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2 Processing of natural rubber (NR) and NR-based PSAs

2 Processing of natural rubber (NR) and NR-based PSAs

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Application of epoxidized NR in pressure sensitive adhesives



355



process during which the adhesive will form a bond with the surface

or substrate with which it is in contact within a short time and with the

application of low pressure; the second is the debonding process during which

the adhesive is separated from the surface of the substrate via peeling. This

latter process involves a significantly higher localized force and a shorter

timescale for deformation.

The factors governing the bond formation between the adhesive and its

surface are as follows:





Properties of the adherent surface material, wettability or surface energy,

roughness and porosity.

∑ Preparation for PSA cleanliness, pre-treatments, coating weight and

uniformity, adhesive application, open or drying time and environmental

conditions experienced prior to bonding.

∑ Physical and chemical properties of the PSA – type, functional groups,

flow properties and surface energy.

∑ Bonding process – contact pressure, duration of contact, rate of pressure

change, thermal history, and penetration into the surface.

Debonding, on the other hand, is dependent on the following factors:





Separation process: rate of separation, angle of peel and specimen

clamping.

∑ Mechanical properties of adherents: flexibility, modulus and cohesive

strength of surface layers.

∑Mechanical properties of adhesive: rigidity, cohesive strength, extension

to failure, viscoelastic properties, creep and stress relaxation.

A loop tack test is one of the methods used to measure the tackiness of the

adhesive tape. During this test, a loop of pressure sensitive product (PSP)

is formed with the PSA layer facing out and is brought into contact with

a substrate at a defined rate; for example, 300 mm/min if a tensile tester

is used. The force required to immediately remove the loop with a defined

speed is measured as the loop tack. The value is reported in force per area

of tape width where the width is 1 inch. Figure 13.3 shows the schematic

diagram of a loop tack test.



13.3.2 Shear test

Shear tests are generally carried out to measure the ability of an adhesive tape

to resist creep under a constant load, applied parallel to the surface of the tape

and substrate. The purpose of this test is to compare the performance of an

adhesive in a joint and to determine its mechanical response. The shear test

measures the time required to pull a defined area of PSA from the test panel

under a constant load. Shear strength is the internal or cohesive strength of



356



Chemistry, Manufacture and Applications of Natural Rubber

Force



Test fixture



PSA (usually

25.4 ¥ 25.4 mm)

Test surface



1 inch

Support



13.3 Schematic diagram of loop tack test (Everaerts and Clemens,

2002).

Force



Force



13.4 Schematic diagram showing lap joint shear test.



the adhesive mass. The crosshead speed used depends on the type of shear

test; for example, during a lap joint shear test the crosshead speed used is

1.27 mm/min. The value is expressed in terms of failing stress per shear

area; i.e. megapascals. Usually, tack and adhesion decrease as shear strength

increases. Figure 13.4 shows a schematic diagram of a shear test.



13.3.3 Peel test

A peel test is designed to measure the average load per unit width of bond

line required to separate bonded materials. There are many variations of the

test; when one of the substrates is rigid, the flexible one is peeled at a defined

angle, but, where both bonded materials are flexible, such as laminated plastic

film, a T-peel test is usually carried out. Figure 13.5 shows a schematic

diagram of some types of peel test.

Peel strength is strongly governed by the extent of dissipation within the

materials of the adhesive bond. The factors governing peel resistance are as

follows:







Preparation of PSA: chemical composition and crosslinking nature and

density, viscoelastic properties and miscibility between the PSA and

other formulation components.

Coating of the PSA onto carrier material: modulus of the carrier material,



Application of epoxidized NR in pressure sensitive adhesives



357



Force

Force

Force



(a)



(b)

Force

(c)



13.5 Schematic diagram of (a) 90° peel test; (b) 180° peel test and (c)

T-peel test (Packham, 2005; Satas, 1982).



PSA thickness, thickness of the backing material and surface properties

of the carrier material.

∑ Preparation of sample: sample width.

∑ Cleaning substrate: surface properties of substrate (surface energy and

roughness), surface treatment and degree of pollution.

∑ Bonding step and sample stabilization: bonding pressure and bonding

time.

∑ Peeling test: peeling angle or geometry and peeling tool, peeling rate

and temperature and humidity.

The following discussion will focus on the studies carried out by various

researchers on the performance of PSAs, namely their tack, shear and peel

properties.



13.4



The use of epoxidized NR as an adhesive



NR was first epoxidized by Pummer and Burkhard in as early as 1922.

