GB2181735A - Maleic acid polymer for use as scale inhibitor - Google Patents

Abstract

Water service such as city water and industrial water is prevented from scaling of metal salts by use of a scale-inhibitor comprising a polymaleic acid, a salt thereof or a copolymer of maleic acid and another monomer such as an acrylic acid, each having an alcohol group or a ketone group at at least one terminal thereof. The polymer is suitably prepared by free radical polymerisation in a water/alcohol or water/ketone mixture, the alcohol or ketone behaving as a chain-terminating agent.

Description

SPECIFICATION Scale inhibitor of maleic acid polymer This invention relates to a scale inhibitorforan aqueous system. More particularly, it relates to a scale inhibi torfor an aqueous system which mainly comprises a specified polymer of maleic acid, including a homopolymer and a copolymerthereof.

Statement ofpriorarts Many aqueous systems including natural water, tap water and industrial water contain metal salts such as magnesium and calcium salts as well as silica dissolved therein. When water in a boiler or condenser system is heated or seawater is desalted by distillation, the dissolved salts are liable to become hardly soluble and stick to the surface of a heat exchanger or a heating surface to thereby form scale.

The scale thus formed not only significantly lowers the cooling or heat transfering effect but also brings about various troubles such as corrosion oftubes or heating surfaces.

Various scale inhibitors have been employed in orderto inhibit the formation of scale. Conventional scale inhibitors include inorganic and organic phosphate compounds, polysodium (meth) acrylate and homopolymers and copolymers of maleic acid.

However phosphate compounds are undesirable since they might cause eutrophication when eliminated from a system by, for example, blowing to thereby bring about environmental pollution. Furthertheytend to be hydrolyzed in water, which makes the application thereof in a system of a prolonged residence time ora high temperature difficult. On the other hand, polysodium (meth)acrylate exhibits an insufficient effect.

Awellknown example of a scale inhibitorcomprising a maleicacid homopolymer is one obtained bypolymerizing maleic anhydridewith the use oftoluene orxylene as a solventfollowed by hydrolyzing theobtained polymaleic anhydride (cf. Japanese Patent Publication No. 20475/1978).Taking into account the decarbonization and groups of the catalyst and the solvent as terminal groups, the above Publication suggests thatthe polymaleicanhydridewould have the following structure:

wherein Rand R' represent each a hydrogen atom or a group derived from the solvent or the catalyst used for the polymerization such as a C5H5CH2- group derived from toluene as solvent, a CH3C6H4CH2- group derived from xylene as solvent, a C6H5- group derived from benzene as solvent or benzoyl peroxide as catalyst or a (CH3)3C- group derived from tert-butyl peroxide as catalyst; and m + n is from 2 to 50.

Such a maleic acid homopolymer exhibits an excellent scale inhibiting effect. However it is required to use not only an organic solvent as described above but also a large amountofan expensive initiatorforthe polymerization of the same, which makes the product very expensive. Therefore it has been urgently required to develop an excellent scale inhibitorwhich can be readily and economically prepared in an aqu- eous solvent system.

Summary of the invention We have studied to overcome these disadvantages as described above and consequently found that a polymer having a specific structure ofthefollowing formula (I) and containing a large amount of maleic acid is an excellent scale inhibitor, thus completing the present invention.

A scale-inhibitor of the invention comprises a polymer or copolymer of maleic acid having the formula (I) or a salt thereof.

in which R1 and R2 are hydrogen, hydroxyl, an alcohol redidue having 1 to4carbon atoms ora ketonegroup having 3 or4carbon atoms, R1 and R2 not being hydrogen or hydroxyl atthesametime, M is hydrogen, NH4, an alkali metal oran alkaine earth metal, m 2to 40, n is zero ora positive number,thetotal of m and n being3 to 80 when n is positive, a ratio of n to m being 0/100 to 100/100, R3 is hydrogen or methyl, A is -COOM, -CONH2, -CH20H, -CH2SO3M, -SO3M, phenyl, -COOR4 or SO3M, R4 is a saturated hydrocarbon group having 1 to 6 carbon atoms.

