Tuning the mechanical properties of UV coatings towards hard and flexible systems

Progress in Organic Coatings 32 (1997) 191–196

Tuning the mechanical properties of UV coatings towards hard and flexible systems R. Schwalm a ,*, L. Ha¨ußling a, W. Reich a, E. Beck b, P. Enenkel b, K. Menzel b a

BASF AG, Kunststofflaboratorium, Ludwigshafen/Rhein, Germany b Marketing Dispersions, 67056 Ludwigshafen/Rhein, Germany

Abstract The standard resins of radiation-curable coatings provide either hard or flexible coatings dependent on the type of chemistry used. Whereas aromatic epoxide acrylates usually give hard and brittle coatings, urethane acrylates are known for their flexibility. Since the radiation curable systems should not contain solvents, the desired low viscosity for the specific application is adjusted with reactive monomers. This normally prevents the use of flexible high-molecular-weight polymers. On the other hand, the viscosity of dispersions is determined by the solid content only and not by the molecular weight of the polymers used. Thus, waterbased UV-curable coatings are one strategy out of this dilemma in order to combine the flexibility of higher-molecular-weight polymers with the hardness of highly crosslinked acrylates. The mechanical data of conventional and waterbased UV coatings are discussed in dependency on glass transition temperature and elastically effective chain length between crosslinks.  1997 Elsevier Science S.A. Keywords: Conventional UV coatings; Waterbased UV coatings; Glass transition temperature; Crosslinking

1. Introduction Radiation curable coatings are mostly used for protective purposes. Therefore, the films must be able to withstand the desired use without damage. Often the requirements ask for high flexibility and hardness, for instance the coating of a wooden table must tolerate the expansion and contraction of the wood, depending on humidity and temperature, and must be hard enough to withstand mechanical or chemical attack on the surface. Factors mostly affecting the mechanical properties are the elastically effective chain length between crosslinks (Mc) and the glass transition temperature (Tg) of the crosslinked resin. Generally, the higher the crosslink density, i.e. the lower the elastically effective chain length, the harder the film, whereas flexibility is best in non crosslinked resins. The mechanical properties of coatings are discussed in the literature [1]. Most coatings are viscoelastic films, whose elastic properties can be described by the elastic modulus (E′). Since almost all polymer properties change to a large extent at the glass transition temperature (Tg), it is very important to consider this transition. In linear polymers * Corresponding author.

0300-9440/97/$17.00  1997 Elsevier Science S.A. All rights reserved PII S0300-9440 (97 )0 0060-X

another transition temperature exists, the brittle–ductile transition temperature (Tb) [2]. Between the lower Tb and Tg the polymer is hard and ductile. Whereas this is valid for linear polymers, the limited data available for crosslinked systems indicate, that these two transitions still exist at low crosslink densities, but Tb disappears as crosslink density increases. Thus, highly crosslinked coatings are hard and brittle below Tg and increasingly soft and flexible above Tg. Therefore, the mechanical behaviour of hardness and flexibility is diametrical dependent on Tg. The Tg of crosslinked polymers depends mainly on the structure of the segments between crosslinks, on the crosslink density and on network defects (chain ends, cyclization, etc.). It has been shown by Stutz et al. [3] that Tg is not a linear, but a hyperbolic function of the crosslink density, thus higher functionality normally increases Tg significantly. Other factors determining the flexibility even in crosslinked systems are the width of the Tg transition and low temperature loss peaks (b transitions). As a general rule, one can deduct that materials with broad Tgs have better flexibility. Furthermore, flexibility can be enhanced by increasing the linear molecular weight between crosslinks (Mc). However, this feature is hard to adjust in radiation curing systems, since reactive monomers are used instead of sol-

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tubes with a power of totally 120 W/cm. Radiation energy was varied most conveniently by varying the speed of a conveyor belt moving the samples below the lamps. Testing of elastic moduli, pendulum hardness and Erichsen cupping were performed with standard equipment of Koenig, Hemer Sundwig, Germany. Vickers hardness was measured with a Fischer Ha¨rtemessung H 100. Mc was determined with a Solid Analyzer RSA II of Rheometrics Comp. The conversion was followed by the decrease of the 810 cm−1 acrylate band in real-time infra-red spectroscopy on a Perkin-Elmer 1320 IR spectrometer, and the value reported in Table 1 is taken at 20 s exposure time. Glass transition temperatures were determined with a Mettler DSC 30; given are the values for Tg and the width of the glass transition DT.

