JPH0443365B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing an antistatic film or sheet that is suitable for packaging items susceptible to static electricity damage and whose interior can be seen through.

Ordinary plastic film or sheet is
Its surface resistance value (measuring method is based on JIS K 6911, the same applies hereafter) is 10 ¹⁴ Ω or more, and the object itself easily becomes electrically charged, and it also strongly charges other objects that come in contact with it and rub against it. In order to package items that are susceptible to electrostatic damage, antistatic treatment is necessary. The present inventors conducted research on transparent plastic packaging materials with low surface resistance, and previously filed a patent application for an antistatic plastic film in Japanese Patent Application Laid-Open No. 115591/1983. The above patent application is
An important technical means is to provide a conductive layer of ionic conductive resin on the surface of a plastic film or sheet, but the electrical resistance of ionic conductive materials changes relatively largely due to changes in relative humidity. As a result, conductive performance deteriorates in low humidity environments. We conducted various research on technology to obtain transparent packaging materials by using conductive carbon black without using ionic conductive materials as the main conductive means, and published Japanese Patent Application Laid-Open No. 58-42456. I filed an application for the publication. The above-mentioned application uses a technical means to obtain a transparent conductive coating film by applying a conductive paint containing conductive carbon black, and it has good performance even in dry air with low relative humidity. Although a packaging material with a low drop in color can be obtained, it has low light transmittance and large haze, making it relatively difficult to confirm the contents.
In addition, when the content of carbon black in the coating film is increased, the carbon black tends to be easily detached, and when the surface is rubbed with white paper, black stains are observed.

The present invention is a method for manufacturing a see-through antistatic plastic film or sheet that uses carbon black, whose conductive performance is less likely to change due to relative humidity, as the main conductive means, and which improves transparency and reduces haze. It is an object of the present invention to provide a method for producing a film or sheet which has improved transparency and improved usability by preventing contamination due to detachment of carbon black.

The present invention involves coating a transparent plastic film or sheet with a conductive paint containing carbon black and drying it to form a transparent conductive layer, then applying a transparent synthetic resin paint on top of the conductive layer and drying it. This is a method for producing a see-through antistatic film or sheet, which is characterized by forming a surface layer with a thickness of 10 μm or less.

The transparent plastic film or sheet used in the present invention is selected from those that can be seen through and are suitable as packaging materials. For example, they are selected from films or sheets of polystyrene, polyvinyl chloride, polyester, polypropylene, polyethylene, cellulose plastics and the like. In order to improve the adhesion of the paint film, it is often better to perform corona discharge treatment.

This film or sheet serves as a base material for packaging materials, and is also used as a base material in the manufacturing method of the present invention. That is, a transparent layer of carbon black is preferably formed on this substrate.
Of course, if desired, on the surface layer described in detail later,
A layer of carbon black can also be provided. That is, by providing a layer of carbon black on the base material,
Rather than providing a surface layer thereon, it is also possible to provide a layer of carbon black on a separately prepared surface layer and then bond this to the base material. Usually, a transparent layer of carbon black is obtained by coating a conductive paint containing carbon black on a substrate. The conductive paint contains carbon black as an essential component, and may also contain a binder, a solvent, or a dispersion medium as required. Carbon black should be selected from those used as conductive fillers. Various brands are available, but particle size, dispersibility,
No criteria for selection that necessarily uniquely determines the degree of graphitization caused by firing, the degree of structure development, etc. has been found. It is a good idea to test several types of conductive fillers and select a brand that is suitable for each process and has as good transparency as possible for the same conductivity. The binder should be selected from among those that have a high adhesive strength with the base material. Examples of binders include latexes such as EVA latex, acrylic latex, and SB latex, PVA, cellulose derivatives, starch derivatives, and resins that are dissolved in solvents such as acrylic resins, EVA resins, and styrene resins. selected from. Particular attention should be paid to the amount of binder, and it should not be too large. That is, the proportion of carbon black in the dried coating should be relatively large.

This may seem at first glance to contradict the purpose of obtaining a see-through paint film, but if the carbon black concentration in the dried paint film is relatively low,
Although the mechanical strength of the coating film is increased and contamination due to detachment of carbon black is reduced, the balance between electrical conductivity and transparency deteriorates. That is, when the film thickness is increased to increase conductivity, the transparency becomes extremely poor. The concentration of carbon black in a coating film varies depending on the type of carbon black used, dispersion method, film formation method, etc., so it is difficult to specify a specific concentration range, but it is usually around 8% or higher. It is preferable to use 100% carbon black if you choose a film-forming method that is darker. That is, a coating film can be formed by agglomeration of carbon particles without using any binder.

