CN115058914A

CN115058914A - Coating composition for paper for food and medicine contact and preparation method thereof

Info

Publication number: CN115058914A
Application number: CN202210657968.6A
Authority: CN
Inventors: 喻力; 何占飞
Original assignee: UPM China Co Ltd
Current assignee: UPM China Co Ltd
Priority date: 2022-06-10
Filing date: 2022-06-10
Publication date: 2022-09-16

Abstract

The application provides a coating composition for wet-strength coating on the surface of paper, a preparation method thereof, and paper for food and medicine contact or base paper thereof produced by the coating composition. The paper for food and medicine contact or the raw paper thereof comprises a paper base material layer and a strengthening layer positioned on at least one surface of the paper base material layer, wherein the strengthening layer is formed by using the coating composition, and the coating composition contains a sizing agent, a film forming agent, a rheological agent and an aqueous solvent. By specifically designing the composition of the coating composition, excellent wet strength and excellent bonding ability between the hydrophobic layer and the strengthening layer are achieved while avoiding the introduction of toxic ingredients.

Description

Coating composition for paper for food and medicine contact and preparation method thereof

Technical Field

The present invention relates to a paper material having high strength in a wet state, and more particularly to a contact paper or a base paper product thereof for food and pharmaceutical packaging and kitchen or industrial scale for food and pharmaceutical use.

Background

Many different materials have been developed so far for use in direct contact with products for direct ingestion such as foods, beverages, and medicines, and paper materials are one of the most commonly used materials for the above applications due to advantages such as renewable resources, environmental protection and easy degradation of waste, appropriate price, and wide application range, under a large background of reducing the scale of use of non-renewable fossil materials in various countries throughout the world. Examples of such uses include soft packing paper, paper bags, paper capsules, cardboard packaging boxes, disposable lunch boxes, wiping tissues used in the kitchen or food industry fields, oil absorbing paper, steamer paper, cook-resistant paper bags, etc. for containing food ingredients, semi-finished products or cooked foods, and may include various containers for containing cosmetics, medicines, granules, drinks, beverages, etc., such as cartons of milk or fruit juice, tea bags, herbal packages, cosmetic packaging bags or packaging boxes, etc. A common feature of these applications is that the paper materials will come into contact with moisture or liquids in the food or ambient environment, such as steam, water, wine, lotion, cosmetics, beverages, soups, grease, dairy products, etc., to varying degrees during use, while the wet strength of the paper materials in contact with steam or liquids and/or wetted by liquids is a paramount concern to manufacturers and consumers alike, and there is a need for such paper materials to be able to achieve sufficient resistance to all normal or incidental mechanical, heating or cooking, etc., effects experienced during product production, packaging, storage, transportation, sale and consumption in a cost effective, process simple manner while maintaining the overall structural integrity of the product.

The practice of ensuring the wet strength of paper products commonly used in the prior art can be broadly divided into physical and chemical means, wherein the physical means includes adjusting the structure, size, morphology or relative content of fiber bundles, fibers, microfibrillated fibers or fillers, and changing the number of bonding sites or entanglement between fibers or between fibers and fillers at an appropriate level; chemical means typically include the addition of strengthening agents, particularly wet strength agents, to the pulp or wet end at one or more stages of the paper production process to adsorb onto various materials in the pulp, such as fillers, sizes, fines, etc., to achieve an improvement in the wet strength of the paper product. Although the polyamide epichlorohydrin resin is widely used as a wet strength agent in the field of papermaking, and the wet strength agent is generally commercially considered to have the advantages of no toxicity, no odor, wide application range and good strengthening performance, recent research finds that the process of adding the wet strength agent can introduce toxic substances, such as 3-chloro-1, 2-propanediol (chloroglycerol), 1, 3-dichloro-2-propanol and the like, into paper pulp in the production process and finally prepared paper products, has great stimulation on eyes, skin and mucous membranes, can generate toxicity after being swallowed in a body, and can possibly damage fertility, so that relevant regulations are improved in various countries, and the application of the toxic and harmful wet strength agent in the paper products for the purposes of food, beverage, medicine and the like is strictly limited, so as to avoid the toxic and harmful substances from being transferred from the paper products to the food, beverage, medicine and the like, The surface or inside of the beverage, medicine, and further cause injury to the consumer. Accordingly, it remains desirable to develop a novel paper product for food use in which toxic strengthening agents are minimized or eliminated altogether, yet excellent wet strength is maintained, while still exhibiting improvements in other properties, such as higher dry strength, etc. To date, no new technical development capable of solving the above problems has been reported in the prior art.

Disclosure of Invention

In view of the above problems, the inventors of the present application have conducted intensive studies and succeeded in developing a novel coating composition capable of forming a strengthening layer on the surface of a paper substrate to provide wet strength properties to the paper substrate, avoiding the use of toxic and harmful strengthening additives in the pulp, and solving the technical problems in the prior art which have been urgently solved for a long time in a simple and cost-saving process. Meanwhile, the strengthening layer also reduces the oil absorption value of the paper substrate, and is beneficial to the coating and combination of the subsequent hydrophobic layer.

According to a first aspect of the present application, there is provided a coating composition for increasing the wet strength and reducing oil absorption of the surface of paper, said paper being paper for food and pharmaceutical contact, said coating composition being an aqueous coating composition comprising:

sizing agents, film forming agents, rheological agents, and aqueous solvents. According to one embodiment of the present application, the sizing agent is preferably a sizing agent having hydroxyl groups and the film former is preferably a film former having double bonds capable of reacting with hydroxyl groups. According to another embodiment of the first aspect of the present application, the sizing agent is selected from one or more of the following: starch, modified starch, polyvinyl alcohol (PVA), and combinations thereof; the film forming agent is selected from one or more of the following: acrylic polymers, glyoxal, and combinations thereof. According to another embodiment of the first aspect of the present application, the sizing agent, film former, rheological agent and aqueous solvent are mixed together prior to use of the aqueous coating composition; in other words, the components are applied as a coating composition mixed together, rather than applying the components separately.

According to another embodiment of the first aspect of the invention, the rheological agent is selected from: carboxymethyl cellulose, salts of carboxymethyl cellulose, esters of carboxymethyl cellulose, and combinations thereof.

According to another embodiment of the first aspect of the present invention, the coating composition further comprises an inorganic pigment selected from the group consisting of: talc powder, china clay, calcined china clay, and combinations thereof.

According to another embodiment of the first aspect of the present invention, the weight ratio of the sizing agent to the film former is ≧ 1.

According to a preferred embodiment of the first aspect of the present invention, the polyvinyl alcohol has a degree of alcoholysis of 96 to 99% and a 4% aqueous viscosity of 10 to 12cp when measured at 20 ℃; the acrylic polymer is water-based polyacrylic resin, the concentration of the acrylic polymer is 40-46 wt%, and the viscosity at 25 ℃ is 4000-8000 cp; the glyoxal is in the form of an aqueous glyoxal solution having a concentration of 37-40% by weight, and has a specific gravity of 1.20-1.30 kg/L.

According to another embodiment of the first aspect of the present invention, the sizing agent is polyvinyl alcohol and the film former is glyoxal or a polyacrylic resin.

According to another embodiment of the first aspect of the present invention, the sizing agent is present in an amount of 30 to 95 wt.%, the film-forming agent is present in an amount of 5 to 40 wt.%, and the inorganic pigment is present in an amount of 0 to 70 wt.%, based on the total weight of components of the coating composition, excluding the aqueous solvent.

According to another embodiment of the first aspect of the present invention, the coating composition is free of cationic polyalkyleneamine-based wet strength agents. According to another embodiment of the first aspect of the present invention, the coating composition is free of 1, 3-dichloro-2-propanol and 3-chloro-1, 2-propanediol or the content of each of 1, 3-dichloro-2-propanol and 3-chloro-1, 2-propanediol is <12 μ g/L.

According to a second aspect of the present application, there is provided a base paper, in particular a base paper for food and pharmaceutical contact paper, comprising a paper substrate layer and a strengthening layer located on at least one surface of the paper substrate layer, the paper substrate layer being prepared using wood pulp and one or more of the following wet end chemicals: filler, sizing agent, reinforcing agent, retention aid, charge regulator, filter aid, defoaming agent and bactericide,

the wood pulp used by the paper base material layer is selected from: softwood pulp, hardwood pulp, or mixtures thereof;

the softwood pulp comprises: sulfate softwood pulp, bleached sulfate softwood pulp, chemi-mechanical softwood pulp and bleached softwood chemi-mechanical pulp;

the hardwood pulp comprises: natural color sulfate hardwood pulp, bleached sulfate hardwood pulp, natural color hardwood chemico-mechanical pulp, bleached hardwood chemico-mechanical pulp,

the strengthening layer is formed from the coating composition of any one of the embodiments of the first aspect of the present application.

