WO2005111091A1 - Reduction of discoloration in polyolefin resins - Google Patents

Abstract

The invention relates to a process for producing polyolefin-based pellets, where the polyolefin is prepared in the presence of a chromium catalyst system. This process comprises blending one or more additives into a polyolefin resin, and adding water to the resin, prior to pelletizing, in an amount sufficient to improve the color of the resulting pellets, and where wherein water is added in an amount from 100 ppm to 1000 ppm. The invention also relates to pellets formed by such process, and to a process of reducing color in such polyolefin-based pellets, by blending one or more additives into a polyolefin resin, and adding water to the resin, prior to pelletizing, in an amount sufficient to improve the color of the resulting pellets, and where wherein water is added in an amount from 100 ppm to 1000 ppm.

Description

REDUCTION OF DISCOLORATION IN POLYOLEFIN RESINS

The present invention relates to polyolefm resins having improved color as evidenced by a lower Yellowness Index value. More particularly, the present invention relates to a process in which water is added to the resin during production of resin pellets, in an amount sufficient to reduce the color of the resulting pellets.

REFERENCE TO PRIOR APPLICATION This application claims the benefit of U.S. Provisional Application No. 60/566,693, filed on April 30, 2004, incorporated herein, in its entirety, by reference.

BACKGROUND OF THE INVENTION Polyolefm based resins are used in a wide variety of applications. The resins are commonly formulated with various additives, including UV-stabilizers and antioxidants in order to protect the resins from degradation. In many of these applications, it is desirable that the resins be substantially clear. In some situations it has been observed that some of the commonly used additives lead to increased color (or decreased clearness) of the resins.

Discoloration is a complex phenomenon, and appears to depend on many factors, including catalyst residues, vinyl end groups and antioxidants. One method for determining the color of resin pellets is known as the Yellowness

Index ("YI"). The method for determining YI for pellets is set forth in ASTM D6290.

Using this method it was observed that certain formulated high density polyethylene resins had a Yellowness Index of about -1, whereas a YI of less than -3 was considered desirable.

Furthermore, the YI has been observed to increase upon storage times and exposure to increased temperatures. Accordingly, methods of improving the color of these and other polyolefin-based resins, and maintaining such improvement over time, as evidenced by a reduced Yellowness Index, are desired. Current methods for improving the color of polyolefin-based resins typically involve the use of one or more additives. Common additives for improving color include anti- oxidants such as organophosphites and organophosphonites, sterically hindered phenolic antioxidants and amines. While such additives may be successful for many applications, it would be desirable to reduce or eliminate the need for such additives to limit costs and to limit use of new materials, particularly in food packaging applications, which would require extensive re-qualification processes. Food packaging applications are also especially sensitive to taste and odor which can be imparted by either the additive itself, or the reaction products of these additives with catalyst residues. It has been observed that the presence of certain commonly used additives, particularly in combination with polymerization catalyst residue, can lead to increased color. Accordingly, in one aspect, the present invention relates to an improved process for producing polyolefin-based pellets in which one or more additives are blended into a polyolefin resin in an extruder prior to pelletizing, the improvement comprising adding a hydroxyl-containing species (most preferably water) in the extruder, in an amount sufficient to improve the color of the pellets. The present invention is believed to have particular applicability when the one or more additives include at least one phenolic additive, such as a hindered phenol.

