
and/or an acrylate prepolymer and/or an N-vinyl monomer. 
The magnetic recording media predominantly used at present, in the form of 
audio, video and computer tapes and other magnetic computer media 
possessing flexible or rigid bases, eg. floppy disks or hard disks, are 
generally produced as follows: magnetic dispersions are prepared from 
magnetic pigments and binders possessing good mechanical properties, the 
dispersions are applied to bases, the magnetic particles are, if required, 
oriented magnetically, drying is carried out physically by evaporating, in 
general, large amounts of solvents, and the surface is smoothened and, if 
required, then chemically cross-linked to produce hard-wearing layers. 
In the case of particularly high quality flexible recording media, these 
binders predominantly consist of tough high molecular weight 
thermoplastics, eg. polyurethanes, and thermal crosslinking agents, eg. 
polyfunctional isocyanates. In this procedure, drying and partial curing 
are carried out in a drying oven, directly after application of the layer 
to the base. Complete curing takes place slowly in the course of from a 
few days to weeks. It is true that such binder systems which are selected 
for the production of magnetic recording media capable of withstanding 
severe mechanical stresses give magnetic layers which have excellent 
mechanical properties, eg. low abrasion, good adhesion to the base, the 
ability to withstand the various changes in the direction of the tape path 
without breaking or sticking, a long service life even under different 
climatic conditions, etc.; however, they are also expensive to 
manufacture. 
For example, the high molecular weights make it necessary to use a large 
amount of solvent for processing. 
Another disadvantage of using high molecular weight thermoplastics is that 
this procedure leads indirectly to a deterioration in the magnetic 
properties of the magnetic materials. During the dispersing procedure, a 
certain proportion of the acicular magnetic particles are comminuted, and 
this leads to tape noise and a poorer signal to print-through ratio. The 
high molecular weight of the binder necessary to achieve very high 
mechanical quality imposes a certain limit with regard to more rapid 
dispersing under milder conditions. There is in principle also a limit in 
connection with the magnetic orienting of the pigments in the still liquid 
layer shortly after the coating procedure, the binder offering resistance 
to orientation. The better the orientation ratio, the better is the 
sensitivity and the maximum output level for a particular pigment type and 
a given pigment concentration. 
The use of isocyanates for thermal crosslinking of high molecular weight 
thermoplastics for the production of magnetic layers likewise entails 
disadvantages or is very expensive. As a 2-component system, it is 
possible to prepare only relatively small amounts if pot-life problems are 
to be avoided. In order to achieve uniform production of very high quality 
products, a large number of influences in the course of production have to 
be monitored and precisely controlled. The process is therefore sensitive 
to any disturbances which may occur, and this can result in production 
losses. 
It has therefore been proposed to use binders which can be cured by an 
electron beam and are based on special polyurethane acrylate polymers, 
with which it is possible to provide dispersions of magnetic particles for 
the production of magnetic recording media, the said dispersions requiring 
a substantially shorter dispersing time and only about half the amounts of 
solvents used in the present-day very high quality magnetic recording 
media, and, after curing by irradiation of the binders, possessing much 
better mechanical strength than the previously used electron-beam-curable 
surface coating binders and hence meeting the particular requirements for 
magnetic recording media, as had been met up to then only by chemical 
crosslinking of high molecular weight thermoplastic polymers. These 
polyurethane acrylates have a broad molecular weight distribution and, for 
the most part, have a higher molecular weight than the 
electron-beam-curable binders used up to then. However, the molecular 
weights are lower than those of the conventional binders used in magnetic 
dispersions. These polyurethane acrylates are straight-chain and contain 
polymerizable acrylate double bonds at each end of the chain of molecules. 
For curing with virtually complete crosslinking, they require an electron 
beam energy dose of about 50-70 kGray. Depending on the intensity of the 
electron beam unit used, this permits transport speeds of about 40-80 
m/min. Hence, for the production of magnetic recording media at production 
speeds of up to 200 m/min, it is necessary to use a plurality of beam 
apertures in succession, which entails correspondingly high capital costs. 
Attempts have therefore been made to improve the electron-beam-curable 
binders so that they require a very low curing dose and hence have a 
higher curing rate. It is known that higher curing rates can be obtained 
by using compounds which contain more than two acrylate double bonds in 
the molecule, eg. trimethylolpropane triacrylate and pentaerythritol tri- 
or tetraacrylate. However, these compounds generally have to be used in 
amounts of more than about 30% by weight so that the radiation dose 
required for complete curing is no more than about 25-40 kGray. Another 
possibility is the use of branched prepolymers containing three or more 
double bonds per molecule and having molecular weights of not more than 
1,000-2,000. The use of the conventional types and amounts of branched 
monomers and prepolymers for improving the reactivity or decreasing the 
curing dose in binders for magnetic recording media resulted in a 
deterioration in the extensibility of these binders, which as a rule 
should be greater than 100%. Contrary to expectations, even the tensile 
strength of such binders deteriorated when an attempt was made to so 
adjust the hardness that very high extensibility was still obtained. 
It is an object of the present invention to further develop magnetic 
recording media which comprise a non-magnetic base and one or more 
magnetic layers consisting of an organic binder matrix containing finely 
divided magnetic material, the said matrix being obtained by electron beam 
curing of a mixture of from 60 to 100% by weight of a polyurethane 
acrylate polymer possessing polymerizable acrylate double bonds and from 0 
to 40% by weight of one or more acrylate monomers and/or acrylate 
prepolymers and/or N-vinyl monomers, so that the advantages of a binder 
matrix produced by electron beam curing, resulting in recording media 
having very good mechanical and magnetic properties, and in the high 
elasticity necessary for magnetic layers, are retained but a higher curing 
rate can nevertheless be achieved. 
We have found that this object is achieved, and that magnetic recording 
media of the said type meet the set requirements if the polyurethane 
acrylate polymer is branched and consists of 
(1) one or more diisocyanates, 
(2) one or more oligomeric or polymeric diols, 
(3) one or more low molecular weight diols, 
(4) one or more monohydroxyalkyl acrylates and 
(5) one or more low molecular weight triols, and the amount of NCO groups 
of the diisocyanate is from 95 to 105% of the number of equivalents of OH 
groups of components 2, 3, 4 and 5 and the molar amounts of the 
OH-containing components are chosen so that the concentration of urethane 
groups is from 2.4 to 3.7 moles per kg of polyurethane acrylate polymer 
and the mean number of acrylate groups per average molecule is greater 
than two and less than four, with the proviso that the polymer has a broad 
molecular weight distribution, the number-average molecular weight is from 
2,000 to 10,000 and the ratio of the weight-average to the number-average 
molecular weight is from 2.2 to 3.5. 
The present invention furthermore relates to a process for the production 
of these magnetic recording media. 
Because of its method of preparation, the polyurethane acrylate polymer 
which alone or predominantly constitutes the binder matrix of the novel 
magnetic recording media which is formed by means of electron beam curing, 
contains not only molecules which have a very low molecular weight and, in 
the simplest case, are formed only from one diisocyanate molecule and two 
hydroxyalkyl acrylate molecules, but also molecules which have a very high 
molecular weight, ie. above 100,000. Furthermore, there is also a small 
amount of compounds which contain only one type of diol in the molecule 
and are not branched. Although for number-average molecular weights of, 
for example, 6,000 it is still possible to detect molecular weights of 
about 110,000-120,000 by means of high pressure gel chromatography, the 
weight-average molecular weight of the entire polyurethane acrylate 
mixture is substantially lower and is about 14,000-18,000 for a 
number-average molecular weight of about 5,000-6,000. 
The magnitude of the molecular weight can be used to influence, within 
certain limits, not only the mechanical properties and flow behavior, but 
also the dispersing behavior, solvent requirement and magnetic properties. 
The mechanical properties generally improve as the molecular weight 
increases. For high toughness requirements, as in the case of, for 
example, highly pigmented magnetic tapes, number-average molecular weights 
of about 5,000-8,000 are preferred. For rigid and flexible disk coatings 
which have to meet comparatively low elasticity requirements and, in some 
cases, more stringent requirements with regard to flow behavior or 
abrasion resistance, molecular weights of about 2,000-6,000 are preferred. 
To obtain the desired degree of branching, an alcohol having more than two 
hydroxyl groups in the molecule is used. In the simplest case, aliphatic 
triols, such as trimethylolpropane or glycerol, are employed, but cyclic 
triols are also very useful. Triols modified with ethylene oxide or 
propylene oxide can also be used. 
