High heat resistance hot melt pressure sensitive adhesive is most commonly needed for automotive interiors or products that can remain inside an automobile. This is so because the temperature of air and some auto parts inside an automobile may exceed 180F. At such high temperature, most Styrenic block copolymer (SBC)-based hot melt pressure sensitive adhesives start to soften and/or flow under certain shear forces. This is because the surrounding temperature is close to the softening point of the styrene domain.

How to design or develop an extremely high heat resistant HMPSA is always a major challenge for most adhesive formulators. In recent years - many UV and/or E-beam curable HMPSAs have been developed and made available for various markets requiring high heat resistant particularly for use in both automobile and electronics.

Many adhesive users request high heat resistance of a hot melt pressure sensitive adhesive when shipping products in containers. What is the actual temperature inside a container during summer time? What is the actual temperature of an adhesive within those bonded goods? A lot of people believe it might be 160F and many feel it is 180F. To practically verify the actual temperature variations inside a container, day and night, companies can use a floating thermometer inside a container and record entire temperature history.

Most results indicated that the highest temperature inside the container is about “150F” instead of 160F to 180F. Please not that 150F is simply the temperature of air inside the container, but not the temperature of adhesive within those bonded goods, which are further packed inside various cartons and boxes. It is likely that the actual temperature of those adhesives bonded on goods is lower than 130F.

To improve the high temperature resistance of HMPSAs, more percentage of high molecular weight SBC or less tackifier is needed in the formulation. As a result, formulators need to sacrifice processibility due to the high melt viscosity of formulated adhesives.

We recommend that users of hot melt pressure sensitive adhesive look closely at the actual temperatures their package or product will need and tailor the adhesive accordingly.

For more information call or email Pierce Covert,
Glue Machinery Corporation
1(888)202-2468 info@gluemachinery.com

Many hot melt adhesive producers and users are under the impression that hot melt adhesive with high temperature and low temperature resistances are not possible. They consider these two properties conflicting and difficult to produce. In reality, this perception is completely incorrect and without any scientific support.

Most thermoplastic elastomers used for making hot melt adhesives, e.g. Styrenic Block Copolymers (SBC), possess both rubbery and plastic features. These polymeric materials normally exhibit two significant transition zones: a glass transition zone from glassy state to rubbery plateau for the rubbery domain, and a softening zone from rubbery plateau to fluid (or melt flow) state for the plastic domain. These two transition zones are independent of each other and are tailored by the polymer’s molecular structure, such as molecular weight, molecular weight distribution, volume fraction of rubber-plastic components and other factors. Moreover, the blend of thermoplastic elastomers with various tackifying resins can also greatly affect the status of glass transition and softening zones.

The high temperature and low temperature resistances of hot melt adhesives are relevant to the status of both transition zones. For a Styrene-Isoprene-Styrene (SIS) block copolymer-based hot melt pressure sensitive adhesive, the high temperature resistance can be correlated with the flow point, where the Styrene (plastic) predominate phase is softened from a rubbery state to a fluid state; and the low temperature resistance, independent of the styrene predominate phase, is related to the glass transition temperature of Isoprene (rubber) predominate phase.

In real world applications not every hot melt adhesive requires both high temperature and low temperature resistances. Most hot melt adhesives are developed or designed as general purposed products with specific adhesion performances at room temperature. The heat resistance of hot melt adhesives can only be improved to certain extents by introducing some specific reinforcement materials such as an aromatic hydrocarbon resin.

For more information call or email Pierce Covert,
Glue Machinery Corporation
1(888)202-2468 info@gluemachinery.com

Most hot melt pressure sensitive adhesives (HMPSAs) are based on Styrenic block copolymers (SBCs). SBCs consist of two consecutive domains: a plastic phase (styrene, S) and a rubber phase (di-ene). Typical di-ene phases are Isoprene (I), Butadiene (B), and their hydrogenated forms, Ethylene-Butylene (EB), and Ethylene-Propylene (EP).

Besides those differences in di-ene phases, SBCs are synthesized purposely into various ratios of styrene/di-ene (S/D) and di-blocks/tri-blocks; and melt flow rate (MFR, a test used to measure the flow properties or viscosity of thermoplastic high molecular weight polymers.) or melt index (MI). The typical ratio for S/D is in the range of 15/85 to 45/55. The ratio of di-block/tri-block is between 0/100 (a pure tri-block) and 80/20. The MFR varies from 3.0 to 100.

When SIS and SBS were initially developed in the early 1970, scientists tried to match their tensile strengths and elongations as those of Isoprene (IR) and Styrene Butadiene (SBR) rubbers. Those early-developed SBCs, e.g. 15% styrene and 20% di-block of a typical SIS, remain the most versatile and popular thermoplastic elastomers for use in the hot melt industry to date.

