High char yield silazane-modified phenolic resins

The hydrolytic polycondensation of a monomer of ##STR1## wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are independently alkyl groups containing from 1 to 4 carbon atoms, R.sup.a, R.sup.b, R.sup.c, and R.sup.d are independently alkenyl or alkyl groups containing from 2 to 6 carbon atoms with the proviso that at least one of R.sup.a, R.sup.b, R.sup.c, or R.sup.d group is an alkenyl group; can be carried out directly in the presence of a phenolic resin. This in situ polymerization reaction results in the formation of a new family of polysilazoxane-modified phenolic resins. Modified phenolic resins may also be prepared by the controlled hydrolysis of the above monomers in the presence of initiators. The polysilazoxane so formed is blended into a phenolic resin. The organic soluble thermosetting resins exhibit synergistic char yields of 63 to 80 wt. % (1500.degree. C./N.sub.2).

FIELD OF THE INVENTION 
This invention relates to thermally stable high char yield, 
polysilazoxane-modified phenolic resins. More specifically, this invention 
relates to a composition of matter that is a reaction product between a 
phenolic resin (containing residual water and phenol normally present as a 
result of its manufacture) and a silazane, as well as a phenolic resin 
(with residual water and phenol) that is modified by blending in a 
controlled hydrolysis product of a silazane monomer. 
BACKGROUND 
Phenolic resins have utility in both the uncarbonized and carbonized forms, 
the choice of forms being dependent upon the performance criteria required 
for specific applications. In the uncarbonized form, phenolic resins can 
be used, for example, as adhesives, coatings, and matrix resins. Although 
phenolic resins are typically thermally stable and resistant to chemical 
attack, most unfilled and/or unmodified phenolic resins have a maximum use 
temperature of &lt;200.degree. C. The utility of phenolic resins in the 
uncarbonized form would be extended and enhanced if the thermal stability 
of the resins were improved. In the carbonized form, phenolic resins are 
useful, for example, as prepreg and densification resins. As a prepreg 
resin, the phenolic resin, in the uncarbonized form, serves to bind 
together or laminate various articles. 
Japanese patent application No. 81,454, filed Apr. 23, 1984, relates to the 
production of beta type SiC by heating a silicon and carbon containing raw 
material in a non-oxidizing atmosphere using catalysts such as mineral 
acids. 
The present invention describes inherently oxidation resistant, high char 
resins. The resins of this invention are based on ceramic precursor 
polymers, i.e., metal-organic polymers which yield ceramic/refractory 
phases upon pyrolysis. This patent application describes silazane-modified 
phenolic resins and their utility in resin matrix composites and 
adhesives. 
SUMMARY OF THE INVENTION 
This invention is directed to a modified phenolic resin prepared by the in 
situ polymerization of a monomer of 
##STR2## 
wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are independently alkyl 
groups containing from 1 to 4 carbon atoms, R.sup.a, R.sup.b, R.sup.c, and 
R.sup.d are independently alkenyl or alkyl groups containing from 2 to 6 
carbon atoms with the proviso that at least one of R.sup.a, R.sup.b, 
R.sup.c, or R.sup.d group is an alkenyl group; with a phenolic resin.

DETAILED DESCRIPTION OF THE INVENTION 
The silazane modified phenolic resins of the present invention are prepared 
by the in situ polymerization of a cyclic silicon-nitrogen monomer with a 
phenolic resin. 
The cyclic silicon-nitrogen monomer is represented by the structure 
##STR3## 
wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are independently alkyl 
groups containing from 1 to 4 carbon atoms, and preferably 1 to 2 carbon 
atoms. R.sup.a, R.sup.b, R.sup.c, and R.sup.d are independently alkenyl or 
alkyl groups containing from 2 to 6 carbon atoms, preferably 2 to 4 carbon 
atoms, and most preferably 2 to 3 carbon atoms, with the proviso that at 
least one of R.sup.a, R.sup.b, R.sup.c, and R.sup.d is an alkenyl group. 