The polymer or copolymer is called thereinafter as polymer.

The polymer is characterized by having an alcohol group or a ketone group at one or both terminals thereof, preferably in an amount of 20 mole percent or larger.

The polymerto use in the invention includes three groups of embodiments. One is the homopolymer.

Another is a copolymer containing up to 5 percent by weight of a vinyl co-monomer. The other is a copolymer of maleic acid or a salt thereof and a vinyl co-monomer at a monomer ratio of 100/5 to 100/100.

The polymerandthe invention will be below illustrated. The description aboutthe polymerappliesto all the three embodiments.

The fact that the polymaleic acid ora saltthereofofthe present invention has the structure as shown bythe general formula (I) is proved by 1H-NMR. For example, a rH-NMRspectrum of the same obtained by using water/isopropanol as a solvent shows a signal of the methylene hydrogen in the terminal carboxyl group at 2.5to 3.0 ppm (8) and that of the methyl hydrogen at 1 .0to 1.5 ppm (8). As forthe homopolymer, it shows no peak around 1.6 ppm (â) which obviously indicates that maleic acid has not been decarbonated and no acrylic acid group is present in the homopolymer. The copolymerwith acrylic acid has such a peak.

Accordingly the present invention provides a scale inhibitor comprising a polymaleic acid polymer ofthe general formula:

wherein R1 and R2 represent each -H or -OH, an alcohol residue having one to four carbon atoms such as

ora ketone residue having three orfour carbon atoms such as

provided that R1 and R2 do not represent -H or -OH at the same time; R3 represents -H or-CH3; A represents -COOM, -CONH2, -CH2-OH,-CH2SO3M,-SO3M,

or-COOR4wherein R4 is a saturated hydrocarbon having one to six carbon atoms; M represents H, NH4, an alkaline metal such as Na or K or an alkaline earth metal such as Ca or Mg; and m and n show a molarcopolymerization ratio in combination, as defined above.

The polymer of maleic acid ofthe present invention may be obtained by polymerizing maleic acid with a copolymerizable monomer in a solvent mixture comprising water and a monohydric alcohol having one to fourcarbon atoms ora ketone having three orfourcarbon atoms.

Examples ofthe alcohol to be used are methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol and tert-butanol, while examples of the ketone to be used are acetone and methyl ethyl ketone.

A preferable ratio by weight of water the alcohol or ketone is from 1:1 to 5 : 1.

As will be shown in Referential Examples hereinbelow, the polymaleicacid polymerofthe presentinvention may be prepared by adding Oto 120 moles of an alkaline metal hydroxide or aqueous ammonia to a solution of maleic acid (100moles) in a mixture of water and an alcohol or a ketone,adding a monomer(s) corresponding to the moiety of

in the general formula (l)thereto and adding an initiatordropwisethereto under heating to 50to 1200Cto thereby polymerize the mixture.

A co-monomerto use in the invention includes those to form the above defined units. Awater-soluble co-monomer is preferred for the purpose.

In the embodimentofthe co-polymer containing upto 5wt.% of a vinyl co-monomer, the co-monomer includes unsaturated carboxylic acids such as acrylic acid, methacrylic acid and itaconic acid; unsaturated amides such as acrylamide; unsaturated (meth)acrylates such as methyl acrylate and ethyl acrylate; allyl alcohol; unsaturated ally ethers such as allyl alcohol EO adduct; unsaturated sulfonic acid such as styrenesu Ifonic acid and vinyl-sulfonic acid; vinyl esters such as vinyl acetate; unsaturated nitriles such as acrylonitrile and aromatic vinyl compound such as styrene.

The above listed monomerforthe copolymerization may be applied to the embodiment ofthe copolymer having a monomer ratio of 100/5 to 100/100.

Awater-soluble initiator such as a persulfate or hydrogen peroxide ora mixture thereof are particularly suitable as the polymerization initiator. The initiator may be employed in an amount of 0.5 to 200 moles per 100 moles of the monomer. The reaction period may be from 2 to 12 hours.

