Presented by Shrikant Athavale On 30 th Oct 2010 Hot Melt Adhesives and Applications
The name itself suggests these type of adhesives. A adhesive which melts on application of heat is called as Hot Melt Adhesive Hot melt adhesives are thermoplastics, based on polymers that become liquid between temperatures of 80 – 220 °C and solidify again by cooling down. They consist of 100 % dry substance and are applied in liquid state without using water or solvents. With respective pre-melt systems, pumps and application units (slot die, hot roller and powder scattering) an exact dosage and adjustment is possible. Due to the process only a short binding and setting time is requested in comparison with dispersions or solutions.
1. Thermoplastic Rubber (Block Copolymers) a) Styrene Isoprene Styrene b) Styrene Butadiene Styrene 2. Poly amide 3. Poly acrylate 4. Ethylene Vinyl Acetate Copolymer 5. Poly ethylene etc. 6. Poly Urethane POLYMERS USED Hot melts can basically be divided between different adhesives depending on the working temperature and the chemical composition like. The main differences are regarding properties and handling, which determinate the final application.
Hot melts can be further divided into <ul><li>Pressure Sensitive </li></ul><ul><ul><ul><li>Mainly used for the manufacture of self adhesive tapes, labels and stickers </li></ul></ul></ul><ul><li>Non pressure sensitive </li></ul><ul><ul><ul><li>Mainly used for the manufacture of specialty laminates, barrier coatings , heat seal applications </li></ul></ul></ul><ul><li>sealants </li></ul><ul><ul><ul><li>Mainly used for sealing applications ,filling the gaps, and other industrial assembly jobs </li></ul></ul></ul>
<ul><li>have excellent adhesion properties to a wide range of substrates. </li></ul><ul><li>processing machine is very compact and space saving with great degree of automation. </li></ul><ul><li>no fire hazards, as no solvent is used for coating process. </li></ul><ul><li>very high production levels. </li></ul><ul><li>environmentally friendly due to water and solvent-free </li></ul><ul><li>easier to adjust adhesive viscosity by temperature variation. </li></ul><ul><li>relatively easy cleaning operations. </li></ul><ul><li>lower Electrical installation cost. </li></ul>Advantages of HMA
<ul><li>change of adhesive type / cleaning of pre-melt system, </li></ul><ul><li>hoses, die, etc. </li></ul><ul><li>limited application due to low softening point of thermoplastics </li></ul><ul><li>3. pot life (limited time of coating in case of reactive systems) </li></ul><ul><li>4. setting time (time between coating and bonding effect = storage) </li></ul>dis Advantages of HMA
<ul><li>Hot melt adhesives are available ready to use from number of MNC’s , locally as well as from abroad </li></ul><ul><li>However these adhesives can be manufactured in house also by compounding </li></ul>Availability of HMA
<ul><li>The main Raw materials that are used for the manufacture of </li></ul><ul><li>Hot Melts are </li></ul><ul><ul><li>The Thermoplastic polymer </li></ul></ul><ul><ul><li>The Tackifing / Reinforcing Resins </li></ul></ul><ul><ul><li>Plasticizers </li></ul></ul><ul><ul><li>Antixidants </li></ul></ul>COMPOUNDING OF HMA
The Thermoplastic Rubber / polymer is heated , in a closed vessel/ reactor , equipped with uniform heat all around , to temperatures , above glass transition temperature of end block i.e. above 90 deg C . This unlocks the physically Cross Linked polymer or Rubber network. Domains soften at temperature above 90 deg c but do not melt or flow and the polymer remains viscous. Mechanical Energy is applied ( that is a slow speed stirrer is used for mixing ,along with the addition of Resins and plasticizers , to reduce the viscosity and help the processibility. 120 deg C to 180 deg C, is normal temperature range for mixing/compounding of these adhesives. COMPOUNDING OF HMA
COMPOUNDING OF HMA Temperature above 180 deg C will lead to excessive oxidation and above 220 deg C to thermal degradation. Therefore at this stage the antioxidant is added to protect the adhesive against oxidation. Since oxidation degradation risk is lower at lower temperature the preferred mixing method is high shear / low speed equipment at 135 deg C to 160 deg C temperature. As a further step of precaution to minimize oxidation , the mixing is done under Nitrogen gas blanket , by replacing the atmosphere air inside the reaction vessel by Nitrogen gas , which is a inert gas. As we all know the air contains oxygen and by replacing this air the oxygen supply is cut off , which helps to a great extent in minimizing oxidation risk.
