Study of the degradation of typical HVAC materials, filters and components irradiated by UVC energy--part I: literature search.
The literature search presented herein describing the different degradation mechanisms of the materials and nonmetallic components typically subjected to long-term UVC [254 nanometers (nm), 0.01 mil] exposure in HVAC systems is a summary of the project final report (Kauffman 2011). Two books published by J. Rabek (1995 and 1996) were the basis for the conducted literature search and contain thousands of references on research performed to study polymer photodegradation mechanisms. Although the majority of the reported UV studies were performed with UVA [320-400 nm (0.013-0.016 mil)] and UVB (280-320 nm (0.011-0.013 mil)], a significant number of the studies explored the effects of UVC on polymer degradation. The results of the conducted literature search were broken down into the following subcategories for discussion.
1. UVC absorption by the fresh polymeric surface
2. Photodegradation reaction mechanism
3. Effects of UV wavelength on degradation products
4. Effects of contaminants and additives on degradation rate
5. Effects of UV irradiance on degradation rate (Reciprocity Law)
6. Correlation between laboratory and HVAC UVC results
7. Identification of laboratory photoreactors
8. Identification of photodegradation monitoring techniques
UVC ABSORPTION BY THE FRESH POLYMERIC SURFACE
In order for UV to degrade a polymeric surface, chromophoric groups (chemical groups capable of absorbing UV light) must be contained in the chemical structure of the polymer or in compounds dispersed in the polymer matrix. Examples of chromophoric groups present in common polymers are carbonyls (polyesters, nylon, acrylates, polyimide, etc.) and phenyl rings [polystyrene, epoxy, poly (ethylene terephthalate), etc].
Since they do not contain chromophoric groups, polyethylene and polypropylene do not absorb UV light above 220 nm (0.0087 mil) (Rabek 1996), and therefore, should be totally resistant to sunlight (UVA and UVB) as well as UVC. Poly (vinyl chloride) does not absorb light above 290 nm (0.011 mil) (Rabek 1996), and therefore, should only be slightly susceptible to UVA and UVB degradation.
However, in practice, polyethylene, polypropylene and poly (vinyl chloride) are all highly susceptible to UVA-UVC degradation (Denizligil and Schnabel 2003, Hamid 2000, Rabek 1995 and 1996, Schnabel 1981, Scott 1965 and Hamid 2000). The UV reactivities of the polymers are attributed to the presence of UV absorbing additives and impurities formed during polymerization, processing and/or storage. The impurities can be both internal (polymerization) and external (processing/storage) and include hydroperoxides, carbonyl and unsaturated bonds, catalyst residues, additives, metal traces, etc. (Rabek 1995 and 1996). Consequently, the capability of a polymer to absorb UV energy is affected by both the chemical structure of the polymer as well as the presence and location of the chromophoric impurities.
PHOTODEGRADATION REACTION MECHANISM
Once the organic compound absorbs the UV light, the produced excited state must be capable of producing radicals and other reactive species for the UV degradation mechanism to continue. The reaction scheme in Figure 1 is basically the same whether the polymer free radicals (P*) are generated by thermal or UV energy (photolysis).
