JP7474825B2 - Glass plate protection sheet - Google Patents

Description

The present invention relates to a protective sheet for glass plates made of a resin foam sheet.

Resin foam sheets are used for various purposes because they have excellent light weight and cushioning properties. Resin foam sheets are sometimes used as protective sheets for glass plates instead of slip sheets because of their excellent cushioning properties. In addition, resin foam sheets that are composed of only one foam layer or have non-foamed layers on both sides of the foam layer are widely used. The latter resin foam sheets are also called laminated foam sheets, and these laminated foam sheets are also used as protective sheets for glass plates (see Patent Document 1 below). Resin foam sheets used as protective sheets for glass plates are made of a resin composition containing polyethylene resin, which has better flexibility than polyester resin, polystyrene resin, etc. Resin foam sheets exhibit good cushioning properties because at least the surface that contacts the glass is made of a resin composition containing polyethylene resin.

JP 2021-791 A

Resin foam sheets used as protective sheets for glass plates are required to keep the surface of the glass plate clean. Specifically, protective sheets for glass plates are required to be antistatic so that dust and other particles do not adhere to the glass plate due to static electricity. In addition, when protecting this type of glass plate with a protective sheet, it is required to prevent substances that cannot be easily washed off with water, etc. from adhering to the surface of the glass plate.

To satisfy such demands, a surfactant that can be easily washed off with water is incorporated into the resin foam sheet, and the surfactant is quickly exuded to the interface with the glass plate to provide antistatic properties while preventing substances other than the surfactant from adhering to the glass plate.

It is required that an appropriate frictional force acts between the glass plate protective sheet and the glass plate so that the glass plate placed on the glass plate protective sheet does not move inadvertently. However, the use of a surfactant to prevent static electricity can make the glass plate slippery. For this reason, it is difficult for the glass plate protective sheet to suppress excessive slippage between the glass plate and the protective sheet while satisfying the demand to keep the glass plate clean. Therefore, the object of the present invention is to provide a glass plate protective sheet that can suppress excessive slippage between the glass plate and the protective sheet.

The present inventors have thoroughly investigated the above-mentioned problems and have found that when the amount of sodium ions and lithium ions extracted from the resin foam sheet by water exceeds a certain amount, the cleanliness of the surface of the glass plate may be reduced. The present inventors have also found that by reducing the moisture permeability and moisture absorption of the resin foam sheet, it is possible to provide a protective sheet that is less likely to cause excessive slippage of the glass plate, and have completed the present invention.

To solve the above problems, the present invention provides:
A glass plate protective sheet made of a resin foam sheet,
The resin foam sheet is
At least the surface is made of a resin composition containing a polyethylene resin,
The thickness is 0.1 mm or more and 0.5 mm or less,
The total amount of sodium ions and lithium ions extracted in the metal ion extraction test is 0.1 mg/ ^m2 or less,
Moisture permeability is 100 g/( ^m2 ·24 h) or less, and
Provided is a glass plate protective sheet having a water absorption rate of 2.0 g/ ^m2 or less.

According to the present invention, the surface of the glass plate can be kept clean without actively utilizing the seepage action of the surfactant, and excessive slippage between the glass plate and the surface can be suppressed.

FIG. 1 is a schematic view showing one embodiment of a use of a resin foam sheet. FIG. 2 is a schematic cross-sectional view showing the structure of a resin foam sheet according to one embodiment. FIG. 3 is a schematic cross-sectional view showing the structure of a foamed sheet according to another embodiment. FIG. 4 is a schematic cross-sectional view showing the structure of a foamed sheet according to another embodiment. FIG. 5 is a schematic diagram showing an example of a profile curve obtained by measuring the surface roughness.

One embodiment of the present invention will be described below with reference to the drawings. The protective sheet for glass plates (hereinafter also simply referred to as "protective sheet") of this embodiment is composed of a resin foam sheet. The protective sheet of this embodiment is used, for example, in the state shown in FIG. 1. The protective sheet 1 of this embodiment is used by being interposed between adjacent glass plates 2 when a plurality of glass plates 2 are stacked vertically to form a laminate 100. The protective sheet 1 in this embodiment is composed of a resin foam sheet produced by an extrusion foaming method or the like.

The protective sheet 1 of this embodiment can be used in other modes than those shown in FIG. 1. The protective sheet 1 can be used not only to protect the glass plate 2 in a horizontally placed state (lying down state) as shown in FIG. 1, but also when arranging multiple glass plates 2 in a vertically placed state side by side. The protective sheet 1 can be used not only to protect the glass plates 2 overlapping with their edges aligned in the vertical direction as shown in FIG. 1, but also to protect the glass plates 2 stacked in a stepped shape with slight positional deviations from the top to the bottom. The same is true when the glass plates 2 are placed vertically, and when multiple glass plates 2 tilted from the vertical direction are arranged side by side, the edges of the glass plates 2 are not aligned, but the protective sheet 1 of this embodiment can be used in such modes as well. Furthermore, the protective sheet 1 of this embodiment can be used when the glass plates 2 are arranged in a diagonal direction, not vertically or horizontally.

When the glass plates 2 are arranged without aligning their edges, a protective sheet 1 of the same size as the glass plate 2 will have its edges extend beyond the glass plate 2. The protective sheet 1 shown in FIG. 1 is slightly smaller than the glass plate 2, and can therefore be used without extending beyond the glass plate 2 even when the glass plates 2 are arranged in various ways. The size of the protective sheet 1 is naturally not limited to the example shown in FIG. 1. In the sense of more reliably protecting the entire glass plate 2, the protective sheet 1 may be the same size as the glass plate 2 or slightly larger than the glass plate 2.

The protective sheet 1 in this embodiment is composed of, for example, a resin foam sheet as shown in Figures 2 to 4. As shown in Figure 2, the resin foam sheet according to one embodiment is composed of a resin foam sheet having a single-layer structure, with a foam layer 10 exposed on the surface on both sides. Since the protective sheet 1 in this embodiment is composed only of the foam layer 10, both sides of the foam layer 10 serve as contact surfaces that come into contact with the glass plate 2.

The protective sheet 1 in this embodiment is not limited to such a single-layer structure. As shown in FIG. 3, a resin foam sheet according to another embodiment has a non-foamed layer 11 on one side of a foamed layer 10. The protective sheet 1 shown in FIG. 3 has one of its two surfaces constituted by a foamed layer 10, and the other by a non-foamed layer 11. That is, in the protective sheet 1 shown in FIG. 3, the foamed layer 10 and the non-foamed layer 11 each constitute a contact surface that contacts the glass plate 2.

The protective sheet 1 in this embodiment may be as shown in FIG. 4. The protective sheet 1 shown in FIG. 4 is a resin foam sheet having a foam layer 10 and non-foamed layers 11 on both sides of the foam layer 10. Both surfaces of the protective sheet 1 shown in FIG. 4 are the surfaces of the respective non-foamed layers 11. That is, in the protective sheet 1 shown in FIG. 4, each of the two non-foamed layers 11 constitutes an abutment surface that contacts the glass plate 2.

The protective sheet 1 in this embodiment is not limited to these examples, and may have a laminated structure of four or more layers, and may have multiple foam layers 10.

The glass plate 2 in this embodiment may be, for example, a glass plate for use as a substrate or cover glass in a display panel such as a liquid crystal display or an organic EL display.

The surface of the resin foam sheet (protective sheet 1) of this embodiment is composed of a resin composition containing a polyethylene resin. The resin composition in this embodiment contains the polyethylene resin and an additive. The resin composition of this embodiment does not need to contain the polyethylene resin or the additive alone, and may contain multiple polyethylene resins or multiple additives. The resin composition contains a polymeric antistatic agent and a surfactant as the additives. The resin composition of this embodiment may contain another resin, such as a polypropylene resin, in addition to the polyethylene resin and the polymeric antistatic agent, if necessary.

Resin compositions containing polymer-type antistatic agents and surfactants can prevent static electricity from being generated between the mating material and the mating material, and are therefore suitable as materials for forming a contact surface with a glass plate. In the protective sheet 1 shown in FIG. 3 and FIG. 4, the foamed layer 10 and the non-foamed layer 11 may use the same or different resin compositions. In the protective sheet 1 shown in FIG. 4, the one non-foamed layer 11 and the other non-foamed layer 11 may use the same or different resin compositions. In the protective sheet 1 shown in FIG. 4, the foamed layer 10 does not constitute the surface of the protective sheet 1, so the amount of polymer-type antistatic agent or surfactant in the resin composition constituting the foamed layer 10 may be less than that in the non-foamed layer 11. In the protective sheet 1 shown in FIG. 4, the resin composition constituting the foamed layer 10 may not contain one or both of the polymer-type antistatic agent and the surfactant.

