An Overview of Water Repellent & Waterproof

We all familiar with water repellent or waterproof. I did some research and arranged today’s blog about water and oil repellent finishing. It’s very common to see in natural. For example the droplets on lotus leaves. 

lotus leaves water repellent waterproof

Humans have long history to made waterproof clothing to resist the rain. For example, the tarpaulin impregnated with tung oil in China can be made into umbrellas, rain boots and raincoats. In modern times, rubber-coated fabrics are used to make raincoats and tarpaulins to form a uniform film on the surface of the fabric, relying on physical methods to prevent water from penetrating.

The development of science and technology has also promoted the continuous development of the textile industry. The clothing in the new century will develop in the direction of comfortable wearing, functionalization and returning to nature. Various intelligent weaving and functional fabrics have received extensive attention and development. Waterproof and breathable fabric is one of them.

The finishing that makes fabric waterproof can be divided into two categories: one is called waterproof finishing, which is a continuous film coated on the surface of the fabric to block the pores of the fabric so that neither water nor air can penetrate. The other type is called water-repellent finishing, which is to apply a layer of water-repellent film on the fabric fibers that cannot be wetted by water, but does not close the gaps of the fabric, so that the fabric has both water-repellent and breathable characteristics.

1. Waterproof finishing

1.1 Principle of waterproof finishing

Waterproof finishing is to coat a layer of airtight continuous film on the surface of the fabric, such as rubber, which uses physical methods to block the penetration of water, so that the fabric is impermeable but it’s uncomfortable to wear.

1.2 Waterproof finishing agent

There are two types of water repellents used in water repellent finishing: hydrophobic and hydrophilic, both of which are used to block the gaps in the fabric to achieve the purpose of waterproofing. Such finishing agents include grease, wax, paraffin and various rubbers or various types of thermoplastic resins.

1.1.1 Grease, wax and paraffin

Natural solid substances such as oils, waxes and paraffin wax are often used in waterproof finishing. The melt coating method is often used, that is, they are heated and eutectic, and they are coated on the fabric by waxing or spraying. Then dried and heated to make it penetrate into the fabric. Paraffin wax can be mixed with animal fats and oils, or used alone, usually by emulsification. The disadvantage of paraffin coating is its low melting point, which melts at 56~60℃. The method of applying certain greases for airtight finishing is still in use. For example, tent cloth, tung oil, etc. are added to the fabric and coated with a metal desiccant. After being oxidized by air, it will form a soft, impermeable transparent film after being fully dried. In order to prevent the fabric from adhering to other objects, another layer can be coated A dilute ammonia solution of shellac. But its disadvantages are slow drying, yellowing, and sometimes an unpleasant smell.

1.1.2 Rubber

Use natural rubber or synthetic rubber, latex with fillers, pigments, vulcanizing agents, anti-aging agents, etc. to be coated on the fabric and then vulcanized. The fabric has a good waterproof effect. Because natural rubber is prone to aging easily, so synthetic rubbers are currently used a lot. Including neoprene, nitrile rubber, isobutylene rubber, styrene butadiene rubber, butyl rubber, etc., which can be selected according to the type of fabric, usage requirements and price. Since polyisobutylene rubber has no double bond structure and is difficult to vulcanize. It is necessary to add a small amount (less than 3%) of isoprene for copolymerization to form butyl rubber. The rubber is the least permeable and breathable, and it is widely used as a waterproof rubber. Styrene butadiene rubber is the cheapest kind of synthetic rubber, but it has poor adhesion to fabrics.

1.1.3 Thermoplastic resin

The olefinic resin has large output, low price, and can be made into solution type or emulsion type. It is easy to process, so it has a wide range of applications. Generally, it can be processed by scraping, extrusion or film welding. Polyvinyl chloride (PVC) generally does not use a solution type, but uses a slurry to coat it by extrusion. Polyvinyl chloride can be mixed with other resins, such as polyvinyl acetate or polyacrylate, or form copolymer applications to improve its performance and is widely used as a waterproofing agent.

Polyacrylate has the characteristics of heat resistance and solvent resistance, and can often be used as a waterproof coating for fabrics instead of vinyl chloride and rubber. After polyacrylate coating, drying can improve its water resistance, adding amino resin can improve its abrasion resistance, adding polyvinyl chloride can improve its softness.

