Hot-melt adhesive

Glue applied by heating

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Hot-melt adhesive (HMA), also known as hot glue, is a form of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters designed to be applied using a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, which the user pushes through the gun either with a mechanical trigger mechanism on the gun, or with direct finger pressure. The glue squeezed out of the heated nozzle is initially hot enough to burn and even blister skin. The glue is sticky when hot, and solidifies in a few seconds to one minute. Hot-melt adhesives can also be applied by dipping or spraying, and are popular with hobbyists and crafters both for affixing and as an inexpensive alternative to resin casting.

In industrial use, hot-melt adhesives provide several advantages over solvent-based adhesives. Volatile organic compounds are reduced or eliminated, and the drying or curing step is eliminated. Hot-melt adhesives have a long shelf life and usually can be disposed of without special precautions. Some of the disadvantages involve thermal load of the substrate, limiting use to substrates not sensitive to higher temperatures, and loss of bond strength at higher temperatures, up to complete melting of the adhesive. Loss of bond strength can be reduced by using a reactive adhesive that after solidifying undergoes further curing, whether by moisture (e.g., reactive urethanes and silicones), or ultraviolet radiation. Some HMAs may not be resistant to chemical attacks and weathering.[citation needed] HMAs do not lose thickness during solidifying, whereas solvent-based adhesives may lose up to 50–70% of layer thickness during drying.[1]

Properties

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Melt viscosity

One of the most noticeable properties. Influences the spread of applied adhesive, and the wetting of the surfaces. Temperature-dependent, higher temperature lowers viscosity.

Melt flow index

A value roughly inversely proportional to the molecular weight of the base polymer. High melt flow index adhesives are easy to apply but have poor mechanical properties due to shorter polymer chains. Low melt flow index adhesives have better properties but are more difficult to apply.

Pot life stability

The degree of stability in molten state, the tendency to decompose and char. Important for industrial processing where the adhesive is molten for prolonged periods before deposition.

Bond-formation temperature

Minimum temperature below which sufficient wetting of substrates does not occur.[2]

General terms

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Open time

The working time to make a bond, where the surface still retains sufficient tack, can range from seconds for fast-setting HMAs to infinity for pressure-sensitive adhesives.

Set time

Time to form a bond of acceptable strength.

Tack

The degree of surface stickiness of the adhesive; influences the strength of the bond between wetted surfaces.

Surface energy

Influences wetting of different kind of surfaces.

Materials used

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Hot-melt glues usually consist of one base material with various additives. The composition is usually formulated to have a glass transition temperature (onset of brittleness) below the lowest service temperature and a suitably high melt temperature as well. The degree of crystallization should be as high as possible but within limits of allowed shrinkage. The melt viscosity and the crystallization rate (and corresponding open time) can be tailored for the application. Faster crystallization rate usually implies higher bond strength. To reach the properties of semicrystalline polymers, amorphous polymers would require molecular weights too high and, therefore, unreasonably high melt viscosity; the use of amorphous polymers in hot-melt adhesives is usually only as modifiers. Some polymers can form hydrogen bonds between their chains, forming pseudo-cross-links which strengthen the polymer.[3]

The natures of the polymer and the additives used to increase tackiness (called tackifiers) influence the nature of mutual molecular interaction and interaction with the substrate. In one common system, EVA is used as the main polymer, with terpene-phenol resin (TPR) as the tackifier. The two components display acid-base interactions between the carbonyl groups of vinyl acetate and hydroxyl groups of TPR, complexes are formed between phenolic rings of TPR and hydroxyl groups on the surface of aluminium substrates, and interactions between carbonyl groups and silanol groups on surfaces of glass substrates are formed.[4] Polar groups, hydroxyls and amine groups can form acid-base and hydrogen bonds with polar groups on substrates like paper or wood or natural fibers. Nonpolar polyolefin chains interact well with nonpolar substrates. Good wetting of the substrate is essential for forming a satisfying bond between the adhesive and the substrate. More polar compositions tend to have better adhesion due to their higher surface energy. Amorphous adhesives deform easily, tending to dissipate most of mechanical strain within their structure, passing only small loads on the adhesive-substrate interface; even a relatively weak nonpolar-nonpolar surface interaction can form a fairly strong bond prone primarily to a cohesive failure. The distribution of molecular weights and degree of crystallinity influences the width of melting temperature range. Polymers with crystalline nature tend to be more rigid and have higher cohesive strength than the corresponding amorphous ones, but also transfer more strain to the adhesive-substrate interface. Higher molecular weight of the polymer chains provides higher tensile strength and heat resistance. Presence of unsaturated bonds makes the adhesive more susceptible to autoxidation and UV degradation and necessitates use of antioxidants and stabilizers.

