CA2256225A1 - Rotomolding resin - Google Patents

Abstract

A process to prepare a rotomolded container using a thermoplastic ethylene-alpha olefin copolymer ("polyethylene resin") having a narrow molecular weight distribution. The polyethylene resin is prepared in a dual reactor solution polymerization process. The dual reactor process provides advantages in the manufacture of the polyethylene resin, particularly with respect to the utilization of the alpha olefin monomer.
Polyethylene resin prepared in a dual reactor process enables rotomolded containers having excellent physical properties to be molded in a fast molding cycle.

Description

FIELD OF THE INVENTION
This invention relates to a rotomolded container which is prepared from a narrow molecular weight distribution polyethylene resin that is manufactured in a dual reactor polymerization process.
BACKGROUND OF THE INVENTION
Rotational molding, also known as "rotomolding" or "rotational to casting", is a well known fabrication process which is used to prepare hollow plastic parts such as toys, drums, chemical storage tanks, septic tanks, gasoline "cans" and the like. Parts which are manufactured by rotomolding are comparatively large. A nominal volume of at least one litre is typical, though some parts may be several thousand litres. (The term "nominal volume" refers to the actual volume of a container, or the volume defined by a toy or the like, as the context requires.) This process is typically initiated by placing a known amount of finely ground plastic particles in the open mold at ambient temperature.
The mold "shell" is then closed, rotated and heated. This causes the polymer power to cascade through the mold; melt, and form a polymer coating on the mold walls. The mold is subsequently cooled and opened to allow removal of the finished part. Further details of rotational molding so processes are widely available in the literature.
The so-called "blow molding" process (also well known to those skilled in the art) may also be used to prepare containers. However, as disclosed in EP 126,248, containers which are prepared by rotational molding often have superior physical properties in comparison to those prepared by blow molding.
G:\Scott\PSCSpec\9173can.doc 2 The use of a solution polymerization process to prepare polyethylene thermoplastics is also well known. It is also known to use a "dual reactor" solution polymerization process.
However, it is conventional practice to use a dual reactor polymerization process to produce a polymer having a broad molecular weight distribution. In direct contrast, the rotomolding process of this to invention uses polyethylene having a narrow molecular weight distribution.
SUMMARY OF THE INVENTION
The present invention provides a molded part having a volume of at least 1 litre which is prepared by rotational molding of thermoplastic polyethylene, wherein said polyethylene is characterized by:
a) having a melt index 12, as determined by ASTM standard D1238, condition 190/2.16, of from 2 to 15 grams per 10 minutes;
b) a density of from 0.905 to 0.950 grams per cubic centimeter;
and c) a polydispersity of from 2 to 3.5, and wherein said polyethylene is prepared by the catalytic copolymerization of ethylene with at least one alpha olefin having from 3 to 12 carbon atoms in a medium pressure solution polymerization process which uses at least two 3 o polymerization reactors.
As used herein, the term catalytic copolymerization means that the copolymerization is catalyzed by an organometallic-containing catalyst system (i.e. the term excludes polymerizations which are initialized by free radical generators such as peroxides). Preferred organometallic catalysts are described below in the Detailed Description.
G:\Scori\PSCSpec\9173can.doc DETAILED DESCRIPTION
A distinctive feature of this invention is that a "dual reactor"
polymerization process (i.e. a polymerization process which uses at least two polymerization reactors) is used to prepare a polyethylene resin having a narrow molecular weight distribution.
Polyethylene having a narrow molecular weight distribution is to desirable for rotational molding applications because the rheological properties of "narrow MWD" resins are generally superior in comparison to those having a broader molecular weight distribution. This is illustrated in the examples.
