CA1072710A

CA1072710A - Stretching process

Info

Publication number: CA1072710A
Application number: CA258,650A
Authority: CA
Inventors: Heinz Vetter
Original assignee: Roehm GmbH Darmstadt
Current assignee: Roehm GmbH Darmstadt
Priority date: 1975-08-16
Filing date: 1976-08-09
Publication date: 1980-03-04
Also published as: SE7609104L; JPS5281373A; SE419717B; GB1531608A; IT1062243B; DE2536462A1; FR2321380A1; FR2321380B1; NL7608682A; CH612877A5

Abstract

ABSTRACT OF THE DISCLOSURE

The Specification describes a process for producing transversely stretched hollow strips of thermoplastic synthetic resin having a non-circular cross-section which comprises passing a hollow strip of a thermo-plastic synthetic resin in the thermoplastic state, said strip having a non-circular cross-section, through a stretching chamber, the transverse cross-sectional area of which increases, in the direction of travel of the said strip, to a size corresponding to the desired degree of transverse stretching of the strip, the hollow strip containing an inert gas under a pressure serving to press the strip against the walls of the said chamber whereby the strip is transversely stretched according to the cross-sectional area of the chamber; the stretched strip being subse-quently cooled to a temperature below the softening temperature of the said resin.

Description

The present invention relates to a process for pro-ducing stretched hollow strips of synthetic resin having a non-circular cross-section.
The mechanical properties and resistance to weather-ing and corrosion of thermoplastic synthetic resins can often be decidedly improved if they are stretched biaxially while in the thermoelastic state, Whereas the synthetic resin is fluid in the thermoplastic state and it undergoes non-elastic changes in shape under the in-fluence of relatively small forces, in the thermoelastic state it resists with an elastic resiliency any changes in shape, An imposed change of shape, for example the biaxial stretching of a planar synthetic resin sheet, can be "frozen" by cooling the resin to a temperature below the thermoelastic temperature range of the resin, ~ The "frozen" molecular alignment caused by the stretching -~, is responsible for improvements in the properties of the stretched material. However9 these improvements tend to ` disappear if the stretched resin is subsequently reheated , i .
to the thenmoelastic state and contracted back to its original form. Most thermoplastic synthetic resins `^Y~

which have a sufficiently marked thermoelastic phase range between the glass transition temperature and the start of the thermoplastic phase range can be stretched in the above-described manner. Particular examples of such thermoplastic synthetic resins include those with a high molecular weight, such as polymethyl methacrylate, polystyrene, ABS plastics, polyvinyl chloride and cellu-lose ester plastics.
The production of stretched plastics in sheet form r has long been known. In "Kunststoffe" Vol 50, pages 371 to 375 (1960) K. Richard, G Diedrich and E. Gaube also . .:
` described the biaxial stretching of tubes. The tubes are .-;
given an enlarged cross section by means of a mandrel or ~` by means of compressed air and simultaneously stretched in the longitudinal direction. The wall of the tube is .
. .
thus stretched biaxially and its thickness is reduced, just as in the case of a planar synthetic resin sheet.
The stretching of tubes with compressed air is described by the authors as the preferred process, but hitherto it 20 has been used very little in practice, apparently owing to technical difficulties connected with the process.

