Shellac as filler in sheet moulding compound

Transcription

1 Indian Journal of Chemical Technology Vol. 11, January 2004, pp Shellac as filler in sheet moulding compound D N Goswami*, P C Jha & K Mahato Division of Lac Processing and Product Development, Indian Lac Research Institute, Namkum, Ranchi , India Received 10 September 2002; revised received 29 July 2003; accepted 21 August 2003 Use of the natural resin shellac in general grade based sheet moulding compound (SMC) has been examined in the manufacture of fibre-glass reinforced sheets (FRP). Use of small quantity of shellac resulted in some improvement in the mechanical properties of FRP sheets and the sheets did not deteriorate in sunlight and showed resistance towards various chemicals. The FRP sheets were also prepared based on isophthalate and bisphenol s. Shellac being natural and possessing UV ray absorbing properties, its use in the manufacture of FRP sheets is expected to make sheets more eco-friendly and last longer. IPC Code: B29C70/00; C09F1/00 Fibre glass reinforced plastics have now become an integral part in different engineering applications, because of their high strength, thermal and dimensional stability, comparatively lighter than metals and resistance towards corrosion. Polyester resin based sheet moulding compounds are widely used for preparing fibre-glass reinforced sheets. FRPs find extensive applications in structural fabrication, electrical housing, automobiles etc 1. FRPs used for roofing, are expected to be resistant towards UV rays. Shellac is known to be UV resistant 2. Since shellac based coating is likely to be removed due to weathering (shellac does not possess sufficient resistance towards water) and cannot last longer, it was attempted to use shellac along with the sheet moulding compound in the manufacture of FRP sheets, in which shellac will remain embedded within the system and does not run the risk of its removal due to weathering. The present paper includes various properties of shellac containing FRP sheets. The study indicated that FRP sheets containing shellac might be used for commercial exploitation with additional advantages as mentioned above. Experimental Procedure Commercial variety of fresh shellac (lemon/orange/tn, pure, without any additive) with flow value not less than 50 was used in this study. General grade, isophthalate, bisphenol s were used along with usual accelerators and catalysts for preparing the FRP *For correspondence ( rch_dipne747@sancharnet.in; Fax: ) sheets by hand lay-up technique and curing at room temperature. Needle type fibre glass mats (450 g/m 2 ) and surface mats used in this study, were manufactured either by M/s Vetrotex Industries, Hyderabad or M/s Binani Industries, Goa. Fibre glass and resin ratios were maintained such that the glass content falls in the range 33-36%. Powdered shellac (10-30%) was added in the SMC. FRP sheets of thickness of 3 mm and size, were prepared and sent to CIPET, Lucknow for characterization of the mechanical properties. The properties were determined as per ASTM D638 (tensile strength and modulus), ASTM D790 (flexural strength and modulus), ASTM D258A (Izod impact resistance), ASTM D459 (arc resistance) standards. Dielectric strength was determined following the procedure described by Lakshminarayanan et al 3. Resistance towards tracking was investigated with the help of a Beckmann (USA) Tracking Insulation Test Set (DT 1-25) following IS: /IS: , and dissipation factor was determined with a General Radio (USA) capacitance bridge with samples of 4 dia. Volume and surface resistivities were determined with the help of a megohm meter (Sivananda Electricals, Nasik, Maharashtra). Viscosities of shellac containing and shellac free SMCs, were investigated with the help of a Haake (Germany) cone and plate Rheometer. Different parameters were evaluated with the help of the software supplied along with the instrument. Shellac containing SMC was filtered to separate out the undissolved shellac before measurements. Resistance towards different chemicals was investigated by dipping pieces of FRP sheets of

2 68 INDIAN J. CHEM. TECHNOL., JANUARY 2004 Fig. 1(a,b) FRP sheets prepared with non-pigmented shellac filled SMC and (c) tea coasters (FRP) prepared, based on pigmented shellac-filled SMC Table 1 Characteristics of the shellac-free and shellac-filled fibre glass reinforced sheets Properties Shellac-free polyester Shellac (10%) filled Shellac (30%) filled based FRP sheets polyester based FRP polyester based FRP sheets sheets Tensile strength Tensile modulus 17,680 22,190 16,340 Flexural strength Flexural modulus 37,016 48,630 18,160 Izod impact strength (kg cm/cm of notch) Arc resistance (s) suitable sizes in the chemicals kept in glass beakers and observations were recorded after the lapse of the period mentioned in the Table 4 (Chemicals were chosen on the basis of the Chemical resistance data Plastic Library Institute, U.K.). Tests were carried out both before and after sealing the sides of the pieces with wax. Thermal resistance of the FRP sheets was determined by the usual procedure. Both pigmented and non-pigmented FRP sheets were prepared, all of which yielded very attractive glossy finish (Fig. 1). Results and Discussion Mechanical properties of shellac-free and shellac filled FRP sheets are shown in the Table 1. Tensile

