Abstract:
Polyurethane molding compositions characterized in providing tough, high temperature resistant cross-linked products during the in-mold curing cycle thereby obviating the need for post curing in order to obtain potentially optimum physical strength properties.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to cross-linkable polyurethane molding compositions. 
     2. The Prior Art 
     Essentially linear polyurethane resins resulting from the step-growth reaction of a polyisocyanate and a diol have been employed extensively in the past for fabricating both cellular and non-cellular shaped articles. The underlying step-growth polymerization reaction is particularly adapted for fabricating such articles by the reaction injection molding (RIM) technique. The latter operation has now accordingly become the general molding procedure of choice in the relevant industry because of the substantial inherent economic advantages associated therewith. However, the linear polyurethane resins, in spite of being particularly adapted for RIM processing, have only rather limited end-use industrial applications due to the lack of structural strength properties. The ever expanding use of structural plastic members in the manufacture of automobiles and like assemblies has accordingly prompted efforts to upgrade the mechanical strength properties of the conventional polyurethane resins. 
     One way to achieve improved properties in this regard is to increase the cross-link density of the resultant polymeric structure. This can be realized in the most straightforward economical manner by employing as part of the conventional diol content a polyol having a hydroxyl functionality in excess of two. Representative recent patents following this tack for upgrading the mechanical strength properties of a typically composed polyurethane resin include U.S. Pat. Nos. 4,260,718; 4,376,834 and 4,385,133. This relevant prior art demonstrates the degree of improvement that can be realized in accordance with this approach. A notable disadvantage of this prior art is only tacitly indicated therein. It is to be accordingly noted that an extended post-cure cycle is required in order to attain the improvement in mechanical strength properties residing in the practice of this prior art. When this disadvantage is considered in context of RIM processing it can be seen how seriously the efficiency of such a molding operation is compromised. It is therefore the object of this invention to obviate the need for post-curing as called for in the practice of the aforementioned prior art. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention polyurethane molding compositions are provided which are particularly suited for RIM operations. The contemplated compositions are most singularly composed in respect of their overall polyol contents. An applicable polyol content consists essentially of a combination of propylene glycols and an oligomeric polyol of from about 3,000-10,000 average molecular weight. Of such combinations the propylene glycols constitute at least 50% by weight of the total polyol content. Of the indicated glycol component, specifically a combination of propylene glycol and dipropylene glycol, the latter; viz., the dipropylene glycol, is present in not in excess of 50% by weight of the total content of said propylene glycols. The polyisocyanate is present in an amount to provide an excess of equivalents of isocyanate for effecting the underlying urethane reaction and under conditions serving to trimerize the available excess. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As pointed out hereinabove the foremost object of this invention is to provide polyurethane molding compositions capable of developing into tough, high temperature resistant molded articles during the molding cycle. An important feature of the contemplated compositions contributing to the toughness of the resultant moldings, as measured by impact resistance, resides in the amount and make-up of the propylene glycol constituent of the total polyol content. As regards amount, the propylene glycol constituent ranges weight-wise from about equal to twice that of the oligomeric polyol constituent, the latter being discussed in detail more appropriately hereinbelow. As regards the make-up of the propylene glycol constituent same ranges, again weight-wise, from an equal amount of propylene glycol to that of dipropylene glycol to a combining ratio of about 6:1, respectively. It has been noted that a minor amount of a lower primary diol, ranging up to about 40% of the propylene glycol constituent, can be included without adversely affecting physical strength properties or the curing rate. The use of such diols may be resorted to for economy, increased compatability characteristics, or a like reason. Representative primary diols for this purpose include ethylene glycol, 1,3-propane diol, 1,4-butane diol, 1,6-hexane diol, and 1,4-cyclohexanedimethanol. 