SUMMARY OF THE INVENTION The invention relates to a process for producing polyolefin-based pellets, and pellets produced, where in the process, the polyolefin is prepared in the presence of a chromium catalyst system. This process comprises blending one or more additives into a polyolefin resin, and adding water, prior to pelletizing, in an amount sufficient to improve the color of the resulting pellets, and where wherein water is added in an amount from 100 ppm to 1000 ppm. Color is improved by a reduced discoloration in the pellets. The invention also relates to a process of reducing color in such polyolefin-based pellets, comprising blending one or more additives into a polyolefin resin, and adding water, prior to pelletizing, in an amount sufficient to improve the color of the resulting pellet, and where wherein water is added in an amount from 100 ppm to 1000 ppm. In one embodiment, the one or more additives include a hindered phenolic antioxidant. In another embodiment, water is added in an amount from 200 ppm to 300 ppm. In further embodiments, the one or more additives do not include aliphatic hydroxyl compounds, epoxide containing compounds and sulfur compounds.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of a suitable line pelletization process for use in the present invention. This figure adds an additional mode of addition of water (via stream 1), which was not present in the "Figure 1" submitted with corresponding Provisional Application No. 60/566,693. Figure 2 is a block diagram of a resin manufacturing process. Figure 3 is a distribution profile of the YI - prior to water injection improvement implemented for resin production (1227 samples; mean YI of -1.555). Figure 4 is a distribution profile of the YI - after water injection improvement implemented for resin production (120 samples; mean YI of -4.784). Figure 5 is a comparison of YI distributions as shown in Figures 4 and 5, using the same index scaling (the top profile is the YI distribution prior to water injection and the bottom profile is the YI distribution after water injection).

DETAILED DESCRIPTION OF THE INVENTION The present invention is suitable for use with any polyolefin-based resin. This includes homopolymers and copolymers or interpolymers (both terms "copolymers" and

