Chapter 8
Role of Homopolymer Suppressors in UV and Radiation Grafting in the Presence of Novel Additives Significance of Processes in Analogous Curing Reactions Downloaded by UNIV OF CINCINNATI on November 10, 2014 | http://pubs.acs.org Publication Date: April 13, 1993 | doi: 10.1021/bk-1993-0527.ch008
P. A. Dworjanyn, J. L. Garnett, M. A. Long, Y. C. Nho, and M. A. Khan Departments of Chemistry and Industrial Chemistry, University of New South Wales, Kensington, New South Wales 2033, Australia Recently developed novel additives for accelerating grafting reactions initiated by ionising radiation and UV are summarised. These include mineral acids, inorganic salts like lithium perchlorate and polyfunctional monomers, also combinations of these. Using the grafting of styrene in methanol to cellulose/polyolefins as model systems, homopolymerisation is shown to be a competing detrimental reaction in the presence of these additives. Inclusion of salts such as copper and ferrous sulfates retard homopolymerisation and lead to unexpected enhancement in grafting under some experimental conditions. A novel mechanism involving partitioning of reagents between monomer solution and substrate is proposed to explain both the accelerative effect of additives in grafting and also the role of salts in the retarding of homopolymerisation. The importance of this work in the related fields of EB and UV curing including bothfreeradical and cationic processes is evaluated particularly the mechanistic role of multifunctional acrylates as common components in both processes. The significance of concurrent grafting during curing is discussed, particularly its relevance to commercial processing involving recycling of radiation cured products. The use of novel additives for enhancing yields in grafting reactions initiated by ionising radiation and UV is of interest both practically and fundamentally (1-5). Mineral acids (1), inorganic salts (6) and polyfunctional monomers (PFMs) have been used for this purpose (7). Synergistic effects involving acids and salts with PFMs, particularly multifunctional acrylates (MFAs) and methacrylates (MFMAs) have also been reported (7). A variety of backbone polymers and monomers have been investigated in these studies (1,7-10,11). A novel mechanism to explain the behaviour of these acid and salt additives involving partitioning effects has been developed (12). This work is of value in a preparative context since, in the presence of additives, lower doses of radiation and UV are required to achieve a particular percentage graft. Thus the additive technique is particularly useful for grafting reactions involving radiation sensitive backbone polymers typically cellulose, the polyolefins and PVC, and also monomers of low reactivity. In all of these additive studies homopolymer formation remains a significant competing detrimental reaction. Methods for overcoming this problem involve the use of 0097-6156/93/0527-0103$06.00/0 © 1993 American Chemical Society
In Irradiation of Polymeric Materials; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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IRRADIATION OF POLYMERIC MATERIALS
copper, iron (12-15) and eerie salts (16). Such compounds generally reduce both homopolymer and grafting yields, the former preferentially, thus overall there is an increase in grafting efficiency in the presence of these homopolymer suppressors. However under these conditions, higher doses of radiation are generally required to achieve a particular percentage graft and compensate for the lower yields in the presence of the suppressors. In the present paper the effect of these inorganic salts as homopolymerisation suppressors in the presence of acid, salt and PFM additives is reported using the grafting of styrene in methanol to both polypropylene and cellulose as model systems. UV and ionising radiation are used as initiating sources. Under some conditions, an actual enhancement in grafting yield with homopolymer reduction is observed. From the results, a refinement in the original partitioning mechanism to explain the additive effect is proposed. In addition the partitioning concept is extended to include an explanation of the role of homopolymerisation suppressors in these grafting processes. The possible significance of the work in analogous radiation curing processes including cationic systems is discussed. Experimental The procedures used for grafting were the following modifications of those previously reported (1). For the irradiations, polypropylene film (isotactic, 0.10 cm thickness, 5.0 χ 3.0 cm) or strips of cellulose (Whatman No 41 double acid washed chromatography filter paper) of comparable size were fully immersed in the monomer solutions and irradiated in either a 1200 Ci cobalt 60 source or with a 90 W medium pressure mercury vapour lamp. Homopolymerisation was determined by previously published methods (7J2). Percent grafting efficiency was then estimated from the ratio of graft to graft plus homopolymer χ 100. For the radiation curing experiments, appropriate resin mixtures containing oligomers, monomers, flow additives and photoinitiators (UV) were applied to the substrate as thin coatings, the material placed on the conveyor belt and exposed to EB and UV sources. Two UV systems were used namely a Primarc Minicure unit and a Fusion unit with lamps of 200 W/inch. Two EB facilities were utilised namely a 500 KeV Nissin machine and a 175 KeV ESI unit. Results Radiation Grafting Using Acid Additives with Homopolymer Suppressors. The results in Table I for the grafting of styrene in methanol to polypropylene show that when sulfuric acid is present in the monomer solution, the well known acid enhancement effect is observed at certain concentrations, particularly in solutions corresponding to the Trommsdorff peak (1,12). When copper sulfate is used as additive the following observations can be made, (i) In the absence of acid, grafting yields are reduced at all styrene concentrations studied, particularly in the more dilute solutions. Homopolymer yields are also strongly reduced from both the visual observation of the monomer solutions after irradiation and also from the polystyrene precipitation studies from these same solutions, (ii) In the presence of acid a novel result is observed with C u . Not only is homopolymer reduced, grafting is enhanced at all monomer concentrations studied and there is a synergistic effect with acid and copper sulfate up to monomer concentrations of 40% (the gel peak). This synergistic effect is independent of the salt structure in terms of the salts studied (cupric sulfate, ferrous sulfate and Mohr's salt), although copper appears to be marginally the best for the enhancement process (Figure 1). For these three salts optimum grafting yields for 30% monomer solutions occur at salt concentrations of between 5xl0 M and lxlΟ" M, with 5xl0 M a compromise (Table II) in acid concentrations of 2xl0 M (Table ΠΙ). 2+
-4
-3
2
_1
In Irradiation of Polymeric Materials; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
8.
