Thermoplastic elastomer composition with improved overmolding adhesion

Abstract: The present invention relates to A thermoplastic elastomer composition comprising: a hydrogenated elastomeric block copolymer having at least two resinous endblocks of polymerized monovinyl aromatic compound and an elastomeric midblock of polymerized conjugated diene; a plasticizer; a olefin-acrylate polymer; and a polyolefin. The present invention further relates to a process for the preparation of an overmoulded polar polymer shape; to an overmoulded polar polymer shape; to an overmoulded polar polymer shape; and to articles containing the same.

Description: This invention relates to with good overmolding adhesion. In particular, this invention relates to a thermoplastic elastomer composition with good adhesion to polar substrates such as polycarbonate or acrylonitrile-butadiene-styrene (ABS). Although elastomeric block copolymers are known to be compatible with polyolefins, their use on polar matrices/substrates, such as polycarbonate or ABS, has been limited due to poor compatibility between the block copolymer and the substrate. In particular, it is desirable in many instances to mold a soft rubber material directly onto a hard substrate, thus providing a soft cover. However, the use of many attractive soft, rubbery elastomers is limited due to incompatibility between the elastomer and the substrate. The rubber coating will not adhere well to the substrate and sloughs off with use. EP-A-0 409 155 discloses a thermoplastic elastomer composition comprising a) 25-60 % by weight of a hydrogenated block copolymer having at least two endblocks of polymerized monovinyl aromatic compound and an elastomeric midblock of polymerized diene, like Kraton 1650, b) 1-20 % by weight of a plasticizer, c) 5-35 % by weight of an ethylene-acrylate polymer and d) 20-40 % by weight of polypropylene (see claims 1, 2, example 1-4). The composition is used for a multi bag system for the storage of blood platelets. The plasticizer is eluted into the blood platelets to prevent the shape of the platelets from changing. EP 0 203 425 A1 describes a composition comprising an olefin/acrylate copolymer and a block copolymer of vinylaromatic and aliphatic diene monomers which is useful as a tie layer between an aromatic polycarbonate and a polyolefin. However, the adhesion between the tie layer and the aromatic polycarbonate could still be improved upon. It would be desirable to have a soft, rubbery thermoplastic elastomer composition that will adhere more strongly to hard (polar) substrates such as polycarbonate and ABS. Such a composition has now surprisingly been found. Therefore, the present invention relates to a thermoplastic elastomer composition comprising: 15-60% by weight of a hydrogenated elastomeric block copolymer having at least two resinous endblocks of polymerized monovinyl aromatic compound and an elastomeric midblock of polymerized conjugated diene; 15-45% by weight of a plastizer selected from paraffinic oils; vegetable oils and animals oils or their derivatives; and hydrogenated oils; 5-20% by weight of a olefin-acrylate polymer; and 0.1 to 20% by weight of a polypropylene. Applicants have found that such compositions, and in particular, the combination of an ethylene-acrylic ester copolymer and a hydrogenated elastomeric block copolymer yields a thermoplastic elastomer composition with improved adhesion characteristics, especially to polar substrates such as polar polymers, i.e. those polymer being more polar than polyolefins. The elastomeric block copolymers suitable for use herein are known in the art, as disclosed for instance in U.S. 5,194,530. The elastomeric block copolymers have at least two resinous endblocks of polymerized monovinyl aromatic compound, thus giving a resinous segment, and an elastomeric midblock of polymerized conjugated diene, thus giving an elastomeric segment. The block copolymers can be linear, such as A-B-A, or (A-B) n X or radial, such as (A-B) p X. In these formulae A stands for a polymerised monovinyl conjugated diene block; X stands for a coupling agent; n is 2 and p is more than 2, preferably from 3 to 20, more preferably from 3 to 6. Suitable monovinyl aromatic compounds include those having 8 to 20 carbon atoms as exemplified by styrene and styrene homologs such as alpha-methylstyrene and para-methylstyrene. Styrene is especially preferred. Suitable conjugated dienes include those having 4 to 8 carbon atoms. Illustrative of such conjugated dienes are 1,3-butadiene (butadiene), 2-methyl-1,3-butadiene (isoprene), 1-3-pentadiene (piperylene), 1,3-octadiene, and 2-methyl-1,3-pentadiene. Preferred conjugated dienes are butadiene and isoprene, most preferably butadiene. The conjugated diene blocks may also be formed of two or more different conjugated diene monomers. The weight average molecular weight of the block copolymer will generally be at least 50,000. For linear polymers, the weight average molecular weight will generally be within the range of 50,000 to 300,000. Actually, the upper limit is dictated by viscosity considerations and can be as high as can be tolerated and still be processable. The most preferred weight average molecular weight for linear block copolymers is 50,000 to 185,000. With radial polymers, the molecular weight can be much higher since these polymers have a lower viscosity for a given total molecular weight. Thus, for radial polymers the weight average