Over the last thirty years or so, the use of contact angle measurement in the study of solid-liquid interactions and problems of adhesion has become very frequent. Methods have been developed based on the concepts 1 2 of authors such as Zisman¹ (critical surface tension, γ_c) and Fowkes² (polar and apolar interactions), to name but two examples. The essence of the method of contact angle measurement is that, unless considering a very thin substrate³, the triple line where the solid, S, liquid 1 and second, immiscible fluid 2 meet can be described by Young’s equation relating the three interfacial tensions, γ_ij, and the contact angle θ:

$${\gamma _{s2}} = {\gamma _{s1}} + {\gamma _{12}}\cos \theta$$
(1)

Provided the surface characteristics of two of the phases are known, those of the third may be assessed using the theoretically unique value of θ. This contact angle will be unique provided the three phases are strictly homogeneous and smooth. Unfortunately, in practice, it is very common to obtain experimentally a whole range of contact angles for a given three- phase system. The largest angle obtainable corresponds to that observed just after a drop of liquid has advanced on the solid surface and is therefore known as the advancing angle, θ_A. Similarly the smallest angle is obtained just after the arrest of a receding liquid drop and the corresponding angle is θ_R.

The equilibrium at the triple line where a liquid and a fluid (either vapour or a second liquid immiscible with the first) meet on a solid surface was originally described nearly two centuries ago¹. By using a simple vectorial argument, the well-known Young equation may be obtained by resolution of the three interfacial tensions, γ, parallel to the solid surface:

$${\gamma _{s2}} = {\gamma _{s1}} + {\gamma _{12}}\cos \theta$$
(1)

where 1, 2 and S represent respectively the liquid, the fluid and the solid and θ is the contact angle measured in phase 1. Nevertheless, an objection has on occasion been presented. Although everything is balanced parallel to the solid surface, nothing would seem to counteract the vertical component γ₁₂ sin θ.^2,3 When the solid is treated as perfectly rigid, it is possible to apply variational calculus and the criterion of minimum free energy at equilibrium. The result is that equation 1 is perfectly correct.^4–9 When the solid is considered to be elastic, but very thin, a variational treatment leads us to take into account, in addition to interfacial effects, those due to elastic strain energy and (implicitly) gravity¹⁰. The approach invokes the modelling of the solid either by thin plate or membrane theory. This treatment leads to modified equilibrium conditions although in practice the effect will be very small except for very thin solids (cell walls?).

The sessile drop contact angle measurement is a useful and reliable method for surface energy determination and cleanliness verification. A review of the available methods, commercial instruments, patents, and literature describing the state of the art in contact angle measurement is followed by a description of contact angle measurement techniques that have been modified for use on large surfaces. The negative effects of these changes on accuracy and precision are discussed, and remedies are proposed including the use of standard reference objects that mimic the size and shape of sessile drops. The combination of these validation tools and the modified contact angle measuring techniques fills a need for robust, production-line capable cleanliness verification methods.

This chapter describes an application of plasma in printed substrates and the influence of plasma treatment on polymers and polymeric composites, their printability and prints quality. Plasma is one of the physical methods of surface modification that includes, among others, corona, flame, and laser treatment. Contrary to the corona treatment, plasma activation enables very uniform modification. Due to the attributes of polymers, particularly their thermal sensitivity, their modification is almost exclusively done using cold plasma, which described in depth in this chapter. Additionally, the changes induced in the material are explained. Especially substrate wettability and its roughness are of paramount importance, impacting printability significantly. Moreover, the influence of selected parameters of plasma treatment on surface modification is presented. Due to the fact that changes induced in the material surface are not permanent, the chapter also goes into more detail about the aging process in relation to the type of polymer, conditions of plasma activation, and storage.

There are many current and emerging wetting and adhesion issues which require an additional surface processing to enhance interfacial surface properties. Materials which are non-polar, such as polymers, have low surface energy and therefore typically require surface treatment to promote wetting of inks and coating. One way of increasing surface energy and reactivity is to bombard a polymer surface with atmospheric plasma. When the ionized gas is discharged on the polymer, effects of ablation, crosslinking and activation are produced on its surface. In this paper we will analyse the role of plasma and its use in increasing the surface energy to achieve wettability and improve adhesion of polymeric surfaces.

