Ethylene Acrylic Acid Copolymer - High Performance EAA Copolymer Dispersions for Applications in Coatings by Michelman

Topics Covered

Background
Development of High Performance Ethylene Acrylic Acid (EAA) Copolymers
Chemical Structure of Ethylene Acrylic Acid (EAA) Copolymers
Physical Properties of Ethylene (E) and Acrylic Acid (AA) Copolymer
Ethylene Acrylic Acid Copolymer's Resistance to Light and High Temperature Exposure
Stability of Ethylene Acrylic Acid Copolymers
Wetting and Adhesion Properties of EAA Copolymer Dispersions
Modification to EAA Copolymer-based Coating
     Uncrosslinked Ethylene Acrylic Acid Copolymers
     Crosslinking Agents or Polymers for Ethylene Acrylic Acid Copolymers
Thermoplasticity of Ethylene Acrylic Acid Copolymers
Typical Coating Applications of Ethylene Acrylic Acid Copolymer Dispersions
Compatibility of Typical Coating Ingredients with Ethylene Acrylic Acid Copolymer Dispersions
Compatibility of Two or More Binder Systems
Main Applications of EAA Copolymer Dispersions
     Flexible Film Coatings of EAA Copolymers
     Metal Coatings of EAA Copolymers
     Paper coatings of EAA Copolymers
     Textile Coatings of EAA Copolymers
Summary

Tag Links : EAA copolymer | Ethylene Acrylic Acid CopolymerHeat-seal coating | Heat-seal coating

Background

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Development of High Performance Ethylene Acrylic Acid (EAA) Copolymers

Aqueous dispersions of random Ethylene Acrylic Acid (EAA) copolymers provide a unique combination of properties that take advantage of respective comonomers (E and AA), used in their preparation. The presence of AA reduces the crystallinity of E segments. Increasing the AA content in a copolymer, will not only result in a more amorphous molecule, but will also significantly improve the adhesion performance.

This article will describe how new dispersion developments have evolved and discuss the high performance properties now delivered on a wide variety of substrates. Requirements for adhesion to non-polar substrates, faster line times and lower heat seal temperatures are just a few of the developments explored in this paper.

Chemical Structure of Ethylene Acrylic Acid (EAA) Copolymers

The basic chemical structure of a random Ethylene Acrylic Acid copolymer is described hereunder:

Physical Properties of Ethylene (E) and Acrylic Acid (AA) Copolymer

A copolymer of ethylene (E) and acrylic acid (AA) combines characteristics from both E and AA to the molecule. E segments provide properties such as water resistance, flexibility, crystallinity, chemical resistance and barrier. Long straight PE chains are highly crystalline (>90% crystalline) in nature, which results in a regular packing. This is easily explained by the facility with which long PE chains can align with each other (See Fig 1a).

On the other hand, acrylic acid brings polarity, toughness, crosslinkability, hot tack strength, low temperature heat seal. The presence of AA reduces the crystallinity of E segments. Increasing the AA content in an EAA copolymer will result in the formation of more amorphous regions, disrupting the formation of crystals, as illustrated in Fig 1b. Just as the increased branching in LDPE reduces the crystallinity of PE, so will the AA behaves.

Furthermore, increasing the AA content will also increase adhesion to polar substrates, decrease softening and melting point, improve optics and increase strength.

Figure 1a. Non branched PE.

Figure 1b. Branched PE.

Fig 2 illustrated the properties brought by AA in the copolymer, whilst Table 1 summarizes the properties of each individual monomer.

Table 1. Typical properties of individual monomers.

Modifying the ratio between the components can provide the balance needed to tailor the copolymer properties in order to meet specific end-use application requirements. Those properties and performances are reviewed in more details in the following sections.

Ethylene Acrylic Acid Copolymer's Resistance to Light and High Temperature Exposure

The absence of unsaturated >C=C< bonds in EAA copolymers allows molecules to resist both light and high temperature exposure. The decomposition temperature - above 300°C - will depend on exposure conditions such as time, degree of O₂ level, and the presence of substances acting as a catalyst. Oxidation is driven by a combination of time and temperature. Above 170°C, appreciable oxidation should occur as long as oxygen is present. The combination of time and temperature is usually determined by the needs of the process or other process issues. For example, if a line must be run with a 2 minute oven dwell time for production reasons, a temperature of 225°C may work. However, if a part has a heat resistance limitation of 175°C, the part may need 30 minutes in the oven to reach the same level of oxidation (and that length of time may cause the coating to flow and deform). Not only do EAA copolymers resist discoloration, but in addition, they are not sensitive to hydrolysis.

