Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-08-08
2002-08-13
Szekely, Peter (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S349000, C524S351000, C524S352000, C524S736000
Reexamination Certificate
active
06433074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for producing a vinyl chloride polymer, and more particularly to a process for producing a vinyl chloride polymer having less uneven plasticizer absorption and having both good anti-initial-discoloration properties and good thermal stability.
2. Description of the Prior Art
In a processes for producing a vinyl chloride polymer by polymerizing in an aqueous medium a vinyl chloride monomer alone or a monomer mixture of a vinyl chloride monomer and a monomer copolymerizable with it, an antioxidant is conventionally added in order to, e.g., improve the anti-initial-discoloration properties and thermal stability of the vinyl chloride polymer obtained and to keep the polymerization conversion constant to lessen any uneven plasticizer absorption of the polymer. In some cases, this antioxidant is added in order to terminate reaction urgently at the time of abnormal reaction.
With regard to the time at which the antioxidant is to be added, it is added, e.g., before polymerization is initiated, before the pressure in a polymerization vessel (hereinafter “internal pressure”) changes during polymerization, at the time the internal pressure has dropped at the last stage of polymerization, or before, in the course of or after the recovery of residual unreacted monomers after polymerization is completed. In particular, since the antioxidant has the action to terminate polymerization, it is usually added to a polymerization mixture at the time the internal pressure has dropped at the last stage of polymerization, also aiming at the termination of polymerization. As methods of adding the antioxidant, it is proposed to use, e.g., (1) a method in which an antioxidant which is liquid at normal temperature (20 to 25° C.; the same applies hereinafter) is directly added in a polymerization mixture (Japanese Post-Examination Publication (Kokoku) No. 7-113041), (2) a method in which an antioxidant which is powdery at normal temperature is dissolved in an organic solvent such as methanol or toluene and the resultant solution is press-added by means of a pump, and (3) a method in which the powdery antioxidant is press-added as an aqueous dispersion by means of a pump, by the aid of an emulsifying agent or a suspending agent (Japanese Post-Examination Publication (Kokoku) No. 5-86407).
The antioxidants used in these methods of addition are proposed in a large number. However, these antioxidants have performances individually specific to themselves. For example, some have an insufficient ability of polymerization termination but are well operable as being liquid and provide a low COD (chemical oxygen demand) in waste water after polymerization; some have a sufficient ability of polymerization termination but lowers the anti-initial-discoloration properties and thermal stability of polymers; and some are powdery and must be dissolved in an organic solvent, resulting in a high COD in polymerization waste water. Thus, they have merits and demerits. Under such existing circumstances, no antioxidant is available which satisfies all the requirements when used alone.
More specifically, in the method (1), in which an antioxidant which is liquid at normal temperature is directly added in a polymerization mixture, problems have remained unsettled such that, when added in a small quantity, the antioxidant has an insufficient ability of polymerization termination and lowers the anti-initial-discoloration properties and thermal stability of polymers.
In the method (2), in which a solution prepared by dissolving in an organic solvent an antioxidant which is powdery at normal temperature is added, there is a problem that, when a vinyl chloride monomer is polymerized in an aqueous medium, the organic solvent used to dissolve the antioxidant is discharged in the waste water after polymerization, resulting in a high COD in waste water. The organic solvent is contained in the resulting vinyl chloride polymer, causing an unpleasant odor of products or in processing the polymer, or it is incorporated into unreacted monomers in recovery thereof. Also, some types of antioxidants have a low solubility in various organic solvents. In such a case, the organic solvent must be used in a large quantity, and this makes the above problems more serious.
In the method (3), in which an antioxidant which is powdery at normal temperature is added as an aqueous dispersion by the aid of an emulsifying agent or a suspending agent, it is usually difficult to obtain two or more antioxidants as a single stable aqueous dispersion. Accordingly, in the use of two or more antioxidants, there has been a problem that some installation (tanks, piping, pumps, etc.) used exclusively for their addition is required, resulting in a high cost.
In any methods of addition, it has also been difficult to keep polymerization conversion constant, and has been difficult to obtain vinyl chloride polymers having less uneven plasticizer absorption.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a process for producing a vinyl chloride polymer which process is well operable, enables polymerization to be sufficiently terminated using an antioxidant in a small quantity and enables production of vinyl chloride polymers having less uneven plasticizer absorption and having both good anti-initial-discoloration properties and good thermal stability.
To achieve the above object, the present invention provides a process for producing a vinyl chloride polymer; the process comprising the steps of polymerizing in an aqueous medium a vinyl chloride monomer alone or a mixture of a vinyl chloride monomer and a monomer copolymerizable with the vinyl chloride monomer, and adding an antioxidant to starting materials or a polymerization reaction mixture;
the antioxidant comprising a mixed antioxidant comprised of an antioxidant A which is liquid at 20° C. and an antioxidant B which is powdery at 20° C.
The vinyl chloride polymer production process of the present invention is well operable, enables polymerization to be sufficiently terminated using an antioxidant in a small quantity and enables production of vinyl chloride polymers having less uneven plasticizer absorption and having both good anti-initial-discoloration properties and good thermal stability, without requiring any installation used exclusively for the addition of antioxidants and any organic solvent.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described below in detail.
In the present invention, “initial discoloration” means any color change that may occur when a kneaded product prepared by adding a plasticizer and so forth to a vinyl chloride polymer is molded into a sheet under application of heat and pressure. Thus, the term “anti-initial-discoloration properties” means a resistance the vinyl chloride polymer has, to such color change.
In the vinyl chloride polymer production process of the present invention, an antioxidant is added to starting materials or a polymerization reaction mixture, and a mixed antioxidant comprised of an antioxidant A which is liquid at 20° C. and an antioxidant B which is powdery at 20° C. is used as the antioxidant.
The antioxidant A which is liquid at 20° C. may be an antioxidant having a melting point of 20° C. or below. Such an antioxidant may preferably be a compound which is liquid at 20° C. and also represented by the following general formula (I):
wherein R
1
represents a straight-chain or branched-chain alkyl group having 1 to 5 carbon atoms; and R
2
and R
3
each independently represent a hydrogen atom or a straight-chain or branched-chain alkyl group having 1 to 5 carbon atoms.
In the general formula (1), as examples of the group represented by R
1
, it may preferably be a t-butyl group, a t-amyl group or an isopropyl group. Also, as examples of the groups represented by R
2
and R
3
, they may each preferably be a hydrogen atom, a t-butyl group or a sec-butyl group.
What is preferred as the compound which is liquid at 20° C. a
Amano Tadashi
Ooura Makoto
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Shin-Etsu Chemical Co. , Ltd.
Szekely Peter
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