In the trendiest boutiques and even in more conservative stores, plastic eyewear is seeing a revival. Let's take a look at two of the more commonly used plastic frame materials-cellulose acetate and cellulose propionate. Both are from cellulose, but what's the difference?

	Guess Eyewear, courtesy of Viva International Group.

Cellulose acetate is better known as zyl, and is the most commonly used plastic frame material. It is made from cotton seed fibers and wood flakes, and stabilizers and plasticizers.

Cellulose propionate is used for injection molded frames, meaning temples and frame fronts are created individually. Injection molding is a technique whereby resin is melted, and subsequently forced into a mold. Then it's cooled, so it can hold the frame shape.

Cellulose acetate frames can also be injection molded. However, it is formed from sheets of block acetate, from which temples and frame fronts are cut. Industry experts seem to agree that the sheet method is usually better for creating the zyl frames.

"Sheet stock is the better method," says one vendor. "When you inject mold there is more to worry about, such as microscopic bumps, dust, or dirt within the mold. If it cannot be removed, you will have bumps, dots, and indentations on the frame itself, in the same spot on each and every frame."

Acetate Origins

According to a spokeperson for Mazzuchelli, there are four ways to create sheet stock acetate:

Block: Colors are hand laid onto a cake and then pressed with heat and pressure.

Extrusion: Machines called extruders melt down colored resins and then feed it to a dye. The dye regulates the flow of the color to make an overlay or demi or any other pattern.

Laminate: Extruded products and block products are laminated together or with the other.

Ceblox: Extruded components or even a mix of block components, are placed into a mold and turned into a sheet by compression molding. 

One advantage of cellulose propionate frames is their light density.

This material can also be made thinner and can hold its shape well, but there is less room for expansion of the frame to insert lenses. One vendor says cellulose propionate is often used for clear or transparent materials.

"Making it more useful as a traditional product, or for mature styles," she says.

Choose your plastic

Some say cellulose acetate is more "optician friendly." In the words of one insider, "It is most desired by the optician due to ease of handling and perceived value. There is more room for expansion and constriction to fit the lenses to the frame, and more adjustments can be made to the material."

So, is one really better than the other? It depends on the application. Acetate has a tremendous advantage when it comes to shape, according to the Mazzucchelli spokesperson. Acetate in sheet form is more friendly to manufacture as well as to insert lenses, as opposed to propionate. Regardless of the differences, one thing is certain: Plastics are hot. EB