Abstract:
Oxalic acid, alone or in combination with phenothiazine or with phenothiazine and 4-chloro-2-nitrophenol, increases the storage life of vinyl ester resins or unsaturated polyesters without adversely suppressing the exotherm or affecting their curability.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of copending U.S. Ser. No. 265,473 filed May 20, 1981, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     Thermosettable resins generally have limited shelf lives, i.e., upon storage at ambient temperatures the resins undergo premature gelation which, of course, is undesirable. Generally, the tendency of a resin to gel increases as the storage temperature increases and is also dependent on the choice of reactants and on the kind and amount of monomer which is generally mixed with the resin. 
     Thermosettable vinyl ester resins, unsaturated polyesters or mixtures thereof have similar shelf life or storage stability problems. In the past, attempts with known stabilizing materials to prevent gelation have also resulted in an undesirable increase in the catalyzed gel times of the resins and adversely affected the curability of the resin. Typical of inhibitors taught by the art are certain hydroxyamines as proposed in U.S. Pat. No. 3,408,422 and phenothiazine taught in U.S. Pat. No. 3,683,045. 
     SUMMARY OF THE INVENTION 
     This invention relates to the unexpected finding that oxalic acid when added in small proportions, greatly increases the storage life of thermosettable vinyl ester resins without suppressing thge exotherm or adversely affecting their curability. A combination of oxalic acid and phenothiazine exhibits a synergistic stabilizing effect. Still, additional benefits result from the use of a combination of oxalic acid, phenothiazine and 4-chloro-2-nitrophenol. 
     DESCRIPTION OF THE INVENTION 
     The thermosettable polymer compositions include a vinyl ester resin or an unsaturated polyester or blends and mixtures of those two materials. 
     Vinyl esters resins are described in U.S. Pat. No. 3,367,992 wherein dicarboxylic acid half esters of hydroxyalkyl acrylates or methacrylates are reacted with polyepoxide resins. Bowen in U.S. Pat. Nos. 3,066,112 and 3,179,623 describes the preparation of vinyl ester resins from monocarboxylic acids such as acrylic and methacrylic acid. Bowen also describes alternate methods of preparation wherein a glycidyl methacrylate or acrylate is reacted with the sodium salt of a dihydric phenol such as bisphenol A. Vinyl ester resins based on epoxy novolac resins are described in U.S. Pat. No. 3,301,743 to Fekete et al. Fekete et al. also describe in U.S. Pat. No. 3,256,226 vinyl ester resins wherein the molecular weight of the polyepoxide is increased by reacting a dicarboxylic acid with the polyepoxide resin as well as acrylic acid, etc. Other difunctional compounds containing a group which is reactive with an epoxide group, such as an amine, mercaptan, and the like, may be utilized in place of the dicarboxylic acid. All of the above-described resins, which contain the characteristic linkages ##STR1## and terminal, polymerizable vinylidene groups, are classified as vinyl ester resins, and are incorporated herein by reference. 
     Briefly, any of the known polyepoxides may be employed in the preparation of the vinyl ester resins of this invention. Useful polyepoxides are glycidyl polyethers of both polyhydric alcohols and polyhydric phenols, epoxy novolacs, epoxidized fatty acids or drying oil acids, epoxidized diolefins, epoxidized diunsaturated acid esters as well as epoxides of unsaturated polyesters, as long as they contain more than one oxirane group per molecule. 
     Preferred polyepoxides are glycidyl polyethers of polyhydric alcohols or polyhydric phenols having weights per epoxide group of about 150 to 2000. These polyepoxides are usually made by reacting at least about two moles of an epihalohydrin or glycerol dihalohydrin with one mole of the polyhydric alcohol or polyhydric phenol, and a sufficient amount of a caustic alkali to combine with the halogen of the halohyrin. The products are characterized by the presence of more than one epoxide group per molecule, i.e., a 1,2-epoxy equivalency greater than one. 
     Unsaturated monocarboxylic acids include acrylic acid, methacrylic acid, halogenated acrylic or methacrylic acid, cinnamic acid and the like and mixtures thereof. Also included within the term &#34;unsaturated carboxylic acids&#34; are the hydroxyalkyl acrylate or methacrylate half esters of dicarboxyl acids as described in U.S. Pat. No. 3,367,992 wherein the hydroxyalkyl group preferably has from 2 to about 6 carbon atoms. 
     A wide variety of unsaturated polyesters which are readily available or can be prepared by methods well-known to the art are also stabilized by oxalic acid. Such polyesters result from the condensation of polybasic carboxylic acids and compounds having two or more hydroxyl groups. Generally, in the preparation of suitable polyesters, an ethylenically unsaturated dicarboxylic acid such as maleic acid, fumaric acid, itaconic acid or the like is interesterified with an alkylene glycol or polyalkylene glycol having a molecular weight of up to 2000 or thereabouts. Frequently, dicarboxylic acids free of ethylenic unsaturation such as phthalic acid, isophthalic acid, adipic acid, succinic acid and the like may be employed within a molar range of 0.25 to as much as 15 moles per mole of the unsaturated dicarboxylic acid. It will be understood that the appropriate acid anhydrides when they exist may be used and usually are preferred when available. 
