Polymer xxx (2010) 1e15 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer Feature Article The fundamentals of ﬂame treatment for the surface activation of polyoleﬁn polymers e A review Stefano Farris a, *, Simone Pozzoli a, Paolo Biagioni b, Lamberto Duó b, Stefano Mancinelli c, Luciano Piergiovanni a a b c DiSTAM, Department of Food Science and Microbiology, Packaging Laboratory, University of Milan, Via Celoria 2 e 20133 Milan, Italy LNESS, Department of Physics, Politecnico di Milano, Piazza L. da Vinci 32 e 20133 Milan, Italy esseCI srl Company, Via Flaminia Ternana n. 386 e 05035 Narni, Italy a r t i c l e i n f o a b s t r a c t Article history: Received 25 March 2010 Accepted 19 May 2010 Available online xxx This paper aims to provide an exhaustive and comprehensive overview on ﬂame treatment as a valuable technique for improving the surface properties of polymers, especially polyoleﬁns. It starts with a brief historical excursus on the origin of ﬂame treatment, and the second section deals with the major fundamentals of ﬂame chemistry, with a special focus on the combustion process and mechanism of surface activation. The most important parameters inﬂuencing the extent of the oxidation reaction along with relevant practical notes are discussed in the third section. The concluding section outlines how the most signiﬁcant features of ﬂame treatment can be proﬁtably used to improve the wettability and adhesion properties of polyoleﬁn surfaces, especially from the perspective of developing novel composite solutions such as polyoleﬁns/bio-based coating pairs intended for many different applications. Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Coatings Polymer science and technology Surface energy 1. Introduction Surface properties play a pivotal role in deﬁning the performance of materials. Among these properties, wettability and adhesion are sought after in several industrial ﬁelds such as automotive, aerospace, building, engineering, biomedical, and biomaterials . For this reason, they have been extensively studied by different branches of science such as polymer chemistry, physics, and rheology. Adhesion and wettability are of critical importance for polymers intended for packaging applications, since they can greatly affect relevant and practical attributes such as the printability of a ﬁlm, the strength of a laminate, and the anti-fog property of boxes, as well as the processability, convertibility, recyclability, and biodegradability of the ﬁnal materials. Worldwide attention has long been focused on those applications requiring the deposition of a layer or coating (e.g., adhesives, paints, and varnishes) onto a polymeric substrate, especially when the adhesion at their interfaces is difﬁcult to accomplish due to the inherent chemical surface differences of the two contacting polymers. As a consequence, the establishment of both interatomic and intermolecular interactions governing the adhesion phenomenon at the substrate/coating interface is totally frustrated . To make these surfaces prone to printing and coating processes, different * Corresponding author. Tel.: þ39 0250316654; fax: þ39 0250316672. E-mail address: [email protected] (S. Farris). strategies have been developed including using an adhesion promoter (e.g., chlorinated polyoleﬁn, CPO) , blending ethylenepropylene rubber to form thermoplastic polyoleﬁn (TPO) , and exploiting physical-chemical phenomena at the base of plasma , corona , laser , and ﬂame treatments . Although all of them have been suggested as suitable approaches for enhancing polymer adhesion strength, which is the most effective and feasible one is still the subject of debate . However, it is generally agreed that ﬂame treatment, together with corona discharge, is the most widely used for the surface activation of polyoleﬁn substrates . The development of ﬂame treatment has proceeded hand in hand with that of polyoleﬁns . After the early pioneer work of W.H. Kreidl, a considerable drive towards industrial implementation arose after the discovery of isotactic polypropylene (PP) by Giulio Natta in 1954. At that time, researchers belonging to the Montecatini Company located in the chemical district of Terni started working on MoplenÒ in an attempt to ﬁnd a solution to the high recalcitrance of such a polymer to printing and coating . In those same years, the electrical corona discharge process was being set up by Kreidl’s assistant, Kritchever, with the same goal of improving the surface properties of polyoleﬁns. Thereafter, the use of such a process grew tremendously and has become the primary method of treating polymer ﬁlms for two main reasons: ﬁrstly, because of concerns about the safety of open ﬂames in industrial environments and secondly, as a consequence of the recognised sensitivity of ﬂame treatments to small changes in process 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.05.036 Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 2 S. Farris et al. / Polymer xxx (2010) 1e15 conditions . As a result, although originally developed to treat ﬁlms, up to the beginning of the new century ﬂame treatment has chieﬂy been used for cellulosic (paper and paperboard) or relatively thick polyoleﬁn materials (e.g., automobile body parts and blowmoulded bottles) under the common misconception that corona treatment is more suitable for polyoleﬁnic ﬁlms, whereas ﬂame treatment is preferred for tridimensional symmetrical shapes. Over the past two decades many remarkable innovations, which will be discussed later in this review, have contributed to the renewed interest in ﬂame treatment, making it a recognised technique for modifying ﬁlm surfaces as well as tridimensional objects. However, to fully exploit the potential of this technique, it seems of primary importance to acquire a deep knowledge of the overall process. For this purpose, this review has been conceived as ﬁrstly a collection of the most relevant basic principles and key concepts of ﬂame treatment, with special emphasis on the fundamental chemistry governing both the ﬂame and surface activation phenomena. Secondly, this paper aims to illustrate the main practical parameters to make the process successful. The conclusion is dedicated to a brief discussion on the future trends in this ﬁeld, illustrating how ﬂame treatment can help in the design of new high performance packaging materials. simultaneous transfer of two electrons (divalent reduction). Since paired electrons are common in organic molecules, singlet oxygen is much more reactive towards organic molecules than its triplet counterpart. At this point, the so-called hydrogen abstraction from the fuel to oxygen can take place and hydroperoxide ( OOH) and hydroxyl ( OH) radicals are formed: RH þ 1O2 / R þ OOH (1a) RH þ 1O2 / RO þ OH (1b) 2.2. Chain branching This step can be schematically represented by the following mechanism, where M and M0 are the reactant molecules, R the radical species, a a multiplicator factor and K2 the reaction rate: k2 R$ þ M / bR$ þ M0 (2) Many different radical species (radical pool) are formed primarily by a general oxyhydrogenation reaction pattern: 2. Flame chemistry In 1848, Michael Faraday inaugurated the ‘Christmas Lectures’ at the English Royal Institute with a talk entitled “The chemical history of a candle”, starting with the following words: “There is no better, there is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a candle” . Approximately 150 years later, worldwide scientists can only agree with this leading opinion, since an apparently trivial process indeed governs many modern human activities. In addition, such a process paved the way for theoretical research topics that, in most cases, found remarkable applications in many ﬁelds. One example is the treatment of plastic objects in a ﬂame, which makes them suitable adherends. Combustion is a complex process involving many chemical reactions between a fuel (generally a hydrocarbon) and an oxidant (e.g., the oxygen in the air) with the production of heat and (although not always) light in the form of a ﬂame. Migration of chemical species within the ﬂame results in a subsonic wave (40e45 cm s1 in air/hydrocarbon systems) supported by combustion . Although a huge variety of chemical reactions take place during combustion, leading to many active radical species, it is generally recognised that the overall process can be summarised in few main steps, as schematically displayed in Fig. 1. 2.1. Initiation This ﬁrst step is represented by the general following reaction, where M is the reactant molecule, R the radical species and K1 the reaction rate: H þ O2 / O þ OH O þ H2 / H þ OH (2a) (2b) H2 þ OH / H2O þ H (2c) O þ H2O / OH þ OH (2d) Among them, Reaction (2a), which is promoted by H radicals arising from the dissociation of hydrogen at temperatures above 400 C, seems to be the most important since it generates all the successive reactions [(2b)e(2d)]. It has to be pointed out that, since the rate of Reaction (2a) is smaller than the rate of the reaction between a hydrocarbon and the hydrogen radical, the presence of the hydrocarbon actually inhibits the formation of the radical pool . 