Calcined kaolins (calcined clays) are used as antiblocking agents for polyolefin films. Initial research suggests their widespread use is due to the excellent balance of key properties, combined with low additive interactions. A comparison with other antiblocking agents illustrates the physical properties of such additives in LLDPE films. A detailed description of the surface chemistry of calcined kaolin (calcined clay) further enhances our understanding of possible additive interactions. The data includes information concerning a recently commercialised calcined clay and a development product – both of which imparted good antiblocking in LLDPE film with minimal effects on optical properties.
Polyolefin films are used extensively in many applications including packaging and agricultural coverings. Several key additives are required to achieve usable film, these being: antiblocking agents; antioxidants and stabilisers; slip aids and polymer processing aids (PPAs) (the last being polymer dependent).
Imerys Performance Minerals supplies hydrous and calcined kaolins (calcined clays) as antiblocking additives for polyolefin and polyester films. Calcined (heat treated) kaolins are added to low density polyethylene (LDPE) and linear LDPE (LLDPE), either in the post-reactor phase of production or as a masterbatch during downstream processing. Calcined kaolins offer the benefits of low additive interactions and a good balance of film properties.
This feature gives a brief introduction into possible surface interactions of calcined kaolins. A comparative study then demonstrates the merits of such additives as antiblocking agents. The concluding data show new developments we are pursuing in this field.
Materials – See Table 1 below:
Table 1. Summary of antiblocking additives and their properties.
|Product||Mean psd (µm)*||Surface Area (m2g-1)||Coating?|
|Calcined kaolin (CaK)||2.3||6.9||N|
|Hydrous kaolin (HyK)||5.1||5.1||N|
|Calcium carbonate (CaC)||1.6||4.6||N|
|Natural silica (NSi1)||6.6||2.9||N|
|Natural silica (NSi2)||4.6||3.6||N|
|Diatomaceous Earth (DiE)||12.0||1.0||N|
|Synthetic silica (SSi)||4.5||> 400||Y|
Calcined kaolin – InFilm 200, Hydrous Kaolin – Polwhite B, Calcium carbonate – Carbital 110S
* Measured in aqueous suspension with a Cilas laser particle sizer.
Polymer – Innovex LL0209 AA butene LLDPE with antioxidant only (BP Amoco).
Slip aid – Crodamide ER (Croda Universal).
PPA – Polybatch AMF 705 (A. Schulman Plastics).
Part 1 - Surface Interactions of Hydrous and Calcined Kaolin determined by Inverse Gas Chromatography (IGC)
The surface characteristics of a hydrous and a calcined kaolin (InFilm 200) were anlaysed by IGC at infinite dilution, using a Perkin Elmer Autosystem XL. The minerals were prepared by the compaction technique into a stainless steel column. For determining the non-polar interactions a series of linear alkanes were used. For polar interactions selected polar solvents were used. 0.1µl aliquots of the vapour phase were injected by Hamilton syringe through the autosampler system. Injector temperature was maintained at 150° C, with the detector at 250° C.
Methane was used as a non-interacting probe to determine the void volume of the column. Oven temperature, pressure drop across the column and atmospheric pressure were measured independently. Attainment of the infinite dilution region was confirmed by eluting pentane at a range of flow rates and injection volumes. Data were collected and variations in the retention volumes with repeated injections were noted.
The net retention volume, Vn, was calculated by
where: f is the carrier gas flow rate; tr is time taken for the probe to travel through the column; t0 is time taken for a non-interacting marker, such as methane or air, to pass through the column; tn is the net retention time; and J is the correction factor for pressure drop across the column giving rise to gas compressibility.
The standard free enthalpy of adsorption, ΔGa 0, was calculated from net retention volumes for each probe at each temperature from equation (2),
where ps,g is the standard surface pressure, given as 1.013 x 105 Pa, and , the two dimensional surface pressure of the gas, is given as 3.38 x10-4 Nm-1. Ws is the mass of the stationary phase and Sa is its surface area. These components were combined as k, given by
ΔGa0 was thus calculated from (4)
When ΔGa0 (calculated from equation 4) is plotted against T, ΔGa0 (the enthalpy of adsorption) is seen graphically as the intercept and ΔSa0 (the entropy of adsorption) is determined from the slope.
The non-specific or dispersive component of the substrate’s surface energy, γsd, can be calculated from the elution data for hydrocarbon vapours. The free energy change for the adsorption of a methylene group in an alkane series is ΔGa0CH2 and is found as the difference in free energies of adsorption for succeeding alkanes in an homologous series. gsd is calculated from
where N is Avogadro’s number; γCH2 is the surface tension of hypothetical surface of polyethylene containing only methylene groups5, calculated from:
and aCH2 is the cross-sectional area of a methylene group (≈ 0.06 nm2).
