EP3394333B1

EP3394333B1 - Method for making a woven polyester article

Info

Publication number: EP3394333B1
Application number: EP16822400.4A
Authority: EP
Inventors: Zahir Bashir
Original assignee: SABIC Global Technologies BV
Current assignee: SABIC Global Technologies BV
Priority date: 2015-12-23
Filing date: 2016-12-14
Publication date: 2021-09-15
Anticipated expiration: 2036-12-14
Also published as: CN108603313A; EP3394333A1; WO2017108540A1

Description

The present invention relates to a method for the manufacture of a woven article comprising a step of weaving tapes of a polyethylene terephthalate composition.
Generally, polyethylene terephthalate (PET) articles such as fibres, films, bottles and tapes are uniaxially or biaxially stretched and then heat set. The finished articles of PET are made from an amorphous PET precursor article which may be an unoriented filament, a cast sheet or an injection moulded preform. The amorphous precursor article is stretched uniaxially or biaxially above the glass transition temperature (Tg) at about 85-105 °C. At this stage the article attains a moderate crystallinity of about 15%. Such an article would be prone to gross shrinkage if used above the Tg, which is about 78 °C for PET homopolymer. Hence, it is general practice to heat set the PET article at a temperature of at least 150 °C but below the melting point. Heat setting is performed under tension to counter retraction tendency (i.e. shrinkage). The heat setting increases the crystallinity to about 30%, and the hot shrinkage, i.e. shrinkage at high temperature, is significantly reduced. Thus, the conventional practice to make dimensionally stable articles of PET is to start with an amorphous precursor article, stretch the article above the Tg at about 85-105 °C, and then heat set the stretched article at a temperature of at least 150 °C but below the melting point, always under tension.
Unidirectionally-oriented films of polyethylene terephthalate (PET) are disclosed in US 3,627,579 . This document discloses that unidirectionally oriented films have a high tendency to fibrillation (i.e. split in longitudinal direction) upon high stretch ratios even in absence of high temperature heat setting. US 3,627,579 further discloses that heat setting the films more intensely, such as at higher temperature while keeping the speed of manufacture constant, significantly reduces the elongation at break in transverse direction; the present inventors regard this as synonymous with splitting along the draw direction. Thus both the stretching as well as heat setting of PET adversely affect the fibrillation or splitting behaviour of PET films. US 3,627,579 proposes that a higher intrinsic viscosity (IV) allows the manufacture of a film of high tensile strength, which is resistant to shrinkage at elevated temperatures and which is of superior durability with respect to fibrillation or tear along the axis of direction of orientation. US 3,627,579 is not at all concerned with tapes for weaving applications, let alone with a method for the manufacture of a woven article.
In industrial weaving processes the tapes, in particular the weft tapes, are subject to strong forces and deformation. In particular the tapes may be subject to high and/or fast tensile loads, or tensile impact, and twisting motions. Hence in such a process the tapes are subject to off-axis loadings generally causing a higher risk for fibrillation or splitting in longitudinal direction.
Consequently pure PET tapes for weaving applications manufactured using conventional technology, i.e. containing the steps of preparing an amorphous tape, stretching the tape up to 6 times at a temperature above the Tg followed by a heat set at a high temperature such as from 170 - 240°C, were found to lack sufficient splitting resistance and solutions were explored wherein additives were added in order to improve the splitting resistance.
