Polymer Photochemistry 1 (1981) 103-109

I N F L U E N C E O F S A M P L E T H I C K N E S S O N

U L T R A - V I O L E T I N I T I A T E D P O L Y M E R I S A T I O N

STEPHEN C'LARKEt and ROBERT A. SI-IA~KS~:

Department of Applied Chemistry, Royal Melbourne Institute of Technology, Melbourne, Australia

(Received: 27 August, 1980)

ABSTRACT

The extent of photoinitiated polymerisation of butyl acrylate has been determined using a range of sample thicknesses. For each of the sample thicknesses a range of photoinitiator (benzoin) concentrations were used. It was found that the extent of polymerisation was greatest when the sample thickness was least. For each sample thickness there is a maximum in the graph of extent of polymerisation versus photoinitiator concentration. The optimum concentration of photoinitiator, for maximum extent of polymerisation, increases as the path length increases. As the photoinitiator concentration becomes greater the extent of polymerisation becomes less susceptible to changes in the sample thickness.

INTRODUCTION

In order to produce radicals from initiator molecules the ultra-violet radiation must first be transmitted through the polymerisation medium. On passing into the medium some of the radiation will be absorbed by the photoinitiator (which is chosen in part, for its ability to absorb strongly at the wavelength of irradiation), some by the monomer and some by any other components which may be in the reaction mixture. Therefore, in going deeper into the polymeris- ing medium the intensity of the radiation will decrease and, hence, so will the polymerisation rate. 1

Ri = cbloesSe -~l (1)

t Present address: Nonporite Pty. Ltd, Melbourne, Australia. Author to whom correspondence should be addressed.

103

Polymer Photochemistry 0144-2880/81/0001-01031/$2.50 © Applied Science Publishers Ltd, England, 1981 Printed in Northern Ireland

104 STEPHEN CLARKE, R O B E R T A. SHANKS

where Ri = rate of initiation, d~ = quantum yield, Io = incident radiation, e, = molar absorbance of photosensitiser, S = concentration of photosensitiser, e = molar absorbance of the mixture and l = depth in mixture at which rate is being considered.

The objective in formulating photoinitiated systems is to achieve the greatest mean rate of polymerisation throughout the system. That is to avoid having a rapid surface reaction at the expense of reaction rate deeper in the system.

Holman and Rubin 2 developed a theoretical model for the interaction of ultra-violet radiation with the active components of a simple photopolymer coating. Expressions for radiation absorption by the surface and bottom layers were obtained. Some of the predictions from this model will be used in interpreting the results of the present study.

The effect of sample thickness on ultra-violet initiated polymerisation is usually overshadowed by oxygen inhibition at the surface. Since getting com- plete surface cure is often the overriding problem, high concentrations of photoinitiator are used to the detriment of radiation transmission and hence cure in the lower layers. The degree of below surface curing will affect scratch resistance, hardness and adhesion to the substrate, a A high concentration of photoinitiator would also be expected to produce lower molar mass polymer and to leave high levels of initiator by-products in the cured film giving further depletion of properties. In this study oxygen was excluded by polymerising between layers of glass and quartz.

E X P E R I M E N T A L

The reactions were carried out using a Rayonet RPR-100 photochemical chamber reactor using 254 nm lamps. The temperature of the reactor was 45 ± I°C, and the reaction solutions were warmed to this temperature prior to being placed in the reactor.

Butyl acrylate was used as monomer for all reactions. The butyl acrylate was distilled under vacuum prior to use. The reactions were carried out in quartz tubes. The various sample thicknesses were obtained by placing glass rods down the centre of the tubes. Sample thicknesses were determined by measur- ing the internal diameter of the tubes (microcalipers) and diameter of the rods (micrometer). The degree of polymerisation was measured by refractometry, as described previously. 4 Data reported are from duplicate measurements. The standard deviation of all data was 1-7% giving 90% confidence limits of +1.9%. Each measurement of extent of polymerisation was made after the sample had been allowed to polymerise for 10 rain.

RESULTS AND DISCUSSION

Values for the extent of polymerisation after 10 rain are listed in Table 1 for eight sample thicknesses; each sample thickness having values for five different

I N F L U E N C E OF SAMPLE THICKNESS ON U V I N I T I A T E D P O L Y M E R I S A T I O N 105

k '/, i~mer ,zo t ,

,-,,, o b 20.

