Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in reactor of specified material – or in reactor...
Reexamination Certificate
2001-05-22
2003-01-14
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in reactor of specified material, or in reactor...
C526S072000, C526S315000, C526S316000, C526S344000
Reexamination Certificate
active
06506854
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a terpolymer for inhibiting the formation of scale on reactor surfaces that are in contact with polymerizing vinyl monomers. In particular, it relates to a method of making the terpolymer without an acid catalyst and to a solution of the terpolymer, a salt, and a base.
When vinyl monomers are polymerized, scales of the polymer adhere to the reactor walls and to reactor components, such as stirrers. The deposition of this scale reduces the yield of the polymer, reduces heat transfer efficiency during heating and cooling of the reactor, and lowers monomer efficiency and general process control. Some of the scale flakes off during polymerization of the monomer and mixes with the rest of the polymer, which makes the polymer less processable and reduces its quality. For example, when vinyl chloride monomer (VCM) is polymerized, scales of polyvinyl chloride (PVC) can flake off and form “fish eyes” &pgr;in the PVC.
Cleaning the reactor to remove this scale is normally required after each batch, resulting in considerable reactor downtime as well as increased cost in producing the polymer. Because VCM can become entrapped in the scale, the physical removal of the scale by an operator could result in the exposure of the operator to the harmful monomer.
EP 343,706 teaches that polythioureas of the general formula [NHRNHC(S)]n (where R is an arylene group substituted by a carboxylic acid group) can be used to reduce scale formation in vinyl chloride polymerization reactors. JP 01,217,016 discloses a scale preventing compound comprising an alkali metal xanthogenate, polyethyleneimine, and a dialdehyde. In JP 59,32,482, dithiocarboxylic acids or salts containing OC(═S)S or NC(═S)S groups and/or (RO)2P(S)SR derivatives are disclosed as scale preventing agents. U.S. Pat. No. 4,431,783 discloses that naphthol/formaldehyde condensates formed by a base-catalyzed process are effective scale-preventing agents vinyl chloride polymerization reactors. While many of these scale prevention agents are effective in preventing scale, they can alter the color of the resulting PVC, giving it a yellowish tinge. Customers of high quality PVC prefer a pure white or colorless PVC and regard any color in the PVC as an indication of inferior quality.
SUMMARY OF THE INVENTION
We have discovered that a terpolymer of an aromatic compound, a carbonyl compound, and a thiourea can be made without using an acid catalyst. By omitting an acid catalyst, the terpolymer can be made with less cost as not only is the cost of the acid catalyst avoided, but subsequent neutralization and disposal of the acid catalyst is also avoided. We have further found that the terpolymer is more effective in reducing scale when a solution of it, a base, and a salt is applied to the reactor components. The scale prevention agent of this invention is very effective in preventing the formation of scale on reactor components that come in contact with polymerizing VCM. Because there is little or no scale, there are fewer fish eyes. It is easily prepared from commercially available, inexpensive, non-toxic materials and can be applied in an aqueous medium, thereby avoiding the use of potentially harmful and flammable organic solvents. Quite unexpectedly, the scale prevention agent of this invention imparts little or no color to the resulting PVC.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The terpolymer used in the process of this invention is the condensation reaction product of an aromatic compound, a carbonyl compound, and a thiourea. The aromatic compound contains the group:
where Q is OR or SR and R is hydrogen, alkyl from C
1
to C
15
, or aryl, alkaryl, or aralkyl from C
6
to C
15
. The Q group is preferably OR and the R group is preferably hydrogen as phenols are more readily available and work well. Preferred aromatic compounds include:
where Z is 0 or NOH, each R
1
is independently selected from OH, R, SO
3
R, and CO
2
R, and each R
2
is independently selected from R
1
, SH, SR, haloalkyl from C
1
to CO
15
, and alkoxyaryl, naphthyl, and polycyclic aryl from C
6
to C
15
. Examples of suitable aromatic compounds include salicylic acid, 1 -naphthol, thiosalicylic acid, hydroxybenzenesulfonic acid isomers, 4-hydroxybenzoic acid, 2′-hydroxypropiophenone oxime, and 2′-hydroxypropiophenone; 2′-hydroxypropiophenone and 1-naphthol are preferred due to their low cost, low toxicity, and good performance in the terpolymer condensate. Mixtures of the aromatic compounds can also be used.
