# Patent

## Title
.beta.-keto-ester-esterified polyol with polymerized monomers

## Abstract

This is a divisional application of application Ser. No. 08/527,092, filed Sep. 12, 1995, now abandoned, which is a division of application Ser. No. 08/285,998, filed Aug. 4, 1994, now abandoned, which is a division of application Ser. No. 08/184,290, filed Jan. 21, 1994, now abandoned.

## Background
The present invention relates to a heated/melted polypropylene lining, a lining powder, a polypropylene resin composition for lining and a method for preparing the polypropylene lining, which are applicable to metal (carbon steel) materials for use in an exhaust gas desulfurization device, a gas purification plant, a seawater-fresh water converter, a thermal power plant and the like.

Heretofore, as the polypropylene lining, there is known the so-called sheet lining in which the surface of a sheet or a pipe of the polypropylene is treated by corona discharge or with a mixed solution of concentrated sulfuric acid and chromic acid to impart adhesive properties to the surface, coated with an adhesive such as an epoxy resin, and then stuck on a material to be lined. However, the so-called heated/melted polypropylene lining has not been suggested at all in which a material to be lined is heated up to a temperature more than the melting point of the polypropylene, and the heated surface of the material is then sprinkled with a polypropylene powder to form a film thereon, as in the case of the lining of a polyethylene and a fluororesin. In addition, there has been neither marketed nor suggested a lining powder for heated/melted polypropylene lining in which a material to be lined is sprinkled with the powder, and this powder is then heated/melted.

Furthermore, there have not been suggested a polypropylene sheet lining in which a material to be lined is heated up to a temperature more than the melting point of a polypropylene, a sheet or a stick of the polypropylene having adhesive properties is then fused to the heated surface of the material to form a lining and to thereby secure the adhesion of the polypropylene sheet or stick to the metal.

In addition, another lining method has not been suggested at all in which a metal surface is sprinkled with a polypropylene powder, and the powder is then heated/melted, and then cooled/solidified.

The above-mentioned polypropylene lining, in which the surface of a sheet or a pipe is treated, coated with an adhesive, and then used for lining, has the following problems, and so it is scarcely applied on an industrial scale:

(1) Since the adhesive is used for the lining, a defect such as an adhesive failure tends to occur.

(2) The polypropylene sheet or pipe is pressed against a material to be lined by worker's hands or by the use of a suitable tool to achieve the adhesion, and therefore it is difficult to apply the polypropylene lining to a material having a complicate shape such as a bent portion of a pipe or a T-shaped pipe, or a material having a small diameter.

(3) When metal pipes are lined, polypropylene pipes which have been previously extruded by an extruder are used. Therefore, in a plant in which the metal pipes having various diameters are used, the polypropylene pipes which are fit for the diameters of these metal pipes are required. Thus, for the manufacture of these polypropylene pipes having the various diameters, many molds are necessary, which increases the cost of the polypropylene pipes.

In general, the polypropylene has been used for injection-molded articles, extruded articles, films and the like, but the commercially available polypropylene can adhere to the metal surface while it is in a heated/melted state, but it peels therefrom inconveniently when it is cooled/solidified. Therefore, as described above, one of the polypropylene lining techniques has been only put to practical use, and this practical polypropylene lining comprises treating the surface of the extruded sheet or pipe to modify the surface, i.e., to impart adhesive properties to the surface, and then lining a metal material with the sheet or the pipe by the use of an adhesive.

As described above, the commercially available polypropylene cannot be stuck on the metal material by a heating/melting method, because the solidification shrinkage ratio of the polypropylene is large; creep stress caused by the case where the solidification shrinkage remains for a long period of time even at a relatively low temperature (ordinary temperature) is much larger than when a polyethylene or a fluororesin is used; and there is the influence of an additive such as an aggregate which is added to the polypropylene in order to improve mold release properties in the step of the injection molding or the extrusion, but detail factors are indefinite.

In view of the above-mentioned technical level, the present invention has been completed, and an object of the present invention is to provide a heated/melted polypropylene lining material and its lining powder which can maintain excellent chemical resistance, heat resistance and mechanical properties of the polypropylene and which has excellent adhesive properties.

Another object of the present invention is to provide a lining material and its lining powder for metals which can maintain excellent chemical resistance, heat resistance and mechanical properties of the polypropylene and which can inhibit the deterioration of the resin.

Still another object of the present invention is to provide a polypropylene resin composition for lining which can maintain excellent chemical resistance, heat resistance and mechanical properties of the polypropylene and which has excellent adhesive properties, and a method for preparing this composition.

A further object of the present invention is to provide a method for extremely simply lining a metal (carbon steel) with a polypropylene.

The present inventors have intensively conducted experiments and investigations with the intention of developing a heated/melted polypropylene lining technique using a powder which can be applied to metal materials or similar materials having intricate shapes as well as large and small diameters, and as a result, they have found that after a heating/melting step, the polypropylene, when used singly, peels at the time of cooling/solidification, but when the polypropylene is mixed with a maleic acid-modified polypropylene in a suitable ratio, the resin does not peel to obtain a good lining film having strong adhesive force.

That is, the inventions are directed to:

(1) a polypropylene lining characterized by comprising a heated/melted mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/10,

(2) a polypropylene lining powder characterized by comprising a mixture of a maleic acid-modified polypropylene powder having an average particle diameter of 500 .mu.m or less and a polypropylene powder in a weight ratio of the maleic acid-modified polypropylene powder/the polypropylene powder=10/1 to 1/10, and

(3) a polypropylene lining powder obtained by melting and kneading a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/10, and then milling the mixture so that an average particle diameter may be 500 .mu.m or less.

A metal which is useful in applying the present inventions is usually carbon steel. Furthermore, the content of maleic acid in the maleic acid-modified polypropylene which can be used in the present invention is 0.05 wt% or more, preferably in the range of from 0.1 to 1.0 wt%.

In the present invention, a mixing ratio of the maleic acid-modified polypropylene to the polypropylene depends upon the kind and the grade of selected maleic acid-modified polypropylene and polypropylene. If the mixing ratio of the maleic acid-modified polypropylene is too high, chemical resistances such as acid resistance and alkali resistance deteriorate, and if it is too low, adhesive force declines and the lining tends to peel. Hence, the proper mixing ratio is in the range of from 10/1 to 1/10.

