Scale inhibition agent and method for using same

An agent for preventing scale formation and a method for preventing scale formation include a polymer which has at least 50% of N-vinyl formamide units or N-vinyl acetamide units and a phosphorus compound added to water in a cooling water system, a boiler water system, or the like. The scale preventing agent and scale preventing method of the present invention are effective in preventing the adherence of various types of scales which are generated in cooling water systems, boiler water systems, and the like. In particular, the present invention is effective in preventing the adherence of silica and calcium scale.

BACKGROUND OF THE INVENTION 
The present invention relates to an agent for preventing scale formation 
and a method for preventing scale formation. More particularly, the 
present invention relates to an agent for preventing scale formation and a 
method for the same which is effective in preventing scale formation in 
water systems, such as cooling water systems, boiler water systems, and 
the like. The present invention is particularly effective in preventing 
calcium and silicate scale formation. 
Scale obstructions are generated on heat exchange surfaces and inside pipes 
which are in contact with water in a water system, such as a cooling water 
system, a boiler water system, or the like. The types of scales which are 
deposited include calcium carbonate, calcium sulfate, calcium sulfite, 
calcium phosphate, calcium silicate, magnesium silicate, magnesium 
hydroxide, zinc phosphate, zinc hydroxide, basic zinc carbonate, and the 
like. Scale formation is particularly troublesome in water systems that 
employ high concentration operation. In, for example, an open circulating 
cooling water system, high concentration operation is preferred to 
conserve resources and energy. However, when a high concentration 
operation is performed by reducing the amount of cooling water blow to the 
outside of the system, dissolved salts become concentrated in the water. 
In addition to the heat exchange surfaces becoming more likely to corrode, 
the solubility of the dissolved salts declines, resulting in scale 
formation. The resulting scale obstructions can become major impediments 
to the operation of boilers or heat exchange devices by reducing heat 
transfer efficiency and by narrowing pipes. 
Polymers which contain carboxyl groups, such as polymerized maleic acid, 
acrylic acid, itaconic acid, and the like are often effective in 
inhibiting formation of calcium or magnesium scale. Furthermore, 
copolymers which combine monomers with a carboxyl group and monomers which 
contain a sulfonic acid group, such as vinyl sulfonic acid, allyl sulfonic 
acid, and 2-acrylamide-2-methylpropane sulfonic acid, are often used as 
scale preventing agents, depending on the water quality. To inhibit 
formation of silica scale, scale preventing agents such as acrylamide 
polymers, cation polymers, polyethylene glycol, and the like, have been 
proposed. Thus, different polymers have been used to inhibit scale 
formation, depending on the type of scale. However, to date no universal 
scale formation preventing agent has been described. 
The water used in water systems is typically industrial water or tap water. 
These types of waters contain numerous and various ions. Particularly when 
conducting high concentration operations in a cooling water system or a 
boiler water system, for example, there is a need for a scale preventing 
agent which can effectively respond to all scale types. Currently, there 
is no scale preventing agent which satisfies this requirement. Of 
particular concern is the fact that there is no scale preventing agent 
which effectively prevents the adhesion of silica scale. 
In Japanese Laid Open Patent Publication Number 61-107998, there is 
proposed a scale preventing agent which has an excellent effect in 
preventing silica scale formation. The scale preventing agent proposed in 
this publication contains acrylamide polymer and acrylic acid polymer. 
When the silica concentration is low, acrylamide polymer has a good scale 
preventing effect. However, when the silica concentration is high, the 
effect is reduced. In Japanese Laid Open Patent Publication Number 
7-256266, there is disclosed a water treatment method in which a water 
soluble cationic polymer, halogenated aliphatic nitro alcohol, a low 
molecular weight carboxylic acid polymer, or phosphonic acid are added. 
However, because the proposed cation polymer is a quaternary ammonium 
salt, the cationic property is extremely strong. Under these conditions, 
the components of the water treatment method can form gel reaction 
products with silica. Furthermore, slime from microorganisms can also form 
easily. As a result, the use of the proposed water soluble cationic 
polymer is disadvantageous, because of problems arising from the clogging 
of pipes. 
