The polypropylene films are pretreated in a nitrogen or ammonia low-pressure plasma in order to improve their adhesive properties towards an in-situ deposited aluminium coating. The treatment conditions are similar to industrial ones and treatment times as short as 23 ms allow a considerable improvement of the adhesion between the polypropylene and the aluminium. The aim of this work is to understand better the mechanisms involved in the adhesive phenomena. Indeed, the modifications created by the plasma (for very short treatment times) are not easily detected. SSIMS has revealed the presence of a thin non-homogeneous film of light-weight hydrocarbons on the non-pretreated polymer. This film is responsible for the non-adhesion of the aluminium coating onto the polymer. Actually when this film is removed by a cleaning process induced by the plasma, the interactions between the aluminium and the polypropylene are strong enough to allow a good adhesion. This explains one of the effects of the plasma and more experiments will be carried out in order to determine the key factor of the phenomenon: the role of the oxygen at the interface on the treated polymer will be investigated as well as the diffusion depth of the treating gas.

The mechanics of adhesion have been investigated both theoretically and experimentally, using model adhesive joints consisting of a crosslinked amorphous rubber bonded to a variety of rigid polymeric substrates.

An adhesive failure energy, θ, is defined which is characteristic of the bond but independent of test-piece geometry. Both theory and experiment show that θ has the form, θ=θ₀f(R) where θ₀ is the “intrinsic adhesive failure energy” which depends only on the physical and chemical nature of the adhesive-substrate interface, and f is a function of R, the “reduced” rate of failure propagation obtained from rate and temperature data using the WLF equation.

θ₀ is the work of bond fracture across the interface and, for clean interfacial failure, is equal to the thermodynamic work of adhesion w_A. Where failure is not purely interfacial, θ₀ can be expressed as θ₀=iI+r𝒯₀+sF where i, r, and s are respectively the area fractions of interfacial, cohesive-in-rubber and cohesive-in-substrate failure, and I, θ₀, and F are the intrinsic failure energies for the interface, rubber, and substrate, respectively.

It is believed that this work is the first to demonstrate explicitly and quantitatively the separate contributions of interfacial properties and bulk rheological behavior to the strength of adhesive joints.

One of the most important claims for the plasma technique as a surface treatment is that it modifies only a few atomic layers of materials. However, with polymers, this assumption must be carefully verified to keep the bulk mechanical properties constant. Besides the oxidation of the film, with specific plasma conditions such as high power and duration, the polypropylene film structure is also modified in the bulk through vacuum ultraviolet absorption and thermal relaxation. This change is associated with smectic- and amorphous-phase transformation into an α-monoclinic phase, with a rapid rate for the smectic transformation and a slower rate for the amorphous transformation. At the same time, the crystallite size increases, and the polypropylene film texture is planar and moderated (1.7 mrd at the maximum of the distribution, with a discharge power of 100 W and a treatment duration of 10 min). © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2007–2013, 2004
https://onlinelibrary.wiley.com/doi/abs/10.1002/polb.20071

The polypropylene modification in CO₂ plasma mainly contributes to degradation, functionalization, and cross-linking. The degradation, whose rate is depending on CO₂ dissociation and oxygen atom formation, is a quite slow reaction and it is associated with surface topography alteration, especially of the amorphous phase of the polypropylene. The surface roughness increases with the treatment duration and the amorphous phase is more degraded than the crystallized part. The functionalization, corresponding to an increase of the surface energy (57.3 mJċ m ^{− 2} in 30 s), and to an oxidation (23 oxygen at.%) with the appearance of alcohol, ketone, and acid functions is a much faster phenomenon. Cross-linking takes also place during this type of treatment and will reinforce the stability of the modified surface.

This paper deals with the plasma surface treatment of polymers in a low frequency bell jar reactor with non-symmetrical configuration of electrodes. The highly energetic character of this discharge due to its low excitation frequency and electrode configuration, as well as its small discharge volume makes it a very efficient and fast functionalization process. Amongst the different plasma gases used for the adhesion improvement of polypropylene to aluminum, ammonia has shown to be the most suitable one for this application. Since the NH and NH ₂ radicals play an important role in the kinetics of nitrogen incorporation in polymers, mixtures of N ₂ and H ₂ were also used as possible substitutes for ammonia. The former are more environmentally friendly and easier to handle in industry than ammonia. The efficiency of nitrogen rich mixtures in the case of the second application, i.e. adhesion improvement of copper to fluoropolymers has been compared to that of ammonia which still shows faster nitrogen incorporation. The last part of this paper is devoted to the study of the energetic character of plasmas of mixtures of He+NH ₃ by OES and electrical measurements in the whole range of composition of the two gases. The results show that an ammonia percentage ranging from 5 to 10% in plasmas of mixtures of He/NH ₃ represents a transition between two different discharge regimes. Plasmas of mixtures of He+2% NH ₃ , characterized by highly energetic electrons, ions and probably metastables of helium give rise to enhanced adhesion of PP to aluminum which remains stable with time.

