Molding of urethane resin composition

A radiation crosslinked molding of a urethane resin composition comprising a thermoplastic urethane resing having incorporated therein a polyfunctional monomer selected from the group consisting of trimethylolpropane triacrylate, trimethylolpropane trimethacrylate and triacrylformal., is disclosed. The molding according to the present invention has excellent resistance to hot water and heat.

FIELD OF THE INVENTION 
The present invention relates to a radiation crosslinked molding of a 
urethane resin composition having improved resistance to hot water and 
heat. 
BACKGROUND OF THE INVENTION 
Thermoplastic urethane resins have superior mechanical strength and wear 
resistance on account of which these resins find many uses such as for 
hoses, belts, coatings for electric wires, pipes, shoe soles and various 
other moldings. However, urethane resins have easily hydrolyzable ester 
urethane bonds and are not suitable for use in areas where they are 
exposed to moisture for a prolonged period of time or in applications 
where steam or hot water is used. Efforts are being made to improve the 
resistance of water of urethane resins by using polyols having ether bonds 
[e.g., poly(1,4-oxybutylene)glycol], or caprolactam-based polyols (e.g., 
.epsilon.-lactone ester polymers instead of the aliphatic esters having 
easily hydrolyzable ester bonds. However, the inherent problem of 
hydrolysis still exists in urethane resins. A further problem with 
urethane resins is that they melt at temperatures of 180.degree. C. or 
higher and cannot be used in applications where they are exposed to high 
temperatures as 150.degree. C. or above, such as in the operation of 
dipping electric wires in a solder bath, without causing deformation of 
the resin coat. 
With the rapid increase in the use of NC controlled machine tools, the 
field in which materials having high mechanical strength (e.g., high wear 
resistance) are used is expanding and the development of urethane resins 
having high resistance to heat and hot water has been sought. 
From the view point of preventing fire and other disasters, the 
requirements for flame retardancy are becoming increasingly strict and 
there is a great need to offer a molding of a urethane resin composition, 
such as a coated electric wire, that is flame-retardant and exhibits 
superior resistance to water and heat. 
One conventional method for improving the resistance to heat of 
high-molecular weight materials is to crosslink individual polymer 
molecules as is frequently practiced with polyethylenes. Crosslinking is 
commonly achieved by chemical crosslinking with organic peroxides, by 
radiation crosslinking with electron beams or gamma rays, or by water 
crosslinking with a reactive silane. However, chemical crosslinking and 
water crosslinking are unsuitable for thermoplastic urethane resins 
because the temperature for molding is at least 180.degree. C., which is 
higher than the decomposition temperature of organic peroxides, and the 
addition of a reactive silane is uncontrollable. 
The common technique for effecting radiation crosslinking is to add 
reactive polyfunctional monomers, to thereby cause accelerated 
crosslinking. It is generally held that a higher crosslinking efficiency 
is attained by polyfunctional monomers that have many functional groups 
and have a low molecular weight of monomers per functional group. 
Polyfunctional groups that are commonly employed include diacrylates such 
as diethylene glycol diacrylate; dimethacrylates such as ethylene glycol 
dimethacrylate; triacrylates such as trimethylolethane triacrylate and 
trimethylolpropane triacrylate; trimethacrylates such as trimethylolethane 
trimethacrylate and trimethylolpropane trimethacrylate; as well as 
triallyl cyanurate, triallyl isocyanurate, diallyl phthalate, 
trimethylmethacryl isocyanurate, trimethylacryl cyanurate, trimethylacryl 
isocyanurate and triacrylformal. 
The present inventors added these polyfunctional monomers to thermoplastic 
urethane resins and studied their effectiveness in radiation crosslinking. 
To their great surprise, the urethane resin compositions having 
incorporated therein polyfunctional monomers other than trimethylolpropane 
trimethacrylate, trimethylolpropane triacrylate, and triacrylformal 
experienced total deformation in a thermal deformation test conducted at 
180.degree. C. 
