Environmental stress cracking affected factors in pp
Answers
Explanation:
Environmental stress cracking is considered to be the most frequent mode of failure of plastic materials.73,74 It was estimated in 1996 that 20% of plastics failed because of environmental stress cracking. By environmental stress cracking, we understand a failure in which materials in contact with a fluid are subjected to stress.74 It is quite difficult to simulate environmental stress cracking in the laboratory, and for this reason many methods were developed, frequently giving conflicting results, which is a part of the problem in finding solutions.
PVC was tested by several methods, including monotonic creep method,73 microhardness,73 crazing,75 crack growth,76,77 and three point bending.78 The monotonic creep method is particularly useful as an accelerated test for materials which can induce fracture after a few years.73 The method generates critical time, critical strain, and critical stress data. Microhardness is used for general screening and generation of data on plastic/fluid compatibility.73 Initiation time for PVC varied from 360 s for methyl ethyl ketone to 45900 s for water in the range of fluids studied.73 There is a relationship between the reduction of glass transition temperature caused by absorption of liquid and the reduction of craze initiation stress.75 Flory-Huggins interaction constant can be used for the prediction of the craze initiation stress. Also, solubility parameters can be used for such predictions.71 Crack growth rate is accelerated by the presence of vapors causing environmental stress cracking.76 Failures are caused by plasticization and/or plastic deformation around the cave
Answer:
Environmental Stress Cracking (ESC) is one of the most common causes of unexpected brittle failure of thermoplastic (especially amorphous) polymers known at present. According to ASTM D883, stress cracking is defined as " an external or internal crack in a plastic caused by tensile stresses less than its short-term mechanical strength." This type of cracking typically involves brittle cracking, with little or no ductile drawing of the material from its adjacent failure surfaces.[1] Environmental stress cracking may account for around 15-30% of all plastic component failures in service.[2] This behavior is especially prevalent in glassy, amorphous thermoplastics.[3] Amorphous polymers exhibit ESC because of their loose structure which makes it easier for the fluid to permeate into the polymer. Amorphous polymers are more prone to ESC at temperature higher than their glass transition temperature (Tg) due to the increased free volume. When Tg is approached, more fluid can permeate permeation into the polymer chains.[4]
ESC and polymer resistance to ESC (ESCR) have been studied for several decades.[5] Research shows that the exposure of polymers to liquid chemicals tends to accelerate the crazing process, initiating crazes at stresses that are much lower than the stress causing crazing in air.[5][6] The action of either a tensile stress or a corrosive liquid alone would not be enough to cause failure, but in ESC the initiation and growth of a crack is caused by the combined action of the stress and a corrosive environmental liquid. These corrosive environmental liquids are called 'secondary chemical agents', are often organic, and are defined as solvents not anticipated to come into contact with the plastic during its lifetime of use. Failure is rarely associated with primary chemical agents, as these materials are anticipated to come into contact with the polymer during its lifetime, and thus compatibility is ensured prior to use. In air, failure due to creep is known as creep rupture, as the air acts as a plasticizer, and this acts in parallel to environmental stress cracking.[7]
It is somewhat different from polymer degradation in that stress cracking does not break polymer bonds. Instead, it breaks the secondary linkages between polymers. These are broken when the mechanical stresses cause minute cracks in the polymer and they propagate rapidly under the harsh environmental conditions.[8] It has also been seen that catastrophic failure under stress can occur due to the attack of a reagent that would not attack the polymer in an unstressed state. Environmental stress cracking is accelerated due to higher temperatures, cyclic loading, increased stress concentrations, and fatigue.[7]
Explanation: