- Email: [email protected]
Effect of stabilizers in photodegradation of thermoplastic elastomeric rubber–polyethylene blends—a preliminary study
Polymer Degradation and Stability 74 (2001) 513–521 www.elsevier.com/locate/polydegstab
Effect of stabilizers in photodegradation of thermoplastic elastomeric rubber–polyethylene blends—a preliminary study Anil K. Bhowmicka, J. Heslopb, J.R. Whiteb,* a Rubber Technology Centre, Indian Institute of Technology, Kharagpur-721302, India Department of Mechanical, Materials and Manufacturing Engineering, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK
b
Received 23 May 2001; received in revised form 9 July 2001; accepted 16 July 2001
Abstract The effect of adding stabilizers to a family of thermoplastic elastomeric blends of natural rubber and polyethylene has been investigated using UV exposures of samples both unstressed and under tensile elongation. Samples were prepared both with dicumyl peroxide crosslinking agent and with no crosslinking agent. The stabilizers used were (i) isopropyl paraphenylene diamine (IPPD); (ii) a high molecular weight phenolic anti-oxidant; and (iii) a commercially available blend of two high molecular weight hindered amine stabilizers. The crosslinked samples showed better photo-resistance than the compounds with no dicumyl peroxide. All of the stabilizers produced considerable improvement in photo-resistance. The most effective one was IPPD (a stabilizer for rubber) even though the continuous phase in these blends is polyethylene. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Natural rubber–polyethylene blends; Photo-degradation; Thermoplastic elastomers; UV-stabilizers
1. Introduction Thermoplastic elastomers (TPEs) based on rubber– plastic blends have potential for many applications in engineering and consumer goods [1,2]. Applications under consideration such as automotive window seals and footwear involve outdoor service and it is necessary to develop satisfactory stabilization systems to ensure prolonged life. Stabilizers are required to protect the compounds from thermal degradation (at the elevated temperatures used in processing) and photo-degradation caused by the ultraviolet irradiation (UV) in sunlight. Although some stabilizers have a wide application and can be used with many different polymers, some additives may act as a stabilizer with one polymer and as a prodegradant for another. Therefore, when choosing a stabilizing system for a polymer blend it is necessary to know the effect of the stabilizer on both polymeric components. Similarly any possible pro-degradant action of any other additive must be considered. Ideally a stabilizer will have a beneficial effect on both component polymers but in many cases it will be present * Corresponding author. Tel.: +44-191-2227906; fax: +44-1912228563. E-mail address: [email protected] (J. R. White).
to protect one of the components only. In this case it is unfortunate that it will usually become distributed into both polymeric components during compounding and subsequent fabrication. This is a problem common to all polymer blends, not just the thermoplastic elastomers. In the study reported here the effect of including commercial stabilizers on the photo-oxidation behaviour of a family of TPEs based on natural rubber-low density polyethylene blends was investigated using methodology described elsewhere [3]. We have reported previously on the ageing and photo-oxidation behaviour of these materials [3,4]. The unprotected materials degraded very rapidly under laboratory exposures with UV intensities set at tropical levels [3]. The unprotected natural rubber degraded at least as rapidly under these conditions. When natural rubber is compounded with anti-oxidants and fillers such as carbon black it can have very acceptable weatherability. So it can be anticipated that significant improvements can be achieved with the TPEs. With natural rubber the chemical attack is often attributed to ozone [5–9], particularly when the component is loaded in tension. Although ozone is undoubtedly aggressive when present, other studies indicate that photo-oxidation often dominates over ozone attack under conditions such as those found in our laboratory and in many service environments [10–14]. Our earlier
0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00188-4
514
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
studies point to photo-oxidation as the main mechanism of degradation [3,15]; further evidence for this was sought in the study of the family of TPE blends described here. There are several stabilizers available, commercially, that give protection against oxidation and which are very effective in extending outdoor lifetime. Polyethylene also degrades rapidly when exposed to UV [16–19]. Again, there are commercial stabilizers that give spectacular increases in outdoor lifetime of polyethylene. The work described here was part of a broader study aimed at developing a class of inexpensive TPEs with good weatherability. It is a preliminary investigation to determine whether or not significant lifetime improvements could be achieved using readily accessible, relatively cheap commercial additives.
2. Experimental 2.1. Materials Details of the preparation of the materials used here are given elsewhere [3]. They are blends of natural rubber and low density polyethylene with compositions given in Table 1. Natural rubber (NR) was supplied by Rubber Board, Kottayam, India. This had a molecular mass 780103, intrinsic viscosity (benzene 30 C m3/kg []=0.44) and Wallace Plasticity=59.0. The polyethylene (PE) was Indothene 16 MA 400, supplied by IPCL, Baroda, India, melt flow index (MFI) 40 g/10 min. Dicumyl peroxide (DCP) was supplied by Hercules Inc., Wilmington, DE, USA. The stabilizers used were: (a) IPPD: isopropyl paraphenylene diamine, an antioxidant for rubber, supplied by Polyolefines India Ltd., Mumbai, India. (b) IRG: tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane (Irganox 1010: Ciba Table 1 Compositions of the rubber–polyethylene blends (in phr) Code
NR:LDPEa
DCPa
IPPDa
IRGa
TINa
70NR30PE 70NR30PE/DCP 70NR30PE/IPPD 70NR30PE/DCP/IPPD 70NR30PE/IRG 70NR30PE/DCP/IRG 70NR30PE/TIN 70NR30PE/DCP/TIN 70NR30PE/IRG/TIN NR/DCP PE/DCP
70:30 70:30 70:30 70:30 70:30 70:30 70:30 70:30 70:30 100:0 0:100
0 1 – 1 – 1 – 1 – 1 1
– – 1 1 – – – – – – –
– – – – 1 1 – – 0.5 – –
– – – – – – 1 1 0.5 – –
a NR, natural rubber; PE, low density polyethylene; DCP, dicumyl peroxide; IPPD, isopropyl paraphenylene; IRG, Irganox 1010; TIN, Tinuvin 783.
