What is the raw material for biodegradable plastic bags?

I see brands race to go green, yet buyers still feel lost. Plastic bans grow. Cost worries rise.

Most biodegradable plastic bags¹ start with plant starch², polylactic acid (PLA)³, or poly-butylene adipate terephthalate (PBAT⁴) blended with calcium carbonate. These feedstocks break down under industrial compost⁵, leaving water, CO₂, and biomass.

I mix these resins every week while testing BagMec® film lines. I watch how each pellet melts, cools, and seals. PLA gives high clarity. PBAT adds softness. Starch cuts price. The right ratio keeps tensile strength⁶ above 18 MPa and melt flow near 3 g/10 min, so the film will run on standard bag machines with few changes.

I often guide buyers through three steps.

1. Confirm end-of-life: home compost, industrial compost⁵, or landfill decay.
2. Match resin type to bag style: T-shirt, courier, or zipper.
3. Adjust machine zones: screw temp, cooling roll speed, knife dwell. When these points align, film waste drops by 12-20 %, and certification goes smoothly.

How is polylactic acid (PLA) turned into film for bags?

Plant sugars hold the key.

I ferment corn, cassava, or sugarcane into lactic acid, polymerize it to PLA resin, then extrude it into film between 160 °C and 190 °C. Quick chill rolls lock clarity and keep shrink below 3 %.

From plant to pellet

1. Mill crop starch to glucose.
2. Add bacteria. They eat glucose and release lactic acid.
3. Purify, dry, and combine lactic monomers. A catalyst links them into long PLA chains.
4. Cut solid strands into 3 mm pellets.

Extrusion window

I feed pellets into a 38∶1 L/D screw. Shear melts resin fast yet avoids thermal spots. Barrel zones sit at 165 / 175 / 185 / 185 °C. Melt pumps hold 0.3 % pressure swing. A 200 µm die forms the web. Chilled rolls at 20 °C fix the amorphous state.

Step	Core parameter	Good range	What goes wrong outside range
Drying	Moisture	< 250 ppm	Hydrolysis cuts MFI, film splits
Melting	Barrel temp	160-190 °C	Yellow color, low gloss
Cooling	Roll temp	18-22 °C	Warp, waviness
Winding	Tension	3-5 kg	Telescoping rolls

Seal strength tests

I seal at 145 °C, 1.5 s dwell, 2 bar pressure. Peel force hits 8 N/15 mm. That beats many HDPE bags. Yet PLA alone cracks in cold trucks. I blend 20 % PBAT⁴ when frozen-food clients ask.

Why does feedstock choice impact machine settings⁷?

Fear of line downtime keeps managers up.

Each bio-resin⁸ melts, cools, and seals at its own pace, so I must tweak screw speed, nip tension, and knife heat to stop gels, curling, or weak seals.

I run side-seal, wicket, and courier bag machines daily. When starch content rises above 40 %, gels appear. I drop screw speed from 80 to 65 rpm. I lift back pressure with a smaller filter screen. Curling edges tell me cooling air is too warm. A five-degree change fixes it.

Fine-tuning cheatsheet

Resin blend	Ideal MFI (g/10 min)	Barrel temp (°C)	Seal bar (°C)	Notes
100 % PLA	3-5	160-190	140-150	Clear, stiff
60 % PLA / 40 % PBAT	5-6	150-180	125-135	Soft, tough
40 % starch / 30 % PBAT / 30 % PLA	6-8	140-175	120-130	Cheap, opaque

Bag type versus blend

T-shirt bags need dart drop > 150 g. I keep PBAT⁴ high.
Courier bags need stretch. I add 10 % elastomer.
Bread bags need clarity. I stay near pure PLA.

Quick swap SOP

1. Purge extruder with 30 kg LDPE.
2. Load new bio-resin⁸.
3. Run 15 min at 50 % speed.
4. Dial temps minus 10 °C if PBAT rises.
5. Cut first 200 m, check gauge.

These steps cut changeover loss to 2 % of daily output.

Which raw material fits my bag making budget?

Money drives every deal.

