Attenuation of polymer substrate degradation due to ultraviolet radiation

Improved methods and polymer compositions are provided whereby ultraviolet radiation degradation of substrates formed of the compositions is attenuated. In accordance with the invention, ultraviolet scattering and absorbing particles having a particle size in the range of from about 0.001 micrometer to about 0.20 micrometer in diameter are dispersed in at least a surface layer of a polymer substrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to improved methods and polymer compositions 
wherein degradation due to ultraviolet radiation is attenuated. 
2. Description of the Prior Art 
Metal oxide particles, e.g., titanium dioxide and zinc oxide particles, 
have heretofore been dispersed in polymer substrates for the purpose of 
minimizing ultraviolet radiation degradation of the substrates. The metal 
oxide particles either scatter or absorb harmful ultraviolet radiation 
thereby reducing damage to the polymer substrate. An example of a plastic 
product which has included titanium dioxide particles is polyvinyl 
chloride siding used in the construction or renovation of buildings and 
homes. 
While the above described metal oxide particles dispersed in polymer 
substrates have functioned to reduce ultraviolet radiation degradation, 
the particles have generally been relatively large, i.e., approximately 
0.2 micrometer in diameter and larger, and consequently they have not 
achieved maximum attenuation of the damaging effects of ultraviolet 
radiation. 
Another desirable property of particles used for reducing ultraviolet 
radiation degradation is low opacity in the visible portion of the 
spectrum. This property is required so that the particles do not mask 
colored pigments which are added to paints and plastics. If the particles 
used for reducing ultraviolet radiation have such low opacity, lower 
quantities of the expensive colored pigments used in colored products or 
less expensive colored pigments with low tinting strengths can be used. As 
stated above, the metal oxide particles used heretofore have generally 
been relatively large and have not provided the minimum opacity possible. 
By the present invention improved methods and polymer compositions 
containing ultraviolet attenuating particles are provided whereby 
ultraviolet radiation degradation is reduced to a minimum and relatively 
low particle opacity in the visible part of the spectrum is attained. 
SUMMARY OF THE INVENTION 
Improved methods of reducing the ultraviolet radiation degradation of 
polymer substrates are provided. In accordance with the methods, 
ultraviolet light scattering and absorbing particles having relatively low 
opacity in the visible part of the spectrum are dispersed in at least a 
surface layer of a polymer substrate to attenuate ultraviolet radiation 
degradation. The particles are formed of a material having a band gap in 
the range of from about 2.8 electron volts (eV) to about 4.1 eV, such as 
rutile, anatase or amorphous titanium dioxide or wurtzite or amorphous 
zinc oxide, and are of a size in the range of from about 0.001 micrometer 
to about 0.2 micrometer in diameter, more preferably from about 0.01 to 
about 0.15 micrometer. The loading of the particles is generally in the 
range of from about 0.1% to about 30% particles by weight of the polymer 
substrate layer and particles contained therein, more preferably from 
about 1% to about 15% by weight. 
Polymer compositions containing ultraviolet attenuating particles of 
relatively low opacity whereby the compositions have reduced 
susceptibility to degradation as a result of ultraviolet radiation, and in 
which less colored pigment or less expensive colored pigment can often be 
utilized, are also provided. 
It is, therefore, a general object of the present invention to provide 
improved methods and polymer compositions containing ultraviolet 
attenuating particles whereby degradation due to ultraviolet radiation is 
reduced and the particles are of relatively low opacity. 
Other and further objects, features and advantages of the present invention 
will be readily apparent to those skilled in the art upon a reading of the 
description of preferred embodiments which follows when taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In accordance with the improved methods of the present invention, the 
ultraviolet radiation degradation of a polymer substrate is reduced by 
dispersing ultraviolet scattering and absorbing particles in at least a 
surface layer of the polymer substrate, the particles being of a size in 
the range of from about 0.001 micrometer to about 0.20 micrometer in 
diameter, more preferably from about 0.01 to about 0.15 micrometer in 
diameter. 