molecular weight generally will be in the range of 50,000 to 1 million, preferably 100,000 to 500,000. The block copolymer used in the present invention may be a single block copolymer or a blend of block copolymers. Adhesion has been seen to improve when the block copolymer of the present invention comprises at least one block copolymer having a weight average molecular weight less than 75,000. When used herein in reference to linear copolymers, the term "molecular weight" is the weight average molecular weight as determined by gel permeation chromatography (GPC) where the GPC system has been appropriately calibrated with polystyrene standards. The polymer in anionically polymerized linear polymers is essentially monodispersed and it is both convenient and adequately descriptive to report the peak molecular weight of the narrow molecular weight distribution observed. Such methods are well known and described in patents including U.S. 5,229,464. Since styrene (as polystyrene) is used as the calibration standard, the measurement directly gives the absolute molecular weight of the styrene endblocks. Knowing the molecular weight and percentage of styrene present, the absolute molecular weight of the midblock segment can be calculated. The molecular weight is measured on an aliquot which is removed after the first step of the polymerization and then terminated to deactivate the initiator. It is not as straightforward to measure the true molecular weight of a final coupled star or radial copolymer by using GPC. The star-shaped molecules do not separate and pass through the packed GPC columns in the same manner as do the linear polymers used for the calibration. Hence, the time of arrival at a UV or refractive index detector is not a good direct indicator of the molecular weight. Therefore, instead of GPC, light scattering techniques, may be used to measure the weight average molecular weight of a star copolymer. The sample is dissolved in a suitable solvent to a concentration of less than 1 gram of sample per 100 milliliters of solvent and then filtered directly into the light scattering cell using a syringe and porous membrane filters of less than 0.5 micron pore size. The light scattering measurements are performed as a function of scattering angle and of polymer concentration using standard procedures. The differential refractive index (DRI) of the sample is measured at the same wave length and in the same solvent used for the light scattering. This results in a molecular weight value which approximates the weight average molecular weight of the radial copolymer. For the purposes of this specification, molecular weight values and ranges referred to herein are weight average molecular weight values and ranges. The elastomeric block copolymers utilized in this invention are hydrogenated to such a degree that the unsaturation of the elastomeric block is greatly reduced without significant change in unsaturation of the resinous block component. Generally, at least 90 percent of the unsaturation in the diene midblock is hydrogenated and no more than 25 percent, preferably less than 10 percent, of the aromatic unsaturation is hydrogenated. Such hydrogenation techniques are known in the art and disclosed, for instance, in US Reissue 27,145. Since the block copolymers utilized in this invention are hydrogenated to remove the aliphatic unsaturation, they can be viewed as S-EB-S polymers, or (S-EB) (n or p) X as the case may be, where the S refers to the monovinyl aromatic, generally styrene, endblocks and the EB represents ethylene/butylene, which is the structure resulting from the hydrogenation of polymerized 1,3-butadiene, and n,p and X as herein before described. The composition of this invention contains in addition an ethylene-acrylate polymer which is compatible both with the elastomeric block copolymer and the hard substrate, such as polycarbonate or ABS. Where maleic anhydride or acrylate combinations are present, ethylene-acrylate polymers include copolymers and terpolymers of ethylene-methyl acrylate, ethylene-ethyl acrylate, ethylene-butyl acrylate, ethylene-methyl methacrylate, ethylene-acrylic acid, ethylene-methacrylic acid, ethylene-glycidyl methacrylate, and mixtures thereof. An ethylene-acrylate copolymer or terpolymer containing methyl acrylate has provided good results when used with the block copolymers utilized in the invention. An example of an ethylene-methyl acrylate copolymer is OPTEMA TC-221 (OPTEMA is a trademark), an ethylene-methyl acrylate copolymer containing approximately 30 %w methyl acrylate and having a density of about 0.948 g/cc. An example of an ethylene-methyl acrylate terpolymer is IGETABOND 7M (IGETABOND is a trademark), an ethylene-glycidyl methacrylate-methyl acrylate terpolymer containing approximately 3 %w glycidyl methacrylate and approximately 30 %w methyl acrylate. IGETABOND 7M has a density of about 0.964 g/cc. The acrylate content is typically greater than 10 %w. The plasticizer used is typically an oil which is compatible with the elastomeric mid-block segment of the elastomeric block copolymer and which does not tend to go into the aromatic endblock portions to any significant degree. Therefore, the plasticizer is selected from paraffinic oils; vegetable oils and animal oils or their derivatives; and hydrogenated oils. Paraffinic oils which may be used in the elastomeric composition should be capable of being melt processed with other