INK VEHICLES

Vehicle- liquid portion of ink.
Vehicle composition determines: stiffness, drying rate, gloss, rub resistance, appropriateness to printing processes
Vehicle: carries pigment, controls flow, binds the pigment to substrate
To bond pigment particles to one another, to substrate- resin (binder).
All of ink characteristics (except color) determined by vehicle.
Vehicle: Resin + Polymer and/or Drying oil + Solvent
Often, two or more components (resin + polymer, polymer + polymer, polymer + drying oil) will be combined to capture desired characteristics of each.

RESINS
Natural:
- Rosin Based (lithography, letterpress, gravure, flexo)
- Cellulosic : Nitrocellulose, CMC, EHEC (flexo, gravure)
- Cyclized rubber (screen printing, offset duplicator)

Synthetic:
-  Acrylic (flexo, gravure, screen printing)
-  Vinyls (Screen inks)
-  Maleics (Litho, flexo)
-  Polyamides (Flexo)
-  Epoxies (Metal decorating)

Synthetic- polymerization (condensation, addition of monomers). Controlled reaction- product with closely controlled properties.

NATURAL RESINS
Rosin
Pine trees- wood rosin- crushing stumps of pine trees, extracting rosin and rosin oil with petroleum solvent.
Gum rosin- tapping live trees, sometimes with adding aid to artificial simulation to accelerate the flow of extruded sap.
Tall oil - kraft pulping by-product
Wood rosin, gum rosin- similar, gum rosin- higher melting point.
Rosin- amber in color, melting point about 60oC,saponification # 172, acid # 168.
Saponification #: total amount of -COOH groups (free –COOH and esterified with triglycerides).
Acid #: amount of free –COOH groups in molecule. Titration of –COOH with KOH, Acid# is number of milligrams of KOH required to neutralize 1g of resin.
Rosin- inexpensive, ecologically sound.
90% acids, 10% neutral material.
Abietic acid- main component of rosin, converts to levopimaric when heated.

Rosin: Used in alcohol base lacquers, in combination with mineral oils- newspaper inks. Raw material for chemically modified rosin- rosin soap, metallic rosinate, ester gum, rosin maleic adduct, rosin-fumaric adduct, modified with phenolic, and alkyd resins

Ester Gum and Ester of Rosin
Triglyceride derived from rosin. Reaction of rosin and glycerol.
Varnishes- good pigment wetting properties.

R.COO-CH2
             I
R.COO-CH2                                    R- rosin acid residue
             I
R.COO-CH2

Pentaerythritol ester rosin – higher softening point 112oC, quicker solvent release, better alkali and water resistance- more popular
             CH2-OOC.R
             I
R.COO-CH2-C-CH2-OOCR              R- rosin acid residue
             I
             CH2-OOC.R

Maleic resin and ester- wide use in inks, steam set, letterpress inks, water reducible letterpress inks, water based flexo inks.
Dimerised or polymerised rosin- by heating with suitable catalyst.
Cheap gloss overlacquers, letterpress black inks.
Rosin modified fumaric resin – flexo and gravure inks, in combination with other resins- high gloss.
Fumaric- isomer of maleic.

Nitrocellulose

Wide application in flexo and gravure inks (Paper, film, foil, board), in combination with maleic, polyamide, acrylic resins .
Cellulosic- excellent scratch, and rub resistance. Heat resistant, withstand heat-seal temperatures. Used for overprint lacquers.
Cellulose nitrate- made by treatment of purified cellulose (C6H10O5)n with excess of nitric and sulfuric acid at OoC.
Nitration complete, product is centrifuged and washed with water.
2.25 -OH groups are nitrated- it corresponds to 12.2 % N in solid cellulose nitrate.
Dry nitrocellulose- explosive- dangerous to handle. Legal minimum is 25 % alcohol- dampened nitrocellulose.
Grades of nitrocellulose – characterized by their N content.
For each N content several viscosities available.
White fibrous material, excellent thermal stability at room temperature.
Elevated T- decomposes violently.
Compatible with shellac, rosin derivatives, epoxies, short oil alkyd resins.
Outstanding film former.

