Coating Plastics - Some Important Concepts from a Formulators Perspective

 

Lawrence C. Van Iseghem, President

Van Technologies, Inc.

Introduction

In the automobile industry, the trend is to produce less expensive, lighter, and stronger components that appeal to the aesthetic tastes of the consumer.  Appliance manufacturers produce units that have non-metallic casings and components that provide superior durability and function relative to their older counterparts.  Composite materials are currently used to manufacture furniture including the use of laminate sheeting that provides the beauty and feel of real wood.  Cassettes and enclosures are manufactured for the safe storage of static sensitive electronic components.  Eyeglass lenses are produced and sold to wearers that are lighter, safer upon impact, and highly resistant to scratching.  From automobiles, to appliances, to furniture, to electronics, to eyeglasses, and beyond, plastics represent materials that increasingly impact many facets of everyday life.

 

Often, plastic materials will require coatings or markings to enhance their function or appeal.  Designers and manufacturers of technical coatings routinely experience unique challenges when dealing with plastics due to their chemical and physical nature. 

 

This discussion will present concepts pertaining to the coating of plastics including the nature of plastic surfaces, wetting and adhesion, and adhesion promotion.  With increased awareness of these topics, the formulator of coatings for plastic materials will be better able to design coatings having superior properties.

 

The Nature of Plastics as Coating Substrates

Although, the term, “plastic” denotes materials that can be deformed, shaped, or molded, it is more common today to apply the term to synthetic high polymers that are thermally deformable.  It is also quite common to see references to various thermosetting polymers and synthetic composites within the context of plastic materials.  Plastics, therefore, encompasses a diverse family of polymeric materials. 

 

For successful coating design, the coatings formulator must consider the physical and chemical characteristics of the polymer substrate or plastic surface to be coated.  Surface tension, modulus, coefficient of thermal expansion, response to coating drying and cure, as well as the chemical structure and conformation of the polymer are just a few factors that influence the type of coating required for a particular application.  The following will discuss these factors and offer some insight into their management.

 

A.  Surface Tension:

Surface tension will directly influence a coating’s ability to wet out, to penetrate, and to adhere to  the porous structure of a surface .  Although plastics may be present in a porous structure (as used in filtration media), details discussed below will be limited to the phenomenon of wetting and adhesion and the role that surface tension plays. 

 

It is generally seen that the lower the surface tension, the more problematic it is to coat the surface uniformly with good adhesion.  Within the family of plastic materials, there is a considerable range of surface tension that the formulator must consider when examining various types of polymers (Table 1.).  It is also noteworthy that the surface tension of a given polymer or plastic material will vary upon changes in molecular weight and temperature.  Fortunately, once the molecular weight (Mw) of a polymer reaches approximately 2000 - 3000, the surface tension will reach within 1 dyne/cm of the surface tension at infinite molecular weight.  As temperature fluctuates between 10o C and 50o C (normal process temperatures),  the surface tension is fairly constant but is seen to decrease at significantly elevated temperatures.  It is not uncommon to see a reduction in surface tension of 20% to 30% at 150o C for particular plastic material.  Therefore, for practical considerations, the formulator need not be overly concerned with polymer molecular weight, but should consider any elevated process temperatures that may impact surface tension factors especially when applying thermoset coatings1.  Table 1. illustrates the surface tensions of various polymers.

 

TABLE 1.  Surface Tension of Various Polymeric Materials1

 

Polymer                             Surface Tension (dyne/cm)@ 20o C

 