Hot melt application machines <ul><li>Three Roller applicator system </li></ul><ul><li>Slot die applicator system </li></ul><ul><li>Extrusion coating system </li></ul><ul><li>Application guns </li></ul>
<ul><li>a metering roller, </li></ul><ul><ul><li>a applicator roller and </li></ul></ul><ul><ul><li>a dipping / pickup roller. </li></ul></ul><ul><li>for constant low coating wt., the applicator roller can be an engraved roller. </li></ul><ul><li>hot applicator roller system is good for coated substrate with open or closed surface. </li></ul><ul><li>a wide range of coating wt. can be reached. </li></ul><ul><li>The coating wt is determined by the gap between </li></ul><ul><ul><li>the dipping roller and applicator roller </li></ul></ul><ul><ul><li>applicator roller and counter roller </li></ul></ul><ul><ul><li>Or </li></ul></ul><ul><ul><li>the variable line speed </li></ul></ul>The hot applicator roller system bases on a standard three roller applicator system.
The integration of a lamination station a second substrate can be laminated. Working with thermoplastic hot-melts it is important that the lamination point is very close to the coating point, because a cooling down of the hot melt should be prevented due to viscosity changes in order to ensure a good and sufficient penetration into the fabric.
The slot die system is suitable to coat continuously hot melts with a wide range of viscosity. In combination with the right “mask”, which determines the coating weight and width, a closed film > 10 g/m² or a porous film < 10 g/m² can be coated. The thermoplastic polymer is heated via extruder / tank melting / barrel melting equipment and pumped to the coating head. In the die itself has, depending on the working width, up to four pneumatic operated valves, which supplies the hot melt from the tank. In addition there are distributors and ducts to spread the adhesive to the die outlet equally over the width. The slot die system
The position of the slot die to the counter roller is flexible. A manual/motoric adjustment enables the movement of the die in horizontal and vertical position as well as angle adjustment. The main advantage of this “closed system” is, that the hot melt is protected against any contact with oxygen and air humidity as well as temperature lost from the pre-melt system to the application point. So the hot melt has no chance to crack or to react if they would get in contact with air humidity, which is excluded by using the slot die system. The coating weight is determined by the pump, web speed and distance of die to substrate or web tension. With this process it is important, that the substrate has a certain stability, because of the shear rate between Because of the closed system, this coating method via slot die is most suitable to high speed coating of hot melts.
Slot Die section view displaying internal manifold Transfer flow of slot-bead coating Slot-bead Coating with applied vacuum.