Epoxidized natural rubber (ENR) can be prepared using several methods,

which employ different types of peroxides and peracids. One method is to

obtain ENR from the reaction of a peracid and NR. This reaction introduces

the epoxirane ring onto the backbone of the NR and at the same time reduces

the number of double bonds meaning that various degrees or mol% epoxidation

is possible. The peracid can be prepared separately or generated in situ during

the epoxidation process. Figure 13.6 shows the epoxidation of NR where the

peroxy acid was prepared prior to the epoxidation process.

There are two commercial grades of ENR available: 25 mol% (ENR 25)

and 50 mol% (ENR 50) of epoxidation. The epoxidation disrupts the stereoregularity of the NR backbone and subsequently inhibits strain-induced

crystallization, especially where the epoxidation is higher than 50%. As the

number of epoxirane rings increases, changes to the raw rubber, including



358



Chemistry, Manufacture and Applications of Natural Rubber

Epoxidation involving peroxy acid and NR:

Step 1:

Peroxy acid (rcoooh) preparation from hydrogen peroxide

(HOOH) and carboxylic acid (RCOOH):

HOOH + RCOOH s RCOOOH + H2O

Step 2:

Epoxidation involving peroxy acid (RCOOOH) and NR:

Rcoooh

NR



O



ENR



13.6 Epoxidation of NR using peroxy acid (Gelling, 1991).



increases in density, polarity and the glass transition temperature (Tg), are

reported. The silica reinforcement of ENR without the aid of a coupling agent

is another advantage for the material in terms of its mechanical and physical

properties. The higher Tg, polarity and resistance to oxidation of ENR, as

compared to NR, plus its ability to undergo strain-induced crystallization

suggest a wide range of applications including tyres, adhesives and vibration

isolation mounting. The low rolling resistance and high wet grip properties

of ENR make it an attractive material to use for tyres and its high damping

behaviour makes it very suitable for use in vibration isolation mounting.

Due to its polarity, ENR is also highly suitable for adhesive applications.

Traditionally, ENR is used as an adhesive with substrates such as nylon,

brass-coated steel and glass.

The application of ENR in the area of adhesives has recently expanded

to include PSAs. NR alone is not sufficient to provide the adhesion and tack

performance required in a PSA. NR is, therefore, normally blended with

tackifier to improve its wettability and adherent performance. ENR is more

effective than NR as a PSA due to its higher polarity. ENR is therefore also

expected to be more suitable and, chemically speaking, more compatible

than NR for polar substrates. When combined with functionalized tackifiers

and fillers, it should provide a better adhesive performance. Both ENR and

NR have a high molecular weight, which can benefit the cohesive strength.

This chapter will focus on the application of ENR in PSAs.



13.5



Effect of coating thickness



Various studies carried out by Poh and co-workers on ENR-25 and ENR-50

(Poh and Kwo, 2007; Poh et al., 2008; Poh and Chew, 2009; Poh and Yong,

2010; Khan and Poh, 2010a) showed that loop tack, peel strength and shear

strength increased with the higher coating thickness. Poh and co-workers

investigated different tackifiers; i.e. coumarone–indene (CI) resin, gum



Application of epoxidized NR in pressure sensitive adhesives



359



rosin, and petro resin as well as different fillers; i.e. kaolin, silica, calcium

carbonate. In each case, the higher coating thickness resulted in a more

adhesive presence which led to an improved wettability. In all studies the

coating thicknesses evaluated were 30 mm, 60 mm, 90 mm and 120 mm and

the solvent used was toluene.

In a study by Poh and Saari (2011), the adhesion properties of an ENR50-based adhesive were studied in the presence of magnesium oxide. CI resin

and toluene were used as the tackifier and solvent, respectively. The effect of

the loop tack and peel tests on the range of coating thickness at 30 mm, 60

mm, 90 mm and 120 mm showed that the 60 mm coated sample consistently

exhibited the highest loop tack and peel strength. This suggests that the

maximum viscoelastic property was achieved at this coating thickness. The

shear strength, however, decreases with the increasing filler loading for all

coating thicknesses.

In a study by Poh and Lai (2010), the filler, tackifying agent and solvent

combination of barium chloride, CI resin and toluene, respectively, were used.

