Why Thorough Surface Preparation in Plastics Bonding is Important

Posted on 3/9/2021 12:19:54 PM By Masterbond

To ensure the strength of the bond when bonding plastics, preparing the surface of the substrates is vital to make certain it’s not affected by contaminants or loose dirt. This can be achieved by using a standard pattern of abrasion degreasing and cleaning, or by using physical or chemical treatments.

Standard Surface Preparation

The standard three-part system involves one round of degreasing and cleaning before the abrasion, to remove any pre-existing loose dirt or contaminants, and another after abrasing to remove any debris left behind by it.  However, while adopting that three-step process every time plastics are bonded is essential, making sure that the right products and techniques are used is essential. Plastics such as PEEK, polycarbonates and nylon, surfaces can be roughened and abraded successfully using sandpaper, but some other plastics may require a different approach dependant on the particular materials involved. 

When it comes to cleaning and degreasing, the methods used get a bit more complicated. The most common methods of degreasing, cleaning and rinsing make use of volatile solvents chosen according the substrate’s material, as well as to meet the requirements of any health, safety and environmental regulations. Most commonly used are acetone, toluene, methyl ethyl ketone, methyl or isopropyl alcohol, and trichloroethylene. 

Solvents can be used to degrease, clean and rinse either in vapor form; or in a three step immersion process in which substrates are submerged in a fresh solvent bath for a degreasing wash, then transferred to a second bath for rinsing before being cleaned and dried.

Physical and Chemical Treatments

The physical approach takes advantage of the plastic substrates’ surface reactivity and uses various physical treatments to modify surface chemistry to increase its adhesion without adding chemicals. Depending on the substrate and the treatment used, it may sometimes needed to treat the parts in line, as some can lead to a shortened shelf life. 

Types of treatments used include using ionized air generated by a Corona Discharge to form free radicals which increase surface energy by reacting with oxygen; Fame treatment during which the substrate surface is exposed to a gas flame for a couple of seconds to oxidize the surface and increase energy by forming higher surface energy functional groups, but are must be taken to avoid warping during the process; and Plasma treatment. Under partial vacuum gas plasma is activated to produce excited species which react with the plastic substrate which often provides more stable substrates than result from either of the physical or any chemical treatments do.

Chemical treatments using different acids and alkalies aim at changing both the chemical and physical properties of the substrate so as to increase its adhesion. The chemical/s are usually placed in a container which is chemically resistant before being put into a bath, and heated to the required temperature. The substrate is then submerged in it for a set time length before being rinsed with DI water and then dried. Those dealing with these chemical treatments should be wearing PPE and have good chemical handling and laboratory skills.