In the beginning, man turned to nature for his needs. All of the materials he used were directly taken from natural sources in the early stages of the synthesis of human civilization. Manmade synthetic materials appeared lately. The earliest inks incorporated materials like gum arabic and egg albumin with carbon black. Thus materials from plant and animal sources found immediate application due to their ready availability.
Resins are materials obtained from natural sources such as bark of trees, essential oils, and insects. The sap oozing out of a cut bark of pine or fir is a common source of resins. The well known rosin is the solid residue obtained from turpentine. Resins are not pure substances and they contain a range of compounds from organic acids to esters. Abietic acid is a chief component in rosin. Fossilized tree resins called amber find artistic and ornamental uses. Shellac is a resin made from the secretions of the lac insect, a tiny scale insect. In fact, it contains a thermoplastic polymer.
Resins found application in household coatings for a long time. For example, alcoholic solutions of shellac were used as varnishes for priming and finishing furniture. Resins from various sources such as pine trees and tall oil formed an important constituent of inks until recently. Possible search for resins was quoted as one of the reasons for colonizing continents by Europeans.
The advances in chemical sciences that followed as a direct manifestation of mimicking nature inaugurated the era of synthetic chemistry. Polymer chemistry also earned an independent existence in the first half of the twentieth century, which produced a plethora of novel materials that had impact on all walks of life. Ink chemistry was also a beneficiary of these developments. In this article the term polymers will be used in lieu of resins since most of the modern inks utilize the polymer technology in fine tuning the properties of inks.
Polymers are basically large molecules with certain restrictions applied to their constitution[1-4]. They are molecules that contain a repeating unit in them, characterized by high molecular weight. This repeating unit is called a mer that will be closely related to the chemical species called monomer in structure from which the polymer is formed. Linking of mer units yields the polymer. The process of polymer formation is called polymerization. Low molecular weight polymers, which contain few mers, are often called as oligomers. The polymer formation from a monomer M may be represented as follows:
nM ﬁ [M]n
As an example, in polyethylene, monomeric ethylene molecules are joined together to form the polymer.
There are many naturally occurring polymers. For example, starch and cellulose contain glucose molecules linked in two different fashions widely seen in plants and animals. Other biologically important polymers (biopolymers) such as nucleic acids (DNA and RNA) and proteins (polypeptides) are formed from nucleotide and amino acid monomers, respectively. Many technologically important polymers are popular by their trade names as PVC (polyvinyl chloride), Teflon (polytetrafluoroethylene), Plexiglas or Perspex (polymethylmethacrylate), Nylon (polyamide), Styrofoam (polystyrene), etc.
In the beginning, polymers were referred to as macromolecules and giant molecules. Since many large molecules such as cyanocobalamine (Vitamin B12) and chlorophyll that would come under the purview of this nomenclature are not polymers, the term polymer is reserved to show its special significance with respect to repeating units.
Polymers differ in their structural properties. It is not necessary that the monomers should be joined in a linear fashion. They may grow sideways or may form a network structure. Accordingly, polymers are classified as linear, branched and cross-linked (Figure 1). Obviously a linear polymer that is not usually a rigid rod is unbranched. In cross-linked polymers, some of the polymer backbones are linked together, giving them rigidity.
Polymers may contain two or more different monomers. They are known as copolymers to distinguish them from homopolymers that contain only one type of monomer. In a copolymer, monomers of the same type may cluster together in blocks or the different monomers may be arranged without any apparent order. They are respectively termed as block copolymers and random copolymers. If one homopolymer is attached to the backbone of another homopolymer, graft copolymer results. Figure 2 shows these cases for two monomers X and Y.
Polymers are often distinguished by the mechanism of polymerization process itself. Two mechanisms are universally observed: in the first one, a monomer unit adds to another monomer, forming a dimer that in turn adds to a third one forming a trimer, which further adds to more monomer molecules. Repetition of this process results in a molecule of high molecular weight. Such a process is called addition polymerization.
In another process, two difunctional molecules combine with the elimination of small molecules such as water leaving behind functionality at the ends, which further continues the condensation process to yield the polymers. This process is called condensation polymerization. These may be illustrated with the examples of ethylene for the addition polymerization and that of a diamine-diacid system for the condensation polymerization. Large number of ethylene monomers combine together to form polyethylene as follows:
nCH2=CH2 ﬁ [CH2-CH2]n
The condensation polymerization may be exemplified by the following process:
nH2N-R-NH2 + nHOOC-R’-COOH
ﬁ H-[HN-R-NHCO-R’-CO]n-OH + (2n-1) H2O
The addition polymerization identifies three major sequences that lead to the end polymer. They are the initiation, propagation and termination. In the case of a vinyl monomer like ethylene, the double bond opens up to form a radical, anion or cation depending on the action of the initiator that propagates the chain by adding to more of monomers. The growing chain terminates by combining two of them or by eliminating a hydrogen atom by a disproportionation mechanism or by transferring the chain to another molecule like solvent. Polystyrene and polyacrylic acid are produced by this mechanism.