The polymer substrates in which the sized particles of this invention can 
be included to reduce ultraviolet radiation degradation are paints, 
coatings, plastic articles and the like. Typically the polymer substrates 
are comprised of any of the well-known resin materials such as polyolefin, 
polyvinylaromatic, acrylic, polycarbonate, polyester, polyamide, epoxy and 
polyvinylhalide resins. Representative, but nonlimiting examples of 
specific polymeric resin materials include polyolefin resins such as 
polyethylene and polypropylene, polyvinylhalide resins such as poly(vinyl 
chloride) and poly(vinyl chloride) copolymers, polyvinylaromatic resins 
such as polystyrene and polystyrene copolymers, and acrylic resins such as 
poly(methyl acrylate). A variety of diluents and additives which are well 
known to those skilled in the art are usually admixed with the polymer 
resins including, but not limited to, water, oils, fillers, reinforcement 
materials, coupling agents and the like. 
The present invention is particularly useful in paints, coatings and 
plastic articles which are subjected to solar radiation, i.e., are 
utilized outdoors. 
The ultraviolet light scattering and absorbing particles which are useful 
in accordance with this invention are particles formed of a material 
having a band gap within the range of from about 2.8 eV to about 4.1 eV, 
and having a size in the range of from about 0.001 micrometer to about 
0.20 micrometer in diameter. Particularly suitable such particles are 
formed of metal oxides such as rutile, anatase or amorphous titanium 
dioxide having band gaps of about 3.0 eV, wurtzite or amorphous zinc oxide 
also having band gaps of about 3.0 eV and mixtures thereof. Preferably, 
the particles are selected from the group consisting of rutile, anatase, 
and amorphous titanium dioxide particles, wurtzite and amorphous zinc 
oxide particles and mixtures thereof, the particles being of a size in the 
range of from about 0.01 micrometer to about 0.15 micrometer in diameter. 
Such particles, in addition to providing greater reduction of polymer 
substrate degradation due to ultraviolet radiation than the particles used 
heretofore, as a result of being smaller provide a lower opacity in the 
visible part of the spectrum which reduces the quantities or strengths of 
the colored pigments required to produce colored products containing the 
particles. 
The above described particles can be dispersed throughout a polymer 
substrate to thereby scatter and absorb ultraviolet radiation, or the 
particles can be dispersed in a surface layer of the polymer substrate 
whereby ultraviolet radiation is substantially scattered and absorbed in 
the layer prior to being transmitted to the underlying polymer substrate. 
The particle loading of the polymer substrate or layer thereof is 
preferably within the range of from about 0.1% to about 30% particles by 
weight of the polymer substrate or layer and the particles contained 
therein. 
When titanium dioxide and/or zinc oxide particles are utilized in 
accordance with the method of this invention, the particles have diameters 
in the range of from about 0.001 micrometer to about 0.20 micrometer, more 
preferably from about 0.01 to about 0.15 micrometer, and are dispersed in 
the polymer substrate or layer thereof at a loading in the range of from 
about 0.1% to about 30% by weight, more preferably from about 1% to about 
15% by weight. 
A polymer composition of this invention which has reduced susceptibility to 
degradation as a result of ultraviolet radiation comprises a polymer resin 
having ultraviolet scattering and absorbing particles dispersed therein. 
The particles are formed of a material having a band gap in the range of 
from about 2.8 eV to about 4.1 eV and are of a size in the range of from 
about 0.001 micrometer to about 0.20 micrometer, more preferably from 
about 0.01 micrometer to about 0.15 micrometer. Preferred such particles 
are formed of rutile, anatase or amorphous titanium dioxide, wurtzite or 
amorphous zinc oxide and mixtures thereof. 
The polymer resin can be any resin suitable for use in paints, coatings or 
articles which are exposed to solar radiation. Generally, the polymer 
substrates can be comprised of any of a great variety of resin materials. 
Examples are polyolefin, polyvinylaromatic, acrylic, polycarbonate, 
polyester, polyamide, epoxy and polyvinylhalide resins. 
In order to illustrate the beneficial effects of the present invention, the 
transmittance, reflectance and absorptance of ultraviolet radiation by 
particles formed of materials and sized in accordance with the invention 
dispersed in a polymer layer were calculated. As mentioned above, the 
particle bearing layer protects the underlying polymer substrate from an 
ultraviolet radiation source such as sunlight, and prevents or 
substantially reduces ultraviolet radiation degradation damage thereto. 
