Development of a Novel Super-active Ziegler-Natta Polyethylene Catalyst: Study on Structure, Performance and Application

In this research, the synthesis process of a novel super-active catalyst, named SAC-510 is investigated at experimental scale. The authors compared the analyzed data against those derived from a popular catalyst, Finix-112 , and several other available alternatives. The data indicated that SAC-510 catalyst derived the optimal activity from its spherical particles. Titanium and other chemical elements were less uniformly distributed in SAC-510 than in Finix-112 samples, with the mean particle size being slightly larger than that found in Finix-112 . The pores’ dispersion and sizes in SAC-510 were not as uniformly distributed as those in Finix-112 catalyst. Lastly, both SAC-510 and Finix-112 catalysts were equally adaptable for use in high-density polyethylene pipes (grade 100). Compared to other commercially available catalysts, the major advantages of SAC-510 are the economical use of hydrogen and monomers, and low purging of its valuable gases during the polymerization process. The results obtained are as follows: the increase in oxidative induction time efficiency with SAC catalyst compared to Finix catalyst by 5.8%, activity by 35.7%, hydrogen responsibility by 24.39%, 1-butene consumption by 8.38% and triethylaluminium consumption by 27.27%.


INTRODUCTION 1
Society, in general, and the industry, in particular, have significantly included plastics for changing different materials.High density polyethylene (HDPE) is a semicrystalline polymer widely used for the transport of drinking water supplies (1,2).These are often exposed to different weathering conditions, for example in outdoor applications or when the freshly extruded pipes are stored in the open before their installation (3).Polyethylene (PE) is known as one of the most commonly-employed *Corresponding Author Email: Hossein.Mazaheri@iau.ac.ir (H.Mazaheri) thermoplastic polymer, which have demonstrated its brilliant advantages in different industrial-based activities like wires/cables insulation, agricultural mulch, membrane-based gas separation and medical engineering due to its ease of installation, absence of corrosionrelated problems, low cost and projected 50 to 100-year service life (4)(5)(6).Ziegler-Natta catalyst is a commonlyused catalyst in a bimodal slurry system for HDPE production.This promising catalyst is made based on the mixture of different chemical components that are prominently applied in the synthesis process of 1-alkenes polymers (i.e., alpha-olefins) (7,8).Titanium, and to a lesser extent, chrome and vanadium are regarded as important metallic elements in Ziegler-Natta catalysts (9).They have enabled scientists to polymerize many organic compounds, such as alkenes and dienes with superior activity and selectivity, under a wide variety of experimental conditions.Since their advent, these catalysts have also played a significant role in the development of valuable products, highly needed in petrochemical and plastic industries (9).In this context, the role of titanium and magnesium is particularly well known in the production and use of polyethylene products (10).Currently, scientists are conducting much research with a focus on the activity, particle morphology and distribution, hydrogen response, and performance of catalysts and co-catalysts (11)(12)(13).
Among the commonly-applied polymerization processes, ethylene slurry has attracted much scientific attention.In addition to improving the activity of catalysts, controlling the particle size and its distribution in polyethylene products are of significant importance (14)(15)(16)(17).In the recent published research articles, various aspects of catalysts and their interactions with cocatalysts and intermediate metals have been investigated.These include, a) controlling titanium oxidation; b) effect of external and internal donors (18,19); c) the role of cocatalysts (20); d) effect of newly developed catalysts (21,22); and, e) synthetic and modelling studies in the polymerization field (23)(24)(25)(26).In addition to the molecular structures of catalysts, the polymerization and industrial process have an essential role in the production of polymers with a variety of properties, and promoting scientific research on these products (27)(28)(29)(30).During 1980's and 1990's, a series of patented methods on the preparation of new Ziegler-Natta catalysts has been introduced (31,32).Later studies proposed and examined the role of titanium alkoxide to replace the use of Titanium tetrachloride (TiCl4).In these products at least one alkoxide ligand connects Ti (OR) 4 to tetra-titanium alkoxide.A good example of this subject is the polymerization of olefin compounds (33)(34)(35).
To introduce a wide range of molecular weights in manufacturing resins, Ziegler-Natta catalysts require different reactors that operate under specific conditions of temperature, monomers, and hydrogen concentration, used in series or singly, which involve high costs (17,(36)(37)(38).In this context, there exists an urgent need for new catalysts with better performance to produce bimodal resins with added market value.For this purpose, two methods are popular for the preparation of the main components of the catalyst.The first method involves a solid carrier, consisting of a magnesium dihalide reaction with titanium or vanadium in a suspended hexane environment.This method provides the components of the catalyst, although it is difficult to control the polyethylene product, the particle size, and its distribution (39).In the second method, a magnesium compound, such as magnesium dichloride, is dissolved in a solvent, so that homogeneous solid particles of magnesium or titanium precipitate after adding a titanium compound.This process can provide the main component of the catalyst upon its solid processing with another liquid titanium compound and a co-catalyst (40).
This study aims to synthesize and manufacture a novel and efficient super-active Ziegler-Natta catalyst, named SAC-510 toward the production of a new generation of high-density polyethylene pipes.Subsequently, the authors analyzed the particle morphology and its performance, compared to a popular catalyst, Finix-112, and several other commercially available alternatives.We have obtained some promising results, which are presented and discussed in the present article.

