The rapid decrease in viscosity of liquids with increasing temperature is well-known. Ideally, for many applications (automobile lubricants, etc.) it would be desirable to solve this problem so that liquid viscosity would be insensitive to temperature. Alternatively, it might be desirable to provide liquid systems whose viscosities actually increase with temperature. It is true that with selected polymeric additives, it has been possible to reduce substantially the viscosity change with temperature which does occur with most oils and similar systems. These polymer additives, known as viscosity index improvers (or V.I. Improvers), are generally high molecular weight polymers.
The way in which these additives function can be summarized very briefly. In effect, they perform two functions, i.e., thickening, which merely increases fluid viscosity; and Viscosity Index (V.I.) improvement, which corresponds to limited thickening at ambient temperatures and a corresponding greater thickening at elevated temperatures. This can be accomplished by utilizing a polymeric additive which is poorly solvated by the liquid at ambient temperatures; however, at elevated temperatures, the polymer is more highly solvated such that the polymer expands and is a relatively more effective thickener.
While these V.I. Improvers have proven successful commercially, it is important to note that their effect at reducing viscosity changes with temperatures is rather mild. For a typical base oil containing a suitable V.I. Improver, the kinematic viscosity will still decrease by a factor of from 5 to 10 as the temperature increases from 30.degree. to 100.degree. C. Obviously, if it is desired to hold the viscosity roughly constant with such temperature changes, current technology has not offered an appropriate additive system.
U.S. Pat. No. 3,396,136 describes how copolymers of alkenylaromatic sulfonic acids, when properly neutralized, can be employed as thickeners for nonpolar solvents. Those metal sulfonate systems have been shown to be very effective; however, when employed as 2 component systems (i.e., ionic polymer plus nonpolar solvent), the variation of viscosity with increased temperature is very conventional and predictable. That is, the solution viscosity decreases markedly as temperature is increased.
U.S. Pat. No. 3,396,136 further teaches "in situ" neutralization of the sulfonic acid polymer which, under some conditions, can result in the availability of a small amount of polar cosolvent, i.e., a solvent for the sulfonate groups about equal in amount to the amount of sulfonate groups which are present. This amount of polar cosolvent is not within the critical limits of the instant invention, which require amounts of the third component (which interacts with the ionomeric groups of the ionomer copolymer) at levels which range from 10 to 600 times the molar equivalence of ionic groups. This level of cosolvent is about one to two orders of magnitude or more higher than employed in the cited art. This amount of such cosolvent is required to obtain the unusual and unexpected viscosity-temperature behavior which is observed. In addition, the cited patent is restricted to aromatic sulfonate polymers. The instant invention describes other polymers such as co- or terpolymers of acrylates or methacrylates with a sulfonate-containing monomer.
U.S. Pat. No. 3,666,430 teaches the gelling of organic liquids by the interaction of polar "associative bonds" which includes hydrogen bonding and "ionic crosslinking." Again this patent specifies that two components are necessary--the associating polymer (or polymers in some cases) and the nonpolar organic liquid. There is no mention of a third polar cosolvent except to point out that such polar liquids should not be present. Specifically, this patent states (column 2, line 7) that the hydrocarbon liquids to which this invention is to be applied should not contain a substantial portion of a miscible protolytic liquid such as methanol. It is clear that the language of this patent limits this invention to gels and further, that any amount of polar liquids which are present to an extent where they disrupt those gels are undesirable. The instant invention is distinct from that cited in that amounts of such polar compounds, as will break up gel at ambient conditions are required, and in fact, the most preferred state is free of any said gel at ambient temperatures.
U.S. Pat. No. 3,679,382 teaches the thickening of aliphatic hydrocarbons with synthetic organic polymers which contain olefinically unsaturated copolymerizable acids, amides, hydroxyacrylic esters, sulfonic acids, etc. It is emphasized in this patent (column 3, line 72) that it is critical in the preparation of such polymers that no surface active agent, catalyst or other additive be employed which introduces a metallic ion into the system. Therefore, it is preferred to employ ammonium or amine salts. It is clear that this invention (U.S. Pat. No. 3,679,382) specifically precludes the use of metallic counterions and is directed towards amine or ammonium derivatives. Only metallic counterions are effective in the instant invention and, in fact, attempts to employ amine derivatives have not resulted in the products which are the objective of this invention. Finally, this cited patent does describe (column 7, lines 13-19) that the addition of alcohols will reduce the viscosity of the thickened hydrocarbon and alter flow characteristics thereof.
One class of oil soluble polymers known to possess good low temperature viscometrics and to be practically of great interest in the V.I. additive area is that of the acrylate or methacrylate polymers. These long chain acrylate systems (alkyl groups of about 10 to 12 carbons or so) are sold commercially and have been very successful in achieving a suitable balance of thickening behavior and a combination of good low temperature fluidity and high temperature viscosity at reasonable polymer levels. This invention proposes a class of materials that will have comparable or superior viscometrics as compared to the acrylate or methacrylate family of materials. This preferred class of polymers is based on combining sulfonate containing monomers with acrylates and methacrylates of suitable chain length by appropriate polymerization techniques.
The instant invention represents an improvement over the process as described in U.S. Pat. No. 3,931,021, wherein the class of co- or terpolymers, as described in the instant invention, show improved low temperature viscometric properties. The co- and terpolymers of the instant invention are restricted to a class of polymers comprising copolymers and terpolymers derived from a metal sulfonate monomer and an alkyl acrylate or alkyl methacrylate, wherein the alkyl group has at least eight carbon atoms.