PROCESS FOR MAKING LIGHTLY CROSSLINKED LINEAR POLYMER FOAMS

Abstract: A substantially closed-celled linear polyolefin foam is made by melt processing under pressure a linear polyolefin together with a volatile blowing agent and a cross-linking agent and then expanding the polymer. Suitable cross-­ linking agents, including azido and vinyl functional silanes, organic peroxides, multifunctional vinyl monomers, and oxygen, increase the melt tension and melt viscosity of the polymer by lightly cross-linking the polymer with itself to enable the production of foams having improved properties in comparison with branched polyolefin foams.

Description: This invention relates to a method for preparring a lightly cross-linked linear polymer foam materials. Closed-cell branched ethylenic polymer foams may be manufactured by the process of extrusion foaming, wherein a normally solid thermoplastic ethylenic polymer resin, such as low density polyethylene, is melt processed and mixed under pressure with a volatile blowing agent to form a flowable gel. The gel is then passed through a shaping orifice or die opening into a zone of lower pressure. As the pressure is lowered, the volatile constituent of the gel vaporizes forming cells in the resin structure, which cools to a cellular foam material. Linear olefinic polymers such as linear low density polyethylene, high density polyethylene, and polypropylene have several properties which make the use of these polymers in foams desirable. For example, these linear olefinic polymers have a higher modulus of elasticity, greater toughness, higher heat distortion temperature, and lower permeability to blowing agents than other branched olefins. However, previous attempts to produce low density foams of these linear olefins by an extrusion process have been unsuccessful. Linear polyolefins, when foamed by an extrusion process suffer from small foam cross-section, high levels of open cells, flow instability, and a narrow range of foaming temperatures. The exact cause of problems of foaming linear polyolefins is not believed to be known. However, it is generally believed that poor melt strength together with a sharp change in melt viscosity near the transition temperature makes extrusion foaming of linear polyolefins difficult. With these properties, it is difficult to control bubble expansion during extrusion and under conditions of heat and high shear stresses. This results in many broken bubbles (open cells) during expansion and consequent poor foams. Previous efforts to cure these problems with the foamability of linear polyolefins have centered around blending a linear polyolefin with another olefin polymer having good extrusion foamability. For example, Park et al, US-A-4,226,946, blended a linear polyethylene with a low density branched polyethylene to improve extrusion foamability of the linear polyethylene. Watanabe et al, US-A-4,102,829, blended an ionomer resin with a linear polyethylene for the same purpose. However, such known methods offer only partial solutions because of one or more deficiencies in the range of operating parameters, cost of materials, and final foam properties. GB-A-928 656 refers to the preparation of closed-cell polyolefin foam in the presence of peroxides or hydroperoxides as crosslinking agent by different methods, for example, melt extrusion using a solid blowing agent. This reference does not specify the rate of crossslinking nor is there any mention of the die pressure of the extruder. From US-A-3,452,123 a melt extrusion of crosslinked polyethylene is known whereby a closed cell foam is achieved. The difficulties of controlling the process conditions are removed by the presence of an organic solvent for the ethylene polymer. Thus introduction of propellant and crosslinking agent is possible at temperatures below the starting point of the crosslinking agent. It is also known that relatively lightly to moderately cross-linked thermoplastic polymers have melt properties that are suitable for foam expansion. However, such cross-linked polymers are difficult to process on conventional melt processing apparatus such as extruders because of flow instability. Accordingly, there is a need in the art for a process which can be utilized to foam or expand linear polymers in conventional melt processing apparatus. This object is attained by a process for preparing a lightly crosslinked linear polymer foam having a substantially closed-cell structure, comprising the steps of: (a) melt blending in an extruder under pressure at least one linear polymer selected from the group consisting of linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP) polystyrene (PS) and copolymers and blends thereof together with a volatile blowing agent or mixtures of two or more such blowing agents and an amount of crosslinking agent sufficient to initiate crosslinking so that the lightly crosslinked polymer melt has a melt index greater than 0.1 (determined by ASTM D-1238 Condition N 190°C, 10 kg) and raise the pressure of the admixture at the die opening to 2759 kPa (400 psig) or above but insufficient to cause melt fracture of the polymer to occur wherein the crosslinking agent is selected from the group consisting of azido functional silanes, vinyl functional silanes, organic peroxides, multifunctional vinyl monomers, and oxygen, and forming a flowable mixture by maintaining the temperatures of the feed zone at 160° to 200°C, the melting, making and mixing zones at 190° to 230°C and of the cooling and temperature control zone at 100° to 140°C (b) extruding said admixture through a die to a zone of lower pressure and activating said blowing agent to expand at least one said linear polymer to form said lightly crosslinked foam. The present invention meets that need by providing a process for the preparation of a lightly cross-linked linear polymer foam having a substantially closed-cell structure. The term "linear polymer" as used in this specification includes both polymers and copolymers of linear olefins such as linear low density polyethylene (LLDPE), high density polyethylene (HDPE) and polypropylene (PP), as well as polystyrene (PS). The cross-linking agent in step (a) can be admixed together with the blowing agent and the linear polyolefin or can be added after a flowable admixture of the linear polyolefin and the blowing agent is formed. The amount is sufficient to initiate cross-linking of the linear polyolefin and increase the pressure of the admixture at the die to at least 2759 kPa (400 psig). A linear polymer or blend of linear polymers is melt processed in a conventional manner by feeding, melting, and metering it in a conventional melt processing apparatus such as an extruder. A volatile blowing agent and cross-linking agent are mixed with the linear polymer under pressure to form a flowable gel or admixture. The cross-linking agent is added in an amount which is sufficient to initiate cross-linking and raise the pressure of the gel or admixture at the die opening to 2759 kPa (400 psig) or above but insufficient to cause melt fracture of the polymer to occur. "Melt fracture" is a term used in the art to describe a melt flow instability of a polymer as it is extruded through a die which flow instability causes voids and/or other irregularities in the final product. The cross-linking agents is selected from azido and vinyl functional silanes, organic peroxides, multifunctional vinyl monomers, and oxygen. As the flowable gel or admixture is extruded through the die, opening to a zone of lower pressure, the volatile blowing agent activates to expand the linear polymer to a substantially closed-cell foam structure which is lightly cross-linked to itself. Foams having densities of between 9.6 and 240.3 kg/m³ (0.6 and 15 pounds per cubic foot, pcf ) may be produced. The linear polymer foams have excellent dimensional stability and higher compressive strengths and heat distortion temperatures than branched low density polyethylene foams of the same density. The linear polymer foams preparred by the method of the present invention may be produced using conventional melt processing equipment by extrusion and injection molding. The compositions are useful as structural foams, cross-linked jackets for cables and wires, thermocollapse resistant films, and cross-linked polyolefin sheet for polyolefin foam production. Brief Description of the Drawings Figure 1 is a graph of die pressure versus die gap for a linear low density polyethylene (having a melt index of 1.0 and a density of 0.935 g/cm³) foam cross-linked with an azido silane and using FC-12 (dichlorodifluoromethane) as a blowing agent; open and filled symbols indicate good and poor foams, respectively; Figure 2 is a graph of die pressure versus die gap for an 80/20 by weight blend of linear low density polyethylene (1.0 melt index, 0.935 g/cm³ density) and polystyrene (average molecular weight of 200,000) foam cross-linked with an azido silane and using a 95/5 by weight blend of FC-12 and ethanal as a blowing agent; filled symbols indicate prefoaming; Figure 3 is a graph of die pressure versus die gap for a linear low density polyethylene (1.0 melt index, 0.935 g/cm³ density) foam cross-linked with dicumyl peroxide; filled symbols indicate prefoaming; Figure 4 is a graph of the thermocollapse resistance (percent volume retention versus temperature) for linear low density polyethylene (2.3 melt index, 0.935 g/cm³ density) foams cross-linked with different levels of an azido silane and using a 80/20 by weight blend of FC-12 and ethanol as a blowing agent; Figure 5 is a graph of heat distortion characteristics (percent volume retention versus temperature) for a series of linear polymer foams cross-linked with an azido silane; Figure 6 is a graph of compressive strengths of linear polymer foams made with a linear low density polyethylene having a melt index of 1.0 and a density of 0.935 g/cm³ compared with a low density polyethylene foam (2.3 melt index, 0.923 g/cm³ density); and Figure 7 is a graph of compressive strengths of linear olefinic polymer foams and a low density polyethylene foam. The process of the present invention permits the production of a low density foam material having a substantially closed-cell structure from a linear polymer or blend of linear polymers by an extrusion process. The linear polymers which can be used include linear low density polyethylene, high density polyethylene, polypropylene, polystyrene, and blends thereof. Copolymers of linear olefins may also be utilized. Such linear polymers have been, heretofore, difficult to expand by conventional melt processing apparatuses into suitable foamed materials. Blowing agents useful in the practice of the present invention are well-known and may comprise solids which decompose into gaseous products at extrusion temperatures or volatile liquids. A preferred class of blowing agents is the group of halogenated hydrocarbon compounds having from 1 to 4 carbon atoms. Dichlorodifluoromethane (FC-12) is most preferred as it is inexpensive and readily available. When halogenated hydrocarbon compounds are used as the blowing agent, there can be from 0.013 to 0.50 gram mole, and preferably 0.040 to 0.30 gram mole of such blowing agent per 100 parts of linear polymer resin in the flowable gel or admixture. Although use of a single blowing agent is preferred, mixtures of two or more of such blowing agents may also be used. The blowing agent is compounded into the flowable gel in proportions to make the desired degree of expansion in the resulting foamed cellular product to make products having foamed densities down to 9.6 kg/m³ (0.6 pcf) . Depending on the amount of blowing agent added, the resulting foamed materials may have densities from 9.6 to 240 kg/m³ (0.6 to 15.0 pcf). Cross-linking agents useful in the practice of