EVOLUTION of COMPOSITES

Jan. 1, 2020
The demand for more fuel-efficient vehicles has sparked advances in plastics since the 1930s. As plastics change, so do the adhesive repair products and techniques that are vital to easy applications and reliable repairs.

Plastic automobile parts first appeared in the 1930s and 1940s in the form of fiberglass, the material we now refer to as fiber-reinforced plastic. Back then, plastic parts interested automotive engineers because they were extremely light compared to their steel counterparts, and, also, because they wouldn't rust.

By the 1950s, fiberglass production was simplified, and changed from a difficult and time-consuming hand lay-up to a much faster production process of chop-gun application. In 1953, the first all-fiberglass car, the Corvette, went into production, and the use of automotive plastic (now called composite plastic) began to develop.

Composite plastics have changed greatly since that time, though.

In the 1960s, plastic bumpers were first developed. Then in the 1980s, fiberglass changed to the more stable material, sheet moldable compound (SMC), and technicians faced a major change in repair procedures. The methods to repair SMC were different from the methods of their predecessor, fiberglass, and when technicians naively used old fiberglass repair methods, these repairs failed.

Plastic bumpers, introduced in the 1960s, were soon being manufactured from a staggering array of plastics, many with unpronounceable names. Such a vast array of plastics became available, each with their own differing repair recommendations, that repair technicians were unable to remain up-to-date on the recommended repair procedures for all of them. As a result, plastic parts that could have been repaired were usually just replaced.

Plastic (or composite) parts continued to improve, though, and engineers, faced with rising gas prices, developed more and more plastic automobile parts to lower the weights of vehicles, thus improving their gas mileages. Repairers began to ask the question, "How can we repair these parts successfully?" With the improvement of plastic composite repair materials, technicians were now able to make plastic repairs without needing a staggering array of repair materials on hand.

Plastic composites and their use in automotive construction will continue to evolve. The repair procedures will also continue to evolve, and repair technicians must continue to learn about, understand and adapt to these new products and procedures.

Categories of plastics

Plastics or composite materials refer to a wide range of man-made synthetics components. Composite plastics are combinations of two or more plastics, formulated to achieve specific performance characteristics. They are created from substances such as oil, coal, natural gas, soybeans and corn, just to name a few. These plastics fall into one of two categories: thermoplastics and thermoset plastics.

Thermoplastics can be repeatedly softened and shaped when heated without changing their chemical makeup. These plastics soften when heated and, when placed into a mold, they can be shaped. Then, when the molded plastic is allowed to cool, it will harden. This group of plastics can be repaired by plastic welding or with adhesives.

Plastic welding, which is similar to brazing, uses either hot air or airless heat to soften the thermoplastic material, and then adds a filler rod to fuse the softened plastic. When cooled, the parts have become fused together or "welded." To accomplish this, the technician must identify the original plastic, so the proper filler rod can be chosen for the repair. When using adhesives to repair thermoplastics, the precise type of plastic is less critical.

Thermoset plastics are plastics that, following the melting and forming of the plastic part during manufacturing, will no longer soften or melt with heat. They cure by using a catalyst, which may be added or sped up by the application of heat, or by radiation such as ultraviolet or infrared light. Following catalyzation, thermoset plastics will not soften or reflow by adding heat or radiation. This means they must be repaired by using adhesives.

Adhesive repairs of flexible plastic

Chemical adhesive bonding has become popular with collision repair technicians, due to the advances made in the application, sandability and durability of the products. These adhesives can be epoxy, urethane, acrylic or cyanoacrylate ("super glue"), though cyanoacrylate may not be strong enough to be used alone. These adhesives, which are generally two-part adhesives, cannot and should not be intermixed. Mixing the two agents (filler and bonding agent) is extremely critical, and if the two ratios are not precisely measured, the adhesive will not perform as designed, resulting in failure.

To aid the technician with this precise measuring, manufacturers have developed a two-tube dispenser, where each tube is sized to hold and dispense the precise amount of each agent. Mixing is also critical, and a mixing tube has been developed that produces a properly mixed adhesive when the precisely measured materials are forced through it.

