Matting solvent-free UV-curable coatings · 2018. 2. 14. · Matting solvent-free UV-curable coatings Technical Information 1399 15-01-139, 15-01-037 vND TI 1399 Matting solvent-free

Matting solvent-free UV-curable coatingsTechnical Information 1399

15-01-139, 15-01-037 vND TI 1399 Matting solvent-free UV-curable coatings, us.indd 1 21.07.15 14:45

Content

Introduction � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 4

Interaction between particle size of the matting agent and film thickness of the coating � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 4

Matting of thick coating � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 6

Matting of thin coating � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 9

Recommendations for choosing suitable matting agents � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 11

Influence of polymerization-related volumetric shrinkage � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 11

The importance of the “gel point” � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 13

Influence of the coating formulation on matting � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 14

Influence of the monomers � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 15

Influence of the photoinitiator � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 18

Influence of rheological parameters � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 20

Influence of system and process parameters on matting � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 22

The physics of light absorption � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 22

Influence of the light spectrum � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 23

Curing with the “dual-cure” method � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 26

Influence of the temperature � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 27

Influence of the substrate type � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 28

Influence of the reflector design � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 29

Influence of the reflector coating � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 29

Illumination with LED lights � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 29

Results at a glance � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 30

Schematic presentation of the influencing parameters � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 31

Evaluation of the influencing parameters as regards use � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 32

References � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 33

List of tables � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 34

List of figures � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 35

2


3


IntroductionConventional coating systems with volatile compo-nents, including water and solvent-based UV-curable coatings, are easy to matte because of their emission-related vertical film shrinkage [1]� On the other hand, solvent-free coatings, such as 100 % UV-cur-able coatings are difficult to mat because there is no film shrinkage by evaporation of volatile compounds�

The task of the coating formulator is to make up for the lack of vertical film shrinkage with suitable mechanisms to achieve a roughening of the coating film surface and, consequently, an adequate degree of matting� One option is to select the particle size of the matting agent so that it largely corresponds to the thickness of the applied coating [2]� The second possibility is to strengthen the volumetric shrinkage

of the coating film caused by radical polymerization of the acrylate oligomers and monomers and make optimum use of this� Important in this context is that the matting agent matrix, consisting of silica particles distributed evenly in the liquid coating, shrinks less than the surrounding binder matrix during the curing process�

In this publication it will be shown that it is possible to influence the degree of matting significantly by choosing suitable matting agents, through “clever” coating formulation, and by targeted control of cur-ing and equipment parameters� With optimum utili-zation of all the available impacting parameters it is even possible to produce flat matte coatings�

Interaction between particle size of the matting agent and film thickness of the coating

A model will be presented to describe the matting of coatings with low and high coating film thicknesses� The coating films > 18 g / m² were applied to BYK charts with wire-wound rods while the coating thick-nesses < 18 g / m² were created through roller appli-cation on black painted glass panels� In the following figures it must be considered that the matting silicas used in the model (Table 1) ACEMATT® OK 607 and ACEMATT® HK 440 are shown in an idealized form as evenly sized spheres in the densest possible sphere packing� As well as the agglomerate distri-bution, the morphology (inner and outer surfaces, pore volume, and fine structure) of the silicas are not considered� The high degree of filling shown here cannot be achieved in this form in practical applica-tions� The degree of shrinkage shown later also has other values in practice� Nevertheless, the model is extremely suitable for visualizing the interaction�

Apart from the matting silicas used in the model – ACEMATT® OK 607 and ACEMATT® HK 440, which have been on the market for some time – ACEMATT® 3600 is also presented here� This is a newly developed matting agent based on a precipi-tated silica that is subsequently treated with a side group-modified polydimethylsiloxane (PDMS)� Its significantly better matting effect compared to the slightly finer matting silica ACEMATT® OK 607 can also be attributed to the special surface treatment and the higher concentration possible in the coating which is due to the better rheological properties� It must also be mentioned that the matting silica ACE-MATT® HK 440 is not primarily recommended for matting UV-curable coatings but is used only within the scope of the model�

4


Table 1 Matting silicas used

Unit ACEMATT® OK 607 ACEMATT® 3600 ACEMATT® HK 440

Production process Precipitated silica

Drying loss % ≤6 ≤6 ≤6

Surface treatment organic PDMS none

pH 6�3 6�6 6�0

Agglomerate particle sized50 (laser diffraction)

µm 4�4 5�0 14�5

DOA absorption ml / 100 g 250 255 270

Sulfate content as SO4 % ≤1 ≤1 ≤1

SiO2 content % ≥98 ≥98 ≥98

Carbon content % 5�5 3�0 ‒

Table 2 Formulations A

Formulation Characterization

Raw material #1 #2 #3

Oligoether acrylate, amine-modifiedg

68�03 67�43 69�90Molar weight 600 g / mol, Functionality: approx� 3�0Viscosity: approx� 110 mPas

Hexanediol diacrylate g 16�99 16�84 17�45

α-hydroxy ketone g 2�58 2�56 2�65 Melting point: approx� 47 °C

Bis-acylphosphine oxide g 1�29 1�28 1�32 Melting point: approx� 130 °C

ACEMATT® OK 607 g 11�11 ‒ ‒

ACEMATT® 3600 g ‒ 11�89 ‒

ACEMATT® HK 440 g ‒ ‒ 8�68

Total g 100.00 100.00 100.00

Viscosity, D = 0�4 / s mPas 2600 2800 10500

Viscosity, D = 4 / s mPas 614 685 1560

Viscosity, D = 1000 / s mPas 238 254 150

Matting solvent-free UV-curable coatings

5


Figure 1 Influence of the application weight and particle size of the matting agent on the gloss level

70

60

50

40

30

20

10

0

60°-

Refle

ctom

eter

val

ueApplication weight [g/m2]

10 20 30 40 50 60 70 800

ACEMATT® OK 607ACEMATT® 3600ACEMATT® HK 440

Formulations: #1, #2 and #3Hg radiator: 100 W / cm 5 m / minDose: 850 mJ / cm2, Peak: 1480 mW / cm2

Below 18g/m2 the coating was applied by a roller coater�

= Shrinkage of the entire coating [μm] – Shrinkage of the silica matrix [μm] x 100 % Shrinkage of the entire coating [μm]

Matting of thick coatingWhen conventional, solvent-based coatings are being formulated, in general it is difficult to matte thick lay-ers because as the film thickness increases, the gloss of matte surfaces increases [1]� On the contrary, this behavior is less pronounced or even reverses on the graph when 100 % UV-curable coatings are used� Examples can be seen in the graph of coatings that were applied with application weights of 18 g / m² to 64 g / m² (Figure 1)� In the case of the fine-particle matting agents ACEMATT® OK 607 and ACEMATT® 3600 one can already observe a continuous decrease in gloss as the layer thickness increases in the range from below 20 g / m²� In the case of the very coarse-particle matting agent ACEMATT® HK 440 the gloss increases to about 36 g / m²� The gloss is also decreasing with this silica, if the film thickness is increased further�

The reason for the reduction in gloss as the layer thickness increases is the desired volume shrinkage of the coating film during UV-curing�

Volume and density determination of uncured and cured coatings have shown that when all formulation and system parameters are utilized, that a volumetric shrinkage of 12 % is achievable in practice �

Figure 2 shows the influence of volumetric shrink-age when matting agents with different particle sizes are used� As examples, ACEMATT® OK 607 (very fine particles) and ACEMATT® HK 440 (very coarse particles) are used� The model is based on volumet-ric shrinkage of 8 %� The shrinkage efficiency takes account of the relative shrinkage of the matting agent matrix in relation to the shrinkage of the entire coating�

Shrinkage efficiency of 0 % means that the silica matrix shrinks to the same degree as the entire coat-ing matrix, consequently, neither the desired rough-ening of the coating film surface nor matting of the coating occur�

In earlier publications [18] [19] [20], shrinkage efficiency was defined the other way round� To give a better understanding, shrinkage efficiency is now defined as meaning better matting is achieved with the higher efficiency�

Shrinkage efficiency

6


Application thickness: 55 μmVolumetric shrinkage: approx. 8 %Shrinkage efficiency: approx. 50 %Effective shrinkage: approx. 2 μm