2. Water and oil repellent finishing

Water repellent finishing uses the finishing agents with low surface energy, relying on low surface tension to prevent water from wetting the fabric. The biggest advantage of this method is that it can still maintain good air and moisture permeability, help the microclimate adjustment between human skin and clothing, and increase wearing comfort. In addition, it will not affect the feel of the fabric, and help to improve the style of clothing. The characteristics of water repellent finishing fabrics are not only rainproof and windproof, but also perspiration and breathability, and comfortable to wear. During the wearing process, water does not penetrate the fabric under a certain pressure, while the sweat emitted by the human body can be in the form of water vapor. It is transmitted to the outside through the fabric, and does not condense and accumulate between the surface of the human body and the fabric, keeping the wearer dry and warm, and is mainly used for wearing fabrics.

2.1 Principles of water and oil repellency

Water and oil repellency are based on the premise of low wettability. Wetting is very important in the processing and application of textiles. The cleaning, water repellency, absorption and other properties of textiles are all affected by the wetting properties of the fabric. Wetting is a composite process, which becomes more complicated due to the influence of the fiber structure of the textile.

The wetting of textiles consists of the following main processes: impregnation, capillary absorption, adhesion and spreading.

Young proposed that the droplet lying on the solid plane is affected by the following balancing forces (Figure 7-1):

force of droplets on solid surface

Figure 7-1 Force of Droplets on Solid Surface

σSG– σSL= σLGcosθ

cosθ=( σSG– σSL)/ σLG

Where θ is the contact angle at the solid-liquid-gas three-phase boundary; σSG is the solid-gas interfacial tension; σLG is the liquid-gas interfacial tension; σSL is the solid-liquid interfacial tension.

When θ=0, the liquid droplet is completely flat on the solid surface, indicating that the solid surface is completely wetted by the liquid drop; when θ=180°, the liquid droplet is in the shape of a bead, which is an ideal non-wetting state; When 0>90°, it means that the solid has a water repellent effect. In water-repellent finishing, the surface tension (σLG) of the liquid can be regarded as a constant. Therefore, whether the liquid can wet the solid surface depends on the solid surface tension (σSG) and the liquid-solid interfacial tension (σSL). In terms of water repellency requirements, σSGSL should be a negative value, that is, θ>90°.

When the droplet presents a certain shape on the solid surface and shows a certain angle θ, it is used to express the characteristics of a certain liquid-solid interface. Here, the equilibrium condition must be emphasized, because the effective conclusion is derived from the contact angle at equilibrium.

The Young equation is derived from thermodynamics, and research shows that it is only applicable to the equilibrium state of an ideal system. The surface of the ideal system must be a solid plane that is smooth, uniform, impervious and deformable. The equilibrium contact angle in the ideal system is different from the contact angle measured in the actual system. The equilibrium contact angle in the actual system is not a single value. Generally speaking, the advancing contact angle is greater than the receding contact angle. This hysteresis is due to the adsorption of the solid to the liquid and the change in the surface energy of the solid, the unevenness of the surface or the roughness of the solid surface.

Table 7-1 shows the contact angle of water droplets on various untreated fibers.

The contact angle of various fibers with water

Table 7-1 The Contact Angle of Various Fibers with Water

It can be seen from Table 7-1 that different types of fibers have different contact angles. Among them, cotton and viscose fibers are customarily called hydrophilic fibers, which have a small contact angle with water, and synthetic fibers have a large contact angle with water, so they are called hydrophobic fibers. Among them, acrylic fibers have some exceptions. Among fibers, fibers with low hygroscopicity and swelling properties have larger contact angles. In addition, the larger contact angle of wool is related to the scaly layer structure on its surface. However, the θ of water on the surface of various fibers is less than 90°, so they can be partially wetted by water.

2.2 Water and oil repellent conditions

The liquid-solid interaction determines the wettability, which can be expressed by the adhesion work WA. The so-called adhesion work refers to the work required to separate the unit liquid-solid contact area. It is a characterization of the liquid-solid interface binding capacity and the interaction force between the two-phase molecules:

WASGLGSL

It shows that during this process, two new surfaces are appearing, the tensions of which are σSG and σLG respectively, and σSL no longer exists after tearing. But in fact, the adhesion work cannot be measured, and can only be calculated by σSG, σLG, and σSL:

WA= σLG(1+cosθ)

The above formula shows that the adhesion work is a function of the contact angle θ. If the value of θ is small, the WA value is large, that is, the solid is easily wetted by droplets; on the contrary, the solid has different degrees of anti-wetting properties.

