Hot Melt Adhesives: Ingredient Selection & Formulation Tips

What are Hot Melts and How Do They Work?

Hot melts are thermoplastics that are used after melting the polymer beyond its melting point. Hot melt adhesives majorly comprise of three components:

A polymer/ blend of polymers
A tackifying resin
A wax/oil

Wax and oil are used for the same purpose, the only difference is that oil is liquid, and waxes are solid. Apart from these ingredients, hot melts may include an antioxidant, filler, UV stabilizer, or a pigment, and others. The hot melt should have very low viscosity in its molten form in order to achieve wetting. Also, it must not cool too rapidly else it will not have time to completely wet the substrate. Special dispensing equipments are used for the application of hot melt adhesives.

The main advantage of hot melts over other forms of adhesives is that they do not require a solvent and, hence, associated environmental issues are avoided.

They also set rapidly
The dispensing is easy to automate
A smaller space is required for storage and use
There is wide formulation latitude to meet a variety of costs and performance needs

Hot melts are used in packaging, paper laminates, non-wovens, bookbinding, labeling, textiles, and other industries.

They are particularly used in high-volume industries due to their short setting time. Hot melt adhesives are available in many forms (pellets, slugs, blocks, sticks…).
They can be applied from a molten tank using heated hoses and dispensed with coaters or nozzles.
They also can be applied directly at the point of assembly using a continuous rope or slug that is dispensed through a heated handgun.
Films are also available for high area assembly and continuous laminating.

Attention must be paid to the components making up the hot melt formulation. The hot melt adhesive must:

Have a low viscosity in the melt
Wet the substrate,
Solidify into a cohesively strong material, and
Maintain adequate adhesion properties in service.

Before discussing different components used in HMA in detail, let’s first understand how hot melt adhesives work.

Working of Hot Melt Adhesives

The hot melt adhesive is applied from the melt, gaining strength upon solidification and crystallization. Although certain types of hot melt adhesives can crosslink over time or with an outside energy source, general-purpose hot melt adhesives remain thermoplastic after application and gelling.

The working mechanism of holt melts includes the steps listed below.

Step 1: A hot melt adhesive when heated, melts and becomes a liquid. It is then applied by bringing it between two substrates. Once cooled, it will set and solidify. The cooling of the drop largely depends on the size of the droplet, nature, and temperature of the substrate. In case the substrate is not insulated, it may take a lot of time for the hot melt to cool down.

For example; if the hot melt is applied on a cold surface, the hot melt would cool faster, but the adhesion imparted would be weak as the adhesive will not get enough time to fill in the cavities of the substrate. On the contrary, if the hot melt was applied on a pre heated surface and then allowed to cool gradually along with the substrate, the adhesion would be stronger.

Step 2: The second substrate is applied to the adhesive, in order to join the two substrates with the help of hot melt applied between them.

Step 3: The second substrate is pressed, and one can see the adhesive oozing out, in case the contact surface with the substrate is large. The large surface area of the substrate allows for quicker cooling of the hot melt. This hot melt after cooling sets, making it impossible to separate the two substrates.

The horizontal axis describes the temperature, which decreases with time. The relationship between temperature and time is never the same.
The vertical axis exhibits the viscosity or the cohesion [modulus] for a period when the hot melt is liquid and cohesion for the later stages of the hot melt.

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Composition of Hot Melt Adhesives

Hot melt adhesives require a delicate balancing of formulation components relative to their performance and processing properties. As mentioned above, the major components of a hot melt adhesive and their role are:

The base polymer is the molecular backbone of the systems, and it is used to provide the inherent strength and chemical resistance as well as the application characteristics.

Tackifiers are added to improve initial adhesion and to modify the base polymer.

Processing oils and waxes are used to adjust viscosity and set times. Both tackifiers and processing materials will affect the glass transition temperature (Tg) and melting point of the final product.

Fillers are used to fine-tune certain properties such as melt viscosity, thermal expansion coefficient, set time, etc.

Antioxidants are used to provide oxidation resistance - more for the polymer in the application state rather than in the final joint.

UV stabilizers or inhibitors are added to provide stability against exposure to light.

Polymer Selection for Hot Melt Adhesives

Polymers are the workhorse for hot melt adhesives. Any/every polymer could serve the purpose. Polymers impart benefits like flexibility, strength, improved adhesion, and much more.

The base polymer provides the main framework for the adhesive’s overall physical properties.

Since a low melt viscosity is required, most polymers used as bases for hot melt adhesives are semi-crystalline in character. The most common general-purpose hot melt adhesive is based on ethylene vinyl acetate (EVA) resins. These are used in the packaging, furniture, bookbinding, and footwear industries.

For pressure-sensitive hot melt formulations, the base polymer is often a block copolymer, such as styrene-isoprene-styrene (SIS) or styrene-butadiene-styrene (SBS). These products go primarily into tapes and labels. Other polymers commonly used in hot melt adhesives are listed below.

Low-density polyethylene
Polyamides
Ethylene acrylic copolymers
Polypropylene (atactic)
Phenoxy resins
Polyesters
Polyesteramides
Polyurethanes
Butyl rubbers
Polyvinyl acetate and copolymers
Paraffin waxes

The main types of hot melt adhesive polymers that have generally been used in the manufacture of hot melt adhesives are discussed in the table below.

Hot Melt Base Polymer	Characteristics
Ethylene vinyl acetate (EVA)	Most frequently used base polymer Very versatile adhesives Type and amount of wax and resin can control set time and tack Filler can be added in some cases Good compatibility with a wide variety of tackifiers and waxes Various melt indices and vinyl acetate concentrations
Styrene Block Copolymer (SBC)	Low-temperature flexibility High heat resistance Used for PSAs and non-PSAs Fast set Styrene-butadiene-styrene, styrene-isoprene-styrene, styrene-ethylene-butylene-styrene, blends of these, and blends with other polymers
Polyolefin (PO)	Good, general-purpose adhesives Moderate temperature resistance Well suited for porous substrates but relatively rigid Good thermal stability (color, gel) Good resistance to acid, grease, oil Polyethylene and polypropylene with various molecular weights and branching, blends with other polymers
Amorphous polyolefin (APO)	Low cost with good acid and fuel resistance Moderate heat resistance Soft, tacky, and flexible Long open times and good adhesion Low surface energy and capability to wet almost every substrate Amorphous/crystalline concentration, blends with other polymers
Metallocene polyolefin (mPO)	Wider temperature range than EVA Light color, clear, and odorless Good thermal stability Fast set and low density Metallocene-catalyzed polyethylene and metallocene-catalyzed polypropylene
Polyamides (PA)	Considered to be high-performance hot melts Lower melting points than the polyamides used for engineering plastics Good temperature resistance and needs less additives Quite expensive
Polyurethane (PUR)	For reactive hot melt with terminal isocyanate groups Cross-links after application with humidity When applied to the substrate, the terminal isocyanate groups tend to react with the ambient moisture and in turn, forms a thermoset from being a thermoplastic

The application, performance, and cost properties of hot melt adhesives and sealants can vary significantly depending on the base polymer and the specific formulation employed. Typical properties of several common fully formulated hot melt adhesives are listed in the table below.