Figure 1 Photodegradation reaction scheme of polymeric materials. Photolysis PH (polymer) + UV [right arrow] P + P (backone) or P + H (side chain) Propagation P + [O.sub.2] [right arrow] POO (peroxy radical) (Step 1) POO + PH [right arrow] POOH (hydroperoxide) + P (back to Step 1) H + [O.sub.2] [right arrow] HOO (hydrogen peroxy radical) HOO + PH [right arrow] HOOH (hydrogen peroxide) + P (back to Step 1) Scission HOOH + UV [right arrow] HO + OH (hydroxide radical) PH + OH [right arrow] [H.sub.2]O + P (back to Step 1) POOH + UV [right arrow] PO (polymer oxy radical) + OH PO [right arrow] rearrangement to ketone/aldehyde and P (back to Step 1) PH + OH [right arrow] [H.sub.2]O + P (back to Step 1) Cross-linking P + P [right arrow] P - P (inactive products, dominant in vacuum) P + PO [right arrow] P - OP (rearrangement to inactive products) P + POO [right arrow] P - OOP (rearrangement to inactive products) POO + POO [right arrow] P - OOP + [O.sub.2] (dominant in air)
In the presence of oxygen, the scission steps are generally dominant with regard to the cross-linking steps (Rabek 1995 and 1996). If the scission step occurs in the polymer back bone, the UV exposure reduces the tensile strength of the exposed polymer and the carbonyl groups (ketone/aldehyde) attached to the end of the severed polymer molecules increase the UV absorption of the degraded polymer products. Alternately, if the scission step occurs at the end or in a short side chain of the original polymer, the UV exposure produces volatile carbonyl products (ketones/aldehydes, carbon dioxide, carbon monoxide) and a shortened polymer chain with a free radical end. In contrast to scission, the cross-linking steps cause the molecular weight of the polymer to increase and the flexibility of the polymer to decrease.
In addition to the general reaction scheme in Figure 1, several polymers have reaction schemes specific to their chemical structure. For example, poly (vinyl chloride) also undergoes dehydrochlorination when exposed to UV (Denizligil and Schnabel 2003, Kaczmarek 2009 and Rabek 1996) as shown in Figure 2.
[FIGURE 2 OMITTED]
The resulting double bonds (C = C) are chromophoric (increase the UVC reactivity of the reaction product) and are responsible for the yellow-red coloring of the exposed polymer. The fact that the poly (vinyl chloride) and any other chlo-rinated polymers would outgas hydrochloric acid gas indicates UVC exposure would have the potential to promote corrosion of surrounding metallic surfaces during polymer degradation.
Polycarbonates, polyurethanes, poly (phenyl acrylate) and epoxy resins undergo the photo-Fries rearrangement (responsible for yellowing of aged polymers) shown in Figure 3 when exposed to UV (Rabek 1995 and 1996). The rear-ranged polymers produced in Figure 3 are more susceptible to UV degradation than the original polycarbonate polymer.
[FIGURE 3 OMITTED]
EFFECT OF UV WAVELENGTH ON DEGRADATION PRODUCTS
Although the focus of the literature search was on the specific UVC wavelength of 254 nm (0.01 mil) [253.7 nm (0.00999 mil) to be precise] photodegradation of polymers, the majority of the UV experiments in the identified references were performed in the UVA and UVB region or with an unspecified wavelength. However, several authors (Rabek 1995) have noted that the reaction products obtained with UVA/UVB light are lower in quantity (produced radicals have less energy) but similar in composition to those produced with 254 nm (0.01 mil). For instance, both poly (vinyl chloride) and polycarbonate produce yellow compounds (Figure 2 and 3) when exposed to either UVA/UVB or UVC light (Rabek 1996). Under 254 and ~300 (UVB) nm (0.01 and 0.012 mil) light, polypropylene decomposition produced similar products such as ketones, peroxy acids, peroxy esters, etc. (Aslan-zadeh and Kish 2005).
Any differences in the UVA/UVB and 254nm (0.01 mil) (UVC) photodegradation products arise from the higher energy of the UVC light (able to break bonds stable to lower energy of UVA/UVB). Polyacetal undergoes depolymerization to only produce formaldehyde when exposed to 360 nm (0.014 mil) UV light but produces ethyl alcohol and ethylene glycol as well as formaldehyde when exposed to 254 nm (0.01 mil) (Chiang and Huang 1999). In addition to the Photo-Fries rearrangement causing the polycarbonate to yellow, the higher energy of UVC is able to directly break (C-O) bonds in the original polycarbonate structure to form reactive free radicals and volatile products (Hamid 2000).
Regardless of the UV energy, once the photodegradation is initiated, the scission degradation products increase the UV absorption of the polymer surface due to the presence of carbonyl groups (C=O) and double bonds (C=C). Based on the experimental results reported in the identified references (Rabek 1995 and 1996, Rabney and Rabek 1975 and Hamid 2000), Table 1 was compiled to compare the susceptibilities of different materials to photo-initiated scission and gas production [dependent on the efficiencies of both the photolysis (UV absorption) and scission steps in Figure 1]. The materials are listed in order of decreasing yield, i.e., decreasing scission (gas production) with same level of UV exposure indicates increasing resistance to UV degradation.
