Process for the Preparation of a Propylene Glycol Antifreeze Containing an Alkali Metal Silicate

The invention relates to a process for preparing an improved propylene glycol antifreeze containing silicate which is resistant to gellation of the silicate.

Antifreeze compositions comprising ethylene glycol and a wide variety of corrosion inhibitors are well known in the prior art as evidenced by U.S. Pat. Nos. 3,341,469, 3,337,496, 4,241,014 and 4,664,833. Although prior art patents disclose the use of a wide variety of glycols in antifreeze compositions, such patents primarily deal with and provide working examples for ethylene glycol based antifreeze compositions. Further, the prior art has not generally disclosed any criticality of the mixing order of the corrosion inhibitors to the ethylene glycol. Two exceptions to this are found in U.S. Pat. Nos. 3,282,846 (Scott) and 4,149,985 (Wilson). Scott discloses a method for manufacturing an antifreeze involving the mixing order of ethylene glycol, borate, silicate, and sodium mercaptobenzothiazole. Wilson discloses a method for manufacturing an antifreeze involving the mixing order of ethylene glycol, borate, basic inorganic compound (until a pH of from 9.5 to 10.5) and then silicate.

It has been found that the order of mixing disclosed in the Scott and Wilson patents is not applicable to all types of alkylene glycols and is particularly inapplicable for manufacturing propylene glycol antifreeze compositions. U.S. Pat. No. 5,422,026 (Greaney) discloses a phosphate-free corrosion inhibited antifreeze prepared by admixing an aqueous solution comprising silicate, silicate stabilizer (polysiloxane), molybdate, and a relatively small amount of a basic compound, with a solution of an alkylene glycol, preferably, propylene glycol, and the various corrosion inhibitors typically employed, e.g., mercaptobenzotriazole, borate, nitrate, sebacate, tolyltriazole, and the like. Greaney does not recognize or disclose the need for a high pH in the initial silicate premix in achieving long term silicate stability without gel formation. Applicants' invention overcomes the prior art limitations and provides a process for preparing gel resistant propylene glycol antifreeze compositions containing alkali metal silicate.

It has been discovered that the preparation of propylene glycol antifreeze containing an alkali metal silicate can be improved so as to provide enhanced gel resistance by controlling the amount of basic compound and water of the silicate mixture at the time the propylene glycol and alkali metal silicate are admixed. Such compositions are useful in the manufacture of antifreeze/coolant compositions for use in the cooling systems for internal combustion engines. The invention correlates the concentration of basic compound(s) and water to the alkali metal silicate in a premixed additive ("silicate premix") whereby the silicate has improved gel resistance when added to the propylene glycol.

The instant invention achieves this improvement in resistance to gel formation by use of a "silicate premix" which is employed to prepare compositions containing 85 to 98 weight percent propylene glycol, a corrosion inhibiting amount of an alkali metal silicate, basic compound, water and a silicone. The process of the invention comprises:

(a) preparing a "silicate premix" by admixing an alkali metal silicate, a basic inorganic compound (preferably as an aqueous base solution) to provide a silica to alkali metal oxide mole ratio (SiO.sub.2 :M.sub.2 O) where M is an alkali metal cation, of less than about 0.25, preferably less than about 0.22, and adding an effective amount of water to said premix to provide at least about 60 weight percent water in the silicate premix; and

(b) adding the silicate premix of step (a) to said propylene glycol. The above process provides compositions that have improved resistance to gel formation sometimes observed in alkylene glycol-containing antifreeze compositions containing alkali metal silicates.

The order of mixing of the ingredients described herein is significantly different than that taught in the prior art so that the pH and water content associated with the alkali metal silicate can be controlled. This is to be contrasted with the prior art. For example, U.S. Pat. No. 4,149,985 teaches the importance of adding borate to an alkylene glycol while maintaining a neutral, acidic or slightly alkaline pH, then raising the pH to from 9.5 to 10.5 by adding a basic inorganic compound, followed by addition of an alkali metal silicate.

The instant invention teaches that it is not necessary to add alkali metal borate to a neutral alkylene glycol as taught by U.S. Pat. No. 4,149,985, if an alkali metal borate or other corrosion inhibitors are to be employed. In fact, it is preferred to add borate, if present, after addition of a silicate premix to the propylene glycol, as will be further discussed hereinafter.

The instant invention provides compositions which are resistant to silicate gel formation by providing the alkali metal silicate in a form wherein it may be added to the propylene glycol with improved resistance to silicate gel formation. The stable form of the silicate employed herein is achieved by forming an "over-alkalized" silicate premix, i.e., an alkali metal silicate solution containing sufficient base to achieve a silica to alkali metal oxide mole ratio (SiO.sub.2 :M.sub.2 O), where "M" is an alkali metal cation, of less than about 0.25, preferably less than about 0.22, with an effective amount of water to solubilize the alkali metal silicate and inorganic basic compound so as to provide the total weight percent (wt %) of water in the premix of at least about 60 wt. % or higher, and preferably, at least about 63 wt. %.

