JPH0139903B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a polyethylene sheet that has significantly improved impact strength, particularly impact resistance at low temperatures, is free from anisotropy, and has excellent formability and dimensional stability at high temperatures. [Prior Art] Polyethylene sheets produced mainly from polyethylene by melt extrusion have a wide variety of properties depending on a wide variety of changes in the molecular weight, crystallinity, crystal structure, density, etc. of the polyethylene used. Widely used in various containers, automobile parts, electrical appliance parts, packaging materials, etc. For example, sheets made of low-density polyethylene have low crystallinity and are excellent in processability and impact resistance, but have the drawbacks of low elastic modulus, strength, and heat resistance. On the other hand, sheets made of high-density polyethylene have good crystallinity and therefore have good elastic modulus and strength, but poor impact resistance. Sheets made of so-called ultra-high molecular weight polyethylene, which has a molecular weight higher than 1 million, have attracted attention for their high impact resistance, especially at low temperatures.
Its uses are gradually expanding. However, since the ultra-high molecular weight polyethylene has a high molecular weight, it has the disadvantage that it is extremely difficult to mold. [Problems to be Solved by the Invention] The object of the present invention is to provide a sheet that has strength, elastic modulus, and formability that are at least higher than conventional sheets made of high-density polyethylene, and that has impact resistance that is at least as good as sheets made of ultra-high molecular weight polyethylene. The objective is to provide a polyethylene sheet that is equivalent to the above. [Means for Solving the Problems] The present inventors conducted various studies to solve the above problems, and found that the sheet has specific physical properties that can be obtained under conditions completely different from conventional sheet manufacturing methods. The inventors have recognized that the sheet achieves the above object, leading to the present invention. That is, the present invention provides the following sheet. 1) (a) Melt index is 0.05 to 10g/10
(b) When the sheet is heated above its melting point, the force parallel to the sheet surface is generated. Maximum value (Ftmax) of the value (Ft) integrated over the time
is 0.5Kg・min/cm ² or more and 20Kg・min/cm ² or less,
(c) The ratio of Ft (RFt) measured in mutually orthogonal directions within the plane of the sheet is within 10, and (d) The melting point (Tms) of the sheet and the recrystallization by heating and melting the sheet and leaving it to stand. The melting point difference (△
A polyethylene sheet characterized in that Tms = Tms - Tmr) is at least 3°C or higher. 2) The above-mentioned sheet, characterized in that the average notched Izo impact strength at 0°C is 40 kg·cm/cm or more. 3) The above-mentioned sheet having an average notched isot impact strength at -30°C of 30 kg·cm/cm or more. 4) The sheet described above is characterized in that the rate of change in thickness when heated to 85°C is 5% or less. 5) The above-mentioned sheet having a notched Izo impact strength ratio (RIzo) of 3 or less at 0° C. measured in mutually orthogonal directions within the sheet plane. Note that the physical property values described above are based on the measurement method defined below. (a) Melt index (hereinafter sometimes abbreviated as MI) Based on the method specified in JIS K6760. (unit:
g/10 minutes) (b) Melting point (Tm) Measure 5 to 15 mg of the sample using a differential scanning calorimeter (DSC).
The peak of the endothermic peak based on the melting of the resin crystals was measured by increasing the temperature at a rate of 10°C/min in a nitrogen atmosphere (if there are multiple peaks, the peak indicating the maximum endothermic amount The temperature corresponding to the peak of (c) Product of the force parallel to the sheet surface generated during heating and the time during which that force is generated (time integral value)
(Ft) Cut out a strip-shaped test piece 10 mm wide, 90 mm long, and as thick as the sheet from the sheet and use it as a sample. Using a tensile testing machine (Instron Universal Testing Machine) equipped with a constant temperature chamber that maintains the inside of the tank at 150°C, the sample was inserted with a distance between chucks of 30 mm, and the test was performed without tension (by moving the movable crosshead). Measures the force that causes the sample to shrink in the longitudinal direction due to heat and the time during which that force is generated.On the Instron universal test chart, the time during which the force is generated on the horizontal axis, and the contraction on the vertical axis. The peak area corresponding to the force is calculated, and the value obtained by dividing the peak area by the cross-sectional area of the test piece is Ft.The number of test pieces to be measured is at least 3.
or more, and Ft is expressed as the average value. (d) Maximum value of Ft (Ftmax) Obtain the Ft value for each direction within the sheet plane, and calculate the value showing the highest Ft among them.
