Polyolefin composition resistant to hot alkaline solutions
United States Patent 1191 McInerney 14 1 Apr. 15, 1975 POLYOLEFIN COMPOSITION RESISTANT TO HOT ALKALINE SOLUTIONS [75] Inventor: Edward J. Mclnerney, Louisville,  
 [73] Assignee: General Electric Company, New  
 York, NY.  
 [22] Filed: Nov. 12, 1973 21 Appl. No.: 414,915  
 [52] US. Cl ..260/897 A; 260/4575 T; 260/45.85 B  
 [51] Int. Cl. C08f 45/62 [58] Field of Search 260/4575 K, 45.85 B, 897 A Primary ExaminerV. P. Hoke [57] ABSTRACT A composition characterized by resistance to hot alkaline solutions comprising a polymer formed from monomers including propylene, a stabilizer system including tetrakis [methylene 3-(3,5-di-t-butyl-4- hydroxyphenyl) propionate] methane and a tin maleate.  
 9 Claims, No Drawings POLYOLEFIN COMPOSITION RESISTANT TO HOT ALKALINE SOLUTIONS BACKGROUND OF THE INVENTION The present invention relates to a propylene-polymer composition which is particularly characterized by resistance to discoloration or deterioration in a hot alkaline aqueous environment such as that existing in an automatic dishwasher.  
  The design environment for a dishwasher tub is direct exposure to 180F, pH 1 2 dishwasher detergent solution with spot temperatures exceeding 180F during the drying cycle. Because of these conditions, the usual tub materials have been porcelain enamel on steel, vinyl plastisol on steel, and stainless steel. Polypropylene compounds have been used in overseas dishwashers. However, the usual polypropylene compounds, including those containing various stabilizer combinations or systems used or proposed for imparting heat, light or strength stability to polypropylene resins, do not meet 7 the high United States reliability requirements for dishwasher tubs. In fact, as disclosed for example in US pat. No. 3,409,587-Mills, the industry has experienced great difficulty in achieving the stabilization of polypropylene compositions for various applications and it is impossible to predict the combined effect of a plurality of stabilizers from their effects individually. Specifically, the Mills patent teaches that a fourcomponent stabilizer system comprising an organic phosphite such as dioctyl phosphite, a thioester such as dilaurylthiodipropionate, and two alkylphenol antioxidants such as 1,1 ,3-tris(2-methyl-4-hydroxy-5-tertbutylphenyl)butane and tetrakis [3-(3,5-ditert-butyl-4- hydroxyphenyl)propeonyloxymethyl] methane is a superior polypropylene stabilizer system for improving resistance to discoloration and embrittlement upon exposure to air and light at normal and high temperatures.  
  Other prior art patents concerned with the polypropylene stabilization problems include US. pat. No. 3,310,509-Fukumoto et al. which teaches that in the production of dyeable polyolefin composition containing a transition metal compound to promote dyeability, heat discoloration can be prevented by incorporating a tin compound along with the transition metal compound, and photostability can be imparted by the further addition of an alkylphenol type antioxidant.  
  US Pat. No. 3,285,855-Dexter is also concerned with this problem and contains a rather comprehensive list of alkylphenol compounds suitable for stabilizing organic materials, such as polypropylene resins, subject to oxidative deterioration or heat or light deterioration.  
 SUMMARY OF THE INVENTION It is a primary object of the present invention to provide a polypropylene composition containing a stabilizer system particularly adapted to withstand, without significant strength or color deterioration, the rather unique and drastic environmental conditions found in a dishwasher.  
  In accordance with the presently preferred embodiment of the invention, this objective is obtained by providing a polypropylene composition essentially containing as the primary alkylphenol antioxidant and thermal stabilizer, tetrakis [methylene 3-(3&#39;,5-di-tertbutyl-4&#39;-hydroxyphenyl) propionate] methane along with selected secondary antioxidants and stabilizers plus an effective amount of a tin maleate, specifically di-n-butyl tin maleate as a color stabilizer.  
  The presently preferred formulation in which all additives are given as parts by weight is given in the following table:  
 At least one and preferably a mixture of alkyl or aryl thio dipro ionules are used. It distearyl thin dipropionate is utilized alone, the amount utilized will he not greater than 0.6 parts. It dilauryl thio dipropionate is utilized alone. the amount utilized will be not greater than 1.0 part.  
  The poly(propylene-ethylene) copolymer has an isotactic polypropylene center segment and contains approximately 1 to 6 percent ethylene by weight. Similar isotactic polypropylene homopolymers which may be modified as known with ethylene propylene rubber or ethylene propylene diene modified rubber or as ternary blends of these rubbers with high density polyethylene, such as are disclosed in US. Pat. No. 3,562,790Coover et al., may be used. Calcium stearate is a commonly used lubricant. The bisphenol-A- epoxy is commonly used as a crazing resistant additive while the phosphite has been generally used as a processing stabilizer to prevent discoloration during processing of polypropylene compounds.  
