The attraction between molecules of dissimilar materials forms the principle of adhesion. For strong bonding with a substrate, an adhesive needs to ‘wet out’ the substrate by flowing across the substrate surface. This increases the surface area of the adhesive, thereby enabling it to achieve optimum bond strength.
The surface energy of a substrate is used to determine the adhesion strength. High surface energy supports adhesion and allows the liquid droplet to wet out to the surface (Figure 1). Figure 2 illustrates a low surface energy (LSE) and the liquid droplet is adhering to itself instead of the solid surface.
Figure 1. This image shows a High Surface Energy that supports adhesion and the droplet of liquid is wetting out to the surface.
Figure 2. This image shows a low surface energy and the droplet is clinging to itself rather than the solid surface.
Knowing the surface energy of a substrate at an early stage is helpful in order to achieve a successful bonding solution, though most material manufacturers will not reveal the surface energy values of their materials. The unit of surface energy is dynes/cm.
A contact angle measuring device is typically used in order to measure the surface energy of a substrate. However, this approach is not feasible in a typical production environment. As a result, surface energy measurements are more commonly performed by applying a dyne solution to a material surface. Table 1 summarizes typical surface energies in dynes/cm.
Table 1. Typical surface energies (dynes/cm).
||250 - 500
Adhesion to LSE Substrates
Knowing the dyne level of a substrate is crucial in the selection of an appropriate bonding solution, especially considering the widespread adoption of LSE engineering plastics in several fields.
Pressure Sensitive Adhesive (PSA) technologies which are capable of forming a strong bond with LSE substrates are the subject of intense research. Consulting an adhesive application engineer during the design stage of a project involving adhesion is highly recommended. Tecman Speciality Materials provide design engineers with the technical support and knowledge in order to select the appropriate adhesive product.
One approach of forming adhesion to a LSE substrate is the application of a surface treatment process in order to improve the substrate’s surface energy. The surface can be treated either mechanically, by corona or plasma treatment, or chemically. Many different chemical treatments are available.
Choosing an appropriate surface treatment is beneficial in many ways. It allows the use of a wide variety of PSAs in an application, thereby providing more adhesive options for design engineers in terms of cost and performance.
In areas where surface treatment is not feasible, the required adhesion performance can be achieved by selecting an LSE compatible adhesive. The presence of additional polymers in the core construction of this adhesive provides superior a adhesion to LSE substrates. Rubber based adhesives such as solvent rubber and hot-melt rubber exhibit better adhesion performance with LSE substrates. However, they have their own drawbacks which hinder their selection.
The following are the advantages of a typical rubber PSA:
- Forms a strong bond with LES substrates
- Provides superior adhesion to porous or uneven surfaces
- Soft adhesive wets out to the surface rapidly for quick adhesion build up
- Very high initial bond (high tack) up to 80% of total bond strength on contact
The following are the drawbacks of a typical rubber PSA:
- Lack of cohesive strength as it is soft adhesive, thus providing low shear adhesion values
- Low temperature resistance
- Poor resistance to chemicals, acids and alkalis
- Not suitable for external environments as it has little or no resistance to UV light
In its natural state, pure acrylic PSA will not adhere to LSE substrates and requires a surface treatment. Blending special tackifier additives into the pure acrylic PSA construction can address this problem. Compared to rubber-based PSAs, modified acrylic PSAs deliver superior adhesion performance.
The following are the key advantages of a typical pure acrylic PSA:
- Hard adhesive exhibits superior shear properties and good cohesive strength
- Very high ultimate bond
- High temperature resistance (up to 280? for short term)
- Superior UV resistance
- Superior resistance to chemicals and solvents
The shortcomings of a typical pure acrylic PSA are as follows:
- Inferior adhesion to LSE substrates
- Hard adhesive provides slow wet-out
- Low initial tack with down to 20% of bond strength on contact
Modified acrylic adhesives, consisting of carefully selected additives, offer the advantages of both rubber and acrylic adhesives. It is recommended to get the advice of an application engineer when selecting a suitable bonding solution because not all modified acrylic adhesives will form a strong bond with LSE substrates.
Tecman’s PSA Tapes
Tecman offer a variety of PSA tapes, such as film tapes, transfer adhesives, and Very High Performance Bond (Vhpb) tapes, for adhesion to LSE substrates. Table 2 lists the range of adhesive tapes offered by Tecman for bonding LSE substrates.
Table 2. Tecman’s range of adhesive tape for bonding LSE substrates.
||Modified LSE Acrylic
||Modified LSE Acrylic
||Modified LSE Acrylic
Adhesive tapes are increasingly used in design and engineering projects. Application engineers at Tecman recommend adhesive tapes to bond LSE substrates. Adhesive tapes are lightweight, clean and provide quick adhesion. They are cost effective, invisible and spread stress uniformly. They can tolerate vibration and movement, and eliminate structural weak points and investment in machinery.
About Tecman Speciality Materials
Tecman Speciality Materials is a leading adhesive tape converter specialising in bonding, fastening, cushioning and insulating, sealing, masking and protection applications, part of an International Group supplying self- adhesive solutions to manufacturers across the global.
Tecman has advanced manufacturing capabilities in both the UK and USA, manufacturing a diverse range of gaskets, seals and insulators, focusing on improving manufacturing efficiency, reducing assembly process time and unit costs.
This information has been sourced, reviewed and adapted from materials provided by Tecman Speciality Materials.
For more information on this source, please visit Tecman Speciality Materials.