Acid/Oxidizer Catalyst System for Storage Stable, Quick-Cure Phenolic Resins of the Resole or Benzylic Ether Resole Type

The present invention relates to phenolic resins and more particularly to a catalyst system which imparts improved storage stability while retaining rapid cure characteristics.

Phenol or A-stage resins are well known to be prepared from a phenolic part and an aldehyde part which are catalytically reacted. A variety of ionizable catalytic agents are known in the art for the preparation of resole resins, including various alkali metals, alkaline earth metals, and amines. Residual ionizable catalyst permits subsequent heat cure of the resole resin, though at the expense of storage stability. A particular preferred class of resole resins are benzylic ether resins, such as described in U.S. Pat. Nos. 3,409,579 and 3,676,392. Benzylic ether resins can be formed by the reaction of a phenol and an aldehyde under substantially anhydrous conditions at temperatures below about 130.degree. C. In the presence of a catalytic concentration of a metal ion dissolved in the reaction medium. Benzylic ether resins also can be formed using catalysts, which the art has used in forming non-anhydrous resoles, by the aqueous reaction of a phenol and formaldehyde at low temperature, subsequently neutralizing the reaction medium to a pH of about 3.8-5.3 to form insoluble, non-ionizable salts, and then stripping the reaction medium under vacuum at temperatures ranging up to 130.degree. C. The insoluble, non-ionizable salts may be removed by filtration or other common means prior to the dehydration reaction in order to lower the residual ionizability even more. The preparation and characterization of some of these resins is disclosed in greater detail in U.S. Pat. No. 3,485,797. Such catalytic agents include sodium, zinc acetate, lead acetate, lithium naphthenate, lead napthanate, lead oxide, and the like. The benzylic ether resins as formed in the references above also are known as high ortho-ortho resins in that the resin is characterized by ortho-ortho linkages, compared to conventional resole resins, whether anhydrously formed or not, wherein ortho-para linkages predominate. Benzylic ether resins formed during low ionic dehydration possess advantages over conventional resoles in that they are slow to cure with heat. Consequently, they are very stable at room temperature due to the lack of ions present.

Again, the benzylic ether resins can be converted to a cured network in the presence of hydrogen ion-type catalysts, such as typified by strong inorganic and organic acids. Though such benzylic ether resins can be readily cured in the presence of strong acid catalyst, storage stability suffers by dint of such strong acid catalysts. Thus, there is a need in the resole art for maintaining, and even increasing the speed of cure of strong acid-catalyzed resole resins, while improving the shelf life thereof.

The present invention is addressed to providing a catalyst system which dramatically can improve the cure time of acid-catalyzable resole resins, while providing extended shelf life thereof. Broadly, the present invention is directed to a method for formulating an acid-catalyzed, nonaqueous resole resin and also to a method for its heat curing. The improvement comprises using a catalyst system comprising an acid catalyst and an oxidizing agent in an amount adequate for cure in at least about the same time that occurs by use of at least twice the amount of said acid catalyst alone. The shelf life of the catalyzed resole resin is greater with the inventive catalyst system than with an equivalent resole resin containing only the acid catalyst in twice the amount. For example, in uncut base form, the shelf stability of 3-5 weeks have been observed under ambient shelf storage conditions, while stability for over three months has been demonstrated when the base resole is cut in organic solvent. At the same time, Sunshine gel times of 1-4 minutes at 121.degree. C. and hot plate cure at 150.degree. C. of 1-15 seconds have been recorded. A variety of modifiers additionally can be added to the basic resole resin formulation for achieving a vareity of special effects.

Accordingly, advantages of the present invention include a catalyst system which imparts improved shelf life to resole resin formulations. Another advantage is that the speed of cure of the shelf stable resole formulations also is hastened. A further advantage is the ability to impart improved shelf stability to standard base-catalyzed resole resin used in the industry for acid curing while maintaining cure speed. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.

