GB2099831A

GB2099831A - Foundry sand coated with unsaturated polyester and method of preparing same

Info

Publication number: GB2099831A
Application number: GB8213722A
Authority: GB
Original assignee: Nissan Motor Co Ltd; Mitsui Toatsu Chemicals Inc
Current assignee: Nissan Motor Co Ltd; Mitsui Toatsu Chemicals Inc
Priority date: 1981-05-13
Filing date: 1982-05-12
Publication date: 1982-12-15
Also published as: KR860002160B1; KR830009820A; JPS57187142A; AU8362782A; DE3217650A1; FR2505689A1; AU532257B2

Abstract

A resin coated sand for use in sand mould casting processes, for example aluminium alloy casting process, comprises a foundry sand coated with a binder of which the principal material is a crystalline unsaturated polyester which is an untacky solid at room temperature, optionally together with the addition of a cross-linking agent. To cure the binder during the formation of sand moulds and cores for use in sand mould casting processes the binder contains from 0.1 to 5 parts by weight, per 100 parts by weight of crystalline unsaturated polyester, of a mixed radical polymerization catalyst comprising a first organic peroxide having a half-life of 10 hours at a temperature not lower than 130 DEG C, e.g. cumene hydroperoxide, and a second organic peroxide having a half- life of 10 hours at a temperature lower than 130 DEG C but not lower than 100 DEG C, e.g. dicumyl peroxide, the first peroxide forming from 10 to 90% by weight of the total weight of first and second peroxides. The resin coated sand is prepared by first coating the particles of the foundry sand with the binder, excluding the catalysts, by a hot-melt process and adding the catalysts after reducing the sand temperature to a suitable level. <IMAGE>

Description

SPECIFICATION Foundry sand coated with unsaturated polyester and method of preparing same This invention relates to a resin coated foundry sand for forming molds and cores for use in sand mold casting processes and a method of preparing the resin coated sand. A crystalline unsaturated polyester is used in this resin coated sand.

In conventional shell-mold sand casting processes, a phenolic resin has commonly been used as the binder for the preparation of a resin coated sand. In fact, a phenolic resin coated sand has high fluidity and accordingly convenient for handling and, moreover, gives sand molds and cores of sufficiently high physical strength after the baking procedure. In practice, however, sand mold casting processes using phenolic resin have encountered some problems that are attributed to the use of phenolic resin. First, the resin liberates gases, which are irritating in odour and unfavorable to environmental hygiene, when heated during preparation of the resin coated sand, during baking of the shaped molds and cores and/or during pouring of molten metal into the sand molds.Second, in aluminum allow casting processes that are characterized by very low pouring temperatures compared with gray iron casting processes, the molds and cores remain in a very strong and tough state even at the shake-out stage after solidification of the poured molten metal because of partial carbonization of the phenolic resin, so that the shake-out operation encounters difficulty. Particularly, disintegration of the sand cores needs to be preceded by a firing process in which the mold assemblies containing aluminum alloy castings and sand cores are heated at high temperatures to thereby decompose the carbonized phenolic resin.

In view of such problems inherent to phenolic resin coated sand, U.S. Patent No. 4,246,1 65 proposes a resin coated sand in which a crystalline unsaturated polyester is used in place of a phenolic resin. The proposed resin coated sand does not emit an irritating odour even when heated, and, as a more important advantage, not only molds but also cores formed on this resin coated sand and used in an aluminum alloy casting process can easily be disintegrated at the shake-out stage without performing the aforementioned firing process.

However, unsaturated polyester resins are inherently lower in physical strength than phenolic resins. Therefore, sometimes molds or cores formed of a resin coated sand prepared by using an unsaturated polyester have insufficient of their physical strength, and particularly in the case of the cores having relatively thin-wall portions an increase in the number of defective cores constitutes a serious problem from an industrial viewpoint. Although it is possible to enhance the strength of the molds and cores by increasing the proportion of the unsaturated polyester resin to the sand to be coated, this is rather unfavorable because of an inevitable increase in the production cost and an increased probability of casting defects such as blow holes by reason of considerable amounts of gases liberated from the increased amount of resin.

In preparing a resin coated sand by using a crystalline unsaturated polyester resin, it is necessary to add an organic peroxide, which serves as a radical polymerization catalyst, to the unsaturated polyester resin to enable curing of the resin at the baking stage during forming of molds and cores. In this regard, Japanese Patent Application Primary Publication No. 55 (1980)-i 65250 proposes to jointly use at least two kinds of radical polymerization catalysts on condition that the half-life of each catalyst becomes 10 hr at a temperature below 1300C and different from the similarly defined temperature(s) of the other catalyst(s).According to this Japanese patent application, a resin coated sand prepared by using a crystalline unsaturated polyester and a combination of the specified catalysts can be uniformly cured even in the cases of forming molds and cores having both relatively thick-wall portions and relatively thin-wall portions.

