Surface Modification Of Polyethylene For The Packaging Industry Biology Essay

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Abstract: Although polyethylene has desirable bulk properties, suitable costs and appropriate processability, the surface of the material is chemically inert and non-porous with low surface tensions. As with more plastic materials, polyethylene exhibits a very low surface energy, resulting in poor surface adhesion properties. In order to improve the surface adhesion of polyethylene and to make it more receptive to bonding with other chemicals, surface modification has to be introduced. Surface treatments can serve different purposes such as surface functionalization, surface cleaning or etching, and surface deposition. Surface functionalization is used to modify the surface layer of a polymer by inserting some functional groups onto the surface in order to improve its wettability, sealability, printability, dye uptake, its resistance to glazing, or its adhesion to other polymers or metals, while maintaining the desirable bulk properties of the polymer. The adhesion quality of the surface towards specific coatings strongly depends on the nature of the chemical groups attached and on the surface of the substrates. In this paper, different surface modification techniques will be investigated.

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Introduction

History

In 1936 the polymerization process of ethylene to polyethylene was introduced. A relatively low molecular weight polymer was born. Under a relatively high pressure, it was possible to polymerize Low Density polyethylene (LDPE), with a density of 0.915-0.94 g/cm3.

It took nearly 20 years to produce a higher density variant. In 1953, after introduction of the Ziegler-Natta catalysts, High Density polyethylene (HDPE) was born. The density of this polymer was slightly higher than that of HDPE, namely 0.945 g/cm3 [1].

Properties

Polyethylene is a thermoplastic polymer, with a glass transition temperature of -78°C and a melting temperature around 100°C.

The mechanical properties depend on the molecular weight of the polymer and the degree of branching. Polyethylene has a high elasticity, good cold resistance and a good water vapor barrier.

LDPE has a relatively low gas, aroma and fat barrier. However, with increasing density, the quality of the barrier increases, and the hardness, stiffness and strength of the material increase due to higher crystallinity.

Increasing density also has some adverse effects: toughness, impact resistance, stress-cracking resistance, cold resistance and transparency decrease.

Polyethylene is processed at temperatures up to 300°C. The polymer is stable at these temperatures under inert atmosphere, with an oxygen concentration as low as possible, to prevent from oxidation.

In cases oxidation is necessary, e.g. to improve adhesion of printing inks on coated paper, flame or corona treatment can be used [1].

Price

The current market price of both low and high density polyethylene is around 1300 U.S. dollars per ton, the price however is highly depending on the crude oil and natural gas prices [2-3].

Annual production

The annual production of polyethylene was 40 million tons in the early 1990s. The production is estimated to be doubled in the last two decades, resulting in a current yearly production of 80 million tons [1].

Applications

In order to explore the applications of polyethylene, four types have to be distinguished: LDPE, LLDPE, MDPE and HDPE.

LDPE is mainly used for production of films with a thickness of several micrometers. Typical applications are plastic bags, foils, films for industrial purposes, films for laminating, coatings for drink packages and shrink films.

Linear LDPE (LLDPE) has a much higher flexibility, conformability and tensile strength than LDPE. On top of that, LLDPE is softer. For applications where a thin film needs to have a high amount of strength, LLDPE is very well suited. Applications range from plastic sheets to toys and covers around cables.

Medium Density Polyethylene (MDPE) has properties of both LDPE and HDPE. It is stronger than LDPE, more chemically resistant and more tear resistant. It is commonly used for pipes, screw closures and food tanks.

HDPE is mainly processed in plastic containers and injection molded products. Gas/oil tanks, pipes and food packaging are frequently made of polyethylene [1].

Modification methods

Gamma irradiation

The surface modification method first discussed is the Gamma irradiation method. For this irradiation method, Co60, a synthetic radioactive isotope of cobalt is used. Co60 decays by beta decay to the stable Ni60, emitting two gamma rays (Energies: 1,17 and 1,33 MeV).

