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Electron spin resonance spectra of short-lived free radicals formed by addition of HO. to vinyl acetate, vinyl propionate, and 1,3-butadiene in aqueous solution were studied by means of a rapid flow technique. The observed spectra were assigned to be due to free radicals, CH2CHOCOCH3, CH2CHOCOC2H5, and CH2CH(CHCHS2), respectively. The observed hyperfine coupling constants give the spin density of 0.855, 0.848, and 0.629. Taking previously reported spin densities of propagating type free radicals of acrylic acid, methyl acrylate, and ethylene, as well as the present results into consideration, the order of the spin density is ethylene > methyl acrylate ≈ acrylic acid ≈ vinyl acetate > butadiene, which is in agreement with the order of the reactivity of the propagating radicals in addition polymerization. This is interpreted as an effect of resonance stabilization due to the side-chain substituent of the vinyl monomers. In addition, there are differences between vinyl ester and acrylic ester radicals which tentatively are interpreted as related to the hindered rotation of the endgroups of the growing chain radicals.

Trimethylolpropane monoallyl ether (TMP) was studied with an x-band electron spin resonance (ESR) spectrometer by applying a flow method, developed by Dixon and Norman, in which TMP was subjected to hydroxyl radical attack. The observed ESR spectra were assigned to two radicals formed by addition of hydroxyl radical to TMP and one by abstraction of hydrogen from the allylic group of TMP. The ESR spectra from copolymerizing systems of vinyl acetate (VAc) and methyl acrylate (MA) with TMP were also studied. In the system VAc-TMP, the ESR spectrum showed in addition to VAc radicals the presence of the three TMP radicals. In the system MA-TMP, the ESR spectrum was mainly due to TMP radicals in which the allylic radicals were dominant. This result is consistent with the observation that polymerization of MA is strongly retarded by the addition of TMP, which is interpreted as due to the high reactivity of MA radicals to TMP monomer.

Styrene (St) was photografted onto LDPE substrates with different methods including a one-step method, pre-irradiation method, and pre-grafting method. Percent conversion (CP) and grafting efficiency (GE) of LDPE/St polymerization systems were measured by the gravimetric method. The results show that the one-step method is not very effective, no matter which photoinitiator (BP, BPO, Irgacure 651 and ITX) is used. The pre-irradiation technology, by which the dormant groups are introduced onto the substrates in advance, is favorable relatively. If the pre-grafting method is employed, AA is the most preferred one among the monomers examined (AA, MA and MAH). In addition, it is found that in the absence of photoinitiators and at lower temperatures (lower than 50°C), no grafted polymer is produced; but at a high temperature (e.g., 80°C), a remarkable amount of grafted polymer of St is formed (GE=44.9%). The phenomena indicate that St is prone to be grafted onto LDPE film by auto-initiation.

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.

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.

Vinyl acetate (VAc) was grafted onto low-density polyethylene (LDPE) substrates by UV irradiation with benzophenone (BP) as the photoinitiator. BP preabsorbed film samples and BP precoated film samples were prepared in advance and applied as the substrates onto which VAc was photografted, together with the method in which BP was dissolved in VAc directly. In addition, the efficiency of the polymerizations applying the preirradiation technology was examined. The conversion percent, grafting percent, and grafting efficiency were determined by a gravimetric method. The contact angles of the grafted films against water were also measured. The results show that BP preabsorbing and precoating were favorable to grafting polymerization, especially the BP precoating method, which was due to its simple operation and the ease of controlling the amount of BP. The diffusion of BP and VAc through the substrates proved to be an important factor for grafting polymerization. Through UV irradiation, dormant groups can be introduced onto LDPE film, which may be activated again by UV irradiation or by heating, leading to the formation of free radicals. Grafting polymerization can be initiated during the activation process in the presence of monomer. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1426–1433, 2001

To photograft vinyl acetate (VAc) onto low density polyethylene (LDPE) film, suitable solvents should be determined in advance. Different reagents were used as solvents of VAc (acetone, benzene, carbon tetrachloride, cyclohexane, cyclohexanone, ethyl acetate, methanol, tetrahydrofuran (THF) and N,N-dimethylformamide (DMF). All graft polymerizations were completed by two-steps, i.e., benzophenone (BP) pre-coated film samples were prepared onto which VAc was grafted. To prepare film samples containing BP, BP should be dissolved in a suitable solvent. The effects of various solvents of BP on grafting were examined, including acetone, benzene, chloroform, 2,4-dioxane, DMF, ethyl acetate, n-hexane, methanol and THF. Percent conversion (CP), grafting efficiency (GE) and grafting percent (GP) determined by gravimetry and contact angles of grafted films against water showed that reagents have different effects on the graft polymerizations. Acetone is the most suitable as solvent of BP or VAc. The behavior of solvents seems closely related to the affinity with the substrate.

