Reduction of acidity of polyesters by melt reaction endcapping with dialkyloxalate

Abstract: A method for the melt reaction of polyesters, particularly polyethylene terephthalate, with an endcapping agent which reacts with terminal carboxyl groups of the polyester. The endcapping agents are selected from the group consisting of dialkyloxylates and tetraalkylureas.

Description: BACKGROUND OF THE INVENTION Polyesters utilized in fiber formation are generally produced by a heated reaction of one or more dibasic acids such as terephthalic acid with one or more polyhydroxy compounds such as ethylene glycol, propylene glycol, and 1,4-cyclohexane dimethanol, until a product of desired viscosity is obtained. The formed polyesters are characterized in that they contain both terminal hydroxy and carboxy groups. Terminal hydroxy groups are generally more predominant due to the incorporation of an excess of polyol in the reactive mixture. Polyesters are of great importance in the manufacture of tire cords, and as reinforcement for belts, hoses and many other useful articles. In many of these commercial applications the presence of excessive carboxyl groups in the polymer molecule is detrimental. Previous attempts at acid group reduction in polyesters have resulted in a loss of average molecular weight in the polyester product due to substantial cleavage in the polyester backbone. It is an objective of the instant invention to provide improved polyester materials in which the pendant carboxyl groups are either greatly reduced in number or are completely removed. It is a further object of the invention to endcap free carboxyl groups on polyesters without producing water as a byproduct and while maintaining the molecular weight of the polyesters. It is a further object of the instant invention to provide polyester materials having reduced sensitivity to water. SUMMARY OF THE INVENTION The instant invention relates to melt reaction of a carboxyl group containing polyester with a carboxyl group reactive endcapping agent selected from dialkyloxalates, in which the alkyl groups are lower alkyl radicals; to provide a polyester having a substantially reduced number or no carboxyl group while maintaining the approximate molecular weight of the carboxyl group containing polyester precursor. DETAILED DESCRIPTION OF THE INVENTION In the practice of the present invention the polyester material is first produced in any state of the art commercial manner. A typical process for production of a polyester is the heated reaction of a basic difunctional organic acid with a polyol, preferably a diol, optionally together with any other desired components. Suitable polyesters for treatment in the instant invention are prepared from difunctional organic acids including terephthalic acid, 4,4'-dicarboxydiphenyl, 1,5-,1,4-, or 2, 6-naphthalic acid. Suitable polyols are preferably diols such as ethylene glycol, propylene glycol, and butylene glycol. The preferred polyesters of the instant invention are homopolyesters such as polyethylene terephthalate, poly-1,4-cyclohexylenedimethylene terephthalate, polyethylene-2, 6-naphthalate, polyester ethers such as polyethylene hydroxybenzoate, poly-p-phenylene bis-hydroxyethoxy-benzoate, poly-p-phenylene bis-hydroxyethoxy-terephthalate; copolyesters or copolyester ethers which comprise mainly ethylene terephthalate units or tetramethylene terephthalate units and other copolymer components such as tetramethylene or ethylene isophthalate, 1,4-cyclohexylenedimethylene terephthalate units, or tetramethylene or ethylene p-hydroxybenzoate units. The preferred polyester for treatment in the instant invention is polyethylene terephthalate. Polyesters for treatment in the instant invention have an acid value ranging from 40 to 10 equivalents of CO₂H per 10⁶ grams of polyester. Polyesters for treatment in the instant invention should have an average molecular weight ranging from 10,000 to about 60,000. In the practice of the instant invention a formed polyester is melt reacted with a carboxy reactive group or endcapping agent selected from dialkyloxalates in which the alkyl groups are lower alkyl radicals. The use of these endcapping agents permits the treated polyesters to retain their approximate molecular weight and viscosity as significant amounts of water which would promote polymer degradation are not generated during endcapping reaction. The dialkyloxalates which can be utilized in the instant invention are represented by the following structural formula: R₁OOCCOOR₂ (I) wherein R₁ and R₂ represent the same or different C₁ - C₅ alkyl radicals. The preferred dialkyloxalates are dimethyloxalate and diethyloxalate. The dialkyloxalates of the present invention are utilized to endcap carboxy terminated polyesters by the following reaction: R₁-OOCCOOR₂ + Polyester-COOH→Polyester-COO(R₁ or R₂) + HOOCCOO(R₁ or R₂) wherein R₁ and R₂ are as defined above. The alkyl group which esterifies the carboxylpolyester is, of course, replaced in the dialkyloxalate by a hydrogen atom to yield an alkyl-oxalic acid monoester. In the process of the present invention the melt extrusion reaction of the polyester and the appropriate endcapping agent should occur in a temperature range between 270°C and 320°C. The endcapping agent feed rate into the melt extruder should range between 1 and 50 millimoles per minute per 100 grams per minute of polyester feed. The reaction residence time of the polyester and the endcapping agent in the melt reaction must be at least 10 seconds to provide for substantial endcapping of the carboxyl groups present on the untreated polyester which is fed into the melt reactor. This residence time allows for endcapping of the acid group thereby effecting acid number reduction of the polyester to an acid number below 10 equivalents of CO₂H per 10⁶ grams of polyester, preferably below 3 eq. CO₂H/10⁶ grams of polyester. The polyesters produced in accordance with the instant procedure having less than 10 equivalents of CO₂H per 10⁶ grams of polymer are accorded the status of having substantially all of their carboxyl groups endcapped. The following examples are presented for the purposes of clarifying the present invention. The following are specific examples for each of the above groups of the endcapping agents and their use in capping the free carboxyl groups in polyesters. In all of the following examples the treated polyester is polyethylene terephthalate. In each of the following examples the polyethylene-terephthalate (PET) melt was prepared as follows. Tirecord grade PET was continuously prepared from terephthalic acid and ethylene glycol to give an intrinsic viscosity, [η], of 0.94 dl/grams at 25°C in 1:1 ratio of phenol:tetrachloroethane. The PET in chip form was dried at 110°C for at least twelve hours in a rotary dryer under a vacuum of 13,33 Pa (0.10 mm of Hg). The recovered dry PET polymer was transferred to an Acrison No. 1015Z-C feeder under a nitrogen atmosphere and fed to a Werner-Pfleiider ZSK-30 twin screw compounding extruder which had all zones heated to either 280°C or 300°C. At a polymer feed rate of 40 grams/min or 100 grams/min the PET polymer had a melt residence time of 85 or 35 seconds, respectively, in the extruder. EXAMPLE 1 The compounding extruder zones were heated to 280°C and the feed rate of the PET polymer was 87 grams/min. During the PET melt residence time of about 40 seconds the last part of zone one of the compounding extruder was continuously injected with 10.4 millimoles (mM)/min (1.07 cm³/min) of the capping agents dimethyloxalate using a BIF microfeeder No. 1180-07 piston pump. The extruded polymer was cooled, chopped and analyzed to display an intrinsic viscosity [η] = 0.73 dl/grams and 1.8 eq CO₂H/10⁶ grams of PET polymer. In comparison, a control PET polymer passing through the compounding extruder without the added dimethyloxalate displayed [η] = 0.72 dl/grams and 23.8 eq.CO₂H/10⁶ grams of polymer.