the present invention include azido and vinyl functional silanes, organic peroxides, multifunctional vinyl monomers, and oxygen. The cross-linking agents of the present invention are added to the polymer gel with the blowing agent and serve to lightly cross-link the linear polymer with itself. This cross-linking increases the melt tension and melt viscosity of the polymer gel, while permitting the polymer to remain flowable. This light cross-linking of the polymer also permits pressure to build up in the extruder to (2759 kPa) (400 psig) or above without prefoaming which is important in producing a closed cellular structured foam. An important aspect of the present invention is the control of the amount of cross-linking agent to the polymer gel. Too much cross-linking agent causes melt fracture and poor foam quality. Too little cross-linking agent does not provide good foamability. The cross-linked melt must remain flowable without suffering serve melt fracture and should contain less than 10 percent gels by dissolution test in boiling xylene. In terms of melt index (ASTM D-1238 Condition N 190°C, 10 Kg), the lightly cross-linked polymer melt should have a melt index greater than 0.1. While all of the cross-linking agents utilized in the present invention are useful for producing linear polymer foams, the most preferred cross-linking agents are the azido functional silanes of the general formula R R'SiY₂, in which R represents an azido functional radical attached to silicon through a silicon to carbon bond and composed of carbon, hydrogen, optionally sulfur and oxygen, each Y represents a hydrolyzable organic radical, and R' represents a monovalent hydrocarbon radical or a hydrolyzable organic radical. While not wishing to be bound by any particular mechanism, it is believed that the azido silane cross-linking agent acts via two separate mechanisms, one of which forms reversible cross-linked bonds. In this reversible reaction, the azido silane compound grafts onto an olefinic polymer through a nitrine insertion reaction. Cross-linking develops through hydrolysis of the silanes to silanols followed by condensation of silanols to siloxanes. Since the condensation reaction of silanols to siloxanes is catalyzed by the presence of certain metal catalysts such as dibutyl tin dilaurate or butyl tin maleate, it is preferred that when azido silanes are used as the cross-linking agent in the present invention, that a small amount of such catalyst also be added to the polymer melt. The cross-linking reaction is self-controlled in the extruder by the presence of a gaseous reaction product, namely an alcohol, which limits the reaction. However, the cross-linking reaction proceeds during foam expansion at the exit of the die as the alcohol diffuses into the gaseous phase with the volatile blowing agent. In this manner, cross-linking of the polymer gel in the extruder is controlled so that the gel remains flowable until it exits the die to a zone of lower pressure. There, the cross-linking reaction proceeds, which stabilizes gas bubble and cell formation as the olefinic polymer is expanded. Because the degree of cross-linking in the extruder can be controlled, a greater proportion of azido silane cross-linking agent may be added and, a higher degree of cross-linking in resultant polymer foam may be obtained. Accordingly, the resultant polymer, cross-linked by azido silanes, may no longer be flowable and may contain a large amount of gels. Suitable azido-functional silane compounds include the group of azido trialkoxysilanes such as 2-(trimethoxysilyl) ethyl phenyl sulfonyl azide (commercially available from Petrarch Systems, Inc., Bristol, Pennsylvania) and (triethoxy silyl)hexyl sulfonyl azide (commercially available as Azcup D-98 from Hercules, Inc., Wilmington, Delaware). The azido functional silane cross-linking agent is added in an amount between 0.01 to 2.0 parts per hundred (pph), by weight, of linear olefinic polymer. An especially preferred range of addition is between 0.02 to 1.0 pph of azido silane cross-linking agent. Other suitable silane cross-linking agents useful in the practice of the present invention include vinyl functional alkoxy silanes such as vinyl trimethoxy silane and vinyl triethoxy silane. These silane cross-linking agents may be represented by the general formula RR'SiY₂ in which R represents a vinyl functional radical attached to silicon through a silicon carbon bond and composed of carbon, hydrogen, and optionally oxygen or nitrogen, each Y represents a hydrolyzable organic radical, and R' represents a hydrocarbon radical or Y. Further examples of such silanes are found in US-A-3,646,155. Suitable organic peroxide cross-linking agents include dicumyl peroxide, commercially available under the Trademark DICUP R from Hercules, Inc., Wilmington, Delaware. The organic peroxide cross-linking agents are preferably added in an amount between 0.001 to 0.5 pph, and most preferably between 0.005 to 0.1 pph, of linear polymer. Suitable multifunctional vinyl monomer cross-linking agents for the present invention include trimethylolpropane triacrylate (TMPTA) and pentaerythritol triacrylate (PETA), both commercially available from Celanese Corp. The multifunctional vinyl monomer cross-linking agents are preferably added in an amount between 0.01 to 2.0 pph, and most preferably between 0.1 to 1.0 pph, of linear polymer. Additionally, it has been found that the introduction of oxygen will initiate self-cross-linking of linear polymers by a free radical mechanism. The cross-linking of linear polymers by organic peroxides, multifunctional vinyl monomers, and oxygen is believed to take place via a free radical mechanism in which tertiary hydrogens on the polymer are removed. However, since this type of reaction is irreversible, the amounts of cross-linking agent added should be closely controlled to prevent over-cross-linking of the polymer