Coarse‐particle matting agentACEMATT® HK 440/d50 = 14.5 μm

Fine‐particle matting agentACEMATT® OK 607/d50 = 4.4 μm

Volumetric shrinkage has a considerableinfluence on the matting effect

Figure 2 Model for matting thick-layered coatings through high volumetric shrinkage

ACEMATT® HK 440/d50 = 14.5 μm

ACEMATT® OK 607/d50 = 4.4 μm

Derived from the model:Height difference: approx. 2.0 μmwith low rougheningHigh gloss in both measurement angles

Derived from the model:Height difference: approx. 2.0 μmwith low rougheningLow gloss at measuring angle 60°but high gloss at measuring angle 85°

Values from practice60°-reflectometer value: 46.185°-reflectometer value: 86.0Maximum roughness profile height Rz: 0.77 μm Arithmetic mean roughness value Ra: 0.08 μm


Figure 3 Virtual surface structures of the model shown in Figure 2 with technical application data of an associated cured coating

Figure 3 shows the surface structures in a cross-sec-tion of the film� As a result of the assumed effective volumetric shrinkage of 2 µm, the maximum height differences in the surface of the coating films should be about 2 µm� Because of the large particle size of 14�5 µm in ACEMATT® HK 440 has a large particle size of 14�5 µm, a low number of associated mat-ting particles, very few pronounced structures, and very long-wave structures which are created on the surface of the coating film� This results in high gloss values with the 60° and 85° measuring geometries�

As opposed to this, there are many more roughen-ing particles when the fine-particle matting agent ACEMATT® OK 607 is used� Since ideally shrink-

age would be almost half of the particle diameter of 4�4 µm, the much higher number of matting particles leads to much more pronounced short-wave struc-tures� This is noticeable with a much lower gloss level at the 60 ° angle� However, the gloss level is also high at the 85 ° angle because of the low height difference in the surface of the coating film of < 2 µm� The comparison of the theoretical assump-tions in Figure 3 with the values determined in prac-tice would suggest the theory is valid� The roughness values Rz and Ra [4] [5] determined with the tactile roughness tester Hommeltester T 8000 [3] (Figure 4) supports the model�


7


Maximum roughness profile height Rz Arithmetic mean roughness value Ra

DIN EN ISO 4287Sum of the roughness profile derived from the height of the highest peak Rp and the lowestvalley Rv within a single measuring length.

As the vertical distance from the highest to thelowest profile point, Rz is a measure of the range of roughness values in the profile.As Rz is generally determined as the mean of 5single measuring lengths Ir of the roughnessprofile it corresponds to the mean peak-to-valleyheight according to DIN 4768. Rp is equivalentto the height of the single highest peak abovethe mean line previously defined in DIN 4762.

Statistically Ra is the arithmetic mean of thedeviations of the roughness profile about thecentre line. Ra provides only very limitedinformation and is intensive to extreme profilepeaks and valleys.

DIN EN ISO 4287The arithmetic mean of the sum of roughnessprofile values.

Ra = | Z (x) | dx1lr

lr

0∫

Figure 4 Definition of the roughness parameters Rz and Ra

The maximum roughness profile height Rz provides information about the (average) maximum height differences of a roughness profile� The arithmetic mean roughness value Ra describes the height of a rectangle whose surface area corresponds to the cut

profile over the full material, where the second side length is specified by the length of the profile� Hence it is possible to differentiate between the height-related roughness value Rz and what is effectively a surface area roughness value Ra�

8


Application thickness: 10 μmVolumetric shrinkage: approx. 8 %Shrinkage efficiency: approx. 50 %Effective shrinkage: < 0.5 μm

ACEMATT® HK 440/d50 = 14.5 μm ACEMATT® OK 607/d50 = 4.4 μm

Volumetric shrinkage is of secondary importance

Figure 5 Model for matting thin-layered coatings by approximating the particle size of the matting agent to the dry layer thickness of the coating

ACEMATT® HK 440/d50 = 14.5 μm

ACEMATT® OK 607/d50 = 4.4 μm

Derived from the model:Height difference: approx. 2.0 μmwith high rougheningLow gloss in both measurement angles

Derived from the model:Height difference: < 1.0 μmwith low rougheningHigh gloss in both measurement angles



Figure 6 Virtual surface structures of the model shown in Figure 4 with technical application data of an associated cured coating

Matting of thin coatingThe above-mentioned mechanism of polymerization-related volumetric shrinkage is suitable for matting of thin coating films only to a limited extent because this is only of secondary importance due to the thin layers (Figure 5)�

When the surface structures obtained from the vir-tual film cross sections are compared (Figure 6) the difference to the higher coating thicknesses becomes obvious�


9


Figure 7 Surface topograms of thickly applied, matted UV-coatings

Figure 8 Surface topograms of thinly applied, matted UV-coatings

The large-particle matting agent ACEMATT® HK 440 results in very pronounced structures because of the single layer of coarse particles, shown in an ideal-ized form in the model� This results in comparatively low gloss values in both the 60 ° and 85 ° angles� The fine-particle matting agent ACEMATT® OK 607 exhibits the opposite behavior� Here, only flat struc-tures are visible, which results in high reflectometer values in both measuring angles� The roughness values also meet expectations� Apart from deter-mining roughness parameters, the Hommeltester T 8000 [3] can also be used to show topograms of coating film surfaces� Figure 7 and Figure 8 show the surfaces of thickly and thinly applied coatings that were matted with matting agents ACEMATT® OK 607 and ACEMATT® HK 440� To rule out influences of the substrate on the topogram, the coatings were applied to smooth PMMA sheets� The squares have a side length of 0�5 x 0�5 mm, the values of the z-axis (height axis) reflect the maximum height difference within the aligned topogram surface� When the topograms are being evaluated, it must be remem-bered that the z-axis is increased by a factor of about 10 for better visualization�

The correlation between the particle size of the mat-ting agent and the thickness of the coating exhibits that the importance of shrinkage decreases as the film thickness decreases, while the influence of the particle size of the matting agent increases�

10


Recommendations for choosing suitable matting agentsBased on the model and the results of the practical experiments it is possible to make clear recommen-dations for practical applications� To matte thin film coatings matting silicas with an average agglomer-ate particle size (determined with laser refraction) of about 0�5 to 1 times the height of the coating thickness are recommended� When a certain layer thickness is not reached, in case of lower or higher film-thickness compared to particle size, the matting efficiency becomes insufficient�

On the other hand, fine-particle matting silicas are needed to matte thick film coatings� Products with average agglomerate particle sizes of < 5�5 µm showed best matting efficiency in that context� The newly developed polydimethylsiloxane-treated mat-ting agent ACEMATT® 3600 is suitable for a wide range of thicknesses and was used in all the matting examples described below�

Influence of polymerization-related volumetric shrinkageTable 3 shows two matted coating formulations with very different gloss levels� On the one hand, these differences were achieved through the choice of coating raw material and, on the other, by vary-ing the application and curing conditions� With the exception of the matting agent and its – very slightly

changed – concentration, variations were carried out to prevent matting (Table 3, #6) and to support the formation of a matte coating film surface (Table 3 #7)�

Figure 9 SEM images of film cross sections (top) and topograms of the associated coating film surfaces (bottom part of the figure)


