Abstract:
Purified aromatic dicyanates are cured with a catalyst which is an Mn(II), Ag(II), Fe(III), Bi(III), In(III) or Hf(IV) salt at 150°-200° C. and then at 200°-300° C. for a time sufficient to cure the dicyanate without causing color formations to the extent of a Yellowness Index over 30. In some forms the dicyanate is combined with the catalyst and a thermoplastic.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of Ser. No. 213,530, filed Dec. 5, 1980 now U.S. Pat. No. 4,334,045. 
    
    
     BACKGROUND OF THE INVENTION 
     Aromatic dicyanates are a known class of thermosetting monomers which trimerize on heating and/or upon the action of a variety of catalysts to form branched polymers having s-triazine linkages (sometimes called polycyanurates or cross-linked cyanurates). Such aromatic dicyanates, including 2,2-bis(4-cyanatophenyl) propane [dicyanato derivative of bisphenol A or bisphenol A dicyanate or BCP], are commercially available in moderately pure forms as monomers or oligomers with molecular weights of about 2000. A representative process for preparing such monomers is described in U.S. Pat. No. 4,046,796 to Rottloff et al. by reaction of bisphenol A and similar bisphenols with a cyanogen halide in isopropanol with a tertiary amine catalyst. 
     It is known that aromatic dicyanates such as BCP can be heated or catalyzed by zinc chloride or other catalysts to set as a cross-linked polymer which is reasonably translucent, but quite yellow or brown in color. U.S. Pat. No. 4,026,913 to Tanigaichi et al. describes the polymerization of polycarbonates having hydroxy ends converted to dicyanato ends by cyanogen halide which are then polytrimerized with themselves or with monomeric aromatic dicyanates such as BCP to form cross-linked resins. Again, such compositions may be reasonably translucent but are normally quite yellow or brown in color. 
     U.S. Pat. No. 4,157,360 to Prevorsek and Chung describes thermoformable polymers which are an intimate mixture of a cross-linked polycyanurate network polymer and high molecular weight thermoplastic polymer such as a poly(ester-carbonate). It is described that intimate mixtures of aromatic dicyanate monomers such as BCP with thermoplastic polymers such as poly(ester-carbonates) can be set in fashions conventional for aromatic dicyanates to form interpenetrating network polymers of the thermoplastic finely dispersed in the cross-linked polycyanurate. While this patent describes such materials as having high impact strength, high Vicat softening temperature and good tensile properties, there is no description in the reference of the optical properties of the interpenetrating network polymer. Uses proposed for the interpenetrating network polymer include windscreens such as windshields, canopies, door windows and wire housings. In addition, it is described to deposit the interpenetrating network polymer as a coating on an article such as a wire, a conducting material, glass, poly(ester-carbonate) windshield and structural parts such as supporting beams. It is indicated that the cured composition will improve the abrasion and solvent resistance of the article. 
     For many applications of polycyanurates, and especially of polycyanurate networks with thermosetting polymers dispersed therein as described in U.S. Pat. No. 4,157,360, it is desirable that the polycyanurate not detract from the optical properties of either the thermosetting material dispersed therein or the substrate upon which the network is deposited. In this regard, conventional polycyanurates contribute excessive color that detracts from their use in windscreens and the like, even as a thin coating deposited on glass or on clear plastics such as polycarbonates and poly(estercarbonates). 
     BRIEF DESCRIPTION OF THE INVENTION 
     It has been discovered that careful purification of the aromatic dicyanate monomer can remove materials that contribute color to the polycyanurate network, and especially to the interpenetrating network of thermoplastic polymer dispersed in polycyanurate network. It has also been discovered that the selection of curing catalyst is critical in avoiding color formation. It has been further discovered that the dicyanate should be initially cured at between about 150° and 200° C. to avoid color formation as occurs if cure is initiated above about 200° C., and that cure time at higher temperatures (especially above 200° C.) should be limited. 
     Accordingly, the present invention includes a process for preparing a cured composition which comprises: 
     (a) combining a purified monomeric aromatic dicyanate with an effective amount of a catalyst selected from the group consisting of manganese(II), silver(I), iron(III) bismuth(III), indium(III) and hafnium(IV) salts and with a thermoplastic polymer; 
     (b) curing the aromatic dicyanate initially at at least one temperature at least about 150° C. and below about 200° C.; and 
     (c) further curing the aromatic dicyanate at at least one temperature between about 200° C. and about 300° C., for a time sufficient to cure the dicyanate without causing such color formation that the Yellowness Index of the cured composition exceeds about 30. 
     The present invention also includes a process for preparing a cured composition which comprises: 
     (a) combining a purified monomeric aromatic dicyanate with an effective amount of a catalyst selected from the group consisting of manganese(II), silver(I), iron(III) bismuth(III), indium(III) and hafnium(IV) salts; 
