Compositions – Frost-preventing – ice-thawing – thermostatic – thermophoric,...
Reexamination Certificate
1999-09-21
2002-05-21
Green, Anthony (Department: 1755)
Compositions
Frost-preventing, ice-thawing, thermostatic, thermophoric,...
C047S002000, C106S013000, C252S071000, C252S073000, C252S077000
Reexamination Certificate
active
06391224
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of inhibition of ice growth. More specifically, the invention relates to a method for inhibition of ice growth using polyvinyl alcohol and related compounds.
BACKGROUND OF THE INVENTION
Preventing the freezing of water, and solutions that contain water, is a problem of substantial environmental, agricultural, industrial, and biomedical interest. Ice on walkways, roads and aircraft wings constitute an environmental hazard to transportation. Ice formation on and inside plants causes expensive damage to crops and gardens. Freezing of antifreeze solutions, pipeline contents, paints, wet concrete and other aqueous solutions subjected to cold temperatures are issues of concern for industry. Avoiding ice formation during cold storage of tissue is also an important problem in cryobiology.
Below a critical temperature (the equilibrium freezing point), the crystallization of water into ice becomes thermodynamically favored. The freezing point of water can be lowered by adding solutes that interact with water, thereby interfering with the ability of water molecules to organize into ice crystals. The resulting freezing point depression is termed “colligative” freezing point depression. Colligative freezing point depression is the physical basis on which essentially all currently used antifreeze agents (such as glycols and salts) operate. The disadvantage of colligative freezing point depression is that large quantities of solutes (10% or more) are required to lower the freezing point by even a few degrees Celsius.
Beyond colligative freezing point depression, there is another approach that can be used to prevent ice formation and growth. At temperatures above −40° C. water cannot freeze unless the freezing is catalyzed by ice that has already formed, or by impurities called ice nucleating agents (INAs). It is therefore possible for water and water solutions to exist as “supercooled” liquids at temperatures significantly below the freezing point. In practice, significant supercooling is rarely observed in nature (with the exception of microscopic water droplets in rain clouds). This is because INAs are ubiquitous in the environment, causing water to almost invariably freeze if it is cooled slightly below the freezing point. Even highly purified laboratory grade water contains significant background concentrations of INAs. If INAs can be removed or inhibited, water and water solutions can be supercooled to temperatures many degrees below the freezing point without actually freezing.
Cold-hardy plants, insects, and fish have evolved antifreeze proteins that selectively adsorb onto the surface of ice or INAs, thereby preventing water molecules from coming into contact with surfaces that trigger ice growth (Devries, A. L., and Wohlschlag, D. E. “Freezing resistance in some Antarctic fishes” Science 163, pp. 1074-1075, 1969). Antifreeze proteins thus act as non-colligative antifreeze agents, and very small concentrations (less than 1%) are able to suppress the temperature at which ice forms by several degrees. Soon after the original discovery of antifreeze proteins, it was speculated that “many polymeric molecules (not just proteins) ought to be able to inhibit nucleation (of ice) in this way” (Klotz, I. M. in “The Frozen Cell” pp. 5-26. J. & A. Churchill, London, 1970). These speculations opened the door to the possibility that inexpensive synthetic compounds might be found with non-colligative antifreeze activity.
SUMMARY OF THE INVENTION
The present invention provides compounds that adsorb onto ice and especially ice nucleating agents, thereby inhibiting said agents, and thereby permitting supercooling of water and water solutions to temperatures below the freezing point without actually freezing.
The invention further provides additives that in small concentrations prevent water from freezing at temperatures below 0° C. Such additives are useful in industry and agriculture to prevent irrigation water from freezing in pipes or on plants subjected to cold weather. Such additives may also be useful in preventing water inside plants from freezing when irrigation water or soil contain the additives. Such additives may also be further useful in preventing water from freezing on surfaces such as roads or aircraft wings in cold weather.
Still further, the invention provides additives that in small concentrations prevent solutions of water from freezing at temperatures below the solution freezing point. Such additives will augment the performance of antifreeze solutions used as engine coolants and deicing solutions by permitting said solutions to endure temperatures below their rated freezing point without freezing. Such additives also augment the cold tolerance of paints, cements, concretes, and other aqueous media that are stored or cured under conditions that entail a risk of freezing.
The additives of the present invention reduce or inhibit the formation of ice in biological materials undergoing cryopreservation. Such additives are of particular utility for cryopreservation by vitrification, a cryopreservation method which requires complete suppression of ice formation.
The additives of the present invention can facilitate hypothermic preservation of biological materials in a supercooled state below 0° C. These additives reduce the likelihood of freezing occurring in the supercooled state and/or make lower preservation temperatures possible.
The invention additionally provides compounds that are able to substitute for biologically-derived antifreeze proteins in diverse applications, as well as compounds that inhibit growth or recrystallization of ice. Compounds that adsorb onto ice nucleating agents for purposes of extracting ice nucleating agents from water and water solutions are also provided. The compounds of the present invention can also be dispersed in the atmosphere to alter precipitation in rain clouds by inhibiting atmospheric ice nucleating agents.
These and other aspects of the present invention will be apparent to those of ordinary skill in the art in light of the description below and appended claims.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides methods for preventing the freezing of water and solutions that contain water using inexpensive compounds with non-colligative antifreeze activity.
Polyvinyl alcohol (PVA) is a water-soluble polymer consisting of an alkane backbone with hydroxyl groups attached at every second carbon. It thus consists of repeating CH2CHOH units. It is an inexpensive high tonnage industrial chemical used in adhesives, textile manufacturing, food packaging, cosmetics, and pharmaceutical preparations. It is non-toxic, environmentally friendly, and biodegradable.
It has been discovered in this invention that PVA and related compounds act as non-colligative antifreeze compounds that preferentially adsorbs onto ice nucleating particles and surfaces in a manner similar to natural antifreeze proteins. As the examples below show, very small concentrations of PVA (as little as 1 part per million) significantly enhance the ability of water and water solutions to supercool without ice formation.
However, we find that PVA is very effective as an ice inhibiting agent at concentrations ranging from 100 parts per billion to tens of percent. Concentrations ranging from 0.01% to 10% w/w are preferred. Concentrations ranging from 0.1% to 2% w/w are more preferred. Concentrations ranging from 0.3% to 1% w/w are most preferred. It will be understood by those skilled in the art that the choice of PVA concentration in any antifreeze application will also depend on factors other than maximum ice inhibition, including cost and solution viscosity considerations.
PVA is commercially available in molecular weights ranging from approximately 5 kDa (kilo daltons) to hundreds of kDa. Lower molecular weights are preferred because smaller polymer molecules are more mobile in solution. Greater mobility results is lesser viscosity increase when PVA is added to solutions, and greater ability
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Green Anthony
Hendrickson Jay P.
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