     The glycol or polyhydric alcohol component of the polyester is usually stoichiometric or in slight excess with respect to the sum of the acids. The excess of polyhydric alcohol seldom will exceed 20-25 percent and usually is about 10-15 percent. 
     These unsaturated polyesters may be generally prepared by heating a mixture of the polyhydric alcohol with the dicarboxylic acid or anhydride in the proper molar proportions at elevated temperatures, usually at about 150° to 225° C. for a period of time ranging from about 1 to 5 hours. The condensation reaction is contained until the acid content drops to about 2 to 12 percent as COOH and preferably from 4 to 8 percent. 
     Polymerization inhibitors, commonly called process inhibitors, such as t-butyl catechol, monomethyl ether of hydroquinone (MEHQ) or hydroquinone, are advantageously added to prevent premature polymerization during the preparation of the vinyl ester resin or the unsaturated polyester. 
     Vinyl ester/unsaturated polyester resin blends are also effectively stabilized. The blends may be prepared either by physically mixing the two resins in the desired proportions or by preparing said vinyl ester resin in the presence of said unsaturated polyester. 
     Preferably, the thermosettable resin phase comprises from 40 to 70 weight percent of the vinyl ester or polyester resin and from 60 to 30 percent of a copolymerizable monomer. 
     Suitable monomers include vinyl aromatic compounds such as styrene, vinyl toluene, divinyl benzene and the like. Other useful monomers include the esters of saturated alcohols such as methyl, ethyl, isopropyl, octyl, etc., with acrylic acid or methacrylic acid; vinyl acetate, diallyl maleate, dimethyallyl fumarate; mixtures of the same and all other monomers which are capable of copolymerizing with the vinyl ester resin. 
     Oxalic acid is sold commercially as the dihydrate and the acid or its dihydrate the equally effective as the stabilizer. When oxalic acid or dihydrate is used as the sole stabilizer it is usually effective at a concentration of from 50 to 1000 parts per million (ppm). Higher amounts than 1000 ppm can be used and will improve the stability but may extend the gel time. Preferably, the amount to be used is from 50 to 400 ppm. 
     Phenothiazine is usually employed as an inhibitor at a concentration of from 10 to 500 parts per million (ppm), preferably from 50 to 400 ppm. A synergistic inhibition, which is observed when using a mixture of oxalic acid and phenothiazine, is apparent with all ratios of the two ingredients that contain a significant amount of each. When 4-chloro-2-nitrophenol is additionally included in the gelation inhibitor mixture, advantageously in an amount from 10-500 ppm and preferably from 50 to 400 ppm, further benefits are observed. 
     The stabilizing agent is usually added to the prepared resin either before or after admixture with the vinyl monomer. Oxalic acid can extend the shelf life of the resins 1.5 times or more. Also, it is less air dependent and is effective as a stabilizer under both aerobic and anaerobic conditions. 
     Catalysts that may be used for the curing or polymerization are preferably the peroxide and hydroperoxide catalysts such as benzoyl peroxide, lauroyl peroxide, cumen hydroperoxide, t-butyl hydroperoxide, methyl ethyl ketone peroxide (MEKP), t-butyl perbenzoate, and the like. The amount of the catalyst added will vary preferably from 0.1 percent to about 5 percent by weight of the resin phase. 
     Preferably, the cure of the resin can be initiated at room temperature by the addition of known accelerating agents or promoters, such as lead, potassium or cobalt naphthenate, N,N-dimethyl aniline, N,N-dimethyl-p-toluidine and the like usually in concentrations ranging from 0.1 to 5.0 weight percent. The promoted/catalyzed composition of resin/monomer, when uninhibited, will usually be converted to at least a gel state in a few minutes and cured to a solid state in 30 minutes to one or two hours. That time may vary widely depending upon the particular resin and vinyl monomer used, the catalyst/promoter system employed and its concentration, the temperature and other known factors. Similar results are observed when the usual process inhibitors are present that were added during the preparation of the resin. In amounts necessary to achieve that objective, such process inhibitors do not improve the storage stability of the composition to a significant extent. When used in amounts sufficient to provide storage stability, the gelation and cure rates are increased to unacceptable levels. It is an advantage of this invention that the process inhibitor can be employed in that minimum amount needed during resin preparation and that the stabilizing inhibitor systems of this invention can be employed to provide the desired storage stability without unacceptable effects on gelation and cure. 
    