Specialty Chemicals), a high molecular weight phenolic anti-oxidant (1178 g/mol 1) recommended for use as a thermal (processing) stabilizer for plastics. (c) TIN: a commercially-available blend (Tinuvin 783: Ciba Specialty Chemicals) of two high molecular weight hindered amine stabilizers: poly[6-[(1,1,3,3,-tetramethylbutyl) amino]-s-triazine-2,4-diyl][2,2,6,6-tetramethyl-4piperidyl)imino]hexamethylene[(2,2,6,6 - tetramethyl - 4 piperidyl)imino] (Chimassorb 944, 2000--3100 g mol 1) and dimethyl succinate polymer with 4-hydroxy-2,2,6,6tetramethyl - 1 - piperideneethanol (Tinuvin 622, 31004000 g/mol 1) recommended for use both as a thermal stabilizer and a light stabilizer for plastics. The blends were prepared in a Brabender Plasticorder by melt mixing the plastic and the rubber at 150 C at 60 rpm using a cam type rotor for about 5–6 min. The polyethylene component was melted first and the rubber was then added and blended. The stabilizer was incorporated into the blends at this stage. The curative (DCP) was added at a level of 1 phr as required and the mixing continued until the torque increased by 3–4 units. The hot mass was pressed into a sheet of 3 mm thickness from which test slabs of 2 mm thickness were prepared by compression molding in a hydraulic press at a temperature of 150 C for 3 min at 5 MPa pressure. The slabs were subsequently cooled under pressure. 2.2. Ultraviolet exposure arrangements Samples were exposed to ultraviolet irradiation (UV) in a constant temperature room set at 30 1 C. The illumination source consisted of a pair of fluorescent tubes type UVA-340 (Q-Panel Company) for which the UV output matches the spectrum of solar radiation at the Earth’s surface fairly closely [20]. The output at higher wavelengths is very much lower than solar radiation levels and does not cause serious heating of the samples, which remain almost at the room temperature. The total intensity used was about 1.8 Wm 2 in the wavelength range 295–320 nm, that is the total radiation below 320 nm wavelength, comparable to levels in hot sunny climates [21,22]. The illumination provided by the tubes was checked regularly using a Bentham Instruments spectroradiometer. The samples were supported on a wooden base and exposed on one side only. UV exposures were also conducted with samples loaded in uniaxial tensile stress relaxation using simple frames that could accommodate up to 15 separate strips simultaneously. This means that load cannot be monitored on individual specimens but this is not regarded as a serious problem in the context of this rather exploratory study. From the results of creep experiments, however, it is certain that a large fraction of the initial applied stress will have relaxed during the exposure in the blends studied here. Stress relaxation-UV exposures were conducted at strains set at 25 and 50% respectively. Creep
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
under dead weight loading was found to be quite rapid except at low stresses (well below 1 MPa). Samples partially crosslinked by the presence of DCP were found to be less prone to creep, as expected, but still deformed rapidly at stresses near to 1 MPa. A limited number of experiments were conducted using dead weight loading at stresses sufficiently small to reduce creep to an acceptably low level during a 2 week UV exposure for comparison with the stress relaxation–UV exposure trials. The ozone levels were checked at various sites within the room in which the UV trials were conducted, including positions close to the samples under test, and found to be below the measurement threshold for the equipment used (< 0.02 ppm). In the earlier studies conducted by Marcos Maillo and White [15] it was discovered that when a simple shield made from aluminium foil prevented UV reaching part of the sample surface there was a very sharp demarcation between the degradation displayed by the exposed and unexposed zones respectively, indicating that photo-oxidation rather than ozone attack was principally responsible for the degradation. In the trials conducted for the current investigation this simple yet effective device (of shielding part of the sample from direct UV exposure) was adopted as routine. 2.3. Surface degradation analysis Samples were inspected using light optical microscopy and scanning electron microscopy (SEM) at intervals. Although it is possible to view rubber samples in the SEM without modification [23] a sputtered gold coating was applied to improve the image quality. Cracks or fissures that appeared as the result of degradation were rendered more visible under the light optical microscope by applying a strain, using a miniature straining device which can be placed on the microscope stage [3,15]. This device re-opened cracks observed to form during tensile exposures. With samples that were exposed unstrained cracks often appeared (for the first time) when a small strain was applied in this way. Although such cracks could therefore be regarded as an artefact produced by the method of observation, the ease with which such cracks formed and their size and density can be taken as an indication of the level of degradation which had accrued. Oblique illumination was used, adjusted to give greatest visibility of the cracks. Samples which broke during stress relaxation—UV exposures (unprotected blends) were mounted on SEM stubs with the fracture surface upwards, permitting observation of all four sides as well as the fracture surface by appropriate manipulation of the SEM stage controls. For SEM observation of samples which did not break during exposure, a small device was used which stretched them to open any cracks that formed in a similar way to the light microscope jig; samples were
515
mounted in this device for gold-coating and subsequent inspection in the SEM [3,15]. The secondary electron image was used throughout, using 15 kV accelerating potential. 2.4. Mechanical properties After UV exposure, the strips (10012 mm) were tensile tested on an Instron 4500 tensile test machine using a grip separation of 60 mm and a crosshead speed of 500 mm/min. The load-deformation traces were sometimes quite noisy, possibly because the load cell was operating near to its sensitivity limit [3]. This effect and the noise have been smoothed out for the presentation of results in Fig. 1.
3. Results 3.1. Mechanical properties The results of the tensile tests on the conditioned samples containing stabilizers are shown in Fig. 1. Details of the behaviour of (unstabilized) 70NR30PE and 70NR30PE/DCP after similar conditioning treatments are given elsewhere [3]. The maximum strain values from Fig. 1(a)–(g) are summarised in Table 2 and are discussed below. Fig. 1 shows that the effect of UV exposure during stress relaxation at 50% strain is to increase the stiffness: with the exception of 70NR30PE/IRG, each of the compounds that survived this treatment gave the steepest stress–strain relationship for this condition. Table 2 shows the maximum extension recorded in the tensile tests on samples after 14 days conditioning under various combinations of stress relaxation and UV exposure. After stress relaxation at 25% in the absence of UV, compounds based on 70NR30PE displayed an increased extension compared with the corresponding unstrained/unexposed sample when IPPD or IRG were used but a slight decrease when TIN or a combination of IRG and TIN were used. All compounds showed a reduced extension after 14 days stress relaxation at 50% strain in the dark. Stabilized compounds based on 70NR30PE showed poorer performance after stress relaxation at 25% under UV than after unstrained UV exposure with the possible exception of 70NR30PE/IRG. UV exposure during stress relaxation at 50% strain caused a significant loss of extension in every case. Table 3 shows the ratio of the extension observed for stabilized samples to that obtained with the unstabilized sample after the same conditioning treatment. No comparisons were possible for samples preconditioned in stress relaxation at 50% strain under UV exposure because the unstabilized samples failed during the stress relaxation exposure before 7 days had elapsed [3]. Only two of the results shown in Table 3 indicate a lower
516
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
Fig. 1. Engineering stress-deformation curves for blends after conditioning for 14 days under various combinations of strain and UV exposure. (a) 70NR30PE/IPPD; (b) 70NR30PE/DCP/IPPD; (c) 70NR30PE/IRG; (d) 70NR30PE/DCP/IRG; (e) 70NR30PE/TIN; (f) 70NR30PE/DCP/TIN; (g) 70NR30PE/IRG/TIN. (The line for unexposed unstrained 70NR30PE/IPPD in (a) represents the average of 3 tests for which the load-deformation curves superimposed closely up to the point of fracture, which occurred at different values).
