WO1997018913A1

WO1997018913A1 - Cold-box process for preparing foundry shapes

Info

Publication number: WO1997018913A1
Application number: PCT/US1996/018595
Authority: WO
Inventors: Santiago Prat Urreiztieta; Marco Antonio Mendizabel Castellanos
Original assignee: Ashland Inc.
Priority date: 1995-11-21
Filing date: 1996-11-20
Publication date: 1997-05-29
Also published as: AU1078797A

Abstract

The invention relates to a cold-box process for preparing foundry shapes. The process involves curing a foundry shape by subjecting it to carbon dioxide and heat. The foundry shape is made by mixing an aggregate with a foundry binder comprising: (a) an aqueous basic solution of a phenolic resole resin; and (b) preferably a compound which is a source of an oxyanion. The invention also relates to a process of making metal castings with the foundry shapes.

Description

COLD-BOX PROCESS FOR PREPARING FOUNDRY SHAPES

TECHNICAL FIELD

The invention relates to a cold-box process for preparing foundry shapes. The process involves curing a foundry shape by subjecting it to carbon dioxide and heat. The foundry shape is made by mixing an aggregate with a foundry binder comprising: (a) an aqueous basic solution of a phenolic resole resm, and (b) preferably a compound which is a source of an oxyanion. The invention also relates to a process of making metal castings with the foundry shapes.

BAOG-ROUND OF THE INVENTION It is known to prepare foundry shapes by the cold-box process by contacting a foundry shape, made by mixing an aggregate with an aqueous basic solution of a phenolic resole resin, with gaseous methyl formate. Although this process has some advantages from an environmental standpoint when compared to a process using polyurethane-forming binders cured witn volatile amines, the physical properties of the foundry shapes made with these binders are inferior to those made by the cold-box process from polyurethane-forming binders. In particular, the tensile strength and scratch hardness of foundry shapes made by the cold-box process based on methyl formate are lower. Consequently, there s an interest in finding additives or otherwise modifying the formulations of these binders to improve the handling properties of the foundry shapes made with the foundry binders. Because the cold-box process based on methyl formate also creates unwanted stress on the environment, there is an interest m cold-box processes which use carbon cioxide as the curing catalyst. In these processes, foundry shapes are prepared by mixing an aggregate with the alkaline phenolic resole resin, but the foundry shape is cured by contacting the foundry shape with carbon dioxide. This results in a process which creates less stress to the environment, but which produces foundry shapes with inferior tensile strengths and scratch hardness than those produced by the process using methyl formate. Consequently, is necessary to modify these binders based upon aqueous solution of a phenolic resole resin cured with carbon dioxide. One way of improving the tensile strengths and scratch haid-.etss of the foundry shapes made with these binders is by adding a compound capable of forming an oxyanion soluble in the aqueous solution of a phenolic resole resin, for instance a soluble boron containing compound. See for example U.S. Patents 4,985,489, and 4,977,209 which discuss alkaline phenolic resole resins containing oxyanions such as boron.

Although the addition of the oxyanion improves the properties of foundry shapes made with aqueous solution of a phenolic resole resin, the foundry shapes cannot compare to those prepared with polyurethane cold-box binders. In fact, several patents show the use of additives to improve the strength of foundry shapes made by using carbon dioxide cured aqueous alkaline phenolic resoles containing an oxyanion. See for example European Patent Applications 0 503 759 A2 where pyrrolidone is the additive, 0 508 566 A2 where a phenyl ethylene glycol ether is the additive, and U.K. Patent Application GB 2 253 627 where an aliphatic glycol ether is the additive. U.S. patent 5,162,393 also describes improved performance of these binders when a pressurized core box with carbon dioxide is used in the process. In spite of these improvements, there still is a need to improve the physical properties of foundry shapes made with binders cured with carbon dioxide, particularly since these processes require longer gassing times, such as from 30 to 60 seconds to produce workable foundry shapes. These longer gassing time result in slower cycle times and decreased productivity result.

