Composition for rotational molding

Abstract

The rotomolding composition contains a blend of two polyethylenes: 1) a fully stabilized polyethylene; and 2) a poorly stabilized polyethylene with a very low flow rate. The poorly stabilized polyethylene is further characterized by a large particle size (500-3000 microns). Rotationally molded parts prepared from the composition have a rough inner surface. Polyurethane foam adheres well to this rough surface.

Description

[Technical field]

[0001] A composition and process for rotomolding that provides rotomolded parts with improved ability to adhere to polar polymers. [Background technology]

[0002] Rotational molding, also known as rotomolding, is a well-known process widely used to manufacture hollow plastic parts such as gasoline containers, trash cans, agricultural storage containers, septic tanks, and sporting goods such as kayaks. The process involves filling a mold "shell" with a charge of finely divided plastic resin, then rotating the mold (usually on two axes) while heating it to a temperature above the melting point of the plastic resin. The molten plastic flows through the mold cavity under the force induced by the rotation of the device. Rotation continues for a time sufficient to allow the molten plastic to coat the mold's surfaces. The mold is then cooled, freezing the plastic into a solid. The final stage of the molding cycle is removal of the part from the rotational molding machine.

[0003] It will be appreciated that the rotational molding process produces hollow parts, i.e., parts having an interior and exterior surface. In some end uses, it is desirable to coat the interior surface (or even fill the hollow area) to provide an insulating layer or to improve structural strength or buoyancy. In particular, it is known to use polyurethane for this coating or filling. However, effectively adhering polyurethane to polyethylene is difficult due to the different polarities of these two polymers. This problem can be solved by treating the polyethylene to increase its surface polarity, but this is expensive. Summary of the Invention

[0004] In some embodiments, compositions and processes are disclosed herein that provide rotationally molded parts with inner surfaces having high surface roughness. In these embodiments, improved adhesion to polyurethane is observed.

[0005] In one embodiment, there is provided a polyethylene composition comprising: 1) 85 to 99 weight percent stabilized polyethylene having a melt index (I2) of 0.5 to 10 grams / 10 minutes as measured by ASTM D 1238 at 190°C using 2.16 kg of lead; and a density of 0.920 to 0.950 g / cc as measured by ASTM D792; and 2) (i) a high load melt index (I) of 2 to 20 grams / 10 minutes as measured by ASTM D1238 at 190°C using a 21.6 kg load; 21 (ii) a density of 0.950 to 0.965 g / cc as measured by ASTM D792; (iii) 15 to 1 weight percent substantially unstabilized polyethylene having an average particle size of 500 to 3000 microns.

[0006] Definition of Terms Except in the examples or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, extrusion conditions, and the like used in the specification and claims should be understood to be modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and appended claims are approximations that may vary depending upon the desired properties desired to be obtained by various embodiments. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. The numerical values set forth in the specific examples are reported as precisely as possible. However, any numerical value inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0007] It should be understood that any numerical range recited in this specification is intended to include all sub-ranges subsumed therein. For example, the range "1 to 10" is intended to include all sub-ranges subsumed between the recited minimum value of 1 and the recited maximum value of 10. That is, the minimum value is 1 or more and the maximum value is 10 or less. Since the disclosed numerical ranges are continuous, they include all values between the minimum and maximum values. Unless otherwise specified, the various numerical ranges specified in this application are approximate values.

[0008] All composition ranges represented in this specification are actually limited to 100% in total (volume percent or weight percent) and do not exceed it. When multiple components may be present in a composition, the sum of the maximum amounts of each component can exceed 100% understanding that the amounts of the components actually used conform to a maximum of 100%, as will be readily understood by those skilled in the art.

[0009] To form a more complete understanding of the present disclosure, the following terms are defined and should be used throughout in conjunction with the accompanying drawings and the description of the various embodiments.

[0010] As used herein, the term "monomer" refers to a small molecule that can react chemically with itself or other monomers and chemically bond to form a polymer.

[0011] As used herein, the term "α-olefin" is used to represent a monomer having a straight-chain hydrocarbon chain containing 3 to 20 carbon atoms with a double bond at one end of the chain. Examples include butene-1, hexene-1, and octene-1.