Property	Ethylene Vinyl Acetate	Polyamide	Polyester	Polyethylene
Softening point, °C	40	100	-	-
Melting point, °C	95	-	267	137
Crystallinity	L	L	H	H or L
Melt index	6	2	5	5
Tensile strength, psi	2750	2000	4500	2000
Elongation, %	800	300	500	150
Cost	L to M	M	H	L

Ethylene Vinyl Acetate (EVA)

EVA resins are highly flexible products, compatible with many other polymers and additives, and easy to process. The material is essentially a random, amorphous copolymer with regions of crystallinity. Melt viscosity is very dependent on the molecular weight of this material. Melt flow indices ranging from 2 to 200 are possible.

They have high cohesive strength and excellent adhesion to a wide range of substrates. EVA copolymers can be used in soft, permanently tacky pressure-sensitive adhesives or in tough rigid hot melt compositions used for semi-structural applications. Get Quick Tips for Ethylene Copolymers Selection »

Key advantages and limitations associated with EVA based hot melt adhesives are listed in the table below.

Advantages	Disadvantages
Broad formulating latitude is necessary for many different applications and adhesion to a wide variety of substrates Quick setting Retention of properties at low temperatures Pressure-sensitive systems can be formulated Considered to be safe and non-toxic Relatively low cost	Cold flow (creep) Attacked by some greases, oils, and solvents High viscosity needed for maximum performance

Generally, for hot melt adhesives, EVA resins with vinyl acetate concentrations of 18-40% are utilized. The vinyl acetate content can be a significant parameter in varying the properties of the adhesive. The materials with high vinyl acetate concentration exhibit reduced crystallinity and increased polarity. At about 50% vinyl acetate content, all crystallinity is lost. Recrystallization rate or setting speed is greatly influenced by the choice of specific EVA resin.

Melt index or melt viscosity is another important criterion in choosing the correct EVA resins for adhesive formulations. Low melt index EVA grades provide high viscosity, strength, and hot tack. In contrast, high MI grades enable higher polymer content and low application viscosities. Mid-range MI grades provide formulation flexibility.

Property	Change Due to Decreased Crystallinity (Increasing VA Content)
Stiffness modulus	Decreases
Surface hardness	Decreases
Crystalline melting (softening) point	Decreases
Tensile yield strength	Decreases
Chemical resistance	Decreases (generally)
Impact strength (especially at low temperatures)	Increases
Optical clarity	Increases
Environmental stress crack resistance	Increases
Coefficient of friction	Increases
Retention of mechanical strength at high filler loadings	Increases
Compatibility with other polymers, resins, etc.	Variable
Property	Change Due to Increased Polarity (Increasing VA Content)
Dielectric loss factor	Increases
Compatibility with polar resins and plasticizers	Increases
Specific adhesion	Increases
Surface printability	Increases

Changes in Physical Properties of EVA Due to Increasing Vinyl Acetate Content
The higher vinyl acetate copolymers provide better adhesion to polar substrates such as vinyl, aluminum, and steel, while the lower vinyl acetate copolymers are often used for bonding low energy surfaces.

Substrate	Vinyl Acetate (VA) Content 18% VA » 40% VA
Kraft paper	Little effect
Glassine paper	Little effect
Wood	Little effect
Acrylonitrile butadiene styrene (ABS)	Slight trend »
Aluminum	Major improvement »
Steel	Major improvement »
Plasticized vinyl	Major improvement »
Rigid vinyl	Major improvement »
Polypropylene	« Major improvements
High-density polyethylene	« Major improvement

Effect of Vinyl Acetate Content in EVA Copolymer Hot Melts on Adhesion to Various Substrates

EVA resins exhibit miscibility in the melt with a wide range of modifying resins, tackifiers, and waxes. This provides the adhesive formulator with a wide latitude of compounding possibilities. While these polymers, when properly formulated, provide acceptable adhesion, there are a number of drawbacks that detracted from their usefulness. These included:

EVA shows a lack of temperature resistance in the 38°C range (important for resistance to body temperatures)

The pressure-sensitive nature of EVA is generally inferior to other polymers

EVA has a tendency to gel or char when subjected to typical application temperatures such as 150°-175°C resulting in equipment maintenance problems and poor adhesion

These polymers cannot easily be formulated as a multi-purpose adhesive to serve multiple applications.

Ethylene-vinyl acetate (EVA) copolymers are perhaps the most widely used base polymer in general-purpose hot melt adhesives

Styrene-Butadiene Copolymers

Adhesives that are based on styrene-butadiene block copolymers are both useful and unusual. They have the solubility and thermoplastic nature of polystyrene; while at ambient temperature, they possess the toughness and resilience of an elastomer. This characteristic provides SBC adhesives with versatile properties in both pressure-sensitive and non-pressure-sensitive adhesive formulations. A starting formulation for an SIS elastomeric adhesive is:

Components	Parts by Weight
Styrene isoprene styrene	20
Tackifier, polyterpene	60
Oil	10-20
Antioxidant	0.5-1.0

Starting Formulation of an SIS Elastomeric Hot Melt Adhesive
As a hot melt adhesive, the low melt viscosity and fast strength development are significant benefits to converters. Certain SBC polymers are capable of hot melt application temperature in the 150°-170°C range. This is much lower than most EVA hot melt systems.

The four types of SBC resins are commonly available for adhesive formulation:

Of these, SBS offers the lowest cost and high levels of cohesive strength. In general, the saturated block copolymers (SEBS and SEPS) are used where long-term UV, thermo-oxidative, or chemical stability is critical, or where compatibility with other low polarity ingredients is required. SIS copolymers are generally used in pressure-sensitive adhesives where high tack is necessary and cohesive strength is less important.

However, one of the most interesting and valuable properties of SBCs is that they offer a physical form of crosslinking which greatly broadens their applicability. The thermoplastic polystyrene end-segments on the molecule form “pseudo-crosslinking” sites. This results in a superior resistance to creep while maintaining a very high cohesive strength and degree of elongation. Due to the pseudo-crosslinking mechanism associated with SBCs and the glass transition temperatures associated with each phase, SBCs provide very good properties at both high and low temperatures.

