A PROCESS FOR MANUFACTURING CLEAR SHAPED ARTICLES FROM POLYOLEFIN COMPOSITIONS

Abstract: The present invention is a thermoforming process for manufacturing clear shaped articles such as deli containers, lids, cups and the like from blended, nucleated polyolefin compositions.

Description: A PROCESS FOR MANUFACTURING CLEAR SHAPED ARTICLES FROM POLYOLEFIN COMPOSITIONS Technical Field The present invention relates to a process for manufacturing clear shaped articles such as deli containers, lids, cups and the like from blended, nucleated polyolefin compositions. Background of the Invention It is desirable to produce clear or transparent shaped articles such as deli containers, lids, cups and the like so that the mateπal side of the container can be viewed by potential customers before purchasing. Nucleated polypropylene provides for a high degree of claπty or transparency However, shaped articles produced from nucleated polypropylene have a tendency to warp. This is particularly true for flat, shallow-draw parts such as lids. Therefore, shaped articles produced from nucleated polypropylene can be difficult to stack, for example for use in production lines. As a result, non-nucleated mateπals are used for producmg such shaped articles. However, non-nucleated mateπals do not provide for a high degree of claπty or transparency. There are many different techniques for forming shaped articles from polyolefin compositions. The techniques are sometimes referred to as primary processes and secondary processes. Primary processes mclude injection moldmg, compression moldmg, and castmg; while secondary processes mclude melt spun fiber stretching, tubular film ("bubble") forming, blow molding and thermoforming. The primary processes consist of forcing the polyolefin composition into a cavity and duplicating the cavity's shape. In the process of castmg, the cavity may be filled by gravitational flow with a low viscosity liquid (reactmg monomer or prepolymer) and, foiiowmg polymeπzation, the liquid solidifies. In compression molding, a prepolymer sohd mass is heated up or melted and forced to undergo a squeezing flow by hot mold surfaces that close to form a final shape. The prepolymer usually crosslinks and assumes the shape of the closed cavity permanently. In the injection moldmg process a polymer melt is forced through an oπfice or gate into a closed cold mold where it solidifies under pressure m the shape of the mold cavity. The polymer is melted, mixed and injected from the injection unit of the machine. Tadmor, Zehev and Gogos, Costas G., Principles of Polymer Processing, John Wiley & Sons, Inc., New York, 1979, p. 584. With respect to secondary shaping processes, melt spun fiber stretching, tubular ("bubble") forming and blow moldmg occur immediately after die forming. Blow molding involves usmg gas pressure inside an almost molten plastic tube to expand the plastic until it fills the cavity in the mold and solidifies against the walls of the mold. Thermoformmg involves polymer sheets or films that have been extruded and solidified m a separate operation. Tadmor, Zehev and Gogos, Costas G , Principles of Polymer Processing, John Wiley & Sons, Inc., New York, 1979, p. 632. Thermoforming is a geneπc term encompassing many techniques for producmg useful shaped articles from flat sheets. Thermoformmg is one of a family of processes that deal with the pressing or squeezmg of pliable plastic mto a final shape. Thermoformmg is differentiated from the primary processes previously discussed as the primary processes involve melting of the polymer followed by solidification. For example, in mjection moldmg, the initial resin state is pellet or powder and the shaping is done on the polymer as a liquid. Additionally, m compression moldmg pressures are substantially higher than those employed m traditional vacuum or low pressure thermoformmg and the polymer is shaped as a liquid between matched metal molds. Several polypropylene mateπals are currently used to make sheets which can be used m thermoformmg. However, polypropylene is not inherently clear because of its semi- crystalline nature. While it is known to use certain additives to improve the claπty of polypropylene, certain of these known additives result m a high degree of warping. U.S. Patent Nos. 5,300,549 and 5,319,012 to Witco Corporation ("the Witco patents") disclose polyolefin compositions and method and compositions for their preparation. The compositions produced have unproved claπty Further, the Witco patents descπbe that articles may be manufactured from the compositions by castmg, compression moldmg or mjection moldmg; films may be made by flowmg or by extrusion, filament, bars, tapes and the like may