DETAILED DESCRIPTION OF THE INVENTION

[0012] A. Polyethylene Polyethylene is a plastic resin commonly used in the manufacture of rotomolded parts. Although the use of homopolymers of polyethylene is also known, copolymers of polyethylene (generally prepared by copolymerization of ethylene with C 4-10 alpha olefins in the presence of a chromium catalyst or a Ziegler Natta catalyst) are typically used. The comonomer of the alpha olefin produces "short chain branches" (SCBs) in the copolymer. These SCBs reduce the crystallinity of the copolymer (compared to the homopolymer of linear ethylene), and the copolymer typically has improved impact resistance compared to the homopolymer. These conventional polyethylenes can be called "heterogeneous" in the sense that polyethylene is actually a mixture of different polyethylene chains having significantly different molecular weights and comonomer distributions. Most notably, conventional heterogeneous polyethylenes generally contain the following three fractions: i) A low molecular weight fraction having a high comonomer content (or a high level of SCBs) This fraction is often called "wax" or "extract"); ii) A very high molecular weight fraction having little or no comonomer (this fraction is often called "homopolymer"); and iii) A fraction having an intermediate molecular weight and SCB content.

[0013] These conventional polyethylenes are well suited for rotomolding. For example, without wishing to be bound by theory, it is assumed that the high molecular weight "homopolymer" fraction can increase the rigidity or modulus of elasticity of rotomolded parts.

[0014] Recently, homogeneous polyethylenes have been commercially available. These homogeneous polyethylenes have a uniform (or narrow) molecular weight distribution and a uniform comonomer distribution. As a result, homogeneous polyethylenes have a distinct melting point (compared to heterogeneous polyethylenes having a melting point "range" or even multiple melting points). It is also known to use "homogeneous" polyethylenes in the rotomolding process.

[0015] Stabilized Polyethylene The term "stabilized polyethylene" refers broadly to polyethylene (which may be heterogeneous or homogeneous polyethylene) that includes a "stabilizer" package to protect the polyethylene from degradation / oxidation during the rotomolding process. In general, any stabilizer package currently used in rotomolding applications is potentially suitable for use in the present process. Those skilled in the art will recognize that many such stabilizer packages are currently in use.

[0016] In one embodiment, the stabilizer package includes: 1) primary antioxidants (not wishing to be bound by theory, but generally believed to scavenge free radicals); 2) Secondary antioxidants (without wishing to be bound by theory, are generally believed to quench hydroperoxides).

[0017] Suitable (non-limiting) examples of primary antioxidants include hindered phenols; hydroxylamines and lactones, and suitable amounts are 100 to 2000 (particularly 250 to 1500) parts per million ("ppm") by weight based on the weight of the polyethylene.

[0018] Suitable (non-limiting) secondary antioxidants include phosphites and phosphinites, especially in amounts of 100 to 2000 (especially 250 to 1500 ppm).

[0019] It is also desirable to include hindered amine light stabilizers (HALS) in the rotomolding composition, especially in amounts of 500 to 3000 ppm. The optional additional use of zinc oxide (in amounts of 500 to 5000 parts per million by weight) and / or ultraviolet (UV) absorbers can further improve the stability of rotomolded parts exposed to sunlight.

[0020] In one embodiment, the stabilizer package includes: 1) 250 to 1500 ppm or less: 1.1) hindered phenols; and 1.2) Hydroxylamine; at least one primary antioxidant selected from, and 2) 100 - 1000 ppm of the following: 2.1) phosphite; and 2.2) phosphinite; at least one secondary antioxidant selected from, and 3) 500 - 3000 ppm of HALS; and 4) 0 - 5000 ppm of zinc oxide.

[0021] Many other additives are also known to be used with polyethylene. Some of these additives are described in the following Additive section.

[0022] Additives can be incorporated into the polyethylene composition using a mixing device such as an extruder or an internal batch mixer (also known as a Banbury mixer). Additives can be added as a "masterbatch" (i.e., by premixing the additive with a small amount of polyethylene and subsequently mixing this with the bulk of the composition); or as a "preblend" (i.e., a mixture of additives), "neat" (i.e., directly into the resin).

[0023] The stabilized high - density polyethylene composition, in some embodiments, is ground into a powder before being used in a rotational molding process (a conventional practice in rotational molding). A (non - limiting) average particle size suitable for the powder is 200 - 400 microns. In one embodiment, the average particle size of the stabilized polyethylene is lower / smaller than that of the substantially non - stabilized polyethylene.