The double bond functionality of the polyurethane acrylate is controlled by 
means of the ratio of the number of equivalents of triol to that of 
monohydroxyalkyl acrylate. A ratio of 1:1 gives an average polyurethane 
acrylate molecule with a random distribution and a degree of branching of 
three, and hence with one triol unit and three acrylate double bonds in 
the polymer molecule. 
When branching is introduced, the reactivity is improved as the amount of 
triol increases. An exact stoichiometric ratio of triol to hydroxyalkyl 
acrylate is not important. Ratios of from 0:1 to 1:1 give mixtures of 
molecules having a degree of branching of two with those having a degree 
of branching of three. Ratios greater than 1:1 give mixtures of molecules 
having a functionality of three with those having a functionality of four, 
lower and higher degrees of branching also occurring in smaller random 
amounts. 
A mean functionality of from 2.5 to 3.5 is preferred. With lower values, 
the reactivity approaches that of the straight-chain polyurethane 
acrylates. At higher values, there is no further improvement, it is 
difficult to maintain good mechanical properties and, depending on the 
molecular weight of the other hydroxy components selected, the molecular 
weight may increase in an undesirable manner beyond the desired optimum 
calculated range. 
In order to obtain flexible products which have a high tensile strength 
coupled with good abrasion resistance, it is necessary to correlate the 
degree of branching and the molecular weight. The higher the selected 
molecular weight, the higher the degree of branching may be. The amount of 
hydroxyalkyl acrylate in relation to the other polyfunctional hydroxy 
components determines the resulting molecular weight. 
The molecular weight and the functionality fix the crosslinking 
concentration of the acrylate double bonds and hence the network density 
of the irradiated and polymerized magnetic recording layer and, to a 
decisive extent, also the greatest possible elasticity. 
Another possible method of controlling the network density is the addition 
of compounds having fairly high double bond concentrations, in the form of 
prepolymers and monomers. Although the stated polyurethane acrylates can 
be used as the sole binders, the presence of as much as 40% by weight of 
prepolymers and monomers may be advantageous. For magnetic tape 
applications, amounts of from 80 to 95% of polyurethane acrylate are 
preferred, whereas for applications where the flexibility does not have to 
meet such high requirements it is preferable to use from 60 to 85% of 
polyurethane acrylate, the remainder of the binder to 100% preferably 
consisting of other radiation-curable components. 
By adding prepolymers and monomers, the reactivity can be further 
increased. So that the desired effect is obtained and little or no 
deterioration in the mechanical properties results, it is necessary to 
formulate the binder mixture appropriately. 
Great toughness and high reactivity can be achieved simultaneously if the 
concentration of acrylate double bonds, which are present in 
polyfunctional compounds, is from 0.4 to 1.5, preferably from 0.6 to 1.2, 
moles of acrylate double bonds per kg of binder mixture for magnetic tape 
applications, and the distribution of the functionalities of the double 
bonds over large and small molecules in such that the number-average 
functionality is from 1.5 to 3 and the ratio of the weight-average to the 
number-average of the functionality is from 0.8 to 2.3, preferably from 
1.2 to 1.8. Hence, the number-average can be smaller the larger the ratio 
of weight-average to number-average. 
These ratios can be obtained by using, for example, trifunctional 
polyurethane acrylate, small amounts, less than 10%, preferably less than 
5%, of highly crosslinking acrylate monomers, such as trimethylolpropane 
acrylate, bifunctional acrylate prepolymers and/or acrylate monomers, and 
small amounts, not more than 10%, of monofunctional acrylate or N-vinyl 
monomers. The use of prepolymers and monomers which effect less pronounced 
crosslinking is particularly advantageous for increasing the curing rate 
and achieving complete curing with a very low residual monomer content 
coupled with a low curing dose. Hence, it is also possible to obtain 
rapidly curing flexible coatings using fairly small amounts of about 10%, 
but not more than 15%, of bifunctional monomers. 
Rapid curing coupled with a low radiation dose, which is an object of the 
present invention, depends both on the stated parameters, such as the type 
of reactive double bonds and their distribution over molecules of 
different molecular weights and functionalities, and on their mobility in 
the network formed during irradiation. This mobility is influenced by the 
structure and composition of the selected components, the reaction 
temperature, the amount of residual solvents and is characterized by, for 
example, the hardness, the glass transition temperature or the modulus of 
elasticity of the binder formulation. 
For the purposes of the present invention, curing is not the conversion of 
all reactive double bonds but the achieving of properties which do not 
alter significantly on further irradiation. A distinction should be made 
between this and a still smaller curing dose which can frequently be used 
and which, in spite of a lower degree of curing, is sufficient to give 
satisfactory performance characteristics for the particular application 
and is therefore the dose usually stated. However, the use of such a low 
curing dose can be unsatisfactory in that it results in a higher content 
of residual monomers and subsequent reactions, and therefore changes in 
the properties of the finished article may take place during aging. Hence, 
the required curing dose is always defined below as a dose with which the 
level of the mechanical properties achieved, eg. hardness, is not less 
than 80-90% of the final value. 
According to the invention, magnetic recording media which have high 
elasticity and excellent mechanical properties, and are based on 
electron-beam-curable binders can be produced using a curing dose which is 
only 50-70% of the dose required for conventional binders exhibiting good 
mechanical properties, less than 5-10% of prepolymers or monomers which 
are trifunctional or have a higher functionality being needed. In many 
cases, distinctly higher curing rates can be achieved even without such 
compounds. 
In order to obtain novel magnetic recording media which have good 
mechanical properties, it is necessary for the molecular weights of the 
main polymer in the form of the polyurethane acrylate to exhibit a broad 
distribution and for their mean molecular weight to be relatively high 
compared with conventional electron-beam-curable surface coating binders. 
Furthermore, a minimum amount of urethane groups is necessary to achieve 
great toughness; on the other hand, because good flow behavior and very 
good mechanical properties are required, this amount should not be too 
high. These properties can be controlled by means of the ratio of low 
molecular weight diols to polymeric diols. Since the individual components 
of the polyurethane acrylates can be chosen so that they have very 
different equivalent weights, there is a wide range of possibilities for 
varying this ratio of equivalent weights. The ratio should be chosen so 
that when the types of components in the polyurethane acrylate have been 
selected, and the molecular weight and the functionality have been 
predetermined, the resulting concentration of urethane groups is from 2.4 
to 3.7, preferably from 2.7 to 3.5, moles per kg. 
The hardness, the glass transition temperature or the modulus of elasticity 
of the irradiated and polymerized products increases with increasing 
urethane concentration so that, depending on the desired application, it 
is possible to obtain more elastic products for magnetic tapes or harder 
products for magnetic disks. 
Other measures for varying the hardness include the selection of hard or 
soft prepolymers and/or monomers during formulation of the binder mixture, 
and variation of the crosslinking concentration for this mixture within 
the conventional range. 
Diisocyanates (component 1) which are suitable for the preparation of the 
polyurethane acrylate polymers are aliphatic, cycloaliphatic or aromatic 
compounds, such as hexamethylene diisocyanate, 2,2,4- or 
2,4,4-trimethylhexamethylene diisocyanate, isophorone diisocyanate, 
bis(4-isocyanatocyclohexyl)methane, 
bis-(3-methyl-4-isocyanatocyclohexyl)-methane, 
2,2-bis-(4-isocyanatocyclohexyl)-propane, 4,4'-diphenylmethane 
diisocyanate, 2,4- and 2,6-toluylene diisocyanate and 1,5-naphthylene 
diisocyanate. 
The isocyanate components used consist exclusively or predominantly of 
these diisocyanates. For synthesizing branched polyurethane acrylate 
polymers, the triols intended for this purpose (component 5) can 
alternatively be replaced by a small amount of isocyanates having more 
than two isocyanate groups in the molecule, in order to obtain the 
required functionality, as described above. Examples of such isocyanates 
are triisocyanates which are derived directly from the corresponding 
amines, eg. 1,6,11-undecane triisocyanate as described in German Laid-Open 
Application DOS No. 2,924,149, bicyclo(2.2.1)heptane triisocyanates as 
described in German Patent No. 2,515,485, 
4-isocyanatomethyl-1,8-octamethylene diisocyanate as described in German 
Laid-Open Application DOS No. 3,109,276, or triisocyanates of aromatic 
triamines. 