  For a clearer understanding of the invention, the following examples are given:  
 EXAMPLE I Q pprs &#39;JILIIGOLJI parts Propylene-ethylene copolymer alc filler TiO. pigment Primary antioxidant (as indicated) Distearyl thio dipropionate Calcium stearate Distearyl pentaerythritol diphosphite Molded samples in the form of sheets 0.060 inch in thickness containing the different primary antioxidants as listed in Table II were heated in a forced air convection oven to C to determine their resistance to visual craze before and after being subjected to the indicated cycles (DW cycles) of exposure to a dishwasher environment, each cycle involving exposure to F water solution of 0.35% of the dishwasher detergent, commercially available under the name *Cascade&#34; and a drying cycle of one hour at about 180F. The figures given in the table are the average of the tests on two or three samples of each composition.  
 TABLE 11 Days to Visual Craze Following Exposure Nominal D/W The primary antioxidants employed in preparing the compositions for the tests set forth in TABLE 11 are listed and where possible identified by chemical name.  
 EXAMPLE 2 To determine the effect of additive interactions on craze resistance and color stability, polypropylene compounds were prepared having the compositions described in TABLE 111. All compositions are expressed as parts by weight.  
 TABLE 111 C ompos- Phosition* Calcium phite BPA Epoxy Tin Number DSTDP Stearatc (a) Resin maleate *COMMON TO ALL MATERIALS 100 parts propylene-ethylene copolymer 25 parts Talc I 2.5 parts TiO Compohmu&#34; Punk) Am&#39;oxldmt 0.5 part Ciba-Geigy lrganox 1010 tetrakis [methylene 3-3&#39; ,5 &#39;-di-t-butyl-4-hydroxyphenyl )propionate ]me- 1708 tetrakis [methylene 25 m Y y yp y (a) distearyl pentaerythritol diphosphite prop1onate] methane 1710&#39; di-n-octadecyl-2-(3&#39;5&#39;-tli-tert-butyl-4&#39;-hydroxyphenylladi ate The ellowin tendenc of these com ositions was 1711 d4 &#39;1 &#39;1 3&#39;5&#39;d&#39; b 14&#39; g gf &#39;gg g fi n j determined under the 1nd1cated test conditions employ- 1712 zinc dicyclohexyl dithio phosphinate ing a color difference meter per American Society of 22 33:;EZHJiiiiififif&#39; Testing and Materials Standard D1925. Data are re- 1714 l.l,3-tris(5 tert-butyl-4-hydroxy-Z-methylphenyl) Ported as the change in yellowness j q following 771 posure [i.e., Y1 (exposed) Y1 (1n1t1al)].  
  lrinlflhylaAbm31&#39;5Idi ten butyl 4ghydroxy The results of these tests are set forth in TABLE IV.  
  benzyl )benzene 35 1548 Thin diethylene bis-(3.5-di-tert-butyl-4-hydroxy)hydrocinnamate TABLE IV 1549 Octadecyl 3-( 3 ,5 di-te rt-butyl-4 &#39;-hydroxyphenol )propionate Yellowness lndex Change Following Exposure 1550 0,0 D/W di-N-octadecyl-3,5-di-tert-butyl-4-hydroxybenzyl Cycles 750 1500 2160 750 1500 2160 phosphonate 40 Compos- 1558 1,3,5-tris( 2.6-di-tert-hutyl-4-methylene ition phcnohisocyanurate Number 180F 150F 1738 12.7 17.4 18.6 13.7 10.8 18.2 1739 11.9 25.2 32.4 11.9 12.3 22.1 From the results set forth in TABLE [1, 1t W111 be seen 1740 13.7 22.2 25.0 13.7 13.9 that composition 1708 containing the primary antioxi- 1Z1; 1 -8 $3 3-2 3-: :2? dant employed in the practice of the present invention H45 1:; is the only composition exhibiting good craze resis- 1746 1812 21.7 29.8 29.9 14.9 19.5 27.9 tancc after substantlal exposure to a common alkaline dishwasher environment and a craze resistance which declined the least as a result of such exposure. Crazing is a measure of the useful life of a plastic composition in the drastic high temperature aqueous pH 10 to 12 detergent environment ofa dishwasher. However, of all of the compositions listed in TABLE I1, composition 1708 also showed the greatest change in color, a significant yellowing, after exposureto the dishwasher environment, indicating also that the yellowing was caused by the particular primary antioxidant characterizing composition 1708.  
  The present invention is based on the discovery that the yellowing caused by this specific primary antioxidant can be controlled substantially by the addition of a tin maleate, specifically di-n-butyl tin maleate. The test results on various compositions as set forth in the following examples show that while the tin maleate addition causes a slight, but acceptable, drop in oxidative stability, it markedly improves the color stability.  