However, according to the results of our experiments on the resin coated sand of the quoted Japanese patent application, the strength of cores formed of this resin coated sand is still insufficient when the cores are very intricate in shape and nonuniform in wall thickness as typified by a core for casting of a water jacket in the cylinder head of an automobile engine, so that the number of defective cores becomes considerably larger than in the case of using a phenolic resin coated sand for the same purpose.The reason for this fact is that the high temperature strength of this resin coated sand becomes maximal at a specific baking temperature such as about 250 C and is reduced either at lower braking temperatures or at higher baking temperatures, whereas the distribution of temperature in the core under baking becomes considerably nonuniform when the core is very intricate in shape and nonuniform in wall thickness.

It is an object of the present invention to provide an improved resin coated sand for forming molds and cores for use in sand mold casting processes including aluminum alloy casting processes, which resin coated sand utilizes a crystalline unsaturated polyester as shown in U.S. Patent No. 4,246,165 but exhibits enhanced values of high temperature strength over a wide range of baking temperature.

It is another object of the invention to provide a method of preparing a resin coated sand according to the invention.

The present invention provides a resin coated sand comprising a binder in the form of coating on the surfaces of the individual particles of a foundry sand. The binder comprises a crystalline unsaturated polyester which is a scarcely tacky solid at room temperature as a principal component thereof and an organic peroxide component which functions as a radical polymerization catalyst for curing of the unsaturated polyester. The improvement according to the invention resides primarily in that the organic peroxide component is a combination of a first organic peroxide of which the half-life becomes 10 hr at a temperature not lower than 1 30"C and a second organic peroxide of which the half-life becomes 1 0 hrata temperature lower than 1300C but not lower than 1 000 C.The total quantity of the first and second peroxide is from 0.1 to 5 parts by weight per 100 parts by weight of the unsaturated polyester in the binder, and the amount of the first peroxide is from 90 to 10% by weight of the total quantity of the first and second peroxides.

In the present application, "crystalline unsaturated polyester" means a polyester which is at least partially crystalline to such an extent that crystalline domains can be clearly identified by X-ray diffration arslysis, and the crystalline unsaturated polyesters described in U.S. Patent No. 4,246,165 are useful in the present invention.

A typical example of the first organic peroxide as defined above is cumene hydroperoxide of which the half-life becomes 10 hr at 1 580C, and a typical example of the second organic peroxide as defined above is dicumyl peroxide of which the half-life becomes 10 hr at 1 7oC. It is preferred to select the first and second organic peroxides such that the difference between the temperature at which the half life of the first peroxide becomes 10 hr and the temperature at which the half-life of the second peroxide becomes 10 hr is at least 1 OOC.

Optionaily, the binder may comprise a monomer or prepolymer copolymerizable with the unsaturated polyester as a cross-linking agent in an amount not larger than 50 parts by weight per 100 parts by weight of the unsaturated polyester. It is also optional to add a small amount of a silane coupling agent to the binder.

As the effect of the joint use of the above defined first peroxide, which is a high-temperature radical polymerization catalyst, and the second peroxide which is a medium-temperature radical polymerization catalyst, the resin coated sand according to the invention exhibits remarkably enhanced values of its high temperature strength over a very wide range of baking temperature extending to a markedly high temperature such as about 350 C. Therefore, by using the resin coated sand it is possible to form molds and cores having sufficiently high strength even when the molds and cores are very intricate in shape and/or are nonuniform in wall thickness. This resin coated sand retains the advantages attributed to the use of a crystalline unsaturated polyester.That is, the cores formed of this resin coated sand can easily be disintegrated at the shake-out stage even in aluminum alloy casting processes despite the enhancement in the initial strength of the cores. Further, the preparation of the resin coated sand, forming of molds and cores by using the resin coated sand and pouring of molten metal into the formed molds can be performed without suffering from harmful or irritating gases.

A method according to the invention for the preparation of a resin coated sand comprises the steps of mixing a major amount of heated foundry sand with a minor amount of binder comprising a crystalline unsaturated polyester which is a scarcely tacky solid at room temperature as a principal component thereof such that the unsaturated polyester liquefies and adheres to the particles of the foundry sand, adding a combination of a first organic peroxide as defined hereinbefore and a second organic peroxide as defined hereinbefore to the mixture of the foundry sand and the binder while the temperature of the mixture is such that the binder on the sand particles remains in liquid state but does not readily cure even in the presence of the first and second peroxides, stirring the resultant mixture thereby dispersing the first and second peroxides in the mixture, and lowering the temperature of the resultant mixture with continued stirring until solidification of the binder adhering to the sand particles.

Both the total quantity of the first and second organic peroxides relative to the quantity of the unsaturated polyester and the amount of the first peroxide in the total of the first and second peroxides are as defined hereinbefore.

Preferably, the first and second peroxides are added to the sand coated with the liquefied binder in the form of a mixed solution in an organic solvent having a relatively low boiling point as exemplified by ethyl alcohol.

A crystalline unsaturated polyester useful in the present invention is one obtained by esterification reaction between at least one sterically symmetrical unsaturated dibasic acid, such as fumaric acid and/or mesaconic acid, and at least one sterically symmetrical glycol such as ethylene glycol, 1,3propanediol, diethylene glycol, 1,4-butanediol, dipropylene glycol, neopentyl glycol, hydrogenated bisphenol A, 2,2-bis [4-(hyroxyethoxy)phenyl] propane and/or 2,2-bis [4-(hydroxypropoxy)phenyl] propane.

