BACKGROUND OF THE INVENTION

[0001] Many types of thermoplastic compositions are used to manufacture packaging materials, which vary in form or shape (e.g., bags, trays, films, etc.) and purpose (e.g., foods, drinks, consumer goods, etc.). In some applications (e.g., food packaging), it is particularly important that the composition presents good seal properties, such as a low seal temperature and a high performance weld.

[0002] Heat-sealing is a technique where two materials are bonded or fused under the combined effect of three parameters: pressure, temperature, and time. The process involves melting, interdiffusion and crystallization of macromolecules at the interface of the materials to be sealed. In the case of semi-crystallized polymers, such as polyolefins, seal temperature must be close to melting point to allow the mobility of polymer molecules in the interface. The welding strength, stiffness, failure mode and appearance of the weld after cooling at room temperature are important variables in the seal processes.

[0003] Although polyolefins (e.g. , such as polyethylene and polypropylene) are very used in flexible films, they often do not present good seal properties. For example, polypropylene does not generally seal well at low temperatures (below 120°C), which is an important requirement in film applications. Consequently, several studies and papers were developed/written to improve/describe the reduction of heat-seal temperature in polyolefin films.

[0004] Various approaches to reduce the heat-seal temperature of polyolefin compositions, without significantly changing other properties, have been attempted. These include the copolymerization of the use of various comonomers, the substitution of various copolymers, or the introduction of additives. However, known approaches to reduce heat-seal temperature can achieve a reduction of both melting temperature and crystallinity, which can cause undesirable changes of critical physical and mechanical properties.

[0005] In the case of the polypropylene, one way to decrease the seal temperature is via copolymerization of propylene (also known as propene or

methylethylene) with other comonomers, propylene to produce random propylene copolymers or terpolymers. Propylene, butene and ethene terpolymers are used in the seal layer multiple-layers biaxially oriented films. Although such terpolymers present seal temperatures below those obtained for propylene homopolymers, their seal temperatures are still high for most applications in flexible packages.

[0006] In addition, another way to reduce seal temperature is through post-reactor modification, which is an easier and less expensive technology and may include the mixing of one or more polymers that possess a lower melting point and/or lower crystallinity. The combination of these two factors, associated to the use of commodities polymers, gives more versatility to the process. In addition, one can observe a cost reduction compared to reactor technology. U.S. 2007/0287007A1 describes a hot-sealable film, having a seal layer formed of a polymer mixture of propylene copolymers or terpolymers containing a- olefins. The inventors claim that a SIT of 130°C is obtained for a film produced with the referred composition, which is higher than that obtained in this embodiment of the disclosed invention. Moreover, the referenced document does not consider the effect of time in the seal strength. The documents WO 2011/064124, WO 2011/0641 19, and U.S. Pat. 6,849,313 describe the mixture of three polymers (propylene homopolymers or copolymers, ethylene copolymers and thermoplastic elastomers), while the present invention uses just two polymers achieving good optical properties, which is not mentioned by these documents and not expected. Although these combinations can contribute to a reduction of seal temperature the products are distinguishable from the embodiments of the disclosed invention; in addition, they do not mention the effect of time in the seal strength. The use of ethylene copolymers in combination with polypropylene copolymers (terpolymer and copolymer of ethylene and butene and propylene copolymer) in order to improve performance of heat sealable film is mentioned in ES2011285B; besides the fact that they use three polymers in composition, they do not also mention the effect of time in the seal strength. The use of ethylene vinyl acetate copolymer is described in U.S. Pat. 4,333,968 as a coated film exhibiting good heat seal strength; however it is "peelable," different from the present invention.

[0007] U.S. Pat. 6,913,834 discloses another method for post-reactor modification of the polymer, by addition of nucleating agent to a combination of propylene and 1 -butene homopolymers or copolymers, showing a reduction in sealing temperature. U.S. 2005/0197456A1 relates to a composition comprising a random copolymer of propylene and at least a C4-C10 α-olefin and ethylene, and one or more nucleating agent(s) from about 10 ppm to 20,000 ppm, which increases the melting point of the resin at least 3°C. In the present invention, the nucleating agent is used to maintain the seal strength over time, with no observed changes in seal temperature. In U.S. 2005/0197456A1 the nucleating agent broadens the processability window by increasing the melting temperature of the copolymer, and the examples show that it does not cause changes in the sealing temperature of the polymer. These products differ from the embodiments of the disclosed invention, which comprises a composition of propylene copolymer containing ethylene and/or butene comonomers combined with an ethylene copolymer containing at least one a-olefin comonomer having 3 or more carbon atoms and/or one or more polar comonomers, and an additive.