Thus if T is constant, and a mixture of probes is injected into the column, a plot of number of carbon atoms versus RTlnVn should give a straight line graph where ΔG 0CH2 can be found from the slope. This is then inserted into equation (6) to give a value for the surface free energy.
Specific interactions were calculated by plotting ΔGa0 against saturated vapour pressure (readily obtained from literature). Alkanes are taken as the reference line, and the deviation cited as the specific interaction parameter.
Table 2. Adsorption enthalpy results.
|Enthalpy of Adsorption (kJ mol.-1)|
|Probe||Hydrous Kaolin||Calcined Kaolin|
|Pentane||48 ± 3||35 ± 2|
|Hexane||59 ± 4||46 ± 4|
|Heptane||69 ± 4||58 ± 3|
|Octane||81 ± 5||73 ± 4|
Table 3. Surface free energy results.
|Dispersive Component of Surface Free Energy (mJ m-2)|
|Temp. (°C)||Hydrous Kaolin||Calcined Kaolin|
|80||165 ± 5||139 ± 4|
|90||156 ± 6||137 ± 3|
|100||149 ± 5||132 ± 3|
|120||151 ± 4||130 ± 2|
|140||147 ± 5||103 ± 3|
Figure 1. Plot of ΔGa versus log Po for hydrous kaolin to determine specific interaction energies for polar probes.
Figure 2. Plot of ΔGa versus log Po for calcined kaolin to determine specific interaction energies for polar probes.
Table 4. Specific interaction energy results for polar probes on hydrous and calcined kaolin.
|Variation in free energy of adsorption from the alkane line (-ΔGsp) (kJ mol.-1)|
|Probe||Hydrous Kaolin||Calcined Kaolin|
|THF||18.2 ± 0.9||NPD*|
|Chloroform||5.0 ± 0.4||NPD|
|Carbon tet.||-4.9 ± 0.4||-2.1 ± 0.3|
|Cyclohexane||-7.9 ± 0.5||-4.2 ± 0.6|
|Acetone||4.3 ± 0.3||NPD|
|Ethyl acetate||5.1 ± 0.4||NPD|
|Hexene||8.6 ± 0.4||4.0 ± 0.4|
|Octene||8.8 ± 0.4||4.2 ± 0.4|
*NPD = no peak detected with in reasonable experimental time.
Saada et al.7 studied French kaolins in some detail. Comparisons of adsorption enthalpies and surface free energy results (Tables 2 and 3) for our hydrous kaolin with their published data showed some variation. Two of the three French kaolinites were found to have higher values. This was attributed to the higher purity of the hydrous kaolin used in this study and absence of chemicals used during purification and grinding. The French kaolinites were also finer and more porous, as indicted by their higher surface area. This could have resulted in greater probe accessibility. At infinite dilution the probes interact most strongly with the high surface energy sites. Thus if the surface is structurally heterogeneous, the surface free energy results will predominantly reflect these sites, rather than the bulk surface. The values obtained for hydrous kaolin, therefore, compared with those of Saada et al., are an indication of heterogeneity and surface activity, rather than a true representation of surface free energy.
Calcination altered the kaolin’s surface chemistry, reducing both adsorption enthalpies, and the dispersive component of the surface free energy. Kaolin surfaces are known to have strongly acidic properties. These are related to layer defects, and hydroxyl and oxygen atoms in the aluminium structures and silicate layers . Strong Lewis acids are formed by cations of interlayer materials.
On calcination, both of these sites are altered as the interlayered structure collapses. For the calcined kaolin the probe will be subjected to van der Waals forces from a single surface layer, rather than undergoing partial insertion between the layers in the hydrous kaolin. Although the results indicate a reduction in surface activity on calcination, they are higher than expected if the interactive forces were halved. The probes are therefore being adsorbed onto the remaining high energy sites which, although fewer, are still having a significant effect on retention times.
The free energy of adsorption results demonstrated the degree of surface alteration with calcination of the hydrous kaolin. The high values for specific enthalpies of adsorption (Figures 1 and 2, Table 4) for alkenes on the hydrous kaolin were due to very strong specific interactions of the p-electrons of the alkenes with the metallic cations of the kaolin acting as Lewis acid sites. The interactions with calcined kaolin resulted in a halving of the specific adsorption enthalpy. The reduction is due to lessening of the strength of these acidic sites, as the aluminium oxide sites act as weaker Lewis centres than those containing hydroxyl groups. Similar effects have been noted by Bandosz et al. during the intercalation and heat treatment of smectites .