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The principle of gamma irradiation, in this case irradiating polyethylene, results in radical carbon-atoms. These atoms are very susceptible for bonds with other molecules. In figure 1 is an example of a radical reaction shown. The reaction scheme for the reaction between polyethylene and acrylamide is not given in the concerning paper on radiation grafting of polyethylene, however the depicted reaction might very well occur[4].

Figure 1: proposed reaction scheme Polyethylene & Acrylamide

In practice, three variables are considered in gamma irradiation. These factors are briefly described in the following section.

Irradiation dose

An higher irradiation dose implies higher energy absorption per unit mass material. This variable is basically the multiplication of the irradiation dose rate with the irradiation time. As shown in the literature, higher irradiation dose results in higher grafting yield[4]. This effect is fairly predictable, since higher irradiation doses can result in the creation of more radicals, leading to a higher grafting yield. In order to increase the grafting yield, both the irradiation dose and the monomer concentration in the grafting solution can be increased[6-7]. As shown in figure 2, the grafting yield roughly doubles with increasing the irradiation dose from 4 kGy to 10 kGy.

Figure 2: Grafting yield as a function of irradiation dose [4]

Irradiation time

Irradiation time also influences the characteristics of polyethylene. As shown in figure 3, the contact angle of the of the polymer decreases with irradiation time, up to a certain time, in presence of acrylamide [5]. Unmodified polyethylene has a contact angle around 85°, while irradiation for 5 hours with 0.23 kGy/h results in a contact angle around 25°. Lower contact angle implies higher hydrophilicity and might lead to higher surface tension, very important for improving bonding-characteristics with other materials. However, having a low contact angle (hydrophilicity) does not definitely imply high surface adhesion. The chemical structure of the grafted groups on the surface might be very important in this aspect.

Figure 3: Contact angle as a function of irradiation time (PS: â-¡, LDPE: Δ) [5]

Irradiation dose rate

Likewise very interesting for commercial production of polyethylene is the balancing between short irradiation times with high irradiation doses, versus long irradiation times with lower irradiation doses. When producing commercially, time is of essence, resulting in certain time constraints. One might opt for the option with high doses, however research on this subject shows some corresponding disadvantages [4]. When irradiating with a certain dose, lower dose rates lead to higher grafting yields, than higher dose rates. In both cases the same total irradiation dose is applied. Apparently, since the polymer is exposed for a longer time to the gamma-rays, the number of monomers diffusing towards and interacting with the surface is higher. When grafting acrylamide on polyethylene, increasing the irradiation dose rate from 0.56 kGy/h to 1.8 kGy/h results in a decrease in grafting yield of 50%, when applying the same total irradiation dose. The previous implication is highly undesirable, since it results in the necessity for long irradiation times in order to reach higher grafting yield.

Tensile properties

Table 1: Influence of radiation dose on tensile properties of polyethylenes [4].

In commercial production of polyethylene the mechanical properties are of high importance. In order to study the bulk properties of irradiated polyethylenes, several experiments are conducted [4]. The tensile properties of LDPE, HDPE-1 and HDPE-2 are studied under two different irradiation doses, 10 and 80 kGy. Irradiation up to 10 kGy has shown to give no significant bulk property changes.

At higher doses, both tensile strength and elongation increase for LDPE. The increase in tensile strength on the one hand might be attributed to partial crosslinking of the LDPE, the increase in elongation on the other hand might be attributed to breakage of carbon bonds in the chain, leading to lower average chain lengths.

On the contrary, for both HDPE types the elongation at break decreases at higher irradiation dose as the polymer increases in brittleness with overtreatment. Tensile strength of HDPE-1 decreases at higher irradiation dose, while tensile strength of HDPE-2 increases. This probably has something to do with the increase in crystallinity in HDPE-1, while the tensile strength of the higher molecular weight HDPE-2 increases due to crosslinking of the polymers.