[0024] Additives In one embodiment, the rotational molding composition may comprise: A) the following: A. l) hindered phenol (non - limiting examples thereof are described in Sections 1.1 and 1.4 below); and A.2) hydroxylamine (non - limiting examples thereof are described in Section 5 below); at least one primary antioxidant selected from B) at least one secondary antioxidant selected from phosphites and phosphonites (non-limiting examples of which are described in Section 4 below); and C) at least one hindered amine light stabilizer "HALS" (non-limiting examples of which are described in Section 2.6 below).

[0025] Many other additives may also be included in the polyethylene compositions used in rotomolding processes, some of which are briefly described below.

[0026] 1. Antioxidants (also known as "hindered phenols") 1.1 Alkylated monophenols For example, 2,6-di-tert-butyl-4-methylphenol; 2-tert-butyl-4,6-dimethylphenol; 2,6-di-tert-butyl-4-ethylphenol; 2,6-di-tert-butyl-4-n-butylphenol; 2,6-di-tert-butyl-4-isobutylphenol; 2,6-dicyclopentyl-4-methylphenol; 2-(α-methylcyclohexyl)-4,6 dimethylphenol; 2,6-di-octadecyl-4-methylphenol; 2,4,6-tricyclohexylphenol; 2,6-di-tert-butyl-4-methoxymethylphenol; and vitamin E.

[0027] 1.2 Alkylated hydroquinone For example, 2,6-di-tert-butyl-4-methoxyphenol; 2,5-di-tert-butylhydroquinone; 2,5-di-tert-amyl-hydroquinone; and 2,6-diphenyl-4-octadecyloxyphenol.

[0028] 1,3 Hydroxylated thiodiphenyl ether For example, 2,2'-thio-bis-(6-tert-butyl-4-methylphenol); 2,2'-thio-bis-(4-octylphenol); 4,4'-thio-bis-(6-tert-butyl-3-methylphenol); and 4,4'-thio-bis-(6-tert-butyl-2-methylphenol).

[0029] 1,4-Alkylidene-bisphenols (also called "hindered phenols") For example, 2,2'-methylene-bis-(6-tert-butyl-4-methylphenol); 2,2'-methylene-bis-(6-tert-butyl-4-ethylphenol); 2,2'-methylene-bis-(4-methyl-6-(α-methylcyclohexyl)phenol); 2,2'-methylene-bis-(4-methyl-6-cyclohexylphenol); 2,2'-methylene-bis-(6-nonyl-4-methylphenol); 2,2'-methylene-bis-(6-nonyl-4-methylphenol); 2,2'-methylene-bis-(6-(α-methylbenzyl)-4-nonylphenol); 2,2'-methylene-bis-(6-(α,α-dimethylbenzyl)-4-nonyl-phenol); 2,2'-methylene-bis-(4,6-di-tert-butylphenol); 2,2'-ethylidene-bis-(6-tert-butyl-4-isobutylphenol); 4,4'-methylene-bis-(2,6-di-tert-butylphenol); 4,4'-methylene-bis-(6-tert-butyl-2-methylphenol); 1,1-bis-(5-tert-butyl-4-hydroxy-2-methylphenyl)butane 2,6-di-(3-tert-butyl-5-methyl-2-hydroxybenzyl)-4-methylphenol; 1,1,3-tris-(5-tert-butyl-4-hydroxy-2-methylphenyl)butane; 1,1-bis-(5-tert-butyl-4-hydroxy 2-methylphenyl)-3-dodecyl-mercaptobutane; ethylene glycol-bis-(3,3,-bis-(3'-tert-butyl-4'-hydroxyphenyl)-butyrate)-di-(3-tert-butyl-4-hydroxy-5-methylphenyl)-dicyclopentadiene; di-(2-(3'-tert-butyl-2'-hydroxy-5'-methylbenzyl)-6-tert-butyl-4-methylphenyl) terephthalate; and other phenols, for example, monoacrylate esters of bisphenols, such as ethylidene bis-2,4-di-t-butylphenol monoacrylate ester.

[0030] 2. UV Absorbents and Light Stabilizers 2.1 2-(2'-Hydroxyphenyl)-benzotriazole For example, they are 5'-methyl-, 3',5'-di-tert-butyl-, 5'-tert-butyl-, 5'-(1,1,3,3-tetramethylbutyl)-, 5-chloro-3',5'-di-tert-butyl-, 5-chloro-3'-tert-butyl-5'-methyl-3'-sec-butyl-5'-tert-butyl-, 4'-octyloxy-, 3',5'-di-tert-amyl-3',5'-bis-(α,α-dimethylbenzyl)-derivatives.