The photolytic products and a new photoreduction mechanism of benzophenone (BP) as a photoinitiator in the photocrosslinking of polyethylene (PE) and its model compounds (MD) have been studied by means of fluorescence, ESR, 13C and 1H NMR spectroscopy. The fluorescence spectra from the PE and MD systems demonstrate that the main photoreduction product of BP (PPB) is benzpinacol formed by the recombination of two diphenylhydroxymethyl (K•) radical intermediates. The ESR spectrum obtained from the UV irradiation of the MD/BP system gives positive evidence of K• radicals. Two new PPB products: an isomer of benzpinacol with quinoid structure, 1-phenyl-hydroxymethylene-4-diphenyl-hydroxymethyl-2,5-cyclohexa-diene and three kinds of α-alkylbenzhydrols have been detected and identified for the first time by 13C and 1H NMR spectroscopy from the MD systems. The latter could be formed by the reactions of K• radicals with alkyl radicals produced by hydrogen abstraction of the excited triplet state 3(BP)* from polyethylene or its model compounds. These results provide new experimental evidence for elucidating the photoreduction mechanism of BP in the photoinitiated crosslinking of polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 999–1005, 2000

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. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1522–1531, 2000

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.

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.

Based on extensive studies of photoinitiated reactions of synthetic polymers, processes for modification of sheets, films, filaments and yarn have been invented and developed. Using ultra violet irradiation, photocrosslinking of polyethylene melts as a continuous process on line with an extruder has been invented. Effective photocrosslinking has also been obtained for ethylene-propylene-diene (EPDM) elastomers and ultra high strength fibers of high density polyethylene (HDPE). A continuous process for surface modification of polymers by surface photografting has been invented and applied to strips of film, filaments and yarns. The inert surface of polyolefins, polyesters and polyamids, grafted with functional vinyl monomers, show decreased contact angle with water, increased adsorption of dyes and increased adhesion to other polymers. Photoinitiated lamination of polymer films and sheets has been invented and developed as a ”bulk surface photografting process”. Two or more films and sheets are interdispersed with reactive layers of monomer and initiator and then bonded together by photoinitiated grafting with ultraviolet irradiation. The crosslinked network formed is grafted to two adjacent polymer surfaces. This gives laminates of high lamination strength, high barrier properties, and promising commercial applications.

The molecular mechanism, the kinetics and the process development of photocrosslinking of diene elastomers, of high density and low density polyethylenes, and of new linear and dendritic unsaturated polyesters using UV-irradiation are described. New methods for modification of various polymers by surface photografting are presented, the effects on adhesion and adsorption, and the mechanism of the reactions related to the shortlived high energy states of aromatic ketones as initiators are presented and discussed. The development of surface photografting as a new method for UV-initiated lamination of various polymer films and sheets is described, the molecular mechanism and the kinetics of the reactions are discussed, and the mechanical and barrier properties of the laminates are presented.

Grafting of acrylic monomers onto cellulose substrates in acid aqueous medium using complex of Mn3+ ions as a redox initiator is described. Assuming that the /Mn3+/ ions react with aldehydes and vicinal diols, the rate of these reactions has been measured spectroscopically. (ESR and visible light) using model compounds. It is concluded that the /Mn3+/ grafting onto cellulose is a radical reaction initiated mainly by oxidation of aldehydes at reducing end groups and C-C bond scission of vicinal diols at the end groups and along the cellulose chains. The initiation rates of the Ce4+, /Mn3+/ and VO22+ redox ions are related to their oxidation potential.

This paper introduces a set of new concepts related to the macromolecular design and to the practice in the realm of lamination by photografting, with experimental studies of the synthesis, structure, and performance of various systems, including valence bond adhesion of the organic polymeric materials, hyperbranched macromolecules rooted to the surface of organic substrates, macromolecular networks rooted on and bridged between two organic substrates, and a synthetic route combining primary and secondary photografting processes for the preparation of supermolecular architectures. In successful photolamination experiments based on these ideas, hyperbranched macromolecules of large size (10–20 μm) were obtained after initiation with aromatic ketones, which undergo a photoreduction reaction. A range of different organic polymeric films have been laminated. T-peel tests show most of the laminates broke inside the substrates and not at the interface between adhesive and substrate. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1723–1732, 1997

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.

With 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.