An oriented, heat sealable polypropylene film is provided having a metallizable surface. The film includes a core layer derived from isotactic polypropylene containing an effective amount of adhesion promoting agent. A copolyester layer is bonded to the core layer, the adhesion promoting agent protecting against the delamination thereof. A heat sealable layer formed from an ethylene-propylene random copolymer is bonded to the opposite side of the core layer. The film is formed as a coextrudate and is biaxially oriented.

The surface modifications produced by treatment of a synthetic sulfur vulcanized styrene-butadiene rubber with oxidizing (oxygen, air, carbon dioxide) and non oxidizing (nitrogen, argon) RF low pressure plasmas, and by treatment with atmospheric plasma torch have been assessed by ATR-IR and XPS spectroscopy, SEM, and contact angle measurements. The effectiveness of the low pressure plasma treatment depended on the gas atmosphere used to generate the plasma. A lack of relationship between surface polarity and wettability, and peel strength values was obtained, likely due to the cohesive failure in the rubber obtained in the adhesive joints. In general, acceptable adhesion values of plasma treated rubber were obtained for all plasmas, except for nitrogen plasma treatment during 15 minutes due to the creation of low molecular weight moieties on the outermost rubber layer. A toluene wiping of the N{2 } plasma treated rubber surface for 15 min removed those moieties and increased adhesion was obtained. On the other hand, the treatment of the rubber with atmospheric pressure by means of a plasma torch was proposed. The wettability of the rubber was improved by decreasing the rubber-plasma torch distance and by increasing the duration because a partial removal of paraffin wax from the rubber surface was produced. The rubber surface was oxidized by the plasma torch treatment, and the longer the duration of the plasma torch treatment, the higher the degree of surface oxidation (mainly creation of C O moieties). However, although the rubber surface was effectively modified by the plasma torch treatment, the adhesion was not greatly improved, due to the migration of paraffin wax to the treated rubber-polyurethane adhesive interface once the adhesive joint was produced. On the other hand, the extended treatment with plasma torch facilitated the migration of zinc stearate to the rubber-adhesive interface, also contributing to deteriorate the adhesion in greater extent. Finally, it has been found that cleaning of SBS rubber in an ultrasonic bath prior to plasma torch treatment produced a partial removal of paraffin waxes from the surface, and thus improved adhesion was obtained.

An expression is derived, from simple statistical thermodynamical considerations, to relate the cohesive energy density (C.E.D.) to intermolecular interaction parameters. From analogous theoretical relations in the literature for the surface tension and the index of refraction together with the derived expression for the C.E.D., we can obtain relations between the C.E.D. on the one hand and the surface tension and the index of refraction, respectively, on the other. These relations are compared with empirical relations in the literature. The exponents of the surface tension and the index of refraction in the empirical relations are different from those in the relations obtained here. The derived relation between C.E.D. and surface tension is shown to be applicable to experimental data with excellent agreement for both organic liquids and polymers. As surface tension measurements are very simple to perform, another main point of interest of the derived relation is knowledge of the solubility parameters of polymers (the square root of the C.E.D.) from critical surface tension measurements without the necessity of solubility and swelling experiments which are often awkward. The relation between the C.E.D. and the index of refraction derived in this paper implies grave theoretical objections to applications in practice.

A detailed analysis is reported of the chemical and physical modifications which occur to PTFE surfaces exposed to glow discharges in ammonia gas and in air. The analytical methods used were infra-red attenuated total reflectance and differential attenuated total reflectance spectroscopy, X-ray photoelectron spectroscopy, contact angle measurements and scanning electron microscopy. The suitability for bonding with adhesives and the stability of the modified surfaces to attack by oxidizing acids are also reported.

The surface modification of isotactic polypropylene (PP) in a microwave plasma of CO2 is described. The modified PP is characterized in bulk and also at its surface. The mechanism of plasma modification is discussed in terms of degradation and oxidation. The degradation leads to volatile products and to the formation of a layer of oxidized oligomers of PP. The oxidation leads to ketone, acid or ester groups. The degradation and oxidation rates depend on plasma parameters (duration, discharge power, gas flow, pressure, discharge or post-discharge treatment). The oxidation rates vs the various plasma parameters show a maximum. The crosslinking of PP (Crosslinking Activated Species of INert Gases) seems to be negligible.

