JP7792800B2 - Ethylene-vinyl acetate copolymer, and molded article, sheet, and foam containing the same - Google Patents

Description

The present invention relates to an ethylene-vinyl acetate copolymer, and a molded article, sheet, and foam containing the copolymer.

Ethylene-vinyl acetate copolymers offer excellent properties such as transparency, flexibility, mechanical strength, electrical insulation, weather resistance, and durability, and are used in applications such as artificial turf mats, automobile mat guards, and drainage hoses through injection molding and extrusion molding. They are also processed into single-layer or laminated films through inflation and T-die film forming, and are used in a wide range of industrial applications, including agricultural polyolefin films and automobile mudguard covers.

As an example of a film using ethylene-vinyl acetate copolymer, Patent Document 1 proposes an ethylene-vinyl acetate copolymer film that has high tear strength and heat retention despite having a high vinyl acetate content.

Japanese Patent Application Laid-Open No. 2004-161881

However, while ethylene-vinyl acetate copolymers improve in transparency, flexibility, and adhesiveness as the vinyl acetate content increases, the crystallization temperature decreases and releasability tends to decrease.

In recent years, as the range of applications for ethylene-vinyl acetate copolymer has expanded, the market has come to demand even more advanced performance. Specific requirements include high transparency to enhance design, and in applications such as drainage pipes, artificial turf, and automobile mudguard covers, where the material is exposed to stresses such as internal pressure while in contact with various liquids for extended periods, the material must not suffer from environmental stress cracking when left in chemicals under a certain level of stress; in other words, it must have high environmental stress cracking resistance (hereinafter also referred to as "ESCR"), and high falling weight impact strength.

Generally, methods for increasing ESCR include increasing the molecular weight of the ethylene-vinyl acetate copolymer or increasing the vinyl acetate content. However, increasing the molecular weight tends to reduce processability, while increasing the vinyl acetate content tends to reduce mold releasability, as mentioned above. Therefore, there is a demand for ethylene-vinyl acetate copolymers with excellent mold releasability and ESCR.

The ethylene-vinyl acetate copolymer film described in Patent Document 1 has room for improvement in terms of transparency and ESCR, because the vinyl acetate content at each molecular weight of the ethylene-vinyl acetate copolymer decreases as the molecular weight increases.

The present invention was made in consideration of the above-mentioned problems, and aims to provide an ethylene-vinyl acetate copolymer that has excellent impact strength, environmental stress crack resistance, mold releasability, and transparency, as well as molded articles, sheets, and foams containing the copolymer.

The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that the proportion of vinyl acetate units tends to remain constant or increase as the molecular weight distribution shifts from low to high molecular weight components, and that the above-mentioned problems can be solved by using an ethylene-vinyl acetate copolymer with a specified number of branches, which led to the completion of the present invention.

That is, the present invention is as follows.
[1]
Contains 11% by mass or more and 25% by mass or less of vinyl acetate units and ethylene units,
When the molecular weight distribution and the absorbance I _(-CH-) attributed to the methylene group, the absorbance I _(-C=O) attributed to the carbonyl group, and the absorbance I _(-CH) attributed to the methyl group were measured by GPC-FTIR,
a slope P of a least squares approximate linear relationship of the absorbance ratio ( _Ii(-C=O) /Ii _(-CH2-) ) to the logarithm log( _Mi ) of each molecular weight _Mi in the half width region of the molecular weight distribution is 0.00≦P≦1.40;
an average value Q of the absorbance ratio (I _i(-CH3) /I _i(-CH2-) ) of each molecular weight M _i in the half width region of the molecular weight distribution is 23.0≦Q≦30.0;
Ethylene-vinyl acetate copolymer.
[2]
The methyl branch (20.07 ppm) determined by ¹³ C-NMR is less than 0.16/100C.
The ethylene-vinyl acetate copolymer according to [1].
[3]
The heat of fusion (ΔH) determined by differential scanning calorimetry is 43 J/g or more and 90 J/g or less.
The ethylene-vinyl acetate copolymer according to [1] or [2].
[4]
The melt flow rate is 0.1 g/10 min or more and less than 30 g/10 min.
[1] The ethylene-vinyl acetate copolymer according to any one of [1] to [3].
[5]
[1] to [4], comprising the ethylene-vinyl acetate copolymer according to any one of [1] to [4].
Molded body.
[6]
[1] to [4], comprising the ethylene-vinyl acetate copolymer according to any one of [1] to [4].
Sheet.
[7]
[1] to [4], containing the ethylene-vinyl acetate copolymer according to any one of [1] to [4],
Foam.

The present invention provides an ethylene-vinyl acetate copolymer that exhibits excellent falling weight impact strength, environmental stress cracking resistance, mold releasability, and transparency, as well as molded articles, sheets, and foams containing the copolymer.

Schematic diagram for explaining the slope P measured by GPC-FTIR. Schematic diagram for explaining the average value Q measured by GPC-FTIR. Schematic diagram for explaining the temperature from the reactor inlet to the outlet.

The following describes in detail an embodiment of the present invention (hereinafter referred to as the "present embodiment"); however, the present invention is not limited to this embodiment, and various modifications are possible without departing from the spirit of the present invention.

[Ethylene-vinyl acetate copolymer]
The ethylene-vinyl acetate copolymer of the present embodiment contains 11% by mass or more and 25% by mass or less of vinyl acetate units and ethylene units. When the molecular weight distribution and the absorbance I _(-CH2-) attributable to methylene groups, the absorbance I _(-C=O) attributable to carbonyl groups, and the absorbance I _(-CH3) attributable to methyl groups for each molecular weight are measured by GPC-FTIR, the slope P of the least squares approximate linear relational expression of the absorbance ratio ( _Ii(-C=O) /Ii _(-CH2-) ) to the logarithm log(M _i ) of each molecular weight M _i in the half width region of the molecular weight distribution is 0.00≦P≦1.40, and the average value Q of the absorbance ratio (Ii _(-CH3) /Ii _(-CH2-) ) of each molecular weight M _i in the half width region of the molecular weight distribution is 23.0≦Q≦30.0.

After extensive research, the inventors have discovered that the high molecular weight component of the ethylene-vinyl acetate copolymer contains many vinyl acetate units, and that the ethylene-vinyl acetate copolymer has a large number of branches, which further improves the falling weight impact strength, environmental stress crack resistance, mold releasability, and transparency. The reasons for this are not particularly limited, but are thought to be as follows:

When the high molecular weight component of an ethylene-vinyl acetate copolymer contains a large amount of vinyl acetate units, the degree of crystallinity tends to decrease due to steric hindrance in the molecular chain, and the proportion of amorphous components in the ethylene-vinyl acetate copolymer increases. A higher proportion of amorphous components improves the flexibility of the ethylene-vinyl acetate copolymer, further improving the environmental stress cracking resistance of the resulting molded articles and further improving transparency.

In addition, when the high molecular weight component of ethylene-vinyl acetate copolymer contains a large amount of vinyl acetate units, the molecular chains tend to become more strongly entangled, improving the melt tension of the ethylene-vinyl acetate copolymer. It is believed that improved melt tension makes it easier for the molecular chains to orient during injection molding, etc., and further improves the falling weight impact strength of the resulting molded article, etc.

Furthermore, if the number of branches in the ethylene-vinyl acetate copolymer is high, the molecular chains become more entangled, which is thought to reduce the degree of crystallinity and further improve the mold releasability and transparency of the resulting molded article, etc.

