CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No.

60/011,930, filed February 20, 1996.

TECHNICAL FIELD

This invention relates generally to polyvinyl alcohol compositions, and more particularly, to those polyvinyl alcohol compositions suitable for use as an aggregate adhesive or binder. Specifically, this invention relates to a modification of the polyvinyl alcohol composition such that the tendency of the polyvinyl alcohol to become sticky or overly stiff when mixed with sand or other aggregate particles is reduced or essentially eliminated. This reduction in tackiness or stiffness allows the resultant aggregate mixture with a polyvinyl alcohol binder to be used in automated equipment for the production of solid aggregate cores for the foundry, plastics and investment casting industries.

BACKGROUND OF THE INVENTION Polyvinyl alcohol (PVA) has long been used as a binder material in a number of industries. Since at least the 1950's, attempts have been made to use PVA to bind sand or other aggregate for the production of foundry cores. The use of PVA as a foundry core binder is particularly desirable given the known advantages associated therewith. Polyvinyl alcohol exhibits desirable tensile strengths at low addition rates, exhibits exceptional removal of sand after cast metal is poured around the sand core, and exhibits very low gas evolution during casting metals. For these reasons, polyvinyl alcohol binders have been investigated and used in the foundry industry for many years, unfortunately, with oftentimes unsatisfactory results. In attempting to eliminate some of the problems and disadvantages associated with the use of PVA by itself as a core binder, various additives such as, for example, starches, lignins, urea, urea-formaldehydes, and other water soluble resins, have been used to modify PVA and form modified PVA

compositions. While some of the problems and disadvantages were indeed eliminated, the resultant PVA compositions still remained unsatisfactory for use as a foundry core binder.

One problem associated with PVA was its tendency to gel, especially at low temperatures. This problem has since been overcome by adding a small amount of a surfactant to the PVA composition. Extensive research on this gelling tendency resulted in an industry recognized recommendation that up to about 0.2 percent by weight of surfactant, based upon the weight of the PVA, may be needed to control the gelling tendency of the PVA in a PVA-based aggregate adhesive. However, no particular advantage has ever been noticed in the properties of the aggregate adhesive or aggregate mixture which form the foundry core.

The major problem still associated with PVA and/or modified PVA compositions is that, in order to attain an acceptable tensile strength and scratch hardness for a sand or other aggregate core wherein a PVA or modified PVA binder in mixed with the sand or aggregate, a significant amount of PVA is required. However, once the addition rates of PVA and modified PVA are high enough to attain an acceptable tensile strength for the sand core, the sand mixtures themselves become too sticky to use and oftentimes vary widely from a putty-like substance to a wet sand consistency, even within the same sand mix. This lack of consistency within the sand mix and wide variation in green strength makes it all but impossible to use the sand mix in existing automated production equipment which requires a consistent sand mix to blow into cavities.

Thus, the need exists for a modified PVA composition which corrects this problem with the consistency and stickiness of the sand or other aggregate mixes utilizing a PVA binder.

DISCLOSURE OF INVENTION

It is, therefore, an object of the present invention to provide a polyvinyl alcohol composition suitable for use as an aggregate adhesive in an aggregate mix.

It is another object of the present invention to provide an aggregate mixture containing a polyvinyl alcohol adhesive composition which mixture is not sticky.

It is yet another object of the present invention to provide an aggregate mixture containing a polyvinyl alcohol adhesive composition, as above, which mixture is not overly stiff and has a consistent green strength throughout the entire mixture.

It is still another object of the present invention to provide an aggregate mixture containing a polyvinyl alcohol adhesive composition, as above, which mixture is suitable for use in automated production equipment.

At least one or more of the foregoing objects, together with the advantages thereof over the known art relating to aggregate binder materials, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed In general, the present invention provide a modified polyvinyl alcohol composition for binding aggregate particles, the composition including polyvinyl alcohol, from about 1.7 to about 5 percent by weight of a surfactant, based upon the weight of said polyvinyl alcohol and water.