According to a third aspect of the present application, there is provided a paper for food and pharmaceutical contact, comprising a base paper of the second aspect of the present application, and a hydrophobic layer on a surface of the reinforcing layer of the base paper, the hydrophobic layer comprising a hydrophobic material selected from the group consisting of: emulsion silicone coatings, solvent-borne silicone coatings, solventless silicone coatings, Ultraviolet (UV) cured silicone coatings, and combinations thereof.

According to another embodiment of the third aspect of the present application, the paper for food and drug contact comprises: steamer paper, baking paper, wiping paper, oil absorbing paper, cooking-resistant paper bags, cosmetic paper, auxiliary material paper, food packaging paper, beverage packaging paper and medicine packaging paper.

According to a fourth aspect of the present application, there is provided a method of preparing the paper for food and pharmaceutical contact of the present application or the base paper thereof, the method comprising the steps of:

1) separately refining softwood pulp and hardwood pulp and mixing to form a pulp comprising wood fibers in a weight ratio of softwood pulp to hardwood pulp of from 0:100 to 100:0, based on the total weight of the pulp comprising wood fibers;

2) adding a wet-end chemical to the pulp comprising wood fibers to obtain a pulp, the wet-end chemical comprising one or more of: fillers, sizing agents, reinforcing agents, retention aids, charge regulators, filter aids, defoaming agents and bactericides;

3) preparing the paper substrate layer by using the paper pulp;

4) applying a strengthening layer to at least one surface of the paper substrate layer to obtain a base paper; and

5) optionally further applying a hydrophobic layer on the surface of the strengthening layer to obtain the paper for food and drug contact. According to this embodiment, without carrying out the above step 5), the product obtained is a base paper, the surface of which is free of hydrophobic layers; in the case of performing the above step 5), the resulting product is paper for food and medicine contact, the surface of which has a hydrophobic layer.

According to an embodiment of the fourth aspect of the present application, the pulp containing wood fiber is a mixture of softwood pulp and hardwood pulp, the degree of beating of the softwood pulp is 40 to 50 degrees, and the degree of beating of the hardwood pulp is 30 to 40 degrees. According to another embodiment of the fourth aspect of the present application, the paper substrate layer, the strengthening layer and the hydrophobic layer are free of cationic polyalkyleneamine-based wet strength agents and are free of 1, 3-dichloro-2-propanol and 3-chloro-1, 2-propanediol or 1, 3-dichloro-2-propanol and 3-chloro-1, 2-propanediol, respectively, in an amount of <12 μ g/L.

A fifth aspect of the present application provides a method of cooking a food product using a contact paper according to any one of the embodiments of the third aspect of the present application, the method comprising: a. providing a food product, preferably a non-liquid food product; b. placing the food product in a cooking device and lining the contact paper between the food product and the cooking device, the contact paper contacting at least a portion of a surface of the food product such that the food product does not contact the cooking device; c. heating the cooking device.

According to an embodiment of the fifth aspect of the application, the cooking process is a steam process, the method comprising: a. providing a food product, preferably a non-liquid food product; b. the cooking device is a steamer, the food is placed at the upper part in the steamer, the contact paper is lined between the food and the steamer, the contact paper contacts at least one part of the surface of the food, so that the food is not contacted with the steamer, and the liquid generating steam is placed at the lower part in the steamer; c. and closing the steamer, heating the steamer to generate steam in the steamer, and carrying out steam processing on the food.

According to another embodiment of the fifth aspect of the present application, the water vapour produced in step c has a temperature of 50-120 ℃ and a pressure of 50-100 kPa.

Drawings

Various embodiments of the present invention are discussed in the following paragraphs with reference to the accompanying drawings. It is to be noted, however, that the embodiments illustrated in the drawings and described in the following detailed description are only preferred embodiments of the invention, and the scope of the invention is defined by the appended claims rather than by the limitations set forth herein.

FIG. 1 shows a cross-sectional view of a contact paper according to one embodiment of the present invention;

FIG. 2 is a flowchart showing a method for producing a contact paper of the present invention.

Detailed Description

The "ranges" disclosed herein are in the form of lower and upper limits. There may be one or more lower limits, and one or more upper limits, respectively. The given range is defined by the selection of a lower limit and an upper limit. The selected lower and upper limits define the boundaries of the particular range. All ranges that can be defined in this manner are inclusive and combinable, i.e., any lower limit can be combined with any upper limit to form a range. For example, ranges of 60-120 and 80-110 are listed for particular parameters, with the understanding that ranges of 60-110 and 80-120 are also contemplated. Furthermore, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.

In the present invention, unless otherwise stated, the numerical range "a-b" represents a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0 to 5" indicates that all real numbers between "0 to 5" have been listed herein, and "0 to 5" is only a shorthand representation of the combination of these numbers.

In the present invention, all embodiments and preferred embodiments mentioned herein may be combined with each other to form a new technical solution, if not specifically stated.

In the present invention, all the technical features mentioned herein and preferred features may be combined with each other to form a new technical solution, if not specifically stated.

In the present invention, some steps of the present invention may be arbitrarily adjusted in their process order as necessary, if not specifically stated. For example, the steps of preparing wood pulp (refining and mixing of wood pulp) can be performed before or after the step of formulating the coating composition of the present invention, or both steps can be performed simultaneously, so long as there is no conflict with the performance of the process steps of the present invention.

In the present invention, the term "comprising" as used herein means either an open type or a closed type unless otherwise specified. For example, the term "comprising" may mean that other components not listed may also be included, or that only listed components may be included.

The viscosity described in the present invention is a viscosity measured using a Brookfield viscometer (Brookfield viscometer). For example, when viscosity of a material is described herein, the viscosity of the material can be measured using a Brookfield viscometer (Brookfield viscometer) at a suitable test temperature for an aqueous solution, suspension, dispersion or slurry of the material at a suitable concentration, the test temperature can be from 0 ℃ to 60 ℃, such as from 10 ℃ to 50 ℃, or from 20 ℃ to 40 ℃, or from 25 ℃ to 30 ℃, or any combination thereof; the concentration may be in the range of 1 to 60 wt%, or 4 to 50 wt%, or 5 to 45 wt%, or 10 to 40 wt%, or 20 to 30 wt%, or any combination of the two.

In this application, unless otherwise specified, when a component is described as being present in an "absolute dry weight" basis, it is meant that the component content expressed herein is the content of dry matter after excluding liquid components, for example when a component is described as being present in an "absolute dry weight" of 5% by weight, even though the component may be added to the process step in the form of an aqueous solution or aqueous slurry, the content of the component herein has excluded moisture. When the expression "absolute dry weight" is not used to describe the content of one component, it means that the content of the component added herein includes all the material weight in any state, for example, when it is described that 10 wt% of aqueous glyoxal solution having a concentration of 37 wt% is added, the content (10 wt%) of the aqueous glyoxal solution herein is calculated by glyoxal and moisture at the same time.

One technical advance achieved by the present invention is the avoidance of the introduction of any toxic and harmful substances into the raw material pulp of the paper substrate layer by using a wet strength agent by applying a strengthening layer on the surface of the paper substrate layer using a coating composition having a specific composition, so that the resulting paper material has both excellent wet strength and food safety. Another technical advance achieved by the present invention is that the inventors have found that by forming a strengthening layer using a coating composition of a specific composition instead of adding a wet strength agent to the raw pulp, a significant improvement in the wet strengthening of the final paper product and in the effective combination of the strengthening layer and the hydrophobic layer can be achieved.

In the present application, "base paper for contact paper" and "base paper" are used interchangeably to mean that a reinforcing layer has been applied to at least one surface of a paper substrate layer using the coating composition of the present invention but that a hydrophobic layer has not yet been applied. Additionally, in this application, "contact paper" means that a reinforcing layer has been applied to at least one surface of a paper substrate layer using the coating composition of the present invention, and a hydrophobic layer has been applied to the surface of at least one reinforcing layer. Since the contact paper of the present invention has extremely excellent wet strength and structural integrity retention properties during long-term storage under normal temperature and pressure environment and under high temperature and high pressure and high humidity environment, it is very suitable for use in contact with food and medicine, and the specific use is as described below, the "contact paper" of the present invention is also referred to as "food and medicine contact paper" in the present application, but the use of the coating composition and contact paper of the present invention is not limited thereto, but can be used for any arbitrary use and product that can benefit from the unique properties of the coating composition and contact paper of the present invention.

