LASER RADIATION-INDUCED, RESIDUE-FREE, LOCALIZED DECOMPOSITION OF SOME PLASTICS LEO
P A R T S ,
E D G A R
E .
H A R D Y ,
A N D
Dayton Laboratory, Monsanto Research Gorp., Dayton, Ohio
M A R G A R E T
1. R O D E N B U R G
45407
The feasibility of effecting localized degradation of some plastics by continuouswave COZ laser irradiation, with concomitant complete volatilization of products, has been demonstrated. The irradiation times were long in comparison with tinies required for intra- and intermolecular energy dissipation processes of the polymers. Degradation mechanisms that control the thermal decomposition of addition polymers prevail with most materials during laser irradiation. Experimental parameters were investigated in the laser-irradiative degradation of polychlorotrifluoroethylene, polyoxymethylene, poly(methy1 methacrylate), poly( 1 -methylstyrene), and polytetrafluoroethylene. Optimum laser energy utilization efficiency values attained with the last four polymers were: 45, 46, 86, and 6 4 % . The energy utilization efficiency is strongly affected by the absorbence of the plastic at the laser emission wavelength. The localized, residue-free degradation process is applicable for the production of functional and decorative plastic devices and objects.
SOME plastics
undergo very specific chemical changes upon exposure to electromagnetic radiation. Lasers offer a novel means for highly localized and controlled irradiation. In the present study, degradation of plastics by laser irradiation was explored. I t was of interest to effect localized degradation without leaving a decomposition residue in (or near) the irradiated area. The work was devoted primarily to an investigation of localized degradation of plastics composed of polychlorotrifluoroethylene, poly(methyl methacrylate), poly (1-methylstyrene), polyoxymethylene, and polytetrafluoroethylene. These polymers are known to decompose by chain scission, either to the respective monomers or to low-molecular-weight volatile fragments (Grassie, 1956; Madorsky, 1964). Bond strengths, radical stalbilities, and steric factors affect the decomposition mechanism (Jellinek, 1955; Grassie, 1956; Madorsky, 1964). Coherent, continuous-wave radiation from a single-mode laser can be focused to a spot of diameter d = 2.44 h f / D , where h is the wavelength of emitted radiation, f is the focal length of the condensing lens, and D is the diameter of the laseic beam a t the focusing element. To effect heating of the irradiated area, laser energy has to be absorbed by i,he material. Thus, when examined spectroscopically, the material has to exhibit absorption at the laser emission wavelength, A. The latter statement is equally applicable to radiation from ultraviolet, visible, or infrared sources. On the basis of the Beer-Lambert law, the depth of penetration of radiation is inversely proportional to the absorption coefficient. T o generate high temperatures and temperature gradients in the
irradiated material under conditions of high radiation flux density, this material must possess a high absorption coefficient a t the laser emission wavelength. Maximum energy utilization for surface-degradation is attained by meeting these requirements. At present, the most intense, continuously emitting laser source is the infrared radiation-emitting CO, laser ( h = 10.6 microns). Most of the present investigation was conducted with this source. Experiments were also conducted with 4880-A. radiation from an argon laser. However, most polymeric materials do not absorb a t the argon emission wavelength. To attain absorption of radiation at the latter wavelength, dyes or other absorbing materials must be incorporated in the plastics. Focal point diameter is usually reduced by utilizing the shorter wavelength argon laser radiation. Absorption of radiation at the laser emission wavelength provides a mechanism for transferring energy to the plastic. The absorption in the infrared spectral range causes vibrational excitation. Relaxation and energy transfer processes convert the absorbed laser radiation to vibrational and translational kinetic energy of the system. Upon intense, continuous irradiation, the vibrational energy is raised sufficiently to cause bond rupture. Under irradiative conditions such as applied in the present work, the irradiation time is longer than the relaxational and collisional deactivation times. Therefore, equipartition of energy occurs among the vibrational, rotational, and translational modes of freedom, and the decomposition mechanisms would be anticipated to be essentially the same as those found in conventional heating processes. The continuousInd. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
21
