JPS6131235A - Polyethylene sheet having excellent impact resistance - Google Patents

Abstract

PURPOSE:To equalize the impact resistance of polyethylene sheets with that of superhigh molecular weight polyethylene sheets as well as to raise the strength, elasticity modulus, and moldability of the polyethylene sheet beyond those of high-density polyethylene sheet by a method in which a specific ethylenic polymer is stretched at a specific temperature for a specific time period. CONSTITUTION:A raw material polyethylene sheet having 70wt% or more ethylene unit and 0.05-10g/10min melt index is stretched with a high draw ratio for 2-60min after heated in temperature ranges above or below the melting point to form a product sheet. When the product sheet is heated to above the melting point, the maximum value of the value Ft integrated by the time of generating the horizontal force to the sheet face is 0.5-20kg.min/cm<2>, and the ratio of the value Ft in the orthogonal direction becomes less than 10. The melting point of the sheet recrystallined after allowed to stand and melted by heating becomes higher by 3 deg.C or more. The average Izot impact (with notch) strength at 0 deg.C is 40kg.cm/cm or more, and 30kg.cm/cm or more at -30 deg.C, and the changing rate of thickness at 85 deg.C is 5% or less, and Izot impact strength ratio (with notch) is within 3. In this way, the properties of the polyethylene sheets can be increased beyond those of high-density, superhigh molecular polyethylene sheets.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] 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.

[Conventional technology]

Polyethylene sheets, which are mainly manufactured from polyethylene by the melt extrusion method, have a wide variety of properties depending on the molecular weight, crystallinity, crystal structure, density, etc. of the polyethylene used, and can be used for a wide variety of containers, automobiles, etc. Widely used for 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, are attracting attention for their high impact resistance, especially at low temperatures, and their 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 problems]

The present inventors have conducted various studies to solve the above problems, and have found that a sheet with specific physical properties obtained under conditions completely different from conventional sheet manufacturing methods can achieve the above objectives. This recognition led to the present invention.

That is, the present invention provides the following sheet.

1) (a) Melt index is 0.05 to 10
F/10 minutes, and is made of an ethylene polymer whose constituent units are ethylene units of 70 or more by weight, (
b) The value obtained by integrating the force parallel to the sheet surface generated when the sheet is heated above its melting point over the time that the force is generated (the maximum value of F (F t -max) is 0.5 k)
F・min/− or more and 20 ume・min/clA or less, (c)
Ft measured in mutually orthogonal directions within the sheet plane
The O ratio (RFt) is within 10, and (d) the melting point difference between the melting point (h→) of the #sheet and the melting point (Tmr) re-measured for the sheet that was heated and melted while standing and then recrystallized (Tmr). 02) The above-mentioned sheet, characterized in that the average notched Izo impact strength at 0°C is 4Qkf-cm/cIR or more.

3) The sheet described above, having an average notched Izot impact strength at -30°C of 30 stories cfn/cIn or more.

4) The sheet described above, which has a thickness change rate of 5 inches or less when heated to 85°C.

5) Ratio of notched isot impact strengths at 0°C measured in mutually orthogonal directions within the sheet plane (RIz
The above sheet in which o) is within 3.

Note that the physical property values described above are based on the measurement method defined below.

(a) Melt index (hereinafter sometimes abbreviated as MI) According to the method specified in JIS K 6760. (Unit: 2
/10 minutes) (b) Melting point (Tm) Samples 5 to 15 were heated at a heating rate of 1037 minutes in a nitrogen atmosphere F using a differential scanning calorimeter needle (DS) to melt the measured resin crystals. It indicates the temperature corresponding to the apex of the endothermic peak (if there are multiple peaks, the apex of the peak indicating the maximum endothermic amount).

