Environment-friendly white craft paper with functional filler added to bottom layer

Abstract

The invention belongs to the technical field of papermaking, and particularly relates to an environment-friendly white-surface craft paper with a functional filler added to a bottom layer. The invention provides an environment-friendly white craft paper with a functional filler added at a bottom layer, which aims at solving the problems that the white craft paper in the prior art is easy to locally transmit light and the bottom is exposed during printing, and comprises the bottom layer, the bottom layer is prepared from the following components in parts by mass: 80 to 120 parts of bottom layer paper pulp, 25 to 35 parts of modified composite filler, 0.5 to 1.0 part of cationic starch and 1.0 to 1.2 parts of dry strength agent. The modified composite filler is added to replace calcium carbonate and talcum powder filler commonly used in the prior art, so that the problems of local light transmission and printing base exposure of the prepared paper are solved.

Description

Technical Field

[0001] This invention belongs to the field of papermaking technology, and in particular relates to an environmentally friendly white-faced kraft paper with functional fillers added to the bottom layer. Background Technology

[0002] White-faced kraft paper is a packaging material made from bleached wood pulp using a coating or surface-coating process. It is categorized into domestic and imported types based on its origin. Its structure typically consists of a bleached wood pulp surface layer, a deinked pulp lining layer, and a waste paper core bottom layer. It features two-sided coloring, with one side white and the other kraft paper-colored. Current technology typically adds fillers such as calcium carbonate and talc to the bottom layer of white-faced kraft paper. However, adding these fillers to traditional pulp can easily cause flocculation, resulting in localized light transmission and printing defects.

[0003] For example, a Chinese invention patent discloses a white-faced kraft paper and its preparation method [Application No.: CN202310736892.0]. The invention includes the following steps: pulp preparation: Preparation of wet white pulp: Waste milk cartons are used as raw materials. After crushing, high-consistency slag removal and disc milling, wet white pulp is obtained. OCC pulp preparation: Using Class A waste OCC as raw material, the pulping process involves OCC line fragmentation, high-consistency slag removal, coarse screening, and medium-consistency slag removal, followed by sieving to separate short fibers, medium fibers, and long fibers. The long fibers are then finely screened, concentrated in multiple discs, and milled to obtain OCC long fiber pulp. The medium fibers are then lightly slag removed and concentrated in multiple discs to obtain OCC medium fiber pulp. The short fibers are then lightly slag removed and concentrated in multiple discs to obtain OCC short fiber pulp. Preparation of short-fiber bleached wood pulp: Using bleached wood pulp as raw material, short-fiber bleached wood pulp is obtained after crushing, high-consistency slag removal, and disc milling. DIP deinking pulp preparation: DIP deinking pulp is obtained by using domestic waste bleach as raw material and processing it through a DIP deinking pulp line. Pulp preparation: Use 20-30% DIP pulp and 70-80% short fiber bleached wood pulp as the top dressing; Use 20-30% wet white mortar and 70-80% DIP mortar as the core layer mortar; Use 40-45% OCC long fiber pulp and 55-60% OCC short fiber pulp as the base pulp; Copied: The above pulp is dewatered and shaped in the wire section, laminated with the face liner and core bottom, dewatered in the pressing section, dried in the pre-drying section, coated by the film transfer sizing machine, dried in the post-drying section, calendered, and then formed into paper.

[0004] Although this invention patent has the advantages of improving the ink absorption, folding resistance, and tear resistance of white-faced kraft paper, it still does not solve the above-mentioned problems. Summary of the Invention

[0005] The purpose of this invention is to address the above-mentioned problems by providing an environmentally friendly white-faced kraft paper with functional fillers added to the bottom layer.

[0006] To achieve the above objectives, the present invention adopts the following technical solutions: An environmentally friendly white-faced kraft paper with functional fillers added to the bottom layer includes a bottom layer comprising 80-120 parts by weight of bottom pulp, 25-35 parts by weight of modified composite filler, 0.5-1.0 parts by weight of cationic starch and 1.0-1.2 parts by weight of dry strength agent.

[0007] In the above-mentioned environmentally friendly white-faced kraft paper with added functional fillers at the bottom layer, the bottom pulp is waste paper pulp, OCC pulp, or a mixture of equal masses of bleached sulfate softwood pulp and bleached sulfate hardwood pulp.