Cellophane                                                45

Cellulose                                                   36 - 42

Cellulose acetate                                        46

Cellulose acetate butyrate                           34

Epoxy Resins                                             45 - 52

Ethylcellulose                                             32

nitrocellulose                                              38

Nylon 12                                                   36

Nylon 6                                                     38

Nylon 6,6                                                   47

Phenoxy Resins                                         ~ 43

Poly(2-ethylhexylacrylate)                          30

Poly(acrylamide)                                        52

Poly(acrylonitrile)                                       50

Poly(butadiene)                                          43 - 49

Poly(butadiene-acrylonitrile)                       51 - 53

Poly(chloroprene)                                      44

Poly(ethylacrylate)                                     37

Poly(ethylene)                                           33 - 37

Poly(ethylene-acrylic acid)                         41 - 60

Poly(ethylene-propylene)                            30 - 34

Poly(ethylene-propylene-hexadiene)            35

Poly(ethylene-vinyl acetate)                        30 - 36

Poly(ethyleneterephthalic acid)                   45

Poly(ethylmethacrylate)                              36

Poly(hydroxyethylmethacrylate)                  37

Poly(isoprene)                                           31 - 34

Poly(methacrylonitrile)                               39

Poly(methylmethacrylate)                           41

Poly(oxyethylene)                                      43

Poly(propylene)                                         29 - 30

Poly(styrene)                                             39 - 41

Poly(styrene-acrylonitrile)                           37 - 43

Poly(tetrafluoroethylene)                            24

Poly(vinyl acetate)                                     37

Poly(vinyl alcohol)                                      37

Poly(vinyl butyral)                                      38

Poly(vinyl butyrate)                                    31

Poly(vinyl chloride)                                    42

Poly(vinylidene chloride)                             45

Poly(vinylidene fluoride)                             33

Polycarbonate of bisphenol A                     43 - 45

Polyimides                                                 38 - 41

Polyimines                                                 22 - 26

Polysiloxanes                                             19 - 26

Polysulfone                                                47

Polyurethanes                                            36 - 39

Starch                                                       39      

 

It is possible to enhance a coatings performance through chemical modification of the plastic surface.  Chemical modification will alter the surface tension of the plastic material, and is generally done by positioning polar groups on the surface such as pendant hydroxyl, chloro, amino, and carboxyl groups. Notice in Table 1. the influence that polar functionality has on surface tension.  Compare, for example, the relatively high surface tension of epoxy resins with that of poly(propylene).  These two materials will exhibit different properties with the same coating fluid.

 

B.  Modulus:

The modulus characterizes the stiffness or resistance to deformation of a material.  It is common to examine modulus through the relationship between the stress imposed on a material and the resulting strain exhibited by the material.  A material of low modulus exhibits deformation with minimum force applied, whereas a material of high modulus exhibits significant resistance and is typically hard and brittle.  A material having an elastic modulus will show relative ease in elongation with recovery, provided the elongation has not been taken to the break point.  Figure 1. illustrates various types of materials and their stress/strain behavior.  Plastic materials are seen to respond, typically, according to scheme “B” but higher and lower modulus plastics do occur.

 

 

In the design of quality coatings, the stress/strain behavior of the coating composition when dry and/or cured should be consistent with that of the substrate material.  Even though this is true under ideal circumstances, it is often the case that: a.)  high modulus coatings are applied to lower modulus substrates, b.)  low modulus coatings are applied to higher modulus substrates. 

 

1.  High modulus coatings on lower modulus substrates: 

For decorative coatings and other application, it is generally not advised to apply a coating when the modulus of the cured coating is substantially higher than that of the substrate as failure may occur in the form of cracking and/or loss of adhesion.  This statement must be qualified, however, since the formulator must consider the range of stress imposed on the coated product or article during its specified life cycle.  To illustrate this point, take for example the relationship of a very high modulus hardcoat composition used for polycarbonate and other plastic molded articles.  Although the modulus of the hardcoat composition is significantly higher than that of the substrate, during the normal use of the coated article the stress imposed may never be enough to result in failure.  Therefore, the formulator must balance the modulus of the dry and/or cured coating with that of the substrate and the expected stress that will be imposed on them during use.

 

2.  Low modulus coatings on high modulus substrates: 

Many applications occur where a coating composition that exhibits softness and flexibility is applied to a surface that is hard and brittle.  Recent developments in “soft feel” urethane coatings illustrate this point quite well.  The bulk mass of the substrate plastic material will support the dimensional stability of the coating and permit adequate performance in use.  Under certain circumstances, low modulus coatings on a high modulus substrate may provide impact resistance.  A good example of this behavior is seen with glass chemical storage containers that have a chemically resistant vinyl protective coating.

 

C.  Coefficient of Thermal Expansion:

Plastics exhibit various coefficients of thermal expansion and it is important to consider the temperature range that the coated product or article will be exposed to during its life cycle.  A disparity in the coefficient of thermal expansion between the coating and the substrate can result in poor interfacial stability as temperature fluctuates.  It is, therefore, common to cycle coated articles through extremes of temperature.  Failure, when it occurs, will usually take the form of checking, crazing, cracking, and loss of adhesion.  Other non-fatal flaws can also occur, such as curl and wrinkling.  Most often, problems are witnessed when a coating of high modulus and low coefficient of expansion is applied to a plastic of moderate to low modulus and a relatively high coefficient of expansion.