Insofar as the crystalline structure of the product is maintained, a portion of the symmetrical unsaturated dibasic acid may be replaced by at least one unsymmetrical unsaturated dibasic acid such as maleic acid anhydride, citraconic acid and/or itaconic acid, and/or at least one saturated dibasic acid such as phthalic acid, isophthalic acid, tetrahydrophthalic acid an hydride and/or 3,6-endomethylene-A4tetrahydrophthalic anhydride, or addition or substitution derivative(s) thereof. It is also permissible to replace a portion of the sterically symmetrical glycol by at least one unsymmetrical glycol such as propylene glycol and/or 1 ,3-butanediol, and/or at least one polyhydric alcohol such as glycerin and/or trimethylolpropane, or derivative(s) thereof.

By suitably selecting the reactants and controlling the reaction conditions for esterification by a known method, it is possible to obtain a crystalline unsaturated polyester which has an appropriate molecular weight of about 1000-2000 and is a solid which is untacky or scarcely tacky at room temperature and can be divided into small pieces that pass through a 4-mesh sieve (4.76 mm openings). In the synthesis of such a crystalline unsaturated polyester, it is optional to add 100 to 1000 ppm of a known polymerization inhibitor such as a quinone or a phenol to the reactants. Typical examples of compounds useful for this purpose are p-benzoquinone, hydroquinone and catechol.In the present invention it is preferred to use an unsaturated polyester which exhibits a viscosity value below 500 poises, and most preferably below 250 poises at a temperature in the range from 1000C to 1500C.

In the present invention, it is optional to use a resin composition prepared by mixing a crystalline unsaturated polyester in a heated and liquefied state with at least one monomer or prepolymer which is copolymerizable with the unsaturated polyester and serves as a cross-linking agent. Examples of useful monomers and prepolymers are styrene, divinylbenzene, vinyltoluene, cr-methylstyrene, diallyl phthalate, diallyl phthalate prepolymer, diallyl isophthalate, diallyl isophthalate prepolymel ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, neopentyl glycol dimethacrylate, trimethylolpropane trimethylacrylate, triallyl cyanu rate, triallyl isocyanurate, triallyl trimellitate, diacetone acrylamide and N-methylol acrylamide.The total quantity of the cross-linking agent to be added to 100 parts by weight of unsaturated polyester should not exceed 50 parts by weight because the addition of a larger amount of cross-linking agent results in tha; a resin coated sand prepared by using the resin composition becomes tacky and therefore becomes low in its fluidity and in its bulk density when charged into a die cavity, so that the molds and cores formed of the resin coated sand become insufficient in physical strength. Preferably the quantity of the cross-linking agent per 100 parts by weight of unsaturated polyester is limited within the range from 10 to 30 parts by weight.Also it is preferable that the resin composition exhibits a viscosity value below 500 poises, and most preferably below 250 poises, at a temperature in the range from 1000C to 1500C.

With a view to further enhancing the physical strength of molds and cores formed of the resin coated sand, a silane coupling agent may be added to the above described resin composition. For this purpose it is suitable to use a compound expressed by the general formula

where R, represents an alkenyl, alkenylphenylalkyl, acryloxyalkyl, methacryloxyalkyl, glycidoxyalkyl, epoxycyclohexylalkyl halogenated alkyl mercaptoalkyl, aminoalkyl, N-(aminoalkyl)-aminoalkyl or ureidoalkyl group, and each of R2, R3 and R4 represents a hydrolyzable group selected from alkoxy group, alkoxyethyl group, acetoxyl group and halogens.Examples of silanes expressed by this general formula are y-chloropropyltrimethoxy silane, vinyltrichloro silage, vinyl-tris (p-methoxyethoxy) silane, v- methacryloxypropyltrimethyloxy silane, p-(3,4-epoxycyclohexyl) ethyltrimethoxy sila ne, yglycidoxypropyltri methoxy sila ne, y-mercaptopropyltrimethoxy silane, y-a minopropyltriethoxy siiane, N (B-aminoethyl)-y-aminopropyltrimethoxy silane and y-ureidopropyltriethoxy silane. If desired, two or more of these silanes may be used jointly.

In the case of using silane coupling agent(s), the total quantity of the silane coupling agent(s) should be 0.1 to 10 parts by weight per 100 parts by weight of the crystalline unsaturated polyester in the resin composition. The addition of less than 0.1 parts by weight of silane coupling agent is scarcely effective for the enhancement of the strength of the sand molds and cores. However, the effect of the silane coupling agent(s) on the strength of the sand molds and cores does not significantly augment even when more than 10 parts by weight of silane coupling agent(s) is added, and the use of such a large amount of silane coupling agent is unfavorable from an economical viewpoint.

As mentioned hereinbefore, the primary feature of the present invention is to jointly use a radical polymerization catalyst of which the half-life becomes 10 hr at a temperature not lower than 1300C (hereinafter, catalyst of this class will be called Catalyst A) and another radical polymerization catalyst of which the half life becomes 10 hr at a temperature lower than 1 300C but not lower than 1000C (hereinafter, a catalyst of this class will be called Catalyst B).

