A Polymer Matrix Particulate Reinforced Composite

Published Date: 02 Nov 2017

material. The goal is to evaluate the importance of different factors and to suggest a well-balanced post cure mode that

supports the application of the material.

Polymer matrix composites are post cured at elevated temperature to increase the amount of cross linking to achieve

better chemical and heat resistance and mechanical properties. Every material has an individual post cure process that

depends from the raw materials. Post curing variables include temperature, duration of cure, the time between initial

curing and post curing and temperature profile gradient.

There are several ways to determine the cure state of a polymer. It can be evaluated based on the mechanical and

physical properties, residual styrene content, glass transition temperature, residual exotherm or solvent swelling test.

For the determination of the suitable post cure parameters test slabs were casted and post cured with varying time

and temperature. Glass transition temperature, residual exotherm, softening in ethanol, surface hardness, flexural

strength and flexural modulus were determined. It is shown that the material should be cured at 60 °C – 80 °C. With

higher temperature and extended time of cure the glass transition temperature raises but the material becomes too brittle.

Keywords: particulate composites, particle reinforced polymer post curing, cross linking, thermal treatment, unsaturated

polyesters, glass transition temperature.

1. INTRODUCTION*

Thermosetting resins can be reinforced with

continuous or short fibres or particles to form a composite

material. Particulate reinforcement has many favourable

properties. The addition of particles to the matrix material

increases the stiffness, reduces the shrinkage and thermal

expansion, lowers the cost and modifies the rheological

properties [1].

Current study investigates particulate composites that

are used in building and construction industry. More

precisely the material is used for fabricating laboratory and

culinary bench tops, vanity tops and sanitary ware like

washbasins, shower trays and bathtubs.

There are a variety of resins and fillers available for

such application. The most common filler materials for

producing non-gel coated particulate composite products

are alumina trihydrate (ATH), quartz, resin chips and

recycled thermoplastics [2, 3]. To obtain pure white

products ATH is the choice of filler material. Alumina

trihydrate is a non-toxic, non-corrosive, non-cancerogenic,

odourless, flame retardant filler material. It is a mineral

derived from bauxite. ATH has specific gravity of

2.42 g/cm3 and Mohs’ hardness index of 2.5 – 3.5 [4].

Common resin systems used for producing particulate

composites with this kind of application are acrylic,

unsaturated polyester and unsaturated polyester modified

with acrylic. Unsaturated polyester resin modified with

acrylic is the best compromise between cost and properties.

∗ Corresponding author. Tel.: +372-56-227537; fax: +372-6203196.

E-mail address: [email protected] (A. Aruniit)

Unsaturated polyester resin is widely used in a variety

of applications. It has natural resistance to household

chemicals and stains. Because of low viscosities that allow

high filler content and easy casting it is the most common

resin in manufacturing of engineered stone products [5].

Casting applications contribute around 20 % to the overall

unsaturated polyester resin usage [6].

Polyester is formed by the reaction of difunctional acid

and difunctional alcohol (glycol). Properties of polyesters

can be varied by using different diacids and/or glycols

depending from the application. For polyester resins meant

for casting sanitary ware neopentyl glycol (NPG) and

isophthalic acid (ISO) are a good choice. Neopentyl glycol

provides good corrosion and weather resistance.

Isophthalic acid based resins have high heat and chemical

resistance [5, 6].

Styrene is used as comonomer for unsaturated

polyester resins. The mixture will copolymerize many

times faster compared to homopolymerization of polyester.

Styrene also makes the polyester resin an easily handled

liquid [6].

The curing of this kind of polyester resin is initiated by

adding cobalt carboxylates that promote the polymerisation

of monomers. During curing the resin goes through

chemical reactions that finally cause the gelation and

vitrification of the casting dispersion. Curing is an

irreversible reaction where chemical covalent cross-links

are formed that are thermally and mechanically stable. The

curing process plays a major role in achieving the final

mechanical properties and chemical resistance of the

material. Complete cure is rarely achieved at room

temperature. Not completed cure reduces the performance

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of the material. Moreover, residual styrene that remains in

the material causes problems when the material is in

contact with food and by evaporating and causing the

material to smell. The solution to these problems is post

curing of the material at elevated temperature. This is

necessary to obtain extensive cross-linking of the system

[6, 7].