     (b) curing the aromatic dicyanate initially at at least one temperature at least about 150° C. and below about 200° C.; and 
     (c) further curing the aromatic dicyanate at at least one temperature between about 200° C. and about 300° C., for a time sufficient to cure the dicyanate without causing such color formation that the Yellowness Index of the cured composition exceeds about 30. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Some forms of the present invention relate to materials formed by curing an aromatic dicyanate in admixture with a thermoplastic in the manner of U.S. Pat. No. 4,157,360 or the aromatic dicyanate coated on a thermoplastic or the mixture coated on the same or a different thermoplastic. Other forms of the invention relate to the material formed by curing the aromatic dicyanate alone. The following discussion centers upon the mixture alone or on a thermplastic, but would apply similarly to the product of curing the pure dicyanate, except that permissable times for further curing are generally shorter for the pure dicyanate. 
     Monomeric dicyanates useful in the present invention include those of the formula NCO-R-OCN as described in U.S. Pat. No. 4,157,360 in columns 3-5. Preferred dicyanates are those wherein R is one of the following: 
     (a) diphenol moiety; 
     (b) diphenolester moiety, formed from aromatic dicarboxylic acid and diphenol; 
     (c) diphenolcarbonate moiety, formed from diphenol and carbonate precursor; 
     (d) diphenolestercarbonate moiety, formed from aromatic dicarboxylic acid, diphenol and carbonate precursor; or mixtures thereof. 
     Most preferred are those wherein R is ##STR1## It is preferred in these formulas that Ph be 1,4-phenylene and R&#39; be 2,2-propylidine; although Ph may also be 1,3 phenylene and R&#39; may also be 4,4-phthalein. 
     The dicyanate monomer used in the present invention should be purified before curing or mixing with thermoplastic. A preferred method of purification is recrystallization with a ketone having melting point below -25° C., such as acetone or methyl ethyl ketone. The recrystallization should be done without warming to temperatures where any substantial trimerization occurs, e.g. without exceeding 35° C. Other purifications, such as by recrystallizing in a hydrocarbon, may be employed with similar results, except that greater care must be exercised to avoid temperatures warm enough for the impurities to catalyze polymerization and color formation. 
     Unlike the processes of parent application Ser. No. 213,530, the present processes do not employ zinc salts as catalysts. Thus various other salts have been found which give low color products so long as the temperature-time regime of the present process is observed and pure dicyanate materials are used. Suitable salts include manganese(II) chloride, silver nitrate, iron(III) chloride bismuth(III) chloride, indium(III) chloride and hafnium(IV) chloride, and a variety of corresponding salts which are stable and dissolvable in polymer solutions or melts and are generally Lewis Acids. The amount of any catalyst depends on the rate of polymerization desired, but generally levels of 0.001% to 0.1% metal by weight of dicyanate are effective. 
     If a thermoplastic is used, it can be any thermoplastic, but preferably is one that does not react with the dicyanate, and also preferably contains at least some aromatic moieties. 
     If the dicyanate is to be combined with a thermoplastic before curing, then in some methods they are mixed as dry solids with the catalyst. In this case the weight ratio of mixed dicyanate to mixed thermoplastic is preferably about 1:9 to about 9:1. The mixture may be applied to the same or different thermoplastic before curing and then cured as described below. 
     Alternatively the mixture of purified aromatic dicyanate, catalyst and thermoplastic polymer may be coprecipitated from a solution thereof in a solvent for all three such as tetrachloroethane. The solvent may be removed before curing by evaporating at a temperature below either cure temperature or during the cure by heating to the first cure temperature (e.g. 175° C.) and evaporating the solvent. In the latter case, the cure time (generally 0.1-2 h) should be considered to start after the solvent has essentially all evaporated. Where coprecipitation is employed, the weight ratio of coprecipitated dicyanate to coprecipitated thermoplastic is preferably between about 1:9 and about 9:1. In some forms the coprecipitated dicyanate, catalyst and thermoplastic polymer (or dicyanate and catalyst without the thermoplastic polymer) are applied to a thermoplastic substrate before curing, but they may also be applied to other substrates or coprecipitated without a substrate. If applied or coprecipitated on a substrate, the cure generally follows the coprecipitation or application. 
     The purified aromatic dicyanate and thermoplastic polymer may also be melt blended before curing, generally with the catalyst added with one of the two melts. To avoid curing of the dicyanate before melt blending is completed, it is preferred that the powdered thermoplastic polymer be mixed with a major proportion (e.g. 90%) of the total purified aromatic dicyanate used and then heated to elevated temperatures (e.g. 200° to 230° C.) with mixing. When the mixture becomes homogeneous, the catalyst is then added with the remaining aromatic dicyanate (e.g. 10%) with mixing at a desired cure temperature (e.g. 200° C.). It is preferred to employ the melt blending technique with a catalyst level at the low end of the overall range indicated above to minimize premature cure. The weight ratio of melt blended dicyanate to melt blended thermoplastic is preferably between about 1:9 and about 9:1. Once melt blended, the material may be applied to a thermoplastic substrate, or other substrate such as a wire, before curing. 
