A modified fiber fabric and its preparation method

Abstract

This invention discloses a modified fiber fabric and its preparation method, belonging to the field of cellulose technology. It solves the technical problem in existing CMC-H modified fiber products where sodium and potassium residues remain on the absorbent material, affecting its biocompatibility. The modified fiber fabric and its preparation method provided by this invention involve preserving the fibrous raw material, which is then sequentially modified to carboxymethyl cellulose and ammonified with ammonia to obtain a fiber fabric with carboxymethyl cellulose ammonium as the modified product. Specifically, during the modification process, the 1,4-glycosidic bonds in the glucose molecules of the fiber raw material remain unchanged, forming parallel cellulose chains. The modification reaction modifies specific chemical groups occupying specific carbon positions (C6, C2, C3) on the fiber molecular chains, ensuring the softness and physical properties of the resulting paper or nonwoven fabric.

Description

Technical Field

[0001] This invention relates to the field of cellulose technology, specifically to a modified fiber fabric and its preparation method. Background Technology

[0002] Commercially available absorbent materials include ordinary absorbent materials and superabsorbent polymers. Ordinary absorbent materials, such as paper and cotton fabrics, are limited in their water absorption capacity and therefore have limited applications. In contrast, superabsorbent polymers are more widely used in hygiene, medical, industrial, agricultural, and forestry fields. Absorbent mats made from superabsorbent resins, in particular, possess excellent absorbency and water retention properties. However, due to the poor biodegradability of polypropylene-based superabsorbent resins, they can easily cause groundwater and soil pollution. Therefore, researchers have begun to explore environmentally friendly absorbent materials.

[0003] Cellulose is a long-chain natural polymer composed of D-glucose monomers dehydrated and linked by β-1,4-glucosidic bonds. The hydroxyl group at C6 is a primary alcohol hydroxyl group, while the hydroxyl groups at C2 and C3 are secondary alcohol hydroxyl groups. Absorbent materials prepared from cellulose exhibit good degradability and biocompatibility. They are produced by replacing the H-hydroxyl groups (C2, C3, and C6) with carboxymethyl groups to create a relatively less absorbent CMC-H transition material, which is then alkalized to obtain the corresponding absorbent material. However, existing CMC-H materials produced through papermaking or nonwoven fabrication often involve the addition of sodium hydroxide, sodium carbonate, or potassium hydroxide, causing unreacted sodium hydroxide, sodium carbonate, or potassium hydroxide to bind to the CMC-H. This residue remains on the absorbent material, affecting its biocompatibility and hindering its application in hygiene, medical, and other fields.

[0004] Carboxymethyl cellulose ammonium is another derivative material prepared from cellulose. Existing methods for preparing carboxymethyl cellulose ammonium, such as the method disclosed in patent application CN201110457650.5 and the method disclosed in patent application CN202011249366.4 for preparing high-viscosity carboxymethyl cellulose ammonium, are all in powder form and are usually used as a special-purpose binder, mainly for high-quality SCR denitrification catalysts. Furthermore, fiber water absorption relies not only on the fiber material itself but also on the "capillary effect" between fibers. That is, when water comes into contact with the gaps between fibers, water moves along the fiber gaps due to capillary action, and some water fills the fiber gaps. However, the water absorption characteristics of powdered carboxymethyl cellulose ammonium are relatively weak due to this capillary effect. Therefore, in the existing technology, carboxymethyl cellulose ammonium is difficult to utilize as a cellulose water-absorbing material. Summary of the Invention

[0005] This invention addresses the shortcomings and deficiencies of existing technologies by providing a cleverly designed reaction that maintains the smooth progress of the reaction while eliminating the problem of unbound non-volatile sodium and potassium ions remaining. This improves the preparation efficiency and product purity of carboxymethyl cellulose ammonium absorbent materials. The resulting carboxymethyl cellulose ammonium absorbent material exhibits excellent water absorption, water retention, biodegradability, and biocompatibility, and can be well applied to modified fiber fabrics in fields such as hygiene care, medical treatment, industry, agriculture, and forestry, as well as its preparation method and applications.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution: The modified fiber fabric provided by the present invention is based on maintaining the fibrous shape of the fiber fabric raw material, and is subsequently modified into carboxymethyl cellulose and ammonium-modified with ammonia to obtain a fiber fabric with carboxymethyl cellulose ammonium as the modified product.

[0007] The fiber fabric raw material is modified to maintain its fibrous shape. Specifically, during the modification process, the 1,4-glycosidic bonds (i.e., the connection between C1 and C4) in the glucose molecules of the fiber fabric raw material are connected and the parallel cellulose chains are not changed. The specific chemical groups occupying specific carbon positions C6, C2, and C3 on the fiber molecular chain are modified.

[0008] Preferably, the degree of substitution in the carboxymethyl cellulose ammonium fiber fabric treated with ammonia is 0.2-3.0, the terminal modification groups of specific chemical groups at carbon positions C6, C2, and C3 in the carboxymethyl cellulose ammonium molecule include -CH2COONH4, the content of which is limited to 4.3%-58.9%, and -CH2COONH4 is directly linked to the glucose unit of the fiber, and the tensile strength of the fiber in the carboxymethyl cellulose ammonium fiber fabric is not less than 60% of the tensile strength of the fiber in the fiber fabric raw material.

[0009] Preferably, the cellulose fiber is a natural cellulose fiber and / or a regenerated cellulose fiber; the length of the cellulose fiber is not less than 0.5 mm. The cellulose fiber can be a natural cellulose fiber such as cotton, hemp, bamboo, or straw, or a regenerated cellulose fiber such as viscose, Tencel, copper amine, or modal, or a blend of the above two categories of fibers. It has a wide range of applications, abundant raw material sources, and good market prospects.

[0010] Preferably, in the process of modifying cellulose into carboxymethyl cellulose, based on maintaining the fibrous shape of cellulose fibers, alkalization and etherification are carried out in sequence, followed by the addition of acid solution and mixing evenly. The volume percentage concentration of acid in the acid solution is ≥29%. After acidification reaction, the modified product is carboxymethyl cellulose CMC-H.

[0011] A method for preparing modified fiber fabrics, wherein the fiber fabric raw material is kept in fibrous form, and is subsequently modified to carboxymethyl cellulose and then ammonified with ammonia to obtain a fiber fabric with carboxymethyl cellulose ammonium as the modified product.

