Nonwoven fabrics can be described as fibrous webs that are connected together by adding chemicals or by combining some ingredient fibers, to fortify and integrate the final product. Fibres, yarns, rovings etc make these flat textile structures with or without a scrim cloth carrier.

The interfibre adhesiveness is composed by mechanical, chemical, thermal, or solvent means or by various mixtures of these procedures. Table 1 mentions some of the modes that can be employed for blending nonwovens.

                                                                                                            Table 1: Modes of Blending Nonwovens

Nonwovens are being used at the rate of 5-6 percent per year. Clothing, automotive, filtration, geotextiles, roofing, hygiene and home textiles are the chief industries that use nonwovens the most. These fabrics are multitalented and able to get them mixed with a wide range of materials and produce hybrid materials; this is the main feature of these fabrics that makes them popular.

After the production of nonwoven textile, it has to be connected with itself or to another substance. For instance, nonwovens are connected with cloth, leather, and other films for clothes. The potential mixtures and the final product properties are enormously huge and include many industries. Table 2 gives a gist of some of the creative hybrid mixtures that can be produced by these methods:

                                                                                                     Table 2: Selected Hybrid Nonwoven Products

The adhesive (or binder) performs a crucial part in manufacturing nonwoven products and decides the final fabric properties. The mechanical functioning of the nonwoven product is not possible without the contribution of location of the binder, its concentration, and its adhesive qualities. The adhesive may be inserted into the web after it is produced or incorporated into the web as thermoplastic fibers.

This article, nevertheless, will deal with the use of paste dot for the connection of nonwovens to themselves and to other substrates. This connection process is mentioned as paste dot process, because the adhesive is applied in the form of dots, which can later be laminated with all types of textile materials by giving heat and pressure. Generally, a glue-like thermoplastic adhesive diffusion in water is used for the connection of nonwoven fabrics to itself or to other substances. These powders are generally dissolved in water and can be appropriately applied by various processes in a predefined sample and compactness.

Another procedure also exists known as powder dot process wherein the substrate is heated by means of a hot roller and is at the same time pressed against a carved roller containing a thermoplastic powder. The powder is shifted to a particular model onto the substrate by latter roller. The powder dot covering system is considered more non-flexible than paste dot covering system. This system i.e. paste dot covering, also offers least invasion (strike-through) and more control in consistency of length and width of application. This process has all the qualities of greater accuracy in the weight, shape and design of the dot.

Paste dot coverings have countless common applications. Though nonwovens will remain the heart of the article, these coverings can be applicable to all types of textile products. The paste dot mechanism works well in the company of lightweight substrates like nonwoven interlinings, which are not able to tolerate high temperature. These end-uses are:

• Fusible interlinings for woven, knitted, and nonwoven fabrics

• Fusible dot coverings on breathable membranes (e.g., Goretex, Sympatex, Helsapor)

• Printbonding on nonwovens

• Anti-slip properties for automotive fabrics, tablecloth, etc.

    To meet diverse usages, substrates, or property needs, paste dot adhesive dispersal can be expressed. The base polymer is generally a low VOC polymer like a copolyamide or copolyester. With large number of additives like plasticizers, thickeners, dispersing materials etc, formulation can be accomplished. This scope in formulation can afford adhesives good defiance to strike off, and low temperature-fusing capabilities. These adhesives have exquisite bonding features to hydrophobic substrates.

The Paste Dot Bonding Process

The paste dot process is working on the principle that thermofusible pastes are applied directly onto the substrate by means of a rotary coater. The paste is pumped into a rotary screen and applied to the substrate by means of a squeegee. The treated substrate is then led through a drying tunnel to remove the water and any other volatile products.

Achievement of the true spot bonding is the indisputable benefit of applying thermoplastic powders as adhesives. Selecting the size of particle and modes of applications, one can deposit discrete islands of binder; these discrete islands of binder will have the least preventive effect on fibre mobility. Consequently, fabrics with good strength and very soft handle can be manufactured. In this form of bonding, much of the adhesion is due to mechanical interlocking of fibers rather than to chemical bonding.