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The apparatus for the stretching of tubes in this way works continuously and is connected to an extruder. The extruded tubular strip is first cooled to below the soEt-ening temperature of the resin and fed, by means of a pair of rollers, at a definite speed into a heated stret-ching chamber, where it is uniformly heated to a suitable temperature for stretching the resin in the the Doelastic phase range. In the stretching chamber, the strip is pressed against the wall of the chamber by the air pres-sure prevailing inside the strip. In order to allow the strip to slide easily through the stretching chamber, in spite of the friction thus generated, the heating medium, ~`~ which is generally under more or less the same pressure .... . .
`; as the compressed air in the tubular strip, is allowed to -( 15 come into direct contact with the strip. The tubular ~ ;
strip leaves the heating zone through a circular discharge ` opening, which is sealed by the incipient inflation o the tubular strip to prevent the discharge of the heat-ing liquid under pressure. Immediately behind the heat-ing zone 9 the strip is inflated by the internal air pres-sure and in this expanded state passes into a cooling ' `' -~'o~io chamber where it is cooled to below the softening tempera-ture. Subsequently, it is seized by a drawing apparatus which runs faster than the apparatus which feeds the tubular strip into the heating zone. The difference in speed between the two devices causes stretching of the tubular strip in the longitudinal direction. The internal cavity of the strip is sealed off by a pressure-tight sealing stopper connected to the core of the extruder and suspended in sliding manner.
The internal pressure in the tubular strip must be . -, very accurately measured so that the strip is inflated to exactly the circumference required~ after leaving the heating zone and before entering the cooling zone. If the pressure is too low, the necessary expansion of the circum-ference is not achieved. If the pressure is too high, the strip expands so much that it has to be contracted again when entering the cooling chamber. In this case, high friction is obtained in the cooling chamber, since the liquid heat:ing medium acting as a lubricant in the heat-ing chamber is almost totally wiped off when leaving that chc~nber. The process therefore requires continuous and `~ %~

very careful supervision, to ensure uniform stretching.
The difficulties involved in the above process also increase substantially if hollow strips of non-circular cross section, e.g. rectangular or hollow sheets with ~; 5 planar outer surfaces are produced. On leaving the heat-ing chamber, a hollow strip having planar surfaces tends to inflate into a cushion shape, with the result that it .~i can only enter the cooling chamber by being compressed.
;, ` Deviations from the desired geometry can thus arise, par-ticularly when the hollow strip has inner walls, as is the case, for example, with a hollow sheet consisting of two covering layers and cross pieces arranged perpendicularly ,, ~
therebetween.
~ We have now found that stretched hollow strips of non-- 15 circular cross section can be produced from a thermo-plastic synthetic resin by expanding a non-circularhollow ~-strip in the thermoelastic state us~ng a compressed gas enclosed within the cavity of the strip, if the hollow strip passes through a stretching chamber, the transverse cross-section of which increases, in the direction of travel of the strip, to a size corresponding to the ~ -_ 5 _ ~ . ., . - . .. . .

; desired degree of stretching of the str:ip and wherein the expanded hollow strip is cooled, after reaching its desired cross section,to below the softening temperature ; of the resin The expansion in transverse cross section ~: 5 of the strip to the desired extent thus occurs as the re-sult of the conical configuration of the stretching cham-.: ber, independently of the internal pressure in the hollow : strip, provided the internal press~lre is at least of the . magnitude needed Eor expansion :; 10 According to the present invention we provide a pro-cess for producing transversely stretched hollow strips of thermoplastic synthetic resin having a non-circular cross-section which comprises passing a hollow strip of a : thermoplastic synthetic resin in the thermoelastic state, said strip having a non-circular cross-section, through a stretching chamber, the transverse cross-sectional area of which increases, in the direction of travel of the said strip,to a size corresponding to the desired degree of transverse stretching of the strip, the hollow strip con-taining an inert gas under a pressure serving to press the strip against the walls of the said chamber whereby .: . ;

