Method of Recovering the Fibrous Fraction

Method of Recovering the Fibrous Fractionof Glass/Epoxy CompositesL IU Y UYAN,*M ENG L INGHUI,H UANG Y UDONG AND L IU L IXUNDepartment of Applied ChemistryHarbin Institute of Technology,Harbin150001,P.R.ChinaABSTRACT:This study reports the method of recovery of glass fibers from glass/epoxy composites through the solvent method.It is possible to decompose epoxy resin with a yield of more than 99wt%soluble products in a nitric acid solution(8M)at90 C for5h.The decomposition rate of epoxy resin increases with an increase in temperature and concentration of acid solution.The glass fibers can be recovered with very little contamination and they have a tensile strength reduction of 3.5%under decomposition conditions of T¼70 C,t¼250h,C¼6M and ¼6g/100mL.These fibers can be used as reinforcement to prepare new composites.The interlaminar shear strength (ILSS)of recovered short fiber-reinforced composites produce strength reductions of only2.5% compared with pristine composites.KEY WORDS:composites,glass fiber,epoxy resin,recycling.INTRODUCTIONG LASS FIBER/EPOXY COMPOSITES are attractive materials for the automotive andaircraft industry,but one barrier to their increased use is the recycling difficulty.The epoxy resin in the cast-off cannot be melted down and remolded as is done in the thermoplastic industry.With the increasing landfill casts,there is an urgent need to develop recycling technologies for composite materials.The majority of work on composite recycling is concerned with grinding the composites into a suitable size to be used as fillers in new composite materials[1–3].Masatashi[4] found that using glass/epoxy composites as fillers for epoxy resin could yield products with better strength and thermal expansion properties when compared to those made with conventional fillers.Other recycling processes based on pyrolysis have also been considered[5,6].This process breaks polymeric waste into reusable fuel oils and hydrocarbon gasses in the absence of air.A sheet molding compound(SMC)of glass/ polyester composites was pyrolyzed by Torres et al.[7].The liquid obtained can be used as bustion with energy recovery is another means of recovering some value from scrap composites[8].In these processes,carbon-and aramid-reinforced composites destroy valuable materials and can be a source of pollution.A solvent recycling method in which valuable carbon fibers can be recovered has already been described[9].In this study,this method is further described,the recycling*Author to whom correspondence should be addressed.E-mail:liuyy@Figure6appears in color online:Journal of R EINFORCED P LASTICS AND C OMPOSITES,Vol.25,No.14/200615250731-6844/06/141525–9$10.00/0DOI:10.1177/0731684406066748ß2006SAGE Publicationsprocess conditions of glass/epoxy composites are examined,the properties of recovered glass fibers are presented,and the mechanical properties of composites prepared with recovered fibers are compared with those of virgin composites.EXPERIMENTALMaterialsThe epoxy composites with67wt%unidirectional glass fiber were prepared in a laboratory.The bisphenol A type epoxy resin(E-44)and curing agent(IPDA)were used as a matrix.The curing conditions were80 C for2h and150 C for4h.The composites were cast to200mmÂ6mmÂ2mm bar.Decomposition ConditionsEach composite specimen was put into a specially designed glass vessel(D¼100mm, L¼300mm)and then immersed in100mL nitric acid solution.The decomposition temperature was controlled in a water bath.The feedstock ratio is defined as the ratio of resin weight in composite to the volume of the nitric acid solution.The insoluble composites were separated from the solution at a specific decomposition time and washed with acetone.After drying at60 C,the composites were weighed.Preparation of New CompositesThe recovered glass fibers were incorporated into epoxy matrices to produce two different composites.The first sample was made of recovered unidirectional continuous glass fibers with E-44/IPDA at a content level of65%w/w.The next sample was4–5mm short glass fibers which were cut from recovered fibers with E-44/MeTHPA at a content level of20%w/w.These two types of composites were prepared again by utilizing‘virgin’glass fibers,for use as reference samples.Measurement MethodsThe decomposition rate of epoxy resin in composites was determined in each experiment by weighing the amount of solid residue.The decomposition rate is given byDecomposition rateð%Þ¼weight of compositesÀweight of solid residueweight of resin in compositesÂ100:The reclaimed fibers were subjected to visual scrutiny via optical and scanning electron microscopy(SEM)to determine the extent of contamination of the residual surface.The surface of the glass fiber was also observed and pictured by electron probe microscopy using a JEOL JCXA-733instrument.In order to verify the effect of the recycling process on fiber properties,the tensile strength of the single fiber was measured according to ASTM-D3379specification.The cross-head speed of the MTS materials testing system was10mm/min.The interlaminar 1526L.Y UYAN ET AL.Method of Recovering Fibrous Fraction of Glass/Epoxy Composites1527 shear strength(ILSS)testing of new composites was