There are many competing adhesive bonding products to choose from, each with a slightly different repair method that should be followed for maximum strength of repair. Their cleaning, straightening (heat) and setup methods are similar. The types of repairs are also similar, with straightening or stress relieving done with heat, one-sided gouge repairs with primarily filling agents, and two-sided repairs where the plastic part has been damaged completely through the plastic. Two-sided repairs generally require that the part be removed from the vehicle so access to the back is possible.

Single-sided repairs

A single-sided repair is one that repairs a gouge or defect that has penetrated the plastic material, but has not cut through the plastic. Scratches, gouges and scrapes require single-sided repairs. These defects will require cleaning, sanding and tapering of the area to be repaired prior to application of the repair adhesive. Because there are differing types of adhesive repair materials available and each manufacturer may call for a slightly different type of application technique, repair technicians should always read, understand and precisely follow the manufacturer's recommendations.

Most repair procedures call for:

  • Cleaning with a plastic cleaning agent;
  • Sanding to remove paint from the surface of the repair area;
  • Tapering so the adhesive can have a larger surface to bond to;
  • Application of an adhesion promoter, then the adhesive;
  • Curing;
  • Sanding level and smooth.

Two-sided repair

Two-sided repairs often require the technician to remove the part from the vehicle. The technician will do most of the structural repairing from the backside. This type of repair is necessary on damage that has penetrated the plastic part. At times, some of the plastic may even be missing. Often, when a two-sided repair is needed, the plastic has been forced out of shape. In this case, the technician may need to remove some of the part's material, so that the plastic can be returned to its original shape without bulges. When the part is realigned, the repair can be performed.

The edges of the plastic should be gradually tapered from the front of the part to the rear. Repair manufacturers often call for a taper ½ to 1½ inches wide. Next, the front and back sides of the repair should be sanded with the recommended grit and cleaned in preparation for applying the repair material. The repair recommendation might include the application of an adhesion promoter, depending on the type of flexible plastic being repaired.

Typically, instructions for applying the adhesive on the back side of the repair recommend reinforcement of that side. Though reinforcement recommendations vary — with some suggesting a peel-and-stick type of reinforcement and others recommending the application of one or more layers of mesh — the integrity of the repair is assured with back reinforcement.

On the front tapered area, the technician applies the repair material the same way as one-sided repairs. Then, after the plastic has cured, the front is sanded smooth, primed with plastic primer filler and refinished.

Rigid plastic repairs

Though rigid plastic parts can be made from many different types of plastics, their repair methods, both two-sided and single-sided, are similar to soft plastic repairs. The critical component is that the technician follows the repair techniques set out by the manufacturer of the material being repaired.

A discussion of rigid plastics would not be complete without an examination of reinforced plastic parts. Reinforced plastic comes in two basic categories: fiber reinforced plastic (FRP) and sheet moldable plastic. Fiber-reinforced plastic, which was for years called fiberglass, uses rosin, which is spread onto, then allowed to soak into, a fiberglass mat while in a mold. The rosin hardens by use of a catalyst. When cured, the part is then removed from the mold, trimmed and used. Fiber-reinforced plastic has a smooth front and has visible fibers on the back.

SMC is a product that combines both fiber reinforcement and rosin that is placed into a die and heated. The heat activates the SMC, which hardens into the desired part. Manufacturing using SMC is more economical than using fiber-reinforced materials, and thus it has become the material of choice.

SMC is smooth on both sides and requires the use of specific SMC repair materials. The manufacture of Corvettes, previously known for using "fiberglass" (fiber-reinforced plastic), adopted the use of SMC in the 1980s.

The two differing rigid plastics have different repair procedures. After identifying the rigid plastic as either FRP or SMC, the technician should choose an adhesive material suited for this type of material and then follow the manufacturer's recommended procedure.

The use of plastic in the manufacture of automobiles has steadily increased with consumer demand for more fuel-efficient vehicles. There is reason to believe the use of plastics will continue to develop, and the development of adhesive repair techniques will continue, with ease of application and reliability of repairs improving as well.

The demand for technicians who have the skills to repair plastic will continue to grow. Because of this inevitable change, technicians will need to keep current with the ever-changing techniques for plastics repair.

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