The results in Table 1 indicate that the susceptibility of the different polymers to scission are more dependent on polymer composition than on the UV wavelength used in the experiment. Many of the researchers reported that the quantum yields of scission and other reactions (cross-linking, gaseous products, etc.) were independent of the wavelength of UV irradiance.
Regardless of the wavelength used in the studies, the primary volatile product of the UV irradiation was water originating from the hydroxyl radical abstraction of a hydrogen atom from the polymer (Figure 1). Carbon dioxide and carbon monoxide were also common degradation products resulting from the scission reaction in Figure 1 when the carbonyl group was at the end of the polymer radical. Other reported volatile products such as hydrochloric acid (Figure 2), formaldehyde (polyacetal), formic acid [poly (ethylene terephthalate)], etc. were more dependent on the composition of the polymer being irradiated than the wavelength of the UV light.
In addition to the type of degradation products produced, the wavelength of the UV light affects the depth of the polymer surface undergoing photodegradation. For low absorbing polyethylene and polypropylene, UV light in the 290-360 nm (0.011-0.014 mil) range caused significant scission reactions to depths of 1.5-0.4 mm (59-16 mil) (depth decreased with increasing wavelength) (Shyichuk 2005). For high absorbing polystyrene, polycarbonate, acrylonitrile-styrene and poly (methyl methacrylate), (Nagai 2004) reported that the chemical changes due to exposure to UVA/UVB was less than 5 microns (0.20 mil) deep. Comparing the depth penetration of an acrylic resin, 50% of the incident light penetrated to a depth of 10 mm (390 mil) for 364 nm(0.014 mil) light, to 1 mm (39 mil) for 313 nm (0.012 mil) light and to only 0.01 mm [10 microns (0.39 mil)] for 254 nm (0.1 mil) light (Feller 1994).
EFFECTS OF CONTAMINANTS AND ADDITIVES ON DEGRADATION RATE
As previously discussed, the photodegradation of many non-absorbing polymers are attributed to the presence of UV absorbing impurities formed during polymerization, processing and/or storage. The impurities can be both internal (synthesis) and external (processing/storage) and include hydroperoxides, carbonyl and unsaturated bonds, catalyst residues, additives, metal traces, etc. (Rabek 1995 and 1996). Consequently, the capability of a polymer to absorb UV energy is affected by both the chemical structure of the polymer as well as the presence and location of chromophoric impurities. For instance, the susceptibility of polystyrene to photodegradation is highly dependent on how it was synthesized (Rabek 1995). Radically prepared polystyrenes are more susceptible to UV degradation than anionically prepared polystyrenes since they contain double bonds, in-chain peroxide linkages, and other oxygen containing groups, all of which increase the UV absorption/reactivity of the internal and external polystyrene matrix.
In contrast to polystyrene, the UV susceptibilities of poly-ethylene and other extruded polymers are dependent on their thermal history. The hydroperoxides formed at low temperatures [50-90[degrees]C (122-194 degrees]F)] are isolated and have minimal photo-initiating effects (radical produced by impurity is not transferred to polymer). Whereas, the hydroperoxides formed at high [135-160 degrees]C (275-320 degrees]F)], prolonged temperatures on the surfaces of the extruded polymers are associated and have significant photo-initiating capabilities (Rabek 1995). Consequently, low density polyethylene would be expected to be more resistant to UV photodegradation than high density polyethylene due to its lower extrusion temperatures.
In addition to unwanted impurities, additives such as plasticizers are added intentionally to polymers to aid processing and improve the flexibility of the final product. Commercial plasticizers such as di-n-octyl adipate/phthalate esters are capable of acting as photo-initiators for poly (vinyl chloride). Consequently, the degradation products of commercial poly (vinyl chloride) products can originate from both the plasticizer as well as the polymeric matrix (Deni-zligil and Schnabel 2003).