As will be appreciated by those skilled in the art, the use of a "single site catalyst" (such as a so-called "metallocene" catalyst) in a single polymerization reactor is now regarded as a conventional method to prepare polymers having a very narrow molecular weight distribution.
The use of two polymerization reactors to produce a product having a narrow molecular weight distribution requires that the products produced in each reactor have similar molecular weights. This may be achieved, for example, by using similar polymerization conditions (in particular, catalyst concentration, monomer concentration and reaction temperature) in the 3o two reactors. However, the use of the same reaction temperature for two polymerization reactors arranged in series requires either that heat is added to the first reactor or removed from the second reactor (due to the exothermic nature of the polymerization reactor). This may be achieved for example, by refrigeration or through the use of cold feed streams to the second reactor. Alternatively, and as will be appreciated by those skilled G:\Scott\PSCSpec\9173can.doc in the art, molecular weight can be controlled by the use of a chain transfer agent (such as hydrogen) or by changing catalyst concentration (with lower catalyst concentrations typically causing higher molecular weights).
Further details of the polymerization process and catalyst systems are set out below.
Part A Catalysts A.1 Single Site Catalysts The catalysts used in this invention may be either "single site catalysts" or Ziegler Natta catalysts. As used herein, the term "single site catalysts" refers to ethylene polymerization catalysts which, when used under steady state condition (i.e. uniform polymerization conditions -particularly reactor temperature) may be used in a single polymerization reactor to prepare polyethylene having a polydispersity of less than 2.5.
Many polymerization catalysts having one or two cyclopentadienyl-type ligands are single site catalysts. An exemplary (i.e. illustrative, but non-limiting) list includes:
a) monocyclopentadienyl complexes of group 4 or 5 transition metals such as those disclosed in USP 5,064,802 (Stevens et al, to Dow Chemical) and USP 5,026,798 (Canich, to Exxon);
3 o b) metallocenes (i.e. organometallic complexes having two cyclopentadienyl ligands); and c) phosphinimine catalysts (as disclosed in copending and commonly assigned patent applications, particularly Stephan et al and Brown et al - see Canadian patent applications 2,206,944 and 2,243,783).
G:\Scott\PSCSpec\9173can.doc 5 Catalysts having a single cyclopentadienyl type ligand and a single phosphinimine ligand are the preferred single site catalysts for use in this invention, as described below and illustrated in the Examples.
A.2. Description of Cocatalysts for Single Site Catalysts The single site catalyst components described in part 1 above are used in combination with at least one cocatalyst (or "activator") to form an to active catalyst system for olefin polymerization as described in more detail in sections 2.1 and 2.2 below.
A.2.1 Alumoxanes The alumoxane may be of the formula:
(R4)2A10(R4A10)mAl(R4)2 wherein each R4 is independently selected from the group consisting of C1-20 hydrocarbyl radicals and m is from 0 to 50, preferably R4 is a C~_4 alkyl radical and m is from 5 to 30. Methylalumoxane (or "MAO") in which each R is methyl is the preferred alumoxane.
Alumoxanes are well known as cocatalysts, particularly for metallocene-type catalysts. Alumoxanes are also readily available articles of commerce.
The use of an alumoxane cocatalyst generally requires a molar ratio of aluminum to the transition metal in the catalyst from 20:1 to 1000:1.
Preferred ratios are from 50:1 to 250:1.
A.2.2 "Ionic Activators" as Cocatalysts So-called "ionic activators" are also well known for metallocene catalysts. See, for example, USP 5,198,401 (Hlatky and Turner) and USP
5,132,380 (Stevens and Neithamer).