    
     The benefits and advantages of the invention and the best mode for carrying out the same as illustrated in the following examples wherein all parts and percents are by weight unless otherwise specified. 
     EXAMPLE 1 
     The following vinyl ester resins were utilized: 
     Resin A 
     Bisphenol A was catalytically reacted with D.E.R.® 331 epoxy resin at 150° C. under a nitrogen atmosphere for 1 hour to form a polyepoxide having an EEW of 535. After cooling to 110° additional D.E.R.® 331 was added with methacrylic acid and hydroquinone and reacted to a carboxyl content of about 2-2.5 percent. Then maleic anhydride was added to the vinyl ester resin and reacted therewith. The final resin, diluted with styrene, contained approximately 
     
         ______________________________________bisphenol A             7.7%D.E.R.® 331        25.1%methacrylic acid        9.15%D.E.R.® 331        11.6%maleic anhydride        1.45%styrene                45%______________________________________ . 
    
     D.E.R.® 331 is the diglycidyl ether of bisphenol A having an epoxy equivalent weight of 186-192. 
     Resin B 
     Vinyl ester resin was prepared by reacting about 1 equivalent of methacrylic acid with 0.75 equivalent of an epoxy novolac having an epoxide equivalent weight (EEW) of 175-182 (D.E.N.® 438 epoxy novolac) and 0.25 equivalent of a glycidyl polyether of bisphenol A having an EEW of 186-192 (D.E.R.® 331). The above reactants were heated to 115° C. with catalyst and hydroquinone present until the carboxylic acid content reached about 1 percent. The reactants were cooled and then styrene (containing 50 ppm of t-butyl catechol) was added. The final resin contained 
     
         ______________________________________styrene                36%methacrylic acid       20.6%D.E.N.® 438        32.1%D.E.R.® 331        11.3%______________________________________ 
    