[0008] In the above-mentioned environmentally friendly white-faced kraft paper with added functional fillers at the bottom layer, the dry strength agent is cationic polyacrylamide.

[0009] In the above-mentioned environmentally friendly white-faced kraft paper with functional fillers added to the bottom layer, the modified composite filler includes 60-80 parts by weight of anionic polymer-coated modified ultrafine calcium carbonate, 15-30 parts by weight of surface hydroxyl-activated modified kaolin, and 5-10 parts by weight of stabilized modified carbon black.

[0010] In the above-mentioned environmentally friendly white-faced kraft paper with functional fillers added to the bottom layer, the anionic polymer-coated modified ultrafine calcium carbonate is prepared by the following steps: ultrafine calcium carbonate is mixed with sodium polyacrylate aqueous solution, heated to 60-70℃ and stirred at a stirring speed of 3000-4000rpm for 40-60min, and after solid-liquid separation, it is dried to obtain anionic polymer-coated modified ultrafine calcium carbonate.

[0011] Calcium carbonate, rich in calcium ions, exhibits a weakly positive charge or uneven charge in pulp, readily forming rigid, large flocs through calcium bridging or heterogeneous flocculation with negatively charged fibers and anionic waste. This invention utilizes anionic polymers such as sodium polyacrylate, which are firmly adsorbed onto the calcium carbonate surface via carboxyl groups and other functional groups, transforming its surface potential from near-zero or positive to a higher zeta negative potential. This generates strong electrostatic repulsion between particles, effectively preventing homogeneous flocculation of the calcium carbonate particles themselves. Simultaneously, the modified calcium carbonate surface shares the same charge as pulp fibers (which are also typically negatively charged), reducing premature and localized adsorption caused by heterogeneous charge attraction. This allows calcium carbonate to move more freely in the pulp, creating conditions for subsequent uniform distribution.

[0012] In the above-mentioned environmentally friendly white-faced kraft paper with added functional fillers at the bottom layer, the sodium polyacrylate aqueous solution contains 10% sodium polyacrylate by mass; the added sodium polyacrylate is 0.8-1.5% of the mass of ultrafine calcium carbonate.

[0013] In the above-mentioned environmentally friendly white-faced kraft paper with added functional fillers at the bottom layer, the surface hydroxyl-activated modified kaolin is prepared through the following steps: Step 1: Mix kaolin and potassium acetate in water, grind, let stand for intercalation, then wash and peel to obtain finer and thinner flaky kaolin; Step 2: Mix the flaky kaolin obtained in Step 1 with water and adjust the pH to 8-9 with sodium hydroxide aqueous solution. Then add sodium silicate and stir at 65-75℃ for 40-80 minutes to obtain surface hydroxyl-activated modified kaolin.

[0014] Natural kaolin particles easily form "cardboard-like" flocs due to hydrogen bonding and edge-face electrostatic attraction. These flocs are large in size, loosely structured, and unevenly distributed in the paper sheet, failing to utilize their excellent sheet-like covering properties. This invention uses intercalating agents such as potassium acetate to expand the interlayers of kaolin, resulting in thinner flakes with a larger aspect ratio after shearing. The larger flake structure means a larger coverage area per unit mass of filler. The silica species generated by the hydrolysis of sodium silicate undergo a condensation reaction with the Al-OH and Si-OH on the kaolin surface, forming a hydrophilic amorphous SiO2 hydrated layer. The highly dispersed, complete large flakes of kaolin are more easily aligned parallel to the paper surface during the paper forming process by the shear force of the water flow. This arrangement, like roof tiles, efficiently covers and shields the underlying fiber network and pores, achieving optimal light scattering and blocking, which is key to improving overall and local opacity.

[0015] In the above-mentioned environmentally friendly white-faced kraft paper with added functional fillers at the bottom layer, the amount of potassium acetate added is 5-10% of the mass of kaolin; the amount of sodium silicate added is 1.0-2.0% of the mass of flake kaolin.

[0016] In the above-mentioned environmentally friendly white-faced kraft paper with functional fillers added to the bottom layer, the stabilized modified carbon black is prepared by the following steps: mixing carbon black with an aqueous solution of fatty alcohol polyoxyethylene ether phosphate, dispersing by stirring, and adjusting the pH value to 8-9 to obtain a slurry containing stabilized modified carbon black.

