Stress Cracking of Plastics by Gasoline

Transcription

1 .Cj SOCIETY OF AUTOMOTIVE ENGINEERS, INC. 400 Cmmnwealth Drive, Warrendale, Pa Stress Cracking f Plastics by Gasline C. H. M. Jacques and M. G. Wyzgski General Mtrs Research Labratries, General Mtrs Crp. SOCIETY OF AUTOMOTIVE ENGINEERS M&k Autmbile Engineering Meeting Dearbrn, Michigan Octber 18-22, liste*: &^c; - ::;::%:;;;:::;:S:^:::;:^^^;:;: L_,!

2 Stress Cracking f Plastics by Gasline C. H. M. Jacques and M. G. Wyzgski General Mtrs Research Labratries, General Mtrs Crp. GLASSY THERMOPLASTICS are knwn t be highly susceptible t envirnmental stress cracking when expsed t rganic liquids. This type f failure can be prevented by insuring that the level f mechanical stress n the plastic is belw the characteristic critical cracking stress fr the particular liquid envirnment. The mechanism by which liquid cntact initiates craze r crack frmatin is nt presently knwn; hwever, several authrs (1-5)* have shwn that critical stress can be crrelated with the slubility f the liquid in the plymer. Values f critical stress and critical strain have been reprted in the literature fr a variety f glassy plastics expsed t pure liquids (1-9); hwever, the stress cracking behavir f liquid mixtures has received little attentin. The present investigatin was initiated t determine the susceptibility f glassy plymers t cracking when expsed t gaslines r mixtures f gasline cmpnents. The bjectives were t measure the range f critical stresses fr plycarbnate (PC), ply(methyl methacrylate) (PMMA), and cellulse acetate butyrate *Numbers in parentheses designate References at end f paper. (CAB) in cntact with gaslines, t define the effect f chemical cmpsitin n critical stress, and t develp methds fr predicting the stress cracking characteristics f cmplex liquid mixtures frm the physical cnstants f the pure liquid cmpnents. EXPERIMENTAL Tensile specimens cnfrming t ASTM D638, type I, were injectin mlded using standard prcessing cnditins. Materials emplyed were the three plastics mentined abve, namely, plycarbnate, cellulse acetate butyrate, and ply(methyl methacrylate). strains were measured using tw pieces f apparatus. A three pint bending frm (8) instrumented t cntinuusly mnitr strain was used t measure the time fr craze initiatin at varius fixed initial strains. All ther critical strain data were btained using an elliptical bending frm (10) having a strain distributin f 0.3% t 2.3%. stresses were calculated by multiplying the measured critical strains by the apprpriate tensile mdulus. A precisely measured 5 ml quantity f test fluid was pured nt the stressed surface and ABSTRACT The susceptibility f plycarbnate, ply (methyl methacrylate), and cellulse acetate butyrate t gasline-induced stress cracking was evaluated by measuring the critical strains f specimens expsed t a variety f cmmercial gaslines and gasline cmpnents. In general, the critical strain decreased and the severity f cracking f mlded parts increased as the armatic cntent f the gaslines increased. Hwever, lw mlecular weight aliphatics as well as specific armatic cmpnents did als markedly reduce critical strain. The effects f sample gemetry, temperature, and weathering n critical strain were determined. The capability t predict the critical strain f plymers in the presence f cmplex liquid mixtures n the basis f physical cnstants f pure liquid cmpnents was examined.