(c) Product of the force parallel to the sheet surface generated during heating and the time during which the force is generated (time integral value) (Ft) Sheet width: 10 m, length: 901111. A rectangular test piece with the same thickness as the sheet is cut out and used as a sample. Using a tensile testing machine (Instron universal testing machine) with a constant temperature tester that kept the inside of the tank at 150°C, the distance between the chucks was set at 30°C.
Insert the sample as a sample, and measure the force and time during which the sample shrinks in the longitudinal direction due to heat under no tension (without moving the movable crosshead). On the Instron universal test chart, the horizontal axis shows the time during which force is generated, and the vertical axis shows the peak corresponding to the contractile force.The area of this peak was calculated, and the peak area was divided by the cross-sectional area of the test piece. Let the value be Ft. The number of test pieces to be measured is at least 3, and Ft is the average value.〇(d) Maximum value of Ft ('Ftmax) Obtain the Ft value for each direction within the sheet plane, and find the highest value among them. Let Ftmax be a value indicating a high Ft.

(e) Ft ratio (RF't).

Measure the Ft value in the direction perpendicular to the direction indicating the above Ftmax in the sheet plane and calculate it as Ftmin.
RFt is calculated by the following formula.

RF, = ptmax Ftmin (f) Melting point difference (ΔTms) The melting point (Tms) of a part of the sheet is measured according to the above-mentioned method for measuring Tm. On the other hand, the remeasured melting point (Tm) of the sheet
r) The remaining pieces of the sheet are melted by heating to 150°C or higher, and then sufficiently cooled from the top and bottom using a water-cooled cooling plate,
The sheet is sufficiently crystallized by being left at room temperature for at least 48 hours, and then measured in accordance with the TQI measurement method described above. The melting point difference (ΔTms) is calculated using the following formula.

ΔTma = Tma - Tmr) Notched Izot impact strength (Iz) A8TM
Measure according to D 256.

(Average Notched Izot Impact Strength (Iz) The notched Izot impact strength is measured in each direction within the sheet plane. The highest value is taken as Izmax, and the notched Izot impact strength in the direction perpendicular to the sheet plane is determined as Izmax. is defined as Izmin, and its average value is defined as Iz.

(i) Izod impact strength ratio (RIz) Izmax and Izmin are measured by the method described above, and determined by the following formula.

Izmax RIz = - Izmin (5) Rate of change in thickness of sheet The thickness of the sheet is measured at at least four measurement points selected from the points evenly divided over the entire surface of the sample. The cough sheet is then heated at 85° C. for 1 hour, allowed to cool to room temperature, and then left for at least 24 minutes, after which the thickness at the same point is measured. The rate of change in the thickness of the sheet is expressed by the following equation.

In the present invention, the ethylene polymer means that 70% by weight or more, 8 or 85% by weight or more of its constituent units are dried chile 7
An XQ body consisting of units, such as low density polyethylene, high density polyethylene, propylene, 1-butene, l-
Copolymerization of ethylene with other olefins other than ethylene such as 7Sq, or copolymers of ethylene with diene monomers such as butadiene, ethylene and N
- Polar monomers such as vinylcarbazole, raw copolymers. A particularly preferred ethylene polymer is high density polyethylene having a specific gravity of 0.94 or more.

°Here, the melt index of the polymer is 0.0! ~1
It is important that the time is within the range of 0/10 minutes.

In the present invention, the polymer whose impact resistance is significantly improved is required to have a sufficiently high molecular weight, and the melt index is an appropriate measure of this.When using a polymer whose zero melt index is larger than this range, It is not possible to obtain the preferred crystalline and amorphous states of the polyethylene molecules described below, and even if the production method described below is the same, the impact resistance of the sheet cannot be sufficiently increased. On the other hand, when a polymer having a melt index smaller than this range (ultra-high molecular weight polyethylene falls within this range) is used, moldability becomes significantly worse. Furthermore, if the proportion of polyethylene in the polymer decreases beyond 70% by weight, it is clear that it will not 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 that the maximum value (Ftmax) of the time integral value (Ft) of the force parallel to the sheet surface that occurs when the sheet is heated above its melting point is 0.511.
min/d or more and 20 stories/min/cIA or less.

