Enhanced effect of chloroprene on the mechanical properties of polyurethane foams

Abstract

In this research, a new method to increase the mechanical properties of the low-density polyurethane foams (PUF_S) has been described. Chloroprene rubber (CR) is used as reinforcing the mechanical properties of the foam. Polyurethane foam (PUF) containing CR (PUF/CR) and pure PUF were characterized by mechanical testing, Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), Differential scanning calorimeter (DSC), Thermogravimetric analysis (TGA). The results of mechanical properties show the tensile strength of PUF_S containing 0, 1, 2, 3, 4 and 5 wt% CR are 38.2, 42, 56.4, 61.48, 77.3 and 77.18 kPa, respectively. Also the morphological studies show by increasing the amount of CR in the foam structure, the foam cells are smaller and more closed.

Keywords

Chloroprene rubber polyurethane foam low density mechanical properties and thermal stability

Introduction

Polyurethane (PU) polymer compositions have wide physical and chemical properties. These polymers are widely used in the production of fibers, toys, coatings, adhesives, foams and etc. The chemical reaction between polyols and diisocyanates is reaction basis for polyurethane compounds preparation. Presence of one or more blowing agents also is required.¹ Among the various forms of polyurethane (PU_S), the foam forms due to their high surface to volume ratio, their high flexibility, their thermal insulating properties and etc are one of the most useful commercial products of PU_S.² Based on the mechanical performances and core densities, the polyurethane foams (PUF_S) can be classified into three types: flexible foams, semi-rigid foams and rigid foams.³ The first type is the most important because they have wide applications in our daily life. The flexible PUF_S are used for production of furniture, car seats, beds and etc.⁴ These materials have several important advantages and few disadvantages, some of the advantages are below: Wide range of flexibility, High permanence, High chemical resistance, Cost-effectiveness, Thermal insulation, Acoustic insulation and Having low density.^5–7 But this class of foams has not sufficient tensile strength in low densities (less than 17 kg/m³), therefore their use is restricted.⁸ Therefore, during the past two decades, many efforts have been made to overcome this drawback and improved PUF_S mechanical properties especially for the low density foams.^2,9 The density change is one of the simple and usual ways to improve the mechanical properties of the low density flexible PUF_S, but this is a costly method therefore other methods have also been investigated.¹⁰ One of the most important methods is the use of nano and micro fillers in the polymer matrices.^11–14 But this technique has not been widely used in the industrial applications it could be because the uniform dispersion of the fillers in the foam raw materials is very hard on an industrial scale. In another attempt polyols synthesized by using styrene acrylonitrile as co-polymeric polyols introduced. These polyols are commonly used in industry.¹⁵ But despite improving load-bearing capacity, these copolymeric polyols directly decrease the foam resiliency. Also another method is the use of different polymeric materials such as polyethylene terephthalate, polyurethane particles and various rubbers in the polymer matrices.^16–18

Chloroprene rubber (CR) is one of the most important elastomers. CR has good chemical and physical properties and it has the ability to crystallize on stretching.^19,20 Also, CR is resistance to oil, solvents, ozone, weather ageing, oxygen, heat and flame.^20,21 On the other hand, polyurethane foams are very susceptible to fire due to the large amount of oxygen they contain inside their cells. Therefore, the embedding of CR as halogenated compounds among the polymer chains of these compounds can play a significant role in controlling this risk.²² Parallel with these advantages, CR can cause irreparable problems for the environment because these materials are very stable and irresolvable.²³ So providing suggestions for different use of these rubbers can reduce environmental pollutions. CR has electronegative chlorine atoms in its chemical structure that can interact with active hydrogen atoms (see Schematic 1)¹⁶ and increases its mechanical properties. In addition, the non-polar parts of it can also easily interact with non-polar structures. So by embedding of CR molecules among PUF chains, PUF mechanical properties can be improved.

