Segmented copolymers, their manufacture and blends with polypropylene

Abstract: Segmented copolymers of polypropylene with stryene, when made using an initiator having a one-hour half life at a temperature of 110 to 125°C in a process involving a heating rate of at least 1°C per minute, are desirable sag-resistance improvers for polyproplyene.

Description: This invention is concerned with processes for making certain segmented copolymers which are particularly effective in imparting sag resistance to polypropylene, with copolymers so made and with compositions containing them. Our copending application EP-A-0 528 600 discloses contain blends of polyolefin with segmented copolymers. In its broadest aspect it provides a polymer blend comprising: (a) polyolefin; and (b) from 0.1 to 50% by weight of segmented copolymer of non-polar olefin polymer containing at least 75% by weight olefin mers and optionally mers of one or more of vinyl acetate, methyl acrylate, methyl methacrylate and ethyl acrylate and polymer derived from a monomer system containing above 20 to 100% by weight of vinyl aromatic monomer and from 0 to less than 80% by weight of one or more monomers copolymerizable with the vinyl aromatic monomer. The segmented copolymer used in these blends is described as made by a process which comprises: Step (a) mixing non-polar olefin polymer containing at least 75% by weight olefin mers with vinyl monomer comprising above 20 to 200% by weight vinyl aromatic monomer and 0 to less than 80% by weight of one or more monomers copolymerizable therewith, in the presence of an initiator having a 1 hour half life at a temperature in the range of from 110°C to 138°C; Step (b) heating the mixture to a temperature at which the polyolefin completely swells, preferably up to 90°C; and Step (c) continuing heating to below the softening point of the polymer until grafting occurs, preferably to above 100°C. In the process wherein to the non-polar polyolefin, preferably polypropylene or polyethylene, is added a vinyl aromatic monomer which is to be grafted to the polyolefin, such as styrene, methylstyrene, and the like, or mixtures of these with methyl methacrylate and/or butyl acrylate, an initiator is introduced with the monomer either in admixture or in separate feeds. The initiator may also be introduced after the monomer is introduced. The vinyl monomer may act like a solvent to swell the polyolefin. Heating of this mixture may cause the polyolefin to become completely swollen, but there need be no equilibrium swelling reached prior to polymerization. Further heating will cause the grafting reaction to begin. An exothermic reaction occurs. The graft polymer is then blended with one or more suitable polyolefins such as polypropylene or polyethylene, and extruded or otherwise processed into a desired shape, which (for selected segmented copolymers) may be thermoformed with good resistance to sag. Non-polar polyolefins, especially polypropylene and polyethylene and mixtures in various low-density, high-density, and linear low-density form, are major articles of commerce for a wide variety of uses. Nevertheless, there exist specialty needs for which the marketplace has not provided a satisfactory answer. Among these are to overcome the difficulty of thermoforming and processing of the polyolefin, especially unfilled, in a molten or semi-molten form (substantially above its melting point); the polymer tends to sag readily under its own weight because it exhibits an undesirably low stiffness, and to form shapes of grossly non-uniform thicknesses upon thermoforming. Attempts to correct this by increasing the molecular weight lead to difficulties in processing the higher molecular weight polymer not encountered with lower molecular weight polymers. In general terms the process according to EP-A-0 528 600 involves copolymerizing onto a non-polar polyolefin trunk, at least one chain of a polymerized vinyl monomer present in a weight ratio with the polyolefin of from about 1:20 to 4:1. By "polyolefin" we mean polymers derived from the polymerization or copolymerization of monoethylenically unsaturated olefins, such as ethylene, propylene, butene-l, and the like. Segmented copolymers containing blocks whose formal structure is that of a polyolefin, but where the block component is not formed by the direct polymerization (such as Ziegler-Natta catalyzed polymerization) of a monoolefin, are unsatisfactory for improving the melt strength of polyolefins. The segmented copolymer is derived from above 20% of a vinyl aromatic monomer and up to less than 80%, preferably less than about 60%, and most preferably less than 20% based on the total monomer weight, of one or more monomers copolymerizable with the vinyl aromatic monomer such as another vinyl aromatic monomer, substituted and unsubstituted acrylates, methacrylates, acrylic acids, methacrylic acids and anhydrides, for example, maleic anhydride. By "segmented" we mean that the copolymer can be a graft copolymer or a block copolymer or a combination of a graft copolymer and a block copolymer. In one method of manufacturing, when a segmented copolymer is prepared from a polyolefin in solution (for example in tert-butyl benzene), the vinyl monomers are added to the polyolefin together with an initiator which generates a constant low radical concentration, or radical "flux", at the solution temperature. These radicals initiate polymerization of the monomer and cause formation of a covalent bond with the trunk. The earlier copending application describes the use of initiators having a one-hour half life in the temperature range of about 110 to about 138 degrees C. The resulting segmented copolymer (which may include ungrafted