In Japanese Laid Open Patent Publication Number 2-31894, a scale preventing 
agent containing polyethylene glycol and a low molecular weight carboxylic 
acid polymer or phosphonic acid is proposed. However, although 
polyethylene glycol is effective in controlling the adherence of scale 
when the silica concentration is low, it is easily influenced by other 
ions, and the effect is not stable. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a scale formation 
preventing agent and a scale formation preventing method which is 
effective in preventing the adherence of various scales generated in water 
systems, such as cooling water systems, boiler water systems, and the 
like. 
It is another object of the present invention to provide a scale formation 
preventing agent and scale formation preventing method which is effective 
in preventing the adherence of silica and calcium scales in water systems, 
such as cooling water systems, boiler water systems, and the like. 
Briefly stated, an agent for preventing scale formation and a method for 
preventing scale formation include a polymer which has at least 50 mole % 
of N-vinyl formamide units or N-vinyl acetamide units and a phosphorus 
compound added to water in a cooling water system, a boiler water system, 
or the like. The scale preventing agent and scale preventing method of the 
present invention are effective in preventing the adherence of various 
types of scales which are generated in cooling water systems, boiler water 
systems, and the like. In particular, the present invention is effective 
in preventing the adherence of silica and calcium scale. 
According to an embodiment of the present invention, a scale formation 
preventing agent for use in a cooling water system or a boiler water 
system comprises a polymer having at least 50% of N-vinyl formamide units 
or N-vinyl acetamide units and a phosphorus compound. 
According to another embodiment of the present invention, a method for 
preventing scale formation in a cooling water system or a boiler water 
system comprises the step of adding a polymer having at least 50% of 
N-vinyl formamide units or N-vinyl acetamide units and a phosphorus 
compound to water used in the cooling water system or boiler water system. 
The above, and other objects, features and advantages of the present 
invention will become apparent from the following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present inventors have discovered that a scale preventing agent 
comprising a polymer which has at least 50% of N-vinyl formamide units or 
N-vinyl acetamide units and a phosphorus compound has an excellent scale 
formation preventing effect in a water system. Compared to the scale 
preventing agents of the prior art, the scale formation preventing agent 
of the present invention shows an excellent effect in preventing the 
adherence of scale. It is particularly effective for preventing the 
adherence of silica and calcium scale. 
In the present invention, a polymer which has at least 50% of monomer units 
containing N-vinyl formamide as represented by formula [1], or N-vinyl 
acetamide as represented by formula [2], is used. 
EQU HCONHCH.dbd.CH.sub.2 [ 1] 
EQU CH.sub.3 CONHCH.dbd.CH.sub.2 [ 2] 
There are no particular limitations to the manufacturing method for the 
N-vinyl formamide and N-vinyl acetamide monomers. For example, they may be 
made by reacting acetalydehyde with formamide or acetamide. The resulting 
N-(.alpha.-hydroxyethyl)formamide or N-(.alpha.-hydroxyethyl)acetamide is 
etherified by contact with alcohol. Thereafter, the etherified product is 
thermolyzed in the gaseous state at high temperatures. Alternatively, the 
etherified product can also be heated in the liquid state, and then 
thermolyzed. The generated N-vinyl formamide or N-vinyl acetamide can be 
removed during thermolysis. 
In the present invention, polymers with at least 50% of N-vinyl formamide 
units or N-vinyl acetamide units may include: homopolymers of N-vinyl 
formamide; homopolymers of N-vinyl acetamide; copolymers of N-vinyl 
formamide and N-vinyl acetamide; partially hydrolyzed products of an 
N-vinyl formamide homopolymer; partially hydrolyzed products of an N-vinyl 
acetamide homopolymer; partially hydrolyzed products of a copolymer of 
N-vinyl formamide and N-vinyl acetamide; or copolymers of N-vinyl 
formamide or N-vinyl acetamide and a vinyl monomer. All mixed polymers and 
copolymers of N-vinyl formamide and N-vinyl acetamide may be used for the 
scale preventing agent of the present invention, so long as the N-vinyl 
formamide units and the N-vinyl acetamide units total at least 50%. 