2.3. Propagating step forming product The highly reactive free radicals formed can freely interact with the hydrocarbon through the previously mentioned hydrogen/ abstraction mechanism and according to the following general mechanism, where M is the reactant molecule, R the radical species, P the new formed product, and K3 the reaction rate: k3 k1 M/R $ R$ þ M / R$ þ P (1) Firstly, the lowest-energy conﬁguration of the dioxygen molecule (O2), which is a stable, relatively unreactive diradical in a triplet spin state, is forced into a spin-paired state, or singlet oxygen (1O2). This is normally achieved by the absorption of sufﬁcient energy supplied as heat (ignition). The diradical form of oxygen is in a triplet ground state because the electrons have parallel spins. If triplet oxygen absorbs sufﬁcient energy to reverse the spin of one of its unpaired electrons, it will form the singlet state, in which the two electrons have opposite spins. This activation overcomes the spin restriction, and singlet oxygen can consequently participate in reactions involving the (3) The ﬁnal result is the formation of new products as well as additional radical species: RH þ OH / R þ H2O (3a) RH þ OOH / RO þ H2O (3b) RH þ H / R þ H2 (3c) RH þ O / RO þ H (3d) Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 S. Farris et al. / Polymer xxx (2010) 1e15 3 Fig. 1. Schematic overview of the combustion process. Hydrogen, methyl, and ethyl radicals and small alkenes (primarily ethene) can be produced from the fuel degradation occurring during hydrogen abstraction. Subsequent thermal decomposition can give rise to smaller alkyl radicals, small alkenes, and alkynes (acetylene) by thermal decomposition . In these ﬁnal steps, other reactions take place, among which it is worth mentioning the water formation by different pathways. Water forms through the reaction: 2.4. Termination step forming product The termination phase is basically made of two distinct processes: k4 R$ þ M / P0 (4) k5 R$ / P00 (5) In step (4), radicals (R ) react with other molecules (M) at a speciﬁc rate (K4) to give new unreactive species (P0 ), whereas in step (5) the radicals themselves (R ) evolve to new unreactive 00 species (P ) at a deﬁned rate (K5). The two main reactions involved in this ﬁnal step are, respectively, CO formation and its oxidation to CO2. CO formation takes place starting from all those small molecules originating from the previous step. In particular, methyl and ethyl radicals and small alkenes (e.g., ethene) are the most important intermediates leading to the formation of carbon monoxide through an oxidative attack. The oxidation of CO to CO2 is the concluding step of hydrocarbon combustion, according to the main reaction: CO þ OH / CO2 þ H can be asserted that hydrocarbons actually inhibit the formation of CO2. In other words, the rate of the oxidation of CO climbs considerably as soon as both the original fuel and all hydrocarbon intermediates have been consumed, since the hydroxyl radical concentration rises dramatically . (6) Together with the reaction represented by Eq. (2a), the above mechanism (Eq. (6)) plays a dominant role within the combustion of hydrocarbons . The main route to the carbon dioxide is the oxidation of carbon monoxide by OH radicals, whereas the contribution by O atoms is considered negligible . Analogously to the rate of the reaction between the H radical and oxygen in a typical oxyhydrogenation scheme (Eq. (2a)), OH radicals react more rapidly with hydrocarbons than with CO to form CO2. As a consequence, it RHx þ OH / RHx1 þ H2O (7) by the oxidation of formaldehyde (an intermediate of the combustion process): CH2O þ OH / CHO þ H2O (8) starting from hydrogen radicals formed by previous reactions: H þ OH þ M / H2O þ M (9) and through a typical oxyhydrogenation pattern (e.g., Eq. (2c)). 3. Explosive behaviour and the ‘runaway reaction’ It is worth noting that, considering the sequence [(2)e(5)], when: b > bcrit: ¼ 1 þ k4 þ k5 ; k2 (10) the combustion system has reached the explosion condition. This means that if the air/hydrocarbon mix is within its ﬂammability limits (i.e., it has a suitable composition) and within its explosive conditions (i.e., within adequate pressure/temperature boundaries for the same composition), the ﬂame is generated and can spontaneously propagate. Of course, according to Equation (10), the higher the rate of the chain branching step (K2) and the lower the rates of the termination steps (K4 and K5), the higher the probability for the explosion of the combustion system to occur. When so, Reactions (2a)e(2d) continuously increase the number of reactive radicals, allowing the exothermic condition to be approached by the Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 4 S. Farris et al. / Polymer xxx (2010) 1e15 combustion system. Since the rate of the above-reported reactions (and thereby the rate of the heat released) increases exponentially with temperature (according to the Arrhenius law), the fuel/oxidant mixture becomes explosive. Therefore, Reactions (2a)e(2d) are greatly important in the oxidation reaction mechanism of any hydrocarbon because they allow the propagation of the ﬂame. This exothermic reaction is also called the ‘runaway reaction’, which occurs when the reaction rate increases because of an increase in temperature, causing a further increase in temperature and a further increase in the reaction rate. Since direct combustion by atmospheric oxygen in a ﬂame is a reaction mediated by radical intermediates, the conditions for radical production are guaranteed by thermal runaway, where the heat generated by combustion is necessary to maintain the high temperature for radical production. The ‘runaway reaction’ is, therefore, the key condition for radical production. 4. Laminar ﬂame proﬁle A laminar ﬂame (which is ordinarily employed by ﬂame treaters) is deﬁned as a mixture of a fuel and an oxidiser, thoroughly premixed before combustion. The term ‘premixed laminar ﬂame’ is interchangeable with the term ‘deﬂagration’ to indicate the propagation of the combustion process accompanied by a decrease in both density and pressure together with an increase in velocity (contrary to the propagation known as ‘detonation’). Within a laminar ﬂame proﬁle, three main zones can be observed (Fig. 2), which correspond to speciﬁc reactions. As a consequence, different thermal gradients and reactive species can be encountered. These zones are brieﬂy described here. 4.1. Pre-reaction zone This region, also called the ‘dark zone’, has a typical dark bluish colour. It is the coldest region of a ﬂame because even though some of the hydrogen formed is oxidised to water the combustion process has not yet reached the explosion condition, and thereby the amount of net energy released is negligible. In this region, the only abundant free radical is the hydrogen atom, which reacts quickly with hydrocarbons and oxygen, thereby impeding the formation of the radical pool. For this reason, this zone is also known as the ‘reducing zone’. This is an ineffective and unimportant region for surface activation purposes, since it in no way contributes to the oxidation of the plastic substrate. 4.2. Main reaction zone Also called the ‘luminous zone’, the mixed reaction zone is characterised by the highest temperature of the combustion system (for propane-based mixtures the temperature reaches 1900e2000 C). In this zone, radical content increases dramatically to the detriment of the reactant concentration. The high concentration of radical species makes this region strongly oxidising, in contrast to the reducing zone mentioned above. Such an oxidising region is valuable for making effective the ﬂame treatment of polyoleﬁns. The colour of this zone depends on the fuel/air ratio: a deep bluish violet radiation, with the ﬂame becoming almost transparent if the quantity of gas is increasingly reduced, is produced when the mixture is gas-lean (due to excited CH radicals); conversely, a green radiation appears when the mixture is gas-rich (due to excited C2 molecules). When the gas in the mixture increases still further, the radiation turns yellowish because of the carbon particles formed. The observation of the colour of the ﬂame is an empirical tool widely used by the operators of ﬂame treatment plants to keep the right mixture composition throughout the process. 