Effect of Severe Environmental Condition
On Storage Stability of Vinyl Pyridine Latex
 

Dr. Rabin Santra and Dr. Subhra Mohanty
R & D Centre, V.P. Latex Division, Vam Organic Chemicals Limited, Gujarat.

INTRODUCTION

Vinyl Pyridine Latex (V.P. Latex) is a versatile adhesive till date for the treatment of various synthetic fabrics like rayon, nylon, polyester and aramid fiber to improve their adhesion in fabric reinforced composite e.g. carcass in tire, V-belt and conveyor belt etc. ¹ Vinyl Pyridine Latex is a heterogeneous dispersion of ter-polymer of styrene, butadiene and vinyl pyridine in water. In the practical utilization of lattices, either dilute or concentrated, the word stability can be used in many different ways²´³, and stability may be required:

(a) To electrolyte addition
(b) To shear or mechanical work
(c) To freezing
(d) To heat or Sterilization
(e) To Long term Storage
(f) To drying conditions such that instantaneous re-dispersion is obtained on rewetting.

In this present article category (E) will primarily be considered.

All hetero-phase systems are generally affected by different environmental conditions. The various environmental conditions may alter the latex properties as well as the polymer properties. As Vinyl Pyridine latex is also a hetero-phase system hence therefore its final properties during storage may change under environmental conditions. Around the globe storage conditions are different ranging from subzero to as high as 50ºC temperature and hence it was necessary to study the effect of various environmental conditions on VP Latex during storage.

EXPERIMENTAL

(i) Preparation of Latex

The vinyl Pyridine-Styrene-butadiene terpolymer latex is produced by peroxy disulphate-initiated emulsion co-polymerization by using a Rosin acid/Fatty acid emulsifiers at 50-60ºC using reaction system similar to those for the production of styrene butadiene lattices at this temperature. Rate of conversion is monitored by measuring % nonvolatile matter (NVM) from the reactor samples at an interval of two hours. After completion of reaction the latex is subjected to degassing and then final modification is done and packed (I).

(ii) Packing Drums

The latex was packed in clean, virgin UMHDPE drums having 210 kg net latex weight with dimension of 910 x 50mm. Drums were named as A, B and C according to their environmental conditions as given below:

 

Drum No.	Storage Conditions	Duration (in months)	IdentificationMarks graphs
A      B       C	Under direct sunlight (Min 22.0ºC to Max 50.0ºC)  Under Shade (Min 22.0ºC to Max 42.00ºC)  Under Storage (Min 5.0ºC to Max 1.00ºC)	February, ’01-July, ‘01      February, ’01-July, ‘01    February, ’01-July, ‘01	Series –1      Series –2    Series -3

*All the experiments as carried out at vadodara, India.

(iii) Sampling

Sample from each drums were taken (after rolling the drums for 30 minutes) once in every month and subjected to analysis.

(iv) Following Parameters are tasted

(a) Skin formation
(b) %NVM
(c) pH at 20ºC
(d) Brookfield Viscosity in CPS at 25º C With Spindle No. 1 and 60 rpm (Brookfield Engg. Inc., USA, Model O LVDVE A 230)
(e) Surface Tension in dynes /Cm at 25ºC (Fisher Scientific Inc., USA, Model-20)
(f) Mechanical Stability in gms/100 with 14000±200 rpm for 10 minutes (Klaxon Engg., Co., UK, Model-ISO-2006)
(g) Chemical Stability gms/100gms
(h) Particle Size in ºA. (Malvern Instrument Co., USA, Model-2000 MU)
(i) Mooney Viscosity in MU at 100ºC. (Alpha Technology, USA, Model-Mooney MV2000)
(j) Characterization of Polymer by IR Spectroscopic method (Perkin Elmer Instrument., USA, Model-Spectrum One)
(k) Microbial growth in Counts/ml

*Detail of test procedure can be obtained on request.]

RESULTS AND DISCUSSION

All the results are tabulated in Table-1 and plotted in Fig. 1-9.