3 GOSWAMI et al.: SHELLAC AS FILLER IN SHEET MOULDING COMPOUND 69 strength, tensile modulus, flexural strength and flexural modulus of 10% shellac filled FRP sheets were found to be higher than those of shellac free sheets. The results suggest interaction between the polar groups of shellac and s. Incorporation of more shellac (30%) resulted in deterioration in the above properties (Table 1). More shellac might have caused hindrance in the curing of to form the desired rigid threedimensional structure resulting in deterioration in the mechanical properties (Table 1). Izod impact strengths of both shellac-free and shellac-filled sheets were almost same. Both the shellac-free and shellac-filled FRP sheets possessed increased resistance towards tracking. Electrodes were found to sink within the sheets due to the heat generated due to sparking and no tracking was found to occur. Arc resistance of shellac-filled FRP sheets was, however, found to be low compared to that of shellac-free sheets (Table 1). FRP sheets prepared based on general grade possessed low value of dissipation factor (Table 2), compared to that of shellac filled FRP sheets. Bisphenol and isophthalate polyester resins were found to be more polar (Table 2), as the dissipation factors of FRP sheets prepared with these resins were higher than those prepared with general purpose. Addition of 10% shellac as filler resulted in a rise in the dissipation factor of isophthalate resin, but in the case of bisphenol based FRP sheets, addition of shellac resulted in a decrease in the value of dissipation factor, indicating close association of the polar groups of shellac and the synthetic resin. Dielectric strength of isophthalate based FRP sheets was higher compared to other FRP sheets (Table 2). Shellac filled isophthalate polyester resin based FRP possessed lower dielectric strength compared to shellac-free FRPs. Incorporation of shellac in bisphenol did not affect the dielectric strength value. Both the surface and volume resistivity values (Table 3) of shellac (10%) containing fibre glass sheets were low compared to those of shellac-free Table 2 Absorption of water, dissipation factor of shellac containing (10%) and shellac-free based FRP sheets SMC used Density Absorption of water Absorption of water Absorption of water Dissipation Dielectric based on (g/ml) after immersion in after immersion in after immersion in factor at strength water for 24 h after water for 24 h boiling water for 100 khz (kv/mm) sealing sides (%) without sealing 30 min without sides (%) sealing sides (%) Polyester general grade Polyester +10% shellac Polyester +30% shellac No change Bisphenol polyester No change Bisphenol polyester No change % shellac Isophthalate polyester Isophthalate polyester % shellac Table 3 Surface and volume resistivity of shellac free and shellac containing fibre glass sheets (at 500V DC) Material Surface resistivity Volume resistivity (Ohm) (Ohm cm) Polyester resin based sheet Polyester + shellac (10%) based sheet(unpigmented) Polyester + shellac (10%) based pigmented (blue) sheet Polyester + shellac (30%) > based sheet (unpigmented) Bisphenol polyester based sheet Bisphenol polyester + shellac (10%) based sheet Isophthalate polyester based sheet Isophthalate polyester + shellac (10%) based sheet