[0012] The fiber fabric raw material is modified to maintain its fibrous shape. Specifically, during the modification process, the 1,4-glycosidic bonds (i.e., the connection between C1 and C4) in the glucose molecules of the fiber fabric raw material are connected and the parallel cellulose chains are not changed. The specific chemical groups occupying specific carbon positions C6, C2, and C3 on the fiber molecular chain are modified.

[0013] Preferably, the raw material for the fiber fabric is cellulose fiber, and the length of the cellulose fiber is not less than 0.5 mm;

[0014] In the process of modifying carboxymethyl cellulose, alkalization and etherification are carried out sequentially while maintaining the fibrous structure, followed by the addition of acid solution and thorough mixing; the volume percentage concentration of acid in the acid solution is ≥29%, and the modified product obtained after the acidification reaction is carboxymethyl cellulose CMC-H;

[0015] After washing, washed fibers are obtained;

[0016] The obtained washed fibers are then ammonified with ammonia to obtain fiber fabrics with carboxymethyl cellulose ammonium as the modified product.

[0017] The degree of substitution in the ammonia ammoniation treatment of carboxymethyl cellulose ammonium fiber fabric is 0.2-3.0. The terminal modification groups of specific chemical groups at carbon positions C6, C2, and C3 in the carboxymethyl cellulose ammonium molecule include -CH2COONH4, the content of which is limited to 4.3%-58.9%. Furthermore, -CH2COONH4 is directly linked to the glucose unit of the fiber, and the tensile strength of the fiber in the carboxymethyl cellulose ammonium fiber fabric is not less than 60% of the tensile strength of the fiber in the fiber fabric raw material.

[0018] Preferably, the washed fibers are obtained by washing with water until the pH of the residual water is ≥4.0.

[0019] Preferably, the post-treatment drying step in the ammoniation process using ammonia is heating and drying. The heat obtained during the heating and drying of the ammonified fiber fabric removes the uncombined ammonia gas from the reaction. The drying process continues until the moisture content of the carboxymethyl cellulose ammonium fiber fabric is ≤15%. This drying gradually controls the moisture content to ≤15%, which can reduce the deformation of the fiber due to excessive water loss in a short period of time, thereby obtaining a carboxymethyl cellulose ammonium water-absorbing material with better mechanical strength.

[0020] Preferably, in the ammoniation treatment step using ammonia gas, the sheeting is performed first, followed by the ammoniation treatment step. This fabrication method offers greater operability, is applicable to a wider range of processes, has fewer drying steps, and results in higher production efficiency. Specifically:

[0021] First, the washed fibers are made into a preliminary sheet using papermaking or nonwoven processes. Then, the preliminary sheet is ammonified with ammonia to obtain ammonified fiber sheets.

[0022] Preferably, during the process of forming the washed fibers into initial sheets,

[0023] When papermaking is used, in the papermaking process, the washed fibers are first evenly dispersed in pure water or a dispersion of ordinary fibers and water before papermaking. The weight ratio of washed fibers to ordinary fibers is 10-100:90-0. During the papermaking process, the total addition amount of washed fibers and ordinary fibers is 0.3-1.5 wt%, and the single-layer thickness of the paper is 7-30 g / m². 2 If the fiber content is too low and the thickness is too small, the breakage rate of the initial sheet will be high; if the fiber content is too high and the thickness is too large, it will affect the subsequent drying efficiency and effect of the initial sheet. Therefore, when the above-mentioned papermaking parameters are used in the preparation method of the present invention, the product performance is optimal.

[0024] When using a nonwoven fabrication process, in the nonwoven step, washed fibers are first mixed with ordinary fibers before nonwoven fabrication. The weight ratio of washed fibers to ordinary fibers is 10-100:90-0. In the nonwoven step, the washed fibers and ordinary fibers are reinforced into a nonwoven fabric using mechanical, thermal bonding, or chemical methods, with a basis weight of 9-600 g / m². 2 Nonwoven processes can be performed using various methods, such as hot rolling, hydroentangling, and needle punching. Hot rolling results in lower basis weight, while needle punching results in higher basis weight. Therefore, the basis weight of this invention can be adjusted from 9-600 g / m² depending on the specific process requirements. 2 Adjust within the specified range.

[0025] In this invention, the papermaking process involves uniformly dispersing the washed fibers before papermaking; the nonwoven process involves orienting or randomly arranging the washed fibers to form a web structure, which is then reinforced using mechanical, thermal, or chemical methods. Preferably, the common fibers are one or more of bamboo fiber, cotton fiber, wood fiber, hemp fiber, regenerated cellulose fiber, polylactic acid fiber, polyvinyl alcohol fiber, polyethylene fiber, polyamide fiber, polyester, acrylic fiber, polypropylene fiber, and ES fiber. The washed fibers of this invention can be used to prepare pure carboxymethyl cellulose ammonium fiber sheets as needed, or they can be combined with other common fibers to form composite fiber sheets. Pure carboxymethyl cellulose ammonium fiber sheets exhibit superior water absorption and water retention; while composite fiber sheets, due to the incorporation of other common cellulose fibers, possess superior physical properties. The addition of ordinary fibers enhances both physical properties and processability; natural cellulose provides additional support to washable fibers; regenerated cellulose fibers can be cut to a longer length as needed before processing, increasing the tensile properties of absorbent materials; the added polylactic acid fibers, polyvinyl alcohol fibers, polyethylene fibers, polyamide fibers, polyester, acrylic fibers, polypropylene fibers, and ES fibers all possess thermoplasticity, expanding the range of nonwoven processes.

[0026] Preferably, in the ammoniation treatment step using ammonia gas, the ammoniation treatment is performed first, followed by the tableting step, specifically as follows:

[0027] First, the washed fibers are pre-dried, then ammoniated with ammonia to produce CMC-NH4 fibers. Then, the unbound ammonia is removed by heating the air duct. The resulting dry fiber material is then evenly spread and needle-punched to obtain ammoniated fiber sheets.

[0028] Preferably, the acid is a biocompatible acid, specifically one or more of acetic acid, citric acid, hydrochloric acid, nitric acid, and formic acid.

[0029] Preferably, the acidification process involves multiple acidification operations until the fiber acidification reaction is complete, with a total acidification time of 0.8h-1.5h. Multiple acidification operations further ensure that the fiber is effectively and fully acidified in a high-concentration acid, thereby guaranteeing a complete acidification reaction.

