CA1052511A - Extruding and stretching polyester strapping - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention relates to plastic extruded strapping and its processing techniques. More particularly, it is concerned with polyester extruded strapping of high tensile strength and low elongation at break to replace steel strapping. Although not limited thereto, this invention is of primary importance in connection with the technique of crystallization and orientation of polyester predominantly in one direction in the making of strap characterized by two-step stretching.
Furthermore, this invention is related to the design of a multi-purpose extrusion line for the processing of strap of various polymeric materials.

Description

lOS;~511 find their applications in the light packaging field.
The elongation at break of these conventional strappings vary widely from about 13% to about 40~. High strength polypropylene is the best for non-stretching applications but still elongates from 13% to 25~ before breaking, as compared to 3~ for steel strapping. Consequently, steel strapping has so far been fa~ored for heavy bundling and trans-portation and wherever low elasticity is advantageous. In other words, the extruded plastic strapping using the conventional polypropylene, poly-olefins and polyamides as the base materials, are found weak in comparison to steel strapping.
The extruded plastic straps of the prior art are made from easily crystallizable base materials, such as polypropylene, nylon, poly-olefins, wherein the tensile strength of the strap can be readily increased to the maximum by crystallizing and orienting the molecular structure along the length of the strap.
It has been noted that the above-mentioned method of production is not suitable for slowly crystallizable materials, such as polyester, since the extrudate of the latter is primarily amorphous and lacks the strength to withstand high draw ratio. More particularly, it has been noted that the slow crystallization rate of polyester makes it impossible to obtain a polyester strap with a considerable degree of molecular orientation and crystallization in a single stretching step, as taught by the conventional anterior method of production. For example, the single stretching step suggested by the prior art produces an extruded polyester strap which possesses a tensile strength of less than 50,000 p.s.i. and an elongation at break in the order of 20~ to ~5~. Such strap i~ also clear and transparent, which indicates the predominance of the amorphous ~tructure.
It has also been noted that in the production of extruded polyester strap, the presence of moisture greatly reduces the tensile strength of the final strap, due to the reverse hydrolysis of polyester at the elevated temperature employed for the extrusion. With a moisture content in the order of 5%, polyester resins produce a very weak 105;~51~

This invention relates to plastic strapping and, more particularly~ to a method and system for producing plastic strapping of the type used in the packaging art.
Extruded plastic strapping of polypropylene and polyamide base materials has become increasingly impor~ant in the packaging business in the past decade, primarily because o~ their low cost of manufacture and their outstanding properties as compared to steel strapping, such as the ability to absorb shock loads, the high resilience, the great flexibility, the low weight, the corrosion resistance.
Polypropylene anu polyamide strappings are conventionally manufactured at the present time by extruding hot melt polymers through rectangular orifice dies, then the hot melt is quenched, stretched, cooled, and finally rolled in coils. Such straps are easily produced due to the easily crystallizable property of polypropylene and poly-amides. Polymer chains can be oriented along the length of the st~ap in a single short stretching unit in order to promote its strength in that direction.
In general, the prior art concerning the pro~uction of extruded strap can only be applied to easily cyrtallizable polymers, such as polypropylene, polyolefins and polyamides. Straps made Or such types of polymer~ are lacking several important properties in comparison to steel strapping. For instance, polyamide strapping is notch-sensitive, which gives rise to a low breaking point when its edge is cut. It is also susceptible to tearing by delamination initiated by surface cuts or scratches. Polypropylene strapping of high strength i9 prone to longitudin~l splitting induced during sealing or cutting and, thus, inhiblts the proper handling in continuous packaging operations.
In terms of strength, polyamides strapping is limited in the range of 53,000 to 61,000 p.s.i. Polypropylene strapping varies from a low strength of 34,100 p~S~io bo a high strength of 60,000 p.s.i. Therefore, the strappings made with these base materials only