To that extent WO 2013/087200 discloses an unidirectionally-oriented film comprising a composition consisting of a thermoplastic polyester (a) in an amount of 85 to 99.9 wt.%, based on the total composition; a polycarbonate (b) in an amount of 0.1 to 15 wt.%, based on the total composition; and an additive (c) in an amount of 0 to 10 wt.%, based on the total composition. This patent application discloses that these films can successfully be used for weaving the into woven fabric.
Albeit not directed at weaving tapes US 2005/0238897 also addresses a problem of longitudinal splitting and discloses that such can be improved for strapping tapes by incorporating in PET less than 3% by weight of polyolefin such as linear low density polyethylene, branched low density polyethylene, high density polyethylene and combinations thereof.
WO 2012/041482 discloses a uniaxially oriented tape comprising (i) from about 75 wt.% to about 99.9 wt.% of a thermoplastic polyester, (ii) from about 0.1 wt.% to about 25 wt.% of a linear low-density polyethylene and (iii) from 0 wt.% to about 5 wt.% of other components, wherein said tape has a thickness from 5 µm to 300 µm and a width from 0.5mm to 7mm. This patent application also discloses that these tapes can be used for weaving the same into a woven fabric.
EP 0340992 to the contrary discloses that readily splittable tapes can be manufactured by incorporating into PET a minor amount of low density polyethylene. Thus PET compositions having LDPE in ratios of 90:10 to 75:25 are disclosed in this publication. EP 0340992 further discloses that heat setting is carried out such as to create a tape that is highly splittable, a property disclosed to be advantageous to carpet backing applications. The heat setting effect disclosed in EP 0340992 is in line with the teaching of US 3,627,579 .
Hence the prior art discloses that pure PET tapes with the appropriate properties tend to split during weaving causing interruptions of the weaving line and/or pollution of the production environment. The prior art further discloses the use of additives in PET for making woven fabrics, which have the disadvantage that it makes the tape manufacturing process more complex and further the additives adversely affect the transparency of the tapes and hence the woven fabric.
US6510872 discloses heat treatment of a woven article produced using yarns.
Thus, in view of the prior art there remains a need for a method for making a woven fabric based on a PET composition that does not suffer from tape splitting or fibrillation during the weaving and further allows the manufacture of a substantially transparent woven fabric.
To that extent the present inventors found that if the heat setting of the PET tapes is carried out essentially on the woven fabric rather than on the tapes, the weaving process can be carried out without any issues while the final fabric has the desired mechanical properties combined with good dimensional stability, such as a low hot shrinkage. Or, by decoupling the heat setting of the PET tape from its production and transferring it to heat setting of the fabric, the weavability problem is solved while the high temperature shrinkage stability of the fabric is attained.
More in particular the present inventors found that if a tape is manufactured that has not undergone a heat setting or optionally undergone only a moderate heat setting, the resulting tape has a medium crystallinity which provides the tape with sufficient mechanical properties and anti splitting properties for successful weaving.
Next, the so obtained tapes with medium crystallinity are then woven into a fabric after which a (second) heat setting is carried out on the fabric in order to raise the crystallinity resulting in a fabric having good dimensional stability and good mechanical properties. Moreover, since no or only minor amounts of additives have been used the fabric can be made substantially transparent.
Accordingly the present invention is directed at a method for the manufacture of a woven article comprising the steps of