Within the alkenyl group the double bond may be terminal such as vinyl 
(--CH.dbd.CH.sub.2), allyl (--CH.sub.2 CH.dbd.CH.sub.2), 1-butenyl 
(--CH.sub.2 CH.sub.2 CH.dbd.CH.sub.2), 1-pentenyl (--CH.sub.2 CH.sub.2 
CH.sub.2 CH.dbd.CH.sub.2), and 1-hexenyl (--CH.sub.2 CH.sub.2 CH.sub.2 
CH.sub.2 CH.dbd.CH.sub.2). The double bond may also be internal. 
Representative examples of the latter are 2-butenyl (--CH.sub.2 
CH.dbd.CHCH.sub.3) and 3-pentenyl (--CH.sub.2 CH.dbd.CHCH.sub.2 CH.sub.3). 
Additionally, the carbon atoms may be branched such as 3-methyl-1-butenyl 
##STR4## 
2,3-dimethyl-1-butenyl 
##STR5## 
and 3,3-dimethyl-1-butenyl 
##STR6## 
In the preferred embodiment within the trimer, R.sup.1 =R.sup.2 =R.sup.3 
=methyl and R.sup.a =R.sup.b =R.sup.c =vinyl giving the structure 
##STR7## 
(1,3,5-trimethyl-1,3,5-trivinylcyclotrisilazane) abbreviated as TMTVTS. 
Phenolic resins having utility in this application are the reaction product 
of phenol and formaldehyde. The reaction product is of the novolac or 
resole formation. For a discussion of the synthesis and properties of 
phenolic resins, see "Phenolic Resins," Knop and Pilato, Springer-Verlag, 
pages 91-102 (1985), the disclosure of which is herein incorporated by 
reference. The two formations are outlined below. 
NOVOLAC FORMATION 
In the presence of acid catalysts and with the mole ratio of formaldehyde 
to phenol less than 1, the methylol derivatives condense with phenol to 
form first dihydroxydiphenylmethane: 
##STR8## 
and on further condensation and methylene bridge formation, fusible and 
soluble linear low polymers called novolacs with the structure 
##STR9## 
etc , where or and para occur at random. Molecular weights may range as 
high as 1000, corresponding to about 10 phenyl moieties. These materials 
do not themselves react further to give crosslinked resins, but must be 
reacted with more formaldehyde to raise its mole ratio to phenol above 
unity. 
RESOLE FORMATION 
In the presence of alkaline catalysts and with more formaldehyde the 
methylol phenols can condense either to methylene linkages or to ether 
linkages. In the latter case, subsequent loss of formaldehyde may occur 
with methylene bridge formation. 
##STR10## 
Products of this type, soluble and fusible but containing alcohol groups, 
are called resoles. If the reactions leading to their formation are 
carried further, large numbers of phenolic nuclei can condense to give 
network formation. 
The formation of resoles and novolacs respectively leads to the production 
of phenolic resins by one-stage and two-stage processes. 
ONE-STAGE RESIN 
In a production of a one-stage phenolic resin, all the necessary reactants 
for the final polymer (phenol, formaldehyde and catalyst) are charged into 
a resin kettle and reacted together. The ratio of formaldehyde to phenol 
is about 1.25:1, and an alkaline catalyst is used. 
##STR11## 
TWO-STAGE RESIN 
These resins are made with an acid catalyst and only part of the necessary 
formaldehyde is added to the kettle producing a mole ratio of 
approximately 0.8:1. The rest is added later as hexamethylenetetramine 
(HMTA) 
##STR12## 
which decomposes in the final curing step with heat and moisture present 
to yield formaldehyde and ammonia which acts as a catalyst for curing. 
RESIN FORMATION 
The procedures for one and two-stage resins are similar and the same 
equipment is used for both. The reaction is exothermic and cooling is 
required. A formation of a resole or a novolac is evidenced by an increase 
in viscosity. Water is then driven off under vacuum and a thermoplastic 
A-stage resin, soluble in organic solvents, remains. This material is 
dumped from the kettle, cooled, and ground to a fine powder. 