The polymaleic acid polymer of the present invention preferably has a molecularweight of 200 to 5000, more desirably 300 to 1500. The term "molecuiarweight" as used herein means one which is determined in the following manner. Namely, the obtained polymer is converted into an acid form with an ion exchange resin and converted into a methyl ester with the use of, for example, diazomethane followed by the determination in a solvent such as chloroform by vapor osmometry. In the case of a copolymer having a sulfonicacid group in the molecule, the sulfonic acid is chlorinated with, for example, thionyl chloride after the methyl esterification with, for example, diazomethane and then determined.

The polymaleic acid polymer of the present invention contains an alcohol having one to four carbon atoms ora ketone having three orfourcarbon atoms as a solvent in the polymerization. Since these as a solvents are reactive and affect chain propagation and cessation of the polymer, the polymer contains terminal groups selected from among -H, -OH, alcohol residue(s) having one to fourcarbon atoms and ketone residue(s) having three orfourcarbon atoms as shown in the general formula (I).

The mechanism of the formation of the polymaleic acid polymer of the present invention may be assumed as follows. First the a-proton ofthe solvent alcohol or ketone is abstracted by the initiator and the alcohol or ketone radical thus obtained initiates the polymerization of the maleic acid to thereby form a polymaleic acid polymer having alcohol or ketone residue(s) atterminal(s). When hydrogen peroxide is used as the initiator, a polymer having an -OH group derived from the hydrogen at a terminal is formed in addition to the above reaction. Then the a-hydroxycarboxylic acid at the terminal ofthe polymer is oxidized and decarbonization via a-keto acid converts the terminal group into a carboxymethyl group.

Thus the obtained polymer is assumed to be a mixture ofthose having various terminal groups such as alcohol or ketone residue(s) and carboxymethyl groups having a hydrogen atom attheterminal.

The polymer according to the invention is different from the conventional polymer obtained with an or ganic such astoluene and xylene in respect to theterminal groups. The scale inhibiting effect relies on the specific terminal groups in the invention. It should be noted that the scale inhibiting effect is provided inthe invention when the polymer has a low polymerization degree. Becausethefunction oftheterminal groups gets increased.This is caused by the fact that -H and -OH groups, alcohol residues having one to four carbon atoms and ketone residues havingthreeorfourcarbon atoms are more hydrophilicthantheterminal groups of polymers prepared with the use of organic solvents, such as C6H5CH2-, CH3CsH4CHr, C6H5- or (CH3)3Cgroups.

In the case of the polymaleic acid polymer of the present invention, it is particularly desirable to contain at least 20% by mole of alcohol or ketone residue(s) attheterminal(s) from the viewpoint of the gelation stability ofthe polymer against polyvalent metals.

The scale inhibitor ofthe present invention may be added to a system in an amount of 0.5 to 100 ppm, preferably 2 to 50 ppm based on the system. A smaller amount thereof brings about an insufficient effect while a larger amountthereof is undesirable from an economic viewpoint. Thus the above range is preferable.

The scale inhibitor of the present invention exerts a sufficient effect when employed alone. Alternately it may be combined with conventional water treatment agents such as rust preventives, corrosion preventives, chelating agents or dispersants.

Examples ofthe chemicals which may be used simultaneouslywith the scale inhibitor of the present invention include nitrogen-containing compounds such as amines, imidazolines and amides, phosphates, hydroxycarboxylates, lignin sulfonates, formal in condensates (salts) of aromatic sulfonates, formalin condensates (salts) of melaminesulfonates, polymers of acrylic or methacrylic acid (polycarboxylic acids), copolymers of olefins with maleic acid, inorganic salts such as nitrites, sulfates and phosphates, aromatic carboxylates, aliphatic carboxylates, aminopolycarboxylates, thiourea, sulfosuccinic acid, polyethylene glycol, thioglycolates, polyethyleneimine, polyethyleneimide and benzotriazole.

Effect of the invention: The scale inhibitor of the present invention exhibits an excellent effect regardless of the type ofthe scale.

