Identification of off-flavor compounds in high-density polyethylene (HDPE) with different amounts of abscents.
Earlier studies have shown that organoleptic properties of drinking water are essential when using polyethylene pipes. Water plays an important role in the solubility of additives in polyolefins (1). There are three principal causes for organoleptic changes in water: dissolution of additives, oxidation of the internal surface of the pipe, and migration of external contaminants (2, 3). In addition, some studies have shown that odor-causing compounds like aldehydes and ketones, can be released from thermal oxidation of polyethylene, and these most unpleasant odors can be caused by carbonyl compounds in a certain molecular weight range (4-6). Viebke et al. (7) also showed that the degradation of polyolefin pipes is one reason for the formation of carbonyl compounds. Ho et al. (8) considered nonanal as a major contributor of the off- odor due to its abundance and low odor threshold concentration. Rigal has studied the taste of HDPE by organoleptic tests and bad taste was caused by insufficiently controlled cooling conditions followed by formation of oxidation products (9). Berg has also investigated the origin of these off-flavors (10). Anselme et al. (2) have discussed some organoleptic problem sources and possible remedies.
There are many efforts to remove odor-causing compounds. Zambetti et al. (11) have compared the effect of different antioxidants on reducing tastes and odors. Gioffre et al. (12) have eliminated organic odors from air using activated silicalite and zeolite Y, Marcus has sequestered ammonia and its odor using combined zeolites (13). Gustafsson et al. (14) have shown, that off-odor and off-taste compounds in polyolefin packaging materials can be eliminated with the aid of a new molecular sieve, Abscents, which is added to HDPE during compounding, Abscents is a combination of zeolites including (1:1) activated silicalite and zeolite LZ-10 (12). Abscents is a deodorizing powder and it is used in such products where odor is a problem. This molecular sieve is highly selective for adsorbing odor-causing compounds like aldehydes and ketones. Abscents is very effective to decrease the odor of molecules; it has organophilic pores that trap odor molecules within its structure. Abscents is different than other sieves due to its capacity for adsorbing odor-causing compounds even in the presence of water. One of the benefits of Abscents is its ability to remove many odorous compounds below their threshold detection level (14, 15).
The samples were blue high-density polyethylene (HDPE) pellets with four (0.10 wt%, 0.15 wt%, 0.20 wt%, 0.30 wt%) different amounts of Abscents, manufactured by UOP, Inc. (14). Polyethylene pellets were about 4 mm in diameter and 2 mm thick, Each pellet sample (32 g) was placed into 250 mL odor-free water (Ultra High Quality ELGA water) and shaken for four hours at room temperature (25 [degrees] C). These water samples (10 mL of each) were tested using gas chromatography/mass spectrometry/sniffing-methods.
Instrumental analysis was carried out with a VG AutoSpec high resolution mass spectrometer connected to a Hewlett Packard 5890 Series II gas chromatograph. A Tekmar 2000 purge and trap-system (a dynamic headspace) was used to introduce the samples into the GC/MS. The purge and trap operating conditions are shown in Table 1. The GC-column was a 25 m Noribond SE 54 (1 [[micro]meter] phase, i.d. 0.25 mm) and there were two identical columns. The eluted compounds from one column were sniffed at the same time as the peaks of the same compounds were detected by MS and appeared on the chromatogram using the other column (16). An initial temperature for gas chromatography of 40 [degrees] C for 5 minutes was used, followed by increasing the temperature at a rate of 5 [degrees] C/min to 250 [degrees] C and to a final hold for 10 minutes.
Table 1. The Operating Conditions of Purge and Trap Equipment. Function Time/Temperature standby temperature 30 [degrees] C prepurge time 3 min preheat time 3 min sample temperature 100 [degrees] C purge time 12 min dry purge time 6 min moisture control module 0 [degrees] C cryo cooldown -120 [degrees] C desorb preheat 175 [degrees] C desorb time/temperature 4 min/180 [degrees] C inject time/temperature 0.85 min/250 [degrees] C bake time/temperature 7 min/225 [degrees] C
RESULTS AND DISCUSSION
The Results of GC/MS/SNIFF Analysis
In leaching samples with water, 91 volatile compounds with molecular weight ranging from 46-184 were identified. Those compounds belonged mainly to the groups of hydrocarbons (alkanes, alkenes) and carbonyl compounds (aldehydes and ketones). These compounds can partition into water, so they are probably also responsible for off-taste. Some of these compounds, like aldehydes, could be readily identified by their characteristic odors. The special focus in interpretation of mass spectra was concentrated on bad smelling compounds and their behavior with different percentages of Abscents.