The physicochemical modifications of ammonia-treated polypropylene (PP) films have been studied and characterized in terms of acid-b ase properties using the contact angle titration method and X-ray photoelectron spectroscopy in conjunction with a molecular probe technique using chloroform as a reference Lewis acid. These techniques have shown that PP surfaces that have been treated for between 0.7-1 s are basic in character. For longer treatment times, the basic character of the surfaces decreases, as shown by the above techniques and confirmed by time of flight-secondary ion mass spectroscopy (ToF-S IMS). On the other hand, for such treatment times, a degradation of the adhesion and mechanical properties was observed. The ageing of an ammonia-plasma-treated PP was limited by a helium (He) plasma pretreatment known to crosslink the surface, stabilizing in this way the wettability, adhesion and mechanical properties. ToF-S IMS was performed on helium treated High Density Polyethylene (HDPE) in order to point out the structural modifications.

Experimental results on plasma treatments of polysulfone and polyetherimide to improve the wettability of these polymers are presented. The plasma is characterized by optical emission spectroscopy. The wettability of the polymer surfaces were checked by contact angle measurements and ESCA is used to compare the surfaces before and after plasma treatment. Correlations between contact angle, concentration of oxygen at the surface, and optical emission intensity of the OH radical have been established. Optimization of operational plasma parameters leading to the best wettability of the treated samples is reported.

Low-rate dynamic contact angles of water, glycerol and methylene iodide on polystyrene and poly(4-(X = CH₃, (CH₃)₃, Cl, OH)-styrene) films were measured by axisymmetric drop shape analysis-profile (ADSA-P) for sessile drops. It was found that glycerol in the case of poly(4-chlorostyrene) and methylene iodide on all investigated surfaces did not yield constant contact angles: dissolving of the polymer during the contact with the liquid, penetration effects and stick/slip behavior were found. Water and glycerol yielded meaningful constant contact angles (except for glycerol on poly(4-chlorostyrene), for which we used formamide additionally). From these meaningful contact angles the solid surface tensions of the modified polymers were calculated using the equation of state approach [6]. The following values of γ_sv were determined: polystyrene γ_sv = 28.3 mJ/m², poly(4-methylstyrene) γ_sv = 25.8 mJ/m², poly(4-tert-butylstyrene) γ_sv = 22.0 mJ/m², poly(4-hydroxystyrene) γ_sv = 44.1 mJ/m², poly(4-chlorostyrene) γ_sv = 27.2 mJ/m².

The interaction between inks and substrates is critical during printing. Adhesion of the ink film is determined by the reciprocal interactions of polar and nonpolar (dispersive) components between polymer films and inks. The greater the similarity between the polar and dispersive components of inks, coating and substrates, the better the wetting and adhesion on the surface of printing substrate. Various liquid materials in printing such as inks, varnishes, lacquers, and adhesives contain high ratios of water. The highly polar nature of water makes the interaction of these materials unsuitable with predominantly disperse polymer surfaces. Some films with polyolefin structure, especially polypropylene, and polyethylene, are nonpolar and cannot form strong bonds with ink, varnish, or lacquer coatings due to their chemical structure. Increasing surface energy components overcomes the poor wetting and adhesion on polymer surfaces. In this review, the topics of water contact angle measurement and determination of surface energy, surface tension, and using sessile drop method for the wettability and ink adhesion of polymer films are surveyed. Information on structural and chemical processes was given that assists in obtaining wettable film surfaces. Recommendations were made for good adhesion and printability based on surface treatment methods and ink modification.

An absolute method for the determinalion of surface tension of liquids using the pendent drop profiles at conical tips, which has several distinct advantages, has been proposed. For systems with zero contact angle, the dimensionless governing equations for drop profiles at different conical tips have been computer-solved. and the theoretical plots of X_T and Z_T vs. their ratio, where X_T and Z_T are the dimensionless x and z coordinates of the drop profile at a plane at the conical tip perpendicular to the axis of symmetry, are statistically anaJyied to generate suitable tables for using the proposed method.