11


Table 3 Formulations B

Formulation #4 #5 #6 #7 Characterization

Oligoether acrylate, amine-modified

g77�39 ‒ 77�39 ‒

Molar weight 1000 g / mol, Functionality: approx� 3�5Viscosity: approx� 60 mPas


g‒ 77�70 ‒ 77�70

Molar weight 600 g / mol, Functionality: approx� 3�0Viscosity: approx� 110 mPas

TMPTA g 19�33 ‒ 19�33 ‒

HDDA g ‒ 19�40 ‒ 19�40

Benzophenone g 3�28 ‒ 3�28 ‒

ACEMATT® 3600 g ‒ ‒ 11�00 13�00

α-hydroxy ketone g ‒ 2�50 ‒ 2�50 Melting point: approx� 47 °C

Bis-acylphosphine oxide g ‒ 0�40 ‒ 0�40 Melting point: approx� 130 °C

Total g 100.00 100.00 111.00 113.00

Conditions regarding matting capability

bad optimum bad optimum

Substrate Glass3 mm

Glass1 mm

Glass3 mm

Glass1 mm

Application wire- wound rodGap height

µm 150 200 150 200

Radiator type Hg Fe-doped Hg Fe-doped

Radiator output W / cm 100 200 100 200

Conveyor speed m / min 10 2 10 2

Dose mJ / cm2 460 4120 460 4120

Peak mW / cm2 1380 2270 1380 2270

Weight loss during curing % 0�12 0�19 0�10 0�09

Volumetric shrinkage % 10�7 11�3 10�6 11�4

Maximum roughness profile height Rz

µm 0�13 0�14 0�30 1�56

Arithmetic mean roughness value Ra µm 0�03 0�03 0�04 0�19

60 °- reflectometer value 94�2 96�0 82�0 11�9

85 °- reflectometer value 97�6 98�0 95�8 67�3

Electron microscopic investigations (Figure 9, top) have shown that in both formulations the agglomer-ates of the matting silicas are evenly distributed� In no zone of the coating film was there any significant disruption to the arrangement of the agglomerates that could cause the differences in gloss level�

Of course, the very different gloss levels are because of the different roughness characteristics on the sur-faces of the coating films� Measurements with the tactile roughness tester Hommeltester T 8000 [3,4] and the topograms of the coating film surfaces creat-ed with this device show the differences; see Figure

9, bottom half and roughness values from Table 3� When the topograms are being evaluated, it must be remembered that the z-axis is increased by a factor of 10 for better visualization�

On the basis of the very different reflectometer val-ues it can be assumed that this was caused by corre-spondingly different volumetric shrinkage of the two coatings� However, the differences between 10�6 % shrinkage of the almost high gloss formulation #6 and 11�4 % (Table 3) of the deep matt formulation #7 are not sufficient to explain the gloss differences�

12


Model for insufficient mattingEarlier ”gel point” = low shrinkage efficiency factor= insufficient roughening

UV radiator

Model for effective mattingLater ”gel point” = high shrinkage efficiency factor= sufficiently high roughening

UV radiator

Reaction start

Conveyor speed: 6 m/min = 3 sec. exposure

”Gel point” Reaction end

30 cm

Figure 10 Exemplary presentation of the gel point

The importance of the “gel point” A theory is put forward that the conditions during the polymerization process and mainly its timely course to the “gel point” are essentially responsible for the formation of the different degrees of rough-ness and gloss levels� The gel point is described in the literature [7] when coatings cure by various criteria, such as on the basis of solubility, when insoluble material first occurs� The gel point can also be defined via an increase in molar mass (the weight average of the molar mass is infinitely large) or with rheological parameters (liquid and solid matter prop-erties are identical)�

In this publication, analogous to Feig [8], the gel point is defined as the state from when the silica matrix and also the surrounding binder matrix shrink in the same ratio as the coating polymerizes� This means that from this time onward the matting agent agglomerates are fixed in the developing polymer network and can no longer contribute to roughness during the further shrinkage process� With a high degree of probability, the time when the gel point occurs is influenced by various factors, such as the rate at which the molar mass increases, changes in the segment movability of the chain molecules [6], rheological behavior with the effects of temperature change and the increase in molar mass, shift in the glass transition temperature, etc�

The aim of this publication is to indicate how to influence the onset of the gel point by varying the formulations and the system and process parameters� As a result optimum matting is possible�

To determine shrinkage of the coating, density cal-culations were carried out on liquid and dried mat-ted coatings� The absolute shrinkage of the coating is calculated from this data and from the calculated application weights�

Based on the fact that the applied coating film of formulation #7 shrinks by about 11�4 % with a wet film thickness of approx� 110 µm, absolute shrink-age is 12�5 µm� If we put the maximum roughness profile height Rz of 1�56 µm as a measure for the reduced shrinkage of the silica matrix in relation, we get a shrinkage efficiency of approximately 12�5 %� With this thickness, this relatively low shrinkage of the silica matrix is sufficient to achieve the roughness needed to produce a dull matte impression� How-ever, the situation is different in the case of formula-tion #6 which, with a wet layer thickness of 82 µm and 10�6 % shrinkage, exhibits absolute shrinkage of 8�7 µm� With an Rz value of 0�30 µm a shrink-age efficiency of 3�4 % is achieved, which results in insufficient roughness� On the basis of this practical example it can be shown how important it is that the silica matrix shrinks as little as possible in relation to the binder matrix�


13


Table 4 Formulations C

Formulation #8 #9 #10 #11 #12 Characterization


g 68�77 – – – – Molar weight 1000 g / mol, Functionality: approx� 3�5Viscosity: approx� 60 mPas


g ‒ 68�77 – 70�57 70�57 Molar weight 600 g / mol, Functionality: approx� 3�0Viscosity: approx� 110 mPas

Epoxy acrylate, aromatic

g ‒ – 50�98 – – Molar weight 650 g / mol, Functionality: approx� 2�5Viscosity: approx� 4500 mPas

HDDA g 17�17 17�17 ‒ 17�62 17�62

Monomer from Fig� 12 g ‒ – 34�02 – –

α-hydroxy ketone g 2�21 2�21 3�84 – – Melting point: approx� 47 °C

Bis-acylphosphine oxide

g 0�35 0�35 0�45 – – Melting point: approx� 130 °C

Photoinitiator from Fig� 17

g ‒ – – 3�00 –

Photoinitiator from Fig� 18

g ‒ – – – 2�0 – 5�0

ACEMATT® 3600 g 11�50 11�50 10�71 11�81 11�81

Total g 100.00 100.00 100.00 103.00 102.00 – 105.00

Influence of the coating formulation on mattingEnvironmentally friendly, solvent-free UV-curable coatings can also be matted if, as well as choosing suitable matting agents, attention is paid to the influ-ences of the most important coating components, oligomer, monomer and photoinitiator�

Influence of the oligomersAcrylate oligomers have a significant effect on the matting capability of 100 % UV-curable coatings [9]� Decisive parameters include the type of modification of the oligomer, the relative molar mass, functional-ity, reactivity, and viscosity� Because there are so many parameters, some of which affect each other,

it is very difficult to gain any information regarding matting capability from the oligomer characteris-tics� However, the double bond density is especially significant and is calculated from functionality and molar mass as follows:

DB – Density = Functionality [DB / Mol] x 1000 [g / kg Oligomer] = [DB] relative molar mass [g / Mol] [kg Oligomer]

14


Figure 11 Influence of the oligomer and its double bond density on gloss

80

70

60

50

40

30

20

10

0

60°-

Refle

ctom

eter

val

ue

20 µm 40 µm 60 µm 80 µm

Formul. #9 DB density: 5.0Formul. #8 DB density: 3.5

Radiator: Hg Dose: 820 mJ / cm2

Output: 100 W / cm Peak: 1400 mW / cm2

Conveyor: 5 m / min DB density: see key

The following general statement applies to oligo-mers: “The higher the double bond density, the better the matting capability�” The influence of the double bond density of the oligomers is shown in Figure 11 using formulations #8 and #9 (Table 4)�

The differences in the 60 °- reflectometer values of the coatings shown in Figure 11 are only a part of all differences compared to all influencing parameters� However, the difference becomes very obvious based on the energy input required to achieve reflec-tometer values comparable with those of formula-tion #9 under otherwise identical conditions when formulation #8 is used� According to experience, the radiator output (200 W / cm) and the conveyor speed (approx� 2 m / min) must be changed, which corre-sponds to a 4 - 5-factor increase in dosage�

The double bond densities of customary acrylate oligomers are in the range of < 2 to approx� 6 DB / kg oligomer� Experience has shown that oligomers with high double bond densities have low molar weights and also low viscosities� Oligomers with lower DB densities and higher molar weights generally require more monomers� Their DB density can then be conditionally included in the calculation� If suitable

Influence of the monomersAcrylate monomers also have a considerable effect on the matting capability of UV-curable coatings� To investigate them, six different acrylate monomers with different functionalities, double bond densities,

and molecular structures were selected (see Table 5)� Formulation #10 (Table 4) was used to illustrate the effect of the monomers (Figure 12)�

DB – Density = Functionality [DB / Mol] x 1000 [g / kg Oligomer] = [DB]

monomers are chosen, matting is also possible to a certain extent in these coating systems by increasing the DB density�

Table 5 Monomers used

Acrylate monomer Abb� Functionality Molar weight DB density Influence on mech� properties

Isobornyl acrylate IBOA 1 208 g/mol 4�8 flexibilizing

Phenoxyethyl acrylate POEA 1 192 g/mol 5�2 flexibilizing

Hexanediol diacrylate HDDA 2 226 g/mol 8�8 balanced

Dipropylene glycol diacrylate DPGDA 2 252 g/mol 7�9 balanced

Tripropylene glycol diacrylate TPGDA 2 300 g/mol 6�7 balanced

Trimethylolpropane triacrylate TMPTA 3 296 g/mol 10�1 embrittling


15


Figure 12 Influence of the monomer on gloss

80

70

60

50

40

30

20

10

0

60°-

Refle

ctom

eter

val

ue

POEA IBOA HDDA DPGDA TPGDA TMPTA

5.2 4.8 8.8 7.9 6.7 10.1

22 5 27 22 15 33

95 90.5 93.7 93 93 88.1

DB density DB / kg MonomerReactivity m/minConversion %

Formulation: # 10 Conveyor: 2 m / min� Application weight: Dose: 3820 mJ / cm2 approx� 90 g / m2 Output: 200 W / m Radiator: Fe-doped Peak: 2330 mW / cm2

Table 6 Correlation coefficients of monomer, gloss / various parameters

Difunctional monomers n = 3 All monomers n = 6

Parameter Linear model Optimum model Linear model Optimum model

Reactivity 0�96 0�96 0�24 0�47

Double bond density 0�96 0�96 0�02 0�16

Conversion 0�97 0�99 0�87 0�87

When only the double bond densities of the three di-functional monomers hexanediol diacrylate (HDDA), dipropylene glycol diacrylate (DPGDA), and tripro-pylene glycol diacrylate (TPGDA) were analyzed, the result was a correlation coefficient of r = 0�96� How-ever, when the monofunctional monomers phenoxy-ethyl acrylate (POEA) and isobornyl acrylate (IBOA), and the tri-functional trimethylolpropane triacrylate (TMPTA) are included in the evaluation, the result is r = 0�02; Based on this aspect, in this model there is no relation between double bond density and mat-ting capability�

A regression analysis of the relationship between reactivity and gloss level also achieved a similar result� The result is a correlation coefficient of r = 0�96 (di-functional monomers) and r = 0�24 (for all monomers)� This correlation must also be classified as not significant� The previously men-tioned correlation coefficients were calculated with the linear model� If different non-linear models are used, one gets higher coefficients for the respective optimum model, but even these show no significant dependencies�

It is assumed that total conversion of the reactive double bonds with the participation of the different monomers controls matting capability� To record this impacting factor, investigations were carried out on liquid, uncured coatings and on the corresponding cured coating films using FT-Raman spectroscopy� From the intensities of the C=C acrylate bands at 1636 cm–1 in relation to the reference CH2 bands at 1460 cm–1, the conversion rates for the coatings shown in Figure 12 were determined [10]�

The regression analysis of conversion to gloss level again produced a highly significant correlation coeffi-cient of r = 0�97 for the three di-functional acrylates� If the other monomers are included in the evaluation (Figure 13, Table 6), the quality of the correlation coefficients is reduced, but at r = 0�87 (Figure 13) still high enough to support the statement that the conversion of the coatings, determined by the mono-mers, is a much more decisive variable as regards matting capability than, for example, their double bond density or reactivity�

16


Figure 13 Influence of the monomer / regression analysis conversion – gloss

75

65

55

45

35

2560°-

Refle

ctom

eter

val

ue

POEA

IBOA

HDDA

DPGDA

TPGDA

TMPTA

60°-Reflectometer value = –4.595• Conversion + 472,1r=0.87

86 88 90 92 94 96Conversion [%]

Even if one considers that the regression line of the three di-functional monomers within the entire func-tion has a deviating slope and that the model most certainly does not take account of all conventional monomers, it is still suitable for indicating the signifi-cance of conversion as an influencing parameter�

It could be assumed that the monomer conversion that contributes to total conversion is influenced by its molecular structure (Figure 14) and its mobility during the hardening process� Monomers that have mainly a linear, chain-like molecular structure sup-port matting, while spatially structured (IBOA) and branched (tri-functional and probably also higher functional) monomers are less suitable because of the lower mobility due to sterical hindrance and the associated lower conversion�

Monofunctional Difunctional Trifunctional

Hexanediol diacrylate

Dipropylene glycol diacrylate

Tripropylene glycol diacrylate

Trimethylolpropane triacrylate

Isobornyl acrylate

Phenoxyethyl acrylate

CH3 CH3

CH3

CH2CHOOC

H2C C O O

O

(CH2)6 C

O

CH

CH

CH2

H2C C O O

OH2

CH2

CCH

H2C C O

O

(CH O)2

CH3

CH

CH

CH2CH2

C

O

H2C C O

O

(CH O)3

CH3

CH

CH

CH2CH2 C

O

H2C C

H

CH2

CH2C

CH2

C O

O

C2H5 O C

O

HC CH2

OO

CHCH2C

Figure 14 Structural formulas of the monomers used


17


Molecular structure

Mobility

Conversion

Functionality

Number of reaction centers

Reaction kinetics

”Gel point“

Figure 15 Influencing the gel point through the molecular form and functionality of the monomer

Influence of the photoinitiatorApart from the matting agent, the oligomers, and monomers the photoinitiator also plays an important part in terms of matting capability, as shown by the example of benzophenone and bis(acyl)phosphine oxide (BAPO)� Benzophenone, whose absorption maximum of 254 nm is in the shortwave range, can

be considered as an initiator with distinct “surface drying”� On the other hand, BAPO as a “deep dryer”, with its absorption maximums of 295 and 370 nm in the long-wave range, enables hardening of thicker and also pigmented coating layers (Figure 16) [11]�

If we include the results from Table 3 in the analysis of the monomers, it is interesting that the volatility of the monomers HDDA and TMPTA is at the same, very low level and, consequently, plays no part in the matting�

Figure 15 shows how the gel point, which is impor-tant for matting, can be influenced significantly based on the parameters: molecular structure and functionality�

Based on the results, it can be assumed, that in this model the molecular structure has the most influence on the gel point�

18


Figure 16 Absorption spectra of photoinitiator examples

3,0

2,5

2,0

1,5

1,0

0,5

0,0

Abs

orpt

ion

[%]

Absorption spectrum 0.1 % in acetonitrile

280 300 320 340 360 380 400 420 440 460Wavelength [nm]

BAPOBenzophenone

BAPO

Benzophenone

To investigate the contribution of the photoinitiator, 3 % of the named initiators and various mixtures pro-duced in increments of 0�5 % points were added to the formulation #11 (Table 4)� The applied coatings were hardened at a radiator output of 100 W / cm with two different conveyor speeds� Both initiators exhibited the behavior that is to be expected in the model shown in Figure 10� As a dryer affecting the surface of the film, benzophenone causes an early

gel point and, associated with this, lower shrinkage efficiency� The silica matrix shrinks a lot parallel to the binder matrix� But because of its pronounced spectral absorption behavior in the long-wave range, BAPO contributes considerably to a more even and delayed hardening of the entire layer; the later onset of the gel point enables the desired roughness and matting (Figure 17)�

Figure 17 Influence of photoinitiators and their mixtures on gloss

2 m/min=1850 mJ/cm210 m/min=480 mJ/cm2

80

70

60

50

40

30

20

10

0 3.0/0.0 2.5/0.5 2.0/1.0 1.5/1.5 1.0/2.0 0.5/2.5 0.0/3.0

60°-

Refle

ctom

eter

val

ue

Photoinitiator concentration [%]

Formulation: #11 PI conc.: 3% on form. #11 Application weight: 18 g/m2

Radiator: Hg Output: 100 W/cmConveyor: see keyDose: see key Peak: 1440 mW/cm2

Benzophenone (decreasing)BAPO (increasing)


