Bonding Solutions for Low Surface Energy Substrates

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 us discuss different aspects which you should consider while bonding low energy substrates.

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:





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), γ_c. Thus, one of the rules relevant to bonding low energy substrates is that:



Some important implications develop out of this concept about epoxy 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

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

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.

Related Read: Adhesion Myths and Reality - A Complete Guide

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.

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.

Surface Treatment Processes to Optimize Interface

It is also important to note that with a low energy substrate and an adhesive with relatively high surface tension, surface roughening as a pretreatment before bonding generally does not always improve the resulting bond strength. In fact, it usually degrades the bond strength. This is because surface roughening does not change the surface energy, but the many grooves and valleys that it creates on the substrate surface will not fill with adhesive before cure due to lack of wetting. The air remains entrapped between the substrate and the adhesive.

This reduces the effective bond area and creates stress risers at the interface. Thus, the best surface treatment for low energy substrates is to raise the surface energy through chemical or physical surface pretreating processes. Passive surface treatments such as abrasion may actually reduce the bond strength of low surface energy plastics. This way, solvent cleaning will remove soluble contamination without increasing the surface energy.

There are several surface treatment processes that can be used to raise the surface energy of thermoplastics. The specific process will depend on:

• The plastic involved
• The way it was processed, and
• The degree of adhesion required for the end-use application

Active methods used to improve the bonding characteristics of these polymeric surfaces include:

1. Oxidation via chemical or flame treatment
2. Electrical (corona) discharge to leave a more reactive surface
3. Ionized inert gas treatment which strengthens the surface by a chemical change (e.g., crosslinking) or physical change and leaves it more reactive
4. Metal-ion treatment that removes fluorine from the surfaces of fluorocarbons
5. Application of primers, adhesion promoters, and other wettable chemical species

Related Read: Adhesion Promoters - Basics & Material Selection Tips for Adhesives

These processes have been developed over time and are conventionally used in many production applications. The most common of these processes are summarized in Table below for specific thermoplastic substrate families.

Treatment	Polymer	Result
Flame	Polyolefins Nylon, and  Other low surface energy plastics	Oxidizes the surface introducing polar groups
Corona or electrical discharge	Polyolefins Polyethylene terephthalate PVC Polystyrene Cellulose Fluorocarbons	Oxidation and introduction of active groups Increased surface roughness
Plasma discharge	Nearly all low energy surfaces including: Thermoplastics Silicone rubber, and  Other low energy elastomers	Crosslinking of the surface region Surface oxidation with the formation of polar groups Grafting of active chemical species to the surface Halogenation of the surface
UV radiation	Polyolefins Polyethylene terephthalate EPDM rubber, and Other low surface energy polymers	Chain scission of surface molecules followed by crosslinking Surface oxidation
Laser treatment	Polyolefins Engineering plastics Sheet molding compounds	Removal of surface contamination and weak boundary layers Roughening of mineral filled substrates
Oxidizing acids	Polyolefins ABS Polycarbonate Nylon Polyphenylene oxide, and  Acetal	Oxidation of the surface Reactive groups introduced on the surface Cavities formed to provide interlocking sites
Sodium naphthalene etch	Fluorocarbons	Dissolves amorphous regions on the surface and removes fluorine atoms Increases mechanical interlocking by micro-roughening
“Cyclizing” concentrated sulfuric acid	Natural rubber SBR Nitrile rubber Other elastomers	Hairline fractures on the surface create features for mechanical interlocking

Other Surface Preparation Processes

Other, newer surface preparation processes are now being developed specifically for the increased usage of engineering plastics and composites for light weight, energy-saving vehicles in the automotive and aerospace industries.

There are a number of sources of information for plastic surface treating processes. It should be remembered that the major sources for this type of information are the suppliers of the adhesive and the substrate material. There are also several text books^2,3,4 that provide excellent reviews regarding specific surface treatments for a variety of substrates.

ASTM D2093 also describes recommended surface preparations for several plastic adherends. Solvent and heat welding are other methods of fastening plastics that do not require chemical alteration of the surface since conventional adhesives are not used. However, cleaning or degreasing is recommended as a pretreatment for these plastic parts since weak boundary layers are still a matter of concern.

Adhesives for Low Surface Energy Substrates

Even with the difficulties provided by low surface energy, adhesive bonding can be an easy and reliable method of fastening one type of plastic to itself, to another plastic, or to a non-plastic substrate. Pocius, et. al., provides an excellent treatise on the use of adhesives in joining plastics⁵. There are also several articles on joining of plastics that cover adhesive bonding, thermal and solvent welding, and mechanical joining⁶.

Related Read: Hot Melt Adhesives for Low Surface Energy Substrates

The physical and chemical properties of both the solidified adhesive and the plastic substrate affect the quality of the bonded joint. Other than surface energy, major elements of concern of the substrate relative to the adhesive are:

• Thermal expansion coefficient
• Modulus, and
• Glass transition temperature

Table below identifies important characteristics to consider when selecting an adhesive for bonding low energy plastic substrates.

