Introduction
The wide use of polyethylene and polypropylene plastics as well as their copolymers and alloys has increased interest in adhesive bonding of these materials. Polyolefins are very difficult to bond on account of their non-polar, non-porous and chemically inert surfaces. Their high degree of chemical inertness to solvents generally limits the usefulness of solvent bonding as a viable assembly technique. One successful approach to adhesive bonding of these materials involved proper surface pretreatment prior to bonding. The pretreatments which are widely accepted include chemical etching, flame treatments, corona discharge and gas plasma. Recently, specially formulated primers have been used as a pretreatment procedure. A second approach is the use of specialty adhesives such as hot melts (e.g. Master Bond MB514), high tack pressure sensitive adhesives (e.g. Master Bond LTX164) and Master Bond's solvent based specialty adhesives (e.g. Master Bond X17).

Surface Preparation/Pretreatment
Surface preparation and pretreatment is necessary for most adhesives to bond well to polyolefins. The more traditional approach is to pretreat the surface prior to bonding. As a general rule, surface cleanliness is a necessity for achieving good results prior to any pretreatment. All surfaces to be bonded must first be thoroughly cleaned so as to assure the removal of all traces of oils, greases, dirt and any mold releases which may have been used to facilitate fabrication of the polyethylene or polypropylene part. Good degreasing agents include acetone and other ketones as well as selected alkyl acetates etc. Alcohols, gasoline and paint thinners are less effective. MECHANICAL ROUGHENING OR ABRASION is a second necessary process step for enhancing the effectiveness of surface pretreatments and hence bond strength. A second degreasing step after abrasion should be considered to ensure the removal of loose particles.

ACID ETCHING, a very effective surface pretreatment method, can be carried out either at ambient or more quickly at elevated temperatures. The most widely employed acid etches are comprised of sulfuric or chromic acid solutions. Etching times depend on such variables as the exact composition of the plastic part to be bonded, the formulation of the etching solution and the temperature. Typically etching times may vary from 2 to more than 10 minutes. Etching must be followed by thorough water rinsing to remove any residual acid and then drying.

Exposure of polyethylene or polypropylene surfaces to an OXIDIZING FLAME is another simple yet quite successful pretreatment method. A blue flame from an oxyacetylene or propane source is quickly passed over the plastic surface until it becomes glossy or shiny. Care must be taken to avoid warpage of the part due to overheating. After such a flame treatment, the plastic surfaces should be washed with water and detergent and then thoroughly dried. This pretreatment is most successful with parts comprised of thicker sections.

Electrical discharge methods are also employed to increase the bonding characteristics of polyolefinic surfaces. They create free radicals which form oxygenated functional groups for improving bonding to adhesives. CORONA TREATMENT is of particular value for facilitating bonding of thin films and foils. It is accomplished by passing a moving web over a special dielectric covered treated roll and applying a source of high voltage, high frequency power to an electrode spaced over the surface of the traveling web. The resulting change in polarity facilitates adhesive bonding.

GAS PLASMA TREATMENTS are employed for upgrading the bondability of small complex geometric surfaces which are difficult to abrade or flame treat, but require strong bonds. This method has been widely accepted by manufacturers of medical devices and optoelectronic components.

Specially formulated PRIMERS have been developed in order to promote the bondability of polyolefinic surfaces as well as their copolymers and alloys. Master Bond specialty adhesive system X17 can also function as a primer. Primers such as X17 are used in conjunction with other adhesives such as epoxies and urethanes. Another primer-type system involves the use of instant adhesives or cyanoacrylates. Cyanoacrylates will bond to many polyolefinic substrates when used with a special primer. Master Bond offers Primer P to be used in conjunction with cyanoacrylate adhesives. The cyanoacrylate Primer P type system provides for high strength, fast setting bonds.

Following any one of these surface pretreatments, one can then use an appropriate adhesive such as an epoxy, polyurethane, silicone or another adhesive depending upon the conditions the bonded material will be subjected to once in service. For example, Master Bond's EP21 two part epoxy offers excellent strength and chemical resistance as well as convenient handling. Another adhesive widely used in this manner is Master Bond's EP30D12 two part polyurethane which offer excellent flexibility and abrasion resistance. Please consult Master Bond's technical staff or Selector Guides for products that may be used following successful pretreatment of polyolefins.

Adhesive Systems - No Surface Pretreatment Required
Master Bond Inc. offers an impressive array of adhesives for bonding polyethylenes, polypropylenes, their copolymers and various blends to each other as well as to metals, glass, ceramics, elastomers, wood and other plastics.

Master Bond MB514 and MB519 are polyolefin based hot melts with relatively low softening temperatures and provide good adhesion to untreated polyethylene and polypropylene surfaces. The only required processing steps are melting these compositions and then applying the molten hot melt to the surfaces to be bonded. One of the main advantages of using hot melts is the rapid speed of cure which makes systems such as MB514 ideal for high speed assembly operations as the bonds are formed immediately upon cooling. Other advantages of these hot melts are that they are 100% reactive and do not contain any solvents or volatiles as well as excellent chemical resistance properties.

Another kind of system that bonds well to polyolefins are specialty pressure sensitive adhesives, an example of which is Master Bond LTX164. This water based solvent free adhesive offers excellent toughness, high abrasion resistance and durability. It also allows for removability when that feature is needed.

Perhaps the most exciting new development in bonding polyolefins without surface pretreatment is Master Bond's X17, a one part solvent based elastomeric adhesive. X17 cures readily at ambient or more quickly at elevated temperatures. Typical lap shear strength for polypropylene/polypropylene bonds exceed 250-300 psi with 180° peel values up to 10 pli after cures at 75°F for untreated surfaces. Even higher strength values are obtained when heat is applied when curing, e.g. 150°F for 1-2 hours. Since Master Bond X17 system contains a solvent blend it must be employed in accordance with applicable OSHA and environmental regulations. It should be noted that X17 bonds well to other plastics such as PVC, ABS, etc.

Summary
Master Bond, Inc. offers a number of different adhesive systems for bonding polyethylene, polypropylene, their copolymers and blends, both to each other and to different substrates including metals, glass, ceramics, wood and other plastics. These adhesive systems have been designed to best meet specific processing and end product assembly needs. Master Bond's experienced technical staff will be glad to advise on specific processing and application problems and welcomes all such inquires!