Abstract:
Poly(alkylene terephthalates) are stabilized during high temperature processing by neutral esters of phosphorous acid having at least one oxetane group. The stabilized poly(alkylene terephthalates) are resistant to molecular weight degradation and discoloration during melt processing of the polymer when the polymer is being extruded and formed into molded articles.

Description:
This is a continuation-in-part of our copending application Ser. Number 642,841, filed Dec. 22, 1975, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to poly(alkylene terephthalate) polymers and more particularly to poly(alkylene terephthalate) polymers having stability at elevated temperatures. 
     2. Description of the Prior Art 
     Poly(alkylene terephthalate) polymers particularly poly(butylene terephthalate) and poly(ethylene terephthalate) are utilized in molding articles by thermoplastic molding techniques. In processing the poly(alkylene terephthalate) high temperatures are encountered in melting, extruding and forming the polymer which contributes to discoloration making the poly(alkylene terephthalate) molded articles unappropriate for the manufacture of articles which are to be color constant. Further, because of the heating necessary to process the polymer, the poly(alkylene terephthalate) undergoes molecular weight reduction causing a deterioration in the physical and mechanical properties of the molded article. 
     In the synthesis of poly(alkylene terephthalates) linear alkyl phosphites have been used to prevent discoloration of the polymer. The color stability problem during synthesis and the stability problem during processing present different problems to the corresponding manufacturers. The poly(alkylene terephthalate) is typically synthesized in an inert atmosphere and discoloration is due to the heat necessary to perform the esterification or transesterification of the monomers. Whereas during melt processing the polymer is subjected to both heat and air which has made the linear alkyl phosphites ineffective for stabilizing poly(alkylene terephthalates). 
     In accordance with the present invention poly(alkylene terephthalate) polymers are provided with improved resistance to color and molecular weight degradation during melt processing. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Poly(alkylene terephthalate) thermoplastic polymers are provided with improved resistance to heat induced color and molecular weight degradation by the addition of an effective amount of a neutral ester of phosphorous acid with at least one oxetane group thereon (hereinafter referred to as oxetane phosphites). 
     DETAILED DESCRIPTION OF THE INVENTION 
     The oxetane phosphites useful in the practice of the invention are for example tris-[(3-ethyl-oxetanyl-3)-methyl]-phosphite, tris-[(3-pentyloxetanyl-3)-methyl]-phosphites, phenyl-bis-[(3-ethyloxetanyl-3)-methyl]-phosphite, 2-phenoxy-spiro (1,3,2-dioxaphosphorinane-5,3&#39;-oxetane) and 3,3-bis-[spiro(oxetane-3&#39;,5&#34;-(1&#34;,3&#34;, 2&#34;-dioxaphosphorinane-2&#34;)-oxy-methyl]-oxetane, as well as those disclosed in U.S. Pat. No. 3,209,013, column 1, line 28 to column 3, line 10 incorporated herein by reference. 
     Thus a preferred group of oxetane phosphites is those of the formulae: ##STR1## where n is selected from the group consisting of 0 and 1, R 1  is selected from the group consisting of alkyl, aryl, aralkyl, halomethyl, cyanomethyl, alkoxymethyl, aryloxymethyl, aralkyloxymethyl and acyloxymethyl, R 2  and R 3  are ##STR2## or alkyl, aryl, aralkyl, haloaryl, and R 4  is alkyl, aryl, aralkyl, haloaryl or ##STR3## Preferably, n is 1. Preferably, in Formula 1 all three ester groupings attached to the phosphorus atom are the same . R 1  is preferably lower alkyl or halo lower alkyl. 
     A particularly useful oxetane phosphite in stabilizing the poly(alkylene terephthalates) is tris(3-ethyl-3-methyl oxetane) phosphite and is represented by the structural formula: ##STR4## 
     Combinations of the oxetane phosphite may also be utilized in stabilizing the poly(alkylene terephthalates) of the invention. 
     The level of oxetane phosphite needed to stabilize the poly(alkylene terephthalate) is dependent on the amount of phosphorus in the particular oxetane phosphite. Based on the phosphorus level of the oxetane phosphite 0.0013% to 0.0168% by weight of phosphorus is necessary to stabilize the poly(alkylene terephthalate) polymer and more preferably 0.0021% to 0.0084%. Based on the weight of the total oxetane phosphite 0.015 to 0.2% and more preferably 0.024 to 0.1% based on the weight of the poly(alkylene terephthalate) polymer is necessary for adequate stability. 
     The oxetane phosphites can be prepared by methods disclosed in U.S. Pat. No. 3,209,013 incorporated herein by reference. Although the oxetane phosphites have been used to stabilize polyvinylchloride and polycarbonate (see U.S. Pat. Nos. 3,794,629 and 3,209,013) they have not heretofore been used to stabilize the poly(alkylene terephthalates) as herein disclosed. 
     The poly(alkylene terephthalates) to be stabilized are those which are used in molding applications such as poly(ethylene terephthalate) and poly(butylene terephthalate). Although poly(ethylene terephthalate) has been used in molded articles by far poly(butylene terephthalate) has found the most extensive use in this area. 
     Poly(alkylene terephthalate) is synthesized by either the direct esterification of terephthalic acid and the appropriate alkylene diol or by the transesterification of dimethyl terephthalate with the appropriate alkylene diol. In the case of poly(butylene terephthalate) the reaction sequence is as follows: ##STR5## 
     The stoichiometry of the 1,4-butylene glycol to the dimethyl terephthalate and the reaction time is adjusted to obtain the desired degrees of polymerization for adequate molding properties of the polymer. Preferably a minimum intrinsic viscosity of 0.6 measured at 25° C as a 0.1% solution in ortho-chlorophenol is necessary to obtain an adequate article formed from poly(butylene terephthalate). The upper molecular weight is dependent upon processing equipment however a maximum intrinsic viscosity for the poly(butylene terephthalate) is about 1.5 at 25° C in a 0.1% solution with ortho-chlorophenol. 
     Poly(ethylene terephthalate) is synthesized in much the same manner as the poly(butylene terephthalate) except ethylene glycol is substituted for butylene glycol. The intrinsic viscosity of the poly(ethylene terephthalate) should be between 0.5 to 0.8 as above measured and more preferably between 0.6 and 0.7. 
    