components of the elastomeric composition without degrading. Particularly important is the ability of the final composition to be melt extruded. An excellent oil is a white mineral oil sold as DRAKEOL 34, available from the Pennreco Division of the Pennzoil Company (DRAKEOL is a trademark). DRAKEOL 34 has a specific gravity of 0.864-0.878 at 51°C, a flashpoint of 238°C, and a viscosity of 370-420 SUS at 38°C (0.8-0.9 cm 2 /sec). Examples of suitable hydrogenated oils are hydrogenated naphthenics. Materials which are initially aromatic in nature should be hydrogenated to the point where they exhibit no significant levels of aromatic unsaturation or at most, very low levels, typically not more than 5%. It has surprisingly been found that improved adhesion to polar substrates is obtained with the addition of a polymer compatible with the midblock segment, the endblock segments or both, namely polypropylene. The composition of the invention contains 0.1 to 20 weight percent, preferably 3 to 15, even more preferably 5-10, weight percent polypropylene. A flow promoter, or endblock resin, may also be added. A flow promoter provides better "wet-out" of the thermoplastic elastomer and thus, better contact between the elastomer and the substrate, leading to improved adhesion. Preferably, the flow improver is a styrenic resin, more preferably predominantly (i.e. more than 50% by weigh) an α-methylstyrene resin. An example of a flow promoter which has produced good results is KRISTALEX 1120 (KRISTALEX is a trademark). KRISTALEX 1120 is a water-white, low-molecular-weight, thermoplastic hydrocarbon resin derived largely from α-methylstyrene. KRISTALEX 1120 has a specific gravity of 1.07 at 25/25°C, a flashpoint of 236°C, and a viscosity of 50 cm 2 /sec (stokes) at 25°C. Flow promoter additions of 0 to 10 weight percent, preferably 3 to 10 weight percent, have provided good results. Thus, the composition of this invention comprises the hydrogenated elastomeric block copolymer, the ethylene-acrylate copolymer or terpolymer, and the plasticizer. A preferred mix is 35-60 weight percent block copolymer, 15 to 45 weight percent plasticizer, and 10-20 weight percent ethylene-acrylate polymer, all percentages based upon the total weight of the combined additives. Generally, no crosslinking agents are used in the composition. This, of course, does not exclude the presence of materials such as stabilizers which are used for the normal purpose of stabilization. Examples of suitable stabilizers are disclosed in U.S. 4,835,200. Particularly preferred are hindered phenols, particularly less volatile hindered phenols such as tetrakis [methylene (3, 5-di-tert-butyl-4-hydrooxyhydrocinnamate)] methane sold by Ciba Geigy under the tradename IRGANOX 1010 alone or in combination with a thiosynergist such as DLTDP (dilaurylthiodipropionate). Stabilizers are typically present in an amount from 0.1 to 5% by weight, preferably 0.5 to 2.5 % by weight. The thermoplastic elastomer compositions may be prepared according to procedures in general known to those skilled in the art. It is preferred that first the block copolymer and the plasticizer are mixed prior to blending with the other components to prepare the thermoplastic elastomer composition according to the invention. The invention is particularly useful for direct overmolding onto polar matrix/substrate materials such as polycarbonate or ABS. Potential uses include, without limitation, automotive instrument panels, knobs, buttons, pen/pencil grips, cellular phones, toothbrushes, handles, and tool grips. The composition may also be co-extruded, co-extrusion blowmolded, or double-injection molded. It is believed that overmolding onto a warm substrate and/or aging are helpful in improving adhesion. According to a further aspect, the present invention relates to a process for the preparation of an overmoulded polar polymer shape which comprises extruding the thermoplastic elastomer composition to form an extrudate; forming a shape comprising a polar polymer; and molding the extrudate onto the polar polymer to form an overmoulded polar polymer shape. The extrudate is injected and moulded onto a warm polar polymer shape according to procedures known to those skilled in the art. Alternatively, the extrudate may be co-extruded with the polar polymer. According to yet another aspect, the present invention relates to an overmoulded polar polymer shape comprising a first layer containing a polar polymer and a second layer overmoulded onto the first layer containing the thermoplastic elastomeric composition as well as to articles containing the overmoulded polar polymer shape. If desired, the overmoulded polar polymer shape may be subjected to an ageing treatment, to improve adhesion. Examples Fourteen test compositions having varying amounts of block copolymer, oil, ethylene-acrylate copolymer or terpolymer, polypropylene, and flow promoter were prepared. First the block copolymer (in crumb form) and oil were mixed together either by hand or with a mixer. Once the oil was absorbed by the crumb, the remaining ingredients were added and the entire mixture was tumble-mixed. The mixture was then added to an extrusion hopper and feed to a co-rotating, intermeshing twin screw extruder. The mixture was extruded at 300 rpm and a melt temperature of 230°C to 240°C. The extrudate was formed into pellets by strand cutting. The injection molding machine used to make overmolding adhesion