3 the critical strain was established after a ten minute perid. Edge initiated crazing was prevented by cating the edges with a gasline resistant grease. Values reprted in this study represent an average f 3 t 10 separate measurements fr each test liquid. Gasline samples were btained lcally in the Detrit area. Gasline cmpnents, 99 mle percent pure, were btained frm the Phillips Petrleum Cmpany. RESULTS AND DISCUSSION EFFECT OF TIME AND GEOMETRY - Stress cracking characteristics f plycarbnate (PC), ply(methyl methacrylate) (PMMA), and cellulse acetate butyrate (CAB) were evaluated by measuring the minimum strain required t crack r craze samples expsed t gaslines r gasline cmpnents. The time dependency f the crazing phenmena was determined using the three pint bending frm. Crazing strains are presented in Figure 1 fr plycarbnate samples expsed t is-ctane, tluene, and a mixture f these tw gasline cmpnents. Is-ctane is a mild crazing agent; while tluene causes severe cracking f plycarbnate. Each f the tw pure liquids and the mixture exhibit a characteristic critical strain belw which crazing r cracking is nt bserved fr expsure times f up t ne hur. Since the critical strain is reached in a few minutes, a ten minute expsure was chsen fr measurements n the elliptical bending frm. strains fr the entire range f tluene/is-ctane mixtures measured n the elliptical bending frm are presented in Figure 2 fr plycarbnate and in Figure 3 fr ply (methyl methacrylate). Values f critical strain are shwn fr edge initiated crazes as well as fr samples whse edges were cated with grease t prevent edge crazing. It is apparent frm these figures that sharp edges have a significant effect n the measured values f critical strain and that ply(methyl methacrylate) appears t be mre susceptible t edge initiated crazing than plycarbnate. Frm a design standpint, sharp edges and crners shuld be avided when gasline cracking is a pssibility. Subsequent measurements f critical strains fr gaslines were perfrmed n samples with cated edges t minimize the influence f gemetrical factrs n critical strain. EVALUATION OF STRESS CRACKING OF MOLDED PARTS - Stresses which cntribute t envirnmental stress cracking f mlded parts include residual mlding stresses, stresses frm assembly peratins such as mechanical attachment, and stresses which the part encunters in service. When the cmbined stresses are abve a critical level, crazing r cracking f the part will ccur. Fr parts with cmplex gemetries, the stress distributin, mlecular rientatin, and gemetrical factrs generally result in a cmplex crazing and cracking pattern when the w 1.0, I k CC 0.5 ISO-OCTANE 25% TOLUENE 75% ISO-OCTANE A L TOLUENE _L TIME (Minutes) Fig. 1 - Crazing strain as a functin f time f plycarbnate expsed t is-ctane, tluene, and a mixture f these tw liquids 2. r PERCENT TOLUENE Fig. 2 - strains f plycarbnate expsed t tluene/is-ctane mixtures part is expsed t a crazing agent. It is imprtant t establish a crrelatin between critical strains measured n gemetrically simple test samples and the severity f envirn-