The resin foam sheet constituting the protective sheet 1 preferably has a predetermined roughness on the contact surface with the glass plate 2 in order to prevent the glass plate 2 from slipping. It is preferable that both surfaces of the resin foam sheet are in a state suitable for use as contact surfaces. Therefore, it is preferable that both surfaces of the resin foam sheet have a predetermined roughness.

The resin foam sheet preferably has an arithmetic mean roughness (Ra) of 50 μm or more as measured based on JIS B 0601 (2001). The resin foam sheet preferably has a ten-point mean roughness (Rz _JIS ) of 120 μm or more as measured based on JIS B 0601 (2001). The ratio (Rz _JIS /Ra) of the ten-point mean roughness (Rz _JIS ) to the arithmetic mean roughness (Ra) is preferably 3.0 or less.

The arithmetic mean roughness (Ra) is a value obtained by measuring the uneven shape of the surface of a resin foam sheet on the order of microns, for example, when a contour curve RC such as that shown in Figure 5 is obtained, measuring the total area (Sm) obtained by summing the areas of the parts corresponding to the peaks MT above the mean line ML over a section of reference length L, and measuring the total area (Sv) obtained by summing the areas of the parts corresponding to the valleys VL below the mean line ML over a section of reference length L, and dividing the sum (Sm + Sv) by the reference length L (Ra = [(Sm + Sv) / L]).

The ten-point average roughness (Rz _JIS ) is the sum (Rz JIS = |AVR1| + |AVR2|) of the average value of the heights of the five peaks from the highest peak MT to the fifth highest peak MT on _the profile curve RC and the average value of the depths of the five valleys VL from the deepest valley VL to the fifth deepest valley VL.

The arithmetic mean roughness (Ra) and the ten-point mean roughness (Rz _JIS ) are values calculated as described above. The arithmetic mean roughness (Ra) does not change significantly between the presence of extremely high peaks MT and extremely deep valleys VL compared to other peaks MT and valleys VL and the absence of these. On the other hand, the ten-point mean roughness (Rz _JIS ) is larger when high peaks MT and deep valleys VL are present than when they are not present. Therefore, a small ratio (Rz _JIS /Ra) between these means that there are fewer extremely high peaks MT and extremely deep valleys VL, and that the sizes of the peaks MT and valleys VL are uniform.

A resin foam sheet with a large value of arithmetic mean roughness (Ra) is less likely to adhere to the glass plate 2 when placed on it, and can prevent sodium ions and lithium ions from migrating to the glass plate 2. In addition, if the protective sheet 1 and the glass plate 2 are in close contact with each other, when the surfactant bleeds out onto the surface of the protective sheet 1, the surfactant spreads in a film-like form between the protective sheet 1 and the glass plate 2, making the glass plate 2 more likely to slip. Therefore, a resin foam sheet with a large value of arithmetic mean roughness (Ra) is excellent as a constituent material for the protective sheet 1 because it is easier to prevent the glass plate 2 from slipping.

From the above viewpoints, the arithmetic mean roughness (Ra) of the resin foam sheet is preferably 50 μm or more. The arithmetic mean roughness (Ra) of the resin foam sheet may be 55 μm or more, 60 μm or more, or 70 μm or more. The arithmetic mean roughness (Ra) of the resin foam sheet may be 75 μm or more, or 80 μm or more.

If the arithmetic mean roughness (Ra) is extremely large or if there are extremely high peaks MT on the surface of the resin foam sheet, the protective sheet 1 and the glass plate 2 may come into point contact, which may make the glass plate 2 slippery.

From the viewpoint of suppressing slippage of the glass plate 2, the arithmetic mean roughness (Ra) of the resin foam sheet is preferably 150 μm or less. The arithmetic mean roughness (Ra) of the resin foam sheet may be 140 μm or less, 130 μm or less, or 120 μm or less. The arithmetic mean roughness (Ra) may be 108 μm or less, 100 μm or less, or 95 μm or less.

The ten-point average roughness (Rz _JIS ) of the resin foam sheet is preferably 120 μm or more for the same reason as the arithmetic mean roughness (Ra). The ten-point average roughness (Rz _JIS ) of the resin foam sheet may be 130 μm or more, or may be 140 μm or more.

_The ten-point average roughness (Rz _JIS ) of the resin foam sheet is preferably 300 μm or less, and may be 250 μm or less, or may be 200 μm or less.

From the viewpoint of suppressing slippage of the glass plate 2, the ratio (Rz _JIS /Ra) of the arithmetic mean roughness (Ra) to the ten-point mean roughness (Rz _JIS ) is preferably 3.0 or less. The ratio (Rz _JIS /Ra) may be 2.8 or less, or may be 2.5 or less. The ratio (Rz _JIS /Ra) may be 2.2 or less. The ratio (Rz _JIS /Ra) is usually 1 or more. The ratio (Rz _JIS /Ra) may be 1.3 or more, 1.5 or more, or 1.8 or more.

Although only one side of the resin foam sheet may have the preferred surface roughness as described above, it is preferable that both sides have the preferred surface roughness as described above. It is difficult to achieve the preferred surface roughness as described above with a resin foam sheet having a non-foamed layer on the surface. Therefore, it is preferable that the resin foam sheet constituting the protective sheet 1 of this embodiment is composed of foam layers on both surfaces. In forming both surfaces with foam layers, the resin foam sheet may have, for example, a three-layer structure of foam layer/non-foam layer/foam layer, or a two-layer structure of foam layer/foam layer. In order to make the state of both surfaces common, the protective sheet 1 may be a resin foam sheet of a single layer structure composed of only one foam layer.

The arithmetic mean roughness (Ra) and the ten-point mean roughness (Rz _JIS ) can be measured as follows.
(Method of measuring surface roughness)
The resin foam sheet (protective sheet 1) is cut into a size of 40 mm wide x 40 mm long x sheet thickness to prepare a test piece. The planar profile of the smooth surface of the test piece is measured at a magnification of 20 times using a digital microscope (manufactured by Keyence Corporation, product name: VHX-8000). From the planar profile obtained, the surface roughness Ra and Rz _JIS values are calculated based on JIS B 0601:2001.
The cutoff by the phase compensation type low-pass filter is set to λc=8 mm and λs=25 μm.
The number of test pieces shall be five.
For each test piece, the surface roughness is measured at five points on each of the front and back surfaces in the extrusion direction and width direction.
The measured values are arithmetically averaged to obtain the surface roughness Ra and Rz _JIS of the resin foam sheet (protective sheet 1).

In the protective sheet 1 of this embodiment, it is also possible to suppress slippage of the glass plate 2 by electrostatic force. That is, it is advantageous for the resin foam sheet constituting the protective sheet 1 to prevent slippage of the glass plate 2 if the static electricity generated does not disappear immediately but dissipates over time. More specifically, the resin foam sheet (protective sheet 1) preferably has a static attenuation rate of 95% or less. The static attenuation rate of the resin foam sheet may be 90% or less, or may be 85% or less. The static attenuation rate of the resin foam sheet is preferably 40% or more. The static attenuation rate of the resin foam sheet may be 50% or more, 55% or more, or 60% or more.

The static electricity decay rate of the resin foam sheet can be measured as follows.
(Method of measuring electrostatic decay rate)
The static electricity decay rate can be measured in accordance with the method described in JIS L1094 "Test method for electrostatic charge of woven and knitted fabrics."
First, a square test piece with sides of 40 mm is cut out from a protective sheet and left to stand in an atmosphere at a temperature of 22° C. and a relative humidity of 60% for 24 hours to condition the piece.
Using a test device (product name "STATICMETER S-411", manufactured by Nippon Static Co. Ltd.), electrodes are pressed against the conditioned test piece with a load of about 30 N, a voltage of 10 kV is applied, and the change in the charge amount of the test piece after the voltage application is measured.
The charge amount at the moment when the application of the voltage is stopped (initial charge amount) and the charge amount after 30 seconds are read, and the static electricity decay rate is calculated by the following formula.
Static electricity decay rate (%) = [(initial charge amount - charge amount after 30 seconds) / initial charge amount] x 100

In the resin foam sheet (protective sheet 1) of this embodiment, the resin composition as described above is melt-kneaded together with a foaming agent in an extruder, and the molten mixture obtained by the melt-kneading is extruded into a sheet shape from a die attached to the tip side of the extruder in the extrusion direction, and foamed, thereby producing the foam layer 10. The non-foamed layer 11 can be formed by laminating the above-mentioned resin composition on one or both sides of the foam layer 10 by a co-extrusion method. The non-foamed layer 11 can be formed by forming the above-mentioned resin composition into a film shape and then laminating it on one or both sides of the foam layer 10 by a heat fusion method or the like.

