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Adhesives Ingredients
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Adhesives Ingredients
Article

Bonding Solutions for Low Surface Energy Substrates

Edward M. Petrie – Feb 27, 2017

Bonding Low Energy Surface (LSE) Materials Certain polymeric substrates, such as polyolefins, fluoropolymers, and silicone rubber are difficult to bond and present challenges to both the formulator and the end-user. The main reason that these materials present problems is their low surface energy which is unlike metals, ceramics, and most other polymers.

The low surface energy simply prevents conventional adhesives from making intimate contact with the substrate surface and this reduces adhesion.

While finding solutions for better bonding in these substrates, the joint interface becomes the overriding concern. Proper selection of surface treatment and adhesive system for the specific substrate are the main steps involved in improving the process. These bonding mechanisms are well established and represented by important theories of adhesion.

Let’s see them in detail…

Theories of Adhesion


Surface Energy of Substrate


The adsorption theory of adhesion states that adhesion results from molecular contact between two materials and the resulting surface forces that develop. The process of establishing intimate contact between an adhesive and the adherend is known as wetting.

After contact is achieved between adhesive and adherend through wetting, the adhesive solidifies to gain the cohesive strength necessary for a good bond.

Good and Poor Wetting


 − Good wetting occurs if the adhesive spreads out over the substrate in a uniform film (e.g. epoxy adhesive on metal substrate).
 − Poor wetting occurs when the adhesive forms droplets on the surface (e.g., epoxy adhesive on fluoroethylene propylene substrate).


Figure below illustrates good and poor wetting of a liquid adhesive spreading over a surface:


Contact angle of uncured epoxy adhesive

Contact angle of an uncured epoxy adhesive on four surfaces of varying critical surface tension 


Ideally for an adhesive to fully wet a surface, the adhesive should have a lower surface tension, γ, than the substrate’s surface energy (or critical surface tension), γc. Thus, one of the rules relevant to bonding low energy substrates is that:

Good and poor wetting of substrate


Some important implications develop out of this concept about Epoxies and similar adhesives that they would:

 − Bond very well to metal, glasses, and other high-energy surfaces
 − Bond poorly to polyethylene, fluorocarbon, and low energy surfaces


Shown below is the table listing surface tensions of common adherends and adhesive liquids:

Solid Materials
Critical Surface Tension
(dynes / cm)
Acetal   47 
ABS
35 
Cellulose  45 
Epoxy  47 
Fluoroethylene propylene  16 
Polyamide  46 
Polycarbonate  46 
Polyethylene terephthalate  43 
Polyethylene  31 
Polymethylmethacrylate  39 
Polystyrene  33 
Polytetrafluoroethylene  18 
Polyvinyl chloride  39 
Silicone  24 
Aluminum  ~500 
Copper  ~1000 

Critical Surface Tensions for Common Solids 


Liquid Materials 
Critical Surface Tension (dynes / cm)
Epoxy resin  47 
Fluorinated epoxy resin  33 
Glycerol  63 
Petroleum lubricating oil  29 
Silicone Oils  21 
Water  73 

Critical Surface Tensions for Common Liquids 


One would also expect from the above values that polyethylene and fluorocarbon polymers, if used as adhesives, would provide excellent adhesion to a variety of surfaces including low surface energy polymers and metals. In fact, they do provide excellent adhesion. However, commercial polyethylene generally has many low molecular weight constituents. These create a weak boundary layer which prevents practical adhesion, and fluorocarbons cannot be easily melted or put into solution.

Thus, fluorocarbons are difficult to get into a fluid state to wet the surface and solidify without significant internal stresses. However, polyethylene does make an excellent base for hot melt adhesive once the weak low molecular weight constituents are removed. Researchers are attempting to develop epoxy resins with fluorinated chains that can easily wet most surfaces.


It is always interesting to note:

 − Why silicone and fluorocarbon coatings provide good mold release surfaces. Most resins will not easily wet these surfaces.

 − Why mineral oil, oils from touching the substrate, etc. provide weak boundary layers. These contaminants will spread readily on any substrate because of their low surface tension and most adhesives would not wet a surface contaminated by these oils.

 − That by making a coating (or adhesive) more likely to wet a substrate by lowering its surface tension you may inadvertently make it more difficult for subsequent coatings or adhesives to bond to this material once it is cured. Graffiti resistant paints work in this manner.


Adhesive / Substrate Miscibility


Another adhesion theory, the diffusion theory, can be of help in choosing adhesive systems for low surface energy polymers. The fundamental concept of the diffusion theory is that adhesion arises through the inter-diffusion of molecules from one material to another across the interface.

Wetting is followed by inter-diffusion of chain segments across the interface to establish an entangled network of molecules around the joint interface as shown in the figure below. For this to occur, the adhesive and the adherend must be chemically compatible in terms of diffusion and miscibility.

Inter diffusion of poly mermolecules

Interdiffusion of polymer molecules across an interface1 


The diffusion theory explains why certain monomers and solvent that are used in adhesive and primer formulations provide high bond strength on untreated low energy surfaces such as polypropylene. The adhesive or primer molecules diffuse into the substrates and create sites of molecular interlocking. These products offer a method of obtaining high bond strength to low energy substrates without a secondary surface treating process.


 » Continue reading to explore surface treatment processes and some proficient adhesives to overcome problems related to bonding in LSE substrates. 

3 Comments on "Bonding Solutions for Low Surface Energy Substrates"
VINAYAK A Aug 9, 2017
Reading articles by Edward is a great enjoyment. Thanks for sharing Fundamental yet very important information I am searching for such informative article on NON -SILICONIC RELEASE COATS and Manufacturers for the same.
emil s Mar 2, 2017
Corona is good for flat and plane materials.It last for a very limited time. TACKTREAT or Plasma+TACKTREAT are als for 3D materials.It last much longer up to 30 days or even more ,depending on the material.Very important is to use the rigth,adopted and devolped to the process waterbased UNIBOND WB adhesives
emil s Mar 2, 2017
I miss there in the treatment the - PLASMA - TACKTREAT-UVC - Combination of PLASMA and TACKTREAT all VOC-free use with waterbased UNIBOND WB adhesives 1com,blocked or 2 comp. This are already in use and published

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