[0030] Preferably, the acid solution also contains an alcohol with a volume percentage concentration of 0-50%; the alcohol is one or more of methanol, ethanol, and isopropanol. In the CMC-H preparation step, when the acid concentration is low, the water present will cause some swelling and solubility in the insufficiently acidified CMC-Na, making it difficult to maintain the fiber morphology. Considering the characteristic that "fibers are difficult to dissolve in alcohol, thus maintaining their morphology," this invention uses an appropriate amount of alcohol to effectively inhibit fiber swelling and dissolution, thereby better preserving the cellulose fiber morphology. If the acid concentration in the CMC-H preparation step is high, the degree of fiber swelling and dissolution is lower, and the addition of alcohol can be omitted. Methanol, ethanol, and isopropanol are all common low-boiling-point alcohols that have good miscibility with water, causing minimal interference to the cellulose and facilitating subsequent impurity removal.

[0031] Preferably, ammonia is obtained by evaporating liquid ammonia, ammonia water, or ammonia-alcohol water at a temperature of 0-130°C and a pressure of 102 kPa or lower than the liquefaction vapor pressure of ammonia at the corresponding temperature. All of the above liquid ammonia, ammonia water, or ammonia-alcohol water evaporates into ammonia gas at the set temperature and pressure. When the pressure is below 102 kPa, which is slightly lower than atmospheric pressure, even if the ammoniation container is emptied of air, it is difficult to evenly contact the initially formed fibers when ammonia is introduced, leading to uneven reaction. The upper pressure limit is lower than the liquefaction vapor pressure of ammonia at the corresponding temperature because after ammonia liquefies, the fibers dissolve directly into the liquid, making it difficult to maintain the fiber shape and causing the subsequent product to harden. On the other hand, when the temperature is below 0°C, the penetration of ammonia is poor, making it difficult to bind; when the temperature is above 130°C, it is close to the carbonization temperature of cellulose, which easily leads to a decrease in fiber strength.

[0032] This absorbent material can be well applied in hygiene care, medical, industrial, agricultural and forestry fields. The modified fiber fabric obtained by the above method has good water absorption, as well as good biodegradability and biocompatibility, and can be widely used in hygiene care, medical, industrial, agricultural and forestry fields.

[0033] This invention provides a modified fiber fabric and its preparation method. It has the following beneficial effects:

[0034] (1) The modified fiber fabric and its preparation method of the present invention use cellulose fiber as raw material. The entire process is carried out in a fibrous state during the modification treatment reaction. At the same time, the sodium ion removal rate of sodium carboxymethyl cellulose is fully considered. The acid concentration during the acidification process is strictly controlled. Ammonia is used for ammoniation at a suitable temperature and pressure. Then, the product is dried and the unbound ammonia is removed. The ingenious design achieves synergistic effect and ultimately ensures the softness and physical properties of the product paper or nonwoven fabric.

[0035] This invention ensures the smooth progress of the reaction while effectively restoring the excellent water absorption and retention properties of the fiber. During the subsequent drying process, excess ammonia evaporates directly, and the evaporation of ammonia also carries away some moisture, resulting in effective and rapid dehydration and improved drying efficiency. Furthermore, the raw materials have relatively high biodegradability, and since no sodium ions are added to the system, the resulting carboxymethyl cellulose ammonium absorbent material possesses both excellent biodegradability and biocompatibility.

[0036] Compared with the existing method for preparing sodium carboxymethyl cellulose fiber, the modified fiber fabric preparation method of the present invention eliminates the residue of unbound non-volatile sodium and potassium ions while maintaining the smooth progress of the reaction, significantly improving the preparation efficiency and product purity of carboxymethyl cellulose ammonium absorbent material. The obtained carboxymethyl cellulose ammonium absorbent material has excellent water absorption, water retention and biodegradability, and can be well applied in industrial, agricultural and forestry fields.

[0037] (2) The modified fiber fabric of the present invention has a wide range of applications, abundant raw material sources, and good market prospects.

[0038] (3) The method for preparing the modified fiber fabric of the present invention compares the use of alkali metal sodium hydroxide and potassium hydroxide by using the "sheet-ammoniation-drying" or "drying-ammoniation-needling-drying" techniques. Sodium hydroxide and potassium hydroxide are prepared into a water / alcohol solution, which will dissolve the dried washed fiber and make it difficult to make into sheet.

[0039] If CMC-H fibers are first formed into preliminary sheets and then immersed in the solution to generate CMC-Na or CMC-K, although the resulting material can achieve high water absorption after drying, the Na and K that are not bound to the cellulose-COOH will remain on the fiber, causing the fiber to harden or reduce biocompatibility. This invention uses volatile NH3 as the cation to bind to the -COOH. Although the NH3 that does not form CMC-NH4 will also remain on the fiber, its volatility allows it to be removed during drying, resulting in a product with good biocompatibility that can be effectively applied in hygiene, medical, and other fields. Attached Figure Description

[0040] Figure 1 This is a photograph of the modified carboxymethyl cellulose ammonium fiber fabric (cloth) prepared in Example 1 of the present invention. Detailed Implementation

[0041] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0042] Fiber raw materials (cellulose fibers): In this invention, both natural cellulose fibers and regenerated cellulose fibers can be used. This application selects commercially available products of bamboo fiber, cotton fiber, viscose fiber and Tencel as representatives.

[0043] Common fibers: This application mainly uses one or more of the following: bamboo fiber, cotton fiber, wood fiber, hemp fiber, regenerated cellulose fiber, polylactic acid fiber, polyvinyl alcohol fiber, polyethylene fiber, polyamide fiber, polyester, acrylic fiber, polypropylene fiber, and ES fiber. Bamboo fiber and polyester are used as representative fibers in this application.

[0044] Alkalizing agents include sodium hydroxide, potassium hydroxide, etc. This application uses sodium hydroxide as an example only, specifically using commercially available sodium hydroxide product with CAS number 1310-73-2.

[0045] Etherifying agents include chloroacetic acid, sodium chloroacetate, propylene oxide, etc. This application only uses chloroacetic acid as an example, specifically using commercially available chloroacetic acid product with CAS number 79-11-8;

[0046] Alcohols: Commercially available products such as methanol, ethanol, and isopropanol are selected as representatives in this application;

[0047] Acids: Commercially available products of acetic acid, citric acid, hydrochloric acid, and nitric acid are selected as representatives in this application;

[0048] Ammonia: Homemade.

