Flame retardant graft copolymers of polypropylene

Described are polypropylene polymer compositions which have suprisingly good physical properties in combination with flame retardancy, which include graft copolymers represented by the formula: ##STR1## wherein n is >1, P is polypropylene, and S is a grafted side chain having brominated monomeric units of the formula: ##STR2## wherein x=1 to 4, R.sub.1 is H or CH.sub.3, and R.sub.2 is H or a C.sub.1-4 lower alkyl group. Polymer blends including such graft copolymers and methods for making flame retardant polymer compositions are also described.

BACKGROUND OF THE INVENTION 
This invention resides in the field of flame retardant polymers. More 
particularly, it relates to flame retardant polymer compositions which 
include graft copolymers of polypropylene and brominated vinyl aromatics 
such as brominated styrenes, and to methods for making these compositions. 
By way of background, polypropylene has proven to be one of the most useful 
and versatile polymers. Its physical properties make it ideal for many 
applications including molded articles, spun fibers, hot melt adhesives 
and many others. These properties include, for instance, good solvent 
resistance, surface appearance and stain resistance, and low moisture 
absorption. However, polypropylene does not possess adequate flame 
retardancy for certain applications. In view of its other desirable 
physical properties, it has naturally been a matter of great interest to 
provide polypropylene compositions having greater flame retardancy. 
Improvement of flame retardancy has relied on modifications to 
polypropylene, or on additives for the polypropylene, but disadvantages 
have been associated with both approaches. Although a vast number of 
modified polypropylene compositions have been described or theorized in 
the prior art, few if any suitable flame retardant polypropylene 
derivatives have been identified. Similarly, numerous additives for 
increasing the flame retardancy of polypropylene have been studied and 
some are commercially available. Nonetheless, there is at present no 
commercially available flame retardant additive for polypropylene which 
provides adequate retention of polypropylene's physical properties, and 
demonstrates high thermal stability, non-migration of additive to the 
surface, and absence of solids at processing temperatures. The present 
invention contemplates a modification of polypropylene which yields a 
composition that retains the desirable physical properties of 
polypropylene, and avoids the disadvantages of alternate approaches. 
In particular, the modified polypropylene of the invention avoids the 
frequently encountered migration or "bloom" of inert additive-type flame 
retardants to the surface of molded articles. Such bloom leads to 
unsightly surface discoloration on articles molded from the polypropylene 
and thereby effectively limits the amount of additive which can be used. 
Further, these inert additives frequently remain solid at processing 
temperatures, which can damage or foul processing equipment. For example, 
inert additives remaining solid at processing temperatures are known to 
cause problems by clogging spinnerettes used in equipment for producing 
spun fibers. This type of equipment fouling not only reduces the 
efficiency of processing but can also necessitate the costly refurbishment 
or premature replacement of equipment. 
The applicants' preferred modified polypropylenes also avoid many other 
problems encountered in the prior art by having only low levels of 
unreacted styrene monomer, typically less than 1% by weight. For example, 
by this aspect the applicants' invention provides a vehicle to avoid 
monomer juicing problems known to occur in prior art graft modified 
compositions. It is also significant that the compositions of the present 
invention can be efficiently processed without the release of excessive 
volatile monomer into the surrounding environment, which can be hazardous 
to those working with or near the materials. The prior art has failed to 
appreciate these substantial advantages of the compositions of the present 
invention. 
As noted above, known flame retardant additives for polypropylene have 
recognized drawbacks. One such additive is hydrated alumina, which retards 
flame by releasing water under fire conditions. However, high loadings of 
hydrated alumina are necessary to give desired efficacy, and this results 
in very poor physical properties of the polypropylene and articles molded 
therefrom. 
Certain other available additives remain solid at normal polypropylene 
processing temperatures and thus complicate processing. Such additives 
include, for example, a bisimide-containing aliphatic bromine additive 
known as BN-451 from Ethyl Corp. of Sayreville, N.J., and a ring 
brominated polystyrene additive known as Pyro-Chek 68PB from Ferro Corp. 
of Cleveland, Ohio. The latter use of ring brominated polystyrene as an 
additive to polypropylene, rather than as a graft onto polypropylene, is a 
particularly clear demonstration of the failure of the prior art to 
recognize the present invention. Other available additives, such as 
decabromodiphenyl oxide, not only remain solid at processing temperatures 
but also are known to rise or "bloom" to the surface of molded articles. 