In two previous articles, the preparation, modification with epoxy methacrylate and methacrylic anhydride, and properties of dendritic aromatic polyesters with structural symmetry, a high degree of branching, and a high density of terminal functionality were described. The resulting polyester resins were cured by addition of benzildimethylketal (Irgacure 651) as the photoinitiator and trimethylolpropane triacrylate (TMPTA) as the comonomer and irradiated with UV light as laminates with 35 wt % glass fiber mats. After short irradiation times (0.02-0.10 s) at room temperature, the laminates with excellent mechanical properties which are comparable with the thermally cured linear unsaturated polyester in styrene and UV-cured linear epoxy acrylate-modified unsaturated polyester composites were obtained. The addition of 10-40 wt % TMPTA had only small effects on the mechanical properties, except on tensile strength, which decreased about 15%. The dendritic polyesters have low viscosity and a much higher curing rate than those of corresponding linear unsaturated polyesters and are, therefore, of great interest for industrial applications.

The starburst dendritic polyester with about eight double bonds was found to polymerize rapidly in the presence of a 3% photofragmenting initiator, benzildimethylketal, at room temperature in air, reaching 75% conversion after 0.15 s UV irradiation of a μm film. The effects of a photoinitiator and comonomer on the photopolymerization process for methacrylated polyester resins were investigated by measuring the conversion of the unsaturation and physical properties. The degree of copolymerization of the unsaturation decreases in the case of resins with high functionality. A kinetic study of the UV-induced curing of the dendritic polyesters by evaluating gel content and belt speed has shown that the photopolymerization rate increases rapidly with increasing acrylic functionality of the dendritic polyesters, which is in favor of the theory of radical-chain polymerization. High pendulum hardness is obtained with dendritic polyesters of high functionality. However, the addition of 10-40 wt % multifunctional monomer trimethylolpropane triacrylate has a small effect on the curing efficiency after prolonged UV irradiation, which is different from conventional linear acrylated oligomers.

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. © 1996 John Wiley & Sons, Inc.

H- and Y-type crosslinks formed in the benzophenone-photoinitiated crosslinking of various model compounds of low-density polyethylene (LDPE) have been observed and identified for the first time by 13C NMR spectroscopy. The quantitative studies show that both H- and Y-links are formed during the photo crosslinking of dodecane and their concentrations are almost the same; the yield of H-links in the crosslinked isooctane is slightly greater than that of Y-links; in contrast, Y-links are dominant for the crosslinked 1-hexene. These results further support the conclusions from the study of photo crosslinking of LDPE and give new experimental evidence for elucidating the mechanism of the photo crosslinking of LDPE.

Photolysis products of benzophenone (BP) as a photoinitiator in the photo crosslinking of polyethylene and its model compounds have been studied by means of fluorescence spectroscopy, 13C and 1H NMR, etc. The results show that the main photolysis products of benzophenone are benzpinacol and its isomers with quinoid structures. In addition, a small amount of α-alkylbenzhydrols probably exist in photolysis products of BP. These results provide new experimental evidence for elucidating the mechanism of photo crosslinking of polyethylene.

Hyperbranched polymers differ from classical oligomers and polymers by their structural symmetry, high degree of branching and maximized terminal functionality density. The divergent synthesis of a series of polydisperse hyperbranched polyesters based upon pentaerythritol and 1, 2, 4-benzenetricarboxylic anhydride, modified by glycidyl methacrylate and methacrylic anhydride is described, proceeding in a stepwise growth manner starting from an "initiator core", building hyperbranched arms. These materials have about 8, 12 or 16 methacrylate double bonds at the chain extremities on each molecule. The resulting hyperbranched methacrylated polyester resins are expected to have high photocure rate than linear unsaturated polyester by addition of photoinitiator and comonomer.

The(odor) Svedberg* (1884-1971), Uppsala University, was the first professor of physical chemistry in Sweden. He started his research in 1904 as a colloid chemist. He prepared inorganic dispersions by electrical discharge through organic liquids, measured particle size and size distribution of colloids by diffusion and sedimentation rates and invented the ultracentrifuge for convection-free sedimentation of colloidal particles. In 1924 Svedberg applied the ultracentrifuge for studies of haemoglobin in aqueous solution. Unexpectedly he found that the protein was not a gel of associated small molecules. It sedimented as macromolecules of uniform mass (68 000 dalton), i.e. is monodisperse. In subsequent work Svedberg found that other proteins like ovalbumin (from egg white) and haemocyanins from snails and other invertebrates also are monodisperse or contain a few uniform macromolecular components. After extensive studies of soluble protein molecules from a large number of animals and plants, Svedberg classified the species and could trace the relations for the species from the molecular mass of their protein macromolecules. Studies of soluble polysaccharides from bulbs also showed well defined macromolecules of masses that could be related to the various botanical species. Other native substances like cellulose, other wood polysaccharides and starch were found to be of high molecular mass but polydisperse. Svedberg's research in the 1920's and 1930's gave strong experimental support for Staudinger's macromolecular concept although Svedberg, at the time of his own important discoveries, was not influenced by Staudinger. Svedberg's discovery of the protein macromolecules and their uniform molecular mass initiated the macromolecular research in biochemistry.