[0049] The present application will be further described in detail below with reference to the accompanying drawings, embodiments and comparative examples.

[0050] Example 1

[0051] A method for preparing modified fiber fabrics includes the following steps:

[0052] Raw material processing: Take 10kg of dry bamboo strips and cut them into strips 30-40cm long and 2-3cm wide. Soak them in distilled water for 12 hours and then put them into a digester. Pulping is carried out using the sulfate method. The digestion process is as follows: use 20% alkali (calculated as NaOH), 20% sulfidation (calculated as Na2S), and a material-to-liquid ratio of 1:4. Gradually raise the temperature to 170℃ over 2 hours and continue to keep it at that temperature for another 2 hours. After digestion, wash the bamboo pulp with distilled water, filter it through a flat sieve with a sieve opening of 0.15mm to remove impurities, and then dry it to obtain bamboo fiber.

[0053] Preparation of CMC-H: Take 1 kg of bamboo fiber and alkalize and etherify it while maintaining the fiber morphology. First, add it to a 30 wt% sodium hydroxide solution and react. Then, alkalize it at 65℃ for 1 h to obtain alkalized cellulose fiber. Next, add 0.8 kg of etherification solution to the alkalized cellulose fiber for etherification. The specific composition of the etherification solution is 45 wt% chloroacetic acid, 35 wt% ethanol and 20 wt% distilled water. Etherify it at 65℃ for 2.5 h to obtain etherified cellulose.

[0054] Then, add the acid solution and mix evenly. The acid solution consists of 35% acetic acid and 65% purified water by volume percentage. The amount of acid solution added is 100 kg, which is added in two batches of 50 kg each time. After the first addition of acid solution, stir and acidify for 0.5 h. After draining, add the second batch of acid solution and continue stirring and acidifying for 0.5 h to obtain carboxymethyl cellulose CMC-H.

[0055] Washing: CMC-H is washed with pure water until the pH of the residual washing liquid is ≥4 to obtain washed fibers;

[0056] Papermaking: Washed fibers are added to an aqueous dispersion and stirred until evenly dispersed. 1.0 wt% of the dispersion is used for papermaking, with a paper thickness of 15 g / m. 2 A preliminary film was produced;

[0057] Ammoniation: The initial sheet is rolled into a 15cm diameter roll and placed in a sealed container. Ammonia gas is introduced at a temperature of 25±5℃ and the pressure is adjusted to 200-300kPa for ammoniation. The reaction time is 2h.

[0058] Drying and removal of unbound ammonia: The ammonified initial sheet is placed in a drying oven at 65°C and dried for 6 minutes until the moisture content is 10%, thus obtaining the ammonified fiber sheet—carboxymethyl cellulose ammonium water-absorbing material. See the actual product photo. Figure 1 .

[0059] Example 2

[0060] A method for preparing modified fiber fabrics includes the following steps:

[0061] Preparation of CMC-H: Take 1 kg of commercially available viscose fiber with a fiber length of 3.8 cm. Under the premise of maintaining the fiber morphology, perform alkalization and etherification. First, add it to a 27 wt% sodium hydroxide solution and react. Alkalize at 65℃ for 0.8 h to obtain alkalized cellulose fiber. Then, add 0.8 kg of etherification solution to the alkalized cellulose fiber for etherification. The specific composition of the etherification solution is 45 wt% chloroacetic acid, 35 wt% ethanol and 20 wt% distilled water. Etherify at 65℃ for 2.0 h to obtain etherified cellulose. Subsequently, add acid solution and mix evenly. The acid solution consists of 30% citric acid and 70% purified water by volume percentage. The amount of acid solution added is 100 kg, which is added in two batches of 50 kg each time. After the first addition of acid solution, stir and acidify for 0.5 h. After draining, add the second batch of acid solution and continue stirring and acidifying for 0.5 h to obtain CMC-H.

[0062] Washing: CMC-H is washed with pure water until the pH of the residual washing liquid is ≥4 to obtain washed fibers;

[0063] Nonwoven fabric: The carded washed fibers are laid in a straight or cross-overlay web, and high-pressure water is used to puncture the fiber web, causing the fibers to entangle into a fabric. After drying, it forms a 45g / m² fabric. 2 Spunlace fabric initially formed sheets;

[0064] Ammoniation: The initial sheet is rolled into a 10cm diameter roll and placed in a sealed container. Ammonia gas is introduced at a temperature of 65±5℃ and the pressure is adjusted to 600-700kPa for ammoniation. The reaction time is 45min.

[0065] Drying and removal of unbound ammonia: The ammonified initial sheet is placed in a drying tunnel at 65°C and dried for 6 minutes until the moisture content is 15%, thus obtaining ammonified fiber sheet - carboxymethyl cellulose ammonium water-absorbing material.

[0066] Example 3

[0067] A method for preparing modified fiber fabrics includes the following steps:

[0068] Preparation of CMC-H: Take 1 kg of commercially available cotton fiber with a fiber length of 0.3-1.5 cm. Under the premise of maintaining the fiber morphology, perform alkalization and etherification. First, add it to a 27 wt% sodium hydroxide solution and react. Alkalize at 65℃ for 0.8 h to obtain alkalized cellulose fiber. Then, add 0.8 kg of etherification solution to the alkalized cellulose fiber for etherification. The specific composition of the etherification solution is 45 wt% chloroacetic acid, 35 wt% ethanol and 20 wt% distilled water. Etherify at 65℃ for 2.0 h to obtain etherified cellulose. Subsequently, add acid solution and mix evenly. The acid solution consists of 60% acetic acid and 40% purified water by volume percentage. The amount of acid solution added is 100 kg, added in two batches of 50 kg each time. After the first addition of acid solution, stir and acidify for 0.5 h before adding the second batch of acid solution. Continue stirring and acidifying for 0.5 h after adding the second batch of acid solution to obtain CMC-H.