Water repellent finishing and oil repellent finishing is to make the surface of the finished fabric not wet by water and oil, that is, increase its contact angle θ with water and oil, and reduce the adhesion work between them.

Huggins and his team applied the definitions of adhesion work and cohesion work to define the relationship between the “spreading coefficient” of a droplet on a solid and the interfacial tension:

S=WA-WC

In the formula: WC is the work of cohesion, which means the work required to separate the liquid column per unit area. When the work of adhesion is equal to the work of cohesion, the contact angle is zero and the liquid is completely spread on the solid surface. Because cosθ cannot exceed 1. When σSGLG, σSL=0:

WA=2σLG=WC

So:

S=σSGLGSL-2σLGSG-(σLGSL)

When S>0, the droplets will spread on the solid surface, that is, wetting or penetrating. When S<0, the droplets will not spread on the solid surface, that is, bead-like.

Because σSL<<σLG, it can be ignored. Therefore, if water or oil droplets are to be beaded on the solid surface, the surface tension of the solid σSG must be smaller than the surface tension σLG of the droplet.

The surface tension of a solid cannot be measured directly, it is generally measured indirectly by extrapolation. Ziman et al. measured the advancing contact angle of a group of liquid homologues on the surface of low-energy solids such as fluorocarbons and hydrocarbons, and used its cosθ value to act on the surface tension of the liquid, and extrapolated the obtained straight line to make the cosθ close to In 1, Ziman said that the corresponding surface tension is the critical surface tension σc of the solid plane (the interfacial tension of the solid determined by the extrapolation method), and only when the surface tension is lower than σc, the liquid can spread on the solid surface. The liquid with a surface tension higher than σc forms a certain contact angle on the solid.

Figure 7-2 shows the relationship between the cosθ value of n-alkane homologues on the surface of PTFE and its corresponding surface tension. If this straight line is extrapolated to cosθ=1 (that is, the contact angle is zero), the corresponding The surface tension value is about 18×10-5N/cm, and this value is the critical surface tension of the solid surface.

SURFACE TENSION

Figure 7-2 The Relationship Between cosθ and Surface Tension of N-alkane Homologues on Polytetrafluoroethylene

The surface tension of rainwater is 53mN/m, and the surface tension of general edible oil is 32~35mN/m. Therefore, to make the fabric water-repellent, the interfacial tension must be less than 53mN/m. To repel oil, the interfacial tension must be less than 32mN/m.

3 Water repellent and oil repellent finishing agents

3.1 Hydrophobic aliphatic hydrocarbon water repellent

3.1.1 Aluminum soap and zirconium soap:

One of the oldest water-repellent agents is based on the hydrolysis of aluminum acetate on the fiber to form basic aluminum acetate and hydroxides whose structure has not been determined. The disadvantage is poor adhesion and easy dusting. The improved method is to first apply water-soluble soap on the fabric, and then use aluminum salts such as aluminum acetate, aluminum formate or aluminum sulfate to form aluminum soap and deposit on the fabric. Although aluminum soap is insoluble in water, it can be dissolved in alkaline detergent solution. Therefore, the washing fastness of aluminum soap finishing products is poor, and the hydrophobicity and washing resistance of zirconium soap are better than aluminum soap. Therefore, using zirconium acetate or zirconium oxychloride instead of aluminum salt can effectively improve the durability of the finished product. Copper salt can be used as a water repellent, and it also has a bactericidal effect, which can prevent fabrics from decay and deterioration.

3.1.2 Wax and waxy substances:

The oldest and most economical water-repellent finishing method is to use hydrophobic substances, such as paraffin wax, on the surface of the fabric. Paraffin and waxy substances can be applied to fabrics in solid form, and then heated to make them into a molten state. Or apply in the form of organic solvent and emulsion. The paraffin wax emulsion of aluminum acetate and aluminum formate was once the most important water repellent finishing agent for cotton fabrics. Initially, aluminum acetate and paraffin were applied in a two-step method, and later changed to a one-step method. In addition, using protein such as animal glue or white gelatin as a protective colloid can increase the stability of the padding liquid; using aluminum soap together with an emulsion of aluminum acetate and paraffin wax can improve the water repellent effect; using aluminum acetate and trialuminum carbonate The ammonium salt of zirconium and zirconium oxychloride instead of aluminum acetate can enhance the dry cleaning and washing resistance of the finished product; the introduction of polymers, such as polyvinyl alcohol, polyethylene, and polyacrylate into the paraffin dispersion, can improve its stability And the durability of the finished product; the introduction of crosslinking agent can improve the durability of the finishing effect, and can improve the dimensional stability and wrinkle resistance of the cellulose fiber fabric.

