Radiation-crosslinked polyolefin compositions

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DESCRIPTION

Radiation Crosslinked Polyolefin Compositions

FIELD OF THE INVENTION

The present invention relates to radiation-crosslinked polymeric compositions and to coatings and insulating materials containing these compositions.

BACKGROUND OF THE INVENTION

Polypropylenes are ideal for the preparation of coatings and insulating materials designed for use at operating temperatures above those that can be supported by other polyolefins such as polyethylene, which exhibits lower melting and softening temperatures. Polyethylenes have a maximum melting point, measured by differential scanning calorimetry (DSC), of approximately 135 ° C, while polypropylenes can have melting points as high as 175 ° C. As such, polypropylenes can withstand higher operating temperatures without being permanently damaged or deformed.

Other attractive characteristics of polypropylenes are their high stiffness, abrasion resistance, impact resistance, toughness, low cost, and relatively low density. Applications for polypropylene-based coatings and insulation include insulation of electrical wires and cables, heat-shrinkable corrosion protection sleeves for high-temperature transmission pipe jointing elements, heat-shrink tubing or forms for electrical insulation and mechanical protection , or other applications that require greater hardness and rigidity than is provided by polyethylene-based systems.

In order to maximize heat resistance and physical properties, it is necessary to provide some thermosetting characteristic to the material. This is done by crosslinking the polymer to a certain extent necessary. This renders the material resistant to melting and flow when heated to a temperature close to or above the crystalline melting point of the highest melting polymer component of the composition. This feature is necessary for the production of high temperature insulation materials and heat shrink articles. In the latter case the crosslinking provides controlled heat shrink characteristics and renders the material resistant to melting when heated to the temperature necessary to effect shrinkage.

Several examples of crosslinked predominantly polypropylene based polymer compositions are known in the prior art. US Patent No. 6,569,915 describes heat-shrinkable articles comprising a radiation-crosslinked composition predominantly based on polypropylene and also comprising an ethylene-propylene elastomer. US patents nos. 6,455,637, 6,465,547 and 6,794,453, and international application WO 2002/32983 refer to articles composed of compositions based predominantly on heat-shrinkable propylene that also include amounts of silane-grafted polyolefin. The predominantly polypropylene-based compositions described and claimed in these patents all have high stiffness, hardness, and resistance to heat deformation, making them suitable for use in demanding applications as protective sleeves for high-temperature transmission pipes where the operating temperature it is generally in the range of 110-130 ° C, or higher.

However, there are situations where it is desirable to provide compositions and articles with rigidity, toughness, and mechanical integrity similar to that of cross-linked polypropylene-based compositions, but with moderate high temperature capability as determined by those performance-dependent mechanical properties of the materials. constituent polymers above their softening points. However, these compositions should provide similar long-term thermal stability with respect to pot life at the intended operating temperature. Furthermore, there is a need for compositions that economically satisfy these requirements and offer better processability in manufacture.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned shortcomings of the prior art by providing articles comprising a radiation cross-linked polymer composition in accordance with the subject of claim 1 and a process for producing the same in accordance with the subject of claim 17. The inventors have found that compositions according to the invention have similar stiffness and hardness to predominantly polypropylene-based compositions described above, without the need for polypropylene as the predominant ingredient. Furthermore, these compositions offer improved extrusion processability in terms of control and productability.

The articles and compositions of the invention are suitable for use in applications that require a lower degree of heat resistance than is often the case for compositions predominantly comprising polypropylene. More specifically, articles and compositions according to the invention are suitable for use at operating temperatures below the softening point of the medium, linear medium or high density polyethylene component, or where mechanical deformation due to softening above this temperature is not a limiting factor. Examples include radiation crosslinked heat shrink sleeves for

ES 2 355 830 T3 the mechanical and corrosion protection of pipe joints at temperatures up to 110 ° C; radiation crosslinked heat shrink tubing products for mechanical protection, stress relief and insulation of electrical and electronic splices, connections and terminations; and radiation cross-linked electrical insulation for wires and cables where high temperature resistance, stiffness and long-term thermal stability (up to 150 ° C) are important requirements.

DETAILED DESCRIPTION OF THE PREFERRED FORMS OF REALIZATION

As mentioned above, the composition according to the invention is predominantly comprised of high density polyethylene (HDPE), medium density polyethylene (MDPE), or linear medium density polyethylene (LMDPE), referred to herein as the polyethylene. The use of the term predominantly herein with respect to compositions according to the invention is intended to mean at least 50 percent by weight. Consequently, the composition according to the invention comprises at least 50 percent by weight of the polyethylene. Preferably, polyethylene is contained in the composition in an amount of greater than 50 percent to about 90 percent, and more preferably from about 55 to 80 percent.