carried out using an Instron5569 machine with a cross-head speed of2mm/min.RESULTS AND DISCUSSIONDecomposition of CompositesFirst,the decomposition experiment of glass/epoxy composites was carried out in concentrated sulfuric acid.A flake of glass/epoxy composite,approximately 200mmÂ6mmÂ2mm in size,was put into a6M sulfuric acid solution.The solution was transparent even after20h of immersion at60–90 C and the composite appeared swollen and distorted.After removing from the sulfuric acid solution,the composite increased in mass.The fibers were fixed in the matrix and could not be separated.The results showed that concentrated sulfuric acid was not effective in decomposing glass/epoxy composites. The glass/epoxy composite was then immersed in a6M nitric acid solution at60–90 C. The composite started to decompose at an immersion time of about5h.The solution became yellow and a red-brown decomposed compound with a high viscosity could be observed in the bottom of the solution.The color of the solution became deep and the viscous red-brown solid products gradually decreased with the increase in the duration of immersion.When fibers in the composites separated from each other,the solution was cooled down and the insoluble residue was collected.After washing with acetone and drying at60 C,the recovered fibers were obtained.The weight of recovered fibers was about67wt%of the composites,which is close to the content of glass fibers in the composites.It appeared that epoxy resin was completely removed from the glass fibers. The stronger oxidation of nitric acid caused the C–N bond in the network to break and the benzene ring to be nitrated.In order to examine the effects of experimental parameters on the decomposition rate of the resin matrix,the following experiments were carried out.Effect of Reaction Conditions on the Decomposition Rate of the Resin Matrix TEMPERATUREThe decomposition curve for epoxy composites,in8M decomposition solution at 50–90 C for5h,with a2g/100mL feedstock ratio is shown in Figure1.The increase in temperature resulted in improvement of the decomposition rate of epoxy resin.The decomposition rate was about2.1,24.9,and96%,at50,70,and80 C respectively.There is a great increase in decomposition rate after70 C.The epoxy resin kept for6h completely disappeared at90 C.CONCENTRATION OF THE DECOMPOSITION SOLUTIONThe effect of concentration of the decomposition solution on the degree of decompo-sition of the resin matrix at90 C for5h with a2g/100mL feedstock ratio is shown in Figure2.The rate of decomposition increased rapidly with the increase in concentration of the decomposition solution.The resin was entirely decomposed in8M decomposition solutions kept for5h.This could be attributed to the stronger causticity of the decomposition solution,which can open the thermosetting network of the epoxy resin quickly with the increase in concentration of decomposition solution.FEEDSTOCK RATIOThe effect of feedstock ratio on the decomposition rate of the resin matrix in composites under a set of experimental parameters (T ¼90 C,C ¼8M,t ¼5h)is presented in Figure 3.The decomposition rate of the resin decreased slowly as the feedstock ratio increased.It could be seen that the temperature and concentration of the decomposition solution was much more effective than the feedstock ratio in increasing the decomposition rate of the resin matrix.The feedstock ratio can be increased in theory to improve reclaim efficiency.However,because the increase of feedstock ratio results in a decrease in acid solution volume,the composites cannot be soaked and decomposed.Glass Fiber CharacterizationFIBER PROPERTIESTable 1shows the orthogonal experimental results of the decomposition of epoxy composites in which the loss of fiber strength can be calculated by dividing the amount of decrease of single fiber tensile strength of the recovered fiber by the tensile strength of a020406080100D e c o m p o s i t i o n r a t e (%)Reaction temperature (°C)Figure 1.Resin decompositionrate vs reaction temperature.0246810020406080100D e c o m p o s i t i o n r a t e (%)Concentration (M)Figure 2.Resin decomposition rate vs decomposition solution concentration.1528L.Y UYAN ET AL .single fiber of the original fiber bundle.The sample under decomposition conditions of T ¼70 C,t ¼250h,C ¼6M,and ¼6g/100mL has a minimum fiber tensile strength loss of 3.5%,while runs at 80 C with 4M concentration for 240h produced a maximum strength reduction of 15.1%.These results are better than the results of the previous study of the effects of recycling process on the strength of the glass fibers.Pickering et al.