Polymers stabilized against degradation by ultraviolet radiation

Polymers are stabilized against ultraviolet degradation by the use of urea or thiourea derivatives of the formula: ##STR1## Also disclosed is the use of the above compound with specific hydroxybenzoates or titanium dioxide.

This invention relates to novel compositions of matter which comprise a 
polymer normally subject to degradation by ultraviolet radiation, 
particularly a polyolefin such as polypropylene, and a stabilizing amount 
of 
(1) a compound represented by formula (I) 
##STR2## 
wherein X represents oxygen, or sulfur; R.sub.1 and R.sub.2 independently 
represent hydrogen, a cycloalkyl radical of 4 to 6 carbon atoms, an alkyl 
radical of 1 to 18 carbon atoms, an alkenyl radical of 3 to 12 carbon 
atoms, or a hydroxyalkyl radical of 2 to 12 carbon atoms; R.sub.3 is the 
same as R.sub.1 or R.sub.2 with the proviso that it may not be hydrogen; 
or R.sub.2 and R.sub.3 taken together with the N to which they are 
attached form 
##STR3## 
wherein R.sub.4 represents alkyl of 1 to 8 carbon atoms. R.sub.1 is 
preferably cyclohexyl, and R.sub.1, R.sub.2, and R.sub.3 are more 
preferably each cyclohexyl. 
Further stabilization may be obtained by using a mixture of a compound of 
formula (I) and a compound represented by formula (II) 
##STR4## 
wherein R.sub.5 and R.sub.6 are each alkyl of 1 to 8 carbon atoms, at 
least one of which is branched on the alpha carbon atom, and R.sub.7 
represents alkyl to 1 to 20 carbon atoms, C.sub.1 -C.sub.8 
alkyl-substituted phenyl, particularly 2,4-di-t-butylphenyl, 
halo-substituted phenyl, or aryl-substituted phenyl, and 
1,1,3,3-tetracyclohexylurea; and/or an effective amount of titanium 
dioxide. 
It is well-known that sunlight and other sources of ultraviolet radiation 
cause degradation of polymers, particularly polyolefins such as 
polypropylene, as evidenced by embrittlement or yellowing of plastic 
articles made therefrom. It is also well-known that this degradation can 
be inhibited by use of ultraviolet light stabilizers incorporated in or on 
such articles. Various additives, used alone or in combinations, have been 
suggested to inhibit such light degradation in order to prolong the useful 
lives of articles made from polyolefins. Since none has been found to be 
completely satisfactory, research continues in order to find compounds, or 
combinations of compounds, which will be more satisfactory. The present 
invention arose out of such research and resulted in the discovery of 
novel compounds which stabilize polyolefins against degradation by 
ultraviolet light. 
U.S. Pat. No. 2,766,219 relates to a sulfur-vulcanizable rubber vulcanizate 
containing a compound of the structure: 
##STR5## 
where R and R' represent alicyclic hydrocarbon groups and R" represents a 
hydrocarbon group, and X is sulfur or oxygen. The present invention is 
distinguishable in that it does not relate to a sulfur-vulcanizable rubber 
vulcanizate. 
U.S. Pat. No. 2,894,933 relates to a polymerized monovinyl aromatic 
hydrocarbon of the benzene series containing a stabilizing agent selected 
from the group consisting of biuret and urea derivatives having the 
general formula: 
##STR6## 
wherein X represents a member of the group consisting of hydrogen, alkyl 
radicals containing from 1 to 4 carbon atoms, alkenyl radicals containing 
from 2 to 4 carbon atoms, acetyl, and phenyl radicals, and Y is a member 
of the group consisting of phenyl and alkyl radicals containing from 1 to 
4 carbon atoms. The present invention is distinguishable in that Y is 
cyclohexyl and X may not be hydrogen, alkenyl, acetyl or phenyl. 
Belgium Pat. No. 851,575 relates to ureas as thermo-oxidative stabilizers 
for polyolefin molding compositions. However, the ureas which are used are 
structurally different from the ureas of the present invention. 
U.S. Pat. No. 2,960,488 relates to polyolefins containing substituted ureas 
as light stabilizers. However, the ureas employed are structurally 
dissimilar from those of the present invention. 