Flame Treatments

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Flame treatment is widely used in modifying surfaces of polymers. The surface modification is realized by introducing oxygen (containing polar groups) onto surfaces of polymers (such as PE, PP, PET, etc.) which results in enhanced wettability, printability and adhesion. As the polymer of research (PE) has poor adhesion properties, the improved properties are caused by the formation of polar groups on the surface due to the flame treatment. Advantages of flame treatment are the easy construction and the simplicity of the system. The process equipment consists of 2 components; a burner and a fuel tank. The material to be treated determines the required number of burners, as different applications require different geometry of raw material. I.e. plastic blow-molded bottles are commonly flame treated with three burners which results in sufficiently uniform modification throughout the product. Adjustment of the burners and rotation of the bottle combined achieves treatment of the complete bottle surface. The gas mixture that serves as feed for the burners consist of air and gas in specified ratios. The resulting oxidation on the polymer surface provides the modification of surface properties of the polymer.

X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) studies display an increased amount of oxygen containing compounds (carboxyls, carbonyls, hydroxyls) on the surface of flame treated PE. The initiation of flame oxidation by free radicals, which are in significant amounts present in the flame, are 'winning' the oxidation process over the antioxidants present in the polymer [12]. However, there are disadvantages in controlling the process as it requires precise control of;

flame composition (air/gas ratio)

air-gas flow rate

flame contact time

flame temperature

the distance between the flame and the polymer surface to control the extent of the surface modification [13].

After flame treatment, incompletely burnt compounds or chemical residues are a contamination risk. Contamination results in partial or complete loss of the desired adhesion properties caused by the formation of weak boundary layers at the interface. The treated materials are flammable when exposed to a flame with contact times of less than a second.

Corona Discharge Treatments

Corona treatment is regularly used on polymer films (most of them are polyolefins) to improve the printability and adhesion. An example is found where the polymer film moves between an earthed roller and an electrode during a corona treatment where it is subjected to a low energy (typically between 10 and 40 kV) at high frequency [14]. As a result, the air between the 2 surfaces is ionized and radicals, molecules, ions and electrons are interacting with the surface of the polymer film.

With corona treatment it is possible to attach functional polar groups on the surface of the polymer which, as mentioned before, increases the wettability, printability and adhesion. Results from XPS studies showed that with corona treatment it is possible to attach carbonyl, hydroxyl, carboxyl and amide groups on polyolefins [15, 16].

In order to obtain proper adhesion the following must be controlled [15, 17, 18]:

Voltage and frequency of EM field

Exposure time

Geometry of the to be treated material and electrode

Composition of ionized gas between the 2 surfaces

Inaccurate control of these variables has consequences; setting exposure time, voltage and frequency of the EM field too small results in insufficient amounts of functional groups on the polymer surface and decreases the printability and adhesion. Setting the same variables too high leads to decreasing peel strength and surface powdering or the formation of pinholes on the surface; in other words the material becomes (too) brittle. In contrast to the flame treatment there are disadvantages; improved results obtained with corona treatment are temporary especially with corona-treated polyolefins (high surface energy of the polar groups on the surface of the material result in a driving force to transform into a hydrophobic and thermodynamically increased stable structure) [19]. Certain polymers containing plasticizers and lubricants impose additional difficulty; these molecules will reorientate and migrate on the surface resulting in loss of enhanced properties. After corona treatment the material shows sensitivity to the handling, finger marking and dust picking as a consequence of the static electricity in the films. Corona-treated polymer films are most likely immediately used in further application such as adhesion or printability, etc. and will not be stored for a longer period.

UV Treatments

There are limited commercial applications of surface modification of polymers with UV treatment although the amount is increasing (in 1995 10 million lbs. of plastics were UV treated [20]).

UV treatment makes use of photons with high energy that enables several chemical reactions. Common in practice is the use of an initiator, able to absorb UV irradiation, that transforms from a singlet state into a triplet state. In this state it is possible for the initiator to remove hydrogen from the surface resulting in grafting possibilities. When introducing a monomer to the surface, it is possible through graft-polymerization for the monomer to occupy the empty spot of the hydrogen. Without continuous illumination of the initiator it will return to its singlet state and the reactions will diminish as it requires the initiator in the triplet state. Note that the properties of the polymers in the bulk will not change since only the initiator absorbs UV irradiation [21]. Advantages of UV treatment with an initiator are the following:

The costs of equipment and the process with a high selectivity of the photo initiator are reasonable.