SUMMARY OF THE INVENTION The subject invention relates to a cold-box process for preparing a workable foundry shape comprising:

(a) forming a foundry mix by mixing a foundry aggregate with a bonding amount of up to about 10 percent by weight, based upon the weight of the aggregate, of a binder comprising:

(1) an aqueous basic solution of a phenolic resole resin; and

(2) preferably a compound which is a source of an oxyanion capable of forming a stable complex with said resin;

(b) shaping the foundry mix of (a) into a foundry shape;

(c) contacting the foundry shape of (b) with carbon dioxide gas and a source of heat at a temperature of 20°C to 100°C; and (d) allowing the foundry shape to harden into a workable foundry shape. Preferably the source of heat is the tooling of the corebox. Foundry shapes made by this process show improved scratch hardness without sacrificing tensile strengths when compared to foundry shapes made without heating the foundry shape. Improved scratch hardness is particularly important when making bulky cores, for instance cores weighing 0.1 to 100 kilograms and having dimensions of from 5 X 5 X 5 cm³ to 100 X 100 X 100 cm³. Other properties of the cores are not sacrificed even when shorter gassing times are used, for e.g. from 1 second to 30 seconds. Because gassing time can be reduced, productivity can be increased by reducing cycle times.

BEST MODE AND OTHER MODES

Definitions

For purposes of this disclosure, a "foundry shape" is a shape used in pouring metal castings and is made by shaping a mixture of a foundry aggregate and a binder.

Such shapes include cores, molds, and assemblies of cores and molds.

The "cold-box process" refers to a process for making foundry shapes wherein a foundry mix is formed by mixing an aggregate and a binder and cured with a gas curing agent.

The foundry mix is mechanically forced into a corebox where it is cured to form a foundry shape. The cold-box process is particularly useful for smaller foundry shapes such as cores weighing from about 100 g to about 100 kg, typically from about 1 kg to about 50 kg.

Resin Comoonent Of Binder Svstem The aqueous basic solutions of phenolic resole resins used in the subject binder compositions are prepared by methods well known in the foundry art. The specific method for preparing the aqueous solutions of phenolic resole resins is not believed to be critical to the effective practice of this invention. Those skilled in this art will know what conditions to select depending upon the specific application.

The general procedure involves reacting an excess of an aldehyde with a phenolic compound in the presence of a basic catalyst at temperatures of about 40^βC to about 120°C, typically from about 50^CC to about 90°C, to prepare a phenolic resole resin. Generally the reaction will also be carried out in the presence of water. Preferably, the resulting phenolic resole resin is diluted with a base and/or water so that an aqueous basic solution of the phenolic resole resin results having the following characteristics:

1. a viscosity of less than about 2,000 centipoise, preferably less than about 450 centipoise at 25^CC as measured with a Brookfield viscometer, spindle number 3 at number 12 setting;

2. a solids content of 35 percent by weight to 75 percent by weight, preferably 50 percent by weight to 65 percent by weight, based upon the total weight of the aqueous basic solution, as measured by a weight loss method by diluting 0.5 gram of aqueous resole solution with one milliliter of methanol and then heating on a hotplate at 150°C for 15 minutes; and 3. an equivalent ratio of base to phenol of from 0.2:1.0 to 1.1:1.0, preferably from 0.3:1.0 to 0.95:1.0. It has been found that aqueous basic solutions having viscosities outside the cited range are difficult to use in foundry applications. Aqueous basic solutions with a solids content below the cited range will not sufficiently coat the aggregate while those having a solids content above the cited range will not be sufficiently flowable in the molding equipment. The equivalent ratio specified for the base relates co tne need tor having solutions which have adequate shelf stability.

Although these ranges have been specified, it should be pointed out that it is not claimed that these aqueous basic solutions are novel products, or that the ranges are critical. The ranges are set forth to provide guidelines for those who want to make and use the invention. Obviously, the invention will usually be practiced more effectively in the preferred ranges specified. With this in mind, more specific procedures will be set forth for preparing phenolic resole resins.

The phenolic compounds used to prepare the phenolic resole resins can be represented by the following structural formula:

wherein A, B, and C are hydrogen, or hydrocarbon radicals or halogen. The aldehyde used in preparing the phenolic resole resin may also vary widely. Suitable aldehydes include aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, furfuraldehyde, and benzaldehyde. In general, the aldehydes used have the formula RCHO, where R is a hydrogen or a hydrocarbon radical of 1 to 8 carbon atoms. The most preferred aldehyde is formaldehyde.

The basic catalysts used in preparing the phenolic resole resin include basic catalysts such as alkali or alkaline earth hydroxides, and organic amines. The amount υf catalyst. Ubed will vary depending upon the specific purposes. Those skilled in the art are familiar with the levels needed.