be obtained by extrusion. The Witco patents also teach that the invention is useful m all other processes involving meltmg of the polymer followed by solidification. As mentioned, thermoformmg is not a process in which the polypropylene is melted and resolidified. Thermoformmg is an extremely versatile process which is used to make many types of shaped articles well known to many consumers. Therefore, there is a need to produce shaped articles such as deli contamers, lids, cups and the like usmg thermoformmg processes while achieving a high degree of claπty or transparency and a low tendency to warp. Summary of the Invention The present mvention provides a process for manufactuπng a shaped product from a polyolefin composition nucleated with a combination of at least one dicarboxy c acid and at least one aliphatic monocarboxyhc acid, for example oleic acid, steaπc acid, behenic acid, myπstic acid, abietic acid, lauπc acid, linoleic acid, ncmoleic acid, dihydroxysteaπc acid, arachidic acid, eicosenoic acid, erucic acid, tetracosenoic acid, elaidic acid and mixtures thereof, compπsing the steps of pπmaπly processing the polyolefin composition by meltmg and resolidification of the polymer to form a startmg mateπai for a secondary processing step, and secondaπly processing the startmg mateπai mto a shaped article by a process that excludes melting and resolidification. In one embodiment of the present mvention, the step of secondaπly processmg includes the step of thermoformmg Further, the present mvention provides a process for manufactuπng a shaped product from a startmg matenal which consists of a blended nucleated polyolefin composition compπsing forming the startmg mateπai mto a shaped article with a process that excludes melting and resolidification of the startmg mateπai. The present mvention discloses a nucleation system that provides good claπty m thermoformed articles, but with a lower crystallization rate than the thermoformmg grades of polyoleflns currently m use. The present mvention enables nucleated polypropylene, which is clear, to be formed into shaped articles by processes which do not involve meltmg and resolidification of the polypropylene. The resulting shaped articles retain the claπty of the startmg mateπai and exhibit low warpage Those, and other advantages and benefits, will become apparent from the Detailed Descnption of the Invention hereinbelow Bπef Descnption of the Drawings For the present invention to be easily understood and readily practiced, the present invention will be descnbed, for purposes of illustration and not limitation, in conjunction with the foiiowmg figures wherem. FIG. 1 illustrates a thermoformmg device on which the method of the present mvention may be practiced; and FIG. 2 illustrates an in-line sheet extrusion and thermoformmg lme on which the method of the present invention may be practiced. Detailed Descnption of the Invention FIG. 1 illustrates a thermoformmg device 10 The illustrated device 10 is disclosed for purposes of illustration. The reader will understand that the present mvention may be earned out on many different types of thermoformmg equipment such that the illustration of one particular type of device 10 should not be construed as limiting the present invention As shown tn FIG 1, a roll of polypropylene mateπai 12 is supplied to a take-off roll 14 The matenal 12 is then fed from the takeoff roll 14 through the thermoformer and forming press 16 to form a shaped article. Tension is mamtamed on the rolled mateπai 12 by a take-up roll 18 The shaped article may then be transferred to a tπm die (not shown) where excess mateπai is trimmed from the shaped article. The shaped articles are stacked and counted by a stacking/counting device 20. FIG. 2 illustrates an in-line sheet extrusion and thermoformmg lme 22. As shown in FIG. 2, a sheet extruder 24 is placed m-lme with a thermoformer 26, thus obviatmg problems associated with handling cut-sheet mateπals. The sheet is contmuously fed from the extruder 24 to a down-roll stack 28 and coolmg table 30 where the sheet is cooled. The sheet then travels to the salvage tπm 32 where excess mateπai trimmed from the sheet is salvaged for reuse The sheet passes to a hold-down table 34 and edge clamp engagement 36 The sheet then travels to a reheatmg zone 38 where it is reheated and then travels to the thermoformmg station 26 where it is formed mto a shaped article. The shaped article then travels to the inline trimming station 40 where excess mateπai is trimmed from the shaped article. FIG. 2 is provided to illustrate that the present mvention may be incorporated mto vaπous types of existing process lmes tn addition to bemg used m a stand-alone environment