vents to obtain the low application viscosity needed, and high amounts of monofunctional monomers, which increase Mc, are undesirable. In this paper we will discuss the results obtained with different types of UV-cured acrylate coatings relating to hardness and flexibility, and our concept to adjust the viscosity of radiation curable resins with water instead of monomers, which enables us to use higher-molecular-weight resins, contributing better to higher flexibility.

2. Experimental Acrylates used are those available commercially from BASF coatings raw materials division under the Laromer proprietary names. They were used as received, and no additional purification was performed. All mechanical data were obtained with formulations containing the specified acrylate with 4 wt.% photoinitiator at a coating thickness of 50 mm. Photoinitiation was carried out either with the commercially available BASF Lucirin TPO type photoinitiator or with commercially available Irgacure 500, which is a mixture of Irgacure 184 with Benzophenone (Ciba-Geigy). Irradiations were carried out in a PPG irradiation assembly containing two high pressure Hg–Quartz

3. Results and discussion 3.1. Materials The following crosslinkable binder materials and reactive diluents were used. Polyether and -ester acrylates are condensation products of aliphatic di- or triols with polycarboxylic acids and/or acrylic acid. We used preferably polyester acrylates based on polyetherol, since the resulting

Table 1 Mechanical data of various radiation cured acrylates Laromer

Type

Diluent (%)

Func.

PD (s)

Vickers (N/mm2)

Erichsen (mm)

HDDA (20) PO 33F (20) –

2 2 2

186 147 31

194 204 8.8

– – – – DPGDA (25)

2.5 3.5 3.5 3.5 2.5 3.5

42 45 116 36 29 97

Polyether acrylates PO 84F LR 8894 PO 83F

– – –

3 3 3

Urethane acrylates LR 8739 Aliphatic LR 8861 Aliphatic LR 8862 Aliphatic

TPGDA (35) DPGDA (30) DPGDA (30)

Waterbased systems LR 8895 LR 8949 X

Solids: 50% Solids: 40%

Epoxy acrylates EA 81 Aromatic LR 8819 Aromatic LR 8765 Aliphatic Polyester acrylates PE 55F Aliphatic LR 8799 Aliphatic LR 8800 Aliphatic LR 8828 Aliphatic LR 8907 Aliphatic LR 8912 Aliphatic

Mc (g/mol)

1/Mc *E − 3

1 1.2 4.1

209 275 182

11.9 16.7 95 6 3 89

6.5 4.5 2.7 6 6.8 2.6

52 73 55

19.6 47.5 22.9

2 2.5 2

43 168 46

3.5 2.2

154 192

DT (°C)

Conv. (%)

Tg (°C)

4.78 3.64 5.49

59 61 93

(−10) 54 79 46 51 (−8) 24 46

234 176 145

4.27 5.68 6.90

615 143

1.63 6.99

70 72 58 83 90 61

4.6 4.7 2

164 91 161

6.10 10.99 6.21

82 81 78

(−10) 19 73 (−9) 26 70 (−15) 25 74

12 181 17.6

8 1.7 6.8

1100 249 579

0.91 4.02 1.73

60 67 79

26 55 51 87 (−10) 23 54

157 225

5 6.8

121 793

8.26 1.26

66 57

19 14 34 4 −3 27

56 57

35 58 59 35 27 60

60 70

All acrylates were used pure or with the specified diluent containing 4 wt.% of the photoinitiator Irgacure 500. Coating thickness was 50 mm. The films were exposed with 1280 mJ/cm2. Diluents: HDDA, hexandiole diacrylate; PO 33F, oligoetheracrylate; D(T)PGDA, di (tri) propylene glycol diacrylate; Func., functionality; PD, pendulum hardness; Vickers, Vickers hardness; Conv., conversion of acrylates; Tg, glass transition temperature (a second Tg in brackets).