For example, it is highly effective on scale of calcium carbonate, calcium phosphate, magnesium hydroxide and silica. The scale inhibitorofthe present invention is highly useful in inhibiting theformation of scale in a water supplying boiler, a condenser, a desalting system of seawater by evaporation, a geothermal hotwater and petroleum recovery.

Preferred embodiments of the invention: To further illustrate the present invention, and not by way of limitation, the following Examples are given.

Referential Example 1 196 g of maleic anhydride and 300 g ofwaterwere introduced into a flask equipped with a thermometer, a stirrer, a dropping funnel, a nitrogen inlet and a refluxcondenserand heated to 60 C understirring.After stopping the heating, 138 g of 30% caustic soda was added thereto. Then 140 g of isopropanol was added thereto. Subsequentlythetemperature of the system was raised to the refluxing point and 0.25 g of a 1% (as Fe) aqueous solution of FeSO4 was added thereto. Then 40 g of 60% aqueous hydrogen peroxide was added dropwise thereto over six hours. Afterthe completion of the addition, the reaction mixture was heated for additional two hours.The residual isopropanol was removed in vacuo and the moisture content of the mix- ture was adjusted to thereby give an aqueous solution of a polymer having a solid content of 40% (pH 3.8), which will be referred to as a polymer (1).

Referential Examples 2 The procedure of Referential Example 1 was followed except that the isopropanol was replaced with npropanolto give an aqueous solution of a polymer having a solid contentof40% (pH 3.7), which will be referred to as the polymer (2).

Referential Example 3 The procedure of Referential Example 1 was followed except that the isopropanol was replaced with eth- anon to give an aqueous solution of a polymer having a solid content of 40% (pH 3.7), which will be referred to as the polymer (3).

Referential Example 4 196 g of maleic anhydride and 300 g ofwaterwere introduced into a flask equipped with a thermometer, a stirrer, a dropping funnel, a nitrogen inlet and a reflux condenser and the mixture was heated to 60'C under stirring. After stopping the heating, 138 g of 30% caustic soda was added thereto. Then 200 g of methyl ethyl ketone was added thereto. Subsequently the temperature of the system was raised to the refluxing point and 0.25 g of a 1% (as Fe) solution of FeSO4 was added thereto. The 40 g of 60% aqueous hydrogen peroxide was added dropwise thereto over six hours. Afterthe completion of the addition, the reaction mixture was heated for additional two hours.After removing the residual methyl ethyl ketone in vacuo, the moisture content of the mixture was adjusted to thereby give a polymer solution of the solid content of 40% (pH 3.8) which will be referred to as the polymer (4).

Referential Examples 5 The procedure of Referential Example 4was followed except that 100 g of methyl ethyl ketone was used to give an aqueous solution of a polymer having a solid content of 40% (pH 3.6), which will be referred to as the polymer(5).

Referential Example 6 Polymaleic acid was prepared with the use oftoluene as a solventforcomparison.

50 g of maleic anhydride and 100 g of toluene were introduced into a flask equipped with a stirrer, a thermometer and a reflux condenser and the mixture was dissolved by heating to 700C. 10g of benzoyl peroxide dissolved in 50 g oftoluene was added dropwise to the maleic anhydride/toluene solution over 15 minutes.

Then the mixture was allowed to react at 90 to 950C for five hours and then cooled to 30 C. After decanting the upper layer comprising toluene, the polymer obtained as a precipitate was dissolved in 100 g of water at 50 to 60 C. Then it was stripped in vacuo to thereby remove the residual toluene and give an aqueous solution of hydrolyzed polymaleic acid, which will be referred to as the polymer(6).

Example 1 The following scale inhibition test was carried out in orderto examine the scale inhibiting effect of aqueous solutions containing the polymers ofthe present invention as prepared in Referential Example 1 to 3.

300 i of a 1% aqueous solution of each polymer as prepared in Referential Example 1 to 3 was added to 150 g of a 0.176% aqueous solution of calcium chloride dihydrate and 150 g of a 0.168% aqueous solution of sodium hydrogencarbonate was further added thereto. After adjusting the pH value to 8.0, the mixture was introduced into a glass bottle, selaed and allowed to stand at 500C for 6 hours. Then it was cooled and the formed precipitate was filtered through a membrane filiter of 0.1 .Thenthecalciumconcentrationinthe Then the calcium concentration in the filtrate was determined by titration with EDTA. Table 1 shows the result.