[0017] In the above-mentioned environmentally friendly white-faced kraft paper with added functional fillers at the bottom layer, the amount of fatty alcohol polyoxyethylene ether phosphate added is 3-5% of the mass of carbon black.

[0018] Carbon black has an extremely small primary particle size, a huge specific surface area, and extremely high surface energy, exhibiting very strong van der Waals attraction. It readily undergoes irreversible aggregation, forming aggregates several micrometers or even larger. These aggregates form black spots in the paper, while the surrounding areas appear white due to the lack of carbon black, causing severe color unevenness and differences in light transmission. This invention uses fatty alcohol polyoxyethylene ether phosphate adsorbed onto the carbon black surface, with its hydrophilic long chains extending into the water to form a thick polymer adsorption layer. This adsorption layer generates a strong steric hindrance effect, preventing carbon black particles from approaching the distance where van der Waals forces are effective even during intense collisions, allowing the carbon black to exist in a stable colloidal form close to its primary particle size. This ultrafine and uniform dispersion can be evenly distributed in the pulp like a dye and adsorbed onto the surfaces of all fibers and fillers. It provides a uniform, spotless base color in the paper, ensuring consistent light absorption in any localized area. This eliminates the problem of localized darker colors and lighter surrounding colors caused by carbon black aggregation, leading to easy light transmission and exposure of the underlying material.

[0019] Compared with existing technologies, the advantages of this invention are: 1. This invention avoids the problems of local light transmission and printing exposure in the paper by adding modified composite fillers to replace the calcium carbonate and talc fillers commonly used in the prior art.

[0020] 2. The modified composite filler of this invention is obtained by compounding anionic polymer-coated modified ultrafine calcium carbonate, surface hydroxyl-activated modified kaolin, and stabilized modified carbon black. The anionic polymer-coated modified ultrafine calcium carbonate has discrete nano- to micron-sized particles, which effectively fill the smallest gaps between fibers and eliminate microporous light transmission points. The large-sized flake-shaped surface hydroxyl-activated modified kaolin, based on the filling of anionic polymer-coated modified ultrafine calcium carbonate, provides macroscopic coverage and shielding, blocking light penetration over a larger area. The stabilized modified carbon black can dye the entire fiber-filler network with a uniform dark base color, absorb the residual scattered light that passes through the first two barriers, and homogenize the base color of the paper. Therefore, the three form a three-in-one synergistic protective structure of "point-surface-color" in the paper, preventing the problems of local light transmission and printing exposure. Detailed Implementation

[0021] The present invention will now be described in further detail with reference to specific embodiments. Example 1

[0022] This embodiment provides an environmentally friendly white-faced kraft paper with a functional filler added to the bottom layer, including a bottom layer comprising 120 parts by weight of bottom pulp, 25 parts by weight of modified composite filler, 0.5 parts by weight of cationic starch and 1.0 part by weight of cationic polyacrylamide.

[0023] The bottom pulp is OCC pulp, and the modified composite filler includes 80 parts by weight of anionic polymer-coated modified ultrafine calcium carbonate, 15 parts by weight of surface hydroxyl-activated modified kaolin, and 5 parts by weight of stabilized modified carbon black.

[0024] Anionic polymer-coated modified ultrafine calcium carbonate is prepared through the following steps: Ultrafine calcium carbonate was mixed with an aqueous solution of sodium polyacrylate, heated to 60°C, and stirred at 3000 rpm for 40 min. After solid-liquid separation, the mixture was dried to obtain anionic polymer-coated modified ultrafine calcium carbonate. The sodium polyacrylate aqueous solution contained 10% sodium polyacrylate by mass, and the added sodium polyacrylate was 0.8% of the mass of ultrafine calcium carbonate.

[0025] Surface-activated kaolin is prepared through the following steps: Step 1: Mix kaolin and potassium acetate in water, grind, let stand for intercalation, then wash and peel to obtain finer and thinner flaky kaolin; Step 2: Mix the flaky kaolin obtained in Step 1 with water and adjust the pH to 8-9 with sodium hydroxide aqueous solution. Then add sodium silicate and stir at 65℃ for 40 minutes to obtain surface hydroxyl-activated modified kaolin.