G:\Scott\PSCSpec\9173can.doc Whilst not wishing to be bound by any theory, it is thought by those skilled in the art that "ionic activators" initially cause the abstraction of one or more of the activatable ligands in a manner which ionizes the catalyst into a cation, then provides a bulky, labile, non-coordinating anion which stabilizes the catalyst in a cationic form. The bulky, non-coordinating anion coordinating anion permits olefin polymerization to proceed at the 1o cationic catalyst center (presumably because the non-coordinating anion is sufficiently labile to be displaced by monomer which coordinate to the catalyst). Preferred ionic activators are boron-containing ionic activators described in (i) - (iii) below:
(i) compounds of the formula [R5]+ [B(R')4]- wherein B is a boron atom, R5 is a aromatic hydrocarbyl (e.g. triphenyl methyl cation) and each R' is independently selected from the group consisting of phenyl radicals which are unsubstituted or substituted with from 3 to 5 substituents selected from the group consisting of a fluorine atom, a C,_4 alkyl or alkoxy radical which is unsubstituted or substituted by a fluorine atom; and a silyl radical of the formula -Si-(R9)a;
wherein each R9 is independently selected from the group 3o consisting of a hydrogen atom and a C1_4 alkyl radical; and (ii) compounds of the formula [(R8)t ZH]+[B(R')4]- wherein B is a boron atom, H is a hydrogen atom, Z is a nitrogen atom or phosphorus atom, t is 2 or 3 and R8 is selected from the group consisting of C1_8 alkyl radicals, a phenyl radical which is unsubstituted or substituted by up to three C1_4 alkyl G:\Scott\PSCSpec\9173can.doc radicals, or one R8 taken together with the nitrogen atom may form an anilinium radical and R' is as defined above;
and (iii) compounds of the formula B(R')3 wherein R' is as defined above.
In the above compounds preferably R' is a pentafluorophenyl to radical, and R5 is a triphenylmethyl cation, Z is a nitrogen atom and Rs is a C1_4 alkyl radical or R8 taken together with the nitrogen atom forms an anilium radical which is substituted by two C1_4 alkyl radicals.
The "ionic activator" may abstract one or more activatable ligands so as to ionize the catalyst center into a cation but not to covalently bond with the catalyst and to provide sufficient distance between the catalyst and the ionizing activator to permit a polymerizable olefin to enter the resulting active site.
Examples of ionic activators include:
triethylammonium tetra(phenyl)boron, tripropylammonium tetra(phenyl)boron, tri(n-butyl)ammonium tetra(phenyl)boron, trimethylammonium tetra(p-tolyl)boron, so trimethylammonium tetra(o-tolyl)boron, tributylammonium tetra(pentafluorophenyl)boron, tripropylammonium tetra (o,p-dimethylphenyl)boron, tributylammonium tetra(m,m-dimethylphenyl)boron, tributylammonium tetra(p-trifluoromethylphenyl)boron, tributylammonium tetra(pentafluorophenyl)boron, G:\Scott\PSCSpec\9173can.doc tri(n-butyl)ammonium tetra (o-tolyl)boron N,N-dimethylanilinium tetra(phenyl)boron, N,N-diethylanilinium tetra(phenyl)boron, N,N-diethylanilinium tetra(phenyl)n-butylboron, N,N-2,4,6-pentamethylanilinium tetra(phenyl)boron di-(isopropyl)ammonium tetra(pentafluorophenyl)boron, 1o dicyclohexylammonium tetra (phenyl)boron triphenylphosphonium tetra)phenyl)boron, tri(methylphenyl)phosphonium tetra(phenyl)boron, tri(dimethylphenyl)phosphonium tetra(phenyl)boron, tropillium tetrakispentafluorophenyl borate, triphenylmethylium tetrakispentafluorophenyl borate, benzene (diazonium) tetrakispentafluorophenyl borate, tropillium phenyltris-pentafluorophenyl borate, triphenylmethylium phenyl-trispentafluorophenyl borate, benzene (diazonium) phenyltrispentafluorophenyl borate, tropillium tetrakis (2,3,5,6-tetrafluorophenyl) borate, triphenylmethylium tetrakis (2,3,5,6-tetrafluorophenyl) borate, benzene (diazonium) tetrakis (3,4,5-trifluorophenyl) borate, 3o tropillium tetrakis (3,4,5-trifluorophenyl) borate, benzene (diazonium) tetrakis (3,4,5-trifluorophenyl) borate, tropillium tetrakis (1,2,2-trifluoroethenyl) borate, triphenylmethylium tetrakis (1,2,2-trifluoroethenyl) borate, benzene (diazonium) tetrakis (1,2,2-trifluoroethenyl) borate, tropillium tetrakis (2,3,4,5-tetrafluorophenyl) borate, G:\Scott\PSCSpec\9173can.doc triphenylmethylium tetrakis (2,3,4,5-tetrafluorophenyl) borate, and benzene (diazonium) tetrakis (2,3,4,5-tetrafluorophenyl) borate.
Readily commercially available ionic activators include:
N,N- dimethylaniliumtetrakispentafluorophenyl borate;
triphenylmethylium tetrakispentafluorophenyl borate; and trispentafluorophenyl borane.
to A.3. Description of Ziegler Natta Catalyst The term "Ziegler Natta catalyst" is well known to those skilled in the art and is used herein to convey its conventional meaning. A Ziegler Natta catalyst may be used in this invention. Ziegler Natta catalysts comprise at least one transition metal compound of a transition metal selected from groups 3, 4, or 5 of the Periodic Table (using IUPAC
nomenclature) and an organoaluminum component which is defined by the formula:
AI(X')a (OR)b (R)c wherein: X' is a halide (preferably chlorine); OR is an alkoxy or aryloxy group; R is a hydrocarbyl (preferably an alkyl having from 1 to 10 carbon atoms); and a, b, or c are each 0, 1, 2, or 3 with the provisos text a+b+c=3 and b+c>_1.
so It is highly preferred that the transition metal compounds contain at least one of titanium or vanadium. Exemplary titanium compounds include titanium halides (especially titanium chlorides, of which TiCl4 is preferred);
titanium alkyls; titanium alkoxides (which may be prepared by reacting a titanium alkyl with an alcohol) and "mixed ligand" compounds (i.e.
compounds which contain more than one of the above described halide, G:\Scott\PSCSpec\9173can.doc alkyl and alkoxide ligands). Exemplary vanadium compounds may also contain halide, alkyl or alkoxide ligands. In addition vanadium oxy trichloride ("VOC13") is known as a Ziegler Natta catalyst component and is suitable for use in the present invention.
It is especially preferred that the Ziegler Natta catalyst contain both of a titanium and a vanadium compound. The Ti/V mole ratios may be to from 10/90 to 90/10, with mole ratios between 50/50 and 20/80 being particularly preferred.
The above defined organoaluminum compound is an essential component of the Ziegler Natta catalyst. The mole ratio of aluminum to transition metal {for example, aluminum/(titanium + vanadium)} is preferably from 1/1 to 100/1, especially from 1.2/1 to 15/1.
As will be appreciated by those skilled in the art of ethylene polymerization, conventional Ziegler Natta catalysts may also incorporate additional components such as an electron donor - for example an amine;
or a magnesium compound - for example a magnesium alkyl such as butyl ethyl magnesium and a halide source (which is typically a chloride such as tertiary butyl chloride).
Such components, if employed, may be added to the other catalyst 3 o components prior to introduction to the reactor or may be directly added to the reactor.
The Ziegler Natta catalyst may also be "tempered" (i.e. heat treated) prior to being introduced to the reactor (again, using techniques which are well known to those skilled in the art and published in the literature).
G:\Scott\PSCSpec\9173can.doc 11 Part B Description of Dual Reactor Solution Polymerization Process Solution processes for the copolymerization of ethylene and an alpha olefin having from 3 to 12 carbon atoms are well known in the art.
These processes are conducted in the presence of an inert hydrocarbon solvent typically a C5_12 hydrocarbon which may be unsubstituted or to substituted by a C1_4 alkyl group, such as pentane, methyl pentane, hexane, heptane, octane, cyclohexane, methylcyclohexane and hydrogenated naphtha. An example of a suitable solvent which is commercially available is "Isopar E" (C8_12 aliphatic solvent, Exxon Chemical Co.).
The solution polymerization process of this invention must use at least two polymerization reactors. The polymer solution resulting from the first reactor is transferred to the second polymerization (i.e. the reactors must be arranged "in series" so that polymerization in the second reactor occurs in the presence of the polymer solution from the first reactor).