Although there are many processes by which the paste dot adhesive can be applied, rotary screen-printing will be described here because it is a popular method. It has surpassed the dry powder processes and ranks first among powder applications with about 60% of the total volume.

In the rotary screen-printing process, an aqueous suspension of finely divided thermoplastic powder adhesives and additives (the paste) is pressed through the holes of a rotating, perforated cylinder (the screen stencil) onto a cold web of fabric. This procedure is gentle to the substrates, and the wide range of options for formulating the paste gives the user flexibility in the application procedure. The rotary screen-printing process is characterized by high and economical coating rates ranging between 30 and 60 m/min. In certain cases, rates of up to 90 m/min can be achieved.

The aqueous adhesive dispersion is pumped through a hollow doctor blade into the interior of the rotating screen stencil. The viscosity of the paste can be adjusted to allow stencils of either course or fine screens to be used depending on the application and type of fabric. The internal adjustable doctor blade presses the paste through the holes of the stencil and onto the web of fabric, which is running over a counter-roller coated with hard or soft rubber. The paste dots are then dried, and circulating air or infrared radiations are next used to sinter the textile web.

Several manufacturers specialize in this type of applicator. Schaetti AG, Switzerland, for example, supplies complete coating and laminating systems based on thermofusible adhesives. Their equipment can be used to apply paste dots but can also be designed for other processes such as powderdot, doubledot, and scatter coating (see Table 3).

                                             Table 3: Processes Similar to Paste Dot that Employ Thermoplastic Powder to Bond Nonwovens

Formulation

Base resins in paste dot adhesive can be almost any thermoplastic. Frequently, polyamide or polyester copolymers are used for garment applications where wash-and-wear properties are required. Even dispersions of polyethylene in paraffin waxes are used to provide good flow properties and low melting points. Other candidates include copolymers of ethylene with either acrylic acid or vinyl acetate.

The melting point of the paste dot adhesive is a significant parameter in this bonding process and will depend on the type, heat resistance, and ruggedness of the substrates. The base thermoplastics that are used in paste dot systems can have widely varying melting points. Further adjustments in melting point of the paste dots can be made by additives. The paste has two purposes: (1) it functions as a carrier for the thermoplastic and (2) it can alter the melting range.

The primary components for a paste dot adhesive are the thermoplastic powder and the water matrix. Water is an inexpensive carrier, and it emits no volatile organic chemicals. The boiling point of water is generally close to the melting point of most hot melt powders. Additives, other than the water, that change the character of the paste dot include:

• Thickeners

• Dispersing and wetting agents

• Plasticizers

• Processing aids (running conditioners)

• Solvents and other viscosity-influencing substances.

Hot Melt Thermoplastic

The various hot melt thermoplastics that can be used in formulating paste dot adhesives are described in Table 4. Paste dot powders are generally ground to less than a 0-80 micron range, although slightly coarser powders can be used for special applications. The various types of thermoplastics have different melting points, and blends are used as well for even greater variation and range in melting point and melt viscosity.

                                                                  Table 4: Common Thermoplastic Powders Used in Paste Dot Adhesive Formulations

Thickeners

Thickeners are used for controlling the pastes' viscosity during application and reducing the tendency of the powder to separate from the water matrix. Thickeners are generally high molecular weight polymers that become water swollen gels. Most thickeners provide thixotropic characteristics so that the viscosity is shear rate dependent.

                                                                   Table 5: Characteristics of Common Thickeners Used in Paste Dot Adhesives

Common thickeners used in paste dot formulation include acrylates, cellulosic derivatives (cellulose esters), polyurethanes, and mineral systems such as silicates. The characteristics of these thickeners are summarized in Table 5. Thixotropy (the dependence of viscosity on shear rate) is an important characteristic for paste dot adhesives. The paste viscosity must be low enough to easily exit the screen holes on application, but must increase immediately after exit to avoid excessive penetration of the substrate or "strike-through". The shear rate depends not only on running speed but also on the size of the blade and its angle to the print screen.
Dispersing and Wetting Agent

Dispersing agents ensure that powder is uniformly distributed in the water. Ethoxylated fatty alcohols are preferred for their low fogging properties. Those with 3-5 units of ethylene oxide also disperse hydrophobic powders. The salts of stearic acid and palmitic acid also are recommended.

