Glass Transition

GLASS TRANSITION - Tg

Have you ever left a plastic bucket or some other plastic object outside during the winter, and found that it cracks or breaks more easily than it would in the summer time? What you experienced was the phenomenon known as the glass transition. This transition is something that only happens to polymers, and is one of the things that make polymers unique. The glass transition is pretty much what it sounds like. There is a certain temperature(different for each polymer) called the glass transition temperature, or Tg for short. When the polymer is cooled below this temperature, it becomes hard and brittle, like glass. Some polymers are used above their glass transition temperatures, and some are used below. Hard plastics like polystyrene and poly(methyl methacrylate), are used below their glass transition temperatures; that is in their glassy state. Their Tg's are well above room temperature, both at around 100 ^oC. 

Amorphous and Crystalline Polymers:

We have to make something clear at this point. The glass transition is not the same thing as melting. Melting is a transition which occurs in crystalline polymers. Melting happens when the polymer chains fall out of their crystal structures, and become a disordered liquid. The glass transition is a transition which happens to amorphous polymers; that is, polymers whose chains are not arranged in ordered crystals, but are just strewn around in any old fashion, even though they are in the solid state.

But even crystalline polymers will have a some amorphous portion. This portion usually makes up 40-70% of the polymer sample. This is why the same sample of a polymer can have both a glass transition temperature and a melting temperature. But you should know that the amorphous portion undergoes the glass transition only, and the crystalline portion undergoes melting only.

On cooling, rubber undergoes a liquid-glass transition, which has also been called a rubber-glass transition. For example, the Space Shuttle Challenger disaster was caused by rubber O-rings that were being used well below their glass transition temperature on an unusually cold Florida morning, and thus could not flex adequately to form proper seals between sections of the two solid-fuel rocket boosters.

Note:  If anyone interested in theoritical calculation of Tg, please email me.  I will send you Tg calculator excel file.

Unsaturated Polyesters

UNSATURATED POLYESTERS

         Unsaturated polyesters are produced from Propylene glycol and Maleic anhydride. Some specialised resins that need high chemical resistance use Fumeric acid.

         Other acids in conjunction with the unsaturated Maleic type to prevent resins from being too reactive.  Ortho-phthalic anhydride is most common for general purpose formulations.  Iso-phthalic acid is being used where better chemical resistance is required.  Adipic acid is used where flexibility is required.  Halogenated acid (tetra bromo phthalic anhydride) can be used to produce reduced flammability in mouldings.

         Other glycols like Dipropylene glycol, Diethylene glycol give some degree of flexibility.  Neopentyl glycol offers better chemical resistance.

         Methylmethacrylate can be used as part replacement for Styrene in the monomer portion of the resin.

         

         Applications:

       Unsaturated polyester resin is used by  fibre-glass industry to build boats and car bodies, for encapsulating electrical components. There is huge production of synthetic marble and buttons manufacturing with UPR.   

        In all of these applications the UPR is poured into a mould and a free radical initiator such as MEKP (methyl ethyl ketone peroxide) or benzoyl peroxide added to initiate cross-linking.  In general a release agent is necessary to apply on mould before pouring resin.

Anion and Cation

ANION AND CATION

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge.

Ions can be created by both chemical and physical means. In chemical terms, if a neutral atom loses one or more electrons, it has a net positive charge and is known as a cation. If an atom gains electrons, it has a net negative charge and is known as an anion.

An anion (−)  is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged).

A cation (+)  is an ion with fewer electrons than protons, giving it a positive charge.

Urea formaldehyde resin

UREA FORMALDEHYDE RESIN

Reactions producing Amino resins:

The first stage is addition of Paraformaldehyde and Urea to produce mono and dimethylol urea.  This reaction takes place under mildly alkaline or neutral conditions.

The second stage continues when the alkali conditions are changed into acidic conditions.  A branched highly polar polymer is produced.  This polymer is insoluble in solvents and incompatible with other resins, therefore a third stage is required in which an alcohol, commonly Butanol, reacts with the resin.  This stage is known as etherification or specially butylation and in fact proceeds alongside the second stage and in direct competion with it.