    
     The invention can be further understood by the following examples. 
     EXAMPLE I 
     A poly(butylene terephthalate) polymer having an intrinsic viscosity of 1.2 measured in a 0.1% solution in o-chlorophenol at 25° C was admixed with tris(3-ethyl-3-methylol oxetane) phosphite represented by the structural formula: ##STR6## 
     The homogeneous admixture was formed by melt extrusion in a single screw extruder. The above composition was tested for stability by extrusion through a capillary rheometer at 250° C. The extrusion temperature was above the melting point of the polymer. The time necessary for a given weight of polymer to flow through the capillary rheometer after various pre-conditioning time intervals was measured and the melt flow half life was calculated. The melt flow half life was an extrapolated value representing the time necessary to degrade the poly(butylene terephthalate) at 250° C to the extent that the extrusion rate would be double that of a five minute treatment period. The longer the melt flow half life is, the more stable is the poly(butylene terephthalate). The results of the tests for melt flow half life are reported in Table I. 
     EXAMPLES II-VI 
     Examples III-VI were prepared as was described in Example I, except that various phosphites other than oxetane phosphites were used as additives. The phosphites in Examples III-VI are phosphites which are conventionally used as stabilizers in thermoplastic polymers. Example II was a control having no stabilizer therein except a nominal amount for providing stability in the synthesis of the polymer. The control was the base polymer for all of Examples I and III through VI. The results of the testing of Examples II through VI are reported in Table I. 
     
                                           TABLE I__________________________________________________________________________              Concentration parts                         Concentration parts                                    Melt Flow              phosphite/hundred                         P/hundred  Half LifeExample  Phosphite   parts polymer                         parts polymer                                    (min)__________________________________________________________________________I    Tris(3-ethyl-3-methylol              0.08       0.0062     40.5 oxetane) phosphiteII   None                                33III  Mixture of decyl diphenyl              0.22       0.0062     18.5 phosphite and an ali- phatic carbonate con- taining tetrahydrofuran groupsIV   Distearyl pentaerythritol              0.07       0.0062     25 diphosphiteV    Diisodecyl pentaerythritol              0.05       0.0062     26 diphosphiteVI   Tri(mixed mono and dinonyl              0.11       0.0062     22 phenyl) phosphite__________________________________________________________________________ 
    
     In Examples I through VI, the level of stabilizer was based on the amount of phosphorus added to the poly(butylene terephthalate) polymer. Thus all of the phosphites other than the oxetane phosphites contributed to the deterioration of the polymer. 
     EXAMPLE VII 
     Example I was repeated except that a poly(butylene terephthalate having an intrinsic viscosity of 0.82 was used for testing. Further the poly(butylene terephthalate) was tested for color change (APHA color) after contact with heat and air at 250° C for 31/2 hrs. Yellowness was tested by exposing molded plaques for 3 weeks to 165° C. air. The plaques were molded from unaged poly(butylene terephthalate) polymer. The test results are reported on Table II. 
     EXAMPLES VIII AND IX 
     Example VII was repeated except in Example VIII no phosphite was added and in Example IX a mixture of a diaryl, monoaryl phosphite in an aliphatic carboxylate containing oxetane groups was admixed with the poly(butylene terephthalate). The results of these tests are reported in Table II. 
     
                                           TABLE II__________________________________________________________________________               Concentration parts                          Melt Flow                                 Melt Color               phosphite/hundred                          Half Life                                 250° C for                                         Yellowness.sup.1 IndexExample  Phosphite    polymer    (min)  31/2 hrs. APHA                                         Original                                              Aged__________________________________________________________________________VII  Tris(3-ethyl-3-methylol               0.08       95     550     9.3  28.5 oxetane) phoshiteVIII None           --         81.0   850     14.4 30.3IX   Mixture of a decyl diphenyl               0.22       53.5   1000    11.9 33.1 phosphite and an ali- phatic carbonate con- taining tetrahydrofuran groups__________________________________________________________________________ .sup.1 Tested in accordance with ASTM D-1925-63T. 
    
     In both the APHA color test and the Yellowness Index color test the lower the number is the better the color is. 
     Examples VII through IX are demonstrative of the improved color properties obtained by the use of an oxetane phosphite in a poly(alkylene terephthalate) polymer. 
     The admixture of the oxetane phosphites and the poly(alkylene terephthalate) may also contain fillers, dyestuffs and/or pigments without influencing the effect of reduced degradation of the polymer provided by the oxetane phosphite. 
     The invention has been described with reference to specific materials and blending techniques for purposes of illustration only. Thus, the invention is not to be limited by these details but as is set forth in the accompanying claims.