19


Influence of rheological parametersIt is almost impossible to observe the influence of viscosity on its own without considering flow anomalies, because all measures that change the viscosity also shift the formulations� This results in the significant changes in the matting level that was described above� However, experience values are available that suggest lower viscosities have a favor-able effect on matting efficiency� This could also be brought into line with the gel point theory, as lower viscosities will lead to faster and stronger shrinkage of the binder matrix�

It is possible to make a clearer statement as regarding the influence of structural viscosity� To show the influence of structural viscosity, a formulation

(Table 7, #13) was chosen that has average matting capability but, at the same time, almost Newtonian flow behavior� A rheological additive based on modi-fied urea was gradually added to the reference for-mulation (Table 7, #14 to #17)� This substance was especially chosen to avoid using other solid material-based rheological additives and, consequently, to avoid the input of additional roughening and matting solids�

As a measure for structural viscosity, the structural viscosity quotient SVQ is calculated from the viscosi-ties of the coatings at shear rates of D = 1000 / s and D = 1/ s�

Figure 18 Influence of the concentrations of photoinitiators on gloss

Application weight 18 g/m2Application weight 65 g/m2

50

40

30

20

10

0

2.0 2.5 3.0 3.5 4.0 4.5 5.0

60°-

Refle

ctom

eter

val

ue

Photoinitiator concentration [%]

Formulation: #12 PI: α-HK/BAPO PI ratio: 2:1Radiator: Hg Output: 150 W/cmConveyor: 10 m/minDose: 695 mJ/cm2 Peak: 1835 mW/cm2

Since different initiators influence the gel point to different extents, it could be assumed that the gel point and, hence, the matting could also be con-trolled via the concentration of the photoinitiator� For this purpose, formulation #12 (Table 4) was impinged with increasing concentrations of a mixture of α-hydroxy ketone and BAPO (in a ratio of 2:1)� Figure 18 shows a significant increase in gloss when the photoinitiator concentration increases� This result is also in line with the gel point model (Figure 10)�

In addition to the known advantages of initiator mix-tures as regards hardening and cost efficiency, mix-tures of suitable photoinitiators, such as α-hydroxy ketone and BAPO, offer another potential to increase the level of matting compared to using a single initiator� In concentration series it has often been observed that the optimum matting level is reached then exceeded�

20


Table 7 Formulations D


Oligoether acrylate, amine-modified g 71�30 71�30 71�30 71�30 71�30 Molar weight 550 g / mol, Functionality: approx� 3�0Viscosity: approx� 100 mPas

HDDA g 17�80 17�80 17�80 17�80 17�80

ACEMATT® HK 440 g 8�20 8�20 8�20 8�20 8�20

α-hydroxy ketone g 2�30 2�30 2�30 2�30 2�30 Melting point: approx� 47 °C

Bis-acylphosphine oxide g 0�40 0�40 0�40 0�40 0�40 Melting point: approx� 130 °C

Rheology additive (solution of a modified urea)

g 0�00 0�30 0�60 0�90 1�20 Solvent: N-ethylpyrrolidoneNon-volatile components: 52 %

Total g 100.00 100.30 100.60 100.90 101.20

Rotation viscosity, D = 1 / s mPas 700 2000 5140 12800 18300




Structural viscosity quotient SVQ D @ 1/s/ D @ 1000/s

1�46 3�85 9�55 20�58 27�11

Figure 19 shows the connection between the struc-tural viscosity quotient SVQ and the gloss of the coatings� As the structural viscosity rises, the system becomes increasingly mat� This effect can also be explained with the gel point theory� The structural viscosity is based on the formation of a reversible

network that hampers the relative shrinkage of the silica matrix and thus promotes matting� Although it was not investigated in this model, it can be pos-tulated that increasing yield strength also supports matting�

Figure 19 Influence of structural viscosity

Stru

ctur

al v

iscos

ity q

uotie

nt (S

VQ

)

Formulation

60°-

Refle

ctom

eter

val

ue

60°-Reflectometer valueSVQ

30

25

20

15

10

5

0#13 #14 #15 #16 #17

60

55

50

45

40

35

30

Application weight: 65 g/m2 Radiator: Hg Output: 200 W/cmConveyor: 2 m/min Dose: 3500 mJ/cm2

Peak: 2200 mW/cm2

Coatings with high structural viscosities, such as shown in formulations #16 and #17, cannot be applied without problems and, consequently, in practice can be used only to a limited extent� But by

implication, the results also mean that the lack of flow anomalies, such as structural viscosity and yield strength, hamper the formation of roughened sur-faces and, hence, matting�


21


The physics of light absorptionThe Beer-Lambert Law states that when light passes through a coating, its intensity has a logarithmic dependence on decadic extinction coefficients, the concentration of the absorbing substance and the thickness of the layer� Figure 20 shows the calculated progression of light absorption within a coating in relation to the photoinitiator, its concentration, and the resulting extinction coefficients for the light

wavelength that was used as a basis� To calculate the absorption, the 50 µm thick coating is split into 10 equal layers of 5 µm, the calculations are carried out separately for each of the 10 individual layers [12]� The progression of the light absorption within the coating has a significant influence on matting accord-ing to the gel point model�

Figure 20 Example of a theoretical progression of light absorption within a coating

80

70

60

50

40

30

20

10

0

Ligh

t abs

orpt

ion

[%]

Film thickness [µm]

Formulation: UV clear varnishLayer thickness: 50 µmPhotoinitiator: α‐amino ketonePI concentration: 2 %Radiator type: Medium‐pressure Hg lampWavelength: 313 nm

99.99 % of the light is absorbed in complete formulation, 68 % of the light is absorbed in the top 5 µm of the layer, and only 1% of the light is absorbed in the range between 25 µm and 50 µm

0–5 10–15 20–25 30–35 40–455–10 15–20 25–30 35–40 45–50

A steep fall in the absorption graph leads to pro-nounced surface hardening with the corresponding negative effect on matting, while a flatter gradient results in more even hardening of the layer and bet-ter matting�

Influence of system and process parameters on mattingIn addition to the influence of the individual coat-ing ingredients and the resulting formulations, the system and process parameters, which mainly have an effect on the formation of roughened and, hence,

matte surfaces via the timely onset of the gel point, offer other options to influence the matting level in a targeted manner�

22


Figure 21 Spectral distribution graphs of various high-pressure mercury radiators

Radiator type: Hg radiator Fe‐dopedOutput: 200 W/cm

Radiator type: Hg radiator undopedOutput: 200 W/cm

100

90

80

70

60

50

40

30

20

10

0

Rela

tive

spec

tral

radi

atio

n de

nsity

Rela

tive

spec

tral

radi

atio

n de

nsity

Wavelength [nm] Wavelength [nm]

100

90

80

70

60

50

40

30

20

10

0 200 250 300 350 400 450 500 550 600 200 250 300 350 400 450 500 550 600

(Source: IST Metz GmbH)

Influence of the light spectrumTo cure UV-coatings, apart from electrodeless, microwave-excited UV-radiators, mainly low-pres-sure and high-pressure mercury radiators are used� Undoped high-pressure mercury radiators emit a characteristic spectrum with main lines at 254, 302,

313, 366, 405, and 436 nm [6]� Through doping with metal halides, such as iron iodide, the spectral parts shift clearly towards the long-wave range, especially in the area of the UVA spectrum marked in grey in Figure 21�

If a Fe-doped radiator is used instead of an undoped Hg radiator, while all the other conditions remain the same, a better matting effect is achieved (Figure 22)� Because of the clear shift of the spectral distribution of the radiation towards the long-wave range, a more even penetration of the coating with less gradient formation during polymerization than in Figure 20 is exhibited� Through the relatively high shrinkage efficiency, this leads to more roughening and, con-sequently, significantly better matting� However, it must be remembered that when Fe-doped radiators are used, the coatings have lower reactivities and, because of this, is may be necessary to adjust the concentration of the photoinitiator system�

When UV-curable coatings are being formulated and the process components being chosen, it is a matter of course that the absorption graphs of the photoinitiator system and the spectral line of the radiator used will overlap, at least in parts, to induce adequate hardening� As regards optimum matting it is recommended that special attention is paid to the long-wave UV range (to 400 nm) by choosing suit-able photoinitiators and radiators�


