Understanding Flame Retardant Textiles - begoodtex

Abstract: This article mainly introduces the combustion mechanism, thermal cracking characteristics, types and mechanisms of flame retardants in flame-retardant textiles, as well as the production methods and testing methods of flame-retardant fibers and fabrics. It covers various aspects from flame retardant principles to production processes, and even testing standards, and looks forward to the future development trend of flame retardant textiles, especially the research and development of low toxicity and low smoke flame retardants and multifunctional flame retardant fabrics. The article also lists some relevant standards and regulations from multiple countries and regions, as well as the flame retardant products and technologies developed by BEGOODTEX company.

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1. Development Background of Flame Retardant Fabrics

Throughout history fire has been a factor in shaping human progress and development of technology; however it also presents a major threat through the occurrence of fires themselves.The European Flame Retardant Association (FERA) reports that over individuals lose their lives to fires in Europe with substantial socio economic ramifications.Germany sustains losses of up to 6.5 billion marks due to fires while the economic impact of fires, within the European Union represents 1% of the regions GDP. In China each year sees an average of 30 000 to 40 000 fire incidents leading to 2 000 to 3 000 fatalities and economic losses ranging from 200 million to 300 million yuan that are increasing over times.

Flame retardant technology had its early roots back in the nineteen thirties initially with non permanent treatments before progressing to the use of longer lasting flame retardant materials, like those used in military tents during World War II. During the s era nations like Europe the United States, and Japan each created flame guidelines, for textiles mandating certain locations and products to utilize flame retardant materials.

2. Importance of Flame Retardant Fabrics

Flame retardancy refers to the property of a material that can slow down or prevent combustion, which can be inherent or achieved through post-processing. The mechanism of action of flame-retardant textiles is to prevent chain reactions during the combustion process, such as heat absorption, changing the thermal degradation mode, and reducing the production of combustible gases, in order to achieve flame-retardant effects.
Research has shown that flame-retardant fabrics can significantly improve safety. For example, compared to untreated fabrics, flame-retardant fabrics can extend escape time by 10 to 15 times, reduce the heat and toxic gases released during combustion, and avoid the production of thick smoke.

3. Regulations on Flame Retardant Fabric Combustion Technology

Currently, in the world of textiles, testing for flame retardancy involves methods internationally recognized by different countries, such as the UK’s BS standard, Germany’s DlN standard, Canada’s GCSB standard, the US’s FS standard, Japan’s JlS standard, France’s ANF standard, Sweden’s SlS standard, China’s GB standard, and international standards by lSO. Different areas and institutions in nations like well-known urban centers or states such as New York and California in the USA as well as departments like Commerce (DOCFF), Transportation (DOT), and military organizations have their own unique testing standards and methodologies that are followed by various groups or associations like the National Fire Protection Association (NFPA) the Association of Textile Chemists and Dyers (AATCC) the Society, for Testing and Materials (ASTM) among others.

United States

Since ,the United States has enacted the Flammable Fabrics Act (FFAP) which mandates that textiles adhere to flammability technical requirements.Some associated standards include;

• NFPA 701: A fire testing standard for textiles and film materials developed by the National Fire Protection Association, primarily testing the combustion characteristics of materials when exposed to flames.
• NFPA focuses on the guidelines for flame apparel in industrial settings like the oil and gas sector where protective clothing is crucial for shielding against brief bursts of intense heat, from flames.
• CFR /: Federal regulations set out fire safety standards for kids pajamas, in America by detailing what materials can be used and how fast flames can spread on them.

Canada

Canada has passed the Hazardous Products Regulations and related regulations (such as children’s sleepwear, carpets, tents, etc.), which are implemented by Health Canada to ensure that all textiles meet flame retardant requirements. Partial related standards:

• CAN/ULC-S102: Fire testing methods for building materials and components, including home decor.
• CAN/CGSB 4.2 No. 27.5: Burning performance of bedding.

Japan

Japan does not have specific flame retardant requirements for clothing products, but has established flame retardant standards for carpets and curtains in buildings, requiring textiles used in specific locations to meet the prescribed flame retardant performance and be labeled with “fire prevention labels”. For example, JIS L applies to household textiles (curtains, bed sheets).

Australia

Each state in Australia has different technical regulations, with Western Australia enacting the Fair Trade Act and the Children’s Evening Dress Standards ; Tasmania has the Flammable Clothing Act and the Flammable Clothing Regulations ; New South Wales has enacted the Fair Trade (General Requirements) Regulations . These regulations stipulate that the flame retardancy and testing methods for children’s evening wear (such as pajamas, bathrobes, etc.) numbered 00-14 must comply with the AS/NZS standard.

UK

The United Kingdom has rules regarding flame retardant safety for evening attire. Back in the Evening Wear (Safety Regulations came into effect as a replacement for the Womens Sleepwear (Safety Regulations). In there were amendments made extending these regulations to cover all types of evening wear. According to these regulations childrens evening wear for ages ranging from 3 months, to 13 years must adhere to the standard BS and should have a permanent label specifying if it meets the combustion standard. Evening attire that has been treated with flame chemicals should come with labels warning about wash instructions and the specific detergents to use for cleaning purposes as per the guidelines outlined in BS before conducting any tests or assessments, on its properties. Partial list of standards;

• BS is predominantly employed to assess the fire resistance of furniture to guarantee that materials offer safety in case of a fire outbreak.
• BS CRIB 5 is a testing standard that assesses the fire resistance of furniture and filling materials at a level of fire safety requirements.
• BS TYPE C is a fire resistance standard specifically designed for curtains and interior decorative fabrics; a Type C rating signifies that the material demonstrates fire resistance when exposed to flames.
• BS Source 7: Evaluating the fire resistance of bedding, Source 7 is a high standard fire protection requirement commonly used for bedding in public places.

4. Thermal Decomposition of Textiles

The burning of fabrics is influenced by their type, structure and composition. Can be categorized into various groups such as non flammable fire resistant fire retardant, flammable and combustible. The process of burning requires three elements; a source of heat, oxygen and flammable materials. Fabrics ignite due, to sources of heat. Once the temperature of the heat source reaches a level the fibers start to break down and release flammable gases that combine with oxygen and catch fire. The burning of fabrics involves stages like warming up the material first before it melts and cracks open to decompose and eventually catch fire due to oxidation.

Types of fibers Name of fiber Close to the flame In the flames Leave the flame Residual form Cellulose fiber Bamboo pulp fiber Non-melting and non-shrinking Burn quickly Keep burning A small amount of soft dark gray Bamboo fiber Non-melting and non-shrinking Burn quickly Keep burning .A small amount of soft gray Adhesive Non-melting and non-shrinking Burn quickly Keep burning A small amount of soft grayish-white gray Cotton and kapok Non-melting and non-shrinking Burn quickly Keep burning A small amount of soft gray-black gray Flax Non-melting and non-shrinking Burn quickly Keep burning A small amount of silk strip-shaped gray-white gray .Protein fiber Soy protein fiber Contract There is black smoke in the burning. Keep burning Crispy black and gray, a small amount of hard pieces Milk protein fiber Melt and curl Curl, melt, burn Burning, sometimes self-destructing Black, basically crispy, Shell cord fiber Non-melting and non-shrinking Burn quickly, do not melt and keep the original circle bundle. Keep burning .Black and gray, fragile Wool, silk Contraction or curling Gradually burn Not easy to burn Crispy black gray Synthetic fibre Polyester fibre Contraction, melting Melt first and then burn There is a lot of black smoke, and there are molten liquid dripping, Glassy, dark brown hard ball The melt drops are dark brown. Polyamide fibre Contraction, melting Melt first and then burn .There are melt drips, and the melt drops are brown. Glassy, dark brown hard ball Acrylic fibres Contraction, micro-melting, scorching Melting combustion There are small glowing sparks. Crispy, black hard pieces Polyvinyl alcohol fibre Contraction, melting Burning Continue to burn Crispy, black hard pieces Polypropylene fibre Slow contraction Melting combustion There are melt drops, and the melt drops are milky white. Hard yellow-brown ball

Cellulosic Fibers

Cellulose fiber is a material that changes when heated and can result in solid remnants as well as liquids and combustible gases being released. The way in which the fiber breaks down under heat determines if it will keep burning or not. When cellulose burns down it goes through two types of combustion. One with flames and the other without (smoldering).

