Parylene Dimer FAQ: Dimer Purity and Testing

Parylene Dimer FAQ’s 

Questions and Answers about parylene dimer and testing

 

What are the differences between the types of parylene variants found in parylene C and why does this matter? 

 

The chemical name of pure parylene C dimer is Symmetrical dichloro[2.2]paracyclophane with the chemical structure shown below. During the manufacturing process, the chlorine atom can end up in different locations on the molecule and create variants. The variants can affect the performance of the parylene film.

 Each of the dimer variants are expected to give different properties and are described below. 

	Symmetrical dichloro[2.2]paracyclophane Desired version of type ‘C’ with uniform and predictable deposition behavior Two monomers with 1 chlorine each
	Unsymmetrical dichloro[2.2]paracyclophane Not the desired dichloro version and it has a less predictable deposition behavior 1 Monomer with two chlorines This is Parylene D-type High “sticking coefficient” meaning the monomer stays where it lands with minimal bounce Faster deposition Can exhibit shadowing around taller components, especially when not rotating a rack in the deposition chamber Can have issues with coating thickness uniformity, which can improve when heating rack and/or chamber Slightly higher thermal resistance than Parylene C 1 Monomer with no chlorines This is Parylene N-type Low “sticking coefficient” meaning the monomer will bounce several times before staying in a location Slower deposition/extended time to reach or approach base pressure towards the end of the deposition process Can improve deposition time by cooling the rack and/or chamber  Slightly lower thermal resistance than Parylene C
	Trichloro[2.2]paracyclophane Not a desired version 1 Monomer has one chlorine This is Parylene C-type Lower “sticking coefficient” than Parylene D monomer More uniform deposition 1 Monomer has two chlorines This is Parylene D-type Higher “sticking coefficient” than Parylene C monomer Less uniform deposition
	Tetrachloro[2.2]paracyclophane This is Parylene D-type dimer 2 Monomers with two chlorines each Slightly higher thermal resistance than Parylene C Much more problematic in achieving uniform thickness and conformality of a coating

 

How does the symmetrical dichloro[2.2]paracyclophane purity correlate to Water Vapor Transmission Rate (WVTR)?

 

Key Material properties

Parylene Type	Chemical name	Dielectric Strength (V/mi)	Water Vapor Transmission Rate (lower value is better barrier) (g*mm)/(m2*day)
C	Symmetrical dichloro[2.2]paracyclophane	5,600	.08
N	[2.2]paracyclophane	7,000	.59
D	Tetrachloro[2.2]paracyclophane	5,500	.09

 

The symmetrical dichloro paracyclophane is true “Parylene C” and the higher the content the better since Parylene C has the best WVTR of the Parylenes, assuming proper deposition parameters. 

VSi’s testing has shown a direct relationship between the improved WVTR and higher symmetrical dichloro content. Data shows an appreciable WVTR change when the purity of the Parylene C dimer drops to below 95%.

 

 

Should symmetrical and unsymmetrical dichloro paracyclophane both be considered “pure” parylene C?

 

No, the symmetrical and unsymmetrical versions have different chemical structures and different deposition properties.  Pure parylene C is symmetrical dichloro paracyclophane. The symmetrical version has one chlorine attached to each of the two monomers.  This produces the most uniform and predictable deposition behavior and film.  The unsymmetrical version has one monomer with two chlorine atoms and one monomer with no chlorine atoms, so it is a combination of parylene D and Parylene N.  The unsymmetrical version will have less predictable deposition behavior and mixed film properties, this can be a problem for highly regulated technologies like medical devices.   

 

 

How pure of symmetrical dichloro is pure enough?

 

Manufacturing parylene C dimer with a higher symmetrical dichloro content requires a well-controlled process. There can be a relationship between purity and cost so achieving extremely high purity levels can have diminishing returns. Our testing has shown that there are noticeable changes in barrier properties as symmetrical dichloro content purity levels decrease. VSi dimer is produced to achieve a >98% symmetrical dichloro content.

 

 

Is HPLC testing of dimer considered a valid test method? This method is frequently used to report dimer purity, but it doesn’t seem to tell the whole story.

 

High Pressure/Performance Liquid Chromatography (HPLC) is very common and can be used to estimate purity of parylene dimers. 

The main issue with this method is that to get accurate values or percent composition for each component, you should calibrate the detector response [typically absorbance peaks with an ultraviolet light (UV) detector] with pure compounds for each of the component’s individual peaks that you’re expecting. 

This means you should have 100% pure symmetrical and unsymmetrical dichloro, monochloro, trichloro, and tetrachloro dimers to get the most valid numbers for your HPLC results.  The challenge is that 100% pure samples don’t exist.

Another test method that provides more accurate and repeatable results is Gas Chromatography-Mass Spectrometry (GC-MS).  The advantage with GC-MS is that the MS detector is measuring the mass (or more accurately the mass-to-charge ratio) or the compounds that are separated into individual peaks. 

GC-MS gives actual chemical structural information, while a UV detector is much more relative since one compound may absorb much more strongly at a lower concentration as compared to another compound.  VSi only uses GC-MS testing performed by a third party for all of our dimers.

 

 

Can dimer be further refined to obtain a more pure end product?

 

Dimer can be purified with multiple methods, each of which takes more time and effort, therefore more cost. Dimer can be purified through sublimation, recrystallization or column chromatography.  It’s more economical to acquire dimer that is manufactured with a process that results in symmetrical dichloro purity over 95%. 

 

VSi Parylene Dimer

Click here to learn more about our VSi parylene dimer and the importance of GC-MS purity testing to limit unwanted parylene variants. Compare your current parylene dimer, before you switch with our free head-to-head dimer analysis.

To learn more about the benefits of parylene coatings, the coating process, and parylene material properties, visit our Parylene Knowledge Base.