Fast and nonpolluting process

Suitable for heat sensitive polymers

Requires little space

Limitations of the UV treatment include interference and scatter of UC radiation due to the presence of pigments in polymers and coatings, non or little as possible exposure of external UV sources and the emission of UV should be avoided to prevent exposure to eyes and skin. In order to obtain the desired properties with UV treatment the following variables need to be controlled:

Wavelength of UV

Type of initiator and monomer

Irradiation time

Selection of appropriate solvent in liquid phase grafting or controlling the atmospheric composition in vapor phase grafting can substantially affect the surface modification [22, 23].

According to[24, 25] typical wavelengths used to ensure grafting vary between 200 and 400 nm. The interaction between initiator and monomer determine the rate of grafting with as consequence that appropriate selection of an initiator can significantly increase the yield in grafting. Typical initiators used for graft polymerization are benzophenone and its derivatives due the capability of removing hydrogens; creating active graft sides [19, 26]. Examples of monomers used in graft polymerization are acrylonitrile, acrylamide and acrylic acid but also glycidyl methacrylate and glycidyl acrylates which contain epoxy groups. The increase in wettability and adhesion properties of i.e. PE (but also PP) are a result of all mentioned grafted monomers. The monomers containing epoxy groups allow reactions with multiple compounds and have multiple possible functionalities that can be incorporated on the polymer surface [21]. Attachment of functional groups in liquid phase grafting directly occur on the top surface layers of the polymer and therefore careful consideration should be taken when selecting a solvent as it determines the facilitation of attachment. By attaching functional groups with use of UV treatment on polymers as PE, PC, PVC, PP, PET and PS it is possible to increase adhesion and mechanical properties of these polymers. Figure 13 (discussion) illustrates this fact, adapted from a study of [22], where shear strengths of HDPE films are measured with a cyanoacrylate-based adhesive on UV irradation at 335 nm in the presence of an aqueous isopropanol solution.

Electron Beam Treatments

With electron beam treatment, the polymer is subjected to a vacuum process in which accelerated electrons, which possess high amounts of kinetic energy, form free radicals to initiate chemical reactions on the surface of the polymer. The energy from the electrons causes ionization or excitation of molecules. The process consists of a work chamber and an electron beam generating system, which both require vacuum conditions. The accelerated electrons move through a thin metal foil window, almost travelling with the speed of light, and form free radicals the moment they collide with the gaseous environment. The radicals are formed by the molecules that came in contact with the high energy electrons which initiates and propagates polymerization, providing the possibilities to deposit functional groups on electron beam treated polymers.

The extent and type of reactions occurring on the polymer surface depends on the following [27]:

Acceleration voltage required by the electron beam

Composition of the gaseous environment within the vacuum

Type of polymer

Temperature in the work chamber

Pressure

In contrast with UV treatment, electron beam treatment generates X-rays when the high energy electron comes in contact with matter. Therefore electron beam treatments require adequate shielding to prevent hazards, is more expensive, requires stricter safety demands and more space. Advantages of electron beam treatment compared with UV treatment are the following [28]:

Photoinitiators are not required

Curing of materials in the form of a curtain of high energy electrons with desired degree of shielding

No limitation of polymer film thickness

The treatment is applied on materials such as laminates and packaging materials containing pigments where no risk of residual monomer is tolerated [29]. The voltages of commercial electron beam treatments range from 20 kV up to 5 MV which is determined by the purpose. Currently, electron beam treatments have been employed to crosslink polymers, graft polymerization and curing of plastic protective and decorative coatings [30]:

Crosslinking: increase resistance to mechanical failure, thermal stability in higher temperature processes and shrinkability in heat shrinkable applications

Grafting: reduces permeability and increases adhesion of polymers

Curing: improves adhesion, printability, wettability and antistatic behavior of the treated surface and reduces swelling

Electron beam treatment in coating industry

Nowadays, in coating systems there are advantages of electron beam evaporators over conventional coating systems: there is a range of materials that can be evaporated, the process allows deposition of thick films (10 μm) on polymer surfaces and achieves a more uniform film thickness with wrinkle- and crack-free properties. The process of electron beam coating systems used in the packaging industry consists of two steps [31]: the evaporation of liquid aluminum from a crucible inside the coating equipment and deposition of evaporated aluminum onto polymer surfaces. By using a number of small area electron beam evaporators with controlled evaporation rates it is possible to achieve a higher degree of uniform aluminum film thickness over the whole coated polymer film. Recently, the treatment has been employed to deposit a thin film of glass on polymer surfaces to decrease permeability and reduce interaction of the polymer with the content of the coating.