     In all forms of the invention in which a mixture of aromatic dicyanate, thermoplastic polymer and catalyst are applied to a thermoplastic substrate, many preferred forms are those in which the thermoplastic polymer and thermoplastic substrate are formed from the same monomers. This similarity can improve compatability and adhesion. The monomers may be present in the same or similar proportion, but this is not required. The polymer and substrate may be of similar molecular weights, monomer distributions or degrees or branching, but this is not required. 
     Preferred thermoplastics used either for admixture with the purified aromatic dicyanate, for use as the thermoplastic substrate or for both are aromatic polyesters, aromatic polycarbonates, aromatic poly(ester-carbonates) and aromatic polysulfones. More preferred are the aromatic polycarbonates and poly(ester-carbonates) (also called polyester-carbonates). Preferred poly(ester-carbonates) include those formed from phosgene, bisphenol A and a monomer selected from the group consisting of terephthalic acid, terephthaloyl chloride, isophthalic acid, isophthaloyl chloride and mixture thereof. The monomer is preferably terephthaloyl chloride. Preferred poly(ester-carbonates) also include those formed from phosgene, bisphenol A, phenolphthalein, and terephthalic acid or terephthaloyl chloride. With poly(ester-carbonates) including the preferred ones, the most preferred aromatic dicyanate is 2,2-bis(4-cyanatophenyl)propane (&#34;BCP&#34;). 
     In general it is preferred that the weight ratio of combined aromatic dicyanate to thermoplastic polymer be between about 9:1 and about 1:9, with between about 2:1 and about 1:2 being more preferred. 
     In addition to being applied as a coating to a thermoplastic, the compositions of the present invention may be used as coatings for wires, structural parts and other materials requiring a tough coating which is not highly yellowed. For these applications, any of the above coating methods, i.e., dry blending, salt blending or precipitation, may be employed. Such coatings may take the form of dicyanate, thermoplastic and catalyst, or may take the form of dicyanate and catalyst. 
     The curing process used in the present invention, whether or not a thermoplastic polymer or substrate is present, should include cure at at least one temperature at least about 150° C. and below about 200° C., which may be quite short (e.g. 5-10 minutes) or quite long (e.g. several hours), but is preferably in the range of 30 minutes to 2 hours. It is contemplated to apply additional material and repeat the low temperature cure. Following the low temperature cure, the article should be cured at at least one temperature between about 200° C. and 300° C. The times for this second cure depend upon the system being cured, the temperature chosen, the color level desired and the desired degree of curing to achieve a particular hardness, abrasion resistance, mar resistance or other physical property. Generally less than 3 h and more than 1 min total cure time above 200° C. is employed, and times under about 30 minutes are most preferred when no thermoplastic is present. 
     As shown by Examples 1 and 2, performing a high temperature cure without first performing a low temperature cure increases the discoloration of the thermoplastic. While this Example is based on zinc catalysts, similar results are obtained with the present catalysts. Material which has undergone both cures is also more cured, and thus has better physical properties, than material that has undergone only the same high temperature cure. 
     As indicated by several of the Examples, and especially Examples 24 and 25, the longer the further curing above about 200° C., the more yellow the cured composition becomes. Since, however, curing occurs faster at these temperatures than color formation (provided than the other steps are followed such as aromatic dicyanate purification and initial cure below about 200° C.), a limited time is available to obtain desirable physical properties without excessive color formation. In many cases these physical properties are equal to the physical properties of highly colored materials which have cured for significantly longer times. 
     The improved optical properties of materials prepared in accordance with the present invention can be observed by determining a Yellowness Index in accordance with ASTM D1925-70. The physical properties can be measured by a variety of techniques, but for the coatings are generally determined by the Mar Resistance Of Plastics or Falling Grit test of ASTM D673. Other properties such as solvent resistance should also improve as the degree of cure increases. For any system trade-offs between better physical properties and inceased yellowness are encountered. In general, however, either improved physical properties with equivalent optical properties or improved optical properties with equivalent physical properties are achieved with the recrystallization and two-stage cure steps (including limited time above about 200° C.) of the present invention, compared to methods in which one or more of these are not employed. 
    