Representation of an SBC structure in bulk

SBC adhesive manufacturers can tailor properties to match a wide range of applications. Besides being strong, highly extendable, cost-effective, processable, and easily formulated, the SBCs have other attributes that are very beneficial:

No vulcanization is required (results in high processing speeds)
Precise molecules with a wide range of structure (results in great formulation latitude)
Clean and non-toxic – many formulations are FDA approved
Low melt viscosity
Formulations can be clear
Aggressive adhesion to most substrates including plastics
Cohesive strength adjustable with diblock content
Resistant to water, and most acids and bases

The drawbacks associated with EVA hygienic hot melt adhesives noted above opened the door for new hygienic adhesives and styrene-butadiene copolymers in the market. The SBC hot melt adhesive formulations provided an improvement over the previously employed adhesives; however, they also did not have all the properties required for maximum usefulness. The primary drawbacks associated with SBC adhesive compositions are:

Poor storage stability when left in the adhesive applicator for an extended period of time (increase in viscosity and ultimately a gel would form due to thermal oxidation)

Creep resistance at elevated temperature, although better than EVA, was still lacking

Certain low coat styrene-butadiene copolymers, such as styrene-butadiene styrene (SBS) did not have the elongation required for an elastic attachment adhesive. Styrene isoprene styrene (SIS), was found to be more flexible, softer, and better suited for elastic substrates.

However, SIS compositions were found to display a low modulus and poor elevated temperature performance even when formulated with various tackifying resins. When end-block reinforcing resins were used to improve the heat resistance, the adhesion to polyolefin substrates would suffer.

The SIS based adhesive compositions also showed undesirably high viscosities for certain applications and could not be used reliably as a multi-purpose adhesive.

Polyolefins and Amorphous Polyolefins (APO)

Hot melt adhesives that employ amorphous polypropylene (APP), amorphous polyalpha-olefin (APAO), and low-density metallocene and/or single-site polyolefin elastomers have more recently become well known in the art. Due to their low crystallinity, adhesives made from these polyolefin systems generally show good compatibility and long-term thermal aging performance with plasticizing and tackifying agents commonly used in hot melt formulations.

Due to their low crystallinity, however, these polyolefin species tend to develop properties only slowly after application that can make them unsuitable for certain construction applications. In generating laminate structures using porous substrates such as nonwovens, a slow set up characterized by the slow development of modulus upon cooling can lead to over-penetration of the adhesive leading to blocking, equipment fouling, and even compromised mechanical performance of the final article. Additionally, adhesives generated solely from polyolefins with limited crystallinity can also display poor long-term shear strength performance. Adhesives based solely on polyolefin elastomers offer little resistance to such failure modes.

Hot melt adhesives based on higher crystallinity polyolefins can offer a different set of potential drawbacks. Polypropylene polymers containing low levels of comonomer can be employed to provide hot melt adhesive formulations that develop rapidly upon cooling in coating applications. These more crystalline materials, however, tend to exhibit poor compatibility in hot melt adhesive formulations.

Additionally, hot melt adhesives generated from higher crystallinity polyolefins tend to possess lower tack due to the higher modulus of these systems when the polypropylene polymer is added at levels required to provide suitable cohesive strength to provide strong bonds.

Amorphous polyolefins, formed by the copolymerization of alpha-olefins such as ethylene, propylene, and 1-butene with Ziegler-Natta catalysts have also been found to be useful for the production of hygienic products. Although they have lower adhesion properties than EVA-based adhesives, they are known to have better thermal stabilities. Other than thermal stability, the problems with amorphous polyolefin in hygiene hot melt formulations are similar to those described above for EVA adhesives.

High viscosity hot melt systems based on atactic polypropylene have been generally used for the end seal in diaper construction. The composition of an atactic polypropylene-based hot melt adhesive is:

Components	Parts by Weight
Atactic polypropylene	70
Tackifier, C-5 hydrocarbon resin	10
Plasticizer, microcrystalline wax	20
Antioxidant	0.5-1.0

Starting Formulation for an Atactic Polypropylene Hot Melt Adhesive for End-Sealing in Diaper Construction
A blend of amorphous polyolefin and styrene ethylene butadiene styrene (SEBS) copolymer was found to provide thermally stable and more processable hot melt adhesive which could be used as alternatives for EVA and SBC adhesives. SEBS copolymers as noted above have saturated rubber midblocks and when formulated with saturated resins, plasticizers, and stabilizers can obtain a good balance of adhesive properties as well as resistance to degradation by oxidation or UV light. When SEBS is added to an amorphous polyolefin, the toughness of the hot-melt adhesive was enhanced, and the viscosity was also increased.

Components	Parts by Weight
Components	Formulation A	Formulation B
SEBS Elastomer	20	10
Tackifier	60	60
Mineral oil	20	20
Antioxidant	0.15	0.15
Amorphous polyolefin	0	10
Properties	Value
Properties	Formulation A	Formulation B
T- peel adhesion, pli	0.05	0.11
Creep resistance, 12 specimens, 8 hrs at 38°C	All 12 passed	All 12 passed

SEBS Addition to APO in a Hot Melt Adhesive Improves T-peel Strength

Metallocene polyolefin (mPO)

Further development of olefinic polymers has been based on metallocene catalyst technology. This technology results in long-chain branching and improved elastomeric and processability characteristics. Metallocene catalysts offer unique advantages versus conventional catalysts for the production of polyolefin resins. They allow producing consistent, controllable molecular structures that can be designed to:

Improve toughness and impact resistance
Provide low off-taste and odor
Allow tailoring of processing characteristics to fit the conversion process
Eliminate non-targeted molecular weight species in resins
Offer a greater control of molecular weight distribution (MWD).

Hygiene construction adhesives formulated using this new technology provide broad temperature sprayability (good balance of low- and high-temperature sprayability) and excellent adhesion with excellent thermal and viscosity stability. The narrow molecular weight ranges provide hygiene construction hot melt adhesives with fast setting times, low-odor, low-color, and clean running characteristics.

The tables below show two metallocene-catalyzed polypropylene formulations that can produce high-quality nonwoven laminates using either contact or spray application techniques. And hence, provides property data of these formulations compared to commercial metallocene-catalyzed polyethylene and SBC formulations.

Components	Parts by Weight
Components	A	B
Metallocene polypropylene	50	70
Hydrocarbon tackifier	35	20
Oil	15	10

Starting Formulations for a Metallocene Catalyzed Polypropylene Hygiene Adhesive

Properties	Value
Properties	Formula A	Formula B	Commercial mPE	Commercial SIS
Brookfield viscosity, cps
at 120°C	27000	-	18000	-
at 140°C	12500	-	6000	-
at 160°C	4000	-	3000	-
Peel force, gms, measured at 40°C on nonwoven after spiral spray application
at 120°C	100	-	72	-
at 140°C	88	-	102	-
at 160°C	101	-	110	-
Peel force, gms, after:
at 120°C	100	95	105	60
at 140°C	80	80	110	70
at 160°C	85	85	30	145

Property Comparison of Metallocene Polypropylene Adhesives with Other Commercial Hygiene Adhesives

Polyamides (PA)

Polyamide hot melt resins are similar to those used in nylon plastics, but they have a lower melting point. They are also chemically similar to the polyamide curing agents that are used for epoxy adhesives; however, those used in hot melt adhesives are unreactive.