Aside from these inert additives, reports exist in the literature of 
attempts to chemically bond or graft flame retardants to polypropylene. To 
the applicants' knowledge, none of these techniques has been 
commercialized. For instance, M. Hartmann, et al., Z. Chem., 20(4), 146-7 
(1980), report preparing graft copolymers of atactic polypropylene and 
four respective vinylphosphonic acid derivatives. Two of the four 
copolymers prepared were reported as self extinguishing when containing 
greater than 3% by weight phosphorous. 
P. Citovicky et al., Thermochim. Acta., 93, 171-4 (1985), disclose a 
two-step procedure in which glycidyl methacrylate was grafted to isotactic 
polypropylene followed by reaction with various flame retardants including 
bromoacetic acid, 3,3',5,5'-tetrabromo-2,2'-dihydroxybiphenyl, 
dichloroacetic acid, or phenyldihydrogen phosphate. The copolymer reacted 
with Ph dihydrogen phosphate gave the highest limiting oxygen index value 
and was also reported the most thermally stable. In general, this 
technique is not particularly advantageous since it requires two steps and 
the flame retardant must be a functionalized molecule capable of reaction 
with an epoxide. 
B. J. Hill et al., Comm. Eur. Communities [Rep.] EUR, EUR 6718 (1980), 
report irradiation grafting of bis(2-chloroethyl)vinylphosphonate to 
polyester and polypropylene fabrics to render them self-extinguishing. The 
authors report that the bis(2-chloroethyl)vinylphosphonate had poor 
reactivity toward the fabrics. Comonomers were therefore required which in 
some instances diminished flame retardancy and/or stiffened the fabrics. 
K. Nakatsuka et al., Japan JP 44/3965 (Feb. 19, 1969), report air oxidizing 
polypropylene at elevated temperatures to introduce peroxy groups to the 
polymer followed by graft polymerization with CH.sub.2 CClCO.sub.2 Me. The 
product was reported to be self-extinguishing. 
Outside of the field of flame retardancy, various modifications to 
polyolefins have been proposed. For example, U.S. Pat. No. 4,179,401, 
issued to Garnett et al. in 1979, relates to a process for producing a 
heterogenous catalyst for the hydrogenation, hydroformylation, 
isomerization, cracking or dehydrogenation of organic molecules. The 
Garnett process comprises the steps of radiation grafting a monomer having 
an alpha- unsaturated bond to a metal or an organic polymer and complexing 
a nitrogen, halogen, or phosphorous containing group to the monomer. The 
Garnett et al. patent lists many possible polymer/monomer combinations. 
Identified polymer substrates included polyvinyl compounds, polyolefins, 
polyvinylidenes, polysiloxanes, polydienes, polyethers, polyimides, 
polysulphones, polyesters, polyamides, polyurethanes, polycarbonates and 
polyureas. Listed as possible monomers for use in the described process 
were p-nitrostyrene, p-amino styrene, p-chlorostyrene, 
vinyldiphenylphosphine, cis-bis (1,2-diphenylphosphino) ethylene, 
triallylphosphine, divinylphenylphosphine and many more. 
Similarly, U.S. Pat. No. 3,177,270, issued to Jones et al. in 1965, 
describes a method for modifying polyethylene and other substrates for the 
purpose of improving tensile strength, elongation and/or flexural modulus. 
The Jones et al. patent specifically described the preparation of ethylene 
polymer modified with styrene, a styrene/acrylonitrile mixture, 
dichlorostyrene or a mixture of isomeric vinyltoluenes. The Jones et al. 
patent additionally lists other possible polymeric substrates for use in 
the described method as including polypropylene, polyisobutylene, 
polybutene, and copolymers of ethylene and propylene, ethylene and butene, 
ethylene and styrene, ethylene and vinyl acetate, and ethylene and methyl 
methacrylate. Possible graft monomers are listed as including styrene, 
vinyltoluene, vinylxylene, ethylvinylbenzene, isopropyl styrene, 
para-tert-butyl styrene, dichlorostyrene, bromostyrene, fluorostyrene, or 
mixtures thereof with acrylic acid, methacrylic acid, acrylonitrile, 
methacrylonitrile, methyl methacrylate or maleic anhydride. 
As is evident from the foregoing, past efforts to provide a polypropylene 
composition with improved flame retardancy have not been fully 
satisfactory. Available inert flameproofing additives have exhibited 
drawbacks such as bloom and interference with desired physical properties. 
Additionally, polypropylene materials have not been provided with grafted 
fire retardants which perform as well as the present inventive 
compositions. Accordingly, there has remained a need for fire retardant 
polypropylene compositions demonstrating good physical properties, and the 
applicants' invention addresses this need. 