The paper products made of the contact paper comprise all paper packaging materials used for packaging products which are ingested by people or directly applied to any part of the body surface, or the paper materials which are directly contacted with the products which are ingested by people or directly applied to any part of the body surface. The products for human body to take comprise various foods, drinks, beverages, medicines, health-care products and the like, and the products directly applied to any parts of the body surface comprise various external medicines, cosmetics and the like. In the present application, for the sake of simplicity, the above products are collectively referred to as "food or medicine", or collectively referred to as "food and medicine". Thus, the coating composition, the base paper and the contact paper of the present invention are referred to as "coating composition for food and drug contact use, base paper or paper for food and drug contact use", or may be referred to as "food grade or drug grade coating composition, base paper or contact paper". These products include wrapping paper, bags, capsules, boxes, disposable paper lunch boxes for containing the above-mentioned food or medicine, and also paper for various uses such as baking paper, wiping paper, oil absorbing paper, steamer paper, cook-resistant paper pack, cosmetic paper, auxiliary paper, etc., which may come into contact with the above-mentioned product during its use (e.g., production, sterilization, packaging, storage, transportation, cooking of food or medicine). In addition to the above-mentioned uses, the coating composition, the base paper or the contact paper of the present application can be very effectively used for any other uses requiring both wet strength and safety and harmlessness, and benefiting from the above-mentioned technical advantages brought by the present application. According to a preferred embodiment, the coating composition, the base paper and the contact paper of the present invention are particularly suitable for applications requiring high temperature and high humidity environments, such as packaging of foods or pharmaceuticals requiring pasteurization processes, or steamer paper for steaming cooking foods, and the like. More preferably, the contact paper of the present invention can be used to make the following products: food wrapping paper, medicine wrapping paper, cosmetic wrapping paper, steamer paper, baking paper, steamed bread paper, wiping paper, oil absorbing paper, cooking-resistant paper bags, domestic cooking paper pads, fruit wrapping paper and beverage container wrapping paper.

The base paper for the contact paper comprises a paper substrate layer and a strengthening layer positioned on at least one surface of the paper substrate layer. The strengthening layer is used to provide sufficient mechanical strength to the paper substrate, including mechanical strength in the dry state and mechanical strength in the wet state, particularly mechanical strength in the wet state.

In one embodiment of the present application, the paper substrate layer has the reinforcing layer applied to only one surface. According to another embodiment of the present application, the paper substrate layer has the reinforcing layer applied to only one surface and a hydrophobic layer applied to the surface of the reinforcing layer opposite the paper substrate layer.

According to another embodiment of the present application, the paper substrate layer has the reinforcing layer applied to both surfaces. For example, in the embodiment shown in FIG. 1 of the present application, the paper substrate layer has the reinforcing layer applied to both surfaces, and a hydrophobic layer is applied to the surface of the two reinforcing layers opposite the paper substrate layer.

According to other embodiments of the present application, one or more other layers, such as an adhesive layer, a protective layer, a decorative layer, a pigment layer, etc., may also be added between the paper substrate layer and the reinforcing layer, between the reinforcing layer and the hydrophobic layer (for the case where a hydrophobic layer is present), on the outer surface of the contact paper (e.g., on the outer surface of the reinforcing layer, or on the outer surface of the hydrophobic layer).

In the present application, when it is stated that "a component is not contained" it means that the component is not intentionally added thereto, and although it is not excluded that there may be trace amounts of the component which may be introduced therein by accident other than in the operation of the process or as trace impurities in other raw materials, even if it is accidentally present, the content of the component is lower than the upper limit specified by the relevant laws or regulations at home and abroad. For example, it is explicitly stated that the coating composition of the invention is free of toxic, harmful substances, such as cationic polyalkyleneamine-based wet strength agents, in particular free of 1, 3-dichloro-2-propanol and 3-chloro-1, 2-propanediol, which are not deliberately added to the coating composition of the invention, even if traces of such toxic, harmful substances are accidentally included in the coating composition of the invention, the content of such toxic, harmful substances being less than 12 μ g/L, such as less than 10 μ g/L, or less than 8 μ g/L, or less than 6 μ g/L, or less than 5 μ g/L, based on the total weight of the coating composition, or less than 4. mu.g/L, or less than 2. mu.g/L, or less than 1. mu.g/L, or less than 0.5. mu.g/L, or less than 0.1. mu.g/L. In one embodiment according to the application, the paper substrate layer is free of intentionally added wet strength agent, or is completely free of intentionally added strengthening agent, or is completely free of strengthening agent. For example, the total content of toxic or harmful substances therein originating from the strengthening agent, in particular the wet strength agent, is less than 12 μ g/L, such as less than 10 μ g/L, or less than 8 μ g/L, or less than 6 μ g/L, or less than 5 μ g/L, or less than 4 μ g/L, or less than 2 μ g/L, or less than 1 μ g/L, or less than 0.5 μ g/L, or less than 0.1 μ g/L, based on the total weight of the paper substrate layer.

According to a preferred embodiment of the present application, the paper for food and drug contact and the base paper thereof do not transfer or release toxic or harmful substances exceeding the standards set by food safety regulations in China or all countries around the world to food, external environment or human body during use. It is particularly preferred that the paper for food and drug contact and its base paper are totally free of cationic polyalkyleneamine-based wet strength agents, preferably free of crosslinked cationic polyalkyleneamines, such as polyamide epichlorohydrin resins, for increasing wet strength. For example, the paper for food and pharmaceutical contact and the base paper thereof are preferably completely free of 1, 3-dichloro-2-propanol and 3-chloro-1, 2-propanediol.

Fig. 2 shows a method of manufacturing a paper for food and medicine contact of the present application and a base paper thereof according to an embodiment of the present application, the method comprising the steps of: 1) separately refining softwood pulp and hardwood pulp and then mixing to form a pulp containing wood fibers, wherein the weight ratio of the softwood pulp to the hardwood pulp is 0:100 to 100:0 based on the total weight of the pulp containing the wood fibers; 2) adding an additive to the pulp comprising wood fibers to obtain a pulp; 3) preparing the paper base material layer by using the paper pulp; 4) applying a reinforcing layer on at least one surface of the paper substrate layer using the coating composition of the present invention, thereby forming a base paper; and 5) optionally, applying a hydrophobic layer on the surface of the reinforcing layer in the base paper to form a contact paper. According to another embodiment of the present application, in certain applications where a hydrophobic layer is not required, the hydrophobic layer may not be applied to the surface of the reinforcing layer, but rather the base paper may be used directly. These steps, in addition to the hydrophobic layer coating, can be performed using conventional paper machines of the papermaking process, e.g., conventional paper machines can include a wet end, wire, press, dryer, coating, etc. The step of applying the hydrophobic layer is performed by off-machine coating, such as with a laboratory coating apparatus or a separate coater. According to another embodiment of the present application, the application of the reinforcing layer can also be performed by off-machine coating, such as with a laboratory coating apparatus or a separate coater.

According to one embodiment of the present application, the pulp containing wood fiber includes various wood pulps commonly used in the paper industry, and may include softwood pulp including early wood pulp of softwood and late wood pulp of softwood, hardwood pulp including early wood pulp of hardwood and late wood pulp of hardwood, or a mixture thereof, classified according to the wood fiber in the wood pulp. In the present application, the softwood includes pine, fir, cedar, etc., such as masson pine, larch, Korean pine, spruce, fir, cypress, phellodendron, etc.; the broadleaf wood includes birch, poplar, basswood, eucalyptus, maple, etc. According to one embodiment of the present application, the wood fibers contained in the wood pulp used may be long fibers, short fibers, or a mixture thereof. According to one embodiment of the present application, the wood pulp concentration may be 0.5 to 5 wt%, or 1 to 4.5 wt%, or 1.5 to 4 wt%, or 2 to 3.5 wt%, or 2.5 to 3 wt%. The wood pulp can be concentrated or diluted as needed to achieve the target wood pulp consistency. For example, according to one preferred embodiment of the present application, the consistency of the wood pulp after the refining step may be 3 to 5% by weight, such as 3 to 4% by weight, or 4 to 5% by weight, and the wood pulp may be further diluted, such as to a consistency of 0.5 to 2% by weight, such as 0.5 to 1.5% by weight, or 1 to 1.2% by weight, by adding water thereto and stirring sufficiently. In the present invention, the "wood pulp concentration" means the total proportion of the components other than water in the wood pulp, based on the total weight of the wood pulp.

Where a wood pulp manufacturing process is contemplated, the wood pulp may include unbleached kraft pulp, bleached kraft pulp, chemi-mechanical pulp, and the like. Pulp, as used herein, may include unbleached kraft softwood pulp, bleached kraft softwood pulp, unbleached or bleached softwood chemi-mechanical pulp, unbleached kraft hardwood pulp, bleached kraft hardwood pulp, unbleached or bleached hardwood chemi-mechanical pulp, or mixtures of any two or more of the above, taking into account both the type of raw material and the manufacturing process. The first step of the process of the present invention is that softwood pulp and hardwood pulp are separately refined and mixed to form a pulp comprising wood fibers, and the weight ratio of softwood pulp to hardwood pulp is from 0:100 to 100:0, based on the total weight of the pulp comprising wood fibers. Here, "the weight ratio of softwood pulp to hardwood pulp is 0:100 to 100: 0" means that the present invention includes technical solutions in which softwood pulp alone (weight ratio of 100:0) and hardwood pulp alone (weight ratio of 0:100) are used, and for these technical solutions in which only one kind of wood pulp is used, the softwood pulp or hardwood pulp is independently refined and then directly used to obtain pulp without mixing.