 

The formulator should recognize that expansion in volume is a three dimensional phenomenon.  An applied coating at equilibrium is fixed in two dimensions by the surface area of the substrate.  The third dimension is determined by the application thickness (Figure 2).  As the temperature changes to induce expansion, the coating and substrate will respond accordingly.  Flexible substrates and coatings usually respond to stresses by exhibiting curl (Figure 2a. and 2b.).  In the case of more rigid substrates and hard, non-flexible coatings, differing coefficients of expansion will cause significant internal stress to be localized at the interface of the two layers.  This may result in failure of the coating as shown in Figure 3.

 

Figure 2.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


The expansion due to applied heat under ideal circumstances will occur in three dimensions, but with thin film flexible substrates, a mismatch in the coefficient of thermal expansion will result in interfacial stress causing curl.

 

 

 

 

 

 

a.)  Case 1.  The coating has a higher coefficient of thermal expansion vs. the substrate:

 

 

 

 

 

 


b.)  Case 2.  The coating has a lower coefficient of thermal expansion vs. the substrate:

 

 

 

 

 

 

 

 

 

 

Figure 3.

 

 

 

 

 

 

 

 

Coatings of relatively higher modulus may respond to interfacial stress to the extent that any substrate thermal expansion will cause cracking, crazing, and loss of adhesion.  Such failure may not occur until the system has been repeatedly cycled through extremes of temperature.

 

 

D.  Response to Coating Drying and Cure:

The action of the drying and cure of a coating composition on the surface of a plastic is also very important for the formulator to consider.  In many aspects, the elements presented in the previous section (Coefficient of Thermal Expansion) will apply to the concepts of drying and cure.  This is due to the fact that shrinkage is the primary phenomenon to control.  Coatings that are less than 100% non-volatile will experience shrinkage.  Again, since this is a volume relationship, three dimensional forces will be active.  Not only does the thickness change from wet thickness to dry, but stress will be imposed on the coating interface due to shrinkage forces in the plane of applied coating.  This usually results in curl as shown in figure 2b.  In dealing with this situation, higher solid coatings are preferred, soft flexible coatings work reasonably, and at times the plastic can be back coated.

 

Curl can also occur due to the cure of a coating composition, especially if by a condensation type reaction.  Addition reactions show better resistance to curl.  Back coating on the opposite side of the plastic may be necessary or the plastic may need to be thicker or supported to maintain flatness.

 

E.  Chemical Structure and Conformation:

1.  Structure: 

As indicated above, the chemical structure of a polymer will influence the ability of a coating to wet out and adhere to the polymer surface uniformly.  The adage that, “Like - Likes - Like” applies, however, many instances require coatings of dissimilar chemistry to be applied to a plastic surface.  Knowledge of the plastic in both its structure as well as conformation will guide the formulator to successfully develop coatings.

 

Polar functionality, when present in a polymer structure will promote ease in coating application.  For example, it is typical for the formulator to find that epoxy resin products show better coating application properties versus polyolefin products, given the same coating formulation.  The presence of polar functionality is especially beneficial when working with waterborne coatings.  Not only does water tend to wet out more readily due to surface tension forces, but polar groups will promote adhesion through potential hydrogen bonding of suitable functionality of the coating polymer composition.

 

The converse situation is also important to recognize, where polar functionality present in the chemistry of the coating composition will influence the performance of the final coated product.  The higher the polar functionality of the plastic surface or substrate and/or the coating composition, the greater the probability of good interfacial stability.

 

The formulator needs to recognize that chemical reactions occur between polymeric materials just as they do with low molecular weight, monomeric materials.  The difference, however, often is in the slower rate of reaction due to steric hindrance that is evident with high polymers.  The reactions most commonly utilized at a coating/substrate interface are condensation reactions.  For these chemical reaction to occur, the reactant functionality must be oriented appropriately.  Any catalyst used to speed the rate of reaction must be present at the reaction site.  With low molecular weight materials, this is typically not a problem.  With high molecular weight materials, the coating/substrate interface is, at best, a liquid/semi-solid system.  Molecular mobility is limited in the substrate, thus limiting the potential for reaction.  Further complicating the situation is the viscosity of the coating composition and the potential for viscosity increase as volatiles are evaporated during drying.  The coating composition, therefore, may be or become restricted in molecular mobility preventing the reaction from proceeding at the desired rate.