The polymerization temperature in the first reactor is from about 80°C to about 180°C (preferably from about 120°C to 160°C) and the hot reactor is preferably operated at a slightly higher temperature as a result of 3 o the enthalpy of polymerization in the second reactor. Both reactors are preferably "stirred reactors" (i.e. the reactors are well mixed with a good agitation system). Preferred pressures are from about 500 psi to 8,000 psi. The most preferred reaction process is a "medium pressure process", meaning that the pressure in each reactor is preferably less than about G:\Scott\PSCSpec\9173can.doc 1 2 6,000 psi (about 42,000 kiloPascals or kPa), most preferably from about 1,500 psi to 3,000 psi (about 14,000-22,000 kPa) Suitable monomers for copolymerization with ethylene include C3_12 alpha olefins which are unsubstituted or substituted by up to two C1_6 alkyl radicals. Illustrative non-limiting examples of such alpha-olefins are one or more of propylene, 1-butene, 1-pentene, 1-hexene, 1-octene and 1-to decene.
The polyethylene polymers which may be prepared in accordance with the present invention are ethylene copolymers which typically comprise not less than 60, preferably not less than 75 weight % of ethylene and the balance one or more C4_io alpha olefins, preferably selected from the group consisting of 1-butene, 1-hexene and 1-octene.
The polyethylene prepared in accordance with the present invention has a density from about 0.905 to 0.950 g/cc (preferably from about 0.935 to 0.950 g/cc).
The polyethylene also has a melt index ("12" as determined by ASTM standard D1238, condition 190/2.16) of from 2 to 15, preferably from 2 to 7 "grams per 10 minutes". (The units may also be referred to as dg/min.) so The monomers are dissolved/dispersed in the solvent either prior to being fed to the first reactor (or for gaseous monomers the monomer may be fed to the reactor so that it will dissolve in the reaction mixture). Prior to mixing, the solvent and monomers are generally purified to remove potential catalyst poisons such as water, oxygen or metal impurities. The feedstock purification follows standard practices in the art, e.g. molecular G:\Scott\PSCSpec\9173can.doc 13 sieves, alumina beds and oxygen removal catalysts are used for the purification of monomers. The solvent itself as well (e.g. methyl pentane, cyclohexane, hexane or toluene) is preferably treated in a similar manner.
The feedstock may be heated or cooled prior to feeding to the first reactor. Additional monomers and solvent may be added to the second reactor, and it may be heated or cooled.
to Generally, the catalyst components may be premixed in the solvent for the reaction or fed as separate streams to each reactor. In some instances premixing it may be desirable to provide a reaction time for the catalyst components prior to entering the reaction. Such an "in line mixing" technique is described in a number of patents in the name of DuPont Canada Inc (e.g. USP patent 5,589,555, issued Dec. 31, 1996).
The residence time in each reactor will depend on the design and the capacity of the reactor. Generally the reactors should be operated under conditions to achieve a thorough mixing of the reactants. In addition, it is preferred that from 20 to 60 weight % of the final polymer is polymerized in the first reactor, with the balance being polymerized in the second reactor. As previously noted, the polymerization reactors are arranged in series (i.e. with the solution from the first reactor being 3 o transferred to the second reactor). Thus, in a highly preferred embodiment, the first polymerization reactor has a smaller volume than the second polymerization reactor. On leaving the reactor system the solvent is removed and the resulting polymer is finished in a conventional manner.
Further details of the invention are illustrated in the following, non limiting, examples. The examples are divided into three parts.
G:\Scott\PSCSpec\9173can.doc 14 The first part illustrates the copolymerization of ethylene and octene-1 in a dual polymerization reactor system using a Ziegler Natta catalyst. A comparative polymerization using a single reactor is also demonstrated.
The second part illustrates the use of a single site catalyst in "single" and "dual" reactor copolymerizations.
to The third part illustrates the preparation of gasoline containers ("ferry cans") in a rotomolding process using the ethylene copolymers from parts 1 and 2.
Test Procedures Used In The Exama~les Are Briefly Described Below 1. Colour Testing (Whiteness Index or "W Index" and Yellow Index or "Y Index") was completed using a spectraphotometer according to ASTM
E313.