SUMMARY OF THE INVENTION 
Accordingly, a first preferred embodiment of this invention provides a 
flame retardant graft copolymer composition comprising: 
##STR3## 
in which n is an integer &gt;1, P is polypropylene, and S is a side chain 
grafted to the polypropylene and having brominated monomeric units of the 
formula: 
##STR4## 
wherein x=1 to 4, R.sub.1 is H or CH.sub.3, and R.sub.2 is H or a 
C.sub.1-4 lower alkyl group. In an alternate embodiment, the composition 
additionally includes a homopolymer of the brominated monomeric units. 
Another preferred embodiment of this invention provides a flame retardant 
polymer composition comprising a blend of (i) polypropylene, and (ii) a 
polymer composition including a graft copolymer according to the first 
embodiment above and constituted about 10% to about 60% bromine by weight. 
Such a blend can be prepared by diluting or "letting down" the 
bromine-concentrated polymer composition (ii) with a desired amount of 
polypropylene. After let down, the blend preferably comprises about 1% to 
about 20% bromine by weight of the blend. 
Another preferred embodiment of this invention provides a method for 
producing a flame retardant polymer composition which comprises the step 
of graft polymerizing polypropylene with a monomer having the formula: 
##STR5## 
wherein x=1 to 4, R.sub.1 is H or CH.sub.3, and R.sub.2 is H or a 
C.sub.1-4 lower alkyl group. The invention provides a flame retarding 
amount of bromine in the graft polymerization product. 
One object of this invention is to provide flame retardant 
polypropylene-based polymer compositions. 
Another object of this invention is to provide a method for Producing flame 
retardant polypropylene-based polymer compositions. 
Additional objects and advantages will be apparent from reading the 
description which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For the purposes of promoting an understanding of the principles of the 
invention, reference will now be made to preferred embodiments of the 
invention and specific language will be used to describe the same. It will 
nevertheless be understood that no limitation of the scope of the 
invention is thereby intended, such alterations, further modifications and 
applications of the principles of the invention as described herein being 
contemplated as would normally occur to one skilled in the art to which 
the invention relates. 
The present invention provides compositions which have physical properties 
comparable to that of polypropylene, but which have improved flame 
retardancy. In the broadest sense, polypropylene is modified by grafting a 
ring-brominated vinyl aromatic onto the polypropylene. The presence of the 
bromine contributes to the flame retardancy of the resulting polymer. In 
contrast to the prior art, a significant portion of the bromine present in 
the compositions of the present invention is grafted onto, i.e. attached 
to, the polypropylene through the monomeric unit. In addition, it is 
contemplated that the final compositions may also include bromine in the 
form of homopolymers of the monomer used in grafting the polypropylene. 
Although not to be considered limiting of the present invention, it is 
believed that the compatibility of the grafted polypropylene and the 
homopolymer contributes to the desirable physical properties of the 
resulting composition. 
In accordance with the above discussion, one embodiment of this invention 
includes a flame retardant polymer composition comprising a graft 
copolymer represented by the formula: 
##STR6## 
in which n is an integer &gt;1, P is polypropylene, and S is a side chain 
grafted to the polypropylene and having monomeric units of the formula: 
##STR7## 
wherein x=1 to 4, R.sub.1 is H or CH.sub.3, and R.sub.2 is H or a 
C.sub.1-4 lower alkyl group. 
The form of the polypropylene used in the present invention is not critical 
so long as it undergoes suitable graft polymerization to yield the 
indicated compositions. Thus the polypropylene base in the graft copolymer 
can include crystalline polypropylene homopolymer in isotactic, 
syndiotactic, or atactic (also commonly known as "amorphous") form. 
Further, polypropylene materials with melt indices of about 0.1 to about 
200 grams per 10 minutes (as measured by ASTM D-1238) can be employed. 
The polypropylene is graft polymerized with a ring-brominated vinyl 
aromatic which is also optionally alpha- or ring-substituted with one or 
more aliphatic groups including lower alkyl groups such as methyl, ethyl, 
and propyl and butyl isomers. This monomer is preferably a styrene having 
1 to 4 ring-substituted bromines. However, it will be appreciated that 
monovinyl aromatics, including for instance styrenes which are alpha- or 
ring-substituted with one or more lower aliphatic groups as described 
herein, function similarly to styrene in grafting procedures and are 
accordingly also within the scope of this invention. In this vein, methyl 
is a preferred optional alpha-substituted alkyl group and C.sub.1-4 lower 
alkyls are preferred optional ring-substituted alkyl groups. 
