**4. Properties of the loose-fill foams**

Properties of the loose-fill foams highly depend on raw materials and process parameters used in production. Addition of synthetic polymers, plasticisers or other additives, different temperatures of processing are causing changes in products properties.

#### **4.1 Cell structure**

Open cells in foams occur if at least part of one wall is missing, creating an opening onto adjacent cells. Tatarka and Cunningham (1998) compared properties of two expanded polystyrene (EPS) based foams (PELASPAN PAC and FLO-PAK S) and six commercially

modified during extrusion (Moscicki, 2011). The rheological properties of the starch plastic are in turn reliant on these physicochemical properties (Mercier et al.,1998), which affect the quality attributes of the foamed product. Extrusion process parameters, such temperature, screw speed, feed rate and moisture have direct influence on density, expansion ratio and other physical properties of extruded foams. Extrusion temperature depends on processed blend composition and ranged from 100°C to 180°C (Bhatnagar & Hanna, 1995; Cha et al.,

Moulded foam trays have been developed based on baking technology (Shogren et al., 2002) and are commercially available. Batter is foamed up and dried within heated moulds to foam thin-shelled containers similar to the process for making ice cream cones. The foam structure is featured by a highly porous centre sandwiched by much denser skin layers. The technology is somewhat limited by the slow processing rate necessary to dry off the moisture in the batter, which in turn restricts the maximum wall thickness of the foams.

Technologies for producing bulk starch forms have also been developed. Corrugated foam planks (Lye et al., 1998) made by extrusion foaming of modified cornstarch have been shown to have good cushion performance. The high foam density and cost of the materials, however, have somewhat restricted their widespread applications in packaging. Block foams have been made by combination of extrusion foaming and adhesion technology (Wang et al., 2001). The foams are of lower density and made from low cost wheat flours. When combined with other materials to form lightweight sandwich composites, mechanical

Moulded starch block foams are highly desirable in order to provide biodegradable counterparts to moulded polymeric foams. Recently, a microwave foaming process has been described for making moulded starch foams from extruded pellets (Zhou et al., 2006). This involves converting starch-based raw materials into pellets by extrusion processing and foaming the extruded pellets by microwave heating. The microwaveable starch pellets are compact for transportation and storage and can be expanded using microwave when needed. They may be formulated to produce microwaveable snacks in food industry. In non-food applications, free-flowing foamed balls may be produced for loose-fill packaging. When the pallets are foamed in a mould, lightweight mouldings can be produced in forms of containers, end caps, edge or corner cushion pads for protective packaging, which are

Properties of the loose-fill foams highly depend on raw materials and process parameters used in production. Addition of synthetic polymers, plasticisers or other additives, different

Open cells in foams occur if at least part of one wall is missing, creating an opening onto adjacent cells. Tatarka and Cunningham (1998) compared properties of two expanded polystyrene (EPS) based foams (PELASPAN PAC and FLO-PAK S) and six commercially

properties and resistance to water attack can be drastically enhanced (Song, 2005).

difficult to produce with extrusion foaming technology.

temperatures of processing are causing changes in products properties.

**4. Properties of the loose-fill foams** 

**4.1 Cell structure** 

2001; Shogren et al., 2002).

available starch-based foams (CLEAN GREEN, ENVIROFIL, ECO-FOAM, FLO-PACK BIO 8, RENATURE and STAR-KORE), manufactured in extrusion cooking process. They reported that all starch-based foams have higher open-cell content than either EPS-based foam. Considering the manufacturing process used, it is not surprising that starch-based foams have more open cells. The expansion is attributable to the escape of water as steam during the extrusion process, resulting between 96 and 99% open cells. Steam can easily rupture the cell walls because thermoplastic starches have poor melt strength. After exposure to high humidities and temperatures, most foams exhibited a statistically significant, but trivial increase, about 1,0%, in open-cell content. Commercial starch-based foams have an open cellular structure. This differs from patents that claim hydroxypropylated high amylose foams as having a closed-cell structure, but the method used to make this assessment was not disclosed.

Bhatnagar and Hanna (1995) tested commercial polystyrene (PS) loose-fills, commercial starch-based loose-fills and starch-based plastic loose-fills with addition of polystyrene and poly(methyl methacrylate) (PMMA). Tests showed that the PS loose-fills and the starchbased loose-fills had uniform cell size. The commercial starch foams and starch-based PS foams had cells larger than the commercial PS foams. The commercial starch foams also had fewer large-diameter cavities. These cavities can help to reduce the density, but also affect other functional characteristics of the foams.

