POLYETHYLENE AND POLYPROPYLENE SUBSTRATE

Polyethylene and Polypropylene Substrate



Polyethylene and Polypropylene Substrate

Introduction

Many plastics can be readily bonded with a wide range of adhesives to give strong structural bonds. Plastics have much lower tensile strengths than other materials, such as metals, and adhesive bonds can be achieved that often exceed the strength of the plastic. However, attempts to bond certain plastics, such as polyethylene, result in weak assemblies.

Polyethylene and Polypropylene Substrate

Typical low surface energy polymer substrates include plastics such as polyethylene and polypropylene, fluoroplastics like polyetrafluoroethylene (PTFE), polyethylene/rubber composites and thermoplastic olefins (TPOs). These polymers are of considerable interest to the design engineer because of their desirable properties, such as, for example, the chemical and heat resistance of PTFE and the low cost and easy molability of polyolefine.

Pretreatment

Thermal Methods: Flame treatment is widely used to prepare polyolefin surfaces for adhesive bonding, particularly in labeling operations. According to Silverman, et al (2001) “this method is purported to burn off contaminants and weak boundary layers, and it produces surface oxidation. Although flame treatment can be readily automated on a container labeling line, it is very impractical for most product assembly operations” (Silverman, et al, 2001).

Polyolefins are thermoplastic polymers that can be easily melted, and several welding techniques can be utilized successfully to give strong bonds. These include spot welding, ultrasonic welding, ultrasonic staking, vibration welding, spin welding, hot plate welding and hot plate staking. All these methods can be successful, but often have severe limitations such as high capital requirements, restrictions on part geometry, inability to bond dissimilar materials or slow production cycles (often taking minutes per part compared to the seconds of adhesive bonding).

Chemical Surface Treatment

The chemical modification of low energy polymer surfaces is carried out by treatment with metallic sodium complex dispersions (for fluoroplastics), chromic acid etching, bleach/detergents, potassium iodate/sulfuric acid and other mixtures. Dutch authors have shown that pretreatment with oxidizing acids can increase the adhesive strength of polyethylene tapes bonded with epoxy adhesives from 45 psi to 275 psi. “Surface analysis of their treated specimens indicates the generation of functional oxygen containing groups, i.e., hydroxyl carbonyl and carboxyl. Similar work using chromosulfuric acid and chromyl chloride has been reported by Czech researchers” (Yang, et al, 2001).

Treatment of PTFE with sodium complexes also gives strong evidence of surface oxidation and results in up to 10-fold increases in bond strength with epoxy adhesives. Although bond strength can be improved significantly by these chemical methods, they are time-consuming batch processes, often taking hours to treat, wash and dry parts, and are thus only suitable for small, high value parts. There is also the problem of disposing of hazardous waste from these processes.

Plasma Treatment: Gas plasma treatment involves exposing the plastic to a gas activated by radio frequency energy to clean it and make it receptive to adhesives. Oxygen plasma treatment of polyethylene appears to remove weak boundary layers and oxidize the surface, leading to several-fold increases in bond strength with adhesives such as urethanes and ...