2. "Instrumented Impact Testing" was completed using a commercially available instrument (sold under the tradename "DYNATUP") according to ASTM D3763.

3. Melt Index: 12, I6, 121 and Melt Flow Ratio (which is calculated by dividing 121 by 12) were determined according to ASTM D1238.

4. Stress exponent is calculated by to I~/12 log 3 5. Number average molecular weight (Mn); weight average molecular weight (Mw) and polydispersity (calculated by "~'"IMn) were determined by Gel Permeation Chromatography "GPC").

6. Flexural Secant Modulus and Flexural Tangent Modulus were determined according to ASTM D790.
G:\Scott\PSCSpec\9173can.doc 15 7. Elongation and Yield measurements were determined according to ASTM D636.

8. Hexane Extractables were determined according to ASTM D5227.

9. Environmental Stress Cracking Resistance (ESCR) was determined according to ASTM D1693.

10. Branch Frequency was determined by Fourier Transform Infra Red to ("FTIR") analysis.

11. Melting Point was determined by Differential Scanning Calorimetry ("DSC").

12. Density was determined using the displacement method according to ASTM D792.
EXAMPLES
Part 1 This example illustrates the continuous flow, solution copolymerization of ethylene at a medium pressure using a two reactor system using a Ziegler Natta catalyst. Both reactors are continuously stirred tank reactors ("CSTR'S"). The first reactor operates at a relatively low temperature. The contents from the first reactor flow into the second reacto r.
3 o The first reactor had a volume of 12 litres. Monomers, solvent and catalyst were fed into the reactor as indicated in Table 1.A. The solvent used in these experiments was methyl pentane. The contents of the first reactor were discharged through an exit port into a second reactor having a volume of 24 litres. Flow rates to the second reactor are also shown in Table 1.A.
G:\Scott\PSCSpec\9173can.doc 16 The catalyst employed in all experiments was one known to those skilled in the art as a "Ziegler Natta" catalyst and consisted of titanium tetrachloride (TiCl4), dibutyl magnesium (DBM) and tertiary butyl chloride (TBC), with an aluminum activator consisting of triethyl aluminum (TEAL) and diethyl aluminum ethoxide (DEAD). The molar ratio of the components was:
1o TBC:DBM (2-2.2:1 );
DEAO:TiCl4 (1.5-2:1 ); and TEAL: TiCl4 (1-1.3:1 ).
All catalyst components were mixed in methyl pentane. The mixing order was DBM, TEAL (5:1 molar ratio) and TBC; followed by TiCl4;
followed by DEAD. The catalyst was pumped into the reactor together with the methyl pentane solvent. The catalyst flow rate had an aim point as shown in the table and was adjusted to maintain total ethylene conversions above 90%.
Table 1.B provides data from a comparative experiment which was conducted using only a single polymerization reactor (as indicated in Table 1.B, this was done by providing feeds only to the second reactor).
TABLE 1.A
3 o Reactor 1 Reactor 2 Eth lene k /h 50 50 Octene k /h 7 7 H dro en /h 4.5 4.5 Solvent k h 295 375 Reactor tem . C 188 189 TiCl4 to reactor 3.7 3.1 (ppm) ( G:\Scott\PSCSpec\9173can.doc 1 7 TABLE 1.B-c Reactor 1 Reactor 2 Eth lene k /h - 90 Octene k h - 18 H dro en /h - 3.5 Solvent k /h - 520 Reactor tem . C - 188 TiCl4 to reactor ~ - 4.1 (ppm) ** c: comparative Table 2 provides data which describe the physical properties of the thermoplastic ethylene-octene resins produced in Part 1. The sample labeled PS-8500 is the single reactor (comparative) resin.

