UV stable compositions

This invention relates to a method for increasing the ultraviolet light stability of articles made from macromolecular materials comprising: forming a flame-retarded formulation containing macromolecular material and a mixture of (i) a first alkylenebis(tetrabromophthalimide) flame retardant having a yellowness index (YI) value ranging from about 20 to about 40 as determined by ASTM D-1925; and (ii) a second alkylenebis-(tetrabromophthalimide) flame retardant having a yellowness index (YI) value (ASTM D-1925) ranging from about 2 to about 15 whereby the uv stability of a test plaque made from the flame-retarded formulation is greater than the arithmetic average of the uv stabilities of a test plaque made from a flame-retarded formulation containing (i) but not (ii) and of a test plaque made from a flame-retarded formulation containing (ii) but not (i).

BACKGROUND 
This invention relates to a unique blend of flame retardant components 
which provide a synergistic increase in ultraviolet (uv) stability of 
articles made from polymeric formulations containing the flame retardant 
components. 
Typically, the uv stability of polymeric materials is provided by uv 
stabilizers which are colorless or nearly colorless organic substances. 
Ultraviolet radiation contains a quantum of energy sufficient to exceed 
the dissociation energy of covalent bonds found in polymeric materials. 
Without the use of uv stabilizers, visible light containing uv radiation 
tends to cause polymeric materials to discolor and become brittle; 
protective coatings to crack, chalk, and delaminate; and dyes and pigments 
to fade. Accordingly, additives which increase the light stability of 
flame-retarded formulations are generally required. For the purposes of 
this invention, the term "uv stability" shall also refer to the light 
stability of the formulation, blend, or articles made from such 
formulations or blends. 
Classes of compounds used to reduce light-induced degradation of polymeric 
materials, include uv absorbers, hindered amines, nickel chelates, 
hindered phenols, and aryl esters. Commercially available uv absorbers 
include derivatives of 2-hydroxybenzophenones, 
2-(2'-hydroxyphenyl)benzotriazoles, diphenylacrylates and oxalanilides. 
Hindered amine light stabilizers (HALS) include 
bis(2,2,6,6,-tetramethyl-4-piperidinyl) sebacate, CHIMASSORB.RTM. 944, and 
TINUVIN.RTM. 622. Depending on their structures, nickel chelates, such as 
CYASORB.RTM. UV 1084, IRGASTAB.RTM. 2002, and nickel 
dialkyldithiocarbamates, contribute to light stabilization of polymeric 
substances by decomposing hydroperoxides, scavenging free radicals, 
absorbing uv radiation, and quenching photoexcited chromophores. Suitable 
aryl esters include resorcinol monobenzoate, phenyl salicylate, 
substituted aryl salicylates, diaryl terephthalates, and isophthalates. 
While the use of uv stabilizer additives provides a significant increase in 
the uv stability of polymeric materials containing them, there continues 
to be a need to increase the uv stability of macromolecular formulations 
without increasing the amount of uv stabilizer additive required. 
THE INVENTION 
It has been discovered, inter alia, that the uv stability of articles made 
from macromolecular formulations can be enhanced by forming a 
flame-retarded formulation containing macromolecular material and a 
combination or mixture of (i) a first alkylenebis(tetrabromophthalimide) 
flame retardant having a yellowness index (YI) value ranging from about 20 
to about 40 as determined by ASTM D-1925; and (ii) a second 
alkylenebis(tetrabromophthalimide) flame retardant having a yellowness 
index (YI) value (ASTM D-1925) ranging from about 2 to about 15. 
Surprisingly, the uv stability of a test plaque made from the resulting 
flame-retarded formulation has been found to be greater than the 
arithmetic average of the uv stabilities of a test plaque made from a 
flame-retarded formulation containing (i) but not (ii) and of a test 
plaque made from a flame-retarded formulation containing (ii) but not (i). 
This more than proportionate increase in uv stability was totally 
unexpected, since the YI value of a test plaque made from a macromolecular 
formulation containing (i) and (ii) is substantially the same as the 
arithmetic average of the YI values of a test plaque made from a 
flame-retarded formulation containing (i) but not (ii) and of a test 
plaque made from a a flame-retarded formulation containing (ii) but not 
(i). 