wave CO, laser therefore offers a unique means for effecting thermal decomposition in a volume segment or in a predetermined area in a highly controlled manner. When dyes are used to cause absorption of radiation by an otherwise transparent plastic, the initial process consists of electronic excitation of dye molecules. Suhsequent procesges again lead to intermolecular energy transfer, causing a temperature rise in the entire irradiated area. Incorporation of radiation-absorbing additives or chemical tailoring of materials can often be used to attain the required absorption characteristics for the monochromatic laser radiation. Experimental
The Korad Model K-G3 CO2 laser used in this work provides 0 to 75 watts of continuous-wave multimode radiation of 10.6-micron wavelength. The power output is continuously variable. A spherical, 6-inch focal length, water-cooled germanium condensing lens from the Korad Corp. was used for focusing the radiation onto the upper surface of plastic specimens traversing the beam. The laser energy output was calibrated with a Korad K-J10 liquid absorption calorimeter. The specimen transport system was desigued and built around an Optometrix micrometer cross slide. Positioning in the r-direction was accomplished manually; a variablespeed motor provided the drive in the y-direction a t speeds from 0.4 to 4.2 mm. per second. T h e lens-to-sample surface distance remained constant during a scan. The entire system was aligned with a He-Ne laser. I n laser irradiation experiments, conducted with commerical uplasticized materials, specimens (size 25 x 50 x 3.2 mm.) were traversed through the laser beam under normal atmospheric conditions. For experiments with polyoxymethylene, however, a protective nitrogen atmosphere was used to prevent ignition of the evolving vapors. Sample transport speed and laser output power were varied. The engraved groove characteristics and the extent of degradation were subsequently estahlished. Increas'ed ventilation was provided during the experiments with halogenated polymers. Results
C O , Laser Irradiation of Poly(methy1 Methacrylate). Initial experiments with poly(methy1 methacrylate) and polystyrene demonstrated contrasting behavior of plastics upon laser irradiation. A well-defined groove was formed with the sample of poly(methy1 methacrylate) traversed through the focused laser beam and no degradation product was deposited on the surface of the specimen (Figure 1). Poly(methy1 methacrylate) is known t o decompose essentially quantitatively to the monomer upon heating by conventional means (Clark and Jellinek, 1965; Grassie, 1956; Madorsky, 1964). When polystyrene was irradiated in an analogous manner, the high-molecular-weight degradation products were deposited along the edges of the groove. Polystyrene has been reported to yield -65% monomer upon thermal degradation, and -35% of higher molecular weight scission products (Grassie, 1956). Perforation of polyethylene film by laser irradiation also results in deposit formation around the irradiated zone (Silvus and Bond, 1968). The quantitative experiments were conducted a t laser power output levels of 0.3, 1.75, 10.5, and 22.4 watts. The effect of the scanning rate on groove width, groove 22
Ind. Eng. them. Prod. Res.
Develop.,Vol. 9,No. 1, March 1970
Figure 1. Cross sections of COS laser-engraved specimens A. Poly(methy1 methocrylate) B. Polyrtyrene
depth, and depolymerization efficiency a t each power level was established. The results of poly(methy1 methacrylate) laser irradiation experiments are summarized in Table I and in Figures 2 to 6. The depolymerization efficiency (in milligrams per joule) is a measure of the extent of material vaporization in terms of the expended energy. The effective heat of depolymerization is defined as the amount of laser energy required to depolymerize a quantity of polymer corresponding t o the molecular weight of monomer, under the indicated conditions. A comparison of the effective heats of depolymerization with the reported values of the heats of polymerization provides a measure of the thermodynamic efficiency for the laser radiation-induced degradation processes. As indicated in Figure 2, the groove width increased rapidly with laser output power at levels ranging up to 2 watts. At the very low energy flux densities (laser output power per irradiated surface area), the temperature of the specimen is raised sufficiently to degrade the polymer only in the central region of the irradiated area, in which
Table 1. Effect of Laser Output Power on Depolymerization of Poly( methyl Methacrylate) at Constant Sample Transport Speeds
Laser Output Powr,
Waits 0.3 1.75 10.5 22.4 0.3 1.75 10.5 22.4
Sample A". Rate Tmnsport of DepolySpeed, merization, Mm.lSec. Mg.lSec. 2.6 2.6 2.6 2.6 3.9 3.9 3.9 3.9
0.06 0.79 3.2 6.7 0.01 0.82 3.7
1.8
Efiiency, Mg.fJaule
Effectiue Heat of Depolymerization, KdfMole
0.18 0.45 0.30 0.30 0.03 0.47 0.35 0.35
147 53 80 80 700 51 69 69
A". Depoly-
merimtion
the flux density is highest. When the laser output power was increased, degradation occurred essentially in the entire area onto wlhich the beam was focused. The engraved groove diameter increased relatively slowly, and the higher energy flux caused primarily an increase of groove depth. These statements apply to irradiation a t fixed scanning rates.