According to one embodiment of the application, in step 1) described above, the wood pulp is first subjected to a refining operation such that it has a degree of beating of 30-50 degrees, for example 30-48 degrees, or 30-45 degrees, or 35-42 degrees, or 40-42 degrees, or the degree of beating of the wood pulp after refining can be within a range of values combining any two of the above ranges. In this application, the refining may be performed using refining equipment conventional in the papermaking art.

In this application, the terms "degree of beating" and "freeness" are used interchangeably to refer to the degree of refining, such as chopping, splitting, swelling and hydration of the fibers in the wood pulp after it has passed through the refiner. In the present application, the degree of beating of wood pulp is determined according to ISO 05267-1.

According to one embodiment of the application, the softwood pulp is refined with a degree of beating of 40-50 degrees, for example the degree of beating of the softwood pulp may be in the range consisting of any two of the following values: 40 degrees, 41 degrees, 42 degrees, 43 degrees, 44 degrees, 45 degrees, 46 degrees, 47 degrees, 48 degrees, 49 degrees, 50 degrees. According to another embodiment of the present application, hardwood pulp has a degree of beating of 30-40 degrees after refining, for example the degree of beating of hardwood pulp can be within the range of any two of the following compositions of values: 30 degrees, 31 degrees, 32 degrees, 33 degrees, 34 degrees, 35 degrees, 36 degrees, 37 degrees, 38 degrees, 39 degrees, 40 degrees.

According to one embodiment of the application, a mixture of two or more wood pulps is used as the raw material, in which case the above step 1) comprises a pulp mixing step after refining, i.e. a step of homogeneously mixing two or more different wood pulps. The slurry mixing step can be carried out by adopting slurry mixing equipment commonly used in the papermaking field or other slurry mixing equipment used. Examples of wood pulps used to form the mixture may include mixtures using two or more of the above softwood pulps (including unbleached sulfate softwood pulp, bleached sulfate softwood pulp, unbleached or bleached softwood chemi-mechanical pulp), or mixtures using two or more hardwood pulps (including unbleached sulfate hardwood pulp, bleached sulfate hardwood pulp, unbleached or bleached hardwood chemi-mechanical pulp), or mixtures using one or more of the above softwood pulps (including unbleached sulfate softwood pulp, bleached sulfate softwood pulp, mechanical softwood pulp, bleached softwood chemi-mechanical pulp) with one or more of the above hardwood pulps (including unbleached sulfate hardwood pulp, bleached sulfate hardwood pulp, mechanical hardwood pulp, bleached hardwood chemi-mechanical pulp). According to a preferred embodiment of the present application, a mixture of softwood pulp (e.g. unbleached sulphate softwood pulp or bleached sulphate softwood pulp) and hardwood pulp (e.g. unbleached sulphate hardwood pulp or bleached sulphate hardwood pulp) is used. For example, the softwood pulp may be present in an amount ranging from 30 to 55 weight percent, based on the total weight of the mixture, and may, for example, be within the range consisting of any two of the following values: 30 wt%, 31 wt%, 32 wt%, 33 wt%, 34 wt%, 35 wt%, 36 wt%, 37 wt%, 38 wt%, 39 wt%, 40 wt%, 41 wt%, 42 wt%, 43 wt%, 44 wt%, 45 wt%, 46 wt%, 47 wt%, 48 wt%, 49 wt%, 50 wt%, 51 wt%, 52 wt%, 53 wt%, 54 wt%, 55 wt%; the hardwood pulp may be present in an amount of from 40 to 80% by weight, based on the total weight of the mixture, and may be, for example, within the range consisting of any two of the following values: 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80 wt%. In the case of the wood pulp mixture, softwood pulp and hardwood pulp therein may have the same degrees of beating as described above, respectively.

According to one embodiment of the present application, in the above step 2), an additive is added to the slurry prepared in step 1) to form a pulp for manufacturing the paper base layer. In the present application, these added additives may also be referred to as "wet end chemicals" or "wet end additives," and may include, for example, one or more of the following: fillers, sizing agents, reinforcing agents, retention aids, charge control agents, filter aids, defoamers, biocides, and the like. This step may be performed in the wet end of the paper machine.

According to some embodiments of the present application, the wet end chemical may include an amount of filler, such as one or more of the following: kaolin, talcum powder, Ground Calcium Carbonate (GCC), terra alba, sodium stearate modified GCC, Precipitated Calcium Carbonate (PCC), titanium dioxide/titanium dioxide, barium sulfate and the like. According to a separate embodiment of the present application, no inorganic filler is used in the "wet end chemical". According to another independent embodiment of the present application, no filler is used in the "wet end chemical". In embodiments according to the present application where a filler is used, the filler may be used in an amount of 0-100 kg/ton paper, such as 1-50 kg/ton paper, or 2-40 kg/ton paper, or 3-20 kg/ton paper, or 4-10 kg/ton paper, or 5-8 kg/ton paper, based on the weight of the paper substrate produced.

According to some embodiments herein, the wet end chemicals may include an amount of sizing agents primarily for imparting a certain water resistance to the paper substrate layer, examples of sizing agents may include one or more of the following: rosins (e.g., anionically dispersed rosin, cationically dispersed rosin, saponified rosin), polyaluminum chloride (PAC), Alkyl Ketene Dimer (AKD), Alkenyl Succinic Anhydride (ASA), preferably ASA, AKD or mixtures thereof are used as the wet end chemicals herein. According to a preferred embodiment of the present application, the sizing agent may be used in an amount of 0.01-1.5 kg/ton paper, such as 0.1-1.2 kg/ton paper, or 0.2-1.0 kg/ton paper, or 0.5-0.8 kg/ton paper, based on the weight of the paper substrate produced.

According to some embodiments of the present application, the wet end chemicals may include an amount of a strengthening agent, particularly a dry strength agent, to provide some dry strength (dry tensile strength) improvement to the paper substrate layer, examples of which may include natural dry strength agents such as native starches (e.g., corn starch, potato starch, tapioca starch, wheat starch, rice starch, sago starch, high amylopectin starch, etc.), modified starches (e.g., cationic starches, amphoteric starches, enzyme modified starches), carboxymethyl cellulose/salts/derivatives (CMC); synthetic dry strength agents such as polyacrylamides (including anionic, cationic, amphoteric polyacrylamides) may also be included. According to one embodiment of the present application, the natural dry strength agent may be present in an amount of 4 to 10 kg/ton of paper, or 4 to 8 kg/ton of paper, or 5 to 8 kg/ton of paper, based on the weight of the paper substrate produced. According to another embodiment of the present application, the synthetic dry strength agent may be present in an amount of 2-10 kg/ton paper, or 3-9 kg/ton paper, or 5-8 kg/ton paper, based on the weight of the paper substrate produced. According to one embodiment of the application, the dry strength agent used comprises cationic starch in an amount of 5-8 kg/ton of paper.

According to some embodiments of the present application, the wet-end chemical does not include a wet strength agent, in particular, does not contain polyamide polyepichlorohydrin resins commonly used in the paper industry, thereby avoiding the introduction of toxic miscellaneous chloroglycerols, dichloropropanols and other toxic harmful substances in the paper products.

According to some embodiments of the present application, the wet end chemical comprises one or more retention aids, such as silica sol, bentonite, or mixtures thereof, preferably silica sol is used as retention aid. According to one embodiment of the application, the wet end chemical comprises polyacrylamide as a retention aid, together with a silica sol retention aid as an aid. According to another embodiment of the present application, the retention aid may be present in an amount of 2-10 kg/ton paper, or 3-5 kg/ton paper, based on the weight of the paper substrate produced.

In addition, the wet-end chemicals may also contain other various reagents as needed, for example, NaOH or mineral acids may be contained to adjust the pH value appropriately; a proliferation comprising a biocide effective to control the growth of microorganisms in the wet end system; an antifoaming agent is included to reduce foam generation in the system.

According to an embodiment of the present application, in the above step 3), the pulp is used to prepare the paper substrate layer, which may be performed using conventional paper making equipment and process steps, for example, may sequentially comprise dewatering in the wire section of a paper machine, dewatering in the press section, and then front-end drying in the drying section (front-end drying section) to obtain the paper substrate layer. In particular, the pulp produced in the wet end is applied to the wire in the wire section, for example by spraying onto the wire, and dewatered under atmospheric pressure, low pressure, vacuum, suction or centrifugation as the wire moves. Depending on the particular type of paper machine used, the configuration of the wire section may include fourdrinier wire, cylinder wire, nipped wire, lapped wire, etc. The press section may employ a configuration known in the papermaking art to apply mechanical compression to the web produced by the wire section to squeeze water from the web, solidify the sheet, and improve sheet properties (e.g., reduce wire marks and two-sided differences in the sheet, increase the contact area of the wet paper sheet fibers, and increase sheet smoothness, tightness, and strength). The dryness of the paper web after pressing is typically only 30-50%, for example 35-45%, or 40-50%, or 40-45%, and the paper web after pressing is subsequently transported to a drying section where it is dried. For example, the drying section may use drying cylinders to heat the web to evaporate the water, so that the paper dryness increases to 92-95% or more.

