Heating a polymer can cause the amorphous sections of the chain to crystalize. When that happens, the way the molecules scatter light changes and the plastic turns white.
Simply bending something out of shape, or making the repeated bending motions needed to break it entirely think about that Wiffle bat , can also cause molecular changes that lead to whitening. Two types of polymer testing are particularly useful when getting to the bottom of whitening: differential scanning calorimetry DSC and optical birefringence. DSC helps us understand the thermal transitions that occur within a material, including residual reactivity, evaporation of solvents, melting, glass transition temperature and — you guessed it — crystallization.
This analytical technique uses heating and cooling in extremes ranging from to degrees centigrade to help define these transitions. This graph shows the effect of four different masterbatches on change in lightness DL before and after impact testing.
Resin selection using the same color masterbatch has an impact on stress whitening. Part thickness can have a key effect on stress-whitening potential. Thinner parts generally show more stress whitening and require more pigment to hide it. In large parts, such as car door panels, nucleating agents can affect mold shrinkage, which may require more pack-out to fill the mold.
That, in turn, can require more ejection force, which can cause stress whitening where the ejector pins contact the part. Pigment and resin together have a significant effect on stress-whitening potential. Shown here: the effect of different masterbatches in combination with different PP copolymers on change in lightness DL before and after impact testing.
Steps frequently need to be taken to ensure that color will not become compromised over time as a result of environmental factors. Stress whitening is one type of discoloration that can occur, particularly in certain plastics, such as polypropylene—one area of focus for Americhem, particularly in automotive.
How can we best prevent stress whitening? The key is really a combination of color masterbatch, PP copolymer, and the molding process. There are certain factors that affect stress whitening, as discussed below.
How can we best prevent it? The good news is there are a number of ways you can test materials early in the design process to minimize the risk of whitening. Based on your findings, you can tweak the copolymer composition, processing methods, or product design.
Ultimately, the key is doing comprehensive testing regarding the color masterbatch, PP copolymer, and molding process. Does testing show that a certain masterbatch formulation is causing stress whit- ening?
Does resin choice seem to be the culprit? Whatever the case may be, testing is crucial to isolate material variables making a product most prone to stress whitening. Stress whitening starts when stress is created by impact or tension upon a polymeric surface.
Unlike surface cracks, you cannot feel micro- crazes or microvoids, but you can see them. This is because these tiny aberrations reflect light slightly differently than the surfaces around them, which in turn gives them different color- ation as perceived by the human eye. When crazing occurs the addition of variously sized holes reduces the refractive index of the stressed area, causing it to scatter and reflect all visible light wavelengths. This makes it appear white despite the presence of added pigment.
It is interesting to note that if all the holes were the same diameter and spaced just right, you could make the surface appear any color you want. Mechanical properties of plastics vary greatly with temperature and strain rate.
At high strain rates and low temperatures polymers are brittle because there is less time for the molecules to yield to accommodate the load. Less yielding means less whitening. Rapid forced motion also causes internal friction, so a higher stress is required to deform the material. Plastics become measurably stronger as loads are applied more quickly. This is called strain rate sensitivity.
Some silicone polymers behave really weird when the strain rate is varied. Strain rate sensitivity works the other way too. When given time to comply, plastics will slowly deform in the presence of very small loads. This is called plastic creep. To some extent plastics can recover from small creep deformations given time, but not if they have started to turn white. Thermoplastics, by definition, can be remelted any number of times without harm to the structure of the plastic.
So all it takes is the proper application of heat to fix it. Too often I see people try to repair toys or motorcycle body panels with a heat gun or hair dryer. A better option is to use boiling water, which can repair whitened plastic without distorting it. When applied with care! Just for fun I used my new microscope and Testrbot to make a video showing the whitened areas appear in real time.
This info is provided at the end here so I could present the most eye catching stuff first! All plastics are polymers , but not all polymers are plastics. Typical engineering polymers are made of very long chains of molecules.
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