The negative values for carbon tetrachloride on hydrous kaolin were also noted for kaolinites by Saada et al.7. They suggested this was due to limited interactions between the polar surface of the kaolin and also hindering of access to these sites by bulky probe molecules. Negative values were also found with cyclohexane probes. This may be attributed to the same steric hindrance effects, especially when considered relative to the result for hexane. After calcination, the results for carbon tetrachloride and cyclohexane were also negative. The accessibility of the surface sites to these bulky probes has therefore increased. However, the polarity of surface sites on calcined kaolin is significantly lower than those on hydrous kaolin, as indicted by the difference in alkane-alkene results. The relatively small difference in specific free energy of adsorption between hydrous and calcined kaolin for the bulky probes appears less than would be predicted if both steric and polar contributions were equally important. The greatest effect must therefore arise from a change in specific surface polarity, rather than accessibility of the probes. This also suggests that these probes may not be inserted between the silicate layers on the hydrous kaolins, and are subjected predominantly to forces from the outer layers.
IGC has been used to quantify the surface free energy of hydrous and calcined kaolin. Using polar probes the interaction with certain polar species has also been measured. Model polar probes may be used to predict the interactions of additives used in film production. The technique is, however, limited by the volatility of the probe. Thus, for example, ethylamine may be used to indicate possible interactions between HALS (hindered amine light stabilisers) and antiblocking agents. From the evidence thus far the calcined kaolin was found to have limited interactions with both acidic and basic species. These were significantly less than the hydrous kaolin, and thus the reactivity between calcined kaolin antiblocking agents and other additives would be expected to be relatively minor. As yet this study has not been extended to other antiblocking agents.
Part 2. A Comparison of Calcined Kaolin with Alternative Antiblocking Agents
All antiblocking agents were oven dried at 80 °C before tumble mixing with the LLDPE polymer at 50 wt.% (the exception was synthetic silica. This was compounded at 20 wt.% due to its low bulk density). The mixes were compounded with a Baker Perkins (APV) MP2030 twin screw extruder, with a die temperature of 220 °C and at constant torque. Masterbatches were dried in a Conair Churchill desiccant drier overnight, before being diluted to 10 wt.% to improve dosing accuracy in the film premixes. The 10 wt.% masterbatches were dried at 80 °C for 12 hours before tumble mixing with the virgin polymer to the desired loading level.
Films were blown on a Betol BK32 single screw extruder with a 75mm die diameter and 0.8 mm die gap. The die temperature was 240 °C, haul off was 11m min-1, blow-up ratio was 2.25:1. The mean film thickness was 30 µm with a layflat of 250 mm.
Films were prepared at 1500 ppm antiblocking agent, with no other additives.
After conditioning for 5 days at 23 °C, 50 % rh the physical properties were measured.
Haze was measured with a BYK Gardner Haze-Gard Plus spectrophotometer according to ASTM D1003-97. Blocking force was measured using a Dynisco D9046 instrument according to ASTM D3354-89.
Figure 3. A comparison of haze for antiblocking agents.
Figure 4. A comparison of blocking force for antiblocking agents.
Figure 5. A comparison of haze versus blocking force.
Observations and Discussion
From Figure 3, the haze values for the films were dependent on the type of antiblocking agent used. The lowest haze values were seen for the calcined kaolin. The highest values were observed for the synthetic silica. No direct relationship was found between particle size and haze results.
With the exception of the calcium carbonate, all antiblocking agents resulted in a film blocking force of < 30 g (Figure 4). Hydrous kaolin, silicas and talc appeared to be most efficient at antiblocking, with calcined kaolin, the calcined clay, giving a blocking force similar to the diatomaceous earth.
Film producers are looking for an idealised film of low haze and low blocking force. This can be represented by plotting these two parameters, as in Figure 5. In this case, the calcined kaolin gave a good balance between the two.
Part 3. New Developments in Calcined Kaolin-based Antiblocking Agents
Masterbatches containing calcined kaolin, a commercial product (InFilm 200) and RLO, a new development product (RLO 6990) were prepared as described in Part 2. These were compared with Talc, Synthetic silica and Talc2 (an alternative new development talc) at 1500 and 3000 ppm with a 2:1 ratio of antiblocking agent to slip aid plus 500 ppm PPA. Blown films were prepared as described previously.
After conditioning for 5 days at 23 °C, 50 % rh the physical properties were measured. Optical properties and blocking force were measured as detailed previously. Coefficient of friction was measured with a Hounsefield tensometer, fitted with a base plate and sliding sled according to ASTM D1894-90. Tensile strength was measured on the same equipment following ASTM D882-91.
Figure 6. Plot of haze for various antiblocking agents at 1500 ppm.
Figure 7. Plot of haze for various antiblocking agents at 3000 ppm.
Figure 8. Plot of antiblock concentration versus haze for various antiblocking agents.
Figure 9. Plot of blocking force for various antiblocking agents at 1500 ppm.
Figure 10. Plot of blocking force for various antiblocking agents at 3000 ppm.
Figure 11. Plot of coefficient of friction for various antiblocking agents at 1500 ppm.
Figure 12. Plot of coefficient of friction for various antiblocking agents at 3000 ppm.
Figure 13. Plot of tensile breaking force for various antiblocking agents at 1500 ppm.