Stability of Ethylene Acrylic Acid Copolymers

Despite the total absence of surfactants or post-stabilising agents, neutralised aqueous dispersions of EAA copolymers are very stable, due to the electrostatic repulsion imparted by the neutralized carboxylic groups. As illustrated in Fig 3, those functional groups maintain copolymer chains separated but still capable of interacting with each others.

Upon drying, both water and volatile neutralizing agent - such as ammonia or amine - evaporate, leaving a surfactant - free EAA copolymer film on the coated substrate. Such "clean" dispersions are less prone to water sensitivity, recoatability problems, optic defects and reduced adhesion than polymer films deposited from surfactant -containing system. Furthermore, EAA copolymers neutralized with aqueous or volatile amines are less water sensitive compared to fixed alkali neutralized - EAA. Fixed alkali - neutralized EAA dispersions result in the formation of ionomers. Ionomer films exhibit poorer adhesion and increased water sensitivity, greater solvent and chemical resistance, and also higher melting points. Modifying the type of neutralizing agent and also the degree of neutralization, allows an EAA dispersion to be designed to meet specific properties.

Figure 3. Illustration of neutralised EAA copolymer chains in water and upon drying.

In house manufacturing experience and expertise in EAA copolymer dispersions give rise to stable, small particle size colloidal dispersions. The dilution stability of EAA dispersions is good, particularly with soft water, where an EAA content of 10 % solids remains stable

Whilst EAA copolymers with higher molecular weight resist freeze-thaw cycling better, care should be taken with the lower molecular weight or low AA content species, which are much more sensitive to low temperatures. One possible explanation could be that a higher number of carboxylic functions present in high molecular chains, remains operational for the electrostatic repulsion, as the electrolyte concentration increases on freezing.

Wetting and Adhesion Properties of EAA Copolymer Dispersions

Adhesion is the result of interactions that develop between two dissimilar "bodies" when they come into contact. It is a quantitative description of the work required to detach 2 surfaces. For a good adhesion to occur, good wetting of the substrate, high flexibility polymer chain and maximized chemical interactions are necessary. EAA copolymer dispersions meet all those requirements, which result in energetically good adhesion conditions, as expressed by the surface energy.

• Wetting: Without going into a detailed explanation, it is generally recognized that applied coating - such as EAA copolymers - should at least have a surface tension approximately 10 points lower than the free surface energy of the substrate to be coated (1). However, it is often difficult to lower the surface energy of the wetting fluid, without using additives such as surfactants. As mentioned previously, the addition of surface active agents can lead to reduced water resistance and poor adhesion performance. The use of ethanol or isopropanol is recommended particularly when water resistance of the final coating is important. Another common alternative is to treat the substrate (corona, ozone, gas flame or primer application), in order to raise its surface energy, as explained below.
• Flexibility: The flexibility of a copolymer is its ability to adapt to the surface topography of the substrate (2). At the micro level, the surface is rough, because of irregularities (pores, asperities, depressions...). Hence, there are many more sites available to interact with a coating compared to a completely planar surface. If the coating has sufficient mobility (or flexibility), and providing good wetting conditions, the extent of adhesion may be increased by surface roughening. Thanks to the flexibility of the E segments - the dominant monomer in EAA copolymers - this condition exists and flexibility ensures a good fit with irregular surfaces. It is also worth mentioning that by treating the substrates - particularly with corona treatment - not only the surface energy increases, but also, the surface roughness. This can be a secondary assistance in achieving good adhesion performances.
• Interactions: Besides "burning off" surface contaminants, oxidative treatments (Corona, ozone ...) raise the surface energy of the substrate, by creating various chemical functions like -OH, -COOH, >CO.... Those polar groups not only interact with the water (hence, better wetting) but also contribute to increase the number of possible strong chemical interactions between the surface of the substrate and the AA segments of the EAA copolymer. Table 2 presents the main chemical interactions that could occur between a corona-treated plastic substrate and a neutralized acrylic copolymer (3). As some interactions are distance dependent, values of interaction energy of intermolecular forces are given for a distance of 0.5 nm, except for H-bonding interactions. (Contact distance).

Table 2. Energy of typical intermolecular forces.

Corona treatment typically increases the surface energy of the surface by 5 to 20 mN/m, depending on the treatment level. Although not an absolute number, a difference of at least 10 mN/m is recommended to have good wetting / adhesion characteristics.

