Reaction injection molding compositions and process

Replacement of one-third to two-thirds of the short chain diols in a polyisocyanate/polymeric polyol/short chain diol molding composition with a hydroxyalkyl-melamine provides shaped articles of increased high temperature stiffness and reduced heat sag. The hydroxyalkylmelamine is strongly catalytic for the isocyanatepolyol reaction.

This invention relates to novel compositions useful in injection molding 
techniques involving reaction of the ingredients. More particularly, this 
invention relates to such compositions comprising a multi-functional 
isocyanate, a polyol extender, and a hydroxyalkyl melamine. 
A major use of urethane chemistry is to produce elastomers by reaction 
injection molding techniques. These elastomers are conveniently obtained 
by reacting a liquid diisocyanate stream with a liquid polyol stream in a 
heated mold under moderate pressure and in the presence of a catalyst. The 
resulting shaped article exhibits desirable properties for many 
applications. 
In many applications, it is desirable that the molded article possess low 
heat distortion and high flexural modulus at high temperatures. The heat 
distortion temperature and flexural modulus of the conventional 
diisocyanate-polyol elastomers obtained by reaction injection molding are 
not as high as is desired for many applications and improvements in these 
properties have long been desired. 
Previous attempts to obtain good stiffness at high temperature involved the 
addition of chopped glass fibers to the molding compositions as 
reinforcement. This requirement for reinforcing agent adds to the 
complexity of the formulating and molding operations and is undesirable. 
In addition, use of organo-metallic catalysts to promote the reaction of 
diisocyanate with polyols and diols can result in migration of the 
catalyst to the surface of the molded object where it creates odor and 
toxicity problems. 
Accordingly, it would be desirable to provide a polyurethane elastomer of 
good stiffness at high temperatures without the need for added reinforcing 
agent and, without the need for potentially toxic catalysts to promote the 
reaction during injection molding. The provision for such elastomers would 
fulfill a long-felt need and constitute a notable advance in the art. 
In accordance with the present invention, there is provided a reaction 
injection molding composition comprising a polyisocyanate, a polymeric 
polyol, a short chain diol and a hydroxyalkylmelamine, the total hydroxyl 
content of the polymeric polyol, the short chain diol, and the 
hydroxyalkylmelamine being about equal to the stoichiometric requirements 
of said isocyanate, up to an isocyanate index of about 300 the polymeric 
polyol comprising the major weight proportion of the combination of 
polymeric polyol, short chain diol and hydroxyalkylmelamine, and said 
hydroxy-alkylmelamine comprising from about one-half to about twice the 
weight content of short chain diol. 
By polymeric polyol is meant any long-chain hydroxy terminated resin which 
in the finished polyurethane constitutes the soft-segment portion of the 
polymer as is understood by those skilled in the art. By short chain diol 
is meant the chain extender part of the polyurethane formulation which 
through reaction with the isocyanate forms the hard-segment portion of the 
final polymer. It is further understood that reaction injection molding 
composition as understood herein may also contain a minor proportion of 
other multifunctional polyols other than short-chain diols such as 
trimethylolpropane and the like. 
There is also provided a process for manufacturing a shaped article which 
comprises preparing a reactive mixture of a diisocyanate, a polymeric 
polyol, a short chain diol and a hydroxyalkyl melamine wherein the total 
hydroxyl content of the polymeric polyol, the short chain diol and the 
hydroxyalkylmelamine is about equal to the stoichiometric requirements of 
the diisocyanate, the polymeric polyol comprises the major weight portion 
of the combination of polymeric polyol, short chain diol and 
hydroxyalkylmelamine, and the hydroxyalkylmelamine comprises from about 
one-half to about twice the weight content of said short chain diol, 
shaping and reacting said mixture in a heated mold under pressure, and, if 
desired, post-curing the shaped article. 
Use of the hydroxyalkylmelamine in place of a portion of the short chain 
diol normally employed in the molding composition results in a shaped 
article having increased stiffness (flexural modulus) at high temperature 
and reduced heat sag. The use of the hydroxyalkylmelamine eliminates the 
need for an added catalyst and provides very fast cure times which are 
highly desirable for reaction injection molding. 