[0069] Washing: CMC-H is washed with pure water until the pH of the residual washing liquid is ≥4 to obtain washed fibers;

[0070] Papermaking: Washed fibers are added to an aqueous dispersion and stirred until evenly dispersed. 1.5 wt% of the total weight of the dispersion system is used for papermaking, resulting in a paper thickness of 30 g / m². 2 A preliminary film was produced;

[0071] Ammoniation: The initial sheet is rolled into a 12cm diameter roll and placed in a sealed container. Ammonia gas is introduced at a temperature of 125±5℃ and the pressure is adjusted to 1400-1500kPa for ammoniation. The reaction time is 30min.

[0072] Drying and removal of unbound ammonia: The ammonified initial sheet is placed in a drying tunnel at 65°C and dried for 6 minutes until the moisture content is 12%, thus obtaining the ammonified fiber sheet - carboxymethyl cellulose ammonium water-absorbing material.

[0073] Example 4

[0074] A method for preparing modified fiber fabrics includes the following steps:

[0075] Preparation of CMC-H: Take 1 kg of commercially available Tencel fiber with a single filament fineness of 3.0 dtex and a fiber length of 0.5-0.8 cm. Under the premise of maintaining the fiber morphology, perform alkalization and etherification. First, add it to a 28 wt% sodium hydroxide solution and react. The reaction is carried out at 65℃ for 1.2 h to obtain alkalized cellulose fiber. Then, add 0.7 kg of etherification solution to the alkalized cellulose fiber for etherification. The specific composition of the etherification solution is 45 wt% chloroacetic acid, 35 wt% ethanol and 20 wt% distilled water. Etherify at 65℃ for 1.2 h to obtain etherified cellulose. Subsequently, add acid solution and mix evenly. The acid solution consists of 37% acetic acid, 3% hydrochloric acid and 60% purified water by volume percentage. The amount of acid solution added is 100 kg, which is added in two batches of 50 kg each time. After the first addition of acid solution, stir and acidify for 0.5 h before adding the second batch of acid solution. After the second addition of acid solution, continue stirring and acidifying for 0.5 h to obtain CMC-H.

[0076] Washing: CMC-H is washed with pure water until the pH of the residual washing liquid is ≥4 to obtain washed fibers;

[0077] Papermaking: Washed fibers are added to an aqueous dispersion and stirred until evenly dispersed. 1.2 wt% of the dispersion is used for papermaking, resulting in a paper thickness of 20 g / m². 2 A preliminary film was produced;

[0078] Ammoniation: The initial sheet is rolled into a 15cm diameter roll and placed in a sealed container. Ammonia gas is introduced at a temperature of 25±5℃ and the pressure is adjusted to 200-300kPa for ammoniation. The reaction time is 2h.

[0079] Drying and removal of unbound ammonia: The ammonified initial sheet is placed in a drying tunnel at 65°C and dried for 6 minutes until the moisture content is 11%, thus obtaining the ammonified fiber sheet - carboxymethyl cellulose ammonium water-absorbing material.

[0080] Examples 5-6

[0081] Examples 5 and 6 are based on the method of Example 1, except that the acidification concentration in the CMC-H preparation step is adjusted. In Example 5, the acetic acid concentration is reduced to 28% and the proportion of purified water is increased to 72%; in Example 6, the acetic acid concentration is reduced to 16% and the proportion of purified water is increased to 84%.

[0082] Example 7

[0083] Example 7 is based on the method of Example 1, except that the acidification time in the CMC-H preparation step is adjusted. In Example 7, the acidification time is 0.4 h.

[0084] Examples 8-9

[0085] Examples 8 and 9 are based on the method of Example 1, except that the ammonia gas is replaced with an ammonia solution for soaking. Specifically, Example 8 uses 33% ammonia water; Example 9 uses a 10% ammonium chloride aqueous solution.

[0086] Examples 10-11

[0087] Examples 10 and 11 are based on the method of Example 1, except that in the papermaking process, the washed fibers are added to a dispersion of ordinary fibers and water and stirred until evenly dispersed. In Example 10, the ordinary fibers are wood fibers prepared from the above-mentioned raw materials; in Example 11, the ordinary fibers are hemp fibers prepared from the above-mentioned raw materials. The weight ratio of washed fibers to ordinary fibers is controlled at 50:50, and the total weight of washed fibers and ordinary fibers is 1.0 wt% of the dispersion system. The papermaking thickness is 15 g / m². 2 The initial film was completed.

[0088] Examples 12-13

[0089] Examples 12 and 13 are based on the method of Example 2, except that in the nonwoven process, washed fibers and ordinary fibers are mixed and carded in a 50:50 ratio. In Example 12, the ordinary fiber is the regenerated cellulose fiber prepared from the above raw materials; in Example 13, the ordinary fiber is the ES fiber prepared from the above raw materials. The carded fibers are then laid in a straight or cross-overlapping web, and high-pressure water is used to puncture the fiber web, causing the fibers to entangle into a fabric. After drying, the fabric has a density of 45 g / m². 2 The spunlace fabric is initially formed into sheets.

[0090] Comparative Example 1

[0091] A method for preparing modified fiber fabrics includes the following steps:

[0092] 1 kg of bamboo fiber powder, passed through a 60-mesh sieve, was subjected to alkalization and etherification. First, it was added to a 30 wt% sodium hydroxide solution and reacted, then alkalized at 65°C for 1 hour to obtain alkalized cellulose fibers. Next, 0.8 kg of etherification solution (45 wt% acetic acid and 55 wt% distilled water) was added to the alkalized cellulose fibers for etherification at 65°C for 2.5 hours to obtain etherified cellulose. Subsequently, an acid solution was added and mixed thoroughly. This acid solution, by volume percentage, consisted of 35% acetic acid, 30% ethanol, and 35% purified water. The acid solution was added in two batches of 100 kg, 50 kg each time. After the first addition of acid, the mixture was stirred and acidified for 0.5 hours before the second addition, followed by another 0.5 hours of stirring and acidification. During the acidification process, the bamboo fiber powder could not maintain its fiber morphology and dissolved, making subsequent washing, papermaking, ammoniation, and drying processes impossible. Therefore, it can be seen that the modified fiber fabric carboxymethyl cellulose ammonium water-absorbing material of the present invention requires raw materials based on fiber morphology to be prepared.

[0093] Performance testing

[0094] The absorbent materials prepared in Examples 1-13 and Comparative Example 1 were subjected to the following performance tests. The blank group was the viscose fiber raw material of Example 2. The test results are shown in Table 2.