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193

Scheme 1. Polyether/ester acrylates.

polyester acrylate scarcely contains any acrylic monomers derived from transesterification reactions (Scheme 1). Epoxy acrylates are based on Bisphenol A and butanedioldiglycidethers. Urethane acrylates used are polyaddition products of polyols and hydroxyalkyl acrylates with di- or trifunctional isocyanates (Scheme 2). 3.2. Mechanical data We have determined hardness and flexibility data from various types of radiation cured resins, as shown in Table 1 and Fig. 1. 3.2.1. Hardness Hardness relates to scratch and abrasion resistance. Hard coatings give better scratch resistance, whereas abrasion resistance is also affected by surface friction. Hardness measurements can be considered as single temperature modulus determinations and were done with an indentation (Vickers) and pendulum test. 3.2.2. Flexibility Flexibility relates to the requirement that a coating should not crack under the fabrication or application caused distortion, thus the elongation at break should be greater than these extensions. The Erichsen cupping reflects the extent of elongation in mm. The hardness and flexibility data from various types of radiation curable resins reveal that with aromatic epoxy acrylates, very hard but brittle coatings are obtained. By changing the backbone to an aliphatic structure, a more flexible epoxy acrylate results, but at the expense of hardness. Polyester acrylates cover almost the whole spectrum, from hard (PD: 116) to very flexible (Erichsen: 6.8), but the coatings are either hard or flexible. Polyether acrylates are

Fig. 1. Pendulum hardness versus Erichsen cupping for various types of cured acrylate films (Table 1).

in between and the urethane acrylate LR 8739 is the most flexible, but soft; whereas LR 8861 is hard (PD: 168) and much less flexible (Erichsen: 1.7). The waterbased coatings which will be discussed in more detail show a trend toward hard and flexible systems. 3.3. Glass transition temperature During cure, the glass transition of the coating increases. It is important to consider Tg, since the curing reaction stops when Tg approaches or exceeds the curing temperature for about 20°C. Some Tgs reported in Table 1 seem to be too high for a room temperature curing. However, the temperature profiles of photocuring as measured by Decker [4] resulted in a temperature increase DT of up to 70°C. We also found surface temperatures up to 80°C and thus, the highest Tgs of the cured coatings are in the same range. The glass transition temperature has been related to crosslink density [5]: Tg = Tg∞ − K =Mn + Kx X

(1)

Tg∞

where is the limiting glass temperature of the uncrosslinked polymer at infinite molecular weight; the term K/Mn characterizes the decrease in Tg due to the remaining end groups of the uncrosslinked polymer chains; Kx is a constant characterizing the increase of Tg due to the presence of crosslinks; and X is the crosslink density. Eq. (1) holds only for linear polymers with low crosslink densities, since it assumes that the reference glass transition temperature of the linear crosslinked polymer does not change with degrees of cure (end group conversion). A better fit of data with higher crosslinked systems was derived with an equation published by DiBenedetto [6]: (Tg − Tg0 )=Tg = K p X =(1 − X )

(2)

where Tg0 is the glass temperature of the uncrosslinked polymer. This equation has been modified by Stutz [3] by substituting the reference glass transition temperature Tg0 or Tg∞ by Tgp , the glass temperature of the linear reference polymer at conversion p. This equation takes into account that the reference glass temperature is not a constant, but dependent on the cure conversion [3]: Scheme 2. Epoxy and urethane acrylates.

Tg = [Tg∞ − K1 (1 − p)][1 + K2 (X =(1 − X ))]

(3)

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Fig. 2. Conversion of acrylate double bonds followed by real time IR spectroscopy (examples).