The breakdown process can be seen in three stages:

1.The initial breakdown happens at temperatures below 370 ℃

2.The main breakdown occurs between 370 ℃ to 430 ℃

3.The final breakdown stage happens above 430 ℃

At the cracking phase (with temperatures surpassing 430 ℃) the combustion performance is dictated by the cracking products, research findings indicate that diminishing the production of flammable elements can effectively lower combustion hazards. For instance; During pyrolysis processes of cotton fibers approximately 28 flammable substances are generated; conversely with flame retardant treated cotton fibers the types and quantities of pyrolysis products are notably reduced.

Polyester Fibers

The way polyester fiber burns is like how other synthetic polymer materials burn as well. When polyester fibers are exposed to heat they break down. Give off flammable gases that help the fire spread faster. To prevent the fire from spreading it’s important to minimize the release of these gases, during decomposition slow down the reactions happening in the air soak up the heat produced by the fire or limit how long the fire lasts by cutting off oxygen from the environment.

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5. Understanding Flame Retardants Mechanisms

Flame Retardant Mechanisms in Textiles

1. Melting theory (surface coverage theory)

Some substances, such as borax and boric acid, melt and form a glassy film covering the surface of fibers when heated, isolating air and suppressing combustion. Phosphides can promote carbonization, while bromides decompose to produce non combustible gases, further isolating air or diluting combustible gases, thereby producing flame retardant effects.

2. Heat absorption effect

Flame retardants reduce the temperature of polymer surfaces and combustion zones through heat absorption, dehydration, phase change, or decomposition, thereby slowing down the thermal decomposition process.

3. Dehydration theory

Phosphorus based flame retardants generate pyrophosphate upon contact with flames, which has a strong dehydration effect and helps to carbonize fibers. The formed carbonized film can effectively isolate air and reduce the release of flammable gases.

4. Condensed phase flame retardant

The flame retardant effect of condensed phase is achieved by delaying or interrupting the thermal decomposition process of materials, and common methods include:

• Flame retardants delay or prevent the thermal decomposition of flammable gases and free radicals in the solid phase.
• The use of inorganic fillers makes it difficult for the material to reach the thermal decomposition temperature through heat storage and conduction.
• Flame retardants decompose and absorb heat when heated, slowing down the temperature rise.
• The surface of flame-retardant materials forms a porous carbon layer, which provides insulation and oxygen barrier, preventing flammable gases from entering the gas phase and interrupting combustion.

5. Gas phase flame retardant

Gas phase flame retardancy suppresses gas-phase combustion reactions by capturing and eliminating free radicals such as H · and HO ·, effectively controlling the combustion process.

6. Dust particles or wall effects

Free radicals may lose their activity when in contact with dust particles or vessel walls, reducing the rate of gas-phase reactions and thus inhibiting combustion.

7. Droplet effect

When thermoplastic fibers are heated, they melt, which decreases their surface area in contact with air and can lead to droplets detaching from the flame, thus lowering the rate of combustion. To optimize flame retardancy, various mechanisms typically collaborate through synergistic interactions to enhance the overall flame-retardant performance.

Principles of Various Flame Retardants

There are various types of flame retardants, mainly divided into halogen flame retardants, phosphate flame retardants, inorganic flame retardants, and expansion flame retardants. The flame retardant mechanism of each type of flame retardant is different.

1. Flame retardant mechanism of halogenated flame retardants

Upon heating halogen flame retardants decompose and produce non combustible gases, in most cases an hydrogen halide that get to the surface of material covering it with a blanket which isolates oxygen from combustion reaction. Both hydrogen halides and free radicals combine to form low activity chlorine or bromine radicals which further lowers the combustion rate.

2. Flame retardant mechanism of inorganic phosphates

Phosphorus flame retardants act by the mechanism of dehydration and carbonization. Phosphates are able to form polyphosphate glass bodies at high temperatures, which encase the material and prevent oxygen from reaching its surface and supporting combustion. Ion pairs can also improve the flame retardant effect when combined with metal phosphates and chlorides.

3. Flame retardant mechanism of phosphate ester flame retardants

Phosphate ester fire retardants mitigate the flammability of materials by forming non-volatile phosphoric and metaphosphoric acids that catalyze dehydration, as well as an insulating carbon protection layer.

4. Synergistic effect of antimony trioxide and halogenated flame retardants

Antimony trioxide and halogen flame retardants can work together to absorb heat, consume free radicals that form during combustion of the resin, reduce surface temperature or flammable gas release rate in separation fire stage one side,optimize synergist effect on another direction.

5. Flame retardant mechanism of phosphorus nitrogen flame retardants

Phosphorus/Nitrogen Flame Retardant Will Generate The Carbonized Foam Layer By Expansion As Well, Such Content Of Last One Features Are Just About Heat Insulation, Oxygen And Smoke Disconnection And Molten Drops Preclusion. The foam carbon layer, as a kind of porous material produced by the polyurethane rigid foams can isolate and prevent the source from firing, It will inherently slow down combustion to overcome this problem.

6. Production Pathways for Flame Retardant Fabrics

There are essentially two approaches to making fibers and textiles flame retardant. Modifying the fibers themselves for permanent flame resistance or using flame retardant finishes on the surface of the material. When it comes to fibers like cotton wool and linen, post-finishing methods are employed for flame retardancy by either adsorption deposition or chemical bonding to fix the flame retardant, on the fabric or yarn ensuring it provides flame-resistant properties. Synthetic fibers like polyester and acrylic may have flame retardants incorporated during spinning. Then altered through copolymerization or blending to enhance their flame retardant properties. Alternatively flame retardancy in fibers could be achieved through post finishing treatments for added fire resistance. Compared to the methods applying flame retardants after manufacturing is simpler requires less investment and yields quicker outcomes, which makes it a more feasible option, for introducing new product lines. Post processing techniques can influence the fabrics strength and appearance well as affect its flame retardant properties compared to the untreated silk fabric modification.

Production Pathways for Flame Retardant Fibers

Flame retardant fibers obtain flame retardant properties by directly adding flame retardants during the fiber production process. The methods mainly include copolymerization, blending, graft copolymerization, flame retardant absorption, fiber surface halogenation, and post finishing.

1. Co polymerization: Addition of compounds containing flame retardant elements (phosphorus, halogen, sulfur, etc.) as co monomer of the polymer chains to improve fiber flame retardancy. This method has the advantages of long-lasting flame retardancy of the fiber, but the high temperature polymerization temperature would cause side reactions and be detrimental to the performance of the polymer.
2. Blending method: This method involves adding flame retardant at the melt (mixture of fibers in the molten state.) This necessitates flame retardants to undergo thermal stability, polymer compatibility, and non-non-performance to fibers. It needs high-temperature flame retardants that can get along with polymers and will not affect the performance of fibers.
3. Grafting copolymerization: Compounds with phosphorus and halogen is grafted onto the molecular chains of the fibers using chemical methods or high energy radiation to improve flame retarded [9–12]. Grafting copolymerization: Phosphorus and halogen compounds are grafted onto the molecular chains of fibers using chemical methods or high-energy radiation for enhanced flame retardancy.
4. Flame retardant absorption method: adsorbing flame retardants onto fibers, which is simple but less effective.
5. Halogenation of fiber surface: By radiation-induced chlorination treatment, the fiber surface obtains flame retardancy.
6. Post finishing method: Apply flame retardant evenly on the surface of fibers or fabrics. This method is simple and easy to implement, but the flame retardant effect is not long-lasting and can affect the texture and color of the fabric.

Production Pathways for Flame Retardant Fabrics

Flame retardant fabrics are usually made by post finishing the fabric surface and applying different finishing methods to make the fibers flame-retardant. Common flame retardant finishing methods include padding and baking, exhaustion dyeing, coating, spray, etc.

1. Dip rolling baking method: The most common method of flame retardant finishing, soaking flame retardants, drying and baking. The same bath as the rest of finishing methods (i.e., soft finishing) can be done in. Can be done in the same bath of finishing process (like Soft finishing)
2. Exhaustive dyeing method: The fabric is soaked in a flame retardant solution and then dried. It is suitable for hydrophobic synthetic fibers and is usually dyed in the same bath. This method has poor flame retardant effect.
3. Coating method: Mix flame retardants with crosslinking agents or adhesives and apply them to fabrics. Common coating methods include scraper coating, casting coating, and rolling coating.
4. Spray method:It is used for heavy fabrics area, the equipment is not suitable for general finishing equipment, manually or mechanical spray flame-retardant finishing. Spray type: it is suitable for heavy fabrics that are not suitable for traditional finishing equipment. Flame retardant finishing is carried out by manual or mechanical spray.