Ion Beam Treatments

Ion beam treatment is somewhat related to electron beam treatments; ions are created by an anion source, in vacuum conditions, and are subjected to positive voltage that accelerates the ions which form a beam of ions. The ion beam enters a process chamber where the ions collide with atoms of the polymer to be treated, forcing them to leave the matrix which results in vacancies. The ions take the place of the vacancies and provide the desired surface properties, thus the purpose of the treatment determines the used ions (oxygen, nitrogen, argon and aluminum are commonly used in ion beam treatment).

Ion beam treatment offers the opportunity to improve surface characteristics as adhesion, sealability, printability and wettability. The production of ion beams differ in difficulty based on type of ions; oxygen, argon and nitrogen are relatively easy and inexpensive whereas production of ion metals are more difficult. Commonly applied voltages in ion beams range from 20kV up to 5 MV which is determined by the purpose of the treatment and the type of polymer. In polymer surface modification the voltages are roughly between 20 and 200kV. The applied voltage is of critical importance; too low results in ions travelling too slow to penetrate the polymer surface, too high results in ions travelling that fast they can penetrate (or even go through in case of thin polymer films) up to multiple micrometers into the polymer surface. Besides the voltage applied (which results in the kinetic energy of an ion) is the mass of the ions crucial in penetration depth. In practice, the ions with the highest kinetic energy and smallest mass have the deepest penetration depth [32]. Also affecting the penetration depth is the characteristics of the treated polymer (molecular weight, cross-linking, crystallinity, etc). In order to control the extent of the surface modification the following needs to be controlled [33]:

Type of gas in processing chamber

Polymer characteristics

Amount of ion energy

Beam dose

Currently, ion beam treatments have been employed on composites, minerals, ceramics, metals and polymer surfaces [33]. Ion beam treatments are able to be operated at relatively low processing temperatures which makes the treatment suitable for heat sensitive polymers. However, the following disadvantages have been reported:

Ion beams are so high in kinetic energy, often resulting in excessive surface modification and possible surface damage

High energy ion beams induce unwanted chemical reactions (strong interaction with polymer surface atoms and processing chamber walls)

More expensive and requires more space than i.e. flame, corona discharge or UV treatments

As mentioned before, the type of polymer also determines the outcome of the treatment: significant surface oxidation and possible cross-linking of PP and PS films occurred by ion implantation in the presence of oxygen and argon, whereas surface cross-linking of PET displayed little surface oxidation under the same processing conditions [36, 37]. PE surface modifications with ion beam treatments are quite similar; oxygen and nitrogen beam treatments result in the deposition of polar functional groups on the surface of PE films via ion implantation which increases the adhesion [38].

Sputtering (ion-assisted metal deposition)

Sputtering is generally done in case electron beam evaporation is unsuitable (i.e. evaporation of the metal is impossible) or when better coating adhesion properties and pinhole-free films are desired [30, 34]. The process is similar to regular ion beam treatments; ion beams of an inert gas are generated by an anion source under vacuum conditions followed by acceleration of the ionized matter onto the surface of the to be treated material. After collision, atoms of the target surface are ejected followed by the deposition of the ejected atoms on a substrate surface in the form of a thin film. Successful coating of PEs, PET, PVC, PC and PP with metals as copper, aluminum and titanium in ion-assisted metal deposition have been reported by [39]. With use of an ion beam deposition method, substantial improvements in the deposition of aluminum films onto polymer surfaces (in terms of adhesion and sealability) can be achieved in contrast to conventional heat-applied vapor deposition.

Figure 8: comparison of residual on Aluminum films after conventional or Ion beam treatment.

