Hot melt adhesives are solvent-free chemical compounds that are used to join materials.  They can be repeatedly softened by heat and hardened or set by cooling, allowing parts to be removed or repositioned during assembly.  Most hot melt adhesives solidify at temperatures below 180° F and become low viscosity fluids at temperatures above 180° F.  They form strong bonds, set rapidly upon cooling, and are relatively easy to handle. In general, hot melt adhesives store well and are less sensitive to water, moisture, and humidity than thermosetting or chemically curing adhesives. 

Some products contain:

• Acetals
• Acrylics
• Glues and gums
• Natural or synthetic rubbers
• Epoxy resins
• Water-based resins
• Wax-based binders
• Styrene compounds
• Vinyl esters

Commonly used chemical systems also include: 

• Polypropylene (PP)
• Polysulphone
• Polyurethane (PUR)
• Polyetherimide
• Polyolefin

Filled or reinforced compounds consist of resins with additional modifiers such as fillers, pigments, or chopped fibers.

Features

Hot melt adhesives vary in terms of material compatibility and features. Some products adhere to substrates made of: 

• Ceramics
• Glass
• Concrete
• Masonry
• Metal
• Paper
• Paperboard
• Plastic

Products that can bond dissimilar substrates such as metal to rubber are also available. In terms of features, hot melt adhesives that are designed for electrical and electronics applications often provide protection against the following:

• Electrostatic discharge (ESD)
• Electromagnetic interference (EMI)
• Radio frequency interference (RFI)

Materials that are electrically conductive, resistive, insulating, or suitable for high voltage applications are also available.

Flame retardant materials reduce the spread of flames or resist ignition when exposed to high temperatures.

Dampening materials are used to form layers that can bend or flex without cracking or delaminating.

Thermal compounds and thermal interface materials absorb heat from electronic devices or electrical components. Threadlocking adhesives bind threads that are subject to transverse and axial loads against vibrational loosening.

Specifications

Selecting hot melt adhesives requires an analysis of mechanical, thermal, electrical and optical properties. Mechanical properties include: 

• Tensile strength (break)
• Tensile modulus
• Elongation

Thermal properties include: 

• Use temperature
• Thermal conductivity
• Coefficient of thermal expansion (CTE)

Thermal conductivity is the linear heat transfer per unit area for a specific temperature. CTE is the amount of linear expansion or shrinkage that occurs in a material with a change in temperature.

Resistivity, dielectric strength, and dielectric constant are important electrical properties for hot melt adhesives. Optical properties include index of refraction, a measure of the speed of light in a material.

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Applications

Hot melt adhesives are used in many industries and applications. For example, some products are used in: 

• Aerospace
• Automotive
• Marine
• Military
• Photonics

  

• Optical applications
• Tooling

Others are designed for use with: 

• Electronic devices
• Electrical power products
• High voltage applications
• Semiconductors
• Integrated circuit (IC) packaging

Materials that are suitable for medical, pharmaceutical and food processing applications meet requirements established by the Food and Drug Administration (FDA) and the U.S. Department of Agriculture (USDA).

Original equipment manufacturers (OEMs) and repair, maintenance and overhaul (MRO) organizations also use hot melt adhesives.

Standards

9981252 - Adhesive, hot melt, polyamide, supported vinyl to hardboard. 

ASTM D1994 - Standard test method for determination of acid numbers of hot-meltadhesives. 

ASTM D4499 - Standard test method for heat stability of hot-melt adhesives. 

BS 5350-H3 - Methods of test for adhesives - part h3: determination of heat resistance of hot-melt adhesives.

DIN EN ISO 10363 - Determination of thermal stability of hot-melt adhesives. 

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Kanishaka Overseas | Tex Year | 3M Industrial Adhesives & Tapes