[10]measured a 50%reduction in the strength of glass fibers after heating at 450 C in a fluidized-bed process.OBSERVATION OF RECOVERED FIBERSThe SEM was used to evaluate the fiber surfaces and the residual resin present on the composites.Two samples were studied based on their varying extent reaction.The first one was processed at 60 C for 5h with 8M concentration and 2g/100mL feedstock rge flakes of residual resin appeared among the fibers and some fibers were surrounded by a sheath of resin (Figure 4).The residual resin along the fibers could not be separated from the fiber surface.The sample under decomposition conditions of T ¼70 C,t ¼5h,C ¼8M,and ¼2g/100mL was evaluated with SEM (Figure 5).The resin can be removed from the composites in large chunks and some of the fiber bundles wereseparate.708090100D e c o m p o s i t i o n r a t e (%)Feedstock ratio (g/100ml)Figure 3.Resin decomposition rate vs feedstock ratio.Table 1.Orthogonal experiment of decomposition of epoxy composites.LevelResults No.Temperature ( C)Concentration (M)Feedstock ratio (g/100mL)Decomposition time (h)Strength loss (%)1908612 5.2290642210.3390428214.7480841710.4580629511.76804624015.177082459.187066250 3.59908671011.3Method of Recovering Fibrous Fraction of Glass/Epoxy Composites1529An optical graph of recovered glass fibers is given in Figure 6.The fibers are free from residues and arranged in an orderly manner.A careful recycling process could eliminate fiber damage and have a highly regular orientation.Figures 7and 8show the electron probe microscopic analysis of virgin fibers and recycled glass fibers after washing and drying separately.When compared with virgin fibers,the surface of the recycled fibers was smooth and showed little contamination.The glass fibers were recovered undamaged and were reusable.Reuse of Recovered FibersTo obtain a high performance of recovered fiber-reinforced composites,the selection of the decomposition process is necessary.Through orthogonal experimentation,the recycling conditions for retaining the fiber strength were T ¼70 C,C ¼8M,and ¼6g/100mL [11].But this process needed longer immersion time.In this study,the (a)(b)Figure 4.SEM micrographs of composites under T ¼60 C,t ¼5h,C ¼8M,and ¼2g/100mL (1000Â):(a)cross section and (b)side.(a) (b)Figure 5.SEM micrographs of composites under T ¼70 C,t ¼5h,C ¼8M,and ¼2g/100mL (1000Â):(a)cross section and (b)side.1530L.Y UYAN ET AL .recovered fibers available from all of the recycling processes (Table 1)were employed as reinforcement to prepare new composites.Table 2presents the ILSS of new composites.The composites were prepared by using recovered glass fibers as reinforcement or virgin fibers as reference.The recycled unidirectional continuous fiber-reinforced composites showed a reduction of 4.7%in ILSS and recycled short fiber-reinforced composites produced a strength reduction of only 2.5%.Knowing the wettability of fibers is useful to predict their adhesion potential to the matrices.The contact angle of the fiber can characterize its wettability by the contacting liquid in a given ing glycol as a contacting liquid,the contact angle of virgin fibers was 59.4 and that of recycled fibers was 56.1 .After oxidation of nitric acid in the recycling process,the contact angles of the fibers decrease and their adhesion to the matrices improves.It is likely that acid digestion increases surface roughness ofthe Figure 6.Optical graph of the recovered glassfibers.Figure 7.Electron probe micrographs of virgin glass fibers.Method of Recovering Fibrous Fraction of Glass/Epoxy Composites 1531fiber and improves interfacial bond strength,thus annulling the negative effects of loss in tensile strength of the single fiber.CONCLUSIONSRecycling of thermoset epoxy composite containing glass fibers was studied and the following results were obtained.1.Concentrated sulfuric acid was not effective for decomposing glass/epoxy composites.2.When the reaction temperature was 90 C,the concentration of the acid solution was 8M,the feedstock ratio was 2g/100mL,and the reaction time was 5h,the decomposition rate of epoxy resin reached 99%.3.The temperature and concentration of the decomposition solution were much more effective than the feedstock ratio in increasing the decomposition rate of the resin matrix.4.Glass fibers from reinforced epoxy composites can be recovered undamaged without any residue left.The recovered glass fibers have a tensile strength reduction of3.5–15.1%under a set of decomposition conditions considered in this study.5.Interlaminar shear strength (ILSS)of the recovered short fiber-reinforced composites produced a strength reduction of only 2.5%compared with pristinecomposites.Figure 8.Electron probe micrographs of recycled glass fibers.Table 2.Interlaminar shear strength of the new composites.Fiber typesILSS (MPa)Strength loss (%)Virgin fibers/epoxy composites Recycled fibers/epoxy composites Unidirectional continuous glass fibers58.5355.76 4.74mm short glass fibers 21.5020.96 2.51532L.Y UYAN ET AL .Method of Recovering Fibrous Fraction of Glass/Epoxy Composites1533ACKNOWLEDGMENTSThe authors gratefully acknowledge the financial support of the Study Abroad Returnee Foundation of Hei Longjiang Province under Project No.LC03C09.REFERENCES1.Ralph,J.,Graham, B.and David,W.(1991).Recycling SMC Scrap as a Reinforcement,PlasticsEngineering,47(5):13–14.2.Pannkoke,K.,Oethe,M.and Busse,J.(1998).Efficient Prepreg Recycling at Low Temperatures,Cryogenics(UK),38(1):155–159.3.Kouparitsas,C.E.,Kartalis,C.N.,Varelidis,P.C.,Tsenoglou,C.J.and Papaspyrides,C.D.(2002).Recycling of the Fibrous Fraction of Reinforced Thermoset Composites,Polymer Composites,23(4): 682–689.4.Masatashi,I.(1998).Recycling of Epoxy Resin Compounds for Moulding Electronic Components,Journalof Materials 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