KR20040071992A - Degradable mulching film coated water-absorbent material - Google Patents

Abstract

PURPOSE: A biodegradable mulch film obtained by coating a hydrophilic and degradable material on a degradable resin film is provided. It is biodegradable within several months, provides an excellent weed control value and permits the mass production within a short time while being lightweight and cost effective. CONSTITUTION: The biodegradable mulch film is obtained by the steps of: preparing an inflation film by copolymerizing or blending thermoplastic resin and a hydrophilic compound(H-(O-R1)l-OH in which L is a recurring unit and Ri is an aliphatic hydrocarbon containing 2C) and then extruding with an inflation extruder; preparing a compressed film by compressing the inflation film with a compression roll; spray-coating an absorbable polymer on one surface of the film with a spray coater while transferring the film to an induction roll, drying and winding it. In the coating process, the degradable polymer is coated in bilayer or multilayer structures.

Description

Degradable mulching film coated water-absorbent material

The present invention relates to a degradable mulching film, and more particularly to a degradable mulching film produced by coating a resin that is decomposed in soil or water for the purpose of weed control.

In the last few centuries, weed control for rice farming has led to a decrease in productivity and an increase in production costs depending on manpower, but the development of chemical synthetic herbicides in the early 20th century has resulted in labor savings and productivity gains. However, excessive use of herbicides has contaminated the soil and water quality, destroying the ecosystem and distrusting the safety of the produce itself.

The demand for agricultural products using organic farming is increasing due to the increasing interest in environmental protection and safe farm products. As a result of organic farming, farmers are replacing synthetic fertilizers with organic fertilizers, which can be cultivated with a slight yield reduction using varieties that are resistant to pests. As a means for controlling weeds, various farming methods (weeding methods) using chaff, thunder, duck, etc. have been developed, but the disadvantages of using the method are cumbersome, additional labor and cost, and yields are further reduced.

Recently, mulching cultivation technology has been used to overcome the disadvantages of organic weed control. Mulching cultivation technology is an agricultural technology to cover the crops with transparent or opaque plastic film or paper to suppress the occurrence of various weeds and soil parasitic pests, to promote the growth of crops and to improve the yield. However, mulching using black or transparent plastic film, which is currently used, is not only cumbersome to remove after use, but also impossible to remove completely even by manual removal. It has a problem that causes it. On the other hand, paper mulching is decomposed after use, so there is no additional environmental pollution. However, if decomposition occurs before the colony of rice is formed in fresh water, it is not effective for weed control, and the production cost increases due to the high price of wood pulp and imported algae. There is this.

In the related art, there is a mulching material which gives a decay property in a natural state by mixing a photodegradation catalyst or a biodegradable plastic with a non-degradable plastic, and a mulching material manufactured by adding paper, organic material or inorganic material.

An example of a material developed as a collapsible mulching film is described in Korean Patent Application No. 1999-0003319, a mulching film developed based on polyethylene. Polyethylene has excellent intrinsic properties in terms of light transmittance, temperature rising effect, weed control and strength. In addition to the mulching film of the patent is to have a differentiated characteristics by adding an ultraviolet scattering agent, a red wavelength light-retaining agent, etc. for the purpose of additional pest prevention. However, the problem of its own decomposability cannot be solved, and fatal weakness remains in the soil and water environment, which has a big limitation as a mulching film for disposable crops. As an improved mulching film, Korean Patent Application No. 1996-064953 and US Patent No. 5,384,183 describe bio / photodegradable agricultural mulching film. The mulching film is a mixture of biodegradable aliphatic polymers with hardly decomposable polyethylene or a photodegradable titanium oxide catalyst is used to enhance the overall degradability while maintaining the advantages as a mulching film. However, there is a problem in that it is not possible to completely remove the hardly decomposable polyethylene remaining after use in agriculture for one year. In addition, due to the nature of the plastic, there is a disadvantage that it is difficult to expect the weed control effect because it does not adhere to the soil in the fresh water paddy. Therefore, there is an urgent need for the development of a fully degradable mulching film, which replaces the hardly decomposable polyethylene material and has good soil adhesion.