The width and thickness of the resin foam sheet are not particularly limited, but the thickness is preferably 0.1 mm or more. The thickness of the resin foam sheet is more preferably 0.2 mm or more. The thickness of the resin foam sheet is preferably 0.5 mm or less, and more preferably 0.4 mm or less.

In the case of a resin foam sheet having a non-foamed layer 11, the thickness of the non-foamed layer 11 is preferably 4 μm or more, and more preferably 5 μm or more. The thickness of the non-foamed layer 11 is preferably 30 μm or less, and more preferably 20 μm or less. The non-foamed layer 11 is a layer that does not substantially have air bubbles.

The average bubble diameter of the foamed layer 10 is about 100 μm to 500 μm. The average bubble diameter can be measured in accordance with the test method of ASTM D2842-69. The bubble thickness of the foamed layer 10 is preferably 4 μm or more, more preferably 5 μm or more. The bubble thickness of the foamed layer 10 is preferably 15 μm or less, more preferably 12 μm or less. The bubble thickness can be determined by photographing the cross section of the resin foam sheet with an SEM, drawing lines perpendicular to the resin foam sheet in the thickness direction at any 20 points using the image, measuring the thickness of the bubble film on the lines, and calculating the arithmetic average value from the measured values. The thickness of the non-foamed layer 11 can also be measured in the same manner as the bubble thickness.

The thickness of the resin foam sheet can be determined as follows.
(Thickness Measurement)
The measurement is performed using a constant pressure thickness gauge (stand type, manufactured by Teclock, digital indicator PC series model number "PC-440J") and a cylindrical weight.
The measurement is performed in a state where a load of 95 g (including the resin foam sheet's own weight) is applied to a circular surface having a diameter of 3.0 cm (area: 7.1 cm ² ) of the resin foam sheet.
Measurements are then taken at 50 points every 5 cm in the width direction, and the arithmetic mean value of the measurements is taken as the thickness of the resin foam sheet.
In addition, if it is not possible to obtain measurements at 50 points, measurements are made as many as possible, and the arithmetic mean value of the measurements is taken as the thickness.

As described above, in order to provide both sides of the protective sheet 1 with an appropriate surface roughness, it is preferable that the resin foam sheet is composed of only one foam layer. The surface roughness of the resin foam sheet can be adjusted by the expansion of the bubbles present in the outermost layer of the resin foam sheet. The expansion of the bubbles present in the outermost layer of the resin foam sheet can be adjusted by the melt viscosity of the resin composition extruded from the die and the cooling conditions immediately after extrusion. In general, the higher the melt viscosity during extrusion, the less likely the expansion of the bubbles will vary. The melt viscosity of the resin composition during extrusion can be adjusted by the temperature setting of the extruder. The melt viscosity of the resin composition during extrusion can also be adjusted by selecting the polyethylene resin and the polymer-type antistatic agent. The melt viscosity of the resin composition during extrusion can also be adjusted by the type and amount of the foaming agent.

For protective sheet 1 and the resin foam sheet that constitutes protective sheet 1, the thickness determined by the above measurement is the "standard thickness", and in this specification, when "thickness" is simply mentioned without any special mention, this "standard thickness" is meant. Note that the "unloaded thickness" described below means this "standard thickness".

If a resin foam sheet whose thickness decreases significantly when pressure is applied is used as the protective sheet 1, the weight of the glass plate 2 may cause the protective sheet 1 to adhere to the glass plate 2 when the protective sheet 1 is placed on the glass plate 2. Therefore, it is preferable that the thickness of the resin foam sheet (protective sheet 1) when pressure is applied (hereinafter referred to as "thickness when pressure is applied") is close to the "unloaded thickness". Specifically, when the "unloaded thickness" of the resin foam sheet (protective sheet 1) is "T0" and the "thickness when pressure is applied" when a pressure of 4 kPa is applied is "Ta", the ratio of the "thickness when pressure is applied (Ta)" to the "unloaded thickness (T0)" measured in a state where no load is applied ("Ta/T0 x 100 (%)", hereinafter also referred to as "thickness retention rate") is preferably 65% or more. The thickness retention rate may be 70% or more, or may be 75% or more. In order to allow the protective sheet 1 to exhibit good cushioning properties, the thickness retention rate is preferably 95% or less. The thickness retention rate may be 90% or less, or may be 85% or less.

The resin foam sheet in the present embodiment preferably has an apparent density of 10 kg/ ^m3 or more. The apparent density is more preferably 15 kg/ ^m3 or more. The apparent density of the resin foam sheet is preferably 200 kg/ ^m3 or less. The apparent density is more preferably 150 kg/ ^m3 or less, and even more preferably 100 kg/ ^m3 or less.

The apparent density of the resin foam sheet can be determined as follows.
The apparent density can be calculated from the thickness (T: mm) of the resin foam sheet calculated as above and the basis weight (Y: g/m ² ) described below using the following formula.
Apparent density (kg/m ³ )=Y/T

In the resin foam sheet of this embodiment, in order to ensure a certain amount of fatty acid compounds retained in the surface layer, it is preferable that the amount of resin in the surface layer is larger than that in the center in the thickness direction. Specifically, when the resin foam sheet is divided into three in the thickness direction and divided into a sheet in the center in the thickness direction and sheets in both surface layers, it is preferable that the apparent density of the sheet in the surface layer is 1.1 times or more, and more preferably 1.2 times or more, the apparent density of the sheet in the center. The apparent density of the sheet in the surface layer may be 1.3 times or more, or may be 1.5 times or more, the apparent density of the sheet in the center. In addition, it is preferable that the apparent density of the sheet in the surface layer is 3 times or less than the apparent density of the sheet in the center. A non-foamed layer 11 may be formed on one or both sides of the foam layer 10 to increase the amount of resin in the surface layer of the resin foam sheet.

If a protective sheet made of a resin foam sheet extracts a large amount of sodium ions or lithium ions in a metal ion extraction test, there is a risk that the protective sheet may cause adhesion to the glass plate 2 that is difficult to remove simply by washing with water, thereby reducing the cleanliness of the glass plate 2. The resin foam sheet (protective sheet 1) of this embodiment has a total extracted amount of sodium ions and lithium ions of 0.1 mg/ ^m2 or less in a metal ion extraction test.

The metal ion extraction test for determining the amount of extracted sodium ions and lithium ions can be carried out as follows.
(Metal ion extraction test)
For extraction of metal ions, a multi-type ICP emission spectrometer ICPE-9000 manufactured by Shimadzu Corporation is used.
The equipment settings are as follows:
Observation direction: Axial direction Exposure time: 30 seconds High frequency output: 1.20 kW
Carrier flow rate: 0.7 mL/min
Plasma flow rate: 10.0 mL/min
Auxiliary flow rate: 0.6 mL/min
The sample to be measured is prepared as follows.
(1) A test piece of 10 cm x 10 cm is cut out from any part of the resin foam sheet.
(2) Place the test piece in a thick zipper bag measuring 12 cm x 17 cm.
(3) Add an additional 50 mL of ultrapure water to the zip-lock bag containing the test piece, and close the zip-lock while removing as much air as possible.
(4) The test piece and ultrapure water are placed in a ziplock bag and placed in a dryer at 70°C.
(5) After 20 minutes, shake the zipper bag by hand three times and return it to the dryer, turning the test piece upside down and upside down from when it was taken out.
(6) After another 20 minutes, shake the bag by hand three times again, and then collect the ultrapure water in the zipper bag as a sample for ICP measurement.
(7) The total amount of extracted sodium ions and lithium ions obtained by the measurement is divided by the surface area of the test piece (200 ^cm2 = 10 cm x 10 cm x 2 (surfaces)) to obtain the amount of extracted metal ions.
Three test pieces are taken from the resin foam sheet, a sample is obtained from each test piece, and the average value of the three measurements is regarded as the amount of metal ions extracted from the resin foam sheet.