Polytetrafluoroethylene (PTFE) was treated with hydrogen and ammonia microwave plasmas and the effects of treatment were evaluated by means of advancing and receding contact angle measurements, X-ray photoelectron spectroscopy, secondary-ion mass spectroscopy and atomic force microscopy analysis. Hydrogen plasma downstream treatment principally leads to defluorination and creation of CC and CH groups. This surface modification results in a slight decrease of the water contact angle and a large decrease of the methylene iodide contact angle. No evolution of the surface properties occurs over a period of at least two months following treatment. Ammonia plasma downstream treatment leads to defluorination and creation of CC and CH groups, as already observed with the H₂ plasma, but also to the introduction of nitrogen-containing groups. The modification produces a decrease of both water and methylene iodide contact angles. A large hysteresis is found with water contact angles due to the reorientation of the polar groups when the surface is in contact with a polar liquid. The surface modifications that result after a NH₃ plasma treatment are less stable than after a H₂ treatment. Nevertheless, after two days of ageing the water contact angle reaches a constant value, which is largely inferior to that of the untreated PTFE.

The surface modification of polytetrafluoroethylene (PTFE) by microwave plasma treatment was investigated by means of contact angle measurement and e.s.c.a. studies. Various gases (e.g. O₂, O₂N₂, NH₃) were used. The influence of the various plasma parameters, such as power, gas flow, distance between the sample and the centre of the discharge, treatment time, etc., has been evaluated. No modification was induced by O₂ and O₂N₂ treatment, whatever the treatment conditions. NH₃ plasma irradiation, however, rendered the PTFE surfaces more hydrophilic, leading to an increase of the polar component of the surface energy from 4.5 to ∼ 57 mJ m⁻² under optimized treatment conditions. NH₃ treatment led to defluorination, crosslinking, hydrocarbon (CC,CH) bond formation, and incorporation of nitrogen-containing groups, as confirmed by e.s.c.a. Oxygen was also detected at the surface of treated PTFE. Correlations between the contact angle, defluorination rate, and surface nitrogen and oxygen contents, have been established. Optimization of operational NH₃ plasma parameters, leading to the best wettability of the treated samples, is also reported.

A method is described for measuring dynamic surface and interface tension. The technique is essentially a variation of the pendant drop method in which the drop is allowed to oscillate after sudden formation at the tip of a syringe. Immediately after the oscillation stops but before the drop detaches, there is an instant of rest. At this moment, the profile of the drop is obtained using high‐speed photography. The boundary tension is then calculated from the profile using established methods. The technique is demonstrated on systems consisting of aqueous solutions of sodium stearate or oleate on one hand and mineral oil or air on the other hand. Surface or interface tensions may be obtained within 0.25 to 5 s after surface formation.

Scanning electron microscope and goniometer were used to investigate morphology and wetting property of polypropylene membrane surfaces modified by plasma treatment using different reagents. Surface morphology was significantly affected by the types of reagents. X-ray photoelectron spectroscopy and attenuated total refection-Fourier transform infrared spectroscopy were used to characterize the chemical structure of polypropylene membrane surfaces modified by Freon-116 gas plasma treatment. Many fluorine atoms were observed on the polypropylene surface, and its concentration increased to saturation with increasing plasma treatment time. The wetting behavior of plasma treated polypropylene membrane was well explained in relation with morphology and chemical structure.

Flock coating is a widely used process to create a textile-like texture on substrates of various shapes and materials. In the process, flock fibers—short fibers typically 1–3 mm long—are oriented and accelerated towards the substrate by means of an electric field. Impacting fibers are stuck to the substrate surface by an appropriate adhesive. Primary quality criteria are adhesion of the flock fibers to the adhesive, and also the so-called flock density, ie number of fibers per unit area, and evenness. The influential physical and chemical factors refer to interfacial adhesion, but also charging effects by the impacting fibers. The system presently under investigation is based on aliphatic polyamides as material for a molded car component, hot-melt adhesive, and flock fibers. Experiments reported here refer to the application of an air plasma pretreatment of the polyamide (PA) substrate, mainly in order to increase the adhesion of the hot-melt layer. It was found that the plasma treatment affects the polar energy of the PA surface with a related increase in wettability due to a reduction of C–C and C–H bonds and an increase of carboxylic groups. Surface carbonization occurred at higher plasma doses. The effect on hot-melt adhesion was rather small, however two types of failures were observed in these experiments, either due to insufficient adhesion of the hot-melt or due to a break of one of the PA plates with the bond still intact. The characterization of flock coatings on these samples showed no effect on flock fiber adhesion in pull-out as well as on abrasion resistance, but an increased flock density was observed. This is assumed to be due to enhanced dissipation of charges by the conductive water layer adsorbed on the substrate surface.