It is well known by, for example, U.S. Pat. No. 3,624,045 that to the 
thermoplastic urethane resin, N,N'-methylene-bis-acrylamide or 
N,N'-hexamethylene-bis-maleimide is added as a polyfunctional monomer, 
followed by effecting the radiation crosslinking. Although these 
polyfunctional monomers are an effective crosslinking monomer, urethane 
resins crosslinked with such a polyfunctional monomer are greater in terms 
of reduction in strength in hot water at 100.degree. C. than those 
crosslinked with trimethylolpropane trimethacrylate, trimethylolpropane 
triacrylate, or triacrylformal and, therefore, the use of such 
polyfunctional monomers is not suited for attaining the object of the 
present invention. 
Several of the polyfunctional monomers tested had the following molecular 
weights per functional group: 112.6 for trimethylolpropane trimethacrylate 
(molecular weight: 338), 98.7 for trimethylolpropane triacrylate 
(molecular weight: 296), 83 for triacrylformal (molecular weight: 249), 
and 83 for each of triallyl cyanurate and triallyl isocyanurate (molecular 
weight: 249). In consideration of the generally held view about the 
reltionship between the number of moles of a functional group and the 
degree of crosslinking, triallyl cyanurate would be expected to achieve a 
higher degree of crosslinking than trimethylolpropane trimethacrylate 
added in the same amount. Accordingly, there was much reason to expect 
that urethane resin compositions having incorporated therein triallyl 
cyanurate and triallyl isocyanurate would experience less deformation at 
180.degree. C., than those tested after incorporation of 
trimethylolpropane trimethacrylate, trimethylolpropane triacrylate and 
triacrylformal. Curiously enough, however, the improvement in resistance 
to thermal deformation which was attainable by radiation crosslinking was 
observed only with the urethane resin compositions having incorporated 
therein trimethylolpropane trimethacrylate, trimethylolpropane 
triacrylate, or triacrylformal. 
A hot water test which was subsequently conducted at 100.degree. C. showed 
that the urethane resin compositions that had been radiation crosslinked 
suffered from a smaller decrease in tensile strength than non-crosslinked 
urethane resins. 
SUMMARY OF THE INVENTION 
The present invention has been accomplished on the basis of these findings, 
and a principlal object of the invention is to solve the aforementioned 
problems of the prior art by a radiation crosslinked molding of a urethane 
resin composition comprising a thermoplastic urethane resin having 
incorporated therein a polyfunctional monomer selected from the group 
consisting of trimethylolpropane triacrylate, trimethylolpropane 
trimethacrylate and triacrylformal. The urethane resin composition of the 
present invention having trimethylolpropane trimethacrylate, 
trimethylolpropane triacrylate or triacrylformal incorporated therein 
retains high strength and elongation even if it is aged in hot water at 
100.degree. C.

DETAILED DESCRIPTION OF THE INVENTION 
In a preferable embodiment, the composition of the present invention 
contains 0.1 to 50 parts by weight of the polyfunctional monomer based on 
100 parts by weight of the thermoplastic urethane resin. If less than 0.1 
part by weight of the polyfunctional monomer is used based on 100 parts by 
weight of the thermoplastic urethane resin, the addition of the 
polyfunctional monomer is insufficient to ensure complete radiation 
crosslinking. If the amount of the polyfunctional monomer exceeds 50 parts 
by weight, the resulting composition will experience a considerable drop 
in mechanical strength. 
The dose of radiations to which the urethane resin composition is exposed 
varies depending on the amount of the polyfunctional monomer added and is 
preferably at least 3 Mrad and not more than 50 Mrad. Exposure in a dose 
of 3 Mrad or more is particularly effective for attaining the desired 
degree of crosslinking in that the composition will undergo minimal 
deformation in a thermal deformation test conducted at 180.degree. C. No 
great drop in mechanical strength will occur if the dose of radiations is 
not more than 50 Mrad. The radiations which may be used as a crosslinking 
initiator consist of either electron beams or gamma rays. 