Plasma Treatments

In plasma treatments, a glow discharge provides ionization of gas species containing radicals, ions, electrons and excited molecules. The ionized species are contacted to the polymer at the surface at low temperature and low pressure. By reacting with the surface of by crosslinking with the polymer, the species are incorporated in the surface. A plasma reactor is shown in figure 9. The most frequently used gases for the modification are air, Ar, H2, He, O2, CO2, N2, NH2, F2, and SO2.

Figure 9: Plasma reactor [10].

Plasma treatments are commonly used increase adhesion properties, wettability and printability of polyethylene. Additionally, it helps the polymer preventing from mechanical failure. Plasma treatments are also used for applying a thin layer of polymer on packaging materials, e.g. glass or metal. These treatments have several benefits over other physical modification techniques as ion beam, flame and corona discharge. According to the literature, plasma treatments have to following benefits [11]:

Modification only on surface, leaving bulk properties unaffected. Modification typically up to a depth of 0.05µm.

Higher intensity of treatment at the surface, compared to more penetrating methods. This yields low treatment times.

Treatments are environmentally friendly.

The formed layer has a uniform thickness.

Ability of treating heat-sensitive materials.

Ability of treating three-dimensional objects.

Along with the benefits, some drawbacks are identified:

Vacuum required during treatment.

It is difficult to control the amount of functional groups created on the surface.

Processing parameters are heavily depending on a certain application.

In the area of food packaging polymers, adhesion properties of the polymers is the most extensively investigated area of research in plasma treatments. The low surface energies of polyethylene and other polymers (PP/PET) undermines the adhesion to other materials. Plasma treatments can be used to overcome this problem by increasing the adhesion properties of the polymers.

Creating and also sustaining strong adhesion is of high importance in food packaging, since contamination and leakage should be minimized [40].

Figure 10: Adhesion strength of PE, PP and PET after plasma treatment with oxygen[40].

Petasch et al. investigated the bond strength of low density polyethylene, along with polypropylene (PP) and polyethylene terephthalate(PET) [40]. The adhesion strength between polymer and adhesive is measured by pull-off tests with aluminum/epoxy/polymer/epoxy/aluminum joints. As shown in figure 10, after only 6s of plasma treatment the adhesion strength of LDPE increased from 0.4 N.mm-2 to 8.2 N.mm-2.The increased adhesion strength of polyethylene by oxygen plasma treatment is related to the incorporation of hydroxyl, carboxyl, carbonyl and ester groups in the surface layer of polyethylene. Incorporation of these groups results in higher polar interaction between the polymer and the epoxy.

Even without epoxy the adhesion between polyethylene and metal can be improved by plasma treatments. In 1995 O'Kell et al. examined the adhesion strength of plasma treated aluminum/polyethylene/aluminum laminates [41]. The laminates were prepared by direct melting and pressing the polymer onto the aluminum in absence of adhesives. Samples treated with air showed an increase in adhesion strength of around 7000%, while samples treated with nitrogen showed an increase of about 300 - 600%. As in the research of Petasch et al., incorporation of oxygen containing groups in the surface was the main reason for the increase in adhesion strength.

Increasing the adhesion strength between polyethylene and metal without the incorporation of adhesives, is commercially very attractive. Using plasma treatments might limit the use of conventional adhesives, containing volatile organic solvents. This modification yields two positive results: A reduction in environmental damage and a decrease in health risk for people involved in the production of the materials.

Another application for plasma treatment is the enhancement of barrier properties (for food packaging). Rossi et al. showed a significant reduction in O2, CO2, and N2 permeability after plasma treatment of both LDPE and HDPE in the presence of argon [42]. The permeability was found to be stable in time. The decrease in permeability was attributed to an increase in crosslinking in the polyethylene. The degree of altering the barrier properties was shown to be highly dependent on the treatment time and the crystallinity of the polymer.

Laser Treatments

The basic principle of laser treatment system is as follows; atoms are excited under high voltage from its ground state to a higher energy level. The atoms that lost electrons, causing positive ionization, move to a negatively charged cathode and the lost electrons to a positively charged anode. The energy lost when the atom returns to its ground state is emitted in the form of light with a certain photonenergy and frequency.

