Background of the Invention The formation of ice on roads can present a safety hazard, as a well as a potential economic impediment, in climates where road ice formation can occur during part or all of the year. Chemical means are known in the art and widely used for preventing the formation of ice on roads, or for aiding in the removal of ice from roads, e. g. by melting or loosening ice.

The ideal choice of chemical agent to be employed depends on a combination of factors, including the conditions under which the agent will be employed--e. g. temperature, type of surface to be treated, and whether ice is already present or not--and the inherent properties of the chemical agent.

The impact of the chemical agent on the enviroment must also be taken into consideration.

The aforementioned inherent properties can be classified into three groups: the ability of the agent to melt ice, i. e the volume of melt water which can be produced during a given time by application of a given amount of deicer at a specified rate; the ability of the agent to penetrate ice, i. e. its ability to melt ice when operating in a downward mode; and the ability of the agent to loosen ice from the road surface, or its undercutting ability, i. e. its ability to melt ice at the interface between a layer of ice and a pavement or road substrate. A given ice-melting agent may not be efficient in all three capacities.

Chemical deicers may be used in combination, and with other materials, to meet different needs. For example, a solution of 30-32 wt. % CaCl2 in water, so-called"liquid"calcium chloride, is often used in combination with sodium chloride (sometimes referred to, in its crude form, as"rock salt"), or with abrasives such as sand. It has been known to spray rock salt with liquid CaCl2 prior to application of the rock salt; this method reduces the amount of rock salt required for application at low temperatures (~-7°C or less).

US describes a process for the preparation of sodum chloride coated with calcium chloride powder or particles, whereby the sodium chloride is coated with an adhering solution (usually water or water with some potassium acetate mixed in, optionally containing calcium chloride as well), and then dry calcium chloride is mixed in with the NaCl which has been treated with the adhering solution.

Various drawbacks exist in the methods known in the art. By their nature, preventive measures must be taken prior to, or shortly after, the onset of ice formation; the ability of the agent to be applied to prevent ice formation, as well as the temperature and surface to be treated, will dictate how much material need be applied, and whether the material must be applied in advance of ice formation, or how soon after commencement of ice formation the agent may be applied. If, however, the weather forecast is erroneous, and no freezing is forthcoming, a deicing agent applied prior to the fall of precipitation will be wasted. If the potency of the agent could be in increased, this would enable a longer wait prior to application of the agent, so that the accuracy of weather forecasts could be verified or invalidated, thus sparing unnecessary applications of the agents.

Furthermore, increasing the efficacy, and/or lengthening the duration of efficacy, of the chemical agent would enable application of a smaller amount of agent, thus saving money and creating less impact on the environment.

It is an object of the invention to provide a deicing composition with good penetrating, melting and undercutting abilities.

It is another object of the invention to provide a deicing composition which can act quickly.

It is a further object of the invention to provide a deicing composition which, by virtue of the aforesaid properties, can decrease the total amount of deicing agent necessary to melt, loosen, or prevent the formation of ice on roads relative to the total amount of deicing agent required in techniques known in the art.

It is still another object of the invention to provide a process for the preparation of compositions of the invention.

Other objects, purposes and advantages of the invention will become apparent as the description proceeds.

Summary of the Invention The improved deicing composition according to the invention comprises (a) an inner layer containing a secondary ice melting agent, and (b) an outer layer containing a primary ice melting agent, wherein said primary ice melting agent has been melt-coated onto said secondary ice melting agent.

Throughout this description, by"secondary melting agent"is meant potassium chloride or sodium chloride."Primary melting agent"will be understood to refer to calcium chloride complexed with water or magnesium chloride complexed with water; each molecule of CaCl2 or MgCl2 will have from two to six molecules of water complexed around it. By"melted primary melting agent"is meant a primary melting agent which, when part of the composition of the invention, is either in a melted state or was in a melted state immediately prior to formation of the composition of the invention.

The invention also comprises a process for the preparation of a composition for melting ice or preventing the formation of ice on roads, comprising the steps of (a) forming a first layer containing a secondary ice melting agent; and (b) forming a second layer on top of said first layer, said second layer containing a primary ice melting agent which has been melt-coated onto said secondary ice melting agent.

Brief Description of the Figures Fig. 1 compares the rate of ice penetration ability at-10°C of NaCl and a composition of the invention containing NaCl coated with MgC12* (H20) 6, 26% by weight (Salmag 26).

Fig. 2 compares the rate of ice penetration ability at 14°C of NaCl and a composition of the invention containing NaCl coated with 50% MgCl2* (H20) 6 (Salmag 50).

Detailed Description of Preferred Embodiments The composition according to the invention may comprise from about 30wt. % to about 92 wt. % secondary ice melting agent, and from about 8 wt. % to about 70 wt. % primary ice melting agent. The secondary melting agent is selected from potassium chloride or sodium chloride. The primary melting agent agent is selected from calcium chloride complexed with water or magnesium chloride complexed with water; each molecule of CaCl2 or MgCl2 will have from two to six molecules of water complexed around it.

Other ingredients, such as abrasives, may be incorporated into the composition of the invention if desired, although this is not necessary. The relative amounts by weight of secondary ice melting agent and primary ice melting agent may be varied in accordance with intended conditions for use, but will usually fall in the range of from about 11: 1 to 1: 2.