Vinyl monomers which are capable of polymerizing with N-vinyl formamide or 
N-vinyl acetamide may be used without any particular limitations, so long 
as the copolymer or the copolymer obtained by further hydrolysis is 
water-soluble. Examples of such vinyl monomers include nonionic monomers 
such as (meta)acrylamide, (meta)acrylonitryl, N-methyl (meta)acrylamide, 
N, N-dimethyl (meta)acrylamide, methyl (meta)acrylate, ethyl 
(meta)acrylate, styrene, allyl alcohol, hydroxyethyl (meta)acrylate, mono 
(meta)acrylate of (poly)ethylene oxide of mole number 1-30, monoallyl 
ether of (poly)ethylene oxide of mole number 1-30, and monovinyl ether of 
(poly)ethylene oxide of mole number 1-30; anionic monomers such as 
(meta)acrylic acid, alpha-hydroxyacrylic acid, crotonic acid, maleic acid, 
fumaric acid, and itaconic acid; and alkali metal salts of these. Further, 
such vinyl monomers include vinyl sulfonic acid, styrene sulfonic acid, 
2-acrylamide-2-methyl propane sulfonic acid, 2-hydroxy-3-allyloxypropane 
sulfonic acid, isoprene sulfonic acid, and alkali metal salts of these. 
Further, such vinyl monomers include cationic monomers such as allyl 
amine, quaternary ammonium salts or tertiary salts of dimethyl amino ethyl 
(meta)acrylate and diethyl amino ethyl (meta)acrylate, and diallyl 
dimethyl ammonium chloride. 
In the present invention, there are no particular limitations on the method 
of homopolymerization or polymerization of N-vinyl formamide or N-vinyl 
acetamide monomeric units. Polymerization may be conducted as solution 
polymerization or bulk polymerization, but because the homopolymers of 
N-vinyl formamide or N-vinyl acetamide or the copolymers of N-vinyl 
formamide or N-vinyl acetamide used in the present invention are 
preferably water soluble or hydrophilic, water-based polymerization using 
water as the solvent is preferred. In water-based polymerization, an 
aqueous solution or water dispersion solution of the monomer is prepared, 
and the pH may be adjusted as required. After replacing the atmosphere 
with an inactive gas, the solution is heated to 50-100.degree. C. 
Polymerization is then conducted by addition of a water soluble 
polymerization initiation agent. Examples of suitable polymerization 
initiation agents include azo compounds, such as 2, 2'-azobis (2-amidino 
propane)dihydrogen chloride, azobis-N, N'-dimethylene isobutyl amidine 
dihydrogen chloride, and 4,4'-azobis (4-cyano valerianic acid)-2-sodium; 
persulfate compounds, such as ammonium persulfate, sodium persulfate, and 
potassium persulfate; and peroxides, such as hydrogen peroxide, sodium 
periodate, and the like. Polymerization is normally completed after 3-6 
hours. After cooling, an aqueous solution or water dispersion solution of 
a polymer can be obtained. Polymerization of N-vinyl formamide or N-vinyl 
acetamide monomeric units is not limited to being performed in an aqueous 
solvent, and may be conducted as solution polymerization, suspension 
polymerization, or emulsion polymerization in a general organic solvent. 
In the present invention, homopolymers or copolymers of N-vinyl formamide 
or N-vinyl acetamide may also be hydrolyzed. Polymers containing a portion 
of N-vinyl formamide units or N-vinyl acetamide units hydrolyzed to 
N-vinyl amine units may be used, so long as at least 50% remain as N-vinyl 
formamide units or N-vinyl acetamide units. There are no particular 
limitations to the conditions of hydrolysis. For example, hydrolysis may 
be conducted under basic conditions by adding ammonia, primary amine, 
secondary amine or the like to an aqueous solution or water dispersion 
solution of the polymer, followed by addition of sodium hydroxide or the 
like. Alternatively, hydrolysis may be conducted under acidic conditions 
by adding inorganic acids, such as hydrochloric acid or the like, to the 
aqueous solution or water dispersion solution of the polymer, followed by 
heating. 