The Ft value is a physical property value expressing 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. Does the Fxmax value increase to 20 when the rolling ratio is increased in a normal extrusion molded sheet where the value is substantially zero or in so-called cold rolling where the rolling temperature is below the softening point of the polymer?
・If it exceeds minutes/-, the impact resistance will be extremely reduced. The Ftmax value is 05 to 15 kg・min/clA, especially 1 to
It is preferable that the S content be in the range of 10 Um/-.82 The characteristic of the sheet of the present invention lies in its melting point.

The sheet of the present invention has a melting point difference (ΔTm5) of at least 3 when compared with a resin sheet obtained by melting and recrystallizing the sheet.
℃ or more, 5℃ or more for resins with sufficient performance,
Preferably, the temperature is 8°C or higher. That is, the crystals of the resin in the sheet of the present invention have a characteristic that their melting point decreases when melted.

Another feature of the sheet of the present invention is that the sheet has little directionality (anisotropy) as defined by the RFt value. RFt
A sheet with a large value can be obtained by the conventional roll rolling method, but although the roll rolled sheet has sufficient impact strength in a specific direction, the performance in the direction perpendicular to the general RC is extremely insufficient. Shika, JIS K
7211 K falling ball impact test and JIS T
, 8131K has extremely low strength against forces perpendicular to the plane in penetration tests and other tests. The directionality (anisotropy) of the sheet can also be easily determined by the ratio RIz of the notched isot impact strength mentioned above. can be recognized.

In the present invention, as mentioned above, the materials are limited and Ftma
0℃ to specify x value, ΔTms value, RFt value
The average notched isot strength σ cage) is 40 Ume・
c! It is possible to obtain a sheet with markedly improved impact strength particularly at low temperatures, with an average notched isot strength (1) of n/cIn or more at -30 DEG C. of 30 to .mu.m. In the present invention, by limiting those physical property values to more preferable conditions, the Izo value can be reduced to 70).
$-α2 or more, especially 90 hakama/o++/cm,
■I value is 35kIl・m,/m or more, especially 45kg・
An/cm or more, which is unimaginable based on conventional common sense, is possible.

A more preferred embodiment of the sheet of the present invention is achieved by defining the rate of change in thickness to be 5 inches or less when heated to 85°C. 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 period of time after being stretched or rolled,
The sheet or its molded product has the disadvantage of very poor dimensional stability due to its tendency to return to the thickness before stretching or rolling (so-called spring bank phenomenon). The thickness change rate 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 thickness change rate is, the smaller the springback is. As a result of the present invention (when Ft is small and ΔTm shows a large value, the rate of change in thickness is generally significantly small, and the morphological stability at room temperature is
Dimensional stability at high temperatures is improved. In addition, sheets that satisfy the above physical property values have good formability, and "sag", which is a serious obstacle when manufacturing large molded products by thermoforming such as vacuum forming and pressure forming, is a problem with ordinary polyethylene sheets.
Moreover, there is no defect in pattern alignment 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 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, it is common to reduce the crystallinity of one aggregate to improve impact resistance, but in the sheet of the present invention, the above-mentioned physical property values 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 degree of crystallinity is generally 55% or more. The degree of crystallinity of the sheet was also found to be significantly improved when the ΔTm was determined by heating and melting the sheet under the same conditions and comparing the degree of crystallinity after being allowed to cool. There are often

The present invention does not require special additives or compounding agents to increase impact resistance, and does not require modification of the molecular structure of polyethylene. It has the characteristic that properties such as properties, water resistance, and chemical resistance are maintained as they are.

In the present invention, the term "sheet" refers to a sheet with a thickness of 100 μm to 100 sheets, which is used in the usual sense.
In the present invention, 200 μm to 3Qw, especially 200 μm to
A sheet having a length of 10111 m is preferable because it is easy to obtain the above-mentioned specific physical property values.

