Application of Organic Coatings to Titanium Dioxide Pigment and the Effect on the Processing and Properties of PVC

Transcription

1 Application of Organic Coatings to Titanium Dioxide Pigment and the Effect on the Processing and Properties of PVC Application of Organic Coatings to Titanium Dioxide Pigment and the Effect on the Processing and Properties of PVC B.C. Lim, N.L. Thomas* and B.C. Noble** Singapore Polymer Corp. (PTE Ltd), 41, Shipyard Road, Singapore, *Department of Materials, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK **Huntsman Tioxide, Haverton Hill Road, Billingham, TS23 1PS, UK Received: 5 July 2010, Accepted: 7 September 2010 Summary This study investigates the effect of two different types of organic coating applied to titanium dioxide pigment on the processing and properties of rigid PVC. The first pigment was coated with polyol and siloxane, whereas the second pigment was coated just with polyol. The surface energies of the two coated pigments were determined by contact angle measurements. It was found that the pigment coated with both polyol and siloxane was more hydrophobic and gave better dispersion behaviour than the other pigment. The coated TiO 2 pigments were dry blended in a calcium-zinc stabilised PVC window profile formulation and extruded on a laboratory-scale extruder. Fusion properties and impact strength were measured and there was found to be a small but significant improvement in these properties for the formulation containing the first pigment compared with that containing the second. Hence the improvements in processing and properties correlated with the hydrophobicity and dispersability of the two different pigments. Keywords: Titanium dioxide; PVC; poly(vinylchloride); surface energy; contact angle; organic coating 1. INTRODUCTION Titanium dioxide, TiO 2 is added to PVC formulations as a pigment because of its opacity, whiteness and stability. Virtually all commercial TiO 2 pigments are Smithers Rapra Technology, 2011 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 1,

2 B.C. Lim, N.L. Thomas and B.C. Noble treated with some form of surface coating. Alumina is the most widely used coating and many general-purpose grades are treated with alumina at levels between 0.5 and 3.5 weight % [1,2]. This treatment promotes dispersion of the pigment and retards unwanted photodegradation reactions between the pigment and the polymer matrix. Although TiO 2 absorbs strongly in the ultraviolet from nm and therefore protects polymers from photochemical degradation, it is also photoactive and can cause photocatalytic degradation of polymers [3-5]. This so-called chalking process gives rise to erosion of the polymer surface. Where enhanced resistance to weathering is required, TiO 2 pigments are coated with silica in addition to alumina [1,2,6]. This combination creates an effective barrier to reduce the photodegradative effect of TiO 2 on plastics for exterior use. Organic coatings are also applied to TiO 2 pigments to reduce agglomeration of pigment particles and to prevent absorption of water during storage. Organic compounds are coated onto the hydrophilic inorganic-coated pigments, thus creating hydrophobic surfaces. These hydrophobic surfaces improve compatibility of the pigment with organic substrates and hence promote better dispersion of the pigment in the polymer matrix. The types of organic compounds commonly used as surface treatments on TiO 2 are polyols, amines, siloxanes and phosphated fatty acids [2]. In the current study, two different organic coating types have been applied to titanium dioxide pigment and their effect on the processing and properties of PVC formulations has been investigated. 2. EXPERIMENTAL 2.1 Preparation and Characterisation of Coated Rutile Pigment Preparation of Coated TiO 2 A rutile TiO 2 coated with an inorganic coating (silica and alumina) was provided by Huntsman Pigments. It had a surface area determined by BET analysis of m 2 /g. Two types of organic coatings were applied. The first batch of pigment, Pigment 1 (P1), was treated with polyol followed by siloxane, whereas the second batch of pigment, Pigment 2 (P2), was just treated with polyol. The polyol was applied to the pigment by spray coating from a solution in industrial methylated spirit (IMS). In the case of P1, the siloxane was applied by stirring. After being dried, the coated pigments were micronised. 22 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 1, 2011