 

To enhance molecular mobility, a few things may be done.  First, solvent selection for the coating composition is vital.  A blend of solvents of varying evaporation rate, in the coating composition, usually promotes coating uniformity and can provide ample molecular mobility at the latter stages of drying.  Secondly, the solvent that has the lowest rate of evaporation will be the last solvent out of the coating composition during drying.  Ideally, this solvent should be selected from those that are good solvents for the resins of the coating composition, and for the polymer constituting that of the plastic surface.  Solubility parameters of solvents should always be referenced for proper formulation.  Careful attention should be given to the units in which the parameter values are expressed in, as the various tables available for solubility parameter reference do not all use the same unit of measure.

 

2.  Polymer conformation of the plastic surface: 

The formulator needs to keep in mind that although chemical functionality may be present, it may not be readily available to provide the anticipated benefit that the formulator desires.  Polymer conformation may be such that the available functionality is “hidden” simply by the way the polymer is situated in space.  Furthermore, interpolymer chain interactions may occur, including hydrogen bonding, and may effectively prevent the expression of chemical functionality.  In these instances, the formulator should consider the use of selective solvents or co-solvents in the coating formulation to “open-up” the plastic surface and promote final coating properties.  Also, the effect of drying and cure temperature may be examined to induce improved coating performance.  For plastics of high modulus, it is recommended to bring the drying temperature above the glass transition temperature (Tg) of the plastic.  This will promote both molecular mobility and increase the potential for coating polymer interpenetration with that of the plastic surface.  It is possible that the application of heat may only need to be localized at the plastic surface via IR radiant energy, and may be applied to the surface just prior to coating application to be effective. 

 

The formulator of coatings for plastic applications inevitably will encounter materials having varying degrees of crystallization.  In general, the higher the degree of crystallization, the harder, less flexible, more brittle, and more solvent resistant the polymer or plastic will be.  Also, with higher degrees of crystallization, more difficulties may be experienced in coating design.  Polymers are commonly classified according to their degree of crystallization and are termed amorphous when having a low degree, semi-crystalline for a moderate degree, and crystalline for a high degree of crystallization.  Polymer chain to chain packing influences the tendency for a polymer to crystallize where linear structure promotes tight chain to chain packing and higher degrees of crystallization.  Any polymer chain branching and the presence of bulky substituents will decrease the likelihood of crystallization.  When working with plastic coatings, knowledge of polymer structure will guide the formulator in the usage of the appropriate solvent balance and the temperatures required to effectively and efficiently process a given coating composition.

 

Wetting and Adhesion

Regardless of the coating and the surface it is applied to, the coating must adequately wet out, and usually adhere well to that surface.  The formulator will find challenges in determining the correct balance of coating ingredients to achieve the desired wetting and coating uniformity.  There are three (3) principle wetting phenomenon that apply to coatings.  These are spreading, adhesional, and penetrational or immersional wetting.  Spreading and adhesional wetting will directly impact the application of a coating to a particular surface.  Penetrational or immersional wetting will impact the application of a coating to a porous surface structure or the dispersion of particulate matter (including the possibility of a plastic or polymer media, pigment, or filler) into a coating composition.  The discussion that follows will address only spreading and adhesional wetting2.

 

One key concept that the formulator needs to keep in mind is that in the action of wetting, one fluid displaces another from a surface.  In the action of coating a substrate, usually the displaced fluid is air.  We find that surface tension guides the action of wetting, both that of the substrate (plastic) and of the coating fluid.

 

A.  Spreading Wetting:

The spreading of a liquid over a solid is defined by Equation 1.

 

Eq. 1

 
SL/S = gSA - (gLA + gSL)

 

Where, gSA denotes the surface tension of the substrate under air, gLA denotes the surface tension of the liquid coating under air, and gSL denotes the interfacial tension or free energy of the substrate/liquid coating interface.

 

When SL/S is:

a.  positive, coating fluid spreading is spontaneous,

b.  zero, coating fluid spreading is spontaneous,

c.  negative, coating fluid spreading is not complete.