[0031] 2.2 2-Hydroxy-benzophenone For example, they are 4-hydroxy-4-methoxy-, 4-octyloxy-, 4-decyloxy-, 4-dodecyloxy-, 4-benzyloxy, 4,2',4'-trihydroxy- and 2'-hydroxy-4,4'-dimethoxy derivatives.

[0032] 2.3 Hindered amine light stabilizer (HALS) For example, bis(2,2,6,6-tetramethylpiperidyl)-sebacate; bis-5(1,2,2,6,6-pentamethylpiperidyl)-sebacate; n-butyl-3,5-di-tert-butyl-4-hydroxybenzyl malonic acid bis(1,2,2,6,6-pentamethylpiperidyl) ester; condensate of 1-hydroxyethyl-2,2,6,6-tetramethyl-4-hydroxy-piperidine and succinic acid; condensate of N,N’-(2,2,6,6-tetramethylpiperidyl)-hexamethylenediamine and 4-tert-octylamino-2,6-dichloro-1,3,5-s-triazine; tris-(2,2,6,6-tetramethylpiperidyl)-nitrilotriacetate, tetrakis-(2,2,6,6-tetramethyl-4-piperidyl)-1,2,3,4 butane-tetra-carbonic acid; and 1,1’(1,2-ethanediyl)-bis-(3,3,5,5-tetramethylpiperazinone). These amines, typically called HALS (hindered amine light stabilizers), include 2,2,6,6-tetramethylpiperidinol esters of butanetetracarboxylic acid. Such amines include hydroxylamines derived from hindered amines such as di(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl) sebacate; 1-hydroxy 2,2,6,6-tetramethyl-4-benzyloxypiperidine; 1-hydroxy-2,2,6,6-tetramethyl-4-(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyloxy)-piperidine; and N-(1-hydroxy-2,2,6,6-tetramethyl-piperidin-4-yl)-epsilon caprolactam.

[0033] 3. Phosphites and phosphonites For example, triphenyl phosphite; diphenyl alkyl phosphate; phenyl dialkyl phosphate; tris(nonyl-phenyl) phosphite; trilauryl phosphite; trioctadecyl phosphite; distearyl pentaerythritol diphosphite; tris(2,4-di-tert-butylphenyl) phosphite; diisodecyl pentaerythritol diphosphite; 2,4,6-tri-tert-butylphenyl-2-butyl-2-ethyl-1,3-propanediol phosphite; bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite; tristearyl sorbitol triphosphite; and tetrakis(2,4-di-tert-butylphenyl) 4,4'-biphenylene diphosphonite.

[0034] 4. Peroxide scavenger For example, esters of beta-thiodipropionic acid, such as lauryl, stearyl, myristyl or tridecyl esters; zinc salts of mercaptobenzimidazole or 2-mercaptobenzimidazole; zinc dibutyldithiocarbamate; dioctadecyl disulfide; and pentaerythritol tetrakis-(β-dodecylmercapto)-propionate.

[0035] 5. Hydroxylamine and amine oxide For example, N,N-dibenzylhydroxylamine; N,N-diethylhydroxylamine; N,N-dioctylhydroxylamine; N,N-dilaurylhydroxylamine; N,N-ditetradecylhydroxylamine; N,N-dihexadecylhydroxylamine; N,N-dioctadecylhydroxylamine; N-hexadecyl-N-octadecylhydroxylamine; N-heptadecyl-N-octadecylhydroxylamine; and N,N-dialkylhydroxylamine derived from hydrogenated tallow amine. Similar amine oxides (disclosed in USP 5,844,029 (Prachu et al.)) are also suitable.

[0036] 6. Nitrone For example, nitrones derived from N-benzyl-α-phenylnitrone; N-ethyl-α-methylnitrone; N-octyl-α-heptylnitrone; N-lauryl-α-undecylnitrone; N-tetradecyl-α-tridecylnitrone; N-hexadecyl-α-pentadecylnitrone; N-octadecyl-α-heptadecylnitrone; N-hexadecyl-α-heptadecylnitrone; N-octadecyl-α-pentadecylnitrone; N-heptadecyl-α-heptadecylnitrone; N-octadecyl-α-hexadecylnitrone; and N,N-dialkylhydroxylamines derived from hydrogenated tallow amine.