Step 4) of the present application comprises applying a strengthening layer on at least one surface of the paper substrate layer using a coating composition of the present invention comprising a sizing agent, a film former, a rheological agent, and an aqueous solvent, and may optionally further comprise an inorganic filler. For example, for some applications where water repellency is highly desired, the coating composition may include an inorganic filler; for some applications where oil repellency is highly desired, the coating composition does not contain inorganic fillers. This application step can be carried out using equipment or processes conventional in the papermaking art, for example, in the coating section of a paper machine, or by off-machine coating.

According to one embodiment of the present application, a reinforcement layer is applied to at least one surface, preferably both surfaces, of the paper substrate layer using the coating composition of the present invention to form the base paper of the present invention, in particular the base paper for food and pharmaceutical contact paper of the present invention. The coating composition of the present invention is an aqueous coating composition. In the present invention, the "aqueous coating composition" means that the solvent used to form the coating composition is aqueous, for example, the solvent may be water at one hundred percent, or may also contain an amount of other solvents or co-solvents in water as desired. For example, the solvent in the aqueous coating composition may comprise 90 wt.% or more water, or 95 wt.% or more water, or 98 wt.% or more water, or 99 wt.% or more water, or 99.9 wt.% or more water, or 100 wt.% or more water. The coating composition of the present invention has a solids content of 15 to 40 wt.%, alternatively 15 to 35 wt.%, alternatively 15 to 32 wt.%, alternatively 17 to 30 wt.%, alternatively 18 to 25 wt.%, alternatively 20 to 24 wt.%, alternatively 21 to 22 wt.%, based on the total weight of the coating composition. According to one embodiment of the present application, the pH of the coating composition may be adjusted with an acid or base according to the specific requirements, for example, from 4 to 12, alternatively from 5 to 10, alternatively from 6 to 8, alternatively from 6 to 7.

According to one embodiment of the present application, the coating composition may form a reinforcing layer having a total coating weight of from 1.5 to 5 grams per square meter, alternatively from 1.8 to 4 grams per square meter, alternatively from 2 to 3 grams per square meter, over both sides of the paper substrate.

According to one embodiment of the present application, the rheological agent for the coating composition may include one or more of the following: carboxymethylcellulose (CMC), salts of CMC, esters of CMC, or other derivatives of CMC; nanocellulose (NFC); nanocrystalline cellulose; modified nanocrystalline cellulose; and hemicellulose. The rheology agent may be present in an amount of from 0.1 to 20 wt.%, such as from 0.5 to 15 wt.%, or from 1 to 12 wt.%, or from 1.2 to 10 wt.%, or from 1.5 to 8 wt.%, or from 1.8 to 5 wt.%, or from 2 to 4 wt.%, based on the total weight of components in the coating composition other than the solvent, or within a range of values that can be obtained by combining any two of the above endpoints with each other.

One technical improvement of the present application resides in the formulation of the coating composition. Without wishing to be bound by any particular theory, a further technical improvement of the present application is that a specially designed combination of film former and sizing agent is employed in the coating composition, by which the film former and sizing agent interact to collectively increase the wet strength of the base paper and reduce the oil absorption value of the base paper, thereby facilitating the application of a subsequent hydrophobic layer (e.g., a silicon-containing layer), and the incorporation of a strengthening layer with the hydrophobic layer (e.g., a silicon-containing layer). Thereby eventually giving the contact paper sufficient wet strength and excellent bonding properties between the strengthening layer and the hydrophobic layer.

Without wishing to be bound by any particular theory, sizing agents are used to form a flexible film on the paper surface to improve water resistance. According to one embodiment of the present application, sizing agents for coating compositions may include one or more of the following: starch, modified starch, polyvinyl alcohol (PVA), and the like, and any combination thereof. The polyvinyl alcohol (PVA) can be a commercially available white powdery product, has good water solubility, and has an alcoholysis degree of 92-99.7%, or 92-95%, 94-98%, or 95-99.7%. And the viscosity measured at 20 ℃ is 3-60cp, can be 4.5-20cp,10-30cp,20-40cp,30-50cp,40-60cp, preferably 10-12cp, or can be within a numerical range obtained by combining any two of the above endpoints. According to one embodiment of the present application, the viscosity of the polyvinyl alcohol is measured using a Brookfield viscometer (Brookfield viscometer) on a 4% by weight aqueous solution of polyvinyl alcohol at a temperature of 20 ℃. The sizing agent may be present in an amount of 30 to 95 weight percent, such as 34 to 93 weight percent, or 35 to 90 weight percent, or 38 to 80 weight percent, or 39 to 70 weight percent, or 40 to 60 weight percent, or 45 to 50 weight percent, based on the total weight of components in the coating composition other than the solvent, or may be within a range of values that combine with each other at any two of the above endpoints. According to a preferred embodiment of the present application, the sizing agent used in the coating composition is polyvinyl alcohol, and for embodiments where polyvinyl alcohol is used alone as the sizing agent, the polyvinyl alcohol sizing agent is present in an amount of 30 to 95 wt.%, such as 30 to 93 wt.%, or 30 to 90 wt.%, or 30 to 85 wt.%, or 30 to 83 wt.%, or 30 to 75 wt.%, or 30 to 70 wt.%, or 30 to 68 wt.%, or 40 to 68 wt.%, or 50 to 68 wt.%, or 55 to 65 wt.%, or any combination thereof, based on the total weight of the components of the coating composition, excluding the solvent. According to a preferred embodiment of the present application, the sizing agent used in the coating composition is a mixture of polyvinyl alcohol and starch, in which embodiment the polyvinyl alcohol sizing agent is present in an amount of 30 to 60 wt.%, such as 35 to 55 wt.%, or 38 to 52 wt.%, or 40 to 50 wt.%, or 42 to 48 wt.%, or 44 to 46 wt.%, or any combination thereof, based on the total weight of the components of the coating composition other than the solvent; and the starch is present in an amount of 1 to 25 wt.%, such as 2 to 22 wt.%, or 3 to 20 wt.%, or 5 to 18 wt.%, or 6 to 15 wt.%, or 7 to 14 wt.%, or 10 to 14 wt.%, or any combination thereof.

The function of the film former is to increase the film forming efficiency of the surface sizing system, for example glyoxal forms covalent bonds with the hydroxyl groups of the sizing agent through its aldehyde groups, thereby increasing the degree of crosslinking of the sizing agent. The film forming agent containing other functional groups can react with a target structure in the sizing agent through active groups (hydroxyl, carboxyl, amino and the like) in the molecular structure of the film forming agent, so that the film forming property is improved. For example, the film former may include an acrylic polymer, glyoxal, and combinations thereof, and may be, for example, an acrylic polymer or glyoxal.

According to one embodiment of the present application, the film-forming agent is a water-soluble acrylic polymer. In the present invention, the acrylic polymer includes polyacrylic resin, poly (meth) acrylic acid, poly (meth) acrylate salt, poly (meth) acrylate ester, or any copolymer or mixture thereof. The poly (meth) acrylate may include poly (meth) acrylic acid C ₁ -C ₆ Alkyl esters, or (meth) acrylic acid C ₁ -C ₄ Alkyl esters such as methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, or butyl (meth) acrylate; according to a preferred embodiment of the present application, the acrylic polymer may be a polyacrylic resin. According to one embodiment of the present application, the acrylic polymer may have a viscosity of 2000-1 when measured at 25 ℃0000cp, or 2000-. According to another embodiment of the present application, the viscosity of the acrylic polymer is measured using a Brookfield viscometer (Brookfield viscometer) on a 45% by weight aqueous solution of the acrylic polymer at a temperature of 25 ℃. According to one embodiment of the present application, the acrylic polymer is a water-based polyacrylic resin, and may be used as a solution (or dispersion, in the case where the acrylic polymer is not completely dissolved) in water. According to one embodiment of the present application, the solid content (i.e., the acrylic polymer content) may be in the range of 10 to 60 wt%, such as 20 to 55 wt%, or 30 to 50 wt%, or 40 to 48 wt%, or 44 to 46 wt%, or any combination thereof, based on the total weight of the water-based polyacrylic resin. In the case where the film former is selected from the above components, the film former may be present in an amount ranging from 5 to 40 percent by weight, such as from 5 to 30 percent by weight, from 10 to 38 percent by weight, or from 12 to 35 percent by weight, or from 15 to 30 percent by weight, or from 18 to 25 percent by weight, or from 20 to 22 percent by weight, or any combination thereof, based on the total weight of the components of the coating composition other than the solvent. In the case where the film former is a water-based polyacrylic resin, the content of the film former is the dry weight of the water-based polyacrylic resin (i.e., water not taken into account in the water-based polyacrylic resin).