4.3. Post-combustion zone This is the largest of the three regions found in a typical laminar ﬂame proﬁle. The temperature here remains high due to the exothermic oxidation reaction (partial or complete) of CO into CO2, with a release of heat. Although intermediate species such as CH3, C2H2, and CH2O are typical of the luminous region only, radicals such as H , OH , and O can also be detected in the post-combustion zone . Generally speaking, the concentration of radicals in a laminar ﬂame proﬁle accounts for approximately 103 relative to the reactants, whereas ion species (among which the H3Oþ is the most abundant) are decidedly less (106 relative to the reactants). Normally, they lie slightly beyond the luminous portion of the ﬂame . The existence of a proﬁle of compositional differences over a laminar ﬂame can be explained in terms of the convective ﬂows of unburned gases from the dark zone to the luminous zone and the diffusion of radical species from the high temperature zone to the pre-heating region, in the opposite direction to the convective ﬂow. In particular, the diffusion of radical species is dominated by hydrogen atoms, which do not participate to the chain branching step described by Equation (2a) because of the lower temperature in the dark region. Instead, H atoms combine with oxygen radicals in the pre-heating zone to yield a large amount of HOO radicals. These then form hydrogen peroxide (H2O2), which does not dissociate because of the low temperatures in the dark zone. H2O2 is then conveyed to the luminous zone by convective ﬂows, where the temperature conditions make possible the formation of OH radicals. This explains the high concentration of OH radicals relative to O and H in the early part of the luminous zone and the very high temperature reached there, with the OH radicals-forming reaction highly exothermic (w85 kcal mol1). In addition, it explains why the OH attack on the fuel is the primary route for fuel degradation. Finally, it is worth noting that combustion processes are never complete. In the combustion of hydrocarbons, both unburned carbon and carbon compounds (such as CO and others) are always present. In addition, when air is the oxidant, like in a typical ﬂame treater plant, some nitrogen can be oxidised to various nitrogen oxides (NOx) . For example, Pijpers and co-workers observed a signiﬁcant amount of N at the surface of PP samples at air/propane ratios between 26 and 18 . Although different mechanisms can lead to the formation of NOx compounds, in commonly used burners the high temperature oxidation of molecular N2 seems to be the preferred way to form NOx, among which nitrogen monoxide (NO) is the most abundant. The term ‘thermal NO’ is widely accepted to indicate the formation of NO from the N2 present in the combustion air. This process requires very high temperatures (w1500 C) to break the covalent triple bond in the N2 molecule by the attack of the O radical produced during the combustion process. The formation of NO is in an inverse proportion to CHx intermediates and CO emissions when varying the air/fuel ratio. In particular, NO formation is promoted by increased temperatures, residence times, and O2 concentrations. Therefore, controlling NO formation during treatment operations can be easily achieved by burning under lean conditions and ﬂame quenching using a secondary air stream. Besides NO, nitrogen dioxide (NO2) is a minor product of the combustion process . However, since the NO oxidises to NO2 in the atmosphere NO is a potential precursor of NO2. 5. Laminar ﬂame speed and stability As previously stated, in a combustion system the ﬂame is a subsonic wave characterised by a velocity called laminar ﬂame speed, which is deﬁned as the velocity at which unburned gases move throughout the combustion wave in the direction normal to the wave surface . Different theories have been developed over time to provide an insightful description and quantiﬁcation of ﬂame Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 S. Farris et al. / Polymer xxx (2010) 1e15 5 Fig. 2. Main zones in a laminar ﬂame proﬁle. speed. Some of them (e.g., the TanfordePease theory ) are based on the diffusion of the huge variety of chemical species produced during combustion throughout the front of the ﬂame. Accordingly, such diffusion depends on the species’ molecular weights, meaning that low mass species (H, H2, O, and OH) will diffuse more rapidly than the heavier ones. In particular, besides its dominant role in Reaction (2a), hydrogen atom diffusion is especially important since its high diffusion rate is responsible for the main phenomena connected with laminar ﬂames . Other theories, generally called ‘thermic’, are instead based on the diffusion of heat rather than chemical entities. Among them, the theories of ZeldovicheFrankKamenetskii , Semenov [23,24], and MallardeLe Chatelier  deserve to be mentioned because they similarly contribute to the chemical kinetic modelling of hydrocarbon combustion. A generalisation arising from the combination of these theories has been suggested as the most appropriate approach to model laminar burning velocity, since it makes possible ﬁxing the most important practical parameters in laminar ﬂame propagation, which are otherwise difﬁcult to interpret in more complex analyses . Accordingly, it is assumed that there are two main mechanisms governing ﬂame propagation e the convection of heat and the diffusion of chemical species e in a back-and-forward modality, namely from the combustion zone to the zone of unburned gas and vice versa. Thus, the ﬂame can be seen as an array of adjacent waves formed by unburned gas at always higher temperatures until the ignition of the gas is reached. For the assumption that the premixed combustion is onedimensional and steady (contrary to turbulent, non-premixed ﬂames), the temperature proﬁle along a ﬂame can be schematically split into three different regions, as qualitatively depicted in Fig. 3, where the enthalpy of formation diagram is also reported. In the ﬁrst zone, the initial temperature (T0) rises exponentially, whereas the enthalpy of formation ðh0i Þ remains at the same values of the starting mix. This means that in this ﬁrst region the combustion conditions have not yet been reached. Heat-releasing reactions of low entities can anyhow occur, such as oxygen attacks on the hydrocarbon, hydrogen abstraction onto the hydrocarbon backbone (due to radicals diffusing from the main reaction zone), and scission/condensation reactions of the fuel. In this ﬁrst zone, therefore, the temperature is controlled by both diffusion and convection. The boundary between zone I and zone II is the point where the ignition takes place. At this point, the temperature registered is called the ‘mixture ignition temperature’ (Ti). In the second zone, temperature and enthalpy behave similarly, i.e., both increase linearly within a very narrow spatial range. It is assumed that in this zone the convection and generation of new species are the most important reactions, with the diffusion contribution negligible. The boundary between zone I and zone II is called the ‘ﬂame temperature’ (Tf), i.e., the temperature of burning. Finally, in the third region both the temperature and enthalpy increase slowly because of the almost total absence of radicals. In this last step, carbon monoxide is oxidised to carbon dioxide and radical species combine into more stable molecules. Finally, the system reaches the so-called adiabatic temperature (Tad), i.e., the temperature at which the heat release to the surroundings stops. The theoretical treatment for the computation of the ﬂame speed starts with the assumption that within zone I the heat coming from Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 6 S. Farris et al. / Polymer xxx (2010) 1e15 ZONE I ZONE II which assumes the total mass per unit area entering the reaction zone is equal to the mass consumed in that zone for the steady ﬂow problem being considered. In Equation (15), u is the reaction rate in terms of concentration (g cm3) per unit time. Equation (14) can, therefore, be rewritten as: ZONE III Tad Tf Temperature SL ¼ h0 i a m2 s1 2 3 4 5 6 7 Distance (mm) Fig. 3. Spatial evolution of temperature and enthalpy of formation in a premixed laminar ﬂame. zone II by convection equals the heat required to raise the temperature of the unburned gases to the ignition temperature (Ti). Secondly, it is assumed that the increase in temperature between adjacent gas layers is constant. In other words, this means that the slope of the temperature curve is linear, and thereby can be approximated by the expression [(Tf e Ti)/d], where d is the thickness of the reaction zone. From the enthalpy balance the following equation can be obtained: mcp ¼ l Tf Ti d A (11) where l is the thermal conductivity, m is the mass rate of the unburned gas mixture into the combustion wave, and A is the crosssectional area assumed as unity . According to the one-dimensional feature of the problem, the mass rate m can be expressed as: m ¼ rAu ¼ rASL ; (12) where r is the unburned gas density, u is the velocity of the unburned gases, and SL is the symbol for laminar ﬂame velocity. As unburned