Polymer Chemistry and Surface Preparation

Adhesive Polymer Chemistry Adhesion technology has expanded at an increasing pace in its areas of application from the past to the present. Products can be grouped in various ways according to their use or formulation. By its nature as a joining method, adhesives must be compatible with the surfaces they join together. While adhesives provide the function of joining different surfaces through sophisticated surface treatments, it is possible to find a wide range of adhesives suited to the relevant chemical bases in polymer materials on the market. The classification of adhesive products according to various needs is shown in Figure 1. In this type of classification, it should be kept in mind that some categories overlap. For example, polyurethane (PU or PUR) as an adhesive can be integrated into multiple groups with the following properties: • It is a synthetic organic polymer, • It exhibits a cross-linked thermoset structure through a chemical reaction in the form of addition polymerization, • It is composed of isocyanates and polyols, • It is available in single or two-component packaging, • It can be moisture-cured or reactive. Chemical-Curing-Mechanism Adhesives: The final adhesive product can undergo a chemical process cycle from monomers to polymers together with the process to which it is applied, or a ready-made polymer system can be used to hold surfaces together through various effects. Adhesives undergoing a chemical process are created when repeating single types of monomers are arranged sequentially or different monomers are combined in various configurations to form chains. The chain structures examined in three main groups are shown in Figure 3. [caption id="attachment_128644" align="aligncenter"] Figure 2 – Polymer Chain Structures

[/caption] Thermoplastic polymers, classified according to chain structures, consist of linear chains formed by repeating monomers. The most common examples are polyethylene, polyamide, polystyrene and PVC. They are easily shaped by heat and have high surface quality. Elastomer polymers consist of network-structured chains. They carry high elastic and mobility properties. Thermoset polymers show high temperature resistance due to their 3-dimensional bonding structures. Depending on the cross-link density in the structure and aromatic content, they exhibit high chemical and thermal resistance, and their mechanical properties and hardness are more advanced compared to other polymer types. The strength properties of the adhesive polymer chain under variable loads such as heat, weight, chemical media and stresses are regulated by the following parameters:

• - Monomer configuration (singular-plural, regular, sequential, alternating), - Chain length (controlled with initiator and terminator additives) and degree of crystallization, - Chain mobility, - Physical interactions between different chains.

As an example of a hybrid joining method, a reinforced adhesion concept can be shown where a thermoset (epoxy) adhesive and a thermoplastic welding method are used together (Figure 3). Here, a safe design against failure was attempted with a physical barrier positioned around the connection to prevent delamination. [caption id="attachment_128645" align="aligncenter"] Figure 3 – Hybrid adhesion concept incorporating thermoplastic and thermoset materials[/caption] Physical Phase-Change-Mechanism Adhesives In adhesives obtained through physical phase change, ready-made polymer chains are mostly dissolved in water-based or chemical-based solvents. The two basic functions of the solvent are: 1. These solvents, which temporarily penetrate between chain structures and reduce the viscosity of the adhesive, provide ease of application. Solvent-based adhesive products are homogenized with the aid of manual or automatic mixers prior to application and are applied with auxiliary systems to ensure uniform thickness distribution on the application surfaces. After the solvent is removed from the surfaces, the adhesive in the form of a thin layer on the surface is ready for joining at optimum thickness. 2. Another function of the solvent is to prepare the surfaces for the adhesive polymer. To join two surfaces made of polyvinyl chloride (PVC) base material, a PVC-based adhesive dissolved in an appropriate solvent can be used. The solvent that effectively dissolves the PVC base both reduces the viscosity of the adhesive and penetrates between the polymer chains on the surface to which it is applied, "welding" the polymer chains on the surface of the material with the polymer chains of the adhesive.1 Toluene, which is hazardous for health and safety reasons, is frequently encountered in adhesives that are soluble in solvent. In the 1990s, it was determined that two workers suffered acute toluene poisoning in a swimming pool during pool construction. Due to toluene-based solvent adhesive used in the assembly of border stones, the dangerous chemical exposure at concentrations higher than 7,000 mg/m3 at the bottom of the pool over a 3-hour period revealed the importance of the issue. Over time, toluene-based solvents were replaced with chemicals that have solvent performance at least as high as this group of products and are safer for human health and the environment. [caption id="attachment_128647" align="aligncenter"] Figure 4 – Respiratory Poisoning[/caption]

Pressure-Sensitive Adhesives (PSA)

Another group that does not undergo chemical reaction is pressure-sensitive tape-type adhesives (Pressure Sensitive Adhesive, PSA). These adhesive tapes are not clearly classified as solid, they are applied with an appropriate pressure value to a specific surface area and provide adhesion by wetting the surface upon contact. If sufficient pressure is not applied, air gaps and separations occur between the surface and tape. They are commonly available in chemical bases such as acrylic, rubber, and silicone. [caption id="attachment_128648" align="aligncenter"] Figure 5 – Pressure-Sensitive Tapes (PSA)[/caption] They can be produced in film format, double or single-sided. Double-sided versions, where layers with critical properties can be used, are employed on the back of products such as labels, foil, and hook-and-loop fastener tapes. Single-sided versions can be used for masking and insulation purposes. - Products are supplied rolled on themselves or between two protective films. - Tape systems may require surface preparation chemicals specific to them.

