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The Structure and Performance Property Relationship in UV Inks



Ink manufacturers and their raw material suppliers have been evolving new technologies to meet these demands by providing new polymers, oligomers, monomers, photoinitiators, pigments and inks



By Zubair Khan, Environmental Inks and Coatings



Published November 2, 2009
Related Searches: efi water-based flexo ink
Editor’s Note: “The Structure and Performance Property Relationship in UV Inks” was presented during the National Printing Ink Research Institute’s (NPIRI) 47th Annual Technical Conference in October 2003.

UV flexo is making strides both in narrow web and wide web printing. UV flexo inks are being tested for new applications such as shrink labels, pouches, bottle wrap, in-mold labels, lamination, cold foil and other packaging applications.

The advances in the pre-press, color separation techniques, computer to plate, higher line screens, anilox rolls such as Nd:YAG laser engraved anilox, channeled anilox, digital plates, better designed lamps and fast changeover presses have further advanced the quality of UV printing.

Process printing has seen the evolution of Hexachrome and now Opaltone to expand the color gamut and quality of print. All this has placed more demands on the inks. Inks need to be higher strength, faster curing and provide more integrity to the label/package and end application.

Combination printing has become the norm, as new presses are being equipped to run UV inks with flexo, rotary screen, flatbed screen, letterpress, offset, gravure and now ink jet or other digital print techniques to take advantage of each printing process.

Each of these printing processes call for different properties from inks. Properties in UV inks are directly attributed to the vehicle system comprised of oligomers and monomers.

Ink manufacturers and their raw material suppliers have been evolving new technologies to meet these demands by providing new polymers, oligomers, monomers, photoinitiators, pigments and inks. The next generation products are getting better and better.

Thanks to the acrylation by grafting, without which it would not have been possible to make epoxy, urethane, polyester and polybutadiene to become radiation-cured materials suitable for the ink industry. The acrylation of these chemistries has made the reaction kinetics very similar though the backbone differs so widely as some are condensation polymers and some addition polymers.

The structure, physical and chemical properties of oligomers and monomers have direct relationships with performance properties of the inks such as printability, adhesion, transfer, density, dot gain, misting, tack, viscosity, curing, post-curing (in cationic), migration, odor, gloss, residuals and product resistance.

Effort has been made to understand the kinetics, structure performance property relation in UV inks. This study focuses on the structure of monomers and oligomers, the kinetics of curing and performance properties.

Chemistry of UV Inks



Commercial UV inks are of two types depending on chemistry:
1. Free radical
2. Cationic

In order to understand the structure property relations better, we need to understand the kinetics of the curing process in both free radical and cationic inks.

The kinetics of curing helps us understand how the concentration of monomer/oligomer, concentration of PI, extinction coefficient of PI, thickness of ink, intensity of radiation, reactivity ratios of monomer/oligomer, temperature, activation energies of monomer/oligomer and chain transfer reactions affect the curing of ink.

The Kinetics of Free Radical UV Inks



Free radical inks go through step polymerization sequence of three steps. When the ink goes through UV lamps:
1. Initiation:
Iki >2R*
ki is the rate constant for initiation
R*+M>M1*

2. Propagation:
M1* + Mkp > M2*
M2* + M> M3*
Mn* + M> Mn+1*
kp= is rate constant for propagation

3. Termination:
Termination in step polymerization occurs through one of the two ways:

• Coupling or combination of monomer radicals or oligomer radicals
Mn* + Mm*ktc> Mn+m
ktc is rate constant for termination by combination
H2C—CHY* + *YHC_CH2>H2C—CHY——CHY—CH2
• Termination by disproportionation
Mn* + Mm*ktd> Mn + Mm,
ktd is rate constant for termination by disproportionation

This termination is rare but happens depending on the reaction conditions. Here, it results in two polymer molecules. One, Mn, is saturated polymer whereas Mm is unsaturated.

H2C—CHY* + *YHC–CH2>H2C—CYH2 + YHC=CH

Rate Expression



Monomer disappears both in initiation and propagation. The rate of polymerization (curing) could be given as
-d[M]/dt = Ri + Rp

The number of monomer/oligomer molecules forming a polymer in initiation step is negligible in the beginning. Ri could be neglected in the above expression.
-d[M]/dt = Rp

The rate of propagation is the sum of all the propagation steps. Since the rate constants for propagation steps are the same, the polymerization rate (curing rate) could be expressed as
Rp = kp [M*] [M]


It is difficult to find out the exact concentration of monomer or oligomer radical. It is better to express rate of polymerization (curing) without the concentration of radicals. In order to do this, we may have to take steady state assumption.