Other objects of the present invention which will become apparent herein are attained by an aggregate mixture for use in the production of aggregate cores, the aggregate mixture including aggregate particles; and an aggregate adhesive for bonding said aggregate particles together, said aggregate adhesive containing polyvinyl alcohol; from about 1.7 to about 5 percent by weight of a surfactant, based upon the weight of said polyvinyl alcohol; and water. The adhesive composition preferrably further includes up to about 100 percent by weight lignin sulfonate, based upon the weight of said polyvinyl alcohol; less than about 4 percent by weight of at least one acid selected from the group consisting of citric acid and boric acid and mixtures thereof, based upon the weight of said polyvinyl alcohol; and up to about 1 percent by weight of a preservative, based upon the weight of said polyvinyl alcohol

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

The present invention is directed toward a modified polyvinyl alcohol (PVA) composition having a substantially increased amount of surfactant added thereto. Surfactants have long been used with PVA to reduce the gelling tendency thereof. However, only small amounts of surfactant on the order of from about 0.05 percent by weight to about 0.2 percent by weight based upon the weight of the PVA are recommended and used in any polyvinyl alcohol composition. That is, from about 0.05 percent to about 0.2 percent by weight of a surfactant, based upon the weight of the PVA, is required to control the gelling tendency of polyvinyl alcohol in an aggregate adhesive.

In contrast, the present invention employs at least ten times the recommended amount of surfactant to control gelling. This significant increase in the amount of surfactant employed is not used primarily to control the gelling tendency of PVA, but rather is used to reduce the stickiness and to provide consistency in the green strength of the aggregate mixtures employing PVA. Thus, the tendency of the wet sand or aggregate mixtures to vary from putty like substances to wet sand consistency is substantially reduced and essentially eliminated. Accordingly, the sand mixtures can be used in existing automated production equipment which requires a consistent sand mix to blow into cavities in the formation of foundry cores and the like.

Importantly and unexpectedly, it is not until one approaches ten times the recommended amount of surfactant to control gelling, that an appreciable affect on the particular properties of the wet sand mixture is attained.

A modified PVA composition of the present invention suitable for use as a modified PVA adhesive to bond sand or other aggregate particles for use as, for example, foundry cores, may include PVA, a surfactant and a variety of optional additives to modify the PVA, with the oalance of the composition being water. A preferred embodiment of a PVA composition of the present invention includes PVA, a surfactant, a lignin sulfonate, one or more acids, and a preservative.

Preferably, PVA is used in amounts ranging from not less than about 8 percent by weight based on the total weight of the binder solution to not more than about 12 percent by weight based on the total weight of the binder solution.

Amounts of PVA less than 8 percent by weight are not desirable because of the excess water which will become involved in the sand mixtures. Amounts of PVA more than 12 percent by weight are not desirable due to the extreme viscosity and gelling tendencies of the PVA. Moreover, it will be appreciated that PVA preferably constitutes at least about 50 percent by weight of the binder solids, with an amount ranging from about 70 to about 99 percent being more preferred and about 80 percent being most preferred.

Any surfactant which can provide the desired effect of reducing the stickiness and overstiffness of the sand mixture, and of minimizing the inconsistency of the sand mixture, as well as aid in reducing the gelling tendency of PVA can be utilized in the present invention. In order to provide the desired effects described hereinabove, the surfactant must be used in amounts of from about 1.7 percent by weight to about 5 percent by weight based upon the weight of the PVA. Lesser amounts may increase the problems associated with variation and inconsistency, particularly in the ability to blow a sand or aggregate particle mixture into a cavity on automated equipment, while greater amounts tend to increase cost and foaming tendency without many further beneficial effects. The surfactant, Octoxynol-9, available from Union Carbide under the tradename Triton X-100, has been found to very suitable for the purposes of this invention. About 2 percent by weight of this surfactant, based upon the weight of the PVA, appears to provide optimum results.

Lignin sulfonate, if used, may be used in amounts of up to about 100 percent by weight based upon the weight of PVA, although amounts ranging from about 20 to about 65 percent by weight are more preferred, and about 20 percent by weight based upon the weight of the PVA is most preferred. Lignin sulfonate is used as an extender resin which allows the PVA binder to coat completely the sand or other aggregate particles in the mixture. In the context of core formation, the use of an extender resin may be necessary so as to prevent some of the particles from crumbling away from the core during handling or during processing. However, it will be appreciated that even though they are coated, the particles remain sufficiently capable of being blown through automated production equipment as may be necessary.