"interpolymers" should be understood to mean polymers containing two or more monomers) of mono-olefins and di-olefins. Suitable polyolefin-based resins include, but are not limited to, polyethylene, polypropylene, polyisobutylene, polybutadiene, polyisoprene, cyclopentene, and norbornene. Polyethylene based polymers are the most preferred resins for use in the present invention, and include, but are not limited to, high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), homogeneously branched linear or substantially linear low density polyethylene, and heterogeneously branched linear low density polyethylene. Of these, HDPE is the most preferred resin. Suitable comonomers useful for polymerizing with ethylene, or another base monomer include, but are not limited to, ethylenically unsaturated monomers, conjugated or nonconjugated dienes and polyenes. Examples of such comonomers include the C₃-C₂₀ α- olefins such as propylene, isobutylene, 1-butene, 1-hexene, 4-methyl-l-pentene, 1-heptene, 1-octene, 1-nonene, 1-decene, and the like. Preferred comonomers include propylene, 1- butene, 1- hexene, 4-methyl-l-pentene and 1-octene, with 1-hexene being especially preferred. Other suitable monomers include styrene, halo- or-alkyl-substituted styrenes, tetrafluoroethylenes, vinylbenzocyclobutanes, butadienes, isoprenes, pentadienes, hexadienes, octadienes and cycloalkenes, for example, cyclopentene, cyclohexene and cyclooctene. Typically, the heterogeneously- or homogeneously-branched linear ethylene interpolymer is a copolymer in which ethylene is copolymerized with one C₃-C₂₀ α-olefin. These resins, and methods to make them, are generally known in the art. This includes gas-phase, solution phase and slurry-phase polymerization processes. Of particular interest to the present invention are resins made in the gas-phase process, particularly those made using a titanated chromium catalyst, such as those described in EP0640625. Typical, useful catalysts consists of a chromium (VI) compound (typically as the oxide) supported on a high surface area refractory oxide support. Generally the support is an amorphous microspheroidal silica, silica alumina, silica titania or aluminophosphate. The supports are typically produced by spray drying or precipitation. Supports prepared by spray drying typically have a narrower particle size distribution than the precipitated supports, however either type of support may be subsequently sized using known techniques to further control the final support particle size. The catalyst is prepared by activating the chromium-containing support at temperatures of 400-1000°C, in a dry, oxygen-containing atmosphere. Modifying materials such as titanium and fluoride are generally added prior to the activation. Generally, catalysts are prepared by using commercially available silica to which a chrome source has been added. The silica substrate may be treated with a titanium ester (titanium tetraisopropylate or titanium tetraethoxide are typically used) either, after the Cr compound is deposited, or prior to this deposition. The support is generally pre-dried at about 150-200°C to remove physically adsorbed water. The titanate may be added as a solution to a slurry of the silica in isopentane solvent or directly into a fluidized bed of support. If added in slurry form, the slurry is dried. Generally, the Cr compound which is convertible to Cr +6 has already been added to the support. The support is then converted into active catalyst by calcination in air at temperatures up to 1000°C. During activation, the titanium is converted to some type of surface oxide. The chromium compound (generally chromium (III) acetate) is converted to a Cr+6 oxide of some kind. Fluoriding agents may also be added during the activation process to selectively collapse some pores in the support, modifying the molecular weight response of the catalyst. The activated catalyst may also be treated with reducing agents prior to use, such as carbon monoxide in a fluidized bed, or other reducing agents, such as aluminum alkyls, boron alkyls, lithium alkyls and the like. Catalysts of this type are described in numerous patents, such as WO2004094489, EP0640625, US4100105, and the references cited within these references. Each of these references is incorporated, in its entirety, by reference. Catalyst such as those described in US6022933, also containing a Cr+6 component, are also useful in the invention. This reference is also incorporated herein, in its entirety, by reference. It is noted that Ziegler type catalysts, which are typified by a transition metal halide supported on magnesium chloride, contain significant quantities of halogens, especially chlorides, which can increase resin color by causing, for example, mold staining and corrosion. In Ziegler type catalysts, the halogen species need to be sequestered or rendered inactive by various additives, such as calcium or zinc stearate, zinc oxide and the like. These catalyst systems also contain significant amounts of organic species which are bound to catalyst residues. Water is frequently added to the polymers, produced by Ziegler type catalyst systems, to react with the organic residues of the catalyst system and any reactive halogen containing compounds (that is free TiCl_x for example), and to allow these materials to be purged from the polymer, or react with other scavengers, prior to use. The addition of water to a Ziegler type catalyst system can produce acidic species, such as, hydrochloric acid, by hydrolysis of titanium chloride, which can further react in the resin, if not purged from the resin, or sequestered by reaction within the resin. A specific feature of the chromium based catalysts is that their residues (contained within the polymer) contain essentially no. halogens, particularly no metal halides. Catalyst residues for chromium based catalysts include the respective catalyst and corresponding catalyst decomposition products. Other catalyst systems that contain essentially no metal halides are also useful in the invention. The catalyst residues are also essentially free of any HC1 which might react with moisture or oxygen. However, the catalyst residues that do exist in these systems, contribute to increased discoloration in the formulated resin. There is a need for an efficient, cost-effective means of inhibiting discoloration. In addition, there is a need for an efficient means to deactivate catalytic residues, without significantly altering the processing properties and/or the final chemical and physical properties of the original resin formulation. Additionally, the material added to deactivate catalyst residues and inhibit color formation should not increase the taste or odor imparted by the polymer to foodstuffs packaged using the polymer. Accordingly, the present invention meets such needs by providing an improved process for producing polyolefin-based pellets in which one or more additives are blended into a polyolefin resin, and water is added prior to pelletizing, in amount sufficient to improve the color of the resulting pellets. The polyolefins and additives can be blended in an extruder prior to pellitizing, or prior to an injection molding, film extrusion, or other melt