1. Materials
All of the materials for the polymerization processes of both the support catalyst, and the final super-active catalyst (SAC-510) were obtained from the Shazand Petrochemical Technology Company (Arak, Iran) and Pooyesh Co. (Tehran, Iran).Hexane (C6H14) was utilized as the solvent and copolymerization agent.Ethylene was used as the main feed for the polymerization process.Lastly, 1-butylene and hydrogen were used as the components required for the polymerization.Triethylaluminium (97%) was purchased from Fluka Chemie GmbH (Buchs, Switzerland).Oxygen 4.5 (purity 99.995%) and nitrogen 5.0 (99.999% purity) were obtained from the Shazand Petrochemical company .

2 . Equipment 2 .1 . Glove Box
To store and preserve the catalysts and co-catalysts in this study, we used a glove box unit (Cleatech; Orange, CA, USA).This unit prevented the active sites in catalysts and sensitive chemicals from getting contaminated and oxidized, under a nitrogen atmosphere .

Scanning Electron Microscopy/ Energy Dispersive X-ray Analysis (SEM/EDX)
The surface morphology of the catalyst was examined by Scanning Electron Microscope (SEM) (model JSM-5910, JEOL Inc., Peabody, MA, USA) under high vacuum microprobe.This microscope provided scanned images, analyzed the X-ray emitted from the particles, and determined the accurate weight and percentage of the elements contained in the samples [40].

3. Malvern Analyzer
In addition to the SEM analysis, the particle sizes and molecules contained in the samples were also measured on a Malvern analyzer (Entegris; Dresden, Germany).The images obtained from this analyzer were based on the principle of dynamic light scattering .

4 Buchi Reactor
Made of stainless steel with a volume of 1 liter is used for slurry polymerization.The volume of the reactor jacket is 300ml.Addition of catalyst and co-catalyst is by injection under pressure .

5. Gel Permeation Chromatography (GPC)
To analyze the different particle species of large molecular sizes, such as polymers, we measured the particle's molecular weights on a gel permeation chromatography unit (GPC-150-II+IR5; Cambridge Polymer Group, Inc.; Woburn, MA, USA) (41) .

6. Differential Scanning Calorimeter (DSC)
The objective of the test is to represent the duration in time of the heat stabilizer until its deterioration through oxygen and heat.The period of time from the switching over to oxygen until the onset of the oxidation reaction (point of intersection of the basis line with the tangent) is referred to as the induction period (Oxidation Induction Time, OIT).A long induction period shows good stabilization and vice versa.The described test method is carried out in dependence on DIN EN728 and we used Differential Scanning Calorimeter unit (Mettler-Toledo DSC 822e.)(42).

7. Huber Circulator
We utilized a Huber circulating bath (Ministat CC3 Huber UK; Derbyshire, UK) to control the temperature within the reactor's jacket, ranging from -50 °C to +350 °C .
The required materials and laboratory steps to prepare the 4th generation of Ziegler-Natta super-active catalyst are shown in Flowchart 1:

3. 1. 1. Magnesium Ethoxide Powder
This material was necessary to make the base product.The construction of super active Ziegler-Natta catalysts for ethylene polymerization requires substrates for the stability of the active centers of the catalyst.Metal halides and oxides have been proposed as suitable bases for catalysts, and in the meantime, magnesium ethoxide is very suitable for making the super active Ziegler-Natta catalyst.To prepare the super active catalyst base, commercial magnesium ethoxide (Mg(OC2H5)) is first prepared in pure form with the conditions presented in Table 1.The average particle diameter of the primary magnesium ethoxide powder is 500 microns, so that more than 90% of the particles have a diameter in the range of 200 microns to 1200 microns.Titanium sits better on the substrate when the size of the primary powder particles is reduced and their distribution is uniform.Magnesium ethoxide with a diameter of about 500 microns forms a suspension in an inert saturated hydrocarbon after mechanical crushing by a jet mill to a size of about 8 microns using a Jet pulverizer (Jet Mills; Moorestown, NJ, USA).Table 1 presents aims to present different features of the magnesium ethoxide for the catalyst bed.