It is possible to add compounds such as lignin and urea when preparing the phenol formaldehyde resole resins as long as the amount is such that it will not detract from achieving the desired properties of the aqueous basic solutions. Urea is added as a scavenger to react with unreacted formaldehyde and decrease the odor caused by it. Although urea may be added for these purposes, it is believed that lower long term tensile strengths may result by the addition of urea. Therefore, if long term tensile strengths are of paramount importance, the urea should be avoided. The phenolic resole resins used in the practice of this invention are generally made from phenol and formaldehyde at a mole ratio of formaldehyde to phenol in the range of from about 1.1:1.0 to about 3.0:1.0. The most preferred mole ratio of formaldehyde to phenol is a mole ratio in the range of from about 1.4:1.0 to about 2.2:1.0. As was mentioned previously, the phenolic resole resin is either formed in the aqueous basic solution, or it is diluted with an aqueous basic solution. The base used in the aqueous basic solution is usually a dilute solution of an alkali or alkaline earth metal hydroxide, such as potassium hydroxide, sodium hydroxide, calcium hydroxide, or barium hydroxide, preferably potassium hydroxide or mixtures of sodium hydroxide and potassium hydroxide, in water such that the solution typically contains from about 25 to about 55 percent water by weight.

It should again be mentioned that the aqueous basic solutions described herein are not novel products, nor is their method of preparation. The parameters set forth pertaining to their preparations are merely guidelines for those who want to make the aqueous basic solutions. There may be other effective ways to make them which are not described herein.

The Oxvanion Comooner.t of the Binder

The oxyanions present in the binder composition act as cross-linking agents for the resin by forming complexes with adjacent resole phenol-aldehyde chains. The cross- linking action of the oxyanions is promoted by the carbon dioxide gas which is passed through the foundry shape formed with the aggregate and binder composition. As a result much larger, more highly cross-linked resole phenol- aldehyde molecules are formed when the resin is cured. The exact mechanism by which the carbon dioxide promotes curing of the resin is not certain but the carbon dioxide forms carbonic acid by the reaction with water in the binder composition, thus lowering the pH of the binder. The oxyanions form stable complexes with the resin molecules at the reduced pH. The alkalinity of the binder composition must be such that the oxyanions remain largely in the uncomplexed state before gassing with carbon dioxide. Complexing and hence curing of the resin on the passage of carbon dioxide takes place when the pH is reduced.

Examples of suitable oxyanions for use in the process and binder composition of the invention include borate, stannate and aluminate ions. Borate ions are preferred. The oxyanion may be introduced into the binder composition by the addition of for example alkali metal oxyanion salts such as sodium tetraborate decahydrate, potassium tetraborate tetrahydrate, sodium metaborate, sodium bentaborate, sodium stannate trihydrate or sodium aluminaLe, or an ammonium oxyanion salt such as ammonium borate. Borate ions may also be introduced by the addition of boric acid or they may be formed by reaction between boric oxide and alkali in the binder solution. The mole ratio of oxyanions (expressed as boron, tin, etc.) to phenol is preferably in the range of from 0.1:1 to 1:1. When the oxyanion is borate the mole ratio of boron to phenol is more preferably in the range of from 0.1:1 to 0.5:1.

Foundry Aggregate Any foundry aggregate can be used to prepare the foundry mix. Generally the aggregate will be sand which contains at least 70 percent by weight silica. Other suitable sand includes zircon, olivine, alumina-silicate sand, chromite sand, and the like. Generally, the particle size of the sand is such that at least 80 percent by weight of the sand has an average particle size between 50 and 150 mesh (Tyler Screen Mesh) . Mixtures of sand and reclaimed sand can used.

Optional Constituents Several optional constituents can be used in the binder system. A particularly useful additive to the binder compositions in certain types of sand is a silane such as those having the general formula:

wherein R' is a hydrocarbon radical and preferably an alkyl radical of 1 to 6 carbon atoms and R is an alkyl radical, an alkoxy substituted alkyl radical, or an alkyl amine substituted alkyl radical in which the alkyl groups have from 1 to 6 carbon atoms. Such silanes, when employed in concentrations of 0.1% to 2%, based on the phenolic binder and hardener, improve the humidity resistance of the system.