In addition, the result obtained by using no scale inhibitorandthose obtained by using polymaleicacid synthesized by an organic method and a commercially available polysodium acrylate are also shown in Table 1 for comparison.

Table 1 Scale inhibitor Amount pHofaq. Cacao3 in M-alkalinity Inhibition (ppm) soln. filtrate offiltrate ratio after test (ppm) (ppm) (%) Polymer(1) 10 8.1 595 495 98 Sample of Polymer (2) 10 8.1 590 490 97 invention Polymer (3) 10 8.1 580 480 94 Comm.polysodium 10 7.9 505 390 72 acrylate* Sample for None 0 7.1 260 135 0 comparison Polymer (6) 10 8.0 570 465 91 Note) *: Molecularweight = 3000.

**: Inhibition ratio = (B - C)/(A - C) x 100 A: Ca concentration before heating.

B: Ca concentration in the filtrate after the examination.

C: Ca concentration in the filtrate after the examination with the use of no additive.

Example2 The following scale inhibiting test was carried out in orderto examine the scale inhibiting effect of aqueous solutions containing the polymers of the present invention as prepared in Referential Examples 4 and 5.

300 pwl of a 1% aqueous solution of each polymer as prepared in Referential Examples 4 and 5was added to 150 g of a 0.264% aqueous solution of calcium chloride dihydrate and 150 g of a 0.168% aqueous solution of sodium hydrogencarbonate was further added thereto. After adjusting the pH value to 8.0, the mixture was introduced into a glass bottle, sealed and allowed to stand at50 C for 6 hours. Then itwascooled and the formed precipitate was filtered through a membrane filter of 0.1 CL. Then the calcium concentration in the filtrate was determined by titration with EDTA. Table 2 shows the result.

In addition, the result obtained by using no scale inhibitorandthose obtained by using polymaleicacid synthesized with a solvent method and a commercially available polysodium acrylate are also shown in Table 2forcomparison.

Table2 Amount pHofaq. CaCO3in M-alkalinity Inhibition Scale inhibitor (ppm) soln. filtrate offiltrate ratio after test (ppm) (ppm) (%) Polymer (4) 10 8.1 884 482 96 Sample of invention Polymer (5) 10 8.0 875 470 94 Comm. polysodium 10 7.8 814 410 79 acrylate* Sample for None 0 7.2 491 91 0 comparison Polymer (6) 10 8.1 867 462 92 Note) *: Molecular weight = 3000.

**: Inhibition ratio = (B - C)/(A- C) x 100 A: Ca concentration before heating.

B: Ca concentration in the filtrate after the examination.

C: Ca concentration in the filtrate after the examination with the use of no additive.

Example 3 The following test was carried out in orderto examine the scale inhibiting effect of the polymers of the present invention prepared in Referential Example 1 to 3 on a system of seawater heating.

Artificial seawater concentrated two-fold was circulated in a device wherein the seawater was circulated through a stainless pipe of an inner diameter of 6 mm while being heated to 110 C at a rate of 320 1/hrfor20 hours. The scale sticking to the surface of the pipe in the heaterwas dissolved by washing with an acid and determined by atomic absorption spectroscopy to thereby examine the scale inhibiting effect of each inhibitor.

Table 3 shows the result.

Table3 Scale inhibitor Amount Sticked scale* (ppm) (mg/m2) Polymer(1) 10 380 Sample of Polymer (2) 10 425 invention Polymer (3) 10 450 Comm. polysodium 10 1280 acrylate Sample for Non 0 6300 comparison Polymer (6) 10 530 Note) *: The total amount of CaCO# and Mg(OH)2 scale per unit area of the heattransfertube.

Example 4 The following test was carried out in order to examine the scale inhibiting effect of the polymers ofthe present invention prepared in Referential Examples 4 and Son a system of seawater heating.