In contrast to plasticizers, carbon black has been reported to inhibit the UV photodegradation of different polymers such as polyacetal (Chiang and Huang 1999) and polyethylene (Scott 1965). All of the references identified during the literature search were concerned with the protective effects of carbon black with regard to weathering (UVA/UVB), not UVC exposure. The presence of other inorganic fillers capable of forming a protective coating (chalking) on the surface of the polymer or acting as an energy sink for the UV energy absorbed by the polymer would be expected to have an inhibitory effect on the 254 nm (0.01 mil) photodegradation of polymers.
EFFECTS OF UV IRRADIANCE ON THE DEGRADATION RATE (RECIPROCITY LAW)
One of the primary purposes of the literature search was to identify research that indicated that short-term, high irradiance UVC experiments could be used to accurately predict the performance of polymers exposed to long-term, low irradiance UVC, i.e., UVC exposure obeys the reciprocity law. The reciprocity law is obeyed if the photodegradation of the polymer is dependent only on the total energy of UV exposure (irradiance x time) and is independent of the time or irradiance level taken separately. In other words, the degree of photodegradation would be the same whether produced by 10,000 microwatts per square centimeter ([micro]W/[cm.sup.2]) [64,500 microwatts per square inch ([micro]W/[in.sup.2])] for 100 hours or 500[micro]W/[cm.sup.2] (3220 [micro]W/[in.sup.2]) for 2000 hours.
When dealing with materials without fillers such as poly-ethylene, researchers have reported that the rate at which UV (wavelength unspecified) irradiated polyethylene absorbs oxygen (oxidation) is proportional to the square root of the light irradiance (Scott 1965). Also, the researchers determining the quantum yields for different polymers in Table 1 reported that the quantum yields of scission and other reactions (cross-linking, gaseous products, etc.) were independent of the irradiance.
Table 1. Quantum Yields (a) of Chain Scission and Gas Evolution for UV Irradiation of Different Polymers Polymer UV Wavelength Quantum Yield (nm) of Scission (Gas) Poly(phenyl isopropyl 254 0.17 - 0.22 ketone) Poly(methyl vinyl ketone) 254 0.025 Poly(methyl methacrylate) 254 0.02 - 0.04 300 0.016 - 0.005 (0.0002) Poly(methyl phenyl 313 0.017 silane) Poly(vinyl chloride) 254 - 400 (0.015) (loss of HCl) Poly(methyl acrylate) 254 0.013 Poly(a-methyl styrene) 254 0.007 Poly(vinyl acetate) 254 0.005, 0.05 (0.0065) EPDM Rubber < (b) 0.003 Poly(ethylene 254 0.0016 terephthalate) (0.0002) 280 - 360 0.0005 (0.0002) Polystyrene 254 0.0015 -0.0005 (0.000045) Cellulose 254 0.001 - 0.0007 Poly(vinyl pyrrolidone) 254 0.00043 Natural Rubber 254 0.0004 (cis-1,4-polyisoprene) (0.001) Polysulphones 254 0.00084 Nylon 6 254 0.0007 Polycarbonate 260 -300 0.0007 - 0.0001 Polyacrylonitrile 254 0.0002 - 0.00077 Polyurethanes 254 (0.00014) Mixed phenyl - methyl 254 (0.000026) Polysiloxanes (a) Quantum Yield = Number of molecules decomposed/Number of photons absorbed b Unspecified (b)Unspecified
More to the point of this project, research with UV light in the wavelength and irradiance ranges of 290 to 400 nm (0.011 to 0.015 mil) and 3,600 to 32,200 [micro]W/[cm.sup.2](23,200 to 207,700 [micro] W/[cm.sup.2]), respectively, demonstrated that the photo-degradation of acrylic-melamine coatings obeyed the reciprocity law (Chin 2005). Fourier Transform Infrared (FTIR) analyses of the irradiated coatings determined that the rates of scission, photo-oxidation and mass loss were directly proportional to dosage regardless the time of irradiance. The references identified by Chin (2005) also stated that the photodegradation of acrylic coatings, poly (vinyl chloride), polycarbonate, poly-?-methylstyrene, acrylonitrile butadiene-styrene and poly (butylene terephthalate) obeyed the law of reciprocity (wavelength unspecified).