Next, an example of a good method for producing the sheet of the present invention will be described.

'First, a polyethylene polymer with an MI of 0.05 to 10 F/10 minutes is processed into a raw material sheet with very little orientation using a sheet forming machine consisting of an extruder equipped with a T-shaped die, a cooling roll, and a take-up device. Manufacture. In order to obtain a raw material sheet with low orientation, it is necessary to set the ratio between the lip distance 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. It is between 5 and 2.0. In order to obtain a raw material sheet with low 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 ΔTm are small;
The notched isot impact strength is also low. By heating the raw material sheet (or raw material block) to a temperature in a very narrow range above the softening point, especially near the melting point, and rolling it at a high rolling ratio for a sufficient period of time, desired physical properties, impact resistance, etc. A sheet with improved physical properties 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 resulting sheet, will not be sufficiently large, and therefore a sheet with high notched Izo impact strength will not be obtained. On the other hand, when the rolling temperature is low, the Ft value is too large or the ΔTm is too small, resulting in low impact resistance and poor dimensional stability, formability, and shape stability. It is extremely difficult to obtain the sheet of the present invention under the conditions defined by cold rolling, where the rolling temperature is below the temperature at which crystalline melting of the polymer begins as determined by DSC analysis. ,be. 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 preferable melting point, it is extremely difficult to obtain a sheet of the present invention that has both a preferable Ft value and a preferable melting point difference (ΔTmq). follow''-'C#
The sheet not only does not have sufficient impact strength, but also has poor dimensional stability and formability. To obtain the sheet of the present invention, a substantial rolling time of 1 minute or more is required, preferably 2 minutes or more. It is.

If the time required for rolling is long, it is easier to obtain a sheet according to a certain fundamental invention. In particular, F below 10F/min/cm2
A rolling time of 3 minutes or more is required to obtain a sheet with a tmax value. In general, a certain range should be selected in consideration of economics or sheet deterioration, and in this respect the preferred rolling time is between 2 minutes and 60 minutes, particularly between 3 minutes and 30 minutes.

The rolling ratio, which is defined by the thickness of the sheet before rolling 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 multiplier is increased unnecessarily, the Ft value will increase, resulting in poor isot strength, dimensional stability,
Formability decreases. Since rolling tends 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 plate 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.

When the sheet of the present invention is exposed to a temperature higher than its melting point during rolling or for a long time in a rolled body, the fine structure of the sheet is destroyed, which inevitably causes a decrease in physical properties such as impact resistance. I end up.

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, small amounts, specifically up to about 203 amounts, of fillers, colorants, additives, flame retardants, heat stabilizers, anti-degradants, clarifying agents, Antistatic agents, lubricants, fluorescent agents, bactericidal agents, etc. may also be added within the range that does not impair the effects of the present invention.

The sheet of the present invention can be processed as required by a known sheet forming method, so it can be used as is for conventional polypropylene sheet applications, but it can be used in fields where its impact resistance is particularly utilized. Common examples include helmets, containers, transportation equipment parts including pallets and automobile parts, electrical parts, and building materials.

〔Example〕

The present invention will be explained in more detail below using examples and comparative examples, and the test methods and physical property values used in the examples are as follows, except for the measurement methods defined above.

Tensile strength: Based on ASTM D 638.

Bending strength: Based on AS'l'M D 790 K1.

Flexural modulus: Based on ASTM D790.

Penetration resistance: Compliant with JIS T 8131.

The weight completely penetrated while failing. Items that were damaged were marked as destroyed.

Formability: “sagging” of the sheet when heated by air forming.
The moldability was evaluated based on three items: conformity to the mold and uneven thickness of the molded product.