After SBCs were introduced to the hot melt industry - new applications were discovered and developed. Several new SBC grades with various combinations of S/D and di-block/tri-block are gradually developed in order to satisfy diverse market needs.

Tough SBC suppliers provide lots of technical data tables including chemical structures, physical properties, and typical adhesion performances based on reference formulations for their produced SBCs, very few of them can clearly explain why each individual grade of SBC offers its own unique adhesion performance. Two major questions are frequently asked by many adhesive formulators.

1. What is the correlation of these molecular structures and adhesion performance?
2. What molecular structure can affect processibility?

To address these two questions, again, one should have some background in viscoelasticity or rheology. We will discuss the correlation of molecular structures and adhesion performances more thoroughly in later articles. However, three important findings that answer the above questions are summarized as follow.

1. S/D ratio: the higher the %S - the stronger, harder and greater cohesion of adhesive blend that can be observed by a higher storage modulus (G’) at a rubbery plateau zone.
2. Di-block/tri-block ratio: the higher the di-block ratio - the better wetting of adhesive blend that is presented by higher Tan delta values at a rubbery plateau zone.
3. MFR value: the smaller the MFR value - the higher the molecular weight and heat resistance of adhesive blend, but the higher the melt viscosity (which may affect processability). The magnitude of MFR can be easily distinguished by a flow point, where Tan delta equals one.

More tips related to the viscoelasticity of SBCs and various tackifiers blends, and their correlations to adhesion performances and processability, will be discussed in future articles.

For more information call or email Pierce Covert,
Glue Machinery Corporation
1(888)202-2468 info@gluemachinery.com

HMPSAs are typically composed of the following ingredients:

1. Styrenic Block Copolymers (SBCs)
SBCs offer cohesion, strength, and heat resistance for HMPSAs. Styrene phase forms a physically cross-linked network in the adhesive at room temperature. SBC melts and becomes flowable at the temperatures beyond the glass transition temperature of styrene domain, about 200 to 230°. Four typical SBCs are available in the hot melt adhesive market: Styrene-Isoprene-Styrene (SIS), Styrene-Butadiene-Styrene (SBS), Styrene-(Ethylene-Butylene)-Styrene (SEBS, the hydrogenated SBS), and Styrene-(Ethylene-Propylene)-Styrene (SEPS, the hydrogenated SIS). Each SBC has its own specific molecular structure and is able to formulate into specific formulations for diverse applications. Bound styrene (% styrene) and degree of coupling (% tri-block) included in the SBC; and melt flow rate (MFR) or Melt Index (MI) are three key molecular structures affecting adhesion performances and processability of HMPSAs.

2. Tackifiers
Tackifiers are low molecular weight oligomers based on petroleum or natural feedstock with softening points ranging from below room temperature to 320°. Tackifiers can provide specific adhesion and lower melt viscosity for adhesives.

Two families of tackifiers are commonly used for HMPSAs:

a) Petroleum hydrocarbon resins: C5 (aliphatic), C9 (aromatic), C10 (Di-Cyclo-PentaDiene, DCPD), C5/C9 (co-tackifiers), and C10/C9 (co-tackifiers).

b) Natural resins: Rosin, Terpene, and their derivatives.

The selection of tackifiers is primarily dependent upon the SBCs used and application markets. HMPSAs are clear or transparent when tackifiers and SBCs are compatible. For less or incompatible SBC-tackifier blends, they are cloudy or opaque.

3. Plasticizers
Plasticizers can efficiently reduce hardness, lower viscosity and improve low temperature resistance of HMPSAs. They can also lower adhesive cost. Two types of plasticizers are used in HMPSA formulations: mineral oils and butene oil. Mineral oil is a mixture containing various percentages of paraffinic, naththenic, and aromatic component. Different mineral oils may greatly affect adhesion performances, particularly for low temperature and high temperature resistance.

4. Antioxidants
Antioxidants are used to prevent or minimize degradation resulted from heat aging, mechanical shearing, and long term storage.

All the components used in the HMPSA formulations are 100% solid without volatile organic compounds. (Note: mineral oil is also considered as a 100% solid because it does not evaporate or heat loss during production and application.) They are safe, free of fire and explosion concerns, during production, storage, and application. Below are some physical properties of those used components.

For more information call or email Pierce Covert,
Glue Machinery Corporation
1(888)202-2468 info@gluemachinery.com

Hot melt adhesives (HMAs) and hot melt pressure sensitive adhesives (HMPSAs) have been widely used in manufacturing for over 40 years. Almost every industry including packaging, bookbinding, woodworking, hygiene, construction, automotive, electronics, shoemaking, textile lamination, product assembly, tapes and labels use hot melt adhesives extensively. What are these materials?

HMA is a 100% solid adhesive which is applied in a molten state to achieve flow and wetting. HMA relies on cooling to a solid to give a serviceable bond. HMAs generally remain as thermoplastics after application.