At this point, fillers, colorants, lubricants, and (if a two-stage resin) 
enough hexamethylenetetramine to give a final formaldehyde:phenol mole 
ratio of 1.5:1 are added. The mixture is rolled on heated mixing rolls 
where the reactions are carried further, to the point where the resin is 
in the B-stage, nearly insoluble in organic solvents, but still fusible 
under heat and pressure. The resin is then cooled and cut into final form. 
The C-stage, the final, infusible, crosslinked polymer is reached on 
subsequent fabrication, for example by molding. Numerous other types of 
phenolic resins known to the art and in the literature may also be 
employed in the practice of this invention. 
Commercially available phenolic resins generally contain both residual 
water (usually 1-20, preferably 1-10 and most preferably 1-5 wt. %) and 
residual phenol (normally about 10-20 wt. %). Borden's SC-1008 phenolic 
resin, for example, normally contains about 1 to 3 wt. % water and about 
15 wt. % phenol in addition to about 25 wt. % isopropyl alcohol and about 
1 wt. % formaldehyde. These residuals, specifically water and phenol, 
enter into the synthesis of a new family of thermosetting, preceramic 
polymers. 
The hydrolytic polycondensation reaction involving silazane, as described 
in my said copending application Ser. No. 07/447,931, filed on 12/8, 1989, 
can be carried out directly in the presence of the phenolic resin. The 
silazane readily reacts with the residual water and phenol in a phenolic 
resin. Thus, upon mixing various wt/wt ratios of a silazane with a 
phenolic resin at room temperature, an initial, mild exotherm (40.degree. 
C.) is observed, followed by a stronger exotherm (60.degree. C.) several 
seconds later. Stirring, either mechanical or by hand, is maintained 
throughout the reaction. During the reaction, which is complete within 
minutes at ambient temperature, foaming occurs and ammonia is evolved. The 
resulting polysilazoxane-modified phenolic resin is essentially identical 
in appearance (color and viscosity) to the unmodified phenolic resin. And 
most importantly, with the proper adjustment of the reactant ratio, the 
polysilazoxane-modified phenolic resin retains the processability (i.e., 
B-staging and thermosetting characteristics) of the unmodified phenolic 
resin. 
The order of addition at room temperature has an impact on shelf life. A 
longer shelf life is obtained for the modified phenolic resin when the 
silazane monomer is added to the phenolic resin at room temperature. 
However, order of addition has no effect on char values. 
The reaction of the monomer and phenolic resin is generally carried out at 
a temperature of from ambient up to about 125.degree. C. and preferably 
from ambient up to about 100.degree. C. It is important to remain below 
125.degree. C. in order to avoid premature B-staging, i.e., partial cure. 
The weight ratio of monomer:phenolic resin is typically from 4:1 to 1:4, 
preferably from 3:1 to 1:3.2, and most preferably from 2:1 to 1:3.2. 
This invention also contemplates a composition of a blend of polysilazoxane 
prepared by the hydrolytic polymerization of a monomer of 
##STR13## 
wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are independently alkyl 
groups containing from 1 to 4 carbon atoms, R.sup.a, R.sup.b, R.sup.c, and 
R.sup.d are independently alkenyl or alkyl groups containing from 2 to 6 
carbon atoms with the proviso that at least one of R.sup.a, R.sup.b, 
R.sup.c, or R.sup.d group is an alkenyl group; with a phenolic resin. The 
various substituents on the monomers are as earlier disclosed. 
Hydrolyzing trimethyltrivinylcyclotrisilazane (TMTVTS) provides an organic 
soluble polymethylvinylsilazoxane (PMVS). A controlled hydrolysis of the 
monomer is carried out in the presence of initiators, specifically a dual 
initiator system or optionally in the presence of water alone. The 
controlled hydrolysis of the monomer can be effected by utilizing a dual 
initiator system of an aqueous persulfate/thiosulfate catalyst system. The 
Group I persulfate/Group I thiosulfate initiators are present in a mole 
ratio of from 1:0.85 to 0.85:1, preferably 1:0.95 to 0.95:1, and most 
preferably from 1:1. 