Accordingly, preferred brominated monomers suitable for the graft 
polymerization process have the formula: 
##STR8## 
wherein x=1 to 4, R.sub.1 is H or CH.sub.3, and R.sub.2 is H or a 
C.sub.1-4 lower alkyl group. 
In accordance with this formula the preferred styrene monomer has 1 to 4 
bromines per styrene, or can also include mixtures containing these mono-, 
di-, tri-, and/or tetrabromostyrenes. Pentabromostyrene is not a preferred 
styrene monomer as in the applicants work it has failed to efficiently 
graft to polypropylene either alone or in combination with a 
lesser-brominated styrene comonomer (see for instance Examples 4-6 below). 
It is desirable that the monomer material used for the graft 
polymerization be liquid at room temperature (about 25.degree. C.). 
Mixtures of styrenes may accordingly be used which are liquid at room 
temperature and which have varying degrees of bromination to achieve a 
high percentage of bromine in the monomer material. 
In another aspect, it is preferred that the stated levels of bromine be 
achieved with at least about 85% of the brominated monomeric units being 
di-, tri- or tetrabromo units or mixtures thereof, and more preferably 
with at least about 80% of the brominated monomeric units being dibromo 
units. The most preferred monomer is dibromostyrene. In commercial form 
dibromostyrene commonly includes minor levels of mono- and 
tribromostyrene. For example, the applicants have used dibromostyrene 
available from Great Lakes Chemical Corporation of West Lafayette, Ind., 
which normally contains about 15% monobromostyrene and about 3% 
tribromostyrene by weight. The preferred styrene monomer can also contain 
storage stabilizers as known and used in this field to inhibit premature 
Polymerization. As examples, these commonly include phenols and compounds 
of sulfur, nitrogen and phosphorous. 
The ring-brominated vinyl aromatic is grafted to the polypropylene base 
using suitable known graft polymerization techniques which may be 
performed, for example, in solution, suspension, emulsion or bulk 
procedures. Grafting techniques include for instance irradiation, 
peroxidation by exposure to oxygen at elevated temperatures, and 
abstraction of protons by free radical initiators. Among these, the latter 
technique is preferred, with appropriate free radical initiators including 
dicumylperoxide, benzoylperoxide, t-butylperbenzoate, 
2,2'-azobis(isobutyronitrile), cumenehydroperoxide or like initiators. 
In a typical grafting procedure, the free radical initiator is dissolved in 
the brominated monomer at suitable levels, generally from about 0.1% to 
about 5% and preferably from about 1% to about 3% on weight of the 
monomer. The resulting solution is then added to agitated molten 
polypropylene. In this regard, the grafting is preferably performed in a 
kneading type mixer such as a Banbury mixer, or in an extruder or a 
two-roll mill, although other suitable mixers known in the art can also be 
used. 
Additionally, the grafting is carried out at a suitable pressure and 
elevated temperature and for a duration sufficient to yield the desired 
end product. Generally, the temperature will be sufficiently high to 
reduce the viscosity of the molten polypropylene and to ensure thorough 
mixing. Moreover, where free radical initiators are used, this temperature 
will be high enough to promote decomposition of the initiator resulting in 
rapid polymerization of the monomer. In any case, preferred temperatures 
for the grafting procedure are from about 120.degree. C. to about 
230.degree. C., with more preferred temperatures being about 170.degree. 
C. to about 200.degree. C. 
The grafting proceeds readily at atmospheric pressure as well as at the 
elevated pressures encountered in commonly used plastics processing 
equipment. The duration of the grafting procedure will depend upon the 
temperature as well as the grafting technique used. In free radical 
initiated grafting, the duration will also depend upon the chosen 
initiator and the efficiency of mixing. Generally, however, durations 
ranging from about 1 second to several hours can be used, with about 10 to 
about 300 seconds resulting in an efficient polymerization and thus being 
preferred. 
Graft polymerization will typically result in both grafted polypropylene 
and homopolymer of the selected monomer. It has been found that the 
grafted polypropylene and any homopolymer present remain well intermixed, 
even during processing. The homopolymer could alternatively be removed, 
but this is not necessary and the preferred composition therefore includes 
both grafted polypropylene and homopolymer. 