Essentially any lignin sulfonate which is compatible with PVA may be used in the present invention. Preferred lignin sulfonates include calcium lignin sulfonate and ammonium lignin sulfonate, since they appear to provide the most beneficial effect with respect to the properties sought in the use of such resins. Various acids such as citric acid and/or boric acid may be used in the present invention. Although only minimal amounts of less than 4 percent by weight, based upon the weight of the PVA, are necessary to provide the desired effects to be carried out by the acids, too little citric acid may produce variations in the hot tensile strength of the binder, while too little boric acid may, in some instances, result in a sticky mixture. Preferably, only up to about 500 grams citric acid per 55 gallons of PVA binder solution is employed. More than that tends to increase corrosion of equipment during use. On the other hand, only up to about 100 grams boric acid per 55 gallons of PVA binder solution is preferred. More than that tends to produce a gelling effect of the solution. Preservatives may be added to prevent mold and spoilage of the binders during storage. Of course, amounts will vary depending upon the preservative employed, but generally, amounts up to about 1 percent by weight based upon the weight of the PVA are considered sufficient. One preferred preservative is sodium benzoate, available under the tradename Buscan 90. Essentially any preservative which is compatible with the PVA and various other additives and which is environmentally safe can be used in the present invention.

It will be appreciated that, while PVA and its modifying additives and extenders have been explored and used as a sand aggregate adhesive in the past, the prior art only suggests controlling gelling tendency by using surfactants in amounts of between 0.05 percent and 0.20 percent based on the weight of the polyvinyl alcohol. Unexpectedly, it has been found that by increasing the amount of these surfactants to 1.7 percent up to 5.0 percent by weight based on the weight of the polyvinyl alcohol, which range is ten times or more greater in addition ranges suggested by the prior art, the advantages of the present invention, including reduced stickiness or tackiness, are noted. Notably, this result appears to be in conflict with accepted practice in that a surfactant would normally be expected to increase wetting properties and, therefore, increase stickiness. In fact, the amount of boric acid can be increased to control

stickiness by increasing gelling tendency, but green strength also increases. Therefore, the present invention is contrary to conventional wisdom in that a significant wetting agent can be added to control both stickiness and green strength. Other advantages associated with the use of the modified PVA composition include improved synergistic effects from traditional polyvinyl alcohol modifiers, a reduction in the low temperature gelling tendencies of polyvinyl alcohol binders, and increased tensile and scratch hardness due to the ability to increase the rates of addition. Furthermore, it believed that the PVA composition could also be used in the investment casting industry such that the composition of the present invention could be used to make waxless casting components.

A modified PVA composition of the present invention can be prepared using any method known in the art, but is preferably prepared by first mixing the PVA and the citric and boric acids in approximately half of the water to be employed in a suitable container for heating. The mixture is then heated to a temperature greater than 200 °F, such as via live steam. The heat is then removed and the lignin sulfonate is added until dissolved. The surfactant, the preservative and the rest of the water, preferably cold, is added, and the mixture is thoroughly mixed and placed in an appropriate container.

In order to demonstrate the present invention, a 55 gallon batch of the

PVA adhesive solution was prepared essentially as described hereinabove. The amount of each ingredient used is set forth in Table I hereinbelow.

TABLE I MODIFIED POLYVINYL ALCOHOL COMPOSITION - 55 Gallon batch size

Polyvinyl alcohol (DuPont grades 90-50 Elvanol or 20,500 grams

52-22 elvanol)

Ammonium lignin sulfonate (Lignotech USA, Norlig TSD) 4,100 grams Citric Acid 300 grams

Surfactant 0 iton X-100) 950 grams

Boric Acid 65 grams

Preservative (Buscan 90) 200 grams

Balance Water

The production of this 55 gallon batch of the PVA composition had 12.3 percent total solids (Briks meter) and a pH of 4.1. The 55 gallons weighed 475 lbs. This PVA adhesive composition was tested and has been used in existing automated production equipment to produce sand cores for the foundry industry. A PVA composition according to the concepts of the present invention preferably is used in amounts up to about 3 percent by weight, based upon the weight of the aggregate such as sand, when it is used as an aggregate adhesive in the production of cores.

Thus it should be evident that the PVA composition of the present invention is highly effective in providing an aggregate binder material. The invention is particularly suited for foundry, investment casting and plastic industry usage, but is not necessarily limited thereto. Based upon the foregoing disclosure, it should now be apparent that the use of the composition described herein will carry out the objects set forth hereinabove.