With regard to the particle diameter of the powder suitable for the heated/melted lining, if it is less than 500 .mu.m or less, a film having a uniform thickness can be obtained, but preferably, it is in the range of from 100 to 300 .mu.m. Even if the powder having an average particle diameter of more than 500 .mu.m is used, the lining film can be formed, but a long time is taken to melt the powder for the film formation, and the surface of the film becomes a rough state, which makes it impossible to form the lining film having a uniform thickness.

The lining material of the present invention is formed from the maleic acid-modified polypropylene and the polypropylene in a weight ratio of from 10/1 to 1/10. When the lining material is in a heated/melted state, both of these components are melted, and when the lining material is cooled/solidified, a carbonyl group contained in the maleic acid-modified polypropylene is strongly bonded to a metal, whereby the lining film having strong adhesive force can be obtained.

The heated/melted polypropylene lining material and the lining powder can be provided which can be applied to a high-temperature region to which a conventional polyethylene lining and rubber lining cannot be applied. In addition, this lining powder is excellent in chemical resistance and can also be applied to pipes having a complicate shape and different diameters.

Heretofore, in a device which is used in a high-temperature region and in severely corrosive circumstance to which the polyethylene lining and the rubber lining cannot be applied, a precious high-grade stainless steel or an expensive fluororesin-based lining material has been used. The polypropylene lining material and the lining powder of the present invention can be applied to such a device under the severe conditions, and the raw materials of the lining material and the powder are the fairly inexpensive maleic acid-modified polypropylene and polypropylene. Thus, the very inexpensive lining material and powder can be provided, and they have large values from industrial and economical viewpoints.

Furthermore, the present inventors have found that a good lining film which does not peel and which has strong adhesive force can be obtained by mixing a modified polypropylene and an ethylene propylene rubber with a polypropylene in a suitable ratio.

That is, the inventions are directed to:

(1) a polypropylene lining for a metal characterized by comprising a heated/melted mixture of 1 to 20 wt% of an ethylene propylene rubber and a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100,

(2) a polypropylene lining powder obtained by adding 1 to 20 wt% of an ethylene propylene rubber powder having an average particle diameter of 500 .mu.m or less to a mixture of a maleic acid-modified polypropylene powder having an average particle diameter of 500 .mu.m or less and a polypropylene powder in a weight ratio of the maleic acid-modified polypropylene powder/the polypropylene powder=10/1 to 1/100, and then mixing them, and

(3) a polypropylene lining powder obtained by adding 1 to 20 wt% of an ethylene propylene rubber to a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100, mixing them, melting and kneading the mixture, and then milling the same so that an average particle diameter may be 500 .mu.m or less.

A metal which is useful in applying the present inventions is usually carbon steel. Furthermore, the content of maleic acid in the maleic acid-modified polypropylene which can be used in the present invention is 0.01 wt% or more, preferably in the range of from 0.1 to 1.0 wt%.

The content of the propylene in the ethylene propylene rubber which can be used in the present invention is usually in the range of from 10 to 40 wt%. In the present invention, a mixing ratio of the maleic acid-modified polypropylene to the polypropylene depends upon the kind and the grade of selected maleic acid-modified polypropylene and homopolypropylene, but the proper mixing ratio is in the range of from 10/1 to 1/100. The higher the mixing ratio of the maleic acid-modified polypropylene is, the stronger the adhesive force of the lining to the metal is. A ratio of the ethylene propylene rubber which is mixed with the above-mentioned mixture depends upon the kind and the grade of selected ethylene propylene rubber, but the proper mixing ratio is in the range of from 1 to 20 wt%. If the mixing ratio of the ethylene propylene rubber is too high, the strength of the lining declines.

Moreover, if the particle diameter of the powder suitable for the heated/melted lining is 500 .mu.m or less, the film having a uniform thickness can be formed. Even if the powder having the average particle diameter more than 500 .mu.m is used, the lining film can be formed, but a long time is taken to melt the powder for the film formation, and the surface of the film becomes a rough state, which makes it impossible to form the lining film having the uniform thickness.

The lining material of the present invention can be formed from the substance in which 1 to 20 wt% by outer percentage of the ethylene propylene rubber is added to the mixture of the maleic acid-modified polypropylene and the polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100. When the lining material is in a heated/melted state, both of the maleic acid-modified polypropylene and the polypropylene are melted, and when the lining material is cooled/solidified, a carbonyl group contained in the maleic acid-modified polypropylene is strongly bonded to a metal, whereby the lining film having strong adhesive force can be obtained.

On the other hand, when the lining material comprising the substance in which the ethylene propylene rubber is added to the mixture of the maleic acid-modified polypropylene and the polypropylene is in the heated and melted state, the maleic acid-modified polypropylene, the polypropylene and the ethylene propylene rubber are melted. The mixture of the maleic acid-modified polypropylene and the polypropylene (hereinafter referred to as "modified polypropylene" sometimes) is different from the ethylene propylene rubber in fluidity and solidification temperature, and therefore, when the lining material is cooled/solidified, the ethylene propylene rubber is solidified in a dispersed state in the modified polypropylene.

Since the creep stress of the ethylene propylene rubber is very small, the ethylene propylene rubber stretches at the time of solidification shrinkage and the stress is instantly relived to a level smaller than adhesive force. Hence, the lining which does not peel can be obtained.

In the lining powder for obtaining the lining of the present invention, the ethylene propylene rubber is added to the modified polypropylene, and therefore, owing to the stress relief function of the ethylene propylene rubber, the amount of the maleic acid-modified polypropylene can be reduced, as compared with a case where no ethylene propylene rubber is added.

The lining powder of the present invention can be melted and kneaded by the use of a melting kneader, and then milled so that an average particle diameter may be 500 .mu.m or less to obtain a fine lining powder having the further improved adhesive properties. The mixing state of the thus obtained fine lining powder is more uniform than the mixture of the modified polypropylene, the ethylene propylene rubber having the small particle diameter.

