Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-02-20
2003-12-02
Harlan, Robert (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S348600, C525S240000, C264S210700, C264S210800
Reexamination Certificate
active
06657033
ABSTRACT:
The present invention relates to thermal bondable fibers comprising a random copolymer of propylene with one or more olefin comonomers different from ethylene, to the process for preparing said fibers, and to the thermally bonded articles obtained from said fibers.
Fibers of certain thermoplastic materials are used widely in the manufacturing of thermally bonded products, such as nonwoven articles, by various processes. Said processes are mainly staple carding/calendering, through air-bonded, spunbonding, melt-blown, and any combination of them for composite structures of nonwovens.
There have been various attempts made to improve the thermal bondability (i.e. the bond strength) of fibers and/or the calendering speed, among which the use of random copolymers of propylene has been contemplated.
In particular, according to EP-A-416 620 fabric laminates having layers made of fibers formed from olefin copolymers, terpolymers, and blends of polymers having a crystallinity less than 45% provide improved thermal bonding and therefore improved fabric characteristics. However this document provides a concrete disclosure of propylene-ethylene copolymers only, and points out that said copolymers produce fibers with lower tenacity and lower modulus than those formed from polypropylene.
According to U.S. Pat. No. 4,211,819 heat-melt fibers are obtained by spinning a crystalline propylene terpolymer consisting of specified amounts of propylene, butene-1 and ethylene. However such fibers are used as binder material only, the mechanical properties being conferred by other materials. In fact, when nonwoven fabrics are prepared in the examples, the said fibers are mixed with rayon fibers before calendering.
Therefore it would be advantageous to provide fibers containing olefin copolymers and having an improved thermal bondability associated with high mechanical properties. In the typical process of melt spinning, the polymer is heated in an extruder to the melting point and the molten polymer is pumped under pressure through a spinneret containing a number of orifices of desired diameter, thereby producing filaments of the molten polymer. The molten polymer filaments are fed from the face of the spinneret into a cooling stream of gas, generally air, where these filaments of molten polymer are solidified as a result of cooling to form fibers.
In processes of this kind it would be advantageous to be able to operate with the highest possible spinning speed without impairing the final properties of the so obtained fibers. It has now been found that all the said advantages are obtained by spinning specific random copolymers of propylene.
Accordingly, the present invention provides thermal bondable polyolefin fibers comprising 1% by weight or more, in particular 20% by weight or more, of a random copolymer A) of propylene with one or more comonomers selected from &agr;-olefins of formula CH
2
═CHR, wherein R is a C
2
-C
8
alkyl radical, preferably a C
2
-C
6
alkyl radical, the amount of said comonomer or comonomers being from 3% to 20% by weight with respect to the total weight of the random copolymer A).
From the above definitions it is evident that the term “copolymer” includes polymers containing more than one kind of comonomers.
It has been unexpectedly found that the said fibers have Tenacity values comparable to or higher than the tenacity obtainable by spinning propylene homopolymers under substantially the same conditions, while achieving particularly high values of bond strength at unusually low thermal bonding temperatures.
In particular, the thermal bondable fibers of the present invention are preferably characterized by Tenacity values equal to or higher than 10 cN/Tex (measured as explained in the examples), specially equal to or higher than 15 cN/Tex, for instance from 10 to 60 cN/Tex or from 15 to 60 cN/Tex.
Moreover, the fiber retraction tends to increase with the amount of random copolymer A). This is very important to enhance the self-crimping effect of the fiber. The so obtained high level of self-crimping induces bulkiness in the final nonwovens with higher soft feeling. Also the higher softness contributes, with the soft touch, to improve the final nonwoven quality, in particular for the hygiene applications where the market appreciates very soft nonwovens with clothlike appearance.
Preferred amounts of &agr;-olefins of formula CH
2
═CHR (R being a C
1
-C
8
alkyl) in the random copolymer A) are from 5% to 16% by weight, in particular from 5.5% to 13% by weight. Examples of &agr;-olefins of the above reported formula, present as comonomers in the random copolymer A), are 1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene. Preferred are 1-butene and 1 -hexene; particularly preferred is 1-butene.
The presence of substantive amounts of ethylene (indicatively, more than 0.5-1% by weight) in the random copolymer A) is excluded; particularly preferred is a random copolymer A) wherein the comonomer or comonomers present are selected exclusively from the said &agr;-olefins of formula CH
2
═CHR, wherein R is a C
1
-C
8
alkyl radical.
Preferably the Melt Flow Rate (MFR, measured according to ISO 1133 at 230° C. with a load of 2.16 Kg) of the random copolymer A) used for preparing the fibers of the present invention is within the range from 5 to 2000 dg/min., more preferably from 10 to 1000 dg/min.
In the fiber the MFR of the random copolymer A) or of the polymer composition comprising the copolymer A) can be higher, depending upon the degree of thermal degradation occurring during the spinning process.
Such values of MFR can undergo even significative variations from the center to the surface of the fiber, depending upon the formation of skin-core structures where the skin, i.e. a more or less thick layer of polymer on the surface of the fiber, has high MFR values caused by the said thermal degradation.
However it has been surprisingly found that the fibers of the present invention do not necessarily require the formation of skin-core structures to achieve high levels of bond strength, even if the formation of a skin-core structure further enhances this property.
It has been found that a particularly good balance of bond strength and mechanical features is obtained when the fibers of the present invention are prepared from random copolymers A) having values of Tensile Strength at yield (measured according to ISO R 527) equal to or higher than 24 MPa, in particular from 24 to 35 MPa, preferably equal to or higher than 25 MPa, more preferably higher than or equal to 26 MPa, in particular from 25 or 26 to 35 MPa.
Even better properties are achieved when the fibers of the present invention are prepared from a polymeric material obtained by subjecting to chemical degradation (visbreaking) a random copolymer A) having the said values of Tensile Strength at yield, or a polymer composition containing the same.
Other preferred features of the random copolymer A) used for preparing the fibers of the present invention are:
a melting temperature from 135 to 156° C., and a crystallization temperature from 85 to 120° C., both measured by DSC (Differential Scanning Calorimetry) with a temperature variation of 20° C. per minute;
fraction insoluble in xylene at 25° C. higher than or equal to 93% by weight, more preferably higher than or equal to 95% by weight;
Polydispersity Index (PI, measured with the method described in the examples) from 2 to 5;
Flexural Modulus (measured according to ISO 178) from 500 to 1500 MPa;
Izod Impact Strength (notched) at 23° C. (measured according to ISO 180/1) equal to or higher than 20 KJ/m
2
;
Elongation at yield (measured according to ISO R 527) from 8 to 14%;
The ratio of the value of Tensile Strength at yield to the value of Elongation at yield for the random copolymer A), either before or after the said polymer degradation (when occurring) is preferably from 2 to 4, more preferably from 2.1 to 4.
Particularly preferred values of Tenacity for the fibers of the present invention are equal to or higher than 20 cN/Tex, in particular from 20 to 60 cN/Tex;
Braca Giancarlo
Sartori Franco
Basell Poliolefine Italia S.p.A.
Harlan Robert
LandOfFree
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