Medical Design Briefs - May 2021 - 20

TECHNOLOGY LEADERS Materials/Coatings/Adhesives

A stress cracking
agent as in

Mechanical
stresses

Absorption
of solvents
in plastic
*
*
*
*

Softening
Swelling
Permeation
Dissolution

Reduction in
physical
properties
*
*
*

IPA, lipids,
disinfectants,
drugs, and/or
carrier solvents

(tensile, impact,
flexural)

Oxidation
Depolymerization
Reaction of
functional groups
in or on the chain

Fig. 2 - Sequence of events induced in the polymer
material upon chemical attack. Chemical resistance
enables endurance to the polymer against chemical
attack.

tors and handling devices, IV disposable
luer lock connectors and accessories ,
blood plasma oxygenators, drainage sets
for the thorax, medical filter housings,
and disposable luer lock connectors
(see Figure 1).7,8
Polymer Mechanical Behavior: What Is
Chemical Resistance in Medical
Plastics?
The mechanical properties measure
the material's response to a finite strain
by an applied stress. The intermolecular
arrangement of a polymer, among other
attributes, largely governs its mechanical
behavior. However, chemical resistance
measures the ability of a polymeric material to retain its original properties post
exposure to a chemical agent for a specified time period.9 Smart design of the
polymer backbone, polymer blending,

The inherent
susceptibility of
the plastic to
stress crack

Fig. 3 - Simultaneous presence of these three factors results in the surface embrittlement of plastic parts
by ESC.

and formulating with specialty additives
to alter the degree of crystallinity,
crosslinking, or molecular weight are
critical factors that influence the chemical resistance of the blend.10-12
The inability of a polymer to resist
chemical attack could impair physical
and/or chemical properties such as surface aesthetics, or color, of polymer to
modulus, ductility, impact toughness,
and dimensional stability depending on
the chemical agent and the duration of
exposure. The extent of these detriments depends on the polymer design
and the degradation pathway. Common
agents leading to chemical attack are
detergents, surfaces active or oxidizing
chemicals such as disinfectants, alcohols, lipids, processing aids, lubricants,
contaminants, or even ultra-pure water
(see Figure 2).13
Furthermore, crazing and environmental stress cracking (ESC) are distinct
types of failure phenomena in plastics.
The material under critical external
(mechanical) or residual molded-in
stresses, when exposed to a stress
cracking agent, can experience localized weakening at the surface of the
material, permitting crack propagation.

CYROLITE®
Performance attributes

G-20

G-20 HiFLO GS-90

CG-97

Med 2

BPA free

✓

✓

✓

✓

✓

Transparency

++

++

+++

++

+

Color stability post EtO/Gamma/E-beam

+

+

+++

++

++

Bonding with PVC tubing

+++

+++

+++

+++

+++

Flow - Multicavity tools/Thin walls

++

+++

++

+

+

Lipid Resistance

++

++

++

+++

++

Alcohol Resistance

+

+

++

++

+++

®

Table 1. Roehm America's product portfolio offering CYROLITE in eight grades designed for performance in
healthcare applications. Performance rating: Good (+), Very Good (++), Excellent (+++).

20

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Cov

ToC

Depending on the material, macroscopic crazing occurs, sometimes visible as
stress whitening in the region of maximum stress. Crazing is physical embrittlement that is induced by the simultaneous presence of critical stress and a
chemical agent that can lower the cohesive bond energies of the polymer surface layers (see Figure 3).9 Strategies to
incorporate functional moieties such as
block, graft, or amphiphilic block copolymers, or impact modifiers, in the
polymer design can mitigate crack formation and crack propagation yielding
environmental stress crack resistance
(ESCR).11,12
Conclusion
Foremost, patient safety from diseasecausing pathogens is propelling the
industry to design for ESCR utility in
medical assemblies. Designing for ESCR
begins with understanding the chemical
exposure environment before the polymer material selection. The mechanical
behavior of the polymeric material
drives the subsequent ESCR performance. Herein, we showcase CYROLITE®'s product portfolio, for example,
offering performance attributes for
applications in healthcare and medical
devices. The design process for CYROLITE® involves understanding the market landscape, customer input, and cognizance for potential indications for use.
Furthermore, it combines a methodical
assessment of design factors, such as
material selection and processing, providing value-added solutions in terms of
optical transparency, mechanical performance, chemical resistance, sterilizability, and large-volume processability.
Moreover, the application of aggressive disinfectants is interfering with the
structural integrity of medical device
assemblies and causing premature failures. It is essential to integrate an objective screening matrix to enable medical

Medical Design Briefs, May 2021
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Medical Design Briefs - May 2021

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