Let it be Ftmax. (e) Ratio of Ft (RFt) When the Ft value is measured in the direction perpendicular to the direction showing the above Ftmax in the sheet plane and is set as Ftmin, RFt is calculated by the following formula. RFt＝Ftmax／Ftmin (f) Melting point difference (△Tms) Tm above the melting point (Tms) of a part of the sheet
Measure according to the measurement method. On the other hand, the remeasured melting point (Tmr) of the sheet was 150°C.
After heating and melting the sheet as described above, the sheet was sufficiently cooled from the top and bottom using a water-cooled cooling plate, and left at room temperature for at least 48 hours to fully crystallize the sheet. Measure. The aromatic difference (ΔTms) is calculated by the following formula. △Tms=Tms−Tmr (g) Notched Izot impact strength (z) Measured according to ASTM D256. (h) Average notched isot impact strength (z) Measure the notched isot strength in each direction within the sheet plane. the highest value among them
Let Izmax be the notched izot impact strength in the direction orthogonal to it in the sheet plane.
Define Izmin and its average value as z. (i) Izod impact strength ratio (RIz) Measure Izmax and Izmin using the method described above, and calculate by the following formula. RIz=Izmax/Izmin (j) Rate of change in sheet thickness Measure the thickness of the sheet at at least four measurement points selected from the points evenly divided over the entire surface of the sample. The sheet is then heated at 85° C. for 1 hour, then allowed to cool to room temperature, and after at least 24 hours, the thickness is measured at the same point. The rate of change in sheet thickness is expressed in % using the following formula. Thickness change rate (%) = Thickness after heating - Thickness before heating / Thickness before heating x 10
0 The ethylene polymer in the present invention refers to a polymer in which 70% by weight or more, preferably 85% by weight or more of the constituent units thereof are ethylene units, such as low density polyethylene, high density polyethylene, propylene, 1- Random or block copolymers of olefins with values other than ethylene such as butene and 1-hexene and ethylene, copolymers of ethylene and diene monomers such as butadiene, and polar monomers such as ethylene and N-vinylcarbazole. It is a copolymer with a polymer. A particularly suitable ethylene polymer is high density polyethylene having a specific gravity of 0.94 or more. Here, the melt index of the polymer is 0.05~
It is important that it is within the range of 10g/10 minutes. The polymer whose impact resistance is significantly improved in the present invention is required to have a sufficiently high molecular weight, and melt index is suitable as a measure of this. If a polymer with a melt index larger than this range is used, it will not be possible to obtain the preferred crystalline and amorphous states of polyethylene molecules described below, and the impact resistance of the sheet will not be sufficiently increased even if the manufacturing method described below is used. I can't let you. On the other hand, when a polymer having a melt index smaller than this range (ultra-high molecular weight polyethylene falls under this category) is used, moldability becomes significantly poor. Furthermore, it is clear that if the proportion of ethylene units in the polymer decreases beyond 70% by weight, it will no longer be possible to satisfactorily satisfy the parameters considered to be due to the crystal structure of polyethylene, which will be described later in the present invention. The first feature of the sheet of the present invention is the maximum value (Ftmax) of the time-integrated value (Ft) of the force parallel to the sheet surface that occurs when the sheet is heated above its melting point.
is 0.5Kg・min/cm ² or more and 20Kg・min/cm ² or less. The Ft value is a physical property value that expresses the state of the amorphous portion of polyethylene molecules in the sheet, and a low value indicates that the orientation of the molecules in the amorphous portion is small. In other words, it is considered that the molecular structure of the amorphous portion has a structure that easily absorbs impact energy. Originally, it is thought that the smaller this value is (that is, the smaller the molecular orientation value), the greater the impact resistance, but in reality there is a lower limit, and below that value, the impact resistance decreases. The cause is not clear. In other words, the Ftmax value is 0.5Kg・
If it is less than min/cm ² , the impact resistance of the sheet will decrease. Furthermore, if the Ftmax value exceeds 20 Kg·min/cm ² , the orientation of molecules in the amorphous portion becomes large and the impact resistance of the sheet is extremely reduced. The Ftmax value is 0.5~
It is preferably in the range of 1.5 Kg·min/cm ² , particularly 1 to 10 Kg·min/cm ² . The second feature of the sheet of the present invention is its melting point. The sheet of the present invention has a melting point difference (△
Tms) is at least 3°C or higher, and for resins with sufficient performance it is 5°C or higher, preferably 8°C or higher. That is, the resin crystals in the sheet of the present invention are characterized in that their melting point decreases when melted. If the melting point difference is less than 3° C., dimensional stability at high temperatures will not be improved. Another feature of the sheet of the present invention is that the sheet has little directionality (anisotropy) as defined by the RFt value. Sheets with a large RFt value can be obtained by conventional roll rolling methods, but although the roll rolled sheets have sufficient impact strength in a specific direction, their performance in the direction perpendicular to that direction is generally extremely insufficient. Or, the strength against the force perpendicular to the plane is extremely low in the falling ball impact test specified in JIS K 7211 and the penetration test specified in JIS T 8131. Directionality (anisotropy) of the sheet
is the ratio of the notched isot impact strength mentioned above RIz
It can also be easily confirmed by In the present invention, the materials are limited as described above.