The depolymerization efficiency as a function of laser output power exhibited a maximum near the 2-watt level (Figure 5 ) . The observed decrease of the depolymerization efficiency a t high powers is believed to be caused by absorption of laser radiation by the rapidly evolving monomer vapors. The amount of energy required to decompose a quantity of the polymer corresponding to a mole of monomer ranged from 51 to 700 kcal. As expected, the lowest efficiency was attained by rapid scanning of the plastic specimen a t a very low laser output power level. The highest efficiency was achieved in present experiments by scanning at an intermediate power level (1.75 watts; sample transport speed, 3.9 mm. per second), a t which the escaping vapors did not attenuate the beam greatly. I t is of interest to note in the infrared spectrum of poly(methy1 methacrylate) that the absorbance of this polymer a t 10.6 microns is not high (cy = 180 cm.-'). Thus, radiation of this wavelength would not be absorbed within a thin surface layer, high temperature gradients would not be established in the sample, and very efficient energy utilization would not be anticipated in the laser-irradiative degradation of this polymer with 10.6-micron radiation. The heat of polymerization of poly(methy1 methacrylate) is -13.57 kcal. per mole (McCurdy and Laidler, 1964), and the heat of vaporization of the monomer is 9.3 kcal. per mole (Brockham and Jenckel, 1956; Bywater, 1952). I n the present experiments, the laser energy utillization efficiency ranged up to 45% in effecting the reaction:
R -'
Laser Output Power ( w a t t s )
Figure 2. Dependence of groove width ( 0 ) and groove depth ( A ) on laser output power in irradiation of poly(methy1 methacrylate) at constant scanning rate of 3.9 mm. per second
+ R * +R!
n CH,=C
\C 00CH
:j
(d
Figure 3. Dependence of groove width on scanning rate in laser irradiation of poly(methy1 methacrylate) Laser power output, watts 0 0.3 A 1.75 10.5 22.4
Scanning Rate ( m m h e c ) Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
23
Figure 4. Dependence of groove depth on scanning rate in laser irradiation of poly(methy1 methacrylate) laser power output, watts
0.3
A
1.75
8 10.5 22.4
1
0
2
3
4
Scanning Rate (mm/sec)
COn Laser Irradiation of Polyoxymethylene. Polyoxymethylene undergoes depolymerization to formaldehyde upon heating (Dudina and Yenikolopyan, 1963, 1964; Grassie, 1956; Madorsky, 1964). The activation energy of this degradation process is very low [26 kcal. per mole for unstabilized polymer and 32 kcal. per mole for acetylend-blocked polymer (Dudina and Yenikolopyan, 1964)l. The strongest absorption band in the near-infrared region is centered a t 10.6 microns (CY = 3000 cm.-'). Thus, this polymer possesses the desired characteristics for residuefree degradation by CO? laser irradiation, and for efficient energy utilization in this process. The experimental findings in the CO, laser radiationI induced degradation of polyoxymethylene were analogous . to those described above for poly(methy1 methacrylate). The results are summarized in Table 11. At equivalent 0 10 20 30 powers, the groove formed in polyoxymethylene was 7%; narrower and -40": shallower than the groove in Laser Output Power (watts) poly(methy1 methacrylate). The depolymerization efficiency, in terms of the polymer mass loss, based on Figure 5. Effect of laser output power on deploymerization expended energy, was 30% lower for polyoxymethylene efficiency in irradiation of poly(methy1 methacrylate) at concompared with poly(methy1 methacrylate). The relatively stant scanning rates lower values of mass removal efficiency, compared with 2.6 mm./sec. the degradation of poly(methy1 methacrylate), stem from A 3.9 mm./sec.