In carrying out the present invention, the components used to prepare the 
reaction injection molding composition are the same as those 
conventionally employed except for the provision of the 
hydroxyalkylmelamine. Thus, the useful polyisocyanates, polymeric polyols, 
and short chain diols are well known to those skilled in the art and do 
not require further detailed description herein. Conventional compositions 
used in the art comprise a polyisocyanate and a combination of polymeric 
polyol and short chain diol, the total hydroxyl content of which is about 
equal to that of the polyisocyanate up to an isocyanate index of about 
300. The conventional composition will also comprise a catalyst, such as 
an organo-metallic compound. The resulting composition will contain a 
major proportion of polymeric polyol as the hydroxyl-containing component. 
It is further understood that by conventional composition is meant a 
reaction injection moldable composition which through choice of catalyst 
and isocyanate to hydroxyl ratio will produce a cured article containing 
isocyanurate groups as well as polyurethane hard segments. Thus, although 
compositions in which the isocyanate to hydroxyl ratio is about one are 
more commonly employed, it is well known that increasing this ratio can be 
beneficial in some cases and this is considered within the scope of this 
invention. 
In accordance with the present invention, part of the normal content of 
short chain diol is replaced by a hydroxyalkylmelamine, the incorporation 
of which usually eliminates the need for any added catlyst. Generally, the 
hydroxyalkylmelamine will replace from about one-third to two-thirds of 
the amount of short chain diol normally employed. Stated alternatively, 
the hydroxyalkylmelamine will comprise from about one-half to about twice 
the weight content of short chain diol conventionally employed. 
Suitable hydroxyalkylmelamines have the general formula: 
##STR1## 
wherein R.sub.1 is C.sub.1 -C.sub.8 linear or branched chain alkyl, aryl, 
NH.sub.2, or equal to R.sub.2 where R.sub.2 is selected from NH.sub.2, 
NHCH.sub.2 CH.sub.2 OH, 
##STR2## 
NHCH.sub.2 CH.sub.2 CH.sub.2 OH, and NHCH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 
OH and at least one of said R.sub.2 's is a hydroxyalkylamine group. 
Preferably both R.sub.2 groups are hydroxyalkylamine groups and R.sub.1 is 
NH.sub.2 or a hydroxyalkylamine group. These compounds are generally in 
the form of mixtures in which the individual species vary as to the number 
of hydroxyalkylamine groups they contain. For convenience, the hydroxyl 
number of the hydroxyalkylmelamine composition is determined and this 
number indicates the nature of the mixture. The hydroxyl number is the 
number of milligrams of potassium hydroxide equivalent to the hydroxyl 
content of 1.0 gram of polyol. Preferred mixtures generally contain on the 
average about two to three hydroxyalkyl substituents per triazine ring. 
In preparing the molding composition, the polymeric polyol, the short chain 
diol and the hydroxyalkylmelamine are thoroughly mixed and degassed. The 
polyisocyanate is then added with high-speed stirring and the resulting 
mixture is immediately poured in a pre-heated mold and placed in a press 
at elevated temperature under suitable pressure. Curing of the composition 
in the mold and subsequent post-curing will be in accordance with 
conventional procedures. 
In addition to the unexpected benefits in stiffness at high temperatures of 
the molded compositions and in the elimination of the need for added 
catalyst, the hydroxyalkylmelamine can also reduce the flammability of the 
molded composition.

The invention is more fully illustrated in the examples which follow 
wherein all parts and percentages are be weight. 
COMATIVE-EXAMPLE 
A molding composition of the following amounts of ingredients is prepared: 
______________________________________ 
Material Parts 
______________________________________ 
Modified diphenylmethane diisocyanate.sup.1 
98 
Polymer polyol.sup.2 70 
Short chain diols.sup.3 30 
Dibutyltin dilaurate.sup.4 
2.4 
______________________________________ 
Notes: 
.sup.1 Isonate 191, Upjohn, isocyanate index 105, 
.sup.2 Niax 3128, Union Carbide 
.sup.3 Niax 1180, Union Carbide 
.sup.4 T12, M and T Chemicals 
All components except the diisocyanate are thoroughly mixed and degassed. 
The diisocyanate is added with high-speed stirring (10-20 seconds at 5,000 
rpm). The mixture is immediately poured into a pre-heated mold and the 
mold is placed in a press under 15,000 psi for 5 minutes at 100.degree. C. 
The molded samples are removed from the mold and post-cured at 120.degree. 
C. for 1 hour. The samples are conditioned for one week at room 
temperature and 50% relative humidity prior to testing. Properties of the 
resulting elastomer are given in TABLE I. 