It is suitable to measure the half-life of each peroxide useful as radical polymerization catalyst at a predetermined temperature by dissolving the peroxide in a solvent that is relatively inactive to the radicals liberated by decomposition of the peroxide to obtain a solution having a peroxide concentration of 0.1-0.2 mole/litre, and allowing the peroxide in the solution to undergo thermal decomposition at the predetermined temperature in a glass tube which is gastiyhtly closed after substitution of nitrogen gas for air in the tube.

Examples of Catalysts A are p-menthane hydroperoxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyl- 3,1,1 ,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide and 2,5-dimethylhexyl-2,5- dihydrnpernxide. Examples of Catalysts B are methylethyl ketone peroxide, t-butyl hydroperoxide, t butylcumyl peroxide, di-t-butyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-butyl peroxyacetate, 2,2-bis(t-butylperoxy)butane and t butylperoxy benzoate.

These Catalysts A and B can variously be combined, but in the present invention it is particularly preferred to jointly use cumene hydroperoxide as Catalyst A and dicumyl peroxide as Catalyst B.

Usually it is sufficient that a mixed catalyst in the present invention consists of one Catalyst A and one Catalyst B, but if desired the mixed catalyst may contain two or more Catalysts A and/or two or more Catalysts B. In any case, the total quantity of Catalysts A and B used in the present invention is limited within the range from 0.1 to 5 parts by weight per 100 parts by weight of the crystalline unsaturated polyester used in preparing the resin coated sand. If the total quantity of the mixed catalyst is less than 0.1 parts by weight, it will be difficult to aford a satisfactorily high strength to molds and cores formed of the resultant resin coated sand.However, the effect of the mixed catalyst on the strengh of the sand molds and cores does not significantly augment even when the total quantity of the mixed catalyst is increased beyond 5 parts by weight, and the use of such a large amount of catalyst is unfavorable from an economical viewpoint. It is preferable that the total quantity of Catalysts A and B falls in the range from 1 to 3 parts by weight per 100 parts by weight of the unsaturated polyester.

To obtain a resin coated sand that exhibits satisfactory high temperature strength values over a wide range of baking temperature in the forming of sand molds and cores, Catalysts A and B should be used in such proportions that Catalyst A amounts to 10 to 90% by weight, and preferably 20 to 80% by weight, of the total quantity of the mixed catalyst.

Preferably, the selection of Catalysts A and B from the above named exemplary compounds is made such that the difference between the temperature at which the half-life of the selected Catalyst A, i.e. the high temperature type radical polymerization catlayst, becomes 10 hr and the temperature at which the half-life of the selected Catalyst B, i.e. the medium-temperature type radical polymerization catalyst, becomes 10 hr is at least 100C. The reason is as follows. During baking of a mold or core formed of a resin coated sand according to the invention, it is inevitable that the outer surface of the mold or core in contact with a metal die is more intensely heated than the inner region of the mold or core, so that the temperature of the inner region remains considerably lower than the temperature at the outer surface.To afford a sufficiently high strength to the sand mold or core, it is necessary to fully cure the unsaturated polyester used as the binding material not only on the outer surface but also in the inner region of the mold or core despite the lowness of the temperature in the inner region. Therefore, it is favorable that the aformentioned temperature difference between the Catalyst A (high-temperature type catalyst) and the Catalyst B (medium-temperature type catalyst) in the mixed catalyst is as great as possible and is at least 1 OOC. When the temperature difference is smaller than 1 OOC, the effect of the joint use of Catalysts A and B on the degree of curing in the inner region of the sand mold or core does not greatly differ from the effect of using Catalyst A alone or Catalyst B alone, so that sometimes it becomes difficult to afford a sufficient strength to the sand mold or core.

A resin coated sand according to the invention is prepared by a known hot-melt process except for the above described joint use of Catalysts A and B. In a preferred mode, the hot-melt process is performed in the following manner.

At first, a silica sand, useful as foundry sand, is preheated to a temperature above the softening temperature of the crystalline unsaturated polyester selected for this process, e.g. to 1 50--2000C, and charged into a conventional sand mixer with the stirrer in operation. Soon the unsaturated polyester, or a resin composition containing the unsaturated polyester and a cross-linking agent, in the form of small solid pieces is charaged into the mixer, and stirring is continued so that the unsaturated polyester is melted by the heat of the sand and the sand particles are coated with the liquefied polyester.When the temperature of the coated sand has lowered to a suitable level such as 120-1 000C, a selected combination of Catalysts A and B is added to the coated sand while stirring is continued to achieve uniform dispersion of the catalysts. In this process, it is suitable to use a catalyst solution prepared by dissolving both Catalysts A and B in an organic solvent having a relatively low boiling point such as a lower alcohol. It will be understood that the addition of Catalysts A and B is performed at a stage where the sand temperature is still high enough to keep the unsaturated polyester coated on the sand particles in liquid or softened state but is not so high as to cause rapid curing of the polyester by the action of the added catalysts.Stirring of the resultant mixture is contined until the sand temperature becomes well below the softening temperature of the unsaturated polyester with the result that the polyester-coated sand particles separate from one another. Optionally, a small amount of wax or a higher fatty acid or its salt may be added to the sand-polyester mixture after the addition of the catalysts for the purpose of augmenting the fluidity of the resin coated sand. Also it is optional to add auxiliary additive(s) such as disintegratability improving agents, crystallizing agents and/or viscosity adjusting agents to the unsaturated polyester or the aforementioned resin composition. A suitable range of the weight ratio of the unsaturated polyester to the foundry sand to be coated is from 1:100 to 7:100.