Polymers in inks have multiple functions: they act as stabilizers in pigment dispersion by adsorbing onto active sites in pigments and thus preventing their flocculation; emulsion polymer letdown vehicle plays a great role in the adhesion of ink onto the substrate; their film forming ability at a given temperature adds to the mechanical properties of the ink coating. Special polymers help in improving the abrasion resistance of the ink film and in enhancing properties such as print quality, mar resistance and resolubility.
Ink manufacturers look upon polymeric resins for fine tuning many of the ink properties, be it for minimizing misting or for regulating coating viscosity or modifying other rheological properties by the use of thickeners. Ink chemists depend on the resin properties to improve even properties like blocking. When the economic pressure mounts with the increasing cost of petroleum products, resin manufacturers turn to polymer chemistry to deliver novel classes of polymers that depend less on hydrocarbon sources. Moreover, the waterborne ink systems are still being challenged with the need for superior polymeric resins.
Polymers are high molecular weight compounds. The characteristic properties of polymers such as high mechanical strength and hardness compared to small molecules are owing to this high molecular weight. Due to its relative importance in polymer chemistry, the molecular weight aspect of polymers will be detailed further.
The termination step in polymer synthesis can take place from a growing chain at different stages. Accordingly, a mixture of polymer molecules with a range of molecular weights results during synthesis. In other words, one does not observe the usual molecular purity that normally follows the synthesis of small molecules. Polymer chemists express the molecular weight of polymers as an average number to reflect the distribution of molecular weights. They are the number average and weight average molecular weights, designated as Mn and Mw.
If Nx is the number of polymer molecules of molecular weight Mx, then the number average molecular weight is given by,
Mn = (∑NxMx/∑Nx)
where the summation is over all different sizes of polymer molecules from x=1 to x= infinity. Similarly, the weight average molecular weight is expressed as,
Mw = (∑WxMx/∑Wx)
where Wx is the weight of polymer molecules of molecular weight Mx. It follows that
Mw = (∑NxMx2/∑NxMx).
Another way of expressing the molecular weight of polymers is based on the viscosity measurement of polymer solution. Thus, viscosity average molecular weight is defined as,
Mv = (∑NxMxa+1/∑NxMx)1/a
where ‘a’ is a constant. When ‘a’ is unity, then Mw = Mv. Polymer samples with a distribution of molecular weight are referred to as polydisperse. In a polydisperse sample, Mw>Mv>Mn.
Another term to represent the distribution of molecular weights is the average degree of polymerization. Here, the molecular weight term is substituted with the number of mers, i.e., the molecular weight divided by the monomer molecular weight. Thus the number average degree of polymerization Pn is given by,
Pn = (∑NxPx/∑Nx)
where Pn is the degree of polymerization.
Ink chemists characterize the polymer properties such as polymer composition using solubility parameter and atomic ratio. Solution behavior of polymers can be investigated with parameters like conductivity, pH and viscosity, and most importantly from the molecular weight.
A number of methods are available for the determination of molecular weight of polymers. Methods based on colligative properties like osmotic pressure,and Raoult’s law yield the number average molecular weight, whereas light scattering methods yield the weight average molecular weight. Viscosity determination of the polymer fractions provides the viscosity average molecular weight. Out of these, the viscosity method is the simplest and quickest to determine the molecular weight. This method is briefly described here.
The specific viscosity of a polymer solution is defined as,
ηsp = (η -η0)/η0
where η0 is the viscosity of the solvent and η is the viscosity of the polymer solution. Obviously ηsp will be dependent on the concentration of the polymer in solution. It may be noted that the higher the molecular weight, the higher the viscosity. A value independent of the concentration, but characteristic of a given polymer in a given solvent is defined as
Lt (Cﬁ0) [ηsp/C] = [η]
where [η] is known as the intrinsic viscosity and C is the concentration. Intrinsic viscosity is related to the molecular weight as,
η] = KMα
where α and K are constants for a given polymer in a given solvent. Once α and K are evaluated for a polymer in a solvent, M, the molecular weight can be determined routinely with great ease from viscosity data.
Gel permeation chromatography, also known as size exclusion chromatography, is routinely used these days in the determination of molecular weight of polymers. This technique is an offshoot of high performance liquid chromatography (HPLC) where polymers of known molecular weights serve as the standards. Polymers are separated in a solid column by eluting with a liquid under pressure according to the molecular weight, the largest molecules coming out first. Analysis of chromatograms can yield different types of molecular weights. But primary methods based on light scattering and vapor pressure osmometry give weight average and number average molecular weights respectively.
The choice of right range of molecular weight of polymers is essential in fixing the end properties of ink film. Usually high molecular weight polymers provide high tensile strength to the film. But they result in high viscosity of the fluid. Present day high solids type inks favor the use of low molecular weight polymers, but some of the film properties are to be compromised.