The terms HDPE and MDPE as used herein are defined in accordance with the 1248 D standard of the American Society for Testing and Materials (ASTM). By definition, MDPE has a density ranging from 0.926 to 0.940 g / cm³ and HDPE has a density of at least 0.941 g / cm³. The density of LMDPE falls within the same range of densities as MDPE. By way of contrast, low density polyethylene (LDPE) is defined by the ASTM D 1248 standard with a density of 0.910 to 0.925 g / cm³. Furthermore, lDpE has a crystalline melting point of no more than 115 ° C.

The polyethylene used in the present invention preferably has a density of about 0.93 to 0.97 g / cm³, a melt index of approximately 0.1 to 10 dg / min and a crystalline melting point of at least 120 ° C. Polyethylene includes homopolymers of ethylene and copolymers of ethylene with higher alpha olefins such as butene, hexene, and octene and is of a predominantly linear molecular structure. Polyethylene can preferably be made using metallocene catalysts, also known as single-site, stereospecific, or restricted geometry catalysts, and can also comprise a bimodal molecular weight distribution. In some cases, the inventors have found that these materials provide the necessary crosslinking sensitivity for compositions according to the invention without the need for additional crosslinking promoters.

The composition according to the invention also comprises a polypropylene which is selected from one or more members of the group comprising homopolymers of polypropylene and copolymers of propylene with an olefin other than propylene. Preferred polypropylene copolymers are propylene and ethylene copolymers. Furthermore, the polypropylene component can preferably be modified with reactive functional groups, such as silanes, acrylic acids, methacrylic acids, acrylates, methacrylates, glycidyl methacrylates, and anhydrides. When polypropylene is a copolymer, it contains at least about 80 weight percent propylene.

The polypropylene component is present in the composition in an amount greater than about 5 percent and less than 50 percent by weight. Preferably, the polypropylene content of the composition is from about 5 to 40 percent by weight, and more preferably from about 10 to 30 percent by weight.

Polypropylene is preferably isotactic in nature, with a density of about 0.85 to 0.91 g / cm³ and a melt index of about 0.1 to 10 dg / min. Preferably, the polypropylene has a melt viscosity, measured by the melt index, which is similar to that of the HDPE component at the same temperature and under the same shear conditions required for processing the mixture, to ensure optimum compatibility. mix. The crystalline melting point of polypropylene is typically in the range of about 160-170 ° C, with about 165 ° C being typical.

The composition according to the invention optionally comprises one or more ethylene-propylene elastomers selected from the class of materials known as ethylene-propylene elastomers or copolymers (EPM), more preferably from those known as ethylene elastomers or terpolymers. propylene diene (EPDM), and most preferably of those ethylene-propylene diene elastomers or terpolymers polymerized using metallocene or single-site catalysts (mEPDM), or mixtures thereof. The ethylene-propylene elastomer component is preferably selected to have a melt viscosity as close as possible to polypropylene and polyethylene components at the same temperature and under the same shear conditions required for processing the mixture. The ethylene-propylene elastomer component preferably has a density of about 0.85 to 0.92 g / cm³ and a Mooney viscosity (ML 1 + 4 at 125 ° C), which is indicative of melt viscosity, of about 5 to 50.

Preferably, the ethylene-propylene elastomer comprises about 40 to 95% by weight of ethylene, more preferably about 70 to 95% by weight of ethylene, even more preferably about 75 to 95% by weight of ethylene, and most preferably about 85 to 95% in

ES 2 355 830 T3 weight of ethylene. Ethylene-propylene-diene terpolymers further comprise about 0.5 to 10% by weight of diene monomer usually selected from 5-ethylidene-2-norbornene, dicidopentadiene, or 1,4-hexadiene, and preferably 5- ethylidene-2-norbornene.

The most preferred terpolymers of mEPDM are prepared by copolymerizing propylene with ethylene and one or more of the diene monomers listed above using a highly stereospecific, single-site, restricted geometry, or a so-called metallocene catalyst. These preferred mEPDM materials differ from EPDM materials produced using standard Ziegler-Natta coordination catalysts in that the amount and position of comonomers within the polymer structure can be more precisely controlled to provide a more precise molecular weight distribution. and a more regular molecular architecture, resulting in higher crystallinity, for example, and superior material properties. Most importantly with respect to the present invention, it is possible to adjust comonomer levels for optimal sensitivity of mEPDM materials for electron beam irradiation crosslinking. In some cases, the inventors have found that these materials provide the necessary crosslinking sensitivity for compositions according to the invention without the need for additional crosslinking promoters.

