Abstract:
The invention relates to silicate mineral filled polyolefin compositions which are stabilized against color deterioration during processing by incorporting therein a synergistic color stabilizer composition consisting of zinc dibutylidthiocarbamate and zinc stearate in critical ratios.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns stabilization against color deterioration of filled polyolefin compsoitions during processing. More particularly, the invention relates to reduction of discoloration during processing of polyolefins compositions caused by incorporation of mineral fillers into the polyolefin. 
     It is a common practice to use fillers in polyolefin compositions. The fillers are used to reduce cost and to impart or enhance a desired property in the finished product required in certain fields of application. Particularly useful are mineral fillers, such as talc, kaolin clay and pyrophylltie. The polyolefin resin and filler are milled together until the filler is thoroughly dispersed. Commonly present in the polyolefins are long-term stabilizers against degradation by oxygen, heat, and light and proecssing aids. The milling process of the filler and polyolefin causes shearing in the resin and an increase in temperature. The result is a tendency to cuase color deterioration of the polyolefin. The polyolefin composition shows a distnict hue, commonly called the &#34;pinking effect&#34;. In some instances the pinking effect in filled polyolefins ins increased by the presence of epoxy-type additives and stabilizers of the phenol type. The normal amounts of commmon long-term stabilizers that protect the finished article during its life-time are ineffective to prevent the color deterioration cuased by the filler during the relatively short processing stage, when heat may be generated internally or applied externally. The addition of larger amounts of the stabilizer would be economically impractica and in some instances, detrimental to the overall properties of the product. 
     Therefore, it is desirable to add to filled polyolefin compositions, a particular stabilizer for reduction of color development, i.e., pinking effect caused by exposure to elevated temperatures during processing and aging. 
     It is known that talc has a degrading effct on the properties of polyolefins. U.S. Pat. No. 3,553,158 describes the degrading effect on talc on polyolefins and teaches the use of certain &#34;talc deactivators&#34;. It further teaches that certain compounds, including zinc dibutylidthiocarbamate are not acceptable because they cause discoloration of talc filled polyolefins during processing or aging. The compoudn, however, are disclosed as long term stabilizers for polypropylene articles containing no fillers in U.K. Pat. No. 929,435. 
     Surprisingly, it has been discovered that a certin combination of zinc dibutylidthiocarbamate and zinc stearate affords the desired protection of rmineral filled polyolefins during processing and reduces pinking effect in the finished product. 
     SUMMARY OF THE INVENTION 
     The invention relates to polyolefin compositions filled with a silicate mineral filler and stabilized against color deterioration during processing which comprises a major portion of polyolefin, at least 10 percent by weight of mineral filler, and 0.03 to 1.0 percent by weight of a composition consisting of zinc dibutyldithiocarbamate and zinc stearate in the ratio of 1:4 to 4:1. 
     Another aspect of the invention concerns the color stabilization during processing, of compositions comprising polyolefins and a mineral filler by adding a stabilizer composition consisting of zinc dibutyldithiocarbamate and zinc stearate and mixing under shear conditions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The polyolefins that are generally formulated with mineral fillers are thermoplastic synthetic resins. They consist of monomeric 1-olefin units having 2 to 8 carbon atoms and higher. The monomeric units have 1-olefin unsaturation and can be staight chain or brached chain. Representative polyolefins include, among others, polyethylene, polypropylene, poly(6-methyl-1-heptene) and the like. Another group of polyolefins include copolymers of ethylene and propylene. Other compounds which are polymerized with ethylene or propylene include, among others, 1-butene, 1-pentene and similar units having 1-olefinic unsaturation. The polyolefins can be of low or high density. Such resins are well knwn and available commercially. 
     The mineral fillers which generally cause color deterioration during processing, namely the pinking effect, are silicate minerals. Particularly serious pinking effect is caused by talc, kaolin clay and pyrophyllite fillers. The degree of the pinking effect depends somewhat on the source of the crude ore and the properties of the mineral filler, as for example contaimination with other metal ions and paticle size. Minerals frm different depoists will contain varying amounts of different impurities in the form of metal ions. Some mineral refining processes will remove a significant portion of these impurities. The amount and type of impurity remaining in the filler is not critical to the invention. 
     The pinking caused by the silicate fillers is further enchanced by the presence of certain additives and antioxidants. Particularly detrimental are epoxy type mineral deactivators such as polyeposides derived from bispheonl A and epichlorohydrin or from diglycidyl ether of disphenol A and their esters with fatty acids, rosin acids and tall oils. Similarly, phenolic antioxidants ehnance the pinking effect in silicate filler polyolefins. The phenol stabilizer may contain one or more hydroxy groups. Representative phenol stabilizer include, among other, 2,6-t-butyl-p-cresol, 3,5-di- t-butyl-4-hydroxyhydrocinnamic acid triester of 1,3,5-tris(2-hydroxyethyl)-s-triazine-2,4,6-(1H,3H,5H)-trione, 2,2&#39;-ethylidenebis(4,6-di-t-butylphenol), and tetrakis(methylene (3,5-di-t-butyl-4-hydroxyhydrocinnamage) methane. Particularly preferred is tri(3,5-di-t-butyl-4-hydroxygenzyl) isocyanurate. 
     In the instant polyolefin composition, the filler will be present from about 10 to 60 percent by weight and preferably 20 to 50 percent by weight based on the weight of the composition. 
     The present stabilizee composition consits of zinc dibutyldithiocarbamate and zinc stearate. Moreover, certain critical ratios of the two components produce a synergistic action towards inhibition of the pinking effect during processing. the synergistic composition contains zinc dibutyldithiocarbamate and zinc stearate in the ratio of 1:4 to 4:1 and preferably in the ratio of 1:3 to 3:1. A particularly preferred ratio of zinc stearate to zinc dibutylidthiocarbamate is 3:1. 