Other polyisocyanates are those which are derived from the said 
diisocyanates by biuretization, for example the biuretization of 
hexamethylene diisocyanate with water, etc., as described in, inter alia, 
German Pat. Nos. 1,101,394, 1,174,759 and 1,931,055, and those which can 
be prepared by catalytic trimerization of some of the isocyanate groups to 
isocyanurate compounds, for example those prepared from hexamethylene 
diisocyanate according to German Pat. No. 1,201,992 and German Laid-Open 
Applications DOS No. 1,644,809, DOS No. 2,325,826, DOS No. 2,724,914 and 
others, or from isophorone diisocyanate 
(3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate) according to 
British Pat. No. 1,291,066 and German Laid-Open Application DOS No. 
2,325,826, or similar isocyanurate-containing polyisocyanates based on 
1,4-diisocyanatocyclohexane, 4,4'-diisocyanatodicyclohexylmethane or 2,4- 
or 2,6-toluylene diisocyanate. 
Branching of the polyurethane acrylates can furthermore be effected by 
using polyisocyanates prepared from diisocyanates and polyols, in 
particular those based on toluylene diisocyanate and trimethylolpropane 
and other alcohols, for example as described in German Pat. No. 1,090,196. 
The oligomeric or polymeric diols (component 2) have molecular weights of 
from 400 to 2,500, preferably from 700 to 2,000. Lower molecular weights 
result in poorer mechanical properties, while diols having higher 
molecular weights lead to poorer flow characteristics in some cases. 
Molecular weights of from 800 to 1,800 are generally preferred. The 
substances known from the chemistry of the elastomeric polyurethane high 
polymers can be employed, examples of these being polyesterols, such as 
adipates based on glycols, polycaprolactones, polyethers and 
polycarbonates. Polyethers, eg. polytetrahydrofuran, polypropylene glycol 
and others, are less suitable as the only polydiol components, since they 
give products which are too soft and more moisture-sensitive, and have an 
adverse effect on the coefficients of friction. They can therefore 
generally only be used in the form of mixtures with other polydiols in 
order to modify properties such as dispersion behavior or adhesion. 
Preferred polydiols are adipates with glycol (molecular weight 2,000), 
with diethylene glycol (molecular weight 2,000), with butane-1,4-diol 
(molecular weight from 900 to 1,100), with hexane-1,6-diol (molecular 
weight from 800 to 2,500) and with 2,2-dimethylpropane-1,3-diol (molecular 
weight from 900 to 1,100 ) and modified adipates of these diols, in which 
as much as 30 mol % of the aliphatic diol is replaced by a cyclic diol, as 
also described for component (3). Other preferred polydiols are 
polycaprolactones (molecular weight from 830 to 2,000) and polycarbonates 
(molecular weight 2,000). The stated molecular weights are not intended as 
limits but only as examples for commercial polydiols which have been 
tested within the scope of the invention. 
Some of the polydiols listed are also used as components in high molecular 
weight polyurethane binders for magnetic recording media, these binders 
being incapable of further curing. However, in contrast to their use in 
these high polymers, they are employed here not for building up a 
two-phase binder system with a soft phase and a hard phase but are used 
together with other components, in a single-phase binder system having 
only one glass transition temperature, only for achieving the desired 
molecular weight with a broad distribution and for controlling the 
hardness. 
Suitable low molecular weight diols (component 3) are those having 
molecular weights of up to about 500, preferably less than 320. Examples 
of suitable compounds are ethylene glycol, propane-1,2-diol, 
propane-1,3-diol, butane-1,4-diol, hexane-1,6-diol, pentane-1,5-diol, 
decanediol, methylpropane-1,3-diol, 2,2-dimethylpropane-1,3-diol, 
2,2-dimethylbutane-1,4-diol, 2-methyl-2-butylpropane-1,3-diol, 
neopentylglycol hydroxypivalate, diethylene glycol, triethylene glycol, 
tetraethylene glycol, dipropylene glycol, tripropylene glycol, etc. Cyclic 
diols are also suitable for this purpose, and are preferably used, eg. 
1,4-dihydroxymethylcyclohexane, 4,4'-dihydroxybiscyclohexylmethane, 
3(4),8(9)-dihydroxymethyltricyclo(5.1.1.0)decane, 
1,4,3,6-dianhydrosorbitol, anhydrosorbitol, oxyethylated or oxypropylated 
derivatives thereof and oxyethylated or oxypropylated bisphenols, eg. such 
derivatives of bisphenol A. 
Monohydroxyalkyl acrylates (component 4) employed are acrylates which have 
molecular weights of from 116 to about 350 and which are formally derived 
from diols or triols and acrylic acid, eg. hydroxyethyl acrylate, 
1,2-hydroxypropyl acrylate, 1,3-hydroxypropyl acrylate, butane-1,4-diol 
monoacrylate, hexanediol monoacrylate, n-butylglycerol ether monoacrylate, 
2-ethylhexylglycerol ether monoacrylate or versatic acid glycerol ester 
monoacrylate. These acrylates can be prepared by a conventional method, 
either from the corresponding diols by direct esterification or 
transesterification, or can be obtained in the form of 2-hydroxyalkyl 
acrylates by reaction of acrylic acid with epoxides, glycidyl ethers or 
glycidyl esters, eg. ethylene oxide, propylene oxide, butylene oxide, 
n-butyl glycidyl ether, 2-ethylhexyl glycidyl ether or glycidyl versatate 
(cf., inter alia, British Pat. No. 1,375,177). 
Examples of triols (component 5) are predominantly aliphatic alcohols 
having molecular weights of up to about 500, preferably less than 300. 
Both simple triols, such as trimethylolpropane, hexanetriol or glycerol, 
and oxyethylated or oxypropylated derivatives of these are suitable. 
Oxyethylation or oxypropylation of amines also gives suitable triols, eg. 
triethanolamine. However, owing to the catalytic effect of the amines on 
various isocyanate reactions, a modified method of preparation is 
necessary. Other suitable compounds are alicyclic and heterocyclic triols, 
eg. trishydroxyethyl isocyanurate (THEIC). 
The triol can be employed not only in pure form but also as a component in 
polyfunctional isocyanate prepolymers, as used for commercial isocyanate 
crosslinking agents in 2-component polyurethane coatings. 
Because of the better and more uniform definition of functionality in the 
alcohols, branching of the polyurethane acrylate with the aid of triols is 
simpler to control than when polyisocyanates are used, and is therefore 
preferred. For the purposes of the present invention, it may be 
advantageous to use small amounts of tetraols and pentols, eg. erythritol, 
pentaerythritol, arabitol, adonitol, xylitol, etc. 
The polyurethane acrylate polymers are prepared predominantly in solution. 
Suitable solvents are those which are free of groups which can react with 
isocyanates, eg. ketones, ethers and esters, and, if appropriate, also 
aromatic hydrocarbons or chlorohydrocarbons or mixtures of these. Acetone, 
methyl ethyl ketone, tetrahydrofuran, dioxane, ethyl acetate and methylene 
chloride are preferred. However, a minor amount of any other solvent 
employed today for the production of magnetic recording media using high 
molecular weight thermoplastic polymers can also be used concomitantly to 
control particular properties, eg. flow properties. Where ethers are used, 
care should be taken to ensure that these are free from peroxide so that 
premature polymerization of the acrylate double bonds does not take place. 
For molecular weights up to about 2,000, the polyurethane acrylate polymers 
can also be prepared in the absence of a solvent. This may be 
advantageous, for example, if further processing is not carried out 
immediately but it is intended to store the product temporarily for a 
relatively long time or to transport it over a relatively long distance. 
In the preferred temperature range, the reaction of the isocyanate groups 
with the hydroxyl groups can also be carried out in the absence of a 
catalyst. Since the reaction at high conversions is very slow when one 
reactant is not used in excess, the use of a catalyst is more 
advantageous. The catalysts which can be used here are those which are 
known from polyurethane chemistry, eg. tertiary amines, metal salts of 
fatty acids and other organic metal compounds. The catalysts chosen are 
preferably those which predominantly catalyze urethane formation and which 
promote reactions such as allophanate formation of isocyanurate formation 
as little as possible. Compounds of tetravalent tin, eg. dibutyl-tin 
dilaurate, are particularly useful. In general, it has proven advantageous 
to use from 0.005 to 0.3, preferably from 0.01 to 0.15, part by weight per 
100 parts by weight of polyurethane acrylate. The catalyst can be 
initially charged with the starting materials at the beginning of the 
reaction, can be added to the reaction mixture gradually together with the 
feed, or can be added in the subsequent stirring phase. 