    
     EXAMPLES 1 AND 2 
     Samples of 2,2-bis(4-cyanatophenyl)propane (BCP) were placed in an aluminum dish to a level of about 1/16 inch (1.6 mm). The BCP used was purified by dissolving in acetone at room temperature, then filtering the solution and cooling it in Dry Ice, filtering the crystals at -60° C. and washing with -60° C. acetone. This process was usually repeated several (3-5) times. BCP used in later examples was similarly purified. Zinc octoate was added to a level of 0.008% Zn. 
     One dish was then heated in an oven at 200° C. for 30 minutes. The other dish was heated at 150° C. for 30 minutes and then 200° C. for thirty minutes. Yellowness indices (YI1) were then measured by ASTM D1925-70. The plaque from the first dish had a yellowness index of 88.4. The plaque from the second dish showed a yellowness index of 10.1. Thus discoloration at 200° C. occurs far less if the initial cure is below 200° C. 
     EXAMPLES 3-6 
     Comparison of Zinc Catalysts 
     Four plaques were prepared of purified BCP and 0.01% zinc as zinc chloride (ZnCl) or zinc octoate (ZnOc). The first two were subjected to cure at 150° C. for 60 minutes, then 200° C. for 30 minutes (then YI-1 measured), then 200° C. for 120 minutes (then YI-2 measured), and then 250° C. for 120 minutes (and then YI-3 measured). The results were: 
     