[0037] 7. Basic co-stabilizer For example, antimony pyrocatecholate or zinc pyrocatecholate containing neutralizing agents such as melamine; polyvinylpyrrolidone; dicyandiamide; triallyl cyanurate; urea derivatives; hydrazine derivatives; amines; polyamides; polyurethanes; alkali metal salts and alkaline earth metal salts of higher fatty acids, such as Ca stearate, stearoyl calcium lactate, calcium lactate, Zn stearate, Mg stearate, Na ricinoleate and K palmitate; hydrotalcite and synthetic hydrotalcite; and Li, Na, Mg, Ca, Al hydroxycarbonates.

[0038] 8. Nucleating agent For example, 4-tert-butylbenzoic acid; adipic acid; diphenylacetic acid; sodium salt of methylenebis-2,4-dibutylphenyl; cyclic phosphate ester; sorbitol tris-benzaldehyde acetal; and sodium salt of bis(2,4-di-t-butylphenyl) phosphate or Na salt of ethylidenebis(2,4-di-t-butylphenyl) phosphate. The nucleating agent can improve the rigidity of the rotational molding parts.

[0039] 9. Filler and reinforcing agent For example, calcium carbonate; silicate; glass fiber; asbestos; talc; kaolin; mica; barium sulfate; metal oxides and metal hydroxides; carbon black and graphite.

[0040] 10.Other For example, plasticizers; epoxidized vegetable oils, such as epoxidized soybean oil; lubricants; emulsifiers; pigments; optical brighteners; flame retardants; antistatic agents; blowing agents such as dilauryl thiodipropionate or distearyl thiodipropionate, and thiocyanides.

[0041] Substantially unstabilized polyethylene The term "substantially unstabilized" means that the polyethylene is susceptible to oxidation during the rotomolding process. This can be confirmed by observing the formation of oxidized species (particularly carbonyl groups) on the interior surface of the rotomolded part after the rotomolding process. The oxidized species can be detected using conventional Fourier transform infrared ("FTIR") equipment.

[0042] As a general guideline, the substantially unstabilized polyethylene should contain less than 100 ppm of primary and / or secondary antioxidants, and may contain no primary or secondary antioxidants. In one embodiment, the substantially unstabilized polyethylene does not contain any primary or secondary antioxidants.

[0043] In some embodiments, substantially unstabilized polyethylene has a very low flow rate. Flow rate is determined by measuring the amount of polymer that flows through a capillary tube at a specific temperature and load. ASTM D1238 is used to measure the flow rate. In one embodiment, unstabilized polyethylene has no meaningful / measurable flow rate when using a 2.16 kg load at ASTM D1238 (190°C). A "high load" flow rate using a 21.6 kg load is 2-20 grams / 10 minutes, or for example, 3-6 grams / 10 minutes. Flow rate is also commonly referred to as "melt index."

[0044] In one embodiment, the substantially unstabilized polyethylene is prepared using a heterogeneous catalyst, such as a Zeigler Natta catalyst or a chromium catalyst.

[0045] Inner surface of molded part Without wishing to be bound by theory, it is believed that rotomolded parts prepared by this process have an outer surface made primarily of stabilized polyethylene (described above), with substantially unstabilized polyethylene tending to be located away from the outer surface (particularly on the inner surface).

[0046] In one embodiment, the inner surface of the part has a higher surface roughness than the outer surface. Surface roughness can be measured with a mechanical or optical profilometer. The inventors have used a laser confocal microscope (a type of optical profilometer) to measure the roughness of the inner surface of parts produced by this process and observed an average surface roughness "Ra" of greater than 10 microns. In comparison, the outer surface has been observed to have a much lower surface roughness (e.g., Ra of 2-4 microns).

[0047] Additionally, the inventors have observed the presence of carbonyl groups on the interior surfaces and, without wishing to be bound by theory, believe that these carbonyl groups are formed substantially by oxidation of the unstabilized polyethylene.

[0048] The inventors have performed adhesion tests (described in the Examples) to measure the strength of the bond between the surfaces of the rotomolded parts in the polyurethane layer. These tests show that high strength bonds are formed when the surface of the rotomolded part has both a) high surface roughness and b) carbonyl groups.