Willet and Shogren (2002) reported that surface of starch-based foams have many small holes, suggesting that the starch outer wall burst during extrusion foaming. This reflects the low melt strength and elasticity of starch melts and is consistent with previous studies indicating that starch foams have open cells (Tatarka & Cunningham, 1998). Surface of starch foams containing poly(lactic acid) (PLA) or poly(hydroxyester ether) (PHEE) had fewer or no holes. This suggests that these foams have greater melt strength and resistance to bubble rupture. The average cell size is much larger for foams containing addition of polymer, reflecting the higher volume expansion of these foams than the starch-based foams.

According with Zhang and Sung (2007a) foams of PLA/starch were successfully prepared by using water as a blowing agent in the presence of talc, which acted as an effective nucleation agent. Water concentration, foaming temperature, nucleation agent concentration, screw speed, and die nozzle diameter were factors influencing foam forming, cell size, and cell distribution. Water was a good blowing agent for the PLA/starch system. The foam structure was dramatically affected by solid inorganic fillers that acted as nucleation agents. With 0,5% talc the expansion ratio of the foam was dramatically reduced by almost 50%. The expansion ratio was further reduced with talc concentration. At 3% talc, the foam expansion was 11,2, <25% of the expansion ratio of the foam without talc. In contrast, the bulk density remained the same and increased slightly at a talc concentration of 3%. With a reduction in the expansion ratio by talc, the foam cellular structure was also changed dramatically. Compared with foams without talc, the texture of the foam with talc became uniform and fine. Cell size distribution became narrower as talc content increased from 0,5 to 2,0%. Majority cell size (MCS) was also reduced. For example, the MCS was between 0,6 and 1,2 mm for the foam with 0,5% talc, whereas the MCS was about 0,4 mm with 2% talc. At 3% talc, the cell size becomes larger and distribution becomes broader again.

Starch Protective Loose-Fill Foams 87

Fang and Hanna (2001) investigated the effect of blending Z class Mater-Bi (MBI) resin with corn starches on the properties of the starch-based foam materials. They demonstrated that waxy starch had better foaming capabilities than regular starch mainly due to their differences in amylopectin content. Foams made from the waxy starch had slightly lower unit density (83,4 kg/m3) and lower bulk density (49,2 kg/m3) than the foams made from regular starch (83,7 and 56,6 kg/m3 respectively). However, the bulk densities were still much higher than the 8,9 kg/m3 of commercial EPS foams. The unit density increased as MBI and moisture content increased, indicating less expansion of the extrudates at higher MBI and moisture content. The bulk density data also showed a similar trend as the unit density. The relationship between the unit and bulk densities and the MBI and moisture

Bhatnagar and Hanna (1996) extruded five types of starch (normal corn, wheat, potato, rice and tapioca) with polystyrene ( starch to polystyrene ratio 70:30) and addition of three additives: azodicarbonamide, magnesium silicate and polycarbonate. Unit density was dependent on source of starch and type of additive. Foams made from tapioca and corn starches had the unit densities ranging from 36 to 39 kg/m3 and from 30 to 86 kg/m3 respectively. Results indicate that corn starch performed better than the wheat starch. The autors reported unit density of 23 to 59 kg/m3 for commercial starch or starch-based plastic foams, which compares favorably with tapioca and corn starch-based foams in their study. The unit densities of all the foams including commercial starch foams were much higher

Compressive stress is the maximum force required to compress the foam by 3 mm. High compressive stress implies foams resist compression. Tatarka and Cunningham (1998) reported that the compressive stress of commercial starch-based foams does not significantly differ from EPS-based foams. As-received FLO-PAK BIO-8, STAR-KORE, ECO-FOAM and ENVIROFIL have lower values between 0,0565 and 0,0853 MPa, whereas CLEAN GREEN and RENATURE have higher values of 0,0927 and 0,1051 MPa. Compressive stress of starch-based foams was generally insensitive to changes in relative humidity. At 20% relative humidity (r.h.) and 23°C, only the chemically modified starchbased foams, ECO- FOAM and STAR-KORE, significantly increased compressive stress by 17 and 28%, respectively. At 80% r.h., 23°C, FLO-PAK BIO-8, RENATURE, and CLEAN GREEN significantly decreased compression stress by 22 to 32%. The higher moisture content in these products was sufficient to lower their resistance to compression. Although the chemically modified STAR-KORE and ECO-FOAM and the unmodified ENVIROFIL absorbed 13 to 16 % water at this condition, compressive stress did not significantly change. Chemically modified starches produced foams with good resistance to compression over a

Fang and Hanna (2001) found that compressibility of the waxy starch samples were significantly lower than those of the regular starch samples, indicating that the regular starch foams were more rigid than the waxy starch foams. Foam compressibility increased with increases in the MBI content, but compressibility increased more rapidly as the moisture content increased. High moisture content in the processed materials resulted in

contents were relatively linear.