3. 1. Magnesium Ethoxide Suspension Gel
The magnesium ethoxide powder was then suspended in a neutral, aliphatic hydrocarbon until the mixture was ready to convert into gel.Ideal hydrocarbons for this purpose are butane, hexane, cyclohexane, heptane, isooctane, and several others including toluene and xylene.Also, hydrogenated oils devoid of oxygen and sulphur would be appropriate.About 200 grams of the sample is taken under N2 atmosphere and injected into a 5-liter reactor containing two liters of hexane.Then carbon tetrachloride is added and after mixing(100rpm), the temperature of the reactor is increased to about 60 degrees, and this process is allowed to continue for 24 hours.In this case, the solvent penetrates into the crystal layers and causes the particles to take a gel-like shape.If the gelling process has progressed correctly, after stopping the mixer, it takes about 30 minutes for the particles to settle.
Injecting 7 ml of titanium tetrachloride into the gel produced at 80 degrees Celsius to place titanium on the gel bed.

3. 1. 3. Catalyst Assisted Preactivation
The choice of co-catalyst has a great influence on the performance of titanium-based Ziegler Nata catalysts.Catalyst assistance affects activity, molecular weight and molecular weight distribution.By increasing the amount of catalyst aid, the activity of all active centers decreases to almost one level.In order to produce active sites, which Mr. Kissin divided into five types by fluorescent distribution, the catalyst must be placed in the vicinity of triethyl aluminum (TEA) for a certain period of time.

3. 1. 4. Determination Of The Catalyst Content
A 10ml aliquot of the catalyst was removed from its container, using a syringe, and poured in a small glass dish with known weight.After the sample was cooled down, the weight of the empty dish is deducted from the total weight of the dish containing the catalyst 10ml aliquot.The net weight of the catalyst sample varies depending on the solvents, such as hexane, used for the polymerization processes.

3. 1. 5. Polymerization
Prior to this step, the reactor was purged by running nitrogen (≤0.1 bar) through it at 100°C for 60 minutes.Subsequently, the oil circulator pump of the reactor was set at 4500 rpm, and the temperature reset to 60°C.At this temperature, the reactor was cleared with ethylene gas three times at 3-bar pressure.Then, 500 ml hexane at 0.1 bar, and 400 mg/ml triethylaluminium (TEAL) were injected into the reactor and the agitator turned on at 200 rpm.The TEAL was mainly used for the purpose of removing contaminations from the solvent and activation of sites in the catalyst.After 30 minutes from the injection of TEAL, 1-butylene was injected to the reactor at a set amount, and allowed to mix with TEAL and the solvent for 15 minutes with the agitator set at 200rpm.During a 30-min preactivation time, the concentration of catalyst diluted in hexane reached 2mg/ml.
After determining the reactor's pressure at optimal temperature, the catalyst was injected in the reactor under pressure.Next, the reactor's software program was set to reaction mode and the polymerization synthesis plots being printed.During this process, the reaction temperature was closely monitored at 83°C (±1°C) while ethylene was being injected into the reactor at 8 bar.The estimated time for the polymerization of ethylene with 1butylene in the presence of the catalyst SAC-510 was approximately 180 minutes.At the completion of this final step, the reactor's software program was set to "Reaction Off".This terminated the ethylene entry into the reactor and the circulator's temperature was set at 60°C.After the reactor cooled down, the lid was opened, the chamber was cleaned thoroughly with nitrogen, and the catalyst and solvents residues were wiped out.

3. Surface Area Analysis (SSA)
The surface area and pore size analysis of the catalyst were carried out by Surface Area Analyzer (Quanta chrome Nova Station A), using nitrogen adsorption method.The sample was initially degassed at 200 ºC for 4 h.Surface area of the catalysts was calculated from the nitrogen adsorption isotherm data, using Brunauer, Emmett and Teller (BET) equation.The pore size and pore volume of the catalysts were calculated from N2 adsorption isotherm data with the help of Barrett, Joyner, Halenda (BJH) model (44).