The controlled hydrolysis of the monomer can be effected by utilizing a 
dual initiator system of an aqueous Group I metal persulfate/Group I metal 
thiosulfate catalyst system. Group I metals having utility as the 
persulfate/thiosulfate salts are lithium, sodium and potassium. In the 
practice of this embodiment, the initiator system may be potassium 
persulfate with potassium thiosulfate, sodium persulfate with sodium 
thiosulfate, potassium persulfate with sodium thiosulfate or sodium 
persulfate with potassium thiosulfate. Lithium may be substituted with 
sodium or potassium or may be used in addition to sodium and/or potassium. 
The dual initiator system takes its name from the fact that there is at 
least one persulfate and at least one thiosulfate. In practicing this 
embodiment, a persulfate salt mixture of lithium, sodium, and potassium 
may be utilized along with a thiosulfate salt mixture of lithium, sodium 
and potassium. 
In order to effect the controlled hydrolysis, the water:initiator mole 
ratio is from 150:1 to 400:1, preferably 200:1 to 300:1, and most 
preferably 200:1 to 250:1, and the water:monomer mole ratio is from 1.5:1 
to 4:1, preferably 2:1 to 3:1, and most preferably 2:1 to 2.5:1. 
In order for the polymerization to begin, heat is applied. Generally the 
reaction temperature is from about 100.degree. C. to about 190.degree. C. 
and preferably from about 130.degree. C. to about 150.degree. C. Ammonia 
is evolved during the reaction. Generally, temperatures in excess of those 
noted are avoided inasmuch as they tend to produce an intractable product. 
Inasmuch as the polymerization is only slightly exothermic, heat must be 
applied thereto. The polymerization is generally a hydrolytic condensation 
polymerization with some ring opening occurring. The number average 
molecular weight (M.sub.n) of the formed polymer is at least about 3,000 
and desirably from about 10,000 to about 100,000. Generally, it appears 
that the length of time of polymerization and the water:monomer mole ratio 
is directly proportional to the amount of siloxane units produced. The 
amount of siloxane in the end polymer is generally from about 65 percent 
to about 98 percent, preferably from 80 percent to about 98 percent, and 
most preferably from 85 percent to about 98 percent by weight. Some 
silazane groups, within the polymer structure, are always desirable 
inasmuch as they are easily charred and yield better char properties. 
On the basis of IR spectroscopy, NMR and elemental analysis, the PMVS 
produced from the hydrolysis of TMTVTS is believed to have the following 
formulation (Formula I): 
EQU --[Si(CH.sub.3) (CH.dbd.CH.sub.2)0--].sub.x --[Si(CH.sub.3) 
(CH.dbd.CH.sub.2)NH--].sub.y (I) 
The values of x (siloxane content) and y (silazane content) in the above 
formulation will, of course, vary somewhat as a function of the extent of 
hydrolysis. The x and y components together make up the polysilazoxane. 
The above structure is not meant to imply that the polysilazoxane is 
exclusively linear. Some cyclic and branched moieties may also be present 
within the polysilazoxane. Moreover, the polysilazoxane is a random 
copolymer containing siloxane and silazane moieties. 
The preceramic polymers described in this invention can be used as high 
temperature resistant phenolic resins and can be used in the manufacture 
of ablative compositions, high temperature adhesives and flame retardant 
materials. 
The following examples are illustrative of the preparation of the 
polysilazoxane polymers. Unless otherwise indicated, all parts and 
percentages are by weight. 