Polyamides are mainly generated through reactions of difunctional materials. The reactions of a diamine with a diacid or the homo-polymerization of an amino acid are common routes to polyester production. Additionally, polyamides can be formed through the ring-opening polymerization of caprolactam. There are a great many possible resins in the polyamide family. They can be varied to provide hot melts of almost any desired temperature over a span of several hundred degrees.

Polyamide hot melt adhesives have better heat resistance than EVA or SBC types and use fewer additives. However, the base polyamide resin generally costs more than more conventional hot melt resins that rely on the formulation for optimum performance. Certain types of polyamide hot melts can withstand 200°C over the short term without degradation, although creep is generally a problem. Polyamide adhesives generally exhibit sufficient properties, such as tack, without the use of additives.

Modification of the physical properties of these adhesives can be achieved by controlling the synthesis of the polyamide (reaction product of diacids and diamines). Polyamides of differing molecular weights and chemical structures are often employed to provide specific application and performance properties.

The polyamides that are used for adhesives generally fall into three groups that are defined by molecular weight:

Low molecular weight polymer can be applied at low temperature using simple, inexpensive equipment

Intermediate molecular weight polymer's application equipment ranges from simple to moderately sophisticated

High molecular weight polymer requires very sophisticated extruder-type equipment for application. These are used as high-performance structural adhesives where high-temperature performance is not required.

The table below summarizes the typical properties of these polyamide groups. In all the above cases it is important to minimize the time when the molten resin is exposed to oxygen.

Property	Polyamide Molecular Weight
Property	Low	Intermediate	High
Softening point (ball and ring), °C	95-180	95-200	135-200
Melt viscosity, poise
at 160°C	5 - Solid	120 - Solid	-
at 210°C	1 - 10	20 - 110	250-50,000
at 260°C	-	5 - 25	20-1000
Tensile strength, psi	160-1600	450-3000	3400-6500
Elongation, %	5-100	25-1000	25-1000
Adhesive tensile-shear strength, psi
Al-Al	200-1000	700-1900	2200-3700
Steel-Steel	200-1000	500-1800	1900-3300

Typical Properties of Polyamide Adhesives of Various Molecular Weight
The use of mixed resins in polyamide hot melt formulations increases the molecular disorder. This lowers the extent of hydrogen bonding and the melting point. For example, the addition of small percentages of a high molecular weight polyamide raises the melting point of lower molecular weight polyamides so that a flexible, tacky material could be achieved. In this way, polyamide hot melts can be formulated that are extremely tough and resistant to impact.

Polyamide hot melt resins offer highly polar groups in the polymer chain which give interchain hydrogen bonding. This results in high strength at low molecular weights, which is a characteristic that is not present in many conventional hot melt adhesive resins. A result of this polarity is the retention of much of the adhesive strength at temperatures up to just below the melting point. However, polyamides are much more susceptible to moisture permeation than polyesters, since hydrogen bonds can break when water is absorbed by the polymer.

Polyamide hot melt adhesives are very versatile and are capable of bonding many different materials. In addition to metal-to-metal bonding, polyamide hot melt adhesive formulations are used to bond plastics, foils, and papers.

Major application areas for polyamide hot melt adhesive include:

Shoe
Automotive
Packaging
Electrical / electronic
Woodworking

Polyamide adhesives are available in a variety of forms including pellets, cylinders, film, rod, powder, and solution.

Polyesters

Thermoplastic polyesters that are used in hot melt adhesives are chemically similar to those used in the synthetic fiber industry. These resins are also similar to polyamide hot melt resins in that the monomers in their reaction products are generally adjusted to provide the fine-tuning of application and performance properties. Thus, the use of additives is minimized. However, at times stabilizers, plasticizers, and tackifiers are added for special purposes. Occasionally as with the polyamides, different polyesters are blended for optimal properties.

Polyester hot melt resins are based on the reaction of difunctional acids and diols. Mainly terephthalic acid but other diacids, such as isophthalic, adipic, and azelaic are also used. The melting points and glass transition temperature of the polyesters depend on both the acid and diol components and their concentration.

The chain length of the diol used to form the polyester has a major effect on physical properties:

As the diol chain length decreases so does the melting point of the copolymer.
The percent crystalline material in a polyester is dependent on the chain length of the diol. Crystallization decreases as diol chain length increases, but the rate of crystallization increases with chain length

The rate of crystallization is an important criterion in determining the speed at which the hot melt sets to a reasonable strength.

Polyesters do not have as high a melting point or strength as the polyamides of similar structures. However, polyesters are much more resistant to moisture permeation and degradation. Other than in applications where these properties are important, polyesters and polyamides compete for the same applications.

One of the earliest applications for hot melt polyester adhesives was in the manufacture of shoes
Hot melt polyester adhesives are also commonly used for bonding fabrics such as decorative trim, draperies, etc.

Polyester hot melt formulations are commonly found in preformed sticks or coils that are used with hot-melt extrusion guns. The adhesive is pushed through the heated nozzle of the gun, and the molten product is applied to the substrate. Polyester adhesives can be used in their natural state, but often they are compounded heavily to provide for increased toughness, peel strength, and open time.

Polyester hot melt resins have a relatively high melting point of approximately 260°C. Thus, it is often used as a heat-activated film. Polyester hot melts can be formulated to have high tensile strength and are often employed where high strength and high-temperature resistance are required.

Read: Biobased Components in Hot Melt Adhesive Formulation (Soy Polymers, Modified Starch and much more)

Hot Melt Polyurethane Reactive Adhesives

Although hot-melt polyurethane reactive (HMPUR) adhesives have been available for 25 years, their novel properties and the development of diverse products have accelerated their recent growth. They are now found in many applications as products are developed to fit specific needs.

In general, HMPUR adhesives have these characteristics:

They are solid at room temperature, similar to other hot melt adhesives that are thermoplastic polymers, such as ethylene-vinylacetate (EVA), polyalphaolefin (PAO), polyester, and polyamide.

The temperature for application as a melt ranges from 85°C to 140°C (185°F to 284°F), which is less than that for typical thermoplastic hot melt products. The viscosity at the application temperature can be designed in a range from 2,000 to 60,000 cPs depending on the specific requirements.

HMPUR adhesives are being produced with open times varying from 10 seconds to 10 minutes to match application requirements.