Results showed that the filled ENR-25 exhibited the greatest peel strength at

a coating thickness of 120 mm for the range of coating thickness studied. In

a separate study by Poh and Khan (2012), the same trend was shown when

the shear strength of silica-filled ENR-25 was compared to the silica-filled

ENR-50 for which the tackifying agent was CI resin. Figures 13.7, 13.8 and

Tables 13.1 and 13.2 summarize the adhesive performance of ENR-25 and

900

T-Peel strength

90° Peel strength



700



180° Peel strength



600

500

400

300

200



Petro resin

20 phr

magnesium

oxide



Gum rosin

30 phr kaolin



CI resin

40 phr silica



0



CI resin

10 phr barium

chloride



100

CI resin –



Peel strength (N/m)



800



13.7 Peel strength of ENR-25 using various tackifiers and fillers at

coating thickness of 120 mm (Poh and Kwo, 2007; Poh and Chew,

2009; Poh and Gan, 2010; Poh and Lai, 2010; Khan and Poh, 2010c).



360



Chemistry, Manufacture and Applications of Natural Rubber

700

T-Peel strength



Peel strength (N/m)



600



90° Peel strength

180° Peel strength



500

400

300

200

100

0





120 micron



40 phr silica

120 micron



30 phr magnesium

oxide 60 micron



13.8 Adhesion performance of ENR-50 using CI resin as tackifier at

respective optimum coating thickness (Poh and Kwo, 2007; Khan and

Poh, 2010c; Poh and Saari, 2011).

Table 13.1 Loop tack of ENR-based PSAs using various fillers and tackifiers at

specific coating thicknesses

Rubber



Filler



Tackifier



Coating

Loop tack Reference

thickness (mm) (N/m2)



ENR-25



20 phr kaolin



Gum

rosin



120



~200



Poh and Chew,

2009



ENR-25



10 phr barium

chloride



CI resin



120



~6700



Poh and Lai,

2010



ENR-25



40 phr silica



CI resin



120



~0.3



Khan and Poh,

2010a



ENR-50



20 phr silica



CI resin



120



~0.6



Khan and Poh,

2010a



ENR-50



30 phr magnesium CI resin

oxide



60



~0.5



Poh and Saari,

2011



Table 13.2 Shear strength of ENR-based PSAs using CI resin as tackifier at 120 mm

coating thicknesses

Rubber



Filler



Tackifier



Shear strength Reference

(N/m2)



ENR-25



None



CI resin



~20



ENR-50



None



CI resin



~4



ENR-25



40 phr silica



CI resin



ENR-50



40 phr silica



CI resin



ENR-25



10 phr barium chloride CI resin



Poh and Kwo, 2007

Poh and Kwo, 2007



~8.4 ¥ 10



−4



Khan and Poh, 2010a



~8.3 ¥ 10



−4



Poh and Lai, 2010



~8.9 ¥ 10



−4



Poh and Lai, 2010



Application of epoxidized NR in pressure sensitive adhesives



361



ENR-50 using a different tackifier and filler for their respective loading at

the optimum coating thickness of 120 mm.



13.6



Effect of tackifier and filler



The tack and peel strength increases with the increase in tackifier and filler

up to the optimum loading capacity since better mechanical interlocking and

anchorage of the adhesive in pores and irregularities in the adherent can be

expected. Post optimum loading capacity, however, a poorer performance can

be expected with the increase in tackifier and filler. When the filler loading

increases, the wettability of the adhesive will increase up to an optimum point.

Further increases in the filler loading will then lead to poorer miscibility and

wettability of the adhesive due to the diluting effect of the filler.

Shear strength decreases gradually with the increase in resin loading

because of the decrease in the cohesive strength of the adhesive. When the

tackifier loading increases, rubber (i.e., the matrix) gradually decreases due

to the decreasing rubber content. This subsequently weakens the adhesive’s

ability to resist flow during shearing action. This weakening effect is very

significant at higher coating thickness.

Table 13.3 shows the comparison loop tack performance of some ENRbased PSAs. The ENR-25-based PSA using a CI resin as the tackifier showed

that barium chloride resulted in the highest loop tack. This might be due to

the higher wettability of the PSAs achieved for ENR-25 + CI resin + barium

Table 13.3 Loop tack of ENR-based PSAs

Filler



Tackifier



Optimum

Loop tack Reference

coating

(N/m2)

thickness (mm)