Modification to EAA Copolymer-based Coating

The AA content suggests that considerable modification of an EAA copolymer-based coating is possible through the use of carboxylic acid-reactive crosslinking agents or polymers. The presence and the density of so-created covalent chemical bonds between molecules - particularly macromolecules - have a profound influence on both the chemical and mechanical properties of the materials in which they occur. (4)

Uncrosslinked Ethylene Acrylic Acid Copolymers

For example, uncrosslinked EAA copolymers exhibit already good resistance to both solvent and water. However, they will usually dissolve in a so-called "good" solvent.The dissolution process may well be lengthy, but given sufficient time and depending on the force and the extent of interchain covalent bonds, the polymer will dissolve eventually. More durable applications - such as nonwovens and textile coatings - may require additional resistance properties (e.g. washability) to permit repeated cleanings. This additional durability can be achieved by crosslinking all or a portion of the available carboxyl groups.

Crosslinking Agents or Polymers for Ethylene Acrylic Acid Copolymers

Crosslinking reactions will also significantly influence the thermoplasticity behaviour of EAA copolymers. Uncrosslinked EAA copolymers are thermoplastic by nature and will generally melt flow at sufficiently high temperatures. By contrast, fully crosslinked molecules cannot melt because of the constraints on molecular motion introduced by the covalent bonds. Instead, at temperatures well above the normal soft / melt points, fully crosslinked EAA tend to be more brittle and - unlike thermoplastics - begin to undergo irreversible degradation rather than be flexible again. Table 4 presents the main carboxylic-reactive crosslinking agents or polymers for EAA copolymers.

Table 4. Crosslinking agents or polymers for EAA copolymers.

Thermoplasticity of Ethylene Acrylic Acid Copolymers

Some particular applications require large quantities of heat-seal film which can be sealed to itself at a temperature which will not impair the film integrity. This is particularly the case for flexible packaging. Polyolefins, particularly polypropylene, are in demand as film materials owing to their strength and clarity but, in general, exhibit relatively poor heat seal characteristics. This is due to the high crystallinity of such polymers. Introducing AA segments in an ethylene chain will reduce the crystallinity, hence making the copolymer softer and lowering the melting temperatures.

Typical Coating Applications of Ethylene Acrylic Acid Copolymer Dispersions

Because of its process ability and low viscosity; EAA copolymers can be applied to various substrates using traditional coating equipment, such as gravure, flexo, air knife and rod applicators (See Fig 4). Although they will form a surfactant-free film at ambient temperatures, moderately elevated drying temperatures (e.g. 40°C) are recommended for optimal film formation. Depending on the film thickness and substrate porosity, higher temperatures may be required to avoid trapping moisture.

Figure 4. Typical application equipment for EAA dispersions.

Application rates vary, based on intended use. As a primer on non-porous films or foil, EAA copolymers should be applied at coating (dry) weights of 0,5 - 1,0 g/m² . For porous substrates, such as paper, the coating should be applied at slightly larger weights e.g. 0,8 - 1,2 g/m².

Compatibility of Typical Coating Ingredients with Ethylene Acrylic Acid Copolymer Dispersions

Although it is often difficult to predict compatibility between the different components of a formulation, EAA copolymers dispersions are miscible with many emulsions, solutions and dispersions having similar pH. The compatibility of EAA copolymer dispersions with pigment, resins and typical paint additives are given in Table 5. The information should be carefully interpreted as there are, for instance, so many different grades of additives, in particular pigments whose properties can significantly differ due to the surface treatment they were submitted to. In all cases, compatibility and long-term stability should be thoroughly evaluated in small laboratory batches.

Table 5. Compatibility of typical coating ingredients with EAA copolymer dispersions.

Compatibility of Two or More Binder Systems

To determine the compatibility of two or more binder systems, it is recommended to cast a film of each binder system (neat) onto a transparent substrate and to repeat the experiment using the combined binder system. Comparisons of clarity, homogeneity and toughness of the blended binder system to the pure binders can indicate whether or not the two binders are compatible. A hazy or soft film is a sign of poor compatibility.

Main Applications of EAA Copolymer Dispersions

The relatively low melt point of EAA copolymers may cause occasional blocking problems during the coating process or with freshly coated items that are dried and immediately stacked or rolled up for storage or shipment. EAA-containing coatings may stick to hot guide or carrier rolls, or may stick to itself at the rewinder stage when painted surfaces are rolled up. This phenomenon is called blocking. One way to control blocking is to incorporate in the coating formulation additives such as carnauba or paraffin wax emulsions. (5)
Main applications of EAA dispersions are described in the next sections.

Flexible Film Coatings of EAA Copolymers

EAA copolymers adhere to many flexible packaging substrates including foil, paper, LDPE and polyamides. Applied as a tie layer between two substrates, crosslinked EAA copolymers can provide a clear, FDA-compliant coating which is resistant to solvents, grease and water. Film converters can apply lower coating weights with EAA copolymer than with extrusion coatings, making them a cost-effective alternative to extrusions.