4 1.0 PMMA 0.8 feo.6 N " \ D A PREMIUM D REGULAR A UNLEADED REGULAR b A X) PERCENT TOLUENE Fig. 3 - strains f ply(methyl methacrylate) expsed t tluene/is-ctane mixtures 0 MILD I i I SEVERE SEVERITY OF CRACKING Fig. 4 - strain as a functin f severity f cracking f cmplex plycarbnate parts expsed t gaslines mental stress cracking bserved in cmplex parts. strains f plycarbnate expsed t thirteen cmmercial gaslines are presented as a functin f severity f cracking f cmplex mlded parts in Figure 4. strains were measured n the elliptical bending frm and are summarized fr each f the gaslines in Table 1. Severity f cracking was determined by visually ranking parts expsed t each f the gaslines. As shwn in Figure 4, there is an excellent crrelatin between severity f cracking and critical strain. EFFECT OF GASOLINE COMPOSITION - strains and critical stresses fr plycarbnate and ply(methyl methacrylate) expsed t a variety f cmmercial gaslines as well as the verall cmpsitin f each f the gaslines are presented in Tables 1 and 2. strains fr bth materials decrease as the armatic cntent f the gaslines increases. This is mst clearly shwn in Figure 5. Ply(methyl methacrylate) exhibited a greater resistance t gasline-induced crazing and/r cracking than plycarbnate as shwn by its higher values f critical strain. Cellulse acetate butyrate was nt cracked r crazed by any f the gaslines at strains as high as tw percent. N crrelatin was bserved between critical strain and gasline type, i.e., premium, regular, r unleaded. Althugh there was a general crrelatin between critical strain and armatic cntent f cmmercial gaslines, a detailed cmpsitinal analysis f selected gaslines suggested that Table 1 - Strains and Stresses fr Plycarbnate Expsed t Cmmercial Gaslines Strain, Stress, Gasline Grade Armatic MPa (PSI) A Premium (2540) B Unleaded (2340) C Regular (2240) D Premium (2050) E Premium (1950) F Unleaded (1760) G Regular (1720) H Regular (1500) I Regular (1460) J Unleaded (1300) K Unleaded (1300) L Premium (1200) M Premium (1060) Table 2 - Strains and Stresses fr Ply(Methyl Methacrylate) Expsed t Cmmercial Gaslines Gasline A G M Grade Premium Regular Premium Armatic Strain, Stress, MPa (PSI) 67 (9700) 51 (7400) 30 (4400) specific armatic as well as lw mlecular weight aliphatic cmpnents tended t reduce critical strain. In rder t determine the effect f individual gasline cmpnents n critical strain, artificial gasline mixtures cnsisting f 75% aliphatic and 25% armatic cmpnents by vlume were tested. As shwn in

5 2.0 Table 3 - Strains fr Plastics Expsed t Artificial Gasline Mixtures _ 1.6 Liquid Mixture Strain, PC Stress, MPa (PSI) Strain, PMMA Stress, MPa (PSI) w 1.2 C3 Z N O 0.8 IT O 0.4 PMMA 25% Tluene + 75% Is-ctane 75% N-Pentane 75% Is-ctane + 25% Benzene 25% Tluene 25% 0-Xylene 25% Ethyl Benzene (2110) (1110) (2730) (2110) (1170) (1140) (7960) (3210) (9430) (7960) (5710) (4800) 25% 1,2,4 Trimethyl Benzene (1300) (7960) Table 4 - Strains fr Plastics Expsed t Gasline Cmpnents Strain (%) Liquid PC PMMA CAB PERCENT AROMATIC Fig. 5 - strains f plycarbnate and ply(methyl methacrylate) as a functin f gasline armatic cntent Table 3, a lw mlecular weight aliphatic hydrcarbn significantly reduces critical strains f bth plycarbnate and ply(methyl methacrylate). These results are cnsistent with the literature (11,12) in that increasing mlecular weight r branching decreases srptin equilibria and srptin rate. Cntrary t expected results, hwever, crazing strains appear t^ decrease as the mlecular weights f armatic cmpnents f the mixture increase. The absrptin characteristics f these mixtures are currently being determined. PREDICTION OF CRITICAL STRAINS OF COMPLEX MIXTURES - strains f plycarbnate, ply(methyl methacrylate), and cellulse acetate butyrate expsed t pure cmpnents f gasline are summarized in Table 4. Ply(methyl methacrylate) exhibits unifrmly higher critical strains than plycarbnate when expsed t aliphatic r armatic hydrcarbns. Althugh cellulse acetate butyrate was swllen by the armatics, this material was nt cracked r crazed by any f the gasline cmpnents at strains f up t tw percent. Several authrs (1,7-9) have shwn crrelatins between critical strain and liquid parameters such as penetrant size, surface tensin, slubility parameter, and hydrgen bnding parameter. strains f plycarbnate are pltted as a functin f slubility parameter and hydrgen bnding parameter fr a brad range f rganic liquids in Figure 6. strains fr liquids which are nt gasline cmpnents were taken frm the liter- Aliphatics 1-Pentene 0.77 N-Pentane N-Heptane 0.89 N-Octane 0.99 Is-Pentane Is-Octane Armatics Benzene Tluene O-Xylene Ethyl Benzene 1,2,4-Trimethyl Benzene Others Methanl Nitrmethane >0.3 ature (2,9). The slubility map in Figure 6 predicts critical strains fr gaslines t range frm 0.35% t 1.0%, which is apprximately what was bserved experimentally. Hwever, the slubility mapping technique des nt accunt fr differences in the critical strains fr individual aliphatic cmpnents, nr des it adequately accunt fr the effects f different armatic cmpnents n the critical strains fr artificial gasline mixtures. The difficulty f quantitatively predicting critical strains fr cmplex mixtures is illustrated in Figures 2 and 3. The critical strains fr these mixtures are nt defined by simple functins f the critical strains fr the pure liquid cmpnents. In cnclusin, methds presently available fr predicting critical strains fr pure liquids frm liquid prperties may be used successfully as a guide fr estimating critical strains fr cmplex mixtures, but direct measurements f slubility appear t be necessary t fully accunt fr the critical strain behavir in liquid mixtures.