ENR-25-based PSAs

40 phr zinc oxide 80 phr CI resin



60



~85



Poh and Chow, 2007



30 phr calcium

carbonate



80 phr CI resin



60



~16



Poh et al., 2008



30 phr kaolin



40 phr Gum

resin



120



~205



Poh and Chew, 2009



10 phr barium

chloride



40 phr CI resin 120



~6750



Poh and Lai, 2010



40 phr silica



40 phr CI resin 120



~0.33



Khan and Poh, 2010a



ENR-50-based PSAs

30 phr calcium

carbonate



80 phr CI resin 120



20 phr silica



40 phr CI resin 120



30 phr

40 phr CI resin

magnesium oxide



60



~15



Poh et al., 2008



~0.64



Khan and Poh, 2010a



~0.52



Poh and Saari, 2011



Chemistry, Manufacture and Applications of Natural Rubber



362



chloride. Barium chloride and ENR-25 also has a better interaction compared

to the other fillers. This enhances the dispersion and anchorage of barium

chloride in the PSA.

A comparable loop tack was observed between the ENR-25 and ENR-50based PSAs with 30 phr of calcium carbonate and 80 phr of CI resin. This

showed that the matrix base is insignificant to the tack performance. When

silica filler is used, however, an improved silica and ENR-50 interaction

was observed since the optimum silica loading in the ENR-50-based PSA is

lower. A higher loop tack was reported in the ENR-50-based PSA, however,

in comparison to the ENR-25-based PSA.

Figure 13.9 shows the peel strength performance of several ENR-25-based

PSA systems with varying coating thicknesses. As previously mentioned,

the presence of filler will lead to a higher peel strength. The results in Fig.

13.9 contradict this, however, because, for the system of ENR-25 with CI

resin only, the specimen dimension used during the peel test was larger than

that of the filler system.

The fillers studied were: zinc oxide, calcium carbonate, kaolin, barium

chloride and silica. Of these, barium chloride was shown to provide the best

peel strength. Barium chloride also required the lowest loading level to obtain

the optimum peel strength compared to the other fillers. This suggests that



800



T-Peel strength



700



90° Peel strength



600



180° Peel strength



500

400

300

200

100

120 micron

40 phr CI

40 phr petro resin





120 micron

40 phr petro resin

20 phr magnesium oxide



120 micron

40 phr CI resin

40 phr silica



120 micron

40 phr CI resin

10 phr barium chloride



120 micron

40 phr gum rosin

30 phr kaolin



60 micron

80 phr CI resin

30 phr calcium carbonate



60 micron

80 phr CI resin

40 phr zinc oxide



0

120 micron

40 phr CI resin





Peel strength (N/m)



900



13.9 Peel strength of various tackifiers and fillers at respective

optimum coating thickness of ENR-25-based PSAs (Poh and Kwo,

2007; Poh and Chow, 2007; Poh et al., 2008; Poh and Chew, 2009; Poh

and Firdaus, 2010; Poh and Gan, 2010; Poh and Lai, 2010; Khan and

Poh, 2010c).



Application of epoxidized NR in pressure sensitive adhesives



363



barium chloride would also provide a higher wettability compared to the

other fillers used.

The presence of petro resin in the ENR-25 and CI resin resulted in a

poorer peel strength. This might be due to the poorer wettability of the PSA

obtained due to the presence of the petro resin.

Figure 13.10 shows the peel strength performances of an ENR-50-based

PSA with a different tackifier and filler. As with the ENR-25-based PSA,

the presence of filler resulted in a poorer peel strength. This is due to the

fact that a larger test specimen was used. No obvious trend was observed

when different fillers were used.

When comparing the ENR-25-based and ENR-50-based PSAs filled with

calcium carbonate and silica, the ENR-50-based PSAs resulted in a poorer

peel strength for both types of fillers. This is due to the higher epoxidation

on the NR leading to a poorer wettability. ENR-25 also showed a greater

flexibility and compatibility with the tackifier.

Table 13.4 shows the shear strength performance of various ENR-based

PSAs can also decrease due to the presence of a filler and/or tackifier, as

they decrease the rubber loading level. Rubber contributes to the cohesive

strength of an adhesive. The results for both ENR-25 and ENR-50-based

PSAs contradict this theory since the specimen dimensions used vary for

filled and unfilled PSAs. Results also showed that ENR-50-based PSAs

have a poorer cohesive strength since they contain more epoxirane rings.

No obvious trend was observed for different types of filler.