the strip is transversely stretched according to the cross-sectional area of the chamber, a lubricant being introduced between the outer surface of the tubular strip and the inner walls of the stretching chamber, said lubricant having a viscosity of at least 5000 c.p. under the conditions in which it is employed in the stretching chamber; the stretched strip being subsequently cooled to a temperature below the softening temperature of the said resin.
The increases in the overall size of the strip and in the ; dimensions of particular walls of the strip are not necessarily strictly 0 proportional with one another in every case. In other words, for example, the internal walls of a multi-chamber strip may be stretched not only by an ~-~ increase in the diameter of the strip, but also by the reduction in thickness of the outer walls. Thus, there are cases where a flat multi-chamber hollow ~;strip is stretched only in length and width, whilst its thickness remains . -.
~ unchanged, but with the inner walls being stretched, only by the reduction j in thickness of the outer walls, to the same extent as the outer walls.
`~ In a preferred embodiment of the process according to the invention, the hollow strip is passed through three tempera~ure ones: in the first zone, the hollow strip is heated to bring the synthetic resin into the 2a thermoelastic state for stretching, in the second æone, _7_ ~Z~10 ;.e. in the conical stretching chamber, stretching is effected at a substantially constant temperature, and in the third zone, which9 ~ike the first zone, has a unifo~
internal cross section, the strip is cooled to below the softening temperature of the resin.
- In the process of the invention, a lubricant is : used to facilitate passage of the thenmoelastic strip under pressure through the stretching zone. Lubri-cation can be effected, for example; by means of a heating ~; 10 or cooling medium, as in prior processes, which medium is - allowed to come into direct contact with the strip through openings in the wall of the heating zone mentioned above.
However9 lubrication is preferably effected independently ~~
of the heating or cooling medium. Preferably a film of lubricant is applied to the strip which draws the lubri-cant through the stretching zone. Thus9 in the stretching chamber, this film is spread out to the same extent as the strip and accordingly becomes thinner. At the same .
time, the strip is accelerated. All these factors in-crease the friction resistance betweea the strip and the wall oE the stretching zone~ Friction has a considerable effect on the course of the stretch~ng process. As soon 8 ~

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as irregularities in the friction resistance occur at ~;;! various points on the strip, irregularities in stretching , result and these are apparent in the finished strip as optically unstable zones, mechanical weak point or dis-torted intermediate walls. Whilst these problems can -~: .
easily be overcome in the case of a circular tubularstrip, they increase in the case of non circular extruded strips according to the complexity of the latter With non-circular cross sections, particularly at the points of i 10 attac~nent of inner walls of the hollow strip, different resiliencies arise and consequently different pressures between the strip and the walls of the stretching chamber.
The lubricant preferentially collects in the zones of re-duced pressure and there alters the friction resistanceO
It has been found that the local differences in stretching caused by varying frictional forces can sub-stantially be avoided if a lubricant with a viscosity of at least 5000 cP under operational conditions is used.
Such a lubricant system can be independent of the heating or cooling system and the lubricant may be applied to the surface of the strip via for example an oil groove at the _ g _ .. .. ...

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entry to the heating zone of the stretching apparatus. In contrast to the low viscosity oils and glycols used as lubricants in known processes, the high viscosity lubri-cants used according to the present invention form a ~` 5 hydrodynamic lubricant film even at the low speed of ad-vance of the strip, whilst this lubricating film is not perceptibly affected by fluctuations in the pressure ex-erted by the hollow strip. Therefore, at every point in ;
the process, substantially the same flow and friction characteristics may be obtained, with the result that distortions in the cross sectional geometry and in the position of the intermediate walls are reduced, The lubricant should be liquid at the operational temperature, but can be solid at room temperature, mak-ing it easier to remove mechanically from the surface of the strip. Suitable lubricants are materials which have a sufficiently high viscosity at the operating temperature and which do not have a corrosive effect either on the extruded resin or on the stretching apparatus. Examples of such lubricants include, for example, waxes,paraffins, polyethylene glycols or solutions of high molecular - 10- ~:

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materials in non-corrosive solvents. Also, aqueous solu-tions of water-soluble polyrners may be used, for example 0.1 to 10% solutions with a viscosity oE at least SOOO, preferably between 5000 and 150,000 cP. Water-soluble polymers with a molecular weight of about 50,000 toseveral millions may also be used for the preparation of the aqueous lubricant solutions. Preferably, such polymer~
are synthesised wholly or predominantly from acrylic acid, methacrylic acid, itaconic acid, maleic acid or salts thereof, or from acrylamide, methacrylamide, vinyl pyrro-lidone, vinyl alcohol, hydroxy alkyl esters or dialkyl-aminoalkyl esters or acrylic or methacrylic acid, the salts or quaternary products of these esters or from mix-tures of such monomers. Cellulose ethers and other natural or synthetic water-soluble polymers may also be used. Since the solutions are used under pressure, the - water used as solvent does not evaporate under the oper-ational conditions. On leaving the stretching chamber, the lubricallt film rapidly dries and can be removed, if desired, from the surface of the resin, e.g. by brushing.
If it is intended to stick protective paper or foil on `

