PC Summitry:Acrylic-Based Powder Coatings

By Kevin Biller

One of the highest quality, but most overlooked powder coating technologies is based on acrylic resins. Acrylic resins and powder technology based on them have been around since the 1970s. In fact some of the first thermosetting powders were based on acrylics. Acrylics offer a panoply of impressive performance. Depending on formulation technique, they can offer amazing exterior durability, excellent chemical resistance and outstanding smoothness. The following is a summary of acrylic chemistry available in powder coatings and the expected performance of each.

GMA Acrylics

GMA acrylic powders are based on glycidyl methacrylate (GMA) functional acrylic polymers. These chemical groups react readily with carboxyl groups. The most common curing agent is DDDA (dodecanedioic acid) which is somewhat crystalline in nature and allows this type of formula to provide excellent smoothness. GMA acrylics are well known for their excellent weathering resistance which can provide coatings resistant to over 10 years Florida exposure. This type of chemistry has been used as a clear topcoat for BMW automobiles since 1998. As you can surmise, not only does this technology possess excellent UV durability, but also outstanding resistance to environmental agents (e.g. acid rain), cleaners and gasoline.

GMA acrylic powders are typically cured around 350 to 375°F; however, chemists have formulated versions that can cure at 300°F and below.

Hydroxyl Acrylics – Acrylic Urethanes

One of the more interesting acrylic based technologies uses hydroxyl functional polymers that can be crosslinked with the same blocked isocyanates used in polyurethanes (polyester-urethanes). These systems are normally high temperature curing (> 375°F) and can offer the highest chemical resistance and hardness of exterior durable powder coating choices. Acrylic urethanes were quite popular in the 1980s as the appliance industry used them mainly for range (stove) side panels and in some cases for refrigerator doors for their incredible stain resistance. Some other acrylic urethanes were used on vending machines, especially those intended for outdoor use. In most cases lower cost powders have supplanted acrylic urethanes albeit with lesser performance.

Carboxyl Acrylics – Acrylic Hybrids

An acrylic chemistry most suited for indoor applications is comprised of a carboxyl functional acrylic polymer that is cured with an epoxy resin. The epoxy resins are typically the ones normally used in polyester hybrids and pure epoxy powder formulas. These systems are commonly referred to as acrylic hybrids.

Because of the epoxy content acrylic hybrids can only be considered for applications that do not require exterior durability as the epoxy component will fade and discolor when exposed to sunlight and the elements.

Performance-wise acrylic hybrids offer higher hardness and chemical resistance compared to their polyester hybrid counterparts. This is the reason they have been used as a finish for laundry grade appliances (i.e., washers and dryers).

Acrylic Polyesters

This chemistry sounds somewhat confusing however it is possible to react a GMA functional acrylic resin with a carboxyl-polyester. Typically this system produces a rather low gloss coating with relatively poor flexibility and impact resistance.

Common Drawbacks

The biggest issues with acrylic powder technology fall into two categories: incompatibility with other powder coatings and high cost. Both of these issues are real. Most acrylic formulas will cause crater defects in a typical polyester or epoxy based powder coating. The reason is the significant difference in surface tension between acrylic polymers and epoxy and polyester resins. For this reason it is requisite to isolate acrylic based powders from other non-acrylic varieties. This can be accomplished by dedicated application systems and/or thorough cleanup when switching from an acrylic to a more traditional powder chemistry.

Another issue is cost. Acrylic systems normally command a premium price compared to polyester-based powder coatings. The improvement in performance however can often justify the additional cost.

Markets and Applications for Acrylics

Acrylics are essentially the only powder chemistry that can meet the demanding specifications of any exterior automotive application including clear topcoats, trim parts and alloy wheels. Likewise any high-end farm implement application would be a natural for acrylic powder technology.

An untapped but obvious end use is the AAMA 2604 architectural specified applications. Acrylics can easily meet the 5-year Florida requirement as well as all the chemical resistance and mechanical performance specifications.

The Future of Acrylic Powder Coatings

Acrylic powder coatings can have a promising future if finishing engineers can recognize the high performance possible with this technology. The use of powder for exterior automotive applications should expand beyond BMW’s assembly plants in Germany. All metal and engineered plastic trim parts can be powder coated with acrylics that can surpass even the most demanding specifications.

The architectural market is an obvious place to use acrylic powders. Their demonstrated exterior automotive performance should provide the confidence to transfer the technology to architectural applications including AAMA 2604 (5-year Florida) and possibly AAMA 2605 (10-year Florida).

Finally, the use of acrylic powder technology should be explored for any application requiring exterior durability and excellent chemical resistance.

Kevin Biller is technical editor for Powder Coated Tough magazine. He can be reached at kevinbiller@yahoo.com.