[0095] 1. Substitutability

[0096] The degree of substitution of etherified cellulose in Examples 1-13 was determined according to GB 1886.232-2016.

[0097] 2. Water absorption and water retention

[0098] The absorbent materials prepared in Examples 1-17 were cut into test samples with a diameter of 10 cm. The mass m1 (g) of the test sample before soaking was weighed first. The test sample was then soaked in deionized water. After 12 hours, it was taken out and drained thoroughly, and its mass m2 (g) was weighed. The test sample that had fully absorbed water was then placed in an oven at 65°C to dry. The drying time (h) required for the weight of the test sample to recover to m1 (g) was recorded. The water absorption rate of the absorbent material was calculated simultaneously as [(m2-m1) / m1]*100%. The higher the water absorption rate, the better the water absorption. The longer the drying time, the better the water retention.

[0099] 3. Fiber mechanical strength and flexibility

[0100] According to GB / T14337-2022, the dry breaking strength, wet breaking strength and elongation at break of the fiber raw material in the absorbent material are determined. The higher the dry breaking strength, wet breaking strength and elongation at break, the better. Among them, the dry breaking strength and wet breaking strength represent mechanical strength, and the elongation at break represents flexibility.

[0101] 4. Tensile strength test of sheet metal

[0102] According to GB / T12914-2008, the test piece is cut into 100mm x 100mm pieces, with the unit being m / N.

[0103] 5. Determination of the breaking strength of nonwoven sheets

[0104] Tested according to the method of textile standard FZ / T 60005.

[0105] 6. Method for determining the content of -CH2COONH4

[0106] The content of w(-CH2COONH4) is calculated using the following formula:

[0107] w(-CH2COONH4)=N%*76 / 14*100%, where N% is the nitrogen content in the carboxymethyl cellulose ammonium product determined by elemental analysis, Kjeldahl method, or Kjeldahl method.

[0108] Table 2 Performance test results of Examples 1-13 and Comparative Example 1

[0109]

[0110]

[0111] Referring to Table 2, it can be seen that: In Example 1, the raw material is cellulose bamboo cellulose. After implementing the method and process of the present invention, the modified fiber fabric carboxymethyl cellulose ammonium water-absorbing material product has a degree of substitution of 0.4 and a water absorption rate of 176 times its own weight, which is excellent. The tensile strength of the sheet is 230 N / m, and the fiber still maintains excellent physical properties after treatment.

[0112] Example 2 uses viscose fiber made from regenerated cellulose fibers. After implementing the method and process of this invention, the degree of substitution of the obtained product is 0.33. Due to the new structure of the regenerated cellulose fiber sheath, the degree of substitution is slightly lower than that of Example 1. Its water absorption rate is 201 times its own weight, indicating excellent water absorption performance. The fiber dry breaking strength is 2.1 cN / dtex, and the fiber breaking elongation is 22%. Compared with the blank control group, the fiber dry breaking strength after treatment did not decrease significantly, indicating that the raw material fiber did not degrade during the preparation process of this invention. The nonwoven sheet breaking strength is 165 N, indicating good nonwoven fabric strength.

[0113] Example 3: The raw material is cotton fiber. After implementing the method and process of the present invention, the degree of substitution of the obtained product is 0.55; the water absorption rate reaches 170 times its own weight, which is excellent; the tensile strength of the sheet is 310 N / m, and it still maintains the excellent physical properties of the fiber raw material.

[0114] Example 4 uses Tencel fiber, made from regenerated cellulose fiber. After implementing the method and process of this invention, the degree of substitution of the obtained product is 0.22. Due to the new structure of the regenerated cellulose fiber sheath, the degree of substitution is slightly lower than that of Example 1. The water absorption rate is 217 times its own weight, indicating excellent water absorption performance. The dry breaking strength of the fiber is 4.97 cN / dtex, and the fiber breaking elongation is 18%. Compared with viscose fiber, Tencel fiber has superior physical properties compared to the raw material fiber. The wet breaking strength of the fiber is 1.9 cN / dtex. The nonwoven sheet breaking strength is 115 N. Due to the smooth surface of Tencel fiber, the breaking strength of the fabric is slightly lower than that of viscose fiber fabric.

[0115] In summary, the performance test results of the carboxymethyl cellulose ammonium absorbent materials prepared in Examples 1-4 of this invention show that the modified fiber fabrics prepared by the method of this invention have little impact on the original physical properties of the original fibers. While maintaining the excellent physical properties of the raw fiber, the special groups brought by carboxymethyl cellulose ammonium also give it excellent absorbency, water retention, and biodegradability. Furthermore, because it has no potential cytotoxicity in its own structure, it has excellent biocompatibility and can be well applied in fields such as hygiene care, medical treatment, industry, agriculture, and forestry. Regarding the relationship between the water absorption ratio and -COONH4 of the carboxymethyl cellulose ammonium absorbent material prepared by this invention, through the process of this invention, cellulose has three groups at the C2, C3, and C6 positions: -OH, -COOH, and -COONH4. First, from the perspective of the interaction between water molecules and hydrophilic groups, like dissolves like. According to the polarity of the groups, the order of interaction with water molecules from strongest to weakest is -COONH4, -COOH, and OH. Therefore, -COONH4 has better hydrophilicity, and the water absorption performance of the -COONH4 group is significantly better than that of the -COOH and -OH groups.

[0116] On the other hand, the -COONH4 group is an ionic group and will ionize into -COO- and NH4 in aqueous solution. + Ions. -COO- are fixed on the cellulose chain, while NH4+ is... + However, it can move freely, precisely because of NH4 + The presence of ions causes differences in osmotic pressure. When the material comes into contact with water, the microscopic cellulose fiber bundles unfold, releasing NH4+. + Ionization creates a concentration difference between the inside and outside of the cellulose network, causing water molecules to permeate into the network structure through osmotic pressure. Therefore, the higher the content of -COONH4 groups, the more water molecules permeate into the cellulose. However, -OH and -COOH groups do not exhibit osmotic pressure, so their water absorption rate is lower than that of -COONH4 groups.