homopolymers) may be blended with polyolefin either as a result of the manner by which it is made, or after it is made. The resulting blend may be extruded into a desired shape either directly, or after pelletization. The resulting blended product exhibits high sag resistance without an increase in melt viscosity, as compared with similar ungrafted polymers, viz.: polyolefins without a high molecular weight chain or chains covalently bonded thereto. The segmented copolymer may also be blended with polymers other than polyolefins, and particularly with mixtures of two or more polymers which are poorly compatible with one another, and which may or may not include polyolefins, to improve the compatibility of the resulting mixture. The blends may be made by admixing the polyolefin, the vinyl aromatic monomer and an initiator, and heating to above 100°C but below the softening point of the polyolefin (to about 140°C in the case of polypropylene) to decompose the initiator to begin the grafting reaction. The radicals initiate polymerization of the monomer and formation of a covalent bond therewith on the polyolefin trunk. The resultant product consists of the polyolefin with the chain grafted thereto, unreacted polymer, and ungrafted polyvinyl monomer (the amount of which is small, as the grafting efficiency is high). The product may be directly blended with the polyolefin (since the process directly yields pellets). In this preferred method, it is believed a graft copolymer is formed by abstraction of a hydrogen atom from the polyolefin and polymerization of the monomer at that site. However, the possibility of some chain scission of the polyolefin to create polymerization sites at chain ends, and thus to form block copolymers, cannot be overlooked. The product from the solution process may be pelletized, blended with another polyolefin and extruded into desired shape. Alternatively the reaction mixture may be extruded directly in a devolatilizing extruder to volatilize residual monomer, and thereafter blended with a polyolefin and the blend extruded to form article in such forms as sheets, tubes and the like. In general the polymer blends comprise or consist essentially of: a) a polyolefin and b) from about 0.1 to about 50% and preferably from 0.2 to less than about 10% of a segmented copolymer of a non-polar polyolefin (for example a polymer or copolymer of polyethylene, polypropylene, polybutylene, or poly(4-methylpentene)), covalently bonded to a polymer derived from greater than 20 to about 100% (preferably, from about 40 to about 100% and more preferably, from about 80 to about 100%) of a vinyl aromatic monomer such as styrene, methylstyrene, dimethylstyrene, ethylstyrene, isopropylstyrene, chlorostyrene, dichlorostyrene, trichlorostyrene, a-methylstyrene, bromostyrene, tribromostyrene, and the like and from about 0 to 80% (preferably from about 0 to about 60%, and more preferably from about 0 to about 20%) based on the total monomer weight, of one or more monomers copolymerizable with the vinyl aromatic monomer such as other vinyl aromatic monomers as defined above, substituted and unsubstituted acrylates and methacrylates, such as methyl methacrylate or lauryl methacrylate, N-vinyl pyrrolidone, acrylic acids, methacrylic acids and anhydrides such as maleic anhydride and the like. The copolymerizable monomer may also be a crosslinker containing at least two vinyl groups such as divinyl benzene, glycol dimethacrylate and the like. Articles prepared from the above polymer blends, may be in extruded, calendered, thermoformed or molded form, or may be in the form of a fiber, sheet, or hollow container. In the following, "LDPE" is low-density polyethylene, usually branched, of density of about 0.91 to about 0.94 g/cc; "HDPE" is high-density polyethylene of a density above about 0.95 g/cc; "LLDPE" is linear low-density polyethylene of density about 0.91 to about 0.95 g/cc; "EPDM" includes rubber terpolymers of ethylene, propylene, and a non-conjugated diene monomer, such as 1,4-hexadiene or ethylidenenorbornene. The term "polar" or "non-polar" polymer, as used herein, is all relative to polyolefins. Generally "non-polar" means polymers which are predominantly formed from monomer units of mono- or di-olefins. "Polar", as generally understood in the polymer art, refers to monomers or polymers which contain an oxygen, nitrogen, or sulfur functionality. However, when compared to polyolefins other materials are also "polar" such as polystyrene. Methyl methacrylate, acrylonitrile, and vinyl phenyl sulfone are "polar" monomers, whereas polypropylene is a "non-polar" polymer. The segmented copolymers to be added to the polyolefins to form the blends may be block or graft copolymers or a mixture of graft and block copolymers. The polymers to be modified in the grafting process include the non-polar olefin polymers and copolymers. Included are polypropylene, polyethylene (HDPE, LDPE, and LLDPE), polybutylene, ethylene-propylene copolymers at all ratios of ethylene and propylene, EPDM terpolymers at all ratios of ethylene and propylene and with diene monomer contents up to 10%, poly(l-butene), polymethylpentene, ethylene-vinyl acetate copolymers with vinyl acetate contents up to 25%, ethylene-methyl acrylate copolymers, ethylene-methyl methacrylate copolymers, and ethylene-ethyl acrylate copolymers. Also included are mixtures of these polymers in all ratios. Preferred is polypropylene or polyethylene. A polyolefin trunk having a molecular weight of from about 200,000 to about 800,000 M w (weight-average molecular weight) is especially preferred, but polyolefins having a molecular weight of about 50,000 to about 200,000 can be used with some beneficial effect. In