By using a resin coated sand according to the invention, molds and cores for sand mold casting can be formed by a conventional method. For example, the coated sand is blown into a preheated metal die assembly and then baked at 200-3500C for 0.5 to 5 min.

The invention will be illustrated by the following examples and comparative experiments.

EXAMPLE 1 In a 2-liter four-necked flask,1137.5 g of fumaric acid, 33.2 g of isophthalic acid and 651.7 g of ethylene glycol were subjected to esterification reaction by a usual method to give about 1400 g of an unsaturated polyester which had an acid value of 25.

After cooling of the unsaturated polyester to 1 400C, 20 parts by weight of diallyl phthalate, 2 parts by weight of 1o-methacryloxypropyltrimethoxy silane and 0.8 parts by weight of ultrafine particles of silicic acid anhydride (AEROSIL 200 of JAPAN AEROSIL Inc.) were,added to and thoroughly mixed with 1 00 parts by weight of the unsaturated polyester. The resultant mixture was allowed to cool down to room temperature and left standing for a while. A resin composition obtained by this process was an untacky solid containing a crystalline unsaturated polyester. The resin composition was pulverized into fine grains that passed through a 10-mesh sieve (2.00 mm openings).

To prepare a resin coated sand by a hot-melt method using the above described resin composition, 4 kg of a commercially available silica sand for foundry use was preheated to 1 700C and charged into a speed mixer. While continuing stirring of the sand, 122.8 g of the pulverized resin composition was added to the sand so in that the resin composition was melted by the heat of the sand and the sand particles were coated with the liquefied resin composition. While the temperature of the thus treated sand under further stirring was in the range from 120 to 1 000C, a catalyst solution prepared by dissolving 1.5 g of cumene hydroperoxide (Catalyst A) and 1.5 g of dicumyl peroxide (Catalyst B) in 20 g of ethyl alcohol was poured into the mixer, and the mixture was further stirred to uniformly disperse the catalysts in the resin coating on the individual sand particles.When the sand temperature lowered to about 700 C, 5 g of calcium stearate was added to the resin coated sand in the mixer, and stirring was further continued until the once agglomerated sand became so loose and fluid that the individual sand particles separated from one another. Then the resin coated sand was taken out of the mixer.

The resin coated sand of this example was subjected to a high temperature strength test together with resin coated sands of the following references in a comparative experiment described hereinafter.

REFERENCE 1 Using 4 kg of the silica sand mentioned in Example 1, a resin coated sand was prepared generally in accordance with Example 1 but by using the following catalysts instead of the combination of the Catalysts A and B in Example 1. In Reference 1, use was made of a catalyst solution prepared by dissolving 1.0 g of dicumyl peroxide (Catalyst B), 1.0 g of t-butyl peroxybenzoate of which the half-life becomes 10 hr at 1 040 C (Catalyst B) and 1.0 g of t-butyl peroxy-2-ethylhexanate (the half-life of this compound becomes 10 hr at 720C) in 20 g of ethyl alcohol.

REFERENCE 2 A resin coated sand was prepared generally in accordance with Reference 1, but a combination of 1.5 g of dicumyl peroxide (Catalyst B) and 1.5 g of t-butyl peroxybenzoate (Catalyst B) was used instead of the combination of three peroxides in Reference 1.

COMPARATIVE EXPERIMENT 1 In this experiment, each of the three kinds of resin coated sand respectively prepared in Example 1 and References 1 and 2 was sampled and subjected to a high temperature strength test, which was carried out by using a high temperature tensile strength tester for shell-type casting sand. Each test piece for this test was formed in a set of upper and lower metal dies provided with heaters by filling the cavity in the die set (the cavity had a horizontai sectional shape like a dumb-ball and a small depth relative to its horizontal cross-sectional area) with a resin coated sand to be tested and baking the sand in the die cavity for 70 sec. The baking temperature (measured on the metal die surface) was varied within the range from 230 to 3300C as shown in the following Table 1.Each test piece was subjected to the test immediately after completion of the baking. For each resin coated sand, the test was carried out on five test pieces at each baking temperature to represent the high temperature strength by the average value of the five measurements. The results of the comparative experiment are presented in Table 1.

TABLE 1 High Temperature Strength (unit: kg/cm2)

Baking Temperature 'Coated Sand 2300C 2500C 2700C 2900C 3100C 3300C Example 1 13.8 14.2 14.7 15.2 14.9 13.7 Reference 1 13.0 13.3 13.1 12.0 11.1 9.5 Reference 2 13.4 13.7 13.5 12.8 11.0 9.3 As is apparent from the data in Table 1, the resin coated sand of Example 1 prepared by jointly using Catalysts A and B was remarkably higher in high temperature strength than the resin coated sands of References 1 and 2 prepared by using Catalysts B but without using any Catalyst A over the entire range of the baking temperature employed in this experiment.Furthermore, the experimental results demonstrate that the combination of Catalysts A and B used in Example 1 was highly effective for enhancement of the high temperature strength of the resin coated sand at considerably high baking temperatures.