517
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521 Table 2 Maximum extension in tensile tests (mm) after conditioning for 14 days under various combinations of stress relaxation and UV exposure Code
0/noa
0/yesa
25/noa
25/yesa
50/noa
50/yesa
70NR30PE 70NR30PE/DCP 70NR30PE/IPPD 70NR30PE/DCP/IPPD 70NR30PE/IRG 70NR30PE/DCP/IRG 70NR30PE/TIN 70NR30PE/DCP/TIN 70NR30PE/IRG/TIN
85 78 147 113 114 145 135 175 100
52 55 119 105 121 81 100 67 125
85 91 157 113 132 – 121 67 91
57 – 108 133 126 135 75 58 110
62 74 96 116 90 111 85 90 45
– – 69 80 41 30 42 – 76
a
0, unstrained; 25, stress relaxation under 25% extension; 50, stress relaxation under 50% extension; no, in dark; yes, exposed to UV.
Table 3 Normalised extension in tensile testsa Code
0/no
0/yes
25/no
25/yes
50/no
70NR30PE 70NR30PE/DCP 70NR30PE/IPPD 70NR30PE/DCP/IPPD 70NR30PE/IRG 70NR30PE/DCP/IRG 70NR30PE/TIN 70NR30PE/DCP/TIN 70NR30PE/IRG/TIN
1.0 1.0 1.73 1.44 1.34 1.85 1.59 2.23 1.18
1.0 1.0 2.27 1.91 2.30 1.48 1.91 1.23 2.38
1.0 1.0 1.85 1.23 1.55 – 1.43 0.73 1.07
1.0 – 1.89 _ 2.21 – 1.32 – 1.93
1.0 1.0 1.54 1.57 1.44 1.51 1.36 1.22 0.72
a Results for compounds based on 70NR30PE (containing no DCP) are normalised by dividing by the result for 70NR30PE after treatment under the corresponding condition. Results for compounds based on 70NR30PE/DCP (containing DCP) are normalised by dividing by the result for 70NR30PE/DCP after treatment under the corresponding condition.
extension for the stabilized sample than for the corresponding unstabilized sample. The two anomalous results were obtained in single trials because of the shortage of specimens; in view of the overwhelming trend towards significant improvement in the stabilized grades it is likely that repeat tests would have eliminated this anomaly. Table 3, therefore, indicates that the introduction of stabilizers to both 70NR30PE and 70NR30PE/DCP improved performance even in samples that were not exposed to UV, presumably because all of the stabilizers used in this study afford some protection against thermal oxidation (during processing). If averages are taken across the rows in Table 3 it is found that, under the conditions applied in these experiments, the least effective stabilizer is TIN (alone or in combination with IRG). The improvements in performance caused by including stabilizers in compounds based on 70NR30PE were on average slightly better than those in 70NR30PE/DCP. Taking averages of columns in Table 3 it is found that the condition in which the presence of stabilizer appears to be most beneficial is unstrained exposure. 3.2. General observations The UV degradation behaviour of blends containing no stabilizer has been described in detail elsewhere [3].
The basic blend (70NR30PE) was light brown in colour. The appearance did not change markedly when DCP, Irganox or Tinuvin were added (one at a time or in combinations) but the compounds containing IPPD took on a darker grey hue. Samples containing stabilizers (IPPD, IRG, TIN) were shiny and tacky in the unexposed state, as with the unstabilized compounds, and also became dull and less sticky after UV exposure. When exposed to UV in the stress relaxation frames most of the unprotected samples developed cracks perpendicular to the strain axis and similar behaviour was observed with samples containing stabilizers. The unprotected samples were curved after releasing from the frames; samples containing stabilizer tended to show very little curvature after 7 days exposure but generally had developed significant curvature when inspected after 14 days (see below). The curvature was convex on the exposed surface with all samples. The rippling effect often seen on the surface of unprotected blends was absent or barely visible in samples containing stabilizer. Cracks developed in the unexposed surface with some unprotected samples, but this did not occur when a stabilizer was included in the compound. It should be noted that 70NR30PE/DCP is much stiffer than 70NR30PE and that, although crosslinked compounds (70NR30PE/DCP and stabilized compounds
518
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
based on this composition) failed at lower failure strains than 70NR30PE and the stabilized compounds based on it, the crosslinked compounds failed at higher stresses than the corresponding uncrosslinked blends. 3.3. Surface degradation 3.3.1. Unprotected samples Observations with unprotected NR/PE blends, including 70NR30PE and 70NR30PE/DCP, and the homopolymers, NR/DCP and PE/DCP were described in detail elsewhere [3]. 3.3.2. Effect of IPPD After UV exposure, whether unstrained or in stress relaxation, the appearance of samples containing IPPD was quite different from any of the other samples. The most severe cracking pattern was that obtained with 70NR30PE/DCP/IPPD after 7 days exposure in the stress relaxation rig at 25% strain. The cracking pattern in Fig. 2(a) was obtained with the specimen held strained 50% on the microscope stage and the pattern
Fig. 2. Surface of 70NR30PE/DCP/IPPD after exposure in the stress relaxation rig at 25% strain for (a) 7 days; (b) 14 days. Specimens held strained 50% on the microscope stage. (b) shows the full width of the sample; (a) has a magnification four times higher than that of (b).
may have been produced by this rather than solely by the strain-UV exposure treatment. Samples exposed for 7 days unstrained and 14 days strained at 25% respectively showed dense hairline cracks when strained on the microscope stage [Fig. 2(b)]. Samples observed under other conditions showed even less evidence of cracking. Samples of blend 70NR30PE/IPPD showed still less cracking perpendicular to the strain axis than the equivalent blend containing DCP but in some cases revealed a dense pattern of striations parallel to the axis [Fig. 3(a)]. A cross-hatched pattern was observed on the surface of a sample exposed for 14 days under 25% strain [Fig. 3(b)]. Details of fine cracks of the kind shown at low magnification in Fig. 3(a) were revealed in the scanning electron microscope: Fig. 4 shows the surface of a 70NR30PE/IPPD sample after 7 days UV exposure unstressed. 3.3.3. Effect of IRG Inclusion of IRG seemed to provide good short term protection of 70NR30PE with only moderate cracking after 7 days exposure under stress but after 14 days
Fig. 3. Surface of 70NR30PE/IPPD after exposure to UV for 14 days (a) unstressed; (b) strained 25%. Specimens held strained 50% on the stage. (a) shows the full width of the sample; (b) has a magnification four times higher than that of (a).
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
519
Fig. 5. Surface of 70NR30PE/IRG after exposure to UV for 14 days strained 25%. Specimen held strained 50% on the stage.