gases enter normal to the wave, by deﬁnition it can be written SL ¼ u. Therefore, Equation (11) becomes: rSL cp ðTi T0 Þ ¼ l Tf Ti . d (13) Thus the equation for the computation of the ﬂame speed can be easily inferred: SL ¼ l Tf Ti 1 rcp Ti T0 d (14) where cp is the speciﬁc heat capacity of the fuel. From Equation (14) it is possible to observe the direct relationship between ﬂame speed (SL) and ﬂame temperature (Tf), i.e., the higher the ﬂame speed, the higher the ﬂame temperature. It allows us to talk about ﬂame temperature and ﬂame speed interchangeably. Unfortunately, in the above equation, the term d (the reaction zone thickness) is unknown; nevertheless, it can be related to ﬂame speed by the following expression: ru ¼ rSL ¼ ud; (15) r 1 1 (16) W m K l ¼ rcp kg m3 J kg1 K1 Ti 1 1=2 u w a where r is the unburned gas density and a is the thermal diffusivity. More speciﬁcally: 0 1=2 l Tf Ti u rcp Ti T0 r (17) The denominator in Equation (17) is known as the volumetric heat capacity (J m3 K1). Thermal diffusivity can ultimately be deﬁned as the ratio of thermal conductivity to volumetric heat capacity. In practice, thermal diffusivity is a measure of the ability of a given substance (or a mixture, as in the case of a ﬂame) to rapidly adjust its temperature to that of the surroundings. Since the mass of reacting fuel mixture consumed by the laminar ﬂame is given by: rSL w l cp 1=2 u (18) combining Equations (15) and (18) yields the following expression: dw a SL (19) From Equation (19) the average thickness of the luminous zone for a laminar ﬂame can easily be drawn. Since, for hydrocarbon ﬂames, the value of a (at a mean temperature of 1300 K) and SL can realistically be approximated to 5 cm2 s1 and 35e40 cm s1, respectively, d assumes values close to 1.0e1.5 mm. As will be discussed later, this aspect has a valuable practical consequence to fully exploiting the beneﬁt of a ﬂame treatment during the surface activation of polyoleﬁn substrates. Equation (19) also highlights the inverse proportion between the thickness of the luminous zone and ﬂame speed. Thus, ﬂame speed (i.e., ﬂame temperature) should always be adjusted to a certain value of d to treat the samples in a feasible fashion. This can also be achieved by setting the value of thermal diffusivity a, since increasing thermal diffusivity leads to an increase in ﬂame speed, as inferred from Equation (16). Therefore, for high values of a the quality of the combustion system will be enhanced due to an increase in ﬂame temperature, which corresponds to an increase in ﬂame treatment yield. An adequate value of a can be achieved by reducing the volumetric heat capacity of the mixture (i.e., the denominator of Equation (17)), which can be obtained by decreasing the speciﬁc heat capacity of the fuel (cp). To do so, common practice is to replace nitrogen in the fuel mixture with other lower cp diluents such as argon or helium. It has been reported that when helium is added to a stoichiometric methane/ air mixture, the ﬂame speed is roughly threefold higher than using nitrogen (w125 cm s1 vs. w40 cm s1) [26e28]. Another aspect that should be pointed out is the effect of pressure on the ﬂame speed of a stoichiometric air/gas mixture. The pressure dependence of ﬂame speed is described by the following equation : 1=2 SL w pðn2Þ (20) where n is the overall order of the reaction. Therefore, for a given second order reaction, ﬂame speed seems to be independent of Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 S. Farris et al. / Polymer xxx (2010) 1e15 pressure. However, by contrast, hydrocarbon/air reactions are rarely second order. Indeed, experimental data collected by several investigators suggest that the order of a general combustion process mostly falls around 1.75 . This is why a reduction in ﬂame speed is encountered with increasing pressure. A deeper comprehension of this phenomenon can be achieved by looking at the most important oxyhydrogenation reaction governing the formation of the radical pool, i.e.: refer reaction (2a) Any reaction that inhibits the formation of H atoms or competes with the above mechanism will hinder the oxidation process, and thereby the combustion rate. For instance, the reaction: H þ O2 þ M / OOH þ M (21) clearly competes with Reaction (2a). Moreover, since it is a third order reaction, it is much more pressure-dependent than Reaction (2a). The ultimate relevant consequence is that when increasing pressure, Reaction (21) tends to slow down the overall combustion process and, thus, ﬂame speed. Results from analytical calculations of ﬂame speeds under different temperature/pressure conditions with detailed kinetic aspects can be found in the literature [30e33]. Moreover, it has to be mentioned that the decrease in SL with increasing pressure becomes more pronounced for pressures above atmospheric conditions (1e5 atm). This is because, contrary to what happens at high pressures, below 1 atm Reaction (21) does not compete with Reaction (2a), and any decrease owing to Reaction (21) is balanced by a rise in temperature due to chain branching step reactions such as (2a). At the end of this section, a ﬁnal remark deserves to be stressed as far as laminar ﬂame propagation is concerned. It is nowadays accepted that although diffusion phenomena dominate in initially unmixed fuel/oxidiser systems, reaction rate mechanisms prevail in premixed homogeneous mixtures. It is worth emphasising that ﬂame propagation is mostly because of the diffusion of heat and mass, i.e., it is made possible by a diffusion mechanism predominantly. The role of the reaction rate is instead intimately related to the thermal proﬁle of the laminar ﬂame, since it governs the thickness of the reaction zone and temperature gradient. In other words, although the strong effect of the temperature is undisputable, ﬂame propagation has to be primarily attributed to the diffusion of heat and mass. It is deﬁnitively expressed by the following expression: SL wðaRRÞ1=2 (22) This states that the propagation rate is proportional to the square root of the diffusivity and the reaction rate . 6. Flame treatment of polyoleﬁns The term polyoleﬁn encompasses all those polymers produced by an oleﬁn as a starting monomer, whose general formula is CnH2n. Most common polyoleﬁns in the packaging ﬁeld are polyethylene (PE) and PP. Although they have different speciﬁc properties, it is recognised that both polymers are inherently hydrophobic, which is in turn responsible for their typical poor wettability, especially to waterborne systems. For this reason, polyoleﬁns generally need to be surface-activated before the deposition of inks, paints, adhesives, metals, and coatings. Flame treatment is a valuable technique to improve the surface energy of polyoleﬁns, although it has been exploited to a minor extent with respect to corona treatment so far. However, because of improvements in safety conditions as well as in some technical aspects, it is receiving renewed attention, especially by those sectors (e.g., packaging) that historically lagged behind in the exploitation of the technique. It has been reported that the surface activation of polyoleﬁns by ﬂame treatment is based on the free radical degradation 7 mechanism, which occurs at the tertiary carbon of the PP chain and according to a random attack in the case of PE . Two main steps are involved in the oxidation process of PP: 1) the breakage of the CeH links along the polymer surface by the high temperature generated by the combustion process (w1700e1900 C); and 2) the insertion of oxygen-based groups corresponding with the broken links, leading to newly available hydrophilic sites for the interaction between coating and substrate. In particular, the oxidation of methyl groups (eCH3) into eCH2OH groups following treatment has been judged the most relevant surface chemistry change affecting both the wettability and adhesion properties of polyoleﬁn substrates . The generally accepted scheme is reported below: RH/R$ þH (23) R$ þ O2 /ROO$ /ROOH/oxidised products (24) It seems that the oxidation process is principally mediated by the OH radicals in the ﬂame. To elucidate the chemical changes onto the polyoleﬁns’ surface following ﬂame treatment, several techniques have been used. In particular, X-ray photoelectron spectroscopy, also called ESCA (electron spectroscopy for chemical analysis), and static secondary ion mass spectroscopy (SSIMS) have conﬁrmed an increased level of oxidation, as demonstrated by new functionalities formed on the polyoleﬁns’ surface, such as hydroxyl, carbonyl, and carboxyl groups [35e37]. However, it has been ascertained that, working conditions being equal, more oxygen is incorporated onto PE ﬁlms than PP ﬁlms after ﬂame treatment. In addition, it has been proven that the majority of the oxygen added to PP by the ﬂame is in the form of hydroxyl species, which account for approximately 