Claims: A method for reducing the acidity of polyesters to an acid content below 10 equivalents of CO₂H per 10⁶ gm of polyester which comprises melt reacting a carboxyl group containing polyester having an acid value ranging from 40 to 10 equivalents of CO₂H per 10⁶ gm of polyester with dialkyloxalates in which the alkyl groups are lower alkyl radicals having from 1 to 5 carbons as an endcapping agent; in an amount ranging from 1 to 50 millimoles per minute for each 100 grams per minute of polyester feed at a temperature in the range of 270°C to 320°C. The method according to claim 1 wherein the melt reaction of the polyester with the endcapping agent occurs for a period of time of at least 10 seconds. The method of claim 1 wherein the carboxyl group containing polyester is polyethylene terephthalate. The method of claim 1 wherein the endcapping agent is dimethyloxalate. A polyethylene terephthalate resin composition having an acid number below 10 equivalents of CO₂H per 10⁶ grams of polymer comprised of the reaction product of a carboxyl group containing polyethylene terephthalate resin having an acid content greater than 10 equivalents of CO₂H per 10⁶ grams of polymer and dialkyloxalates in which the alkyl groups are lower alkyl radicals having 1 to 5 carbon atoms as a carboxyl group reactive endcapping agent. The polyethylene terephthalate resin composition of claim 5 wherein the endcapping agent is dimethyloxalate.

Inventors: DOSHAK JOHN M, HERGENROTHER WILLIAM L, DOSHAK, JOHN M., HERGENROTHER, WILLIAM L.
Applicants: BRIDGESTONE FIRESTONE INC, BRIDGESTONE/FIRESTONE, INC.
Application year: 1990
Main classification: C08G63
IPC codes: C08G63, C08L67