GB2218036A - Biaxially stretched poly- epsilon -caproamide film - Google Patents

Abstract

In the manufacture of a biaxially stretched poly- epsilon -caproamide film, which is uniform in thickness, excellent in transparency, and low in crystallization, the degree of crystallization is lessened by fixing the homogenized polyamide poly - epsilon -caproamide sheet which uses a single polymer of - epsilon - caprolactam above its relative viscosity of 3.0, desirably, within the limits of 3.0 to 4.5. The degrees of polymerization are fixed by an electrostatic adhesion method in which a 4.0 to 10.0 kV high voltage and low current adheres the resin material impressed between the surface of a roll-shaped rotary cooler anti wire-shaped single system electrode to the cooler.

Description

Process for manufacturing poly -E- caproamide film This invention relates to a process for manufacturing homogenized poly -C- caproamide film using a single polymer of - caprolactam. More particularly, it relates to a process which results in a film which has uniform thickness, excellent transparency, and is low in its degree of crystallization.

Various methods have been proposed as biaxial stretching methods for unoriented film. These may broadly be classified as simultaneous biaxial stretching methods and consecutive biaxial stretching methods.

In simultaneous biaxial stretching methods, the apparatus holding the tenter clip is very complicated since the unoriented film is stretched biaxially (vertically and horizontally) simultaneously in a tenter. This method is very expensive since this device is not yet in generalized use. It also shows a drop in productivity since it is difficult to change the stretching rate.

In consecutive biaxial stretching methods, a roll-shaped vertical stretcher and a tenter-type horizontal stretcher are used in turn after an unoriented sheet is melted and extruded through a T die extruding machine. The sheet is then routinely cooled and hardened on the surface of movable cooler such as a roll-shaped rotary cooler. The product obtained using this method however has poor thermal conduction because a thin layer of air is interposed between the melted but unstretched sheet and the movable cooler resulting in a sheet which is higher in degree of crystallization and poor in transparency. This is because the melted sheet is hardened for crystallization at a rapid rate. The stretchability of the unstretched sheet is poor and the transparency of the stretched film is also poor.

We have found that if an unstretched sheet of molten polymer is adhered to and cooled on the surface of a cooler by a direct movement without including a thin layer of air, a poly -- caproamide polymer sheet with a reduced crystallization rate and improved stretchability results.

In order to enable the extruded melted sheet to glue itself on the surface of the cooler, the present invention uses an electrostatic impression device which in effect adheres the corresponding sheet material on the surface of the cooler and cools it by impressing high voltage with low current between the surface of the electrode and cooler with either a knife edge-shaped, wire-shaped or multineedle electrode between the T die and cooler, or as far as possible, near the cooler, and passing an unstretched sheet between the electrode and cooler. Desirably, a wireshaped electrode impression device is used rather than an expensive knife edge-shaped or a multineedle impression device.

In regard to the degree of polymerization of polyamide resin used as a raw material, a resin having a relative viscosity of more than 3.0 or, more desirably, 3.0 to 4.5 is required.

The homogenized poly -- caproamide resin having a degree of polymerization with its relative viscosity amounting to less than 3.0 has been found to deteriorate easily under the influence of temperature. Therefore in the film manufacturing process, such resins contaminate the deuice with oligomer, failing to produce the desired various stabilized physical properties of film stretched biaxially. When the degree of polymerization is greater than 4.5, it is more difficult to manufacture films because greater loads are caused in the manufacturing process.

The present invention uses homogenized poly -C-caproamide and makes it possible to produce a biaxially stretched poly -E- caproamide film which is excellent in transparency, uniform in thickness, low in the degree of crystallization and small in deviation of the degree of crystallization due to an electrostatic impression device as previously described.

This thermoplastic film has improved physical properties such as gas interception, pinhole resistance, impact resistance, breaking strength, and transparency because of stretching its unoriented sheeting biaxially (vertically and horizontally).

The method of present invention uses as raw material a single polymer of -C- caprolactam generally called nylon 6. In order to improve the activity, blocking-resisting property, stretchability, heatresistance and electricity-removing property of stretched film polymer, such additives as active agent, antioxidant, ultraviolet ray absorbent, and static electricity preventative agent may be blended in the corresponding polymerization reaction to the extent that the nature of the film is not adversely effected.