In this embodiment, GPC-FTIR is used to determine whether the high molecular weight component of the ethylene-vinyl acetate copolymer contains a higher content of vinyl acetate units and whether the ethylene-vinyl acetate copolymer has a high number of branches.

GPC-FTIR is an instrument equipped with an FTIR device downstream of the column of a GPC device, which enables molecular weight measurement by GPC while simultaneously analyzing the composition of each molecular weight sample eluted from the column using IR measurement. This makes it possible to measure changes in the copolymerization ratio of monomers from the high molecular weight side to the low molecular weight side of the GPC chart.

In GPC-FTIR, quantitative analysis of sample composition is generally performed based on the absorbance intensity ratio. For example, the absorbance ratio of the absorbance I _(-CH2-) attributed to methylene groups to the absorbance I _(-C=O) attributed to carbonyl groups in a certain molecular weight fraction can be used to evaluate whether the fraction contains a large amount of carbonyl groups relative to the methylene group. Furthermore, by comparing the absorbance ratios of multiple fractions with different molecular weights, it is possible to evaluate whether the high-molecular weight or low-molecular weight fraction contains a relatively large amount of carbonyl groups.

The ethylene-vinyl acetate copolymer of this embodiment is described in detail below.

(tilt P)
Figure 1 shows a schematic diagram illustrating the slope P measured by GPC-FTIR. In Figure 1, the left vertical axis represents the detected intensity of the copolymer in GPC, the right axis represents the absorbance ratio (I _(-C=O) /I _(-CH2-) ) in FTIR, and the horizontal axis represents the molecular weight of the copolymer expressed in logarithm.

The slope P is defined as the slope of the least-squares linear approximation of the absorbance ratio ( _Ii(-C=O) /Ii _(-CH2-) ) to the logarithm log( _Mi ) of each molecular weight _M in the half-width region of the molecular weight distribution. The absorbance ratio ( _Ii(-C=O) /Ii _(-CH2-) ) is the ratio of the absorbance Ii _(-CH2-) attributable to methylene groups to the absorbance _Ii(-C=O) attributable to carbonyl groups, and indicates the proportion of carbonyl groups in each molecular weight of the ethylene-vinyl acetate copolymer.

Here, for example, a slope P of 0 or positive means that the proportion of vinyl acetate units in the copolymer of this embodiment tends to remain constant or increase as the molecular weight distribution shifts from low molecular weight components to high molecular weight components. In other words, the slope P can be considered a parameter that indicates the distribution of vinyl acetate units (carbonyl groups) in the molecular weight distribution.

As described above, a slope P of 0 or positive means that vinyl acetate has been selectively introduced into the high-molecular-weight component of an ethylene-vinyl acetate copolymer, which is important for the development of mechanical properties. Because vinyl acetate has a higher molecular weight than ethylene, a high content of vinyl acetate units in the high-molecular-weight component tends to reduce crystallinity due to steric hindrance of the molecular chain. Therefore, even if the overall content of vinyl acetate units is the same, an ethylene-vinyl acetate copolymer with a slope P of 0 or positive has a higher proportion of amorphous components. This results in improved environmental stress crack resistance and transparency of the resulting molded article. Furthermore, an ethylene-vinyl acetate copolymer with a slope P of 0 or positive has stronger entanglement of molecular chains, which improves melt tension and thereby orients the molecular chains during injection molding, improving the drop weight impact strength of the resulting molded article.

Here, the half-width refers to the width of the peak in the molecular weight distribution that has an intensity equal to half (1/2A) of the peak-top height A. By defining the slope P in the half-width region, it is possible to express the increasing trend of vinyl acetate units without being affected by detection errors in the region at the base of the molecular weight distribution peak. The same applies to the average value Q, which will be described later.

The slope P is expressed as the slope of the following least squares linear approximation equation, and by expressing the relationship between the logarithm log(M _i ) of each molecular weight M _i and the absorbance ratio (I _i(-C=O) /I _i(-CH2-) ) with an approximation line in this way, it becomes possible to express the tendency of increase or decrease in vinyl acetate units in the molecular weight distribution. In the following equation, "a" is a constant.
Absorbance ratio ( _Ii(-C=O) /Ii _(-CH2-) ) = slope P × log( _Mi ) + a

In this embodiment, the slope P is 0.00≦P≦1.40, preferably 0.05≦P≦1.30, and more preferably 0.10≦P≦1.20. When the slope P is 0.00 or greater, the high molecular weight component of the ethylene-vinyl acetate copolymer contains a large amount of vinyl acetate units, which further improves the falling weight impact strength, environmental stress crack resistance, and transparency of the resulting molded article. Furthermore, when the slope P is 1.40 or less, the demolding properties of the resulting molded article are further improved.

While there are no particular limitations on the method for adjusting the slope P within the above range, one possible method is to polymerize ethylene-vinyl acetate copolymer in a tube reactor and gradually lower the polymerization peak temperature from upstream to downstream of the tube reactor. Figure 3 shows a schematic diagram illustrating the temperature from the reactor inlet to outlet. When ethylene and vinyl acetate are added to the reactor and then an initiator is added, the temperature inside the reactor rises due to the heat of polymerization. This temperature rise is reduced by cooling the reactor. If the highest polymerization temperature at this time is defined as the polymerization peak temperature, the downstream polymerization peak temperature is lower than the upstream polymerization peak temperature. This is thought to be because, at high polymerization temperatures, ethylene polymerization proceeds preferentially over vinyl acetate, and as the polymerization temperature decreases, vinyl acetate polymerization proceeds more easily. Therefore, by gradually lowering the polymerization peak temperature from upstream to downstream of the tube reactor, ethylene polymerization can proceed preferentially upstream, while vinyl acetate polymerization can proceed preferentially downstream, where the molecular weight increases.

In addition, one way to achieve the above temperature conditions is to use a double-tube reactor, and the temperature of the ethylene, vinyl acetate, ethylene-vinyl acetate copolymer, etc. flowing through the inner tube can be adjusted by using steam flowing through the outer tube.

Other methods for adjusting the slope P within the above range include, but are not limited to, reducing the temperature difference caused by introducing raw materials into the front and middle sections of the reactor, and/or increasing the temperature difference in the rear sections compared to the temperature difference in the middle section. Specifically, it is preferable to introduce raw materials such as ethylene and vinyl acetate into the front and middle sections of the reactor. Another method is to preheat the ethylene and vinyl acetate introduced into the front and middle sections of the reactor. The temperature drops temporarily when raw materials such as ethylene and vinyl acetate are introduced midway through the tube reactor, but then rises again due to the heat of polymerization. While the temperature within the reactor varies depending on the location of raw material introduction, in this embodiment, preheating the ethylene and vinyl acetate before introducing them reduces the temperature difference between the front and middle sections of the reactor, thereby maintaining a state in which ethylene polymerization in the front and middle sections proceeds preferentially. It is also preferable to increase the temperature difference in the rear sections compared to the temperature difference in the middle section.

In GPC-FTIR, the absorbance I _(-CH-) attributed to the methylene group is measured as an absorption at 2928 cm ^-1 , and the absorbance I _(-C=O) attributed to the carbonyl group is measured as an absorption at 1741 cm ^-1 . Specific measurements by GPC-FTIR can be performed by the method described in the Examples.