The unique property that gives HMPUR adhesives their performance advantage compared to thermoplastic hot melts is that they cure to a thermoset material that resists melting. This curing process is the reaction with moisture found in the air or in typical substrates to produce a strong, tough, temperature-resistant adhesive.

Formulating Advanced Reactive Hot Melt adhesives

The advantages of HMPUR adhesives result from the properties are discussed below.

The cured adhesive has excellent temperature and environmental resistance. Many of the HMPUR adhesives can withstand exposure to temperatures from -40°F to +200°F while maintaining strong bonds.

Minimal fixturing is needed due to the controlled set time and rapid development of green strength that can be designed into these products. They are typically applied by roll coating, slot-die coating, gravure printing, screen printing, or spray coating (swirl or fiberized) and then briefly nipped or pressed to provide a bond with handling strength. The final cure and ultimate strength are achieved in a few hours to a few days depending on the adhesive, substrate, and conditions in the plant.

HMPUR adhesives can be designed and developed with widely varying properties for specific applications. How this is done and some of these applications are described later in this article.

HMPUR adhesives are 100%-solids and therefore have no volatile organic compounds (VOCs) and do not require drying ovens. This eliminates many of the environmental issues associated with solvent-based adhesives and the energy requirements for drying water-based and solvent-based products.

How Specific Properties are Incorporated into HMPUR Adhesives

HMPUR adhesives are manufactured by reacting mixtures of polyols with an excess of diisocyanate. The reaction of an isocyanate group (-NCO) with an alcohol group (-OH) produces the urethane group. The fact that there are excess diisocyanates present means that the molecular weight of the resulting product is not too high and that the HMPUR product will have a controlled melt viscosity.

These adhesives are then transferred from the reactor into the container package as a melt. The container is sealed to prevent exposure to moist air while the adhesive solidifies on cooling. Typical containers are drums (400 lb), pails (40 lb), slugs (4.4 lb) and cartridges (0.6 lb).

Manufacturing HMPUR adhesives

The adhesive is melted using a specifically designed pre melter and then applied to the article to be bonded. During the next several hours or days after application, the reaction described in the figure takes place. This reaction of isocyanate with water forms the highly stable polyurea structure that gives the cured HMPUR its temperature and environmental performance. The polyurea structure effectively crosslinks the adhesive to prevent its remelting or dissolving as can occur with standard thermoplastic hot melt adhesives.

Specific properties required for varying applications are built into the adhesive by the use of a variety of polyols, isocyanates, and additives.

Polyols are used to vary the open time, set time, and ability to wet effectively various surfaces to produce effective bonds. Some typical polyols used are:

Polyesters, which can be crystalline or amorphous. The crystalline polyols can be used to give short set times and rapid green-strength development. Amorphous polyesters can improve adhesion to specific substrates and increase the open time if required.

Polyethers, which are low-Tg, amorphous liquids. This helps lengthen the open time, reduce viscosity and provide good low-temperature flexibility.

Vinyl-polymerized polyols, which are typically high-molecular-weight, glassy solids. These materials can assist in building green strength and tack while maintaining extended open times.

A mixture of polyols to give the desired performance is reacted with an excess of a diisocyanate. Methylene diphenyl isocyanate (MDI) is used for most HMPUR adhesives. It has a highly reactive isocyanate group and has a relatively low vapor pressure. Since polyurethanes made with MDI tend to yellow when exposed to sunlight, a method using HMDI (hydrogenated MDI), which resists yellowing very effectively, has been developed. It tends to be much slower reacting, so there exists a process to effectively speed its reaction and make it useful for HMPUR applications.

Tackifiers / Tackifying Resins in Hot Melt Adhesives

Tackifiers is an important class of materials used in hot melt adhesives in terms of their effect on both cost and performance properties. They normally have low molecular weights and are resinous, but they have glass transition temperatures and softening temperatures that are often significantly above room temperature.

It is this combination of the properties that make these materials useful for imparting “tack” and desirable viscoelastic properties in the adhesive formulation.

Tackifiers are a principal component used to vary and refine both the performance and processing properties of the adhesive.

Standard tackifiers are either based on natural or petroleum-based products.

Examples of natural tackifiers are rosin acid derivatives and their esters. Rosin resins (rosin acids and rosin esters) are sourced from pine tree by-products such as gum rosin. They offer good tack to most of the polymer types. They are majorly used to tackify natural rubber, ethylene vinyl acetate, acrylic, styrene-butadiene rubber, styrene-butadiene copolymers, and polyurethanes. By esterifying it, you get rosin ester.

Synthetic tackifiers are based on either aromatic or aliphatic petroleum-based resins.
- The aromatic resins are further classified into coumarone-indene resins, aromatic petroleum resins, and other types.
- The aliphatic resins are known as C-5 resins since much other chemistry revolves around polymerized pentene and cyclopentene.
Hydrocarbon resin tackifiers on the other hand are manufactured from petroleum-based feedstock, and therefore, they have the limitation of being linked to the high prices of oil. When compared to rosin resins, hydrocarbon resins have a lower compatibility range with base polymers.

Another natural product is that based on a class of materials known as the terpenes. Terpenes are often referred to as "universal" tackifiers due to their compatibility with numerous polymers like EVA, polyethylene, natural rubber, styrene-butadiene rubber, styrene-butadiene copolymers and others. Terpene resins show good compatibility with polyolefins and the mid-block of styrene-isoprene-styrene resins. Terpene tackifiers offer improved adhesion and outstanding initial color for almost all polymer types. These resins are also compliant with food contact regulations. As a result of their properties and natural source, terpene tackifiers have a relatively high use but limited supply. This combination leads to a somewhat higher price.

How Tackifiers Work?

Tackifiers will raise the glass transition temperature of the adhesive formulation and they also provide a relatively high modulus for a low molecular weight component. As a result, tackifiers are often used to adjust the Tg and storage modulus in order to optimize properties within a certain temperature range as illustrated in the figure below.

Tg (glass transition temperature) and G’ (storage modulus) affect the PSA’s application window

A pressure-sensitive adhesive will typically possess optimum tack properties at 20°C above the adhesive Tg. For non-woven adhesives used in applications where they are positioned close to the skin, the adhesive Tg should range from 15°C and 20°C, corresponding with a body temperature of 37°C. Targeting these values also ensures that the adhesive remains pressure-sensitive even at high process speeds.

Tackifiers modify the "quick grab" and viscoelastic properties of the adhesive. Once the adhesive has solidified, the tackifier also modifies (generally increases) the Tg of the solid adhesive. Tackifiers must have a moderate molecular weight which imparts some cohesive strength and prevents the formation of weak boundary layers at the interface, a phenomenon which happens often with low molecular weight plasticizers. Tackifiers should also have a relatively low surface tension so as to readily wet the substrate.

Tackifiers promote adhesion and wetting and contribute to initial adhesion or tack.