EXAMPLE 2 Resin coated sand was prepared by using the silica sand mentioned in Example 1 and the unsaturated polyester resin composition prepared by the process described in Example 1 using the same materials. Stirring 4 kg of the silica sand preheated to 1 700 C, 122.8 g of the pulverized resin composition was added to the sand in the mixer to result in that the sand particles were coated with the resin composition melted by the heat of the sand.While the temperature of the sand under further stirring was in the range from 1 20 to 1000C, a catalyst solution prepared by dissolving 1.5 g of pmethane hydroperoxide of which the half-life becomes 10 hr at 1 330C (Catalyst A) and 1.5 g of dicumyl peroxide (Catalyst B) in 20 g of ethyl alcohol was poured into the mixer, and the mixture was further stirred to uniformly disperse the catalysts. When the sand temperature was lowered to 80--700C, 5 g of calcium stearate was added to the resin coated sand. The resin coated sand was taken out of the mixer when the individual sand particles separated from one another.

A comparative experiment relating to this example, another example and a reference will be described hereinafter.

EXAMPLE 3 Resin coated sand was prepared generally in accordance with Example 2 except that 1.5 g of 2,5dimethylhexyl-2,5-dihydroperoxide of which the half-life becomes 10 hr at 1 540C (Catalyst A) was used in place of 1.5 g of p-menthane hydroperoxide (Catalyst A) in Example 2.

REFERENCE 3 Resin coated sand was prepared generally in accordance with Example 2, but in this case use was made of 3.0 g of p-menthane hydroperoxide (Catalyst A) instead of the combination of the Catalysts A and B in Example 2.

COMPARATIVE EXPERIMENT 2 Each of the three kinds of resin coated sands respectively prepared in Examples 2 and 3 and Reference 3 was sampled and subjected to the high temperature tensile strength test described in Comparative Experiment 1. The test results are shown in Table 2.

TABLE 2 High Temperature Strength (unit: kg/cm2)

Baking Temperature Coated Sand 2300C 2500C 2700C 2900C 3100C 3300C Example 2 13.2 13.8 14.0 14.1 13.7 12.9 Example 3 13.6 13.9 14.4 14.8 14.1 13.0 Reference 3 7.3 9.8 12.5 13.3 12.6 11.0 The experimental data in Table 2 demonstrate that, compared with the use of only Catalyst A, a joint use of Catalysts A and B is remarkably effective for enhancement of the high temperature strength values of the resin coated sand and enlargement of the baking temperature range in which the resin coated sand exhibits satisfactorily high values of the high temperature strength.

EXAMPLE 4 In a 2-liter four-necked flask, 1102.6 g of fumaric acid, 83.1 g of terephthalic acid, 614.5 g of ethylene glycol and 147.6 g of neopentyl glycol were subjected to esterification reaction by a usual method to give an unsaturated polyester having an acid value of 30.

After cooling of the unsaturated polyester to 1 350C, 10 parts by weight of diallyl phthalate, 3 parts by weight of y-glycidoxypropyltrimethyloxy silane and 0.8 parts by weight of ultrafine particles of silicic acid anhydride (AEROSIL 200) were added to and thoroughly mixed with 90 parts by weight of the unsaturated polyester. The resultant mixture was allowed to cool down to room temperature and left standing for a while. A resin composition obtained by this process was an untacky solid containing a crystalline unsaturated polyester. The resin composition was pulverized into fine grains that passed through a 10-mesh sieve.

By using 4 kg of the silica sand mentioned in Example 1 and 125 g of the resin composition prepared in Example 4, the preparation of resin coated sand was started in the same manner as in Example 1. While the temperature of the sand coated with the melted resin composition was in the range from 120 to 1000C, a catalyst solution prepared by dissolving 0.3 g of cumene hydroperoxide (Catalyst A) and 2.7 g of dicumyl peroxide (Catalyst B) in 20 g of ethyl alcohol was poured into the mixer, and the mixture was further stirred to uniformly disperse the catalysts. When the sand temperature lowered to about 700C, 5 g of calcium stearate was added to the resin coated sand, and when the individual sand particles separated from one another the resin coated sand was taken out of the mixer.

A comparative experiment relating to this example, additional examples and references will be described hereinafter.

EXAMPLE 5 Resin coated sand was prepared generally in accordance with Example 4, but 0.6 g of cumene hydroperoxide and 2.4 g of dicumyl peroxide were used in this case. That is, the mixed radical polymerization catalyst in this example consisted of 20% of Catalyst A and 80% of Catalyst B by weight, whereas the mixed catalyst in Example 4 consisted of 10% of Catalyst A and 90% of Catalyst B.

EXAMPLE 6 Resin coated sand was prepared generally in accordance with Example 4 except that 2.4 g of cumene hydroperoxide and 0.6 g of dicumyl peroxide were used, meaning that the mixed catalyst consisted of 80% of Catalyst A and 20% of Catalyst B.

EXAMPLE 7 Resin coated sand was prepared generally in accordance with Example 4 except that 2.7 g of cumene hydroperoxide and 0.3 g of dicumyl peroxide were used, meaning that the mixed catalyst consisted of 90% of Catalyst A and 1 0% of Catalyst B.