Application Guns that can melt polyester and polyamide like an extruder, but without the disadvantages of such. The application by a PA-PET melter can be controlled, even used for intermittend application. There is no burning and polluting of the glue and purchase costs are really low. Application Guns
Application guns There is a choice of different application guns, for example pneumatic guns, circulating and uncirculating, coating applicators with and without metering pumps. They are used for Dots
The thermoplastic coating material as powder can be applied with a powder scattering device equally over the working width. After that, the powder needs to be melted, cooled down and/or smoothed. For the application the powder is supplied in a pre-dosing unit and picked up by an engraved dosing roller. In front of the dosing roller there is either an oscillating brush bar or a rotating brush roller, which clear the dosing roller so that the powder falls onto the substrate. The engravure of the dosing roller and its rotation speed determines the amount of powder application. The advantage of this system is the flexible working width up to 5 m. Curtain coating system
<ul><li>Some very interesting applications where this technology can be used: </li></ul><ul><ul><li>Flexible packaging </li></ul></ul><ul><ul><li>Coating for graphic industry </li></ul></ul><ul><ul><li>Labelling </li></ul></ul><ul><ul><li>Coating for paper, film and textile </li></ul></ul><ul><li>Nevertheless for every coating and laminating process whether contact or curtain you’ll find the slot die that can continuously be adjusted either manually or computerized and watched on a display. </li></ul><ul><li>It can even been integrated in the whole machine process. </li></ul><ul><li>Thus the slot die utilizes the production of different width without changing the die at all. </li></ul><ul><li>Price advantage coming from time saving and glue saving as well as quality advantage is a win-win situation. </li></ul><ul><li>I would like to show you some interesting coating units that we realized in 2002 and 2003: </li></ul>
You well know that glue application in contact has some disadvantages when producing even films whether very thin or with higher application weight. The surface of the material to be coated might be very irregluar with peaks and valleys. The plate of the coater, however, is straight and accurate. The glue therefore can only fill the valleys. Thus the thickness of the applied glue is not smooth. In order to have enough glue on the peaks there is only one possibility, namely put more glue onto the substrate.
In Curtain Coating the glue film falls out of the die without having any contact. It just follows the profile of the substrate to be coated with alway the same distance to it. Thus the glue film shows the same thickness allover the application width and length – no peaks, no valleys, no weak points. You’ll also save a lot of money as you only use the amount of glue that is really needed. As there is no contact minor abrasion and other pollutions lying on the substrate remain on the same spot and are not transported through the machine thus not wasting the whole material as the die is not pushing the dirt forward. The glue encloses the dirt and just a little spot can be seen. There is no fringing.
Films and Mats Hot melt adhesives can be applied as free standing films (Figure 1) and mats. There are generally two types of hot melt adhesive films: unsupported and supported. If the adhesive film itself has sufficient cohesive strength to be handled as a free film then it can be used without reinforcement or support. If it has low cohesive strength or is used as a very thin film then it must be supported. The “support” is often a reinforcing textile web or fabric. Figure 1: Hot melt adhesive can be made into film and placed on a release liner Unsupported film of pure adhesive is cast onto release paper, usually in thickness of 0.05 to 0.08 mm. This adhesive form is used for bonding thin, lightweight, flat materials and parts that conform to the substrate to which they are bonded. Uses include attaching small components to printed circuit boards, bonding gaskets in place, and attaching nameplates and instruction panels to various types of equipment. These films can also be used as a protective film to cover scratch-susceptible components during production, transportation, or handling.
Hot Melt Laminating Adhesives Hot melt laminating adhesives can either be in film form as described above, or they can be applied to a substrate as either a solution or extruded coating. Once applied to a substrate the adhesive is activated by mating with another substrate and applying heat and pressure (e.g., via a hot nip rolling operation). The temperature and pressure are sufficient to cause the adhesive to flow and create an instantaneous bond when it cools and gels. Hot melt adhesives used in the laminating process are 100% solid polyester, polyamides, ethylene vinyl acetates (EVAs), polypropylene, and urethane adhesives. For many applications, such as flexible packaging, the use of a single material may not satisfy all of the properties demanded of the product. In these cases, a composite consisting of two or more layers of material may provide the desired performance. A particularly common means of creating such a composite is to laminate various polymeric films to other films, foils, papers, etc. with a hot melt adhesive. Multilayer polymeric films have become valuable packaging materials from applications ranging from food preservation to pharmaceuticals. Along with the recognition of the importance of multi-layer films was the recognition that an effective adhesive system would be required in their manufacture. As most of the polymers are incompatible, an adhesive layer (also called a tie layer) is needed to obtain sufficient cohesion. Figure 2 shows an example of a generalized multi-layer structure. The use of multi-layer materials is not limited to packaging. Industrial applications such as fuel tanks, pipes, and electrical cables are using adhesive bonded layers to guarantee performance properties.