In addition to the polyurethane acrylate polymer, the novel binders 
preferably contain further radiation-curable components in the form of 
resins and/or monomers which have a relatively low molecular weight and in 
general also a relatively narrow molecular weight distribution. Such 
polymerizable compounds are state of the art in the field of UV-curable or 
electron-beam-curable coating materials. Reference may be made to, inter 
alia, German Laid-Open Applications DOS No. 2,049,714, DOS No. 2,049,715, 
DOS No. 2,064,701, DOS No. 2,232,822, DOS No. 2,249,446, DOS No. 
2,358,948, DOS No. 2,441,148, DOS No. 2,452,322, DOS No. 2,636,425 and DOS 
No. 2,636,426, in which such radiation-curable components are described 
for other applications, such as surface coatings, printing inks, 
photoresists, etc. 
Among the many substances listed here, products which are particularly 
suitable for use in magnetic recording media are those whose molecules 
contain, as double bonds which can undergo free radical polymerization, 
acrylyl or N-vinyl double bonds. 
Instead of acrylyl compounds, it is also possible to use methacrylyl 
compounds. Because of their substantially lower polymerization rate, 
however, they should be used in no more than a small amount, in 
combination with acrylyl compounds. Compounds containing N-vinyl double 
bonds should be used in an amount such that there is not more than 50, 
preferably not more than 40, mol %, based on the total concentration of 
all polymerizable double bonds, of N-vinyl double bonds. 
The choice of compounds of this type which undergo copolymerization with 
the polyurethane acrylate polymers during irradiation with electrons 
principally comprises prepolymers having two or more polymerizable double 
bonds in the molecule and molecular weights of less than 1,500, preferably 
from 500 to 1,100. These are monofunctional, bifunctional or trifunctional 
acrylates having molecular weights greater than 180, preferably from 220 
to 400, in the form of derivatives of simple aliphatic, cycloaliphatic or 
aliphatic-aromatic alcohols and modifications of these, and monofunctional 
N-vinyl monomers in the form of N-vinyllactams, or N-vinylurethanes 
obtained from N-vinyl isocyanate and monofunctional alcohols. 
Specific examples of these are epoxy resin acrylates as reaction products 
of epoxy resins with acrylic acid, optionally modified with further 
saturated mono- or dicarboxylic acids, or fatty acids; preferably, they 
are reaction products with bisphenol A epoxy resins. Other prepolymers are 
polyester acrylates prepared from hydroxyl-containing polyesters of 
aliphatic and/or cycloaliphatic or aliphatic-aromatic diols, optionally 
combined with triols, and saturated dicarboxylic acids, esterified with 
acrylic acid. Suitable urethane acrylate prepolymers are the conventional 
reaction products of di- or polyisocyanates with hydroxyalkyl acrylates, 
which reaction products have a narrow molecular weight distribution and as 
a rule contain only one, two or three isocyanate molecules in the urethane 
acrylate molecule, and whose chains can furthermore be extended with 
specific diols or triols. 
Acrylate monomers which can be used are monofunctional monomers, eg. 
acrylates, oxyethylated phenols and their methyl-substituted or 
methoxy-substituted derivatives, such as phenoxyethyl acrylate, o- or 
p-cresyloxyethyl acrylate, 2,3-dimethylphenoxyethyl acrylate or 
3,5-dimethylphenoxyethyl acrylate, or acrylates of derivatives of benzyl 
alcohol, eg. 2-(2,5-dimethylphenyl)-ethyl acrylate, 
2-(o-methylphenyl)-ethyl acrylate or 2-(p-methylphenyl)-ethyl acrylate. 
Other monomers are isobornyl acrylate, dicyclopentadienyloxyethyl acrylate 
or 4-tert.-butylcyclohexyl acrylate. Because they are generally lower in 
odor because they have a lower vapor pressure, the acrylates having 
molecular weights of above 200, in particular above 220, are preferred. 
Examples of acrylates having more than one double bond in the molecule are 
the esterification products of diols or triols, as are also used as 
components (3) and (5) for the synthesis of the polyurethane acrylate 
polymers, with acrylic acid. Examples of these are hexanediol diacrylate, 
neopentylglycol diacrylate, triethylene glycol diacrylate, tetraethylene 
glycol diacrylate, tripropylene glycol diacrylate, trimethylolpropane 
triacrylate, hexanetriol triacrylate, cyclohexanediol diacrylate, the 
diacrylate of oxyethylated bisphenol A and the diacrylate of hydrogenated 
bisphenol A. 
Principal examples of N-vinyl monomers are N-vinylpyrrolidone, 
N-vinylimidazole, N-vinylcaprolactam, and the reaction products of N-vinyl 
isocyanate with ethyl diglycol, phenylglycol or alkyl-substituted 
phenylglycols or benzyl alcohols. Among the N-vinyllactams, 
N-vinylcaprolactam is particularly preferred because of its fairly low 
water solubility and the consequent insensitivity to moisture of magnetic 
recording layers produced with it. 
Examples of modified monofunctional acrylates are the reaction products of 
the monohydroxyalkyl acrylates, as are also used for the preparation of 
the polyurethane acrylate polymers, with monofunctional isocyanates, such 
as methyl, propyl, isopropyl, n-butyl or phenyl isocyanate. 
The preparation of the complete binder mixture for the binder matrix of the 
novel magnetic recording media is carried out by simply mixing the 
individual components. 
In addition to the radiation-curable components, it is also possible to add 
minor amounts of higher molecular weight prior art thermoplastic magnetic 
binders as non-reactive components in order to optimize specific 
properties, eg. the anchorage or coefficient of friction of the magnetic 
coating. However, this generally has an adverse effect on the particularly 
advantageous characteristics according to the present invention, such as 
very rapid and gentle dispersing, the saving of a large amount of solvent, 
and outstanding magnetic properties such as a high orientation ratio, high 
remanence and increased maximum output levels. 
Because of the strongly polar nature of these binders, wetting and flow 
problems frequently occur on certain substrates. Hence, flow improvers 
based on, for example, oligomeric organic fluorine compounds, siloxanes, 
etc., are usually added to the conventional radiation-curable coating 
materials. In this context, particularly preferred compounds are 
commercial organic fluorine assistants, in amounts of from 0.1 to 0.5% by 
weight, based on the binder mixture without solvents. 
The novel magnetic recording media are produced in a conventional manner. 
To do this, the magnetic material, eg. gamma-iron(III) oxide, finely 
divided magnetite, undoped or doped ferromagnetic chromium dioxide, 
cobalt-undoped gamma-iron(III) oxide, barium ferrite or finely divided 
ferromagnetic metals or metal alloys, eg. Fe-Co, Co-Ni, Fe-Co-Ni, Fe-Co-B, 
Fe-Co-Cr-B, Mn-Bi, Mn-Al, Fe-Co-V, etc., in a solution of the 
electron-beam-curable binder in an organic solvent is processed into a 
dispersion in a dispersing apparatus, using other conventional assistants. 
The magnetic dispersion is then applied to the non-magnetic base with the 
aid of a coating apparatus, eg. a knife coater, a roll coater, a reverse 
roll coater or a spray coater. The conventional bases, in particular 
polyester films from 6 to 75 .mu.m thick, or aluminum disks can be used as 
the non-magnetic base. It is also possible to use polyimide films for 
special applications. 
Before the still liquid coating mixture is dried on the base, an operation 
which is advantageously carried out at from 50.degree. to 100.degree. C. 
for from 15 to 120 seconds in a conventional dryer, the magnetic particles 
are, if required, oriented in the intended recording direction by the 
action of a magnetic field. This can, if required, be followed by very 
slight partial polymerization of the coating with electron beams, using a 
dose of less than 6, preferably less than 2, kGray, or with UV light. In 
the latter case, it is also necessary to use a conventional highly 
absorbing photoinitiator. The magnetic layers are then calendered on 
conventional apparatus by passing the coated base material between 
polished rollers optionally heated to a temperature of from 30.degree. to 
80.degree. C., preferably from 50.degree. C. to 70.degree. C. 
Advantageously, milder conditions are employed than for conventional 
magnetic tapes, such conditions being sufficient in this case. The 
thickness of the magnetic layer is then in general from 0.5 to 20 .mu.m, 
preferably from 5 to 10 .mu.m, for tapes and from 0.5 to 3 .mu.m for 
magnetic disks. 