as shown m FIG. ' The roll of polypropylene mateπals 12 of FIG. 1 and the sheet of polypropylene mateπai of FIG. 2 are two examples of a startmg matenal for a secondary process. In preparing such startmg mateπals, it is known to blend several types of mateπals, including polypropylene powder or pallets, additives, and nucleatmg agents. Witco Corporation located m Memphis Tennessee ("Witco"), markets a nucleating agent named MoldPRo 931 for molded polypropylene having improved claπty. The MoldPro 931 nucleatmg agent is believed to be compπsed of a combination of at least one dicarboxylic acid and at least one aliphatic monocarboxy c acid selected from the group consistmg of oleic acid, steaπc acid, behenic acid, myπstic acid, abietic acid, launc acid, lmoleic acid, ncmoleic acid, dihydroxysteaπc acid, arachidic acid, eicosenoic acid, erucic acid, tetracosenoic acid, elaidic acid and mixtures thereof. In the moldmg process, Witco asserts this nucleator provides its effect by interactions of the product with aluminum catalyst residues in the polymer (approximately 50 ppm of aluminum) Witco also asserts the need to use 2500-3750 ppm of the MoldPro 931 agent m the moldmg process. To test the efficacy of the MoldPro 931 nucleatmg agent, tests were performed usmg as a starting matenal for a thermoformmg process polypropylene havmg 200-1200 ppm MoldPro 931 nucleatmg agent. That startmg matenal was then formed into cups and lids usmg a thermoformmg process. As shown in Table 1, tests were performed usmg varying amounts of the MoldPro 931 agent to determme the optimum level (of the MoldPro 931 agent) for producing 22-mil or 022 inch sheet samples on a 2-inch HPM sheet extrusion line. Table 1: Clarity of Sheet for Various Levels of MoldPro 931 Nucleating Agent Table I displays the claπty of the vaπous sheets produced when differing amounts of the MoldPro 931 nucleating agent were used. The formulations used a 2 meltflow homopolymer polypropylene produced by Ziegler-Natta type catalyst. The formulations also included the following components in the additive package: Irganox 1010 (500 ppm) available from CIBA Irganox 1076 (100 ppm) available from CD3A; Irganox 168 (1000 ppm) available from CIBA; and DHT4A (300 ppm) available from Mitsui USA. These components were chosen for convenience, and do not contribute to the effectiveness of MoldPro 931. As shown in Table 1, the haze percentage of the sheets decreased, that is, the clarity of the sheets increased as the amount of MoldPro 931 agent used in producing the sheets mcreased. For example, when the level of MoldPro 931 agent was 200 ppm, the haze % of the sheet was 73.1%. When the level of MoldPro 931 agent was 800 ppm, the haze % of the sheet was 50.3%. Table 2: Data from OMV Thermoforming Line As shown on Table 2, a 48-mιl sheet was compared to 16-oz. cups produced using an OMV Thermoformmg Line. The precursor sheets were thermoformed to form 16 oz. cups. That is, the precursor sheets were an intermediate product in that the sheets were fed into a thermoformer and formed into the 16 oz. cups. The formulations of Table 2 used the 2 meltflow homopolymer polypropylene and components in the additive package as identified in Table 1. As shown in Table 2, the haze % of both the sheets and the cups lowered as the amount of MoldPro 931 agent used was increased. Further, Table 2 also shows that the level of gloss on the 16-oz. cups increased as the amount of MoldPro 931 agent used increased. The following table has data from testing on the OMV thermoforming line. This evaluation was performed on a 16-oz. deli cup. The precursor sheets were thermoformed to form 16 oz. cups. FT021N is available from Aristech Chemical Corporation and uses nucleating agent NA-11A, which is available from Amfme Chemicals. FF021B uses no nucleation and is available from Aristech Chemical Corporation. The formulations which used the MoldPro 931 agent also used a 2 meltflow homopolymer polypropylene produced by Ziegler-Natta type catalyst. The formulations using MoldPro 931 also included the following components in the additive package: Irganox 1010 (500 ppm) available from CEBA; Irganox 1076 (100 ppm) available from CIBA; Irgafos 168 (1000 ppm) available from CIBA; and DHT4A (300 ppm) available from Mitsui USA. Table 3: Data from OMV Thermoforming Line, 16-oz. Deli Cup As shown in Table 3, the haze value of the MoldPro 931 formulations, in particular the 800 ppm formulations, drastically reduce in going from t e sheet to the cup. More specifically, the 800 ppm MoldPro 931 48 mil sheet had an average haze value of 68.8%, while the 800 ppm MoldPro 931 cup had an average haze value of 8.4%. This can be attributed to a number of factors, including