Fig. 4. Surface of 70NR30PE/IPPD after exposure to UV for 7 days unstressed, viewed in the scanning electron microscope; (a) high magnification; (b) low magnification of the area with (a) at its centre.
exposure the cracks were larger and more numerous (Fig. 5). With 70NR30PE/DCP/IRG cracking developed readily after 7 days exposure (Fig. 6). Fig. 6 includes part of the sample that was beneath the shield; the boundary between the exposed and the unexposed zones is clearly demarcated.
Fig. 6. Surface of 70NR30PE/DCP/IRG after exposure to UV for 14 days unstressed: specimen held strained 50% on the stage.
3.3.6. Unexposed side Very little sign of cracking was found on the underside of samples containing any of the stabilizers used in this study.
4. Discussion 3.3.4. Effect of Tinuvin On the basis of the microscopical evidence, the Tinuvin stabilizer appears to be least effective of all. Cracks formed easily in samples exposed to UV both in the unstrained and the strained state in both 70NR30PE/ TIN and 70NR30PE/DCP/TIN [Fig. 7(a) and (b)]. Samples containing a mixture of IRG and TIN also cracked quite easily after exposure to UV either unstrained or in tension. 3.3.5. Effect of the shield The region under the aluminium foil shield was almost devoid of degradation (for example, see Fig. 6). This was a common observation and showed that the extensive degradation found in samples after UV irradiation required direct exposure.
The formation of the cracking patterns in the unstabilized blends has been discussed elsewhere [3]. No significant departure from the mechanisms of formation has been indicated to occur in the stabilized compounds. The formation of striations parallel to the tensile axis in 70NR30PE/DCP coincided with a relatively large axial contraction after stress relaxation and a correspondingly large Poisson expansion in the transverse directions, in agreement with the explanation suggested previously [3]. Shielding part of the sample showed that degradation was largely caused by direct UV impingement and that ozone attack contributed little or nothing to the degradation observed in this study, as with the unstabilized materials [3]. Thus all of the stabilizers used in this study are seen to give significant protection
520
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
70NR30PE blend [24] and this means that mechanical properties may depend strongly on the degradation state of the PE. It is possible that photodegradation of the rubber phase will sensitise photodegradation of the PE, as can occur in polypropylene-rubber blends [25]. IRG was found to be almost as effective as IPPD whereas TIN, generally recommended for conventional thermoplastics, including polyolefins, was the least effective. The ranking order of the stabilizers obtained from the mechanical tests is consistent with observations of the surface degradation.
5. Conclusions Addition of IPPD to a 70NR30PE thermoplastic elastomer improved the resistance of the material to UV photo-oxidation very considerably. Introduction of Irganox or Tinuvin, (stabilizers for the LDPE component) also gave some improvement in photo-resistance but was less effective. The inclusion of stabilizers improved the photo-resistance of samples exposed to UV under tensile strain as well as in the unstressed state. Inclusion of DCP to introduce crosslinks (at low concentration, so that the thermoplastic nature of the blends was retained) improved the resistance to photooxidation. The benefit of including a stabilizer was also observed with the crosslinked compounds. Fig. 7. Surface of 70NR30PE/TIN after exposure to UV at 25% strain for (a) 7 days; (b) 14 days. Specimens held strained 50% on the stage.
against UV degradation; their effectiveness against ozone attack cannot be deduced from the observations made here. The cracking patterns were different with different stabilizers. For example, in samples exposed to UV for 14 days at 25% strain the cracks were broad and fairly well spaced out in the TIN-stabilized compound [Fig. 7(b)], somewhat narrower and closer together in the IRG-stabilized compound (Fig. 5) and very fine and densely packed in the IPPD-stabilized compound [Fig. 3(b)]. The unstabilized material displayed a cracking pattern intermediate between those shown in Figs. 5 and 7(b) [3]. The presence of a stabilizer improved the mechanical performance under all conditions. Since rubber is the major component it might be expected that the degradation of the rubber would dominate the behaviour of the blends, especially as the NR homopolymer (+DCP) was found to be much more sensitive to UV degradation than homopolymer PE (+DCP). Hence it might be expected that IPPD would give the best results since it is regarded primarily as a stabilizer for rubbers. On the other hand, PE is the continuous phase even in a
Acknowledgements The UK-India Science and Technology Research Fund (UISTRF) is gratefully acknowledged for a grant to A.K.B. and J.R.W. to support these studies.
References [1] Bhowmick AK, Stephens HL, editors, Handbook of elastomers. New York: Marcel Dekker, 2001. [2] De SK, Bhowmick AK, editors, Thermoplastic elastomers from rubber-plastic blends. Chichester: Ellis Horwood, 1990. [3] Bhowmick AK, Heslop, J, White JR, submitted for publication. [4] Bhowmick AK, White JR, submitted for publication. [5] Davis A, Sims D. Weathering of polymers. Barking: Applied Science, 1983. [6] Braden M, Gent AN. J Appl Polym Sci 1960;3:90. [7] Braden M, Gent AN. J Appl Polym Sci 1960;3:100. [8] Andrews EH. J Appl Polym Sci 1966;10:47. [9] Doyle M. J Rubber and Plastics News, September 1997, p. 69. [10] Dunn JR. J Appl Polym Sci 1960;4:151. [11] Adam C, Lacoste J, Lemaire J. Polym Degrad Stab 1989;24:185. [12] Adam C, Lacoste J, Lemaire J. Polym Degrad Stab 1990;29:305. [13] Adam C, Lacoste J, Lemaire J. Polym Degrad Stab 1991;32:51. [14] Adam C, Lacoste J, Siampiringue N, Lemaire J. Eur Polym J 1994;30:433. [15] Marcos Maillo C, White JR. Plast Rubb Compos 1999;28:27. [16] White JR, Turnbull A. J Mater Sci 1994;29:584.
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521 [17] Hulme AJ, Mills NJ. Plast Rubb Compos, Proc Applic 1994; 22:285. [18] Gijsman J, Hennekens J, Janssen K. Polym Degrad Stab 1994; 46:63. [19] Khan JH, Hamid SH. Polym Degrad Stab 1995;48:137. [20] O’Donnell B, White JR. J Mater Sci 1994;29:3955. [21] Qayyum MM, White JR. J Mater Sci 1985;20:2557.
521
[22] Qayyum MM, Davis A. Polym Degrad Stab 1984;6:201. [23] White JR. In: Bhowmick AK, De SK, editors. Fractography of rubbers. Barking: Elsevier Applied Science, 1991. p. 33 (Chapter 2). [24] Choudhury RN, Bhowmick AK. J Appl Polym Sci 1989;38:1091. [25] Meligi G, Yoshii F, Sasaki T, Makuuchi K, Rabie AM, Nishimoto S-I. Polym Degrad Stab 1997;57:241.