resulting in melt fracture. In accordance with the process of the present invention, linear polymer foams may be made on conventional melt processing apparatus such as by continuous extrusion from a screw-type extruder. Such an extruder typically comprises a series of sequential zones including a feed zone, compression and melt zone, metering zone, and mixing zone. The barrel of the extruder may be provided with conventional electric heaters for zoned temperature control. An inlet, such as a straight-through injection nozzle, is provided for adding a mixture of fluid blowing agent and cross-linking agent under pressure to the polymer in the extruder barrel between the metering and mixing zones. Cross-linking agent is pumped, in a controllable manner, into the stream of fluid blowing agent upstream of the injection nozzle. The blowing agent and cross-linking agent are compounded into the starting polymer in a conventional manner to form a flowable gel or admixture, preferably in a continuous manner. Thus, the polymer, blowing agent, and cross-linking agent may be combined in the mixing zone of an extruder using heat to plastify the polymer resin, pressure to maintain the blowing agent in a liquid state, and mechanical mixing to obtain thorough mixing. The discharge end of the mixing zone of the extruder is connected, through a cooling and temperature control zone, to a die orifice. The hot polymer gel is cooled and then passed through the die orifice into a zone of lower pressure (e.g., normal ambient air atmosphere) where the blowing agent is activated and the polymer gel expands to a lower density, cellular mass. As the foamed extrusion forms, it is conducted away from the die and allowed to cool and harden. In practice, the temperature of the feed zone is maintained at 160°C to 200°C, preferably 180°C, the temperature of the melting, metering, and mixing zones are maintained at 190°C to 230°C, preferably 210°C ,and the temperature in the cooling and temperature control zone is maintained at 100°C to 140°C, preferably 120°C. The temperature of the polymer gel as it expands through the die orifice is preferably just above the temperature at which solid polymer would crystallize out of the gel and will vary depending upon the particular olefinic polymer utilized. The resulting linear polymer foams comprise substantially closed-cell structures and are flexible to bending and shaping. The foams have excellent dimensional stability and high compressive strengths and heat distortion temperatures than branched low density polyethylene foams having an equivalent foam density. As is conventional, finely divided solid materials such as talc, calcium silicate and zinc stearate can advantageously be incorporated with the polymer gel prior to expansion. Such finely divided materials aid in controlling the size of the cells and may be employed in amounts up to five percent by weight of the polymer. Numerous fillers, pigments and lubricants well-known in the art can also be incorporated as desired. Antioxidants may be added to retard or suppress the cross-linking reaction. In such an instance where antioxidant is present in or added to the polymer gel, an additional amount of cross-linking agent may be required to achieve the desired degree of cross-linking. The specific working examples that follow are intended to illustrate the invention as claimed but are not to be taken as limiting the scope thereof. In the examples, parts and percentages are by weight unless otherwise specified or required by the context. Example 1 The apparatus used in this and other examples was 3.17 cm (1-1/4 inch) screw type extruder having two additional zones for mixing and cooling at the end of usual sequential zones for feeding, melting, and metering. An opening for blowing agent injection was provided on the extruder barrel between the metering and mixing zones. A small syringe-type pump was connected to the blowing agent stream for injection of cross-linking agent. At the end of the cooling zone there was attached a die orifice having an opening of rectangular shape. The height of the opening, called die gap hereinafter, was adjustable while its width was fixed at 0.63 cm (0.25 inch). A granular linear low density polyethylene (LLDPE), having 1.0 melt index (ASTM D-1238-79 Condition E) and 0.935 g/cm³ density was uniformly mixed with 0.1 pph dibutyl tin dilaurate condensation catalyst (commercially available under the designation T-12, from M&T Chemical Inc.) and 0.1 pph talcum powder. The mixture was fed into the extruder at an essentially uniform rate of about 12.6 x 10⁻⁴ kg/s (10 pounds per hour). FC-12 blowing agent (dichlorodifluoromethane) was injected into the extruder at a rate of 19.9 pph. The temperatures maintained at the extruder zones were: 170°C at feeding zone, 220°C at melting and metering zone, and 220°C at mixing zone. The temperature of the cooling zone was adjusted so that the gel could be cooled down to 123°C throughout the tests. For each test, the die gap was varied to determine its effect on foam appearance. A predetermined amount of an azido functional silane was injected into the extruder to determine its effect on foaming performance. The test results are summarized in Table I and Figure 1. As can be seen from Figure 1, a key to successful foam extrusion of a linear low density polyolefin is the ability to build the die pressure to approximately 2759 kPa (400 psig) or above. This appears to be an approximate line of demarcation between good and poor foams in appearance. Without addition of azido silane, the LLDPE foam partially collapsed at a larger die gap. The die had to be closed to 0.15 cm (0.06 inch) to substantially prevent the foam from collapsing. At this small die gap, the foam had a very small cross-section, a high level of open cells, a high density, some voids in the cross-section, and ripple on the edges. The foam was not of satisfactory