As another technique, a manufacturing technique for the purpose of a paper mulching sheet is known. 1997 Japanese Journal of Crop Science, 66 (4) pp. 710 ~ 713, 1997 Korean Journal of Weed Science 17 (4), 368 ~ 374, 2000 Korean Crop Science Association, 2000 Journal of Crop Science) 45 (3), pp. 216-219, Japanese Patent Application Laid-Open No. 4-264258, Korean Patent No. 20-0207043 and the like are well reported. The advantage of this paper mulching technology is that it has the advantage of recycling resources, and there is an advantage that the existing recycled paper can use the sheet as it is. Some have used rosin, polyacrylamide, or cooking oil on their surfaces to reinforce paper strength. In the end, however, paper using recycled paper has not overcome two critical drawbacks, making it a major defect as a mulching material. First, when applied to field soils, the thickness is more than 100 microns because of the weak strength of the sheet. Because of this, the drapey of paper sheets is very poor, which leads to problems in workability and coating properties. Second, wet strength is necessarily maintained to treat fresh water in paddy soil, and some elongation properties for mechanical workability. Despite this, paper showed very poor properties compared to vinyl film.

In order to supplement the above two technical disadvantages, Korean Patent Laid-Open No. 2001-0111469 provides a method of manufacturing mulching paper by mixing rice straw and pulp to maintain wet strength in fresh water. When mulching the area of 1000㎡ with the weight of 100-150g / m2, the amount required is 100-150kg. Recently, the Republic of Korea Patent Publication No. 2002-0079217 has developed a multi-layered mat laminated with a decomposable resin film and paper, but also has a disadvantage that the weight of the product per unit area of 65 ~ 100g / ㎡ is heavy and can not be easily transplanted. . In addition, the two patents also have a disadvantage in that they are difficult to use in general farms because of the high price of raw materials of wood pulp and imported paper in terms of using paper.

Therefore, the object of the present invention is to improve the above problems and can be applied to rice paddies and straight field paddy soil, can be significantly improved in the mechanical workability, mulching and transportability of agriculture by standardizing in the form of a roll of a certain size. In addition, as a mulching film, it can show basic weed control effect and temperature rise effect at the same time, and it can reduce production cost and product weight without using paper, and it is biodegradable within a few months after mulching. The present invention provides a mulching film suitable for agriculture and a method of manufacturing the same.

1 is a mulching film manufacturing process of the present invention.

2 is a block diagram of a mulching film of the present invention.

In the present invention to achieve the above object, in the mulching film, copolymerization or blending of the thermoplastic resin of the whole or partially decomposable and the hydrophilic compound of the formula (1) and then extrusion molding with an inflation molding machine to produce an inflation film, the inflation Compressing the film with a compression roll to produce a compressed film, while transporting the film to the induction roll spray coating the absorbent polymer with a spray coater on one surface of the film and dried in a drying room to provide a decomposable mulching film prepared by winding in a winder. .

H- (OR ₁ ) _ℓ- OH

Wherein l is a repeating unit and R ₁ is an aliphatic hydrocarbon having 2 carbon atoms.

In this case, the amount of the hydrophilic compound of Formula 1 is preferably 1.0 to 30.0% by weight of the resin, and if less than 1.0% or more than 30.0%, it is possible to manufacture, but the natural weed control effect, geothermal rise, mechanical workability as a mulching mat There is a problem that does not satisfy at the same time. In addition, the compound of Formula 1 preferably has a molecular weight of 400 to 20,000, and if the molecular weight is less than 400, there is a concern that the loss due to vacuum in the esterification reaction (esterification), if the molecular weight exceeds 20,000 branding As a result, there is a problem that it is difficult to have suitable film properties due to the phase separation problem.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

In the present invention, the term degradable is used to mean all of biodegradable, biodegradable, photodegradable and hydrolyzable.

1 is a process chart of manufacturing a decomposable mulching film of the present invention, as shown in FIG. 1, after copolymerizing or blending a thermoplastic resin, which is fully or partially decomposable, with a hydrophilic compound of Chemical Formula 1, through an biaxial extruder 10, an inflation molding machine 20. To produce a conventional inflation film 22, compress the inflation film with a compression roll 24 to produce a compressed film 26, and the compressed film 26a is an induction roll 32a, 32b. , 32c, 32d, 32e). At this time, the absorbent polymer 31 is coated on one surface of the film 26 by the spray coater 30 containing the absorbent polymer. The film coated with the absorbent polymer 31 is dried in a drying chamber 40 having a blowing temperature of 60 to 70 ° C., and the dried film is wound by a winder 50 to complete the present invention. Here, organic and / or inorganic additives may be further included in the copolymerization or branding.