     The amount of stabilizer that is added to the polyolefin composition will depend somewhat on the amount and type of filler present. Generally, about 0.03 to 1.0 percent of the stabilizer is added to the polyolefin composition. The preferred range is 0.1 to 0.5 percent of the stabilizer composition based on the weight of the polyolefin composition. 
     The polyolefin composition may contain other conventional additives. Such additives include long term stabilizer against deterioration by light, heat or oxygen, processing aids, internal lubricants and the like. 
     Suitable stabilizers include, among others, phenols arlamines, organic phosphites, phosphates, thiophosphites, or thiophosphates of dialkyl hydroquinone. Common secondary antioxidants include thio esters as for example distearyl thiodipropionate and dilauryl thiodipropionate and dilauryl thiodipropionate. The composition may include impact modifiers as for example ethylene propylene diene monomers. In addition, other cnventional additives such as light stabilizers, plasticizers, internal lubricants, emulsifiers, antistatic agent, flameproffing agents, antimicrobial agents and pigments can be empolyed in compounding the filled polymers. 
     The polyolefin composition can be compounded in any suitable manner with conventional mixing eqipment such as a mill, Banbruy mixer or compounding extruder. Mixing is generally conducted under shear condtions at bout 160° C. to about 260° C. Shear is mesured as melt viscosity ranging from about 500 to 5000 meter-grams torque. Mixing is continued until a homogeneous composition is obtained. Thereafter, the polyolefin composition is formed into a desired shape and solidified at elevated temperatures. Typically the temperatures range from about 100° C. to 145° C. depending on the type of resin used. 
     The following examples are given for further illustration of the invention and are not intended in ay way to limit the invention. All percentages and parts are based on weight unles otherwise indicated. 
     EXAMPLE 1 
     Powdered polypropylene resin Profax®6501 (Manufactured by Hmont Inc.) was dry-blended with a filler epoxy filler deactivator (epichlorohydrin-bisphenol A resin), a long term antioxidant tri(3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate, a secondary antioxidant distearyl thiodipropionate and the stabilizer composition of the invention as given in Table I hereinbelow. Formulation 1 contained no stabilizer and served as control. 
     The dry blend was charged on a two-roll mill at 165° C. The compound was fluxed for approximately five minutes at which time it was sheeted off at 0.125 cm and allowed to cool. The milled sheet was compression molded at 182° C. for five minutes at 20,000 psi to a thickness of approximately 0.0625 cm. From this molded slab, 5×5 cm specimens were cut and placed on a glass plate for aging i an air circulating oven at 160° C. 
     The color of the specimens was determined by the L,a,b, 45°O° colorimetry according to the TAPPI Standard Method T524 cm-86. The ratings represent the following color characteristics. 
     L=degree of lightness, a=red (+a) to green (-a), and b=yellow +b) to blue (-b). ΔE is the change in color from the unages to the aged samples. 
     The test data compiled in Table I demonstrate that formulations 4,5 and 6 of the present invention provide good color degradation inhibition which is synergistic in nature. Formulations 2 and 3 containing the individual components of the present composition showed discolration. 
     
                       TABLE I______________________________________Color Characteristics After Aging at 160° C.Formulation,in parts 1       2       3     4     5     6______________________________________Profax 6501    60.0    60.0    60.0  60.0  60.0  60.0Talc     40.0    40.0    40.0  40.0  40.0  40.0Tris(3,5-di-t-    0.075   0.075   0.075 0.075 0.075 0.075butyl-4-hydroxy-benzyl)isocyanurateDistearyl    0.225   0.225   0.225 0.225 0.225 0.225thiodi-propionateEpoxy resin    0.3     0.3     0.3   0.3   0.3   0.3Zinc stearate    --      --      0.1   0.075 0.05  0.025Zinc dibutyl-    --      0.1     --    0.025 0.05  0.075dithio-carbamateColor CharacteristicsBrightnessUnaged   52.6    54.6    50.7  53.9  57.0  54.4Aged 17  43.2    53.1    49.9  55.4  56.2  55.8hoursAged 37  41.0    50.0    47.7  54.5  52.0  53.8hoursAged 100 35.6    49.7    40.7  51.5  53.9  51.6hoursL-ValueUnaged   75.4    76.2    73.6  75.9  77.9  76.1Aged 17  71.9    78.2    75.5  78.2  79.1  79.1hoursAged 37  70.8    77.1    74.9  78.4  76.7  78.8hoursAged 100 67.5    77.8    70.7  77.0  79.9  78.5hoursa-ValueUnaged   -0.6    -0.9    -0.8  -0.9  -0.6  -0.9Aged 17  +2.0    -1.3    +0.6  -1.2  -1.2  -1.4hoursAged 37  +2.4    -1.4    +1.1  -1.0  -1.3  -1.4hoursAged 100 +3.7    -1.5    +2.0  -1.1  -1.5  -1.6hoursb-ValueUnaged   +4.3    +3.5    +3.7  +3.6  +3.7  +3.6Aged 17  +8.6    +7.6    +7.1  +5.4  +6.2  +6.5hoursAged 37  +9.2    +9.1    +8.1  +6.7  +6.8  +7.9hoursAged 100 +10.6   +10.3   +9.5  +7.5  +9.5  +9.5hoursΔE - ValueAged 17  6.12    4.58    4.14  2.94  2.84  4.20hoursAged 37  7.36    5.69    4.97  3.98  3.40  5.10hoursAged 100 10.98   7.01    7.06  4.06  6.11  6.41hours______________________________________ 
    
     The above embodiments have shown various aspects of the present invention. Other variations will be evident to those skilled in the art and such modifications are intended to be within the scope of the invention as defined by the appended claims.