quality. When 0.05 pph of an azido silane was added, the foam quality improved dramatically with the pressure increased both at the die and extruder discharge. At a die gap as large as 0.28 cm (0.110 inch), a good quality foam having a larger cross-section, low density, and low open cell content was achieved. The foam strand was straight with no sign of flow instability. The foam looked even better at 0.075 pph silane level. Excellent quality foam was produced at a die gap as large as 0.37cm (0.145 inch). At 0.10 pph silane level, an even larger cross-sectioned foam was achieved. The foam looked good but there developed a slight sign of over-cross-linking as exhibited by a slightly higher level of open cells. The LLDPE foams thus produced with low cost FC-12 blowing agent had good dimensional stability during aging. This feature is remarkable in that a foam produced from a branched chain low density polyethylene using the same blowing agent shrinks as much as 30 percent during aging. The LLDPE foams also had a heat distortion temperature of 115°C, which is 15°C higher than that for a typical low density polyethylene foam (LDPE). The heat distortion temperature is the highest temperature at which a foam shrinks no more than 5 percent in volume during one hour aging. Examples 2 The same apparatus, polymer, and operating procedure as in Example 1 were also employed in this example. A small amount (0.05 pph) of organotin catalyst (T-12) was added to all formulations is this example. The talc level was varied from 0.1 to 0.7 pph for cell size control. A 90/10 by weight mixture of FC-12/MeCl₂ was used as the blowing agent. A predetermined amount of an azido functional silane was mixed in the blowing agent. The extruder zones were maintained at 170°C, 200°C, and 200°C for feeding zone, melting and metering zone and mixing zone, respectively. The gel temperature was maintained at 128°C. The die gap was fixed at 2.54 mm (0.100 inch) for all tests. Without the crosslinking agent, the foam was of poor quality with small cross-section, some voids, rough skin, and rippled edges in spite of its substantially closed-cell structure. When a small amount (0.125 pph) of an azido functional silane was added, an excellent quality foam was obtained. At the silane level of 0.15 pph, melt fracture developed. Cutting the level of nucleator did not improve foam quality. When the alcohol-containing blowing agent was injected, the melt fracture disappeared and an excellent quality foam of large cross-section was obtained. Te LLDPE foams produced in this example showed good dimensional stability during ambient aging and a high heat distortion temperature of 115°C. Example 3 The apparatus used in this example and its operating procedure were the same as used in Example 1. A high density linear polyethylene (HDPE) having 0.6 melt index (ASTM D-1238-79 Condition E) and 0.963 g/cm³ density was used in this example. The polymer granules were mixed with 0.05 pph talc and 0.05 pph organotin catalyst (T-12). The mixture was fed into the extruder at 12.60 x 10⁻⁴ kg/s (10 pounds per hour). Extruder zones were maintained at 160°C, 200°C and 200°C for feeding, melting and metering, and mixing zone, respectively. The gel temperature was fixed at 132°C for all tests. The test results are set forth in Table III The effect of an azido silane on the foaming was even more dramatic during the tests in this example. Without silane, a good foam could not be produced, but with 0.15 pph silane excellent quality substantially closed-cell foam was achieved. The foams expanded by low cost FC-12 blowing agent showed excellent dimensional stability during ambient aging with shrinkage no more than 3 percent. This is a remarkable improvement over a LDPE foam made with FC-12 blowing agent. The heat distortion temperature of the HDPE foams was 125°C which is 25°C higher than that of typical LPDE foam. Example 4 In this example, the same apparatus, operating conditions, and polymer composition were used, as in Example 3, except for gel temperature and blowing agent. The gel temperature was maintained at 130°C and a 90/10 mixture of FC-12/EtOH was used as the blowing agent. The test results are presented in Table IV. Again, the silane cross-linking agent aided in foam processing and alcohol suppressed development of cross-linking in the extrusion line. At a low silane level, improvements were seen in one or more performance areas. For example, even at 0.05 pph silane level, foam density dropped significantly from the control. At 0.15 pph silane level, good quality foams were produced with a noticeable increase in the die pressure. Interestingly, the pressure at extruder discharge increased little at this silane level. This is an advantage in foam extrusion process since a high die pressure prevents prefoaming but a low extruder discharge pressure facilitates polymer extrusion. The trend indicates that the alcohol-containing blowing agent called for a silane level higher than 0.15 pph for the optimum results. Example 5 The apparatus used in this example was the same as in Example 1. A Hercules Porofax® 6523 polypropylene resin having 4.0 melt index (ASTM D-1738 Condition L) was used. The polymer was mixed with 0.1 pph talc and 0.05 pph organotin catalyst (T-12) and fed into the extruder at a uniform rate of 12.60 x 10⁻⁴ kg/s (10 pounds per hour) . A 90/10 by weight mixture of FC-12 and methylene chloride was injected into the extruder at a rate of 27.8 pph. Extruder zones were maintained at 180°C, 200°C and 210°C for feeding, melting and metering, and mixing zone, respectively. The gel temperature was kept at an essentially constant level of 146°C. The data are summarized in Table V . In this example also, addition of an azido functional silane improved foam processability of polypropylene. The polypropylene foams did not shrink at all during aging and had a heat distortion temperature as high as 165°C. Example 