Patent Application
20060194892
The invention relates generally to polyolefin foams and more particularly to foamed articles comprising a blend of polypropylene and low density polyethylene.
Polyolefin foams such as polyethylene are used to produce foam sheets from which a variety of articles can be created. One application includes the production of foamed expansion joint fillers that can be used in repairing cracks in an existing concrete surface or in creating a new concrete surface. In new concrete construction, the expansion joint filler is typically used to divide the concrete surface into discreet regions. The resiliency of the foam joint allows it to expand and contract with the concrete. The flexibility of the foam also allows the expansion joint filler to be used to fix an existing crack. Typically, most cracks are non-linear and may have several sharp turns or bends. The flexible foam joint can be positioned within the crack so that it follows the contour of the crack. Concrete filler may then be added on opposing sides of the foam to complete the repair.
Polyethylene (PE) is one of the most widely used polyolefin foams. While polyethylene possesses a number of beneficial physical and chemical properties when used to produce a foamed sheet, a disadvantage of PE is that extruded foam sheets made therefrom have a flexural modulus that is lower than would otherwise be desired for certain applications, such as expansion joint fillers. Additionally, polyethylene foam typically has lower temperature resistance than desired for certain applications requiring exposure to relatively high temperatures, such as construction applications where hot sealant may be used in combination with an expansion joint filler.
Polypropylene (PP) is one possible alternative to polyethylene. PP foams are typically stiffer and have greater temperature resistance than PE foam. However, molten PP generally has poor melt strength, which may make it difficult to produce acceptable quality foam, i.e., one having a uniform array of fully-formed, closed cells. Further, PP foams are often brittle and allow cracks to propagate readily through the foam. In addition, PP foams generally exhibit poor thermoformability such that it is difficult to thermoform such foams into desired shapes.
The invention is a composition comprising a blend of polyethylene, polypropylene, and a rubber component having specific advantages for sub-freezing applications. The composition includes a foam blend has improved flexibility at sub-freezing temperatures without sacrificing the desirable physical characteristics that are commonly associated with polypropylene foams. As a result, the foam is particularly useful for producing articles that require flexibility at sub-freezing temperatures.
In some embodiments, the foam blend may comprise at least 50 percent by weight polypropylene and up to about 45 percent by weight polyethylene. The rubber component in the blend may be from about 3 to 10 weight percent, with 5 weight percent being particularly useful. A particularly useful rubber component includes di-block and tri-block styrene-elastomer block copolymers. A preferred styrene-elastomer block copolymer comprises a tri-block copolymer structure which includes styrene end-blocks and a mid-block of a saturated olefin elastomer.
The foam blends may be suitable for producing a variety of articles where it is desirable to have improved flexibility at cold temperatures. In a particularly useful application, the foam blend may be used in concrete expansion joint fillers or totes.
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention is shown. Indeed, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.
The foam blend of the invention may comprise polypropylene that is blended with polyethylene and a rubber component that is present in the blend in the range from about 3 to 10 weight percent. The presence of the rubber component helps to improve the flexibility of the polypropylene foam without sacrificing the desired stiffness of the foam.
Suitable polypropylenes for use in the blend may include atactic, isotactic, syndiotactic, linear and long-chain branched PP homopolymers and copolymers, such as propylene/ethylene copolymer, and combinations thereof. Useful polypropylene homopolymers may have a melt flow index ranging from about 1 to 20 and a density ranging from about 0.87 to 0.915 g/cc. Further, a high melt strength/long-chain branched polypropylene is particularly useful. Such polypropylenes exhibit higher extensional viscosity when compared to other polypropylenes, resulting in beneficial strain hardening when the cells are expanded during the foaming process. A particularly useful polypropylene includes PP homopolymers and copolymers having a melt tension of greater than about 20 centinewtons at 200° C. (“high melt tension PP” or “HMT-PP”). Such HMT-PPs preferably have a melt flow index ranging from about 1 to 20 and a density ranging from about 0.88 to 0.910 g/cc. In some embodiments, the polypropylene may comprises a blend of high melt tension and low melt tension polypropylene. Low melt tension polypropylene refers to polypropylenes having a melt tension of about 20 centinewtons at 200° C. or less, including polypropylenes having a melt tension of less than about 10 centinewtons at 200° C. In some embodiments, the polypropylene in the foam blend may include at least about 40 percent high melt tension polypropylene, based on the total weight of the blend.