general, a graft copolymer imparts greater melt-rheology improvement to a high-molecular-weight polyolefin. Melt flow rate (mfr) is known to correlate with weight-average molecular weight. The preferred range of mfr values for the polyolefin trunks used in preparing the graft copolymers of the present invention are from about 20 to about 0.6 g/10 minutes as measured by ASTM Standard Method D-1238 (condition 230/2.16). The vinyl aromatic monomers that can be employed to prepare the vinyl polymers are preferably styrene type monomers such as styrene, methylstyrene, dimethyl styrene, ethylstyrene, a-methylstyrene, bromostyrene, chlorostyrene, and the like. Styrene is the most preferred. Preferred articles are prepared from the polymer blends wherein the vinyl aromatic monomer is styrene, the polyolefin is polyethylene or polypropylene, preferably polypropylene, and the non-polar polyolefin component of the segmented copolymer is formed by the polymerization of ethylene or propylene, i.e., is polyethylene or polypropylene, preferably polypropylene. These articles may be in extruded, calendered, thermoformed or molded form, such as in the form of a fiber, sheet, or hollow container. The compounding portion of the thermoplastic polymer particles, vinyl aromatic monomer and polymerization initiator in the system prior to polymerization is (a) 50-99.9 weight parts, preferably 50-90 weight parts, of the thermoplastic polymer particles, (b) 50-0.1 weight parts, preferably 50-10 weight parts, of the vinyl monomer and (c) 0.05-20 weight parts, preferably 0.1-10 weight parts, of the polymerization initiator per 100 weight parts of the vinyl monomer. A thermoplastic polymer particle content of less than the stated range (a vinyl monomer content greater than the range) results in agglomeration of the polymer particles during the reaction although multi-step addition of the monomers may be employed to avoid such agglomeration. A content of more than the stated range (a vinyl monomer content of less than the stated range) results in an inadequately modified product. A polymerization initiator content of less than the range cannot fully polymerize the vinyl monomer, whereas a greater content leads to reactions other than polymerization such as deterioration of the polymer or gelation. A typical method for combining the thermoplastic polymer particles with a vinyl monomer involves mixing the thermoplastic polymer particles with the vinyl monomer containing the polymerization initiator (and other additives as required). Another method is by mixing the thermoplastic polymer particles with the polymerization initiator and then adding the vinyl monomer. Another method is by adding monomer and initiator throughout the reaction. Another method is by adding monomer, heating, and then adding initiator. Since combination of monomer, polymer, and initiator should be efficiently done by heating under conditions where the polymerization initiator is not decomposed, the temperature should ordinarily be less than 100°C, preferably about room temperature to 90°C. It is preferred for ease in handling the resultant product that, when polypropylene is used as the polymer particle, it be in a form with high surface; however, polyolefin pellets without enhanced surface area may be employed. The combination time for admixture and heat-up is ordinarily on the order of from 1/2 to 8 hr. In mixing the thermoplastic polymer particles with a vinyl monomer (and polymerization initiator), auxiliary materials such as plasticizer, lubricant, crosslinker and the like may be simultaneously combined (these auxiliary materials may already have been added to the thermoplastic polymer particles or can be compounded after polymerization). It has been found that with styrene, no additional impregnation time need be involved beyond that of heating the monomer and polymer to the desired temperature. Sufficient combination of monomer and polyolefin occurs during the mixing and heat-up cycle. In order to control the molecular weight of the polymer produced by the polymerization of the vinyl monomer, a chain transfer agent such as n -butyl mercaptan, n -dodecyl mercaptan or tert -dodecyl mercaptan may be added. By increasing the temperature of the impregnated thermoplastic polymer particles or of the well-mixed monomer/polyolefin combination obtained in the above-stated manner to a level at which the polymerization initiator is decomposed at a suitable rate, the impregnated or well-mixed vinyl monomer is polymerized to form the modified thermoplastic polymer particles. Because radical polymerization is involved, heating should be done in an essentially oxygen-free atmosphere and suitable mixing provided during polymerization. Since it is essential to polymerize under conditions where the thermoplastic polymer particles are not dissolved or fused, the polymerizing temperature should be in the range of from about 60° to about 150°C. Exceeding 150°C not only favors gelation but also melt fusion or aggregation of the particles. The polymerizing time is ordinarily on the order of from 0.5 to about 10 hr and the polymerizing pressure ordinarily on the order of ordinary pressure to 10 kg/cm 2 . The mixing, impregnating, and polymerizing processes are implemented preferably in the absence of water. One means employs of a single apparatus provided with means for heating and mixing without recourse to stirring vanes. In order to impregnate uniformly and distribute the heat evenly, it is desirable that the components of the system be continuously mixed throughout the impregnation and polymerization processes. Mixing by means of stirring vanes is undesirable because the thermoplastic polymer particles are often marred