6 The same apparatus used in Example 1 was used in this example. The polymer feedstock employed in this example was an 80/20 mixture of a linear low density polyethylene that was used in Example 1 and a polystyrene having weight average molecular weight of 200,000. Two granular polymers were blended by use of a tumbler, mixer with 0.1 pph talc and 0.05 pph organotin catalyst (T-12) and fed to the extruder at an essentially uniform rate of 12.6 x 10⁻⁴ kg/s (10 pounds per hour) . A 95/5 by weight mixture of FC-12/ethanol was injected into the extruder at a rate of approximately 21.0 pph. Extruder zones were maintained at 145°C, 195°C and 205°C, respectively, for feeding, melting and metering, and mixing zone, respectively. The gel was cooled down to an essentially uniform temperature of 122°C. The test results are seen in Table VI and Figure 2. This particular polymer blend provided reasonably good foam without cross-linking when the die gap was closed down to 1.27 mm (0.050 inch). The foam had a small cross-section and a relatively high level of open cells. At a larger die gap, foam collapsed partially. Foam improved progressively with the azido functional silane level. At a silane level of 0.15 pph or higher, superb-looking foams were obtained having a lower density, larger cross-section, and lower open cell content. At a silane level as high as 0.25 pph, there was no sign of over-cross-linking. Again, a die pressure of approximately 2759 kPa (400 psig) was the line of demarcation between good and poor foams. The foams thus produced also had excellent dimensional stability during aging without suffering any shrinkage. The heat distortion temperature of the blend foam was 110°C. Example 7 The apparatus, operating procedure, and polymer used in this example were the same as in Example 1. Polymer granules were adhered with 0.05 pph talc by use of a wetting agent and fed into the extruder. FC-12 blowing agent was injected into the extruder at an essentially uniform rate of 19.7 pph. A predetermined amount of dicumyl peroxide (Dicup R (Trademark), from Hercules Inc.) was injected into the blowing agent stream in 10 percent by weight solution in methylene chloride. Extruder zone temperatures were maintained at 140°C, 180°C, and 200°C, respectively. The gel was cooled down to an essentially uniform temperature of 124°C. The resultant data are given in Table VII and are illustrated in Figure 3. The control LLDPE foam was not satisfactory. Addition of a small amount (0.0054 pph) of dicumyl peroxide improved foam processability and quality. The best result was obtained at a peroxide level of 0.01 pph. Excellent quality foams were produced at a wide range of die gaps. When the peroxide level was raised to 0.0123 pph, symptoms of over-cross-linking developed. At a high peroxide level, the foam strand fractured. Again, a die pressure of approximately 2759 kPa (400 psig) was the line of demarcation between good and poor foams. Example 8 The apparatus used in this example was the same as Example 1. A granular polystyrene, polystyrene with a weight average molecular weight of 200,000, was uniformly mixed with a small amount of dibutyl tin dilaurate condensation catalyst (T-12, available from M & T Chemicals, Inc.), barium stearate and talcum powder. The mixture was fed into the extruder at an essentially uniform rate of 12.60 x 10⁻⁴ kg/s (10 pounds per hour). A premixed blowing agent was injected into the extruder at a predetermined rate. Blowing agents used in this example were a 50/50 by weight mixture of fluorocarbon 12 (FC-12) and methyl chloride (MeCl) for Test No. 1 and a 70/30 by weight mixture of fluorocarbon 12 and fluorocarbon 11 (FC-11) for the other tests. An azido functional silane, 2-(trimethoxysily ethylphenylsulfonyl) azide (CT2905 distributed by Petrarch Systems, Inc.), was injected to the blowing agent stream at a uniform predetermined rate. The extruder zones were maintained at 170°C, 190°C and 200°C for feeding, melting, metering, and mixing zone, respectively. The temperature of the cooling zone was adjusted so that the gel could be cooled down to a uniform temperature for optimum foam expansion. At a fixed die opening for a given blowing agent system where all formulations provided good quality foams, both pressures at the die and extruder discharge were recorded and samples were taken. The foam samples were aged for about one month at ambient temperature prior to property determination and secondary foaming tests. Secondary foaming was conducted both by atmospheric steam and by hot air. Foam slugs of 6.35 mm (0.25 inches) thickness were slided out of foam strands and aged for about one day at ambient temperature prior to secondary expansions. After exposure to atmospheric steam for varying lengths of time, foam specimens were aged at ambient temperature while their weights and volumes were monitored. Highly expanded foam specimens shrank when taken out of steam but recovered to the final steady state volumes in about two days. Expansion tests in hot air were conducted similarly with the exception that expansion or shrinkage of a foam specimen in the oven was permanent not needing ambient aging for volume recovery. Table VIII A shows that incorporation of an azido functional silane gradually raises both the die pressure and the extruder discharge pressure. Other noticable effects of silane include enlargement in cell size and cross-sectional size which are desirable in foam extrusion. The remarkable effect of silane on steam expandability of extruded polystyrene foam is seen in Table VIII B. Addition of silane enhances both the rate and extent of foam expansion in steam. At the silane level of 0.2 pph, the foam expands about twice as much as the control to an expansion ratio exceeding 280 in two hours. At this long exposure to steam, the control and one with a low level (0.05 pph) of silane suffered thermal collapse; the expansion