There are several parameters that define the post-cure

process. Two biggest variables are temperature and time,

but also the time between initial curing and post curing and

temperature profile gradient play a role. In literature it is

stated that the post cure temperature is the most important

factor that influences the extent of cross linking [5, 8, 9].

The goal of the study is to determine a post cure mode

that brings out the desired properties of the material that

suit best to this kind of products and to investigate the

parameters that influence the post curing process.

2. EXPERIMENTAL

2.1. Sample preparation

The properties of composite plastics depend a lot on

the manufacturing process. In order to get adequate results

the material samples were prepared in the production

facility.

For the fabrication of specimens an unsaturated

polyester casting resin based on isophthalic acid and

neopentyl glycol was used. The resin is developed to

produce non-gel coated products and contains methyl

methacrylate. It is pre-accelerated, medium reactive, low

viscosity resin. The styrene content in the resin is 36 %.

For curing a peroxide mixture based on methyl ethyl

ketone peroxide was added with ratio of 1/100 wt%. The

peroxide is intended for room temperature cure of UP

resins and it has low peak exotherm (suitable for thick

parts) and good final cure (low residual styrene content).

60 wt% of ATH with medium particle size was used as

filler material.

(500 × 1,000 × 10) mm slabs were casted. The slabs

were casted with a closed mould with a special vacuum

assisted casting machine. A closed mould guarantees equal

thickness and flatness of the slab. That is necessary to get

test specimens like specified in testing standards. Vacuum

chamber of the machine removes air from the casting

dispersion and helps to achieve non-porous material. The

proportion of filler and other components is controlled by

the machine.

Preliminary cure of the composite was done at room

temperature (23 °C }2 °C) for 12 h. That was followed by

post cure. There are several different oven types that are

used for post curing composite plastics – infrared oven,

microwave oven and conventional thermal oven. A

conventional thermal oven was used in current study to

imitate accurately the production process. From all

materials 5 specimens were cut for all tests. The test

specimens were cut from the slabs with water jet.

2.2. Cure characteristics

As the cure characteristics are resin specific and depend

on various variables these are usually obtained heuristically.

Nevertheless, there are some numerical methods

to simulate the curing process of composites, but these are

not widely used [10]. From literature one can find the

following suggestions to the post curing cycle of neopentyl

glycol and isophthalic acid based polyester resin:

• 80 °C – 90 °C, 4 h – 6 h, not over 107 °C – the material

begins to degrade and discolour [11];

• 24 h cure at room temperature (RT) followed by post

cure at 60 °C for 8 h [12];

• 24 h cure at RT followed by post cure at 80 °C for 4 h

[12];

• After demoulding 120 °C for 1 h [12];

• Post cure temperature should be at glass transition

temperature (TG) or slightly above it (TG of current

resin is 108 °C); [5];

• 80 °C for 4 h to achieve optimum stain resistance

properties [13];

• 50 °C for 5 h – 15 h or 90 °C for 2 h (resin

manufacturers suggestion).

It was decided that the tests would be made as shown

in Table 1.

Table 1. Post cure modes

2.3. Thermal analysis

Differential scanning calorimetry (DSC) is a thermo

analytical technique in which the difference in the amount

of heat required to increase the temperature of a sample

and reference is measured as a function of temperature or

time. The technique provides qualitative and quantitative

information about physical and chemical changes that

involve endothermic or exothermic processes or changes in

heat capacity. It is a common tool in the research of post

curing mode [9, 14 – 17]. The calorimetric measurements

were conducted with a PerkinElmer Instruments DSC-7

differential scanning calorimeter. All samples were cured

in a nitrogen atmosphere. The weight of the samples was

3 mg. Heating was performed from 20 °C to 200 °C with

heating rate of 10 °C/min. The glass transition event is

observed as an endothermic stepwise increase in the heat

flow. Glass transition temperature represents the region in

which the resin transforms from a hard glassy solid to a

viscous liquid. With a further increase in the sample

temperature, the resin eventually undergoes curing and this

is observed as an exothermic peak. At the completion of

cross linking the DSC heat flow returns to a quasilinear

response [18].