or fractured while the stirring may be inadequate. Mixing should be done by means of a self-moving apparatus without depending on vanes such as the self-rotating type, vibration type or self-rotating and vibration type unit. A unit may also be provided with a baffle plate for increasing the degree of mixing among others, a system in which the unit freely rotates on its own axis is desirable, especially a rotating axis that is set at an angle of less than 80 degrees with respect to the horizontal axis. Any unit having a horizontal axis of rotation which is commonly used is satisfactory. Heating methods involve the circulation of a suitable medium such as steam, hot water or oil or a means for providing electrical heat from within or without the unit may be used. After polymerization, even when simple drying or deodorization is required, post-processings such as ventilation and decompression can be done within the unit without recourse to other means. The process may be one shot, or when conducted in solution, run in a semi-continuous or continuous manner. Vinyl monomer and initiator may be added by means similar to those described above. Additives may also be added when the graft copolymer is blended with the matrix polymer. Such additives may include stabilizers against light or heat, such as benzotriazoles, hindered amines, alkyl polysulfides such as dialkyl disulfides, and the like, lubricants, or plasticizers; flame retardants; and the like. The product is isolated by stranding, cooling, chopping, drying, and bagging, or other known collection techniques. In the preferred one-shot process conducted without solvent, the product is already in pelletized form and need merely be collected. The polyolefin and the graft copolymer may be blended by mixing the dry feed materials and extruding either directly to form a film, sheet or the like, or by collecting the blend and reprocessing it into the desired article, or by adding the polyolefin in the course of the devolatilization. Polyolefins are often produced with one or more stabilizers to prevent degradation of the polymer appearance or physical properties during processing and/or end use. Such stabilizers may include metal salts such as metal stearates, which act as acid acceptors, hindered phenols, or phosphites which act as antioxidants, and sulfur-containing organic esters or derivatives, added as heat stabilizers. Examples of such additives, which are usually proprietary to the supplier, are metal stearates, 2,6-dimethylphenolic compounds, Irgafos 168 and thiodiesters of long-chain alcohols. Polyolefins may also contain light stabilizers, such as hindered amines, hydroxy benzotriazoles, and the like. All of the polyolefins used in the present examples are thought to contain small amounts of these proprietary stabilizers. One way to specify the blend composition is that preferably at least about 0.2% of the total formulation (polyolefin plus segmented copolymer) should be a chemically segmented polymer or copolymer within the molecular weight limits specified. The preferred maximum amount is about 10% grafted polymer, with up to about 5% segmented polymer being especially preferred for cost optimization and optimization of most properties of the blend. Optionally, the blend of concentrate i.e., the segmented copolymer plus any ungrafted non-polar polyolefin or polymer of the vinyl aromatic monomer, and polyolefin, may be further modified by the introduction of fillers (both inorganic and organic), fibers, impact modifiers, colorants, stabilizers, flame retardants, and/or blowing agents. Blowing agents may be gases, such as nitrogen or carbon dioxide, admixed with the polymer melt in the extruder and allowed to expand upon extrusion. More often, blowing agents are solids which liberate gases, usually nitrogen, at a specific melt temperature, and which are mixed into the melt, or blended from a pre-compounded mixture of the blowing agent dispersed in a polymeric matrix. The melt temperatures for the polyolefins are typically in the range of about 200° to about 230°C, although other temperatures may be used, depending on the specific blowing agent. Solid blowing agents include azo compounds such as azodicarbonamides, azoisobutyronitriles, hydroazo compounds, or compounds containing the nitroso group. The processing of the polyolefin/ /segmented copolymer blend for improved sag resistance is performed in the melt. The sag resistance test described herein is an appropriate measure of achievement of the desired resistance, and is quantified by stating that acceptable and novel improvement of melt strength requires a sag slope value for the blend (which contains 5 weight percent of segmented polymer, based on total content of polyolefin and segmented copolymer) below 20% of the value for the unmodified polypropylene under similar test conditions. This value converts to a melt strength ratio, defined as sag resistance of matrix polymer sag resistance of blend containing 5% of segmented copolymer of at least 5. The blend of the segmented copolymer and polyolefin is useful in thermoforming, especially of large objects which sag prior to thermoforming, such as containers, cups, panels and the like. Thermoforming of polypropylene, including a list of commercial uses and a description of test methods, such as sag testing, is described in J.L. Thorne, " Thermoforming ", Munich/Vienna/New York (1987). The blend is also useful'in film making (especially blowing and extruding), blow molding, such as for forming drums, coolers industrial parts, automotive parts, and the like, fiber spinning, acid and basic dyeing, foaming, extrusion (sheet, pipe, and profile), coextrusion (multilayer film, sheet, preforms, and