Curing of the magnetic recording layer is completed by means of accelerated 
electrons, the method employed being the same as that usually used for 
curing other kinds of surface coating. Suitable electron accelerators are 
those having an acceleration voltage of from 150 to 300 kV. Both scanner 
and linear or planar cathode apparatuses having a radiation output of more 
than 500, preferably more than 1,000, kGray/sec are suitable. The 
irradiation time or web speed is controlled so that the magnetic coating 
is irradiated with an energy dose of from 15 to 90, preferably from 25 to 
60, kGray. The dose required for substantially complete polymerization of 
the double bonds is in most cases about 30-45 kGray in the case of binders 
which also possess good mechanical properties. A lower curing dose may be 
advantageous for magnetic tape applications, and higher curing doses for 
disk applications.

The Examples which follow illustrate the invention; the Comparative 
Experiments represent the state of the art. In the Examples and 
Comparative Experiments, parts and percentages are by weight, unless 
stated otherwise. The number of equivalents of the components in the 
polyurethane acrylate are based on 10 equivalents of hydroxyl in order to 
permit comparison. 
EXAMPLES 
Preparation and testing of polyurethane acrylate polymers and binders 
prepared from these 
The amounts, stated in the Examples below, of the components constituting 
the polyurethane acrylate are reacted by the following general method: 
All the hydroxyl-containing compounds are initially charged together with 
the solvent in a dry, stirred vessel possessing a feed means and a reflux 
condenser, and, in order to stabilize the acrylate double bonds against 
premature polymerization, 2,6-di-tert.-butyl-p-cresol and 
tris(n-cyclohexyldiazoniumdioxy)aluminum are added, each in an amount of 
1,000 ppm, based on the total weight of all hydroxy and isocyanate 
compounds; 1,000 ppm of dibutyl-tin-dilaurate are added as a catalyst. 
The mixture is heated to 50.degree. C. under an atmosphere of dry nitrogen. 
Feeding in of the isocyanate or, in the case of solid isocyanates, of a 
solution of the isocyanate is begun, and the reaction temperature is then 
brought to about 60.degree.-65.degree. C. while stirring and cooling. The 
addition takes about 30-60 minutes for the amounts of mixture described 
below. For large-scale production (tonnes), the feed time can be increased 
to 2-3 hours. When the addition is complete, stirring is continued for 
about 4-6 hours at this reaction temperature until the residual isocyanate 
content is less than 0.1% by weight, based on solid product, of NCO. 
Finally, 3,000 ppm of an oligomeric organic fluorine compound are added to 
the mixture, as a flow improver, and the resulting polymer solution is 
filtered through a 5 .mu.m filter. 
The parameters specified in the Examples and relating to molecular weight, 
crosslinking concentration and NH concentration have been calculated on 
the basis of analytically obtained parameters for the starting materials, 
such as number of double bonds from the hydrogenation, isocyanate content 
and hydroxyl number. The functionality data are theoretical data for the 
assumed ideal structures of the compounds; these data approach the true 
values fairly closely but can be exactly determined in practice only at 
very great expense, if at all. Ideal conditions have also been assumed for 
the reaction of the components; in practice, these conditions are only 
roughly achieved owing to the fact side reactions occur on a small scale, 
for example as a result of the presence of varying trace amounts of water. 
In particular, the crosslinking concentration and the molecular weight of 
the polyurethane acrylate polymer can therefore differ from the calculated 
values by from 10 to 20%. 
The magnitude and distribution of the actual molecular weight were 
determined by various methods in specific cases, and are roughly the same 
as those of the linear polyurethane acrylate systems cited above. The 
Fikentscher K value, which is easier to determine, is used below as a 
measure of the molecular weight. The possible molecular weight range is 
equivalent to K values (3% strength in tetrahydrofuran) of from 17 to 40, 
preferably from 28 to 36 for flexible recording media and from 19 to 25 
for rigid ones. 
To assess the curing rate and the mechanical properties, coatings of the 
binders were produced on sheets of glass so that the resulting dry film 
was from 50 to 70 .mu.m thick, the solvent being removed by drying in the 
air overnight and for one hour under reduced pressure, in each case at 
room temperature. This was followed by irradiation with electrons under 
nitrogen having a residual oxygen content of less than 200 ppm, using an 
acceleration voltage of 175 kV, on a CB 150/15/10 type linear cathode 
apparatus from Energy Sciences Inc., USA. 
To determine the curing rate, the web transportation speeds were chosen so 
that irradiation was carried out in each case with series of curing doses 
of 10, 20, 30, 50, 70, 100 and 150 kGray. 
Any necessary slight post-curing was effected by storing the irradiated 
films exposed to the air for 1 day, and small residual amounts of solvent 
were removed by heating for one hour at 100.degree. C. under reduced 
pressure. 
In the case of the binder films produced on sheets of glass, the Konig 
pendulum hardness was determined directly at 23.degree. C. according to 
DIN 53,157, and, by plotting the pendulum hardness as a function of the 
curing dose, the curing doses were determined at which 80 and 90% of the 
maximum pendulum hardness achieved with a curing dose of 150 kGray were 
attained. In general, the material is substantially completely polymerized 
when it exhibits about 80-90% of the maximum hardness. 
Accordingly, the curing doses given in the Examples below are ones which 
give films exhibiting 80 and 90% of the maximum pendulum hardness and 
will, for brevity's sake, be referred to hereinafter as 80 and 90% doses 
respectively. Other methods of measuring the hardness may lead to somewhat 
different values for the curing rate or curing dose. 
To determine the mechanical properties, such as tensile strength, 
elongation at break or modulus of elasticity, the irradiated films were 
carefully removed from the sheets of glass with the aid of a knife. Where 
a fluoro-organic flow improver was used, the films could in general be 
detached without problems. In isolated cases, films could be prepared only 
by applying a coating to aluminum foil and then dissolving the aluminum 
with dilute sodium hydroxide solution. 
The modulus of elasticity was determined according to DIN 53,457, at 
23.degree. C., in general as a secant modulus at 0.1% elongation. 
The tensile strength and the elongation at break were determined in 
accordance with DIN 53,504, likewise at 23.degree. C. 
EXAMPLE 1 
A polyurethane acrylate (PUA 1) was prepared which had the following 
characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average 
3 
crosslinking concentration (moles of 
0.5 
acrylate per kg) 
NH concentration (moles of NH per kg) 
3.1 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) Isocyanate: TDI 80 
10.00 404.6 
(2) polymeric diol: PBA 510 
2.86 677.3 
(3) monomeric diol: Dianol 
3.90 286.2 
(4) OH--alkyl acrylate: HPA 
1.62 98.1 
(5) triol: TMP 1.62 33.8 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 32.4. 
A binder BM 1 was prepared by mixing the following: Composition: 
87.2% by weight of PUA 1 
4.6% by weight of TMPTA 
8.2% by weight of VCp 
Characteristic data: 
______________________________________ 
crosslinking concentration (moles of acrylate per kg) 
0.85 
functionality (number average N) 
1.78 
functionality (weight average W) 
2.87 
functionality W/N 1.61 
______________________________________ 
Electron beam curing gave the following results: 
80% dose: 10 kGray 
90% dose: 21 kGray 
The Figure shows the Konig pendulum hardness (in sec) as a function of the 
radiation dose (in kGray) and serves to illustrate the determination of 
the curing doses as 80 and 90% doses. 
______________________________________ 
Mechanical properties at 
20 30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
450 440 440 
Tensile strength (N/mm.sup.2) 
40 41 470 
Elongation at break (%) 
110 96 106 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PBA 510=polybutane-1,4-diol adipate, OH equivalent weight 510 
dianol=bisoxyethylated bisphenol A 
TMP=trimethylolpropane 
HPA=2-hydroxypropyl acrylate, isomer ratio 75:25 
TMPTA=trimethylolpropane triacrylate 
VCp=N-vinylcaprolactam 
EXAMPLE 2a 
A polyurethane acrylate (PUA 2a) was prepared which had the following 
characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average) 
3 
crosslinking concentration (moles of acrylate per kg) 
0.5 
NH concentration (moles of NH per kg) 
3.5 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) isocyanate: TDI 80 
10.00 456.8 
(2) polymeric diol: PBA 510 
2.84 759.3 
(3) monomeric diol: CHDM 
4.16 157.2 
(4) OH--alkyl acrylate: HEA 
1.50 91.5 
(5) triol: TMP 1.50 35.3 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 28.8. 