The interaction of ultraviolet radiation with the scattering and absorbing 
particles was modeled using an expression for the two-stream theory 
presented by C. F. Bohren in his paper "Multiple Scattering of Light and 
Some of its Observable Consequences," Am. J. Phys., 55(6), 524 (1987). The 
interaction assumed for the calculations is illustrated in FIG. 1 wherein 
a polymer substrate 10 is shown having a surface layer 12 containing the 
scattering and absorbing particles 14 of this invention. Incident 
ultraviolet radiation is represented in FIG. 1 by the letter I and the 
arrow 16. The reflectance of the ultraviolet radiation is indicated by the 
letter R and the arrow 18, the transmittance of ultraviolet radiation 
through the layer 12 is indicated by the letter T and the arrow 20 and the 
absorptance of ultraviolet radiation by the particles 14 within the layer 
12 is indicated by the letter A. 
The reflectance R and transmittance T of the polymer substrate layer 12 are 
calculated from the complex refractive index of the particles 14, their 
size, and the wavelength of the ultraviolet radiation, I. Four quantities 
are required for these calculations, namely, the single-scattering albedo, 
.omega..sub.o ; the reflectance of an infinitely thick layer of particles, 
R.sub..infin. ; the optical thickness, .tau.; and the dimensionless 
attenuation coefficient, k. Also, as illustrated by the relationships 
which follow, the reflectance R and transmittance T are calculated from 
the scattering efficiency, Q.sub.sca ; the extinction efficiency, 
Q.sub.ext ; and asymmetry parameter, g, for the particles 14 using Mie 
theory. 
The reflectance of an infinitely thick layer of particles, R.sub..infin., 
is calculated in accordance with the following relationship, 
##EQU1## 
where the single-scattering albedo .omega..sub.o is defined as, 
##EQU2## 
The reflectance R and transmittance T are 
##EQU3## 
where the optical thickness is 
##EQU4## 
The particle 14 radius is denoted by r, f is the particle 14 volume 
fraction in the layer 12 and h is the thickness of the layer 12. 
The dimensionless attenuation coefficient in the equations for R and T is 
##EQU5## 
Finally, the absorptance A is determined from an energy balance as shown 
by, 
EQU A=1-(R+T) 
The quantity fh is the particle loading in layer 12 expressed as volume of 
solid per unit area of surface of the layer. The mass loading is 
determined by multiplying fh by the density of the particles. The mass 
loading is expressed as mass per unit area of layer 12 surface, i.e., 
milligrams per square meter (mg/m.sup.2). 
The complex refractive indices of rutile titanium dioxide and wurtzite zinc 
oxide used in the calculations were obtained from Ribarsky, M. W., 
Titanium Dioxide (TiO.sub.2 -rutile): Handbook of Optical Constants of 
Solids, Palik, E. W. (Ed.), Academic Press, New York, N.Y., 795-804 (1985) 
and Burgiel, J. C., Chen, Y. S., Vratny, F., and Smolinsky, G., 
"Refractive Indices of Zinc Oxide, Zinc Sulfide, and Several Thin-Film 
Insulators," J. Electrochem. Soc., 115, 729-732 (1968), respectively. All 
calculations were performed for monosize spherical particles, and 
scattering was calculated as a function of particle diameter and 
wavelength of the incident ultraviolet radiation. The refractive index of 
the polymer used in the calculations was 1.55. 
The most favorable conditions for reducing damage to the polymer substrate 
by ultraviolet radiation are those which minimize the transmittance of 
ultraviolet radiation. This principle was used to identify the optimum 
conditions for attenuation of ultraviolet radiation in the calculations. 
The computational results obtained for rutile titanium dioxide 
transmittance are given in FIG. 2. FIG. 2 shows the fraction of the 
incident ultraviolet radiation, I, transmitted by the protective layer 12 
on the surface of the polymer substrate 10 (FIG. 1). The transmittance is 
shown as a function of particle diameter and wavelength. The wavelength of 
radiation used in the calculations ranges from 0.3 to 0.4 micrometer which 
corresponds to solar ultraviolet radiation. 
As shown in FIG. 2, the particle diameter that provides the greatest 
attenuation depends on the wavelength of the incident ultraviolet 
radiation. This diameter varies from 0.05 micrometer for 0.30-micrometer 
radiation to 0.12 micrometer for 0.4-micrometer radiation. At the optimum 
particle size, the incident ultraviolet radiation is attenuated 
effectively. That is, the transmittance of 0.3-micrometer radiation is 
only 5% for a 0.05-micrometer particle. 