Given that, in some applications (such as sofa handles, etc.), the density of the PUF must be low. Therefore, the possibility of using high density PUF instead of low density PUF is limited in terms of functionality (not economically). Therefore, the main goal in this study is to increase the strength of low density PUF. For this purpose, in this study, CR is applied instead of part of the raw material of PUF and the tensile strength of PUF is improved at low density with minimum cost. To accomplish this goal, the foam formulation is modified with the adding of different weight percentages of CR (1, 2, 3, 4, 5 and 6 wt%) for the first time. FT-IR, SEM, DSC and TGA measurements are used to characterize of the foams. Finally, the results of mechanical testing showed, by adding CR to the low density PUF formulation the tensile strength of foam can be improved more than twice (38.2 to 77.18 kPa).

Experimental

Materials

Commercial grade products were used as raw materials. Polyether based polyol (KONIX FA-717) with 46-50 mgKOH/g hydroxyl values was supplied by KPX Chemical Co., Ltd. Toluene diisocyanate (2, 4- and 2, 6-toluene diisocyanate (TDI) in the ratio 80/20) was provided from Karoon Company, Iran. Niax silicone L-620 (as surfactant from General Electric Company), dabco 33-LV (as an amine catalyst from Air Products) and kosmos 29 (as co-catalyst from Degussa) were obtained. CR chips (Baypren 243) were supplied by Bayer Material Science and methylene chloride was purchased from Samsung Fine Chemicals. All other chemicals were analytical reagent grade. The distilled water used throughout the research.

Preparation of PUF and PUF containing various percentages of CR (PUF/CR)

Two types of PUFs (PUF and PUF/CR) were synthesized on a laboratory scale by one-shot method²⁴ as formulations presented in Table 1. The foams were synthesized with 14 kg/m³ density according to the standard slab stock flexible foam formulation of Air Products and Chemicals, Inc.²⁴ For the preparation of PUF samples, polyol, distilled water, dabco 33-LV as amine catalyst, kosmos 29 as tin catalyst and silicone surfactant were premixed and stirred at a speed of 600 r/min for 2 min as A component. TDI and methylene chloride mixture as B component added to the A component and mixed at a speed of 3000 r/min for 6 s to attain the cream time. Afterward, the final mixture was transferred into an open mold (15 cm × 15 cm × 15 cm) and allowed to rise freely. The cream time and rise time for the preparation of PUF were recorded. To optimize the foam formulations, different amounts of CR were incorporated into PUF structure as tensile strengthener. The prepared PUF post-cured at room temperature for 72 h. For the preparation of PUF/CR, required CR was firstly dissolved in methylene chloride and then solution added to the A component.

Table 1.

Basic Formulation for Prepared PUF_S.

Component	Polyol	Distill water	Dabco 33-LV	Kosmos 29	Silicone	TDI	Methylene chloride	CR
PUF	100	5.4	0.24	0.34	1.9	68	13	0
PUF/CR	100	5.4	0.24	0.34	1.9	68	13	1-6 % (w/w)

The cream time, defined as the time from the addition of TDI to the creation of a creamy liquid. At this time the polyol and isocyanate blend commence to change from the liquid state to a creamy and begin to expand eventually. The point when the foam stops rising and attain to its maximum height is Rise time.²⁵ The cream time and rise time were measured as reported by ASTM D 7487-13.

Characterization of PUF and PUF/CR

Chemical structure of CR, PUF, and PUF/CR were studied by using a Fourier transform infrared spectrophotometer (FT-IR, Perkin- Elmer), in region of 400–4000 cm⁻¹ with the resolution of 4 cm⁻¹.

Scanning electron microscopy (SEM) (VEGA\\TESCAN, Czech Republic) was applied to investigate the cellular structure and morphology of the samples. The samples were cut into small pieces (0.5 cm × 0.5 cm × 0.5 cm) using hot wire. In the following, cut samples were covered with a thin layer of gold. Also the SEM pictures were produced in parallel to the direction in the foams growth.

The differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA) were used to verify the thermal resistance of PUF and PUF/CR. Thermal analysis under nitrogen flow was performed with a DSC-PT10 (Linseis, Selb, Germany) differential scanning calorimeter and a L81/1550 thermogravimetric analyzer. To obtain the glass transition temperatures (Tg), DSC was used. DSC scans conducted at 10°C/min heating rate. Also for investigate of the samples thermal stability, dynamic TGA was employed. All the samples were first dried before TGA characterization according to the literature method.⁵

The density of the foams was measured according to ASTM D3574. The samples were cut into small pieces (10 mm × 10 mm × 10 mm) and weighted. Then the ratio of weight (g) to volume (cm³) of samples calculated as stated by ASTM D3574. For each result, five samples were tested and the average of five measurements was taken.²⁶

Tensile strength tests were performed on the foam samples with a TCS-2000 Universal Testing Machine (GOTECH, Taiwan) according to the specifications of ASTM D638. Thickness, width and gage length of foam Specimens were 4, 12.7 and 64 mm respectively. The crosshead speed was set at 20 mm/min. The stress-strain curves were drawn simultaneously with 2.5 kN load cell. For each data point, five samples were tested and the average value was taken.⁹

Results and discussion

FT-IR spectroscopy

FT-IR spectra were used to verify the spectral changes and characterize the interaction between PU and CR chains. For PUF and PUF/CR, FT-IR spectrum is shown in Figure 1(a) and (b) respectively. According to Figure 1, it can be inferred that there is a chemical interaction between CR chains and polymer matrix. Because there is almost 20 cm⁻¹ positive shift in the N-H and O-H bond peaks, confirming the interaction between CR chains and polyurethane groups.²⁷ Also there is slight positive shift in the carbonyl group bond peak. According to Schem 1, the presence of an electronegative chlorine atom in the structure of chloropyrene causes the hydrogen atom of the urethane group to stretch. As a result, the bond between nitrogen and hydrogen is weakened and causes a positive shift from 3273 cm⁻¹ to 3292 cm⁻¹ (see Figure 1). In addition, the decrease in the strength of the bond between hydrogen and nitrogen causes the accumulation of negative charge on the nitrogen atom. As a result, resonance is created between the three atoms of nitrogen, carbon and oxygen (on urethane functional group). The creation of this resonance reduces the bond strength of the carbonyl group, and as a result, a Slight positive shift occurs from 1718 cm⁻¹ to 1727 cm⁻¹ (see Figure 1).

The effect of CR content on cream time, rise time and density of PUF

The effect of CR content on cream time, rise time and density of foam samples were measured and the results are shown in Table 2. The influence of CR content on cream time and rise time was considered by measuring these parameters during foaming. It was seen that with increasing CR content cream time and rise time increase. The increase of cream time and rise time is primarily due to the increase of the reaction rate between polyols and TDI against the reaction rate between water and TDI. This process was laid to the fall the TDI reactivity due to in folding by the solution of dispersed CR in methylene chloride.

Table 2.

Some Physical Properties of Prepared PUF_S.

CR wt%	0	1	2	3	4	5	6
Cream times (s)	9	10	12	12	14	15	15
Rise times (s)	78	84	96	105	117	123	133
Cure times (h)	24	24	24	24	24	24	24
Density (Kg/m³)	14.10	14.03	14.08	14.11	14.08	14.19	18.57

According to foam density measurements, at lower values than 6 wt% of CR the foams are produced with densities between 14 and 14.20 kg/m³, but at higher than 6 wt% of CR, the density suddenly increases from 14 to 18.57 kg/m³. This phenomenon can be imputed to the shrinkage in foam cell structure. In this case, due to prolong the curing time, stretching properties of CR effects on the weak and brittle cell structure. You know before curing time the foam structure is very weak and breakdown.

Scanning electron microscopy (SEM)

SEM analysis was used to investigate the effects of CR on cellular morphology of the ‘prepared foams. The cellular morphology of the prepared foams is shown in Figure 2. The difference in cellular morphology of PUF and PUF/CR can be attributed to the presence of CR in foam structure. According to Figure 2, in PUF the foam cells are partially open (Figure 2(a)) while with the addition of CR to the foam structure, the foam cells are completely closed specially in the foam formulation with 5 wt% from CR (Figure 2(b)–(d)). So it is seems by increasing of CR content in PUF structure, the foam cells are closed.