Characteristic	Function
Low surface energy	Ideally the adhesive should have a surface energy that is less than that of the plastic substrate
Flexibility	To accommodate differences in thermal expansion To accommodate stress concentration due to peel forces and joint design Modulus should be similar to the substrates
Compatibility with Substrate During Application and Service	The plastic substrate could be temperature sensitive prohibiting high curing temperatures Solvent and monomers in the adhesive could result in stress cracking of the substrate Must be resistant to migration of additives (e.g., plasticizers) within the substrate
Glass transition temperatures	For structural adhesives, the glass transition temperature should be greater than that of the substrate For toughness, peel strength, and low temperature resistance the glass transition temperature should be low but appropriate for the service temperature of the application

Reducing Stress at the Interface

Significant differences in thermal expansion coefficient between the substrate and the adhesive can cause severe stress at the interface. This is common when plastics are bonded to metals because of the approximately 10X difference in thermal expansion coefficients between the substrates. Residual stresses are compounded by thermal cycling and low temperature service. Selection of a resilient adhesive or adjustments in the adhesive's thermal expansion coefficient via fillers or additives can reduce such stress.

Bonded plastic substrates are commonly stressed in peel because the part thickness is usually small and the modulus of the plastic is low. Tough adhesives with high peel and cleavage strength are usually recommended for bonding plastics.

Get Inspired: Bonding Dissimilar Materials - How to Reduce Internal Stress Levels

Structural Adhesives

Structural adhesives are one or two component thermosetting system. Whereas, non-structural adhesives are generally hot melt or pressure sensitive adhesives. Although flexible epoxy and polyurethane adhesives are often used for structurally bonding plastics, they are not considered to be the best adhesives for low surface energy plastics. Adhesives generally used for these harder to bond substrates include:

• Thermosetting acrylic
• Cyanoacrylate, and
• Light curing acrylic and Cyanoacrylate structural adhesives

Structural adhesives must have a glass transition temperature higher than the operating temperature or preferably higher than the part that is being bonded. This is so to avoid a cohesively weak bond and possible creep problems at elevated temperatures. Some engineering plastics, such as polyimide or polyphenylene sulfide, can have very high glass transition temperatures.

Table below lists the structural adhesives that are commonly used for bonding plastics:

Structural Adhesives	Non-Structural Adhesives
Flexible Epoxy: Ployamide cure Epoxy-polysulfide Epoxy-Polyurethane Polyurethane Two component reactive One component moisture cure (one substrate must be porous) Thermosetting Acrylic Cyanoacrylate Light Curing Adhesives Cyanoacrylate Acrylic	Natural and Synthetic Elastomer Solvent based Waterborne latex Acrylate Solvent based Waterborne latex Thermoplastic Hot Melt Ethylene vinyl acetate Styrene butadiene copolymer Polyolefin RTV Silicone Polysulfide

1. Thermosetting Acrylic Adhesives

Without any special surface preparation, thermosetting acrylic adhesives can bond directly with many low surface energy plastics like:

• Polypropylene
• Polyethylene, and
• Other polyolefin

This characteristic is believed to be due to diffusion of the acrylic monomer into the substrate before cure. The bonds formed on low density polyethylene can be high enough to result in substrate failure.

Thermosetting acrylic adhesives for bonding plastics are generally rubber-toughened, two-component systems that cure rapidly at room temperature. This provides a crosslinked structural adhesive suitable for bonding metals, engineering plastics, and many other substrates.

In this respect they compete for applications with two-part room temperature curing epoxy and polyurethane adhesive systems. Shear strengths of a commercial thermosetting acrylic adhesive on a variety of substrates is shown in the table below:

Substrate Type	Specific Joint Materials	Lap shear Strength (psi)
Polyolefin	High density polyethylene	1658
	Low density polyethylene	632
	Polypropylene	239
Metals	Steel-to-high density polyethylene	657
	Aluminum	423
Plastics	Epoxy	1045
	Polycarbonate	726
	Polyvinyl chloride	1237

2. Cyanoacrylate Adhesives

Cyanoacrylate adhesives are generally methyl or ethyl cyanoacrylate-base, single component liquids. When bonding metals and other rigid surfaces, methyl cyanoacrylate bonds are stronger and more impact resistant than ethyl cyanoacrylate bonds. However, on rubber or plastic surfaces, ethyl cyanoacrylate is preferred.

Cyanoacrylate adhesives generally do not wet or adhere well to polyolefins. The surface tension of the adhesive is much higher than that of the substrate. However, polyolefins can be primed for adhesion with cyanoacrylates by certain chemical compounds normally considered to be activators for cyanoacrylate polymerization.

These primers are simply sprayed or brushed onto the substrate. After drying of the primer, the cyanoacrylate adhesive is conventionally applied and bonds extremely well to the substrate.

Table below shows the significant strength improvements that can be realized with cyanoacrylates on primed low energy plastic substrates.