i) preparing a plurality of uniaxially oriented tapes of a polyethylene terephthalate composition, optionally comprising a step of heat setting said tapes, to obtain tapes having a first crystallinity,
ii) weaving at least two of said tapes into a woven article,
iii) heat setting the woven article to obtain tapes having a second crystallinity higher than the first crystallinity.

Preferably the first crystallinity is a medium crystallinity meaning a crystallinity of preferably from 15 - 25%. More preferably the first crystallinity is from 18 - 23 %.
Preferably the second crystallinity is a high crystallinity meaning a crystallinity of at least 25%. More preferably the second crystallinity is at least 18% or at least 30%. The upper limit for the second crystallinity depends on the PET and the process conditions selected for the heat setting of the fabric in step iii). Generally the second crystallinity will be at most 40%.For example the second crystallinity may be from 30 - 35%. Very high crystallinity materials, i.e. materials having a crystallinity of 50- 60% may be obtained only upon more extreme heat setting conditions such as high temperature in combination with long heat setting times. In tape or fabric manufacture processes such conditions are generally unfeasible from a cost perspective..

Step i)

The process of step i) is generally known to a skilled person and described in the prior art. Generally molten PET is extruded from a film extrusion die, or flat die, and quenched to a substantially amorphous film, i.e. a film having a crystallinity of at most about 5%, preferably at most 3%, more preferably at most 2%. Quenching can be done using known methods. Preferably the molten PET film is cast onto one or more cooled drum(s) or chilled roller(s), which are preferably polished, to better control surface smoothness of the film, at a temperature of about 10 °C to about 30 °C, preferably of about 12 °C to about 20 °C. Other means of quenching can also be applied.
The quenched film can then be slit using a series of parallel cutting devices, such as for example cutting blades into a series of tapes having a desired width, after which the tapes are first drawn uniaxially so as to induce uniaxial orientation as a means to improve the mechanical properties of the tape. After the uniaxial drawing the tapes are optionally heat set to a first crystallinity so as to impart a certain dimensional stability. The slitting of the tapes is preferably carried out prior to drawing, yet the present invention is not limited to such an embodiment and slitting may also be carried out after the drawing step or after the heat setting step.
Drawing of the quenched tape is performed by heating the quenched tapes to a temperature above the glass transition temperature such as from 75°C to 130°C and by drawing the tapes, at that temperature, applying a draw ratio of from 4:1 to 7.5:1, preferably from 4:1 to 7:1, more preferably from 4.5: 6. The draw ratio is defined as the ratio of the length of the plastically deformed film or tape in the direction of stretching to its original length in the same direction before stretching. Preferably the temperature applied in the drawing step is from 80 - 110 °C, more preferably from 85 - 100°C.
Drawing of the tapes will increase the tenacity of the tapes in the direction in which the tape is drawn. Higher draw ratios generally give higher modulus and tenacity, yet a too high a draw ratio would lead to breakage on the line. Suitably, the drawing, or sometimes referred to as stretching, is conducted by passing the slit tapes or by passing the film through a heating zone maintained at a certain temperature from feed rolls to take up rolls, with the latter rotating faster than the former to provide the desired degree of stretching, or draw ratio. Typical heating zones may include an oven, a heated surface or other suitable means, preferably an oven.
Thus, it is preferred that step i) of the method of the invention step further comprises, before the optional step of heat setting the tapes, extruding a polyethylene terephthalate composition into a molten film or tape, quenching said molten film or tape and drawing said quenched film or tape at a drawing temperature of from 75°C - 130°C at a draw ratio of at least 4:1, preferably from 4:1 to 7:1, more preferably to 6:1.
Heat setting of the tape (orfilm) to a first level of crystallinity may be carried out by guiding the tape or film through a heated environment, preferably under tension so as to avoid shrinkage.
The heat setting is carried out at temperatures of 140°C or higher, such as from 170 to 240 °C preferably from 190 to 230 °C. The crystallinity obtained by the heat setting depends on both the temperature of the heat setting step as well as the residence time of the tape or film in the heat setting step. These variables are known to the skilled person and well described in literature so that by routine experimentation a skilled person is able to find the appropriate settings in order to get the desired level of medium crystallinity.
During the heat setting the tape is kept under tension so as to prevent shrinkage in machine direction. It is also possible to apply a low draw ratio during the heat setting such as at most 1.30:1, preferably at most 1.20:1 or 1.10:1, such as from 1.05:1 - 1.15:1. Preferably the tapes are not stretched during heat setting.
It is preferred that the polyester tapes have a width of from 0.5 - 15mm and a thickness of from 5 - 300µm. The width may be from 0.5 - 10mm, such as from 0.7 - 5mm, 1 - 3mm. The thickness may be from 10 - 250µm, such as from 50 - 200 µm or 55 - 100 µm. The tenacity of the tapes obtained in step i) is preferably from 4 - 8 g/denier, such as from 5 - 6 g/denier.
In step I it is preferred that the slit tapes along the stages of the process are prevented from building up static. This may be carried out by incorporating into the PET composition ant-static additives. More preferably however anti static means are provided along the tape production line so as to reduce static build up. Anti static bars are known to a skilled person.

Step ii)

Weaving of PET tapes is generally known, for example from WO 2012/041482 and WO 2013/087200 . Weaving may be performed in a flat loom or a circular loom. A flat loom produces a flat fabric, whereas a circular loom produces a tubular fabric. The essence of the processes is however the same, i.e. a so called weft tape is interlaced (i.e. woven) through a series of parallel warp tapes. During the process the warp tapes run substantially in a machine direction whereas the weft tapes generally move substantially in a direction perpendicular to the machine direction, i.e. in a transverse direction.
The present invention is directed at automated and/or industrial weaving processes. Manual weaving processes are generally excluded.

Step iii)