In another embodiment, this invention provides a flame-retarded 
macromolecular formulation having enhanced uv stability. The formulation 
comprises: (a) macromolecular material and (b) a flame retardant amount of 
a flame retardant combination or mixture containing (i) a first 
alkylenebis(tetrabromophthalimide) flame retardant having a yellowness 
index (YI) as determined by ASTM D-1925 ranging from about 20 to about 40; 
and (ii) a second alkylenebis(tetrabromophthalimide) flame retardant 
having a yellowness index (YI) value (ASTM D-1925) ranging from about 2 to 
about 15 provided the uv stability of test plaques made from the 
flame-retarded formulation is greater than the arithmetic average of the 
uv stabilities of a test plaque made from a flame-retarded formulation 
containing (i) but not (ii) and of a test plaque made from a 
flame-retarded formulation containing (ii) but not (i). 
The macromolecular material used in the methods and formulations of this 
invention may be cellulosic materials or polymeric materials. Illustrative 
polymers are: olefin polymers, cross-linked and otherwise, for example, 
homopolymers of ethylene, propylene, and butylene; copolymers of two or 
more of such alkylene monomers and copolymers of one or more of such 
alkylene monomers and any other copolymerizable monomers, for example, 
ethylene/propylene copolymers, ethylene/ethyl acrylate copolymers and 
ethylene/vinyl acetate copolymers; polymers of olefinically unsaturated 
monomers, for example, polystyrene, e.g. high impact polystyrene, and 
styrene copolymers; polyurethanes; polyamides; polyimides; polycarbonates; 
polyethers; acrylic resins; polyesters, especially 
poly(ethyleneterephthalate) and poly(butyleneterephthalate); epoxy resins; 
alkyls; phenolics; elastomers, for example, butadiene/styrene copolymers 
and butadiene/acrylonitrile copolymers; terpolymers of acrylonitrile, 
butadiene and styrene; natural rubber; butyl rubber; and polysiloxanes. 
The polymer may also be a blend of various polymers. Further, the polymer 
may be, where appropriate, cross-linked by chemical means or by 
irradiation. Particularly preferred macromolecular materials are high 
impact polystyrene, and acrylonitrile/butadiene/styrene terpolymers. 
For the purposes of this invention, the alkylene group of the 
alkylenebis(tetrabromophthalimides) contain from 2 to 6 carbon atoms. 
Commercially available alkylenebis(tetrabromophthalimides) include 
N,N'-1,2-ethylenebis(3,4,5,6-tetrabromophthalimide). Accordingly, the 
first alkylenebis(tetrabromophthalimide) having a YI value ranging from 
about 20 to about 40 may be made in accordance with U.S. Pat. No. 
4,092,345 or obtained commercially from Ethyl Corporation as SAYTEX.RTM. 
BT-93.RTM. flame retardant. The second alkylenebis(tetrabromophthalimide) 
having a YI value ranging from about 2 to about 15 may be made in 
accordance with U.S. Pat. Nos. 4,125,535; 4,914,212; 4,992,557; or 
4,990,626 or obtained commercially from Ethyl Corporation as SAYTEX.RTM. 
BT-93.RTM.W flame retardant. 
When preparing the formulations of this invention, conventional blending or 
mixing techniques can be used. Such techniques include the use of a single 
or twin screw extruder, a high intensity mixer, or a continuous mixer. 
Typically, the macromolecular formulation will contain from about 75 to 
about 99 wt.% macromolecular material, preferably from about 80 to about 
99 wt.% macromolecular material, and from about 1 to about 25 wt.% of 
flame retardant combination or mixture, preferably from about 1 to about 
20 wt.% of the flame retardant combination or mixture. Other additives may 
be used in the formulation such as flame retardant synergists, 
antioxidants, plasticizers, fillers, pigments, UV stabilizers, 
dispersants, melt flow improvers, and the like. 
The flame retardant combination or mixture of (i) and (ii) may be formed 
and then added in a flame retarding amount to the macromolecular material 
or both flame retardants may be added to the macromolecular material 
individually to form the flame-retarded macromolecular formulation in 
situ. Accordingly, the order of introduction of the flame retardants (i) 
and (ii) is not critical to the invention as they can be added in any 
order. 