1. Particles Morphology
Understanding the morphology of micro particles can assist us predict their mass transfer and heat as they go through polymerization processes (45).Initially, monomers penetrate into the catalyst's pores and start numerous active reaction sites, forming the basic polymer particles and clumps as they combine with the catalyst and continue to grow.Later, layers of the polymer are deposited on the catalyst's spherical macro particles, leading to the final polymerized product.Thus, examining the particles reflects the true physical nature of the combined polymer and catalyst (46).Results from this study's tests and experiments are presented below followed by the pertinent discussion under each subsection.

2. Energy Distribution Analyses of the Catalysts' Powders
Table 2 represents the results of energy distribution in the two powdered catalysts, Finix-112 and SAC-510, under scanning X-ray analysis (SEM-EDXA).As shown in Table 2, four and three zones were examined, respectively, under SEM-EDXA using the powdered samples of Finix-112 and SAC-510 catalysts.The results indicated that there were relatively uniform percentages of the chemical elements, especially titanium, in the Finix-112 samples.However, much less uniformity was observed in the SAC-510 samples.Figure 1 shows the electron microscopic images of powdered Finix-112 and SAC-510 catalysts, and the differences in the distribution of the particles for both samples.
Evidently, the differences in the percentage and uniformity of the elemental distributions in both catalysts' samples arise from their chemical nature and formulation.Based on the images illustrated in Figure 1, it is perceived that the presence of smaller catalyst particles makes the mobility of larger particles easier, thereby facilitates the polymerization reaction.A major advantage of SEM-EDXA is its better capability of determining the compositional elements in powdered catalysts than that achieved by either XRF or ICP method.The SEM-EDXA method is able to recognize such elements as oxygen, carbon and halogen.Also, it  provides the distribution patterns and variations in the concentration of elements in different areas of various samples at large magnification.

2. Particle Size Distribution -Malvern's Method
Figure 2 represents the particles' volume of distribution and their sizes in the micron scale for both catalysts, based on Malvern's method.As seen in Figure 2, the particles in both catalysts occurred similarly in one uniform population each (range: 2-11 µm vs 3-30 µm).The single peak (50%) for Finix-112 catalyst occurred approximately at 6.8µ while that of SAC-510 was about 8.6 µm.The data for the particle size distribution in both groups of catalysts are summarized in listed in Tables 3   and 4.
The data in Tables 3 and 4 indicate that the average particle size for SAC-510 catalyst was slightly larger than that of Finix-112.In this context, 10% of the SAC-510     particles fell within 3.9 µm in size while the same percentage of Finix-112 particles had a 2.5 µm size.Further, about 90% of the particles in SAC-510 group had a size of 16.2 µm while those in Finix-112 group were about 12.1 µm in size.The mechanical measurement of catalysts' particle size and their distribution are performed broadly in industries, which is a critical parameter for manufacturing numerous products.In this respect, Malvern is a highly accurate method for measuring particle sizes ranging from low nanometer to several millimeters.

BET Test
Using the popular (BET) test ( 23), we analyzed the features and dimensions of the pores in both Finix-112 and SAC-510 catalysts.The data from this analysis are presented in Table 5.The overall results obtained from BET test revealed that the specific area for the pores in SAC-510 was 9% less than that of Finix-112.Also, the mean dimension and volume of the pores in SAC-510 catalyst were greater than those determined for Finix-112 catalyst.
Considering the above data, it can be argued that the dimensional features of the porosity in SAC-510, compared to those in Finix-112, decreases the ability of this catalyst to respond to physical forces.Also, it is likely that larger pores may be deeper in SAC-510 with smaller lateral surfaces, which further contribute to its lower tolerance against shocks, vibrations and other destructive forces.

4. Barrett-Joyner-Halenda Test
Also, we used another popular test, Barrett-Joyner-Halenda (BJH), to determine the volume and distribution of pores in the catalysts (23).The data from our BJH test on both catalysts are presented in Table 6.Specifically, Table 6 reflects accurate information about three important variables for each of the two catalysts: a), the pore volume (Vp); b) the pore peak radius (rp); and c) specific pore area (ap).Figure 3 schematically presents the scatter diagram of pores and dimensions of super active SAC-510 and Finix-112 catalysts.6, the pore volume in catalyst SAC-510 was greater than that of Finix-112.This had an opposite effect on the specific surface and reduced this variable.Also as shown in Figure 3, the dVp/dRp ratios for SAC-510 indicate that the dispersion of the pores and sizes did not occur as uniformly as those found for Finix-112.
To assess the validity of the findings obtained through the BET analysis, we used BJH test (33).The advantage of BJH test is that it measures the volume and distribution of the pores while BET test provides a reliable measure of the areas associated with the solid material in the catalysts.Also, BJH test assists in obtaining approximate data regarding the absorption and release of isotherms, the pores' dimension, and their distribution within the catalysts.