The adhesives are usually clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions are also made and even versions with glittery sparkles.[5] Materials containing polar groups, aromatic systems, and double and triple bonds tend to appear darker than non-polar fully saturated substances; when a water-clear appearance is desired, suitable polymers and additives, e.g. hydrogenated tackifying resins, have to be used.[6]

Increase of bond strength and service temperature can be achieved by formation of cross-links in the polymer after solidification. This can be achieved by using polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), exposure to ultraviolet radiation, electron irradiation, or by other methods.

Resistance to water and solvents is critical in some applications. For example, in textile industry, resistance to dry cleaning solvents may be required. Permeability to gases and water vapor may or may not be desirable. Non-toxicity of both the base materials and additives and absence of odors is important for food packaging.

Mass-consumption disposable products such as diapers necessitate development of biodegradable HMAs. Research is being performed on e.g., lactic acid polyesters,[7] polycaprolactone with soy protein,[8] etc.

Some of the possible base materials of hot-melt adhesives include the following:

The usual additives include the following:

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Fugitive glues and pressure-sensitive adhesives are available in hot-melt form. With a tack-like consistency, PSA are bonded through the application of pressure at room temperature.[40]

Additives and polymers containing unsaturated bonds are highly prone to autoxidation. Examples include rosin-based additives. Antioxidants can be used for suppressing this aging mechanism.

Addition of ferromagnetic particles, hygroscopic water-retaining materials, or other materials can yield a hot-melt adhesive which can be activated by microwave heating.[41]

Addition of electrically conductive particles can yield conductive hot-melt formulations.[42]

Applications

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Hot-melt adhesives are as numerous as they are versatile. In general, hot melts are applied by extruding, rolling or spraying, and the high melt viscosity makes them ideal for porous and permeable substrates.[43] HMA are capable of bonding an array of different substrates including: rubbers, ceramics, metals, plastics, glass and wood.[40]

Today, HMA (hot-melt adhesives) are available in a variety of different types, allowing for use in a wide range of applications across several industries. For use with hobby or craft projects such as the assembly or repair of remote-control foam model aircraft, and artificial floral arrangements, hot-melt sticks and hot-melt glue guns are used in the application of the adhesive. For use in industrial processes, adhesive is supplied in larger sticks and glue guns with higher melting rates. Aside from hot-melt sticks, HMA can be delivered in other formats such as granular or power hot-melt blocks for bulk melt processors. Larger applications of HMA traditionally use pneumatic systems to supply adhesive.[43]

Examples of industries where HMA is used includes:

• Closing the flaps of corrugated boxes and paperboard cartons in the packaging industry.[44]
• Spine gluing in the bookbinding industry[44]
• Profile-wrapping, product assembly and laminating applications in the woodworking industry[44]
• Disposable diapers are constructed through the use of HMA, bonding the non-woven material to both the backsheet and the elastics.
• Many electronic device manufacturers may also use an HMA to affix parts and wires, or to secure, insulate, and protect the device's components.

Format

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Hot-melt adhesives are often sold in sticks or cartridges suited to the intended glue gun. Bulk pellets are also used: these are dumped or transported to an adhesive reservoir for subsequent application. Large open-head drums are also used for high volume application. Hot-melt drum pumps have a heated platen which melts the adhesive for pumping through heated hoses.

References

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