Quantifying Plastic on Plastic Friction Behavior to Optimize Material Selection for

Single-Use Drug Delivery Devices

Ben GerjetsProduct Development Engineer

RTP Companywww.rtpcompany.com

Josh BlackmoreGlobal Healthcare Manager

RTP MEDICAL PLASTIC TECHNOLOGY

Unmatched Resin & Additive Selection• Virtually All Resins• Thousands of Additives• Unbiased Material Selection

Biocompatibility• Internal list of known biocompatible

compounds• Resin and additives that meet ISO 10992 or

USP Class VI• RTP pre-tested ISO 10993 Compounds

• Colors – Conductive – TPEs

Change Management• Traceablility• ISO 9001 Certified

• Extreme population growth driving need for low cost/safe device development.

• No industry wide standard test is recognized to characterize start up friction.

• Sterilization, UV degradation, human skin oil and shelf life cause “sticktion” concerns.

• External lubrication is messy, imprecise and very difficult to evenly spread and QC.

• You only get One Shot in drug delivery to work and be effective.

MATERIAL CHALLENGES IN DRUG DELIVERY

PROBLEMS SOLVED with INTERNAL LUBRICATION

Improved Selection of Materials• Tribological data driven decisions• Improve SPEED to correct solution

Eliminate Secondary Lubricant• Low start-up friction• Eliminate squeaks• Improve quality & consistency• Fewer rejects, faster cycle times• Reduce finished part cost

Technology Combinations Possible• Can be combined with laser marking, color,

UV stabilization etc.

INTERNAL LUBRICATION EXAMPLES

Lubricated/glass filled PC cover slides.

High speed gears –eliminate 2°

lubrication step

Lubricated lead screws and slides – easy quiet

smooth operation

Lubricated HDPE v PC

INTERNAL PLASTIC LUBRICATION TESTING OVERVIEW

Development of a New Friction Test Method

FRICTION STUDY OVERVIEW

Objectives• Develop a tribology test to measure start up friction to

better predict performance

• Characterize plastic on plastic friction behavior –provide friction pairing

• Eliminate stick slip phenomenon

• Demonstrate internally lubricated plastic performance

FRICTION: TERMS AND DEFINTIONS

µ=F/N

Coefficient of Friction (µ)• Ratio of the force of friction between two

bodies and the force pressing them together.

Static Coefficient of Friction (µs)• Static Coefficient of Friction (µs) = Fx/Fy

• Fx = Force to initiate motion• Fy = Normal force holding surfaces together

TYPES of SLIDING FRICTION

Dynamic Coefficient of Friction (µk)• Dynamic Coefficient of Friction (µk) = Fx/Fy

• Fx = Force to sustain motion• Fy = Normal force holding surfaces together

TRADITIONAL ADDITIVE TECHNOLOGY

PTFE Silicone PFPE

Solid Additives FibersAPWAPLUS

TRADITIONAL ADDITIVE TECHNOLOGY

RESIN PAIRS INVESTIGATED

PC Against POM PC against PC POM against POM

2% Standard Viscosity Silicone 2% Standard Viscosity Silicone 2% Standard Viscosity Silicone

2% Low Viscosity Silicone 2% Low Viscosity Silicone 2% Low Viscosity Silicone

Blended Viscosity Silicone Blended Viscosity Silicone Blended Viscosity Silicone

PFPE PFPE PFPE

APWA Plus APWA Plus

PBT against PC-ABS ABS against ABS PC against HDPE

2% Standard Viscosity Silicone 2% Standard Viscosity Silicone 2% Standard Viscosity Silicone

APWA Plus APWA Plus PFPE

APWA Plus/Silicone APWA Plus/Silicone

PFPE PFPE

DESCRIPTION OF NEW FRICTION TEST METHOD

Modified ASTM D3702 Thrust Washer Test• Specimens Injection molded per ASTM

dimensions• Oscillating friction measurement

Test Method1. Initial oscillating friction measurement -

30°sweep angle at 0.5 Hz frequency for 10 cycles (Initial friction measurement)

2. Standard rotation - 30 minutes single direction rotation (25 ft/min, 20psi, 500PV)

3. Final oscillating friction measurement 30°sweep angle at 0.5 Hz frequency for 10

cycles (Final friction measurement)

NEW FRICTION TEST OUTPUT

Test Outputs:• Initial static friction coefficient, Initial

dynamic friction coefficient• RMS dynamic friction coefficient over 30

min wear test• Final static friction coefficient, final dynamic

friction coefficient

Delta value• Difference between static and dynamic

coefficients of friction• Delta values are calculated from the initial

routine and the final routine• Low delta values are desired to lower

“sticktion”

SPECIMEN PREPARATION

Specimens Molded in house per ASTM D3702 Thrust Washer dimensions

Specimen Care:• Important to mold consistent surfaces• Important to observe care throughout molding,

assembly, and testing• Clean dirt free environment is important• Contaminant free molding environment

IMPORTANCE OF HANDLING METHODS

Specimens Must Be Clean:• Gloves must be used to eliminate human skin oil

contamination• Specimens stored in sealed bags• Other handling?