Effect of stabilizers in photodegradation of thermoplastic elastomeric rubber–polyethylene blends—a preliminary study Anil K. Bhowmicka, J. Heslopb, J.R. Whiteb,* a Rubber Technology Centre, Indian Institute of Technology, Kharagpur-721302, India Department of Mechanical, Materials and Manufacturing Engineering, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK
b
Received 23 May 2001; received in revised form 9 July 2001; accepted 16 July 2001
Abstract The effect of adding stabilizers to a family of thermoplastic elastomeric blends of natural rubber and polyethylene has been investigated using UV exposures of samples both unstressed and under tensile elongation. Samples were prepared both with dicumyl peroxide crosslinking agent and with no crosslinking agent. The stabilizers used were (i) isopropyl paraphenylene diamine (IPPD); (ii) a high molecular weight phenolic anti-oxidant; and (iii) a commercially available blend of two high molecular weight hindered amine stabilizers. The crosslinked samples showed better photo-resistance than the compounds with no dicumyl peroxide. All of the stabilizers produced considerable improvement in photo-resistance. The most effective one was IPPD (a stabilizer for rubber) even though the continuous phase in these blends is polyethylene. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Natural rubber–polyethylene blends; Photo-degradation; Thermoplastic elastomers; UV-stabilizers
1. Introduction Thermoplastic elastomers (TPEs) based on rubber– plastic blends have potential for many applications in engineering and consumer goods [1,2]. Applications under consideration such as automotive window seals and footwear involve outdoor service and it is necessary to develop satisfactory stabilization systems to ensure prolonged life. Stabilizers are required to protect the compounds from thermal degradation (at the elevated temperatures used in processing) and photo-degradation caused by the ultraviolet irradiation (UV) in sunlight. Although some stabilizers have a wide application and can be used with many different polymers, some additives may act as a stabilizer with one polymer and as a prodegradant for another. Therefore, when choosing a stabilizing system for a polymer blend it is necessary to know the effect of the stabilizer on both polymeric components. Similarly any possible pro-degradant action of any other additive must be considered. Ideally a stabilizer will have a beneficial effect on both component polymers but in many cases it will be present * Corresponding author. Tel.: +44-191-2227906; fax: +44-1912228563. E-mail address: [email protected] (J. R. White).
to protect one of the components only. In this case it is unfortunate that it will usually become distributed into both polymeric components during compounding and subsequent fabrication. This is a problem common to all polymer blends, not just the thermoplastic elastomers. In the study reported here the effect of including commercial stabilizers on the photo-oxidation behaviour of a family of TPEs based on natural rubber-low density polyethylene blends was investigated using methodology described elsewhere [3]. We have reported previously on the ageing and photo-oxidation behaviour of these materials [3,4]. The unprotected materials degraded very rapidly under laboratory exposures with UV intensities set at tropical levels [3]. The unprotected natural rubber degraded at least as rapidly under these conditions. When natural rubber is compounded with anti-oxidants and fillers such as carbon black it can have very acceptable weatherability. So it can be anticipated that significant improvements can be achieved with the TPEs. With natural rubber the chemical attack is often attributed to ozone [5–9], particularly when the component is loaded in tension. Although ozone is undoubtedly aggressive when present, other studies indicate that photo-oxidation often dominates over ozone attack under conditions such as those found in our laboratory and in many service environments [10–14]. Our earlier
0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00188-4
514
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
studies point to photo-oxidation as the main mechanism of degradation [3,15]; further evidence for this was sought in the study of the family of TPE blends described here. There are several stabilizers available, commercially, that give protection against oxidation and which are very effective in extending outdoor lifetime. Polyethylene also degrades rapidly when exposed to UV [16–19]. Again, there are commercial stabilizers that give spectacular increases in outdoor lifetime of polyethylene. The work described here was part of a broader study aimed at developing a class of inexpensive TPEs with good weatherability. It is a preliminary investigation to determine whether or not significant lifetime improvements could be achieved using readily accessible, relatively cheap commercial additives.
2. Experimental 2.1. Materials Details of the preparation of the materials used here are given elsewhere [3]. They are blends of natural rubber and low density polyethylene with compositions given in Table 1. Natural rubber (NR) was supplied by Rubber Board, Kottayam, India. This had a molecular mass 780103, intrinsic viscosity (benzene 30 C m3/kg []=0.44) and Wallace Plasticity=59.0. The polyethylene (PE) was Indothene 16 MA 400, supplied by IPCL, Baroda, India, melt flow index (MFI) 40 g/10 min. Dicumyl peroxide (DCP) was supplied by Hercules Inc., Wilmington, DE, USA. The stabilizers used were: (a) IPPD: isopropyl paraphenylene diamine, an antioxidant for rubber, supplied by Polyolefines India Ltd., Mumbai, India. (b) IRG: tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane (Irganox 1010: Ciba Table 1 Compositions of the rubber–polyethylene blends (in phr) Code
NR:LDPEa
DCPa
IPPDa
IRGa
TINa
70NR30PE 70NR30PE/DCP 70NR30PE/IPPD 70NR30PE/DCP/IPPD 70NR30PE/IRG 70NR30PE/DCP/IRG 70NR30PE/TIN 70NR30PE/DCP/TIN 70NR30PE/IRG/TIN NR/DCP PE/DCP
70:30 70:30 70:30 70:30 70:30 70:30 70:30 70:30 70:30 100:0 0:100
0 1 – 1 – 1 – 1 – 1 1
– – 1 1 – – – – – – –
– – – – 1 1 – – 0.5 – –
– – – – – – 1 1 0.5 – –
a NR, natural rubber; PE, low density polyethylene; DCP, dicumyl peroxide; IPPD, isopropyl paraphenylene; IRG, Irganox 1010; TIN, Tinuvin 783.