A photocrosslinking precess for polyethylene and its industrial irradiation apparatus has recently been developed and used successfully in the manufactre of thin wall crosslinked polyethylene-insulated wires. Polyethylene (PE) resin with the desired amounts of additives, such as photoinitiator, multifunctional crosslinker, and antioxidant is mixe homogeneously. The amixture of PE is granulated, and then extruded on a conductor wire. The coated PE layer in the melt is subsequently irradiated in a specially designed UV irradiation apparatus for 5 to 10 s. The studies show that the photocrosslinked PE-insulation high voltage television wires produced by this new technique possess excellent electrical and mchanical properties and much lower operating cost compared with wires produced by the electron beam crosslinking technique.

Surface photografting onto polymers is a new method for surface modification of sheet, film, filament and yarn. It was invented in our laboratories in 1985 and was developed as two processes: A batchwise process where photoinitiator and monomer are transferred to the substrate through vapor phase and the monomer is grafted to the surface of the substrate by UV irradiation; and a continuous process where photoinitiator and monomer in solution are transferred to the substrate in liquid phase by 'presoaking' a strip of film, a filament or a yarn in the solution and the monomer is grafted to the substrate surface by UV irradiation on line. The batch process has been applied to film, sheet and molded plates of various commercial polymers which have been surface photografted with acrylic acid (AA), acrylamide (AM), 4-vinyl pyridine (4-VP) and glycidyl acrylate (GA) using benzophenone (BP) as initiator. The grafted layers are analyzed by electron spectroscopy for chemical analysis (ESCA) and reflection IR spectr...

“Surface-photografting” with UV-irradiation of polypropylene (PP) fiber and film and high-strength polyethylene (HSPE) yarn has been made with acrylic acid (AA) and acrylamide (AM) as monomers and benzophenone (BP), 4-chlorobenzophenone (4-BCP) and hydroxylcyclohexylacetophenone (HHA) as photoinitiators using a new continuous method. The grafting reaction occurs in a thin liquid layer on the fiber or film substrate, which is presoaked in a solution containing initiator and monomer. After irradiation with a highpressure mercury lamp, HPM 15 (2 kW) from Philips, for 5–20 sec at about 50°C the fiber and film surface is completely covered with a 2–8 nm thick layer of grafted polymer, analyzed by electron spectroscopy for chemical analysis. The grafting efficiency is 70–80%, i.e. only 20–30% of the polymer is homopolymer which can be removed by extraction. The grafted layers are so thin that they cannot be analyzed as weight increase (

Electron spectroscopy (ESCA) and reflectance infrared spectroscopy (ATR) were used to measure surface oxidation of photocrosslinked polyethylene. The deterioration of bulk properties during an artificial weathering test was also measured. It was found that the UV-irradiation during crosslinking process decreases the stability of the material considerably. Three kinds of antioxidants and photostabilizers (hindered phenols, hindered amines, and organic sulfides) which have no absorption in the UV region of 300 to 400 nm were added before crosslinking to improve the stability of the crosslinked material. Neither the rate nor the degree of photocrosslinking at different depths of the sample were affected significantly by these additives. It was found possible to prevent the decrease in stability due to the photocrosslinking by using small amounts of a stabilizer, e.g. 0.05% Tinuvin 770 (hindered amine).

Synthesis and photocrosslinking of glass fiber-reinforced composites, based on epoxy acrylate-modified unsaturated polyester, have been investigated. The efficiency of the photocrosslinking process for glass fiber laminates of the polyesters that contain different comonomers has been evaluated by measuring mechanical properties. The data show (i) that 2 mm thick laminates, containing 30–40% glass fiber mats, are cured with UV irradiation for 15 sec at room temperature in air; (ii) that multifunctional acrylate or acrylether monomers added to the polyester resin improve the tensile and flexural properties of the photocured product to a greater extent than do added allylic monomers, due to the different photocrosslinking mechanisms; (iii) that the physical properties of the photocrosslinked laminates are well correlated with the molecular weight of the polyester, the amount of multifunctional monomer added, and the glass fiber content. © 1994 John Wiley & Sons, Inc.

The supermolecular structure of photocross-linked polyethylene (XLPE) has been studied by small-angle light scattering (SALS). The data show that the spherulitic structure of XLPE gradually deteriorates with increasing degree of cross-linking and increasing irradiation temperature from well-developed spherulites to rodlike aggregates and disordered lamellar structures. A photocross-linked sample of PE has lower crystallinity, smaller crystallites, and smaller spherulites than does the original sample. At high degrees of cross-linking, the SALS patterns show little or no spherulitic structure. Results with photocross-linked polyethylene demonstrate that the overall effect of cross-linking on the morphological structure is similar to that of an increase in molecular weight of the polymer. © 1993 John Wiley & Sons, Inc.

It has been known for hundreds of years that native organic materials like textile fibers from cotton and flax are degraded in sunlight and lose strength by outdoor exposure. This phenomenon was studied in the 1800s when fibers and textiles were bleached to increase their whiteness. The molecular mechanisms involved were not known at that time. The chain structure of cellulose was established much later through the macromolecular concept, formulated by Herman Staudinger in 1920, and generally accepted by scientists in the 1930s. On a macromolecular basis, important advances in understanding the chemical reactions involved in photodegradation of polymeric materials have been made since about 1940. Plastics, rubber, fibers, paint, and lacquers are all degraded in sunlight by basically the same free radical mechanisms extensively studied for 50 years [1]. Polymers show different inherent photostabilities when they are used as materials. Depending on their resistance to photodegradation, polymers can...

The kinetic characteristics and the reaction conditions of photocross-linking of low-density polyethylene (LDPE) in the melt have been studied using benzophenone (BP) and its derivatives such as 4-chlorobenzophenone (4-CBP) as photoinitiator and triallyl cyanurate (TAC) as cross-linker. The efficiency of the photoinitiated cross-linking system LDPEBP-TAC and various factors affecting the cross-linking process, such as photoinitiator and cross-linker and their concentrations, irradiation time, temperature, and atmosphere, and UV light intensity were examined extensively by determining gel content, IR, and UV spectra. It has been found (i) that LDPE samples of 2–3 mm thickness are easily cross-linked to a gel content of about 70% with a UV-irradiation time of about 15 s under optimum conditions; (ii) that the photoinitiating system of a suitable initiator combined with a multifunctional cross-linker such as 4-CBP–TAC can enhance the efficiency of photocross-linking reactions, especially by increasing the initial rate of cross-linking; (iii) that photocross-linking of LDPE should be carried out in the melt, which increases the penetration of UV light by decreasing the scattering by crystallites, and (iv) that the rate of cross-linking is proportional to the square of the light intensity. An increase in light intensity can decrease the concentration of photoinitiator required to reach a certain rate of cross-linking. The photocross-linking of polyethylene is promising for industrial applications. © 1993 John Wiley & Sons, Inc.

Morphological structure and other properties of polyethylene photocross-linked in the melt (XLPE) have been studied by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), density, and swelling measurements. The WAXD measurements show that the crystallinity of polyethylene decreases with increasing gel content in the XLPE samples and that the regularity and the size of crystals are affected by photocross-linking. DSC data confirm that photocross-linking occurs preferentially in the amorphous and interfacial regions and causes depression of the melting point and decreases the heat of fusion. The density of XLPE apparently decreases with increasing gel content, which is due to decreased crystallinity and lattice expansion. The cross-link density determined by the swelling measurements and evaluated using the Flory–Rehner equation correlates well with the changes in crystallinity and melting point. © 1993 John Wiley & Sons, Inc.

Two methods for modification of polymer surfaces by photoinitiated grafting have been developed and applied to films and fibers of synthetic polymers, e.g. polyethylene and polypropylene with acrylic monomers. In the batch process the substrate is enclosed in a cell containing initiator and monomer vapor. UV light through a quartz window initiates the grafting reaction by exciting the initiator (e.g. benzophenone). The grafting reaction is slow (1 to 3 min) due to the inefficient transfer of initiator and monomer through vapor phase. In the continuous process the substrate is presoaked in a solution of initiator and monomer and then drawn into a reactor “on line” where the substrate is irradiated by UV light through a quartz window. The grafting takes place in the very thin surface layer of solution on the substrate. The grafting efficiency is high (70–80% of the polymer formed is grafted) and the process is rapid (5–15 s due to the efficient transfer of initiator and monomer through the liquid phase. The continuous surface grafting process is promising for industrial applications.