Examples of some commercially available silanes are Dow Corning Z6040 and Union Carbide A-187 (gamma glycidoxy propyltrimethoxy silane) ; Union Carbide A-1100 (gamma aminopropyltriethoxy silane); Union Carbide A-1120 (N- beta (aminoethyl)-gamma-amino-propyltrimethoxy silane); and Union Carbide A-1160 (Ureido-silane) .

The binders may also contain optional components such glycols, methanol, and pyrrolidone in an amount of 1-15 percent by weight based upon the weight of the resin. See for instance European Patent Applications 0 503 759 A2 where pyrrolidone is the additive, 0 508 566 A2 where a phenyl ethylene glycol ether is the additive, and U.K. Patent Application GB 2 253 627 where an aliphatic glycol ether is the additive. Amounts Of Components Used In making foundry shapes, the aggregate constitutes the major (typically more than 90 percent by weight of the total weight of the foundry shape) constituent and the binder constitutes a relatively minor amount. The amount of binder, which includes the resin and oxyanion, is generally no greater than about ten percent by weight and frequently within the range of about 0.5 to about 7 percent by weight based upon the weight of the aggregate. Most often, the binder content ranges from 0.6 to about 5.0 percent by weight based upon the weight of the aggregate.

In general the weight ratio of resin component to oxyanion component is from about 5:1 to about 50:1, preferably from about 10:1 to 40:1, most preferably from about 15:1 about 30:1.

Preparing Foundry Shapes

A foundry mix is typically prepared by mixing the aqueous solution of the phenolic resole resin containing the oxyanion with the aggregate and other optional components. Foundry shapes are prepared with the foundry mixes by blowing the foundry mix into a heated corebox at pressures from 0.3 to 6.0 kg/cm², according to techniques well known in the art to form a foundry shape. The foundry shape is then gassed with carbon dioxide and left in the corebox until it is workable, typically from 1 second to

10 seconds. A workable founαry shape is one which can be handled without breaking when it is removed from the pattern. Curing with carbon dioxide is carried out according to techniques well known in the art.

Curing the foundry shapes

Curing is affected by subjecting the foundry shapes to carbon dioxide gas in the presence of heat. Preferably, the source of heat is the corebox equipment itself. The temperature of the corebox is from 30°C to 120°C, preferably

40°C to 100°C, most preferably 60°C to 80°C until the foundry shapes can be handled without breaking and damaging their surface, typically for 1 second to 60 seconds. Heating time is a function of the temperature and the heating process used. Other sources of heat include warm air, a conventional oven, or a microwave.

Typical gassing times for the carbon dioxide are from 1 second to 60 seconds. Typical concentrations of carbon dioxide are 10 to 100%. The foundry shape is subjected to temperatures of 30°C to 120°C , preferably from 60°C to 80°C. Metal castings are produced from the workable foundry shapes in a conventional manner. Essentially, molten metal (ferrous or non-ferrous) is poured into and around the workable foundry shape and allowed to harden. The workable foundry shape is then removed.

DEFINITIONS AND ABBREVIATIONS

Cold tensiles Tensiles of foundry shapes measured after they have cooled for about 60 minutes.

Hot tensiles Tensiles of foundry shapes measured within 1 minute after gassing with CO2.

RH Relative humidity.

SH Scratch hardness of foundry shape.

EXAMPLES The binder used in the examples was NOVANOL-100 binder which is sold by Ashland Suedchemie Kernfest. NOVANOL-100 is a borate (about 4% by weight of the resin) containing phenol-formaldehyde base catalyzed resole condensate prepared by reacting phenol, paraformaldehyde, and water in the presence of dilute alkali hydroxide bases (45% to 50% in water) at increased temperatures. The resin also contains glycols in the amount of about 12% by weight based upon the weight of the resin. The resin component has a solids content of about 60 percent, a viscosity of about 300 centipoise at 25°C. Sand Tests Foundry mixes were prepared with various two component binder systems by mixing 2.5 weight percent, based upon the weight of sand, of NOVA OL-100 with round silica ECHAVE C55 sand. Foundry shapes were made with the foundry mixes by conventional cold-box techniques.

The foundry mixes were prepared by first mixing the sand with the aqueous basic phenolic resole resin solution containing the oxyanion. The foundry mix was then forced into a standard core box (dog bone shape/3.5 cm²) and gassed with C0₂ for 30 seconds. The scratch hardness of foundry shapes made with the binders were measured according to the Dietar Machine using standard testing procedures used for cold-box foundry binders. In all of these examples, the same components and amounts were used unless otherwise specified.