Figure 2: Generalized design of a multilayer laminate Current laminate adhesives include acrylic polymers and copolymers, polyesters, vinyl acetate - vinyl chloride copolymers, waterborne polyvinylidene chloride, polyolefin copolymers, and polyurethanes. Coextrusion is gaining popularity for manufacturing multi-layer films because of its lower overall cost, single-step processing, and environmental benefits. The materials best suited for coextrusion are low-density polyethylene, polypropylene, polyvinyl chloride, polyamides, and polystyrene. The coextrusion of polyethylene and ethylene vinyl acetates produces an especially low gas permeation rate required for many health care packaging applications. Sometimes an adhesive may not be required in the coextrusion process. However, usually a tie-layer is employed because of incompatibility between polymeric films.
Hot Melt Sealants Hot melt sealants are a case where there has been a continuing evolution of both technology and market demands. Hot melt sealants have been available for decades, but until recent years the markets had been rather limited because of the well known deficits with thermoplastic hot melt adhesives (sag at elevated temperatures, equipment costs, low bond strength to certain substrates, etc.) However, developments over the last several years indicate that these materials will be seeing much larger, growing markets. One of the primary developments in hot melt sealants has been the development of product in continuous rope form. In this form the hot melt can be extruded through a heated gun and applied in an outdoor environment. The immediate set time of the sealant eliminates problems due to traffic (e.g., pedestrian or automotive travel over road seals) or environment (e.g., rain or dust).
Foam able Hot Melt Adhesive and Sealants Perhaps one of the most unique forms that a hot melt adhesive can take is as a cellular material. Foamable hot melt adhesives have been available since the 1980s. These adhesives use nitrogen or carbon dioxide gas to increase the volume of the adhesive by 20-70% as it is applied to the substrate. Processing equipment mixes the base polymer material with an inert gas that expands during dispensing, thereby forming a resilient adhesive or sealant. Because this process has no effect on the chemical properties of an adhesive, it works well with hot melt formulations of many types. The foaming operation increases hot melt open time and provides for good gap filling properties. The elastic foam also tends to relieve stresses that might develop internally within the joint. Foamable hot melt formulations have been used as both adhesives and sealants. Foamable hot melt adhesives can be used as formed-in-place gaskets as well as in many general purpose bonding applications. The typical benefits of foamed hot melt adhesives are shown in Table 3, but as thermoplastics, their service temperature range is limited.
Foamable hot melt adhesives are often preferred over standard hot melt adhesives because of their longer open times, stronger bond strengths, and lower material cost. These adhesives also produce less thermal distortion and can be used on many heat sensitive materials. Recently, these adhesives are enjoying success in several industries. In the construction industry, for example, foamable hot melt adhesives are used to join standing-seam metal roofs, porous particleboard cabinet walls, and attachment of insulation board. They are also commonly used for bookbinding and the production of filters and packaging. Equipment and materials have also been developed that produce closed cell foam gaskets from a range of hot melt applied materials. Foam-in-place gaskets are more economical than conventional gasketing and provide more consistent quality and higher production rates. These foam-in-place gaskets are replacing many die-cut gaskets due to their lower costs (associated with waste, inventory, and labor) and more consistent quality. While foam-in-place gasketing systems can handle many different sealing geometries, the most common configurations consist of a free standing exposed bead sandwiched between flat surfaces (Figure 3, left) or one that is dispensed into a groove with a tongue on the mating part (Figure 3, right). An application of a hot melt sealant being applied to an engine mounting is illustrated in Figure 4.
