- Functional Additives
- Colorants
- Fillers
- Reinforcements
Functional Additives
- The most important stabilization method is structural modification of the polymer by, for example, copolymerization with cyclic ethers and blocking the end groups.
- Stabilization is carried out by salts of carboxylic acids.
- Polyacetals are not UV resistant
- Without UV stabilization, POM has a tendency to surface cracking and chalking after a short period of weathering outdoors, Provided black is acceptable, carbon black can be used to achieve excellent UV stabilization, A combination of sterically hindered amines (HALS) and UV absorbers, e.g. 2-2’hydroxy-5’methylphenyl) benzotriazole, is suitable for light colors.
Colorants
- The pigments used to produce colored molding compounds from natural POM in house must be able to withstand processing temperatures without decomposing or changing color.
- Furthermore they must not impair the thermal stability of POM. Pigment concentrates are supplied as granules.
Fillers
- Filling with MoS2 reduces the difference between the static and dynamic coefficient of friction and thus the tendency to stick-slip.
- Addition of chalk improves the unlubricated abrasion resistance, a property valuable for gears and bearings. The flexural fatigue strength is also significantly increased.
- Addition of PTFE exploits the good slip properties of this material and the high mechanical strength of POM. Maintenance-free bearings without stick-slip are important applications.
- The slip characteristics of standard POM grades can be improved by the addition of oil concentrates in a ratio 1:10 Aluminium and bronze can be used to increase the heat distortion temperature and electrical conductivity.
Reinforcements
- The good heat distortion characteristics of reinforced grades can only be exploited for a short time.
- The maximum service temperature even for glass- reinforced grades is only just above 100°C, 20 to 30% w/w of chopped and continuous strand doubles the tensile strength and triples the flexural modulus of elasticity.
- Creep behavior at elevated temperatures is also improved.
- The downside is lower notched impact strength and higher price per volume.
- Glass beads can be added up to 80% w/w without significantly affecting processing conditions.
Grades of POM
The polyacetals are available in the following grades.
- Injection grade
- Extrusion grade
- Extrusion blow grade
- Rotational grade
In addition to that the following special grades are available,
- Improved processability grade.
- Low friction grade.
- Glass filled grade
- Mineral filled grade
- UV-Stabilized grade
Processing considerations of Polyacetals
- While processing polyacetal following precautions to be taken.
1. Stepwise thermal or based catalyzed hydrolytic depolymerization initiated from the hemiformal chain end with the evolution of formaldehyde.
2. Oxidative attack at random along the chain leading to chain scission and subsequent depolymerization.
3. Acid catalysed cleavage of the acteal linkages.
4. Thermal depolymerization through scission of C-O bonds can occur catastrophically above 270°C and care must be taken not to exceed this temperature during processing.
- The homopolymer is moulded at melt temperature of 200-210°C while the copolymer would be moulded at melt temperature of 190-205°C.
- Therefore end capping is done during polymerization and antioxidants and acid acceptors are added
Surface Finishing of POM
Hot Stamping
- The hot stamping of POM molding is of increasing importance, e.g. in the manufacture of counter rolls. No pretreatment is necessary.
- The surface must be clean and the embossing stamp must be applied evenly.
Metallizing
- High vacuum metallizing imparts a reflective metal surface to polyacetal moldings.
Printing Painting, Lacquring
These finishes are after pretreating the surface as for polyolefins.
Machineability of POM
Joining
- All joining methods apart from high frequency welding are suitable for POM moldings.
Welding
- Heat tool, friction and ultrasonic welding.
Bonding
- Contact and solvent adhesives. The peel strength of bonded joints is surprisingly high even with unprepared surfaces, e.g. with hot milt adhesives, based on vinyl copolymers.
Cyanoacrylate one pack polymerization adhesives, EP resins, PU adhesives. Hexafluoroacetone sesquihydrate has been used as an adhesive for some years.
Screws
- Self-tapping, threaded inserts are made.
Rivets
- Hot and cold riveting systems are made.
Others
- Used as snap and press connectors.
Applications of Polyacetal
- Appliances
- Agriculture & Irrigation
- Consumer Products
- Industrial
- Electrical
- Plumbing & Hardware
Application of Polyacetal
Appliances: Housing for business machine, gears, cams, friction pads, rollers, pulleys, nuts, chain links and shelf support brackets, detergent pumps, refrigerator clips and brackets, bearing, wear strips and instrument housing in washers and dryers, spray nozzies and soap dispensers in dishwares , bowls, mixing blades and bearings in counter-top appliance bodies, tops and cups in water boilers.
Agriculture & Irrigation: Pop-up sprinklers (nozzles arms, gears, housing and water ways), pumps(housing, impellers, pistons) metering valves, tractor components (shift lever housing, hydraulic connectors, seed applicators, bearings and gears)
Automotive: Fuel level indicators, pump components, gas caps, cooling fans, trip clips, colour co-ordinated bucket housings, window cranks, shift lever handles, knobs, lever and mounting brackets, instrumental cluster gears, bearings, housing and dials, exterior door pulls, mirror housing and brackets.
Consumer Products : Toys soap dispensers , combs, filter bodies and valves, aerosol containers and valves, pen and pencil barrels and tips, mascara wands & containers, sprayer pumps, nozzles and pump components for dental cleaners.
Industrial: Valves, springs, bearings, cams, material handling components such as conveyors, chain links gears, pumps and hose connectors.
Electrical: Key tops pluggers, switches, buttons, cassette tape rollers and hubs, base plates in computer keyboards, springs in telephones and connectors in modular components.
Plumbing & Hardware: Water –meters, cams, gears, dials and pressure plates, pressure regulator valves, drapery and venetian blind guide rollers, furniture , casters, slid plates and locks, tool holders, bearing in adapters, shower heads, sprayers, garden hoses and nozzles, irrigation gates, impellers, pumps and hangers.
Blends of POM:
- Although unmodified polyacetals already have high inherent toughness they exhibit a certain notch sensitivity. That is by virtue of the stress concentration that arises, sharp notches effect a substantial drop in impact strength.
- Blending polyacetal with elastomer brings a noticeable improvement not only in the notched impact strength, but also in the ability to bear multiaxial impact loading.
Polyacetal/TPU
- Among the elastomers, thermoplastic polyurethane have the greatest significance for the impact modification of polyacetals. Such blends are formulated for extrusion, injection, blow, compression and transfer moulding.
They show excellent processability, rigidity high impact strength, high fatigue, flexural and tensile strength, low water absorbency and resistance to chemicals. Mostly contain 10-30 wt of TPU, and have co continuous morphology for especially good performance.
Polyacetal/ Butadiene
- Elastomers based on polybutadiene or poly acrylates are important modifiers.
- The essential advantage of modification with polybutadiene is better cold impact strength.
Polyacetal/Acrylate
- In the case of modification of polyacetal with polyacrylate the advantages are better heat aging and better weathering.
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