         ______________________________________Example   Catalyst   YI-1       YI-2 YI-3______________________________________3         0.01% ZnCl 2.8        21.4 53.44         0.01% ZnOc 9.5        27.0 64.1______________________________________ 
    
     The other two were cured at 140° C. for 120 minutes, then 170° C. for 120 minutes and then 250° C. for 120 minutes. The results were: 
     
         ______________________________________Example         Catalyst YI______________________________________5               ZnCl     47.06               ZnOc     79.4______________________________________ 
    
     Under these regimes, zinc chloride outperformed zinc octate. 
     EXAMPLES 7-13 
     Various Final Cures 
     Seven plaques of purified BCP with 0.01% zinc as zinc chloride were cured at 150° C. for 60 minutes, then 200° C. for 30 minutes. The first three plaques were then cured for varying times at 250° C. The last three plaques were then cured for 5 minutes at varying temperatures. The results are shown in Table 1. 
     
                       TABLE 1______________________________________Example       Cure After 200° C.                       YI______________________________________7             250° C. for 5 min.                       14.68             250° C. for 10 min.                       21.49             250° C. for 15 min.                       29.610            none           7.711            250° C. for 5 min.                       10.512            275° C. for 5 min.                       28.413            300° C. for 5 min.                       45.5______________________________________ 
    
     These results indicate a rapid increase in color at 250° C. and a more rapid increase at higher temperatures. Nevertheless, if a high degree of cure is required, these short high temperature cures may be desired after precuring at lower temperatures. 
     Systems With Thermoplastic Present 
     Based upon zinc chloride catalyzed mixtures of thermoplastics and dicyanates, it has been shown in parent application Ser. No. 213,530 that yellowness indices rise with cure times above 200° C. in a manner similar to the above, except that periods are extended (e.g., a yellowness index may be reached at 250° C. in 30-50 minutes instead of in 15 minutes). It is believed that similar correspondence will occur using the non-zinc catalysts described below. 
     EXAMPLES 14-21 
     Non-Zinc Catalysts 
     To test the ability of metal salts to give low color cured dicyanates when used in this process, three 5 g samples of BCP in aluminum dishes (diameter about 5 cm) were prepared for each salt and 2, 4 and 8 drops (respectively) of a 0.5% solution of the metal salts in DMF was added. The dishes were put in a 150° C. oven and mixed when the BCP had melted. The samples were checked periodically and usually they were removed when the DCB had gotten hard enough that it was no longer tacky, or seven hours had elapsed. They were then put in a 200° C. oven for approximately 1/4 of the time they were in the 150° C. oven. The samples that had a YI of less than 30  are listed below. The samples were about 2 mm thick. 
     
         ______________________________________                     Time in                            Time in                     150° C.                            200° C.            Drops of Oven   OvenEx   Catalyst    Cat. Sol.                     (Min)  (Min)  YI______________________________________14   MnCl.sub.2  2-8      120    60     12-1515   Co(NO.sub.3)            2-8      240    60      9-16*16   AgNO.sub.3  2,4      120    30     19-2317   Ni(OOCCH.sub.3).sub.2            4,8      420    100     19-24*18   FeCl.sub.3  2        140    30     24*______________________________________ *These samples had a color other than yellow so the YI understates their color. 
    
     Some metal salts that gave high color under these conditions are FeCl 2 , SnCl 2 , SnCl 4  and Cr(NO 3 ) 3 . Some metal salts which were not active enough to be considered useful are HgCl 2 , MgCl 2 , AlCl 3  and PbCl 2 . Also not active enough was ethanol. 
     To test the ability of metal salts other than Zn to give low color cured dicyanates when used in this process, three 10 g samples of BCP in aluminum dishes (diameter 5 cm) were prepared for each salt and 1, 2 and 4 drops (respectively) of a 2% solution of the metal salt in DMF was added. The dishes were put in a 150° C. oven and each was mixed when the BCP had melted. The samples were checked periodically and were removed when the BCP had gotton hard, or 12 hours had elapsed. They were then put in a 200° C. oven for about 1/4 the time they were in the 150° C. oven. The samples were about 4 mm thick. Samples whose YI, adjusted to a 2 mm thickness, was less than 30 are listed below. 
     
         ______________________________________           Time in   Time in      YI           150° C.                     200° C.                            YI    AdjustedEx. and  Drops of Oven      Oven   Meas- to 2 mmCatalyst  Cat. Sol (Min)     (Min)  ured  Thickness______________________________________19 BiCl.sub.3  1-4      420 to 720                     30-95  11-20 5-1120 InCl.sub.3  1-4      240 to 420                     30-65  17-37 9-2121 HfCl.sub.4  4        560       90     29    16______________________________________ ##STR2##-   Thus YI(5 g)=[1-(1-0.01 YI(10 g)).sup.1/2 ]100 An example of metal a salt that gave high color under these conditions was MoCl.sub.5. Examples of metal salts that were not active enough to be tested for low color are TiCl.sub.4, TeCl.sub.4, OsCl.sub.3 and RhCl.sub.3. 
    
     EXAMPLES 22-24 
     Based upon mechanical testing, it was determined that dicyanates cured only at 200° C. or only at 150° C. and 200° C. were not as fully cured as materials cured at 250° C. or at 200° C. and at 250° C. (for zinc catalyzed systems as well as for some of these other systems). Accordingly, the four catalysts giving the lowest color and heat cure at 200° C. were tested further: MnCl 2 , Co(NO 3 ), FeCl 3  and Ag(NO 3 ). While nickel acetate produced products with low yellowness indices, these materials were too blue for use in applications where low color was required. 
     EXAMPLE 22 
     AgNO 3   
     Purified dicyanate bisphenol A (5 g) was melt blended with 2,4 or 8 drops of freshly prepared silver nitrate in dimethyl formamide (one weight %) and placed in 5.5 cm diameter aluminum weighing dish and then cured for 4 hours at 150° C., then a variable period at 200° C. and finally for 5 minutes at 250° C. The yellowness index was then measured and a sample analyzed by IR as a KBr pellet (comparing absorbance at 2270 against 2220 cm -1 ) to determine the percent unreacted cyanate. The results were as follows: 
     
