Flame retardant polycarbonate having improved critical thickness

Polycarbonates are provided having improved critical thickness values based on an aromatic diphenol, a halogenated dihydric phenol and an aromatic diphenol thioether. By incorporating an effective amount of the aromatic thiodiphenol based on the total diphenol content into the polymer the critical thickness and flame retardant characteristics of the polycarbonate is substantially improved.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to polycarbonate resins and more particularly to 
flame retardant polycarbonateterpolymers having improved critical 
thickness values. 
2. Description of the Prior Art 
Polycarbonates derived from reactions involving organic dihydroxy compounds 
and carbonic acid derivatives have found extensive commercial application 
because of their excellent mechanical and physical properties. These 
thermoplastic polymers are particularly suited for the manufacture of 
molded article products for which impact strength, rigidity, toughness, 
thermal and dimensional stability as well as excellent electrical 
properties are required. 
However, one deficiency of polycarbonate when used in molded articles is 
the low critical thickness values of polycarbonate polymer, which 
deficiency tends to limit wall thickness to a value below the critical 
thickness. 
It is known that polycarbonate plastics exhibit high notched Izod (ASTM 
test D-256) impact values. This value, however, is dependent upon the 
thickness of the test specimen. Typical notched Izod impact values for a 
one-eighth inch specimen are about 16 ft.-lbs. per in. These high Izod 
values result because specimens of one-eighth inch thickness are thinner 
than the critical thickness of the polymer and therefore upon impact a 
hinged or ductile break occurs. On the other hand, one-fourth inch 
specimens exhibit a clean or brittle break and give notched Izod impact 
values of only about 2.5 ft.-lbs. per in. The one-fourth inch specimens 
are said to be above the critical thickness of the polymer. "Critical 
thickness" has been defined as the thickness at which a discontinuity in 
Izod impact values occurs. In other words, it is the thickness at which a 
transition from a brittle to a ductile break or vice versa occurs. Thus a 
standard impact specimen of polycarbonate polymer thicker than the 
critical thickness exhibits brittle breaks and those thinner than the 
critical thickness exhibit hinged or ductile breaks. Further, the critical 
thickness of a polycarbonate bsed on bisphenol A with a melt flow of 3 to 
6 grams/10 minutes at 300.degree. C (ASTM D1238) has a critical thickness 
of about 225 mils. 
The critical thickness problem is further complicated when the 
polycarbonate article is to meet a specified requirement for flammability 
in applications where high temperature and/or exposure to fire may be 
encountered. Polycarbonate copolymers based on an aromatic diol and a 
halogenated diol reacted with a carbonic acid derivative are accepted as 
effective fire retardant polymers. These polymers exhibit generally 
acceptable physical properties along with complying with flammability 
requirements. However, the critical thickness of copolymers employing 
halogenated diols is very low for example about 130-140 mils with a 
polymer containing 5 to 6 percent by weight bromine in the form of a 
halogenated diol. 
Thus in accordance with the present invention a flame retardant 
polycarbonate is provided with improved critical thickness values and is 
highly transparent. 
BRIEF DESCRIPTION OF THE INVENTION 
A terpolycarbonate having the flame retardant characteristics along with 
improved critical thickness values is provided which is comprised of the 
reaction product of an aromatic diol, a halogenated dihydric phenol and a 
thiodiphenol, and a carbonic acid derivative such as phosgene or carbonyl 
bromide. 
DETAILED DESCRIPTION OF THE INVENTION 
When used herein 37 terpolycarbonate resin" means the neat resin without 
additives; "polycarbonate" means the polycarbonate resin, copolycarbonate 
resin, or terpolycarbonate resin with additives therein. "Aromatic diol" 
means an aromatic diol having no halogen or sulfur therein and primarily 
having only carbon, hydrogen and oxygen molecules. "Aromatic hydroxy 
compound" means any or all compounds which are aromatic diols, halogenated 
aromatic diols or thiodiphenols. 
