(Chemical properties )Stain-resistance test , Solvent stress-cracking resistance , Environmental stress-cracking resistance..........S.S

(01) IMMERSION TEST (ASTM D 543, ISO 175)

The method of measuring the resistance of plastics to chemical reagents by simple 

immersion of processed plastic specimens is a standard procedure used throughout 

the plastics industry. The method can only be used to compare the relative resis￾tance of various plastics to typical chemical reagents. The test results do not provide 

a direct indication of suitability of a particular plastic for end-use application in 

certain chemical environments. The limitation infl uencing the results, such as dura￾tion of immersion, temperature of the test, and concentration of reagents should 

be considered when studying the test data. For applications involving continuous 

immersion, the data obtained in short-time tests are useful only in screening out 

the most unsuitable materials.

The test equipment consists of a precision chemical balance, micrometers, 

immersion containers, an oven or a constant-temperature bath, and a testing device 

for measuring physical properties. The dimensions and type of test specimens are 

dependent upon the form of the material and tests to be performed. At least three 

test specimens are used for each material being tested and each reagent involved. 

For studying the weight and dimension change, each specimen is weighed and its 

thickness is measured. The specimens are totally immersed in a container for seven 

days in a standard laboratory atmosphere, in such a way that no contact is made 

with the wall or the bottom of the container. After seven days, the specimens are 

removed from the container and weighed. The dimensions are remeasured. The 

procedure remains unchanged for studying the mechanical property changes after 

immersion of the test bars in reagents. The mechanical properties of nonimmersed 

and immersed specimens are determined in accordance with standard methods for 

tests prescribed in the specifi cations and comparisons are made. Observations such 

as loss of gloss, swelling, clouding, tackiness, crazing, and bubbling are also reported 

in the test results.

( 2 )  STAIN RESISTANCE OF PLASTICS

Plastics have deeply penetrated the household products market in the last two decades. Determination of stain resistance of plastic materials has become increas-ingly important since such household products come in contact with many types of chemicals and staining reagents everyday. The test developed for determining stain resistance applies only to the incidental contact of plastic materials with miscella￾neous staining reagents. Any long-term intimate contact of the reagent with plastics must be dealt with in a different manner. Certain types of additives in plastic mate￾rials seem to contribute substantially to the staining process.

The test requires an oven, an applicator, and closed glass containers for low￾viscosity liquids. A wide variety of staining reagents are used. The most common ones are found among food, cosmetics, solvents, detergents, pharmaceuticals, bev￾erages, and cleansing agents. Jelly, tea, blood, coffee, bleach, shoe polish, crayons, lipstick, and nail-polish remover are some examples of staining reagents.

A test specimen of any size may be used as long as it has a fl at, smooth surface and is large enough to permit the test and visual examination. It is recommended that all thermosetting decorative laminates be wet-rubbed with a grade FF or equivalent grade of pumice to remove the surface gloss and then washed with mild soap. The staining reagent is applied onto the specimen with an applicator, forming a thin coat. In the case of low-viscosity liquids, the specimen is immersed in a liquid-staining reagent kept in a glass container. The container is then closed and the specimens are placed in an oven at 50 ± 2°C for 16 hr.

Excess staining material is removed from the surface after exposure and the specimen is visually observed for residual staining. Depending upon the specifi c requirement, the residual staining may or may not be acceptable. The color of plastic has a signifi cant bearing on the noticeability of stains and, therefore, one must consider testing end-use color specimens.

( 2.1 ) Resistance of Plastics to Sulfi de Staining (ASTM D 1712).    

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Many plastic compositions contain salts of lead, copper, and antimony in the form of pigments, stabilizers, fi llers, and other additives. When these materials come in contact with external materials containing sulfi de, such as hydrogen sulfi de, they stain easily. For example, if a lead-stabilized PVC compound is mixed with a tin￾stabilized PVC compound that contains sulfi de, the staining is quite evident. Indus￾trial fumes and rubber are two other major sulfi de-containing external agents.The test to determine the resistance of plastics to sulfi de staining is simple and requires only a freshly prepared solution of hydrogen sulfi de and a test specimen of any size or shape. The specimen is partially immersed in a saturated hydrogen sulfi de solution for 15 min along with a control specimen with a known tendency to sulfi de stain. After 15 min, the specimens are removed and examined for stain￾ing. The comparison between control, unexposed, and exposed specimens is made to determine the relative degree of staining.

(2) SOLVENT STRESS-CRACKING RESISTANCE.  

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One of the most diffi cult challenges a design engineer faces is selecting the right plastic for the right application. The chemical resistance of plastics is a prime con￾sideration in selecting the proper material. The chemical resistance data published by material suppliers is the most convenient source of information. Such published  data is usually derived from a simple immersion test, such as described earlier in this chapter. Most polymers will undergo stress cracking when exposed to certain chemical environments under high stress for a given period of time. Such cracking will occur even though some chemicals have no effect on unstressed parts and, therefore, simple immersion of test specimens is an inadequate measure of che￾mical resistance of polymers (6). At this point, it is important to understand how solvent stress cracking occurs in a polymer. Initially, the polymer-to-polymer bond is replaced by a polymer–solvent bond by lowering the cohesive bond energies of the surface layers of the affected materials. These new polymer–solvent bonds cannot contribute to the overall strength of the material. If the stresses pre￾sent exceed the cohesive strength of the weakened polymer, rupture occurs. 