Preferably, extrusion for inflation starts from copolymerization or branding of the various types of degradable resins with the hydrophilic compounds of Formula 1 or from organic and / or inorganic additives. For this purpose, a master chip may be manufactured through a twin screw extruder after kneading using Kneader, or may be manufactured using a Henssel mixer. In total, it is transported from a hopper loader to a hopper and melted and transported by being administered to an extruder. After preparing a conventional inflation film, a non-farming mulching film is prepared according to the process shown in FIG. 1 to coat an absorbent polymer. In this case, the spray coating machine used is a form in which a drying chamber is attached.

In addition, the coating and drying process may be repeated when the absorbent polymer is coated, or by repeatedly installing a spray coater to form a coating layer of two layers or multiple layers.

Degradable mulching mat of the present invention prepared as described above forms a coating layer 60 of the absorbent material on one surface of the film 26 as shown in FIG.

Preferred examples of the degradable thermoplastic resin used in the present invention prepared as described above are starch and derivatives thereof, derivatives such as cellulose, methylcellulose, hydroxymethylcellulose, and natural products such as polyhydroxybutylate and polyhydroxyvalate. Biodegradable materials such as recycled pulp, polylactic acid and copolymers thereof, polycaprolactone, biodegradable aliphatic polyesters of formulas (2) and (3), polybutylene adipate terephthalate copolymers, and polyester urethane copolymers Or a mixture of one or more selected from the group consisting of photodegradable resins comprising a photocatalyst in polyethylene. Although these resins show different processability and degradation behavior, they all have environmentally friendly characteristics, and when used in mulching cultivation, they have a great advantage of being naturally decomposed and reduced after application.

Wherein m is a repeating unit, R ₂ is an aliphatic hydrocarbon of 2-6 carbon atoms, and R ₃ is an aliphatic hydrocarbon of 2-8 carbon atoms.

Wherein n is a repeating unit and R ₄ is an aliphatic hydrocarbon of 2-6 carbon atoms.

The biodegradable aliphatic polyester represented by Chemical Formula 2 is a polycondensation aliphatic polyester, and the biodegradable aliphatic polyester represented by Chemical Formula 3 is a ring-opening polymerization aliphatic polyester.

Monomers commonly used in the preparation of the polycondensation aliphatic polyester of Chemical Formula 2 include ethylene glycol, propylene glycol, 1,4-butanediol, 1,6 Dihydric alcohols such as hexanediol (1,6-hexanediol), cyclohexanedimethanol, succinic acid, adipic acid, pimelic acid, azelaic acid, sebacic acid, fumaric acid, 2,2-dimethylglutaric acid, suberic acid, 1,3-cyclopentanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, diglycolic acid, itaconic acid And divalent carboxylic acids such as maleic acid and 2,5-norbornanedicarboxylic acid, preferably succinic acid and adipic acid.

Monomers commonly used in the preparation of the ring-opening polymerized aliphatic polyester of Formula 3 include caprolactone, lactide, glycolide, and the like.

The biodegradable aliphatic polyesters represented by Chemical Formulas 2 and 3 have different decomposition properties, but all have similar decomposition behaviors, so that all of them have the advantage of being decomposed and reduced after application to mulching plants.

In addition, in the present invention, the mulching film may further include an organic and / or inorganic compound as an additive to add various functionalities. Preferred examples thereof include plant fertilizer components such as calcium carbonate, talc, elvan, starch, palm bark, eosinophils, clam shells, ginkgo biloba, extenders, charcoal, urea, ammonium salts, nitrates, phosphates, molybdenum salts, borate salts, zirconium , One or two or more additives selected from the group consisting of far-infrared emitting components such as germanium are used. When the additive is included in the mulching film, it can be usefully used to control the degree of decomposition and the increase in strength of the mulching film in the soil.