The main objective of this experimental study is the validation of the technique of atmospheric plasma with the aim of improving the surface energy of the polylactic acid (PLA) for further adhesion uses. The wettability of PLA has been improved with the application of an atmospheric plasma surface treatment. This method provides good adhesion properties with the optimizing the process parameters in terms of the nozzle–substrate distance and sample advance rate. In order to achieve that goal, a new and environmentally friendly technology has been used which is based on the use of air atmospheric plasma. The effects of the surface treatment on this type of substrates have been analyzed. The macroscopic effects of the process parameters have been determined using contact angle measurements and subsequent surface free energy (SFE) calculation. In addition, the chemical changes at the topmost layers have been studied using X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared spectroscopy (FTIR). Surface topography changes due to the plasma-acting mechanisms have been evaluated with scanning electron microscopy (SEM) and atomic force microscopy (AFM). The obtained results show a remarkable increase in surface free energy from 37.1 mJm⁻² up to values of 60 mJm⁻² thus indicating the effectiveness of the air plasma treatment. The main advantage of this technology is that the industrial process is continuous, it is easy to establish in current production systems and it does not generate wastes.

The influence of the pulsed CO₂ laser irradiation on the surface structure of the LDPE film was investigated. Significant changes were observed on the surface of laser treated films as it was verified by the attenuated total reflectance Fourier transform infrared (ATR–FTIR) spectroscopy, scanning electron microscopy and contact angle-measurement. Formation of polar functional groups onto the LDPE surfaces exhibited by the ATR–FTIR spectra was shown to be strongly dependent on the number of the CO2 laser pulses. The intensity of the polar groups increased with increasing the number of pulses up to two and then slightly decreased at three laser pulses. This was also confirmed with the contact angle measurements in which the sample subjected to two laser pulses showed the highest wettability i.e. the lowest water drop contact angle. The concentration of peroxide groups formed on the surface of the laser treated films was determined quantitatively by UV spectroscopic method using iodide procedure. The latter results showed a similar trend with the results obtained using FTIR spectroscopy.

In this work, low pressure glow discharge O₂ plasma has been used to increase wettability in a LDPE film in order to improve adhesion properties and make it useful for technical applications. Surface energy values have been estimated using contact angle measurements for different exposure times and different test liquids. In addition, plasma-treated samples have been subjected to an aging process to determine the durability of the plasma treatment. Characterization of the surface changes due to the plasma treatment has been carried out by means of Fourier transformed infrared spectroscopy (FTIR) to determine the presence of polar species such as carbonyl, carboxyl and hydroxyl groups. In addition to this, atomic force microscopy (AFM) analysis has been used to evaluate changes in surface morphology and roughness. Furthermore, and considering the semicrystalline nature of the LDPE film, a calorimetric study using differential scanning calorimetry (DSC) has been carried out to determine changes in crystallinity and degradation temperatures induced by the plasma treatment. The results show that low pressure O₂ plasma improves wettability in LDPE films and no significant changes can be observed at longer exposure times. Nevertheless, we can observe that short exposure times to low pressure O₂ plasma promote the formation of some polar species on the exposed surface and longer exposure times cause slight abrasion on LDPE films as observed by the little increase in surface roughness.

The changes of contact angle (θ) and surface free energy (γS) under low-temperature air plasma in the polymers of different chemical structure and polarity (polyethylene, PE; polypropylene, PP; poly(ethylene terephtalate), PET and poly(methyl methacrylate), PMMA) pointed out to the greater effect of short-time plasma action (5–15 s) on these parameters as compared to longer times of exposure.

The non-reversion effect of θ changes caused by plasma in PE and PP suggests that the oxidation processes mainly decide about values in nonpolar polymers. The significantly greater θ changes in PE than those in PP indicate that the side groups present in the main chains impede oxidation of such a polymer by plasma.

The reversion of θ changes in PET and in PMMA, and return of these values to almost the initial ones after 10 min storage proves that the main reason for θ changes in polar polymers is a certain alteration of the chain conformation.