[0049] B. Rotational Molding Process As described above, rotational molding is a well-known process which involves filling a mold "shell" with a finely divided plastic resin filler and then rotating the mold (usually on two axes) while heating it to a temperature above the melting point of the plastic resin. The molten plastic flows through the mold cavity under the forces induced by the rotation of the apparatus. The rotation continues for a time sufficient to allow the molten plastic to cover the surface of the mold. The mold is then cooled to freeze the plastic into a solid. The final stage of the molding cycle is to remove the part from the rotational molding machine.

[0050] The time required to complete the molding cycle is a function of the bulk properties of the plastic being molded; the size of the part being molded and the molding temperature. Also, the plastic resin filled into the mold is preferably finely divided (i.e., ground into a powder) to promote "free flow" of the resin and has a high bulk density and a narrow particle size distribution.

[0051] It will also be understood that the physical properties of rotational molded parts are affected by the use of an appropriate molding cycle time for "unheated" parts having insufficient strength properties and "overheated" parts suffering from poor appearance ("burned" color) and / or reduced strength properties. It is desirable to have a short molding cycle (to improve the productivity of the expensive rotational molding machine) and a wide "process window" (i.e., the rotational molding composition ideally provides "properly heated" parts in a short period of time but does not become an "overheated" part over a long period of time).

[0052] Furthermore, the properties of rotational molded parts are affected by the molecular structure of the polymer used to prepare the parts. Important physical properties include stiffness (indicated by the modulus of elasticity of the part), environmental stress crack resistance (or "ESCR"), impact resistance and warp resistance.

[0053] Therefore, the shaping time and temperature of the process are affected by many variables. The specific / preferred conditions can also be affected by the choice of machine. However, those skilled in the art will be able to optimize the conditions without undue difficulty. Appropriate conditions for rotomolding the present composition are exemplified.

[0054] Example 1 Polyethylene The polyethylene used in the examples is shown in Table 1.

[0055] PE-1 is SURPASS® RMs539-UG (NOVA Chemicals). PE-2 is NOVAPOL® TR-0735-UG (NOVA Chemicals). PE-4 is SCLAIR® 19A (NOVA Chemicals).

[0056] PE-1 and PE-2 are examples of "stabilized polyethylene" suitable for use in the compositions and processes of the present invention respectively. PE-1 and PE-2 are also suitable for use in conventional rotomolding processes.

[0057] PE-3 is substantially unstabilized polyethylene and contains no antioxidant as shown in Table 1. Further, the flow rate of PE-3 is very low and has no meaningful I2 (i.e., it does not show a meaningful flow rate when measured by ASTM D1238 at 190 °C using a 2.16 kg load), and the "high load melt index, I 21 " (measured by ASTM D1238 at 190 °C using a 21.6 kg load) is only 4.5 grams / 10 minutes. Also, the average particle size of PE-3 is large (700 microns).

[0058] PE-4 is included for comparison purposes. This has a high load melt index I of 64 grams / 10 minutes 21 Therefore, the I 21 of PE-4 is the I 21Since it is much higher, PE-4 "flows" more easily than PE-3. However, the I2 of PE-4 is less than 1 gram per 10 minutes (as shown in Table 1). The I2 flow rate of the polyethylene used in the conventional rotational molding process is usually higher than this. In summary, PE-4 has a lower flow rate than the polyethylene used in the conventional rotational molding process, but this flow rate is "too high" for use as a substantially non-stabilized polyethylene of this composition. PE-4 has an average particle size of 600 microns.

[0059] The average particle size is measured by ASTM D1921.

[0060] A series of blends were made using these polyethylenes. The blend compositions are shown in Table 2.

Table 1

[0061] Both PE-1 and PE-2 are suitable for use in conventional / typical rotational molding processes and are suitable as the "stabilized polyethylene" of this technology. PE-1 is prepared using a single-site catalyst and PE-2 is prepared using a conventional Ziegler Natta catalyst.

[0062] The stabilizer additives added to the PE-1 used in these examples are reported below as parts per million by weight, based on the weight of the polyethylene ("ppm"). The trade names of the stabilizers are provided for convenience, together with an explanation of the chemical name or Chemical Abstracts Registry Number. 1.250 ppm of IRGASTAB™ FS042 (n,n,-di(alkyl)hydroxylamine) 2.450 ppm of DOVERPHOS™ 9228 (diphosphite: CAS Registry Number 154862-43-8) 3.750 ppm of TINUVIN™ 622 (HALS: CAS Registry Number 65447-77-0) 4.750 ppm of CHIMASSORB® 944 (HALS: CAS Registry Number 70624-18-9) 5.750 ppm of zinc oxide