Preferred examples of the absorbent material used in the present invention include natural polymers such as starch, cellulose, agar, glucomannan, derivatives thereof, poly (vinyl alcohol), and crosslinked polyacrylic acid sodium salt (polyacrylic). synthetic polymers such as acid sodium salt) and polyethyleneoxide; and starch-acrylonitrile graft copolymer gelate, starch-acrylic acid graft 1) A suspension composed of one or a mixture of two or more selected from the group consisting of a polymer, a polysaccharide-acrylic acid graft polymer, and the solid content of the suspension is 0.1-30%, more preferably 10-15% It is better to use a suspension. In the preferred coating method using the suspension, when the viscosity is low, air spray coating is performed, and when the viscosity of the suspension is high, airless coating may be performed at high pressure. The physicochemical properties of the absorbent polymer have absorption and water repellency, so that the soil adsorption power is not only increased, but the moisture of the mulching film lower layer is maintained due to the characteristics of rice farming where moisture is always required, which is a great help in water shortage.

In addition, to physically improve the properties of the mulching film of the present invention, one or more perforations of a suitable size may be formed. The purpose of drilling is for seed sowing and to improve adhesion by removing internal air during mulching.

Hereinafter, the present invention will be described in detail by way of examples, but the following examples are merely illustrative and the present invention is not limited to the following examples.

Example 1

555 kg of succinic acid (HOOC (CH ₂ ) ₂ COOH), 217 kg of adipic acid (HOOC (CH ₂ ) ₄ C00H), 1,4-butanediol (1,4-butanediol) , 782 kg of HO (CH ₂ ) ₄ OH), 87 kg of polyethylene glycol-1,000, a compound having the structure of Formula 1, and 0.43 kg of tetrabutyl titanate, a catalyst, and the temperature was raised to 200 ° C. in a stream of nitrogen. The reaction was allowed to flow out of the theoretical amount. After the transesterification reaction, 0.43 kg of tetrabutyl titanate as catalyst, 0.41 kg of trimethyl phosphate as stabilizer and 0.13 kg of cobalt acetate were added to slurry in 1,4-butanediol, and the mixture was mixed well at 230 ° C., and then the temperature was 250. The biodegradable aliphatic polyester copolymer having a number average molecular weight of 20,000 was obtained by performing a polycondensation reaction for 4 hours at a pressure of 0.3 mmHg while gradually raising the temperature. The biodegradable resin thus obtained was made into a film having a width of 1900 mm and a thickness of 30 μm with an inflation film molding machine having a processing temperature of 170 ° C. A starch aqueous solution having a solid content of 13% in the upper part of the film was coated to have a coating thickness of 10 μm, and then a mulching film having a total thickness of 40 μm and a basis weight of 43 g / m 2 through a drying chamber having a feed rate of 100 m / min and a blowing temperature of 60 to 70 ° C. Was prepared.

Example 2

A compound having the structure of Formula 1, 5% by weight of polyethylene glycol-1,000 and 95% of polybutylenesuccinate (SK Chemical SG-100) having the structure of Formula 2 in a kneading Henschel mixer Chips were prepared. The prepared master chip was made into a film having a width of 1900 mm and a thickness of 30 μm with an inflation film molding machine having a processing temperature of 180 ° C. A starch aqueous solution having a solid content of 13% in the upper part of the film was coated to have a coating thickness of 10 μm, and then a mulching film having a total thickness of 40 μm and a basis weight of 40 g / m 2 was passed through a drying chamber at a feed rate of 100 m / min and a blowing temperature of 60 to 70 ° C. Was prepared.

Example 3

Polybutylene succinate having 7% polyethylene glycol-10,000 by weight and 30% polycaprolactone (UCC tone-787) having the structural formula (3) and the structural formula (2) (polybutylenesuccinate, SK Chemical SG-100) 63% was kneaded sufficiently in a kneader for kneading to prepare a polymer blend, and a master chip was prepared using a twin screw extruder. The prepared master chip was made into a film having a width of 1900 mm and a thickness of 30 μm with an inflation film molding machine having a processing temperature of 150 ° C. A starch aqueous solution having a solid content of 13% in the upper part of the film was applied so that the coating thickness was 10 μm, and then a mulching film having a total thickness of 40 μm and a basis weight of 46 g / m 2 was passed through a drying chamber at a feed rate of 100 m / min and a blowing temperature of 60 to 70 ° C. Was prepared.