Methods for Flame Retardant Finishing of Different Fiber Fabrics

1. Polyester fiber. Polyester is a flammable material that is treated in a flame retardant manner through copolymerization, blending, composite spinning and post finishing methods. The flame retardance with copolymerization method is superior, but expensive; while with the blending method, the process is simple and economical, but the flame retardant effect is relatively poor at the same time of flame retardance with copolymerization, this is because that the blending method lacks the synergistic effect of flame retardant and polymer.
2. Nitrile chlorinated fiber: As a bonded flame-retardant fiber by means of copolymerization methods with blending (copolymer) method. A good flame retardant fibers is prepared from the copolymerization of monomers containing chlorine (for example, vinylidene chloride with acrylonitrile) As in the case of example copolymerization of chlorinated monomers such as vinylidene chloride with acrylonitrile, better flame-retardant functionalities are incorporated into the fibers.
3. Cotton fabric:It is primarily an easy catching fire fabric, some flame retarding finishing is necessary. There are two kinds of flame retardant finishing: one is non capable (take phosphate, ammonium chloride and other methods as example) and the other is capable finishing (for example: tetra hydroxymethyl phosphate chloride are fire retardants). incorporate a flame retardant finishing.
4. Wool fabric: Wool itself has a relatively high flame retardancy, but when higher flame retardancy performance is required, flame retardant finishing needs to be carried out. Conventional technology applies the finish as complexes and/or free complexes of titanium, zirconium or hydroxy acids to increase the flame retardancy at almost no change of hand of wool. Typical wool finishes are titanium and zirconium muds or hydroxy acids that form complexes with fiber and enhance the level of flame retardancy without changing the feel of the wool.
5. Hemp fabrics: Cellulose (a carbohydrate polymer that makes up the bulk of hemp fibers) is highly combustible and ignites rapidly. Meanwhile, the hemp fiber has the lowest crack temperature so it is essential to treat the hails of phosphorus containing flame retardants and obtain the flame retardant effect by increasing the temperature of carbonization and decreasing the generation of combustible gases. Hemp Fabrics: Hemp fibre which be spinnable on textile is burning nature and easily break because of low cracking temperature, phosphorus containing flame retardants is common used in order to promote the carbonization process and the release of ash to reduce flame and combustible gases to achieve flame-retardant effect.
6. Nylon fabric: Flame retardant finishing of nylon fabric is more complicated than cotton fabric, and preferred flame retardants are sulfur-containing flame retardants such as thiourea and ammonium thiocyanate which have high flame retardant effects on nylon. Nylon fabric Flame retardant finishing of nylon fabric are more complex, sulfur-containing flame retardants such as thiourea and ammonium thiocyanate have good flame retardant effect on it.
7. Polyester / cotton blended fabric: The flame retardant finishing of polyester cotton blended fabric is more difficult, to be the attribution to the characteristics of the two fibers are different. Each needs different flame retardant treatment, and complementary flame retardants reinforced. The flame retardant treatment is usually required for each of the components though this can be done using synergistic flame retardants.

7. Methods for Testing Flame Retardant Fabrics

Limiting Oxygen Index (LOI) Method

This technique identifies the minimum oxygen concentration required for fabrics to ignite in a blend of oxygen and nitrogen gases. A higher LOl value indicates flame retardant properties. While this approach is valuable, for investigations it is not widely utilized in everyday manufacturing practices.

Vertical Burning Method

Assess the effectiveness of flame properties by examining how fabrics burn and how long it takes for them to ignite and the extent of damage caused under particular flame settings.This approach is commonly employed to test a range of fire fabrics and is particularly prevalent, in Chinese standards where it plays a significant role.

45° Incline Burning Method

Assess how well the fabric resists flames by measuring how long it burns for and the size of the damage area when placed at a 45 degree angle.

Surface Burning Method

Lets test the fabrics fire resistance by measuring how and for how long the flame spreads on a flat surface.

8. Development Trends of Flame Retardant Fabrics

The Current Status of Global Flame Retardant Textiles

In the few years there have been notable advancements in global flame retardant technology for textiles. Various research organizations and businesses have been working on materials and methods to improve flame retardancy such as polypropylene flame retardant masterbatch and composite solutions that offer both flame retardant and anti static properties. The main focus of the research project is on developing high performance flame retardant fibers and exploring their usage specifically looking at fibers with high flame retardant properties and their use in blended fabrics. These fibers have an oxygen index ranging from 45 to 50.

Various nations have also created a range of flame retardants, with exceptional fireproof qualities concurrently. For instance, BEGOODTEX has developed Aquafyreguad™, a line of flame retardants designed for different types of natural and synthetic fibers.

Development Trends of Flame Retardant Textiles

1. Strengthen the development of flame-retardant fibers

Flame retardant fibers have low production and application, and more high-performance and multifunctional flame-retardant fibers should be developed in the future as they can be used in special industries such as the military and firefighting. The yield of flame-retardant fiber and the extent of application are low, and in the future, a great deal of research and production work must be done on flame-retardant fiber research, development and production work, the high performance and high-function flame-retardant fiber particularly small number of special technology including the task of the military and fight flame field prospect.

2. Multi functional research

Currently, most of the flame-retardant textiles can only play the flame-retardant function. BEGOODTEX company of China has announced flame-retardant multifunctional fabrics, like: Flame retardant & Antibacterial (FRANtiBact ™), Flame retardant & Waterproof (FRANTiAqua ™), Flame retardant GRS (GRSFRTex ™), Flame retardant & UV resistant (FRANTIUV ™), and Flame retardant & Light block (AntiLightFR ™), Flame retardant & anti-static (FRStaticGuard ™), Flame retardant & medical grade (FRMediGuard ™).

3. Research and development of low toxicity and low smoke flame retardants

The future trend is to develop flame retardants with low toxicity, low smoke, and pollution-free. Recently BEGOODTEX has launched ECO natural fibres like FR 100% Cotton and FR 100% Viscose which are eco-friendly, biodegradable, formaldehyde free, chemical free, non-irritating and non-allergenic. ECO natural fibers like FR 100% Cotton and FR 100% Viscose have recently been introduced by BEGOODTEX and are environmentally friendly, biodegradable, formaldehyde free, chemical free, non irritating and non fatiguing.

Islam, M. S., and van de Ven, T. G. M. (). "Cotton-based flame-retardant textiles: A review," BioResources 16(2), -.

Abstract

Biodegradable textiles made from cellulose, the most abundant biopolymer, have gained attention from researchers, due to the ease with which cellulose can be chemically modified to introduce multifunctional groups, and because of its renewable and biodegradable nature. One of the most attractive features required for civilian and military applications of textiles is flame-retardancy. This review focuses on various methods employed for the fabrication of cellulose-based flame-retardant cotton textiles along with their developed flame-retardant properties over the last few years. The most common method is to merge N, S, P, and Si-based polymeric, non-polymeric, polymeric/non-polymeric hybrids, inorganic, and organic/inorganic hybrids with cellulose to fabricate flame-retardant cotton textiles. In these studies, cellulose was chemically bonded with the flame-retardants or in some cases, cotton textiles were coated by flame-retardants. The flame-retardant properties of the cotton textiles were investigated and determined by various methods, including the limiting oxygen index (LOI), the vertical flame test, thermal gravimetric analysis (TGA), and by cone calorimetry. This review demonstrates the potential of cellulose-based flame-retardant textiles for various applications.

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Full Article

Cotton-Based Flame-Retardant Textiles: A Review

Md. Shahidul Islam and Theo. G. M. van de Ven *

Biodegradable textiles made from cellulose, the most abundant biopolymer, have gained attention from researchers, due to the ease with which cellulose can be chemically modified to introduce multifunctional groups, and because of its renewable and biodegradable nature. One of the most attractive features required for civilian and military applications of textiles is flame-retardancy. This review focuses on various methods employed for the fabrication of cellulose-based flame-retardant cotton textiles along with their developed flame-retardant properties over the last few years. The most common method is to merge N, S, P, and Si-based polymeric, non-polymeric, polymeric/non-polymeric hybrids, inorganic, and organic/inorganic hybrids with cellulose to fabricate flame-retardant cotton textiles. In these studies, cellulose was chemically bonded with the flame-retardants or in some cases, cotton textiles were coated by flame-retardants. The flame-retardant properties of the cotton textiles were investigated and determined by various methods, including the limiting oxygen index (LOI), the vertical flame test, thermal gravimetric analysis (TGA), and by cone calorimetry. This review demonstrates the potential of cellulose-based flame-retardant textiles for various applications.

