question: Why do rubber bands "melt" and stick to other plastics after a long time?
comment: Just an educated guess: Rubber is mostly made of polyisoprene, and stabilized by disulfide bridges. Polyisoprene is nice because it has double bonds that can be used for cross-linking. After a long time on the air, the oxygen could break down these junctions, giving other plastics the opportunity to connect to the isoprene molecules. This could create a "glue-like" effect.
passages: Hard, plastic thermosets may undergo permanent or plastic deformation under load. Elastomers, which are soft and springy or rubbery and can be deformed and revert to their original shape on loading release.
   A more common and related problem is "swelling", where small molecules infiltrate the structure, reducing strength and stiffness and causing a volume change. Conversely, many polymers (notably flexible vinyl) are intentionally swelled with plasticizers, which can be leached out of the structure, causing brittleness or other undesirable changes.
   The parallel chains of stretched rubber are susceptible to crystallization. This takes some time because turns of twisted chains have to move out of the way of the growing crystallites. Crystallization has occurred, for example, when, after days, an inflated toy balloon is found withered at a relatively large remaining volume. Where it is touched, it shrinks because the temperature of the hand is enough to melt the crystals.
   Polymer degradation by galvanic action was first described in the technical literature in 1990. This was the discovery that "plastics can corrode", i.e. polymer degradation may occur through galvanic action similar to that of metals under certain conditions and has been referred to as the "Faudree Effect". In the aerospace field, this finding has largely contributed to aircraft safety, mainly those aircraft that use CFRP and has resulted in a wide body of follow-up research and patents. Normally, when two dissimilar metals such as copper (Cu) and iron (Fe) are put into contact and then immersed in salt water, the iron will undergo corrosion, or rust. This is called a galvanic circuit where the copper is the noble metal and the iron is the active metal, i.e., the copper is the positive (+) electrode and the iron is the negative (-) electrode. A battery is formed. It follows that plastics are made stronger by impregnating them with thin carbon fibers only a few micrometers in diameter known as carbon fiber reinforced polymers (CFRP). This is to produce materials that are high strength and resistant to high temperatures. The carbon fibers act as a noble metal similar to gold (Au) or platinum (Pt). When put into contact with a more active metal, for example with aluminum (Al) in salt water the aluminum corrodes. However, in early 1990, it was reported that imide-linked resins in CFRP composites degrade when bare composite is coupled with an active metal in salt water environments. This is because corrosion not only occurs at the aluminum anode, but also at the carbon fiber cathode in the form of a very strong base with a pH of about 13. This strong base reacts with the polymer chain structure degrading the polymer. Polymers affected include bismaleimides (BMI), condensation polyimides, triazines, and blends thereof. Degradation occurs in the form of dissolved resin and loose fibers. The hydroxyl ions generated at the graphite cathode attack the O-C-N bond in the polyimide structure. Standard corrosion protection procedures were found to prevent polymer degradation under most conditions.
   Some elastomers which are amorphous in the unstrained state undergo rapid crystallization upon stretching.
  
  Section::::Crystallization mechanisms.:Crystallization from solution.
   Section::::Chemical degradation.:Oxidation.
  
  The polymers are susceptible to attack by atmospheric oxygen, especially at elevated temperatures encountered during processing to shape. Many process methods such as extrusion and injection moulding involve pumping molten polymer into tools, and the high temperatures needed for melting may result in oxidation unless precautions are taken. For example, a forearm crutch suddenly snapped and the user was severely injured in the resulting fall. The crutch had fractured across a polypropylene insert within the aluminium tube of the device, and infra-red spectroscopy of the material showed that it had oxidized, possible as a result of poor moulding.
   There are two types of processing defects that can occur during coextrusion. The first defect is interface instability, causing unintended interface shapes. This can cause "encapsulation" of the higher viscosity melt by the lower viscosity melt, leading to poor final performance of the extruded part. The severity of this type of defect is proportional to the difference in viscosities between the two polymer melts. The other type of defect forms from oscillations in the melt flow, causing small wavelike patterns on the surface of the melt and reducing optical transparency.
   One common example of chemically assisted degradation is the degradation of rubber by ozone particles. Ozone is a naturally occurring atmospheric molecule that is produced by electric discharge or through a reaction of Oxygen with solar radiation. Ozone is also produced with atmospheric pollutants reacted with ultraviolet radiation. For a reaction to occur, ozone concentrations only have to be as low as 3-5 parts per hundred million and when these concentrations are reached, a reaction occurs with a thin surface layer (5 x10-7 metres) of the material.
   Crazing can take place in glassy polymers under environmental effects. It is troublesome because it requires a much lower stress state and sometimes it happens in a long delay, which means it's hard to detect and avoid. For example, the PMMA containers in daily use is quite resistive to humidity and temperature without any visible defects. But after they are machine-washed and then left in air for one or two days, they will shutter abruptly when wet with gin. During the process, the stress applied is negligible, but crazing is still found in the containers.
   Hysteresis heating and Chain scission
   While many ideas from the linear elastic fracture mechanics (LEFM) models are applicable to polymers there are certain characteristics that need to be considered when modeling behavior. Additional plastic deformation should be considered at crack tips as yielding is more likely to occur in plastics.
  
