Bengt Rånby

Bulk surface photografting process and its applications. II. Principal factors affecting surface photografting

The photografting polymerization reactivity of various monomers which can undergo free-radical chain polymerization is examined with benzophenone (BP) as the photoinitiator and LDPE film as the substrate. The results show that acrylate monomers have the highest photopolymerization and photografting reactivities, whereas methacrylate monomers have low reactivity due to the allylic hydrogen atoms in the monomer and no tertiary hydrogens on the polymer formed. Acrylonitrile has a significantly higher photografting efficiency than other acrylic monomers but a low polymerization reactivity. Vinyl acetate and N-vinyl-2-pyrrolidinone have a certain photografting potential, while styrene and 4-vinylpyridine have quite low photopolymerization and photografting reactivities. Using LDPE as the cover film and acrylic acid (AA) and BP as the monomer and photoinitiator, respectively, the photografting reactivity of seven polymer substrates was evaluated. The results show that these polymer substrates can be arranged in a sequence of decreasing photografting reactivity in the order nylon > PET > PP > LDPE > HDPE > OPP > PC. This sequence can be interpreted in terms of differences in surface hydrogens and differences in surface polarity. The self-screening effect plays a significant role in the intertface react ion system and is the main reason for the negative effect of increased photoinitiator concentration on the photografting process. An increase in the thickness of the liquid layer of the monomer and photoinitiator solution between the two substrates has a negative effect on the photografting process, whereas an increase in polymerization temperature in the range of 25-70°C has a positive effect. Added water favors the photografting polymerization of AA on the surface of polyolefins but acetone has a negative effect due to the different solvation of PAA. Adding multifunctional acrylate monomers increases the photografting efficiency to a large extent, whereas adding PVAc or acetyl-cellulose greatly reduces the photografting efficiency.

Surface modification and lamination of polymers by photografting

This paper describes experimental processes for UV initiated “surface photografting” of polymers: the batch process with vapor-phase transfer of initiator and monomer and the continuous process with liquid-phase transfer of initiator and monomer in a thin layer of solution on the surface of the substrate. The grafting effiiency is about 80% and the resulting grafted layers are very thin (less than 10 nm). The mechanism of “surface photografting” is discussed and proposed to be based on excitation of benzophenone (BP) as initiator to an S2 state by absorption of far UV irradiation (≈250 nm) and transition to a “superheated” “BPS1” state, which transfers its excess energy to CH groups at the substrate surface and forms BP triplet states (T1 and T2). The excited CH groups and the BPT1 and BPT2 states react by hydrogen abstraction and form radicals at the substrate surface. These radicals add monomer to grafted chains. “Surface photografting” is developed as a method for “photoinitiated lamination” of plastic films. A thin layer of initiator and monomer between two films is grafted to the two film surfaces by UV irradiation. Short linear grafted chains are formed which by continued UV-irradiation grow to branched chains which eventually fill the space between the two substrate films. The chain ends are terminated by addition of ketyl radicals from the benzophenone. Some growing chain ends may terminate by radical combination. With addition of a small amount of a multifunctional monomer, a network is formed which is grafted to the two film surfaces by main valence bonds. The “bulk surface photografting” gives laminate of high mechanical strength. When a functional film is grafted between two carrier films, laminates of selected and high barrier properties are formed which are of interest for application as packageing materials.

Comparative studies of reactions of commercial polymers with molecular oxygen, singlet oxygen, atomic oxygen and ozone—I. Reactions with cis-1,4-polybutadiene

Abstract The oxidation of cis -1,4-polybutadiene by molecular oxygen, singlet oxygen, atomic oxygen and ozone has been studied using u.v. and i.r. spectroscopic methods. Some possible implications of the results of oxidation in the presence of singlet oxygen (parallel free radical oxidation) and atomic oxygen (formation of NO 2 and its reaction with polymer) are discussed. Chain scission was observed during all types of oxidation. A new mechanism involving opening of double bonds and formation of biradicals has been considered in detail.

Photoinitiation performance of some ketones in the LDPE–acrylic acid surface photografting system

Abstract The photoinitiating performance of four kinds of ketones (alkyl-, alkyl phenyl-, phenyl- and di-ketones) and two aldehydes (formaldehyde and acetaldehyde) were examined by measuring their polymerization reactivity, grafting efficiency and surface wetting time with a simple technique, film interlayer photopolymerization (FIP), with LDPE as film substrate, acrylic acid as monomer and an HPM-15 Lamp (2 kW) as UV radiation source. Benzophenone, xanthone, acetophenone and 9-fluorenone are shown to be efficient photografting initiators for obtaining thick grafted layers, while anthraquinone, 4,4-dichlorobenzophenone, 4-benzylbenzophenone, benzoylformic acid and biacetyl are suitable photoinitiators for obtaining thin grafted layers. These results and subsequent analysis show that high triplet state energy, strong UV absorption, stable molecular structure and low initiating reactivity of the resulting ketyl free radical are decisive factors making the ketones efficient initiators for grafting applications.

Auto-initiating performance of styrene on surface photografting polymerization

Investigating the surface photografting polymerization of styrene (St), it was found that in the absence of photoinitiator and at high temperatures, a remarkable amount the St-grafted polymer is formed. The initiation performance of St was further confirmed by surface photografting polymerization of acrylic acid, where St was used as the photoinitiator. This finding is useful to investigate the reaction mechanism of aromatic compounds under UV radiation and develop photoinitiator-free polymerization systems.