Ethyl Cellulose
R-O-Na + C2H5Cl  --->  R-O-C2H5 +NaCl
Reaction of alkali cellulose with ethyl chloride under controlled conditions. 2.2-2.6 ethoxyl groups per anhydroglucopyranose unit.
Solubility varies according to degree of ethoxylation.
Lower content ethoxy- groups: plastic films. Compatible with rosin, esters, alkyds, methacrylates, styrenated alkyds, polyvinylchloride, nitrocellulose.
Lacquers, toughness, flexibility. Flexo and gravure formulations.

Cyclized rubber- rubber treated with acid, lost elasticity. Readily soluble in aliphatic or aromatic hydrocarbons, and esters. Compatible with drying oils, resinates, cellulose derivatives, hydrocarbon resins, maleics, modified phenolics.
Excellent adhesion and rub resistance (offset duplicators- prevents ink from oxidation “stay open”). Litho inks, in combination with isophtalic alkyd.  Softening point 110-140oC.

SYNTHETIC POLYMERS
Acrylic: The range of acrylic and methacrylic polymers are based upon monomers of acrylic acid CH2=CH-COOH and methacrylic acid:
         CH3
          I
CH2=C-COOH

Easily polymerized and co-polymerized – because of highly reactive double bonds and miscibility with oil soluble and water-soluble monomers.
Polymerization in bulk, in solutions, suspension, emulsion using variety of catalysts.
Polyacrylic acid:
                                                 COOH
                                                 I
n CH2=CH-COOH  --> -(-H2C-CH--)n

Polymethacrylic acid:
               CH                                     CH3
               I                                            I
 n  CH2=C-CH2-COOH --->     (-H2C-C--)n
                                                            I
                                                            COOH
Acrylic: clarity, chemical inertness, very good light fastness.
Softening points vary depending on chemical formula, molecular weight.
Range of acrylic resins so wide- soluble in almost any solvent system used in modern printing inks. Ability to co-polymerize, cross-link with epoxy resins, amines, urea, melamine resins.  Adhere to most foils, fade-resistant (flexo, gravure, screen inks, high gloss lacquers)- packaging inks.

Vinyls:
Polyvinylacetate
Acetylene with acetic acid

CH=CH + CH3COOH ---> CH2=CH-O-CO-CH3
(Triple bond, I could'n find it)
Frequently copolymerised with other monomers. Many viscosity grades commercially available. High molecular weights- tough films. Soluble in many solvents, except aliphatic hydrocarbons.
Compatibility with other resins not good, partially compatible with nitrocellulose, phenolic resins, chlorinated rubber.
Vinyls: Screen inks, flexographic transparent lacquers for foil, heat sealing lacquers. Poor pigment wetting – not pigmented.

Polyvinylbutyral

Reaction of polyvinylalcohol and butyraldehyde under controlled conditions.
White powders-characteristic rancid odor.
Soluble in alcohols, esters, ketones, glycols,  glycol esters.
Non-solvents such as aromatic hydrocarbons – diluents.
Compatible with epoxy resins, maleic and acrylic resins, ethylcellulose and nitrocellulose.
Flexible films. Adhesion to metals, glass, plastics, paper, textiles.

Phenolic resins
Used in conjunction with tung oil –varnishes for letterpress and litho inks.
Pigment wetting – good. Low softening point80-130oC, soluble in aromatic hydrocarbons, partially soluble in aliphatic solvents.
Tung oil phenolic varnishes excellent drying speeds, very tough glossy films.
Tendency to yellow on UV exposure.
Made by heating a para substitued phenol with aqueous formaldehyde solution under reflux with alkaline catalyst.
        OH                                     OH
n R-Ø-OH + (n+1)HCHO --->   -Ø-CH2- Ø-
       R                                          R

Ø- benzene ring
Acid catalyst- phenolic resin can be dissolved in drying oils (tung oil) without reaction (heating 200-250oC).

Polyamides:
Large family of polymers- amide grouping (-CO-NH-) in the main polymer chain. Nylon group- largest group- large molecular weight and poor solubility. Extrusion, molding.
Lower molecular weight- reactive polyamides- used in combination with other polymers. Used in coatings and solventless inks.
Thermoplastic polymers- non-reactive- soluble in solvents, used in liquid inks. In conjunction with other polymers to improve film- forming properties. Molecular weights 4000-7000 alcohol/hydrocarbon solvent, soluble in hydrocarbons, higher alcohols.
2000-4000 MW- improved alcohol solubility- readily blend with nitrocellulose. Compatible with rosin, maleic acid, phenolics, polypropylene, polystyrene, ketone resins- excellent.
Flexo and gravure inks- excellent adhesion to PE, OPP, polystyrene and films coated with PVDC (polyvinylidene chloride).
High gloss, resistance to fats, grease, fast solvent release, clean printing.
Aqueous polyamide dispersions-used in adhesives, heat seal and barrier coatings, acrylic or vinyl emulsions.

