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TitleDevelopment and Application of Polylactic Acid-based Coating Material

s with High Durability

Author(s) 森田, 晃充

Editor(s)

Citation

Issue Date 2014-02

URL http://hdl.handle.net/10466/14149

Rights

Development and Application of

Polylactic Acid-based Coating Materials

with High Durability

Koji Morita

February 2014

Doctoral Thesis at Osaka Prefecture University

Development and Application of Polylactic Acid-based Coating Materials

with High Durability

Contents

1. General introduction

2. Anti-hydrolysis Performance of Cured Coating Films of Acrylic Polyols

with Pendant Polylactic acids

2.1. Introduction

2.2. Experimental

2.2.1. Synthesis of PLA macromonomer modified with HEMA

2.2.2. Synthesis of acrylic polyols with pendant polylactic acids

2.2.3. Preparation of cured coating films

2.2.4. Evaluation method of anti-hydrolysis performance

2.2.5. Evaluation method of the coating performances

2.3. Results and discussion

2.3.1. Synthesis of PLA macromonomer modified with HEMA

2.3.2. Anti-hydrolysis performance of the acrylic polyols

with pendant polylactic acids coatings

2.3.3. Evaluation of basic coating performances of the coatings

of acrylic polyols with pendant polylactic acids

2.4. Summary

2.5. References

3. Synthesis and application of star-shaped polylactic acids to two

component and UV curable coatings

3.1. Introduction

3.2. Experimental

3.2.1. Synthesis of star-shaped polylactic acids polyols

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18

22

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24

28

33

35

36

38

39

3.2.2. Synthesis of star-shaped polylactic acids

with reactive double bonds

3.2.3. Preparation of two component cured coatings

3.2.4. Preparation of UV cured coatings

3.2.5. Evaluation method of the coating performances

3.3.Results and discussion

3.3.1. Synthesis of star-shaped polylactic acids

and their application to two component coatings

3.3.2. Synthesis of star-shaped polylactic acids with reactive

double bonds and their application to UV curable coatings

3.4. Summary

3.5. References

4. Synthesis of monodispersed silica nanoparticles and hybrid with

star-shaped polylactic acids

4.1. Introduction

4.2. Experimental

4.2.1. Preparation of silica nano-particles

4.2.2. Materials characterization of the silica nano-paeticles

4.2.3. Preparation of silica particles modified with reactive

double bonds

4.2.4. Preparation of hybrid coating film of the star-shaped

polylactic acid and the silica nano-particles

modified with reactive double bond

4.2.5. Evaluation method of the coating performances

4.3.Results and discussion

4.3.1. Preparation of SiO2 particles with various molar ratios

in the starting materials

4.3.2. Preparation of SiO2 particles modified with reactive

double bonds

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4.3.3. Evaluation of hardness and heat resistance of the hybrid

coating films

4.4. Summary

4.5. References

5. General conclusions

Acknowledgements

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1

Chapter 1

General Introduction

In recent years, construction of ‘the sustainable recycling society’ utilizing recyclable

resources, such as solar, wind, and biomass, is strongly desired because of global

environment problem and fossil resource shortage. Then, development of petroleum

alternative technologies and carbon-neutral materials is widely studied for maintaining

petroleum resources and reduction of a greenhouse gas, such as carbon dioxide (CO2).

Coating materials are composed of resins, pigments, solvent, additive agents, e.g.

surfactant, ultra violet absorber, light stabilizer, and thickener. In the automotive

coatings business, many petroleum derived materials, like acrylic resins and

polyurethane resins, are still widely employed. The mitigation of global environment

problems through developing the bio-based coatings from bio-based materials is

expected. Carbon-neutral materials are materials which do not affect the change in CO2

amount emitted absorbed in the material’s cycle. Especially, bio-based materials, such

as polylactic acids (PLA), polybutylene succinates (PBS), denatured starch, and

soybean polyols, attract attention [1-10].

When the products are discarded after being used, coating films are disposed together.

This means that the components of coating materials would be finally turned into CO2

after incineration. If coating materials are prepared from bio-based materials, instead of

petroleum-based materials, this would lead to considerable reduction of the amount of

petroleum consumption. The efforts to promote reduction in the quantity of petroleum

consumption have been made in all fields with the goal of reducing the CO2 amount

emitted in a life cycle.

From this point of view, there is a development agenda for environmental plans in the

coating material field. It is important to develop coating materials by using bio-based

materials as renewable starting materials.

2

If attention should be paid to all the products around us to reduce emissions of CO2,

the present study could lead the bio-based material field and the environmentally

friendly products could be developed.

People have used biomass since ancient times. A way to use it was physical

manufacturing such as furniture, tableware, paper, and so on, made from woods or

grasses. On the other hand, when biomass instead of a petroleum resource is used, it is

decomposed into low molecular weight chemicals by microorganism or chemical

reactions. This process differs from traditional ways. That is, bio-based materials are

synthesized by combination of chemical and biological techniques using a renewable

biomass. Bio-based materials also contain polymers which are only partly bio-based

such as polypropylene terephthalate and PBS. Most of the obtained monomers by

microorganism with using biomass are aliphatic acid or alcohol. Bio-based materials

prepared by these monomers are polyester, and thus, they have a hydrolysis property.

Therefore, when they are used as biodegradable plastics, this characteristic is very

useful. In their first stage of development, bio-based polymers were used for the

products which biodegradability was necessary, such as agricultural films, medical films,

packing materials, wrapping films and fishing line. The properties of biomass-based

polymers are basically the same as those of petroleum-based polymers. In particular, the

market for bio-based polymer is growing in developed countries mainly in Europe and

the United States, because composting systems are put into many places and many

people are environmentally minded in their countries compared with many other

countries in the world [11].

On the other hand, when they are used as durable plastics such as in automobiles or

consumer electronics, the hydrolysis property prevents them extensive application.

Therefore, there is a strong demand for the development of a bio-based polymer with a

high performance and functionality, especially high hydrolysis resistance, in order to

accelerate the widespread use of bio-based materials. Bio-based polymers should have

3

the same or further performances in comparison with petroleum-based polymers.

Most of polymeric materials depend on petroleum resource. Therefore, the amount of

CO2 in the atmosphere is increased after incineration disposal of waste products.

Moreover, petroleum resource will also run out in the near future. On the other hand, the

products which are made from bio-based renewable resources like plants as starting

materials cause no increase of CO2. That is, the total amount of CO2 is fixed because the

biomass is derived from CO2 in the air by plants via photosynthesis, and thereby, the use

of biomass is considered to be effective for reducing the increase in atmospheric amount

of CO2 even after incinerating them. The construction of ‘the sustainable recycling

society’ is desired now and the petroleum-based materials are being switched to the

bio-based materials. It is important to use not petroleum resources but renewable

biomass [11].

PLA is actively studied as one of the bio-based plastics and is put in practical use in

recent years [12-19]. PLA is a typical biodegradable plastic and is synthesized by

dehydration of the lactic acids obtained to saccharifying starch, such as corn and so on,

and fermenting it [20-24]. PLA is superior in molding. Therefore, the development of

PLA has been promoted remarkably and mainly used as structural materials, such as

cases of mobile phone, interior parts of automobiles, and so on.

The petroleum-derived materials are mostly used in coating materials. Application of

the bio-based materials to the coating materials is in a developmental stage today. As the

development on PLA has been progressing, it has been desired for PLA to be used as

coating material which has the high hydrolysis resistance. The coating material using

PLA has already been developed by dispersing micronized PLA into water [25], coating

resin consisting of lactic acid, dicarboxylic acid and glycol [26], and coating paint

produced by using PLA emulsion [27] have also been proposed in the previous report.

These materials are environmentally friendly because they are produced by using

bio-based materials. However, they do not have hydrolysis resistance. When a polymer

4

main chain consists of PLA, the number of repetitions of an ester bond increases. If

hydrolysis takes place, the PLA chain is decomposed and the properties of the coating

materials are considered to be decreased with aging. The most difficult challenge is to

restrain hydrolysis of PLA in order to use for the coating material derived from PLA.

A possible solution to reduce the problem could be to synthesize grafted polymer

(comb type polymer) which is composed of acrylic chain with high hydrolysis

resistance as main chain and PLA as side chain. Even if PLA as side chain was

hydrolyzed, the original properties of coating materials might be maintained due to the

main chain which consists mainly of acrylic chain. This technique can contribute to a

reduction of the amount of petroleum consumption and limited emissions of CO2 due to

use of PLA derived from biomass. Grafted polymer is branched polymer, in which the

side chains are structurally distinct from the main chain, and is used widely in various

fields such as coating material, adhesive material, and compatibilizer [11].

As a process to introduce PLA to side chain, the polymer can be synthesized by using

polymer with a number of hydroxyl groups. For example, polyvinyl alcohol-graft-lactic

acid was synthesized by melt polycondensation of polyvinyl alcohol and lactic acid

using stannous chloride as a catalyst [28]. However, it is difficult to control PLA chain

length as side chain and polymer structure in this synthesis method.

On the other hand, Grafted polymer can be also prepared by using macromonomer,

which is polymer or oligomer that has a reactive functional group at the one-end and

can take a part in polymerization. The grafted chains can be introduced easily by using a

macromonomer. Therefore, the use of this must be useful in order to design the grafted

polymer having well defined structure. The macromonomer has been used as not only

precursors used to synthesize grafted polymers but also building block of functionalized

polymers like network polymer or gel [29].

Macromonomer has been studied since early times and various synthesis methods of

macromonomer were reported. For example, synthesis process of macromonomer via

5

radical polymerization and that of macromonomer via addition-cleavage reaction have

been reported. In one of the method, polymerization of methyl methacrylate was firstly

carried out by using ,’-azobisisobutyronitrile as initiator in the presence of

thioglycolic acid, and the prepolymer having a carboxyl group at one end was obtained

by chain transfer reaction. Then, the carboxyl group of the prepolymer was reacted with

glycidyl methacrylate to prepare the macromonomer modified with a methacryloyl

group at one end [30]. In other method, polymerization of styrene in the presence of 2,

4-diphenyl-4-methyl-1-penten (α-MSD) was performed. α-MSD is an effective chain

transfer agent. The macromonomer having vinylidene group at one end of polystyrene

was synthesized through addition-cleavage reaction [31]. There are also several reports

on macromonomer synthesized by using lactide, which is a cyclic dimer of lactic acid

[11, 32-34].