[0008] Although the prior art refers to compositions comprising polymer blends and additives to achieve better sealing properties, none deals with the maintenance of seal strength over time.

SUMMARY OF THE INVENTION

[0009] The purpose of this invention is to provide improved sealable layers in films comprising a composition of at least a first component consisting of propylene copolymer with ethylene and/or butene comonomers; a second component comprising ethylene and a second component comonomer consisting of an α-olefin having 3 or more carbon atoms and/or one or more polar comonomers.

[0010] In an embodiment of the invention, the inventive compositions also contain additives.

[0011] In an embodiment of the invention, the films may have one or more layers.

[0012] In an embodiment of the invention, compositions exhibit an initial heat- seal temperature between about 105° and 110°C, and a heat-seal strength at a force above 1.5 N.

[0013] In an embodiment of the invention, the seal strength is maintained for at least 15 days.

[0014] In an embodiment of the invention, the one or more layers can be applied in a multilayer cast or biaxially oriented film. [0015] In another embodiment the invention comprises a film having greater than two layers having two outer layers for low heat-seal initial temperature applications.

[0016] In yet another embodiment, the invention comprises a manufactured package comprising any if the inventive compositions described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The foregoing summary, as well as the following detailed description of the disclosure, will be better understood when read in conjunction with the appended Figures. For the purpose of illustrating the disclosure, shown in the Figures are embodiments which are presently preferred. It should be understood, however, that the disclosure is not limited to the precise arrangements, examples and instrumentalities shown.

[0018] Figure 1 depicts a melting endotherm of a propylene copolymer;

[0019] Figure 2 depicts a melting endotherm of a propylene copolymer mixed with 7% ethylene copolymer 1 , and a nucleating agent; and

[0020] Figure 3 depicts a melting endotherm of a propylene copolymer + 7% ethylene copolymer 2.

DETAILED DESCRIPTION OF THE INVENTION

[0021] For purposes of the description hereinafter, it is to be understood that the embodiments described hereinafter may assume alternative variations and

embodiments. It is also to be understood that the specific articles, compositions, and/or processes described herein are exemplary and should not be considered as limiting.

[0022] The films disclosed herein are advantageous over the prior art as they provide improved properties, such as low seal temperature and high seal strength. The following terminology is used to describe these inventive features.

[0023] As used herein, "copolymer" is intended to mean polymer derived from two (or more) monomeric species. Copolymerization refers to methods used to chemically synthesize a copolymer. The copolymers of the present invention include alternating copolymers, block copolymers, periodic copolymers, and/or random copolymers.

[0024] As used herein, "seal temperature" is intended to mean the temperature at which two or more films fuse at an interface.

[0025] As used herein, "seal strength" or "weld strength" is intended to mean a measure of opening force and package integrity, as well as the packaging processes' ability to produce consistent seals.

[0026] As used herein, "melt flow index" (MFI) is a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures. The method is described in the similar standards ASTM D1238 and ISO 1 133.

[0027] As used herein, "additive" is intended to include nucleator or

nucleation/nucleating additive, and includes silicate additives such as hydrotalcite, fumed silica, fine particle silica, silica rock, diatomites, clay, kaolin, diatomaceous earth, silica gel, calcium silicate, sericite, kaolinite, flint, feldspar powder, vermiculite, attapulgite, talc, mica, minesotite, pyrophyllite, silica, salts of aliphatic or aromatic carboxylic acids, aromatic salts, metallic salts of aromatic phosphorus compounds, quinaridones, aromatic amides; and crystal nucleators such as a nanitol derivative or sorbitol derivative like dibenzylidene sorbitol, bis(p-methylbenzylidene) sorbitol, bis(p- ethylbenzylidene) sorbitol, disodium bicyclo[2.2.1]heptane-2,3-dicarboxylate, etc.