According to another embodiment of the present application, the film former is glyoxal and is present in an amount ranging from 5 to 40 wt.%, such as 2 to 35 wt.%, or from 3 to 30 wt.%, or from 4 to 20 wt.%, or from 5 to 30 wt.%, or from 5 to 10 wt.%, or from 5 to 8 wt.%, or from 8 to 10 wt.%, or any combination thereof, based on the total weight of the components of the coating composition, excluding the solvent.

According to one embodiment of the application, the weight ratio of the sizing agent to the film former may be at least 1:1, such as 3:2 or 2:1 or 3:1 or 4:1 or 5:1 or 6:1 or 8:1 or 10:1 or 15:1 or 1:1 to 30:1 or 1:1 to 25:1 or 1:1 to 20:1 or any combination thereof.

According to one embodiment of the present application, the inorganic pigments used in the coating composition of the present invention may include one or more of the following: talc, china clay, calcined china clay, or any mixture thereof. Preferably, the inorganic pigment is china clay. The inorganic pigment is an optional component, that is, the coating composition used to form the strengthening layer may or may not contain an inorganic pigment. According to a separate embodiment of the present application, the inorganic pigment is present in an amount of 0 wt.%, or the inorganic pigment is present in an amount of 10 to 70 wt.%, such as 15 to 68 wt.%, or 20 to 65 wt.%, or 25 to 63 wt.%, or 30 to 58 wt.%, or 30 to 63 wt.%, or 35 to 50 wt.%, or 38 to 45 wt.%, or 40 to 42 wt.%, or any two of the above values are combined with each other to form a value within the range, based on the total weight of the components of the coating composition, excluding the solvent. The inorganic pigment not only helps to control the oil absorption value, but also can reduce the cost.

According to one embodiment of the present application, after the application of the strengthening layer using the coating composition of the present invention, the paper substrate with the strengthening coating may be allowed to dry in a back end dryer section, which may be identical to the front end dryer section, or the front end dryer section may employ a different drying apparatus or process than the back end dryer section. A calender section may be included after the back end dryer section, which calender section may use a calender conventional in the papermaking art, such as a calender roll.

According to one embodiment of the present application, after the calendering operation described above, a hydrophobic layer may be applied on the surface of the strengthening layer of the paper in a separate coating machine. It can be seen that in this case the reinforcing layer and the hydrophobic layer are applied and dried independently of each other, the hydrophobic layer being applied on the already dried reinforcing layer. . According to one embodiment of the present application, the hydrophobic layer is formed using a hydrophobic agent selected from the group consisting of: silicone, polyolefin, hydrophobic acrylic resin, and mixtures of two or more of the foregoing hydrophobic agents, preferably the hydrophobic layer is formed using silicone. According to one embodiment of the application, only one side of the paper is coated with a hydrophobic layer. According to another embodiment of the application, both sides of the paper are coated with a hydrophobic layer. According to one embodiment of the present application, the hydrophobic layer may be applied to the reinforcing layer at a single side coating weight of from 0.5 to 2 grams per square meter, alternatively from 0.6 to 1.8 grams per square meter, alternatively from 0.7 to 1.6 grams per square meter, alternatively from 0.8 to 1 gram, square meter, alternatively from 0.8 to 1.5 grams per square meter, alternatively from 1 to 1.2 grams per square meter.

According to one embodiment of the application, the product after leaving the calender is crimped to obtain a base paper and after application of a hydrophobic layer is crimped to obtain a contact paper. According to one embodiment of the present application, the resulting paper substrate has a grammage of 40 grams per square meter, but paper substrates of different grammage, such as 30-90 grams per square meter, may also be produced according to the methods of the present application. In the present application, the grammage of paper may be measured according to ISO 638. The base paper obtained after coating the 40 g/square meter paper base material with the strengthening layer (double-sided coating, coating amount is 2 g/square meter) has the longitudinal wet tensile strength of more than or equal to 0.3KN/m, the transverse wet tensile strength of more than or equal to 0.12KN/m and the transverse wet tensile strength of less than or equal to 3g/m ² Unger oil absorption number of (d); the contact paper obtained after further coating a hydrophobic layer (single-side coating, coating weight is 1 g/square meter) has transverse wet tensile strength of more than or equal to 0.4kN/m, preferably more than or equal to 0.44kN/m, more preferably more than or equal to 0.5kN/m, meets the requirements of food packaging and medicine packaging, and is particularly suitable for paper for food steamers. Since the wet tensile strength in the machine direction is generally greater than the wet tensile strength in the cross direction, the wet tensile strength in the cross direction is generally used to characterize the minimum requirements for contact paper.

As noted above, the contact paper or base paper thereof prepared in accordance with the present invention may be used in a wide variety of applications, in a preferred but non-limiting embodiment, for cooking food products in a cooking device. In the present application, the "food" may include various raw materials of food such as raw or half-cooked meat, fish, eggs, flour, rice flour, grains, and the like; or raw food or semi-finished products such as dough, raw dumplings, raw noodles, etc.; or may be various foods that have been cooked, such as cooked pasta, rice, vegetables, meat, and the like. The cooking device may include a steamer (e.g., a steamer), a microwave oven, an electric oven, a wok, etc., preferably a steamer; the cooking process may include steaming, microwave heating, baking, etc., preferably steaming.

The present application is described below by way of specific examples for the purpose of better understanding the content of the present application. It is to be understood that these examples are illustrative only and not limiting. The reagents used in the examples are, unless otherwise indicated, commercially available. The methods and conditions used in the examples are conventional methods and conditions, unless otherwise specified.

Examples

The technical solution of the present invention can be illustrated by examples 1 to 7. The effect of the composition design of the paper substrate and reinforcing layers on the properties of the final contact paper product was examined in these examples. All slurries and chemicals used in the following examples are conventional raw materials in the papermaking art that are commercially available, but meet the food grade product requirements for food contact.

China clay is an aqueous dispersion with a solids content of 70% and a pH of 7; polyvinyl alcohol was a commercially available water-soluble white powder with an alcoholysis degree of 98% and a 4% aqueous viscosity of 10cp measured at 20 ℃. (ii) a The starch was a commercially available white powdered size-applied coating starch with a specific gravity of 1.25 kg/L; the water-based polyacrylic resin is a commercially available CAS 24938-16-7 product with a concentration of 45% and a viscosity of 6000cp at 25 ℃; glyoxal is provided as a 38% by weight aqueous solution having a pH of 3 to 5 and a specific gravity of 1.20 to 1.30 kg/L. In the examples which follow, the glyoxal content is expressed in terms of "not absolute dry weight", i.e. the glyoxal content includes moisture; the other component contents are in "absolute dry weight".

In examples 1 to 7, the production of the paper base layer was carried out using a paper machine available from the Foitt, a main facility. Commercially available wood pulp is first refined to the desired degree of beating using a vemide commercial refiner. Mixing the ground pulp in an absolute dry weight ratio, feeding the mixture into a paper machine, sequentially adding cationic starch, alkenyl succinic anhydride, polyacrylamide and silica sol into a wet part of the paper machine, conveying paper pulp prepared by mixing the wet part to a net part, and dehydrating until the water content is about 40%; then conveyed to a press section where it is pressed to a moisture content of about 20%; then conveyed to a drying section where it is dried to a moisture content of about 3%; a paper substrate layer with a gram weight of 40 grams was produced. In the present application, the grammage is determined according to ISO536 and the moisture content is measured according to ISO 638.

In examples 1-7, aqueous coating compositions for the strengthening layer were prepared by: an aqueous china clay suspension having a solid content of 70 wt% is first provided, an aqueous solution of carboxymethyl cellulose (CMC) is precooked to a concentration of about 7 wt%, and then the precooked aqueous CMC solution is added to the aqueous china clay suspension while stirring at 1500 rpm so that the CMC content is 2 wt% based on the total oven dry weight in the finally-produced coating composition. Polyvinyl alcohol and starch (cationic modified starch) were dissolved in water simultaneously under heating so that the concentration of polyvinyl alcohol in the aqueous solution was 18% by weight and the concentration of cationic modified starch was 12% by weight, respectively, and then the solutions were added to the above mixture. Glyoxal and water-based acrylate are added to the mixture separately in the amounts used. After all the raw materials were added, water was added to the mixture to adjust the concentration to give an aqueous coating material having a solid content of 21.5. + -. 1.0% by weight (coating solid content test method ISO638), and the pH of the coating material was adjusted to 6.5. + -. 0.5 using a small amount of 0.1M aqueous sodium hydroxide solution (the pH was measured using a pH meter at a temperature of 60 ℃). In examples 1-7, if an example does not contain one or more of the specified components, the step of adding the component described above is omitted. For example, for examples 1-3, which did not contain china clay, the step of adding an aqueous china clay suspension was omitted directly; for examples 1-5, which did not contain starch (cationically modified starch), the step of adding starch was omitted; for examples 1 and 4-6, which did not contain a water-based acrylate, the step of adding a water-based acrylate was omitted.