[0063] The stabilizer package added to PE-2 is described similarly below. 1.500 ppm of IRGANOX® 1076 (hindered phenol: octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionaldehyde 2.1000 ppm of IRGAFOS® 168 (phosphite: tris-(2,4-di-tert-butylphenyl) phosphite) 3.1000 ppm of CYASSORB® 3529 (HALS: CAS Registry Number 193098-40-7)

[0064] PE-3 has a very low flow rate (the I2 test does not give significant results; the flow rate at a 21.6 kg load is 4.5 grams / 10 minutes). This is prepared using a chromium-based catalyst. Polyethylene with this flow rate is generally not used in the rotational molding process (instead, this type of polyethylene is suitable for blow molding). PE-3 does not contain stabilizer additives and is not stabilized.

[0065] PE-4 has a relatively low flow rate (I2 is less than 1 gram / 10 minutes, I 21 is 64 grams / 10 minutes). However, the flow rate of PE-4 is too high to be suitable as the substantially unstabilized polyethylene of the present disclosure. This is prepared using a Z / N catalyst. This type of polyethylene is suitable for the preparation of films. PE-4 contains 300 ppm of a hindered phenol (IRGANOX® 1010; CAS Registry Number 6683-19-8) as the only stabilizer.

Table 2

Table 3

[0066] The rotational molding parts were prepared from the compositions shown in Table 2 in a rotational molding machine sold by Ferry Industries Inc. under the trade name Rotospeed RS3-160. The machine has two arms that rotate around a central axis. Each arm has a plate that rotates on an axis that is approximately perpendicular to the axis of rotation of the arm. Three cast aluminum molds for producing plastic cubes with dimensions of 12.5 inches (31.8 cm) × 12.5 inches × 12.5 inches are attached to each plate. These molds produce parts with a nominal thickness of approximately 0.25 inches (0.64 cm) when initially filled with a standard charge of about 3.7 kg of polyethylene resin.

[0067] A gas-fired furnace capable of providing 2 million British Thermal Units (Btu) per hour is used to provide hot air that is circulated around the molds by a fan. Generally, the temperature inside the enclosed oven is maintained at a temperature of 250°C to 400°C for a specified period while the machine rotates the arms (typically at about 8 revolutions per minute (rpm)) and the plates (typically at about 2 rpm). The specific molding conditions for different compositions are reported in Table 2.

[0068] The "heated parts" are then cooled by opening the oven. Water spray can also be used to facilitate cooling. The "heating time", rotation speed, temperature, and cooling cycle are computer-controlled with appropriate software that also includes a data collection system.

[0069] Surface roughness The parts were cut to obtain test specimens. The surface roughness data for the inner and outer surfaces are shown in Table 3.

[0070] Adhesion test The test specimens were prepared by applying a layer of polyurethane foam with a thickness of 1 inch (2.5 cm) between two pieces of rotationally molded polyethylene (prepared as described above).

[0071] The foam was "conditioned" for 24 hours at 23°C before adhesion testing.

[0072] The specimen was placed in a tensile tester (sold under the trademark Instron 4204) and the grips were attached to the two polyethylene layers. The test speed was set at 8 inches per minute. The test continued until the part failed. The results are reported in Table 4 as "Peel Strength" (units of pounds force). [Table 4]

[0073] The results show that rotomolded parts prepared with a conventional polyethylene composition and having a conventional stabilizer package produce molded parts that do not adhere well to polyurethane foam when the parts are prepared according to the rotomolding conditions of this example (see Comparative Experiments 1, 4, and 7).

[0074] The use of substantially unstabilized polyethylene (i.e., PE-3) with low flow rate and large particle size was observed to improve / increase the inner surface roughness of the molded parts (Inventive Experiments 2, 5, 8, 12 and 15 in Table 3).

[0075] Comparative experiments 3-C, 6-C, 9-C, 11-C, 13-C and 16-C (using PE-4 as the minor component) show that the use of polyethylene with a relatively high flow rate is less effective in producing a rough inner surface.

[0076] The adhesion data in Table 4 shows that a higher surface roughness can improve adhesion to polyurethane.

[0077] Finally, without wishing to be bound by theory, it is hypothesized that there may be optimal heating time / temperature conditions that maximize adhesive bonding, as higher temperatures and / or longer heating times may increase the oxidation levels of the polyethylene to a greater extent.

