Failure Analysis of Plastic and Rubber Products

There are many reasons why plastic and rubber products fail. Some causes occur at the material selection phase of a development program while others originate from the manufacturing process. Of course, the end-use environment itself or product abuse can also lead to failure. It is important to determine the root cause of failure so that similar future failures may be prevented.

There are chemical analysis, physical testing, and microscopy methods available to objectively perform failure analysis of plastic and rubber products.
> Read more about failure analysis

Weak Weld Lines or Knit Lines

Extruded and injection molded plastic parts may contain weld lines or knit lines because of the shape of the product and the required features of the die or mold. This zone of a plastic part may have physical properties that are distinctly different from the bulk of the part. For example, low process temperatures may cause poor consolidation of the converging flow fronts. Alternatively, the carrier resin for the additives may cause a weakening of the weld line.

An example of the detrimental effect that the masterbatch can cause in physical properties is our analysis of a plastic strap that was used on a folding fin aircraft rocket. The plastic strap was manufactured using a high strength polypropylene resin. A wide variety of PP resins were evaluated, and the few candidates having the proper balance of strength, dimensional stability, and suitable functioning of the snap fasteners were selected for production. After verification of the performance of the sample straps it was determined that a bright red colorant package was needed so that the presence of the strap would be strikingly obvious. A time-delayed series of cracking failures ensued.

Optical imaging revealed that the failures were occurring exclusively at weld lines. Destructive tensile testing of weld line and non weld line zones verified the drastic difference in ultimate physical properties between these two distinct regions. Infrared spectroscopy verified that the weld line was populated by polyethylene resin, not polypropylene. The necessary red colorant masterbatch was switched to a PP-based carrier resin and the cracking failures stopped.

The ability to document a common failure location, determine the chemical composition at that location, and verify the contrast in ultimate physical properties in different locations brought this project to a rapid, objective, and firm conclusion.

If you want more information about these methods, click on the links below.
> Read more about imaging
> Read more about physical testing
> Read more about infrared spectroscopy

Wrong Plastic

It is true that the wrong plastic is sometimes used to manufacture products and as a result failures occur.

The "wrong plastic" generally is wrong for one of two typical reasons. In one case, a completely different base plastic resin was used to manufacture products. In the second case, the correct type of plastic resin was used but it was the wrong grade or contained the wrong additive package.

Analytically, the first scenario is easily verified by using infrared spectrometry. Modern sampling accessories even allow for non-destructive testing. Non-destructive testing is not always necessary but is important in litigated matters and it leaves the product intact for additional tests. Different plastic resins have distinguishable infrared signatures called spectra.

In some cases, one more test is required to more definitely identify the plastic resin, differential scanning calorimetry. For example, using infrared spectroscopy, polyamides are readily contrasted with other polymers, and most polyamides are readily contrasted with each other. In a few cases though, it is best to add the melting temperature range information to the data set to make a conclusive identification. The combination of FTIR spectroscopy and DSC is also a powerful protocol to assess the composition of blends.

If you want more information about these methods, click on the links below.
> Read more infrared spectroscopy
> Read more differential scanning calorimetry

Variable Crystallinity

Many plastic parts are manufactured from materials that are known to be semi-crystalline. This term does not mean "half crystalline" but rather means that the plastic is partly amorphous and partly crystalline. An enormous amount of material selection, process optimization, and end-use function is a direct consequence of the fact that plastic materials are semi-crystalline. Physical properties of plastic products are greatly influenced by their level of crystallinity. The level of crystallinity can change over time, and two polymers with the same level of crystallinity may have different distributions of crystallite sizes and types.

To address variable crystallinity at one of the more basic levels usually involves acquiring differential scanning calorimetric analysis scans of materials. The DSC method of analysis will quickly document if differences in crystallinity are a potential root cause for a plastics failure issue. Because the DSC method uses small specimens, the crystallinity of various specific regions of a single manufactured part can be determined. This is a powerful approach to optimize molding or annealing conditions.

For example, one can determine if the threaded portion of a screw and the head of the same screw have different levels of crystallinity. Although higher crystallinity generally provides higher strength it may also impart brittle failure characteristics too. So, understanding and intentionally controlling the distribution of crystallinity within the product may be important.

If you want more information about the DSC method of analysis, click on the link below.
> Read more differential scanning calorimetry

In one case, we deformulated a specialty protective coating.
> Read more

We bring you chemical analysis, physical testing and molecular systems information you can use to better understand how analysis may help you fully characterize the materials you work with! This issue focuses on five of the top causes of failure.

• Weak Weld lines or Knit Lines
• Wrong Plastic
• Variable Crystallinity
• Degraded Molecular Weight
• Contamination

Of course, we also have

• Jay´s Cool Microscopy Pics of the Month

• Cool Chemistry Links

Molecular weight and molecular weight distribution of the polymers that comprise a plastic product strongly influence the properties. For this reason, it is common that some determination of the molecular weight be made. This can be done using melt flow rate or index, dilute solution viscosity, or gel permeation chromatography.

The MFR, MFI, IV, and GPC methods each have their own pros and cons. Having baseline values for these parameters can accelerate a failure analysis program. This is true because a common failure mode of plastic materials is a reduction in molecular weight. Unless the reduction is drastic, it is important to compare the distribution in the failed product with the original distribution.

If reliable historical data sets are not available to serve as a benchmark, comparison testing with an exemplar part is usually performed ("good" versus "bad" analysis).

If you want more information about these methods, click on the links below.
> Read more about MFR or MFI
> Read more about GPC

Plastic failure caused by a contaminant can be very straight-forward or highly challenging.

Some molded plastic or rubber products may have obvious occlusions or discolored zones. The chemical composition of these zones, even if the zones are microscopic in size, is accomplished using infrared spectroscopy for organic contaminants and energy dispersive spectroscopy for inorganic contaminants.

The cause of failure in other cases is more complex, involving an additive package for example. It is common in these cases to separate the low molecular weight components from the plastic itself and to then analyze these low molecular weight components. The typical analysis uses gas chromatography coupled with mass spectrometry (GC-MS) or high performance liquid chromatography (HPLC).

The separation and identification of the components of a failed plastic part, especially when a good comparison part is available for comparison, quickly solves many plastic and rubber failure analyses.



If you want more information about these methods, click on the links below.
> Read more about infrared spectroscopy
> Read more about energy dispersive spectroscopy
> Read more about GC-MS
> Read more about HPLC

Or, check out the Case Studies
> Read more about additives analysis
> Read more about defect analysis