The ethylene-propylene elastomer content in the composition according to the invention is up to about 40%, more preferably up to about 30%, even more preferably about 5 to 20% by weight of the composition.

The composition may further comprise one or more optional ingredients selected from the group comprising radiation sensitizers (also known as crosslinking promoters), compatibilizers, pigments, antioxidant stabilizers, heat stabilizers, ultraviolet (UV) stabilizers, mineral fillers, halogenated flame retardants, processing aids, and the like.

The optional compatibilizer may be selected from the group comprising: any of the polyethylenes and polypropylenes described above; one or more members of the group comprising ethylene-propylene copolymers; ethylene-propylene diene elastomers; crystalline propylene-ethylene elastomers; thermoplastic polyolefin elastomers; metallocene polyolefins; cyclic olefin copolymers; polyoctenamers; copolymers of ethylene with vinyl acetate, vinyl alcohol, and / or alkyl acrylates; polybutenes; hydrogenated and non-hydrogenated polybutadienes; butyl rubber; polyolefins modified with reactive functional groups selected from the group consisting of silanes, alcohols, amines, acrylic acids, methacrylic acids, acrylates, methacrylates, glycidyl methacrylates, and anhydrides; polyolefin ionomers; polyolefin nanocomposites; and block copolymers selected from the group consisting of styrene-butadiene, styrene-butadiene-styrene, styrene-ethylene / propylene, and styrene-ethylene / butylene-styrene.

In any given composition according to the invention, the compatibilizer is different from the polyethylene, polypropylene and ethylene-propylene elastomer components of that particular composition. Furthermore, when the compatibilizer comprises a polyethylene, polypropylene or ethylene-propylene elastomer as defined above, the amount of compatibilizer present in the composition is such that the total amounts of the polyethylene, polypropylene and ethylene-propylene elastomers in the composition fall within the ranges described above for these components.

The compatibilizer is preferably added to the composition in an amount of up to about 25 percent by weight, more preferably up to about 15 percent, and even more preferably from about 5 to 10 percent by weight of the composition, of so that the composition remains predominantly polyethylene based. The function of the compatibilizer is primarily to improve the miscibility of the polyethylene and polypropylene components when mixed with each other, so that the mixture exhibits a homogeneous behavior during use. The addition of the compatibilizer may not be required when the other components of the composition show satisfactory performance or natural miscibility.

The radiation sensitizer is preferably selected from the family of multifunctional monomers that are generally used as crosslinking promoters for polyolefin-based polymers. Preferred monomers include trimethylol propane triacrylate, trimethylol propane trimethacrylate, tetramethylol tetraacrylate, ethylene glycol dimethacrylate, triallyl cyanurate, and triallyl isocyanurate. The radiation sensitizer is preferably added in an amount of about 0.25 to 2.5%, more preferably 0.5 to 1.5% by weight of the mixture. The function of the radiation sensitizer is to make the polymer composition more susceptible to crosslinking by electron beam radiation (beta) or gamma radiation, thus allowing a certain level of crosslinking to be achieved with less radiation dose and energy than if not. a sensitizer is used. The addition of the crosslinking promoter is not necessary when the composition is sufficiently sensitive to irradiation to achieve the necessary degree of crosslinking.

The antioxidant stabilizer can be selected from any suitable antioxidant or mixture of antioxidants designed to prevent degradation of the composition during melt processing and subsequent heat aging of the final product. Examples of suitable antioxidants and thermostabilizers include those classes of chemicals known as hindered phenols, hindered amines, phosphites, bisphenols, benzimidazoles, phenylenediamines, and, dihydroquinolines. These are preferably added in an amount of about 0.1 to 5%

ES 2 355 830 T3 by weight of the mixture, depending on the required aging properties and the type and amount of optional destabilizing ingredients in the composition, for example halogenated flame retardants or mineral fillers. It should also be noted that these antioxidants and stabilizers, if added in excessive amounts, can become radiation scavengers, which act to reduce the effectiveness of radiation to induce the desired crosslinking reaction and the resulting degree of crosslinking that can be obtained. for a given radiation dose.

The composition according to the invention is formed by melting polyethylene and polypropylene, together with any of the optional components described above. The melting of the components can be carried out as a separate step before the melt processing of the finished article, or it can be carried out simultaneously with the melt processing of the finished article.

When carried out as a separate pre-step, the components are preferably melt mixed using a machine specifically designed for that purpose, such as a continuous single or twin screw extrusion mixer, kneader, or internal batch mixer. . The mixed composition can then be pelleted and stored for further melt processing into the desired finished article.