The rate of change of initiator concentration becomes zero during the course of curing. At steady state, Ri = Rt, the rate of initiation is equal to rate of termination.
Ri = 2kt [M*]2

The expression on the right in the above equation is the rate of termination. Since the termination involves disappearance of 2 M* radicals, hence the factor 2 represents the two radicals. It is generally accepted convention for reactions destroying radicals in pairs.

Rearranging the above expression as,
[M*] = ( Ri/2kt )1/2

substituting this in Rp rate of polymerization expression,
Rp= kp [M] (Ri/2kt)1/2……Equation I

The significance of this conclusion is that:
• Doubling the rate of initiation does not double the polymerization rate (curing rate). When the rate of initiation is doubled, the polymerization (curing) rate is increased by √2.

The rate of producing radicals of initiator Rd is given by
Rd=2fkd [I]=Ri

where f is initiator efficiency which is the fraction of radicals produced that initiate polymer chains. The value of f is less than 1, as there is wastage of initiators.

Ri = 2fkd[I]

Substituting Ri value in equation I,
Rp=kd [M] ( fkd[I]/kt )1/2

This shows that the polymerization rate (curing rate) is dependent on the square root of the initiator concentration. This has been confirmed by many monomer, initiator combinations.

In UV inks, the kinetics of curing is similar to the kinetics of radical copolymerization. Let’s consider a UV formulation with one monomer (M1) and one oligomer (M2):

M1* + M1K11>M1* ………........1
M1* + M2K12> M2* ……...… 2
M2* + M1K21>M1* ……....….3
M2* + M2K22>M2* …….....….4

where K11 is the rate constant for a propagating chain ending in M1.
Similarly K12, K21, K22 are the rate constants for a propagating chain ending in M1, M2, M2 radicals.
Equation 1 represents the disappearance of monomer M1. Equation 3 represents the disappearance of oligomer, which are synonymous with their rates of entry into the copolymer (cured film).
The rate of disappearance of monomer and oligomer is given as
d[M1]/dt = K11[M1*][M1] + K21[M2*][M1].......5

d[M2]/dt = K12 [M1*][M2] + K22 [M2*][M2]….6

Dividing 5 by 6
d[M1]/d[M2] =K11[M1*][M1] + k21 [M2*][M1]
K12[M1*][M2] + K22[M2*][M2]

Since we can’t calculate the M1*, M2* concentration, we may have to take the steady state assumption. At steady state
K21[M2*][M1] = K12[M1*][M2]

Rearranging
d[M1]/d[M2] =[M1](r1[M1] +[M2]) .......................7
[M2]([M1] +r2[M2])

r1 = K11/K12
r2 = K22/K21

Equation 7 is copolymerization composition equation. r1, r2 are referred as reactivity ratios of monomer, oligomer. The rate of curing of a UV ink made with a monomer and an oligomer depends on the reactivity ratios of monomer and oligomer.
r1 = K11/K12 =A11/A12e(E12 - E11)/RT..8
where E11, A11 are the propagation activation energy and frequency factor for M1 radical adding M1 monomer; respectively E12 and A12 are the corresponding values for M1 radical adding M2 oligomer.

The activation energies for free radical propagation are relatively small. The effect of temperature on r1 is not large. The temperature has greatest effect on cationic propagation. Increase in temperature has little effect on free radical propagation, but in cationic, the propagation is increased by the increase in temperature.



Chain Transfer



Chain transfer is chain breaking reaction; it affects the curing of ink adversely. It results in the decrease in the chain length. The relative decrease depends on the magnitude of transfer constant.

The degree of polymerization (the extent of cure) can be defined as polymerization rate (curing rate) divided by sum of all chain breaking reactions.

Xn=Rp
(Rt/2) + Ktr, M [M*][M] + Ktr, s [M*] [S] + Ktr, I [M*][I]

• (Rt/2) represents the termination by coupling.
• Ktr, M [M*][M] represents the chain transfer to monomer.
• Ktr, s [M*] [S] represents the chain transfer to solvent.
• Ktr, I [M*][I] represents the chain transfer to initiator.