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105ZS~l.
strap. Besides, the use of dried polyester resin ls also disadvantaged by the rapid or high rate of moisture pick-up~ which results in uneven dryness of the supplied polyester, and thence, in the uneven quality of the strap from the beginning to the end of each load of polyester which is supplied to the extruder.
It is a general object of the present invention to provide an extruded plastic strap that obviates the foregoing disadvantages of the anterior extruded plastic straps, as comp~red with steel strapping and, in particular, to provide an extruded plastic strap which possesses high tensile strength and low elongation at break, and which is otherwise adapted to effectivel~ replace steel ~trapping.
It is another general object of the present invention to proNide a suitable m0thod and a suitable sgstem to form a polyester strapping.
It is a further object of the nresent invention to produce a strapping which is particlllarly suitable for packaging in medium and heavy duty applications and which can be mechanically fastened, such as by heat welding or by ultrasonic sealing for continuous packaging operations.
It is another object of the present lnvention to provide a method and a system for producing an extruded polyester strap which is characterized by high tensile strength in the longitudinal direction, low elongation before breaking, non-splitting in the transverse direction, good resistance to low and elevated temperatures and resistance to corrosion, weathering, bacteriological attack and moisture absorption.
It is still another general object of the present invention to provide a system adapted for multi-purpose extrusion of different polymers for extended and efficient use of the system.
It i5 another object of the present invention to provide an e~trusion system with interchangeable stretching units and cooling units adapted to produce different combinations of these units and as many distinct systems.
It is another general object of the present invention to -provide a method and a system producing the required cyrstallization and lQS'~
orientstion of slowly crystallizable polymers predo~inantly in one direction along the length of the strap.
It is a further general object of the present invention to provide a method and a system to produce an extru~ed polyester strapping which is superior to anterior plastic strappings and comparable to steel strapping in terms of tensile strength and of elongation bsfore breaking.
It is a more specific object of the present invention to provide a method and a system based on a new stretching technlque suit-able for polyester strap production and, more particularly, to produce polyester strap with optimum tensile strength.
It is a spe¢ific object of the present invention to provide an appropriate method of ùrying polyester while avoiding degradation oP the latter.
It is a still Iurther specific object of the present invention to provide a method and a system to prouuce an extruded linear aliphatic and aromatic polyester strap containing dicarboxylic acids and polymethylene glycols or an extruded strap of polyethyleno terephthalate.
The above and other objects and advantages of the present invention will be better understood with the following detailed description of preferred embodiments thereof which are illustrated, by way of example, in the accompanying drawings, wherein:
Figure 1 is a schematic side elevation view of a two-step stretching system a~cording to a first embodiment of the present invention;
Figure 2 is a schematic elevation view of a drying device according to the present invention and adapted to supply dried polyester to the extruder;
Fig~e 3 is a schematic view of the end of a roll to explain the coiling ~f the produced strap thereon in accordance with the present invention;
Figure 4 is a schematic side elevation of a Iirst embodiment of ~05'~S~l 8 ~trap prod~cin8 sy~te~ accardi~g to the prssent invention, and Figure 5 i3 a ~chema~lc slde elevation Or a ~econd e~bodi~ent of a a~r~p produclDg ~st6~ accordiDB to thc pres~nt iDvantlon.
The polyoster o~ployed i8 f Q ed by the pol~erization o~
dlc~rbo~ylic acid ~ith a polymethylene ~lycol, whorein th~ aoid ha~
the f or~ula HOOC - ~ - COOH wh~reln ~
a~ (CH2)n, n being a ~hole numbffr;
" b) a~ sromatic r~dical~ or c) a~ hydroryl or chloride sub~tituted aro~atic rRdiCU
and ~herein the poly~ethylsne glycol ha~ the f or~ula HO (C~2) n~ OH
Wherein ~n'~ i~ a wholc nu~b~r rrom 2 to 6 inclu~lve.
I~ acoor~anoe with the pr~ent i3vontion~ the e~ploysd polyest~r ca~ be either a ~ingle t~pe Or polyester or B copolyester.
The latter 18 u~ed to ~nbsnce certaln desirable propertle~ ~or a strap.
copolyo~ter Or polyethylen~ tcrephthalate and Or an allphatlc polye~ter tend~ to in~rease the heat ~ealabil~t~ of tho strap. Chlarln~tod polre9t~r onchance~ the ro~is~anco of a str4 to ~levated t~mporature. Hydro~yl ~20 d~riratl~o polyoat~r i9 anployed to incr-~e the dieabllity to cert~ln ch d cal ~y~a.
The strappi~g of tho present inrention may th~u advanta-goou~ly be ~de o~ a polye~tor r6~ult~ng ~roa the poly~erizstion or a dicsrbo~yllc acid and o~ a poly~et~yl~no glycol of t~e for~ulae not~d abovo. Th~ pol~e~t~r csn bo po}rethyl~ tsrephthalRt~ or a copoly~ffr af polyothylone t~rephth~l~te ~ith another ono~ or aga~ a copol~er o~ a ~ilturo o~ t~o or ~orc of bh~ above-~ot~d poly~t~r~.
Thc t~o-stop s~retchiDg ~yst~ o~ Flg~r~ provlded to produco ~tretching o~ pol~rcster lrhioh ~coeed~ ~e r;O(~ ar 5 to 1 ratio which is co~ventionall~ obtained by a Jinglo stap o~ stI ctchiDg. Th~
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two-st~p stretohing of the pre~en~ ve~tion allows to produc~ Qtretchi~g bet~roen 5 to 1 ~nd 12 to 1 relativo to the fro3h o~trudate.
retching 3yst~ of Fig~re 1 illu~trates 8 rirst ,.