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the hollow strip produced, this paper or foil may be ` applied to the lubricant filrn before drying, in which case the lubricant will act as an adhesive for the protective layer.
~` 5 The gas used to expand the strip may be introduced into the cavity of the strip thro-ugh the core portion of the extruder nozzle~ The gas pressure required depends on the degree of stretching desired and the resiliencies occurring. In the case of a linear increase in the cir-cumference of the strip by 50 to 200%, a pressure of be-tween 1 and 10 bars is generally preferred. Compressed air may be used for example as the pressure gas. In order to cool the hollow strip rapidly and intensively after it has left the extrusion die, liquid nitrogen may also be introduced.
So that the gas used to expand the strip does not es-cape from the end of the hollow strip, the cavity of the strip should be sealed off against the atmosphere. This may be effected, for example, with a closure stopper in-serted in the hollow strip, this stopper being attached to the core portion of the extruder die with a chain and , , .

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i held in place more or less level with the end of the , cooling zone of the stretching apparatus. However, since it is often difficult to introduce such a closure stopper into the hollow strip as it moves up, it may be prefer-able to heat-seal the expanded hollow strip behind the exit from the stretching apparatus, each time that a :~
; sufficiently long portion of the expanded strip has been obtained, and to remove the portion behind the heat-seal-ing point. The thermoplastic synthetic resins used to -. ~
produce the hollow strips can generally be heat-sealed .
in a pressure-tight manner by bringing heated open dies : together.
If the hollow strip is heat-sealed at its free end in the manner described above J the gas pressure prevail-ing inside not only causes an enlargement of the cross-section, but at the same time causes stretching in the longitudinal directlonO If desired, the gas pressure ~.
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can be increased to such an extent that the stretching in the longitudinal direction is as extensive as the stretching in the transverse direction, with the result that the preferred biaxially uniform stretching is .... .. ...... .. . . . .
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obtained. However, the gas pressure required for this is in any case so high that very considerable pressure forces are transmitted to the walls of the stretching chamber an~
these walls must be made correspondingly thick. Therefore, it is preferable to apply only such gas pressure as will cause the desired transverse stretching and give rise to at most a slight longitudinal stretching. Nevertheless, in order to obtain biaxially uniform stretching, the free end of the strip may be pulled away, after or upon cooling, by a pair of feed rollers, at a greater speed than the strip enters the stretching chamber. The difference in speed results in the stretching of the strip in thP
longitudinal direction. If the pressure gas in the hollow profile is sealed off from the surrounding atmos-lS phere by means of a drag stopper, longitudinal stretchingcan be obtained only by means of such a pulling deviceO
The extruded strip offers the least resistance to longitudinal stretching in the thermoplastic range direc-tly behind the extrusion die and would be expanded vir-tually only at this point if this were not prevented byspecial precautions, Expansion in the thermoplastic ''`- .

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range does not lead to the improvement in properties which is obtained by stretching in the thermoelastic range, In the known process described hereinbefore, the expansion in the thermoplastic range is prevented by first cooling the strip to below the softening temperature and allowing it, by means of a braked pair of rollers9 to travel into a heating apparatus where the strip is heated up again to the thermoelastic state. It can then be stretched in the longitudinal and transverse directions 9 the longitudinal stretching resulting from the difference in speed between the pair of braking rollers and the pair of feed rollers.
The use of a highly viscous lubricant is a particu- -larly simple way of preventing undesirable longitudinal expansion of the strip in the thermoplastic range. The highly viscous lubricant film has a ~raking effect on the ;--strip flowing through the stretching apparatus such that the tensile force leading to stretching in the longitud- `
inal direction is opposed by an equal friction force;
which becomes effective only in the thermoelastic phase of the strip between this strip and the walls of the stretching chamber. Thus, the strip is gradually . .