ratios are lower than those at one hour exposure. The type of blowing agent also has a significant effect on steam expandability of the extruded polystyrene foams. The foam expanded with FC-12/MeCl blowing agent shows expansions lower than one half of that made with FC-12/FC-11 blowing agent. Loss of methyl chloride during aging is one probable cause of the relatively poorer performance of the FC-12/MeCl blown foam. FC-11, having a high solubility in polystyrene, effectively plasticizes polystyrene for faster and greater expansion. The effect of crosslinking is also seen in hot air expansion tests. As Table VIII C shows, foams lightly crosslinked with an azido functional silane expaned more and withstand hot air longer than the control. For example, a foam crosslinked with 0.15 pph or higher level of azido silane keeps expanding during 90 minutes exposure to 100°C air while the control collapses within 60 minutes at the same temperature. Although less pronounced, the cross-linking effect persists at a lower level of silane as well. Example 9 The apparatus and its operating procedure used for tests in this example are the same as in Example 1. The polymer feedstock employed for tests in this example was a 50/50 by weight blend of polystyrene with Mw 200,000 and polystyrene with Mw 300,000. Approximately 0.05 pph T-12, 0.1 pph barium stearate and 0.1 pph talcum powder were mixed in the granular blend of two polystyrenes and fed into the extruder at a uniform rate of 12.60 x 10⁻⁴ kg/s (10 pounds per hour). In test 1, there was used a 50/50 by weight mixture of FC-12 and methyl chloride as the expanding agent and a 70/30 mixture of FC-12 and FC-11 in the rest of the tests. The level of azido silane cross-linking agent was varied up to 0.45 pph. The extruder zones were maintained at about 170°C, 200°C and 200°C for feeding, melting, metering, and mixing zone, respectively. The temperature of the cooling zone was adjusted so that the gel could reach an essentially uniform temperature of 135°C. When the operating condition reached an essentially steady state, the effects of die opening on foam appearance and line pressure were studied. Foam samples were taken both at the threshold die gap for prefoaming and at a fixed die gap for a given blowing agent system. Property determination and secondary foaming tests were conducted as described in Example 8 In general, silane cross-linking has a similar effect on this polymer blend. As shown in Tables IX A, IX B and IX C, azido functional silane increased die pressure and cell size permitting achievement of a larger foam cross-section and greater expansion in both steam and hot air. The die may be opened wider at an azido functional silane level 0.25 pph or higher. For this polymer blend also, the type of expanding agent has a pronounced effect on steam expandability. FC-12/FC-11 is superior to FC-12/MeCl blowing agent. This is ascribed to the low permeability and high solubility of FC-11 in polystyrene. Example 10 The apparatus used in this example was the same as used in the other examples of this invention. An "S" shaped die orifice for Dow Pelaspan-Pac loose-fill packing was attached at the discharge end of the extruder replacing the gap-adjustable die used in the other examples. A 60/40 by weight mixture of polystyrene with Mw 200,000 and polystyrene with Mw 300,000 was mixed with 1.2 pph fire retardant made by Dow Chemical Co. (FR-651), 0.02 pph magnesium oxide and 0.02 pph calcium stearate by the use of a small amount of wetting agent. For tests incorporating azido silane, a small amount of (0.05 pph) of dibutyl tin dilaurate (T-12) was additionally mixed in. The mixture was fed into the extruder at a uniform rate of about 12.6 x 10⁻⁴ kg/s (10 pounds per hour). A predetermined amount of azido functional silane was premixed in the blowing agent and injected into the extruder. A 95/5 by weight mixture of FC-11 and methylene chloride was the blowing agent used for Test 1 and 2 and small amount of methanol was mixed to the blowing agent for Test 3. The extruder zones were maintained at 160°C, 180°C and 200°C for feeding, melting and metering, and mixing zone, respectively. The temperature of the cooling zone was adjusted so that the extruder strand could become essentially free of voids upon quenching in cold water. The solid strand was cut to granules of approximately 6.35 mm (1/4 inch) in length. The granules were about 7.9 mm (5/16 inch) in width and about 2.4 mm (3/32 inch) in thickness. The as-extruded granules were annealed in 65°C water for about 30 minutes and aged in ambient air for one day prior to steam expansion tests. The granules were expanded by atmospheric steam for 10 minutes and then aged for a day. This first expansion was followed by the second, third and fourth expansion in a day interval between the expansions. Expansion times were 10, 1 and 1 minutes for the second, third and fourth expansion, respectively. Foam densities were determined after the foams were aged for one day after each expansion. As shown in Table X, addition of an azido silane results in a slight increase in the line pressure and leads to lower foam densities. The expanded material cross-linked with azido silane not only showed lower densities after each expansion but better appearance after third and fourth expansions. The control (Test 1) developed splits and cracks on the surface but the cross-linked materials showed few such deficiencies. Example 11 The heat distortion temperature of foams was tested by aging them in a convective oven for one hour. Figure 5 illustrates the results of the tests. A control branched low density polyethylene (LDPE) foam with no azido silane cross-linking showed substantial heat distortion at between 105°C and 110°C. The heat distortion temperatures of foam produced from polypropylene (PP), high density polyethylene (HDPE), and linear low density polyethylene (LLDPE) are 165°C, 