Furthermore, the present inventors have found that when an antioxidant is added to an adhesive polypropylene prepared by mixing a polypropylene with a modified polypropylene and an ethylene propylene rubber, a tough film having good surface properties can be obtained without impairing adhesive properties and without deteriorating the resins.

That is, the inventions are directed to:

(1) a polypropylene lining for a metal characterized by comprising the steps of adding 1 to 20 wt% of an ethylene propylene rubber to a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100, further adding 0.1 to 1.0 wt% of an antioxidant to the mixture, heating/melting the resulting powder, and then cooling/solidifying the same to form a lining layer,

(2) a polypropylene lining powder obtained by adding 1 to 20 wt% of an ethylene propylene rubber powder having an average particle diameter of 500 .mu.m or less to a mixture of a maleic acid-modified polypropylene powder having an average particle diameter of 500 .mu.m or less and a polypropylene powder having an average particle diameter of 500 .mu.m or less in a weight ratio of the maleic acid-modified polypropylene powder/the polypropylene powder=10/1 to 1/100, further adding 0.1 to 1.0 wt% of an antioxidant to the mixture, and then mixing them, and

(3) a polypropylene lining powder obtained by adding 1 to 20 wt% of an ethylene propylene rubber to a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100, further adding 0.1 to 1.0 wt% of an antioxidant to the mixture, melting and kneading the resulting mixture, and then milling the same so that an average particle diameter may be 500 .mu.m or less.

A metal which is useful in applying the present inventions is usually carbon steel.

The content of maleic acid in the maleic acid-modified polypropylene which can be used in the present invention is 0.01 wt% or more, preferably in the range of from 0.1 to 1.0 wt%.

The content of the propylene in the ethylene propylene rubber which can be used in the present invention is usually in the range of from 10 to 40 wt%. Moreover, in the present invention, the melt flow rate of the polypropylene is preferably between 0.1 and 40.

In the present invention, a mixing ratio of the maleic acid-modified polypropylene to the polypropylene depends upon the kind of selected maleic acid-modified polypropylene and polypropylene, but the proper mixing ratio is in the range of from 10/1 to 1/100. The higher the mixing ratio of the maleic acid-modified polypropylene is, the stronger the adhesive force of the lining to the metal is. A ratio of the ethylene propylene rubber which is mixed with the above-mentioned mixture depends upon the kind of selected ethylene propylene rubber, but the proper mixing ratio is in the range of from 1.0 to 20 wt%. If the mixing ratio of the ethylene propylene rubber is too high, the strength of the lining declines.

In the present invention, preferable examples of the antioxidant include high-molecular weight phenol-based antioxidants and phosphites (and phosphonites), and the amount of the antioxidant to be used depends upon the kind of antioxidant, but the proper amount is in the range of from 0.1 to 1.0 wt% by outer percentage based on the weight of the modified polypropylene.

A typical example of the antioxidant is a combination of a phosphorus-based working stabilizer such as tris(2,4-di-t-butylphenyl) phosphite and at least one selected from the group consisting of hindered phenol-based pentaerythrityltetrakis3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate!, octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene and tris-(3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate.

If the particle diameter of the powder is 500 .mu.m or less suitable for the heated/melted lining, a film having a uniform thickness can be obtained. Even if the powder having the particle diameter larger than this size is used, the lining film can be formed, but a long time is taken to melt the powder for the film formation, and the surface of the film becomes a rough state, which makes it impossible to form the lining film having the uniform thickness.

The lining material of the present invention can be formed by adding 1.0 to 20 wt% of the ethylene propylene rubber to the mixture of the maleic acid-modified polypropylene and the polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100, and further adding the antioxidant to the mixture. When the lining material is in a heated/melted state, both of the maleic acid-modified polypropylene and the polypropylene are melted, and when the lining material is cooled/solidified, a carboxyl group contained in the maleic acid-modified polypropylene is strongly bonded to a metal, whereby the lining film having strong adhesive force can be obtained.

On the other hand, when the lining material is in a heated/melted state, the ethylene propylene rubber added to the mixture of the maleic acid-modified polypropylene and the polypropylene is melted in the polypropylene, and when the lining material is cooled/solidified, the ethylene propylene rubber is dispersed in the polypropylene and the modified polypropylene, because the maleic acid-modified polypropylene, the polypropylene and the ethylene propylene rubber are different from each other in fluidity and solidification temperature. Since the creep deformation stress of the ethylene propylene rubber is very small, the ethylene propylene rubber stretches at the time of solidification shrinkage and the stress is instantly relived to a level smaller than adhesive force. Hence, the lining which does not peel can be obtained.

In addition, the antioxidant has a function for inhibiting the main chain of the polypropylene from thermally decomposing, and therefore the employment of the antioxidant permits thermal deterioration in the heating/melting step to decrease. In the lining powder of the present invention the antioxidant is added to the mixture of the polypropylene, the modified polypropylene and the ethylene propylene rubber, so that the thermal deterioration can be inhibited in the heating/melting step and so a tough film can be formed.

The lining powder of the present invention can be melted and kneaded by the use of a melting kneader, and then milled so that an average particle diameter may be 500 .mu.m or less to obtain a fine lining powder having the further improved adhesive properties. The mixing state of the thus obtained fine lining powder is more uniform than the mixture of the modified polypropylene, the ethylene propylene rubber and the antioxidant having the small particle diameter.

Furthermore, the present inventors have intensively conducted experiments and investigations with the intention of developing a lining material which can be manufactured from a polypropylene sheet or a polypropylene stick having good adhesive properties, and as a result, they have found that when an antioxidant is added to an adhesive polypropylene prepared by mixing a polypropylene with a modified polypropylene and an ethylene propylene rubber, a tough film having good surface properties can be obtained without impairing adhesive properties and without deteriorating the resins.

That is, the inventions are directed to:

(1) a polypropylene lining sheet or stick comprising a resin composition obtained by adding 1 to 20 wt% of an ethylene propylene rubber to a mixture of a maleic acid-modified polypropylene and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=10/1 to 1/100, and further adding 0.1 to 1.0 wt% of an antioxidant to the mixture, and

(2) a polypropylene lining method characterized by comprising the step of thermally fusing a sheet or a stick of a polypropylene resin composition of the above-mentioned (1) to a member to be lined.