Rotational Molding Resins Material Name PS-8500 PS-11715 Properties 2 0 Rheology/Flow Properties Melt Index 12 (g/10 min) 1.97 1.939 Melt Index I6 (g/10 min) 8.77 7.69 Melt Index 121 (g/10 min) 45.9 Melt Flow Ratio 23.7 Stress Exponent 1.3598 1.2543 Crossover Frequency (rad/s)181.73 242.05 Zero Shear Viscosity (Pa-s)5761.4 4739.8 Flexural Testing Flex Secant Mod. 1% (MPa) 820 734 Flex Secant Mod. 1% Dev. 10 24 (MPa) Flex Secant Mod. 2% (MPa) 650 30 Flex Secant Mod. 2% Dev. 21 (MPa) Flex Tangent Mod. (MPa) 882 853 Flex Tangent Mod. Dev. 228 255 (MPa) Flexural Strength (MPa) 27.1 25.1 Flexural Strength Dev. 0.4 0.6 (MPa) Tensile Testing Elong. at Yield (%) 10 9 Elong. at Yield Dev. (%) 0.3 1 Yield Strength (MPa) 22.9 19 Yield Strength Dev. (MPa) 0.3 0.6 Ultimate Elong. (%) 983 979 G:\Scott\PSCSpec\9173can.doc 1 Ultimate Elong.Dev. (%) 25 20 Ultimate Strength (MPa) 34.3 34 Ultimate Strength Dev. 3 1.3 (MPa) Impact Properties Dynatup Maximum Load ~ 833.49 23C(Ibf) Dynatup Total Energy C~ 41.04 23C(ft-Ibf) Dynatup Maximum Load C~ 1399.12 -40C(Ib,) Dynatup Total Energy C~ 62.87 -40C(ft-Ibf) Tensile Impact (ft-Ib/in ) GPC

No. Ave. Mol. Wt. (MN) 20.9 29.5 x 10~

Wt. Ave. Mol. Wt. (MW) 87 84.1 x 10~

Z Ave. Mol. Wt. (MZ) x 267.3 211.5 Polydispersity Index 4.16 2.85 Branch Frequency FTIR

Branches per 1000 carbons 2.8 Comonomer ID Octene Octene DSC

Melting Point ( C) 128.32 New Crystallinity (%) 65.3 Old Crystallinity (%) N/A

ether Hexane Extractables (%) 0.34 0.29 ESCR Cond. B at 10 % (hrs) ESCR Cond. B at 100 % (hrs)>1000 >1000 ESCR CTL (hrs) Density (g/cm3) I -- 0.944 0.9397 Part 2 This example illustrates the preparation of ethylene-octene 3 o copolymers using a single site phosphinimine catalyst.
The catalyst used in each example is a titanium complex having one cyclopentadienyl ligand; one tri(tertiary butyl) phosphinimine ligand;
and two chloride ligands ("Cp T NP(tBu)3 C12). The cocatalyst used was a combination of a commercially available methylalumoxane (sold under the G:\Scott\PSCSpec\9173can.doc tradename MMAO-7 by Akzo Nobel) and trityl borate (or Ph3CB(C6F3)a, where Ph represents phenyl, purchased from Asahi Glass).
The same polymerization reactions described in Part 1 were used.
The catalyst and trityl borate were co-fed through a common line (thus permitting some contact prior to the reaction) and the MMAO-7 was added directly to the reactor.
to Two comparative experiments (single reactor) and one dual reactor experiment are described in Table 3. Experiment PS 13004 shows that it was not necessary to add further octene-1 to reactor 2. This is because of the relative reactivity ratios of ethylene and octene (i.e. the ethylene in the first reactor is preferentially polymerized. This means that there is sufficient octene flowing from the first reactor to the second reactor to permit proper ethylene-octene copolymerization in the second reactor. In aggregate, this also means that less octene must be subjected to the process conditions to produce a given amount of ethylene-octene copolymer in the dual reactor mode (compared to the single reactor mode). This is highly advantageous, as some unreacted octene is lost when subjected to process conditions - for example, by isomerization).
G:\Scott\PSCSpec\9173can.doc Sam le # PS8835- PS12965- PS13004 M I 5.9 2.2 2.4 I 104 44.6 45.5 MFR I /MI 17.6 20.1 18.8 Stress Ex onent 1.15 1.21 1.18 Densit 0.935 0.943 0.942 Mn/1000 18.5 - -Mw/1000 59.2 - -Mw/M n 3.2 - -Reactor 1 Eth lene k /hr - - 50 1-octene k /hr - - 7.7 H dro en /hr - - 0 Tem erature C - - 193 Total Flow k /hr - - 328 Ti micromol/I - - 0.5 AI/Ti mol/mol - - 42 BlTi mol/mol - - 1.0 Reactor 2 Eth lene k /hr 90 95 50 1-octene k /hr 20 12.6 0 H dro en /hr 2 2.2 2.3 Tem erature C 194 195 194 Total Flow k /hr 605 617 703 Ti micromol/) 1.1 1.1 0.9 2 0 AI/Ti mol/mol 46 41 48 ~BITi (mol/mol) 1.5 1.1 1.4 ** c: comparative Part 3 Preparation of Rotomolded Container This example illustrates the preparation of containers using a rotomolding apparatus. A commercially available apparatus (sold under the tradename Ferry RS-169) was used.
The polyethylene resins used in this example were initially ground to -35 mesh. The resins were compounded with conventional antioxidant/stabilizers prior to grinding.
The mold was a standard, two piece mold ("clamshell-type"). The molded part was a container having a nominal value of about 20 litres (in the shape of a gasoline container or "ferry can").
G:\Scott\PSCSpec\9173can.doc The mold included a vented aluminum insert (to form the threaded neck of the gasoline container) and was coated with polytetrafluorethylene to facilitate de-molding of the finished part.
The amount of resin added to the mold was 1.7 kg (to produce a container having a wall thickness of from 0.12 to 0.13 inches).
Conventional oven temperatures for this apparatus typically range to from 450 to 650°F. Conventional temperatures were used, as shown in Table 4.
As will be appreciated by those skilled in the art, rotomolding devices have rotating pieces referred to as "arms" and "plates". The speed of rotation of these parts (revolutions per minute) is also shown in Table 4. The rotation ratio (arm speed/plate speed) is also shown.
In a conventional rotomolding cycle, the temperature of the mold is increased (or "ramped"), then optionally held constant ("hold time") then cooled. The mold is opened when the cooling cycle is complete to allow removal of the finished part. It is desirable to minimize the total cycle time so as to maximize the productivity of these expensive machines. In order to minimize total cycle times, the resin must have excellent rheological properties (i.e. so that the resin flows sufficiently to completely fill the 3o mold).
In order to establish a "processing window" for a given resin, different heating times are experimented with (where an "undercooked"
part may be usually recognized by the presence of small amounts of incompletely melted resin powder and an "overcooked" part may be G:\Scott\PSCSpec\9173can.doc 22 recognized by the presence of glossy yellow/brown surfaces and a distinct odor).
Table 4 provides data which show that the dual reactor resin from Example 2 has an optimal cycle time of about 28 minutes for the ferry can mold used in this example. The resulting ferry can had excellent impact resistance and color. Moreover, the excellent color indicates good thermal 1o resistance (and, therefore, offers the possibility of increasing the molding temperatures to further reduce cycle time).
The dual reactor resin from Example 1 was also successfully used to fabricate a "ferry can" using rotomolding conditions similar to those described above.