 

The surface tension of the plastic is given by the nature of the polymer used to construct it.  Table 1. lists several types of plastics and their respective surface tensions.  When evaluating coating compositions, it will be quite evident whether the coating fluid has a comparable surface tension.  If the coating does not flow out well, pinholes or fisheyes occur, or if thick edges or picture framing occurs, the coating surface tension is probably too high relative to that of the plastic surface.  Of the terms in Equation 1, the formulator has control of only the surface tension of the coating fluid.  Interfacial tension is not typically determined accurately with ease but is assumed to be made minimal when the surface tension of the substrate and the coating fluid are the same or near the same.  Therefore, for best coating uniformity the formulator should keep gLA and gSL as low as possible relative togSA. 

 

Another method of determining spreading wetting is through contact angle measurement.  The contact angle is simply the angle tangent to the edge of a droplet of fluid in contact with a surface as shown in figure 4.  Equation 2 describes spreading wetting in terms of the contact angle that the fluid makes with the substrate.

 

Eq. 2

 
SL/S = gLA (cos q - 1)

 

The contact angle must be zero for spontaneous spreading wetting, unless the surface is rough.  Equations exist which account for substrate roughness or rugosity, and we find that spontaneous spreading can occur, provided that the contact angle is less than 90o .

 

Contact angle measurements are very useful in situations where (as is usually the case) gSA < gLA, and when gSA > gLA, the contact angle will always be zero.

 

Figure 4.

 

 

 

 

 

 

 

 

 

 


The contact angle that a liquid makes with the surface it is sitting on provides a means of determining surface tension relationships.  A contact angle of zero implies spontaneous spreading wetting, although spontaneous spreading wetting can occur provided that the contact angle is less than 90o.

 

The formulator needs to keep in mind the following guidelines for good spreading wetting:

1.  Adjust the fluid surface tension to match that of the substrate.

2.  Check the contact angle and maintain it below 90o.

3.  When using surfactants to adjust fluid surface tension, select surfactants to perform to the highest potential at the lowest concentration.

 

B.  Adhesional Wetting:

In spreading wetting, the area wetted by the fluid increases over time, whereas, adhesional wetting brings two surfaces together in intimate contact.  Adhesional wetting is defined by Equation 3.

 

Eq. 3

 
WA = gSA + gLA - gSL

 

This equation is key to the understanding of why plastics are difficult to adhere to relative to other types of coating substrates.  Equation 3 tells the formulator that coating adhesion is best when the surface tension of the substrate and the coating fluid are both as high as possible.  Plastics, according to our discussion above, have generally low surface tension and , therefore, contradict the requirements for tight adhesion, as stated in Equation 3.  Therefore, the conditions for best adhesion of a coating to a substrate are to keep the interfacial tension (gSL ) as low as possible, and both gSA and gLA as high as possible.

 

At times, however, it is desirable to design a coating that will release from the substrate (protective purposes, free cast films, etc.).  Since substances do not adhere well to surfaces of low energy, equation 3 directs the formulator to minimize the surface tension of the substrate and maximize the surface tension of the coating composition (there will likely be an increase in the interfacial tension under these circumstances).  This will provide a good release coating condition.

 

Adhesion Promotion

Adhesion promotion is a topic of great interest to the plastic coating industry.  This discussion will provide an overview of some of the methods used to enhance adhesion to plastics, thus providing the formulator with some insights to integrate with other coating design factors.  The following lists the more common approaches to the topic of adhesion.

 

1.  Solvent and co-solvent composition of the coating fluid

2.  Primers and tie-coats

3.  Adhesion promoters

            a.  covalent bonding

            b.  chemical similarity

c.  other forces of attraction - hydrogen bonding, electrostatic, van der Waals

4.  Corona and plasma treatment

5.  Surface roughening

 

A.  Solvent and co-solvent composition of the coating fluid:

Plastics of low surface energy present difficult surfaces for coating adhesion.  Also, as discussed above, plastics may have a polymer conformation that is not conducive to good coating adhesion.  To accommodate these types of plastics, one approach to promoting adhesion is through careful formulation of the coating composition.  In doing so, significant emphasis should be given to the balance of solvents and/or cosolvents used in the formulation.  Consideration of the demands of environmental regulations, to reduce volatile organic compound (V. O. C.) usage, presents a significant challenge to the formulator.

