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Hot Melt Adhesives for Low Surface Energy Substrates

Edward M. Petrie & Edward Petrie – Feb 7, 2018

Hot Melt Adhesives for LSE Substrates Low energy polymeric surfaces, such as polyolefins, fluorocarbons, acetals, and polystyrene, are notoriously difficult to bond with adhesives. Wetting becomes a challenge when using conventional adhesives to bond these low surface energy (LSE) substrates.

Traditionally, LSE plastics have been primed or pretreated with flame, corona, or plasma to raise the surface energy so that they can be bonded with conventional adhesives. While finding solutions for better bonding in these substrates, the joint interface becomes the overriding concern.

Thus, in order to find bonding solutions for LSE substrates; first ensure that you have a clear understanding of LSE, difficulties associated with them and hence their respective adhesive bonding requirements.

To ease your problems! We have come up with a solution - Hot melt adhesives for bonding LSE substrates. Certain hot melt adhesives are capable of offering moderate bond strengths to these substrates without expensive and time consuming substrate pretreatment. For this reason, they are primarily used in:

 − Packaging
 − Automotive
 − Component assembly
 − Medical and electronic assembly, and
 − Textiles

Check out the strengths and weaknesses of using hot melt adhesives for bonding LSE substrates from the table below:

Advantages Disadvantages
 − Forms bonds rapidly (high assembly speeds and short fixturing time)
 − Clean, easy handling
 − Little waste
 − Easy disassembly and repair of joints
 − No problems with solvent or solvent vapors
 − Good storage life and simpler storage requirements
 − Precise bond line control can be achieved through temperature and pressure
 − Equipment available for automated assembly
 − Easily maintained equipment
 − Minimal floor space required
 − Bonds lose strength at elevated temperatures
 − Some bonds may exhibit creep under stress and moderate temperatures
 − Adhesive may be sensitive to moisture and chemicals
 − Some substrates may be sensitive to heat of application
 − Hot melt used in bulk form (heated tanks) may be subject to oxidation and require a nitrogen blanket


Explore some interesting tips on requirements for formulating optimal hot melt adhesive for your specific application.


Hot Melt Base Polymers for Formulating Adhesives for LSE Substrates


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

The table below summarizes the types of hot melt adhesive polymers that have generally been used in the manufacture of plastic products. (Continue reading or click on to go on specific polymer technology details)

Hot Melt Base Polymer Formulation Characteristics Variations
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
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 (APAO)   − Low cost with good acid and fuel resistance
  − Moderate heat resistance
  − Soft, tacky, and flexible
  − Long open times and good adhesion
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


Let's explore them in detail:


Ethylene Vinyl Acetate


The first copolymer to be used as a base for hot melt adhesives was ethylene vinyl acetate (EVA).

 − EVA resins are highly flexible products, compatible with many other polymers and additives, and easy to process.
 − 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.

The table below identifies the major advantages and limitations associated with EVA based hot melt adhesives.

Advantages Disadvantages
  − Broad formulating latitude necessary for many different applications
  − 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
  − Many formulations have FDA approval
  − Cold flow (creep); temperature resistance only to about 40°C
  − Attacked by some greases, oils, and solvents
  − High viscosity needed for maximum performance
  − The pressure sensitive nature of EVA is generally inferior to other polymers
  − EVA have a tendency to gel or char when subjected to typical application temperatures such as 150°-175°C
  − Cannot easily be formulated as a multi-purpose adhesive


EVA Resins in Hot Melt Adhesive Formulations


Generally, for hot melt adhesives EVA resins with vinyl acetate concentration of 18-40% are utilized. The vinyl acetate content can be a significant parameter in varying the properties of the adhesive.

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


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 (MI) 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.

Review the table given below for understanding: a starting formulation for a general purpose, EVA hot melt adhesive formulation for bonding difficult substrates (e.g., polyethylene and polypropylene).

Components Parts by Weight
Ethylene vinyl acetate copolymer resin
(28% vinyl acetate, melt index of 25)
50
Microcrystalline wax (190°F to 195°F melting point) 30
Tackifier (Zonarez B-125, Talas) 20
Properties Value
Brookfield viscosity, cps
 − @ 350°F
 − @ 390°F

 − 11870
 − 6760
Sealing temperature, °F 177
Adhesion, oz / inch:

 − Kraft to Kraft
 − Kraft to polyethylene
 − Kraft to aluminum foil

 − 33
 − 33
 − 39

The adhesive is stable at high temperatures. It will adhere to most packaging foils and films, including polypropylene. It has increased tackiness and heat sealable properties.

EVA Copolymer Resins for Packaging Applications


Packaging applications are a primary market for hot-melt adhesives that are based predominantly on EVA copolymer resins. A tackified EVA is necessary for bonding plastic substrates that have a relatively low surface energy.

A starting formulation for a low cost, general purpose EVA hot melt formulation for bonding plastic substrates is given in the table below, have a look!

Components Parts by Weight
EVA polymer (Elvax® 250, Dupont) 35.0
EVA polymer (Elvax® 260, Dupont) 10.0
Tackifier – Polyterpene resin, 115°C m.p. 30.0
Filler – Calcium carbonate (15 μm) 24.8
Antioxidant - Butylhydroxytoluene 0.2
Properties Value
Viscosity at 149°C, cps ~1500 - 2500
Melting temperature by DSC, °C ~150

Adhesive application temperature is critically important in bonding LSE substrates. This is because it directly affects how long the hot melt remains “open” during the manufacturing process. If the hot melt pot is not hot enough, open time will be too short, and the substrates will fail to adhere properly. If it’s too hot the adhesive may have a detrimental effect on temperature sensitive substrates, and the adhesive’s viscosity will be too thin creating excessive seepage.


 » Continue reading to explore more hot melt bonding solutions for low surface energy substrates.


2 Comments on "Hot Melt Adhesives for Low Surface Energy Substrates"
RAJENDRAN P Feb 13, 2018
Good introduction to formulation of Hot melt adhesives for diff substrates and raw materials of hot melt adhesives.
Larry S Feb 12, 2018
Your point, "Hot melt used in bulk form (heated tanks) may be subject to oxidation and require a nitrogen blanket" is well taken, but there are some new alternatives to this including 'tankless' (low residence time) systems and reduced degradation hoses (http://www.nordson.com/en/divisions/adhesive-dispensing-systems/products/hoses/pureflow-hoses ). Much of the oxidation takes place in the hose, so this is actually often more effective than a nitrogen blanket on the tank!

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