Low density polyethylene (LDPE) is extruded, stretched, and photocrosslinked at different temperatures with different stretch rations in a continuous production line. Three different sequences are adopted: crosslinking before drawing, crosslinking after drawing, and crosslinking during drawing. The effect of drawing is studied by measuring the tensile properties. The increase of tensile strength and the decrease of elongation at break after stretching are related to chain orientation in crystalline morphology measured by X-ray diffraction. Thermal analysis by differential scanning calorimetry (DSC) supports these results. Owing to chain relaxation, the effects of orientation by stretching are significant only when the samples are drawn at a temperature not much higher than the melting point. Annealing of the drawn samples at a temperature above the melting point shows that the orientation in a crosslinked sample is retained for a much longer time than in an uncrosslinked sample. For the crosslinked samples, it is found that crosslinking after or during drawing gives a material with lower shrinkage upon heating; however, crosslinking before drawing gives a heat-shrinkable material.

High-strength polyethylene (HSPE), polypropylene (PP), poly(ethylene terephthalate) (PET), and poly(vinyl alcohol) (PVA) textile yarns have been surface-photografted with various functional monomers, such as acrylic acid (AA), acrylamide (AM), glycidyl acrylate (GA) and 4-vinyl pyridine (VP), by means of the continuous presoaking process developed. The dyeing of these surface-modified yarns with various textile dyes has been investigated. In general, considerable improvements of dyeability have been observed. The dye adsorption of the surface-photografted fibers is influenced by many factors, such as type of fiber, amount and properties of the functional monomer grafted on the surface of the fibers, type of textile dye, etc. The fibers surface-grafted with a monomer containing basic groups, such as acrylamide and 4-vinyl pyridine, are efficiently dyed with an acid dye. Conversely, a fiber surface-grafted with acidic functional monomer is easily dyed to deep shades with basic dyes. The dye adsorption increases monotonically with increasing grafting measured in ESCA spectra as relative intensities of relevant lines. The ungrafted HSPE, PP and PET fibers can be dyed to some extent with certain dyes. In the present work, the dye adsorption increased by 3.4 times for HSPE fiber grafted with GA and dyed with the metal complex dye IO, by 7.9 times for PP fiber grafted with AA and dyed with the basic dye MB, by 6.1 times for PET with AM and with the direct dye SL, and by about 15.3 times for PVA with VP and with the acid dye TE. Hochfeste Polyethylen- (HSPE), Polypropylen- (PP), Polyethylenterephthalat-(PET) und Polyvinylalkohol-(PVA) Textilgarne wurden unter Anwendung des entwickelten kontinuierlichen Einweichprozesses mit verschiedenen funktionellen Monomeren, wie beispielsweise Acrylsaure (AA), Acrylamid (AM), Glycidylacrylat (GA) und 4-Vinylpyridin (VP) oberflachenfotogepfropft. Die Einfarbbarkeit der oberflachenmodifizierten Garne mit verschiedenen Textilfarbstoffen wurde untersucht. Im allgemeinen wurde eine erhebliche Verbesserung der Einfarbbarkeit beobachtet. Die Farbstoffaufnahme der oberflachenfotogepfropften Fasern wird von vielen Faktoren beeinflust, beispielsweise von der Art der Faser, der Menge und den Eigenschaften des auf die Oberflache der Fasern gepfropften funktionellen Monomeren, der Art des Textilfarbstoffs usw. Die Faseroberflachen, auf die ein Monomeres mit basischen Gruppen wie Acrylamid und 4-Vinylpyridin gepfropft wurde, lassen sich mit einem sauren Farbstoff wirkungsvoll einfarben. Dagegen last sich eine Faseroberflache, auf die ein saures funktionelles Monomeres gepfropft wurde, leicht mit basischen Farbstoffen in tiefen Farbtonen einfarben. Die Farbstoffaufnahme steigt stetig mit zunehmender Pfropfung, in ESCA-Spectren als relative Intensitaten relevanter Linien gemessen. Die nicht gepfropften HSPE-, PP-, und PET-Fasern konnen mit bestimmten Farbstoffen in gewissem Mase eingefarbt werden. Bei der vorliegenden Arbeit erhohte sich die Farbstoffaufnahme bei HSPE-Fasern, die mit GA gepfropft und mit dem Metallkomplexfarbstoff IO eingefarbt wurden, um das 3,4fache, bei PP-Fasern, die mit AA gepfropft und mit dem basischen Farbstoff MB eingefarbt wurden, um das 7,9fache, bei PET mit AM und dem Direktfarbstoff SL um das 6,1fache, bei PVA mit VP und dem sauren Farbstoff TE um das 15,3fache.