but not limited to the fact that the sheet is approximately 4 times as thick as the cup, and also that the precursor sheet undergoes a stretching process as it is thermoformed into the cup. Further, as shown in Table 3, none of the tested materials provided sheet as clear as FT021N. However, the MoldPro 931 agent formulations produced thermoformed cups with surprisingly good clarity and gloss. In fact, cups with 800 ppm MoldPro 931 agent had an average haze value of 8.4% vs. 8.2% for FT021N. This difference cannot be seen by the naked eye. This OMV data suggests that the MoldPro 931 agent might work well for any clear application. Tests were performed to determine whether the MoldPro 931 agent might work well for specifically producing clear lids. As previously discussed, clear lids are difficult to process because nucleated polypropylene has a tendency to warp. As a result, OMV lid lines typically use non-nucleated materials, which do not provide good clarity. An analysis of three different homopolymer material which are used in the formation of lids are given in the following table. Table 4: Analysis of Thermoformed Lids e-crystallizauon temperature •• Weight */* of crystalinity ••• Rate of crystalinity As shown in Table 4, the nucleated lid has a low haze value, but too much post-mold warping. The high Tc and high crystallization rate of this material is believed to cause the bottom surface of the lid (which contacts the cold mold) to crystallize in the mold, whereas the top surface crystallizes later. The non-nucleated lid has a low crystallization rate and does not warp, but also has a very high haze value. The 50/50 blend provided a reasonable compromise between clarity and warping, but still had some tendency to warp. To determine if a nucleation system using the MoldPro 931 agent would produce better results, formulations using the MoldPro 931 nucleating agent were studied. The following table has Differential Scanning Caloπmetry ("DSC") data for five formulations evaluated on an OMV thermoformmg lme. Table 5: Thermal Analysis of Potential Lid Materials FT021N is available from Aπstech Chemical Corporation and uses nucleating agent NA-l 1A, which is available from Amfine Chemicals. FF021B uses no nucleation and is available from Anstech Chemical Corporation. The formulations which used the MoldPro 931 agent also used a 2 meltflow homopolymer polypropylene produced by Ziegler-Natta type catalyst. The formulations usmg MoldPro 931 also mcluded the foiiowmg components m the additive package: Irganox 1010 (500 ppm) available from CIBA; Irganox 1076 (100 ppm) available from CIBA; Irgafos 168 (1000 ppm) available from CIBA; and DHT4A (300 ppm) available from Mitsui USA. As shown in Table 5, the MoldPro 931 agent formulations looked promising because they had lower Tc and lower crystallization rates than even the 50/50 blend matenal shown in Table 4. Therefore the MoldPro 931 agent formulations were expected to exhibit less warpage than even the 50/50 blend matenal. This DSC data suggests that the MoldPro 931 agent should work well for producmg clear lids with minimal warpage. A tπal was subsequently performed to produce lids on an OMV thermoformmg line. The precursor sheets were thermoformed to form hds. The mateπals tested were a standard non-nucleated homopolymer polypropylene and a homopolymer polypropylene containing 800 ppm MoldPro 931. As the following table shows, the clarity of the lids was greatly unproved by usmg MoldPro 931. Table 6: Data from OMV thermoforming Line, Lids % HAZE GLOSS Pre-cursor Sheet Non-nucleated: 66.2 11.5 800 ppm MoldPro 931: 45.6 79.8 Thermoformed Lids Non-nucleated: 71.3 87.2 800 ppm MoldPro 931: 50.2 85.3 There were no problems with warping or stacking with either matenal. This assessment was easily made by visual inspection The present mvention involves the discovery that polyoleflns nucleated with the MoldPRo 931 agent may be subsequently shaped by thermoformmg processes with improved claπty and no increase in warpage as compared to a non-nucleated matenal. It was also discovered that 50 ppm alummum residue was not necessary and that levels of less than 1000 ppm of the MoldPro 931 agent are sufficient. The MoldPro 931 nucleation system provides for good claπty in the thermoformed articles, but with a lower crystallization rate than the thermoformmg grades of polyoleflns currently in use which leads to less warpage. While the present invention has been disclosed in conjunction with preferred embodiments thereof, those of ordinary skill m the art will recognize that modifications and vaπations are possible. This disclosure and the foiiowmg claims are intended to cover all such modifications and vaπations.