  From these test results, it will be seen that variations in different additives had no significant effect except for the addition of the tin maleate (compositions 1745 and 1746) which materially improved resistance to yellowing.  
  For comparison, the craze resistance of these compositions are given in TABLE V.  
 TABLE V-Continued TABLE VIII-Continued Days To Visual Craze Following Exposure The effect of the tin maleate addition was further proven by the results of tests on additional compositions which along with the test results on certain of the compositions previously described are set forth in TA- BELS VI and VII while the compositions are described in TABLE VIII.  
 TABLE VI Days To Visual Craze Following Exposure D/W Cycles 0 750 1500 Composition Days To Craze at 150C (0.060 inch thick) Number 180F 150F The following TABLE VII gives the yellowing test results.  
  The ingredients in the various systems listed in TA BLES VI and VII are listed in the following TABLE VIII.  
 TABLE VIII Composition Number 1741 System V nothing 1745 System V 0.20 phr di-n-butyl tin maleate 1743 System VII nothing 1746 System VII 0.20 phr di-n-butyl tin maleate 1744 System VIII nothing 1747 System VIII 0.20 phr di-n-butyl tin maleate 1760 System IX nothing 1756 System IX 0.20 phr di-n-butyl tin maleate BASIC COMPOSITIONS:  
 SYSTEMS V VII VIII IX Propylene-ethylene copolymer 100 100 100 100 Talc (filler) 25 25 25 TiO 2.5 2.5 2.5 2.5  
 Composition Number Tetrakis [methylene-3-(3&#39;,5-di-tertbutyl-4&#39;-hydroxyphenyl) propionate] methane Distearyl thio dipropionate 0. Calcium stearate 0.  
 Disteuryl Pentaerythritol diphosphite Bisphenol A epoxy resin N.N&#39;-di-acetyladipic acid dihydrazide 1.3 .5-tris( 2.6-di-tert-butyl 4- methylene phenol) isocyanurate From the test results of TABLE VII it will be seen that l,3,5-tris(2,6-di-tert-butyl-4-methylene phenol) isocyanurate (Composition 1756) may be advantageously substituted for part of the Irganox 1010. However, the primary stabilizer tetrakis [methylene 3- (3&#39;,5-di-t-butyl-4-hydroxyphenyl) propionate] methane is essential for the mechanical life of the composition under dishwasher conditions and this antioxidant appears to be unique in this respect. It is also presently conceived that the anti-yellowing contribution of the tin maleate may be obtained by equivalent organo tin maleates and particularly by similar alkyl derivatives such as diisobutyl tin maleate and di-n-oetyl tin maleate. It is further contemplated that changes can also be made in the remaining ingredients including the secondary antioxidants. Accordingly, it is intended by the appended claims to cover all such compositions as fall within the true spirit and scope of the invention.  
 I claim:  
  1. A composition having improved resistance to aging and discoloration in a high-temperature aqueous alkaline environment comprising a polymeric resinous material, a major portion of which is a moiety selected from propylene homopolymer, propylene-ethylene copolymer and propylene and ethylene polymer blends; and a stabilizer system comprising tetrakis (methylene 3-(3&#39;,5-di-t-butyl-4&#39;-hydroxyphenyl)propionate) methane as an antioxidant and a dialkyl tin maleate as its principal constituents, said tetrakis methane constituent being present in an amount in the range of between about 0.20 and 1.50 percent by weight and said tin maleate being present in an amount in the range of between about 0.05 and 0.25 percent by weight.  
  2. A composition according to claim 1 also including a second antioxidant.  
  3. A composition according to claim 1 in which said di-alkyl tin maleate is di-n-butyl tin maleate.  
  4. A composition according to claim 1 in which said secondary antioxidant is distearyl thiodipropionate.  
  5. The composition of claim 1 wherein the alkyl groups of said dialkyl tin maleate contain from one to eight carbon atoms.  
  6. The composition of claim 1 wherein said polymeric resinous material is a homopolymer of polypropylene modified with a rubber compound selected from ethylenepropylene rubber, ethylene propylene diene modified rubber, and combinations thereof.  
  7. A composition particularly characterized by its resistance to hot alkaline environment comprising a copolymer of propylene and ethylene and a stabilizer system consisting of tetrakis (methylene 3(3&#39;,5-di-tbutyl-4&#39;-hydroxyphenyl)propionate) methane, disteabeing present in an amount in the range of between about 0.05 and 0.25 percent by weight.  
  8. A composition according to claim 7 in which said tin maleate is di-n-butyl tin maleate.  
  9. The composition of claim 6 wherein said propylene ethylene copolymer includes an isotactic polypropylene center segment and contains about one to about six percent ethylene by weight.