Polivinil asetat kökenli termoplastik yapıştırıcıların radyasyonla çapraz bağlanarak iyileştirilmesi

Abstract

ÖZET Yapıştırmanın diğer birleştirme yöntemlerine göre önemli üstünlüklerinin bulunması nedeniyle yapıştırıcı maddelerin geliştirilmesi kimya endüstrisinin öncelikli konuları arasında yer almaktadır. Yapıştırıcıların da üyesi olduğu polimer malzemeler grubu üzerinde, radyasyon yardımıyla bir çok yapı değişikliği oluşturulabileceği bilinmektedir. Çalışmada sıvı faz içinde bulunan, termoplastik tür yapıştırıcılardan polivinilasetat ve polivinilalkol grupları üzerinde radyasyon yolu ile çapraz bağlanma sağlanarak özellik iyileştirilmesi gerçekleştirilmiştir. Co-60 kökenli gama ışınları kullanılarak 5-75 kGy aralığında ışınlanan 5 tür yapıştırıcı ve PE daha sonra çeşitli fiziksel, kimyasal ve yapışma deneylerine tabi tutulmuş ve özellik değişimleri saptanmıştır. Yapılan deneylerde viskozite, çapraz bağlanma oranı, IR spektrumları, NMR spekrumları, elementel analiz değerleri, pH değerleri, E modülü, tan8 değerleri, ıslak tutma yeteneği, yapışma gelişimi, kopma mukavemeti, çentik-darbe tokluğu, soyulma mukavemeti, yumuşama sıcaklığı bölgesi gibi özellikler incelenmişlerdir. Sonuçta, artan dozlara paralel olarak yapışma hızının arttığı, yapıştırıcı mukavemet özelliklerinin yükseldiği, sıcaklık ve çözünme dirençlerinin arttığı belirlenmiş, özellikle 15-25 kGy bölgesinde optimum kullanım değerlerinin ortaya çıktığı saptanmıştır. VI

UPGRADING OF PVAC BASED THERMOPLASTIC ADHESIVES BY RADIATION CROSSLINKING SUMMARY Crosslinking of thermoplastic polymers in order to enhance their material properties like strength, heat and solvent resistance is a well known practice. Analogously crosslinking of thermoplastic adhesives may yield products with better adhesive qualities such as improved cohesive strength, better wet tack, shorter bonding time and higher blocking temperature range. Crosslinking of polymers can be achieved by chemical as well as physical means, whereas usage of ionising high energy radiation, represents the most effective method of the latter. This dissertation comprises experimental studies on radiation crosslinking and performance evaluation of two types of thermoplastic adhesives, namely the polyvinylacetate (PVAC) group and its hydrolysis product the polyvinyl alcohol (PVAL) group and also on polyethylene (PE) dispersions. The latter being a compound and not an adhesive in its own right. It is aimed and shown that radiation crosslinking is a versatile method for obtaining better adhesive qualities for the mentioned species. Experiments include irradiation of the specimens with and without hydrogen peroxide enclosure, evaluation of the changes in their physical and chemical nature and consequently, appraisal of their adhesive properties. The present work is unique in such a way that it treats adhesive crosslinking by irradiating formulated aqueous adhesive dispersions and solutions. Similar previous works either employed solid hot melt granules or readily spread dry films of liquids. Literature of radiation crosslinking on bulk adhesives in aqueous medium, prior to application was not encountered.The chosen types of adhesives are very popular and primarily used by the consumer in liquid form, so it is technically interesting to determine crosslinkability in liquid medium and consequences of it. EXPERIMENTAL Materials Six types of samples have been employed. PVAC homopolymer (PVAC), PVAC dibutyl maleate copolymer (PVAC-MA), PVAC - ethylene copolymer (PVAC-E) and polyethylene (PE) are emulsions with a solid content of approximately, 55-57 % for PVAC group and 40 % for PE. The remaining two samples are 12 % aqueous solutions of polyvinyl alcohol, a plain version (PVALl)and another with addition of 9 % of dextrin(PVAL2). Irradiation Formulated and prepared sets of adhesive samples have been irradiated in small boxes at a (Co-60) gamma irradiation facility with a dose rate of l kGy / hr and at 11 different dose levels. Additionally some samples have been irradiated under enclosure of H2O2 ( wt. % ranging 3-10) at 10 different dose levels. Also control dried adhesive films have been irradiated at 4 different doses. Physical and Chemical Tests - Gas emission determination - pH values measurement - Measurement of dielectric loss tan5 - Sol and gel percentage measurements - IR spectra - Viscosity measurement and visual assessment - Shear modulus evaluation - NMR spectra - Elemental analysis Adhesive Performance Tests -Clarity and continuity of dry film (TAPPI Spec.) - Wet tack test (TAPPI Spec.) - Rate of bond formation (TAPPI Spec.) - T - Peeling test (ASTM D - 1876)- Shear strength test (ASTM D - 0905) - Impact strength test (ASTM D - 0950) - Blocking test (ASTM D - 1146) Results and Discussion Weighing of samples before and after irradiation in order to detect any gas emission didn't show significant weight differences. Also no gas pressure in the Erlenmeyer