The content of maleic acid in the maleic acid-modified polypropylene which can be used in the present invention is 0.01 wt% or more, preferably in the range of from 0.1 to 1.0 wt%.

The content of the propylene in the ethylene propylene rubber which can be used in the present invention is usually in the range of from 10 to 40 wt%. Moreover, in the present invention, the melt flow rate of the polypropylene is preferably between 0.1 and 40.

The sheet of the polypropylene resin composition according to the present invention is linable on the surface of a metal irrespective of its thickness, and when the sheet is thermally fused on the metal, a film having good adhesive properties can be formed. Furthermore, the stick of the polypropylene resin composition according to the present invention is linable on the surface of a metal irrespective of its thickness, and when the stick is arranged in one direction and thermally fused on the metal, a film having good adhesive properties can be formed. Thus, the stick is applicable for the repair of the lining.

The sheet or the stick of the polypropylene resin composition of the present invention can be fused on the surface of a metal to obtain a lining film having good adhesive properties to the metal.

Furthermore, the inventions are directed to:

(1) a lining method of a polypropylene characterized by comprising the steps of sprinkling the surface of a metal with a maleic acid-modified polypropylene powder, heating/melting the powder, sprinkling the melted maleic acid-modified polypropylene powder layer with a polypropylene powder, heating/melting the powder, and then cooling/solidifying the same to form a lining layer of the polypropylene, and

(2) a lining method of a polypropylene characterized by comprising the steps of sprinkling the surface of a metal with a maleic acid-modified polypropylene powder, heating/melting the powder, cooling/solidifying the same, sprinkling the resulting maleic acid-modified polypropylene film layer with a polypropylene powder, heating/melting the powder, and then cooling the same to form a lining layer of the polypropylene.

The content of maleic acid in the maleic acid-modified polypropylene which can be used in the inventions in claims 12 and 13 is 0.05% or more, preferably in the range of from 0.1 to 1.0%.

The maleic acid-modified polypropylene has a carbonyl group, and if heated/melted and then cooled/solidified, the maleic acid-modified polypropylene can be bonded to a metal. Thus, the present invention intends that the maleic acid-modified polypropylene film layer is first formed on the surface of the metal, and the polypropylene film is then formed on the maleic acid-modified polypropylene film layer.

The maleic acid-modified polypropylene has compatibility with the polypropylene, and so they can be bonded to each other by the heating/melting. Thus, the polypropylene lining of the present invention having the polypropylene film on its outer periphery can be obtained.

The film layer of the maleic acid-modified polypropylene interposed between the metal surface and the polypropylene film layer is preferably as thin as possible in order to improve the heat resistance of the polypropylene film. However, the minimum thickness of the film layer of the maleic acid-modified polypropylene depends upon the particle diameter of the maleic acid-modified polypropylene powder to be used for the film formation, and so the practical thickness of the film layer is in the range of from 50 to 200 .mu.m.

FIG. 1 is a sectional view of a matrix lined with a polypropylene.

A preferable embodiment of the inventions will be described.

The pellets of a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.01 to 1.0 wt% and the pellets of a polypropylene having a melt flow index of 1.0 were separately milled by means of a refrigeration mill using liquid nitrogen, and each powder was classified by sieves into portions having average particle diameters of 1,000 .mu.m, 500 .mu.m, 300 .mu.m and 100 .mu.m. Afterward, the maleic acid-modified polypropylene powder was mixed with the polypropylene powder in blend ratios in Table 1 to obtain lining powders.

Furthermore, the pellets of the above-mentioned maleic acid-modified polypropylene was mixed with the pellets of the above-mentioned polypropylene in blend ratios shown in Table 1, and each mixture was then kneaded by means of a melting kneader, milled by the refrigeration mill, and then classified by the sieves to obtain lining powders having average particle diameters of 1,000 .mu.m, 500 .mu.m, 300 .mu.m and 100 .mu.m.

Next, a 70-mm-long, 200-mm-wide, 5-mm-thick carbon steel plate was degreased, shot-blasted, heated on a heated plate so that its surface temperature might be 250.degree..+-.20.degree. C., sprinkled with the above-mentioned powder, and then heated/melted to obtain a lined steel plate having a film thickness of 1.5 mm. Afterward, the observation of the surface state of each lining, the measurement of adhesive force by a 90.degree. peel method in a wide of 10 mm, and a durability test for 6 months under conditions of 5% H.sub.2 SO.sub.4 and 100.degree. were carried out. The results are set forth in Table 1. ______________________________________ Maleic Acid Modification Ratio Weight Ratio of of Maleic Acid- Maleic Acid-Modified Kind of    Modified Polypropylene Polypropylene No.  Lining Powder (%)            to Polypropylene ______________________________________ 1    Mixed powder 0.01           10/1 2    "          0.05           1/1 3    "          0.5            1/5 4    "          1.0             1/10 5    "          0.5            20/1 6    "          0.5             1/20 7    "          0.5            1/5 8    "          0.5            1/5 9    "          0.5            1/5 ______________________________________ Average Particle Surface Diameter of State   Adhesive Force Lining Powder after   (kgf/    State after No.  (.mu.m)     Lining  10 mm width) Durability Test ______________________________________ 1   500         Good     4       Color changed slightly 2   500         "        7       No problem 3   500         "        8       " 4   500         "        7       " 5   300         "       12       Color changed, fine cracks occurred 6   300         Peeled  --       -- 7   300         Good    10       No problem 8   100         "       11       " 9   1000        Rough    6       " ______________________________________ Maleic Acid Modification Ratio Weight Ratio of of Maleic Acid- Maleic Acid-Modified Kind of    Modified Polypropylene Polypropylene No.  Lining Powder (%)            to Polypropylene ______________________________________ 10   Melted &   0.01           10/1 kneaded 11   Melted &   0.05           1/1 kneaded 12   Melted &   0.5            1/5 kneaded 13   Melted &   1.0             1/10 kneaded 14   Melted &   0.5            20/1 kneaded 15   Melted &   0.5             1/20 kneaded 16   Melted &   0.5            1/5 kneaded 17   Melted &   0.5            1/5 kneaded 18   Melted &   0.5            1/5 kneaded ______________________________________ Average Particle Surface Diameter of State   Adhesive Force Lining Powder after   (kgf/    State after No.  (.mu.m)     Lining  10 mm width) Durability Test ______________________________________ 10   500         Good     5       Color changed slightly 11   500         "        8       No problem 12   500         "       10       " 13   500         "        9       " 14   300         "       12       Color changed, fine cracks occurred 15   300         Peeled  --       -- 16   300         Good    11       No problem 17   100         "       12       " 18   1000        Rough    7       " ______________________________________

A preferable embodiment of the inventions will be described.