From the above, it is difficult for the coating films containing PLA to have high

durability. Grafted polymer having PLA side chain introduced by macromonomer is one

of the good solutions to this problem.

Although the PLA macromonomer is synthesized by ring-opening polymerization

from lactide, lactide is high cost and poor handling by ring-opening from water

absorbency. Thus, it is necessary to synthesize coating materials from cheap lactic acid.

The way of synthesizing polylactic acids form lactic acid is dehydration condensation of

lactic acid [35, 36]. In this doctoral thesis, preparation of coating materials synthesized

from lactic acid is examined.

Another problem in the coating films containing PLA is that PLA is inferior to heat

resistance and the durability compared with petroleum derived materials. Although

increasing crystallinity of PLA or formation stereo-complex of PLA from poly(L-lactic

acid) and poly(D-lactic acid) are suggested as means of improving hydrolysis

resistance[37], these approaches are unsuitable for paint materials due to its low solvent

solubility and low storage stability. The hybrid with inorganic material and organic

6

material is actively studied as technique of the heat resistance improvement of organic

materials [38-41]. It should be interesting to examine heat resistance and durability of

hybrid coatings with PLA and inorganic materials by using the solvent dispersion of

silica nanoparticles which is used well as a coating material. However, the silica

particles added to the coating must be smaller than a few tens of nanometer to avoid

scattering and the coating layer should be transparent. Stability of silica particle

dispersion is needed too. Those silica nanoparticles are usually prepared under highly

diluted conditions. However, for the use of those nanoparticles to the hard-coating for

the polymer substrates, high concentration of silica is preferred.

Preparation of mono-disperse inorganic nanoparticles has attracted a considerable

attention because of their potential importance in technological applications in the fields

of optical devises, catalysts, filler for polymers, and so on. Among synthesis techniques

of particles, a liquid phase process, especially sol–gel process, is a superior one to

prepare monodispersed particles [42–46]. The so-called “Stöber method’’ is an

excellent process to prepare monodispersed silica particles using sol-gel process [47],

and silica particles with a diameter of about 100 nm to a few μm are easily prepared by

this method [48]. There are plenty of studies on the preparation of silica nanoparticles

by the Stöber method. However, not so many studies are reported on the preparation of

high-concentration of silica nanoparticles without using surfactants or dispersing agent.

In this study, polylactic acid-based bio-polymers for coating materials with high

durability were developed. UV curable coating materials were also designed by

introducing reactive double bonds to the polylactic acid-based bio-polymers. Properties

of coating films and anti-hydrolysis performance for these bio-polymers were evaluated.

Moreover, hybrid coating films with the polylactic acid-based bio-polymers and silica

nanoparticles by sol-gel process, and heat resistance and coating performances were

evaluated.

7

The doctoral thesis consists of 5 chapters indicated below;

Chapter 1

This chapter describes the background, the objectives and the contents of the thesis.

Chapter 2

In this chapter, graft copolymers were prepared using PLA macromonomer, and

properties of the product polymers were examined. Anti-hydrolysis performance of

cured coating films of the polymer was also examined.

In the section 2.3.1, PLA macromonomer modified with HEMA was synthesized.

In the section 2.3.2, the comb type polymers were synthesized and anti-hydrolysis

performance of the cured coating films of the polymer were evaluated and discussed.

In the section 2.3.3, basic coating performances of the cured coating films of the

polymer were also evaluated.

Chapter 3

This chapter describes synthesis of star-shaped polylactic acids and the application to

two component and UV curable coatings.

In the section 3.3.1, synthesis of star-shaped oligomeric lactic acids and their

application to two component coatings were examined.

In the section 3.3.2, preparation of star-shaped oligomeric lactic acids with reactive

double bonds and their application to UV curable coatings are described.

Chapter 4

In this chapter, synthesis of monodispersed silica nanoparticles with high

concentration by the Stöber process and their application to hard-coating with

star-shaped polylactic acids were examined.

8

In the section 4.3.1, preparation of silica particles with various molar ratios in the

starting materials ware examined. In the section 4.3.2, preparation of silica particles modified with reactive double

bonds were examined.

In the section 4.3.3, hybrid coatings of the star-shaped polylactic acids and the silica

nano-particles were examined about hardness and heat resistance.

Chapter 5

The chapter summarizes the results and the conclusions in this thesis.

9

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11

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12

Chapter 2

Anti-hydrolysis Performance of Cured Coating Films of Acrylic Polyols

with Pendant Polylactic acids

2.1 Introduction

Carbon-neutral material is a material which does not affect the change in CO2 amount

emitted and absorbed in the material’s cycle. Especially, bio-based materials, such as PLA,

PBS, denatured starch, and soybean polyols, attract attention [1-10]. In automotive coatings

business for which many petroleum derived materials, like acrylic resins and polyurethane

resins, are currently employed in much amount, and hence, the mitigation of global

environment problems through developing the bio-based coatings from bio-based materials

has been aimed.

PLA is a typical biodegradable plastic and is synthesized by dehydration of the lactic acids

obtained to saccharifying starch, such as corn and so on, and fermenting it [11-15]. Although

PLA is actually put in practical use in recent years [16-23], it is not yet for commodity use.

The lack of heat resistance, impact resistance, and hydrolysis resistance compared with

petroleum derived materials, and high cost of lactide, a starting material of PLA, are the main

reasons [14, 15]. In particular, hydrolysis resistance is an important point. In other words,

because PLA contains a lot of ester bonds in its molecular structure, it is the biggest issue that

degradation by hydrolysis must be controlled. Although increasing crystallinity of PLA or

formation stereo-complex of PLA from poly(L-lactic acid) and poly(D-lactic acid) are

suggested as methods of improving hydrolysis resistance, these approaches are unsuitable for

paint materials due to its low solvent solubility and low storage stability.

Moreover, when a polymer main chain consists of PLA, the number of repetitions of an

ester bond increases. If hydrolysis takes place, the decrease in molecular weight is caused as

shown in Figure 2-1. Thus coating film will degrade quickly. In the grafted type structure

shown in Figure 2-2 (a), where the main chain is acrylics and side chains are PLA,

degradation by hydrolysis could be controlled by introducing PLA into side chains. That is, as

13

Figure 2-1 The model of PLA chain hydrolysis.

14

Figure 2-2 The model of acrylic polyols with pendant PLA: (a) before

hydrolysis and (b) after hydrolysis.

15

seen in Figure 2-2 (b), if the length of acrylic main chain is longer enough than that of the

PLA side chains, and even if the PLA side chains are hydrolyzed, acrylic main chain will

remain without hydrolysis. Therefore, coating film degradation due to decrease of molecular

weight by hydrolysis can be controlled.

In order to synthesize grafted type polymers like Figure 2-2 (b), PLA macromonomer

having acryloyl group is needed. PLA macromonomer polymerizable from 2-hydroxyethyl

methacrylate (HEMA), as shown in Figure 2-3, has been synthesized, and acrylic polyols with

pendant polylactic acids are designed by copolymerization of this PLA macromonomer and

acrylic monomer.

The reason why we use HEMA as a source of polymerizable group is that HEMA can be

introduced at the end of PLA by ring-opening polymerization of lactide [24-30]. Two

component paint coatings of the grafted type polyols obtained by copolymerization of HEMA

and PLA macromonomer cured by polyisocyanate hardner can control coating film

degradation, because HEMA cross-linking parts are hard to be hydrolyzed even if PLA

cross-linking parts were hydrolyzed as shown in Figure 2-4.

A bio-based emulsion has been prepared by the emulsion polymerization of PLA

macromonomer modified HEMA and n-butyl methacrylate (BMA) in water, and we measured

physical properties of its cast film has been studied [24, 25]. Although this polymer emulsion

containing PLA can be expanded into resins for water borne paints, physical properties of this

emulsion coating film were not enough because this film was formed by melting among these

emulsion particles and cross-linking structure was not introduced. On the other hand, the

acrylic polyols with pendant polylactic acids designed here are resins for solvent borne paints,

and physical properties are expected to be improved by introducing cross-linking structure by

curing with polyisocyanate hardner because the end of side chains are OH groups.

In this chapter, preparation of acrylic polyols with pendant polylactic acids by

copolymerization of PLA macromonomer modified with HEMA and acrylic monomer is

described. Anti-hydrolysis performance and other basic coating performances of the coatings

of acrylic polyols with pendant polylactic acids cured by polyisocyanate were evaluated.

16

Figure 2-3 PLA manromonomer derived from HEMA.

17

Figure 2-4 The model of copolymer of HEMA and the PLA macromonomer

cured with polyisocyanate: (a) before hydrolysis and (b) after hydrolysis.

18

2.2 Experimental

2.2.1 Synthesis of PLA macromonomer modified with HEMA

L-Lactide (PURASORB L; Purac Co.), 2-hydroxyethyl methacrylate (HEMA; Mitsubishi

Rayon Co., Ltd.), and tin octoate (Sn(Oct)2; Nacalai Tesque Inc.) were commercial reagents

and used as received.

PLA macromonomer with ring-opening polymerization of lactide (lactide method) was

synthesized as the previous report [24].

A macromonomer having a methacryloyl group (MM6.0) was synthesized as follows.

L-Lactide (35.9 g) was placed in a 200 mL flask with three necks and dried under vacuum for

1 hour. HEMA (10.8 g) and Sn(Oct)2 (0.20 g) were added into the flask filled with dry N2 gas,

and the mixture was heated at 110 °C, using an oil bath, for 3 hours with stirring. MM6.0 was

almost quantitatively synthesized [24]. 1H NMR measurements were recorded on a spectrometer ARX-500 (500 MHz, Bruker Co.).

ESI-TOF-MS analysis was performed by using a micro-TOF instrument (ESI-TOF-MS;

BRUKER DALTONICS, Germany). Molecular weight of polymers was measured by a gel

permeation chromatography (GPC) (GL-7400 Series; GL Science Inc.) with a refractive index

(RI) detector using tetrahydrohuran eluent at a column temperature 40 °C, in which

polystyrene standards (molecular weight; 2,200-650, 000) were employed.

2.2.2 Synthesis of acrylic polyols with pendant polylactic acids

Acrylic polyols with pendant polylactic acids (Figure 2-5) were synthesized by radical

copolymerization of PLA macromonomer derived from HEMA and n-butyl acrylate (nBA;

Mitsubishi Rayon Co., Ltd.). t-Butyl peroxy 2-ethylhexanoate (Kaya Ester O; Kayaku Akzo

Co., Ltd.) was used as a peroxide type polymerization initiator. The molecular weight Mw of

the copolymer was about 10,000-20,000. The monomer unit ratio (wt.%) is shown in Table

2-1.

