JPS6192827A - Polyoxymethylene oriented molded shape - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel stretched polyoxymethylene article.

We have submitted the crystalline polymer to a heat treatment under conditions that substantially reduce the likelihood that more than a given number of molecular chains (i.e., likely more than two) will be incorporated into crystalline lamellae. discovered that a high modulus polymeric material having a Young's modulus of greater than 3 X 1 O''N/m2 can be obtained by stretching the polymer at a speed and temperature such that the molecules become almost perfectly linear.

That is, the present invention relates to a high modulus oriented polyoxymethylene stretched molded product having a weight average molecular weight of less than 300,000 and a Young's modulus of greater than 3XIO"N7m".

As used herein, a crystalline polymer refers to a polymer that can form a crystalline or semi-crystalline structure when its melt is cooled.

The crystalline polymer should have a suitably high weight average molecular weight so that the shaped article after stretching retains favorable physical properties in terms of strength and elongation at break.

However, a high concentration of long chain moieties increases the chances that a given molecular chain will be introduced into -more crystal lamellae before stretching. Hereinafter, in this specification, such a molecular chain will be referred to as an interlamellar molecular chain.

Although the present invention is not limited by any particular theory, it is believed that an excess of interlamellar chains reduces the degree of molecular orientation and linearization obtained by stretching due to increased permanent morphological entanglement, resulting in an optimal It is thought that this prevents the acquisition of physical properties.

The preferred average heavy metal molecule ffi (Mw) of the polymer is 5
0°000 to 250,000, more preferably 50.0
00 to 150,000, and the number average molecular weight (Mn) is 5,000 to 25,000, more preferably 5,000.
~]'5,000. Usually, in order to make the deformation at the molecular level more uniform during the elongation process, M
It is advantageous to avoid increasing the ratio of Mw to n. A preferable ratio of Mw to Mn is 30 or less. In particular, relatively narrow molecular weight distributions, e.g. Mnh'
10' or more, Mw/Mn is 10 or less (preferably 8 or less, most preferably 6 or less), and Mn is 104
When Mw/Mn is 25 or less (most preferably 15
Good results can be obtained using the following polymers:

Molecular weights quoted herein are determined by gel permeation chromatography.

Heat treatment causes, for example, a substantial portion of molecules of intermediate molecular weight to crystallize into discontinuous, highly ordered folded chain lamellae, while a small amount of very large and very small molecular weight molecules crystallize. In separating and, in rare cases, interconnecting crystalline regions, molecular weight fractionation has the effect of reducing the possibility of introducing excess molecular chains into one or more lamellae.

This effect can be obtained, for example, by the following three methods.

(1) A method of cooling a polymer at a predetermined rate from a temperature at or above its melting point to an ambient temperature at which a desired crystal structure can be obtained. In this case, the cooling rate depends on the molecular weight characteristics of the polymer. It is generally necessary to cool the polymer at a rate of less than 40°C per minute, preferably less than 20°C per minute. The most preferred cooling rate is 10° C. per minute or less.

(2) A method in which the polymer is cooled at a predetermined rate from a temperature at or above its melting point to a temperature below its crystallization temperature, and then rapidly cooled to ambient temperature. The cooling rate is preferably between 1 and 15 liters per minute.
°C, most preferably 2-10 °C per minute. Preferably, the polymer is rapidly cooled when the temperature reaches 5 to 20°C below the crystallization temperature. Quenching should preferably be at a rate of no more than 100,000°C per minute.

(3) A method of rapidly cooling a polymer from a temperature at or above its melting point to near its crystallization temperature and holding the polymer at that temperature for a sufficient period of time to cause crystallization. Preferably, the polymer is heated to a temperature within 15°C of the crystallization temperature.
Hold for 5 to 10 minutes.

The polymer is then preferably rapidly cooled to ambient temperature.

Optical micrographs of samples prepared by methods (1), (2) and (3) each show that a polymer with a given molecular weight characteristic has a macrostructure containing uniformly oriented regions of various sizes throughout the polymer. It shows that it is forming.

The conditions used in methods (1), (2) and (3) are selected based on the molecular weight characteristics of the polymer.

However, in general, if the polymer has a particularly narrow molecular weight distribution, extremely slow cooling (@ min 1'C or less) will not produce a crystalline structure suitable for elongation. Very fast cooling of more than 40°C per minute in methods (1) and (2) and holding longer than necessary at the crystallization temperature in method (3) have also been found to be detrimental. .

As an alternative method of forming a crystalline structure that substantially reduces the likelihood that a given molecular chain will be incorporated into one or more crystalline lamellae, the polymer can be heated from a temperature at or above its melting point to a temperature well below its crystallization temperature. There is a method of rapidly cooling to a low temperature to obtain a relatively low degree of crystallinity. This method, namely method (4)
This significantly reduces the size of the spherulites and produces a crystal-containing structure surrounded by large amorphous regions, thereby reducing the number of interlamellar chains. Preferably, the cooling rate is at least t/min, ooo°C, most preferably,
The temperature is 5000°C or more per minute throughout the crystallization temperature range, and the polymer is cooled to ambient temperature or below.

Depending on the molecular weight characteristics of the polymer, it may be advantageous to reheat the cooled polymer to increase the size of the crystalline regions prior to attenuation.

The optical micrograph of the sample produced by method (4) clearly shows that it has a string-like spherulite structure as in the conventional method, but the spherulites are clearly distinguished by small points. I can do it.