- 6a _ iOS~S~l stretching unit 1 and a second stretching unit 2 positioned between drawing roll sets 3, 4~ and 5 provided to draw the str~p S through the stretching units 1 and 2. The stretching units 1 and 2 may be of any conventional type and so are the drawing roll sets 3, 4, and 5. Ths draw ratios may be adjusted by varying the speed of the intermediate drawing roll set 4 and of the drawing roll set 5.
The pre-drying device or system of Figure 2 includes a supply tank or container 6 of fresh polyester 7, or other appropriate polymers.
A vacuum pump 8 transfers the fresh polyester 7 fron the tank 6 to the dryer 9 through a tube or hose 10. A heater 11 takes the air from the top of the dry~r 9 through a tube 12 and after heating thereof feeds it through tubes 13 to dessicant towers 14. The dried hot air passes by t~bes 15 from the towers 14 to a collector 16 frDm which it i5 expelled by tubes 17 to the bottom Or the dryer 9. Thus, a continuous upward flow of dried hot air may be maintained in the dryer 9 to dry the polyester 18 which passes in th~ latter.
The dried polyester 18 is transferred from the dryer 9 to a hopper 19 through a tube 20 and a vacuum pump 21. A heater 22 collects air by an intake 23 and feeds it through a tube 2~ to the bottom of the hopper 19 which is provided with a vent 25 for exit of th~
air. This hot air from the heater 22 prevents moisture reabsorption by the polyester 26 in the hopper 19. This is made necessary due to the high rate of moisture absorption of the dried polyester. The polyester 26, or other polymer plastic which may be instead placed in the hopper 26, is dispensed in any known manner to an extruder 27 which is of appropriate type to extrude a strap of the known type for packaging.
Figure 3 serves to explain a preferred winding technique according to which the finished strap is coiled in hexagonal pattern on large core rolls. As shown at ~, B, C, D, E, and F, the starting point of each additional layer is angularly offset from the previous one by an angle of 60 degrees around the ciroumference of a roll 28.

In other words, the first ~trap layer starts at A; the second at B, lOS'~S~ -8-etc. The pattern repeats itqel~ a~ter co~plet~on of ~ ucoo~lve lsyer~. With this he~gonal coiling pattern, each etra~ laysr i~ ~up-ported regularly by si~ underlying la~er~.
The strap e2tru~on sy~t~m~ shoNn .~n ~i~ure~ 4 and 5 include each the hopp~r 19 and the axtruder 27 u~ed in operative aæsoci~tion with tha dr~ing ~yste~ of ~igurs 2.
In the ~bodi~ent o~ Figure 4~ the extrlldate i~9uing ~rom the s~truder 27 i3 pa3sed through A ~uenching unit 29, of ~ny ap-.~ ~ropriate construction, before passing throueh the two-~tep stretching a99e~bly. In thi9 embodi~ent, the stretch~ng unita 1 ana 2 are of the ~et or fluia type in partlcularl~ usiDg hot ~ater batbs 30 and idldr roller~ 31. The strap S i~ tretched a rirst time by tha roll ~ets

