DE2519075B2 - PROCESS FOR STRETCHING A MOVING, CONTINUOUS WEB MADE OF A THERMOPLASTIC PLASTIC - Google Patents

Description

The invention relates to a method of stretching a moving, continuous web from a thermoplastic material given initial thickness and initial width in the longitudinal direction of the Web with a reduction in the initial thickness with a minimal reduction in the initial width, where the (, 0 Web through a nip between two spaced apart, rotating in opposite directions Rolls is moved whose distance at the nip is smaller than the initial thickness of the web.

Such a method is described in US Pat. No. 3,083,410. There, the two counter-rotating rollers driven by the web running through the nip. This means that there is only one in the longitudinal direction of the web

low stretching effect associated.

From CH 4 59 545 it is also known in the above-mentioned stretching process to make a web a plastic material thereby additionally stretching in the longitudinal direction by the web after the nip around a faster running roller and thus out of the nip with a predetermined stretching force is pulled.

The invention is based on the object of proposing a method of the type mentioned at the beginning, which is further developed to the effect that the stretching effect of the web subjected to the process is improved in the longitudinal direction of the web without additional longitudinal stretching device.

The object is achieved according to the invention in that both rollers run under friction.

As a result, the stretching is improved, while at the same time only low compression forces for the Stretching in the longitudinal direction must be applied. The process can also be carried out with a relative carry out a wide range of output parameters cheaply. This is especially true for the selection of the Draw ratio and the temperature in a subsequent transverse orientation. Another advantage of the method consists in that the pressure exerted in the nip causes the enlargement of incipient material defects addiction to prevent. This is especially true for very small cracks, generally in the machine direction are aligned.

Preferred configurations of the subject matter of the invention emerge from the subclaims.

The invention is explained in more detail below on the basis of exemplary embodiments. The figure shows schematically in a cross section a pair of counter-rotating rollers, between their roller gap a path made of a thermoplastic material is passed through.

A continuous web 10 made of a thermoplastic film with an initial thickness η is fed to a first roller 12 in the direction of arrow a. The film thickness is exaggerated in the drawing for better clarity. The first roller 12 is driven at a peripheral speed Si. The web 10 is guided around the first roller 12 to a roller nip 14 between the first roller 12 and a second roller 16. The first roller 12 and the second roller 16 are at a distance d from one another, the distance d being smaller than the initial thickness n. The distance d is the height of the roller gap 14. The second roller 16 rotates at a peripheral speed S2, which is less than the peripheral speed 5i of the first roller. The moving web 10 is stretched in the area of the nip 14 between the two rollers 12 and 16. This stretching reduces the thickness of the web 10 to a final thickness fc. After the web 10 has passed through the nip 14 between the two rollers 12 and 16, it is stripped from the second roller 16 and wound onto a take-up roll, not shown. The distance fl is preferably less than approximately half the initial thickness ti. The advantages of the invention are progressively improved as the ratio ti Id increases.

The peripheral speed of the first roller 12 should be at least 1.5 times greater than the peripheral speed of the second roller 16. However, it is more advantageous if they are at least twice the Peripheral speed of the second roller 16 is. The greatest speed difference that drove

may depend in part on factors such as the value ^s ^ (for example, the point at which a type ^he o _the web enters) and the desired reduction ^^ train. As a general rule

It should be stated that the relative speeds ¹⁰¹¹¹¹ -j "rolls should rarely differ by more than one factor of ^the "? ia8bis9different _e a
^VO c ground sheets made of thermoplastic polymers

Let us give examples of such thermoplastic poly ^ve used individually or in mixtures

10

Polyolefins, in = »!» »'™ *" - "··"', -

low density polyethylene, polypropylene,

Polybutene !, polystyrene, uncrosslinked

ctvrol / butenediene copolymers and.

Polyisobutylene; Polyethylene oxide and

Mixed polymers of ethylene oxide with

^ polymerizable monomers such as

ProDvlenoxid; Acrylic polymers like

Poly (methyl methacrylate) and poly (ethyl acrylate); 2

RoDolymers of ethylene and one or more

co-polymerizable monomers such as propylene,

Vinyl acetate acrylic acid, ethyl acrylate

Methyl methacrylate and vinyl chloride;

Vinyl polymers such as homopolymer

and copolymers of vinyl chloride, vinyl acetate, and

Vinylidene chloride; Polyurethane polymers like that

linear polymer produced by reaction of

Polyethylene adipate with butanediol and

Bis 4-isocyanatophenyl methane; Polycarbonates;

Polysulfones; Polyformaldehyde; Polyester, like

Polyethylene terephthalate and others. During the time in which the web is in the region of the gap, it must be kept at a temperature at which the stretching and thus the alignment of the molecular chains occurs Stretch can be used. These temperatures are known or can easily be derived for any thermoplastic film, whether it consists of a single polymer or a mixture of polymers. While the stretching temperature is theoretically any temperature between the freezing temperature of the thermoplastic polymer and the crystalline melting point or softening point « In can, the selected temperature is normally slightly below the softening point, for example about 5 to 20 ^° C.