**4.3 Compressive stress** 

than commercial polystyrene foams (8,9 kg/m3)

broad humidity range (Tatarka & Cunningham, 1998).

more rigid products, which was due to low expansion.

### **4.2 Foam unit density and bulk density**

Foam unit density describes the density of an individual expanded loose-fill foam specimen. Bulk density is more complex than specific density. Bulk density takes into account not only material and foam densities, but also packing efficiency, which depends on size, shape, and uniformity of the loose fill. Packing efficiency describes how well the loose fill fills the voids among adjacent foam specimens and can be measured by the ratio of bulk density to foam density. If this ratio is equal to one, the efficiency is very high because no voids exits among adjacent foam specimens. A low packing ratio can be achieved from irregular shaped foams. A loose-fill product with low packing is most desirable because the end-user reduces material consumption and saves transport costs.

The foam density of starch-based products was much higher than EPS-based ones (Tatarka & Cunningham, 1998). These values ranged between 16,7 and 22,6 kg/m3. These products are approximately two to three times more dense than EPS-based foams. This difference is attributable to the large difference in density between polystyrene and starch and a lower expansion factor. Dry, unmodified granular starch has a nominal density of 1500 kg/m3. During the extrusion process, the starch density has been reduced by factors ranging between 60 and 90. Open cells created during expansion will prevent the foam from continuing the expansion.

All commercial starch-based foams have a significantly higher bulk density by a factor of two to three than EPS-based foams or commercial PS loose-fills (Bhatnagar & Hanna, 1995; Tatarka & Cunningham, 1998). Starch-based foams have bulk densities between 8,8 and 11,3 kg/m3. Foam density of starch-based foams correlated well with bulk density. The correlation coefficient for this relationship is 0,97. The packing ratio of the starch-based foams is between 0,435 and 0,538. Irregular cylindrical shapes impart products with lower packing ratios than does uniform cylindrical or dual cylindrical shapes with two similar dimensions (Tatarka & Cunningham, 1998).

Generally, the bulk density of starch-based foams decreased as extrusion temperature increased, whereas it was not significantly changed as moisture content increased. This might be reasonably explained by changes in cell wall thickness and viscosity caused by extrusion temperature and the changes in the volume and weight of cells after absorbing sufficient water (Cha et al., 2001).

The microwave-foamed extruded pellets have lower densities, ranged between 114 kg/m3 and 145 kg/m3 and are significantly denser than commercial extruded starch-based loosefills. This disparity is attributable to the difference in expansion ratio during the foaming process and the existence of a denser skin layer in the microwave-foamed pellets (Zhou et al., 2006). The formation of this denser skin layer is likely to have resulted from moisture loss from the surface of a pellet during heating and hence a reduced driving force for foaming in this region.

Investigations made by Willet and Shogren (2002) indicated that density of foam prepared from normal corn starch had average density 61,4 kg/m3, from wheat starch 58,6 kg/m3 and from potato starch 40,6 kg/m3. Foam densities for commercial starch foams are approximately 20 kg/m3. Addition of PLA or PHEE to starch yields foam densities comparable to commercial starch products.

Foam unit density describes the density of an individual expanded loose-fill foam specimen. Bulk density is more complex than specific density. Bulk density takes into account not only material and foam densities, but also packing efficiency, which depends on size, shape, and uniformity of the loose fill. Packing efficiency describes how well the loose fill fills the voids among adjacent foam specimens and can be measured by the ratio of bulk density to foam density. If this ratio is equal to one, the efficiency is very high because no voids exits among adjacent foam specimens. A low packing ratio can be achieved from irregular shaped foams. A loose-fill product with low packing is most desirable because the end-user reduces

The foam density of starch-based products was much higher than EPS-based ones (Tatarka & Cunningham, 1998). These values ranged between 16,7 and 22,6 kg/m3. These products are approximately two to three times more dense than EPS-based foams. This difference is attributable to the large difference in density between polystyrene and starch and a lower expansion factor. Dry, unmodified granular starch has a nominal density of 1500 kg/m3. During the extrusion process, the starch density has been reduced by factors ranging between 60 and 90. Open cells created during expansion will prevent the foam from

All commercial starch-based foams have a significantly higher bulk density by a factor of two to three than EPS-based foams or commercial PS loose-fills (Bhatnagar & Hanna, 1995; Tatarka & Cunningham, 1998). Starch-based foams have bulk densities between 8,8 and 11,3 kg/m3. Foam density of starch-based foams correlated well with bulk density. The correlation coefficient for this relationship is 0,97. The packing ratio of the starch-based foams is between 0,435 and 0,538. Irregular cylindrical shapes impart products with lower packing ratios than does uniform cylindrical or dual cylindrical shapes with two similar

Generally, the bulk density of starch-based foams decreased as extrusion temperature increased, whereas it was not significantly changed as moisture content increased. This might be reasonably explained by changes in cell wall thickness and viscosity caused by extrusion temperature and the changes in the volume and weight of cells after absorbing

The microwave-foamed extruded pellets have lower densities, ranged between 114 kg/m3 and 145 kg/m3 and are significantly denser than commercial extruded starch-based loosefills. This disparity is attributable to the difference in expansion ratio during the foaming process and the existence of a denser skin layer in the microwave-foamed pellets (Zhou et al., 2006). The formation of this denser skin layer is likely to have resulted from moisture loss from the surface of a pellet during heating and hence a reduced driving force for

Investigations made by Willet and Shogren (2002) indicated that density of foam prepared from normal corn starch had average density 61,4 kg/m3, from wheat starch 58,6 kg/m3 and from potato starch 40,6 kg/m3. Foam densities for commercial starch foams are approximately 20 kg/m3. Addition of PLA or PHEE to starch yields foam densities

**4.2 Foam unit density and bulk density** 

material consumption and saves transport costs.

dimensions (Tatarka & Cunningham, 1998).

comparable to commercial starch products.

sufficient water (Cha et al., 2001).

foaming in this region.

continuing the expansion.

Fang and Hanna (2001) investigated the effect of blending Z class Mater-Bi (MBI) resin with corn starches on the properties of the starch-based foam materials. They demonstrated that waxy starch had better foaming capabilities than regular starch mainly due to their differences in amylopectin content. Foams made from the waxy starch had slightly lower unit density (83,4 kg/m3) and lower bulk density (49,2 kg/m3) than the foams made from regular starch (83,7 and 56,6 kg/m3 respectively). However, the bulk densities were still much higher than the 8,9 kg/m3 of commercial EPS foams. The unit density increased as MBI and moisture content increased, indicating less expansion of the extrudates at higher MBI and moisture content. The bulk density data also showed a similar trend as the unit density. The relationship between the unit and bulk densities and the MBI and moisture contents were relatively linear.

Bhatnagar and Hanna (1996) extruded five types of starch (normal corn, wheat, potato, rice and tapioca) with polystyrene ( starch to polystyrene ratio 70:30) and addition of three additives: azodicarbonamide, magnesium silicate and polycarbonate. Unit density was dependent on source of starch and type of additive. Foams made from tapioca and corn starches had the unit densities ranging from 36 to 39 kg/m3 and from 30 to 86 kg/m3 respectively. Results indicate that corn starch performed better than the wheat starch. The autors reported unit density of 23 to 59 kg/m3 for commercial starch or starch-based plastic foams, which compares favorably with tapioca and corn starch-based foams in their study. The unit densities of all the foams including commercial starch foams were much higher than commercial polystyrene foams (8,9 kg/m3)

### **4.3 Compressive stress**

Compressive stress is the maximum force required to compress the foam by 3 mm. High compressive stress implies foams resist compression. Tatarka and Cunningham (1998) reported that the compressive stress of commercial starch-based foams does not significantly differ from EPS-based foams. As-received FLO-PAK BIO-8, STAR-KORE, ECO-FOAM and ENVIROFIL have lower values between 0,0565 and 0,0853 MPa, whereas CLEAN GREEN and RENATURE have higher values of 0,0927 and 0,1051 MPa. Compressive stress of starch-based foams was generally insensitive to changes in relative humidity. At 20% relative humidity (r.h.) and 23°C, only the chemically modified starchbased foams, ECO- FOAM and STAR-KORE, significantly increased compressive stress by 17 and 28%, respectively. At 80% r.h., 23°C, FLO-PAK BIO-8, RENATURE, and CLEAN GREEN significantly decreased compression stress by 22 to 32%. The higher moisture content in these products was sufficient to lower their resistance to compression. Although the chemically modified STAR-KORE and ECO-FOAM and the unmodified ENVIROFIL absorbed 13 to 16 % water at this condition, compressive stress did not significantly change. Chemically modified starches produced foams with good resistance to compression over a broad humidity range (Tatarka & Cunningham, 1998).

