Titania (TiO2) is used as an opacifier in many polymer applications, including thermoplastic
sheets and films. Many consumer products comprise single or multilayer pigmented sheets
with TiO2 added for colour and opacity. Although it is acknowledged as the most effective
opacifier available, the cost of TiO2 has lead to the search for more cost-effective
alternatives. Calcium carbonates have been used in combination with TiO2, but these are primarily as extenders and dispersion aids.
Opacilite™ Ultra is a high whiteness calcined kaolin produced by a new, patented rapid heating process. This technique creates large numbers of tiny sealed voids within the clay particles, resulting in excellent opacifying properties, high whiteness and low specific gravity. When used in combination with TiO2, the colour and opacity of thermoplastics can be maintained and cost savings achieved.
The partial replacement of TiO2 has been studied in three polymer systems – LLDPE, HDPE and PS . In each, masterbatch blends were prepared in a twin-screw extruder. Films were produced on a blown film line and the optical properties were measured. Key physical properties, such as tensile strength and Elmendorf tear strength, were also measured.
Table 1: Film formulation details.
|Opacilite Ultra wt.%||0.4||0.8||1.2|
|Total wt.% pigment||4||4||4.4||4.8|
LLDPE and HDPE film gauge was 40 µm; PS film gauge was 20 µm.
Figure 2: Opacity of LLDPE films versus Opacilite Ultra concentration.
The solid blue line is the reference film containing 4 wt.% TiO2, with the dashed line showing the standard deviation. In LLDPE a 2:1 partial replacement of TiO2 with Opacilite Ultra gave equivalent opacity.
In HDPE and PS a 1:1 partial replacement of TiO2 with Opacilite Ultra gave equivalent opacity
(within the deviations of the test method).
Figure 3: Opacity of HDPE films versus Opacilite Ultra concentration.
Figure 4: Opacity of PS films versus Opacilite Ultra concentration.
Table 2: Colour data for films.
|LLDPE 4% TiO2||97.0||-0.39||2.01||3.65|
|LLDPE 3.6% TiO2 0.4% Opacilite Ultra||97.2||-0.37||2.04||3.48|
|LLDPE 3.6% TiO2 0.8% Opacilite Ultra||97.1||-0.40||2.02||3.58|
|LLDPE 3.6% TiO2 1.2% Opacilite Ultra||96.9||-0.38||2.08||3.75|
|HDPE 4% TiO2||98.3||-0.46||1.90||2.59|
|HDPE 3.6% TiO2 0.4% Opacilite Ultra||97.9||-0.44||2.03||2.96|
|HDPE 3.6% TiO2 0.8% Opacilite Ultra||97.6||-0.39||2.10||3.24|
|HDPE 3.6% TiO2 1.2% Opacilite Ultra||97.3||-0.37||2.19||3.50|
|PS 4% TiO2||96.8||-0.34||1.73||3.66|
|PS 3.6% TiO2 0.4% Opacilite Ultra||96.4||-0.37||1.76||4.03|
|PS 3.6% TiO2 0.8% Opacilite Ultra||96.3||-0.37||1.80||4.16|
|PS 3.6% TiO2 1.2% Opacilite Ultra||95.9||-0.37||1.80||4.16|
The colour of the LLDPE films was not affected significantly by the addition of Opacilite Ultra. A small reduction in L (whiteness) and an increase in yellowness (b*) were seen in the HDPE and PS films at the highest Opacilite Ultra addition.
Table 3: Gloss data for films
|Description||Gloss 20°||SD||Gloss 60°||SD||Gloss 85°||SD|
|LLDPE 4% TiO2||49||6||91||2||100||3|
|LLDPE 3.6% TiO2 0.4% Opacilite Ultra||36||6||88||2||99||2|
|LLDPE 3.6% TiO2 0.8% Opacilite Ultra||27||8||72||2||94||3|
|LLDPE 3.6% TiO2 1.2% Opacilite Ultra||20||5||66||2||90||2|
|HDPE 4% TiO2||1.3||0.3||15||3||44||4|
|HDPE 3.6% TiO2 0.4% Opacilite Ultra||1.0||0.2||9||1||35||6|
|HDPE 3.6% TiO2 0.8% Opacilite Ultra||1.1||0.2||10||2||33||2|
|HDPE 3.6% TiO2 1.2% Opacilite Ultra||1.9||0.6||16||4||41||5|
|PS 4% TiO2||53||16||105||1||99||1|
|PS 3.6% TiO2 0.4% Opacilite Ultra||40||20||101||1||96||4|
|PS 3.6% TiO2 0.8% Opacilite Ultra||46||15||97||1||94||2|
|PS 3.6% TiO2 1.2% Opacilite Ultra||33||12||94||1||92||1|
In HDPE, due to the high degree of crystallinity and surface roughness of the films, gloss was not greatly affected by Opacilite Ultra addition. In PS and LLDPE gloss was reduced with increasing levels of Opacilite Ultra in the film. The reduction in gloss values was larger for LLDPE films, with 20° values changing from 49 to 20 at the highest Opacilite Ultra addition. In PS the reduction was less significant, with gloss values being reduced from 55 to 35.
Figure 5: MD tensile strength results.
Figure 6: Elmendorf tear strength results.
The thick, horizontal lines represent values for unpigmented film. Within experimental error the addition of Opacilite Ultra did not significantly affect the mechanical properties when compared with the reference film. The Elmendorf tear strength results of the PS films were very low and thus not included in the data.
QUV weathering was also carried out on the LLDPE films . No differences were found between the films tested.
Opacilite Ultra has an average refractive index (RI) of 1.39 compared with 2.7 for rutile TiO2 and 1.51 for LLDPE, 1.54 for HDPE and 1.59 for PS. Opacity is generated by light scattering around the particles. Opacity can be predicted by determining the difference in RI of the opacifier and the matrix. Thus for LLDPE-TiO2 the difference is 1.19 compared with 0.12 for LLDPE-Opacilite Ultra. In PS the differences between the TiO2 and Opacilite Ultra mixtures is smaller, being 1.11 and 0.20 respectively. Thus Opacilite Ultra is a more efficient opacifier in polymers with a higher RI.
Guidelines for using Opacilite Ultra in polymer films and sheets to achieve maximum cost savings, without significant loss of colour or opacity are as follows:
- In LLDPE, LDPE and PP a 2:1 replacement of 10 % of rutile TiO2 is recommended.
- In HDPE, PS and higher RI polymers a 1:1 replacement of 10 % rutile TiO2 is recommended.
- Greater efficiencies may be achieved in specific formulations. Thus further work is advised to optimise the balance of colour and opacity for different polymer systems and film gauge.
For detailed formulation advice please contact us.