What are the Properties of Polylactic Acid?

Testing the tensile properties of polylactic acid
Testing the tensile properties of polylactic acid
Resource: https://www.youtube.com/watch?v=uAoZCpXoPWo

Based on the required performance, the properties of polylactic acid can make it suit some applications more than others. The right choice must, therefore, be made. That, in turn, requires an understanding of the various PLA properties and how these can be modified to suit a manufacturing process of product application. This definitive guide will help you learn all that.

What does Polylactic Acid Look Like?

Polylactic acid is generally clear and transparent. However, it’s also common practice for manufacturers to produce it in the form of colored granules or pellets, ready for processing into different products.

Apart from the physical appearance, some properties of PLA make it what it is: a bioplastic that you can use in a variety of applications.

And while some of these properties are based on the PLA chemical structure and cannot be changed, others are modifiable during manufacture; which is why its production is such a controlled process.

That aside, some properties of polylactic acid make it usable in some applications than others. Understanding these characteristics is, therefore, a crucial step when you intend to use it in your project.

Testing polylactic acid thermal properties
Testing polylactic acid thermal properties
Resource: https: https://www.youtube.com/watch?v=mwS_2R2mIvo

Properties of Polylactic Acid

The properties of polylactic acid determine its performance characteristics when used in different industries. They also help researchers find new ways of using the plastic, such as blending it with other polymers to increase mechanical strength, or annealing to raise its melting point. PLA properties are broadly classified as physical, mechanical, chemical, and thermal.

1. Physical Properties of Polylactic Acid

These mean the characteristics that can be observed in the material without a change to its chemical structure or identity. The physical properties of polylactic acid include its density, melting point, viscosity, glass transition (Tg), and heat transfer characteristics.

PLA Density

Polylactic acid has a density that varies from 1.25 to 1.28g/cm3. The specific value depends on various factors as type of raw material and manufacturing method, or if other materials have been added to it. For example, additives introduce voids and decrease its density, while fillers serve to increase it.

Generally, the low density of polylactic acid makes it easier to process into different products using different manufacturing processes that include extrusion, 3D printing, and injection molding.

Higher density polylactic acid (PLA with fillers) is used when the product must be tough and rigid. On the other hand, low density PLA (with additives) is used in applications where the flexibility or a raw material or final product is paramount. These include packaging and textiles.

Melting Point

Being a thermoplastic, polylactic acid has a low melting point of about 176°C. The generally low melting point of polylactic acid is especially useful in some industrial processes, such as 3D printing and injection molding. In 3D printing, for instance, it means a raw material that melts and cools quickly and does not warp.

Annealing is often used to improve the melting point of PLA whenever necessary. In this process, the material is first heated to a heat level just above its Tg or glass transition, then gradually cooled. Other ways include blending it with higher melting point polymers.

Intrinsic Viscosity

Intrinsic viscosity is a crucial factor when it comes to the physical properties of polylactic acid. It defines its behavior, both during processing and in the final product. In essence, it reflects a polymer’s molecular weight and thus, its physical and mechanical characteristics such as melting point and strength.

Polylactic acid has a low intrinsic viscosity (IV) of 0.2-8 dL/g. This low IV makes it a good material for any process that requires a material that will melt and flow with ease. Examples include extrusion and injection molding.

The low intrinsic viscosity is also one of the PLA properties acid also one of the reasons for its application in the packaging and textile industries, as you can easily extrude it into thin films and fibers. Whenever needed, the PLA intrinsic viscosity can be modified using special additives known as chain extenders.

Glass Transition Temperature

Glass transition, usually abbreviated Tg, is defined as the temperature level at which a material changes from solid state to a less rigid or viscous state. The glass transition of PLA is low, at around 60°C to 65°C. This is desirable in some applications, such as 3D printing and extrusion.

The glass transition of polylactic acid also makes it an excellent material for the making of shrink wrap. This property has its downsides, though. It makes the material less applicable in situations where it will likely be subjected to high heat conditions.

Heat Transfer Coefficient

The heat transfer coefficient of PLA is measured at 0.025 λ(w/m k). This is generally low, and can either an advantage or disadvantage based on the type of manufacturing process or product application.

During injection molding, for example, the low HTC means good mechanical properties of the finished product, as the heat transfer between the material and the mold is greatly minimized. But it also means a longer cooling time and limited production speeds.

In some applications that require heat insulation, the low HTC properties of polylactic acid are an advantage. It not only provides insulation benefits but also a biodegradable material for green application needs.

2. Mechanical Properties of Polylactic Acid

Polylactic acid exhibits certain mechanical properties, which determine its strength and ability to withstand various conditions. In general, these are comparable to the physical properties of PE and PET. The mechanical properties of polylactic acid for tensile strength, elastic or flexural modulus, elongation-at-break, Izod impact strength, and Rockwell hardness are as below.