Fang and Hanna (2001) found that compressibility of the waxy starch samples were significantly lower than those of the regular starch samples, indicating that the regular starch foams were more rigid than the waxy starch foams. Foam compressibility increased with increases in the MBI content, but compressibility increased more rapidly as the moisture content increased. High moisture content in the processed materials resulted in more rigid products, which was due to low expansion.

Starch Protective Loose-Fill Foams 89

Resiliency describes the ability of the foam to recover to its original form after deformation. Resiliency less than 100% implies that the polymer was strained beyond its elastic limit, for example, by cell wall rupture, which prevents the foam from recovering to its original state. The resiliency of comercial starch-based foams with values between 69,5 and 71,2% are, as a group, about 10% lower on a relative basis than EPS-based foams (Tatarka & Cunningham, 1998). After conditioning, the resiliency of all starch-based foams were significantly lower, with values between 60 and 70%. Although starch-based foams absorbed 13 to 16% moisture after conditioning at 80% r.h. and 23°C, the 62 to 67% resiliency retained is sufficient for the

Fang and Hanna (2001) showed that the resiliency value of the regular starch samples was 89,5% compared to 86,6% for the waxy starch foams. The resiliency increased with increases in MBI and moisture content. The addition of Mater-Bi resin improved the mechanical properties of the starch foams. However, moisture content had a more significant positive effect than the MBI content on resiliency. The achieved results ranged between 80 and 95%

According to Lin et al. (1995) extruded starch foams at water activity levels below 0,23 are brittle and easily crushed, disintegrating upon compression. It was evident that, in these conditions, they are not suitable for packaging use. The resiliency of foams increased with increasing water activity levels (0,33 – 0,53). This was probably due to the increase in starch chain mobility of extruded foams at the higher water content. Further increases in water

Bhatnagar and Hanna (1995) reported that the resiliency of starch-based PS loose-fill is not different from the resiliency of PS loose-fill or starch loose-fill. The ideal elastic body will have a resiliency of 100%, whereas the foams had resiliency of the order of 80 to 90%. The resiliency of starch-based PMMA foam was significantly lower than the PS loose-fill. They found resiliency values of 97% and 96% for commercial polystyrene and commercial starch foams, respectively. They found that, resiliency was significantly affected by the type of starch but not by the type of additive (Bhatnagar & Hanna, 1996). For corn and tapioca starches, the resiliency was quite comparable to that of commercial polystyrene loose-fill.

Nabar et al. (2006) investigated influence of polymer addition into starch on foam resiliency. The control starch foams provided a resiliency of 69,7%. The addition of poly(butylene adipate-co-terephthalate) (PBAT) improved the resiliency considerably, from 69,7% to 85,9% at a PBAT content of 7% of the starch used. Polycaprolactone and cellulose acetate helped increase the spring index up to ~78%, while poly(vinyl alcohol) and methylated pectin barely increased the resiliency to ~71–72%. When glyoxal was added as a crosslinker, it increased the

Zhang and Sung (2007b) tested the properties of PLA/starch foams. The foam samples dried at 135°C for 2 h, then immediately undergoing mechanical compression testing, showed no recovery at all, and all inside structure fractured into pieces, regardless of starch/PLA ratios.

rigidity of the starch foams and, thus, the resiliency of the foams decreased by ~3%.

activity (0,53 – 0,75) caused the foams to soften and thus decreased the resiliency.

Potato starch extrudates had good resiliency with all the additives.

**4.4 Resiliency** 

product to function.

which indicated a good deformation resiliency.

