Fibers and fabrics prepared with propylene impact copolymers

Details Fibers and fabrics prepared with propylene impact copolymers Fibers and fabrics prepared with propylene impact copolymers

2000-02-29

2001-06-19

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...

C525S240000

Reexamination Certificate

active

06248833

ABSTRACT:

FIELD
This invention relates generally to novel fibers and fabrics. Specifically, these fibers are prepared using propylene impact copolymer compositions. Fabrics, particularly nonwoven fabrics, formed from these fibers exhibit improved elongation properties.
BACKGROUND
The use of various thermoplastic resins to make fibers and fabrics is well known. Examples of such resins include polyesters, polyetheresters, polyamides and polyurethanes. Polyolefins, particularly propylene homopolymers and copolymers, are thermoplastic resins commonly used to make fibers and fabrics.
Propylene impact copolymers are a type of thermoplastic resin commonly used in applications where strength and impact resistance is desired such as in molded and extruded automobile parts, household appliances, luggage and furniture. Propylene homopolymers are often unsuitable for such applications because they are too brittle and have low impact resistance particularly at low temperature, whereas propylene impact copolymers are specifically engineered for applications such as these.
Though sometimes used to make films, propylene impact copolymers have not been used to make fibers and fabrics because impact resistance is not a required property for such applications. For fibers and fabrics, manufacturers focus on properties such as strength, processability, softness and breathability.
The use of propylene homopolymers, copolymers and various blends to make nonwoven fabrics is well known. For example, U.S. Pat. Nos. 5,460,884, 5,554,441 and 5,762,734 describe the use of polypropylene blends to prepare nonwoven fabrics. U.S. Pat. No. 5,994,482 describes the use of a polypropylene alloy for making soft nonwoven fabrics.
Though a variety of properties can be obtained, the use of blends such as these has the primary disadvantages associated with the additional processing steps required to make and use blended materials. We have discovered that many of these same properties can be obtained using a propylene impact copolymer which is not post reactor blended.
A typical propylene impact copolymer contains two phases or components, a matrix component and a copolymer rubber component dispersed in the matrix. These two components are usually produced in a sequential polymerization process wherein the homopolymer produced in one or more initial reactors is transferred to one or more subsequent reactors where copolymer is produced and incorporated within the matrix component. The copolymer component has rubbery characteristics and provides the desired impact resistance, whereas the matrix component provides overall stiffness.
Many process variables influence the resulting impact copolymer and these have been extensively studied and manipulated to obtain various desired effects. For example U.S. Pat. No. 5,166,268 describes a “cold forming” process for producing propylene impact copolymers where finished articles are fabricated at temperatures below the melting point of the preform material, in this case, the propylene impact copolymer. The patented process uses a propylene impact copolymer comprised of either a homopolymer or crystalline copolymer matrix, or first component, and at least ten percent by weight of an “interpolymer” of ethylene and a small amount of propylene (the second component). Adding comonomer to the first component lowers its stiffness. The ethylene/propylene copolymer second component is said to enable the finished, cold-formed article to better maintain its shape.
U.S. Pat. No. 5,258,464 describes propylene impact copolymers with improved resistance to “stress whitening.” Stress whitening refers to the appearance of white spots at points of impact or other stress. These otherwise conventional propylene impact copolymers have first and second components characterized by a numerical ratio of the second component intrinsic viscosity to the first component intrinsic viscosity which is near unity.
In U.S. Pat. No. 5,362,782, nucleating agent is added to propylene impact copolymers having a numerical ratio of the intrinsic viscosity of the copolymer rubber phase (second component) to the intrinsic viscosity of the homopolymer phase (first component) which is near unity and an ethylene content of the copolymer phase in the range of 38% to 60% by weight. These propylene impact copolymers are described as producing articles having good clarity as well as impact strength and resistance to stress whitening. The nucleating agents increase stiffness and impact strength.
Propylene impact copolymers are also used to produce films as described in U.S. Pat. No. 5,948,839. The impact copolymer described in this patent contain a conventional first component and 25 to 45 weight percent ethylene/propylene second component having from 55 to 65 weight percent ethylene.
We have discovered that fibers and fabrics prepared with impact copolymers have distinct advantages, particularly over similar products prepared with homopolymers and random copolymers.
SUMMARY
This invention relates generally to fibers and fabrics comprising reactor produced propylene impact copolymer compositions comprising from about 40% to about 95% by weight Component A based on the total weight of the impact copolymer, Component A comprising propylene homopolymer; and from about 5% to about 60% by weight Component B based on the total weight of the impact copolymer, Component B comprising propylene/comonomer copolymer, preferably propylene/ethylene copolymer, wherein the copolymer comprises at least 20% by weight isotactic propylene and at least 30% comonomer. The fibers may be spunbond or meltblown to create nonwoven fabrics.
DESCRIPTION
The propylene impact copolymers (“ICPs”) useful for making the fibers and fabrics of this invention comprise at least two major components, Component A and Component B. Component A is preferably an isotactic propylene homopolymer, though small amounts of a comonomer may be used to obtain particular properties. Typically such copolymers contain 10% by weight or less, preferably less than 6% by weight or less, comonomer such as ethylene, butene, hexene or octene. Most preferably less than 4% by weight ethylene is used. The end result is usually a product with lower stiffness but with some gain in impact strength compared to homopolymer Component A.
Component A preferably has a narrow molecular weight distribution Mw/Mn (“MWDI”), i.e., lower than 4.5, preferably lower than 4.0 more preferably lower than 3.5, and most preferably 3.0 or lower. These molecular weight distributions are obtained in the absence of visbreaking using peroxide or other post reactor treatment designed to reduce molecular weight. Component A preferably has a weight average molecular weight (Mw as determined by GPC) of at least 100,000, preferably at least 200,000 and a melting point (Np) of at least 145° C., preferably at least 150° C.
Component B is most preferably a copolymer comprising propylene and comonomer, preferably ethylene, although other propylene copolymers or terpolymers may be suitable depending on the particular product properties desired. For example propylene/butene, hexene or octene copolymers may be used. In the preferred embodiment though, Component B is a copolymer comprising at least 20% by weight isotactic propylene, more preferably from about 20% by weight to about 70% by weight propylene, even more preferably from about 30% by weight to about 60% by weight propylene; and from about 30% to about 80% by weight comonomer, more preferably from about 40% to about 70% by weight comonomer, preferably ethylene. Most preferably Component B consists essentially of propylene and from about 20% to about 80% by weight ethylene, more preferably from about 30% to about 70% by weight ethylene, even more preferably from about 40% to about 60% by weight ethylene.
Component B preferably has an intrinsic viscosity greater than 1.00 dl/g, more preferably greater than 1.50 dl/g and most preferably greater than 2.00 d/g. The term “intrinsic viscosity” or “IV” is used conventionally herein to mean the viscosity of a solution of polymer such as Comp

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