NtO 2 benzoin concentration/mot

Fig. 1. Effect of benzoin concentration and thickness on per cent polymerisation.

benzoin concentrations. These data are plotted on a three dimensional graph (Fig. 1). The results show that the extent of polymerisation was greatest when the sample thickness was least. This has also been observed with pigmented compositions. 5 Increasing sample thickness caused a rapid drop in the extent of polymerisation as predicted by eqn (1).

A shallow maximum can be observed in the curves of extent of polymerisa- tion versus photoinitiator (benzoin) concentration. The curves rise steeply to this maximum and then gradually decrease for higher concentrations of in- itiator. The effect becomes less pronounced as the thickness of polymerising solution increases. Holman and Rubin 2 observed similar maxima in plots of power absorption versus photoinitiator concentration.

TABLE 1 EXTENT OF POLYMERISATION OF BUTYL ACRYLATE WITH BENZOIN PHOTO-

INITIATOR

Benzoin concentration % ra/m 0.10 0'37 1"00 3"00 5"00

raoldra-3x 102 0.42 1"58 4-21 12.7 21-2

Sample thickness Extent of polymerisation (rata) after 10 rain

6"57 0-5 4.5 7.6 9.7 11-2 4"70 0"8 7"6 11"5 13"3 11"7 3.91 1-6 13.0 15-3 18.6 16"3 3"27 1"6 15"6 30-9 24.8 17.8 3"03 1"9 25-8 35"9 25-8 22"1 1-73 18"6 57.0 54.1 41.1 40.4 0"80 45"3 82.4 77.4 72.3 70.2 0-43 72"1 97.7 96"9 92"6 89-9

106 S T E P H E N C L A R K E , R O B E R T A. S H A N K S

14

optimum 12

benzoin 10 concentrotior), / mo[ dm- 3 o

.10 2 6

4

2

(3 I 2 3 4 5 thickness /ram

Fig. 2. Influence of thickness of optimum benzoin concentration.

It is proposed that these maxima arise from an uneven distribution of initiator radicals through the solution. At low initiator concentrations radicals are generated fairly evenly throughout the solution. 6 At higher initiator con- centrations most of the radiation would be absorbed near the surface of the solution, resulting in rapid polymerisation in surface layers but reduced polymerisation in lower layers. This non-uniform distribution would result in an overall lowering of the extent of polymerisation because of the half-order dependence of free radical polymerisation rates on the rate of initiation.

Values obtained from Fig. 1 for the maximum extent of polymerisation and the corresponding benzoin concentration for each sample thickness are listed in Table 2. When this optimum concentration of benzoin was plotted against thickness of polymerising solution a smooth curve was obtained (Fig. 2). This curve shows that the optimum concentration of photoinitiator increases as the sample thickness increases. Holman and Rubin 2 observed their maxima for power absorption to shift to lower values of initiator concentration as the film thickness increased. Since quantitative relationships for the formation of the maxima are not available this discrepancy cannot be explained. Previous

T A B L E 2 MAXIMUM EXTENT OF POLYMERISATION ( P ~ ) AND CORRESPONDING OPTIMUM BEN-

ZOIN CONCENTRATION FOR EACH SAMPLE THICKNESS. '

Sample thickness Pm~, after 10 min Benzoin concentration (ram) (%) (tool dm -3 × 102)

0-43 97-8 0-30 + 0.05 0-80 82-4 0-35±0.05 1-73 57.3 0.45 + 0.05 3-03 35.9 1.0 ±0.1 3.27 31.1 1.1 ±0.1 3.91 18.8 2-4 ±0.5 4-70 13.6 3-1 ±0-5 6.57 <11 >5

INFLUENCE O F SAMPLE THICKNESS ON U V INITIATED P O L Y M E R I S A T I O N 107

('i[]0'I~) Z[

I,[

log('/, polyrn.}

0

-0/. 0 1 2 3 4 5 6

thickness /mm

Fig. 3. Relationship between per cent polymerisation and thickness. Benzoin concentration, mol d m -3 x 1 0 2 : 1 = 0 . 4 2 ( + ) ; 2 = 1 - 5 8 ( O ) ; 3 = 4-21(~7); 4 = 12 .7 ( I I ) ; and 5 = 2 1 . 2 ( O ) .

results 4 for methyl acrylate photoinitiated with benzophenone-triethylamine showed a maximum in the rate of polymerisation when either benzophenone or triethylamine concentrations were independently varied. Since triethylamine does not absorb the wavelengths of radiation being considered, the observa- tions must be due to a more complex mechanism than complete absorption of radiation at the surface by high concentrations of photoinitiator.