     An oligomeric polyol having an average molecular weight between about 3,000 and 10,000 and having a hydroxyl functionality preferably in excess of 2 is employed in conjunction with the propylene glycols in the practice of this invention. A wide variety of these polyols are available for this purpose. These polyols are commercially marketed specialty products of a number of chemical manufacturers. In view of this, the best overview of these available commercial offerings can be accomplished by identifying the various general categories of the polymers and listing representative proprietary products falling within such categories. Also given in several instances are certain relevant specifications of the listed products supplied by the manufacturer. 
     Polymer/Polyether Polyols 
     Niax 31-28 (Union Carbide)--6000 MW triol/polyacrylonitrile dispersion. 
     Niax 34-28 (Union Carbide)--6000 MW triol/polyacrylonitrile-styrene dispersion. 
     Polyether Polyols 
     Thanol SF5505 (Texaco)--5000 MW poly(propylene oxide/ethylene oxide triol. 
     Voranol 2148 (Dow)--7200 MW poly(propylene oxide/ethylene oxide) triol. 
     Voranol 2103 (Dow)--3000 MW poly(propylene oxide/ethylene oxide) triol. 
     Pluracol TP4040 (BASF Wyandotte)--4000 MW poly(PO/EO) triol. 
     Terathane (Union Carbide)--polytetramethylene ether polyols. 
     Polyester Polyols 
     Formrez (Witco)--polyester polyols. 
     Tone (Union Carbide)--polycaprolactone polyols. 
     Polybutadiene Polyols 
     R45HT (Arco Polymers). 
     Polybutadiene--co--Acrylonitrile Polyols 
     Hycar HTBN (B. F. Goodrich). 
     Certain minor constraints largely imposed by RIM practices narrow the preferable choices of polyisocyanates for the general implementation of this invention. The aliphatic polyisocyanates are not particularly desirable because of the relatively low order of reactivity associated with the use thereof. The aromatic polyisocyanates on the other hand suffer for the reason of being relatively toxic in nature. The latter, particularly exemplified by toluene diisocyanate, can however be advantageously used but only under safeguard conditions. The most universally preferred polyisocyanate for molding operations of the type herein concerned is methylene-4,4&#39;-diphenyldiisocyanate (MDI) as well as the various polymeric analogs thereof having an isocyanate functionality of from 2.1-2.7. An especially suitable polyisocyanate includes the various proprietary liquified MDI products. These products are chemically modified versions of MDI such that the melting point is lowered by the modification effected. Mixtures of the liquified MDI products and the polymeric isocyanates are particularly preferred in the practice of this invention. 
     The polyisocyanate is employed in an amount to provide an isocyanate index of from about 1.1-1.6; i.e., a 10-60% excess of NCO equivalence based on the total hydroxyl equivalents present in the molding composition. A catalyst for the urethane reaction is customarily utilized with the various tin salts and tertiary amines serving advantageously for this purpose. In order to achieve a higher degree of crosslinking in accordance with this invention attributable to the presence of excess isocyanate, a trimerization catalyst is indicated. A variety of bases are useful for this purpose. A particularly preferred catalyst of this type is N,N&#39;,N&#34;-tris(dimethylaminopropyl) hexahydrotriazine which conveniently serves to catalyze effectively both the urethane reaction and the trimerization reaction. 
    
    
     The following working examples illustrate the manner in which the present invention can be implemented and further includes the best mode contemplated for carrying out same. All parts and percentages are by weight unless otherwise indicated. 