Additives may also include other clarifying or coloring agents. Additives may also include a phenolic antioxidant and phosphorous-based antioxidant that can prevent crystalline polymer compounds containing a crystal nucleator composition from undesired coloring. Additives may also include anti-blocking agents and flow

auxiliaries.

[0028] As used herein, the meanings of "melting temperature," "crystallization temperature" and "heat of fusion" are intended to fall within American Society for Testing and Materials (ASTM): ASTM E793 , ASTM D3418 and ASTM E794, which are incorporated herein by reference. [0029] As used herein, "heat-seal", "heat-sealing", "hot seal", and the like are intended to refer to processes to join bond or fuse (and thereby seal), one or more thermoplastic monolayers and/or multilayers (e.g., films).

[0030] As used herein, weld is intended to mean a seal between layers of film. For purposes herein, welds are generated by the process of heat-sealing.

[0031] As used herein, heat-seal initiation temperature or "initial heat-seal temperature" are intended to mean the minimum value at which a heat seal is achieved.

[0032] As used herein, biaxially oriented film refers to a polymeric composition that is extruded then stretched in both the machine direction and across machine (thereby being biaxially oriented). Biaxially oriented polypropylene (BOPP) is intended to fall within this definition.

[0033] As used herein, heat of fusion is intended to mean the change in enthalpy resulting from heating one mole of a substance to change its state from a solid to a liquid. The temperature at which this occurs is the melting point.

[0034] The propylene copolymer used in the present invention is a propylene copolymer with a-olefin comonomers, more specifically ethylene and/or butene, in amounts that may vary between 2-5% in mass and 3-8% in mass, respectively.

[0035] The composition initial heat-seal temperature is reduced due to the addition of other polymers and/or additives, which can be used together or separately, to achieve an initial heat-seal temperature less than 1 10°C. Among the polymers that can be used for reducing the initial seal temperature of a propylene copolymer are: an ethylene copolymer having a-olefin comonomers; and/or polar comonomers such as vinyl acetate, acrylates, etc. Among the additives that can be used, flow auxiliaries, nucleating agent, and antiblocking agents stand out.

[0036] In an embodiment of the invention, the composition initial heat-seal temperature is 105°C.

[0037] The blends of the present invention are prepared in the polymerization post-reactor stage. The propylene copolymer spheres or pellets, activated or not, are mixed with a second and/or third compound, ethylene copolymers and/or additives, respectively, in a single-screw or a twin-screw extruder, with processing temperatures between 180 and 290°C. [0038] After being prepared, the blends can be used, for example, in the manufacture of biaxially oriented multilayer films or BOPP film with a multi -layered structure, in which the blend is mainly used in the seal layer, besides the possibility of being used in the layer that undergoes treatment and metallization processes.

[0039] In an embodiment, the blends are prepared by blending a first component consisting of a propylene copolymer with ethylene and/or butene comonomers, or a mixture of both, and a second component consisting of an ethylene copolymer. The first and second components are present at between 80 to 97 weight percent (wt%) and 3 to 20 wt%, respectively. The second component (e.g., an ethylene copolymer) contains a comonomer in an amount between 3-30 wt%, the comonomer consisting of the following: a-olefins, alkyl acetates, alkynyl acetate, alkyl acrylates, dienes, etc. At least one additive selected from anti-blocking agents, nucleating agents, flow

auxiliaries, etc., is present in a quantity ranging between 500 to 3000 ppm.

Method for determining seal strength

[0040] The test for determining seal strength is performed in a universal test machine, using a charge cell of IkN and dislocation speed of 200 mm/min after seal, as described below.

[0041 ] Specimens with 7.5cm length and 1.5cm width were cut with scissors from biaxially oriented original films and individually sealed at 1 10°C in Teller Model EB equipment, according to ASTM F2029 standard, with a contact time of 1 second and a pressure of 76 psi. The time established for cooling the weld was 5 minutes in room temperature. After this time, the weld bar was open and film thickness (near the weld area) was measured. After 10 minutes of seal, test specimens were fixed in the test machine, the weld tensioning was started with a 200mm/min speed, in room

temperature, and the strength used to open the weld was measured. The analyses were performed in triplicates.