 

The primary action that must take place is to relax or partially solubilize the surface polymer molecules, thus, providing the potential for coating polymer or binder resin to penetrate the plastic surface and form an interpolymer entanglement.  Temperature will also improve this action, especially if the temperature can be elevated to a point well above the Tg  of the plastic without detrimental effects on the plastic or coating composition.  This solvent/temperature action will result in exceptional adhesion of the coating. 

 

In practice, the formulator must consider the solubility parameters of the solvent used in the coating composition and both the coating polymer or binder resin and the plastic or polymer substrate.  Solubility parameter theory3 provides numeric three dimensional coordinates for solvents according to Equation 4.

 

Eq. 4

 
dO2 =  dd2 +  dp2 + dh2

 

Where:

dO =  Solubility parameter

dd = Component due to dispersive forces

dp = Component due to polar forces

dh = Component due to hydrogen bonding

 

It is, however, common practice to use only a two dimensional approach to permit easy plotting of parameter coordinates according to standard X-Y graphing.  The most frequently used parameters used in such plots are the polar (dp) and the hydrogen bonding (dh) according to the Hanson solubility parameter theory3.    A solvent will be located at a specific point according to its parameter value, but polymers will have a range of solubility and will cover an area within a plot of dp vs. dh.  The center of the polymer solubility area may be designated the solubility parameter average and may be used to determine those solvents that are most likely to exhibit high potential for dissolution of the polymer based on proximity to the polymers’ average value.

 

Also it is important to recognize that binary, tertiary, etc. mixtures or blends of solvents will exhibit a summed solubility parameter value based on the weight fraction of each solvent in the mixture according to Equation 5.

 


Eq. 5

 
dO mixture2 =  åWi ddi2 + åWidpi2 + åWidhi2

 

Where:

Wi =  Weight fraction of solvent i

ddi = Component due to dispersive forces of solvent i

dpi = Component due to polar forces of solvent i

dhi = Component due to hydrogen bonding of solvent I

 

And:

dp mixture = åWidpi

 


dh mixture = åWidhi

 

will provide the parameters for two dimensional plotting of the solvent mixture or blend.  Furthermore, as the solvent mixture of the coating composition begins to evaporate during drying and cure, the parameter values will shift in the direction of the solvent(s) having the lowest rate of evaporation.  This leads to a very important realization.  The solvent having the lowest rate of evaporation (the last one out of the coating during drying) should be a very good solvent for the coating polymer or binder resin and for the polymer constituting that of the plastic substrate.  This will promote the best conditions for coating adhesion.

 

One last concept in the use of solvents is the fact that, as solvent evaporation progresses during drying, the surface tension of the coating or fluid layer may drift and present problems with uniformity and adhesion.  Consideration of the surface tension of the solvent of lowest rate of evaporation is also important to maintain the optimum coating performance.

 

B.  Primers and Tie-Coats:

There are many times where the coating chemistry desired will never be compatible with the plastic surface, even in consideration of the solvents used.  Under these circumstances, primers or tie-coats serve to act as an intermediate layer for adhesion purposes.  The basic premise is to apply a coating to the plastic surface that exhibits good adhesion and will also act as a compatible intermediate surface for the final topcoat composition.

 

It is often seen that primers and tie-coats can be specific for the type of topcoat composition to be applied to a particular plastic.  “Universal” primers have been claimed with some materials but caution is warranted when examining them for any application.

 

Primers may be clear or filled to provide added surface area or roughness for maximum adhesion.  Tie-coats are typically clear and may simply be an adhesion promoting agent solubilized or dispersed in low concentration in a particular solvent or water.  These will be addressed in the next section.

 

C.  Adhesion Promoters:

Adhesion promoters may be added to either the primer, tie-coat, or coating compositions and are primarily of three main types, those that bond covalently, those that bond by other forces of attraction, and those that promote adhesion by chemical similarity (“like-sticks to-like”).  In all instances, these types of adhesion promoters use the described mode of adhesion to the plastic substrate.  They all, additionally, possess specific functional groups that are presented to the coating for subsequent bonding or binding of that layer.  In application, it is typical to use a very low concentration of an adhesion promoter, thus, providing a near mono-molecular layer over the surface of the plastic.  Effectiveness may decrease with greater applied thickness of the adhesion promoter.  It is, therefore, essential to fully discuss and/or review the performance mechanism of the adhesion promoter through respective vendor communications.

