The water-dependent decay mechanism of biaxially-oriented corona-treated polyethylene terephthalate films

Abstract

In moist environments biaxially-oriented corona-treated polyethylene terephthalate (BOPET) film undergoes a decay in surface energy with time. This decay is a significant and well-known problem and it considerably restricts the industrial application of BOPET film. In the present study the decay effect and the dynamics of corona-treated BOPET film in an aqueous environment have been studied using water contact angle and variable angle X-ray photoelectron spectroscopy (XPS) measurements. In addition the surface decay mechanism of the corona-treated BOPET film in aqueous environments was analyzed and a molecular moving model for the decay mechanism is proposed.

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Introduction

Biaxially-oriented polyethylene terephthalate (BOPET) film is an important polymeric material used in a number of manufacturing applications due to its excellent mechanical strength, good thermal stability, high transparency, low cost, good gas barrier properties, good appearance and relatively light weight.^1–4 Unfortunately its intrinsically low surface energy and high chemical inertness produce poor wetting properties and adhesion, and these have a negative impact in subsequent industrial manufacturing processes,^5–7 including printing, coating and gold stamping. A variety of methods have been introduced to modify the surface characteristics of BOPET film, including graft polymerization, plasma and corona treatment,^8–12 among which corona treatment has the advantage of being easy to handle and environmentally friendly.^13–17

Corona treatment is an electrical process employing ionized air to increase the surface energy of nonporous substrates.¹³ In the case of BOPET film, corona treatment does not affect its bulk properties. During treatment, the surface of the film becomes etched, the molecular chains are interrupted^18,19 due to free radical oxidation,^10,19–21 and the surface energy is improved in line with the demands of downstream industrial applications. However, a major problem with the corona treatment is that the effect quickly decays, and the treatment may need to be repeated before use. An investigation of the mechanism of surface decay of the corona-treated film, leading to delay or elimination of the decay process, is therefore of importance both in industrial applications and in scientific research.

A number of related studies have been reported regarding the decay mechanism of the corona-treated BOPET film. For example Pandiyaraj and coworkers¹⁰ studied the aging process of the plasma-treated film, and ascribed the decay to the migration of surface polar groups into the polymer bulk. Morra and his coworkers²² showed that the decay could be attributed to the orientation of polar groups, the blooming of additives and the absorption of widespread contamination. Geyter and his group found that the decrease in wettability of corona-treated films after storage in air was due to migration of induced polar groups from the surface into the bulk of the material. Yang²⁴ and Donnio¹³ reported that aging of the corona-modified surface took place in the sequence: (1) reorientation of polar groups in the surface layer, (2) diffusion of non-polar groups from the subsurface to the surface, and (3) free radical reactions at the surface. In addition, in our previous work,²⁵ we too found that the molecular mechanism for the decay effect involved migration of polar groups from the surface to the interior, and that the decay process was highly dependent on temperature.

Although some studies have mentioned the process of decay, no systematic work has been reported in the literature about the influence of water on the decay process of corona-treated BOPET film surface properties. In the present study the effect of water has been systematically investigated by analyzing the changes in water contact angle and surface chemical composition. In addition, a model of the movement of molecules on the surface of corona-treated films in an aqueous environment has been established and the dynamics of the decay of the film under these conditions have been analyzed in detail.

Experimental

Preparation of the corona-treated BOPET film

Corona-treated BOPET films of 12 μm thickness were provided by Fuwei Films Co Ltd. (Shandong, China). These were commercial films obtained from the biaxially-oriented stretch production line of Brückner, Germany. The experimental conditions for the corona treatment were as follows. The output voltage and power were 8.2 kV and 5.7 kW, respectively, and the processing time was 1 s. A schematic representation of the molecular structure of the corona-treated film is shown in Fig. 1.¹⁵

Fig. 1 Schematic representation of the molecular modification of the BOPET film following corona treatment.

Decay of corona-treated BOPET film in water

Pieces of corona-treated BOPET film were placed in water at 25, 50, and 90 °C for various time periods. After removal from the hot water the films were dried in a block of ice in order to freeze the surface configuration.

Water contact angle

Water contact angle measurements were conducted by the sessile drop method using a contact-angle goniometer (JY125; Beijing United Test Co Ltd). Distilled water was used as the working liquid and the volume of the water drops was regulated to 0.5 μl. Each contact angle value was the average of five readings on the same sample surface. The standard deviation in the average contact angle was less than 0.5°.