[0042] Samples were sealed and elongated right after the bi-orientation

(immediate), and also in different time intervals after the biaxial orientation to evaluate the seal strength maintenance. Results and Discussion

[0043] The tables herein present examples of formulations, where ethylene copolymer 1 is an ethylene/vinyl acetate copolymer (vinyl acetate 15 wt%), ethylene copolymer 2 is an ethylene/a-olefin copolymer and the propylene copolymer is a propylene/ethylene/butene copolymer (i. e. , terpolymer), and ethylene copolymer 3 ethylene/vinyl acetate copolymer (vinyl acetate 28 wt%).

Table 1 - Characterization of samples

MFI - melt flow index (*condition used: 230°C/21.6 Kg); Tc - crystallization temperature; AHc - heat of crystallization (crystallization enthalpy); Tm2 - melting temperature (at the second heating cycle); AHf- heat of fusion, "s" denotes the shoulder temperature, which is indicative of crystallization temperature.

[0044] Table 1 shows that the addition of ethylene copolymer to propylene copolymer with ethylene and/or butene comonomers can increase melt flow index, being established that the final product has an MFI about 4-20 g/10 min.

[0045] Significant variations in the crystallization temperature of mixtures were not observed, related to "propylene copolymer" (variation up to 7°C). The exception is the mixture of propylene copolymer with 7 wt% ethylene copolymer 1 + nucleating agent demonstrated a shoulder of about 76°C. Similarly, the crystallization enthalpy (AHc) was also much lower (62 J/g). Reductions in the melting temperatures and the heat of fusion were observed in the exemplary compositions (compared to "propylene copolymer"). The decrease was higher for the mixture with ethylene copolymer 1 + nucleating agent, where, besides a decrease of 3°C at endotherm maximum (melting point), a small shoulder in about 90°C was also observed. However, it should be highlighted that endotherm profile was not significantly different from the profile presented by pure propylene copolymer (as can be seen in Figure 1).

[0046] In the composition with ethylene copolymer 2, a decrease of 2°C in the melting point and a second melt peak at 114°C are observed. Similar properties were observed for mixtures of propylene copolymer with ethylene copolymer 2 + nucleating agent and propylene copolymer with ethylene copolymer 2 + silica. Figures 1 to 3 show melting endotherms of "propylene copolymer" and the two blends of ethylene copolymers.

[0047] The compositions presented in Table 1 were used in a sealing layer (an outermost or external layer) of a multi-layered BOPP film, wherein the core layer was a propylene homopolymer and in both outermost layers (one of them being a sealing layer), the compositions of the present invention were used. During sealing tests, the compositions presented an sealing strength above 1.5N with an initial heat-seal temperature of 1 10°C.

[0048] Three-layered BOPP films manufactured from the compositions herein were also subjected to heat-seal tests to determine the initial heat- seal temperature and the corresponding seal strength, as well as the maintainence of the seal strength over time.

Examples

[0049] Table 2 shows values of average initial heat-seal strength at 1 10°C, obtained for different compositions over time. It is very important to monitor the heat- seal strength over time because alterations on the microstructure of the material caused, among others, by molecules' mobility, can change the surface to be sealed. This change can prevent the film from presenting enough resistance to hold the weld over time. The monitor of the initial heat-seal strength over time (as the method described before) was assessed in this study. Table 2 - Initial seal strength at 1 10°C, over time.

average value for 3 measures

[0050] Table 2 shows that mixtures with 5 to 10 wt% of ethylene copolymers present a desirable seal strength, right after the stretching and over time. The three different ethylene copolymers also provide a reduced initial heat-seal temperature of the whole composition .

[0051 ] The use of additives (e.g. , silica), combined with the use of an ethylene copolymer, can improve the maintenance of the seal power over time. One possible mechanism is that additives increase the stability of composition microstructures, thereby decreasing the molecular mobility and, thus, promoting seal strength. The results of Table 3 and Table 4 show that the use of additives promote seal strength over time, in the disclosed compositions. Table 3 - Initial seal power at 110°C over time.

* average value for 3 measures