Furniture Coatings Past and Present

1
Furniture Coatings - Past and Present

• Mark Taylor
• Mirotone (NZ) Ltd.

2
Introduction

• The purpose of this presentation is to summarise
  the history of furniture coatings from French
  polishing, through N/C based lacquers, to the two
  pack acid catalysed, urethane, polyester, and UV
  light cured finishes.
• Most people outside of the furniture industry
  take the appearance and performance of furniture
  for granted, generally assessing appearance and
  style, but seldom the finish or feel of the
  coating. To the lacquer manufacturer it is an
  art that has enhanced the appearance of furniture
  for centuries, with ironically the older French
  polish systems being generally better known
  compared with modern day finishes.

3
French Polish

• Introduced into Europe in the early 1600s.
• As late as 1935, 80 of furniture was still being
  finished with French polish or related products.
• Based on Shellac, which makes high quality
  furniture coating due to its various intrinsic
  properties such as
• quick set times,
• excellent adhesion to wood and metal,
• unsurpassed in its time for resistance to oil,
  grease and hydrocarbon solvents,
• together with its brilliantly clear finish.
• Sap is extracted from any of 60 varieties of tree
  (commonly the Palas, Kusum and Ber trees) by the
  lac insect, reconstituted and excreted as a
  sticky deposit which is collected and processed
  into various forms of Shellac.

4
Shellac - Disadvantages

• Disadvantages are
• thermoplasticity,
• water sensitivity
• and a multicoat system, hence taking a long time
  to finish an article.
• In recent times of course has been the gradual
  erosion of French polishers in the workplace and
  concerns that the trade is becoming a dying art.
• The successor to French polishing were
  nitrocellulose lacquers and latterly
  pre-catalysed lacquers.

5
Nitro-cellulose

• Discovered by a German/Swiss named Schonbein in
  1846, as a somewhat unstable explosive.
• In 1868 a method was perfected to stabilise
  nitrocellulose and its use in the production of
  celluloid in the late 1800s and early 1900s was
  considerable.
• However it was not until around 1907 when spray
  guns were invented that cellulose lacquers gained
  any real popularity as a coating.
• (In the 1914 18 war, cellulose solutions were
  used as dope for aircraft production, and thence
  as a coating for furniture.)

6
NC - Manufacturing

• Nitrocellulose itself is produced from natural,
  cellulosic based materials such as wood and
  cotton linters predominantly.
• Wood or linters is chemically treated to remove
  lignin, then nitrated with a nitric
  acid/sulphuric acid mix, washed and stabilised.
• The degree of nitration determines the solubility
  with the lower nitration levels (10.2 11.2)
  tending to be alcohol soluble varieties, whereas
  the higher nitrogen levels (12.4 13.0) exhibit
  diminishing solubility in alcohol, while being
  soluble in ketones and ester solvents.

7
NC - Properties

• Nitrocellulose confers the following properties
• High mechanical strength
• Hardness
• Transparency
• Good solvent release
• Good durability
• Reasonable chemical resistance
• Reasonable water resistance
• Easy solubility in active solvents
• Good flexibility when plasticised
• Good alkyd and resin compatibility
• Polishable
• Moderate cost

8
NC - Disadvantages

• Disadvantages are
• Highly flammable
• Brittle when unplasticised
• Poor inherent adhesion when unmodified
• Adhesion, toughness and build can be bettered by
  newer coatings
• Nitrocellulose is a dangerous explosive and
  should never be allowed to dry out. It is
  supplied in two forms, namely linters grade, a
  fibrous grade, or dense grade made from timber
  which is purchased in small chips. All grades
  are damped 25 or 30 with ethanol or IPA for
  safety reasons.

9
NC Lacquers

• The formulation of N/C furniture lacquers
  involved dissolving the N/C in a suitable solvent
  mix, consisting of
• ester and ketone solvents,
• latent solvents,
• non solvents or diluents,
• then modifying with plasticisers, and
• alkyds
• Lacquers are virtually always spray applied and
  are complimented and enhanced by the use of a
  variety of timber stains and sealers.
• Available as clear and pigmented, and in a range
  of gloss levels.
• Drying is very fast, approximately 10-15 mins so
  an article can be stained, sealed, sanded and
  topcoated in an hour, if so desired.

10
Pre-Catalysed NC Lacquers

• Addition of amino resin(s) to a Nitro-cellulose
  laccquer creates a so called pre-catalysed
  lacquer system. This encourages some cross
  linkage within the system, through reaction with
  a weak acid (eg ABP), enhancing properties like
  alcohol and water resistance.
• The array of permutations possible by varying
  alkyd type, amino (MF or UF), plasticiser type
  and quantity, solvent selection, etc are
  enormous. Incorporated into these are flow
  additives, waxes to improve surface feel, UV
  absorbers and possibly re-odourants.

11
In general

• Over the past 10-15 years the volumes of
  pigmented colour products, compared to clear, has
  increased with the advent of painted kitchen
  units, as opposed to the previously dominant
  timber preferences. This ratio swings with the
  fashion depending whether pigmented or clear
  timber units are in vogue.
• As more and more demands have been put on kitchen
  units, commercial and motel situations etc, the
  single pack pre-catalysed, and N/C based lacquers
  have been partially replaced by acid catalysed
  and / or polyurethanes systems.
• Because of perceived concerns over the spraying
  of isocyanates, where in many locations
  extraction conditions left something to be
  desired, acid catalysed lacquers were considered
  more favourable.

12
Acid Catalysed Systems

• Acid catalysed coatings are two pack pigmented or
  clear systems, based on alkyd-amino blends, which
  are catalysed with an acid solution.
• These systems are similar in many respects to
  stoving or baking enamels but as they are applied
  to timber furniture components, baking is out of
  the question as heat distortion would result.
• Cure is therefore effected at room temperature by
  the addition of an acid catalyst just prior to
  use and the resultant applied film will cure
  quite rapidly to a hard, firmly adherent,
  abrasion resistant film.

13
ACs - Some technical bits

• Non acid component (traditionally Part A) is
  comprised of a blend of short oil alkyd or
  alkyds, with urea, and / or melamine formaldehyde
  resins.
• Alkyd/amino ratios can vary considerably from
  55-45 to 75-25, depending on film properties
  required. Normally a 60-40 blend provides a very
  good compromise.
• UF/MF blends tend to favour the UF along a range
  of 5-1 down to 3-1 with higher melamine
  contributing to better moisture resistance but
  higher cost.
• Simple mineral acids such as HCL and H2SO4,
  organic acids such as PTSA, or the use of the
  many proprietary sulphuric acid derivatives such
  as DNNDSA Di Nonyl Naphthalene Di Sulphonic
  Acid (NACURE 155), are used as catalysts.

14
AC Systems

• Topcoats are used in conjunction with an
  appropriate acid catalysed U/C or sealer to
  attain firmly adhering, chip resistant coatings
  which offer a good coating life.
• Acid catalysed finishes can be buffed to give a
  very high gloss finish, though approximately 90
  of furniture in NZ is finished in low gloss
  levels with a satin (30) gloss being the most
  popular.
• (Buffing process is to sand back with 1200 grit
  paper, then applying buffing and polishing
  compound with a mechanical buff and polishing up
  a nice high gloss level.)
• In any acid catalysed reaction, formaldehyde is
  evolved, but in latter years determined efforts
  to reduce free and emitted formaldehyde have
  resulted in a considerable reduction in emission
  levels.

15
Polyurethane Systems

• Two pack fast drying polyurethane clear sealers
  and topcoats are used in considerable quantity by
  furniture applicators.
• These systems are modified to give near lacquer
  dry characteristics, so as to reduce dirt pick up
  to an absolute minimum. Touch dry times of 20
  min are generally achievable and quick, sanding
  turnaround of two pack sealers is possible.
• Normal urethane systems are based on modified oil
  free polyester/aromatic or aliphatic isocyanate
  mixture, opting for the most reactive types to
  ensure rapid speed of dry. Where good colour
  retention is required, predominantly aliphatic
  isocyanates are used, but drying speed is
  retarded.
• A range of gloss levels are provided in this
  range also.

16
PUs - Some important bits

• Proper air extraction systems, and equipment such
  as air fed masks are mandatory and are well worth
  the inconvenience when the finished properties
  are considered.
• Two pack urethane sealers are particularly
  popular in damp areas such as kitchens and
  bathrooms where a highly moisture resistant seal
  is required. Once this efficient moisture
  barrier is in place, single pack lacquers or acid
  catalysed coatings can be applied.
• It is particularly important to apply a two pack
  urethane sealer to rimu as the dimensional
  instability of this indigenous timber will lead
  to a whitening effect, particularly along open
  grain door edges, if moisture ingress and egress
  is not curtailed.

17
Polyesters

• Polyester systems are naturally of significance
  where high build coatings are required.
• These are relatively normal, styrene thinned
  products catalysed with peroxide catalysts in the
  presence of cobalt accelerators.
• A high proportion of casket manufacturers apply
  polyester clears because of the unsurpassed deep
  shine and build that can be obtained. Wax free
  polyesters are used in these high gloss systems.

18
UV Light Cured

• Also known as radiation cure, with the first
  industrial use in 1967.
• Huge use of this technology in printing (inks and
  associated coatings).
• Extremely fast reacting product that solidifies
  only when three reactive components are
  simultaneously present.
• 1. Organic resin(s) with the ability to
  chemically cross link to produce a tough durable
  coating.
• 2. Photo-initiators which produce the active
  radicals which trigger polymerisation.
• 3. The energy derived from the high intensity
  ultra-violet light source.

19
UV and -

• For UV curable coatings
• short cure time
• non-polluting
• low VOC content
• excellent coating quality
• less material and less energy.
• Against UV curable coatings
• Expensive investment in equipment
• Difficulties with pigmented systems (not
  insurmountable)
• Exterior durability (e.g. typically lower than 2K
  PU)

20
UV vs. Conventional coatings

• Conventional coatings fall roughly into two
  categories
• Non Convertible / Thermoplastic, e.g.
  Nitro-cellulose lacquers. Typically fast drying
  and moderate chemical resistance.
• Convertible / Thermoset, e.g. ACs and PUs.
  Slower drying / curing speed, but greatly
  improved chemical resistance properties.
• UV cured coatings have both the benefits of very
  fast dry / cure speeds, and excellent chemical
  resistance properties.

21
Some Effects

• Stains
• Oiled Timber
• Hammer Finish
• Texture
• Metallic
• Crackle Finish