A new continuous method is developed to crosslink polyethylene, based on a photoinitiated radical reaction. An experimental production line for low density polyethylene (LDPE) extrudate is constructed with a Brabender® extruder and an “Irradiation Unit” on line. Some new initiators of benzophenone derivatives are prepared to overcome the volatility of benzophenone and increase its compatibility with the polymer matrix. Extraction and gel swelling experiments are performed to characterize the crosslinking of the product and the results are quite satisfactory: under the conditions used, crosslinking can be optimized to about 65% gel content within 5 to 15 s irradiation time, corresponding to a production rate of about 1.5 to 0.5 m/min, respectively. The dynamic mechanical properties of the crosslinked polymer are measured through the melting region and the effects of crosslinking are demonstrated. Without affecting the rate of crosslinking, the stability against thermal oxidation of the crosslinked products is greatly improved by adding stabilizers to the compounds before crosslinking.

Photoinitiated surface grafting of acrylic acid (AA), acrylamide (AM) and 4-vinyl pyridine (VP) onto poly(ethylene terephthalate) (PET) fibers (a commercial textile yarn) has been studied using benzophenone (BP) as photoinitiator. A continuous process as previously described has been applied, which involves presoaking of the PET yarn in a solution of initiator and monomer in acetone and UV irradiation in nitrogen atmosphere. The resulting grafted polymer on the fiber surface has been analyzed by ESCA, titration of carboxy groups (grafted AA), and dye absorption. The relative ESCA intensities (RI) of O1s/C1s and N1s/C1s are used as measure for grafted AA, AM and VP, respectively, after recording the RI-values for ungrafted fibers. For grafting with AA, the RI-values increased from 32.8% (background) to 48.6% after 20 s irradiation time. The amount of carboxy groups measured by titration increased from 0.045 to 0.106 mmol/m2. Assuming an even coverage of grafted AA polymer, this means a grafted layer of 4.8 nm thickness. After grafting, the adsorption of the dye Crystal Violet (CV) from aqueous solution increased by about 3 times. With AM as monomer, the RI-values increased from 2.6 (background) to 14.8% and the adsorption of a direct dye Sirius Lichtbordo B-LL increased by about 6 times. With VP as grafted monomer, the RI-values increased from about 2.6 (background) to 5.1% and the adsorption of the direct dye increased by about 4 times. Die lichtinduzierte Oberflachenpfropfung von Acrylsaure (AA), Acrylamid (AM) und 4-Vinylpyridin (VP) auf Polyethylenterephthalatfasern (PET) (handelsubliches Garn) mit Benzophenon (BP) als Initiator nach einem bereits beschriebenen, kontinuierlichen Verfahren, bestehend aus der Impragnierung der Fasern mit einer Losung aus Initiator und Monomerem in Aceton und anschliesender UV-Belichtung, wurde untersucht. Die auf der Faseroberflache gebildeten Pfropfcopolymeren wurden mittels ESCA, Titration der Carboxygruppen (gepfropfte AA) und Farbstoffadsorption charakterisiert. Dabei dienten die relativen ESCA-Intensitaten (RI) O1s/C1s und N1s/C1s als Mas fur die Pfropfung von AA, AM und VP, unter Berucksichtigung der Intensitaten der ungepfropften Fasern. Fur AA stiegen die RI-Werte von 32,8% (Blindprobe) auf 48,6% nach 20 h Bestrahlung. Die titrimetrisch bestimmte Konzentration der Carboxygruppen stieg von 0,045 auf 0,106 mmol/m2. Unter der Annahme einer gleichmasigen Pfropfcopolymerschicht ergibt sich daraus eine Beschichtungsdicke von 4,8 nm. Nach der Pfropfung stieg die Adsorption von Kristallviolett (CV) aus wasriger Losung um etwa das Dreifache. Fur AM stiegen die RI-Werte von 2,6 (Blindprobe) auf 14,8% und die Adsorption des Direktfarbstoffs Sirius Lichtbordo B-LL um etwa das Sechsfache. Fur VP stiegen die RI-Werte von 2,6 (Blindprobe) auf 5,1% und die Adsorption des Direktfarbstoffs um etwa das Vierfache.