(Average value Q)
Figure 2 shows a schematic diagram illustrating the average value Q measured by GPC-FTIR. In Figure 2, the left vertical axis represents the detected intensity of the copolymer in GPC, and the right axis represents the absorbance ratio (I _(-CH3) /I _(-CH2-) ) in FTIR. The horizontal axis represents the molecular weight of the copolymer expressed in logarithm.

The average value Q is defined as the average value of the absorbance ratio (I _i(-CH3) /I _i(-CH2-) ) for each molecular weight M _i in the half-width region of the molecular weight distribution. The absorbance ratio (I _i(-CH3) /I _i(-CH2-) ) is the ratio of the absorbance I _i(-CH2-) attributable to methylene groups to the absorbance I _i(-CH3) attributable to methyl groups, and indicates the methyl group content at each molecular weight of the ethylene-vinyl acetate copolymer. Because a high proportion of ethylene-vinyl acetate copolymers are terminated with methyl groups, a high methyl group content means a large number of branches. Therefore, a high average value of the absorbance ratio (I _i(-CH3) /I _i(-CH2-) ) means that the copolymer of this embodiment has a large number of branches.

The absorbance ratio ( _Ii(-CH3) /Ii _(-CH2-) ) determined as described above indicates the number of carbon atoms in " _-CH3 " per 1000 carbon atoms in " _-CH2- ". The average value Q means the average number of carbon atoms in " _-CH3 " per 1000 carbon atoms in " _-CH2- " in the half width region of the molecular weight distribution.

In this embodiment, the average value Q is 23.0≦Q≦30.0, preferably 23.5≦Q≦28.0, and more preferably 24.0≦Q≦26.0. When the average value Q is 23.0 or more, the ethylene-vinyl acetate copolymer exhibits increased entanglement of molecular chains, resulting in a decrease in crystallinity and improved transparency. Furthermore, when the average value Q is 30.0 or less, demolding properties are further improved.

The method for adjusting the average Q value to fall within the above range is not particularly limited, but can be controlled by adjusting the amounts of chain transfer agent, vinyl acetate, initiator, etc. For example, one method is to set the amount of propylene used as a chain transfer agent to 0.30 mol% or less, preferably 0.28 mol% or less, and more preferably 0.25 mol% or less.

When more than 0.3 mol% of propylene is added, hydrogen abstraction from the molecular chains of the ethylene vinyl acetate resin increases, generating free radicals. These free radicals combine with each other and undergo a termination reaction, which tends to reduce the number of branches.

It is also preferable to feed raw materials such as ethylene and vinyl acetate into the reactor from the front and middle stages. By feeding raw materials such as ethylene and vinyl acetate into the reactor inlet, polymerization progresses, and a polymer layer and a gas layer coexist inside the reactor. By newly feeding raw materials such as ethylene and vinyl acetate into this stage, the polymer layer and the gas layer are agitated, promoting polymerization and making it possible to appropriately adjust the number of branches.

However, if raw materials such as ethylene and vinyl acetate are added only through the reactor inlet, the amount of polymer obtained will be small, which is undesirable from the standpoint of productivity.

In GPC-FTIR, the absorbance I _(-CH-) attributed to the methylene group is measured as an absorption at 2928 cm ^-1 , and the absorbance I _(-CH3) attributed to the methyl group is measured as an absorption at 2960 cm ^-1 . Specific measurements by GPC-FTIR can be performed by the method described in the Examples.

(Methyl branched)
The methyl branching of the ethylene-vinyl acetate copolymer of this embodiment, as determined by ^13C -NMR, is preferably 0.00/100C or more and less than 0.16/100C, more preferably 0.00/100C or more and 0.13/100C or less, and even more preferably 0.00/100C or more and 0.10/100C or less. When the methyl branching is less than 0.16/100C, the resulting molded article or the like tends to have further improved falling weight impact strength, environmental stress cracking resistance, mold releasability, and transparency.

The amount of methyl branching can be adjusted by changing the type and amount of chain transfer agent. The amount of methyl branching can be measured according to the method described in the Examples.

(heat of fusion)
The ethylene-vinyl acetate copolymer of this embodiment has a heat of fusion (ΔH) determined by differential scanning calorimetry of preferably 43 J/g to 90 J/g, more preferably 50 J/g to 88 J/g, and even more preferably 61 J/g to 83 J/g. When the heat of fusion is within the above range, the resulting molded article or the like has an excellent balance of falling weight impact strength, environmental stress cracking resistance, mold releasability, and transparency.

The heat of fusion can be adjusted by adjusting the vinyl acetate concentration, molecular weight distribution, etc. The heat of fusion can be measured according to the method described in the Examples.

(Melt Flow Rate)
The melt flow rate of the ethylene-vinyl acetate copolymer of this embodiment is preferably 0.1 g/10 min or more and less than 30 g/10 min, more preferably 0.6 g/10 min or more and 10 g/10 min or less, and even more preferably 0.8 g/10 min or more and 4.0 g/10 min or less. When the melt flow rate is within the above range, the falling weight impact strength and environmental stress crack resistance of the resulting molded article and the like tend to be further improved.

The method for adjusting the melt flow rate of the ethylene-vinyl acetate copolymer is not particularly limited, but examples include adjusting the reaction temperature and/or reaction pressure when polymerizing the ethylene-vinyl acetate copolymer. More specifically, when polymerizing the ethylene-vinyl acetate copolymer, increasing the reaction temperature tends to increase the melt flow rate of the ethylene-vinyl acetate copolymer, and increasing the reaction pressure tends to decrease the melt flow rate of the ethylene-vinyl acetate copolymer.

Melt flow rate can be measured in accordance with JIS K7210:1999 Code D (temperature = 190°C, load = 2.16 kg).

(vinyl acetate unit)
The content of vinyl acetate units is 11% by mass or more and 25% by mass or less, preferably 12% by mass or more and 23% by mass or less, and more preferably 13% by mass or more and 20% by mass or less, based on the total amount of the ethylene-vinyl acetate copolymer. When the content of vinyl acetate units is within the above range, the resulting molded article or the like has an excellent balance of falling weight impact strength, environmental stress cracking resistance, mold releasability, and transparency.

The method for adjusting the vinyl acetate unit content is not particularly limited, but examples include appropriately adjusting the amount of vinyl acetate monomer added in the process of polymerizing the ethylene-vinyl acetate copolymer, as well as the polymerization temperature and polymerization pressure.

The vinyl acetate unit content can be measured in accordance with JIS K7192:1999, using a calibration curve prepared by saponification and potentiometric titration as the reference test method, and converting the content into vinyl acetate using infrared spectroscopy as the control test method. Specifically, it can be measured using the method described in the examples below.

(ethylene units)
The content of ethylene units is preferably 75% by mass or more and 89% by mass or less, more preferably 77% by mass or more and 88% by mass or less, and even more preferably 80% by mass or more and 87% by mass or less, based on the total amount of the ethylene-vinyl acetate copolymer. When the content of ethylene units is within the above range, the resulting molded article or the like has an excellent balance of falling weight impact strength, environmental stress cracking resistance, mold releasability, and transparency.

The ethylene-vinyl acetate copolymer of this embodiment may contain monomer units other than ethylene units and vinyl acetate units. The other monomer units are not particularly limited, but examples include units derived from propylene, etc. From the viewpoints of drop weight impact strength, environmental stress crack resistance, and transparency, the amount of monomer units other than ethylene units and vinyl acetate units is preferably 0.3 mol % or less relative to ethylene.