The surface resistivity of the resin foam sheet constituting the protective sheet 1 is preferably 1×10 ¹⁴ Ω or less. The surface resistivity of the resin foam sheet can be 1×10 ⁹ Ω or more. The surface resistivity of the resin foam sheet may be 1×10 ¹² Ω or more. When only one of the two surfaces of the protective sheet 1 contacts the glass plate 2, only the side that contacts the glass plate 2 may have such a preferred surface resistivity. It is preferable that both surfaces of the protective sheet 1 have the above-mentioned surface resistivity.

The surface resistivity of the resin foam sheet can be measured as follows.
(Measurement of surface resistivity)
The surface resistivity can be measured by the method described in JIS K6911:1995 "General testing method for thermosetting plastics."
The test equipment used may be a digital ultra-high resistance/microcurrent meter, model number "R3840", and a resistivity chamber, model number "R12702A", both manufactured by Advantest Corporation.
The surface resistivity can be calculated by the following formula, in which electrodes are pressed against a test piece taken from a resin foam sheet with a load of about 30 N, the resistance value is measured after charging at 500 V for 1 minute, and the surface resistivity is calculated by the following formula.
ρs = (π(D+d)/(D-d)) × Rs
ρs: surface resistivity (MΩ)
D: inner diameter of the surface annular electrode (cm)
d: outer diameter of the inner circle of the surface electrode (cm)
Rs: Surface resistance (MΩ)
The test piece had a width of 100 mm, a length of 100 mm, and a thickness T (the thickness of the resin foam sheet as is).
The measurement is carried out after conditioning the test piece for at least 24 hours in an environment of a temperature of 20±2° C. and a relative humidity of 65±5% RH.
The test environment is a temperature of 20±2°C and a relative humidity of 65±5% RH.
The number of test pieces (n number) shall be five, and in principle, both the front and back sides of each shall be measured.

When the protective sheet 1 is in a state where moisture can easily permeate, ion diffusion can occur actively. Furthermore, when the protective sheet 1 is in a state where it is easy to absorb moisture and where moisture can easily permeate, low molecular weight components can also easily move. Therefore, the resin foam sheet used as the protective sheet 1 in this embodiment has a moisture permeability of 100 g/( ^m2 ·24 h) or less and a water absorption rate of 2.0 g/ ^m2 or less.

The moisture permeability of the resin foam sheet constituting the protective sheet 1 is preferably 90 g/( ^m2 ·24 h) or less, and more preferably 75 g/( ^m2 ·24 h) or less. The moisture permeability may be, for example, 10 g/( ^m2 ·24 h) or more.

The moisture permeability of the resin foam sheet can be determined by the following method.
(Measurement of Moisture Permeability)
The moisture permeability is measured in accordance with JIS Z0208:1976 Moisture permeability test method for moisture-proof packaging materials (cup method) condition B.
A moisture permeability cup (JIS standard φ60 mm) and a low temperature and constant temperature and humidity chamber HPAV-120-20 (manufactured by ISUZU SEISAKUSHO LTD.) are prepared, and three test pieces with a diameter of 70 mm and a thickness T (the thickness of the resin foam sheet itself) are cut out from three randomly selected locations on the resin foam sheet.
Use approximately 15 g of anhydrous calcium chloride (special grade) and perform the measurement under condition B (temperature (40±0.5°C), humidity (90±2%)).
Measurements are taken at intervals of 24, 48, 72 and 96 hours, and the moisture permeability is calculated from the following formula.
Moisture permeability (g/m ² ·24 h) = 24 × increased mass (g) / (time interval (h) × moisture permeability area 2.827 × 10 ^-3 (m ² ))
(The amount of water vapor (g) passing through the boundary surface in 24 hours converted to 1 ^m2 of test piece)

The water absorption rate of the resin foam sheet constituting the protective sheet 1 is preferably 1.8 g/ ^m2 or less, more preferably 1.5 g/ ^m2 or less, even more preferably 1.2 g/ ^m2 or less, and particularly preferably 1.0 g/ ^m2 or less. The water absorption rate may be, for example, 0.1 g/ ^m2 or more.

The water absorption rate of the resin foam sheet can be determined by the following method.
(Measurement of water absorption rate)
The measurement is carried out in accordance with JIS K6767:1999 (Method B).
Test piece: 120 mm length x 120 mm width x thickness T (the same thickness as the resin foam sheet)
Number of tests: 3 (with skin on both sides)
Measurement environment: The test piece is kept in a standard atmosphere of JIS K7100:1999, code "23/50", grade 2 for 24 hours or more, and then measured in the same environment.
Measuring method:
After allowing the test piece to absorb water for 24 hours, it is taken out and immersed in ethanol with a concentration of 95% or higher for 5 seconds, and then dried at 60°C for 5 minutes.
After drying, cut the test piece into a 100 mm square and immediately measure its mass.
Furthermore, the test piece is dried at 60°C for 24 hours and the mass is measured.
The water absorption rate (Qs) is calculated using the following formula.
Qs (g/ ^m2 ) = (W1 - W2) / (2A)
W1 (g): Mass immediately after cutting into 100 x 100 mm W2 (g): Mass after drying at 60°C for 24 hours A ( ^cm2 ): Area of test piece (0.1 x 0.1 = 0.001 ^m2 )

If the protective sheet 1 does not contain any moisture, it may be difficult for the protective sheet 1 to exhibit antistatic properties. Therefore, it is preferable that the resin foam sheet constituting the protective sheet 1 can contain a certain amount of moisture under normal temperature and humidity conditions. It is preferable that the resin foam sheet has a moisture content of more than 4000 ppm. The moisture content of the resin foam sheet is preferably 6000 ppm or more, and more preferably 8000 ppm or more. The moisture content of the resin foam sheet may be, for example, 50,000 ppm or less, 40,000 ppm or less, or 25,000 ppm or less.

The moisture content of the resin foam sheet can be calculated from the water absorption rate and the mass per unit area (basis weight) of the resin foam sheet. That is, when the water absorption rate of the resin foam sheet is Xg/ ^m2 and the basis weight is Yg/ ^m2 , the moisture content can be calculated by the following formula.
Moisture content (ppm) = [2X/Y] x ¹⁰⁶

The basis weight of the resin foam sheet can be determined by the following method.
(Measurement of Basis Weight)
A strip-shaped test piece having a width of 20 cm is taken from the resin foam sheet.
The test pieces are taken, for example, so that the direction perpendicular to the extrusion direction is the length direction.
The mass W (g) and area S (cm ² ) of the test piece are obtained, and the basis weight is calculated using the following formula.
Basis weight (g/ ^m2 ) = W/S x 10000
If it is difficult to cut a test piece with a width of 20 cm in the direction perpendicular to the extrusion direction, cut it into a rectangle of the possible size, and calculate the basis weight from its mass W (g) and area S (cm ² ) using the above formula.

The water absorption rate and moisture content of the resin foam sheet can be adjusted by adjusting the components contained in the resin composition. In other words, by increasing the amount of a highly hydrophilic additive used as a material for forming the resin foam sheet, the water absorption rate and moisture content of the resin foam sheet can be increased, and by decreasing the amount of the additive, the water absorption rate and moisture content can be decreased.

The water absorption rate and moisture content of the resin foam sheet can also be adjusted by the polyethylene resin. Moisture absorption in polyethylene resin does not usually occur in the crystalline region, but in the amorphous region. Therefore, using a polyethylene resin with low crystallinity can increase the water absorption rate and moisture content of the resin foam sheet, and using a polyethylene resin with high crystallinity can decrease the water absorption rate and moisture content.

Generally, hydrophilic functional groups such as hydroxyl groups and carboxyl groups may be present at the molecular ends of polyethylene resins. Therefore, using a polyethylene resin with a small average molecular weight (a polyethylene resin with a large number of molecules per unit mass) can increase the water absorption rate and moisture content of the resin foam sheet, while using a polyethylene resin with a large average molecular weight can decrease the water absorption rate and moisture content.

Even if the molecular weight of polyethylene resin is the same, the more branches there are, the more molecular ends there are. Therefore, even if the polyethylene resin has the same average molecular weight, using a polyethylene resin with more branches (lower melt mass-flow rate value) can increase the water absorption rate and moisture content of the resin foam sheet.

The moisture permeability of a resin foam sheet is affected by the speed of movement (diffusion speed) of water molecules in the resin composition that constitutes the resin foam sheet. Therefore, the moisture permeability of the resin foam sheet can be increased by using a polyethylene resin with low crystallinity, and the moisture permeability can be decreased by using a polyethylene resin with high crystallinity. The moisture permeability of a resin foam sheet is also affected by the open cell rate of the resin foam sheet. Therefore, the moisture permeability of a resin foam sheet increases when the open cell rate is increased, and decreases when the open cell rate is decreased.