parisons, with or without the use of tie layers), hot melt adhesives, calendering, and extrusion coating (for the preparation of polymer/fabric, carpet, foil, and other multilayer constructions). Such graft copolymers, especially with small amounts of copolymerized acid functionality, are useful when blended with polyolefins for improved printability. The grafts themselves may be used as tie layers between otherwise incompatible polymers. In extrusion, the copolymer is useful, especially with LLDPE, at reduction of melt fracture without an effect on the melt flow rate. The blends, copolymers, articles and processes described above are the subject of EP-A-0 528 600 which discloses, as initiators suitable for use in the production of the segmented copolymers, those having a one-hour half life in the temperature range 110 to 138°C. However,it is noted from EP-A-0 528 600 that a wide range of sag resistance values can be obtained, even when these suitable initiators are used. The object of the present invention, therefore, is to produce selected thermoformable segmented copolymers which have the optimum sag resistance. Surprisingly, we have now found that when the object of the polymerisation process is the production of polyolefin/styrene segmented copolymer intended specifically to impart sag resistance to polypropylene, the degree of sag resistance can be controlled by using, in combination, a selected initiator and a specific heating rate. This particular heating rate for the production of the segmented copolymer is neither recommended nor disclosed in prior application EP-A-0 528 600, and it is now appreciated that it is crucial to ensure that the resulting process has the optimum constancy of product for this use. The initiator used according to the invention has a one-hour half life in the temperature range 110 to 125°C and the rate of heating is at least 1 C° per minute. Accordingly therefore this invention provides a process for preparing a segmented copolymer which is substantially free from ungrafted polystyrene and which imparts improved melt strength to polypropylene which comprises: (a) mixing a non-polar polyolefin with styrene (and optionally up to 20% by weight, based on total monomer content, of one or more monomers copolymerizable with styrene) in the presence of an initiator, (b) heating the mixture to a temperature of at least 85°C at which the non-polar polyolefin completely swells; and (c) continuing heating to below the softening point of the non-polar polyolefin until grafting occurs between the non-polar polyolefin and a polymer formed by the polymerisation of styrene (and optionally the copolymerization with the one or more monomers copolymerizable with styrene), wherein the initiator has a one-hour half life at a temperature of from 110°C to 125°C, and the rate of heating in steps (b) and (c) is at least 1 C° per minute. Preferably the polymer formed by the polymerisation of styrene (and optionally the copolymerization with the one or more monomers copolymerizable with styrene), is derived from 40 to 100% by weight of styrene and from 0 to 60% by weight of one or more monomers copolymerisable therewith. The invention also extends to polymer blends comprising (a) polypropylene and (b) from 0.1 to 50% by weight (of (a) plus (b)) of segmented copolymer made by the process of the invention and to the use of said segmented copolymer to impart improved sag resistance to molten polypropylene. Preferred embodiments are given in the subclaims. The initiators employed in the swell process of this invention should have a one hour half life at a temperature in the range of from 110°C to 125°C. Such initiators include tert -butyl peroxybenzoate (125°C), di- tert -butyl diperoxy phthalate (123°C), tert -butyl peroxycrotonate (118°C); 2,2 bis-tert-butyl(peroxybutane) (119°C); tert-butylperoxy isopropyl carbonate (119°C); 2,5 dimethyl-2,5-bis(benzoylperoxy)-hexane (118°C); tert-butyl peracetate (120°C); di-tert-butyldiperoxyphthalate (123°C). The figures in parentheses are the 1 hr. half-life temperatures. When polypropylene is modified with the segmented copolymers made by the process of the present invention, it may be employed in the manufacture of many useful objects, such as extrusion- or injection-blown bottles for packaging of foodstuffs, aqueous solutions such as intravenous feeds, hot-filled items such as ketchup, or extruded articles in profile form such as clips, scrapers, window and door casings and the like. The foamed articles may be used as substitutes for wood in moldings, for packaging materials, for insulation or sound-deadening materials, for food containers, and other rigid-article applications. Films may be used in many protective or wrapping applications, such as for food packaging, blister packaging of consumer goods, and the like. The copolymers are useful in preparing polypropylene fibers; they are especially useful when the graft copolymer is formed from a polypropylene trunk. Polypropylene is relatively easy to process into fibers having high strength and toughness. Polypropylene fibers show certain deficiencies which include difficulty in dyeing and poor long-term dimensional stability. Grafts containing functional sites capable of accepting dye may be prepared by the present process by incorporating low levels of dye-accepting monomers, such as methacrylic acid, dimethylaminoethyl methacrylate and, N-vinylpyridine. The improved sag resistance noted for the present segmented polymers in a polypropylene matrix should correspond to improvements in creep resistance of the fiber. Polypropylene may be formed into fibers by slitting tape from extruded film to form large-denier, coarse fibers, by extruding monofilaments