flasks was detected. pH value of low crosslinkable samples moved slightly towards neutral with increasing doses for (PVAC, PVAC-MA, PVAL2). Whereas for high crosslinkable samples (PVAC-E and PVAL1) the pH values deflected towards higher acidity or alkalinity. This can be explained as: In the low crosslinkable species more electrons, ions and radicals remain unreacted and as a result of these neutralising of the radiolysis products of water occur (i.e., OH, H, and peroxide radicals) and thus the general ion concentration is lowered. H2O2 added samples didn't show any significant change. Dose measurement with PMMA dosimeters showed following irradiation doses: (in kGy) 4.6, 10.5, 12.1, 16, 21.4, 24.5, 28.8, 31.5, 46.7, 57, 77; for dry control samples doses were 10.5, 16, 28.8 and 46.7 kGy. For H202 added samples 6.0, 8.6, 13.6, 16.2, 23.4, 28.9, 36.6, 59.4, 61.5, 95.3 kGy. Dielectric loss tanS measurements have been carried out only for PVAC group samples. The values went through a maximum at dose ranges between 10-20 kGy, coinciding with the shear strength maximum. This condition denotes that increasing dipole activities also indicates higher adhesive strength, which indeed is in compliance with the adhesion theory. Gel measurement was carried out by a soxhlet extractor. Ethyl acetate for dissolving the PVAC group samples and water for PVAL group. At 77 kGy PVAC was cross linked to 45 %, PVAC-MA to 29%, PVAC-E to 95%, PVAL plain to %88 and PVAL + Dextrin to 35%. Dextrin acts partly as an inhibitor for crosslinking. Dry irradiated control samples had slightly higher gel percentages (10-12 % higher) than the gel values of the wet specimens of the same dose. This fact denotes that the shielding effect of water is not significant.IR Spectrograms showed every characteristic absorbencies of the relating molecular subgroups, however a particular absorbency difference for a single group was not detectable. Instead it could be observed clearly that general absorbency decreased with increasing crosslinking degree at PVAC group adhesive. This can be attributed to the more rigid cross-linked and steric, structure which somehow restricts the atomic vibrations. Surprisingly the opposite happened for the PVAL's where the absorbency increased with croslinking degree. Visual assessment revealed an elastomeric hydrogel structure for irradiated PVAL's. Such an elastomeric structure conversion leads to higher torsional vibrations at the subgroups of the polymer chain and hence higher absorbency. For hydrogen peroxide added specimens NMR spectra were obtained and elemental analysis conducted. NMR spectra have showed essential hydroxylation about (2.7-6.0 %) of the protective colloid and / or the polymer. Elemental analysis also indicated an increase of hydroxyl groups though lesser accurate. Viscosity measurement with Brookfield viscometer showed in general a viscosity increase with increasing crosslinking degree. Beginning with a crosslinking degree of 25% the liquid is gradually converted into a pseudoplastic hydrogel structure which seems to be rubbery elestomeric for the PVAL group, stiffer and less rubbery elastomeric for the PVAC group. Applicability as adhesive on a substrate begins to be impossible beyond 40-45 % crosslinking degree, where the consistency is interrupted and wettability worsened. For H2O2 added specimens viscosity broke down to a tiny fraction of the initial value. The stability of the dispersions also deteriorated. Shear modulus evaluation was attempted by interpretation of the shear stress-strain diagrams. The moduli were calculated for the PVAC group of adhesives and a general decrease of G values with increasing degree of crosslinking has been observed. It is generally assumed that with an increase of crosslinking degree the viscosity component of the shear values decreases and elastic shear modulus increases. Evaluating the stress strain gradient confirms this assumption until a certain cross link degree (app. 20%) is established. Afterwards a definite decrease is observed which can be explained with conversion to low modulus rubber elasticity.Appraisal of adhesive properties Clarity and film continuity was not impaired at all until 20- 30kGy doses, beyond that range hazy and discontinuous films occur. Wet tack refers to quick grip of the adhesive in wet condition and is an indispensable quality for high speed machine application. As wet tack is more function of viscosity and wettability, tests showed a sharp increase in the (0-30 kGy) range and further a slight decrease because of worsened wettability for PVAC's. At PVAL's however no subsequent decrease appeared because of elastomeric structure change. Rate of bond formation analogously increased in the (0 -40 kGy) range and then fell slightly, probably attributed to deterioration in wettability. It can be said that setting speed of the adhesive rises sharply with crosslinking degree, provided the adhesive