Weather testing of plastics parts is becoming a crucial step to avoid potential product failures - particularly in automotive plastics. There is often inadequate communication between the OEM, processor, compounder and additive supplier regarding the service environment to which the product will be exposed. Misunderstandings regarding how or where a product is to be used, is definitely a recipe for product failure. As OEMs increasingly turn to processors for design and material selection, the responsibility for in-service weathering failures is also being passed down to the processors. Processors, under constant pressure to improve product performance and simultaneously reduce costs, are resorting to new materials and additives as potential solutions. However, introduction of these unknowns without adequate testing, explains why the incidence of weathering-related failures is on the rise.

Plastics materials and products are being introduced in markets globally, for applications they may not have been designed or tested. New polymers, variations on traditional polymers (such as metallocene grades) and new copolymer blends and alloys are marketed aggressively for their cost-performance or processing benefits. However, in the absence of adequate testing, durability of these new materials remains a question mark. Similarly, colorant systems based on new organic pigments and pigment blends are replacing heavy-metal based colorants. These new colorants can have unexpected color stability and light-fastness problems.
"Same-spec" resin from different plants, even different reactors of the same supplier, can differ in crosslinking, molecular-weight distribution, side-chain branching, etc. All of these can affect processing and performance - including weatherability. This is especially true for semi-crystalline polymers such as Polypropylene.
Purchase of fillers and other bulk additives are frequently price based, but when non-technical buyers switch vendors or grades to save money, it can result in a compromise on product performance. For example, talc absorbs HALS UV stabilizers and reduces their effectiveness and metal content in various silica fillers, and can catalyze degradation of PP unless it is adequately stabilized.

All polymer additives can be thought of as contaminants. Despite their positive value, they can also have undesirable side effects. For example, titanium dioxide (TiO2) is used to protect resins like rigid vinyl from UV radiation. However, it is well known that TiO2 causes polymer degradation in the presence of ultra violet rays and moisture, resulting in chalking of Vinyl window and door profiles. Fortunately, chalking can be avoided by using a weatherable grade of TiO2 with an appropriate surface coating.
Carbon black is another pigment often added for UV protection. However, carbon black comes in many grades and forms. Some of them have higher levels of surface functional groups that can absorb antioxidants and thereby decrease, instead of increase overall stability.
Metal chelates such as calcium and zinc stearates are often added as processing stabilizers. However, they can have antagonistic effects on other additives, such as hindered amine light stabilizers (HALS). The result can be diminished weatherability.
Hindered phenolic antioxidants are added to many polymers as processing stabilizers. However, they may react with atmospheric nitrogen oxides during the product's service life, resulting in yellowing or pinking. This effect is accentuated when the material is exposed to excessively high temperatures. Secondary stabilizers are needed to minimize this "gas fade" problem.
Addition of reground process scrap or post-consumer recyclables can severely affect product durability, particularly physical performance. These materials' previous heat histories can deplete levels of protective antioxidant additives. The end result is higher UV photosensitivity and decreased molecular weight, often accompanied by yellowing. Mechanical properties can also be compromised.
The risks associated with adding already processed degraded material to virgin resin are greater when it is not done on a consistent basis. Some processors start adding regrind only when the scrap pile builds up too high and many a times the parts processed out of the machine could contain nearly 100% regrind. Moreover, processors don't always take the same care in drying regrind as they do with the virgin resin. The added moisture content can have significant effects on polymers such as nylon, PET and polycarbonate (PC). This moisture, together with heat from processing and oxygen from the air, can form hydroperoxides, which can accelerate thermal degradation and sensitize the plastic to light exposure.

Government regulations, such as the European Community's new "end-of-life" recycling directive for automotive materials, will soon start to have an impact on American manufacturers. As the amount of recycled content increases, so will the potential consequences for long-term durability.
Another reason to submit products to weathering tests is to ensure that improper processing does not compromise in-service durability. Premature yellowing of vinyl siding processed at overly high temperatures is just one example. Many product failures are traced to practices such as increasing the molding temperature to process a "difficult" batch of resin or increasing the extruder speed and die-head temperature to boost output. Such steps negatively affect the polymer by initiating free-radical oxidation and autocatalytic degradation mechanisms, depleting antioxidants and making the product more sensitive to UV radiation.

It is neither feasible to design products to be "fail-safe" nor to test for all possibilities. However, there are usually a limited number of identifiable variables that affect product durability. Therefore, it is prudent for material producers and processors to conduct basic designed experiments to gauge the sensitivity of the product to formulation, processing and environmental factors. Statistically designed experiments are far more productive and cost-effective than the "test it and see what happens" approach.
Even a modest durability testing program using accelerated weathering equipment can usually pay for itself by decreasing warranty costs and allowing processors to optimize formulations and processing methods without compromising weatherability of the product.