Polymeric materials constitute an integral ingredient in any ink composition. However, ink chemists, due to a strong historical link, prefer to call them resins. Here the chemistry connection reflecting the chemical nature of these compounds takes a back stage in front of the rigid web of habit of calling it by a trivial name.
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.