Surface Preparation

Prior to adhesion, it is important that surfaces have appropriate wetting potential, and the surface energies of the adhesive and surface are made favorable for bonding. Surfaces are first inspected and prepared. Then cleaning and pre-treatment are performed as appropriate to the adhesive. In this context, surfaces are physically, chemically and mechanically prepared for adhesion.

Inspection

Surface inspection begins with the storage conditions of the material planned for adhesion. It is important that the material reaches appropriate atmospheric conditions. Surfaces that have been transported or stored for long periods and under different seasonal conditions must absolutely be brought to equilibrium in the pre-storage area or adhesion station under specified conditions. It is common practice to keep adhesive products under critical transport and storage conditions, and if cold chain requirements exist, to ensure the cold chain is not broken with special labels. Taking a glass bonding example from the automotive sector, it is important that the glass is in the same atmospheric conditions as the application area. If present, resistance control should be performed first to securely begin this difficult-to-repair process. Another parameter is dimensional control. Material-specific tolerances and the physical volume of the bonding area must be checked. When necessary, the bonding area is marked and demarcated with appropriate consumables. Care must be taken that temporary marking pens or masking tapes used in these areas do not leave residue as permanent contaminants in the bonding area.

Cleaning

The most critical point for adhesion quality is the removal of oil, dirt, dust and other contaminants from surfaces. In the error types mentioned in the previous section (Introduction: Introduction to Adhesion Technology), adhesion failure, which is explained as the inability to establish a connection between the adhesive and the surface, is the primary cause. In addition to various solvents and cleaning agents, pressurized cleaning systems can be employed. [caption id="attachment_128649" align="aligncenter"] Figure 6 – Surface Cleaning[/caption]

Pre-Treatment

Pre-treatment methods can be examined in 3 groups: 1. Mechanical abrasion techniques increase surface roughness. Through applications such as sanding, sandblasting, or peel ply on structural composite parts, a mechanical lock mechanism is created through a larger surface area where the adhesive can penetrate the surface. More contact area means more effective adhesion. [caption id="attachment_128650" align="aligncenter"] Figure 7 – Surface Roughness through Mechanical Abrasion[/caption] 2. Chemical bond strength constitutes the most important steps of the adhesion mechanism. Active bonding sites required for the adhesive to match the surface are created through various methods. Chemical products such as adhesion promoters and primers are the most commonly employed examples. [caption id="attachment_128651" align="aligncenter"] Figure 8 – Chemical Bond Strength through Adsorption, Chemisorption and Diffusion[/caption] 3. Electrostatic theory is based on the principle of electronegative differences in materials intended to be bonded. An adhesion force arises from the transfer of electrons by creating positive and negative charges that attract each other across the interface. For example, when an organic polymer comes into contact with a metal, electrons are transferred from the metal to the polymer and form an electrical double layer (EDL). In a sufficiently darkened environment, the electron transfer in this event can be seen as small blue flashes. [caption id="attachment_128652" align="aligncenter"] Figure 10 – Electrical Double Layer (EDL)[/caption]

1In the welding method, one of the joining methods, metals must be brought close together to establish atomic bonds.

In the fusion welding method, metals are made to pass into the liquid state and form this bond by solidifying, while in the cold welding method, this union is accomplished under pressure and below the melting temperature without melting.

The welding methods mentioned above and commercially available as "cold welding paste" should be kept separate from the group of welding methods.

2Dangerous chemicals are listed by the European Chemicals Agency (ECHA) pursuant to the REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) Regulation, registered, chemicals with circulation exceeding specified annual consumption quantities are reported, restricted, and banned.

Similarly, the Regulation on the Registration, Evaluation, Authorization and Restriction of Chemicals (KKDİK) is in effect and applied in our country.

Sources

• A hybrid bondline concept for bonded composite joints (sciencedirect.com), https://www.sciencedirect.com/ science/article/abs/pii/S0143749616300732 • Difference Between Linear & Branched Polymers (sciencing.com), https://sciencing.com/difference-between-linear-branched-polymers • Elastomer (sciencedirect.com), https://www.sciencedirect.com/topics/nursing-and-health-professions/elastomer • Thermoset Plastics (sciencedirect.com), https://www.sciencedirect.com/topics/materials-science/thermosetplastics • Two cases of acute toluene intoxication (pubmed), https://pubmed.ncbi.nlm.nih.gov/2378819/ • Fumigation Safety, https://www.uky.edu/Ag/Entomology/PSEP/fumsafety.html • Pressure Sensitive (adhesives.org), https://www.adhesives.org/adhesives-sealants/adhesives-sealants-overview/adhesive-technologies/pressure-sensitive • European Chemicals Agency, https://echa.europa.eu/ • Cleaning Strategies for Great Adhesion, https://www.btglabs.com/blog/cleaning-strategies-for-greatadhesion 14 • Diagrams Source: http://ffden-2.phys.uaf.edu/webproj/212_spring_2014/Connor_ Mattson/connor_mattson/physics.html   A.Tuğçe Onur Adhesive Engineer, EAE Materials Engineering Turkish Aerospace Industries Inc. /