Referring initially to the inventive catalyst system, a variety of conventional strong acids are known to catalytically cure resole resins. These catalysts include, for example, mineral acids including sulfuric acid, phosphoric acid, boric acids, halogen acids such as hydrochloric acid, and sulfonic acids such as para-toluene sulfuric acid; and organic acids including chlorinated acetic acids, oxalic acid, maleic acid, and chlorendic acids. Salts of acids that produce the acid upon heating also can be used, e.g. ammonium sulfate and ammonium chloride. Typical acid concentrations in the past have ranged from about 2 to 20 percent (actives basis). Addition of an oxidizing agent in accordance with the precepts of the present invention, however, permits the level of acid catalyst to be at least cut in half without loss of cure times, thereby increasing shelf life of the catalyzed system by decreasing the presence of destabilizing ions of the acids.

Presently preferred oxidizing agents include potassium permanganate and potassium dichromate. Additional oxidizing agents which may be appropriate include, for example, persulfates, organic peroxides, perborates, and the like, and even mixtures thereof. The proportion of oxidizing agent can range from as low as about 1% on up to 25% or higher of the active acid catalyst. In practical terms, it has been determined that the proportion of oxidizer is determined by the degree of solubility of the oxidizing agent of choice in the resole formulation. For the preferred resoles described herein, the proportion of oxidizing agent typically will not exceed about 5% and often will range from about 2 to 5%.

Referring now to the resole resins, it will be appreciated that resole resins common in acid-catalyzed systems generally are low in water content, i.e. contain less than about 10% water as is standard in resole preparation chemistry. The phenols employed in the formation of the phenolic resins generally include any phenol which has heretofore been employed in the formation of phenolic resins and which are not substituted at either the two ortho positions or at one ortho and the para position, such unsubstituted positions being necessary for the polymerization reaction to occur. Phenols substituted in these positions may be used in lesser quantities (e.g. up to about 30%) to control molecular weight by a capping reaction. Any one, all, or none of the remaining carbon atoms of the phenol ring can be substituted in conventional fashion. The nature of these substituents can vary widely, and it is only necessary that the substituent not interfere in the polymerization of the aldehyde with the phenol at the ortho and/or para positions thereof (except for molecular weight control as noted above). Substituted phenols employed in the formation of the phenolic resins include: alkyl substituted phenols, aryl substituted phenols, cycloalkyl substituted phenols, alkenyl-substituted phenols, alkoxy substituted phenols, aryloxy substituted phenols, and halogen-substituted phenols, the foregoing substituents containing from 1 to 26, and preferably from 1 to 9, carbon atoms.

Specific examples of suitable phenols include: phenol, o-cresole, m-cresole, p-cresole, 3,5-xylenol, 3-4-xylenol, 3,4,5-trimethylphenol, 3-ethyl phenol, 3,5-diethyl phenol, p-butyl phenol, 3,5-dibutyl phenol, p-amyl phenol, p-cyclohexyl phenol, p-octyl phenol, 3,5-dicyclohexyl phenol, p-phenyl phenol, p-crotyl phenol, 3,5-dimethoxy phenol, 3,4,5-trimethoxy phenol, p-ethoxy phenol, p-butoxy phenol, 3-methyl-4-methoxy phenol, and p-phenoxy phenol.

The aldehydes reacted with the phenol component can include any of the aldehydes heretofore employed in the formation of phenolic resins including, for example, formaldehyde, acetaldehyde, propionaldehyde, furfuraldehyde, and benzaldehyde. In general, the aldehydes employed have the formula R'CHO wherein R' is a hydrogen or hydrocarbon radical of 1-8 carbon atoms. It will be appreciated that some aldehydes, e.g. acetaldehyde and butaraldehyde, improve toughness of the resole resin at the express of lowering the HDT thereof (heat distortion temperatures, American Society for Testing and Materials ASTM D-648).

A variety of modifiers can be cooked into the resole resin in order to improve toughness. These modifiers include, for example, chain terminating phenols, glycols, with or without halogen substitution for additional fire retardency, polyester polyols, alkyl phenols, lactams, polyamides, polyamines, hydroxy-containing acrylates, and the like and mixtures thereof. The proportion of modifier incorporated into the resole resin typically ranges from about 5 to 35 weight percent (based upon the phenol component).