Preferably, the composition is in pelletized or spheronized form, with pellets (spheres) of a size between about 2 and about 11 mm average diameter (although pellets (spheres) with sizes larger or smaller than this may in principle be obtained); more preferably, the size of the pellets (spheres) is between about 2 and about 5 mm average diameter. The radius of the inner layer is preferably from about 1 to about 4 mm. The thickness of the outer layer is preferably between 0.5 and 3 mm. It is known (see, e. g.,"Handbook of Test Methods for Evaluating Chemical Deicers", Strategic Highway Research Program, National Research Council, Washington, D. C., 1992, and protocol SHRP H-205.1 therein for measuring ice-melting ability) that the ice-melting properties of NaCl and CaCl2 are similar at-3. 9°C, but at-15°C CaCl2 is a markedly better ice-melter. Similarly, while the ice undercutting ability (cm2 ice undercut/g deicer applied, determined by protocol SHRP H-205.5, ibid.) of CaCl2 is better then NaCl at-3. 9°C, at-15°C CaCl2 is noticeably better at ice-undercutting.

One of the advantages of the present invention is that the compositions of the invention combine properties of both the primary ice melting agent and the secondary ice melting agent. Thus, as shown in Fig. 1, a composition according to the present invention, containing sodium chloride coated with magnesium chloride, initially acts more quickly than NaCl alone (approximately twice as effective over the first 30 minutes), and over the next several hours closely approximates the efficacy of NaCl alone (approximately 80% as effective as NaCl). Melting ability was determined using the aforementioned protocol SHRP H-205.1.

The compositions of the invention may be used in conjunction with other methods, e. g. dispersion of abrasives on road surfaces or incorporation of abrasives into the compositions of the invention. As stated, the amounts of material to apply will depend on a variety of conditions, including but not limited to the surrounding temperature, amount of ice to be melted or prevented from forming, nature of the surface to be treated, density of traffic on the surface to be treated, and the size of the pellets used.

The compositions of the invention may be formed by any convenient method known in the art. For example, the compositions may be prepared by drum-coating NaCl granules with magnesium chloride, although other methods of forming the compositions of the invention, particularly other methods of melt-coating the primary ice-melting agent onto the secondary ice melting agent, can readily be envisioned by practitioners skilled in the art, and are contemplated within the scope of the invention. Unlike the compositions and process disclosed in US 5,599,475, the compositions of the present invention are formed by coating the secondary ice melting agent with melted primary ice melting agent, without the use of additional adhering components and the like. The outer coating is essentially homogeneous, as it is not composed of powder or particles which are glued onto a foreign surface, as is done in prior art materials.

The foregoing characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative examples of preferred embodiments thereof.

Example 1 Drum coating of NaCl crystals NaCl crystals of 5 mm average diameter g each) were placed in a revolving drum and heated to about 150°C. At the same time, a weighed amount (about 25 wt. % relative to NaCl) of MgCl2 hexahydrate flakes were heated at 160°C until molten. The clear molten liquid was carefully poured into the revolving drum containing the rock salt crystals, using a baffle to help mix the components. Visual inspection indicated that the NaCl crystals became coated almost immediately. The heat source was removed and the coated crystals were allowed to cool while mixing was continued.

After a few minutes the coated particles stopped adhering to one another, and were allowed to continue to cool in the rotating drum. The particles obtained contained an average of about 74 wt. % NaCl, about 26 wt. % MgCl2. 6H20.

In a similar fashion, smaller NaCl particles were coated with MgCl2. The ratio of sodium chloride to magnesium chloride in the coated particles was approximately the same as when 5 mm NaCl was used.

Example 3 Ice Penetration Test This example was conducted in accordance with SHRP H-205.3 ("Handbook of Test Methods for Evaluating Chemical Deicers", Strategic Highway Research Program, National Research Council, Washington, D. C., 1992).

The ice penetration abilities (at-10°C) of pellets of NaCl and MgCl2-coated NaCl (Salmag 26), prepared as per Example 1, were compared. The results are plotted in Fig. 1. These results confirm that the coated material behaves initially in a manner similar to the behavior of the coating material, but over the course of several hours behave in a manner more similar to the core material.

A similar test was conducted to compare the rate of ice penetration ability at 14°C of NaCl and a composition of the invention containing NaCl coated with 50% MgCl2* (H20) 6 (Salmag 50). The results are plotted in Fig. 2.

Example 4 Preparation of CaCl2-coated NaCl To CaCl2D2H20 was added water to obtain CaCl2D4H20. This slurry was then heated until a clear liquid was obtained at 45°C. This was then added to a revolving drum, also heated to 45°C and containing NaCl (15-50 wt. % relative to CaCl2-4H20), and the temperature of the drum and its contents was then raised to 175°C. The coated particles obtained were allowed to cool. The coated particles contained, on average, about 75 wt. % NaCl and about 25 wt. % CaCl2 complexed with water. The coated particles so obtained behave similarly to the particles of Example 1 in the de-icing tests described above.

The foregoing description and examples have been provided for illustrative purposes only, and are not intended to limit the invention in any way. It will be apparent that many modifications, variations and adaptations may be made to the invention by persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.