Equation (1) predicts that Ri is proportional to e -~, i.e. - log Ri is propor- tional to I. The measured extent of polymerisation in this work was the mean for the reaction mixture of specified thickness, and not the value at a specific distance, l, into the sample. However, the plots shown in Fig. 3 show that the proportionality exists for the data obtained. A summary of a linear correlation of the data by least squares is presented in Table 3. The correlation coefficients show that there is a proportionality between log(per cent extent of polymerisa- tion) and thickness of polymerising solution, although the fit of data is not exceptionally good. Each of the correlations predict that for an infinitely small

T A B L E 3 DATA FROM PLOT OF LOG (PER CENT EXTENT OF POLYMERISATION) VERSUS SAMPLE THICKNESS

Benzoin concentration Intercept a Slope Correlation (tool dm-3x 102) (log (% polymerisation)) (tara -1) coeff~ient

0 " 4 2 2 ' 0 2 - 0 . 4 9 - 0 - 9 7 8 1"58 2 " 0 9 - 0 " 2 4 - 0 . 9 8 5 4"21 2 . 0 6 - 0 . 1 9 - 0 - 9 8 1

12"7 1"95 - 0 " 1 6 - 0 . 9 7 9 2 1 - 2 1 -90 - 0 - 1 6 - 0 . 9 4 3

a Note: intercept of 2.00 corresponds to 100% polymerisation.

108 STEPHEN CLARKE, ROBERT A. SHANKS

O~

Slope of plot -0,1

Iog(% polym._)O. 2 versu=: thickness.

-03

i i . . . . . i0 . . . . .3,10 2

benzoin concentration / tool

Fig. 4. Effect of benzoin concentration on the slope of the plot of log(per cent polymerisation) versus thickness.

path length 100% polymerisation is approached (i.e. intercept -~log (100%)= 2). Another observation is the variation of the slope of the correlation with benzoin concentration. The slope of these plots is greatest for the lowest concentration of initiator. A plot of the slope of these correlations versus benzoin concentration is shown in Fig. 4. The curve obtained shows that as the photoinitiator becomes more concentrated the extent of polymerisation be- comes less susceptible to the effect of sample thickness.

CONCLUSIONS

By measuring the degree of polymerisation of butyl acrylate photoinitiated with benzoin, and using a range of sample thicknesses and benzoin concentra- tions it has been found that:

(1) The extent of polymerisation is greatest when sample thickness is least. (2) There is a maximum in the curve of extent of polymerisation versus

initiator concentration. (3) The optimum concentration of benzoin increases as the sample thickness

increases. (4) - log(per cent extent of polymerisation) is proportional to the thickness of

polymerising solution, and approaches 2, i.e. 100% as sample thickness approaches zero.

(5) As the photoinitiator becomes more concentrated the extent of polymer- isation becomes less susceptible to the effect of sample thickness.

ACKNOWLEDGEMENT

The authors are grateful to the Royal Melbourne Institute of Technology for a research grant.

INFLUENCE OF SAMPLE THICKNESS ON U V INrrIATED POLYMERISATION 109

REFERENCES

1. COOK, W. D., Proc. 11th AustraUan Polym. Syrup., Lorne, Australia, 1980, p. 14. 2. HOLMAN, R. J. and RUBIN, H., Z Oil Colour Chem. Assoc., 61 (1978) 189. 3. DEPOORTERE, M., DUCARME, A., DUFOUR, P. and MERCK, Y., J. Oil Colour Chem. Assoc., 61

(1978) 195. 4. CLARKE, S. R. and SHANKS, R. A., J. Macromol. Sci.- Chem., A14 (1980) 69. 5. HULME, B. E. and MARRON, J. J., The curing of coaangs by ultraviolet radiation, Tioxide

Technical Service Report, 1977, p. 8. 6. HUTCHISON, J. and LEDWrrH, A., Polymer, 14 (1973) 405.