Furthermore, the enhanced thermoplasticity behaviours of EAA make them attractive heat-seal coatings. The presence of AA segments in the copolymer, will reduce the heat-seal temperatures (sealing 20 - 30 °C lower than LDPE) and the high degree of hot tack will improve line speeds and cut costs in most flexible packaging applications. EAA copolymers can be used either in extrusion or in laminate coatings. Strong adhesive bonds will enable EAA to be used in lamination of foil - foil, paper - foil and paper - paper. When EAA copolymers are used as laminating coatings, the recommended coating (dry) weight is 1,6 - 2,4 g/m². Fig 5 illustrates the influence of AA content on the heat-seal temperatures and bond strength.

Figure 5. Influence of AA content on the heat-seal temperatures and bond strength.

Metal Coatings of EAA Copolymers

• Metal coatings are another typical application where EAA dispersions can be very useful. This includes coatings for cans, coils, industrial equipment, automotive …, all applications where high adhesion properties are required. The literature reports the use of Zn-crosslinked acrylate oligomers in UV-cured coatings for metals (6). The ability of EAA copolymers to crosslink with metal ions, its excellent adhesion properties to metal surfaces, and the flexibility and barrier properties obtained with the E segments definitely open doors to numerous applications in metal coatings.
• Can coatings are one area where waterborne paints are finding some growth. Water-based coatings are easier to be approved in critical application areas such as food packaging. Furthermore, the metal packaging area for food and beverage is facing tough competition from PET bottles, due to restrictions on the use of certain components such as dioxins and Bisphenol A. It is important for can coating formulators to find acceptable alternatives, while maintaining the high level of technical requirements. Coatings for cans have to have enough flexibility and adhesion strength in demanding process conditions. Based on the EAA performances as mentioned previously, it is worth thinking about the potential that EAA copolymers can offer in this application. Furthermore, the fact that EAA copolymers comply with BfR XIV and BfR XXXVI chapters as well as with most FDA sections constitutes another non-negligible argument for their use in food packaging.

Paper coatings of EAA Copolymers

Paper coatings are another area where EAA copolymers are use on a commercial scale. Its excellent adhesion to cellulose fibers combined with its compatibility to pigments and fillers such as clay, calcium carbonate, titanium dioxide or some carbon black types make them a good choice for a wide range of applications in the paper industry. By taking advantage of the facility to apply EAA copolymers with conventional coating equipment, formulators can quickly modify a recipe to improve barrier properties and prevent moisture damage to paper.

In this application, EAA copolymer dispersions are frequently formulated with pigments or fillers such as clay, Calcium carbonate, Titanium dioxide or Carbon black. For optimal performance, the filler - typically 65 to 75 % solids - should be pre-dispersed with 0.1 to 0.3 % dispersing aids (polyacrylates) in water. An additional dispersant might be necessary to minimize pigment shock, particularly with very fine particle size fillers.

Textile Coatings of EAA Copolymers

The textile industry uses much of the same materials and technology common to the coatings industries. The coating industry normally thinks of a substrate as a solid, continuous and planar surface, but textiles are open and flexible structures that have many types of coatings applied to them. Although coating mechanism, formulations and test methods are different, the same considerations exist in textile coatings as those used on more solid surfaces. (7)

Textiles come in many forms. Structurally, they are termed knits, weaves or nonwovens to distinguish the process by which the fibers are arrayed in their final presentation. Thanks to their particular properties, EAA copolymer dispersion can be used in textile application for:

• Back adhesive on a tufted carpet. EAA copolymers could be applied directly to the back of tufted carpet that is made from rough fibers (e.g. jute), causing the tufted fibers to adhere strongly to each other, and making the overall structure much more stable. The backcoating formulation includes thickener, filler and additives. The backcoating is applied by a wetted roller applicator and the coated carpet is then dried. If a warranty for washability is required, the use of a crosslinker - such as melamine / formaldehyde resin - can be envisaged.
• Fusible interlining in garment manufacture. Fusible interlinings comprised of a textile sheet which is coated with a thermoplastic adhesive, such as copolyamide, copolyester and polyethylene, both LD and HD types. Fusing is the bonding of the interlining to the outer fabric. It can be done with a hand-iron or with continuous fusing presses with double pressing units. By application of heat over a certain period of time, the thermoplastic adhesive turns soft or melts and penetrates the yarn and woven structure of the textile.

Summary

In this article, we have provided a description of how EAA copolymer dispersions are used in some industrial coating areas. Incorporating AA segments in a PE chain brings new properties such as adhesion, whilst still offering barrier properties. Our intent herein is to give readers a broader understanding of EAA copolymer technology applications in coatings and to stimulate new ideas in research and development.

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Source: High Performance EAA Copolymer Dispersions for Applications in Coatings

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