Keywords: Cellulose; Cotton textile; Flame-retardancy

Contact information: Department of Chemistry, McGill University, Montreal, QC, Canada; Quebec Centre for Advanced Materials, and Pulp and Paper Research Centre, University Street, Montreal, QC, Canada; *Corresponding author:

INTRODUCTION

The textile markets are currently dominated by synthetic polymer fibers such as polyester and nylon, and natural polymer fibers such as cotton and rayon. The cost of cotton fibers has increased due to limited arable land on which it can be grown. Cotton also requires extensive irrigation and use of pesticides. Increasing concerns regarding the environmental impact of non-biodegradable synthetic polymer fibers prepared from non-renewable sources are the driving force to find suitable alternatives. Biomass contains large quantities of cellulose, which is biodegradable and unusable as food or feed. Therefore, cellulose has become an extremely suitable candidate as a sustainable alternative to natural or synthetic polymer fibers in textile markets. It is estimated that the production of cellulosic textile fibers in was 5.2 million tons (approximately 5% of total filament products), which is projected to reach 10 million tons in (Carmichael ).

Textiles play a significant role in the everyday life of human beings. Textiles are primarily made of organic polymers, which are flammable in nature. The annual UK fire statistics demonstrates that most of the fire accidents that occur in houses involve upholstering furniture, bedding, and nightwear (Salmeia et al. ). The inclusion of flame-retardants can prevent or delay the appearance of a flame and can reduce the flame-spreading rate of the textile (Salmeia et al. ; Babu et al. ; Dai et al. ; Holdsworth et al. ; Thi et al. ; Xu et al. ; Yin et al. ).

The transmission of heat and oxygen can be prevented by a low heat permeable char layer, which is produced from a flame-retardant textile during burning. Non-flammable gases that are produced during the process, such as H2O and CO2, assist in diluting the concentration of the flammable gases and minimizing the absorption of heat energy. In principle, non-flammable gases of a flame-retardant textile can resist flames by functioning in condensed and gaseous phases simultaneously during the burning process (Horrocks et al. ; Salmeia et al. ; Yusuf ; Zhang et al. a). A schematic diagram of a possible flame-retardant mechanism for a flame-retardant textile can be seen in Fig. 1.

Fig. 1. A schematic representation of the fire-resistant mechanism of flame-retardant textiles

The limiting oxygen index (LOI), the vertical flame test, the thermalgravimetric analysis (TGA), and cone calorimetry are the most common methods employed for investigation of the flame-retardant properties of textiles (Horrocks et al. ; Tata et al. ; Tata et al. ; Lyon et al. ; Walters et al. ). Several LOI standard methods, such as the ISO standard and the ASTM standard D, are employed to evaluate the flame-retardancy of textiles. Basically, the LOI signifies the least volume percentage of the O2 in a mixture of O2 and N2 that is capable to just sustain flaming combustion of a material, in the same way a candle burns. Literature demonstrates that textiles that have LOI values up to 21% (by volume) burn quickly, while those with LOI values between 21% and 25% burn slowly. Once a the LOI value of a textile goes above 25%, it starts to become flame-retardant (Horrocks et al. ).

The ASTM standard D () is used to investigate the flame-retardant properties of textile materials. A test sample is placed vertically above a controlled flame and exposed for a specified period before the flame source is removed. The length of time of the flame exposure to the specimen and the time for which afterglow continues after the flame source has been removed are both recorded. Afterwards, the char length and the visible damage of the test sample after applying a defined tearing force are determined (Zhang et al. b; Kundu et al. ).

A thermogravimetric analyzer records the mass of a substance while its temperature changes with time. A conventional thermogravimetric analyzer is made up of a precise balance and a sample holder located inside a furnace in which the temperature is controlled automatically. This instrument starts its measurements at room temperature and then the temperature is increased at a constant rate to cause thermal degradation of the substance used for testing.

The thermogravimetric analyzer can be operated under a variety of atmospheres, such as, ambient air, vacuum, inert gas, oxidizing/reducing gases, corrosive gases, carburizing gases, vapors of liquids, and at various pressures. (Coats and Redfern ; Liu and Yu ).

A fire test instrument called a cone calorimeter measures the amount of heat released during the combustion process, which is directly related to the oxygen consumption in the combustion process. The generation of heat is directly proportional to the fire growth rate of a material exposed to an external radiation heat source, which is a measure of the flammability of the material. Usually a sample is exposed to 35 kW/m2 generated by external flux-forming cone-shaped radiant heaters. However, for more fire-retardant materials, the heater frequently is increased to 50 kW/m2. This calorimeter can measure the heat release rate by the oxygen consumption, the mass loss rate, the smoke production rates, and the CO2/CO production rates (Beyler et al. ). A low CO2/CO ratio implies incomplete burning and is an indication of flame-retardancy.

Cellulose-based textiles, with superior quality and distinctive features, have carved a niche for themselves in the world of fashion. Countries all over the world are involved in developing innovative cellulose-based fabrics. In comparison with synthetic fibers, cellulose fibers, such as cotton and rayon, have important advantages, as they are abundant, biodegradable, and can be recyclable. The number of applications for cellulose-based flame-retardant textiles increases day by day world-wide (Gaan et al. ; Horrocks ). Flame-retardants textiles are used as protective clothing for people in many chemical industries, as uniforms for fire-fighter, as gear for soldiers and used in many other places, where there is a chance of causing accidents due to contact with flames.

Textiles are made flame-resistant by the inclusion of flame-retardant chemicals. A chemical additive in the fiber or treatment on the fabric is used to provide some level of flame-retardancy. This review will provide information on the different types of flame-retardants that can be employed to fabricate cotton-based flame-retardant textiles. Flame-retardants that are effective for cotton are likely to be equally effective in rayon, and other cellulose-based materials.

FLAME-RETARDANT PROPERTIES OF NON-TREATED COTTON

The flame-retardancy efficiency of cotton fabrics treated with flame-retardants can be judged by comparing them with the flame-retardant properties of non-treated cotton. Results for non-treated cotton from various studies are summarized in Table 1.

The cotton fabrics burned completely without any residue formation. The LOI values of the pure cotton ranged from 16% to 22%, the percentage of the remaining char ranged from 0% to 16%, the [CO2]/[CO] ratio ranged from 33 to 143, and the percentage residue as measured by cone calorimetry ranged from 0% to approximately 7%. These variations are mainly due to the natural variability of cotton and somewhat different test conditions. Flame-retardant cotton should have higher LOI values, more remaining char, a lower [CO2]/[CO] ratio, and higher residues.

FLAME-RETARDANT COTTON FABRICS OBTAINED BY TREATMENT WITH ORGANIC FLAME-RETARDANTS

Organic polymeric, nonpolymeric, and polymeric/nonpolymeric hybrid materials that are composed of one or more of the elements such as N, S, P, Si, B, or Cl work as flame-retardant materials. These types of materials are currently used to make flame-retardant cellulosic textiles.

Cotton Fabrics Treated with Polymeric Flame-Retardants

Polymers that contain N, S, and P atoms can work as flame-retardant materials for cellulosic textiles, such as cotton or rayon. Organic polymers can work as a flame-retardant due to the presence of one type or all these three types of elements. These atoms can be found in the original polymers or they can be incorporated by chemical modification.

Cotton fabrics treated with N-based organic polymers

Poly(amidoamine) (PAMAM) can be synthesized by a reaction of N,N′-methylenebis (acrylamide) (MBA) with (4-aminobutyl) guanidine. An aqueous solution of the synthesized PAMAM was added drop-wise to the cotton fabric uniformly, followed by drying for 5 min at 100 °C. The chemical reaction and all the steps for fabricating the PAMAM-treated cotton fabric are presented in Fig. 2 (Manfredi et al. a).

Fig. 2. Steps for fabricating cotton fabric treated with PAMAM (Manfredi et al. a)

Manfredi et al. (a) found that the remaining char tested by TGA at 600 °C and the [CO2]/[CO] ratio tested by cone calorimetry were approximately 12% and 39, respectively, for pristine cotton fabric (Table 1). This study also demonstrated that pure cotton fabric burned fully in a short period of time, and no residue was visibly formed in a vertical flame test. However, cotton fabrics impregnated with PAMAM (add-on 19%) showed a higher char production (approximately 30%, tested by TGA), a lower [CO2]/[CO] ratio (approximately 9, tested by cone calorimetry) than those of pure cotton fabric, and the vertical flame test of the treated fabric resulted in a maximum damaged length (char length) of 2.3 cm (Table 2) (Manfredi et al. a).