3 Application of Organic Coatings to Titanium Dioxide Pigment and the Effect on the Processing and Properties of PVC TEM Analysis of Coated TiO 2 The coated TiO 2 samples were examined in a JEOL 2000 FX transmission electron microscope Dispersion Tests A tiny amount of each coated pigment was dispersed in liquid paraffin and then viewed in transmitted light in a Leica optical microscope Contact Angle Measurements Contact angle tests were carried out using Dataphysics OCA-20 contact angle equipment. Water and diiodomethane (DIM) were used as the two testing liquids. Five pressed discs were made from each sample of coated pigment and these were taken as the substrates upon which contact angles of the liquid drops were measured. An advancing sessile drop method was used for the contact angle measurements. Conventional contact angle measurement methods rely on surfaces being perfectly flat and smooth. However, in these studies contact angles were measured on compacted particles rather than on a perfectly flat surface and so it was necessary to employ a correction factor to allow for this. A relationship between the apparent contact angle, f, which is the macroscopic value observed in the low power microscope, and the true Young s contact angle, (q), was derived in an earlier publication by Lim et al [7]. The relationship derived is given in equation 1. This relationship is independent of particle size and was used to determine true values of contact angle from measured values. cos φ = π 2 3.( 1+ cosθ) 2 1 (1) Software based on the Owens and Wendt equation [8] was used to calculate surface energies of coated titanium dioxide pigment from the corrected contact angle measurements. Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 1,

4 B.C. Lim, N.L. Thomas and B.C. Noble 2.2 Preparation and Testing of PVC Samples Preparation of PVC Dry Blend Dry blends were prepared with the formulation shown in Table 1 for both of the coated TiO 2 samples. As indicated in the table, formulations were prepared with TiO 2 pigment at both 4 and 10 parts per hundred parts of resin (phr). The PVC resin and other additives were blended simultaneously in a high speed Henschel mixer from 80 to 125 C at 2000 rpm and then cooled to room temperature in the cooling chamber. Table 1. PVC Window Profile Formulation Ingredient Concentration Parts per hundred parts of polymer (phr) Poly(vinyl chloride) K Ca/Zn Thermal Stabiliser 4.5 Acrylic Impact Modifier 7 Acrylic Processing Aid 1.0 Calcium Carbonate Filler 10 Titanium Dioxide Pigment 4 or Study of Fusion Behaviour The fusion behaviour of the PVC dry blends was studied using a Polylab Haake Rheomix 600 torque rheometer. For each test 60 g of dry blend was charged into the chamber and mixing was carried out at a rotor speed of 20 rpm and a set temperature of 190 C Extrusion of PVC Dry Blend A Krauss Maffei KMD 2-25 twin-screw extruder fitted with conical counterrotating screws was used to extrude the dry blends into profiles. A strip die was used with cross-section 5 mm x 30 mm. Extrusion was carried out using the set conditions shown in Table 2. Table 2. Extrusion conditions on the Krauss Maffei KMD 25KK-L Die Temperature 190 C Front Barrel Temperature 190 C Rear Barrel Temperature 160 C Screw Temperature 140 C Feed Rate 20 rpm Screw Speed 40 rpm 24 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 1, 2011

5 Application of Organic Coatings to Titanium Dioxide Pigment and the Effect on the Processing and Properties of PVC DSC Measurements on Extruded Profiles Differential Scanning Calorimetry (DSC) is a useful technique for assessing PVC fusion level [9]. Figure 1 illustrates a typical DSC plot. Endothermic peaks on the DSC plot allow measurement of the proportions of primary (peak B) and secondary (peak A) crystallinity in a processed sample, and hence the degree of fusion (equation 2). ΔH %Fusion = A ΔH A + ΔH x100% B (2) The true processing temperature can also be determined see Figure 1. DSC thermograms were recorded using a DuPont 2010 Thermal Analyser. Specimens of mg were taken from the core of each extrudate. Specimens were heated from 40ºC to 240ºC at 20ºC per minute. Five samples were tested from each extrudate Impact Testing Impact testing was performed according to British Standard 7413 using a pendulum Charpy Impact tester. Specimens were prepared with a milling Figure 1. Typical DSC plot of extruded PVC Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 1,