Tg∞

where is the true backbone Tg without endgroups or crosslinks, K1 characterizes the influence of endgroups, thus reflecting the degree of cure, and K2 is another constant accounting for the influence of crosslinks. Thus, the Tg of the linear polymer is lowered by endgroups if the degree of cure is not 100%, and increased due to the crosslinks. Note, that this equation represents a hyperbolic increase of Tg on crosslink density. In order to verify the degree of cure of the resins, we measured the conversion of the acrylic double bonds as a function of cure time (Fig. 2 and Table 1). Since, according to Eq. (2), the Tg is dependent on the conversion of the functional groups, the conversion of the acrylate groups was determined by real time infrared spectroscopy as described by Decker [8]. It is exemplified in Fig. 2 and in more detail in Table 1 that the conversions of most of the coatings are relatively high (.80%), however not at all complete. The limitation of acrylate conversion is often due to vitrification of the system, analogous to thermosetting systems published by Gillham [9]. Thus, there is a considerable influence of remaining endgroups on Tg. The waterbased coatings, in particular, exhibit only moderate conversions. The hardness and flexibility data of the various acrylate resins were plotted against the glass transition temperature. In the case where two distinct Tgs were determined, the higher one is taken for the plot in Fig. 3. Hardness is mainly a function of modulus. It is well known from the literature that the modulus drops significantly at the glass transition. This behaviour is also reflected in the hardness measurements of the coatings. From the data in Fig. 3 it is very

Fig. 4. Flexibility as a function of Tg, measured with Erichsen cupping method.

obvious that there is a significant increase in hardness for coatings with Tgs above room temperature, which appears with both hardness measurement methods. The hardness data of the waterbased systems fit in well with this trend, since their Tgs are above room temperature (RT). The data for flexibility show nearly the opposite behaviour (Fig. 4). There is a drop of flexibility when Tg rises above room temperature. The step is not as pronounced as in the hardness diagram, probably due to the existence of a second Tg below room temperature in several coatings, which indicates phase separations. However, there are two exceptions with the waterbased systems, which exhibit considerable flexibility at high Tgs. 3.4. Elastically effective chain length between crosslinks (Mc) The crosslink density X, or the inverse, the elastically effective chain length between crosslinks Mc has been tried to relate to Tg. The proposed simple relationship [6] Tg = Tg∞ + K =Mc

does not give a satisfactory correspondence with experimental data. Thus, to calculate Tg under consideration of Mc (or crosslink density X) one should use Eq. (3). Anyway, it is very useful to determine the elastically effective chain length between crosslinks (Mc), since it can be related to flexibility. Considering the network structure, it is obvious that the higher the molecular weight between crosslinks, the more flexible the coating (Fig. 5). However, the absolute value of flexibility is also determined by the chemical structure of the chains, reflected in the Tg∞ . The elastically effective chain length between crosslinks (Mc) has been determined experimentally from measurement of the elastic modulus in the rubber state according to Eq. (5) [7]. 3 p r p RT (5) E′ There is no exact linear correlation between flexibility and Mc (Fig. 6). The aromatic epoxyacrylates and LR 8861 are less, LR 8894 and LR 8895 are more flexible in relation to Mc. This may relate to a more rigid backbone structure in

Mc =

Fig. 3. Hardness as a function of Tg, measured with indentation (Vickers) and pendulum tests.

(4)

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Fig. 5. Influence of Mc on flexibility.

195

Fig. 7. Concept for the use of waterbased coatings: Viscosity increases with molecular weight in 100% radcure coatings. In dispersions the viscosity is independent of molecular weight. The reduced hardness of dispersions is compensated by UV curving.

Our concept to obtain hard and flexible systems is thus based on the use of higher-molecular-weight polymers, which should give more flexibility. To overcome the constraints imparted by the viscosity, we designed radiationcurable systems which are dispersed in water. It is known from conventional dispersions that the viscosity can be adjusted independently from the molecular weight just by choosing the appropriate solids content. For most of the applications of radiation-cured coatings, these dispersions are not hard enough to be useful. Thus we looked at dispersions where the required hardness can be introduced by