In accordance with the present invention, it has been discovered that the 
above-described stabilizers of formula (I) when used alone or with those 
of formula (II) and/or titanium dioxide provide effective stabilization of 
polymers against deterioration by ultraviolet radiation. They are 
particularly useful in stabilizing polypropylene. These stabilizers may be 
incorporated in or on such plastic materials by any of the various 
procedures known in the art for such purpose, such as by dry blending the 
additive with the polymer in powder or granular form followed by milling, 
Banbury mixing, molding, casting, extruding, swelling, and the like; by 
immersing the polymer as film, sheet, fibers, and the like, in a solution 
of the additive in an appropriate solvent (as in a dyeing process). 
Illustrative of polymers which may be used as the polymeric material in the 
subject invention are the following: 
polyethylene, 
polypropylene, 
poly(vinyl chloride), 
poly(methyl methacrylate), 
polystyrene, 
high impact polystyrene, 
polycarbonate, 
polycaprolactam, 
poly(hexamethylene adipamide), 
poly(hexamethylene terephthalamide), 
poly(methyl acrylate), 
poly(ethylene terephthalate), 
cellulose acetate, and the like, and blends thereof. 
The preferred material is polypropylene. 
The compounds of formula (I) can be prepared by methods well-known in the 
art. See Beaver et al., J. Am. Chem. Soc. 79, pgs, 1236-1245 (1957), 
incorporated herein by reference. 
The amount of the stabilizers needed to be an effective amount for 
stabilizing the polymer against degradation will depend on the nature of 
the polymer and the amount of exposure to ultraviolet radiation to which 
the composition will be subjected. 
For most purposes it is sufficient to use an amount of the compound of 
formula (I) within the range of about 0.1 to about 5 percent by weight, 
preferably 0.2 to 2 percent by weight, based on the weight of untreated 
polymer. 
The following compounds are illustrative of the stabilizers represented by 
formula (I): 
1,1,3,3-tetracyclohexylurea, 
1,1,3-tricyclohexylurea, 
1,1-dicyclohexyl-3,3-dimethylurea, 
1,3-dicyclohexyl-1-methylurea, 
1-cyclohexyl-3,3-diallylurea, 
1,1,3,3-tetracyclohexylthiourea, 
1,1,3-tricyclohexylthiourea, 
1-cyclohexyl-3,3-dimethylurea, 
1-cyclohexyl-3,3- dimethylthiourea, 
1,1-dicyclohexyl-3-methylurea, 
1,3-dicyclopentyl-1-(2-hydroxyethyl)urea, 
1-cyclohexyl-3-(1,1-dimethyl-2-hydroxyethyl)urea, 
1-cyclopentyl-3,3-dimethylurea, 
4-cyclohexylcarbamoylmorpholine, 
1-cyclohexylcarbamoylpiperidine, 
1-cyclohexylcarbamoyl-2,2,6,6-tetramethylpiperidine, 
1-cyclohexylcarbamoyl-4-methyl-piperazine, 
1-cyclohexylcarbamoyl-4-n-octyl-piperazine, and the like. 
For most purposes when the stabilizers of formula (I) are used along with 
those of formula (II), it is sufficient to use about 0.05 to about 2.5 
percent by weight, of the compound of formula (I) and about 0.05 to about 
2.5 percent by weight of the compound of formula (II), each based on the 
weight of untreated polymer. 
The following 3,5-dialkyl-4-hydroxybenzoic acid esters are exemplary of the 
compounds of formula (II): 
n-octadecyl 3,5-di-t-butyl-4-hydroxybenzoate, 
n-dodecyl 3,5-di-t-butyl-4-hydroxybenzoate, 
n-hexadecyl 3,5-di-t-butyl-4-hydroxybenzoate, 
n-hexyl 3,5-di-t-butyl-4-hydroxybenzoate, 
methyl 3,5-di-t-octyl-hydroxybenzoate, 
cyclohexyl 3-octyl-5-t-butyl-hydroxybenzoate, 
o-tert-butylphenyl 3,5-di-t-butyl-hydroxybenzoate, 
o-chlorophenyl 3,5-di-t-butyl-hydroxybenzoate, 
2,4-di-t-butylphenyl 3,5-di-t-butyl-hydroxybenzoate, 
2-bromo-4-t-butylphenyl 3,5-diisopropyl-4-hydroxybenzoate, 
biphenyl 3-octyl-5-t-butyl-4-hydroxybenzoate, and the like. 
The preparation of compounds of formula (II) can be found in the 
literature, such as U.S. Pat. No. 3,681,431, incorporated herein by 
reference. 