Example 4

Polybutylene succinate having a structure of Formula 1, 5% polyethylene glycol-10,000 by weight and 15% polylactide (Boehringer Ingelheim Resomer R-206) having a structure of formula 3 and a structure of formula 2 80% of polycarbonate (polybutylenesuccinate, SK Chemical SG-100) was kneaded sufficiently in a kneader for kneading to prepare a polymer blend, and a master chip was prepared using a twin screw extruder. The prepared master chip was made into a film having a width of 1900 mm and a thickness of 30 μm with an inflation film molding machine having a processing temperature of 190 ° C. A starch aqueous solution having a solid content of 13% in the upper part of the film was coated to have a coating thickness of 10 μm, and then a mulching film having a total thickness of 40 μm and a basis weight of 53 g / m 2 through a drying chamber having a feed rate of 100 m / min and a blowing temperature of 60 to 70 ° C. Was prepared.

Example 5

Polybutylenesuccinate adipate copolymer (polybutylenesuccinate-co-adipate, SK Chemical SG-200) having 5% polyethylene glycol-1,000 by weight and a structural formula (2) as a compound having the structure of Formula 1 at a processing temperature of 180 It was made into the film of width 1900mm and thickness 30micrometer by the inflation film forming machine of ° C. A starch aqueous solution having a solid content of 13% in the upper part of the film was coated to have a coating thickness of 10 μm, and then a mulching film having a total thickness of 40 μm and a basis weight of 41 g / m 2 through a drying chamber having a feed rate of 100 m / min and a blowing temperature of 60 to 70 ° C. Was prepared.

Comparative Example 1

The biodegradable resin used in Example 1 was processed by a melt coating method that is extruded through T-Dies at 250 ° C. on a rolled recycled paper (thickness: 65 μm, basis weight: 50 g / m 2, PanAsia Korea), and width 1,900 mm, A mulching mat having a thickness of 70 μm and a basis weight of 122 g / m 2 was prepared.

Comparative Example 2

The master chip used in Example 2 was processed by a melt coating method extruded through T-Dies at 230 ° C. on recycled paper (thickness 65 μm, 50 g / m 2, PanAsia Korea) in roll form, and width 1,900 mm and total thickness 75. A mulching mat having a basis weight of 174 g / m 2 was prepared.

Comparative Example 3

The master chip used in Example 3 was processed by a melt coating method extruded through T-Dies at a temperature of 230 ° C. on a roll-type recycled paper (thickness 65 μm, 50 g / m 2, PanAsia Korea), width 1,900 mm, and total thickness 75. A mulching mat having a basis weight of 170 g / m 2 was prepared.

[Comparative Example 4]

The master chip used in Example 4 was processed by a melt coating method extruded through a T-Dies at 270 ° C. on a roll-shaped recycled paper (thickness 65 μm, 50 g / m 2, PanAsia Korea), width 1,900 mm, and total thickness 75. A mulching mat having a basis weight of 183 g / m 2 was prepared.

[Comparative Example 5]

The same procedure as in Example 1 was repeated except that it was not coated with aqueous starch solution.

Comparative Example 6

The same procedure as in Example 2 was repeated except that it was not coated with aqueous starch solution.

Comparative Example 7

The same procedure as in Example 3 was repeated except that it was not coated with an aqueous solution of starch.

Comparative Example 8

The same procedure as in Example 4 was repeated except that it was not coated with aqueous starch solution.

After the mulching film of Examples 1 to 5 and Comparative Examples 1 to 8 were cut into a width of 1.9 m and a length of 200 m, respectively, and the rice transplanter was subjected to rice transplanting treatment using a rice transplanter to which the finished film can be mounted, a machine operation was performed. Sex (trouble count), soil adhesion of mulching mat and weed control rate (%) were evaluated by the following method. The evaluation results are presented in Table 1.

* Machine workability due to mulching mat (number of troubles): The efficiency of machine work is evaluated by the number of work troubles that occur when simultaneously coating and cutting using a rice transplanter. According to the hydrophilic property of the soil contact surface of the mulching mat and the coating property of the mat, there is a difference in adhesion to the soil and lifting. Measure the number of troubles that occur during 10a treatment and use it as a measure of machine workability by mulching mat.

* Weed control: Weed in Formula 1 by comparing the dry weight of weeds generated from weed control areas (untreated areas) without mulching mat coating and weeds generated from weed control areas (treatment areas) coated with mulching mats. The control price is calculated (%).