The ethylene-vinyl acetate copolymer of this embodiment can also be a dry blend or melt blend of two or more ethylene-vinyl acetate copolymers in any ratio. When two or more ethylene-vinyl acetate copolymers are used, it is preferable that the content of vinyl acetate units in the entire resin be within the above-mentioned range.

[Method for producing ethylene-vinyl acetate copolymer]
The ethylene-vinyl acetate copolymer of the present embodiment is not particularly limited, but can be obtained, for example, by polymerizing ethylene and vinyl acetate under pressure and heat in the presence of a polymerization initiator. A chain transfer agent may be added to the polymerization system as needed.

The polymerization method for ethylene-vinyl acetate copolymer is not particularly limited, but examples include the autoclave method and the tubular method. Among these, it is preferable to use a tube reactor with a long ring structure. By using a tube reactor, it is possible to appropriately adjust the polymerization temperature and polymerization pressure in each region from upstream to downstream.

The average polymerization temperature is preferably 150°C or higher and 280°C or lower, more preferably 180°C or higher and 240°C or lower. The average polymerization pressure is preferably 100 MPa or higher and 275 MPa or lower, more preferably 120 MPa or higher and 270 MPa or lower, and even more preferably 180 MPa or higher and 265 MPa or lower.

The reactor may have multiple points for feeding ethylene, vinyl acetate, and a polymerization initiator. The temperature drops temporarily near each feed point when raw materials are introduced, but then the temperature inside the reactor rises again due to the heat of polymerization. In this embodiment, the raw materials supplied from the front and middle stages are pressurized before being introduced into the tube reactor, so the temperature rises to some extent due to the pressurization. However, it is preferable to preheat the raw materials to be supplied. This prevents the temperature at the upstream side from dropping due to the supply of relatively high-temperature raw materials. Therefore, from the perspective of controlling the gradient P, the polymerization temperature can be gradually lowered from the upstream to downstream of the polymerization.

In this embodiment, the front section of the reactor refers to a position (inlet) that is 0% to 10% of the total length of the reactor, the middle section of the reactor refers to a position from the inlet that is 10% to 40% of the total length of the reactor, and the rear section of the reactor refers to a position from the inlet that is 40% to 70% of the total length of the reactor.

The ethylene and vinyl acetate supplied to the reactor may be in gaseous or liquid form, but are preferably in gaseous form.

Polymerization initiators are not particularly limited, but examples include free radical generators such as peroxides. Free radical generators such as peroxides are not particularly limited, but examples include t-butyl-peroxy-2-ethylhexanoate, t-butyl peroxyacetate, t-butyl peroxypivalate, and di-t-butyl peroxide.

The chain transfer agent is not particularly limited, but examples include alcohols such as methanol, ethanol, normal propyl alcohol, isopropyl alcohol, normal butyl alcohol, and isobutyl alcohol; alkanes or alkenes such as ethane, propane, propylene, butane, 1-butene, and 2-butene; and ketones or aldehydes such as acetone, methyl ethyl ketone, 2-pentanone, 3-pentanone, 2-hexanone, 3-hexanone, diisobutyl ketone, methyl isopropyl ketone, formaldehyde, acetaldehyde, normal butyraldehyde, isobutyraldehyde, normal valeraldehyde, and isovaleraldehyde.

The ethylene-vinyl acetate copolymer polymerized as described above is preferably separated from the raw materials and granulated into pellets using an extruder. For example, when using a tubular system, it is preferable to separate the raw materials and the ethylene-vinyl acetate copolymer while reducing the pressure using a high-pressure separator and a low-pressure separator, and then granulate the molten ethylene-vinyl acetate copolymer into pellets using an extruder.

In the high-pressure separator and low-pressure separator, molten ethylene-vinyl acetate copolymer and unreacted gases such as raw ethylene gas and vinyl acetate gas exist as a gas-liquid mixture. The unreacted gases can be recovered from the top of each separator vessel and reused for polymerization.

After the ethylene-vinyl acetate copolymer is pelletized in an extruder, it is preferable to dry the pellets in a silo.

The ethylene-vinyl acetate copolymer of this embodiment may contain known additives, such as antioxidants, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments, as needed.

Antioxidants are not particularly limited, but examples include phenolic antioxidants such as 2,6-di-t-butyl-4-methylphenol, pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], and octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate; phosphorus-based antioxidants such as tris(2,4-di-t-butylphenyl)phosphite and tetrakis(2,4-di-t-butylphenyl)-4,4-biphenylene-diphosphonite; phosphorus/phenolic antioxidants such as 6-tert-butyl-4-[3-(2,4,8,10-tetra-t-butyldibenzo[d,f][1,3,2]dioxaphosphepin-6-yloxy)propyl]-o-cresol; and sulfur-based antioxidants such as dilauryl-thio-dipropionate.

[Molded body]
The molded article of this embodiment contains the above-mentioned ethylene-vinyl acetate copolymer. The molded article of this embodiment can be obtained by, but is not particularly limited to, injection molding, extrusion molding, or stretch molding, and can be suitably used in a variety of applications. Specific examples include, but are not particularly limited to, artificial turf mats, automobile mat guards, mudguard covers, drainage hoses, and the like. It can also be used as fibers, etc.

[Sheet]
The sheet of this embodiment contains the above-mentioned ethylene-vinyl acetate copolymer. Examples of methods for producing the sheet include T-die molding, inflation molding, calendar molding, and skiff molding. T-die molding and inflation molding are particularly preferred.

The sheet of this embodiment can be suitably used for automobile mudguards and mudguard covers. Note that the term "sheet" refers to a thin plastic plate with a thickness of 250 μm or more. The thickness of the sheet of this embodiment is preferably 250 μm or more, more preferably 300 μm to 10 mm, and more preferably 0.5 to 10 mm.

The sheet of this embodiment may also have a laminated structure including other layers in addition to the layer made of the ethylene-vinyl acetate copolymer described above. The method for producing such a laminated sheet is not particularly limited, but examples include a method in which layers are bonded together using a lamination process, or a method in which layers are produced using a laminate extrusion process.

[Foam]
The foam of this embodiment contains the above-mentioned ethylene-vinyl acetate copolymer. The foam of this embodiment is not particularly limited, but can be obtained using, for example, foamed microparticles, and is suitable for various applications such as impact-absorbing flooring materials.

The present invention will be explained in more detail below using examples and comparative examples. The present invention is not limited in any way by the following examples.

[GPC-IR measurement (carbonyl group gradient P)]
GPC-IR measurement (carbonyl group gradient P) was performed under the following conditions. For sample pretreatment, the sample was weighed, a solvent (0.05% BHT-added TCE) was added, and the sample was dissolved by shaking at 110°C for 1 hour. Molecular weight calibration was performed using a cubic approximation curve using standard polystyrene manufactured by Tosoh. The molecular weight was converted to polyethylene molecular weight using a factor.

The average molecular weight was calculated from the elution curve based on the absorbance peak area between 2850 and 2940 ^cm using SEC-FTIR software (Thermo Nicolet). The vinyl acetate content was calculated by creating a calibration curve using EVA of known composition from the absorbance ratio between the C=O stretching vibration at 1741 ^cm and the _CH stretching vibration at 2928 ^cm .