Specialty Chemicals), a high molecular weight phenolic anti-oxidant (1178 g/mol 1) recommended for use as a thermal (processing) stabilizer for plastics. (c) TIN: a commercially-available blend (Tinuvin 783: Ciba Specialty Chemicals) of two high molecular weight hindered amine stabilizers: poly[6-[(1,1,3,3,-tetramethylbutyl) amino]-s-triazine-2,4-diyl][2,2,6,6-tetramethyl-4piperidyl)imino]hexamethylene[(2,2,6,6 - tetramethyl - 4 piperidyl)imino] (Chimassorb 944, 2000--3100 g mol 1) and dimethyl succinate polymer with 4-hydroxy-2,2,6,6tetramethyl - 1 - piperideneethanol (Tinuvin 622, 31004000 g/mol 1) recommended for use both as a thermal stabilizer and a light stabilizer for plastics. The blends were prepared in a Brabender Plasticorder by melt mixing the plastic and the rubber at 150 C at 60 rpm using a cam type rotor for about 5–6 min. The polyethylene component was melted first and the rubber was then added and blended. The stabilizer was incorporated into the blends at this stage. The curative (DCP) was added at a level of 1 phr as required and the mixing continued until the torque increased by 3–4 units. The hot mass was pressed into a sheet of 3 mm thickness from which test slabs of 2 mm thickness were prepared by compression molding in a hydraulic press at a temperature of 150 C for 3 min at 5 MPa pressure. The slabs were subsequently cooled under pressure. 2.2. Ultraviolet exposure arrangements Samples were exposed to ultraviolet irradiation (UV) in a constant temperature room set at 30 1 C. The illumination source consisted of a pair of fluorescent tubes type UVA-340 (Q-Panel Company) for which the UV output matches the spectrum of solar radiation at the Earth’s surface fairly closely [20]. The output at higher wavelengths is very much lower than solar radiation levels and does not cause serious heating of the samples, which remain almost at the room temperature. The total intensity used was about 1.8 Wm 2 in the wavelength range 295–320 nm, that is the total radiation below 320 nm wavelength, comparable to levels in hot sunny climates [21,22]. The illumination provided by the tubes was checked regularly using a Bentham Instruments spectroradiometer. The samples were supported on a wooden base and exposed on one side only. UV exposures were also conducted with samples loaded in uniaxial tensile stress relaxation using simple frames that could accommodate up to 15 separate strips simultaneously. This means that load cannot be monitored on individual specimens but this is not regarded as a serious problem in the context of this rather exploratory study. From the results of creep experiments, however, it is certain that a large fraction of the initial applied stress will have relaxed during the exposure in the blends studied here. Stress relaxation-UV exposures were conducted at strains set at 25 and 50% respectively. Creep
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
under dead weight loading was found to be quite rapid except at low stresses (well below 1 MPa). Samples partially crosslinked by the presence of DCP were found to be less prone to creep, as expected, but still deformed rapidly at stresses near to 1 MPa. A limited number of experiments were conducted using dead weight loading at stresses sufficiently small to reduce creep to an acceptably low level during a 2 week UV exposure for comparison with the stress relaxation–UV exposure trials. The ozone levels were checked at various sites within the room in which the UV trials were conducted, including positions close to the samples under test, and found to be below the measurement threshold for the equipment used (< 0.02 ppm). In the earlier studies conducted by Marcos Maillo and White [15] it was discovered that when a simple shield made from aluminium foil prevented UV reaching part of the sample surface there was a very sharp demarcation between the degradation displayed by the exposed and unexposed zones respectively, indicating that photo-oxidation rather than ozone attack was principally responsible for the degradation. In the trials conducted for the current investigation this simple yet effective device (of shielding part of the sample from direct UV exposure) was adopted as routine. 2.3. Surface degradation analysis Samples were inspected using light optical microscopy and scanning electron microscopy (SEM) at intervals. Although it is possible to view rubber samples in the SEM without modification [23] a sputtered gold coating was applied to improve the image quality. Cracks or fissures that appeared as the result of degradation were rendered more visible under the light optical microscope by applying a strain, using a miniature straining device which can be placed on the microscope stage [3,15]. This device re-opened cracks observed to form during tensile exposures. With samples that were exposed unstrained cracks often appeared (for the first time) when a small strain was applied in this way. Although such cracks could therefore be regarded as an artefact produced by the method of observation, the ease with which such cracks formed and their size and density can be taken as an indication of the level of degradation which had accrued. Oblique illumination was used, adjusted to give greatest visibility of the cracks. Samples which broke during stress relaxation—UV exposures (unprotected blends) were mounted on SEM stubs with the fracture surface upwards, permitting observation of all four sides as well as the fracture surface by appropriate manipulation of the SEM stage controls. For SEM observation of samples which did not break during exposure, a small device was used which stretched them to open any cracks that formed in a similar way to the light microscope jig; samples were
515
mounted in this device for gold-coating and subsequent inspection in the SEM [3,15]. The secondary electron image was used throughout, using 15 kV accelerating potential. 2.4. Mechanical properties After UV exposure, the strips (10012 mm) were tensile tested on an Instron 4500 tensile test machine using a grip separation of 60 mm and a crosshead speed of 500 mm/min. The load-deformation traces were sometimes quite noisy, possibly because the load cell was operating near to its sensitivity limit [3]. This effect and the noise have been smoothed out for the presentation of results in Fig. 1.
3. Results 3.1. Mechanical properties The results of the tensile tests on the conditioned samples containing stabilizers are shown in Fig. 1. Details of the behaviour of (unstabilized) 70NR30PE and 70NR30PE/DCP after similar conditioning treatments are given elsewhere [3]. The maximum strain values from Fig. 1(a)–(g) are summarised in Table 2 and are discussed below. Fig. 1 shows that the effect of UV exposure during stress relaxation at 50% strain is to increase the stiffness: with the exception of 70NR30PE/IRG, each of the compounds that survived this treatment gave the steepest stress–strain relationship for this condition. Table 2 shows the maximum extension recorded in the tensile tests on samples after 14 days conditioning under various combinations of stress relaxation and UV exposure. After stress relaxation at 25% in the absence of UV, compounds based on 70NR30PE displayed an increased extension compared with the corresponding unstrained/unexposed sample when IPPD or IRG were used but a slight decrease when TIN or a combination of IRG and TIN were used. All compounds showed a reduced extension after 14 days stress relaxation at 50% strain in the dark. Stabilized compounds based on 70NR30PE showed poorer performance after stress relaxation at 25% under UV than after unstrained UV exposure with the possible exception of 70NR30PE/IRG. UV exposure during stress relaxation at 50% strain caused a significant loss of extension in every case. Table 3 shows the ratio of the extension observed for stabilized samples to that obtained with the unstabilized sample after the same conditioning treatment. No comparisons were possible for samples preconditioned in stress relaxation at 50% strain under UV exposure because the unstabilized samples failed during the stress relaxation exposure before 7 days had elapsed [3]. Only two of the results shown in Table 3 indicate a lower
516
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
Fig. 1. Engineering stress-deformation curves for blends after conditioning for 14 days under various combinations of strain and UV exposure. (a) 70NR30PE/IPPD; (b) 70NR30PE/DCP/IPPD; (c) 70NR30PE/IRG; (d) 70NR30PE/DCP/IRG; (e) 70NR30PE/TIN; (f) 70NR30PE/DCP/TIN; (g) 70NR30PE/IRG/TIN. (The line for unexposed unstrained 70NR30PE/IPPD in (a) represents the average of 3 tests for which the load-deformation curves superimposed closely up to the point of fracture, which occurred at different values).