3 ana 4 and a sscond ti~e by the roll sets 4 and 5. Finall~, the strap 18 cooled in the cooling tank 32 under tension by the driven rolls 33.
The strap is then coilod as aroredoscribed on the roll 28.
1~ tbe s~bodim~nt Or Figure 5, the extrudste issuing from t~o e~truder 27 i~ ¢oolsd through a chill roll as~e~bly~ or ~et 34, to fl~ the shape of the strap S before passing the latter through tho t~o 3tretching stsges. In thiA case, the stretc~in~ units 1 and 2 are of the dry typo and, more psrticularly~ include e~ch a radiant ,~, ...
heater ov~n 35~through ~hich the 3trap S i9 drawo. The 3trap S ~
therea~ter cooled in th- open B~ under teDsion during travel frcu the second radiant heater o~on 35 to the dri~en rolls 33. Fina.l:4r, the strap S ~ co~led on the ~oll 28.

E~a~ple 1: Manu~acturing of Polge~t~r ~strud~d Strap by _ Wet Pro¢e~A~Dg Nethod Po~yeeter poly~thyleDe tsrsphthalato employed in thl~
~x~pl~ i5 dried and crystallized to the ~oist~re content Or about 0.2~ according to the drying techni~ue mentioned above. The drying temp~rature is about 300F (149~C) in four hour~ for th~ fir~t ~00 lb~ (172 Kg) oR rs~in. The dri~d re~n is tak~n out gradually fro~ the bottom Or the ll)S~511 dryer and trans~erred to the heated hopper and the fresh resin is loaded at the top to assure a continuous supply of dried resin for processing. Dried resin is being kept from the moisture reabsorpti~n by circulating hot air at 300F (149C) in the heated hopper.
The extrusion technique is well known by those skilled in the art. However, for polyester as in this invention, satisfactory extrudate can only be achieved through the precise temperature control, adequate heating and cooling, as well as satisfactory extrusion pressure.
The melting temperature of polyester is relatively higher than other polymers from 245C to 250C (473F to 428F) and the degrading temperature o~ polyester is about 300C to 310C (572F to 590F); thus, in general, the extruder temperatures should be limited in this range~
The extrudate is quenched in water to obtain adequate strength as well as the definite form of strap. The water level, the water temperature and the travelling length in the quenching tank have to be adjusted in connection with the extrusion rate in order to avoid strap deformation~ strap size variation and the strap wavy motion. The freshly formed polyester strap in the quenching tank is transparently clear, which indicates predominance of the amorphous unoriented nature. In this example, the quenching water temperature is kept at about 50C
(122F) and the travelling length is about 3 feet (91.5 cm) for an extrusion rate of about 65 feet per minute (19.8 m/min).
Then the quenched strap is passed through the first set of rolls 3, which rolls are of 9 inches ~22.8 cm) in diameter and running at about 28 RPH. In the liquid stretching tank, the common form of liquid employed is water. However, through the knowledge of those skilled in the art, other liquids, such as ethylene dioxide, ethylene glycol, glycerol or their mixtures with water, can be used. In this example, water at its boiling temperature is employed. With a travel-ling length of about 10 feet (3m~, polyester strap can be stretched up to 400~ without breaking. The stretching at the boiling water temperature 100C (212F) induces and accelerates simultaneously crystallization and molecular orientation along the strap.

_ 9 _ lOS;Z511 The stretched strap, then, is passed through the second stretching tank containing boiling water to prcmote the molecular crystal-lization and orientation. Additional stretching~ orientation and crystal-lization can be successfully achieved. In this example, the additional dr~wratlo varies from 2 to 1, to 2.5 to 1. This makes the total draw ratio of the two stretching units from 8 to 1, to 9 to 1. The arrangement of the stretching ratio in the two stretching units is dependent upon the travelling length ss well as the stretching temperature. This stretching technique provides an efficient means to achieve the highest strength in the axial direction by opti~ized orientation ana crystal-lization of the polyester.
The strap leaving the second stretching tank is cooled either in water or in air, and finally rolled in coils.
Polyethylene terephthalate strap obtained by this production method possesses a tsnsile strength of above 75,000 p.s.i. and elongation at break of 8%. The strap is milky white, strong in the transverse direction and flexible.