Structures formed with different heat treatment methods do not necessarily deform in the same way under a given condition. There may be an optimal elongation method for each structure. If there is an apparent molecular orientation in the unstretched material, elongation may be undesirably limited.

Polymer crystallization has been widely studied, and books such as "Crystallization of Polymers" (Chapter 8: Crystallization Kinetics and Mechanisms) by Elle Mandelkaan, published by McGraw-Hill in 1964. There is an explanation. By observing changes in properties such as density and specific volume, it is understood that crystallization occurs in stages. There is a time delay until crystallization is observed, but as soon as it is observed, it proceeds naturally with almost autocatalytic acceleration. Eventually, the crystallization reaches a state of pseudo-equilibrium, after which crystallization proceeds very slowly over a long period of time to a small but discernible amount. This crystallization process is continuous, and a plot of the rate of crystallization versus time is sigmoidal, with no abrupt changes or discernible discontinuities.

The rapid crystallization is called primary crystallization, and the slow crystallization stage is called secondary crystallization. Secondary crystallization (i) is believed to be disadvantageous in obtaining high modulus, and while some secondary crystallization can occur in some polymers and still provide favorable properties upon elongation. , it has been found that the highest modulus is obtained when the process is dominated by secondary crystallization.At least for methods (1), (2) and (3), it is usually rapidly It has been found to be advantageous to allow the crystallization to proceed until the cut-off crystallization is substantially complete.The extent of crystallization progress can be tracked by measuring the density of the polymer.

In methods (1), (2) and (3), the maximum elongation achievable (maximum modulus obtained) for a given heat treatment and given polymer is an increase in density until the polymer density reaches an optimum. increases with However, increasing the polymer density above the optimum density will reduce the maximum elongation obtained.

By attenuating samples of different densities under the same conditions, it is possible to determine the optimal polymer density for a given attenuation condition.

After heat treatment, the polymer is stretched at a temperature and at a rate such that the deformation is at least 15, preferably at least 20. It is believed that the high degree of elongation necessary to obtain high modulus is achieved by uniform stretching and consequent orientation of the polymer, which corresponds at the molecular level to the degree of unfolding of the molecules in the crystalline lamellae.

A particularly preferred method of elongation is one in which the tensile force is less than or equal to the bow length force of the polymer, but at a rate sufficient to cause linearization of the molecules by causing deformation in the plus deck greater than the elongation that would result from flow stretching. The polymer is stretched to a high stretching ratio at a temperature of . Preferably the draw ratio is at least 20.

Optimum stretching conditions depend to an extent on the nature of the polymer and its thermal history. In general, polymers are preferably stretched relatively slowly, for example, at a rate of 1 cm or more per minute between relatively movable clasps, typically on the order of lO to 20 cR per minute, at a temperature of at least 40°C below the melting point of the polymer. or stretched on a stretching frame at a temperature of 5 to 20° C. below the melting point at a speed of 30 to 150 cm per minute.

The physical properties of the polymeric material may be further improved by stepwise stretching, with pauses in the polymer between each step, and intermittent stretching.

Preferably, polymers with relatively small cross-sections are subjected to a stretching process, and the invention is particularly suited to the production of fibers and films. In particular, continuous filaments are produced by melt spinning or by drawing in a drawing frame-1. For convenience, the fiber diameter or film thickness in the stretching 11q is preferably 1 mm or less.

In this specification, the deformation ratio or stretching ratio is defined as the ratio of the length after treatment to the length of the treated surface, or the ratio of the cross-sectional area after stretching to the cross-sectional area of the stretched acid part.

According to the method of the present invention, the Young's modulus defined below is 3X I
O1ON/m” or more, in some cases at least 6 X
It is possible to produce a polyoxymethylene stretched molded product of l 01 ON / m'. Since the Young's modulus of a polymeric material depends to a large extent on the measurement method, in this specification, the Young's modulus is determined by a dead-loading creep test.
It is defined as the measured modulus at 21'C when % tension is applied for 10 seconds. This test method was developed by Gupta and Ward: Journal of Macromolecular Science and Φ Physics, B1.373 (1967).
It is written in ゛.

It can be seen that the method of the invention results in polymer molecules that are substantially perfectly linearly arranged upon plastic deformation. This molecular orientation is often -axial, but with appropriate stretching a biaxial orientation can also be obtained in the polymer. Substantially complete orientation can be determined by physical measurement methods such as X-ray diffraction analysis and nuclear magnetic resonance analysis.

The polymeric material according to the invention is manufactured in a monolithic structure. The present invention will be explained below with reference to Examples.

Example 1 Polyoxymethylene (Delrin 500, a product of DuPont, Mn: 45.000. Mw/Mn+ slightly larger than 2) was heated in air at 180° C. or below at a ram speed of 0.
, 6 cm 1 minute, winding speed 36 cm/min, diameter 0
．． Isotropic filaments with a diameter of 0.04-0.06 cm are obtained by melt spinning through a 2cIR die. The take-up spool was positioned at a distance of about 7 cm from the die.

The resulting filament was drawn on a drawing frame in two stages at a winding speed of about 50 ca+/min. The conditions at that time are shown in Table 1.

Table-1 *) Measured from the movement of marks placed on the surface of the specimen.

Table 2 shows the 10 second Young's modulus determined by dead-loading creep tests at various temperatures.

Table-2 Patent applicant: National Research Development・

Claims (1)

[Claims] 1. High modulus oriented polyoxymethylene stretched molded product having a weight average molecular weight of less than 300,000 and a Young's modulus of greater than 3×10^1^0 N/m^2.