Laser treatment for polymer surface modification, until recently, has not been popular due to the cost of capital compared to other surface modification methods. With the upcoming of industrial scaled laser treatment systems this is an emerging sector as it became economically sustainable. The reason that in spite the high cost laser treatment is sustainable is caused by the superior precision in the surface modification. This resulted in employment of laser treatment in sectors requiring a high degree of precision such as electronics, medical applications, microchips production, aerospace, optoelectronics, automotive industry, imaging purposes, analysis, testing and materials processing.

The following advantages of laser treatment have been identified:

Laser beams can easily be manipulated and directed efficiently onto any location even from long distances without any energy being lost.

Laser beams can be focused to smaller spots that provide high photon fluxes in small volumes depending on the available power.

Tunability and monochromotic properties of laser beams offer precise control over the surface being modified.

Lasers with a wide variety of powers and pulse durations allow a wide range of materials to be treated with minimal heat damage.

Figure 11: Characteristics of some commercial lasers.

According to Folkes, there are 3 types of lasers employed for surface modification: CO2, Nd:YAG and excimer lasers (see figure 11)[43]. The Nd:YAG and CO2 lasers are not appropriate for polymer surface modification as this equipment dissociates molecules of a material by thermal means. As a variety of polymers is heat sensitive, laser treatment with CO2 and Nd:YAG results in possible combustion, charring, melting, flow and boiling of surrounding material caused by the temperature increase during the treatment. Only the excimer laser is suitable for polymer surface modification as it operates in the UV region of the electromagnetic spectrum. As mentioned before, UV radiation can be employed to promote photochemical processes that modify the surface of a polymer without causing excessive heat damage.

Discussion

Gamma irradiation

Very important for the effect of gamma irradiation, is the irradiation time. It needs to be mentioned, the applied irradiation times in research are that high, they are commercially not attractive. In other words, the real world value of this effect might be marginal. Simply increasing the irradiation dose rate to apply a certain amount of radiation on a shorter amount of time does not result in an comparable grafting yield. However, in industrial production, where time is of essence, one might opt for higher irradiation dose rates in order to decrease the modification time.

Effect of type of PE (gamma irradiation vs mechanical abrasion and chemical etching)

In the industry, many types of polyethylene are used. Recently, research is conducted on the adhesion strength of different types of polyethylene [4]. In this research, three types of polyethylene were considered: LDPE, and two types of HDPE with different Melt Flow Indices (HDPE-1 > HDPE-2). The adhesion towards a sputtered Cu metallization layer is measured.

Figure 12: Adhesion strength of treated polyethylenes metallized by sputtering process.

Not only gamma irradiation in presence of acrylamide (AAm) is considered, also Chemical etching with 10 minutes submersion, hydroxylation (gamma irradiation in presence of water), and mechanical abrasion (grinding with SiC 1200 paper). The results are shown in figure 12. As can be seen, gamma irradiation in presence of acrylamide yields the best results for LDPE. The values for HDPE-1 and HDPE-2 are not significantly different from the chemical etching method. Both grinding and hydroxylation give no real improvements over the untreated polyethylene. Even though hydroxylation results in a lower contact angle than gamma irradiation in presence of acrylamide, the adhesion strength towards the metallic layer is much lower. Apparently the chemical structure of incorporated groups on the surface is very important for adhesion strength.

The higher density polyethylenes seem to have lower adhesion strength towards the metallic layer. This tendency corresponds with the lower grafting yields of HDPE-1 and HDPE-2. The grafting yield of LDPE might be higher due to lower crystallinity of the polymer. Lower crystallinity corresponds with higher amorphous regions, associated with a higher grafting yield [8].

On an industrial level the choice between chemical etching and gamma irradiation is easily made. Since the irradiation dose rates are fairly limited, irradiation times are incredibly high. In the described research irradiation times are in the order of magnitude of 2 - 4 hours. Comparing these outcomes with the submersion times for chemical etching, around 10 minutes, shows the tremendous advantage of chemical etching, even though adhesion strength might be significantly lower.