20e30% . Nitrogen ﬁxation has also been detected as a consequence of treatment, although it seems to occur on PE samples rather than PP. Nevertheless, the ﬁxation of nitrogen is quantitatively less important than oxygen ﬁxation, as revealed by ESCA measurements (N/C atomic ratios < 0.03; O/C atomic ratios > 0.1e0.2) . The mechanism responsible for the modiﬁcation of the PP surface caused by the hydrocarbon ﬂame has been brilliantly elucidated by Strobel and co-workers . Arising from their work, it seems that the polymer radical formation occurs primarily by hydrogen abstraction because of the free radicals in the ﬂame, such as O atoms, H atoms, and OH radicals, according to the reactions (3a) and (3c) and following reaction : RH þ O/R$ þ OH (25) where R is an alkyl radical. Not only can the radical species in the ﬂame provoke polymer radical formation, but also so can the thermal effect according to the mechanism: RH/R$ þ H (26) Based on the results obtained using a combustion mode , and considering that the reactivity of the H atom for hydrogen abstraction is three to ﬁve orders of magnitude inferior than the reactivity of O and OH , the authors concluded that, at a speciﬁc equivalence ratio of 0.93, OH radicals, O atoms, and heat are the driving forces for polymer radical formation. Most alkyl radicals formed during the previous steps (Eqs. (25) and (26)) react with oxygen atoms, generating polymer alkoxy radicals : R$ þ O/RO$ (27) It is well established as such polymer alkoxy radicals (RO ) are the main species involved in the chain backbone scission of PP during oxidation through the well-known b-scission reaction (Fig. 4). Surface oxidation can also take place by additional routes; however, these tend to be less important than the aforementioned Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 8 S. Farris et al. / Polymer xxx (2010) 1e15 direct reaction with atomic oxygen. For example, the alkyl radicals (R ) can be attacked by molecular oxygen (O2), yielding peroxy polymer radicals (ROO ), which in turn can abstract hydrogen from other polymer chains to produce polymer hydroxyperoxides (ROOH). All of these intermediates (alkoxy, peroxy, and hydroperoxy) can originate a large variety of oxidised species reacting with atomic oxygen, OH radicals, or even through cross-reaction with intramolecular polymer radicals . Arising from these different reaction mechanisms, a wide range of new chemical groups can be inserted onto the polyoleﬁn backbone. In particular, the formation of hydroxyl, carboxyl, and carbonyl groups is the most relevant concerning the increase in the wettability ad adhesion properties. Finally, it is worth stressing the heterogeneity of oxidation on the polyoleﬁn surface. This has been attributed to the different physical domains in a typical semi-crystalline polymer such as PP. More speciﬁcally, it seems that the regions most susceptible to treatment are those amorphous rather than crystalline. This fact would justify the scarce homogeneity in the extent of the oxidation, which is the basis of the hysteresis phenomenon that can be observed during contact angle measurements on ﬂame-treated PP ﬁlms. 7. Flame treatment equipment Although conceptually similar, ﬂame treaters used in packaging industries for polyoleﬁn surfaces show obvious differences depending on whether the sample to be treated has a twodimensional or three-dimensional geometry. In both cases, three main components can be recognised. For 3D objects, the plant typically consists of (Fig. 5a): (3) a nip roll, which is usually rubber-coated. Its function is to exert a certain pressure on the ﬁlm to ensure the necessary contact between the web and the cooled roll. This prevents the formation of bubbles and/or blisters, which might otherwise impede the right thermal exchange between the web and the treater roll. Certainly, the core of a typical ﬂaming system is the burner. Nowadays, burners are complex parts affecting strongly the outcome of the whole process. Despite the wide range of burners available on the market, a common feature is the system that delivers the gas/air mixture to the burner nozzle (head) by exploiting the still valid principles developed by Venturi and Bunsen. Such a system, generally known as Venturi mixer, is located a few metres upstream of the burner. Burners fall into two main groups. Atmospheric burners are so called because part of the air used to generate the premixed fuel/air laminar ﬂame is from the surrounding atmosphere, and is thereby at atmospheric pressure. This is because the gas entering the oriﬁce at the base of the mixing tube is at low pressure (only a few inches of water column), providing only approximately 50% of the required air for the combustion. Consequently, the remainder is drawn from the environment around the nozzle, where the free air is usually conveyed by openings near the burner. An example of atmospheric air is the Bunsen burner. Contrary to atmospheric burners, power burners provide a powerful (1) a conveyor belt, which allows a continuous loop of material, i.e., the polyoleﬁn objects, which are normally mounted on heatresistant holders; (2) a cleaning device, such as a stream of compressed air or a brush-like system. This is normally placed a few centimetres in front of the burners to assure the removal of all small particles (e.g., dust) that might negatively affect successful ﬂame treatment; and (3) a burner, i.e., the basic part of the equipment that produces the oxidising ﬂame. A typical plant for the ﬂame treatment of polyoleﬁn ﬂexible ﬁlms (Fig. 5b) is instead conceived as follows: (1) a burner, which should produce a suitable ﬂame for treating the surface of the web; (2) a treater roll, which is normally water-cooled. This enables the rewinding of the treated ﬁlm and prevents any unwanted damage due to overheating; and Fig. 4. Schematic representation of a b-scission reaction on a polyoleﬁn backbone. Fig. 5. Schematic representation of a ﬂame treatment station for polyoleﬁn a) tridimensional objects and b) ﬂexible ﬁlms. Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 S. Farris et al. / Polymer xxx (2010) 1e15 source of combustion air, making it possible to achieve higher energy output compared with atmospheric burners. In an attempt to fulﬁl market requirements, different burners have been designed and developed over time, and a large variety of conﬁgurations are currently available. Gun-type nozzles were especially developed for the ﬂame treatment of three-dimensional objects, where part of the gas/air mixture is deviated into small holes at a speed that is gradually reduced until continuous ignition is provided to the main gas/air ﬂux coming out of the central oriﬁce. This makes it possible to increase the velocity of the laminar ﬂame out of the head of the burner, thereby achieving the targeted heat output. The burners used for ﬂaming ﬂexible ﬁlms, e.g., polyoleﬁns for the packaging industry, are based on a similar principle. In this case, the need to spread the ﬂame on a wider front (i.e., equal to the width of the roll) led to developing pipe-like nozzles with a long array of drilled holes emitting the ﬂame. On each side of this main row of drilled holes are smaller oriﬁces, above which deﬂectors control the speed of the ﬂame. So-called ribbon burners represent the last generation of burners available on the market. They consist of a regular shaped slot mounted with a dimpled geometry ribbon stack. Such a design can reduce the speed of part of the gas/air mixture without needing devices such as deﬂectors or ignition rails. To date, the ribbon burner is the most widely adopted solution at an industrial level because it can attain customised ﬂame patterns by adjusting the width of the slot and conﬁguring the ribbons . 