The present invention provides polymer compositions having a flame 
retarding amount of bromine, about 1 weight % or more, based on the weight 
of the overall composition. This bromine may be present either in the 
grafted polypropylene or in a homopolymer mixed with the grafted 
polypropylene. In any event, however, the grafted polypropylene copolymer 
includes at least about 0.5%, and more preferably at least about 1%, 
bromine by weight. As processed (e.g. molded or spun), the preferred flame 
retardant polymer compositions of the invention will generally include 
about 1% to about 20% bromine by weight of the composition, and more 
preferably about 3 to about 15% bromine by weight. 
In another preferred embodiment of the invention, a bromine-concentrated 
polymer composition is provided having about 10% to about 60% bromine by 
weight, or more preferably about 30% to about 50% bromine by weight, of 
the overall product. In this embodiment, the grafted polypropylene 
copolymer preferably includes at least about 5% bromine by weight for the 
broad range, and at least about 15% for the more preferred range. This 
composition can thereafter be let down with polypropylene prior to 
processing to yield a resulting composition having an appropriate level 
within the 1% to 20% or more preferred 3% to 15% bromine range. 
A certain amount of bromine may also be present as a part of unreacted 
monomer, but this form is not preferred and the amount of unreacted 
monomer is desired to be relatively low. This will prevent or minimize 
juicing, i.e., migration of the monomer to the surface. The amount of 
unreacted monomer is preferably at most about 3% by weight, and more 
preferably at most about 1% by weight, of the overall composition. These 
low levels are generally achieved in the preferred products without the 
need for further processing steps. However, when desired the unreacted 
monomer can be removed, for example, by placing the graft polymerization 
products under vacuum. 
The let down blends and other polymer compositions of the applicants' 
invention have demonstrated excellent flame retarding properties as, for 
instance, the specific Examples and Table 3 below demonstrate. In 
addition, the compositions of the invention, particularly the let down 
blends, have demonstrated excellent physical properties. This can be seen 
for example from the high values reported in Table 2 for impact strength 
and percent elongation of a blend prepared in Example 11. The combination 
of improved flame retardancy and desirable physical properties, especially 
without bloom, juicing, etc. found in prior art approaches, provide a 
significant and unexpected advantage, and highlight the magnitude of the 
applicants' discoveries. Additional materials which do not significantly 
interfere with the grafting procedure or the resulting products can also 
be used as known in the art or determined by routine experimentation. For 
instance, reactive additives such as chain transfer agents can be 
dissolved into the brominated monomer prior to grafting to control the 
molecular weight of the brominated polymer content of the graft procedure. 
Alkyl halides and mercaptans, for example, are suitable chain transfer 
agents for limiting the extent of styrene polymerization and thus the 
molecular weight of the styrene polymer chains. As will be understood, the 
product of the graft polymerization will normally contain brominated 
styrene polymer grafted to the polypropylene as well as brominated styrene 
homopolymer resulting from separate polymerization of the monomer. The 
chain transfer agent can thus be used to regulate the molecular weight of 
each. 
Other reactive unsaturated comonomers can also be included during the 
grafting process to modify the properties of the resultant polymer 
composition. These can include for example maleic anhydride, styrene, 
chlormethylstyrene, acrylonitrile, methylmethacrylate, acrylic acid, 
butene, butadiene, acrylamide and many others as known in the art. 
Modifications which can be achieved by addition of other materials during 
the grafting process include alterations in color, clarity, lubricity, 
dyability, melt viscosity, softening point, thermal stability, ultraviolet 
stability, viscoeleastic behavior, polarity, biodegradability, static 
charge dissipation, strength and stiffness. 
Nonreactive materials can also be included in the grafting procedure to 
modify product properties. As examples, antioxidants, ultraviolet 
absorbers, antistatic agents, pigments, dyes, nucleating agents, fillers, 
slip agents, lubricants, antiblocking agents, plasticizers, and/or 
antimicrobials can be included. These materials can be incorporated into 
the polypropylene prior to or during the grafting process Alternatively, 
these materials can be added in a separate compounding step which provides 
the advantage of avoiding possible interference by these additives with 
the grafting chemistry. 
Additional flame retardants (aside from the brominated styrene) can also be 
included in the graft polymerization product when desired to improve 
efficiency and lower costs. These may be reactive flame retardants such as 
bis(2-chloroethyl)vinylphosphonate or acrylic acid esters of halogenated 
alcohols, or inert flame retardants such as antimony oxide, 
triphenylphosphate, or hexabromocyclododecane. 
The invention will be further described with reference to the following 
specific Examples. It will be understood that these Examples are 
illustrative and not restrictive in nature. In the following Examples, 
percents indicated are percents by weight unless indicated otherwise. 