In the present paper, the following topics are reviewed in detail: (a) the available adhesives, as well as their recent advances, (b) thermodynamic factors affecting the surface pretreatments including adhesion theories, wettability, surface energy, (c) bonding mechanisms in the adhesive joints, (d) surface pretreatment methods for the adhesively bonded joints, and as well as their recent advances, and (e) combined effects of surface pretreatments and environmental conditions on the joint durability and performance. Surface pretreatment is, perhaps, the most important process step governing the quality of an adhesively bonded joint. An adhesive is defined as a polymeric substance with viscoelastic behavior, capable of holding adherends together by surface attachment to produce a joint with a high shear strength. Adhesive bonding is the most suitable method of joining both for metallic and non-metallic structures where strength, stiffness and fatigue life must be maximized at a minimum weight. Polymeric adhesives may be used to join a large variety of materials combinations including metal-metal, metal-plastic, metal-composite, composite-composite, plastic-plastic, metal-ceramic systems. Wetting and adhesion are also studied in some detail in the present paper since the successful surface pretreatments of the adherends for the short- and long-term durability and performance of the adhesive joints mostly depend on these factors. Wetting of the adherends by the adhesive is critical to the formation of secondary bonds in the adsorption theory. It has been theoretically verified that for complete wetting (i.e., for a contact angle equal to zero), the surface energy of the adhesive must be lower than the surface energy of the adherend. Therefore, the primary objective of a surface pretreatment is to increase the surface energy of the adherend as much as possible. The influence of surface pretreatment and aging conditions on the short- and long-term strength of adhesive bonds should be taken into account for durability design. Some form of substrate pretreatment is always necessary to achieve a satisfactory level of long-term bond strength. In order to improve the performance of adhesive bonds, the adherends surfaces (i.e., metallic or non-metallic) are generally pretretead using the (a) physical, (b) mechanical, (c) chemical, (d) photochemical, (e) thermal, or (e) plasma method. Almost all pretreatment methods do bring some degree of change in surface roughness but mechanical surface pretreatment such as grit-blasting is usually considered as one of the most effective methods to control the desired level of surface roughness and joint strength. Moreover, the overall effect of mechanical surface treatment is not limited to the removal of contamination or to an increase in surface area. This also relates to changes in the surface chemistry of adherends and to inherent drawbacks of surface roughness, such as void formations and reduced wetting. Suitable surface pretreatment increases the bond strength by altering the substrate surface in a number of ways including (a) increasing surface tension by producing a surface free from contaminants (i.e., surface contamination may cause insufficient wetting by the adhesive in the liquid state for the creating of a durable bond) or removal of the weak cohesion layer or of the pollution present at the surface, (b) increasing surface roughness on changing surface chemistry and producing of a macro/microscopically rough surface, (c) production of a fresh stable oxide layer, and (d) introducing suitable chemical composition of the oxide, and (e) introduction of new or an increased number of chemical functions. All these parameters can contribute to an improvement of the wettability and/or of the adhesive properties of the surface.

The physical significance of contact angles has been interpreted on the basis of a model derived from known surface energy relationships. The degrees of non-spreading and spreading have been expressed in terms of the magnitude of contact angles. On the basis of the physical picture, hysteresis of contact angle has been calculated from the experimental values of equilibrium contact angle and surface tension of the liquid. It has been suggested that it is not necessary to assume that hysteresis of contact angles is due to surface roughness of solids. The picture also explains why apparent contact angle on a non-flat solid surface is more than that on a flat solid.

The effects of cross-linking and crystallinity on the aging of plasma-treated high-density polyethylene (HDPE) have been investigated. In the case of mixed argon and oxygen, aging has been found to be reduced with an increased amount of argon in the mixture owing to an increased degree of cross-linking. A similar decrease in hydrophobic recovery has been achieved by increasing the crystallinity of HDPE. Diffusion of polar functional groups from the surface into the bulk has been observed to be lowered by both increasing the degree of cross-linking and crystallinity. The samples were analyzed by angle-resolved XPS, contact angle measurements and SEM investigations.