A HMPSA pressure sensitive hot melt is a HMA which retains the ability to form a serviceable bond under light pressure at room temperature. Pressure sensitive adhesives are very tacky and have unlimited open time - meaning that they can bond to another substrate at most any time. HMPSAs are commonly used to manufacture pressure sensitive tapes and labels.

HMA can be categorized into two major families: non-formulated and formulated HMAs. Non formulated HMAs are intentionally synthesized as serviceable adhesives without further modifications by other materials such as tackifiers. Typical non-formulated HMAs are Poly-Esters (PET), Poly-Amides (PA), Poly-Urethanes (PU), and Poly-Olefins. They offer appreciable “hot tack” or the ability of hot melt adhesives to hold substrates together prior to solidification or set or bonding strength while they are heated and bonded at elevated temperatures.

Formulated HMAs are composed of thermoplastic elastomers, tackifiers and other ingredients. Unlike those non-formulated HMAs, these basic thermoplastic elastomers alone are not tacky at either room temperature or elevated temperatures. Three commonly used basic thermoplastic elastomers are Styrenic Block Copolymers (SBCs), Ethylene Vinyl-Acetates (EVAs), and Amorphous Poly-Olefins (APOs). These thermoplastic elastomers are modified by various types of tackifiers (natural and synthetic resins) which generate diverse adhesion performances according to specific market needs.

Most HMAs are normally based on EVAs. These products exhibit relatively short open time (typically less than 10 seconds) and fast set speed. Only very minimal tack is detectable on the adhesive surface at room temperature. HMPSAs are primarily based on SBCs. They are permanently tacky at room temperature and offer good bonding strength under a light finger pressure. APO-based HMAs offer very long open time after they are applied and cooled from their molten stage. However; they are not permanently open and will lose most of the surface tack once they are completely set. This unique characteristic is very useful for those bonding processes requiring long open time but low surface tack after bonding. The low residual surface tack will avoid future contamination at the edge of those bonding areas.

What is the perfect HMA and/or HMPSA? In reality, there is no such perfect product. All adhesives must be designed or formulated according to actual needs. How do we select an appropriate HMA or HMPSA for actual needs? Before one can select an optimum product for a specific application, both the end use adhesion performances and application techniques must be clearly defined. More discussions for each individual market’s requirements will be addressed in future articles.

For more information call or email Pierce Covert,
Glue Machinery Corporation
1(888)202-2468 info@gluemachinery.com

EVA is a 100% solid, transparent, flexible Ethylene Vinyl-Acetate copolymer. It is normally categorized by percent VA content and melt flow rate (MFR) or melt index (MI). Most commonly used EVAs contain between 19% and 28% VA. The MI values are mostly ranging from 3 to 2500.

In an EVA-based HMA formulation, a greater VA content can result higher transparency, more polar, extended flexibility, and better wettability to substrates. The MI value is a simple indicator of molecular weight. A lower MI value reflects a higher molecular weight, cohesion, and internal strength.

Most EVA-based HMAs do not have significant surface tack and are not pressure sensitive at room temperature. They need to be bonded at elevated temperatures by hot tack. They normally impart relatively short open time (typically less than 10 seconds) and fast set time. Open time and set time are primarily determined by the wax used - if it is included in the formulation, VA content, and the compatibility of EVAs and tackifiers. Typical waxes used in hot melt adhesive formulations are: synthetic, micro-crystalline and poly-ethylene wax. Waxes can significantly shorten open time and speed up set time. EVAs with more VA content will offer longer open time and slower set time.

Hot tack is a term relating to the ability of HMAs to hold substrates together prior to solidification or set.

Open time is the time after adhesive is applied during which a serviceable bond can be made. Many factors affect open time, including temperature, substrate, adhesive, and amount of adhesive applied.

Set time is the time it takes to form an acceptable bond when two or more substrates are combined with an adhesive.

When EVAs are blended with various tackifiers, they may exhibit a different open time, set time, cloud point and specific adhesion performance - depending upon the type of tackifiers used or their compatibilities.

Cloud point is the temperature at which a HMA becomes cloudy upon cooling from a molten to a solid stage. Cloud point is a good indication of compatibility of the formula components.

Generally, when EVAs and tackifiers are more compatible, the blended HMAs are more transparent. Moreover, they can exhibit lower cloud point, longer open time, slower set time and better adhesion to varied substrates.

In practice, a balance of open time and set time are key parameters to ensure satisfactory bonding and therefore obtain optimum adhesion performances. Beside those effects resulting from VA content, the polarity of tackifiers is another important factor which will significantly affect the degree of physical adsorption between the interface of hot melt adhesives and substrates. Hot Melt Adhesives with higher physical adsorption will result in a better wetting and adhesion.