A cotton fabric treated G2-PAMAM (second generation PAMAM dendrimer) is also fire-retardant. It can be produced by first mixing an aqueous solution of citric acid (CA) with sodium hypophosphite (SHP, a catalyst) and dipping a cotton fabric into it, to produce a chemical link between the fabric and the CA. This chemical link was produced by the chemical reaction between the –OH groups of the fabric and the –COOH groups of the CA under heating for 4 min at 160 °C. The cotton fabric chemically linked with the CA was added to a second-generation poly(amidoamine) (G2-PAMAM) dendrimer to allow the chemical reaction between these two chemicals. Basically, amine groups (–NH2) of the G2-PAMAM reacted with carboxyl groups (–COOH) of the CA-treated cotton fabric under heating for 4 min at 160 °C to form amide bonds. The chemical reactions for the entire process are exhibited in Fig. 3. (Taherkhani and Hasanzadeh ).

The flame-retardant properties of cotton fabric covalently bonded with G2-PAMAM through CA (as a crosslinker) were also studied by Taherkhani and Hasanzadeh (). The LOI and the remaining char, tested via TGA at 600 °C, were approximately 23% and 25.1%, respectively, of the treated fabric. These values were lower than the control fabric, which had an LOI of 18% and a remaining char at 600 °C tested via TGA of 1.2%) (Tables 1 and 2). A char length of only 0.32 cm was obtained for the treated fabric, while the control fabric burned completely as was shown by the vertical flame test (Tables 1 and 2). The inclusion of nitrogen-containing PAMAM and G2-PAMAM resulted in excellent flame-retardancy of the treated fabrics.

Fig. 3. Chemical reactions for fabricating cotton fabric chemically linked with through CA (Taherkhani and Hasanzadeh )

Cotton fabrics treated with N, S-based organic polymers

Cotton fabrics treated with polyamidoamine containing disulfide-groups in the main chain (SS-PAMAM), prepared by Michael polyaddition of 2,2-bis(acrylamido) acetic acid (BAAA) with L-cystine were shown to have flame-retardant properties (Emilitri et al. ; Manfredi et al. b). Double-bonded carbon of BAAA reacted with the –NH2 group of L-cystine to produce SS-PAMAM. An aqueous solution of the synthesized SS-PAMAM was added to the cotton fabric, after which it was dried for 10 min at 100 °C.

Fig. 4. Steps for fabricating cotton fabric treated with SS-PAMAM (Emilitri et al. ; Manfredi et al. b)

The chemical reaction and all the steps for fabricating the SS-PAMAM treated cotton fabric are exhibited in Fig. 4. The remaining char at 600 °C (tested via TGA), the [CO2]/[CO], and the residue (investigated by cone calorimetry) were approximately 13%, 33.2, and 0%, respectively. The non-treated fabric burned completely in a very short time without forming any residue (Table 1) (Emilitri et al. ; Manfredi et al. b). An increase in the flame-retardancy was demonstrated by the char formation (24% by TGA and 5.5% by cone calorimetry), by the [CO2]/[CO] ratio (approximately 30.8) and by the vertical flame test (maximum char length 0.7 cm). The flame-retardancy was caused by the incorporation of nitrogen and disulfide containing PAMAM (SS-PAMAM) (add-on 12%) into the cotton fabric (Table 2).

Cotton fabrics treated with P-based organic polymers

Mixing a cotton fabric with HBPOPN, the cationic NH4+ ions of which reacted with –OH groups of cellulose, renders it flame-retardant. This reaction is catalyzed by dicyandiamide. The HBPOPN is produced by mixing a hyperbranched polymer (HBP) with phosphoric acid (H3PO4) to obtain the phosphate esterification product of HBP (called HBPOP), which reacted with urea (H2N–CO–NH2) to produce the ammonium salt of HBPOP (called HBPOPN). The chemical reactions of this entire process are shown in Fig. 5 (Ling and Guo ).

Fig. 5. Chemical reactions for fabricating cotton fabric (Cell-OH) modified by HBPOPN (Ling and Guo )

In principle, the HBPOPN can link to 12 glucose units of cellulose. However, the cotton yarn made from cellulose chains is porous, with a pore size of the order of microns, which is much larger than the size of a HBPOPN macromolecule. Therefore, the HBPOPN usually links to only one cellulose molecule, except for chain crossings or chains in close proximity, in which case it is able to bridge two cellulose chains. The inclusion of phosphorus-containing HBPOPN increased the flame-retardancy of the cotton significantly. Ling and Guo () showed that the incorporation of 28.1% HBPOPN enhanced the LOI (42%) and char formation (approximately 35%, tested by TGA at 600 °C), decreased the [CO2]/[CO] ratio (3.11), and gave a maximum char length of 5.6 cm for the cotton fabric. The untreated cotton fabric had a lower LOI (17.2%) and TGA char formation (2.5% at 600 °C) (Tables 1 and 2). The non-treated cotton fabric also showed a higher [CO2]/[CO] ratio (56.5) and burned completely in the vertical flame test (Table 1).

Cotton Fabrics Treated with Non-polymeric Flame Retardants

Non-polymeric organic compounds that contain the elements N, P, Si, B, or Cl can work as flame-retardant materials for cellulosic textiles. These types of organic compounds can work as flame-retardants due to the presence of one type or more than one type of these five categories of elements. They can be found in the original organic compound or they can be incorporated by chemical modification.

Cotton fabrics treated with P-based non-polymeric organic compounds

An example of a non-polymeric organic compound that contains phosphor atoms is the ammonium salt of 1-hydroxyethylidene-1,1-diphosphonic acid (AHEDPA). It is produced by mixing 1-hydroxyethylidene-1,1-diphosphonic acid (HEDPA) with urea. To render the cotton flame-resistant, it was treated with AHEDPA. The cationic NH4+ ions of the AHEDPA reacted with the OH groups of the cellulose to produce a covalent bond between them. This reaction was catalyzed by dicyandiamide. The chemical reactions of this process are shown in Fig. 6 (Lu et al. ).

Fig. 6. Chemical reactions for fabricating cotton fabric treated with AHEDPA (Lu et al. )

Another example is ammonium phytate (APA), produced by mixing phytic acid (PA) with urea. As seen in cotton, mixed with APA, the cationic NH4+ ions of the APA reacted with the –OH groups of the cellulose to produce a flame-retardant cotton fabric. This reaction was also catalyzed by dicyandiamide. The chemical reactions of this process are shown in Fig. 7 (Feng et al. ). The flame-retardant properties of the cotton fabric were developed due to covalent linkages of the cellulose chains with phosphorous containing non-polymeric compounds, such as AHEDPA and APA. By incorporating 20.11% AHEDPA, a significant increase in the flame-retardancy was achieved in which the LOI, the TGA char formation at 600 °C, and the cone calorimeter residue increased from 18.4% to 41.5%, approximately 10% to approximately 45%, and 1.3% to 38.9%, respectively, as reported by Lu et al. () (Tables 1 and 2). In that study, it was also found that the [CO2]/[CO] ratio decreased from approximately 83 to 3.77, and for the treated fabric a damaged length of only 5.3 cm was obtained (Tables 1 and 2). Feng et al. investigated APA-treated textile and found that the LOI, the remaining char at 600 °C (tested by TGA), and the [CO2]/[CO] ratio and residue (tested by cone calorimetry) were 17.8%, approximately 0.8%, and approximately 86.18 and 1.31%, respectively, for the control fabric, which burned completely with no residue (Table 1). In contrast, the LOI, the remaining char at 600 °C (tested by TGA), and the [CO2]/[CO] ratio and residue (tested by cone calorimeter) were 36.1%, approximately 40%, and 3.05 and 36.24%, respectively, for the fabric treated with APA (add-on 14.49%) (Table 2). Similar results were obtained those of the AHEDPA-treated samples (Feng et al. ; Lu et al. ). This treated fabric also promptly self-extinguished after ignition, and the maximum damaged length (char length) was 3.5 cm (Table 2).

Fig. 7. Chemical reactions for fabricating cotton fabric treated with APA (Feng et al. )

Cotton fabrics treated with N, P-based non-polymeric organic compounds

An example of a nitrogen containing compound is the ammonium salt of ethylenediamine tetramethylenephosphonic acid (AEDTMPA). The AEDTMPA is produced by reacting ethylenediamine tetramethylenephosphonic acid (EDTMPA), prepared from the reaction that occurs among the reactants ethylenediamine, formaldehyde, and phosphorous acid (H3PO3), with urea. After adding a cotton fabric, the –OH groups of the cellulose molecules reacted with the phosphonic groups in the AEDTMPA to form P–O–C covalent bonds. The chemical reactions of the entire process are presented in Fig. 8 (Zheng et al. ).

