Improved phosphite stabilizer compositions.

0001This application is a continuation-in-part of appli­cation Serial No. 802,894, filed November 29, l985.

0002This invention relates to improved stabilizers for polyolefins, and more particularly to stabilizer compo­sitions comprising tris-alkylphenyl phosphites and minor amounts of a dialkyl thiodipropionate. Still more parti­cularly, the invention relates to stabilizer compositions comprising a tris-alkylphenyl phosphite and from l to 55 parts by weight, per hundred parts by weight phosphite, of a dialkyl thiodipropionate. These low levels of dialkyl thiodipropionate synergistically enhance the stabilizing effectiveness of the phosphite, affording improved melt-­processing stability to polyolefin resins with less de­terioration in color.

0003Tris-alkylphenyl phosphites have long been known for use in stabilizing a variety of resins, and are known to be particularly beneficial for stabilizing polyolefins during melt processing operations. Although these phosphites may be used alone for such purposes, they are often employed in combination with a variety of co-stabilizers such as hinder­ed phenols and the like to provide additional stabili­zation, as is disclosed for example in U.S. 3,558,554.

0004Thioester compounds have also been used in combination with phosphites and phosphite-hindered phenol stabilizer compositions to impart long-term heat aging stability. However, U.S. 4,l87,2l2 discloses that thioesters contribute to yellowing, particularly when used in polyolefins and that such combinations should therefore be avoided.

0005Although the combination of hindered phenols and phosphites is well established for use in polyolefin stabilization, further improvements have long been sought. The addition of further co-stabilizers has been of some benefit, and the use of combinations of particular hindered phenols, as well as the use of alternative phosphite structures, has gained some acceptance in the art. How­ever, in as much as tris-alkylphenyl phosphites are readily prepared and have become widely available commercially in large quantities and at relatively favorable prices, a method for increasing their effectiveness as melt proces­sing stabilizers would be a useful advance in the stabiliz­ing art. Improved stabilizer compositions based on tris­alkylphenyl phosphites would increase the range of low-cost melt processing stabilizers available to the resin com­pounder.

0006This invention relates to improved stabilizer compo­sitions comprising tris-alkylphenyl phosphites and small amounts of dialkyl thiodipropionates and to polyolefins containing these stabilizer mixtures.

0007The tris-alkylphenyl phosphites useful in the practice of this invention include tris-monoalkylphenyl phosphites, tris-dialkylphenyl phosphites and tris-trialkylphenyl phosphites, wherein the alkyl groups are C₁-C₁₂ alkyl radicals. Examples of such phosphites include tris-(2,4-di-­t-butylphenyl) phosphite, tris-nonylphenyl phosphite, tris-(2-t-butyl-4-methylphenyl) phosphite, tris-(2,4 di-t-­butyl-6-methyl-phenyl) phosphite and the like, as well as mixtures thereof. A great variety of these and analogous tris-alkylphenyl phosphites are well-known and widely available from commercial sources.

0008The effectiveness of tris-alkylphenyl phosphites as melt processing stabilizers for polyolefins is synergis­tically enhanced by including a minor but effective amount of a dialkyl thiodipropionate wherein the alkyl portion is a long chain, about C₈-C₂₀, aliphatic group, such as octyl, decyl, lauryl, tridecyl, myristyl, palmityl, stearyl and arachidyl. Representative examples of such compounds are distearyl thiodipropionate, dipalmityl thiodipro­pionate, ditridecyl thiodipropionate, dimyristyl thiodi­propionate and dilauryl thiodipropionate. Alkyl moieties having about l0 to about 20 carbon atoms are preferred, with C₁₂-C₁₈ alkyl moieties being particularly pre­ferred. Most preferred are dilauryl thiodipropionate and ditridecyl thiodipropionate. These and other such compounds are known and are readily available from commercial sources, or may be prepared by means known in the art.

0009The improved stabilizer composition of this invention will comprise from l to 55, preferably about 5 to about 45, and more preferably from about l0 to about 40 parts by weight of dialkyl thiodipropionate per hundred parts by weight of the tris-alkylphenyl phosphite compound. At levels within this range, the effectiveness of the phosphite as a melt processing stabilizer, particularly for polyolefins, is enhanced, and improvement in color stability is shown. At levels below this range, little or no improvement is realized, while at greater levels, further improvement in melt stabilization is minimal or not existent at all, while color stability possibly may decrease. This is particularly unexpected due to the fact that the level of thioester normally employed to provide long term heat aging stability to resins generally is l0 to 50 fold greater than the amount needed to provide synergistic enhancement of the stabilizing character of the phosphite.

0010The stabilizing compositions of this invention are effective in improving the melt processing stability of polyolefins such as polyethylene, polypropylene and the like when compounded therewith in amounts of up to 0.5 wt%, and usually less than 0.l wt%. Generally, from about 0.0l to about 0.2 wt% are sufficient for the purpose of stabiliz­ing the olefin resins against deterioration during melt processing. Although considerably greater amounts may also be effective, such large amount afford little additional benefit and are therefore not preferred.

0011Conventional combinations of tris-alkylphenyl phos­phites with co-stabilizers such as antioxidants are also known and widely used as polyolefin stabilizers. For example, the combination of tris-alkylphenyl phosphites with a variety of hindered phenols has been disclosed in U.S. patents 4,l87,2l2 and 4,290,94l. The addition of l to 55 parts by weight dialkyl thiodipropionate per hundred parts by weight of tris-alkylphenyl phosphite may also synergistically improve the effectiveness of other com­positions for the stabilization of polyolefins in melt processing operations.