Wetting agents are used to provide better wetting of hydrophobic surfaces and to improve adhesion to the substrate. Reduction of the surface tension will result in saturation of hydrophobic substrate fibers and improved adhesion. One commonly used wetting agent in paste dot adhesives is ethoxylated polysiloxane. This is generally used at a concentration of 0.5-1.0 percent by weight.

Plasticizers

Plasticizers are used to reduce the melt temperature and melt viscosity of the paste dot adhesive and improve the adhesive strength of the joint. Only amorphous polymers, such as polyamides, are easy to plasticize. Polyesters, which have medium crystallinity, are difficult to plasticize; highly crystalline polymers, such as polyethylene, cannot be pasticized.

Both solid and liquid plasticizers are used in paste dot adhesive formulations. Plasticizers should have low diffusion rates if they are being used to improve joint adhesion (flexibility of the base polymer). The rate of plasticizer diffusion out of the paste dot formulation is less important if only melt temperature and viscosity are of primary concern.

Para-toluenesulfonamide, a solid plasticizer with a melt point of about 140⁰C, is often used with polyamide hot melt powders. These plasticizers tend to have very low diffusion rates. Other suitable plasticizers for polyamides are: dicyandiamide, urea, hydroxybenzoic acid (either substituted or not substituted), and phenoldicarbonic acids. Dioctylphthalate can be used as a plasticizer for polyamide or polyesters; however, they tend to diffuse out of the coating.

Processing Aids

Processing aids or running conditioners are used to prevent clogging of the screen after the paste dot coating line is run for a time. These also help to control the dot geometry and precision. A solution of high molecular weight polyethylene oxide (1% in water) is generally employed. This will reduce drying speed, keeping the paste moist during coating.

There are several viscosity grades of polyethylene oxide useful for paste dot application. High viscosity grades are used for screen with large holes, and the lower viscosity grades are used for screens with small holes. The preparation of polyethylene oxide for addition to the paste dot formulation is somewhat tricky.

Other Additives

Various auxiliary agents may be used as additives in the paste formulation for specific properties. These include the following:

• Anti-blocking agents: Used to reduce adhesion.

• Antioxidants: Not used in the paste dot formulation, but generally added to the thermoplastic formulation to prevent thermal degradation.

• Antistatic agents: Organic quaternary phosphate compounds or specific organic sulphates are commonly used.

• Antifoaming agents: Generally a non-silicone antifoaming agent is used.

• Lubricants: Used as a processing agent to control the speed through the screen. The lubricant prevents particle build-up on the surface of the screen and machine parts.

• Solvents: Not typically used because of environmental regulations and potential to cause clogging.

Paste Production

Table 6 shows typical paste formulation for a polyamide based and a polyester based paste dot adhesive. The optimal paste viscosity will depend on the screen being used in the coating process. A viscosity of 3000-8000 mPa-s is recommended for low mesh screens. High mesh screens require lower viscosities (1500-3000 mPa-s).

                                                Table 6: Typical Paste Dot Adhesive Formulations for Polyamide and Polyester Based Systems

Suppliers:
(1) - Stockhausen
(2) - Bohme
(3) - Th. Goldschmidt
(4) - Degussa AG

The paste should be prepared by adding the dispersing agents and anti-foaming agents to the water while stirring at 500 rpm. Stirring is then continued at the same speed while adding the adhesive powder. The plasticizers and wetting agents are then added while continuing the stirring. The thickeners are then added in portions until the desired viscosity is achieved. The stirring speed is then reduced to less than 500 rpm, and finally the polyethylene oxide is incorporated into the formulation. Stirring is continued for 30 mins or until the paste is homogeneous.