into large-denier fibers with a controlled cross-sectional size, or by extruding multifilaments through a spinnerette to produce bundles of small-denier fibers. In all cases, the fibers may be draw-textured. Polypropylene fibers may be used for, among other things, strapping, netting (including fish nets), slit tape, rope, twine, bags, carpet backing, foamed ribbon, upholstery, rugs, pond liners, awnings, swimming-pool covers, tarpaulins, lawn-furniture webbing, shades, bristles, sutures, cigarette filters, nonwoven fabrics, such as for tea bags, bed sheets, bandages, diaper liners and the like, and for doll hair, apparel and the like. In a preferred process of the invention the non-polar polyolefin is polypropylene and then heating is carried out directly (i.e. without pause or hold time) to a temperature of about 140°C. Although tested most thoroughly with the non-solvent process described herein, the process restriction is applicable to solution and aqueous heterogenous processes also. Unlike the grafting process described for methacrylic monomers in US-A-4,957,974, wherein the radical flux during polymerization was important to achieving an additive which improved the melt strength of polyolefins, for processes involving vinyl aromatic monomers, the radical flux per se does not seem to be critical. Upon further study of the process, it has been found that reactors equipped with stirrers and with baffle or mixing vanes may be employed without as severe degradation of the resulting segmented copolymer's physical shape as had been first anticipated. In the examples which follow, polymer concentrates (i.e., the segmented copolymer plus any ungrafted components) and polymer blends are tested using standard procedures which are summarized below. The polypropylene, polymer concentrate, and any additives are blended in the melt on a 7.6 cm by 17.8 cm electric mill with a minimum gap of 3.8 mm set at 190°C. Once the material had fluxed, it is mixed an additional 3 minutes. Higher temperatures are used for higher viscosity materials (for example, mfr=0.5-2 material is done at 195-210°C). While still hot, the material is either compression molded or cut into small chunks (about 1-2 cm in each dimension) for granulation (5 mm screen). It is of interest that the additives of the present invention contribute to easy release from hot metal surfaces, such as mill rolls, Haake Rheocord bowls, etc. The blends are compression molded in an electrically heated Carver press 15 x 15 cm or Farrel press 30.5 x 30.5 cm. The samples are molded between stainless steel with an appropriate spacer to provide the required thickness 0.25-3.8 mm. In one method the hot melt is taken directly from the mill roll and placed between two stainless steel sheets. This is then placed in the press set at 190°C and pressed at high pressure (68-91 metric tonnes for the Farrel press and 6820 kg for the Carver press). After three minutes the mold is placed in an unheated press at high pressure for three minutes. In the other procedure, granulated material or pellets produced from an extrusion, Haake, or milling operation are dried and then compression molded. The procedure used is the same as for molding a melt except that a 5 minute preheat is used while maintaining a slight pressure on the press. This is followed by the high pressure molding in the hot and cold presses. A hot press of 190°C is usually sufficient for mfr=4 polypropylenes, but higher viscosity (lower mfr) polypropylenes would split during sag testing unless higher molding temperatures are used (195-210°C). The sag tests are performed on a compression molded sheet 10 x 10 x 0.15 cm. This sheet is clamped in a frame with a 7.6-cm-square opening. There are metal rulers attached to the front and back of the frame for use in measuring sag. The frame and sheet are placed in a hot, forced air oven (typically at 190°C). The amount of sag of the center of the sheet is then recorded as a function of time. Typically, the sag is first recorded at 2.5 cm but for slow sagging materials sags as low as 16 mm are recorded. Data is recorded up to 10.2 cm of sag or for 30 minutes, whichever occurred first. The term "slope" refers to the slope of a plot of the natural logarithm of the sag in centimeters versus time, resulting in a straight line. A high slope indicates that the material sags quickly while a low slope indicates that it sags slowly. The advantage of comparing slopes in this manner is that it eliminates any differences in oven cooling when the sample is introduced. The recoverable compliance test is carried out on a Rheometrics Stress Rheometer. The sample is placed between the parallel plates (1 mm gap) of the rheometer at 190°C and subjected to a shear stress of 1000 dynes/sq.cm. The stress is applied for 1000 seconds during which time the compliance versus time is recorded. The shear stress is then removed and the sample allowed to recover. The compliance versus time is recorded again and the final compliance at 1000 seconds is determined. The recoverable compliance is the difference between the final compliance of the stress cycle and the final compliance from the recovery cycle. The sample is a small disk (25 mm diameter) cut from a 60 mil (2.12 mm.) plaque, which is prepared by melt blending the sample into the polyolefin on an electric mill at 190°C, then compression molding the plaque in an electrically heated Carver press at 190°C for three minutes and at 70 metric tonnes with a 60 mil spacer followed by cold pressing for three minutes. The Examples are intended to illustrate the present invention. All percentages are by weight unless otherwise specified and all reagents are of good commercial quality unless otherwise