[0117] Based on the hydrogen bonds formed by water molecules and the highly electronegative oxygen atoms within the material (H exhibits a +1 valence, N a -3 valence), the O atom in -COONH4 is more electronegative than the hydrogen atoms in -OH and -COOH. When the -COONH4 content is high, free water forms hydrogen bonds with O and is thus trapped, forming frozen water, which in turn forms a gel state with excellent water retention. Therefore, in the carboxymethyl cellulose ammonium water-absorbing material of this invention, the terminal modifying groups at the C6, C2, and C3 carbon positions of the carboxymethyl cellulose ammonium molecule include -CH2COONH4, with a content range limited to 4.3%–58.9%. Furthermore, -CH2COONH4 is directly linked to the glucose units of the fiber, all of which contribute to the high water absorption performance of this invention.

[0118] The preparation method of this invention uses cellulose fiber as raw material and is applicable to natural cellulose fiber and / or regenerated cellulose fiber. First, the acid concentration is strictly controlled during the acidification process. Then, ammonia is used for ammoniation under conditions of "temperature 0-130℃ and pressure 102kPa to below the liquefaction vapor pressure of ammonia at the corresponding temperature." Afterward, drying is performed to remove unbound ammonia, thereby ensuring that the cellulose maintains its fibrous form, guaranteeing the physical properties of the product fiber, and consequently ensuring the softness and physical properties of the corresponding paper or nonwoven fabric. While maintaining the smooth progress of the reaction, it eliminates the need for organic solvents and the incorporation of impurities such as sodium and potassium ions, thus improving the preparation efficiency and product purity of the modified carboxymethyl cellulose ammonium absorbent material.

[0119] Examples 5 and 6 show that when the acid concentration used in the acidification method of the present invention is below 29%, the high water content causes the fibers to swell or dissolve significantly during acidification. Because the fibers in Examples 5 and 6 swelled or dissolved significantly, they could not be detected.

[0120] The product performance test results in Example 7 show that, compared with Example 1, the basic methods and parameters are the same, the difference is that the acidification time is shortened, and the water absorption rate decreases due to the shortened acidification time, which is 119 times its own weight. The tensile strength of the sheet is 90 N / m.

[0121] The product performance test results in Examples 8 and 9 are basically the same as those in Example 1 in terms of method and parameters. The difference is that ammonia solution is used for ammonia treatment. Since the acidified fiber dissolves in ammonia solution, the fiber will swell or dissolve in large quantities, making it impossible to test.

[0122] The preparation methods of Examples 10 and 11 are based on the method of Example 1, except that in the papermaking process, the washed fibers are added to a dispersion of ordinary fibers and water and stirred until uniformly dispersed. Specifically, the ordinary fiber added in Example 10 is wood fiber; and the ordinary fiber added in Example 11 is hemp fiber. The weight ratio of washed fibers to ordinary fibers is controlled at 50:50. The water absorption rates were measured to be 67 times and 77 times their own weight, respectively; the tensile strength of the paper sheets was enhanced due to the addition of ordinary fibers, reaching 260 N / m and 330 N / m, respectively.

[0123] Examples 12 and 13 are based on the preparation method of Example 2, except that the washed fibers and ordinary fibers are mixed and carded in a 50:50 ratio during the nonwoven process. The measured water absorption rates were 114 times and 121 times their own weight, respectively, and the breaking strengths of the nonwoven sheets were 209 N and 480 N, respectively. In Example 13, the ordinary fiber was ES fiber, and due to the properties of ES fiber, the breaking strength of the nonwoven sheet was significantly enhanced.

[0124] Application examples

[0125] The modified fiber fabric absorbent materials prepared in Examples 1-4 of this invention all have excellent water absorption, water retention, biodegradability, and excellent biocompatibility. Therefore, they can be well applied in fields such as hygiene care, medical care, industry, agriculture and forestry. Specifically, they can be made into absorbent pads, absorbent paper, absorbent strips, etc., and further applied to sanitary napkins, wound dressings, face masks, etc.

[0126] The above are merely embodiments of the present invention. For example, the cellulose fiber can be natural cellulose fibers such as cotton, hemp, bamboo, and straw, or regenerated cellulose fibers such as viscose, Tencel, copper amine, and Modal, or a mixture of the above two categories of fibers; the common fiber is one or more of bamboo fiber, cotton fiber, wood fiber, hemp fiber, regenerated cellulose fiber, polylactic acid fiber, polyvinyl alcohol fiber, polyethylene fiber, polyamide fiber, polyester, acrylic fiber, polypropylene fiber, and ES fiber; the acid is a biocompatible acid, specifically one or more of acetic acid, citric acid, hydrochloric acid, nitric acid, and formic acid. The washed fiber of the present invention can be used to prepare pure carboxymethyl cellulose ammonium fiber sheets as needed, or it can be combined with other common fibers to make composite fiber sheets, both of which can realize the modified fiber fabric and its preparation method of the present invention.

[0127] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A modified fiber fabric, characterized in that, The raw material for the fiber fabric is cellulose fiber. The raw material is alkalized and etherified in sequence while maintaining its fibrous shape, and then acid solution is added and mixed evenly. After the acidification reaction, the modified product is carboxymethyl cellulose CMC-H. After washing, washed fibers are obtained; the washed fibers are then ammonified with ammonia to obtain a fiber fabric with carboxymethyl cellulose ammonium as the modified product. In the ammoniation treatment step using ammonia, the operation involves first preparing the fiber sheet and then performing the ammoniation treatment. Specifically, the washed fiber is first made into a preliminary sheet using a papermaking or nonwoven process, and then the preliminary sheet is ammoniated with ammonia to obtain an ammoniated fiber sheet. Alternatively, in the ammoniation treatment step using ammonia, the operation involves first performing the ammoniation treatment and then preparing the fiber sheet. Specifically, the washed fiber is first pre-dried, and then ammoniated with ammonia to generate CMC-NH4 fiber. Then, the unbound ammonia is removed by heating the air duct, and the resulting dry fiber material is evenly spread and needle-punched to obtain an ammoniated fiber sheet. The degree of substitution in the ammonia ammonification treatment of the carboxymethyl cellulose ammonium fiber fabric is 0.2-3.0; The fiber fabric raw material undergoes a fibrous modification treatment process. Specifically, during the modification treatment, the 1,4-glycosidic bonds (i.e., the connection between C1 and C4) in the glucose molecules of the fiber fabric raw material remain unchanged, forming parallel cellulose chains. Specific chemical groups occupying specific carbon positions C6, C2, and C3 on the fiber molecular chains are modified. The terminal modification groups of the specific chemical groups at carbon positions C6, C2, and C3 in the carboxymethyl cellulose ammonium molecule include -CH2COONH4.