As used herein the term "propylene glycol" shall be understood to mean propylene glycol and mixtures of propylene glycol with one or more of dipropylene glycol, ethylene glycol and diethylene glycol, wherein the propylene glycol comprises greater than about 95 weight percent of any such mixture. As is well known, however, propylene glycol and the propylene glycol mixtures referred to herein can contain up to about 5 weight percent added water.

The basic inorganic compound can be an alkali metal hydroxide, alkali metal carbonate, alkali metal phosphate, and the like, and is preferably sodium or potassium hydroxide.

Alkali metal silicates useful herein include, for example, sodium metasilicate, potassium metasilicate, lithium metasilicate, the corresponding orthosilicates, and the like, and mixtures thereof.

Optional additives can be employed in minor amounts of less than 10 weight percent based on the total weight of the antifreeze composition. Typical optional additives can include, for example, known corrosion inhibitors for metals such as, for example, tungstates, selenates, carbonates and bicarbonates, monocarboxylic acids, sebacates and other dicarboxylic acids, molybdates, borates, phosphates and benzoates, hydroxy benzoates or acids thereof, silicones, alkali metal nitrates, alkali metal nitrites, tolyltriazole, mercaptobenzotriazole, benzotriazole, and the like, or mixtures thereof. If one or more of the known inhibitors are employed together with the inhibitors present in the instant invention, the sum total of all inhibitors should be used in an effective inhibitory amount, i.e., an amount sufficient to provide some corrosion inhibition with respect to the metal surfaces to be protected. Other typical optional additives would include buffers, such as alkali metal borates, phosphates and carbonates; wetting agents and surfactants, such as, for example, known ionic and non-ionic surfactants such as poly (oxyalkylene) adducts of fatty alcohols; anti-foaming agents and/or lubricants such as the well known polysiloxanes and the polyoxyalkylene glycols, as well as any other minor ingredients known in the antifreeze/coolant art.

For the purposes of this invention, the gel resistance of the composition is determined by observing the presence or absence of gel formation immediately upon mixing or after sitting quiescently in a closed bottle at 55.degree. C. for a determinate period of time (until gel formation is observed).

As can be seen from the examples, the order of addition of an over-alkalized aqueous silicate premix is a critical feature of the invention, and is not taught by the prior art. The order of addition is related to maintaining silicate solubility in the propylene glycol. First, silicate requires a high pH to be stable. The premix of the present invention maximizes the pH during silicate addition and never allows the pH to be less than the final buffer pH. Second, silicate will condense if dehydrated. The premix of the present invention ensures that a sufficient amount of water is present during admixture with the propylene glycol. While the order of mixing is especially advantageous with propylene glycol, which is less soluble than ethylene glycol, this order of mixing also can be useful in the preparation of other alkylene glycol antifreezes.

Once the silicate is in solution in the premix, further silicate stabilization can be achieved through use of a conventional silicate stabilizer, e.g., the organic and/or inorganic silanes and/or siloxanes disclosed in U.S. Pat. Nos. 4,772,408, 4,725,405, 4,414,126, 5,064,552 and the patents referred to therein, the relevant portions of which are incorporated herein by reference. The silicate stabilizer can be added directly to the propylene glycol or to the premix or to the resulting admixture obtained upon adding the premix to the propylene glycol solution.

The present invention involves the stabilization of the alkali metal silicate with basic inorganic compound and water to form a silicate premix which is then added to the propylene glycol.

Although it is preferred to add corrosion inhibitor compounds to the resultant silicate-containing propylene glycol mixture, the corrosion inhibitor compounds can, if desired, be added to the propylene glycol solution prior to the addition of the silicate premix. Since the corrosion inhibitor compounds can lower the pH, it is preferred that in such instance, additional basic inorganic compounds be added to bring the pH of the propylene glycol solution at least back to the normal pH of propylene glycol, i.e., about 6.0. In this manner, the pH in the final silicate-containing propylene glycol mixture will be at least about 9.0 and, preferably, between about 9.5 and about 11.5.

In one embodiment a mixture is prepared containing between about 92 and 93 weight percent propylene glycol and between about 3.7 and about 3.9 weight percent of a silicate premix.

The rate of addition of the silicate premix to the propylene glycol is related to the pH and water content of the propylene glycol and silicate premix. It is preferred to add the silicate premix either slowly or stepwise in two or more aliquots to the propylene glycol, allowing any cloudiness which may form to clear before continuing the addition of the silicate premix to the propylene glycol. This rate of addition becomes more important where the pH of the propylene glycol or propylene glycol/inhibitors mixture may be low, e.g., having a pH below that of the propylene glycol.