processes. Water added in an amount from 100 ppm to 1000 ppm, and particularly, in an amount from 200 ppm to 300ppm, has been shown to be sufficient to reduce the color of the resulting pellets. Preferably, one or more additives do not include aliphatic hydroxyl containing compounds. Water may be added directly to a mixer system, such as an extruder or pelleter. Here, the water may be added directly to the molten resin, for example, at an injection port along an extruder, or may be added into a feed throat with the resin and other additives. In addition, water may be added as a vapor or mist to a resin bin, for the example, the introduction of steam into a granular resin purge bin, upstream from an extruder or pelleter. Moreover, water may be added as a continuous on-line addition to a mixer system, such as an extruder or pelleter, or as a continuous on-line addition to a resin purge bin. The invention is particularly useful in a non- vented, or unvented mixer system, such as an unvented extruder. When water is directly added to a pelleter, it is preferred that a slip stream be obtained from water already being used in the pelletization process. In this way, no requalification of the resin would be needed for customer use. The current invention has particular utility for reducing resin color when the polymerization catalyst contains chromium compounds, and particularly, both chromium and titanium compounds. When a water treatment is used, the final resin product maintains properties of the non-treated resin, including maintaining the same taste and odor standards. Water can be used without the addition of an organic base, such as, an amine, a metal salt of a carboxylic acid, a trialkyl phosphate or a metal alkoxide. Water can also be used without the addition of radical intiators or curing agents. The current invention appears to have particular utility when one or more of the additives is a phenolic compound. Such compounds include Irganox™ 1010 (tetrakis(methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate) methane, CAS 006683-19-8), a commonly used antioxidant. Although not intending to be bound by theory, it is hypothesized that phenolic compounds may react with residual catalyst residues to form a precursor compound. This precursor compound then may react with oxygen in the pelletization process to form a colored species. Thus it is theorized that adding a material capable of either excluding oxygen or capable of deactivating the catalyst residue, so that it does not react with the phenolic compounds, will result in a resin having a lower yellowing index. For polymer systems that incorporate a hindered phenol, such as Irganox™ 1010, it is hypothesized that discoloration in the final resin product is due to the oxidation of the hindered phenol, catalyzed by polymerization catalyst residues (Ti and/or Cr). Accordingly, the rate of discoloration is a function of the phenol concentration, catalyst residue, temperature, and oxygen in the pelletization step. Although the polymerization of the polymer should terminate when a particle of resin, containing catalyst, leaves the reactor, it is hypothesized that catalyst remaining in these resin particles may still be "active." Oxygen (50 - 100 ppb) may be added as a catalyst poison in the reactor to control molecular weight; however, the catalyst may still be "active" in terms of polymer oxidation reactions. Catalyst residues can be deactivated via incorporation of an appropriate catalyst kill. The variation seen in resin color during polymer pelletizing is mainly due to the variation of oxygen ingression and extruder temperature. Although not intending to be bound by theory, a proposed reaction mechanism for color formation and prevention is suggested in the following scheme and description: MR + PhOH ^ M-OPh (1) M-OPh + O2 -> M-O-O-Ph -> color (fast) (2) PhOH + 02 ^ color (slow) (3) MR + ROH -^ M-OR -_ no color (4) MR = catalyst residue (Ti &/or Cr) PhOH = Irganox 1010 ROH = hydroxyl-containing species In this mechanism, Irganox™ 1010 forms a complex, M-OPh (discoloration precursor), with the catalyst residue (1 ) which can then be oxidized to form a color species (2). The oxidation of Irganox™ 1010 alone, (3), also results in discoloration, however at a slower rate than reaction (2). A catalyst kill, ROH, can complex with the catalyst residue (4), thereby competing with the Irganox 1010™ - catalyst residue reaction (1), resulting in less discoloration precursors available for oxidation (2). Thus, the hydroxyl containing species are able to deactivate chromium and titanium residues derived from the chromium based catalysts, as discussed above, without significant formation of acidic species, which can further react within the resin. It has been found that the addition of a hydroxyl-containing species at some point in the pelletization process reduces the color of the resulting resin, as evidenced by a lower YI. Preferred hydroxyl-containing species include water, alcohols and glycols. Water is the most preferred hydroxyl-containing species. It should be understood that the water may be in the form of liquid or gas, depending on where in the process the water is added. However, it has been found that a relatively low amount of water (100-1000 ppm) added directly into a resin mixer is very effective in reducing the Yellowness Index of the resulting pellets. Water can be used without the addition of other hydroxyl-containing species, such as aliphatic alcohols and glycols. It is believed that water can be used efficiently, even at low levels (100-1000 ppm), to deactivate catalyst residues, without leaving a significant amount of residual water behind in the final resin. It has been found that the internal pressures used in a high pressure, melt process, such as an extrusion, have been sufficient to force the diffusion of a sufficient amount of the water into the resin, where, once within the resin, it is believed the water can then complex with catalytic residues. Thus, water provides a balance of reactivity and diffusion that results in a resin with improved Yellowness Index, without an excess of residual water remaining in the resin. Thus, it has been found that water provides an efficient means of deactivating catalyst residuals, even at relatively low levels. Thus, the chemical and physical properties of the resin are substantially preserved with the use of water. Moreover, increased levels of water (> 1000 ppm) may result in extensive hydrolysis of additives, for example, the hydrolysis of Irganox™ 1010, which would deteriorate the oxidative stability of the final resin product. In addition, too much residual water remaining in the resin melt may result in the formation of a significant amount of pockets, such as, for example, water vapor and/or air pockets, in the extrudate, also known as "foamy pellets." These foamy pellets, or pellets with internal voids, can cause defects in the final end-use products, that is, gels or blow holes. Also, trapped air within the resin would further decrease the oxidative stability of the final resin product. Therefore, it is desired to reduce the level of water in a formulated resin to minimize the potential for hydrolysis and oxidative products. It