Coating Plastics - Some Important Concepts from a Formulators
Perspective
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.