     In the following examples the compositions were evaluated according to the following procedures. 
     Stability at 55° C.--(a) Two ounces (59.1 cc) narrow mouth round amber bottles were filled to 70 percent volume with the resin composition. The bottles were capped with 20 mm plastic tin lined caps. The bottles were placed in an air circulated oven maintained at 55° C. Sample stability was checked daily by inverting the bottles. A sample is considered gelled and is removed from the oven when a layer of solid resin remains at the bottom of the bottle or when the sample appears stringy in consistency. 
     (b) The test is repeated with other samples except that before capping the head space is sparged with nitrogen. 
     (c) The tests (a) and (b) are repeated with other samples except that the bottles are filled to 94 percent volume with the resin composition. 
     (d) Tests of (c) are repeated at 110° F. 
     (e) Gel time at 180° F.--(82.22° C.) One gram of benzoyl peroxide is mixed with one gram of styrene in a 4 ounce (108.2 cc) wide mouth bottle. One hundred grams of the resin composition is introduced and mixed on a shaker for 45 minutes. The catalyzed resin was poured into a test tube to a level of 3 inches (7.6 cm). The test tube was corked and allowed to stand for 5 to 10 minutes to be rid of air bubbles. A thermocouple is inserted into the center of the resin composition to one inch (2.54 cm) below the resin surface. The test tube is placed in a constant temperature bath maintained at 180° F. (82.22° C.) and the timers started when the resin reaches 150° F. (65.56° C.). One timer is stopped when the resin temperature has risen to 10° F. (5.56° C.) above the constant temperature bath. That time is taken as the gel time. The test is continued until the recording pyrometer has run two minutes past the maximum temperature. The maximum temperature is the Peak Exotherm. The Peak Time is that time required for the resin temperature to rise from 150° F. (65.56° C.) to the Peak Exotherm. 
     (f) Gel time at 77° F. (25° C.)--One hundred grams of resin is placed into a 4 ounce (108.2 cc) wide mouth glass jar and held in a constant temperature bath for 45 minutes. 0.5 cc of 6 percent cobalt naphthenate in petroleum spirits is then mixed in thoroughly. To that is added 1.5 cc of 60 percent methyl ethyl ketone peroxide (MEKP) in dimethyl phthalate. The jar is placed on the gelometer stand with the probe extending into the resin. The timer is started and the time recorded when the timer stops and alarm is sounded. That time is the Gel Time. The jar is removed from the gelometer and a thermocouple placed about 3/4 inch (1.9 cm) from the bottom of the jar. The temperature at which the sample breaks away from the side of the jar is the Peak Exotherm. The total elapsed time is the Peak Time. 
     (g) The tests were repeated using 0.2 cc cobalt naphthenate and 0.75 MEKP. 
     EXAMPLE 2 
     To Resin A there was added 200 parts per million (ppm) oxalic acid dihydrate. Various stability evaluations and gel time and exotherm temperatures were determined by the aforementioned procedures. 
     A sample of Resin A containing only the 250 ppm MEHQ was subjected to the identical determinations. 
     The results are listed in Table I. 
     By identical procedures, compositions of Resin A containing 150 ppm Ptz and various inorganic and organic acids were evaluated. The acids employed were concentrated HCl; 20 percent H 2  SO 4  ; 85 percent H 3  PO 4  ; acetic acid and trichloroacetic acid included at 100 and 200 ppm. All of those compositions failed to provide the advantageous benefits of oxalic acid. 
     When concentrated nitric acid was substituted for the oxalic acid, the compositions appeared to exhibit good stability without apparent adverse effect on curability. 
     
                       TABLE I(a)______________________________________                           43.3° C.Stabilizing Inhibitor             55° C. Stability                           Stability(ppm)             (Days)        (Days)       Oxalic             70%      94%      94%Sample Ptz.sup.1        MEHQ.sup.2                 Acid  Air  N.sub.2                                Air  N.sub.2                                         Air  N.sub.2______________________________________1     150    250      --     9    5  4    4   14   122     150    --       200   11   13  8    8   25   --3     150    --       200   --   --  --   --  17   17______________________________________ .sup.1 Ptz = phenothiazine .sup.2 MEHQ = monomethyl ether of hydroquinone 
    
     
                                           TABLE I(b)__________________________________________________________________________25° C. Gel           25° C. Gel(f)             (g)        82.2° C. Gel    Gel Peak       Peak           Gel Peak                   Peak                      Gel Peak                              PeakSample    (min)   (min)       (°C.)           (min)               (min)                   (°C.)                      (min)                          (min)                              (°C.)__________________________________________________________________________1   19  35  167.2           75  141 128.3                      12.8                          15.6                              188.93   15.5   28.1       155 69.3               99.5                   162.2                      10.5                          13.1                              197.8__________________________________________________________________________ 
    
     EXAMPLE 3 
     To Resin B was added 200 ppm oxalic acid dihydrate. The same gel determinations were made as in Example 2 with the results listed in Table II. 
     When the oxalic acid was substituted with HCl; HNO 3  ; 20 percent H 2  SO 4  ; or acetic acid the benefits of oxalic acid were not realized. When H 3  PO 4  was substituted, storage stability was increased but gel times were extended beyond practical working limits. 
     The stability tests were repeated with compositions wherein maleic acid was substituted for the oxalic acid. The results are listed in Table III. 
     