These changes, taking place after longer plasma treatment, suggest that the side ester groups in PMMA retard the above-mentioned conformational transformations. Then, in both kinds of polymers (polar and nonpolar) the structure of macrochain decides about the efficiency of reaction caused by plasma, and at the same time the side groups retard not only the oxidation processes but the conformational changes as well.

The surface of medical devices is of great importance for biocompatibility. Surface properties can evolve with a material treatment, time, and storage conditions. In this work, poly(urethane) catheters sterilised by cold nitrogen plasma treatment, were subjected to air and temperature aging in order to evaluate the influence of humidity and temperature on surface recovery. The surface of catheters was analysed by contact angle measurements and XPS. Faster surface changes upon aging were observed at high temperature (45 °C) and relative humidity (90%). For the commercial poly(urethane) catheters analysed in this work, the importance of the nature and polymorphism of additives added to the polymer (lubricant, antioxidant) in the recovery process was demonstrated. Indeed, DSC and TSC showed that additive transitions (relaxation, melting…) could govern the aging process.

A hexamethyldisiloxane (HMDSO)-RF plasma was used to treat polypropylene (PP) fabrics to achieve an inorganic type surface. The properties of the plasma modified PP were investigated through demand wettability and contact angle techniques. ESCA and ATR-IR spectroscopy indicated the presence of Si  O  Si and Si  O  C based structures. The influence of treatment time on the level of deposition and surface atomic composition was established. Plasma induced molecular fragmentation of HMDSO was determined through GC-MS and high resolution MS analyses of the molecular structures produced from recombination of active species, in the cold trap.

A method is described for measuring the level of pretreatment of polyethylene films in terms of critical surface tensions. Drops of water-dioxan mixtures of various surface tensions are placed upon the pretreated films and their critical surface tensions assessed from the spreading behaviour of the liquids.

A suggestion is made for using this method as a process control test.

Polyester fabric was treated by corona discharge irradiation at different voltages. The treated fabric showed increased wicking and hydrophilic properties and the properties can be preserved for a long time. Dyeing of the treated fabric showed that dyeing speed and the dye-uptake were improved. Surface affinity between the treated fabric surface with modified starch sizing was also confirmed to be increased. This is generally useful for the sizing of polyester staple yarn and the polyester fabric dyeing. All the results are supposed due to the improved hydrophilic properties produced by the corona discharge treatment.

This study aims to verify the effectiveness of a biodegradable PLA coating on biaxially oriented poly ethylene terephthalate (BOPET) films substrates in order to develop a biodegradable sealant layer for improving the functional properties of polyester films. In particular, this work focuses on the evaluation of different surface treated BOPET films to understand the effect of typical polyester substrate treatments on the adhesion of the PLA coating. At this purpose the film’s surface properties and topography have been characterized respectively by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and by atomic force microscopy (AFM); while their surface tension has been analyzed by contact angle tests. Furthermore, the PLA coating adhesion to different BOPET webs has been assessed by delamination tests and dynamical mechanical analysis. In addition, to evaluate the optical properties haze and transparency measures have been performed. It was found that corona treatment proves to be more effective in increasing surface energy and roughness of substrate with respect to a chemical treatment. The coating adhesion to corona treated BOPET was higher compared to the neat and the chemical treated BOPET films. Improvement in adhesion of PLA on corona treated BOPET was interpreted on the basis of surface oxidation due to the electrical discharge as showed by AFM and ATR-FTIR analyses. Indeed the increase of both contact surface and functional groups able to form hydrogen bonds and other molecular interactions, allow to a better interface interaction between the two layers.

Polyolefins and fluoropolymers have a major drawback: they don't adhere to polymer surfaces properly. If these substrates do have to be coated, however, there are three different approaches to improving adhesion. One is to change the coating, another is to modify the polymer surface, and the third is to apply some kind of adhesion promoter. One successful process which could solve this problem comprises a combination of a plasma pre-treatment and an additional thin acrylated photoinitiator coating. The result is that the purely physical bond gives rise to a covalent chemical bond between the polymer surface and the coating. Further advantages offered by this process are discussed in this article.