In the present invention, the molecular weight of the polymer with at least 
50% of N-vinyl formamide units or N-vinyl acetamide units is preferably 
between about 2,000 and about 1,000,000. If the molecular weight of the 
polymer is less than about 2,000, the scale formation preventing effect 
may be inadequate. If the molecular weight of the polymer exceeds 
1,000,000, the viscosity of the solution becomes too great, and the 
solution may be difficult to handle. The molecular weight of the polymer 
can be determined by any appropriate means, such as gel permeation 
chromatography. 
In the present invention, one type of polymer with at least 50% of N-vinyl 
formamide units or N-vinyl acetamide units may be used alone, or two or 
more types may be combined and used. 
The scale preventing agent of the present invention also contains a 
phosphorus compound. There are no particular limitations on the phosphorus 
compound to be used. The following may be used with good results: sodium 
tripolyphosphate; sodium hexametaphosphate; nitro trimethylene phosphonic 
acid (NTMP); hydroxy ethylidene diphosphonic acid (HEDP); ethylene diamine 
tetramethylene phosphonic acid (EDTP); phosphono butane tricarboxylic acid 
(PBTC); amino methylene phosphonate (AMP); polyamino polyether methylene 
phosphonate (PAPEMP, Corrosion '96, The NACE Annual International 
Conference and Exposition, Mar. 24-29, 1996, Denver, Colo., U.S.A., paper 
number 575); phosphono polycarboxylic acid (POCA, CAS Registry No. 
110224-99-2); and bis(poly-2-carboxyethyl)phosphinic acid (CAS Registry 
No. 71050-62-9). These phosphorus compounds may be used individually or in 
any combination. 
There are also no particular limitations as to the form of the scale 
preventing agent. For example, a powder which is a mix of polymer with at 
least 50% of N-vinyl formamide units or N-vinyl acetamide units and the 
phosphorus compound may be dissolved in water before use and then used. 
Alternatively, an aqueous solution containing the polymer and the 
phosphorus compound may be used. Alternatively, a two-fluid type 
preparation containing an aqueous solution of a polymer and a separate 
aqueous solution of a phosphorus compound may be used. 
In the present invention, there are no particular limitations on the ratio 
of the amount of the polymer with at least 50% of N-vinyl formamide units 
or N-vinyl acetamide units to the phosphorus compound. However, a weight 
ratio of polymer to phosphorus compound is preferably between 1:9 and 9:1. 
A weight ratio of polymer to phosphorus compound between 5:5 and 8:2 is 
even more preferable. 
The scale formation preventing agent of the present invention can be added 
as an aqueous solution which is prepared at a concentration which is 
appropriate for the water system for which is to be used. Normally, it is 
preferable for the total concentration of polymer and phosphorus compound 
to be such that a concentration of 1-100 mg/liter in the water is 
achieved. 
It is possible to use the scale formation preventing agent of the present 
invention together with one or more other scale preventing agents as 
needed. Examples of scale preventing agents which may be used in 
conjunction with the present invention include: polyacrylic acid, 
polymaleic acid, copolymers of acrylic acid and 2-acrylamide-2-methyl 
propane sulfonic acid, a reaction product of N, N, N', N'-tetramethyl 
ethylene diamine with epichlorohydrin, and a reaction product of N, N, N', 
N'-tetramethyl ethylene diamine with dichloroethyl ether. The scale 
formation preventing agent of the present invention may be further mixed 
with corrosion preventing agents, such as organic phosphonate, and/or 
antibacterial agents, such as hydrazine, and added as needed. 
Alternatively, these chemicals may be added separately. 