Handling Methods for Production• Fingerprints on devices can cause variation in

friction performance and attract contaminants. Dirt increases friction

SELECTED RESULTS – POM against PC

-0.008

0.012

0.032

0.052

0.072

0.092

POM - Si (2%SV) vs PC - SI

(2% SV)

POM - PFPEvs PC - Si (2%

SV)

POM - Si (2%SV) vs Neat

PC

POM - Si (2%SV) vs PC -Blended Si

POM - Si (2%SV) vs PC - SI

(2% LV)

POM - PFPEvs PC - Si (2%

LV)

POM - Si (2%LV) vs PC -Blended Si

POM - PFPEvs PC - PFPE

Neat POM vsPC - Si (2%

LV)

POM - PFPEvs neat PC

Neat POM vsPC - Blended

Si

Neat POM vsPC-PFPE

Initial Static µ 0.031 0.033 0.045 0.06 0.061 0.072 0.077 0.087 0.088 0.091 0.093 0.096

Initial Dynamic µ 0.024 0.029 0.036 0.056 0.067 0.07 0.076 0.084 0.081 0.09 0.086 0.092

Δ 0.007 0.004 0.009 0.004 -0.006 0.002 0.001 0.003 0.007 0.001 0.007 0.004

SELECTED TEST RESULTS - HDPE against Neat PC

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

HDPE - PFPE vsNeat PC

HDPE - Si (2% SV)vs Neat PC

Neat HDPE vs NeatPC

Initial Static µ 0.07 0.09 0.16

Initial Dynamic µ 0.066 0.084 0.142

Δ 0.004 0.006 0.018

HDPE against Neat PC – Varying Pressure

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

HDPE - PFPE vs Neat PC HDPE - Si (2% SV) vs NeatPC

Neat HDPE vs Neat PC

Initial Static µ 0.07 0.09 0.16

Initial Dynamic µ 0.066 0.084 0.142

Δ 0.004 0.006 0.018

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

HDPE - PFPE vs Neat PC HDPE - Si (2% SV) vs NeatPC

Neat HDPE vs Neat PC

Initial Static µ 0.085 0.09 0.097

Initial Dynamic µ 0.073 0.075 0.075

Δ 0.012 0.015 0.022

4 lb load (0.8psi) 10 lb load (2.0psi)

Large Increase in friction with addition of pressure.

SELECTED TEST RESULTS – PBT against PC-ABS

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

PC/ABS - Si (2%SV) vs Neat PBT

PC/ABS - Si (2%SV) vs PBT - Si

(2% SV)

Neat PC/ABS vsPBT - APWAPlus/Silicone

PC/ABS -APWA+ vs PBT -

Si (2% SV)

Neat PC/ABS vsNeat PBT

Initial Static µ 0.067 0.074 0.077 0.077 0.093

Initial Dynamic µ 0.061 0.065 0.069 0.07 0.096

Δ 0.006 0.009 0.008 0.007 -0.003

SELECTED TEST RESULTS – SELF against SELF

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

ABS - Si (2%SV) vs Neat

ABS

PC - Si (2% LV)vs PC -

Blended Si

Neat PC vs PC- Blended Si

ABS -APWA+Si vs

ABS APWA+/Si

Neat POM vsNeat POM

Initial Static µ 0.083 0.095 0.12 0.151 0.26

Initial Dynamic µ 0.075 0.103 0.114 0.145 0.21

Δ 0.008 -0.008 0.006 0.006 0.05

• Selecting optimal “friction pairs” is important.

• Self vs self should be avoided unless lubrication is applied.

• Internal lubricants reduce friction in self vs self friction pairings.

• Topical lubricants can wear off over time.

SELF vs. SELF OBSERVATIONS

POM

vs

POM

PC

vs

PC

DISSIMILAR FRICTION PAIR OBSERVATIONS

• Dissimilar friction pairs show improved low-friction performance over self v self.

• Properly selected friction pairs have improved low-friction performance.

• The best low-friction performance is obtained when combined with internal lubricants even after short period of wear.

₋ Examples: PC/POM - PBT/PC/ABS - others

INTERNAL LUBRICANTS REDUCE CLARITY

SUMMARY TEST OBSERVATIONS

Resin Selection is Key to Achieve Low Friction Performance1. Lubricated dissimilar friction pairs – best performance2. Lubricated self v self pairs – good performance3. Neat dissimilar pairs – poor performance4. Neat self v self pairs – worst performance

All Lubricants Improve Friction Performance to Some Degree1. Silicone has very good performance across the resin systems2. Silicone viscosity has little effect on performance3. APWA Plus with silicone performs well in some pairs4. PFPE has acceptable performance in some friction pairs

Pressure or Load Increases Friction Dramatically1. APWA Plus with silicone is good in some pairs2. Internal lubricants reduce friction sensitivity

• Understand the influence of pressure and velocity on friction

• Measure PTFE additive performance

• Further silicone investigation₋ High concentration silicone loadings at high pressures₋ Viscosity/loading level affect on long term aging performance

• Evaluation of expanded friction angle

• One pass testing on multiple samples

• Additional Friction Pairs to be tested₋ POM against PC/ABS and others

FUTURE TESTING AND INVESTIGATION

CONCLUSIONS on OBJECTIVES• New friction test measurements were consistent,

duplicable and consistent with long term wear testing direction. Further testing on pressure variants needed.

• Friction test results allow engineers to improve material and lubricant section.

• Plastic on plastic friction behavior is characterized and useful to predict actual performance. More work is needed to characterize all the friction variables.

• Internal plastic lubrication can eliminate stick slip

Thank you for your attention.

Questions?

Ben Gerjets

Product Development Engineer

bgerjets@rtpcompany.com

(507) 474-5381

Josh Blackmore

Global Healthcare Manager

jblackmore@rtpcompany.com

(989) 835-6493