For comparison, linear chain-like PLA and polyglycerol copolymer (linear chain PLA; Mw

7,200, Mn 6,000, OH value 150 mg KOH/g) as shown in Figure 2-6 was used as a model.

19

Figure 2-5 A model structure of aclyric polyols with pendant polylactic acids.

20

Table 2-1 The monomer ratio for the preparation of copolymers of acrylic

polyols with pendant polylactic acids.

21

Figure 2-6 Linear chain-like PLA and polyglycerol coplymer

22

Molecular weight of the polymers was measured by a gel permeation chromatography (GPC)

(HLC-8220GPC; TOSOH Co.)with a refractive index (RI) detector using tetrahydrohuran

eluent at a column temperature 40 °C, in which polystyrene standards (molecular weight;

2,200-650, 000) were employed.

2.2.3 Preparation of cured coating films

Cured coating films were prepared by reaction of acrylic polyols with pendant polylactic

acids (Table 2-1, No.1) or linear chain PLA with biuret type polyisocyanate hardner

(NCO/OH=1/1 [mol ratio]) by air spray painting on polypropylene sheets. Baking condition

was set at room temperature for 10 minutes and heating at 80 °C for 60 minutes. After baking,

the coating films were peeled from polypropylene sheets, and their free films were obtained.

The free film thickness was 40-45 μm.

2.2.4 Evaluation method of anti-hydrolysis performance

The free films were cut to small pieces in 10 mm wide and 700 mm long as test pieces. The

hydrolysis test was carried out at 50 °C and 95% R.H..

The test piece was kept for 0, 5, 10, 20, 30, and 40 days under the hydrolysis test conditions

and was taken out for drying at room temperature for 3 hours. Then, tensile strength and

cross-linking density of the test piece were measured.

The tensile strength of the test piece was measured by using Auto Graph (AG-IS; Shimadzu

Co., Ltd.). As for the tensile measurement, the test piece was cut in 10 mm wide and 50 mm

long, and tensile-loading speed was 5 mm/min. The cross-linking density of the test piece was

measured with dynamic viscoelastic measurement using rheospectror (Rheogel E4000 ; UBM

Co., Ltd.) and computed from Equation 1. As for the dynamic viscoelastic measurement

condition, the test piece size was 5 mm wide and 20 mm long, and the measuring temperature

range is from 20 °C to 150 °C.

n=E'min/(3RT) [mol/cc] (Equation 1)

23

n: Cross-linking density [mol/cc]

E'min: Flat area storage modulus [dyne/cm2]

T: The absolute temperature which gives E'min [K]

R: The gas constant [8.314*107 erg/deg/mol]

Moreover, in order to examine hydrolysis behavior of ester bond in the coatings, Fourier

transform infrared (FT-IR) measurements were also performed with attenuated total reflection

(ATR) method using NICOLET 4700 (Thermo Electron Co., Ltd.).

2.2.5 Evaluation method of the coating performances

The clear coatings which consisted of acrylic polyols with pendant poly(lactic acid)s (Table

2-1 No.1-5) and biuret type polyisocyanate hardner (DURANATE 24-100; Asahi Kasei

Chemicals Co.), NCO/OH = 1/1 [mol ratio], were prepared by air spray painting on solvent

borne two component black base paint coatings (R-241MB 202; Nippon Bee Chemical Co.,

Ltd.) applied to poly(acrylonitrile/butadiene/styrene) (ABS) sheets. Baking condition was at

room temperature for 10 minutes and heating at 80 °C for 60 minutes. Film thickness was

30-35 μm.

The coating performances, such as an initial adhesion property, a humidity resistance

property, and chemical resistance properties for alkaline, water, and acid, were evaluated.

An initial adhesion property of the clear coating was evaluated by cross-cut adhesion test;

the surface of the clear coating was cut in 100 meshes to 20 mm around square with a cutter,

scotch tape was sticked on it and peeled off, and the number of meshes peeled off was

counted.

A hydrolysis resistance property was evaluated by the appearance and cross-cut adhesion

test of the clear coating after the humidity test. The method of hydrolysis resistance test is that

the clear coating was placed under high humidity condition such as 50 °C and 95% R.H. for

240 hours.

Chemical resistance properties were evaluated by spot tests. The appearance of the clear

coating observed after the spot tests that a teflon ring (diameter; 4 mm) was sticked on the

24

coating, a reagent (10 ml) was put into the ring, and the test piece was settled on 55 °C for 4

hours after the ring was sealed. 0.1N NaOH aqueous solution was used as an alkaline reagent,

distilled water as a neutral reagent, and 0.1N H2SO4 aqueous solution as an acid reagent.

2.3 Results and discussion

2.3.1 Synthesis of PLA macromonomer modified with HEMA

The synthesis route is given in Scheme 2-1 [24]. First, ring-opening polymerization of

L-lactide was carried out, initiated by HEMA, with a Sn(Oct)2 catalyst, to synthesis PLA

macromonomer modified with a methacryloyl group (MMm).

The Sn catalyzed ring-opening polymerization of L-lactide initiated by HEMA has been

reported [24-30]. The PLA chain length (m value) could be controlled by the feed molar ratio

of L-lactide/HEMA. Figure 2-7 [24] shows the 1H NMR (500 MHz) spectrum of (a) PLA

macromonomer, which was prepared by the reaction of L-lactide/HEMA = 3.0/1.0 (the feed

molar ratio), so that the lactic acid units might become statistically 6.0 and, hence, designated

as MM6.0. The monomer conversion was almost quantitative (>98% from the NMR analysis).

In comparison with the spectra of (b) L-lactide and (c) HEMA, the spectrum of (a) PLA

macromonomer clearly supports the structure of MM6.0, in which the plausible assignments

of peaks are given. From the peaks integration ratio due to methine protons and also to methyl

protons, the average m value of MMm was obtained to be m = 6.0, with the average m value

being equal to the feed molar ratio of L-lactide/HEMA. Further, from the integration ratio of

the olefin proton and the methine proton, the methacryloyl group content of MM6.0 was

calculated as higher than 96%.

Figure 2-8 demonstrates the ESI-TOF-MS chart of the PLA macromonomer (MM6.0) [24],

indicating that MM6.0 contains a mixture of 2-10 lactic acid units (peak top at 6) linked to

HEMA. The peak top value of m/e 585.2 corresponds to the sum of the mass due to the

structure of MM6.0 of 562 (130 of HEMA + 6×72, the repeat unit mass, plus 23 of Na+). In

addition to the major peak, the minor peak observed at 601 is due to the mass of 562 plus 39

25

Scheme 2-1 The synthesis route of PLA macromonomer by ringo-pening

polymerization of L-lactide and HEMA.

26

Figure 2-7 500 MHz 1H-NMR spectra (CDCl3 with TMS) of (a) PLA

macromonomer (MM6.0), (b) L-lactide, and (c) HEMA.

27

Figure 2-8 ESI-TOF-MS chart of MM6.0.

28

(K+). The major and minor peaks situation is similar for other 8 peaks, which appear at every

72 intervals. MM6.0 is composed of not only even numbered repeat units but odd numbered

ones as well. According to the accepted Sn catalyzed reaction mechanism of the lactide

ring-opening polymerization, only even numbered units are to be produced [31-33]. Therefore,

the present reaction suggests that the transesterification reaction between lactide-ester bonds

took place extensively. Following the PLA macromonomer preparation reaction by

ESI-TOF-MS measurements revealed that at the beginning of the reaction only the even

numbered products were formed. With progress of the reaction, production of the odd

numbered ones was also observed, indicating that the transesterification started. However,

there was no product observed derived from the ester bond cleavage of HEMA under the

present reaction conditions.

PLA macromonomer derived from HEMA was obtained by ring-opening polymerization of

L-lactide, of which the rate of functional group is >98% and the degree of polymerization is

from 4 to 10.

2.3.2 Anti-hydrolysis performance of the acrylic polyols with pendant polylactic acids

coatings

The acrylic polyols with pendant polylactic acids was prepared by radical copolymerization

of HEMA, nBA, and the PLA macromonomer based on the feed monomer ratio of Table 2-1.

The Mw of the polymer is about 10,000-20,000, and the Mn is about 3,200-10,000. The

hydrolysis behavior of the coating films prepared from the cured acrylic polyols with

polyisocyanate was evaluated.

Figure 2-9 shows tensile strength of the coatings as a function of the time of hydrolysis

resistance test. The increase of the coating tensile strength by post-curing was observed for

the coatings of the test until 10 days and for the linear chain PLA coating until 20 days.

However, the tensile strength of the linear chain PLA coating decreased sharply for the

coating of hydrolysis after 20 days. In contrast, the tensile strength of the acrylic polyols with

pendant polylactic acids coating was kept with hydrolysis test for 40 days. These results show

that the acrylic polyols with pendant polylactic acids coating is not easier than the linear chain

29

Figure 2-9 Tensile strength of the coatings as a function of time of hydrolysis

resistance test; the acrylic polyols with pendant polylactic acids (■; Table2-1

No.1), linear chain PLA (●).

30

PLA coating to be hydrolyzed under high temperature and high humidity.

Figure 2-10 shows cross-linking density of the coatings as a function of the time of

hydrolysis resistance test. It can be observed that the increase of cross-linking by post-curing

until 10 days in about both the acrylic polyols with pendant polylactic acids coating and the

linear chain PLA coating. However, like the result of tensile strength, the cross-linking density

of the linear chain PLA coating was decreasing sharply after 10 days, and the cross-linking

density of the acrylic polyols with pendant polylactic acids coating was keeping initial

cross-linking density almost until 40 days.

As mentioned above, it was showed that the degradation by hydrolysis could be controlled

more easily with the acrylic polyols with pendant polylactic acids than the linear chain PLA.

In order to examine the hydrolysis behavior of the ester bond in coating at the time of a

hydrolysis resistance test, FT-IR spectra was measured with the ATR method. The result is

shown in Figure 2-11. Figure 2-11 (a) and (b) include the spectra of the sample after the

hydrolysis resistance test of 10 and 30 days.