By specifying the Ftmax value, △Tms value, and RFt value, the average notched isot strength ( _zp ) at 0℃ is 40Kg・min/cm2 or ^more , and especially the average notched isot strength ( _z-30 ) at -30℃ is 30Kg・
A sheet with significantly improved impact strength, especially at low temperatures, of cm/cm or more can be obtained. In the present invention, by limiting these physical property values to more preferable conditions, _{the zp} value is 70 Kg·cm/cm or more, particularly 90 Kg·cm/cm or more, and _{the z-30} value is 35 Kg·cm/cm.
cm/cm or more, especially 45Kg/cm/cm or more, which is impossible to imagine with conventional common sense. A more preferred embodiment of the sheet of the present invention is achieved by defining the rate of change in thickness when heated to 85° C. to be 5% or less. One of the major drawbacks of products produced by conventional stretching and rolling techniques is that if they are left at room temperature for a long period of time or exposed to high temperatures for a short time after being stretched or rolled, they tend to return to their original thickness before stretching or rolling. (so-called springback phenomenon), and the sheet or its molded product has a drawback of very poor dimensional stability. The rate of change in thickness as used in the present invention is a measure by which the springback phenomenon can be quantitatively understood, and it can be said that the smaller the rate of change in thickness is, the smaller the springback is. As seen in the present invention, when Ft is small and ΔTm exhibits a large value, the rate of change in thickness generally becomes significantly small, and the morphological stability at room temperature and dimensional stability at high temperatures are improved. In addition, sheets that satisfy the above physical property values have good formability, and do not have "sag", which is a serious obstacle when manufacturing large molded products by thermoforming such as vacuum forming or pressure forming, in ordinary polyethylene sheets. Furthermore, there is no problem with poor mold conformation as seen in stretched and rolled sheets other than those of the present invention. The sheet of the present invention is further characterized in that the degree of crystallinity of the polymer constituting it is unexpectedly relatively high. Generally, in order to improve the strength and elastic modulus of sheets and molded products, the crystallinity of the polymer is usually lowered to improve impact resistance, but the sheet of the present invention has the above-mentioned physical properties. To the extent that it is satisfactory, the impact resistance is very high despite the higher than usual crystallinity. Therefore, the sheet of the present invention is characterized by improved impact resistance as well as improved strength and elastic modulus. The crystallinity is generally 55% or more. The degree of crystallinity is also improved when the △Tm is determined and compared with the degree of crystallinity after heating and melting the sheet under the same conditions and allowing it to cool.It has improved by more than 3% or 5%. There are often The present invention does not require special additives or compounding agents to increase impact resistance, nor does it require modification of the molecular structure of polyethylene, so it has the mechanical strength and electrical properties originally possessed by high-density polyethylene. It has the characteristic that properties such as water resistance and chemical resistance are maintained as they are. The sheet in the present invention is a sheet used in the usual sense, and refers to a sheet with a thickness of 100 μm to 100 mm, but in the present invention, a sheet with a thickness of 200 μm to 30 mm, particularly
A sheet having a thickness in the range of 200 μm to 10 mm is preferable because it can easily obtain the above-mentioned specific physical property values. Next, an example of the manufacturing method of the sheet of the present invention will be described. First, a raw material sheet with very little orientation is produced from a polyethylene polymer having an MI of 0.05 to 10 g/10 minutes using a sheet forming machine consisting of an extruder equipped with a T-shaped die, a cooling roll, and a take-off device. In order to obtain a raw material sheet with low orientation, it is necessary to set the ratio between the lip interval of the die and the thickness of the resulting raw material sheet (generally called the draw ratio) within a very specific range, and this draw ratio is 1.5 to 1.5. It is between 2.0. In order to obtain a raw material sheet with less orientation, it is desirable to laminate several to several dozen raw material sheets and melt them at a temperature above the melting point to form a flat raw material block. Since the raw material sheet (or raw material block) is simply an extruded sheet, the above-mentioned Ft and △