6 B.C. Lim, N.L. Thomas and B.C. Noble machine and notched with a single tooth cutter. A minimum of 10 specimens was tested for each formulation. 3. RESULTS AND DISCUSSION 3.1 Characterisation of Coated Rutile Pigment TEM Analysis of Coated TiO 2 Samples of the coated and micronised TiO 2 pigments were examined in the transmission electron microscope. A thin non-uniform shadow was found encapsulating the TiO 2 pigment particles, as shown in Figure 2. No significant differences were found between the two coated pigment types. In fact it was concluded that the shadows were probably due to the inorganic coating rather than the organic coating. It was difficult to image the organic coating because of the low level applied (<1 weight %). Other workers have also come to this conclusion [10] Dispersion Tests Figure 3 shows optical micrographs taken of the two differently coated pigment samples dispersed in liquid paraffin. The TiO 2 pigment particles Figure 2. Transmission electron micrograph of coated pigment 26 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 1, 2011

7 Application of Organic Coatings to Titanium Dioxide Pigment and the Effect on the Processing and Properties of PVC Figure 3. Optical micrographs of pigments dispersed in liquid paraffin appear as dark spots against the light background. It is seen that P1 gives good dispersion whereas P2 appears agglomerated Contact Angle Measurements Contact angle measurements are given in Table 3. This table shows the measured values, observed in the low power microscope, as well as the true values (Young s contact angle), derived from using equation (1) above [7]. It is observed that the contact angles measured using diiodomethane (DIM) remain relatively constant whereas the contact angles measured using water are quite different for the two coated pigments. For P1, the contact angle with water is greater than 90 ; showing that the water does not wet the surface and therefore the surface is hydrophobic. In the case of P2, the contact angle with water is below 90, indicating that the surface is hydrophilic. Table 3. Contact angle test results Pigment P1 P2 Testing Liquid Water DIM Water DIM Measured Contact Angle, 126 ± 4 50 ± 2 48 ± 6 42 ± 4 Young s Contact Angle, 109 ± 3 70 ± 2 69 ± 1 68 ± 1 Surface energies, mjm -2 γs γd γp 22.2 ± ± ± ± ± ± 2.4 Surface energy values derived from the corrected contact angle measurements are given in Table 3. Following the method of Owens and Wendt [8], the Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 1,

8 B.C. Lim, N.L. Thomas and B.C. Noble dispersive (γ d ) and polar (γ p ) components of the surface free energy have been determined. The total surface energy (γ s ) is the sum of the dispersive and polar components. It is seen that the polar surface energies of the two coated pigments are quite different. P1 has a value of polar surface energy approaching zero, indicating that the coated surface is non-polar and therefore hydrophobic. The polar surface energy of P2 is 13.4 ± 2.4 mjm -2, which shows that the surface is more polar and therefore more hydrophilic. These results are in agreement with the dispersion tests: namely that the batch of pigment coated with polyol followed by siloxane (Pigment 1) has a hydrophobic surface and disperses well in liquid paraffin, whereas the pigment coated just with polyol (Pigment 2) has a more hydrophilic surface and shows some agglomeration in liquid paraffin. 3.2 Testing of PVC Samples Fusion Behaviour in the Torque Rheometer Typical plots of torque as a function of time were obtained for each of the PVC dry blends. Time to fusion of the PVC resin is taken as the peak in the torque/time curve. Fusion times are tabulated in Table 4. It is seen that the formulation containing 4 phr of P1 fuses faster than the formulation containing 4 phr of P2. This difference is small but statistically significant. When the level of pigment is increased to 10 phr, it is found that there is a much greater accelerating effect on fusion by Pigment 1 compared with Pigment 2. Table 4. Results from processing on the torque rheometer and extruder Pigment Fusion time (Seconds) Torque Rheometer % Fusion Extrusion Charpy impact strength (kj/m 2 ) P1 (4 phr) 152 ± 3 89 ± ± 3.0 P2 (4 phr) 160 ± 3 86 ± ± 3.3 P1 (10 phr) 148 ± 7 91 ± ± 1.0 P2 (10 phr) 188 ± 9 84 ± ± DSC Measurements on Extruded Profiles DSC measurements were carried to assess the fusion levels of the extruded PVC profiles. The percentage fusion level of each sample was calculated using 28 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 1, 2011