The terpolycarbonate resins of the invention may be prepared by 
conventional methods of preparation for polycarbonate resins and may have 
a weight average molecular weight of 10,000 to 200,000 and preferably a 
melt flow rate of 1 to 24 gram/10 min at 300.degree. C. ASTM (1238). 
Any suitble processes, reactants, catalysts, solvents, conditions and the 
like for the production of the terpolycarbonate resins of this invention 
which are customarily employed in polycarbonate resin syntheses may be 
used such as disclosed in German Patent Nos. 1,046,311 and 962,274, and 
U.S. Pat. Nos. 3,248,414; 3,153,008; 3,215,668; 3,187,065; 3,028,365; 
2,999,846; 2,964,974; 2,970,137; 2,991,273; and 2,999,835 all incorporated 
herein by reference. The preferred process is the interfacial 
polycondensation process. 
According to the interfacial polycondensation process copolycarbonate 
resins are obtained by reacting the aromatic dihydroxy compounds with an 
alkali metal hydroxide or alkaline earth metal oxide or hydroxide to form 
the salt of the hydroxy compounds. The salt mixture is present in an 
aqueous solution or suspension and is reacted with phosgene, carbonyl 
bromide, or bischloroformic esters of the aromatic dihydroxy compounds. An 
organic solvent is provided in the rection admixture which is a solvent 
for the polymer but not for the aromatic dihydroxy salts. Thus chlorinated 
aliphatic hydrocarbons or chlorinated aromatic hydrocarbons are used as 
the organic solvent which dissolves the condensation product. In order to 
limit the molecular weight one may use monofunctional reactants such as 
monophenols, for example the propyl-, isopropyl- and butyl-phenols, 
especially p-tert.-butyl-phenol and phenol itself. In order to accelerate 
the reaction, catalysts such as tertiary amines, quaternary ammonium, 
phosphonium or arsonium salts and the like may be used. The reaction 
temperature should be about -20.degree. to +150.degree. C., preferably 
0.degree. C to about 100.degree. C. 
According to the polycondensation process in a homogeneous phase, the 
dissolved reaction components are polycondensed in an inert solvent in the 
presence of an equivalent amount of a tertiary amine base required for 
absorption of the generated HCl, such as e.g. N,N-dimethyl-aniline, 
N,N-dimethyl-cyclohexylamine or preferably pyridine and the like. In still 
another process, a diaryl carbonate can be transesterified with the 
aromatic dihydroxy compounds to form the polycarbonate resin. 
It is to be understood that it is possible to combine in the processes 
described above in a chemically meaningful way both the aromatic dihydroxy 
compounds, and the monohydroxycompounds in the form of the alkali metal 
salts and/or bis-haloformic acid esters, and the amount of phosgene or 
carbonyl bromide then still required in order to obtain high molecular 
products. Other methods of synthesis in forming the polycarbonates of the 
invention such as disclosed in U.S. Pat. No. 3,912,688 incorporated herein 
by reference, may be used. 
The aromatic diphenols useful in the practice of the invention are those 
represented by the structural formula 
##STR1## 
wherein Y is a single bond, an alkylene or alkylidene radical with 1 to 7 
carbon atoms, a cycloalkylene or cycloalkylidene radical with 5 to 12 
carbon atoms, --O--, --CO--, --SO--, or --SO.sub.2 --, preferably 
methylene or isopropylidene. 