In step iii) of the method of the present invention heat setting is performed on the woven fabric. The method for heat setting in step iii) is essentially similar to the method for heat setting in step i). Since the purpose of the heat setting step on the fabric is to increase the crystallinity generally a higher temperature and/or a higher heat setting time is applied as compared to the heat setting in step i). To avoid shrinkage of the fabric as a result of exposure to increased temperature it is preferred that during the heat setting the fabric is dimensionally stabilised, meaning that the dimensions of the fabric during the heat setting are fixed. Or it is preferred that during the heat setting the woven article is prevented from shrinking. This may be performed by clamping the fabric between (co) rotating caterpillars, in the embodiment where the fabric is a flat fabric, or by guiding the fabric over a mandrel having dimensions similar or slightly larger to the fabric's tubular diameter, in the embodiment where the fabric is a tubular fabric, i.e. a fabric manufactured with a circular loom. The clamping and mandrel respectively prevent shrinkage in transverse direction, whereas the windup equipment imposing a windup tension on the fabric prevents shrinkage of the fabric in machine direction. Prevented from shrinking as used herein means that shrinkage in machine direction and transverse direction is less than 3%, preferably less than 1%, more preferably less than 0.5%, most preferably less than 0.1%. Preventing the woven article from shrinkage thus prevents the tapes of the woven article to become distorted or corrugated. The heat setting in step iii) is preferably carried out at temperatures of from 155 - 240 °C, such as from 175 - 230 °C.
Similar as the optional heat setting in step i) the tapes in the fabric may be slightly stretched ruing the heat setting. Or, a draw ratio of at most 1.30:1, preferably at most 1.20:1 or 1.10:1, such as from 1.05:1 - 1.15:1 may be applied during the heat set of the fabric. Preferably the tapes are not stretched during heat setting of the fabric.
In order for the stretching to be carried out the diameter of the mandrel may be selected larger than the diameter of the tubular diameter of the fabric. At the same time the take up speed is slight higher than the speed at which the fabric is fed to the heat setting step. Thus, by stretching the fabric in two directions both the warp as the weft tapes are stretched longitudinally. No stretching occurs in transverse direction of the tapes.
In the embodiment where the fabric is a flat fabric the stretching in transverse direction thereof can be carried out in known manner, for example using a tenter frame. Such devices are known to the skilled person.
Generally the dimensions of the tapes obtained in step i) do not change in step iii). Or the thickness and width of the tapes in the fabric obtained after step iii) preferably differs at most 5%, more preferably at most 3%, even more preferable at most 1% or 0.5% with respect to the thickness and the width of the tapes obtained after step i).
In the fabric obtained after step iii) the tapes have a high tenacity and a shrinkage at 130°C of at most 5%. Thus the tenacity of the tapes in the fabric may be from 5 - 9 g/denier, typically 6-7 g/denier. The shrinkage at 130 °C is preferably 3% at most.
It is preferred that the tapes in the fabric have a tensile strength of at least 500MPa, preferably at least 600 MPa, such as from 600 to 1200 MPa.
In the fabric the tapes of the invention have a density of at least 1.333 kg/m³ and do not contain voids or micro voids. Thus, the PET in the tapes in the fabric forms a continuous phase. Typically the density of the tapes in the final fabric is from 1.390 - 1.400.
The PET in the method of the invention is preferably a polyethylene terephthalate homopolymer. The PET preferably has a an intrinsic viscosity of at least 0.6 dl/g, preferably at least 0.8 dl/g. For example, the PET may have an intrinsic viscosity of at least 0.6 dl/g and at most 2.5 dl/g, more preferably of at least 0.8 dl/g and at most 2.0 dl/g. A PET copolymer having up to 15 wt.% of comonomer may also be used. For example, when the PET is a PET copolymer, the PET may comprise up to 15 wt% of polymeric units derived from a comonomer, preferably up to 10 wt%, more preferably up to 5 wt%, with regard to the total weight of the PET. More preferably, when the PET is a copolymer, the PET comprises more than 0.1 wt% up to 15 wt% of polymeric units derived from a comonomer, preferably more than 0.2 wt% up to 10 wt%, more preferably more than 0.5 wt% up to 5 wt%, with regard to the total weight of the PET. In particular, when the PET is a copolymer, the PET comprises more than 0.1 wt% up to 15 wt% of polymeric units derived from a comonomer, preferably more than 0.2 wt% up to 10 wt%, more preferably more than 0.5 wt% up to 5 wt%, with regard to the total weight of the PET, where the comonomer is isophthalic acid.
The PET composition further comprises preferably at least at least 95 wt.%, preferably at least 98, more preferably at least 99 wt.% of PET. Most preferably the PET composition essentially consists of PET meaning that apart from common stabilisers no further additives are present in the PET composition. A high amount of PET is advantageous for obtaining a fabric that has a high transparency. Optionally the PET composition may contain colourants, such as dyes, opacifiers and UV stabilisers.
Notwithstanding the foregoing the PET composition generally contains anti-blocking additives like silica. Such additives are advantageous for winding the tape of step i) onto a bobbin. Particularly preferred is that the polyethylene terephthalate composition further comprises an anti-blocking additive, preferably a silica anti-blocking additive. Silica antiblock additives are commercially available, for example from Grace under the tradename SYLOBLOC® Antiblocking Agents or from Evonik Industries under the tradename AEROSIL®. The amount of antiblock agents may be from 0.05 to 1 wt.%, such as from 0.1 to 0.8 wt.% or 0.2 - 0.5 wt.% on the basis of the weight of the composition. The anti-block additive may be present in the polymerised PET or may be added during the film extrusion. In an embodiment the PET film is of the A-B-A type with A being PET layers containing the anti-block additive and the middle B layer being a PET layer not containing anti-block additives. The A layers may be from 5 - 20 micrometer in thickness, typically these are 10 micrometer.
In a preferred embodiment the PET composition does not contain anti-splitting additives. Thus in a preferred embodiment the PET composition does not contain any one or more of polyolefins such as polypropylene, polyethylene or polycarbonate.