Artificial seawater concentrated two-foid was circulated in a device wherein the seawater was circulated through a stainless pipe of an innerdiameter of 6 mm while being heated to 1 100C at a rate of 320 1/hrfor 10 hours. The scale sticking to the surface of the pipe in the heater was dissolved by washing with an acid and determined by atomic absorption spectroscopyto thereby examine the scale inhibiting effect of each inhibitor.

Table4showsthe result.

Table 4 Scale inhibitor Amount Sticked scale* (ppm) (mg/m2) Sample of Polymer (4) 10 220 invention Polymer (5) 10 250 Comm.polysodium 10 710 acrylate Sample for None 0 4000 comparison Polymer (6) 10 285 Note) *: The total amount ofCaCO3and Mg(OH)2scale per unit area ofthe heattransfertube.

Referential Example 7 196 g of maleic anhydride and 300 g ofwaterwere introduced into a flask equipped with a thermometer, a stirrer, a dropping funnel, a nitrogen inlet and a reflux condenser and heated to 60 C under stirring. After stopping the heating, 138 g of 30% caustic soda was added thereto. Then 140 g of isopropanol was added thereto followed by 63.2 g of sodium methallylsulfonate. Subsequently thetemperature of the system was raised to the refluxing point and 0.25 g of a 1% (as Fe) solution of FeSO4 was added thereto. Then 40 g of 60% aqueous hydrogen peroxide was added dropwise thereto over six hours. After the completion of the addition, the reaction mixture was heated for additional two hours.The residual isopropanol was removed in vacuo and the moisture content of the mixture was adjusted to thereby give an aqueous solution of a polymer having a solid content of 40% (pH 3.8), which will be referred to as the polymer (7).

Referential Examples Band 9 Polymers (8) and (9) were prepared in the same manner as the one described in Referential Example7 exceptthatthe sodium methailylsulfonatewas replaced with copolymerizable monomers as shown in Table 5.

Table 5 Ref. Monomer composition (mole) Molecular Polymer Ex. Maleic Copolymerizablemonomer weight No.

No. acid 7 100 sodiummethallylsulfonate 20 770 (7) 8 100 acrylic acid 25 900 (8) 9 100 acrylamide 40 1050 (9) Referential Example 10 196 g of maleic anhydride and 300 g of water were introduced into a flask equipped with a thermometer,a stirrer, a dropping funnel, a nitrogen inlet and a reflux condenser and the mixture was heated to 600sunder stirring. After stopping the heating, 138 g of 30% caustic soda was added thereto. Then 100 g of methyl ethyl ketone was added thereto followed by 63.2 g of sodium methallysulfonate. Subsequentlythe temperature of the system was raised to the refluxing point and 0.25 g of a 1% (as Fe) aqueous solution of FeSO4 was added thereto.Then 40 g of 60% aqueous hydrogen peroxide was added dropwise thereto over six hours. Afterthe completion of the addition, the reaction mixture was heated for additional two hours. After removing the residual methyl ethyl ketone in vacuo, the moisture content ofthe mixture was adjusted to thereby give a polymer solution of a solid content of 40% (pH 3.8) which will be referred to as the polymer (10).

Referential Examples 11 and 12 Polymers (11) and (12)were prepared in the same manner as the one described in Referential Example 10 except that the sodium methallylsulfonate was replaced with copolymerizable monomers as shown in Table 6.

Table 6 Ref. Monomercomposition (mole) Molecular Polymer Ex. Maleic Copolymerizablemonomer weight No.

No. acid 10 100 sodiummethallylsulfonate 20 770 (10) 11 100 acrylicacid 25 900 (11) 12 100 acrylamide 40 1050 (12) Example 5 The following scale inhibiting test was carried out in orderto examine the scale inhibiting effect of aqueous solutions containing the polymers of the present invention as prepared in Referential Examples 7 to 9.

