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Hot-Melt Adhesives: Advances in Raw Materials

SpecialChem – Feb 13, 2020

TAGS:  Hot-melt Adhesives    

Raw Material Advances in Hot-Melt AdhesivesThe global hot-melt adhesives industry is driven by several evolving trends, such as:

  • Low odor
  • Low VOC
  • Solutions for hard-to-bond substrates, etc.

In addition, governments and regulatory bodies are placing limits on trace chemicals produced during the processing of raw materials used in HMA formulations.

Thus, the market increasingly demands solutions that deliver bond stability over time with minimal environmental impact.

Last year in Dubrovnik, FEICA brought together specialists during one breakout session “Developments in thermoplastic hot-melts” to discuss insightful developments which can provide a strong response to market needs.

Specialists from the key industry players presented raw material advances to help formulators and manufacturers develop new high-performance formulations to meet end-use application needs today. The key highlights include:

  1. Eastman’s low odor hydrocarbon resins
  2. Evonik’s APAO for foamed hot-melt applications
  3. Dow’s polar and non-polar ethylene copolymers

Let’s discuss these developments in detail.

#1. The Development of Low Odor Hydrocarbon Resins

Hot-melt adhesives (HMA) are used in the hygiene industry for non-woven materials such as diapers. These HMAs are majorly based on styrene block copolymers (SBCs) and contain hydrocarbon resins as tackifiers.

However, hydrocarbon resins have a very distinctive odor. During the processing of these resins, breakdown products may lead to the formation of odorous chemicals. The undesirable odors often cause an unfavorable perception and attract negative attention around product safety. Thus, leading to high demand to reduce odor, VOCs, and trace chemicals.

Odor Analyzing Techniques

Eastman has developed a technology that makes it possible to manufacture hydrocarbon resins with low odor and low amounts of trace chemicals. Eastman thoroughly analyzes the odor of raw materials during new hydrocarbon resin development. The techniques used to determine odor include:

Technique Name and Description
Headspace Analysis GC-MS Headspace Analysis – It is a sampling technique that involves the indirect determination of volatile constituents in liquid or solid samples. It includes analysis of vapor phase that is in thermodynamic equilibrium with the sample in a closed system.
GC-MS Sniffing (GC-olfactory Detection) – This technique combines the chemical information gained from gas chromatography with the sensory information provided by the human sensory system.
Odor panel testing Odor Panel Testing – It involves a group of 5-15 volunteers where each member is trained with olfactory awareness, sniffing techniques, standardized descriptors, and olfactometry responses.

Techniques Used During New Hydrocarbon Resin Development by Eastman

Eastman utilizes olfactory measurements and analytical techniques to determine, characterize, and measure the odor and VOC content of a final product.

Using the results from these techniques, chemical components such as aldehydes, alcohols, ketones, aliphatic hydrocarbons, and aromatic hydrocarbons can be significantly reduced from the:

  • Pure resin
  • Resin after aging for times applicable in the hot-melt industry

The resins developed with this technology are found to have improved odor profile and very low levels of trace chemicals and volatiles.

According to the company, combined GC-MS headspace analysis, GC sniffing, and odor panels with sensory profiling have proved to be powerful tools during product development.

#2. Amorphous Poly-alpha-olefins (APAOs) for Foamed Hot-melt Applications

Amorphous Poly-alpha-olefins (APAOs) are polymers of α-olefins and are especially useful to produce hot-melt adhesives. Recently, foamed hot-melt formulations have gained more and more interest. By increasing the volume of the hot-melt adhesive and thus reducing the material usage, foaming has become an attractive alternative in various applications.

In the past years, Evonik has been working to get an in-depth understanding of the relation between the technical properties of VESTOPLAST® such as viscosity & crystallinity content and different application systems, for example - foam or spray application. Evonik conducted a detailed evaluation of performance, application and machine parameters to get a better understanding of the conditions for foaming applications.

Developing Foamed Adhesives

First, the relation between flexibility and cohesion must be balanced to develop a perfectly foamed adhesive.

  • Good flexibility of the adhesive allows the nitrogen bubbles to expand once it is released out of the dispense head.
  • The cohesion keeps the bubbles inside the adhesive to create an excellent foam.
Hot Melt Adhesive Foaming Process
Hot-Melt Adhesive Foaming Process
(Source: Evonik)

The crystalline and amorphous parts of Evonik’s VESTOPLAST® APAO were found to provide the right balance. It combines amorphous and crystalline structural elements on the polymer chain leading to excellent wetting and adhesion properties as well as providing strong bonds on various substrates.

  • The amorphous parts are important for the flexibility and adhesion to different substrates.
  • The crystalline parts are responsible for the inner strength as well as the good cohesion, which keeps the nitrogen bubbles inside the hot-melt when it comes to the cool down of the foamed material to room temperature.

Up to 65% of nitrogen can be added for example to VESTOPLAST® to create a foam. Results show that the application temperatures can be lowered down to 130°C thus being especially suitable for temperature-sensitive material.
Unfoamed and Foamed VESTOPLAST® Under 125x Magnification (Source: Evonik)
Unfoamed and Foamed VESTOPLAST® Under 125x Magnification
(Source: Evonik)

Further different polymers (other APAOs and metallocene polyolefins (mPO)) were tested to study the effects of polymer structure on application systems. The foaming behavior of each material was evaluated step by step with a design of experiment.

  • The typical temperature of 150°C was tested, as well as lower temperatures of 130°C for more sensitive substrates and resource-efficient product and material handling.
  • The foaming process is started by adding nitrogen to a melt in a tank under pressure.
  • After releasing the mixture from the applicator, the nitrogen will expand and form bubbles, thus creating the foam.

Most machine parameters remained the same, to ensure reproducibility, reduce complexity and to check and balance the different behavior of hot-melt formulations depending on polymers identity and additive influences.

The extensive tests have shown that both physical properties of the polymer and the polymer structure influence the foaming behavior. It is necessary to have enough flexibility to allow the volume expansion as well as enough cohesion and crystallinity to keep the bubbles inside until the foam is hardened. 

Overall, VESTOPLAST® grades have shown excellent foamability results. These APAO grades could be considered for applications where the following properties are required:

  • High bond strength
  • Prolonged open time compared to the unfoamed product
  • Faster set times
  • Low application temperature for temperature-sensitive materials
  • Bond to porous or irregular surfaces
  • Reduced adhesive consumption
  • White color

Application Scope of Foamed Adhesives

Some well-known applications for foamed adhesives can be found in the air filter market as well as for mattresses.

Application Industry Requirements Benefits of APAO
  • Automotive air filter
  • Air condition: household, humidifier
  • Industrial air filter: air conditioning, clean rooms, hospitals, gas turbines
Foamed Adhesive air filter
  • Low odor and light color are crucial
  • Need for optimized production processes supported by the adhesive e.g. adjustable open time, control of crystallization time
  • Multipurpose adhesive
  • Foam able adhesive especially for pleat stabilization
  • Huge variety of different physical properties
  • Excellent adhesion and hot tack properties
  • Bonding to a variety of substrates (e.g. paper, polypropylene, glass fiber, metal)
  • Chemically inert
  • Various grades with different open and setting time
  • Easy to formulate
  • Mattresses
Foamed Adhesive Mattresses
  • Flexibility
  • Good adhesion to various substrates: latex, foams, coconut fiber, etc.
  • Fast bonding and green strength
  • High cohesion
  • Adjustable open time
  • Variable open time due to a broad spectrum of formulations with tackifier and waxes
  • Solvent-free
  • High green strength
  • Re-positioning possible
  • Low odor, white color, no bleeding
  • Low viscosity grades in combination with low softening point allow for low application temperature

#3. Polar and Non-Polar Ethylene Copolymers for High-performance Bonding

Dow offers a broad range of polar and non-polar ethylene copolymers solutions for the hot-melt adhesive industry. At FEICA, Dow showcased how these solutions can be used on their own or in combination to address key industry trends such as the increased usage of hard-to-bond surfaces.

HMAs are used in many industries such as packaging solutions, non-woven hygiene products, bookbinding, and furniture & woodwork, among others. Packaging solutions of HMAs accounts for nearly 40% of the HMA market.

Dow offers a broad range of polar and non-polar ethylene copolymer technologies for high performance and general-purpose packaging segments.

The main technologies include:

These solutions offer efficient bonding in the case and carton sealing packaging industry.
Hard-to-Bond Substrates – Carton Structures
Hard-to-Bond Substrates – Carton Structures
(Source: Dow)

Further, innovative combinations can address bonding challenges, such as higher temperature resistance and hard-to-bond substrates. These solutions also offer advantages, such as:

  • Superior thermal/oxidative stability
  • Balanced performance across a temperature range
  • Reduced equipment maintenance
  • Less down-time

FEICA 2020 - Meeting Place for the Adhesive and Sealant Industry

The 2020 FEICA European Adhesive and Sealant Conference & EXPO will take place from 9-11 September 2020 in Warsaw, Poland. Get ready to gain essential insights into these key issues affecting the industry, as well as great networking opportunities for formulators, customers, and suppliers through high quality break-out sessions & presentations and much more!
FEICA 2020

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