has been found that the hydrolysis of Irganox™ 1010 can result in the formation of HCA (3-(3,5-di-tert-butyl-4-hydroxyphenyl) propanoic acid), which can create processing difficulties, such as, residual collection of this compound in mold vents, and which can decrease properties in the final resin, such as taste and odor properties. Water provides an additional advantage in that it is nontoxic, and appears not to result in significant amounts of reaction by-products. Thus, water does not diminish the biocompatibility or the original resin formulation. Moreover, water provides improved resin color stability, without adversely affecting taste and odor properties. Also, as discussed above, the addition of water, in an amount sufficient to affect the Yellowness Index, does not adversely affect the original processing parameters of the original base resin or the originally formulated resin. Moreover, resin color is improved, without altering the resin composition to such an extent, as to require a requalification of the resin for its particular use. Water is preferably used without the addition of aliphatic hydroxyl containing compounds, and is preferably used without the addition of epoxide containing compounds. In addition, water is preferably used without other additives that significantly impact the odor and taste properties of the final resin product, such as thiol containing compounds. Water can also be used without the addition of sterates. The water should be added in an amount sufficient to improve the color of the resulting pellets. The color of the pellets can conveniently be determined, for example, by measuring the Yellowness Index according to the procedure set out in ASTM D6290. Preferred ranges for water include 100 to 1000 ppm amounts, including all individual amounts and subranges between 100 and 1000 ppm (including endpoints where required). More preferably water is added in an amount of at least 200 ppm, more preferably 300 ppm, and most preferably at least about 350 ppm, and less than 900 ppm, and more preferably less than 800 ppm. The amount of water is based on the weight of the resin. Too little water results in more color, whereas too much water may result in the additive being entrapped in the resin, which may result in foamy pellets. This can occur in vented and unvented extrusion systems, as venting in a large commercial extruder (that is, one which processes 30-50 tons of polymer per hour) are largely ineffective due to the very small surface area available for venting. Thus, it is important to use the minimum, useful amount of water to achieve the technical effect of color improvement (yellowness reduction) without initiating other processing problems. For a chromium based catalyst system, catalyst residues are typically present in an amount from 2-10 ppm (total Cr and Ti, by weight). When added directly to a resin mixer, for example in an extrusion or pelletization process, water may be added at any convenient place in the mixing system. Suitable places are indicated by the number 2 in Figure 1. It should be understood that the addition sites indicated in Figure 1 are examples of suitable sites, and are not meant to be exhaustive. It should also be understood that although several addition sites, are indicated in Figure 1, it is expected that in the normal mode of operation, only one site will be used, although this is not mandatory. Water may be introduced into the mixing system together with the resin and any additives prior to the polymer becoming molten, or may be added directly to a molten resin. When water is added prior to a mixer unit, for example, prior to the feed throat of an extruder, it is most advantageously added as part of the purge gas which strips off unpolymerized hydrocarbons from the granular polymer. While the water may be added at any point in the purging process, it is particularly preferred to add the water as a vapor dispersed in the purge gas at the base of a counter-current purge bin. Suitable bins are described in US4372758, and a particular example is described in US4758654, both references are incorporated, in their entirety, by reference. When the water is added in this manner, additional amounts may also be added in the extrusion process. The resins for use in the present invention are all compounded with one or more additives in a mixing device. Any such mixing device known in the art may be used for this process, with a melt extruder being preferred. Figure 1 shows a schematic of how a mixing device 3 might be positioned in a suitable line pelletization process. In that Figure, the number 1 indicates a suitable place for the addition of resin pellets together with additives, number 2 indicates suitable places for the addition of water, number 3 indicates a mixing device and number 4 indicates the compounded resin pellets resulting from the pelletization process. Figure 1 provides that water may also be introduced into the mixer, as a premix with the resin, at position 1. The unformulated resin together with the desired additives, and/or any other resin to be blended into the final material, will be mixed together using devices, such as shown in Figure 1, and formed into pellets. The choice and amount of additives used, is not limited by the present invention. Such additives include, but are not limited to, antioxidants, ultraviolet light absorbers, antistatic agents, pigments, dyes, nucleating agents, fillers slip agents, fire retardants, plasticizers, processing aids, lubricants, stabilizers, smoke inhibitors, viscosity control agents, crosslinking agents, catalysts, boosters, tackifiers and anti-blocking agents. The following examples are provided for the purpose of illustrating the invention, and are not to be construed as limiting the scope of the invention. EXPERIMENTAL EXAMPLES Experiment 1 - Extrusion of Granular Resin at 250°C under Nitrogen In each of the examples below, the base resin was a high density polyethylene resin made using a titanated Cr catalyst, and having a density of 0.96 g/cm³, as determined by ASTM D-792, and a melt index of 0.8 g/10 minutes, as determined by ASTM D 1238 (190 °C and 2.16 kg (also known as I₂)). Other examples of catalysts of this type are also disclosed in WO2004094489, or equivalent US Publication No. 20041104, the entire contents of which are incorporated by reference. This resin was combined with the various additives listed in Table 1 (additional information was added to Table 1 since filing of the provisional application), and extruded at 250°C under N . The extruder was not vented. The hydroxyl-containing species was added to the mixing system in the amount indicated in Table 1. For samples 3 and 4, water was added via injection into mixer (feed throat) with the granular resin and additives. For example 5, water was added as steam in a purge bin, upstream of the mixer used in this process The color of the resulting pellets was then determined via the procedure outlined in ASTM D6290, using a BYK-Gardner Colorview instrument. A positive number refers to a resin measuring a higher yellowness, and a more negative number is indicative of a less yellow resin. The resulting YI (Yellow Index) measurements are reported in Table 1.