EXAMPLE 1 
TMTVTS (122.34 g, 0.48 mole) was placed in a 500 mL three-neck, round 
bottom flask equipped with thermometer, reflux condenser, blade stirrer 
and addition funnel. An aqueous persulfate/thiosulfate "catalyst" solution 
[0.38 g (0.0024 mole) Na.sub.2 S.sub.2 O.sub.3 and 0.65 g (0.0024 mole) 
K.sub.2 S.sub.2 O.sub.8 dissolved in 19.8 mL (1.1 mole) water] was then 
added drop-wise to the TMTVTS with rapid stirring under a nitrogen 
blanket. The reaction mixture was then heated at 130.degree. C. for 10 
hours/N.sub.2. During the initial stage of the reaction, it was necessary 
to use a slow rate of heating (two hours to reach 130.degree. C.) in order 
to control foaming due to ammonia evolution. The reaction mixture was then 
cooled to 22.degree. C., and the resulting pale yellow, viscous liquid was 
dissolved in 250 mL of dichloromethane; the polymer solution was then 
dried over CaSO.sub.4 for 16 hours. After filtering to remove the 
CaSO.sub.4, the polymer solution was freed of dichloromethane by rotary 
evaporation (70.degree. C., water aspirator). After vacuum drying 
(60.degree. C./3 hours), polymethylvinylsilazoxane (PMVS) was obtained as 
a pale yellow, viscous liquid; yield 107 g. PMVS is soluble in 
dichloromethane, THF and toluene. The resulting PMVS has a molecular 
weight M.sub.n =42,100 and M.sub.w =89,400. Elemental analysis (in weight 
percent): carbon--41.97, hydrogen 7.27, nitrogen--3.33, silicon-29.65, and 
oxygen (by difference)--17.78. 
EXAMPLE 2 
The procedure of Example 1 was followed except that the reaction time was 
increased from 10 hours to 16.5 hours. The resulting PMVS had a molecular 
weight M.sub.n =51,700 and M.sub.w =132,000. 
EXAMPLE 3 
The procedure of Example 1 was followed except that the reaction time was 
increased from 10 hours to 20 hours. The resulting PMVS had a molecular 
weight M.sub.n =87,000 and M.sub.w =334,000. 
EXAMPLE 4 
The procedure of Example 1 was followed except that half the level of both 
water and TMTVTS were utilized 0.55 moles and 0.24 moles, respectively, 
and the reaction time was 8.5 hours. The resulting PMVS had a molecular 
weight M.sub.n =59,500 and M.sub.w =181,500. 
PMVS/SC-1008 blends can be prepared by simply mixing (hand-stirring) 
various wt/wt ratios of the two components, followed by a B-staging 
operation (heating the mixture in air to effect a lightly crosslinked 
resin). The materials produced from these experiments (using mixtures 
containing 10-75 wt. % phenolic resin) varied from heterogeneous to 
homogenous in appearance, depending upon the PMVS content. 
These blends showed char yields significantly greater than those calculated 
on the basis of the thermal properties of the component resins. For 
example, a 3:1 wt/wt blend (SC-1008:PMVS), after B-staging in air for 30 
minutes at 150.degree. C., had a 63% char yield at 1500.degree. C./N.sub.2 
(TGA). The char yields obtained on the individual component resins, under 
identical TGA conditions, were found to be 43% and 60% respectively for 
SC-1008 and PMVS. More pronounced synergistic char yields were observed 
for blends containing higher concentrations of PMVS. For example, a 1:4 
wt/wt blend (SC-I1008:PMVS) showed a 77% char yield at 1500.degree. 
C./N.sub.2 ; compared to a calculated char yield of 57% based on the 
thermal properties of the component resins. 
PREATION OF PMVS/PHENOLIC RESIN BLENDS 
PMVS and SC-1008 phenolic resin, in various wt/wt ratios (10-75 wt. % 
phenolic resin), were blended together at 22.degree. C. (hand-mixed for 10 
minutes). These blends appear in Table I as items d and e. The resulting 
mixtures were then B-staged at 150.degree. C./30-75 minutes in an air 
circulating oven. 
PREATION OF POLYSILAZOXANE-MODIFIED PHENOLIC RESINS 
(IN-SITU-POLYMERIZATION) 
TMTVTS (1.0 g) was added in one portion to 3.2 g of SC-1008 phenolic resin 
(the SC-1008 resin, as supplied by Borden Chemical Co., contained 2-3 wt. 