The pellets of a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.01 to 1.0 wt%, the pellets of a polypropylene having a melt flow index of 1.0 and the pellets of an ethylene propylene rubber having a polypropylene content of from 10 to 40% were separately milled by means of a refrigeration mill using liquid nitrogen, and each powder was classified by sieves into portions having average particle diameters of 1,000 .mu.m, 500 .mu.m and 100 .mu.m. Afterward, the resulting powders were mixed in blend ratios in Table 2 to obtain lining powders.

Furthermore, the pellets of the above-mentioned maleic acid-modified polypropylene, the above-mentioned polypropylene and the above-mentioned ethylene propylene rubber were mixed in blend ratios shown in Table 2, and each mixture was then kneaded by means of a melting kneader, milled by the refrigeration mill, and then classified by the sieves to obtain lining powders having average particle diameters of 1,000 .mu.m, 500 .mu.m and 100 .mu.m.

Next, a 70-mm-long, 200-mm-wide, 5-mm-thick carbon steel plate was degreased, shot-blasted, heated on a heated plate so that its surface temperature might be 250.degree..+-.20.degree. C., sprinkled with the above-mentioned powder, and then heated/melted to obtain a lined steel plate having a film thickness of 1.5 mm. Afterward, the observation of the surface state of each lining, the measurement of adhesive force by a 90.degree. peel method in a wide of 10 mm, and a durability test for 6 months under conditions of 5% H.sub.2 SO.sub.4 and 100.degree. were carried out. The results are set forth in Table 2. ______________________________________ Weight Ratio Maleic Acid Propylene of Maleic Modification Content in Acid-Modified Ratio of Maleic Ethylene Polypropylene/ Acid-modified Propylene Ethylene Kind of    Polypropylene Rubber Propylene Rubber/ No.  Lining Powder (%)        (%)    Polypropylene ______________________________________ 1   Mixed powder 0.01       25     1/0.4/1 2   "          0.05       25     1/0.4/1 3   "          0.10       25     1/0.4/1 4   "          0.50       25     1/0.4/1 5   "          1.00       25     1/0.4/1 6   "          0.50       10     1/0.4/1 7   "          0.50       40     1/0.4/1 8   "          0.50       25     1/0.2/1 9   "          0.50       25      1/0.025/1 10   "          0.50       25      10/1.2/1 11   "          0.50       25     5/0.7/1 12   "          0.50       25      1/0.21/1 ______________________________________ Average Particle Diameter of Lining Powder Surface State Adhesive Force State after No.  (.mu.m)     after Lining (kgf/10 mm .phi.) Durability Test ______________________________________ 1   100         Good       4       No problem 2   100         "         11       " 3   100         "         13       " 4   100         "         14       " 5   100         "         16       " 6   100         "         12       " 7   100         "         11       " 8   100         "         12       " 9   100         "         14       " 10   100         "         17       " 11   100         "         15       " 12   100         "         14       " ______________________________________ Weight Ratio Maleic Acid Propylene of Maleic Modification Content in Acid-Modified Ratio of Maleic Ethylene Polypropylene/ Acid-modified Propylene Ethylene Kind of    Polypropylene Rubber Propylene Rubber/ No.  Lining Powder (%)        (%)    Polypropylene ______________________________________ 13   Mixed powder 0.50       25      1/1.2/10 14   "          0.50       25      1/5.7/50 15   "          0.50       25       1/12.2/100 16   "          0.05       25       1/23.3/200 17   "          0.50       25      1/1.2/10 18   "          0.50       25      1/1.2/10 19   Melted &   0.01       25     1/0.4/1 kneaded 20   Melted &   0.05       25     1/0.4/1 kneaded 21   Melted &   0.10       25     1/0.4/1 kneaded 22   Melted &   0.50       25     1/0.4/1 kneaded 23   Melted &   1.00       25     1/0.4/1 kneaded 24   Melted &   0.50       10     1/0.4/1 kneaded ______________________________________ Average Particle Diameter of Lining Powder Surface State Adhesive Force State after No.  (.mu.m)     after Lining (kgf/10 mm .phi.) Durability Test ______________________________________ 13   100         Good      14       No problem 14   100         "         13       " 15   100         "         12       " 16   100         Peeled    --       -- 17   500         Good      14       No problem 18   1000        Rough     11       " 19   100         Good       6       " 20   100         "         12       " 21   100         "         15       " 22   100         "         15       " 23   100         "         18       " 24   100         "         12       " ______________________________________ Weight Ratio Maleic Acid Propylene of Maleic Modification Content in Acid-Modified Ratio of Maleic Ethylene Polypropylene/ Acid-modified Propylene Ethylene Kind of    Polypropylene Rubber Propylene Rubber/ No.  Lining Powder (%)        (%)    Polypropylene ______________________________________ 25   Melted &   0.50       40     1/0.4/1 kneaded 26   Melted &   0.50       25     1/0.2/1 kneaded 27   Melted &   0.50       25      1/0.025/1 kneaded 28   Melted &   0.50       25     10/1.2/1 kneaded 29   Melted &   0.50       25     5/0.7/1 kneaded 30   Melted &   0.50       25     1/0.2/1 kneaded 31   Melted &   0.50       25     1/1.2/10 kneaded 32   Melted &   0.50       25     1/5.7/50 kneaded 33   Melted &   0.50       25      1/12.2/100 kneaded 34   Melted &   0.50       25      1/23.3/200 kneaded 35   Melted &   0.50       25     1/1.2/10 kneaded 36   Melted &   0.50       25     1/1.2/10 kneaded ______________________________________ Average Particle Diameter of Lining Powder Surface State Adhesive Force State after No.  (.mu.m)     after Lining (kgf/10 mm .phi.) Durability Test ______________________________________ 25   100         Good      12       No problem 26   100         "         13       " 27   100         "         15       " 28   100         "         19       " 29   100         "         17       " 30   100         "         15       " 31   100         "         15       " 32   100         "         14       " 33   100         "         12       " 34   100         Peeled    --       -- 35   500         Good      16       No problem 36   1000        Rough     12       " ______________________________________

A preferable embodiment of the inventions will be described.