517
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521 Table 2 Maximum extension in tensile tests (mm) after conditioning for 14 days under various combinations of stress relaxation and UV exposure Code
0/noa
0/yesa
25/noa
25/yesa
50/noa
50/yesa
70NR30PE 70NR30PE/DCP 70NR30PE/IPPD 70NR30PE/DCP/IPPD 70NR30PE/IRG 70NR30PE/DCP/IRG 70NR30PE/TIN 70NR30PE/DCP/TIN 70NR30PE/IRG/TIN
85 78 147 113 114 145 135 175 100
52 55 119 105 121 81 100 67 125
85 91 157 113 132 – 121 67 91
57 – 108 133 126 135 75 58 110
62 74 96 116 90 111 85 90 45
– – 69 80 41 30 42 – 76
a
0, unstrained; 25, stress relaxation under 25% extension; 50, stress relaxation under 50% extension; no, in dark; yes, exposed to UV.
Table 3 Normalised extension in tensile testsa Code
0/no
0/yes
25/no
25/yes
50/no
70NR30PE 70NR30PE/DCP 70NR30PE/IPPD 70NR30PE/DCP/IPPD 70NR30PE/IRG 70NR30PE/DCP/IRG 70NR30PE/TIN 70NR30PE/DCP/TIN 70NR30PE/IRG/TIN
1.0 1.0 1.73 1.44 1.34 1.85 1.59 2.23 1.18
1.0 1.0 2.27 1.91 2.30 1.48 1.91 1.23 2.38
1.0 1.0 1.85 1.23 1.55 – 1.43 0.73 1.07
1.0 – 1.89 _ 2.21 – 1.32 – 1.93
1.0 1.0 1.54 1.57 1.44 1.51 1.36 1.22 0.72
a Results for compounds based on 70NR30PE (containing no DCP) are normalised by dividing by the result for 70NR30PE after treatment under the corresponding condition. Results for compounds based on 70NR30PE/DCP (containing DCP) are normalised by dividing by the result for 70NR30PE/DCP after treatment under the corresponding condition.
extension for the stabilized sample than for the corresponding unstabilized sample. The two anomalous results were obtained in single trials because of the shortage of specimens; in view of the overwhelming trend towards significant improvement in the stabilized grades it is likely that repeat tests would have eliminated this anomaly. Table 3, therefore, indicates that the introduction of stabilizers to both 70NR30PE and 70NR30PE/DCP improved performance even in samples that were not exposed to UV, presumably because all of the stabilizers used in this study afford some protection against thermal oxidation (during processing). If averages are taken across the rows in Table 3 it is found that, under the conditions applied in these experiments, the least effective stabilizer is TIN (alone or in combination with IRG). The improvements in performance caused by including stabilizers in compounds based on 70NR30PE were on average slightly better than those in 70NR30PE/DCP. Taking averages of columns in Table 3 it is found that the condition in which the presence of stabilizer appears to be most beneficial is unstrained exposure. 3.2. General observations The UV degradation behaviour of blends containing no stabilizer has been described in detail elsewhere [3].
The basic blend (70NR30PE) was light brown in colour. The appearance did not change markedly when DCP, Irganox or Tinuvin were added (one at a time or in combinations) but the compounds containing IPPD took on a darker grey hue. Samples containing stabilizers (IPPD, IRG, TIN) were shiny and tacky in the unexposed state, as with the unstabilized compounds, and also became dull and less sticky after UV exposure. When exposed to UV in the stress relaxation frames most of the unprotected samples developed cracks perpendicular to the strain axis and similar behaviour was observed with samples containing stabilizers. The unprotected samples were curved after releasing from the frames; samples containing stabilizer tended to show very little curvature after 7 days exposure but generally had developed significant curvature when inspected after 14 days (see below). The curvature was convex on the exposed surface with all samples. The rippling effect often seen on the surface of unprotected blends was absent or barely visible in samples containing stabilizer. Cracks developed in the unexposed surface with some unprotected samples, but this did not occur when a stabilizer was included in the compound. It should be noted that 70NR30PE/DCP is much stiffer than 70NR30PE and that, although crosslinked compounds (70NR30PE/DCP and stabilized compounds
518
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
based on this composition) failed at lower failure strains than 70NR30PE and the stabilized compounds based on it, the crosslinked compounds failed at higher stresses than the corresponding uncrosslinked blends. 3.3. Surface degradation 3.3.1. Unprotected samples Observations with unprotected NR/PE blends, including 70NR30PE and 70NR30PE/DCP, and the homopolymers, NR/DCP and PE/DCP were described in detail elsewhere [3]. 3.3.2. Effect of IPPD After UV exposure, whether unstrained or in stress relaxation, the appearance of samples containing IPPD was quite different from any of the other samples. The most severe cracking pattern was that obtained with 70NR30PE/DCP/IPPD after 7 days exposure in the stress relaxation rig at 25% strain. The cracking pattern in Fig. 2(a) was obtained with the specimen held strained 50% on the microscope stage and the pattern
Fig. 2. Surface of 70NR30PE/DCP/IPPD after exposure in the stress relaxation rig at 25% strain for (a) 7 days; (b) 14 days. Specimens held strained 50% on the microscope stage. (b) shows the full width of the sample; (a) has a magnification four times higher than that of (b).
may have been produced by this rather than solely by the strain-UV exposure treatment. Samples exposed for 7 days unstrained and 14 days strained at 25% respectively showed dense hairline cracks when strained on the microscope stage [Fig. 2(b)]. Samples observed under other conditions showed even less evidence of cracking. Samples of blend 70NR30PE/IPPD showed still less cracking perpendicular to the strain axis than the equivalent blend containing DCP but in some cases revealed a dense pattern of striations parallel to the axis [Fig. 3(a)]. A cross-hatched pattern was observed on the surface of a sample exposed for 14 days under 25% strain [Fig. 3(b)]. Details of fine cracks of the kind shown at low magnification in Fig. 3(a) were revealed in the scanning electron microscope: Fig. 4 shows the surface of a 70NR30PE/IPPD sample after 7 days UV exposure unstressed. 3.3.3. Effect of IRG Inclusion of IRG seemed to provide good short term protection of 70NR30PE with only moderate cracking after 7 days exposure under stress but after 14 days
Fig. 3. Surface of 70NR30PE/IPPD after exposure to UV for 14 days (a) unstressed; (b) strained 25%. Specimens held strained 50% on the stage. (a) shows the full width of the sample; (b) has a magnification four times higher than that of (a).
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
519
Fig. 5. Surface of 70NR30PE/IRG after exposure to UV for 14 days strained 25%. Specimen held strained 50% on the stage.
