From 60 samples 55% (i.e. 33 samples in total) passed the Oddy test and get a rating for permanent use. 33% (20 samples) are for temporary use as slight corrosion occurred on at least one metal coupon. Only 11% (7 samples) of the tested materials are unsuitable for use as they fail the Oddy test producing significant amount of corrosion due to a high concentration of harmful volatile components. Obviously, by checking the table of results (Table 1) no group of cellulose derivates entirely passes the Oddy test as there are always some products that release harmful volatile components.
A total of 18 Klucel® HPC samples had been tested whereas 11 were freshly purchased and seven were historical or undated products from the materials’ collection. All historical samples failed the Oddy test with a T or and F rating (Table 1). Klucel® MF (Hercules, 2006, F rating) generated significant amount of corrosion on both copper and lead coupon. The phases were identified by means of Raman spectroscopy and include cuprite (Cu2O), massicot (PbO), hydrocerussite (Pb3(CO3)2(OH)2) and lead formate (Pb(HCOO)2). Cuprite yields Raman bands at 147, 215, 494 and 623 cm−1 which are often only visible as weak bands or shoulders (cf. Fig. 2a) [79, 80]. Massicot and its polymorph litharge are strong Raman scatterer. Both have their strongest band around 145 cm−1 and an additional main feature at 285 (massicot) and 340 cm−1 (litharge) [81, 82]. The main bands of hydrocerussite are located at 1048, 1051 cm−1 (shoulder) and c. 3540 cm−1 and can be used to differentiate it from plumbonacrite (Pb5O(OH)2(CO3)3) (see further discussion in this section and [83]). The Raman spectrum of lead formate is shown in Fig. 2c. Lead formate can be easily differentiated from lead acetates by its typical band pattern. The C–H stretching band doublet is located at 2843 and 2873 cm−1, the strong C–O stretching is centered at 1345 cm−1 and various spectral features occur at 760, 1075, 1375 and 1530 cm−1 [84, 85]. The identification of lead formate may not clearly proof the presence of formic acid as formaldehyde can be also oxidized during the test conditions, inducing the growth of metal formates [86, 87].
Regarding the fresh HPC products, Klucel® M and H passed the Oddy test with no visible corrosion on all metal coupons. The low viscosity type E generated little corrosion on the lead coupon leading to a T rating. Klucel® G was tested multiple times as ambivalent results occurred, causing at first a veritable confusion. A sample from 2017 (purchased from GMW) passed the Oddy test with no corrosion on any coupon (Fig. 1a). That was not surprising as Korenberg et al. [72] reported the same result for Klucel® G. However, the Oddy test from a freshly purchased product from 2021 (purchased from Deffner) shows some amount of corrosion on the lead coupon, yielding a T rating (Fig. 1c). The same result was achieved from Klucel® G purchased from Kremer (Fig. 1b). In both cases Raman spectroscopy revealed the presence of massicot/litharge (PbO), hydrocerussite and basic lead acetate (Pb3(CH3COO)2(OH)4). Basic lead acetate yields a plenty number of bands, with several bands (e.g. 370, 447, 612, 641, 648, 666, 912, 922, 929, 1345, 1410, 1428, 2923, 2970, 2996 cm−1) being diagnostic for this phase (cf. Fig. 2d). A detailed discussion and band assignment can be found elsewhere [84]. Especially the spectral feature at 370 cm−1 and the bands between 910 and 930 cm−1 can be used to differentiate it from lead acetate trihydrate (Pb(CH3COO)2·3H2O) [84]. Generally, lead acetates indicate the presence of acetic acid in the test tube. One gram of Klucel® G (Deffner) was more detailed tested utilizing the BEMMA scheme with a quantification of the outgassing VOC and VVOC. A high amount of acetic acid can be detected (about 4000 µg/m3). Furthermore, fragments of the hydroxypropyl cellulose can be detected with the VOC Tenax method and a small amount of formaldehyde was analyzed. This material does not fulfil the BEMMA scheme. A sum of VOC higher than 500 µg/m3 and the high acetic acid concentrations and the following high amounts of VVOCs were the reasons.