Lin et al. (1995) show direct influence of water activity index on a compressive strength of extruded starch foams. A maximum compressive strength occurred at water activity of 0,53 which is close to normal ambient conditions. Further increase in water activity levels result in a decrease in compressive strength. Water acts as a plasticizer, which lubricates the starch chains. As the water activity increased, the starch chain mobility is also increased. This chain softening results in lower resistance to compression and lowers compressive strength values. It was observed that the foams become more flexible at higher water activity levels, probably due to the plasticization of foam walls by water. At lower water activity levels the starch chain mobility is decreased. This chain stiffening might result in higher resistance to compression and cause a higher compressive strength values. Examination of the samples showed that samples contain tiny fissures or fractures and, with the lower water activity came the more fissures. The effect of fissure or fracture formation in compressive strength of extruded foams at lover water activity levels is more important than the effect of the chain mobility of starch in compressive strength. In other words, fissure or fracture formations controls the compressive strength. The maximum compressive strength at water activity level of 0,53. It seems to be caused by the combination effect of starch chain mobility and fissure or fracture formation.

During measurements a power-law relationship was observed between compressive strength and foam density (Lin et al., 1995, Willett & Shogren, 2002; Zhou et al., 2006). Denser foams tend to have thicker cell walls and hence resist deformation better than lower density foams with thinner cell walls. A strong correlation exists between foam density and compressive strength, regardless of the type of polymer blended with the corn starch.

According to Zhou et al. (2006), some of the mechanical properties such as compressive modulus of elasticity, compressive stress and deformation energy at 40% strain of foamed pellets made from the Superfine flour and purified wheat starch were close to that of EPS block. This suggests that by further optimizing the cell structure and flexibility of cell wall material, it is possible to produce foam by a microwave process so that its mechanical properties match that of EPS block. The starch loose-fill produced by extrusion foaming have very low density, and thus they are suitable for cavity filling in packaging for light weight goods under low compressive stress. While the microwave processed starch foams have relatively high density, they are more suitable for packaging under high compressive stress levels or for heavy goods.

Greater compressibility values correspond to a sample which is more difficult to compress, while easily compressible samples have lower values of compressibility. Researches of Bhatnagar and Hanna (1995) indicated, that the starch-based PMMA foam had the lowest value of compressibility, which was consistent with its low resiliency. This was expected because this product also had the largest cell size. The starch-based PS loose-fill had more compressibility than the commercial PS foam. They found that compressibility was significantly affected both by the type of starch and type of additive (Bhatnagar & Hanna, 1996). For foams made from corn and tapioca starches compressibility values were quite comparable to that of commercial polystyrene foams. Compressibility values of potato starch extrudates were dependent on type of additive, with magnesium silicate and polycarbonate giving comparatively better products.

#### **4.4 Resiliency**

88 Thermoplastic Elastomers

Lin et al. (1995) show direct influence of water activity index on a compressive strength of extruded starch foams. A maximum compressive strength occurred at water activity of 0,53 which is close to normal ambient conditions. Further increase in water activity levels result in a decrease in compressive strength. Water acts as a plasticizer, which lubricates the starch chains. As the water activity increased, the starch chain mobility is also increased. This chain softening results in lower resistance to compression and lowers compressive strength values. It was observed that the foams become more flexible at higher water activity levels, probably due to the plasticization of foam walls by water. At lower water activity levels the starch chain mobility is decreased. This chain stiffening might result in higher resistance to compression and cause a higher compressive strength values. Examination of the samples showed that samples contain tiny fissures or fractures and, with the lower water activity came the more fissures. The effect of fissure or fracture formation in compressive strength of extruded foams at lover water activity levels is more important than the effect of the chain mobility of starch in compressive strength. In other words, fissure or fracture formations controls the compressive strength. The maximum compressive strength at water activity level of 0,53. It seems to be caused by the combination effect of starch chain mobility and

During measurements a power-law relationship was observed between compressive strength and foam density (Lin et al., 1995, Willett & Shogren, 2002; Zhou et al., 2006). Denser foams tend to have thicker cell walls and hence resist deformation better than lower density foams with thinner cell walls. A strong correlation exists between foam density and compressive strength, regardless of the type of polymer blended with the corn starch.

According to Zhou et al. (2006), some of the mechanical properties such as compressive modulus of elasticity, compressive stress and deformation energy at 40% strain of foamed pellets made from the Superfine flour and purified wheat starch were close to that of EPS block. This suggests that by further optimizing the cell structure and flexibility of cell wall material, it is possible to produce foam by a microwave process so that its mechanical properties match that of EPS block. The starch loose-fill produced by extrusion foaming have very low density, and thus they are suitable for cavity filling in packaging for light weight goods under low compressive stress. While the microwave processed starch foams have relatively high density, they are more suitable for packaging under high compressive

Greater compressibility values correspond to a sample which is more difficult to compress, while easily compressible samples have lower values of compressibility. Researches of Bhatnagar and Hanna (1995) indicated, that the starch-based PMMA foam had the lowest value of compressibility, which was consistent with its low resiliency. This was expected because this product also had the largest cell size. The starch-based PS loose-fill had more compressibility than the commercial PS foam. They found that compressibility was significantly affected both by the type of starch and type of additive (Bhatnagar & Hanna, 1996). For foams made from corn and tapioca starches compressibility values were quite comparable to that of commercial polystyrene foams. Compressibility values of potato starch extrudates were dependent on type of additive, with magnesium silicate and

fissure or fracture formation.

stress levels or for heavy goods.

polycarbonate giving comparatively better products.

Resiliency describes the ability of the foam to recover to its original form after deformation. Resiliency less than 100% implies that the polymer was strained beyond its elastic limit, for example, by cell wall rupture, which prevents the foam from recovering to its original state.

The resiliency of comercial starch-based foams with values between 69,5 and 71,2% are, as a group, about 10% lower on a relative basis than EPS-based foams (Tatarka & Cunningham, 1998). After conditioning, the resiliency of all starch-based foams were significantly lower, with values between 60 and 70%. Although starch-based foams absorbed 13 to 16% moisture after conditioning at 80% r.h. and 23°C, the 62 to 67% resiliency retained is sufficient for the product to function.

Fang and Hanna (2001) showed that the resiliency value of the regular starch samples was 89,5% compared to 86,6% for the waxy starch foams. The resiliency increased with increases in MBI and moisture content. The addition of Mater-Bi resin improved the mechanical properties of the starch foams. However, moisture content had a more significant positive effect than the MBI content on resiliency. The achieved results ranged between 80 and 95% which indicated a good deformation resiliency.

According to Lin et al. (1995) extruded starch foams at water activity levels below 0,23 are brittle and easily crushed, disintegrating upon compression. It was evident that, in these conditions, they are not suitable for packaging use. The resiliency of foams increased with increasing water activity levels (0,33 – 0,53). This was probably due to the increase in starch chain mobility of extruded foams at the higher water content. Further increases in water activity (0,53 – 0,75) caused the foams to soften and thus decreased the resiliency.

Bhatnagar and Hanna (1995) reported that the resiliency of starch-based PS loose-fill is not different from the resiliency of PS loose-fill or starch loose-fill. The ideal elastic body will have a resiliency of 100%, whereas the foams had resiliency of the order of 80 to 90%. The resiliency of starch-based PMMA foam was significantly lower than the PS loose-fill. They found resiliency values of 97% and 96% for commercial polystyrene and commercial starch foams, respectively. They found that, resiliency was significantly affected by the type of starch but not by the type of additive (Bhatnagar & Hanna, 1996). For corn and tapioca starches, the resiliency was quite comparable to that of commercial polystyrene loose-fill. Potato starch extrudates had good resiliency with all the additives.

Nabar et al. (2006) investigated influence of polymer addition into starch on foam resiliency. The control starch foams provided a resiliency of 69,7%. The addition of poly(butylene adipate-co-terephthalate) (PBAT) improved the resiliency considerably, from 69,7% to 85,9% at a PBAT content of 7% of the starch used. Polycaprolactone and cellulose acetate helped increase the spring index up to ~78%, while poly(vinyl alcohol) and methylated pectin barely increased the resiliency to ~71–72%. When glyoxal was added as a crosslinker, it increased the rigidity of the starch foams and, thus, the resiliency of the foams decreased by ~3%.

Zhang and Sung (2007b) tested the properties of PLA/starch foams. The foam samples dried at 135°C for 2 h, then immediately undergoing mechanical compression testing, showed no recovery at all, and all inside structure fractured into pieces, regardless of starch/PLA ratios.