However, if the carbon black concentration is 70% or more,
If it approaches 100%, the coating will become weaker and more likely to break before the next step, so care must be taken. Also,
When the carbon black concentration increases to over 70%,
Adhesive strength to the base material is also often insufficient. However, advantageously, as the concentration of carbon black increases, the thickness of the conductive carbon black layer that can be seen through becomes thinner, and it is thought that the film becomes mechanically imperfect. When a synthetic resin liquid is applied on top of the carbon black to form a surface layer, it is thought that the conductive layer of carbon black will not be destroyed and will allow the synthetic resin liquid to penetrate, allowing the surface layer synthetic resin to adhere to the base material. By selecting from among those with good properties, it is possible to improve the adhesion between the conductive layer and the base material. The coverage of the transparent layer of carbon black must be determined to provide the desired light transmission and conductivity. Generally speaking,
If the carbon black concentration is high and the binder concentration is low, the amount of coating required to obtain the desired conductive performance will be smaller, and the light transmittance will be higher, but the strength of the intermediate coating will be lower, so it may be difficult to tolerate depending on the manufacturing process used. The strength of the intermediate coating film that can be created should be considered when determining the formulation and amount of application. When applying a surface layer by spray coating, etc., which rarely comes into contact with the paint film, it is possible to use less binder and reduce the amount of coating.

If you compare paints with a certain blend and dispersion,
Since the larger the coating amount, the higher the conductivity and the lower the transparency, the coating amount should be determined to meet the required performance. If the coating amount is too small, the carbon black will form a layer and will no longer exhibit conductivity, making it impossible to obtain conductivity.

Examples of coating amounts and examples of conductive performance and transparency of the resulting products are shown in Examples. Regarding conductive performance, although a surface resistance of about 10 ¹⁰ Ω may be useful, the material of the present invention can have a resistance as low as about 10 ³ Ω. In addition, the see-through performance allows the contents to be confirmed in a bright place even with light transmission of about 10%, but the material of the present invention can achieve much higher light transmission and lower haze. Conventional see-through coatings containing carbon black as a conductive component had drawbacks such as low light transmission, large haze, and poor visibility, and black staining when rubbed with white paper. By providing a transparent synthetic resin surface layer with a volume resistivity of 1×10 ¹³ Ωcm or less and a thickness of 10μ or less,
A good antistatic film or sheet with low haze, improved transparency, and no visible staining even when rubbed with white paper is obtained. This surface layer can be provided by applying a transparent synthetic resin paint directly on top of the conductive paint film. Since this surface layer forms the surface layer of the antistatic plastic film or sheet, it should be selected to satisfy the various surface properties required for packaging materials, such as hardness, gloss, slipperiness, and anti-blocking properties. It is. Since this surface layer is related to the conduction between the conductive layer made of conductive material and the surface of the packaging material, the volume resistivity of the layer is 1×
10 ¹³ Ωcm, thickness must be less than 10μ.
If the layer has a volume resistivity of 1×10 ¹³ Ω or more and a thickness of 10 μ or more, the antistatic performance of the packaging material will be insufficient. However, even though the resin itself used for this surface layer has a high volume resistivity of 10 ¹⁴ Ωcm or more, the layer coated directly on the conductive layer made of conductive paint often has a volume resistivity of 10 ¹³ Ωcm or less. It was found that the material exhibits a low volume resistance. The cause of this is unknown, but
It is also conceivable that part of the carbon black, which is a conductive component, moves during film formation, alters the newly formed surface layer, and lowers the volume resistivity measured in the thickness direction of the surface layer. Therefore, whether or not the plastic can be used as a material for the surface layer should not be determined from the normal volume resistivity of the plastic.In order to make the film or sheet of the present invention,
This can be easily determined by forming a film by linear coating, bringing an electrode into contact with the resulting coated surface, and measuring the surface resistance. The relationship between this surface resistance and volume resistance is as follows.

Surface resistance value: Based on a method based on JIS K 6911. The measurement electrodes are a mercury electrode with a diameter of 5 cm and a donut-shaped mercury electrode with an inner diameter of 7 cm and an outer diameter of 8 cm placed concentrically with the mercury electrode.
11.8cm ² , and the circular pole is 19.6cm ² .

Calculation of the volume resistance of the surface layer Thickness of the surface layer T (cm) Measured on the surface of the conductive paint film r ₁ (Ω) Resistance value measured on the surface layer layered on the conductive paint film r ₂ (Ω) of the surface layer Volume resistance R (Ω·cm) Δr=(r ₂ −r ₁ ) (Ω) R=Δr/0.136T (Ω·cm) However, 0.136 is a constant determined by the electrode.

Calculation of surface resistance is based on the following formula.

Resistance value r (Ω) measured on the coating surface or surface layer surface Surface resistance (Ω) = 18.3r (Ω) However, 18.8 is a constant determined by the electrode.

Light transmittance and haze: Based on a method based on JIS K 7105.