Suitable aromatic diphenols are for example 
(4,4'-dihydroxy-diphenyl)-methane, 2,2'-dihydroxy-diphenyl)-propane, 
1,1-(4,4'-dihydroxy-diphenyl)-cyclohexane, 
1,1-(4,4'-dihydroxy-3,3'-dimethyl-diphenyl)-cyclohexane, 
1,1-(2,2'-dihydroxy-4,4'-dimethyl-diphenyl)-butane, 
2,2-(2,2'-dihydroxy-4,4'-di-tert.-butyl-diphenyl)-propane or 
1,1'-(4,4'-dihydroxy-diphenyl)-1-phenyl-ethane, furthermore methane 
derivatives which carry besides two hydroxyaryl groups an alkyl residue 
with at least two carbon atoms and a second alkyl residue with one or more 
carbon atoms, such as 2,2-(4,4'-dihydroxy-diphenyl)-butane, 
2,2-(4,4'-dihydroxy-diphenyl)-pentane, 
3,3-(4,4'-dihydroxy-diphenyl)-pentane, 
2,2-(4,4'-dihydroxy-diphenyl)-hexane, 
3,3-(4,4'-dihydroxy-diphenyl)-hexane, 2,2-(4,4'-dihydroxy-diphenyl)-4-meth 
yl-pentane, 2,2-(4,4'-dihydroxy-diphenyl)-heptane, 
4,4-(4,4'-dihydroxy-diphenyl)-heptane (melting point 
148.degree.-149.degree. C) or 2,2-(4,4'-dihydroxy-diphenyl)-tri-decane. 
Suitable di-(monohydroxyaryl)-alkanes, the two aryl residues of which are 
different are, for example, 
2,2-(4,4'-dihydroxy-3'-methyl-diphenyl)-propane and 
2,2-(4,4'-dihydroxy-3-methyl-3'-isopropyl-diphenyl)-butane. Suitable 
di-(monohydroxyaryl)-alkanes, the alkyl residue of which, linking the two 
benzene rings, is substituted by an aryl residue are for instance 
(4,4'-dihydroxy-diphenyl)-phenyl-methane and 
1,1-(4,4'-dihydroxy-diphenyl)-1-phenyl-ethane. 
Suitable dihydroxybenzenes and substituted dihydroxybenzene are 
hydroquinone, resorcinol, pyrocatecol, methyl hydroquinone and the like. 
Other suitable dihydroxyaromatic compounds are 4,4'-dihydroxy-diphenylene, 
2,2'-dihydroxy-diphenylene, dihydroxynaphthalene and dihydroxyanthracene. 
The halogenated phenolic diols are any suitable bis-hydroxyaryl components 
such as for example the halogen containing bisphenols such as 
2,2-(3,5,3',5'-tetrachloro-4,4'-dihydroxydiphenyl)-propane, 
2,2-(3,5,3',5'-tetrabromo-4,4'-dihydroxydiphenyl)-propane; 
2,2-(3,3-dichloro-4,4'-dihydroxydiphenyl)-propane; 
2,2-(3,5-dichloro-4,4'-dihydroxydiphenyl) propane; 
2,2-(3,3'-dichloro-5,5'-dimethyl-4,4'-dihydroxydiphenyl)-propane; 
2,2-(3,3'-dibromo-4,4'-dihydroxydiphenyl) propane and the like and are 
represented by the structural formula : 
##STR2## 
wherein Z is a single bond, an alkylene or alkylidene radical with 1 to 7 
carbon atoms, a cycloalkylene or cycloalkylidene radical with 5 to 12 
carbom atoms, 'O--, --CO--, --SO--, or --SO.sub.2 --, preferably 
methylene, isopropylidene or --So.sub.2 --, and X is a halogen, preferably 
chlorine or bromine, most preferably bromine and n is 1 to 4. 
These halogenated diols are incorporated into the polycarbonate at levels 
sufficient to impart flame retardant characteristics. For example, a 
halogen content of about 3 to 10 percent by weight of the polymer is 
normally sufficient. 
The thiodiphenols useful in the practice of the invention are those 
represented by the structural formula 
##STR3## 
wherein R.sub.1 and R.sub.2 are hydrogen or lower alkyl selected from the 
group consisting of methyl, ethyl, propyl and butyl; and n is equal to 
from 0 to 2. 
As is well known the halogenated dihydric phenols hereinbefore discussed 
are known to impart flame retardant characteristics to polycarbonates 
which are synthesized therefrom. Further, it is known that sulfur 
containing bisphenols when used in combination with the halogenated 
bisphenols to synthesize polycarbonates produce a flame retardant product 
which is superior to polycarbonates based on sulfur containing bisphenols 
or halogen containing bisphenols alone. The sulfur containing 
polycarbonates taught by the prior art to produce these synergistic 
flammability characteristics are the bis-hydroxysulfones (see U.S. Pat. 