% water and 14.9 wt. % phenol). Immediately upon mixing the components at 
22.degree. C., an initial exotherm was noted (temperature increased to 
40.degree. C.) followed by an additional exotherm (to 60.degree. C.) 
within a few seconds. Ammonia evolution, accompanied by foaming, also was 
observed. The reaction mixture was hand-stirred for 5 minutes, at which 
time the temperature of the system had returned to ambient. The reaction 
mixture was then heated at 75.degree. C./2 hrs/N.sub.2. (This additional 
heating was found to improve the shelf-life of the polysilazoxane-modified 
phenolic resin). The resulting polysilazoxane-modified phenolic resin was 
essentially identical in appearance (color and viscosity) to the 
unmodified SC-1008 resin. The polysilazoxane-modified resin was then 
B-staged at 125.degree.-150.degree. C./30 minutes in an air circulating 
oven. This polysilazoxane-modified phenolic resin appears in Table I as 
item f. 
Two additional polysilazoxane-modified phenolic resins were prepared as 
above wherein 1 and 2 grams of TMTVTS were added to 2 and 1 grams, 
respectively, of SC-1008 resin. The polysilazoxane-modified phenolic resin 
appears in Table I as items g and h respectively. 
This invention also contemplates the use of other additives in combination 
with the modified phenolic resins. Such additives include, for example, 
refractory materials and fibers. 
Refractory materials impart suitable oxidation resistance to the modified 
phenolic resins. Suitable refractory materials are boron carbide, boron 
nitride, boron metal, silicon carbide, and silicon nitride. Refractory 
materials, when utilized, are usually present up to about 40% by weight. 
Typically the refractory material is present from about 5 to about 30 
percent. 
Suitable fiber materials include conventional refractory fibers such as 
SiC. Normally, composites contain from 35 to 65 volume percent of a fiber 
material. 
TABLE I 
______________________________________ 
Thermal Analysis (TGA) of Silazane-Modified Phenolic Resins 
Temp. Wt. % 
B-Stage (.degree.C.) 
Char at 
Sample Conditions of Initial 
1500.degree. C./ 
Item Composition (air) wt. loss/N.sub.2 
N.sub.2 
______________________________________ 
a SC-1008.sup.a 15 150 43 
min/150.degree. C. 
b TMTVTS.sup.b -- 110 0 
c PMVS -- 150 60 
d SC-1008:PMVS 75 250 77 
(1:4 wt/wt) min/150.degree. C. 
e SC-1008:PMVS 30 200 63 
(3:1 wt/wt) min/150.degree. C. 
f SC-1008:TMTVTS 
30 175 80 
(3.2:1 wt/wt) min/150.degree. C. 
g SC-1008:TMTVTS 
40 400 80 
(2:1 wt/wt) min/125.degree. C. 
h SC-1008:TMTVTS 
20 200 68 
(1:2 WT/WT) min/125.degree. C. 
______________________________________ 
.sup.a Phenolic resin (Borden Chemical Co.) 
.sup.b Trimethyltrivinyltrisilazane (Petrarch Systems) 
As shown in Table I and FIG. 1, the thermal stability of the 
TMTVTS-modified phenolic resins is quite good. TGA experiments show onset 
of weight loss occurring in the range of 175.degree.-400.degree. C., with 
char yields of 68-80 wt. % (1500.degree. C./N.sub.2) for several 
formulations of the TMTVTS-modified phenolic resins. These modified 
phenolic resins exhibit a pronounced synergistic charring effect; the 
reactants, SC-1008 and TMTVTS, having char yields of 43 wt. % and 0 wt. %, 
respectively, under identical TGA conditions. For comparison, char yield 
data for several of the PMVS/phenolic resin blends are also shown in Table 
I. FIG. 1 depicts actual TGA profiles (N.sub.2) obtained on the following 
B-staged samples: SC-1008 phenolic resin, a 3:1 wt/wt (SC-1008: PMVS) 
blend, and a 3.2:1 wt/wt (SC-1008:TMTVTS) polysilazoxane-modified phenolic 
resin. 
While in accordance with the Patent Statutes, the best mode and preferred 
embodiment has been set forth, the scope of the invention is not limited 
thereto, but rather by the scope of the attached claims.