accelerated in the thermoelastic range~ whilst the degree of longitudinal stretching results from the difference in speed between the discharge rollers and the speed of the strip in the range where the thermoelastic state is attained. This difference ln speed is determined by the friction forces between the strip and the walls of the stretching chamber. The friction is directly proportional to the viscosity of the lubricant at the operation tempera-ture, the speed of the strip and the frictional surface area oE the thermoelastic part of the strip in contact with the wall of the stretching chamber and inversely pro- ~ `
portional to the thickness of the film of lubricant.
Since the flow rate and the frictional surface area in-crease during the stretching process and the thickness of ; 15 the ~ilm of lubricant decreases, the friction in the `
stretching chamber is sharply increased and prevents the force applied for longitudinal stretching from being transmitted over the stretching zone into the thermo-plastic zonle of the strip located in front of it. In general, thle thickness of the lubricant film is automati-cally fixed, in commercial operations, depending on the ' '.

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viscosity, by the quantity of lubricant which the strip picks up from the oil groove. If the viscosity oE the lubricant is correctly measured, the thermoplastic strip is drawn into the stretching chamber without any expan-sion in the longitudinal direction and is not stretched until friction increases. Owing to the strong effect of -~
the viscosity of the lubricant on the course o the stretching process, it is important to keep this viscosity constant.
The outer walls of the hollow strip9 and in the case of several chambers the intermediate walls, are generally between 0.5 and 2 mm thick in the stretched state. In the case of stretching by 70% in each direction in space, this wall thickness is obtained from a starting thickness of 1.5 to 6 mm Hollow strips of every shape can be produced with a high degree of accuracy by the process of the invention, They may have one or more chambers. Whereas the shaping of the contour form of the cross section is virtually free from any restrictions~ the intermediate walls assume the position and form which results from the elastic tension characteristics during inflationO In addition, it is also possible to create different pressures in the individual chambers of a hollow profile, in order to pro-duce convex intermediate walls9 for example-The process of the invention is applicable to any resin which can be extruded in its thermoplastic state and which passes through a marked thermoelastic phase on cooling, e.g.
polycarbonates, polymethyl methacrylate, polystyrene, polyvinyl chloride or ABS resins. Preferably, polymethyl methacrylate is used and may also contain flame-retardant additives.
For a better understanding of the present invention, a preferred embodiment will be described with particular reference to the accompanying drawings wherein:-Figu-re 1 shows the cross section of a hollow strip after extrusion Figure 2 shows the cross section of the same hollo~ strip after stretching, to the same scale.
Figure 3 is a vertical section and ~ is a horizontal section through the plane A-A of Figure 3 through an extruding and stretching apparatus, including the heat-sealing and separating apparatus~

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The thermoplastic moulding compound 3 is fed into the extrusion die 1 through the opening 2 from an extruder (not shown). Between the lips 4, 5 of the die and the core portions 6 is formed a hollow strip, which consists of two layers 7 and 8 and cross pieces 9 arranged at right angles between them.
Upon commencement of the process, the cavities 10 of the strip, which are closed at the front end by heat-sealing in the thermoplastic state, are supported with a slight overpressure of air, which passes through the pipe 11 and the channels 12 into the chambers 10. The stretch-ing apparatus made up of parts 13 to 20 is divisible along the plane A - A and the upper parts 13, 15, 17 and 19 are detached from the lower parts 14, 16, 18, 20 during oper-ation and raised so that the strip emerging can travel through as far as the end of the calibrating apparatus without being stretched. The upper parts 13, 15~ 17 and 19 are then placed on the lower parts and bolted to them.
The pressure in the chambers 10 is gradually increased to about 5 bars, so that the strip is inflated in the stretching zone 17/18. At the same time, the air pressure in ~he aperture 21/22 is increased to the same exten~ as in the chambers 10. As it travels past the channel 23, the hollow strip is enveloped in a film of lubricant 24 and travels into the zone 13/14 with this film.
The zone 13/14 is surrounded by a cooling casing, through which cold water flows. On leaving this zone, the outsides of the covering layers 7 and 8 are cooled to a temperature below 100, whilst the surface of the cover-ing layers on the inside and the cross pieces are at a temperature of about 140. The subsequent zone 15/16 is kept at about 140C by means of water under pressure.
i At the end of this zone, the strip is at a unifonm temperature of 140C, so that it is uniformly stretched in three dimensions in the following zone 17/18.
After stretching is completed, the strip is cooled to below the softening temperature in the zone 19/20, which is washed over by a cooling medium at about 25C.
The layer of lubricant 24 is wiped off the emerging strip by means of rubber lips 25/26 and the lubricant is re-cycled for re-use. The residue of lubricant film is dried .