The pellets of a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.01 to 1.0 wt%, the pellets of a polypropylene and the pellets of an ethylene propylene rubber having a polypropylene content of from 10 to 40% were separately milled by means of a refrigeration mill using liquid nitrogen, and each powder was classified by sieves into portions having average particle diameters of 1,000 .mu.m, 500 .mu.m and 100 .mu.m. Afterward, the resulting powders were mixed with a high-molecular weight phenol-based antioxidant and another antioxidant (IRGANOX B215, made by Ciba-Geigy) containing a phosphite (or a phosphonite) in blend ratios in Table 3 to obtain lining powders.

In this connection, the antioxidant (IRGANOX B215) used herein was a mixture of pentaerythrityltetrakis3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate! and tris(2,4-di-t-butylphenyl) phosphite in a ratio of 1:2.

Furthermore, the above-mentioned maleic acid-modified polypropylene, the polypropylene, the ethylene propylene rubber and the antioxidant were mixed in blend ratios shown in Table 4, and each mixture was then kneaded by means of a melting kneader, milled by the refrigeration mill, and then classified by the sieves into portions having average particle diameters of 1,000 .mu.m, 500 .mu.m and 100 .mu.m to obtain lining powders.

Next, a 70-mm-long, 200-mm-wide, 5-mm-thick carbon steel plate was degreased, shot-blasted, heated on a heated plate so that its surface temperature might be 250.degree..+-.20.degree. C., sprinkled with the above-mentioned powder, and then melted to obtain a lined steel plate having a film thickness of 1.5 mm.

Afterward, the observation of the surface state of each lining, the measurement of adhesive force by a 90.degree. peel method in a wide of 10 mm, and a durability test for 6 months under conditions of 5% H.sub.2 SO.sub.4 and 100.degree. were carried out.

The results are set forth in Tables 3 and 4. __________________________________________________________________________ Maleic Acid Modification Ratio of PP Content   Weight Ratio Kind of Modified PP* in EPR** Melt Flow of Modified No. Lining Powder (%)      (%)   Rate of PP PP/EPR/PP __________________________________________________________________________ 1  Mixed Powder --       25    0.5     --/0.1/1.0 2  "      0.10     25    0.5    1.0/0.2/1.0 3  "      0.50     25    0.5    1.0/0.2/1.0 4  "      1.00     25    0.5    1.0/0.2/1.0 5  "      0.50     25    0.5    1.0/0.2/1.0 6  "      0.50     25    0.5    1.0/0.2/1.0 7  "      0.50     25    0.5    1.0/0.2/1.0 8  "      0.50     25    5.0    1.0/0.2/1.0 9  "      0.50     25    15.0   1.0/0.2/1.0 10  "      0.50     25    30.0   1.0/0.2/1.0 __________________________________________________________________________ Amount of Antioxidant to Mixture Average of Modified Particle Diameter PP, EPR and PP of Lining Powder Surface State Adhesive Force State after No. (%)     (.mu.m) after Lining (kgf/10 mm width) Durability Test __________________________________________________________________________ 1 0.25    100     Good  Peeled   -- 2 0.25    100     "     12       No problem 3 0.25    100     "     14       " 4 0.25    100     "     13       " 5 --      100     Air bubbles 15       Air bubbles 6 0.50    100     Good  15       No problem 7 1.00    100     "     14       " 8 0.25    100     "     16       " 9 0.25    100     "     15       " 10 0.25    100     Partial liquid 15       " suspension __________________________________________________________________________ Maleic Acid Modification Ratio of PP Content   Weight Ratio Kind of Modified PP* in EPR** Melt Flow of Modified No. Lining Powder (%)      (%)   Rate of PP PP/EPR/PP __________________________________________________________________________ 11  Mixed powder 0.50     10    5.0    1.0/0.2/1.0 12  "      0.50     40    5.0    1.0/0.2/1.0 13  "      0.50     25    5.0    5.0/0.6/1.0 14  "      0.50     25    5.0     10.0/1.1/1.0 15  "      0.50     25    5.0     1.0/2.6/25.0 16  "      0.50     25    5.0      1.0/10.1/100.0 17  "      0.50     25    5.0    1.0/0.4/1.0 18  "      0.50     --    5.0     1.0/--/1.0 19  "      0.50     25    5.0    1.0/0.2/1.0 20  "      0.50     25    5.0    1.0/0.2/1.0 __________________________________________________________________________ Amount of Antioxidant to Mixture Average of Modified Particle Diameter PP, EPR and PP of Lining Powder Surface State Adhesive Force State after No. (%)     (.mu.m) after Lining (kgf/10 mm width) Durability Test __________________________________________________________________________ 11 0.25    100     Good  14       No problem 12 0.25    100     "     16       " 13 0.25    100     "     18       " 14 0.25    100     "     17       " 15 0.25    100     "     15       " 16 0.25    100     "     14       " 17 0.25    100     "     15       " 18 0.25    100     "      2       Peeled 19 0.25    500     "     14       No problem 20 0.25    1000    Rough 15       Rough __________________________________________________________________________ Note *means a polypropylene, and **means an ethylene propylene rubber. __________________________________________________________________________ Amount of Average Maleic Acid PP            Antioxidant Particle Modification Content Melt Weight to Mixture of Diameter Surface Adhesive Ratio of in  Flow Ratio of Modified PP, of Lining State Force State after Kind of  Modified PP* EPR** Rate Modified EPR and PP Powder after (kgf/10 Durability No. Lining Powder (%)    (%) of PP PP/EPR/PP (%)   (.mu.m) Lining width) Test __________________________________________________________________________ 21  Melted & kneaded --     25  0.5 --/0.1/1.0 0.25  100  Good  Peeled -- 22  "        0.10   25  0.5 1.0/0.2/1.0 0.25  100  "     14    No problem 23  "        0.50   25  0.5 1.0/0.2/1.0 0.25  100  "     17    " 24  "        1.00   25  0.5 1.0/0.2/1.0 0.25  100  "     16    " 25  "        0.50   25  0.5 1.0/0.2/1.0 --    100  Rough 16    " 26  "        0.50   25  0.5 1.0/0.2/1.0 0.50  100  Good  17    " 27  "        0.50   25  0.5 1.0/0.2/1.0 1.00  100  "     17    " 28  "        0.50   25  5.0 1.0/0.2/1.0 0.25  100  "     18    " 29  "        0.50   25  15.0 1.0/0.2/1.0 0.25  100  "     17    " 30  "        0.50   25  30.0 1.0/0.2/1.0 0.25  100  Partial liquid 18    " suspension 31  Melted & kneaded 0.50   10  5.0 1.0/0.2/1.0 0.25  100  Good  17    No problem 32  "        0.50   40  5.0 1.0/0.2/1.0 0.25  100  "     18    " 33  "        0.50   25  5.0 5.0/0.6/1.0 0.25  100  "     20    " 34  "        0.50   25  5.0 10.0/1.1/1.0 0.25  100  "     18    " 35  "        0.50   25  5.0 1.0/2.6/25.0 0.25  100  "     17    " 36  "        0.50   25  5.0 1.0/10.1/100.0 0.25  100  "     17    " 37  "        0.50   25  5.0 1.0/0.4/1.0 0.25  100  "     18    " 38  "        0.50   --  5.0 1.0/--/1.0 0.25  100  "      3    Peeled 39  "        0.50   25  5.0 1.0/0.2/1.0 0.25  100  "     17    No problem 40  "        0.50   25  5.0 1.0/0.2/1.0 0.25  100  "     17    " __________________________________________________________________________ Note: * means a polypropylene, and ** means an ethylene propylene rubber.