Tackifiers that are commonly used in solvent-borne pressure-sensitive adhesives are also applicable for hot melt pressure-sensitive adhesive applications.

Selecting the Right Tackifier for your HMA Formulation

The first and most important criterion in the selection process is the compatibility between the tackifier and the base polymer in the adhesive formulation. Certain classes of tackifiers work well with certain types of polymers. Unless the tackifier is compatible there is no need to extend the selection process.

Solubility parameters, molecular weight, and molecular weight distribution determine compatibility. Materials that possess similar solubility parameters are generally assumed to be compatible with each other. The following table provides a general guide to the selection of tackifiers with various common polymers used in hot melt and pressure-sensitive adhesives.

Polymer	SIS	SBS	SEBS	EVA	PE
Natural Tackifiers
Polyterpene	✔⁽¹⁾	✔	✔		✔
Terpene phenolics	✔⁽¹⁾			✔
Styrenated polyterpene	✔^(1,2)	✔	✔	✔
Rosin esters	✔	✔		✔
Hydrocarbon Tackifiers
Aliphatic (C5)	✔		✔	✔	✔
Aliphatic / Aromatic (C5/C9)	✔	✔		✔
Aromatic and pure monomer	✔⁽²⁾	✔⁽²⁾	✔⁽²⁾
⁽¹⁾ Midblock compatible ⁽²⁾ Endblock compatible

This grouping is based primarily on compatibility, and it is a good starting point in the selection of a tackifier. The formulator will want to contact the suppliers of these types of tackifiers for information on the specific resins that they supply, their properties and prices, and any specific recommendations they might make.

Care must be taken in the formulation of block copolymer pressure-sensitive hot melt adhesives to ensure that the tackifier does not dissolve in the polystyrene phase. The polystyrene phase must remain glassy for optimum cohesive properties. The C-5 resins, which are more compatible in the non-styrene phase, therefore are more likely to be used in block copolymer-based adhesives. Consideration must also be given to the heat stability of the tackifier in the melt. Tackifiers with unsaturation could potentially gel while the adhesive is in the melt.

Selecting Tackifiers and Plasticizers for PSA Formulations

Plasticizers for Hot Melt Adhesives

In addition to the base polymers and the tackifiers, the other most common additive in hot melt adhesive formulations are plasticizers.

When the base resin is excessively stiff, it is often blended with an elastomeric hydrocarbon, thereby obtaining a tough material with:

Improved energy dissipation, extrudability, flexibility, workability, and stretchability
Reduced glass transition temperature

These materials act opposite to the tackifiers in that they decrease Tg. The plasticizer must be completely soluble in the base resin and be sufficiently non-volatile to result in a weak boundary layer.

Plasticizers or flexibilizers are usually chosen as a second base polymer to improve flexibility and toughness. The addition of a plasticizer promotes wetting and reduces the melt viscosity of the formulation. The criteria used while selecting plasticizers that are used in adhesive formulations include key considerations as listed below.

Compatibility with a given polymer or set of component ingredients
Compounding characteristics
Effect of plasticizer on the rheological properties of the polymer
Desired mechanical and thermal properties of the end formulation
Resistance to water, chemicals, UV, weathering, dirt, microorganisms, general aging
Toxicity
Cost analysis (volume required or plasticizing efficiency, price/pound, etc.)
Mineral Oil & Wax

The most commonly used plasticizers are oils which are primarily hydrocarbon oils, low in aromatic content, and are paraffinic or naphthenic in character. The oils are preferably low in volatility, transparent, and have as little color and odor as possible.

Related Read: Achieve desired flexibility by understanding the fundamentals of plasticizers

Waxes Used in Hot Melt Adhesive

Mineral oil and wax are commonly employed as diluents. When added to the matrix polymer, wax and oils can negatively affect the adhesion properties because of the shrinkage and hardening of the adhesive. Waxes are often used in hot melt formulations to lower surface tension and decrease melt viscosity. Certain waxes such as microcrystalline waxes also reinforce the hot melt by forming crystallites that resist deformation under load. These are used in formulations that require a relatively high degree of creep strength.

Adding waxes to hot melts increases the setting speed, improves the heat resistance and lowers the viscosity of hot melts leading to excellent bonding.

Given their semi-crystalline nature, even mixed polyolefin systems can show lower than required set up times for end-use applications. For this reason, higher crystallinity materials such as waxes are often added to polyolefin-based hot melt adhesives to assist the rapid development of properties after application.

Optimizing Hot Melt Open Time vs. Setting Speed

Despite the benefits offered, systems employing low molecular weight, crystalline waxes have significant limitations.

They may increase the setting speed but can also reduce the wet-out and adhesion of the hot melt.

Additionally, the use of low molecular weight, crystalline waxes at even relatively low levels can compromise the mechanical properties such as elongation required for hot melt adhesives employed in elastomeric constructions.

Therefore, there exists a need in the art for hot melt adhesive formulations that display rapid set, a good balance of mechanical properties, and excellent long-term aging performance.

Waxes used in hot melt adhesive formulations could be:

Natural (carnauba and montan wax),

Petroleum-based (paraffins, microcrystalline wax) or

Synthetic, derived from petroleum distillates or residues (polyethylene, polypropylene, polytetrafluoroethylene, Fischer-Tropsch wax)

EVA hot melts make good use of Fisher-Tropsch wax to adjust the setting time and improve the thermal resistance of the hot melt.

	CP (°C)	DMP (°C)	Pen at 25°C (dmm)	Viscosity at 150°C (mPas)
PE-like	102	117	1	10
PE-like	117	112	1	6
PE-like	117	116	2	11
PP		128	>1	40
PE		145		40

Antioxidants in Hot Melt Adhesives

Antioxidants are used in a variety of adhesive formulations to protect against degradation caused by reaction with atmospheric oxygen. The antioxidant or stabilizer maintains viscosity, color, and physical properties as well as preventing thermal degradation. The introduction and type of antioxidant will depend on factors, such as:

The nature of the base polymer
The processing parameters
The end-use application

Excessive oxidation generally results in undesirable changes in the adhesive’s mechanical, aesthetic, or bonding properties. 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.

Oxidation Sensitive Components in Hot Melts

Adhesive components especially susceptible to oxidation are listed below.

Base synthetic polymers such as Ethylene-vinyl acetate, Styrene block copolymers, Polyolefins, Polyamides, Natural rubber, Polychloroprene, Polyurethane, Butyl rubber

Hydrocarbon additives, such as tackifiers and waxes, are also vulnerable to oxidation and can actually contribute to the oxidation of the base polymer

Metallic and other impurities in the adhesive can accelerate the oxidation process

Depending on the aging environment, most adhesives can benefit from antioxidants.

