Today's KNOWLEDGE Share:Yellowing of Polycarbonate

Today's KNOWLEDGE Share:

Yellowing of Polycarbonate

Because of the molecular structure of polycarbonate, the polymer is susceptible to yellowing through a variety of degradation mechanisms. Discoloration of polycarbonate can occur through oxidation, either during processing or as a result of elevated temperature exposure in air. Yellowing can also take place through sterilization, such as autoclaving, radiation, or ethylene oxide. Further, polycarbonate is highly susceptible to yellowing through ultraviolet radiation (UV) exposure.

When polycarbonate is exposed to ultraviolet radiation as sunlight, yellowing can take place rapidly. This discoloration is principally a surface phenomenon, approximately 25 micrometers deep, due to the penetration depth of the UV radiation into the polycarbonate.

 

Yellowing within polycarbonate occurs as a result of the formation of degradation breakdown products that absorb in the yellow range of the visible light spectrum. This includes substituted ortho-quinones, quinone methides, phenones and phenone derivatives, and bisphenol-A derivatives. Analytical techniques, including gas chromatography-mass spectroscopy (GC-MS) and Fourier transform infrared spectroscopy (FTIR), have been used to study the degradation mechanisms and characterize the breakdown products. 

 

The yellowing of the polycarbonate is accentuated by the high level of transparency, which results in a strong yellow hue.

The effects of UV exposure, as well as oxidation and hydrolytic degradation, can be assessed through accelerated aging followed by color evaluation and physical/mechanical testing. Contact me to see how this can benefit you.

Souce:The Madison Group

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#plastics #failureanalysis #uv #degradation #discoloration #yellowing #polycarbonate #testing #ftir #phenone

Today's KNOWLEDGE Share:Lightning damages the fuselage of a Boeing 787-9!

Today's KNOWLEDGE Share:

Lightning damages the fuselage of a Boeing 787-9! 

An American Airlines Boeing 787 Dreamliner needs repairs after a lightning strike significantly damaged its fuselage. The incident happened Monday, February 20th, when N839AA, a 787-9, was traveling from Tokyo to Dallas/Fort Worth. The aircraft is reportedly being worked on to be restored back to operational condition! 

Despite a strong and thick fuselage, the 787 reportedly has a known issue with lightning strikes. In 2019, Boeing reduced lightning protection in the wings of some 787s to reduce costs and speed up deliveries, but the company reportedly said that safety was not compromised. In May, a Jetstar 787 was grounded after sustaining extensive damage from a lightning strike.

Source: Simpleflying/ #managingcomposites #thenativelab

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#aerospace # #aircraft #airlines #boeing #safety #composites #boeing787

Today's KNOWLEDGE Share: Shear Rate

Today's KNOWLEDGE Share:

Shear Rate

Sorry to disappoint some of you, but there is NO such a thing as Maximum admissible Shear Rate for a material in Injection Molding.

 

30+ years back, Colin Austin (Moldflow founder) wanted to put a recommendation in the code documentation and, having no clear answers from suppliers ,he decided to list some values that were loosely based on a wild guess exercise. Actually an estimated typical shear rate (at some standard T) for a critical Shear Stress equal to an arbitrary fraction (abt 10%) of a Stress at break in the solid state. How wild is that ??

Shear rate does not destroy a polymer.

If chains break, that is due to Shear Stress, not shear rate. If they degrade thermally, it is the result of accumulated shear-heating along the flow. In both cases, shear rates alone CANNOT resolve the risk of damaging the material.

At low temperature (high viscosity hence higher stress) a lower shear rate can be more dangerous than a higher rate at a higher temperature (lower viscosity so potentially much lower stress).

So if you want to say something about the injection rate being too fast, check the stresses or the melt temperature increase. A high shear rate will be a warning signal, at best.

Source:VITO LEO

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#plastics #injectionmolding #shear #stress #melt #viscosity #temperature

Today's KNOWLEDGE Share: How Strong is Your Material?

Today's KNOWLEDGE Share:

How Strong is Your Material?

Do you know the true strength of your material? With plastics, that is a trick question.

 

Because of their molecular structure, thermoplastic materials have different properties compared to other materials, like metals.