Substrate		Block Shear Strength (psi)
		Ethyl  Cyanoacrylate	Ethyl  Cyanoacrylate with  Primer	Rubber Toughened  Ethyl Cyanoacrylate
Fluorocarbon	ETFE	100	>1650	50
	FEP	<50	<50	50
	PTFE	300	1050	250
Polyolefin	HDPE	<50	2000	50
	LDPE	150	500	<50
	PP	50	>1950	50

3. Light Curing Acrylic and Cyanoacrylate Adhesives

Typically acrylic monomers and oligomers have been formulated for use with UV light sources. New formulations are now available that will also cure on exposure to visible light sources. These acrylic formulations cure to form thermoset resins. Thus offer better chemical and thermal resistance than uncrosslinked adhesives such as conventionally cured cyanoacrylates.

Depending on the formulation and curing conditions, light curing acrylic formulations can be varied to provide adhesives ranging from hard, high modulus to slightly flexible, moderate elongation materials. Formulations can come in a range of viscosities from thin water-like liquids to thixotropic gels.

Table below shows typical bond strengths on hard-to-bond substrates using a standard UV or visible light curing acrylic adhesive.

Substrate	Block Shear Strength (psi), per ASTM 4501
Acetal	250
Fluoropolymer	150
Polyethylene	350
Polypropylene	100
Thermoplastic vulcaniizate	120

Light Curing Acrylic Adhesive Strength on Low Energy Substrates⁸

Recently, light-curing cyanoacrylate adhesives have been developed that offer the rapid light-cure properties of a thermosetting acrylic adhesive coupled with the ease and speed of a secondary cyanoacrylate cure. Light-curing cyanoacrylates are ethyl-based products that have photoinitiators added to the formulation. These allow them to set rapidly on exposure to low intensity light, and to cure in shadowed areas.

A major benefit that light-curing cyanoacrylate adhesives offer is that the liquid adhesive can be cured to a tack free surface in less than three seconds through exposure to a low intensity light.

Light curing cyanoacrylate adhesives provide many of the same benefits to the manufacturer as do light curing acrylic adhesives such as rapid cure and high bond strength. However, light curing cyanoacrylate adhesives differ from light curing acrylics, in that they bond well to polyolefins and fluorocarbons. They do this by making use of the specialty primers that have been previously developed for standard cyanoacrylates.

Light curing cyanoacrylates also offer excellent bond strengths to unprimed higher energy plastics and elastomers as do conventional cyanoacrylate adhesives. This is shown in table below:

Substrate	Block Shear Strength (psi), per ASTM 4501
	Low Viscosity	High Viscosity
ABS	4750	4895
Acrylic	1410	1550
Aluminum (etched)	3390	3360
Neoprene	110	115
Phenolic	1880	1670
Polycarbonate	1870	1660
PVC	660	830
Steel (grit blasted)	2310	2490

4. Primers and Adhesion Promoters

There are several instances where primers have provided excellent adhesion without having to go through the process of surface preparation. This is a distinct advantage because surface treatment methods may be:

• Hazardous
• Inconvenient
• Time consuming, and
• Often expensive

The use of a surface primer, although an extra step in the bonding process, is a more desirable alternative for use on the production line. It appears that one of the main reasons for improved adhesion by primers is that the solvents in the primer system wet-out and swell the low surface energy plastic. This then facilitates interpenetration of the low viscosity adhesive.

• Solvent-based Chlorinated polyolefins - these are often used for priming low energy surfaces such as polyolefin plastics. These primers are based on either chlorinated polyethylene or polypropylene and are usually used as a solvent based solution.
  
  They are generally used to improve the adhesion of paint to polyolefin substrates, but they can also be utilized for adhesive bonding. A primer based on a solution of chlorinated polypropylene has been used to adhere paint to polypropylene automobile bumpers with some interdiffusion of primer into the part.


• Solvent free, water borne chlorinated polyolefin - such primers based on emulsions and dispersions of chlorinated polyolefin have also been developed. They provide an increase in bond strength and water resistance for polypropylene and other thermoplastic polyolefin joints. View Several Chlorinated Polyolefins Available Today >>


• Primers for Cyanoacrylate Adhesion to Polyolefin Substrate – cyanoacrylate requires a primer for optimal adhesion to polyolefin substrates as discussed above. Polyolefins can be primed for adhesion to cyanoacrylates by certain chemical compounds normally considered to be activators for cyanoacrylate polymerization.
  
  Materials such as long chain amines, quaternary ammonium salts and phosphine can be applied in either pure form or in solution to the surface of the polyolefin. These primers are simply sprayed or brushed onto the substrate. After drying of the primer, the cyanoacrylate adhesive is conventionally applied and bonds extremely well to the substrate.


• Triphenylphosphine or cobalt acetylacetonate primers - these used with cyanoacrylate adhesives produce adhesive bonds with polypropylene and low-density polyethylene that are sufficiently strong to exceed the bulk shear strength of the substrate. They are also sufficiently durable as to withstand immersion in boiling water for long periods of time.