 |
Feature |
Originally Published MDDI August 2002
COATING TECHNOLOGY
|
Return to Article: |
Transparent parylene film is applied to substrates in a vacuum chamber by means of vapor deposition polymerization. A dry, powdered precursor known as dimer is converted by heat in the coating system to form a dimeric gas, and heated further to generate a monomer gas that is passed to a deposition chamber. Within the chamber, it polymerizes at room temperature as a conformal film on all exposed substrate surfaces.
Parylene deposition has no liquid phase, uses no solvent or catalyst, and generates no gaseous by-products. Consequently, there are no cure-related hydraulic or liquid surface-tension forces in the coating cycle, and coated objects remain free of mechanical stress. The resulting film is a high-molecular-weight, linear, crystalline polymer with an all- carbon backbone. With the absence of polar entities, and substantial crystallinity, the finished film is stable and highly resistant to chemical attack.
The static and dynamic coefficients of friction for parylenes are in the range of 0.25 to 0.33. This dry-film lubricity is an important chracteristic for certain device applications, such as catheter and guide-wire coatings.
PARYLENE VARIANTS
There are four primary variants of the polymer: Parylenes N, C, D, and HT. Although they all have the same essential coating properties and are applied in the same manner, each has a unique molecular form that results in specialized performance characteristics. Parylenes N and C are the most commonly used variants in medical coating applications. Table I describes the key properties of these parylenes.
|
Property
|
Parylene N
|
Parylene C
|
|
| Dielectric constnat |
60 Hz
|
2.65
|
3.15
|
|
1 KHz
|
2.65
|
3.10
|
|
|
1 MHz
|
2.65
|
2.95
|
|
| Dissipation factor |
60 Hz
|
0.0002
|
0.020
|
|
1 KHz
|
0.0002
|
0.019
|
|
|
1 MHz
|
0.0006
|
0.013
|
|
| Secant modulus (psi) |
350,000
|
400,000
|
|
| Tensile strength (psi) |
6000-11,000
|
10,000
|
|
| Yield strength (psi) |
6100
|
8000
|
|
| Elongation to break (%) |
20-250
|
200
|
|
| Yield elongation (%) |
2.5
|
2.9
|
|
| Density (gm/cm3) |
1.10-1.12
|
1.289
|
|
| Index of refraction (nD23) |
1.661
|
1.639
|
|
| Water absorption (% after 24 hr) |
<0.1
|
<0.1
|
|
| Rockwell hardness |
R85
|
R80
|
|
| Static coefficient of friction |
0.25
|
0.29
|
|
| Dynamic coefficient of friction |
0.25
|
0.29
|
|
| Melting point (°C) |
420
|
290
|
|
| T5 point (°C) |
160
|
125
|
|
|
Gas permeability at 25°C |
N2
|
7.7
|
1.0
|
|
O2
|
39
|
7.2
|
|
|
CO2
|
214
|
7.7
|
|
|
H2
|
540
|
110
|
|
| Moisture vapor transmission at 90% RH, 37 °C 0.21 |
1.5 | ||
| Table I. Key physical and mechanical properties displayed by Parylene N and Parylene C. | |||
Parylene N offers the highest penetrating power of the variants. Because of its greater molecular activity in the monomer phase, it can be used to coat relatively deep recesses and blind holes. This form of parylene also provides slightly higher dielectric strength than C, and a dielectric constant that is independent of frequency. The lower dissipation factor and dielectric constant of this parylene form enable it to be used for protecting high-frequency substrates where the coating is in the direct electromagnetic field.
Parylene C differs from N in that it has a chlorine atom on the benzene ring, providing a useful combination of electrical and physical properties. Among these are very low permeability to moisture and corrosive gases. Compared to Parylene N, C displays less crevice-penetrating ability.
Copyright ©2002 Medical Device & Diagnostic Industry
|
Most Recent |
|
Related articles |