The area under the exothermic peak can be integrated

to give the heat of the cure ΔHcure (J/g). The heat of cure

maybe used to determine the percentage of cure. It is the

heat of cure of a post cured sample (ΔHc) compared to the

uncured sample (ΔHuc) (Eq. 1) [16 – 18].

Percentage cure (%) ×100

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2.4. Indirect assessment of cure

Polymer softening in ethanol solution was used as an

indirect evaluation of the degree of cross-linking. Firstly

the specimens were stored in air at 37 °C for 24 h and the

Barcol hardness was measured. After that the specimens

were placed into 80 % ethanol-water solution at 37 °C for

24 h and the Barcol hardness was determined again. The

measurements were done promptly after the treatment.

2.5. Indentation hardness

The indentation hardness of the material was measured

with GYZJ 934-1 Barcol impressor according to ASTM D

2583-99. Barcol hardness can be used as a basic determination

of how cured a material is, or as an indication of the

wear resistance of a surface.

2.6. Flexural properties

The flexural properties of the material were determined

by 3 point bending test (Fig. 1) as specified in ISO

178 "Plastics – Determination of flexural properties". The

dimensions of the test specimens were (50×300×10) mm.

A test speed of 4 mm/min was used. The support span was

250 mm. The tests were conducted with Instron 5866.

Fig. 1. 3-point bending test

3. RESULTS AND DISCUSSION

3.1. Thermal analysis

Glass transition temperature (TG) is a thermodynamic

and thermo mechanical characteristic. TG is a

characteristic, which indicates the softening point at

elevated temperatures, effectiveness of curing agents and

percentage of cure. The TG depends on different factors,

including composition of the resin molecule, cross-link

density, curing agent, cure time and temperature. The cure

time and temperature have a considerable effect on the TG.

Therefore, TG can be a measure of the cure of a material.

The TG increases progressively in thermosetting resins

during curing. A general rule is that the TG achieved

during a post cure will increase with increasing post cure

temperature but will not exceed the cure temperature itself.

When TG reaches the cure temperature the curing reaction

stops [14, 19, 20].

As can be seen from Fig. 2 the TG increases rapidly

between temperatures 40 °C till 80 °C. The trend line

suggests that the increase in TG slows down at 80 °C,

although the TG achieved with post curing at 90 °C is

higher. As the TG reflects the mobility of polymer chains it

can provide an estimate of the crosslink density. So the

decrease in TG increase suggests that the polymer reaches

its maximum crosslink density. The post cure temperature

of the material under observation should be above 80 °C to

reach high TG and thus high cross linking.

The time of post cure seems to play a role in the TG

value at lower temperatures. The TG achieved with post

curing the material for 12 hours at 60 °C is 13 °C higher

than post curing it for 6 hours at the same temperature.

Nevertheless, independent of the time the TG of the

sample post cured at 60 °C for 12 h does not reach the TG

that is achieved at higher temperatures. At higher

temperature the system receives more energy and the TG is

higher. This verifies the assumption that temperature plays

more important role in post curing than time. The same has

been observed by Lipovsky and Groenendaal [5, 21].

Fig. 2. Test materials TG dependence on post cure temperature

The physical significance of TG is that at temperatures

above TG the values of physical and mechanical properties

like tensile strength reduces and the coefficient of thermal

expansion increases [19 – 22]. From practical view point it

means that the TG has to be considerably higher than the

materials service temperature because reduction in

materials physical and mechanical properties start below

the glass transition temperature. Heat deflection

temperature is a method that is used for assessing the

practical temperature where a polymer deforms under a

specified load. The HDT is determined by the test

procedure outlined in ASTM D648 and ISO 75. It has been

proved by several authors that there exists a good

correlation between the TG and HDT [5, 9, 23]. The HDT

for sanitary ware would have to be around 70 °C because

the maximum temperature of tap hot water is in the range

of 60 °C – 65 °C. In kitchen environment the temperatures

are up to 100 °C or even more, especially around heat

emitting sources like cooker and kettle. Based on resin

manufacturers tests the HDT of the UP resin is 20 °C –

25 °C lower than the TG. This means that the composite

has to be cured at least at 60 °C to achieve the necessary

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HDT for bathroom environment and at 90 °C for kitchen

environment. It has been observed that also the stain

resistance of the material is related to the TG. Materials

with low TG will pick up dirt more easily compared to

ones with higher TG (too tacky). It is related to the lower

cross linking and thus lower chemical resistance.