  Section::::Fracture Mechanics in Polymers.:"Yielding Mechanisms".
   UV-C radiation is able to break down chemical bonds. This leads to rapid aging of plastics, insulation, gaskets, and other materials. Note that plastics sold to be "UV-resistant" are tested only for UV-B, as UV-C doesn't normally reach the surface of the Earth. When UV is used near plastic, rubber, or insulation, care should be taken to shield these items; metal tape or aluminum foil will suffice.
   In general, the process of microbial plastic biodegradation results in a considerable decrease in polymer molecular weight, causing the plastic to lose its structural integrity. There are several different ways in which microorganisms can carry out the process of plastic degradation, and the mechanism differs slightly depending on the environmental conditions. 
  
  Section::::Mechanism of biodegradation.:Direct Action.
   Lüders band
  
  Lüders bands, also known as "slip bands" or "stretcher-strain marks," are localized bands of plastic deformation in metals experiencing tensile stresses, common to low-carbon steels and certain Al-Mg alloys. First reported by Guillaume Piobert and W. Lüders, the mechanism that stimulates their appearance is known as "dynamic strain aging," or the inhibition of dislocation motion by interstitial atoms (in steels, typically carbon and nitrogen), around which "atmospheres" or "zones" naturally congregate.
   Polymer oxidation has caused accidents involving medical devices. One of the oldest known failure modes is ozone cracking caused by chain scission when ozone gas attacks susceptible elastomers, such as natural rubber and nitrile rubber. They possess double bonds in their repeat units which are cleaved during ozonolysis. Cracks in fuel lines can penetrate the bore of the tube and cause fuel leakage. If cracking occurs in the engine compartment, electric sparks can ignite the gasoline and can cause a serious fire. In medical use degradation of polymers can lead to changes of physical and chemical characteristics of implantable devices.
   Section::::Properties.:Phase behavior.:Inclusion of plasticizers.
   BULLET::::- Improved closure strength during shipping
  
  BULLET::::- Regulatory requirement (sometimes)
  
  Section::::Roll-on metal closures.
  
  When an aluminum or plastic bottle cap has an integral band, it is usually connected by thin bridges. They can be molded along with the cap or slit afterwards. When unscrewing the cap, the frangible ring breaks: the ring can separate from the cap, and two separate pieces remain, the sealing piece and the leftover ring; or the broken ring can form a "pigtail" still attached to the cap.
  
  Section::::Shrink bands.
   Section::::Corrosion in nonmetals.:Corrosion of polymers.
  
  Polymer degradation involves several complex and often poorly understood physiochemical processes. These are strikingly different from the other processes discussed here, and so the term "corrosion" is only applied to them in a loose sense of the word. Because of their large molecular weight, very little entropy can be gained by mixing a given mass of polymer with another substance, making them generally quite difficult to dissolve. While dissolution is a problem in some polymer applications, it is relatively simple to design against.
   The original shaped silicone rubber bands were created in 2002 by the Japanese design team Passkey Design, Yumiko Ohashi and Masonar Haneda. They made the bands in cute animal shapes to encourage sustainability by discouraging people from treating the rubber bands as disposable. Sold under the brand name Animal Rubber Bands, they won the Best Design award at the 2003 Japanese National Competition. These bands were not widely distributed in the United States, but in 2005 they were local hits in Guilford, Connecticut and at the Design Store at the New York City Museum of Modern Art.
   In a finished product, such a change is to be prevented or delayed. Failure of safety-critical polymer components can cause serious accidents, such as fire in the case of cracked and degraded polymer fuel lines. Chlorine-induced cracking of acetal resin plumbing joints and polybutylene pipes has caused many serious floods in domestic properties, especially in the US in the 1990s. Traces of chlorine in the water supply attacked vulnerable polymers in the plastic plumbing, a problem which occurs faster if any of the parts have been poorly extruded or injection molded. Attack of the acetal joint occurred because of faulty molding, leading to cracking along the threads of the fitting which is a serious stress concentration.
   The ozone molecules react with the rubber which in most cases is unsaturated (contains double bonds), however a reaction will still occur in saturated polymers (those containing only single bonds). When reaction occurs, scission of the polymer chain (breaking of double covalent bonds) takes place forming decomposition products:
   Section::::Physics.:Wear.:Rubbing Wear or Fretting.
  
  The rubbing wear occurs in systems subject to more or less intense vibrations, which cause relative movements between the surfaces in contact with the order of the nanometer. These microscopic relative movements cause both adhesive wear, caused by the displacement itself, and abrasive wear, caused by the particles produced in the adhesive phase, which remain trapped between the surfaces. This type of wear can be accelerated by the presence of corrosive substances and the increase in temperature.
  
  Section::::Physics.:Wear.:Erosion Wear.
   Plastics and plastic-based composites may suffer swelling, debonding and loss of strength when exposed to organic fluids and other corrosive environments, such as acids and alkalies. Under the influence of stress and environment, many structural materials, particularly the high-specific strength ones become brittle and lose their resistance to fracture. While their fracture toughness remains unaltered, their threshold stress intensity factor for crack propagation may be considerably lowered. Consequently, they become prone to premature fracture because of sub-critical crack growth. This article aims to give a brief overview of the various degradation processes mentioned above.
  
  Section::::Stress corrosion cracking.
   Plasticizers also function as softeners, extenders, and lubricants, and play a significant role in rubber manufacturing.
  
  Section::::For plastics.:Antiplasticizers.
   Above its glass transition temperature and below its melting point, the physical properties of a thermoplastic change drastically without an associated phase change. Some thermoplastics do not fully crystallize below the glass transition temperature, retaining some or all of their amorphous characteristics. Amorphous and semi-amorphous plastics are used when high optical clarity is necessary, as light is scattered strongly by crystallites larger than its wavelength. Amorphous and semi-amorphous plastics are less resistant to chemical attack and environmental stress cracking because they lack a crystalline structure.
  