The following examples were conducted to demonstrate the improved gel resistance of silicate-containing alkylene glycol compositions formed according to the process of the instant invention as compared to the processes of the prior art.

In the following examples, the following nomenclature is employed.

Examples A and C are comparative examples. Examples B, D, E, F, G, H, I and J were carried out according to the instant invention. The compositions discussed in the following examples are set forth in Table 1 and Table 2.

Example A is a comparative example which demonstrates the effect of adding inhibitors to an alkylene glycol in a "conventional" manner as taught in the prior art. This order of mixing may work for ethylene glycol in some instances but is not generally applicable to other alkylene glycols. This example will be contrasted with Example B, below, where the inhibitor content is identical to that used in this Comparative Example A, but the inhibitors are added according to the process of the instant invention.

A composition was formed by placing propylene glycol in a mixing vessel. Under constant stirring, 20 wt % borax in propylene glycol and phosphoric acid were added. The required amount of basic inorganic compound, 45% KOH in this case, was then added to neutralize acids and provide a pH of about 10.8. Next the silicone is added. Upon addition of the high mole ratio silica/alkali metal oxide mixture (SiO.sub.2 :M.sub.2 O.apprxeq.1.81) the mixture turns cloudy as a silicate gel forms. Although some clearing is noted as additional silicate mixture is added, the mixture is never completely clear of a silicate gel. The final pH of the mixture is about 11.3. Additional components were then added as shown in Table 1.

Example B demonstrates an alkali metal silicate-containing alkylene glycol made according to the process of the instant invention. the chemical concentrations are substantially the same as employed in Example A, but the mixing order is according to the instant invention.

The selected amount of propylene glycol was placed in a mixing vessel. Next, under constant stirring, a silicate premix consisting of alkali metal silicate, basic inorganic compound, and water was added slowly. (The SiO.sub.2 :M.sub.2 O mole ratio was 0.13). Addition of the silicate premix to the propylene glycol was achieved by adding a small aliquot of the silicate premix and waiting for the mixture to substantially clear before adding the next silicate premix aliquot. Addition of the silicate premix took about 2 hours and about 12 aliquot additions in substantially equal amounts. The silicone was then added. The remaining components, e.g., the inhibitors and any additional components were then added. No silicate gel formation was observed after completing the silicate premix addition. No silicate gel was observed after storage at 55.degree. C. for one year.

Comparative Example C demonstrates how silicate gel formation can arise if the process of the instant invention is not employed in the manufacture of silicate-containing propylene glycol compositions. Comparative Example C exhibited a persistent silicate gel upon addition of the silicate to the propylene glycol.

In Comparative Example C, the selected amount of propylene glycol was placed in a mixing vessel. The benzoic acid and phosphoric acid were mixed and then added to the mixture, followed by addition of the silicone. The silicate premix contained alkali metal silicate, all required basic inorganic compound (with a SiO.sub.2 : M.sub.2 O ratio of 0.12 as shown in Table 1), and water. The pH was observed to rise from less than 3 initially to about 10.8 after all the silicate premix was added. Comparative Example C was always cloudy and contained a silicate gel after storage.

Example D has the same composition as Comparative Example C, however, the composition is prepared according to the process of the instant invention.

The selected amount of propylene glycol was placed in a mixing vessel. The silicate premix was added over time in a number of aliquots. The initial aliquot was added and produced a slight cloudiness which quickly cleared. Subsequent aliquots produced less and less cloudiness upon addition. The silicone, corrosion inhibitors, and additional components were then added. No cloudiness was observed for the mixture and no silicate gels are detected after storage at 55.degree. C. for 3 months.

Examples E to I, Table 2, demonstrate the gel resistance benefits derived by use of the instant invention for a wide variety of antifreeze formulations. The compositions of Examples E to I, inclusive, were prepared according to the instant invention by adding an aqueous mixture of highly alkalized silicate directly to the propylene glycol.

The above compositions can be employed as antifreeze compositions for the cooling systems of internal combustion engines and were observed to be gel resistant both during their preparation and after storage at 55.degree. C. for 3 months.

The corrosion protection properties of the compositions prepared in Examples B and E were evaluated according to ASTM D 4340-89, "Standard Test Method for Aluminum alloys in Engine Coolants Under Heat-Rejecting Conditions" and ASTM D 1384-93, "Standard Test Method for Corrosion Test for Engine Coolants in Glassware" to demonstrate the improved corrosion protection of compositions prepared according to the instant invention.

The ASTM D 4340 and D 1384 results for Examples B and E, as compared to the ASTM D 3306 and D 5216 requirements for new coolants, demonstrates the excellent protection of aluminum under heat transfer conditions and the overall general corrosion protection afforded to the other metals typically found in an engine cooling system. This excellent protection can be attributed directly to the stable, fully formulated nature of the coolant.