Typically, the base polymers and hydrocarbon raw materials used in formulating adhesives have a minimal level of stabilization to withstand processing and warehouse storage. Additional levels of stabilization are often required by the adhesive formulator to provide the necessary levels of compounding, storage, application, and end-use properties.

Stabilizers Commonly Used in Hot Melts

Hindered phenols, amines, phosphites, and thioester are commonly used stabilizers for hot melt adhesives. The chemical types of common antioxidants that are most often used in hot melt adhesive applications are shown in the table below. The hot melt adhesive composition includes from about 0.1% to about 1.0% by weight antioxidant. There are numerous antioxidants and blends of antioxidants that have proven beneficial for various types of adhesives.

Types	Common Resin Applications	Comments
Amine	Rubber, some pigmented polymers, and polyurethane polyols	Arylamines tend to discolor and cause staining
Phenolic	Polyolefins, styrenics, and most engineering resins	Phenolics are generally stain-resistant and include simple phenolics (BHT), various polyphenolics, and bisphenolics
Organo-phosphite	Polyolefins, styrenics, and most engineering resins	Phosphites can improve color stability and property retention but can be corrosive if hydrolyzed
Thioester	Polyolefins and styrenics	The major disadvantage with thioesters is their odor which is transferred to the host polymer

There are other properties in addition to preventing oxidation that must be considered when choosing an antioxidant for a specific adhesive formulation. These properties include:

Volatility
Compatibility
Color stability
Taste and odor
Regulatory issues

Cost is generally not an overriding issue since most antioxidants are used in small concentrations.

It is important that the stabilizer and its transformation products (which also may provide stability) not volatilize from the polymer. Many commercial antioxidants have been designed with higher molecular weights with this in mind. Antioxidants should be soluble in the polymeric matrix or at least diffuse slowly throughout the matrix. The migration of the additive out of the adhesive could result in a weak boundary layer and poor adhesion.

Related Read: Advances in Raw materials for Hot Melt Adhesives

Efficient Mixing of Hot Melt Adhesive Components

The various types of equipment and processes that can be used to mix hot melt adhesive components are particularly diverse, and they are usually determined by either familiarity with existing practice or the availability of equipment. Typical compounding ingredients include elastomers (30-50%), thermoplastic resins (20-40%), plasticizers (10-40%), fillers (1-10%), color pigments (0.1-3%), and stabilizers against oxidation and UV (0.1-3%).

Low softening temperature thermoplastics and their additives and modifiers are generally mixed without difficulty. These hot melt adhesive formulations can be compounded in several types of mixers such as:

Vertical mixers
Horizontal mixers (Sigma blade or Kneader)

The most economic method is the vertical mixer. Its advantages and disadvantages are:

Advantages	Disadvantages
Easier to pull vacuum during mixing Adhesives offer better aging performance Material change sequence is flexible No need for skilled labor	Difficult to produce very high viscosity products due to its lower shearing torque Heat exchange is somewhat slow Total mixing time is longer than those made with horizontal mixers

The horizontal mixers are normally accompanied by an extruder for easy adhesive mixing and discharge. Generally, the elastomers are metered individually or as a premix into the feed barrel. The resins (solid or liquid) and plasticizers are added downstream. For larger quantities, the liquids can be fed at several locations along the extruder using multiple kneading and homogenizing stages.

The main advantage of the horizontal mixers is that the entire operation can be done on a continuous basis. This enhances both productivity and quality. A continuous twin-screw mixing process is illustrated below.

Multi-stage Feed Continuous Extruder for Compounding Hot Melt Adhesives

Continuous mixers such as these are claimed to provide 30 percent cost savings. This is primarily due to lower operating costs and energy savings. Processing aids can also be added to the formulation to even further improve mixing capability and achieve efficient mixing with less energy.

Typical processing agents that can be used are:

Organosilane or organotitanate dispersing agents
Polyethylene glycol interfacial agents

Continuous mixers provide compounding flexibility as they are capable of utilizing a wide range of resins and melt temperatures. Slight changes in formulations or complete changes in the product can be accomplished with relatively little effort and minimal cleaning or purging.

The quality of the product is improved due to a higher melt quality and improved process control. Oxidation of the formulation is significantly reduced since the continuous extruder generates high shear without high temperatures. The residence time is also short minimizing exposure of heat-sensitive components to high temperatures.

Hot Melts Adhesives Optimization for Quality at Lower Costs

Hot Melt Adhesive Working Properties

The three working properties of hot melts are hot tack, open time, and setting speed.

Hot Tack

Tack as the word says, refers to the stickiness of the adhesive. Hot tack here points to the high temperature of the hot-melt applied. In the process of hot tack, wetting occurs. It happens when the surface tension of the liquid is lower than the surface tension of the substrate.

Then occurs wet adhesion which is a blend of capillary [SP] and viscous forces. The viscous forces play a significant role in the wet adhesion process.

Open Time

Open time is the time taken by the adhesive to create a bond. It can also be defined as the time wherein wetting can still occur to create a bond. Another way of defining it could be the time from the application of the hot-melt till the cooling of the hot-melt where it loses its initial adhesive properties. There are various factors that can influence open time like environment, usage conditions, and the type of hot melt adhesive used.

How to measure open time?

We present before you a very simple way of measuring open time. Place different hot melts on paper strips. Place these strips in the oven for about 150° C for the temperature of these hot melts to reach on a higher side.

After this new paper strips are placed perpendicularly on the hot melt strips in time intervals of 4 seconds, 8 seconds, 12 seconds, and so on. These strips are then cooled for at least 24 hours till crystallization has completed. These strips are then removed, and it can be observed that at some places, strips are successfully removed, but at other places not. Therefore, one gets to know that the open time was over because you couldn't wet that hot melt or the strip anymore. If you have a standard hot melt-over and you know the open time, then you can estimate the open time of other hot melts.

The Setting Speed

Setting the speed of a hot melt can be defined as the time taken by the hot melt to form a bond of acceptable strength.

HMA Requirements for Hygiene Products

Hot Melt Adhesive Requirements for Hygiene Products

Hot melt adhesives play a strong enabling role in the construction of disposable hygienic products such as:

Baby diapers (nappies)
Feminine care products
Adult incontinence articles

However, there are also a number of other hygienic applications for adhesives including medical dressings, hospital bed pads, and surgical drapes.

All of these hygienic products have a similar basic function – reliable absorption of liquids and/or solids with no leakage while promoting comfort and health. These products are also alike in that they are produced in high-speed production processes, and the substrates generally consist of non-woven sheets and polymeric film.

However, not all applications within the manufacture of a hygiene product are alike as illustrated for baby diapers as shown below. Specific applications include:

Construction
Core (fluff pad) adhesive
Positioning adhesive
Elastic attachment
Frontal tape adhesive
Side tape adhesive

It has been found that a hot melt adhesive used for a particular use must have a specific set of properties and may be completely unsuitable for other uses or applications.

