Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Extrusion molding
Reexamination Certificate
2000-07-19
2002-11-26
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Extrusion molding
C264S211170, C264S234000, C264S473000, C264S477000, C528S048000, C528S074000, C528S083000, C528S906000
Reexamination Certificate
active
06485665
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for producing polyurethane elastomer fibers, and it also relates to the fibers that are produced in accordance with the inventive method.
Polyurethane elastomers are block polymers with a regular structure of soft and hard segments. The soft segments consist of long, flexible chains lacking any order of arrangement, which give the required rubber-like elasticity to the fiber. The properties of the fiber in regard to stretching and tensile strength can be varied depending on the molar mass and the kind of soft segment. The hard segments serve to fixate the soft segments. After a deformation, the return of the molecular chains of the soft segments to their former length occurs in an entropy-elastic manner. The hard segments consist of partially crystalline domains with a structure of short chains. The main task of the hard segments is to serve as anchor points to prevent a slippage of the polymer chains under the influence of mechanical forces. Following an elongation, the shape-restoring forces present in the elastomer will cause a contraction to nearly the original length. The remaining difference in length is called permanent elongation.
In general, polyurethane elastomers are produced in a single step or in a two-step process. In the first step of the two-step process, pre-polymers are formed in a reaction between diols of a higher molecular order and diisocyanates. In a second step, high-molecular end products are formed as the pre-polymers react with so-called chain extenders. Excess quantities of diisocyanate are used in the reaction of the first step, so that the pre-polymer molecule is terminated by isocyanate groups at both ends. The chain extenders are bi-functional, low-molecular compounds terminated by hydrogen atoms, mostly dihydroxy and diamino compounds capable of entering into reactions. The dihydroxy and diamino compounds react with the pre-polymers in the formation of the corresponding carbamic acid derivatives, i.e., the polyurethane elastomers, specifically poly-urea urethane elastomers. In the macro-molecular chains, the soft segments of higher-molecular diols alternate with the rigid hard segments resulting from the reaction of the chain extenders terminated by iso-cyanate groups. The single-step process (also called one-shot process) bypasses the pre-polymer stage, as the diisocyanate reacts simultaneously with the macro-diol and the chain extender.
The difference in the chemical composition of hard and soft segments as well as the difference in their polarities and molecular weights will cause the hard and soft segments to segregate from each other. The formation of hydrogen bridges between neighboring chains has the effect that the hard segments congregate in parallel alignment with each other. The long movable molecule chains in between develop cross-linked interconnections that are loosened and stretched when the loose-knit network is subjected to a tensile deformation. The interaction between the hard segments prevents plastic flow of the molecular chains in the distended state. The stretching of the macro-molecules is associated with a transition into a more highly ordered state and a decrease in entropy. When the mechanical stress load is removed, the thermal motion of the molecules will cause them to return to the cross-linked state that corresponds to a higher level of entropy. Strong mechanical stress loads, however, will break the interaction between the hard segments, causing irreversible structural rearrangements of the hard segments. This has a negative effect on the mechanical hysteresis properties. Especially melt-extruded polyurethane fibers exhibit a large hysteresis loss of elastic energy and force as well as a high amount of permanent elongation. In order to improve the properties of the fibers, it will therefore be necessary to provide a better fixation of the hard segments.
The known methods of producing melt-extruded polyurethane elastomer fibers primarily use a polyurethane polymer based on aromatic diisocyanates, mostly di-phenyl methane-4,4′-diisocyanate (MDI). The polymer coming out of the reaction is melted and processed into a fiber by way of a melt-extrusion process. However, polyurethane polymers based on aromatic diisocyanates are becoming less acceptable, because their decomposition releases aromatic amino groups that are suspected of being carcinogens. In addition, polyurethane polymers based on aromatic diisocyanates have a tendency of yellowing.
OBJECT OF THE INVENTION
It is therefore the object of the present invention to provide a method of producing polyurethane elastomer fibers based on non-aromatic diisocyanates with improved properties, particularly with respect to tear strength, tear elongation, permanent elongation and heat distortion temperature (HDT).
SUMMARY OF THE INVENTION
In accordance with the present invention, the objective is met by a method that has the following steps:
(a) A segmented polyurethane polymer is produced on the basis of i) a macrodiol with a molecular weight of approximately 500 to 10,000, ii) an aliphatic, cyclo-aliphatic and/or aliphatic-cycloaliphatic diisocyanate and iii) a chain extender with at least two hydroxy and/or amino groups. As a percentage of the sum of the hydroxy and amino groups, the polymer has a molar excess of isocyanate groups of at least approximately 0.2% over the hydroxy and/or amino groups from the macrodiol and chain extender.
(b) The polyurethane polymer is melt-extruded to form a fiber. Steps (a) and (b) are carried out under temperature conditions and within a time interval where essentially no allophanate will be formed.
(c) The fiber is subjected to a post-treatment under temperature conditions and within a time interval in which the polyurethane polymer is cross-linked through the formation of allophanate.
The polyurethane polymer must melt and be in the liquid phase at a suitable temperature. The polyurethane polymer is produced under the pre-polymer or the one-shot method, either through a reaction between macro-diol, chain extender and diisocyanate with the optional addition of a catalyst, essentially without a solvent, or by melting a preliminary form of polyurethane polymer containing iso-cyanate groups in stoichiometric proportion or in deficit proportion to hydroxy- and amino groups and by allowing the melted material, possibly after it has cooled down, to react with a diisocyanate and/or an isocyanate-terminated pre-polymer, again essentially in the absence of a solvent.
In preferred embodiments, the polyurethane polymer has a molar excess of iso-cyanate groups in relation to hydroxy- and amino groups of about 0.2% to 15%, the range from 1% to 10% being especially preferred. Polyurethane-polymer chains can be cross-linked through the formation of allophanate- or biuret bonds (Subsequently, only the term “allophanate” will be used. Depending on the context, this is meant to include “biuret”). In this case, an excess iso-cyanate group reacts with an already formed urethane or urea group, causing a branch in the molecular structure. The applicant has made the observation that allophanate-cross-linked polyurethane polymers based on aliphatic diisocyanates are unsatisfactory for the melt-extrusion process. The extrusion temperatures required for allophanate-cross-linked polyurethanes are in the area of 230° C. The extruded fibers have a high degree of stickiness and inadequate strength. At the high extrusion temperature required, there is furthermore a strong decay in the molar mass of the polymer. The inventive method makes use of the difference in the reaction kinetics between the polyurethane chain formation and the allophanate formation. The formation of the allophanate links occurs more slowly than the build-up of the linear polyurethane chains. Consequently, the polyurethane polymer is produced with a defined excess proportion of isocyanate and is extruded even before the formation of allophanate cross-links sets in. The not yet cross-linked polyurethane can be extruded at relat
Hermanutz Frank
Hirt Peter
Oess Oliver
Oppermann Wilhelm
Darby & Darby
Rhodianyl S.N.C.
Tentoni Leo B.
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