In this way, the moisture permeability, water absorption rate, and water content of the resin foam sheet constituting the protective sheet 1 of this embodiment can be appropriately adjusted depending on the foaming state of the resin foam sheet and the resin composition used to form the resin foam sheet.

The resins contained in the resin composition of this embodiment include a polyethylene resin as a base resin and a polymer-type antistatic agent. The resin composition may contain other resins. The ratio of the total amount of the polyethylene resin and the polymer-type antistatic agent to the total amount of the resins contained in the resin composition may be, for example, 85% by mass or more. The ratio may be 90% by mass or more, 95% by mass or more, or 98% by mass or more.

The resins contained in the resin composition may be only polyethylene resin and polymeric antistatic agent. The content of the resins contained in the resin composition that are different from the polyethylene resin and the polymeric antistatic agent may be less than 15% by mass, less than 10% by mass, or less than 5% by mass. The content of the resin may be less than 2% by mass or substantially zero.

The content of the polyethylene resin in the resin composition can be, for example, 75% by mass or more. The content of the polyethylene resin in the resin composition may be 80% by mass or more, or may be 85% by mass or more.

The content of the polymeric antistatic agent in the resin composition can be, for example, 1% by mass or more. The content of the polymeric antistatic agent may be 2% by mass or more, or 3% by mass or more. The content of the polymeric antistatic agent can be, for example, 15% by mass or less. The content of the polymeric antistatic agent may be 14% by mass or less, or 13% by mass or less.

The content of the polymer-type antistatic agent can be, for example, 2 parts by mass or more when the content of the polyethylene resin contained in the resin composition is 100 parts by mass. The content of the polymer-type antistatic agent can be 4 parts by mass or more, or 6 parts by mass or more when the content of the polyethylene resin in the resin composition is 100 parts by mass. The content of the polymer-type antistatic agent can be, for example, 20 parts by mass or less per 100 parts by mass of the polyethylene resin. The content of the polymer-type antistatic agent can be 18 parts by mass or less, or 15 parts by mass or less per 100 parts by mass of the polyethylene resin.

The content of the surfactant in the resin composition can be, for example, 0.01% by mass or more. The content of the surfactant may be 0.03% by mass or more, or 0.05% by mass or more. The content of the surfactant in the resin composition is, for example, 2% by mass or less. The content of the surfactant may be 1% by mass or less, or 0.5% by mass or less.

The content of the surfactant can be, for example, 0.01 parts by mass or more when the content of the polyethylene resin contained in the resin composition is 100 parts by mass. The content of the surfactant can be 0.05 parts by mass or more, or 0.1 parts by mass or more when the content of the polyethylene resin in the resin composition is 100 parts by mass. The content of the surfactant can be, for example, 1.5 parts by mass or less per 100 parts by mass of the polyethylene resin. The content of the surfactant can be 1.0 parts by mass or less, or 0.5 parts by mass or less per 100 parts by mass of the polyethylene resin.

Other additives contained in the resin composition other than the surfactant and the polymer-type antistatic agent include, for example, lubricants, weather resistance stabilizers, light stabilizers, antioxidants, antibacterial agents, deodorants, pigments, and inorganic fillers. The content of other additives in the resin composition is, for example, 5% by mass or less. The content of other additives may be 2% by mass or less, or 1% by mass or less. The content of other additives may be substantially zero.

In this embodiment, the resin composition preferably does not contain a fatty acid compound that is used as a lubricant or surfactant. Examples of the fatty acid compound include fatty acids, fatty acid metal salts, fatty acid amides, and fatty acid esters.

Specific examples of fatty acids include saturated fatty acids such as lauric acid, palmitic acid, stearic acid, and behenic acid, unsaturated fatty acids such as oleic acid, erucic acid, linoleic acid, and linolenic acid, and in addition to these monocarboxylic acids, dicarboxylic acids such as dimer acid. Metals that constitute fatty acid metal salts include calcium, magnesium, aluminum, and zinc.

Fatty acid amides are acid amides derived from fatty acids. Examples of such fatty acid amides include those derived from aliphatic amines.

Specific examples of fatty acid amides include stearic acid amide, palmitic acid amide, oleic acid amide, erucic acid amide, methylene bisstearic acid amide, ethylene bisstearic acid amide, ethylene bisoleic acid amide, and ethylene bishydroxystearic acid amide.

Specific examples of fatty acid esters include butyl stearate, stearic acid monoglyceride, oleic acid monoglyceride, behenic acid monoglyceride, linoleic acid monoglyceride, ricinoleic acid monoglyceride, hydroxystearic acid triglyceride, sorbitan fatty acid ester, polyoxyethylene (5) glycerin monostearate, polyoxyethylene (20) glycerin monostearate, polyoxyethylene (5) monooleate, pentaerythritol tetrastearate, polyadipate pentaerythritol stearate, stearyl stearate, 1,2-oxystearic acid, and hydrogenated castor oil.

The total content of fatty acid compounds in the resin composition constituting the resin foam sheet of this embodiment is preferably 10,000 ppm or less, more preferably 1,000 ppm or less, even more preferably 100 ppm or less, and particularly preferably 10 ppm or less. The total content of fatty acid compounds may be 7 ppm or less, or may be 5 ppm or less. The total content of fatty acid compounds in the resin composition constituting the resin foam sheet of this embodiment may be 1 ppm or more, or may be 3 ppm or more. If necessary, the total content of fatty acid compounds may be 5 ppm or more, 10 ppm or more, or 100 ppm or more.

The content of fatty acid compounds in the resin foam sheet can be measured by a known method. Examples of such methods include the following.

The fatty acid metal salt is measured, for example, as follows.
The resin foam sheet is immersed in boiling methanol for extraction, cooled, and the precipitate that has formed is filtered off. The precipitate is suspended in dilute hydrochloric acid and subjected to solvent extraction using ether, whereby the fatty acids are liberated into the ether phase and the metals are extracted into the aqueous phase. The fatty acids can be analyzed by the method described below, and the metals can be identified and quantified using an ICP (inductively coupled plasma) emission analyzer or the like.

The fatty acids, fatty acid amides, and fatty acid esters can be quantified using a liquid chromatography-tandem mass spectrometer (LC-MS/MS) (e.g., "ACCELA" manufactured by Thermo Fisher Scientific).

The calibration curve used for quantifying the fatty acid compounds is prepared by diluting a 1000 ppm standard solution (methanol solution) of the fatty acid compound (fatty acid, fatty acid amide, fatty acid ester) to be quantified with methanol to prepare five different standard solutions (5 ppm, 2 ppm, 1 ppm, 0.5 ppm, 0.2 ppm) of different concentrations.
Moreover, the sample to be measured by LC-MS/MS is prepared as follows.
- Cut the resin foam sheet into a size of about 2 mm square and collect an extraction sample of about 0.15 g.
The extracted sample is accurately weighed, and the extracted sample is placed in a PTFE (polytetrafluoroethylene) pressure-resistant container together with 10 ml of methanol, and the pressure-resistant container is sealed.
- Heat the sealed pressure-resistant container in an oven at 120°C for 2 hours, then remove it from the oven and allow it to cool naturally in a room at room temperature.
- Open the pressure vessel that has been cooled to room temperature and filter the extract inside the vessel through filter paper (No. 5A).
The filtrate obtained by the above filtration is used as a measurement sample for LC-MS/MS.
-From the measurement results, the amount of fatty acid compounds in the filtrate is calculated using the calibration curve obtained earlier.
The content (ppm) of fatty acid compounds in the resin foam sheet is calculated from the amount of fatty acid compounds contained in the filtrate.

The polyethylene resin contained in the resin composition may be, for example, a high density polyethylene resin (PE-HD) produced by a catalytic method (medium-low pressure polymerization method) and having a high density of 942 kg/m ³ or more with almost no branching in the molecular structure. The polyethylene resin contained in the resin composition may be a linear low density polyethylene resin (PE-LLD) produced by a catalytic method (medium-low pressure polymerization method) and produced to have a density of 910 kg/m ³ or more and 925 kg/m ³ or less by introducing a comonomer. The polyethylene resin may be a medium density polyethylene resin (PE-MD) produced to have a density between the linear low density polyethylene resin (PE-LLD) and the high density polyethylene resin (PE-HD). The resin composition may contain a very low density polyethylene resin (PE-VLD) as the polyethylene resin. The polyethylene resin may be, for example, a low density polyethylene resin (PE-LD) produced by a high pressure polymerization method and having long chain branches in the molecular structure. The polyethylene resin may be a polyethylene resin derived from a fossil fuel from the viewpoint of easy procurement of raw materials. From the viewpoint of reducing the environmental load, the polyethylene resin may be a polyethylene resin derived from plants.