Claims: What is claimed is. 1 A process for manufactuπng a shaped article from a polyolefin composition nucleated with a combination of at least one dicarboxylic acid and at least one aliphatic monocarboxyhc acid selected from the group consistmg of oleic acid, steaπc acid, behemc acid, myπstic acid, abietic acid, lauπc acid, linoleic acid, ncmoleic acid, dihydroxysteaπc acid, arachidic acid, eicosenoic acid, erucic acid, tetracosenoic acid, elaidic acid and mixtures thereof, compπsing the steps of: pπmaπly processing said polyolefin composition by meltmg and resolidification of the polymer to form a startmg matenal for a secondary processing step; and secondaπly processing said startmg matenal mto a shaped article by a process that excludes melting and resolidification. 2. The process as claimed m claim 1 wherem said step of secondaπly processmg includes the step of thermoformmg 3 The process as claimed m claim 1 wherem said combmation of at least one dicarboxylic acid and at least one aliphatic monocarboxyhc acid is from 200-1200 ppm. 4 A process for manufactuπng a shaped article from a startmg matenal which consists of a blended nucleated polyolefin composition compnsmg: forming the startmg matenal mto a shaped article with a process that excludes meltmg and resolidification of the startmg matenal. 5 The process as claimed m claim 4 wherem said forming step mcludes the step of thermoformmg. 6. The process as claimed m claim 4 wherem said startmg matenal consists essentially of an olefin polymer containing residual polymerization catalyst and an effective amount of a combmation of at least one dicarboxylic acid and at least one aliphatic monocarboxyhc acid selected from the group consistmg of oleic acid, steaπc acid, behemc acid, myπstic acid, abietic acid, lauπc acid, linoleic acid, ncmoleic acid, dihydroxysteaπc acid, arachidic acid, eicosenoic acid, erucic acid, tetracosenoic acid, elaidic acid and mixtures thereof wherem the weight ratio of ahphatic dicarboxylic acid to aliphatic monocarboxyhc acid is about 1:1, wherem the startmg matenal is formed by meltmg of the polymer followed by solidification. 7 The process as claimed m claim 4 wherem said effective amount of a combmation of at least one dicarboxylic acid and at least one ahphatic monocarboxyhc acid is from 200-1200 ppm. 8 A process for manufacturing a shaped article from a nucleated polyolefin composition, wherem the improvement compπses: forming the starting matenal mto a shaped article by a process that includes heating the matenal to a temperature less than its meltmg temperature. 9. The process as claimed in claim 8 wherein said forming step includes the step of thermoforming.

Inventors: ROBERTS TIMOTHY N, ROBERTS, TIMOTHY, N.
Applicants: ARISTECH CHEMICAL CORP, ARISTECH CHEMICAL CORPORATION
Application year: 2001
Main classification: C08K5
IPC codes: B29C51, C08K5