Melt processing of the composition can preferably be carried out by techniques commonly used in industry such as extrusion or molding. Examples of extruded articles include sheet, tube, and electrical insulation. In some preferred embodiments, the composition can be co-extruded or laminated with another material of similar or different composition to form a laminated structure with discrete but well-bonded layers, each layer having different functional properties. For example, an adhesive coated polymeric sheet can be produced by co-extruding or laminating the composition with an adhesive. In other examples, the composition can be laminated with a less expensive or non-crosslinkable layer. Injection molded, compression or air injection molded articles can be produced and examples include electrical insulating articles such as plugs and heat shrink boots.

Once formed, the article is cross-linked by irradiation, preferably with electron beam, UV or gamma radiation. Crosslinking is the formation of permanent covalent bonds between individual polymer chains that act to link the polymer chains together and prevent them from irreversibly separating during subsequent heating. It is this cross-linked structure that, while preserving the elastomeric nature of the material, makes the material thermoset and melt resistant which, in turn, is a desirable property for producing heat shrinkable articles, as discussed below. Crosslinking also provides the article with high heat resistance, allowing it to maintain mechanical strength and integrity at high service temperatures.

Preferably, the article is irradiated by electron beam radiation at a dose of about 1 to 20 megarads in an electron beam accelerator, for example a Dynamitron manufactured by Radiation Dynamics Inc. The desired dose is dependent on the desired properties of the article. . Too low a dosage will result in the article having a low degree of crosslinking, poor mechanical strength, and a tendency to soften or melt prematurely at elevated temperatures. Too high a dose can result in degradation of the polypropylene component with unacceptable resulting deterioration of mechanical properties. It has been found that a preferred dose is between 5 and 10 megarads for the manufacture of heat shrinkable articles according to the invention. Higher rates may be more suitable for wire and cable insulation applications.

The radiation dose is sufficient to provide the article with a level of crosslinking, as measured by the gel fraction, of approximately 20 to 90 percent. Preferably, the gel fraction of the cross-linked article is about 30 to 70 percent, and more preferably about 40 to 70 percent.

As mentioned above, articles produced according to the invention can be made heat shrinkable as they exhibit the property of not melting when heated to a temperature close to or above the crystalline melting point of the higher melting point component, which it is usually polypropylene. This is important because the cross-linked structure allows the article to stretch with minimal force and without melting, and to maintain its mechanical integrity when heated to this temperature. The article is fixed in this stretched state by rapidly cooling it below the crystalline melting point while maintaining the article in its stretched position, the reformed rigid crystalline regions of the polymeric components of the material preventing the article from spontaneously regaining its original dimensions. The stretching of the article can be done by mechanical, pneumatic or hydraulic means. Cooling of the article in its stretched state can be accomplished by a cooling medium such as air, water, or other heat transfer medium.

Subsequent reheating of the stretched article above the melting point of the higher melting component will cause the crystalline regions to re-melt and the structure elastomerically return to its original unstretched dimensions. The crosslinked structure provides the initial restoring force and again ensures that the article does not melt and that it maintains its mechanical integrity.

The invention is further illustrated by the following examples:

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EXAMPLE 1

An isotactic polypropylene copolymer of density 0.90 g / cm³ and melt index 0.45 dg / min (Profax 7823 from Basell Polyolefins), an hDpE of density 0.947 g / cm³ and melt index 0.28 dg / min and a metallocene-catalyzed ethylene propylene diene terpolymer of density 0.908 g / cm³ and melt index 1.0 dg / min (Nordel IP 4820P from DuPont Dow Elastomers) were melt blended with an antioxidant masterbatch of 15% hindered phenol and phosphite stabilizers in polyethylene (Irganox B225 from Ciba Specialty Chemicals) and a pigment masterbatch of 25% carbon black in polypropylene in the amounts shown in Table 1 using a Buss Kneader tilting screw continuous extrusion mixer, at a temperature of approximately 180 ° C, the dispersed mixture then being fed through a hot cut pelletizer and dryer attachment.

The composite pellets produced in this way were fed through a 24: 1 L / D single screw extruder equipped with a monolayer sheet die cutter, and extruded into sheet at a melt temperature of about 220 ° C. The sheet was set to the required width, thickness and orientation dimensions by running it through a cooled 3-roll calendering stack.

The extruded sheet was then cross-linked at a dose of approximately 5 megarads using an electron beam accelerator "Dynamitron" from Radiation Dynamics, and was then tested for the degree of cross-linking achieved and for the mechanical properties indicated in the Table. 2.