A suitable blend of HMT-PP and low density polyethylene is described in U.S. Pat. No. 6,462,101 to Ramesh et al., the contents of which are hereby incorporated by reference. In some embodiments, the foam blend may comprise about 60 percent by weight high melt polypropylene having a high melt tension of greater than about 20 centinewtons at 200° C., about 20 percent by weight low melt polypropylene having a low melt tension of less than about 20 centinewtons at 200° C., about 15 percent by weight ethylene vinyl acetate, and about 5 percent by weight rubber component.
In accordance with the present invention, “melt tension” may be determined by stretching a strand of polymer between two counter-rotating wheels and maintaining the temperature of the polymer at 200° C. The frequency of rotation increases linearly and the resultant pulling force increases as the filament is stretched. The force is recorded in centinewtons (cN) until the polymer strand breaks. The maximum force obtained before break is recorded as the melt tension of the polymer. The foregoing procedure may be performed as described by M. B. Bradley and E. M. Phillips in the Society of Plastics Engineers's ANTEC 1990 Conference paper at page 718, the disclosure of which is hereby incorporated herein by reference. A suitable device for performing the test is a Rheotens Melt tester.
The foam blend may include polyethylene homopolymers or copolymers. Examples of useful polyethylene homopolymers include low density polyethylene and high density polyethylene. Polyethylene copolymers may include, e.g., ethylene vinylacetate copolymers, homogeneous ethylene/alpha-olefin copolymers (i.e., metallocene/single-site catalyzed copolymers of ethylene and, e.g., one or more C3 to C10 alpha-olefin comonomers) or heterogeneous (i.e., Ziegler-Natta catalyzed) ethylene/alpha-olefin copolymers. A preferred polyethylene is low density polyethylene (LDPE) having a melt flow index ranging from about 1 to about 40 and a density ranging from about 0.912 to about 0.930 g/cc.
A suitable rubber component may include thermoplastic elastomers comprising a copolymer or terpolymer including a styrenic component and a rubbery component, with the rubbery component having at least one carbon-carbon double bond and comprising at least about 70 wt. % of the thermoplastic elastomer. A preferred thermoplastic elastomer comprises a block copolymer or terpolymer, wherein the rubbery component is distributed in the copolymer or terpolymer between styrenic end-blocks. Preferred examples of such block copolymers or terpolymers that are useful in accordance with the present invention include the following: styrene-ethylene-butylene-styrene block copolymer (SEBS), styrene-butadiene-styrene block copolymer (SBS), and styrene-isoprene-styrene block copolymer (SIS). As an alternative to block copolymers and terpolymers, random copolymers and terpolymers comprising styrene and a rubbery component may be employed, such as polybutadiene/styrene rubber.
It may be possible to employ other rubber components in the foam blend, such as, e.g., polybutadiene rubber, butyl rubber, polychloroprene rubber, acrylonitrile-butadiene rubber, vinylpyridine rubber, ethylene-propylene rubber, etc., provided that such rubber components can be processed into the foam blend. Thermoplastic elastomers comprising a styrenic component and a rubbery component as described above have been found optimally suited to achieve the foregoing objectives in accordance with the present invention.
A particularly useful rubber component includes di-block and tri-block styrene-elastomer block copolymers. A preferred styrene-elastomer block copolymer comprises a tri-block copolymer structure which includes styrene end-blocks and a mid-block of a saturated olefin elastomer. Typically, in these tri-block copolymer structures, the saturated olefin elastomer mid-block may comprise butadiene, isoprene, ethylene-butylene, or ethylene-propylene.
Particularly preferred for the present invention is a styrene-isoprene-styrene (SIS) block polymer having greater than 80 wt. % isoprene (i.e., the rubbery component), and most desirably a SIS block copolymer which is predominately a linear tri-block copolymer structure. A useful rubber component includes SIS block copolymer having a styrene to rubber (elastomer) ratio of from 30 to 70 with an Average Molecular Weight (Mn) in the range of 50,000 to 300,000, most desirably about 200,000.