Tm is small and notched isot impact strength is also low. The raw material sheet (or raw material block) is heated to a temperature in a very narrow range above the softening point, especially near the melting point, and then rolled at a high rolling ratio over a sufficient period of time to achieve desired physical properties such as impact resistance and other properties. An improved sheet is obtained. If the rolling temperature is too high, even if the rolling time and other conditions are selected, the desired physical properties, especially the Ft value of the obtained sheet, will not be sufficiently large, and therefore a sheet with high notched Izo impact strength will not be obtained. On the other hand, if the rolling temperature is low, the Ft value may become too large or △Tm
is too small, resulting in low impact resistance and poor dimensional stability, moldability, and shape stability. The rolling temperature
Under the conditions defined by so-called cold rolling, which is below the temperature at which crystalline melting of the polymer begins as determined by DSC analysis, it is extremely difficult to obtain the sheet of the present invention no matter what other conditions are set. The preferred rolling temperature is within an extremely narrow temperature range of several degrees Celsius above and below the melting point (Tm) of polyethylene. Rolling time is an important factor to obtain the sheets of the present invention. In the case of instantaneous rolling such as conventional roll rolling, even if the rolling temperature is near the most preferred melting point, the preferred Ft value and melting point difference (△Tms)
It is extremely difficult to obtain a sheet of the present invention that has both of the following. Therefore, the sheet not only does not have sufficient impact strength, but also has poor dimensional stability and moldability. To obtain the sheet of the present invention, a substantial rolling time of 1 minute or more is required, preferably 2 minutes or more. The longer the time required for rolling, the easier it is to obtain the sheet of the present invention. In particular, in order to obtain a sheet with an Ftmax value of 10Kg・min/ ^cm2 or less, 3
A rolling time of more than 1 minute is required. In general, a certain range should be selected in consideration of economics and sheet deterioration, and in this respect the preferred rolling time is 2 minutes to 60 minutes.
minutes, especially between 3 and 30 minutes. The rolling ratio defined by the thickness of the sheet before forming and the thickness of the sheet after rolling is also an important condition.
If the sheet is not rolled, the desired sheet will not be obtained no matter how long it is left near the melting point. There is a lower limit to this point rolling ratio. On the other hand, if the rolling ratio is increased unnecessarily, the Ft value will increase, resulting in a decrease in isot strength, dimensional stability, and formability. Since rolling has a tendency to elongate molecular chains, the rolling ratio should be kept to the minimum necessary. From these viewpoints, the desired sheet can be obtained by setting the amount in the range of usually 2 to 15 times, preferably 3 to 10 times. A flat press such as a hydraulic press or a mechanical press is preferred as an apparatus for manufacturing the sheet as described above, and roll rolling is not preferred for reasons such as temperature control and insufficient rolling time. If the sheet of the present invention is exposed to a temperature higher than its melting point for a long time during or after rolling, the microstructure of the sheet will be destroyed, and physical properties such as impact resistance will inevitably deteriorate. The sheet of the present invention can be improved by conventionally known methods as long as the physical properties of the present invention are not impaired. For example, by blending with other resins,
In addition, small amounts, specifically about 20% by weight or less, of fillers, colorants, pigments, flame retardants, heat stabilizers, anti-degradants, clarifying agents, antistatic agents, lubricants, fluorescent agents, bactericidal agents, etc. It is also possible to incorporate these ingredients within a range that does not impair the effects of the present invention. The sheet of the present invention can be used as is for conventional polypropylene sheet applications by being processed as necessary using known sheet forming methods, but it is especially suitable for use in fields that utilize its impact resistance properties. For example, there are helmets, containers, pallets, transportation equipment parts including automobile parts, electrical parts, building materials, etc. [Example] The present invention will be explained in more detail with reference to Examples and Comparative Examples below. The test methods and physical property values used in the Examples are as follows, except for the measurement method defined above. be. Tensile strength: Compliant with ASTM D638. Bending strength: Compliant with ASTM D790. Flexural modulus: Compliant with ASTM D790. Penetration resistance: Compliant with JIS T8131. Among those that failed, those in which the weight completely penetrated were indicated as broken. Formability: Pressure forming was performed, and the formability was evaluated based on three items: ``sagging'' of the sheet during heating, conformity to the mold, and thickness unevenness of the molded product. Examples 1-5 and Comparative Examples 1-5 MI=0.5g/10min high-density polyethylene (specific gravity