REFERENCE 4 Resin coated sand was prepared generally in accordance with Example 4 except that 0.15 g of cumene hydroperoxide and 2.85 g of dicumyl peroxide were used, meaning that the mixed catalyst consisted of 5% of Catalyst A and 95% of Catalyst B.

REFERENCE 5 Resin coated sand was prepared generally in accordance with Example 4 except that 2.85 g of cumene hydroperoxide and 0.15 g of dicumyl peroxide were used, meaning that the mixed catalyst consisted of 95% of Catalyst A and 5% of Catalyst B.

COMPARATIVE EXPERIMENT 3 Each of the six kinds of resin coated sands respectively prepared in Examples 4-7 and References 4 and 5 was sampled and subjected to the high temperature tensile strength test described in Comparative Experiment 1. The test results are shown in Table 3.

TABLE 3 High Temperature Strength (unit: kg/cm2)

Baking Temperature Coated Sand 2300C 2500C 2700C 2900C 3100C 3300C Example 4 (Catalyst A 10%) 13.0 13.2 13.7 14.3 14.0 13.1 Example 5 (Catalyst A 20%) 13.1 13.7 14.2 14.7 14.4 13.2 Example 6 (Catalyst A 80%) 12.2 12.8 13.5 14.3 13.6 12.9 Example 7 (CatalystA90%) 11.7 12.3 13.0 13.8 13.1 11.6 Reference 4 (Catalyst A 5%) 13.3 t 13.6 13.2 12.5 10.9 9.8 Reference 5 (Catalyst A 95%) 9.7 10.2 12.8 13.4 12.5 11.0 As can be seen in Table 3, the joint use of Catalysts A and B in Examples 4-7 by limiting the amount of Catalyst A in the mixed radical polymerization catalyst within the range from 10 to 90% by weight resulted in considerably enhanced values of the high temperature strength of the resin coated sand over a very wide range of the baking temperature. The results of the tests on the samples of References 4 and 5 demonstrate that the effect of joint use of Catalysts A and B particularly on the width of the baking temperature range in which the resin coated sand exhibits fairly high strength values can hardly be obtained when the amount of either Catalyst A or Catalyst B in the mixed catalyst exceeds 95% by weight

Claims

1. A resin coated sand for forming moulds and cores for use in sand mould casting processes, and comprising a foundry sand the particles of which are coated with a binder comprising a crystalline unsaturated polyester which is scarcely tacky solid at room temperature as a principal component thereof together with an organic peroxide component which acts as a radical polymerization catalyst for curing the unsaturated polyester, in which the organic peroxide component comprises a combination of a first organic peroxide having a half-life of 10 hours at a temperature not lower than 1 300C and a second organic peroxide having a half-life of 10 hours at a temperature lower than 1 300C but not lower than 1000C, the total quantity of said combination of first and second peroxides being from 0.1 to 5 parts by weight per 100 parts by weight of the unsaturated polyester, and the amount of the first peroxide being form 90 to 10% by weight of the total quantity of the first and second peroxides.

2. A resin coated sand as claimed in claim 1 in which the difference between the temperature at which the half-life of the first organic peroxide is 10 hours and the temperature at which the half-life of the second organic peroxide is 10 hours is at least 1 OOC.

3. A resin coated sand as claimed in claim 1 or claim 2 in which the total quantity of the first and second peroxide is from 1 to 3 parts by weight per 100 parts by weight of the unsaturated polyester.

4. A resin coated sand as claimed in any one of the preceding claims in which the first peroxide forms from 80 to 20% by weight of the total quantity of the first and second peroxides.

5. A resin coated sand as claimed in any one of the preceding claims in which the first organic peroxide is p-methane hydroperoxide, cumene hydroperoxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyl- 3, 1 1,3,3-tetramethylbutyl hydroperoxide or2,5-dimethylhexyl-2,5-dihydroperoxide; and the second organic peroxide is methylethyl ketone peroxide, t-butyl hydroperoxide, di-t-butyl peroxide, dicumyl peroxide, t-butylcu myl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexa ne, 2,5-dimethyl-2,5- di(benzoylperoxy)hexane, t-butyl peroxyacetate, 2,5-bis(t-butylperoxy)butane or t-butylperoxy benzoate.

6. A resin coated sand as claimed in claim 5 in which the first organic peroxide is cumene hydroperoxide and the second organic peroxide is dicumyl peroxide.

7. A resin coated sand as claimed in any one of the preceding claims in which the binder further comprises at least one cross-linking agent which is a monomer or prepolymer copolymerizable with the unsaturated polyester, the total quantity of the cross-linking agent(s) being not larger than 50 parts by weight per 100 parts by weight of the unsaturated polyester.

8. A resin coated sand as claimed in claim 7 in which the total quantity of the cross-linking agent(s) is from 10 to 30 parts by weight per 100 parts by weight of the unsaturated polyester.

9. A resin coated sand as claimed in claim 7 or claim 8 in which the cross-linking agent comprises diallyl phthalate.

10. A resin coated sand as claimed in any one of claims 7-9 in which the binder further comprises at least one silane coupling agent, the total quantity of said silane coupling agent(s) being from 0.1 to 10 parts by weight per 100 parts by weight of the unsaturated polyester.