Fabric

The fabric that results from the method of the present invention is preferably a transparent fabric consisting of woven PET tapes wherein the PET does not contain additives.
Transparency is desirable for making sacks containing for example sugar rice, lentils and food stuffs, where customers may want to see the product.
For FIBCs, there is a tendency for the filled bag to bulge with time, because the material inside causes a distension of the side wall. This bulging causes stacking instability and makes the FIBCs difficult to fit into containers. For these applications it is preferred that the heat setting of the fabric is carried out such that the shrinkage at 130°C is from about 10 - 15%, i.e. is significant higher compared to fabrics that are to be used in high temperature applications. Surprisingly the present inventors found that the higher shrinkage provides the FIBC with more resistance to bulging, i.e. provides a FIBC that, in filled state, has improved dimensional stability when used at normal, i.e. ambient, temperatures of up to 50 °C.
The fabric obtained with the method according to the invention is especially suitable for sacks, FIBCs, geotextiles and all room temperature uses. Moreover if, in use, the fabric would be subjected to high temperature and therefore shrinkage, the invention also provides for fabrics based on PET that are both transparent, i.e. contain no or only minimal additives, and exhibit a low shrinkage.
A specific high temperature application is that of carpet backing, where the fabric is usually coated with a latex and cured at high temperature, such as 130°C. For such applications it is essential that the fabric does not shrink too much, levels of at most 3% at 130°C being acceptable.
The present invention in one particular embodiment relates to a method for the manufacture of a woven article comprising the steps of

i) preparing a plurality of uniaxially oriented tapes of a polyethylene terephthalate composition, optionally comprising a step of heat setting said tapes, to obtain tapes having a first crystallinity,
ii) weaving at least two of said tapes into a woven article,
iii) heat setting the woven article to obtain tapes having a second crystallinity higher than the first crystallinity;

X_{c} = (\frac{ρ_{c}}{ρ_{sample}}) \cdot \frac{(ρ_{sample} - ρ_{a})}{(ρ_{c} - ρ_{a})} \times 100

_sample

_a

_c

All references cited herein are hereby completely incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms "a" and "an" and "the" and similar referents in the context of describing the invention (especially in the context of the claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms "comprising", "having", "including" and "containing" are to be construed as open-ended terms (i.e., meaning "including, but not limited to") unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. For the purpose of the description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.
Preferred embodiments of this invention are described herein. Variation of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein.
The invention will now be further illustrated by the following non-limiting examples and the sole figure.
The sole figure schematically shows steps ii) and iii) of an embodiment according to the inventive method. In circular weaving loom 1 the tapes previously prepared and heat set in a step i) are woven into a fabric 2. The skilled person will understand that a circular weaving loom will result in a woven fabric 2 that is tubular. After weaving in loom 1 woven fabric 2 is transported to mandrel 3 where the woven fabric 2 will be heat set according to step iii) of the inventive method. In the figure mandrel 3 is shown to consist of three portions. A first sloped portion at the side of loom 2 is connected to a middle portion, which in turn is connected to another sloped portion in a direction away from the loom. The slopes in the sloped portions allow the fabric to be easily guided to and from the middle portion of mandrel 3. Middle portion 3 has a substantially constant diameter, i.e. is not sloped. The middle portion is connected to heating means (not shown) that allow heat to be transferred via the middle portion of mandrel 3 to woven fabric 2. For example the mandrel may be provided with small openings allowing hot air to be blown from the inside of mandrel 3 onto fabric 2. Alternatively the mandrel itself is heated to a temperature sufficient for the heat setting of woven fabric 2. In another alternative the heat is supplied from the outside by means of hot air or infrared lamps. In yet a further alternative embodiment the mandrel 3 is enclosed by an oven. Combinations of the foregoing heating methods is also possible. The essential function of mandrel 3 is to prevent or at least reduce to a minimum the shrinkage of fabric 2 during the second heat set operation. Thus the mandrel prevents shrinkage in a transverse direction, and to some extent even in a machine direction. In addition thereto the woven fabric is kept under a certain tension in machine direction, for example via take up means (not shown), sufficient to prevent or at least reduce to a minimum shrinkage of fabric 2 in machine direction. After the heat set on mandrel 3 the woven fabric is transported and wound up and a winding unit (not shown). The arrow on the upper side of the figure indicates the process direction.
The sole figure is intended to clarify and exemplify an embodiment of the present invention. By no means however is the present invention restricted by this embodiment.