Examples 1 to 5 and Comparative Examples 1 to 5 MI = 0.5 f/10 min high density polyethylene (specific gravity 0
．． 960) to a sheet forming machine consisting of a single-screw extruder with a screw of diameter 4 o x φ, a T-shaped die with a lip interval adjusted to 3.5 mm, and a sheet take-up machine with a cooling roll adjusted to about 80°C. Sa2-(draw ratio=1.75
), a material sheet with a width of 180 m+ was obtained. Two to 13 pieces of the raw material sheets were cut to a length of 180 mK and laminated to obtain a raw material block with a thickness of 3 mm to 30 cm using a hydraulic press. The melting point of this raw material block was 132°C.

The raw material block! After rolling using a hydraulic press under the conditions shown in Table I11, the sheet was cooled for 5 minutes while being pressurized using a water-cooled cooling press to obtain a sheet. The results are shown in Table 1,
In all of the examples satisfying the Ftmax value, the RF' value, and the ΔT m s, sheets were obtained in which not only the average notched Izot impact strength at room temperature but also the impact resistance particularly at low i1 was significantly improved. The sheet exhibited good moldability, with no decrease in strength or flexural modulus, and no ``sag'' during ripening.

When the material sheet before rolling (the sheet shown in Comparative Example 1, which is the same as a normal extrusion-molded sheet) or the rolling temperature is extremely high (Comparative Example 2), the Ftmax value and ΔTms are almost O, and the impact resistance is poor. , the penetration resistance is low and sagging occurs during molding.

The sheet of Comparative Example 3 whose rolling temperature was 138°C was the sheet of Comparative Example 2.
Although the ΔTma is satisfactory compared to the ΔFtmax value, the ΔFtmax value is still low, and although the impact strength is slightly increased compared to the material C), the value is not sufficient and the formability is poor.

In Comparative Example 4 where the rolling temperature is 110° C., although the impact strength is improved, ΔTms is small, so the extent of the change is small and the rate of change in thickness is high, resulting in poor formability.

Comparative Examples 5 to 6 Using the same material block as in Example 1, using a Daito Seisakusho DBR-5Q small roll rolling machine instead of the flat plate press machine, the rolls were passed 10 times (actual rolling time within 10 seconds)
After rolling with a cooling press, the sheet was cooled with a cooling press to obtain a sheet having a thickness of 21111 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. Also, the rolling temperature was set to 135
In Comparative Example 6 where the temperature was set at .degree. C., the sheet adhered to the rolling roll and a good sheet could not be obtained.

Example 6.7 and Comparative Example 7 Sheets with a thickness of 2 gm were produced according to Example 1 using polyethylene exhibiting various MI values. The physical properties of the sheet are shown in Table 2. Example 8 Ml = 2.0 P/1/10 Example except that a 1/10-butene-ethylene copolymer (ethylene content: 95% by weight) was used. A sheet with a thickness of 21 m was produced in the same manner as in Example 1.

The results are shown in Table 2.0

Claims (1)

[Claims] 1) (a) Melt index is 0.05 to 10 g/1
(b) The force parallel to the sheet surface that occurs when the sheet is heated above its melting point is The value (Ft
) maximum value (Ftmax) is 0.5kg・min/cm^2
(c) The ratio of Ft measured in mutually orthogonal directions within the sheet plane (
RFt) is within 10, and (d) the melting point (T
ms) and the melting point (Tmr) re-measured on a sheet obtained by heating and melting the sheet while standing, and recrystallizing the sheet (ΔTms=Tms-Tmr), which is at least 3°C higher. sheet. 2) The sheet according to claim 1, wherein the sheet has an average notched Izod 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 Izod impact strength at -30°C of 30 kg·cm/cm or more. 4) The thickness change rate when the sheet is heated to 85°C is 5%.
A sheet according to claim 1, 2 or 3, characterized in that: 5) Ratio of notched Izod impact strengths at 0°C measured in mutually orthogonal directions within the sheet plane (RI
Claims 1 and 2, in which zo) is within 3;
The sheet described in paragraph 3 or 4.