(a) Skin formation
As it is shown in Table-1, skin formation more in case of drum with is stored in direct sunlight. But in other case there is no such observation. This skin formation is attributed to the rapid evaporation of latex at higher temperature as it is continuously exposed to the direct sunlight but in higher temperature as it is continuously exposed to the direct sunlight but in other cases this is not true and hence skin formation is not possible.

(b) %NVM

All the observations in different environmental conditions are reported in Table-1 and plotted in Fig. 1. As it is seen in Fig. 1 the % NVM in case of drum B and C is almost constant and whatever little variation is there which is because of the experimental error- But on other hand in drum C it shows a slight downward trend. This trend is observed because f the thick skin formation in the drum where this like amount of solid goes out of phase
 

Sample Drums	Parameters		February ‘01	March ‘01	April ‘01	May ‘01	June‘01	July ‘01
A Under Direct Sunlight	Skin Formation % NVM pH at 25 C Brookfield Viscosity in CPS Mechanical Stability in gms Surface Tension in Dynes/cm Particle Size in OA Mooney Viscosity in MU Chemical Stability in gms/100 gs Characterization of polymer Bacterial Growth in Counts/ml	NA 40.7 10.99 40.3 0.018 52 920 42 0.008 given in Fig. 9 NA	Slight Skinning 40.71 10.72 40 0.028 51.8 920 42.2 0.0092   Nil	Thick Skin 40.68 10.6 39.7 0.036 51.4 920 42.5 0.012   Nil	Thick Skin 40.72 10.43 40.1 0.045 51.9 920 41.7 0.015   Nil	Thick Skin 40.68 10.43 39.85 0.053 52 920 41.4 0.019   Nil	Thick Skin 40.7 10.22 39.9 0.068 51.9 920 42.3 0.023   Nil	Thick Skin 40.66 10.1 35 0.088 51.5 920 42.5 0.03 given in Fig.9 Nil
B Under Shade	Skin Formation % NVM pH at 25C Brookfield Viscosity in CPS Mechanical Stability in gms/100 gms Surface Tension in dynes/cm Particle Size in OA Mooney Viscosity in Mu Chemical Stability in gms/100 gms Characterization of polymer from IR Bacterial Growth in Counts/ml	Same as Above	Nil 40.73 10.9 40 0.0187 52.1 920 42.36 0.008   Nil	Nil 40.69 10.87 39.8 0.02 51.9 920 42.43 0.0097   Nil	Nil 40.74 10.75 39.9 0.023 51.87 920 41.83 0.021   Nil	Nil 40.63 10.68 38.5 0.031 52.1 920 42.18 0.022   Nil	Nil 40.85 10.6 37.5 0.026 52.2 910 41.8 0.032   Nil	Nil 40.62 10.55 38.1 0.032 50.2 920 42.23 0.025 given in Fig.9 Nil
C In Cold Storage	Skin Formation %NVM pH at 25 C Brookfield Viscosity in CPS Mechanical Stability in gms/100 gms Surface Tension in dynes.cm Particle Size in OA Mooney Viscosity in MU Chemical Stability Characterization of Polymer from IR Bacterial Growth in Counts/ml	Same as Above	Nil 40.73 10.93 39.7 0.0182 52.1 920 42.4 0.009   Nil	Nil 40.69 10.88 38.2 0.022 52.3 920 41.9 0.011   Nil	Nil 40.65 10.85 38 0.025 51.6 930 41.84 0.014   Nil	Nil 40.62 10.7 38 0.0282 51.4 910 42.3 0.021   Nil	Nil 40.58 10.62 37.4 0.031 51.6 910 42.5 0.025   Nil	Nil 40.55 10.6 37 0.036 51.5 920 42.1 0.026 given in Fig.9 Nil

© pH
Variation in pH in different environmental conditions are reported in Table-1 and plotted in Fig. 2. As it is clearly seen from the graph that in all the three different conditions it shows a downward trend. This trend in the latex is mainly attributed to two factors:

1. As few millimeter of empty space is left out during packing of the drums so that much of space is occupied by air, Air contains CO2 Which gets converted to mild acid e.g., H2CO3 in presence of moisture by Characteristic adsorption – desorption phenomenon. This mild acid in turn reduce the pH of lattices during storage.