Fig. 8. Chemical reactions for fabricating cotton fabric treated with AEDTMPA (Zheng et al. ).

Fig. 9. Chemical reactions for fabricating cotton fabric treated with ATPMPA (Wan et al. )

A second example is the ammonium salt of tris˗(hydroxymethyl)˗ amino-methane˗penta (methyl phosphonic acid) (ATPMPA), which was prepared by mixing tris˗(hydroxymethyl)˗aminomethane (THAM) with formaldehyde (H-CHO), H3PO3, and H3PO4 to obtain the phosphate esterification product of THAM, which reacted with urea (H2N-CO-NH2) to produce ATPMPA. When added to cotton, the NH4+ cations of the ATPMPA reacted with the –OH groups of the cellulose to produce a flame-retardant cotton fabric. This reaction was catalyzed by dicyandiamide. The chemical reactions of this entire process are shown in Fig. 9 (Wan et al. ).

A third example is the ammonium salt of tetraethylenepentamine heptamethyl-phosphonate (ATEPAHP), prepared from the reaction occurring among the reactants tetraethylenepentamine (TEPA), formaldehyde, and H3PO3. The produced TEPAHP was then mixed with urea to produce ATEPAHP. When added to cotton, the –OH groups of the cellulose react with the phosphonate groups of the ATEPAHP to form P–O–C covalent bonds, a reaction catalyzed by dicyanodiamide. The chemical reactions of the entire process are presented in Fig. 10 (Tian et al. ).

Fig. 10. Chemical reactions for fabricating cotton fabric treated by ATEPAHP (Tian et al. )

In a fourth example, diethanolamine (DEA) was mixed with formaldehyde (H-CHO), H3PO3, and H3PO4 to obtain the phosphate esterification product of DEA, which reacted with urea (H2N-CO-NH2) to produce the ammonium salt of cholamine (methylenephosphonic acid) ethylene-organic phosphate acid (ACMPEP). Finally, cotton fabric was treated with ACMPEP, which resulted in reaction of NH4+ ions with the –OH groups of the cellulose to produce a flame-retardant cotton fabric. This reaction was catalyzed by dicyandiamide. The chemical reactions of this entire process are shown in Fig. 11 (Li et al. ).

Fig. 11. Chemical reactions for fabricating cotton fabric treated by ACMPEP (Li et al. )

Fig. 12. Chemical reactions for fabricating cotton fabric treated by AATMP (Huang et al. )

A fifth example is ammonium amino trimethylene phosphonate (AATMP), produced from amino trimethylene phosphonic acid (ATMPA) mixed with urea. When mixing a cotton fabric with AATMP, the NH4+ cations reacted with the –OH groups of the cellulose to produce a flame-retardant cotton fabric. As in previous cases, this reaction was catalyzed by dicyandiamide. The chemical reactions of this entire process are presented in Fig. 12 (Huang et al. ).

A final example of a nitrogen-containing compound is the ammonium salt of melamine hexa(methylphosphonic acid (AMHMPA), synthesized by the reaction between urea and melamine hexa(methylphosphonic acid) (MHMPA), obtained by a reaction between melamine (MA), formaldehyde (H–CHO), and H3PO3. After adding a cotton fabric, NH4+ cations of AMHMPA react with –OH groups of cellulose to produce a flame-retardant cotton fabric. This reaction was also catalyzed by dicyandiamide. The chemical reactions of this entire process are presented in Fig. 13 (Zhang et al. a).

An example of a compound containing both nitrogen and phosphor atoms is AEDTMPA (ammonium salt of ethylenediamine tetramethylenephosphonic acid). Zheng et al. () demonstrated that the inclusion of N, P-based AEDTMPA into cotton increased its flame-retardancy. The LOI, the TGA char formation, and the cone calorimeter residue increased from approximately 20% to 43.6%, approximately 8% to approximately 43.4% (at 600 °C) and 1.3% to 42.5%, respectively (Tables 1 and 2). Pristine cellulosic textiles burned completely while the AEDTMPA-treated textiles had a damaged length of just 3.5 cm obtained by the vertical flame test. The [CO2]/[CO] ratio decreased from 78 to 2.1 after treatment.

Fig. 13. Chemical reactions for fabricating cotton fabric treated by AMHMPA (Zhang et al. a)

Another example of a fire-retardant containing both N and P atoms is ATPMPA (ammonium salt of tris˗(hydroxymethyl)˗aminomethane˗penta (methyl phosphonic acid)). Wan et al. () showed that although the pure cotton fabric burned fully, the inclusion of 26.13% of ATPMPA decreased the damage length significantly to 2.5 cm, as determined by the vertical flame test (Tables 1 and 2). The LOI, the TGA char formation (at 600 °C), and the cone calorimeter residue increased from 18.4% to 43.6%, approximately 10% to 38%, and 1.2% to 30.8%, respectively, and the [CO2]/[CO] ratio decreased from 78 to 28.5 after the incorporation of the N, P-based ATPMPA flame-retardant. (Tables 1 and 2).

Other studies examined the flame-retardant properties of cellulose textiles by incorporating the N-P-based flame retardants ATEPAHP (ammonium salt of tetraethylene pentaamine heptamethylphosphonate), ACMPEP (ammonium salt of cholamine (methylene phosphonic acid) ethylene-organic phosphate acid), AATMP (ammonium amino trimethylene phosphonate), and AMHMPA (ammonium salt of melamine hexa(methylphosphonic acid), respectively (Zhang et al. a; Huang et al. ; Li et al. ; Tian et al. ). The vertical flame tests demonstrated that the pristine cellulose textiles burned fully, but damage lengths of 4.8 cm, 3.8 cm, 3.0 cm, and 5.3 cm were obtained for the cellulose textiles treated with ATEPAHP, ACMPEP, AATMP, and AMHMPA, respectively (Tables 1 and 2). In each study, the LOI, the TGA char formation (at 600 °C) and the residue studied by cone calorimeter increased, and the [CO2]/[CO] ratio decreased (Tables 1 and 2). This demonstrates that these compounds have excellent fire-retardant properties.

Cotton fabrics treated with N, P, Si-based non-polymeric organic compounds

Flame-retardant cotton can be obtained in a two-step process, in which the cotton is first treated with 3-mercaptopropyltriethoxysilane (MPTES) and subsequently with dimethyl-[1,3,5-(3,5-triacryloylhexahydro)triazinyl]-3-oxopropylphosphonate (DHTP). In the reaction with MPTES, a covalent link is formed between cellulose and MPTES through an O–Si linkage that results from the reaction between an oxyethyl group on one end of MPTES and a hydroxyl group of the fiber surface (Sun et al. ). In the reaction with DHTP, a thiol group of the MPTES-treated cotton fabric reacted with DHTP to produce a non-halogenated organophosphorous based flame-retardant cotton fabric (Yoshioka-Tarver et al. ; Xu et al. ). The protocol for the synthesis of DHTP is described elsewhere (Weil ; Weil ; Yoshioka-Tarver et al. ; Xu et al. a). The reactions for fabricating the DHTP-based flame-retardant cotton fabric are shown in Fig. 14.

Fig. 14. Chemical reactions for fabricating DHTP based flame retardant cotton fabric (Yoshioka-Tarver et al. ; Sun et al. ; Xu et al. a,b)

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Another example of this class is (3-glycidyloxypropyl triethoxysilane modified N-(phosphonomethyl) iminodiacetic acid (PGPTES). Cotton can be rendered flame-resistant by immersing it in a PGPTES solution, followed by drying at 90 ◦C for 5 min followed by curing at 170 ◦C for 5 min. The entire process for the fabrication of the cotton fabric coated by a PGPTES solution is shown schematically in Fig. 15.

Fig. 15. Schematics of fabrication of cotton fabric coated by PGPTES (Castellano et al. )

Another example is the H-DPTA, obtained by the hydrolysis of N-(diphenylphosphino)-1,1-diphenyl-N-(3-(triethoxysilyl)propyl) phosphinamine (DPTA), produced by the reaction of 3-triethoxysilylpropylamine with chlorodiphenylphosphine (Ph2PCl). The chemical reactions of this process are shown in Fig. 16 (Zhao et al. ).