The importance of scattering can be assessed by considering the reflectance 
of the particle-loaded polymer layer. FIG. 3 shows the reflectance of the 
layer as a function of particle diameter and wavelength. Reflectance is 
greatest for optimum-size particles at the long wavelength end of the UV 
spectrum, indicating that scattering is an important mechanism for 
attenuation of ultraviolet radiation at such wavelengths. 
As shown in FIG. 4, absorption is more important at short wavelengths. That 
is, FIG. 4 shows the absorptance of a polymer substrate layer containing 
rutile titanium dioxide particles as a function of incident radiation 
wavelength and particle diameter. For titanium dioxide, both scattering 
and absorption are important in attenuating ultraviolet radiation in 
polymer substrate layers, each mechanism predominating at different ends 
of the solar ultraviolet spectrum. 
The product of film thickness and titanium dioxide loading, fh, was chosen 
to be 0.05 micrometer. This value provides sufficient transmittance of 
ultraviolet radiation so that the performance of the titanium dioxide 
particles could be compared readily across a large range of size and 
wavelength. This value of fh corresponds to a titanium dioxide loading of 
210 mg/m.sup.2. In terms of the thickness of the layer 12, this fh value 
corresponds to a one micrometer thick layer containing five volume percent 
titanium dioxide particles. 
Referring now to FIG. 5, the transmittance for a wurtzite zinc oxide 
particle containing layer with an fh of 0.05 micrometer corresponding to a 
loading of 280 mg/m.sup.2 is illustrated as a function of particle 
diameter and wavelength. The transmittance of ultraviolet radiation by a 
polymer substrate layer containing zinc oxide particles is different from 
a polymer substrate layer containing titanium dioxide particles. As 
illustrated by FIG. 5, small zinc oxide particles provide the highest 
attenuation, and the effectiveness of the particles declines rapidly as 
size increases. Since the real portion of the refractive index of wurtzite 
zinc oxide is smaller than that for rutile titanium dioxide, absorption 
plays a more important role for zinc oxide in attenuation of ultraviolet 
radiation than does scattering. This is consistent with the information 
shown in FIGS. 6 and 7 which illustrate the absorptance and reflectance, 
respectively, of zinc oxide as a function of particle size and wavelength. 
Comparison of the data in FIGS. 6 and 7 indicates that absorption 
predominates over the range of particle sizes and wavelengths considered. 
In accordance with the present invention, the operable size of spherical 
particles of rutile, anatase or amorphous titanium dioxide, wurtzite or 
amorphous zinc oxide, or other materials of similar band gap for 
attenuating ultraviolet radiation is a diameter in the range of from about 
0.001 micrometer to about 0.20 micrometer. As shown by the calculation 
results for rutile titanium dioxide particles, the optimum size is in the 
range of from about 0.05 to about 0.12 micrometer over the wavelength 
range of from about 0.3 to about 0.4 micrometer. When titanium dioxide 
spherical particles are utilized, scattering plays an important role in 
the attenuation of ultraviolet radiation at long ultraviolet wavelengths 
and absorption plays an important role at short ultraviolet wavelengths. 
For wurtzite zinc oxide particles, the optimum size for attenuation of 
ultraviolet radiation is in the range of from about 0.05 micrometer to 
about 0.06 micrometer for the wavelength range of from about 0.3 to about 
0.4 micrometer. Absorption is the dominant mechanism for attenuation of 
ultraviolet radiation by zinc oxide particles in polymer substrates in the 
aforementioned ultraviolet wavelength range. 
While monosized particles are generally preferred for use in accordance 
with this invention, particles of varying diameter can also be utilized 
provided that the diameters of the particles in the distribution are 
within the broad range of diameters given herein, i.e., from about 0.001 
micrometer to about 0.20 micrometer and at least 50% by weight of the 
total particles used are in the size range of from about 0.01 micrometer 
to about 0.15 micrometer in diameter. 
Thus, the present invention is well adapted to carry out the objects and 
attain the ends and advantages mentioned as well as those which are 
inherent therein. While changes in the invention may be able to be made by 
those skilled in the art, such changes are encompassed within the spirit 
of this invention as defined by the appended claims.