The chain transfer constant C for monomer/oligomer, solvent, initiator can be represented as:
CM = Ktr, M/Kp
Cs= Ktr, s/Kp
Ci= Ktr, i/ Kp

1/Xn = KtRp/Kp2[M]2 + CM + Ci {KtRp2/Kp2fKd[M]}
Cs=0 in UV inks.

This shows the quantitative effect of the various transfer reactions on the number average degree of polymerization(extent of curing).

Initiators in UV inks



Both aliphatic and aromatic ketones are used. The aromatic ketones are more useful in commercial practice. Benzophenone and acetophenone and their derivatives, alpha amino ketones, alpha hydroxy ketones, substituted thioxanthones and phosphine oxide derivatives are the commonly used photoinitiators in the UV ink applications.

Ketones undergo homolysis by either fragmentation or photo cleavage and hydrogen abstraction.

The second and most common in UV inks is hydrogen abstraction in the presence of hydrogen donor (RH).

Amines are the most efficient hydrogen donors. The efficiency depends upon their oxidation potential.

Among the two, the photocleavage initiation results in better curing compared to hydrogen abstraction type, as one of the radicals generated is terminating radical in the type II may not result in the propagation of the chain. In flexo ink applications, the combination of type I, type II gives better surface and through curing. In type II, the efficiency depends on the reduction potential of the PI.

Once the ink or coating goes through UV curing unit, the benzophenone generates an excited state triplet followed by an efficient intersystem crossing from the singlet state. This excited state triplet can be quenched by monomer (desirable), oxygen (inhibition, not desirable), decay due to non-radioactive decay, accepting hydrogen from the hydrogen donors like amines, ethers, alcohols or thiols.

Among the hydrogen donors, tertiary amines, amino benzoates are the choice for ink makers.

The following is the reaction mechanism with BP, amine synergist MDEA:

The result is the generation of ketyl radical, the radical centered on the hydrogen donor. The ketyl radical does not initiate polymerization; it is a terminating radical. The other radical initiates polymerization. As a result of formation of one terminating and one initiating radical, the hydrogen abstraction type initiation is not as efficient as photocleavage. But, recent studies with the addition N-substituted maleimides, along with tertiary amines, BP, the efficiency of initiation is as good as photocleavage without the byproduct formation.

The photocleavage type are more expensive and generates byproducts like benzaldehyde.

Ex: DMPA (2,2-dimethoxy-2-phenylacetophenone) undergoes photocleavage.

Most excitations of ketones involve n-----Π* excited states. These radicals are much more stable compare to Π ----Π*.

The rate of photochemical initiation is expressed as:
Ri = 2φIa………………..Equation 2

where Ia is the intensity of absorbed light in moles; and φ is the number of propagating chains initiated per light photon absorbed. φ is referred to as quantum yield for initiation.

Substituting Ri value in Equation I:
Rp=kp[M](φIa/kt)_ ……………………Equation 3

It is often convenient to express the absorbed light intensity by:
Ia= εIo[A]b……………………Equation 4

Io is the incident light intensity, A is the species which undergoes photo excitation (PI), ε is the molar absorptivity or extinction coefficient of A at the particular frequency of radiation absorbed and b is the thickness of the ink or coating being irradiated.

Equations 1,2 and 3 allows Ri and Rp to be expressed as
Ri = 2øεIo[A]b……………………….Equation 5

Rp = kp[M] (øεIo[A]b/kt)1/2 ……..Equation 6

This expression assumes the incident light intensity does not vary appreciably throughout the thickness of the ink film. This will be true only if the ink film thickness is small (using 1000/1.2 bcm anilox or higher).

Io,Ia will vary with the thickness of ink or coating. Under these circumstances, the light intensity absorbed by the ink or coating Ia can be obtained from Lambert-Beer’s Law
I = Io e-ε[A]b ………………….Equation 7

where I is the incident light intensity at a distance b in the ink film. The light absorbed by the ink film is given by
Ia = Io[ 1- e-ε[A]b……………………Equation 8

The polymerization rate can be given by combining the equation 3 and 8 results,

Rp = kp [M] ( φ Io[1- e-ε[A]b]/kt)_


Conclusion



Rp is first order in [M], 1/2 order in light intensity and 1/2 order in [A] for the case where there is negligible attenuation through the thickness of the ink. In practice, there is appreciable amount of attenuation of light intensity occurs.