For most purposes when the stabilizers of formula (I) are used along with 
titanium dioxide, it is sufficient to use about 0.1 to 5 percent by 
weight, preferably about 0.2 to 2 percent by weight, of the compound of 
formula (I) and about 0.25 to 5 percent by weight, preferably about 1 to 3 
percent by weight, of titanium dioxide, each based on the weight of 
untreated polymer. 
The stabilizers can be used as described or in combination with other 
conventional additives, such as fillers, antioxidants, flame retardants, 
heat stabilizers, anti-slipping and anti-static agents, supplemental light 
stabilizers, pigments, dyes, lubricants, and the like. Any further 
additive is advantageously employed in conventional amounts.

The following examples, in which parts and percentages are by weight unless 
otherwise stated, are presented to further illustrate the present 
invention. 
EXAMPLES 1-10 
Evaluation of Light Stabilization Properties 
The resistance to degradation by ultraviolet light is determined by 
dry-blending and milling 0.5 part of the additive under test, 100 parts of 
unstabilized polypropylene (Profax.RTM. 6401, Hercules Incorporated, 
Wilmington, Del.) and 0.2 part of 2,4,6-tri-t-butylphenol, a processing 
antioxidant, on a standard two-roll mill at 350.degree.-370.degree. F. for 
4-5 minutes and compression molding the milled composition at 400.degree. 
F. into films 4-5 mils thick. The film is then exposed in a carbon arc 
Atlas Fade-Ometer.RTM., hereafter abbreviated FOM, and in a xenon arc 
Atlas Weather-Ometer.RTM., without using a water spray, hereafter 
abbreviated WOM, until the carbonyl content of the film increases by 0.10 
percent by weight, as determined by infrared spectrophotometric 
measurement. A control film identically prepared, without the additive 
under test, is similarly prepared and tested. The time in hours required 
to increase the carbonyl content of the film by 0.1 percent coincides with 
the time required to reach the point of embrittlement. 
The results obtained with various compositions of this invention are shown 
in Table I. 
TABLE I 
______________________________________ 
Time (Hr.) 
to .DELTA. Wt. % 
Ex- Carbonyl of 0.10% 
ample Additive FOM WOM 
______________________________________ 
##STR7## 1600 1800-2000 
2 
##STR8## 600 1300 
3 
##STR9## 500 1200 
4 
##STR10## 500 1150 
5 
##STR11## 500 1150 
6 
##STR12## 500 1150 
7 
##STR13## 600 1100 
8 
##STR14## 400 1100 
9 
##STR15## 400 950 
10 None 0-300 600-800 
______________________________________ 
EXAMPLES 11-16 
The procedure of Examples 1-10 is repeated using a total amount of 0.5 part 
of the additive, or combination of additives, under test together with 100 
parts of unstabilized polypropylene and 0.2 part of 
2,4,6-tri-t-butylphenol in the composition. The results obtained are shown 
in Table II. 
TABLE II 
______________________________________ 
Time (Hr.) 
to .DELTA. Wt. % 
Carbonyl of 0.10% 
Example 
Additive Concentration 
FOM WOM 
______________________________________ 
11 A.sup.a 0.5% 1800 1800-2000 
12 B.sup.b 0.5% 2100 3200 
13 C.sup.c 0.5% 1100 2000 
14 A + B 0.25% each 3100 3800 
15 A + C 0.25% each 2300 3400 
16 None -- 0-300 600-800 
______________________________________ 
.sup.a 1,1,3,3-tetracyclohexylurea 
.sup.b 2,4-di-t-butylphenyl 3,5di-t-butyl-4-hydroxybenzoate 
.sup.c n-hexadecyl 3,5di-t-butyl-4-hydroxybenzoate 
The results in Table II show that the stabilizing activities of the 
mixtures of A+B, and A+C, are higher than those of A, B, or C alone at the 
same total concentrations. 
EXAMPLES 17-20 
The procedure of Examples 1-10 is repeated using the additives and 
concentrations shown in Table III. 
TABLE III 
______________________________________ 
Time (Hr.) to .DELTA. Wt.% 
Carbonyl of 0.1% 
Example 
Additive Concentration 
FOM Xenon WOM 
______________________________________ 
17 A.sup.a 0.5% 1000 1400 
18 TiO.sub.2 2.0% 300 1000 
19 A + TiO.sub.2 
0.5% and 2.0% 
1800 2400 
20 None -- 0-300 600-800 
______________________________________ 
.sup.a 1,1,3,3-tetracyclohexylurea The results in Table III show that 
excellent stabilization to ultraviolet radiation is imparted to the 
polypropylene by the combination of additive A and titanium dioxide.