Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-11-28
2003-07-22
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S199000, C525S222000, C525S240000
Reexamination Certificate
active
06596814
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to the production of synthetic films and more specifically to polypropylene films made by cold drawing a high melt strength, high &bgr;-cystalline polypropylene. The resultant films are opaque, porous and have good physical properties.
BACKGROUND OF THE INVENTION
Opaque polypropylene film can generally be obtained by stretching polypropylene containing a filler or colorant such as talc, clay, calcium carbonate or TiO
2
. It is known to those skilled in the art that &bgr;-crystalline polypropylene can be used to produce opaque films without adding any filler or colorant. Such films owe their opacity to the presence of numerous pores throughout the film matrix that are formed by stretching the film.
Opaque films from &bgr;-crystalline polypropylene can be prepared via “cold drawing”. That is, drawing the material at a relatively low temperature, such as more than 40° C. below the melting temperature, e.g., <120° C., as opposed to about 20 to 25° C. below the melting temperature, e.g., 140° C. for normal drawing, at a stretch ratio of 2-10 times in two directions. The first direction being the machine direction and the second direction being the transverse direction. However, it is difficult to produce a film with uniform thickness throughout because of the localized stress differences in the drawing directions, especially if thin gauge films (<2 mil) are produced. As a result, the thickness of a film made from &bgr;-crystalline polypropylene has been limited to >2 mil and the porosity limited to 30-40%. Another drawback to using a low stretching temperature is that it may lead to relatively high stretching forces that cause web breakage and machine limitation.
Therefore, a need exists in the art for a polypropylene capable of cold drawing to a thin gauge film which overcomes the above-mentioned drawbacks. The present invention provides a &bgr;-polypropylene including a small amount of a very high molecular weight component. This &bgr;-polypropylene exhibits high melt strength and is useful in making thin gauge films with high opacity and porosity at low drawing temperatures.
SUMMARY OF THE INVENTION
The present invention provides a composition of a &bgr;-crystalline polypropylene with a K value of at least about 0.5 and a melt tension at 230° C. of at least 5 cN, and 0.01 wt % to 10 wt % of a high molecular weight polymer having a molecular weight of at least about 1,000,000.
The present invention further provides a film made from a composition comprising a &bgr;-crystalline polypropylene with a K value of at least about 0.5 and a melt tension at 230° C. of at least 5 cN, and 0.01 wt % to 10 wt % of a high molecular weight polymer having a molecular weight of at least about 1,000,000.
The present invention yet further provides a method of making a porous film, the method comprising, incorporating 0.01 wt % to 10 wt % of a high molecular weight polymer having a molecular weight of at least about 1,000,000 into a polypropylene, the polypropylene having a K value of at least about 0.5 and a melt tension of at least about 5 cN at 230° C., blending the polypropylene with a &bgr;-nucleating agent, and drawing the polypropylene at a temperature about 20° C. to about 50° C. below the melt temperature of &bgr;-crystals therein to a film having a thickness of about 0.1 to about 10 mil.
DETAILED DESCRIPTION OF THE INVENTION
As used herein, the term “polypropylene” generally includes homopolymers, copolymers of polypropylene, such as, for example, block, graft, impact, random and alternating copolymers, terpolymers, etc., and blends with other polymers, preferably polyolefins, such as high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), poly-1-butene, ethylene-vinyl alcohol copolymer or ethylene-methylmethacrylate copolymer and modifications thereof. Comonomers useful in the present invention include ethylene, 1-butene, 1-hexene, and other alpha-olefins.
Isotactic polypropylene is capable of crystallizing in several crystal forms. The &agr;, or monoclinic, form is the most prevalent one. The &bgr;, or hexagonal, form is occasionally found in commercially available polypropylene, usually at low levels. The relative proportion, defined as the K value, of the &bgr; form in a polypropylene specimen can be determined by X-ray diffraction and is expressed by the empirical expression:
K
=
H
β
H
β
+
H
110
+
H
040
+
H
130
wherein H
110
, H
040
and H
130
are the heights of the three strong peaks (100), (040) (130) of the &agr; form respectively and H
&bgr;
is the height of the strong &bgr;-peak (300). From the above expression, it will be apparent that in the absence of the &bgr;-form the K value will be zero. Alternatively the K value will be one if only the &bgr;-form is present.
The &bgr; crystalline content of the polypropylene used in the present invention should preferably be at least 50% (i.e., having a K value of 0.5 or more), more preferably at least 60% (K value of 0.6 or more) and most preferably at least 70% (K value of 0.7 or greater). Polypropylene with a high &bgr; content can be produced by any number of methods known in the art including, but not limited to, using the following &bgr;-nucleators: Q-dye (the gamma-crystalline form of a quinacridone colorant, Permanent Red E3B), described in U.S. Pat. No. 5,310,584 issued to Jacoby et al., and the amides, such as N,N′-dicyclohexane-2,6-naphthalene dicarboxamide, described in U.S. Pat. No. 5,491,188 assigned to the New Japan Company. In U.S. Pat. No. 5,231,126 Shi, et al describe &bgr;-nucleating agents that are particularly preferred for use in the present invention. The &bgr;-nucleating agents described by Shi et al are mixtures of organic dibasic acids with oxides, hydroxides or acid salts of Group II metals. Suitable dibasic acids include pimelic, suberic, azelaic, o-phthalic, iso-phthalic and terephthalic. Examples of suitable Group II metals are magnesium, calcium, strontium and barium.
“Drawing ratio” as used herein, also referred to as “stretching ratio”, means the ratio of the area of the film after drawing versus the area of the film, or sheet, before drawing and can be expressed as:
Drawing Ratio
=
Area
[
After
Drawing
]
Area
[
Before
Drawing
]
This ratio can also be expressed in terms of the product of the ratio of the area in one drawing direction, such as the machine direction, before drawing, versus the area after drawing, times the ratio of the area in a second direction, such as the transverse direction, after drawing versus the area before drawing, and can be expressed as:
Drawing Ratio
=
(
Area
[
Before
Drawing
]
)
(
Area
[
After
Drawing
]
)
MD
×
(
Area
[
After
Drawing
]
)
(
Area
[
After
Drawing
]
)
TD
wherein MD refers to the machine direction and TD refers to the transverse direction.
Stretching ratios for the films of the present invention are preferably 2 to 50, more preferably 3-45 and most preferably 4-35. The films of the present invention have a porosity of preferably 10% to 75%, more preferably 15% to 50% and most preferably 25% to 45%. The films of the present invention can be stretched, or drawn down, to 0.1 to 10 mil, preferably 0.2 to 5 mil, more preferably to 0.5 to 2 mil and most preferably to about 1 mil.
High melt strength (HMS) polypropylene has been commercially available since at least the early 1980's. “Melt strength” as used herein means the resistance of elongational flow of a polymer melt and is characterized either by melt tension, i.e., the tensile stress of molten specimen or by elongational viscosity. Extensibility, which measures melt strength and drawability of a polymer melt can be determined by using a Rheoten extensional viscometer, in which a melt strand is extruded through a capillary die and pulled down with increasing velocity by a pair of wheels. The force necessary to pull the melt strand is measured to the breaking poi
Fujii Masaki
Kim Se-hyun
Buchanan & Ingersoll PC
Koons, Jr. Robert A.
Nutter Nathan M.
Richardson Karen C.
Sunoco Inc. (R&M)
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