The catalyst used in preparation of the resole can be critical in subsequent use of the resole. As noted above, the preferred catalysts are those which result in the formation of benzylic ether phenols, which are characterized by having a high ortho-ortho content. It will be appreciated, however, that conventional catalyst which result in conventional ortho-para linkage-containing resoles also find wide use with respect to the inventive catalyst system disclosed herein as do the low ionic types discussed above.

Finally, it will be appreciated that a variety of modifiers can be included in the resole formulation, such as fire retardant modifiers typified by melamines, ureas, and dicyandiamides. Additional modifers include fillers, though such fillers should be restricted to neutral or acid fillers in order that the acid/oxidizer system is not spent on neutralization of basic fillers incorporated in the formulation. Reactive modifiers also can be added to the resole resin after it has been formulated. These reactive modifiers include, for example, di and higher polyols, e.g. di or polyhydric phenols, resorcinol, phloroglucinol, and the like. Finally, modifiers to achieve special effects in particular applications may be appropriate, e.g. the addition of polyvinyl acrylate-N-methylol acrylamide latex or the like in oil filter paper saturant uses; various nylons or polyvinyl butyrals, or epoxies for better impact strength in composites.

A final general component for the phenol system is a solvent. A variety of solvents can be used in cutting the phenol resin to a desired solids concentration or viscosity for application. These solvents include a variety of aromatic solvents including benzene, toluene, xylene, ethyl benzene, and the like. Additional solvents include polar solvents such as furfural, furfuryl alcohol (which co-reacts under acid conditions) various Cellsolves, carbitols, ketones, and various alcohols. It was discovered during the course of research on the present invention, however, that shelf stability of the catalyst resole formulation may be improved by utilizing a solvent containing hydroxyl groups. However, the speed of cure can be improved by excluding hydroxy solvents, since it is believed that hydroxy solvents may form intermediates during the curing reaction which require additional time and/or heat in order to push the reaction to completion. This same intermediate reaction occurs during storage but at a much slower rate.

It is not unusual to find acid catalyzed casting resins used in soil stabilization in oil wells, laminating resins for fiberglass reinforcement, as well as a binder resin for molded articles. However, it is unique to find an acid catalyzed resin and an acid catalyst-based system that will exhibit the following characteristics. Because of the low ionic character of the catalyst system, the catalyst system will contribute to the preferred resole resins described herein having a higher dielectric constant than conventional resole resins, suggesting its used as a coating resin for electrostatic grain orientation in coated abrasive uses and in electrical grade laminates where the material exhibits good shelf stability and fast cure at elevated temperature. The unexpectedly low corrosivity on cellulosic paper of the catalyzed resole system suggests its use in areas where standard phenolic resins are used. Because of its fast cure at elevated temperature and low residual acidity, the inventive system may find use in pultrusion formation of fiberglass reinforced sucker rods for oil wells which exhibit higher ultimate temperature performance than polyester sucker rods. The reactivity of the inventive system also makes it a prime candidate for reaction injection molded articles containing fiberglass and other reinforcement.

Because of its low corrosiveness on cellulosic paper, its low hardness when modified, and its fast cure, the inventive catalyzed resole system suggests its use on metal coil coatings used for sanitary can and other filled coatings wherein the fillers are not basic enough to detract from the acid cure.

The acceleration of the acid cure by inclusion of an oxidizing agent suggests that certain free radical foundry resin mechanisms may be improved when sulfur dioxide is used as the gasing system. This statement is based on the fact that SO.sub.2 is a sulfurous acid precursor and sulfurous acid is related to sulfuric acid which finds utilization in the catalyst system of the present invention and is known to catalyze ethylenically unsaturated double bond polymerization.

The examples also will show that the present resole system exhibits very low corrosive activity on cellulosic paper, suggesting it as an impregnate for non-woven media and as a laminate of woven and non-woven materials. Finally, the system may find use with fluorocarbon blowing agents in heated molding operations to yield closed cell phenolic foams at lower catalyst levels and with higher boiling fluorocarbons.

