Page 31 - Plastics News April 2019

FeAtures

Failure Analysis of a Thermoplastic Elastomer

Failure Analysis of a Thermoplastic Elastomer;
Melt Flow Index (MFI) Method Provided Critical Data Sets
A case study for companies that manufacture rubber and plastic parts as processing issues are a serious
contributor to reduced profit and increased waste. Rapid evaluations that can pin-point the root cause of a
Background
For companies that manufacture rubber and plastic parts, processing issues are a serious contributor to reduced profit
process issue are vital to maintain productivity.
and increased waste. Rapid evaluations that can pin-point the root cause of a process issue are vital to maintain
productivity. In addition, third-party materials evaluations often serve to swiftly identify raw material compliance issues.
Based on scientific data, the business issues caused by the raw material can be properly addressed and the production
n addition, third-party materials evaluations often serve samples in terms of their measured MFI values. It is clear
process can get back on track.
Ito swiftly identify raw material compliance issues. Based that a noteworthy difference in MFI values is present
As is typically the case, Polymer Solutions Incorporated received a pair of samples that were described as “good” and
“bad.” It was critical to our client and the rapid resumption of their manufacturing process that an objective science-based
on scientific data, the business issues caused by the raw between the two samples as shown in the inset table
determination be made as to whether or not there were variations in the raw material. Comparative testing of the two
material can be properly addressed and the production of data. The Bad sample shows a much higher MFI than
samples was performed. From a root cause analysis perspective, three analytical methods were proposed and
implemented. This was considered to be a technically appropriate, robust, and cost-effective first step. The three
process can get back on track. As is typically the case, the Good sample. This finding indicates that the Bad
analytical methods included:
Polymer Solutions Incorporated received a pair of samples sample has a much lower molecular weight compared
 Thermogravimetric Analysis (TGA) to Determine Inorganic Filler Content
 Differential Scanning Calorimetry (DSC) to Document Thermal Transitions
that were described as “good” and “bad.” It was critical to to the Good specimen. The actual molecular weight
 Melt Flow Index (MFI) Tests to Compare Melt Flow Characteristics
our client and the rapid resumption of their manufacturing distributions can be further documented by using gel
Melt Flow Index (MFI)
process that an objective science-based determination be permeation chromatography (GPC) or size exclusion
Small portions of Good and Bad extrudates were cut using scissors and a razor blade. These specimens were then dried
made as to whether or not there were variations in the chromatography (SEC) to evaluate the molecular weight
at 120°C in a convection oven. Preliminary melt flow index (MFI) tests were performed. Two critical observation resulted.

raw material. Comparative testing of the two samples parameters (molecular weight averages and polydispersity,
First, after initial drying for three hours, the samples exhibited significant bubbling when MFI testing was carried out.
was performed. Therefore, the specimens were dried for 24 hours at 120°C. Second, it was noted that at the initial standard MFI
for example).
conditions of 224°C and 1.2 kg (435°F and 2.65 lb), the resin flowed so quickly that highly accurate determinations of melt
flow were difficult. Therefore, the test temperature was lowered to 199°C (390°F) and the modified protocol, agreed to by
From a root cause analysis perspective, three analytical Thermogravimetric Analysis (TGA) In order to compare the
the client, was included as part of the formal report.
methods were proposed and implemented. This was degradation profile of the two samples, thermogravimetric
A significant difference was noted between the two samples in terms of their measured MFI values. It is clear that a
considered to be a technically appropriate, robust, and analysis (TGA) was carried out using a TA Instruments Q500
noteworthy difference in MFI values is present between the two samples as shown in the inset table of data. The Bad
instrument. Approximately 15 mg of sample was placed in
cost-effective first step. sample shows a much higher MFI than the Good sample. This finding indicates that the Bad sample has a much lower
molecular weight compared to the Good specimen. The actual molecular weight distributions can be further documented
by using gel permeation chromatography (GPC) or size exclusion chromatography (SEC) to evaluate the molecular
a tared platinum TGA pan and heated from 30°C to 900°C
The three analytical methods included: weight parameters (molecular weight averages and polydispersity, for example).
First, Thermogravimetric Analysis (TGA) to Determine
Inorganic Filler Content, Second, Differential Scanning
Calorimetry (DSC) to Document Thermal Transitions and
Third, Melt Flow Index (MFI) Tests to Compare Melt Flow
Characteristics
Melt Flow Index (MFI) Small portions of Good and Bad
extrudates were cut using scissors and a razor blade. These

specimens were then dried at 120°C in a convection oven.
Preliminary melt flow index (MFI) tests were performed. Fig.1
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Two critical observations resulted. at 10°C per minute in an atmosphere of air.
First, after initial drying for three hours, the samples The inset Figure 2. shows the TGA curves that were
exhibited significant bubbling when MFI testing was recorded for this pair of samples. The degradation profiles
carried out. Therefore, the specimens were dried for 24 and the amount of inorganic residue were compared
hours at 120°C. Second, it was noted that at the initial between the two samples. No significant differences
standard MFI conditions of 224°C and 1.2 kg (435°F and were noted between the two samples. A different
2.65 lb), the resin flowed so quickly that highly accurate thermal degradation profile would have indicated a
determinations of melt flow were difficult. Therefore, the substantially different polymer or additive package. If a
test temperature was lowered to 199°C (390°F) and the different degradation profile had been observed, chemical
modified protocol, agreed to by the client, was included identification of the base polymer followed by an analysis
as part of the formal report. of the stabilizer package would have been performed.
A significant difference was noted between the two Analytical methods employed would include

31 April 2019 Plastics News