From Figure 2-11, after 30 days of the hydrolysis resistance test passed, the peak at 3300

cm-1 is assigned to stretching vibration of OH groups and the peak at 1710 cm-1 is assigned to

C=O stretching vibration of COOH groups in the linear chain PLA coating, the ratio of peak

intensity became larger than the ratio of the peaks of the acrylic polyols with pendant

polylactic acids coating.

Hydrolysis of the ester bond included in PLA is shown in Reaction 1.

R-COO-R' + H2O → R-COOH + R'-OH (Reaction 1)

Therefore, the linear chain PLA coatings can be concluded that there are more amounts of

generation per unit time of OH groups and COOH groups than the acrylic polyols with

pendant polylactic acids coatings. That is, it is thought that the linear chain PLA coating

degradation speed was relatively high and it led to a rapid reduction of tensile strength and

cross-linking density.

31

Figure 2-10 Cross-linking density of the coatings as a function of time of

hydrolysis resistance test; the acrylic polyols with pendant polylactic acids (■;

Table 2-1 No.1), linear chain PLA (●).

32

Figure 2-11 FT-IR spectra of the coatings of the time of humidity resistance test;

(a) linear chain PLA, (b) acrylic polyols with pendant polylactic acids.

33

2.3.3 Evaluation of basic coating performances of the coatings of acrylic polyols with

pendant polylactic acids

An initial adhesion property, a humidity resistance property, and chemical resistance

properties of alkaline, water, and acid of the coatings of acrylic polyols with pendant

polylactic acids were evaluated. The results are shown in Table 2-2. Weight percentage of the

bio-based constituent occupies in coating containing curing agent is "Bio-content."

In the coating of homopolymer of PLA macromonomer modified with HEMA (No.5),

white blooming was observed by the alkaline resistance spot test, and the coating was

hydrolyzed in this condition. On the other hand, in No.1 and No.2, in which HEMA was

copolymerized like No.1 and the cross-linking points were introduced with HEMA, alkaline

resistance was satisfied, and the basic coating performances were passing. Moreover, in No.3

and No.4, even if it carried out the copolymerization of HEMA and introduced cross-linking

points with HEMA, when the OH value was 88 mg KOH/g like No.4, alkaline resistance was

not enough. On the other hand, when the OH value was 120 mg KOH/g like No.3, alkaline

resistance was satisfied. It was found that the 51 wt% bio-content coating satisfied the basic

coating performances when HEMA was copolymelyzed and total OH value was more than

120 mg KOH/g like No.3. These results demonstrate that the PLA macromonomers derived

from lactic acid can be used for good coating materials.

34

Table 2-2 The basic coating performances of the cured coatings of the acrylic

polyols with pendant polylactic acids.

35

2.4 Summery

The acrylic polyols with pendant polylactic acids was synthesized by copolymerization of

PLA macromonomer and acrylic monomer, such as HEMA and nBA.

The clear coating of acrylic polyols with pendant polylactic acids which was cured by

polyisocyanate had better anti-hydrolysis property than the coating of linear chain PLA.

The coating of acrylic polyols with pendant polylactic acids, which is about 50 wt.%

bio-content included curing agent, satisfied basic coating performances, such as an adhesion

property, a hydrolysis resistance property, and chemical resistance properties. The acrylic

polyols with pendant polylactic acids can be applied to two component coatings.

36

2.5 References

[1] P. T. Anastas, J. C. Warner, Green Chemistry: Theory and Practice; Oxford University

Press (1998).

[2] S. Kobayashi, J. Polym. Sci. Part A: Polym. Chem., Vol.37, pp.3041-3056 (1999).

[3] S. Kobayashi, H. Uyama, S. Kimura, Chem. Rev., Vol.101, pp.3793-3818 (2001).

[4] S. Kobayashi, H. Uyama, T. Takamoto, Biomacromolecules, Vol.1, pp.3-5 (2000).

[5] S. Kobayashi, A. Makino, Chem. Rev., Vol.109, pp.5288-5353 (2009).

[6] T. Tsujimoto, R. Ikeda, H. Uyama, S. Kobayashi, Macromol. Chem. Phys., Vol.202,

pp.3420-3425 (2001).

[7] A. Mahapatro, A. Kumar, R. A. Gross, Biomacromolecules, Vol.5, pp.62-68 (2004).

[8] J. E. Puskas, M. Y. Sen, J. R. Kasper, J. Polym. Sci. Part A: Polym. Chem., Vol.46,

pp.3024-3028 (2008).

[9] S. Kobayashi, Macromol. Rapid Commun., Vol.30, pp.137-166 (2009).

[10] J. E. Puskas, M. Y. Sen, K. S. Seo, J. Polym. Sci. Part A: Polym. Chem., Vol.47,

pp.2959-2976 (2009).

[11] Y. Kimura, Nature Material Plastics, KYORITSU SHUPPAN CO., LTD. (2006).

[12] H. Tsuji, Poly lactic acids, KOUBUNSHIKANKOUKAI CO., LTD. (1997).

[13] I. Inomata, High function and recycle technology of bio-based plastics, Science &

Technology (2007).

[14] K. Yoshida, H. Matsuda, T. Tochioka, Mazda Technical Review, Vol.25, pp.157-160

(2007).

[15] Y. Kimura, Polymers, Vol.57, pp.430-433 (2008).

[16] B. Gupta, N. Revagade, J. Hilborn, Prog. Polym. Sci., Vol.32, pp.455-482 (2007).

[17] H. Tsuji, Y. Ikada, Polym. Deg. Stab., Vol.67, pp.179-189 (2000).

[18] J. Lunt, Polym. Deg. Stab., Vol.59, pp.145-152 (1998).

[19] K. Fukushima, Y. Kimura, Macromol. Symp., Vol.224, pp.133-144 (2005).

[20] K. Fukushima, Y. H. Chang, Y. Kimura, Macromol. Biosci., Vol.7, pp.829-835 (2007).

37

[21] S. I. Moon, C. W. Lee, I. Taniguchi, M. Miyamoto, Y. Kimura, Polymer, Vol.42,

pp.5059-5062 (2001).

[22] S. I. Moon, C. W. Lee, M. Miyamoto, Y. Kimura, J. Polym. Sci.,Part A: Polym.Chem.,

Vol.38, pp.1673-1679 (2000).

[23] K. Fukushima, Y. Furuhashi, K. Sogo, S. Miura, Y. Kimura, Macromol. Biosci., Vol.5,

pp.21-29 (2005).

[24] K. Ishimoto, M. Arimoto, H. Ohara, S. Kobayashi, M. Ishii, K. Morita, H. Yamashita, N.

Yabuuchi, Biomacromolecules, Vol.10, pp.2719-2723 (2009).

[25] K. Ishimoto, M. Arimoto, T. Okuda, S. Yamaguchi, Y. Aso, H. Ohara, S. Kobayashi, M.

Ishii, K. Morita, H, Yamashita, N. Yabuuchi, Biomacromolecules, Vol.13, pp.3757-3768

(2012).

[26] S. J. Huang, J. M. Onyari, J. Macromol. Sci.:Pure Appl. Chem., Vol.A33, pp.571-584

(1996).

[27] I. Burakat, Ph. Dubois, Ch. Grandfils, R. Jerome, J. Polym. Sci., Polym. Chem. Ed.,

Vol.34, pp.497-502 (1996).

[28] D. W. Lim, S. H. Choi, T. G. Park, Macromol. Rapid Commun., Vol.21, pp.464-471

(2000).

[29] J. A. Wallach, S. J. Huang, Biomacromolecules, Vol.1, pp.174-179 (2000).

[30] H. Shinoda, K. Matyjaszewski, Macromolecules, Vol.34, pp.6243-6248 (2001).

[31] H. R. Kricherdorf, I. Kreiser-Saunders, A. Stricker, Macromolecules, Vol.33, pp.702-709

(2000).

[32] A. Kowalski, A. Duda, S. Penczek, Macromolecules, Vol.33, pp.7359-7370 (2000).

[33] O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev., Vol.104, pp.6147–6176

(2004).

38

Chapter 3

Synthesis and application of star-shaped polylactic acids to two component

and UV curable coatings

3.1 Introduction

In chapter 2, the acrylic polyols with pendant polylactic acids was synthesized by

copolymerization of PLA macromonomer modified with HEMA and acrylic monomer.

Anti-hydrolysis performance and other basic coating performances of the coatings of acrylic

polyols with pendant polylactic acids cured by polyisocyanate were evaluated [1-3]. The

hydrolysis property and basic coating performances are good. The PLA macromonomer was

synthesized from L-lactide and HEMA. L-lactide is high cost and poor handling by

ring-opening from water absorbency.

Then, to achieve high hydrolysis resistance and low cost, star-shaped polylactic acids were

designed by dehydration condensation of lactic acid and polyols [4]. Because star-shaped

polylactic acids are synthesized from cheap lactic acid, lowering of the cost was achieved.

And because star-shaped polylactic acids are multi-functional, their cured coatings have high

cross-linking density and high hydrolysis resistance. As these star-shaped polylactic acids

have hydroxyl groups, they were developed as two component thermal curing type coatings

with a polyisocyanate hardner. If oil and lactic acid were starting materials to synthesize

branched polylactic acids [5], the end of the branched polymer were carboxylic acid groups

and the branched polymer could not be used for general two component type coatings with a

polyisocyanate hardner.

Reactive double bonds were introduced to ends of the star-shaped polylactic acids polyols,

and they were applied to UV curable coatings. UV curable coatings are environmentally

friendly coating because of low generation of CO2 and volatile organic compounds.

In this chapter, multi-functional star-shaped polylactic acids with hydroxyl groups or

reactive double bonds were synthesized from the polylactic acids polyols, and they were

applied to bio-based two component or UV curable coatings. Moreover, their basic coating

39

performances were evaluated.

3.2 Experimental

3.2.1 Synthesis of star-shaped polylactic acids polyols

Tetra-functional star-shaped polylactic acids polyols were synthesized by dehydration

condensation of L-lactic acid (Purac HS88; Purac Co.) and pentaerythritol (Kishida Chemical

Co.) in a molar ratio of 14/1 as shown in Scheme 3-1. 0.2 wt% of p-toluenesulfonic acid

(Kishida Chemical Co.) to solid content as a reaction catalyst and 5.0 wt% of xylene (Kishida

Chemical Co.) as an azeotropic solvent were used. The reaction was carried out under the

atmosphere pressure and the reaction temperature was raised from the room temperature to

175 °C within 2 hours, and maintained for 5 hours at 175 °C. The reaction was stopped when

acid value of the reaction mixture became less than one. The acid value was determined by

titration with a 2-propanol solution of 0.1 mol/L potassium hydroxide (Kishida Chemical

Co.).

In the same way, hexa-functional star-shaped polylactic acids polyols were synthesized by

dehydration condensation of L-lactic acid and dipentaerythritol (Dipentarit; Koei Chemical

Co.) in a molar ratio of 14/1.

The star-shaped polylactic acids polyols were diluted with butyl acetate (Kishida Chemical

Co.) to become 70 wt% solutions.

Molecular weight of the polymers was measured by a gel permeation chromatography

(GPC) (HLC-8220GPC; TOSOH Co.) with a refractive index (RI) detector using

tetrahydrohuran eluent at a column temperature 40 °C, in which polystyrene standards

(molecular weight; 2,200-650, 000) were employed. ESI-TOF-MS analysis was performed by

using a microTOF instrument (ESI-TOF MS; BRUKER DALTONICS, Germany).

40

Scheme 3-1 Synthesis of star-shaped polylactic acids polyols.

41

3.2.2 Synthesis of star-shaped polylactic acids with reactive double bonds

Multi-functional star-shaped polylactic acids with reactive double bonds were synthesized

by addition of succinic anhydride and glycidyl methacrylate (GMA) as shown in Scheme 3-2.

At first, the star-shaped polylactic acids polyols and succinic anhydride (Kishida Chemical

Co.) were dissolved at 90 °C. The temperature was lowered to 60 °C and triethylamine

(Kishida Chemical Co.) was added as a reaction catalyst. The temperature was maintained for

5 hours. The amounts of succinic anhydride and triethylamine were 90 mol% and 10 mol% to

the hydroxyl groups in the polylactic acids, respectively. Then, GMA (GE-510; Mitsubishi

Gas Chemical Co.) was added to the solution, and the temperature was raised to 80 °C and

was maintained for 7 hours. Two thousand ppm of p-methoxyphenol (Kishida Chemical Co.)

was added as a polymerization inhibitor at once. In this way, the methacryloyl groups were

introduced to the star-shaped polylactic acids. The amount of GMA was 85 mol% based on

the carboxyl groups of the polylactic acids. The degree of addition reaction of succinic

anhydride was evaluated with Fourier transform infrared spectroscopy by the disappearance

of the peak at 1865 cm-1 assigned to C=O antisymmetric stretching vibration. The end point of

GMA addition reaction was estimated by the use of acid value, and the amount of unreacted

GMA was determined with liquid chromatography. Molecular weight of the polymers was

measured by GPC (HLC-8220GPC; TOSOH Co.) with RI detector using tetrahydrohuran

eluent at a column temperature 40 °C, in which polystyrene standards (molecular weight;

2,200-650, 000) were employed.

42

Scheme 3-2 Synthesis of star-shaped polylactic acids modified with

methacryloyl groups.

43

3.2.3 Preparation of two component cured coatings

Bio-based two component cured coatings were prepared by formulating the star-shaped

polylactic acids polyol and acrylic polyol. The acrylic polyol was prepared by radical

copolymerization of HEMA (Mitsubishi Rayon Co., Ltd.), methacrylic acid (MAA;

Mitsubishi Rayon Co., Ltd.), methyl methacrylate (MMA; Mitsubishi Rayon Co., Ltd.),

n-butyl methacrylate (nBMA; Mitsubishi Rayon Co., Ltd.), isobornyl methacrylate (IBXMA;

Light Ester IB-X, Kyoeisha Chemical Co., Ltd.), HEMA containing 2 mol of caprolactone

(FM2D; PLACCEL FM2D, DAICEL Co., Ltd.). The monomer weight ratio of the acrylic

polyol is HEMA/MAA/MMA/nBMA/IBXMA/FM2D=25.7/0.7/10.6/16.4/21.6. The Mw of

the polymer is about 8,200, and the Mn is about 4,000. The OH value is 150 mg KOH/g and

the acid value is 4.6 mg KOH/g. The glass transition temperature (Tg) is about 40 °C. The

t-Butyl peroxy 2-ethylhexanoate (Kaya Ester O; Kayaku Akzo Co., Ltd.) was used as a

peroxide type polymerization initiator.

The clear coatings which consisted of the star-shaped lactic acids poliols, the acrylic

polyols, Tinuvin 384-2 (BASF Co.) as an ultra violet absorber, Tinuvin 123 (BASF Co.) as a

hindered amine light stabilizer, dibutyltin dilaurate (Wako Pure Chemical Industries Co., Ltd.)

as a curing catalyst, BYK-310 (BYK Co.) as a levelling additive, CARBODILITE V-05

(Nisshinbo Chemical Inc.) as a carbodiimide hardner and CORONATE HX (NIPPON

POLYURETHANE INDUSTRY CO., LTD.) isocyanurate type polyisocyanate hardner,

NCO/OH = 1.2/1.0 [mol ratio], were prepared by air spray painting on solvent borne two

component black base paint coatings (R-241MB 202; Nippon Bee Chemical Co., Ltd.)

applied to ABS sheets. The formulations of the clear coating are shown in Table 3-1. Baking

condition was at room temperature for 10 minutes and heating at 80 °C for 30 minutes. Film

thickness was 30-35 μm. Free coating films of the clear coating was applied to polypropylene

sheets by air spray directly. After baking at 80 °C for 30 minutes, the free coating films were

peeled from the polypropylene sheets, and their free films were obtained. The free film

thickness was 40-45 μm.

‘Bio-content’ is defined as weight percentage of the bio-based component in the star-shaped

polylactic acids or their coating films. The bio-content of the star-shaped polylactic acids

44

Table 3-1 The formulations of the two component cured coatings.

45

polyols is about 88 wt.%.

3.2.4 Preparation of UV cured coatings

Bio-based UV curable coatings were prepared by formulating the star-shaped polylactic

acids having reactive double bonds with photo-initiators and urethane acrylates (or

dipentaerythritol hexaacrylate [DPHA]). DPHA (Aronix M-402; TOAGOSEI Co.) and

urethane hexaacrylate (UN3320HC; Negami Chemical Industrial Co.) were used as acrylate

oligomers. The formulation of UV curable coatings is shown in Table 3-2. Five wt% of

Irgacure 184 (BASF Co.) as a photo-initiator and 0.8 wt% of BYK-333 (BYK Co.) as a

levelling additive based on total resins were added.

The bio-based UV curable coating films were prepared by air spray coating on

polycarbonate (PC) substrates. The coatings were pre-heating at 50 °C for 10 minutes and

irradiated with a high pressure mercury vapor lamp (240 W/cm, Fusion UV Systems Inc.).

Irradiation energy was 1000 mJ/cm2 at 365 nm, and the intensity was 750 mW/cm2. Film

thickness was about 15 μm.

‘Bio-content’ is defined as the same way in the section 3.2.3. The bio-content of the

star-shaped polylactic acids polyols with double bonds is shown in Table 3-2.

46

Table 3-2 The formulations of UV curable coatings.

47

3.2.5 Evaluation method of the coating performances

Performances of the cured films were evaluated such as an initial adhesion property, a

humidity resistance property, chemical resistance properties for alkaline, water, and acid,

accelerated weatherability, pencil hardness, and abrasion resistance.

The initial adhesion property, the humidity resistance, and the chemical resistance were

evaluated by the same way in the section 2.2.5.

The accelerated weatherability was evaluated with a fade meter (FOM; UV Auto Fade

Meter FAL-AUH, Suga Test Instruments Co., Ltd.) under the condition at 83 °C and 50%

R.H. for 400 hours.

The pencil hardness was determined based on JIS standard K5600-5-4.

The abrasion resistance was evaluated by rubbing with steel wool (BonStar No.0000;

Nippon Steel Wool Co.) under the load of 100 g/cm2.

The cross-linking density and of the free film was measured by the same way in the section

2.2.4.

3.3 Results and discussion

3.3.1 Synthesis of star-shaped polylactic acids and their application to two component

coatings

The GPC chart of the star-shaped polylactic acids polyol is shown in Figure 3-1. Figure 3-2

demonstrates the ESI-TOF-MS chart of the star-shaped polylactic acids polyol. From Figure

3-1 and 3-2, about 4% of free lactic acid oligomers which were not dehydration

condensation with pentaerythritol contained the star-shaped polylactic acids polyol, and

most existed as it. The yield of star-shaped polylactic acids polyol was about 96%.

Table 3-3 shows the results of coating performances of the two component cured coatings.

Bio-content is weight percentage of the bio-based constituent occupies in coating containing

curing agent. Table 3-4 shows the cross-linking density and dynamic Tg of the cured coating.

From Table 3-3, the coating performances of the star-shaped poly lactic acids polyol were

48

Figure 3-1 GPC chart of star-shaped polylactic acids polyol.

49

Figure 3-2 ESI-TOF-MS chart of star-shaped polylactic acids polyol.

50

51

very good, except accelerating weatherability (Table 3-3 No.1). The coating of cured

star-shaped poly lactic acids polyol alone became its gloss down after accelerating

weatherability test. On the other hand, the coating of cured star-shaped poly lactic acids

polyol with the acrylic polyol was good accelerating weatherability (Table 3-3 No.2). The

bio-content of the coating of star-shaped PLA and acrylic polyol was 30 wt.%.

The basic performances satisfied even the coating of cured star-shaped PLA polyol alone.

However, because cross-linking density increased by formulating the acrylic polyol together

(Table 3-4), its accelerating weatherability property was improved.

These results demonstrate that the star-shaped polylactic acid polyol derived from cheep

lactic acid can be used for two component type coating materials.

Test adoption was done in grip parts of ‘i-REAL’s that are personal mobilities of TOYOTA

Motor Co., as shown in Figure 3-3. The bio-content of its star-shaped PLA coating was 40

wt.%.

52

Table 3-4 Cross-linking density and dynamic Tg of the cured coating.

53

Figure 3-3 TOYOTA personal mobility ‘i-REAL’; (a) its whole image, (b) its

grip part.

54

3.3.2 Synthesis of star-shaped polylactic acids with reactive double bonds and their

application to UV curable coatings

The GPC chart of the star-shaped polylactic acids is shown in Figure 3-4. The peak-top

shifts to high molecular weight side after each addition reaction (Scheme 3-2). Moreover,

peaks at low molecular weights were hardly seen and the amount of remaining GMA was

about 0.32%. These results suggest that the star-shaped polylactic acids modified with

methacryloyl groups were synthesized in a high yield. The theoretical equivalent of reactive

double bond of the star-shaped polylactic acids was about 634 or 416 when pentaerythritol or

dipentaerythritol was used as starting material, respectively.

The results of the UV cured film performances are shown in Table 3-5. The initial adhesion,

the alkaline resistance, and the abrasion resistance of the film consisted of the star-shaped

polylactic acids that used pentaerythritol as the starting material were inferior (Table 3-5

No.1). It is thought that the initial adhesion was dissatisfied because of high curing shrinkage

and the abrasion resistance was not good because of low hardness. Moreover, it is thought

that the alkaline resistance was dissatisfied. Because the double bond equivalent of the

tetra-functional star-shaped polylactic acids was high, that is, the cross-linking density was

low. On the other hand, the alkaline resistance of the star-shaped polylactic acids that used

dipentaerythritol as the starting material was improved (Table 3-5 No.2). It is thought that the

hydrolysis resistance was improved because the double bond equivalent of the star-shaped

polylactic acids is low, that is, the cross-linking density was high. However, the improvement

of the abrasion resistance was not observed, because the initial adhesion degree was low due

to high curing shrinkage and the hardness did not increased. The pencil hardness of the

coating film that was formulated with DPHA to raise its hardness was improved (Table 3-5

No.3). However, its initial adhesion degree was dissatisfied because of its high curing

shrinkage. Then, a urethane acrylate was formulated to achieve the initial adhesion by

relaxing curing shrinkage and increasing cohesion energy of the coating film (Table 3-5 No.4).

The initial adhesion and the abrasion resistance of the coating film were satisfied. Bio-content

of the coating film was about 44wt%. Moreover, it was found that the Table 3-5 No.4 coating

film had hard-coating property, because the pencil hardness of Table 3-5 No.4 was higher

55

than that of the PC substrate.

56

Figure 3-4 GPC chart of star-shaped polylactic acids modified with

methacryloyl groups.

57

Table 3-5 The coating performances of the UV cured coatings.

58

3.4 Summery

The multi-functional star-shaped polylactic acids polyol was synthesized from cheap lactic

acid and polyols. The multi-functional star-shaped polylactic acids with reactive double bonds

were synthesized by the addition of succinic anhydride and GMA to the star-shaped polylactic

acids polyols. Moreover, the star-shaped polylactic acids polyols were applied to bio-based

two component type coatings. And, the multi-functional star-shaped polylactic acids with

reactive double bonds showed good UV curability, and they were applied to bio-based UV

curable coatings. The achievement of high bio-content and performances of the coatings will

be aimed at practical use.

59

3.5 References

[1] K. Ishimoto, M. Arimoto, H. Ohara, S. Kobayashi, M. Ishii, K. Morita, H. Yamashita, N.

Yabuuchi, Biomacromolecules, Vol.10, pp.2719-2723 (2009).

[2] K. Ishimoto, M. Arimoto, T. Okuda, S. Yamaguchi, Y. Aso, H. Ohara, S. Kobayashi, M.

Ishii, K. Morita, H, Yamashita, N. Yabuuchi, Biomacromolecules, Vol.13, pp.3757-3768

(2012).

[3] K. Morita, H. Yamashita, N. Yabuuchi, M. Ishii, M. Arimoto, K. Ishimoto, H. Ohara, and S.

Kobayashi, Progress in Organic Coatings, submitted.

[4] K. Morita, H. Yamashita, N. Yabuuchi, Y. Hayata, M. Ishii, K. Ishimoto, H. Ohara, and S.

Kobayashi, RadTech Asia 2011 Proceedings, pp.126-129 (2011).

[5] T. Tsujimoto, N. Imai, H. Kageyama, H. Uyama, M. Funaoka, J. Network Polym., Jpn.,

Vol.29, pp.192-197 (2008).

60

Chapter 4

Synthesis of monodispersed silica nanoparticles and hybrid with

star-shaped polylactic acids

4.1 Introduction

In chapter 3, the multi-functional star-shaped polylactic acids polyol was synthesized from

cheap lactic acid and polyols. The star-shaped polylactic acids with reactive double bonds

were synthesized by the addition of succinic anhydride and GMA to the star-shaped polylactic

acids polyols [1]. Anti-hydrolysis performance and other basic coating performances of the

coatings of the star-shaped polylactic acids were evaluated, and the hydrolysis property and

basic coating performances are good for the actual applications. In addition, the star-shaped

polylactic acids with reactive double bonds showed good UV curability and hard-coating

property. However, to achieve superior hard-coating property and heat resistance, the

star-shaped polylactic acids hybrid with silica particles will be attractive.

UV paints are generally used for hard coating. Hybrid materials with inorganic material and

PLA material have been reported [2-6]. However, they were not for UV curable coating

material. In the present study, the star-shaped polylactic acids with reactive double bonds are

used as an UV coating material, and silica particles are modified with reactive double bonds

and cured with the star-shaped PLA acid by UV radiation. Accordingly, bio-based hybrid

coating with the star-shaped PLA material and silica particles can be prepared.

Monodispersed silica nanoparticles were prepared by the sol-gel process based on the

Stöber method. Very small silica particles are required for some applications. For example, in

a hard-coating for polymer substrates, the coating layer should be transparent, and thus the

silica particles added to the coating must be smaller than a few tens of nanometer to avoid

scattering. However, the Stöber method is believed that the preparation of uniform-sized silica

nanospheres with a size of below 50 nm is rather difficult [7, 8]. To obtain much smaller

nanoparticles, e.g., about 10 nm in diameter, preparation of silica nanoparticles by modified

Stöber method using basic amino acid such as L-lysine [7, 8] or by water in oil

61

microemulsion process [9, 10] have been reported. In the industrial scale, colloidal silica with

small diameter has been prepared using water glass [11]. To introduce reactive double bond to

silica particles, silane coupling agent was used which has reactive double bond such as

acryloyl and methacryloyl groups.

In this chapter, monodispersed silica nanoparticles were prepared by the sol-gel process

based on the Stöber method, by the control of the preparation conditions to increase its

concentration. Effects of composition of starting materials on the size and distribution of

silica particles were examined. In addition, the silica particles were modified by sol-gel

process with a silane coupling agent which has reactive double bonds. Moreover, coating

performances of bio-based hybrid coating with the star-shaped PLA material and silica

particles were evaluated.

4.2 Experimental

4.2.1 Preparation of silica nano-particles

Silica particles were prepared through a process based on the Stöber method, using

tetraethoxysilane (TEOS) (Shin-Etsu Chemical Co.) as a starting material. TEOS was

dissolved in ethanol (Wako Pure Chemical Industries Co.). Separately, ammonia water (25

wt.%) and ethanol were mixed. Then, two solutions were mixed, and then the obtained sol

was stirred for 24 hours. The particles in the sols were collected by centrifugation and dried in

vacuo for 12 hours. The mole ratios of TEOS:H2O (25 wt% NH4OHaq):ethanol were (a)

x:126:9 (1 < x < 15), (b) 5:y:9 (63 < y < 126), and (c) 5:79:z (9 < z < 14).

To increase the concentration of silica, the solvent in the prepared sol was removed under a

reduced pressure using a rotary evaporator at 55 °C.

The concentration of SiO2 in the sol was determined from the weight of the collected and

heat-treated (at 1,000 °C) particles versus the total weight of the prepared sol. The yield of the

SiO2 was calculated from the weight of the collected and heat-treated (at 1,000 °C) particles

versus the theoretical weight of SiO2 obtained from TEOS.

62

4.2.2 Materials characterization of the silica nano-paeticles

A transmission electron microscope (TEM, JEOL JEM2010) was used for the observation

of shape and size of the particles. Size distribution of particles in the sol was determined using

a dynamic light scattering (DLS6000, Otsuka Denshi). Thermal properties of the obtained

particles were determined using DTA-TG (Themo Plus TG8120, Rigaku), with a heating rate

of 10 °C/min.

4.2.3 Preparation of silica particles modified with reactive double bonds

Silica particles were prepared and increased the concentration through the same way in the

section 4.2.2. 3-Methacryloxypropyltrimethoxysilane (MPTMS; KBM-503, Shin-Etsu

Chemical Co.) was used to introduce methacryloyl groups as reactive double bonds to the

silica particles.

The silica particles and MPTMS were refluxed at room temperature or 70 °C for 24 hours,

and the silica particles modified with MPTMS were obtained. The mole ratios of

TEOS:MPTMS:H2O (25 wt.% NH4OHaq):ethanol were 1:x:8.6:125.6 (x=0.1, 0.7, 1.0). The

pH condition is under 7.2, 8.0 or 10.0.

Thermal properties of the obtained particles were determined using DTA-TG (Themo Plus

TG8120, Rigaku), with a heating rate of 10 °C/min. In order to examine state of surface

modification of the silica particles modified with MPTMS were measured with diffuse

reflectance FT-IR measurements using NICOLET 4700 (Thermo Electron Co., Ltd.). Size

distribution of particles in the sol was determined using a dynamic light scattering (DLS6000,

Otsuka Denshi).

4.2.4 Preparation of hybrid coating film of the star-shaped polylactic acid and the silica

nano-particles modified with reactive double bonds

The star-shaped polylactic acids modified with methacryloyl groups (ULAO) were

synthesized by the same way in the section 3.2.2. The star-shaped polylactic acids modified

with methacryloyl groups were ULAO-1 or ULAO-4 when pentaerythritol or

63

dipentaerythritol was used as starting material, respectively.

The silica particles modified with methacryloyl groups were synthesized from commercial

silica (MEK-ST: Nissan Chemical Industries, LTD., particle size; 10-20 nm, SiO2 content; 30

wt.%) and MPTMS. MEK-ST and MPTMS were refluxed at 70 °C for 12 hours, and the silica

particles modified with MPTMS (MPTMS-modified MEK-ST) were obtained. The mole

ratios of SiO2:MPTMS:H2O or 1wt.% NH4OHaq were 1:0.1:0.3 or 0.6.

Bio-based organic-inorganic hybrid UV curable coatings were prepared by formulating

ULAO and MPTMS-modified MEK-ST with photo-initiators. The solid weight ratio of

ULAO and MPTMS-modified MEK-ST was ULAO/MPTMS-modified MEK-ST=100/0,

70/30, 50/50, and 30/70. Five wt% of Irgacure 184 (BASF Co.) formulated as a

photo-initiator based on total resins were added. The coating materials were stirring for 10

minutes at room temperature.

The hybrid UV curable coating films were prepared by coating with doctor blade on

polymethyl methacrylate (PMMA) substrates. The coatings were pre-heating at 80 °C for 5

minutes and irradiated with a high pressure mercury vapor lamp. The intensity was 10

mW/cm2. Exposure time was 5 minutes. Film thickness was about 3-5 μm.

4.2.5 Evaluation method of the coating performances

Performances of the cured films were evaluated by techniques such as pencil hardness, heat

resistance and optical transmittance.

The pencil hardness was determined based on JIS standard K5600-5-4.

The heat resistance was evaluated using DTA-TG (Themo Plus TG8120, Rigaku), with a

heating rate of 10 °C/min under air flow.

The optical transmittance was measured with UV-visible spectroscopy (JASCO V-570).

64

4.3 Results and discussion

4.3.1 Preparation of SiO2 particles with various molar ratios in the starting materials

Silica nanoparticles were prepared using the Stöber process and according to the Ref. [11],

where the molar ratio of TEOS:EtOH:H2O (25 wt.% NH4OHaq) is 1:126:9. Figure 4-1 shows

a TEM image of the nanoparticles obtained, and size distribution of the nanoparticles

determined by dynamic light scattering (DLS). From the TEM image, the diameter of the

primary particles is about 10 nm. The average particle diameter in the sol and geometric

standard deviation, determined by the DLS, were 8.5 and 1.22 nm, respectively. These results

are in good agreement with the TEM observation, and indicate the rather good monodispersity

of the particles. In this composition, the SiO2 concentration in the sol is <1 %. Under such a

dilute condition, silica nanoparticles with a diameter of about 10 nm were obtained.

To increase the SiO2 concentration in the sol, the molar ratio of TEOS was increased,

according to the molar ratio series (a), where TEOS:EtOH:H2O (25 wt.% NH4OHaq.) was

x:126:9 (1 ≤ x ≤ 15). Figure 4-2 (a) shows average particle size and geometric standard

deviation, and Figure 4-2 (b) shows particle concentration and yield, with changing TEOS

ration in the starting materials. With an increase in x (TEOS ratio) up to 10, the concentration

of SiO2 is increased, but the average particle size is <10 nm. Concentration of SiO2 was about

3 wt.% with x = 10. When the x was larger than 10, the water for the hydrolysis of TEOS was

not enough. Thus, unreacted TEOS may remain in the sol, and the concentration of SiO2 was

leveling off with larger TEOS content.

To increase the SiO2 concentration in the sol, the molar ratio of EtOH was decreased,

according to the molar ratio series (b), where TEOS:EtOH:H2O (25 wt.% NH4OHaq.) was

5:y:9 (63 ≤ y ≤ 126). Figure 4-3 shows (a) the average particle size and geometric standard

deviation and (b) particle concentration and yield, with changing EtOH ratio in the starting

materials. With a decrease in y (EtOH ratio), the concentration of the obtained SiO2 is

increased, but the average particle size is increased. With the smallest y in the series (b) (y =

63, largest TEOS concentration), the concentration of the obtained SiO2 was about 5 % and

the average particle size is about 18 nm.

65

Figure 4-1 (a) TEM image of silica nanoparticles and (b) size distribution of

silica nanoparticles determined by DLS. The molar ratio of TEOS:EtOH:H2O

(25 wt% NH4OHaq) is 1:126:9.

66

Figure 4-2 (a) average particle size and geometric standard deviation and (b)

particle concentration and yield, with changing TEOS ratio in the starting

materials. The molar ratio of TEOS:EtOH:H2O (25 wt% NH4OHaq) is x:126:9

(1 ≤ x ≤ 15).

67

Figure 4-3 (a) average particle size and geometric standard deviation and (b)

particle concentration and yield, with changing EtOH ratio in the starting

materials. The molar ratio of TEOS:EtOH:H2O (25 wt% NH4OHaq) is 5:y:9

(63 ≤ y ≤ 126).

68

To promote the hydrolysis and condensation reaction of TEOS, which should result in the

increase in the SiO2 concentration, the H2O molar ratio was increased. The sol was prepared

in the molar ratio series (c), where TEOS:EtOH:H2O (25 wt.% NH4OHaq.) was 5:79:z (9 ≤ z

≤ 14). Figure 4-4 shows (a) average particle size and geometric standard deviation and (b)

particle concentration and yield, with changing H2O (25 wt.% NH4OHaq.) ratio in the starting

materials. With an increase in z (H2O ratio), the concentration of the obtained SiO2 is

increased, but the average particle size is largely increased up to about 60 nm.

To increase the concentration of silica, the solvent in the prepared sol was removed under a

reduced pressure using a rotary evaporator at 55 °C. In this experiment, the sol prepared with

TEOS:EtOH:H2O (25 wt.% NH4OHaq) = 5:79:9 was used, and initial SiO2 concentration was

about 4 %. By controlling the heating time, the sols with SiO2 concentration of 7, 11, 15, and

31 were obtained. The particle concentration and size of silica nanoparticles are shown in

Table 4-1. In the condensed sol with SiO2 concentration of 7, 11, and 15, the average particle

size determined by DLS is <10 nm, and the sol was almost transparent. In addition, the sol

was confirmed to be stable for a few weeks. These results suggest that the particles are not

aggregated during concentration of the sol up to 15 wt.%. The pH of the sol was changed

from pH of 11 to pH of 7 with the removal of solvent, because NH3 is also removed with the

solvent. This change of pH (elimination of NH3 from the sol) must contribute to the stability

of the concentrated sol. However, when the sol was concentrated to 30 wt.%, the particle size

is more than 10 nm, and the sol became translucent. This suggests that the particles are highly

aggregated during concentration of the sol. Figure 4-5 shows the FE-SEM image of silica

nanoparticles (a) before and (b) after the concentration to 15 wt.% SiO2. The size of the

primary particle is almost the same before and after the concentration, and is less than about

20 nm. DLS results supported the FE-SEM observation, where the average particle size was

about 10 nm and the geometric standard deviation was 1.2. The aggregated particles observed

in the FE-SEM images must be formed during centrifugation and drying for the FE-SEM

observation.

It is found that silica sol with 4 wt.% SiO2 concentration and with an average diameter of

<10 nm can be obtained by the control of chemical composition in the starting materials. By

69

Figure 4-4 (a) average particle size and geometric standard deviation and (b)

particle concentration and yield, with changing H2O (25 wt.% NH4OHaq) ratio

in the starting materials. The molar ratio of TEOS:EtOH:H2O (25 wt%

NH4OHaq) is 5:79:z (9 ≤ z ≤ 14).

70

Table 4-1 Particle concentration and size of silica nanoparticles in concentration

process.

71

Figure 4-5 FE-SEM images of silica nanoparticles (a) before and (b) after the

concentration to15 wt.% SiO2.

72

the combination of chemical composition control and solvent evaporation, silica sol with SiO2

concentration of 15 wt.% and with an average particle size of <10 nm. In this process,

surfactant or dispersing agents were not used. Thus, the surface of SiO2 particles in the sol

must be easily modified with a silane-coupling agent. In addition, the sol with such high

concentration and with small particle size is very attractive for the use of filler for the

hard-coating.

4.3.2 Preparation of SiO2 particles modified with reactive double bonds

Figure 4-6 shows TG curves of the modified SiO2 particles with different reaction

temperatures. When the reaction temperature was room temperature, the TG curve was about

the same with the one of the non-modified silica nano-particles. On the other hand, when the

reaction temperature was 70 °C, the weight loss was larger than that of particles reacted at

room temperature. Figure 4-7 shows diffuse reflectance FT-IR spectra of the modified SiO2

particles with different reaction temperatures. The peak at 1710 cm-1 is assigned to C=O

stretching vibration and the peak at 2900 cm-1 is assigned to C-H stretching vibration. The

peak intensity of the sample reacted at 70 °C was larger than the one at room temperature.

These results indicate that the modification reaction of MPTMS to the silica nano-particles is

hardly proceeds at room temperature.

Figure 4-8 shows TG curves of the modified SiO2 particles prepared under different pH

values. With an increase in the pH value, the weight loss was increased. Figure 4-9 shows

diffuse reflectance FT-IR spectra of the modified SiO2 particles prepared under different pH

values. With an increase in the pH value, the peak intensity at 1710 cm-1 and 2900 cm-1 is

increased. These results indicate that the more MPTMS was reacted to the silica

nano-particles surface under larger pHs and higher reaction temperatures.

Figure 4-10 shows TG curves of the modified SiO2 particles prepared with different molar

ratio of MPTMS/TEOS. Even if the molar ratio became larger, the weight loss was almost the

same. Figure 4-11 shows diffuse reflectance FT-IR spectra of different molar ratio of

MPTMS/TEOS. Because the peaks at 1710 cm-1 and 2900 cm-1 are observed in every

spectrum, the molar ratio MPTMS/TEOS=0.1 is assumed to be enough amount to modify the

73

Figure 4-6 TG curves of the modified SiO2 particles with different reaction

temperatures.

74

Figure 4-7 Diffuse reflectance FT-IR spectra of the modified SiO2 particles with

different reaction temperatures.

75

Figure 4-8 TG curves of the modified SiO2 particles prepared under different pH

values.

76

Figure 4-9 Diffuse reflectance FT-IR spectra of the modified SiO2 particles

prepared under different pH values.

77

Figure 4-10 TG curves of the modified SiO2 particles prepared with different

molar ratio of MPTMS/TEOS=x; x=0.1, 0.7, and 1.0.

78

Figure 4-11 Diffuse reflectance FT-IR spectra of different molar ratio of

MPTMS/TEOS=x; x=0.1, 0.7, and 1.0.

79

silica particle.

From Figure 4-12, the silica particles modified with MPTMS were proved to have good

monodispersity.

4.3.3 Evaluation of hardness and heat resistance of the hybrid coating films

The modification conditions of MPTMS to MEK-ST were determined by the results in the

section 4.3.2. Figure 4-13 shows TG curves of MPTMS modified-MEK-ST and weight loss

was observed. Figure 4-14 shows diffuse reflectance FT-IR spectra of MPTMS

modified-MEK-ST. There are peaks at 1710 cm-1 and 2900 cm-1 are observed in the spectra.

From Figure 4-13 and 4-14, silica nano-particles containing in MEK-ST were proved to be

modified with MPTMS.