Starch blends cost $1.2 – 1.4 / kg, PLA around $1.8 / kg, and PBAT near $2.0 / kg; a 30 % starch mix drops bag cost by 18 % yet keeps OK strength for retail sacks.

I sit with buyers, open spreadsheets, and plug numbers. Film yield, bag weight, scrap, power, labor—all enter. Below is an average case for a 200 mm × 400 mm T-shirt bag.

Item	30 % starch blend	60 % PLA blend	100 % PLA
Resin cost/kg	$1.30	$1.60	$1.80
Bag weight (g)	7.0	6.3	6.1
Resin cost/bag	$0.0091	$0.0101	$0.0110
Power & labor/bag	$0.0015	$0.0015	$0.0015
Total/bag	$0.0106	$0.0116	$0.0125

A million-bag run saves $1,900 with starch. But third-party compost tests fail when starch exceeds 50 %. So I guide clients to 30-40 % to balance price and certificates.

Hidden costs

1. Dryer electricity rises 0.15 kWh/kg for high starch.
2. Knife life drops 10 % with chalk filler.
3. Shelf life shortens; PLA loses strength after 12 months. I label cartons with month-year to avoid recall.

Revenue upside

Eco-logo bags sell 8-15 % higher at retail. Many supermarkets agree to long-term supply deals when they see EN 13432 or ASTM D6400 stamps.

Can current bag machines handle biodegradable resins without upgrades?

I meet this doubt in almost every call.

Most servo-driven side-seal, T-shirt, and courier bag machines run PLA or PBAT blends after swapping Teflon coatings, adding low-inertia nip rolls, and tuning PID seal bars; hardware cost stays under $3,000.

I swap chrome seal belts for PTFE to avoid sticking. I replace hard-coated take-up rolls with silicone sleeves. I mount extra cooling air rings on film blowers. Control software already stores recipes, so I log new temps and speed.

Upgrade cost table

Part	Change	Price (USD)
PTFE seal belt set	Replace standard	450
Silicone nip roll sleeves	Add	320
Extra air ring	Install	880
Exhaust hood	Install	540
PID module firmware	Update	Free (BagMec)
Total		2,190

Return on that spend comes fast. A 50 kg film break can waste $90 in resin and one hour of labor. Avoid four breaks per month and payback arrives in two weeks.

Safety notes

PLA fumes stay mild, yet PBAT can emit small aldehydes. I fit carbon filters. Screw pull force rises 8 %, so I grease gearbox weekly.

Certification path

I partner with TÜV and DIN CERTCO labs. They test disintegration at 58 °C for 12 weeks. Bags must leave no fragment over 2 mm. My latest batch hit 90 % mass loss in 10 weeks. Data tables impress auditors and help clients win food-contact licenses.

Why should I mix biodegradable and recycled content?

I can blend 20 % post-consumer PLA regrind into fresh PLA without hurting seal strength, cutting resin cost by 12 % and carbon footprint⁹ by 18 %.

I grind edge trim on site. I dry it to 150 ppm moisture. I vacuum-load it with virgin pellets at a 1 : 4 ratio. Melt flow stays within spec. Yet color drifts a shade yellower. For cosmetic bags I add 0.2 % blue masterbatch. For courier sacks I skip the dye.

Carbon math

Source	CO₂ e/kg	Share	Weighted CO₂ e
Virgin PLA	1.65	80 %	1.32
Recycled PLA	0.35	20 %	0.07
Blend			1.39 kg

That saves 0.26 kg CO₂ per kg film. Over 500 t per year, a plant cuts 130 t CO₂, equal to 30 gas cars off the road. Buyers in the EU reward that with green-label tenders.

Machine impact

Regrind raises dust. I add cyclone separators. I set screw vent vacuum to −0.95 bar. Screen change jumps from 8 hours to 6 hours. Still, extra labor is minor.

Conclusion

Plant starch, PLA, PBAT, and smart blends fuel biodegradable bag lines. I match them to budget, machine tweaks, and certificate rules, so clients shift to green film with low risk.

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