23


Table 8 Formulations E


Oligoether acrylate, amine-modified g 68�52 68�52 68�52 67�32 68�77 Molar weight 600 g / mol, Functionality: approx� 3�0Viscosity: approx� 110 mPas

HDDA g 17�11 17�11 17�11 17�65 17�17

ACEMATT® 3600 g 11�47 11�47 11�47 12�21 11�50

Photoinitiator or photoinitiator mixture from Fig� 22

g ‒ 2�90 ‒ ‒ ‒ Benzophenone, α-hydroxy ketone, bis-acylphosphine oxide, see Figure 22

Various photoinitiators or photoini-tiator mixtures

g ‒ ‒ 2�90 ‒ ‒ Benzophenone, α-hydroxy ketone, bis-acrylphosphine oxide, see Figure 24

Benzophenone g 1�45 ‒ ‒ ‒ ‒

α-hydroxy ketone g 1�45 ‒ ‒ 2�35 2�21 Melting point: approx� 47 °C

Bis-acylphosphine oxide g ‒ ‒ ‒ 0�47 0�35 Melting point: approx� 130 °C

Total g 100.00 100.00 100.00 100.00 100.00

Figure 22 Influence of different radiator types on the matting level

Hg radiator Fe‐dopedHg radiator undoped

80

70

60

50

40

30

20

10

0

60°-

Refle

ctom

eter

val

ue

Benzophenone BP/α-hydroxy ketone 1:1

Hydroxy ketone BAPO

Formulation: #19 Application weight: 64 g/m2 Output: 200 W/cmConveyor speed: 10 m/min Dose Hg: 880 mJ/cm2

Peak Hg: 2190 mW/cm2

Dose Fe: 990 mJ/cm2 Peak Fe: 2380 mW/cm2

24


Figure 23 Gloss level development in relation to various doses with the same intensity

5 m/min 10 m/min2 m/min

60

50

40

30

20

10

0

60°-

Refle

ctom

eter

val

ue

1860 880 480

18 g/m2

1860 8801860 880 480

64 g/m2

Formulation: #19 Application weight: see key Radiator: HgOutput: 100 W/cm Peak: 1430 mW/cm2

Dose [mJ/cm2]

Figure 23 shows the dependence of the 60 °- reflec-tometer values on the dose with the same intensity based on two different application weights� As the dose increases, an increase in matting level can be observed� This dependence can be regarded as generally applicable� Experience has shown that it applies to all matte, radically curable 100 % UV-coatings� It is only the achievable gloss level and the degree of change in gloss that can be influenced by the raw materials used in the formulation�

Another aspect is the development of the gloss level with the same dose and different intensities� Figure 24 shows this on the basis of the statistically evaluated data from an experiment series of 42 data pairs� Within this series of experiments, various photoinitiators and mixtures of these were used in a concentration of 3 % each time� The coating was applied with 20 µm and 80 µm wire-wound rods� The respective doses were set by varying the lamp output and the conveyor speed�

Influence of dose and intensityThe emission capacity of UV-radiators is defined with two variables, intensity and dose� The intensity or irradiance (in this series of articles, also described as the “peak”) is the maximum radiator power that the coating to be cured receives� The intensity is characteristic for the type of radiator and the reflec-tor which are used� The intensity is independent of the speed of the curing system and is inversely pro-portional to the square of the distance between the substrate and the radiator�

The dose is the surface area-related total radiation energy that acts upon the coating while it is passing through the curing equipment� The dose is the prod-uct of the intensity and the radiation time [6] and is inversely proportional to the speed of the conveyor�

Intensity (H) = W / cm²

Dose (E) = H•t = W•s / cm2 = J / cm2


25


It is interesting that the variation with low lamp out-put (100 W / cm at 5 m / min) in the average value of all experiments has a 60 °- reflectometer value that is almost 8 units lower than the setting at 200 W / cm at 10 m / min� The extent to which this effect can be attributed to the increase in intensity cannot be

determined� With a high degree of probability, the more intense matting level is based on the previously described timely shift of the gel point and the associ-ated change in thermodynamic conditions during curing�

Curing with the “dual-cure” methodIn literature the term dual-cure (also known as dou-ble-cure) is used in different contexts� On the one hand, it describes various curing mechanisms, such as the combinations of peroxide and UV-curing [13], or polyaddition and UV-curing [14]� Dual-cure has another meaning when different radiator types are used for pre-gelling and final curing [15], or with a standard light source and using different doses�

Regarding matting of unsaturated polyester resins, it is reported that with the dual-cure method lower gloss levels are achieved through pre-gelling with a dose of 700 mJ / cm² and subsequent final cur-ing with 2100 mJ / cm² than through simple radia-tion with 2800 mJ / cm² and the other conditions unchanged [16]�

A coating based on an acrylate oligomer behaves exactly the opposite� Figure 25 shows the gloss level of a matted formulation, in which pre-gelling was carried out at reducing conveyor speeds before simple curing under maximum conditions (200

Figure 24 Gloss level development in relation to system operation with the same dose

40

35

30

25

20

15

10

5

0

60°-

Refle

ctom

eter

val

ue, a

vera

ge

Evaluation of 42 data pairsFormulation: #20PI: various PI conc.: 3 % of form. Applicatin weight: from 18 g/m2 to 64 g/m2 Radiator: Hg

200 W/cm 10 m/min

8802170

8771445

100 W/cm 5 m/min

Average dose [mJ/cm2]Average peak [mW/cm2]

W / cm at 2 m / min)� The coating that was pre-gelled with the lowest dose of 253 J / cm² has the lowest level of matting� As the dose is increased during pre-gelling, the matting level also increases� These results also support the gel point theory� Even very small amounts of energy during the pre-gelling step are sufficient to largely achieve the fixing of the silica agglomerates in the developing polymer network, suggested in this theory� The other energy input dur-ing the final curing step then leads to largely equal shrinkage of the silica and polymer matrices� The increasing matting level observed with a higher dose during pre-gelling is probably due to the change in thermodynamic conditions, such as temperature influence, increase in molar mass, and the shift in the glass transition temperature�

Since, under the named conditions, the reactivity of the system is 14 m / min�, it can be said that the gel point that is important for matting is already reached at speeds well above the reactivity and, consequent-ly, well before the onset of vitrification�

26


Figure 25 Influence of pre-gelling with a high-pressure mercury radiator on the matting level

30

25

20

15

10

5

0

60°-

Refle

ctom

eter

val

ue

none‒

50253

40307

30401

20577

15752

101070

51975

24250

2 m/min without dual cureReactivity of the coating: 14 m/min

Formulation: #21 Application weight: 64 g/m2 Radiator: Fe-dopedOutput: 200 W/cm Curing in 2nd step Conveyor: 2 m/min Dose: 4250 mJ/cm2

Peak: 2450 mW/cm2

Conveyor pre‐gelling m/minDose pre‐gelling mJ/cm2

Derived from the results presented in the previous section, to achieve a low matting level it is recom-mended that matted coatings are cured with just one lighting unit (maximum possible power input) at the highest possible energy input and the slowest possible conveyor speed� Since these recommen-dations are opposite to the practiced operation of UV- systems from economic and ecological aspects, one should find a reasonable balance for the curing conditions�

Curing of acrylate-based, matte UV-coatings by the dual-cure method – in the sense of “pre-gelling at high speed” – with the final curing only with high-pressure radiators is not practical� In the case that several radiator units in series are used, it is advisable to overlap their radiation areas as much as possible to satisfy the aforementioned recommendations�

Influence of the temperatureHeating of the coating layer is essentially determined by two factors:

• absorption of radiator heat from the IR part of the spectrum of the UV-radiator, and

• the reaction heat of approx� 80 kJ / mol that is released during polymerization of the acrylate double bonds

From real-time infrared spectroscopic investigations it is known that the conversion rate of a UV-curing coating increases in relation to the reaction time and also as the temperature increases [12]�

It has also been reported that pre-heating the coated object and temperature control in a heating oven is favorable for matting [9]� Figure 26 shows the gloss

development of a matted coating in relation to its temperature directly before it is put into the UV-curing channel� Glass plates (h = 4 mm) were coated and heated to show this� While the temperature of the coating was measured, the substrate was heated to the respective temperature in a stable manner and cured with no significant delay� The results are subject to the relationship shown in the diagram to a very significant extent� The 60 °- reflectometer values of 21�8 (T = 20 °C) and 17�0 (T = 25 °C) regressively calculated from this relationship exhibit obvious differences even for this narrow temperature range� Even if this data was gained from a UV-coat-ing that is very easy to matte, it can be said that the temperature has a major influence on the resulting gloss level and should be considered in the operation of UV-curing systems�


27


Figure 26 Influence of pre-heating

20

15

10

5

0

60°-

Refle

ctom

eter

val

ue

23°C 40°C 60°C 85°CTemperature control before curing

Formulation: #22 Application weight: 85 g/m2 Radiator: HgOutput: 100 W/cm Conveyor: 5 m/min Dose: 890 mJ/cm2

Peak: 1460 mW/cm2

60°-Reflectometer value = –2.42484.4Temperature

r=0.9983

Figure 27 Influence of the substrate on the matting level

16

14

12

10

8

6

4

2

0

100

90

80

70

60

50

40

30

20

60°-

Refle

ctom

eter

val

ue

carton glass 1mm glass 3mm

Peak temperature60°-Reflectometer value

PMMA 3 mm

Peak

tem

pera

ture

[°C]

Formulation: #22 Application weight: 85 g/m2 Radiator: HgOutput: 200 W/cm Conveyor: 2 m/min Dose: 3500 mJ/cm2

Peak: 2200 mW/cm2

Influence of the substrate typeThe temperature of the coating and its timely prog-ress during curing is influenced not only by the quan-tity of radiated energy but also by the substrate prop-erties, such as specific heat capacity, specific thermal conductivity, and solidness [9]� Figure 27 shows the 60 °- reflectometer value in relation to the peak tem-perature reached during curing of the coated sub-

strates� This corresponds to the highest temperature that was reached within the coating layer during the curing process� The correlation coefficients resulting from this experiment, which were evaluated accord-ing to several non-linear models, are in the range of r > 0�99�

28


Influence of the reflector designReflectors with various geometric shapes are used to focus the UV-rays on the coating� Apart from ellipti-cal reflectors that focus the light on a caustic curve, parabolic reflectors are also used� They do not reflect light to a caustic curve but over a much wide area; the cross section is almost a triangle with the wider area facing towards the conveyor� All experiments discussed in this series of articles were carried out with this type of radiator� No comparative experi-ments were carried out�

Influence of the reflector coatingApart from the design of the reflector, its coating also likely plays a part in the formation of the mat-ting level� Within the scope of attempting to improve energy efficiency, coatings have recently been devel-oped that reflect only the UV-light; the IR-radiation is allowed to pass through into a heat absorbing layer� This layer removes the heat very quickly and effectively to the cooled carrier profile� In line with the gel point theory, it is assumed that slightly lower matting levels are achieved because of the lower heat input during the curing process�

Based on the gel point theory, a hypothesis is put forward that the strongly focusing elliptical radiator will have a negative influence on the formation of optimum matting levels because of the very short time that is available for curing�

IR

UV

Figure 28 Schematic presentation of an IR-absorbing reflector (source: IST METZ)

Illumination with LED lightsA lot of effort is being put into developing energy-saving LED lights� Modern UV-LED elements emit light in the wavelength range of 365 nm to 405 nm with an energy output of 10 W / cm² and higher [17]� Because there is no shortwave part < 365 nm, this causes drying problems, especially on the film surface� The raw materials industry is attempting to

resolve this by developing new coating raw materi-als adapted to the spectral distribution of LED lights, such as special photoinitiators and oligomers with modified reactivity� In regard to optimum matting of UV-curable coatings, it must be questioned whether the reduced or lack of temperature input will not have a negative effect�


29


Results at a glance • As a result of the lack of film shrinkage from the release of solvent, matting of 100 % UV-curable coatings is based on other rules in contrast to solvent-based coatings�

• To matte thickly applied coatings, fine-particle (average agglomerate particle size < 5�5 µm) mat-ting silicas with special surface treatment should be used�

• To matte thinly applied coatings matting sili-cas with an average agglomerate particle size (determined with laser refraction) of about 0�5 to 1 times the height of the coating thickness are recommended�

• Important for matting is the onset of the gel point during curing� The following applies: the longer this can be delayed, the more matting is promoted�

• Because of the large number of oligomer char-acteristics, some of which influence each other, such as the type of modification of the oligomer, relative molar mass, functionality, reactivity, and viscosity, it is difficult to assess their suitability as regards matting�

• A suitable variable is double bond density, which is calculated from molar mass and functionality� Generally it can be said: the higher the DB densi-ty, the better the matting capability� The influence of the double bond density increases as the layer becomes thicker�

• Oligomers with low DB densities and high viscos-ity can be matted if suitable monomers are used�

• Monomers that have mainly a linear, chain-like molecular form support matting, while spatially formed (IBOA) and branched (tri-functional and probably also higher functional) monomers are less suitable because of the lower mobility and the associated lower conversion�

• The choice of photoinitiator is very important� “Surface dryers”, such as benzophenone, hamper matting, whereas dryers with longer wave absorp-tion spectra, such as BAPO, favor matting� Mix-tures of suitable photoinitiators can also optimize matting�

• As the photoinitiator concentration increases, so does the gloss level�

• Increasing structural viscosity promotes the mat-ting level�

• Radiators with higher spectral parts in the lower wave range, such as iron-doped high-pressure mercury lamps, promote matting�

• If single radiators are used, the matting level improves as the dose increases�

• When the required dose is set via (variable) radia-tor output and conveyor speed, “low dose at low speed” is the preferable mode of operation�

• Curing with the dual-cure method (with the exclusive use of high-pressure radiators) is not advisable�

• Irradiating the required radiation energy via a single radiator is recommended; if several radia-tors in series are used, their radiation areas should overlap�

• The type of substrate has a considerable influence on the matting level of the coating�

• Pre-heating the substrate to be coated, the coated area, and the coating has a positive effect on matting�

30


Influencing parameters Effect on gloss level

Application weight of the coatingwhen a fine-particle matting agent is used

glossy

Application weightmattlow high

Application weight of the coatingwhen a coarse-particle matting agent is used

glossy

Application weightmattlow high

Double bond density of the oligomer glossy

Double bond densitymattlow high

Molecular structure of the monomer glossy

Molecular structure monomermattlinear, mobile branched, sterical

Type of photoinitiator glossy

PhotoinitiatormattSurface dryer Complete dryer

Concentration of the photoinitiator glossy

Concentration of photoinitiatormattlow high

Viscosity (without flow anomalies) glossy

Viscosity (Newtonian)mattlow high

Structural viscosity glossy

Structural viscositymattlow high

Schematic presentation of the influencing parameters The following diagrams show the effect of various parameters on the matting level and indicate how they exert this influence� The shape and angle of the direction arrows are not relevant, only the direction

of the change is reflected� Interactions between the individual parameters are also not taken into consid-eration in this type of presentation�


31


Influencing parameters Effect on gloss level

Type of light spectrum glossy

Light spectrummattshort wave long wave

Conveyor speed glossy

Conveyor speedmattlow high

Dose during curing glossy

Dose [mJ / cm2]mattlow high

Peak during curing (with the same dose)

glossy

Peak [mW / cm2]mattlow high

Temperature of the coating glossy

Temperaturemattlow high

Evaluation of the influencing parameters as regards use As well as choosing the matting agent adapted to the thickness of the application, suggestions as to the choice of oligomers, monomers, and photoinitiators are the easiest to put into practice� This also applies under the assumption that the formulator does not have complete leeway since he or she must fulfill the required film properties�

It is very likely that the influence with the help of the system and process parameters is the most difficult to put into practice, because under certain circumstanc-

es, this would have economic effects or may require investment� Even if the parameters described here can be used only to a limited extent in certain cases, the results should be an incentive to use the instru-ments that are available for optimum matting�

With consideration of all experience values, the influencing parameters described here give coat-ings formulators a certain scope for the control and optimum configuration of matting solvent-free UV-curable coatings�

32


References1 Evonik Industries AG, Technical Bulletin No� 21 “ACEMATT® Matting Agents for the Coatings

Industry”

2 D� J� Kent, H� Schneider, Fatipec Kongreßbuch 1986, Volume 2 A, p� 353

3 Jenoptic AG, hommel-etamic�com, stationary roughness measurement, http://www�jenoptik�com / de_40674_hommel-etamic_t8000_r

4 Jenoptic AG, hommel-etamic�com, roughness characteristics flyer http://www�jenoptik�com / cms / jenoptik�nsf / res / Rauheitskenngroessen_DE_2009�pdf/$file / Rauheitskenngroessen_DE_2009�pdf

5 DIN EN ISO 4287

6 Prieto Jorge, Kiene Jürgen, Holzbeschichtung, Vincentz-Verlag, Hannover, 2007

7 Georg Meichsner, Thomas Mezger, Jörg Schröder, Lackeigenschaften messen und steuern, Vincentz Network GmbH & Co KG, 2003

8 Feig P, Zur Frage der Formulierung von Mattlacken, Farbe & Lack, 74th volume, September 1968, p� 877 - 880

9 Schottenloher, Kai Philipp, Mattierung von strahlenhärtenden Lacken, thesis, Esslingen University of Applied Sciences, Applied natural science department, presented on 11 / 25 / 2004

10 F� Dollish, W� Fateley, F� Bentley, Characteristic Raman Frequencies of Organic Compounds, John Wiley & Sons July 1973, Chapter 6, Article 6�6, p� 74 - 75, Table 6�8

11 Company publications and product information, BASF SE D-67056 Ludwigshafen

12 Dietliker, Kurt, BASF Schweiz AG, unpublished speech documents; presentation «Licht für die Lack-härtung» presented on 6 / 29 / 2010 at Evonik Degussa GmbH, Hanau

13 Kremer, Wolfgang; Deckend pigmentierte UV-härtbare Lacke nach dem Double-Cure-Prinzip, Farbe & Lack, March 1988, Vincentz-Verlag

14 BASF SE, Ludwigshafen, brochure “High lights!” EDF 2909 e (no publication date)

15 Unknown, Die unendliche UV-Geschichte, Industrie Lackierbetrieb 1992, 60th volume No� 6, p� 228

16 Garratt, Peter R�; Die Mattierung von UV-härtenden Lacken auf Basis ungesättigter Polyesterharze, Farbe & Lack, August 1990, Vincentz-Verlag

17 Presentation documents: First LED-UV Symposium at IST METZ; December 15, 2010

18 Behl, Reinhard and Christian, Hans-Dieter; Radikal matt – Zur Mattierung lösemittelfreier UV-här-tender Lacke – Part 2, Farbe & Lack, April 2011, Vincentz-Verlag

19 Behl, Reinhard and Christian, Hans-Dieter; Radikal matt – Zur Mattierung lösemittelfreier UV-här-tender Lacke – Part 2, Farbe & Lack, June 2011, Vincentz-Verlag

20 Behl, Reinhard and Christian, Hans-Dieter; Radikal matt – Zur Mattierung lösemittelfreier UV-här-tender Lacke – Part 3, Farbe & Lack, July 2011, Vincentz-Verlag


33


List of tablesTable 1 Matting silicas used �� 5

Table 2 Formulations A �� 5

Table 3 Formulations B �� 12

Table 4 Formulations C �� 14

Table 5 Monomers used �� 15

Table 6 Correlation coefficients of monomer, gloss / various parameters �� 16

Table 7 Formulations D �� 21

Table 8 Formulations E �� 24

34


List of figuresFigure 1 Influence of the application weight and particle size of the matting agent on the gloss level �� 6

Figure 2 Model for matting thick-layered coatings through high volumetric shrinkage �� 7

Figure 3 Virtual surface structures of the model shown in Figure 2 with technical application data of an associated cured coating �� 7

Figure 4 Definition of the roughness parameters Rz and Ra �� 8

Figure 5 Model for matting thin-layered coatings by approximating the particle size of the matting agent to the dry layer thickness of the coating �� 9

Figure 6 Virtual surface structures of the model shown in Figure 4 with technical application data of an associated cured coating �� 9

Figure 7 Surface topograms of thickly applied, matted UV-coatings �� 10

Figure 8 Surface topograms of thinly applied, matted UV-coatings �� 10

Figure 9 SEM images of film cross sections (top) and topograms of the associated coating film surfaces (bottom part of the figure) �� 11

Figure 10 Exemplary presentation of the gel point �� 13

Figure 11 Influence of the oligomer and its double bond density on gloss �� 15

Figure 12 Influence of the monomer on gloss �� 16

Figure 13 Influence of the monomer / regression analysis conversion – gloss �� 17

Figure 14 Structural formulas of the monomers used �� 17

Figure 15 Influencing the gel point through the molecular form and functionality of the monomer �� 18

Figure 16 Absorption spectra of photoinitiator examples �� 19

Figure 17 Influence of photoinitiators and their mixtures on gloss �� 19

Figure 18 Influence of the concentrations of photoinitiators on gloss �� 20

Figure 19 Influence of structural viscosity �� 21

Figure 20 Spectral distribution graphs of various high-pressure mercury radiators �� 22

Figure 21 Spectral distribution graphs of various high-pressure mercury radiators �� 23

Figure 22 Influence of different radiator types on the matting level �� 24

Figure 23 Gloss level development in relation to various doses with the same intensity �� 25

Figure 24 Gloss level development in relation to system operation with the same dose �� 26

Figure 25 Influence of pre-gelling with a high-pressure mercury radiator on the matting level �� 27

Figure 26 Influence of pre-heating �� 28

Figure 27 Influence of the substrate on the matting level �� 28

Figure 28 Schematic presentation of an IR-absorbing reflector (source: IST METZ) �� 29


35


07-2

015

This information and any recommendations, tech-nical or otherwise, are presented in good faith and believed to be correct as of the date prepared. Recipients of this information and recommenda-tions must make their own determination as to its suitability for their purposes. In no event shall Evonik assume liability for damages or losses of any kind or nature that result from the use of or reliance upon this information and recommenda-tions. EVONIK EXPRESSLY DISCLAIMS ANY REPRESENTATIONS AND WARRANTIES OF ANY KIND, WHETHER EXPRESS OR IMPLIED, AS TO THE ACCURACY, COMPLETENESS, NON-INFRINGEMENT, MERCHANTABILITY AND/OR FITNESS FOR A PARTICULAR PUR-POSE (EVEN IF EVONIK IS AWARE OF SUCH PURPOSE) WITH RESPECT TO ANY INFORMA-TION AND RECOMMENDATIONS PROVIDED. Reference to any trade names used by other companies is neither a recommendation nor an endorsement of the corresponding product, and does not imply that similar products could not be used. Evonik reserves the right to make any changes to the information and/or recommenda-tions at any time, without prior or subsequent notice.

ACEMATT® is a registered trademark of Evonik Industries or its subsidiaries.

Europe / Middle-East / Africa / Latin AmericaEvonik Resource Efficiency GmbH Business Line Silica Rodenbacher Chaussee 4 63457 Hanau-Wolfgang Germanyphone +49 6181 59-8118 fax +49 6181 59-78118 [email protected] www.acematt.com

North America

Evonik Corporation 299 Jefferson Road Parsippany, NJ 07054 USAphone +1 800 233-8052 fax +1 973 929-8502 [email protected]

Asia / Pacific

Evonik (SEA) Pte. Ltd. Business Line Silica 3 International Business Park #07 – 18 Nordic European Centre Singapore 609927phone +65 6809-6877 fax +65 6809-6677 [email protected]

L+L 15-01-139, 15-01-037 vND TI 1399 Matting solvent-free UV-curable coatings, us.indd ––– 21. Jul 2015; 14:44 Uhr


Documents

Matting solvent-free UV-curable coatings · 2018. 2. 14. · Matting solvent-free UV-curable coatings Technical Information 1399 15-01-139, 15-01-037 vND TI 1399 Matting solvent-free