A preferable embodiment of the inventions will be described.

The pellets of a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.01 to 1.0 wt%, the pellets of a polypropylene, the pellets of an ethylene propylene rubber having a polypropylene content of from 10 to 40%, a high-molecular weight phenol-based antioxidant and another antioxidant (IRGANOX B215, made by Ciba-Geigy) containing a phosphite (or a phosphonite) were mixed with in blend ratios shown in Tables 5 and 6, and each mixture was then kneaded by means of a melting kneader, and then molded by the use of a sheet molding machine to obtain sheets having film thicknesses of 200 .mu.m, 500 .mu.m, 1,000 .mu.m and 2,000 .mu.m.

In this connection, the antioxidant (IRGANOX B215) used herein was a mixture of pentaerythrityltetrakis3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate! and tris(2,4-di-t-butylphenyl) phosphite in a ratio of 1:2.

Next, a 70-mm-long, 200-mm-wide, 5-mm-thick carbon steel plate was decreased, shot-blasted, and then heated on a heated plate so that its surface temperature might be 250.degree..+-.20.degree. C., and the above-mentioned sheet was pressed against the heated steel plate to fuse it thereon, thereby obtaining the steel plate lined with the polypropylene sheet.

Afterward, the observation of the surface state of each sheet lining, the measurement of adhesive force by a 90.degree. peel method in a wide of 10 mm, and a durability test for 6 months under conditions of 5% H.sub.2 SO.sub.4 and 100.degree. were carried out.

The results are set forth in Table 5. __________________________________________________________________________ Amount of Maleic Acid PP            Antioxidant Thickness Modification Content Melt      to Mixture of of Sheet Surface  Adhesive Ratio of in  Flow Modified Modified PP, of PP Resin State    Force State after Modified PP* EPR** Rate PP/EPR/PP EPR and PP Composition of       (kgf/10 mm Durability No. (%)    (%) of PP (weight ratio) (%)   (.mu.m) Lining   width) Test __________________________________________________________________________ 1  --     25  0.5 --/0.1/1.0 0.25  500   Metal surface Peeled -- peeled 2  0.10   25  0.5 1.0/0.2/1.0 0.25  500   Uniform and good 14    No problem 3  0.50   25  0.5 1.0/0.2/1.0 0.25  500   "        12    " 4  1.00   25  0.5 1.0/0.2/1.0 0.25  500   "        13    " 5  0.50   25  0.5 1.0/0.2/1.0 --    500   Air bubbles 13    Air bubbles 6  0.50   25  0.5 1.0/0.2/1.0 0.50  500   Uniform and good 14    No problem 7  0.50   25  0.5 1.0/0.2/1.0 1.00  500   "        14    " 8  0.50   25  5.0 1.0/0.2/1.0 0.25  500   "        15    " 9  0.50   25  15.0 1.0/0.2/1.0 0.25  500   "        16    " 10 0.50   25  30.0 1.0/0.2/1.0 0.25  500   "        16    " 11 0.50   10  5.0 1.0/0.2/1.0 0.25  500   Uniform and good 15    No problem 12 0.50   40  5.0 1.0/0.2/1.0 0.25  500   "        15    " 13 0.50   25  5.0 5.0/0.6/1.0 0.25  500   "        16    " 14 0.50   25  5.0 10.0/1.1/1.0 0.25  500   "        16    " 15 0.50   25  5.0 1.0/2.6/25.0 0.25  500   "        17    " 16 0.50   25  5.0 1.0/10.1/100.0 0.25  500   "        14    " 17 0.50   25  5.0 1.0/0.4/1.0 0.25  500   "        14    " 18 0.50   --  5.0 1.0/--/1.0 0.25  500   Partially peeled 2    Peeled 19 0.50   25  5.0 1.0/0.2/1.0 0.25  200   Uniform and good 15    No problem 20 0.50   25  5.0 1.0/0.2/1.0 0.25  1000  "        16    " 21 0.50   25  5.0 1.0/0.2/1.0 0.25  2000  "        15    " __________________________________________________________________________ Note: * means a polypropylene, and ** means an ethylene propylene rubber.