2. During Processing few ingredients (such as initiator) is left out in ppm level after completion of reaction. These ingredients get converted into very mild acids in a long run which again contributes to the lower or pH in due course of storage and which is prominent in elevated temperature.

(d) Brookfield Viscosity
Brookfield Viscosity is one of the important parameter of any liquid material. This is directly related to particle size and % NVM of the Material. The change is Viscosity is reported in Table –1 and plotted in Fig. 3. As it is seen in Fig. 3 the BPV in case of drum B and C is slight varying and whatever little Variation is there is because of the experimental error. But in other hand in drum C it shows a slight downward trend. This trend is observed because of the thick skin formation in the drum where little amount of solid goes out of phase. Due to the low solid compared to other the drop in viscosity is clearly observed.

(e) Surface Tension

Surface Tension is one of the important Parameter of any emulsion polymerized latex. This a directly related to the emulsifier concentration in the latex. As it is seen in Fig. 4, Surface tension is almost found constant in all the three different case of drums A little bit of difference which is observed this is may be due to the experimental error.

(f) Mechanical Stability
\
Mechanical Stability is very important in case of the latex as it is mainly used in dipping of synthetic fabrics In dipping operation the latex is subjected to a very high shearing force and mechanical stability testing is a simulation test for the same. The variation in mechanical stability is shown in Fig, 5. As it is seen from the figure that in all the cases there is a constant deterioration with respect to storage time. But this is more prominent in case of drum, A which was kept under direct sunlight. This is directly related to pH during storage.

(g) Chemical Stability

Chemical Stability is another important factor for VP latex dipping. In dipping operation the latex is subjected to different chemicals e.g., resorcinol and formaldehyde etc. Chemical stability is done to verify the stability of the lattices against those chemical stability is shown in Fig. 6. As it is seen from the figure that in all the cases the trend is almost same with respect to storage time.

(h) Particle Size

Particle size is he controlling factor of any lattices for all their intrinsic properties. As it is shown in the graph (Fig. 7) there is no such variation in the particle size of the latex during storage as it is directly related to micelles which are not liable to change during this small storage time without any external addition.

(i) Mooney Viscosity

Mooney Viscosity is polymeric property which is related to the molecular aspects of dry polymer. It can be seen from the Fit. 8 that the polymer exits in the heterogeneous dispersed phase which does not come in direct contact with the environmental condition. This is further supported by the infrared spectroscopic results as given in Fig. 9.

(j) IR Spectroscopy

Fig 9. (a) Shows the infrared spectrographs of vinyl pyridine-styrene butadiene polymer in 15.15.70 ratio just after packing. The characteristics peak at 1470 cm-¹ is attributed to the presence of vinyl pyridine respectively in the main chain polymer. Fig.9 (b.c and d) shows the IR curves after 6 months of latex storage. As the graph shows there is no change in the characteristic curve of the polymers which shows that no functional group develops in the main chain during storage.

As it can be seen in the IR graphs that there is no change in the composition of polymer when it is tested in initial stage and after six months of storage. This confirms the money viscosity trend as shown in Fig. 8.

(k) Microbial Growth

Bacterial growth is very harmful to any emulsion system. Particularly it is very prominent to VP latex as because the system contains fatty acid and rosin acid which are very good food for bacteria. With the advent of bacterial growth the emulsifiers may be consumed which in turn will remove the emulsifier layer in the particles. The result will be coagulation of latex. In present system no microbial growth was observed even after six months in all cases because the system contains an effective bactericide.

CONCLUSION

From the above study it is very clear that exposure to direct sunlight deteriorate the latex properties drastically. But the latex can be stored under shade with a temperature varying from – 5 to 40ºC Without any drastic change in various properties for six months.

REFERECES

1. Vinyl Pyridine latex – A Versatile adhesive for synthetic fabrics – Dr. Rabin Santra, Rubber India January 2001.
2. Blackely, D.C., “Polymer Lattices-Science and Technology” Volume-1, Chapman and Hall London (1997).
3. Blackely, D.C., “Polymer Lattices-Science and Technology” Volume-1, Chapman and Hall London (1997).