As described above, the resin composition of the present embodiment does not need to contain only one of these polyethylene resins, and may contain a plurality of these polyethylene resins. The resin composition preferably contains a polyethylene resin derived from plants. By containing a polyethylene resin derived from plants, the glass plate protection sheet can become a product that can contribute to reducing the environmental load.

The lower limit of the content of the plant-derived polyethylene resin in the resin composition may be, for example, 5% by mass or more, 10% by mass or more, 15% by mass or more, 20% by mass or more, or 25% by mass or more. The upper limit of the content of the plant-derived polyethylene resin in the resin composition may be, for example, 95% by mass or less, 90% by mass or less, 85% by mass or less, 80% by mass or less, or 75% by mass or less. All of the polyethylene resin in the resin composition may be plant-derived polyethylene resin.

The polyethylene resin in the resin composition of the present embodiment preferably has a density (density at 23° C.) of 925 kg/m ³ or more, and more preferably 926 kg/m ³ or more. The polyethylene resin in the resin composition preferably has a density of 935 kg/m ³ or less, and more preferably 934 kg/m ³ or less. When the resin composition contains multiple polyethylene resins, the density of the polyethylene resin in the resin composition means the density of the multiple polyethylene resins in a mixed state.

When the resin composition contains multiple polyethylene resins, the density of the polyethylene resin in the resin composition can be determined by, for example, preparing a mixture in which the multiple polyethylene resins are blended in the mass ratio in which they are contained in the resin composition, melt-kneading the mixture, preparing a sample from the resulting melt-kneaded product, and measuring the density of the sample.

The density of the polyethylene resin can be measured, for example, by the method described in JIS K 7112, and can be determined by the D method (density gradient tube).

The polyethylene resin in this embodiment is preferably a low-density polyethylene resin (PE-LD), which is a polymerized product by a high-pressure polymerization method. Crystalline and amorphous regions are formed in the resin foam sheet by the polyethylene resin. In many cases, the deterioration of the cleanliness of the surface of the glass plate 2 occurs when low-molecular-weight components contained in the resin foam sheet migrate from the inside of the resin foam sheet to the interface with the glass plate 2. The low-density polyethylene resin (PE-LD) has long chain branches, which is advantageous in forming large amorphous regions in the resin foam sheet. When low-molecular-weight components are contained inside the resin foam sheet, the low-molecular-weight components tend to remain in the amorphous regions rather than in the crystalline regions. Therefore, it is considered that the inclusion of low-density polyethylene resin (PE-LD) in the resin foam sheet makes it difficult for low-molecular-weight components to migrate to the surface of the glass plate 2.

The long chain branching of polyethylene can be confirmed, for example, by ¹³ C-NMR, and if it is confirmed that an alkyl group having a length equal to or longer than a hexyl group is present, it can be determined that the polyethylene has a long chain branching. The low-density polyethylene resin (PE-LD) preferably has an alkyl group having a length equal to or longer than a heptyl group, and more preferably has an alkyl group having a length equal to or longer than an octyl group.

The polyethylene resin as described above preferably has a molecular structure with a certain degree of bulkiness, for example, preferably has a moderately low melt mass flow rate. The melt mass flow rate (MFR) of the polyethylene resin is preferably 7 g/10 min or less. The melt mass flow rate (MFR) of the polyethylene resin is more preferably 6 g/10 min or less, and even more preferably 5 g/10 min or less. In order to reduce the equipment load when producing a resin foam sheet by the extrusion foaming method, the melt mass flow rate (MFR) of the polyethylene resin is preferably 0.1 g/10 min or more. The melt mass flow rate (MFR) of the polyethylene resin is more preferably 0.2 g/10 min or more, and even more preferably 0.3 g/10 min or more.

The melt mass flow rate (MFR) can be measured according to JIS K7210:1999 "Plastics - Test method for melt mass flow rate (MFR) and melt volume flow rate (MVR) of thermoplastic plastics." The melt mass flow rate (MFR) can be measured according to "b) Method for measuring the time it takes for a piston to move a specified distance" described in Method B of the same standard. Specifically, the melt mass flow rate (MFR) can be measured using, for example, the "Semi-Auto Melt Indexer 2A" manufactured by Toyo Seiki Seisakusho Co., Ltd.

The measurement conditions for the melt mass-flow rate (MFR) can be as follows.
Sample: 3 to 8 g
Preheat: 270 seconds Load hold: 30 seconds Test temperature: 190°C
Test load: 21.18N
Piston travel distance (interval): 25mm
The test is performed on each sample three times, and the average of the three tests can be used as the melt mass-flow rate (g/10 min).

The polymer-type antistatic agent may, for example, be a polyalkylene oxide-type antistatic agent, which is a copolymer of an olefin block and a hydrophilic block such as ethylene oxide or propylene oxide; or an ionomer-type antistatic agent, which is a high molecular weight compound having a carboxylate or sulfonate in the molecule. The resin composition of this embodiment preferably contains an ionomer-type antistatic agent as the polymer-type antistatic agent.

Ionomer-type antistatic agents are also believed to be effective in preventing the migration of lithium ions or sodium ions to the interface with the glass plate 2 when the resin composition contains these ions.

The salt contained in the molecular structure of the ionomer type antistatic agent may be a carboxylate or a sulfonate. The ionomer type antistatic agent of the present embodiment is preferably a copolymer containing ethylene and an unsaturated carboxylic acid as constituent units. The copolymer may be a random copolymer, a block copolymer, or a graft copolymer.

As the ethylene-unsaturated carboxylic acid copolymer, for example, one having a structure in which a part or all of the carboxyl groups are neutralized with an alkali metal or the like can be used. Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic anhydride, monomethyl maleate, and monoethyl maleate. As the unsaturated carboxylic acid, acrylic acid or methacrylic acid is preferable. In addition, examples of other monomers that can be structural units in the ethylene-unsaturated carboxylic acid copolymer apart from ethylene and unsaturated carboxylic acid include vinyl esters such as vinyl acetate, unsaturated carboxylic acid esters such as methyl acrylate, ethyl acrylate, isobutyl acrylate, n-butyl acrylate, isooctyl acrylate, and methyl methacrylate, and carbon monoxide.

The ethylene-unsaturated carboxylic acid random copolymer may contain, for example, an ethylene component in a ratio of 60% by mass or more and 90% by mass or less. The ratio of the ethylene component may be 70% by mass or more and 88% by mass or less. The ethylene-unsaturated carboxylic acid random copolymer may contain, for example, an unsaturated carboxylic acid component in a ratio of 10% by mass or more and 40% by mass or less. The ratio of the unsaturated carboxylic acid component may be 12% by mass or more and 30% by mass or less. The ethylene-unsaturated carboxylic acid random copolymer may contain, for example, another unsaturated monomer component in a content of 30% by mass or less. The content of the other unsaturated monomer component is preferably 20% by mass or less. The ethylene-unsaturated carboxylic acid random copolymer of this embodiment is preferably an ethylene-methacrylic acid random copolymer.

As the ion source of the ionomer, an alkali metal, i.e., lithium, sodium, potassium, rubidium, or cesium, is preferably used. As the ion source of the ionomer, potassium is preferred. The content of the alkali metal cation in the ionomer is desirably in the range of 0.4 to 4 moles, preferably 0.6 to 2 moles, per kg of the ionomer type antistatic agent.

From the viewpoint of dispersibility in the resin foam sheet, the ionomer type antistatic agent preferably has a melt mass flow rate of 10 g/10 min or more. The melt mass flow rate of the ionomer type antistatic agent is more preferably 11 g/10 min or more. The melt mass flow rate of the ionomer type antistatic agent may be 25 g/10 min or less. The melt mass flow rate of the ionomer type antistatic agent may be 20 g/10 min or less, or 15 g/10 min or less.

From the viewpoint of increasing the melt viscosity of the resin composition and making it easier to suppress the variation in bubble expansion at the surface layer of the resin foam sheet, an ionomer type antistatic agent having a small melt mass flow rate may be used. The ionomer type antistatic agent used for such a purpose may have a melt mass flow rate of, for example, less than 10 g/10 min. The melt mass flow rate of the ionomer type antistatic agent may be 8 g/10 min or less, or 6 g/10 min or less. The melt mass flow rate of the ionomer type antistatic agent may be 5 g/10 min or less. The melt mass flow rate of the ionomer type antistatic agent may be, for example, 1 g/10 min or more.