[0078] This technique can "broaden" the molding conditions that provide appropriate adhesive strength, because, as shown in the data of Table 4, it was observed that a higher surface roughness can be compensated for by stronger adhesion.

[0079] The inventors observed that the inner surface of the rotomolded parts prepared according to this process can have carbonyl (C=O) groups detectable by Fourier transform infrared (FTIR) analysis.

[0080] The inventors also observed that the use of substantially unstabilized polyethylene having a low flow rate and large particle size can produce rotomolded parts having a high surface roughness.

[0081] The inventors observed that polyurethane adheres well to the rough surface having carbonyl groups.

[0082] Therefore, although not wishing to be bound by theory, the following is considered: 1) Stabilized polyethylene (having a small particle size and high flow rate compared to substantially unstabilized polyethylene) melts in the heated mold and preferentially flows to the mold surface, thereby forming the outer surface of the molded part); 2) Substantially unstabilized polyethylene (having a large particle size and low flow rate) does not flow to the mold surface like stabilized polyethylene, and as a result, substantially unstabilized polyethylene is present on the inner surface of the molded part; 3) The large particles of substantially unstabilized polyethylene, together with this low-flow material, contribute to the high surface roughness of the inner surface of the parts prepared according to this process; 4) Substantially unstabilized polyethylene is partially oxidized during the rotomolding process, thereby resulting in the formation of oxidation species on the inner surface of the parts, and in particular, carbonyl groups (C=O) are formed in an amount sufficient to be observed by FTIR analysis; and 5) The combination of A) high surface roughness and B) the presence of carbonyl improves the adhesion between the surface of the rotomolded part and polyurethane.

Industrial Applicability

[0083] A rotational molding composition is provided that comprises two polyethylenes: 1) a well-stabilized polyethylene; and 2) a blend of a poorly stabilized polyethylene having a very low flow rate. Rotational molded parts prepared from the composition have a rough inner surface to which polyurethane foam adheres well.

Claims

Claim 1 1) A melt index (I) of 0.5 to 10 grams / 10 minutes measured by ASTM D 1238 at 190 °C using a 2.16 kg load; ２ a density of 0.920 to 0.950 g / cc measured by ASTM D792; and 85 to 99 wt% of a stabilized polyethylene having an average particle size of 200 to 400 microns; and 2) (i) A high load melt index (I) of 2 to 20 grams / 10 minutes measured by ASTM D1238 at 190 °C using a 21.6 kg load ２１ ); (ii) a density of 0.950 to 0.965 g / cc measured by ASTM D792; and (iii) 15 to 1 wt% of substantially non-stabilized polyethylene having an average particle size of 500 to 3000 microns comprising wherein the stabilized polyethylene comprises a primary antioxidant selected from the group consisting of hindered phenols, hydroxylamines, and lactones in an amount of 100 to 2000 parts per million by weight, and a secondary antioxidant selected from the group consisting of phosphites and phosphinites in an amount of 100 to 2000 parts per million by weight; wherein the substantially non-stabilized polyethylene comprises less than 100 ppm of a primary antioxidant and / or a secondary antioxidant; a polyethylene composition. Claim 2 The polyethylene composition according to claim 1, wherein the substantially non-stabilized polyethylene has an average particle size of 500 to 1000 microns. Claim 3 The substantially non-stabilized polyethylene has a high load melt index I of 3 to 6 grams per 10 minutes as measured by ASTM D1238 at 190 ° C using a load of 21.6 kg. ２１ The polyethylene composition according to claim 1, having Claim 4 wherein the stabilized polyethylene 1.1) the primary antioxidant in an amount of 250 to 1500 parts per million by weight, and 1.2) the secondary antioxidant in an amount of 250 to 1500 parts per million by weight and comprises a stabilizer package. The polyethylene composition according to claim 1. Claim 5 a) preparing a polyethylene composition according to any one of claims 1 to 4, and b) subjecting the polyethylene composition to rotational molding, A process for preparing a rotationally molded part comprising. Claim 6 The process according to claim 5, wherein the rotational molding is carried out at a temperature of 250°C to 400°C. Claim 7 A process according to claim 5 or 6 for producing a rotationally molded part having an inner surface with a surface roughness of 10 microns to 30 microns. Claim 8 The process according to claim 7, wherein the surface roughness is 12 to 20 microns. Claim 9 The process according to claim 7, further characterized in that the inner surface has a carbonyl functional group detectable by Fourier transform infrared equipment.

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