Like PET, polylactic acid can be extruded into sheet and then thermoformed, or injection molded into preforms and blown into bottles and processed on the same machinery as PET. But PLA preparation is critical, through proper crystallizing, drying and blending of the bioresin, according to machinery makers.
Uncrystallized PLA becomes sticky and clumps when it reaches 140° F, known as its glass-transition temperature, or the point at which the amorphous portion of the pellet begins to soften.
That means PLA requires lower temperatures than PET, for crystallizing and drying, according to Universal Dynamics Inc. of Woodbridge, Va. Una-Dyn has a line of crystallizers and related dehumidifying hopper dryers with air diffusers that are specially designed for the properties of PLA.
Companies that change back and forth between PET and PLA should ensure the two materials don't mix. ``Any pipes and vessels that are shared will require complete cleaning,'' said Una-Dyn President Bill Goldfarb.
Before it can be dried, PLA pellets or flake must be crystallized. Pellets must reach a uniform crystallinity, with no amorphous pellets left over.
``You first have to crystallize it, so that it doesn't stick together, before you can bring it up to the high temperature you need to put it in a dehumidifying dryer, because you can't get the moisture out at the low temperature before it crystallizes,'' said Frank Nissel, chairman of sheet extruder manufacturer Welex Inc.
Nissel said extruding PLA sheet is no problem. ``It runs essentially on the same equipment you run PET on, with minor modifications,'' he said in an interview at his company's headquarters in Blue Bell, Pa.
According to Nissel, Welex machines can extrude sheet both PLA and PET with no need for pre-drying or crystallizing, using a vented barrel. Thermoformed cups, clamshells or other packaging that all stack together need some special treatment so they don't stick together, he said.
``Welex has always run PET vented, and in many cases, we just take the material as it comes, and run it vented,'' Nissel said. ``But, there's one issue that we have. All of this sheet, PET and PLA both, need to be silicone-coated before you thermoform. Because the stuff is very tacky, PET and PLA both, and if you don't silicone-coat them, you'd never get them apart.''
Nissel estimates that 70-80 percent of PLA goes into thermoformed food containers and cups.
Operations that thermoform thin-wall packaging generate a lot of scrap what's left over after parts are formed. The material is ground up and returned to the process. But Una-Dyn said PLA regrind is amorphous, and since it will soften at very low temperatures, cannot be treated the same as virgin, crystalline pellets. The material has to be mixed with virgin PLA and then crystallized, which happens when PLA is slowly heated.
But during that change, PLA resin particles become tacky, so they will clump in a crystallizer and won't flow through the system, Una-Dyn said.
Una-Dyn's crystallizer for PLA steadily agitates the material while it's heated, using stirring bars. The operator can see the process by looking through sight-glass openings in the vessel. If any large clumps reach the lower parts of the crystallizer, the equipment blocks their discharge so they can be removed manually.
Una-Dyn operates its crystallizers at 190°-210° F air-inlet temperature to convert amorphous resin to crystalline.
``You want to run a low-temperature process'' to prevent clumping, Goldfarb said. ``PET is known for developing clumps that can be broken up easily, but PLA clumps can't be broken via mechanical means.''
When amorphous PLA is heated above the glass-transition temperature, its surface becomes tacky just like PET, said Bob Crawford, Una-Dyn vice president of engineering. ``In order to dry the resin adequately, we will use the crystallizer to make the initial conversion to a surface crystalline chip, developing a hard- surface particle,'' he said.
Una-Dyn officials stress proper conveying methods for PLA. ``You want to use a low-velocity handling system and use degradation-abatement measures because PLA is more sensitive to degradation,'' Goldfarb said.
Despite the need to use different handling and processing procedures for PLA, lower temperature requirements give companies a cost advantage, according to Goldfarb. PLA can be dried and crystallized at 66 percent the cost of PET. Since melt temperatures are more than 100° F lower than PET, there is a net reduction in heating costs of one half for PLA resin compared to PET.
``It is a huge opportunity to save costs,'' he said.