Process Design Step 1:
Variables to consider when designing a surface modification program
I lied, partially.
I said that my next blog would be a trip report (sadly, Dyana said it was overcast) and power of plasma for modification of materials for the life sciences industry. I’m not ready to jump into specific applications, rather want to start with some of the basics to a successful surface modification program.
I’ll start with Step 1. Q&A.
The beginning of any lab development program typically involves a thorough Q&A session. At the minimum, I want to know:
2. Product environment
3. Desired surface performance goal
The success, based on my experience, of a surface modification program relies on a thorough (if possible) understanding of these three items as 1 and 2 greatly impact 3.
For each question, there are 10s if not 100s of sub-questions that can shift outcome considerably. I spoke about some of these recently at Hantel Technology http://www.youtube.com/watch?v=gZemVc790oQ and am summarizing a partial list of variables for each question below.
- SUBSTRATE. Tell me about (I’m polymer focused):
- Resin selection/Metal properties
- Composite properties
- Manufacturing practice (molded, extruded, cast). Are you starting with a machined part for R&D and then possibly considering molding for production. We may talk about molecular weight distribution as well.
- Cure mechanism
- Cure temperatures
- Hardness (durometer)/crystallinity
- Additives (stabilizers, pigments, nucleating agents, plasticizers, etc)
- Propensity for migration of additives
- Propensity for molecular rotation
- Mold release materials
- Machining debris
- Moisture retain/absorption/adsorption
- Cleanliness (and how is the substrate cleaned prior to plasma)
- Manufacturing controls for said substrate
- Throughput targets
- ENVIRONMENT. Once treated, please tell me about the next steps in processing and environment as these variables may impact surface performance and stability:
- See Item #1.9 above. Bloom, migration of Internal impurities
- Adhesive technique (if bonding) and cure mechanism
- Potential for oxidation
- Chemical exposure
- Sterilization technique
- Subsequent assembly step (are you heat sealing?)
- Subsequent cleaning steps and techniques (are you IPA wiping part 100X times during assembly?)
- Handling (glove selection and practices)
- Storage (Packaging materials, Temperatures)
- SURFACE PROPERTIES. What do you want as we have many variables to consider to provide the desired outcome. Rather than listing the myriad of applications we practice, I’ll focus on variables that we consider in designing an experimental plan.
- Type of equipment (Corona, Atmospheric, Low Pressure)
- Steps and type of process (Cleaning, Etching, Activation, Functionalization, PECVD, Grafting, Crosslinking)
- Chemistry (gas, liquid vapor, sublimated solids, combinations).
- Temperature of substrate, chamber, liquid/solid
- Pressure (flow driven, throttled, pumping capacity)
- Fixturing and fixture materials (does it contribute to dark space?)
- Power (continuous, pulsed, duty cycle, frequency)
- Time (3o seconds or 10 minutes)
BUT WAIT. There is more!
VALIDATION TECHNIQUE/SURFACE ANALYSIS
Even the choice of how to validate the surface can impact the results. Our chief technologist, Steve Kaplan, loves to say “don’t throw the baby out with the bathwater.” It is not unusual for a customer to overlook the success of the process by improper selection of the validation method. Test the product in the ultimate application. Techniques used and considered at our laboratories include:
- Surface energy testing
- Dyne-cm, contact angle
- fluid choice
- Adhesion testing
- Wear and abrasion testing
- Friction testing
- Hardness testing
- Surface analysis
- X-ray Photoelectron Spectroscopy (XPS)
- Scanning Electron Microscopy (SEM)
- AFM Atomic Force Microscopy
- Fourier Transform Infrared Spectroscopy (FTIR)
- Chemical resistance
- Gas permeation / vapor barrier testing
This list isn’t to overwhelm. I don’t expect answers to all of these questions nor do we screen every possible combination of variables. We know where to start if you can provide us with the basics about 1 (Substrate), 2 (Environment of use) and 3 (Desired surface performance) so that we can design efficiently and effectively the best surface for your application.
Next blog…no promises.