2. The modified fiber fabric according to claim 1, characterized in that, The terminal modification group of the specific chemical groups at carbon positions C6, C2, and C3 in the ammonium carboxymethyl cellulose molecule includes -CH2COONH4, the content of which is limited to 4.3%–58.9%, and -CH2COONH4 is directly connected to the glucose unit of the fiber, and the tensile strength of the fiber in the ammonium carboxymethyl cellulose fiber fabric is not less than 60% of the tensile strength of the fiber in the fiber fabric raw material.

3. The modified fiber fabric according to claim 2, characterized in that, The raw material for the fiber fabric is cellulose fiber, which is natural cellulose fiber and / or regenerated cellulose fiber; the length of the cellulose fiber is not less than 0.5 mm.

4. A modified fiber fabric according to claim 1 or claim 3, characterized in that, In the process of modifying carboxymethyl cellulose, based on maintaining the fibrous shape of cellulose fibers, alkalization and etherification are carried out in sequence, followed by the addition of acid solution and mixing evenly. The volume percentage concentration of acid in the acid solution is ≥29%. After acidification reaction, the modified product is carboxymethyl cellulose CMC-H.

5. A method for preparing a modified fiber fabric, characterized in that, The fiber fabric raw material is based on maintaining its fibrous shape, and is successively modified into carboxymethyl cellulose and ammonium treatment to obtain a fiber fabric with carboxymethyl cellulose ammonium as the modified product; The fiber fabric raw material is subjected to a fibrous modification treatment reaction. Specifically, during the modification treatment reaction, the 1,4-glycosidic bonds (i.e., the connection between C1 and C4) in the glucose molecules of the fiber fabric raw material are connected and the parallel cellulose chains are not changed. The specific chemical groups occupying specific carbon positions C6, C2, and C3 on the fiber molecular chains are modified. In the process of modifying carboxymethyl cellulose, the cellulose is maintained in its fibrous state by sequentially alkalizing and etherifying, followed by the addition of acid solution. After acidification reaction, the modified product is carboxymethyl cellulose CMC-H. After washing with water, washed fibers are obtained; the washed fibers are then subjected to ammoniation treatment with ammonia gas to obtain fiber fabric with carboxymethyl cellulose ammonium as the modified product. In the ammonia treatment step, the sheeting process is performed first, followed by the ammoniation process. Specifically, the washed fibers are first made into a preliminary sheet using a papermaking or nonwoven process, and then the preliminary sheet is ammoniated with ammonia to obtain an ammoniated fiber sheet. Alternatively, the ammonia treatment step can be performed by first ammoniation followed by the sheeting process. Specifically, the washed fibers are first pre-dried, and then ammoniated with ammonia to generate CMC-NH4 fibers. Unbound ammonia is then removed by heating the air duct, and the resulting dry fiber material is evenly spread and needle-punched to obtain an ammoniated fiber sheet.

6. The method for preparing a modified fiber fabric according to claim 5, characterized in that, The raw material for the fiber fabric is cellulose fiber, and the length of the cellulose fiber is not less than 0.5 mm; the volume percentage concentration of the acid in the acid solution is ≥29%. The degree of substitution in the carboxymethyl cellulose ammonium fiber fabric after ammonia ammoniation treatment is 0.2-3.

0. The terminal modification groups of specific chemical groups at carbon positions C6, C2, and C3 in the carboxymethyl cellulose ammonium molecule include -CH2COONH4, the content of which is limited to 4.3%-58.9%. Furthermore, -CH2COONH4 is directly connected to the glucose unit of the fiber. The tensile strength of the fiber in the carboxymethyl cellulose ammonium fiber fabric is not less than 60% of the tensile strength of the fiber in the fiber fabric raw material.

7. The method for preparing a modified fiber fabric according to claim 6, characterized in that, After washing with water until the pH of the residual water is ≥4.0, water-washed fibers are obtained.

8. The method for preparing a modified fiber fabric according to claim 6, characterized in that, The post-treatment drying step in the ammonia treatment step is heating and drying. The heat obtained from heating and drying the ammonium-treated fiber fabric removes the uncombined ammonia gas in the reaction, and the heating and drying is carried out until the moisture content of the carboxymethyl cellulose ammonium fiber fabric is ≤15%.

9. The method for preparing a modified fiber fabric according to claim 6, characterized in that, When the ammoniation treatment step using ammonia gas involves first preparing the sheet and then performing the ammoniation treatment step, during the process of forming the initial sheet from the washed fibers, When papermaking is used, in the papermaking step, the washed fiber is first put into pure water or ordinary fiber-water dispersion and dispersed evenly before papermaking. The weight ratio of the washed fiber to the ordinary fiber is 10-100:90-0. When using a nonwoven process to produce sheets, in the nonwoven step, the washed fibers are first mixed with ordinary fibers before nonwoven sheet production. The weight ratio of the washed fibers to the ordinary fibers is 10-100:90-0.

10. The method for preparing a modified fiber fabric according to claim 9, characterized in that, The common fiber is one or more of the following: bamboo fiber, cotton fiber, wood fiber, hemp fiber, regenerated cellulose fiber, polylactic acid fiber, polyvinyl alcohol fiber, polyethylene fiber, polyamide fiber, polyester, acrylic fiber, polypropylene fiber, and ES fiber.

11. The method for preparing a modified fiber fabric according to claim 6, characterized in that, The acid is a biocompatible acid, specifically one or more of acetic acid, citric acid, hydrochloric acid, nitric acid, and formic acid.

12. The method for preparing a modified fiber fabric according to claim 6, characterized in that, The acidification process involves multiple acidification operations until the fiber acidification reaction is complete, and the total acidification time is 0.8h-1.5h.

13. The method for preparing a modified fiber fabric according to claim 11, characterized in that, The acid solution also contains an alcohol with a volume percentage concentration of 0-50%; the alcohol is one or more of methanol, ethanol, and isopropanol.

14. The method for preparing a modified fiber fabric according to claim 6, characterized in that, The ammonia gas is obtained by evaporating liquid ammonia, ammonia water, or ammonia-alcohol water at a temperature of 0-130°C and a pressure of 102 kPa to below the liquefaction vapor pressure of ammonia at the corresponding temperature.

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