The nature of the final etherified UF resin depends on the proportions of each of the  starting raw materials, the amount and type of acid, and which particular alcohol is used in the etherification stage.

When the desired molecular weight and degree of etherification has been obtained (by measuring the water evolved), the reaction is stopped by neutralizing the acid and cooling the mixture.  Vacuum distillation removes unreacted materials although the alcohol is often returned to be used as a solvent.  Hydrocarbon solven Xylene is being used as a carrier.

The reactivity of an amino resin can, to a certain extent, be controlled by the ratio of the main functional groups present on the resin.  The more formaldehyde used in the polymerization, the less ‘N-H’ function will be present.  The more alcohol used the more ether groups that will be present.  In addition, the choice of alcohol used is very important.  The lower the molecular weight of the alcohol, the more reactive is the resin.

The N-H group is polar, as are the methylol groups.  They produce hydrogen bonding which increases viscosity and decreases compatibility with other less polar resins.  By increasing the formaldehyde content we reduced the amount of N-H function and hence, viscosity decreases and compatibility increases.

Acrylamide Acrylic Resin

ACRYLAMIDE ACRYLIC RESIN

Acrylamide acrylics contain the same reactive groups as Amino resins, namely amide, methylolamide and butylated methylol groups.

                       NH2                                                  CH2OH

                 Amine group                                     Methylol group

Can self polymerise.  Acrylamide acrylics cured by any resin which will cure amino resins.  React with resins containing –OH groups such as Epoxy, Alkyd and Polyesters.  As with amino resins the proportions of these reactive groups determine their reactivity and compatibility with other resin systems.

As with amino resins, the presence of acid speeds the curing rate.  Self cured films are not brittle and they don’t need the addition of other resins just to make them more flexible.  This gives the formulator more tolerance in ratio of hydroxyl polymer and acrylamide acrylic which he blends.

Acrylamide acrylic / Epoxy systems have excellent chemical resistance and flexibility.  These are pale coloured and have good enough colour retention.  These are commonly stoved for 30 minutes at 160°C or 20 minutes at 170°C but can be stoved for 12 minutes at 180°C if necessary.  Widely used as white appliance finishes where excellent detergent and stain resistance is important.  These are also used on aluminium sheeting for caravan exteriors although here they will suffer from chalking.  Both these applications can utilize pre-painted coil strip.  In this case flexibility and adhesion which these blends have become important.  Acrylamide acrylic / Epoxy blends also used on roller-coated strip.  An example here is white exteriors for cans.  The white surface is usually overprinted and so the ability to remain flexible and not to yellow on double stoving is important.

As acrylamide acrylics are self polymerized,  if formulated correctly, produce excellent films.  Acrylamide acrylics are expensive, so they are often blended with alkyds or polyesters.  These blends are used for factory applied car finishes, usually for the solid or non-metallic colours where a water white colour is not necessary.  Darker shades tend to use alkyds, where any yellowing tendency is less noticeable, whereas for whites and pale shades, the acrylic is usually blended with polyester.  In both cases a clear coat can be applied over the colour coat, as with metallics, for better durability and gloss.

Acrylated VS Styrenated alkyds

Specific gravity of solid resins

SPECIFIC GRAVITY OF SOLID RESINS

W1 = Empty SG bottle weight

W2 = SG bottle  +  Water weight

W3 = SG bottle  +  10 grams Resin sample weight

W4 = SG bottle  +  10 grams Resin sample  +  Water weight

                                           W3 – W1

Calculation       =  ----------------------------------

                               (W2 – W1) – (W4 – W3)

Example:

W1 = 47.62 gm.

W2 = 147.36 gm.

W3 = 57.66 gm.

W4 = 152.22 gm

                                               57.66 – 47.62

Calculation       =  ---------------------------------------------

                              (147.36 – 47.62) – (152.22 – 57.66)

                                           10.04

                        =  -----------------------------

                                    99.74 – 94.56

                                   10.04

                        =  -----------------

                                     5.18

                        =  1.918

Note: 

This method can be used to find out SG of any high viscous resin or any solid resin