specified. EXAMPLES The following procedure and apparature is used to prepare the segmented copolymers. APPARATUS and GENERAL PROCEDURE A 2 part 1 liter resin kettle with a 4 neck top is equipped with a Teflon paddle stirrer, a thermocouple with a temperature control unit, a heating mantle and a Allihn condenser. (All joints should be ground glass.) The interface between the top and the bottom of the reactor must have a Teflon gasket. The stirrer adaptor apparatus must maintain a vacuum when it is applied to the reaction vessel. The reactor should be able to maintain the vacuum at the same level at which is supplied, 30 inches of Hg. for at least 15m. Charge polypropylene into the reactor. Degas the reactor and replace with nitrogen 5 times. After the last degassing, allow nitrogen to slowly free flow through the reactor. Mix the styrene - initiator solution and degas with nitrogen for 20 minutes. Pump the solution into the reactor. Commence the swell phase at the recommended temperature. When the swell is completed, set temperature controller to 115°C. Reaction will begin. The resulting exothermic event will cause the temperature to reach approximately 140°C. As the temperature tends to drop maintain it at 140°C. Collect the product and dry overnight at 80°C in a vacuum oven. Example 1 - Step A - Segmented Copolymer Polypropylene (175g) beads are added to a 1L kettle type reactor and the system evacuated and refilled with nitrogen. The styrene monomer (30 ml) containing tert -butylperoxy benzoate initiator (500mg) is degassed by bubbling nitrogen through for 20m. The monomer - initiator solution is then rapidly pumped into the polypropylene (5m). While the polypropylene and styrene is stirred the temperature is raised to 85°C (swell temperature) and then, without holding at that temperature, to140°C and held there for 2h. The heating rate is more than 2°C per minute throughout. The heating is then removed and the still free flowing pellets are dried overnight at 60° C in a vacuum oven to yield segmented copolymer mixture. Step B - Blend of Polypropylene and Segmented Copolymer of Step A The sag slope of this material is determined as described above and shown to be below 0.02 when blended into polypropylene at 2%. The segmented copolymer is extracted continuously with ethyl acetate for 16h in order to remove the polystyrene homopolymer. Example 2 A blend is made of 5 weight percent of the segmented copolymer of Example 1 with Aristech Ti 4007F, believed to be a copolymer of propylene with 7.4% ethylene, mfr 0.7. The polymer is chosen because it has good impact strength at room and refrigerator temperatures and contains additives which qualify it for food packaging uses. Properties are shown below: the higher extrusion rate (for a 0.10 inch (2.5 mm.) sheet appears to be a consequence of the improved melt strength which permits faster extrusion without breaking. The wider thermoforming window allows commercial production of parts without "webbing" (holes or tears formed by exceeding the thermoformability of the polymer at a set temperature) over a wider temperature range. To determine the thermoforming window, the values are normalized to the behavior of an unmodified polyolefin sheet, and what is determined in a standardized thermoforming operation is the relative temperature range over which an acceptable part can be formed. A sheet of polypropylene, modified or unmodified, is produced by a 2.5 inch (63.5 mm) extruder, and cut to a 0.100" x 21.5" x 27" sheet (2.54 mm. x 0.546 m x 0.686 m). The sheets are then formed in a 4-cavity mold in a Brown thermoformer. The mold has depressions of various sizes, which allows the modified polyolefin to be tested for thermoformability under a variety of stretch and orientation conditions, simulating practical thermoforming of commercial articles. The thermoforming window is the temperature range over which a good part can be formed. A good part is one that does not contain any warpage or "folding over" in the part. These windows are reported as "normalized values" relative to the window of the unmodified material. Test Control PP 5% Blend Vicat Softening Temp. (ASTM D-1525) 142.9°C 145.4 °C DTUFL (ASTM D-648) 48.7°C, 264 psi (1.81 mPa) 52.7°C, 264 psi (1.81 mPa) Melt Flow rate (ASTM 1238) 0.74g/10 min. 0.56g/10 min. Extrusion Rate 72.6 lb/hr (33 kg/hr) 85.7 lb/hr (38.9 kg/hr) 82.5 lb/hr (37.5 kg/hr) (at 3% additive) Relative Thermoforming Window 1 2.08 1.44 (at 3% additive) Recoverable Compliance 8.9 x 10 -5 cm 2 /dyne 19.5 x 10 -5 cm 2 /dyne The recoverable compliance appears to relate directly to the ability to resist sag; a higher value for the compliance of the blend correlates with a lower sag value and with better thermoforming properties. Example 3 The preparation of Example 1 is repeated but with substitution of 1,1-bis(tert-butylperoxy)3,3,5-trimethylcyclohexane, half-life 18.6 minutes at 125°C. In this example, the temperature is raised directly to 125°C. at a rate exceeding 2°C./minute. The resulting segment copolymer exhibits good resistance to sag at the 2% level in polypropylene. The following examples illustrate preparation of the segmented copolymer by the process of the invention. In all cases, the polypropylene homopolymer used in the synthesis had a melt flow rate ca. 4; where noted, the polypropylene as supplied had a porous structure. The polypropylene used in the sag test also had a mfr of 4. Example 4 The initiator tert-butyl peroxybenzoate (0.5g, 2.57 mmoles) is added to polypropylene (175g, 4.13 moles, porous) in a 500ml round bottomed flask followed by the styrene monomer (18.4g, 0.177 moles). The flask is then placed on a rotary evaporator and the system purged with nitrogen for 