5. Gel Permeation Chromatography
GPC-IR has been used to characterize the copolymer and the consequences are given in Figure 4 it is observed that the The polymer properties are determined by the characteristics of the polymer including molecular weight, its distribution, and degree of branching.The breath of the molecular weight distribution, Mw/Mn, additionally affects the process ability of the polymer.The degree of short chain branching strongly influences a few variables together with crystallinity and density, which in flip determines the last properties of the material.The slurry polymerization processes give resins, which have great mechanical properties preserving wonderful process ability.It can be found out via bimodal high molecular weight HDPE.The low molecular weight component product in a single reactor offers suitable process ability, even as the high molecular weight component created with inside the different reactor offers exceptional mechanical strength.

6. Comparison of both SAC-510 and Finix-112 with Industrial Catalysts and Efficiency
Table 7 represents major important factors and performance about some currently used catalysts in chemical engineering science, as well as in industries, compared with the novel, super-active catalyst, SAC-510.All of the currently available catalysts have been used for the manufacturing of polyethylene pipes grade 100.
Hydrogen is the most broadly used chain-transfer agent for molecular weight control with Ziegler-Natta structures in industry.Hydrogen is the simplest commercially relevant chain-transfer agent in the lowpressure olefin polymerization process over the Ziegler-Natta catalysts.
Valuable and important points aspects that are suggested from the data analysis, as shown in Table 7 in support of the super-active SAC-510 catalyst application toward the manufacturing of grade 100 polyethylene pipes are as follows: a) superior performance and durability; b) little consumption of triethylaluminum, 1-Butylene and hydrogen; c) better hydrogen response; and, d) ideal strength of the polyethylene pipes made of HDPE-100 product compared to other currently available industrial catalysts.
As can be seen in Table 8, the efficiency with the Sack catalyst has a considerable advantage over other catalysts in almost all parameters, such as reducing 1-butene consumption, reducing hydrogen consumption, reducing triethylaluminum consumption and increasing activity, and using this Catalyst is recommended in the production of polyethylene 100 product.

7. Oxidation Induction Time
In Table 9, some characteristics of super catalyst product are listed with the products of other catalyst.The comparison of OIT in the product of the novel catalyst with the product of other catalyst shows an acceptable OIT and indicates the successful performance of the SAC-510 catalyst.
Oxidative induction time (OIT) is widely used for the determination of the thermal oxidative resistance of polyethylene materials The OIT testing only provides a relative measure of the level of antioxidants remaining in the pipe material but is a suitable analytical technique to monitor the degree of depletion of antioxidants from the pipe surface.Oxygen induction times (OIT) indicate the stabilities of the products (47).The concentration of effective antioxidant was assessed by determining the oxidation induction time (OIT) at 210 ºC.The data presented on the deterioration of polyolefin pipes exposed to water containing chlorine dioxide have been mostly concerned with lifetime issues.Choosing a constant testing temperature which delivers OIT times of around 60 min for the unaged reference samples can be too high for the aged samples.On the other hand, a much lower temperature for OIT tests could result in high differences between the unaged and aged samples (48).

LIMITATIONS OF THE STUDY
This study faced several challenges due to limited or lack of laboratory equipment and organic chemicals when it was being conducted as follows: 1. Lack of nuclear magnetic resonance facility to quantify the extent that butylene-1 was incorporated into the copolymer chains at 125°C. 2. Difficulty with separating 1-butylene from the solvent due to their close boiling points.

Figure 3 .
Scatter diagram of pores and dimensions of a) super active Finix-112 and b) SAC-510 catalysts.Key: Vp = Pore volume; Rp = Pore peak radius; ap = Specific pore area As seen in Table

TABLE 1 .
Strouhal number for different geometric cases

TABLE 2 .
Scanning electron microscopic features of the Finix-112 and SAC-510 powders

TABLE 4 .
Particle distribution in the catalysts based on Malvern method

TABLE 5 .
Results of the SAC-510 and Finix-112 powders based on BET test

TABLE 6 .
Results of the SAC-510 and Finix-112 powders based on BJH test

TABLE 7 .
(32)lts of the SAC-510 performance versus other industrial catalysts at our site(32)

TABLE 8 .
(32)lts of the SAC-510 efficiency versus other industrial catalysts at our site(32)