The pellets of a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.01 to 1.0 wt%, the pellets of a polypropylene, the pellets of an ethylene propylene rubber having a polypropylene content of from 10 to 40%, a high-molecular weight phenol-based antioxidant and another antioxidant (IRGANOX B215) containing a phosphite (or a phosphonite) were mixed with in blend ratios shown in Table 6. Each mixture was then kneaded by means of a melting kneader, and then extruded through a cylinder head to obtain strands, and sticks were prepared from the strand.

Next, a 70-mm-long, 200-mm-wide, 5-mm-thick carbon steel plate was degreased, shot-blasted, and then heated on a heated plate so that its surface temperature might be 250.degree..+-.20.degree. C., and the above-mentioned stick was pressed against the heated steel plate to fuse it thereon, thereby obtaining the steel plate lined with the polypropylene.

Afterward, the observation of the surface state of each lining, the measurement of adhesive force by a 90.degree. peel method in a wide of 10 mm, and a durability test for 6 months under conditions of 5% H.sub.2 SO.sub.4 and 100.degree. were carried out.

The results are set forth in Table 6. __________________________________________________________________________ Amount of Maleic Acid PP            Antioxidant Size of Modification Content Melt      to Mixture of Stick of Surface Adhesive Ratio of in  Flow Modified Modified PP, PP Resin State Force State after Modified PP* EPR** Rate PP/EPR/PP EPR and PP Composition of  (kgf/10 mm Durability No. (%)    (%) of PP (weight ratio) (%)   (mm)  Lining width) Test __________________________________________________________________________ 22 0.50   25  0.5 1.0/0.2/1.0 0.25  3     Good 15    No problem 23 0.50   25  5.0 1.0/0.2/1.0 0.25  3     "   14    " 24 0.50   25  15.0 1.0/0.2/1.0 0.25  3     "   16    " 25 0.50   25  30.0 1.0/0.2/1.0 0.25  3     "   14    " __________________________________________________________________________ * means a polypropylene, and ** means an ethylene propylene rubber.

A preferable embodiment of the inventions will be described in reference to FIG. 1.

In FIG. 1, reference numeral 1 is a metal matrix (carbon steel) having a diameter of 120 mm and a thickness of 5 mm in which the surface of the metal matrix is cleaned by degreasing, washing and blasting, numeral 2 is a maleic acid-modified polypropylene film formed by heating/melting (250.degree..+-.20.degree. C.) a maleic acid-modified powder having a melt index of 10 and an average diameter of 100 .mu.m on the surface of the metal matrix 1. Numeral 3 is a polypropylene film formed by heating/melting (250.degree..+-.20.degree. C.) a polypropylene powder having a melt index of 5 and an average diameter of 100 .mu.m on the maleic acid-modified polypropylene film 2. The formation of the polypropylene film 3 can be carried out at a time when the previously formed maleic acid-modified polypropylene film 2 is still in a melted state or in a cooled/solidified state. In Table 7, there are shown the results of the adhesive force and the durability test (for 6 months under conditions of 5% sulfuric acid and 100.degree. C.) of the polypropylene lining materials which lining has been made by two methods of the present invention. __________________________________________________________________________ Thickness of Thickness of Thickness of Thickness of Film of 0.05% Film of 0.5% Film of 1.0% Film of 0.01% Thickness Maleic Acid- Maleic Acid- Maleic Acid- Maleic Acid- of Poly- Modified Modified Modified Modified propylene      Adhesive Force Polypropylene Polypropylene Polypropylene Polypropylene Film  Surface State after Lining State after No. (.mu.m) (.mu.m) (.mu.m) (.mu.m) (.mu.m) after Lining (kgf/10 mm Durability __________________________________________________________________________ Test 1  None   None   None   None   2000  Peeled from --       -- metal surface 2  None   None   None   None    200  Peeled from --       -- metal surface 3  200    None   None   None   1000  Uniform & good 7       No problem 4  None    50    None   None   2000  "        12       " 5  None    200   None   None   2000  "        11       " 6  None    500   None   None   2000  "        10       " 7  None   1000   None   None   1000  "        10       Wavy wrinkles occured slightly 8  None   2000   None   None    500  Wrinkles occured 10       Wavy wrinkles slightly on poly- occured slightly propylene film layer 9  None   None   200    None   1000  Uniform & good 12       No problem 10 None   None   None   200    1000  "         2       Peeled __________________________________________________________________________

## Claims

1. A method for preventing corrosion of a metal piece under severe operating conditions comprising the steps of obtaining a metal piece whose surface will come into contact with a corrosive fluid of an exhaust gas desulfurization device, a gas purification plant, a seawater-fresh water converter or a thermal power plant, sprinkling the surface of the metal piece with a polypropylene lining powder which comprises a mixture of a maleic acid-modified polypropylene powder having an average particle diameter of 100 to 500 .mu.m and having a maleic acid modification ratio of from 0.05 to 1.0 wt% and a polypropylene powder having an average particle diameter of 100 to 500 .mu.m in a weight ratio of the maleic acid-modified polypropylene powder/the polypropylene powder=1/1 to 1/10, heating/melting the polypropylene lining powder, and then cooling/solidifying the same to form a lining layer of the polypropylene.

2. A method of claim 1 including the step of placing the surface of the metal piece into contact with the corrosive fluid at a temperature of about 100.degree. C.

3. A method for preventing corrosion of a metal piece under severe operating conditions comprising the steps of obtaining a metal piece whose surface will come into contact with a corrosive fluid of an exhaust gas desulfurization device, a gas purification plant, a seawater-fresh water converter or a thermal power plant, sprinkling the surface of the metal piece with a polypropylene lining powder having an average particle diameter of 100 to 500 .mu.m which comprises a maleic acid-modified polypropylene having a maleic acid modification ratio of from 0.05 to 1.0 wt% and a polypropylene in a weight ratio of the maleic acid-modified polypropylene/the polypropylene=1/1 to 1/10, heating/melting the polypropylene lining powder, and then cooling/solidifying the same to form a lining layer of the polypropylene.

4. A method of claim 3 including the step of placing the surface of the metal piece into contact with the corrosive fluid at a temperature of about 100.degree. C.