Table 1 : YI Results using Polyethylene, Prepared by Cr Catalyst, as a Base Resin

Experiment 2 - Extrusion of Granular Resin at 195 - 205°C The polyolefin of Experiment 1 was extruded in the presence of water. A second series of samples was prepared in which the water was injected, as a liquid, into the melt pump suction. A nominal 64 Flow Index resin was produced during this series of experiments. Antioxidant (Irganox™ 1010) was added at an aim rate of 350-400 ppm. All samples in Table 2 were produced with an extrusion rate of 40-45 metric tons/hour of operation. Resin temperature at melt pump suction averaged between 195 and 205°C. In . general, no vent was utilized. Table 2 Sample Water Level N? in vent zone (kg hr) YI A 0 (control) 0-no vent -1.6 B 100 ppm 0-no vent -3.93 C 250 ppm 0-no vent -4.64 D 454 ppm 0-no vent -5.6 E 750 ppm 0-no vent -5.26 F 1000 ppm 48 -5.14 G 500 ppm 48 -5.27 H 0 (control) 0-no vent -2.13 Experiment 3 - Extrusion of Base Resin (granular form) (a), 250°C under N?. To mimic the discoloration during a plant pelletization process, the resin color change before and after one pass extrusion at 250°C under N₂ was measured. Materials, sample preparation and testing procedures Materials: Resin FX, is a polyethylene-α-olefin granular resin, based on ethylene with hexene as the comonomer. The FX resin was obtained, and stored in a polyethylene lined drum without nitrogen purge.