Fig. 1Lead coupons after the Oddy test of various Klucel® G samples. The lower part of the coupon was stuck into the stopper and may not be used for rating purpose. a Klucel® G 2017 (GMW), b Klucel® G 2021 (Kremer), c Klucel® G 2021 (Deffner), d Klucel® G IND (Ashland), e Klucel® GF (Ashland), f Klucel® GF Pharm (Ashland). Please note that the metallic reflections in f may lead to misleading interpretations regarding the metal corrosion
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Fig. 2Raman spectra of common corrosion phases. Offset was applied for better comparison. a Cuprite and tenorite (Tn) (Cu coupon of the Hydroxyethylcelluose test); b anglesite (Pb coupon of the Blanose® 7H4Xf test); c lead formate (Pb coupon of the Klucel® MF test); d massicot (M), plumbonacrite (Pl), basic lead acetate (Pb coupon of the Hydroxyethylcelluose test); e plumbonacrite (Pl), lead acetate trihydrate (Pb coupon of the Hydroxyethylcelluose test)
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In a next step, Klucel® G samples with different purity grades were obtained directly from the producer. The testing of the industrial quality (i.e. the same quality as for the Kremer and Deffner sample) shows unsurprisingly the same amount of corrosion as the two samples before (Fig. 1d). For Klucel® GF (food grade, i.e. higher purity) hardly any corrosion is visible, only some small white spots can be found on the edge of the coupon (Fig. 1e). Raman measurements show the presence of litharge and plumbonacrite. Plumbonacrite can be differentiated from hydrocerussite by means of Raman spectroscopy. It is characterized by a strong band at 1048 cm−1 with two additional sub-bands at 1052 and 1056 cm−1 that are visible as shoulders [83]. Additionally, among other band shifts in the fundamental modes the OH stretching results in two bands for plumbonacrite, whereas there is only one band for hydrocerussite [83]. Klucel® GF Pharm (pharmaceutical quality) passes the test with no corrosion on the lead coupon (Fig. 1f). The coupon even kept its metallic gloss after testing. The results for Klucel® G show that the amount of corrosion depends on the purity grade, hence, not the product itself releases the harmful volatile components but an impurity or a leftover from the production process causes the problems. The producer states upon request, that acetic acid is used in the production process for neutralization and that the acetate is removed during purification. Unfortunately, there is no data available how much acetate is left in the products after the purification. However, it is not surprising that the acetate amount is the highest in the industrial grade product. Moreover, according to the producer, there was an expand in production capacity for Klucel® in 2016 that may explain the different results for the older and new Klucel® G of industrial quality.
Only few HEC and only one EC sample were included in this study. EC (Ethylcellulose ET 200 purchased in 17.11.2015) failed the Oddy test, generating significant amount of corrosion on the lead coupon. The corrosion phases include massicot, litharge, hydrocerussite and basic lead acetate indicating the presence of acetic acid. For HEC, 2 of 5 samples passed the test, whereas one got a T rating (Tylose® H 100000 YP2, purchased in 11.2016) and two failed the test (Natrosol 250 MHBR purchased in 05.03.2004, Hydroxyethylcelluose (unknown date of purchase) (Table 1). The corrosion phases of the lead coupon from Hydroxyethylcelluose (Fluka) are particularly interesting as they include not only massicot and plumbonacrite but also basic lead acetate and lead acetate trihydrate. The Raman spectra of these phase associations are shown in Fig. 2d, e. Both lead acetates are good Raman scatterers and a direct comparison of their spectra reveal the differences. Lead acetate trihydrate yields its typical bands at 464, 617, 657, 930, 1345, 1429, 2930 and c. 2980 cm−1 [84]. The 370 cm−1 band is completely missing and there are significant differences in the 600–660 cm−1 and the 910–950 cm−1 ranges respectively. Both lead acetates indicate the generation of acetic acid during the Oddy test. Most of the MC products passed the Oddy test. Four freshly purchased samples were tested, three of them passed the test with no sign of corrosion. The only fresh sample that got a T rating was Methyl cellulose (Sigma) as little corrosion occurred on the lead coupon. Among the fresh samples, special emphasis was laid on Methocel® A4M as it is the most common cellulose ether in conservation practice. It was tested as industrial grade and passed the test. The same result was obtained from historical equivalents from 1997 and 2003. Generally, 9 historical or undated samples were included in this study of which 7 passed the Oddy test. Two low viscosity grade products got a different rating. Metolose® SM-15 (Shin Etsu) generated slight corrosion on the copper and lead coupon, yielding a T rating. Raman measurements reveal the presence of cuprite; litharge and hydrocerussite as main phases. Culminal® MC 15 S is the only tested MC that truly failed the Oddy test as heavy corrosion occurred on the lead coupon. Cuprite, massicot, hydrocerussite, plumbonacrite and lead formate could be identified by means of Raman spectroscopy.
Nine MHEC products were included in this study of which five were freshly purchased. The results show that MHEC products mainly pass the Oddy test. Only two of the nine samples did not pass the test with a T rating for Tylose® MH 50 (Hoechst) and Tylose® MH 30000 YP4 (Shin Etsu). Raman measurements on the copper and lead coupon of the historical Tylose® MH 50 sample reveal the presence of cuprite, litharge, hydrocerussite and basic lead acetate. The corrosion phases of the high viscosity MHEC Tylose® MH 30000 YP4 include cuprite; massicot, hydrocerussite and lead formate. Most of the MHPC products got a P rating as no corrosion occurred during the tests. Culminal® MHPC 20000 P was tested as fresh and historical (2005) sample and both passed the Oddy test. A temporary rating was assigned to two historical products. The Oddy test of Methocel® K4MFG from 2003 yielded slight corrosion on copper (cuprite) and lead (hydrocerussite). Methocel® E5 Premium LV from 2000 created slight corrosion on the lead coupon with hydrocerussite as main phase.
Eight Na-CMC products were tested in this study and most of them passed the test. The two freshly purchased samples (Cekol® 700, Blanose® ref CMC 7M65) passed the Oddy test with no sign of corrosion. Slight corrosion on the lead coupon was observed from three historical or undated products. Raman measurements reveal the presence of hydrocerussite (Blanose® 7LF), anglesite (PbSO4) (Blanose® 7H4Xf; cf. Fig. 2b) or massicot, hydrocerussite and anglesite (Tylose® C6000) as main phases. Anglesite yields a characteristic Raman spectrum with a main band at 976 cm−1 and some weak to medium intensity bands at 439, 451 and 609 cm−1 [88].
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