Measurement methods

Density

Density of a sample, ρ_sample, was measured in a density gradient column.

Crystallinity

Crystallinity was determined by means of density measurements following the equation: $X_{c} = (\frac{ρ_{c}}{ρ_{sample}}) \cdot \frac{(ρ_{sample} - ρ_{a})}{(ρ_{c} - ρ_{a})} \times 100$
Wherein

Xc: = crystallinity of the tape
ρsample: = density of the sample [kg/m³]
ρa: = density of amorphous PET [kg/m³]
ρc: = density of 100% crystalline PET [kg/m³]

For PET homopolymers ρ_a is 1333 kg/m³ and ρ_c = 1455 kg/m³.

Intrinsic viscosity (IV)

The IV was measured with a dilute solution of the polyester resin in a 3:2 mixture of phenol-1, 2 dichlorobenzene solution, at 25°C (single measurement). The I.V. was calculated from the measurement of relative viscosity η_r for a single polymer concentration (c = 0.5%) by using the Billmeyer equation: $IV = [η] = 0.25 (η_{r} - 1 + 3 \ln η_{r}) / c$

Tenacity and Elongation at break

The tenacity and the elongation of the uniaxially oriented tapes was measured according to ISO 2062 (DIN 53834) on Basic Line Z005 from Zwick/Roell, with a 500 mm free clamping length for the tape, and a testing speed of 500 mm/min.

Haze

Haze is the percentage of the total transmitted light that after passing through a film sample is scattered by more than 2.5° (see ASTM D-1003-97). The tapes were too narrow for direct haze measurements. Hence, it was measured on the cast amorphous film before tape slitting with a Haze Gard Plus instrument from BYK Gardner. Surface and bulk contributions were not separated.

Hot Shrinkage

Residual shrinkage measured by a hot air method following ASTM D - 4974 - 93 and DIN 53866. The sample length was about 600 mm. One end of the sample was fixed in the hot air oven by means of a clamp. The tape rested freely on the drum (which was directly attached to the scale) and one of the prepared weights (1 g/100 den) was fastened to the other end of the tape. The distance from the drum to the weight was approximately 100 mm. The test temperature used was 132°C with an exposure time of 2 minutes. The residual shrinkage was indicated directly on the scale in percent.

Tensile impact test (qualitative)

A piece of tape is wound on both hands, slackened first by moving the hands closer to each other and then giving a sudden tensile yank along the tape axis. An un-weavable tape will break into fibrils with tears along the long axis, while a weavable tape would break across the TD with little or no axial tearing.

Tensile impact test (quantitative)

Tensile impact measurements were performed at room temperature (23°C) by hitting the tapes along the drawn direction with a 1J pendulum using a Zwick/Roell tensile-impact machine, applying a gauge length of 25mm and a sample width of from 2 -3 mm. The energy loss of the pendulum after impacting to complete fracture was recorded as impact strength of the samples. For each sample 10 samples were tested and the average of these measurements was calculated. The tensile impact is reported in kJ/m².
Both the value of the tensile impact strength and the type of fracture indicates whether the tape will be weavable. If the tape fails by splintering into fibrils, it will not be weavable. If the tape fails predominantly by break across the TD with a puckered fracture surface with little or no axial tearing, it would be weavable.

Tape folding test

A part of the tape was folded along its longitudinal axis and the folding crease was examined. If there was a folding crease, but the tape was not broken, then weaving was considered possible. If there is breakage or tear along the fold which occurs with a popping sound, than weaving of the thin PET tapes was considered not possible.
Both the tensile impact test as the tape folding test correlated with weaving trials and were found to be a reliable tests to assess weavability of the tape.

Comparative Example 1

PET homopolymer resin without any additives having an IV of 0.84 dL/g. was dried to less than 50 ppm of moisture and a cast amorphous film was extruded onto a chill roll. The transparent film was 105 mm wide after trimming the edges. The film was slit into 22 tapes, each having a width of 4.82 mm and a thickness of 62 microns (before drawing). After drawing at 90°C under application of a draw ratio of 5.5:1, the width of the tapes was 2.1 mm and the thickness was 29 µm. Next, the drawn tape was heat set at 230°C in a second oven.
The tape had a tenacity of 5.65 cN/dtex (6.4 g/denier). It was transparent and had a hot shrinkage at 132°C of 4.5%. Unfortunately, in the tape tensile impact test, it splintered into fibrils with cracks along the tape axis. Table 1 shows the tensile impact strength. In the folding test, the tape cracked with a popping sound and on reopening a tear was found along portions of the fold axis. Such tensile impact and folding behaviour was found to result in formation of cotton like fluff in the weaving equipment, in particular the loom guides.