Fig. 4. Surface of 70NR30PE/IPPD after exposure to UV for 7 days unstressed, viewed in the scanning electron microscope; (a) high magnification; (b) low magnification of the area with (a) at its centre.
exposure the cracks were larger and more numerous (Fig. 5). With 70NR30PE/DCP/IRG cracking developed readily after 7 days exposure (Fig. 6). Fig. 6 includes part of the sample that was beneath the shield; the boundary between the exposed and the unexposed zones is clearly demarcated.
Fig. 6. Surface of 70NR30PE/DCP/IRG after exposure to UV for 14 days unstressed: specimen held strained 50% on the stage.
3.3.6. Unexposed side Very little sign of cracking was found on the underside of samples containing any of the stabilizers used in this study.
4. Discussion 3.3.4. Effect of Tinuvin On the basis of the microscopical evidence, the Tinuvin stabilizer appears to be least effective of all. Cracks formed easily in samples exposed to UV both in the unstrained and the strained state in both 70NR30PE/ TIN and 70NR30PE/DCP/TIN [Fig. 7(a) and (b)]. Samples containing a mixture of IRG and TIN also cracked quite easily after exposure to UV either unstrained or in tension. 3.3.5. Effect of the shield The region under the aluminium foil shield was almost devoid of degradation (for example, see Fig. 6). This was a common observation and showed that the extensive degradation found in samples after UV irradiation required direct exposure.
The formation of the cracking patterns in the unstabilized blends has been discussed elsewhere [3]. No significant departure from the mechanisms of formation has been indicated to occur in the stabilized compounds. The formation of striations parallel to the tensile axis in 70NR30PE/DCP coincided with a relatively large axial contraction after stress relaxation and a correspondingly large Poisson expansion in the transverse directions, in agreement with the explanation suggested previously [3]. Shielding part of the sample showed that degradation was largely caused by direct UV impingement and that ozone attack contributed little or nothing to the degradation observed in this study, as with the unstabilized materials [3]. Thus all of the stabilizers used in this study are seen to give significant protection
520
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521
70NR30PE blend [24] and this means that mechanical properties may depend strongly on the degradation state of the PE. It is possible that photodegradation of the rubber phase will sensitise photodegradation of the PE, as can occur in polypropylene-rubber blends [25]. IRG was found to be almost as effective as IPPD whereas TIN, generally recommended for conventional thermoplastics, including polyolefins, was the least effective. The ranking order of the stabilizers obtained from the mechanical tests is consistent with observations of the surface degradation.
5. Conclusions Addition of IPPD to a 70NR30PE thermoplastic elastomer improved the resistance of the material to UV photo-oxidation very considerably. Introduction of Irganox or Tinuvin, (stabilizers for the LDPE component) also gave some improvement in photo-resistance but was less effective. The inclusion of stabilizers improved the photo-resistance of samples exposed to UV under tensile strain as well as in the unstressed state. Inclusion of DCP to introduce crosslinks (at low concentration, so that the thermoplastic nature of the blends was retained) improved the resistance to photooxidation. The benefit of including a stabilizer was also observed with the crosslinked compounds. Fig. 7. Surface of 70NR30PE/TIN after exposure to UV at 25% strain for (a) 7 days; (b) 14 days. Specimens held strained 50% on the stage.
against UV degradation; their effectiveness against ozone attack cannot be deduced from the observations made here. The cracking patterns were different with different stabilizers. For example, in samples exposed to UV for 14 days at 25% strain the cracks were broad and fairly well spaced out in the TIN-stabilized compound [Fig. 7(b)], somewhat narrower and closer together in the IRG-stabilized compound (Fig. 5) and very fine and densely packed in the IPPD-stabilized compound [Fig. 3(b)]. The unstabilized material displayed a cracking pattern intermediate between those shown in Figs. 5 and 7(b) [3]. The presence of a stabilizer improved the mechanical performance under all conditions. Since rubber is the major component it might be expected that the degradation of the rubber would dominate the behaviour of the blends, especially as the NR homopolymer (+DCP) was found to be much more sensitive to UV degradation than homopolymer PE (+DCP). Hence it might be expected that IPPD would give the best results since it is regarded primarily as a stabilizer for rubbers. On the other hand, PE is the continuous phase even in a
Acknowledgements The UK-India Science and Technology Research Fund (UISTRF) is gratefully acknowledged for a grant to A.K.B. and J.R.W. to support these studies.
References [1] Bhowmick AK, Stephens HL, editors, Handbook of elastomers. New York: Marcel Dekker, 2001. [2] De SK, Bhowmick AK, editors, Thermoplastic elastomers from rubber-plastic blends. Chichester: Ellis Horwood, 1990. [3] Bhowmick AK, Heslop, J, White JR, submitted for publication. [4] Bhowmick AK, White JR, submitted for publication. [5] Davis A, Sims D. Weathering of polymers. Barking: Applied Science, 1983. [6] Braden M, Gent AN. J Appl Polym Sci 1960;3:90. [7] Braden M, Gent AN. J Appl Polym Sci 1960;3:100. [8] Andrews EH. J Appl Polym Sci 1966;10:47. [9] Doyle M. J Rubber and Plastics News, September 1997, p. 69. [10] Dunn JR. J Appl Polym Sci 1960;4:151. [11] Adam C, Lacoste J, Lemaire J. Polym Degrad Stab 1989;24:185. [12] Adam C, Lacoste J, Lemaire J. Polym Degrad Stab 1990;29:305. [13] Adam C, Lacoste J, Lemaire J. Polym Degrad Stab 1991;32:51. [14] Adam C, Lacoste J, Siampiringue N, Lemaire J. Eur Polym J 1994;30:433. [15] Marcos Maillo C, White JR. Plast Rubb Compos 1999;28:27. [16] White JR, Turnbull A. J Mater Sci 1994;29:584.
A.K. Bhowmick et al. / Polymer Degradation and Stability 74 (2001) 513–521 [17] Hulme AJ, Mills NJ. Plast Rubb Compos, Proc Applic 1994; 22:285. [18] Gijsman J, Hennekens J, Janssen K. Polym Degrad Stab 1994; 46:63. [19] Khan JH, Hamid SH. Polym Degrad Stab 1995;48:137. [20] O’Donnell B, White JR. J Mater Sci 1994;29:3955. [21] Qayyum MM, White JR. J Mater Sci 1985;20:2557.
521
[22] Qayyum MM, Davis A. Polym Degrad Stab 1984;6:201. [23] White JR. In: Bhowmick AK, De SK, editors. Fractography of rubbers. Barking: Elsevier Applied Science, 1991. p. 33 (Chapter 2). [24] Choudhury RN, Bhowmick AK. J Appl Polym Sci 1989;38:1091. [25] Meligi G, Yoshii F, Sasaki T, Makuuchi K, Rabie AM, Nishimoto S-I. Polym Degrad Stab 1997;57:241.