Fig. 16. Chemical reactions for fabricating cotton fabric treated by H-DPTA (Zhao et al. )

Finally, DHTP (dimethyl-[1,3,5-(3,5-triacryloylhexahydro)triazinyl]-3-oxopropyl phosphonate), PGPTES ((3-glycidyloxypropyl triethoxysilane modified N-(phosphono-methyl) iminodiacetic acid), and DPTA (N-(diphenylphosphino)-1,1-diphenyl-N-(3-(triethoxysilyl) propyl) phosphinamine) are all N, P, and Si-based non-polymeric organic compounds, which can enhance the flame-retardancy of cotton fabrics. Studies have examined the flame-retardancy of cotton fabric covalently-linked with DHTP, and found that the LOI and the TGA char formation (at 600 °C) increased from 26.1% to 34% and 31.2% to 43%, respectively. For pure cotton fabrics the LOI was between 18.3% and 21% and the TGA char formation was 5% to 13%. (Tables 1 and 2) (Yoshioka-Tarver et al. ; Sun et al. ; Xu et al. a, b). It was also found that the vertical flame test gave a maximum damage length of 5 cm to 9.2 cm for the treated fabrics, while the non-treated fabrics burned completely.

For the cotton treated with PGTES (add-on 25.2%), Castellano et al. () showed that the TGA char formation at 600 °C, the [CO2]/[CO] ratio and the residue were 38% and 20 and 26%, respectively, with a maximum char length of 5 cm. For the control fabric, which burned completely within a very short time, these values were 5.8%, 143, and 1%, respectively (Table 1). For the cotton treated with DPTA, the LOI and the remaining TGA char at 600 °C of a pure cotton fabric, which burned entirely, increased to 25.4% and approximately 42% from 18.4% and approximately 15%, respectively, with maximum damage length of 8.1 cm (Tables 1 and 2) (Zhao et al. ).

Cotton fabrics treated with N, P, and Cl-based non-polymeric organic compounds

A phosphorous trichloride (PCl3)-dimethylformamide (DMF) adduct was prepared according to a protocol described elsewhere by Smith (), as seen in Fig. 17. A cotton fabric was then treated with the prepared PCl3-DMF adduct to form a covalent link between cotton fabric and the adduct through O–P and O–C linkages resulting from the reaction of the P– and double-bonded C– sites of the adduct with hydroxyl groups of cellulose of the cotton fabric (Fig. 17) (Vigo et al. ).

Fig. 17. Chemical reactions for fabricating cotton fabric covalently linked with PCl3–DMF adduct (Smith ; Vigo et al. )

Cotton fabrics that were treated with 5% adduct for 5 min exhibited flame-retardancy. For a 30 cm original sample length, the char formation was 8.9 cm (Table 2) (Vigo et al. ). However, the entire length (30 cm) of the control fabric was burned quickly by the vertical flame without the formation of any char (Table 1). The incorporation of PCl3–DMF helped to achieve a good flame-retardancy of the treated cotton fabric.

Cotton fabrics treated with N, B, Cl-based non-polymeric organic compounds

Tri-HTAC (2,4,6-tri[(2-hydroxy-3-trimethyl-ammonium)propyl]-1,3,5-triazine chloride) was added to boric acid (H3BO3) and a cotton fabric, which resulted in a covalent bond between the cotton and the B-containing Tri-HTAC. The chemical reaction for this process is shown in Fig. 18.

The flame-retardant properties of the cotton produced this way were studied by Xie et al. (). It was found that the cotton fabric’s resistance to fire increased due to the incorporation of the N, B, and Cl– based non-polymeric organic compound Tri-HTAC. The data is shown in Tables 1 and 2, which show that the LOI and the TGA char formation at 600 °C increased from approximately 22% to approximately 27.5% and from 6.3% to approximately 40.5%, respectively, after incorporating B-containing Tri-HTAC in the cotton fabric.

Fig. 18. Chemical reactions for fabricating cotton fabric covalently linked with B-containing Tri-HTAC (Xie et al. )

Cotton Fabrics Treated with Polymeric/Non-Polymeric Hybrid Organic Flame Retardants

Polymeric/non-polymeric hybrid organic compounds that contain N, P, S, and CNT can work as flame-retardant materials for cellulosic textiles. These types of organic compounds can work as flame-retardants due to the presence of one or more types of these four categories of materials (N, P, S, and CNT). These materials can be added in their original organic form or they can be incorporated via chemical modification of cotton fabrics.

Cotton Fabrics Treated with N, P-Based Polymeric/Non-Polymeric Hybrid Organic Flame-Retardant

Layer-by-layer deposition on cotton of alternating cationic polyelectrolytes and anionic compounds can be used as a method to render cotton flame-retardant. Cotton with bilayers of cationic polyethylenimine (PEI) and anionic phytic acid (PA) was produced by Zhang et al. (b). The cotton was immersed in a PEI solution, removed from the solution, rinsed with water, and then dried at 80 °C. Subsequently, the fabric coated by the cationic PEI was dipped into a PA solution, taken out of the solution, rinsed with water, and then dried at 80 °C. After such a cycle, the cotton fabric was coated by one PEI/PA bilayer. This procedure was repeated eight times to obtain eight bilayers on the cotton fabric. The layer-by-layer process is shown schematically in Fig. 19.

Fig. 19. Schematic representation of the fabrication of the cotton fabric coated by PEI/PA bilayers (Zhang et al. b). Multilayers can be formed by repeating this process multiple times.

The cotton fabric coated by PEI/PA bilayers exhibited flame-retardant properties (Zhang et al. b). The pure cotton fabric had an LOI of 18.5% and a TGA residue (at 600 °C) of approximately 16.3%, which increased to 37% and approximately 35%, respectively, for the treated cotton fabric. A vertical flame produced a damage length of approximately 7 cm in the treated sample but burned the entire length of the non-treated cotton fabric (Tables 1 and 2).

Cotton Fabrics Treated with N, P, S, and CNT-based Polymeric/Non-Polymeric Hybrid Organic Flame-Retardant

Cotton with bilayers of cationic polyhexamethylene guanidine phosphate (PHMGP) and anionic VBL-CNT (fluorescent whitening agent (VBL)-carbon nanotube) was produced by Ding et al. (). The cotton fabric was dipped into a PHMGP solution, taken out of the solution, rinsed with water, and then dried at 60 °C. Subsequently, the fabric coated by cationic PHMGP was dipped into anionic solution containing fluorescent whitening agent VBL (4,4′-bis[(hydroxyethylamino-6-anilino-1,3,5-triazin-2yl) amino] diphenylethylene-2,2′-sodium disulfonate) and carbon nanotubes (CNT), taken out of solution, rinsed with water, and then dried at 60 °C. The cotton fabric was coated by one bilayer after this cycle. Ten cycles were performed to obtain 10 bilayers on the cotton fabric. The layer-by-layer process is similar to what is shown in Fig. 19. Testing the coated cotton showed that the flame-retardancy of the cotton fabric improved due to coating by the N, P, S, and CNT-based polymeric/non-polymeric hybrid organic, PHMGP/VBL-CNT (Ding et al. ). After making the coating, the fabric promptly self-extinguished after ignition, and the maximum damage length was approximately 15 cm. However, the non-treated fabric burned completely within a very short time without the formation of any residue in the vertical flame tester (Tables 1 and 2). Moreover, the TGA char formation (at 600 °C) for the treated fabric increased to 21.7% from 3.3% for the pure fabric (Tables 1 and 2).

FLAME-RETARDANT COTTON FABRICS OBTAINED BY THE TREATMENT WITH INORGANIC FLAME-RETARDANTS

An example of an inorganic flame-retardant is ammonium polyphosphate (APP), a P-based inorganic flame retardant, which was applied as a coating on cotton fabric to develop flame-retardancy (Yin et al. ; Lin et al. ). To produce the APP-coated cotton, a cotton fabric was dipped into an aqueous solution of APP and then dried at 80 °C. The APP was attached to cellulose due to formation of hydrogen bonds between a hydrogen of a cellulose hydroxyl group and an oxygen of the polyphosphate group. A high number of -OH groups of cellulose chains can strongly interact with N-H, P=O and P-O groups in APP through hydrogen bond formation (Yin et al. ). A schematic representation of the fabrication of the cotton fabric coated by APP is shown in Fig. 20.

Fig. 20. Schematic representation on fabrication of cotton fabric coated by APP (Yin et al. ; Lin et al. ).

The flame retardant properties of APP-coated cotton, such as TGA char formation increased significantly and the damage length tested by the vertical flame tester decreased (Tables 1 and 2).

FLAME-RETARDANT COTTON FABRICS OBTAINED BY THE TREATMENT WITH ORGANIC/INORGANIC HYBRID FLAME-RETARDANTS

A multicomponent fire-retardant was developed by Lessan et al. ().