The above equation helps in calculating the maximum ink film that can be cured. The maximum thickness b can be calculated
e-ε[A]b = 0

The maximum thickness increases with increasing ε (extinction coefficient of PI), [A] the concentration of PI.

Initiator wastage: There is a wastage of initiator due to two reactions:
1. Induced decomposition of initiator:

This reaction is called chain transfer to initiator. Here, theMn—OCOO formation resulted in the wastage of initiator radical.
2. Cage effect :

C6H5COO---OOCC6H5<------------> [2C6H5COO*]
[2C6H5COO*]------------> [C6H5COOC6H5 + CO2]

This wastage of radical by reverse reaction to form neutral molecules is called cage effect.

Inhibition reaction kinetics:
Mn* + Z(inhibitor) ------------> Mn + Z* will terminate without regeneration of inhibitor radical.

The steady state assumption for radical concentration leads to
d[M]/dt = Ri- 2kt[M*]2 – kz [Z] [M*] =0

which can be combined with Rp= kp[M*][M], resulting in

2Rp*Rpkt/kp*kp [M][M] + Rp[Z]kz/kp[M] – Ri = 0

which shows that Rp is inversely proportional to the ratio kz/kp. This ratio is called inhibition constant z.

z=kz/kp for the case where retardation is strong, this ratio kz/kp >>1.

Oxygen, sulfur, cucl2, Fecl3, p-benzoquinone, nitrobenzene, DPPH( diphenyl picryl hydrazyl ) could scavange the radical used in the inks and coatings.

The Oligomers



Epoxy Acrylate:
Urethane Acrylate:
Polyester Acrylate:
Polybutadiene Acrylate:

The acrylation of these polymers/prepolymers/oligomers make the epoxy, urethane and polyester useful in UV ink applications.

The base resin or prepolymer of epoxy resin, urethane, polyester is used as protective colloid to carry radical graft copolymerization with monomers such as AA, MAA, MMA, BMA, HEMA, EA, BA, BDDA, IBMA, EMA styrene using persulfate or peroxide initiators until desired acid value, viscosity and molecular weight is achieved. The selection of monomers in the acrylation has direct impact on the performance properties of the ink. The reactivity of acrylate monomers is higher than methacrylate monomers or styrene. So, the acrylated oligomer with no methacrylic groups or styrene cure faster than oligomer acrylated with more methacrylic monomers and styrene.

One important caution is that if we add chain transfer aid to the monomer mix in the acrylation, care should be taken in the selection of chain transfer aid so that when this product is used in the ink application, the residual chain transfer aid should not terminate the polymerization and hence affect curing of ink or coating adversely. The popular chain transfer aids are hydrogen donors.

Cationic UV Inks



Aryldiazonium (ArN2+Z-), Aryliodonium (Ar2I+Z-), Arylsulfonium (Ar3S+Z-) and Arylselenonium(Ar3Se+Z-) salts containing nonnucleophilic and photostable anions such as BF4-, SbF6-, PF6- are effective photoinitiators.

Initiation:
Ar2I+(PF6)-hv> ArI+. (PF6)- +Ar*
ArI+. (PF6)- + MH> ArI + HF. PF5 +M*
where MH is monomer.

Propagation:
The initiator ion pair proceeds to grow by successive addition of monomer molecules. This addition of monomer results in monomer being in between carbenium ion and its negative gegenion.
HMn+(IZ)- + Mkp> HMn M+(IZ)-


The propagation step involves H: shift and :CH3 group shift to form tertiary carbenium ion which is more stable and abundantly available during propagation.

In propagation, intramolecular rearrangements due to hydride(H:-) ion or carbanion (R:-)shifts. The extent of rearrangement during cationic propagation will depend on the relative stabilities of propagating and rearranged carbenium ions and relative rates of propagation and rearrangement. Vinyl ethers follow similar reaction mechanism as that of olefins in rearrangement. The olefin rearrangement occurs as following:

The first carbenium ion I undergoes hydrogen shifts to form carbenium ions II, III, V and rearranges to IV by methyl shift. Carbenium ion V is most stable.

Termination



• Chain transfer to monomer
HMnM+(IZ)- +M Ktr,M > Mn+1 + HM+(IZ)-

Another chain transfer to monomer involves hydride ion abstraction from monomer to the propagating species.