From the viewpoint of dispersibility in the resin foam sheet, the melt mass flow rate of the ionomer type antistatic agent is preferably 25 g/10 min or less. The melt mass flow rate of the ionomer type antistatic agent may be 20 g/10 min or less, or may be 15 g/10 min or less. From the viewpoint of easily suppressing the variation in bubble expansion in the surface layer portion of the resin foam sheet, the melt mass flow rate of the ionomer type antistatic agent is preferably 1 g/10 min or more. The melt mass flow rate of the ionomer type antistatic agent may be 2 g/10 min or more, or may be 3 g/10 min or more.

Examples of the surfactant contained in the resin composition include nonionic surfactants, anionic surfactants, cationic surfactants, and amphoteric surfactants.

Examples of the nonionic surfactants include ester surfactants in which a polyhydric alcohol such as glycerin or sugar is ester-bonded to a fatty acid; ether surfactants such as polyoxyethylene alkyl ether and polyoxyethylene alkylphenyl ether; ester/ether surfactants in which an alkylene oxide is added to a fatty acid or a polyhydric alcohol fatty acid ester; and amide surfactants such as fatty acid alkanolamides in which a hydrophobic group and a hydrophilic group are bonded via an amide bond.

Examples of the anionic surfactants include sulfonate type surfactants such as alkyl sulfonates, dialkyl sulfosuccinates, alpha olefin sulfonates, linear alkyl benzene sulfonates, naphthalene sulfonate-formaldehyde condensates, alkyl naphthalene sulfonates, and N-methyl-N-acyltaurate salts; carboxylate type surfactants such as aliphatic monocarboxylates, polyoxyethylene alkyl ether carboxylates, N-acylsarcosinates, and N-acyl glutamates; sulfate ester salt type surfactants such as alkyl sulfates, polyoxyethylene alkyl ether sulfates, and oil and fat sulfate ester salts; and phosphate ester salt type surfactants such as alkyl phosphates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkyl phenyl ether phosphates. Examples of metals constituting the salt include alkali metals such as sodium, potassium, and lithium, and alkaline earth metals such as calcium and magnesium.

Examples of the cationic surfactant include quaternary ammonium salt surfactants such as alkyl ammonium salts and alkyl benzyl ammonium salts; and amine salt surfactants such as N-methyl bishydroxyethylamine fatty acid ester hydrochloride.

Examples of the amphoteric surfactant include betaine type surfactants such as alkylbetaines; amino acid type surfactants such as alkylamino fatty acid salts; and amine oxide type surfactants such as alkylamine oxides.

The resin composition of this embodiment preferably contains an anionic surfactant or a nonionic surfactant among the surfactants. The resin composition of this embodiment preferably contains an anionic surfactant, such as sodium alkylsulfonate or sodium linear alkylbenzenesulfonate.

The resin foam sheet of this embodiment, which is made of the resin composition, is used as a protective sheet 1 for a glass plate 2 in place of interleaf paper as described above, and is used in such a way that it is in surface contact with the glass plate 2 and then peeled off from the surface of the glass plate 2, and therefore antistatic properties are imparted by the polymeric antistatic agent and surfactant. The resin composition of this embodiment may not contain only one of these, or may not contain both.

The resin foam sheet of this embodiment has the above-mentioned characteristics and can be suitably used as a protective sheet for a glass plate. In this embodiment, a specific resin foam sheet is exemplified as a protective sheet for a glass plate, but the protective sheet for a glass plate of the present invention is not limited to the above-mentioned examples.

The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these.

First, the following polyethylene resins, designated by abbreviations "PE1" and "PE2", were prepared.
PE1: low-density polyethylene resin derived from fossil fuels produced by high-pressure polymerization, density: 931 kg/m ³ , MFR=4.0 g/10 min, plant-derived content: 0%, fossil fuel-derived content: 100%.
PE2: low-density polyethylene resin derived from plants produced by high-pressure polymerization, density: 923 kg/m ³ , MFR=2.7 g/10 min, plant-derived content: 95%, fossil fuel-derived content: 5%.
In addition, four types of polymer-type antistatic agents, designated by the abbreviations "AS1-1,""AS1-2,""AS2," and "AS3," were prepared.
AS1-1: An ionomer type antistatic agent which is an ethylene-methacrylic acid random copolymer containing potassium as an ion source. MFR = 12 g/10 min
AS1-2: An ionomer type antistatic agent which is an ethylene-methacrylic acid random copolymer containing potassium as an ion source. MFR = 4 g/10 min
AS2: A polyalkylene oxide type antistatic agent having a polyolefin block and a hydrophilic block composed of polyoxyalkylene. MFR=28 g/10 min
AS3: A polyalkylene oxide type antistatic agent having a polyolefin block and a hydrophilic block composed of polyoxyalkylene. MFR = 33 g/10 min
Furthermore, four types of surfactants, designated by the abbreviations "SA1,""SA2,""SA3," and "SA4," were prepared.
SA1: Sodium alkylsulfonate SA2: Sodium dodecylbenzenesulfonate SA3: Polyethylene glycol SA4: Stearic acid monoglyceride Furthermore, one type of foam regulator, designated by the abbreviation "NA1", was prepared.
NA1: Azodicarbonamide-containing masterbatch

(Rating 1)
Example 1
(Preparation of resin foam sheet)
(Production Example a)
A mixture (blending No.: #E1) was prepared by blending a fossil fuel-derived polyethylene resin "PE1", a polymeric antistatic agent "AS1-1", and an air bubble regulator "NA1" in the ratios shown in Table 1.
A circular die with an outlet diameter of 222 mm (slit width 0.04 mm) was attached to the tip (tip of the second extruder) of a tandem extruder in which two extruders, a first extruder and a second extruder, were connected in that order in the extrusion direction.
The mixture was supplied to a first extruder (cylinder diameter: φ90 mm) of a tandem extruder, and melt-kneaded in the first extruder so that the maximum temperature reached was 210° C. While melt-kneading, mixed butane (isobutane/normal butane=50/50 (molar ratio)) was injected as a foaming agent from the middle of the first extruder, and further melt-kneading was performed.
The amount of the mixed butane injected was adjusted so that the ratio was 18 parts by mass per 100 parts by mass of the polyethylene resin.
After the melt-kneading in the first extruder, the melt-kneaded product was cooled to a temperature range suitable for foaming (111° C.) in a second extruder (cylinder diameter: φ150 mm) connected to the first extruder, and extruded and foamed into the atmosphere through the circular die.
The resin temperature at that time was 112°C.
The extruded tubular foam was cooled by blowing air onto it, and then cooled by placing it on a cooling mandrel with a diameter of 770 mm and a length of 650 mm. The tubular foam was cut along the extrusion direction with a cutter provided on the rear side of the cooling mandrel, and wound up at a speed of 40 m/min to obtain a long strip-shaped foam sheet, thereby obtaining a resin foam sheet (protective sheet) of Production Example (a) of Example 1. In addition, a resin foam sheet (protective sheet) of Example 5 was obtained from a material (mixture No.: #E5) in which "AS1-2" was used instead of "AS1-1" as the polymer-type antistatic agent. Furthermore, a resin foam sheet (protective sheet) of Example 6 was obtained from a material (mixture No.: #E6) in which the amount of "AS1-1" used was changed to 25 parts by mass. The basis weight, thickness, and apparent density of the obtained resin foam sheet were measured, and the results are also shown in Table 1.

(Production Example b)
The resin foam sheets of Examples 2 to 4 were produced using materials #E2 to #E4 prepared in the same manner as in Production Example (a), except that a surfactant was further used and the amounts of the surfactant and the polymer-type antistatic agent used were as shown in Table 1 below.

(Production Example c)
The resin foam sheet of Example 7 was produced using the material #E7 prepared in the same manner as in Example 1, except that the fossil fuel-derived polyethylene resin "PE1" was changed to the plant-derived polyethylene resin "PE2."

(Comparative Examples 1 to 4)
Comparative Examples 1 to 4 were prepared in the same manner as in Example 1, except that the materials used were changed to those shown in Table 1 below (mixture Nos.: #C1 to #C4).

As described above, except for Example 4, which produced a resin foam sheet with a high apparent density (low degree of expansion), the other Examples and Comparative Examples produced roughly similar resin foam sheets.