Antioxidants (Irganox™ 1010 and Irgafos™ 168 (a phosphite)), supplied by Ciba Specialty Chemicals, were used as additives. A masterbatch containing 10 wt% water and the FX resin was prepared. Portions of this masterbatch were mixed with dry FX resin to obtain resin samples containing 200, 500 and 1000 ppm water. A Thermo Haake Polylab System with a Bench 300p single screw extruder with a Rod Die 3/1-0 was used in the pelletzation process. This extruder was not vented. The blended ingredients were extruded as follows.

1) The ingredients were weighed out into a 3 gallon plastic bag, and the bag was purged with nitrogen 5 or 6 times, and then a slight purge was allowed to inflate the bag and bleed offfor 2 hours.

2) The extruder was equipped with a nitrogen purge located in the throat, just above the screw. A nitrogen flow of 20 SCFH, sufficient to fluidized the granular resin, when put into the hopper, was used to keep air out of system.

3) Extrusion conditions are given in Table 3 below.

4) The dry blends were carefully poured into the hopper, with the nitrogen flow, to avoid air incorporation. 5) The resin was strand chopped in to pellets, and saved for further experimentation. Table 3: Lab Experiment Extrusion Conditions

The effect of antioxidant and water on the discoloration of the resin during pelletization is shown in Table 4 below. A comparison of the samples with and without Irganox™ 1010, indicates that the Irganox™ 1010 significantly increases the YI. This data supports the theory that color forms as a result of a reaction (oxidation) involving the Irganox™ 1010. The addition of water is shown to improve the YI in samples containing Irganox™ 1010.

Table 4: Effect of additives on the Yellow Index - Extrusion at 250°C

It should be noted that the effect of the invention is still observed, even though the original granular resin had been stored in air, as opposed to the preferred method of contacting the resin with water, before any exposure to oxygen. Experiment 4 - Extrusion of Pellets (a), 200°C in Air to Mimic Blow Molding Process To mimic the discoloration due to a blow molding processing, the color change of extruded pellets was measured before and after a one pass extrusion at 200°C, in air. Materials, sample preparation and testing procedures Materials: DH pellets (polyethylene-α-olefin resin based on ethylene, with hexene comonomer) obtained from The Dow Chemical Company, and based upon the feedstock FX resin, used in Experiment 3, described above. The Antioxidants (Irganox™ 1010 and hrgafos™ 168) were used as additives. The DH pellets were extruded from granular resin containing 100-1000 ppm, approx. 350 ppm water. These pellets were extruded at 200°C, in air, in the same extruder used in Experiment 3. The extruder conditions are shown in Table 5.

The samples produced with the water addition showed better initial and final color over the baseline control. The results are shown in Table 6.

Table 6: Effect of water addition on discoloration during re-extrusion at 200°C

Experiment 5 - Discoloration Reduction Study on Resin Samples from a Gas Phase Production Plant Process A general gas phase production process is shown in Figure 2. Materials: FX resin, as described above. Antioxidants (Irganox™ 1010 and Irgafos™ 168) were used as additives. Water was injected into the mixer feed throat for samples C, E and G-I. For sample B, water was added, as steam, to a purge bin, upstream from the pelleter. Water was injected into the mixer feed throat of the extruder (not vented) via a slip stream of demineralized water to one of the liquid injection ports on the mixer. The water flow was adjusted and calibrated initially to 6 kg/hr. The amount of water added, was estimated to be around 200 ppm (mixer rates 30t/hr). An immediate improvement in YI (-2 to -4.5) was observed. As the amount of water, injected into the mixer, was increased incrementally (water level added estimated between 300 - 1000 ppm), the resins had YI values near - 4.0, and eventually near - 5.0. No detrimental effects (that is, foamy pellets) were observed at these levels of water addition. Table 7 lists average YI values for this study. The YI distribution profiles before and after water addition are shown in Figures 3 and 4. Figure 5 shows the comparison of YI distributions using same index scaling.

Table 7: The Effect Water Injection on Resin YI

Experiment 6 - Taste and Odor Evaluations on the DH Resins Produced with Water Addition Antioxidants (Irgariox™ 1010 and Irgafos™ 168) were used as additives. Water was injected into the mixer feed throat at an estimated amount of 350 - 500 ppm. Evaluations of taste and odor properties of samples were conducted via the standard PO&E Organoleptic Lab protocol summarized below.