The type and number of such fractures depend upon the stress pattern present in the material. The solvent penetrates deeper and cracks becomes more extensive with time 

The solvent stress-cracking phenomenon occurs in all plastics at varying degrees. However, the presence of stress, internal or external, is essential. The internal or molded-in stresses pose the biggest problem since complete removal of such stresses is practically impossible. The internal stresses can be minimized through proper design, optimizing processing conditions, and annealing the parts after fabrication. 

When a polymeric material is exposed simultaneously to a chemical and a stress, it can be characterized as exhibiting “critical stress,” below which chemical media has no apparent effect. Critical stress is defi ned as the stress at which the fi rst sign of crazing is observed when a specimen is exposed to a chemical environment. Two different tests have been developed to determine critical stress. One test, often referred to as the calibrated solvent test, employs a tensile testing machine along with a standard tensile test bar. The test is carried out by stressing the tensile bar specimen to a known stress level and immediately exposing it to a chemical envi￾ronment. This is accomplished by either spraying the chemical onto the specimen of continuously wetting the specimen using a wick. The specimen is exposed to the chemical for 1 min and is examined by any sign of crazing with the naked eye. If no such crazing is evident, the experiment is repeated at a higher level of stress using a fresh specimen each time until crazing is observed. The material is consid￾ered safe to use in that particular chemical environment if no crazing is observed at the yield point of the material.

One of the disadvantages of the calibrated solvent test is that it requires a large number of specimens to determine the critical stress level. One other factor is how long the specimen is exposed to chemicals. It is quite possible that the chemical may attack the polymer if exposed for a long time period. Because it is no practical to expend a long time for such visual testing, an accelerated method of testing must be developed. This is generally accomplished by carrying out the test at elevated temperature and high stress. As always, there is no substitute for testing an actual part by simulating the service condition; however, this test does provide some useful information regarding the behavior of the polymer exposed to a chemical environment at different stress levels. The critical stress value established for a particular polymer–solvent combination is very useful in determining the level of molded-in stresses in a part.

An alternate method for measuring solvent stress cracking, developed several years ago, has a few advantages over the previous method (8). This method employs a specimen of size 4 × 1 × 0.03 in. strapped to an elliptical jig. The entire assembly, as is immersed in a reagent. Because of the elliptical design of the jig, the stress at the high end of the jig is extremely low. Conversely, the stress at the  low end of the jig is extremely high. The level of stress in the specimen at different  points on the jig can be calculated. After 1 min, the specimen is observed for crazing.  

The point at which the crazing stops is considered the critical stress point. The critical stress value at this point is determined from a previously calculated value. If no  crazing is observed after 1 min, the test is continued for several hours. The test may  also be carried out at elevated temperatures to accelerate the stress-cracking process.  

The biggest advantage of this method is that one can look at the stress-cracking process over the entire range of stress values using only one specimen. 

( 3 )ENVIRONMENTAL STRESS-CRACKING RESISTANCE (ASTM D 1693, ISO 4599) 

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Environmental stress cracking is the failure in surface-initiated brittle fracture of a polyethylene specimen, or a part under polyaxial stress, in contact with a medium  in the absence of which fracture does no occur under the same conditions of stress.  Combinations of external and/or internal stresses may be involved, and the sensi￾tizing medium may be gaseous, liquid, semisolid, or solid. 

Several conditions must be present for environmental stress cracking to occur. First, the presence of a “stress riser” or a “notch” is a very important factor. The  need for some type of stress, “molded-in” or external, is inevitable. Finally, without  the presence of an external sensitizing agent environmental stress cracking is  impossible ( Environmental stress cracking should not be confused with other types of stress cracking, such as solvent stress cracking and thermal stress cracking. 

Environmental stress cracking describes the tendency of polyethylene products to prematurely fail in the presence of detergents, water, sunlight, oil, or other active environments, usually under conditions of relatively high strain. It is a purely phy￾sical phenomenon that involves no swelling or similar mechanical weakening of the material. Polyethylene products are most susceptible to such cracking or crazing under load when exposed to certain chemicals and environments. This pheno￾menon was fi rst recognized in polyethylene-coated wire, which often was lubricated with surface-active materials to facilitate installation in conduits. Under these conditions, polyethylene, which appeared to perform satisfactorily in the laboratory, rapidly developed severe cracks that propagated completely through to the conductor  The stress-cracking resistance of polyethylene can be improved by increasing molecular weight, reducing stresses by proper fabrication practices, and incorporating elastomers in the formulation. It is further observed that narrow molecular-weight distributions considerably improve the resistance of a polymer of given density and average molecular weight. Large crystalline structures and molecular orientations appear to aggravate the problem 

 Test Procedure

The test specimens of size 1.5 × 1 in. are cut very precisely. The rectangular specimen is nicked to a fi xed length and depth using a sharp blade mounted in the nicking jig (Figure 9-2). The nicked specimen is then bent through 180° so that the nick is on the outside of the bend and at a right angle to the line of bend. The samples are mounted onto the holder. The holder is inserted in the test tube. Immediately after that, the test tube is fi lled with fresh reagent to submerge the samples. The reagent can be a surface-active agent, soap, or any other liquid organic sub￾stance. One of the most commonly used reagents is Igepal C0–630, manufactured by Rhone-Poulenc, Cranbury, NJ. The tube is placed in a constant-temperature bath maintained at 50 ± 0.5°C or 100.0 ± 0.5°C, depending upon the conditions selected for the test. Test specimens are removed after a specifi ed time and observed for crazing

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