600



Peel strength (N/m)



500

400



T-Peel strength

90° Peel strength

180° Peel strength



300

200

100

0



120 micron

120 micron

120 micron

60 micron

40 phr CI resin 80 phr CI resin 40 phr CI resin 40 phr CI resin



30 phr Calcium 40 phr Silica

30 phr

carbonate

Magnesium

oxide



120 micron

40 phr CI

40 phr Petro

resin





13.10 Peel strength of various tackifiers and fillers at respective

coating thickness of ENR-50-based PSAs (Poh and Kwo, 2007; Poh

et al., 2008; Poh and Firdaus, 2010; Khan and Poh, 2010c; Poh and

Saari, 2011).



364



Chemistry, Manufacture and Applications of Natural Rubber



Table 13.4 Shear strength of ENR-based PSAs at 120 mm coating thickness

Filler



Tackifier



Shear strength Reference

(N/m2)

~21



ENR-25-based PSAs

None



40 phr CI resin



40 phr silica



40 phr CI resin



None



Poh and Kwo, 2007



~8.4 ¥ 10



−4



Khan and Poh, 2010a



40 phr CI & 40 phr ~1.1 ¥ 10

petro resin



−2



Poh and Firdaus, 2010



ENR-50-based PSAs

None



40 phr CI resin



30 phr magnesium

oxide



40 phr CI resin



~3.6 ¥ 10



40 phr silica



40 phr CI resin



~8.3 ¥ 10−4



None



40 phr CI & 40 phr

petro resin



13.7



~4



~16 ¥ 10



Poh and Kwo, 2007

−4



−3



Poh and Saari, 2011

Khan and Poh, 2010a

Poh and Firdaus, 2010



Effect of molecular weight



As the molecular weight of the ENR increases, so does the cohesive strength

of the adhesive until it reaches its optimum value for tack, peel strength

and shear strength performances. This is due to improved wettability. At

the optimum loading point, the mechanical interlocking and anchorage of

the adhesive in pores, as well as the irregularities in the adherent are at

their maximum condition. Further increases in the molecular weight will

subsequently result in a poorer tack and peel strength. The decrease in peel

strength is due to the increasing effect of chain entanglement. In order to

achieve the best performance during shearing action, the rubber needs to

have an optimum rubber chain length to provide maximum cohesive and

adhesive strength. If the ENR is below the optimum molecular weight,

cohesive failure occurs since the rubber adhesive strength is weakened, which

contributes to the lower shear strength. A higher than optimum molecular

weight, on the other hand, results in a poor adhesive strength as a result of

poor wettability.

Studies carried out by Poh and Yong (2009a, 2009b, 2010) on ENR-25

and ENR-50-based PSAs using a tackifier of either CI resin, gum rosin or

petro resin at 40 phr loading showed that the loop tack and peel strength

are at their optimum performance at molecular weight for ENR-25 and

ENR-50 of 6.8 ¥ 104 g/mol and 3.9 ¥ 104 g/mol, respectively. The overall

performances are shown in Figs 13.11 and 13.12.

All tackifiers showed the molecular weight required to achieve the optimum

shear strength performance is 6.63 ¥ 104 g/mol and 4.14 ¥ 104 g/mol for

ENR-25 and ENR-50, respectively. The results are summarized in Fig. 13.13.



Application of epoxidized NR in pressure sensitive adhesives



365



110

100



ENR-25



ENR-50



Loop tack (¥10–2 N/m2)



90

80

70

60

50

40

30

20

10

0



40 phr CI resin



40 phr Petro resin



40 phr Gum rosin



13.11 Loop tack of ENR-based PSAs using different tackifier at their

respective optimum molecular weight (Poh and Yong, 2009a; 2009b,

2010).

350

ENR-25



Peel strength (N/m)



300



ENR-50



250

200

150

100

50

0



40 phr 40 phr

CI resin Petro

resin

T-peel



40 phr 40 phr 40 phr

Gum CI resin Petro

rosin

resin

90°



40 phr 40 phr 40 phr

Gum CI resin Petro

rosin

resin

180°



40 phr

Gum

rosin



13.12 Peel strength of ENR-based PSAs using different tackifier at

their respective optimum molecular weight (Poh and Yong, 2009a,

2009b, 2010).



A lower critical molecular weight was reported for ENR-50-based PSA’s

loop tack, peel strength and sheer strength as compared to ENR-25-based

PSAs due to the improved intermolecular interaction between ENR-50 and

the tackifiers.

Further studies by Khan and Poh (2010b, 2011) using ENR-25 and

ENR-50-based PSAs and CI resin levels, which varied between 10 phr to



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