6 EC a. O z a z DO z til u cr > I it.3 I 1-37 / \.53 f^35l \.37 jtsa y'l.7 1.1/ \ //SSI/.5 /.85 /.89.86V V46 /v K1.01- Xj^ [>TÖ ^ \ \ \ \ \ 6!!.3 - X. *J 1 _L _L X SOLUBILITY PARAMETER (5) Fig. 6 - strains f plycarbnate expsed t a variety f pure liquids as a functin f liquid slubility parameter and hydrgen bnding parameter 14 SUMMARY The susceptibility f plycarbnate (PC), ply(methyl methacrylate) (PMMA), and cellulse acetate butyrate (CAB) t gasline-induced stress cracking was evaluated by measuring the critical strains f specimens expsed t a variety f cmmercial gaslines and gasline cmpnents. strains f these materials expsed t gaslines are as fllws: PC: PMMA: CAB: 0.3 t 0.8% 0.8 t 1.9%.0% Gemetrical factrs, such as sharp edges, reduced critical strain significantly. A general crrelatin was bserved between the armatic cntent f the gaslines and critical strain. Hwever, lw mlecular weight aliphatic as well as specific armatic cmpnents markedly reduced the critical strains f gaslines. N crrelatin was bserved between critical strain and gasline type, i.e., premium, regular, r unleaded. ACKNOWLEDGEMENTS The authrs wish t thank Dr. R. L. Furey, E. G. Malzahn, B. E. Nagel and M. W. Jacksn f the General Mtrs Research Labratries fr the cmpsitinal analysis f gaslines. REFERENCES 1. G. A. Bernier, R. P. Kambur, "Macrml," 1, 393 (1968). 2. R. P. Kambur, C. L. Grüner, E. E. Rmagsa, "Macrml," 7, 248 (1974). 3. R. P. Kambur, E. E. Rmagsa, C. L. Grüner, "Macrml," 5, 335 (1972). 4. G. L. Earl, R. J. Lneragan, J. H. T. Jhns, M. Crk, "Plym. Eng. Sei.," 13, 390 (1973). 5. E. H. Andrews, L. Bevan, "Plymer," 13, 337 (1972). 6. E. E. Ziegler and W. E. Brwn, "Plastics Tech.," 1, 409 (1955). 7. R. L. Bergen, Jr., "SPE Jurnal," 24, 77 (1968). 8. P. I. Vincent, S. Raha, "Plymer," 13, 283 (1972). 9. L. F. Henry, "Ply. Eng. Sei.," 14, 167 (1974). 10. R. L. Bergen, Jr., "SPE Jurnal," 18, 667 (1962). 11. R. H. Hlley, H. B. Hpfenberg, V. Stannett, "Ply. Eng. Sei.," 10, 376 (1970). 12. H. B. Hpfenberg, R. H. Hlley, V. Stannett, "Ply. Eng. Sei.," 9, 242 (1969).