If flame retardancy is particularly required for the urethane resin 
composition, it preferably contains two additional components, i.e., 
decabromodiphenyl ether and antimony trioxide. Decabromodiphenyl ether is 
the most resistant to water of all the halogen compounds known today. 
Antimony trioxide, when combined with halogen compounds, serves to provide 
significantly enhanced flame retardancy. 
The following examples are provided for the purpose of further illustrating 
the present invention but should in no sense be taken as limiting. 
EXAMPLES 1 TO 4 
A thermoplastic urethane resin (Elastollan E 385 PNAT of Nippon Elastollan 
Industries Ltd.) was blended by employing 180.degree. C.-hot rolls with 
one of the polyfunctional monomers shown in Table 1 in the amount 
indicated in the same table. The blend was compression molded into 1 
mm-thick sheets of a test sample by applying a pressure for 10 minutes 
with a 180.degree. C.-hot press. Thereafter, the sheets were exposed to 
electrom beams (2 MeV) in doses of 2.5, 5 and 15 Mrad. The exposed sheets 
were set in the apparatus shown in the FIGURE and, while being given a 
load of 0.5 kg, they were subjected to preheating for 10 minutes and 
pressed for 10 minutes. The resulting deformation of each test sample was 
calculated by the following equation: 
##EQU1## 
The samples that were exposed to electron beams in a dose of 15 Mrad in 
Examples 1, 2 and 4 were aged for 3 or 7 days in hot water (100.degree. 
C.) and subsequently tested to check for changes in tensile strength. The 
test samples were blanked with dumbbells (No. 3, JIS) and set in an 
Instron tester for testing at a tensile speed of 500 m/min. The results of 
the thermal deformation test and the tensile test are shown in Table 1. 
COMATIVE EXAMPLES A TO C 
Additional test sheets were prepared as in Examples 1 to 4 in accordance 
with the formulations shown in Table 1. Apart from those prepared in 
Comparative Example C, the sheets were exposed to electron beams (2 MeV) 
in doses of 2.5, 5 and 15 Mrad. A thermal deformation test and a hot water 
aging test were subsequently conducted as in Examples 1 to 4, except that 
the only sheets that were given an exposure in a dose of 15 Mrad were 
subjected to the hot water aging test. 
EXAMPLES 5 TO 7 
A thermoplastic urethane resin (Elastollan E385 PNAT of Nippon Elastollan 
Industries Ltd.) was blended by employing 180.degree. C.-hot rolls with 
one of the polyfunctional monomers shown in Table 1, plus a flame 
retardant (decabromodiphenyl ether, DBDP) and antimony trioxide in the 
amounts also indicated in Table 1. Each of the blends was compression 
molded into 1 mm-thick sheets of a test sample by applying a pressure for 
10 minutes with a 180.degree. C.-hot press as in Examples 1 to 4. 
Thereafter, the sheets were exposed to electron beams (2 MeV) in doses of 
2.5 and 15 Mrad. 
The percentage of deformation and the change in tensile strength were 
measured for each sample by the same methods as employed in Examples 1 to 
4. The results are shown in Table 2. The flame retardancy of each test 
sample was evaluated by determining its oxygen index (JIS K 7201). The 
results are also shown in Table 2. 
COMATIVE EXAMPLES D TO F 
A thermoplastic flame-retardant resin (Elastollan E585 FUOO, the trade name 
of Nippon Elastollan Industries Ltd. for a caprolactam-based polyurethane) 
was blended by employing 180.degree. C.-hot rolls with one or more of the 
components shown in Table 2 in the amounts also indicated in the same 
table. The blend was compression molded into 1 mm-thick sheets of a test 
sample by applying a pressure for 10 minutes with a 180.degree. C.-hot 
press. Thereafter, the sheets were exposed to electron beams (2 MeV) in 
doses of 2.5 and 15 Mrad. A thermal deformation test, measurements of 
oxygen index and a hot water aging test were conducted as in Examples 5 to 
7. The results are shown in Table 2. The samples prepared in Comparative 
Examples D and E became too brittle in 7 days of aging in the hot water 
test to be subjected to a tensile test. The sheets prepared in Comparative 
Example F were not capable of being crosslinked. 