The amount of rubber component in the blend may be in the range from about 3 to 10 weight percent, based on the total weight of the blend. It has been discovered that the foam blends comprising at least 3 weight percent rubber component have improved flexibility and reduced brittleness at sub-freezing temperatures. In particular, foam blends having a rubber component of at least 4 weight percent showed improved flexibility at sub-freezing temperatures. Typically, the amount of rubber component in the blend should be sufficient to prevent the foam from breaking at sub-freezing temperatures, and more typically at temperatures approaching −20° F. Foam blends having a rubber component that exceeds 10 weight percent may have increased softness that is less than desirable for expansion joint filler applications and other articles. However, it should be recognized that in some embodiments the foam blend may include a rubber component in excess of 10 percent, although not necessarily with equivalent results.
The amount of polypropylene in the blend is typically at least about 50 weight percent, based on the total weight of the blend, with a weight percentage of about 70 to 90 being somewhat more typical. Polyethylene may be present in the blend with polypropylene at a weight percentage ranging from about 1 to about 45, based on the total weight of the blend. More typically, the weight percentage of PE in the blend ranges from about 5 to about 25, such as from about 10 to about 20. In a particularly useful embodiment, the foam blend comprises about 80 percent by weight polypropylene, about 15 percent by weight low density polyethylene, and about 5 percent by weight rubber component.
The density of the foam in some embodiments may range from about 0.5 to about 15 pounds/ft3. Somewhat more typically, the density ranges from about 1.5 to about 10 pounds/ft3. The foam may be in the form of a sheet or plank having a thickness ranging from about 0.025 to about 4 inches and, more typically, from about 0.06 to about 3 inches.
In some embodiments, the foam blend may comprise a blend of polypropylene and ethylene vinylacetate. It has been discovered by the Applicants, that the flexibility of polypropylene foams can be improved by preparing a blend of polypropylene and ethylene vinylacetate. The amount of ethylene vinylacetate in the blend may range from about 5 to 25 percent by weight, with a range from about 10 to 20 weight percent being somewhat more typical. In some embodiments, the flexibility may also be improved by preparing a blend comprising polypropylene, ethylene vinylacetate, and the rubber component. In another useful embodiment, the foam blend may comprise about bout 80 percent by weight polypropylene, about 15 percent by weight ethylene vinyl acetate, and about 5 percent by weight rubber component.
The foam blend of the invention is particularly useful for producing foam sheets and articles. In some embodiments, the foam blend is ideally suited as a concrete forming material. In this regard,
With reference to
With reference to
Referring to
With reference to
The foam blend is also particularly useful for producing foamed structures such as totes. In this regard,
Totes prepared in accordance with the invention are particularly useful for the transportation and storage of a variety of parts, such as automotive and airplane parts. The resilience of the foam blend may help prolong the life of the tote. The resiliency of the foam blend also may help prevent fracture or breakage of the totes in cold weather applications.
In producing the foam sheets and articles described herein, any conventional chemical or physical blowing agents may be used. Preferably, the blowing agent is a physical blowing agent such as carbon dioxide, ethane, propane, n-butane, isobutane, pentane, hexane, butadiene, acetone, methylene chloride, any of the chlorofluorocarbons, hydrochlorofluorocarbons, or hydrofluorocarbons, as well as mixtures of the foregoing.
The blowing agent may be mixed with the polymer resin (i.e., the blend of PE, PP, and the rubber component) in any desired amount to achieve a desired degree of expansion in the resultant foam. Generally, the blowing agent may be added to the polymer resin in an amount ranging from about 0.5 to 80 parts by weight, based on 100 parts by weight of the polymer. More preferably, the blowing agent is present at an amount ranging from 1 to 30 and, most preferably, from 3 to 15 parts per 100 parts by weight of the polymer.
If desired or necessary, various additives may also be included with the polymer. For example, it may be desirable to include a nucleating agent (e.g., zinc oxide, zirconium oxide, silica, talc, etc.) and/or an aging modifier (e.g., a fatty acid ester, a fatty acid amide, a hydroxyl amide, etc.). Other additives that may be included if desired are pigments, colorants, fillers, antioxidants, flame retardants, stabilizers, fragrances, odor masking agents, and the like.