specimens was equipped with an insert mold which had the ability to mold two different thickness plaques. First, the desired substrate material (e.g., polycarbonate, ABS) was molded into 0.32 cm (0.125 inch) thick plaques. The insert mold was then modified to allow molding of a thicker plaque. A strip of masking tape was laid across one end of the premolded plaque parallel to the molding direction. The premolded substrate plaque was then placed back into the insert mold and the run composition was injected into the mold cavity, forming an approximately 0.25 cm (0.100 inch) thick covering over the substrate material. The overmolded plaque was then cut perpendicular to the direction of flow into 2.54 cm (1 inch) wide strips for a 90° peel adhesion test. The area where the making tape was placed on the plaque acted as a site for gripping the compound portion of the overmolded plaque during testing. Testing was conducted per ASTM D-903 with modifications made for specimen size (1" x 6" (2.54 x 15.24 cm) as opposed to 1" x 8") and test speed (2 inches/minute 5.08 cm/min). Tests were conducted on an INSTRON (INSTRON is a trademark) testing machine at ambient temperature (approximately 75°F - 78°F (23.9 - 25.6°C)). The initial peel and average peel were measured. The initial peel was measured at the point where the curve on the adhesion test curve suddenly changed slope due to the beginning of the peel. The average peel was the average of the substantially horizontal slope during the majority of the peel. Example 1 A first set of test runs were conducted on compositions having either a medium or high molecular weight block copolymer and varied additions of ethylene-acrylate copolymer or terpolymer. The test run compositions and adhesion test results are given in Table 1. It can be seen that significantly improved adhesion is obtained with the addition of a ethylene-acrylate copolymer or terpolymer and polypropylene (Compositions 4 and 6-8). Note also that the use of less oil (Runs 4, and 6-8) and a higher melt flow copolymer (Compositions 7 and 8) tends to lead to higher and more uniform adhesion values. Example 2 A second set of tests were conducted on compositions having varied copolymer mixtures and of flow promoter additions. The test run compositions and results of the adhesion tests are given in Table 2. It can be seen that improved flow of the compound is realized through use of a low to medium molecular weight block copolymer (Compositions 9-14), or functionalized block copolymer (Compositions 9 and 10), or endblock resin (Composition 14). The improved flow leads to improved overmolding adhesion to the ABS and polycarbonate.

Claims: A thermoplastic elastomer composition comprising: 15-60% by weight of a hydrogenated elastomeric block copolymer having at least two resinous endblocks of polymerized monovinyl aromatic compound and an elastomeric midblock of polymerized conjugated diene; 15-45% by weight of a plasticizer selected from paraffinic oils; vegetable oils and animals oils or their derivatives; and hydrogenated oils; 5-20% by weight of a olefin-acrylate polymer; and 0.1 to 20% by weight of a polypropylene. A thermoplastic elastomer composition according to claim 1, wherein the olefin-acrylate polymer is an ethylene-acrylate polymer. A thermoplastic elastomer composition according to claim 2, wherein the ethylene-acrylate polymer is a copolymer or terpolymer selected from ethylene-methyl acrylate, ethylene-ethyl acrylate, ethylene-butyl acrylate, ethylene-methyl methacrylate, ethylene-acrylic acid, ethylene-methacrylic acid, ethylene-glycidyl methacrylate and mixtures thereof. A composition according to claim 3, wherein the ethylene-acrylate polymer is an ethylene-methyl acrylate copolymer or terpolymer. A thermoplastic elastomer composition according to any one of claims 1-4, wherein said resinous endblocks comprise polymerized styrene and said elastomeric midblock comprises polymerized 1,3-butadiene hydrogenated to remove the aliphatic unsaturation, thus appearing as ethylene/butylene. A thermoplastic elastomer composition according to any one of claims 1-5, wherein said block copolymer has a weight average molecular weight in the range 50,000 to 500,000. A thermoplastic elastomer composition as claimed in any one of the preceding claims, further comprising a flow promoter. A thermoplastic elastomer composition according to any one of the preceding claims wherein the hydrogenated elastomeric block copolymer is a blend of block copolymers wherein at least one block copolymer has a weight average molecular weight of less than 75,000. Process for the preparation of an overmoulded polar polymer shape which comprises extruding the thermoplastic elastomer composition according to any one of the preceding claims to form an extrudate; forming a shape comprising a polar polymer; and molding the extrudate onto the polar polymer to form an overmoulded polar polymer shape. Overmoulded polar polymer shape comprising a first layer containing a polar polymer and a second layer overmoulded onto the first layer containing the thermoplastic elastomeric composition according to any one of the preceding claims 1-8. Articles containing the overmoulded polar polymer shape according to claim 10.

Inventors: MODIC MICHAEL JOHN, MODIC, MICHAEL JOHN
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH
Application year: 1997
Main classification: C08L53
IPC codes: C08L33, C08J5, C08L53, C08L23, B29C45