A binder BM 2a was prepared by mixing the following: 
______________________________________ 
Composition Characteristic data 
______________________________________ 
88.8% by weight of PUA 2a 
crosslinking concentration 
0.85 
(moles of acrylate per kg) 
11.2% by weight of EPA 520 
functionality 2.42 
(number average N) 
functionality 2.89 
(weight average W) 
functionality W/N 1.19 
______________________________________ 
Electron beam curing gave the following results: 
______________________________________ 
80% dose 24 kGray 
90% dose 42 kGray 
Mechanical properties at 
30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
228 104 
Tensile strength (N/mm.sup.2) 
41 36 
Elongation at break (%) 
98 104 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PBA 510=polybutane-1,4-diol adipate, OH equivalent weight 510 
CHDM=cyclohexane-1,4-dimethanol 
TMP=trimethylolpropane 
HEA=2-hydroxyethyl acrylate 
EPA 520=bisphenol A epoxy resin acrylate, molecular weight 520 
EXAMPLE 2b 
As a comparison with PUA 2a, a polyurethane acrylate PUA 2b without 
branching was prepared which had the following characteristic data: 
______________________________________ 
number of average molecular weight 
6,000 
acrylate functionality (number average) 
2.0 
crosslinking concentration 0.33 
(moles of acrylate per kg) 
NH concentration (moles of NH per kg) 
3.50 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) isocyanate: TDI 80 
10.00 456.8 
(2) polymeric diol: PBA 510 
2.79 747.3 
(3) monomeric diol: CHDM 
6.20 234.2 
(4) OH--alkyl acrylate: HEA 
1.01 61.8 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 28.9. 
A binder BM 2b was prepared by mixing the following: 
Composition: 
84.3% by weight PUA 2b 
15.7% by weight EPA 520 
Characteristic data: 
______________________________________ 
crosslinking concentration (moles of acrylate per kg) 
0.85 
functionality (number average N) 
2.00 
functionality (weight average W) 
2.00 
functionality W/N 1.00 
______________________________________ 
Electron beam curing showed the following differences in curing behavior, 
the comparative binder BM 2b having the same composition as the binder BM 
2a, with the exception of branching: 
______________________________________ 
(Comparison) 
BM 2b BM 2a 
______________________________________ 
80% dose 50 kGray 24 kGray 
90% dose 75 kGray 42 kGray 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PBA 510=polybutane-1,4-diol adipate, OH equivalent weight 510 
CHDM=cyclohexane-1,4-dimethanol 
HEA=2-hydroxyethyl acrylate 
EPA 520=bisphenol A epoxy resin acrylate, molecular weight 520 
EXAMPLE 3a 
A polyurethane acrylate PUA 3a was prepared which had the following 
characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average) 
3 
crosslinking concentration 0.5 
(moles of acrylate per kg) 
NH concentration (moles of NH per kg) 
3.5 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) isocyanate: TDI 80 
10.00 456.8 
(2) polymeric diol: PBA 510 
2.79 747.6 
(3) monomeric diol: CHDM 
4.33 163.7 
(4) OH--alkyl acrylate: HPA 
1.44 98.1 
(5) triol: TMP 1.44 33.8 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 32.2. 
A binder BM 3a was prepared by mixing the following: 
Composition: 
88.8% by weight PUA 3a 
11.2% by weight EPA 520 
Characteristic data: 
______________________________________ 
crosslinking concentration (moles of 
0.85 
acrylate per kg) 
functionality (number average N) 
2.42 
functionality (weight average W) 
2.89 
functionality W/N 1.19 
______________________________________ 
Electron beam curing gave the following results: 
______________________________________ 
80% dose 27 kGray 
90% dose 44 kGray 
Mechanical properties at 
30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
33 34 
Tensile strength (N/mm.sup.2) 
37 43 
Elongation at break (%) 
168 148 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PBA 510=polybutane-1,4-diol adipate, OH equivalent weight 510 
CHDM=cyclohexane-1,4-dimethanol 
TMP=trimethylolpropane 
HPA=2-hydroxypropyl acrylate, isomer ratio 75:25 
EPA 520=bisphenol A epoxy resin acrylate, molecular weight 520 
EXAMPLE 3b 
As a comparison with PUA 3a, a polyurethane acrylate PUA 3b without 
branching was prepared which had the following characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average) 
2.0 
crosslinking concentration 0.33 
(moles of acrylate per kg) 
NH concentration (moles of NH per kg) 
3.50 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) isocyanate: TDI 80 
10.00 456.8 
(2) polymeric diol: PBA 510 
2.77 740.6 
(3) monomeric diol: CHDM 
6.28 237.2 
(4) OH--alkyl acrylate: HPA 
0.96 65.4 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 36.1. 
A binder BM 3b was prepared by mixing the following: 
Composition: 
84.3% by weight PUA 3b 
15.7% by weight EPA 520 
Characteristic data: 
______________________________________ 
crosslinking concentration (moles of 
0.85 
acrylate per kg) 
functionality (number average N) 
2.00 
functionality (weight average W) 
2.00 
functionality W/N 1.00 
______________________________________ 
Electron beam curing showed the following differences in curing behavior, 
the comparative binder BM 3b having the same composition as the binder BM 
3a, with the exception of branching: 
______________________________________ 
BM 3b BM 3a 
______________________________________ 
80% dose 40 kGray 27 kGray 
90% dose 70 kGray 45 kGray 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PBA 510=polybutane-1,4-diol adipate, OH equivalent weight 510 
CHDM=cyclohexane-1,4-dimethanol 
HPA=2-hydroxypropyl acrylate, isomer ratio 75:25 
EPA 520=bisphenol A epoxy resin acrylate, molecular weight 520 
EXAMPLE 4a 
A polyurethane acrylate PUA 4a was prepared which had the following 
characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average) 
3 
crosslinking concentration 0.5 
(moles of acrylate per kg) 
NH concentration (moles of NH per kg) 
3.5 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) isocyanate: TDI 80 
10.00 456.8 
(2) polymeric diol: PBA 510 
2.74 734.0 
(3) monomeric diol: CHDM 
4.28 161.8 
(4) OH--alkyl acrylate: BMA 
1.49 112.7 
(5) triol: TMP 1.49 35.0 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 27.9. 
A binder BM 4a was prepared by mixing the following: 
Composition: 
95.9% by weight PUA 4a 
4.1% by weight TMPTA 
Characteristic data: 
______________________________________ 
crosslinking concentration (moles of 
0.85 
acrylate per kg) 
functionality (number average N) 
3.00 
functionality (weight average W) 
3.00 
functionality W/N 1.00 
______________________________________ 
Electron beam curing gave the following results: 
______________________________________ 
80% dose 17 kGray 
90% dose 28 kGray 
Mechanical properties at 
30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
70 82 
Tensile strength (N/mm.sup.2) 
36 35 
Elongation at break (%) 
145 128 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PBA 510=polybutane-1,4-diol adipate, OH equivalent weight 510 
CHDM=cyclohexane-1,4-dimethanol 
TMP=trimethylolpropane 
BMA=butane-1,4-diol monoacrylate 
TMPTA=trimethylolpropane triacrylate 
VCp=N-vinylcaprolactam 
EXAMPLE 4b 
As a comparison with PUA 4a, a polyurethane acrylate PUA 4b without 
branching was prepared which had the following characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average) 
2.0 
crosslinking concentration 0.33 
(moles of acrylate per kg) 
NH concentration (moles of NH per kg) 
3.50 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) isocyanate: TDI 80 
10.00 456.8 
(2) polymeric diol: PBA 510 
2.73 731.0 
(3) monomeric diol: CHDM 
6.28 237.3 
(4) OH--alkyl acrylate: BMA 
0.99 75.2 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 28.9. 