No. 3,912,688). However, the terpolycarbonates synthesized from the 
aromatic diphenol, the halogenated dihydric phenol and the 
bis-hydroxysulfones exhibit low critical thickness values. 
Surprisingly, the terpolycarbonate resins of the invention not only exhibit 
the flame retardant synergism of sulfur and halogen but also demonstrate 
critical thickness values similar to the critical thickness values 
demonstrated by polycarbonates based solely on an aromatic diol. Such an 
improvement in critical thickness is quite surprising and unexpected 
because it is known that the critical thickness of polycarbonate 
copolymers based on an aromatic diol and a halogenated dihydric phenol is 
significantly lower than the critical thickness of polycarbonates based 
solely on an aromatic diol. This difference in critical thickness is 
attributable to the detrimental effect of the halogenated dihydric phenol. 
Thus, the combination of the thiodiphenol with the aromatic diol and the 
halogenated dihydric phenol unexpectedly overcomes this detrimental effect 
of the halogenated dihydric phenol and results in polycarbonate terpolymer 
resins having critical thickness values similar to the critical thickness 
values of polycarbonates based only on an aromatic diol. 
The invention will further be illustrated but is not intended to be limited 
by the following examples.

EXAMPLE I 
An aqueous solution was prepared by simultaneously charging to a suitable 
premix vessel with agitation 67.6kg of water, 13.29kg of bisphenol A, 
0.34kg of 4,4'-thiodiphenol, 9.7kg of 50% aqueous sodium hydroxide 
solution and 0.30kg of tert-butyl phenol. 33.75kg/hour of the above 
solution was continuously phosgenated with 2.87kg of phosgene/hour in 
50.38kg of 1:1 methylene chloride:chlorobenzene solvent. 2.5kg/hour of 25% 
aqueous sodium hydroxide solution was continuously added to the reaction 
mixture to provide the proper basicity for the interfacial 
polycondensation. The phosgenated solution as obtained above was then 
reacted with 0.56kg/hour of tetrabromobisphenol A dissolved in 4.2kg of 
1:1, methylene chloride:chlorobenzene solvent. The precondensate as above 
obtained was then mixed with a solution/hour of 25g of triethylamine and 
2.8kg of 25% aqueous sodium hydroxide solution and was further reacted in 
a stirred kettle cascade over the course of an average dwell time of 30 
minutes. The organic phase was separated from the aqueous phase and the 
organic phase washed with 1% aqueous sodium hydroxide solution, separated 
and then the organic phase was washed with 1% aqueous phosphoric acid 
solution and separated. The organic phase was simultaneously washed and 
separated 3 times with water. The polymer was recovered from the organic 
phase by concentrating the polymer by solvent evaporation and subsequently 
passing the polymer through a devolatilizing extruder. The polymer as 
above prepared had the following composition. 
______________________________________ 
Compound Weight Percent 
Mole Percent 
______________________________________ 
Bisphenol A 89.63 93.14 
4,4'-thiodiphenol 
2.25 2.45 
Tetrabromobisphenol A 
10.13 4.41 
______________________________________ 
the polymer was pelletized and tested for physical, optical and rheological 
properties. The polymer was found to be highly transparent. The test 
results are reported on Table I with the test results of the following 
examples. 
EXAMPLES II-V 
Example I was repeated except that the stoichiometric amounts of bisphenol 
A, tetrabromobisphenol A and thiodiphenol were varied. Table I shows both 
the weight and mole percent of the constituents. Each of these polymers 
were found to be highly transparent. 
EXAMPLE VI 
Example VI is a comparison between thiodiphenol and sulfonyl diphenol 
demonstrating the contrasting critical thickness values. 
EXAMPLE VII 
Example VII is a control having no thiodiphenol therein. 
