Study on the Migration and Control of Slip Agents on the Surface of High Density Polyethylene Beverage Closers

The migratory slip additives permeate from the polymer matrix and modify the surface characteristics. This enables customisation of slip performance of the polymer products. To date, the migration behaviour of  slip additives has primarily been studied on polyolefins with low crystallinity. Several factors such as the physicochemical properties and bulk loading of slip additives, crystallinity of polymer matrix, and storage temperature and time are found to affect the migration of these additives. Once the slip additives migrate to the surface, the extent of lubrication they produce depends on their surface concentration, distribution and morphology. Therefore, a similar study on the migration of industrially important slip additives such as erucamide, behenamide, oleamide and Incroslip Q (slip Q) is required in a highly crystalline polymer product such as high-density polyethylene (HDPE) screw cap.

This thesis evaluates the migration of slip additives through HDPE screw cap matrix to its surface to control their application and removal torque during bottling. The research investigates the dependence of migration on physicochemical nature and bulk concentration of slip additives. Similarly, the interaction of slip additives with the polymer has also been quantified and explained. The effect of the most pertinent process variables such as temperature, concentration, time, and their interactions on the surface concentration and performance of slip additives has been determined. Finally,  the optimum values of bulk concentration, storage temperature and storage time of above mentioned slip additives required to produce the industrially desired amount of surface concentration and the required torque for HDPE screw cap are identified.

HDPE specimens (plaques and screw caps) with different slip additives were prepared by injection moulding and studied at different times after storing at different temperatures. The migration of slip additives to the surface was examined using an array of surface profiling techniques. The functional groups at a detection depth of 800 nm and the elemental composition on the surface at the penetration depth of 10 nm were measured using attenuated total reflection Fourier transform infrared (ATR¿FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Static contact angle was measured on the HDPE specimens as a function of storage temperature and time to determine the change in their surface energy. Gas chromatography with flame ionisation detector (GC-FID) was used to measure the amount of surface slip additive migrated from the bulk. The morphology of slip additives on the surface of HDPE was also captured using optical microscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM).

Erucamide, behenamide, oleamide and slip Q (a blend of erucamide and behenamide) produced different surface concentration on HDPE plaques and caps at the same storage condition. Of these four slip additives, slip Q produced the highest surface concentration which was followed by oleamide and erucamide while behenamide produced the least. The surface concentration of erucamide was found to be higher than that of behenamide at an identical bulk loading and storage condition. The non-invasive FTIR method was found to be effective in quantifying the surface concentraton of slip additives. Increased intensity of spectral peaks of C=O and N-H correlated well with increase in surface concentration of slip additives.  The surface concentration of each slip additive could be attributed to its physicochemical properties, bulk concentration and the storage condition.

Surface concentration of slip additives determined by their physicochemical properties:

The migration was significantly affected by the physicochemical property such as molecular structure of slip additives. The extent and the pattern of migration  of each slip additive were explained well by their diffusion coefficients which were 5.63 × 10-15 cm2/s for behenamide, 1.53 × 10-13 cm2/s for erucamide, 1.16 × 10-13 cm2/s for oleamide, and 8.05 × 10-13 cm2/s for slip Q, respectively. These slip addtives were found to follow a Fickian diffusion behaviour while migrating through HDPE specimen. The diffusion coefficient of oleamide (18-C chain amide) was higher than that of erucamide (22-C chain amide) because of its smaller molar volume.

The diffusion coefficient of behenamide was much lower than erucamide; even though the molar volume of the former  is smaller than than that of the latter. This discrepancy on diffusion coefficient  is  can be attributed to higher interaction of linear behenamide molecules amongst themselves and with HDPE chains. Both of these interactions were weaker in the case of erucamide and oleamide due to the bend created by a cis -C=C- bond present in their carbon chains. The diffusion coefficient of slip Q was found to be the highest which was explained by the presence of erucamide. The kink induced by the double bond of erucamide disrupted the interaction of highly compatible behenamide with itself and with HDPE resulting in higher migration of slip Q.

The morphological features of slip additives on HDPE surface depended on their surface concentration, physicochemical properties and storage conditions. Behenamide formed numerous asperities in HDPE surface while erucamide and oleamide formed flat placoid structures. The blend of erucamide and behenamide produced both asperities and flat plate-like structures. Numerous asperities were formed by behenamide and slip Q irrespective of  temperature used, and they increased in size when more slip additives migrated to the surface. At higher temperatures, erucamide and oleamide collapsed into flat plate-like structures being softer than behenamide. The soft and detachable sheets of erucamide were more effective to reduce the torque of caps than the asperities of behenamide.

The contact angle of HDPE specimens containing slip additives, used in this study, increased with time indicating the decrease in the surface energy. The increase in contact angle was consistent with an increase in the surface concentration of slip additives. However, the contact angle of HDPE loaded with slow migrating slip additive (behenamide) decreased from its initial value up to 6 h and then gradually increased. This decrease was caused by the exposure of polar amide groups on the air-solid interface with their hydrophobic chains embedded in the polymer. With subsequent migration, double layered crystalline structures were formed with their hydrophobic carbon chain exposed to the air-solid interface, which increased the contact angle to 135°.

The diffusion of the slip additives through a highly crystalline HDPE matrix was found to be a complex process. The migration of all the slip additives used in this study followed Fick¿s law of diffusion and Arrhenius equation for their dependence on temperature. This study shows that the migration of slip additives and their performance in HDPE products can be tailored by utilising their material properties (initial concentration and physicochemical properties) and storage conditions (temperature and time). The molecular structure of each slip additive  greatly affected its surface characteristics and morphologies. These morphologies ranged from softer plate-like structures to a harder asperities which affected the efficiency of the lubrication. Antislip behaviour, i.e.  increase in surface energy on HDPE surface, observed in the case of slow migrating slip additive at the initial stage of migration could reduce the lubrication. Although relatively large number (three) independent variables were included in this study, it was possible to predict their optimum level  to achieve required level of lubrication (torque). The holistic approach documented in this thesis to study the migration behaviour of slip additives through HDPE can be adopted  to study other migratory additives in different polymers. Further, the models developed to predict the optimum level of variables which were needed to determine the required lubrication in HDPE screw caps can be applied in other migratory slip additives as well.