What Are Biodegradable Plastics?

Your plant injection molds disposable cutlery, and a food service customer asked you to switch from polypropylene to a biodegradable polylactic acid (PLA) resin, in green, for its new packaging line. Your team loaded the bags straight into the hopper, used the usual green masterbatch and adjusted the temperature profile until the parts filled the mold. The cutlery came out with good color, passed visual inspection and shipped on the committed date. Two weeks later the complaint arrived: the spoons bent in hot soup and the forks snapped on first use. The PLA had gone into the machine wet and lost strength in the barrel, and on top of that the part softened at a temperature polypropylene handles without trouble. Your customer pulled the cutlery from its locations, returned the inventory and asked you to explain in writing what material you had sold them.

Biodegradable plastics are polymers that microorganisms convert into water, carbon dioxide and biomass under defined conditions. They are processed with their own rules for drying, temperature and color.

The confusion about origin that ends in the wrong material

The word "biodegradable" describes the end of a material's life. Bacteria and fungi consume the polymer chains and turn them into water, carbon dioxide and biomass. That conversion depends on five conditions: temperature, moisture, oxygen, microbial activity and time. A polymer that biodegrades in weeks inside a composting facility can last for years in dry soil, at the bottom of a landfill or in the ocean.

That is why, when your customer asks for "a biodegradable plastic," the first useful question is where the part will end up. The answer defines the resin, the way to process it and what your customer can print on its packaging.

Bio-based and biodegradable: two independent properties

The origin of the raw material and the behavior at the end of life are separate properties. A bio-based plastic is made wholly or partly from renewable feedstocks, such as corn or sugarcane. A biodegradable plastic breaks down through the action of microorganisms. PLA has both properties. PBAT is made from petrochemicals and is biodegradable. Sugarcane polyethylene is bio-based and, at the end of its life, behaves just like conventional polyethylene. If your customer uses the word "bioplastic," confirm which of the two properties it needs.

Oxo-degradables: fragmentation triggered by an additive

A third category often gets mixed up with the other two: conventional polyethylene or polypropylene with a pro-degradant additive that speeds up oxidation and fragmentation. Its mechanism, its regulation and its evidence differ from those of a biodegradable resin, and several markets restrict its use. It is best to treat it as a separate family from the quoting stage.

Four families of biodegradable resin and the typical mistake with each

In the Mexican market, most projects arrive with one of these four families, alone or blended. Each has a specific weakness worth knowing before you accept the order.

PLA: rigid, clear and sensitive to heat

Polylactic acid is obtained by fermenting plant sugars. It produces rigid, clear parts and is used in cold cups, thermoformed trays, film, fiber and injection molded parts. Its glass transition temperature is around 60 °C: above it, the part softens and deforms. That limit explains the bent spoons in the opening example. For hot applications there are crystallizable grades and modified formulations, which call for different mold and process conditions.

PBAT: flexible and almost always blended

Polybutylene adipate terephthalate is a flexible polyester, with an elongation similar to that of low-density polyethylene. It is rarely processed on its own. It is blended with PLA or starch to adjust stiffness, tear resistance and cost, and its main application is film for bags and flexible packaging. Every commercial blend has its own processing window, so the data sheet for the exact grade takes precedence over any general rule.

Thermoplastic starch and PHA: moisture and a narrow window

Thermoplastic starch readily absorbs moisture from the air and its properties change with it; that is why it is almost always used in blends. Polyhydroxyalkanoates (PHA) are produced by bacterial fermentation, and some of their grades have been assessed for biodegradation in soil or water. In exchange, their processing window is narrow and their cost is high. With both, moisture and temperature control decide whether the part keeps its properties.

What changes on your machine when you process a biodegradable plastic

A molder that has mastered polypropylene already has almost everything it needs to process PLA or PBAT. What changes are three plant habits, and all three show up in the cutlery case.