The base papers of examples 1 to 7 were prepared by coating both sides of a paper base with a water-based coating for a reinforcing layer. In examples 1-6, a laboratory coater, specifically, a paper substrate was cut to the size of a4 paper, and the coating composition was applied to both sides of the paper using a laboratory flatbed coater, respectively. The coating rod of the plate coater is selected to be 3#, the coating speed is 8 m/min, and the coating amount is controlled to be 1 g/square meter on one side. After one side of the paper substrate was coated, it was placed in a laboratory dryer, the paper was tightly attached to a stainless steel hot plate by the dry net of the dryer, and dried at 90 ℃ for 45 minutes. The above steps were then repeated to coat the other side of the panel on the lab plate coater. The two-side coated and dried paper pattern was finally calendered using a laboratory roll calender at a calender roll temperature of 70 ℃ and a pressure of 50 KN/m. The base paper is obtained after calendaring, and the moisture content of the prepared base paper is about 6.5 wt%.

The paper substrate of example 7 was directly fed to a coating section of a paper machine to be subjected to roll film coating, passed through a drying section after the coating section to be dried, and then calendered in a machine calender. The pressure of the coating roller is 155kN/m, and the coating amount of the strengthening layer is controlled to be 1 g/square meter on one side. The subsequent calender pressure was 100kN/m and the temperature 105 ℃. The moisture content of the base paper produced was about 6.5 wt%.

In examples 1-7, a laboratory coater was used to apply a hydrophobic layer to one or both sides of the base paper. Specifically, a solvent-free silicone coating (commercially available, containing vinyl silicone oil, a small amount of crosslinking agent and a catalyst, for short, referred to as silicone oil) for forming a hydrophobic layer is placed in a feed pipe of a laboratory coater, and the base paper is fed into the coater to be subjected to single-side silicon coating. The coating amount of the hydrophobic layer was controlled to 1 g/m. And placing the silicon-coated paper in a drying box for free drying to obtain the contact paper coated with the hydrophobic layer. The moisture content of the contact paper produced was about 7 wt%.

Various properties of the various reinforcing base papers prepared in examples 1 to 7 were tested before and after application of the hydrophobic layer. Wherein the grammage is determined according to ISO536, the moisture content is determined according to ISO638, the dry tensile strength is determined according to ISO 1924-3, the wet tensile strength is determined according to GB/T465.2-2008, and the Unger oil absorption value is determined according to SCAN P37: 77.

Specific process conditions for each of examples 1 to 7 are described in detail below.

Example 1:

the bleached kraft pine wood pulp and the bleached kraft eucalyptus wood pulp are respectively ground to 45 degrees and 35 degrees and then mixed according to the proportion of 40:60 to form the pulp containing wood fibers. To the pulp comprising wood fibres the following additives were added per ton of oven dried paper: 5kg of starch, 0.8 kg of alkenyl succinic anhydride, 0.2 kg of polyacrylamide and 3 kg of silica sol to obtain paper pulp; preparing a paper substrate layer on a paper machine by using the paper pulp, and preparing a coating composition required by a reinforcing layer according to the following formula: 100 parts of an oven dry coating composition comprises 2 parts (by dry weight) of a rheological agent CMC, 93 parts (by dry weight) of PVA and 5 parts (by non-dry weight) of glyoxal; coating a strengthening layer on two sides of the paper base material layer to form base paper; and coating organic silicone oil on one surface of the base paper to form the contact paper. Through tests, the dry tensile strength of the base paper is 4.71kN/m in the longitudinal direction and 2.29kN/m in the transverse direction; the wet tensile strength was 0.3kN/m in the machine direction and 0.12kN/m in the transverse direction, and the Unger oil absorption number was 2.5. The wet tensile strength of the contact paper was 0.44kN/m in cross direction.

Comparative example 1

This comparative example 1 was conducted using exactly the same procedure as in example 1 except that the reinforcing layer coating material used was a general coating material containing 98 parts of polyvinyl alcohol and 2 parts of CMC in 100 parts of the oven dried coating composition. The base paper prepared in this comparative example 1 was finally measured to have a dry tensile strength of 4.05kN/m in the machine direction and 2.05kN/m in the cross direction; the wet tensile strength was 0.2kN/m in the machine direction, 0.10kN/m in the transverse direction, and the oil absorption value was 3.2, and after coating one side with silicone oil, the wet tensile strength of the contact paper obtained was 0.32kN/m in the transverse direction.

Example 2:

the effect of increasing the amount of film former used on the product properties was examined in example 2. Bleached kraft pine pulp and bleached kraft eucalyptus pulp are ground to 40 degrees and 40 degrees respectively and then mixed according to the proportion of 40:60 to form the pulp containing wood fibers. Adding to the pulp comprising wood fibers, per ton of oven dried paper, additives: 7 kg of starch, 1.05 kg of alkenyl succinic anhydride, 0.4 kg of polyacrylamide and 4.2 kg of silica sol to obtain pulp; preparing a paper base material layer on a paper machine by using the paper pulp, and preparing a coating composition required by a reinforcing layer according to the following formula, wherein 100 parts of an absolute dry coating composition comprises 2 parts (absolute dry weight) of a rheological agent CMC, 83 parts (absolute dry weight) of polyvinyl alcohol and 15 parts (absolute dry weight) of water-based polyacrylic resin; coating a reinforcing layer on both surfaces of the paper base material layer to form a base paper; and coating organic silicone oil on one surface of the base paper to form the contact paper. Through tests, the dry tensile strength of the base paper is 4.98kN/m in the longitudinal direction and 2.47kN/m in the transverse direction; the wet tensile strength was 0.34kN/m in the machine direction and 0.19kN/m in the transverse direction, and the Unger oil absorption number was 2.9. The wet tensile strength of the contact paper was 0.52kN/m in the cross direction.

Example 3:

bleached kraft pine pulp and bleached kraft eucalyptus pulp are ground to 45 degrees and 38 degrees respectively and then mixed according to the ratio of 55:45 to form the pulp containing wood fibers. Adding to the pulp comprising wood fibers, per ton of oven dried paper, additives: 5.5 kg of starch, 1.02 kg of alkenyl succinic anhydride, 0.3 kg of polyacrylamide and 3.2 kg of silica sol to obtain pulp; preparing a paper base material layer on a paper machine by using the paper pulp, and preparing a coating composition required by a reinforcing layer according to the following formula, wherein 100 parts of an absolute dry coating composition comprises 2 parts (absolute dry weight) of a rheological agent CMC, 68 parts (absolute dry weight) of polyvinyl alcohol and 30 parts (absolute dry weight) of water-based polyacrylic resin; coating a reinforcing layer on both surfaces of the paper base material layer to form a base paper; and coating organic silicone oil on one surface of the base paper to form the contact paper. Through testing, the dry tensile strength of the base paper is 5kN/m in the longitudinal direction and 2.5kN/m in the transverse direction; the wet tensile strength was 0.39kN/m in the machine direction and 0.12kN/m in the transverse direction, and the Unger oil absorption number was 3. The wet tensile strength of the contact paper was 0.55kN/m in the cross direction.

Example 4:

this example examined the effect of adding a specific amount of china clay to the strengthening layer on the performance of the product. Bleached kraft pine pulp and bleached kraft eucalyptus pulp are ground to 44 degrees and 34 degrees, respectively, and then mixed in a ratio of 40:60 to form a pulp containing wood fibers. Adding to the pulp comprising wood fibres an additive per ton of oven dried paper: 6 kg of starch, 1 kg of alkenyl succinic anhydride, 0.3 kg of polyacrylamide and 3.3 kg of silica sol to obtain pulp; preparing a paper base material layer on a paper machine by using the paper pulp, and preparing a coating composition required by a strengthening layer according to the following formula, wherein 100 parts of an absolute dry coating composition comprises 2 parts (absolute dry weight) of a rheological agent CMC, 63 parts (absolute dry weight) of china clay, 30 parts (absolute dry weight) of polyvinyl alcohol and 5 parts (absolute dry weight) of glyoxal; coating a strengthening layer on two sides of the paper base material layer to form base paper; and coating organic silicone oil on one surface of the base paper to form the contact paper. Through tests, the dry tensile strength of the base paper is 4.05kN/m in the longitudinal direction and 1.97kN/m in the transverse direction; the wet tensile strength was 0.37kN/m in the machine direction and 0.14kN/m in the transverse direction, and the Unger oil absorption number was 2.7. The wet tensile strength of the contact paper was 0.5kN/m in the cross direction.