When using the scale formation preventing agent and scale formation 
preventing method of the present invention, there are no particular 
limitations to the water quality or the operating conditions of the boiler 
or heat exchanging device. The present invention may be altered as 
described above to suit the operation requirements and water quality of 
any standard boiler and heat exchange device. The scale preventing agent 
and the scale preventing method of the present invention is effective in 
preventing scale such as calcium carbonate, calcium sulfate, calcium 
phosphate, zinc phosphate, zinc hydroxide, magnesium silicate, silica and 
the like which are generated in boilers and cooling water systems and 
which adhere to the heat transfer surface and the pipe walls. In 
particular, the present invention is effective in preventing adherence of 
silica scales. 
The mechanism of how the scale preventing agent and scale preventing method 
of the present invention is particularly effective in preventing silica 
scale formation is not known. However, it is thought that the amide group 
of the N-vinyl formamide unit or the N-vinyl acetamide unit and the 
phosphate group or the phosphonic acid group of the phosphorus compound 
act synergistically against the silica hydroxide group, and effectively 
prevent adherence of silica scales to the wall surface. 
EMBODIMENTS 
The present invention is described below in further detail. The present 
invention is not limited to these embodiments. 
The polymers and the phosphorus compounds used in the Embodiments and in 
the Comparative Examples are described in Tables 1 and 2, respectively. 
TABLE 1 
______________________________________ 
Designation 
Polymer Molecular weight 
______________________________________ 
A1 poly-N-vinyl formamide 
80,000 
A2 hydrolyzed product of poly-N-vinyl 
90,000 
formamide (10 mole %) 
A3 copolymer of N-vinyl formamide and 
45,000 
acrylamide (mole ratio 6:4) 
A4 copolymer of acrylic acid and 
9,000 
hydroxyallyloxypropane sulfonic acid 
(mole ratio 8:2) 
A5 Copolymer of N-vinyl formamide and 
25,000 
acrylamide (mole ratio 4:6) 
A6 sodium polyacrylamide 
5,000 
A7 copolymer of N-vinyl acetamide and 
38,000 
acrylamide (mole ratio 6:4) 
______________________________________ 
TABLE 2 
______________________________________ 
Designation 
Phosphorus compound 
______________________________________ 
P1 sodium hexametaphosphate 
P2 hydroxy ethylidene diphosphonic acid 
P3 phosphono butane tricarboxylic acid 
P4 polyamino polyether methylene phosphonate [Calgon Co.] 
P5 sodium tripolyphosphate 
P6 nitro trimethylene phosphonic acid 
P7 ethylene diamine tetramethylene phosphonic acid 
P8 amino methylene phosphonate 
P9 phosphono poly carboxylic acid 
P10 bis(poly-2-carboxyethyl)phosphinic acid 
______________________________________ 
Scale Adherence Test Method 
An open, circulating, cooling water system having a heat exchange area of 
approximately 0.25 m.sup.2 and a water capacity of 0.45 m.sup.3 was used. 
Distilled water and salts were added to Atsugi City water. This water was 
added to the cooling water system as circulating water and make up water. 
The cooling water system was operated for 30 days while maintaining the 
water quality at a constant value. The heat exchange device tube was made 
of material SUS304 (equivalent to 18-8 stainless steel, having 18% 
chromium component and 8% nickel component), and the outer diameter was 19 
mm. The temperature of the circulating water at the inflow was 30.degree. 
C., the temperature of the circulating water at the outflow was 50.degree. 
C., and the circulating water flow rate was 0.5 m/s. 
A fixed amount of scale formation preventing agent was added to the water 
system. Scale which adhered to the heat exchange tube after 30 days was 
collected. After washing the collected scale at 600.degree. C., the CaO 
content and SiO.sub.2 content were measured through standard analytical 
chemistry techniques. The scale adherence rate was expressed in units of 
mg/cm.sup.2 /month. 