• Spontaneous termination

This involves regeneration of initiator-coinitiator complex by expulsion from the propagating ion pair with the polymer molecule left with terminal unsaturation.
HMnM+(IZ)-kts> Mn+1 + H+(IZ)-

This is also called chain transfer to counterion.
• Combination with counterion
The propagating carbenium ion combines with counterion

HMnM+(IZ)-kt> HMnMIZ

Retardation



Amines, trialkyl or triarylphosphines, thiophenes act as retarder for cationic polymerization.
HMnM+(IZ)- + :NR3>HMnMN+R3(IZ)-

Termination by amines involves formation of stable quarternary ammonium ions which are unreactive to propagation. This is the precise reason why you can’t have cationic inks or coatings over the water-based inks or coatings.

p-benzoquinone acts as an inhibitor as the transfer of protons from the propagating carbenium ion and/or initiator-protonated species to form p-hydroquinone.

Free Radical vs. Cationic



Kp/kt ratio is strong for best curing. The extent/degree of curing in cationic = kp/kt. The extent of curing in free radical = √kp/kt.

The degree of curing in free radical inks is lower compared to cationic as termination is higher in free radical. The concentration of propagating species is much higher in the cationic than free radical.

Monomer Structure, Physical



Properties – Effect on Ink Properties

• Straight Chain vs. Branched: Straight chain monomers are flexible compared to branched.
• Migration: Straight chain or branched, temperature, gauge and type of substrate determine the migration.
• Odor: The alkoxylated monomers have less odor compared to non-alkoxylated monomers.
• Curing: The curing was slightly slower for the alkoxylated monomers. Higher functional cures faster but tends to curl on light weight paper substrates. Higher functional also contributes to brittleness.
• Resistance Properties: The resistance properties were poor for the alkoxylated monomers.
• Pigment wetting: The alkoxylated monomers were better pigment wetting compared to the ones without. The printability was slightly better for the alkoxylated.
• Static and Dynamic Surface Tension: The static and dynamic surface tension decreases with increase in the chain length of monomers. The surface tension of alkoxylated monomers is slightly higher than their straight homologues.
• Diluency: The viscosity reduction of inks is higher using non-alkoxylated monomers compare to the alkoxylated monomers.
• Shrinkage: Alkoxylated monomers gave lower shrinkage compare to the non-alkoxylated.
• Double Bond Density: The higher the DBD, the higher the reactivity, and the higher the shrinkage.
• Gloss: The refractive index of monomers plays a critical role in the gloss of the ink and coating. The higher the RI, the higher the gloss. The alkoxylated monomers gave higher gloss than the non-alkoxylated ones.
• Water Pickup: The hydrophilic monomers increase the water pickup in the range of >60, whereas hydrophobic monomers lower the water pickup.
• Tack: The viscosity of oligomer contributes to the tack of the ink.
• Viscosity: The alkoxylated monomers gave higher viscosity inks.
• Yellowing: Some photoinitiators cause yellowing. Yellowing is pronounced if the aromatic urethane acrylate is used.

Rheological Properties


UV flexo inks are shear thinning with some yield value. Some colors when properly formulated exhibit near Newtonian behavior. The rheological properties of future inks are going to be near Newtonian as more and more customers are demanding easy to use inks that could perform on higher press speeds.

The viscosity and the stability aspects are very critical to the ink maker and their customers. UV inks are sensitive to metal contaminations in the pigments. As more and more cheaper pigments are available to the pigment manufacturers from Southeast Asian sources, we have to be careful about the impurities in those pigments, as the contaminants could cause the premature curing.

References



1. George Odian, Principles of Polymerization. John Wiley & Sons, Inc.; 1981.
2. Fouassier, J.P., Photoinitiation, Photopolymerization and Photocuring. Verlag; New York, NY, 1995.
3. Radiation Curing: Science and Technology. Pappas, S.P. Plenum Press; New York, NY, 1992.
4. Murov, S.L., Carmichael, I. and Hug,G.L, Handbook of Photochemistry. Dekker: New York, NY, 1993.
5. Anbu Natesh and Ramesh Narayan, Matthias Fies and Holger Endres, RadTech 2002, Technical Conference Proceedings.
6. Chau K. Nguyen, T. Brian Cavitt and Charles E. Hoyle, RadTech 2002, Technical Conference Proceedings.
7. J.-L. Birbaum, S. Ilg, T. Bolle, E.V. Sitzmann and D.A. Wostratzky, RadTech 2002, Technical Conference Proceedings.


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