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with the hot air blower 27/28 and removed from the sur~
face of the strip with the brush rollers 29/30~ The pair of rollers 31/32 causes longitudinal stretching into zone 17/18 of the calibrating apparatus and feeds the strip into the separating apparatus 33/34, which contains two heated welding dies 35/36, which are brought together hydraulically and heat-seal the hollow strip. The portion 37 of the hollow strip is pulled away at the heat-seal point 38 in a the Doplastic state and separated from the strip. The chambers of the portion 37 burst open in the region of the still thermoplastic weld seam, so that the pressure gas can escape. During the heat-sealing process 9 the separating apparatus 33/34 runs on the rails 39~40 with the strip.

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Claims

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

1. A process for producing transversely stretched hollow strips of thermoplastic synthetic resin having a non-circular cross-section which comprises passing a hollow strip of a thermoplastic synthetic resin in the thermoplastic state, said strip having a non-circular cross-section, through a stretching chamber, the transverse cross-sectional area of which increases, in the direction of travel of the said strip, to a size corresponding to the desired degree of transverse stretching of the strip, the hollow strip con-taining an inert gas under a pressure serving to press the strip against the walls of the said chamber whereby the strip is transversely stretched accord-ing to the cross-sectional area of the chamber, a lubricant being introduced between the outer surface of the tubular strip and the inner walls of the stretching chamber, said lubricant having a viscosity of at least 5000 c.p.
under the conditions in which it is employed in the stretching chamber; the stretched strip being subsequently cooled to a temperature below the soften-ing temperature of the said resin.

2. A process as claimed in claim 1 wherein the strip is longitudinally stretched simultaneously with the transverse stretching in the stretching chamber.

3. A process as claimed in claim 2 wherein the longitudinal stretching is effected by drawing the strip from the stretching chamber at a higher speed than the speed at which the strip enters the stretching chamber.

4. A process as claimed in claim 3 wherein the said lubricant comprises an aqueous solution of a polymer, said solution having a viscosity of 5000 to 150,000 cP.

5. A process as claimed in claim 4 wherein a 0.1 to 10% aqueous solu-tion of the polymer is employed.

6. A process as claimed in claim 5, wherein the said polymer has a molecular weight of at least 50,000.

7. A process as claimed in claim 6 wherein the said synthetic resin is a polycarbonate, polystyrene, polyvinyl chloride or an ABS resin.

8. A process as claimed in claim 7 wherein the said synthetic resin is polymethyl methacrylate.

9. A process as claimed in claim 8 wherein the walls of the stretched strip are 0.5 to 2 mm in thickness.

10. A process as claimed in claim 9 wherein the hollow strip has been produced by extrusion.

11. A process as claimed in claim 10 wherein the strip emerging from the extruder is cooled to a temperature below the softening point of the resin.

12. A process as claimed in claim 11 wherein the end of the strip remote from the extruder has been sealed by hot-welding.

13. A process as claimed in claim 12 wherein the gas is present in the strip under a pressure of 1 to 10 bars.

14. A process as claimed in claim 13 wherein the said gas is air.