Using GC/MS/SNIFF analysis, seven ketones and four aldehydes were identified that caused off-odors. Some of those compounds only appeared and caused odors when the added percentage of Abscents was the lowest (0.10%) as seen in Fig. 1. These bad-smelling compounds were heptanal, octanal, nonanal of aldehydes, and five ketones: 3,3-dimethylcyclobutanone ([C.sub.6]-ketone), methyl isobutyl ketone (MIK), 3, 4-dimethyl-2-hexanone ([C.sub.8]-ketone) and isopropylcyclohexanone and 4,6-dimethyl-5-hepten-2-one ([C.sub.9]ketones). The calculated concentrations of these carbonyl compounds with different amounts of Abscents are shown in Table 2.
In leaching water with a minimum percentage of Abscents there were many bad smells compared with the leachate of pellets when the maximum amount of Abscents (0.30%) was used. As shown in Fig. 1, the concentrations of off-flavor compounds and the intensifies of odors decreased or disappeared thoroughly when the maximum percentage of Abscents was added. The strongest odor was caused by [C.sub.9]-ketone; isopropylcyclohexanone (pungent). The intensity of this odor decreased notably using the maximum amount of Abscents. The odor was then described as very weak and solvent-like.
The off-odors of other carbonyl compounds (nonanal, octanal, heptanal, MIK, 3, 3-dimethylcyclobutanone) and their behavior with increasing amount of Abscents are indicated in Fig. 1. The odor descriptions became more pleasant. Using the maximum amount of Abscents, the concentrations of some ketones were below the threshold odor concentration (TOC) and thereby caused no odor. Some of these ketones were removed completely with a higher percentage of Abscents (0.20%, 0.30%). Additionally, one aldehyde, decanal, appeared only when a minimum amount (0.10%) of Abscents was used.
Table 2. Concentration of Off-Flavor Compounds in Leachates (mg/L). Compound 0.10 wt% 0.15 wt% 0.20 wt% 0.30 wt% methytbutenone 0.002 0.001 - - dimothylbutanone 0.008 0.006 0.005 0.003 MIK 0.002 0.0012 0.0013 0.0006 methylheptadienone 0.003 0.001 0.001 - heptanal 0.05 0.03 0.03 0.01 dimethylhexanone 0.002 0.001 0.001 0.001 dimethylheptenone 0.009 0.005 0.006 0.004 octanal 0.13 0.06 0.07 0.03 isopropylhexanone 0.010 0.005 0.007 0.003 nonanal 0.27 0.12 0.11 0.04 decanal 0.036 - - - TOTAL 0.52 0.23 0.23 0.09
As seen in Fig. 2, the total amounts of carbonyl compounds were very low (0.09 mg/L), being 0.30% of Abscents. When the amount of Abscents was minimum (0.10%) the total carbonyl concentration was 0.52 mg/L. Such a high total content of off-flavor compounds was especially caused by aldehydes. This was particularly noticed in tile case of decanal; this aldehyde appeared only in the sample with 0.10% of Abscents.
The amounts of all ketones (Table 2) were also the largest with the minimum percentage of Abscents. The concentration of methyl isobutyl ketone (MIK) was twice as big (0.002 mg/L) when the added amount of Abscents was 0.10% compared with amounts of 0.15% and 0.20% Abscents. The concentration of MIK was the lowest (0.0006 mg/L) when the amount of Abscents was the highest. The concentration of isopropylcyclohexanone ([C.sub.9]-ketone), the source of the strongest odor, was three times smaller (0.003 mg/L) with a maximum amount (0.30%) of Abscents compared to samples with an added minimum percentage (0.10%) of Abscents (0.010 mg/L). Furthermore, some ketones were removed completely when the percentage of Abscents was more than 0.20%.
A purge and trap-system connected to the GC/MS with simultaneous sniffing is a very useful technique for solving the off-flavor problems of HDPE used in pipe applications. Bad smelling areas were chosen for more detailed studies and closer investigations showed that the unpleasant odors in waters were due to carbonyl compounds.
The combined zeolite, Abscents, was used to reduce carbonyl compounds. Four different percentages (0.10%, 0.15%, 0.20% and 0.30%) of Abscents were investigated. For 0.10% of Abscents, off-flavor compounds appeared. There was no significant difference in concentrations of off-flavor compounds when the percentage of Abscents was varied between 0.15% and 0.20%. When the added amount of Abscents was the highest (0.30%), almost all carbonyl compounds disappeared. The intensities of odors also decreased or disappeared thoroughly.
The acceptability for pipes made for drinking water for less off-odor in water appears to be possible. This study unquestionably showed that higher percentages of Abscents in HDPE pellets lead to lower concentrations of off-flavor compounds.
This study was financially supported by Neste Foundation, Finland.
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|Author:||Villberg, Kirsi; Veijanen, Anja; Gustafsson, Irma|
|Publication:||Polymer Engineering and Science|
|Date:||Jun 1, 1998|
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