Photoinitiated surface grafting of acrylic monomers has been carried out onto high strength polyethylene (HSPE) yarn by means of a continuous process. The grafting reaction is initiated by UV irradiation of the yarn after presoaking in an acetone solution of initiator and monomer. Four initiators, benzophenone (BP), 4-chlorobenzophenone (4-CBP), 2-hydroxy-2-cyclohexylacetophenone (HHA), and 2,2-dimethoxy-2-phenylacetophenone (DMPA), and two monomers, acrylic acid (AA) and acrylamide (AM), have been used. After short irradiation time (10 to 20 s) successful grafting is obtained, as shown by ESCA and IR-ATR spectra, dye adsorpotion from aqueous solution, and measurements of adhesion of single filaments to epoxy resin. Grafting efficiency of 74% has been reached for AA as monomer (26% is homopolymer). The tensile strength and modulus of the HSPE yarn are retained in the grafting process. The degree of surface grafting is mainly a function of structure and concentration of monomer and photoinitiator in the presoaking solution and of the irradiation conditions used. Increasing irradiation time gives increasing amounts of grafted polymer up to a certain limiting value. The reactivities of the four initiators have been compared showing the highest grafting yields of AA with BP and of AM with 4-CBP as photoinitiator. AA grafted HSPE yarn can be dyed to rather deep color by dipping in an aqueous solution of Crystal Violet. Increased dye absorption by a factor of up to seven has been measured for yarn grafted with AA to the maximum level obtained. The filaments of the grafted yarn show increased adhesion to epoxy resin by a factor of up to five compared with the ungrafted filaments. Die photoinitiierte Oberflachen-Pfropfung von Acrylmonomeren auf hochfeste Polyethylen-Fasern (HSPE-Fasern) wurde in einem kontinuierlichen Prozes durchgefuhrt. Die Pfropfreaktion wurde durch UV-Bestrahlung der Fasern initiiert, nachdem diese mit einer Losung des Photoinitiators und des Monomeren in Aceton vorimpragniert worden waren. Es wurden vier Initiatoren, Benzophenon (BP), 4-Chlorbenzophenon (CCBP), 2-Cyclohexyl-2-hydroxyacetophenon (HHA) und 2,2-Dimethoxy-2-phenylacetophenon (DMPA), sowie die zwei Monomeren Acrylsaure (AA) und Acrylamid (AM) untersucht. Bereits nach kurzer Bestrahlungszeit (10 bis 20 s) konnte eine erfolgreiche Pfropfung mittels ESCA und IR-ATR sowie anhand von Farbstoffadsorption aus wasriger Losung und Adhasions-Versuchen an Epoxidharz mit Einzelfasern nachgewiesen werden. Mit AA als Monomerem konnte eine Pfropfausbeute von 74% erreicht werden (26% Homopolymer). Zugfestigkeit und Modul der HSPE-Fasern wurden durch die Pfropfung nicht verandert. Der Oberflachen-Pfropfungsgrad hangt im wesentlichen von Struktur und Reaktivitat des Monomeren und des Photoinitiators in der Vorimpragnierungslosung und von den Bestrahlungsbedingungen ab. Durch Verlangerung der Bestrahlungszeit konnte die Menge an aufgepfropftem Polymerem bis zu einem bestimmten Grenzwert gesteigert werden. Der Vergleich der vier untersuchten Photoinitiatoren zeigte die groste Pfropfausbeute von AA mit BP und von AM mit 4-CBP. Mit AA gepfropfte HSPE-Fasern konnten durch Eintauchen in eine wasrige Kristallviolett-Losung um bis zu siebenfach starker eingefarbt werden, als dies ohne Pfropfung moglich ist. Auch die Adhasion an Epoxidharz konnte durch die Pfropfung um das funffache gesteigert werden.

ESR spectroscopy combined with spin-trapping technique is a selective and powerful method for studies of reaction mechanisms and intermediates in free radical reactions of polymer systems. The method has been applied to several problems in our research program of which three are presented in this paper. - The mechanism of photo-initiation with aromatic ketone/tert. amine and subsequent photocuring of acrylic coatings has been largely resolved. - The mechanism of photocrosslinking of ethylene-propylenediene copolymers (EPDM elastomers) studied by model compounds has been shown to involve formation of allyl radicals in the diene groups and crosslinking by allyl radical combination. - The photoinitiated crosslinking of polyethylenes (LD, LLD and HD) has been studied using model compounds. It involves formation of primary alkyl and allyl radicals which crosslink the polymer by radical combination. - The mechanism of photocrosslinking indicates a second order reaction, i.e. rate of crosslinking depends of the square of irradiation intensity which has been verified experimentally. This result is the basis for development of rapid crosslinking processes of technical and industrial interest.

The kinetics of the reaction in photocrosslinking of the low density polyethylene-benzophenone-triallyl cyanurate (LDPE-BP-TAC) system was studied by theoretically deriving a kinetic equation and comparing it with the results from crosslinking, extraction and swelling experiments. It was found that the rate of crosslinking is proportional to the square of light intensity. The roles of the crosslinker (TAC) were to capture the excited initiators efficiently to form free radicals and to act as a carrier of the active radicals. The ratio of degradation/ crosslinking was extrapolated to be around 0.28, and compared with 0.44 in the γ-irradiation process for the same material.