EXAMPLE 1 
Dibromostyrene Grafted Onto Polypropylene 
63 g of dicumylperoxide were dissolved into 3157 g of dibromostyrene (also 
containing 15% by weight monobromo- and 3% by weight tribromostyrene) from 
Great Lakes Chemical Corporation of West Lafayette, Ind. In a continuous 
process the monomer was metered at 3.2 lbs/hr to a Werner & Pfleiderer 
twin screw extruder while simultaneously feeding polypropylene homopolymer 
(AMOCO 10-5219) at 17.6 lbs/hr. Temperature in the extruder was graduated 
from 170.degree. C. in Zone 1 to 185.degree. C. in Zone 5, and screw speed 
was 160 revolutions per minute (rpm's). The molten product was stranded 
into a water bath and was then granulated. The graft copolymer was found 
to contain 8.4% bromine and had a residual monomer content of 0.52%. 
EXAMPLE 2 
Concentrate Dibromostyrene/Polypropylene Graft Copolymer 
24 g of dicumylperoxide were dissolved into 1212 g of Great Lakes 
Dibromostyrene. In a batch process, 250 g of the monomer plus peroxide 
were added to 105 g of molten polypropylene homopolymer (AMOCO 10-5219) in 
a Brabender Prep Center (a Banbury-type mixer). Bowl temperature was 
maintained at 180.degree. C. while mixing at 50 rpm's as the monomer was 
being added over a 10 minute period. Following the addition of the last of 
the monomer, the mixture was held an additional 5 minutes at 180.degree. 
C. The product was emptied from the bowl, cooled to room temperature and 
granulated. The graft concentrate was found to contain 40.5% bromine with 
a residual monomer content of 0.57%. 
EXAMPLE 3 
Tribromostyrene Grafted to Polypropylene 
A mixture of 11.1 g of powdered tribromostyrene and 0.22 g of 
dicumylperoxide were dry blended. Separately, 48.9 g of polypropylene 
(AMOCO 10-5219) were melted in a small Brabender Plasticorder mixer at 
180.degree. C. and 60 rpm's, and the tribromostyrene/peroxide mixture was 
spooned in over a 2 minute period. The resulting graft copolymer was held 
an additional 5 minutes at 180.degree. C./60 rpm's before cooling and 
granulating. The product was found to contain 10.6% bromine with a 
residual monomer content of 0.06%. 
EXAMPLE 4 
COMATIVE EXAMPLE 
Attempt to Graft Pentabromostyrene to Polypropylene 
A mixture of 9.9 g of powdered pentabromostyrene and 0.20 g dicumylperoxide 
were dry blended. Separately, 50.1 g of polypropylene (AMOCO 10-5219) were 
melted in a small Brabender Plasticorder mixer. Using the same conditions 
as in Example 3, the pentabromostyrene plus peroxide mixture was added. On 
removing the product from the bowl, powdery white residue was observed on 
the surface of the bowl and on areas of the product. The product was found 
to contain 13.1% bromine and 11.1% residual pentabromostyrene monomer, 
thus indicating that of the monomer charged failed to polymerize. 
EXAMPLE 5 
COMATIVE EXAMPLE 
Attempt to Graft Pentabromostyrene to Polypropylene 
8.3 g of powdered pentabromostyrene were dry blended with 0.16 g 
t-butylperbenzoate. The mixture was added in a single dose to 41.7 g 
polypropylene (AMOCO 10-5219) which was mixing at 190.degree. C./60 rpm's 
in a Brabender Plasticorder. After 5 minutes at 190.degree. C. the product 
was removed and granulated. The product was found to contain 12.6% bromine 
and 10.7% residual pentabromostyrene monomer, and thus 64% of the 
pentabromostyrene failed to polymerize. 
EXAMPLE 6 
COMATIVE EXAMPLE 
Attempt to Co-graft Pentabromostyrene and Dibromostyrene 
A slurry of 4.8 g of pentabromostyrene, 4.8 g of dibromostyrene and 0.19 g 
of dicumylperoxide was prepared. Using the conditions from Example 5, the 
slurry was added to 40.4 g of polypropylene (AMOCO 10-5219). The product 
was found to contain 11.9% bromine and 5.3% residual pentabromostyrene 
monomer. It also contained 1.6% residual mono- and dibromostyrene. Thus, 
based upon the amount of monomers added, 55% of the pentabromostyrene and 
17% of the mono- and dibromostyrene failed to react. 