The approximate softening points of some preferred ones thermoplastic polymers are shown in the following table:

polymer

Approximate softening point in ⁰ C

Polyethylene oxide
Low density polyethylene
High density polyethylene
Polyoxymethylene

Polypropylene
Vinylidene chloride copolymer

Polyvinyl acetate
Polyethylene terephthalate
Polystyrene (syndiotactic)
Unplasticized polyvinyl chloride, polymethyl methacrylate
Polycarbonate

65 98 135 185 165 195 175 255

105-110 212 160 230 If mixtures of polymers are used, then the temperature selected is usually a compromise, depending on the softening point of each ingredient. The stretching temperature is, in almost all cases, in the case of the egg. a mixture of polymers is used becomes higher than the glass transition temperature of any polymer present in significant quantities in the mixture. It is reasonable to start at a temperature slightly above the highest freezing temperature the polymer lies in the mixture and gradually increases the temperature until the desired degree of stretching is achieved and orientation wheel is reached

Means for maintaining the web at the appropriate temperature while it is in the nip are known. For example, the web may be in contact with a significant part of the perimeter of the slower roller, as shown in the drawing, with the slower roller on a temperature is maintained which is sufficient to heat the web to the desired stretching temperature. Alternatively, the web can either be extruded or otherwise directly onto the slower roll be shaped, in which case the slower roller is kept at a temperature such that the s web is cooled to the desired stretching temperature as it enters the nip. In any case the solid thermoplastic sheet has a temperature below its crystalline melting point, when it enters the nip between the slower and faster rolls.

It is preferred to hold the web on both rollers, as shown in the drawing. It is possible to lead a web straight through the nip without it on one of the two rollers Is held for 3s. During this tour, the However, the advantages mentioned are not achieved to the same extent as in the preferred embodiment shown.

The treatment of the web after it has been subjected to the method according to the invention is allowed determined to a certain degree by the nature of the orbit. Highly crystalline polymers, for example high density polyethylene, only need to be cooled down before winding. Because amorphous Polymers, such as polystyrene, have a strong tendency to retreat to their previous dimensions Adopting orientation, precaution must be taken to prevent shrinkage prior to winding impede. These considerations are known to those skilled in the art and conventional means can be used for these 50 purposes can be used.

Example I.

A sheet of polyethylene oxide 0.1397 mm thick with a molecular weight of about 600,000 (determined by theological measurements)

ho wui ^ e in a nip with two opposing directions rotating rollers, as shown in the drawing, stretched. The experiment was carried out several times, with different nips set between the two rolls for each attempt

<> 5 were. Table I, below, shows the experimental conditions for every attempt. Table II, below, shows the respective properties of those obtained in each experiment stretched tracks.

<s

Table I.

Test conditions and data for example 1

Trial no. 1

Initial width (mm)

Final width (mm)

Initial thickness (mm)

Final thickness (mm)

Slower roller, temp, in ° C

Faster roller, temp, in ⁰ C

Slower roller, speed (m / min.)

Faster roller, speed (m / min.)

Nip opening (mm)

Separation force (kp) ^# )

*) The force-indicating system for the two-roll stretching device was designed for a maximum load of 113,000 kp. the The accuracy of the system is ± 226 kp.

Table Il

Physical properties of the samples of Example 1

355.6 355.6 355.6 355.6 327 331.8 335 335 0.1397 0.1397 0.1397 0.1397 0.0216 0.0193 0.0183 0.019558 59 59 59 59 57 57 57 57 0.6 0.6 0.6 0.6 3.7 3.7 3.7 3.7 2 1 0.12 between 0.025 and 0.135 0 0 0 Π3

Starting Trial no. 2 1 3 4th material Nip opening (mm) 1 2 1237 0.12 0.025-0.05 64 1230 no yield 1235 1226 Machine direction properties: 62 <1 59 62 Tensile stress (kp / cm ² ) 118 no yield 85,433 no yield no off. % Elongation 752 <1 0.0183 <1 <1 Yield tension (kp / cm ² ) 81 89.233 87.666 85.733 % Elongation of the yield 10 0.0203 1540 0.0185 0.0188 Secant modulus of elasticity (kp / cm ² ) 33,000 690 Average thickness (mm) 0.137 1505 1540 1510 1538 Properties in the transverse direction: 836 7th 754 796 Tensile stress (kp / cm ² ) 122 1505 7250 1510 1538 % Elongation 816 5 0.0181 8th 8th Yield tension (kp / cm ² ) 90.8 7400 0.0228- 7180 6770 % Elongation of the yield 11th 0.0206 0.0318 0.0181 0.0203 Secant modulus of elasticity (kp / cm ² ) 3070 0.0254- 0.0228- 0.0228- Average thickness (mm) 0.14 0.0343 0.0292 0.0252 Total thickness fluctuation (mm) *) -

*) At the edges, due to the build-up of the edge bed, which is characteristic of the contraction, there are higher Thick.