9 Application of Organic Coatings to Titanium Dioxide Pigment and the Effect on the Processing and Properties of PVC equation 2. These values are given in Table 4. It is found that with pigment loading levels of both 4 and 10 phr, there is an increase in the percentage fusion of the PVC when the formulation contains P1 as opposed to P2. The effect is again more marked at the higher loading level. An explanation for the effect of P1 in promoting PVC fusion in both the torque rheometer and extruder is that, being hydrophobic, P1 disperses better in the PVC matrix than P2, which is more hydrophilic and hence has a greater tendency to agglomerate. A highly dispersed inorganic pigment will act as a processing aid and promote fusion of the PVC. The mean particle size of the pigments of 0.2 µm is comparable in size with primary particles of PVC ( µm) and hence the pigment particles are capable of generating interparticle friction with PVC particles, increasing shear and promoting fusion Impact Testing Results of the Charpy Impact tests are given in Table 4. The standard BS7413 has a pass mark of 12.0 kjm -2 and it can be seen that all samples have passed the standard. Again it is found that with pigment loading levels of both 4 and 10 phr, there is an increase in the Charpy Impact strength of the PVC when the formulation contains P1 as opposed to P2. These increases are statistically significant at the 95% confidence level when the results are assessed using t testing. The improvement in impact strength can be attributed both to the increase in fusion and to the effect of well dispersed ultra-fine filler particles in enhancing the toughness of rigid PVC by micro-crazing [11]. 4. CONCLUSIONS The effect of the organic coating applied to titanium dioxide pigment on the processing and properties of rigid PVC was investigated. Two types of organic coating were studied: the first consisted of polyol overcoated with siloxane and the second was polyol alone. It was found that TiO 2 pigment treated with the first coating type (P1) gave good dispersion in liquid paraffin and its contact angle with water was greater than 90, showing that the surface was hydrophobic. The total surface energy of this pigment was found to be 22.2 ± 1.8 mjm -2, with a polar component approaching zero, which again confirmed that the coated surface was non-polar. The TiO 2 pigment treated with polyol alone (P2) appeared agglomerated in liquid paraffin and its contact angle with water was below 90, indicating that the surface was hydrophilic. The total surface energy of this pigment was 38.9 ± 1.6 mjm -2, with a polar component of 13.4 ± 2.4 mjm -2, showing the surface to be more polar than that of P1. Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 1,

10 B.C. Lim, N.L. Thomas and B.C. Noble The coated TiO 2 pigments were dry blended in a calcium-zinc stabilised PVC window profile formulation. It was found that the dry blended formulations containing P1 (at both 4 and 10 phr) fused faster in the torque rheometer than the formulations containing P2. Also DSC measurements on specimens taken from extruded profiles showed that the PVC formulations containing P1 had higher fusion levels than those containing P2. Similarly there were found to be small but statistically significant improvements in Charpy impact strength for PVC profiles containing P1 compared with those containing P2. These improvements were attributed to the first pigment being better dispersed in the PVC matrix than the second, due to its lower surface energy. A well-dispersed inorganic pigment can act as a processing aid in PVC formulations to enhance fusion and also can increase toughness by promoting micro-crazing. ACKNOWLEDGEMENTS The authors would like to acknowledge funding for this work from Huntsman Pigments and the EPSRC. REFERENCES 1. Murray H.H., Chapter 2 in Chemistry of Pigments and Fillers, Eds. D.H. Solomon & D.G. Hawthorne, John Wiley & Sons, Canada (1983) 2. Valente S. and Butler R., Mod. Plast. Int., 24 10, (1994) Voelz H.G, Koempf G., Fitzky H.G., Farbe und Lack, 78, (1972) Balfour J.G., JOCCA, vol.73, no.12, (1990) Braun J.H., Baidins A. and Marganski R.E., Prog. Org. Coat., 20, (1992) Day R.E., Poly. Deg. & Stab., 29, (1990) Lim B.C., Thomas N.L. and Sutherland I., Prog. Org. Coat., 62, (2008) Owens D.K. and Wendt R.C., J. Appl. Poly. Sci., 13, (1969) Fillot L-A., Hajii P., Gauthier C. and Masenelli-Varlot K., J Vinyl & Additive Technol, 12, (2006) Day R.E. and Egerton T.A., Colloids and Surfaces, 23, (1987) Fernando N.A.S. and Thomas N.L., J Vinyl & Additive Technol, 13, (2007) Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 1, 2011