Tensile Strength

Tensile strength is an important factor when it comes to a material’s application in different situations. It determines its ability to withstand or resist breaking under tensional loads. The tensile strength of polylactic acid lies between 40-60 MPa and varies depending on various factors such as PLA material type and grade.

When an application demands a higher tensile strength, manufacturers normally use PLA that has had other materials incorporated. These may vary but most include a number of esters. Some physical processes, such as annealing, can also be used on PLA to improve its tensile strength.

Elongation at Break

Also called percent elongation, this property shows the extent to which a material can stretch or lengthen before it can break, and usually indicated as a percentage. The elongation-at-break of polylactic acid ranges from 4% and 10%.

This is generally low, and shows the plastic to be less ductile. However, when needed, this property can be improved. This usually involves adding citrate esters or PEG, or blending it with polymers such as polycaprolactone.

Polycaprolactone is a type of polyester with a very high elongation-at-break percentage (about 600). When blended with polylactic acid, it improves its strength considerably.

Flexural Modulus

Flexural modulus defines a material’s flexural strength or ability to resist bending stress before experiencing permanent deformation. It determines the structural integrity of finished parts, and is one of the most critical properties of polylactic acid.

When subjected to bending forces, polylactic acid shows a flexural modulus of between 100-150 MPa, depending on the grade and other factors.

By selecting the appropriate value of PLA flexural strength (usually based on grade), manufacturers can effectively ensure products that meet application requirements. These include packaging containers, medical devices, and even prototypes.

Elastic Modulus

Unlike flexural modulus, the PLA modulus of elasticity shows its behavior when under tensile forces or stress. In other words, it indicates its response to tensile stress as gotten from the strain-stress curve of a recommended test.

The elastic modulus of polylactic acid is measured at between 3000 and 4000 MPa. In terms of application, it means manufacturers can use it for products that will require resistance to elastic deformation such as rigid packaging products.

Izod Impact Strength

PLA is a brittle type of plastic with low ductility properties. As such, it exhibits low impact strength when compared to several other plastics. PLA impact strength is measured using both notched and unnotched Izod tests, as defined by the ASTM D256 standard for plastics.

In the unnotched test, the material gives impact strength of 150-300 J/m, while the notched test provides readings between 20 and 60 J/m. While the low impact strength of PLA may limit its application in certain situations, adding materials to it can significantly improve performance.

One of the ways to do so is the addition of mineral fillers such as ground calcium carbonate. Blending PLA material with impact modifiers like PHAs (polyhydroxalkanoate) also produces great results, making it less brittle and more resistance to impact forces.

Rockwell Hardness

Rockwell hardness measures a material’s ability to resist deformation by compressional stress or forces. The Rockwell hardness of pure polylactic acid is measured at 88. This makes it a tough type of polyester that can withstand various forms of compressional stress.

To increase the harness properties of polylactic acid, the material may be added with reinforcing fillers such titanium or blending it with different types of polymers to produce PLA composites. This is especially seen when making 3D printing PLA, since it is one of the most used plastics.

3. Chemical Properties of Polylactic Acid

The chemical properties of polylactic acid describe its behavior when subjected to conditions that could alter its chemical structure or composition. Generally, the material shows various characteristics. For instance, it is biodegradable and will naturally break down in a reaction called hydrolysis.

Other PLA degradation methods include thermolysis (200–500° C) and alcoholysis. In terms of toxicity, polylactic acid is considered a safe material, being derived from naturally occurring raw materials and from lactic acid as its direct source.

Polylactic acid is also flammable and will readily burn. In the recent past, however, several research studies have discovered several flame retardants to use with PLA. These include nanoparticles or nitrogen, phorsphorus, and cellulose-based retardants.

4. Thermal Properties of Polylactic Acid

The thermal properties of PLA determine its many other characteristics, including strength and processability. Generally, this material has a low thermal resistance, and is mostly used in low heat applications.

Due to the nature of its aliphatic ester chain, polylactic acid will thermally degrade. PLA thermal properties can be improved when an application demands it, such as when making parts for automotive and other high temperature applications.

For example, using a thermoset polymer introduces, into the PLA structure, a highly cross-linked structure that reduces the movement of the ester chain. This serves to enhance both the PLA thermal properties and its mechanical strength.

Conclusion

Over the years, polylactic acid has been the subject of numerous research studies. These have enabled scientists to predict the material’s behavior in different conditions and environments. Understanding the properties of polylactic acid is, therefore, a crucial step when using it to manufacture parts or products. We hope the guide has been helpful in providing information about the different—and most important—PLA characteristics.

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