Hot Melt News

Open time and set time are two key processing parameters for EVA-based hot melt adhesives (HMA) which will significantly affect the extent of instantaneous bonding and eventual fracture energy. It is not necessary to offer long open time and fast set time for any application. The optimum open time and set time are determined by the application system, actual bonding process, and bonded substrates.

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.

It is easy to define open time and set time described above. In most HMA labs, however, the open time and set time are often determined by touch, which is a very fast but subjective test. Test methods are described as follows…Read Article>>

Open Time and Set Time of EVA-Based Hot Melt Adhesives
- by Pierce Covert, Glue Machinery Corporation

What is the mechanism of bringing two similar or dissimilar materials together? Below are five commonly discussed adhesion mechanisms.

1. Physical Adsorption
2. Chemical Reaction
3. Inter-diffusion
4. Electrostatic Attraction
5. Mechanical Interlocking

Physical Adsorption:
All materials have different polarities resulting from the density of electron clouds surrounding their chemical element or functional group. The magnitude of force attributed from these electron clouds, from low to high forces are Van der Waals force, dipole-dipole moment, hydrogen bonding, and acid-base interaction. The greater force is the spit of electron clouds or polarity difference; the better force is the attraction force when two materials are brought together. In general, polar substrates and adhesive offer higher polarity and cause higher adhesion forces… Read Article>>

Principles of Pressure Sensitive Adhesion
- by Pierce Covert, Glue Machinery Corporation

Hot Melt Pressure Sensitive Adhesives (HMPSAs) have been widely used for making medical pads and tapes in the past couple decades. Prior to introducing HMPSAs to this industry, both calendared and solvent-borne natural rubber (NR) based PSAs dominated this market. NR PSA is normally tackified by rosin or its derivatives. This type of PSA generates very good adhesion to human skin because of its high acidity or polarity.

Beginning in 1990, HMPSA and Acrylic water-borne PSA were introduced to skin contact applications. Disregarding those common adhesion performances, three major requirements must be met for medical pad and tape applications… Read Article>>

Skin Contact Hot Melt Pressure Sensitive Adhesives
- by Pierce Covert, Glue Machinery Corporation

Selection of an adequate mineral oil for a Styrenic Block Copolymer (SBC)-based adhesive is very important. Although most mineral oils look alike, they impart very different compatibilities to the SBC used and will greatly affect both adhesion and aging performances of Hot Melt Pressure Sensitive Adhesives (HMPSAs).

Mineral oils are a necessary component for most SBC-based HMPSAs. Below are some of the major advantages contributed by mineral oils.
1. Significantly reduce the viscosity for easy mixing and processing.
2. Lower the hardness of adhesive for better deformation and wetting.
3. Decrease glass transition temperature (Tg) to improve low temperature resistance.
4. Lower the total raw material cost of HMPSAs. Among all used ingredients in most SBC-based HMPSAs, mineral oil is normally the cheapest raw material.

Mineral oils are extremely complicated mixtures including various ratios (contents) of the following major hydrocarbon species… Read Article >>

Selection of Mineral Oils for Hot Melt Pressure Sensitive Adhesives
- by Pierce Covert, Glue Machinery Corporation

Hot Melt Pressure Sensitive Adhesives (HMPSAs) are composed of Styrenic Block Copolymers (SBC), tackifier, mineral oil, a small amount of antioxidant, and other special additives - such as filler, colorant, and low molecular weight polymer if needed. Most of these ingredients are thermoplastic materials and require a heated environment to mix them together.

Many different types of mixers are available for mixing HMPSAs. Following are two commonly used techniques.
1. Vertical mixer. This is the most economic technique for making HMPSAs. Low molecular weight ingredients, oil and/or tackifiers, must be charged first for this type of mixer. Otherwise, the high torque generated from SBC may damage agitators. Commonly used charging sequence is… Read Article>>

Production of Hot Melt Pressure Sensitive Adhesives
- by Pierce Covert, Glue Machinery Corporation

“Cloud Point” is the temperature at which a hot melt adhesive becomes cloudy on cooling and is a measure of compatibility of the formula components. Hot Melt Adhesives (HMA) with a lower cloud point normally impart longer open time and better hot tack. A hot melt adhesive possessing a very low cloud point may present a certain degree of undesired surface tack.