The resin foam sheets of each Example and Comparative Example were measured for surface resistivity, moisture permeability, water absorption rate, moisture content, and ion extraction amount (Na, K, Li). In addition, the resin foam sheets of each Example and Comparative Example were examined for their effect on glass plates. The results are shown in Table 2.

The impact on glass plates was evaluated as follows:

(Contact angle)
The resin foam sheets obtained in the Examples and Comparative Examples were used to simulate a case where a glass plate for display was wrapped and stored for a long period of time, and the effect on the cleanliness of the surface of the glass plate was evaluated by the contact angle.
First, the resin foam sheet was cut into a size of 5 cm x 10 cm to prepare a test piece.
The test piece was placed on a cleaned and dried glass plate (alkali-free glass OA-11, manufactured by Nippon Electric Glass Co., Ltd.), and a weight of 1 kg was placed on the test piece so that the entire test piece was subjected to a load. The test piece was then heated in a thermo-hygrostat (manufactured by ISUZU SEISAKUSHO, product name "HPAV-120-40") according to the following cycle.
(Cycle Conditions)
The heating in the thermo-hygrostat was carried out in one cycle consisting of the following steps (1) to (4).
(1) Raise the temperature from 20°C and 60% RH to 60°C and 90% RH in 1 hour (2) Hold at 60°C and 90% RH for 1 hour (3) Lower the temperature from 60°C and 90% RH to 20°C and 60% RH in 1 hour (4) Hold at 20°C and 60% RH for 1 hour After the above cyclic heating test, the test specimen was removed from the surface of the glass plate, washed with cleaning water containing 0.4% household alkaline detergent (manufactured by Kao Corporation, product name "Attack"), rinsed with distilled water, and then dried at a temperature of 30°C and a relative humidity of 0% for 24 hours.
The contact angle of purified water on the surface of the glass plate that had been in contact with the sample was measured using a solid-liquid interface analyzer (product name "DROP MASTER 300") manufactured by Kyowa Interface Science Co., Ltd.
The contact angle was calculated by averaging 20 measurements.

A glass plate (glass size 50 mm x 40 mm) was sandwiched between the resin foam sheets, and cycle tests were performed at 50°C, 70% RH, and room temperature. After that, the glass was rinsed with 100 mL of pure water, and the number of foreign particles with a size of 10 μm or more adhering to the surface of the glass plate was measured using a microscope.
Based on the results, the staining of the glass plate was evaluated according to the following four-level scale.
(Glass contamination)
◎: 3 or less foreign objects ○: 4 to 10 foreign objects △: 11 to 25 foreign objects ×: 26 or more foreign objects

(Glass sliding properties)
Glass sliding property evaluation: To evaluate glass sliding property, the dynamic friction coefficient was measured.
Pretreatment: First, a glass plate and a resin foam sheet were each cut into pieces measuring 50 mm x 70 mm to prepare a plurality of pieces.
Glass plates and resin foam sheets were stacked alternately to create a rectangular parallelepiped laminate.
A weight of 1 kg was placed on the laminate, and it was kept in an environment of 60° C. and 90% RH for 72 hours.
After 72 hours, a glass plate was removed from the laminate and used as a measurement sample.

(Measurement of coefficient of friction)
The dynamic friction coefficient of the obtained measurement sample was measured in accordance with the friction coefficient test (JIS K7125).
The test equipment used was a universal testing machine "Autograph AG-X plus 100kN" manufactured by Shimadzu Corporation and a universal testing machine data processing machine "TRAPEZIUM X" manufactured by the same company.
The opposing material was the resin foam sheet produced in each of the Examples and Comparative Examples, and the sliding piece was a fixed jig with a glass plate (50 mm x 70 mm) attached to one side thereof so that the total mass was 20 g. The dynamic friction coefficient was measured under the following measurement conditions.
Test speed: 100 mm/min
Number of tests: 3
Dynamic friction coefficient (μD): integral average load (N) / normal force 0.215 (N)
No springs are used, PE line and hooks are standard

From the dynamic friction coefficient obtained in the above measurement, the glass sliding property was judged as follows.
◯: Dynamic friction coefficient is 3.5 or more and 9 or less △: Dynamic friction coefficient is 2.0 or more and 3.4 or less ×: Dynamic friction coefficient is 1.0 or more and 1.9 or less

Each evaluation of the resin foam sheet was assigned a score as shown below, and an overall evaluation was made based on the total score of (1) to (4).
The results are shown in Table 2 above.
(1) Apparent density: 100 kg/ ^m3 or less, 1 point. Over 100 kg/ ^m3 , 0 point.
(2) Contact angle: 7 degrees or less, 2 points; more than 7 degrees but less than 10 degrees, 1 point; 10 degrees or more, 0 point.
(3) Glass staining: 3 points for "◎", 2 points for "〇", 1 point for "△", and 0 points for "×".
(4) Glass sliding property: 2 points for "good", 1 point for "good", 0 point for "bad".

Overall rating: 7 points or more: ◎, 5-6 points: 〇, 3-4 points: △, 2 points or less: ×

As can be seen from the above table, the present invention provides a glass plate protective sheet that can prevent excessive slippage between the glass plate and the protective sheet.

(Rating 2)
The resin foam sheets (protective sheets) of Examples 8 to 14, which are made of materials #E1 to #E7 and have surface roughnesses as shown in Table 3 below, were investigated for properties such as the ratio (Ta/T0 x 100 (%)) of "thickness under pressure (Ta)" to "no-load thickness (T0)" and static electricity decay rate. In addition, the resin foam sheets (protective sheets) of Comparative Examples 5 to 8, which are made of materials #C1 to #C4 and have surface roughnesses as shown in Table 3 below, were similarly evaluated. The results are shown in Table 3 below.

From the above table, it can be seen that the present invention provides a protective sheet for glass plates that can suppress excessive slippage between the glass plate and the protective sheet.

1: Protective sheet, 2: Glass plate, 10: Foam layer, 11: Non-foam layer

Claims (8)

A glass plate protective sheet made of a resin foam sheet,
The resin foam sheet is
At least the surface is made of a resin composition containing a polyethylene resin,
The thickness is 0.1 mm or more and 0.5 mm or less,
The total amount of sodium ions and lithium ions extracted in the metal ion extraction test is 0.1 mg/ ^m2 or less,
Moisture permeability is 100 g/( ^m2 ·24 h) or less, and
A glass plate protective sheet having a water absorption rate of 2.0 g/ ^m2 or less. The resin foam sheet is
The surface resistivity of at least one of the two surfaces is 1×10 ¹² Ω or more and 1×10 ¹⁴ Ω or less,
The electrostatic decay rate is 40% or more and 95% or less,
The resin composition contains an ionomer type antistatic agent,
2. The glass-plate protective sheet according to claim 1, wherein the content of the ionomer-type antistatic agent in the resin composition is 5 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the polyethylene resin. The resin foam sheet is
The thickness under pressure, which is the thickness measured when a pressure of 4 kPa is applied in the thickness direction, is 65% to 95% of the unloaded thickness, which is the thickness measured without applying pressure;
2. The glass plate protective sheet according to claim 1, wherein the arithmetic mean roughness of at least one of the two surfaces is 50 μm or more and 120 μm or less, and the value obtained by dividing the ten-point mean roughness of the surface by the arithmetic mean roughness is 3.0 or less. The resin foam sheet is
The thickness under pressure, which is the thickness measured when a pressure of 4 kPa is applied in the thickness direction, is 65% to 95% of the unloaded thickness, which is the thickness measured without applying pressure;
3. The glass plate protective sheet according to claim 2, wherein the arithmetic mean roughness of at least one of the two surfaces is 50 μm or more and 120 μm or less, and the value obtained by dividing the ten-point mean roughness of the surface by the arithmetic mean roughness is 3.0 or less. 4. The glass plate protective sheet according to claim 1, wherein the resin composition contains an ionomer type antistatic agent, and the ionomer type antistatic agent has a melt mass flow rate of 25 g/10 min or less as measured by JIS K7210:1999 Method B. 5. The glass plate protective sheet according to claim 2, wherein the ionomer type antistatic agent has a melt mass flow rate of 25 g/10 min or less as measured by JIS K7210:1999 Method B. The resin composition includes an anionic surfactant, and
5. The glass plate protective sheet according to claim 1, wherein the anionic surfactant is contained in an amount of 0.01 parts by mass or more and 0.5 parts by mass or less with respect to 100 parts by mass of the polyethylene resin. The glass plate protective sheet according to any one of claims 1 to 4, wherein the resin composition contains the polyethylene resin derived from plants.