How to bond Low Surface Energy (LSE) Surfaces?

New Methods for Bonding LSE Plastics

Technology has advanced to the point where adhesives are available that are capable of high performance bonding to LSE substrates such as TPO, PP and PE without surface treatment. Easy-to-use adhesion promoters are also available as a companion to some adhesive product types to increase the strength and broaden the selection.

Structural Adhesives

Structural Adhesive is formulated to bond to LSE plastics (as well as high surface energy plastics and metals). These are two-part, solvent-free, room temperature curing adhesives that come in a convenient duo-pak format or, for large applications, in bulk. They resist many chemicals, water, humidity and corrosion. Generally surface preparation is limited to solvent cleaning (to remove surface contaminants). Sometimes, light abrasion or a matte finish on the bonded surfaces can increase bond strengths.

Adhesion strength of structural adhesives are usually characterized using an overlap shear test. Substrates are bonded together with a controlled overlap and the adhesive is allowed to cure. After cure the adhesive bond is pulled in the shear mode at a constant rate and the peak force to break is measured. By convention an adhesive is considered structural if it is capable of achieving greater than 1000 psi break strength in the overlap shear test. To achieve this level of break strength the adhesive must have high adhesion strength to the substrates.

Thoes kind of adhesives create structural (greater than 1000 psi overlap shear) bonds to low surface energy plastics without pre-treatment. Below are some representative overlap shear bond strength data on common plastic substrates including LSE plastics. Note that with several substrates tested the substrate itself was not strong enough to support the 1000 psi load and the substrate failed before the adhesive-to-substrate bond did.

Because of their high bond strengths to untreated polyolefins, ease of use (in Duo-Pak cartridges with static mix nozzles) and favorable environmental characteristics, those adhesives have met great success in bonding low surface energy plastics in a variety of applications. Typical applications include bonding of molded or thermoformed parts for interior and exterior fascia, liquid containers, decorative panels, appliance and sporting goods trim and accessories, protective equipment and electronic components wire potting and housings.

Hot Melt Adhesives

Hot melt adhesives may also be used to bond lightweight thermoplastic pieces. These adhesives have the advantage of providing quick tack and handling strength, thus speeding production. These adhesives combine high heat resistance with relatively high strength and low creep. These products can provide benefits to manufacturers who can trade off heavy duty bonding for faster production speed in applications such as POP displays; sample boards and tabletop displays; exhibitor booths; foam inserts to carrying cases; fabric or paneling to foam; and molded reinforced plastic to fabric or fascia for furniture and automotive interiors.

Pressure Sensitive Adhesives

Pressure sensitive is a unique category of adhesives in that they do not cure or undergo a chemical change when applied. Pressure sensitive adhesives are viscoelastic materials that exhibit both viscous (flow) and elastic (resistance) properties at the same time. When the adhesive is put on the substrate typically in tape form and pressure is applied the adhesive makes immediate contact for initial adhesion but continues to flow onto the surface to achieve increased contact and a higher level of strength over time. One advantage of pressure sensitive bonding tapes is that the bond is immediate so there is no clamp or cure time. They are also unique in that you do not have to bond the adhesive to both substrates at the same time. The tape can be applied to the first substrate one day and to the second substrate the same day, the next day, or weeks later. This brings added convenience and can be a benefit for many applications including assembly line processes.

Acrylic pressure-sensitive adhesives often provide the best balance of adhesion and performance properties for many applications, including the type Pressure Sensitive Adhesive family of products that have been developed to bond low surface energy plastics. High-Strength Acrylic Adhesive is a thin PSA designed specifically for bonding difficult LSE substrates such as polypropylene, TPO, and a variety of other plastics while providing excellent resistance to high
temperatures and chemicals, and high peel strength. It also bonds well to metals and other more friendly substrates, thus providing a PSA bonding solution for applications such as durable graphic bonding, dissimilar material bonding, foam and rubber bonding, plastics assembly and more.

Very high strength bonding tapes are available and are used for a variety of applications previously reserved for mechanical fasteners or structural adhesives. These tapes have an acrylic foam construction and have viscoelastic characteristics throughout the product. The foam absorbs energy to provide high strength and relaxes stress to protect the bond. The tape allows, rather than fights, movement between parts. A high level of tape-to-substrate adhesion is required for the foam to allow relative movement of the parts without coming debonded at the tape-to-substrate interface.

Tape LSE tapes are also designed to adhere to PP, TPO, TPE, and composite substrates without the use of primer. Other acrylic foam tapes do not have high enough adhesion strength to LSE plastics without additional surface modification. Easy-to-use brush-on primer is available to give very high tape-to-substrate adhesion on some LSE plastics. For even lower surface energy plastics such as PE, flame, corona or plasma treatment is suggested prior to bonding. Strength of pressure sensitive bonding tapes is typically characterized using a peel adhesion test. Below is 90-degree peel adhesion data for a variety of Tapes on four grades of TPO with different surface preparation techniques.

Although required bond strength varies by application, typically an adhesion level of 20 lb/inch or greater in this test is desired for most general applications. High strength bonding tapes are typically used for bonding panels to frames, bonding stiffeners to panels, and bonding decorative overlays and scuff and rub strips.

Summary

New tape and adhesive technologies that bond to LSE plastics offer increased efficiency, reduced costs, and improved design flexibility when using these versatile and popular plastics for manufacturing a variety of products. Examples include bonding thermoformed bumpers to metal; vehicle seats, toppers and accessories; binning strips; architectural panels; plastic lumber; signage; transport cases; protective armament, and many others.