As a result, the selection of the proper adhesive formulation is a complex and difficult task that depends on the end-use requirements of the specific application as well as the manufacturing method employed. In addition, adhesive formulations used are constantly undergoing changes as the performance requirements change.

Adhesive Use with Non-wovens in Diaper Manufacture
(Credit: Nordson)

Disposable hygiene products consist of an absorbent filler (sometimes referred to as a super absorbent polymer, SAP) protected on its outer face by a liquid-proof polyolefin film and covered internally with a film of non-woven fabric which is generally polyethylene. The non-woven fabric comes into contact with the skin and allows body fluids to flow toward the filler. More elaborate products may in addition contain a leak-proof barrier, an elastic band, a wetness indicator, or be made completely of biodegradable products.

The hygiene adhesive must possess a high degree of adhesion since it is often applied in the form of a number of stripes such as a spray pattern. The adhesive can be a pressure-sensitive or non-pressure-sensitive product depending on the application.

To function in manufacturing equipment for hygiene product applications, the hot melt adhesives must have suitable:

Viscosity
Set speed
Open time

The temperature/viscosity relationship of the adhesive must be controlled to permit easy application and surface wetting of the liner without wrinkling the liner material. It must not be excessively absorbed into the non-woven substrate. Yet, low surface energy is also needed in order to properly wet out the non-wovens as well as polymeric film (e.g., polyethylene) used in construction. Once the article is made:

The adhesive and the adhesive joint must remain flexible after long periods of storage

It is also desirable that the adhesive remain white or clear in color

The adhesive must also possess sufficient adhesive and cohesive strength to provide high bond strength values when subjected to stress so the constructions cannot be separated

Tensile strength and heat resistance of the final bond are important because the diaper must not come apart and release the fluff wadding (the absorbent material) which may harm the child

Also, the adhesive is either in direct contact or near-contact with the skin and, thus, the adhesive will see temperatures in the 40°C range. The adhesive may also experience warehouse temperatures in the 40°-55°C range, and the bond strength and flexibility of the adhesive must be preserved during this period

Other basic requirements for the adhesive are:

Low Odor Suitable for skin contact (non-toxic, good adhesion)

Suitable for skin contact (non-toxic, good adhesion) Low color and thermal stability for high temperature application

Low color and thermal stability for high-temperature application No staining or bleed-through

No staining or bleed-through

Excellent initial and aged adhesion to low energy (e.g., polyolefin film) substrates Good retention of shape, high softness, and drape

Good retention of shape, high softness, and drape High mileage (low cost)

High mileage (low cost) Robust application (stable spray application patterns and slot die coating)

Robust application (stable spray application patterns and slot die coating)

Multipurpose adhesives which can be used for more than one application within a single product are desirable in that they can reduce the number of different adhesive products which must be held in the manufacturer’s inventory. Furthermore, less adhesive products will reduce the likelihood of the wrong adhesive being used. However, as explained above, a multipurpose adhesive may be difficult to develop due to the unique requirements of each application.

Classification of Hygiene Adhesive Applications

The various hygiene adhesive applications can be classified into three main categories: Construction, Elastic attachment, and Garment attachment.

Construction

The construction adhesive is used to bond the primary layers of the article together. These layers generally consist of various non-woven materials and thin polyolefin films. The adhesive that is most commonly used is a non-pressure sensitive hot melt. Hot melt adhesives having low surface energy are preferred to assure proper wetting of these low-energy substrates.

Very small amounts of adhesive are needed (e.g., 1 gm/m²), and the adhesive covers only a portion of the substrate. To deliver this requirement while still providing adequate bond strength, adhesives are often sprayed on, requiring a very 'short' formulation (e.g., one with low melt elasticity). Narrow molecular weight distribution polymers, such as styrene block copolymers and metallocene-catalyzed polyolefins, are ideal.

Spray coatings apply stringent requirements on hot melt adhesives. The adhesive must have a sufficiently low melt viscosity (generally less than 15,000 cps at the application temperature). The adhesive must also be applied at a temperature low enough to avoid heat distortion of the substrate (e.g., thin gauge polyethylene film). Many other physical factors, especially rheological properties, come into play in determining the sprayability of the hot melt. A long open time is also needed, as the small volumes of dispensed adhesive will cool rapidly. Formulations are designed to maximize open time while maintaining adequate heat resistance.

Elastic Attachment

The trend in baby diaper construction has definitely been toward form-fitting leg bands which are stretchable. This requires that the diaper adhesive both adhere to the polyolefin liner and be elastic or adhere to the elastic band that is inserted in the leg area.

The elastic materials that are added to the hygienic product may be made of Lycra or Spandex fibers, natural rubber, polyurethane foam, or other elastic laminates. The elastic adhesive will need high adhesive strength but also high cohesive strength in order to hold onto the stretched elastic. Like construction adhesives, elastic attachment adhesives also need a low surface tension for good wetting and low melt temperature viscosity for sprayability.

Garment Attachment

The adhesive strip on the outside of sanitary napkins must bond to panty liners to hold the pad in place. The pressure-sensitive adhesive is usually applied by a hot-melt transfer coating. This is done in-line with the production of the napkin itself. The substrates include the napkin (e.g., low energy polyethylene) and the panty liner. The bond must be sufficient to secure holding and not “snap-back” or debond after the consumer presses the product in place. As with construction and elastic adhesives, the garment attachment adhesive must be stable on storage for up to two years without change in its pressure-sensitive adhesive properties.

Because of the manufacturing processes and end-use, the adhesive must provide functions other than merely supplying a secure attachment. Other requirements include:

Clean-cut off because the adhesive is applied to a defined area
High tack
Light color and no staining of the release liner
Low odor
Non-toxic, as the adhesive will be in close proximity to the skin
No residual adhesive after peeling

A sometimes-overlooked end-market where non-wovens and adhesives come together is as intermediate attachments used in manufacturing. Here, double-sided pressure-sensitive adhesive tapes or sheets based on nonwoven carriers impregnated with adhesive provide highly reliable attachment, with good workability. Solvent-borne or waterborne pressure-sensitive adhesives can be used in these adhesive products as they are produced off-line to the hygiene product manufacturing operation. However, environmental regulations and production speed are still requirements that mean hot melts will maintain leadership in this end-market area.

Enhancing possibilities for assembly, hygiene and packaging hot melt adhesives – Download Brochure

Additives and Polymers Used in Hot-Melt Adhesives

View a wide range of additives and polymer grades available today for use in HMA, analyze technical data of each product, get technical assistance or request samples.

Key Applications

Hot Melt Adhesives: Basics, Ingredient Selection & Formulation Tips