The carbonyl group slope P was measured by GPC (apparatus: HLC-8121GPC/HT manufactured by Tosoh Corporation) using an Avatar 370 detector manufactured by Thermo Nicolet, and the vinyl acetate ratio (VAc ratio) in each molecular weight region was plotted and calculated from a square law approximation line. For the GPC-IR elution curve, the value of the component amount (d(W)/d(logM)) at half the peak top value in the GPC measurement (d(W)/d(logM)) was calculated as 1/2A, and this was taken as the VAc ratio in the molecular weight range of 1/2A.
(Measurement conditions)
GPC device: HLC-8121GPC/HT (manufactured by Tosoh)
FT-IR device: Avatar370 (manufactured by Thermo Nicolet)
Column: TSKgel GMH _HR -H(20)HT (7.8 mm ID x 30 cm) x 2 (Tosoh)
Eluent: Tetrachloroethylene (Fujifilm Wako Pure Chemical Industries, special grade)
Flow rate: 0.7mL/min
Sample concentration: 2.0 mg/mL
Injection volume: 0.3mL
Column temperature: 110°C
Detector temperature: 110°C
Measurement wave number: 4000 to 650 cm ^-1
Resolution: 4cm ^-1
Number of scans: 8 times/1 point

Based on the obtained IR data, the absorbance ratio ( _Ii(-C=O) /Ii _(-CH2-) ) for each molecular weight was calculated, and the relationship between the logarithm log( _Mi ) of each molecular weight _Mi and the absorbance ratio ( _Ii(-C=O) /Ii _(-CH2-) ) in the half-width region was expressed by an approximate linear equation using the least squares method (see Figure 1). The slope P was obtained from the approximate linear equation thus obtained.

The absorbance I _(-CH2-) attributed to the methylene group was measured as the absorption at 2928 cm ^-1 , and the absorbance I _(-C=O) attributed to the carbonyl group was measured as the absorption at 1741 cm ^-1 .

[GPC-IR measurement (average value Q of terminal methyl groups)]
GPC-IR measurement (average value Q of terminal methyl groups) was performed under the following conditions. As sample pretreatment, the sample was weighed, and the solvent orthodichlorobenzene was added and the sample was dissolved by shaking at 140°C for 90 minutes. Molecular weight calibration was performed using 12 points in the range of MW (molecular weight) of standard polystyrene manufactured by Tosoh Corporation from 1,050 to 20,600,000. The MW of each standard polystyrene was multiplied by a coefficient of 0.43 to obtain the polyethylene-equivalent molecular weight. A primary calibration line was created from a plot of elution time and polyethylene-equivalent molecular weight, and the weight-average molecular weight (Mw) and number-average molecular weight (Mn) were determined.

The amount of terminal methyl groups was measured using a Polymer Char Composition Calibration Kit (Octene) _CH3 /1000C at six points in the range of 2.6 to 45.9 to create a calibration curve, and then measured by GPC (apparatus: Polymer Char GPC-IR) using an FT-IR (Polymer Char IR5) as the detector.

The amount of terminal methyl groups was determined by determining 1/2A of the amount of the component (d(W)/d(logM)) when the peak top value of the amount of the component (d(W)/d(logM)) measured by GPC was defined as A, and the average amount Q of _CH3 per 1000C was calculated within the molecular weight range of 1/2A.
(Measurement conditions)
Apparatus: GPC-IR manufactured by Polymer Char
Detector: Polymer Char IR5
Column: UT-807 (1 column) manufactured by Showa Denko K.K. and GMHHR-H(S)HT (2 columns) manufactured by Tosoh Corporation connected in series Mobile phase: orthodichlorobenzene Column temperature: 140°C
Flow rate: 1.0 ml/min Sample concentration: 16 mg/8 mL

Based on the obtained IR data, the absorbance ratio (Ii _(-CH3) /Ii _(-CH2-) ) for each molecular weight was calculated, and the average of the absorbance ratios (Ii _(-CH3) /Ii _(-CH2-) ) in the half-width region was calculated to obtain the average value Q.

The absorbance I _(-CH2-) attributed to the methylene group was measured as the absorption at 2928 cm ^-1 , and the absorbance I _(-CH3) attributed to the methyl group was measured as the absorption at 2960 cm ^-1 .

[Measurement of vinyl acetate unit content]
In accordance with JIS K7192:1999, a calibration curve was prepared using a VAc standard sample of an ethylene-vinyl acetate copolymer with a known content of vinyl acetate units by saponification and potentiometric titration as a reference test method, and the content of vinyl acetate units (VA content) in the ethylene-vinyl acetate copolymers obtained in the examples and comparative examples was measured by infrared spectroscopy as a control test method.

[Melt flow rate (MFR) measurement]
The melt flow rate of the ethylene-vinyl acetate copolymers obtained in the examples and comparative examples was measured in accordance with JIS K7210:1999 Code D (temperature = 190°C, load = 2.16 kg).

[ ¹³ C-NMR Measurement (Methyl Branch)]
^C -NMR measurements (methyl branches) were carried out under the following conditions: As sample pretreatment, the sample was weighed, and the solvent orthodichlorobenzene was added and the sample was dissolved by shaking at 140°C for 180 minutes. Methyl branches were measured using a Bruker AVANCE 500HD nuclear magnetic resonance spectrometer, with the methylene carbon signal at 29.9 ppm as the reference.

The ratio of methyl branches was calculated from the ratio of the area intensity of the methylene carbon signal (29.9 ppm) observed in ^the C-NMR spectrum measured above to the area intensity of the methyl branch (20.07 ppm). The ratio of methyl branches was determined from the ratio of the signal around 20.07 ppm in the C-NMR spectrum (Assignment of branched species in low-density polyethylene by C- ¹³ NMR, Analytical Chemistry, Nishimura Atsuo et al., p. 774 (29) 1980), assuming the intensity of the methylene signal to be 100.
(Measurement conditions)
Measuring device: AVANCE-500HD manufactured by Bruker
Observed nucleus: ^13C
Observation frequency: 125.77MHz
Pulse width: 5.0 μsec
PD: 5 sec
Measurement temperature: 120℃
Number of integrations: 8,000 times Reference: PE (-eee-) signal, 29.9 ppm
Solvent: orthodichlorobenzene- _d4
Sample concentration: 5 wt/vol%
Melting temperature: 140℃

[Differential scanning calorimetry (heat of fusion)]
The heat of fusion ΔH was measured using a DSC (manufactured by PerkinElmer, trade name: DSC8000). Specifically, 8 mg of the ethylene-vinyl acetate copolymer obtained in the examples and comparative examples was weighed out as a sample and placed in an aluminum sample pan. An aluminum cover was attached to this aluminum sample pan and placed in a differential scanning calorimeter. While purging with nitrogen at a flow rate of 20 mL/min, the measurement sample and reference sample were held at 0°C for 1 minute, then heated to 150°C at a rate of 200°C/min, held at 150°C for 5 minutes, cooled at a rate of 10°C/min, held at 0°C for 5 minutes, and then heated from 0°C to 150°C. The total heat calculated from the melting peak area was divided by the mass of the sample to determine the heat of fusion (ΔH).