CORRELATION BETWEEN LABORATORY AND HVAC UVC RESULTS
The main goal of this project is to allow HVAC designers to select the best material for use in an UVC application and to allow maintenance personnel to estimate the useful life of an in-service HVAC component based on its UVC dosage. For the laboratory test results to be the most useful for the designer or maintenance personnel, the law of reciprocity must be obeyed by both the laboratory test and HVAC results. The reciprocity law is obeyed if the photodegradation of the material is dependent only on the total energy of UV exposure (irradiance x time) and is independent of the irradiance time, geometry, chamber, environment, etc.
Various factors such as:
* wind, particle erosion, rain,
* large temperature variances between day and night,
* air polluants (ozone, nitrogen oxides, etc.),
* wavelengths/angle of sunlight versus UVA/UVB used in lab test,
* light and dark cycles, etc
are listed in ASTM G151 and ISO 4892-1 to explain the poor correlation between accelerated laboratory (ASTM G154) and standardized outside (D1435) weathering tests (Scott 1965 and Rabek 1995 and 1996). Even with their lack of correlation, studies have shown that the susceptibilities of different materials to photodegradation are ranked in similar orders by the accelerated UVA/UVB laboratory and standardized outside weathering tests (Hamid 2000).
The surface photodegradation results produced by UVC lighting in the accelerated laboratory tests and HVAC systems are expected to have better correlation than the results of the UVA/UVB laboratory tests and weathering tests since the exposure environments of the laboratory photoreactors and HVAC systems are controlled, surface erosion is minimal and sun light is not involved. The sample vibration (rotating plat-form) and the frequent air flow (maintain constant temperature) during the accelerated laboratory tests are expected to help simulate the effects of HVAC operation on the photodegradation rates (enlarge microcracks, remove particles, etc.) of exposed materials. The expected UVC laboratory test correlation with the HVAC systems is further improved when the exposed materials are at a controlled temperature in the HVAC system.
To further increase the probability of reciprocity between the photodegradation rates produced by the laboratory and HVAC systems, the UVC irradiances of the accelerated laboratory tests were selected to be with in an order of magnitude of those used in HVAC systems (Kauffman 2011). For instance, the UVC irradiances for air and cooling coil treatments range from 1 to 20,000 [micro]W/[cm.sup.2] (6.4 to 129,000 [micro] W/[in.sup.2]) (IUVA 2005) and 10 to 500 [micro]W/[cm.sup.2] (64.5 to 3220 [micro]W/[in.sup.2]) (Kowalski 2009), respectively. Consequently, the irradiance range of the developed accelerated laboratory tests was between 900 and 14,000 [micro]W/[cm.sup.2] (5800 and 90,300 [micro]W/[in.sup.2]). Even for the deeper penetrating UVA/UVB, in which the rate of photodegradation is limited by oxygen diffusion, the laboratory tests obeyed reciprocity (Chin 2004) when the lower UV irradiance level was within 10x of the upper irradiance level used. The range of UVC irradiance levels that obey reciprocity should be even wider since UVC mainly produces scission reactions (Rabek 1995 and 1996) and its depth of penetration is reported to be only 10 microns (0.39 mil) (Feller 1994 and Shyichuk 2005). Consequently, the UVC scission step (Figure 1) will always occur in the presence of air, regardless of the UVC irradiance level, so that the rate of photodegradation will only depend on the level of UVC irradiance (criteria for reciprocity) independent of the oxygen diffusion rate.
One factor that could lead to large differences in the laboratory and HVAC results is the wavelength spectrum of the UVC light used by the HVAC system. According to the IUVA Draft Guideline IUVA-G01A-2005, the two most common types of UV germicidal lamps are the medium and low pressure mercury lamps. The emission spectra of the low and medium pressure lamps are quite different as shown in Figure 4. The emission spectrum of the UVC lamps to be used in the accelerated laboratory tests matched the spectrum of the low pressure lamp in Figure 4.