EXAMPLES VIII - XI 
Example I was repeated except that tetrabromobisphenol S 
(tetrabromosulphonylphenol) was substituted for tetrabromobisphenol A. 
Additionally the stoichiometric amounts of bisphenol A, 
tetrabromobisphenol S and thiodiphenol were varied. Each of these polymers 
were found to be highly transparent. The test results are reported in 
Table II. 
TABLE I 
__________________________________________________________________________ 
Monomer Composition 
of terpolymer Izod Impact* 
Critical Ul-94-1/6" Specimen 
wt % (mol %) (Notched) 
Thickness 
Melt Index 
Oxygen Index 
Thickness Average 
Example 
BPA.sup.1 
TDP.sup.2 
SDP.sup.3 
TBBPA.sup.4 
1/8" 
1/4" 
(mils) 
g/10 min 
% Burn Time, 
__________________________________________________________________________ 
Seconds) 
I 89.63 
2.25 10.3 14.46 
2.47 
136 3.1 28.5 V-0 (3.3) 
(93.14) 
( 2.45) ( 4.41) 
II 85.36 
4.51 10.13 
14.22 
2.30 
146 2.6 28.2 V-0 (2.2) 
(90.50) 
( 5.00) ( 4.5) 
III 71.5 
18.8 9.7 12.24 
3.01 
197 1.19 28.5 V-0 (3.6) 
(75.09) 
(20.64) ( 4.27) 
IV 61.5 
28.8 9.7 12.29 
5.60 
232 1.8 31.3 V-0 (1.7) 
(64.27) 
(31.48) ( 4.25) 
V 51.5 
38.9 9.6 12.95 
11.28 
&gt;255 2.5 32.2 V-0 (2.4) 
(53.53) 
(42.29) ( 4.18) 
VI 84.8 5.14 
10.06 
9.53 
2.13 
132 2.5 29.1 V-0 
(90.50) (5.00) 
( 4.5) 
VII 90 10 14.53 
2.2 130 2.9 28.0 V-2 
(95.55) ( 4.45) 
__________________________________________________________________________ 
.sup.1 BPA = Bisphenol A 
.sup.2 TDP = 4,4'-thiodiphenol 
.sup.3 SDP = Sulfonyldiphenol 
.sup.4 TBBPA = Tetrabromobisphenol A 
*Expressed in ft. lb/in. 
TABLE II 
__________________________________________________________________________ 
Monomer Composition 
of Terpolymer Izod Impact.sup.4 
Critical UL-94-1/16" Specimen 
Wt. % (notched) Thickness 
Melt Index 
Oxygen Index 
Thickness (Average 
Example 
BPA TDP.sup.2 
TBBPS.sup.3 
1/8" 1/4" (mils) 
g/10 min. 
% Burn Time, 
__________________________________________________________________________ 
Seconds) 
VII 90 -- 10 12.1 2.40 147 3.7 30.2 V-0 
IX 75 15 10 11.97 
2.97 215 2.5 31.4 V-0 
X 65 25 10 11.21 
2.81 205 4.0 34.5 V-0 
XI 55 35 10 12.06 
12.32 
255 2.2 34.1 V-0 
__________________________________________________________________________ 
.sup.1 BPA = Bisphenol A 
.sup.2 TDP = 4,4'-Thiodiphenol 
##STR4## 
.sup.4 Expressed in ft. lbs./in 
As is demonstrated by the foregoing examples the thiodiphenol based 
polycarbonates exhibit both flame retardant characteristics and improved 
critical thickness values. 
A minimum of about 4 mole percent of the thiodiphenol based upon the total 
diphenol content in the polymer is necessary in the polymer to impart 
improved critical thickness values. While there is no upper limit to the 
amount of thiodiphenol which can be incorporated into the polymer, an 
upper limit of 50 mole percent has been shown to be useful. Also the 
halogenated dihydric phenol content may be reduced while maintaining fire 
retardancy due to the synergism of the sulfur and halogen in the polymer. 
Although the invention has been described with reference to specific 
materials, the invention is only to be limited so far as is set forth in 
the accompanying claims.