Embodiment 1 
15 mg/liter of polymer A-1 and 5 mg/L of phosphorus compound P-1 were added 
to a water having the following water quality: pH 9.0, calcium hardness 
350 mg/liter, M alkalinity 350 mg/liter, silica 150 mg/liter, and 
magnesium hardness 150 mg/liter. Scale adherence was tested as described 
above. The scale adherence rate was 4 mg/cm.sup.2 /month for CaO and 2 
mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 2 
15 mg/liter of polymer A-2 and 5 mg/L of phosphorus compound P-2 were added 
to water of the same water quality as Embodiment 1. Scale adherence was 
tested as described above. The scale adherence rate was 6 mg/cm.sup.2 
/month for CaO and 1 mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 3 
15 mg/liter of polymer A-7 and 5 mg/L of phosphorus compound P-4 were added 
to water having the same water quality as Embodiment 1. Scale adherence 
was tested as described above. The scale adherence rate was 6 mg/cm.sup.2 
/month for CaO and 2 mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 4 
12 mg/liter of polymer A-1 and 3 mg/L of polymer A-4, and 5 mg/liter of 
phosphorus compound P-2 were added to water having the same water quality 
as Embodiment 1. Scale adherence was tested as described above. The scale 
adherence rate was 4 mg/cm.sup.2 /month for CaO and 1 mg/cm.sup.2 /month 
for SiO.sub.2. 
Comparative Example 1 
20 mg/liter of polymer A-1 was added to water having the same water quality 
as Embodiment 1. Scale adherence was tested as described above. The scale 
adherence rate was 21 mg/cm.sup.2 /month for CaO and 16 mg/cm.sup.2 /month 
for SiO.sub.2. 
Comparative Example 2 
20 mg/liter of polymer A-2 was added to water having the same water quality 
as Embodiment 1. Scale adherence was tested as described above. The scale 
adherence rate was 25 mg/cm.sup.2 /month for CaO and 13 mg/cm.sup.2 /month 
for SiO.sub.2. 
Comparative Example 3 
20 mg/liter of polymer A-4 was added to water having the same water quality 
as Embodiment 1. Scale adherence was tested as described above. The scale 
adherence rate was 48 mg/cm.sup.2 /month for CaO and 38 mg/cm.sup.2 /month 
for SiO.sub.2. 
Comparative Example 4 
15 mg/liter of polymer A-6 5 mg/liter of phosphorus compound P-2 were added 
to water having the same water quality as Embodiment 1. Scale adherence 
was tested as described above. The scale adherence rate was 39 mg/cm.sup.2 
/month for CaO and 59 mg/cm.sup.2 /month for SiO.sub.2. 
The results from Embodiments 1-3 and Comparative Examples 1-4 are shown in 
Table 3. 
TABLE 3 
______________________________________ 
Polymer Scale adhesion rate 
Phosphorus 
Concentration 
(mg/cm.sup.2 /month) 
Example compound (mg/L) CaO SiO.sub.2 
______________________________________ 
Embodiment 1 
A-1 15 4 2 
P-1 5 
Embodiment 2 
A-2 15 6 1 
P-2 5 
Embodiment 3 
A-7 15 6 2 
P-4 5 
Embodiment 4 
A-1 12 4 1 
A-4 3 
P-2 5 
Comp. Ex. 1 
A-1 20 21 16 
Comp. Ex. 2 
A-2 20 25 13 
Comp. Ex. 3 
A-4 20 48 38 
Comp. Ex. 4 
A-6 15 39 59 
P-2 5 
______________________________________ 
Embodiment 5 
15 mg/liter of polymer A-1 and 5 mg/L phosphorus compound P-1 were added to 
a water having the following water quality: pH 9.0, calcium hardness 350 
mg/liter, M alkalinity 350 mg/liter, silica 200 mg/liter, and magnesium 
hardness 200 mg/liter. Thus, this water contained higher levels of silica 
and magnesium hardness than that tested in Embodiments 1-4. Scale 
adherence was tested as described above. The scale adherence rate was 5 
mg/cm.sup.2 /month for CaO and 4 mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 6 
15 mg/liter of polymer A-2 and 5 mg/L of phosphorus compound P-2 were added 
to water having the same water quality as Embodiment 5. Scale adherence 
was tested as described above. The scale adherence rate was 6 mg/cm.sup.2 
/month for CaO and 3 mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 7 
15 mg/liter of polymer A-3 and 5 mg/L of phosphorus compound P-3 were added 
to water having the same water quality as Embodiment 5. Scale adherence 
was tested as described above. The scale adherence rate was 6 mg/cm.sup.2 
/month for CaO and 4 mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 8 
12 mg/liter of polymer A-1, 3 mg/liter of polymer A-4, and 5 mg/L of 
phosphorus compound P-2 were added to water having the same water quality 
as Embodiment 5. Scale adherence was tested as described above. The scale 
adherence rate was 3 mg/cm.sup.2 /month for CaO and 2 mg/cm.sup.2 /month 
for SiO.sub.2. 