* Soil adhesion by mulching mat: Observed and evaluated the soil adhesion one month after suppression on the pavement. Evaluation grade: ◎ Very good, O Good, △ Normal, X Bad

division Composition of Mulching Film Effect of Mulching Film Film thickness (㎛) Weight per roll (kg) Machine workability (trouble count / 10a) Soil adhesion Weed control (%) Example 1 40 8.6 0 ◎ 96 Example 2 40 8.0 One △ 91 Example 3 40 9.2 0 ◎ 98 Example 4 40 10.6 2 △ 92 Example 5 40 8.2 One O 94 Comparative Example 1 70 24.4 3 O 93 Comparative Example 2 75 34.8 4 ◎ 95 Comparative Example 3 75 34.0 2 △ 90 Comparative Example 4 75 36.8 6 △ 89 Comparative Example 5 30 6.4 15 X 32 Comparative Example 6 30 5.8 12 X 36 Comparative Example 7 30 7.0 13 X 39 Comparative Example 8 30 8.4 17 X 28

※ Weight per roll is 200m in length

As described above, the mulch film for use in rice fields coated with a hydrophilic and degradable material on the degradable resin film according to the present invention is easy to be mechanized and provides excellent weed control without generating environmental pollution by completely biodegrading within a few months. It keeps moisture in the lower part of mulching film and has hydrophilic property on the surface, which improves mechanical workability and soil adhesion at the time of coating.It can be produced in large quantities in a short time by using various extrusion methods, and it is inexpensive and light in weight. There are advantages such as ease of use.

In addition, when the present invention is applied, not only weed control effects can be obtained during mulching cultivation, but additionally, mechanical workability improvement effect, geothermal synergy effect, and moisture moisturizing effect can be obtained at the same time.

Claims (6)

In the mulching film, Copolymerization or blending of the total or partially degradable thermoplastic resin and the hydrophilic compound of Formula 1 and extrusion molding with an inflation molding machine to produce an inflation film, compressing the inflation film with a compression roll to produce a compressed film, inducing the film A decomposable mulching film, which is produced by spray coating an absorbent polymer with a spray coater on one surface of a film while transferring to a roll, drying the film in a drying chamber maintaining a constant temperature, and winding the film. [Formula 1] H- (OR ₁ ) _ℓ- OH Wherein l is a repeating unit and R ₁ is an aliphatic hydrocarbon having 2 carbon atoms. The decomposable thermoplastic resin according to claim 1, wherein the degradable thermoplastic resin comprises starch and derivatives thereof, derivatives such as cellulose, methylcellulose, hydroxymethylcellulose, natural materials such as polyhydroxybutylate and polyhydroxyvalate, and recycled pulp and poly Biodegradable materials such as lactic acid and copolymers thereof, polycaprolactones, biodegradable aliphatic polyesters of formulas 2 and 3, polybutylene adipate phthalate copolymers, polyester urethane copolymers, and photocatalysts in polyethylene Decomposable mulching film, characterized in that one or a mixture of two or more selected from the group consisting of a photodegradable resin. [Formula 2] Wherein m is a repeating unit, R ₂ is an aliphatic hydrocarbon of 2-6 carbon atoms, and R ₃ is an aliphatic hydrocarbon of 2-8 carbon atoms. [Formula 3] Wherein n is a repeating unit and R ₄ is an aliphatic hydrocarbon of 2-6 carbon atoms. The method of claim 1, wherein the absorbent material is a natural polymer such as starch, cellulose, agar, glucomannan and derivatives thereof, and synthetic polymers such as poly (vinyl alcohol), cross-linked polyacrylic acid sodium salt, polyethylene oxide and the like. Decomposable mulching film, characterized in that a suspension made of one or a mixture of two or more selected from the group consisting of ronitrile graft copolymer gelling, starch-acrylic acid graft polymer, and polysaccharide-acrylic acid graft polymer . 4. The decomposable mulching film according to claim 3, wherein the solid content of the suspension is 0.1 to 30%. The decomposable mulching film of claim 1, wherein the decomposable polymer is coated in two layers or multiple layers. The decomposable mulching film of claim 1, wherein one or more perforations of a suitable size are formed.