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Claims (5)

1. A molded part having a nominal volume of at least 1 litre which is prepared by rotational molding of polyethylene, where said polyethylene is characterized by:
a) having a melt index 12, as determined by ASTM standard D1238, condition 190/2.16, of from 2 to 15 grams per 10 minutes;
b) a density of from 0.905 to 0.950 grams per cubic centimeter;
and c) a polydispersity of from 2 to 3.5, and wherein said polyethylene is prepared by the catalytic copolymerization of ethylene with at least one alpha olefin having from 3 to 12 carbon atoms in a medium pressure solution polymerization process which uses at least two polymerization reactors.

2. The part of claim 1 wherein each of said at least two polymerization reactors has independent feed addition streams for said catalyst system and wherein said catalyst system is added to each of said reactors.

3. The part of claim 1 wherein said catalyst system is a Ziegler Natta catalyst system comprising a magnesium component, a titanium component and an aluminum alkyl.

4. The part of claim 3 wherein said magnesium component is defined by the formula:

R a MgX b wherein R is an alkyl having from 2 to 4 carbon atoms; X is a chloride; a is 1 or 2; b is 0 or 1 and a+b=2; said titanium component is selected from TiCl3 and TiCl4; and said aluminum alkyl is defined by the formula:

5. The part of claim 1 wherein said catalyst system is a single site catalyst system which comprises:
a) a catalyst which is an organometallic complex of a group 4 metal, wherein said organometallic complex is characterized by having a cyclopentadienyl ligand and a phosphinimine ligand; and b) an activator.