The flame retardant combination or mixture may contain from 1 to about 99 
wt. % of the first flame retardant, and from about 99 to about 1 wt.% of 
the second flame retardant. Preferably, the flame retardant combination 
contains from about 50 to about 85 wt.% of the first flame retardant, and 
from about 50 to about 15 wt.% of the second flame retardant, and most 
preferably there is about a 3:1 weight ratio of the first flame retardant 
to the second flame retardant in the flame retardant combination or 
mixture. Notwithstanding the ratio of the first flame retardant to the 
second flame retardant selected for any flame-retarded macromolecular 
formulation, it is critical that the uv stability of a test plaque made 
from the flame-retarded formulation be greater than the arithmetic average 
of the uv stabilities of a test plaque made from a flame-retarded 
formulation containing (i) but not (ii) and of a flame-retarded 
formulation containing (ii) but not (i). In the most preferred embodiment, 
the uv stability of a test plaque made from the flame-retarded formulation 
is greater than the uv stability of test plaques made from flame-retarded 
formulations containing either (i) or (ii). 
The uv stability of macromolecular formulation may be determined using any 
one or more techniques typically used in the polymer industry. Such 
techniques include the 100 hour and 300 hour xenon arc test according to 
ASTM D-4459, the Hewlett Packard uv (HPUV) test according to ASTM D-4674, 
and the like. The test for determining uv stability is not critical to the 
invention, provided the test is generally accepted in the industry and 
provides reasonably reproducible results. 
The Y.I. values for test plaques made from the flame-retarded formulations 
as disclosed herein were obtained in accordance with ASTM D 1925 using a 
HunterLab Model Colorquest.RTM. integrated sphere. Y.I. values are 
calculated values, i.e. Y.I.=100(0.72a+1.79b)/L, wherein the values for 
"a","b", and "L" are observed values. The "a" value measures redness when 
a plus value, gray when zero and greenness when a negative value. The "b" 
value measures yellowness when a plus value, gray when zero and blueness 
when a minus value. The "L" value measures lightness and varies from 100, 
for perfect white, to 0, for black. The "L" value is used by some in the 
industry to convey a sense of the degree of whiteness of a material. The 
Y.I. values for the powdered flame retardants and mixtures thereof were 
obtained generally in accordance with ASTM D 1925 using a Hunter-Lab model 
Colorquest.RTM. 45.degree./0.degree.. The only (A) modification to the 
ASTM procedure, is that once the powdered sample is placed in the quartz 
sample cup, the cup is tapped on a paper note pad about 100 times within 
about one minute by raising the cup about 5 cm above the note pad and 
tapping it flat against the pad. This procedures is used so as to provide 
reproducible results. 
While not desiring to be bound by theory, it is believed that there is a 
synergistic effect which increases the uv stability of flame-retarded 
macromolecular formulations containing the mixture of first and second 
flame retardants described herein. This synergistic effect results in 
greater uv stability of a test plaque made from a formulation containing 
(i) and (ii) than would have been expected from an arithmetic average of 
the uv stabilities of a test plaque made from a flame-retarded formulation 
containing (i) but not (ii) and a test plaque made from a flame-retarded 
formulation containing (ii) but not (i). By "arithmetic average" is meant 
the calculated average of the sum of the actual uv stabilities of test 
plaques made from the individual formulations, the first formulation (A) 
containing the first flame retardant (i), and the second formulation (B) 
containing the second flame retardant (ii). Accordingly, the arithmetic 
average can be calculated by the following formula: 
##EQU1## 
For example, a test plaque containing only flame retardant (i) has a uv 
stability of 35. A second test plaque containing only (ii) has a uv 
stability of 13. The arithmetic average uv stability for a test plaque 
made from a flame retardant combination containing 50 wt.% (i) and 50 wt.% 
(ii) based on the total weight of flame retardant is therefore 24 
according to the above formula. (uv stability (avg.)=(0.5 wt.% 
.times.35)+(0.5 wt.%.times.13)=24) 
For the purposes of this invention the uv stability of a formulation is 
indicated by the .DELTA.E values which are obtained by measuring the 
initial Hunter "L", "a" and "b" values of a test plaque containing flame 
retardant(s) by placing the test plaque made from a macromolecular 
formulation containing flame retardants (i) or (ii), or both (i) and (ii) 
in a HunterLab Colorquest.RTM. intergrated sphere. After exposing the test 
plaque to light in the xenon arc or HPUV tests for the prescribed period 
of time designated in the ASTM test procedure selected, the final Hunter 
"L", "a", "b" values are determined and the .DELTA.E is calculated by the 
following formula: 
##EQU2## 
Decreasing .DELTA.E values indicate increasing uv stabilities which are 
more desirable. In order to further illustrate the advantages of this 
invention, the following example is given: