Synthesis of bio-based low viscosity curable polyesters for composite applications T. a
a Porubský ,
J.
a Petrůj ,
M.
b Skrifvars ,
D.
b Åkesson
Department of material chemistry, Brno University of technology, Purkyňova 118, 61200 Brno, Czech Republic b University of Borås, Allégatan 1, S-501 90 Borås, Sweden Contact:
[email protected]
Material and methods
Introduction Unsaturated polyester (UP) resins are well known for many years. UP’s find their use in composite manufacture in many areas spanning from building and ship constructions, electrical applications and automotive industry. UP’s are also used without reinforcement as coatings and paints. Feedstock for UP synthesis basically consists of diacid and glycol, where unsaturation is provided by addition of unsaturated diacid such as maleic or fumaric anhydride. Phthalic acid anhydride is also added to the resin to improve thermomechanical properties. Conventionally styrene or other reactive monomer is added to the UP to increase its reactivity through copolymerization. However, one issue with such formulations arises from the use of styrene, which is highly volatile and has been identified as a hazardous air pollutant by the environmental protection agency. Lactic acid based thermoset resins were synthesized by Bakare [1] as the potential matrix for automotive aimed composites. Glycerol-lactic acid based suffers from brittleness.
Unsaturated polyester synthesis Itaconic acid (IA) and maleic acid (MA) used as diacid components were reacted with 1,2-propylene glycol (PG12) or 1,3-propylene glycol (PG13) and lactic acid (LA) which was added into the reaction mixture in various ratios related to the amount of diacid and glycol part. Methylenesulfonic acid (MSA) and 2,4,6-tri-tert-butylphenol were added to the reaction mixture as a catalyst and stabilizator, respectively. Polycondensation was run at 150 °C under nitrogen flow. Reaction was controlled by measuring acidity number (AN). Obtained polyester had AN value under 40 mg KOH/g. After this step, polyester was alcoholysed with 8,25 wt. % of allyl alcohol (AA), what caused decrease in molecular weight and introduced additional unsaturation at the same time. AN decreased by the value of 10 mg KOH/g after this step. Free hydroxyl groups formed during alcoholysis were further end-caped with methacrylic anhydride (MAA) to form reactive resin functionalized at both ends of the molecule. Reaction pathway is illustrated in Fig. 1.
Preparation of cured resin
fdvd
UP resin sample was mixed with 2 % wt. of benzoyl peroxide and stirred until homogeneous solution had formed. Samples were then poured to the moulds and heated to 130 °C and left to cure at this temperature for 5 hours. Cured samples were cooled down slowly to room temperature.
Glycerol-lactic acidbased thermoset resin (LA_GLY_MAA)
AIM: 1) Incorporation of bio-based feedstocks into the unsaturated polyester resin and formulation of non-volatile reactive resin without the use of reactive diluent. 2) Comparation of thermal and mechanical properties of novel synthesized reactive resins with previously synthesized lactic acid based branched thermosetting resin (LA_GLY_MAA).
Fig 1: Reaction pathway
Material characterization Cured UP resin properties
Uncured UP resin properties Gel permeation chromatography (GPC)
1,6
showed broad distribution of MW compared to the reference resin LA_GLY_MAA In both IA, MA based resins little extent of crosslinking took place altougth stabilizator was used during the synthesis. This unwanted crosslinking can be seen as high MW parts in GPC spectrum. No crosslinking has been seen in the reference LA_GLY_MAA
1,4
IA_PG12 IA_PG12_LA30 LA3_GLY_MAA MA_PG12_LA30 MA_PG12_LA50
Intensity
1,2 Branched: LA3_GLY_MAA Mn = 550 g/mol D = 1,3 Linear UPs: Mn = 800-1000 g/mol D = 2-4
1,0 0,8 0,6 0,4 0,2 0,0 100
1 000
10 000
100 000
M (g/mol)
Dynamic Mechanical Analysis (DMA) - Effect of LA and different diacid component on thermomechanical properties of cured samples
62,3
Viscosity (Pa.s) 25C Viscosity (Pa.s) 50C
60
IA based UP resin showed higher modulus and also Tan δ than MA based resin with the same molar composition. Biobased thermosetting resin IA_PG12_LA50 showed comparable glass transition temperature as LA_GLY_MAA reference, value close to Tg of commercial UP styrene-based resin.
Dynamic viscosity (Pa.s)
50
40
30
27,1
20 13,7
10
1H NMR Confirmed presence of both allylic and methacrylic end unsaturated groups in the structure of synthesized UP resin
2,8
1,0
2,4
0
2,4
4,3 0,3
1,3 0,2
0,4
Sample
Rheometry • Great reduction in viscosity has been seen when 1,3-PG was used instead of 1,2-PG • At 50°C all resins have low viscosity favorable for processing • Increasing portion of the LA caused the dynamic viscosity to raise – 20 % increase caused almost 5-times higher viscosity at 25°C.
Tensile properties of cured resins LA increasing content in the UP probably caused decrease in density of unsaturation per mole of resin but tensile strength did not change significantly. Toughening of the materials was observed with LA content – increase in the elongation at break. Elongation increased for IA resins more than 2-times and more than 3times for MA based resins with 50 % of LA in resin composition Sample LA_GLY_MAA IA_PG12 IA_PG13_LA30 IA_PG12_LA50 MA_PG12 MA_PG12_LA30 MA_PG12_LA50
Tg (°C) DMA - tan δ 96,5 112,0 113,9 96,5 78,0 69,2
Tensile strength (MPa) Elongation at break (%) MA content (%) 31,5 ± 5,7 39,5 ± 7,9 50,0 ± 1,6 40,7 ± 2,8 33,5 ± 3,0 50,5 ± 6,8 30,6 ± 7,0
1,8 ± 1,3 1,6 ± 0,4 2,5 ± 0,1 3,9 ± 1,2 1,2 ± 0,5 2,9 ± 0,3 4,4 ± 1,2
34,2 6,5 7,6 8,9 4,4 7,4 8,3
FTIR – curing of the resin
Thermogravimetric
Decrease in intensity of the absorption bands at 1650 and 945 cm–1 proved that present unsaturation was reacted to form CH2 groups what can be seen in the increase of 2850 cm–1 absorption band intensity in cured resin
Effect of LA and glycol on thermodegradation of cured samples – presence of LA in the polyester chain decreased degradation temperature. Degradation temperature, measured as 5 % weight loss, raised when 1,3PG was used in resin formulation
analysis
(TGA)
Conclusions • Biobased UP resins with low non-biobased content were synthesized from renewable feedstocks: itaconic acid, lactic acid and propylene glycol. No reactive solvents were used for curing. • Improvement in mechanical properties of cured biobased UP was achieved with lower amount of methacrylic unsaturation compared to other lactic acid based thermosets • Modification of the UP with AA showed potential for low viscosity resin formulation with low volatile contents • Assumption of the use of lactic acid modified UP for bio-composite fabrication with enhanced interface adhesion
Refferences: [1] Bakare, F. O., Skrifvars, M., Åkesson, D., Wang, Y., Afshar, S. J., Esmaeili, N. Synthesis and characterization of bio-based thermosetting resins from lactic acid and glycerol. Journal of Applied Polymer Science. 2014, 131(13), n/a-n/a. DOI: 10.1002/app.40488. Acknowledgements: This work has been supported by project “Materials Research Centre at FCH BUT – Sustainability and Development” No. LO1211 of the Ministry of Education, Youth and Sports of the Czech Republic (MŠMT) and by FreeMovers international development programme of the MŠMT.