remains liquid. T-peeling test was conducted on elastic but hard to wet surfaces. PVAC and PVAC-E lost their wettability (i.e., lower peeling strength) slightly with rising crosslinking degree, probably because having got a less flexible structure, whereas PVAL and PV AC-MA gained in peeling strength. This can be explained in the new structural formation. PVAL have been converted to a typical elastomeric substance with low modulus where molecules have grater freedom of rotation that permits them to conform to the substrate ; and for PVAC-MA no cross-bond occurred at the long dibutyl maleate side chains which can move freely and adhere to any substrate. T-peeling test was also applied for aged (one year) bonds and for peroxide containing samples. Aged bonds proved to have gained in strength up to 30% whereas poor results were obtained for H2O2 added samples (25-60% of the normal peel strength values). Shear strength test carried out on block compression test samples for PVAC group and on lap shear test samples for PVAL, showed a rise in shear strength until reaching a maximum (at 16 - 25 kGy) range and subsequently a decrease. The rise can be attributed to the increased cohesive strength created by crosslinking bonds and also to a rise in dipole movements (better adhesion). Decrease may be due to less wettability and weakening chain scission effect of radiation. XIImpact strength was measured on a Charpy impact tester for the PVAC's. Fluctuating rise could be generally observed. There are several components of toughness which simultaneously increase and decrease creating a fluctuation in the sum. Crosslinking bonds raise the shear modulus without necessarily embrittlement of the material. Viscosity component of the impact work is decreasing with crosslinking degree and elastomeric toughness is being added up at higher doses. Blocking test revealed a better heat resistance with rising croslinking degree. Blocking temperature ranges could be increased (30-70 K) depending on the dose. Adding a few percent peroxide has contributed to the degradation of the protective colloid and destabilised the dispersion. Adhesive properties have also been adversely effected. It remains still to be investigated whether adding of much smaller amounts of peroxide might enhance crosslinking properties. PE for its part remained unaffected by irradiation and peroxide presence. It might also be tested as a crosslinking inhibitor for some adhesive dispersions. As a result, it can be said that up to a crosslinking degree of 25% the material is suitable for adhesive application. Moreover a general rise in shear strength and toughness is measured. Peel strength also rises for PVAC -MA and PVAL. Adhesives have become significantly faster and blocking range is delayed to higher temperatures. Thus radiation crosslinking in a range of (15-25 %) proves to be an effective way of upgrading of the PVAC and PVAL adhesives.NOTASYON LİSTESİ c Cn Cx D D D d d E f G G` AG AD AHAD h Io I* Al lo M! M2 MA m ma n Pn Po q0 Rx r ASAD Tg Wad Xj/2 Y yks Işık hızı Schering köprüsü bilinen kapasite Ölçülen kapasite Işınlama dozu (Charlesby-Pinner eşitliği) Dielektrik kayıp Dedektör (Schering köprüsü) Stefan bağıntısında tutkal film kalınlığı Viskoelastik kayıp Jelleşme derecesi Islak tutma kuvveti Elastiklik (kayma) modülü Elastiklik (kayma) kayıp modülü Serbest enerji değişimi (adsorpsiyona bağlı) Entalpi değişimi (adsorpsiyona bağlı) Planck sabiti Başlangıç ışın şiddeti x malzeme kalınlığına bağlı ışın şiddet fonksiyonu ma alan kütlesine bağlı ışın şiddet fonksiyonu Şekil değişimi Başlangıç uzunluğu Başlangıç numune miktarı (ekstraksiyon) Çözünmeyen madde miktarı (ekstraksiyon) Molekül ağırlığı Elektron durağan kütlesi Alan kütlesi Stefan bağıntısında dinamik viskozite Makro molekülün polimerizasyon sayısı Birim dozda parçalanan monomer sayısı Birim dozda çapraz bağlanan monomer sayısı Schering köprüsü bilinen direnç Temas yarıçapı Entropi değişimi (adsorpsiyona bağlı) Camsı geçiş sıcaklığı Bağ enerjisi Gelen ışınların yarısının soğurulduğu kalınlık Yüzey gerilimi Katı-sıvı arasındaki yüzey gerilimi xmykg ysg 8 8as 8 8 îl rjdis TS1V1 Tizafi 9 0 K X X ' v v v' Vas P a a T 9 9 Katı-gaz arasındaki yüzey gerilimi Sıvı-gaz arasındaki yüzey gerilimi Dielektrik kayıp ve viskoelastik kaybın arctanjantı Asimetrik deformasyon titreşimi Dielektrik katsayısı Şekil değişim oranı Dinamik viskozite Dispersiyonun dinamik viskozitesi Sürekli (sıvı fazın) viskozitesi Tjdis' nun T] sıvı' ya oranı Sıvı damlasının temas açısı Foton saçılma açısı Yutma katsayısının ikiz teşekkülüne bağlı bileşeni Dalga boyu Saçılan ışın dalga boyu Kütlesel yutma katsayısı Dalga sayısı (İR spektroskopisi) Işın frekansı Saçılan ışın frekansı Asimetrik gerilme titreşimi Yoğunluk Yutma katsayısı Compton olayı bileşeni Gerilme Yutma katsayısı Fotoelektrik olay bileşeni Elektron saçılma açısı Sıvı dağılımda tanecik konsantrasyonu Elektrik akımı frekansı XIV