The DSC graphs have a further value besides

determining the TG. Since the DSC is measuring the heat

flow then any heat flow from unreacted material of the

sample is recorded on the graph. It is expressed as Jouls of

energy left in a gram of material (J/g). Less residual energy

means that the material is closer to fully cured state [24].

The DSC results are depicted in Figure 3 and are expressed

as percentage of cure.

Fig. 3. Percentage of cure achieved with different post cure

modes based on DSC graphs

The samples that were cured at room temperature or at

40 °C reach around 90 % of cure. The percentage of cure

improves as the temperature rises. For a more than 99 %

cure the system needs post curing temperatures above

80 °C. This correlates with the obtained TG results.

3.2. Indirect assessment of cure

Polymers properties are largely related to its crosslink

density. Highly cross-linked material tends to be harder,

stiffer, more heat and fracture resistant.

To assess the state of cure of the composite (crosslink

density) the glass transition temperature and residual

exotherm were determined with DSC. In addition to that a

swelling test was conducted. Ethanol has softening effect

on polymers. It is assumed that a more linear polymer

softens more than a cross-linked polymer. It is explained

by the interaction between solvent and polymer. A suitable

solvent that is able to constitute secondary bonds with

polymers chains replaces interchain secondary bonds and

dissolves linear and branched polymers. However, the

secondary bond cannot overcome primary valence cross

links, so cross-linked polymers do not dissolve.

Nevertheless, the cross linked polymers may swell and

become soft depending from the cross link density. The

swelling and softening effect gets smaller as the cross-link

density increases [11, 25].

Figure 4 presents how much percentage a sample lost

in indentation hardness when samples that were stored in

air in 37 °C for 24 h were compared to samples that were

placed into 80 % ethanol-water solution at 37 °C for 24 h.

The samples that were post cured in elevated temperatures

lost less than 20 % in indentation hardness. Samples that

were not post cured or were treated at lower temperatures

lost up to 36 % of their hardness. Post curing at 90 °C gave

the highest hardness number after swelling test. When the

hardness numbers of samples that were stored in air at

37 °C for 24 h were compared to hardness values of regular

samples than a similar loss in hardness was observed. The

loss is much smaller but the pattern is the same.

Fig. 4. The cross-link density increases with increased post cure

temperature (smaller loss of hardness in swelling test)

If the cross link density is evaluated based on this test

than it can be seen that post curing at higher temperature

gives higher cross link density, although the difference gets

smaller as the temperature rises. The time of post curing

plays a role at lower temperatures. Similar trends can be

observed from the DSC graphs that present the TG and

residual exotherm.

3.3. Indentation hardness

The Barcol hardness values of differently post cured

samples are presented in Fig. 5. The composite was

manufactured with a vacuum assisted casting machine that

produces air void free casting dispersion. That should

assure a homogeneous material. When the hardness test

was conducted this assumption was verified by measuring

the hardness on both sides of the test specimen. No

discrepancy was found. The hardness values show that the

increase of post curing temperature increases the hardness

of the surface. A bigger step can be observed when the

hardness of at room temperature and at 40 °C post cured

samples are compared to the ones treated at 60 °C. The

next step is much smaller. The hardness value increases

only 1.5 % when the temperature is raised from 60 °C to

80 °C. At 90 °C for 2 h post cured material shows even a

minor loss in hardness when compared to the sample post

cured at 80 °C for 12 h.

The application of the material demands great scratch

and wear resistance so the indentation hardness of the

material is of great importance to the manufacturer. The

recommended Barcol reading by the ICPA is between 45

and 65. A lower number might indicate an under cured

material and higher number too brittle material [2]. The

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surface hardness also depends on the resin and filler

materials, from the concentration of filler material and

other factors. The Barcol values obtained are in the upper

range of the suggested numbers. This indicates that there is

not a necessity to post cure the material above 60 °C

because of the surface hardness.