         ______________________________________  Drops of Time at           % UnreactedRun    AgNO.sub.3           200° C.*                        YI   Cyanate______________________________________22 A   2        30 min.      21.3 18.222 B   4        10 min.      22.7 12.922 C   4        20 min.      27.0 15.422 D   4        30 min.      28.9 NM22 E   8        10 min.      25.8 10.722 F   8        20 min.      25.8 12.322 G   8        30 min.      30.8 NM______________________________________ *after 4 h at 150° C. and before 5 min. at 250° C. NM = not measured 
    
     EXAMPLE 23 
     FeCl 3   
     Example 22 was repeated using 1% FeCl 3  in dimethyl formamide (1,2 or 4 drops) and various times at 150° C., 200° C. and 250° C. The results were as follows: 
     
         ______________________________________  Minutes at              %Run  Drops   150° C.                200° C.                      250° C.                            YI      Unreacted______________________________________23 A 1       5.5     22    5.5   25.1    13.723 B 1       5.5     40    5.5   28.6    12.723 C 1       5.5     22    10    34.5    11.123 D 2       4       22    5.5   28, 34.7*                                    NM23 E 2       4       40    5.5   33.9    NM23 F 2       4       40    10    36.8    NM23 G 4       4       22    10    55.5, 49.6*                                    NM23 H 4       4       40    5.5   34.0    NM23 I 4       4       40    10    49.6    NM______________________________________ *duplicate readings 
    
     It appears that 5 minutes at 250° C. is near the maximum that can be used with this FeCl 3  catalyst without exceeding a 30 yellowness index. 
     EXAMPLE 24 
     MnCl 2   
     When Example 22 was repeated with 8 drops of MnCl 2  (1% dimethyl formamide) and the plaques were cured for 4 hours at 150° C., 10-30 minutes at 200° C. and 0, 5, 7.5 or 10 minutes at 250° C., YI values below 30 were uniformly obtained. Infrared showed 15% or more of the cyanate to be unreacted, however. Selected runs are summarized below: 
     
         ______________________________________Time atRun    150° C.          200° C.                    250° C.                          YI     % Unreacted______________________________________24 A   4 h     10        5     15.7   18.724 B   4 h     20        7.5   20.1   15.324 C   4 h     20        10    25.2   13.124 D   4 h     30        5     20.7   17.324 E   4 h     30        7.5   19.8   15.724 F   4 h     30        10    23.1   12.7______________________________________ 
    
     To improve total cure, the catalyst was increased to 10 drops, the time at 150° C. was then extended to 6 hours and the time at 200° C. was also extended. The results were then: 
     
         ______________________________________Time atRun    150° C.          200° C.                    250° C.                          YI     % Unreacted______________________________________24 G   6 h     20        0     11.0   --24 H   6 h     63        0     15.9   --24 I   6 h     20        5     18.3   --24 J   6 h     20        7.5   23.4   --24 K   6 h     63        7.5   23.6   14.524 L   6 h     20        10    25.2   11.124 M   6 h     63        10    26.0   13.724 N   6 h     63        12.5  26.3   13.624 O   6 h     63        15    31.8   --24 P   6 h     63        17.5  37.7   --24 Q   6 h     63        20    36.0   --______________________________________ 
    
     It is apparent that after 6 hours at 150° C. and 63 minutes at 200° C., the time permitted at 250° C. is between 12.5 and 15 minutes for this MnCl 2  catalyst. Other runs with 20 hours at 150° C., 16, 36 or 46 hours at 210° C. and 2.5, 5.5, 7.5 or 10.0 hours at 270° C. produced yellowness indices from 17.6 to 49. 
     COMPARATIVE EXAMPLE 25 
     When the procedures of Example 22 were repeated with cobalt (I) nitrate, a strong color appeared after the 200° C. cure, such that further curing was not performed.