The following examples show how the present invention has been practiced, but should not be construed as limiting. In this application, all citations are expressly incorporated herein by reference.

A resole resin was synthesized from the following ingredients and possessed the following properties:

In oreder to evaluate its cure response shelf life, samples of resole 185 were catalyzed with different catalysts, and subjected to cure response and stability testing as follows:

The above-tabulated results demonstrate the effectiveness of the novel catalyst system. With reference to the H.sub.2 SO.sub.4 series of tests, it will be observed that the oxidizing agent permitted use of less than one-third of the acid catalyst to achieve the same quickness of cure (Nos. A and C). Improved stability also was noted for inventive No. A compared to standard No. C. Test No. B shows the effect of the oxidizing agent on cure and stability at equal acid concentrations.

The PTSA data shows it to require a greater concentration than the H.sub.2 SO.sub.4 catalyst. Still, the enhancement of its catalytic activity by the oxidizing agent is evident from Nos. G and H where cure times are equivalent. Stability for the novel catalyzed system also was better than with an equivalently curing acid-only catalyzed system.

Use of the zinc catalyst in Example 1 yield a resole high in ortho-ortho linkages. In this example, a conventional base catalyzed resole (ortho-para linkages, catalyst partially neutralized but not removed) was prepared with the following properties.

The resole resin (800 g) was cut to 1985 cps viscosity with methoxy propylene glycol solvent (75 g) and subjected to testing (125 g samples) as described in Example 1.

These data show that the presence of the oxidizing agent contributed to improved stability, but apparently did not significantly improve cure speed. However, it should be realized that the resole was base catalyzed so that a certain amount of the acid was needed just to neutralize residue base in the resole. No attempt was made to determine the amount of acid required for this task. Thus, the true effect of the novel catalyst system was not experimentally determined. These data do, however, establish the operability of the novel catalyst system with respect to conventional base catalyzed resole resins.

The following base resole resins were prepared.

Each of the resins was cut in solvent and catalyzed as follows:

The properties of the catalyzed resoles then were determined.

Finally, several of the resins were drawn down on clean, mill finished steel (0.030 in. panels) to give a 10 mil wet film (about 0.7 mil dry film). Barcol hardness values 4 hours after cure and again after 24 hours of ambient water soak were taken.

The foregoing data establishes the inventive catalyst system for a variety of resole resins and the relative equivalency in cure compared to a standard acid only cure resole system. The near equivalency of the low ionic resins to the standard base-catalyzed resin shows that hardness can be achieved without use of large quantities of residual acidity. The inventive resole resins, then, can be electrostatically deposited, for example, on metal and display less corrosive behavior thereon.

Resole 185 was evaluated as a paper impregnant for making oil or air filters. Control filter paper samples (90 lb/resin) were soaked in the resole for 10 minutes at 300.degree. F. to establish the desired 20% pick-up rate (cured resin weight add-on to the paper). The resole was catalyzed with 1 wt-% of H.sub.2 SO.sub.4 /K.sub.2 Cr.sub.2 O.sub.7 (30%/5%) catalyst system. The impregnated paper was cured for 2.5 minutes at 350.degree. F. The following test data was recorded.

These data demonstrate the relatively acceptable performance of the system as a paper impregnant.

Resole 190 was evaluated for filter paper impregnation with and without the addition of 35% (solids basis) of a polyvinyl acrylate-N-methylolacrylamide latex modifier. The catalyst system (0.15%, solids basis) was 30% H.sub.2 SO.sub.4 /2% K.sub.2 Cr.sub.2 O.sub.7. The paper was 90 lb/ream oil filter paper. The saturaing solvent was methanol. The following results were recorded.

Again, the relative efficacy of another resole catalyzed with the novel catalyst system is demonstrated.

A resole resin was synthesized from the following ingredients.

The resole was cut to about 9% n.v. solids with a solvent blend of methanol/methyl ethyl ketone (90/10 wt) to form Solution A. A final pick-up of 19.1-19.6% was noted when the paper (90 lb/ream) was cured at 300.degree. F. for 10 minutes. The final pH of the saturating solution was 1.6.