Foam in accordance with the present invention is preferably made by an extrusion process that is well known in the art. In such a process, the polymeric components are added to an extruder, preferably in the form of resin pellets. Any conventional type of extruder may be used, e.g., single screw, double screw, and/or tandem extruders. In the extruder, the resin pellets are melted and mixed. A blowing agent is preferably added to the melted polymer via one or more injection ports in the extruder. Any additives that are used may be added to the melted polymer in the extruder and/or may be added with the resin pellets. The extruder pushes the entire melt mixture (melted polymer, blowing agent, and any additives) through a die at the end of the extruder and into a region of reduced temperature and pressure (relative to the temperature and pressure within the extruder). Typically, the region of reduced temperature and pressure is the ambient atmosphere. The sudden reduction in pressure causes the blowing agent to nucleate and expand into a plurality of cells that solidify upon cooling of the polymer mass (due to the reduction in temperature), thereby trapping the blowing agent within the cells.
The foregoing, as well as other, aspects and advantages of the invention may be further understood by reference to the following examples, which are provided for illustrative purposes only and are not intended in any way to be limiting. In general, flexibility of the foam blends at sub-freezing temperatures were determined by chilling the foam blends at an appropriate temperature for at least 4 hour followed by bending the foam at least 90 degrees. Foams that did not break or fracture were considered acceptable.
The foam blends in example 1 were prepared with a twin-screw extruder. The following ingredients were used.
The above resin and additives were added into the feed hopper. Isobutane was mixed with the molten resin and additives and the melt was allowed to cool. The cooled mixture was extruded through an annular sheet die. The extrusion conditions and foam density are given in the following Table 1.
Table 2 shows the tensile, tear, and elongation properties for the foam blends in Table 1. In these examples, flexibility was determined by placing the sample in a cold chamber at −20° F. for at least 24 hours. A high velocity piston struck the surface of the foam at a speed of 12 ft/sec. If the foam breaks or shatters, it is considered unacceptable. Samples where the foam traveled through the foam without breakage, were considered acceptable.
Tensile and elongation properties were evaluated as per ASTM D412-98 test method. Tear strength was determined using ASTM D624-00 test method.
A twin-screw extruder was used to make PP/LDPE/rubber blend foam. The extrusion conditions are shown below in Table 3. The line output rate was held at 500 lbs./hr. The composition details are given in Table 3. For example, for sample 6, the percentages of polypropylene (PP), low density polyethylene (LDPE), styrene-isoprene styrene (SIS) rubber, ultraviolet inhibitor (UVI), and black colorant are 75.6%, 20%, 0%, 2.4%, and 2.0%, respectively. In addition to the mentioned additives, a talc masterbatch was added at 0.4% by weight to nucleate fine cells. The chemical ingredients used are listed below:
styrene-isoprene-styrene rubber component Model No. Europrene Sol T 190 available from Enichem.
The extruded foams were tested to evaluate their properties. The results are shown in Table 4. Sample 6 shows the data for the “Control” sample with no rubber content. For brittleness test, the foam samples were immersed in a cold bath at −4° F. for 5 hours and then the samples were bent and tested for breaking. Samples comprising less than 3% rubber broke. Samples having a rubber component greater than 3% showed the desired flexibility at −4° F. The resulting flexibility at sub-freezing temperatures is very useful for expansion joint filler or automotive tote applications.
*MD and cMD represent machine direction and cross-machine directions, respectively.
The density was checked in accordance with ASTM D3575-00 test method. The compression and recovery were done in accordance with ASTM D545-99 test method. Tensile and elongation properties were evaluated as per ASTM D412-98 test method.
It is interesting to note that the percent elongation in the machine direction increases with the increase of rubber concentration in the formulation. For example, at 5% rubber concentration (sample 8), the percent elongation at break in the machine direction improved by 28%. Such improvements are desirable for the foam to remain flexible in filling the concrete expansion joint filler without tearing. Also, the 25% compression gradually improved with the addition of rubber component, which may indicate that the foam has a desired stiffness needed to serve for the expansion joint filler application.