     EXAMPLE I 
     In this Example a series of neat moldings were prepared illustrating various embodiments of the invention. The compounding of the molding compositions and the subsequent casting of the moldings were the same in each instance. Dipropylene glycol, propylene glycol and the oligomeric polyol and catalyst were initially mixed in the desired proportions and degassed hot on a rotary evaporator. The mixture was cooled under vacuum and mixed by hand with the appropriate amount of isocyanate in an open container for about 15-20 seconds. The mixture was poured into a mold formed by two glass plates coated with a mold release and separated by a gasket and spacers to provide a 1/8&#34; cavity. The mold in each instance was heated to 60°-90° C. before introducing the molding composition. After 5 minutes the mold was open and the solid plaque removed and divided into two parts. Part of the panel was tested as molded while the other diol was postcured for 1 hour at 130° C. The panels were tested for heat distortion temperature (HDT-ASTM D 648-56) and notched izod impact strength (ASTM D 256). Details concerning the compositional make-up of the various experimental molding experiments together with the indicated test data obtained for these samples are given in the following Table I: 
     
                                           TABLE I__________________________________________________________________________Prop.    Diprop.         Polyol.sup.1             Poly NCO.sup.2                      HDT - °C.                                NII - ft/lbs/inchExper.    Glyc.    Glyc.         Type             Type  Cat..sup.3                            post      postNo. (g)  (g)  (g) (g)   (g)                      as molded                            cured                                as molded                                      cured__________________________________________________________________________1.  (20) (10) A(20)             P-I(123)                   (1)                      102   116 1.1   1.42.  (15) (10) A(14)             P-I(97)                   (1)                      105   114 1.3   2.03.  (25) (10) A(24)             P-I(144)                   (1)                      102   113 1.1   2.04.  (30) (10) A(26)             P-I(168)                   (1)                      109   114 1.0   1.65.  (20) (10) A(20)             P-I(53)                   (1)                      109   121 0.9   0.9             P-II(74)6.  (28) (18) B(30)             P-I(178)                   (1)                      100   115 1.0   1.27.  (15) (10) C(14)             P-I(48.5)                   (1)                      122   118 1.2   1.2             P-II(44.5)8.  (15) (10) B(14)             P-I(48.5)                   (1)                      115   115 1.3   1.2             P-II(44.5)9.  (15) (10) A(93)             P-I(48.5)                   (1)                      111   123 0.9   1.0             P-II(44.5)10. (15) (10) A(18.7)             P-I(48.5)                   (1)                      104   124 1.3   1.3             P-II(44.5)11. (15) (10) D(14)             P-I(48.5)                   (1)                      104   --  1.6   1.3             P-II(44.5)12.  (20) +    (10) A(30)             P-I(176) 100   116 1.1   1.8    10 g EG13..sup.4    (15) (10) A(14)             P-I(44)  105   128 1.1   1.4             P-II(48)14. (15) (10) E(14)             P-I(44)  106   120 1.5   1.4             P-II(48)15. (15) (10) F(14)             P-I(44)  109   127 1.1   1.1             P-II(48)__________________________________________________________________________ .sup.1 Polyol Type A -- Niax 3128 (Union Carbide) B -- Niax 3428 (Union Carbide) C -- Thanol SF5505 (Texaco) D -- Voranol 2148 (Dow) E -- Voranol 2103 (Dow) F -- Pluracol TP4040 (BASF Wyandotte) .sup.2 Polyisocyanate Type PI Isonate 143L (Upjohn) PII Mondur MR (Mobay) .sup.3 Catalyst PC 41 (Abbott Laboratories) N,N&#39;N&#34;--tria(dimethylaminopropyl)hexane hydrotriazine .sup.4 2 minute demold 
    
     EXAMPLE II 
     The composition of experiment No. 6 set forth in Table I of Example I was processed in an Accuratio VR-75 RIM machine. One feed to the machine consisted of the diols, polyol and catalyst while the other feed consisted of the polyisocyanate. A mold temperature of 90° C. was observed. Three random roving glass mats were placed into the mold cavity (M-8610 Owens Corning). The plaque was removed after 4 minutes and divided into two parts. One part was tested as molded whereas the other part was post-cured for 1 hour at 130° C. Various physical properties were determined for the respective test samples and the results obtained are set forth as follows: 
     
         ______________________________________        AS MOLDED  POST CURED______________________________________Flex strength (at yield)          30,900 psi   31,000 psiFlex Modulus   955,000 psi  845,000 psiTensile Modulus          1.05 × 10.sup.6 psi                       946,000 psiTensile Strength          20,200 psi   18,500 psiHDT            133° C.                       139° C.NII            11.6 ft-lbs/in.                       9.5 ft-lbs/inUnnotched Izod impact          18.0         16.3______________________________________