4 70 INDIAN J. CHEM. TECHNOL., JANUARY 2004 based sheets, indicating better resistivity of the latter. Incorporation of 30% shellac in based SMC, resulted in considerable rise in both the resistivity values. In corporation of shellac in bisphenol caused a decrease in the above parameters. Shellac, however, resulted in an increase in the surface and volume resistivity values upon incorporation in isophthalate polyester resin based SMC, contributing improvement in the electrical property of the latter (Table 3). Shellac was found to be partially soluble and dispersible in the. Thermal resistance of the FRP sheets, where undissolved shellac was allowed to disperse on the surface, was found to be 110 C. Higher thermal resistance (168 C) was, however, obtained when undissolved shellac was allowed to remain within the under layers of the prepared FRP sheets. The thermal resistance was found to be little lower compared to that (180 C) obtained for shellac-free polyester based FRP sheets (Thus, thermal resistance of shellac containing FRP sheets was intermediate between those of natural resin shellac (65-75 C) and based sheets). The thermal resistance of the shellac filled FRP sheets was found to depend on the period of curing. The variation of thermal resistance with the period of curing has been shown in the Fig. 2. Inertness towards water and various chemicals of FRP sheets is an important factor, which makes their multipurpose application in a variety of fields. Knowledge of the effect of various chemicals on FRP sheets (about surface erosion/defect) is essential, when these sheets come in contact, either for short or long periods. The results of the study (Table 4) are encouraging. Incorporation of shellac in the SMC did not cause much adverse effect in the resistance towards various chemicals of based FRP sheets. Bisphenol and isophthalate polyester resins are generally used, where high resistance towards different chemicals is desired. Incorporation of shellac (10%) in these synthetic resins did not affect their chemical resistance property (Table 4). Thus, shellac can be used as an extender for the bisphenol as well as isophthalate s for manufacture of FRP sheets. It has been found that both polymerized seedlac and shellac can also be used as extender. Since these forms of the natural resin are cheaper than the synthetic resins, use of these is likely to reduce the cost of final products, with additional advantage that the products will be semi-synthetic and more eco-friendly. Fig. 2 Variation of thermal resistance of shellac filled FRP sheets with time of curing in the cold Rheological study with sheet moulding compound (SMC) Study was made to characterize and understand the nature of shellac-free and shellac-containing sheet moulding compounds. Shellac was found to be partially soluble and dispersible in the general purpose grade matrix used for sheet moulding, as indicated by the change in colour of the (colourless syrupy liquid changed to pale brown orange when shellac was added). The material was filtered and subjected to rheological measurements with the help of a Haake cone and plate Rheometer. All measurements were carried out with the help of the software supplied along with the instrument by Haake. The viscosity/flow curves of shellac-free and shellac-containing are shown in the Fig. 3. The higher viscosity of the shellac (10%) containing compared to that of shellac-free synthetic resin, confirmed partial solubility of shellac in the polymer resin matrix. The higher viscosity of shellac containing did not cause any inconvenience in the workability in the lamination process (unfiltered shellac containing was used for lamination). The little difference in the enclosed area between the ramp up and down profiles of the flow curves (Fig. 3b), is also indicative of not so different leveling properties and finish of the product. The hysteresis generally indicates the inability of the material to recover from the disarrangement caused due to application of shear. The small area in the hysteresis loop suggests presence of shellac has not destroyed the ability of the polymer to recover from the deformation of the system undergone due to the application of shear. Shellac containing moulding composition resulted in higher thickness of the moulded sheet compared to that of the shellac free SMC based sheet. Little inconvenience was experienced in the spreadability/leveling of the moulding composition due to the addition of 30% sticklac/seedlac/shellac in the polymer matrix. Addition of little medium into the system solved the problem in the lamination work.

5 GOSWAMI et al.: SHELLAC AS FILLER IN SHEET MOULDING COMPOUND 71 Table 4 Resistance to different chemicals of various fibre glass sheets with and without shellac Material Polyester Polyester + Isophthalate Isophthalate Bisphenol Bisphenol (general grade shellac (10%) polyester polyester + polyester polyester + resin) resin 10% shellac resin 10% shellac Water No effect No effect No effect No effect No effect No effect H 2 SO 4 Surface turns Surface turns No effect up to Slight turbidity No effect up to No effect (50%) light brown light brown 7 days after 7 days 7 days up to 7 days after 96 h after 96 h HNO 3 Surface turns Surface starts Surface turns Surface turns Surface turns Surface turns (50%) to light yellow yellowing yellow after yellow after darker after darker after after 24 h after 24 h 72 h 72 h 72 h 72 h Acetic acid Surface becomes Surface becomes No effect No effect No effect Acid turns (50%) rough after 48 h rough after 96 h light yellow HCl No effect No effect up to No effect No effect No effect No effect (10%) 192 h Chromic acid No effect No effect No effect No effect No effect No effect (10%) Acetone No effect No effect No effect No effect No effect No effect (10%) NaOH No effect Solution changes No effect Solution changes No effect Solution turns (10%) to light brown to yellow colour light brown after 3 days CH 3 OH No effect up to Solution turns No effect up to No effect up to No effect up to No effect up (100%) 7 days light yellow 7 days 7 days 7 days to 7 days after 48 h Ethanol No effect Surface turns No effect Surface turns No effect Surface turns (50%) yellow after yellow after 24 h light yellow after darker after 4 days 4 days KMnO 4 Surface turns Surface turns Surface turns Surface turns Surface turns Surface (25%) purple after purple after purple after purple after purple after purple after 24 h 24 h 24 h 24 h 24 h 24 h Transformer No effect No effect No effect No effect No effect No effect oil Kerosene - do - - do - - do - - do - - do - - do - oil NH 4 Cl - do - - do - - do - - do - - do - - do - (5%) NaCl - do - - do - - do - - do - - do - - do (Saturated) The tests were carried out up to 7 days unless otherwise mentioned Similar results were obtained when shellac was added in either isophthalate or bisphenol polyester matrices. In both the cases, higher viscosity was obtained of the filtered shellac (10%) containing resins (Table 5). The flow/viscosity curves for the above moulding compositions are shown in the Figs 4 and 5 respectively. The yield point (minimum force required to initiate flow) and thixotropy values were determined and have been shown in the Table 5. The yield points of shellac containing s were higher than those of alone, which was obvious due to higher viscosity of the former. The yield points for both isophthalate s were almost same. Addition of shellac in bisphenol polyester increased the yield point like general grade. Undissolved shellac floats on top of the polyester and isophthalate (shellac is lighter compared to the synthetic resins). For bisphenol, undissolved shellac settles at the bottom of the test tube indicating density of shellac is higher than that of the synthetic resin. It is known that the coating materials need to possess some thixotropy for obtaining good leveling