[0026] The amount of potassium acetate added is 5% of the mass of kaolin; the amount of sodium silicate added is 1.0% of the mass of flaky kaolin.

[0027] Stabilized modified carbon black is prepared through the following steps: Carbon black and an aqueous solution of fatty alcohol polyoxyethylene ether phosphate were mixed, and the pH was adjusted to 8-9 after stirring and dispersion to obtain a slurry containing stabilized modified carbon black; the amount of fatty alcohol polyoxyethylene ether phosphate added was 3% of the mass of carbon black.

[0028] The printing exposure rate of the white-faced kraft paper prepared in the example was measured to be 1.1%. Example 2

[0029] This embodiment provides an environmentally friendly white-faced kraft paper with a functional filler added to the bottom layer, including a bottom layer comprising 80 parts by weight of bottom pulp, 35 parts by weight of modified composite filler, 1.0 part by weight of cationic starch and 1.2 parts by weight of cationic polyacrylamide.

[0030] The bottom pulp is OCC pulp, and the modified composite filler includes 60 parts by weight of anionic polymer-coated modified ultrafine calcium carbonate, 30 parts by weight of surface hydroxyl-activated modified kaolin, and 10 parts by weight of stabilized modified carbon black.

[0031] Anionic polymer-coated modified ultrafine calcium carbonate is prepared through the following steps: Ultrafine calcium carbonate was mixed with an aqueous solution of sodium polyacrylate, heated to 70°C, and stirred at 4000 rpm for 60 min. After solid-liquid separation, the mixture was dried to obtain anionic polymer-coated modified ultrafine calcium carbonate. The sodium polyacrylate aqueous solution contained 10% sodium polyacrylate by mass, and the added sodium polyacrylate was 1.5% of the mass of ultrafine calcium carbonate.

[0032] Surface-activated kaolin is prepared through the following steps: Step 1: Mix kaolin and potassium acetate in water, grind, let stand for intercalation, then wash and peel to obtain finer and thinner flaky kaolin; Step 2: Mix the flaky kaolin obtained in Step 1 with water and adjust the pH to 8-9 with sodium hydroxide aqueous solution. Then add sodium silicate and stir at 75℃ for 80 minutes to obtain surface hydroxyl-activated modified kaolin.

[0033] The amount of potassium acetate added is 10% of the mass of kaolin; the amount of sodium silicate added is 2.0% of the mass of flaky kaolin.

[0034] Stabilized modified carbon black is prepared through the following steps: Carbon black is mixed with an aqueous solution of fatty alcohol polyoxyethylene ether phosphate, and after stirring and dispersing, the pH value is adjusted to 8-9 to obtain a slurry containing stabilized modified carbon black; the amount of fatty alcohol polyoxyethylene ether phosphate added is 5% of the mass of carbon black.

[0035] The printing exposure rate of the white-faced kraft paper prepared in the example was measured to be 0.5%. Example 3

[0036] This embodiment provides an environmentally friendly white-faced kraft paper with a functional filler added to the bottom layer, including a bottom layer comprising 100 parts by weight of bottom pulp, 30 parts by weight of modified composite filler, 0.8 parts by weight of cationic starch and 1.1 parts by weight of cationic polyacrylamide.

[0037] The bottom pulp is OCC pulp, and the modified composite filler includes 70 parts by weight of anionic polymer-coated modified ultrafine calcium carbonate, 22 parts by weight of surface hydroxyl-activated modified kaolin, and 8 parts by weight of stabilized modified carbon black.

[0038] Anionic polymer-coated modified ultrafine calcium carbonate is prepared through the following steps: Ultrafine calcium carbonate was mixed with an aqueous solution of sodium polyacrylate, heated to 65°C, and stirred at 3500 rpm for 50 min. After solid-liquid separation, the mixture was dried to obtain anionic polymer-coated modified ultrafine calcium carbonate. The sodium polyacrylate aqueous solution contained 10% sodium polyacrylate by mass, and the added sodium polyacrylate was 1.0% of the mass of ultrafine calcium carbonate.