The present invention manufactures an unstretched sheet which is not oriented, being amorphous, by heating the poly -- caproamide polymer above its melting point, extruding it and electrostatically adhering it between the surface of roll-shaped rotary cooler and an electrode impressed with 4.0 to 10.0 kV high voltage and low current, followed by hardening upon cooling. When the voltage is higher than 10.0 kV, a spark takes place and the wire is broken, tending to be impressed electrostatically.When the voltage is lower than 4.0 kV, the polyamide film stretched biaxially fails to attain its productivity because the electrostatic impression power is low. (In the case of polyamide film stretched biaxially, the drawing speed of unstretched sheeting should generally be more than 20m/min to attain its production.) As to the unstretched sheet manufactured under such an electrostatic adhesence method, the degree of crystallization amounting to less than 5% is desirable because it facilitates stretching biaxially in the subsequent stretching process.

The invention may be more readily understood by reference to the accompanying drawings in which embodiments of the invention are schematically illustrated. In the drawings, Figure 1 shows schematically a manufacturing process using a consecutive biaxial stretching method.

Figure 2 shows in more detail the cooling process of an unstretched sheet using electrostatic adhesion.

An electrostatic impression device of the type detailed is shown schematically in Figure 2. A wireshaped single system electrode (5) is installed near the surface of a roll-shaped rotary cooler (2) and 4.0 to 10.0 kV high voltage with low current are impressed between the electrode (5) and the earthed cooler (2). Electrostatic power is produced between the electrode and the surface of the cooler. When a melted extrusion advances in between it is satisfactorily adhered on the surface of the cooler.

Then, as shown in Figure 1, the unstretched sheet (S) is stretched vertically above its vertical stretch rate (=v2) of- 2.0 in the vertical stretcher (3) and is stretched horizontally above its horizontal stretch rate (= 12 of 2.0) in the tenter-type horizontal stretcher (4). Aster it is drawn to a specified width by being fixed by the heat fixing part (6), both of its edges are slit. Thus, a completed film is wound.

Although biaxially stretched poly -E- caproamide film (F) obtained in such a manner as described is satisfactory by nature, it may be further stretched vertically or horizontally to improve a specific physical property.

A method of measuring the peculiar value of the film (F) obtained under the present invention is as follows: (Maximum thickness (Minimum thickness (A) Thickness in the direction - in the direction Deviation Rate = of width) of width) (Average thickness in the direction of width) x 100 (B) Relative viscosity : Resin is dissolved in 96.3% sulphuric acid in the concen-tration of 1.0 gr/100 ml and measured in a 200C thermostat with an Oswald's viscosimeter.

The invention will now be more particularly described in the following non-limiting Examples.

Examples 1 to 6 Samples of polyamide poly -F- caproamide resin with relative viscosities between 2.0 to 4.5 are desiccated at 1100C in a rotary-type vacuum drier.

After the moisture content of the resin in each case is adjusted to 0.1 wt%, it is melted and extruded at 2600C by a 90 mm diameter extruding machine leading to extrusion via a T die (1). Melted polymer resin is extruded between a roll-shaped rotary cooler (2) cooled at 250C and a wire-shaped single system electrode (5) grounded to this cooler with 5.0 kV high voltage and low current. The polymer resin is impressed on the cooler and an amorphous and unstretched sheet (S) with a thickness of about 150 jim and a width of 1 m is obtained. This unstretched sheet (S) is vertically stretched using a roll-shaped vertical stretcher (3) in the ratio three times more than a normal vertical stretcher and horizontally stretched in a tenter-type horizontal stretcher (4) in a ratio 3.5 times more than a normal tenter-type horizontal stretcher.When its heat is adjusted, a biaxially stretched film (F) with a thickness of 15 ym is wound.

Table 1 shows the stretchability of each manufactured film. The thickness deviation rate of each strecthed film depends on its relative viscosity which is produced by adjusting the voltage of the electrostatic impression device at 5.0 kV and the drawing speed of the roll-shaped rotary cooler at 20 or 40m/min according to Examples 1 to 6.

In Table 1, a relative viscosity ranging from 3.0 to 4.5 produced a film. having a satisfactory stretchability and a thickness deviation rate amounting to less than 10% Table 1 Electrostatic impression voltage @ 5.0 KV

Thickness Example Relative Drawing speed of roll- Stretch- variation rate Notes viscosity shaped rotary cooler ability of streiched (m/min) film % 1 2.0 20 Broken 40 " 2 2.5 20 Satifactory 23.5 Eroken 3 3.0 20 Satifactor 12.8 40 " 9.9 4 3.5 20 " 5.3 40 " 4.2 5 4.0 20 " 3.8 40 " 7.9 6 4.5 20 " 6.7 More 40 " 13.1 loads * hhen the thickness deviation rate of the stretched film amounts less than 10%, it is satisfactory.