Figure 4-15 shows pencil hardness of the hybrid coating films with changing SiO2

nano-particle weight ratios. The hardness of the hybrid coating prepared with addition of 1

wt.% NH4OHaq is larger than that of the films with non-modified silica or addition of H2O, in

the both case of using ULAO-1 and ULAO-4. By using 1 wt.% NH4OHaq, silica particles

were modified with more amounts of MPTMS, as discussed in the previous section. Thus,

more cross-linking between ULAO and MPTMS modified-MEK-ST was developed. The

pencil hardness of the hybrid coating with more than 50 wt.% silica particles was higher than

that of the PMMA substrate. Thus, the hybrid coating with ULAO and MPTMS

modified-MEK-ST is found to have hard-coating property. The pencil hardness of the hybrid

coating using ULAO-4 was larger than that of the hybrid coating using ULAO-1. Because the

double bond equivalent of hexa-functional ULAO-4 was lower than that of tetra-functional

ULAO-1, that is, the cross-linking density was high.

Figure 4-16 shows optical transmission spectra of the hybrid coating of ULAO and

MPTMS modified-MEK-ST. The transmittance of the hybrid coating prepared with H2O

showed good transparency. However, because of aggregation of the MPTMS

modified-MEK-ST in the coating solution during drying, the transmittance of the hybrid

coating prepared with NH4OHaq was lower than that of the hybrid coating with H2O.

80

Figure 4-12 Size distribution of silica nanoparticles modified with MPTMS

determined by DLS. The molar ratio of TEOS:MPTMS:H2O is 1:0.1:0.3.

81

Figure 4-13 TG curves of MPTMS modified-MEK-ST prepared with H2O or

1 wt.% NH4OHaq.

82

Figure 4-14 Diffuse reflectance FT-IR spectra of MPTMS modified-MEK-ST

prepared with H2O.

83

Figure 4-15 Pencil hardness of the hybrid coating films with changing SiO2

nano-particle weight ratios.

84

Figure 4-16 Optical transmission spectra of the hybrid coating of ULAO and

MPTMS modified-MEK-ST; (a) ULAO-1, (b) ULAO-4.

85

Figure 4-17 shows TG curves of the hybrid coating of ULAO and MPTMS

modified-MEK-ST. In the both case of using ULAO-1 and ULAO-4, the decomposition

temperature was shifted to higher temperature with an increase in SiO2 contents. That

indicates that the hybrids with ULAO and silica particles have good heat resistance property.

86

Figure 4-17 TG curves of the hybrid coating of ULAO and MPTMS

modified-MEK-ST; (a) ULAO-1, (b) ULAO-4.

87

4.4 Summery

Silica nanoparticles with high concentration were prepared by the Stöber method using

TEOS as a starting material. It was found that about 4 wt% of silica nanoparticles with a

diameter of about 10 nm were obtained by controlling the reaction conditions in the Stöber

process. By removing solvent under a reduced pressure, the particle concentration was

increased up to 15 wt.% without aggregation.

Silica nanoparticles modified with methacryloyl groups were prepared by modification

reaction of MPTMS. Much more MPTMS was reacted to the silica nano-particles surface

under large pHs and higher reaction temperatures.

The bio-based hybrid coatings of star-shaped polylactic acids and silica particles modified

with reactive double bonds showed good hard-coating property and heat resistance property.

In particular, the hardness of the hybrid coating became larger with an increase in the

MPTMS amount in the modified silica particles. Hard-coating and heat resistance property

were also improved with an increase in the amounts of silica particles in the hybrid coating.

88

4.5 References

[1] K. Morita, H. Yamashita, N. Yabuuchi, Y. Hayata, M. Ishii, K. Ishimoto, H. Ohara, and S.

Kobayashi, RadTech Asia 2011 Proceedings, pp.126-129 (2011).

[2] T. Tsujimoto, M. Kuwabara, H. Uyama, S. Kobayashi, M. Nakano, A. Usuki, J. Adhesion

Soc. Jpn., Vol.46, pp.131-136 (2010).

[3] H. Uyama, M. Kuwabara, T. Tsujimoto, M. Nakano, A. Usuki, S. Kobayashi, Macromol.

Biosci., Vol.4, pp.354-360 (2004).

[4] H. Uyama, T. Tsujimoto, S. Kobayashi, Network Polymer, Vol.25, pp.124-130 (2004).

[5] H. Uyama, M. Kuwabara, T. Tsujimoto, M. Nakano, A. Usuki, S. Kobayashi, Chem.

Mater., Vol.15, pp.2492-2494 (2003).

[6] T. Tsujimoto, H. Uyama, S. Kobayashi, Macromol. Rapid Commun., Vol.24, pp.711-714

(2003).

[7] T. Yokoi, Y. Sakamoto, O. Terasaki, Y. Kubota, T. Okubo T. Tatsumi, J. Am. Chem. Soc.,

vol.128, pp.13664-13665, 2006.

[8] T. Yokoi, J. Wakabayashi, Y. Otsuka, W. Fan, M. Iwama, R. Watanabe, K. Aramaki, A.

Shimojima, T. Tatsumi, T. Okubo, Chem. Mater., Vol.21, pp.3719-3729 (2009).

[9] FJ. Arriagada, K. Osseo-Asare, J. Colloid Interface Sci., Vol.221, pp.210-220 (1999).

[10] Y. Naka, Y. Komori, H. Yoshitake, Colloids Surf. A, Vol.361, pp.162-168 (2010).

[11] N. Masuda, S. Ota, World Patent, WO/2008/015943 (2008).

89

Chapter 5

General Conclusions

In this thesis, development of polylactic acid-based coating materials was examined, and

application of the polylactic acid-based materials to two component and UV curable coatings

with high durability was investigated. The results and knowledge obtained in this thesis are

summarized as follows;

1. The acrylic polyols with pendant polylactic acids was synthesized by copolymerization of

PLA macromonomer and acrylic monomer, such as HEMA and nBA. The clear coating of

acrylic polyols with pendant polylactic acids which was cured by polyisocyanate had

better anti-hydrolysis property than the coating of linear chain PLA. The coating of acrylic

polyols with pendant polylactic acids, which is about 50 wt.% bio-content included curing

agent, satisfied basic coating performances, such as an adhesion property, a hydrolysis

resistance property, and chemical resistance properties. The acrylic polyols with pendant

polylactic acids can be used as two component type coating materials.

2. The multi-functional star-shaped polylactic acids polyol was synthesized from cheap

lactic acid and polyols. The coating of star-shaped polylactic acids polyol, which is about

30 wt.% bio-content included curing agent, satisfied coating performances, such as an

adhesion property, a humidity resistance property, chemical resistance properties, and

accelerated weatherability. The star-shaped polylactic acids polyols can be applied to

bio-based two component type coatings.

3. The multi-functional star-shaped polylactic acids with reactive double bonds were

synthesized by the addition of succinic anhydride and GMA to the star-shaped polylactic

acids polyols. The star-shaped polylactic acids with reactive double bonds showed good

UV curability, and they can be applied to bio-based UV curable coatings. The coating of

star-shaped polylactic acids polyol with reactive double bonds, which is about 44 wt.%

90

bio-content included succinic anhydride, satisfied coating performances, such as an initial

adhesion, a humidity resistance, an alkaline resistance, an abrasion resistance, and

hardness. In particular, the UV cured coating films had hard-coating property.

4. Silica nanoparticles with high concentration were prepared by the Stöber method using

TEOS as a starting material. It was found that about 4 wt% of silica nanoparticles with a

diameter of about 10 nm were obtained by controlling the reaction conditions in the

Stöber process. By removing solvent under a reduced pressure, the particle concentration

was increased up to 15 wt.% without aggregation.

5. Silica nanoparticles modified with methacryloyl groups were prepared by modification

reaction of MPTMS. More amounts of MPTMS are modified at higher reaction

temperature and under larger pH value condition.

6. The bio-based hybrid coatings of star-shaped polylactic acids and silica particles both

modified with reactive double bonds showed good hard-coating property and heat

resistance property. In particular, the hardness of the hybrid coating became larger with an

increase in the MPTMS amounts in the modified silica particles. Hard-coating and heat

resistance property were also improved with an increase in the amounts of silica particles

in the hybrid coating.

In conclusion, hydrolysis resistance of PLA materials for coating was achieved by

designing the graft type structure and star-shaped structure. Heat resistance and hard-coating

property was obtained by hybridizing with silica nano-particles. The acrylic polyols with

pendant polylactic acids and the star-shaped polylactic acids polyols can be used as coating

materials.

The author sincerely wishes that these PLA coating materials will be used all over the

world near the future and be successful for reduction of a greenhouse gas.

91

Acknowledgements

The author wishes to express his deep gratitude to Professor Masahiro Tatsumisago of

Osaka Prefecture University for his useful direction, suggestion, discussion, and continuous

encouragement during the course of this work.

The author would like to express his deep appreciation to Professor Kenji Kono and

Professor Masaya Matsuoka of Osaka Prefecture University for their valuable suggestion and

useful comments to this thesis.

The author would like to express his sincere gratitude to Professor Kiyoharu Tadanaga of

Hokkaido University for his helpful advice, valuable discussion, and continuous

encouragement in carrying this work.

The author would like to express his sincere thanks to Professor Hitomi Ohara and

Professor Shiro Kobayashi of Kyoto Institute of Technology for their technical advice,

valuable discussion, and explanation of extensive knowledge on biobased polymer.

The author is sincerely grateful to Professor Tsutomu Minami and Associate Professor

Akitoshi Hayashi of Osaka Prefecture University for their helpful suggestion and guidance.

The author would like to express his deep appreciation to Mr. Masahiko Ishii and Yuki

Hayata of TOYOTA MOTOR CORPORATION, Dr. Naoya Yabuuchi and Hirofumi

Yamashita of Nippon Bee Chemical Co., Ltd., Dr. Kiyoaki Ishimoto of Rajamangala

University of Technology Thanyaburi, Ms. Maho Arimoto of Kyoto Institute of Technology,

and Mr. Keisuke Mori of Osaka Prefecture University for their continuous guidance, useful

discussion and comments.

The author wishes to express his thanks to all of undergraduate and graduate students,

graduates, and researchers in Professor Tatsumisago’s laboratory and Professor Ohara’s

laboratory for their cooperation and assistance.

The author expresses his deepest gratitude to his parents, wife, and sons for their support

and hearty encouragement through this study.