The carbonyl compound can be an aliphatic mono or dialdehyde, an aromatic aldehyde, or an aliphatic mono or diketone. Compounds, such as trioxane, that form the carbonyl compound under the reaction conditions, are also contemplated. Preferred carbonyl compounds have the general formula:
where each R
2
is independently selected as hereinabove defined and n is an integer from 1 to 10. Examples of suitable carbonyl compounds include acetaldehyde, glyoxal, benzaldehyde, dialkyl ketones such as acetone and methyl ethyl ketone, diaryl ketones such as benzophenone and ring-hydroxylated and sulfonated benzophenones, alkyl aryl ketones such as acetophenone, ring-hydroxylated acetophenones, and ring-sulfonated acetophenones, and formaldehyde and oligomers thereof, such as trioxane. Preferably, the carbonyl compound is formaldehyde or an oligomer thereof as those compounds have low cost, high reactivity, and are readily available. Mixtures of carbonyl compounds are also contemplated.
The thiourea has the general formula
where each R
3
is independently selected from NH
2
and R
2
. Examples of suitable thioureas include thiourea, thiocarbamide, monomethyl thiourea, 1,3-dimethyl-2-thiourea, 1-phenyl-2-thiourea, semicarbazide, thiosemicarbazide, and thiocarbohydrazide. The preferred thioureas are thiourea and thiosemicarbazide because they work the best, are inexpensive, and are readily available. Mixtures of thioureas can also be used.
The terpolymer is made by preparing a mixture of its three components. The most preferred combination of the three components is a 1 -naphthol-formaldehyde-thiosemicarbazide (NFTS) terpolymer as it has the best combination of properties. For every mole of the aromatic compound, about 0.1 to about 10 moles of the carbonyl compound and about 0.1 to about 10 moles of the thiourea can be used. If less than 0.1 moles of the carbonyl compound is used, the terpolymers produced will have a lower molecular weight, and if more than 10 moles of the carbonyl compound is used, the terpolymer will have lower solubility. If less than 0.1 moles of the thiourea is used, the terpolymer will be less effective in preventing scale formation and, if more than 10 moles of the thiourea is used, the terpolymer will be less soluble. Preferably, for each mole of the aromatic compound, about 0.1 to about 5 moles of the carbonyl compound and about 0.1 to about 5 moles of the thiourea are used.
While the condensation reaction of the terpolymer components can be carried out in an acidic or basic medium, which also acts as the catalyst, preferably no acidic or basic catalyst is used as it has been found that none is needed. Mineral acids, such as hydrochloric acid and sulfuric acid, and organic acids, such as acetic acid, can be used as an acid catalyst and bases such as sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, barium hydroxide, sodium carbonate, potassium carbonate, and organic tertiary amines can be used as a basic catalyst. The strength of the acidic or basic catalyst can vary from about 1M to about 6M. If a catalyst is used, typically about 50 to about 500 wt % of it is used, based on the total weight of the terpolymer components.
The condensation reaction of the terpolymer components can be performed in water or in a polar organic solvent such as an alcohol (e.g., methanol, ethanol, propanol), a ketone (e.g., acetone, methyl ethyl ketone), an ester (e.g., ethyl acetate), or a dipolar aprotic solvent, such as sulfolane, tetrahydrofuran, N,N
Hichri Habib
Krishnamurti Ramesh
Nagy Sandor
Wang Qi
Fuerle Richard D.
Harlan R.
Occidental Chemical Corporation
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