A binder BM 4b was prepared by mixing the following: 
______________________________________ 
crosslinking concentration (moles of 
0.85 
acrylate per kg) 
functionality (number average N) 
2.43 
functionality (weight average W) 
2.06 
functionality W/N 0.85 
______________________________________ 
Electron beam curing showed the following differences in curing behavior, 
the comparative binder BM 4b having the same composition as the binder BM 
4a, with the exception of branching: 
______________________________________ 
BM 4b BM 4a 
______________________________________ 
80% dose 35 kGray 17 kGray 
90% dose 70 kGray 28 kGray 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PBA 510=polybutane-1,4-diol adipate, OH equivalent weight 510 
CHDM=cyclohexane-1,4-dimethanol 
TMP=trimethylolpropane 
BMA=butane-1,4-diol monoacrylate 
TMPTA=trimethylolpropane triacrylate 
EXAMPLE 4c 
A binder BM 4c was prepared by mixing the following: 
Composition: 
87.2% by weight PUA 4a 
4.6% by weight TMPTA 
8.2% by weight VCp 
Characteristic data: 
______________________________________ 
crosslinking concentration (moles of 
0.85 
acrylate per kg) 
functionality (number average N) 
1.67 
functionality (weight average W) 
2.84 
functionality W/N 1.70 
______________________________________ 
Electron beam curing gave the following results: 
______________________________________ 
80% dose 13 kGray 
90% dose 26 kGray 
Mechanical properties at 
30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
352 426 
Tensile strength (N/mm.sup.2) 
40 42 
Elongation at break (%) 
116 96 
______________________________________ 
Abbrevations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PBA 510=polybutane-1,4-diol adipate, OH equivalent weight 510 
CHDM=cyclohexane-1,4-dimethanol 
BMA=butane-1,4-diol monoacrylate 
TMPTA=trimethylolpropane triacrylate 
EXAMPLE 5 
A polyurethane acrylate (PUA 5) was prepared which had the following 
characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average) 3 
crosslinking concentration (moles of 
0.5 
acrylate per kg) 
NH concentration (moles of NH per kg) 
3.5 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) Isocyanate: TDI 80 
10.00 456.8 
(2) polymeric diol: PBA 510 
2.92 781.1 
(3) monomeric diol: Hexanediol 
4.21 130.4 
(4) OH--alkyl acrylate: HPA 
1.44 98.1 
(5) triol: TMP 1.44 33.8 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 32.0. 
A binder BM 5 was prepared by mixing the following: 
Composition: 
81.3% by weight PUA 5 
10.5% by weight DIANOLDA 
8.2% by weight VCp 
Characteristic data: 
______________________________________ 
crosslinking concentration 
0.85 
(moles of acrylate per kg) 
functionality (number average N) 
1.53 
functionality (weight average W) 
2.73 
functionality W/N 1.78 
______________________________________ 
Electron beam curing gave the following results: 
______________________________________ 
80% dose 5 kGray 
90% dose 15 kGray 
Mechanical properties at 
30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
128 155 
Tensile strength (N/mm.sup.2) 
38 48 
Elongation at break (%) 
106 100 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PBA 510=polybutane-1,4-diol adipate, OH equivalent weight 510 
TMP=trimethylolpropane 
HPA=2-hydroxypropyl acrylate, isomer ratio 75:25 
DIANOLDA=bisoxyethylated bisphenol A diacrylate 
VCp=N-vinylcaprolactam 
EXAMPLE 6 
A polyurethane acrylate (PUA 6) was prepared which had the following 
characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average) 
3 
crosslinking concentration (moles of 
0.5 
acrylate per kg) 
NH concentration (moles of NH per kg) 
3.1 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) Isocyanate: MDI/TDI 1:1 
10.00 492.9 
(2) polymeric diol: PBA 510 
3.29 780.2 
(3) monomeric diol: Hexane-1,6-diol 
3.46 95.0 
(4) OH--alkyl acrylate: HPA 
1.62 98.1 
(5) triol: TMP 1.62 33.8 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 33.7. 
A binder BM 6 was prepared by mixing the following: 
Composition: 
87.0% by weight PUA 6 
6.3% by weight DIANOLDA 
6.7% by weight VCp 
Characteristic data: 
______________________________________ 
crosslinking concentration 
0.85 
(moles of acrylate/kg) 
functionality (number average N) 
1.57 
functionality (weight average W) 
2.80 
functionality W/N 1.78 
______________________________________ 
Electron beam curing gave the following results: 
______________________________________ 
80% dose 40 kGray 
90% dose 66 kGray 
Mechanical properties at 
30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
52 91 
Tensile strength (N/mm.sup.2) 
37 42 
Elongation at break (%) 
116 128 
______________________________________ 
Abbreviations: 
MDI/TDI=mixture of 1 equivalent of diphenylmethane diisocyanate with 1 
equivalent of toluylene diisocyanate 
PBA 510=polybutane-1,4-diol adipate, OH equivalent weight 510 
TMP=trimethylolpropane 
HPA=2-hydroxypropyl acrylate, isomer ratio 75:25 
DIANOLDA=bisoxyethylated bisphenol A diacrylate 
VCp=N-vinylcaprolactam 
EXAMPLE 7 
A polyurethane acrylate (PUA 7) was prepared which had the following 
characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average) 
3 
crosslinking concentration (moles of 
0.5 
acrylate per kg) 
NH concentration (moles of NH per kg) 
3.33 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) Isocyanate: TDI 80 
10.30 434.0 
(2) polymeric diol: PCL 415 
3.20 645.0 
(3) monomeric diol: Dianol 
3.70 283.2 
(4) OH--alkyl acrylate: BMA 
1.55 104.3 
(5) triol: TMP 1.55 33.6 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 32.6. 
The polyurethane acrylate PUA 7 Pwas used directly as binder BM 7, without 
further electron-beam-curable additives. 
Electron beam curing gave the following results: 
______________________________________ 
80% dose 46 kGray 
90% dose 81 kGray 
Mechanical properties at 
30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
228 384 
Tensile strength (N/mm.sup.2) 
37 35 
Elongation at break (%) 
174 146 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanates 
PCL 415=polycaprolactonediol, OH equivalent weight 415 
Dianol=bisoxyethylated bisphenol A 
TMP=trimethylolpropane 
BMA=butane-1,4-diol monoacrylate 
EXAMPLE 8 
A polyurethane acrylate (PUA 8) was prepared which had the following 
characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average) 
3 
crosslinking concentration (moles of 
0.5 
acrylate per kg) 
NH concentration (moles of NH per kg) 
2.96 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) Isocyanate: TDI 80 
10.00 386.1 
(2) polymeric diol: PHA 905 
1.65 645.0 
(3) monomeric diol: Dianol 
4.86 331.2 
(4) OH--alkyl acrylate: BMA 
1.74 104.3 
(5) triol: TMP 1.74 33.6 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 31.6. 
The polyurethane acrylate PUA 8 was used directly as binder BM 9, without 
further electron-beam-curable additives. 
Electron beam curing gave the following results: 
______________________________________ 
80% dose 41 kGray 
90% dose 71 kGray 
Mechanical properties at 
30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
67 131 
Tensile strength (N/mm.sup.2) 
41 34 
Elongation at break (%) 
168 146 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PHA 905=polyhexane-1,6-diol adipate, OH equivalent weight 905 
dianol=bisoxyethylated bisphenol A 
TMP=trimethylolpropane 
BMA=butane-1,4-diol monoacrylate 
EXAMPLE 9 
A polyurethane acrylate (PUA 9) was prepared which had the following 
characteristic data: 
______________________________________ 
number average molecular weight 
6000 
acrylate functionality (number average) 
3 
crosslinking concentration (moles of 
0.5 
acrylate per kg) 
NH concentration (moles of NH per kg) 
2.66 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) Isocyanate: TDI 80 
10.30 346.8 
(2) polymeric diol: PHA 905 
2.22 776.6 
(3) monomeric diol: Dianol 
3.91 239.0 
(4) OH--alkyl acrylate: BMA 
1.94 104.3 
(5) triol: TMP 1.94 33.6 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 32.0. 
A binder BM 9 was prepared by mixing the following: 
Composition: 
85.0% by weight PUA 9 
4.8% by weight EPA 520 
4.8% by weight TMPTA 
8.2% by weight VCp 
Characteristic data: 
______________________________________ 
crosslinking concentration 
0.80 
(moles of acrylate/kg) 
functionality (number average V) 
1.62 
functionality (weight average W) 
2.80 
functionality W/N 1.73 
______________________________________ 
Electron beam curing gave the following results: 
______________________________________ 
80% dose 35 kGray 
90% dose 53 kGray 
Mechanical properties at 
30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
116 168 
Tensile strength (N/mm.sup.2) 
29 32 
Elongation at break (%) 
100 84 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PHA 905=polyhexane-1,4-diol adipate, OH equivalent weight 905 
Dianol=bisoxyethylated bisphenol A 
TMP=trimethylolpropane 
BMA=butane-1,4-diol monoacrylate 
EPA 520=bisphenol A epoxy resin acrylate, molecular weight 520 
TMPTA=trimethylolpropane triacrylate 
VCp=N-vinylcaprolactam 
EXAMPLE 10 
A polyurethane acrylate (PUA 10) was prepared which had the following 
characteristic data: 
______________________________________ 
number average molecular weight 
6,000 
acrylate functionality (number average) 
3 
crosslinking concentration (moles of 
0.5 
acrylate per kg) 
NH concentration (moles of NH per kg) 
2.82 
______________________________________ 
Ratio of Amount 
number of in mixture 
Components in PUA equivalents 
(g) 
______________________________________ 
(1) Isocyanate: TDI 80 
10.30 368.6 
(2) polymeric diol: PHC 1000 
1.68 650.0 
(3) monomeric diol: Dianol 
4.67 303.8 
(4) OH--alkyl acrylate: BMA 
1.82 104.3 
(5) triol: TMP 1.82 33.6 
______________________________________ 
The product was prepared as a 60% strength solution in tetrahydrofuran, the 
Fikentscher K value (3% strength in tetrahydrofuran) being 31.5. 