Role of Homopolymer Suppressors
DWORJANYN ET AL.
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Effect of Copper Sulfate and Sulfuric Acid on Radiation Grafting of Styrene to Polypropylene
Table I
a
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Styrene (%v/v) 20 30 40 50 60 Cu Table II.
2+
Graft (%) H+ Cu 4.6 14 25 40 41 108 32 73 27 40
Control 10.6 42 67 64 51
2+
C u + H+ 11.5 64 94 41 45
2+
= CuS0 .5H 0 (5 χ 10-3M); H+ = H S 0 (0.2M). 4
2
2
4
Effect of Canonic Salt Concentration on Radiation Grafting of Styrene to Polypropylene in Presence of Acid 3
Graft (%) M+H+ + H+ F e + H+ 53 59 47 64 56 35 70 36 3.3 67 5.4 1x10-1 Conditions as in Table I; styrene in methanol (30% v/v).
Canonic Salt (M) 5x10^ 5x10-3 lxlO"
Cu
2+
2+
2
3
Table ΙΠ. Effect of Acid Concentration on Radiation Grafting of Styrene to Polypropylene in Presence of Cationic Salts
3
H S0 (M) 0 5xl0lxlO2X10-
Graft (%) M Fe Cu 34 41 43 29 29 52 40 50 64 56 68 63 58 49 3x10-1 Conditions as in Table I; styrene in methanol (30% v/v). 2
4
2+
2+
2
1
1
a
UV Grafting with Acid Additives and Homopolymer Suppressors. Two categories of UV grafting are relevant to this discussion. The distinction involves the use of photoinitiator to accelerate the rate of grafting. Although the rates of grafting are slower in the absence of photoinitiator, under these conditions the possibility of photoinitiator contaminating the final product copolymer is eliminated. For some applications this is important. Photoinitiator Absent. The data in Table IV show that photografting styrene in methanol to polypropylene without photoinitiator is low under the UV conditions used confirming previous work (17). Inclusion of mineral acid in these grafting solutions leads to an increase in photografting at almost all monomer concentrations studied,
In Irradiation of Polymeric Materials; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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IRRADIATION OF POLYMERIC MATERIALS
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1004
Styrene
(%
VW)
Figure 1 Comparison of Copper Sulfate with Ferrous Sulfate and Mohr's Salt in Radiation Grafting Styrene in Methanol to Polypropylene in Presence of Acid, (o) control; ( · ) C u + H+; (a) F e + H ; (D) Mohr's Salt + H+ Radiation dose of 2.5x103 Gy at 5.2x1ο Gy/hr; Cationic salts (5xl0' M); H+ = H S 0 (0.2 M). 2+
2+
+
2
2
3
4
particularly with the monomer solution corresponding to the Trommsdorff peak (50%). Addition of copper sulphate to these acidified monomer solutions lowers the grafting yield marginally at the lower concentrations up to 50%, except for the monomer solution corresponding to the Trommsdorff peak (30%) where a significant enhancement is observed. In the presence of acid the position of the Trommsdorff peak shifts from 50% to 30% monomer. Table IV. Effect of Photoinitiator, Acid and Cationic Salt in Photografting Styrene in Methanol to Polypropylene 2
Graft (%) Styrene Without Β With Β (%v/v) N.A. H+ C u + H+ 20