photoinitiated crosslinking, therefore these systems should be comparable to standard radcure coatings in terms of hardness, but much more flexible (Fig. 7). Dealing with waterbased systems, three approaches are possible: acrylates which can be diluted with water, emulsions and dispersions. We focused on emulsions and dispersions. The advantages of radiation-curable emulsions have also been published recently [10]. Out of the usable stabilizers, we relied on nonionic polymeric surfactants in combination with emulsions and self-emulsifying ionic stabilizers for the dispersions (Fig. 8). As a polymeric surfactant, a copolymer based on N-vinylpyrrolidone (Tg ∼94°C) is used, which usually gives in 50% solids systems emulsion droplets of 1 mm diameter. The mechanical data of such an emulsion (LR 8895) are given in Table 1. By emulsifying a polyesteracrylate with such a polymeric surfactant, the hardness as well as the elasticity increases. This can be attributed to the formation of an acrylate network which is interpenetrated by the flexible polymeric surfactant. In the case of the ionic stabilized polyurethane dispersion, the dispersions are stable up to a solid content of about 50% and the particle sizes are in the range of 100 nm. The elastic part in these dispersions is contributed by a soft block within the polyurethane, and the hard segments are formed by the urethane structures as well as additionally by the crosslinked acrylates. The mechanical data of the waterbased systems are listed in Table 1 and included in Figs. 1, 3, 4 and 6. It has been demonstrated that the emulsion as well as the dispersion exhibit high hardness in agreement with the high Tgs; however, considerably more flexibility was expected according to a pure Tg consideration. Thus, it has been shown that it is

Fig. 6. Flexibility in dependence on the elastically effective chain length between crosslink (Mc).

Fig. 8. Different approaches to incorporate higher-molecular-weight polymers in radiation-curable systems.

the former case and a more flexible one with 8894. In the case of LR 8895 Mc is too low in relation to its flexibility. However, in such heterogeneous systems, consisting of acrylate crosslinking reaction and addition of a linear polymeric surfactant, the value of Mc may not reflect the flexibility behaviour of the total system. Generally there is a trend that higher Mc results in higher flexibility. Concluding from the available data in literature and from our conventional UV coatings (see Table 1), it is recognized that in order to get hard coatings Tg should be high, in order to get flexible ones Tg should be low. That is the reason why most of the coatings are either hard or flexible. Another possibility to increase flexibility is to increase Mc. This, however, is difficult to realize in 100% coatings, since viscosity also increases with molecular weight. Therefore we chose to look to waterbased systems, where the viscosity is independent from molecular weight and low application viscosities can easily be obtained by adjusting the solid content. 3.5. Concept of waterbased UV coatings

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Fig. 9. Advantages of radiation-curable dispersions.

possible to design UV-curable coatings with improved mechanical properties towards hard and flexible systems. From the point of view of UV coatings, the use of water may be one step back (an additional drying is required); from the point of view of conventional dispersions this combination is one step forward (faster blocking resistance and higher hardness). In any case, these systems are a good supplement to UV coatings, since they offer new opportunities in several application areas as well as environmental and handling advantages [11] (Fig. 9). The mechanical data of radiation-cured coatings presented in this paper have shown that Tg of a coating is of primary importance for the mechanical properties. Tg can be influenced by the true backbone Tg of the uncrosslinked polymer, thus the Tg of the elastically effective chains between crosslinks (Mc), and the crosslink density. Since the hardness and elastic properties of coatings require either high or low Tg, it is advantageous not to increase Tg too much by crosslinking, which results in brittleness, but rather by a combination of moderate Tg∞ , moderate crosslinking and broad Tg range.

4. Conclusions The standard resins of radiation curable coatings provide either hard or flexible coatings dependent on the type of chemistry used. Whereas aromatic epoxide acrylates usually

give hard and brittle coatings, urethane acrylates are known for their flexibility. Since the radiation-curable systems should not contain solvents, they have to exhibit the desired low viscosity for the specific application. This normally prevents the use of flexible high-molecular-weight polymers. On the other hand, the viscosity of dispersions is determined by the solid content only and not by the molecular weight of the polymers used. Thus, waterbased UVcurable coatings are one strategy out of this dilemma, in order to combine the flexible properties of higher-molecular-weight polymers with the hardness of highly crosslinked acrylates.

Acknowledgements The authors would like to thank R. Horn for the measurement of mechanical data, A. Zosel and M. Trivunov for determining the Mc values, B. Buck for contributing the RT–IR spectra, and H. Stutz for valuable discussions.

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