Starch Protective Loose-Fill Foams 91

homopolymer and to the direct water-to-steam expansion process, which creates a predominately open cellular structure that stops foam expansion. Starch-based foam loosefill is very hygroscopic. Foam density of starch-based products is significantly increasing

The compressive stress of most starch-based foams does not differ significantly from EPS products. Chemically modified starches gives foams with good retention of compressive

The resiliency of starch-based foams with values between 69,5 and 71,2% are, as a group, about 10% lower on a relative basis than EPS foams. Although starch-based foams absorbs 13 to 16 wt % moisture after conditioning at 80% r.h. and 23°C, these products are retaining

Both starch- and EPS-based foam fragmentation amounts to 2 to 6 wt %, but starch-based

All starch-based foams have a significantly higher foam and bulk density and open cell and moisture content than EPS-based foam. Both product types have similar compressive stress, resiliency, and friability. Starch-based foams are more sensitive to changes in relative humidity and temperature than EPS-based foam, but the higher amount of absorbed

Generally, extrusion technique can be successfully employed for starch-based foams production. The physical properties of loose-fills, such as density, porosity, cell structure, water absorption characteristics and mechanical properties are highly dependent on the raw materials and additives. Mechanical behaviour of foamed pellets can be adjusted effectively by controlling the cell structure through using different additives. At room temperature and 50% relative humidity, some mechanical properties, such as compressive strength or

Starch-based foams can be prepared from different starch sources replacing 70% polystyrene with biopolymer starch. Functional starch-based plastic foams can be prepared from

Starch-based foams with polymer addition (for example: PS, PMMA, PHEE) exerts improved properties in comparison with 100% starch foams. The addition of polymers significantly increases radial expansion and gives low density foams. Compressive strength is depending primarily on foam density, and not on starch type or polymer structure.

Foams of PLA/starch can be successfully prepares by using water as a blowing agent in the presence of talc, which acts as an effective nucleation agent. Water is a good blowing agent for the PLA/starch system. Talc at 2% gives the PLA/starch foam fine foam cell size and

The addition of Mater-Bi is affecting the foam expansion characteristic. High levels of MBI are resulting in low radial expansions and high densities. The resiliency is improving as the levels of MBI and moisture contents are increasing. The MBI-starch foams have the potential

breaks down into a fine dust, whereas EPS-based foams breaks into large fragments.

compressive modulus of elasticity are comparable to commercial EPS foams.

between 10 and 30% after conditioning at high humidity.

moisture does not compromise its mechanical integrity.

different starch sources depending on their availability.

Friability is reduced when polymer is present in the foam.

to be used as an environmentally friendly loose-fill packaging material.

uniform cell size distribution.

stress over a broad humidity range.

between 62 and a 67% resiliency.

The foam samples conditioned at 50% relative humidity for one week at room temperature and then tested yielded up to 73% recovery after 1 h of force removal. The recovery was reduced with the reduction in starch content. Water can be a good plasticizer for starch, and PLA could reduce water diffusion into the starch phase. After immersing the foams in distilled water at room temperature for one week, however, foams with PLA showed instant recovery to their original shape after one minute of force removal regardless of PLA/starch ratio.

The addition of poly(hydroxyamino ether) (PHAE) to starch improved the foam resiliency considerably from 69,7% (only starch) to 93,5% at a PHAE content of 7% of the starch used (Nabar & Narayan, 2006).

## **4.5 Friability**

Friability is a measure of the fragmentation of foam during handling. Fragmentation of loose-fill during handling and use is an important product quality concern among endusers. Tests made by Tatarka and Cunningham (1998) indicated that the friability of commercial starch-based foams ranged between 0,003 and 2,3%. Although these values are lower than EPS-based foams, they are not significantly different. After conditioning, the friability of these starch-based foams increased significantly when exposed to 80% r.h. and 23°C 50% r.h. and 35°C. Quantitatively, starch- and EPS-based foams fragmented similarly at 2 to 6 % of the total weight, but starch-based foams broke down into a fine dust, whereas virgin EPS-based foams broke into large fragments.

Willet and Shogren (2002) founded that friability of starch/polymer foams is high for all tested formulations and starches at 10% r.h. Of the control starch foams, only the high amylose starch has friabilityof less than 95%. Under these conditions, the starch matrix is well below its glass transition temperature and its brittle. As the relative humidity increases to 50%, the equilibrium moisture content of the starch rises, and the foams are consequently less brittle. The friability decreases significantly, but only the high amylose starch foam displays negligible friability (0,4%). The addition of polymer generally reduces friability at 50% r.h. Corn starch loss-fills with the greatest density also exhibited greater friability, while corn starch foams with low densities had insignificant levels of friability. Friability decreases as polymer surface concentration increases. The presence of a ductile polymer on the foam surface may retard the formation of cracks and fragments under impact by the wooden blocks. One cannot, however, rule out indirect effects of the polymer on foam structure, i.e. lower foam density and thinne cell walls as contributing to the greater flexibility of the foam structure.