EXAMPLE 7 
A second graft copolymer was prepared using the procedure described in 
Example 1. A slightly faster rate of dibromostyrene addition was used, 
however, so that the product this time contained 9.1% bromine and 0.68% 
residual monomer. 
EXAMPLE 8 
150 g of atactic polypropylene (Ring and Ball softening point=136.degree. 
C., Brookfield Viscosity at 149.degree. C. (300.degree. F.)=9900 
centipoise) were melted in a Brabender Prep Center at 160.degree. C. A 
mixture of 142.5 g Great Lakes Dibromostyrene, 3.0 g dicumylperoxide, and 
4.5 g 1-Dodecanethiol was added over a 5 minute period while mixing at 50 
rpm's. The temperature was increased to 180.degree. C. and the product 
removed from the bowl. The product was a leathery solid containing 29.4% 
bromine and 0.16% residual monomer. 
EXAMPLE 9 
The graft copolymer from Example 1 was molded into test specimens using a 
Newbury Injection Molding Machine (Model HI-30 RS, Newbury Industries, 
Inc., Newbury, Ohio). Molding conditions are shown in Table 1: 
TABLE 1 
______________________________________ 
Conditions for Injection Molding 
______________________________________ 
Injection Pressure, psi 
500 
Cycle time, sec. 30 
Rear Temperature, .degree.F. 
370 
Front Temperature, .degree.F. 
380 
Mold Temperature, .degree.F. 
75 
Screw Speed, rpm 100 
Injection Time, sec 
10 
______________________________________ 
EXAMPLE 10 
The graft copolymer concentrate from Example 2 was dry blended with base 
polypropylene at a ratio of 700 g of graft concentrate per 1500 g of 
polypropylene (AMOCO 10-5219). The dry blend was then melt blended by 
passing it through a 30 mm twin screw extruder (Werner & Pfleiderer Model 
ZSK 30) at 180.degree. C. The let down mixture was found to contain 12.9% 
bromine and 0.25% residual monomer. 
EXAMPLE 11 
The let down mixture from Example 10 was molded into test specimens using 
the procedures and conditions shown in Example 9. 
EXAMPLE 12 
COMATIVE EXAMPLE 
390 g of polypropylene (AMOCO 10-5219) were dry blended with 11 g of 
polydibromostyrene homopolymer containing 58.5% bromine. The mixture was 
melt blended as in Example 10 to obtain a composition containing 12.9% 
bromine. 
EXAMPLE 13 
COMATIVE EXAMPLE 
The composition from Comparative Example 12 was molded into test specimens 
using the procedures and conditions shown in Example 9. 
EXAMPLE 14 
COMATIVE EXAMPLE 
Unmodified polypropylene (AMOCO 10-5219) was molded into test specimens 
using the Procedures and conditions shown in Example 9. 
EXAMPLE 15 
The composition from Example 7 was molded into test specimens using the 
procedures and conditions shown in Example 9. 
EXAMPLE 16 
Injection molded test specimens from Example 11 were maintained at 
75.degree. C. to 80.degree. C. for 53 days in a gravity oven. Surfaces 
remained perfectly glossy with no trace of bleed, thus demonstrating the 
ability to achieve low levels of residual monomer in preferred 
compositions and by preferred methods of their preparation. 
EXAMPLE 17 
9.8 g of a molded specimen from Example 11 were dissolved in 279 g of 
boiling xylenes (Mallinckrodt #8664). The warm solution was added dropwise 
to 2 liters of vigorously stirring methanol. The precipitated polymer was 
removed by filtration and dried. Yield was 100%. A portion of the polymer 
(4.9 g)--now having greatly increased surface area--was treated with 
approximately 500 ml of methylene chloride in a Soxhlet Extractor for 6 
hours. The methylene chloride solution was evaporated to dryness to obtain 
0.94 g of solid which was found to contain 44.53% bromine. The polymer 
after extraction still contained 4.32% bromine, or 36% of the bromine 
present before extraction, thus evidencing in a conventionally accepted 
manner that the graft copolymerization product is not a simple blend of 
homopolymers but rather a new graft copolymer composition of matter which 
has been formed. 
EXAMPLE 18 
COMATIVE EXAMPLE 
9.7 g of molded specimens from Comparative Example 13 were dissolved, 
precipitated and extracted as described in Example 17. 0.749 g of extract 
were recovered which contained 52.1% bromine. In contrast with the results 
of Example 17, the bromine content of the polymer after extraction was 
"none detected" with a detection limit of 0.2%. Before extraction (but 
after precipitation) it contained 12.19% bromine. 