0012Other co-stabilizers include compounds such as acid neutralizers, UV stabilizers and antioxidants, such as hindered phenolic compounds like, for example, alkyl esters of (dialkyl hydroxyphenyl) propionic acid, and tris(dialkyl hydroxybenzyl) isocyanurates, In the preferred embodiment, at least one hindered phenolic compound is included.

0013The present invention may also be understood froma consideration of the following illustrative examples.

Examples l - 2

0014Unstabilized polyethylene, obtained as LLDPE GRSN 7042 from Union Carbide Corporation, was compounded with 0.08 parts by weight Weston® 447, tris(2,4-di-t-butylphenyl) phosphite from Borg-Warner Chemicals, Inc., and varying amounts of dilauryl thiodipropionate (DLTDP). All compositions also contained 0.05 parts by weight calcium stearate processing aid per hundred parts by weight polyethylene. Control Composition A-C were also prepared and tested for comparison purposes.

0015The compositions were first prepared by dry blending the components, then tested for melt-flow stability by carrying out multiple extrusions at 525.F, (stock temperature), using a l in. single screw extruder having a 24:l L:D ratio, fitted with a ¼ in. rod die. The melt flow properties, measured by a modification of the method of ASTM D l238, condition 'E' and Yellowness Index were measured. The compositions and results are summarized in Table I. <tables id="tabl0001" num="0001"><img file="EP0227948A2_D0001.tif" /></tables>

0016It will be apparent that although the addition of the phosphite to polyethylene affords improvement in melt stability and color development (compare Control B with Control A), the further addition of a minor amount of DLTDP affords a substantial improvement in melt stability. The conventional combination of phosphite and hindered phenol (Control C) actually provides slightly less effective stabilization and exhibits poorer color than do the compositions stabilized with phosphite and thioester according to the practice of this invention.

Examples 3 - 7

0017Further compositions were prepared using 0.08 pbw W-447 phosphite, 0.03 pbw U-276 stabilizer, an alkyl ester of a (dialkyl hydroxphenyl) propionic acid, an varying levels of DLTDP. The compositions and test results are summarized in Table II. <tables id="tabl0002" num="0002"><img file="EP0227948A2_D0002.tif" /></tables>

0018It will be apparent from a comparison of Examples 3-7 with Control Example C that the addition of a minor but effective amount of dilauryl thiodipropionate to the conventional polyolefin stabilizer combination of a hindered phenol and a tris-alkylphenyl phosphite signi­ficantly both enhances the melt flow stability and reduces color development during melt processing.

Examples 8 - ll

0019The melt flow and color stability characteristics of the composition of Examples 8-ll, consistent with various embodiments of the invention, may be compared with those of the Control Compositions wherein the trialkylphenyl phosphite and the dialkyl thiodipropionate are not both present (Control Examples E-J) and Control Compositions containing both, but wherein the amount of dialkyl thiodipropionate exceeded 55 pbw relative to the phosphite (Control Examples K-O). The data for these examples are indicated below in Table III. The base resin was linear low density polyethylene (LLDPE) (Union Carbide G-7042). All compositions additionally include 500 ppm calcium stearate, and were processed and tested as described for Examples l-2, except that the ASTM method for melt flow was unmodified. <tables id="tabl0003" num="0003"><img file="EP0227948A2_D0003.tif" /></tables>

Examples 9 - l2

0020Different dialkyl thiodipropionates were incorporated at the same level into the same LLDPE as Examples 8-ll, along with 500 ppm calcium stearate, 300 ppm U-276 and 560 ppm W-447 and processed and tested as described for Examples l-2, except that seven extrusions were performed. These compositions may be compared against a Control Composition wherein U-276, W-447 and dialkyl thiodi­propionate were absent (Control Example P). The data for these examples are indicated below in Table IV. <tables id="tabl0004" num="0004"><img file="EP0227948A2_D0004.tif" /></tables>

0021The dialkyl thiodipropionates were: <tables id="tabl0005" num="0005"><img file="EP0227948A2_D0005.tif" /></tables>

Examples l3-l9

0022Compositions were made to demonstrate the use of different antioxidants. The compositions included the same LLDPE as Examples 9-l2 and included 500 ppm calcium stearate. These compositions were processed and tested as in Examples 9-l2. The antioxidants were U-276, GR-3ll4, tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate, availa­ble from the B. F. Goodrich Corporation, and Irganox® l0l0 (I-l0l0), tetrakis [3-(4-hydroxy-3,5-di-t-butylphenyl)pro­panoyloxy] methane, available from the Ciba- Geigy Corporation. The LLDPE without antioxidant, phosphite or thio-synergist was also tested (Control Example Q). The data for these examples are indicated below in Table V. <tables id="tabl0006" num="0006"><img file="EP0227948A2_D0006.tif" /></tables>

0023The instant invention will thus be seen to be an improved stabilizer composition comprising a tris­alkylphenyl phosphite and from l to 55 pbw, preferably from 5-45 pbw, and more preferably l0-40 pbw of a dialkyl thiodipropionate, per hundred pbw of phosphite, and a polyolefin composition which includes that stabilizer composition.

0024It will be understood that various changes and modifications may be made in the embodiment outlined above without departing from the spirit of the invention, which includes all equivalents and modifications thereof, and is limited only by the following claims.