Example 2: Manufacturing of Polyester Extruded Strap by Dry Processing Method In the dry processing method according to this invention, polyester resin is driedg transferred to the heater hopper and melted in the extruder in the same way as mentioned in the Example 1. The extrudate is cooled by a set of chill rolls made with 8 highly smooth chromium surface to avoid sticking of the hot extrudate. The chill rolls are cooled by circulating cold water or air inside their cores.
The hot extrudate is properly chilled to form a strap with adequate strength for subsequent stretching, orientation and crystallization.
Then, the chilled strap is passed through the two radiant heater ovens to obtain satis~actory stretching orientation and crystallization.
Finally, the strap is cooled under tension in the ambient air.

Example 3: Polypropylene Strap Production in Multi-Purposed Extrusion System Polypropylene extruded strapping can be manu~actursd by the cOEmbination of the wet and dry stretchlng techni~ue. Equipment em~loyed 10~'~5~1 is that employed in the ~xamples 1 and 2.
In the polypropylene strap processing, resin is loaded in the heated hopper to facilitate the melting in the extruder. Since the presence of moistllre does not affect the quality of strap, rior drying is not necessary. The resin is melted in the extruder in the temperature range of about 420oFto 430F (216C - 221C) and is quenched in water at a temperature of about 110F (43C~ for an e~trusion rate of about 26 feet per minute (8 ~Jmin~. Then, the quenched strap is passed through the two stretching radiant heater ovens at a temperature of about 250F (121C) to acquire a total stretchlng from 9 to 1 to 12 to 1~ depending on the desired strength of the final strap. Since the crystallization of polypropylene is fast, and the crystallization rate is quite appreciable, even at the temperature of the quenching tank, the strap getting into the stretching section has s~fficlent strength for any stretching ratio. The radiant heater stretchings employed in connection with water quenching, as in this case, is due to the hydro-phobic nature of polypropylene. Water droplets cannot attach to the sur-face of the newly formed polypropylene strap. Thus, there is no pos-sibility of forming an uneven, crater-like strap surface due to the evaporation of ~ater drops in the radiant heater stretching section.
This last example is used to show the multi-purpose use of the e~trusion system wherein the cooling may be done by either of the two assemblies 29 and 34 and the stretching may be done by either the dry or the wet stretching technique.

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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED, ARE DEFINED AS FOLLOWS:

1- A method to produce a polyester extruded strap having a tensile strength of at least 60,000 pounds per square, inch and an elongation at break of less than 12%, comprising drying a batch of granular polyester feed material in a container by circulating hot dried air through the material in said container to bring down the moisture content of said material to a maximum of about 0.2%, transferring said dried batch of polyester feed material to an extrusion hopper to allow refilling the container with a fresh batch, circulating hot dried air through the batch in said extrusion hopperto keep the dried granular polyester from moisture absorption, directly discharging the dried polyester feed material into an extruder without contact with ambient air, melting and extruding the polyester feed material by said extruder, quenching the extrudate, stretching the extrudate in two consecutive steps with a major stretching ratio in a first stretching step and a minor stretching ratio in the second stretching step for a total stretching ratio of between 8 to 1 and 12 to 1, and cooling the extruded strap.

2- A method as claimed in claim 1, wherein the stretching ration in the first stretching step is about 4 to 1 and in the second stretching step, varies between 2 to 1 and 3 to 1.

3- A method as claimed in claim 1 or 2, wherein the drying of the polyester in the container and in the extrusion hopper is effected between 212 degrees Fahrenheit and 401 degrees Fahrenheit.

4- An extruding system for plastic extruded strap comprising an extruder, an extrusion hopper in direct communication with the extruder, a container, piping connecting said container to said extrusion hopper, said container adapted to receive a batch of granular plastic feed material, means to transfer said granular material from said container to said hopper through said piping, means to circulate hot dried air through said container and through said extrusion hopper to keep the moisture content of the granular material in said container and extrusion hopper to a minimum, a quenching unit arranged to receive the extrudate issuing from the extruder and two consecutive stretching devices constructed and arranged to produce a total stretching ratio of between 8 to 1 and 12 to 1 of the extrudate with the major stretching ration being effected in the first stretching device.

5- An extruding system as defined in claim 4, wherein the quenching unit is a cooling water tank and each of the two consecutive stretching devices includes stretching rollers in a hot liquid tank.

6- An extruding system as defined in claim 4, wherein the quenching unit is a chill roll set and each of the two consecutive stretching devices includes stretching rollers in a radiant heater oven.