Following is the simplest method to determine the cloud point of HMA…(Read Article)

…Although this is a cost free, very simple and subjective test, the repeatability is good and relatively accurate with a careful observation.

If the VA (vinyl acetate) domain of EVA exhibits better compatibility with the incorporated tackifying resins, the blend remains clear for a longer time during cooling and results in a lower value of cloud point. A blend of EVA with high VA content and rosin esters may form transparent HMAs even though they are fully set at room temperature. Normally, a more polar tackifying resin blended with an EVA will present a lower cloud point… More >>

Cloud Point for an EVA-Based Hot Melt Adhesive
- by Pierce Covert, Glue Machinery Corporation

Although many specifications are listed on the technical data sheet (TDS), the diameter of glue sticks is the most important specification for a successful glue gun application. Most glue guns are designed to accept glue sticks with certain diameters. Although a tolerance is permitted for the glue sticks, e.g. 11-12 mm, a consistent diameter is always desired for long term processing. Glue sticks with a larger diameter may not fit properly and may not be smoothly inserted into the gun. Conversely, a smaller diameter will be too loose upon triggering the stick. An accurate diameter of glue stick will provide a consistent adhesive output without damaging the glue gun. An inconsistent output, or bead size, can also result in poor bond strength.

All the polymeric materials impart a certain degree of shrinkage upon cooling from high temperatures to low temperatures. When hot melt adhesives are made in a mixer at elevated temperatures, the polymeric molecular chains are fully extended and orientated. Upon cooling at the holding tank before extrusion, the polymeric molecular chains can relax and coil back to a certain extent. When the adhesive is extruded… Read Article >>

Key Requirements for Hot Melt Glue Stick Production
- by Pierce Covert, Glue Machinery Corporation

Many HMPSA (Hot Melt Pressure Sensitive Adhesives) users frequently request good products at low prices without knowing or telling suppliers what they actually need. In reality, there is no ideal product for a variety of applications. Each application may need a specific performance of the HMPSA.

How does a manufacturer select an appropriate HMPSA for a specific use? Adhesive users should always consider the following subjects:

Price: Price is not always related to the raw material cost, processibility and adhesion performance. However, it is directly related to the volume purchased, color, and lot-to-lot consistency. Most water white HMPSAs contain hydrogenated hydrocarbon tackifying resins, which are generally more expensive than non-hydrogenated yellow tackifying resins. Quality raw materials normally offer narrower specifications and result in more consistent products. These quality materials are normally more expensive.

Quality: Quality is not equivalent to processibility and adhesion performance. Instead, quality is a reflection of aging stability and the consistency of color, clarity, and other physical and adhesion performances. Adhesives with the correct bonding and machining characteristics do no always have to be the most expensive product to be considered of high quality. It is worth emphasizing … Read Article >>

What Are “Good” Hot Melt Pressure Sensitive Adhesives?
- by Pierce Covert, Glue Machinery Corporation

Shear adhesion, or holding power, is one of the most important pressure sensitive adhesives (PSA) performances. Shear adhesion is the ability of a tape to resist the static forces applied in the same plane as the backing. It is usually expressed in time required for a given weight to cause a given amount of tape to come loose from a vertical panel.

What is the determining molecular parameter that can significantly affect shear adhesion? Most adhesion researchers propose that Me (Molecular weight between two entanglements) is the controlling factor of shear adhesion. In practice, it is found that all the following parameters can affect shear adhesion to certain extent… Read Article >>

Shear Adhesion of Pressure Sensitive Adhesives
- by Pierce Covert, Glue Machinery Corporation

Tack, or quick stick, is one of the most important properties of pressure sensitive adhesives (PSAs). The definition of tack is the property of a pressure sensitive adhesive that allows it to adhere to a surface under very slight pressure. It is determined by the ability of the adhesive to quickly wet the surface it contacts.

There are four standard test methods that are used globally to evaluate a pressure sensitive hot melt’s tack. They are Loop Tack, Probe Tack, Rolling Ball Tack, and Quick Stick. Although these test methods do not result in the identical values for a fixed PSA, they can distinguish relative adhesion performance for different PSAs. The descriptions of various test methods are listed below.

Loop tack has recently become the most reliable test method because it can generate reproducible and consistent data. Most tape and label manufacturers now prefer…. Read Article >>

Tack of Pressure Sensitive Adhesives (PSAs) - by Pierce Covert, Glue Machinery Corporation