Sample Preparation Taste Test medium Ozarka brand drinking water (900 ml) Sample Pellets (10 grams) Contact time 16 hours at room temperature Serving temperature Room temperature

Odor Test medium Air in 16 ounce glass bottle Sample Pellets (10 grams) Contact time 16 hours at 60°C Serving temperature Room temperature

Test Method

Water rinses were used between taste samples. Panelists smelled the backs of their hands between odor samples. The replicate set of samples provided a measure of test reproducibility. Number of panelists Taste: 20 Odor: 20 Test type Ranking and Hedonic Acceptability Scale Sample codes Random 3 digit Test design Balanced block Fatigue minimization Taste: Ozarka water Odor: Smell back of hand Replicate served Yes Taste and odor evaluations showed no adverse taste and no adverse odor in the water injection production samples. Additionally, the GC Mass Spec analyses for aldehydes and ketones show all samples to be odorless (< 4.0 ppb). The color improvements implemented via this project appear to have no adverse effect on the taste and odor of the resin.

Experiment 7 - Effect of the Addition of Steam on Yellowness Index A high density polyethylene resin was obtained from a production line, in which steam was added to the precursor granular resin, at an estimated amount of 200-460 ppm steam, based on a 65 MPPH (thousand pounds per hour (lib = 0.454 kg)) production scale. Prior to the addition of steam, the resin sample had a YI of about -0.5. After the addition of steam, the average YI of sixteen resin samples was about -1.9.

Claims

WHAT IS CLAIMED IS:

Claim 1 : A process for producing polyolefin-based pellets, comprising blending one or more additives into a polyolefin resin, and adding water, prior to pelletizing, in an amount sufficient to improve the color of the resulting pellets, and wherein the polyolefin was prepared in the presence of a chromium catalyst system, and wherein water is added in an amount from 100 ppm to 1000 ppm.

Claim 2: The process of Claim 1, wherein the water is added in an amount from 200 to 300 ppm.

Claim 3: The process of Claim 1, wherein the one or more additives do not include aliphatic hydroxyl compounds.

Claim 4: The process of Claim 1, wherein the one or more additives include a hindered phenolic antioxidant.

Claim 5: The process of Claim 1, wherein the water is added into a mixer system.

Claim 6: The process of Claim 5, wherein the water is added to the feeder of an unvented mixer system.

Claim 7: The process of Claim 6, wherein the water is added at the same point in the mixer system as a hindered phenolic antioxidant.

Claim 8: The process of Claim 7, wherein the hindered phenolic antioxidant is IRGANOX™ 1010. Claim 9: The process of Claim 5, wherein a slip stream from this process is used as the source of the water, added to the mixer system.

Claim 10: The process of Claim 1 , wherein the water is added as a vapor or mist in a granular resin purge bin.

Claim 11: The process of Claim 1, wherein the water is added as a continuous on-line addition to a mixer system or to a purge bin.

Claim 12: The process of Claim 1, wherein the one or more additives do not include epoxide containing compounds.

Claim 13: The process of Claim 1, wherein the one or more additives do not include sulfur containing compounds.

Claim 14: A resin pellet made by the process of Claim 1.

Claim 15: A resin pellet made by the process of Claim 2.

Claim 16: A resin pellet made by the process of Claim 7.

Claim 17: The resin pellet of Claim 14, wherein the pellet is characterized by having a yellowing index at least one unit lower, according to the method set out in ASTM D6290, than a similar pellet made without the addition of water.

Claim 18: The resin pellet of Claim 14, wherein the pellet is characterized by having a yellowing index at least two units lower, according to the method set out in ASTM D6290, than a similar pellet made without the addition of water. Claim 19: A process for reducing the color in polyolefin-based pellets, comprising blending one or more additives into a polyolefin resin, and adding water, prior to pelletizing, in an amount sufficient to improve the color of the resulting pellets, and wherein the polyolefin was prepared in the presence of a chromium catalyst system, and wherein the water is added in an amount from 100 ppm to 1000 ppm.

Claim 20: The process of Claim 19, wherein the water is added in an amount from 200 to 300 ppm.