Hydrolysis: the moisture that cuts chains in the barrel

PLA, PBAT and PHA are polyesters. At melt temperature, the water they have absorbed breaks the ester bonds in their chains. Molecular weight drops, viscosity falls and the part comes out brittle, even if it looks perfect. That is why these resins are dried in a desiccant dryer at the temperature, time and residual moisture specified on their data sheet. An open bag sitting in the hopper during a humid shift can undo that drying.

Thermal window and residence time

These resins process at lower temperatures than most conventional thermoplastics, and the margin between melting well and degrading is short. A line stop with material held in the barrel, a poorly calibrated heater zone or excessive shear will yellow the part and reduce its strength. It is best to purge before and after production with a compatible material and to plan stops so the barrel can be emptied.

Storage of the resin and the finished part

Moisture and heat also act outside the machine. A hot warehouse speeds up water absorption in the resin and, over time, can affect the properties of some finished parts. Rotate inventory by receiving date, keep bags closed and ask your resin supplier for the recommended shelf life.

The color that weakens the part or compromises its claim

Masterbatch goes into a biodegradable part the same way it goes into any other, and with it come its pigment, its carrier and its moisture. Each one can affect the part or what your customer wants to claim about it.

Pigments, fillers and absorbed moisture

The lower processing temperature widens the list of pigments that can withstand the heat. On the other hand, some inorganic pigments and mineral fillers carry absorbed moisture, and in a polyester that moisture adds to hydrolysis. Dispersion also needs attention, because these resins run with less shear. If your customer is going to claim its part as compostable, the pigments must also meet the heavy metal limits of the applicable standard.

A carrier compatible with the biodegradable resin

A masterbatch with a polypropylene carrier can melt poorly at PLA temperatures and leave color specks or streaks. Traditional masterbatch in biodegradable resins works in some cases, and in others a compatible carrier is the better choice. The practical rule is to choose the carrier based on your resin, your temperature and the claim your customer will make.

Dosing and drying the concentrate

If the masterbatch carrier is also a polyester, the concentrate is dried along with the resin or separately, with the same discipline. Stable dosing matters more than in a polyolefin, because any excess carrier or pigment alters a formulation that already works with little margin.

How we develop color for a biodegradable resin at Pigmentos Químicos

At Pigmentos Químicos we formulate custom masterbatch and routinely work with PE, PP, PS, PC, PET and ABS. When your project uses a biodegradable resin, we assess compatibility with your exact grade before quoting. We review with you the resin data sheet, your process, your barrel temperature, how the part will be used and whether your customer will claim it as compostable. With that information we define the possible pigments and the right carrier.

A custom development starts at 100 kg and includes a free 1 kg sample so you can validate it on your line, with your dried resin and your temperature profile. Every order comes with a quality certificate, technical data sheet and safety data sheet, and we operate under an ISO 9001:2015 certified quality management system.

A biodegradable plastic performs when it is chosen for where the part will end up, processed dry and within its thermal window, and colored with pigments and a carrier designed for that resin. At Pigmentos Químicos we assess with you how compatible your color is with your biodegradable resin before formulating, and we send you a free kilogram to test it on your line. Send us your resin data sheet, your process and your color at our color development page

Frequently asked questions

Does a biodegradable plastic break down on its own in a landfill or outdoors?

It depends on the resin and the environment. Many biodegradable resins need the temperature, moisture and oxygen of a composting facility to break down in short time frames. In a landfill, with little oxygen, or in dry soil, the process can take much longer.

Can I process PLA on the same machine I use for polypropylene?

Yes, with three precautions: purge the barrel with a compatible material, adjust the temperature profile to the PLA data sheet and dry the resin before loading it. Polypropylene residue in the hopper or on the screw contaminates the first parts and reduces their strength.

Does the masterbatch affect whether my part stays biodegradable?

At low let-down ratios, its effect on degradation is usually small. If your customer will claim the part as compostable, the masterbatch is part of the formulation the certification evaluates.