[Falling weight impact strength]
Using the ethylene-vinyl acetate copolymers obtained in the Examples and Comparative Examples, 100 x 100 x 2.0 mm flat test pieces were produced using a Sumitomo all-electric injection molding machine SE130DUZ-C360 (cylinder temperature set to 200°C, mold temperature set to 40°C) with an injection time of 20 seconds and a cooling time of 30 seconds. The obtained flat test pieces were subjected to a drop weight impact test under the following conditions using a graphic impact tester (manufactured by Toyo Seiki Co., Ltd.), and the total absorbed energy was measured.
(conditions)
Holder diameter: 76 mm
Striker diameter: 12.7 mm
Striker weight: 6.5 kg
Height: 100cm
Temperature: 23℃

Environmental Stress Crack Resistance (ESCR)
To evaluate environmental stress crack resistance, the ethylene-vinyl acetate copolymers obtained in the examples and comparative examples were subjected to measurement of b-ESCR (constant strain environmental stress cracking test) in accordance with ASTM D 1693. A 10% by mass aqueous solution of IGEPAL (registered trademark) CO-630 manufactured by Rhodia Nikka Co., Ltd. was used as the test liquid, and the time required for the probability of crack occurrence due to environmental stress to reach 50% (hereinafter referred to as F50 value) was measured, and this was taken as the environmental stress crack resistance (ESCR) value of the ethylene-vinyl acetate copolymer. The unit is hr (hours). The larger this value, the better the environmental stress crack resistance.

[Transparency Measurement (Haze)]
A 0.1 mm thick aluminum plate was placed on a 5 mm thick smooth iron plate, and a 50 μm thick polyethylene terephthalate film (Lumirror, manufactured by Toray Industries, Inc.) was placed on top of the aluminum plate. A mold measuring 200 mm in length, 200 mm in width, and 3.1 mm in thickness was placed on top of this, and 130 g of the ethylene-vinyl acetate copolymer obtained in the Examples and Comparative Examples was placed into this mold. Then, the same polyethylene terephthalate film as above, the same aluminum plate as above, and the same iron plate as above were further placed on top of that.

This was placed in a compression molding machine (SFA-37, manufactured by Shinto Metal Industries Co., Ltd.) with a temperature setting of 180°C, preheated at 180°C and 0.1 MPa for 180 seconds, then air was removed for 5 seconds (10 MPa), and compression was performed at 180°C and 15 MPa for 120 seconds.

After pressing, the sample was removed and, 5 seconds after removal, placed in a compression molding machine (SFA-37, manufactured by Shinto Metal Industries Co., Ltd.) whose temperature had been adjusted to 25°C. It was then cooled while being pressed at 25°C and 10 MPa for 300 seconds to produce a pressed sheet. After cooling, the pressed sheet was removed from the mold and left to stand in an environment at a temperature of 23°C and humidity of 50% for at least 24 hours.

The thickness of the pressed sheet obtained as described above was measured using a constant pressure thickness gauge (manufactured by TECLOCK CORPORATION, model PG-02, minimum display 0.001 mm), and a portion of the pressed sheet with a thickness of 3.1 mm was selected. The haze value of the 3.1 mm thick portion was then measured in accordance with ASTM D1003 using a HAZE METER HM-150 manufactured by Murakami Color Research Laboratory Co., Ltd. The smaller the haze value, the better the transparency.

[Mold releasability]
Using the ethylene-vinyl acetate copolymers obtained in the examples and comparative examples, pin-gate flat test pieces of 200 × 200 × 4.0 mm were prepared using a Sumitomo all-electric injection molding machine SE130DUZ-C360 (cylinder temperature set to 200°C, mold temperature set to 40°C) with an injection time of 20 seconds and a cooling time of 30 seconds. The mold releasability from the mold was evaluated according to the following criteria when the ejector was set to 4 points (electric), ejector ejection force: 10 KN, ejector speed: 10 mm/s, and ejector count: 1 time.
◯: Out of 10 molded plates, 0 plates did not peel off from the mold. △: Out of 10 molded plates, 1 or more but less than 5 plates did not peel off from the mold. ×: Out of 10 molded plates, 5 or more plates did not peel off from the mold. ◯ and △ were considered to be acceptable.

Example 1
Polymerization was carried out using a tubular reactor having multiple feed ports. Specifically, ethylene and vinyl acetate were preheated to 160°C through a feed port on the front side of the tubular reactor and then introduced into the reactor. Subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate preheated to 160°C were additionally introduced through a feed port on the middle side of the tubular reactor, and subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator from the middle and rear sides. The overall amount of vinyl acetate was 7 mol% relative to ethylene.

The average polymerization temperature in the tubular reactor was 221°C, and the polymerization pressure was 236 MPa. The polymerization temperature in the tubular reactor was adjusted so that the initial peak top temperature was 250°C, the middle peak top temperature was 246°C, the latter peak top temperature was 230°C, and the reactor outlet temperature was 171°C. The difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator at the middle feed port was 10°C, and the difference between the bottom temperature and peak top temperature before and after the introduction of polymerization initiator at the latter feed port was 66°C.

The ethylene-vinyl acetate copolymer discharged from the tubular reactor was introduced into a high-pressure separator, where unreacted gases and other components were separated. The ethylene-vinyl acetate copolymer discharged from the high-pressure separator was then introduced into a low-pressure separator, where remaining unreacted gases and other components were separated, yielding a molten ethylene-vinyl acetate copolymer.

The resulting molten ethylene-vinyl acetate copolymer was fed into an extruder and pelletized to obtain ethylene-vinyl acetate copolymer pellets. The physical properties and characteristics of the resulting ethylene-vinyl acetate copolymer were measured using the methods described above. The measurement results are shown in Table 1.

Example 2
An ethylene-vinyl acetate copolymer of Example 2 was obtained in the same manner as in Example 1, except that ethylene, vinyl acetate, and propylene were preheated to 160°C and then introduced into the tubular reactor through feed ports on the front and middle stages, and the overall amount of propylene was 0.11 mol% relative to ethylene.

Example 3
An ethylene-vinyl acetate copolymer of Example 3 was obtained in the same manner as in Example 1, except that ethylene, vinyl acetate, and propylene were preheated to 160°C and then introduced into the tubular reactor through feed ports on the front and middle stages, and the overall amount of propylene was 0.17 mol% relative to ethylene.

Example 4
Ethylene and vinyl acetate were preheated to 160°C through a feed port on the front side of the tubular reactor and then introduced into the reactor. Subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate preheated to 160°C were additionally introduced through a feed port on the middle side of the tubular reactor, and subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator from the middle and rear sides. The overall amount of vinyl acetate was 7 mol% relative to ethylene.

The average polymerization temperature in the tubular reactor was 222°C, and the polymerization pressure was 264 MPa. The polymerization temperature in the tubular reactor was adjusted so that the initial peak top temperature was 255°C, the middle peak top temperature was 237°C, the latter peak top temperature was 230°C, and the reactor outlet temperature was 175°C. The ethylene-vinyl acetate copolymer of Example 4 was obtained in the same manner as in Example 1, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator at the middle feed port was 20°C, and the difference between the bottom temperature and peak top temperature before and after the introduction of polymerization initiator at the latter feed port was 69°C.

Example 5
Ethylene and vinyl acetate were preheated to 160°C through a feed port on the front side of the tubular reactor and then introduced into the reactor. Subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate preheated to 160°C were additionally introduced through a feed port on the middle side of the tubular reactor, and subsequently, t-butyl-peroxy-2-ethylhexanoate and di-t-butyl peroxide were introduced as polymerization initiators from the middle side. The overall amount of vinyl acetate was 7 mol% relative to ethylene.