Example 1 
Following the procedure of the comparative example, a molded elastomer is 
prepared from the following composition: 
______________________________________ 
Material Parts 
______________________________________ 
Modified diphenylmethane diisocyanate.sup.1 
72.4 
Polymer diol.sup.2 70 
Hydroxypropylmelamine.sup.5 
15 
Short chain diols.sup.3 15 
______________________________________ 
Notes: 
.sup.1, .sup.2, and .sup.3, see comparative example 
.sup.5, hydroxyl number = 570 
Properties of the resulting elastomer are also given in TABLE I. 
TABLE I 
______________________________________ 
Comparison of Conventional and Melamine Polyol Elastomers 
Comparative 
Example Example 1 
______________________________________ 
Processing 
Pot Life, sec. 65 15 
Cure Time, min/.degree.C. 
5/100 5/100 
Post Cure, min/.degree.C. 
60/120 60/120 
Physical Properties 
Shore Hardness.sup.1 
A 98 93 
D 82 75 
Tensile Strength.sup.2, psi 
6551 5684 
Elongation.sup.2, % 
30 45 
Flexural Modulus.sup.3 psi- 
20.degree. F. 317,100 264,500 
70.degree. F. 215,800 149,000 
158.degree. F. 53,200 97,700 
Flexural Ratio-20/158 
6.0 2.7 
Heat Sag.sup.4, in. 
0.6 0.3 
______________________________________ 
Notes: 
.sup.1 ASTM D2240-75 
.sup.2 ASTM D412-75 
.sup.3 ASTM D790-71 (Method I) 
.sup.4 The test specimen (1 in .times. 5 in) is set in a 4 in hang at 
250.degree. F. for 1 hour and the decline distance is measured. 
The data show that the use of hydroxyalkylmelamine almost doubles the high 
temperature stiffness of the elastomer and also greatly reduces the heat 
sag. 
EXAMPLES 2-4 
Molded elastomers were prepared according to the following compositions: 
______________________________________ 
Parts 
Material 2 3 4 
______________________________________ 
Modified diphenylmethane 
96 82 71 
diisocyanate.sup.1 
Polyol.sup.2 70 70 70 
Hydroxypropylmelamine 
5 10 15 
1,4 Butanediol 25 20 15 
Dibutyltin dilaurate.sup.3 
0.04 0.02 -- 
______________________________________ 
Notes: 
.sup.1 Isonate 191, Upjohn isocyanate index 115 
.sup.2 Pluracol 380, BASF 
.sup.3 T12, M & T Chemicals 
Properties of the resulting elastomers are given in TABLE II. 
TABLE II 
______________________________________ 
Melamine Polyol Elastomers 
2 3 4 
______________________________________ 
Processing 
Mixing Temp, .degree.C. 
23 23 23 
Pot Life, sec 10 7 5 
Cured Mold, min/.degree.C. 
10/120 60/120 60/120 
Physical Properties 
Shore Hardness.sup.1 
A 95 96 96 
D 60 62 63 
Tensile Strength.sup.2, psi 
2577 3072 3280 
Elongation, % 62 49 28 
Flexural Modulus.sup.3, psi- 
20.degree. F. 316,800 156,600 376,500 
70.degree. F. 304,800 101,800 337,900 
160.degree. F. 127,600 73,300 210,400 
Flexural Modulus Ratio, 
2.5 2.0 1.6 
20/160 
Heat Sag, in 1.6 1.1 0.8 
______________________________________ 
Notes: 
.sup.1 ASTM D2240-75; 
.sup.2 ASTM D412-75 
.sup.3 ASTM D790-71 (Method 1) 
.sup.4 Copy from Table I 
The data show the beneficial effect of hydroxyalkyl melamine on improving 
the stiffness retention at high temperature, reducing heat sag, and also 
demonstrate the catalytic effect of hydroxyalkylmelamine when used to 
replace part of all of the conventional tin catalyst. 
EXAMPLE 5 
A molded elastomer is prepared according to Example 1 except that the 
amount of diisocyanate is adjusted to give an isocyanate index of 150. The 
resulting elastomer has a heat sag value of 1.7 inches compared to a value 
of 2-8 for a control containing no hydroxypropylmelamine and twice the 
amount of short chain diol. The modulus ratio is similarly reduced from 
5.5 to 5.1 with the addition of hydroxypropylmelamine.