[0039] Surface-activated kaolin is prepared through the following steps: Step 1: Mix kaolin and potassium acetate in water, grind, let stand for intercalation, then wash and peel to obtain finer and thinner flaky kaolin; Step 2: Mix the flaky kaolin obtained in Step 1 with water and adjust the pH to 8-9 with sodium hydroxide aqueous solution. Then add sodium silicate and stir at 70℃ for 60 minutes to obtain surface hydroxyl-activated modified kaolin.

[0040] The amount of potassium acetate added is 8% of the mass of kaolin; the amount of sodium silicate added is 1.5% of the mass of flaky kaolin.

[0041] Stabilized modified carbon black is prepared through the following steps: Carbon black and an aqueous solution of fatty alcohol polyoxyethylene ether phosphate were mixed, and the pH was adjusted to 8-9 after stirring and dispersion to obtain a slurry containing stabilized modified carbon black; the amount of fatty alcohol polyoxyethylene ether phosphate added was 4% of the mass of carbon black.

[0042] The printing exposure rate of the white kraft paper prepared in the example was measured to be 0.7%.

[0043] Comparative Example 1 This comparative example provides an environmentally friendly white-faced kraft paper with a functional filler added to the bottom layer, including a bottom layer comprising 100 parts by weight of bottom pulp, 30 parts of ultrafine calcium carbonate, 0.8 parts of cationic starch and 1.1 parts of cationic polyacrylamide.

[0044] The bottom layer pulp is OCC pulp.

[0045] The printing exposure rate of the white-faced kraft paper prepared from the bottom layer in the comparative example was measured to be 7.8%.

[0046] Comparative Example 2 This comparative example provides an environmentally friendly white-faced kraft paper with a functional filler added to the bottom layer, including a bottom layer comprising 100 parts by weight of bottom pulp, 30 parts by weight of modified composite filler, 0.8 parts by weight of cationic starch and 1.1 parts by weight of cationic polyacrylamide.

[0047] The bottom pulp is OCC pulp, and the modified composite filler includes 70 parts by weight of anionic polymer-coated modified ultrafine calcium carbonate, 22 parts by weight of kaolin, and 8 parts by weight of carbon black.

[0048] Anionic polymer-coated modified ultrafine calcium carbonate is prepared through the following steps: Ultrafine calcium carbonate was mixed with an aqueous solution of sodium polyacrylate, heated to 65°C, and stirred at 3500 rpm for 50 min. After solid-liquid separation, the mixture was dried to obtain anionic polymer-coated modified ultrafine calcium carbonate. The sodium polyacrylate aqueous solution contained 10% sodium polyacrylate by mass, and the added sodium polyacrylate was 1.0% of the mass of ultrafine calcium carbonate.

[0049] The printing exposure rate of the white-faced kraft paper prepared from the bottom layer in the comparative example was measured to be 7.1%.

[0050] Comparative Example 3 This comparative example provides an environmentally friendly white-faced kraft paper with a functional filler added to the bottom layer, including a bottom layer comprising 100 parts by weight of bottom pulp, 30 parts by weight of modified composite filler, 0.8 parts by weight of cationic starch and 1.1 parts by weight of cationic polyacrylamide.

[0051] The bottom pulp is OCC pulp, and the modified composite filler includes 70 parts by weight of ultrafine calcium carbonate, 22 parts by weight of surface hydroxyl-activated modified kaolin, and 8 parts by weight of carbon black.

[0052] Surface-activated kaolin is prepared through the following steps: Step 1: Mix kaolin and potassium acetate in water, grind, let stand for intercalation, then wash and peel to obtain finer and thinner flaky kaolin; Step 2: Mix the flaky kaolin obtained in Step 1 with water and adjust the pH to 8-9 with sodium hydroxide aqueous solution. Then add sodium silicate and stir at 70℃ for 60 minutes to obtain surface hydroxyl-activated modified kaolin.

[0053] The amount of potassium acetate added is 8% of the mass of kaolin; the amount of sodium silicate added is 1.5% of the mass of flaky kaolin.

[0054] The printing exposure rate of the white-faced kraft paper prepared from the bottom layer in the comparative example was measured to be 5.3%.

[0055] Comparative Example 4 This comparative example provides an environmentally friendly white-faced kraft paper with a functional filler added to the bottom layer, including a bottom layer comprising 100 parts by weight of bottom pulp, 30 parts by weight of modified composite filler, 0.8 parts by weight of cationic starch and 1.1 parts by weight of cationic polyacrylamide.