Example 1 Styrene-butadiene latex with 50% non-volatile content
1.2 parts by weight, 1.4 parts by weight of carbon black, and water.
A conductive paint containing 97.4 parts by weight was prepared. This paint contains 2% non-volatile content and 70% of the dry film is carbon black. The above-mentioned conductive paint was applied in a dry amount of 0.12 gr/m ² onto the corona-treated surface of a biaxially stretched sheet of polystyrene, one side of which had been corona-treated, and dried. The light transmittance of the coating is 52%, haze
At 19%, the sheet has transparency, but when this coating is rubbed against white paper, it easily stains the paper black and cannot be used as a practical packaging material. The surface resistance of the coated surface was 4×10 ⁵ Ω. Lay it on top of this,
15% ethyl alcohol solution of nitrified cotton in dry weight
1.6gr/ ^m2 coated and dried. The specific gravity of nitrified cotton is 1.6
Then, the thickness of the coating film is 1×10 ^-4 cm. The surface resistance of the coated surface is 6×10 ⁵ Ω, and the light transmittance of the coated material is
54%, haze was 7%, and had good transparency and low surface resistance. Even when the coated surface was rubbed with white paper, no staining occurred. The volume resistivity of the thin film coated with nitrified cotton is calculated to be 8×10 ⁸ Ωcm,
Although the value of volume resistivity of nitrified cotton is low,
This is considered to be because the thin film of nitrified cotton is denatured by the components of the underlying conductive layer due to direct application. When a film was separately formed from this nitrified cotton coating solution and the volume resistivity was measured, it was 3×10 ¹¹ Ω·cm. This sheet can be thermoformed to make trays and containers, which are useful for packaging items that are sensitive to static electricity.

Example 2 0.4 parts by weight of styrene-butadiene latex with a nonvolatile content of 50%, 1.8 parts by weight of carbon black, and water.
A conductive paint containing 97.8 parts by weight was prepared. This paint contains 2% non-volatile content and 90% of the dry film is carbon black. The conductive paint described above was applied in a dry amount of 0.12 gr/m ² onto the corona-treated surface of a polypropylene film having a thickness of 60 μm, which had been corona-treated on one side, and dried. The coated material has a light transmittance of 38% and a haze of 25%, making it possible to see through it, but when this coating is rubbed against white paper, it easily stains the paper black and cannot be used as a practical packaging material. . The surface resistance of the coated surface was 5×10 ⁵ Ω.A 15% toluene solution of polymethyl methacrylate was applied thereon in a dry amount of 1 gr/m ² and dried. If the specific gravity of polymethyl methacrylate is 1, the coating layer is 1.
The thickness is ×10 ^-4 cm. The surface resistance of the coated surface is 7×
It was 10 ³ Ω. The volume resistivity of the applied thin layer of polymethyl methacrylate is calculated to be 8×10 ⁶ Ωcm;
The volume resistivity of polymethyl methacrylate is low, but this is because it is directly applied.
It is thought that the thin layer of methyl methacrylate is denatured by the components of the underlying conductive layer. When this polymethyl methacrylate coating liquid was separately formed into a film and its volume resistivity was measured, it was 2×10 ¹⁴ Ω·cm. The light transmittance of the coating was 40%, the haze was 8%, and no staining was observed even when the coating was rubbed with white paper. This film is useful in the form of bags for packaging items that are susceptible to static electricity damage.

Example 3 Styrene-butadiene latex with 50% non-volatile content
A conductive paint was prepared consisting of 18 parts by weight, 1 part by weight of carbon black, and 81 parts by weight of water. This paint contains 10% non-volatile matter and 10% of the dry film is carbon black. The conductive paint was applied in a dry amount of 0.5 gr/m ² to the corona-treated surface of a 60 μm polypropylene film, one side of which had been corona-treated, and dried. The light transmittance of the coated material was 35% and the haze was 15%, making it possible to see through it. Slight staining was observed when the coated surface was rubbed with white paper. The surface resistance of the coated surface was 2×10 ⁷ Ω, and it had sufficient antistatic performance. Layer polystyrene 15 on top of this coating.
% toluene solution at a dry weight of 1 gr/m ² and dried. If the specific gravity of polystyrene is 1, the thickness of the coating layer will be 1×10 ⁴ cm. The surface resistance of the coated surface is 6
It was ×10 ⁷ Ω. The volume resistivity of the applied polystyrene layer is calculated to be 2×10 ¹¹ Ω·cm. This polystyrene coating solution was separately formed into a film and the volume resistivity was measured to be 1×10 ¹⁵ Ω·cm. Even when the coating film was rubbed with white paper, no staining was observed. Light transmittance was 38%, haze was 7%, and visibility was improved. This film is useful when made into bags and used to package items that are susceptible to static electricity damage.

As detailed above, the plastic film or sheet produced by the method of the present invention contains carbon black as a conductive component whose conductive performance does not change depending on relative humidity, but it has improved light transmittance and
An antistatic film or sheet with reduced haze, improved transparency, low surface electrical resistance, and from which carbon black does not come off and contaminate objects, which is useful for packaging items susceptible to electrostatic damage. Packaging materials can be manufactured.

Claims (1)

[Claims] 1 Apply a conductive paint containing carbon black to a transparent plastic film or sheet and dry it to form a transparent conductive layer, then apply a transparent synthetic resin paint on top of the conductive layer and dry it to increase the thickness. A method for producing a see-through antistatic film or sheet, characterized by forming a surface layer of 10μ or less.