X-ray photoelectron spectroscopy

The chemical composition of the surface of the BOPET film and its corona-treated version were investigated by X-ray photoelectron spectroscopy (XPS) on a PHI 5300 XPS spectrometer (Perkin-Elmer) using monochromatic MgK_α (1253.6 eV) photon radiation at an anode voltage of 10 kV, anode current 20 mA, and pressure in the analytical chamber of 10⁻⁷ Pa. Data collection, processing and handling were all carried out using the Apollo Series 3500 workshop. Samples of the corona-treated film were allowed to stand in water at room temperature for different periods of time, and analysis of the surface elements was carried out at take-off angles of 20, 45 and 70°.

Results and discussion

Decay of the corona treatment effect in water

Fig. 2 shows the evolution of water contact angle on the surface of the corona-treated BOPET film. In water the surface hydrophilicity of the film initially declined but then eventually reached equilibrium, similarly to the decay process in air determined in our earlier study,²⁵ but the speed of the decay in water was much more rapid, and equilibrium was achieved in 2 min. The higher the temperature, the more rapid the decay. The water contact angle of the decay in water was 65°, 62° and 60°, less than that in hot air,²⁵ corresponding to hot air temperatures of 50, 70 and 90 °C, respectively.

Fig. 2 Change in water contact angle of corona-treated BOPET film in water at various temperatures.

Water fulfils two roles in the decay process of the corona-treated film:^13,23,24 (1) it accelerates the decay in surface hydrophilicity, and (2) it improves the surface hydrophilicity balance. In the first role, the surface energy of water is higher than that of the corona-treated film. When the latter was immersed in water the surface macromolecules would be subject to the effect of water molecules, promoting the movement of surface molecules and polar groups into the bulk of the material. On the other hand, the polar groups on the surface of the BOPET corona-treated film were subjected to solvation by water, and some water molecules would be absorbed physically and chemically around the polar groups. These water molecules act as a lubricant and undermine the original hydrogen bonds between the BOPET macromolecules, reducing the forces and increasing the activity of the units in the macromolecules and assisting the migration of polar groups to the bulk of the film.

In addition, there is both a physical and chemical etching effect on the BOPET film during corona treatment, and these may each generate highly polar substances. These substances will be removed by water, leading to a reduction in surface hydrophilicity.

With regard to the second role, hydrogen bonds are formed by the effect of water molecules and polar groups, such as carbonyl, carboxyl, and hydroxyl, on the surface of the corona-treated BOPET film. This means that the polar groups become attached to the surface of the film, improving the conservation ratio of hydrophilicity.

Microscopic decaying mechanism of the BOPET corona-treated film in water

Variable angle XPS was conducted to characterize the changes in surface composition of the corona-treated film after treatment in water at room temperature. The variable test angle was correlated to the sample depth of the BOPET corona film. Different depths, d, can be obtained by changing the take-off angle of the measurements, according to eqn (1):

d = 3λ sin θ, (1)

where λ is the non-elastic scattering mean free path, and θ is the take-off angle (the smaller the angle, the smaller the sample depth). Following corona treatment both the oxygen and nitrogen content increased, due to the formation of polar groups. On the other hand, the carbon content did not change greatly during this process. We therefore used the relative content of oxygen to carbon atoms, O_1S/C_1S, to characterize the changes in the polar groups on the surface of the film (Table 1).

Table 1 Changes in O_1S/C_1S ratio on the BOPET film surface during decay in water at room temperature

Take-off angle	Untreated film, %	Corona film, %	Soaked in water 30 s, %	Soaked in water 60 s, %	Soaked in water 120 s, %
20°	38.0	46.6	41.8	40.4	37.5
45°	38.0	45.6	45.7	37.9	39.2
70°	38.0	46.2	46.1	45.4	40.9

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After corona treatment, the oxygen content naturally increased. When the take-off angle was 20° the content of O atoms increased by 8.6%, as expected from the fact that the water contact angle had decreased and the surface tension increased. The regularity is different at different depths from the surface of the film. The first, second and third surface layers correspond to take-off angles of 20°, 45° and 70°, respectively. The O atom content in the first surface layer gradually decreased as the duration of the water soaking increased. On the whole, the second and the third surface layers showed the same trend as the first, but the rate of decline progressively decreased. After soaking in water for 30 s, the O atom content in the first surface layer had declined by 3.6%, whereas those in the second and the third surface layers were almost unchanged. After soaking in water for 60 s, the O atom content of the second surface layer had declined by 7.7%, while the third surface layer was little changed. Not until the soaking time had reached 120 s did the O atom content of the third decline significantly.