Comparative Example 5 
20 mg/liter of polymer A-1 was added to water having the same water quality 
as Embodiment 5. Scale adherence was tested as described above. The scale 
adherence rate was 26 mg/cm.sup.2 /month for CaO and 14 mg/cm.sup.2 /month 
for SiO.sub.2. 
Comparative Example 6 
20 mg/liter of polymer A-3 was added to water having the same water quality 
as Embodiment 5. Scale adherence was tested as described above. The scale 
adherence rate was 21 mg/cm.sup.2 /month for CaO and 23 mg/cm.sup.2 /month 
for SiO.sub.2. 
Comparative Example 7 
15 mg/liter of polymer A-5 and 5 mg/L of phosphorus compound P-2 were added 
to water having the same water quality as Embodiment 5. Scale adherence 
was tested as described above. The scale adherence rate was 42 mg/cm.sup.2 
/month for CaO and 32 mg/cm.sup.2 /month for SiO.sub.2. 
Comparative Example 8 
20 mg/liter of polymer A-6 was added to water having the same water quality 
as Embodiment 5. Scale adherence was tested as described above. The scale 
adherence rate was 28 mg/cm.sup.2 /month for CaO and 94 mg/cm.sup.2 /month 
for SiO.sub.2. 
The results of Embodiments 5-8 and Comparative Examples 5-8 are shown in 
Table 4. 
TABLE 4 
______________________________________ 
Polymer Scale adhesion rate 
Phosphorus 
Concentration 
(mg/cm.sup.2 /month) 
Example compound (mg/L) CaO SiO.sub.2 
______________________________________ 
Embodiment 5 
A-1 15 5 4 
P-1 5 
Embodiment 6 
A-2 15 6 3 
P-2 5 
Embodiment 7 
A-3 15 6 4 
P-3 5 
Embodiment 8 
A-1 12 3 2 
A-4 3 
P-2 5 
Comp. Ex. 5 
A-1 20 26 14 
Comp. Ex. 6 
A-3 20 21 23 
Comp. Ex. 7 
A-5 15 42 32 
P-2 5 
Comp. Ex. 8 
A-6 20 28 94 
______________________________________ 
Embodiment 9 
15 mg/liter of polymer A-1 and 5 mg/L of phosphorus compound P-5 were added 
to water having the same water quality as Embodiment 1. Scale adherence 
was tested as described above. The scale adherence rate was 5 mg/cm.sup.2 
/month for CaO and 2 mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 10 
15 mg/liter of polymer A-2 and 5 mg/L of phosphorus compound P-6 were added 
to water having the same water quality as Embodiment 1. Scale adherence 
was tested as described above. The scale adherence rate was 6 mg/cm.sup.2 
/month for CaO and 2 mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 11 
15 mg/liter of polymer A-3 and 5 mg/L of phosphorus compound P-7 were added 
to water having the same water quality as Embodiment 1. Scale adherence 
was tested as described above. The scale adherence rate was 4 mg/cm.sup.2 
/month for CaO and 1 mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 12 
15 mg/liter of polymer A-1 and 5 mg/L of phosphorus compound P-8 were added 
to water having the same water quality as Embodiment 1. Scale adherence 
was tested as described above. The scale adherence rate was 8 mg/cm.sup.2 
/month for CaO and 4 mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 13 
15 mg/liter of polymer A-2 and 5 mg/L of phosphorus compound P-9 were added 
to water having the same water quality as Embodiment 1. Scale adherence 
was tested as described above. The scale adherence rate was 7 mg/cm.sup.2 
/month for CaO and 3 mg/cm.sup.2 /month for SiO.sub.2. 