Fig. 5. Necessary Barcol hardness can be achieved with post

curing at 60 °C

3.4. Flexural properties

The flexural strength is considered to be a

demonstrative parameter for brittle materials like ceramics

and particulate composites. The tensile strength tests of

brittle materials are difficult to conduct and the results

have a large deviation. From Figure 6 one can see that the

flexural strength of the material changes a bit differently

than TG or cross link density.

Fig. 6. Flexural strength rise comes to a halt at 60 °C

The flexural strength raises from 40 MPa to 52 MPa

when the material is post cured at 60 °C for 6 h. Post

curing at higher temperature than 60 °C does not give any

remarkable rise in the flexural strength of the material.

There is a drop in the flexural strength when the material is

post cured for 12 h at 60 °C, but it does not seem to have

any correlation with the temperature nor time and should

be treated as aberrancy. From the manufacturer’s point of

view higher flexural strength is better because the product

does not require elasticity on the contrary the products are

expected to be rigid.

The other flexural property that was measured was

flexural modulus. The modulus of the material increases

remarkably after post curing it at 40 °C, from 3300 MPa to

7700 MPa. After that the rise is not as rapid and there is

only 6.5 % rise in flexural strength when the post cure

temperature is raised from 60 °C to 80 °C. The increase of

flexural modulus shows that the stiffness of the composite

increases. That trend was confirmed by the deflection

values that were obtained during the testing. Stiffness is

generally a good property when considering the application

of the material. On the other hand the material gets more

brittle as the flexural modulus increases. Brittleness is not

the material property a manufacturer of bench tops or

washbasins is looking for because it makes the products

prone to cracks and breakage by falling objects.

Fig. 7. Materials stiffness increases with higher post cure

temperature

It is clear that post curing influences the stiffness and

brittleness of the material but it is also largely related to the

filler material. Particulate fillers increase the flexural

modulus of the composite, while flexural strength remains

the same or decreases [26]. The stiffness and impact

strength are dependent on the particle size. Smaller particle

size provides higher stiffness [27]. In this case it might be

necessary to increase the particle size or decrease the filler

wt% to decrease the stiffness.

A correlation exists between the content of unpolymerised

material and the mechanical and physical properties of

the material. Unpolymerised material is residual styrene

and phlegmatizers. The unpolymerised material has

negative effect on hardness, flexural strength, flexural

modulus and chemical resistance. In the first days and up

to a month the styrene will build very quickly. After that

the polymerization reaction will slow down or come to a

standstill that is caused by the increasing rigidity of the

polyester network [5, 19]. The reaction continues when the

network is flexibilised again. An optimal cure can only be

achieved at elevated temperature. As test data shows high

cross link density of ISO/NPG based UP resin is

achievable at temperatures above 60 °C. The curing is less

time dependent as the temperature rises. The rise of

temperature increases the heat resistance and chemical

resistance of the material but has unfavourable effect on

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the flexural modulus and small positive effect on the

hardness and flexural strength. Temperature of 60 °C and a

cure time of 12 h seem to be optimum balance between

cross link density and mechanical properties. Moreover,

higher temperatures demands powerful post curing

equipment. Nevertheless, the statement that the material

needs post curing at 80 °C for 4 h to achieve optimum stain

resistance has to be looked into.

4. CONCLUSIONS

The research was carried out to study the effect of

different post curing modes to mechanical and physical

properties of particle reinforced composite. Experimental

part included fabrication of material specimens, heat

treatment, DSC analysis, ethanol swelling test and testing

of mechanical properties.

The test data acquired from DSC analysis and ethanol

swelling test showed that increased post curing

temperature increases the heat resistance and cross link

density. The mechanical tests showed an ascending trend

of the mechanical properties with increased post cure

temperature. From the experimental data one can conclude

that the manipulation of post cure temperature influences

many variables. Aspects like brittleness and impact

strength need to be considered.

The test results obtained betoken a prospect for the

tested material to be used commercially as a material for

laboratory, culinary, marine or agricultural products.

Acknowledgments