Examples 7 to 13 Desiccation, electrostatic impression and stretcher conditions are as shown in Table 1. An unstretched sheet and a stretched film are respectively manufactured by adjusting the impression voltage at 10.0 kV and the drawing speed of the roll-shaped rotary cooler at 5 to 70m/min for homogenized polyamid poly -C- caproamide resin having a relative viscosity of 3.5. The extent, to which melted polymer resin is adhered on the surface of the roll-shaped rotary cooler determines the thickness deviation rates, strechability of unstretched sheeting and the thickness deviation rates of stretched film (Table 2). The drawing speed of unstretched sheet amounting to more than 20m/min within the limits of 4.0 to 1.0 kV is shown.Satisfactory stretchability and the thickness deviation rates of stretched film amounting to less than 108 are also exhibited.

Table 2 poly -#- caproamid resin having a relative viscosity amounting to 3.5 is used.

Drawing speed condition in which Thickness devia- Thickness deviation Example voltage of roll-shaped it is glued to the tion cate of un- Stretchability rate of stretched Notes rotary cooler surface of coll- stretched sheet film (%) (m/min) shaped rotary cooler (%) 5 o 5.9 Broken 7 3.0 kv 10 x 15,8 " 20 x 29.2 " 5 x 6.2 Satisfactory 19.2 8 3.5 kv 10 o 5.8 " 16,3 15 x 7.2 " 15.2 20 x 9.9 Broken 10 o 2.3 Satisfactory 5.2 20 o 3.6 " 6.1 9 4.0 kv 30 o 2.1 " 5.7 40 o 4.1 " 7.2 50 x 8.3 " 9.1 60 x 8.2 Broken 10 o 2.1 Satisfactory 4.1 20 o 4.2 " 4.9 10 6.0 kv 30 o 2.6 " 3.8 40 o 3.1 " 7.8 50 o 3.2 " 6.2 60 x 7.8 Broken 10 o 3.2 Satisfactory 6.6 20 o 2.2 " 5.1 11 8.0 kv 30 o 4.5 " 7.3 40 o 4.1 " 7.8 50 o 3.8 " 8.1 60 x 3.9 " 8.8 70 x 8.3 Broken

10 o 6.4 Broken 20 x 12.1 " 30 o 4.6 Satisfactory 9.6 12 10.0kv 40 x 8.8 " 20.2 50 x 13.5 Broken 25.1 A spark took plac 60 x 20.3 " - " 70 x 25.2 " - " No ele13 ctric 5 - 30 x More than 2 Broken charge * The thickness deviation nate of unstretched sheet is satisfactory if it amounts to less than 5.0% and the thickness deviation rate of stretched film is satisfactory if it amounts to less than 10.0%.

The present invention is thus very effective for the manufacture in a highly efficient way of a biaxially stretched poly -c- caproamide film, which is uniform in thickness, excellent in transparency, and low in crystallization. The degree of crystallization is lessened by fixing the homogenized polyamide poly -c- caproamide sheet which uses a single polymer of -c- caprolactam above its relative viscosity of 3.0, desirably, within the limits of 3. to 4.5.

The degrees of polymerization are adjusted by the electrostatic adhesion method in which a 4.0 to 10.0 kV high voltage and low current adheres the resin material impressed between the surface of a roll-shaped rotary cooler and wire-shaped single sytem electrode to the rotary cooler.

Claims (6)

Claims:

1. A process for the manufacture of a biaxially stretched poly - E- caproamide film wherein, prior to biaxial stretching by a consecutive biaxial stretching method, an unstretched sheet of poly -s- caproamide film is formed by heating the poly - E- caproamide polymer above its melting point, extruding it and electrostatically adhering it to the surface of rollshaped rotary cooler by means of an electrode impressed with 4.0 to 10.0 kV high voltage and low current, followed by hardening upon cooling, the relative viscosity of the poly - E- caproamide polymer of -caprolactam being above 3.0.

2. A process as claimed in claim 1 wherein the relative viscosity of the poly - E- caproamide is from 3.0 to 4.5.

3. A process as claimed in claim 1 or claim 2 wherein the electrode is a single system wire electrode.

4. A process as claimed in claim 1 or claim 2 wherein the electrode is a knife-shaped or multineedle impression electrode.

5. A process as claimed in claim 1 substantially as hereinbefore described.

6. A process as claimed in claim 1 substantially as hereinbefore described and with reference to the accompanying drawings.

6. A process as claimed in claim 1 substantially as hereinbefore described and with reference to any of the Examples.