Embodiment 14 
15 mg/liter of polymer A-7 and 5 mg/L of phosphorus compound P-10 were 
added to water having the same water quality as Embodiment 1. Scale 
adherence was tested as described above. The scale adherence rate was 6 
mg/cm.sup.2 /month for CaO and 4 mg/cm.sup.2 /month for SiO.sub.2. 
The results from Embodiments 9-14 are shown in Table 5. 
TABLE 5 
______________________________________ 
Polymer Scale adhesion rate 
Phosphorus 
Concentration 
(mg/cm.sup.2 /month) 
Example compound (mg/L) CaO SiO.sub.2 
______________________________________ 
Embodiment 9 
A-1 15 5 2 
P-5 5 
Embodiment 10 
A-2 15 6 2 
P-6 5 
Embodiment 11 
A-3 15 4 1 
P-7 5 
Embodiment 12 
A-1 15 8 4 
P-8 5 
Embodiment 13 
A-2 15 7 3 
P-9 5 
Embodiment 14 
A-7 15 6 4 
P-10 5 
______________________________________ 
Referring to Tables 3-5, it can be seen from these results that embodiments 
which used polymer A-1 having 100 mole % of N-vinyl formamide units, 
polymer A-2 having 90 mole % of N-vinyl formamide units, polymer A-3 
having 60 mole % of N-vinyl formamide units or polymer A-7 having 60 mole 
% of N-vinyl acetamide units together with phosphorus compounds had a low 
adherence rate of CaO and SiO.sub.2 scales, and there were excellent scale 
formation preventing effects. 
In contrast, Comparative Examples 1 and 5 included only polymer A-1 without 
any phosphorus compound. Comparative Example 2 included only polymer A-2, 
and Comparative Example 6 included only polymer A-3. Scale adherence rates 
for these comparative examples were relatively high. Therefore, scale 
formation was not prevented if a phosphorus compound was not included with 
the polymer, even when the polymers contained at least 50% of N-vinyl 
formamide units. 
Comparative Example 3 included a copolymer of acrylic acid and 
hydroxyallyloxypropane sulfonic acid, which is widely used in the prior 
art as a scale preventing agent. Comparative Example 8 included sodium 
polyacrylamide, which is also a prior art scale formation preventing 
agent. The scale adherence rate for these comparative examples were high 
compared to the embodiments of the present invention. Therefore, the scale 
preventing agents of the present invention demonstrate an excellent scale 
formation preventing effect, compared to the scale preventing agents of 
the prior art. 
Furthermore, the scale adherence rate of Comparative Example 4, which 
included polymer A-6 and phosphorus compound P-2 jointly, was high 
compared to Embodiment 2, which included polymer A-2 and phosphorus 
compound P-2 jointly. Thus, the excellent scale preventing effect which is 
obtained by the joint use of polymers and phosphorus compounds in the 
present invention is demonstrated only with polymers which include N-vinyl 
formamide or N-vinyl acetamide units. Furthermore, as shown in Comparative 
Example 7, there is not an adequate scale preventing effect by polymer 
A-5, which has 40 mole % of N-vinyl formamide units, even when used in 
conjunction with phosphorus compounds. Therefore, the N-vinyl formamide 
units and the N-vinyl acetamide units must total at least 50% to create an 
adequate scale formation preventing effect. 
Having described preferred embodiments of the invention with reference to 
the accompanying drawings, it is to be understood that the invention is 
not limited to those precise embodiments, and that various changes and 
modifications may be effected therein by one skilled in the art without 
departing from the scope or spirit of the invention as defined in the 
appended claims.