3.1.3 Metal complex:

Quilon Werner type complex water repellent of Dupout Company is used in the form of isopropanol solution. Use this finishing agent on natural or synthetic fibers to obtain semi-durable water repellent finished products with good initial water repellency and soft hand feeling.

3.1.4 Pyridine compound:

Chlorinated stearamide methyl pyridine can be applied to the cellulose fiber fabric through a rolling-baking-baking process. Pyridine is released during the reaction and produces an unpleasant odor. Therefore, the finished fabric must be cleaned after baking. The joint application of pyridine water repellent and fluorine-containing water repellent produces a synergistic effect, so it produces a long-lasting rainproof effect and has good washing resistance. However, due to toxicity, its dosage has been reduced.

3.1.5 Hydroxymethyl compounds:

N-methylol chemicals used in the cross-linking finishing of cellulose fibers have also been successfully applied to the durable water-repellent finishing of cellulose fibers.

3.2 Organic silicon (polyoxane) water repellent

The organosilicon used for textile water repellent finishing is a polymer with silicon oxygen as the main chain. These polymers are called polysiloxanes. The substituent R in polysiloxanes can be hydrogen, hydroxyl, alkyl, or aromatic. The substituents on the polymer backbone can be the same or different. The substituents of organosilicon used in textile finishing are usually methyl or hydrogen.

Polydimethylsiloxane can form a flexible film on the surface of the fiber, giving the finished product a soft touch. In order for the finished product to achieve sufficient water repellency, a very high melting temperature is required unless a catalyst is used to reduce the reaction temperature. When polydimethylsiloxane is cross-linked on cotton fabric in the presence of organic peroxides, it can produce high water repellency and a soft hand.

The water repellency of the silicone-finished fabric is due to its fiber surface is covered with a polysiloxane film, the oxygen atoms point to the fiber surface, and the methyl group is aligned away from the fiber surface. When the silicone film is washed, for example, due to fiber swelling and damaged, although the polymer is not reduced, it may lose water repellency. Appropriate alignment of the silicone polymer on the fiber surface is also a necessary condition for water repellency. In order to improve the bonding performance of polysiloxane on the fiber surface, it is necessary to apply additives to achieve proper alignment and accelerate crosslinking.

The polysiloxane is padding, then dried, and baked at 120~150℃ for a few minutes or by padding process. Since the surface potential of most fabrics in water is negative, cationic surfactants should be used for the emulsified polysiloxane in the dip dyeing process. In the padding method or the dip dyeing method, about 1% to 2% polysiloxane is deposited on the fabric.

3.3 Fluorine-containing compound water and oil repellent

Some properties of fluorine-containing compound water and oil repellents are different from those of silicone and aliphatic hydrocarbon water repellents. The most important difference is oil repellency. Fluorine-containing compounds can repel both water and oil, while silicone and aliphatic hydrocarbon compounds only have a water repellent effect. The oil repellency of fluorocarbon compounds is related to their low surface energy.

The water and oil repellency of fluorocarbon compounds depends on the structure of the fluorocarbon segment and the non-fluorocarbon segment in the molecule, the orientation of the fluorocarbon segment end, the number and distribution of fluorocarbon groups on the fiber, and the fabric Composition and geometric shapes.

Most of the industrially used fluoropolymer water and oil repellent agents are acrylic or methacrylate side-branched fluoropolymers. Fluoropolymers are suitable for the water and oil repellent finishing of fabrics composed of synthetic fibers and natural fibers. In the finishing prescription of cellulose fiber fabrics, it contains crosslinking agents that improve the durability of fluoropolymer finishing, improve wrinkle resistance, and improve washability and durability. The commonly used crosslinkers are melamine, triazine or Denatured triazine, carbamate or glyoxal type compounds. Because fluoropolymers and silicones are different, they cannot impart softness to the fabric, so a softener must be added.

The water and oil repellent of fluoropolymer can be finished by padding method, spray method or exhausting dyeing method, usually by padding method. Dry at 120~180℃, and then bake at 150~180℃ for 1~3m. When used in the same bath with resin, it needs to be washed and dried at 150~175℃.

Conclusion

It’s a big professional overview about water repellent and oil repellent. Hope you will have some new thoughts after reading it.

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