·    The polymer molecules consist of very long chains – high molecular weight.

·    The individual polymer chains are entangled in each other.

·    The polymer chains are mobile and can slide past each other because they do not share chemical bonds with the other chains around them.

 

Because of this, plastics exhibit viscoelastic behavior, displaying aspects of both elastic and viscous performance. Attributed to their viscoelastic character, the properties of plastics, including tensile strength, will vary depending on the conditions of the stress loading. This means the “strength” of the material will vary over temperature, time under load, duration of dynamic loading, strain rate, and more.

 

Unfortunately, this is not captured on plastic material datasheets. For a particular grade of polycarbonate, the tensile strength as determined by standard tensile testing was determined to be 64 MPa, a good match with the datasheet value. However, this is not how the component manufactured from the polycarbonate will be stressed – it is expected to last longer in field use than the 90 seconds it took for the tensile test, and it will be under continuous static loading or dynamic loading depending on the specific application. Based upon realistic conditions, creep and fatigue tests were performed. The testing determined that the actual material strength was 20 MPa for 5 years under continuous loading and 31 MPa at 750,000 cycles, the expected service life. Obviously these are far less than the 64 MPa identified as the tensile strength.

 

Source:The Madison Group

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#plastics #plasticsengineering #polycarbonate #failure #engineering #design #failureanalysis #fatigue #stess #creep #tensile #loading

Today's KNOWLEDGE Share:THIN RUNNER

Today's KNOWLEDGE Share:

THIN RUNNER

I am saying that sometimes, you will avoid thermal degradation in runners by going thinner !

How is that possible ?

Materials like RPVC have a strong tendency to degrade in extrusion and even more in Injection Molding. Some lubricants are typically present in the compounds to promote the slip of the melt on the reciprocating screw surfaces.

Such slip will also be present in runners, in particular in hot runners (considered as a dangerous option in PVC molding by most).

Slip is very well described by a power law relating Slip Velocity to the applied Shear Stress.

So the onset of slip is triggered by a sufficiently high stress, very much like motion is triggered in a static friction coefficient context.

This means that in a larger runner, where Shear Stress is much lower, the melt may well NOT slip, resulting in a very long (infinite) residence time of the top melt layers in contact with the hot runner chamber.

By making your hot runners thinner, you increase stress and thus promote slip and consequently lower the melt residence time and the associated risk of thermal degradation.

Years ago, we took advantage of Flow Analysis (with a Slip model included in the constitutive laws) and the above idea to design a hot runner system for PVC that proved to work for weeks without any melt degradation.

This may not apply to many other materials and will not be very relevant in cold runners where residence time will never exceed cycle-time, but I thought it was good food for thought anyway. It could apply to the nozzle of a molding machine by the way.

Source:VITO LEO

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#plastics #polymers #injectionmolding #thermal #extrusion #shear #hotrunner #meltflow #meltdegradation #PVCmolding # #design

Today's KNOWLEDGE Share: Comparing the properties of composite materials!!!

Today's KNOWLEDGE Share:

Comparing the properties of composite materials!!!

A very useful method of doing this is by plotting them as ''Ashby charts'', which represent each material on the chart as ellipses or ''bubbles'', whose width and height are determined by the range of the value of the properties. 

This Ashby plot shows the comparison of the impact strength at room temperature of polymer composites reinforced by glass fibers, carbon fibers, plant fibers and silk fibers. The corresponding composites are respectively abbreviated as GFRP (Glass Fiber Reinforced Plastics), CFRP (Carbon Fiber Reinforced Plastics), PFRP (Plant Fiber Reinforced Plastics) and SFRP (Silk Fiber Reinforced Plastics). 

We find it amazing how this type of plot can make our life much easier!

Source: Article "Enhancing the Mechanical Toughness of Epoxy-Resin Composites Using Natural Silk Reinforcements", written by Kang Yang, SuJun Wu, Juan Guan, Zhengzhong Shao and Robert O Ritchie.

Credit:#managingcomposites #thenativelab

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#composites #cfrp #gfrp #nfrp #impact #strength #mechanical #properties #fibers #resin #silk