The web produced in Run A was uniformly clear and transparent. After stretching under the test conditions of Run 4, the nip was opened to 0.254-0.279 mm during the run. With the nip open, a slight but noticeable contraction developed and the web had run-up lines which generally ran in the transverse direction. At least about 85-90% of the web was covered with these guide lines. The guide lines indicated non-uniform stretching. In order to obtain a clear, transparent web by the conventional methods of drawing the web through an open nip, the polyethylene oxide web of Example 1 would have to be drawn at a draw ratio between 8: 3/4 and 9: 1.

Example Π

A sheet of polyethylene oxide 0.254 mm thick and having a molecular weight of about 600,000 was stretched similarly to Example 1. Table III below shows the experimental conditions used in the stretching. Table IV below shows the respective properties of the stretched webs as well as representative properties of a similar polyethylene oxide web conventionally drawn at a 6: 1 draw ratio .

Table III

Test conditions and data for example 2

VerMidis no.
1 2 Initial width (mm) 254 254 Final width (mm) 250.8 250.8 Initial thickness (mm) 0.254 0.254 Final thickness (mm) 0.13 0.13 Temperature of the slower ones 59 59 Roller ( ⁰ C) Temperature the faster 58 58 Roller ( ⁰ C) Speed of the long 1.8 1.29 sameren roller (m / min.) Speed of the fast 3.33 3.6 empty roller (m / min.) Gap opening (mm) 0.127 0.0762 Separation force (kp) ¹ ) 907 3402

*) The star indicating the force was for a maximum l.asi constructed from 113,400 kp. The (ienauigkeil des Systems is ± 227 kp.

Table IV

Physical properties of the samples in the example

Machine direction properties:

Tensile stress (kp / cm ² ) ⁰ O elongation

Yield tension (kp / cm-)% Yield elongation Elasticity secant modulus (kp / cm ² ) Average thickness (mm)

Properties in the transverse direction:

Tensile stress (kp / cm ² ) '"<' Elongation

Yield stress (kp / cm ² )% Yield elongation Elastic modulus (kp / cm ² ) Average thickness (mm)

Exit- Example 2 2 Conventional material Versui'hs no. 756.4 liches 68 stretch 1 no yield Example no 103.1 460.9 1 1096.7 682 134 5303 64 91.4 no yield 0.0922 no off 15th 1 156.8 1 2793 3791 708 3705 0.2545 0.127 156.8 0.053 125.1 169.4 11th 138.5 770 936 6545 862 105.5 149.7 0.0942 138.5 14th 14th 8th 3501 5160 5751 0.2545 0.1328 0.038 Here / u 1 Bkiit drawings

Claims (7)

• i patent claims: 1. Method of stretching a moving, continuous. Sheet made from a thermoplastic Plastic given initial thickness and initial width in the longitudinal direction of the web with reduction the initial thickness with a minimal reduction in the initial width, the path being replaced by a to Roller gap between two spaced apart rollers rotating in opposite directions is moved whose distance at the nip is smaller than the initial thickness of the web, thereby characterized in that both rollers (12,16) run under friction. 2. The method according to claim 1, characterized in that the web before entering the Roller gap (14) is guided around the faster running roller (12). 3. The method according to claim 1 or 2, characterized in that the web after leaving of the roller gap (14) is guided around the slower running roller (16). 4. The method according to claim 3, characterized in that that the peripheral speed of the faster running roller (12) at least that 1.5 times the circumferential speed of the slower running roller (16). 5. The method according to claim 3, characterized in that the peripheral speed of the faster running roller (12) at least twice the peripheral speed of the slower one running roller (16). 6. The method according to any one of claims 1 to 5, characterized in that the distance (d) between the rollers (12, 16) at the roller gap (14) is smaller than half the initial thickness (u) of the web. 7. The method according to any one of claims 1 to 6, characterized in that the temperature of the web when passing through the nip (14) is approximately (5 to 20 ⁰ C) below the softening point of the thermoplastic material of which the web is made.