Tests were conducted to determine the effect of using a heated and unheated corebox on tensile strengths and scratch hardness.

In the tables which follow, examples designated by the A-C are controls. The controls show the effect of curing the foundry shapes with carbon dioxide at room temperature.

TABLE I

SCRATCH HARDNESS MEASUREMENTS

FOR FOUNDRY SHAPES MADE WITH NOVANOL-100

CURED WITH CO_∑ IN THE PRESENCE AND ABSENCE OF HEAT

IMMEDIATE SCRATCH HARDNESS

Table I indicates that curing the foundry shapes the presence of heat improved the scratch hardness of the foundry shape, and that acceptable cores were made with a gassing time of 30 seconds. Improved scratch hardness is particularly important when making bulky ceres which can be roughly handled in an industrial setting, for instance those weighing up to 100 kilograms and having dimensions of from 100 X 100 X 100 cm³. Improvements m scratch hardness result in a better surface finish for cas^tings which reduces casting defects and machining.

TABLE II EFFECT OF SHORTER GASSING TIMES ON SCRATCH HARDNESS

The data in Table II further confirm that curing the foundry shapes in the presence of heat improves the scratch hardness of the foundry shape. The data also show that these benefits can be achieved when shorter gassing times are used. Example 5 shows that an acceptable core can be made with a gassing time as little as 5 seconds. Shorter gassing times result in shorter cycle times.

Castings

Improved scratch hardness was obtained for larger cores made by this process handled under use conditions. Inspection of castings made made cf such cores under use conditions n a foundry showed less veinmg for both grey ron and mild steel even when gassing times were reduced.

Claims

CLAIMS We claim:

1. A process for preparing a workable foundry shape comprising:

(a) forming a foundry mix by mixing a foundry aggregate with a bonding amount of up to about 10 percent by weight, based upon the weight of the aggregate, of a binder comprising an aqueous basic solution cf a phenolic resole resin;

(b) shaping the foundry mix of (a) into a foundry shape;

(c) contacting the foundry shape of (b) with carbon dioxide gas and a source of heat at a temperature of 20°C to 100°C; and

(d) allowing the foundry shape to harden into a workable foundry shape.

2. The process of claim 1 wherein the binder contains a compound which is a source of an oxyanion capable of forming a stable complex with said resin.

3. The process of claim 2 wherein the aqueous basic solution of phenolic resole resin has (a) a viscosity of less than about 1,000 centipoise at 25^CC; (b) a solids content of about 45 to about 65 percent by weight, said weight based upon the total weight of the basic solution; and (c) an equivalent ratio of base to phenol of 0.2:1.0 to 1.1:1.0.

4. The process of claim 3 wherein the mole ratio of phenol in the phenolic resin to oxyanion is from 0.1:1 to 0.8:1.0.

5. The process of claim 4 wherein the phenolic resole resin is prepared by reacting formaldehyde and phenol in a mole ratio of formaldehyde to phenol of about 1.1:1.0 co about 2.2:1.0 in the presence of an effective amount of a basic catalyst at elevated temperatures of about 50°C to about 120°C.

6. The process of claim 5 wherein the equivalent ratio of base to phenol used in preparing the aqueous basic solution is from 0.3:1.0 to 0.95:1.0.

7. The process of claim 6 wherein the base is selected from the group consisting of sodium hydroxide, potassium hydroxide, and mixtures thereof.

8. The process of claim 7 wherein the gassing time for the carbon dioxide is from 1 to 30 seconds.

9. The process of claim 8 wherein the source of heat is from the corebox equipment.

10. A foundry shape prepared in accordance with claim 2.

11. A foundry shape prepared in accordance with claim 9.

12. A process for preparing a metal casting comprising: (a) fabricating a shape in accordance with claim 2;

(b) pouring said metal into and around said shape; (c) allowing said metal to cool and solidify; and (d) then separating the molded article.

13. A process for preparing a metal casting comprising: (a) fabricating a shape in accordance with claim o•

(b) pouring said metal into and around said shape;

(c) allowing said metal to cool and solidify; and

(d) then separating the molded article.

14. A metal casting prepared in accordance with claim 12,