                                           TABLE II__________________________________________________________________________StabilizingInhibitor(ppm)      25° C. Gel (f)                 25° C. Gel (g)                            82.2° C. Gel  Oxalic      Gel Peak              Peak                 Gel Peak                         Peak                            Gel Peak                                    PeakSample    Ptz  Acid      (min)          (min)              (°C.)                 (min)                     (min)                         (°C.)                            (min)                                (min)                                    (°C.)__________________________________________________________________________4   256  200 11.4          16.7              202                 91.7                     105.2                         185.6                            12.5                                14.2                                    203.9__________________________________________________________________________ 
    
     
                       TABLE III______________________________________Stabilizing Inhibitor(ppm)               55° C. Stability   Oxalic         Maleic    70%        94%Sample Ptz     Acid    Acid    Air  N.sub.2                                    Air  N.sub.2______________________________________4     256     200     --      10   8     6    55     256     --      100      5   2     2    2______________________________________ 
    
     EXAMPLE 4 
     Resin B was formulated into various compositions and tested for stability at 55° C. by the previously described procedures. The resin in all cases contained hydroquinone which had been added as a process inhibitor during preparation of the resin. The results are shown in Table IV. 
     
                       TABLE IV______________________________________Stabilizing Inhibitor(ppm)                  55° C. Stability         Oxalic   (Days)  Oxalic   Acid       70%      94%Sample Ptz    Acid.2HOH  (ANHYD)  Air  N.sub.2                                     Air  N.sub.2______________________________________6     256    --         --       5    3   3    37     256     50        --       6    4   4    48     --     100        --       3    --  --   --9     256    100        --       9    6   6    610    --     200        --       2    --  --   --11    256    200        --       9    6   6    --12    256    --         200      9    7   5    513    256    300        --       10   6   6    514    256    400        --       9    5   6    5______________________________________ 
    
     EXAMPLE 5 
     Resins A and B were formulated with various inhibitor combinations and tested according to the previously described procedures. The results are shown in Table V. 
     EXAMPLE 6 
     A general purpose polyester of 4 moles propylene glycol, 3 moles orthophthalic acid and 1 mole fumaric acid was diluted with 35 percent styrene and evaluated for stability according to the procedures of the preceding examples. No additional inhibitor was added. 
     All of the samples run at 43.3° C. and 55° C. at 70 percent and 94 percent in air and nitrogen gelled in one day. 
     To other samples there were added 100 ppm 4-chloro-2-nitrophenol, 200 ppm oxalic acid and 150 ppm phenothiazine. When tested under the same procedures, none had gelled in 18 days. 
     
                                           TABLE V__________________________________________________________________________                              43°                    55° Stability                              StabilityInhibitor (ppm)          70%  94%  94%  Barcol Hardeners.sup.1Resin    Ptz  MeHQ      4Cl.sub.2 NP.sup.2           Oxalic acid.2HOH                    Air                       N.sub.2                         Air                            N.sub.2                              Air                                 N.sub.2                                   after 24 hours__________________________________________________________________________A   150  250 --   --        9  5                         4  4 14 12                                    3A   150  --  100  200      14 17                         10 10                              26 26                                   23B   256  --  --   --        5  3                         3  3  9 10                                   18B   256  --  100  200      10 10                         6  5 17 17                                   20__________________________________________________________________________ .sup.1 cured at room temperature with 0.3 percent of a 6 percent cobalt naphthenate solution and 1.5 percent of a 60 percent MEKP solution. .sup.2 4Cl--2NP = 4chloro-2-nitrophenol