The polyurethane acrylate PUA 10 was used directly as binder BM 10, without 
further electron-beam-curable additives. 
Electron beam curing gave the following results: 
80% dose: 44 kGray 
90% dose: 64 kGray 
______________________________________ 
80% dose 44 kGray 
90% dose 64 kGray 
Mechanical properties at 
30 70 kGray 
Modulus of elasticity (N/mm.sup.2) 
481 579 
Tensile strength (N/mm.sup.2) 
38 42 
Elongation at break (%) 
142 146 
______________________________________ 
Abbreviations: 
TDI 80=80:20 mixture of 2,4- and 2,6-toluylene diisocyanate 
PHC 1000=polyhexane-1,6-diol carbonate, OH equivalent weight, 1,000 
dianol=bisoxyethylated bisphenol A 
TMP=trimethylolpropane 
BMA=butane-1,4-diol monoacrylate 
EXAMPLE 11 
436 parts of a 55% strength binder solution in tetrahydrofuran, as 
described in Example 1, 928 parts of tetrahydrofuran, 1,200 parts of 
chromium dioxide having a coercive force of 49.0 kA/m, 1.2 parts of 
silicone oil and 30 parts of zinc oleate were dispersed for 46 hours in a 
ball mill which had a capacity of 6,000 parts by volume and was charged 
with 8,000 parts by weight of steel balls having a diameter of from 4 to 6 
mm. The resulting dispersion was passed under pressure through a filter 
having 5 .mu.m pores, and then applied to a 12 .mu.m thick polyethylene 
terephthalate film by means of a conventional knife coater. 
The coated film was passed through a conventional magnetic field to orient 
the magnetic particles and was then dried at from 60.degree. to 80.degree. 
C., after which the magnetic layer was calculated by passing the coated 
film twice between heated rollers at 70.degree. C. under a nip pressure of 
35 kg/cm. The magnetic layer was then 5.1 .mu.m thick. The coating was 
then cured by means of an electron beam, the dose used being 20 kGray. The 
coated film was then slit into 3.81 mm wide audio tapes, which were 
subjected to magnetic, electroacoustic and mechanical tests. The roughness 
of the surface was determined as the average peak-to-valley height 
R.sub.z, in accordance with DIN No. 4768, sheet 1. The electroacoustic 
properties of the resulting tapes were determined according to DIN Nos. 
45,401, 45,403 and 45,512 (sheet 12), in comparison with reference tape C 
401 R. 
______________________________________ 
Magnetic properties: R.sub.z 0.21 .mu.m 
H.sub.c (kA/m) 
48.6 Microhardness: 
10.2 kp 
M.sub.r (mT) 178 
______________________________________ 
Orientation ratio: 3.9 
Electroacoustic properties: 
E.sub.T (sensitivity at long wavelengths)+0.6 dB 
E.sub.H (sensitivity at short wavelengths)+3.2 dB 
A.sub.T (maximum output level at long wavelength)+1.5 dB 
A.sub.H (maximum output level at short wavelengths)+5.5 dB 
RG.sub.a (reference level-to-noise ratio+0.2 dB 
K.sub.o (signal-to-print-through ratio)+4.0 dB 
The mechanical properties of the tape in respect of abrasion, coefficient 
of friction, anchorage, etc., ranged from very good to excellent. 
EXAMPLE 12 
(a) The experiment described in Example 11 was repeated, except that the 
electron radiation dose was increased to 70 kGray. No significant changes 
in the magnetic and electroacoustic properties were observed. 
Microhardness: 10.7 kp 
(b) The experiment was repeated using a radiation dose of 100 kGray. 
Microhardness: 10.7 kp 
EXAMPLE 13 
374 parts of a 53.5% strength binder solution in tetrahydrofuran, as 
described in Example 5, 1,039 parts of tetrahydrofuran, 1,200 parts of 
chromium dioxide having a coercive force of 39.0 kA/m, 1.2 parts of 
silicone oil and 30 parts of zinc oleate were dispersed for 50 hours in a 
ball mill which had a capacity of 6,000 parts by volume and was charged 
with 8,000 parts by weight of steel balls having a diameter of from 4 to 6 
mm. The resulting dispersion was passed under pressure through a filter 
having 5 .mu.m pores, and then applied to a 12 .mu.m thick polyethylene 
terephthalate film by means of a conventional knife coater. 
The coated film was passed through a conventional magnetic field to orient 
the magnetic particles and was then dried at from 60.degree. to 80.degree. 
C., after which the magnetic layer was calendered by passing the coated 
film twice between heated rollers at 70.degree. C. under a nip pressure of 
35 kg/cm. The magnetic layer was then 5.1 .mu.m thick. The coating was 
then cured by means of an electron beam, the dose used being 20 kGray. The 
coated film was then slit into 3.81 mm wide audio tapes, which were 
subjected to magnetic, electroacoustic and mechanical tests, as described 
in Example 11. 
______________________________________ 
Magnetic properties: R.sub.z 0.32 .mu.m 
H.sub.c (kA/m) 
38.5 Microhardness: 
8.8 kp 
M.sub.r (mT) 200 
______________________________________ 
Orientation ratio: 3.5 
Electroacoustic properties: 
E.sub.T (dB)+2.0 
E.sub.H (dB)+1.0 
A.sub.T (dB)+4.0 
A.sub.H (dB)+0.5 
RG.sub.A (dB)-0.2 
K.sub.o (dB)+1.2 
Good abrasion properties and coefficients of friction were obtained. 
EXAMPLE 14 
The experiment described in Example 13 was repeated, except that the 
electron radiation dose was increased to 70 kGray. No significant changes 
in the magnetic and electroacoustic properties were observed. 
Microhardness: 8.9 kp 
EXAMPLE 15 
3,207 parts of a 48.5% strength binder solution in tetrahydrofuran, as 
described in Example 9, 2,549 parts of tetrahydrofuran and 4,200 parts of 
dioxane, 7,000 parts of chromium dioxide having a coercive force of 8 
kA/m, 7.0 parts of silicone oil and 175 parts of zinc oleate were 
dispersed for 52 hours in a ball mill which had a capacity of 30,000 parts 
by volume and was charged with 40,000 parts by weight of steel balls 
having a diameter of from 4 to 6 mm. The resulting dispersion was passed 
under pressure through a filter having 5 .mu.m pores, and then applied to 
a 10 .mu.m thick polyethylene terephthalate film by means of a 
conventional knife coater. 
The coated film was passed through a conventional magnetic field to orient 
the magnetic particles and was then dried at from 60.degree. to 80.degree. 
C., after which the magnetic layer was calendered by passing the coated 
film twice between heated rollers at 70.degree. C. under a nip pressure of 
35 kg/cm. The magnetic layer was then 3.0 .mu.m thick. The coatings were 
then cured by means of an electron beam, the dose used being 30 kGray. The 
coated film was then slit into 0.5 inch wide video tapes, which were 
subjected to magnetic, video and mechanical tests on a VHS recorder. 
______________________________________ 
Magnetic properties: R.sub.z 
0.15 .mu.m 
H.sub.c (kA/m) 48.1 
M.sub.r (remanence in mT) 
165 
Orientation ratio 
3.7 
______________________________________ 
Video properties in comparison with a CrO.sub.2 reference tape: 
______________________________________ 
Output at 4.5 MHz 
+2.4 dB 
chroma output +2.4 dB 
S/N +1.8 dB 
chroma S/N +0.6 dB 
______________________________________ 
Still life: longer than 60 minutes 
Durability: tape still OK after 200 hours. 
EXAMPLE 16 
The experiment described in Example 15 was repeated and the coating was 
irradiated with 70 kGray. No significant differences in the video and 
mechanical properties were found.