Comparative Example 2

This example is based on the prior art WO 2013/087200 and uses 2% of polycarbonate as anti-splitting agent in a PET homopolymer having an IV of 0.84 dl/g. The PET homopolymer was dried to less than 50 ppm of moisture and a cast amorphous film was extruded onto a chill roll. During the extrusion 2wt.% of the (linear) polycarbonate (PC 2200 available from SABIC) was added. The cast film and PET tape were transparent and had a haze comparable to the pure PET film of CE1. The extension to break was 15.9% and the hot shrinkage at 132°C was 4.4%. Both properties were comparable to the pure PET tape heat set at 240°C. The tape also passed the tensile yanking (tensile impact) test, as it broke without splintering. Table 1 shows the tensile impact strength was higher than that of Comparative Example 1. However, in the folding test, the tape split and a tear in longitudinal direction appeared. Thus, the inventors found that indeed the tape performed better than the tape of CE1 with regards to weavability, but it nonetheless needed improvement.

Comparative Example 3

This example is also based on the prior art WO 2013/087200 and uses 2% of a linear polycarbonate and further 2% of a C6 LLDPE (grade 6318BE LLDPE available from SABIC) as an anti-splitting agent in a PET homopolymer having an IV of 0.84 dl/g. The PET was dried to less than 50 ppm of moisture and a cast amorphous film was extruded onto a chill roll; during extrusion the anti-splitting agent was added. Due to the LLDPE, the cast film was no longer transparent. The film was 105 mm wide after trimming the edges. It was slit into 22 tapes, each having a width of 4.82 mm and a thickness of 62 microns (before drawing). After drawing at a temperature of 90°C applying a draw ratio of 5.5:1, the width was 2.1 mm and the thickness was 29 microns. The drawn tape was heat set at 230°C in a second oven. The tape had a tenacity of 5.83 cN/dtex (6.6 g/denier) and the hot shrinkage at 132°C was 4.4%; both properties were similar to the pure PET tape heat set at 240°C in CE1. Despite the elongation to break being similar to CE1, in the tensile yank (tensile impact) test, the tape broke across the transverse direction (instead of splintering), and the broken halves sprung back elastically, to form accordion like folds. Table 1 shows the tensile impact strength was higher than that of the pure PET tape of Comparative Example 1. Thus, with the combination of PC and LLDPE, the fracture did not propagate axial cracks. In the folding test, the tape did not crack with a popping sound; on reopening, there was a crease but no tear along the axis. Thus, with a combination of PC and LLDPE as additives, the tape has suitable weaving characteristics. Thus even though the prior art solution to tape splitting was confirmed it still has the disadvantage of increased cost due to the complexity of the formulation and the introduction of haze in the material. In addition to the foregoing the skilled person will recognise that recycling of these prior art formulations is more troublesome in view of the added components to the PET.

Comparative Example 4 (CE 4)

This example is based on the prior art WO 2012/041482 A1 and uses 2.8 wt.% C8 LLDPE as additive in a PET homopolymer having an IV of 0.84 dl/g.
The PET was dried to less than 50 ppm of moisture and a cast amorphous film was extruded onto a chill roll; during extrusion the additive was added. Due to the LLDPE, the cast film was hazy (haze value of 13.7%). Drawing was performed at a temperature of 90°C applying a draw ratio of 6.3:1. The drawn tape was heat set at 230°C in a second oven. The tape had a tenacity of 6.45 cN/dtex (7.3 g/denier) and the hot shrinkage at 132°C was 6.2%.
The tape passed the tensile yanking (tensile impact) test as it broke without splintering. It also passed the folding test. Consequently this tape had excellent weavability in the loom. The disadvantage however is that the LLDPE introduced haze (13.7%) and reduced the gloss, so that at best the woven fabric was translucent. Similar to CE3 recycling of these prior art formulations is more troublesome.