[FIGURE 4 OMITTED]
Consequently, the photodegradation rates of the UVC laboratory tests and HVAC systems are expected to have good correlation and reciprocity when the HVAC systems employ low pressure mercury lamps. Poor correlation is expected between the photodegradation rates of UVC laboratory tests and HVAC systems employing medium pressure lamps [depth of penetration and accelerated reactions dependent on wave-length (Feller 1994, Nagai 2004 and Shyichuk 2005)].
One other significant wavelength that is produced by a low pressure lamp is the 185 nm (0.0073 mil) line. The quartz envelope of the UVC lamp in Figure 4 absorbs the 185 nm (0.0073 mil) so it is not transmitted to the environment. If the lamp envelope is produced from synthetic quartz (Heraeus 2009) or has a defect, then both the 185 and 254 nm (0.0073 and 0.01 mil) lines are transmitted resulting in ozone production [185 nm (0.0073 mil) line]. A large portion of the generated ozone is converted into diatomic oxygen ([O.sub.2]) and a singlet oxygen (O*) (Jones and Wayne 1969 and Qu 2005) by the 254 nm (0.01) line. The quantum yield of ozone photolysis by 254 nm (0.01 mil) is nearly unity, i.e., ratio (number of molecules converted/number of photons absorbed) is almost 1 for ozone compared to <0.02 for most polymer photodegradation (Rabek 1995 and Table 1). In the presence of water [water treatment (Spartan water treatment 2009)] or water vapor [surface cleaning systems (Kim 1996)], the ozone photolysis by 254 nm (0.1 mil) results in the production of highly reactive hydroxide radicals (Figure 1) and oxygen.
IDENTIFICATION OF LABORATORY PHOTODEGRADATION CHAMBERS
In order to compare the photodegradation rates of different type materials in the irradiation range of 900 and 14,000 [micro]W/[cm.sup.2] (5800 and 90,300 [micro]W/[in.sup.2]) photoreactors designed to perform chemical and biochemistry experiments involving UVA-UVC exposure were identified. The photoreactors are instrumented to monitor the irradiance reaching the surface of the test solutions and the test chambers are temperature regulated to ensure repeatable experimental conditions. The LZC-ICH2 photoreactor (Luzchem Research, Inc. Ottawa, Ontario) has an inside chamber that is 30 cm wide, 30 cm deep and 22 cm high) (12 in. x 12 in. x 8.5 in.) that is lined with an aluminum alloy (Al 5052-H32) to maximize UVC reflections and is equipped with 16 UVC lamps (Figure 5) to ensure the UVC irradiance is consistent throughout the reaction zone. A rotating octagon turntable 21.6 cm wide with 8.9 cm sides (8.5 in., 3.5 in. sides) can be added to the photoreactor to ensure all of the samples receive equal levels of UVC exposure to allow the photodegradation rates of successive tests to be compared. The spectral output of the UVC lamps employed by the ICH2 photoreactor is well characterized with ~ 96% of the UV energy (92.5% UV, 7.2% visible and 0.5% infrared) produced by the 254 nm (0.01 mil) line [similar to 254 nm (0.01 mil) lamps in Figure 4].
[FIGURE 5 OMITTED]
Even though the ICH2 lamps are designed to produce only 254 nm (0.01 mil), ozone measurements were made inside the photoreactor to ensure ozone was not being produced by a lamp defect [185 nm (0.0073 mil) transmitted through the defect]. Measurements determined that the ozone level inside the photoreactor was below the instrument detection limit of 0.03 parts per million by volume confirming the proper functioning of the photoreactors. Since varying ozone concentration is one of the factors contributing to poor correlation between accelerated laboratory tests and weathering tests (Scott 1965), ozone destruction by the UVC lamps in both the accelerated laboratory tests and HVAC systems should further aid in their mutual reciprocity.
IDENTIFICATION OF PHOTODEGRADATION MONITORING TECHNIQUES
The final topic of the literature search was to identify the analytical technique(s) best suited to monitor the photodegradation of the non-metallic samples exposed to 254 nm (0.01 mil) UV light. Although 254 nm (0.01 mil) exposure causes the irradiated molecules to undergo free radical induced oxidation (Figure 1) followed by scission reactions, and to a lesser extent cross-linking reactions (Rabek 1995 and 1996), the affected molecules are concentrated close to the surface (Feller 1994, Rivaton 2002). Also, the oxidation of the surface molecules (C=O and C=C bonds increase) increases the 254 nm (0.01 mil) absorption by the surface further decreasing the depth of the UVC penetration into the material (Rabek 1995 and 1996). Consequently, even though photodegradation strongly affects the mechanical properties and chemical composition of the exposed surface, the bulk properties and composition of the polymer remain unaffected so that structural integrity tests such as tensile strength, impact and flexibility are considered impractical for monitoring UVC photodegradation during the accelerated laboratory tests.
Since the identified literature was not focused on HVAC applications, several scrap samples (Witham 2009) from HVAC material UVC testing were obtained for initial study (dosage levels unknown). The scrap samples were analyzed with different analytical techniques to identify the type of surface photo-oxidation mechanisms occurring and to identify the analytical test(s) with the best potentials for monitoring the accelerated 254 nm (0.01 mil) photo-oxidation tests. Since the original, unexposed material was not available for comparison, a portion of each sample's surface was scraped away to reveal unexposed material for analysis.
For polymers with fillers, the FTIR spectra of the unexposed materials (inside) had distinctive organic peaks (C-H, C=O, C-O, etc.) not present in the spectra of the UVC exposed surfaces as illustrated by the representative spectra in Figure 6, i.e., UVC volatilization of polymer surface left behind protective layer of inorganic filler.
[FIGURE 6 OMITTED]
The Energy Dispersive Spectrometric (EDS) elemental analyses of the polymer samples containing fillers also indicated that the UVC exposed surface contained a much higher inorganic content than the interior material (Kauffman 2011).
As opposed to Figure 6, the FTIR and EDS spectra of polymers not containing fillers were virtually identical for the interior material and UVC exposed surface (Kauffman 2011). Surface analyses indicated that the UVC exposed organic surface had a distinctive pattern, i.e., mass loss of clean surface concentrated at susceptible locations (grain structure, crystallinity, etc.) as previously discussed (Rabek 1995 and 1996).
Consequently, all of the samples underwent organic mass loss during UVC exposure in agreement with Figure 1 indicating that photolysis (direct bond breaking) and photooxidation scission reactions producing volatile products (mass loss) occur at a much higher rate than cross-linking reactions (harden surface). The polymers without fillers would be expected to undergo constant mass loss regardless of dosage while the polymers containing fillers would lose mass until an exterior layer of inorganic fillers formed to protect the interior molecules from further UVC exposure and photodegradation reactions (Figure 6). Several references were identified which stated that surfaces exposed short-term to 254 nm (0.01 mil) had mass loss and other changes in morphology (Kaczmarek 2005 and 2009, Rabek 1996 and Soto-Oviedo 2002). Longer-term 254 nm (0.01 mil) tests with linear polyethylene with out fillers (Ranby and Rabek 1975) demonstrated that mass loss occurred for the entire 800 hour test as indicated by the constant rate of oxygen adsorption by the exposed surface (fresh material exposed by mass loss reacts with oxygen).
In addition to the HVAC scrap material analyses, additional polymers without fillers were exposed to UVC for different lengths of time and then visually inspected for color changes as performed in ASTM methods such as D6290 or International Standard ISO 4582. The surface of a polyurethane sealer turned light orange after only 10 hours of UVC exposure [~4,000 [micro]W/[cm.sup.2] (25,800 [micro]W/[in.sup.2]) with UVC pen light]. After 50 and 200 hours of UVC exposure, the surface remained light orange in color, i.e., extended UVC exposure did not further darken the exposed areas since the oxidized species were not accumulating due to mass loss. In contrast to the polyurethane sealer, the polished surface of a polyacetal rod did not change color (exposed area remained white but became dull) even after 200 hours of UVC exposure.