[0056] The bottom pulp is OCC pulp, and the modified composite filler includes 70 parts by weight of ultrafine calcium carbonate, 22 parts by weight of kaolin, and 8 parts by weight of stabilized modified carbon black.

[0057] Stabilized modified carbon black is prepared through the following steps: Carbon black and an aqueous solution of fatty alcohol polyoxyethylene ether phosphate were mixed, and the pH was adjusted to 8-9 after stirring and dispersion to obtain a slurry containing stabilized modified carbon black; the amount of fatty alcohol polyoxyethylene ether phosphate added was 4% of the mass of carbon black.

[0058] The printing exposure rate of the white-faced kraft paper prepared from the bottom layer in the comparative example was measured to be 4.9%.

[0059] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. An environmentally friendly white-faced kraft paper with a functional filler added to the bottom layer, comprising a bottom layer, characterized in that: The bottom layer comprises 80-120 parts by weight of bottom pulp, 25-35 parts by weight of modified composite filler, 0.5-1.0 parts by weight of cationic starch and 1.0-1.2 parts by weight of dry strength agent.

2. The environmentally friendly white-faced kraft paper with functional filler added to the bottom layer as described in claim 1, characterized in that: The underlying pulp is waste paper pulp, OCC pulp, or a mixture of equal masses of bleached sulfate softwood pulp and bleached sulfate hardwood pulp.

3. The environmentally friendly white-faced kraft paper with functional filler added to the bottom layer as described in claim 1, characterized in that: The dry strength agent is cationic polyacrylamide.

4. The environmentally friendly white-faced kraft paper with functional filler added to the bottom layer as described in claim 1, characterized in that: The modified composite filler comprises 60-80 parts by weight of anionic polymer-coated modified ultrafine calcium carbonate, 15-30 parts by weight of surface hydroxyl-activated modified kaolin, and 5-10 parts by weight of stabilized modified carbon black.

5. An environmentally friendly white-faced kraft paper with functional filler added to the bottom layer as described in claim 4, characterized in that: The anionic polymer-coated modified ultrafine calcium carbonate is prepared by the following steps: ultrafine calcium carbonate is mixed with an aqueous solution of sodium polyacrylate, heated to 60-70℃, and stirred at a stirring speed of 3000-4000 rpm for 40-60 minutes. After solid-liquid separation, the mixture is dried to obtain anionic polymer-coated modified ultrafine calcium carbonate.

6. The environmentally friendly white-faced kraft paper with functional filler added to the bottom layer as described in claim 5, characterized in that: The sodium polyacrylate aqueous solution contains 10% sodium polyacrylate by mass; the added sodium polyacrylate is 0.8-1.5% of the mass of the ultrafine calcium carbonate.

7. The environmentally friendly white-faced kraft paper with functional filler added to the bottom layer as described in claim 4, characterized in that: The surface hydroxyl-activated modified kaolin is prepared through the following steps: Step 1: Mix kaolin and potassium acetate in water, grind, let stand for intercalation, then wash and peel to obtain finer and thinner flaky kaolin; Step 2: Mix the flaky kaolin obtained in Step 1 with water and adjust the pH to 8-9 with sodium hydroxide aqueous solution. Then add sodium silicate and stir at 65-75℃ for 40-80 minutes to obtain surface hydroxyl-activated modified kaolin.

8. An environmentally friendly white-faced kraft paper with functional filler added to the bottom layer as described in claim 7, characterized in that: The amount of potassium acetate added is 5-10% of the mass of kaolin; the amount of sodium silicate added is 1.0-2.0% of the mass of flaky kaolin.

9. An environmentally friendly white-faced kraft paper with functional filler added to the bottom layer as described in claim 4, characterized in that: The stabilized modified carbon black is prepared by the following steps: mixing carbon black with an aqueous solution of fatty alcohol polyoxyethylene ether phosphate, dispersing by stirring, and adjusting the pH value to 8-9 to obtain a slurry containing stabilized modified carbon black.

10. An environmentally friendly white-faced kraft paper with functional filler added to the bottom layer as described in claim 9, characterized in that: The amount of fatty alcohol polyoxyethylene ether phosphate added is 3-5% of the mass of carbon black.