11. A resin coated sand according to claim 10, wherein the total quantity of the silane coupling agent(s) is from 0.5 to 5 parts by weight per 100 parts by weight of the unsaturated polyester.

12. A resin coated sand as claimed in claim 10 or claim 11 in which the silane coupling agent(s) have the general formula:

in which R, is an alkenyl, alkenylphenylalkyl, acryloxyalkyl, methacryloxyalkyl, glycidoxyalkyl group, epoxycyclohexylalkyl, halogenated alkyl, mercaptoalkyl, aminoalkyl, N(aminoalkyl)aminoalkyl or ureidoalkyl group, and each of R2, R3 and R4 represents a hydrolyzable group which is alkoxy group, aikoxyethoxy group, acetoxyl group or a halogen.

13. A resin coated sand as claimed in any one of the preceding claims in which the unsaturated polyester has a viscosity below 500 poises at a temperature of from 100 to 1 500 C.

1 4. A resin coated sand as claimed in claim 13 in which the unsaturated polyester has a viscosity value below 250 poises at a temperature of from 100 to 1 500C.

1 5. A resin coated sand as claimed in any one of the preceding claims in which the weight ratio of the unsaturated polyester to the foundry sand is from 1:100 to 7 100.

1 6. A resin coated sand as claimed in claim 1 substantially as hereinbefore described with reference to the examples.

1 7. A method of preparing a resin coated sand for forming moulds and cores for use in sand mould casting processes, the method comprising the steps of: mixing a major amount of a heated foundry sand with a minor amount of a binder comprising a crystalline unsaturated polyester which is a scarcely tacky solid at room temperature as a principal component thereof such that the unsaturated polyester liquefies and adheres to the particles of the foundry sand;; adding a combination of a first organic peroxide, which functions as a radical polymerization catalyst for curing the unsaturated polyester and has a half-life of 10 hours at a temperature not lower than 1 300 C, and a second organic peroxide. which functions as a radical polymerization catalyst for curing the unsaturated polyester and has a half-life of 10 hours at a temperature lower than 1 300C but not lower than 1 OO"C, to the mixture of said sand and said binder while the temperature of the mixture is such that said binder remains in the liquid state but does not readily cure even in the presence of the first and second peroxides, the total quantity of the first and second peroxides being from 0.1 to 5 parts by weight per 100 parts by weight of the unsaturated polyester and the amount of the first peroxide being from 90 to 10% by weight of the total quantity of the first and second peroxides; stirring the resultant mixture to disperse the first and second peroxides in the mixture; and lowering the temperature of said resultant mixture with continued stirring until solidification of the binder adhering to the particles of said sand.

1 8. A method as claimed in claim 1 7 in which the first and second organic peroxides are selected such that the difference between the temperature at which the half-life of the first peroxide is 10 hours and the temperature at which the half-life of the second peroxide is 10 hours is at least 1 OOC.

1 9. A method as claimed in claim 1 7 or claim 1 8 in which the total quantity of the first and second peroxides is from 1 to 3 parts by weight of the unsaturated polyester.

20. A method as claimed in any one of claims 1 7-19 in which the amount of the first peroxide forms from 80 to 20% by weight of the total quantity of the first and second peroxides.

21. A method as claimed in any one of claims 17-20 in which the first and second organic peroxides are as defined in claim 5 or claim 6.

22. A method as claimed in any one of claims 17-21 in which the combination of the first and second peroxides is added to the mixture of the sand and binder as a solution in an organic solvent.

23. A method as claimed in claim 22 in which the solvent is a lower alcohol.

24. A method as claimed in claim 23 in which the solvent is ethyl alcohol.

25. A method as claimed in any one of claims 1 7-24 in which the binder further comprises at least one cross-linking agent which is a monomer or prepolymer copolymerizable with the unsaturated polyester, the total quantity of the cross-linking agent(s) being not larger than 50 parts by weight per 100 parts by weight of the unsaturated polyester.

26. A method as claimed in claim 25 in which the total quantity of the cross-linking agent(s) is from 10 to 30 parts by weight per 100 parts by weight of the unsaturated polyester.

27. A method as claimed in claim 25 or claim 26 in which the cross-linking agent comprises diallyl phthalate.

28. A method as claimed in any one of claims 25-27 in which the binder further comprises at least one silane coupling agent, the total quantity of the silane coupling agent(s) being from 0.1 to 10 parts by weight per 100 parts by weight of the unsaturated polyester.

29. A method as claimed in claim 28 in which the total quantity of the silane coupling agent(s) is from 0.5 to 5 parts by weight per 100 parts by weight of the unsaturated polyester.

30. A method as claimed in claim 28 or claim 29 in which the silane coupling agent(s) are as defined in claim 12.

31. A method as claimed in any one of claims 17-30 in which the unsaturated polyester has a viscosity below 500 poises at a temperature of from 100 to 1 500C.

32. A method as claimed in claim 31 in which the unsaturated polyester has a viscosity below 250 poises at a temperature of from 100 to 1 500C.

33. A method as claimed in any one of claims 17-32 in which the weight ratio of the unsaturated polyester to the foundry sand is from 1:100 to 7 :100.

34. A resin coated sand prepared by a method as claimed in any one of claims 1 7-34.