As you know, during the generation of cellular structures, at first the reaction between water and isocyanate causes the bubbles production, then these gas bubbles are growth and separated from each other by thin layers. Under the influence of internal pressure, the wall can be destroyed. In this case the foam cell structure will be open.^28,29 But it can be seen that by increasing the amount of CR in PUF structure, the cell wall resistance has been increased. It can be related to the elasticity of CR.³¹ So in high amount of CR the gas pressure inside the bubble is not enough to rupture the cell walls then the cellular structure will be more closed. Also, as you can see in Figure 2(e) the cellular structure of the foam is shrinkage and disrupted. As mentioned earlier, due to prolong the curing time, stretching properties of CR effects on the weak and brittle cell structure. You know before curing time the foam structure is very weak and breakdown.

Thermal stability

For the thermal stability of samples, DSC and TGA analysis were employed. All the samples were first dried before TGA characterization according to the literature method.⁵ CR glass transition temperature (Tg) is consistent with reported literature (see Figure 3(a)).³¹ As is clear in Figure 3, Tg values for CR, PUF/CR and PUF are 372, 333 and 323°C, respectively. According to Figure 3(b) and (c), Tg value of PUF has been improved (10°C) by adding 5 wt% from CR in PUF structure. It can be related to the interaction between CR chlorine groups and active hydrogen groups of PUF chains. In fact, CR acts as a hydrogen bond generator and restricts the movement of PUF polymer chains (see Schem 1).³²

Also the samples thermal degradation behaviors were investigated by TGA analysis. As seen in Figure 4, there is a single-step thermal decomposition for all samples. The onset degradation temperatures for PUF and PUF/CR (5 w%t) are at 239 and 263°C. TGA date show that the degradation rates of PUF/CR (5 w%t) become slightly slower compared to that of PUF. This represents the positive effect of CR on the thermal stability of PUF. This can be related to two reasons: First, CR is more stable than flexible PUF_S in higher temperature,³³ second, because the CR acts as a hydrogen bond generator and prevent PUF chains moving. Also from TGA curves can be found the speed and weight loss of every stage.

Tensile properties of the PUF/chloroprene foams

The measurement results of tensile tests for prepared samples exhibited in Figure 5. These results confirm that the amount of the tensile strength increased with increasing CR content in the foam structure. It can be related to three reasons:

First, the tensile strength and elongation at break values for CR are higher than flexible PUF.³⁴ Therefore the tensile strength increases with increasing CR in flexible PUF structure.

Second, it can be related to the interaction between CR chlorine groups and active hydrogen groups of PUF chains (see Scheme 1). As you know, there are hydrogen bonds in the polymer matrix, increases the strength of the polymer.³⁵ Increase in the amount of CR increases the number of chlorine groups. The result is an increase in the number of hydrogen bond and the foam strength. Third, in accordance with Figure 2 by increasing the amount of CR in the foam structure, the foam cells are smaller, as you know the large cell size of foam is less strength.³⁶ In the case of foam containing 6% CR, the increase in mechanical properties is much higher than in the rest of materials. This is mainly due to the high density of this material (4.5 kg/m³ more than the rest of foams.

The elongation at break for samples is also increased by increasing CR content and shifted to higher values (see Figure 6).

Conclusions

In the present research, PUF_S with different percentages of CR (0, 1, 2, 3, 4, 5 and 6 wt%) were prepared. Characterization of foams was carried out by FT-IR, SEM, DSC, TGA, mechanical test and some other testing methods. SEM results indicated PUF_S containing CR have smaller cell size than neat PUF. Thermal stability analysis showed with the addition of CR to the PUF formulation, the foam thermal stability increases. The mechanical test results demonstrated that the tensile strength increases with the increasing CR content in PUF/CR structure. The tensile strength was obtained for PUF/CR (wt/wt: 95/5 or 5 wt%) equal to 77.18 kPa. In addition, with this method, CR can be recycled and reused. It was also perceived that the increasing CR content caused to obtain the closed cell structure in the prepared foam.

Footnotes

Acknowledgements

This paper is published as part of a research project supported by the Research Affairs Office of the Ahar branch Islamic Azad University. The authors are grateful to the Ahar branch Islamic Azad University for the financial support.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Mir Saeed Seyed Dorraji

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