Figure 3: Application of a hot melt sealant to an engine mounting
<ul><li>Non-Reactive Hot Melt Sealants </li></ul><ul><li>Traditional hot melt sealants are formulated very much like their adhesive counterparts. However, since they are used as a sealant, their elongation or movement capability and their resistance to fluids, gases and so forth must be significantly greater. As a result not all hot melt adhesive systems can be used as sealants. </li></ul><ul><li>Traditional hot melt sealants were limited to operating environments of 100°C or less. Backbone base polymers used in hot melt sealant formulations include butyl rubber, EVA and its copolymers, LDPE, atactic polypropylene, polyamides, polyesters, and styrene block copolymers. Of these the butyl and styrene block copolymers have found the greatest acceptance in hot melt sealant formulations. </li></ul><ul><li>In the formulation of hot melt sealants, tackifying resin contribute wetting and tack, and wax lowers the melt viscosity and controls the setting speed. </li></ul><ul><li>A typical hot melt sealant is composed of three primary components: </li></ul><ul><ul><li>Polymers (30-40 weight percent) </li></ul></ul><ul><ul><li>Tackifying resin (30-40 weight percent) </li></ul></ul><ul><ul><li>Petroleum wax (20-30 weight percent). </li></ul></ul><ul><li>Antioxidants, fillers, plasticizers, and blowing agents can also be used to enhance other properties. </li></ul>
Reactive Hot Melt Sealants A reactive hot melt adhesive is any thermoplastic sealant that can be applied at elevated temperatures as a liquid melt, cools to become a solid at room temperature, and then subsequently reacts to become a thermosetting polymer with enhanced physical properties. Hot melt sealants can be made to be reactive in several ways. Reactive hot melt adhesives and sealants have been available since the 1980s. Due to their thermoplastic nature during application, they have many of the desirable processing characteristics of conventional hot melts, such as no solvents present, no mixing requirements, and immediate green strength. Although conventional, nonreactive hot melts (e.g., ethylene vinyl acetate, polyalphaolefin, polyester, and polyamide) are widely used in many industrial applications, they have certain performance limitations, such as poor heat resistance, water or solvent permeation, and creep. These limitations generally prevent their use in many critical or structural sealing applications. For certain applications requiring unusually high bond strength and fast set time, reactive polyurethane hot melt sealants have been developed6,7. More recently reactive silicone hot melt adhesives and sealants have also been introduced8. These systems are applied like a hot melt, and the parts can be rapidly assembled within seconds. As a hot melt, this applied formulation has a high degree of initial handling strength.
One of the major advantages of a hot melt adhesive (being able to be applied as a molten liquid) is also one of its disadvantages. The heat required can cause oxidation of the adhesive before the bond is made. Excessive oxidation can result in short pot life, discoloration, viscosity changes, char formation, and loss of adhesion. Hot melt adhesives are primarily made from thermoplastic polymers including ethylene vinyl acetate (EVA), block copolymers such as styrene butadiene styrene (SBS) or styrene isoprene styrene (SIS), and polyolefins. Of these, EVA is the most popular due to their high versatility and low cost. Unfortunately, these same resins are prone to oxidation, and this limits the amount of time that a hot melt adhesive can be held in the molten state. This article will review the mechanism of oxidation in hot melt adhesives and their effect on both application and performance properties. Resolutions to this problem will also be identified. These primarily encompass: (1) the use of proper adhesive formulations (e.g., with antioxidants), (2) adhesive compounding methods, and (3) adjustments to the end-users' application processes. Heat Stability of Hot melts
The Mechanism of Oxidation Methods of Reducing Oxidation in Hot Melt Adhesive Systems Oxidation can occur at all stages of an adhesive's life from synthesis to final end-use. It is usually recognized at high processing temperatures such as during mixing, compounding, or extrusion (in the case of hot melt adhesives). However, oxidation can also occur at relatively low temperatures including ambient storage and also on exposure to UV light
<ul><ul><li>In extrusion coating, resin is melted and formed into thin hot film. The MP50 measures the extruded film as it is coated onto a moving, flat substrate such as paper, paperboard, metal foil, or plastic film. The coated substrate then passes between a set of counter rotating rolls, which press the coating onto the substrate to ensure complete contact and adhesion. The extrusion coated layer may protect a printed substrate or act as a moisture barrier. </li></ul></ul>extrusion coating