UV treatment

Figure 13: Lap shear strength (virgin HDPE, treated with oxygen-plasma, plasma + UV-treated with a cyanoacrylate adhesive) [26].

Plasma treatment of polyethylene results in enhanced lap shear strength over virgin samples. Irradiation with plasma, followed by UV irradiation results in even higher lap shear strengths (shear strength of adhesive for bonding materials, tested on single-lap-joint specimen).

Since elongation at break deteriorates faster in HDPE with the UV irradiation time, LDPE or MDPE might be a more convenient choice if this property is important. However, the order of magnitude of irradiation is of hours, making the process commercially unattractive. On top of that, using chemicals in UV treatments is not allowed (in the US), due to the risk of interaction of the chemicals with the packaged food. Most common applications in food packaging are graft polymerization, crosslinking activated by photons and curing of coatings to increase surface adhesion, mechanical strength and printability.

Moreover, UV treatment of coatings can be effectively applied to reduce microbial bonding to the polymer and to decorate the exterior of the packaging.

Packaging industry

In current literature, very limited research is available on the costs of the various treatments with respect to packaging applications. Due to this limitation in available knowledge for managers, several treatment methods are relatively unknown. Even in these modern times, the industry is a little unaware of the possible benefits of the advanced treatment methods. Comprehension of benefits of advanced treatment methods over conventional treatments will probably increase commercial applications the coming years. Thus, conducting an analysis on the costs of these treatment methods with respect to packaging industry would be highly valuable for increasing their application in the industry. In terms of food packaging, advanced treatments will conduce to packages with better properties: longer shelf life, higher integrity and higher stability.

Economic aspects

Still, there is recent progress in the promotion of advanced treatment methods. Corona discharge and flame treatments are still the most widespread techniques in the packaging industry, caused basically by their low cost.

Advanced treatment methods as plasma irradiation, ion beam irradiation and laser treatments are already technologically possible, however economic factors determine the commercial applicability. Selecting an applicable treatment method, requires examination of both technological and economic factors of the proposed methods. While technological factors are widely available in literature, economic factors are highly depending on the industry.

Several factors can be considered: throughput, (quality-)requirements, investments, variable costs, competitiveness with other processes and automation of the process. After considering several of these factors, whether or not to implement the process has to be decided.

While being relatively new, advanced treatment methods might be both economically and technologically viable. Investments for a laser process for instance, are higher than for conventional machinery used in the microelectronics industry [44]. However, operational costs are on the same level and the laser process is suitable for long-term use.

Also the use of plasma treatment might be economically viable. Plasma gas relatively high costs of capital, however the operation costs are lower compared to conventional processes [45].

Already in 1994 an extensive cost calculation for an ion beam laser process was executed [46]. At high throughput (5000 up to 80.000 m2 per year), implementation costs decreases from $1 to ~$0.02 per cm2. This enormous decrease in costs was caused by the high equipment capacity and the higher treated area at a time.

Conclusion

The surface modification of polyethylene can be achieved by physical or chemical treatments. Physical treatments have shown to provide slight advantages over chemical treatments. Strikingly, physical treatments provide very accurate surface modification of the polymer, without the need for expensive process control equipment. This advantage leads to a decrease in surface roughening, lower damage to the surface and less problems with improper surface modification. On top of that, physical treatments are environmentally friendly because no chemicals are used in these treatments. Therefore, there is no need for disposal of waste after the treatments.

Notorious surface treatments vary from relatively simple flame and corona treatment, to highly complicated and advances techniques as gamma-, UV-, electron beam-, ion beam- and plasma- irradiation and laser treatments.

In the packaging industry, the most applied treatments are flame and corona treatment. These methods are relatively inexpensive, however they are not the best treatment methods in the world of polymers because of the relatively short time frame of the enhanced functionality of the treated polymer.

Plasma treatment is an auspicious technique for applying polyethylene or other polymers on food packaging like glass, metal or plastics. Due to high crosslinking density of the treated layer, aging of treated films is not a big issue and gives plasma treatment advantages over alternative treatment methods.

References

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Appendix A: overview of papers