A twin-screw extruder was used for this experiment. The foam was extruded in a cylindrical rod shape. Glycerol monostearate was used as an aging modifier to stabilize the foam and was added 1.6% by weight. Talc masterbatch was added 1.5% by weight to nucleate fine cells. The resin and additives were fed through the hopper and the molten polymer was mixed with the isobutane foaming agent. The mixture was then allowed to cool and pass through a capillary nozzle to result in a cylindrical rod shaped foam for evaluation and testing. The extrusion conditions are shown in Table 5 below. The ingredients used are given below.
EVA—0.925 g/cc, 4.6 by wt % Ethylene/vinyl acetate copolymer
The above foam samples were tested for density, cell count and percentage elongation at break to check their elasticity.
The data in Table 6 indicates that percent elongation increases as the amount of EVA in the blend is increased. It is interesting to note that when 5% rubber was added the percent elongation improved from 30.3 to 51.2 (69% improvement). Replacing 4.6% of the LDPE with ethylene/vinyl acetate copolymer further improved the % elongation of the foam blend
A twin-screw extruder was to make the foam blends in example 4. The total resin (blend) rate was set at 718 lb/hr. Isobutane was added at 63 lb/hr. Glycerol monosterate was added as an aging modifier at 3.6 lbs/hr or 0.5%. Talc masterbatch was added at 4 lb/hr or 0.6%. UVI stabilizer and black color additives were added at 2% and 1.4% respectively. The extruded foam thickness ranged from 0.226 to 0.263 in thickness as shown in Table 7 below. The sheets were cured for a few days and then heat laminated close to ½″ approximately in thickness before the cold temperature test. The brittleness of the foam samples at cold temperature was evaluated by placing the sample in a 35° F. environment for 20 hours and then, the samples were bent up to a 90 degree angle. The flexibility of the foam is acceptable if the samples do not break when they are bent. The experimental data is summarized in Table 7.
The following materials were used: 2.3 MI, 0.918 density LDPE, EVA containing 9% vinyl acetate (VA) content, Rubber containing 84% SIS, polypropylene resins: (PP1 (PF814, high melt strength), PP2 (high melt strength, 0.902 g/cc), and PP3 (HL 783H low melt tension, 2.0 MI,0.902g/cc) available from Basell Polyolefins. Annular sheet die was used for foam extrusion.
Notably, the only sample that failed the flexibility test included no rubber component or EVA. As a result, it can be seen that the presence of the rubber component, EVA, or combination thereof may help improve the flexibility of the foam at cold temperatures. In addition, the presence of the rubber component also helps provide resiliency and long-term flexibility without undesirable degradation of properties. Sample E, comprising 80 weight percent polypropylene and 20 weight percent EVA was additionally tested for flexibility at sub-freezing temperatures. Sample E was place in a cold bath at 23° F. for 7 hours. Sample E was then removed from the bath and bent up to a 90 degree angle. The sample did not break or fracture.
In the following example, a foam blend comprising polypropylene and low density polyethylene (Sample G) was compared to two foam blends (Samples H and I) that were prepared in accordance of the invention. Sample H comprises a blend of polypropylene, EVA, and a rubber component. Sample I comprises a blend of polypropylene and EVA.
The foam blends were prepared in a similar fashion as described above. The following ingredients were used in the foam blends: 2.3 MI, 0.918 density LDPE, EVA having 4.6% vinyl acetate content, rubber component containing 84% SIS, polypropylene resin(PP1) (PF814, high melt strength) available from Basell Polyolefins; and polypropylene resin (PP3) (low melt tension, 2.0 MI, 0.902 g/cc) available from Basell Polyolefins.
Tensile Elongational was determined with ASTM D412-98 test method. The percentage recovery and compression were determined with ASTM D545-99.
The data in Table 8 indicates that the addition of a rubber component helps improve the elongational properties of the foam bent. Improved elongation properties may help when the foam is stretched or bent during use. The data also indicates that the PP/EVA blend may be better suited in concrete joint filler applications than PP/LDPE foam blends. In addition, the data suggests that the rubber component may help improve the recovery of the foam after compression. It is also interesting to note that Samples H and I have better strength at 25% compression.
Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which the invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