EXAMPLE 19 
84.6 g of graft concentrate from Example 2 and 5.4 g Atactic Polypropylene 
(diluent) were blended in a Brabender Plasticorder for 10 minutes at 
190.degree. C. A portion of the mixture was put into a test tube and 
placed in an oven at 190.degree.-200.degree. C. for 7 hours. The mixture 
was cooled, the glass broken away, and samples were taken from near the 
top and bottom. Bromine content near the top was 36.5%, and near the 
bottom it was 37.2%. Based on the materials added, the bromine content 
should have been 38.1%. Thus, the blend of this Example demonstrated 
excellent ability to remain substantially homogenous under melt 
conditions. 
EXAMPLE 20 
COMATIVE EXAMPLE 
The following were blended as in Example 19: 
21.6 g Polypropylene (AMOCO 10-5219) 
63.0 g Polydibromostyrene Homopolymer 
5.4 g Atactic Polypropylene (diluent) 
Part of the mixture was put into a test tube, heated and sampled as in 
Example 19. In contrast to the results in Example 19, bromine content near 
the top was 12.8% while it was 48.6% near the bottom. Calculated bromine 
content was 40.9% based on the materials charged. 
EXAMPLE 21 
Known graft polymerization procedures are used to graft Great Lakes 
Dibromostyrene to polypropylene in amounts whereby the graft 
polymerization products comprise about 1% to about 60% by weight bromine. 
EXAMPLE 22 
The graft polymerization products of Example 21 having about 10% to about 
60% by weight bromine are blended with polypropylene using known 
techniques to achieve bromine levels in the final blend ranging from about 
1% to about 20% by weight bromine. 
EXAMPLE 23 
Physical properties were determined for several of the molded compositions. 
Results are shown in Table 2. 
TABLE 2 
______________________________________ 
Physical Property Test Results 
Compar. Compar. 
Ex. 14 Ex. 9 Ex. 11 Ex. 13 
______________________________________ 
Flex. Strength, psi 
6600 8290 7570 8000 
Flex. Mod. psi .times. 10.sup.5 
2.13 2.81 2.64 2.95 
Tensile Strength, psi 
5250 5660 5530 5300 
Elong. @Peak, % 
8.4 6.8 7.1 3.6 
Tensile Mod., psi .times. 105 
2.24 2.73 2.59 2.84 
Izod Impact (Unnotched), 
21.7 5.9 17.9 8.5 
ft. lbs./in. 
______________________________________ 
As Table 2 demonstrates, the preferred compositions retain desirable 
physical properties. For instance, the preferred compositions prepared in 
Examples 9 and 11 demonstrate superior tensile strengths as compared to 
the unmodified polypropylene of Example 14 as well as the 
polypropylene/polydibromostyrene blend of Example 13. Additionally, the 
preferred compositions avoid the excessive stiffening which occurred in 
the blend of Example 13 as evidenced by their respective elongation 
percent values. In another aspect the blend of Example 11 in particular 
proved to be surprisingly tough and durable as measured by impact 
strength. 
EXAMPLE 24 
Molded specimens were tested for flammability using the Underwriters 
Laboratories Standard UL-94 and the ASTM D 2863 Oxygen Index Test. Results 
are shown in Table 3: 
TABLE 3 
______________________________________ 
Flammability Test Results 
Comp. Comp 
Ex. 14 
Ex. 9 Ex. 15 Ex. 11 
Ex. 13 
______________________________________ 
Bromine Content, % 
0.0 8.4 9.1 12.9 12.9 
UL-94, 1/16 inch 
Fail Fail 94V-2 94V-2 Fail 
Oxygen Index, %02 
19.0 24.0 24.5 25.0 22.5 
______________________________________ 
As illustrated in Table 3, the let down as well as other preferred 
compositions demonstrate surprising and superior efficiency in resistance 
to flame. Specifically, for example, despite having bromine levels equal 
to or even less than the polypropylene/polydibromostyrene blend of Example 
13, preferred compositions prepared in Examples 9, 11 and 15 showed 
unexpectedly greater oxygen index values. These results combined with the 
results set forth in Table 2 demonstrate the magnitude of the applicants 
discovery, which addresses head on the long felt desire and need for 
improved fire retardant polypropylene compositions. 
While the invention has been described in detail in the foregoing 
description and its specific Examples, the same is to be considered as 
illustrative and not restrictive in character, it being understood that 
only the preferred embodiments have been described and that all changes 
and modifications that come within the spirit of the invention are desired 
to be protected.