The average polymerization temperature in the tubular reactor was 240°C, and the polymerization pressure was 264 MPa. The polymerization temperature in the tubular reactor was adjusted so that the initial peak top temperature was 275°C, the middle peak top temperature was 268°C, the latter peak top temperature was 255°C, and the reactor outlet temperature was 189°C. The ethylene-vinyl acetate copolymer of Example 5 was obtained in the same manner as in Example 1, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator at the middle feed port was 27°C, and the difference between the bottom temperature and peak top temperature before and after the introduction of polymerization initiator at the latter feed port was 64°C.

Example 6
Ethylene and vinyl acetate were preheated to 160°C through a feed port on the front side of the tubular reactor and then introduced into the reactor. Subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate preheated to 160°C were additionally introduced through a feed port on the middle side of the tubular reactor, and subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator from the middle side. The overall amount of vinyl acetate was 8.1 mol% relative to ethylene.

The average polymerization temperature in the tubular reactor was 195°C, and the polymerization pressure was 264 MPa. The polymerization temperature in the tubular reactor was adjusted so that the initial peak top temperature was 238°C, the middle peak top temperature was 220°C, and the reactor outlet temperature was 150°C. The ethylene-vinyl acetate copolymer of Example 6 was obtained in the same manner as in Example 1, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator into the middle feed port was 7°C.

Example 7
Ethylene and vinyl acetate were preheated to 160°C through a feed port on the front side of the tubular reactor and then introduced into the reactor. Subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate preheated to 160°C were additionally introduced through a feed port on the middle side of the tubular reactor, and subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator from the middle side. The overall amount of vinyl acetate was 8.7 mol% relative to ethylene.

The average polymerization temperature in the tubular reactor was 196°C, and the polymerization pressure was 264 MPa. The initial peak-top temperature was adjusted to 238°C, the middle peak-top temperature to 220°C, and the reactor outlet temperature to 155°C. The ethylene-vinyl acetate copolymer of Example 7 was obtained in the same manner as Example 1, except that the difference between the bottom temperature and peak-top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator into the middle feed port was 7°C.

Example 8
Ethylene and vinyl acetate were preheated to 160°C through a feed port on the front side of the tubular reactor and then introduced into the reactor. Subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate preheated to 160°C were additionally introduced through a feed port on the middle side of the tubular reactor, and subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator from the middle and rear sides. The overall amount of vinyl acetate was 5.8 mol% relative to ethylene.

The average polymerization temperature in the tubular reactor was 212°C, and the polymerization pressure was 236 MPa. The polymerization temperature in the tubular reactor was adjusted so that the initial peak top temperature was 245°C, the middle peak top temperature was 227°C, the latter peak top temperature was 218°C, and the reactor outlet temperature was 155°C. The ethylene-vinyl acetate copolymer of Example 8 was obtained in the same manner as in Example 1, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator at the middle feed port was 19°C, and the difference between the bottom temperature and peak top temperature before and after the introduction of polymerization initiator at the latter feed port was 50°C.

Example 9
Ethylene, vinyl acetate, and propylene were preheated to 160°C through a feed port on the front side of the tubular reactor and then introduced into the reactor. Subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate preheated to 160°C were additionally introduced through a feed port on the middle side of the tubular reactor, and subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator from the middle and rear sides. In total, the amount of vinyl acetate relative to ethylene was 7 mol %, and the amount of propylene relative to ethylene was 0.27 mol %.

The average polymerization temperature in the tubular reactor was 221°C, and the polymerization pressure was 236 MPa. The polymerization temperature in the tubular reactor was adjusted so that the initial peak top temperature was 252°C, the middle peak top temperature was 247°C, the latter peak top temperature was 229°C, and the reactor outlet temperature was 170°C. The ethylene-vinyl acetate copolymer of Example 9 was obtained in the same manner as in Example 1, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator at the middle feed port was 26°C, and the difference between the bottom temperature and peak top temperature before and after the introduction of polymerization initiator at the latter feed port was 65°C.

Example 10
Ethylene and vinyl acetate were preheated to 160°C through a feed port on the front side of the tubular reactor and then introduced into the reactor. Subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate preheated to 160°C were additionally introduced through a feed port on the middle side of the tubular reactor, and subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator from the middle side. The overall amount of vinyl acetate was 7.0 mol% relative to ethylene.

The average polymerization temperature in the tubular reactor was 224°C, and the polymerization pressure was 236 MPa. The polymerization temperature in the tubular reactor was adjusted so that the initial peak top temperature was 252°C, the middle peak top temperature was 221°C, and the reactor outlet temperature was 173°C. The ethylene-vinyl acetate copolymer of Example 10 was obtained in the same manner as in Example 1, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator into the middle feed port was 25°C.

Comparative Example 1
Ethylene and vinyl acetate were introduced into the feed port on the front side of the tubular reactor, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate as a polymerization initiator to initiate polymerization. Subsequently, additional ethylene and vinyl acetate were introduced into the feed port on the middle side of the tubular reactor, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate and di-t-butyl peroxide as polymerization initiators from the middle and rear sides. The overall amount of vinyl acetate was 7 mol% relative to ethylene.

The average polymerization temperature in the tubular reactor was 235°C, and the polymerization pressure was 264 MPa. The polymerization temperatures in the tubular reactor were adjusted so that the peak top temperature in the front stage was 243°C, the peak top temperature in the middle stage was 255°C, and the peak top temperature in the rear stage was 275°C, with the reactor outlet temperature being 191°C. The ethylene-vinyl acetate copolymer of Comparative Example 1 was obtained in the same manner as in Example 1, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator at the feed port on the middle stage was 64°C, and the difference between the bottom temperature and peak top temperature before and after the introduction of polymerization initiator at the feed port on the rear stage was 44°C.

Comparative Example 2
An ethylene-vinyl acetate copolymer of Comparative Example 2 was obtained in the same manner as in Example 1, except that ethylene, vinyl acetate, and propylene were preheated to 160°C and then introduced into the reactor through a feed port on the upstream side of the tubular reactor, and the total amount of propylene was 0.34 mol% relative to ethylene.

Comparative Example 3
Ethylene, vinyl acetate, and propylene were introduced into the feed port on the front side of the tubular reactor, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate were additionally introduced into the feed port on the middle side of the tubular reactor, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate and di-t-butyl peroxide as polymerization initiators from the middle and rear sides. The overall amount of vinyl acetate relative to ethylene was 2 mol %, and the amount of propylene relative to ethylene was 0.5 mol %.

The average polymerization temperature in the tubular reactor was 237°C, and the polymerization pressure was 264 MPa. The polymerization temperatures in the tubular reactor were adjusted so that the peak top temperature in the front stage was 260°C, the peak top temperature in the middle stage was 265°C, and the peak top temperature in the rear stage was 275°C, with the reactor outlet temperature being 198°C. The ethylene-vinyl acetate copolymer of Comparative Example 3 was obtained in the same manner as in Example 1, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator at the feed port on the middle stage was 67°C, and the difference between the bottom temperature and peak top temperature before and after the introduction of polymerization initiator at the feed port on the rear stage was 44°C.