Based on these initial results and the literature search, surface compositional changes were not pursued for monitoring UVC photodegradation damage. Although the FTIR spectrum peak heights are well-suited for quantification, the produced spectra did not change with dosage due to surface mass loss (no fillers) and became independent of the polymer (filler accumulation due to polymer loss). Color changes, although simple to perform with a colorimeter, are strongly affected by surface finish and do not change with exposure time (surface craters observed), and consequently, do not correlate well with degree of photodegradation. Finally even though monitoring the rates of oxygen absorption (Ranby and Rabek 1975) and/or volatile compounds produced during mass loss could be used to monitor the rate of UVC degradation of a polymer surface, the large numbers of samples to be irradiated in the accelerated UVC laboratory tests made the monitoring of oxygen consumption and/or produced gases from individual samples impractical.
Consequently, an analytical technique capable of monitoring surface mass loss was selected as the best technique for monitoring surface UVC photodegradation. The most direct technique for monitoring mass loss from a surface is measuring the sample weight loss using an analytical balance [sensitive to 0.1 milligram (3.5e-06 ounce) changes]. The primary drawback of measuring weight loss is that small changes in the water content of the sample (due to heating, room air humidity changes, etc.) will affect the sample weight, and consequently, the accuracy of the UVC mass loss calculations. The second technique of monitoring surface mass loss is masking the surface of the sample so that the surface exposed to the UVC suffers mass loss/height reduction compared to the original surface protected from the UVC (crater formed in exposed area compared to protected area). Scanning electron micro-scope microphotographs, which are informative as to the morphology changes of the exposed surface, are limited by expense and inability to measure crater depth. Optical micro-scopes using multiple focusing depths are able to monitor crater depths but are laborious and time-consuming. Consequently, surface profilometers were used to quantitate the depths of the UVC formed craters by sequentially scanning across the protected then exposed surface areas. The primary drawback of the surface profilometers is that the sample surface has to be polished to a level finish prior to masking and UVC exposure.
The results of the literature search indicate that even though the rates of UVC photodegradation are affected by a wide range of factors (processing parameters, contaminants, fillers, additives, etc.) and vary by several orders of magnitude for different polymers (Table 1), the primary photodegradation mechanisms are photolysis (direct bond breakage) and photooxidative scission (Figure 1). The results of the literature search and initial UVC irradiated samples further indicate that the primary change in the surface of the UVC irradiated samples is mass loss as opposed to the compositional changes observed during UVA and UVB tests. For polymers without fillers, the mass loss due to UVC exposure is expected to obey the law of reciprocity, i.e., photodegradation of the polymer is dependent only on the total energy of UV exposure (irradiance x time) and is independent of the time or irradiance level taken separately. For polymers with fillers, the mass loss is expected to obey the law of reciprocity until the layer of protective inorganic particles/fibers forms on the exposed surface inhibiting further mass loss. Analytical balances for measuring sample weight loss or surface profilometers for measuring the depths of craters formed in masked sample surfaces were identified as the best techniques for monitoring the degree of surface UVC photodegradation.
The literature search also identified photoreactors with a rotating turntable and intermittent air flow (simulate HVAC environment) that were capable of irradiating multiple small samples under controlled temperature conditions as required by the accelerated UVC laboratory test to be developed. According to the literature search, selecting irradiance levels within an order of magnitude of those used in HVAC systems would further increase the probability of reciprocity between the photodegradation rates produced by the developed laboratory tests in Parts 2 and 3 of this project and the rates experienced in HVAC systems.
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This paper is based on findings resulting from ASHRAE Research Project RP-1509.
Robert E. Kauffman is a distinguished research chemist at the University of Dayton Research Institute, Dayton, OH.
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|Author:||Kauffman, Robert E.|
|Date:||Jul 1, 2012|
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