8. Flame treatment variables 8.1. Process variables 8.1.1. Gas/air ratio The molar ratio of the fuel to the oxidiser is probably the most important parameter within the ﬂame treatment process. For this reason, particular care must be paid to setting it adequately before the ﬂame treatment is started. For each gas there exists a speciﬁc and well-deﬁned amount of oxidiser at which the fuel is completely burnt. This precise ratio is known as the stoichiometric ratio, which relies on the chemical structure of the gas. For example, the stoichiometric ratio methane/air by mass is equal to 1:17.2, whereas for a propane/air ﬂame it is 1:15.5, i.e., 15.5 kg of air is needed for the complete combustion of 1 kg of propane. However, in practical applications it is unlikely that the stoichiometric ratio can be veriﬁed. Most probably, the ﬂame obtained will be below or above this value. Therefore, the concept of the equivalence ratio (f), deﬁned as the actual mass gas/air ratio used during treatment divided the stoichiometric fuel-to-oxidiser ratio , is widely accepted: 8.1.2. Mixture ﬂow Based on the previous discussion, it is necessary to expose the polyoleﬁn surface to a certain amount of thermal energy (heat) to achieve the desired activation of the web surface. Deﬁning this quantity is not an easy task because the thermal energy required during the ﬂaming process strongly relies on other parameters. Among them, it is worth mentioning ﬂame power (i.e., the product of the volume of fuel burned per unit time and the heat content of the fuel, expressed in W), the exposure time of the ﬁlm to the ﬂame, the conﬁguration of the burner, and the gap between the ﬂame and ﬁlm surface. However, a practical way to control the energy supplied to the web is to adjust the mixture ﬂow (m3 h1). Increasing the mixture ﬂow leads to a corresponding increase in the treatment efﬁcacy to a certain level (Fig. 7, Q1). Any mixture ﬂow setting beyond this boundary value (Fig. 7, Q2) is proﬁtless and causes unnecessary energy waste and thermal stress on the plastic ﬁlm. Based on these principles, it has been possible to set down the relationship between mixture ﬂow and ﬂame treatment efﬁcacy in terms of the surface energy of the treated surface. In Fig. 8, the results obtained by our team for bi-oriented polypropylene (BOPP) at low and high line speeds are reported (per unit (28) where m is the mass. The most common parameter is the reciprocal of the equivalence ratio, which is called the lambda factor and is expressed by the formula: l ¼ f1 This is because although excess fuel (or oxidiser ﬂame) could never participate chemically in the combustion reaction, it does affect the system from a physical point of view since, depending on its speciﬁc heat value, such an excess tends to draw heat from the combustion system, thereby causing the aforementioned decrease in yield. In practice, the most widely adopted conﬁguration foresees a fuel/air ratio slightly shifted to an oxidising ﬂame composition (i. e., fuel-lean), because, as mentioned previously, the web surface activation strictly depends on both ﬂame temperature and oxygen radical concentration. Thus, the best working condition can often be a compromise between high ﬂame temperature and oxygen radical content in the ﬂame. It has been proven by many authors that oxidising ﬂames (0.75 < f < 1) lead to the best surface activation of polyoleﬁn substrates [37,38,44,45]. More recently, a detailed report by Strobel and co-workers  suggested the best performing equivalence ratio was 0.93 for all combinations of ﬂame-to-ﬁlm distance, ﬂame power, and ﬁlm speed using a methane/air mixture. At this optimum value, a maximum surface energy of approximately 62 mJ m2 (according to the ASTM wetting test standard method ) was achieved. Accordingly, the highest ESCA O/C atomic ratio of ﬂame-treated PP was recorded for equivalence ratio values ranging between 0.92 and 0.94, thereby following the same trend as the wettability measures. The authors concluded that such a high level of oxidation is the main reason for the increased wettability of the ﬂame-treated PP surface. (29) As a consequence, fuel-lean (oxidising) ﬂames will have f < 1 and fuel-rich ﬂames f > 1 (vice versa as far as the l factor is concerned). Unambiguously, both l and f will be equal to the unit at the stoichiometric ratio. It is worth pointing out that, for a given combustion system, the maximum yield (expressed in terms of ﬂame temperature) is achieved at the stoichiometric ratio, where neither excess fuel nor excess oxidiser can be veriﬁed. Conversely, as f veers from the stoichiometric value (below and above), the ﬂame temperature drops correspondingly (Fig. 6). Flame temperature mfuel =moxidizer mfuel =moxidizer stoichiometric f¼ 9 φ<1 fuel-lean 1.0 φ>1 fuel-rich Fig. 6. Flame temperature trend as a function of the equivalence ratio (f). Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 S. Farris et al. / Polymer xxx (2010) 1e15 Surface energy (dyne cm-1) 8.1.3. Flame/surface gap It is widely recognised that the gap between the ﬂame and web surface (i.e., the distance between the tips of the luminous ﬂame cones and polyoleﬁn surface) is a key factor in determining the extent of activation accomplished by the treatment. As a general trend, it has been observed that when the ﬁlm passes through the ﬂame, a rapid depletion in the wettability of the treated surface occurs. As the distance between the cone of the ﬂame and ﬁlm surface increases, surface activation decreases, although a beneﬁcial effect arising from the treatment is still appreciable up to approximately 20 mm. Many researchers have carried out empirical tests to set the optimum distance between the ﬂame and ﬁlm surface. Ayers and Shofner suggested that the optimum distance was 0e6 mm above the luminous ﬂame front . Sheng and co-workers pointed out that the most effective ﬂame treatment on the activation of PP webs is achieved 5e10 mm ﬁlm-to-ﬂame distance . Other authors concluded that to achieve the best wettability and oxidation of polyoleﬁn surfaces, the distance between the tips of the ﬂame cones and web surface should be less than 10 mm . The conclusions by Strobel and co-workers conﬁrm further the tendency to position the ﬁlm slightly beyond the luminous cone . The authors ﬁxed the right ﬁlm-to-ﬂame gap at 2 mm for a PP ﬁlm treated with a methane/air mixture at a 0.93 equivalence ratio. These ﬁndings are consistent with the ﬂame proﬁle theory discussed above. Indeed, to maximise the beneﬁt from the treatment, the ﬂame should work in tandem with its luminous zone, which is the richest in active oxidising species (OH radicals and O atoms) and the one at the highest temperature within the whole combustion system. Conversely, when the ﬁlm-to-ﬂame distance is set below 1.5e2.0 mm, the part of the ﬂame involved is the ‘dark zone’. Here, the contribution by the ﬂame temperature is negligible and the reactive oxidising species are almost absent. Rather, this zone has plenty of hydrogen radicals, which tend to recombine with oxygen radicals and thereby act as a limiting factor in the oxidation mechanism of the ﬁlm surface. Analogously, placing the ﬁlm surface further than 1.5e2.0 mm from the tips of the luminous ﬂame cones would mean the ﬂame treatment would be less effective than in the luminous zone. However, since both the ﬂame temperature and oxygen radical concentration are higher in this region (post-combustion) than in the dark zone, some positive effect because of the ﬂame is still detectable on the treated ﬁlm surface. This fact explains the typical aspect of the curve obtained by plotting the surface energy values as a function of the ﬁlm-toﬂame distance. As shown in Fig. 9, this curve is asymmetric with respect to the maximum surface energy value found at a ﬁlm-to- Q1 Q2 Mixture flow (m3 h-1) Fig. 7. Surface energy evolution as a function of the gas/air mixture ﬂow. a Surface energy (dyne cm-1) of burner width). Such types of plots are useful tools for pinpointing the best conditions for each speciﬁc application. 54 52 50 48 46 44 150 m/min 200 m/min 250 m/min 300 m/min 42 40 38 15 30 45 60 75 90 Mixture flow (Nm3 h-1 m-1)* b Surface energy (dyne cm-1) 10 54 350 m/min 400 m/min 450 m/min 500 m/min 52 50 48 46 44 42 40 38 30 45 60 75 90 3 105 -1 120 135 -1 * Mixture flow (Nm h m ) Fig. 8. Inluence of the mixture ﬂow on the surface energy of treated BOPP at low (a) and high (b) speeds. *Normal cubic metres per hour, equal to one cubic metre under “normal” conditions, deﬁned as 0 C and 1 atm (101.3 kPa). ﬂame distance of approximately 2 mm, indicating that the positive effect of ﬂame treatment is still somehow evident in the postcombustion zone, whereas it quickly drops to zero in the dark region. 8.1.4. Temperature and relative humidity external conditions The temperature (T) and relative humidity (RH) of the surroundings are often underestimated parameters during ﬂame treatment, but these can greatly affect the ﬁnal outcome of the process because increases in either can cause a shift in the gas/air mixture towards a fuel-rich composition, thereby provoking a dramatic change in the properties of the treated surface of the polyoleﬁn ﬁlm. The inﬂuence of both temperature and relative humidity is schematically displayed in Fig. 10. Here, it is possible to observe that the value of l diverges from its initial setting (w1.04) owing to an increase in temperature and relative humidity. This diagram was obtained from natural gas (relative density ¼ 0.59) under the hypothesis that the mixture is in the stoichiometric condition at T ¼ 20 C and RH ¼ 0%. Based on these considerations, a systematic check of the gas/air mixture is deemed necessary to keep it constant, regardless of the inﬂuence of external conditions. For this purpose, a wide variety of portable and online devices enabling the measurement of any variation in l due to changes in room conditions, are available on the market. 