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Figure 6. Depolymerization efficiency of poly(methy1 methacrylate) as a function of scanning rate laser power output, watts 0.3 A 1.75 10.5
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Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
8
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24
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Table 11. Effect of Laser Output Power on Depolymerization of Polyoxymethyleneat Constant Sample Transport Speeds
Laser Output Pouer Watt, 0.3 1.75 10.5 22.4 0.3 1.75 10.5 22.4
Sample A L Rate Transport o f DepoljSpeed merization, Mg See M m See 2.4 2.4 2.4 2.4 3.8 3.8 3.8 3.8
0.08 0.64 2.5 5.0 0.06 0.63 2.5 5.3
Mp Joule
Effectiw Heat of Depoljmeruation, Kcal Mole
0.26 0.36 0.24 0.22 0.21 0.36 0.24 0.23
28 20 30 33 34 20 30 31
A L Depoljmerization Efficient\,
the lower molecular weight of the repeating unit in the polymer chain. The minimum amount of energy required to degrade a quantity of polymer corresponding to a mole of monomer was 20 kcal. per mole. The reported heat of polymerization of formaldehyde is --17.2 kcal. per mole (Melia, 1966; Thompson, 1964). Thus, 86% utilization of laser energy was attained in the degradation of polyoxymethylene. It was noted during the irradiative degradation of polyoxymethylene that the vapors formed in the process ignited very readily when no protective atmosphere was used. The ignition of formaldehyde vapors could be prevented by maintaining an inert gas atmosphere over the irradiated area. I n contrast, a protective atmosphere was not required in experiments with poly(methy1 methacrylate). COL Laser Irradiation of Polychlorotrifluoroethylene, Poly(1-methylstyrene),, and Polytetrafluoroethylene. Poly(1methylstyrene) and polytetrafluoroethylene depolymerize essentially quantitatively to the respective monomers upon heating to moderately high temperatures (Grassie, 1956;
Madorsky, 1964): polychlorotrifluoroethylene yields some other volatile compounds, in addition to the monomer that constitutes the major thermal degradation product (Madorsky, 1964; Madorsky and Straus, 1955). Upon CO, laser irradiation with a focused beam, anticipated residuefree degradation was observed. The results are summarized in Table 111. Some loosely adhering fibrous resin particles remained in the groove during the irradiative degradation of polytetrafluororethylene when the samples were transported through the focused laser beam a t a rapid rate. Solid resin particles were then also propelled with the evolving gases from the irradiated surface in aerosol form. The incompleteness of polytetrafluoroethylene degradation under such conditions is attributed to scattering of radiation at the amorphous-crystalline phase interfaces and to the absence of fluid flow below the thermal degradation temperature at atmospheric pressure. Discussion
The feasibility of causing residue-free, localized degradation of polymers upon CO, laser irradiation was demonstrated with some selected systems. A prerequisite for the process is that the thermal degradation of the polymer produce only volatile species. The thermal degradation mechanism of polymers prepared from olefinic monomers is strongly affected by steric effects. As a class, many addition polymers synthesized from 1,l-disubstituted olefins undergo thermal degradation essentially quantitatively to the monomeric species. Thus, in addition to the representative polymers incorporated in the present investigation, other structurally related homopolymers and copolymers could be utilized for the residue-free laserirradiative degradation process. Table IV summarizes results listing the dimensions of the zones in which degradation occurred. The minimum
Table Ill. COz laser Radiation-Induced Depolymerization of PolycMorotrifluoroethylene, Poly( 1 -methylstyrene), and Polytetrafluoroethylene
Material Polychlorotrifluoroethylene Poly(1-methylstyrene) Polytetrafluoroethylene
Laser Output Pouer, Watts
Sample Transport Speed, M m See
1.75 0.3 1.75 10.5 0.3 1.75 10.5
4.2 4.0 4.0 4.0 4.0 4.0 4.0
A0
Rate of Depolymerization, Mg Sec
Depol? merzzatzon Efficiency, Mg Joule
Effectiw Heat of Depolymerization, Kcal. i Mole
1.4 0.08 1.2 4.1
0.80 0.28 0.67 0.39
...
...
34 103 42 71
0.58 1.75
0.33 ,os7
72 144
'Extent of depolymerization under conditions o f this experiment was wry loui Table IV.. Comparative Summary of Experimental Results for Polychlorotrifluoroethylene, Poly( methyl Methacrylate), Poly( 1 -methylstyrene), Polyoxymethylene, and Polytetranuoroethylene" Au. Degraded Zolw Width, Mm.*
Material
L75
10.5
Polychlorotrifluoroethylene Poly(methy1 methacrylate) Poly(1-methylstyrene) Polyox ymethylene Polytetrafluoroethylene
0.62 0.66 0.77 0.59 0.48
...
...
0.94 0.82 0.91 0.65
1.13
22.4
...
1.05
...
Au. Degmded Zorx Depth, Mm.* 1.75 10.5 22.4 0.50 0.46 0.65 0.35 0.25
...
...
2.34 2.05 1.23 1.oo
3.99
...
2.27
...
'Results obtained at uwrage scanning rate of -3.9 mm. see. *Numerical ualues at column heads refer to laser power output values in u3att.s.
Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 1, March 1970
25
~~
~~
Table V. Optimum Depolymerization Efficiencies Attained in CO?laser-Irradiative Degradation
Polymer
Minimum Efectiw Heat of Depolymerization, Kcal./Mole
Reported Heat of Condensation and Polymerization, Kcal./Mole
Laser Energy Uti1ization Efficiency, 70
Depolymerization Efficiency, Mg./ Joule
Polychlorotrifluoroethylene Poly(methy1 methacrylate) Poly (1-methylstyrene) Polyoxymethylene Polytetrafluoroethylene
34 51 42 20 72
-22.gKb -19.3”d -17.2‘ -46‘
45 46 86 64
0.08 0.47 0.67 0.36 0.33
a McCurdy and Laidler, 1964. Bywater, 1952; Bmckham and Jenckel, 1956. ‘Roberts and Jessup, 1951. Heat o f vaporization calculated from data reported by Klages (1902) and Matsuraba and Perkins (1905). ‘Thompson, 1964; Melia, 1966. Patrick, 1958.
’
irradiated zone width attainable with an optimized system (diffraction-limited optics, single-mode laser emission; Kogelnik, 1966) that would possess the same optical parameters as the laser and lens combination is 0.26 mm. At high power output levels of the multimode laser, and a t the indicated sample transport speeds, the degraded zone width ranged up to four times the diffraction-limited size. A degraded zone width of 0.23 mm. was attained a t low laser output power and high scanning speed [poly(methyl methacrylate), laser output power 0.3 watt, sample transport speed 3.9 mm. per second]. I t is reasonable to presume that engraved lines or dots of diffractionlimited or lower widths can be produced reproducibly with a TEMW-mode laser beam of low divergence by controlling laser output power and sample transport speed. Data in the first three columns of Table V indicate the thermodynamic efficiency of the laser radiationinduced degradation process. The values in the first column represent the amount of emitted laser energy required, under present optimum conditions, to depolymerized and vaporize a quantity of polymer corresponding to the molecular weight of the monomer. The second column lists corresponding reported values for the heats of polymerization, based on the reactions of gaseous monomers. Laser energy utilization efficiencies for the residuefree localized degradation process ranged from 4570 with poly(methy1 methacrylate) to 86% with polyoxymethylene. The thermodynamic efficiency of the degradation process is a function of the absorbance of the plastic a t the laser emission wavelength, its thermal diffusivity, the mechanism and kinetic parameters of the degradation reaction, the energy flux density of incident radiation, and the irradiation time. With the high energy flux density applied in the present work, the first factor assumes a major significance. Most efficient energy utilization was attained in the degradation of polyoxymethylene. Among the materials used, this plastic exhibits the highest absorbance at 10.6 microns. Acknowledgment
The authors are indebted to John T. Miller for capable
26
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
experimental assistance, and to J. M. Butler and I. 0. Salyer for stimulating discussions. literature Cited
Brockham, A., Jenckel, E., Makromol. Chem. 18/19, 262 (1956). Bywater, S., J . Polymer Sci. 9, 417 (1952). Clark, J. E., Jellinek, H. H. G., J . Polymer Sci. A3, 1171 (1965). Dudina, L. A., Yenikolopyan, H . S., Polymer Sci. (USSR) 4, 1580 (1963)’. Dudina, L. A., Yenikolopyan, N. S., Polymer Sci. (USSR) 5 , 36 (1964). Grassie, N., “Chemistry of High Polymer Degradation Processes,” pp. 4, 24, 50, 80, Interscience, New York, 1956. Jellinek, H . H. G., “Degradation of Vinyl Polymers,” p. 161, Academic Press, New Yosk, 1955. Klages, A., Ber. 35, 3506 (1902). Kogelnik, H., in “Lasers,” A. K. Levine, ed., Vol. 1, p. 295, Marcel Dekker, New York, 1966. McCurdy, K. G., Laidler, K . J., Can. J . Chem. 42, 818 (1964). Madorsky, S. L., “Thermal Degradation of Organic Polymers,” pp. 26, 61, 130, 176, 293, Interscience, New York, 1964. Madorsky, S. L., Straus, S., J . Res. Natl. Bur. Std. 55, 223 (1955). Matsuraba, K., Perkins, W. H., Jr., J . Chem. SOC.London 87,672 (1905). Melia, T. R., Polymer 7, 640 (1966). Patrick, C. R., Tetrahedron 4, 26 (1958). Roberts, D. E., Jessup, R. S., J . Res. Natl. Bur. Std. 46, 11 (1951). Silvus, H. S., Jr., Bond, R. L., Final Report on Contract DAAA21-68-C-0692,Sept. 19,1968 (AD-677121). Thompson, J. B., in “Formaldehyde,” J. F. Walker, ed., 3rd ed., p. 180, Reinhold, New York, 1964. RECEIVED for review September 19, 1969 ACCEPTED November 17, 1969 Division of Organic Coatings and Plastics Chemistry, 158th Meeting, ACS, New York, N. Y., September 1969.