125°C, and 115°C, respectively. Blending in 20 percent polystyrene (PS) slightly reduces the heat distortion temperature of linear low density polyethylene. On the other hand, incorporating as much as 40 percent LDPE does not cause a drop in the heat distortion temperature of the linear low density polyethylene. All of the linear olefinic polymer foams cross-linked with azido silanes would be serviceable at temperature higher than that with a branched low density polyethylene foam. Example 12 As shown in Figures 6 and 7, and also in Table XI below, the toughness and strength of the linear polyolefins are also well reflected in their foam properties. A 1.64 26.27 kg/m³ (pounds per cubic foot) density LLDPE foam outperforms an equivalent density LDPE foam in all strength categories. The LLDPE foam was twice as strong and twice as tough as the LDPE foam. The 26.27 kg/m³ (1.64 pcf) density LLDPE foam almost matches a much higher density 40.53 kg/m³ (2.53 pcf) LDPE foam in compressive strength and actually outperformed the LDPE foam in tensile properties. The heat distortion temperature of the LLDPE foams were 10°C to 15°C higher than that of the LDPE foam. Foams made from high density polyethylene and polypropylene also reflect superior compressive strengths and heat distortion temperatures in comparison to the LDPE foam. Example 13 The apparatus used in this example and its operating procedure were the same as used in Example 1. A linear low density polyethylene having a 1.0 melt index and 0.935 gm/cm³ density was used in this example. The polymer granules were mixed with 0.05 pph talc and 0.05 pph organotin catalyst (T-12). A dichlorodifluoromethane (FC-12) blowing agent was added at the rate of 2.29 x 10⁻⁴ to 2.37 x 10⁻⁴ Kg/s (1.82 to 1.88 lb/hr). As the cross-linking agent, a vinyltrimethoxy silane (VTMO) was injected into the blowing agent stream in the amounts indicated in Table XII. The extruder zones were maintained at 160°C, 180°C, and 200°C for feeding, melting and metering, and mixing, respectively. The gel temperature was fixed at 125°C for all tests. The test results are set forth in Table XII. As can be seen, the control sample with no cross-linking agent collapsed at larger die openings. Only at the smallest die opening was a satisfactory foam produced; however, this foam had a very small cross-section and some open cells. The addition of VTMO as the cross-linking agent permitted the use of wider die openings resulting in good foams having larger cell and cross-sectional sizes while exhibiting a very low open cell content. Example 14 The apparatus, procedure, polymer, additives, and blowing agent used in this example were the same as used in Example 13. As the cross-linking agent, a 30:1 by weight mixture of vinyltrimethoxy silane and dicumyl peroxides was injected into the blowing agent stream in the amounts indicated in Table X II. As can be seen, the VTMO/peroxide mixture was quite effective and produced excellent quality foams having large cell and cross-sectional sizes and very low open cell content.

Claims: A process for preparing a lightly crosslinked linear polymer foam having a substantially closed-cell structure, comprising the steps of: (a) melt blending in an extruder under pressure at least one linear polymer selected from the group consisting of linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP) polystyrene (PS) and copolymers and blends thereof together with a volatile blowing agent or mixtures of two or more such blowing agents and an amount of crosslinking agent sufficient to initiate crosslinking so that the lightly crosslinked polymer melt has a melt index greater than 0.1 (determined by ASTM D-1238 Condition N 190°C, 10 kg) and raise the pressure of the admixture at the die opening to 2759 kPa (400 psig) or above but insufficient to cause melt fracture of the polymer to occur wherein the crosslinking agent is selected from the group consisting of azido functional silanes, vinyl functional silanes, organic peroxides, multifunctional vinyl monomers, and oxygen, and forming a flowable mixture by maintaining the temperatures of the feed zone at 160°to 200°C, the melting, making and mixing zones at 190° to 230°C and of the cooling and temperature control zone at 100° to 140°C (b) extruding said admixture through a die to a zone of lower pressure and activating said blowing agent to expand at least one said linear polymer to form said lightly crosslinked foam. The process of claim 1 in which said cross-linking agent is 2-(trimethoxysilyl ethyl phenyl sulfonyl) azide, and a catalyst which catalyzes the condensation of silanols is added with the cross-linking agent. The process of claim 1 in which said cross-linking agent is dicumyl peroxide. The process of claim 1 in which said cross-linking agent is trimethylolpropane triacrylate. The process of claim 1 in which said cross-linking agent is vinyl trimethoxy silane. The process of claim 1 wherein in step (a) said cross-linking agent is added after a flowable admixture of said at least one linear polymer and said blowing agent is formed. The process of claim 1 in which said cross-linking agent is from 0.01 to 2.0 parts per hundred by weight of linear polymer (pph) of an azido functional silane, and a catalyst which catalyzes the condensation of silanols is added. The process of claim 1 in which said cross-linking agent is from 0.001 to 0.5 parts per hundred by weight of linear polymer (pph) of an organic peroxide. The process of claim 1 in which said cross-linking agent is from 0.01 to 2.0 parts per hundred by weight of linear polymer (pph) of a multifunctional vinyl monomer. The process of any of claims 1 to 9 wherein the crosslinking is controlled by the inclusion of an alcohol in the volatile blowing agent.

Inventors: PARK, CHUNG POO
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1985
Main classification: C08J9
IPC codes: C08J9, B29C47, B29K105, B29K23