0.960) to a thickness of 2 mm (draw ratio = 1.75), and a material sheet with a width of 180 mm was obtained. The material was cut to a length of 180 mm and 2 to 13 sheets were laminated to obtain a raw material block having a thickness of 3 mm to 30 mm using a hydraulic press. The melting point of this raw material block was 132°C. The raw material block was rolled using a hydraulic press under the conditions shown in Table 1, and then cooled for 5 minutes under pressure using a water-cooled cooling press to obtain a sheet. The result is the first
As shown in the table, in all of the examples that satisfied the Ftmax value, RFt value, and △Tms, sheets with significantly improved not only the average notched Izot impact strength at room temperature but especially the impact resistance at low temperatures were obtained. There is. and the sheet has other strength,
The sheet had good moldability, with no decrease in flexural modulus and no "sagging" during thermoforming. When the material sheet before rolling (the sheet shown in Comparative Example 1 is the same as a normal extrusion sheet) or the rolling temperature is extremely high (Comparative Example 2), the Ftmax value and △Tms are almost 0, and the impact resistance is poor. It has low properties and penetration resistance, and sagging occurs during molding. The sheet of Comparative Example 3, whose rolling temperature was 138℃, is satisfactory in △Tms compared to Comparative Example 2, but the △Ftmax value is still low, and although the impact strength is slightly increased compared to the material sheet, the value is not sufficient and it is difficult to form. Sex is also bad. In Comparative Example 4 where the rolling temperature was 110° C., although the impact strength was improved, ΔTms was small, so the extent of the change was small and the rate of change in thickness was high, resulting in poor formability. Comparative Examples 5 to 6 Using the same material block as in Example 1, using Daito Seisakusho's DBRR-50 small roll rolling machine instead of the flat plate press, rolling was completed 10 times (within 10 seconds of actual rolling time). After rolling, it was cooled in a cooling press to obtain a sheet with a thickness of 2 mm. Table 1 shows the physical properties of the obtained sheet. The sheet obtained in Comparative Example 5 had large anisotropy and poor impact strength and formability. Further, in Comparative Example 6 where the rolling temperature was 135°C, the sheet adhered to the rolling rolls and a good sheet could not be obtained. Examples 6 and 7 and Comparative Example 7 Sheets with a thickness of 2 mm were produced according to Example 1 using polyethylene exhibiting various MI values. Table 2 shows the physical properties of the sheet. Example 8 A sheet with a thickness of 2 mm was produced in the same manner as in Example 1, except that MI = 20 g/10/1-butene-ethylene copolymer (ethylene content: 95% by weight) was used. The results are shown in Table 2.

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Claims (1)

[Scope of Claims] 1 (a) The sheet is made of an ethylene polymer having a melt index of 0.05 to 10 g/10 minutes and in which 70% by weight or more of the constituent units are ethylene units, (b) The sheet has a melting point of The maximum value (Ftmax) of the value (Ft) that is obtained by integrating the force parallel to the sheet surface generated when the sheet is heated over the time that the force is generated is 0.5Kg・min/
cm ² or more and 20 Kg・min/cm ² or less, (c) the ratio of Ft (RFt) measured in mutually orthogonal directions within the sheet plane is within 10, and (d) the melting point (Tms) of the sheet
and the melting point (Tmr) re-measured on a sheet recrystallized by heating and melting the sheet and allowing it to stand, the melting point difference (△Tms=Tms−Tmr) is at least 3°C or more. sheet. 2. The sheet according to claim 1, wherein the sheet has an average notched isot impact strength at 0° C. of 40 Kg·cm/cm or more. 3. The sheet according to claim 1 or 2, wherein the sheet has an average notched isot impact strength at -30°C of 30 kg·cm/cm or more. 4 The rate of change in thickness when the sheet is heated to 85℃ is 5
% or less, the sheet according to claim 1, 2, or 3. 5. Claims 1, 2, 3, or 4 in which the ratio (RIzo) of notched Izo impact strength at 0°C measured in directions perpendicular to each other within the sheet plane is within 3. Sheet with section description.