Process of making highly oriented polyolefin fiber

Details Process of making highly oriented polyolefin fiber Process of making highly oriented polyolefin fiber

2001-04-30

2004-04-20

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Forming continuous or indefinite length work

Shaping by extrusion

C264S205000, C264S210700, C264S210800, C264S211140, C264S211160

Reexamination Certificate

active

06723267

ABSTRACT:

The invention relates to a highly oriented polyolefin fibre containing polyolefin with an intrinsic viscosity of at least 5 dl/g, which fibre has a tensile strength of at least 26 cN/dtex and a modulus of tension of at least 700 cN/dtex, a process for the preparation thereof and the use in ropes or anti-ballistic shaped articles. The invention also relates to improved ropes and anti-ballistic shaped articles.
The said highly-oriented polyolefin fibres are known from EP-A-0,205,960. The highly oriented polyolefin fibres described there have a very high tensile strength and modulus of tension and a low creep rate, making them particularly suitable for use in, inter alia, ropes and anti-ballistic shaped articles. The fibres are prepared by spinning a solution of a polyolefin into a gel fibre, extracting the solvent from the fibre, and drawing the extracted and dried fibre in one or more steps.
However, there is an ongoing need for further improvement of the quality of such fibres, or at least for optimization of the properties of the fibres to such an extent that the quality of the products, such as ropes and anti-ballistic shaped articles, made from these fibres can be improved. The aim of the invention therefore is to provide highly oriented polyolefin fibres with improved properties in said applications.
Surprisingly, this aim is achieved in that the fibre contains 0.05-5 wt. % of a solvent for the polyolefin (relative to the fibre's total weight).
It has been found that the fibres according to the invention are eminently suitable for use in anti-ballistic shaped articles since shaped articles on the basis of these fibres have a high Specific Energy Absorption (SEA), which means that less fibre, and hence less weight, is needed to obtain the same level of protection. It has also been found that the fibres according to the invention are suitable for use in ropes, inter alia because their compactness is better without any loss in flexibility and because the strength of the ropes is enhanced.
The improved quality of the fibres is particularly surprising since up to now the presence of a significant amount of solvent in the fibre has been considered undesirable as this reduces the mechanical properties of the fibre, in particular because the fibre's creep rate is higher and its strength and modulus are lower. It is also surprising that solvent-containing fibres have a higher anti-ballistic quality than “dry” fibres of comparable strength and modulus, for in itself the solvent cannot contribute to the level of protection, while it does increase the areal density.
Fibres that contain solvent are known in the state of the art. However, these fibres are not highly oriented and they are unsuitable for the desired applications as their mechanical properties are not good enough. Within the context of the present application, highly oriented is understood to mean that the fibre has a modulus of tension of at least 700 cN/dtex and a tensile strength of at least 26 cN/dtex (as determined according to the method specified below). The known solvent-containing fibres are intermediates in a process in which the fibre is prepared from a solution. The description makes it clear that the solvent is undesirable in the end product and therefore still needs to be removed. U.S. Pat. No. 5,213,745, for example, describes optimum extraction agents for the removal of mineral oil solvent from an undrawn gel fibre. This publication does not describe solvent-containing, highly oriented polyolefin fibres. EP-A-0,115,192 describes fibres having a high solvent content and a low tensile strength and modulus of tension. These fibres, too, are intermediates, and as such unsuitable for use in the said applications.
The tensile strength (or strength) and the modulus of tension (or modulus) are defined and are determined as specified in ASTM D885M, using a nominal gauge length of the fibre of 500 mm, a crosshead speed of 50%/min and Instron 2714 clamps. Before the measurement the fibre is twisted at 31 turns per meter. On the basis of the measured stress-strain curve the modulus is determined as the gradient between 0.3 and 1% strain. For calculation of the modulus and strength, the tensile forces measured are divided by the titer, as determined by weighing 10 meters of fibre. Creep is here and hereinafter understood to be the elongation as a percentage of the original length after 5 hours under a load of 8.11 gr/dtex at 50° C. The elongation includes the elastic elongation.
A fibre is understood to be a continuous or semi-continuous object such as a monofilament, multifilament yarn, tapes or staple fibre yarn. In principle, the filaments may have any cross-sectional shape and thickness. Preferably, the filament titer is at most 5, more preferably at most 3 denier per filament. The advantage of such a low filament titer is that the fibre has better anti-ballistic properties.
Varying polyolefins can be used in the fibre according to the invention. Particularly suitable polyolefins are homo- and copolymers of polyethylene and polypropylene. In addition, the polyolefins used may contain small amounts of one or more other polymers, in particular other alkene-1-polymers. Good results are achieved if linear polyethylene (PE) is chosen as polyolefin. Linear polyethylene is here understood to be polyethylene with fewer than one side chain per 100 carbon atoms, and preferably fewer than one side chain per 300 carbon atoms, which may moreover contain up to 5 mol % of or more alkenes that can be copolymerized with it, such as propylene, butene, pentene, 4-methylpentene or octene. Besides the polyolefin and the solvent the fibre may contain small amounts of the additives that are customary for such fibres, such as anti-oxidants, spinfinish, thermal stabilizers, colourants, etc.
Preferably, the polyolefin fibre, in particular the polyethylene fibre, has an intrinsic viscosity (IV) of more than 5 dl/g. Because of their long molecule chains, polyolefin fibres with such an IV have very good mechanical properties, such as a high tensile strength, modulus, energy absorption at break. This is also the reason why even more preferably the polyolefin is a polyethylene with an IV of more than 10 dl/g. The IV is determined according to method PTC-179 (Hercules Inc. Rev. Apr. 29, 1982) at 135° C. in decalin, the dissolution time being 16 hours, the anti-oxidant is DBPC, in an amount of 2 g/l solution, and the viscosity at different concentrations is extrapolated to zero concentration.
To ensure a good anti-ballistic effect, the tensile strength of the fibre is at least 26 cN/dtex and the modulus at least 700 cN/dtex. Preferably, the modulus is at least 880 cN/dtex, more preferably at least 1060 cN/dtex, and most preferably at least 1235 cN/dtex. The strength is preferably at least 31 cN/dtex, more preferably at least 33 cN/dtex, and most preferably at least 35 cN/dtex. Surprisingly, it has been found that at relatively low, but for the purpose of the invention effective, solvent concentrations, the creep of such a highly oriented fibre is only to a very low extent adversely affected by the solvent. Preferably, the fibre according to the invention has a tensile strength of at least 26 cN/dtex, a modulus of at least 700 cN/dtex, a solvent content of 0.05-2 wt. % and a creep of at most 20%, more preferably at most 15%, even more preferably at most 10% and most preferably at most 5%. Such a low creep is favourable in particular for use in ropes. When use is made of copolymer with more than 2 short side chains per 1000 carbon atoms, the creep can be reduced further. Preferably, the creep then is at most 10% and more preferably at most 5%.
Solvent is here and hereinafter understood to be a substance that is capable of dissolving the polyolefin in question. Suitable solvents for polyolefins are known to one skilled in the art. They can, for example, be chosen from the ‘Polymer Handbook’ by J. Brandrup and E. H. Immergut, third edition, chapter VII, pages 379-402. Preferably, use is made of a solvent with a chi-parameter for the polyolef

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