Example 5:

this example examined the effect of adding another specific amount of china clay to the strengthening layer on the performance of the product. Bleached kraft pine pulp and bleached kraft eucalyptus pulp are ground to 48 degrees and 35 degrees respectively and then mixed according to the proportion of 40:60 to form the pulp containing wood fibers. Adding to the pulp comprising wood fibres an additive per ton of oven dried paper: 6.5 kg of starch, 0.9 kg of alkenyl succinic anhydride, 0.2 kg of polyacrylamide and 3.5 kg of silica sol to obtain pulp; preparing a paper base material layer on a paper machine by using the paper pulp, and preparing a coating composition required by a strengthening layer according to the following formula, wherein 100 parts of an absolute dry coating composition comprises 2 parts (absolute dry weight) of a rheological agent CMC, 58 parts (absolute dry weight) of china clay, 30 parts (absolute dry weight) of polyvinyl alcohol and 10 parts (absolute dry weight) of glyoxal; coating a reinforcing layer on both surfaces of the paper base material layer to form a base paper; and coating organic silicone oil on one surface of the base paper to form the contact paper. Through tests, the dry tensile strength of the base paper is 4.46kN/m in the longitudinal direction and 2.01kN/m in the transverse direction; the wet tensile strength was 0.37kN/m in the machine direction and 0.15kN/m in the transverse direction, and the Unger oil absorption number was 2.7. The wet tensile strength of the contact paper was 0.51kN/m in the cross direction.

Example 6:

in this example, the strengthening layer coating is carried out on a paper machine, making the coating more uniform. Bleached kraft pine pulp and bleached kraft eucalyptus pulp are ground to 45 degrees and 35 degrees respectively and then mixed according to the ratio of 35:65 to form the pulp containing wood fibers. Adding to the pulp comprising wood fibres an additive per ton of oven dried paper: 6 kg of starch, 0.95 kg of alkenyl succinic anhydride, 0.35 kg of polyacrylamide and 4 kg of silica sol to obtain paper pulp; preparing a paper base material layer on a paper machine by using the paper pulp, and preparing a coating composition required by a strengthening layer according to the following formula, wherein 100 parts of an absolute dry coating composition comprises 2 parts (absolute dry weight) of a rheological agent CMC, 30 parts (absolute dry weight) of china clay, 46 parts (absolute dry weight) of polyvinyl alcohol, 14 parts (absolute dry weight) of starch and 8 parts (absolute dry weight) of glyoxal; coating a reinforcing layer on both surfaces of the paper base material layer to form a base paper; and coating organic silicone oil on one surface of the base paper to form the contact paper. Through tests, the dry tensile strength of the base paper is 4.2kN/m in the longitudinal direction and 2.2kN/m in the transverse direction; the wet tensile strength was 0.22kN/m in the machine direction and 0.14kN/m in the transverse direction, and the Unger oil absorption number was 2.4. The wet tensile strength of the contact paper was 0.6kN/m in the cross direction. After the two sides are further coated with silicon, the wet tensile strength of the contact paper is 1.3kN/m in the transverse direction, the wet strength packaging requirements of food and medicines are completely met, and the contact paper is particularly suitable for paper for food steamers.

Comparative example 2

This comparative example 1 was carried out using exactly the same procedure as example 1, except that the reinforcing layer coating used was a conventional coating containing 98 parts of starch and 2 parts of CMC. The dry tensile strength of the base paper prepared in this comparative example 2 was finally measured to be 3.9kN/m in the machine direction and 1.9kN/m in the cross direction; wet tensile strength was 0.18kN/m in the machine direction, 0.10kN/m in the cross direction, and oil absorption number was 4, and after one side coating with silicone oil, the wet tensile strength of the paper sheet formed was 0.3kN/m in the cross direction.

As can be seen from examples 1-6 and comparative examples 1-2, the paper products made in the examples of the present application have excellent dry strength, wet strength and Unger oil absorption before the application of the hydrophobic layer, thereby ensuring uniform and effective bonding of the applied hydrophobic layer and sufficient wet strength of the resulting contact paper for use in high temperature and high humidity environments without breaking. The contact paper can completely replace the paper used for the steamer with the wet strength agent added at the wet end in the market, and avoids the addition and the migration of toxic and harmful substances.

Claims

1. A coating composition for increasing the wet strength and reducing oil absorption of the surface of paper, said paper being paper for food and pharmaceutical contact, said coating composition being an aqueous coating composition comprising: sizing agent, film former, rheological agent and aqueous solvent, and is characterized in that:

the sizing agent is selected from one or more of the following: starch, modified starch, polyvinyl alcohol (PVA), and combinations thereof;

the film forming agent is selected from one or more of the following: acrylic polymers, glyoxal, and combinations thereof;

the sizing agent, film former, rheological agent, and aqueous solvent are mixed together prior to use of the aqueous coating composition.

2. The coating composition of claim 1, wherein: the coating composition further comprises an inorganic pigment selected from the group consisting of: talc powder, china clay, calcined china clay and combinations thereof.

3. The coating composition of claim 1, wherein:

the sizing agent is polyvinyl alcohol, and the film forming agent is glyoxal or polyacrylic resin; and is

The weight ratio of the sizing agent to the film forming agent is more than or equal to 1.

4. The coating composition of claim 1, wherein:

the alcoholysis degree of the polyvinyl alcohol is 96-99%, and the viscosity of a 4% aqueous solution is 10-12cp when the polyvinyl alcohol is tested at 20 ℃;

the acrylic polymer is water-based polyacrylic resin, the concentration of the acrylic polymer is 40-46 wt%, and the viscosity at 25 ℃ is 4000-8000 cp;

the glyoxal is in the form of an aqueous glyoxal solution having a concentration of 37-40% by weight, and has a specific gravity of 1.20-1.30 kg/L.

5. The coating composition of claim 2, wherein: the content of the sizing agent is 30-95 wt%, the content of the film forming agent is 5-40 wt%, and the content of the inorganic pigment is 0-70 wt% based on the total weight of the components except the aqueous solvent.

6. The coating composition of claim 1 or 2, wherein: the coating composition is free of cationic polyalkyleneamine-based wet strength agents and is free of 1, 3-dichloro-2-propanol and 3-chloro-1, 2-propanediol or 1, 3-dichloro-2-propanol and 3-chloro-1, 2-propanediol each in an amount of <12 μ g/L.

7. The utility model provides a body paper that is used for food and medicine contact paper, its includes the paper substrate layer and is located at least one strengthening layer on the surface of paper substrate layer, its characterized in that:

the paper substrate layer is prepared using wood pulp and a wet end chemical comprising one or more of the following: filler, sizing agent, reinforcing agent, retention aid, charge regulator, filter aid, defoaming agent and bactericide,

the softwood pulp comprises: natural color sulfate softwood pulp, bleached sulfate softwood pulp, natural color softwood chemi-mechanical pulp, bleached softwood chemi-mechanical pulp;

the strengthening layer is formed from the coating composition of any of claims 1-6.

8. A food and pharmaceutical contact paper comprising the base paper of claim 7, and a hydrophobic layer on a surface of the reinforcing layer of the base paper, wherein the hydrophobic layer comprises a hydrophobic material selected from the group consisting of: emulsion silicone coatings, solvent-borne silicone coatings, solventless silicone coatings, Ultraviolet (UV) cured silicone coatings, and combinations thereof.

9. A method for producing the base paper for food and pharmaceutical contact paper of claim 7 or the food and pharmaceutical contact paper of claim 8, comprising the steps of:

3) preparing the paper substrate layer by using the paper pulp;

4) applying a reinforcing layer on at least one surface of the paper substrate layer using the coating composition of any one of claims 1-6 to obtain a base paper; and

5) optionally, a hydrophobic layer is applied on the surface of the strengthening layer to obtain a paper for food and drug contact.

10. The method of claim 9,

the pulp containing the wood fiber is a mixture of softwood pulp and hardwood pulp, the beating degree of the softwood pulp is 40-50 degrees, and the beating degree of the hardwood pulp is 30-40 degrees.

11. The method of claim 9,

in the paper substrate, strengthening layer and hydrophobic layer, no cationic polyalkyleneamine-based wet strength agent is contained, and no 1, 3-dichloro-2-propanol and 3-chloro-1, 2-propanediol or no 1, 3-dichloro-2-propanol and 3-chloro-1, 2-propanediol are contained, respectively, in an amount of <12 μ g/L.

12. A method of cooking a food product using the contact paper of claim 8, the method comprising:

a. providing a food product, preferably a non-liquid food product;

b. placing the food product in a cooking device and lining the contact paper between the food product and the cooking device, the contact paper contacting at least a portion of a surface of the food product such that the food product does not contact the cooking device;

c. heating the cooking device.

13. The method of claim 12, wherein the cooking process is a steam process, the method comprising:

a. providing a food product, preferably a non-liquid food product;

b. the cooking device is a steamer, the food is placed at the upper part in the steamer, the contact paper is lined between the food and the steamer, the contact paper contacts at least one part of the surface of the food, so that the food is not contacted with the steamer, and the liquid generating steam is placed at the lower part in the steamer;

c. and closing the steamer, heating the steamer to generate steam in the steamer, and carrying out steam processing on the food.

14. The process of claim 13, wherein the water vapor produced in step c has a temperature of 50 to 120 ℃ and a pressure of 50 to 100 kPa.