Alkyd resins
Tailor made resins for different purposes. Primary resins in formulation of paste inks- workable viscosities without the addition of solvent.
Soluble in variety of solvents. Used in screen, letterpress and litho inks.
Excellent compatibility with other resins.
Manufactured by esterification of polyhydric alcohol with polyacids- polyesters.
Combination of drying oil and fatty acids that modify the polyesters to allow them cross-link – make tough film.
Drying and non-drying – depending on amount of drying oil incorporated.
Linseed, tung, dehydrated castor, soya, tall oil.
Also classified as short, medium and long oil alkyds. The higher the oil length, the more soluble is in aliphatic hydrocarbons.
Most alkyds used in printing inks-  long oil alkyds- more than 65% or the oil or fatty acid component.
COMPOSITION
Linseed oil 70                    Pentaerythritol            Isophtalic acid 20
Linseed oil   67                  Glycerol                      Phtalic anhydride 22
Dehydrated castor 65         Glycerol                      Phtalic anhydride  25
 

Water - based inks:

2 types:        Solution resins
                    Emulsion resins
Solution resins - dissolved by water- true solution
Emulsion resins  - do form an emulsion- cannot be dissolved.
Solution resins - enhance ability of water to wet the pigment particles - glossy films - bad drying.
Emulsion resins - excellent drying properties, do not wet pigments.
Both types must be present in the formula.

Vehicle for paste ink:
Based on oils or UV curable ink system
Improve properties of oils- resins may be added (improve printing and drying properties, rub resistance).

Vehicle for liquid ink system:
Resins soluble in given solvent
Gravure: solvency, evaporation, toxicity.
Flexo: no ketones, aromatic solvents dissolving plates.
Epoxies: Offset, metal decoration
Two or more resins combined to capture desired characteristics of each.

OILS
Oldest raw material in printing ink industry, still important role in the formulation of printing inks.
Classification: drying, semidrying, non-drying depending on degree of unsaturation (iodine value).
Chemical structures of drying oils are triglycerides of fatty acids varying from those completely saturated- no double bonds, to those C18 acids - molecules with 2-3 double bonds- (or Unsaturated bonds).
Organic acid structure is –COOH.
The chemical structures responsible for drying are

-HC=CH-HC=CH-  Conjugated system of double bonds- very reactive

-HC=CH-CH2-HC=CH- Isolated system of double bonds- less reactive

The more double bonds or conjugated double bonds the molecule has, more rapid will be the drying (Heat bodying) process

Castor oil has –OH group in molecule. Dehydration of castor (DCO) oil yields into system of conjugated double bonds – very reactive- dehydrated castor oil; dries very rapidly (7min).
         OH
          I                                                Heat, dehydration
-CH- CH2-CH2-CH=CH-  + H2SO ---------------------->  -CH=CH-CH=CH-
                                                         - H2O
 

The mechanism of viscosity increase is cross-linking of unsaturated molecules in the sites of double bonds:

-HC=CH-CH2-HC=CH-  +   O2, energy                      -HC-CH-CH2-HC-CH-
-HC=CH-CH2-HC=CH-         ------------->                 -HC-CH-CH2-HC-CH-
                                                                                    (crosslinked)
Linseed oil – from flax, mechanical (pressing) or solvent extraction (petroleum ether or triclorethylene).
Crushing seeds, cooked, extracted by hydraulic pressure at 110oC. Oil drained to the tanks.
Linolenic acid- C18 acid with 3 isolated -C=C-, linoleic C18 acid with 2 isolated -C=C- bonds.
Heat bodied linseed oil- heat, air is blown Co, Mn added to dry it.

Tung oil- China wood oil from nuts elaeostearic acid with 3 conjugated
-C=C-, very reactive. Heated 290-310oC, during 7 min. Litho inks, tough, glossy finish, water and alkali resistant.