6 72 INDIAN J. CHEM. TECHNOL., JANUARY 2004 Fig. 3a Viscosity curves of shellac-free polyester (lower set) resin and shellac containing (upper set) Fig. 4 Viscosity curves of shellac-free (lower set) isophthalate and shellac containing (upper set) isophthalate Fig. 3b Flow curves (ramp up and down) of shellac free polyester (lower set) resin and shellac containing (upper set) Table 5 Yield point and thixotropic values of different polyester-shellac compositions Material General grade General gr. polyester + shellac (10%) Viscosity (PaS) at γ = (1/s) Yield point (Pa) Thixotropy (Pa/s) Isophthalate polyester Isophthalate polyester + shellac (10%) Bisphenol polyester Bisphenol polyester + shellac (10%) Fig. 5 Viscosity curves of shellac-free (lower set) bisphenol and shellac containing (upper set) bisphenol and smooth surface. Since, addition of shellac in s increased the viscosity, both yield and thixotropy values were higher compared to those of the unmodified resins. Shellac induced larger thixotropy values (larger hysteresis) compared to those of shellac-free polyesters, indicating that the samples needed more time to rebuild structure disrupted by application of shear i.e., shellac containing matrices possessed longer relaxation time. In a similar experiment carried out with gummy mass (a hydrolysed product of shellac), it was observed, that, gummy mass is compatible with 4. The partial dissolution of shellac in polyester matrix noticed in the present case indicated that organic sesquiterpenic acids might have diffused in the system.

7 GOSWAMI et al.: SHELLAC AS FILLER IN SHEET MOULDING COMPOUND 73 Table 6 Cross model parameters for the SMCs Material η o η n General grade General grade % shellac Bisphenol polyester Bisphenol polyester + 10% shellac Isophthalate polyester Isophthalate polyester + 10% shellac Cross model parameters Further characterization of the SMC was done with the help of the Cross 5 model parameters. Cross had formulated the following relationship with the assumption that the pseudo-elastic flow is associated with the formation and rupture of structural linkages, η = η + (η o η )/(1 + α γ n ) where, η is the apparent viscosity of solution at shear rate γ, η o and η are the limiting viscosities at zero and infinite shear rates and α is a constant. An important point of the non-newtonian phenomenon of polymer/resin solutions is an assumption of existence of limiting value η o. Values of η o are dependent on the solvency power of the solvent system, concentration and viscosity of the individual components. Limiting shear viscosity η is also a function of molecular structure, molecular weight, solvent quality and concentration. The value of η for polymer/resin solutions may be lower or higher than η o due to shearing effect. Low η o and η values also indicate good interaction between solute and solvent. Polyester resins are higher viscosity system in styrene. Addition of shellac caused a rise in the η o values (Table 6) of s compared to those of shellac-free resins (due to concentration effect). η values of shellac-free s were zero, higher values of the parameter of shellac containing s (Table 6) indicating good interaction between the solute molecules. Conclusion The results of the present study suggest that the natural resin shellac can be used along with the s to make the FRP sheets more ecofriendly. Such use will increase the domestic consumption of this natural resin opening a new avenue for sustenance of the shellac trade and economic support of a substantial tribal population of the country. Since shellac possesses UV absorption properties, no separate UV resistant additive will be required in the SMC. Acknowledgements The authors are indebted to ICAR, New Delhi for the grant of an Ad hoc Research Scheme under AP Cess Fund for carrying out the research work. They are grateful to Dr K. K. Kumar, Director and Head, Division of LP & PD of the Institute for extending all the facilities for carrying out the research work and also for their keen interest during the course of the work. They are also thankful to Shri D.D.Singh for valuable technical assistance, Shri R.P.Srivastava for photography and Shri R.Ramani, Pr. Sc. for computer processing of the photographs. References 1 Kislev B A, in Polymer Matrix Composites, edited by Shalin R E (Chapman & Hall), Goswami D N, Prasad N & Das R N, J Oil Col Chem Assoc, 64 (1981) 20; Goswami D N & Prasad N, J Oil Col Chem Assoc, 65 (1982) Lakhsminarayanan T R, Sankaranarayanan Y & Gupta M P, Indian J Technol, 12 (1974) Goswami D N, Prasad N, Jha P C & Mahato K, Indian J Chem Technol, in press 5 Cross M M, J Colloid Sci, 20 (1965) 417.