These observations indicated that the polar oxygen-containing groups were migrating step by step from the surface within the bulk. Initially, polar oxygen-containing groups migrated from the first surface layer to the inner surface layer, and migration did not reach the second and the third surface layers. The O atom content of the second and the third surface layer therefore remained constant. As the water soaking continued, the polar oxygen-containing groups began to migrate from the surface layer to a much deeper inner layer. The shorter the path, the more rapid the migration, which is consistent with the O atom content for different surfaces soaked in water for 120 s. The first surface layer declined by 9.1%, the second by 6.4% and the third by 5.3%. Obviously, the essence of the migration of polar groups is the movement of the molecules on the film surface. But this is different from pure thermal movement. If heat is the only driving force, the polar groups should all migrate at the same time, but this was not supported by the experimental results. The results indicated that diffusion could be considered the mechanism by which molecular movement was taking place.

Based on these results a theoretical moving molecular model of the corona-treated BOPET film surface is now proposed, and is shown in Fig. 3. Crystalline and amorphous areas are penetrated by the BOPET molecular chains. The molecular chains in the crystalline area are completely frozen at room temperature, but in the amorphous area a proportion of the chains have the ability to migrate under the action of an external force. When the polar groups migrate, water is the driving force providing the non-axial force. Smaller units need less power, but can only migrate close to the surface. While the larger units need more power, they are able to migrate deeper into the film.

Fig. 3 Sketch of the molecular movement model on the surface of the corona-treated BOPET film.

In Fig. 3, molecules in the amorphous area have the relative ability to migrate, causing the polar groups on the molecular chain to separate from the backbone of the chain into the inner film. The smaller units migrate close to the surface, while the larger units can migrate into deeper areas. Consequently, as the time of soaking in water increases, the polar groups on the surface of the film migrate layer by layer, and the hydrophilicity gradually declines. This model provides a satisfactory explanation for the experimental results.

Dynamics of the decay of the corona-treated BOPET film in water

The chemical reaction controlled by diffusion should satisfy the following equation describing the dynamics:

A_t = A₀t^−k, or

log A_t = log A₀ − k log t, (2)

where A_t is based on the surface physical properties (contact angle, surface tension, O atom content of the surface, etc.), and A₀ is the starting value.

Plotting log A_t against log t a straight line is obtained, of slope k. The variation in the water contact angle of the corona-treated film which had been soaked in water at a variety of temperatures was investigated, and the results are shown in Fig. 4.

Fig. 4 The dynamic features of the change of water contact angle as the corona-treated BOPET film decayed in water.

Fig. 4 shows that the decay curve does not fully satisfy the linear relationship. As the time of soaking in water increased, the water contact angle can be seen to deviate from a straight line. The value of k is derived from the linear section and has a definite physical significance, as it characterizes the dynamics of the polymer surface. The larger the value of k, the easier the diffusion of molecules, the more rapid the decline of hydrophilicity and the more significant the decay that takes place.

The values of k for different temperatures are showed in Table 2, from which it is seen that the degree of correlation is higher in the linear area of the graph of log A_t and log t. The linear regression coefficient is close to unity, confirming that the description of the diffusion dynamics of the corona-treated film decay mechanism was successful. In addition, when the temperature of the water was higher the value of k was larger, confirming that the decay of the corona-treated film in water was related to the temperature. The higher the temperature, the more rapid the decay, in line with the experiment results.

Table 2 Dynamic data of the decay of the BOPET corona-treated film in water at various temperatures

Temperature	Regression coefficient	k
25 °C	0.99218	0.10983
50 °C	0.99844	0.11536
90 °C	0.99090	0.13250

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During the later stages of the decay, the experimental plot deviates from a straight line, which may be attributed to the fact that the driving force produced by the water was unable to overcome the resistance within the film. The movement of molecules on the surface was therefore slow.

Conclusions

Corona-treated BOPET film undergoes significant decay with time in moist environments; the surface decay is a dynamic equilibrium. In water the decay is more rapid, but the conservation ratio of surface hydrophilicity is greater than in air. When the corona-treated film decays in water, water provides the driving force for the moveable units. The mechanism for the movement is the rotation of the moveable molecular units around the rigid molecular chain, resulting in migration of polar groups from the surface to the inner areas of the film. This is a process of molecular diffusion. The dynamic diffusion equation describes the earlier stages of the decay process in water. The present study indicates that when the water temperature is higher, the diffusion of polar groups within the inner areas of the film is easier and the decay effect becomes more serious. A molecular movement model of the BOPET corona-treated film surface has been proposed to describe the decay effect.

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