Comparative Example 4
Ethylene, vinyl acetate, and propylene were introduced through a feed port on the front side of the tubular reactor, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate were additionally introduced through a feed port on the middle side of the tubular reactor, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate and di-t-butyl peroxide as polymerization initiators from the middle and rear sides. The overall amount of vinyl acetate relative to ethylene was 4 mol %, and the amount of propylene relative to ethylene was 0.5 mol %.

The average polymerization temperature in the tubular reactor was 229°C, and the polymerization pressure was 264 MPa. The polymerization temperatures in the tubular reactor were adjusted so that the peak top temperature in the front stage was 250°C, the peak top temperature in the middle stage was 260°C, and the peak top temperature in the rear stage was 275°C, with the reactor outlet temperature being 197°C. The ethylene-vinyl acetate copolymer of Comparative Example 4 was obtained in the same manner as in Example 1, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator at the feed port on the middle stage was 63°C, and the difference between the bottom temperature and peak top temperature before and after the introduction of polymerization initiator at the feed port on the rear stage was 40°C.

Comparative Example 5
Ethylene, vinyl acetate, and propylene were introduced into the feed port on the front side of the tubular reactor, followed by the introduction of t-butylperoxy-2-ethylhexanoate as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate were additionally introduced into the feed port on the middle side of the tubular reactor, followed by the introduction of t-butylperoxy-2-ethylhexanoate and di-t-butyl peroxide as polymerization initiators from the middle and rear sides. The overall amount of vinyl acetate relative to ethylene was 4 mol %, and the amount of propylene relative to ethylene was 0.8 mol %.

The average polymerization temperature in the tubular reactor was 229°C, and the polymerization pressure was 264 MPa. The polymerization temperatures in the tubular reactor were adjusted so that the peak top temperature in the front stage was 250°C, the peak top temperature in the middle stage was 260°C, and the peak top temperature in the rear stage was 275°C, with the reactor outlet temperature being 205°C. The ethylene-vinyl acetate copolymer of Comparative Example 5 was obtained in the same manner as Comparative Example 4, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator at the feed port on the middle stage was 55°C, and the difference between the bottom temperature and peak top temperature before and after the introduction of polymerization initiator at the feed port on the rear stage was 49°C.

Comparative Example 6
Ethylene, vinyl acetate, and propylene were introduced into the feed port on the front side of the tubular reactor, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate were additionally introduced into the feed port on the middle side of the tubular reactor, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate as a polymerization initiator from the middle side. The overall amount of vinyl acetate relative to ethylene was 14 mol%.

The average polymerization temperature in the tubular reactor was 200°C, and the polymerization pressure was 265 MPa. The polymerization temperature in the tubular reactor was adjusted so that the peak top temperature in the front stage was 210°C, the peak top temperature in the middle stage was 223°C, and the reactor outlet temperature was 171°C. The ethylene-vinyl acetate copolymer of Comparative Example 6 was obtained in the same manner as Comparative Example 4, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator into the feed port on the middle stage was 48°C.

Comparative Example 7
Ethylene, vinyl acetate, and propylene were introduced into the feed port on the front side of the tubular reactor, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate were additionally introduced into the feed port on the middle side of the tubular reactor, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate as a polymerization initiator from the middle side. The overall amount of vinyl acetate relative to ethylene was 12.5 mol%.

The average polymerization temperature in the tubular reactor was 210°C, and the polymerization pressure was 265 MPa. The polymerization temperature in the tubular reactor was adjusted so that the peak top temperature in the front stage was 220°C, the peak top temperature in the middle stage was 230°C, and the reactor outlet temperature was 180°C. The ethylene-vinyl acetate copolymer of Comparative Example 7 was obtained in the same manner as Comparative Example 4, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator into the feed port on the middle stage was 59°C.

Comparative Example 8
Ethylene and vinyl acetate were introduced into the tubular reactor through a feed port on the upstream side, followed by the introduction of t-butyl-peroxy-2-ethylhexanoate as a polymerization initiator to initiate polymerization. The total amount of vinyl acetate relative to ethylene was 7.0 mol %.

The average polymerization temperature in the tubular reactor was 228°C, and the polymerization pressure was 236 MPa. The ethylene-vinyl acetate copolymer of Comparative Example 8 was obtained in the same manner as Comparative Example 4, except that the polymerization temperature in the tubular reactor was adjusted so that the first peak top temperature was 255°C and the reactor outlet temperature was 175°C.

Comparative Example 9
Ethylene and vinyl acetate were preheated to 160°C through a feed port on the front side of the tubular reactor and then introduced into the reactor. Subsequently, t-butyl-peroxy-2-ethylhexanoate was introduced as a polymerization initiator to initiate polymerization. Subsequently, ethylene and vinyl acetate preheated to 160°C were additionally introduced through a feed port on the middle side of the tubular reactor, and subsequently, t-butyl-peroxy-2-ethylhexanoate and di-t-butyl peroxide were introduced as polymerization initiators from the middle side. The overall amount of vinyl acetate was 7 mol% relative to ethylene.

The average polymerization temperature in the tubular reactor was 240°C, and the polymerization pressure was 276 MPa. The polymerization temperature in the tubular reactor was adjusted so that the initial peak top temperature was 275°C, the middle peak top temperature was 268°C, the latter peak top temperature was 255°C, and the reactor outlet temperature was 191°C. The ethylene-vinyl acetate copolymer of Comparative Example 9 was obtained in the same manner as in Example 1, except that the difference between the bottom temperature and peak top temperature before and after the introduction of ethylene, vinyl acetate, and polymerization initiator at the middle feed port was 28°C, and the difference between the bottom temperature and peak top temperature before and after the introduction of polymerization initiator at the latter feed port was 64°C.

The ethylene-vinyl acetate copolymer of the present invention has industrial applicability as a resin raw material used in a wide range of industrial fields.

Claims (7)

Contains 11% by mass or more and 25% by mass or less of vinyl acetate units and ethylene units,
When the molecular weight distribution and the absorbance I _(-CH-) attributed to the methylene group, the absorbance I _(-C=O) attributed to the carbonyl group, and the absorbance I _(-CH) attributed to the methyl group were measured by GPC-FTIR,
the slope P of the least squares approximate linear relational expression of the absorbance ratio ( _Ii(-C=O) /Ii _(-CH2-) ) to the logarithm log( _Mi ) of each molecular weight _Mi in the half width region of the molecular weight distribution is 0.014 ≦P≦1.40;
The absorbance ratio (Ii _(-CH3) /Ii _(-CH2) of each molecular weight M _i in the half-width region of the molecular weight distribution
_-) ) the average value Q is 23.0≦Q≦30.0,
Ethylene-vinyl acetate copolymer. The methyl branch (20.07 ppm) determined by ¹³ C-NMR is less than 0.16/100C.
The ethylene-vinyl acetate copolymer according to claim 1. The heat of fusion (ΔH) determined by differential scanning calorimetry is 43 J/g or more and 90 J/g or less.
The ethylene-vinyl acetate copolymer according to claim 1 or 2. The melt flow rate at a temperature of 190°C and a load of 2.16 kg is 0.1 g/10 min or more and less than 30 g/10 min;
The ethylene-vinyl acetate copolymer according to any one of claims 1 to 3. The ethylene-vinyl acetate copolymer according to any one of claims 1 to 4,
Molded body. The ethylene-vinyl acetate copolymer according to any one of claims 1 to 4,
Sheet. The ethylene-vinyl acetate copolymer according to any one of claims 1 to 4,
Foam.