Polymers have gained a central position in technology owing to their plasticity, a property by which polymers can be melted and shaped as desired. In this context, polymers are classified as thermoplastic and thermosetting: thermoplastics can be molded and remolded repeatedly; thermosetting ones cannot be reprocessed upon heating. The molecules in thermoplastics are easily separable and maintain mobility, but the molecules in thermosetting polymers undergo chemical reactions on heating that result in infusible, insoluble networks. Many varieties of polyethylene and polystyrene are thermoplastics and some epoxy polymers used in making rigid sheets undergo crosslinking reaction on molding at a high temperature.
In a solid polymer, the molecules are not always arranged in a symmetric manner. They differ in their morphology (structure and form). Whether they are arranged in a perfect order or not depends on the conditions of their formation. Accordingly, we have both crystalline and amorphous regions in a polymeric solid. Crystalline region corresponds to well-ordered arrangements of chains and the amorphous regions correspond to disordered arrangements of the chains.
The differences in the crystalline and amorphous regions in a polymer will be evident when the polymer is heated. Specifically, the melting points of amorphous and crystalline regions will be different. When heated, the attractive forces holding the polymer chains together in the solid state will be weakened and the molecules become mobile. Amorphous regions naturally need less heat for overcoming the cohesive forces in the solid state rather than those in the crystalline state. Accordingly two transition temperatures are observed in polymers: Tg, the glass transition temperature corresponding to the amorphous region and Tm, the melting temperature corresponding to the crystalline region. Tm is a sharp transition called the first order transition and Tg is a continuous transition called the second order transition. These changes are discernible when a property like specific volume (volume per gram) of the polymer is plotted against the temperature. Most of the polymers have both Tg and Tm, since perfect crystallinity is seldom observed.
Ink chemists utilize the transition temperature concept widely while choosing polymeric resins to impart block resistance and to adjust the minimum film forming temperature (MFFT).
The rheology (science of flow and deformation of materials) of inks dictates the flow properties during the application stage. Polymers have a great say in translating an ink into a Newtonian or non-Newtonian fluid. The structure and morphology of polymeric resins give useful leads to appreciate rheological behavior. In this regard, the binding properties of many amorphous polymers that usually dominate the coating scenario are to be understood.
The most favored polymers used in waterborne inks are based on acrylate chemistry. They are either homopolymers or copolymers of a variety of acrylic monomers that often contain free carboxylic acid groups. Polymers that function as thickeners perform at high pH making use of the conformational variations in polyacrylates that result from the ionization of the acid groups.
Polymers can undergo mutual interactions (often detrimental) with other classes of compounds used in inks. For example, surfactants used for multiple purposes in inks can complex with polymers introducing complications in already tuned ink. They interact with surfactants that otherwise are meant to function as dispersants, wetting agents, foam breakers, etc. Importance of polymer-surfactant interactions is greatly appreciated in recent literature.
Polymers are widely used in the present day inks to stabilize pigment dispersions, an important area in ink chemistry. They attach onto the surface of the solid pigments by adsorption. Adsorbed layers of polymers tend to balance the attraction/repulsion forces among the pigment particles and prevent the coalescence of particles by processes generally known as flocculation and coagulation. They directly affect the interparticle energy. These stabilizing forces are the electrostatic and steric repulsion effects.
Finally, an ink invariably is printed on a substrate which itself very often is a polymer. The most common substrate is paper which is cellulose that contain repeating units of glucose. Synthetic polymers such as polyethylene, polyamides and other plastic materials also serve as popular substrates. Even though not directly related to the ink composition, the structures of these polymers also dictate the printability of inks.
To conclude, polymeric substances are central to the performance of inks and other coatings. They regulate the properties of inks at various stages of production and application as in stabilizing dispersions, modifying the viscosities, and regulating the properties of dried ink film.
1. “Principles of Polymerization,” G. Odian, McGraw-Hill Book company, New York, 1970 (later editions are available).
2. “The Chemistry of High Polymers,” C. E. H. Bawn, Butterworths Scientific Publications Ltd., London, 1948.
3. “Macromolecules – an Introduction to Polymer Science,” F. A. Bovey and F. H. Winslow (Eds.), Academic Press, New York, 1979.
4. “Principles of Polymer Chemistry,” P. J. Flory, Cornell University Press, New York, 1953.
5. Nucleic acids contain phosphoric acid, sugars such as deoxyribose in DNA and ribose in RNA, and organic bases such as adenine, guanine, cytosine, thymine or uracil. They play a major role in determining the genetic features of living things. Proteins are formed by the condensation of α-amino acids such as glycine and alanine. About 20 amino acids are involved in building most of the proteins.
6. G. P. Turco, S. A. Fischer and G. A. Deeter, American Ink Maker, March 2000, p.44.
7. Joy T. Kunjappu, The Emergence of Polyacrylates in Ink Chemistry, InkWorld, February 1999, p.40.
8. Joy T. Kunjappu, Polymer-Surfactant Interactions in Ink Chemistry, American Ink Maker, March 1999, p.34. n