EXAMPLES 8 TO 10 
Each of the urethane resin compositions having the formulations indicated 
for Examples 5, 6 and 7 in Table 2 was extruded for an outer diameter of 7 
mm over a strand (2.5 mm.sup..phi.) of three polyethylene resin-coated 
conductors, and exposed to electron beams (2 MeV) in doses of 2.5 or 15 
Mrad. 
Each of the urethane resin coats was subjected to a horizontal burning test 
in accordance with the JASO specifications. The results are shown in Table 
3. 
COMATIVE EXAMPLES G TO I 
Urethane resin-coated electric wires were fabricated as in Examples 8 to 10 
using the urethane resin compositions having the formulations indicated 
for Comparative Examples D to F in Table 2. After being given an exposure 
to electron beams (2 MeV) in a dose of 2.5 or 15 Mrad, the urethane resin 
coats were subjected to a horizontal burning test as in Examples 8 to 10. 
The results are also shown in Table 3. 
TABLE 1 
______________________________________ 
Comparative 
Example Example 
1 2 3 4 A B C 
______________________________________ 
Urethane Resin 
100 100 100 100 100 100 100 
(parts) 
.sup.1) TAF (parts) 
5 
.sup.2) TMPTM (parts) 
5 10 
.sup.3) TMPTA (parts) 5 
.sup.4) TAIC (parts) 5 
.sup.5) TAC (parts) 5 
Heat Deformation 
(%) 
unirradiated 
100 100 100 100 100 100 100 
2.5 Mrad 100 100 100 100 100 100 -- 
5.0 Mrad 76.6 70.4 62.4 65.7 100 100 -- 
15.0 Mrad 69.3 50.0 34.3 53.7 100 100 -- 
Change in Strength 
(%) during immer- 
sion in hot water 
(100.degree. C.) 
initial 100 100 100 100 100 100 100 
3 days 96.1 81.2 -- 75.3 51.2 46.4 41.1 
7 days 96.7 76.4 -- 67.2 54.0 45.9 34.7 
______________________________________ 
.sup.1) TAF: triacrylformal 
.sup.2) TMPTM: trimethylolpropane trimethacrylate 
.sup.3) TMPTA: trimethylolpropane triacrylate 
.sup. 4) TAC: triallyl cyanurate 
TABLE 2 
______________________________________ 
Comparative 
Example Example 
5 6 7 D E F 
______________________________________ 
Urethane Resin (non- 
100 100 100 100 
flame-retardant) 
(parts) 
Urethane Resin 100 100 
(flame-retardant) 
(parts) 
.sup.1) TAF (parts) 
5 5 
.sup.2) TMPTM (parts) 
5 5 
.sup.3) TMPTA (parts) 5 
.sup.4) TAIC (parts) 5 
.sup.5) DBDP (parts) 
30 30 30 30 
Antimony Trioxide 
10 10 10 10 
(parts) 
Heat Deformation (%) 
unirradiated 100 100 100 100 100 100 
2.5 Mrad 100 100 100 100 100 100 
15.0 Mrad 55 63 70 48 61 100 
Oxygen Index 30.0 30.0 30.0 30.5 30.0 30.0 
(15 Mrad) 
Change in Strength 
(%) during Immersion 
in Hot Water 
(100.degree. C.) 
initial 100 100 100 100 100 100 
7 days 79.1 94.0 67.2 broken 
broken 
43.6 
14 days 62.7 73.9 50.5 broken 
broken 
33.0 
______________________________________ 
.sup.1) TAF: triacrylformal 
.sup.2) TMPTM: trimethylolpropane trimethacrylate 
.sup.3) TMPTA: trimethylolpropane triacrylate 
.sup.4 TAIC: triallyl isocyanurate 
.sup.5) DBDP: decabromodiphenyl ether 
TABLE 3 
______________________________________ 
Comparative 
Example Example 
8 9 10 G H I 
______________________________________ 
horizontal burning 
3 2 3 1 2 3 
time (second) 
______________________________________ 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.