Neat or pristine PLA, while having good flexibility, biocompatibility, and biodegradability, also has limited thermomechanical properties that lead to limited applications and inefficiencies in its processing.
Since PLA parts are more commonly produced using heat processing methods like injection molding, extrusion, or 3D printing, this often necessitates compounding to improve its processibility and performance.
Fillers and additives are widely used in PLA to improve its processibility, particularly during thermal processing. A specific additive or filler can impact more than one property of PLA. The properties of the final PLA compound—or the performance of the final part—are the combined effect of all components and the processing conditions.
In most applications, PLA is rarely used in its neat form. It is either blended/compounded with other polymers or with additives and fillers. For example, in biomedical applications like tissue engineering, PLA might be blended with hydroxyapatite to better mimic bone tissue in terms of mechanical properties.
With PLA compounding, you are using one of two approaches: conventional compounding or injection molding compounding. The following sections discuss both methods.
Conventional Compounding vs Injection Molding Compounding
In the conventional process, compounding is done in a twin-screw extruder. The polymer, additives, and fillers are fed into the hopper in specified quantities. The formulation is mixed and melted under heat and shear, and the output is typically pelletized compound.
These pellets are then fed into a separate injection molding machine, where they are plasticized again and injected into the mold to form the final part.
A twin-screw extruder is required to achieve sufficient shear force for proper mixing. If mixing is insufficient, non-uniformity and phase separation can occur, defeating the purpose of compounding. The goal is to create a uniform material with improved properties.
In injection molding compounding, the compounding and molding stages are combined into a single process. This eliminates the pelletizing step, saving time and energy while reducing thermal degradation.
Both processes have advantages:
Conventional compounding
Easier scale-up
Enables specialization
Uses ready-made pellets
Injection molding compounding
Greater flexibility
Allows formulation changes during production
Suitable for smaller batches
The Purpose of Compounding
Compounding modifies a polymer by adding fillers, additives, or blending with other polymers under heat and shear.
For example, PLA can be modified to achieve toughness similar to polycarbonate while retaining biodegradability and biocompatibility.
This is particularly important in biomedical applications, where materials must balance strength and biodegradability.
Role of Fillers and Additives in PLA Compounding
Fillers are fibres or particles added to polymers to form composites. During processing, the polymer is molten while fillers typically remain solid.
Fillers can:
Reinforce strength
Reduce material usage
Add functional properties (e.g., thermal conductivity)
A composite consists of:
Matrix (continuous phase) → polymer
Dispersed phase → filler
Additives, on the other hand, are typically blended into the polymer matrix.
Difference Between Fillers and Additives
Fillers Additives
Add bulk to polymer Minimal contribution to volume
Used in large amounts (>50 wt%) Used in small amounts (~0.1 wt%)
Create composites Used in neat or composite polymers
Mainly affect physical properties Affect physical & chemical properties
Solid phase Liquid or solid
Dispersed phase Continuous phase
Usually cheaper Usually more expensive
Some Common Fillers and Additives Used in PLA
Fillers can be inorganic, organic, bio-based, or synthetic.
Examples include:
Hydroxyapatite
Carbon fibre
Cellulose fibres
Hemp hurd
Basalt fiber
Calcium carbonate
Glass fibers
Wood fibers
Cork
Keratin
Even eggshells (rich in calcium carbonate) have been explored as sustainable fillers.
Example additive brand:
Incromax
Thermal Stability and Heat Resistance
PLA heat resistance can be improved by:
Blending with polymers like:
POM
PA
PC
ABS
Adding nucleating agents (e.g., zinc phenylphosphonate)
Using fillers like cellulose nanofibers
Adding UV stabilizers and antioxidants
Processibility
Good processibility means:
Moderate melting temperature
مناسب melt viscosity（既能流动又不漏料）
良好的热稳定性
合适的玻璃化温度
PLA本身不完全满足这些条件，因此需要添加：
Plasticizers（增塑剂）
降低熔体粘度
降低熔融温度
降低玻璃化温度
Compatibilizers
Compatibilizers prevent phase separation between components.
它们的作用类似“桥梁”，增强：
polymer ↔ filler 结合
polymer ↔ additive 相容性
Example:
Hexadecyltrimethoxysilane (for glass fiber reinforced PLA)
Nanoparticles and Microparticles
At micro/nano scale, fillers and additives may overlap.
They can:
Improve thermal stability
Improve mechanical properties
Achieve效果 with lower dosage
Examples:
Nanocellulose
Nanoclay
Carbon nanotubes
Biodegradation
PLA biodegradation can be tuned:
Biomedical use → controlled degradation
Packaging → balance shelf life & compostability
Typical fillers:
Calcium phosphate
Hydroxyapatite
Flexibility
PLA is brittle in its neat form.
To improve flexibility:
Add plasticizers such as:
Citrate esters
PEG
Epoxidized oils
Toughness
To improve toughness:
Add elastomers:
EPDM
Acrylonitrile rubber
Use nanofillers:
Nanocellulose
Nanoclay
Carbon nanotubes
Blend with biopolymers:
PBAT
PBS
PA11
Cost
Fillers are usually cheaper than PLA.
Benefits:
Reduce material cost
Reduce polymer usage
Avoid expensive molding processes
Compounding can be more cost-effective than:
Foam molding
Gas-assisted molding
Conclusion
PLA compounding enables a wide range of material formulations for different applications.
By selecting appropriate fillers and additives:
Mechanical properties can be tailored
Processing efficiency can be improved
Costs can be reduced