THE ROLE OF SINGLET OXYGEN IN THE PHOTOOXIDATION OF POLYMERS

Abstract— This paper is a critical review of the singlet oxygen oxidation of polymers in solid state and in solution, referring in particular to polydienes, polystyrene and polyvinyl chloride). The singlet oxygenation of polydienes resulted in formation of allylic hydroperoxide groups with shifted double bonds, according to the “ene”‐type process. The singlet oxygenation of polystyrene and polyvinyl chloride) occurs only when the new double bonds are formed in these polymers. During dye‐photosensitized singlet oxygenation of polydienes in methanol‐benzene solution, a very rapid decrease in the molecular weight was observed. For the chain‐scission which occurs, not only singlet oxygen but several intermediates such as radicals, bi‐radicals and cation‐radicals which are formed during light fading of dyes are responsible. At the end of this paper a short review appears which has been focused on the quenching behavior of stabilizers, particularly interactions with singlet oxygen.

Surface photografting polymerization of vinyl acetate (VAc), maleic anhydride, and their charge transfer complex. II. VAc(2)

On the basis of our earlier work, extensive experiments were conducted to further investigate grafting vinyl acetate onto different films (10 types of films and 20 kinds of film combinations), using various photoinitiators, for example, BP, AQ, ITX, XAN, Irgacure 651 and 184, acetone, and cyclohexanone, irradiated by UV radiation. Several means were applied to confirm the grafted PE-g-PVAc film. The results showed that increasing UV radiation intensity is favorable for grafting polymerization and the far UV radiation plays a decisive role. Among the photoinitiators tested, BP is the best one, because it was found that BP initiates the grafting copolymerization efficiently, and other initiators are less effective. To various combined substrates, the affinity between the monomer and films, the reactivity of hydrogens on the substrates, and the transparence of the top film to UV radiation are main factors affecting polymerization. Both direct method (FTIR) and indirect methods (contact angle, color reaction, absorption of dye and water) verified the true product PE-g-PVAc.

Photopolymerization of dendritic methacrylated polyesters. I. Synthesis and properties

The so-termed starburst dendritic polyesters differ from classical polymers by their structural symmetry, high degree of branching, and high density of terminal functionality. The divergent synthesis of a series of polydisperse dendritic polyesters based upon pentaerythritol and 1,2,4-benzenetricarboxylic anhydride, modified by glycidvl methacrylate and methacrylic anhydride, is described, proceeding in a stepwise growth manner starting from an “initiator core,” building branched arms. These materials have about eight (two samples), 12, or 16 methacrylate double bonds (indicated as D-1-OH, D-1, D-2, and D-3, respectively) at the chain extremities on each molecule. The rheological behavior was investigated by measuring the dynamic viscosity of the dendritic polyesters. The viscosities of the dendritic polyesters largely decrease in the order D-1-OH, D-1, D-2, and D-3 at lower frequency, but the viscosities of D-1-OH and D-1 decrease dramatically at higher frequency. Using a viscometric cup, the flow time of the dendritic methacrylated polyesters was compared with epoxy acrylate-modified unsaturated polyester of a linear type and it was shown that the flow time for the former is only one-third of that for the latter. The thermal mechanical properties and glass transition temperatures of the UV-cured films increase with the degree of terminal functionality.

The role of far UV radiation in the photografting process

SummaryWith low density polyethylene (LDPE) film as substrate, polyethyleneterephthalate (PET) film as filter, and an high pressure mercury (HPM 15) lamp as UV radiation source, the function of far UV radiation was examined. The results show that when the far UV (200–300 nm) was eliminated, the rate of polymerization of acrylic acid in the interlayer between two LDPE films initiated by benzophenone (BP) dramatically decreased, and the grafting efficiency became close to zero. The decisive effect of the far UV is further confirmed in UV-VIS spectra measuring the hydrogen abstraction reaction of the excited BP. For polymerization systems containing allylic hydrogens, this effect is smaller, while hydroxycyclohexyl phenyl ketone (HHPK) and benzoyldimethylketal (BDK), which are typical photocleaving initiators, show little sensitivity to the far UV. Based on an energy graph of the excited states, the bond energies and the relevant photochemical reactions a tentative interpretation of the results has been made.

Melt-photografting polymerization of maleic anhydride onto LDPE film

Abstract Maleic anhydride (MAH) was photografted onto low density polyethylene substrates at temperatures above the melting point of MAH. The effects of some principal factors including irradiation temperature, photoinitiators, the intensity of UV radiation, and the far UV radiation on the grafting polymerization were investigated in detail. Percent conversion and grafting efficiency of the polymerizations were determined by the gravimetric method. The contact angles of the grafted film PE-g-PMAH against water and the FTIR spectrum of the grafted film were measured as characterization. The results show that the photografting polymerization of MAH can proceed smoothly at temperatures higher than the melting point of MAH; the far UV radiation and the intensity of the UV radiation affect the grafting polymerization greatly; the photoinitiators also have influence on the polymerization. According to the FTIR spectra, it is clearly confirmed that the grafted film samples contain anhydride groups. The contact angles demonstrate that the wettability of the grafted films is enhanced obviously, especially to those grafted film samples through hydrolysis.