8.1.5. Number of sequential treatments A ﬁnal aspect that should be carefully taken into consideration is the number of treatments to which the ﬁlm surface has been Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 S. Farris et al. / Polymer xxx (2010) 1e15 8.2. Sample variables Surface energy (dyne cm-1) 8.2.1. Surface contaminations Although often underestimated, the potential presence of contaminants on the plastic surface is an important aspect to face, since it directly inﬂuences the efﬁcacy of ﬂame treatment. Probably because of the high potency associated with a ﬂame, a common misconception is that to activate a polyoleﬁnic surface, ﬂame treating it using a proper fuel/air mixture is the only prerequisite. Instead, the activation step is a necessary but insufﬁcient condition to assure durable adhesion at the polyoleﬁn substrate/coating interface. Contaminations of samples can originate from different causes, for example, the manufacturing processes and storage conditions of the polyoleﬁnic substrates. Even though they are not always easy to detect, typical residuals can be found on the surface of ﬁnished objects, such as spots of the releasing agents commonly used in the injection moulding process (e.g., silicones), additives migrated from the bulk (plasticisers, antioxidants), or, more simply, dust. Irrespective of the origin, the ﬁnal effect will be the inhibition (more or less deeply depending on the extent of the contamination) of the surface activation promoted by the ﬂame. This is because of the ‘shield effect’, whereby the contaminant screens regions of the 0 1 2 3 4 5 6 7 8 9 10 Film-to-flame distance (mm) Fig. 9. General trend of the surface energy values of ﬂame-treated polyoleﬁn ﬁlms as a function of the ﬁlm-to-ﬂame gap. 1,00 1.00 0,90 0.90 λ value submitted. Although it strongly depends on other aspects (i.e., ﬂame temperature, ﬂame ﬂow, ﬂame-to-ﬁlm distance), some general considerations can help carry out the appropriate treatment. Contrary to what common sense might suggest, increasing the number of treatments in the same sample does not imply a proportional increase in the surface properties of the polyoleﬁn surface. Indeed, in particular when high temperatures are reached, over treatment lead to surface reorganisation in the modiﬁed polymer surface. Two different phenomena have been highlighted in this respect . On one hand, as a result of over treatment, the oxygen-containing functional groups inserted in the ﬁrst step of treatment can disappear from the surface. On the other hand, high temperatures can trigger the migration of the additives normally included in polyoleﬁn compounds, such as heat stabilisers, release agents, antistatics, and UV stabilisers. In both cases, the ﬁnal result is the same: the wettability and adhesion properties of the plastic surface are irremediably compromised and the successful deposition of paints, inks, or whatever coating will be hindered. To prevent these detrimental effects, when planning more than one treatment on the same sample it is very important to avoid excessively short time intervals between two sequential ﬂames to allow the heat generated by the ﬂame to dissipate properly. 11 0,80 0.80 RH = 0% RH = 25% RH = 50% RH = 75% RH = 100% 0,70 0.70 0,60 0.60 0,50 0.50 0,40 0.40 0 20 40 60 80 100 Temperature (°C) Fig. 10. Inﬂuence of room conditions (temperature and relative humidity) on the l value of a stoichiometric (T ¼ 20 C; RH ¼ 0%) natural gas (dr ¼ 0.59)/air mixture. polymer susceptible to chemical modiﬁcations mediated by the treatment. Therefore, following the ﬂaming, a lower amount of chemical modiﬁcations will be found per unit of the treated area. As an ultimate consequence, the deposition of whatever coating will be dramatically affected in those zones of the plastic substrate lacking adequate wettability. To counteract these considerations, the proper cleaning step of the polyoleﬁn surface should be always planned, especially for long-term adhesion durability. This can be achieved in different ways. Among them, blow-off dust devices (generally in the form of brush), nitrogen gas steam, and solvent degreasing are the most widely used strategies. The ﬁnal choice greatly depends on the shape of the samples and speciﬁc manufacturing constraints. 8.2.2. Topography of the surface It is well established that the wettability of a polymer surface is strongly affected by its topography. In this respect, two major theories can explain the effect of the roughness of the surface on its wettability behaviour: the Wenzel theory [49,50] and the CassieBaxter theory , which differ from Young’s theory that applies only to perfectly smooth surfaces . Although the surface morphology affects the wettability properties, the extent of the ﬂame treatment also seems to be inﬂuenced by this parameter. Our preliminary results corroborate this hypothesis. Injection-moulded PP (nucleated heterophasic copolymer, Basell Polyoleﬁns srl, Ferrara, Italy) square plates (40 mm width, 3 mm thick) at different topographies (highly rough e H, medium-sized roughness e M, and perfectly smooth e S) were analysed by atomic force microscopy (AFM) before (Fig. 11) and after (Fig. 12) ﬂame treatment. The three untreated samples exhibited a noticeable difference in topography. The smooth PP plates had an RMS roughness of approximately 390 nm, whereas the mean roughness of the M and H samples was in the order of 550 nm and 1.34 mm, respectively. However, apparently out of line with the aforementioned theories, both water contact angle and surface energy values of the three untreated samples (103.5 2.51 and 28.74 0.64 dyn cm1 for S samples, 103.1 2.21 and 29.08 0.72 for M samples, and102.3 2.66 and 29.37 0.88 dyn cm1 for H samples) were quite similar, presumably because the differences in roughness between samples were too narrow to justify statistically signiﬁcant distinctions. When subjected to the same ﬂame treatment (propane/air mixture with l ¼ 1.028; ﬂame contact time ¼ 0.05 s; ﬁlm-to-ﬂame distance ¼ 2.0 mm), all samples revealed a distinct reduction of RMS roughness, which amounted to 270 nm, 340 nm, and 490 nm for samples S, M, and H, respectively. Noticeably, a clear dependence of the surface response to a given ﬂame treatment on the average roughness was found by optical contact angle and surface energy measurements, which amounted Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 12 S. Farris et al. / Polymer xxx (2010) 1e15 Fig. 11. Left column: 100 100 mm2 AFM height images of: a) perfectly smooth e S; b) medium-sized roughness e M, and c) highly rough e H polypropylene untreated (nonﬂamed) samples. Right column: proﬁle along the dashedotted line from the corresponding height image. to 72.98 4.8 and 39.19 0.67 dyn cm1 for S samples, 50.23 3.6 and 44.53 0.58 dyn cm1 for M samples, and 40.43 2.24 and 48.89 0.75 dyn cm1 for H samples. A clear trend is therefore demonstrated, with the roughest surface being also the most sensible to ﬂame treatment (i.e. leading to the largest variations in its own wettability properties). Although the total effective exposed surface area for the untreated rough samples is not considerably larger than that of smooth samples (less than 10% difference), a tentative explanation for this trend should likely consider that the amount of polyoleﬁnic substrate exposed to the ﬂame (per unit area) increased proportionally to the roughness of the sample. According to this hypothesis, the roughest samples would be oxidised to a larger extent than the smoothest ones. It is also worth noting that AFM images of treated samples clearly revealed, within our spatial resolution, that other relevant structural changes occurred at the surface of S samples (Fig. 12a), with the appearance of small, evenly distributed agglomerates on the treated surface, with dimensions in the order of 0.5e1.0 mm in height and few microns in width (Fig. 13). On the contrary, height images captured from samples M and S did not show any apparent evolution from this point of view after the treatment (Fig. 12b and c, respectively). This observation suggests a further likely scenario. Owing to the ﬂame treatment, it might be plausible that the S samples underwent a reorganisation at the surface level, as already postulated in an earlier paper . Whether such modiﬁcations rely on the migration of additives from the bulk to the surface of the polymer because of the high temperature or on the disappearing of oxygen-containing groups from the surface is still unknown. X-ray photoelectron spectroscopy, confocal Raman microscopy, and FTIRATR spectroscopy analyses currently carried out within our group should provide our ongoing research with further elucidations. Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 S. Farris et al. / Polymer xxx (2010) 1e15 13 Fig. 12. Left column: 100 100 mm2 AFM height images of: a) perfectly smooth e S; b) medium-sized roughness e M, and c) highly rough e H polypropylene ﬂame-treated samples. Right column: proﬁle along the dashedotted line from the corresponding height image. Fig. 13. Magniﬁed topography of an aggregate from the height image in Fig. 12a and corresponding section along the dashedotted line. Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 14 S. Farris et al. / Polymer xxx (2010) 1e15 9. Concluding remarks Flame treatment is a powerful technique for enhancing the surface attributes of plastic materials, especially those with a marked inherent hydrophobicity such as polyoleﬁns. However, its potential has not been completely capitalised so far for two main reasons: 1) the lack of familiarity with the principles governing the combustion phenomena; and 2) the high number of parameters affecting the overall ﬂame treatment process, which make the initial tweak of the ﬂame equipment time consuming and frustrating, especially compared with alternative techniques such as the corona discharge, which is nowadays widely used in speciﬁc applications such as the treatment of polyoleﬁn ﬁlms intended for packaging applications. Although it has not been possible to address all topics related to the ﬂame phenomenon, this review has attempted to provide the basic tools to rationally exploit ﬂame treatment at both an industrial and academic level. Our discussion was based on some major guiding principles. Firstly, without knowing the underlying fundamentals of ﬂame chemistry it is difﬁcult to manage the ﬂame phenomena in any application. Secondly, knowing the most important controlling factors of the overall process and being aware of how these parameters can affect the ﬁnal outcome is of utmost importance to gain the maximum beneﬁt from the treatment. Thirdly, it is essential to understand how to control the process variables to keep the ﬂame treatment setting as standardised as possible, because even minimal changes can cause huge deviations in the expected results, i.e., the low surface activation of treated surfaces. Therefore, controlling accurately all parameters throughout the process represents a major task that cannot be procrastinated longer in any industrial application envisaging using ﬂame to activate polymer surfaces. It is important to stress that although generally valid, the concepts outlined in this review do not apply in any circumstance; hence, some aspects need to be faced separately depending on the speciﬁc application. For example, the inﬂuence of the substrate has to be regarded carefully, since different polyoleﬁn types are affected in different ways by modiﬁcation treatment. Therefore, tailored operative conditions have to be pinpointed accordingly. A systematic approach to using ﬂame as a surface-activation technique is not only necessary for obtaining reproducible results but would decisively encourage the future development of new structures. This notion is supported by strong recent research attention on the potential use of biomacromolecules in many applications, such as within the packaging industry, motivated by the growing needs for more sustainable solutions. To address this issue, many researchers have suggested a way of generating new optimised structures, in which the use of plastic resins should be less of a driving force to lighter conﬁgurations without jeopardising the overall performance of the package. This can be attained by replacing multi-layered architectures with high performance thin coatings. In addition, recent advancements in the coatings ﬁeld have provided the opportunity of fabricating composite structures by laying plastic substrates with water-based bio-coatings (i.e., obtained from molecules of natural origin). Among other beneﬁts, this would allow cleaner processes, since the use of organic solvents normally used for synthetic coatings is avoided. However, the deposition of totally waterborne coatings onto polyoleﬁn surfaces is a tough target because of the higher surface tension of water-based coatings compared with current formulations. With this scenario in mind, a remarkable contribution could arise from ﬂame treatment becoming a leading technique for the surface activation of inherently hydrophobic polymers. 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Rosario R, Gust D, Garcia AA, Hayes M, Taraci JL, Clement T, et al. Journal of Polymer Science e Part B: Polymer Physics 2004;108:12640e2.  Wu X, Zheng L, Wu D. Langmuir 2005;21:2665e7.  Morra M, Occhiello E, Garbassi F. Advances in Colloid and Interface Science 1990;32:79e116. Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 S. Farris et al. / Polymer xxx (2010) 1e15 15 Stefano Farris is a post-doctoral researcher in the Department of Food Science and Microbiology (DiSTAM) at the University of Milan e Packaging Lab. He received his M.S. in Agricultural Science & Technology from the University of Sassari, Italy, in 2004. In the same year he joined the Food Packaging Lab (Packlab) led by Prof. L. Piergiovanni at the University of Milan. There, in 2007 he earned his PhD in Food and Microbial Biotechnology in collaboration and by the ﬁnancial support of the University of Sassari. His thesis defense focused on the different packaging strategies to extend the shelf-life of multi-domain foods. From 2007 to 2008 he was a postdoctoral fellow at Rutgers, The State University of New Jersey e Department of Food Science, in the Food Packaging Lab led by Prof. K. Yam, where he worked on the development of new ﬁlms and coatings partially or totally obtained from renewable resources. His current research activity at Packlab is devoted to the development and deposition of new high-performance waterborne bio-coatings. After obtaining his degree, Lamberto Duò worked as a postdoc at the Surface Science Centre of the University of Liverpool (UK) on electron spectroscopies of metal alloys. He was then appointed as a staff scientist to the Physics Department of the Politecnico di Milano, where, in 1999, he became associate professor of physics. Professor Duò worked on highly correlated electron systems, spin resolved electron spectroscopies of magnetic systems with low dimensionality, and scanning probe microscopies with a special emphasis on scanning near-ﬁeld optical microscopy. He is author of over 120 publications. Simone Pozzoli was born in 1986. He is a master student in the Department of Food Science and Microbiology (DiSTAM) at the University of Milan e Packaging Lab (Packlab), under the supervision of Professor L. Piergiovanni. His research activity is focused on the surface activation of polyoleﬁns by ﬂame treatment and new alternative routes. Currently, his activity at Packlab is granted by industrial partners (Mitaca srl and esseCI srl). Paolo Biagioni obtained his Ph.D. in Physics at the Physics Department of the Politecnico di Milano, under the supervision of professor Lamberto Duò. After that, he worked as a post-doc researcher in Milano and then at the Physics Department of the University of Würzburg, in the group of professor Bert Hecht. At present, he holds a research position at the Physics Department of the Politecnico di Milano. His main interests are in scanning probe microscopy and plasmonics. He is author of over 35 publications. Born in 1950, Luciano Piergiovanni is full professor of “Food Science & Technology” in the Department of Food Science & Microbiology (DiSTAM), University of Milan. He is the head of the Packaging Laboratory (Packlab), where he coordinates different research activities dealing with some major topics, such as modiﬁed atmosphere packaging of perishable foods, modeling and forecasting of foods shelf-life in ﬂexible packaging, validation of new packaging materials and techniques. He is responsible for the PhD Program of Food Science Technology and President of the Italian Scientiﬁc Group for Food Packaging (GSICA). He has been visiting professor in the Universities of San Paolo (Brasil) and Santafè de Bogotà (Colombia). He is author of two text books on food packaging, 3 chapters in international text books, 7 patents in the packaging ﬁeld and more than two hundred works among scientiﬁc publications, communications to conferences, technical and popular articles. He coordinated research teams in projects sponsored by National Institutions and European Union. He belongs to the Editorial Board of Packaging Technology and Science, Industria delle Conserve, Croatian Journal of Food Science and Technology and Brazilian Journal of Pharmaceutical Sciences. Please cite this article in press as: Farris S, et al., Polymer (2010), doi:10.1016/j.polymer.2010.05.036 Stefano Mancinelli earned his M.S. in Materials Engineering at University of Perugia in 1998. In his over 11 years employment in esseCI srl company, he has acquired a deep experience in the ﬁeld of ﬂame treatment as process engineer involved in plants commissioning and post sale customer service all over the world. Currently, he is company process manager and pilot plant responsible.
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