 

In the use of adhesion promoters, the adhesion promoter must look for chemical functionality in the plastic to bond or bind to.  With thermoset surfaces, the best performance of an adhesion promoter will be when there is a fair level of residual functionality remaining and available for binding.  Polymers of low or no residual functionality will remain more problematic to promote adhesion, but nonetheless, there are some options available.

 

A very popular class of adhesion promoters are silanes and several are available for specific combinations of plastics and coatings.  For these types of  adhesion promoters to find such functionality certain conditions must be met4.  First, the plastic must have appropriate functional groups.  Second, the adhesion promoter must be mobile enough in the primer or coating composition for the time necessary to migrate to and  find the appropriate functionality on the surface of the plastic.  This implies careful formulation of the solvent balance of the coating composition and an appropriate surface activity in the adhesion promoter itself.  Third, there should be minimal competition between the plastic functionality and the coating polymer or binder resin so that the effectiveness of the adhesion promoter is not diminished.  Fourth, the adhesion promoter must be soluble in the coating composition and miscible with the polymer of the plastic.

 

Of the types of plastics available, polyolefins and other hydrocarbon thermoplastics are difficult substrates for coating adhesion.  When dealing with these substrates, polychlorinated polyolefin adhesion promoters5 have been used to bind the coating to the plastic surface by their chemical similarity.  Additionally, being relatively hydrophobic, they tend to migrate to the coating/substrate interface and orient themselves with their pendant maleic anhydride and/or halogen groups toward the coating layer.

 

D.  Corona and Plasma Treatment;

Corona and plasma treatment of plastic surfaces causes a change in the chemical nature of the surface through possible oxidation or possible microstructure formation so that subsequent coating adhesion is made maximum.  These treatments are quite effective for a number of types of plastics and work well for most types of coatings.  In particular, this form of treatment works remarkably well for polyester,  polyolefin, and other hydrocarbon thermoplastics.

 

Corona discharge is made possible by high voltage across a gap through which the plastic substrate is passed.  Current passes from an electrode to a grounded hardened steel support surface for the plastic.  This hardened steel support may be ceramic coated for longer use.  Plasma discharge is, in simple terms, a high temperature flame impinging on the surface of the plastic for a very short period of time.  The effectiveness is not necessarily permanent and it is best to apply the desired coating shortly after treatment.

 

E.  Surface Roughening:

As introduced earlier, rough surfaces are easier to coat and to adhere to.  Although, the surface tension of the base plastic material does not change, microscopically the surface has varied angles causing the net contact angle that the coating makes appear exceptionally low.  The increase in surface area coated also contributes to adhesion.  When this option is acceptable in the overall process of producing the plastic article, it is advised to take advantage of it, especially in combination with other forms of adhesion promotion.

 

Summary

By no means are the concepts introduced above all inclusive for what the formulator needs to know about plastic surfaces for successful coating design.  There are numerous other concepts and principles that interplay with the science of coating design for plastics.  Obstacles always seem to occur, some of which may be insurmountable.  If the coating cannot be modified to comply with requirements, perhaps the type of plastic can.  It is always helpful to combine a coating formulators skill with that of a polymer or plastic engineer to design the best possible end product.  The plastic coating formulator must always realize that the product is never a fluid composition but always an article composed of a coated plastic.  The coating and the substrate must function synergisticly as a unit to satisfy the requirements of the final consumer.

 

References

1.  J. Brandrup, E. H. Immergut, Polymer Handbook, 3rd Ed., VI/411-426 (1989) John Wiley & Sons, Inc.

2.  M. J. Rosen, Surfactants and Interfacial Phenomenon, 2nd Ed., 240-255 (1989) John Wiley & Sons, Inc.

3.  C. M. Hanson, J. Paint Tech. 39 (505), 104-117, (1967).

4.  A Guide to Dow Corning Silane Coupling Agents, Dow Corning Corporation, Midland, MI (1990).

5.  Chlorinated Polyolefins, Eastman Chemical Company, Kingsport, TN (1995).

 

About the Author

Lawrence C. Van Iseghem is the president and founder of Van Technologies, Inc.  Van Technologies designs, formulates, and manufactures specialty industrial coatings with emphasis on environmentally compliant technology.