The crosslinked sheet was then reheated to a temperature of about 150 ° C, and then stretched about 50% using a machine direction stretch (MDO). While in the stretched state, the sheet was rapidly cooled by feeding it between water-cooled steel rollers to below the crystalline melting point of the composition to fix the sheet in the stretched dimensions. The sheet was subsequently extrusion laminated with a layer of thermoplastic glue.

EXAMPLE 2

In this example, the process of Example 1 was repeated except that the relative amounts of HDPE, polypropylene, and terpolymer were varied as shown in Table 1.

EXAMPLES 3, 4 AND 5

In these examples, the effect of alternative HDPE components was examined. In Example 3, the HDPE had a density of 0.960 g / cm³ and the melt index of 4.9 dg / min. In Example 4, a metallocene catalyzed HDPE of density 0.935 g / cm was used.³ and a melt index of 0.9 dg / min. Example 5 contained the same HDPE used in Examples 1 and 2, above. The polypropylene-HDPE ratio remained constant in all cases.

The Examples were prepared by mixing the components listed in Table 1 using an internal laboratory mixer at a temperature set at approximately 200 ° C. The mixed compositions were then pressed into plates approximately 0.060 in. Thick. and subsequently cross-linked as described in Example 1.

The crosslinked plates were then tested for the degree of crosslinking achieved and for the mechanical properties indicated in Table 2.

EXAMPLE 6

An extrusion cross-linked heat shrink tubing product of the composition of Example 1 was prepared at a melting temperature of 220 ° C in a tubular cross-section, irradiating the extruded tube at a dose of approximately 5 megarads, heating the thus cross-linked tube to 190 ° C, stretching the tube to twice its original internal diameter using internally applied compressed air, finally spraying the stretched tube with cold water to freeze the tube in its stretched state.

EXAMPLE 7

A crosslinked insulated wire was prepared by extruding the composition of Example 5 at a melting temperature of 240 ° C onto a 14 AWG size wire, then irradiating said wire at a dose of 12 megarad.

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TABLE 1

<td colspan="6">Compositions</td>

<td>Ingredient</td><td>Example 1</td><td>Example 2</td><td>Example 3</td><td>Example 4</td><td>Example 5</td>

<td>HDPE</td><td> 60</td><td> 80</td><td> 10</td><td> 20</td><td> 70</td>

<td>Polypropylene (Profax 7823)</td><td> 30</td><td> 15</td><td> 30</td><td> 30</td><td> 30</td>

<td>EPDM (Nordel IP 4820P)</td><td> 10</td><td> 5</td><td> -</td><td> -</td><td> -</td>

<td>Antioxidant Master Mix *</td><td> 12</td><td> 12</td><td> 7,5</td><td> 7,5</td><td> 7,5</td>

<td>Pigment Master Mix **</td><td> 0,5</td><td> 0,5</td><td> 0,5</td><td> 0,5</td><td> 0,5</td>

<td colspan="3">* 15% masterbatch in polyethylene</td><td></td><td></td><td></td>

<td colspan="3">** 25% polypropylene masterbatch</td><td></td><td></td><td></td>

TABLE 2

<td colspan="6">Process Properties and Conditions</td>

<td>Property</td><td>Ex. 1</td><td>Ex. 2</td><td>Ex 3</td><td>Ex. 4</td><td>Ex. 5</td>

<td>Dose (Mrad)</td><td> 5</td><td> 5</td><td> 8</td><td> 8</td><td> 8</td>

<td>Gel Fraction (%)</td><td> 50</td><td> 70</td><td> 40</td><td> 45</td><td> 45</td>

<td>Hot Tensile Strength @ 200 ° C and 100% Elongation (psi)</td><td> 12</td><td> 13</td><td> 3,5</td><td> 5,5</td><td> 13,5</td>

<td>Ultimate Hot Elongation @ 200 ° C (%)</td><td> &gt; 450</td><td> 400</td><td> &gt; 450</td><td> &gt; 450</td><td> &gt; 450</td>

<td>Ultimate Tensile Strength @ 23 ° C (psi)</td><td> 4300</td><td> 3100</td><td> 2900</td><td> 2900</td><td> 3000</td>

<td>Ultimate Elongation @ 23 ° C (%)</td><td> 600</td><td> 550</td><td> 13</td><td> 180</td><td> 10</td>

<td>Flex Modulus @ 23 ° C (psi)</td><td> 63.000</td><td> 56.000</td><td> 65.000</td><td> 55.000</td><td> 62.000</td>

Although the invention has been described with respect to certain preferred embodiments, it will be understood that it is not intended to be limited thereto. Rather, the invention is intended to encompass all embodiments that are within the scope of the following claims.