Optimal Processingof Polypropylene Films
with CO2 Lasers
Polypropylene, commonly abbreviated as PP, is one of the most common andimportant plastics used today. It is a thermoplastic that has practical applications in manydifferent industries. Its strength, flexibility, and resistance to damage means polypropylene film
is an excellent material for various types of storage and labeling applications. In addition,
polypropylene’s ability to withstand heat combined with chemical resistance to organic
compounds makes it a good choice in particular for food and beverage packaging.
In recent years, the digital revolution
has changed the landscape of the flexpackaging and labeling industries, challenging
manufacturers to meet demands for highdesign flexibility and shorter time to market.
Digital laser converting provides the solution
with on-the-fly customization, zero contact
processing, and repeatable results. Synrad highperformance CO2 lasers are ideally suited formany of these applications due to our longwavelength’s superior absorption in many
common film materials. In the specific case ofpolypropylene films, simply changing the CO2
wavelength slightly from the standard 10.6 µm
to more optimally absorb into that particularmaterial can result in significant speed andprocess quality improvements.

Chemical Properties and Absorption Characteristics of PP
To efficiently use the laser energy in the process, the photons of the laser beam must beabsorbed into the material rather than transmitted or reflected back. This leads to efficientvaporization of the material with little energy lost due to passing through the material, melting,or chemical degradation processes.
When absorbing Infrared Radiation (IR), a polymer molecule’s chemical bonds will vibrate.
These vibrations will stretch and bend the bonds within the polymer chain. However, for thatabsorption to occur, the energy level of the IR photons at a specific frequency must matchthe distinct vibrational energy differences within the molecule. This can be expressed by thisequation:v is the frequency of the photons (Hz)k is the force constant of the molecule (N/m)©2021 Novanta Corporation. All rights reserved.

µ is the reduced mass (kg)

Optimal Processing of Polypropylene Films with CO2 Lasers

2In a generalized sense for the stretching case, these vibrational energy differences in the moleculecan be thought of in a classical mechanics sense as a two body system connected by a spring:

The reduced mass constant of this simple system is found by:Long polymerized polypropylene molecular chains are typically linked together through single
bonds between the carbon atoms (C) in the chain: C–C, with a force constant, k, of approximately
335 N/m between them in many common propylene orientations. So in the above diagram thetwo spheres would be carbon atoms.
The mass of a carbon atom is 1.99x10-26 kg, so setting both M1 and M2 to this value gives us a
reduced mass, µ, of 9.97x10-27 kg. Solving for the frequency, v, in the first equation, we find a valueof 29.2 terahertz. Converting this to the wavelength of IR radiation needed through the relationʎ = c / v, where c is the speed of light: 3.00x108 m/s, we find a value of about 10.27 µm. This is
the specific wavelength of light required to provide photons at the energy level that matches thevibrational energy to stretch the C-C bond in the polypropylene chain. These vibrations quickly
heat the molecule until the material vaporizes.
When we experimentally measure the absorption curve of polypropylene film with an FTIR
spectrometer, we find that there is indeed a very strong absorption peak at that wavelength.

©2021 Novanta Corporation. All rights reserved.

Optimal Processing of Polypropylene Films with CO2 Lasers

3

It should be noted that there are also more complex bending vibrations of the molecule that
contribute to this absorption curve as well, but this is outside the technical scope of this paper.

Label Cutting Examples
So polypropylene absorbs the 10.2 µm significantly better, but how does this translate to actual
speed and quality improvements for a real application? To examine this specifically for label
cutting applications, the Novanta Applications Lab looked at a number of different samples
and cut them with Synrad p150 lasers at both the standard wavelength of 10.6 µm and also thealternate 10.2 µm wavelength. This alternate wavelength is achieved with band-select optics
inside the laser to only release the narrow wavelength band centered around 10.25 µm.
Clear and opaque polypropylene films were examined, as well as ones that had additionallamination on top. Each laser tube was set to output the same amount of power so any speed
differences would only be attributable to the wavelength change. The resulting cut speed
differences between the two wavelengths were most notable on the thinner labels, with around
80% speed increase being typical for 80 µm thickness, while a 22% increase was seen on
thicknesses of 140 µm.
T (µm)

Coating

10.6 Velocity(mm/s)

10.2 Velocity(mm/s)% Increase

Polypropylene - Opaque140

Polypropylene Laminate2794

3429

22.73%Polypropylene - Clear

80

None

4318

7620

76.47%Polypropylene - Opaque80

None

4572

8255

80.56%

Base Material

The cut edge quality differences were most obvious on clear PP labels in the unprinted regions.
At 10.6 µm, yellowing of the film was commonly seen and heat affect zones were evident between
the liner and base label materials where residue and hot vapor was trapped. The high transmission
at 10.6 µm also meant that scoring of the bottom liner material was very noticeable. These issues
were completely eliminated when the 10.2 µm wavelength was used instead.

Opaque label cut edge 10.2 µm wavelength

Opaque label cut edge 10.6 µm wavelength©2021 Novanta Corporation. All rights reserved.

Optimal Processing of Polypropylene Films with CO2 Lasers

Clear label cut edge 10.2 µm wavelength

4

Clear label cut edge 10.6 µm wavelength

Food Packaging Examples
Labels are not the only application that benefits from the ability to selectively process
polypropylene film layers. A common food pouch design consists of a two layer stack offilms. This includes an outward facing top PP layer and a bottom LDPE liner. Laser perforating
pouches is commonly done for easy-open tear lines. However, you do not want to penetratecompletely through the bottom liner, exposing the food while laser processing.

By carefully controlling process speed, laser power, and wavelength, selective perforation ofmulti-layer pouches can allow the upper PP layer to be processed while the bottom liners
remains intact. In this example of a multi-layer pouch, a p150 - 10.2 µm laser was carefully chosen
to only perforate the top polypropylene layer. This was impossible to do using the standard 10.6
µm wavelength which blasted through both layers due to its high transmission characteristics
through the PP layer.

©2021 Novanta Corporation. All rights reserved.

Optimal Processing of Polypropylene Films with CO2 Lasers

5Conclusion
In this paper you learned one of the basic mechanisms governing how CO2 laser radiation isabsorbed by polymers, specifically polypropylene, and how this can be optimized to maximize
the absorption through wavelength selection. Using a 10.2 µm wavelength laser can greatly
increase the process speed while improving the edge quality on polypropylene products.
Further, where selective layer processing is needed for fine labeling and packaging applications,the 10.2 µm wavelength offers a much wider successful processing window.
As the markets for polypropylene film grow, especially in the labeling and flexible packaging
spaces, the importance of 10.2 µm processing will increase as well, especially for projects withhighly demanding application requirements. Novanta is there for you with application expertiseand the laser products to help push the limit on your most demanding film applications.

Interested in speaking to one of our knowledgeable
representatives?
Americas & Asia PacificNovanta Headquarters
Bedford, USA
P +1-781-266-5700
Photonics@Novanta.com

Europe, Middle East,
AfricaNovanta Europe GmbH
Wackersdorf, Germany
P +49-9431-7984-0
Milan, Italy
P +39-039-793-710

China

Japan

Novanta Sales & Service Center Novanta Service & Sales OfficeShenzhen, China
Tokyo, Japan
P +86-755-8280-538
P +81-3-5753-2460
Suzhou, China
P +86-512-6283-7080
Photonics.China@Novanta.com

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com