Fig. 21. Schematic representation of the cotton fabric treated with the aqueous dispersion of SHP, MA, TEA, and nano TiO2 (Lessan et al. ).

An aqueous dispersion of sodium hypophosphite (SHP), maleic acid (MA), triethanol amine (TEA), and nano TiO2 was prepared in an ultrasonic bath (Merck Chemical Co. and Evonik Co., Germany). A cotton fabric was then impregnated in the dispersed solution for 1 h. The weight ratio of dispersion to fabric was 20:1. The treated cotton fabric was taken out from the dispersed solution, dried at 80 °C for 2 min, and then cured at 180 °C for 2 min. The treated samples were rinsed in an ultrasonic bath for 5 min to remove non-attached nano TiO2 from the fabric surface (Lessan et al. ). The entire process is presented schematically in Fig. 21. The measured LOI and the residual char (Table 2) show that the treated cotton was flame-retardant.

Another example of a multicomponent flame-retardant consisted of polyphosphoric acid (PPA), PEI, and nanosilica. The cotton fabric was first immersed into an aqueous solution of PPA to produce a negatively charged phosphorylated cotton fabric, which was then immersed into an aqueous dispersion of nano SiO2 coated by PEI, to generate a flame-retardant bilayer on the cotton fabric (Li et al. ). Cationic PEI wrapped nano SiO2 was produced from a SiO2-PEI solution, the pH of which was adjusted to five with acetic acid. The chemical reactions for making a PPA/PEI-SiO2 coating on the cotton fabric are shown in Fig. 22.

Fig. 22. Chemical reactions for fabricating cotton fabric coated by the PPA/PEI-SiO2 solution (Li et al. )

Fig. 23. Schematic representation on the fabrication of the cotton fabric coated by CA, PA, and BA2+ (Zhang et al. a)

Another multicomponent coating of cotton that was flame-retardant consisted of chitosan (CS), phytic acid (PA), and barium ions (Ba2+). A cotton fabric was immersed into a CS solution for 5 min and then padded dry. This dried fabric was then immersed into a PA solution for 5 min, followed again by padding and drying. Finally, this treated fabric was dipped into a Ba+2 solution for 5 min, padded and dried to obtain a cotton fabric coated by CS, PA, and Ba2+ ions. A schematic representation of the process can be seen in Fig. 23 (Zhang et al. a).

A dual component PA and silica solution coating has also been found to be a flame-resistant, coating on cotton. A mixture consisting of tetraethyl orthosilicate (TEOS), ethanol and PA was employed to prepare a PA/silica solution in which a cotton fabric was immersed for a certain period. The impregnated cotton fabric was then taken out from the PA/silica solution, dried at 80 °C, and cured at 160 °C for 3 min. The treated cotton fabric was then washed and air-dried. The entire process for the fabrication of the cotton fabric coated by the PA/silica solution is shown exhibited schematically in Fig. 24.

Studies of the dual and multicomponent coatings on cotton, discussed above, showed that these coatings rendered cotton flame-retardant (Lessan et al. ; Li et al. ; Zhang et al. a; Cheng et al. ). Each study showed that pure cotton fabric burned completely within a very short time without the formation of any residue. In contrast, the treated fabrics promptly self-extinguished after ignition, and the maximum damage length reduced remarkably in each case (Tables 1 and 2). The TGA char formation also increased for the treated fabrics in every case.

Fig. 24. Schematic representation on the fabrication of cotton fabric coated by the PA/silica solution (Cheng et al. )

COMPARISONS BETWEEN VARIOUS FLAME-RETARDANT COTTONS

A major difficulty in comparing various flame-retardants is that in most studies the amount of flame-retardant incorporated in cotton varies tremendously. However, an apparent comparison can be made based on their reported compositions and performances. Table 2 summarizes the flame-retardant properties of cotton treated with flame-retardants, either adsorbed or covalently linked.

This review has shown that textiles that had been prepared with P-based non-polymeric flame retardants, such as AHEDPA and APA (Feng et al. ; Lu et al. ), demonstrated the best performances among all the different types of flame-retardants. Phosphorus-based flame-retardants are quite versatile in their flame-retardant action, in which both the condensed and the gas phase play a role in the efficiency of the flame-retardant (Granzow ; Li et al. ; Salmeia et al. ; Liang et al. ). The inclusion of flame-retardants accelerates the formation of a dense char layer to promote the dehydration and carbonization of the cotton fabric. The formation of a char layer also prevents heat radiation and oxygen transfer that also has a positive effect on the flame-retardancy of the treated fabric.

SUMMARY AND CONCLUDING REMARKS

There are two methods for fabricating cotton-based flame-retardant textiles: coating and covalently-linking flame-retardants. It has been shown that textiles coated by N-based PAMAM, N-S-based SS-PAMAM, N-P-Si-based PGPTES, N-P-based PEI/PA blends, APP-N-P-S-CNT-based PHMGP/VBL/CNT blends, P-N-TiO2-based SHP/MA/ TEA/nanoTiO2 blends, P-N-Si-based PPA/PEI/SiO2 blends, N-P-Ba2+-based CS/PA/Ba2+ blends, and P-SiO2-based PA/silica gels, all have considerable flame-retardancy. The flame-retardancy of textiles was also achieved by a covalent-linkage between the cotton fabric and each of the following materials: N-based G2-PAMAM, P-based HBPOPN, AHEDPA and APA, N-P-based AEDTMPA, ATPMPA, ATEPAHP, ACMPEP, AATMP and AMHMPA, N-P-Si-based DPTA and DHTP, N-P-Cl-based PCl3-DMF adduct, and N-B-Cl-based B-containing tri-HTAC. It is not easy to compare the flame-retardant ability of the textiles prepared by different methods because in most cases, different amounts of flame-retardant material were incorporated into cotton textiles, which were obtained from different sources. However, it was evident that flame-retardant properties of cotton textiles were developed due to the incorporation of the above materials.

This work clearly outlines the wide potentialities of cotton-based flame-retardant textiles fabricated with the use of the flame-retardant materials mentioned above. The materials mentioned above could also be applied to other cellulose-based textiles to make them flame retardant. The concurrent presence of phosphorus and other elements including nitrogen, sulphur, boron, chlorine, barium, and nanomaterials (CNT, silica, and TiO2) could be significant in order to obtain synergistic effects during the exposure of the treated textiles to a flame or to a heat source.

A sustainable flame-retardant textile should also demonstrate durability to washing, which has not been achieved for many of the flame-retardant textiles mentioned above (Xu et al. a; Lu et al. ; Taherkhani and Hasanzadeh ; Zhang et al. a). Some exhibit no washing fastness at all, and some can withstand a very limited number of washing cycles before losing their flame-retardant properties. This is obviously a drawback of the currently reported cotton-based flame-retardant textiles, which use is restricted to applications for which longevity to washing is not required. For this reason, further research is needed to fabricate novel and effective cotton-based flame-retardant textiles that can overcome this drawback. In general, an excellent washing fastness is achieved for the flame-retardant textiles in which covalent bonds are formed between textiles and flame-retardants. During washing, small adsorbed molecules can be washed out easily, whereas adsorbed polymeric compounds remain adsorbed due to multiple linkage with the textiles. Adsorbed flame-retardants can be washed out during washing, but even cottons with covalently linked flame-retardants loose some of their flame-retardancy during washing, as in each washing cycle part of the cotton ends up in the wastewater. For synthetic textiles, this is the main source of microplastics in municipal waste waters. Flame-retardants are dominantly located on the surface of fibers (Lessan et al. ; Li et al. ; Zhang et al. a, b; Cheng et al. ), and thus cotton fibrils removed during washing are expected to have a higher concentration of flame-retardants than the washed material.

Although flame-retardants protect textiles against ignition, they could adversely affect the environment and human health when used above a certain limit. Flame-retardants can be released to the environment and the human body during fabrication and after disposing of the flame-retardant textiles. The flame-retardants can contaminate air, water and soil, and also can affect immune, reproductive, and nervous systems of human beings. Long-term exposure to flame-retardants can even cause cancer in humans (Segev et al. ).

ACKNOWLEDGEMENTS

The authors want to acknowledge financial support from a NSERC Strategic Research Project grant (Grant No. -17) and the industrial partner FPInnovations.

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Article submitted: September 24, ; Peer review completed: January 30, ; Revised version received and accepted: February 22, ; Published: March 1, .

DOI: 10./biores.16.2.Islam

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