Example 4

Uniaxially oriented PET homopolymer tape was manufactured similar to the procedure set out in Comparative Example 1 with the exception that heat set was carried out at much lower temperature, namely 140°C. PET homopolymer resin with IV of 0.84 dL/g was dried to less than 50 ppm of moisture and a cast amorphous film was extruded onto a chill roll, without any additives. The film was 105 mm wide after trimming the edges. It was slit into 22 tapes, each having a width of 4.82 mm and a thickness of 62 microns (before drawing). After drawing at 90°C applying a draw ratio of 5.5:1, the width was 2.1 mm and the thickness was 29µm. The drawn tape was heat set at 140°C in a second oven.
The tape had a tenacity of 5.3 cN/dtex (6.0 g/denier) (strength of 735 MPa). It was transparent and had a hot shrinkage at 132°C of 12.7%. The haze was 1.1%, which is similar to CE1. Most surprisingly in the tape tensile yank test, the tape did not splinter into fibrils with cracks along the tape axis. Surprisingly, it broke in a similar manner to the tape with PC and LLDPE in Comparative Example 3. Table 1 shows the tensile impact strength which was higher than the pure PET of Comparative Example 1.
Also, in the folding test, unlike in CE1, the tape did not crack with a popping sound; on reopening, there was a crease but no tear along the axis. Yet the elongation to break was lower than CE 1 (10.9%).
Thus, the tape of this Example 4 is a weavable, transparent PET tape with no additives. The shrinkage at 132°C was 12.7%, which is desirable for FIBCs and sacks to counteract the bulging as explained herein. Recycling of the material is possible.
For high temperature applications, such as carpet backing, and/or for applications requiring higher strength the tape as such has insufficient properties. Therefore the tape, once woven to a fabric needs to be subjected to a second heat setting during which the fabric is fixed in both machine direction and transverse direction so as to prevent shrinkage of the fabric during such second heat setting. The conditions for heat setting depend on the desired level of shrinkage and mechanical properties. Generally however conditions as applied in the Comparative Examples can be applied; the properties of the tape in the fabric can be measured in the same manner. Thus, heat setting of the fabric can be performed at a temperature of from 150- 240 °C at a time sufficient so as to obtain the desired level of crystallinity, which preferably is at least 30% for fabrics that are used in high temperature applications such as carpet backing.

Table 1 below summarises the experimental data (note: 1.0 g/denier = 0.88 cN/dtex).

	T_Draw (°C)	T_heat set (°C)	Draw ratio	Tenacity (g/denier)	Elongation to break (%)	Shrinkage at 132°C (%)	Tensile Impact test (qualitative)	Tensile impact test (quantitative)	Tape folding test
CE
1	90	240	5.5:1	6.4	15.5	4.5	Splinters	2511	Axial cracking
CE
2	90	140	5.5:1	6.6	15.9	4.4	Does not splinter; puckered break surface	4160	Axial cracking
CE
3	90	230	5.5:1	6.6	16.6	4.4	Does not splinter; puckered break surface	4300	No axial cracking
CE
4	90	230	6.3:1	7.5	12	6.2	Does not splinter; puckered break surface		No axial cracking
E4	90	140	5.5:1	6.0	10.9	12.7	Does not splinter; puckered break surface	3986	No axial cracking

Claims

Method for the manufacture of a woven article comprising the steps of
i) preparing a plurality of uniaxially oriented tapes of a polyethylene terephthalate composition, optionally comprising a step of heat setting said tapes, to obtain tapes having a first crystallinity,

ii) weaving at least two of said tapes into a woven article,

iii) heat setting the woven article to obtain tapes having a second crystallinity higher than the first crystallinity.
The method of claim 1 wherein the first crystallinity is from 15 - 25% as determined via the density of the tape prepared in step i).
The method of claim 1 or 2 wherein the second crystallinity is at least 25% as determined via the density of the tape prepared in step iii).
The method of any one of claims 1 - 3 wherein step i) further comprises, before the step of heat setting the tapes, extruding a polyethylene terephthalate composition into a molten tape, quenching said molten tape and drawing said quenched tape at a drawing temperature of from 80°C - 130°C at a draw ratio of at least 4:1, preferably from 4:1 to 7:1.
The method of any one of claims 1 - 4 wherein the polyethylene terephthalate composition comprises a polyethylene terephthalate homopolymer or a polyethylene terephthalate copolymer and having an intrinsic viscosity of at least 0.6 dl/g, preferably at least 0.8 dl/g.
The method of any one of claims 1 - 5 wherein the polyethylene terephthalate composition comprises at least 95 wt.%, preferably at least 98, more preferably at least 99 wt.% of polyethylene terephthalate.
The method of any one of claims 1 - 6 wherein the polyethylene terephthalate composition does not contain anti-splitting additives.
The method of any one or more of claims 1 - 7 wherein the polyethylene terephthalate composition further comprises an anti-blocking additive, preferably a silica anti-blocking additive.
The method of any one of claims 1 - 8 wherein the polyester tapes have a width of from 0.5 - 15mm and a thickness of from 5 - 300µm.
The method of any one or claims 1 - 9 wherein the tapes in the fabric have a density of at least 1.333 kg/m3 and preferably do not contain voids or micro voids.
The method of any one or more of claims 1 - 10 wherein in step iii) the woven article is prevented from shrinking.