Solution B was made by adding an 8-10% n.v. methanol solution of PVA-N methylolacrylamide latex to Resole 192 (20-21% solids basis of latex). This solution then was cut to 9% n.v. using the noted solvent blend. A cured pick-up of 20% of the pH 5.0 solution for 90 lb/ream paper was recorded.

Papers were saturated, cured, and aged overnight at 70.degree. F. and 50% R.H. Additional papers also were aged for 5 days at 100.degree. F. and 50% R.H. The following results were recorded.

These results set forth in Tables 12 and 13 demonstrate that the aging did not degrade the paper.

The heat histories required in impregnating the paper in the previous examples were longer than desired. In an attempt to speed up the cure of the dilute resole impregnating solutions, phloroglucinol was blended with the resole at 5% (solids basis). A 0.30% (solids basis) of H.sub.2 SO.sub.4 /K.sub.2 Cr.sub.2 O.sub.7 (30%/2%) was used with an 80/20 blend of methanol/methyl ethyl ketone saturating solvent. The paper stock was 60 lb/ream filter paper. The resole was 192M which was 192 (Example 6) with diethylene glycol (10 parts per 100 parts phenol) cooked in. The following results were recorded.

The results do not appear to demonstrate the needed cure speed increase desired by addition of the noted triol, but do demonstrate an increase in moisture resistance performance.

As a paper impregnant, the catalyzed resoles displayed quite acceptable performance, but required a significant heat history. It was theorized that the methanol solvent was participating in the formation of reaction intermediates which were displayed during the final curing reaction to release the methanol. It followed that non-alcoholic solvents should ameliorate the heat history factor. Thus, samples of Resole 184 (Table 5) were cut in methanol and acetone in order to verify the foregoing.

The samples were catalyzed with varying proportions (based on solids of resole) of H.sub.2 SO.sub.4 (30%) and K.sub.2 Cr.sub.2 O.sub.7 (2%) catalyst system. A 10% solids concentration of each sample was used to impregnate felt paper (15% pick-up weight) and then heated at 350.degree. F. The degree of dryness (solvent evaporation) and cure was determined by feeling the impregnated samples as a function of heating time with the following results.

The improved speed of cure using the non-alcoholic solvent certainly is demonstrated.

The performance of the samples then was adjudged from impregnated 90 lb/ream filter paper (15% pick-up weight).

The foregoing data establishes the relative equivalency in performance between the methanol solvent and acetone solvent samples. Relatively good performance values are seen displayed. No corrosiveness on the cellulose fibers was observed despite the relatively low pH (about 5) of the samples.

In this example, Resole 192M of Example 7 again was evaluated alone and with 5% (solids basis) phloroglucinol addition. Also, the proportion of oxidizing agent was 2% in samples A and B, and 10% in samples C and D. The catalyst system used 1% total of 30% H.sub.2 SO.sub.4 and the noted K.sub.2 Cr.sub.2 O.sub.7. The saturating solvent used was acetone. Again, an increase in cure speed of the dilute impregnating solution was the goal.

As before, the triol did not seem to significantly improve cure speed at the lower oxidizer level tested. The increased oxidizer level, however, did improve all properties except for water resistance.

In this example, the efficacy of the novel catalyst system in hot box foundry core operations was evaluated. The following systems were tested.

The hot box operation used a 0.5 sec. blow time, a 60 sec. dwell time, and a 450.degree. F. temperature. The following tensile strengths were measured for fresh mix, mix that had aged for 6 hours (6 Hr. Bench), and mix that had aged for 24 hours (24 Hr. Bench) before use.

These results demonstrate the efficacy of the novel catalyst system in hot box operations. Comparable tensile strengths and longer bench life are exhibited by the inventive catalyst system compared to an acid only system. This is true even for a conventional acid catalyzed water-based resole resin. Note that Resin D exhibited a bad odor, but that Resins A and B exhibited no odor.