5 minutes. The purging is followed by rotation of the flask on the rotary evaporator for 1 minute and then another 5 minute purge with nitrogen. Then while the flask is rotating on the rotary evaporator the temperature of the reaction mixture is raised to 140°C at a rate of 2.1°C/min by means of an oil bath under the 500ml flask.. The temperature is maintained at 140°C for 2 hours and then the still free flowing beads dried overnight at 70°C under vacuum to yield 192g of product. The product is blended into polypropylene at 4% and a 60 mil (1.52 mm) plaque prepared as described previously. A piece from the plaque is tested by the sag test and found to sag only 1/4" in 30 minutes, i.e., its sag slope value would be <<0.02. Example 5: With 1.1-di-(tert-butyl-peroxy)-3,3,5-trimethylcyclohexane : A) Styrene monomer (18.4g, 0.18 moles) and initiator (0.39g, 1.28 mmoles) are mixed together and added to a 500ml round bottomed flask containing the polypropylene (175g, 4.13 moles). The round bottomed flask is placed on a rotary evaporator and evacuated to a vacuum of 20 inches (ca. 500 mm) of Hg. The vacuum is released into nitrogen. The evacuation and releasing is repeated three times and the reaction mixture is heated to 140°C at a rate of 1.7°C/min. The temperature is then maintained at 127°C for a further 2 hours and then the material is dried overnight at 60°C to yield 192.9g of product. The sag slope of this material is 0.028 when blended into polypropylene at 4%. B) Styrene monomer (27.27g, 0.26 moles) and initiator (0.39g, 1.28 mmoles) are premixed and added to the polypropylene (175g, 4.13 moles) followed by evacuation and inertion as before. The temperature is raised to 140°C at a rate of 2.6°C/min and maintained at this temperature for 40 min. The material is dried at 60°C overnight to yield 200g of product. The sag slope of this material is 0.023 when blended into polypropylene at 4%. Example 6 Polypropylene (23.5 Kg, mfr = 4) is added to a 2.2 Cu.Ft. (62.3 l.) Porcupine Processor (The Bethlehem Corporation, Easton, PA, USA), which is a reactor equipped with stirring paddles and many small baffles, followed by styrene monomer (2.45 Kg) and then t-butyl peroxybenzoate initiator (67.1g). The reactor is sealed and then inerted by evacuating to 30 inches Hg (760 mm.) and then filling with nitrogen to atmospheric pressure. The evacuation and release into nitrogen is done three times in succession. The reactor contents are heated at atmospheric pressure at a rate of 3°C/Min to a final temperature of 140°C. The reactor is equipped with a water cooled condenser. Heating is accomplished by means of high pressure steam passing through the stirring paddles. This reaction temperature is maintained for 2 hours. After this time, the steam heating is removed and the reactor pressure is reduced from atmospheric to 30 inches Hg (760 mm.). The reactor contents are dried in this manner for 2.5 hours and then the product (25.5 Kg) collected. The sag slope of this material in a blend with polypropylene is measured in the usual manner and the plaque sags to less than 1/2" (12.7 mm.) in 25 minutes. This value correlates to a melt strength ratio of at least 5.

Claims: A process for preparing a segmented copolymer which is substantially free from ungrafted polystyrene and which imparts improved melt strength to polypropylene which comprises: (a) mixing a non-polar polyolefin with styrene (and optionally up to 20% by weight, based on total monomer content, of one or more monomers copolymerizable with styrene) in the presence of an initiator. (b) heating the mixture to a temperature of at least 85°C at which the non-polar polyolefin completely swells; and (c) continuing heating to below the softening point of the non-polar polyolefin until grafting occurs between the non-polar polyolefin and a polymer formed by the polymerisation of the styrene (and optionally the copolymerization with the one or more monomers copolymerizable with styrene), wherein the initiator has a one-hour half life at a temperature of from 110°C to 125°C, and the rate of heating in steps (b) and (c) is at least 1 C° per minute. A process as claimed in claim 1 wherein the polymer formed by the polymerisation of styrene (and optionally the copolymerization with the one or more monomers copolymerizable with styrene) is derived from 40 to 100% by weight of styrene and from 0 to 60% by weight of one or more monomers copolymerizable with styrene. A process as claimed in any preceding claim wherein the non-polar polyolefin is a homopolymer or co-polymer of ethylene, propylene, butylene and/or, 4-methylpentene. A process as claimed in any preceding claim wherein the non-polar polyolefin is polypropylene, and wherein the mixture is heated directly to a temperature of about 140°C. A polymer blend comprising: (a) polypropylene; and (b) from 0.1 to 50% by weight (of (a) plus (b)) of segmented copolymer made by a process as claimed in any preceding Claim. A polymer blend as claimed in Claim 5 comprising 0.2 to less than 10% by weight (of (a) plus (b)) of the segmented copolymer. An article prepared from a polymer blend as claimed in Claim 5 or 6. An article as claimed in claim 7 which has been extruded, calendered, thermoformed or molded. An article as claimed in claim 8 which is a fiber, sheet, film or hollow container. The use of segmented copolymer made by a process as claimed in any of Claims 1 to 4 to impart improved sag resistance to molten polypropylene.

Inventors: HAMILTON RAYMOND GERARD, MCCARTY MARK THOMAS, HAMILTON, RAYMOND GERARD, MCCARTY, MARK THOMAS
Applicants: ROHM & HAAS, ROHM AND HAAS COMPANY
Application year: 1993
Main classification: C08F255
IPC codes: C08L23, C08J5, D01F6, C08L25, C08F255