3001l1 of a 1% aqueous solution of each polymeras prepared in Referential Examples7 to 9was added to 1 50g of a 0.176% aqueous solution of calcium chloride dihydrate and 150 g of a 0.168% aqueous solution of sodium hydrogencarbonate was further added thereto. After adjusting the pH value to 8.0, the mixture was introduced into a glass bottle, sealed and allowed to stand at 500C for six hours. Then it was cooled and the formed precipitate was filtered through a membrane filter of 0.1 #. Then the calcium concentration in the filtrate was determined by titration with EDTA. Table 7 shows the result.

In addition, the result obtained by using no scale inhibitor and those obtained by using polymaleicacid synthesized by an organic method and a commercially available polysodium acrylate are also shown in Table 7forcomparison.

Table 7 Amount pHofaq. CaCO3in M-alkalinity Inhibition Scale inhibitor (ppm) soln. filtrate offiltrate ratio aftertest (ppm) (ppm) (%) Polymer(7) 10 8.0 580 480 94 Sample of Polymer (8) 10 8.1 585 475 95 invention Polymer (9) 10 8.0 575 475 93 Comm. polysodium 10 7.9 505 390 72 acrylate* Samplefor None 0 7.1 260 135 0 comparison Polymer (6) 10 8.0 570 465 91 Note) *: Molecularweight = 3000.

**: Inhibition ratio = (B - C)/(A - C) x 100.

A: Ca concentration before heating.

B: Ca concentration in the filtrate after the examination.

C: Ca concentration in the filtrate after the examination with the use of no additive.

Example 6 The following test was carried out in orderto examine the scale inhibiting effect of the polymers ofthe present invention prepared in Referential Examples 7 to 12 on a system of seawater heating.

Artificial seawater concentrated two-fold was circulated in a device wherein the seawater was circulated through a stainless pipe of an innerdiameter 6 mm while being heated to 1 100C at a rate of 320 1/hrfor20 hours. The scale sticking to the surface of the pipe in the heater was dissolved by washing with an acid and determined by atomic absorption spectroscopy to thereby examine the scale inhibiting effect of each inhibitor.

Table8showsthe result.

Table 8 Scale inhibitor Amount Stocked scale* (ppm) (mg/m2) Polymer(7) 10 480 Polymer (8) 10 470 Sample of Polymer (9) 10 490 invention Polymer(10) 10 490 Polymer (11) 10 480 Polymer(12) 10 480 Comm. polysodium 10 1280 acrylate Sample for None 0 6300 comparison Polymer (6) 10 530 Note) *: The total amount of CaCO3 and Mg(OH)2 scale per unit area of the heattransfertube.

Claims (8)

1. Ascale-inhibitorcomprising a polymer or copolymer of maleic acid having the formula (1) ora salt thereof:

in which R1 and R2 are hydrogen, hydroxyl, an alcohol redidue having 1 to4carbon atoms ora ketonegroup having 3 or4 carbon atoms, R1 and R2 not being hydrogen or hydroxyl at the same time, M is hydrogen, NH4, an alkali metal or an alkaine earth metal, m 2to 40, n iszero ora positive number, the total ofm and n being3 to 80 when n is positive,a ratio ofnto m being 0/100to 100/100, R3 is hydrogen or methyl,A is a -COOM, R4 is a saturated hydrocarbon group having 1 to 6 carbon atoms.

2. Ascale-inhibitoras claimed in Claim 1, in which said copolymer has a molecularweight of 200 to 5000 on the average.

3. A scale-inhibitor as claimed in Claim 1, in which said alcohol residue is one or more of the followings:

4. Ascale-inhibitoras claimed in Claim 1, in which said ketonegroup is one or more ofthefollowings:

5. Ascale-inhibitoras claimed in Claim 1, in which said copolymer has 20 mole percent or largerofthe alcohol residue and/or the ketone group in R1 and/or R2.

6. Ascale-inhibitorasclaimed in Claim 1, in which said copolymerincludes upto 5 percent byweightof another vinyl monomer units.

7. Ascale-inhibitorasclaimed in Claim 1, in which n is zero.

8. Ascale-inhibitorasclaimed in Claim 1, in which a ratio of nto m is from 5/100to 100/100.