In summary, there is no ideal EVA Hot Melt Adhesive formulation for diversely bonding applications. The optimal formulation must be developed specifically to the substrates to be bonded (polarity and surface roughness), application techniques (applicators, speed, and bonding temperature), and the end users’ servicing environment (temperature and humidity).

For more information call or email Pierce Covert,
Glue Machinery Corporation
1(888)202-2468 info@gluemachinery.com

Polypropylene is one of most versatile plastics used in our daily life. Two types of Polypropylenes are made by chemical manufacturers, i.e. iso-tactic polypropylene (IPP) and a-tactic polypropylene (APP). IPP is a tough, crystalline polymer, which is primarily used for manufacturing of film and molding. APP is a high softening point, soft, and slightly rubbery amorphous polymer. It is a minor by-product of IPP manufacture. Most APP is used as a modifier in asphalts for road pavement and roofing. It has also used in the hot melt industry as a primary polymer or a modifier for many years. Since the 1980s, the availability of this by-product has been reduced due to the high yield of produced IPP. To compensate for the shortage of APP, which has been widely consumed in hot melt applications, the tailor manufactured amorphous poly-olefins (APOs) are therefore developed and mass produced.

Various olefin copolymers are found in commercially available APOs. They are propylene homo-polymer - amorphous propylene (APP); co-polymer (two component) - amorphous propylene/ethylene (APE), amorphous propylene/butene (APB), amorphous propylene/hexene (APH); and ter-polymer (three components) - propylene/ethylene/butene. The order of hardness of various APOs is: APP > APE > APB > APH.

APOs alone provide relatively lower viscosities, 400 cps to 30,000 cps at 375°, and better low temperature adhesion than EVAs (Ethylene Vinyl-Acetate). APOs are non-polar and resistant to acid-base and organic solvent. They exhibit very high Ring and Ball softening points (ranging from 210° to 320°F) but their cohesive strengths are rather low and tend to creep at elevated temperatures.

Some APOs alone could be used as adhesives for certain applications, however, the modification of APOs with various tackifiers, plasticizers (e.g. poly-butene oil and mineral oil), and/or waxes, can greatly extend the usefulness of APOs in diverse application markets. Similar to EVA-based Hot Melt Adhesives, APO-based Hot Melt Adhesives are bonded to substrates while they are heated at elevated temperatures. These adhesives offer relatively long open time and slow set time compared to those of EVA-based HMAs.

Most chemists believe that the low polarity character of APO is the primary reason why APO-based hot melt adhesives can offer good adhesion to low surface energy materials such as PE and PP. In fact, the “like dissolve like” principle is good for chemical mixing, but is not applicable to adhesion behavior. On the contrary to the “like dissolve like” principle, materials with greater polarity differences are the key factors that create higher physical adsorption and result in better adhesion.

What is the major reason why non-polar APO-based hot melt adhesives can offer good adhesion on most substrates? APO-based hot melt adhesives exhibit relatively low cohesion or internal strength. Upon separation, when an external force is applied to the adhesive layer, it is first elongated; then, most applied forces are dissipated within molecular chains by means of disentanglements. Only minimally applied forces can actually reach the interface of the adhesive and substrates. As a result, cohesive failure is commonly observed for APO-based HMAs. The fracture energy resulted from a cohesive failure mode is often much higher than that determined from an adhesive failure mode.

In summary, APO-based HMAs are adhesives with long open time and slow set time. They are chemically inert, offer low cohesion, and can easily adhere to low surface energy materials.

For more information call or email Pierce Covert,
Glue Machinery Corporation
1(888)202-2468 info@gluemachinery.com

Most hot melt users anticipate achieving a high heat resistance with a “high softening point” hot melt adhesive. Softening point, like the viscosity measurement, is simply an indicator used to evaluate if adhesives are consistent between batches or lots.

Softening point is not heat resistance and customers must carefully evaluate their adhesive where temperature resistance is required.

Softening point is the temperature at which a material softens sufficiently to allow significant flow under a small stress. Most hot melt adhesives are measured by a Ring and Ball apparatus according to the ASTM D-2398 Test Method. Softening point is heating rate dependent. If the heating unit cannot provide a consistent heating rate, the determined softening point may be varied. The faster the heating rate - the higher the softening point observed.

For example, any APO-based hot melt adhesives may present very high softening points and yet they actually do not resist flowing in the real world applications. This is because most APO-based adhesives do not have sufficient cohesion to resist both inter and intra-molecular flow upon heating.

To evaluate the heat resistance, one must perform the following tests: shear adhesion fail temperature (SAFT), peel adhesion fail temperature (PAFT), or holding power (shear) at a fixed high temperature.

For more information call or email Pierce Covert,
Glue Machinery Corporation
1(888)202-2468 info@gluemachinery.com